NoteSubstitution reactions in dinuclear molybdenum(III) thiolato-complexes induced by isocyanato ligands
Introduction
We have investigated for some years the influence of metal centres and of the sulfur substituents R or the Cp′ rings (C5H5, C5Me5) on the electrochemical behaviour and the reactivity of complexes possessing {Cp′2M2(μ-SR)n} core (n=1–3, M=Mo, W, V) 1, 2. We were particularly interested to generate substrate-binding sites and to control their selectivity. Very recently we have reported that the substitution of C5H5 (Cp) rings by the C5Me5 (Cp*) ligands in the series of complexes [Cp′2Mo2(CO)4(μ-SR)2](BF4)2 (Cp′=C5H5, C5Me5) affects the life time of intermediates produced in the reduction processes [2]b and lowers the reactivity of these complexes [2]. Cp* compounds show greater resistance to decarbonylation than their Cp analogues, no substitution of carbonyl by acetonitrile is observed when [Cp*2Mo2(CO)4(μ-SR)2](BF4)2 is stirred in refluxing MeCN [2]a. A way to activate such inert carbonyl complexes is to introduce into the metallic framework a halide or pseudo-halide ligand [3]. We have tried to activate toward carbonyl substitution the inhibited pentamethylcyclopentadienyl complex [Cp*2Mo2(CO)4(μ-SR)2](BF4)2 by transforming one carbonyl group into an isocyanate ligand. Here we report the reaction of the dicationic compound [Cp′2Mo2(CO)4(μ-SR)2](BF4)2 with natrium azide NaN3, giving the isocyanate products [Cp′2Mo2(NCO)(CO)3(μ-SR)2](BF4) (Cp′=C5H5, R=Me 1, R=Ph 2; Cp′=C5Me5, R=Me 3). The reactivity of nitrile (CH3CN) and isocyanide (t-BuNC, xylyNC) towards these isocyanate complexes has been investigated and compared to that with the dicationic tetracarbonyl precursor [Cp′2Mo2(CO)4(μ-SR)2]2+. We show that the presence in the complexes of a pseudo-halide ligand labilises these dinuclear compounds to carbonyl substitution.
Section snippets
Results
A red solution of [Cp′2Mo2(CO)4(μ-SR)2](BF4)2 in acetonitrile reacted instantaneously with one equivalent of NaN3 in ethanol to give a brown solution from which were isolated the isocyanate products [Cp′2Mo2(NCO)(CO)3(μ-SR)2](BF4) (Cp′=C5H5, R=Me 1, R=Ph 2; Cp′=C5Me5, R=Me 3) 1–3 in quantitative yields (Reaction 1).
These new compounds have been characterised by various spectroscopic methods (1H and 13C NMR, IR) (Table 1) and elemental analyses. 1H and 13C NMR spectroscopy of 1–3 shows the
Discussion
The reaction of N3− with the dinuclear tetracarbonyl complexes [Cp′2Mo2(CO)4(μ-SR)2]2+ leads to the isocyanate products via a mechanism analogous to the Curtius rearrangement which has been proposed by Beck and coworkers with carbonyl monometallic compounds [LnM–CO] [5]. Some examples of di- or polymetallic isocyanate products have been reported. Gladfelter et al. have described the trinuclear ruthenium complex [Ru3(NCO)(CO)4]− obtained by reaction of [Ru3(CO)12] with N3− [6]. Other polynuclear
General procedures
The reactions were performed under either a nitrogen or an argon atmosphere using standard Schlenk techniques, and solvents were deoxygenated and dried by standard methods. Literature methods were used for the preparation of [Cp′2Mo2(CO)4(μ-SR)2](BF4)2 (Cp′=Cp [4], Cp′=Cp* [2]a).
Infrared spectra were obtained with a Perkin-Elmer 1430 spectrophotometer. NMR spectra were recorded on a Bruker AC300 spectrophotometer. Peak positions were relative to tetramethylsilane as an internal reference.
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