Second sphere coordination of phenols, anilines, and benzoic acids to the hexacyanocobaltate(III) anion

Abstract

Can. J. Chem. 61, 1524 (1983. Complexing between the hexacyanocobaltatc(lI1) anion and ten substitutcd phenols, cight substituted anilines, and eight substituted benzoic acids has been studied in dimethyl sulfoxide solutions by mcans of '"Co nmr. For the phenols substantial changes in the chemical shifts and in the linc widths are obscrvcd and are attributed to hydrogen bonding interactions. Effective equilibrium constants and chemical shifts for the hydrogcn bondcd species have been calculated. Thc equilibrium constants vary with the phcnol substituent, being largest for electron withdrawing substituents and smallest for electron donating substituents. The chemical shifts are virtually independent of substituent. The data show that the lifetimes of the hydrogen bonded complexes are long compared to the rotational correlation time. Thc complex with porn-nitrophenol has also been studied by measuring 'H and '.'C spin-lattice relaxation times. These measurements allow the separation of the increase in 59 Co line width due to increase in correlation time and that due to increase in quadrupolc coupling constant. From the I3C measurements a value of the correlation time for the complex of 2.8 1 0~" ' s is obtained together with a '9Co quadrupole coupling constant of 8.34 MHz. The equilibrium constants for complex formation with the anilincs are small and only the products of the equilibrium constant and the chernical shift of thc hydrogen bonded complex can be derived from the experimental data. The bcnzoic acids show evidence of dinicrization in solution and this again prevcnts the calculation of absolute equilibrium constants. The data, however, do indicate that the ability to form hydrogcn bond complexes does not vary dramatically with thc acid strength of the hydrogen donor. With phenols and benzoic acids therc is considerable linc broadening on complex formation but with anilines only a small line broadening is obscrved. This is consistent with very short hydrogen bond lifetimes for the aniline complexes. DONALD R. EATON, RICHARD J . BUIST et CAROL V. ROGERSON. Can. J. Chcm. 61, 1524 (1983. Faisant appel ii la rmn du ' "0, on a CtudiC la complexation de l'anion hcxacyano cobaltate(lll) avec dix phenols substituks, huit anilines substitukes et huit acides bcnzo'iqucs substituCes dans des solutions dc dimCthylsulfoxide. Dans le cas des phknols on observe des changements importants dans Ics dkplacements chimiques et dans la largeur des raies et on les attribue i des interactions de liaison hydrogknc. On a calculC les constantes effectivcs d'kquilibre et les dCplacements chimiques des espkces likes par des liaisons hydrogknes. Les constantes d'dquilibre varient avcc le substituant du phdnol. cette constante est plus ClevCe dans le cas des substituants dlectro-attracteurs ct plus faible dans le cas des substituants Clectro-donneurs. Les dCplacements chimiques sont virtuellement indkpendants du substituant. Les donnCes montrent que les temps de vie des complexes liCs par des liaisons hydrogknes sont longs par rapport...

Outer sphere complexes as intermediates in coordination chemistry

“…They suggest that hydrogen bonding between the coordinated water and the cyanide groups of the incoming ligand may be important in stabilizing the outer sphere complex. The evidence for hydrogen bonded second sphere complexes with the same hexacyanometalates obtained by 59Co NMR and IR spectroscopy reported in the previous section of this review [71][72][73] provides some support for this interpretation. Further support is provided by the results of Ewen and Darensbourg [6] who obtained IR evidence for hydrogen bonding between cobaloximes and piperidine and suggested that this outer sphere complex could act as an intermediate in an Ia ligand substitution mechanism.…”

“…Similar lifetimes have been obtained for H bond breaking in amines [69,70]. Hydrogen bonding between phenols, anilines and organic acids and the hexacyanocobaltate(HI) anion has been studied by 59Co NMR and by IR spectroscopy [71][72][73]. These studies also provided evidence for very short lifetimes of between 10 -° s and 10 -12 S. At the latter limit broadening of vibrational bands becomes possible.…”