Structure of tetraphenylphosphonium bromide

Solid energy calibration standards for P K-edge XANES: electronic structure analysis of PPh4Br

P K-edge X-ray absorption near-edge structure (XANES) spectroscopy is a powerful method for analyzing the electronic structure of organic and inorganic phosphorus compounds. Like all XANES experiments, P K-edge XANES requires well defined and readily accessible calibration standards for energy referencing so that spectra collected at different beamlines or under different conditions can be compared. This is especially true for ligand K-edge X-ray absorption spectroscopy, which has well established energy calibration standards for Cl (Cs₂CuCl₄) and S (Na₂S₂O₃·5H₂O), but not neighboring P. This paper presents a review of common P K-edge XANES energy calibration standards and analysis of PPh₄Br as a potential alternative. The P K-edge XANES region of commercially available PPh₄Br revealed a single, highly resolved pre-edge feature with a maximum at 2146.96 eV. PPh₄Br also showed no evidence of photodecomposition when repeatedly scanned over the course of several days. In contrast, we found that PPh₃ rapidly decomposes under identical conditions. Density functional theory calculations performed on PPh₃ and PPh₄⁺ revealed large differences in the molecular orbital energies that were ascribed to differences in the phosphorus oxidation state (III versus V) and molecular charge (neutral versus +1). Time-dependent density functional theory calculations corroborated the experimental data and allowed the spectral features to be assigned. The first pre-edge feature in the P K-edge XANES spectrum of PPh₄Br was assigned to P 1s → P-C π* transitions, whereas those at higher energy were P 1s → P-C σ*. Overall, the analysis suggests that PPh₄Br is an excellent alternative to other solid energy calibration standards commonly used in P K-edge XANES experiments.

On the coordination non-innocence of antimony in nickel(ii) complexes of the tetradentate (o-(Ph2P)C6H4)3Sb ligand

In a continuation of our investigations into the unusual properties of antimony-based ligands, we have decided to investigate the structure of the complexes formed by the tetradentate ligands (o-(Ph₂P)C₆H₄)₃E (E = P (1), As (2), Sb (3)) with NiCl₂. We observed that the structure of the resulting complexes depends on the nature of the central pnictogen atom E, with the phosphorus- and arsenic-based systems adopting ionic structures of the type [ClNi(o-(Ph₂P)C₆H₄)₃E][Cl] (E = P ([4][Cl]), As ([5][Cl])) while the known antimony complex ClNi(o-(Ph₂P)C₆H₄)₃SbCl (6-Cl) is molecular, with a chloride anion bound to the antimony atom trans from the nickel atom. Using both structural and computational arguments, we propose that the Lewis acidic behavior of the antimony atom in the cation [ClNi(o-(Ph₂P)C₆H₄)₃Sb]⁺ ([6]⁺) which as been structurally characterized as its [BPh₄]^- salt, originates in part from the steric compression imposed by the rigid ligand buttresses and the larger size of antimony. This steric compression pushes the antimony atom of [6]⁺ toward a geometry that is closer to that of a trigonal-base pyramid, setting the stage for the coordination of a ligand trans from the nickel atom as in 6-Cl. Coordination of the chloride ligand also benefits from an accumulation of positive character at the antimony atom of [6]⁺ which is a assigned to the polarizable and electron-releasing nature of this heavy pnictogen.

Establishing the Coordination Chemistry of Antimony(V) Cations: Systematic Assessment of Ph4 Sb(OTf) and Ph3 Sb(OTf)2 as Lewis Acceptors

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.

Feasibility of arsenic and antimony NMR spectroscopy in solids: an investigation of some group 15 compounds

The feasibility of obtaining (75)As and (1)(21/123)Sb NMR spectra for solids at high and moderate magnetic field strengths is explored. Arsenic-75 nuclear quadrupolar coupling constants and chemical shifts have been measured for arsenobetaine bromide and tetraphenylarsonium bromide. Similarly, (121/123)Sb NMR parameters have been measured for tetraphenylstibonium bromide and potassium hexahydroxoantimonate. The predicted pseudo-tetrahedral symmetry at arsenic and the known trigonal bipyramidal symmetry at antimony in their respective tetraphenyl-bromide "salts" are reflected in the measured (75)As and (121)Sb nuclear quadrupole coupling constants, CQ((75)As)=7.8MHz and CQ((121)Sb)=159MHz, respectively. Results of density functional theory quantum chemistry calculations for isolated molecules using ADF and first-principles calculations using CASTEP, a gauge-including projector augmented wave method to deal with the periodic nature of solids, are compared with experiment. Although the experiments can be time consuming, measurements of (75)As and (121)Sb NMR spectra (at 154 and 215MHz, respectively, i.e., at B0=21.14T) with linewidths in excess of 1MHz are feasible using uniform broadband excitation shaped pulse techniques (e.g., WURST and WURST-QCPMG).

A simple complex on the verge of breakdown: isolation of the elusive cyanoformate ion

Why does cyanide not react destructively with the proximal iron center at the active site of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase, an enzyme central to the biosynthesis of ethylene in plants? It has long been postulated that the cyanoformate anion, [NCCO2](-), forms and then decomposes to carbon dioxide and cyanide during this process. We have now isolated and crystallographically characterized this elusive anion as its tetraphenylphosphonium salt. Theoretical calculations show that cyanoformate has a very weak C-C bond and that it is thermodynamically stable only in low dielectric media. Solution stability studies have substantiated the latter result. We propose that cyanoformate shuttles the potentially toxic cyanide away from the low dielectric active site of ACC oxidase before breaking down in the higher dielectric medium of the cell.

Crystal chemistry of tetraradial species. Part 2. Crystal structures of Et4NI, Ph4PBr•H2O, and Ph4PBr•2H2O

The crystal structure of Et4NI [Formula: see text], a = 8.860(2) Å, c = 6.933(1) Å, Z = 2) has been redetermined and those of Ph4PBr•H2O ([Formula: see text], a = 10.005(3) Å, b = 10.659(2) Å, c = 10.697(4) Å, α = 102.61(2)°, β = 83.39(3)°, γ = 108.09(3)°, Z = 2) and Ph4PBr•2H2O (Pnma, a = 16.255(4) Å, b = 10.810(4) Å, c = 12.667(9) Å, Z = 4) have been determined for the first time. In the Et4NI structure Et4N+ cations in an extended S4 conformation and I− anions are arranged in a zincblende-type packing. The two hydrate structures are ionic and very similar. The anion in the monohydrate is a centrosymmetric cyclic dimer [Br2(OH2)2]2−; in the dihydrate the anion, which is an almost planar infinite [Br−(OH2)4/2]x chain 11b, appears to be of a novel type. If the H2O molecules are not considered, the ion packing in both hydrates may be regarded as of the anti-NiAs type. The ion packing in R4EX structures and the conformation of the Et4N+ ion in crystals are discussed in some detail.