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;
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
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,
4. Snow, M. (1974) Acta Cryst.Sect. B, 1850.
Page - Back to Sheridan Profile -
Back to Faculty Research Interests
- Chemistry Home Page - Colgate Home Page
||Design: Al Rehm and Roger S. Rowlett
Updated September 10, 2009
Questions to: firstname.lastname@example.org
© 1997 Colgate University. All Rights Reserved.