Crystal and Molecular Structures of Divalent Metal Complexes with cis, cis-1,3,5-Cyclohexanetriol and Nuclear Magnetic Resonance Studies of Diamagnetic Metal Complexes in Solution

One of our recent interests has been to explore the structure of cis, cis-1, 3, 5-cyclohexanetriol (triol) complexes of first raw transition metal ions. To date, there has not been extensive study of the coordination of triol to these metals. The reasons may be due to the relative instability of these complexes in aqueous media where even small amounts of water effectively compete for coordination sites of the metal ion, the hygroscopic nature of these complexes, and the difficulties encountered in growing single crystals. In addition, no complete X-ray structure analysis of these complexes, leading to accurate values of bond lengths and bond angles or to geometric configuration, has been carried out previous to this work. Only the arrangement and configuration of a-phloroglucitol dihydrate (triol) molecule in crystalline form has been reported. It has been previously shown that M(triol) 2Y2 complexes (M=Mn+2, Fe+2, eo+2, Ni+2, cu+2, zn+2; Y=p-CH3C6H4so4-l, N03-l, CH3so3-1) are formed in crystalline form from triol and the appropriate hydrated metal salt in 95% ethanol.1 Single crystals of. Ni(triol)2cl2 and Cu(triol)Cl2, prepared as above, were obtained. Both of these crystals were analyzed by single crystal X-ray diffraction which revealed their solid state structure. The most interesting feature of these results is the transformation of the uncomplexed ligand to a species capable of coordinating to the metal ion. Specifically, the equatorial confrontation of triol\u27s three oxygen atoms converts to a sterically hindered axial conformation when coordinated to the metal ion. \u27Ibis unfavorable conformation is stabilized by the chelate effect. 1H and 13c nuclear magnetic resonance (NMR) spectroscopy provide additional information in the study of complexation in solution. The use of NMR in studying the structure and bonding of metal chelate compounds has been increasing in the last decade. A good share of this work has focused on characterizing complexes by NMR. These studies demonstrated that the spectra of complexed and uncomplexed ligand were different. Normally, there was a change in the chemical shift for equivalent atoms. For instance, evidence for coordination of pyridine nitrogen atoms of dipyridyi ketone (DPK) was obtained from carbon-13 and proton NMR spectra of the ligand and the diamagnetic ra+3 and y+3 complexes. 3 In the proton spectra, there are a downfield shift of all proton resonance peaks in the complexes compared to the free ligand. 3 Shifts in carbon resonance signals of the complexes compared to the free ligand were also observed. Therefore, a reasonable conclusion is that observed changes in chemical. shift is a consequence of complexation. In the present study, quantities of DPK and trial in solution were examined by NMR spectroscopy in the absence and presence of diamagnetic metal ions. In addition, the hydration of the carbonyl of DPK was monitored by changes in the resonance from the carbonyl carbon atom. Other endeavors included the synthesis of cis,cis-1,3,5 cyclohexanetriol .and the substitution reaction of a sulfur atom for the oxygen atom of the carbonyl group of DPK. Molecular modeling was also used to try to predict the structure and bond length and bond angles of bis triol complexes of Ni(II), CU(II) , Al, and Zn(II). In addition, comparisons between calculated and experimental values for bond lengths and angles of Ni(triol) 2 and CU(triol)Cl2 were made