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

Abstract

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.

2-Pyridonates: a versatile ligand platform in 3d transition metal coordination chemistry and catalysis

“…The resultant heterobimetallic μ 2 -κ- N -Cu,κ- O -Al pyridonates were studied for the electronic nature of the Cu–Al bond that could adopt 2c1e/2c2e σ-bond character ( 36 ) as well as a non-bonding zwitterionic state ( 37 ). 85…”

Cooperation towards nobility: equipping first-row transition metals with an aluminium sword

“…After this groundbreaking example, a series of complexes featuring Cu-Al bonds was recently synthesised and fully characterised by Crimmin (132 and 135), 125,196 Power (133), 197 Liu (134), 126 Lu (135) 198 and Muñoz-Hernández (137). 199 Synthesis of such heterobimetallic Cu-Al species was achieved through conventional methods, such as ligand substitution and direct complexation of neutral low-valent Al(I) species, while salt metathesis has been more extensively utilised when Cu was paired to Al starting from Al(I) anions, as reported by Hill and McMullin and Aldridge. 200,201 While the species depicted in Scheme 31 were not tested towards small molecule activation, the reactivity of Cu-Al species with X-type Al(I) ligands has seen significant advances during the last couple of years (see below), demonstrating promising potential for catalysis.…”