Well-defined 2:1 and 2:2 arylcopper-copper bromide aggregates and selective biaryl formation

Abstract

Reaction of tetranuclear [Li4(C6H4CH2NMe2-2)4] (1) with 4/3 equiv of the copper(I) arenethiolate [Cu3(SC6H4NMe2-2)3] (2) results in the quantitative formation of the arylcopper compound [Cu4(C6H4CH2NMe2-2)4] (3) and the lithium arenethiolate [Li(SC6H4NMe2-2)]n (4). The arylcopper species 3 is a yellow air-sensitive solid, which is tetrameric in the solid state as well as in solution. The reaction of the dinuclear aryllithium reagent [Li2{C6H4(CH2N(Me)CH2CH2NMe2)-2}2] (5) with 3 equiv of CuBr affords the trinuclear mixed 2:1 arylcopper-copper bromide aggregate [Cu3Br{C6H4(CH2N(Me)CH2CH2NMe2)-2}2] (6). The 1:4 reaction of 5 with CuBr affords the tetranuclear mixed 2:2 aggregate [Cu4Br2{C6H4(CH2N(Me)CH2CH2NMe2)-2}2] (7). As solids, aggregates 6 and 7 are less air-sensitive than 3, but their solutions rapidly turn green upon exposure to air. The 1:2 reaction of the naphthyllithium reagent [Li{1-C10H6(CH2N(Me)CH2CH2NMe2)-2}] (9) with CuBr affords the tetranuclear mixed 2:2 aggregate [Cu4Br2{1-C10H6(CH2N(Me)CH2CH2NMe2)-2}2] (10), in which there is a 1:1 arylcopper to copper bromide ratio. Aggregate 10 is obtained as brownish yellow crystals, and solutions of 10 slowly turn bluish green upon exposure to air. The molecular structures of 6 and 10 both contain an R-Cu-R fragment and can be described as comprising a [CuR2]- unit stabilized by a [Cu2Br]+ or a [Cu3Br2]+ cation, respectively. Heating solutions of 6 (55 C; 15 min) and solutions of 10 (110 C; 19 h) results in quantitative biaryl (6) and moderate binaphthyl (10) formation, respectively. The relevance of neutral aggregates such as 6, 7, and 10 as model complexes both for the interpretation of bonding in higher order cuprate species and for understanding of their reactivity is discussed.