Christina Y. Tang

Rhodium and Iridium Aminoborane Complexes: Coordination Chemistry of BN Alkene Analogues

The chemistry of boron donor ligands, isoelectronic with classical organometallic systems, has received considerable recent attention, not only from a structure/bonding perspective, but also from a desire to develop inherently useful patterns of reactivity (e.g. boryl complexes in hydroand diboration, and in C H activation). Such studies have also allowed comparative assays of ligand properties and reactivity (e.g. relative trans influences, comparative reactivity towards electrophiles/nucleophiles) for isoelectronic carbon/boroncontaining ligand families, e.g. N-heterocyclic carbene/ boryl, 8] vinylidene/aminoborylene, CO/BF, and cyclopentadienyl/azaborolyl pairs. 11] Such comparisons can also be exploited in a predictive capacity, as in the case of the ligating properties of alkanes/amineboranes, with structural data being widely available only for the latter class of donor. 13] By contrast, while transition-metal complexes are well known for alkene ligands, the coordination chemistry of the isoelectronic aminoborane family (R2N= BX2) has yet to be elucidated, [15] despite the potential relevance of such species to boron/nitrogen based hydrogen storage materials (for X = H). The metal-catalyzed dehydrogenation of ammonia borane and related amine derivatives has been the subject of intense recent study, both with a view to unlocking the potential of such systems as hydrogen storage media, and with the aim of developing new Group 13/Group 15 materials by dehydrocoupling methodologies. Typically, in the absence of significant steric bulk, the dehydrogenation of an alkylor dialkylamineborane catalyzed by Group 9 metal systems leads to the formation of oligoor polymeric products. The monomeric nature of R2NBH2 (R = iPr, Cy), [18a, 20] on the other hand, led us to consider the in situ rhodiumor iridiumcatalyzed dehydrogenation of iPr2HN BH3 and Cy2HN BH3 as a potential route to complexes of the type [LnM(H2BNR2)]. Related systems have been proposed as a potential resting state in the catalytic dehydrocoupling of amineboranes, but as yet no structural data have been forthcoming. As part of a recently initiated study into the reactivity of unsaturated group 9 N-heterocyclic carbene (NHC) systems, we hereby report the synthesis of cationic rhodium and iridium aminoborane complexes, together with that of a closely related 1,1disubstituted alkene complex. Structural characterization of these species allows for the first time direct experimental comparison of the modes of binding of these topical ligand systems. Halide abstraction is well established as a synthetic methodology for the generation of highly reactive cationic species stabilized, for example, by weak agostic or solvent molecule derived interactions. We have therefore investigated the reactivity of [M(IMes)2(H)2Cl] (M = Rh (1a), Ir (1b); IMes = N,N-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) towards the potent halide abstraction agent Na[BAr4] (Ar F = 3,5-C6H3(CF3)2) with a view to generating reactive cationic bis(NHC) systems capable of coordinating aminoborane molecules. Surprisingly, the reaction of either 1a or 1b with Na[BAr4] in fluorobenzene at 20 8C over 4 h does not proceed through salt metathesis, but rather to the formation of the sodium-containing cations [M(IMes)2(H)2Cl(Na)] , (M = Rh (2a), Ir (2b)] as the [BAr4] salts (Scheme 1). 2a and 2b represent extremely rare examples of isolated NaCl metathesis intermediates and can

Stable GaX2, InX2 and TlX2 radicals

The chemistry of the Group 13 metals is dominated by the +1 and +3 oxidation states, and simple monomeric M(II) species are typically short-lived, highly reactive species. Here we report the first thermally robust monomeric MX2 radicals of gallium, indium and thallium. By making use of sterically demanding boryl substituents, compounds of the type M(II)(boryl)2 (M = Ga, In, Tl) can be synthesized. These decompose above 130 °C and are amenable to structural characterization in the solid state by X-ray crystallography. Electron paramagnetic resonance and computational studies reveal a dominant metal-centred character for all three radicals (>70% spin density at the metal). M(II) species have been invoked as key short-lived intermediates in well-known electron-transfer processes; consistently, the chemical behaviour of these novel isolated species reveals facile one-electron shuttling processes at the metal centre.

Structures and Aggregation of the Methylamine−Borane Molecules, MenH3−nN·BH3 (n = 1−3), Studied by X-ray Diffraction, Gas-Phase Electron Diffraction, and Quantum Chemical Calculations

The structures of the molecules methylamine-borane, MeH(2)N.BH(3), and dimethylamine-borane, Me(2)HN.BH(3), have been investigated by gas-phase electron diffraction (GED) and quantum chemical calculations. The crystal structures have also been determined for methylamine-, dimethylamine-, and trimethylamine-borane, Me(n)H(3-n)N.BH(3) (n = 1-3); these are noteworthy for what they reveal about the intermolecular interactions and, particularly, the N-H...H-B dihydrogen bonding in the cases where n = 1 or 2. Hence, structures are now known for all the members of the ammonia- and amine-borane series Me(n)H(3-n)N.BH(3) (n = 0-3) in both the gas and solid phases. The structural variations and energetics of formation of the gaseous adducts are discussed in relation to the basicity of the Me(n)H(3-n)N fragment. The relative importance of secondary interactions in the solid adducts with n = 0-3 has been assessed by the semi-classical density sums (SCDS-PIXEL) approach.

Hydrogen shuttling: synthesis and reactivity of a 14-electron iridium complex featuring a bis(alkyl) tethered N-heterocyclic carbene ligand

Solvent dependent double C-H activation in an Ir(NHC)(2) system generates an agostically stabilized 14-electron complex featuring a face-capping bis(alkyl) tethered NHC ligand [NHC = N-heterocyclic carbene]. These activation processes are reversible, and the resulting ligand-derived hydrogen shuttle can be applied to the dehydrogenation of BN-containing substrates.

IR and Raman Characterization of the Zincocenes (η5-C5Me5)2Zn2 and (η5-C5Me5)(η1-C5Me5)Zn

The measured Raman and IR spectra of solid, polycrystalline bis(pentamethylcyclopentadienyl)dizinc, (eta(5)-C5Me5)2Zn2, 1, and bis(pentamethylcyclopentadienyl)monozinc, (eta(5)-C5Me5)(eta(1)-C5Me5)Zn, 8, are reported in some detail. The IR spectra of the vapors of 1 and 8 each trapped in a solid Ar matrix at 12 K confirm the essentially molecular character of the solids. The experimental results have been interpreted with particular reference (i) to the corresponding spectra of (68)Zn-enriched samples of the compounds, and (ii) to the spectra simulated by density functional theory (DFT) calculations at the B3LYP level. The marked differences of structure of 1 and 8 contrast with the relatively close similarity of their vibrational spectra, disparities being revealed only on detailed scrutiny, including the effects of (68)Zn enrichment, and primarily at wavenumbers below 1000 cm(-1). The Zn-Zn stretching motion of 1 features not as a single, well-defined mode identifiable with intense Raman scattering but in several normal modes which respond in varying degrees to (68)Zn substitution. A stretching force constant of 1.42 mdyne A(-1) has been estimated for the Zn-Zn bond of 1.

Molecular structure of trimethylphosphine–gallane, Me3P·GaH3: gas-phase electron diffraction, single-crystal X-ray diffraction, and quantum chemical studies

The structure of the gallane adduct Me3P·GaH3 in the vapour and crystalline states has been investigated. The gas-phase electron-diffraction (GED) pattern has been analysed using the SARACEN method to determine the most reliable structure of the gaseous molecule. Salient structural parameters (rh1 structure) were found to be: r(Ga–H) 159.0(11), r(Ga–P) 244.3(6), r(P–C) 184.0(2), r(C–H) 108.3(7) pm; H–Ga–P 98.4(12) and Ga–P–C 117.7(3)°. The structure of a single crystal at 150 K shows that the adduct retains the same monomeric unit in the crystalline phase, with dimensions generally close to those of the gaseous molecule and an eclipsed conformation of the C3PGaH3 skeleton. The results are discussed and analysed in the light of quantum chemical calculations and of the properties of related adducts of Group 13 metal hydrides.

Gas electron diffraction study of the vapour over dimethylamine–gallane leading to an improved structure for dimeric dimethylamidogallane, [Me2NGaH2]2: a cautionary tale

Dimethylamine-gallane is relatively slow to decompose in a closed system and vaporises at low temperature primarily as Me2(H)N.GaH3 molecules which can be trapped in a solid Ar matrix and characterised by their IR spectrum. Under the conditions needed to secure a useful gas electron diffraction (GED) pattern, however, the vapour was found to consist of dimeric dimethylamidogallane molecules, [Me2NGaH2]2, formed from the secondary amine adduct by elimination of H2, and the most reliable structure for which has been determined. Salient structural parameters (r(hl) structure) were found to be: r(Ga-N) 202.6(2), r(Ga-H) 155.6(8), r(N-C) 148.0(3), r(C-H) 111.2(6) pm; Ga-N-Ga 90.7(1), C-N-C 109.3(5), N-C-H 109.9(10) and H-Ga-H 119.4(42) degrees.

Molecular structure of trimethylamine–gallane, Me3N·GaH3: ab initio calculations, gas-phase electron diffraction and single-crystal X-ray diffraction studies†

The structure of the gallane adduct Me3N·GaH3 has been investigated by ab initio quantum chemical calculations. The results of gas-phase electron-diffraction (GED) measurements, together with earlier microwave measurements, have been reanalysed using the SARACEN method to determine the most reliable structure of the gaseous molecule. Salient structural parameters (rαo structure) were found to be: r(Ga–H) 151.1(13), r(Ga–N) 213.4(4), r(N–C) 147.6(3), r(C–H) 108.4(4) pm; H–Ga–N 99.3(8) and Ga–N–C 108.8(2)°. Unlike the corresponding alane derivative, the adduct is monomeric in the crystalline phase with dimensions very close to those of the gaseous molecule, as confirmed by a redetermination of the structure of a single crystal at 150 K.

Molecular structure of Hf(BH4)4 investigated by quantum mechanical calculations and gas-phase electron diffraction

The structure of the gaseous hafnium tetrakis(tetrahydroborate) molecule, Hf(BH4)4, has been investigated by detailed quantum mechanical calculations and by analysis of its gas electron-diffraction (GED) pattern. The ground-state geometry possesses T symmetry with all of the triply-bridged BH4 groups twisted equally about the Hf...B-H axes. Salient structural parameters (ra distances, r angles) deduced from the GED pattern by the SARACEN method were: r(Hf...B) 231.4(2), r(Hf-Hb) 221.5(7), r(B-Hb) 127.6(5), r(B-Ht) 121(1) pm, Hf...B-Hb 69.4(3), Hb-B-Hb 108.4(4), Hb-B-Ht 110.6(3), B...Hf...B-Hb 166(1) degrees. A notable feature is the large magnitude of the Hf...B and Hf-Hb anharmonicity parameters, attributed to the fluxional hydrogen atom exchange process. The properties are compared with those of related tetrahydroborates..

Dimeric piperidino-alane and -gallane: metal hydrides with a cyclic M(μ-N)2M core (M = Al or Ga)

The crystal structures of piperidino-alane and -gallane at 150 K have each been shown to consist of dimeric molecules [CH2(CH2)4NMH2]2 centred on a planar, nearly square M(μ-N)2M core (M = Al or Ga). The molecular structures thus contrast with the hydrogen-bridged units favoured by the sterically encumbered piperidino derivatives {[CMe2(CH2)3CMe2N]AlH2}3 and {[CMe2(CH2)3CMe2N]2AlH}2 and are also compared with those of other amido derivatives of the Group 13 hydrides.