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}]
(5Me) and [Cu­{MesAl­(OPy-6-Me)3}]
(5Mes) were isolated (73 and 97% yields) from
metalloligands 2 and 3, respectively. [(Cu­{Al­(OPy-6-Me)4})2(μ-Cu)]+ ([6+]­[B­(ArCF3)4]) was isolated via a reaction
that involves alkane elimination and redistribution reacting from 5Me with [H­(OEt2)2]­[B­(ArCF3)4] in benzene solution. Alkane elimination in
benzene of either 5Me or 5Mes with [HNEt3]­[B­(ArCF3)4]
renders [Cu­{(Et3N)­Al­(OPy-6-Me)3}]+ (Et3N-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(ArCF3)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+ > Et3N-5+ > [6+]­[B­(ArCF3)4], 5Me, and 5Mes 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 Et3N-5+ and 5+, hint at the generation in the electrochemical cell of the radical
species 5rad at −1.82 V and the anionic
complex 5– at −2.32 V vs Fc/Fc+, respectively. The proposed species 5rad exhibits 2-center/1-electron (2c/1e) σ bonding whereas 5– a 2-center/2-electron (2c/2e) bond.