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{q-C,( CF3)2}2(q-C5Hs)] with certain dienes give the paramagnetic complexes [MoX,(q-diene)(q-C,H,)][diene = 1,3-butadiene, X = CI ( l a ) , Br ( l b ) , or I ( l c ) ; diene = isoprene, X = CI (2); diene = trans-l,3-pentadiene, X = CI (3)] characterised by elemental analysis, i.r. spectroscopy, and mass spectrometry. The structure of (1 a), determined from 2 71 6 observed reflections measured at 185 K and refined to R = 0.0367 (R' = 0.0532), shows a molecule of near Cs symmetry. The cyclopentadienyl ligand is distorted from regular q5 bonding by slippage of the Mo atom across the ring by 0.1 2 A towards q3 co-ordination. The 1,3-butadiene adopts a cis-endo configuration with internal C-C bonds shorter by 0.045 A 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-Huckel molecular orbital (EHMO) calculations on complex (1 a) are presented and confirm the stability of the endo conformation relative to the ex0 form (85 k J mol-'). A by-product in the formation of complex ( l a ) is [Mo,(p-CI) (p3-O){p3-0,~:q2:q2-C4(CF3),}(q-C,H,),], (4)' which has been structurally characterised as its 0.5C,H6 solvate by X-ray diffraction. The structure has been refined t o R = 0.0377 (R' = 0.0488) using 4 21 5 data recorded at 185 K. A flattened isosceles triangle of metal atoms, base ca. 2.90 A, base-apex ca. 2.58 A, is face-capped by 0 and (open) edge-bridged by CI. The C,(CF,), unit is symmetrically bonded to the opposite Mo, face to the capping 0 atom, in a 2 0 + 47t bonding mode. EHMO calculations suggest some multiple character for the base-apex Mo-Mo bonds, but little direct bonding between the basal atoms. Complex ( l a ) is interconverted with TI(SR) into [Mo(SR),(q-C,H,)(q-C,H,)][R = C,H,Me-4 (5a) (72%) or C,F5 (5b) (1 5%)], characterised by analysis and spectroscopy. Cyclic voltammetric studies on complexes ( l a ) , (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 9 5 p 9 7 M ~ and to ligand nuclei of 35r37Cl, 7g*e1Br, and diene terminal 'H 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 EH M O calculations on complex (1 a). Co-ordinatively unsaturated alkyne complexes of the typeJ and corresponding tungsten(I1) species have been the subjects of a number of studies. It has been established that such complexes undergo substitution reactions at ligand X , ' T ~ to form binuclear complexes with alkyne-bridged metal-metal bonds and are attacked at the co-...

Half-sandwich molybdenum(III) compounds containing diazadiene ligands and their use in the controlled radical polymerization of styrene

“…The Mo-N distances are considerably shorter than in the structurally related half-sandwich Mo(II) complex [CpMo(H2Biim)(CO)2] + (H2Biim = bisimidazole; 2.17(2) and 2.21(2) Å), [29] and also shorter than the higher oxidation state MoO2Cl2(Bu t 2-dad) (2.399(2) and 2.388(2) Å) [30]. This is supporting evidence for the description of the coordination geometry as B. Analogously, the Mo-Cl distances are shorter than those found for CpMoCl2(PMe3)2 (2.468(2) and 2.474(2) Å) [31] and CpMoCl2( 4 -C4H6) (2.435(1) and 2.433(1) Å) [26], in further agreement with the assignment of a higher formal oxidation state to the metal in compound 3.…”

“…This limiting form corresponds to an enediamido complex of Mo(V). There is an interesting similarity between this situation and that of the analogous diene systems [26][27][28] for which contribution of an  4 -diene Mo(III) formulation as well as an enediyl Mo(V) formulation is invoked, though the former seems to predominate [27]. Another interesting point of discussion is the remarkable structural change undergone by the CpMCl2(dad) system on going from Nb and Ta (for which the C-type arrangement is always observed [10][11][12][13][14]) to molybdenum.…”

Half-sandwich complexes of molybdenum-(III), -(IV) and -(V) with P–O and P–N bifunctional ligands Ph2PCH2X (X = 2-oxazolinyl, or C(O)NPh2)

“…and have shown compati-bility with a large array of ligands. Examples with X = halide, 1-5 hydrosulfido, alkane-and aryenethiolato, 6-9 amido, 10 dialkylphosphido, 11 phenyldiazo, 12 isocyanato, 13 azido, 14 alkyl, 15 and hydroborate units, 16 L=phosphine, 17-21 CO 22-26 isocyanides 27, 28 and MeCN, 27,29,30 L2 =diphosphines, [31][32][33] phosphine sulfides 34 and dienes, [35][36][37][38] and XL=phosphine-thiolato 34 and allyl [37][38][39][40] have been reported. Our own work in this area has been summarized in review articles.…”

Different Approaches to Mid-Valent, Paramagnetic Half-Sandwich Complexes of the Heavier Group 6 Metals

“…5 has been extended to the high-yield synthesis of the butadiene complex CpMoCl 2 (h 4 -C 4 H 6 ), 46 previously available only in < 50% yields by a disproportionation procedure. 47 Other derivatives with diamine and with bifunctional ligands (phosphine-amine, phosphine-thioether, etc.) are also being prepared and characterized.…”