Cooperative effects in π-ligand bridged dinuclear complexes: XV. Heterodinuclear FeCo complexes from mononuclear η 5-cyclooctatrienyl iron complexes

Abstract

The reaction of CpFe(1-5-η-C 8H 8-8- exo-R) (R  CH(CO 2Et) 2: 2a; R  H: 2b) with CpCo(C 2H 4) 2 at T ≤ 40°C yields the diamagnetic heterodinuclear, cyclo-C 8 bridged FeCo complexes [(CpFe)(CpCo)μ-(η 7-C 8H 8R)] (R  CH(CO 2Et) 2: 3; R  H: 4a) which are thermally labile. For 3 the substituent R  CH(CO 2Et) 2 is thermally cleaved with increasing temperature yielding the paramagnetic ESR active 35 valence electron (ve) compound [(CpFe)(CPCo) μ-Cot] ( 5). The ESR data of 5 indicate a cobalt localized unpaired electron. 5 is easily oxidized by O 2, [Ph 3C]BF 4 or [FeCp 2]PF 6 yielding the stable diamagnetic 34 ve cation [(CpFe)(CpCo)μ-Cot] + ( 5 +). Crystal structure analysis of 5BF 4 establishes a synfacial η 5 : η 5 coordination with a very long FeCo distance of about 287(1) pm which is assumed to be too long for a FeCo single bond. The cyclic voltammetry stu 5/ 5 +, 5 +/ 5 2+ and one quasi-reversible redox couple 5 −/ 5. Evidence for the formation of the radical dication 5 2+ containing a Co-centred unpaired electron, can be got from in situ ESR measurements. The thermal lability of 4a leads irreversibly to the isomeric complex 4b at elevated temperature. ' The activation energy Δ E a of this molecular transformation was estimated by 1H NMR spectroscopy as 103 kJ/mol. Preliminary crystal structure determinations of the high temperature isomer 4b confirm a synfacial 1,2,6,7-η:3-5-η bonding mode for the cyclooctatrienyl ligand. The cyclooctatrienyl ligand of the low temperature isomer 4a is assumed to coordinate via a 1-η:6,7-η-bonding mode to the Co centre whereas the coordination mode of the cyclo-C 8 ligand to the Fe centre should occur in 2-5-η fashion. C s molecular symmetry for 4a, which can be deduced from the 1H NMR spectra, is explained by a low energy twitching motion of 4a.