Stretching the P–C Bond. Variations on Carbenes and Phosphanes

Abstract

The stability and the structure of adducts formed between four substituted phosphanes (PX3, X:H, F, Cl, and NMe2) and 11 different carbenes have been investigated by DFT calculations. In most cases, the structure of the adducts depends strongly on the stability of the carbene itself, exhibiting a linear correlation with the increasing dissociation energy of the adduct. Carbenes of low stability form phosphorus ylides (F), which can be described as phosphane → carbene adducts supported with some back-bonding. The most stable carbenes, which have high energy lone pair, do not form stable F-type structures but carbene → phosphane adducts (E-type structure), utilizing the low-lying lowest unoccupied molecular orbital (LUMO) of the phosphane (with electronegative substituents), benefiting also from the carbene–pnictogen interaction. Especially noteworthy is the case of PCl3, which has an extremely low energy LUMO in its T-shaped form. Although this PCl3 structure is a transition state of rather high energy, the large stabilization energy of the complex makes this carbene–phosphane adduct stable. Most interestingly, in case of carbenes with medium stability both F- and E-type structures could be optimized, giving rise to bond-stretch isomerism. Likewise, for phosphorus ylides (F), the stability of the adducts G formed from carbenes with hypovalent phosphorus (PXphosphinidene) is in a linear relationship with the stabilization of the carbene. Adducts of carbenes with hypervalent phosphorus (PX5) are the most stable when X is electronegative, and the carbene is highly nucleophilic.

Bond Dissociation Energies of Carbene–Carbene and Carbene–Main Group Adducts

“…The geometry of this PF 3 adduct could also be regarded as a monosolvated transition state for the edge inversion of trigonal pyramidal PF 3 with an inversion barrier of ∼53 kcal/mol (though the short, strong, nature of the C−P interaction perhaps makes "solvation" a bit of a misnomer). 52,53 Our results for the stabilizing effect of carbenes A−D on the central T-shaped PF 3 unit are consistent with prior DFT calculations at the M06-2x/ cc-pVYZ level on the stabilization of this transition state by carbenes B and D. 54 The accessible LUMO of the T-shaped PF 3 is of "p" type in character perpendicular to the PF 3 plane, which readily accepts the σ lone pair of carbenes with strong nucleophilic characters like A−D. In carbenes A and C, the PF 3 binds in plane with the heterocyclic ring.…”

Bending Ferrocenes with Low Coordinated Bridging Units: The Investigation of Carbenes and Their Analogues with a Ferrocenophane Backbone

“…Since the bond angle has significant impact on the singlet–triplet gap (the triplet state is favored at wide angles) and also on the reactivity of carbenes, this feature suggests that the properties of ferrocenophanes 1 correspond better to 4 than to the five-membered rings. Similar conclusions had been drawn before to explain the stability of carbene–phosphine adducts, the increased electrophilicity of 1 NR–P systems compared to 2 NR–P systems, and the varying organocatalytic activity of these systems in the benzoin condensation …”

“…Since the bond angle has significant impact on the singlet−triplet gap (the triplet state is favored at wide angles) and also on the reactivity of carbenes, this feature suggests that the properties of ferrocenophanes 1 correspond better to 4 than to the five-membered rings. Similar conclusions had been drawn before to explain the stability of carbene−phosphine adducts, 92 the increased electrophilicity of 1 NR−P systems compared to 2 NR−P systems, 55 and the varying organocatalytic activity of these systems in the benzoin condensation. 93 Having evaluated the structural aspects of the ferrocenophanes, we also analyzed their electronic structure to establish whether there is a noticeable interaction of the ferrocene moiety with the ansa unit or, in particular, with the low coordinated center.…”

Tweaking the Charge Transfer: Bonding Analysis of Bismuth(III) Complexes with a Flexidentate Phosphane Ligand

“…12 In the past few years, the potential of PnB in structural assembly, supramolecular architecture, anion sensing, (organo)catalysis, and molecular recognition has also been highlighted. 7 , 13 − 21 On the basis of thorough computational studies, 6 , 9 , 22 − 41 pnictogen bonding is chiefly electrostatic in nature (attraction between the oppositely charged regions around the two centers). Moreover, charge transfer effects (donation from the lone pair of the Lewis base into the σ*-antibonding orbitals at the pnictogen center) may also contribute, though to a much lesser extent.…”

“…A relatively new congener of noncovalent interactions is the pnictogen bond (PnB), − which (in analogy with the IUPAC definition of a halogen bond) can be defined as an attractive interaction between the electron-deficient region of a pnictogen (group 15 element) called a pnictogen bond donor and a Lewis base (pnictogen bond acceptor, acting as an electron pair donor) . In the past few years, the potential of PnB in structural assembly, supramolecular architecture, anion sensing, (organo)­catalysis, and molecular recognition has also been highlighted. ,− On the basis of thorough computational studies, ,,− pnictogen bonding is chiefly electrostatic in nature (attraction between the oppositely charged regions around the two centers). Moreover, charge transfer effects (donation from the lone pair of the Lewis base into the σ*-antibonding orbitals at the pnictogen center) may also contribute, though to a much lesser extent.…”