Inter-ligand electronic coupling mediated through a dimetal bridge: dependence on metal ions and ancillary ligands

摘要

A series of Mo-2, Ru-2, Rh-2 and Cu-2 complexes with redox-active NP-R [2-(2-R)-1,8-naphthyridine; R = pyrazinyl (NP-pz, L-1) and thiazolyl (NP-tz, L-2)] ligands have been synthesized and characterized by X-ray crystallography and spectroscopic methods. Two NP-R ligands wrap the dimetal core by occupying four equatorial positions and two axial sites. The remaining four equatorial sites are engaged by bridging acetates in quadruply bonded cis-[Mo-2(L-1)(2)(OAc)(2)][BF4](2) (1), cis-[Mo-2(L-2)(2)(OAc)(2)][BF4](2) (1A), doubly bonded cis-[Ru-2(L-1)(2)(OAc)(2)][ClO4](2) (3), cis- [Ru-2(L-2)(2)(OAc)(2)][ClO4](2) (3A) and singly bonded trans-[Rh-2(L-1)2(OAc)(2)][BF4](2) (5) and trans-[Rh-2(L-2)(2)(OAc)(2)][BF4] (2) (5A). Compounds cis-[Mo-2(L-1)(2)(CH3CN)(4)][BF4](4) (2), cis-[Mo-2(L-2)(2)(CH3CN)(4)][BF4](4) (2A), cis-[Ru-2(L-1)(2)(CO)(4)][OTf](2) (4) and cis-[Ru-2(L-2)(2)(CO)(4)][ClO4](2) (4A) contain acetonitriles or carbonyls as the ancillary ligands. The dicopper complexes trans-[Cu-2(CH3CN)(L-1)(2)][ClO4](2) (6) and trans-[Cu-2(L-2)(2)(ClO4)(2)] (6A) involve no bonding interaction between two Cu(I) units. Cyclic voltammogram studies reveal that two one-electron processes corresponding to each of the two ligands bound to the metal-metal bonded dimetal core result in four reversible one-electron reductions, with the exception of dirhodium(II,II) compounds 5 and 5A which show two one-electron reductions. The highest comproportionation constant (K-c) values are obtained for inter-valence complexes originating from the diruthenium(II,II) compounds 3 and 3A, whereas no electron delocalization is observed for dicopper(I,I) complexes 6 and 6A. The dimetal bridge and the ancillary ligands tune the degree of inter-ligand electronic coupling in these complexes. DFT calculations reveal a pi*(NP)-delta*(M-2)-pi*(NP) orbital conduit for electron delocalization. For diruthenium(II,II) compounds 3 and 3A, an additional pi*(NP) pi*(M-2) pi*(NP) pathway is accessible contributing to high K-c values. The ancillary pi-ligands (acetates and carbonyls) reduce the extent of the electron flow through pi*(NP)-delta*(M-2)-pi*(NP) and thus lower the K-c values. The absence of metal-metal bond orbitals and the reduced metal-ligand covalency in dicopper(I,I) compounds are responsible for the lack of electron delocalization in these systems.