Asymmetric diosmium sawhorse complexes.

Three asymmetric diosmium(I) carbonyl sawhorse complexes have been prepared by microwave heating. One of these complexes is of the type Os2(μ-O2CR)(μ-O2CR')(CO)4L2, with two different bridging carboxylate ligands, while the other two complexes are of the type Os2(μ-O2CR)2(CO)5L, with one axial CO ligand and one axial phosphane ligand. The mixed carboxylate complex Os2(μ-acetate)(μ-propionate)(CO)4[P(p-tolyl)3]2, (1), was prepared by heating Os3(CO)12 with a mixture of acetic and propionic acids, isolating Os2(μ-acetate)(μ-propionate)(CO)6, and then replacing two CO ligands with two phosphane ligands. This is the first example of an Os2 sawhorse complex with two different carboxylate bridges. The syntheses of Os2(μ-acetate)2(CO)5[P(p-tolyl)3], (3), and Os2(μ-propionate)2(CO)5[P(p-tolyl)3], (6), involved the reaction of Os3(CO)12 with the appropriate carboxylic acid to initially produce Os2(μ-carboxylate)2(CO)6, followed by treatment with refluxing tetrahydrofuran (THF) to form Os2(μ-carboxylate)2(CO)5(THF), and finally addition of tri-p-tolylphosphane to replace the THF ligand with the P(p-tolyl)3 ligand. Neutral complexes of the type Os2(μ-O2CR)2(CO)5L had not previously been subjected to X-ray crystallographic analysis. The more symmetrical disubstituted complexes, i.e. Os2(μ-formate)2(CO)4[P(p-tolyl)3]2, (8), Os2(μ-acetate)2(CO)4[P(p-tolyl)3]2, (4), and Os2(μ-propionate)2(CO)4[P(p-tolyl)3]2, (7), as well as the previously reported symmetrical unsubstituted complexes Os2(μ-acetate)2(CO)6, (2), and Os2(μ-propionate)2(CO)6, (5), were also prepared in order to examine the influence of axial ligand substitution on the Os-Os bond distance in these sawhorse molecules. Eight crystal structures have been determined and studied, namely μ-acetato-1κO:2κO'-μ-propanoato-1κO:2κO'-bis[tris(4-methylphenyl)phosphane]-1κP,2κP'-bis(dicarbonylosmium)(Os-Os) dichloromethane monosolvate, [Os2(C2H3O2)(C3H5O2)(C21H21P)2(CO)4]·CH2Cl2, (1), bis(μ-acetato-1κO:2κO')bis(tricarbonylosmium)(Os-Os), [Os2(C2H3O2)2(CO)6], (2) (redetermined structure), bis(μ-acetato-1κO:2κO')pentacarbonyl-1κ2C,2κ3C-[tris(4-methylphenyl)phosphane-1κP]diosmium(Os-Os), [Os2(C2H3O2)2(C21H21P)(CO)5], (3), bis(μ-acetato-1κO:2κO')bis[tris(4-methylphenyl)phosphane]-1κP,2κP-bis(dicarbonylosmium)(Os-Os) p-xylene sesquisolvate, [Os2(C2H3O2)2(C21H21P)2(CO)4]·1.5C8H10, (4), bis(μ-propanoato-1κO:2κO')bis(tricarbonylosmium)(Os-Os), [Os2(C3H5O2)2(CO)6], (5), pentacarbonyl-1κ2C,2κ3C-bis(μ-propanoato-1κO:2κO')[tris(4-methylphenyl)phosphane-1κP]diosmium(Os-Os), [Os2(C3H5O2)2(C21H21P)(CO)5], (6), bis(μ-propanoato-1κO:2κO')bis[tris(4-methylphenyl)phosphane]-1κP,2κP-bis(dicarbonylosmium)(Os-Os) dichloromethane monosolvate, [Os2(C3H5O2)2(C21H21P)2(CO)4]·CH2Cl2, (7), and bis(μ-formato-1κO:2κO')bis[tris(4-methylphenyl)phosphane]-1κP,2κP-bis(dicarbonylosmium)(Os-Os), [Os2(CHO2)2(C21H21P)2(CO)4], (8).