==== Front
J Am Chem Soc
J Am Chem Soc
ja
jacsat
Journal of the American Chemical Society
0002-7863
1520-5126
American Chemical Society

37318758
10.1021/jacs.3c04167
Article
Stability and C–H Bond Activation Reactions of Palladium(I) and Platinum(I) Metalloradicals: Carbon-to-Metal H-Atom Transfer and an Organometallic Radical Rebound Mechanism
https://orcid.org/0000-0001-5842-9553
Krämer Tobias *†‡
https://orcid.org/0000-0002-7565-5154
Gyton Matthew R. §
https://orcid.org/0000-0001-9404-0051
Bustos Itxaso §∥
https://orcid.org/0000-0002-6913-343X
Sinclair Matthew J. G. §
https://orcid.org/0000-0002-3789-8745
Tan Sze-yin §⊥
https://orcid.org/0000-0002-3686-1043
Wedge Christopher J. #g
https://orcid.org/0000-0003-3454-6776
Macgregor Stuart A. †
https://orcid.org/0000-0003-4286-8791
Chaplin Adrian B. *§
† Institute of Chemical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K.
‡ Department of Chemistry, Maynooth University, Maynooth W23 F2K8, Ireland
§ Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, U.K.
∥ Facultad de Química de San Sebastián, Universidad del País Vasco (UPV/EHU), Apartado 1072, 20080 San Sebastián, Spain
⊥ Department of Chemical Engineering, Imperial College London, London SW7 2AZ, U.K.
# Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, U.K.
g Department of Chemical Sciences, University of Huddersfield, Queensgate, Huddersfield HD1 3DH, U.K.
* E-mail: tobias.kraemer@mu.ie
* E-mail: a.b.chaplin@warwick.ac.uk
15 06 2023
28 06 2023
145 25 1408714100
22 04 2023
© 2023 The Authors. Published by American Chemical Society
2023
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/).

One-electron oxidation of palladium(0) and platinum(0) bis(phosphine) complexes enables isolation of a homologous series of linear d9 metalloradicals of the form [M(PR3)2]+ (M = Pd, Pt; R = tBu, Ad), which are stable in 1,2-difluorobenzene (DFB) solution for >1 day at room temperature when partnered with the weakly coordinating [BArF4]− (ArF = 3,5-(CF3)2C6H3) counterion. The metalloradicals exhibit reduced stability in THF, decreasing in the order palladium(I) > platinum(I) and PAd3 > PtBu3, especially in the case of [Pt(PtBu3)2]+, which is converted into a 1:1 mixture of the platinum(II) complexes [Pt(PtBu2CMe2CH2)(PtBu3)]+ and [Pt(PtBu3)2H]+ upon dissolution at room temperature. Cyclometalation of [Pt(PtBu3)2]+ can also be induced by reaction with the 2,4,6-tri-tert-butylphenoxyl radical in DFB, and a common radical rebound mechanism involving carbon-to-metal H-atom transfer and formation of an intermediate platinum(III) hydride complex, [Pt(PtBu2CMe2CH2)H(PtBu3)]+, has been substantiated by computational analysis. Radical C–H bond oxidative addition is correlated with the resulting MII–H bond dissociation energy (M = Pt > Pd), and reactions of the metalloradicals with 9,10-dihydroanthracene in DFB at room temperature provide experimental evidence for the proposed C–H bond activation manifold in the case of platinum, although conversion into platinum(II) hydride derivatives is considerably faster for [Pt(PtBu3)2]+ (t1/2 = 1.2 h) than [Pt(PAd3)2]+ (t1/2 ∼ 40 days).

Ministerio de Ciencia, Innovación y Universidades 10.13039/100014440 NA European Commission 10.13039/501100000780 NA Royal Society 10.13039/501100000288 UF150675 Royal Society 10.13039/501100000288 UF100592 Engineering and Physical Sciences Research Council 10.13039/501100000266 EP/K035681/1 document-id-old-9ja3c04167
document-id-new-14ja3c04167
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pmcIntroduction

With many applications in synthetic organic chemistry,1 the development of methods for enacting the cleavage of C(sp3)–H bonds is an important facet of contemporary organometallic chemistry.2 Building on pioneering work by Bergman and Graham,2,3 the activation of these robust and nonpolar σ-bonds by concerted oxidative addition to electron-rich, low-valent platinum group metals is a well-established and exploited mechanism. These reactions proceed via transient three-center-two-electron M–H–C adducts and involve +2 changes in the formal oxidation state of the metal (ΔOS = +2; Scheme 1).4 Other distinct manifolds include electrophilic activation (ΔOS = 0), σ-bond metathesis (ΔOS = 0), 1,2-addition across polar metal–ligand multiple bonds (ΔOS = 0), and radical oxidative addition (ΔOS = +1).2,5 The last is typically associated with the homolysis of appreciably polar σ-bonds by metalloradicals and is an underdeveloped C(sp3)–H bond activation strategy. The most long-standing precedent emerged from Wayland’s work with rhodium(II) porphyrins in the early 1990s, in which a mechanism involving addition of C(sp3)–H bonds across two metalloradicals was established experimentally (Scheme 1).6 Related bimetallic reactivity has also been invoked in the cyclometalation of a triphenylphosphine-ligated rhodium(II) metalloradical and allylic C(sp3)–H bond activation reactions of MII(cyclooctadiene) complexes (M = Rh, Ir).7 Carbon-to-metal H-atom transfer reactions of a photochemically generated osmium(I) cyclopentadienyl metalloradical with allylic and benzylic substrates is a more recent and notable precedent (Scheme 1).8

Scheme 1 Activation of C(sp3)–H Bonds by Late Transition Metal Complexes

Low-valent paramagnetic derivatives of palladium and platinum are intriguing candidates to participate in radical oxidative addition reactions; however, the chemistry of complexes of this nature is significantly underdeveloped, especially with reference to the advances being made with d9-complexes of nickel.9−11 Mononuclear palladium(I) and platinum(I) complexes have been invoked as transient intermediates or generated electrochemically in situ,12,13 but only a handful have been isolated to date.14−17 Instead adoption of the formal +1 oxidation state for palladium and platinum is almost exclusively limited to bimetallic adducts where formation of a metal–metal bond confers a closed-shell electronic configuration.18

Building upon work by Stalke, Frenking, Roesky, and co-workers, who examined the electrochemical oxidation of two-coordinate cyclic alkyl(amino) carbene complexes of palladium(0) and platinum(0),12 some of us demonstrated the facile and reversible one-electron oxidation of [M(PtBu3)2] (M = Pd, 1; Pt, 2) using cyclic voltammetry in 2016.14 Subsequent reaction of 1 with Fc[PF6] (Fc = FeCp2) in 1,2-difluorobenzene (DFB) enabled isolation of [Pd(PtBu3)2][PF6] (3[PF6]) in high yield, which proved to be stable in solution for prolonged periods of time under an inert atmosphere at room temperature (Scheme 2). Under the same conditions, however, the corresponding platinum(I) metalloradical [Pt(PtBu3)2][PF6] (4[PF6]) could not be isolated, and instead a 1:1 mixture of platinum(II) metallocycle [Pt(PtBu2CMe2CH2)(PtBu3)][PF6] (5[PF6]) and platinum(II) hydride [Pt(PtBu3)2H][PF6] (6[PF6]) was observed. This outcome corresponds to net radical oxidative addition of a C(sp3)–H bond across two transient platinum(I) metalloradicals, although the precise mechanism was not resolved at the time. Zhou, Meyer, Hughes, Ozerov, and co-workers later reported the isolation of both metalloradicals using fluorobenzene as a solvent and [Ph3C][HCB11Cl11] as the one-electron oxidant, viz. 3[HCB11Cl11] and 4[HCB11Cl11].15 Pairing the platinum(I) metalloradical with the weakly coordinating [HCB11Cl11]− counterion curbs onward reactivity but did not prevent complete conversion of 4+ into a 1:1 mixture of 5+ and 6+, which occurred within 48 h at room temperature in fluorobenzene. Underscoring the propensity for cyclometalation, however, dissolution of 4[HCB11Cl11] in acetonitrile at room temperature resulted in instantaneous conversion into a 1:1 mixture of [Pt(PtBu2CMe2CH2)(PtBu3)(NCMe)][HCB11Cl11] and [Pt(PtBu3)2H(NCMe)][HCB11Cl11].

Scheme 2 One-Electron Oxidation of [M(PtBu3)2] and Subsequent Reactivity

Under certain conditions, we have since observed onward reactivity of 3[PF6] that results in formation of [Pd(PtBu2CMe2CH2)(PtBu3)][PF6] (7), most notably upon exposure of the palladium(I) metalloradical to air.19 Similar reactivity has recently been noted by Mirica and co-workers while exploring the use of dithiapyridinophane-ligated palladium(I) complexes in Kumada cross-coupling reactions (Scheme 2).16 Both reactions invoke radical oxidative addition of a C(sp3)–H bond but, compared to cyclometalation of 4+, are less well defined.

We herein present further findings from our work exploring the chemistry of two coordinate palladium(I) and platinum(I) metalloradicals,20 focused on uncovering the mechanism of C(sp3)–H bond activation by late transition metal metalloradicals. Examples with enhanced solution stability are described along with a detailed computational examination of different radical-based mechanisms of C(sp3)–H bond cyclometalation and other onward reactivity of 4+.

Results and Discussion

Isolation and Stability of Tri-tert-butylphosphine-Ligated Metalloradicals

Preceding work has highlighted the instability of 4+, for which isolation requires careful consideration of the counterion and solvent used. We chose to focus our attention on the partially fluorinated tetraarylborate counterion [BArF4]− (ArF = 3,5-(CF3)2C6H3). This weakly coordinating anion finds widespread utility for the stabilization of reactive and low-coordinate metal cations,21 while its use is practically convenient, as the sodium salt is commercially available. Straightforward synthetic procedures that have been optimized for the isolation of solvent-free anhydrous M[BArF4] (M = Li, Na, K) have also been reported recently.22

To probe the role of solvent on the stability of 3+ and 4+, we began by re-evaluating the oxidation of 1 and 2 by cyclic voltammetry using [nBu4N][BArF4] as the electrolyte and DFB and tetrahydrofuran (THF) as solvents (Figure 1). Quasi-reversible one-electron oxidation was observed in all cases and, noting that considerable variance is to be expected when changing the electrolyte and solvent,23 the resulting redox potentials are in line with what we have previously determined under different conditions (E1/2 = −0.44 V, 1; −0.10 V 2; DFB/[nBu4N][PF6], relative to Fc/Fc+). Using deviation of the peak current ratio (iPred/iPox) from unity as a gauge, it is apparent that the platinum metalloradical (iPred/iPox ≤ 0.90) is considerably less stable than the palladium congener (iPred/iPox ≥ 0.95). Moreover, use of THF as the solvent considerably accelerates the onward reactivity of 4+ (iPred/iPox = 0.41 cf. 0.90): to the point that the stability of this metalloradical is limited to seconds at room temperature (vide infra).

Figure 1 Cyclic voltammograms for the oxidation of [M(PtBu3)2] in DFB and THF at room temperature (2 mM complex; 0.2 M [nBu4N][BArF4] electrolyte; glassy carbon working electrode, coiled Pt wire counter electrode, and Ag wire quasi-reference electrode; scan rates = 30, 50, 70, and 100 mV·s–1).

Subsequent to the electrochemical study, 3[BArF4] and 4[BArF4] were prepared by chemical oxidation of 1 and 2 using Fc[BArF4] in rigorously dried DFB at room temperature (Scheme 3).24 Although the palladium metalloradical was readily isolated in high purity and 87% yield, obtaining analytically pure samples of the heavier congener using this method was less reproducible and samples were often contaminated with small amounts of the platinum(II) hydride 6[BArF4].25 This impurity appears to cocrystallize with 4[BArF4], as subsequent recrystallization did not result in improvements in purity. Consistent with this assertion, independently isolated 6[BArF4] obtained from the reaction between 2 and [H(OEt2)2][BArF4] in DFB (Scheme 3) is isomorphous to 4[BArF4] (see Supporting Information).26 In our experience, the most reliable way of obtaining high-purity samples of 4[BArF4] was by performing the reaction in the presence of 0.05 equiv of 2,6-di-tert-butyl-4-methylpyridine (62% yield, >99% purity).

Scheme 3 Synthesis and Onward Reactivity of [M(PtBu3)2][BArF4]

[BArF4]− counterions omitted for clarity.

Complexes 3[BArF4] and 4[BArF4] were extensively characterized in DFB and unsurprisingly show directly comparable spectroscopic and electrochemical signatures to those previously reported for 3+ and 4+.14,15 Most notably, deep blue 3[BArF4] is characterized by a singlet EPR resonance with axial g-tensor, g⊥= 2.343 and g∥ = 1.978, superimposed with a lower intensity sextet arising from isotropic hyperfine coupling to 105Pd (I = 5/2, 22% abundance, a = 25.2 mT; no superhyperfine coupling to 31P was evident; DFB glass @ 200 K), but a 31P NMR resonance could not be observed between +500 and −500 ppm. In contrast, for green 4[BArF4] a paramagnetically shifted 31P NMR resonance could be located at δ −213.9, while an EPR signal could not be detected down to 100 K (DFB glass). The UV–vis spectrum of 4+ has not previously been reported and is most remarkable for a sharp band at 306 nm (ε = 700 M–1·cm–1) ascribed to a metal-centered transition.27 Further details, including analysis by time-dependent density functional theory (TD-DFT) calculations, are provided in the Supporting Information.

The solid-state structures of the new metalloradicals have also been determined and are isomorphic, crystallizing in the cubic P213 space group with the P–M–P vector lying along a 3-fold rotation axis (M = Pt, Figure 2). A disordered mixture of staggered and eclipsed cations in approximately equal ratios is evident in both structures, contrasting the solid-state structures of 3[PF6], 3[HCB11Cl11], and 4[HCB11Cl11], which feature metal centers that lie on an inversion center and crystallographically imposed staggered conformations. There are, however, no statistically significant differences in the associated M–P bond lengths for 3+ (2.3470(6) Å, [PF6]−; 2.3466(5) Å, [HCB11Cl11]−; 2.349(3)/2.353(3) Å, [BArF4]−; avg 2.349(4) Å) and 4+ (2.3362(6), [HCB11Cl11]−; 2.333(4)/2.338(4) Å, [BArF4]−; avg 2.336(6) Å). As previously noted, these bonds are appreciably elongated relative to the corresponding zerovalent precursors 1 (2.285(3) Å) and 2 (2.249(3) Å).28,29 The conclusion of the computational analysis conducted by Ozerov et al. is that the unpaired electrons in 3+ and 4+ belong to nonbonding orbitals and bond elongation results from a perturbation of the attractive electrostatic and repulsive Pauli forces.15 In relation to the bulk purity, the Pt–P distances observed for 4[BArF4] are longer on average than those found in 6[BArF4] (2.314(3)/2.320(3) Å).

Figure 2 Solid-state structure of 4[BArF4]. Anisotropic displacement ellipsoids drawn at 30% probability; hydrogen atoms and anion omitted for clarity. Symmetry equivalent phosphine substituents are generated using the operations: 3/2–z, 1–x, 1/2+y and 1–y, −1/2+z, 3/2–x. Selected data: isomorphous 3[BArF4], Pd1–P2, 2.349(3) Å; Pd1–P3, 2.353(3) Å; P2–Pd1–P3, 180°; 57.5(10)% eclipsed; 4[BArF4], Pt1–P2, 2.333(4) Å; Pt1–P3, 2.338(4) Å; P2–Pt1–P3, 180°; 52.9(11)% eclipsed.

Under an inert atmosphere 3[BArF4] and 4[BArF4] are stable in DFB, with no appreciable onward reactivity observed upon standing at room temperature for 24 h. Formation of green [Pd(PtBu3)2(NCMe)][HCB11Cl11] has previously been established for the palladium metalloradical in acetonitrile,15 but there are no significant changes to the UV–vis spectrum of 3[BArF4] upon dissolution in THF at room temperature that would suggest formation of a similar adduct in this solvent. Consistent with the electrochemical study, the metalloradicals show contrasting stability in THF at room temperature. Partial decomposition of 3[BArF4] (ca. 20%) was observed upon standing in solution for 24 h, while rapid conversion into a 1:1 mixture of platinum(II) metallocycle 5[BArF4] (δ31P 59.1, 1JPtP = 2899 Hz; 25.3, 1JPtP = 1915 Hz; 2JPP = 317 Hz) and platinum(II) hydride 6[BArF4] (δ1H −35.28, 1JPtH = 2540 Hz; δ31P 86.5, 1JPtP = 2631 Hz) was observed upon dissolution of 4[BArF4] (Scheme 3). No deuterium incorporation into the products occurred when the latter was repeated in d8-THF, and the identity of 5[BArF4] and 6[BArF4] was confirmed by comparison to literature data and independent synthesis from 2 (Scheme 3).14,26 While 4[BArF4] persists in DFB, reaction with the 2,4,6-tri-tert-butylphenoxyl radical (•OMes*) resulted in smooth conversion into 5[BArF4] with high selectivity (ca. 90%) at room temperature within 8 days (Scheme 4). Consumption of the metalloradical occurred with apparent first-order kinetics (t1/2 = 49 h) under these conditions, suggesting that •OMes* is acting as an H-atom trap.

Scheme 4 Conversion of 4[BArF4] into 5[BArF4] by Reaction with •OMes* (2,4,6-Tri-tert-butylphenoxy)

[BArF4]− counterions omitted for clarity.

Computational Evaluation of Radical Oxidative Addition Pathways

To further interrogate the mechanism associated with C(sp3)–H bond cyclometalation of 4+, we turned to unrestricted DFT calculations to analyze the viability of possible reaction pathways (Figure 3). Following benchmarking, geometries were optimized in the gas phase using the PBEh-3c composite method,30 and single-point energies were calculated at the B2PLYP-D3(BJ)/def-TZVPP31,32 level of theory with corrections included for London dispersion and solvation effects.33,34

Figure 3 Computed reaction profiles (B2PLYP-D3(BJ)/def2-TZVPP+def-ECP(Pt)//PBEh-3c) for the C–H bond cyclometalation of 4+. Energies corrected for DFB solvent in kcal·mol–1 (selected THF values given in parentheses).

The prospect for Wayland-like bimetallic radical C–H bond oxidative activation was first examined using an antiferromagnetically spin-polarized four-centered transition state of the form Pt(↓)···C(↑)···H(↓)···Pt(↑) derived from two equivalents of 4+ and producing 5+ and 6+ in one step.35 The calculations indicate that this intermolecular process is associated with a prohibitively high activation barrier of ΔG⧧298K = 40.0 kcal·mol–1. The bulky PtBu3 ancillary ligands appear to encumber the approach of the two metal centers, and the hydridic character of the transition state suggests homolysis of the C–H bond occurs with a significant degree of asymmetry (Pt···C = 3.30 vs 2.04 Å in 5+; H···Pt = 1.62 vs 1.50 Å in 6+; Pt···Pt = 6.19 Å). With the former in mind, and in an attempt to reconcile the large solvent dependence of the reaction, the possibility for ligand exchange with THF to generate the less bulky metalloradical [Pt(PtBu3)(THF)]+ as the H-atom acceptor was also considered. In this scenario, cyclometalation of 4+ occurs with more symmetric homolysis of the C–H bond (Pt···C = 2.61 Å; H···Pt = 1.63 Å) and a reduced activation barrier of ΔG⧧298K = 15.4 kcal·mol–1, but initial substitution of PtBu3 to form [Pt(PtBu3)(THF)]+ renders the overall process energetically inaccessible (ΔG⧧298K = 47.2 kcal·mol–1).36 Likewise, conversion of 4+ into 5+ by reaction with •OMes* via a four-centered transition state of the form Pt(↓)···C(↑)···H(↓)···O(↑) can be ruled out on the basis of a large activation barrier of ΔG⧧298K = 33.5 kcal·mol–1.

More promisingly, intramolecular cyclometalation of 4+, resulting in the platinum(III) metalloradical [Pt(PtBu2CMe2CH2)H(PtBu3)]+ (9), is calculated to be a moderately endergonic process (ΔG298K = +9.4 kcal·mol–1). A pathway commencing with concerted C–H bond oxidative addition (ΔG⧧298K = 21.3 kcal·mol–1) to the metalloradical can be located between 4+ and 9, but our calculations suggest stepwise insertion into the C–H bond is considerably more favorable with an activation barrier of only ΔG⧧298K = 18.3 kcal·mol–1 (Figure 3). The latter commences with carbon-to-metal H-atom transfer (ΔG⧧298K = 18.0 kcal·mol–1) and culminates in the formation of 9 following combination of the resulting pendent C-centered radical with the platinum(II) center in [Pt(PtBu2CMe2CH2•)H(PtBu3)]+ (8). Radical rebound sequences of this nature were initially considered, but subsequently ruled out, as part of early mechanistic work on C(sp3)–H bond activation reactions of diamagnetic iridium(I) cyclopentadienyl complexes by Janowicz and Bergman.37 There are, however, strong parallels with the established catalytic action of metal-oxo-based enzymes38 and experimental precedent for the formation of mononuclear Pt(III) complexes.39 Consistent with the higher relative solution stability of 3+ observed, the activation barrier calculated for intramolecular carbon-to-metal H-atom transfer is considerably larger (ΔG⧧298K = 33.5 kcal·mol–1) than for 4+. This difference is attributed to the inherently weaker nature of the PdII–H bond (calcd De = 64.4 kcal·mol–1) compared to the PtII–H bond (calcd De = 78.5 kcal·mol–1).

Experimentally observed conversion of 4+ into 5+ by reaction with •OMes* at room temperature can be reconciled by direct H-atom abstraction from 9 (Figure 4). Noting the challenges associated with accurately predicting the entropic contributions of bimolecular transition states using static DFT calculations,40 the calculated activation barrier (ΔG⧧298K = 24.4, ΔH⧧ = 5.6 kcal·mol–1 vs 4+) is consistent with the suggested role of •OMes* as an H-atom trap.41 In the case of the solvent-induced formation of a 1:1 mixture of 5+ and 6+ from 4+, we propose a reaction sequence commencing with solvent-mediated deprotonation of 9 to give neutral platinum(I) cyclometalated complex [Pt(PtBu2CMe2CH2)(PtBu3)] (10) (Figure 4; solvent modeled as a THF dimer). Oxidation of 10 by 4+ would thereafter give 5+, with the reduced product 2 capturing the proton to afford 6+. This suggestion is consistent with the product stoichiometry, absence of H/D exchange when conducted in d8-THF, known reduction potential of 5+ (E1/2 = −1.90 V relative to Fc/Fc+),14 and synthetic procedures used for preparing 5+ and 6+ from 2 (Scheme 3). Computational analysis suggests that deprotonation of 9 is the rate-determining step, conferring an overall activation barrier of ΔG⧧298K = 32.0 kcal·mol–1 with respect to 4+. When excess THF is factored in, this barrier is lowered to ΔG⧧298K(20 mM 4+ in THF) = 24.3 kcal·mol–1.

Figure 4 Computed reaction profiles (B2PLYP-D3(BJ)/def2-TZVPP+def-ECP(Pt)//PBEh-3c) for H-atom transfer reactions of 9 resulting in formation of 5+. Energies corrected for DFB solvent in kcal·mol–1 (selected THF values given parentheses).

Isolation and Stability of Tri(1-adamantyl)phosphine-Ligated Metalloradicals

Recognizing the requirement for C-atom planarization in carbon-to-metal H-atom transfer reactions, we speculated that phosphine ligands with caged substituents would be less susceptible to cyclometalation and therefore confer enhanced metalloradical stability in solution. Tri(1-adamantyl)phosphine (PAd3) is well suited to test this conjecture and is notable for a similar steric profile to PtBu3 about the metal (%Vbur = 40.5 cf. 40.0%), but appreciably stronger donor characteristics (TEP = 2052.1, cf. 2056.1 cm–1).42 The bis(phosphine) Au(I) complex [Au(PAd3)2]+ is a notable diamagnetic derivative,43 and the nickel metalloradical [Ni(PAd3)2]+ has recently been reported.10

Exploiting the relative donor strength of the phosphine ligands, zerovalent PAd3 complexes [M(PAd3)2] (M = Pd, 11;44 Pt, 12) were obtained as analytically pure white powders from ligand substitution reactions of the PtBu3 analogues 1 and 2 in toluene (>80% isolated yields, Scheme 5). These neutral complexes are highly insoluble in common organic solvents (including CH2Cl2, THF, PhMe, and DFB), presumably resulting from abnormally strong intermolecular dispersion forces,45 but the target and considerably more soluble metalloradical derivatives [M(PAd3)2][BArF4] (M = Pd, 13; Pt, 14) were obtained in >80% yield by treatment of suspensions of 11 and 12 in DFB with Fc[BArF4] and extensively characterized. While the insolubility of 11 and 12 prevented direct measurement of the M(0)/M(I) redox potentials by cyclic voltammetry, these values can be estimated from the half-peak potentials for the reduction of 13 and 14, EP/2 = −0.70 and −0.33 V, respectively (DFB/[nBu4N][BArF4], relative to Fc/Fc+).46,47 These values are ca. 0.15 V more anodic than the corresponding PtBu3 systems under equivalent conditions (E1/2 = −0.50 V, 1/3+; −0.19 V, 2/4+; DFB/[nBu4N][BArF4], relative to Fc/Fc+), congruent with installation of stronger phosphine donors.

Scheme 5 Synthesis and Stability of [M(PAd3)2][BArF4]

[BArF4]− counterion omitted for clarity.

Very broad paramagnetically shifted adamantyl resonances are observed by 1H NMR spectroscopy for both new metalloradicals in DFB solution. No 31P NMR resonance was observed for 13 between +500 and −500 ppm, but a broad signal can be identified for 14 at δ −252.4 (fwhm = 120 Hz), upfield of that observed for 4[BArF4] (δ −213.9). Analysis of 13 by EPR spectroscopy confirms the assignment as a metal-centered radical, with observation of a singlet resonance arising from an axial g-tensor, g⊥= 2.333 and g∥ = 1.979, that is superimposed with a lower intensity sextet arising from isotropic hyperfine coupling to 105Pd (I = 5/2, 22% abundance, a = 24.6 mT; DFB glass @ 200 K). The magnitude of the hyperfine coupling constant is similar to that recorded for 3[BArF4], implying only small changes in the character of the singly occupied molecular orbital. Weak shoulders on the 338 mT hyperfine line could be an indication of an unresolved superhyperfine interaction, but this was not modeled. As for 4[BArF4], no metal-centered EPR spectrum was observed for 14 down to 100 K (DFB glass). The UV–vis spectra of the new metalloradicals (blue, 13; green, 14) are comparable to those of the respective PtBu3 analogues, with the main bands slightly red-shifted.

The formulations of 13 and 14 have been corroborated in the solid state by X-ray diffraction. The structures were obtained using samples recrystallized from DFB/hexane and are isomorphic (monoclinic C2/c) with no crystallographically imposed cation or anion symmetry (M = Pt, Figure 5).48 The cations are well-ordered and adopt near-ideal linear geometries (P2–M1–P3 = 178.94(6)°, 13; 179.29(9)°, 14) and eclipsed phosphine conformations, with the dihedral angles < 11°. Isostructural gold(I) and nickel(I) complexes also adopt this geometry in the solid state.10,43 In line with donor strength arguments the M–P bond lengths for 13 (2.3368(13)/2.3383(14) Å) and 14 (2.309(3)/2.324(2) Å) are on average shorter than those found in the solid state for 3+ (avg 2.349(4) Å) and 4+ (avg 2.336(6) Å), but the difference is not statistically significant.

Figure 5 Solid-state structure of 14. Anisotropic displacement ellipsoids drawn at 30% probability; hydrogen atoms, solvent molecules, and anion omitted for clarity. Selected bond lengths and angles: isomorphous 13, Pd1–P2, 2.3383(14) Å; Pd1–P3, 2.3368(13) Å; P2–Pd1–P3, 178.94(6)°; 14, Pt1–P2, 2.309(3) Å; Pt1–P3, 2.324(2) Å; P2–Pt1–P3, 179.29(9)°.

As for their PtBu3-ligated congeners, both new metalloradicals are stable in DFB with no onward reactivity observed upon standing at room temperature for 24 h. Consistent with our hypothesis, however, 13 and 14 show considerably enhanced stability in THF. The former is fully retained after 24 h in solution, while only ca. 10% decomposition was observed for the latter, in marked contrast to 4[BArF4], which was instantaneously converted into a 1:1 mixture of 5[BArF4] and 6[BArF4] under these conditions.

Carbon-to-Metal H-Atom Transfer Reactions

With a homologous series of isolated examples in hand we sought to systematically evaluate the propensity of late transition metal metalloradicals to activate C(sp3)–H bonds by carbon-to-metal H-atom transfer. Building on the precedent set by Bullock, Fujita, Grills, and co-workers using a transient osmium(I) metalloradical, 3[BArF4], 4[BArF4], 13, and 14 were reacted with 50 equiv of 9,10-dihydroanthracene [De(C(sp3)–H ∼ 77 kcal·mol–1] in DFB at room temperature (Scheme 6).49

Scheme 6 Carbon-to-Metal H-Atom Transfer Reactions of [M(PR3)2]+

[BArF4]− counterion omitted for clarity.

Under these conditions, pseudo-first-order conversion of 4[BArF4] into 6[BArF4] was observed within 5 h (t1/2 = 1.2 h), with concomitant generation of ∼0.5 equiv of anthracene by NMR spectroscopy. An equivalent carbon-to-metal H-atom transfer reaction was, however, not apparent for the palladium congener after 48 h, with only 5% decomposition of 3[BArF4] observed. The corresponding palladium(II) hydride [Pd(PtBu3)2H][BArF4] 15 (δ31P 81.9; δ1H −21.18, 2JPH = 14.1 Hz) is not an inherently unstable product and was independently prepared in 80% isolated yield by reaction of 1 with [H(OEt2)2][BArF4] at low temperature in DFB and shown to be sufficiently persistent to be observed in solution at room temperature (30% decomposition after 24 h). In line with these findings and the enhanced solution stability of the PAd3-ligated metalloradicals, no onward reactivity was observed in the case of 13 after 48 h, while only a very slow carbon-to-metal H-atom transfer reaction was detected for 14. After 6 weeks, ca. 45% conversion of 12 into the new platinum(II) hydride [Pt(PAd3)2H][BArF4] 16 (δ31P 75.0, 1JPtP = 2612 Hz; δ1H −37.32, 2JPH = 8.0 Hz, 1JPtH = 2539 Hz) was observed alongside a small amount of [HPAd3]+ (t1/2 ∼ 40 days). The identity of the hydride was subsequently confirmed by independent synthesis from 12 by reaction with [H(OEt2)2][BArF4] and obtained in 80% isolated yield (see Supporting Information for solid-state structure).

The experimental trends are well reproduced computationally using a direct H-atom transfer mechanism between the metalloradicals and 9,10-dihydroanthracene (Table 1). Reactions of the palladium complexes are characterized by an activation barrier ca. 12 kcal·mol–1 larger than their platinum counterparts. This difference is inversely correlated with the calculated MII–H bond dissociation energies, with those of palladium (64.4/64.1 kcal·mol–1) substantially lower than the bond dissociation energy of the C(sp3)–H bonds in 9,10-dihydroanthracene. The phosphine ligand substituent has a less pronounced effect, but the overall reaction kinetics and thermodynamics are less favorable for the PAd3-ligated metalloradicals. This difference appears to be steric in origin, with the transition states in this case characterized by smaller distortions of the P–M–P angles from linearity.

Table 1 Kinetic and Thermodynamic Parameters for the Carbon-to-Metal H-Atom Transfer Reaction between [M(PR3)2]+ and 9,10-Dihydroanthracene (kcal·mol–1)a

M	R	ΔG‡	ΔGHAT	ΔGrxn	rMC /Å	θ/°	De(MII–H)b	
Pd	tBu	35.5	+13.0	–6.7	3.38	162.0	64.4	
Pt	tBu	22.7	–1.4	–21.0	3.25	160.4	78.5	
Pd	Ad	35.7	+14.0	–5.6	3.32	164.0	64.1	
Pt	Ad	24.2	+0.6	–19.1	3.21	161.9	77.9	
a Calculated at the B2PLYP-D3(BJ)/def2-TZVPP+def-ECP(Pd,Pt)//PBEh-3c level of theory with reaction energies at 298K and corrected for DFB solvent.

b Gas phase values.

Conclusions

One-electron oxidation of palladium(0) and platinum(0) bis(phosphine) complexes enables isolation of a homologous series of linear d9 metalloradicals of the form [M(PR3)2]+ (M = Pd, Pt; R = tBu, Ad), which are stable in 1,2-difluorobenzene (DFB) solution for >1 day at room temperature when partnered with the weakly coordinating [BArF4]− (ArF = 3,5-(CF3)2C6H3) counterion. The metalloradicals exhibit reduced stability in THF solution, decreasing in the order palladium(I) > platinum(I) and PAd3 > PtBu3, especially in the case of [Pt(PtBu3)2]+ (4+), which was converted rapidly into a 1:1 mixture of platinum(II) metallocycle [Pt(PtBu2CMe2CH2)(PtBu3)]+ (5+) and platinum(II) hydride [Pt(PtBu3)2H]+ (6+) upon dissolution at room temperature.

Cyclometalation of 4+ and generation of 5+ can also be achieved by reaction with the 2,4,6-tri-tert-butylphenoxyl radical (•OMes*) in DFB and in situ through reaction with 2,6-di-tert-butyl-4-methylpyridine and ferrocenium in THF. Computational analysis of the onward reactivity of 4+ rules out mechanisms involving Wayland-like four-center-two-electron and concerted C–H bond oxidative activation, but an energetically feasible pathway involving carbon-to-metal H-atom transfer, combination of the resulting C-centered radical with the metal, and formation of the intermediate platinum(III) hydride complex [Pt(PtBu2CMe2CH2)H(PtBu3)]+ (9) was identified (Scheme 7). This organometallic radical rebound mechanism reconciles the experimental observations, with conversion of 9 into 5+ proposed to proceed by H-atom abstraction by •OMes* or sequentially by solvent-mediated deprotonation of 9 to the form neutral platinum(I) complex [Pt(PtBu2CMe2CH2)(PtBu3)] (10), which can be oxidized to 5+ by 4+ or ferrocenium. The platinum(0) byproduct 2 or an added base thereafter mops up the solvated proton, affording platinum(II) complex 6+ in the former case. Carbon-to-metal H-atom transfer is less accessible for the palladium(I) congener [Pd(PtBu3)2]+ (3+), due to an inherently weaker MII–H bond, and the PAd3-ligated metalloradicals, where the caged phosphine substituents encumber C-atom planarization.

Scheme 7 Summary of Mechanistic Proposals

•OMes* = 2,4,6-tri-tert-butylphenoxy.

Reactions of the metalloradicals with 9,10-dihydroanthracene in DFB at room temperature provide direct experimental evidence for carbon-to-metal H-atom transfer in the case of the platinum, although conversion into platinum(II) hydride derivatives is considerably faster for [Pt(PtBu3)2]+ (4+) (t1/2 = 1.2 h) than [Pt(PAd3)2]+ (14) (t1/2 ∼ 40 days). Paralleling their solution stability, the calculated barriers decrease in the order palladium(I) ≫ platinum(I) and PAd3 > PtBu3, reflecting inherent periodic trends in MII–H bond strength and steric constraints, respectively.

These findings demonstrate (a) the synthetic accessibly of low-valent paramagnetic palladium and platinum complexes and (b) the ability of late transition metal-based radicals of this nature to activate C–H bonds in a manner that is mechanistically distinct from their more widely investigated diamagnetic counterparts. Insights of this nature may help inspire the development of new and more effective catalysts for the functionalization of C–H bonds in organic synthesis.

Experimental Section

1 General Methods

All manipulations were performed under an atmosphere of argon using Schlenk and glovebox techniques unless otherwise stated. Glassware was oven-dried at 150 °C overnight and flame-dried under vacuum prior to use. Molecular sieves were activated by heating at 300 °C in vacuo overnight. DFB was predried over Al2O3, distilled from calcium hydride, and dried over two successive batches of 3 Å molecular sieves.24 THF was vacuum distilled from sodium/benzophenone and stored over 3 Å molecular sieves. CD2Cl2 was freeze–pump–thaw degassed and dried over 3 Å molecular sieves. d8-THF was dried over sodium, vacuum distilled, freeze–pump–thaw degassed, and stored over molecular sieves (3 Å). All other anhydrous solvents were purchased from Acros or Sigma-Aldrich, freeze–pump–thaw degassed, and stored over 3 Å molecular sieves. [M(PtBu3)2] (M = Pd, 1; Pt, 2) were purchased from Sigma-Aldrich or Strem Chemicals and recrystallized from hexane before use. [nBu4N][BArF4],50 [FeCp2][BArF4] (recrystallized from Et2O/pentane),51 [H(OEt2)2][BArF4],52 the 2,4,6-tri-tert-butylphenoxyl radical,53 and PAd342 were prepared using literature procedures. 2,6-Di-tert-butyl-4-methylpyridine was purchased from Sigma-Aldrich and used as received.

Cyclic voltammetry (CV) experiments were carried out in an inert atmosphere glovebox under argon using a PalmSens EmStat3+ Blue potentiostat and a three-electrode setup comprising a glassy carbon (CH Instruments, 3.0 mm diameter) working electrode (WE), coiled platinum wire counter electrode (CE), and silver wire quasi-reference electrode (RE). All potentials are calibrated to the ferrocene/ferrocenium (Fc/Fc+) redox couple, which was used as an internal standard. The half-wave potentials, E1/2, were determined from E1/2 = (EPred + EPox)/2, where EPred and EPox are the reduction and oxidation peak potential values, respectively. For the irreversible electrochemical process, the half-peak potential, EP/2, was used as an approximation for E1/2.47

NMR spectra were recorded on Bruker spectrometers under argon at 298K unless otherwise stated. Chemical shifts are quoted in ppm, and coupling constants in Hz. Virtual coupling constants are reported as the separation between the first and third lines.54 NMR spectra in DFB and THF were recorded using an internal capillary of C6D6.24 EPR spectra were acquired on a Bruker EMX spectrometer using a Bruker High Sensitivity cavity (ER 4119 HS). Samples were cooled by nitrogen gas flow through a standard quartz insert from a nitrogen evaporator with a B-VT 2000 temperature control unit. To limit the dielectric loss arising from the solvent, all samples were contained in 2.2 mm i.d. quartz tubes (Wilmad 705-SQ), and the quartz insert was removed for room-temperature operation. The reported g-factors are referenced to a DPPH standard (g = 2.0036(3)).55 EPR data were obtained using a 200 mW microwave power at 9.51 GHz, with a 0.5 mT field modulation at 100 kHz. The simulation was performed using the pepper routine in EasySpin56 with the data fitted directly using a genetic algorithm in the esfit routine. Due to the high spectral width, it was necessary to remove a broad nonlinear cavity baseline prior to fitting by subtraction of a smoothed cubic spline derived from the experimental cavity background recorded under identical conditions. Convolution broadening was applied in the simulations with a combination of Lorentzian and Gaussian line width components necessary to adequately reproduce the observed spectral shape. With this phenomenological line shape model there was no further improvement in the quality of fit from allowing a nonisotropic hyperfine interaction. UV–vis spectra were recorded on an Agilent Cary 3500 UV–vis spectrometer compact Peltier system.

High-resolution (HR) ESI-MS analyses were recorded on a Bruker Maxis Impact instrument. Microanalyses were performed at the London Metropolitan University by Stephen Boyer.

2 Preparation of [Pd(PtBu3)2][BArF4] (3[BArF4])

A solution of [FeCp2][BArF4] (99.6 mg, 94.9 μmol) in DFB (5 mL) was added to a solution of [Pd(PtBu3)2] (53.4 mg, 104 μmol) in DFB (5 mL), and the resulting blue solution stirred at room temperature for 15 min. Volatiles were removed in vacuo, and the residue was washed with hexane (3 × 5 mL) and then recrystallized from DFB/hexane at −30 °C to afford the product as ultramarine blocks. Yield: 113 mg (82.2 μmol, 87%). Spectroscopic data are consistent with literature data for 3[PF6] and 3[HCB11Cl11].14,151H NMR (400 MHz, DFB): δ 19.3 (vbr, fwhm = 950 Hz, 54H, tBu), 8.26 (br, 8H, ArF), 7.62 (s, 4H, ArF). 1H NMR (500 MHz, THF): δ 19.7 (vbr, fwhm = 940 Hz, 54H, tBu), 8.27 (br, 8H, ArF), 8.06 (s, 4H, ArF). 31P{1H} NMR (162 MHz, DFB): No signals observed over the range δ −500 to + 500. 31P{1H} NMR (162 MHz, THF): No signals observed over the range δ −500 to + 500. UV–vis (DFB): λmax 330 (br, ε = 300 M–1 cm–1), 668 (br, ε = 4700 M–1 cm–1) nm. UV–vis (THF): λmax 337 (br, ε = 700 M–1 cm–1), 668 (br, ε = 4400 M–1 cm–1) nm. EPR (15 mM in DFB, 200 K): g⊥= 2.343, g∥ = 1.978, aiso(105Pd) = 25.2 mT. HR ESI-MS (positive ion, 4 kV): 511.2785 ([M + H]+, calcd 511.2818) m/z. Anal. Calcd for C56H66BF24P2Pd (1374.28 g·mol–1): C, 48.94; H, 4.84; N, 0.00. Found: C, 48.86; H, 4.68; N, 0.00.

3 Preparation of [Pt(PtBu3)2][BArF4] (4[BArF4])

Method A: A solution of [FeCp2][BArF4] (127.8 mg, 121.8 μmol) in DFB (5 mL) was added to a cooled (−10 °C) solution of [Pt(PtBu3)2] (80.6 mg, 134.4 μmol) in DFB (10 mL). The resulting green solution was warmed to room temperature and stirred for 5 min. Volatiles were removed in vacuo, and the residue was washed with hexane (4 × 2 mL) and then recrystallized from DFB/hexane at −30 °C to afford the product as baby blue blocks. Yield: 123.0 mg (84.1 μmol, 69%).

Method B: A mixture of [FeCp2][BArF4] (52.5 mg, 50 μmol), [Pt(PtBu3)2] (31.5 mg, 52.5 μmol), and 2,6-di-tert-butyl-4-methylpyridine (1.03 mg, 5 μmol) was dissolved in cold DFB (4 mL, −30 °C). The resulting green solution was layered with an excess of cold hexane (−30 °C) to afford the product as baby blue crystals upon diffusion at −30 °C. Yield: 45.0 mg (30.8 μmol, 62%).

Spectroscopic data are consistent with literature data for 4[HCB11Cl11].151H NMR (400 MHz, DFB): δ 11.4 (vbr, fwhm = 260 Hz, 54H, tBu), 8.16 (br, 8H, ArF), 7.46 (s, 4H, ArF). 31P{1H} NMR (162 MHz, DFB): δ −213.9 (vbr, fwhm = 90 Hz). UV–vis (DFB): λmax 306 (sharp, ε = 700 M–1 cm–1), ∼320 (br shoulder, ε = 200 M–1 cm–1), 680 (ε = 200 M–1 cm–1) nm. EPR (15 mM in DFB, 100 K): observed signal not significantly different from cavity background. Anal. Calcd for C56H66BF24P2Pt (1462.95 g·mol–1): C, 45.98; H, 4.55; N, 0.00. Found: C, 45.92; H, 4.39; N, 0.00.

4 Solution Stability of 3[BArF4] and 4[BArF4]

Solutions of the metalloradicals (10.0 μmol) were prepared by dissolution in DFB or THF/d8-THF (0.5 mL) within J. Young valve NMR tubes at room temperature, and the stability/onward reactivity monitored in situ by NMR spectroscopy. After standing at room temperature for 24 h, no onward reactivity was apparent for 3[BArF4] or 4[BArF4] in DFB. Partial decomposition of 3[BArF4] (ca. 20%) was observed after standing at room temperature for 24 h in THF, while instantaneous formation of a 1:1 mixture of [Pt(PtBu2CMe2CH2)(PtBu3)][BArF4] and [Pt(PtBu3)2H][BArF4] in quantitative spectroscopic yield was apparent upon dissolution of 4[BArF4] in THF at room temperature. The identity of these species was thereafter established in situ by comparison to literature values and analysis of isolated samples in THF. When this experiment was repeated in d8-THF, no D-atom incorporation was apparent by analysis of the product mixture by 2H NMR spectroscopy in CH2Cl2 (with 20 μL of CD2Cl2).

5 Preparation of [Pt(PtBu2CMe2CH2)(PtBu3)][BArF4] (5[BArF4])

A solution of [FeCp2][BArF4] (104.9 mg, 100 μmol) in THF (2 mL) was added to a solution of [Pt(PtBu3)2] (30.0 mg, 50.0 μmol) and 2,6-di-tert-butyl-4-methylpyridine (52.0 mg, 253 μmol) in THF (2 mL). The deep blue solution was stirred at room temperature for 36 h. Volatiles were removed in vacuo, and the residue was washed with hexane (3 × 5 mL) and then recrystallized from CH2Cl2/PhMe at room temperature to afford the product as pale-yellow blocks. Yield: 40.9 mg (28.0 μmol, 56%). Spectroscopic data are consistent with literature data for 5[PF6].141H NMR (500 MHz, CD2Cl2): δ 7.71–7.75 (m, 8H, ArF), 7.56 (br, 4H, ArF), 2.74 (dd′, 3JPH = 10.4, 3JPH = 4.0, 2JPtH = 110, 2H, PtCH2), 1.56 (d, 3JPH = 13.1, 6H, PtBu2CMe2CH2), 1.55 (d, 3JPH = 14.1, 18H, PtBu2CMe2CH2), 1.44 (d, 3JPH = 13.1, 27H, PtBu3). 31P{1H} NMR (202 MHz, CD2Cl2): δ 59.5 (d′, 2JPP = 316, 1JPtP = 2902, 1P, PtBu3), 25.2 (d′, 2JPP = 316, 1JPtP = 1916, 1P, PtBu2CMe2CH2). 31P{1H} NMR (162 MHz, DFB): δ 59.0 (d′, 2JPP = 316, 1JPtP = 2898, 1P, PtBu3), 24.4 (d′, 2JPP = 316, 1JPtP = 1918, 1P, PtBu2CMe2CH2). 31P{1H} NMR (162 MHz, THF): δ 59.1 (d′, 2JPP = 317, 1JPtP = 2899, 1P, PtBu3), 25.3 (d′, 2JPP = 317, 1JPtP = 1915, 1P, PtBu2CMe2CH2). HR ESI-MS (positive ion, 4 kV): 598.3268 ([M]+, calcd 598.3268) m/z.

6 Preparation of [Pt(PtBu3)2H][BArF4] (6[BArF4])

A cold (−30 °C) solution of [H(OEt2)2][BArF4] (45.8 mg, 45.2 μmol) in DFB (2 mL) was added to a cold (−30 °C) solution of [Pt(PtBu3)2] (27.1 mg, 45.2 μmol) in DFB (2 mL). The yellow solution was warmed to room temperature and stirred for 15 min. Volatiles were removed in vacuo, and the residue was recrystallized from DFB/hexane at −30 °C to afford the product as yellow blocks. Yield: 51.0 mg (34.8 μmol, 77%). Spectroscopic data are consistent with literature data.26 This complex is stable in DFB solution at room temperature for 24 h. 1H NMR (400 MHz, CD2Cl2): δ 7.70–7.75 (m, 8H, ArF), 7.56 (br, 4H, ArF), 1.51 (vt, JPH = 13.2, 54H, tBu), −36.53 (t′, 2JPH = 8.5, 1JPtH = 2602 Hz, 1H, PtH). 1H NMR (400 MHz, DFB, selected data): δ −36.41 (t′, 2JPH = 8.6, 1JPtH = 2598, 1H, PtH). 1H NMR (400 MHz, THF, selected data): δ −35.28 (br′, 1JPtH = 2540, 1H, PtH). 31P{1H} NMR (162 MHz, CD2Cl2): δ 86.9 (s′, 1JPtP = 2624). 31P{1H} NMR (162 MHz, DFB): δ 86.5 (s′, 1JPtP = 2623). 31P{1H} NMR (162 MHz, THF): δ 86.5 (s′, 1JPtP = 2631). HR ESI-MS (positive ion, 4 kV): 600.3407 ([M]+, calcd 600.3424) m/z.

7 Reaction of 4[BArF4] with •OMes*

To a J. Young valve NMR tube charged with 2,4,6-tri-tert-butylphenoxyl radical (3.14 mg, 12.0 μmol) was added a solution of 4[BArF4] (10.0 μmol) in DFB (0.5 mL) at room temperature, and the onward reactivity monitored in situ by NMR spectroscopy at room temperature. Smooth and apparent first-order conversion of 4[BArF4] into [Pt(PtBu2CMe2CH2)(PtBu3)][BArF4] with ca. 90% selectivity was observed over 8 days (t1/2 = 49 h).

8 Preparation of [Pd(PAd3)2] (11)

A suspension of [Pd(PtBu3)2] (23.7 mg, 46.4 μmol) and PAd3 (60.8 mg, 139 μmol) in PhMe (5 mL) was heated at 130 °C for 18 h. The precipitate was isolated by filtration at room temperature, washed with PhMe (3 × 2 mL) and hexane (5 × 1 mL), and dried in vacuo to afford the product as a fine colorless powder. Yield: 40.4 mg (41.2 μmol, 89%). Acquisition of NMR data was not possible due to the insolubility of this complex. An alternative procedure has been deposited in a preprint archive.44 Anal. Calcd for C60H90P2Pd (979.75 g·mol–1): C, 73.56; H, 9.26. Found: C, 73.36; H, 9.17.

9 Preparation of [Pt(PAd3)2] (12)

A suspension of [Pt(PtBu3)2] (180 mg, 300 μmol) and PAd3 (288 mg, 660 mmol) in PhMe (5 mL) was heated at 130 °C for 18 h. The precipitate was isolated by filtration at room temperature, washed with PhMe (3 × 5 mL), and dried in vacuo to afford the product as a fine colorless powder. Yield: 265 mg (248 μmol, 83%). Acquisition of NMR data was not possible due to the insolubility of this complex. Anal. Calcd for C60H90P2Pt (1068.41 g·mol–1): C, 67.45; H, 8.49. Found: C, 67.50; H, 8.62.

10 Preparation of [Pd(PAd3)2][BArF4] (13)

A suspension of [Pd(PAd3)2] (99.2 mg, 101 μmol) and [FeCp2][BArF4] (103 mg, 98.1 μmol) in DFB (5 mL) was vigorously stirred at room temperature for 15 min. Volatiles were removed in vacuo, and the residue was washed with hexane (3 × 1 mL) and then extracted with DFB (3 × 1 mL). The combined DFB extracts were layered with excess hexane to afford the product as turquoise blocks upon diffusion at −30 °C. Yield: 167 mg (90.6 μmol, 92%). 1H NMR (400 MHz, DFB): δ 8.27 (br, 8H, ArF), 7.63 (s, 4H, ArF), 4.64 (vbr, fwhm = 140 Hz, 18H, Ad{3-CH}), 1.66 (vbr, fwhm = 230 Hz, 36H, Ad{4-CH2}). The 2-Ad signal was not observed over the range −50 to +50. 1H NMR (400 MHz, THF): δ 8.23 (br, 8H, ArF), 8.00 (s, 4H, ArF), 5.11 (vbr, fwhm = 130 Hz, 18H, Ad{3-CH}). The 4-Ad signal is obscured by the solvent and the 2-Ad signal was not observed over the range −50 to +50. 13C{1H} NMR (126 MHz, DFB): δ 162.7 (q, 1JCB = 50, ArF), 135.3 (s, ArF), 129.9 (qq, 2JFC = 32, 2JCB = 3, ArF), 125.1 (q, 1JFC = 272, ArF), 117.8 (br, ArF). No Ad signals observed. 31P{1H} NMR (162 MHz, DFB): No signals observed over the range δ −500 to +500. 31P{1H} NMR (162 MHz, THF): No signals observed over the range δ −500 to +500. UV–vis (DFB): λmax 310 (ε = 800 M–1 cm–1), 694 (ε = 3100 M–1 cm–1) nm. EPR (15 mM in DFB, 200 K): g⊥= 2.333, g∥ = 1.979, aiso(105Pd) = 24.6 mT. HR ESI-MS (positive ion, 4 kV): 978.5568 ([M]+, calcd 978.5568) m/z. Anal. Calcd for C92H102BF24P2Pd (1842.97 g·mol–1): C, 59.96; H, 5.58; N, 0.00. Found: C, 60.04; H, 5.69; N, 0.00.

11 Preparation of [Pt(PAd3)2][BArF4] (14)

A suspension of [Pt(PAd3)2] (53.4 mg, 50.0 μmol) and [FeCp2][BArF4] (47.2 mg, 45.0 μmol) in DFB (2 mL) was vigorously stirred at room temperature for 15 min. Volatiles were removed in vacuo, and the residue was washed with hexane (3 × 1 mL) and then extracted with DFB (3 × 1 mL). The combined DFB extracts were layered with excess hexane to afford the product as malachite blocks in >99% purity upon diffusion at −30 °C. Yield: 71.3 mg (36.9 μmol, 82%). Trace amounts of [HPAd3][BArF4] can be observed, but removed by recrystallization from CH2Cl2/heptane. 1H NMR (400 MHz, DFB): δ 8.28 (br, 8H, ArF), 7.63 (s, 4H, ArF). A continuous broad resonance in the baseline is observed over the range δ −5 to 15 and attributed to Ad resonances. 1H NMR (400 MHz, THF): Only anion signals observed. 13C{1H} NMR (126 MHz, DFB): δ 162.8 (q, 1JCB = 50, ArF), 135.3 (s, ArF), 129.9 (qq, 2JFC = 32, 2JCB = 3, ArF), 125.0 (q, 1JFC = 272, ArF), 117.8 (sept., 3JFC = 4, ArF). No Ad signals observed. 31P{1H} NMR (162 MHz, DFB): δ −252.4 (vbr, fwhm = 120 Hz). 31P{1H} NMR (162 MHz, THF): δ −245.2 (vbr, fwhm = 95 Hz). UV–vis (DFB): λmax 309 (sharp, ε = 1100 M–1 cm–1), ∼325 (br shoulder, ε = 700 M–1 cm–1), 703 (ε = 200 M–1 cm–1) nm. EPR (15 mM in DFB, 100 K): observed signal not significantly different from cavity background. HR ESI-MS (positive ion, 4 kV): 1068.6242 ([M]+, calcd 1068.6244) m/z. Anal. Calcd for C92H102BF24P2Pt (1931.63 g·mol–1): C, 57.21; H, 5.32; N, 0.00. Found: C, 57.40; H, 5.35; N, 0.00.

12 Solution Stability of 13 and 14

Solutions were prepared by dissolution of the metalloradicals (10.0 μmol) in DFB or THF (0.5 mL) within J. Young valve NMR tubes at room temperature, and thereafter the stability/onward reactivity was analyzed in situ by NMR spectroscopy. After standing at room temperature for 24 h, no onward reactivity was apparent for 13 and 14 in DFB and 13 in THF. Partial decomposition of 14 (10%) was observed after standing at room temperature for 24 h in THF, generating a mixture containing [Pt(PAd3)2H][BArF4] and [HPAd3][BArF4]. The former was subsequently confirmed by independent synthesis.

13 Reactions of 3[BArF4], 4[BArF4], 13, and 14 with 9,10-Dihydroanthracene

Solutions of 3[BArF4], 4[BArF4], 13, or 14 (10.0 μmol) in DFB (0.5 mL) were added into a J. Young valve NMR tube charged with 9,10-dihydroanthracene (92.0 mg, 500 μmol) and the resulting solutions analyzed in situ by NMR spectroscopy over time at room temperature. There was no evidence for the formation of anthracene or hydride derivatives after 48 h for both palladium metalloradicals, but ca. 5% decomposition of 3[BArF4] was observed over this time frame. Pseudo-first-order conversion of 4[BArF4] into [Pt(PtBu3)2H][BArF4] was observed within 5 h (t1/2 = 1.2 h), alongside ca. 0.4 equiv of anthracene. Trace amounts of anthracene and [Pt(PAd3)2H][BArF4] were observed after 48 h in the case of 14, the identity of which was subsequently confirmed by independent synthesis. After 6 weeks in solution, ca. 45% conversion of 14 (t1/2 ∼ 40 days) into [Pt(PAd3)2H][BArF4] was observed alongside anthracene and a small amount of [HPAd3][BArF4].

14 Preparation of [Pd(PtBu3)2H][BArF4] (15)

A cold (−30 °C) solution of [H(OEt2)2][BArF4] (45.8 mg, 45.2 μmol) in DFB (1 mL) was added to a cold (−30 °C) solution of [Pd(PtBu3)2] (22.9 mg, 44.8 μmol) in DFB (2 mL). The yellow solution was stirred for 5 min, then layered with excess cold hexane to afford the product as yellow blocks upon diffusion at −30 °C. Yield: 49.0 mg (35.6 μmol, 80%). Partial decomposition (30%) of this complex was observed after standing in DFB solution at room temperature for 24 h. 1H NMR (500 MHz, DFB): δ 8.13 (br, 8H, ArF), 7.49 (s, 4H, ArF), 1.28 (vt, JPH = 13.6, 54H, tBu), −21.18 (t, 2JPH = 14.1, 1H, PdH). 13C{1H} NMR (126 MHz, DFB): δ 162.6 (q, 1JCB = 50, ArF), 135.1 (s, ArF), 129.7 (qq, 2JFC = 32, 2JCB = 3, ArF), 124.9 (q, 1JFC = 273, ArF), 117.6 (sept., 3JFC = 4, ArF), 39.0 (vt, JPC = 12, tBu{C}), 31.6 (vt, JPC = 5, tBu{CH3}). 31P{1H} NMR (162 MHz, DFB): δ 81.9 (s). HR ESI-MS (positive ion, 4 kV): 511.2798 ([M]+, calcd 511.2818) m/z. Anal. Calcd for C56H67BF24P2Pd (1375.29 g·mol–1): C, 48.91; H, 4.91; N, 0.00. Found: C, 48.98; H, 4.84; N, 0.00.

15 Preparation of [Pt(PAd3)2H][BArF4] (16)

A cold (−30 °C) solution of [H(OEt2)2][BArF4] (20.6 mg, 20.3 μmol) in DFB (1 mL) was added to a cold (−30 °C) suspension of [Pt(PAd3)2] (22.0 mg, 20.6 μmol) in DFB (1 mL). The yellow suspension was warmed to room temperature, stirred for 18 h, and filtered, and the precipitate extracted with additional DFB (3 × 0.5 mL). The combined filtrate and washings were layered with excess hexane, and the product was obtained as yellow blocks upon diffusion at −30 °C. Yield: 31.4 mg (16.3 μmol, 80%). This complex is stable in DFB solution at room temperature for 24 h. 1H NMR (400 MHz, DFB): δ 8.11 (br, 8H, ArF), 7.48 (s, 4H, ArF), 2.37 (br, 36H, Ad{2-CH2}), 1.94 (s, 18H, Ad{3-CH}), 1.60–1.73 (m, 36H, 2 × Ad{4-CH2}), −37.32 (t′, 2JPH = 8.0, 1JPtH = 2539,1H, PtH). 1H NMR (400 MHz, THF, selected data): δ −36.58 (t′, 2JPH = 7.8, 1JPtH = 2531,1H, PtH). 13C{1H} NMR (126 MHz, DFB): δ 162.6 (q, 1JCB = 50, ArF), 135.1 (s, ArF), 129.7 (qq, 2JFC = 32, 2JCB = 3, ArF), 124.8 (q, 1JFC = 272, ArF), 117.6 (sept., 3JFC = 4, ArF), 48.9 (vt, JPC = 13, Ad{1-C}), 43.3 (br, Ad{2-CH2}), 36.2 (s, Ad{4-CH2}), 29.4 (vt, JPC = 8, Ad{3-CH}). 31P{1H} NMR (162 MHz, DFB): δ 75.0 (s′, 1JPtP = 2612). 31P{1H} NMR (162 MHz, THF): δ 75.2 (s′, 1JPtP = 2609). HR ESI-MS (positive ion, 4 kV): 1068.6244 ([M]+, calcd 1068.6244) m/z. Anal. Calcd for C92H103BF24P2Pt (1932.64 g·mol–1): C, 57.18; H, 5.37; N, 0.00. Found: C, 57.26; H, 5.18; N, 0.00.

16 Computational Details

All electronic structure calculations presented in this paper were carried out using the ORCA 5.0.3 program package.57 To find energetically most favorable conformers, initial conformer searches of selected complexes were conducted using the automated crest approach,58 which employs the efficient semiempirical extended tight binding method (GFN2-xTB) with a specially adapted implicit solvation model (ALPB).59 Unconstrained geometry optimizations in C1 symmetry and analytical (or numerical for the largest systems) frequency calculations of all compounds were carried out at the DFT level, using the PBEh-3c composite method.30 The orbitals are expanded in a modified valence double-ζ Gaussian basis corresponding to the Ahlrichs-type def2-mSVP set, in conjunction with the def2/J auxiliary basis set for the RI approximation to the Coulomb term.60 The calculations utilized the def2-ECP for Pd (replacing 28 core electrons) and Pt (replacing 60 core electrons).61 To account for inter- and intramolecular basis set superposition error (BSSE) and long-range London dispersion effects, the geometrical counterpoise correction (gCP)62 and atom-pairwise DFT-D3 (Becke–Johnson damping) schemes are utilized.33 All optimized stationary points were characterized by analysis of their analytical second derivatives, with minima having only positive eigenvalues and transition states having one imaginary eigenvalue. The nature of transition states was confirmed via intrinsic reaction coordinate (IRC) calculations in both forward and reverse direction of the reaction coordinate.63 Subsequent geometry optimizations of the IRC end points yielded the nearest minima linked by a transition state. The frequency calculations also provided thermal and entropic corrections to the total energy in the gas phase at T = 298.15 K and p = 1 atm within the quasi rigid-rotor/harmonic oscillator (QRRHO) approximation.64 Open-shell singlet states (MS = 0) corresponding to antiferromagnetically coupled metal centers were modeled with the spin-unrestricted broken-symmetry (BS) formalism.65 The FlipSpin feature of ORCA was used to generate initial guesses for the BS calculations. Geometries with these states were fully optimized, and convergence to the desired BS solution was confirmed by inspection of magnetic orbitals, spin populations, and the expectation value of the ⟨S2⟩ operator. The energies of the BS states were used without spin projection. Single-point energies were computed using the B2PLYP-D3(BJ) double-hybrid functional31 (including Grimme’s D3 atom-pairwise dispersion correction and Becke–Johnson damping) in combination with the def2-TZVPP basis set.32 Full details of calculations are provided in the Supporting Information. Effects due to the presence of a solvent were treated implicitly with a conductor-like polarizable continuum (CPCM) and Truhlar’s SMD model.34 Solvent parameters corresponded to those of tetrahydrofuran (ε = 7.4, refractive index = 1.000), or, in the absence of defined parameters for DFB solvent, default SMD parameters were selected for fluorobenzene and the dielectric constant adjusted to that of DFB (ε = 13.4, refractive index = 1.443). Geometries were visualized using the ChemCraft software package.68

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.3c04167.NMR, EPR, UV–vis, and ESI-MS spectra of complexes and selected reactions; additional electrochemical data and analysis; further computational details, data, and analysis (PDF)

Optimized geometries of species examined computationally (XYZ)

Animations of selected transition states (MP4)

Supplementary Material

ja3c04167_si_001.pdf

ja3c04167_si_002.xyz

ja3c04167_si_003.mp4

The authors declare no competing financial interest.

Acknowledgments

We thank the EPSRC (EP/K035681/1, T.K., S.A.M.; DTP studentship to M.J.G.S.), European Research Council (ERC grant agreement 637313, M.R.G., A.B.C.), Spanish Ministry of Universities and European Union (Margarita Salas grant funded by the European Union-NextGenerationEU, I.B.), and Royal Society (UF100592, UF150675, A.B.C.) for financial support. T.K. and S.A.M. acknowledge the DJEI/DES/SFI/HEA Irish Centre for High-End Computing (ICHEC) and ARCHER2 UK National Supercomputing Facility for the provision of high-performance computing facilities and support. EPR facilities were provided by the Spectroscopy RTP at the University of Warwick. High-resolution mass-spectrometry data were collected using instruments purchased through support from Advantage West Midlands and the European Regional Development Fund. Crystallographic data were collected using an instrument that received funding from the ERC under the European Union’s Horizon 2020 research and innovation program (grant agreement No. 637313). T.K. thanks Dr. Ragnar Bjornsson (CEA Grenoble) for insightful discussions.
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We suggest that 6[BArF4] results from reaction of the platinum metalloradical with trace phenolic impurities in the DFB solvent, such as 2-fluorophenol.24 Consistent with this suggestion, reaction of 4[BArF4] with 2,4,6-tri-tert-butylphenol in DFB resulted in formation of 6[BArF4] and over time 5[BArF4] also.

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The barrier remains high when concentration factors are taken into account: ΔG⧧298K(20 mM 4+ in THF) = 39.2 kcal·mol–1.

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For comparison, H-atom abstraction from a higher energy isomer of 9 invoked in the concerted C–H bond oxidative addition pathway has a barrier of ΔG⧧298K = 29.9 kcal·mol–1 vs 4+. Full details are provided in the Supporting Information (Figure S82).

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