Atoms to fibers: Identifying novel processing methods in the synthesis of pitch-based carbon fibers.

Understanding and optimizing the key mechanisms used in the synthesis of pitch-based carbon fibers (CFs) are challenging, because unlike polyacrylonitrile-based CFs, the feedstock for pitch-based CFs is chemically heterogeneous, resulting in complex fabrication leading to inconsistency in the final properties. In this work, we use molecular dynamics simulations to explore the processing and chemical phase space through a framework of CF models to identify their effects on elastic performance. The results are in excellent agreement with experiments. We find that density, followed by alignment, and functionality of the molecular constituents dictate the CF mechanical properties more strongly than their size and shape. Last, we propose a previously unexplored fabrication route for high-modulus CFs. Unlike graphitization, this results in increased sp3 fraction, achieved via generating high-density CFs. In addition, the high sp3 fraction leads to the fabrication of CFs with isometric compressive and tensile moduli, enabling their potential applications for compressive loading.

Multistep Fractionation of Coal and Application for Graphene Synthesis.

Despite its complex structure, coal has shown to be a promising precursor for graphene synthesis by chemical vapor deposition (CVD). However, the presence of heteroatoms and aliphatic chains in coal can lead to defects in the graphene lattice, preventing the formation of pristine graphene layers. Therefore, the goal of this study was to formulate a multistep coal fractionation scheme to extract and characterize the most aromatic fractions and explore their potential as graphene precursors. The scheme consisted of direct coal liquefaction under different conditions, Soxhlet extraction with heptane then toluene, and preparative liquid chromatography on silica gel using heptol solutions with different heptane/toluene ratios. The fractions obtained by this process were analyzed by proton nuclear magnetic resonance, thermogravimetric and elemental analyses, and automated SAR-AD (saturates, aromatics, resins-asphaltene determinator) separations. This characterization allowed the identification of two aromatic fractions with and without heteroatoms, which were subsequently used for graphene synthesis by CVD on nickel and copper foils. Raman spectrometry revealed that both fractions primarily formed defect-free multilayered graphene with approximately 11 layers on nickel due to the high solubility of carbon and the defect-healing effect of nickel. On the other hand, these fractions generated amorphous carbon on copper due to the high solubility of hydrogen in copper, which competed with carbon. Molecules in the more aromatic heteroatom-free fraction still contained alkyl pendant substituents and did not share the same planarity and symmetry to form defect-free graphene on copper. Thus, the quality of graphene was governed by the substrate on nickel and by the precursor quality on copper. When deposited directly on lacey carbon-coated copper grids of a transmission electron microscope, the heteroatom-free fraction gave rise to much larger graphene domains. The presence of heteroatoms promoted the formation of small self-assembled agglomerates of amorphous carbon.

Homoleptic Tris-Diphosphine Re(I) and Re(II) Complexes and Re(II) Photophysics and Photochemistry.

The ligand-to-metal charge transfer state (LMCT) of [(dmpe)3Re](2+) (dmpe = 1,2-bis(dimethylphosphino)ethane) has been demonstrated to be a potent oxidant (E(0)(Re(2+*)/Re(+)) = 2.61 V vs standard calomel electrode). This complex has been traditionally prepared by nontrivial routes in low yields, and very little has been achieved in optimizing the ground state and emission energy properties of the general class of complexes [(PP)3Re](2+) (PP = chelating diphosphine) through phosphine modification. Improved syntheses for Re(I) tris-homoleptic diphosphine complexes [(PP)3Re](+) (PP = 1,2-bis(dimethylphosphino)ethane (dmpe), 1,2-bis(diethylphosphino)ethane (depe), bis(dimethylphosphino)methane (dmpm), bis(diphenylphosphino)methane (dppm), Me2PCH2PPh2, 1,3-bis(dimethylphosphino)propane (dmpp), or 1,2-bis(dimethyl-phosphino)benzene (dmpb)) were achieved by single-pot reactions exploiting the reducing potential of the phosphines when reacted with Re(V) oxo-complexes in 1,2-dichlorobenzene at 160-180 °C. Single-electron chemical oxidation of [(PP)3Re](+) yields luminescent Re(II) analogues; appropriate use of Ph3C(+), Cp2Fe(+), or (4-BrC6H4)3N(+) B(C6F5)4(-) salts produced [(PP)3Re](2+) complexes in good yields. Crystallographic trends for the Re(+)/Re(2+) pairs show significantly lengthened Re(2+)-P bonds for [(PP)3Re](2+) relative to the corresponding [(PP)3Re](+) system. The redox and luminescence behavior of the complexes indicates the luminescence is from a ligand P(σ)-to-metal (Re(dπ)) charge transfer ((2)LMCT) state for all the complexes. Structured luminescence at 77 K is postulated to originate from relaxation of the (2)LMCT state into two spin-orbit coupled states: the ground state and a state ∼ 3000 cm(-1) above the ground state. The excited-state reduction potential (Re(II*/I)) for [(depe)3Re](2+) was determined from the free energy dependence of luminescence quenching rate constants. Yields for formation of charge separated ions were determined for three of the complexes with a variety of electron donors. Despite favorable electrostatics, no charge separated ions were observed for radical ion pairs for which the energy of back electron transfer exceeded 1.1 V.

Acceptor pincer Ru(II) chemistry.

A series of new acceptor pincer Ru(II) complexes are reported. The carbonyl complex ((CF(3))PCP)Ru(CO)Cl(2)(-)Et(3)NH(+) is obtained from the reaction of (CF(3))PCPH with [(cod)Ru(µ-Cl)(2)](n). Chloride displacement with (CF(3))PCPH, CO, PPh(3), or C(2)H(4) gave complexes of the type ((CF(3))PCP)Ru(CO)(L)Cl, or in the case of (CF(3))PCPH, the bridged dimeric complex [((CF(3))PCP)Ru(CO)Cl](2)(µ-(CF(3))PCPH). Chloride abstraction from ((CF(3))PCP)Ru(CO)(L)Cl, L = CO or PPh(3) by (Et(3)Si)(2)(µ-H)(+)B(C(6)F(5))(4)(-) followed by Et(3)N addition produced ((CF(3))PCP)Ru(CO)(L)(H) products. Reaction of cis-((CF(3))PCP)Ru(CO)(2)Cl with (Et(3)Si)(2)(µ-H)(+)B(C(6)F(5))(4)(-) in the presence of excess CO afforded ((CF(3))PCP)Ru(CO)(3)(+). The reaction of (CF(3))PCPH with (PPh(3))(3)Ru(H)(O(2)CR) (R = Me or Ph) produced the corresponding carboxylate complexes ((CF(3))PCP)Ru(PPh(3))(O(2)CR).

Acceptor (CF3)PCPH pincer reactivity with (PPh3)3Ir(CO)H.

The syntheses of Ir(I) and Ir(III) complexes incorporating the electron-withdrawing pincer ligand (1,3-C(6)H(4)(CH(2)P(CF(3))(2))(2)) ((CF(3))PCPH) with (PPh(3))(3)Ir(CO)H and subsequent chemistry are reported. Under ambient conditions, reaction of 1 equiv. (CF(3))PCPH with (PPh(3))(3)Ir(CO)H gave the mono-bridged complex [Ir(CO)(PPh(3))(2)(H)](2)(µ-(CF(3))PCPH) (1). Reaction of (PPh(3))(3)Ir(CO)H with excess (CF(3))PCPH and MeI gave the doubly-bridged complex [Ir(CO)(PPh(3))(H)](2)(µ-(CF(3))PCPH)(2) (2), whereas the tetrameric oligomer [Ir(CO)(PPh(3))(H)](4)(µ-(CF(3))PCPH)(4) (2-sq) was obtained from a 1:1 ligand:metal mixture in benzene in the presence of excess MeI. At higher temperatures (165 °C) the reaction of (CF(3))PCPH with (PPh(3))(3)Ir(CO)H afforded the 5-coordinate Ir(I) complex ((CF(3))PCP)Ir(CO)(PPh(3)) (3). Complex 3 shows mild catalytic activity for the decarbonylation of 2-naphthaldehyde in refluxing diglyme (162 °C).

Unexpected formation of ruthenium(II) hydrides from a reactive dianiline precursor and 1,2-(Ph2P)2-1,2-closo-C2B10H10.

Reaction of the new precursor cis, trans-Ru(cod)(anln)2Cl2 with the diphosphine 1,2-bis(diphenylphosphino)-1,2-dicarba-closo-dodecaborane (o-dppc) unexpectedly results in two new ruthenium(II) hydrides, trans-Ru(o-dppc) 2(H)Cl and the neutral, five-coordinate complex Ru(o-dppc)(nido-dppc)(H), depending upon the reaction conditions [anln is aniline and nido-dppc is 7,8-(Ph2P)2C2B9H10(-)]. Chloride abstraction from trans-Ru(o-dppc)2(H)Cl leads to another five-coordinate hydride, [Ru(o-dppc)2(H)](+), which is isolated as either a triflate or hexafluorophosphate salt. On the basis of labeling and reactivity studies, the source of the hydride appears to be the cod ligand.

Synthesis and platinum coordination chemistry of the perfluoroalkyl acceptor pincer ligand, 1,3-(CH(2)P(CF(3))(2))(2)C(6)H(4).

The synthesis of perfluoroalkyl-substituted "pincer"-type PCP ligands, 1,3-C6H4(CH2P(Rf)2)2 (Rf = CF3, C2F5), and platinum coordination studies (Rf = CF3) are reported. 1,3-C6H4(CH2P(CF3)2)2 (CF3PCPH) reacts at ambient temperatures with (cod)Pt(Me)Cl (cod = 1,5-cyclooctadiene) and (cod)PtMe2 to afford unmetalated PCPH-bridged products [(CF3PCPH)Pt(Me)Cl]x and cis-[(CF3PCPH)PtMe2]2, respectively. cis-[(CF3PCPH)PtMe2]2 is soluble and has been spectroscopically and crystallographically characterized. Thermolysis of these compounds results in the loss of methane and the formation of metalated complexes (CF3PCP)PtCl and (CF3PCP)PtMe. Treatment of (CF3PCP)PtCl with MeMgBr provides an alternative route to (CF3PCP)PtMe. The carbonyl cation (CF3PCP)Pt(CO)+SbF6- (nu(CO) = 2143 cm(-1)) was readily prepared by chloride abstraction with AgSbF6 under 1 atm CO. nu(CO) data indicates that RfPCP ligands are electronically analogous to trans acceptor phosphine complexes such as trans-((C2F5)2PMe)2Pt(Me)(CO)+ (nu(CO) = 2149 cm-1).