Electrografting refers to the electrochemical reaction that permits organic layers to be attached to solid conducting substrates. This definition can be extended to reactions involving an electron transfer between the substrate to be modified and the reagent, but also to examples where a reducing or oxidizing reagent is added to produce the reactive species. These methods are interesting as they provide a real bond between the surface and the organic layer. Electrografting applies to a variety of substrates including carbon, metals and their oxides, but also dielectrics such as polymers. Since the 1980s several methods have been developed, either by reduction or oxidation, and some of them have reached an industrial stage. This critical review describes the methods that are used for electrografting, their mechanism, the formation and growth of the layers as well as their applications (742 references).

Electrografting: a powerful method for surface modification

For Sandmeyer hydroxylation and chlorination in aqueous solution, the reduction steps have been investigated by means of correlation analyses of the effects of diazonium ion substitution on the rates of reduction. For simple hydroxylation, a change of behaviour between diazonium ions substituted by electron donor groups and those substituted by electron acceptor groups is interpreted as a change within an inner-sphere process from rate-determining electron transfer to rate-determining association of the reactants. By contrast, for citrate-promoted hydroxylation, a similar change in behaviour may be interpreted as a change between inner- and outer-sphere electron transfers. For chlorination, there is no mechanistic variation within the range of substituents examined but the pattern of behaviour is consistent with an inner-sphere mechanism. The various patterns of behaviour are rationalised in terms of the effects of diazonium ion substitution and catalyst ligation on the reduction
potentials and self-exchange rates of the various reacting redox couples. Comparative correlation analyses of reductions and other electrophilic reactions of diazonium ions are used to support the arguments advanced in respect of Sandmeyer reduction steps. It is suggested that the CuI reductants react via a nucleophilic bridging ligand at the diazonium Nβ to give transient Z-adducts which are the precursor complexes and that activation for electron transfer involves rotation about the N–N bond.

Sandmeyer reactions. Part 7. An investigation into the reduction steps of Sandmeyer hydroxylation and chlorination reactions

Reaction of Mo(N[R]Ar)3 (R = tBu or C(CD3)2CH3) with N2O gives rise exclusively to a 1:1 mixture of nitride NMo(N[R]Ar)3 and nitrosyl ONMo(N[R]Ar)3, rather than the known oxo complex OMo(N[R]Ar)3 and dinitrogen. Solution calorimetry measurements were used to determine the heat of reaction of Mo(N[R]Ar)3 with N2O and, independently, the heat of reaction of Mo(N[R]Ar)3 with NO. Derived from the latter measurements is an estimate (155.3 ± 3.3 kcal·mol-1) of the molybdenum−nitrogen bond dissociation enthalpy for the terminal nitrido complex, NMo(N[R]Ar)3. Comparison of the new calorimetry data with those obtained previously for oxo transfer to Mo(N[R]Ar)3 shows that the nitrous oxide N−N bond cleavage reaction is under kinetic control. Stopped-flow kinetic measurements revealed the reaction to be first order in both Mo(N[R]Ar)3 and N2O, consistent with a mechanism featuring post-rate-determining dinuclear N−N bond scission, but also consistent with cleavage of the N−N bond at a single metal center in a mechan...

On the origin of selective nitrous oxide N-N bond cleavage by three-coordinate molybdenum(III) complexes.

Visible irradiation of a gold surface dipped into a solution of diversely substituted aryldiazonium salts in the presence of a photosensitizer (Ru(bipy)32+ or eosinY) leads to a modification of the surface by attached aryl groups. A grafted nanometer thick polyphenylene film is obtained. The reaction is extended to a polyvinylchloride (PVC) surface. The mechanism proposed for the formation of this film involves the formation of an aryl radical that reacts with the surface and then with the first grafted aryl group. The film is characterized by infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), water contact angles, and ellipsometry.

Sensitized Photografting of Diazonium Salts by Visible Light.

Electrode Surface Modification Using Diazonium Salts

Aryldiazenyl radicals were formed at −104–−116°C by photoirradiation of cyclopropane solutions of 1-aryl-2-(arylthio)diazenes and of l-aryl-3,3-dimethyltriazenes in the presence or absence of olefins, and also by irradiation of cyclopropane solutions of substituted 1,2,3-benzothiadiazoles in the presence of di-t-butyl peroxide. The ESR spectra were analyzed and the hfs constants were assigned by reference to the spectra of methyl, 2H, and 15N-labelled aryldiazenyl radicals and to the results of CINDO calculations. Aryldiazenyl radicals are relatively persistent and the –N=N group rotates about the C–N2 bond axis. The preferred conformations of the radicals were considered.

Formation and Identification of Aryldiazenyl Radicals Using the ESR Technique

The decay processes of aryldiazenyl radicals were studied by using a time-resolved ESR method. The first order rate constants of decomposition were determined at −117–−48 °C in cyclopropane solutions in the presence or absence of olefins. Aryldiazenyl radicals were relatively persistent and the k1 of phenyldiazenyl-d5 at −96 °C in cyclopropane was 34 s−1. The k1 of 20 aryldiazenyl radicals at −96 °C afforded a ρ value of +1.53 (r=0.91) in a Hammett plot. Activation energies ranged from 46.2 to 25.2 kJ mol−1 and the frequency factors, log A, from 13.2 to 8.2 s−1. The second-order decay rates were measured at the initial period of the decay process at lower temperatures, in a batch system (cyclopropane) and in a flow system (isohexane). The rate constant of phenyldiazenyl-d5 in cyclopropane was 9.2·10−5 M−1 s−1 at −93 °C and the Arrhenius parameters were Ea=23.5 kJ mol−1 and logA=12.77 M−1 s−1, respectively.

Akemi Koike

Papers

Formation and Identification of Aryldiazenyl Radicals Using the ESR Technique

Decay Reactions of Aryldiazenyl Radicals in Solution

Formation and Identification of Aryldiazenyl Radicals Using the ESR Technique.