Quantifying Variations in Metal–Ligand Cooperative Binding Strength with Cyclic Voltammetry and Redox-Active Ligands

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.