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Title:Low-temperature protonation studies of an electron-poor osmium methyl complex, and molecular precursors for the construction of graphene nanostructues with atomically precise edge structures
Author(s):Sempsrott, Peter J.
Director of Research:Girolami, Gregory S.
Doctoral Committee Chair(s):Girolami, Gregory S.
Doctoral Committee Member(s):Lyding, Joseph W.; Murphy, Catherine J.; Rauchfuss, Thomas B.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Methane activation
osmium methyl complex
NMR spectroscopy
graphene nanostructures
Abstract:Treatment of Cl2PCH2Cl2 with CF3SiMe3 in the presence of CsF promotor gave the cesium salt Cs[((CF3)2P)2CH], which was converted to the electron-poor diphosphine (CF3)2PCH2P(CF3)2 (dfmpm) by addition of ethereal hydrogen chloride. Treatment of (C5Me5)2Os2Br4 with excess dfmpm in the presence of zinc dust in refluxing ethanol gave the piano stool osmium complex (C5Me5)Os(dfmpm)Br, which was converted to (C5Me5)Os(dfmpm)Me in excellent yield by treatment with excess ZnMe2 in refluxing toluene. The analogous ethyl complex (C5Me5)Os(dfmpm)Et can also be prepared in a similar fashion, but with simultaneous formation of the hydrides (C5Me5)Os(dfmpm)H and (C5Me5)Os(kappa-1-dfmpm)(C2H4)H by beta-hydrogen elimination. Protonation of (C5Me5)Os(dfmpm)Me with (CF3SO2)2NH at –110 °C in CDCl2F yielded the methane coordination complex [(C5Me5)Os(dfmpm)CH4][N(SO2CF3)2]. The analogous 13CH4 complex exhibited a binomial quintet in the 13C NMR with a coupling constant (127 Hz) almost identical to that of free methane (125 Hz). Kinetic analysis revealed that the enthalpy and entropy of activation for methane loss was ΔH‡ = 14.9 ± 1.5 kcal mol-1 and ΔS‡ = 12.3 ± 8.8 cal mol-1 K-1, respectively, and the barrier for methane loss was ΔG‡ = 12.8 ± 0.1 kcal/mol at –100 °C, which is in good agreement with previous DFT calculations. Treatment of (C5Me5)Os(dfmpm)13CH3 and (C5Me5)Os(dfmpm)13CHD2 with (CF3SO2)2NH and (CF3SO2)2ND at –110 °C in CDCl2F gave a mixture of the four methane complex isotopologs [(C5Me5)Os(dfmpm)13CH4][N(SO2CF3)2], [(C5Me5)Os(dfmpm)13CH3D][N(SO2CF3)2], [(C5Me5)Os(dfmpm)13CH2D2][N(SO2CF3)2], and [(C5Me5)Os(dfmpm)13CHD3][N(SO2CF3)2] in one NMR tube, which permitted an isotopic perturbation of resonance (IPR) analysis. The IPR data in conjunction with the DFT analysis indicated that methane is coordinated to osmium in a kappa-1 fashion. A reinvestigation of the protonation of the ruthenium complex (C5Me5)Os(dmpm)Me resulted in rapid generation of free methane, even at low temperature. Several oligophenylene derivatives were investigated as possible precursors for the surface-assisted synthesis of various graphene structures, including graphene sheets, graphene sheets with a regular pattern of holes, and graphene nanoribbons (GNRs). Criteria were specified for determining the likelihood that oligophenylene precursors would form the intended structures. Through this approach, 2,6-bis(2,5-dibromophenyl)biphenyl and 1,4-bis(2,6-dibromophenyl)benzene were identified as attractive candidates for the surface-assisted construction of graphene structures. Solution-based iterative cross-coupling (ICC) of anthracene derivatives was also investigated as an alternative strategy for the construction of GNRs. Synthetic routes and general fabrication methods that could lead to the construction of GNR-based electronic structures were suggested. In initial attempts to synthesize the identified GNR precursor candidates, Suzuki coupling of 2,2′-diiodobiphenyl with (2,5-dibromophenyl)boronic acid failed to yield the desired 2,6-bis(2,5-dibromophenyl)biphenyl product. On the other hand, 1,4-bis(2,6-dibromophenyl)benzene was successfully prepared, although in low yield, by Suzuki coupling of 1,4-phenylenediboronic acid with 1,3-dibromo-2-iodobenzene; the latter compound was synthesized by treatment of 1,3-dibromobenzene with LDA, followed by addition of iodine.
Issue Date:2018-11-28
Rights Information:Copyright 2018 Peter J. Sempsrott
Date Available in IDEALS:2019-02-07
Date Deposited:2018-12

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