σ-carbon versus π-arene interactions in the solid-state structures of trimeric and dimeric copper aryls (CuAr)n (n = 3, Ar = 2,6-Ph2C6H3; n = 2, Ar = 2,6-Mes2C6H3)

Reaction of LiC6H3Ph2-2,6 or LiC(6)H(3)Mes(2)-2,6 with CuOt-Bu in pentane or toluene yields the unsolvated copper aryls (CuC6H3Ph2-2,6)(3) (2) and (CuC(6)H(3)Mes(2)-2,6)(2) (3). Addition of another equivalent of lithium aryl to 3 affords the lithium cuprate LiCu(C(6)H(3)Mes(2)-2,6)(2) (4). The new compounds 2-4 were characterized by H-1 NMR, C-13 NMR, IR spectroscopy, mass spectrometry, and X-ray crystallography. The X-ray structure shows 2 to be trimeric in the solid state. The bridging terphenyl ligands exhibit two different coordination modes. Two of them connect two copper atoms via their ipso C6H3 carbons, while the third bridges two copper centers by either a sigma-C (ipso C6H3) or a pi-arene (ipso/ortho phenyl) interaction. A dimeric unit is observed in the solid-state structure of 3, in which the two aryl-groups are linked by two bridging coppers. The copper atoms mainly interact with the ipso carbons of the C6H3 group. The quasi-linear coordination is completed by two ipso and ortho carbons of a mesityl group. Molecular weight determinations of the copper aryls 2 and 3 by cryoscopy in benzene solution show an equilibrium between monomers and dimers. Compound 4 is the first example of a structurally characterized, monomeric, nonsolvated lithium cuprate. It crystallizes with two independent molecules, showing C-Cu-C angles of 171.1 degrees and 173.8 degrees, respectively. The lithium ions are accommodated between two adjacent mesityl groups of the terphenyl ligands.