Hannah N Lancashire

The University of Manchester, Manchester, ENG, United Kingdom

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Publications (7)29.82 Total impact

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    ABSTRACT: The first examples of vinylidene complexes of the cycloheptatrienyl tungsten system [W(C=CHR)(dppe)(η-C₇H₇)](+) (dppe = Ph₂PCH₂CH₂PPh₂; R = H, 3; Ph, 4; C₆H₄-4-Me, 5) have been synthesised by reaction of [WBr(dppe)(η-C₇H₇)], 1, with terminal alkynes HC≡CR; a one-pot synthesis of 1 from [WBr(CO)₂(η-C₇H₇)] facilitates its use as a precursor. The X-ray structure of 4[PF₆] reveals that the vinylidene ligand substituents lie in the pseudo mirror plane of the W(dppe)(η-C₇H₇) auxiliary (vertical orientation) with the phenyl group located syn to the cycloheptatrienyl ring. Variable temperature ¹H NMR investigations on [W(C=CH₂)(dppe)(η-C₇H₇)][PF₆], 3, estimate the energy barrier to rotation about the W=C(α) bond as 62.5 ± 2 kJ mol⁻¹; approximately 10 kJ mol⁻¹ greater than for the molybdenum analogue. Deprotonation of 4 and 5 with KOBu(t) yields the alkynyls [W(C≡CR)(dppe)(η-C₇H₇)] (R = Ph, 6; C₆H₄-4-Me, 7) which undergo a reversible one-electron oxidation at a glassy carbon electrode in CH₂Cl₂ with E(½) values approximately 0.12 V negative of Mo analogues. The 17-electron radicals [6](+) and [7](+) have been investigated by spectroelectrochemical IR, UV-visible and EPR methods. The electronic structures of representative vinylidene (3) and alkynyl (6) complexes have been investigated at the B3LYP/Def2-SVP level. In both cases, electronic structure is characterised by a frontier orbital with significant metal d(z²)character and this dominates the structural and spectroscopic properties of the system.
    Dalton Transactions 02/2011; 40(6):1267-78. · 4.10 Impact Factor
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    ABSTRACT: The design and study of organometallic mixed-valence complexes is complicated by the mixing of metal d and bridging ligand π orbitals, which often makes the assignment of metal oxidation states ambiguous. However, in the case of complexes based on the cycloheptatrienyl-ligated molybdenum fragment, Mo(dppe)(η-C7H7), the strong ring to metal π bonding and metal to ring δ back-bonding interactions stabilize four of the metal d orbitals, while the dz2 orbital is destabilized by filled−filled interactions with the a-type MO of the C7H7 ring. When the Mo(dppe)(η-C7H7) auxiliary is used in conjunction with a 1,12-bis(ethynyl)-1,12-carbaborane-based bridging ligand, a weakly coupled (Robin and Day class II) mixed-valence system, [{Mo(dppe)(η-C7H7)}2{μ-1,12-(C≡C)2-1,12-C2B10H10}]+ ([2]+), with well-defined molybdenum oxidation states can be prepared. The near-IR region of [2]+ exhibits three intervalence charge transfer (IVCT) transitions and two lower intensity interconfigurational (or dd) transitions, which have been resolved through spectral deconvolution. The band shape of the lowest energy IVCT transition associated with [2]+, which arises from electron exchange between the dz2-type orbitals at the two Mo centers, is in excellent agreement with the predictions of the Hush two-state model for weakly coupled mixed-valence complexes. The half-height bandwidths of the higher energy IVCT transitions, which arise from transitions between lower lying metal orbitals that have symmetry properties that permit more significant mixing with the bridging ligand, are in less good agreement with the Hush model, due to the breakdown of the two-state approximation through the greater involvement of the bridge-based orbitals in those transitions.
    Organometallics. 12/2010; 30(4).
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    ABSTRACT: The paramagnetic aryl-alkynyl complexes [Mo(C≡CAr)(dppe)(η-C(7)H(7))](+) (dppe = Ph(2)PCH(2)CH(2)PPh(2); Ar = C(6)H(5), [1](+); C(6)D(5), [2](+); C(6)H(4)-4-F, [3](+); C(6)H(4)-4-Me, [5](+)) and [Mo(C≡CBu(t))(dppe)(η-C(7)H(7))](+) [4](+), have been investigated in a combined EPR and ENDOR study. Direct experimental evidence for the delocalisation of unpaired spin density over the framework of an aryl-alkynyl ligand has been obtained. The X-band solution EPR spectrum of the 4-fluoro derivative, [3](+), exhibits resolved hyperfine coupling to the remote para position of the aryl group [a(iso)((19)F) = 4.5 MHz, (1.6 G)] in addition to couplings attributable to (95/97)Mo, (31)P and (1)H of the C(7)H(7) ring. A full analysis of the (1)H ENDOR spectra is restricted by the low g anisotropy of the system which prevents the use of orientation selection. However, inter-comparison of the (1)H cw-ENDOR frozen solution spectra of [1](+), [2](+), [4](+) and [5](+), combined with spectral simulation informed by calculated values derived from DFT investigations, has facilitated estimation of the experimental a(iso)((1)H) hyperfine couplings of [1](+) including the ortho, ±3.7 MHz (±1.3 G) and para, ±3.9 MHz (±1.4 G) positions of the C(6)H(5) substituent of the aryl-alkynyl ligand.
    Dalton Transactions 10/2010; 39(47):11424-31. · 4.10 Impact Factor
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    ABSTRACT: A series of molybdenum alkynyl complexes [Mo(C≡CR)(dppe)(η-C7H7)] featuring a range of alkynyl substituents R with varying electron-donating and -withdrawing properties have been prepared. Oxidation of representative members of the series to the corresponding 17-electron radical cations has been achieved through both chemical oxidation and in situ spectroelectrochemical methods. The largely metal-centered character of the HOMO in this class of compounds has been established through a combination of experimental measurements (IR, UV−vis−NIR, EPR spectroscopies) and DFT-based calculations and rationalized in terms of the stabilization of the metal dxy, dyz, dxz, and dx2-y2 through π- and δ-interactions with the C7H7 ring and concomitant destablization of the dz2 orbital.
    Organometallics 03/2010; 29(5):1261-1276. · 4.15 Impact Factor
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    ABSTRACT: A series of metallacumulenylidene complexes of the cycloheptatrienyl molybdenum auxiliary [Mo{(C)nCR2}L2(η-C7H7)]+ (n = 0−2; L2 = P-donor ligand) have been synthesized by reaction of [MoXL2(η-C7H7)] (X = Br, L2 = Ph2PCH2CH2PPh2 (dppe); X = I, L2 = 2P(OMe)3) or [Mo(OCMe2)L2(η-C7H7)]+ (L2 = dppe, Ph2PCH2PPh2 (dppm)) with the terminal alkynes HCCR; use of [MoBr(dppe)(η-C7H7)] as a precursor is facilitated by an improved, one-step synthesis from [MoBr(CO)2(η-C7H7)]. Syntheses of the 2-oxacyclocarbene complexes [Mo(CCH2CH2(CH2)xO)L2(η-C7H7)]+ (x = 1, L2 = dppm (1), dppe (2); x = 2, L2 = dppm (3), L2 = dppe (4)), the vinylidenes [Mo{CC(H)(CH2)2CH2OH}(dppe)(η-C7H7)][PF6] (5), [Mo{CC(H)(CH2)xPh}(dppe)(η-C7H7)]+ (x = 2 (6), 1 (7)), and [Mo{CC(H)Ph}{P(OMe)3}2(η-C7H7)]+ (8), and the first examples of monometallic molybdenum allenylidene complexes, [Mo(CCCRPh)(dppe)(η-C7H7)]+ (R = Ph (9), Me (10)) are reported. X-ray structural studies on complexes 8 and 9 have determined Mo−Cα distances as follows: 8, 1.929(3) Å; 9, 1.994(3) Å. In 8, the vinylidene ligand substituents lie in the pseudo mirror plane of the MoL2(η-C7H7) auxiliary (vertical orientation) with the phenyl group located syn to the cycloheptatrienyl ring, whereas the allenylidene ligand substituents of 9 are perpendicular to the pseudo mirror plane (horizontal orientation). This series of atypical orientations of cumulenylidene ligand substituents, imposed by the cycloheptatrienyl molybdenum auxiliary, has been investigated further by variable-temperature NMR spectroscopy and quantum-chemical theoretical calculations. A DFT analysis of the complexes [Mo(CCH2)(dppe)(η-C7H7)]+ (11), [Ru(CCH2)(dppe)Cp]+ (12), and [Mo(CCCMePh)(dppe)(η-C7H7)]+ (10) concurs with experimentally determined cumulenylidene ligand orientations. A separate analysis of the fragments [Mo(dppe)(η-C7H7)]+ and [Ru(dppe)Cp]+ reveals that the HOMO of the [Mo(dppe)(η-C7H7)]+ fragment includes a significant contribution from the metal dz2 orbital, whereas the HOMO of the [Ru(dppe)Cp]+ fragment is based on a metal dxy orbital, orthogonal to the HOMO of the [Mo(dppe)(η-C7H7)]+ unit. In cumulenylidene complexes of the Mo(dppe)(η-C7H7) auxiliary, interactions between the dz2-based HOMO of the metal fragment and the vacant LUMO of the cumulenylidene ligand dominate the control of ligand orientation and thus account for the observed structures.
    Organometallics 01/2008; 27(5):857-871. · 4.15 Impact Factor
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    ABSTRACT: The reaction of LiCCCCSiMe3 with [MoBr(N–N)(η-C7H7)] in thf affords the diynyl complexes [Mo(CCCCSiMe3)(N–N)(η-C7H7)], [1, N–N = 2,2′-bipyridine (bipy); 2, N–N=1,4-Bu2t-1,3-diazabutadiene (But-dab)]; an analogous procedure yields [Mo(CCCCSiMe3)(CO)(PPh3)(η-C7H7)], 3 starting from [MoBr(CO)(PPh3)(η-C7H7)]. Replacement of bipy in complex 1 with Ph2PCH2CH2PPh2 (dppe) in refluxing toluene gives [Mo(CCCCSiMe3)(dppe)(η-C7H7)], 4. Proto-desilylation of 2 and 4 leads to formation of the unsubstituted diynyls [Mo(CCCCH)L2(η-C7H7)] (5, L2 = But-dab; 6, L2 = dppe). Cyclic-voltammetric studies in CH2Cl2 reveal that each of complexes 1, 2 and 4–6 undergo reversible one-electron oxidation processes on the electrochemical time-scale. The X-ray crystal structures of complexes 1 and 6 are reported; the bipyridine complex 1, exhibits a π–π stacking interaction between the bipy ligands of adjacent molecules leading to a head-to-head interaction between diynyl ligands and a resultant substantial bending of the diynyl chain.
    Journal of Organometallic Chemistry 08/2006; 691(17):3617–3626. · 2.00 Impact Factor
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    Angewandte Chemie International Edition 12/2004; 43(43):5772-5. · 11.34 Impact Factor