Pn Ring Size Dependence on NHC-Induced Ring Contraction Reactions of [CoCp‴(η4-P4)], [Ta(CO)2Cp″(η4-P4)], and [FeCp*(η5-P5)]: A DFT Study.

The facile ring contraction of [CoCp‴(η4-P4)] and [Ta(CO)2Cp″(η4-P4)] to [CoCp‴(η3-P3)][(MeNHC)2P] and [Ta(CO)2Cp″(η3-P3)] [(MeNHC)2P] induced by MeNHC and the absence of the ring contraction of [FeCp*(η5-P5)] under the same conditions are studied by density functional theory (DFT) computations. The latter is estimated to be thermodynamically the least favorable reaction and also has a very high energy barrier. The similar strain energies of P3 and P4 rings and the lower strain energy of the P5 ring play a decisive role in the ring contraction capability of these [TM-cyclo-Pn] complexes. Theoretical approaches involving NBO and IBO analysis have been employed to provide a qualitative picture of the overall reactions. The role of substituents and the nature of transition metals in determining the energetics of these reactions has also been studied and an isolobal perspective on these systems affords a simplified picture.

A Benzodiphosphaborolediide.

The first example of a diphosphaborolediide, the benzo-fused [C6 H4 P2 BPh]2- (12- ), is prepared from ortho-bis(phosphino)benzene (C6 H4 {PH2 }) and dichlorophenylborane, via a sequential lithiation approach. The dilithio-salt can be obtained as an oligomeric THF solvate or discrete TMEDA adduct, both of which are fully characterized, including by X-ray diffraction. Alongside NICS calculations, data strongly suggest some aromaticity within 12- , which is further supported by preliminary coordination studies that demonstrate η5 -coordination to a zerovalent molybdenum center, as observed crystallographically for the oligomeric [{Mo(CO)3 (η5 -1)}{µ-η1 -Mo(CO)3 (TMEDA)}2 ] ⋅ [µ-Li(THF)][µ-Li(TMEDA)].

Structure and bonding in stannadiphospholes and their dianions SnC2P2R2m (R=H, tBu m=0, -2): a comparative study with C5H5+ and C5H5- analogues.

The potential energy surfaces of both neutral and dianionic SnC(2)P(2)R(2) (R=H, tBu) ring systems have been explored at the B3PW91/LANL2DZ (Sn) and 6-311+G* (other atoms) level. In the neutral isomers the global minimum is a nido structure in which a 1,2-diphosphocyclobutadiene ring (1,2-DPCB) is capped by the Sn. Interestingly, the structure established by X-ray diffraction analysis, for R=tBu, is a 1,3-DPCB ring capped by Sn and it is 2.4 kcal mol(-1) higher in energy than the 1,2-DPCB ring isomer. This is possibly related to the kinetic stability of the 1,3-DPCB ring, which might originate from the synthetic precursor ZrCp(2)tBu(2)C(2)P(2). In the case of the dianionic isomers we observe only a 6π-electron aromatic structure as the global minimum, similarly to the cases of our previously reported results with other types of heterodiphospholes. The existence of large numbers of cluster-type isomers in neutral and 6π-planar structures in the dianions SnC(2)P(2)R(2)(2-) (R=H, tBu) is due to 3D aromaticity in neutral clusters and to 2D π aromaticity of the dianionic rings. Relative energies of positional isomers mainly depend on: 1) the valency and coordination number of the Sn centre, 2) individual bond strengths, and 3) the steric effect of tBu groups. A comparison of neutral stannadiphospholes with other structurally related C(5)H(5)(+) analogues indicates that Sn might be a better isolobal analogue to P(+) than to BH or CH(+). The variation in global minima in these C(5)H(5)(+) analogues is due to characteristic features such as 1) the different valencies of C, B, P and Sn, 2) the electron deficiency of B, 3) weaker pπ-pπ bonding by P and Sn atoms, and 4) the tendency of electropositive elements to donate electrons to nido clusters. Unlike the C(5)H(5)(+) systems, all C(5)H(5)(-) analogues have 6π-planar aromatic structures as global minima. The differences in the relative ordering of the positional isomers and ligating properties are significant and depend on 1) the nature of the π orbitals involved, and 2) effective overlap of orbitals.

Electronic structure and bonding in neutral and dianionic boradiphospholes: R'BC2P2R2 (R = H, tBu, R' = H, Ph).

Classical and non-classical isomers of both neutral and dianionic BC(2)P(2)H(3) species, which are isolobal to Cp(+) and Cp(-), are studied at both B3LYP/6-311++G(d,p) and G3B3 levels of theory. The global minimum structure given by B3LYP/6-311++G(d,p) for BC(2)P(2)H(3) is based on a vinylcyclopropenyl-type structure, whereas BC(2)P(2)H(3)(2-) has a planar aromatic cyclopentadienyl-ion-like structure. However, at the G3B3 level, there are three low-energy isomers for BC(2)P(2)H(3): 1) tricyclopentane, 2) nido and 3) vinylcyclopropenyl-type structures, all within 1.7 kcal mol(-1) of each other. On the contrary, for the dianionic species the cyclic planar structure is still the minimum. In comparison to the isolobal Cp(+) and H(n)C(n)P(5-n)(+) isomers, BC(2)P(2)H(3) shows a competition between pi-delocalised vinylcyclopropenyl- and cluster-type structures (nido and tricyclopentane). Substitution of H on C by tBu, and H on B by Ph, in BC(2)P(2)H(3) increases the energy difference between the low-lying isomers, giving the lowest energy structure as a tricyclopentane type. Similar substitution in BC(2)P(2)H(3)(2-) merely favours different positional isomers of the cyclic planar geometry, as observed in 1) isoelectronic neutral heterodiphospholes EtBu(2)C(2)P(2) (E = S, Se, Te), 2) monoanionic heterophospholyl rings EtBu(2)C(2)P(2) (E = P(-), As(-), Sb(-)) and 3) polyphospholyl rings anions tBu(5-n)C(n)P(5-n) (n = 0-5). The principal factors that affect the stability of three-, four-, and five-membered ring and acyclic geometrical and positional isomers of neutral and dianionic BC(2)P(2)H(3) isomers appear to be: 1) relative bond strengths, 2) availability of electrons for the empty 2p boron orbital and 3) steric effects of the tBu groups in the HBC(2)P(2)tBu(2) systems.

Syntheses, structural studies, photoelectron spectra and density functional theory calculations of the "pseudo" tetraphospha-metallocenes [M(eta-P2C3Bu(t)3)(2)], (M = Ni, Pd, Pt).

Syntheses and structural characterization of the complexes [M(eta-P(2)C(3)Bu(t)(3))(2)], (M = Ni, Pd, Pt) are described. The nickel compound has an 18-electron [Ni(eta(5)-P(2)C(3)Bu(t)(3))(eta(3)-P(2)C(3)Bu(t)(3))] structure, whereas the palladium and platinum compounds both have a 16-electron [M(eta(3)-P(2)C(3)Bu(t)(3))(2)] structure (M = Pd, Pt).The electronic structure is examined and discussed using both photoelectron spectroscopy and DFT calculations.

Specific insertion reactions of a germylene, stannylene and plumbylene into the unique P-P bond of the hexaphospha-pentaprismane cage, P6C4tBu4: crystal and molecular structures of P6C4tBu4ER2 (E = Ge, Sn, R = N(SiMe3)2; E = Pb, R = (C6H3(NMe2)2 -2,6).

Treatment of the cage compound P6C4(t)Bu4 with M(N(SiMe3)2)2 (M = Ge or Sn) or Pb(C6H3(NMe2)2- 2,6) at room temperature results in their specific insertion into the P-P bond connecting the two 5-membered P3C2(t)Bu2 rings. The products were fully characterised by multinuclear NMR spectroscopy and single crystal X-ray diffraction studies.

Facile insertion and halogen migration reactions of the hexaphosphapentaprismane cage P6C4tBu4 with zerovalent and divalent platinum complexes.

The hexaphosphapentaprismane P(6)C(4)(t)Bu(4) undergoes specific insertion of the zerovalent platinum fragment [Pt(PPh(3))(2)] into the unique P-P bond between the 5-membered rings to afford [Pt(PPh(3))(2)P(6)C(4)(t)Bu(4)]. A similar reaction with the Pt(ii) complexes [{PtCl(2)(PMe(3))}(2)] and [PtCl(2)(eta(4)-COD)] results in both insertion and chlorine migration reactions. The complexes [Pt(PPh(3))(2)P(6)C(4)(t)Bu(4)], trans-[PtCl(PMe(3))P(6)C(4)(t)Bu(4)Cl], cis-,trans-[{PtCl(2)(PMe(3))}micro-{P(6)C(4)(t)Bu(4)}{PtCl(2)(PMe(3))}], [{PtClP(6)C(4)(t)Bu(4)Cl}(2)] and cis-[PtClP(6)C(4)(t)Bu(4)Cl(P(6)C(4)(t)Bu(4))] have been structurally characterized by single crystal X-ray diffraction and multinuclear NMR studies.

Copper(I) mediated oligomerisation of a phosphaalkyne.

The oligomerisation of tert-butylphosphaalkyne, tBuC[triple bond, length as m-dash]P, mediated by Cu(I) complexes yields an unprecedented C4P5 cage compound, which is stabilised in a matrix of copper(I) iodide.

Synthesis of the 2,4,5-tri-tert-butyl-1,3-diphospholide anion by phosphinidene elimination from 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene on treatment with the amide Li[NPh(SiMe3)].

Treatment of the lithium amide Li[NPh(SiMe3)] with 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene, P(3)C(3)tBu(3), in a 1:2 ratio afforded equimolar amounts of the lithium salt of the five-membered 2,4,5-tri-tert-butyl-1,3-diphospholide anion, LiP(2)C(3)tBu(3) (isolated as its N,N,N',N'-tetramethylethylenediamine (TMEDA) adduct), and the tricyclic compound 6-[phenyl(trimethylsilyl)amino]-3,5,7-tri-tert-butyl-1,2,4,6-tetraphosphatricyclo[3.2.0.0(2,7)]hept-3-ene. Both compounds have been structurally characterised by single-crystal X-ray diffraction studies. The mechanism of this remarkable reaction has been elucidated by theoretical methods at the B3LYP/6-311+G** level of theory. The reaction involves a hitherto unobserved aminophosphinidene, which was formed by abstraction of a phosphorus atom from triphosphabenzene. The intermediate aminophosphinidene, which is further stabilised by the solvent THF, shows, in agreement with previous theoretical predictions, enhanced stability and reacts then with a second molecule of triphosphabenzene.

Electronic structure and bonding studies on triple-decker sandwich complexes with a P6 middle ring.

DFT and hybrid HF-DFT studies of structure and bonding of CpMP6MCp triple-decker sandwich complexes, ranging from 18-28 valence electrons (VE) with M=Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W, show that the middle P6 ring complexes adopt symmetric planar (28 valence electron count [VEC]), asymmetric planar (26 VEC), and puckered (24 VEC) geometries. According to the mno Rule, 50 skeletal electrons are needed for these triple-decker cluster frameworks. For 28 VEC, this corresponds to 10 electrons more than the 50 electrons of the mno Rule if all VE of the metal are included. These additional electrons control the distortion of a P6 middle ring and other finer structural details. Completely filled 2a* and 2b* orbitals in 28 VE complexes lead to a planar symmetrical P6 middle ring, while the occupancy in either 2a* or 2b* alone explains the in-plane distortions (asymmetric) in 26 VE complexes. In comparison with 28 VE complexes, the puckering of P6 middle ring in 24 VE complexes is due to the greater stabilization of 5a and the extra stabilization of the +4 oxidation state of Ti. The quintet state of 22 VE complexes is planar as 2a* and 2b* are half filled. Similar geometrical and bonding patterns of CpScP6ScCp and C2P3H2ScC3P3H3ScC2P3H2 support the carbon-phosphorus analogy further. The 18 VE systems, CpScC3B3H6ScCp+ and CpScP3B3H3ScCp+, have the 50 skeletal electrons as stipulated by the mno Rule. Corresponding anions have 52 skeletal electrons (20 VE); the middle rings here are distorted in the plane.

The molecular structure of [Sn(P2C2But2)] using gas-phase electron diffraction and DFT calculations.

The molecular structure of 2,4-di-tert-butyl-eta4-1,3-diphosphacyclobutadiene tin has been determined in the gas phase by electron diffraction using both the DYNAMITE and SARACEN methods. The suitability of many different theoretical methods for the calculation of structures of half-sandwich main-group metal complexes has been investigated, and, by comparison of the results with the experimental structures, suggestions have been made as to the most suitable methods for this class of compound.

Synthesis of a eta 2-2,3-diphosphabutadiene complex of zerovalent platinum from the corresponding eta 2-phosphaalkyne complex.

Hydrozirconation of the eta 2-phosphaalkyne complex [Pt(dppe)(eta 2-tBuCP)] with [ZrHCl(eta 5-C5H5)2], followed by treatment with the chlorophosphaalkene ClP=C(SiMe3)2 affords the eta 2-2,3-diphosphabutadiene complex [Pt(dppe)(eta 2-tBuC(H)=PP=C(SiMe3)2]. In the presence of [Pt(PPh3)2] the latter undergoes an addition reaction with water to afford the structurally characterised Pt(II) complex [Pt(dppe)(tBuCH2P(O)HPC(SiMe3)2].

Hexaphosphapentaprismane: a new gateway to organophosphorus cage compound chemistry.

Several independent synthetic routes are described leading to the formation of a novel unsaturated tetracyclic phosphorus carbon cage compound tBu4C4P6 (1), which undergoes a light-induced valence isomerization to produce the first hexaphosphapentaprismane cage tBu4C4P6 (2). A second unsaturated isomer tBu4C4P6 (9) of 1 and the bis-[W(CO)5] complex 13 of 1 are stable towards similar isomerization reactions. Another starting material for the synthesis of the hexaphosphapentaprismane cage tBu4C4P6 (2) is the trimeric mercury complex [(tBu4C4P6)Hg]3 (11), which undergoes elimination of mercury to afford the title compound 2. Single-crystal X-ray structural determinations have been carried out on compounds 1, 2, 9, 11, and 13.

The First Delocalized Phosphole Containing a Planar Tricoordinate Phosphorus Atom: 1-[Bis(trimethylsilyl)methyl]-3,5-bis(trimethylsilyl)-1,2,4-triphosphole.

The sterically demanding groups on the tricoordinate phosphorus atom, the π-electron acceptors substituted on the ring, and the dicoordinate phosphorus atoms within the ring are the most significant factors contributing to the planarity and aromaticity of the 1,2,4-triphosphole ring in 1. The Bird aromaticity index for 1 shows that it has the most pronounced aromatic character of all known phospholes.