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Detection of a Novel Intermediate in the Addition of Thiols to Osmium Carbonyl Clusters

(a) Inorganic Chemistry 1, Chemical Center, Lund University, P.O. Box 124, S-22100 Lund, Sweden (b) Dipartimento di Chimica "G.Ciamician", Università di Bologna, via Selmi 2, I-40124 Bologna, Italy

Transition metal carbonyl clusters can be used as catalysts themselves or as models for metal surfaces in the process of chemisorption in catalytic processes.¹ One advantage of using transition metal carbonyl clusters to model surface reactions is that it is possible to study metal-ligand interactions in detail through analytical techniques not available to surface scientists, e.g. NMR and IR spectroscopy, mass spectrometry, and X-ray and neutron crystallography.

As an approach to the modelling of hydrodesulfurization (HDS) reactions, we have investigated the reactions and coordination modes of thiols to trinuclear osmium clusters. Dodecacarbonyl-triosmium clusters are known to react with thiols to produce the trinuclear compounds [HOs₃(CO)₁₀(SR)].² We have studied the reaction between different thiols and the acetonitrile substituted osmium cluster [Os₃(CO)₁₁(CH₃CN)].³

The acetonitrile ligand is more labile than the carbonyls and it is therefore easily substituted by suitable nucleophiles at room temperature. Furthermore, the acetonitrile ligand may be used as a kinetic probe for mass retardation studies and for proton NMR investigations.

We have carried out the above reaction with a variety of different thiols. All the resulting complexes are very similar, their metal-carbonyl cores being basically isostructural. Below is reported the molecular structure of the para-thiocresol adduct.

Systematic kinetic studies for these reactions were performed with different ligand concentrations in pseudo-first order conditions, and were followed spectrophotometrically. In the following picture is reported the UV-visible spectral variation for the two-steps reaction (1) between the triosmium cluster and phenylthiol.

The plot of the rate constant for the para-thiocresol addition at different acetonitrile concentrations shows a saturation shape, with a curvilinear dependence on the concentration of the entering thiol.

We have also carried out kinetic studies with the para-thiocresol ligand by ¹H-NMR spectroscopy in the presence of different amounts of free acetonitrile, which made it possible to follow both processes. The plot of the time dependence of the relative concentration (I %) of the species involved in the consecutive reactions is reported below.

A: [Os₃(CO)₁₁(CH₃CN)] (). B: [Os₃(CO)₁₁(h¹:h¹-SRH)] (). C: [Os₃(CO)₁₀(m-SR)(m-H)] () .

The observed kinetics indicate that both steps in the proposed reaction mechanism are dissociative:

1. Both rate constants are independent of ligand concentration and are quite similar for all thiols [k₁= (1.7 ± 0.3).10^-2 s^-1, k_i= (1.3 ± 0.5).10^-3 s^-1, respectively]. In the second step, the most reactive thiol is the cyclohexylmercaptan (k_i= (2.45 ± 0.3).10^-3s^-1), while the least reactive one is the more sterically hindered 2-naphtalenethiol [k_i = (0.682 ± 0.05). 10^-3 s^-1].

1. The activation parameters obtained from the temperature dependence of k₁ and k_i for reaction (1) are consistent with a dissociative process (k₁: DH^‡ = 98 ± 2kJ mol^-1, DS^‡ = 48 ± 7 J.K^-1mol^-1; k_i: DH^‡ = 90 ± 2 kJ.mol^-1, DS^‡ = 2 ± 8 J.K^-1mol^-1).

1. In the first step the displacement of the acetonitrile by the sulfur donor ligands is retarded by the addition of the free leaving group. The plot of the rate constants for the para-thiocreosol at different acetonitrile concentrations shows saturation kinetics, with a curvilinear dependence on the concentration of the entering thiol.
   

NMR relaxation experiments indicate that the mechanism of thiol addition proceeds via an intermediate in which an agostic Os-S-H interaction is detected. In the case of the para-thiocresol, the relaxation time T₁ is 2.1 ± 0.2 ms for the intermediate and 2.7 ± 0.2 ms for the final species. The position of the h¹-H signal is at lower field than that expected for a terminal hydride (d = - 4.65 ppm), especially if compared to the bridged resonance m-H (d = -17.0 ppm).

This is the first example of such an intermediate and may be related to the initial steps of HDS reactions on metal surfaces.