Fine Tuning of the Electronic Coupling between Metal Centers in Cyano-Bridged Mixed-Valent Trinuclear Complexes

We report the synthesis, characterization, and spectroscopic properties of a family of trinuclear cyano-bridged
mixed-valent compounds, trans-[RuIIL4{NCFeIII(CN)5}2]4-, trans-[RuIIL4{CNFeIII(CN)5}2]4-, and cis-[RuII(bpy)2{NCFeIII(CN)5}2]4- (L = pyridine, 4-tert-butylpyridine, and 4-methoxypyridine). Tetraphenylphosphonium salts of
complexes trans-[RuIIL4{NCFeIII(CN)5}2]4- (L = pyridine and 4-tert-butylpyridine) crystallize in the space groups
C2 and P21/c, respectively, and show a linear arrangement of the metal units and an almost completely eclipsed
configuration of the equatorial ligands. An intense band (ε ∼ 2000−9000 M-1 cm-1) is observed for all of the
compounds in the NIR region of the spectrum, not present in the separated building blocks, and strongly solvent
dependent. We assign it as a metal-to-metal charge transfer (MMCT) from the Ru(II) to the terminal Fe(III) moieties
in the context of a simplified three-center model. The electrochemistry measurements reveal a splitting of the redox
waves for the reduction of the iron centers for some of the complexes with a trans configuration between the metal
units, ranging from 100 to 260 mV, depending on the substituting pyridine ligand and the solvent, suggesting
long-range metal−metal interactions. These interactions are rationalized in terms of the energy matching between
the π-symmetry orbitals of the metals and the cyanide bridge. The one- and two-electron reduced species derived
from compounds trans-[RuIIL4{NCFeIII(CN)5}2]4-,5-,6- were characterized in methanolic solution. The mixed-valent
FeII−RuII−FeIII system exhibits an intense red shifted band in the NIR region of the spectrum, arising from the
superposition of MMCT bands from the central Ru(II) to the terminal Fe(III) fragments and from the 1 nm distant
Fe(II) to Fe(III) centers.