Unsupported Bonds between Copper(I) and Heavier Pnictogens

This study highlights the use of fluorinated poly(pyrazolyl)borate ligands to stabilize copper(I) complexes with heavy pnictogen donors such as SbPh₃ and BiPh₃. It presents two series of valence isoelectronic molecules, enabling a direct and meaningful comparison between the lighter and heavier analogues. Specifically, [HB(3,5-(CF₃)₂Pz)₃]⁻ and [H₂B(3,5-(CF₃)₂Pz)₂]⁻ have been utilized in the stabilization of a series of copper(I) complexes with EPh₃ (E = As, Sb, Bi), along with full characterization including their molecular structures and a detailed analysis of group 15 trends. The isolation of a copper-bismuthine complex, [HB(3,5-(CF₃)₂Pz)₃]Cu(BiPh₃), is especially notable as an example of a direct, unsupported Cu-Bi bond. DFT calculations show that the stabilization energy of Cu-E interactions decreases from P to Bi, illustrating the difficulty in stabilizing bismuthine ligands and the role of relativistic effects in reducing the bonding and Lewis basicity characteristics of the E coordinating center. The Cu-EPh₃ bond mainly relies on electrostatic interactions, but orbital interactions are also significant, with the Ph₃E→Cu σ-donation being much stronger than Ph₃E←Cu π-backdonation. Reactivity studies of [HB(3,5-(CF₃)₂Pz)₃]Cu(BiPh₃) and computational investigations on how fluorinated supporting ligands on Cu(I) affect the formation of the Cu-E adduct are also included.