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{Me 2 Al(OPy-6-Me) 2 }] ( 2 ) was furnished in excellent yield (92%) from the nucleophilic attack of Na(OPy-6-Me) to AlMe 3 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 AlMe 3 with 6-methyl-2-hydroxypyridine. The zwitterionic Cu-Al atranes [Cu{MeAl(OPy-6-Me) 3 }] ( 5 Me ) and [Cu{MesAl(OPy-6-Me) 3 }] ( 5 Mes ) were isolated (73 and 97% yields) from metalloligands 2 and 3 , respectively. [(Cu{Al(OPy-6-Me) 4 }) 2 (μ-Cu)] + ( [6 + ][B(Ar CF3 ) 4 ] ) was isolated via a reaction that involves alkane elimination and redistribution reacting from 5 Me with [H(OEt 2 ) 2 ][B(Ar CF3 ) 4 ] in benzene solution. Alkane elimination in benzene of either 5 Me or 5 Mes with [HNEt 3 ][B(Ar CF3 ) 4 ] renders [Cu{(Et 3 N)Al(OPy-6-Me) 3 }] + ( Et 3 N-5 + ). 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 3 N-5 + > [6 + ][B(Ar CF3 ) 4 ] , 5 Me , and 5 Mes 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 3 N-5 + and 5 + , hint at the generation in the electrochemical cell of the radical species 5 rad at -1.82 V and the anionic complex 5 - at -2.32 V vs Fc/Fc + , respectively. The proposed species 5 rad exhibits 2-center/1-electron (2c/1e) σ bonding whereas 5 - a 2-center/2-electron (2c/2e) bond.