Formation, Reactivity and Decomposition of Aryl Phospha-Enolates.

Two lithium phospha-enolates [RP=C(Sii Pr3 )OLi]2 were prepared by reaction of triisopropyl silyl phosphaethynolate, i Pr3 SiPCO, with aryl lithium reagents LiR (R=Mes: 1,3,5-trimethyl phenyl; or Mes*: 1,3,5,-tri-tertbutyl phenyl). Monomer/dimer aggregation of the enolates can be modulated by addition of 12-crown-4. Substitution of lithium for a heavier alkali metal was achieved through initial formation of a silyl enol ether, followed by reaction with KOt Bu to form the corresponding potassium phospha-enolate [MesP=C(Sii Pr3 )OK]2 . On addition of water, the enolates are protonated to afford RP=C(Sii Pr3 )(OH). For the sterically less demanding system (R=Mes), this phospha-enol rapidly tautomerises to the corresponding acyl phosphine MesP(H)C(Sii Pr3 )(O), which on heating extrudes CO. In contrast, bulkier phospha-enol (R=Mes*) is stable to rearrangement at room temperature and thermally decomposes to RH and i Pr3 SiPCO.

Metal-Ligand Cooperativity to Assemble a Neutral and Terminal Niobium Phosphorus Triple Bond (Nb≡P).

Decarbonylation along with P-atom transfer from the phosphaethynolate anion, PCO- , to the NbIV complex [(PNP)NbCl2 (Nt BuAr)] (1) (PNP=N[2-Pi Pr2 -4-methylphenyl]2 - ; Ar=3,5-Me2 C6 H3 ) results in its coupling with one of the phosphine arms of the pincer ligand to produce a phosphanylidene phosphorane complex [(PNPP)NbCl(Nt BuAr)] (2). Reduction of 2 with CoCp*2 cleaves the P-P bond to form the first neutral and terminal phosphido complex of a group 5 transition metal, namely, [(PNP)Nb≡P(Nt BuAr)] (3). Theoretical studies have been used to understand both the coupling of the P-atom and the reductive cleavage of the P-P bond. Reaction of 3 with a two-electron oxidant such as ethylene sulfide results in a diamagnetic sulfido complex having a P-P coupled ligand, namely [(PNPP)Nb=S(Nt BuAr)] (4).

The "Hidden" Reductive [2+2+1]-Cycloaddition Chemistry of 2-Phosphaethynolate Revealed by Reduction of a Th-OCP Linkage.

The reduction chemistry of the newly emerging 2-phosphaethynolate (OCP)- is not well explored, and many unanswered questions remain about this ligand in this context. We report that reduction of [Th(TrenTIPS )(OCP)] (2, TrenTIPS =[N(CH2 CH2 NSiPri 3 )]3- ), with RbC8 via [2+2+1] cycloaddition, produces an unprecedented hexathorium complex [{Th(TrenTIPS )}6 (µ-OC2 P3 )2 (µ-OC2 P3 H)2 Rb4 ] (5) featuring four five-membered [C2 P3 ] phosphorus heterocycles, which can be converted to a rare oxo complex [{Th(TrenTIPS )(µ-ORb)}2 ] (6) and the known cyclometallated complex [Th{N(CH2 CH2 NSiPri 3 )2 (CH2 CH2 SiPri 2 CHMeCH2 )}] (4) by thermolysis; thereby, providing an unprecedented example of reductive cycloaddition reactivity in the chemistry of 2-phosphaethynolate. This has permitted us to isolate intermediates that might normally remain unseen. We have debunked an erroneous assumption of a concerted fragmentation process for (OCP)- , rather than cycloaddition products that then decompose with [Th(TrenTIPS )O]- essentially acting as a protecting then leaving group. In contrast, when KC8 or CsC8 were used the phosphinidiide C-H bond activation product [{Th(TrenTIPS )}Th{N(CH2 CH2 NSiPri 3 )2 [CH2 CH2 SiPri 2 CH(Me)CH2 C(O)µ-P]}] (3) and the oxo complex [{Th(TrenTIPS )(µ-OCs)}2 ] (7) were isolated.

Cyano(triphenylsilyl)phosphanide as a Building Block for P,C,N Conjugated Molecules.

The cyano(triphenylsilyl)phosphanide anion was prepared as a sodium salt from 2-phosphaethynolate. The electronic structure of this new cyano(silyl)phosphanide was studied via computational methods and its reactivity investigated using various electrophiles and Lewis acids, demonstrating its P- and N-nucleophilicity. The ambident reactivity is in agreement with computations. The silyl group also shows lability and therefore the cyano(silyl)phosphanide can be considered as a phosphacyanamide synthon, [PCN]2- , and serves as building block for the transfer of a PCN moiety.

Phosphorus and Arsenic Atom Transfer to Isocyanides to Form π-Backbonding Cyanophosphide and Cyanoarsenide Titanium Complexes.

Decarbonylation along with E atom transfer from Na(OCE) (E=P, As) to an isocyanide coordinated to the tetrahedral TiII complex [(TptBu,Me )TiCl], yielded the [(TptBu,Me )Ti(η3 -ECNAd)] species (Ad=1-adamantyl, TptBu,Me- =hydrotris(3-tert-butyl-5-methylpyrazol-1-yl)borate). In the case of E=P, the cyanophosphide ligand displays nucleophilic reactivity toward Al(CH3 )3 ; moreover, its bent geometry hints to a reduced Ad-NCP3- resonance contributor. The analogous and rarer mono-substituted cyanoarsenide ligand, Ad-NCAs3- , shows the same unprecedented coordination mode but with shortening of the N=C bond. As opposed to TiII , VII fails to promote P atom transfer to AdNC, yielding instead [(TptBu,Me )V(OCP)(CNAd)]. Theoretical studies revealed the rare ECNAd moieties to be stabilized by π-backbonding interactions with the former TiII ion, and their assembly to most likely involve a concerted E atom transfer between Ti-bound OCE- to AdNC ligands when studying the reaction coordinate for E=P.

Phosphine Carboxylate-Probing the Edge of Stability of a Carbon Dioxide Adduct with Dihydrogenphosphide.

We present a new adduct of carbon dioxide with dihydrogenphosphide, that may be prepared either by direct reaction of NaPH2 with carbon dioxide or by hydrolysis of the phosphaethynolate ion (PCO- ). In this hydrolysis transformation, a new mechanism is proposed for the electrophilic reactivity of the phosphaethynolate ion. Protonation to form phosphine carboxylic acid (PH2 COOH) and functionalization to form esters is shown to increase the strength of the P-C interaction, allowing for comparisons to be drawn between this species and the analogous carbamic (NH2 COOH) and carbonic acids (H2 CO3 ). Functionalization of the oxygen atom is found to stabilize the phosphine carboxylate while also allowing solubility in organic solvents whereas phosphorus functionalization is shown to facilitate decarboxylation. Substituent migration occurs in some cases.

A "Push-Pull" Stabilized Phosphinidene Supported by a Phosphine-Functionalized ß-Diketiminato Ligand.

The use of a bis(diphenyl)phosphine functionalized ß-diketiminato ligand, [HC{(CH3 )C}2 {(ortho-[P(C6 H5 )2 ]2 C6 H4 )N}2 ]- (PNac), as a support for germanium(II) and tin(II) chloride and phosphaketene compounds, is described. The conformational flexibility and hemilability of this unique ligand provide a versatile coordination environment that can accommodate the electronic needs of the ligated elements. For example, chloride abstraction from [(PNac)ECl] (E=Ge, Sn) affords the cationic germyliumylidene and stannyliumylidene species [(PNac)E]+ in which the pendant phosphine arms associate more strongly with the Lewis acidic main group element centers, providing further electronic stabilization. In a similar fashion, chemical decarbonylation of the germanium phosphaketene [(PNac)Ge(PCO)] with tris(pentafluorophenyl)borane affords a "push-pull" stabilized phosphinidene in which one of the phosphine groups of the ligand backbone associates with the low valent phosphinidene center.

Phosphanyl Cyanophosphide Salts: Versatile PCN Building Blocks.

The facile preparation of alkali salts of phosphanyl cyanophosphides [NHP-PCN]- (NHP=N-heterocyclic phosphenium) is reported. Their formation is achieved by isoelectronic replacement of O for [N]- in the phosphaketenes NHP-PCO using alkaline hexamethyldisilazide M[N(SiMe3 )2 ] (M=Na, K) as reagent. The new anionic entities are versatile PCN building blocks which allow the formation of a diversity of new cyanophosphine derivates including the first example of a PCNB hetero-cumulene and a PCN-ligated transition metal complex.

Trapping of a Highly Bent and Reduced Form of 2-Phosphaethynolate in a Mixed-Valence Diuranium-Triamidoamine Complex.

The chemistry of 2-phosphaethynolate is burgeoning, but there remains much to learn about this ligand, for example its reduction chemistry is scarce as this promotes P-C-O fragmentations or couplings. Here, we report that reduction of [U(TrenTIPS )(OCP)] (TrenTIPS =N(CH2 CH2 NSiPri 3 )3 ) with KC8 /2,2,2-cryptand gives [{U(TrenTIPS )}2 {µ-η2 (OP):η2 (CP)-OCP}][K(2,2,2-cryptand)]. The coordination mode of this trapped 2-phosphaethynolate is unique, and derives from an unprecedented highly reduced and highly bent form of this ligand with the most acute P-C-O angle in any complex to date (P-C-O ∡ ≈127°). The characterisation data support a mixed-valence diuranium(III/IV) formulation, where backbonding from uranium gives a highly reduced form of the P-C-O unit that is perhaps best described as a uranium-stabilised OCP2-. radical dianion. Quantum chemical calculations reveal that this gives unprecedented carbene character to the P-C-O unit, which engages in a weak donor-acceptor interaction with one of the uranium ions.

Quantum Chemical Calculations on CHOP Derivatives-Spanning the Chemical Space of Phosphinidenes, Phosphaketenes, Oxaphosphirenes, and COP- Isomers.

After many decades of intense research in low-coordinate phosphorus chemistry, the advent of Na[OCP] brought new stimuli to the field of CHOP isomers and derivatives thereof. The present theoretical study at the CCSD(T)/def2-TZVPP level describes the chemical space of CHOP isomers in terms of structures and potential energy surfaces, using oxaphosphirene as the starting point, but also covering substituted derivatives and COP- isomers. Bonding properties of the P⁻C, P⁻O, and C⁻O bonds in all neutral and anionic isomeric species are discussed on the basis of theoretical calculations using various bond strengths descriptors such as WBI and MBO, but also the Lagrangian kinetic energy density per electron as well as relaxed force constants. Ring strain energies of the superstrained 1H-oxaphosphirene and its barely strained oxaphosphirane-3-ylidene isomer were comparatively evaluated with homodesmotic and hyperhomodesmotic reactions. Furthermore, first time calculation of the ring strain energy of an anionic ring is described for the case of oxaphosphirenide.