Steric Effects in the Reactivity and Bonding of Low Coordinate Germanium and Tin Species

This dissertation describes the reactivity a series of terphenyl-supported heavier group 14 germylenes and stannylenes toward fundamentally important small molecules. The germylenes and stannylenesthat are used in this thesis are Ge(ArMe6)2, Ge(AriPr4)2, Ge(AriPr6)2, Sn(ArMe6)2, Sn(AriPr4)2, and Ge(AriPr6)2 (ArMe6 = C6H3-2,6(C6H2-2,4,6-Me3)2, AriPr4 = C6H3-2,6(C6H3-2,6-iPr2)2 and AriPr6 = C6H3-2,6(C6H2-2,4,6-iPr3)2). The solid-state structures of their reaction procuts were determined by single X-ray spectroscopy. Other characterization method includes nuclear magnetic resonance spectroscopy, UV-visible spectroscopy, infrared spectroscopy, melting point analysis, and electron paramagnetic spectroscopy. All operations were carried out under anaerobic and anhydrous conditions using modified Schlenk techniques. All solvents were dried over alumina columns, stored over a sodium or potassium mirror, and degassed before use. A summary of the previous investigations of the synthesis and reactivity of tetrylenes is described in Chapter 1. Chapter 2: The diarylstannylene, :Sn(AriPr4)2 (AriPr4 = C6H3-2,6-(C6H3-2,6- iPr 2)2), undergoes C-H metathesis with toluene, m-xylene or mesitylene in solutions of these solvents at 80 °C. The products afforded [AriPr4Sn(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, AriPr4H, was also produced in these reactions. The use of EPR spectroscopy indicated the presence of a new type of one-coordinate, tin-centered radical, ꞏSnAriPr4, resulting from Sn-C bond cleavage in :Sn(AriPr4)2. Chapter 3: It is shown that the tin-tin triple bond in the distannyne AriPr4SnSnAriPr4 (AriPr4 = C6H3-2,6-(C6H3-2,6- iPr 2)2), undergoes reversible cleavage in deuterated toluene to afford two ꞏSnAriPr4 radicals as shown by 1H NMR and EPR spectroscopy. Variable temperature 1H NMR data afforded an enthalpy of dissociation of ΔHdiss=17.2±1.7 kcal mol-1 via van‘t Hoff analysis. The EPR and 1H NMR data indicated that the Sn-Sn bond in AriPr4SnSnAripr4 is weak and is consistent with the Sn-Sn bond being a charge-shift bond. Chapter 4: In this Chapter, the detection of tin and germanium radicals during the photolysis/thermolysis of diarylstannylene SnR2 and diarylgermylene GeR2 (R = AriPr4 = C6H3-2,6-(C6H3-2,6- iPr2)2 or AriPr6 = C6H3-2,6-(C6H2-2,4,6- iPr3)2 ), by using EPR spectroscopy complemented with theoretical calculations are described. The trapped tin radical is a one-coordinated S = 1/2 Sn(I) radical, i.e., : Sṅ R (R = AriPr4 or AriPr6), with g tensor values of [2.031, 1.980, 1.940] (giso= 1.984). In contrast, the trapped germanium radical is a pseudo-planar S = 1/2 Ge(III)-hydride species, i.e., ꞏGeHRR’ (R = AriPr4 or AriPr6 , R’ is a quaternary carbon), with g tensor values of [2.029, 2.003, 1.989] (giso = 2.007) and a strong 1H-hyperfine tensor [-23.0, -20.5, -31.5] MHz for the hydride. The generation of this Ge(III)-hydride could be due to the greater reactivity Ge(I) radical intermediate (: Gė R) and the greater strength of a Ge-H bond. The Ge(III)-hydride species arise from the insertion of the active Ge(I) radical intermediate in a C-H bond. This chapter provides insights into the radical mechanistic understanding of the heavier group 14 element tetrylenes chemistry. Chapter 5: It is shown that the diarylstannylenes, Sn(AriPr4)2 and Sn(AriPr6)2, (AriPr4 = C6H3- 2,6-(C6H3-2,6- iPr 2)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 these products were characterized via 1H , 13C and 119Sn NMR, UV-vis and IR spectroscopy, as well as by X-ray crystallography. Quantum mechanical calculations were performed by our collaborator Dr. Guo, and two potential mechanisms were identified. Chapter 6: The diarylgermylenes Ge(ArMe6)2 and Ge(AriPr4)2 (ArMe6=C6H3-2,6-(C6H3-2,4,6- Me3)2), AriPr4=C6H3-2,6-(C6H3-2,6-iPr2)2) were shown to react reversibly with ethylene. It is shown that the more sterically crowded Ge(AriPr4)2 also reversibly binds propylene. The germirane products Ar2GeCH2CHR) (Ar,R = ArMe6, H (1a), AriPr4, H (1b) and AriPr4, Me (1c) were characterized via 1H and 13C NMR spectroscopy and by X-ray crystallography in the case of 1b. Thermolysis of Ge(AriPr4)2 under ethylene resulted in Ge-C bond homolysis and the formation of the digermene [(AriPr4)Ge(Et)]2(1d). The thermodynamic parameters of the reactions were determined by variable temperature 1H NMR spectroscopy. Chapter 7: The diarylgermylene, Ge(ArMe6)2 (ArMe6 = C6H3-2,6-(C6H2-2,4,6-Me3)2), were shown to react reversibly with the four unstrained alkynes: 3-hexyne, diphenylacetylene, trimethylsilylacetylene and phenylacetylene at ambient temperature in toluene. The germirene products, (ArMe6)2GeC(R)=C(R’) (R, R’ = Et, Et (1a), Ph,Ph (1b), H, SiMe3 (1c) and H, Ph (1d) were characterized via 1H and 13C NMR spectroscopy and by X-ray crystallography in the case of 1a and 1d. The thermodynamic parameters of the reactions were determined by variable temperature 1H NMR spectroscopy and the experimental Gibbs free energies indicated their near thermoneutrality. Chapter 8: 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 a pre-catalyst 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, but at an increased rate relative to the less crowded {ArMe6Sn(μ-OMe)}2. Also presented is {ArMe6Sn(μ-NEt2) (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, gives insight into the mechanism of the catalysis. Chapter 9: The diarylgermylene Ge(ArMe6)2 (ArMe6=C6H3-2,6-(C6H2-2,4,6-Me3)2) was shown to react with azobenzene at room temperature to yield the Ge(IV) diamido products, (ArMe6)2Ge{N(H)(Ph)-o(Ph)(H)N}(1a) and (ArMe6)2Ge{N(H)-o-C6H4N(Ph)}(1b) which were characterized via 1H and 13C NMR spectroscopy and by X-ray crystallography. Treatment of Ge(ArMe6)2 with bulkier diazenes,1,2-bis(2,6-diethylphenl)diazene or 1,2-bis(1,3,5- trimetylphenyl)diazene, even at elevated temperature did not afford N-N bond cleavage and only C-H activation of the germylene was observed. Use of the analogous tin species Sn(ArMe6)2 under the same condition afforded no reactions