Cobalt and Nickel Compounds with Pentadienyl and Edge-Bridged Pentadienyl Ligands: Revisited

A series of Co(I), Co(II), and Ni(II) compounds were prepared by employing the open pentadienyl 2,4-(Me3C)(2)C5H5 (Pdl') and some edge-bridged variants (ePdl) such as 6,6'-dimethylcyclohexadienyl (dmch), cycloheptadienyl (chd), and cyclooctadienyl (cod). The sterically encumbered Pdl' ligand allowed the first synthesis of an open cobaltocene derivative [(eta(3)-Pdl'(2))Co] (1), which adopts a low-spin configuration and features two eta(3)-coordinate Pdl' units. For Ni(II), the heteroleptic derivative [eta(3)-Pdl')Ni(acac)] (2) and the open nickelocene [(eta(3)-Pdl'(2))Ni] (3) were isolated. Complex 3 is isostructural to its Co analogue (1). The reactivity patterns, however, change when similar reactions are performed with the edge-bridged pentadienyl derivatives (ePdl = dmch, chd, and cod). For [Co(acac)(2)] the reaction with K(dmch) and K(chd) resulted in reduction of Co(II) to Co(I), concomitant with ligand coupling to form bridging Co(I) complexes 5 and 6 with mu(2)-eta(4):eta(4)-(dmch)(2) and mu(2)-eta(4):eta(4)-(chd)(2) units, respectively. In contrast, the isolation of [(eta(3)-dmch)(2)Ni] (7) and [(eta(3)-cod)(2)Ni] (8) proceeds in excellent yields. In addition, several Co(I) complexes of the type [(eta(3)-cod)Co(L-3)] (L = PMe3; L-2 = dppe/dmpe, L = PMe3/CO) were prepared, which feature low melting and sublimation/distillation points. These complexes may offer potential for ALD/CVD processes.