Thiosemicarbazone complexes of the platinum metals. A story of variable coordination modes

Salicylaldehyde thiosemicarbazone (H2saltsc) reacts with [M(PPh3)3X2] (M = Ru, Os; X = Cl, Br) to afford complexes of type [M(PPh3)2(Hsaltsc)2], in which the salicylaldehyde thiosemicarbazone ligand is coordinated to the metal as a bidentate N,S-donor forming a four-membered chelate ring. Reaction of benzaldehyde thiosemicarbazones (Hbztsc-R) with [M(PPh3)3X2] also affords complexes of similar type, viz. [M(PPh3)2(bztsc-R)2], in which the benzaldehyde thiosemicarbazones have also been found to coordinate the metal as a bidentate N,S-donor forming a four-membered chelate ring as before. Reaction of the Hbztsc-R ligands has also been carried out with [M(bpy)2X2] (M = Ru, Os; X = Cl, Br), which has afforded complexes of type [M(bpy)2(bztsc-R)]+, which have been isolated as perchlorate salts. Coordination mode of bztsc-R has been found to be the same as before. Structure of the Hbztsc-OMe ligand has been determined and some molecular modelling studies have been carried out determine the reason for the observed mode of coordination. Reaction of acetone thiosemicarbazone (Hactsc) has then been carried out with [M(bpy)2X2] to afford the [M(bpy)2(actsc)]ClO4complexes, in which the actsc ligand coordinates the metal as a bidentate N,S-donorformingafive-membered chelate ring. Reaction of H2saltsc has been carried out with [Ru(bpy)2Cl2] to prepare the [Ru(bpy)2(Hsaltsc)]ClO4 complex, which has then been reacted with one equivalent of nickel perchlorate to afford an octanuclear complex of type [Ru(bpy)2(saltsc-H)4Ni4](ClO4)4

Chemistry of 2-(arylazo) phenolate complexes of osmium. Synthesis, structure and redox properties

Reaction of five 2- (arylazo)phenol ligands (abbreviated in general as Hap-R, where H stands for the phenolic proton) with [Os (bpy)<SUB>2</SUB>Br<SUB>2</SUB>] has afforded complexes of type [Os<SUP>II</SUP> (bpy)<SUB>2</SUB> (ap-R)]<SUP>+</SUP>, which have been isolated as the perchlorate salts. The complexes are diamagnetic (low-spin d <SUP>6</SUP>, S=0) and in acetonitrile solution shows several MLCT transitions in the visible region. Structure of the [Os (bpy)<SUB>2</SUB> (ap-Me))ClO<SUB>4</SUB> complex has been determined by X-ray crystallography. The 2- (arylazo)phenolate anion is coordinated to osmium as a bidentate N,O-donor forming a five-membered chelate ring and the OsN<SUB>5</SUB>O coordination sphere is distorted octahedral. Cyclic voltammetry shows a reversible osmium (II)-osmium (III) oxidation in the range of 0.37-0.51 V vs SCE followed by an irreversible osmium (III)-osmium (IV) oxidation in the range of 1.36-1.50 V vs SCE. These oxidation potentials are sensitive to the electronic nature of the substituent R in the 2- (arylazo)phenolate ligands. Three one-electron reductions of the coordinated bpy ligands are also displayed on the negative side of SCE below -1.0 V. Chemical or electrochemical oxidation of the [Os<SUP>II </SUP>(bpy)<SUB>2 </SUB>(ap-R)]ClO<SUB>4</SUB> complexes affords brownish-yellow [Os<SUP>III </SUP>(bpy)<SUB>2</SUB> (ap-R)]fn<SUB>2</SUB> species, which have been isolated as the perchlorate salts. These complexes are one-electron paramagnetic (low-spin d <SUP>5</SUP>, S=&#189;) and in acetonitrile solution show LMCT transitions in the visible region. Reduction of the brownish-yellow [Os<SUP>III</SUP> (bpy)<SUB>2</SUB> (ap-R)] (ClO<SUB>4</SUB>)<SUB>2</SUB> complexes gives back the respective brown [Os<SUP>II</SUP> (bpy)<SUB>2</SUB> (ap-R)]ClO<SUB>4</SUB> complexes

Unusual coordination mode of thiosemicarbazone ligands. A search for the origin

Twelve mixed-ligand thiosemicarbazone complexes of ruthenium and osmium, ten of general formula [M(bpy)<SUB>2</SUB>(bztsc-R)]ClO<SUB>4</SUB>, (M = Ru, Os; bpy = 2,2&#8216;-bipyridine, Hbztsc-R = benzaldehyde thiosemicacbazone) and two of type [M(bpy)<SUB>2</SUB>(actsc)]ClO<SUB>4</SUB> (Hactsc = acetonethiosemicarbazone), have been synthesized and characterized. All the complexes are diamagnetic (low-spin d<SUP>6</SUP>, S = 0) and in acetonitrile solution show several intense metal-to-ligand charge-transfer (MLCT) transitions in the visible region. Structures of Hbztsc-OMe, [Ru(bpy)<SUB>2</SUB>(bztsc-NO<SUB>2</SUB>)]ClO<SUB>4 </SUB>and [Ru(bpy)<SUB>2</SUB>(actsc)]ClO<SUB>4</SUB> have been determined by X-ray crystallography. Benzaldehyde thiosemicarbazone exists in the thione form with the phenyl group trans to the hydrazinic nitrogen. The benzaldehyde thiosemicarbazone ligand coordinates to the metals through the hydrazinic nitrogen and sulfur with a bite angle of &#8764;67&#176;, forming a four-membered chelate ring. However, the actsc ligand coordinates through the imine nitrogen and sulfur, forming a five-membered chelate ring with a bite angle of &#8764;81&#176;. The difference in coordination modes of two types of thiosemicarbazone ligands, viz., bztsc-R and actsc, appears to result from the difference in steric bulk of the aryl and methyl group trans to the hydrazinic nitrogen. In acetronitrile solution they all show a reversible metal(II)-metal(III) oxidation in the range 0.18-0.58 V vs SCE followed by an irreversible oxidation in the range 1.11-1.60 V vs SCE. Two successive one-electron reductions of the coordinated bipyridine are also observed in the range -1.53 to -1.96 V vs SCE