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

The reactions of Os3(CO)9(μ3-η2-(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 Os3(CO)9(μ3-η2-(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 first-order rate constants of (6.42 ± 0.6) × 10-5 and (1.28 ± 0.1) × 10-4 s-1 for 9 → 11 and 10 → 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 Os3(CO)9(μ3-η3-(2-CH3)C7H5NS)(μ-H) (9‘). The complexes Os3(CO)9(μ3-η2-(L-2H))(μ-H)2 (L = tetrahydroquinoline (3),
indoline (4)) also tautomerize to the corresponding μ-amido−aryl complexes Os3(CO)9(μ3-η2-(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.