The crystal and molecular structure of a non-alternant hydrocarbon: heptafulvalene

Tuning the redox profile of the 6,6'-biazulenic platform through functionalization along its molecular axis

The E1/2 potential associated with reduction of the linearly-functionalized 6,6'-biazulenic scaffold is accurately correlated to the combined σp Hammett parameters of the substituents over >600 mV range. X-ray crystallographic analysis of the 2,2'-dichloro-substituted derivative revealed unexpectedly short C-Cl bond distances, along with other metric changes, suggesting a non-trivial cycloheptafulvalene-like structural contribution.

Homoleptic complexes of isocyano- and diisocyanobiazulenes with a 12-electron, ligand-based redox capacity

Oligo- and polyazulenes are attractive π-conjugated building blocks in designing advanced functional materials. Herein, we demonstrate that anchoring one or both isocyanide termini of the redox non-innocent 2,2'-diisocyano-6,6'-biazulenic π-linker (1) to the redox-active [Cr(CO)5] moiety provided a convenient intramolecular redox reference for unambiguously establishing that the 6,6'-biazulenic scaffold undergoes a reversible one-step 2e- reduction governed by reduction potential compression/inversion. Treatment of bis(η6-naphthalene)chromium(0) with six equiv. of 2-isocyano-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene (6) or [(OC)5Cr(η1-2,2'-diisocyano-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene)] (11) afforded homoleptic Cr(0) complexes 13 and 14 with a 12e- (per molecule) ligand-based reduction capacity at mild E1/2 of -1.29 V and -1.15 V vs. Cp2Fe0/+, respectively. The overall reversible redox capacity varies from 15e- for the mononuclear complex 13 to 21e- for the heptanuclear complex 14. The latter "nanocomplex" has a diameter of ca. 5 nm and features seven Cr(0) centers interlinked with six 2,2'-diisocyano-6,6'-biazulenic bridges. The X-ray structure of [(OC)5Cr(2-isocyano-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene)] (7) indicated a 43.5° interplanar angle between the two azulenic moieties. Self-assembly of 11 on a Au(111) substrate afforded an organometallic monolayer film of 11 featuring approximately upright orientation of the 2,2'-diisocyano-6,6'-biazulenic linkers, as evidenced by ellipsometric measurements and the RAIR signature of the C4v-symmetric [(-NC)Cr(CO)5] infrared reporter within 11. Remarkably, comparing the FTIR spectrum of 11 in solution with the RAIR spectrum of 11 adsorbed on Au(111) suggested electronic coupling at a ca. 2 nm distance between the Cr(0) and Au atoms linked by the 2,2'-diisocyano-6,6'-biazulene bridge.

Do Antarafacial Cycloadditions Occur? Cycloaddition of Heptafulvalene with Tetracyanoethylene

The cycloaddition of heptafulvalene (1) with tetracyanoethylene (TCNE) was previously described as an example of an antarafacial cycloaddition, a [π¹⁴ _a +π² _s ] process that afforded only the trans cycloadduct by virtue of the edge-to-face approach of TCNE, facilitated by the S shape of 1. The reaction has been investigated in depth and found not to be a concerted antarafacial process. At low temperature, the reaction is observed to give a mixture of cis and trans cycloadducts as well as a [4+2] cycloadduct. The mixture of products is converted to the trans cycloadduct by equilibration upon warming to room temperature. Studies with diethyl 2,3-dicyanofumarate and -maleate confirmed the formation of cis cycloadducts. DFT studies at the M06-2X/6-311+G(2d,p) SCRF=acetone level of theory show that the originally proposed edge-to-face approach of TCNE to 1 is highly disfavored, whereas a stepwise mechanism involving the addition of TCNE at C2 to form a zwitterion followed by collapse at either C2' or C7' is energetically accessible. The Diels-Alder adduct is also formed in a stepwise reaction by competitive addition of TCNE at C4 of 1. These studies suggest that edge-to-face interactions are prohibitive in even the most favorable cases.

Fulvalenes, Fulvenes, and Related Molecules: An ab Initio Study

Ab initio calculations using conventional (HF/6-31G and MP2/6-31G) and density functional theory (B-LYP/6-31G) methods have been used to determine the structures of the [n]fulvene and [n,m]fulvalene (n, m = 3, 5, 7) series of molecules, with particular emphasis on heptafulvalene (n = m = 7: 12). Calculations have also been performed on the parent cycloalkenes: cyclopropene, cyclopentadiene, and cycloheptatriene (1-3, respectively). All the fulvenes (n = 3, 5, 7: 4-6, respectively) and the smaller fulvalenes (n = 3, m= 3, 5, 7: 7-9, respectively, and n = m = 5: 10) are found to be planar. Pentaheptafulvalene (n = 5, m = 7: 11) adopts a very slightly nonplanar C(s)() arrangement of the five- and seven-membered rings. Heptafulvalene (12) is predicted to have an anti-folded C(2)(h)() structure, in accord with the X-ray crystal structure. We propose that the underlying reason for 11 and 12 adopting nonplanar conformations is the proximity of the H(2) and H(2)(') hydrogen atoms which promotes a distortion of the rings away from planarity at the central fulvalenic C=C double bond. In the process, pi-overlap is lost but this is partially regained by pyramidalization of the carbon centers in the seven-membered ring(s). The degree of folding is substantially more pronounced in 12 than in 11. Our calculated dipole moments, pi-electron distributions, bond alternation parameters, and energy comparisons indicate that the unknown smallest fulvalene, triafulvalene (7), is highly destabilized with localized bonding while triapentafulvalene (8), which is also unknown, is predicted to be stabilized and quite delocalized, consistent with Hückel 4n + 2 considerations.

The metal-mediated conversion of octachlorocycloheptatriene into dodecachloroheptafulvalene: a synthetic, structural, and EHMO study

Octachlorocycloheptatriene, C7Cl8, 3, reacts with a variety of organometallic precursors, e.g., Fe2(CO)9, Co2(CO)8, [(C5H5)Fe(CO)2]−, to give dodecachloroheptafulvalene, C14C12,12, in yields ranging from 54 to 68%. There is no evidence to support the intermediacy of carbene complexes of the type C7Cl6=MLn, and it is suggested that the reaction proceeds by the dechlorination of C7Cl8 to give hexachlorotropylidene, which dimerizes to the observed product. Extended Hückel molecular orbital calculations are used to show that the chlorinated carbene, C7Cl6, should be more stable than the corresponding parent system, C7Cl6. The EHMO calculations also reveal that planar heptafulvalenes are electronically disfavored and should exist preferentially in the S-anti conformation, as is the case for C14H12. The molecule C14Cl12,12, is shown by X-ray crystallography to be formed only as the syn conformer, which isomerizes to the anti structure only at high temperature. The conformations of the two seven-membered rings in 12 closely resemble that found in octachlorocycloheptatriene itself. C7Cl8, 3, crystallizes in the orthorhombic space group Pnma with a = 7.140(1) Å, b = 13.329(3) Å, c = 12.595(3) Å, and V = 1198.7(4) Å3 for Z = 4.3 adopts a severely bent boat structure in which the planes C(1)-C(7)-C(6) and C(2)-C(3)-C(4)-C(5) make angles of 51.8° and 32.4°, respectively, with the C(1)-C(2)-C(5)-C(6) plane. Key words: octachlorocycloheptatriene, heptafulvalenes, metal-mediated dimerization.