Lowering inversion barriers of buckybowls by benzannelation of the rim: synthesis and crystal and molecular structure of 1,2-dihydrocyclopenta[b,c]dibenzo[g,m]corannulene.

[structure: see text]. The title compound (3) has been synthesized by a non-pyrolytic route providing a 36% isolated yield in the final step. X-ray crystal structure determination shows that 3 crystallizes with one solvating molecule of toluene and exhibits slightly lower curvature than the parent cyclopentacorannulene. DFT calculations predict a substantially lower bowl-to-bowl inversion barrier for 3 than for dihydro-cyclopentacorannulene, and this is confirmed by dynamic 1H NMR experiments.

Disappearing and reappearing polymorphs. The benzocaine:picric acid system.

The low-melting polymorphic modification of the 1:1 complex of benzocaine (BC) and picric acid (PA) had earlier been reported to be an example of a "disappearing polymorph". The BC:PA system has been reinvestigated by thermomicroscopy, calorimetry, solid-state NMR, and X-ray crystallography. The phase diagram has been derived, and robust procedures for the crystallization of the two 1:1 complexes, a hydrate of the 1:1 complex, and a 2:1 complex have been devised. The structures of all four phases have been determined and compared using graph set analysis to characterize the hydrogen-bonding patterns. It is shown that the thorough microscopic investigation of the thermal behavior, combined with calorimetric methods, can lead to the development of strategies to crystallize metastable polymorphic forms which may be difficult to obtain once their stable congeners have been obtained.

Reactions of the protonated dinuclear ruthenium complex [[(eta(5)-C(5)H(3))(2)(SiMe(2))(2)]Ru(2)(CO)(4)(mu-H)]+ with nucleophiles.

Complex [[(eta(5)-C(5)H(3))(2)(SiMe(2))(2)]Ru(2)(CO)(4)(mu-H)](+)BF(4)(-) (1H(+)BF(4)(-)), which features a protonated Ru-Ru bond, reacts with F(-) to give (eta(5)-C(5)H(5))(2)Ru(2)(CO)(4) (2), resulting from the cleavage of both SiMe(2) groups, with I(-) to give the Ru-Ru cleaved product [(eta(5)-C(5)H(3))(2)(SiMe(2))(2)]Ru(2)(CO)(4)(H)(I)(3), and with phosphines (PEt(3), PMe(2)Ph) to give [[(eta(5)-C(5)H(3))(2)(SiMe(2))(2)]Ru(2)(CO)(4)(H)(PR(3))](+) (4a-b). Reaction of 1H(+)BF(4)(-) and NaOMe in THF generates [(eta(5)-C(5)H(4))(2)SiMe(2)]Ru(2)(CO)(4) (5), resulting from the cleavage of a single SiMe(2) group, while the reaction of 1H(+)BF(4)(-) and NaOMe in MeOH generates [mu-eta(5):eta(5)-(C(5)H(3)SiMe(2)OMe)(C(5)H(4))SiMe(2)]Ru(2)(CO)(4) (6), resulting from the partial cleavage of a SiMe(2) group. Reaction of 1H(+)BF(4)(-) and NaSR (R = Me, Et) in THF generates [(eta(5)-C(5)H(3))(2)(SiMe(2))(2)]Ru(2)(CO)(4)(H)(SR) (R = Me, Et; 7a-b), which undergoes rearrangement upon contact with neutral and basic alumina or silica to give complexes [mu-eta(5):eta(1):eta(5)-(C(5)H(3)C=O)(C(5)H(4))(SiMe(2))(2)O]Ru(2)(mu-SR)(CO)(3) (R = Me, Et; 8a-b). Molecular structures of 4a, 6, and 8a as determined by X-ray diffraction studies are also presented.

Isomer formation and other issues in the substitution reactions of oxorhenium(V) complexes of 2,2'-bipyridine and related ligands.

Two new oxorhenium(V) compounds were prepared and characterized: MeReO(mtp)(Me(2)Bpy) and MeReO(mtp)(dppb), where mtpH(2) is 2-(mercaptomethyl)thiophenol, Me(2)Bpy is 4,4'-dimethyl-2,2'-bipyridine, and dppb is 1,2-(Ph(2)P)(2)C(6)H(4). The more stable geometric isomer of MeReO(mtp)X forms MeReO(mtp)Y (X, Y = PR(3), NC(5)H(4)R) in two steps, both of which show a first-order dependence on [Y], proceeding through the metastable geometric isomer MeReO(mtp)Y. When Y = PR(3), no MeReO(mtp)Y was detected at equilibrium; with NC(5)H(4)R, however, both isomers were detected. The values of K(PyPy) were 8.5-9.8, largely irrespective of R; for NC(5)H(5), DeltaH degrees = -4.47 +/- 0.29 kJ and DeltaS degrees = 3.9 +/- 1.0 J K(-1). For the more symmetric edt ligand, geometric isomers do not exist, but enantiomers do. The rate of racemization of MeReO(edt)(NC(5)H(4)R) was proportional to [Py]. Values of k(rac) for 16 compounds span the range 135-370 L mol(-1) s(-1) in C(6)H(6) at 25 degrees C (rho = -0.39 +/- 0.07). In toluene-d(8), k(rac) for 4-picoline has DeltaH = 28.9 +/- 0.4 kJ, DeltaS() = -103.6 +/- 0.9 J K(-1). A common mechanism applies to ligand substitution (mtp) and racemization (edt). MeReO(dithiolate)Py complexes react with Bpy, Me(2)Bpy, Phen, and Me(2)Phen to form six-coordinate chelates, with rate constants 0.024-0.74 L mol(-1) s(-1) at 25 degrees C, some 10(3) times smaller than with pyridines, no doubt owing to the bulk of the bidentates. Values of DeltaS are -86 to -138 J K(-1), reflecting substantial orientational barriers as well as the inherent contribution of the associative mechanism. The product is MeReO(mtp)(Me(2)Bpy). The formation of the metastable isomer is consistent with the mechanism assigned to the ligand substitution and racemization reactions. Such compounds, once formed, no longer participate in ligand substitution reactions at reasonable rates. The formation of the metastable isomer is consistent with the mechanism assigned to the ligand substitution and racemization reactions.