Inorganica Ckimica Acta, 66 (1982) 115- 118 115 Synthesis and Structure of a Trinuclear Chromium(III)-Nicotinic Acid Complex zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA ENRIQUE GONZALEZ-VERGARA, JACK HEGENAUER, PAUL SALTMAN Departments of Chemistry and Bzology, University of California, San Diego, La Jolla, Calif 92093, US A. MICHAL SABAT and JAMES A. IBERS Department of Chemistry, Northwestern University, Evanston, Ill. 60201, U.S.A. Received October 31, 1981 The reaction of nicotinic acid (nit), chromic perchlorate hexahydrate, and sodium perchlorate in aqueous solution affords a CP(III)-vitamin complex whose composition has been established to be Na- [~~O(nicH)6(HzO)J][C104]s’nicH~6Hz 0, based on analytical data and a single-crystal X-ray analysis. In the [CL, O(nicH),(H, O/3J’7 ion a central oxygen atom is bonded to three chromium atoms. The nicotinic acid zwitten’ons bridge these chromium centers through the carboxylate oxygen atoms. On each chromium atom a water oxygen, tram to the central oxygen atom, completes the octahedral coordination. The _ ion has cry stallographically imposed symmetry 6. The binding of nicotinic acid in this Ck(III)-nicotinic acid complex is significantly different from that currently proposed for the glucose tolerance factor. Introduction Schwarz and Mertz identified an organic complex of chromium that promotes cellular utilization of glucose in chromium-deficient rats. They designated this complex as a ‘glucose tolerance factor’ (GTF) [l] . Partial characterization of GTF from brewers’ yeast indicated that this ionic complex may contain Cr(III), nicotinic acid (nit), glycine, glutamate, and cysteine [2] . Although a structure has been proposed, GTF is still not a well-defined chemical entity [3, 41. Many attempts have been made to synthesize GTF, but yields have been low and propos- ed structures remain highly speculative [5]. We have undertaken a systematic study of the synthesis of compounds of chromium(II1) with nicotinic acid as part of a program to determine whether N- or O- coordinated nit confers functionality on chromium in biological systems. We have already reported [6] the synthesis and metabolic studies of a chromium- (III)-nit-pyridoxylidene ternary complex in which 0020-1693/82/0000-0000/$02.75 nit is coordinated to the metal through the ring nitrogen atom, as in the structure proposed by Mertz [2]. Although we have not addressed bro- availability (promotion of glucose tolerance) in these studies, we have shown that absorption and biodrstribution of such complexes is similar to GTF analogues. We now report the synthesis and structure of a new Q-coordinated trinuclear chromium complex of nit, the first such Cr(III)- vitamin complex to be fully characterized. Experimental Nicotinic acid (0.246 g, 2 mmol) was dissolved in 50 ml of water at 80 “C, and chromic perchlorate hexahydrate (0.458 g, 1 mmol) was added. The solution was slowly evaporated to dryness on a steam bath. Excess chromic perchlorate was extracted with ethanol. The crude green material was crystal- lized by dissolvmg equal amounts of NaC104 and the complex in a minimal amount of hot water. The solution was immediately filtered and cooled. The resulting small emerald-green crystals were filtered, washed several times with ethanol, and air dried. Crystals sufficiently large and well form- ed for X-ray analysis were obtained by recrystal- lization from 50% methanol. A yield of 60% of the original starting materials was obtained. The material crystallizes in space group P6Jm or P6a of the hexagonal system in a cell of dimen- sions a = 13,565(6), c = 24.89(l) A at -150 “C. A total of 4789 diffracted intensities (+h, tk *l; 28(MoKo1 ) < 48”) was collected at -150 “C by diffractometer methods standard in this laboratory [7]. The crystal used was a hexagonal needle of total volume 0.0061 mm’. No correction for absorp- tion was necessary. Examination of Friedel pairs provided no evidence for the noncentrosymmetric space group. Accordingly, the intensities were 0 Elsevier Sequoia/Printed in Switzerland