RESUMO
Metal-ligand cooperativity (MLC), a phenomenon that leverages reactive ligands to promote synergistic reactions with metals, has proven to be a powerful approach to achieving new and unprecedented chemical transformations with metal complexes. While many examples of MLC are known with a wide range of substrates, experimentally quantifying how ligand modifications affect MLC binding strength remains a challenge. Here we describe how cyclic voltammetry (CV) was used to quantify differences in MLC binding strength in a series of square-pyramidal Ru complexes. This method relies on using multifunctional ligands (those capable of both MLC and ligand-centered redox activity) as electrochemical reporters of MLC binding strength. The synthesis and characterization of Ru complexes with three different redox-active tetradentate ligands and two different ancillary phosphines (PPh3 and PCy3) are described. Titration CV studies conducted using BH3·THF with BH3 as a model MLC substrate allowed ΔGMLC to be quantified for each complex. Compared to our base triaryl ligand, increasing π conjugation in the backbone of the redox-active ligand enhanced MLC binding, whereas increasing π conjugation in the flanking groups decreased the MLC binding strength. Structures and spectroscopic data collected for the isolated MLC complexes are also described along with supporting DFT calculations that were used to illuminate electronic factors that likely account for the observed differences in the MLC binding strength. These results demonstrate how redox-active ligands and CV can be used to quantify subtle differences in the MLC binding strength across a series of structurally related complexes with different ligand modifications.
RESUMO
Metal-ligand cooperativity (MLC) relies on chemically reactive ligands to assist metals with small-molecule binding and activation, and it has facilitated unprecedented examples of catalysis with metal complexes. Despite growing interest in combining ligand-centered chemical and redox reactions for chemical transformations, there are few studies demonstrating how chemically engaging redox active ligands in MLC affects their electrochemical properties when bound to metals. Here we report stepwise changes in the redox activity of model Ru complexes as zero, one, and two BH3 molecules undergo MLC binding with a triaryl noninnocent N2S2 ligand derived from o-phenylenediamine (L1). A similar series of Ru complexes with a diaryl N2S2 ligand with ethylene substituted in place of phenylene (L2) is also described to evaluate the influence of the o-phenylenediamine subunit on redox activity and MLC. Cyclic voltammetry (CV) studies and density functional theory (DFT) calculations show that MLC attenuates ligand-centered redox activity in both series of complexes, but electron transfer is still achieved when only one of the two redox-active sites on the ligands is chemically engaged. The results demonstrate how incorporating more than one multifunctional reactive site could be an effective strategy for maintaining redox noninnocence in ligands that are also chemically reactive and competent for MLC.