Substituent Effects on the Reactions of Diarylgermylenes and Tetraaryldigermenes with Acetic Acid and Other Lewis Bases in Hydrocarbon Solution

Germanium Complexes with ONO Tridentate Ligands: O-H Bond Activation Control According to DFT Calculations

Polydentate ligands are used for thermodynamic stabilization of tetrylenes-low-valent derivatives of Group 14 elements (E = Si, Ge, Sn, Pb). This work shows by DFT calculations how the structure (the presence or absence of substituents) and type (alcoholic, Alk, or phenolic, Ar) of tridentate ligands 2,6-pyridinobis(1,2-ethanols) [^AlkONO^R]H₂ and 2,6-pyridinobis(1,2-phenols) [^ArONO^R]H₂ (R = H, Me) may affect the reactivity or stabilization of tetrylene, indicating the unprecedented behavior of Main Group elements. This enables the unique control of the type of the occurring reaction. We found that unhindered [ONO^H]H₂ ligands predominantly led to hypercoordinated bis-liganded {[ONO^H]}₂Ge complexes, where an E(+2) intermediate was inserted into the ArO-H bond with subsequent H₂ evolution. In contrast, substituted [ONO^Me]H₂ ligands gave [ONO^Me]Ge: germylenes, which may be regarded as kinetic stabilized products; their transformation into E(+4) species is also thermodynamically favorable. The latter reaction is more probable for phenolic [^ArONO]H₂ ligands than for alcoholic [^AlkONO]H₂. The thermodynamics and possible intermediates of the reactions were also investigated.

NH bond activation of ammonia and amines by ditetrelenes: key insights into the stereochemistry of nucleophilic addition

The NH bond activation of ammonia, primary and secondary amines by tetramesityldisilene and -digermene was investigated. In each case, a disilyl- or digermylamine was formed as the only product of amine addition. The mechanism of the addition of ammonia to tetramesityldisilene was computed and revealed a three-step reaction pathway: formation of the anti-ammonia-disilene adduct, inversion at the β-silicon, and syn-transfer of the proton to give the syn-product, where each step follows a distinct stereochemical course. Examination of the reaction landscape also revealed several additional insights: (a) that, in the initial step, the formation of the anti-oriented zwitterionic intermediate is kinetically more preferable than formation of the syn-oriented zwitterionic intermediate, (b) that intermolecular transfer of a proton is not energetically feasible in non-polar solvents, and (c) that the bulk of the substituents can have a profound effect on the stereochemical course of the reaction. With this detailed understanding, nucleophilic additions to ditetrelenes can be exploited in the future.

Fast kinetics studies of the Lewis acid-base complexation of transient stannylenes with σ- and π-donors in solution

The Lewis acid-base complexation chemistry of dimethyl- and diphenylstannylene (SnMe2 and SnPh2, respectively) in hexanes solution has been studied by laser photolysis methods. Complexation of the two stannylenes with a series of nine O-, S-, and N-donors (including cyclic and acyclic dialkyl ethers and sulfides, a primary, secondary, and tertiary amine, ethyl acetate and acetone), two alkenes, and an alkyne proceeds rapidly and reversibly to generate the corresponding stannylene-donor Lewis pairs, which have been detected in each case by time-resolved UV-vis absorption spectroscopy. The complexes exhibit UV-vis absorption maxima in the range of λmax ∼ 310-405 nm depending on the donor and substitution at tin. Bimolecular rate constants for complexation (kC), which could be determined for 14 of the 24 Lewis pairs that were studied, were found to fall within a factor of four of the diffusional limit in all cases, with SnMe2 showing consistently higher reactivity than SnPh2. Equilibrium constants for complexation (KC) could be measured for all but one of the stannylene-π- and O-donor pairs, the values corresponding to (gas phase) binding free energies in the range of +1.1 to -3.9 kcal mol-1. Comparison of the experimental equilibrium constants for complexation of SnMe2 and SnPh2 with methanol and diethyl ether to those measured previously for the homologous silylenes and germylenes indicates that Lewis acidity decreases in the order SiR2 > SnR2 > GeR2 for both the dimethyl- and diphenyltetrylene series, the diphenyl derivatives exhibiting significantly stronger Lewis acidity in all three cases. The experimental trends in the binding energies and UV-vis spectra of the complexes are reproduced well by density functional theory (DFT) calculations, which have been carried out at the (TD)ωB97XD/def2-TZVP level of theory. The experimental data also show evidence of a reaction between tetramethyldistannene (Me2Sn[double bond, length as m-dash]SnMe2, 4a) and amine donors, which is suggested to afford the corresponding amine-stabilized stannylidenestannylene structure. The mechanistic proposal is supported by DFT calculations of the complexation of 4a and SnMe2 with model O-, S- and N-donors.

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.

A combined kinetic and computational study of the reactions of transient germylenes with oxiranes and thiiranes in solution

The reactions of dimethyl- and diphenylgermylene (GeMe2 and GePh2, respectively) with cyclohexene oxide (CHO) and propylene sulfide (PrS) have been studied in hydrocarbon solvents at 25 °C by laser flash and steady-state photolysis methods using appropriately substituted germacyclopent-3-ene derivatives as germylene precursors. GeMe2 reacts with CHO and PrS with rate constants in the range of 1.2–1.7 × 1010 M−1 s−1 in hexanes at 25 °C to form new transient products that are assigned to the corresponding Lewis acid-base complexes of the germylene with the substrates. The complexation reactions were found to be reversible and are characterized by equilibrium constants of KC = (3.7 ± 0.8) × 103 M−1 and (3 ± 1) × 104 M−1 for complexation of GeMe2 with CHO and PrS, respectively. The complexes decay over approximately 10 μs with the concomitant formation of tetramethyldigermene (Ge2Me4), identifiable by its characteristic UV-vis spectrum centered at λmax = 370 nm. Diphenylgermylene behaves analogously, reacting rapidly and reversibly with the two substrates to form the corresponding Lewis acid-base complexes (λmax ≈ 355 nm) that decay over several tens of microseconds with the concomitant growth of the characteristic UV-vis spectrum of tetraphenyldigermene (Ge2Ph4) (λmax = 440 nm). Steady-state photolysis of the germylene precursors in the presence of CHO afforded germanium-containing oligomers but showed no evidence of oxygen abstraction or the formation of substrate-derived product(s). Similar photolyses in the presence of PrS also afforded germanium-containing oligomers, but as well yielded propene in 20%–30% yield and (in the case of the GePh2 precursor) minor amounts of low molecular weight compounds that appear to be derived from the corresponding germanethione. Density functional theory calculations of the chalcogen abstraction reactions of GeMe2 with oxirane and thiirane in the gas phase have been carried out at the B3LYP/6-311+G(d,p) level of theory and are in good qualitative agreement with the experimental data.

Synthesis, structure, and properties of the hexagermane Pri3Ge(GePh2)4GePri3

The synthesis of the hexagermane Pri3Ge(GePh2)4GePri3 was achieved starting from the cyclotetragermane (Ph2Ge)4. Ring-opening of (Ph2Ge)4 with Br2 yielded Br(GePh2)4Br that was converted to H(GePh2)4H, and this material was treated with two equiv. of Pri3GeNMe2 to furnish Pri3Ge(GePh2)4GePri3 via the hydrogermolysis reaction. The X-ray crystal structures of (Ph2Ge)4, Br(GePh2)4Br and Pri3Ge(GePh2)4GePri3 were determined. The hexagermane Pri3Ge(GePh2)4GePri3 represents the longest structurally characterized linear oligogermane reported to date and exhibits physical properties that resemble those of the larger polygermane systems. The hexagermane is luminescent and interacts with polarized light, appearing pale yellow under one orientation of polarized light and deep blue under the opposite orientation. The electrochemistry of Pri3Ge(GePh2)4GePri3 was also explored, and this species exhibits the expected five irreversible oxidation waves.

Kinetic and mechanistic studies of the reactions of diarylgermylenes and tetraaryldigermenes with carbon tetrachloride

The mechanisms of the reactions of diphenylgermylene (GePh2) with CCl4 in hydrocarbon solvents and in THF solution have been studied, employing 3,4-dimethyl-1,1-diphenylgermacyclopent-3-ene (6a) and 1,1-diphenylgermacyclobutane (17) as photochemical precursors to GePh2. In hydrocarbon solvents, the reaction produces Ph2GeCl2 (10) and Ph2Ge(Cl)CCl3 (12) in a ratio of 10:12 ≈ 7, along with a variety of other radical-derived products and small amounts of Ph2GeH(D)Cl (11), which is formed partly by reaction of GePh2 with adventitious HCl. The reaction is much cleaner in THF, where 12 is formed as the major product (10:12 ≈ 0.8); a similar product distribution is obtained in hexanes containing 0.05 mol/L THF, while 12 is the exclusive product in hexanes containing 3 mmol/L NEt3. Rate constants for the reactions of CCl4 with GePh2 and five ring-substituted derivatives were determined by laser flash photolysis, as well as Arrhenius parameters for reaction of the parent (GePh2), in the two solvents. The reactions of GePh2 with CCl4 and CHCl3 have also been studied in 3-methylpentane solution at 78–90 K. Different reaction mechanisms are clearly operative in hydrocarbon and complexing solvents, but both involve modest charge donation from germanium to the substrate in the transition state for the rate-determining step. For the reaction in hydrocarbon solvents, the data are consistent with inner-sphere electron transfer following or in concert with weak Lewis acid–base complexation. A similar mechanism is proposed for the reaction in THF solution, in competition with a second involving nucleophilic attack of the germylene–THF complex at a chlorine atom of the substrate. Rate constants were also determined for reaction of CCl4 with the corresponding tetraaryldigermenes at low halocarbon concentrations in hexanes, along with Arrhenius parameters for the parent (Ge2Ph4). These reactions also proceed via initial Cl-atom abstraction, based on the identity of the products formed in the reaction of CCl4 with tetramesityldigermene.