Carbene complexes. Part 11. Steric and conformational effects upon the transition-metal reactivity of electron-rich olefins and derived species; heteroatom-donor-olefin–metal (Cr0, Mo0, W0, or RhI) and 2-imidazoline-N-donor–metal complexes, and the crystal and molecular structure of tetracarbonyl[NN′N″N‴-tetramethylbi(imidazolidin-2-ylidene)-NN″]-chromium(0)

Abstract

The electron-rich olefin [:[graphic omitted]R]₂, L^R₂, (R = Me or Et), reacts thermally with (a)[Cr(CO)₆], forming [Cr(CO)₅L^R][(1) or (2)] or cis-[Cr(CO)4(L^R₂),][(3) or (4)] or(b)[Cr(η-C₆H₆)(CO)₃], yielding [Cr(η-C₆H₆)(CO)₂L^Me], (5): [Cr(CO)₅L^Me], (1), forms the mixed ligand complexes cis-[Cr(AsPh₃)(CO)₄L^Me], (6) or cis-[Cr(CO)₄{C(OMe)Me}L^Me], (7), upon treatment with AsPh₃, or LiMe and then MeOSO₂F, respectively. The related olefins [:[graphic omitted]Me]₂(L′^Me₂) and [:C(NMe₂)₂]₂(tdae) do not afford carbene complexes with Cr⁰. From [M(CO)₄(bhd)](M = Cr, MO, or W; bhd = bicyclo[2.2.1]hepta-2,5-dtene) each of the three olefins readily affords heteroatom-donor-olefln species [M(CO)₄(olefin-NN″)]. For M = MO, the latter is transformed thermally into the N′N″-carbenemolybdenum complexes, only for olefin = L^Me₂, but not for L′^Me₂ or tdae. Attempted in situ syntheses of metal complexes of L^H₂ or L^H–L^Et[from HC(OMe)₂NMe₂, and H₂NCH₂CH₂NH₂ or H₂NCH₂CH₂NHEt, and a MO⁰ or Rh^I reagent] lead instead to 2-imidazoline [N[graphic omitted]H₂](R = H or Et)N-bonded complexes of Mo⁰ and Rh^I. Carbon-13 n.m.r. (but also i.r. and ¹H n.m.r.) spectra are diagnostic for differentiating the type of complex (carbene, olefln-N,N″, or imidazoline) and its stereochemistry. The crystal and molecular structure of [Cr(CO)₄(L^Me-NN″)] shows the metal to be in an approximately octahedral environment: the mutually-trans CO ligands are bent away from the olefin fragment and have Cr–C bond lengths of 1.90 Å compared to 1.82 Å of those cis; the CC bond length is 1.34 Å and there is probably little interaction with the Cr atom, the distance of closest approach being ca. 3.0 Å.