Synthesis and characterization of heterobimetallic (Pd/B) Nindigo complexes and comparisons to their homobimetallic (Pd2, B2) analogues.

Reactions of Nindigo-BF2 complexes with Pd(hfac)2 produced mixed complexes with Nindigo binding to both a BF2 and a Pd(hfac) unit. These complexes are the first in which the Nindigo ligand binds two different substrates, and provide a conceptual link between previously reported bis(BF2) and bis(Pd(hfac)) complexes. The new Pd/B complexes have intense near IR absorption near 820 nm, and they undergo multiple reversible oxidations and reductions as probed by cyclic voltammetry experiments. The spectral, redox, and structural properties of these complexes are compared against those of the corresponding B2 and Pd2 complexes with the aid of time-dependent density functional calculations. In all cases the low-energy electronic transitions are ligand-centered π-π* transitions, but the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies--and hence the absorption wavelength as well as the oxidation and reduction potentials--are significantly modulated by the moieties bound to the Nindigo ligand.