Light- and Thermal-Induced Spin Crossover in {Fe(abpt)₂[N(CN)₂]₂}. Synthesis, Structure, Magnetic Properties, and High-Spin ↔ Low-Spin Relaxation Studies

Abstract

[Fe(abpt)2(N(CN)2)2] (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) represents the first example of an iron(II) spin-crossover compound containing dicyanamide ligand, [N(CN)(2)](-), as a counterion. It shows an incomplete two-step spin transition with around 37% of HS molecules trapped in the low-temperature region when standard cooling or warming modes, i.e., 1-2 K min(-)(1), were used. The temperature, T(1/2) approximately 86 K, at which 50% of the conversion takes place, is one of the lowest temperatures observed for an iron(II) spin-crossover compound. Quenching experiments at low temperatures have shown that the incomplete character of the conversion is a consequence of slow kinetics. The quenched HS state relaxes back to the LS state displaying noticeable deviation from a single-exponential law. The rate of relaxation was evaluated in the range of temperatures 10-60 K. In the upper limit of temperatures, where thermal activation predominates, the activation energy and the pre-exponential parameter were estimated as E(a) approximately 280 cm(-)(1) and A(HL) approximately 10 s(-)(1), respectively. The lowest value of k(HL) around 1.2 x 10(-)(4) s(-)(1) (T = 10 K) was obtained in the region of temperatures where tunneling predominates. A quantitative light induced excited spin state trapping (LIESST) effect was observed, and the HS --> LS relaxation in the range of temperatures 5-52.5 K was studied. From the Arrhenius plot the two above-mentioned characteristic regimes, thermal-activated (E(a) approximately 431 cm(-)(1) and A(HL) approximately 144 s(-)(1)) and tunneling (k(HL) approximately 1.7 x 10(-)(6) s(-)(1) at 5 K), were characterized. The crystal structure was solved at room temperature. It crystallizes in the triclinic P_1 space group, and the unit cell contains a centrosymmetric mononuclear unit. Each iron atom is in a distorted octahedral environment with bond distances Fe-N(1) = 2.216(2) A, Fe-N(2) = 2.121(2) A, and Fe-N(3) = 2.160(2) A for the pyridine, triazole, and dicyanamide ligands, respectively.

First Dicyanamide‐Bridged Spin‐Crossover Coordination Polymer: Synthesis, Structural, Magnetic, and Spectroscopic Studies

“…Moreover, we are aware of only two examples of dca-containing complexes exhibiting spin-crossover properties. The first one is the mononuclear compound [FeA C H T U N G T R E N N U N G (abpt) 2 A C H T U N G T R E N N U N G (dca) 2 ] (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole), which has an incomplete spin transition with T 1/2 = 86 K. [32] The dinuclear complex {[FeA C H T U N G T R E N N U N G (bztpen) 2 ]A C H T U N G T R E N N U N G (m-dca)}A C H T U N G T R E N N U N G (PF 6 ) 3 (bztpen = N-benzyl-N,N',N'-tris(2-pyridylmethyl)ethylenediamine) shows a complete two-step spin crossover, with transitions approximately centred at 350 K and 200 K. [33] Interestingly, the 2 } (tpm = tris(pyrazolyl)methane) dimer remains in the HS state. [52] The magnetic measurements on 2 reveal a complete twostep spin transition.…”

“…The compound displays a two-step complete spin transition and represents the first example of a spin-crossover polymer with the dca spacer. Although the dca anion has been widely used in the engineering of coordination polymers, [31] to the best of our knowledge there have hitherto been only two spin-crossover systems in which it is involved, the first one mononuclear [32] and the other binuclear with one bridging dca ion. [33] The aqin ligand was chosen for its capacity for p-stacking intermolecular interactions and to be part of an H-bond network in the crystal by virtue of its two amino hydrogen atoms, both of these interactions being considered to improve cooperativity in spin-crossover systems.…”

Multimetastability, phototrapping, and thermal trapping of a metastable commensurate superstructure in a FeIIspin-crossover compound

“…10 Although the vast majority of thermal spin transitions proceed in a single step, there has been an increasing number of two-step transitions reported for polymeric, [11][12][13][14][15][16][17] polynuclear, [18][19][20][21][22][23][24] or mononuclear SCO systems. [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] Such transitions result most frequently from the existence of two crystallographically (symmetrically) independent SCO Fe I I sites with slightly different coordination environments, leading to dissimilar transition temperatures delimiting a plateau (or shoulder), which usually occurs with an approximately equal populations of HS and LS species. Alternatively, the subtle interplay between short-range correlations and long-range interactions may be such that intermediate phases with remarkable structural topologies, characterized by well-ordered patterns of HS and LS Fe I I molecular species, may be stabilized in a more or less wide temperature range, providing a two-step character.…”

Preparation of New Ligand‐Deficient Thiocyanato Compounds with Cooperative Magnetic Phenomena by Thermal Decomposition of Their Ligand‐Rich Precursors

“…The ligand-rich 1:2 hydrate [{Ni(NCS) 2 (bipy)(H 2 O) 2 }· (bipy)] n (1-Ni) is isotypic to its analogues 1-Mn, [7,8] 1-Fe, [11] and 1-Co [9] and crystallizes in the centrosymmetric triclinic space group P1 with one formula unit in the unit cell (Table 8). The crystal structure was refined by the Rietveld method with the structure model of its analogue 1-Co as the starting structure model.…”