Carbene complexes. Part 10. Electron-rich olefin-based mono-, bis-, and tris-carbene-tungsten(0) complexes and some derived six and seven-co-ordinate mono- and bis-carbene-dihalogenotungsten(II) and related molybdenum(II) species

Abstract

Stable mono- and bis-carbene-tungsten(0) complexes [W(CO)₅L^R] or cis-[W(CO)₄(L^R)₂][L^R=:[graphic omitted]R (R = Me, Et, or CH₂Ph)] are obtained by heating [W(CO)₆] with the electron-rich olefin [:[graphic omitted]R]₂, L^R₂. The six-membered chelate olefin [:[graphic omitted]Me]₂, L′^Me₂, is less reactive, only forming [W(CO)₅L′^Me]. The cis-dicarbenetungsten(0) complexes are isomerised photochemically to the trans species, but readily revert thermally to the former. Bases react with (a)[W(CO)₅L^R] to yield cis-[W(CO)₄(L^R)Q](Q = PEt₃ or PBuⁿ₃), fac-[W(CO)₃L^RQ₂][Q = P(OMe)₃ or P(OPh)₃], or {using successively LiR′(R′= Me or Ph) and [OEt₃][BF₄]}cis-[W(CO)₄{C(OEt)R′}L^R], or (b)cis-[W(CO)₄(L^R)₂] to afford fac-[W(CO)₃(L^R)₂Q][Q = C₅H₅N or P(OMe)₃]. The tricarbene complex fac-[W(CO)₃(L^Me)₃], obtained from [W(CO)₃(η-mesitylene)], is rather unstable. The cis-dicarbene complexes cis-[M(CO)₄(L^R)₂](M = Mo or W) with dihalogens give the metal(II) species [M(CO)₂(L^R)₂X₂](M = Mo, R = Et, X = I, Br, or Cl; M = W, R = Me or Et, X = I) and [M(CO)₃(L^R)₂X₂](M = W, R = Me or Et, X = I). The latter provides an example of a CO-carrying system, the interconversion of [W(CO)₃I₂(L^R)₂] and [W(CO)₂I₂(L^R)₂]+ CO being reversible as a function of temperature. The monocarbene complexes [M(CO)₅L^R](M = MO or W) behave differently, yielding respectively [IL^R][Mo(CO)₄I₃] or [W(CO)₄I₂(L^R)](R = Me). The frequency ν(CN₂) is at 1 500–1 480cm^–1for the carbene-tungsten(0), but at 1 530–1 500 cm^–1 for the carbene-molybdenum(II) and -tungsten(II), complexes; values of ν(CO) are generally low, in accordance with the strong σ-donor character of the carbene ligand. The value of ΔG^‡ for W–C_carb. rotation in cis-[W(CO)₄(L^R)₂] is ca. 10 kcal mol^–1 and ca. 14 kcal mol^–1 in [W(CO)₄I₂(L^R)]. Carbon-13 n.m.r. chemical shifts for C_carb. and CO in the tungsten(0) species are comparable, but distinguishable by ¹H decoupling and ¹H off-resonance studies, and are in the range 200–220 p.p.m. downfield from SiMe₄. Values for 1J(¹⁸³W–¹³C) and ²J(¹³C–³¹P) are provided.