Synergy between Spin Crossover and Metallophilicity in Triple Interpenetrated 3D Nets with the NbO Structure Type Ana Galet, † Virginie Niel, ‡ M. Carmen Mun ˜oz, † and Jose ´ A. Real* ,‡ Departament de Fı ´sica Aplicada, UniVersitat Polite ` cnica de Vale ` ncia, Camino de Vera s/n, 46022 Vale ` ncia, Spain, and Institut de Ciencia Molecular/Departament de Quı ´mica Inorga ´ nica, UniVersitat de Vale ` ncia, Doctor Moliner 50, 46100 Burjassot, Spain Received August 4, 2003; E-mail: jose.a.real@uv.es Functional crystalline materials with switch properties and memory transduction have considerable interest in view of their potential technological applications. 1 In this respect, the develop- ment of rational synthetic routes by self-assembly have afforded an important number of coordination polymers with specific topologies. 2 Besides their construction principles, a particular attractive goal has been the synthesis of zeolite-like porous networks of potential ability in catalysis, absorption, and host-guest chem- istry. 3 Incorporation of mechanically, electronically, or optically active building blocks as essential structural components in the construction of such functional materials is further implementation scarcely investigated. 4 In this context, the use of spin-crossover (SCO) building blocks is a suitable strategy as they change reversibly from the low-spin (LS) to the high-spin (HS) state accompanied by drastic structural modifications and color changes in response to stimuli such as variation of temperature or pressure and light irradiation. 5 Hofmann-like clathrate compounds containing iron(II) ions have afforded a number of two-dimensional (2D) 6,7 and 3D 7 polymeric SCO networks {Fe(L) x [M II (CN) 4 ]}‚G [L ) pyridine (x ) 2), pyrazine (x ) 1, G ) 2‚H 2 O), M II ) Ni, Pd, Pt] whose magnetic and chromatic properties change abruptly and display thermal- and pressure-induced hysteresis at temperatures close to room temper- ature. 8 The assembly of [M I (CN) 2 ] - groups (M I ) Cu, Ag, Au), Fe(II), and trans-bispyridylethylene, 4,4′-bipyridine, 8 or pyrimidine (pmd) has resulted in new 2D and double or triple interpenetrated porous 3D SCO polymers with interesting architectures and properties. 9 Continuing this research, we herein report the synthesis of {Fe(3CNpy) 2 [Ag(CN) 2 ] 2 }‚2/3H 2 O(1) (3CNpy ) 3-cyanopyri- dine), 10 a system made up of triple interpenetrated 3D SCO networks with tunable metallophilic interactions. The crystal structure has been solved for the HS (240 K) and the LS (145 K) forms. 11 1 adopts the trigonal P-3 space group irrespective of the spin state. The iron atom lies at the inversion center of an elongated octahedron. The equatorial bond lengths defined by the nitrogen atoms of four [Ag(CN) 2 ] - groups [Fe-N(1) ) 2.133(8), 1.951(3) Å and Fe-N(2) ) 2.133(9), 1.953(3) Å for the HS and LS states, respectively] are shorter than those of the axial positions occupied by the 3CNpy ligands [Fe-N(3) ) 2.203(8) Å (HS), 1.988(3) Å (LS)]. The average change of the equatorial and axial bond distances upon spin change is 0.181 and 0.215 Å, respectively. The quasi-linear bismonoden- tate [Ag(CN) 2 ] - bridges and the iron atoms assemble to form {Fe [Ag(CN) 2 ] 2 } ∞ 4-connected 3D networks, which corresponds to an expanded version of the prototypal NbO net decorated by the 3CNpy groups (Figure 1a). 12 The much larger intraframework spaces are occupied by two other identical but independent networks, which interpenetrate the first and each other (Figure 1b). The closest approach between nets corresponds to Ag‚‚‚Ag contacts, which define triangular Ag arrangements. The Ag‚‚‚Ag distance, 3.256(2) Å (HS), indicates the occurrence of argentophilic interac- tions. 13 The HSfLS spin change increases significantly the homoatomic interactions. The Ag‚‚‚Ag distance, 3.1593(6) Å, decreases by 0.0967 Å in the LS form. Along the c-axis, the nets display cross-sectional trigonal chan- nels where the homoatomic interactions take place. The vertices and edges being defined by the iron atoms and the [Ag(CN) 2 ] - groups, respectively (Figure 1c). Each vertex is shared by an equivalent trigonal motif defining hexagonal shaped voids. The relative orientation of the [FeN 6 ] sites imparts handedness to the trigonal channels so that right- and left-handed channels alternate around the hexagonal voids. The 3CNpy ligands fill partially these voids defining hexagonal channels (diameter ca. 7.5 Å) where water molecules are located. Variation of the unit cell volume with temperature, expressed per iron atom, shows an abrupt change, ∆V SCO ) 45.05 Å 3 , in the surroundings of the SCO (210-170 K, vide infra), which corre- sponds to 91.7% of the total volume variation ∆V T ) 49.1 Å 3 (∆V T - ∆V SCO ) 4.05 Å 3 corresponds to thermal dilation/contraction) † Universitat Polite `cnica de Vale `ncia. ‡ Universitat de Vale `ncia. Figure 1. Fragment of a NbO net (a), interpenetration of three networks (b) (3CNpy groups are omitted for clarity), and perspective view along the c-axis of the framework (c) [code: red (Fe), brown (Ag), orange (C), pink (H 2O)]. Published on Web 11/01/2003 14224 9 J. AM. CHEM. SOC. 2003, 125, 14224-14225 10.1021/ja0377347 CCC: $25.00 © 2003 American Chemical Society