A weak, short metal-metal bond in a chromium(II) amidinate complex

Treatment of chromium(II) chloride with 2 equiv of Li[tBuNC(CH3)NtBu], Li[iPrNC(CH3)NiPr], or Li[tBuNC(CH3)NEt] (prepared from the corresponding carbodiimides and methyllithium) afforded Cr[tBuNC(CH3)NtBu]2 (1, 50%), Cr[iPrNC(CH3)NiPr]2 (2, 54%), and [Cr(tBuNC(CH3)NEt)2]2 (3, 58%) as deep blue, purple, and amber or metallic green crystals, respectively. Cr[tBuNC(CH3)NtBu]2 and Cr[iPrNC(CH3)NiPr]2 possess monomeric structures in the solid state with tetrahedral and square planar geometries about the chromium centers, respectively. [Cr(tBuNC(CH3)NEt)2]2 exists as a dimer in the solid state, with two mu2-amidinate ligands, two eta2-amidinate ligands, and a chromium-chromium bond length of 1.9601(12) A. [Cr(tBuNC(CH3)NEt)2]2 is diamagnetic in the solid state, as determined by magnetic susceptibility measurement and CP/MAS 13C NMR spectroscopy. However, [Cr(tBuNC(CH3)NEt)2]2 dissociates into paramagnetic monomers in solution, as determined by observation of extremely broad peaks in the 1H NMR spectra in cyclohexane-d12 and benzene-d6, a solution magnetic moment measurement, and by solution molecular weight measurements in benzene. The combined results suggest that the chromium-chromium bond strength in [Cr(tBuNC(CH3)NEt)2]2 is weak and cannot exceed the difference in solvation energies between the dimer and two monomers, plus any increase in metal-nitrogen bond strengths in the monomers.