Structural and magnetic studies on mono- and polynuclear chromium ascorbate complexes Birgu¨l Zu¨mreoglu-Karan*, Ahmet N. Ay and Canan U ¨ naleroglu Department of Chemistry, Hacettepe University, Beytepe Campus, 06800 Ankara, Turkey Tezer Firat Department of Physics Engineering, Hacettepe University, Beytepe Campus, 06800 Ankara, Turkey Thomas Ristau and Wolfgang Jabs Institut fu ¨r Chemie u. Biochemie, E. M. A. Universita ¨t Greifswald, D-17489 Greifswald, Germany Received 10 January 2005; accepted 14 January 2005 Abstract Polynuclear chromium ascorbate complexes were isolated and physicochemically analyzed in a comparative manner with their mononuclear analog (1). Characterization by elemental analysis, electronic, vibrational, 13 C-n.m.r and mass spectroscopies, and variable temperature magnetic susceptibility studies, allowed structural proposals for the binuclear, [Cr 2 (l-OH) 2 (H 2 O)(C 6 H 7 O 6 ) 3 (OH)] Æ 4H 2 O (2), and trinuclear, [Cr 3 (l-O) 3 (H 2 O) 6 (C 6 H 7 O 6 ) 3 ] Æ 4H 2 O (3) complexes. The pseudo-octahedral Cr III centers were suggested to be connected through hydroxo bridges in (2) and in (3) by oxo bridges forming a hexocyclic ring. Introduction Chromium is an essential trace element for mammalian carbohydrate and lipid metabolism. The biologically active form of chromium is the trivalent ion while hexavalent chromium is a toxicant. Under physiological conditions, Cr VI is reduced by microsome, mitochondria and by some non-enzymatic cellular components. As a cellular component, ascorbic acid is known to reduce Cr VI to Cr III in vivo [1, 2] and through inner-sphere electron transfer reactions in vitro [3, 4]. The final Cr III ascorbate complexes may be of considerable interest in biocoordination chemistry aiding a better understanding of chromium metabolism and also targeting miscella- neous potential applications. About two decades ago, a chromium ascorbate complex, synthesized directly from Cr III ions, was described by its electronic and i.r. spectra and magnetic moment [5, 6]. A group of complexes, prepared via reduction of several Cr VI species by ascorbate, has since been reported [7–10]. However, the structural information as yet available is still limited due to the problems commonly associated with the chemical instability of ascorbic acid, the low stability constants of the complexes [11] and their reluctance towards crystallization, excluding the few well character- ized mixed ligand platinum ascorbate complexes [12–16]. In recent years, research on the isolation/preparation and characterization of polynuclear Cr III complexes has been stimulated. The olation of Cr III aqua ions and the polynuclear basic chromium carboxylates have been known for nearly a century [17–19]. The interest in these polynuclear complexes stems not only from their special magnetic properties but is also associated with their significance as modeling for naturally occuring Cr III containing proteins like: i. glucose tolerance factor, ‘GTF’, an organic Cr III containing complex isolated from brewer’s yeast [20, 21], ii. low-molecular weight chromium binding substance ‘LMWCr’, or as recently termed ‘chromodulin’, iso- lated from mammals [22–25] and iii. the collagen-binding chromium species in the tan- ning process [26–28]. Polynuclear chromium ascorbate complexes might be regarded as alternative models for such biological systems or potential substituents for dietary chromium supplements (e.g. chromium picolinates) in general use. The results presented here concern some structural correlations between mono-, di- and tri-nuclear chro- mium ascorbate complexes which exhibit intermetallic bridges through hydroxo or oxo ligands. Experimental General L (+)-Ascorbic acid (H 2 A, Merck) and iso-propylidene ascorbic acid (i-H 2 A, prepared as described elsewhere * Author for correspondence Transition Metal Chemistry (2005) 30:451–459 Ó Springer 2005 DOI 10.1007/s11243-005-1035-0