The potential energy surfaces for the insertion reactions of germylene into XH n molecules have been characterized in detail using ab initio molecular orbital theory and density functional theory. The model system Ge(CH3)2 + XH n (X = C, N, O, F, Si, P, S, and Cl; n = 1−4) has been chosen for the present study. All the interactions involve the initial formation of a donor−acceptor complex, followed by a high-energy transition state, and then an insertion product. The agreement between MP2 and B3LYP results indicates that the latter provides an adequate theoretical level for further investigations of molecular geometries, electronic structures, and kinetic features of the germylene reactions. The following conclusions emerge from this work:  (i) the X−H insertion reactions of germylene occur in a concerted manner via a three-membered-ring transition state, and that the stereochemistry at the heteroatom X center is preserved; (ii) the stabilization energies of the germylene−XH n complexes increase in the order NH3 > H2O > PH3 > H2S ∼ HF > HCl ≫ SiH4 ∼ CH4; (iii) the order of reactivity for X−H bonds toward germylene insertion is Cl > F > S > O > P > N ≫ Si > C. In other words, the greater the atomic number of heteroatom (X) in a given row, the easier the insertion reaction of XH n hydrides and the larger the exothermicity. Moreover, the present study demonstrates that both electronic and steric effects play a major role in the course of insertion reactions of germylene into X−H bonds. This work also indicates that the chemical behavior of germylene should be more similar to that of silylene than to that of carbene species.

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Singlet O2 is a key reactive oxygen species responsible for photodynamic therapy and is generally recognized to be chemically reactive towards C=C double bonds. Herein, we report the hydroperoxidation/lactonization of α-ethereal C–H bonds by singlet O2 (1Δg) under exceptionally mild conditions, i.e., room temperature and ambient pressure, with modest to high yields (38~90%) and excellent site selectivity. Singlet O2 has been known for > 90 years, but was never reported to be able to react with weakly activated C–H bonds in saturated hydrocarbons. Theoretical calculations indicate that singlet O2 directly inserts into the α-ethereal C–H bond in one step with conservation of steric configuration in products. The current discovery of chemical reaction of singlet oxygen with weakly activated solvent C–H bonds, in addition to physical relaxation pathway, provides an important clue to a 35-year-old unresolved mystery regarding huge variations of solvent dependent lifetime of singlet O2.

This study describes the first use of a silicon­(II) complex, NHC-parent silyliumylidene cation complex [(IMe)2SiH]­I (1, IMe = :C­{N­(Me)­C­(Me)}2) as a versatile catalyst in organic synthesis. Complex 1 (loading: 10 mol %) was shown to act as an efficient catalyst (reaction time: 0.08 h, yield: 94%, TOF = 113.2 h–1; reaction time: 0.17 h, yield: 98%, TOF = 58.7 h–1) for the selective reduction of CO2 with pinacolborane (HBpin) to form the primarily reduced formoxyborane [pinBOC­(O)­H]. The activity is better than the currently available base-metal catalysts used for this reaction. It also catalyzed the chemo- and regioselective hydroboration of carbonyl compounds and pyridine derivatives to form borate esters and N-boryl-1,4-dihydropyridine derivatives with quantitative conversions, respectively. Mechanistic studies show that the silicon­(II) center in complex 1 activated the substrates and then mediated the catalytic hydroboration. In addition, complex 1 was slightly converted into the NHC-borylsilyliumylidene complex [(IMe)2SiBpin]I (3) in the catalysis, which was also able to mediate the catalytic hydroboration.

Complete geometry optimizations were carried out using density functional theory to study potential energy surfaces for the insertion of germylene into C−H bonds of methane. The GeXY + CH4 (GeXY = GeH2, GeCH2, GeH(CH3), Ge(CH3)2, GeHF, GeF2, GeHCl, GeCl2, GeHBr, and GeBr2) systems are the subject of the present study. All the stationary points were determined at the B3LYP/6-311G* level of theory. Our theoretical findings suggest that the computed structures of germylenes are in good agreement with the available experimental results, with the bond lengths and angles in agreement to within 0.04 Å and 1.0°, respectively. A configuration mixing model based on the theory of Pross and Shaik has been used to develop an explanation for the barrier height as well as the reaction enthalpy. Our theoretical findings suggest that the singlet−triplet splitting (ΔE st = E triplet − E singlet) of the GeXY species can be used as a guide to predict its activity for insertion reactions. Thus, the major conclusion that can be drawn from this work is as follows:  the more strongly π-accepting, the bulkier, or the more electropositive the substituents, the smaller the ΔE st of GeXY, the lower the activation energy, and the larger the exothermicity for the insertion of GeXY into saturated C−H bonds. In other words, it is the electronic factors, rather than steric factors, that play a decisive role in determining the chemical reactivity of the germylene species.

The potential energy surfaces corresponding to the reactions of heavy carbenes with various molecules were investigated by employing computations at the B3LYP and CCSD(T) levels of theory. To understand the origin of barrier heights and reactivities, the model system (CH3)2X+Y (X=C, Si, Ge, Sn, and Pb; Y=CH4, SiH4, GeH4, CH3OH, C2H6, C2H4, and C2H2) was chosen for the present study. All reactions involve initial formation of a precursor complex, followed by a high-energy transition state, and then a final product. My theoretical investigations suggest that the heavier the X center, the larger the activation barrier, and the less exothermic (or the more endothermic) the chemical reaction. In particular, the computational results show that (CH3)2Sn does not insert readily into C-H, Si-H, C-H, Ge-H, or C-C bonds. It is also unreactive towards C=C bonds, but is reactive towards C identical with C and O-H bonds. My theoretical findings are in good agreement with experimental observations. Furthermore, a configuration mixing model based on the work of Pross and Shaik is used to rationalize the computational results. It is demonstrated that the singlet-triplet splitting of a heavy carbene (CH3)2X plays a decisive role in determining its chemical reactivity. The results obtained allow a number of predictions to be made.

We have chosen six ML 2 complexes, with a systematic variation in the ligands and metals, to investigate oxidative additions as well as reductive eliminations by using the MP2/LANL2DZ and the MP4SDTQ//MP2/LANL2DZ levels of theory. A qualitative model based on the theory of Pross and Shaik (Su, M.-D. Inorg. Chem. 1995, 34, 3829) has been used to develop an explanation for the barrier heights. Considering the geometrical effect, the substituent effect, and the nature of the metal center, the following conclusions emerge: for 14-electron ML 2 complexes, a smaller L-M-L angle and a better electron-donating ligand as well as a heavier transition metal center (such as Pt) should be a potential model for the oxidative addition of saturated C-H bonds. Conversely, a linear structure and a better electron-withdrawing ligand as well as a lighter transition metal center (such as Pd) would be a good candidate for reductive coupling of C-H bonds. The results obtained are in good agreement with the available experimental results and permit a number of predictions to be made. S0020-1669(97)00320-0 CCC: $15.00

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