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, Cr2(CHIP)4, where CHIP is the anion of 1′,3′-dihydrospiro[cyclohexane-1,2′-[2H]imidazo[4,5-b]pyridine], has been synthesized and crystallographically characterized. It has a CrCr distance of 2.016(1) Å, 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 Mo2(azin)4, where azin represents the anion of 7-azaindole, has also been characterized. Together with W2(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.