Experimental and Computational Studies of Nucleophilic Attack, Tautomerization, and Hydride Migration in Benzoheterocycle Triosmium Clusters

Abstract

The reactions of Os 3 (CO) 9 (µ 3 -η 2 -(L-H))(µ-H) (L ) benzothiazole (7), benzoxazole ( 8)) with H -/H + are reported, We observe only nucleophilic attack at the 2-position followed by protonation at the metal core to yield the 48e µ-amido-aryl complexes Os 3 (CO) 9 (µ 3 -η 2 -(L-H))(µ-H) 2 (L ) benzothiazole (9), benzoxazole ( 10)). The solid-state structure of 9 is reported. On thermolysis these complexes tautomerize to the corresponding µ-alkylidene-imino complexes 11 (L ) benzothiazole) and 12 (L ) benzoxazole) with equilibrium constants of 11/9 ) 1.6 and 12/10 ) 3.1. The tautomerization takes place with firstorder rate constants of (6.42 ( 0.6) × 10 -5 and (1.28 ( 0.1) × 10 -4 s -1 for 9 f 11 and 10 f 12, respectively. Substitution of a methyl group at the 2-position in the case of 7 changes the site of hydride attack to the 4-position and results in the formation of the σ-π-vinyl complex Os 3 (CO) 9 (µ 3 -η 3 -(2-CH 3 )C 7 H 5 NS)(µ-H) (9′). The complexes Os 3 (CO) 9 (µ 3 -η 2 -(L-2H))(µ-H) 2 (L ) tetrahydroquinoline (3), indoline (4)) also tautomerize to the corresponding µ-amido-aryl complexes Os 3 (CO) 9 (µ 3 -η 2 -(L-2H))-(µ-H) 2 (L ) tetrahydroquinoline ( 5), indoline (6)) at approximately the same rates as 9 and 10 and have similar equilibrium constants. Density functional theory (DFT) calculations reveal that the tautomerization in these systems proceeds via a four-center transition state. The calculated barriers for tautomerization are in reasonable agreement with the experimental data. DFT calculations on 6 reveal that the lower energy transition state for hydride migration is an unsymmetrically face-bridged hydride rather than a terminal hydride. Natural population analysis (NPA) of the optimized structures of the 46e clusters and their corresponding free ligands reveals that the site of nucleophilic attack can be predicted on the basis of the net charge on the C-H groups on the heterocyclic rings and/or the relative stability of the resulting anions.

An Electron-Deficient Triosmium Cluster Containing the Thianthrene Ligand: Synthesis, Structure and Reactivity of [Os3(CO)9(μ3-η2-C12H7S2)(μ-H)]

“…This segregation of metal and ligand participation in the constitution of the molecular orbitals of 2 carries over to the H-1 and H-2 orbitals (Figure 3) until finally at H-3 significant participation of both metal and ligand orbitals is seen (Figure 4). Again this is contrast with the results of previous DFT studies of related electron deficient clusters where participation of both metal core and ligand in the occupied and unoccupied molecular orbitals is pervasive [4,33,34]. The non-planarity of this heterocyclic system in both the solid state and computed structures surely makes a strong contribution to the orbital pictures presented in Figures 2-4 and may also be the route cause of the instability of the radical anion.…”

“…This interaction leads to a more delocalized radical anion with the associated stabilization [33]. This interaction is conspicuously missing in 2 and probably explains why this tricyclic aromatic triosmium cluster does not show reversible oneelectron reductions.…”

Cationic Heterocycles as Ligands: Synthesis and Reactivity with Anionic Nucleophiles of Cationic Triruthenium Clusters Containing C‐Metalated N‐Methylquinoxalinium or N‐Methylpyrazinium Ligands

“…[29] It has been previously shown that the B3LYP/LanL2DZ/6-31GA C H T U N G T R E N N U N G (d,p) approximation used in this paper provides reasonable agreement with available experiments and higher-level methods in analogous systems. [30] No simplified model compounds were used for the calculations. All stationary points were confirmed as energy minima (reactants, products, and intermediates; all positive eigenvalues) or transition states (one imaginary eigenvalue) by analytical calculation of frequencies.…”

Synthesis, structure, photophysical and electrochemical behavior of 2-amino-anthracene triosmium clusters

“…Moreover, the regio-and stereoselectivity can be controlled in a previously unprecedented way [1][2][3]. Subsequent density functional theory (DFT) computational studies revealed that it was not the actual electron deficiency in the carbocyclic ring that lead to the observed novel reaction chemistry but rather the ability of the metal core to stabilize the anionic intermediates that are formed after nucleophilic attack [4].…”