A High‐Pressure Polymorph of Phosphorus Oxonitride with the Coesite Structure

The chemical and physical properties of phosphorus oxonitride (PON) closely resemble those of silica, to which it is isosteric. A new high‐pressure phase of PON is reported herein. This polymorph, synthesized by using the multianvil technique, crystallizes in the coesite structure. This represents the first occurrence of this very dense network structure outside of SiO₂. Phase‐pure coesite PON (coe‐PON) can be synthesized in bulk at pressures above 15 GPa. This compound was thoroughly characterized by means of powder X‐ray diffraction, DFT calculations, and FTIR and MAS NMR spectroscopy, as well as temperature‐dependent diffraction. These results represent a major step towards the exploration of the phase diagram of PON at very high pressures and the possibly synthesis of a stishovite‐type PON containing hexacoordinate phosphorus.     

High‐Pressure Phase Transformations in TiPO4: A Route to Pentacoordinated Phosphorus

Titanium(III) phosphate, TiPO₄ , is a typical example of an oxyphosphorus compound containing covalent P?O bonds. Single‐crystal X‐ray diffraction studies of TiPO₄ reveal complex and unexpected structural and chemical behavior as a function of pressure at room temperature. A series of phase transitions lead to the high‐pressure phase V, which is stable above 46 GPa and features an unusual oxygen coordination of the phosphorus atoms. TiPO₄‐V is the first inorganic phosphorus‐containing compound that exhibits fivefold coordination with oxygen. Up to the highest studied pressure of 56 GPa, TiPO₄‐V coexists with TiPO₄‐IV, which is less dense and might be kinetically stabilized. Above a pressure of about 6 GPa, TiPO₄‐II is found to be an incommensurately modulated phase whereas a lock‐in transition at about 7 GPa leads to TiPO₄‐III with a fourfold superstructure compared to the structure of TiPO₄‐I at ambient conditions. TiPO₄‐II and TiPO₄‐III are similar to the corresponding low‐temperature incommensurate and commensurate magnetic phases and reflect the strong pressure dependence of the spin‐Peierls interactions.     

Nitride Spinel: An Ultraincompressible High‐Pressure Form of BeP2N4

Owing to its outstanding elastic properties, the nitride spinel γ‐Si₃N₄ is of considered interest for materials scientists and chemists. DFT calculations suggest that Si₃N₄‐analog beryllium phosphorus nitride BeP₂N₄ adopts the spinel structure at elevated pressures as well and shows outstanding elastic properties. Herein, we investigate phenakite‐type BeP₂N₄ by single‐crystal synchrotron X‐ray diffraction and report the phase transition into the spinel‐type phase at 47 GPa and 1800 K in a laser‐heated diamond anvil cell. The structure of spinel‐type BeP₂N₄ was refined from pressure‐dependent in situ synchrotron powder X‐ray diffraction measurements down to ambient pressure, which proves spinel‐type BeP₂N₄ a quenchable and metastable phase at ambient conditions. Its isothermal bulk modulus was determined to 325(8) GPa from equation of state, which indicates that spinel‐type BeP₂N₄ is an ultraincompressible material.     

Phenakite‐Type BeP2N4—A Possible Precursor for a New Hard Spinel‐Type Material

BeP₂N₄ was synthesized in a multi‐anvil apparatus starting from Be₃N₂ and P₃N₅ at 5 GPa and 1500 °C. The compound crystallizes in the phenakite structure type (space group R$\bar 3$ , no. 148) with a=1269.45(2) pm, c=834.86(2) pm, V=1165.13(4)×10⁶ pm³ and Z=18. As isostructural and isovalence‐electronic α‐Si₃N₄ transforms into β‐Si₃N₄ at high pressure and temperature, we studied the phase transition of BeP₂N₄ into the spinel structure type by using density functional theory calculations. The predicted transition pressure of 24 GPa is within the reach of today’s state of the art high‐pressure experimental setups. Calculations of inverse spinel‐type BeP₂N₄ revealed this polymorph to be always higher in enthalpy than either phenakite‐type or spinel‐type BeP₂N₄. The predicted bulk modulus of spinel‐type BeP₂N₄ is in the range of corundum and γ‐Si₃N₄ and about 40 GPa higher than that of phenakite‐type BeP₂N₄. This finding implies an increase in hardness in analogy to that occurring for the β‐ to γ‐Si₃N₄ transition. In hypothetical spinel‐type BeP₂N₄ the coordination number of phosphorus is increased from 4 to 6. So far only coordination numbers up to 5 have been experimentally realized (γ‐P₃N₅), though a sixfold coordination for P has been predicted for hypothetic δ‐P₃N₅. We believe, our findings provide a strong incentive for further high‐pressure experiments in the quest for novel hard materials with yet unprecedented structural motives.     

Triazaphospholenium Tetrafluoroborate: A Phosphorus Analogue of a 1,2,3‐Triazole‐Derived Carbene

3H ‐1,2,3,4‐Triazaphosphole derivatives can be selectively alkylated with Meerwein's reagent at the most nucleophilic nitrogen atom. According to the principle of valence isoelectronicity, the corresponding phosphorus heterocycle represents the first formal phosphorus analogue of the well‐known 1,2,3‐triazolylidenes (mesoionic carbenes). Theoretical calculations revealed that the cation in triazaphospholenium tetrafluoroborate is an aromatic system with a high degree of π‐conjugation. First investigations showed that the cationic phosphorus heterocycle can stabilize a [Cu₂Br₄]²⁻ dianion by formation of a neutral coordination compound with an unusual bonding situation between phosphorus and copper(I).     

β‐CuN3: The Overlooked Ground‐State Polymorph of Copper Azide with Heterographene‐Like Layers

An unexpected polymorph of the highly energetic phase CuN₃ has been synthesized and crystallizes in the orthorhombic space group Cmcm with a=3.3635(7), b=10.669(2), c=5.5547(11) Å and V=199.34(7) ų. The layered structure resembles graphite with an interlayer distance of 2.777(1) Å (=1/2 c). Within a single layer, considering N₃^? as one structural unit, there are 10‐membered almost hexagonal rings with a heterographene‐like motif. Copper and nitrogen atoms are covalently bonded with Cu? N bonds lengths of 1.91 and 2.00 Å, and the N₃^? group is linear but with N? N 1.14 and 1.20 Å. Electronic‐structure calculations and experimental thermochemistry show that the new polymorph termed β‐CuN₃ is more stable than the established α‐CuN₃ phase. Also, β‐CuN₃ is dynamically, and thus thermochemically, metastable according to the calculated phonon density of states. In addition, β‐CuN₃ exhibits negative thermal expansion within the graphene‐like layer.     

Pyridyl‐Functionalised 3H‐1,2,3,4‐Triazaphospholes: Synthesis,Coordination Chemistry and Photophysical Properties of Low‐Coordinate Phosphorus Compounds

Novel conjugated, pyridyl‐functionalised triazaphospholes with either tBu or SiMe₃ substituents at the 5‐position of the N₃PC heterocycle have been prepared by a [3+2] cycloaddition reaction and compared with structurally related, triazole‐based systems. Photoexcitation of the 2‐pyridyl‐substituted triazaphosphole gives rise to a significant fluorescence emission with a quantum yield of up to 12 %. In contrast, the all‐nitrogen triazole analogue shows no emission at all. DFT calculations indicate that the 2‐pyridyl substituted systems have a more rigid and planar structure than their 3‐ and 4‐pyridyl isomers. Time‐dependent (TD) DFT calculations show that only the 2‐pyridyl‐substituted triazaphosphole exhibits similar planar geometry, with matching conformational arrangements in the lowest energy excited state and the ground state; this helps to explain the enhanced emission intensity. The chelating P,N‐hybrid ligand forms a Re^I complex of the type [(N^N)Re(CO)₃Br] through the coordination of nitrogen atom N² to the metal centre rather than through the phosphorus donor. Both structural and spectroscopic data indicate substantial π‐accepting character of the triazaphosphole, which is again in contrast to that of the all‐nitrogen‐containing triazoles. The synthesis and photophysical properties of a new class of phosphorus‐containing extended π systems are described.     

A Novel High-Pressure Tin Oxynitride Sn2N2O

We report the first oxynitride of tin, Sn₂N₂O (SNO), exhibiting a Rh₂S₃-type crystal structure with space group Pbcn. All Sn atoms are in six-fold coordination, in contrast to Si in silicon oxynitride (Si₂N₂O) and Ge in the isostructural germanium oxynitride (Ge₂N₂O), which appear in four-fold coordination. SNO was synthesized at 20 GPa and 1200–1500 °C in a large volume press. The recovered samples were characterized by synchrotron powder X-ray diffraction and single-crystal electron diffraction in the TEM using the automated diffraction tomography (ADT) technique. The isothermal bulk modulus was determined as B_o=193(5) GPa by using in-situ synchrotron X-ray diffraction in a diamond anvil cell. The structure model is supported by DFT calculations. The enthalpy of formation, the bulk modulus, and the band structure have been calculated.     

One‐Dimensional Polymers Based on Silver(I) Cations and Organometallic cyclo‐P3 Ligand Complexes

The synthesis and characterization of the first supramolecular aggregates incorporating the organometallic cyclo‐P₃ ligand complexes [Cp^RMo(CO)₂(η³‐P₃)] (Cp^R=Cp (C₅H₅; 1a ), Cp* (C₅(CH₃)₅; 1b )) as linking units is described. The reaction of the Cp derivative 1a with AgX (X=CF₃SO₃, Al{OC(CF₃)₃}₄) yields the one‐dimensional (1D) coordination polymers [Ag{CpMo(CO)₂(μ,η³:η¹:η¹‐P₃)}₂]_n[Al{OC(CF₃)₃}₄]_n ( 2 ) and [Ag{CpMo(CO)₂(μ,η³:η¹:η¹‐P₃)}₃]_n[X]_n (X=CF₃SO₃ ( 3a ), Al{OC(CF₃)₃}₄ ( 3b )). The solid‐state structures of these polymers were revealed by X‐ray crystallography and shown to comprise polycationic chains well‐separated from the weakly coordinating anions. If AgCF₃SO₃ is used, polymer 3a is obtained regardless of reactant stoichiometry whereas in the case of Ag[Al{OC(CF₃)₃}₄], reactant stoichiometry plays a decisive role in determining the structure and composition of the resulting product. Moreover, polymers 3a, b are the first examples of homoleptic silver complexes in which Ag^I centers are found octahedrally coordinated to six phosphorus atoms. The Cp* derivative 1b reacts with Ag[Al{OC(CF₃)₃}₄] to yield the 1D polymer [Ag{Cp*Mo(CO)₂(μ,η³:η²:η¹‐P₃)}₂]_n[Al{OC(CF₃)₃}₄]_n ( 4 ), the crystal structure of which differs from that of polymer 2 in the coordination mode of the cyclo‐P₃ ligands: in 2 , the Ag⁺ cations are bridged by the cyclo‐P₃ ligands in a η¹:η¹ (edge bridging) fashion whereas in 4 , they are bridged exclusively in a η²:η¹ mode (face bridging). Thus, one third of the phosphorus atoms in 2 are not coordinated to silver while in 4 , all phosphorus atoms are engaged in coordination with silver. Comprehensive spectroscopic and analytical measurements revealed that the polymers 2 , 3a , b , and 4 depolymerize extensively upon dissolution and display dynamic behavior in solution, as evidenced in particular by variable temperature ³¹P NMR spectroscopy. Solid‐state ³¹P magic angle spinning (MAS) NMR measurements, performed on the polymers 2 , 3b , and 4 , demonstrated that the polymers 2 and 3b also display dynamic behavior in the solid state at room temperature. The X‐ray crystallographic characterisation of 1b is also reported.     

A Phosphorus Analogue of Bis(η4‐cyclobutadiene)iron(0)

P makes it possible : The convenient oxidative synthesis of the 16‐electron organophosphorus iron sandwich complex [Fe(η⁴‐P₂C₂tBu₂)₂] (see structure) suggests that the elusive all‐carbon complex [Fe(η⁴‐C₄H₄)₂] is a viable synthetic target.

     

Binary Compounds of Boron and Beryllium: A Rich Structural Arena with Space for Predictions

We explore ground‐state structures and stoichiometries of the Be? B system in the static limit, with Be atom concentrations of 20 % or greater, and from P=1 atm up to 320 GPa. At P=1 atm, predictions are offered for several known compounds, the structures of which have not yet been determined experimentally. Specifically, at 1 atm, we predict a structure of R$\bar 3$ m symmetry for the compound Be₂B₃, seen experimentally at high temperatures, which contains interesting BeBBBBe rods; and for the compound BeB₄ we calculate metastability with respect to the elements with a structure similar to MgB₄, which is quickly replaced as the pressure is elevated by a Cmcm structure that features 6‐ and 4‐membered rings in B cages, with Be interstitials. For another high‐temperature compound, Be₂B, we confirm the CaF₂ structure, but find a competitive and actually slightly more stable ground‐state structure of C2/m symmetry that features B₂ pairs. In the case of BeB₂, a material for which the stoichiometry has been the subject of debate, we have a clear prediction of a stable F$\bar 4$ 3m structure at P=1 atm. It has a diamondoid structure that is based on cubic (lower P) or hexagonal (higher P) diamond networks of B, but with Be in the interstices. This Zintl structure is a semiconductor at low and intermediate pressures. At higher pressures, BeB₂ dominates the phase diagram. In general, the Zintl–Klemm concept of effective electron transfer from the more electropositive ion and bond formation among the resulting anions has proven useful in analyzing the structural preferences of many compositions in the Be? B system at P=1 atm and at elevated pressures. An unusual feature of this binary system is that the 1:1 BeB stoichiometry never appears to reach stability in the static limit, although it comes close, as does Be₁₇B₁₂. Also stable at high pressures are stoichiometries BeB₃, BeB₄, and Be₅B₂.     

Functionalization of a cyclo‐P5 Ligand by Main‐Group Element Nucleophiles

Unprecedented functionalized products with an η⁴‐P₅ ring are obtained by the reaction of [Cp*Fe(η⁵‐P₅)] ( 1 ; Cp*=η⁵‐C₅Me₅) with different nucleophiles. With LiCH₂SiMe₃ and LiNMe₂, the monoanionic products [Cp*Fe(η⁴‐P₅CH₂SiMe₃)]^? and [Cp*Fe(η⁴‐P₅NMe₂)]^?, respectively, are formed. The reaction of 1 with NaNH₂ leads to the formation of the trianionic compound [{Cp*Fe(η⁴‐P₅)}₂N]^3?, whereas the reaction with LiPH₂ yields [Cp*Fe(η⁴‐P₅PH₂)]^? as the main product, with {[Cp*Fe(η⁴‐P₅)]₂PH}^2? as a byproduct. The calculated energy profile of the reactions provides a rationale for the formation of the different products.     

Mechanism of Redox‐Active Ligand‐Assisted Nitrene‐Group Transfer in a ZrIV Complex: Direct Ligand‐to‐Ligand Charge Transfer Preferred

The mechanism of the nitrene‐group transfer reaction from an organic azide to isonitrile catalyzed by a Zr^IV d⁰ complex carrying a redox‐active ligand was studied by using quantum chemical molecular‐modeling methods. The key step of the reaction involves the two‐electron reduction of the azide moiety to release dinitrogen and provide the nitrene fragment, which is subsequently transferred to the isonitrile substrate. The reducing equivalents are supplied by the redox‐active bis(2‐iso‐propylamido‐4‐methoxyphenyl)‐amide ligand. The main focus of this work is on the mechanism of this redox reaction, in particular, two plausible mechanistic scenarios are considered: 1) the metal center may actively participate in the electron‐transfer process by first recruiting the electrons from the redox‐active ligand and becoming formally reduced in the process, followed by a classical metal‐based reduction of the azide reactant. 2) Alternatively, a non‐classical, direct ligand‐to‐ligand charge‐transfer process can be envisioned, in which no appreciable amount of electron density is accumulated at the metal center during the course of the reaction. Our calculations indicate that the non‐classical ligand‐to‐ligand charge‐transfer mechanism is much more favorable energetically. Utilizing a series of carefully constructed putative intermediates, both mechanistic scenarios were compared and contrasted to rationalize the preference for ligand‐to‐ligand charge‐transfer mechanism.     

Gas‐Phase Chemistry of Ethynylamine, ‐Phosphine and ‐Arsine. Structure and Stability of their Cu+ and Ni+ Complexes

The Cu⁺ and Ni⁺ binding energies of ethynylamine, ethynylphosphine and ethynylarsine have been calculated at the B3LYP/6‐311+G(2df,2p)//B3LYP/6‐311G(d,p) level of theory. Significant differences between nitrogen‐containing and phosphorus‐ or arsenic‐containing compounds have been found regarding structural effects upon metal cation association. While for ethynylamine the global minimum of the potential energy surface corresponds to the complex in which the metal cation binds to the β‐carbon, for ethynylphosphine the most favourable process corresponds to phosphorus attachment. For ethynylarsine, the conventional π‐complex is the most stable one. This behavior resembles that found for the corresponding vinyl analogues, with the only exception being the arsenic derivative. The calculated Cu⁺ and Ni⁺ binding energies for attachment to the heteroatom follow a different trend, P>As>N, to that predicted for the corresponding proton affinities, P>N>As. Cu⁺ and Ni⁺ binding energies are almost identical when the metal cation binds to the heteroatom. However, Ni⁺ binding energies are slightly larger than Cu⁺ binding energies when the metal cation interacts with the C?C bond.