Solution properties, electrochemical behavior and protein interactions of water soluble triosmium carbonyl clusters

The synthesis and solution chemistry of the water soluble clusters [Os3(CO)9(μ-η2-Bz)(μ-H)L+] (HBz=quinoxaline, L+=[P(OCH2CH2NMe3)3I3], 1) along with its negatively charged analog [Os3(CO)9(μ-η2-Bz)(μ-H)L−] (L−=[P(C6H4SO3)3Na3], 2) are reported. In addition, we have examined the reduction potentials of the complexes [Os3(CO)9(μ-η2-Bz)(μ-H)L] (HBz=phenanthridine, L=L+ (3); HBz=5,6 benzoquinoline, L=L+ (4); HBz=3-amino quinoline, L=L+ (5); HBz=3-amino quinoline, L=L− (6). The neutral analog of 1 and 2 [Os3(CO)9(μ-η2-Bz)(μ-H) PPh3] (Bz=quinoxaline, 7) was also examined for comparison. Both compounds 1 and 2 show pH dependent NMR spectra that are interpreted in terms of the extent of protonation of the uncoordinated quinoxaline nitrogen which impacts the degree of aggregation of the clusters in aqueous solution. Compound 1 undergoes a reversible 1e− reduction in water while 2 undergoes a quasi-reversible 1e− reduction at more negative potentials as expected from the difference in charge on the phosphine ligand. Compound 7 undergoes a marginally reversible CV in methylene chloride at a potential intermediate between the positively and negatively charged clusters. The overall stability of the radical anions of 1, 2 and 7 is somewhat less than the corresponding decacarbonyl [Os3(CO)10(μ-η2-Bz)(μ-H)] (HBz=quinoxaline). While complexes 1 and 2 show reversible 1e− reductions, all the other complexes examined show 1e− and/or two 1e− irreversible reductions in aqueous and non-aqueous solvents. The potentials for these complexes follow expected trends relating to the charge on the phosphine and the pH of the aqueous solutions. The ligand dependent trends are compared with those of the previously reported corresponding decacarbonyls. The interactions of the positively and negatively charged clusters with albumin have been investigated using the transverse and longitudinal relaxation times of the hydride resonances as probes of binding to the protein. Evidence of binding is observed for both the positive and negative clusters but the positive and negative clusters exhibit distinctly different rotational correlation times. Two additional complexes [Os3(CO)9(μ-η2-Bz)(μ-H)L] (HBz=2-methylbenzimidazole, L=L+ (8); L=L− (10) and HBz=quinoline-4-carboxaldehyde, L=L+ (9); L=L− (11)) are reported in connection with these studies.