Stereochemistry of Arylmetal-1B, aryllithium, and arylmetal-1b-lithium clusters Ar4M2Li2 (M = Cu, Ag, or Au). Detection of rotation of three-center two-electron bonded aryl groups around the C(1)-C(4) axis

Abstract

Arylmetal- 1 B, aryllithium, and arylmetal- 1 B-lithium compounds have polynuclear structures consisting of a metal core to which each of the aryl groups is bound via C(1) to two metals by a three-center two-electron (3c-2e) bond. When the metals are unlike and aryl is dissymmetrically substituted C( 1) is a center of chirality. Rotation of the aryl group around the C( 1)-C(4) axis causes a continuous inversion of configuration at C(1). Dynamic 'H and I3C NMR studies revealed that the prochiral methylene group in 2-(Me2NCHz)CsHj-metaI compounds Ar4M2Li2 (M = Cu, Ag, or Au) is an excellent probe for the monitoring of the configuration at C( I ) in each of the 2-(Me2NCH*)ChH4MLi units. In this way the rotation of 3c-2e bonded aryl groups around the C( I)-C(4) axis has been established for the first time. Introduction of a second center of chirality of which the configuration cannot invert allows the detection of the stereochemistry of the Ar4M2Li2 aggregates. The use of the chiral 2-Me>NCH(Me) rather than the prochiral 2-MezNCHz group as a built-in ligand provided unambiguous proof for the occurrence of Cu-N coordination in the Ar4Cu4 compounds at low temperature. Formation of Cu-N coordination bonds in the Ar4Cu4 cluster having Cz symmetry constitutes a second element of chirality which in combination with the chirality at the benzylic C atoms gives rise to two distinguishable stereoisomers of Ar4Cu4. Li-N coordination in [5-Me-2- ( M ~ Z N C H ~ ) C ~ Hw~h]icJhL li~ke~w,is e constitutes an element of chirality, could be detected by 'H and I3C NMR. The novel results concerning the dynamics of 3c-2e bonded aryl groups at the same time provide a rationale for the kinetic stability order of the ortho-substituted arylmetal-l B clusters: 2-MerNC(Z)HChH4 (Z = H or Me) = 2-MerCHCH2C6H4 = 2-MezNChH4 = 2,6-(MeO)lChH3 = 2,6-Me>C6H3 > 2-MeChH4 > 4-MeC6H4 = unsubstituted phenyl-M cluster.