Electronic Structure and Bonding in Neutral and Dianionic Boradiphospholes: R′BC₂P₂R₂ (R=H, tBu, R′=H, Ph)

Abstract

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.

Structure and Bonding in Stannadiphospholes and their Dianions SnC2P2R2m (R=H, tBu m=0, −2): A Comparative Study with C5H5+ and C5H5− Analogues

“…No minima structures with the trigonal bipyramidal or planar tetracoordinated geometries typified by BC 2 P 2 H 3 were found. [19] The structures are listed (Figures 1 and 2, below) in increasing order of energy, with neutral structures being suffixed with "n" and dianionic species with "d". We discuss the isomers that are 40 kcal mol À1 (with respect to the H system) for the neutral and dianionic species to focus more on the comparative study with other isolobally related (isoelectronic) analogues.…”

“…[12,17] We and others have recently explored how potential energy surface (PESs) vary when a CH group is replaced by a P atom or a BH group is substituted by CH + or P + . [18,19] A comparison of the PESs of the series of cations C n H n P 5Àn + , as well as the neutral and dianionic systems R'BC 2 P 2 R 2 (R = H, tBu, R' = H, Ph), with both C 5 H 5 + and C 5 H 5 À established that 1) increasing the number of phosphorus atoms in the C n H n P 5Àn + series favours cluster geometry as the ground state, [18] 2) in BC 2 P 2 H 3 compounds there is competition between p-delocalised vinylcyclopropenyl and cluster-type structures (nido and tricyclopentane, respectively) and 3) in PhBtBu 2 C 2 P 2 the steric bulk of the tBu groups increases the energies of the two low-lying isomers (vinylcyclopropenyl or nido) over the isomer with tricyclopentane geometry. [19] There is a close relationship between the 6p-stabilized stannadiphospholide dianion SnC 2 R 2 P 2 2À , C 5 H 5 À , the neutral stannadiphospholes and C 5 H 5 + .…”

“…[18,19] A comparison of the PESs of the series of cations C n H n P 5Àn + , as well as the neutral and dianionic systems R'BC 2 P 2 R 2 (R = H, tBu, R' = H, Ph), with both C 5 H 5 + and C 5 H 5 À established that 1) increasing the number of phosphorus atoms in the C n H n P 5Àn + series favours cluster geometry as the ground state, [18] 2) in BC 2 P 2 H 3 compounds there is competition between p-delocalised vinylcyclopropenyl and cluster-type structures (nido and tricyclopentane, respectively) and 3) in PhBtBu 2 C 2 P 2 the steric bulk of the tBu groups increases the energies of the two low-lying isomers (vinylcyclopropenyl or nido) over the isomer with tricyclopentane geometry. [19] There is a close relationship between the 6p-stabilized stannadiphospholide dianion SnC 2 R 2 P 2 2À , C 5 H 5 À , the neutral stannadiphospholes and C 5 H 5 + . In this paper we explore the potential energy surfaces of both neutral and dianionic stannadiphospholes to provide a better understanding of the isolobal connections between Sn, P + , BR and CR + (Scheme 2), which might find application in the design of new metal complexes structurally related to those known for the ubiquitous cyclopentadienyl ring anion.…”

“…In the case of the dianionic isomers we observe only a 6p-electron aromatic structure as the global minimum, similarly to the cases of our previously reported results with other types of heterodiphospholes. [1,4,19] The existence of large numbers of cluster-type isomers in neutral and 6p-planar structures in the dianions SnC 2 P 2 R 2 2À (R = H, tBu) is due to 3D aromaticity in neutral clusters and to 2D p 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.…”

“…Experimental work in this area is also ongoing. [35] We followed our previous approach [19] in establishing all possible isomers for neutral and dianionic SnC 2 P 2 R 2 (R = H or tBu) structures. Both PESs were explored thoroughly by optimisation of all possible stable skeletons of three-, fourand five-membered rings, as well as acyclic systems.…”