Kevin Lam

Using toluates as simple and versatile radical precursors

The viability of the toluate moiety as a radical precursor has been examined by studying deoxygenation and cyclization reactions.

An Anodic Method for Covalent Attachment of Molecules to Electrodes through an Ethynyl Linkage

Electroactive organometallic molecules have been covalently attached to electrode surfaces through an ethynyl linkage. The process takes advantage of ethynyl-based radicals generated by anodic oxidation of a lithio-activated terminal ethynyl group. Electrophores containing redox-active ferrocene, cymantrene, or cobaltocenium moieties have been deposited at the one-to-three monolayer level. Both metal-based and ligand-based chemical reactions have been carried out on the surface-modified systems.

Covalent attachment of porphyrins and ferrocenes to electrode surfaces through direct anodic oxidation of terminal ethynyl groups.

One with the surface: A method is presented for electrode modification with terminal alkynes and alkenes. Direct oxidation of these moieties leads to efficient grafting onto glassy carbon, gold, platinum, and indium tin oxide surfaces. Various ferrocenes and 5,10,15,20-(4-ethynylphenyl)porphyrin were attached in this way.

Chemoselective chemical and electrochemical deprotections of aromatic esters.

Alcohols can be easily and chemoselectively deprotected from the corresponding aromatic esters by using either SmI(2)/HMPA or by electrolysis in the presence of a proton source.

Novel Electrochemical Deoxygenation Reaction Using Diphenylphosphinates.

The electrochemical reduction of diphenylphosphinate esters leads smoothly and in high yields to the corresponding deoxygenated products. In comparison with the previously developed methodologies, the electrolysis could be performed at lower temperature and with a higher current density, resulting in a shorter reaction time.

Polyferrocenylsilane homopolymers and diblock copolymers with pendant ruthenocenyl groups by photocontrolled ring-opening polymerisation

The strained, sila[1]ferrocenophane [{Fe(η-C5H4)2}SiMe(η-C5H4)Ru(η-C5H5)] (4) has been prepared by the reaction of Li[(η-C5H4)Ru(η-C5H5)] (8) with [{Fe(η-C5H4)2}SiMeCl] (7). Two different routes for the synthesis of 8 were investigated, which differed predominately in the purity of the product obtained. Although the thermal ring-opening polymerisation (ROP) of 4 failed to yield soluble polymeric material, photocontrolled ROP using Na[C5H5] as the initiator afforded relatively monodisperse, soluble polymers of predetermined molecular weights and the general formula [(η-C5H4)Fe(η-C5H4)SiMe{(η-C5H4)Ru(η-C5H5)}]n (19). This material was stable to both oxygen and moisture. Electrochemical characterisation of 4 revealed unusual, concentration dependent behaviour as a consequence of the oxidation of the ruthenocenyl groups. This phenomenon was not observed upon oxidation of the Ru centres in 19, presumably due to the sterically encumbered structure of the polymer. The photocontrolled ROP protocol was extended to afford the block copolymers polystyrene (PS)n-b-[(η-C5H4)Fe(η-C5H4)SiMe{(η-C5H4)Ru(η-C5H5)}]m (22) by the application of a PS-based macroinitiator. These materials represent rare examples of complex polymer architectures where two different metals are incorporated in the same block.

Anodic oxidation of disulfides: detection and reactions of disulfide radical cations.

The anodic oxidation of five diaryldisulfides have been studied in a dichloromethane/[NBu4][B(C6F5)4] electrolyte. Cyclic voltammetry scans of (p-RC6H4)2S2 (R = Me, 1a; R = F, 1b; R = OMe, 1c) show modest chemical reversibility for the 1(0/+) couple (E1/2 values vs ferrocene: 1.04 V for 1a, 1.21 V for 1b, 0.92 V for 1c), providing the first voltammetric evidence for the radical cation [Ar2S2](+). A dimer dication, [Ar4S4](2+), is proposed as an intermediate in the formation of the electrolysis product, the trisulfide [Ar3S3](+). The chemical reversibility of the one-electron oxidations of Ar2S2 vanishes in [PF6](-)-containing electrolytes. The radical cations of the more sterically constrained ortho-substituted analogues dimesityldisulfide (2a, E1/2 = 1.01 V) and bis(2,4,6-triisopropylphenyl)disulfide (2b, E1/2 = 0.98 V) show less tendency to dimerize. In all cases except 2b, the bulk electrolysis product is [R3S3](+), consistent with earlier literature reports. A mechanism is proposed in which the trisulfide is formed by reaction of the dimer dication [Ar4S4](2+) with neutral Ar2S2 to afford the trisulfide in a net 2/3 e(-) process. Oxidation of Ar2S2, either anodically or by a strong one-electron oxidant, in the presence of cyclohexene gives an efficient synthetic route to 1,2-substituted cyclohexyldisulfides.

Metal-Metal Bond Formation Between [n]Metallocenophanes: Synthesis and Characterisation of a Dicarba[2]ruthenocenophanium Dimer

Feeling the strain: The first example of metal-metal bonding between strained [n]metallocenophanes is reported. A dicarba[2]ruthenocenophanium dimer has been synthesised through the oxidation of a dicarba[2]ruthenocenophane (see figure). The structural and electrochemical characterisation of the dimer is also discussed.

Molecular Oligogermanes and Related Compounds: Structure, Optical and Semiconductor Properties

The optical (UV/Vis absorbance, fluorescence in the solid state and in solution) and semiconducting properties of a number of di- and trigermanes as well as related silicon- and tin-containing germanes, 1-6 ((p-Tol)3 GeGeMe3 (1), Ph3 SnGe(SiMe3 )3 (2), (C6 F5 )3 GeGePh3 (3), (p-Tol)3 GeSiMe2 SiMe3 (4), (p-Tol)3 GeGeMe2 Ge(p-Tol)3 (5), (p-Tol)3 GeSiMe2 SiMe2 Ge(p-Tol)3 (6)) were investigated. Molecular structures of 5 and 6 were studied by X-ray diffraction analysis. All compounds displayed luminescence properties. In addition, a band gap (of about 3.3 eV) was measured for compounds 1-6 showing that those molecules display semiconductor properties.

Influence of Cyclopentadienyl Ring‐Tilt on Electron‐Transfer Reactions: Redox‐Induced Reactivity of Strained [2] and [3]Ruthenocenophanes

In contrast to ruthenocene [Ru(η(5) -C5 H5 )2 ] and dimethylruthenocene [Ru(η(5) -C5 H4 Me)2 ] (7), chemical oxidation of highly strained, ring-tilted [2]ruthenocenophane [Ru(η(5) -C5 H4 )2 (CH2 )2 ] (5) and slightly strained [3]ruthenocenophane [Ru(η(5) -C5 H4 )2 (CH2 )3 ] (6) with cationic oxidants containing the non-coordinating [B(C6 F5 )4 ](-) anion was found to afford stable and isolable metalmetal bonded dicationic dimer salts [Ru(η(5) -C5 H4 )2 (CH2 )2 ]2 [B(C6 F5 )4 ]2 (8) and [Ru(η(5) -C5 H4 )2 (CH2 )3 ]2 [B(C6 F5 )4 ]2 (17), respectively. Cyclic voltammetry and DFT studies indicated that the oxidation potential, propensity for dimerization, and strength of the resulting RuRu bond is strongly dependent on the degree of tilt present in 5 and 6 and thereby degree of exposure of the Ru center. Cleavage of the RuRu bond in 8 was achieved through reaction with the radical source [(CH3 )2 NC(S)SSC(S)N(CH3 )2 ] (thiram), affording unusual dimer [(CH3 )2 NCS2 Ru(η(5) -C5 H4 )(η(3) -C5 H4 )C2 H4 ]2 [B(C6 F5 )4 ]2 (9) through a haptotropic η(5) -η(3) ring-slippage followed by an apparent [2+2] cyclodimerization of the cyclopentadienyl ligand. Analogs of possible intermediates in the reaction pathway [C6 H5 ERu(η(5) -C5 H4 )2 C2 H4 ][B(C6 F5 )4 ] [E=S (15) or Se (16)] were synthesized through reaction of 8 with C6 H5 EEC6 H5 (E=S or Se).