Preparation and Protonation of Fe₂(pdt)(CNR)₆, Electron-Rich Analogues of Fe₂(pdt)(CO)₆

Abstract

The complexes Fe2 (pdt)(CNR)6 (pdt2− = CH2(CH2S−)2) were prepared by thermal substitution of the hexacarbonyl complex with the isocyanides RNC for R = C6H4-4-OMe (1), C6H4-4-Cl (2), Me (3). These complexes represent electron-rich analogues of the parent Fe2(pdt)(CO)6. Unlike most substituted derivatives of Fe2(pdt)(CO)6, these isocyanide complexes are sterically unencumbered and have the same idealized symmetry as the parent hexacarbonyl derivatives. Like the hexacarbonyls, the stereodynamics of 1–3 involve both turnstile rotation of the Fe(CNR)3 as well as the inversion of the chair conformation of the pdt ligand. Structural studies indicate that the basal isocyanide has nonlinear CNC bonds and short Fe–C distances, indicating that they engage in stronger Fe–C π-backbonding than the apical ligands. Cyclic voltammetry reveals that these new complexes are far more reducing than the hexacarbonyls, although the redox behavior is complex. Estimated reduction potentials are E1/2 ≈ −0.6 ([2]+/0), −0.7 ([1]+/0), and −1.25 ([3]+/0). According to DFT calculations, the rotated isomer of 3 is only 2.2 kcal/mol higher in energy than the crystallographically observed unrotated structure. The effects of rotated versus unrotated structure and of solvent coordination (THF, MeCN) on redox potentials were assessed computationally. These factors shift the redox couple by as much as 0.25 V, usually less. Compounds 1 and 2 protonate with strong acids to give the expected μ-hydrides [H1]+ and [H2]+. In contrast, 3 protonates with [HNEt3]BArF4 (pKaMeCN = 18.7) to give the aminocarbyne [Fe2(pdt)(CNMe)5(μ-CN(H)Me)]+ ([3H]+). According to NMR measurements and DFT calculations, this species adopts an unsymmetrical, rotated structure. DFT calculations further indicate that the previously described carbyne complex [Fe2(SMe)2(CO)3(PMe3)2(CCF3)]+ also adopts a rotated structure with a bridging carbyne ligand. Complex [3H]+ reversibly adds MeNC to give [Fe2(pdt)(CNR)6(μ-CN(H)Me)]+ ([3H(CNMe)]+). Near room temperature, [3H]+ isomerizes to the hydride [(μ-H)Fe2(pdt)(CNMe)6]+ ([H3]+) via a first-order pathway.

Rational Design of Fe2(μ‐PR2)2(L)6 Coordination Compounds Featuring Tailored Potential Inversion

“…[77,78] In fact, these characteristics of the isocyanide ligands have allowed the installation of 6 À CNR groups (with R of different nature) in the same Fe 2 S 2 system, while the maximum number of phosphine ligands in the same Fe 2 S 2 complex is 4. [79] We have therefore also studied three models with À CNCH 3 ligands; 24 (one À CNCH 3 ), 25 (two À CNCH 3 ) and 26 (six À CNCH 3 ). Notably, all three cases feature ΔΔG < 0, which becomes more and more negative with increasing number of CO's, and the computed ΔΔG for 26 is as low as À 14.2 kcal/mol (which corresponds to a span of 754 mV).…”

Electrochemical and Theoretical Investigations of the Oxidatively Induced Reactivity of the Complex [Fe2(CO)4(κ2‐dmpe)(μ‐adtBn)] Related to the Active Site of [FeFe] Hydrogenases

“…That implies that t Bu groups can be accommodated quite distant from the diiron core, thus not being subject to destabilizing intramolecular interactions in contrast to PR 3 ligands, where the steric hindrance already affects the first coordination sphere of the iron center (see Figure S18 in the Supporting Information, for the superposition of optimized structures of [ 1 ‐( t BuNC) 4 ] + and the ideal [ 1 ‐(P(OMe) 3 ) 4 ] + , as well as the ones of their bicationic counterparts). Further support for that is lent by the observation that examples of stable Fe 2 S 2 complexes incorporating more than four (and up to six) donor ligands are only known for RNC (R=Me, Ph), but not for phosphines …”

“…DFT calculations have been performed with the pure functional BP86 and an all‐electron triple‐ζ basis set with polarization on all atoms (TZVP), as implemented in the TURBOMOLE 7.1.1 suite . This level of theory has provided good and consistent results in the reproduction of structural and reactivity features in bioinspired compounds and also very recently in other diiron catalysts for hydrogen evolution . In addition, the BP86/TZVP scheme was previously used to study the oxidative behavior of complex 1 in the absence and in the presence of CO .…”

Bond Forming Reactions Involving Isocyanides at Diiron Complexes

“…Finally, a resurged interest in isocyanide coordination in diiron complexes has been more recently stimulated by studies concerning [FeFe] hydrogenase model systems. In particular, molecular mimics of the type [Fe 2 (µ-S 2 C 3 H 6 )(CNR) 6−x (CO) x ] (x = 2, 3; R = Me, t Bu) have been investigated in order to exploit the different donor/acceptor properties of CNR in comparison with CO [46][47][48].…”

“…Indeed, treatment with strong alkylating agents (RSO 3 CF 3 ) of complexes of type 1 (R = Me) transforms isocyanides into the corresponding µ-aminoalkylidyne (aminocarbyne) complexes of the type [Fe 2 (Cp) 2 (CO) 3 (µ-CNRR')] + , (4) (Scheme 2b) [49,50] Finally, a resurged interest in isocyanide coordination in diiron complexes has been more recently stimulated by studies concerning [FeFe] hydrogenase model systems. In particular, molecular mimics of the type [Fe2(μ-S2C3H6)(CNR)6−x(CO)x] (x = 2, 3; R = Me, t Bu) have been investigated in order to exploit the different donor/acceptor properties of CNR in comparison with CO [46][47][48].…”