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

Strong linear relationships between their Ceq-Os-Os-Ceq dihedral angles and their Os-Os bond distances in diosmium sawhorse complexes Os2(u-O2CR)2(CO)4L2 (L = CO and/or PR3) form two trendlines depending upon the presence or absence of terminal phosphines. These trends appear unrelated to the basicity of the bridging ligand or the number of phosphines. The mathematical derivation of the relationship between the O-Os-Os-O dihedral angle and the Os-Os bond distance shows how the other geometric parameters affect this relationship. Optimized density functional theory (DFT) structures reveal a similar strong linear correlation, where more electron-donating ligands render shorter Os-Os bond distances and larger dihedral angles, but these results form a single trendline. Computational scans of individual parameters show that the Os-Os bond responds strongly to changes in the dihedral angles, but the dihedral angles only respond weakly to changes in the Os-Os bond distance because the Os-Os-O bond angle links and modifies their direct coupling. Solid-state analysis of their structures, including DFT geometry optimizations, shows that phosphines protect the Os-Os bond distance from packing influences along the Os-Os axis, while in complexes without phosphines, packing compresses the Os-Os bond and the weak dihedral responses create the second trendline.

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.

Bis(µ4-adamantane-1,3-di-carboxyl-ato-1κO 1:2κO 1':3κO 3:4κO 3')octa-carbonyl-1κ2 C,2κ2 C,3κ2 C,4κ2 C-tetra-kis-[tris-(4-methyl-phen-yl)phosphane]-1κP,2κP,3κP,4κP-tetra-osmium(I)(2 Os-Os).

The title complex, [{Os2(CO)4(C21H21P)2}2(C12H14O4)2], is a centrosymmetric mol-ecular loop consisting of two Os-Os sawhorse units linked by two adamantane di-carboxyl-ato bridges. It was synthesized by the microwave-mediated reaction between Os3(CO)12 and adamantane-1,3-di-carb-oxy-lic acid. In contrast to the related complex [{Os2(CO)6}2(µ4-adamantane-1,3-di-acetate)2], the metal-metal axes within each mol-ecule are oriented parallel rather than perpendicular to one another. The crystal structure exhibits cavities that contain residual electron density peaks, but it was not possible to unambiguously identify the solvent therein. The contribution of the disordered solvent mol-ecules to the scattering was removed using the SQUEEZE (Spek (2015 ▸). Acta Cryst. C71, 9-18) routine in PLATON [Spek (2020 ▸). Acta Cryst. E76, 1-11]. These solvent mol-ecules are not considered in the given chemical formula and other crystal data.

(S)-2-[(4-Fluoro-phen-yl)formamido]-3-phenyl-propanoic acid.

The title compound, C16H14FNO3, was synthesized via solid phase methods; it exhibits monoclinic (P21) symmetry at room temperature. The two independent mol-ecules that comprise the asymmetric unit display distinct torsion angles of 173.2 (2) and 72.6 (2)° along the central sp 3 C-N bond. In the crystal, hydrogen bonding through N-H⋯O contacts couples the asymmetric unit mol-ecules into pairs that align in layers extending parallel to (100) via additional O-H⋯O inter-actions. The phenyl ring of one independent mol-ecule was found to be disordered over two sets of sites in a 0.55 (3):0.45 (3) ratio.

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).