Structural variation in copper(I) complexes with pyridylmethylamide ligands: structural analysis with a new four-coordinate geometry index, τ₄

Four Cu(I) complexes were synthesized with a family of pyridylmethylamide ligands, HL^R [HL^R = N-(2-pyridylmethyl)acetamide, R = null; 2,2-dimethyl-N-(2-pyridylmethyl)propionamide, R = Me₃; 2,2,2-triphenyl-N-(2-pyridylmethyl)acetamide, R = Ph₃)]. Complexes 1–3 were synthesized from the respective ligand and [Cu(CH₃CN)₄]PF₆ in a 2 : 1 molar ratio: [Cu(HL)₂]PF₆ (1), [Cu₂(HL^Me3)₄](PF₆)₂ (2), [Cu(HL^Ph3)₂]PF₆ (3). Complex 4, [Cu(HL)(CH₃CN)(PPh₃)]PF₆, was synthesized from the reaction of HL with [Cu(CH₃CN)₄]PF₆ and PPh₃ in a 1 : 1 : 1 molar ratio. X-Ray crystal structures reveal that complexes 1, 3 and 4 are mononuclear Cu(I) species, while complex 2 is a Cu(I) dimer. The copper ions are four-coordinate with geometries ranging from distorted tetrahedral to seesaw in 1, 2, and 4. Complexes 1 and 2 are very air sensitive and they display similar electrochemical properties. The coordination geometry of complex 3 is nearly linear, two-coordinate. Complex 3 is exceptionally stable with respect to oxidation in the air, and its cyclic voltammetry shows no oxidation wave in the range of 0–1.5 V. The unusual inertness of complex 3 towards oxidation is attributed to the protection from bulky triphenyl substituent of the HL^Ph3 ligand. A new geometric parameter for four-coordinate compounds, τ₄, is proposed as an improved, simple metric for quantitatively evaluating the geometry of four-coordinate complexes and compounds.