Interdependent Metal–Metal Bonding and Ligand Redox-Activity in a Series of Dinuclear Macrocyclic Complexes of Iron, Cobalt, and Nickel

Abstract

This report describes an isostructural series of dinuclear iron, cobalt, and nickel complexes bound by a redox-active macrocyclic ligand. The series spans five redox levels (34–38 e–/cluster core), allowing for a detailed investigation into both the degree of metal–metal interaction and the extent of ligand-based redox-activity. Magnetometry, electrochemistry, UV–vis–NIR absorption spectroscopy, and crystallography were used in conjunction with DFT computational analyses to extract the electronic structures of the six homodinuclear complexes. The isoelectronic, 34 e– species [(3PDI2)­Fe2(PMe3)2(μ-Cl)]­(OTf) and [(3PDI2)­Co2(PMe3)2(μ-Cl)]­(OTf)3 exhibit metal–metal single bonds, with varying amounts of electron density delocalization into the ligand as a function of the effective nuclear charge of the metal ions. One- and two-electron reductions of [(3PDI2)­Co2(PMe3)2(μ-Cl)]­(OTf)3 lead to isolable products, which show successive increases in both the Co–Co distances and the extent of reduction of the ligand manifold. This trend results from reduction of a Co–Co σ* orbital, which was found to be heavily mixed with the redox-active manifold of the 3PDI2 ligand. A similar trend was observed in the 37 and 38 e– dinickel complexes [(3PDI2)­Ni2(PMe3)2(μ-Cl)]­(OTf)2 and [(3PDI2)­Ni2(PMe3)2(μ-Cl)]­(OTf); however, their higher electron counts lead to high-spin ground states that result from occupation of a high-lying δ/δ* manifold with significant Ni–NPDI σ* character. This change in ground state configuration reforms a M–M bonding interaction in the 37 e– complex, but formation of the 38 e– species again disrupts the M–M bond alongside the transfer of electron density to the ligand.

Bis-Ferrocenyl-Pyridinediimine Trinuclear Mixed-Valent Complexes with Metal-Binding Dependent Electronic Coupling: Synthesis, Structures, and Redox-Spectroscopic Characterization

“…Similar observations and conclusions have been made that binding of PDI macrocycles to Lewis acids stabilizes the PDI π* orbitals and leads to better energy matching between metal and organic orbitals, causing greater delocalization. 61 We can thus describe a metal to mixed metal−ligand charge transfer ("MMMLCT") in analogy with mixed metal−ligand to ligand CT ("MMLLCT") excitations. 54a,62 As shown in Figure 6, this assignment explains the present observations of an unperturbed HOMO and a red-shifted HOMO−LUMO transition.…”

Synthesis and Ethylene Reactivity of Dinuclear Iron and Cobalt Complexes Supported by Macrocyclic Bis(pyridine-diimine) Ligands Containing o-Terphenyl Linkers

“…The metal–metal distances for ( 1 – 3 )­M 2 Br 4 are given in Table and range from 5.7600(8) to 6.232(1) Å. Similar distances were observed for macrocyclic bis-PDI complexes in which the PDI units are bridged by m -xylylene linkers, including ([ D ]­Cu 2 Im]­[CF 3 SO 3 ] 3 (Cu–Cu = 5.9181(9) Å), [ E ]­Zn 2 Ox]­[CF 3 SO 3 ] 2 (Zn–Zn = 5.3849(6) Å), ( F )­Zn 2 Cl 4 (Zn–Zn = 5.515(1) Å), and ( F )­Co 2 Cl 4 (Co–Co = 5.501(1) Å), which are shown in Chart . − In contrast, the Co–Co distance in ( A )­Co 2 Cl 4 (R 1 = R 2 = H; 7.731(2) Å) is much longer. While the syn -double-decker complexes ( B )­Fe 2 Cl 4 have not been crystallographically characterized, Groysman has reported a similar [( B )­Co 2 (CH 3 CN) 4 ]­[Co­(CO) 4 ] 2 complex with a Co–Co distance of 4.628(1) Å .…”

Molecular Complexes Featuring Unsupported Dispersion-Enhanced Aluminum–Copper and Gallium–Copper Bonds

“…An understanding of the nature of metal–metal bonding in both main group and transition metal systems is of fundamental importance. , Recently, the discovery of compounds featuring new metal–metal bonding types has been driven not only by curiosity but also by their many potential applications. − These include their use in C–F bond functionalization, CO 2 activation, electrochemistry, and precursors to thin films for photovoltaic applications . Many notable homometallic-dinuclear compounds have been isolated, , but scarcer heterobimetallic species have also generated interest as a result of their potential for improved selectivity and reactivity as catalysts .…”