Toshikazu Kitagawa

Ideal Redox Behavior of the High-Density Self-Assembled Monolayer of a Molecular Tripod on a Au(111) Surface with a Terminal Ferrocene Group

A dyad consisting of a tripod-shaped trithiol with an adamantane core and a terminal ferrocenyl group linked through ap-phenyleneethynylene bridge was synthesized. The trithiol formed a stable self-assembled monolayer (SAM) on Au(111), wherein each molecule is bound to the surface by three-point adsorption using all sulfur atoms, with confirmation by PM-IRRAS and XPS analyses. Cyclic voltammetry of the SAM showed a line shape typical of an ideal adsorbed system, that is, a monolayer with negligible electrostatic interaction among the terminal ferrocenyl groups. Thus, a rare SAM was achieved, in which the component molecules were isolated from adjacent molecules without the coadsorption of nonelectroactive molecules.

Structural effects on valence tautomerism : VII. An NMR and molecular mechanics study of the steric effects of t-butyl substituents on the cycloheptatriene-norcaradiene equilibrium of 7-cyano-7-m

Abstract 7-Cyano-7-methylcycloheptatrienes containing one t-butyl group on the 1-position ( 2 ) or two t-butyl groups on the 1- and 3-, 1- and 4-, or 1- and 5-positions ( 3 , 4 , or 5 , respectively) were synthesized and their cyclo- heptatriene (CHT)-norcaradiene (NCD) equilibria measured by variable-temperature 1 H NMR for CS 2 -CD 2 Cl 2 solutions. The 1 H NMR chemical shifts of the 7-methyl group indicate that these compounds are composed of essentially one CHT and one NCD tautomer with an endo geometry of the methyl group. The introduction of a t-butyl group at the 1-position of 7-cyano-7-methylcycloheptatriene ( 1 ) markedly shifts the equilibrium to the NCD side and the addition of the second t-butyl group further favors the NCD form, with the NCD populations for 2 , 3 , 4 , and 5 at 25 °C 70.9, 96.5, 92.3, and 99.3%, respectively. An application of molecular mechanics (MMPI) calculations to various t-butylated CHT-NCD systems suggests that the t-butyl groups sterically destabilize the CHT form more than the NCD form, bringing about increased NCD populations.

Self-Assembly and Scanning Tunneling Microscopy Tip-Induced Motion of Ferrocene Adamantane Trithiolate Adsorbed on Au(111)

We have studied the self-assembled monolayers (SAMs) of adamantane-based trithiolate, which consists of a ferrocene derivative at the head (ferrocene adamantane trithiolate; ferrocene-ATT), on Au(111) using low temperature scanning tunneling microscopy (STM). It was found that the adsorption behavior of ferrocene-ATT is similar to that of bromine adamantane trithiolate (BATT) adsorbed on Au(111). This indicates that adsorption of adamantane-based trithiol is controlled by three legs (CH2S) connected to bridgehead positions of the adamantane cage. Molecules, which form an ordered structure, are stable under low-bias-voltage scanning, i.e., a sample bias voltage lower than 1 V. STM tip-induced diffusion, however, was observed both for small clustered molecules and for molecules bound around the edge of an ordered molecular island. Furthermore, applying a high bias voltage (5 V) resulted in the destruction of SAMs structures.

Ionically Dissociative hydrocarbons Containing the C60 Skeleton

Abstract The coordination of tert -butylfulleride ion ( t -BuC 60 − ) with tropylium ( 1a + ), 1,4-di- tert -butyltropylium ( 1b + ), and 1,4-dicyclopropyltropylium ( 1c + ) ions yielded 1- tert -butyl-4-(3,6-R 2 -2,4,6-cycloheptatrien-1-yl)-1,4-dihydrofullerenes ( 2a : R = H, 2b : R = t -Bu, 2c : R = cyc -Pr) in isomerically pure form. The structures of 2a–c were determined from the 1 H NMR NOE experiments. Spectrophotometric studies showed that 2a–c are unique hydrocarbons that regenerate t -BuC 60 − and 1 + in polar solvents by heterolysis of a carbon-carbon σ bond. Although this facile heterolysis is due principally to the high thermodynamic stabilities of both t -BuC 60 − and 1a–c + , the free energies of heterolysis are considerably lower than predicted from p K HA of t -BuC 60 − and pK R + of 1a–c + using Arnetts master equation. The results suggest that steric repulsion between the 1- tert -butyl group and the cycloheptatrienyl group on the surface of C 60 plays an important role in enhancing the heterolysis of 2a–c .

Ionic Dissociation of Carbon—Carbon σ Bonds in Hydrocarbons and the Formation of Authentic Hydrocarbon Salts

Publisher Summary This chapter deals with the ionic dissociation of carbon–carbon σ bonds in hydrocarbons and the formation of authentic hydrocarbon salts. Carbon compounds are most commonly constructed using covalent bonds, and the nature of the bond is of prime importance in all organic compounds. Among the compounds of carbon, hydrocarbons, which are made up of only hydrogen and carbon, are the simplest known group of organic compounds having exclusively covalent bonds. The search for such novel hydrocarbons depends primarily on the synthesis and examination of highly stabilized hydrocarbon cations and anions. The details of structural features such as the length of the carbon–carbon bond σ that easily undergoes heterolysis and the spatial arrangement of ions in hydrocarbon salts are intriguing subjects. Electrical characterization of the hydrocarbon salts, though they are not stable enough to air and light, might be useful as new functional materials.

Tailoring electronic states of a single molecule using adamantane-based molecular tripods

Adsorption structures and electronic states of molecular tripods, having a Br atom (BATT) and a ferrocene derivative (Ferrocene-ATT) at the head part of the adamantane-based trithiolate, adsorbed on Au(111) have been investigated using low temperature scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). We found that BATT and Ferrocene-ATT form self-assembled monolayers (SAMs), and their orderings are identical to one another, which suggests that the adsorption structure of adamantane-based molecular tripods is independent of the type of functional substituent attached to the head part. The electronic states originated from the ferrocene group were confirmed in the STS spectrum of Ferrocene-ATT whereas those are absent in the BATT spectrum. We note that the ferrocene part has few interactions with the Au substrate owing not only to the upright geometry of Ferrocene-ATT but also to the insulative properties of the adamantane base. The STS mapping revealed the spatial distribution of the electronic state of Ferrocene-ATT.

Reaction of cyclopropenylium ions with the tert-butyl-C60 anion: carbocation–carbanion coordination vs salt formation

The reaction of the tert-butyl-C60 anion (t-BuC60−) with tricyclopropylcyclopropenylium ion and 1,2-dicyclopropyl-3-(4-methylphenyl)cyclopropenylium ion underwent C–C covalent bond formation to give 1-tert-butyl-4-(2-cyclopropen-1-yl)-1,4-dihydro[60]fullerenes. The newly formed C–C bond of these hydrocarbons was found to undergo reversible heterolysis in polar solvents to give the original ions. A more highly stabilized carbocation tris[1-(5-isopropyl-3,8-dimethylazulenyl)]cyclopropenylium ion did not react with t-BuC60−, and a hydrocarbon with a salt structure was isolated as a solid. Tri-tert-butylcyclopropenylium ion, although less stable than the other cations, underwent no C–C bond formation with t-BuC60−, indicating that steric hindrance effectively suppresses carbocation–carbanion coordination. These results provide a demonstration of a change in reactivity between t-BuC60− and substituted cyclopropenylium ions toward C–C coordination, depending on the thermodynamic stability and steric crowding of the cation.

Structural effects on valence tautomerism. Part 5. A carbon-13 nuclear magnetic resonance. Study on the effects of the π-acceptor ability of aryl substituents on the valence tautomerism of 7-aryl-2,5-di-t-butyl-cyclohepta-1,3,5-trienes

The cycloheptatriene (CHT)–norcaradiene (NCD) equilibria of eight 7-aryl-2,5-di-t-butylcyclohepta-1,3,5-trienes in CS2–CD2Cl2 have been studied with the use of variable-temperature 13C n.m.r. The equilibrium constants (K=[NCD]/[CHT]) below the coalescence temperatures were calculated from signal intensities; those at higher temperatures were obtained by use of the equation K=(δCHT–δ)/(δ–δNCD). The enthalpy (ΔHo) for the conversion of the CHT into the NCD tautomer decreases in the substituent order p-MeO > p-Me > H > p-Br ∼p-Cl > m-Cl > p-CF3 > m,m′-Cl2, with the ΔHo values for p-MeO and m,m′-Cl2 2.64 ± 0.11 and –0.81 ± 0.13 kJ mol–1, respectively. The ΔHo values are linearly correlated not only with the Hammett σ-constants, but also with the π-acceptor ability (the D value) of the aryl carbon atom adjacent to C(7) as estimated by INDO calculations; this supports Hoffmann and Gunthers donor–acceptor theory. The 13C signals of C(3) and C(4) in both the NCD and the CHT tautomer undergo downfield shifts as the D value of the aryl group increases; the shift of the former is 4.3 times more sensitive than that of the latter as evaluated by the Hammett plot. In addition, the difference in the 13C shift of the para-carbon atom between the NCD and the CHT tautomer is larger in the 7-(m,m′-dichlorophenyl) than in the 7-phenyl system. These results suggest that conjugative interaction between the cyclopropane ring in the NCD structure and the aryl group increases with increase in the π-acceptor ability of the aryl group.

Bis(2,6-difluorophenyl)benzoylmethyl cation: α-ketocarbenium ion as a single-electron acceptor

Abstract The title carbenium ion 1 was observed by low temperature NMR in solution. Its strong oxidizing power was demonstrated by the reduction potential and the effective oxidation of methoxy-substituted benzopinacolone 3 .

Generation and reaction of a phenyl-substituted cyclopentadienyl cation annelated with two homoadamantene frameworks.

The generation of the title cation 2(+) and its reaction under solvolytic and non-nucleophilic conditions were investigated. When the precursor chlorocyclopentadiene 5 was reacted with silica gel that contained water or with anhydrous MeOH, the corresponding 5-hydroxy- and 5-methoxycyclopentadienes (7 and 6) were produced in 68 and 81% yields, respectively. This indicates that 2(+) is formed as an intermediate under solvolytic conditions and persists without any rearrangement of the homoadamantane frameworks, at least during the period before capture by the nucleophile. On the other hand, the abstraction of a chloride ion from 5 by Ag(+) in the absence of a nucleophile at -78 degrees C resulted in the quantitative formation of allyl cation 8(+), incorporated in a bicyclo[3.1.0]hexane framework, via the Wagner-Meerwein rearrangement of a homoadamantane framework. Cation 8(+) was isolated as the SbF6(-) salt, and its structure was determined by X-ray crystallography. Quenching this cation with MeOH afforded a methyl ether 14, with a cyclopentadiene structure retained but one of the homoadamantane frameworks had undergone a structural change by a further Wagner-Meerwein rearrangement.