Preparation and studies of paramagnetic diene complexes of molybdenum(III); molecular and electronic structures of [MoCl2(η-C4H6)(η-C5H5)] and [Mo3(µ-Cl)(µ3-O){µ3-σ,σ:η2:η2-C4(CF3)4}(η-C5H5)3]

Abstract

Reactions of complexes [MoX{η-C₂(CF₃)₂}₂(η-C₅H₅)] with certain dienes give the paramagnetic complexes [MoX₂(η-diene)(η-C₅H₅)][diene = 1,3-butadiene, X = Cl (la), Br (1b), or I (1c); diene = isoprene, X = Cl (2); diene =trans-1,3-pentadiene, X = Cl (3)] characterised by elemental analysis, i.r. spectroscopy, and mass spectrometry. The structure of (1a), determined from 2 716 observed reflections measured at 185 K and refined to R= 0.0367 (R′= 0.0532), shows a molecule of near C_s symmetry. The cyclopentadienyl ligand is distorted from regular η⁵ bonding by slippage of the Mo atom across the ring by 0.12 Å towards η³ co-ordination. The 1,3-butadiene adopts a cis-endo configuration with internal C–C bonds shorter by 0.045 Å than terminal C–C bonds; the Mo–C bonds are shorter to terminal than to internal C atoms, and the butadiene is tilted away from a parallel geometry to assume a configuration with Mo which approaches a metallacyclopentene unit. Extended-Hückel molecular orbital (EHMO) calculations on complex (1a) are presented and confirm the stability of the endo conformation relative to the exo form (85 kJ mol^–1). A by-product in the formation of complex (1a) is [Mo₃(µ-Cl)(µ₃-O){µ₃-σ,σ:η²:η²-C₄(CF₃)₄}(η-C₅H₅)₃], (4), which has been structurally characterised as its 0.5C₄H₆ solvate by X-ray diffraction. The structure has been refined to R= 0.0377 (R′= 0.0488) using 4 215 data recorded at 185 K. A flattened isosceles triangle of metal atoms, base ca. 2.90 Å, base–apex ca. 2.58 Å, is face-capped by O and (open) edge-bridged by Cl. The C₄(CF₃)₄ unit is symmetrically bonded to the opposite Mo₃ face to the capping O atom, in a 2σ+ 4π bonding mode. EHMO calculations suggest some multiple character for the base–apex Mo–Mo bonds, but little direct bonding between the basal atoms. Complex (1a) is interconverted with Tl(SR) into [Mo(SR)₂(η-C₄H₆)(η-C₅H₅)][R = C₆H₄Me-4 (5a)(72%) or C₆F₅(5b)(15%)], characterised by analysis and spectroscopy. Cyclic voltammetric studies on complexes (1a), (5a), and (5b) establish a reversible reduction process in both tetrahydrofuran and dichloromethane, and also more complex oxidations. The most easily reduced complex is (5b), whereas (5a) is most readily oxidised. Complexes (1) and (5) all show e.s.r. spectra in solution with hyperfine coupling to ^95,97Mo and to ligand nuclei of ^35,37Cl, ^79,81Br, and diene terminal ¹H atoms. E.s.r. spectra of these complexes in frozen 2-methyltetrahydrofuran are also presented. The e.s.r. results are discussed in relation to the electronic structures of the complexes, especially the nature of the singly occupied molecular orbital, as determined by EHMO calculations on complex (1a).