Asymmetric diosmium sawhorse complexes

Abstract

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)4 L 2, with two different bridging carboxylate ligands, while the other two complexes are of the type Os2(μ-O2CR)2(CO)5 L, 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)5 L 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-1κ2 C,2κ3 C-[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-1κ2 C,2κ3 C-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).

Theoretical Investigation of Linear Relationships between the Dihedral Torsion Angles and Diosmium Bond Distances in Diosmium Sawhorse Complexes

“…The symmetric diosmium carbonyl sawhorse complex, Os 2 (u-O 2 CR) 2 (CO) 6 , features a diosmium center with two cis -carboxylate bridging ligands and six carbonyl ligands occupying the remaining equatorial and terminal positions. − Recent studies have shown an unexpected mechanism for the 12 CO/ 13 CO exchange of the terminal carbonyls, which is kinetically zero order in [ 13 CO] but involves a very weakly bound interchange transition state . One or both weakly bound terminal COs can be easily replaced by other ligands such as phosphines, pyridines, and nitriles. ,− A wide-ranging study of multiple new asymmetric diosmium sawhorse complexes, Os 2 (u-O 2 CR) 2 (CO) 6– n (P­( p -tolyl) 3 ) n ( n = 0, 1, or 2 and R = H, CH 3 , CH 2 CH 3 , or C 6 H 5 ), revealed linear relationships between their C eq –Os–Os–C eq dihedral angles and Os–Os bond distances . These linear relationships appear as two separate parallel trendlines, either with or without terminal phosphine(s).…”

“…The authors’ interest in this study began with the observation of variations in metal–metal bond distance and dihedral angle among newly synthesized diosmium­(I) sawhorse complexes . We were curious about the relationship between M–M bond distance, ligand type, and molecular conformation, initially wondering whether the basicity of the bridging ligands was the defining factor causing differences in M–M bonding interactions.…”