Neil Burford

University of Victoria, Victoria, British Columbia, Canada

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Publications (202)1023.72 Total impact

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    ABSTRACT: A high-yielding and facile synthesis for diphosphane monochalcogenides (1Ch(R)) and their constitutional isomers, diphosphanylchalcoganes (2Ch(R)), was developed, featuring a condensation reaction between chlorophosphanes (R2PCl) and sodium chalcogenides (Na2Ch, Ch = S, Se, (Te)). The optimized protocol selectively yields either 1Ch(R) (R2(Ch)PPR2) or 2Ch(R) (Ch(PR2)2) depending upon the steric demand of the substituents R. Reaction pathways consistent with the distinct reaction outcomes are proposed. The application of 1Ch(R) and 2Ch(R) as an interesting class of ligands is exemplarily demonstrated by the preparation of selected transition metal complexes.
    No preview · Article · Feb 2016 · Inorganic Chemistry

  • No preview · Article · Jan 2016 · Canadian Journal of Chemistry
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    ABSTRACT: A series of phosphinophosphonium cations ([R2PPMe3]+; R = Me, Et, iPr, tBu, Cy, Ph and NiPr2) have been prepared and examined by collision-induced dissociation (CID) to determine the fragmentation pathways accessible to these prototypical catena-phosphorus cations in the gas-phase. Experimental evidence for fission of P-P and P–E (E = P, C) bonds, and β-hydride elimination has been obtained. Comparison of appearance potentials for the P–P bond dissociation fragments [R2P]+ (P–P heterolysis) and [PMe3]+• (P–P homolysis) shows that heterolytic P–P cleavage is more sensitive than P–P homolysis towards changes in substitution at the trivalent phosphorus center. The facility of β-hydride elimination increases with the steric bulk of R in [R2PPMe3]+. A density functional theory (DFT) study modelling these observed processes in gas-phase, counterion- and solvent-free conditions, to mimic the mass spectrometric environment, was performed for derivatives of [R2PPMe3]+ (R = Me, Et, iPr, tBu, Ph and NiPr2), showing good agreement with experimental trends. The unusual observation of both homolytic and heterolytic cleavage pathways for the P–P and P–C bonds reveals new insight into the fundamental aspects of bonding in monocations and undermines the use of simplistic bonding models.
    No preview · Article · Jan 2016 · Chemical Science
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    ABSTRACT: Reactions of polydentate phosphines with sources of Pn(3+) (Pn = P, As, Sb, Bi) yield complexes of Pn(1+) (Pn = P, As) or Pn(3+) (Pn = Sb, Bi) acceptors. The distinction between coordination of a phosphine center to Pn and oxidation of a phosphine ligand is dependent on Pn. The first structurally verified triphosphine complexes of Sb(iii) and Bi(iii) acceptors are reported.
    No preview · Article · Nov 2015 · Chemical Communications
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    ABSTRACT: A series of 2-phosphino-1,3-diphosphonium trifluoromethanesulfonate salts has been prepared and comprehensively characterized. The compounds represent rare examples of salts containing triphosphorus dications and establish important structural and spectroscopic parameters and trends for catenated phosphorus chains.
    No preview · Article · Oct 2015 · Dalton Transactions
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    ABSTRACT: Triflate salts of trications [(bipy)2E]3+ ([6E][OTf]3) and [(tbbipy)2E]3+ ([6’E][OTf]3) (bipy = 2,2’-bipyridine, tbbipy = 4, 4’-di-tbutyl-2,2’-bipyridine; E = P, As, Sb, Bi) have been synthesized and comprehensively characterized. The unique molecular and electronic structures of this new class of complexes involving pnictogen Lewis acids has been assessed in the solid, solution and gas phases to reveal systematic variations in metric parameters, ligand lability and charge concentration. While the Lewis acidity of E3+ has the trend E = Bi < Sb < As < P as determined by gas-phase calculations and 1H NMR spectroscopy, the Lewis acidity of [6E]3+ has the trend E = P < As < Sb < Bi according to gas-phase calculations. Derivatives of [6’E][OTf]3 (E = P, As) are latent sources of E(OTf)3 as demonstrated by their reactions with dmap, which give the corresponding derivatives of [(dmap)3E][OTf]3. The highly oxidizing nature of P(OTf)3 and As(OTf)3 is evidenced in reactions of [6’E][OTf]3 (E = P, As) with phosphines, which give EI-containing monocations [(R3P)2E]1+ and oxidatively coupled dications [R3PPR3]2+, illustrating new P-P and P-As bond forming strategies. Cations [6’E]3+ (E = P, As) are C-H bond activating agents that dehydrogenate 1,4-cyclohexadiene, with higher activity observed for E = P. Combinations of [6’E]3+ and tBu3P activate H2 and D2 under mild conditions, evidencing frustrated Lewis pair activity. Oxidation of [6’P][OTf]3 with SO2Cl2 gives [(tbbipy)2PCl2][OTf]3, containing a PV-trication, but there is no evidence of the analogous reaction with [6’As][OTf]3. The observations highlight new directions in the chemistry of highly charged cations and reveal a rich reactivity for p-block triflates E(OTf)3, which can be accessed through derivatives of [6E][OTf]3 and [6’E][OTf]3.
    Preview · Article · Aug 2015 · Chemical Science
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    ABSTRACT: Reactions of triarylphosphines with fluoroantimony(III) triflates give phosphine antimony(III) complexes, which undergo spontaneous reductive elimination of fluorophosphonium cations. The resulting phosphine antimony(I) complexes catenate to give the first examples of cationic antimony bicyclic compounds, [(R3 P)4 Sb6 ](4+) , featuring a bicyclo[3.1.0]hexastibine framework stabilized by four phosphine ligands. The unprecedented 14-electron redox process illustrates the generality of the reductive catenation method. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    No preview · Article · Jun 2015 · Angewandte Chemie International Edition
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    ABSTRACT: Reactions of triarylphosphines with fluoroantimony(III) triflates give phosphine antimony(III) complexes, which undergo spontaneous reductive elimination of fluorophosphonium cations. The resulting phosphine antimony(I) complexes catenate to give the first examples of cationic antimony bicyclic compounds, [(R3P)4Sb6]4+, featuring a bicyclo[3.1.0]hexastibine framework stabilized by four phosphine ligands. The unprecedented 14-electron redox process illustrates the generality of the reductive catenation method.
    No preview · Article · Jun 2015 · Angewandte Chemie
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    ABSTRACT: The coordination chemistry of the stiboranes Ph4 Sb(OTf) (1 a, OTf = OSO2 CF3 ) and Ph3 Sb(OTf)2 (3) with Lewis bases has been investigated. The significant steric encumbrance of the Sb center in 1 a precludes interaction with most ligands, but the relatively low steric demands of 4-methylpyridine-N-oxide (OPyrMe) and OPMe3 enabled the characterization of [Ph4 Sb(OPyrMe)][OTf] (2 a) and [Ph4 Sb(OPMe3 )][OTf] (2 b), rare examples of structurally characterized complexes of stibonium acceptors. In contrast, 3 was found to engage a variety of Lewis bases, forming stable isolable complexes of the form [Ph3 Sb(donor)2 ][OTf]2 [donor=OPMe3 (6 a), OPCy3 (6 b, Cy=cyclohexyl), OPPh3 (6 c), OPyrMe (6 d)], [Ph3 Sb(dmap)2 (OTf)][OTf] (6 e, dmap=4-(dimethylamino)pyridine) and [Ph3 Sb(donor)(OTf)][OTf] [donor=1,10-phenanthroline (7 a) or 2,2'-bipy (7 b, bipy=bipyridine)]. These compounds exhibit significant structural diversity in the solid-state, and undergo ligand exchange reactions in line with their assignment as coordination complexes. Compound 3 did not form stable complexes with phosphine donors, with reactions instead leading to redox processes yielding SbPh3 and products of phosphine oxidation. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    No preview · Article · Apr 2015 · Chemistry - A European Journal
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    ABSTRACT: Reductive elimination of [R3PPR3]2+, [11(R)]2+, from the highly electrophilic SbIII centres in [(R3P)3Sb]3+, [8(R)]3+, gives SbI containing cations [(R3P)Sb]1+, [9(R)]1+, which assemble into frameworks identified as cyclo-tetra(stibinophosphonium) tetracations, [(R3P)4Sb4]4+, [10(R)]4+. A phosphine catalyzed mechanism is proposed for conversion of fluoroantimony complexes [(R3P)2SbF]2+, [7(R)]2+, to [10(R)]4+, and the characterization of key intermediates is presented. The results constitute evidence of a novel ligand activation pathway for phosphines in the coordination sphere of hard, electron deficient acceptors. Characterization of the associated reactants and products supports earlier, albeit less definitive, detection of analogous phosphine ligand activation in CuIII and TlIII complexes, illustrating that these prototypical ligands can behave simultaneously as reducing agents and σ donors for a variety of hard acceptors. The reactivity of the parent cyclo-tetra(stibinophosphonium) tetracation, [10(Me)]4+, is directed by high charge concentration and strong polarization of the P-Sb bonds. The former explains the observed facility for reductive elimination to yield elemental antimony and the latter enabled activation of P-Cl and P-H bonds to give phosphinophosphonium cations, [Me3PPR’2]1+, including the first example of an H-phosphinophosphonium, [(Me3P)P(H)R’]1+, and 2-phosphino-1,3-diphosphonium cations, [(Me3P)2PR’]2+. Exchange of a phosphine ligand in [10(Me)]4+ with [nacnac]1- gives [(Me3P)3Sb4(nacnac)]3+, [15(Me)]3+, and with dmap gives [(Me3P)3Sb4(dmap)]4+, [16]4+. The lability of P-Sb or Sb-Sb interactions in [10(Me)]4+ has also been illustrated by characterization of heteroleptically substituted derivatives featuring PMe3 and PEt3 ligands.
    Preview · Article · Feb 2015 · Chemical Science
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    ABSTRACT: Three series of phosphine chalcogenide complexes of the antimony(III) halides SbX3 (X = F, Cl, Br, or I) have been synthesized and characterized by spectroscopic and crystallographic methods. Complexes of the generic formulae (Cy3PO)SbX3 (X = F (1a), Cl (1b), or Br (1c)), (Cy3PO)2SbX3 (X = F (2a), Cl (2b), or Br (2c)), and (Cy3PS)SbX3 (X = Cl (3b), Br (3c), or I (3d)) were synthesized via the treatment of solutions of SbX3 with OPCy3 and SPCy3, respectively. Derivatives of (Cy3PO)SbX3 were characterized by single crystal X-ray diffraction for 1a and 1b and crystallize as dimers through symmetry related intermolecular Sb–X interactions, providing the first structurally characterized examples of this class of complex. Derivatives of (Cy3PS)SbX3 (3b–3c) adopt analogous dimeric structures in the solid state. The solid-state structure of (Cy3PO)2SbCl3 (2b) is consistent with the previously reported structures of bis-phosphine oxide complexes of antimony(III), with a square pyramidal Sb center and cis-configured OPCy3 ligands. The phosphine chalcogenide complexes of SbX3 display configurations that are consistent with the perceived trans-labilizing properties of the ligands/substituents.
    No preview · Article · Jan 2015 · Canadian Journal of Chemistry
  • Saurabh S. Chitnis · Neil Burford
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    ABSTRACT: An overview of the synthesis, structures and reaction chemistry of coordination complexes featuring an acceptor with at least one lone pair and at least one phosphine donor is presented. One or more examples of complexes have been structurally-characterized for the majority of p-block elements but few are known for most elements. The unusual condition of a p-block element centre accommodating a lone pair of electrons and offering a low energy LUMO gives the element centre the potential to behave as both a Lewis acid and a Lewis Base. The structural diversity and reactivity of the phosphine complexes highlights new directions in main group chemistry and by comparison with transition metal coordination chemistry, the featured complexes demonstrate significant configurational and stereochemical flexibility. Ligand exchange, oxidation and reduction chemistry at the lone pair acceptor centre reveals unusual reactivity and an interesting class of ligands and inorganic reagents, with new possibilities for catalysis or small molecule activation.
    No preview · Article · Oct 2014 · Dalton Transactions
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    ABSTRACT: The cyclo-diphosphinophosphonium salt [(PtBu)3Me][OTf] (2) has been shown to be highly reactive towards Lewis bases, exhibiting diverse reactivity towards phosphines, 4-(dimethylamino)pyridine (dmap) and chlorophosphines, providing approaches to new open-chain and cyclic catena-phosphorus frameworks. Reaction of 2 with R3P (R = Me or nPr) or dmap led to the ring-opened adducts [R3P-PtBu-PtBu-P(Me)tBu][OTf] (R = Me (4a), nPr (4b)) and [(dmap)-PtBu-PtBu-P(Me)tBu][OTf] (6), respectively. The complicated 31P NMR spectra of the three compounds were simulated, evidencing the presence of two diastereomeric forms of 4a, and single diastereomers of 4b and 6. This ring-opening reactivity of the cation in 2 parallels the reactivity of isolobal epoxides with nucleophiles under acidic conditions. Compound 2 was also shown to react with a 2:1 mixture of Me2PCl and TMSOTf to form the unexpected cyclo-1,2-diphosphino-3,4-diphosphonium salt [(Me2P)2(PtBu)2][OTf]2 (8), which is postulated to result from two consecutive ring-opening and ring closing steps. In contrast, reaction with MePCl2 furnished [(MeP)(PtBu)2P(Me)tBu][OTf] (9), consistent with insertion of a 'MeP' moiety into the cationic phosphorus framework of 2. The importance of ring strain on the reactivity of the cation in 2 was illustrated by comparative studies of the corresponding cyclo-tetraphosphorus cation in [(PtBu)4Me][OTf] (10), which exhibits no reactivity under analogous conditions.
    No preview · Article · Sep 2014 · Journal of the American Chemical Society
  • Saurabh S Chitnis · Marc J Whalen · Neil Burford
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    ABSTRACT: We report a comprehensive and systematic experimental and computational assessment of the P-P bond in prototypical molecules that represent a rare series of known compounds. The data presented compliments the existing solid-state structural data and previous computational studies to provide a thorough thermodynamic and electronic understanding of the P-P bond. Comparison of homolytic and heterolytic bond dissociation for tricoordinate-tricoordinate, tricoordinate-tetracoordinate and tetracoordinate-tetracoordinate P-P bonds frameworks 1-6 provides fundamental insights into covalent bonding. For all types of P-P bond discussed, homolytic dissociation is favoured over heterolytic dissociation, although the distinction is small for 21+ and 61+. The presence of a single cationic charge in a molecule substantially strengthens the P-P bond (relative to analogous neutral frameworks) such that it is comparable with the C-C bond in alkanes. Nevertheless, P-P distances are remarkably independent of molecular charge or coordination number, and trends in values of d(PC) and νsymm(PC) imply that a molecular cationic charge is distributed over the alkyl substituents. In the gas phase, the diphosphonium dication 32+ has similar energy to two [PMe3]+ radical cations, so that it is the lattice enthalpy of 3[OTf]2 in the solid-state that enables isolation, highlighting that values from gas-phase calculations are poor guides for synthetic planning for ionic compounds. There are no relationships or correlations between bond lengths, strengths and vibrational frequencies.
    No preview · Article · Aug 2014 · Journal of the American Chemical Society
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    ABSTRACT: Pniktanderivate können die Rolle von sowohl 2 e‐Donoren (Lewis‐Basen) als auch 2 e‐Akzeptoren (Lewis‐Säuren) übernehmen. Als prototypische Liganden in der Koordinationschemie von Übergangsmetallen bilden Amine und Phosphane auch Komplexe mit Lewis‐Säuren des p‐Blocks, die verschiedene Pniktogen‐zentrierte Akzeptoren umfassen. Die Lewis‐Acidität von Pniktogenzentren kann durch eine kationische Ladung verstärkt werden, was in den letzten Jahren zur Entwicklung von Verbindungen mit koordinativen Pn‐Pn‐ und Pn‐Pn′‐Wechselwirkungen genutzt wurde. Diese Verbindungen bieten die außergewöhnliche Möglichkeit zur homoatomaren Koordinationsbindung und ermöglichen die Entwicklung von Komplexen mit einem freien Elektronenpaar am Akzeptorzentrum. Dieser Aufsatz weist neue Richtungen für die systematische Erweiterung der Koordinationschemie von der Übergangsreihe zum p‐Block. GermanKoordinations‐ oder Donor‐Akzeptor‐Bindungen bilden die Grundlage der Übergangsmetallchemie, sie treten aber auch zwischen Nichtmetallen auf. Dieser Aufsatz betrachtet koordinative Wechselwirkungen zwischen Pniktogenen (Pn) und zeigt deren strukturelle Vielfältigkeit. Verbindungen, die Pn‐Pn‐ oder Pn′‐Pn‐Bindungen enthalten, werden geordnet nach dem als kationischer Akzeptor wirkenden Pniktogenelement behandelt.
    No preview · Article · Jun 2014 · Angewandte Chemie
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    ABSTRACT: The coordination of Me2E(OTf)2 (E = Si, Ge, Sn) acceptors by dmap or 2,2'-bipy furnishes two series of complexes which exhibit distinct structural trends that correlate with the covalent radii of the tetrael elements, and which contrast complexes of these ligands with EX4 (X = Cl or Br).
    No preview · Article · Jun 2014 · Chemical Communications
  • Alasdair P M Robertson · Paul A Gray · Neil Burford
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    ABSTRACT: Pnictine derivatives can behave as both 2e(-) donors (Lewis bases) and 2e(-) acceptors (Lewis acids). As prototypical ligands in the coordination chemistry of transition metals, amines and phosphines also form complexes with p-block Lewis acids, including a variety of pnictogen-centered acceptors. The inherent Lewis acidity of pnictogen centers can be enhanced by the introduction of a cationic charge, and this feature has been exploited in recent years in the development of compounds resulting from coordinate Pn-Pn and Pn-Pn' interactions. These compounds offer the unusual opportunity for homoatomic coordinate bonding and the development of complexes that possess a lone pair of electrons at the acceptor center. This Review presents new directions in the systematic extension of coordination chemistry from the transition series into the p-block.
    No preview · Article · Jun 2014 · Angewandte Chemie International Edition in English
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    ABSTRACT: Complexes of the generic formula [Cln(PR3)mSb]((3-n)+) (n = 1, 2, 3, or 4 and m = 1 or 2) have been prepared featuring [ClSb](2+), [Cl2Sb](1+), Cl3Sb, or [Cl4Sb](1-) as acceptors with one or two phosphine ligands {PMe3, PPh3, PCy3 (Cy = C6H11)}. The solid-state structures of the complexes reveal foundational features that define the coordination chemistry of a lone pair bearing stibine acceptor site. The experimental observations are interpreted with dispersion-corrected density functional theory calculations to develop an understanding of the bonding and structural diversity.
    No preview · Article · Apr 2014 · Inorganic Chemistry
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    ABSTRACT: The syntheses of salts containing ligand-stabilized Ph3Sb2+ and Ph3Bi2+ dications have been realized by in situ formation of Ph3Pn(OTf)2 (Pn=Sb or Bi) and subsequent reaction with OPPh3, dmap and bipy. The solid-state structures demonstrate diversity imposed by the steric demands and nature of the ligands. The synthetic method has the potential for broad application enabling widespread development of the coordination chemistry for PnV acceptors.
    No preview · Article · Mar 2014 · Angewandte Chemie
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    ABSTRACT: The syntheses of salts containing ligand-stabilized Ph3Sb 2+ and Ph3Bi2+ dications have been realized by in situ formation of Ph3Pn(OTf)2 (Pn=Sb or Bi) and subsequent reaction with OPPh3, dmap and bipy. The solid-state structures demonstrate diversity imposed by the steric demands and nature of the ligands. The synthetic method has the potential for broad application enabling widespread development of the coordination chemistry for PnV acceptors. A coordinated effort: The synthesis and characterization of OPPh 3, dmap (4-(dimethylamino)pyridine), and bipy (2,2′-bipyridine) complexes of SbV and BiV are reported. The solid-state structures demonstrate structural diversity driven by the steric demands and the nature of the ligands.
    No preview · Article · Mar 2014 · Angewandte Chemie International Edition

Publication Stats

4k Citations
1,023.72 Total Impact Points

Institutions

  • 2011-2015
    • University of Victoria
      • Department of Chemistry
      Victoria, British Columbia, Canada
    • University of Louisville
      • Department of Chemistry
      Louisville, Kentucky, United States
  • 1989-2014
    • Dalhousie University
      • Department of Chemistry
      Halifax, Nova Scotia, Canada
  • 1986-2010
    • University of New Brunswick
      • Department of Chemistry
      Fredericton, New Brunswick, Canada