ORIGINAL PAPER Dinuclear Fe(III) complexes with spin crossover Ivan Nemec Æ Roman Boc ˇa Æ Radovan Herchel Æ Zdene ˇk Tra ´vnı ´c ˇek Æ Milan Gembicky ´ Æ Wolfgang Linert Received: 1 October 2008 / Accepted: 5 November 2008 / Published online: 4 December 2008 Ó Springer-Verlag 2008 Abstract A series of dinuclear Fe(III) complexes was synthesized in which the Schiff-base blocking ligand L 5 coordinates each of the centers which are linked by a bidentate, bipyridine-type ligand. For these systems, [L 5 Fe III {bridge}Fe III L 5 ](BPh 4 ) 2 , thermally induced spin crossover is observed. The corollary of the systems is that the spin crossover interferes with the magnetic exchange interaction. The overlap of the energy bands of the LL and HH reference states (L, low-spin; H, high-spin) causes the exchange interaction to act against the spin crossover (leading to incompleteness or gradual behavior). Keywords Spin crossover  Exchange interaction  Dinuclear iron(III) complexes  Schiff-base ligands Introduction Thermally induced spin crossover is a well described phe- nomenon, mainly in mononuclear Fe(II) complexes [1–5]. However, other central atoms (d 4 to d 7 ) are also capable of electronic bistability manifesting itself in spin crossover behavior (low-spin to high-spin transition). Whereas there are two relevant spin states for mononuclear octahedral complexes (LS: 2 T 2g , S = 1/2 HS: 6 A 1g , S = 5/2) in com- plexes with less symmetry the intermediate spin state (IS; S = 3/2) could also be involved in the interplay (Fig. 1). In dinuclear complexes, the spin states of the individual centers produce LL, LH, HL, or HH combinations (on abstracting from the IS). The energy separation D ab of these reference states is dominated by the orbital angular momentum (that involves the effects of the crystal field) and it cannot be assumed that the LH (HL) lies just in- between LL and HH (Fig. 2). A new feature encountered in the dinuclear complexes is an exchange interaction. This causes a splitting of the indi- vidual reference states into energy bands formed of the S multiplets. For instance, for the Fe(III) diad, the LL state consists of four magnetic energy levels (S = 0, 1), LH involves 12 levels (S = 2, 3), and the HH state contains 36 levels (S = 0, 1, 2, 3, 4, 5). The width of each band is given by the exchange-coupling constant: whereas J LL and J LH only split the S min and S max , the J HH produces a virtual band between S = 0 and S = 5. Depending on the interplay between D ab (dominated by the orbital angular momentum) and the band width d ab , several different situations can occur (Fig. 3). With ground LL, well separated from the other states, the system stays low-spin over the whole attainable temperature region. When HH (or LH) is thermally popu- lated, the system shows the thermally induced spin crossover (thermodynamic conditions DH [ 0 and DS [ 0 should be I. Nemec  R. Boc ˇa (&) Institute of Inorganic Chemistry, Slovak University of Technology, 812 37 Bratislava, Slovakia e-mail: roman.boca@stuba.sk R. Herchel  Z. Tra ´vnı ´c ˇek Department of Inorganic Chemistry, Palacky ´ University, 77147 Olomouc, Czech Republic M. Gembicky ´ Department of Chemistry, State University of New York at Buffalo, Buffalo, NY 14260, USA W. Linert Institute of Applied Synthetic Chemistry, Vienna University of Technology, 1060 Vienna, Austria 123 Monatsh Chem (2009) 140:815–828 DOI 10.1007/s00706-008-0096-0