Selective Synthesis and Redox Sequence of a Heterobimetallic Nickel/Copper Complex of the Noninnocent Siamese-Twin Porphyrin

Abstract

The Siamese-twin porphyrin (<b>1H</b>_<b>4</b>) is a redox noninnocent pyrazole-expanded porphyrin with two equivalent dibasic {N₄} binding sites. It is now shown that its selective monometalation can be achieved to give the nickel­(II) complex <b>1H</b>_<b>2</b><b>Ni</b> with the second {N₄} site devoid of a metal ion. This intermediate is then cleanly converted to <b>1Ni</b>_<b>2</b> and to the first heterobimetallic Siamese-twin porphyrin <b>1CuNi</b>. Structural characterization of <b>1H</b>_<b>2</b><b>Ni</b> shows that it has the same helical structure previously seen for <b>1Cu</b>_<b>2</b>, <b>1Ni</b>_<b>2</b>, and free base <b>1H</b>_<b>6</b>^<b>2+</b>. Titration experiments suggest that the metal-devoid pocket of <b>1H</b>_<b>2</b><b>Ni</b> can accommodate two additional protons, giving <b>[1H</b>_<b>4</b><b>Ni]</b>^<b>2+</b>. Both bimetallic complexes <b>1Ni</b>_<b>2</b> and <b>1CuNi</b> feature rich redox chemistry, similar to the recently reported <b>1Cu</b>_<b>2</b>, including two chemically reversible oxidations at moderate potentials between −0.3 and +0.5 V (vs Cp₂Fe/Cp₂Fe⁺). The locus of these oxidations, in singly oxidized [<b>1Ni</b>_<b>2</b>]⁺ and [<b>1CuNi</b>]⁺ as well as twice oxidized [<b>1CuNi</b>]²⁺, has been experimentally derived from comparison of the electrochemical properties of the complete series of complexes <b>1Cu</b>_<b>2</b>, <b>1Ni</b>_<b>2</b>, and <b>1CuNi</b>, and from electron paramagnetic resonance (EPR) spectroscopy and X-ray absorption spectroscopy (XAS) (Ni and Cu K edges). All redox events are largely ligand-based, and in heterobimetallic <b>1CuNi</b>, the first oxidation takes place within its Cu-subunit, while the second oxidation then occurs in its Ni-subunit. Adding pyridine to solutions of [<b>1Ni</b>_<b>2</b>]⁺ and [<b>1CuNi</b>]²⁺ cleanly converts them to metal-oxidized redox isomers with axial EPR spectra typical for Ni^III having significant d_<i>z</i>²¹ character, reflecting close similarity with nickel complexes of common porphyrins. The possibility of selectively synthesizing heterobimetallic complexes <b>1MNi</b> from a symmetric binucleating ligand scaffold, with the unusual situation of three distinct contiguous redox sites (M, Ni, and the porphyrin-like ligand), further expands the Siamese-twin porphyrin’s potential to serve as an adjustable platform for multielectron redox processes in chemical catalysis and in electronic applications