Title

Binding site preference and isomerization in pentaamminecobalt(III) linkage isomers

Date of Award

1991

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Chemistry

First Committee Member

William L. Purcell, Committee Chair

Abstract

The coordination of ambidentate tetrazole ligands to the pentaamminecobalt(III) cation was studied using both Visible and Nuclear Magnetic Resonance spectroscopy. Reaction rate and activation data for the linkage isomerization of the N1 bound (5-methyltetrazolato)pentaamminecobalt(III) complex were determined in eleven solvents. Reaction rate constants at 25$\sp\circ$C were as follows: DMSO $1.16\times 10\sp{-6 }{\rm s}\sp{-1},$ Formamide $1.21\times 10\sp{-6}{\rm s}\sp{-1},$ N-Methylformamide $1.17\times 10\sp{-6}{\rm s}\sp{-1},$ DMF $7.07\times 10\sp{-7}{\rm s}\sp{-1},$ Dimethylacetamide $8.24\times 10\sp{-7}{\rm s}\sp{-1},$ Diethylacetamide $9.23\times 10\sp{-7}{\rm s}\sp{-1},$ Acetonitrile $3.41\times 10\sp{-7}{\rm s}\sp{-1},$ Propionitrile $2.59\times 10\sp{-7}{\rm s}\sp{-1},$ HMPA $1.54\times 10\sp{-6}{\rm s}\sp{-1},$ Methanol $8.92\times 10\sp{-7}{\rm s}\sp{-1}$ and Water $1.13\times 10\sp{-6}{\rm s}\sp{-1}.$These data were then fitted according to two different solvent polarity models. The Reichardt (E$\sb{\rm T})$ and Gutmann (D$\sb{\rm N})$ solvent acidity and basicity scales were found to successfully correlate the observed solvent effect data (R$\sp2$ = 0.97).$$\rm ln\ k\sb{25\sp\circ C} = -18.70 + 0.056E\sb{T} + 0.079D\sb{N}\leqno(1)$$Unlike the analogous nitrito to nitro linkage isomerization, the N1 to N2 linkage isomerization for the (5-methyltetrazolato)pentaamminecobalt(III) complex was not significantly slower in the solvents water and methanol than predicted by equation 1. The solvent effect data were also successfully fitted using the Kamlet and Taft model of solvent hydrogen bond acidity and basicity.$$\rm ln\ k\sb{25\sp\circ C} = -17.35 + 0.92\alpha + 2.16\beta + 1.98\pi\sp*\ R\sp2 = 0.98\leqno(2)$$Reaction rates in the hydroxylic solvents water and methanol were not significantly slower than predicted by equation 2. The original solvent effect data for the nitrito linkage isomerization were also re-analyzed using the Kamlet and Taft model and were found to be consistent with the results for 5-methyltetrazolato provided that the anomalous sulpholane data point was deleted.The natural abundance $\sp $N NMR spectra of two N1 bonded and one N2 bonded tetrazole complexes of cobalt(III) were measured as perchlorate salts. Chemical shifts in d$\sb6$ DMSO relative to an external nitromethane reference were as follows: N1 bonded ((5-methyltetrazolato)pentaamminecobalt(III)) perchlorate N1 $-$167.3, N2 0.1, N3 12.4, N4 $-$53.2; N1 bonded (N1,N$\sp\prime$-tetraammine(tetrazolato-$ 5$-carboximidamidato)cobalt(III)) perchlorate N1 $-$146.7, N2 0.10, N3 28.1, N4 $-$53.0; N2 bonded ((5-methyltetrazolato)pentaamminecobalt(III)) perchlorate N1 $-$69.1, N2 $-$98.2, N3 6.8, N4 $-$47.4. These ring nitrogen assignments were made by preparing a specifically $\sp $N enriched N1 bonded 5-methyltetrazole complex such that enrichment occurred specifically at ring positions N2 and N4. Allowing this enriched sample to isomerize to the N2 bonded form permitted the investigation of linkage isomerization transition states.

Keywords

Chemistry, Inorganic

Link to Full Text

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