Intermetallic Sigma And Pi Communication In Heterodinuclear Mu-Cyclooctatetraene Complexes

The reaction of [(C5R5)Cr(eta(6)-Cot)](R = H, Me; Cot = cyclooctatetraene) with [(CO)(2)Fe(eta-cis-cyclooctene)(2)] affords the heterodinuclear complexes [(C5R5)Cr{mu-eta(5)(Cr):eta(3)(Fe)-Cot}Fe(CO)(3)] (R = H: 1, R = Me: 2) in quite good yields. One of the CO ligands in 1 and 2 can be easily substituted by a phosphane ligand, PR3' (R' = Me, Ph, OEt, F) to obtain [(C5R5)Cr{mu-eta(5)(Cr):eta(3)(Fe)-Cot}Fe(CO)(2)PR3'] (R = H, R' = Me: 3a; R = H, R' = Ph: 3b; R = Me, R' = Me: 4a; R = Me, R' = Ph: 4b; R = Me, R' = OEt: 4c; R = Me, R' = F: 4d). The X-ray structure determinations of 2, 3a, and 4c showed exclusively synfacial coordination of the two metal ligand moieties, despite the bulky C5Me5 (Cp*) ligand in 2 and 4c. However, the steric demand of Cp* gives rise to structural distortions in 2 and 4c, compared to the Cp-containing products 1 and 3a, and an elongation of the Cr-Fe distance from 293 pm to 303 and 304 pm, respectively The heterodinuclear complexes were investigated by cyclic voltammetry and ESR spectroscopy in order to elucidate the role of the permethylation of the cyclopentadienyl ligand and the influence of phosphane ligands with different pi-accepting abilities. The ESR spectroscopic results reveal surprisingly large P-31 hyperfine coupling constants (hfcc). These can be explained by a superposition of two different electron spin transfer mechanisms, which include a sigma- and a pi-bonding mode between the Cr and Fe centers.