title: Evidence of crystal packing effects in stabilizing high or low spin states of iron(ii) complexes with functionalized 2,6-bis(pyrazol-1-yl)pyridine ligands.
abstract: The molecular structures and magnetic properties of homoleptic iron(ii) compounds [Fe(bpp-COOMe)2](ClO4)2(1) and [Fe(bpp-triolH3)2](ClO4)2(2) have been investigated to ascertain their spin crossover (SCO) behaviour. In these hexacoordinated complexes, the bpp (2,6-bis(pyrazol-1-yl)pyridine) ligands adopt a mer-mer coordination mode and carry COOMe or C(O)NHC(CH2OH)3para substituents, respectively, on the central pyridyl ring. In spite of the almost equal donor power of the ligands to the iron(ii) centre, the two compounds feature different spin state configurations at room temperature. Compound 1 displays a highly-distorted octahedral environment around the iron(ii) centre, which adopts a high spin (HS) state at all temperatures, even under an external applied pressure up to 1.0 GPa. By contrast, 2 is characterized by a more regular octahedral coordination around the metal ion and exhibits a low spin (LS) configuration at or below room temperature. However, it shows a thermally-induced SCO behaviour at T > 400 K, along with Light-Induced Excited Spin State Trapping (LIESST) at low temperature, with TLIESST= 38 K. Since DFT (U)M06/6-311+G(d) geometry optimizations in vacuo indicate that both complexes should adopt a HS state and a highly-distorted coordination geometry, the stabilization of a LS configuration in 2 is ultimately ascribed to the effect of intermolecular hydrogen bonds, which align the [Fe(bpp-triolH3)2]2+cations in 1D chains and impart profound differences in the geometric arrangement of the ligands.
authors.affiliation: Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Modena e Reggio Emilia and INSTM RU of Modena and Reggio Emilia, via G. Campi 103, 41125 Modena, Italy. luca.rigamonti@unimore.it.
authors.forename: Nathalie
authors.initials: N
authors.lastname: Bridonneau
authors.affiliation: Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Modena e Reggio Emilia and INSTM RU of Modena and Reggio Emilia, via G. Campi 103, 41125 Modena, Italy. luca.rigamonti@unimore.it.
authors.forename: Luca
authors.identifier: http://orcid.org/0000-0002-9875-9765
authors.initials: L
authors.lastname: Rigamonti
authors.affiliation: Laboratory of Molecular Magnetism (LAMM), Dipartimento di Chimica 'Ugo Schiff', Università degli Studi di Firenze and INSTM RU of Firenze, via della Lastruccia 3-13, 50019 Sesto Fiorentino (FI), Italy.
authors.forename: Giordano
authors.initials: G
authors.lastname: Poneti
authors.affiliation: Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland.
authors.forename: Dawid
authors.initials: D
authors.lastname: Pinkowicz
authors.affiliation: Consiglio Nazionale delle Ricerche (CNR), Istituto di Scienze e Tecnologie Molecolari (ISTM), Università degli Studi di Milano and INSTM RU of Milan, via C. Golgi 19, 20133 Milano, Italy.
authors.forename: Alessandra
authors.initials: A
authors.lastname: Forni
authors.affiliation: Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Modena e Reggio Emilia and INSTM RU of Modena and Reggio Emilia, via G. Campi 103, 41125 Modena, Italy. luca.rigamonti@unimore.it.
authors.forename: Andrea
authors.initials: A
authors.lastname: Cornia