Chemistry @ Colgate

Peter Sheridan

Research Projects Description

We are studying a group of coordination compounds containing the biguanide (C2H5N7, abbreviated as HBg) ligand. Single crystal X-ray analysis1 confirms the assymetric structure for the neutral biguanide (C2H5N7), (1). Not surprisingly, biguanide is basic (a stronger base than ammonia), and the structures of the mono (H2Bg+) and diprotonated (H3Bg2+) species are also represented below (2 and 3, respectively)2. In contrast to the assymetric parent, both the mono and diprotonated ions are symmetric, and in the diprotonated species the central nitrogen atom is protonated.

We are interested in biguanide because it is an effective bidentate ligand toward a variety of metals; a most unusual aspect of the biguanide ligand is that stable complexes can also be formed with HBg in the conjugate base form (Bg-). Single crystal x-ray analyses show that biguanide is symmetric and virtually planar in either form; the coordinated, deprotonated form (as seen in [Co(Bg)3]) is represented in 4, while the coordinated proteonated form (the structure is of [Co(HBg)3]Cl3 * H2O) has a protonated central nitrogen (5), reminiscent of the very acidic, diprotonated form (3).
The coordination chemistry of biguanide was an active research area in India between 1935 and 1950, but work slowed markedly in the 60's, and only scattered reports are available in the more modern literature (most of it still in the Indian literature). The chemical reactivity of these complexes (thermal, photochemical, and electrochemical) is virtually unexplored.
Our early work has focused on the acid/base properties of biguanides coordinated to Co(III) or Cr(III), and we have shown that even with an H atom on the central N atom, the protonated ligand (C2H7N5) is not acidic; aqueous solutions of [M(HBg)3]3+ (M = Co, Cr) do not react with added OH-. The anionic ligand (C2H7N5-) is basic, however, as [Co(HBg)3] reacts reversibly with three equivalents of H+: the acidity of the resulting [Co(HBg)3]3+ ion implies that at least two forms of coordinated (C2H7N5) exist: an acidic form, and the more familiar form which is neither acidic nor basic.
This acid-base work is necessary, as we intend to compare the chemistries of analogous complexes with protonated and deprotonated biguanides. Our initial goal is to prepare cis and trans forms of [M(Bg)2V2]-, and compare their rates of substitutions to those of the more thoroughly (but still incompletely) studied [M(Bg)2V2]+ analogues (where X is a halogen or psuedohalogen). (To our surprise, we can find no syntheses of complexes containing deprotonated biguanides and other ligands; [M(Bg)2V2]- complexes have not been reported.) We are also interested in the photo- and electrochemical properties of these unusual complexes; if the thermal reactivity is sufficiently robust, we will initiate such studies.
This is an excellent area for undergraduates, as the complexes are easily prepared and air stable, and se can characterize the complexes with modern instruments (NMR, UV/Vis and IR, for example, rather than the freezing point depressions or conductivities in the early literature). The unorthodox electronic structures and unique acid/base properties of these complexes suggest some interesting, and perhaps, important chemistry could result.

1. Ernst, S. R., & Cagle, F. W. (1977) Acta Cryst. B33, 235.
2. Pinkerton, A. A. & Schwarzenbach, D. (1978) J. Chem. Soc. Dalton Trans. 1978, 989.
3. Cioghi, L., Lanfranchi, M., Pellizzi, G., Tarasconi, P. (1978) Trans. Metal Chem. 3, 69.
4. Snow, M. (1974) Acta Cryst.Sect. B, 1850.

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