Reversible Sn-Sn Triple Bond Dissociation in a Distannyne: Support for Charge-Shift Bonding Character.

The tin-tin triple bond in the distannyne AriPr4SnSnAriPr4, AriPr4 = C6H3-2,6(C6H3-2,6-iPr2)2, undergoes reversible cleavage in deuterated toluene to afford two :SnAriPr4 radicals in solution as shown by 1H nuclear magnetic resonance and electron paramagnetic resonance spectroscopy. Variable temperature data afforded an enthalpy of dissociation of ΔHdiss = 17.2 ± 1.7 kcal mol-1 via van't Hoff analysis.

Germanium Hydride Radical Trapped during the Photolysis/Thermolysis of Diarylgermylene.

Herein, we report the characterization of a novel germanium hydride radical arising from the photolysis/thermolysis of the diarylgermylene GeR2 [R = terphenyl: AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)2 or AriPr6 = C6H3-2,6-(C6H2-2,4,6-iPr3)2] by using electron paramagnetic resonance spectroscopy complemented with theoretical calculations. The trapped germanium radical is a pseudoplanar S = 1/2 germanium(III) hydride, i.e., •GeHRR' (R = AriPr4 or AriPr6; R' is a quaternary carbon), with a g tensor of [2.029, 2.003, 1.990], a 73Ge hyperfine tensor of [-10, -90, -10] MHz, and a strong 1H hyperfine tensor of [-23.0, -20.5, -31.5] MHz for the hydride. The germanium(III) hydride radical is a result of the insertion of a germanium(I) radical intermediate (:GeR) in a C-H bond, due to the greater reactivity of the germanium(I) radical intermediate in comparison with the tin(I) counterpart that we trapped earlier.

Facile C-H Bond Metathesis Mediated by a Stannylene.

The diarylstannylene, :Sn(Ar iPr4)2 (Ar iPr4 = C6H3-2,6-(C6H3-2,6- iPr2)2), undergoes C-H metathesis with toluene, m-xylene, or mesitylene in solutions of these solvents at 80 °C. The products, [Ar iPr4Sn(CH2Ar)]2 (Aryl=C6H5 (1a), C6H4-3-Me (1b), C6H3-3,5-Me2(1c)) were characterized via 1H, 13C and 119Sn NMR, UV-vis and IR spectroscopy, and by X-ray crystallography for 1a and 1b. A stoichiometric amount of the arene, Ar iPr4H, was also produced in these reactions. The use of EPR spectroscopy indicated the presence of a new type of one-coordinate, tin-centered radical, :SnAr iPr4, resulting from Sn-C bond cleavage in Sn(Ar iPr4)2.

P-N Cooperative Borane Activation and Catalytic Hydroboration by a Distorted Phosphorous Triamide Platform.

Studies of the stoichiometric and catalytic reactivity of a geometrically constrained phosphorous triamide 1 with pinacolborane (HBpin) are reported. The addition of HBpin to phosphorous triamide 1 results in cleavage of the B-H bond of pinacolborane through addition across the electrophilic phosphorus and nucleophilic N-methylanilide sites in a cooperative fashion. The kinetics of this process of were investigated by NMR spectroscopy, with the determined overall second-order empirical rate law given by ν = -k[1][HBpin], where k = 4.76 × 10-5 M-1 s-1 at 25 °C. The B-H bond activation process produces P-hydrido-1,3,2-diazaphospholene intermediate 2, which exhibits hydridic reactivity capable of reacting with imines to give phosphorous triamide intermediates, as confirmed by independent synthesis. These phosphorous triamide intermediates are typically short lived, evolving with elimination of the N-borylamine product of imine hydroboration with regeneration of the deformed phosphorous triamide 1. The kinetics of this latter process are shown to be first-order, indicative of a unimolecular mechanism. Consequently, catalytic hydroboration of a variety of imine substrates can be realized with 1 as the catalyst and HBpin as the terminal reagent. A mechanistic proposal implicating a P-N cooperative mechanism for catalysis that incorporates the various independently verified stoichiometric steps is presented, and a comparison to related phosphorus-based systems is offered.

Reversible intermolecular E-H oxidative addition to a geometrically deformed and structurally dynamic phosphorous triamide.

The synthesis and reactivity of geometrically constrained tricoordinate phosphorus (σ(3)-P) compounds supported by tridentate triamide chelates (N[o-NR-C6H4]2(3-); R = Me or (i)Pr) are reported. Studies indicate that 2 (P{N[o-NMe-C6H4]2}) adopts a Cs-symmetric structure in the solid state. Variable-temperature NMR studies demonstrate a low-energy inversion at phosphorus in solution (ΔG(‡)(exptl)(298) = 10.7(5) kcal/mol), for which DFT calculations implicate an edge-inversion mechanism via a metastable C2-symmetric intermediate. In terms of reactivity, compound 2 exhibits poor nucleophilicity, but undergoes oxidative addition at ambient temperature of diverse O-H- and N-H-containing compounds (including alcohols, phenols, carboxylic acids, amines, and anilines). The resulting pentacoordinate adducts 2·[H][OR] and 2·[H][NHR] are characterized by multinuclear NMR spectroscopy and X-ray crystallography, and their structures (which span the pseudorotation coordinate between trigonal bipyramidal and square planar) are evaluated in terms of negative hyperconjugation. At elevated temperatures, the oxidative addition is shown to be reversible for volatile alcohols and amines.

Facile insertion of ethylene into a group 14 element-carbon bond: effects of the HOMO-LUMO energy gap on reactivity.

The diarylstannylenes, Sn(AriPr4)2 and Sn(AriPr6)2, (AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)2, AriPr6 = C6H3-2,6-(C6H2-2,4,6-iPr3)2), undergo a facile migratory insertion reaction with ethylene at 60 °C to afford the alkyl aryl stannylenes AriPr4SnCH2CH2AriPr4 and AriPr6SnCH2CH2AriPr6 which were characterized via1H, 13C and 119Sn NMR, UV-vis and IR spectroscopy, as well as by X-ray crystallography. Quantum mechanical calculations were performed, and two potential mechanisms were identified, with a migratory insertion reaction pathyway being energetically preferred.

Catalytic dehydrocoupling of amines and boranes by an incipient tin(ii) hydride.

The facile heterodehydrocoupling of a range of primary or secondary amines and even ammonia with pinacolborane (HBPin) was accomplished using {ArMe6Sn(µ-OMe)}2 (1, ArMe6 = C6H3-2,6-(C6H2-2,4,6-Me3)2) as pre-catalysts for a catalytically active tin(ii) hydride. The more sterically hindered pre-catalyst 2, {AriPr4Sn(µ-OMe)}2 (AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)2) facilitated the dehydrocoupling only of primary amines with HBPin, and at an increased rate relative to the less crowded {ArMe6Sn(µ-OMe)}2. Also presented is {ArMe6Sn(µ-NEt2)}2 (3), which can be converted into the structurally characterizable {ArMe6Sn(µ-NEt2)(µ-H)SnArMe6} (4) via the addition of pinacol borane. This, alongside stoichiometric studies, give insight into the mechanism of the catalysis.