Rsc_cc_c2cc32893c 1..3

The high-spin/low-spin bistability of spin-crossover (SCO) transition metal complexes is a fascinating property that attracts considerable interest because of its potential applications in molecular switches. The largest class of SCO compounds comprises monometallic, octahedral iron(II) complexes with six N-donor ligands, whose bistability involves interconversions of the high-spin (t2g) 4 (eg) 2 and the low-spin (t2g) 6 (eg) 0 states. SCO has also been observed in five-coordinate iron(II) complexes, a tetrahedral iron(II) complex, and in monometallic complexes of chromium(II), manganese(III) and cobalt(II). Polymetallic, exchange-coupled SCO compounds with bridging N-donor ligands are less common, however they are attractive synthetic targets because interactions between SCO centres could lead to significant enhancements in cooperativity and bistability properties. Although considerable progress has been made with N-donor ligands, the different electronic properties of ligands based on heavier pnictogens such as phosphorus and arsenic could provide an alternative method of influencing the interplay between magnetic exchange and SCO. Thus, we now report the structures and magnetic properties of the phosphorusand arsenic-bridged cyclopentadienyl-manganese(II) dimers [CpMn{m-E(SiMe3)2}]2, with E = P (1) or As (2). In 1 and 2, antiferromagnetic exchange occurs concurrently with thermally induced two-step SCO involving the high-spin S = 5/2 and the rare intermediate-spin S = 3/2 states of manganese(II). Compounds 1 and 2 were synthesized by reacting Cp2Mn with LiE(SiMe3)2 (E = P, As) (Scheme 1). The structures of 1 and 2 were determined by X-ray diffraction,w and are very similar, both consisting of pnictogen-bridged dimers of general formula [(Z-Cp)Mn{m-E(SiMe3)2}]2. The dimers have approximate molecular D2h symmetry, and the {CpME2} coordination environments have approximate C2v symmetry. Assuming that an Z -Cp ligand formally occupies three coordination sites, each metal atom in 1 and 2 is five-coordinate. The formally five-coordinate, 15-valence-electron phosphidebridged dimanganese compound [CpMn{m-P(SiMe3)2}]2 (1) crystallizes with two independent molecules in the unit cell, 1a and 1b, which are structurally similar and lie about independent inversion centres (Fig. 1, S1). In 1a, the twoMn(II) centres are bridged by two m-[(Me3Si)2P] ligands. The Mn(1)–P(1) and Mn(1)–P(1A) bond distances in 1a are 2.5075(5) and 2.5123(5) Å and the P–Mn–P and Mn–P–Mn angles are 93.83(2) and 86.17(2)1, respectively. The arsenide-bridged dimanganese compound 2 has only one independent molecule in the unit cell, which lies about an inversion centre (Fig. 1). The Mn–As bond distances in 2 are 2.5877(7) and 2.5980(8) Å, and the As–Mn–As and Mn–As–Mn angles of 90.81(2) and 89.20(2)1, respectively, Scheme 1 Synthesis of 1 and 2.