High-valent diphenylacetylene complexes of tungsten

Abstract

The W(4f_7//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 d^o 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(4f_7//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(4f_7//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 X_α 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(5d_π)→ HC₂H(π*) back donation as well as HC₂H(π)→ W(5d_σ) and HC₂H(π_⊥)→ W(5d_π) 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(5d_π) 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.