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

Abstract

The Siamese-twin porphyrin (1H(4)) is a redox noninnocent pyrazole-expanded porphyrin with two equivalent dibasic {N-4} binding sites. It is now shown that its selective monometalation can be achieved to give the nickel(II) complex 1H(2)Ni with the second {N-4} site devoid of a metal ion. This intermediate is then cleanly converted to 1Ni(2) and to the first heterobimetallic Siamese-twin porphyrin 1CuNi. Structural characterization of 1H(2)Ni shows that it has the same helical structure previously seen for 1Cu(2), 1Ni(2), and free base 1H(6)(2+). Titration experiments suggest that the metal-devoid pocket of 1H(2)Ni can accommodate two additional protons, giving [1H(4)Ni](2+). Both bimetallic complexes 1Ni(2) and 1CuNi feature rich redox chemistry, similar to the recently reported 1Cu(2), 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 [1Ni(2)](+) and [1CuNi](2+) as well as twice oxidized [1CuNi](2+), has been experimentally derived from comparison of the electrochemical properties of the complete series of complexes 1Cu(2), 1Ni(2), and 1CuNi, 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 1CuNi, the first oxidation takes place within its Cu-subunit, while the second oxidation then occurs in its Ni-subunit. Adding pyridine to solutions of [1Ni(2)](+) and [1CuNi](2+) cleanly converts them to metal-oxidized redox isomers with axial EPR spectra typical for Nil having significant d(z)(21) character, reflecting close similarity with nickel complexes of common porphyrins. The possibility of selectively synthesizing heterobimetallic complexes 1MNi 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.