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

Abstract

A series of Mo2, Ru2, Rh2 and Cu2 complexes with redox-active NP-R [2-(2-R)-1,8-naphthyridine; R = pyrazinyl (NP-pz, L1) and thiazolyl (NP-tz, L2)] 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-[Mo2(L1)2(OAc)2][BF4]2 (1), cis-[Mo2(L2)2(OAc)2][BF4]2 (1A), doubly bonded cis-[Ru2(L1)2(OAc)2][ClO4]2 (3), cis-[Ru2(L2)2(OAc)2][ClO4]2 (3A) and singly bonded trans-[Rh2(L1)2(OAc)2][BF4]2 (5) and trans-[Rh2(L2)2(OAc)2][BF4]2 (5A). Compounds cis-[Mo2(L1)2(CH3CN)4][BF4]4 (2), cis-[Mo2(L2)2(CH3CN)4][BF4]4 (2A), cis-[Ru2(L1)2(CO)4][OTf]2 (4) and cis-[Ru2(L2)2(CO)4][ClO4]2 (4A) contain acetonitriles or carbonyls as the ancillary ligands. The dicopper complexes trans-[Cu2(CH3CN)(L1)2][ClO4]2 (6) and trans-[Cu2(L2)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 (Kc) 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 π*(NP)-δ*(M2)-π*(NP) orbital conduit for electron delocalization. For diruthenium(ii,ii) compounds 3 and 3A, an additional π*(NP)-π*(M2)-π*(NP) pathway is accessible contributing to high Kc values. The ancillary π-ligands (acetates and carbonyls) reduce the extent of the electron flow through π*(NP)-δ*(M2)-π*(NP) and thus lower the Kc 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.