Spin-Crossover Behavior in Cyanide-bridged Iron(II)-Gold(I) Bimetallic 2D Hofmann-like Metal-Organic Frameworks § Gloria Agustí, † M. Carmen Muñoz, ‡ Ana B. Gaspar, † and José A. Real* ,† Institut de Ciència Molecular/Departament de Química Inorgànica, UniVersitat de València, Edificios de Institutos de Paterna, P.O. Box 22085, 46071, València, Spain, and Departament de Física Aplicada, UniVersitat Politècnica de València, Camino de Vera s/n, 46022, València, Spain Received September 20, 2007 The synthesis and characterization of new two-dimensional (2D) cyanide-bridged iron(II)-gold(I) bimetallic coordination polymers formulated, {Fe(3-Xpy) 2 [Au(CN) 2 ] 2 } (py ) pyridine; X ) F(1), Cl (2), Br (3), and I (4)) and the clathrate derivative {Fe(3-Ipy) 2 [Au(CN) 2 ] 2 } · 1/2(3-Ipy) (5), are reported. The iron(II) ion lies in pseudoctahedral [FeN 6 ] sites defined by four [Au(CN) 2 ] - bridging ligands and two 3-Xpy ligands occupying the equatorial and axial positions, respectively. Although only compounds 2 and 4 can be considered strictly isostructurals, all of the components of this family are made up of parallel stacks of corrugated {Fe[Au(CN) 2 ] 2 } n grids. The grids are formed by edge sharing of {Fe 4 [Au(CN) 2 ] 4 } pseudosquare moieties. The stacks are constituted of double layers sustained by short aurophilic contacts ranging from 3.016(2) to 3.1580(8) Å. The Au · · · Au distances between consecutive double layers are in the range of 5.9562(9)-8.790(2) Å. Compound 5, considered a clathrate derivative of 4, includes one-half of a 3-Ipy molecule per iron(II) atom between the double layers. Compound 1 undergoes a half-spin transition with critical temperatures T c V ) 140 K and T c v ) 145 K. The corresponding thermodynamic parameters derived from differential scanning calorimetry (DSC) are ΔH ) 9.8 ( 0.4 kJ mol -1 and ΔS ) 68.2 ( 3JK mol -1 . This spin transition is accompanied by a crystallographic phase transition from the monoclinic P2 1 /c space group to the triclinic P ¯ 1 space group. At high temperatures, where 1 is 100% high-spin, there is only one crystallographically independent iron(II) site. In contrast, the low temperature structural analysis shows the occurrence of two crystallographically independent iron(II) sites with equal population, one high-spin and the other low-spin. Furthermore, 1 undergoes a complete two-step spin transition at pressures as high as 0.26 GPa. Compounds 2-4 are high-spin iron(II) complexes according to their magnetic and [FeN 6 ] structural characteristics. Compound 5, characterized for having two different iron(II) sites, displays a two-step spin transition with critical temperatures of T c 1 ) 155 K, T c 2 V ) 97 K, and T c 2 v ) 110 K. This change of spin state takes place in both sites simultaneously. All of these results are compared and discussed in the context of other {Fe(L) x [M I (CN) 2 ]} coordination polymers, particularly those belonging to the homologous compounds {Fe(3-Xpy) 2 [Ag(CN) 2 ] 2 } and their corresponding clathrate derivatives. Introduction The search for new molecular materials able to manifest a bistable behavior is an issue of permanent interest in the chemistry and physics of materials because they can be envisaged as basic components of new memory and sensory devices. 1 It is well-known that one of the most spectacular examples of bistability in molecular science stems from iron(II) spin crossover (SCO) complexes. 2 They reversibly change between two electronic states, low-spin (LS) and high-spin (HS), induced by an external action such as temperature, pressure, 3 light irradiation, 4 pulsed magnetic fields, 5 and even an analyte 6 (typically solvent molecules). Since some years after the discovery of the first Hofmann- like iron(II) SCO coordination polymer, {Fe(pyridine) 2 [Ni(CN) 4 ]}, 7 we have been systematically investigating the § Dedicated to the memory of Professor Xavier Solans Huguet. * To whom correspondence should be addressed. E-mail: jose.a.real@ uv.es. † Universitat de València. ‡ Universitat Politècnica de València. (1) Lehn, J. M. Science 2002, 295, 2400. Inorg. Chem. 2008, 47, 2552-2561 2552 Inorganic Chemistry, Vol. 47, No. 7, 2008 10.1021/ic701865k CCC: $40.75  2008 American Chemical Society Published on Web 02/23/2008