Andrew K. Hughes

Contemporary boron chemistry

Applications to Polyolefin Catalysis Materials and Polymers Medicinal Applications Cluster Synthesis Carboranes Metallaboranes Metallaheteroboranes Organic and Inorganic Chemistry of Mono- and Di-Boron Systems Theoretical and Computational Studies Subject Index.

Metal–metal and metal–ligand bond strengths in metal carbonyl clusters

Abstract The limited experimental thermochemical information about metal carbonyl clusters, and the more extensive literature on structural studies of such compounds, provide a means of exploring trends in their stabilities. This review surveys that literature for selected metals, showing how the enthalpy of disruption of gaseous Mx(CO)y clusters into gaseous metal atoms and carbon monoxide can be partitioned into two components representing the strengths of metal–metal and metal–ligand bonds. In doing so, it is assumed that the bond enthalpies, E(MM), of metal–metal bonds vary smoothly with their length, d(MM), according to a relationship E(MM)=A[d(MM)]−4.6, for which a justification is provided. The structure of a cluster thus provides a means of determining the total metal–metal bond enthalpy of that cluster. Application of this method to thermodynamically characterised clusters demonstrates that the average metal–ligand bond enthalpy, E(MCO), in carbonyl clusters Mx(CO)y varies slightly with the ligand to metal ratio, y/x; a carbonyl ligand binds more strongly to a metal when it is competing with few other ligands. We demonstrate that for binary osmium carbonyl clusters, Osx(CO)y, the distances d(OsC) and d(CO) are also functions of the ligand to metal ratio, y/x, providing evidence for the familiar synergistic bonding of the carbonyl ligand, and that these distances are a function of the metal–ligand bond enthalpy, E(OsCO). Trends in cluster stability, as determined by the total metal–metal bond enthalpy, ΣE(MM), for anionic and carbonyl hydride clusters of osmium, rhenium and rhodium, [Mx(CO)yHz]c−, are presented. Similar trends for clusters of rhenium and rhodium containing core or interstitial carbon, nitrogen or other atoms are also explored, and partition of the atomisation enthalpy of binary metal carbides, MC and M2C, into metal–metal and metal–carbon components is investigated to provide insight into the strength of binding of core carbon atoms surrounded by octahedral arrays of metal atoms.

Synthesis of monodispersed fcc and fct FePt/FePd nanoparticles by microwave irradiation

A simple microwave heating method has been used for the stoichiometrically controlled synthesis of FePt and FePd nanoparticles using Na2Fe(CO)4 and Pt(acac)2/Pd(acac)2 as the main reactants. By varying the solvents and surfactants, the microwave assisted reactions have shown a significant advantage for the rapid production of monodisperse fcc FePt nanoparticle metal alloys which can be converted to the fct phase at low temperatures (364 °C). Microwave reactions at high pressure (closed system) have led to the direct formation of a mixture of fcc and fct phase FePt nanoparticles. Room temperature structural and magnetic properties of materials have been characterized by X-ray diffraction, HRTEM and magnetic measurements. The onset of ordering has been investigated by in situ high temperature X-ray diffraction studies.

New lanthanide–hydrogen–transition metal compounds: [{(PMe3)3WH5}2Yb·L3] and [{(η-C5H5)2NbH2}2Yb·L3] where L3=(MeOCH2CH2)2O

The new lanthanide–hydrogen–transition metal compounds [{[PMe3)3WH5}2Yb·L3] and [{(η-C5H5)2NbH2}2Yb·L3], where L3=(MeOCH2CH2)2O, are prepared and their crystal structures determined.

LEWIS BASE FUNCTIONALISED CYCLOPENTADIENYL COMPLEXES OF TITANIUM

Abstract The reaction of Ti(NMe2)4 with the amine functionalised cyclopentadiene C5H5CH2CH2CH2N(H)CMe3 gives the titanium cyclopentadienyl—amide complex [Tiη5:σ1-(C5H4CH2CH2CH2NCMe3)(NMe2)2] which reacts with aniline to generate the phenylimido bridged dimer [Ti(η5-C5H4CH2CH2CH2N(H)CMe3)(NHPh)2(μ-NPh)2] whose molecular structure has been determined by X-ray diffraction, revealing a slightly asymmetric Ti(μ-NPh)2Ti core.

Synthesis of η-cyclopentadienyl-polyborane derivatives of molybdenum and tungsten

The reaction of LiBH4 with [Mo(η-C5H4Me)Cl4] gives the dimetallaborane bicapped closo-[{Mo(η-C5H4R)}2B5H9], which has been characterised by X-ray crystallography. The reaction of LiBH4 with [W(η-C5H4R)Cl4], for R = Me, gives closo-[{W(η-C5H4Me)H2}2B3H7] or, for R = Pri, both closo-[{W(η-C5H4Pri)H2}2}B3H7] and nido-[{W(η-C5H4Pri)H3}B4H8]. The reaction between [Mo(η-C5H4Me)(PMe3)2Cl2] and LiBH4 yields bicapped closo-[{Mo(η-C5H4Me)}2B5H9] or nido-[{Mo(η-C5H4Me)(PMe3)H}B4H8], depending on the reaction conditions. Also LiBH4 reacts with [W(PMe3)3Cl4] to give a mixture of nido-[{W(PMe3)2H4}B4H8] and arachno-[{W(PMe3)3H3}B3H8].

Synthesis, X-ray structure and spin crossover in the triple-decker complex [(η5-C5Me5)Cr(µ2:η5-P5)Cr(η5-C5Me5)]+[A]–(A = PF6, SbF6)

The triple-decker chromium complexes [(η5-C5Me5)Cr(µ2:η5-P5)Cr(η5-C5Me5)]+[A]–(A = PF6, SbF6) can be prepared in high yield by oxidation of [(η5-C5Me5)Cr(µ2:η5-P5)Cr(η5-C5Me5)] with either [Fe(η5-C5H5)2]+[A]–(A = PF6, SbF6); the single crystal X-ray structure determination for [(η5-C5Me5)Cr(µ2:η5-P5)Cr(η5-C5Me5)]+[SbF6]– reveals that the Cr–Cr separation is 3.185(8)A which is 0.456 A longer than in the neutral complex; magnetic susceptibility studies show that these 26-electron multidecker cations in these salts undergo spin crossover at 33 and 23 K respectively.

The lanthanide-hydrogen-transition metal compounds: [{(PMe3)3WH5}2Yb·L3] and [{(η-C5H5)2NbH2}2Yb·L3] where L3 = (MeOCH2CH2)2O, and related studies

Abstract The preparation of lanthanide-hydrogen-transition metal compounds [{W(PMe 3 ) 3 H 5 } 2 Yb·diglyme] and [{Nb(η-C 5 H 5 ) 2 H 2 } 2 Yb·diglyme] from the potassium salts [W(PMe 3 ) 3 H 5 ][K] and “[Nb(η-C 5 H 5 ) 2 H 2 ][K]” and YbI 2 are described. Their X-ray crystal structures and NMR data indicate the presence of W(μ-H 3 )Yb or W(μ-H) 2 Nb groups, respectively. The related Group IV metallocene compounds [Hf(η-C 5 H 5 ) 2 ClW(PMe 3 ) 3 H 5 ], [Hf(η-C 5 H 4 Me) 2 ClW(PMe 3 ) 3 H 5 ], [Zr(η-C 5 H 4 Me) 2 ClW(PMe 3 ) 3 H 5 ] and [Zr(η-C 5 Me 5 )Cl 2 W(PMe 3 ) 3 H 5 ] have been prepared and the crystal structure of [Zr(η-C 5 Me 5 )Cl 2 W(PMe 3 ) 3 H 5 ] has been determined.

Synthesis and reactivity of bis(η-cyclopentadienyl) trimethyltin derivatives of niobium: crystal and molecular structure of [{Nb(η-C5H5)2(SnMe3)}2(µ-O)]

The reaction between [Nb(η-C5H5)2H3] and LiBu followed by SnMe3Cl gives [Nb(η-C5H5)2H2(SnMe3)]2, also produced from [Nb(η-C5H5)2H3] and SnMe3H. Heating 2 with styrene, PMe3, isoprene or but-2-yne gives [Nb(η-C5H5)2L(SnMe3)](L =η-H2CCH–C6H5, 3; PMe3, 4; η-H2CCHCMeCH2, 5; or η-C2Me26). Photolysis of complex 3 in the presence of CO, 2,6-Me2C6H3NC, MeCN, or PMe3 gives [Nb(η-C5H5)2L(SnMe3)](L = CO, 7; 2,6-Me2C6H3NC, 8; η2-MeCN, 9a and 9b; or PMe3, 4). Heating 3 with H2 regenerates 2 and releases styrene. Compound 3, 5 or 6 reacts with SnMe3H to give the bis(stannyl) derivative [Nb(η-C5H5)2H(SnMe3)2]10, which is also formed by prolonged heating of [Nb(η-C5H5)2H2(SnMe3)] with SnMe3H. Photolysis of 3 under CO2 gives a mixture of 7 and [{Nb(η-C5H5)2(SnMe3)}2(µ-O)]11 by cleavage of the CO2 molecule. Compound 11 crystallises in the orthorhombic space group Pbca with a= 8.409(3), b= 21.109(4) and c= 15.582(3)A. The bridging oxygen atom sits on a crystallographic inversion centre giving a linear Nb–O–Nb group with Nb–O 1.9434(4) and Nb–Sn 2.8619(5)A.

Reactivity of nido-[2-{Fe(η-C5H5)}B5H10] with some transition-metal hydride complexes

The reaction of nido-[2-{Fe(η-C5H5)}B5H10] with [Mo(PMe2Ph)4H4] gave capped-closo-[1-{Fe(η-C5H5)}-2-{Mo(PMe2Ph)3H}B5H7] which has been characterised by X-ray crystallography as well as by 11B and 1H NMR spectroseopy. The structure determination [monoclinic, space group P21/c, a= 15.586(5), b= 11.523(4), c= 19.186(4)A, β= 106.23(2)°] reveals a capped-closo geometry with one Fe–Mo–B face being capped by a boron atom and the other having a triply bridging hydrogen atom. The isostructural tungsten compound capped-closo-[1-{Fe(η-C5H5)}-2-{W(PMe3)3H}B5H7] was similarly prepared from [W(PMe3)3H6] and nido-[2-{Fe(η-C5H5)}B5H10]. Reaction between [Re(PMe3)5H] and nido-[2-{Fe(η-C5H5)}B5H10] gave a mixture of nido-[2-{Re(PMe3)3}B5H10] and the salt [Re(PMe3)5H2]+[nido-2-{Fe(η-C5H5)}B5H9]–, a product of an acid–base reaction.