Polymorphism in Spin Transition Systems. Crystal Structure, Magnetic Properties, and Mössbauer Spectroscopy of Three Polymorphic Modifications of [Fe(DPPA)(NCS)₂] [DPPA = (3-Aminopropyl)bis(2-pyridylmethyl)amine]

Abstract

Three polymorphic modifications A-C of [Fe(II)(DPPA)(NCS)(2)], where DPPA = (3-aminopropyl)bis(2-pyridylmethyl)amine is a new tetradentate ligand, have been synthesized, and their structures, magnetic properties, and Mössbauer spectra have been investigated. For polymorph A, variable-temperature magnetic susceptibility measurements as well as Mössbauer spectroscopy have revealed the occurrence of a rather gradual HS if LS transition without hysteresis, centered at about 176 K. The same methods have shown that polymorph B is paramagnetic over the temperature range 4.5-295 K, whereas polymorph C exhibits a very abrupt S = 2 if S = 0 transition with a hysteresis. The hysteresis width is 8 K, the transitions being centered at T(c) downward arrow = 112 K for decreasing and T(c) upward arrow = 120 K for increasing temperatures. The crystal structures of the three polymorphs have been solved by X-ray diffraction at 298 K. Polymorph A is triclinic, space group P&onemacr; with Z = 2, a = 8.710(2) Å, b = 15.645(2) Å, c = 7.985(1) Å, alpha = 101.57(1) degrees, beta = 112.59(2) degrees, and gamma = 82.68(2) degrees. Polymorph B is monoclinic, space group P2(1)/c with Z = 4, a = 8.936(2) Å, b = 16.855(4) Å, c = 13.645(3) Å, and beta = 97.78(2) degrees. Polymorph C is orthorhombic, space group Pbca with Z = 8, a = 8.449(2) Å, b = 14.239(2) Å, and c = 33.463(5) Å. In the three polymorphs, the asymmetric units are almost identical and consist of one chiral complex molecule with the same configuration and conformation. The distorted [FeN(6)] octahedron is formed by four nitrogen atoms belonging to DPPA and two provided by the cis thiocyanate groups. The two pyridine rings of DPPA are in fac positions. The main differences between the structures of the three polymorphs are found in their crystal packing. The stabilization of the high-spin ground state of polymorph B is tentatively explained by the presence of two centers of steric strain in the crystal lattice resulting in the elongation of the Fe-N(aromatic) distance. The observed hysteresis in polymorph C seems to be due to the existence of an array of intermolecular contacts in the crystal lattice making the spin transition more cooperative than in polymorph A.

Spin Crossover in a Family of Iron(II) Complexes with Hexadentate ligands: Ligand Strain as a Factor Determining the Transition Temperature

“…We previously attempted such a study with the iron(II) spin-crossover complexes with the two tetradentate ligands DPEA [(2-amino-A C H T U N G T R E N N U N G ethyl)bis(2-pyridylmethyl)amine] [30] and DPPA [(3-aminopropyl)bis(2-pyridylmethyl)amine] [25] through the enlargement of just one chelate ring from five-to six-membered. The two ligands differ by the presence of an additional methylene unit in the amino aliphatic chain of the DPPA ligand.…”

“…The conformational freedom of six-membered chelate rings has been found to be responsible for several polymorphic modifications for the same iron(II) spin-crossover complex [25] or to lead to order-disorder structural transitions in the coordination sphere of complex. [26,27] In this context, polydentate ligands capable of generating one or several six-membered chelate rings in coordination with iron(II), conveying a flexible coordination sphere, are of special interest.…”

Structure and Electronic Configuration of an Iron(II) Complex in a LIESST State: A Pump and Probe Method

“…[18] (3-Mepz = 3-methylpyrazolyl) and [Fe{(3-aminopropyl)bis(2-pyridylmethyl)amine}A C H T U N G T R E N N U N G (NCS) 2 ] [19] all exhibit quite different behavior in various forms of polymorphs. Recently the SCO Fe complex trans-[FeA C H T U N G T R E N N U N G (abpt) 2 (N(CN) 2 ) 2 ] [20] was also found to have different polymorphs; therefore, detailed structural analysis becomes essential to understand the magnetic behavior.…”

Surface induces different crystal structures in a room temperature switchable spin crossover compound

“…[1][2][3][4][5] This setback, which manifests itself in phenomena such as polymorphism and solvatomorphism, is particularly relevant in the drug industry; however, it is common in many materials, such as organic semiconductors, 6,7 supramolecular systems 1 and spin crossover (SCO) compounds. 5,[8][9][10][11][12][13][14][15][16][17][18][19][20] Despite some exceptions, in which polymorphism is exploited as an advantageous property, [21][22][23][24] usually it is thought to be a drawback for many technological applications, which often compromises the development of materials. On the contrary, gaining a detailed knowledge of different crystal forms and their influence on the functionalities can be the manner to select the best performing phase.…”