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)2C5H5 (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
[(η3-Pdl′2)­Co] (1),
which adopts a low-spin configuration and features two η3-coordinate Pdl′ units. For Ni­(II), the heteroleptic
derivative [(η3-Pdl′)­Ni­(acac)] (2) and the open nickelocene [(η3-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 μ2-η4:η4-(dmch)2 and μ2-η4:η4-(chd)2 units, respectively. In contrast, the isolation
of [(η3-dmch)2Ni] (7) and
[(η3-cod)2Ni] (8) proceeds
in excellent yields. In addition, several Co­(I) complexes of the type
[(η3-cod)­Co­(L3)] (L = PMe3;
L2 = dppe/dmpe, L = PMe3/CO) were prepared,
which feature low melting and sublimation/distillation points. These
complexes may offer potential for ALD/CVD processes.