The effect of divergent-bite ligands on metal-metal bond distances in some paddlewheel complexes

In a paddlewheel type bimetal unit, the most frequent arrangement of the bridging ligands has a conformation such that the two donor orbitals are directed along approximately parallel lines. However, when a ligand has one of the two donor atoms derived from a six-membered ring and the other from a fused five-membered ring, the two donor orbitals will be directed divergently. Such divergent-bite ligands might be expected to elongate the M-M bonds. A Cr-Cr core supported by a set of four divergent-bite ligands, Cr-2(CHIP)(4), where CHIP is the anion of 1',3'-dihydrospiro[cyclohexane-1,2'-[2N]imidazo[4,5-b]pyridine], has been synthesized and crystallographically characterized. It has a Cr-Cr distance of 2.016(1) Angstrom, which is much longer than those in paddlewheel complexes with non-divergent dinitrogen ligands. This is the first example of a dichromium complex supported by a set of four divergent-bite ligands and no axial coordination. An analogous complex, namely Mo-2(azin)(4), where azin represents the anion of 7-azaindole, has also been characterized. Together with W-2(azin)(4), which was previously reported, the first series of quadruple bonds wrapped in a sheath of four divergent-bite ligands has been obtained. A comparison of the M-M bond lengths with those in a homologous series having a more flexible ligand, DTolF, which is the anion of di-p-tolylformamidine, reveals that the Cr-Cr quadruple bonds are much more sensitive to the effect of the bridging ligand geometry than are Mo-Mo and W-W quadruple bonds; this is very similar to the trend in susceptibility of quadruple bonds towards axial coordination. (C) 2000 Elsevier Science S.A. All rights reserved.