Stereoinversion of tetrahedral p-block element hydrides

The potential energy surfaces of 15 tetrahedral p-block element hydrides were screened on the multireference level. It was addressed whether stereoinversion competes against other reactions, such as reductive H₂-elimination or hydride loss, and if so, along which pathway the stereomutation occurs. Importantly, stereoinversion transition structures for the ammonium cation (C_4v) and the tetrahydridoborate anion (C_s) were identified for the first time. Revisiting methane's C_s symmetric inversion transition structure with the mHEAT+ protocol revealed an activation enthalpy for stereoinversion, in contrast to all earlier studies, which is 5 kJ mol^-1 below the C-H bond dissociation enthalpy. Square planar structures were identified lowest in energy only for the inversion of AlH₄ ^-, but a novel stepwise C_s-inversion was discovered for SiH₄ or PH₄ ⁺. Overall, the present contribution delineates essentials of the potential energy surfaces of p-block element hydrides, while structure-energy relations offer design principles for the synthetically emerging field of structurally constrained compounds.

Single-Site and Cooperative Bond Activation Reactions with Ylide-Functionalized Tetrylenes: A Computational Study

Due to their transition metal-like behavior divalent group 14 compounds bear huge potential for their application in bond activation reactions and catalysis. Here we report on detailed computational studies on the use of ylide-substituted tetrylenes in the activation of dihydrogen and phenol. A series of acyclic and cyclic ylidyltetrylenes featuring various α-substituents with different σ- and π-donating capabilities have been investigated which demonstrate that particularly π-accepting boryl groups lead to beneficial properties and low barriers for single-site activation reactions, above all in the case of silylenes. In contrast, for the thermodynamically more stable germylenes and stannylenes an alternative mechanism involving the active participation of the ylide ligand in the E-H bond (E=H or PhO) activation process by addition across the element carbon linkage was found to be energetically favored. Furthermore, the boryl substituted tetrylenes allowed for a further activation pathway involving the active participation of the boron element bond. These cooperative mechanisms are especially attractive for the heavier cyclic ylidyltetrylenes in which the loss of the protonated ylide group is prevented due to the cyclic framework. Overall, the present studies suggest that cyclic ylide-substituted germylenes and stannylenes bear huge potential for cooperative bond activations at mild conditions which should be experimentally addressed in the future.

Activation of Protic, Hydridic and Apolar E-H Bonds by a Boryl-Substituted GeII Cation

The synthesis of a boryl-substituted germanium(II) cation, [Ge{B(NDippCH)₂ }(IPrMe)]⁺ , (Dipp=2,6-diisopropylphenyl) featuring a supporting N-heterocyclic carbene (NHC) donor, has been explored through chloride abstraction from the corresponding (boryl)(NHC)GeCl precursor. Crystallographic studies in the solid state and UV/Vis spectra in fluorobenzene solution show that this species dimerizes under such conditions to give [(IPrMe){(HCNDipp)₂ B}Ge=Ge{B(NDippCH)₂ }(IPrMe)]²⁺ (IPrMe = 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene), which can be viewed as an imidazolium-functionalized digermene. The dimer is cleaved in the presence of donor solvents such as [D₈ ]thf or [D₅ ]pyridine, to give monomeric adducts of the type [Ge{B(NDippCH)₂ }(IPrMe)(L)]⁺ . In the case of the thf adduct, the additional donor is shown to be sufficiently labile that it can act as a convenient in situ source of the monomeric complex [Ge{B(NDippCH)₂ }(IPrMe)]⁺ for oxidative bond-activation chemistry. Thus, [Ge{B(NDippCH)₂ }(IPrMe)(thf)]⁺ reacts with silanes and dihydrogen, leading to the formation of Ge^IV products, whereas the cleavage of the N-H bond in ammonia ultimately yields products containing C-H and B-N bonds. The facile reactivity observed in E-H bond activation is in line with the very small calculated HOMO-LUMO gap (132 kJ mol^-1 ).

Excited-State Photolytic Mechanism of Cyclopentene Containing a Group 14 Element: An MP2-CAS//CASSCF Study

The potential energy surfaces corresponding to the photolytic reactions of 1,2-dimethyl-cyclopentene, 3,4-dimethyl-silacyclopent-3-ene, and 3,4-dimethyl-germacyclopent-3-ene were investigated by employing the CAS(6,6)/6-311G(d) and MP2-CAS-(6,6)/6-311++G(3df,3pd)//CAS(6,6)/6-311G(d) methods. Also, six kinds of substituted germacyclopent-3-ene were used as model reactants by way of the CASSCF and MP2-CAS methods to study their photolytic mechanisms. The theoretical findings indicate that the photolysis of the above reactants all adopt the same reaction path as follows: reactant → Franck-Condon region → conical intersection → germylene and 1,3-butadiene. However, the theoretical results demonstrate that no photolysis ((1)(π →π*)) can be observed in the 1,2-dimethyl-cyclopentene system. Above all, the theoretical investigations strongly suggest that both steric effects, originating from the bulky substituents, and the atomic radius of the group 14 element (C, Si, and Ge) play a crucial role in determining the cis/trans selectivity of the conformation of 1,3-butadiene during their photolytic reactions.