A new Co(II) coordination solid with mixed oxygen, carboxylate, pyridine and thiolate donors exhibiting canted antiferromagnetism with TC # 68 K

The synthesis of new metal–organic coordination polymers continues to attract a great deal of interest since such materials have been shown to exhibit highly desirable solid-state properties such as magnetism, nanoporosity, second-harmonic generation and show promise for use in molecular electronics. They may also be doped with other metals to produce active heterogeneous catalysts. In particular, materials that have thermally-accessible magnetically bistable states have the potential to be utilized in data storage on the molecular level. One of the major obstacles impeding progress in this area is the very low temperatures at which magnetic ordering occurs. Extensive study of Prussian blue derivatives has produced materials which order above room temperature. Good metal–ligand orbital overlap is expected to increase the strength of magnetic exchange due to a greater degree of spin delocalisation onto the bridging ligand. Thiolate ligands are attractive candidates for such an approach, but success has previously been limited due to the ease of disulfide formation for such ligands. We have recently shown that hydrothermal synthesis is an effective way of preparing thiolate networks whilst minimising the possibility of disulfide formation, presumably due to the inherently reducing nature of high temperature water. The thiolate-mediated magnetic exchange in these materials is still relatively weak, presumably as a result of unfavorable M–S–M bridging angles. However, we now report strong superexchange and weak ferromagnetism due to spin canting in a highly complex and dense coordination solid containing 5and 6-coordinate Co, [Co4(2-mna)4(OH2)] (1) (2-mna = 2-mercaptonicotinate, (NC5H3(S)(CO2)-2,3) ) (Fig. 1). Spherical clusters of very fine branched (fern leaf-like) deep red needles were initially observed from the hydrothermal reaction of CoCl2 with 2-mnaH2 and KOH (ratio 1 : 1 : 2), although much better samples were prepared when NaOH was employed as the base.{ Single crystal X-ray diffraction§ led to the identification of the low symmetry 3-dimensional polymer 1 (in the non-centrosymmetric orthorhombic space group Pca21, Z = 4). The asymmetric unit contains four highly distorted Co environments. Co1 and Co2 are both within distorted octahedra, Co3 conforms most closely to trigonal bipyramidal coordination geometry, while Co4 is in a square-based pyramid with a single OH2 ligand (O9) in the apical position (Fig. 1 inset). The most striking detail of the asymmetric unit of 1 is the wide range of coordination modes present between metal ions and the four unique 2-mna ligands. There are a total of six different modes: (a) all ligands form planar 4-membered N,Sdonor chelate rings to Co (bite angles in the range 62.4(2)– 64.6(2)u); (b) each ligand also forms a near-planar 6-membered S,O-chelate ring, thus leading to pseudo-linear bridging via RS; (c) two carboxylate groups provide syn,anti-bridges between metal ions; (d) one carboxylate moiety mediates a distorted anti,anti-type bridge; (e) there is a single chelating carboxylate; (f) a single carboxylate-O provides a 1,19-bridge between metals. Octahedral Co1 and Co2 have ‘softer’ coordination environments since both are doubly N,S-chelated while each also coordinates to a third thiolate from an adjacent ligand. Single carboxylate-O donor atoms complete their coordination environments. In contrast, the environments around Co3 and Co4 comprise coordination of four O-donors and a single thiolate-S. University Chemical Laboratory, Lensfield Rd, Cambridge, UK CB2 1EW. E-mail: [email protected]; Fax: +44 1223 336017 Instituto de Ciencia Molecular, Universidad de Valencia. Pol. La Coma s/n, 46980 Paterna, Valencia, Spain { Current address: Department of Chemistry, University of California, Berkeley, CA, USA and the Chemical Sciences Division, Lawrence Berkeley National Laboratory, CA, USA. Fig. 1 The extended asymmetric unit of 1; all unique atoms shown with completed coordination environments. Co–S bonds are shown in blue, Co–O bonds are shown in pink. Inset: polyhedral view of the same four unique Co(II) centers. Selected bond lengths [Å] and angles [u]: Co1–S1 2.652(2), Co1–S2 2.585(2), Co1–S3 2.310(2), Co2–S1C 2.288(2), Co2–S3 2.691(2), Co2–S4 2.739(2), Co3–S2 2.301(2), Co4–S4 2.266(2); S1–Co1–S2 135.62(8), S2–Co1–S3 122.83(8), S1C–Co2–S4 82.35(7), S3–Co2–S4 97.44(8), Co1–S1–Co2E 169.3(1), Co1–S2–Co3 153.4(1), Co1–S3–Co2 158.6(1), Co2–S4–Co4 165.4(1), Co3A–O7–Co4 137.7(2), O7F–Co3–O8F 58.5(2). COMMUNICATION www.rsc.org/chemcomm | ChemComm