Conformationally dynamic copper coordination complexes

Abstract

The interplay between oxidation state and coordination geometry dictates both kinetic and thermodynamic properties underlying electron transfer events in copper coordination complexes. An ability to stabilize both Cu^I and Cu^II oxidation states in a single conformationally dynamic chelating ligand allows access to controlled redox reactivity. We report an analysis of the conformational dynamics of Cu^I complexes bearing dipicolylaniline (dpa^R) ligands, with ortho-aniline substituents R = H and R = OMe. Variable temperature NMR spectroscopy and electrochemical experiments suggest that in solution at room temperature, an equilibrium exists between two conformers. Two metal-centered redox events are observed which, bolstered by structural information from single crystal X-ray diffraction and solution information from EPR and NMR spectroscopies, are ascribed to the Cu^II/I couple in planar and tetrahedral conformations. Activation and equilibrium parameters for these structural interconversions are presented and provide entry to leveraging redox-triggered conformational dynamics at Cu.

Figure 1.

Single crystal XRD ORTEP diagrams of (a) {[Cu(dpa^OMe)]₂(µ-Cl)}{CuCl₂} and (b) [CuCl(dpa^OMe)][PF₆] with thermal ellipsoids shown at 50 % probability. H-atoms and counterions are omitted for clarity.

Figure 2.

X-band EPR spectra (black) and simulations (red) of CuCl₂(dpa^H) (top), and [CuCl(dpa^OMe)][PF₆] (bottom) in frozen solutions of 1:1 toluene:DMF collected at 50 K. Simulations parameters are tabulated in the supporting information.

Figure 3.

CVs of Cu^I (black) and Cu^II (green) complexes with dpa^H (top), and dpa^OMe (bottom). Complexes are 1 mM in DMF containing 0.1 M [Bu₄N][PF₆] supporting electrolyte. Data were collected at a glassy carbon working electrode, with a platinum wire counter electrode, and a silver wire pseudo-reference electrode using a scan rate of 100 mV s⁻¹. Open circuit potentials are indicated by arrowhead.

Figure 4.

Partial ¹H-NMR spectra (500 MHz) of CuCl(dpa^OMe) in d₂-DCM showing the aromatic proton resonances over the temperature range of 233–298 K.

Scheme 1.

Synthetic procedure for [CuCl(dpa^R)]^+/0 complexes. Structures depict those that exist in solution. The preparation of CuCl₂(dpa^H) has previously been reported.

Scheme 2.

Model for conformational equilibria observed.