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The first example of a diphosphaborolediide, the benzo-fused [C 6 H 4 P 2 BPh] 2- (1 2- ), is prepared from ortho-bis(phosphino)benzene (C 6 H 4 {PH 2 }) 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 1 2- , which is further supported by preliminary coordination studies that demonstrate η 5 -coordination to a zerovalent molybdenum center, as observed crystallographically for the oligomeric [{Mo(CO) 3 (η 5 -1)}{μ-η 1 -Mo(CO) 3 (TMEDA)} 2 ] ⋅ [μ-Li(THF)][μ-Li(TMEDA)]., (© 2021 Wiley-VCH GmbH.)

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., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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.

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.

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 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., (© 1998 WILEY-VCH Verlag GmbH, Weinheim, Fed. Rep. of Germany.)