The crystal packing of 4,7‐dichloro‐ and 4,7‐dibromobenzo[c]furazan 1‐oxide

The molecular structures of 4,7‐di­chloro­benzo­[c]­fur­azan 1‐­oxide, C₆H₂Cl₂N₂O₂, (I), and 4,7‐di­bromo­benzo­[c]­fur­azan 1‐oxide, C₆H₂Br₂N₂O₂, (II), are normal. Compound (I) occurs in two polymorphic forms. One polymorph contains one mol­ecule in the asymmetric unit, organized into two‐dimensional sheets involving intermolecular N?Cl and O?Cl inter­actions. The second polymorph has three mol­ecules in the asymmetric unit, organized into two crystallographically different two‐dimensional sheets with similar interactions. Compound (II) is isomorphous with the second polymorph of (I). The three two‐dimensional sheets in the two polymorphs comprise a set of three two‐dimensional polymorphic arrangements.     

Fused s-triazino heterocycles. XVII. 8,13-Dihydro-1,3,7,8,13c-pentaazabenzo[de]naphthacene, 7H-1,3,7,8,11b,12,14-heptaazadibenzo[de,hi]naphthacene and 8H-1,3,7,8,13,14c-hexaazabenzo[4,5]cyclohepta[1,2-a]phenalene,three new ring systems

The reaction of 7,9-dibromo-5-tribromornethyl-2-t-butyl-4-cyano-1,3,6,9b-tetraazaphenalene ( 1a ) with p-toluidine is shown to give 4,6-dibromo-2-t-butyl-8,13-dihydro-13-imino-11-methyl-1,3,7,8,13c-pentaazabenzo[de]naphthacene ( 4 ) in two steps with 7,9 dibromo-2-t-butyl-4-cyano-5-p-toluidino-1,3,6,9b-tetraazaphenalene ( 2b ) as the intermediate product. A related annulation reaction of 1a with N-(5-amino-2,4-dimethylphenyl)trimethylacetamide ( 8 ) leads in two steps to 9,11-dibromo-2,13-di-t-butyl-4,6-dimethyl-7H-1,3,7,8,11b,12,14-heptaazadibenzo[de,hi]naphthacene ( 6 ) with 7,9-dibromo-2-t-butyl-4-cyano-5N-(2,4-dimethyl-5-trimethylacetamidophenyl)amino-1,3,6,9b-tetraazaphenalene ( 2d ) as the intermediate product. In a similar fashion the reaction of 1a with o-phenylenediamine forms 14-amino-4,6-dibromo-2-t-butyl-8H-1,3,7,8,13,14c-hexaazobenzo[4,5]cyclohepta[1,2-a]-phenalene ( 12 ) by way of the intermediate 5-N-(2-aminophenyl)amino-7,9-dibromo-2-t-butyl-4-cyano-1,3,6,9b-tetraazaphenalene ( 2e ). The preparation of N-(2,4-dimethyl-5-nitrophenyl)-trimethylacetamide ( 11 ) and its reduction to N-(5-amino-2,4-dimethylphenyl)trimethylacetamide ( 8 ) is also described.     

The quenching of isopropyl group rotation in van der Waals molecular solids

X-ray diffraction experiments are employed to determine the molecular and crystal structure of 3-isopropylchrysene. Based on this structure, electronic structure calculations are employed to calculate methyl group and isopropyl group rotational barriers in a central molecule of a ten-molecule cluster. The two slightly inequivalent methyl group barriers are found to be 12 and 15 kJ mol(-1) and the isopropyl group barrier is found to be about 240 kJ mol(-1), meaning that isopropyl group rotation is completely quenched in the solid state. For comparison, electronic structure calculations are also performed in the isolated molecule, determining both the structure and the rotational barriers, which are determined to be 15 kJ mol(-1) for both the isopropyl group and the two equivalent methyl groups. These calculations are compared with, and are consistent with, previously published NMR (1)H spin-lattice relaxation experiments where it was found that the barrier for methyl group rotation was 11+/-1 kJ mol(-1) and that the barrier for isopropyl group rotation was infinite on the solid state NMR time scale.     

Lewis pair polymerization by classical and frustrated Lewis pairs: acid, base and monomer scope and polymerization mechanism

Classical and frustrated Lewis pairs (LPs) of the strong Lewis acid (LA) Al(C(6)F(5))(3) with several Lewis base (LB) classes have been found to exhibit exceptional activity in the Lewis pair polymerization (LPP) of conjugated polar alkenes such as methyl methacrylate (MMA) as well as renewable α-methylene-γ-butyrolactone (MBL) and γ-methyl-α-methylene-γ-butyrolactone (γ-MMBL), leading to high molecular weight polymers, often with narrow molecular weight distributions. This study has investigated a large number of LPs, consisting of 11 LAs as well as 10 achiral and 4 chiral LBs, for LPP of 12 monomers of several different types. Although some more common LAs can also be utilized for LPP, Al(C(6)F(5))(3)-based LPs are far more active and effective than other LA-based LPs. On the other hand, several classes of LBs, when paired with Al(C(6)F(5))(3), can render highly active and effective LPP of MMA and γ-MMBL; such LBs include phosphines (e.g., P(t)Bu(3)), chiral chelating diphosphines, N-heterocyclic carbenes (NHCs), and phosphazene superbases (e.g., P(4)-(t)Bu). The P(4)-(t)Bu/Al(C(6)F(5))(3) pair exhibits the highest activity of the LP series, with a remarkably high turn-over frequency of 9.6 × 10(4) h(-1) (0.125 mol% catalyst, 100% MMA conversion in 30 s, M(n) = 2.12 × 10(5) g mol(-1), PDI = 1.34). The polymers produced by LPs at RT are typically atactic (P(γ)MMBL with ～47% mr) or syndio-rich (PMMA with ～70-75% rr), but highly syndiotactic PMMA with rr ～91% can be produced by chiral or achiral LPs at -78 °C. Mechanistic studies have identified and structurally characterized zwitterionic phosphonium and imidazolium enolaluminates as the active species of the current LPP system, which are formed by the reaction of the monomer·Al(C(6)F(5))(3) adduct with P(t)Bu(3) and NHC bases, respectively. Kinetic studies have revealed that the MMA polymerization by the (t)Bu(3)P/Al(C(6)F(5))(3) pair is zero-order in monomer concentration after an initial induction period, and the polymerization is significantly catalyzed by the LA, thus pointing to a bimetallic, activated monomer propagation mechanism. Computational study on the active species formation as well as the chain initiation and propagation events involved in the LPP of MMA with some of the most representative LPs has added our understanding of fundamental steps of LPP. The main difference between NHC and PR(3) bases is in the energetics of zwitterion formation, with the NHC-based zwitterions being remarkably more stable than the PR(3)-based zwitterions. Comparison of the monometallic and bimetallic mechanisms for MMA addition shows a clear preference for the bimetallic mechanism.     

CF3 Rotation in 3-(Trifluoromethyl)phenanthrene: Solid State 19F and 1H NMR relaxation and Bloch-Wangsness-Redfield theory

We have observed and modeled the 1H and 19F solid-state nuclear spin relaxation process in polycrystalline 3-(trifluoromethyl)phenanthrene. The relaxation rates for the two spin species were observed from 85 to 300 K at the low NMR frequencies of omega/2pi = 22.5 and 53.0 MHz where CF3 rotation, characterized by a mean time tau between hops, is the only motion on the NMR time scale. All motional time scales (omegatau < 1, omegatau approximately 1, and omegatau > 1) are observed. The 1H spins are immobile on the NMR time scale but are coupled to the 19F spins via the unlike-spin dipole-dipole interaction. The temperature dependence of the observed relaxation rates (the relaxation is biexponential) shows considerable structure and a thorough analysis of Bloch-Wangsness-Redfield theory for this coupled spin system is provided. The activation energy for CF3 rotation is 11.5 +/- 0.7 kJ/mol, in excellent agreement with the calculation in a 13-molecule cluster provided in the companion paper where the crystal structure is reported and detailed ab initio electronic structure calculations are performed [Wang, X.; Mallory F. B.; Mallory, C. W; Beckmann, P. A.; Rheingold, A. L.; Francl, M. M J. Phys. Chem. A 2006, 110, 3954].     

CF3 rotation in 3-(trifluoromethyl)phenanthrene. X-ray diffraction and ab initio electronic structure calculations

The molecular and crystal structure of 3-(trifluoromethyl)phenanthrene has been determined by X-ray diffraction. The structure of the isolated molecule has been calculated using electronic structure methods at the HF/3-21G, HF/6-31G, MP2/6-31G and B3LYP/6-31G levels. The potential energy surfaces for the rotation of the CF3 group in both the isolated molecule and cluster models for the crystal were computed using electronic structure methods. The barrier height for CF3 rotation in the isolated molecule was calculated to be 0.40 kcal mol(-1) at B3LYP/6-311+G//B3LYP/6-311+G. The B3LYP/6-31G calculated CF3 rotational barrier in a 13-molecule cluster based on the X-ray data was found to be 2.6 kcal mol(-1). The latter is in excellent agreement with experimental results from the NMR relaxation experiments reported in the companion paper (Beckmann, P. A.; Rosenberg, J.; Nordstrom, K.; Mallory, C. W.; Mallory, F. B. J. Phys. Chem. A 2006, 110, 3947). The computational results on the models for the solid state suggest that the intermolecular interaction between nearest neighbor pairs of CF3 groups in the crystal accounts for roughly 75% of the barrier to rotation in the solid state. This pair is found to undergo cooperative reorientation. We attribute the CF3 reorientational disorder in the crystal as observed by X-ray diffraction to the presence of a pair of minima on the potential energy surface and the effects of librational motion.