Group 1B organometallic chemistry. 18. Novel routes to stable hexanuclear copper-silver, silver-gold, and copper-gold cluster compounds starting from polymeric 2-(dimethylamino)phenylcopper and from bis[(2-dimethylamino)pheny l]goldlithium dimer

Mixed hexanuclear copper-silver, silver-gold, and copper-gold complexes of the type R₄M₄M′₂X₂ have been prepared via the reaction of 2-(dimethylamino)phenylcopper (RCu) with CuOTf or AgOTf (OTf = trifluoromethanesulfonato) as well as via the reaction of cuprous halide, of CuOTf, or of AgOTf with the novel bis[2-(dimethylamino)phenyl]goldlithium dimer R₄Au₂Li₂. IR and NMR spectroscopies reveal a structure for the R₄M₄M′₂X₂ compounds which is similar to the structure of R₄Cu₆Br₂ (x ray), i.e., a hexanuclear cluster structure with the (2-dimethylamino)phenyl ligands spanning triangular faces of the mixed M₄M′₂ core by 2e-3c carbon-metal and by 2e-2c metal-nitrogen bonds. The gold atoms in R₄M₄Au₂X₂ (M = Cu or Ag) occupy apical positions which is in line with the strong preference of gold(I) for a digonal coordination geometry. R₄Cu₄Ag₂(OTf)₂ is not stable and decomposes into R-R, R₄Cu₆(OTf)₂, and metallic silver at room temperature. Dynamic NMR spectroscopy reveals that the NMe₂ grouping is an excellent probe for the detection and elucidation of the dissymmetry in the cluster (R groups bridge unlike metal atoms; metal-N(Me₂) coordination renders the NMe₂ grouping a stable prochiral assembly). NMe resonances in R₄Cu₄Au₂X₂ remain anisochronous up to 90°C, whereas those of R₄Cu₆X₂ coalesce at 40°C. This has been explained in terms of increased N-Cu_eq, bond strength in the copper-gold derivative resulting from a contribution to the bonding of canonical structures such as R₂Au^-[Cu₄X₂²⁺] ^-AuR₂, in which 2e-3c RAuCu bonding has been replaced by 2e-2c RAu interactions. IR spectroscopy reveals that in spite of the weak electron-donor properties of the OTf anion the R₄M_6-nM′_n,(OTf)₂ compounds (n = 0, 2) like the halide clusters R₄M_6-nM′_nX₂ (X = Br or I) are 84-electron species with each OTf anion contributing 4 electrons to the cluster by bridging two equatorial metal atoms. The similarity of the NMR patterns of R₄Cu₄Au₂X₂ (X = halide or OTf) derivatives in electron-pair-donating solvents such as pyridine can be explained both by metal core⋯anion dissociation resulting in 84-electron dications R₄M_6-nM′_n(py)₄²⁺ or by cluster rearrangement affording R₄M_4-n,M′_n.