High-valent diphenylacetylene complexes of tungsten

Abstract

The W(4f7//2) binding energy of [{WCl4(PhC2Ph)}2]1 obtained by X-ray photoelectron spectroscopy is similar to that of [{WCl4(NPh)}2] and is consistent with a do tungsten(VI) formulation. The reaction of complex 1 and [NEt4][WCl5(PhC2Ph)]2 with NaOH–EtOH gave cis-stilbene, indicating considerable electron transfer from the metal to the co-ordinated alkyne. Reduction of complex 1 with 2 equivalents of sodium–mercury amalgam in the presence of phosphines gave the complexes [WCl3(PhC2Ph)L2](L = PMe3, PMe2Ph or PMePh2) with magnetic moments and W(4f7//2) binding energies similar to those of the d1 tungsten(V) organoimido complex [WCl3(NPh)(PMe3)2]. Decomposition of the alkyne complexes with NaOH–EtOH again gave cis-stilbene. The crystal structure of [WCl3(PhC2Ph)(PMe3)2]3 has been determined. The W–Cl bond trans to the alkyne ligand is long [2.479(3)Å], and the W–C bond lengths [2.011(13) and 2.038(12)Å] indicate a four-electron-donor alkyne ligand. The geometry is similar to that of [WCl3(NPh)(PMe3)2]. Reduction of [{WCl4(PhC2Ph)}2] using 4 equivalents of sodium–mercury amalgam in the presence of phosphines gave the complexes [WCl2(PhC2Ph)L3](L = PMe3 or PMe2Ph) which again produced cis-stilbene on decomposition with NaOH–EtOH. The acetylenic carbon resonance at δ 223.15 in the 13C-{1H} NMR spectrum of [WCl2(PhC2Ph)(PMe3)3]6 is also indicative of a four-electron-donor alkyne ligand. Its W(4f7//2) binding energy is similar to [WCl2(NPh)(PMe3)3] and is consistent with tungsten(IV). A crystal structure of complex 6 shows a similar ligand geometry to [WCl2(NPh)(PMe3)3], and the W–C bond lengths [2.019(11) and 2.006(11)Å] indicate a four-electron-donor alkyne ligand. Hartree–Fock and scattered wave Xα calculations have been performed on the model complexes [WCl5(HC2H)]–8, [WCl3(HC2H)(PH3)2]9 and [WCl2(HC2H)(PH3)3]10. Molecular orbital and population analyses indicated that the acetylene–tungsten bond in each involves W(5dπ)→ HC2H(π*) back donation as well as HC2H(π)→ W(5dσ) and HC2H(π⊥)→ W(5dπ) forward donation, consistent with a four-electron-donor alkyne formalism. Electron withdrawal from the tungesten to the more electronegative Cl ligand in complexes 8 and 9 is minimised by rotation of the alkyne away from the meridional vectors. In complex 10 the HC2H(π⊥)→ W(5dπ) donation and phosphine contributions compensate and no rotation is observed. The total d atomic orbital population of complex 8 is close to that of WCl6, and populations of complexes 9 and 10 step up linearly from this. The computational results support the experimental evidence that [{WCl4(PhC2Ph)}2]1, [WCl3(PhC2Ph)(PMe3)2]3 and [WCl2(PhC2Ph)(PMe3)3]6 are complexes of tungsten-(VI), -(V) and -(IV) respectively.