Resonance Raman electronic absorption and circular dichroism spectra and pH titration curves are reported for the trianionic ferric complexes of enterobactin, catechol, and N-methyl-2,3-dihydroxybenzamide (MDHB). The spectral signatures of the enterobactin and MDHB complexes are virtually identical and differ from those of the catechol complex in ways that reflect the influence of the amide group on the electronic structure. Excitation in either the visible charge-transfer bands or the near-ultraviolet pi-pi* bands enhances Raman bands associated with benzene ring modes, although the relative enhancements differ markedly in the two regions. The data stronly support a structural model in which iron is bound exclusively to the phenolate oxygen atoms in all three complexes.

Resonance Raman spectra, obtained with 5145,4880 and 3638 excitation, are reported for aquo-, methyl-, 5'-deoxyadenosyl-(coenzyme B,,), thiosolfate and cysteinyl cobdamin, of methyl cobmamide, and of Co(XI) cobalamin (Blzr). Laser-induced photodecomposition was held to acceptable levels with a rapid-%ow technique. A rich assortment of corrin ring vibrations are observed above 600 an-', but the frequencies are constant (within *3cm-') for all the derivatives. Some Raman bands show differential enhancement by the visible and near-ultraviolet electronic transitions, reflecting their different polarizntions. B,, shows large relative intensity changes in bands at 1500 and 1600 cm-', witb respect to the Co(1II) derivatives. Despite an absorption spectrum resembling that of B,,, methyl cobmamide shows Raman intensity behavior typical of the other Co(II1) derivatives. INTRODUCIlONThere is much current interest in the chemistry and biochemistry of vitamin B12 and its derivatives.' All of these contain a cobalt atom coordinated to four nitrogen atoms of the corrin ring. One axial coordinate position is occupied by benzimidazole, bound covalently to the ring, although this can be removed chemically, e.g. in the cobinamides.2 The other axial position can be occupied by any of a number of ligands (sixth ligand). Coenzyme B12 contains a carbon-bound 5'-deoxyadenosyl ligand,3 and complexes with other carbon ligands can also be prepared.The corrins have strong absorption bands in the visible and near-u.v. regions, which are due to T-T* electronic transitions. As with the porphyrins, excitation in these bands produces strong resonance enhancement of Raman bands associated with corrin ring vibrations. Several resonance Raman (RR) studies of B12 derivatives have been rep~rted.~" These have shown very similar RR spectra for cobalamins, the Co(II1) B12 derivatives, regardless of the nature of the sixth ligand. Somewhat different spectra are observed for the reduced forms, B12r, which contains Co(I1) and BlZs, which contains Co(1).Wozniak and Spiro reported,6 however, that methyl cobinamide and 'base off methyl cobalamin, in which the bond between benzimidazole and cobalt is dissociated by protonation, both have spectra identical with B12r, rather than other Co(II1) derivatives. They suggested that the loss of benzimidazole might allow the corrin ring to relax to a conformation close to that of B12r. In retrospect, an alternate explanation is that the observed spectra were in fact due to Co(I1) species t Abbreviations: B,, = Vitamin B,, [cr-(5,6-dimethylbenzimidazolyl)cobamide]; BIZ, = Co(I1) Cobalamin; Coenzyme-B,, = 5'-deoxyadenosyl-B,,; Methyl-cobin = Methyl-cobinamide. @ Heyden & Son Ltd, 1977 generated by photolysis in the laser beam. Photolysis of B12 derivatives is particularly facile when they contain ~a r b o n ,~-~ and to a lesser extent sulfur, ligands, and under anaerobic conditions, the product is B12r. Detection of this artifact is made difficult by the similarity of the B12r and methyl cobinamide (Me-Cobin) or base-off ...

Spectra of Cobalt(IIj-Imidazole Complexes 1105 compared to unity. While E12 does enter x quadratically, the exact value used is not very important unless it is small enough to give x less than, say, unity. The insensitivity of 1 -( ) to variation in x when x » 1 is simply the result of the rapid fall-off of the Boltzmann factor exp(-y) as compared to Pr(t). This behavior was shown in Figure 2 and discussed in section III.

The formation of 1 : 1 complex species between SO,, SOCI,, SO,CI,, and CI-, Br-, and I-is reported. The stability constants of the S0,X-species were determined in acetonitrile. dimethyl sulphoxide. and water. The stability constants of SOCI,,X-and SO,CI,,X-were determined in acetonitrile. The standard enthalpies of formation point to weak association of a charge-transfer type. The nature of association between the complexing components is discussed in relation to the acid-base characters of such components.WITECKOWA and WITOK investigated the reaction between SO, and iodine in the gas phase and in aqueous solutions spectrophotometrically and kinetically. They suggested that the interaction between HI and SO, in aqueous solutions is due to dipole-dipole interaction. Burke and Smith studied the molecular complexes between HF and SO, by i.r. methods. Jander and Tuerk3 studied the adduct of iodine with H,S in dichloroethylene at -95 "C. The low enthalpy of formation (AH" = -7.59 kcal mol-l) was taken as an indication of the charge-transfer nature of the adduct formation. They also prepared SO,,I, and SO,,I-complexes. Burow studied the solvate formation between SO, and C1-, Br-, and I-in liquid sulphur dioxide.Gutmann isolated a number of adducts of SOCI, and SO,CI,. Sandhu et aL6 discussed the tendency of sulphuryl chloride (SO,Cl,) to form adducts with Lewis acids and bases. We have extended our earlier work on the tendency of sulphur compounds to form weak complexes with halide ions. Our work on selenium compounds will be discussed separately. EXPERIMENTALDetection of Complex Species in Solution.- Figure 1 shows the absorbance peaks of mixtures of SO, with A, tetramethylammonium iodide ; B, tetraethylammonium bromide, and C, tetramethylammonium chloride in acetonitrile. Table 1 gives Lx. values. Similar peaks were obtained in dimethyl sulphoxide, nitromethane, water (only for SO,), and other solvents. The spectra of SO, with halide ions confirm work by Jander and Seel * and their co-workers. The same spectra were obtained when alkalimetal, trimethylsulphonium, and trimethylphosphonium halides were used. This confirmed that the new spectra resulted from the interaction of the halide ions with SO,, SOCl,, and SO,Cl,.Stoicheiouzetvy.-Job's @ and Asmus's lo methods were adopted in studying the stoicheiometry of the complex species of sulphur compounds with halide ions. The former gave the empirical formula of the complex and the 1 S. Witeckowa and T. Witok, Zeszyty nauk. Politech. lodz., latter its molecular formula. Since Job's method proves conclusively the existence of a 1 : 1 species, the results on Asmus's method will not be reported. 2.0 ' 4 4 1.2 A/nm FIGURE 1 Spectra of complex halide species in acetonitrile (A, €3, C see text) *TABLE 1 hm,/nm of mixtures in acetonitrile c1-Br-I-280 292 320 380 293 322 382 so2 S02C12 275 293 322 375 SOCI, 280 Evaluation of Stability Constants.-(a) Grafihical method. The complex species are formed between a polar molecule (SO,, SOCl,, and SO,Cl,) and a halide ion (Cl-, Br-, and ...

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