The novel N,P,C-cage complexes 5 a–f and 6 a–f have been obtained by the reaction of the P-pentamethylcyclopentadienylphosphinidene complex 2, generated thermally from 2H-azaphosphirene complex 1, with N-methyl-C-arylcarbaldimines 3 a–f. Li/Cl phosphinidenoid complex 8 reacted with 3 a,b to give N,P,C-cage complexes 6 a,b, whereas with 3 c–f, complexes 6 c–f were obtained in negligible amounts only. Both types of ligand N,P,C-cage structures 5 and 6 were found to be in an unprecedented equilibrium, with 5 a,f as the predominant species. Transient electrophilic terminal phosphinidene complexes 10 a–f serve as intermediates in both ligand interconversions (5 a,f6 a,f), as evidenced through trapping reactions with phenylacetylene and N-methyl-C-phenylcarbaldimine, thus leading to the novel N,P,C-cage complexes 13 b and 15. DFT calculations predicted a small difference in the relative energies of the two types of N,P,C-cage ligands, and a remarkable stabilisation of the aminophosphinidene complex 10 as the common precursor, thereby providing an insight into this surprising 5-ring–3-ring interconversion. In depth analysis of intermediate 10 revealed the occurrence of both through-bond (conventional inductive/mesomeric effects) and through-space (non-covalent interactions) mechanisms, which amount to 67.8 and 14.4 kcal mol⁻¹, respectively, and account for the remarkable stabilisation of this intermediate.

The synthesis and structures of Rh^I and Ir^I complexes bearing 4-phosphinoyl- and 4,5-bis(phosphinoyl)imidazol-2-ylidene ligands (NHC^P) is reported. Deprotonation of the corresponding imidazolium hydrogen sulfate salts by KOtBu in the presence of [M(cod)Cl]₂ afforded complexes of the formula [M(cod)Cl(NHC^P)] [5a/5b: NHC^P = 4-Ph₂P(O)-IiPrMe, 5a: M = Rh, 5b: M = Ir and 7a/7b: NHC^P = 4,5-{Ph₂P(O)}₂-InBuMe]. The Rh^I complexes 5a and 7a were then converted into [Rh(CO)₂Cl(NHC^P)] 8a and 9a via reaction with carbon monoxide; the average CO stretching frequencies [8a: (CO)_av = 2036 cm^–1 and 9a: (CO)_av = 2038.4 cm^–1] led to the conclusion that mono- and bis-functionalized NHC^Ps act as relatively weaker donors than backbone unsubstituted NHC. A first examination of Janus-head ligand properties of complex 7a revealed that reaction with TiCl₄ afforded hetero-dinuclear 10a,a′ as a mixture of isomeric monodentate phosphinoyl titanium(IV) adducts. All products were characterized by elemental analyses, spectroscopic and spectrometric methods and, in addition, complexes 7a and 7b by single-crystal X-ray structure analysis.

An Li/Cl phosphinidenoid complex, obtained by chlorine/lithium exchange from a [dichloro(organo)phosphane]tungsten(0) complex, reacted with aliphatic dicarbonyl derivatives to provide oxaphosphirane complexes, a 1,3,2-dioxaphosphol-4-ene complex, and a P-alkoxyphosphane complex; the latter is formally derived from the enol form of the β-diketone. DFT calculations on the ring-expansion rearrangement support a preferred mechanism involving a pericyclic [1,3] shift of the phosphorus fragment in an oxaphosphirane complex rather than a stepwise diradical or ionic mechanism. The latter is slightly unfavored (ΔΔE^‡ = 2.2 kcal/mol) and involves heterolytic P–C bond cleavage to give a methylene oxonium phosphanide intermediate and cyclization through a low-lying transition state featuring an unusual linear C–O–P geometry.

Während P^V-1,2-Oxaphosphetane von der Wittig-Reaktion her geläufig sind, waren die P^III-Analoga bisher unbekannt. Wir präsentieren hier die Synthese und Reaktionen der ersten 1,2-Oxaphosphetankomplexe, die durch Reaktion des Phosphinidenoidkomplexes [Li(12-Krone-4)(Lsgm)][(OC)₅W{(Me₃Si)₂HC-PCl}] mit unterschiedlichen Epoxiden erhalten wurden. Die Titelverbindungen sind in Toluol bis ca. 100 °C stabil, bevor eine unselektive Zersetzung eintritt. Die säureinduzierte Ringerweiterung mit Benzonitril führt zur selektiven Bildung des ersten Komplexes mit einem 1,3,4-Oxazaphosphacyclohex-2-en-Liganden.

The biradicaloid character of the ground-state structures of N₂P₂ rings is studied by using the high-level ab initio multiconfigurational CASPT2/CASSCF method. In order to obtain accurate descriptors, we combine two criteria: 1) singlet–triplet energy gaps and 2) relative values of the occupation numbers for bonding and antibonding orbitals associated with the radical sites. The singlet–triplet energy gaps, the occupation numbers of antibonding-like orbitals, and the weights of the main configuration state functions (CSFs) of the ground-state wavefunctions, that is, Ψ(¹A₁), are used to derive the biradicaloid character that ranges from 10–15 %.

While P^V 1,2-oxaphosphetanes are well known from the Wittig reaction, their P^III analogues are still unexplored. Herein, the synthesis and reactions of the first 1,2-oxaphosphetane complexes are presented, which were achieved by reaction of the phosphinidenoid complex [Li(12-crown-4)(solv)][(OC)₅W{(Me₃Si)₂HCPCl}] with different epoxides. The title compounds appeared to be stable in toluene up to 100 °C, before unselective decomposition started. Acid-induced ring expansion with benzonitrile resulted in selective formation of the first complex bearing a 1,3,4-oxazaphosphacyclohex-2-ene ligand.

The reaction of Li/Cl P-CPh₃ phosphinidenoid tungsten(0) complex 2 with dimethylcyanamide afforded tricyclic phosphirane complex 4, an unprecedented rearrangement of which led to the novel N,P,C cage complex 6. On the basis of DFT calculations, formation and intramolecular [3+2] cycloaddition of the transient nitrilium phosphane ylide complex 3 to a phenyl ring of the triphenylmethyl substituent to give 4 is proposed. Furthermore, theoretical evidence for terminal N-amidinophosphinidene complex 7, formed by [2+1] cycloelimination from 4, is provided, and the role of the electronic structure and non-covalent interactions of intermediate 7 discussed.

P-H,P-OR-substituted phosphane complexes 3a–e have been synthesized by two methods: (1) the thermal reaction of 2H-azaphosphirene complex 1 with methanol, n-butanol, or ethylene glycol monomethyl ether (3b,c,e) or (2) the reaction of P-chlorophosphane complex 2 with appropriate sodium phenolate salts (3a,d). All the complexes 3a–e were obtained in good yields and fully characterized by NMR, IR, MS, and elemental analysis. Furthermore, the structures of 3a, 3d and 3e were confirmed unambiguously by X-ray analysis. The deprotonation of complexes 3a–e by using lithium diisopropylamide in the presence of 12-crown-4 led to phosphinidenoid complexes 4a–e, which exhibit downfield ³¹P resonances and small tungsten–phosphorus coupling constants. Studies on the reactivity of complexes 4a–c,e revealed a “phosphanido-type” reactivity, and only for complex 4d, a thermally labile complex, was evidence found for a “phosphinidene-type” reactivity.

Aminophosphane complexes 3a–e have been prepared by two different pathways: (1) the thermal reaction of 2H-azaphosphirene complex 1 with primary or secondary amine derivatives at 75 °C (3a,b,d,e) or (2) the reaction of chlorophosphane complex 2 with sodium diphenylamide (3c). In the latter case, complex 3c was obtained together with the diphosphene complex 6, thus providing evidence for the transient formation of Na/X phosphinidenoid complexes 4a,b and/or the terminal phosphinidene complex 5. Preliminary deprotonation studies, carried out on complex 3a by using lithium diisopropylamide in TMEDA in the presence of 12-crown-4 at low temperature, yielded a mixture of the Li/NMe₂ phosphinidenoid complex 7 together with the nonligated phosphane derivative 8, which could not be separated. Complexes 3a,b,d,e and 6 were characterized by NMR and IR spectroscopy, MS, and elemental analysis. The structures of complexes 3c–e and 6 were confirmed by single-crystal X-ray diffraction analysis.

Novel bis(imidazole-2-thion-4-yl)- phosphanes (2a–d) were synthesized via lithiation of the precursor imidazole-2-thiones followed by the phosphanylation reaction. Oxidation of bis(imidazole-2-thion-4-yl)phosphane 2b–d with elemental sulfur and selenium led selectively and in good yields to the P-thio (3b–d) and P-seleno (4c) derivatives of bis(imidazole-2-thion-4-yl)phosphanes, respectively. The treatment of 2a,c with phosphorus trichloride gives the corresponding P-chloro derivatives 5a,c. These compounds were unambiguously characterized by elemental analyses, spectroscopic and spectrometric methods, in addition by single-crystal X-ray structure analysis in the case of 2d. © 2012 Wiley Periodicals, Inc. Heteroatom Chem 00:1–7, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.21043

A first computational insight into the intrinsic strength of exocyclic bonds to phosphorus in oxaphosphirane κP-pentacarbonylmetal(0) complexes 1 a–f (M=Cr, Mo) is provided using a set of PR derivatives (R=Me, tBu, CPh₃). Whereas homolytic cleavage of the exocyclic PR bond was found to be always unfavored (for neutral complexes), heterolytic cleavage leading to a carbocation R⁺ moiety and the oxaphosphiranide complex 2⁻ constitutes the lowest-energy process, especially if R is bulky and can stabilize the positive charge, that is, triphenylmethyl (trityl), efficiently. The energies required for PM bond cleavage are about 30 kcal mol⁻¹, and decrease with the increasing bulk of the R substituent (from Me to trityl) and ongoing from Cr to Mo. The reactivities of complexes 1 a–f towards oxidative and reductive single electron transfer (SET) reactions were analyzed using the facile variation of bond-strength-related descriptors (VBSD) methodology, thus enabling the design of synthetically useful strategies addressing decomplexation and P-functionalization. Reductive SET reactions with sodium naphthalenides enable selective PM bond cleavage (i.e., decomplexation) for the case of P-Me and P-tBu substitution, whereas reductive PR bond cleavage is favored in the case of the P-trityl complexes 1 c,f, and results in the formation of the (anionic) oxaphosphiranide complex 2⁻, which may be regarded as a potential key intermediate for further P-functionalization.

P-Functional (acylphosphane)tungsten complexes 4a–f have been prepared in good yields by the reaction of phosphinidenoid complexes 2a–d with acyl chlorides 3a,b. The reactions of acyl(chloro)phosphane complex 4a at –80 °C with organolithium reagents selectively led to the formation of lithiated phospha-enolate complex 5b. The ambident reactivity of this compound was demonstrated in reactions with electrophiles such as PhC(O)Cl, MeI, and Me₃SiCl, which yielded O-substituted complexes 6a,b and 8a,b and P-substituted complex 7.

Synthesis of 1,3,2,4σ³λ³⁻dioxadiphosphetane complexes 3a,b and 4a,b was achieved via thermal decomposition of 2H-azaphosphirene complexes of chromium (1a) and tungsten (1b) in the presence of tert-butyl isocyanate; as reactive intermediates 3-imino-oxaphosphirane complexes 8a,b are proposed, which decompose to give phosphinidene oxide complexes 9a,b. The intermediacy of the latter is inferred from a cross-dimerization experiment (decomposition of a 1:1 mixture of 2H-azaphosphirene complexes 1a,b with tert-butyl isocyanate), which furnished the mixed metal dinuclear cis and trans complexes 5a/b and 6a/b. ³¹P NMR spectroscopy revealed the formation of [bis(trimethylsilyl)methyl]cyanophosphane complexes 11a,b as by-products. Thermal reaction of complex 1b with the less bulky ethyl isocyanate furnished the novel 1,3,4σ³λ³-diazaphospholidin-5-imin-2-one complex 7 together with complexes 3b and 4b. For the latter case, 3-imino-oxaphosphirane (12) and azaphosphiridin-3-one (13) complexes are proposed as reactive intermediates. All final products were characterized by multinuclear NMR spectroscopy, IR, MS, and single-crystal X-ray crystallography in the cases of complexes 3b and 4a,b. DFT calculations strongly support the transient formation and decomposition of 3-imino-oxaphosphirane complex 12 to form 9b and isonitrile. Azaphosphiridin-3-one complex 13 was characterized computationally as the methyl model. In accordance with the calculations, final products 1,3,2,4σ³λ³-dioxadiphosphetane complexes 3a,b and 4a,b form by dimerization of transient terminal phosphinidene oxide complexes 9a,b. © 2011 Wiley Periodicals, Inc. Heteroatom Chem 22:275–286, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20677

The relative energies of azaphosphiridine and its isomers, the ring stability towards valence isomerization, and the ring strain, as well as the kinetics and thermodynamics of possible ring-opening reactions of P^III derivatives (1–5) and P^V chalcogenides (6–9; O to Te), were studied at high levels of theory (up to CCSD(T)). The barrier to inversion at the nitrogen atom in the trimethyl-substituted P^III derivative 5 increases from 12.11 to 15.25 kcal mol⁻¹ in the P-oxide derivative 6 (P^V); the relatively high barrier to inversion at the phosphorus in 5 (75.38 kcal mol⁻¹) points to a configurationally stable center (MP2/def2-TZVPP//BP86/def2-TZVP). The ring strain for azaphosphiridine 5 (av. 22.6 kcal mol⁻¹) was found to increase upon P-oxidation (6) (30.8 kcal mol⁻¹; same level of theory). Various ring-bond-activation processes were studied: N-protonation of P^III (5) and P^V (6, 7) derivatives leads to highly activated species that readily undergo PN bond cleavage. In contrast, metal chlorides such as LiCl, CuCl, CuCl₂, BeCl₂, BCl₃, AlCl₃, TiCl₃, and TiCl₄ show little PN bond activation in 5 and 7. Remarkably, TiCl₃ selectively activates the CN bond, and induces stronger bond activation for P^V (6, 7) than for P^III azaphosphiridines (5). The ring-expanding rearrangement of P^V azaphosphiridines 6–9 to yield P^III 1,3,2-chalcogena-azaphosphetidines 32 a–d is predicted to be preferred for the heavier chalcogenides 7–9, but not for the P-oxide 6. The first comparative analysis of three bond strength parameters is presented: 1) the electron density at bond critical points, 2) Wiberg’s bond index, and 3) the relaxed force constant. This reveals the usefulness of these parameters in assessing the degree of ring bond activation.

Selective macrocyclization of 1,1′-ferrocenediylbis(aminocarbene complex) 1 was achieved by its reaction with chloro(methylene)phosphane 2 and triethylamine at ambient temperature, thus yielding the novel diaminophosphane-bridged [5]ferrocenophane bis(carbene complex) 3. This trimetallic species, which combines the structural features of both ferrocenophane and Fischer carbene complex functional groups, is prone to undergo facile ring opening under mild conditions with formation of bis(2H-azaphosphirene complexes) 6a,b and 2,3-dihydro-1,2,3-azadiphosphete complex 7. The single-crystal X-ray structures of 3 and 7 as well as the reaction courses are discussed.

The reaction of 3-ferrocenyl-substituted 2H-azaphosphirene complexes 1a–c in the presence of substoichiometric amounts of ferrocenium hexafluorophosphate yields 3,5-diferrocenyl-substituted 2H-1,4,2-diazaphosphole complexes 3a–c and difluoro(organo)phosphane complexes 4a–c. The reaction of 1a,c and [FcH]PF₆ with cyanoferrocene yields 3a,c in a straightforward way. The molecular structures of 3a,c were unambiguously identified by multinuclear NMR spectroscopic experiments, mass spectrometry, and single-crystal X-ray diffraction studies. DFT calculations on model complexes 1d–m and 3d–m reveal a close similarity of Mo and W complexes (vs. Cr) and a strong influence of the ferrocenyl substituent on the geometry, spin, and charge distribution of reactive intermediates and the reaction course. Strong support for the assumption of a dissociation–cycloaddition reaction sequence leading to 3 and thus a surprising “cannibalistic” reaction was obtained.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

2H-Azaphosphirene complex 1a reacted with TfOH, cyclohexyl isocyanide, and subsequently with NEt₃ – P–N-bond-selectively – to give the first 2,3-dihydro-1,3-azaphosphete complex 2. Under the same conditions, 2H-azaphosphirene complex 1b underwent a P–C-bond-selective ring enlargement with phenylacetylene with formation of 2H-1,2-azaphosphole complex 3. Apart from X-ray crystallographic data (2, 3), DFT calculations are discussed, which provide first insight into this surprising dichotomy.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Reaction of C-ferrocenyl-substituted aminocarbene tungsten complex 1 with [bis(trimethylsilyl)methylene]chlorophosphane (2) and triethylamine yielded 2H-azaphosphirene complex 3 in good yield. Reaction of complex 3 with aryl nitriles 4a–c, N-piperidinonitrile (4d), and acetonitrile (4e) in the presence of ferrocenium hexafluorophosphate yieldedregioselectively 2H-1,4,2-diazaphosphole complexes 5a–e through single-electron-transfer-induced ring expansion together with complex 6 in varying amounts; isolation of the latter failed. Apart from the NMR spectroscopic parameters of complexes 1, 3, and 5a–e, cyclic voltammetric (1, 3, 5a–d), and single-crystal X-ray diffraction data (1, 3, and 5d) are presented and discussed.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)