Bipyridyl reduction

Organometallic compounds apart, oxidation states below - -2 are best represented by complexes with tris-bidentate nitrogen-donor ligands such as 2,2 -bipyridyl. Reduction by LiAlH4 in thf yields tris(bipyridyl) complexes in which the formal oxidation state of vanadium is -1-2 to —1. Magnetic moments are compatible with low-spin configurations of the metal but. 

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

Electrochemical reduction of pentatluoronitrobenzene produces an intermediate radical anion that couples at position 4 to form the corresponding biphenyl along with hydroxy derivatives from subsequent nucleophilic substitution meta to the nitio groups [44] (equation 34) Similar reduction of halopyridines such as pen-tafluoropyridine leads mainly to 4,4 bipyridyls [45] (equation 35)... 

The bipyridyl ligand, with 80 %, gives the best catalyst, probably because it affords the best balance between the a-donor and rt-acceptor characters which favours respectively the oxidative addition of the aryl bromide and the reductive elimination of the aryl ether at the nickel center. 

Examination by stopped-flow method of the autoxidation of the bipyridyl complex of Cu(I), Cu(bipy)2, shows that it is first-order both in O2 and in the complex, with k2 (25 °C) = (6.5 0.5)x 10 l.moIe . sec. No was incorporated from labelled water into the product H2O2, indicating the 0-0 bond remains intact during reduction. The authors favour a /wo-equivalent reduction on thermodyamic grounds, proposing a rate-determining formation of a Cu(I)-02 complex which reacts rapidly with a second Cu(I) species, viz. 

A stopped-flow examination of the reduction by the bipyridyl complex of Cu(I), Cu(bipy)2, reveals the kinetics... 

The reductions by ferrous ion and mono- and bis-bipyridyl complexes of Fe(II) are also simple second-order with (for the Fe reaction at zero ionic strength ). 2 = TO X 10 exp(—12.1 X 10 /Rr) l.mole . sec . This reaction generates an intermediate capable of oxidising ethanol but the effect is suppressed by addition of Cl , Br and acrylonitrile, the latter being polymerised. 

The use of triphenylphosphine and 2,2 -bipyridyl disulphide in oxidation-reduction condensations has been extended to the phosphorylation of alcohols and phosphates, and to the preparation of 5 -(2-pyridyl) phosphorothioates (60) which have been used for the synthesis of pyrophosphates (see Chapter 6, Section 1). 

Allegedly, the product of the reduction is a bipyridyl palla-dium(0) complex. This is not likely and our personal opinion is that the obtained material is a supported metal palladium catalyst. This is the reason why the paper was included in this review. 

Figure 17.19 A membianeless ethanol/02 enz3fme fuel cell. Alcohol dehydrogenase and aldehyde dehydrogenase catalyze a stepwise oxidation of ethanol to acetaldehyde and then to acetate, passing electrons to the anode via the mediator NAD+/NADH. At the carhon cathode, electrons are passed via the [Ru(2,2 -bipyridyl)3] and biUverdin/bilimbin couples to bilirubin oxidase, which catalyzes O2 reduction to H2O. (a) Schematic representation of the reactions occruring. (b) Power/cmrent response for the ceU operating in buffered solution at pH 7.15, containing 1 mM ethanol and 1 mM NAD. Panel (b) reprinted from Topcagic and Minteer [2006]. Copyright Elsevier, 2006.

The reductive coupling of allyl halides to 1,5-hexadiene at glassy C electrodes was catalyzed by tris(2, 2,-bipyridyl)cobalt(II) and tris(4,4 -dimethyl-2, 2/-bipyridyl)cobalt(II) in aqueous solutions of 0.1 M sodium dodecylsulfate (SDS) or 0.1 M cetyltrimethylammonium bromide (CTAB).48 An organocobalt(I) intermediate was observed by its separate voltammetric reduction peak in each system studied. This intermediate undergoes an internal redox reaction to form 1,5-hexadiene... 

The intercalated catalysts can often be regarded as biomimetic oxidation catalysts. The intercalation of cationic metal complexes in the interlamellar space of clays often leads to increased catalytic activity and selectivity, due to the limited orientations by which the molecules are forced to accommodate themselves between sheets. The clays have electrostatic fields in their interlayer therefore, the intercalated metal complexes are more positively charged. Such complexes may show different behavior. For example, cationic Rh complexes catalyze the regioselective hydrogenation of carbonyl groups, whereas neutral complexes are not active.149 Cis-Alkenes are hydrogenated preferentially on bipyridyl-Pd(II) acetate intercalated in montmorillonite.150 The same catalyst was also used for the reduction of nitrobenzene.151... 

A diorganogold(m) complex with the aryl substituent attached to a bipyridyl ligand was found to undergo unexpected reductive elimination of the hydrocarbon upon treatment with Ph3P which involves C-C coupling (Equation (32)).121... 

A unique CL reagent, /n.v(2,2 -bipyridyl)rut.hcnium(II) [Ru(bpy)32+] for the postcolumn CL reaction, was applied to HPLC detection. The oxidative-reduction reaction scheme of CL from Ru(bpy)32+ is shown in Figure 17. When the production of light following an oxidation of Ru(bpy)32+ to Ru(bpy)33+ at an electrode surface is measured, this CL reaction is termed electrogenerated chemiluminescence (ECL). The CL intensity is directly proportional to the amount of the reduc-tant, that is, the analyte. 

Systems which fulfil these conditions are tris(2,2 -bipyridyl)rhodium complexes [63] and, more effectively, substituted or unsubstituted (2,2 -bipyridyl) (pentamethylcyclopentadienyl)-rhodium complexes [64], Electrochemical reduction of these complexes at potentials between — 680 mV and — 840 mV vs SCE leads to the formation of rhodium hydride complexes. Strong catalytic effects observed in cyclic voltammetry and preparative electrolyses are... 

Fig. 17. Electroenzymatic reduction of 4-phenyI-2-butanone catalyzed by HLADH with in-situ indirect electrochemical regeneration of NADH using a Cp (2,2 -bipyridyl)aquo rhodium(III) complex as mediator...

The orange complex 83 (57) exhibits a pseudotetrahedral geometry in the solid state, with the chlorine atoms distorted 49° and 63° away from the bipyridyl-Cu plane (Fig. 8). The d-d transition occurs at 919 nm. It seems likely that the large size of the ligand substituent is responsible for the deviation in the structure of the copper complex. Whether this effect is also responsible for the ease of reduction of the corresponding triflate complex by diazoester is not clear. 

Tetramethyl- or tetraphenyl- (cyclobutadiene)nickel dihalides undergo reductive ligand substitution with nitrogen donor ligands such as 2,2 -bipyridine or 1,4-diaza-1,3-dienes with the addition of sodium metal237. The 2,2/-bipyridyl ligand is readily displaced and reaction of this complex with a variety of olefins and alkynes leads to cycloaddition reactions with the cyclobutadiene ligand. 

Reductive dimerization of substituted pyridines to yield bipyridyls by zinc is catalysed by nickel salts under phase-transfer catalytic conditions [25]. 

For instance, the (pentaphenylcyclopentadienyl) cobalt dicarbonyl anion-radical complex [(q-C5Ph5)Co(CO)2] has (n + 1) metal orbital populated with an unpaired electron, according to calculations by Connelly et al. (1986). In contrast, reduction of (bpy)Cr(CO)4 (bpy = 2,2 -bipyridyl) to its anion radical is known to occur without any major change in its structure or composition. 

Reductive elimination on transition metal complexes seems to be enhanced by coordination of electron-withdrawing 71-acids such as cyanobenzene and cyanoethylene. For example, the reductive elimination reaction of NiR2(bpy) (R=alkyl or aryl group bpy =2,2 -bipyridyl) is enhanced by electron-withdrawing olefinic and aromatic compounds [12-16] (Scheme 1). 

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