Dalton Transactions Dynamic Article Links Cite this: Dalton Trans., 2011, 40, 3295 www.rsc.org/dalton PAPER Structure and magnetic properties of an unprecedented syn-anti l-nitrito-1jO:2jO¢ bridged Mn(III)-salen complex and its isoelectronic and isostructural formate analogue† Paramita Kar, a Rituparna Biswas, a Michael G. B. Drew, b Yumi Ida, c Takayuki Ishida* c and Ashutosh Ghosh* a Received 3rd November 2010, Accepted 18th January 2011 DOI: 10.1039/c0dt01521k The preparation, crystal structures and magnetic properties of two new isoelectronic and isomorphous formate- and nitrite-bridged 1D chains of Mn(III)-salen complexes, [Mn(salen)(HCOO)] n (1) and [Mn(salen)(NO 2 )] n (2), where salen is the dianion of N,N¢-bis(salicylidene)-1,2-diaminoethane, are presented. The structures show that the salen ligand coordinates to the four equatorial sites of the metal ion and the formate or nitrite ions coordinate to the axial positions to bridge the Mn(III)-salen units through a syn-anti m-1kO:2kO¢ coordination mode. Such a bridging mode is unprecedented in Mn(III) for formate and in any transition metal ion for nitrite. Variable-temperature magnetic susceptibility measurements of complexes 1 and 2 indicate the presence of ferromagnetic exchange interactions with J values of 0.0607 cm -1 (for 1) and 0.0883 cm -1 (for 2). The ac measurements indicate negligible frequency dependence for 1 whereas compound 2 exhibits a decrease of c ac ¢ and a concomitant increase of c ac ¢¢ on elevating frequency around 2 K. This finding is an indication of slow magnetization reversal characteristic of single-chain magnets or spin-glasses. The m-nitrito-1kO:2kO¢ bridge seems to be a potentially superior magnetic coupler to the formate bridge for the construction of single-molecule/-chain magnets as its coupling constant is greater and the c ac ¢ and c ac ¢¢ show frequency dependence. Introduction Magnetic molecular clusters have attracted much interest in the last few years, due to the fundamental research of magnetic in- teractions and magneto-structural correlations. 1 One of the major challenges to the researchers is the rational design of this type of polymer. There are developments of a whole range of single- molecule magnets 2 and single-chain magnets 3 that bridge the gap between paramagnetism and long-range ordered magnetism and potential applications of data storage and quantum calculations. 1D magnetic chain systems are of special interest because of their importance in lower-dimensional magnetism. 4 The single- chain magnets (SCMs) possess a large uniaxial anisotropy, strong ferromagnetic intrachain exchange interactions without spin compensation between the high-spin magnetic units, and good isolation of the chains in order to avoid two- and three- a Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata, 700 009, India. E-mail: ghosh_59@ yahoo.com b School of Chemistry, The University of Reading, P.O. BOX 224, Whiteknights, Reading, RG6 6AD, U.K. c Department of Engineering Science, The University of Electro- Communications, Chofu, Tokyo, 182-8585, Japan. E-mail: ishi@pc.uec.ac.jp †Electronic supplementary information (ESI) available. CCDC reference numbers 799078–799079. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0dt01521k dimensional ordering. 5 However, it is still a big challenge to design and prepare such ferromagnets due to the weakness of the ferromagnetic coupling compared to the antiferromagnetic interaction. 6 Polynuclear transition-metal complexes are an inter- esting class in this regard and most of the new examples have been found in manganese(III) chemistry. 7 Therefore, preparation of new polynuclear manganese(III) complexes continues to receive a great deal of attention. Employment of appropriate types of bridging ligands which can mediate the magnetic coupling between the local spin carriers has allowed access to a variety of high nuclearity products with interesting structural and magnetic properties. The carboxylate ligands play an important role in this field because they can assume a large palette of coordination modes such as syn-syn, anti-anti, syn-anti, and monatomic to form key building blocks for the synthesis of polynuclear complexes. 8 With extensive knowledge of the coordination characteristics of the carboxylate group, it is clear that the formate ion, being the smallest carboxylate and consequently having a small steric effect, is very effective in joining two or more transition-metal ions as a three-atom connector to form a variety of zero-, 9 one-, 10 two- 11 and three-dimensional 12 complexes. All of the bridging modes of formate usually promote effective magnetic exchange pathways between moment carriers and consequently mediate ferro- or antiferromagnetic coupling between metal ions in different situations. 13 The m 1,3 -azide bridge, This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 3295–3304 | 3295 Downloaded by Calcutta University on 19 April 2011 Published on 23 February 2011 on http://pubs.rsc.org | doi:10.1039/C0DT01521K View Online