Chemical Control on the Coordination Mode of Benzaldehyde Semicarbazone Ligands. Synthesis, Structure, and Redox Properties of Ruthenium Complexes

Reaction of benzaldehyde semicarbazone (HL-R, where H is a dissociable proton and R is a substituent (R =
OMe, Me, H, Cl, NO2) at the para position of the phenyl ring) with [Ru(PPh3)3Cl2] and [Ru(PPh3)2(CO)2Cl2] has
afforded complexes of different types. When HL-NO2 and [Ru(PPh3)3Cl2] react in solution at ambient temperature,
trans-[Ru(PPh3)2(L-NO2)Cl] is obtained. Its structure determination by X-ray crystallography shows that L-NO2
is coordinated as a tridentate C,N,O-donor ligand. When reaction between HL-NO2 and [Ru(PPh3)3Cl2] is carried
out in refluxing ethanol, a more stable cis isomer of [Ru(PPh3)2(L-NO2)Cl] is obtained. The trans isomer can be
converted to the cis isomer simply by providing appropriate thermal energy. Slow reaction of HL-R with [Ru(PPh3)2(CO)2Cl2] in solution at ambient temperature yields 5-[Ru(PPh3)2(L-R)(CO)Cl] complexes. A structure
determination of 5-[Ru(PPh3)2(L-NO2)(CO)Cl] shows that the semicarbazone ligand is coordinated as a bidentate
N,O-donor, forming a five-membered chelate ring. When reaction between HL-R and [Ru(PPh3)2(CO)2Cl2] is
carried out in refluxing ethanol, the 4-[Ru(PPh3)2(L-R)(CO)Cl] complexes are obtained. A structure determination
of 4-[Ru(PPh3)2(L-NO2)(CO)Cl] shows that a semicarbazone ligand is bound to ruthenium as a bidentate N,O-donor, forming a four-membered chelate ring. All the complexes are diamagnetic (low-spin d6, S = 0). The
trans- and cis-[Ru(PPh3)2(L-NO2)Cl] complexes undergo chemical transformation in solution. The 5- and 4-[Ru(PPh3)2(L-R)(CO)Cl] complexes show sharp NMR signals and intense MLCT transitions in the visible region.
Cyclic voltammetry of the 5-[Ru(PPh3)2(L-R)(CO)Cl] and 4-[Ru(PPh3)2(L-R)(CO)Cl] complexes show the Ru(II)−Ru(III) oxidation to be within 0.66−1.07 V. This oxidation potential is found to linearly correlate with the
Hammett constant of the substituent R.