title: Stabilization of the Low-Spin State in a Mononuclear Iron(II) Complex and High-Temperature Cooperative Spin Crossover Mediated by Hydrogen Bonding.
abstract: The tetrapyridyl ligand bbpya (bbpya=N,N-bis(2,2'-bipyrid-6-yl)amine) and its mononuclear coordination compound [Fe(bbpya)(NCS)2 ] (1) were prepared. According to magnetic susceptibility, differential scanning calorimetry fitted to Sorai's domain model, and powder X-ray diffraction measurements, 1 is low-spin at room temperature, and it exhibits spin crossover (SCO) at an exceptionally high transition temperature of T1/2 =418 K. Although the SCO of compound 1 spans a temperature range of more than 150 K, it is characterized by a wide (21 K) and dissymmetric hysteresis cycle, which suggests cooperativity. The crystal structure of the LS phase of compound 1 shows strong NH⋅⋅⋅S intermolecular H-bonding interactions that explain, at least in part, the cooperative SCO behavior observed for complex 1. DFT and CASPT2 calculations under vacuum demonstrate that the bbpya ligand generates a stronger ligand field around the iron(II) core than its analogue bapbpy (N,N'-di(pyrid-2-yl)-2,2'-bipyridine-6,6'-diamine); this stabilizes the LS state and destabilizes the HS state in 1 compared with [Fe(bapbpy)(NCS)2 ] (2). Periodic DFT calculations suggest that crystal-packing effects are significant for compound 2, in which they destabilize the HS state by about 1500 cm(-1) . The much lower transition temperature found for the SCO of 2 compared to 1 appears to be due to the combined effects of the different ligand field strengths and crystal packing.
authors.affiliation: Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, Leiden, 2300 RA (The Netherlands).
authors.forename: Sipeng
authors.initials: S
authors.lastname: Zheng
authors.affiliation: Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, Leiden, 2300 RA (The Netherlands).
authors.forename: Niels R M
authors.initials: NR
authors.lastname: Reintjens
authors.affiliation: Small Molecule X-ray Facility, Department of Chemistry, Johns Hopkins University, Baltimore, MD 21218 (USA).
authors.forename: Maxime A
authors.initials: MA
authors.lastname: Siegler
authors.affiliation: Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC and Universidad de Zaragoza, Plaza San Francisco s/n, 50009 Zaragoza (Spain).
authors.forename: Olivier
authors.initials: O
authors.lastname: Roubeau
authors.affiliation: Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, Leiden, 2300 RA (The Netherlands).
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Bouwman
authors.affiliation: Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen (The Netherlands).
authors.forename: Andrii
authors.initials: A
authors.lastname: Rudavskyi
authors.affiliation: Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen (The Netherlands).|Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands.|Ghent Quantum Chemistry Group, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), 9000 Gent (Belgium).
authors.forename: Remco W A
authors.initials: RW
authors.lastname: Havenith
authors.affiliation: Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, Leiden, 2300 RA (The Netherlands). bonnet@chem.leidenuniv.nl.
authors.forename: Sylvestre
authors.initials: S
authors.lastname: Bonnet
keywords: cooperative effects; hydrogen bonds; iron; ligand design; spin crossover