Isolation of Elusive Phosphinidene-Chlorotetrylenes: The Heavier Cyanogen Chloride Analogues

The elusive phosphinidene-chlorotetrylenes, [PGeCl] and [PSiCl] have been stabilized by the hetero-bileptic cyclic alkyl(amino) carbene (cAAC), N-heterocyclic carbene (NHC) ligands, and isolated in the solid state at room temperature as the first neutral monomeric species of this class with the general formulae (L)P-ECl(L′) (E=Ge, 3 a – 3 c ; E=Si, 6 ; L=cAAC; L′=NHC). Compounds 3 a – 3 c have been synthesized by the reaction of cAAC-supported potassium phosphinidenides [cAAC=PK(THF)_x]_n ( 1 a – 1 c ) with the adduct NHC:→GeCl₂ ( 2 ). Similarly, compound 6 has been synthesized via reaction of 1 a with NHC:→SiCl₂ adduct ( 4 ). Compounds 3 a – 3 c , and 6 have been structurally characterized by single-crystal X-ray diffraction, NMR spectroscopy and mass spectrometric analysis. DFT calculations revealed that the heteroatom P in 3 bears two lone pairs; the non-bonding pair with 67.8 % of s- and 32 % of p character, whereas the other lone pair is involved in π backdonation to the C_cAAC-N π* of cAAC. The Ge atom in 3 contains a lone pair with 80 % of s character, and slightly involved in the π backdonation to C_NHC. EDA-NOCV analyses showed that two charged doublet fragments {(cAAC)(NHC)}⁺, and {PGeCl}⁻ prefer to form one covalent electron-sharing σ bond, one dative σ bond, one dative π bond, and a charge polarized weak π bond. The covalent electron-sharing σ bond contributes to the major stabilization energy to the total orbital interaction energy of 3 , enabling the first successful isolations of this class of compounds ( 3 , 6 ) in the laboratory.     

A Persistent 1,2‐Dihydrophosphasilene Adduct

The reaction of the arylchlorosilylene–NHC adduct ArSi(NHC)Cl [Ar=2,6‐Trip₂‐C₆H₃; NHC=(MeC)₂(NMe)₂C] 1 with one molar equiv of LiPH₂^.dme (dme=1,2‐dimethoxyethane) affords the first 1,2‐dihydrophosphasilene adduct 2 (ArSi(NHC)(H)?PH). The latter is labile in solution and can undergo head‐to‐tail dimerization to give [ArSi(H)PH]₂ 3 and “free” NHC. Further stabilization of 2 by complexation with {W(CO)₅} affords the isolable 1,2‐dihydrophosphasilene–tungsten complex 4 [ArSi(NHC)(H)?P(H)W(CO)₅]. Additionally, the new 1‐silyl‐2‐hydrophosphasilene ArSi(NHC)(H)?PSiMe₃ 5 could be synthesized and structurally characterized. DFT studies confirmed that the Si?P bond in 2 and 4 is mostly zwitterionic with drastically decreased double‐bond character.     

Synthesis and Characterization of Methylzinc Tri(tert‐butyl)silylphosphanide as well as Related Sodium and Potassium Phosphanylzincates

The reaction of dimethylzinc and tri(tert‐butyl)silylphosphane in toluene yielded dimeric methylzinc tri(tert‐butyl)silylphosphanide ( 1 ) which crystallized tetrameric. Compound 1 was deprotonated with sodium in DME and the solvent‐separated dimeric ion pair [(dme)₃Na]⁺ [(dme)Na(MeZn)₂(μ‐PSitBu₃)₂]^? ( 2 ) was isolated. The reaction of 1 in THF with two equivalents of potassium and one equivalent of tri(tert‐butyl)silylphosphane gave dimeric [{tBu₃Si(H)P}{(thf)₂K}₂(MeZn)(PSitBu₃)]₂ ( 3 ). Both of these phosphanylzincates contain Zn₂P₂ cycles with Zn‐P bond lengths of approximately 237 pm, whereas in 1 larger Zn‐P bond lengths of 248.5 pm were found due to the larger coordination numbers of the phosphorus and zinc atoms.     

SiP Double Bonds: Experimental and Theoretical Study of an NHC‐Stabilized Phosphasilenylidene

An experimental and theoretical study of the first compound featuring a Si?P bond to a two‐coordinate silicon atom is reported. The NHC‐stabilized phosphasilenylidene (IDipp)Si?PMes* (IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene, Mes*=2,4,6‐tBu₃C₆H₂) was prepared by SiMe₃Cl elimination from SiCl₂(IDipp) and LiP(Mes*)SiMe₃ and characterized by X‐ray crystallography, NMR spectroscopy, cyclic voltammetry, and UV/Vis spectroscopy. It has a planar trans‐bent geometry with a short Si? P distance of 2.1188(7) Å and acute bonding angles at Si (96.90(6)°) and P (95.38(6)°). The bonding parameters indicate the presence of a Si?P bond with a lone electron pair of high s‐character at Si and P, in agreement with natural bond orbital (NBO) analysis. Comparative cyclic voltammetric and UV/Vis spectroscopic experiments of this compound, the disilicon(0) compound (IDipp)Si?Si(IDipp), and the diphosphene Mes*P?PMes* reveal, in combination with quantum chemical calculations, the isolobal relationship of the three double‐bond systems.     

Stabilization of a Diphosphagermylene through pπ–pπ Interactions with a Trigonal‐Planar Phosphorus Center

N‐Heterocyclic carbenes and their heavier homologues are, in part, stabilized by delocalization of the N lone pairs into the vacant p‐orbital at carbon (or a heavier Group 14 element center). These interactions are usually absent in the corresponding P‐substituted species, owing to the large barrier to planarization of phosphorus. However, judicious selection of the substituents at phosphorus has enabled the synthesis of a diphosphagermylene, [(Dipp)₂P]₂Ge, in which one of the P centers is planar (Dipp=2,6‐diisopropylphenyl). The planar nature of this P center and the correspondingly short P? Ge distance suggest a significant degree of P? Ge multiple bond character that is due to delocalization of the phosphorus lone pair into the vacant p‐orbital at germanium. DFT calculations support this proposition and NBO and AIM analyses are consistent with a Ge? P bond order greater than unity.     

14N and 35Cl NQR studies of organophosphorus compounds

¹⁴N and ³⁵Cl NQR spectra have been investigated for 24 organophosphorus compounds using a pulse technique. The electron populations of the nitrogen lone pair orbital and the N? P bond are calculated according to the Townes and Dailey method. The experimental data are interpreted assuming a partial double bond character of the N? P bond due to the p_π? d_π interaction and p_π? σ conjugation of the lone pair electrons of the nitrogen atoms. The effect of the different nature of substituents X on the N? P bond populations is observed in X ? PR_n (R₂N)_3-n molecules (where X is O, S, Se, or lone pair electrons and n = 0, 1, 2). It can be seen from this dependence that the effective electronegativity of the phosphorus atom is largest in selenophosphoramidates and falls in the sequence P?Se > P?S > P?O > P.     

Electronic Structure Trends in N‐Heterocyclic Carbenes (NHCs) with Varying Number of Nitrogen Atoms and NHC?Transition‐Metal Bond Properties

Carbenes derived from five‐membered heterocycles with different numbers of nitrogen atoms ranging from two to four lead formally either to normal N‐heterocyclic or mesoionic carbenes with, in some cases, the same skeletal structure. The electronic structures of fourteen of these compounds were examined by means of DFT calculations at the B3LYP/aug‐cc‐pVTZ level. The examined parameters include the energies of the σ‐lone pair at C_carbene and the π‐HOMO of the protonated form, which are correlated to the first and second proton affinities. The singlet–triplet energy gap was used as a measure of the stability of the N‐heterocyclic carbene (NHC) towards dimerisation. Natural population analysis provided insight into the variation of the p_π population and the natural charge at C_carbene with NHC structure. Additionally, the transition metal? NHC bond in L‐AuCl and L‐TiCl₄ and the nature of the orbital interactions between the NHC and the transition‐metal fragment were analysed in detail by the extended transition state–natural orbitals for chemical valence (ETS–NOCV) approach at the BP86/TZ2P level. Similarities and differences between the NHC? gold and the NHC? titanium bond are discussed, and trends in key bonding properties can be traced back to the variation of the electronic parameters of the NHC.     

Etude conformationnelle par RMN de phosphines ethyleniques cis et trans: Influence de l'orientation du doublet libre du phosphore sur le couplage 2JP-H transmis a travers un carbone sp

NMR spectra of variously substituted cis and trans ethylene phosphines have been taken at different temperatures. When a bulky group is bound to the double bond, the cis phosphines adopt a preferred conformation, but a conformational equilibrium applies to some trans phosphines.The calculated values of ²P_P-H for the different conformers are dependent on the phosphorus lone pair orientation: ²J_P-H is large and positive when the CH bond and the lone pair direction are parallel; it is small and may become negative when these directions are antiparallel.     

Thermochemistry of N‐heterocyclic carbenes with 5‐, 4‐, 3‐, and 2‐membered rings

N‐heterocyclic carbenes (NHCs) based on imidazole‐2‐ylidene ( 1 ) or the saturated imidazolidine‐2‐ylidene ( 2 ) scaffolds are long‐lived singlet carbenes. Both benefit from inductive stabilization of the sigma lone pair on carbon by neighboring N atoms and delocalization of the N pi lone pairs into the nominally vacant p‐pi atomic orbital at the carbene carbon. With thermochemical schemes G4 and CBS‐QB3, we estimate the relative thermodynamic stabilization of smaller ring carbenes and acyclic species which may share the keys to NHC stability. These include four‐membered ring systems incorporating the carbene center, two trivalent N centers, and either a boron or a phosphorus atom to complete the ring. Amino‐substituted cyclopropenylidenes have been reported but three‐membered rings containing the carbene center and two N atoms are not known. Our calculations suggest that amino‐substituted cyclopropenylidenes are comparable in stability to the four‐membered NHCs but that diazacyclopropanylidenes would be substantially less effectively stabilized. Concluding the series are acyclic carbenes with and without neighboring N atoms and a series of “two‐membered ring” azapropadienenylidene cations of form :C?N?W with W = an electron‐withdrawing agent. We have studied W = NO₂, CH₂(+), CF₂(+), and (CN)₂C(+). Although these systems display a degree of stabilization and carbene‐like electronic structure, the stability of the NHCs is unsurpassed. © 2014 Wiley Periodicals, Inc.     

Theoretical analysis of fluoroglycine conformers

Seven different optimized conformers of α‐fluoroglycine (H₂NCHFCOOH) were obtained from ab initio calculations. Some of these conformers are exceptionally stable compared to similar conformers of glycine. Conformers in which the lone pair of electrons on the nitrogen atom are antiperiplanar to the C F bond are more stable than conformers that do not have such an arrangement. The stability difference between conformers with such an arrangement and conformers that have the lone pair of electrons synperiplanar to the C F bond is about 27 kJ/mol (calculated at the MP2/6‐31+G* level). Conformers that have the lone pair of electrons antiperiplanar to the C F bond possess a longer C F bond, a shorter C N bond, and sp²‐like amino bond angles. For some conformers an unusual hydrogen bond involving the acidic carboxylic acid hydrogen and the electronegative fluorine atom is observed. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 426–431, 2000     

1,2-Metallobridging and the Antiperiplanar Effect. Thermodynamic Preference for the Z-Configuration in Imidoyl(Alkali Metals) and their Phosphorus Analogs

We showed that imidoyl- and phosphaethenyl(alkali metals) would thermodynamically prefer the Z-configuration. The bond model analysis of the electronic structures showed that the Z-preference should originate from 1,2-metallobridging by the delocalization of lone pairs on N or P to vacant p-orbitals of the alkali metals and from the antiperiplanar effect of the delocalization from σ _C—M to σ? _{N(P)—R ²} and from n _N(P) to the C—R¹. The Z-preference increases by more electron-withdrawing groups at the carbon atom of the double bond. However, substitution at the nitrogen/phosphorus results in E-preference because of 1,4-chelation of the lone pair of the substituents to alkali metals. Most of halogen derivatives were not stable and eliminate metal halides.     

Stereoelectronic Effects on the Nucleophilic Addition of Phosphite to the Carbonyl Double Bond: Ab Initio Molecular Orbital Calculations on Reaction Surfaces and the α-Effect

Ah initio molecular orbital calculations have been carried out on adducts of trihydroxy phosphine, P(OH)₃, and formaldehyde, H₂C=0. Stationary points were located and a reaction surface calculated. One stationary point exists as a stable pentacovalent phosphorane, and the other as a 1,3-dipolar transition state. Calculations differing in the conformation about the P-OH bonds of the phosphite reveal that an antiperiplanar (app) lone pair on oxygen to the phosphorus lone pair (acyclic analogue) raises the energy of the molecule by 1.7 kcal/mol relative to a phosphite conformation with no app lone pairs to the phosphorus lone pair (bicyclic analogue). In the transition state, the relative energy between the two conformations reverses with the acyclic analogue transition state 5 kcal/mol lower energy than the bicyclic analogue transition states. The lower energy for the acyclic analogue in the transition state is attributed to the mixing of the app lone pairs on the oxygens of the phosphite mixing with the σ orbital of the newly formed bond between phosphorus and carbon. This kinetic Stereoelectronic effect can explain why acyclic phosphites react much faster in nucleophilic reactions than bicyclic phosphites. This phenomenon suggests that the origin of the α-effect, the enhanced nucleophilicity of a base possessing a heteroatom with an adjacent unshared electron pair arises from the stereoelectronic effect.     

Quantum-Chemical Study of the Structure of Cyanophosphines and Their Oxides

The structure of cyanophosphines and their oxides was studied by ab initio (RHF/6-31G^**) and semiempirical (PM3) methods. Both methods predict that MeOP(CN)₂, (MeO)₂PCN, and (MeO)₂P(O)CN exist in noneclipsed antiperiplanar and synclinal conformations. The calculation results nicely agree with measured dipole moments and Kerr constants of these compounds. The phenyl and diphenyl derivatives PhP(CN)₂, Ph₂PCN, Ph(Et)PCN, and Ph₂P(O)CN prefer forms in which the phenyl ring plane is eclipsing the phosphorus lone electron pair or the phosphoryl bond. The interactions of the phosphorus lone electron pair with the phenyl ring and with the cyano group are lacking in the title compounds.     

Nickel(I) Monomers and Dimers with Cyclopentadienyl and Indenyl Ligands

The reaction of (μ‐Cl)₂Ni₂(NHC)₂ (NHC=1,3‐bis(2,6‐diisopropylphenyl)‐1,3‐dihydro‐2H‐imidazol‐2‐ylidene (IPr) or 1,3‐bis(2,6‐diisopropylphenyl)imidazolidin‐2‐ylidene (SIPr)) with either one equivalent of sodium cyclopentadienyl (NaCp) or lithium indenyl (LiInd) results in the formation of diamagnetic NHC supported Ni^I dimers of the form (μ‐Cp)(μ‐Cl)Ni₂(NHC)₂ (NHC=IPr ( 1 a ) or SIPr ( 1 b ); Cp=C₅H₅) or (μ‐Ind)(μ‐Cl)Ni₂(NHC)₂ (NHC=IPr ( 2 a ) or SIPr ( 2 b ); Ind=C₇H₉), which contain bridging Cp and indenyl ligands. The corresponding reaction between two equivalents of NaCp or LiInd and (μ‐Cl)₂Ni₂(NHC)₂ (NHC=IPr or SIPr) generates unusual 17 valence electron Ni^I monomers of the form (η⁵‐Cp)Ni(NHC) (NHC=IPr ( 3 a ) or SIPr ( 3 b )) or (η⁵‐Ind)Ni(NHC) (NHC=IPr ( 4 a ) or SIPr ( 4 b )), which have nonlinear geometries. A combination of DFT calculations and NBO analysis suggests that the Ni^I monomers are more strongly stabilized by the Cp ligand than by the indenyl ligand, which is consistent with experimental results. These calculations also show that the monomers have a lone unpaired‐single‐electron in their valence shell, which is the reason for the nonlinear structures. At room temperature the Cp bridged dimer (μ‐Cp)(μ‐Cl)Ni₂(NHC)₂ undergoes homolytic cleavage of the Ni?Ni bond and is in equilibrium with (η⁵‐Cp)Ni(NHC) and (μ‐Cl)₂Ni₂(NHC)₂. There is no evidence that this equilibrium occurs for (μ‐Ind)(μ‐Cl)Ni₂(NHC)₂. DFT calculations suggest that a thermally accessible triplet state facilitates the homolytic dissociation of the Cp bridged dimers, whereas for bridging indenyl species this excited triplet state is significantly higher in energy. In stoichiometric reactions, the Ni^I monomers (η⁵‐Cp)Ni(NHC) or (η⁵‐Ind)Ni(NHC) undergo both oxidative and reductive processes with mild reagents. Furthermore, they are rare examples of active Ni^I precatalysts for the Suzuki–Miyaura reaction. Complexes 1 a , 2 b , 3 a , 4 a and 4 b have been characterized by X‐ray crystallography.     

Carbene triel bonds between TrR3 (Tr = B,Al) and N-heterocyclic carbenes

The carbene triel bond is predicted and characterized by theoretical calculations. The C lone pair of N-heterocyclic carbenes (NHCs) is allowed to interact with the central triel atom of TrR₃ (Tr = B and Al; R = H, F, Cl, and Br). The ensuing bond is very strong, with an interaction energy of nearly 90 kcal/mol. Replacement of the C lone pair by that of either N or Si weakens the binding. The bond is strengthened by electron-withdrawing substituents on the triel atom, and the reverse occurs with substitution on the NHC. However, these effects do not strictly follow the typical pattern of F > Cl > Br. The TrR₃ molecule suffers a good deal of geometric deformation, requiring on the order of 30 kcal/mol, in forming the complex. The R(C···Tr) bond is quite short, for example, 1.6 Å for Tr = B, and shows other indications of at least a partially covalent bond, such as a high electron density at the bond critical point and a good deal of intermolecular charge transfer.     

Conformational studies by nuclear magnetic resonance?VI: C?C and C?N rotation barriers in various enamine derivatives

Delocalisation of the nitrogen lone electron pair by an acyl and acylvinylogue group simultaneously, results in a lowering of both C? N rotational barriers. MeC(O)CBr?CHNMe₂ and MeC(O)C(OCOMe)?CHNMe₂ exhibit restricted rotation around what is formally a double bond C?C.     

Bonding/antibonding character of “lone pair” molecular orbitals from their energy derivatives; consequences for experimental data

The derivative of molecular orbitals (MO) energies with respect to a bond length (dynamic orbital force [DOF]) is used to estimate the bonding/antibonding character of valence MOs along this bond, with a focus on lone pair MOs, in a series of small molecules: AH (A = F, Cl, Br), AH₂ (A = O, S, Se), AX₃ (A = N, P, As; X = H, F), and H₂CO. The HOMO DOF agrees with the calculated variation of bond length and force constant in the corresponding ground state cation, and of bond length variation by protonation. These results also agree with available experimental data. It is worthy to note that the p‐type HOMOs in AH and AH₂ are found bonding. The lone pair MO is bonding in NH₃, while it is antibonding in PH₃, AsH₃, and AF₃.     

Bonding Analysis of N‐Heterocyclic Carbene Tautomers and Phosphine Ligands in Transition‐Metal Complexes: A Theoretical Study

DFT calculations at the BP86/TZ2P level were carried out to analyze quantitatively the metal–ligand bonding in transition‐metal complexes that contain imidazole (IMID), imidazol‐2‐ylidene (nNHC), or imidazol‐4‐ylidene (aNHC). The calculated complexes are [Cl₄TM(L)] (TM=Ti, Zr, Hf), [(CO)₅TM(L)] (TM=Cr, Mo, W), [(CO)₄TM(L)] (TM=Fe, Ru, Os), and [ClTM(L)] (TM=Cu, Ag, Au). The relative energies of the free ligands increase in the order IMID<nNHC<aNHC. The energy levels of the carbon σ lone‐pair orbitals suggest the trend aNHC>nNHC>IMID for the donor strength, which is in agreement with the progression of the metal–ligand bond‐dissociation energy (BDE) for the three ligands for all metals of Groups 4, 6, 8, and 10. The electrostatic attraction can also be decisive in determining trends in ligand–metal bond strength. The comparison of the results of energy decomposition analysis for the Group 6 complexes [(CO)₅TM(L)] (L=nNHC, aNHC, IMID) with phosphine complexes (L=PMe₃ and PCl₃) shows that the phosphine ligands are weaker σ donors and better π acceptors than the NHC tautomers nNHC, aNHC, and IMID.     

Molecular Structures of 1-Dibutylboryl-2-Diphenyl-Phosphino-1-Butyl-2-Phenylethene and ITS Derivatives

Abstract

The X-ray crystal structure study of l-dibutylboryl-2-diphenylphosphino-I-butyl-2-phenylethene (I) revealed that this molecule has Z-conformation respective to C=C bond. The P-B distance is 2.104(5) Å and the P-C=C and B-C=C bond angles are decreased to 96.1(3) and 109.3(4)°. This is the evidence of P → B dative interaction. The ab initio calculations of the model borylphosphinoethene H₂P-CH=CH-BH₂with full geometry optimization showed that the phosphorus lone electron pair is shifted towards boron by 0.26 Å.