Dinuclear copper(I) benzoato quinoline complexes as intermediates in the copper‐quinoline decarboxylation reaction

The reaction of both the novel mixed trinuclear organo‐organic copper(I) species [Cu₃(C₆H₂Me₃‐2,4,6)(μ‐O₂CC₆H₅)₂] (1) and [Cu(μ‐O₂CC₆H₅)]₄ (2a) with quinoline in toluene or diethyl ether yields the dimeric 1:2 copper/quinoline complex [Cu(μ‐O₂CC₆H₅)(NC₉H₇)₂]₂ (3a). The reactions of methyl‐substituted copper(I) benzoates, [Cu(μ‐O₂CC₆H₄Me‐x)]₄ (x = 2, 2b; x = 3, 2c; x = 4, 2d), with quinoline in toluene or diethyl ether, however, afford the 1:1 copper/quinoline complexes [Cu(μ‐O₂CC₆H₄Me‐x)(NC₉H₇)]₂ (x = 2, 4b; x = 3, 4c; x = 4, 4d). The structure of complex 4b, determined by a single‐crystal X‐ray diffraction study, shows a dicopper(I) unit doubly bridged by two benzoato units [Cul…Cul* = 2.667(1) Å]. At each copper site, the CuO bonds to the bridging benzoato groups differ considerably [Cul‐Ol = 1.923(5) Å; Cul‐O2* = 2.134(5) Å]. Coordination of a quinoline at each copper site [Cul‐Nl = 1.952(5) R] produces a distorted trigonal arrangement. Crystallization of the 1:2 copper‐quinoline complex 3a led to formation of crystals of the dimeric 1:1 copper/quinoline complex [Cu(μ‐O₂CC₆H₅)(NC₉H₇)]₂, 4a. The structure of 4a is similar to that of 4b [Cul…Cul* = 2.638(2) Å Cul‐Ol = 1.945(6) Å Cul‐O2* = 2.107(6) Å Cul‐Nl = 1.939(7) Å]. Since complexes 3 and 4 can be expected to be similar to the intermediates formed in the copper‐quinoline decarboxylation reaction, the reactivity of 3 and 4 at elevated temperatures in non‐coordinating solvents has been studied. It was found that the decarboxylation reaction requires more than one equivalent of quinoline per copper atom. An improved mechanism is presented for the copper‐quinoline decarboxylation reaction involving dinuclear copper(I) complexes as intermediates.