The acidity of transition-metal dihydrogen complexes of the general type trans-[LM(H 2 -PCH 2 CH 2 PH 2 ) 2 (η 2 -H 2 )] n+ (M ) Ru, Os; L ) H -, CH 3 -, F -, CF 3 -, CN -, Cl -, Br -, CO, NCH, NH 3 , PH 3 ; n ) 1, 2) has been modeled by calculating deprotonation energies using the B3LYP density functional method and effective core potentials. Calculated DP energies correlate well with experimental pK a values. For anionic L with strong σ-donor properties the DP energies are higher and acidities lower, due to significant destabilization of the conjugate hydride. When L is neutral, the complexes are dicationic and have much lower DP energies. The acidities of these complexes are governed by the π-accepting ability of L, as the less positively charged conjugate hydride can be stabilized through metal(d) f π* back-donation. The effect of the metal center on acidity is also dependent upon the electronic properties of L. Dihydrogen complexes of Ru with L that is either a σ-donor or π-acceptor are less acidic when compared with those of Os. The effect of a strong σ-donor leads to greater destabilization of the Ru conjugate hydrides, while the π-accepting property of L preferentially stabilizes the Os conjugate hydrides.

The reaction of [Os4(μ‐H)4(CO)12] with 4‐(2‐pyridylazo)‐N,N‐dimethylaniline (PNNDA) in dichloromethane afforded the new clusters [Os4(μ‐H)4(CO)11{NC5H4(N=N)C6H4NMe2}] (1) and [Os4(μ‐H)4(CO)10{η2‐NC5H4(N=N)C6H4NMe2}] (2) in 34% and 15% yields, respectively. Upon heating in toluene under reflux, compound 1 converted into 2 in 80% yield. The MLCT transition of compound 2 shows strong solvent dependency, displaying unusually large positive solvatochromism in different organic solvents. Treatment of [Os4(μ‐H)4(CO)12] with 2‐(5‐bromo‐2‐pyridylazo)‐5‐(diethylamino)phenol (Br‐PADAP) in dichloromethane gives [Os4(μ‐H)3(CO)10{μ‐η3‐Et2NC6H3(O)(N=N)C5NH3Br}] (3). The O−H bond activation is observed and the azo‐ligand behaves as a five‐electron donor. The clusters [Os4(μ‐H)2(CO)10{μ‐η3‐NC5H4(N=N)C5H4N}] (4) and [Os4(μ‐H)4(CO)10{μ‐η2‐NC5H4(N=N)C5H4N}]2 (5) were isolated from the reaction of [Os4(μ‐H)4(CO)10(NCMe)2] with 2,2′‐azopyridine and 3,3′‐azopyridine, respectively. In compound 4, both pyridine and azo nitrogen atoms were found to coordinate to the cluster core. Dehydrogenation was also observed in this compound. For compound 5, two tetra‐osmium metal cores were connected by two azo ligands through their pyridyl nitrogen atoms to form a novel cyclic structure. The redox properties of the compounds described herein were investigated by means of cyclic voltammetry and controlled potential coulometry. Both compound 2 and 5 exhibit a reversible cathodic wave, which indicates that they undergo addition of one electron without significant structural changes. Furthermore, compound 5 displays an electronic interaction between two redox centers, the extra electrons are believed to delocalize throughout the cyclic structure.

A novel synthetic method has been developed for cardiolipin and its analog via a chlorophosphoramidite coupling reaction followed by oxidation. The reagent, N,N-diisopropylmethylphosphoramidic chloride, couples effectively with 1,2-O-dimyristoyl-sn-glycerol in the presence of an amidite activator to form a phosphoamidite intermediate, which then reacts with 2-O-benzylglycerol in the presence of a basic catalyst followed by in situ oxidation to give the corresponding protected cardiolipin. Deprotection of the protecting groups provides tetramyristoyl cardiolipin in good overall yield of 60%. The synthetic method is applicable to large-scale synthesis of cardiolipin and various analogs with or without unsaturation for liposomal drug delivery.

No abstract