π-Rich σ(2)P-heterocycles: bent η(1)-P- and μ(2)-P-coordinated 1,3-benzazaphosphole copper(I) halide complexes

The reaction of 1-neopentyl-1,3-benzazaphosphole 1 with CuCl, CuBr, or Cu(SMe2)Br in THF at room temperature provides sparingly soluble [Cu7(μ(2)-L6)(μ(2)-X7)](+)[CuX2](-) cluster complexes 2a,b (L indicates coordinated 1, a X = Cl, b X = Br), with loosely bound THF, in high yields. The conversions proceed via transient THF-soluble labile [(L2CuX)2] complexes. Separation before complete conversion, combined with suitable conditions for crystallization, allowed these intermediates to be trapped. Depending on the reactant ratios, crystals of the clusters or of dimeric L2CuX complexes were formed. The crystal structure analyses of 2a·4THF and the dimers 3b [{Cu(η(1)-L)2(μ(2)-Br)}2], 4b [{Cu(μ(2)-L)(η(1)-L)(κBr)}2], 5a·2MeOH, and 5b·2MeOH [{Cu(μ(2)-L)(η(1)-L)(κX···HOMe)}2] generally display μ(2)-P- and/or tilted η(1)-P-coordination, contrasting with the preference for the η(1)-P in-plane coordination mode of phosphinine ligands in their copper(I) halide complexes. DFT studies of geometry-optimized monomers LCuBr, L(CuBr)2, L2CuBr, and the dimers 3b and 4b, calculated at the ωB97xD/cc-PVDZ level, suggest that weak competing interactions with the solvent THF and the entropy factor of the dimerization result in lability and a subtle balance between the different complexes in solution, whereas the particular coordination observed in the crystals is attributable to conservation of the delocalized π-system in the ligand. The HOMO of 4b is composed of Cu d orbitals and the π-type HOMO of the bridging ligand. Interestingly, despite the rather short Cu···Cu interatomic separation (2.726 Ǻ), no bond critical point could be located in 4b, indicating the absence of weak cuprophilic interactions in this compound.