Alkane {u03C3}-complex observation with NMR and IR spectroscopies and the synthesis of rare carbido, carbon monosulfide and carbon disulfide complexes

Abstract

This work describes the generation and observation of alkane {u03C3}-complexes by IR and NMR spectroscopies. Herein is reported the first tungsten alkane {u03C3}-complexes observed with NMR spectroscopy at low temperature. A series of tungsten alkane complexes of the type [({u03B7}{u2076}- C{u2086}Et{u2086})W(C0){u2082}(alkane)] (alkane = c-pentane, c-heptane, n-pentane, iso-butane, 2,2- dimethylbutane) were created by the photolysis of solutions of [({u03B7}{u2076}-C{u2086}Et{u2086})W(C0){u2083}] in alkane solvent below 168 K. {u00B9}H NMR data were obtained for all tungsten alkane complexes. {u00B9}{u00B3}C and {u00B9}{u2078}{u00B3}W NMR data are reported for experiments using either isotopically enriched substrates, or where the concentration of the alkane complex was high enough for observation using naturally abundant solvents and precursor complex. Larger JcH (cf. rhenium analogues) and small JwH values suggest that the tungsten complexes bind alkanes less strongly than rhenium complexes previously reported, consistent with their comparatively short lifetimes. In contrast to reports for rhenium-alkane binding modes where methylene coordination typically prevails, tungsten alkane complexes were found to bind exclusively through methy1 groups on n{u00AD} pentane and 2,2-dimethylbutane, perhaps reflecting the steric constraints imposed by the C{u2086}Et{u2086} co-ligand.

In addition to this, a series of rhenium precursor complexes were synthesised and evaluated as precursors for alkane coordination: [TpRe(C0){u2082}(CS)], [TpRe(C0){u2082}(PMe{u2083})], [nBuTpRe(C0){u2083}], [nBuTpRe(C0){u2082}(PMe{u2083})], [iPrOBpRe(C0){u2083}], [LoEtRe(C0){u2083}]. This library of complexes include the first examples of oxygen donor ligated alkane {u03C3}-complexes ([LoEtRe(C0){u2082}(alkane)], [iPrOBpRe(C0){u2082}(alkane)]), structurally isomeric alkane complexes ([N-trans{u00AD} iPrOBpRe(C0){u2082}(alkane), [O-trans-iPrOBpRe(C0){u2082}(alkane)]) and, what appears to be, the highly stable alkane {u03C3}-complex at comparatively high (~250 K) temperatures, [O-trans{u00AD}iPrOBpRe(C0){u2082}(c-C{u2085}H{u2081}{u2080})]. A correlation between the a-acidity of co-ligands with alkane stability was made possible by the diversity of the precursor complexes [nBuTpRe(C0){u2083}], [nBuTpRe(C0{u2082})(PMe{u2083})], [TpRe(C0){u2083}], [TpRe(C0){u2082}(CS)] and [TpRe(C0){u2082}(PMe{u2083})]. Alkane {u03C3}{u00AD} complexes formed with the photo-products of these complexes indicated that the 'TpRe(CO)L' (L = CS, CO, PMe{u2083}) fragment is less likely to insert into alkane ligands as compared to previously reported 'CpRe(CO)L' fragments. The findings also indicated that {u03C0}-neutral or weakly {u03C0}-acidic ligands tend to stabilise alkane complexes more than strongly {u03C0}-acidic co{u00AD} ligands on the 'TpRe(CO)L' fragment, with the order of longevity following PMe{u2083} >CO>CS.

During the course of this study, the novel bridging carbido species [CpRe(C0){u2082}]{u2082}({u03BC}-C) was synthesised. Its physical properties were probed analytically and computationally interpreted through DFT studies. The calculated characteristics were found to correlate strongly to observed experimental data. To probe the formation of the carbido species, [CpRe(C0){u2082}]{u2082}({u03BC}{u00AD}C), the novel model bridging carbon monosulfide complexes, [TpRe(C0){u2082}][CpRe(C0){u2082}]({u03BC}{u00AD}CS) and [TpRe(C0){u2082}({u03BC}-CS), and the bridging carbon disulfide complexes, [TpRe(C0){u2082}]{u2082}({u03BC}{u00AD}CS{u2082}) and [Tp*Re(C0){u2082}]{u2082}({u03BC}-CS{u2082}), were synthesised. It was found likely that both the bridging thiocarbonyl complexes were bridging in either a linear {u03C3}-C, {u03C3}-S fashion or a {u03BC}-C, {u03B7}{u00B2}-CS fashion, both of which are very rare. The bridging carbon disulfide complex [TpRe(C0){u2082}]{u2082}({u03BC}{u00AD} CS{u2082}) was found to decompose into, inter alia, a novel bridging disulfide species, [TpRe(C0){u2082}]{u2082}({u03C3},{u03C3} -S{u2082}), and the thiocarbonyl complex, [TpRe(C0){u2082}(CS)]. This thiocarbonyl was also produced intentionally for use in photochemical alkane complexation studies.