
==== Front
Inorg Chem
Inorg Chem
ic
inocaj
Inorganic Chemistry
0020-1669
1520-510X
American Chemical Society

39163584
10.1021/acs.inorgchem.4c02275
Article
A Caged Neutral 17-Valence-Electron Iron(I) Radical [Fe(CO)2(Cl)(P((CH2)10)3P)]•: Synthetic, Structural, Spectroscopic, Redox, and Computational Studies
https://orcid.org/0000-0001-6426-568X
Zarcone Samuel R. †
https://orcid.org/0000-0003-0976-1845
Zhang Zihan ‡
Handunneththige Suhashini †
https://orcid.org/0000-0002-8216-3748
Ni Zhen †
Bhuvanesh Nattamai †
https://orcid.org/0000-0003-1091-4677
Nippe Michael *†
https://orcid.org/0000-0002-7844-2998
Meyer Karsten *‡
https://orcid.org/0000-0003-3263-3219
Hall Michael B. *†
https://orcid.org/0000-0002-7012-4872
Gladysz John A. *†
† Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012, United States
‡ Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Egerlandstraße 1, 91058 Erlangen, Germany
* Email: gladysz@mail.chem.tamu.edu.
* Email: mbhall@tamu.edu.
* Email: karsten.meyer@fau.de.
* Email: nippe@chem.tamu.edu.
20 08 2024
02 09 2024
63 35 1631316326
01 06 2024
26 07 2024
23 07 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by/4.0/ Permits the broadest form of re-use including for commercial purposes, provided that author attribution and integrity are maintained (https://creativecommons.org/licenses/by/4.0/).

UV irradiation of yellow CH2Cl2 solutions of trans-Fe(CO)3(P((CH2)10)3P) (2a) and PMe3 (10 equiv) gives, in addition to the previously reported dibridgehead diphosphine P((CH2)10)3P (46%), a green paramagnetic complex that crystallography shows to be the trigonal-bipyramidal iron(I) radical trans-[Fe(CO)2(Cl)(P((CH2)10)3P)]• (1a•; 31% after workup). This is a rare example of an isolable species of the formula [Fe(CO)4–n(L)n(X)]• (n = 0–3, L = two-electron-donor ligand; X = one-electron-donor ligand). Analogous precursors with longer P(CH2)nP segments (n = 12, 14, 16, 18) give only the demetalated diphosphines, and a rationale is proposed. The magnetic susceptibility of 1a•, assayed by Evans’ method and SQUID measurements, indicates a spin (S) of 1/2. Cyclic voltammetry shows that 1a• undergoes a partially reversible one-electron oxidation, but no facile reduction. The UV–visible, EPR, and 57Fe Mössbauer spectra are analyzed in detail. Complex 2a is similarly studied, and, despite the extra valence electron, exhibits a comparable oxidation potential (ΔE1/2 ≤ 0.04 V). The crystal structure shows a cage conformation, solvation level, disorder motif, and unit cell parameters essentially identical to those of 1a•. DFT calculations provide much insight regarding the structural, redox, and spectroscopic properties.

Irradiation of a CH2Cl2 solution of the Fe(CO)3 complex that is encased in a triply trans-spanning diphosphine cage affords the title radical, which is a rare example of a stable pentacoordinate iron(I) species. The structures of the radical and its precursor are compared crystallographically and computationally. The oxidation potentials are surprisingly close, although molecular orbital analyses provide insight. The radical is further characterized by UV−visible, SQUID, EPR, and Mössbauer measurements.

National Science Foundation 10.13039/100000001 CHE-1566601 Friedrich-Alexander-UniversitÃ&#131;Â¤t Erlangen-NÃ&#131;Â¼rnberg 10.13039/501100001652 NA Alexander von Humboldt-Stiftung 10.13039/100005156 NA Welch Foundation 10.13039/100000928 A-0648 National Science Foundation 10.13039/100000001 CHE-1900549 document-id-old-9ic4c02275
document-id-new-14ic4c02275
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pmcIntroduction

For a variety of reasons, the synthesis, isolation, and study of organometallic radicals has lagged behind that of diamagnetic species.1 This dichotomy also extends to computational investigations.2,3 Among the many classes of interest, considerable attention has been given to 17-valence-electron pentacoordinate iron(I) carbonyl radicals (Figure 1). Teams spearheaded by Connelley, Baird, and Krossing have reported the synthesis and extensive spectroscopic and structural characterization of the cationic complexes trans-[Fe(CO)3(PPh3)2]•+PF6– and [Fe(CO)5]•+Al(OC(CF3)3)4–.4,5 Berke has isolated neutral adducts of the formula trans-[Fe(CO)2(X)(PX′3)2]• (X/X′ = OiPr/Br, OiPr/I, Et/Br, Et/I),6 and related species have been detected in situ.6−8 Additional isolable neutral pentacoordinate iron carbonyl radicals are illustrated in Figure 1.9

Figure 1 Some previously isolated pentacoordinate iron(I) carbonyl radicals.

Given the innate reactivity of most types of radicals, chemists often seek to sterically shield them. In this context, the P(OiPr)3-substituted radicals in Figure 1 are much more stable than the P(OMe)3 homologues,6 in accord with the greater phosphite ligand cone angle. Similar relationships have been established for numerous pairs of organometallic radicals.6,10 Some recent dramatic examples involve bulky meta-terphenyl isocyanide ligands that contain multiple isopropyl or trifluoromethyl substituents, which enable the crystallization of the pentacoordinate Group 7 complexes [M(CO)(CNAr)4]• (M = Mn, Tc, Re).10 However, comparably enveloping steric environments have not yet been applied to pentacoordinate Group 8 radicals.

In this paper, we report the (1) serendipitous isolation, (2) structural, spectroscopic, and electrochemical characterization, and (3) computational analysis of the neutral pentacoordinate iron dicarbonyl chloride radical trans-[Fe(CO)2(Cl)(P((CH2)10)3P)]• (1a•). The stability is attributed to a new motif of steric protection afforded by a cage-like, triply trans-spanning diphosphine ligand. Furthermore, the mechanistic sequences involved are believed to involve topologically unusual steps tantamount to turning a molecule inside out. The many unique physical properties of 1a• are thoroughly interpreted, often with the aid of the DFT calculations.

Results

Synthesis of the Title Complex

The iron tricarbonyl complexes 2 in Scheme 1 have been under study in the group of one author for some time.11 They feature trans-spanning diphosphines with three P(CH2)nP tethers, the lengths of which can be varied. It was recently found that irradiation with a Hanovia mercury lamp (450 W) in the presence of excess PMe3 in hexanes or CH2Cl2 afforded the corresponding free dibridgehead diphosphines 3.12,13 These could be isolated as white solids in 46–77% yields. All are capable of homeomorphic isomerization, a topological process that turns the molecules inside out, equilibrating in,in and out,out isomers without the need for pyramidal inversion of the phosphorus bridgeheads. This is illustrated in Scheme 2, and a video is available in the Supporting Information of a recent publication.14

Scheme 1 Photochemical Conversions of 2a–e to Dibridgehead Diphosphines 3a–e and/or the Iron(I) Radical 1a•

Scheme 2 Homeomorphic Isomerization of Dibridgehead Diphosphines 3a–e

As further elaborated on in the Discussion section, the Fe(CO)3 moiety is believed to escape by an initial iron–phosphorus bond cleavage, followed by homeomorphic isomerization of the diphosphine to give an adduct of out,out-3 and Fe(CO)3. Subsequent attack of PMe3 would displace the η1-diphosphine from iron. Although a small amount of trans-Fe(CO)3(PMe3)2 can be detected during photolyses, the conversion never exceeds 5%, presumably because this complex is itself photoactive under these conditions.

When 2b–e were photolyzed in hexane or CH2Cl2, the yellow solutions became cloudy orange.15 However, CH2Cl2 solutions of 2a turned green.16 Workup afforded a paramagnetic green material in 31% yield based upon the structure established below (3a was also produced). The IR spectrum exhibited two strong νCO bands (1944 and 1861 cm–1) at higher frequencies than those of the precursor (1853 and 1841 cm–1), and microanalysis supported the formulation trans-[Fe(CO)2(Cl)(P((CH2)10)3P)]• (1a•). A mass spectrum showed a strong ion corresponding to [M – 2(CO)]+. As illustrated in Figure s12, solid 1a• decomposed over the course of 20 h in air. Solutions decomposed over the course of 1.5 h.

Crystallography

Single crystals of a hexane hemisolvate of 1a• could be grown, and the X-ray structure was solved as described in Table 1 and the Experimental Section. The (CH2)10 segments exhibited two conformations, which could be modeled by a 59:41 occupancy ratio. Thermal ellipsoid plots of the dominant isomer are provided in Figure 2, and bond lengths and angles about iron are summarized in Table 2. The trigonal-bipyramidal geometry was evidenced by a P–Fe–P angle of 178.53(5)°, P–Fe–CO and P–Fe–Cl angles ranging from 89.65(7)° to 91.64(7)°, and OC–Fe–CO and Cl–Fe–CO angles ranging from 107.5(1)° to 126.57(8)°.

Table 1 Summary of Crystallographic Data

 	1a•·0.50C6H14	2a·0.45C6H14	
empirical formula	C35H67ClFeO2P2	C35.7H66.3FeO3P2	
formula weight	673.12	661.37	
temperature [K]	110.0	100.00	
diffractometer	Bruker Venture	XtaLAB Synergy, Dualflex, HyPix	
wavelength [Å]	0.71073	1.54184	
crystal system	monoclinic	monoclinic	
space group	P21/c	P21/c	
unit cell dimensions	 	 	
a [Å]	11.4017(6)	11.45057(5)	
b [Å]	11.7236(6)	11.76785(6)	
c [Å]	27.9438(14)	28.05972(13)	
α [deg]	90	90	
β [deg]	96.4986(19)	96.6447(4)	
γ [deg]	90	90	
V [Å3]	3711.2(3)	3755.61(3)	
Z	4	4	
ρcalc [Mg m–3]	1.205	1.170	
μ [mm–1]	0.593	4.252	
F(000)	1464	1422	
crystal size [mm3]	0.249 × 0.082 × 0.048	0.44 × 0.28 × 0.12	
θ limit [deg]	1.467–25.729	3.171–70.073	
index range (h, k, l)	–13/13, –14/13, –34/34	–14/14, –13/13, –35/35	
reflections collected	70838	69531	
independent reflections	7060	7106	
R(int)	0.0493	0.0350	
completeness to θ [%]	100.0	99.8	
max and min transmission	0.4684 and 0.2960	1.000 and 0.225	
data/restraints/parameters	7060/1284/534	7106/159/633	
goodness-of-fit on F2	0.866	1.052	
R indices (final) [I > 2σ(I)]	 	 	
R1	0.0779	0.0377	
wR2	0.1813	0.0938	
R indices (all data)	 	 	
R1	0.0794	0.0389	
wR2	0.1824	0.0947	
largest diff peak and hole [e Å–3]	0.864 and −0.534	0.579 and −0.287	

Figure 2 Thermal ellipsoid plots of the molecular structures of 1a•·0.5C6H14 (top) and 2a·0.45C6H14 (bottom) (50% probability levels, dominant conformations, hydrogen and solvate atoms omitted).

Table 2 Selected Crystallographic Distances (Å) and Angles (deg) for 1a• and 2a and the Corresponding Computational Data for 1a′•, 2a′, and PMe3-Substituted 7′•, 7′–, and 8′

 	1a•·0.5C6H14	1a′•	2a·0.45C6H14	2a′	7′•	7′–	8′	
P···P	4.517(1)	4.61	4.4323(7)	4.52	4.52	4.39	4.45	
Fe–P(1)	2.2603(10)	2.31	2.2199(5)	2.26	2.27	2.20	2.23	
Fe–P(2)	2.2577(11)	2.31	2.2128(5)	2.27	2.27	2.20	2.23	
Fe–C(31)	1.793(5)	1.78	1.764(2)	1.77	1.79	1.73	1.77	
Fe–C(32)	1.802(5)	1.79	1.763(2)	1.77	1.79	1.73	1.77	
Fe–C(33)	 	 	1.768(2)	1.77	 	 	1.77	
Fe–Cl	2.3128(12)	2.36	 	 	2.36	2.60	 	
Fe–O(1)	2.866(4)	2.94	2.922(1)	2.93	2.94	2.89	2.93	
Fe–O(2)	2.897(4)	2.94	2.925(2)	2.93	2.94	2.89	2.93	
Fe–O(3)	 	 	2.928(2)	2.93	 	 	2.93	
C(31)–O(1)	1.074(5)	1.15	1.157(2)	1.16	1.15	1.18	1.16	
C(32)–O(2)	1.095(5)	1.16	1.162(2)	1.16	1.15	1.18	1.16	
C(33)–O(3)	 	 	1.160(2)	1.16	 	 	1.16	
C(1)–P(1)a	1.814(5)	1.86	1.903(4)	1.86	1.84	1.85	1.84	
C(2)–P(1)a	1.843(5)	1.86	1.837(4)	1.86	1.84	1.85	1.84	
C(3)–P(1)a	1.850(6)	1.85	1.804(4)	1.86	1.84	1.85	1.84	
C(4)–P(2)a	1.835(6)	1.86	1.878(5)	1.86	1.84	1.85	1.84	
C(5)–P(2)a	1.839(6)	1.86	1.821(5)	1.86	1.84	1.85	1.84	
C(6)–P(2)a	1.821(7)	1.85	1.768(4)	1.86	1.84	1.85	1.84	
P(1)–Fe–P(2)	178.53(5)	176.3	178.18(2)	176.9	173.0	170.8	179.9	
P(1)–Fe–C(31)	89.7(1)	 	89.52(6)	 	 	 	 	
P(1)–Fe–C(32)	91.0(1)	 	91.31(6)	 	 	 	 	
P(1)–Fe–C(33)	 	 	89.98(6)	 	 	 	 	
P(1)–Fe–Cl	89.89(4)	88.76	 	 	86.48	85.38	 	
P(2)–Fe–Cl	89.83(4)	88.47	 	 	86.48	85.38	 	
P(2)–Fe–C(31)	89.3(2)	 	88.94(6)	 	 	 	 	
P(2)–Fe–C(32)	90.3(1)	 	90.31(6)	 	 	 	 	
P(2)–Fe–C(33)	 	 	89.97(6)	 	 	 	 	
C(32)–Fe–C(31)	108.3(2)	 	120.63(9)	 	 	 	 	
C(32)–Fe–C(33)	 	 	118.55(10)	 	 	 	 	
C(33)–Fe–C(31)	 	 	120.82(9)	 	 	 	 	
C(31)–Fe–Cl	126.2(2)	 	 	 	 	 	 	
C(32)–Fe–Cl	125.5(1)	 	 	 	 	 	 	
a For these distances, the carbon atom numbers differ from those in the CIF file.

Complex 2a, which differs from 1a• only by a CO/Cl replacement (identical tether lengths), was viewed as an especially valuable reference compound. Accordingly, the crystal structure of a hexane solvate of 2a was solved. Again, the (CH2)10 segments exhibited two conformations, which in this case was best modeled by a 54:46 occupancy ratio. The thermal ellipsoid plots are presented side-by-side with those of 1a• in Figure 2. As can be seen in Table 1, the space group, crystal system, and Z value of 2a and 1a• are identical (P21/c, monoclinic, 4), and the unit cell dimensions vary by less than 0.4%, showing the lattices to be essentially isostructural.17

As would be intuitively expected from the reduced number of valence electrons and presumably attenuated back-bonding in 1a•, the Fe–C bonds [1.793(5) and 1.802(5) Å] are longer than those of 2a [1.764(2), 1.763(2), and 1.768(2) Å], 2c [1.761(3) and 2 × 1.764(2) Å], and other closely related 18-valence-electron Fe(CO)3 adducts.11a The CO bond lengths [1.074(5) and 1.095(5) Å] are in turn shorter [2a, 1.157(2), 1.160(2), and 1.162(2) Å; 2c, 1.162(3) and 2 × 1.164(3) Å]. It merits note in passing that complexes of the formula Fe(CO)3(PPh2R)2 can additionally adopt square pyramidal geometries,18 and that isomerization can take place upon oxidation.19

Additional Physical Characterization

UV–visible spectra of 1a• and 2a were recorded in CH2Cl2. As depicted in Figure 3, 1a• exhibits a moderately intense band at 382 nm (ε 1230 M–1 cm–1) superimposed on an absorption tail, and a broader and weaker band at 692 nm (ε 490 M–1 cm–1). The latter is of course primarily responsible for the green color. The precursor 2a displays a shoulder of modest intensity at 359 nm (ε 510 M–1 cm–1), and did not absorb above 450 nm. The underlying electronic transitions are defined with the aid of TD-DFT calculations below.

Figure 3 UV–visible spectra (8.50–8.60 × 10–5 M in CH2Cl2) of 1a• (λmax (nm) [ε (M–1 cm–1)]: 382 [1,230], 693 [480] and 2a (λmax (nm) [ε (M–1 cm–1)]: 359 [510]).

Next, the magnetic susceptibility (χ) of 1a• was determined by Evans’ method. The value, μeff 1.67 μB, is typical of complexes with a spin (S) of 1/2. Two VT SQUID measurements were made with powdered samples. Both gave identical results, as depicted in Figure 4 (top), indicating a magnetic moment (μB) ranging from 1.76 (300 K) to 1.63 (2 K).

Figure 4 Top: VT SQUID magnetization data for 1a•; Bottom: CW X-band EPR spectrum of a 3 mM CH2Cl2 solution of 1a• at 293 K (black trace), an analogous frozen solution at 95 K (red trace), a powdered sample at 95 K (green trace), and a crystalline sample at 95 K (blue trace). The simulation of the liquid solution data revealed the following EPR parameters: giso = 2.05 and Aiso = (31P, n = 2, I = 1/2, 100%) 23.7 × 10–4 cm–1 (Figure s2).

As shown in Figure 4 (bottom), CW X-band EPR spectra of 1a• were recorded under several conditions. An isotropic spectrum in liquid CH2Cl2 at 293 K (black trace) exhibited a triplet due to 31P coupling with a giso value of 2.05 and a superhyperfine coupling constant (Aiso) of 2.48 mT (23.7 × 10–4 cm–1). A spectrum of the corresponding frozen glass (95 K) also gave a signal with a giso value of 2.05, but without coupling (red trace).

Next, zero-field 57Fe Mössbauer spectra of solid samples of 1a• and 2a were recorded at 77 K. As shown in Figure 5 (top), 1a• exhibits a positive isomer shift (IS or δ) of 0.14 mm s–1, a conspicuously small quadrupole splitting (QS) of 0.29 mm s–1, and a line width (Γ) of 0.34 mm s–1. In contrast, 2a (bottom) displays a negative isomer shift of −0.12 mm s–1, a larger quadrupole splitting of 2.38 mm s–1, and a line width of 0.27 mm s–1. These properties are summarized in Tables 3 and 4, together with the experimental data from the literature20−22 and DFT computational results23 described and interpreted below.

Figure 5 Zero field 57Fe Mössbauer spectrum of solid 1a• (top) and 2a (bottom) at 77 K and referenced to α-iron. Collected data are represented by black circles, and additional data are in Tables 3 and 4.

Table 3 Computed and Experimental 57Fe Mössbauer Quadrupole Splittings (QS) of Selected Complexes

 	complex	calc. QS (mm s–1)	exp. QS (mm s–1)	
1a•	trans-[Fe(CO)2(Cl)(P((CH2)10)3P)]•	0.74	0.29	
2a	trans-Fe(CO)3(P((CH2)10)3P)	2.55	2.38	
7′•	trans-[Fe(CO)2(Cl)(P(CH3)3)2]•	0.71	 	
7′–	trans- [Fe(CO)2(Cl)(P(CH3)3)2]−	2.33	 	
8′	trans-Fe(CO)3(P(CH3)3)2	2.60	 	
 	trans-Fe(CO)3(PCy3)2	 	2.7521	
 	trans-Fe(CO)3(PPh3)2	3.01	2.7620	
 	Fe(CO)5	2.30	2.555	
 	[Fe(CO)5]•+Al(OC(CF3)3)4–	0.47a	0.535	
a Calculated for the cation Fe(CO)5•+.

Table 4 Computed and Experimental 57Fe Mössbauer Isomer Shifts (IS) of Selected Complexes

 	complex	electron density (a.u.–3)	calc. IS (mm s–1)	exp. IS (mm s–1)	
1a•	trans-[Fe(CO)2(Cl)(P((CH2)10)3P)]•	11606.45	0.12	0.14	
2a	trans-Fe(CO)3(P((CH2)10)3P)	11607.58	–0.14	–0.12	
7′•	trans-[Fe(CO)2(Cl)(P(CH3)3)2]•	11606.31	0.16	 	
7′–	trans-[Fe(CO)2(Cl)(P(CH3)3)2]−	11606.87	0.02	 	
8′	trans-Fe(CO)3(P(CH3)3)2	11607.48	–0.12	 	
 	trans-Fe(CO)3(PCy3)2	 	 	–0.12521,22	
TR1a	trans-Fe(CO)3(PPh3)2	11607.47	–0.12	–0.10520,22	
 	 	 	 	–0.15121,22	
TR2a	Fe(CO)5	11607.36	–0.09	–0.085	
TR3a	[Fe(CO)5]•+Al(OC(CF3)3)4–	11606.25b	0.17	0.175	
a TR1,TR2, and TR3 are training sets. Parameters for the isomer shift (δ) were calculated using the equation δ = α[ρ(0) – C] + β, where α, β, and C are constants and ρ(0) is the electron density.23 The electron densities of the compounds comprising the training set were graphed versus their experimental isomer shifts, resulting in α = −0.2356 and β = 0.2296 (C was set at a constant of 11606.00).

b Calculated for the cation Fe(CO)5•+.

Cyclic voltammograms of 1a• and 2a are depicted in Figure 6. These were recorded in CH2Cl2 under the standard conditions summarized in the caption. Each gave a partially reversible oxidation with ic/ia values of 0.42 and 0.75, respectively. The E1/2 values (vs Fc0/+) show that the oxidation of 1a•, which would give the 16-valence-electron species 1a+, is only slightly less thermodynamically favorable (with respect to any arbitrary oxidizing agent) than that of 2a to give 2a•+ (0.02 vs −0.01 V; ΔE1/2 = 0.03 V). A replicate determination on another apparatus but at 200 instead of 100 mV s–1 gave ic/ia values of 0.70 and 0.88 and a ΔE1/2 of 0.04 V (Figure s10). Surprisingly, neither complex underwent reduction when scans were extended to −2.0 V. Computational insight is provided below.

Figure 6 Cyclic voltammograms of 1a• (top) and 2a (bottom). Conditions: 1.0 mM in 0.10 M n-Bu4N+PF6–/CH2Cl2 under argon, 23 ± 1 °C; 3 mm glassy carbon working electrode, Pt wire counter electrode, and Ag wire reference electrode; scan rate, 100 mV s–1. All scans were continued to −2.0 and 1.0 V, but no additional features were observed.

Computational Characterization

To help interpret the preceding data, DFT calculations were carried out using standard functionals and protocols extensively benchmarked for the electrochemical and Mössbauer calculations, as detailed in the Experimental Section. For computational structures and data, molecules are designated with primes (1a′• and 2a′). To provide additional perspective, three acyclic model complexes with PMe3 ligands were also studied: the 17-valence-electron radical trans-[Fe(CO)2(Cl)(PMe3)2]• (7′•), the corresponding 18-valence-electron anion trans-[Fe(CO)2(Cl)(PMe3)2]− (7′–), and the 18-valence-electron adduct trans-Fe(CO)3(PMe3)2 (8′).24 The first two represent experimentally unknown compounds, although 7′• has been proposed as a transient,7 as described in the Discussion section.

As illustrated in Figure 7, gas phase energy minimization afforded structures of 1a′• and 2a′ that were very close to those obtained crystallographically. Bond lengths and angles associated with the central iron atoms and carbonyl ligands are incorporated into Table 2, and the excellent agreement is apparent. The conformations along the P–Fe–P axes (Figure 2, right) were also nicely reproduced. Figure 8 shows that the corresponding PMe3-substituted model compounds 7′• and 8′ have very similar structures, but now with perfectly staggered P–Fe–P conformations. Interestingly, the anionic 18-electron complex [Fe(CO)2(Cl)(P(CH3)3)2]− (7′–) has a much longer Fe–Cl bond than 7′• (2.60 vs 2.36 Å), while the other bonds to iron are somewhat shorter (0.06–0.07 Å).

Figure 7 DFT-optimized structures for (A) trans-[Fe(CO)2(Cl)P((CH2)10)3P]• (1a′•) and (B) trans-Fe(CO)3P((CH2)10)3P (2a′).

Figure 8 DFT-optimized structures for (A) trans-[Fe(CO)2(Cl)(P(CH3)3)2]• (7′•), (B) trans-[Fe(CO)2(Cl)(P(CH3)3)2]− (7′–), and (C) trans-Fe(CO)3(P(CH3)3)2 (8′).

To help understand the Fe–Cl bond length trends, the spin densities of 7′• and 1a′• were calculated. As can be derived from the former (Figure 9A), the electron lost from 7′– comes from the iron 3dxy orbital, which is antibonding with respect to the chlorine 3px orbital (x-axis perpendicular to the FeP2Cl plane). The spin density motif calculated for the full molecule 1a′• (Figure 9B) is, as expected, very close to that of 7′•. Parts C and D of Figure 9 show the corresponding plot for the HOMO. As expected, the spin density is dominated by the unpaired electron in this orbital, but the spin density shows spin polarization with the opposite spin perpendicular to the majority spin.

Figure 9 Spin density diagrams for trans radical complexes (A) 7′• and (B) 1a′• (left). α-HOMO diagrams for (C) 7′• and (D) 1a′• (right).

A reviewer noted that the experimental Fe–P bonds in 1a• were longer than those in 2a, although one might have expected a shortening due to the oxidized iron having a shorter bond radius in 1a•, and asked if this lengthening was due to decreased π-bonding. While parallel trends have been observed with other complexes of iron and phosphorus donor ligands where the corresponding radical cations have been crystallographically characterized25 a more thorough examination of this issue was undertaken as this same trend is reproduced by the calculations for both the actual complexes, 1a′• and 2a′, and the simplified model complexes 7′• and 8′. The Quantum Theory of Atoms in Molecules (QTAIM)26 provides a quantum mechanically accurate method to calculate atomic charges. The QTAIM results for 8′ and 7′• (Table s1) show that, in spite of 7′• having a higher formal oxidation state than 8′, the atomic charges differ by less than 0.10 electrons. This small change occurs because the increased donation from Cl, a strong σ and π donor, offsets the expected electron loss from the increase in formal oxidation state. Thus, in spite of the increase in formal oxidation state, the bond radius of the iron should be similar. One can gain some insight into the evolution of the π-bonding by examining the bond lengths. Replacing a CO in 8′ with Cl– to produce 7′– causes a substantial increase in the back-bonding of the other ligands and both the Fe–C and Fe–P bond lengths decrease substantially and the C–O and P–C bond lengths increase substantially, a characteristic signature of increased back-bonding. When 7′– is oxidized to produce 7′• the bond length changes described above are reversed, an indication that the back-bonding in 7′– is reduced in 7′•. Relative to 8′, 7′• appears to have somewhat less backbonding.

Table 5 compiles the upper valence molecular orbitals (MOs) for the three model complexes, and Figure 10 illustrates how the energies of the MOs evolve from the 18-electron tricarbonyl 8′ to 7′– and then 7′•. It is helpful to include 7′– in this sequence as 7′• is an unrestricted calculation with different MOs for α and β spins, and 7′– shows how the chloride ligand splits the iron 3d orbitals before they are split further by the unrestricted calculation in 7′•.

Table 5 MOs of Model trans PMe3 Complexes 8′, 7′–, and 7′• with Energies in Atomic Units

Figure 10 MO diagrams for model trans PMe3 complexes 8′, 7′–, and 7′•. The orbitals are depicted in Table 5. The orbitals energies of 7′•α, 7′–, 8′ are aligned at HOMO–3 level, as this is the Fe 3dxz MO that does not interact directly with any Cl orbitals. In spite of the reduction in symmetry, this figure maintains the z axis as the P–Fe–P axis and the x axis as perpendicular to the FeP2Cl plane.

As depicted in Figure 10, 8′ displays the typical symmetry and orbital distribution for a 3d8 trigonal-bipyramidal complex.27 The two most stable iron 3d MOs, HOMO–2 and HOMO–3, are only π bonding with the carbonyl ligands. The two higher energy iron MOs, HOMO and HOMO–1, are π bonding with some of the carbonyls and σ antibonding with others. When one carbonyl ligand is replaced with a chloride ligand, the degeneracy of both of these pairs is broken. Comparison of HOMOs of 8′ and 7′– demonstrates that the strong destabilization of two MOs in 7′– is due to the π antibonding character with the chloride ligand. When a β electron is removed from the HOMO of 7′–, the MO splits and the energy of the β MO increases, becoming the LUMO of 7′•β. All of the MOs in 7′•β are higher in energy due to the loss of an exchange integral, but they follow the same order as 7′•α.

Attention was next turned to the cyclic voltammetry data in Figures 5 and s10. First, consider the oxidation of 2a by 1a+ as expressed in eq 1:1

The sum of the E1/2 values for the half-reactions (+0.03 to +0.04 V) predict that it should be exergonic by −0.69 to −0.92 kcal mol–1, as calculated from the Nernst equation. Thus, the energies of each of the four species (i.e., 2a′/1a′+/2a′•+/1a′•) were calculated as described in the Experimental Section. As summarized in Table 6, when the geometries were optimized with solvent corrections, the reaction was computed to be exergonic in CH2Cl2 by −0.68 kcal mol–1 in accord with the experimental data. However, the process was predicted to be slightly endergonic in the gas phase and to have mild solvent dependency. In spite of 1a′+ having only 16 valence electrons, it has a singlet ground state because the strong π donation from the Cl destabilizes the HOMO of 1a′• such that removal of its remaining electron offsets the alternative of removing the electron from dx2–y2 orbital that is strongly stabilized by π-back-bonding of the two CO ligands.

Table 6 Computational Dataa for the Oxidation of 2a′ by 1a′+ (See eq 1)b

description	kcal mol–1	Ecell	
TPSS gas	+0.53	–0.023	
TPSS-SMD correction; CH2Cl2	+0.10	–0.004	
TPSS-SMD correction; MeCN	–0.19	+0.008	
TPSS-SMD optimization; CH2Cl2	–0.68	+0.029	
a The ground state of 1a′+ is a singlet; see Table s2.

b Experimental range: –0.69 to −0.92 kcal mol–1.

Next, attention was focused on modeling the UV–visible spectra in Figure 3 with TD-DFT. The spectra computed for 1a′• and 2a′ are depicted in Figure 11. No intensity is observed for 2a′ in the visible region, consistent with the white to pale yellow color of 2a and its experimental UV–visible spectrum (Figure 3). However, 1a′• exhibits four moderately strong absorptions in the blue and orange/red regions, consistent with the green color of 1a• and its UV–visible spectrum. As expected, the PMe3-substituted model complexes 7′• and 8′ show very similar spectra, as illustrated in Figure s11.

Figure 11 TD-DFT UV–visible spectra for (A) 1a′• and (B) 2a′. The visible transitions are mainly due to the minority spin: 1 is mainly HOMO–1 to LUMO, 2 and 3 are mainly HOMO–2 to LUMO, and 4 is HOMO to LUMO+1; see Table s3 for details.

Discussion

Mechanism of Formation of 1a•

Most of the pentacoordinate 17-valence-electron iron(I) carbonyl radicals in Figure 1 were accessed by simple oxidative or reductive pathways. In contrast, the title complex 1a• is prepared by the photolysis of an iron carbonyl phosphine complex (2a) in the absence of conventional oxidizing or reducing agents (Scheme 1). Such photolyses can labilize either the carbonyl or phosphine ligands,28 and the dichloromethane solvent is the only possible source of the chlorine atom in 1a•. Over the last 40 years, the coordination chemistry of dichloromethane has been extensively developed, evolving from conjecture29 to spectroscopically characterized intermediates30 to isolable adducts.31 Thus, the postulation of an initial adduct 4a (Schemes 1 and 3) seems plausible.

Scheme 3 Mechanism Proposed for Conversions of the Iron Tricarbonyl Complexes 2a-e to the Free Diphosphines 3a-e and the Title Complex 1a•

This would be followed by inner sphere chlorine atom transfer by one of several mechanistic variants,32 generating the iron-centered radical 1a• and the carbon-centered radical •CH2Cl. Indeed, many dichloromethane complexes undergo carbon–chlorine bond cleavage, but with the next observable being an addition product of the type LyM(Cl)(CH2Cl).30b However, this might be suppressed by the cage-like diphosphine ligand. An experimental and computational study involving the transient 18-valence-electron complex (η5-C5H5)Re(CO)2(ClCH2Cl) implicated subsequent thermal conversion to the 17-valence-electron species [(η5-C5H5)Re(CO)2(Cl)]•.33

None of the other dibridgehead diphosphine complexes 2b-e (Scheme 1) give an analogous iron(I) chloride complex upon photolysis. As shown in Scheme 3, the mechanism of demetalation of 2a-e to the free dibridgehead diphosphines 3a-e is believed to involve initial iron–phosphorus bond cleavage and homeomorphic isomerization to yield the η1-diphosphine adducts 5a-e. Since 2a has the smallest diphosphine cage, its conversion to 5a would be expected to be slower. Indeed, the barriers to isomerization from out,out-3a,b,c to in,in-3a,b,c decrease from 27.6 to 14.9 to 13.7 kcal mol–1 (ΔG⧧353 K).12 Thus, it would be no surprise if CO/CH2Cl2 photosubstitution of 2a to give the dichloromethane complex 4a and then 1a• were to become competitive (Scheme 3).

A less direct pathway to 1a• would involve a CO/CH2Cl2 substitution of 5a, followed by intermediates of the types 6a and 7a (Scheme 3). If this sequence were operative, it would seem likely that the 5b-e generated from 2b-e would undergo analogous substitutions, and ultimately form some 1b-e• as byproducts. However, there has been no evidence for the generation of significant quantities of these species.

Redox Properties of 1a•

As illustrated by the cyclic voltammograms in Figures 6 and s10, 1a• can undergo a partially reversible oxidation to the 16-valence-electron cation 1a+ in CH2Cl2. There is seemingly the possibility for dichloromethane coordination to 1a+ as well. The octahedral coordination geometry that would result has abundant precedent.11a,34,35 Interestingly, no evidence was seen for reduction of 1a•, including cathodic scans out to −2.0 V, close to the solvent-imposed limit. Reduction would presumably yield 1a–, an 18-valence-electron species analogous to computationally characterized 7′–.

The E1/2 value for the couple 1a•/1a+ (0.020 V) is also a puzzle. This is only slightly greater than that for the tricarbonyl complexes 2a/2a+ (−0.010 V), each of which have one additional valence electron. Nonetheless, the oxidation of 1a• is only slightly less thermodynamically favorable than 2a, consistent with the DFT data in Table 6. As shown in Figure 10, when the strong π acceptor, CO, is replaced by the strong π donor, Cl–,36 two of the iron d orbitals are markedly destabilized. The HOMO is so strongly destabilized that the 17-electron neutral species becomes more stable than the 18-electron anion. Loss of an electron from the HOMO of the 18-electron anion stabilizes the remaining electron in this MO; now its orbital energy is nearly identical to that of the HOMO of the 18-electron tricarbonyl system. Accordingly, the E1/2 value for the 1a•/1a+ couple is only slightly different from that of the 2a/2a+ couple. However, the E1/2 values for couples involving higher homologues of the tricarbonyl complexes, 2b/2b+ and 2c/2c+, indicate much more thermodynamically favorable oxidations (−0.136 and −0.146 V).11a,37 Thus, there is a significant cage size effect.

Other Pentacoordinate Iron(I) Radicals

Of the isolable iron carbonyl radicals in Figure 1, the four reported by Berke,6trans-[Fe(CO)2(Br)(P(OiPr)3)2]•, trans-[Fe(CO)2(I)(P(OiPr)3)2]•, trans-[Fe(CO)2(Br)(PEt3)2]•, and trans-[Fe(CO)2(I)(PEt3)2]•, are the most relevant. All are especially air-sensitive as solids, whereas 1a• survives more than 6 h (Figure s12). The PEt3 adducts give IR νCO bands (X = Br, 1955/1884 cm–1; I, 1956/1886 cm–1) comparable to those of 1a• (1944/1861 cm–1). Using similar synthetic approaches as Berke, others later attempted the isolation of our computational model complex trans-[Fe(CO)2(Cl)(PMe3)2]• (7′•).7 The green material, characterized by IR (1984/1935 cm–1), was reported to be thermally sensitive and rapidly react with air, and could never be obtained in pure form. These observations vividly illustrate the stabilization imparted by the dibridgehead diphosphine cage of 1a•. Coincidentally, Berke has also studied 7′• by DFT, but communicated only a few structural parameters.38

Berke also reported the EPR spectra of his four radicals, as well as P(OMe)3 analogs that could not be isolated in pure form. The two PEt3 adducts give g and A0 values (2.057/2.55 and 2.078/2.40) very close to those of 1a• (2.05/2.48). The radical trans-[Fe(CO)2(I)(PEt3)2]• could furthermore be crystallographically characterized, and extended Hückel calculations were reported for the PH3 analog.6 In several cases, Berke could reduce his radicals with Na/Hg under nitrogen and obtain diamagnetic adducts with Fe–N2–Fe linkages. Under similar conditions using sodium, no reaction was observed with 1a•, in parallel with the cyclic voltammetry data.

The Mössbauer data for 1a• and 2a in Figure 5 can be compared to those of the 18-valence-electron complexes trans-Fe(CO)3(L)2 (L = CO, PPh3, PCy3), as summarized in Tables 3 and 4. However, among the iron-based radicals in Figure 1, only [Fe(CO)5]•+Al(OC(CF3)3)4– has been characterized by Mössbauer spectroscopy. In all cases, the DFT calculations give quadrupole splittings (QS, Table 3) and isomer shifts (IS, Table 4) that are in excellent agreement. Also, the computational data for the PMe3 model compounds are very close to those of their experimental counterparts.

The most interesting Mössbauer comparisons involve the data for 1a• and 2a versus those of [Fe(CO)5]•+Al(OC(CF3)3)4– and Fe(CO)5. In both pairs of complexes, the paramagnetic species exhibits a dramatically lower quadrupole splitting (0.29 vs 2.38 mm s–1 and 0.53 vs 2.55 mm s–1), with that of 1a• being conspicuously small. The larger splitting of the diamagnetic iron(0) complexes arises from the d8 configurations, which, as compared to a spherical d10 atom, have less electron density along the P–Fe–P axis. When another electron is lost from the d orbital in the (CO)2FeCl plane to give iron(I), the electron density becomes more spherical, reducing the quadrupole splitting.

Also, the isomer shifts of the paramagnetic iron(I) complexes are greater than those of their diamagnetic iron(0) counterparts (0.14 vs −0.12 mm s–1 and 0.17 vs −0.08 mm s–1, respectively). This is opposite to some literature generalizations regarding oxidation state trends [e.g., Fe(0) > Fe(I)].22b,23 However, fuller treatments highlight the independent roles of ligand fields and covalency, coordination numbers, and bond lengths.22b,23a Furthermore, Peters has reported an iron carbonyl triphosphine system that can be isolated in three oxidation states, and his isomer shift trend parallels ours [Fe(II) > Fe(I) > Fe(0)].9c The doublet character and (in the case of 1a•) the strong donation from the chloride ligand would be expected to enhance isomer shifts. So to sum, pairs of closely related pentacoordinate paramagnetic iron(I) and diamagnetic iron(0) complexes appear to give diagnostically different isomer shifts and quadrupole splittings.

Overview and Conclusion

The steric stabilization afforded by the dibridgehead diphosphine cage of the radical 1a• can also be visualized with space-filling representations, as shown in Figure 12. The equatorial CO and Cl ligands are visible only in a “peekaboo” mode, and the tightly fitting methylene chains strongly shield the metal. Despite these favorable factors, at least two potential Achille’s heels remain. One is direct air oxidation (Figure s12), presumably by some outer sphere process. Another would be homeomorphic isomerization, a pathway we consider operative for isosteric 2a as outlined in Scheme 3. This would expose the iron atom, facilitating a variety of possible degradation reactions. For this reason, related complexes with less conformationally flexible phosphorus–phosphorus linkages have been a long-standing synthetic goal of one author.39

Figure 12 Space-filling representations of 1a•.

In conclusion, this work has established a new strategy for the stabilization of organometallic radicals based upon the steric shielding provided by cage-like trans-spanning dibridgehead diphosphine ligands. Importantly, these ligands can also accommodate square planar35,40 and octahedral11a,34,35 coordination geometries, so this approach could have considerable generality. However, such efforts will be facilitated by the development of other synthetic routes, as that used for 1a• (Schemes 1 and 3) is only feasible for one cage size. Nonetheless, the different reaction modes of 2a-e as a function of cage size provide valuable insight regarding homeomorphic isomerization in coordination chemistry. It is also easy to envision substituted dibridgehead diphosphines that provide even more steric shielding for the LyM core, and the analogous dibridgehead diarsines and distibines are also available.13

Safety Statement

Caution! The 450 W photochemical lamp emits considerable heat during use, requiring an external cooling well.12,13 Interruption of the water flow in the quartz cooling well can lead to glass failure, solvent ignition, and other potential hazards. Attached hosing should be checked thoroughly before operation of the lamp. Ultraviolet light produced by the lamp is damaging to biological tissues. When in use, proper PPE should be worn (e.g., UV protective goggles/glasses, lab coat, gloves) to ensure that exposure is minimized. Care should also be taken when using liquid nitrogen for Schlenk line traps to avoid condensation of liquid oxygen from air.

Experimental Section

General Procedures

Reactions and workups were conducted under inert atmospheres. Chemicals were treated as follows: hexanes, CH2Cl2, and toluene, dried and degassed using a Glass Contour solvent purification system; PMe3 (Strem, 98%) used as received; silica gel (40–63 μm mesh, Silicycle) flame-dried and left under vacuum for 1 day before use.

trans-Fe(CO)2(Cl)(P((CH2)10)3P) (1a•)

A flame-dried Schlenk flask was charged with trans-Fe(CO)3(P((CH2)10)3P) (2a; 0.278 g, 0.446 mmol), PMe3 (0.69 mL, 6.69 mmol), and CH2Cl2 (10 mL), and placed in front of a water-cooled quartz immersion well of a Hanovia 450 W lamp. As illustrated with photographs elsewhere,12,13 the sample was irradiated overnight with stirring. The solvent was removed by an oil pump vacuum. The residue was dissolved in hexanes and applied to a small pipet column of silica gel. The column was rinsed with hexanes (eluting 3a) and then CH2Cl2. The dark green fractions were collected, and the solvent was removed by an oil pump vacuum. The residue was washed several times with hexanes and dried under vacuum to give 1a• (0.088 g, 0.140 mmol, 31%) as a green solid, dec pt. 166 °C (open capillary). Anal. Calcd for C32H60ClFeO2P2 (630.07): C, 61.00; H, 9.60. Found: C, 61.24; H, 9.76.

IR (powder film, cm–1): 2922 (m), 2852 (m), 1944 (s, νC≡O), 1861 (s, νC≡O), 1456 (w), 1259 (w), 1074 (w), 1017 (w), 948 (w), 804 (w), 713 (m). HRMS (ESI, m/z): calcd for C30H60P2FeCl [M – 2CO]+: 573.3203, found: 573.3198; calcd for C32H61P2FeO2 [M – Cl + H]+: 595.3491, found: 595.3480.

Computations

Input models were built with Avogadro software. Except as otherwise noted the density functional theory (DFT) calculations were performed with the Gaussian 16 program (revision C.01).41 Geometry optimizations and frequency calculations in the gas phase used the B3LYP42 functional and the 6-311G(d)43 basis set for all atoms. Tight convergence criteria were used for the optimizations. Wave function stability calculations were performed to confirm the absence of lower-energy numerical solutions for all computed structures. Ultrafine grids (99,590 points per atom) were used as implemented in the Gaussian software. Calculations involving redox phenomena were performed with TPSS44 functional with the 6-311G(d) basis set for all atoms. The TPSS functional has performed well for predicting structures and electrochemical properties of other iron and nickel complexes.45 Initially, dichloromethane and acetonitrile solvent corrections with SMD46 were applied to the gas-phase-optimized geometries. The solvent dependency suggested that optimizing the structure in CH2Cl2 would bring its predicted value closer to the experimental one. Mössbauer calculations were done in the gas phase with B3LYP/def2-TZVP47 method/basis set combination in the Orca 5.0.3 software package,48 as recommended.23a

Crystallography

A

A small quantity of 1a• was suspended in hexanes, and CH2Cl2 was added dropwise until the mixture was homogeneous. The sample was allowed to slowly concentrate under argon at −38 °C. After 5 d, green prisms were collected and data obtained per Table 1. Cell parameters were obtained from 90 data frames taken at widths of 1° and refined with 70838 reflections. Integrated intensity information for each reflection was obtained by reduction of the data frames with the program APEX3.49 Lorentz, polarization, and absorption corrections were applied, the last using the program SADABS.50 The space group was determined from systematic reflection conditions and statistical tests. The structure, a hexane hemisolvate, was solved using XT/XS in APEX349,51 and refined (weighted least-squares refinement on F2) to convergence.51,52 Elongated ellipsoids and nearby residual electron density peaks on the methylene chains (C1–C10, C11–C20, and C21–C30) suggested disorder, which was successfully modeled. Further nearby residual electron density peaks, and the larger thermal ellipsoids of C21–C30, indicated additional disorder, but no efforts were made to model this. The occupancy ratios of disordered atoms were first refined individually. Since they were close, they were grouped together, giving a final 59:41 ratio. Appropriate restraints and constraints were added to keep the bond distances, angles, and thermal ellipsoids meaningful. All non-hydrogen atoms were refined anisotropically. Hydrogen atom positions were calculated and refined using a riding model.

B

A hexane/toluene (2:1 v/v) solution of 2a was kept at −20 °C. After 3 d, a yellow block-shaped crystal was collected, which was cut as it appeared to be a multi twin or cracked. Data were obtained as per Table 1. Crystal screening, unit cell determination, and data collection were carried out using a XtaLAB Synergy, Dualflex, HyPix diffractometer. The diffraction pattern was indexed and the total number of runs and images was based on the strategy calculation from CrysAlisPro,53 which was used throughout. The unit cell was refined using 45684 reflections. Integrated intensity information for each reflection was obtained by reduction of data frames within the same software suite, and Gaussian and numerical absorption corrections were similarly applied. A hexane molecule was found, with the C–C midpoint coincident with an inversion center. The occupancy was refined to 0.90, corresponding to 0.45 molecules of solvated hexane per iron atom. A residual electron density peak near C2s suggested disorder of the hexane, which was modeled between two positions with an occupancy ratio of 81:19. Also, elongated or abnormal thermal ellipsoids and/or residual electron density peaks was noted near all the carbon atoms in three hydrocarbon chains except C28. This disorder was modeled between two positions with an occupancy ratio of 54:46. Appropriate restraints and constraints were added to keep the bond distances and thermal ellipsoids meaningful. Systematic reflection conditions and statistical tests afforded the space group (Table 1), as confirmed by ShelXT 2018/254 using dual methods and Olex2–1.5.52 The structure was refined by full matrix least-squares minimization on F2 using version 2019/1 of XL.51 All non-hydrogen atoms were refined anisotropically. Hydrogen atom positions were calculated and refined using a riding model.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c02275.Additional instrumental procedures, spectroscopic, electrochemical, and computational data, and photographs of samples (PDF)

Supplementary Material

ic4c02275_si_001.pdf

The authors declare no competing financial interest.

Acknowledgments

The authors thank the U.S. National Science Foundation (NSF; CHE-1566601 and CHE-1900549 to J.A.G.), Welch Foundation (A-0648 to M.B.H.), and FAU (to K.M.) for financial support. This cooperative interaction between two Texas A&M authors (J.A.G. and S.R.Z.) and two Erlangen authors (Z.Z. and K.M.) was further supported by the Alexander von Humboldt Foundation.
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References

a Kégl T. ; Fortman G. C. ; Temprado M. ; Hoff C. D. Organometallic Radicals: Thermodynamics, Kinetics, and Reaction Mechanisms. In Physical Inorganic Chemistry: Reactions, Processes, and Applications, Bakac A. , Ed.; John Wiley & Sons: New York, 2010; Chapter 10.
b Baird M. C. Seventeen-Electron Metal-Centered Radicals. Chem. Rev. 1988, 88 , 1217–1227. 10.1021/cr00089a011.
Some lead references, diamagnetic complexes:

a Davidson E. R. Computational Transition Metal Chemistry. Chem. Rev. 2000, 100 , 351–818. 10.1021/cr980385s.11749239
b Theoretical and Computational Chemistry, Coord. Chem. Rev. 2003, Volumes 238–239 .
c Vogiatzis K. D. ; Polynski M. V. ; Kirkland J. K. ; Townsend J. ; Hashemi A. ; Liu C. ; Pidko E. A. Computational Approach to Molecular Catalysis by 3d Transition Metals: Challenges and Opportunities. Chem. Rev. 2019, 119 , 2453–2523. 10.1021/acs.chemrev.8b00361.30376310
d Nandy A. ; Duan C. ; Taylor M. G. ; Liu F. ; Steeves A. H. ; Kulik H. J. Computational Discovery of Transition-Metal Complexes: From High-Throughput Screening to Machine Learning. Chem. Rev. 2021, 121 , 9927–10000. 10.1021/acs.chemrev.1c00347.34260198
e Feldt M. ; Phung Q. M. Ab Initio Methods in First-Row Transition Metal Chemistry. Eur. J. Inorg. Chem. 2022, 2022 , e202200024 10.1002/ejic.202200014.
Some lead references, paramagnetic complexes:

a Lin Z. ; Hall M. B. Theoretical Studies of Inorganic and Organometallic Reaction Mechanisms. 5. Rates of Ligand Substitution Reactions of 17e- and 18e- Transition Metal Hexacarbonyl Complexes. Inorg. Chem. 1992, 31 , 2791–2797. 10.1021/ic00039a024.
b Cacelli I. ; Keogh D. W. ; Poli R. ; Rizzo A. Theoretical Study of the 15- and 17-Electron Structures of Cyclopentadienylchromium(III) and Cyclopentadienylmolybdenum(III) Complexes. Dichloride and Dimethyl Compounds. J. Phys. Chem. A 1997, 101 , 9801–9812. 10.1021/jp972920i.
c Cahoon J. F. ; Kling M. F. ; Sawyer K. R. ; Andersen L. K. ; Harris C. B. DFT and Time-Resolved IR Investigation of Electron Transfer between Photogenerated 17- and 19-electron organometallic radicals. J. Mol. Struct. 2008, 890 , 328–338. 10.1016/j.molstruc.2008.05.047.
a Baker P. K. ; Connelly N. G. ; Jones B. M. R. ; Maher J. P. ; Somers K. R. Reduction-Oxidation Properties of Organotransition-metal Complexes. Part 8. Formation and Reactivity of the Radical Cations [Fe(CO)3L2]+, and the Mechanism of the Oxidative Elimination Reaction of [Fe(CO)3L2] with Halogens. J. Chem. Soc., Dalton Trans. 1980, 579–585. and an earlier communication cited therein 10.1039/dt9800000579.
b MacNeil J. H. ; Chiverton A. C. ; Fortier S. ; Baird M. C. ; Hynes R. C. ; Williams A. J. ; Preston K. F. ; Ziegler T. An X-ray Crystallographic and Single-Crystal EPR Investigation of the Cationic, Iron-Centered Radical Tricarbonylbis(triphenylphosphine)iron(I), {Fe(CO)3(PPh3)2+}. A Theoretical Examination of the Structural Preferences of Five-Coordinated Seventeen-Electron Complexes. J. Am. Chem. Soc. 1991, 113 , 9834–9842. 10.1021/ja00026a019.
Rall J. M. ; Schorpp M. ; Keilwerth M. ; Mayländer M. ; Friedmann C. ; Daub M. ; Richert S. ; Meyer K. ; Krossing I. Synthesis and Characterization of Stable Iron Pentacarbonyl Radical Cation. Angew. Chem., Int. Ed. 2022, 61 , e202204080 (identical in Angew. Chem.) 10.1002/anie.202204080.
Kandler H. ; Gauss C. ; Bidell W. ; Rosenberger S. ; Bürgi T. ; Eremenko I. L. ; Veghini D. ; Orama O. ; Burger P. ; Berke H. The Reduction of [Fe(CO)2L2X2] (L = P(OMe)3, P(OiPr)3, PEt3; X = Br, I) – From Iron(II) to Iron(0) via Stable Iron(I) Intermediates. Chem. Eur. J. 1995, 1 , 541–548. 10.1002/chem.19950010808.
Venturi C. ; Bellachioma G. ; Cardaci G. ; Macchioni A. Syntheses and structures of vinyl and aryl derivatives of carbonyl halide iron complexes. Inorg. Chim. Acta 2004, 357 , 3712–3720. 10.1016/j.ica.2004.05.013.
The radical [Fe(CO)4(H)]• has been postulated as a reaction intermediate: Krusic P. J. ESR Study of Paramagnetic Iron Carbonyl Hydrides. J. Am. Chem. Soc. 1981, 103 , 2131–2133. 10.1021/ja00398a060.
a MacLeod M. C. ; Vinyard D. J. ; Holland P. L. A Multi-iron System Capable of Rapid N2 Formation and N2 Cleavage. J. Am. Chem. Soc. 2014, 136 , 10226–10229. 10.1021/ja505193z.25004280
b Murdoch H. D. ; Lucken E. A. C. 163. Einige paramagnetische π-Allyl-carbonyl-Komplexe des Eisens. Helv. Chim. Acta 1964, 47 , 1517–1524. 10.1002/hlca.19640470614.
c Lee Y. ; Peters J. C. Silylation of Iron-Bound Carbon Monoxide Affords a Terminal Fe Carbyne. J. Am. Chem. Soc. 2011, 133 , 4438–4446. 10.1021/ja109678y.21375250
Salsi F. ; Wang S. ; Teutloff C. ; Busse M. ; Neville M. L. ; Hagenbach A. ; Bittl R. ; Figueroa J. S. ; Abram U. A Complete Triad of Zero-Valent 17-Electron Monoradicals of Group 7 Elements Stabilized by m-Terphenyl Isocyanides. Angew. Chem., Int. Ed. 2023, 62 , e202300254 10.1002/anie.202300254.
Salsi F. ; Wang S. ; Teutloff C. ; Busse M. ; Neville M. L. ; Hagenbach A. ; Bittl R. ; Figueroa J. S. ; Abram U. Ein m-Terphenylisocyanid stabilisiert 17-Elektronen-Monoradikale von drei Elementen der Gruppe 7. Angew. Chem. 2023, 135 , e202300254. 10.1002/ange.202300254.
a Lang G. M. ; Shima T. ; Wang L. ; Cluff K. J. ; Skopek K. ; Hampel F. ; Blümel J. ; Gladysz J. A. Gyroscope-Like Complexes Based on Dibridgehead Diphosphine Cages that are Accessed by Three-Fold Intramolecular Ring Closing Metatheses and Encase Fe(CO)3, Fe(CO)2(NO)+ and Fe(CO)3(H)+ Rotators. J. Am. Chem. Soc. 2016, 138 , 7649–7663. 10.1021/jacs.6b03178.27203383
b Lang G. M. ; Skaper D. ; Hampel F. ; Gladysz J. A. Synthesis, Reactivity, Structures, and Dynamic Properties of Gyroscope Like Iron Complexes with Dibridgehead Diphosphine Cages: Pre- vs. Post-Metathesis Substitutions as Routes to Adducts with Neutral Dipolar Fe(CO)(NO)(X) Rotors. Dalton Trans. 2016, 45 , 16190–16204. 10.1039/C6DT03258C.27722339
Zarcone S. R. ; Bhuvanesh N. ; Gladysz J. A. Molecules that Turn Themselves Inside-Out: Tuning in/out Equilibria and Homeomorphic Isomerization in Macrocycle Dibridgehead Diphosphines P((CH2)n)3P Newly Accessible by Earth-Abundant Metal Templates. Chem. Eur. J. 2023, 29 , e202302200 10.1002/chem.202302200.37738010
Corresponding diarsine chemistry: Zarcone S. R. ; Verardi P. J. ; Chu G. M. ; Bhuvanesh N. ; Gladysz J. A. Macrobicyclic Dibridgehead Di(trialkyl)pnictogens E((CH2)n)3E (E/n = As/10, As/12, As/14, Sb/14) and their Cage-Like Metal Complexes: Syntheses, Structures, and Homeomorphic Isomerizations. Organometallics 2024, 43 , 1285–1298. 10.1021/acs.organomet.4c00120.
Zhu Y. ; Stollenz M. ; Zarcone S. R. ; Kharel S. ; Joshi H. ; Bhuvanesh N. ; Reibenspies J. H. ; Gladysz J. A. Syntheses, Homeomorphic and Configurational Isomerizations, and Structures of Macrocyclic Aliphatic Dibridgehead Diphosphines: Molecules that Turn Themselves Inside Out. Chem. Sci. 2022, 13 , 13368–13386. 10.1039/D2SC04724A.36507162
Both hexanes and CH2Cl2 solvents have been investigated in every case. Yields of 3b–e only differ by a few percent, with the better solvent being described in the Experimental Section.10

In hexane, the conversion of 2a to 3a was less efficient, and some black coproduct precipitated from the darkened yellow solution. The photolysis of 2a in toluene gave a vibrant yellow-green solution with a higher conversion to 3a. A scouting experiment in THF gave a cloudy green solution. All of these reactions were conducted with excess PMe3, and the green colors may represent other radicals of the formula trans-[Fe(CO)2(X)(P((CH2)10)3P)]•.

The homology is reflected by a number of quantifiable geometric features. For example, each lattice consists of two sets of molecules. Within each set, the P–Fe–P axes are parallel, but between the sets, the axes are not parallel. A least-squares plane can be derived from four phosphorus atoms in each set (from two molecules). These planes make angles of 82.2° in 2a and 84.1° in 1a•. Alternatively, one can compare the distances between the parallel planes defined by the three CO atoms in 3a (4.774 Å) and the Cl and two CO atoms in 1a• (4.769 Å).

Zarcone S. R. ; Chu G. M. ; Ehnbom A. ; Cardenal A. ; Fiedler T. ; Bhuvanesh N. ; Hall M. B. ; Gladysz J. A. Towards Frameworks with Multiple Aligned and Interactive Fe(CO)3 Rotators: Syntheses and Structures of Diiron Complexes Linked by two trans Diaxial α,ω-Diphosphine Ligands Ar2P(CH2)nPAr2. Inorg. Chem. 2021, 60 , 3314–3330. 10.1021/acs.inorgchem.0c03737.33571407
Synthesis and crystal structure of cis-Fe(CO)3[Ph2P(CH2)3PPh2]: Ringenberg M. R. ; Wittkamp F. ; Apfel U.-P. ; Kaim W. Redox Induced Configurational Isomerization of Bisphosphine-Tricarbonyliron(I) Complexes and the Difference a Ferrocene Makes. Inorg. Chem. 2017, 56 , 7501–7511. 10.1021/acs.inorgchem.7b00957.28598145
Greatrex R. ; Greenwood N. N. Mössbauer Spectra, Structure, and Bonding in Iron Carbonyl Derivatives. Discuss. Faraday Soc. 1969, 47 , 126–125. 10.1039/DF9694700126.
Carroll W. E. ; Deeney F. A. ; Delaney J. A. ; Lalor F. J. Ligand-variation Studies on the Mössbauer Effect in Low-valency Iron Organometallic Complexes: The LFe(CO)4 and L2Fe(CO)3 Series. J. Chem. Soc. Dalton 1973, 718–722. 10.1039/DT9730000718.
The isomer shifts in refs (20) and (21) were referenced to sodium nitroprusside and have been corrected to the α-iron reference used in this work:

a Stevens J. G. Isomer Shift Reference Scales. Hyperfine Interact. 1983, 13 , 221–236. 10.1007/BF01027252.
b Gütlich P. ; Bill E. , Trautwein A. X. Mössbauer Spectroscopy and Transition Metal Chemistry; Springer: Berlin, 2011. See especially Table 3.1 and pp 84–85.
a McWilliams S. F. ; Brennan-Wydra E. ; MacLeod K. C. ; Holland P. L. Density Functional Calculations for Prediction of 57Fe Mössbauer Isomer Shifts and Quadrupole Splittings in β-Diketiminate Complexes.. ACS Omega 2017, 2 , 2594–2606. and the subsequent minor correction, DOI: 10.1021/acsomega.7b01252, 10.1021/acsomega.7b00595.28691111
b Römelt M. ; Ye S. ; Neese F. Calibration of Modern Density Functional Theory Methods for the Prediction of 57Fe Mössbauer Isomer Shifts: Meta-GGA and Double-Hybrid Functionals. Inorg. Chem. 2009, 48 , 784–785. 10.1021/ic801535v.19102678
Wright S. C. ; Baird M. C. Stereochemistry at Iron during Carbonyl ″Insertion″ Reactions of FeMeI(CO)2(PMe3)2. J. Am. Chem. Soc. 1985, 107 , 6899–6902. 10.1021/ja00310a026.
Orpen A. G. ; Connelly N. G. Structural Evidence for the Participation of P–X σ* Orbitals in Metal–PX3 Bonding. J. Chem. Soc., Chem. Commun. 1985, 1310–1311. 10.1039/C39850001310.
Bader R. F. A Quantum Theory of Molecular Structure and Its Applications. Chem. Rev. 1991, 91 , 893–928. 10.1021/cr00005a013.
Frenking G. ; Fröhlich N. The Nature of the Bonding in Transition-Metal Compounds. Chem. Rev. 2000, 100 , 717–774. 10.1021/cr980401l.11749249
Nayak S. K. ; Burkey T. J. Photosubstitution of Iron Carbonyl Phosphine Complexes: Quantum Yield, Kinetic, and Thermochemical Studies. J. Am. Chem. Soc. 1993, 115 , 6391–6397. 10.1021/ja00067a064.
Fischer E. O. ; Wanner J. K. R. Übergangsmetall-Carbin-Komplexe. LXXV. Synthese neuer halogenverbrückter zweikerniger Carbinkomplexe von Chrom und Wolfram. J. Organomet. Chem. 1983, 252 , 175–179. 10.1016/0022-328X(83)80080-1.
a Fernández J. M. ; Gladysz J. A. Synthetic Approaches to the Chiral, Pyramidal, Transition-Metal Lewis Acid [(η5-C5H5)Re(NO)(PPh3)]+ X–. Generation, Characterization, and Reactions of a Dichloromethane Adduct. Organometallics 1989, 8 , 207–219. 10.1021/om00103a027.
b Peng T.-S. ; Winter C. H. ; Gladysz J. A. Generation and Reactivity of Substitution-Labile Dichloromethane and Chlorobenzene Adducts of the Chiral Pentamethylcyclopentadienyl Rhenium Lewis Acid [(η5-C5Me5)Re(NO)(PPh3)]+. Inorg. Chem. 1994, 33 , 2534–2542. 10.1021/ic00090a011.
Crystallographically characterized η1 adducts:

a Butts M. D. ; Scott B. L. ; Kubas G. J. Syntheses and Structures of Alkyl and Aryl Halide Complexes of the Type [(PiPr3)2PtH(η1-XR)]BArf and Analogues with Et2O, THF, and H2 Ligands. Halide-to-Metal π Bonding in Halocarbon Complexes. J. Am. Chem. Soc. 1996, 118 , 11831–11843. 10.1021/ja961836y.
b Huhmann-Vincent J. ; Scott B. L. ; Kubas G. J. Rhenium Complexes with Weakly Coordinating Solvent Ligands, cis-[Re(PR3)(CO)4(L)][BArF], L = CH2Cl2, Et2O, NC5F5: Decomposition to Chloride-Bridged Dimers in CH2Cl2 Solutions. Inorg. Chem. 1999, 38 , 115–124. and an earlier communication cited therein 10.1021/ic980912q.
c Fang X. ; Huhmann-Vincent J. ; Scott B. L. ; Kubas G. J. H2 binding to and silane alcoholysis on an electrophilic Mn(I) fragment with tied-back phosphite ligands. X-ray structure of a Mn-CH2Cl2 complex. J. Organomet. Chem. 2000, 609 , 95–103. 10.1016/S0022-328X(00)00230-8.
d Taw F. L. ; Mellows H. ; White P. S. ; Hollander F. J. ; Bergman R. G. ; Brookhart M. ; Heinekey D. M. Synthesis and Investigation of [Cp*(PMe3)Rh(H)(H2)]+ and Its Partially Deuterated and Tritiated Isotopomers: Evidence for a Hydride/Dihydrogen Structure. J. Am. Chem. Soc. 2002, 124 , 5100–5108. 10.1021/ja0165990.11982375
e Santiso-Quiñones G. ; Brückner R. ; Knapp C. ; Dionne I. ; Passmore J. ; Krossing I. Cyclododecasulfur as a Ligand: From Gas-Phase Experiments to the Crystal Structures of [Cu(S12)(S8)]+ and [Cu(S12)(CH2Cl2)]+. Angew. Chem., Int. Ed. 2009, 48 , 1133–1137. 10.1002/anie.200804021.
Santiso-Quiñones G. ; Bruckner R. ; Knapp C. ; Dionne I. ; Passmore J. ; Krossing I. Cyclododecasulfur as a Ligand: From Gas-Phase Experiments to the Crystal Structures of [Cu(S12)(S8)]+ and [Cu(S12)(CH2Cl2)]+. Angew. Chem. 2009, 121 , 1153–1157. 10.1002/ange.200804021.
a Kochi J. Organometallic Mechanisms and Catalysis; Academic Press, 1978; Chapter 7.
b Halpern J. Oxidative-Addition Reactions of Transition Metal Complexes. Acc. Chem. Res. 1970, 3 , 386–392. 10.1021/ar50035a004.
Yempally V. ; Moncho S. ; Muhammad S. ; Brothers E. N. ; Arndtsen B. A. ; Bengali A. A. Oxidative Addition of Haloalkanes to Metal Centers: A Mechanistic Investigation. Organometallics 2014, 33 , 3591–3595. 10.1021/om5005226.
a Fiedler T. ; Bhuvanesh N. ; Hampel F. ; Reibenspies J. H. ; Gladysz J. A. Gyroscope like molecules consisting of trigonal or square planar osmium rotators within three-spoked dibridgehead diphosphine stators: syntheses, substitution reactions, structures, and dynamic properties. Dalton Trans. 2016, 45 , 7131–7147. 10.1039/C6DT00692B.27007878
b Hess G. D. ; Fiedler T. ; Hampel F. ; Gladysz J. A. Octahedral Gyroscope-like Molecules Consisting of Rhenium Rotators within Cage-like Dibridgehead Diphosphine Stators: Syntheses, Substitution Reactions, Structures, and Dynamic Properties. Inorg. Chem. 2017, 56 , 7454–7469. 10.1021/acs.inorgchem.7b00909.28598610
Estrada A. L. ; Wang L. ; Bhuvanesh N. ; Hampel F. ; Gladysz J. A. Syntheses, Structures, Reactivities, and Dynamic Properties of Gyroscope Like Complexes Consisting of Rh(CO)(X) or Rh(CO)2(I) Rotators and Cage Like trans Aliphatic Dibridgehead Diphosphine Stators. Organometallics 2022, 41 , 733–749. 10.1021/acs.organomet.1c00708.
Lichtenberger D. L. ; Rai-Chaudhuri A. R. ; Seidel M. J. ; Gladysz J. A. ; Agbossou S. K. ; Igau A. ; Winter C. H. Delocalized Electronic Interactions in Chiral Cyclopentadienylrhenium Halide Complexes. Valence Photoelectron Spectra of CpRe(NO)(L)X (Cp = η5-C5H5, L = CO, P(C6H5)3; X = Cl, Br, I). Organometallics 1991, 10 , 1355–1364. 10.1021/om00051a028.
The cyclic voltammetry data in ref (11a) were referenced to Fc0/+ = 0.46 V and are converted to Fc0/+ = 0.00 V in this paper.

Berke H. ; Jacobsen H. Tracing an organometallic early-late transition element relationship. Russ. Chem. Bull. 1998, 47 , 841–850. 10.1007/BF02498150.
Estrada A. L. ; Wang L. ; Hess G. ; Hampel F. ; Gladysz J. A. Square Planar and Octahedral Gyroscope-Like Metal Complexes Consisting of Dipolar Rotators Encased in Dibridgehead Di(triaryl)phosphine Stators: Syntheses, Structures, Dynamic Properties, and Reactivity. Inorg. Chem. 2022, 61 , 17012–17025. 10.1021/acs.inorgchem.2c02855.36264646
a Nawara-Hultzsch A. J. ; Stollenz M. ; Barbasiewicz M. ; Szafert S. ; Lis T. ; Hampel F. ; Bhuvanesh N. ; Gladysz J. A. Gyroscope-Like Molecules Consisting of PdX2/PtX2 Rotators within Three-Spoke Dibridgehead Diphosphine Stators: Syntheses, Substitution Reactions, Structures, and Dynamic Properties. Chem. Eur. J. 2014, 20 , 4617–4637. 10.1002/chem.201304419.24604783
b Kharel S. ; Joshi H. ; Bhuvanesh N. ; Gladysz J. A. Syntheses, Structures, and Thermal Properties of Gyroscope-like Complexes Consisting of PtCl2 Rotators Encased in Macrocyclic Dibridgehead Diphosphines P((CH2)n)3P with Extended Methylene Chains (n = 20/22/30), and Isomers Thereof. Organometallics 2018, 37 , 2991–3000. 10.1021/acs.organomet.8b00345.
Frisch M. J. ; Trucks G. W. ; Schlegel H. B. ; Scuseria G. E. ; Robb M. A. C. J. R. ; Scalmani G. ; Barone V. ; Mennucci B. ; Petersson G. A. ; Nakatsuji H. ; Caricato M. ; Li X. ; Hratchian H. P. ; Izmaylov A. F. ; Bloino J. ; Zheng G. ; Sonnenberg J. L. ; Hada M. E. M. ; Toyota K. ; Fukuda R. ; Hasegawa J. ; Ishida M. ; Nakajima T. ; Honda Y. ; Kitao O. ; Nakai H. ; Vreven T. ; Montgomery J. A. Jr. ; Peralta J. E. ; Ogliaro F. ; Bearpark M. ; Heyd J. J. ; Brothers E. ; Kudin K. N. ; Staroverov V. N. ; Kobayashi R. ; Normand J. ; Raghavachari K. ; Rendell A. ; Burant J. C. ; Iyengar S. S. ; Tomasi J. ; Cossi M. ; Rega N. ; Millam N. J. ; Klene M. ; Knox J. E. ; Cross J. B. ; Bakken V. ; Adamo C. ; Jaramillo J. ; Gomperts R. ; Stratmann R. E. ; Yazyev O. ; Austin A. J. ; Cammi R. ; Pomelli C. ; Ochterski J. W. ; Martin R. L. ; Morokuma K. ; Zakrzewski V. G. ; Voth G. A. ; Salvador P. ; Dannenberg J. J. ; Dapprich S. ; Daniels A. D. ; Farkas Ö. ; Foresman J. B. ; Ortiz J. V. ; Cioslowski J. ; Fox D. J. Gaussian 16, revision C.01; Gaussian, Inc.: Wallingford, CT, 2016.
a Becke A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98 , 5648–5652. 10.1063/1.464913.
b Lee C. ; Yang W. ; Parr R. G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B 1988, 37 , 785–789. 10.1103/PhysRevB.37.785.
c Miehlich B. ; Savin A. ; Stoll H. ; Preuss H. Results obtained with the correlation energy density functionals of Becke and Lee, Yang and Parr. Chem. Phys. Lett. 1989, 157 , 200–206. 10.1016/0009-2614(89)87234-3.
a McLean A. D. ; Chandler G. S. Contracted Gaussian basis sets for molecular calculations. 1. 2nd row atoms, Z = 11–18. J. Chem. Phys. 1980, 72 , 5639–5648. 10.1063/1.438980.
b Krishnan R. ; Binkley J. S. ; Seeger R. ; Pople J. A. Self-Consistent Molecular Orbital Methods. 20. A Basis set for correlated wave functions. J. Chem. Phys. 1980, 72 , 650–654. 10.1063/1.438955.
Tao J. ; Perdew J. P. ; Staroverov V. N. ; Scuseria G. E. Climbing the Density Functional Ladder: Nonempirical Meta–Generalized Gradient Approximation Designed for Molecules and Solids. Phys. Rev. Lett. 2003, 91 , 146401–146405. 10.1103/PhysRevLett.91.146401.14611541
a Hsieh C.-H. ; Ding S. ; Erdem Ö. F. ; Crouthers D. J. ; Liu T. ; McCrory C. C. ; Lubitz W. ; Popescu C. V. ; Reibenspies J. H. ; Hall M. B. ; Darensbourg M. Y. Redox Active Iron Nitrosyl Units in Proton Reduction Electrocatalysis. Nat. Commun. 2014, 5 , 3684 10.1038/ncomms4684.24785411
b Brothers S. M. ; Darensbourg M. Y. ; Hall M. B. Modeling Structures and Vibrational Frequencies for Dinitrosyl Iron Complexes (DNICs) with Density Functional Theory. Inorg. Chem. 2011, 50 , 8532–8540. 10.1021/ic201137t.21819054
c Ghosh P. ; Ding S. ; Chupik R. B. ; Quiroz M. ; Hsieh C.-H. ; Bhuvanesh N. ; Hall M. B. ; Darensbourg M. Y. A Matrix of Heterobimetallic Complexes for Interrogation of Hydrogen Evolution Reaction Electrocatalysts. Chem. Sci. 2017, 8 , 8291–8300. 10.1039/C7SC03378H.29619175
d Ding S. ; Ghosh P. ; Darensbourg M. Y. ; Hall M. B. Interplay of Hemilability and Redox Activity in Models of Hydrogenase Active Sites. Proc. Natl. Acad. Sci. U. S. A. 2017, 114 , E9775–E9782. 10.1073/pnas.1710475114.29087322
Marenich A. V. ; Cramer C. J. ; Truhlar D. G. Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions. J. Phys. Chem. B 2009, 113 , 6378–6396. 10.1021/jp810292n.19366259
Hellweg A. ; Rappoport D. Development of New Auxiliary Basis Functions of the Karlsruhe Segmented Contracted Basis Sets Including Diffuse Basis Functions (DEF2-SVPD, DEF2-TZVPPD, and Def2-QVPPD) for RI-MP2 and RI-Cc Calculations. Phys. Chem. Chem. Phys. 2015, 17 , 1010–1017. 10.1039/C4CP04286G.25410795
Neese F. Software Update: The Orca Program System-Version 5.0. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2022, 12 , e1606 10.1002/wcms.1606.
APEX3, Program for Data Collection on Area Detectors; Bruker AXS Inc.: Madison, WI, 2012.
Sheldrick G. M. SADABS, Program for Absorption Correction of Area Detector Frames; Bruker AXS Inc.: Madison, WI, 1996.
a Sheldrick G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64 , 112–122. 10.1107/S0108767307043930.
b Sheldrick G. M. Crystal structure refinement with SHELXL. Acta Crystallogr. 2015, 71 , 3–8. 10.1107/S2053229614024218.
Dolomanov O. V. ; Bourhis L. J. ; Gildea R. J. ; Howard J. A. K. ; Puschmann H. OLEX2: A Complete Structure Solution, Refinement and Analysis Program. J. Appl. Crystallogr. 2009, 42 , 339–341. 10.1107/S0021889808042726.
CrysAlisPRO Software System; Rigaku Oxford Diffraction/Agilent Technologies UK Ltd., Yarnton, England, 2023.
Sheldrick G. M. SHELXT – Integrated space-group and crystal-structure determination. Acta Crystallogr., Sect. A: Found. Adv. 2015, 71 , 3–8. 10.1107/S2053273314026370.25537383
