Restricting the geometrical relaxation in four-coordinate copper(I) complexes using face-to-face and edge-to-face pi-interactions

A series of [Cu(N,N)(P,P)](+), [Cu(N,N)(P,S)](+), [Cu(N,N)(2)](+) and [Cu(P,S)(2)](+) complexes incorporating the ligands 2,2`-bipyridine, 1,10-phenanthroline, 1,2-bis(diphenylphosphino)ethane, bis(2-(diphenylphosphino)phenyl)ether (1), 2-diphenylphosphinothioanisole (3) and 2-methyl-6`-phenyl-2,2`- bipyridine (4) has been synthesized and structurally characterized. We have assessed the degree of distortion of two bidentate ligands away from an ideal tetrahedral arrangement about the copper(I) ion using the White model. The greatest distortion along a pathway towards square planar coordination is observed in [Cu(4)(2)][PF6] and is a result of intra-cation pi-stacking between phenyl and bpy domains. Each of the complexes which contain the P,S-chelating ligand 3 exhibits significant `rocking` or `wagging` distortions which are associated with intra-cation CHmethyl...pi interactions. The extent of this distortion can also be assessed using a less rigorous approach by measuring the S-Cu-X and P-Cu-X angles where the S and P atoms belong to ligand 3, and X is the midpoint of the backbone of the second ligand. [Cu(3)(2)][PF6] and [Cu(1)(3)][PF6] exhibit embraces between the phenyl substituents that result in the copper(I) ion being sterically protected, and the room temperature H-1 NMR solution spectrum of [Cu(1)(3)][PF6] reveals hindered rotation of the phenyl rings of ligand.