Kinetic and Computational Analysis of CO Substitution in a Dinuclear Osmium Carbonyl Complex: Intersection between Dissociative and Dissociative-Interchange Mechanisms.

The mechanism for the CO substitution reaction involving the diosmium carbonyl sawhorse complex Os2(µ-O2CH)2(CO)6, which contains an Os-Os single bond, two axial CO ligands, and four equatorial CO ligands, was investigated experimentally and theoretically. Kinetic measurements show 13CO axial substitution proceeding by a dissociative reaction that is first-order in the complex and zero-order in 13CO but with an unexpectedly negative entropy of activation. The corresponding electronic structure calculations yield an enthalpy of activation for axial CO dissociation that is much larger than that determined by the kinetic experiments, but in agreement with the complex's stability with respect to CO loss. Additional calculations yield a dissociative interchange transition state whose free energy, enthalpy, and entropy of activation are in good agreement with those obtained from the kinetic measurements for the apparently dissociative substitution. These results point to an exchange reaction mechanism that is surprisingly close to the poorly understood transition from a dissociative mechanism with a CO-loss intermediate to a dissociative interchange mechanism with a transition state involving both the entering and the leaving COs. The key to explain these findings is provided by the vibrational analysis, which shows very low energy wagging motions for the axial COs. Thus, the incoming CO only displaces the outgoing CO when the complex has an outgoing CO near the wag's turning point. This dissociative interchange mechanism predicted by the calculation explains the unexpected combination of kinetics and stability characteristics. Kinetics reveals that the reaction is first-order in the Os dimer with a negative Eyring entropy, while a stability study shows that the Os dimer's decomposition rate is several orders of magnitude slower than CO exchange.

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).

Crystal structure of [µ-1κ2C1,C4:2(1,2,3,4-η)-1,2,3,4-tetra-phenyl-buta-1,3-diene-1,4-di-yl]bis-(tri-carbonyl-osmium)(Os-Os).

In the title complex C34H20O6Os2 or (µ-η4-C4Ph4)Os2(CO)6, one Os atom is part of a metalla-cyclo-penta-diene ring, while the second Os atom is π-bonded to the organic portion of this ring. The distance of 2.7494 (2) Šbetween the two Os atoms is typical of an Os-Os single bond. Three carbonyl ligands are attached to each Os atom and these six carbonyls adopt an eclipsed conformation. There are no bridging or semibridging CO groups. Two carbonyl ligands and all four phenyl groups are disordered over two slightly different positions for which each atom in the minor components is displaced less than 1 Šfrom the corresponding atom in the major components. The refined occupancies of the major com-ponents of the carbonyl ligands are 0.568 (16) and 0.625 (13), while those for the phenyl rings are 0.50 (3), 0.510 (12), 0.519 (18), and 0.568 (12).