Hypervalent hydridosilicates: synthesis, structure and hydride bridging

A range of hydridosilicate anions has been prepared and characterised by spectroscopic, structural and computational methods. The general approach involved reaction of KH with a neutral silane precursor in the presence of [18]crown-6. In this manner, [K([18]crown-6)]+ salts of [Ph3SiH2](-) (1), [Ph3SiF2](-) (9), and [(p-FC6H4)3SiHF](-)/[(p-FC6H4)3SiH2](-) (12) were stabilised and characterized by NMR spectroscopy and X-ray diffraction. In each case, the anion adopts a trigonal bipyramidal (TBP) geometry with three equatorial phenyl groups eclipsing the axial Si-H/Si-F bonds. The Si-H[dot dot dot]K distances, along with DFT calculations on 1, indicate an electrostatic interaction that does not dictate the geometry adopted by the anion. A [H2SiOiPr3](-) salt (7) has also been crystallised in the same way; X-ray diffraction shows in this case a distorted TBP array with axial hydride ligands, and both Si-H[...]K and Si-O[...]K interactions. 1H NMR exchange experiments show 1 to undergo facile hydride exchange with Ph3SiH. Compound 1 acts as a good hydride transfer reagent to a variety of substrates, but its high reactivity often results in redistribution and other side reactions.     

Evidence for a SN2-type pathway for phosphine exchange in phosphine-phosphenium cations, [R2P--PR'3]+

Abstraction of a Cl(-) ion from the P-chlorophospholes, R4C4PCl (R=Me, Et), produced the P--P bonded cations [R4C4P--P(Cl)C4R4]+, which reacted with PPh3 to afford X-ray crystallographically characterised phosphine-phosphenium cations [R4C4P(PPh3)]+ (R=Me, Et). Examination of the 31P-{1H} NMR spectrum of a solution (CH2Cl(2)) of [Et4C4P-(PPh3)]+ and PPh3 revealed broadening of the resonances due to both free and coordinated PPh3, and importantly it proved possible to measure the rate of exchange between PPh3 and [Et4C4P-(PPh3)]+ by line shape analysis (gNMR programmes). The results established second-order kinetics with DeltaS( not equal)=(-106.3+/-6.7) J mol(-1) K(-1), DeltaH( not equal)=(14.9+/-1.6) kJ mol(-1) and DeltaG( not equal) (298.15 K)=(46.6+/-2.6) kJ mol(-1), values consistent with a SN2-type pathway for the exchange process. This result contrasts with the dominant dissociative (S(N)1-type) pathway reported for the analogous exchange reactions of the [ArNCH2CH2N(Ar)P(PMe3)]+ ion, and to understand in more detail the factors controlling these two different reaction pathways, we have analysed the potential energy surfaces using density functional theory (DFT). The calculations reveal that, whilst phosphine exchange in [Et4C4P(PPh3)]+ and [ArNCH2CH2N(Ar)P(PMe3)](+) is superficially similar, the two cations differ significantly in both their electronic and steric requirements. The high electrophilicity of the phosphorus center in [Et4C4P]+, combined with strong pi-pi interactions between the ring and the incoming and outgoing phenyl groups of PPh3, favours the SN2-type over the SN1-type pathway in [Et4C4P(PPh3)]+. Effective pi-donation from the amide groups reduces the intrinsic electrophilicity of [ArNCH2CH2N(Ar)P]+, which, when combined with the steric bulk of the aryl groups, shifts the mechanism in favour of a dissociative SN1-type pathway.     

Reversible SO2 Uptake by Tetraalkylammonium Halides: Energetics and Structural Aspects of Adduct Formation Between SO2 and Halide Ions

Contact with SO₂ causes almost immediate dissolution of tetraalkylammonium halides, R₄NX, (R = CH₃ (Me), X = I; R = C₂H₅ (Et), X = Cl, Br, I; R = C₄H₉ (nBu), X = Cl, Br), with the formation of an adduct, [R₄N]⁺[(SO₂)_nX]^– (n = 1–4). Vapor pressure measurements indicate the proclivity for SO₂ uptake follows the order N(CH₃)₄⁺ < N(C₂H₅)₄⁺ < N(C₄H₉)₄⁺. This trend is in accord with the Jenkins–Passmore volume‐based thermodynamic model. Born–Haber cycles, incorporating the lattice energy and gas phase energy terms, are used to evaluate the energetic feasibility of reactions. Density functional theory calculations (B3PW91; 6‐311+G(3df)) have been used to calculate the energetics of (SO₂)_nX^– (X = Cl and Br) anions in the gas phase. The experimental studies show that tetraalkylammonium halides are feasible sorbents for SO₂. In order to correlate the theoretical model, experimental enthalpy, Δ_rH° and entropy, Δ_rS° changes have been determined by the van't Hoff method for the binding of one SO₂ molecule to (C₂H₅)₄NCl, resulting in the liquid adduct (C₂H₅)₄NCl · SO₂. The structure of the analogous 1:1 bromide adduct, (C₂H₅)₄NBr · SO₂, has been determined by single‐crystal X‐ray diffraction (monoclinic, P2₁/c, a = 9.1409(14) Å, b = 12.3790(19) Å, c = 11.3851(17) Å, β = 107.952(2)°, V = 1225.6(3) ų). The structure consists of discrete alkylammonium cations, bromide anions and SO₂ molecules with short contacts between the anion and SO₂ molecules. The (C₂H₅)₄N⁺ cationadopts a transoid conformation with D_2d symmetry, and represents a rare example of a well‐ordered (C₂H₅)₄N⁺ cation in a crystal structure. The Br^– anions and SO₂ molecules forms a chain, (SO₂Br^–)_n, with bifurcated contacts. Non‐bonding electron pairs on the halide anions engage in electrostatic interactions with the sulfur atoms and charge‐transfer interactions with the antibonding S–O orbitals of the bound SO₂ moiety. Raman and ¹⁷O NMR spectra provide compelling evidence for a charge‐transfer interaction between SO₂ molecules and the halide ions.     

Equilibria between alpha- and beta-agostic stabilized rotamers of secondary alkyl niobium complexes.

The isopropyl chloro complex Tp(Me2)NbCl(i-Pr)(PhC&tbd1;CMe) (2) [Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate] exhibits a beta-agostic structure in the crystal. The conformation of the alkyl group is such that the agostic methyl group lies in the Calpha-Nb-Cl plane and the nonagostic one, in a wedge formed by two pyrazole rings. As observed by solution NMR spectroscopy, restricted rotation about the Nb-C bond allows the observation of an equilibrium between this species, 2beta, and a minor alpha-agostic rotamer 2alpha. A putative third rotamer which would have the secondary hydrogen in the wedge is not observed. Similar behavior is observed for related Tp'NbCl(i-Pr)(R(2)C=CMe) [Tp' = Tp(Me2), R(2) = Me (3); Tp' = Tp(Me2,4Cl), R(2) = Ph (4)]. The two diastereomers of the sec-butyl complex Tp(Me2)NbCl(sec-Bu)(MeC=CMe) (5) have been separated. In the crystal, 5CR-AS has a beta-agostic methyl group with the ethyl group located in the wedge formed by two pyrazole rings. The same single beta-agostic species is observed in solution. The other diastereomer, 5AR-CS has a beta-agostic methylene group in the solid state, and the methyl group sits in the wedge. In solution, an equilibrium between this beta-agostic methylene complex 5AR-CSbeta and a minor alpha-agostic species 5AR-CSalpha, where the ethyl substituent of the sec-Bu group is located in the wedge between two pyrazole rings, is observed. NMR techniques have provided thermodynamic parameters for these equilibria (K = 2beta/2alpha = 4.0 +/- 0.1 at 193 K, DeltaG(o)(193) = -2.2 +/- 0.1, DeltaH(o) = -7.4 +/- 0.1 kJ mol(-)(1), and DeltaS(o) = -27 +/- 1 J K(-)(1) mol(-)(1)), as well as kinetic parameters for the rotation about the Nb-C bond (at 193 K, DeltaG(2)= 47.5 +/- 2.5, DeltaH= 58.8 +/- 2.5 kJ mol(-)(1), and DeltaS = 59.0 +/- 10 J K(-)(1) mol(-)(1)). Upon selective deuteration of the beta-methyl protons in Tp(Me2)NbCl[CH(CD(3))(2)](PhC=CMe) (2-d(6)), an expected isotope effect that displaces the equilibrium toward the alpha-agostic rotamer is observed (K = 2-d(6)beta/2-d(6)alpha = 3.1 +/- 0.1 at 193 K, DeltaG(o)(193) = -1.8 +/- 0.1, DeltaH(o) = -8.3 +/- 0.4 kJ mol(-)(1) and DeltaS(o)= -34 +/- 2 J K(-)(1) mol(-)(1)). The anomalous values for DeltaH(o) and DeltaS(o) are discussed. Hybrid quantum mechanics/molecular mechanics calculations (IMOMM (B3LYP:MM3)) on the realistic model Tp(Me2)NbCl(i-Pr)(HC=CMe) have reproduced the energy differences between the alpha- and beta-agostic species with remarkable accuracy. Similar calculations show that Tp(Me2)NbCl(CH(2)Me)(HC=CMe) is alpha-agostic only and that Tp(5)(-)(Me)NbCl(CH(2)Me)(HC=CMe), which has no methyl groups at the 3-positions of the pyrazole rings, is beta-agostic only. Analysis and discussion of the computational and experimental data indicate that the unique behavior observed for the secondary alkyl complexes stems from competition between electronic effects favoring a beta-agostic structure and steric effects directing a bulky substituent in the wedge between two pyrazole rings of Tp(Me2). All of the secondary alkyl complexes thermally rearrange to the corresponding linear alkyl complexes via a first-order reaction.     

Hydrofunctionalisation of an Aromatic Triphosphabenzene

The aromatic heterocycle 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene reacts with a series of silanes, germanes and stannanes, with weaker E−H bonds reacting in an increasingly facile manner. All react by 1,4-addition to give bicyclic products with diastereomeric ratios varying with the substrate. Density functional theory (DFT) calculations show that activation of the E−H bond occurs across the 1,4-C/P axis of the triphosphabenzene, with the small energetic differences with respect to the stereochemistry of the addition offering insight into the experimentally observed diastereomeric ratios.     

B(C6F5)3-Enabled Synthesis of a Cyclic cis-Arsaphosphene

The synthesis and characterization of an (arsino)phosphaketene, As(PCO){[N(Dipp)](CH₂)}₂ (Dipp=2,6-diisopropylphenyl) is reported along with its subsequent reactivity with B(C₆F₅)₃. When reacted in a stoichiometric ratio, B(C₆F₅)₃ drove the insertion of the P=C bond of the phosphaketene into one of the As−N bonds of the arsino functionality, affording an acid-stabilized, seven-membered, cyclic arsaphosphene. In contrast, when catalytic amounts of B(C₆F₅)₃ were employed, dimeric species, which formed through a formal [2+2] cycloaddition of the cyclic arsaphosphene, were generated. The cyclic arsaphosphene product represents the first example of such a compound in which the two substituents are arranged in a cis-configuration.     

An electronic perspective on the reduction of an n=n double bond at a conserved dimolybdenum core

Density functional theory has been used to assess the role of the bimetallic core in supporting reductive cleavage of the N=N double bond in [Cp2Mo2(mu-SMe)3(mu-eta1:eta1-HN=NPh)]+. The HOMO of the complex, the Mo-Mo delta orbital, plays a key role as a source of high-energy electrons, available for transfer into the vacant orbitals of the N=N unit. As a result, the metal centres cycle between the Mo(III) and Mo(IV) oxidation states. The symmetry of the Mo-Mo delta "buffer" orbital has a profound influence on the reaction pathway, because significant overlap with the redox-active orbital on the N=N unit (pi* or sigma*) is required for efficient electron transfer. The orthogonality of the Mo-Mo delta and N-N sigma* orbitals in the eta1:eta1 coordination mode ensures that electron transfer into the N-N sigma bond is effectively blocked, and a rate-limiting eta1:eta1-->eta1 rearrangement is a necessary precursor to cleavage of the bond.