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Acta Crystallogr C Struct Chem
Acta Crystallogr C Struct Chem
Acta Cryst. C
Acta Crystallographica Section C: Structural Chemistry
2053-2296
International Union of Crystallography

39115534
jx3087
10.1107/S2053229624007058
ACSCGG
S2053229624007058
Research Papers
Formation of a di­iron–(μ-η1:η1-CN) com­plex from aceto­nitrile solution
Formation of a di­iron–(μ-η1:η1-CN) complex
Schlachta Tim P. a
Sauer Michael J. a
Richter Leon F. a
Kühn Fritz E. a*
a Technical University of Munich, School of Natural Sciences, Department of Chemistry and Catalysis Research Center, Molecular Catalysis, Lichtenbergstrasse 4, 85748 Garching, Germany
Turner D. R. Editor
University of Monash, Australia
Correspondence e-mail: fritz.kuehn@ch.tum.de
01 9 2024
08 8 2024
08 8 2024
80 Pt 9 c240900 534537
11 6 2024
18 7 2024
© Schlachta et al. 2024
2024
https://creativecommons.org/licenses/by/4.0/ This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.
A full version of this article is available from Crystallography Journals Online.
A di­iron end-on μ2-η1:η1-CN-bridged com­plex is obtained from a crystallization experiment of an open-chain iron–NHC com­plex. The cyanide appears to originate from the aceto­nitrile (MeCN) solvent by C—C bond cleavage or through carbon–hy­dro­gen oxidation.

The activation of C—C bonds by transition-metal com­plexes is of continuing inter­est and aceto­nitrile (MeCN) has attracted attention as a cyanide source with com­paratively low toxicity for organic cyanation reactions. A di­iron end-on μ-η1:η1-CN-bridged com­plex was obtained from a crystallization experiment of an open-chain iron–NHC com­plex, namely, μ-cyanido-κ2C:N-bis­{[(aceto­nitrile-κN)[3,3′-bis­(pyridin-2-yl)-1,1′-(methyl­idene)bis­(benzimidazol-2-yl­idene)]iron(II)} tris­(hexa­fluoro­phos­phate), [Fe2(CN)(C2H3N)2(C25H18N6)2](PF6)3. The cyanide appears to originate from the MeCN solvent by C—C bond cleavage or through carbon–hy­dro­gen oxidation.

organometallic iron(II) complex
acetonitrile cleavage
N-heterocyclic carbene
NHC
crystal structure
cyanation
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pmcIntroduction

The first iron–NHC (N-heterocyclic carbene) com­plex was developed by Öfele in 1969 (Öfele, 1969 ▸). However, it has taken many years for iron–NHC com­plexes to attract the attention of a wider audience of chemists, but, especially in the last decade, there has been a sharp increase in related publications (Riener et al., 2014 ▸). The open-chain iron–pyridine-NHC com­plex bis­(aceto­nitrile-κN)[3,3′-bis­(pyridin-2-yl)-1,1′-(methyl­idene)bis­(benzimidazol-2-yl­idene)]iron(II) bis­(hexa­fluoro­phos­phate), 1 (Scheme 1), can be successfully employed in homogeneous epoxidation catalysis (Schlachta et al., 2024 ▸). In the present work, a di­iron end-on μ-η1:η1-CN-bridged com­plex, 2 (Scheme 2), is formed from a solution of 1 in deu­ter­ated aceto­nitrile. The activation of C—C bonds by tran­si­tion-metal com­plexes is of continuing inter­est and MeCN has attracted attention as a cyanide source with com­paratively low toxicity for organic cyanation reactions (Ahmad et al., 2020 ▸; Lu et al., 2004 ▸; Spentzos et al., 2020 ▸; Grirrane et al., 2016 ▸).

Experimental

General procedures and analytical methods

Complex 1 was synthesized according to a literature method (Schlachta et al., 2024 ▸). Solvents were purified, dried and de­gassed using standard methods (Armarego, 2017 ▸) or received from a solvent purification system by M. Braun. All other chemicals were obtained from commercial suppliers and were used without further purification. NMR spectra were recorded on a Bruker Avance Ultrashield AV400 (400.13 MHz for 1H NMR and 100.53 MHz for 13C NMR). The chemical shifts are given in δ values in ppm (parts per million) relative to TMS (tetra­methyl­silane) and are reported relative to the residual deuterated solvent signal (Fulmer et al., 2010 ▸). Electrospray ionization mass spectrometry (ESI–MS) data were measured on a Thermo Fisher Ultimate 3000. FT–IR measurements were conducted on a PerkinElmer Frontier FT–IR spec­trom­eter (ATR). The ‘inVia Reflex Raman System’ com­prises a research grade optical microscope [Leica DM2700M, Magnification 5×, 20× and 50× (in this case, 50× was used)] coupled to a high-performance Raman spectrometer (Renishaw). A 633 nm wavelength laser was used (Renishaw RL633 Class 3B).

Crystallization of 2

Single crystals of 2 suitable for X-ray diffraction were obtained by slow evaporation of a solution of 1 in CD3CN over a period of six months at room temperature under an ambient atmosphere near a window with sunlight (see supporting information).

A solution of 1 (around 1–2 mg) in CD3CN (around 0.4 ml, dry and degassed) from an NMR tube (see supporting information) was placed in a 10 ml vial under an ambient atmosphere. A human hair was fixed with adhesive tape to the inside of the vial, reaching into the solution. Heterogeneous nu­cle­ation occurs more frequently than homogeneous nucleation (Sear, 2014 ▸; Pruppacher & Klett, 1997 ▸) and human hair has been used for the growth of nanoparticles or as catalyst-support material (Deng et al., 2016 ▸; Liu et al., 2015 ▸; Haveli et al., 2012 ▸; Walter et al., 2006 ▸). The vial was closed and the cap was punctured with a cannula. The vial was left for six months at room temperature under ambient conditions near a window with sunlight, allowing the solvent to evaporate slowly. Orange crystals suitable for SC-XRD analysis were obtained.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1 ▸. H atoms could not be located in difference Fourier maps and were calculated in ideal positions (riding model), with C—H = 0.98 Å and Uiso(H) = 1.5Ueq(C) for CH3 groups, C—H = 0.99 Å and Uiso(H) = 1.2Ueq(C) for CH2 groups, and C—H = 0.95 Å and Uiso(H) = 1.2Ueq(C) for CH groups. Split-layer position refinement was used for atoms P2, F7, F8, F9, F10, F11 and F12 (PF6− anion), as well as N8 and C28 (bridging cyanide). Restraints were applied to atoms N8 and C28 to ensure reasonable ellipsoids. CD3 has been modelled as CH3 as there is no appreciable difference in SC-XRD.

Results and discussion

When a solution of 1 (Scheme 1 and Fig. 1 ▸) in CD3CN was evaporated slowly over a period of six months under ambient conditions, a di­iron end-on μ-η1:η1-CN-bridged com­plex, [(MeCN)(NHC)Fe]2(μ-η1:η1-CN)(PF6)3 (2) (Scheme 2 and Fig. 2 ▸), was obtained, as determined by X-ray diffraction. The two iron centres are bridged by a cyanide anion, hence three PF6− anions are present in the crystal structure. Under similar conditions, i.e. MeCN solution, room temperature and air, a dinuclear CuII cryptate has been found to form a μ-η1:η1-CN-bridged com­plex by C—C bond cleavage of MeCN (Lu et al., 2004 ▸). A possible mechanism involving the activation of the sp-hybridized C atom of MeCN, bound to one Cu atom (MeCN—Cu), by the second Cu centre has been suggested. The increased electrophilicity of the methyl group would allow cleavage by H2O to form MeOH and the cyanide-bridged com­pound (Lu et al., 2004 ▸; Ahmad et al., 2020 ▸). Another possible mechanism for the formation of 2 might be the carbon–hy­dro­gen oxidation of MeCN by iron com­plex 1 to form glycolo­nitrile, as observed previously for an iron(III) tetra­carbene com­plex, and subsequent release of cyanide upon decay of glycolo­nitrile (Knapp et al., 2012 ▸; Dyckhoff et al., 2021 ▸; Lewis, 2008 ▸). Due to the stronger Me—CN bond (122 kcal mol−1) com­pared to the H—CH2CN bond (93 kcal mol−1) (Spentzos et al., 2020 ▸; Blanksby & Ellison, 2003 ▸; Goebbert et al., 2010 ▸; Miscione & Bottoni, 2014 ▸), the carbon–hy­dro­gen oxidation of MeCN seems to be more likely the origin of cyanide in this case. However, C—C bond cleavage of MeCN by UV irradiation is known (Grirrane et al., 2016 ▸) and, given the fact that the crystallization setup with 1 was also accessible for sunlight during the extensive period of six months, C—C bond cleavage of MeCN cannot be excluded.

The crystal structure of 2 reveals strongly bent equatorial NHC ligands. This finding is in stark contrast to 1, where the NHC ligand is largely planar (Fig. 1 ▸). This sandwich-like structure encapsulates the cyanide ion and is indicative of some noncovalent inter­actions between the equatorial ligands, likely contributing to the stability of 2. Inter­estingly, the py­ri­dine units are bent less towards the centre com­pared to the NHC units, forming a Z-shape or diamond-shape, depending on the viewing angle of 2. The Fe—N—C angle is slightly bent (Table 2 ▸) in a trans fashion, resulting in a ‘zigzag’ vertical axis. Another inter­esting finding is the rotation of the NHC ligands towards each other in an anti conformation, resulting in a dihedral angle (CH2—Fe—Fe—CH2) of 160.7° (Scheme 2). The crystal structure can in principle also be solved as the di­iron–(μ-η1:η1-N2) com­plex (Fig. 2 ▸), which is why we refrain from a detailed structural discussion at this point. However, there are several arguments against a di­iron–(μ-η1:η1-N2) com­plex:

(i) The main argument against a di­iron–(μ-η1:η1-N2) com­plex is the fact that the crystal structure contains three counter-ions. As the crystallization was performed with 1 containing an iron(II) centre, bridging two FeII atoms with a neutral N2 ligand should lead to the presence of four counter-ions. Otherwise, three counter-ions would indicate that a redox process has occurred during the formation of 2, but the nature of a hypothetical reducing agent and the location of reduction are highly speculative. The main com­ponents of the crystallization experiment were 1 and CD3CN, as well as un­reacted ligand precursor as a minor impurity (see sup­porting information). In a cyclic voltammetry study of 1, the first reduction event occurred at −1.78 V (versus Fc/Fc+). A pre­limi­nary experiment measuring 1 in cyclic voltammetry under an N2 atmosphere did not show significant redox processes or electric current. Considering all these facts, the involvement of a redox process appears to be quite implausible.

(ii) Di­nitro­gen is a weak σ-donor and a weak π-acceptor, and substitution of the N2 ligand with CO or nitriles like MeCN is often observed (Crossland & Tyler, 2010 ▸; Sunada et al., 2013 ▸). A di­iron–(μ-η1:η1-N2) version of 2 would be very surprising in this context, since one axial MeCN ligand coordinates with one iron centre each, the crystallization of 2 occurred in (deuterated) MeCN as solvent and the previous occupation of both axial coordination sites by MeCN in 1. The stability of 2 under air is also inter­esting, which would be rather uncommon for a di­iron–(μ-η1:η1-N2) com­plex (Crossland & Tyler, 2010 ▸; Takeshita et al., 2018 ▸; Saouma et al., 2011 ▸; Regenauer et al., 2022 ▸) and an affinity for N2 over O2 would be very unusual considering other Fe com­pounds tending to form di­iron–μ-oxido species (Schlachta & Kühn, 2023 ▸; Schlachta et al., 2021 ▸).

(iii) A di­iron–(μ-η1:η1-N2) com­plex should show a distinctive vNN absorption band in Raman spectroscopy and be IR inactive due to the centrosymmetric structure (Suess & Peters, 2013 ▸; McWilliams et al., 2018 ▸; Gu et al., 2018 ▸). No vNN band was detected in the crude material either by IR or Raman spectroscopy. However, no pronounced vCN stretch could be observed either and, inter­estingly, com­plex 1 also does not show a characteristic vCN band in IR, contrary to similar com­plexes (Raba et al., 2012 ▸), but signals attributable to axial MeCN are visible in the Raman spectrum (see supporting information).

Conclusion

A di­iron end-on μ-η1:η1-CN-bridged com­plex, 2, was obtained from a crystallization experiment with an open-chain iron NHC com­plex 1. The cyanide presumably originates from the MeCN solvent by C—C bond cleavage or through carbon–hy­dro­gen oxidation. The strongly bent NHC ligands are positioned in an anti conformation.

Supplementary Material

Crystal structure: contains datablock(s) I, global. DOI: 10.1107/S2053229624007058/jx3087sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2053229624007058/jx3087Isup2.hkl

Supporting information. DOI: 10.1107/S2053229624007058/jx3087sup3.pdf

CCDC reference: 2326821

Lena Schröck is gratefully acknowledged for Raman measurements. There are no com­peting inter­ests to declare. Open access funding enabled and organized by Projekt DEAL.

Figure 1 The mol­ecular structure of 1. H atoms and hexa­fluoro­phos­phate anions have been omitted for clarity. Displacement ellipsoids are shown at the 50% probability level (Schlachta et al., 2024 ▸).

Figure 2 The mol­ecular structure of 2. H atoms and hexa­fluoro­phos­phate anions have been omitted for clarity. Displacement ellipsoids are shown at the 50% probability level.

Table 1 Experimental details

Crystal data	
Chemical formula	[Fe2(CN)(C2D3N)2(C25H18N6)2](PF6)3	
M r	1465.67	
Crystal system, space group	Monoclinic, C2/c	
Temperature (K)	100	
a, b, c (Å)	25.562 (2), 17.0373 (15), 14.8998 (12)	
β (°)	112.112 (3)	
V (Å3)	6011.8 (9)	
Z	4	
Radiation type	Mo Kα	
μ (mm−1)	0.67	
Crystal size (mm)	0.13 × 0.05 × 0.04	
 	
Data collection	
Diffractometer	Bruker D8 Venture	
Absorption correction	Multi-scan (SADABS; Bruker, 2016 ▸)	
Tmin, Tmax	0.708, 0.745	
No. of measured, independent and observed [I > 2σ(I)] reflections	89835, 5310, 4621	
R int	0.051	
(sin θ/λ)max (Å−1)	0.595	
 	
Refinement	
R[F2 > 2σ(F2)], wR(F2), S	0.031, 0.083, 1.11	
No. of reflections	5310	
No. of parameters	461	
No. of restraints	9	
H-atom treatment	H-atom parameters constrained	
Δρmax, Δρmin (e Å−3)	0.43, −0.27	
Computer programs: APEX4 (Bruker, 2021 ▸), SAINT (Bruker, 2019 ▸), SHELXT (Sheldrick, 2015a ▸), SHELXL2019 (Sheldrick, 2015b ▸), PLATON (Spek, 2020 ▸) and enCIFer (Allen et al., 2004 ▸).

Table 2 Selected geometric parameters (Å, °)

Fe1—N3	2.0703 (16)	Fe1—N8	1.83 (4)	
Fe1—N6	2.0754 (17)	Fe1—C1	1.824 (2)	
Fe1—N7	1.9336 (17)	Fe1—C14	1.824 (2)	
Fe1—C28i	1.98 (5)	N8—C28	1.128 (12)	
 	 	 	 	
N7—Fe1—N8	173.3 (10)	Fe1—N8—C28	171.1 (7)	
Fe1i—C28—N8	177 (4)	N3—Fe1—C14	167.26 (8)	
Symmetry code: (i) .
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