Bipyridine radical cation

Metallopolymer films have also been prepared by oxidative polymerization of complexes of the type [M(phen)2(4,4 -bipy)2]2+ (M = Fe, Ru, or Os phen= 1,10-phenanthroline, 4,4 -bipy = 4,4 -bipyridine).23 Such films are both oxidatively and reductively electrochromic reversible film-based reduction at potentials below —IV lead to dark purple films,23 the color and potential region being consistent with the viologen dication/radical cation electrochromic response. A purple state at high negative potentials has also been observed for polymeric films prepared from [Ru(L13)3]2+.24 Electropolymerized films prepared from the complexes [Ru(L16)-(bipy)2][PF6]22 and [Ru(L17)3][PF6]226,27 exhibit reversible orange/transparent electrochromic behavior associated with the Run/Ruin interconversion. 

The electron spin resonance (ESR) spectra of the radical anion of 2,2 -bipyridine, sometimes in the form of its alkali metal com-plgx, 71.175,177.299-304 radical anion of 3,3 -bipyridine, ° and the radical anion of 4,4 -bipyridine, ° ° usually obtained by reduction of the bipyridines with an alkali metal, have been measured, and hyperfine splitting constants were assigned. Related biradical species have also been investigated. The ESR spectrum of the 4,4 -bipyridinium radical cation, of which... 

The one-electron reduction of diquaternary salts of 2,2 -bipyridine has attracted much attention. When an aqueous solution of diquat dibromide (75) is treated with a one-electron reducing agent, such as zinc dust, the solution acquires an intense green color. This is due to the formation of the stable radical cation 87. The one-electron transfer is completely reversed by air. In theory the radical cation 87 can take up another electron to form the neutral species 88. The stability of the radical cation 87 is due to the ability of the odd electron to be located at any of the nuclear positions because of the near coplanarity of the two pyridine rings. The potential 0 of the first one-electron transfer occurs at — 0.35 V in aqueous solution... 

Very little has been reported regarding 2,4 -bipyridine. As expected, it is quaternized preferentially on the y-pyridyl ring " and with excess methyl iodide the diquaternary salt l,r-dimethyl-2,4 -bipyridinium diiodide is ob-tained. ° The latter salt, like diquat, is reduced to a radical cation by one-electron reducing agents. The potential E of the one-electron transfer in aqueous solution is —0.64 lV-alkyl-2,4 -bipyridinones have been... 

Several reactions of diquaternary salts of 4,4 -bipyridine have attracted attention. The behavior of 1,1 -dialkyl diquaternary salts in strong alkaline solution has long been of interest. They give intense blue-violet colorations on treatment with aqueous potassium hydroxide, and the deep color is discharged on shaking in air. The blue-violet color is largely due to radical cations such as 94. The mechanism of the reaction of paraquat dication (95) with alkali has been elucidated. There are several competing... 

The spectra of the radical cations derived from l,r-dialkyl diquaternary salts of 4,4 -bipyridine have received attention, the UV -997,1029,1036.1056 ig spectfa having been well studied. -1057-1059 jg eyjfjejjt from some of these studies that the radical cations are in equilibrium with dimeric species. The Raman spectra of paraquat and its radical cation adsorbed at a silver electrode have also been investigated, whereas a resonance Raman effect with radical cations of viologens has been noted. Other Raman studies at metal... 

Radical Cations with Biological Relevance. - The radical cation of 1-ben-zyl-l,4-dihydronicotinamide, an analogue of NADH, was generated by oxidation with Fe(bpy)33+ and Ru(bpy)33+ (bpy = 2,2 -bipyridine) in deaerated acetonitrile solutions using a rapid-mixing flow apparatus. The ESR spectra reveal a keto structure of the radical cation, rationalized by DFT calculations. Upon photo-induced electron transfer, also the enol form could be established, which, however, relaxes to the keto form131,132. 

Bauld and coworkers studied the [2+2] cycloaddition of A-vinyl carbazoles 86a and electron-rich styrenes 86b catalyzed by iron(III) catalysts A or B in the presence of 2,2 -bipyridine as a ligand, which was reported originally by Ledwith and coworkers (Fig. 21) [142, 143]. Deuterium-labeling studies provided support for the stepwise nature of the process, consisting of reversible SET oxidation of the electron-rich olefin to a radical cation 86 A. Nucleophilic addition of excess 86 leads to distonic radical cation 86B, which cyclizes to cyclobutane radical cation 86C. Back electron transfer affords cyclobutanes 87 and regenerates the catalyst. Photoelectron transfer catalysis gave essentially the same result, thus supporting the pathway. 

Colorless viologens composed of diquatemary salts of 4,4 bipyridine(I), 2,2 -bipyridine, and 1,10-phenanthroline turn into a violet-blue radical cation(II) by one-electron reduction,5 and are further reduced to the yellowish quinonoid(III ) via the biradical (III) (Scheme 1). 

Ru(II)tris(bipyridine) [Ru(bpy)3 +] as a photosensitizer, triammonium ethylene-diaminetetraacetic acid [(NH4)3EDTA] as a sacrificial electron donor and the enzyme ferredoxin NADP+ reductase (FDR) [215, 216]. Oxidative electron-transfer quenching of the excited Ru(bpy)3 + yields the A,A -dimethyl-4,4 -bipyridinium radical cation (reduced methylviologen, MV+), which mediates the reduction of NADP+ in the presence of FDR as a biocatalyst (Figure 32A). The quantum efficiency for NADH production corresponds to = 1.9 x 10 . A related system that includes Zn(II)wc50-(A-tetramethylpyridinium)porphyrin (Zn-TMPyP +) as a photosensitizer, mercaptoethanol as a sacrificial donor and lipoamide dehydrogenase (LipDH) as a biocatalyst has been applied for the photochemical reduction of NAD+ to NADH (Figure 32B). 

Reduction of the simple AT-methylpyridinium ion, 103, is believed initially to give the expected AT,AT -dimethyltetrahydro-4,4 -bipyridine, but the end product (in the absence of oxygen) is the A/, A/ -dimethyIbipyridine radical cation formed by a formal loss of two hydride ions and one-electron reduction of A/. A/ -dimethylbipyridinium [297,298]. The isolated product, 27,27 -dimethylbipyridinium dication, results from air-oxidation of the radical cation [298] ... 

These chiral viologens were applied to the reduction of metmyoglobin, too [72]. In this reaction, the one-electron reduced viologen radical cations were photochemically produced by tris(2,2 -bipyridine)ruthenium(II) in the presence of disodium salt of ethylene diamine tetra acetic acid (Na2H2edta), and the cation radical reacts with metmyoglobin to reduce it. The (S,S) isomer more rapidly reacts with metmyoglobin than the (R,R) isomer. The reaction rate is analyzed with Michaelis-Menten mechanism, as shown in Scheme 29. 

Organo Metallic Compound/Ketone-Based Systems A nice example of a bond cleavage via an electron transfer reaction was shown in the ruthenium tris bipyridine/ morpholinoketone system the readily formed radical cation on the amine cleaves into an imino cation and a benzoyl radical [214]. 

---