A re-investigation of arsenoacetic acid, (AsCH 2 COOH) n Brian K. Nicholson a, * , Peter S. Wilson a, b , Adelle Nancekivell a a Chemistry Department, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand b Institute for Applied Ecology New Zealand, School of Applied Sciences, Auckland University of Technology, Private Bag 92006, Auckland 1142, New Zealand article info Article history: Received 23 May 2013 Received in revised form 10 July 2013 Accepted 18 July 2013 Keywords: Arsenic Cyclopolyarsine X-ray crystal structure Electrospray ionisation mass spectrometry abstract Detailed spectroscopic data have been obtained for arsonoacetic acid, As(CH 2 COOH)O 3 H 2 , and its barium and sodium salts. The X-ray crystal structure of the free acid is isomorphous with phosphonoacetic acid. Reduction gave the As(I) compound arsenoacetic acid, (AsCH 2 COOH) n which was shown by ESI-MS to contain cyclic species based on AseAs bonds, with n mainly 3e6. The X-ray crystal structure of the hexamer was determined as the pyridine solvate and shown to have a hexacyclic As 6 ring in a puckered chair conformation, with eCH 2 COOH groups in equatorial sites, each H-bonded to a pyridine molecule in the lattice. Ó 2013 Published by Elsevier B.V. 1. Introduction Interest in the chemistry of organo-arsenic(I) chemistry can be traced to the historically important drug Salvarsan (also known as Arsphenamine) which Ehrlich introduced as a cure for syphilis in 1910 [1]. This compound, produced by reduction of 3-amino-4- hydroxyarsonic acid, was originally assigned the structure 1a based on analogies with aryleazo compounds. As]As double bonds, however, are only isolable with extremely bulky substituents [2], therefore polymeric or cyclic structures were subsequently pro- posed [3]. In support of the cyclic forms, recent ESI-MS studies on Salvarsan have shown that it consists of (RAs) n rings 2, n ¼ 3e8, with n ¼ 5 and 6 being the most abundant [4]. For other derivatives, structural studies have now defined cyclic species (RAs) n for n ¼ 4, 5, and 6 which all show puckered rings with AseAs distances of ca 2.46  A and AseAseAs angles of around 90  [5]. A compound of this type that is still unresolved is arsenoacetic acid, the product of reduction of arsonoacetic acid 3. This was first reported in 1923 by Palmer [6] and was assigned the structure 1b. Even now it is represented as such in the Merck Index [7] where it is noted that it has had veterinary use as a tonic for horses. It has been patented as a fire retardant [8a] and as a treatment for premen- strual syndrome in women [8b] and has been used for treating chronic fatigue syndrome in horses [8c]. The original structure 1b is clearly unlikely but there have been no modern studies concerning its chemistry, so we herein report the results of a re-investigation using modern techniques. 2. Experimental ESI-MS was carried out on a Bruker MicrOTOF instrument, operating under standard conditions in negative ion mode, with samples made up in H 2 O or 1:1 CH 3 CN:H 2 O (with the addition of pyridine), immediately before infusion. Peaks are reported as the m/z with the greatest intensity in the isotopic envelope. NMR spectra were recorded on a Bruker Avance 400 machine in D 2 O or pyridine-d 5 , and IR spectra were from a PerkineElmer Spectrum 100 run as KBr disks. Arsenoacetic acid was handled under an at- mosphere of nitrogen using standard Schlenk techniques. 2.1. Synthesis of barium arsonoacetate, Ba 3 [As(CH 2 COO)O 3 ] 2 The barium salt of arsonoacetic acid was prepared using the Meyer reaction [9], as adapted by Palmer [6]. Arsenic trioxide (10 g, 51 mmol) was added to a hot aqueous solution of NaOH (10 g, 0.4 mol in 30 mL H 2 O) and cooled to room temperature. Chloro- acetic acid (4.8 g, 51 mmol) was added and stirred for 1 h. The clear solution was acidified with glacial acetic acid (16 mL) and, after cooling to 40  C, the precipitated excess arsenic trioxide was filtered by suction and washed with water. The filtrate was poured into a hot solution of BaCl 2 $2H 2 O (18.5 g, 76 mmol) in H 2 O (60 mL) and stirred for 5 min. Barium arsonoacetate was filtered by suction after the solution was allowed to stand overnight, and washed thoroughly with water. Yield 30 g, 75%. IR: (cm 1 ) 3403(vs, br) * Corresponding author. Fax: þ64 7 838 4219. E-mail address: b.nicholson@waikato.ac.nz (B.K. Nicholson). Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem 0022-328X/$ e see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jorganchem.2013.07.056 Journal of Organometallic Chemistry 745-746 (2013) 80e85