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

• Published in: Inorganic chemistry 2014-08, Vol.53 (15), p.7876-7885
• Main Authors: Blusch, Lina K, Mitevski, Oliver, Martin-Diaconescu, Vlad, Pröpper, Kevin, DeBeer, Serena, Dechert, Sebastian, Meyer, Franc
• Format: Article
• Language: English
• Subjects: Copper - chemistry; Crystallography, X-Ray; Electron Spin Resonance Spectroscopy; Magnetic Resonance Spectroscopy; Nickel - chemistry; Organometallic Compounds - chemical synthesis; Oxidation-Reduction; Porphyrins - chemistry
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic500460n

The Siamese-twin porphyrin (1H 4 ) 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 1H 2 Ni with the second {N4} 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]+ 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 NiIII having significant d z 2 1 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.