The carbene [Cp*Fe(dppe)(C(OMe)Me)][CF3SO3] [2b, Cp* = η5-C5Me5, dppe = ethylenebis(diphenylphosphine)] was prepared in 97% yield from [Cp*Fe(CO)2(C(OMe)Me)][CF3SO3] upon photochemical displacement of the carbonyl ligands, and the iron methoxycarbene [Cp*Fe(CO)(PMe3)(C(OMe)Me)][CF3SO3] (2c) was obtained (80%) upon alkylation of the acyl derivative Cp*Fe(PMe3)(CO)(COCH3) (4). The iron−methylidene [Cp*Fe(dppe)(C(H)Me)][PF6] (6) was synthesized (95%) by treatment of Cp*Fe(dppe)(CH(OMe)Me) (5) with an aqueous solution of hexafluorophosphate acid. The ethylidene 6 is very stable in solution as in the solid state, and no decomposition reaction was observed by NMR spectroscopy at 50 °C. The X-ray crystal structure of 6 was solved and refined. The Fe−C(37) bond distance (1.787 Å) reveals the double-bond character of the metal−carbon bond. Reaction of the iron carbene complexes 2a−c and 6 with 2 equiv of potassium tert-butoxide in THF gave the corresponding vinyl derivatives 7a−d in high yield (80−90%). CV analyses of the vinyl complexes 7a−d display at 20 °C an oxidation wave at a platinum electrode with the (i p a/i p c) current ratio less than unity. CV and ESR measurements established the stability of the 17-electron vinyl radical in CH2Cl2 at −80 °C. Warming to 20 °C of the 17-electron iron(III) complexes [7a−d]•+[PF6 -] in solid state allows the vinyl−vinyl coupling providing the new binuclear bis(carbene) complexes 8a−d in 60−95% yield. The complex 8a was isolated as a pure diastereoisomer, whereas 8b was a mixture of the meso 8b( RS , SR ) and dl 8b( RR , SS ) isomers in the 2/1 ratio. The diastereoisomers were separated by CH2Cl2 extraction which only solubilizes the dl pair.

The new diiron μ-bis(carbene) complex [Cp*(dppe)FeC(OCH3)-CHCHC−(OCH3)Fe(dppe)Cp*][PF6]2 (4) [Cp* = pentamethylcyclopentadienyl, dppe = ethylenebis(diphenylphosphine)] was obtained by double deprotonation of [Cp*(dppe)FeC(OCH3)CH2CH2C(OCH3)Fe(dppe)Cp*]·2[PF6] (2) with KOBut followed by a two-electron oxidation with 2 equiv of ferrocenium (80%). On the basis of the multinuclear NMR spectroscopy the complex 4 appeared as diamagnetic in solution. Temperature-dependent magnetic susceptibility measurements with a microcrystalline sample of 4 showed antiferromagnetic behavior. Isotropic spin-only models of the data derived magnetic exchange coupling constants of J = − 27.4 cm-1. Variable-temperature Mössbauer spectroscopy allowed the observation of both the singlet and triplet spin isomers and their interconversion. The time scale for the spin flipping is larger than 10-6 s, suggesting the existence of a thermal barrier between the two spin states.

The binuclear iron complexes [Cp*­(PMe3)­(CO)­Fe–C­(OCH3)CH–CHC­(OCH3)–Fe­(PMe3)­(CO)­Cp*] (1 meso and 1 dl ) were prepared by double deprotonation of their known parents [Cp*­(PMe3)­(CO)­FeC­(OCH3)­CH2–CH2–C­(OCH3)Fe­(PMe3)­(CO)­Cp*]­(PF6)2 (5 meso and 5 dl ) and were isolated in good yield (90%). These complexes were characterized by ESI-mass spectrometry, IR and multinuclear NMR spectroscopy, and cyclic voltammetry. The singly and doubly oxidized forms 1 meso (PF 6 ) n and 1 dl (PF 6 ) n (n = 1, 2) were prepared by oxidation of the parent neutral complexes with 1 and 2 equiv of ferrocenium salt (93–100% yield). The related complex [Cp*­(dppe)­Fe–C­(OCH3)CH–CHC­(OCH3)–Fe­(dppe)­Cp*]­(PF6) (2­(PF 6 )) was obtained by reduction of the known dicationic derivative [Cp*­(dppe)­Fe–C­(OCH3)CH–CHC­(OCH3)–Fe­(dppe)­Cp*]­(PF6) (2­(PF 6 ) 2 ) with 1 equiv of cobaltocene (100% yield). Multinuclear NMR spectroscopy allowed us to establish the diiron­(II) conjugated μ-bis­(carbene) structure for 1 meso (PF 6 ) 2 and 1 dl (PF 6 ) 2 . In the case of the meso derivative, 1H NMR revealed the presence of E and Z isomers in a 4:1 ratio, confirming the presence of a CC double bond in the middle of the bridge. The three radicals 1 meso (PF 6 ), 1 dl (PF 6 ), and 2­(PF 6 ), which are thermally stable, were analyzed by IR, Mössbauer, ESR, UV–vis, and NIR spectroscopy. Experimental data, discussed with the support of quantum chemistry calculations performed at the DFT level of theory, indicate that these radical cations exhibit characteristics of oxidation on the butadienediyl bridge rather than on the metal centers.