Neil M. Boag

Alkyl‐, Silyl‐ und Phosphanliganden – klassische Liganden in nichtklassischer Bindungsweise

Alkyl-, Silyl- und Phosphanliganden gehoren sicherlich zu den bekanntesten und gangigsten Liganden in der metallorganischen und der Koordinationschemie. Ihre Donoratome, C, Si bzw. P, sind in allen drei Fallen sp3-hybridisiert; zudem stehen diese Liganden uber die Isolobalanalogie (CR3)−(SiR3)−PR3 in Beziehung. Kurzlich wurde uber einige ungewohnliche Ergebnisse zur Reaktivitat und zur Bindung dieser Liganden berichtet. Auf den ersten Blick scheint kein Zusammenhang zwischen diesen Befunden zu bestehen, doch fugen sie sich zu einem konsistenten Gesamtbild zusammen. Die Charakterisierung stabiler Komplexe, in denen Alkyl-, Silyl- und Phosphanliganden als symmetrische Brucken fungieren, bestatigt, dass diese Bindungssituationen keinesfalls von Natur aus thermodynamisch instabil sind. Reaktivitatsstudien deuten sogar darauf hin, dass diese Liganden in der Lage sein sollten, in Intermediaten oder Ubergangszustanden Brucken zwischen Metallzentren zu bilden.

Evidence for multiple polytypes of semiconducting boron carbide (C2B10) from electronic structure

Boron carbides fabricated via plasma enhanced chemical vapour deposition from different isomeric source compounds with the same C2B10H12 closo-icosahedral structure result in materials with very different direct (optical) band gaps. This provides compelling evidence for the existence of multiple polytypes of C2B10 boron carbide and is consistent with electron diffraction results.

Semiconducting boron-rich neutron detectors

Semiconducting boron-rich boron-carbon alloys have been deposited by plasma-enhanced chemical vapor deposition. Heterojunction diodes made with 276nm thick nanocrystalline layers of these alloys have been used as real-time solid-state neutron detectors. Individual neutrons were detected and signals induced by gamma rays were determined to be insignificant. Linearity of detection was demonstrated over more than two orders of magnitude in flux. The neutron detection performance was unaffected by > 1 x 1015 neutrons / cm2. The source gas closo-1,2-dicarbadodecaborane (ortho-carborane) was used to fabricate the boron carbon alloys with only the natural isotopic abundance of 10B. Devices made of thicker boron carbon alloy layers enriched in 10B could lead to increased detection efficiency.

Hydrosilylation of α,β-unsaturated aldehydes and ketones by trans-DI-μ-hydrido-bis(silyl)bis-(trialkylphosphine)diplatinum complexes

The diplatinum complexes [{Pt(μ-H)(SiR3)(PR′3)}2] readily catalyse the hydrosilylation of the α,β-unsaturated aldehydes and ketones RCH : CHCHO (R = H, Me, or Ph), PhCH : C(Me)CHO, PhCH : CHC(Me)O, Me2C : CHC(Me)O, CH2(CH2)2CH : CHC : O, and (PhCH : CH)2CO. The silanes studied were MePh2SiH, Et3SiH and EtMe2SiH. In all cases, 1,4-addition products were obtained in high yield usually as equi-molar mixtures of two isomers, except for cyclohex-2-enone which is constrained to afford a single product. Tetraphenylcyclopentadienone and diphenylmethylsilane gave the 1,2-adduct Ph4C4CHOSiMePh2. The 1H and 13C NMR spectra of the various products are reported and assigned.

A mass spectrometric investigation of chloro-, bromo- and methyl-ferrocenes by electron and photon impact ionisation

Abstract Various ferrocenes Fe(C 5 H 5- n Cl n ) 2 ( n = 1–5), Fe(C 5 H 4 Br) 2 , Fe(C 5 H 5- n -(CH 3 ) n ) 2 ( n = 1–5) have been investigated by electron and photon impact mass spectroscopy. Ionisation and appearance potentials (IP/AP) have been measured and we have characterized the influence of substitutions of CH 3 , Cl, or Br at the cyclopentadienyl rings upon the IPs, APs, and the fragmentation pathways. In addition, some bond energies are derived.

Selective silver ion transfer voltammetry at the polarised liquid vertical bar liquid interface

Transfer of silver ions across the water/1,2-dichloroethane interface was studied by cyclic voltammetry (CV). In the absence of added neutral ionophore, Ag+ transferred across the interface when the organic phase contained either tetraphenylborate or tetrakis(4-chloro)phenylborate anions, but this transfer was not possible in the presence of organic phase hexafluorophosphate or perchlorate anions. The ion transfer processes observed were independent of the nature of the organic phase cation. The CV in the presence of tetraphenylborate exhibited a shape consistent with an ion transfer followed by chemical reaction; the rate constant for the following chemical reaction was 0.016 s(-1). In the presence of tetrakis(4-chloro)phenylborate, a return peak equivalent in magnitude to the forward peak was observed, indicative of a simple ion transfer reaction uncomplicated by accompanying chemical reactions. The selectivity of the transfer was assessed with respect to other metal cations: no transfers for copper, cadmium, lead, bismuth, cobalt, nickel, palladium or zinc were observed. The selectivity of the transfer suggests this can form the basis of a selective voltammetric methodology for the determination of silver ions.

The synthesis and reactions of some cyclopentadienylpiatinum complexes. Crystal structure of [Pt2{µ-C(Ph)C(Ph)C(O)}(η-C5H5)2]

Treatment of di-µ-chloro-dichlorobis(η-ethylene)diplatinum with Mg(C5H5)2 affords the compounds [Pt(η-C2H4)(σ-C5H5)(η-C5H5)] and [Pt2(µ-C10H10)(η-C5H5)2]. Reaction between [NBun4]2[Pt2Cl4(CO)2] and Mg(C5H5)2 affords [Pt2(CO)2(η-C5H5)2] in good yield, as well as the compound [Pt(CO)(σ-C5H5)(η-C5H5)]. Protonation of [Pt(η-C2H4)(σ-C5H55)(η-C5H5)] yields the salt [Pt(η-C2H4)(η2-C5H6)(η-C5H5)][BF4]. Reaction of [Pt(η-C2H4)(σ-C5H5)(η-C5H5)] with PhCCPh affords (η-cyclopentadienyl)(σ: 2–3-η-2,3-diphenylbicyclo[2.2.1 ] hepta-2,5-dien-7-yl) platinum. The compound [Pt2(CO)2(η-C5H5)2] on treatment with PhCCPh in the presence of Me3NO gives [Pt2{µ-C(Ph)C(Ph)C(O)}(η-C5H5)2], and on similar treatment with ButCCBut and Me3NO affords [Pt2(µ-ButC2But)(η-C5H5)2]. The complex [Pt2{µ-C(Ph)C(Ph)C(O)}(η-C5H5)2] was structurally characterised by a single crystal X-ray diffraction study. The metal–metal bond [Pt–Pt 2.590(1)A] is bridged by the C(Ph)C(Ph)–C(O) ligand, in such a manner that the terminal carbon atoms are each σ-bonded to one platinum centre, while the C(Ph)C(Ph) fragment is η2-co-ordinated to the other. Both metal atoms carry C5H5 ligands, but one is asymmetrically bound. The n.m.r. data (1H, 31C-{1H}, and 195Pt-{1H}) for the new compounds are reported and discussed, and this information used to suggest mechanisms for the dynamic behaviour of [Pt(η-C2H4)(σ-C5H5)(η-C5H5)], [Pt2(CO)2(η-C5H5)2], and [Pt(η-C2H4)(η2-C5H6)(η-C5H5)]+.

Alkyne complexes of platinum. Part 5. Di- and tri-platinum complexes with bridging trimethylsilyl-substituted alkyne ligands; crystal structure of [Pt2(µ-PhC2SiMe3)(C8H12)2]

Bis(trimethylsilyl) acetylene in excess reacts with [Pt(cod)2](cod = cyclo-octa-1,5-diene) to give the diplatinum complex [Pt2(µ-Me3SiC2SiMe3)(Me3SiC2SiMe3)(cod)], whereas [Pt(C2H4)3] and the alkyne afford [Pt2(µ-Me3-SiC2SiMe3)(Me3SiC2SiMe3)2]. Reactions of [Pt(cod)2] with the alkynes RCCSiMe3(R = SiMe3, SiMe2But, CF3, or Ph) in 2 : 1 mol ratio, or excess of alkyne in the case of CF3CCSiMe3, yields the diplatinum complexes [Pt2(µ-RC2SiMe3)(cod)2]. The nickel compounds [Ni2(µ-RC2SiMe3)(cod)2](R = SiMe3 or Ph) have also been prepared. Reaction of PhCCSiMe3 with [Pt(cod)2] in 1 : 1 mol ratio gives the triplatinum complex [Pt3(µ-PhC2SiMe3)2(cod)2], which like [Pt2(µ-PhC2SiMe3)(cod)2] undergoes dynamic behaviour in solution, as revealed by variable-temperature n.m.r. studies. An X-ray crystallographic study on the latter was carried out. Crystals are monoclinic, space group P21/c, Z= 4, in a unit cell of dimensions a= 14.93(1), b= 10.38(1), c= 17.463(8)A, β= 112.79(6)°. The structure has been refined to R 0.038 (R′ 0.038) for 5 464 unique reflections to 2θ⩽ 60°(Mo-Kα, X-radiation) collected at 200 K. The PhC2SiMe3 ligand transversely bridges the Pt ⋯ Pt vector (2.914 A) such that the dihedral angel between the two planes formed by the two Pt–µ–C2 units is 97°. This, together with the long metal–metal distance and the essentially planar geometry about each platinum atom, is taken to imply little or no direct Pt–Pt bonding, with the two halves of the molecule held together by orthogonal π orbitals of the shared PhCCSiMe3 ligand.

New chemistry of 1,2-closo-P2B10H10 and 1,2-closo-As2B10H10; in silico and gas electron diffraction structures, and new metalladiphospha- and metalladiarsaboranes

The molecular structures of 1,2-closo-P(2)B(10)H(10) (1) and 1,2-closo-As(2)B(10)H(10) (2) have been determined by gas electron diffraction and the results obtained compared with those from computation at the MP2/6-31G** level of theory. The level of agreement is good for 2 (root-mean-square [rms] misfit for As and B atoms 0.0297 Å) and very good for 1 (rms misfit for P and B atoms 0.0082 Å). In comparing the structures of 1 and 2 with that of 1,2-closo-C(2)B(10)H(12) (I) it is evident that expansion of the polyhedron from I to 1 to 2 is restricted only to the heteroatom vertices and the B(6) face to which these are bound. Following deboronation (at B3) and subsequent metallation, compounds 1 and 2 have been converted into the new metalladiheteroboranes 3-(η-C(9)H(7))-3,1,2-closo-CoAs(2)B(9)H(9) (4), 3-(η-C(10)H(14))-3,1,2-closo-RuAs(2)B(9)H(9) (5), 3-(η-C(5)H(5))-3,1,2-closo-CoP(2)B(9)H(9) (6), 3-(η-C(9)H(7))-3,1,2-closo-CoP(2)B(9)H(9) (7) and 3-(η-C(10)H(14))-3,1,2-closo-RuP(2)B(9)H(9) (8), the last three constituting the first examples of metalladiphosphaboranes. Together with the known compound 3-(η-C(5)H(5))-3,1,2-closo-CoAs(2)B(9)H(9) (3), compounds 4-8 have been analysed by NMR spectroscopy and (except for 8) single-crystal X-ray diffraction. The (11)B NMR spectra of analogous pairs of metalladiphosphaborane and metalladiarsaborane (6 and 3, 7 and 4, 8 and 5) reveal a consistently narrower (9-10 ppm) chemical shift range for the metalladiarsaboranes, the combined result of a deshielding of the lowest frequency resonance (B6) and an increased shielding of the highest frequency resonance (B8) via an antipodal effect. In crystallographic studies, compounds 3 and 5B (one of two crystallographically-independent molecules) suffer As/B disorder, but in both cases it was possible to refine distinct, ordered, components of the disorder, the first time this has been reported for metalladiarsaboranes. Moreover, whilst the Cp compounds 6 and 3 are disordered, their indenyl analogues 7 and 4 are either ordered or significantly less disordered, a consequence of both the reduced symmetry of an indenyl ligand compared to a Cp ligand and the preference of the former for a distinct conformation relative to the cage heteroatoms. Unexpectedly, whilst this conformation in the cobaltadiphosphaborane 7 is cis-staggered (similar to that previously established for the analogous cobaltadicarborane), in the cobaltadiarsaborane 4 the conformation is close to cis-eclipsed.

Synthesis of ultra-high purity trialkylgalliumMOVPE precursors. Crystal structures of triethylgallium and triisopropylgalliumadducts with macrocyclic tertiary amines

A synthesis and purification route has been developed for base-free trialkylgallium compounds which avoids the use of traditional ether solvents. The R3Ga compound (R = Et, Pri) is synthesised in triethylamine, NEt3, which leads to the adducts R3Ga(NEt3). The NEt3 ligand can be readily displaced by the addition of low volatility bidentate, tetradentate or hexadentate tertiary amines to give adducts of the type (R3Ga)xL (x = 2, 4, 6), from which base-free R3Ga compounds are obtained by mild thermal dissociation. The synthesis and characterisation of these adducts are described and crystal structures reported for the adducts between R3Ga (R = Et, Pri) and the multidentate macrocyclic amines 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane and 1,4,7,10,13,16-hexamethyl-1,4,7,10,13,16-hexaazacyclooctadecane. These are the first reported structural studies of the Et3Ga and Pri3Ga molecules in the solid state.