498
D.A. Brown et al. / Inorganic Chemistry Communications 7 (2004) 495–498
drogens were located in the difference fourier map and
allowed to refine freely including isotropic temperature
factors. In III, all hydrogens were treated as the hy-
drogens attached to disordered carbons in I and II.
Anisotropic temperature factors were used for all non-
hydrogen atoms.
This result suggests that in related enzymes, e.g. ureases,
protonation may occur at the nitrogen atoms of ami-
noacid side chains in proteins as suggested previously [4].
Crystallographic data for the structures of I, II and
III reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre, CCDC.
A comparison of the structures of the trimeric and
dimeric cobalt hydroxamates II and III (Figs. 2 and 3,
respectively) shows the presence of bridging/chelating
hydroxamates in both cases with two bridges in the case
of the dimer III and single hydroxamate bridges between
each pair of cobalt atoms in II with the central cobalt
occupying an inversion centre with O(1)# 1–Co(1)–O(1)
at 180.000°. The Co(1)–Co(2) distance of 3.028(2) in III
is shorter than the corresponding distance Co(1)–Co(2)
of 3.550(5) in II, due to the presence of a double hy-
droxamate bridge in III compared with one hydroxa-
mate bridge for each pair of cobalt atoms in II. Small
lengthenings are present in some comparable distances
e.g. Co(1)–O(1) of 2.048(1) in II and Co(1)–O(21) of
2.080(5) in III reflecting the linear Co–O–Co moiety in
II compared with the bent bridging Co–O bonds in III,
where Co(1)–O(21)–Co(2) is 91.74 (19)°. In both com-
plexes the cobalt atoms are in near octahedral environ-
ments e.g. O(21)–Co(1)–O(31) is 87.2(2)° in III
compared with O(1)–Co(2)–O(5) is 89.83(5)° in II.
The different behaviour between the trifluoroacetate
and acetate bridged dimers is a consequence of their
different electronic properties. Thus OAcF is a weaker
donor then OAc and so the bridging water–carboxylate
bonds are more easily broken by the neutral hydroxamic
acid ligand for the former series, but the resulting tri-
fluoroacetic acid thereby formed is more acidic than
acetic acid and so protonates the tmen nitrogen atoms.
Acknowledgements
We thank the EU COST D21 programme, Project
D21/0001/00 and Dublin City Council for support.
References
[1] D.E. Wilcox, Chem. Rev. 96 (1996) 2435–2458.
[2] E.M.F. Muri, M.J. Nieto, R.D. Sindelar, J.S. Williamson, Curr.
Med. Chem. 9 (2002) 1631–1653.
[3] (a) M.A. Pearson, L.O. Nickel, R.P. Hausinger, P.A. Karplus,
Biochemistry 36 (1997) 8164–8172;
(b) S. Ciurli, S. Benini, W.R. Rypniewski, K.S. Wilson, S. Miletti,
S. Mangani, Coord. Chem. Rev. 190-192 (1999) 331–355.
[4] J. El Yazal, Y.P. Pang, J. Phys. Chem. 104 (2000) 6499–6504.
[5] M. Arnold, D.A. Brown, O. Deeg, W. Errington, W. Haase, K.
Herlihy, T.J. Kemp, H. Nimir, R. Werner, Inorg. Chem. 37 (1998)
2920–2925.
[6] D.A. Brown, W.K. Glass, N.J. Fitzpatrick, T.J. Kemp, W.
Errington, G. Clarkson, W. Haase, F. Karsten, A. Mahdi, Inorg.
[7] G.M. Sheldrick, SADABS, Empirical Absorption Corrections
Program, University of Gottingen, Germany, 1996.
[8] Siemens SHELXTL-PC Version 5.0 Reference Manual, Siemens
Industrial Automation, Inc., Analytical Instrumentation, Madison,
WI, 1994.
[9] G.M. Sheldrick, SHELXL97 Program for Crystal Structure
Refinement, University of Gottingen, Germany, 1997.