Tuning of Cu-Al Interactions in Complexes Derived from Tris(pyridonyl-6-methyl)aluminum Metalloligands

A series of bioinspired polar atrane Cu-Al complexes were studied with a combined experimental and computational approach to assess the range and nature of Cu-Al interactions in these novel species. The aluminum metalloligand [Na{Me₂Al(OPy-6-Me)₂}] (2) was furnished in excellent yield (92%) from the nucleophilic attack of Na(OPy-6-Me) to AlMe₃ and the subsequent alkane elimination reaction with 6-methyl-2-hydroxypyridine. At the same time, the metalloligand [Al(OPy-6-Me)₃] (3) was isolated in an also excellent yield (95%) via alkane elimination of AlMe₃ with 6-methyl-2-hydroxypyridine. The zwitterionic Cu-Al atranes [Cu{MeAl(OPy-6-Me)₃}] (5) and [Cu{MesAl(OPy-6-Me)₃}] (5) were isolated (73 and 97% yields) from metalloligands 2 and 3, respectively. [(Cu{Al(OPy-6-Me)₄})₂(μ-Cu)]⁺ ([6) was isolated via a reaction that involves alkane elimination and redistribution reacting from 5 with [H(OEt₂)₂][B(Ar^CF3)₄] in benzene solution. Alkane elimination in benzene of either 5 or 5 with [HNEt₃][B(Ar^CF3)₄] renders [Cu{(Et₃N)Al(OPy-6-Me)₃}]⁺ (Et). The Lewis base-free cationic complex [Cu{Al(OPy-6-Me)₃}]⁺ (5) was isolated in 68% yield upon reacting 3 with [Cu(COD)₂][B(Ar^CF3)₄] in benzene. Metalloligands and complexes were fully characterized with an array of spectroscopic and analytical techniques that include multinuclear NMR, ATR-IR, ESI-spectrometry, combustion microanalysis, cyclic voltammetry (CV), and, whenever feasible, SCXRD. X-ray and DFT parameters indicate that the strength of the Cu→Al transannular interaction follows the trend 5 > Et > [6, 5, and 5 in a smooth transition from zwitterionic species where the Cu-Al interaction is nonexistent to moderate Cu-Al Z-type interactions. CV, in conjunction with DFT calculations of Et and 5, hint at the generation in the electrochemical cell of the radical species 5 at -1.82 V and the anionic complex 5 at -2.32 V vs Fc/Fc⁺, respectively. The proposed species 5 exhibits 2-center/1-electron (2c/1e) σ bonding whereas 5 a 2-center/2-electron (2c/2e) bond.