Bis(diphenylphosphino)acetylene-bridged ten-membered dimetallacyclic Cu(I) complexes: 2,2′-bipyridine methyl substitution effect on structure and bonding

Four new bis(diphenylphosphino)acetylene-bridged copper(I) bipyridine complexes, [Cu₂(dppa)₂(L)₂](BF ₄)₂; L = 2,2′-bipyridine (1), 4,4′-dimethyl-2, 2′-bipyridine (2), 5,5′-dimethyl-2,2′-bipyridine (3) and 6,6′-dimethyl-2,2′-bipyridine (4), have been prepared and characterized by spectroscopic methods. X-ray crystal structure analyses of complexes 2-4 reveal bis-chelate doubly bridged bimetallic ten-membered metallacycles. The structures of 2 and 3 exhibit a centrosymmetric chair-like configuration. In compound 4, the steric effect of the methyl groups at the 6,6′ positions of the bipyridine derivative induces a severe distortion in the configuration of the metallacycle. The Raman shift of the dppa triple bond stretch is significantly affected upon coordination and shifts to higher wavenumbers relatively to the free ligand. This fact was rationalized by considering the coordination effect on the relative contributions of the resonance forms of dppa. Within the series of complexes, the lowest shift of the complex based on 4,4′-Me₂bipy (2) is consistent with the relatively highest basic strength of the chelating ligand in this series and the relative value of the para methyl Hammett constant. The largest ³¹P NMR downfield shift, relative to free dppa, observed for 1 is consistent with the relatively lowest basic strength of the chelating ligand in this series of 2,2′-bipyridine derivatives.