Coordination polymers undergoing spin crossover and reversible ligand exchange in the solid{ Ana Galet, a M. Carmen Mun ˜oz a and Jose ´ Antonio Real* b Received (in Cambridge, UK) 8th May 2006, Accepted 8th August 2006 First published as an Advance Article on the web 6th September 2006 DOI: 10.1039/b606434e Here we report the synthesis and characterisation of a polymer made up of a system of parallel 2-D grids of Fe(II) ions linked by [Au(CN) 2 ] 2 bridges and its transformation into a new system of three interpenetrated 3-D coordination open frame- works with the NbO topology. Reversibility of this crystal-to- crystal transformation is evidenced by X-ray crystallographic data and from their spin crossover properties. Implementation of coordination polymers 1 with iron(II) spin crossover (SCO) building blocks 2 is a singular approach to construct new advanced materials with potential applications. The SCO building blocks can be switched between the high- (HS) and low-spin (LS) states. This leads to distinctive changes in magnetism, colour and structure, which may be induced by a variation of temperature and/or pressure and by light irradiation. Strong signal generation and hysteresis (memory effect) may occur when rigid linkers, communicating between the SCO centres, propagate the structural changes cooperatively to the whole framework conferring a bistable character to the material. 3 Furthermore, a synergy or interplay between the SCO and the inherent properties of the coordination polymer may be expected in favourable cases. This is particularly true when structural transformations are triggered by host–guest interactions for instance, 1c,4 which have dramatic effects on the SCO properties. 5 Herein we report the reversible structural transformation of a non porous 2-D SCO coordination polymer, {Fe(3- CNpy) 2 (CH 3 OH) 2/3 [Au(CN) 2 ] 2 }(1) (3-CNpy = 3-CNpyridine), { into a triple interpenetrated 3-D microporous SCO framework with the NbO structure type, {Fe (3-CNpy) 2 [Au(CN) 2 ] 2 }(2). This is a new example of a crystalline-state ligand exchange reaction involving substitution-active iron(II) coordination sites able to selectively recognise guest CH 3 OH molecules. The effect of this transformation on the SCO properties is also investigated. Compound 1 crystallizes in the triclinic P1 ¯ space group.§ There are three crystallographically independent Fe( II) atoms, each one lying on an inversion centre (Fig. 1, top). Sites Fe(1) and Fe(2) display strongly distorted [FeN 6 ] elongated octahedrons with the equatorial positions occupied by four [Au(CN) 2 ] 2 anions, which define the short Fe–N distances [Fe(1)–N(1) = 2.13(6) A ˚ , Fe(1)– N(2) = 2.16(6) A ˚ , Fe(2)–N(6) = 2.09(7) A ˚ and Fe(2)–N(7) = 2.19(6) A ˚ ], while the axial positions are occupied by the 3-CNpyridine ligands [Fe(1)–N(3) = 2.28(4) A ˚ and Fe(2)–N(8) = 2.25(3) A ˚ ]. The Fe(3) site lies in a [FeN 4 O 2 ] compressed octahedron. The equatorial plane of the octahedron is defined by two 3-CNpy ligands [Fe(3)–N(11) = 2.22(3) A ˚ ] and two [Au(CN) 2 ] 2 anions [Fe(3)–N(10) = 2.21(6) A ˚ ]. The axial positions are occupied by methanol molecules [Fe(3)–O(1) = 2.10(5) A ˚ ]. There are also three crystallographically different [Au(CN) 2 ] 2 groups: (i) [Au(1)(CN) 2 ] 2 bridges Fe(1) and Fe(2) sites, (ii) [Au(3)(CN) 2 ] 2 links Fe(2) and Fe(3) sites and (iii) [Au(2)(CN) 2 ] 2 does not act as a bridging ligand. Consequently, the [Au(1)(CN) 2 ] 2 and [Au(3)(CN) 2 ] 2 bridges meet at the Fe(2) atom, which constitute the nodes of a slightly corrugated 2-D grid defined by the sharing of [Fe(2)] 4 rhombuses whose edges make 20.738(3) 6 20.888(3) A ˚ (Fig. 1, bottom). The shortest and longest edges correspond to the connection of two Fe(2) sites through the Fe(1) and the Fe(3) atoms, respectively. Interestingly, the Fe(2) and Fe(1) or Fe(3) octahedrons are rotated ca. 90u with respect to each a Departament de Fı ´sica Aplicada, Universitat Polite ´cnica de Vale `ncia, Camino de Vera s/n, 46022, Valencia, Spain. E-mail: mcmunoz@fis.upv.es b Instituto de Ciencia Molecular/Departamento de Quı ´mica Inorga `nica, Universitat de Vale `ncia, Edificio de Institutos de Paterna, P O Box 22085, 46071, Vale `ncia, Spain. E-mail: jose.a.real@uv.es; Tel: 34 963544856 { Electronic supplementary information (ESI) available: Thermal analysis of 1; X-ray powder diffraction patterns of 1 and 2. See DOI: 10.1039/ b606434e Fig. 1 Top: view of the asymmetric unit of the coordination polymer 1 displaying the corresponding atom numbering. Bottom: view of the relative disposition of three consecutive layers. COMMUNICATION www.rsc.org/chemcomm | ChemComm This journal is ß The Royal Society of Chemistry 2006 Chem. Commun., 2006, 4321–4323 | 4321