Trialkylphosphine-Stabilized Copper(I) Gallium(III) Phenylchalcogenolate Complexes: Crystal Structures and Generation of Ternary Semiconductors by Thermolysis

A series of organometallic trialkylphosphine-stabilized
copper
gallium phenylchalcogenolate complexes [(R3P)mCunMe2–xGa­(EPh)n+x+1] (R = Me, Et, iPr, tBu; E = S, Se,
Te; x = 0, 1) has been prepared and structurally
characterized by X-ray diffraction. From their molecular structures
three groups of compounds can be distinguished: ionic compounds, ring
systems, and cage structures. All these complexes contain one gallium
atom bound to one or two methyl groups, whereas the number of copper
atoms, and therefore the nuclearity of the complexes, is variable
and depends mainly on size and amount of phosphine ligand used in
synthesis. The Ga–E bonds are relatively rigid, in contrast
to flexible Cu–E bonds. The lengths of the latter are controlled
by the coordination number and steric influences. The Ga–E
bond lengths depend systematically on the number of methyl groups
bound to the gallium atom, with somewhat shorter bonds in monomethyl
compounds compared to dimethyl compounds. Quantum chemical computations
reproduce this trend and show furthermore that the rotation of one
phenyl group around the Ga–E bond is a low energy process with
two distinct minima, corresponding to two different conformations
found experimentally. Mixtures of different types of chalcogen atoms
on molecular scale are possible, and then ligand exchange reactions
in solution lead to mixed site occupation. In thermogravimetric studies
the complexes were converted into the ternary semiconductors CuGaE2. The thermolysis reaction is completed at temperatures between
250 and 400 °C, typically with lower temperatures for the heavier
chalcogens. Because of significant release of Me3Ga during
the thermolysis process, and especially in case of copper excess in
the precursor complexes, binary copper chalcogenides are obtained
as additional thermolysis products. Quaternary semiconductors can
be obtained from mixed chalcogen precursors.