Dinuclear Spin-Crossover Complexes Based on Tetradentate and Bridging Cyanocarbanion Ligands

Abstract

Spin-crossover (SCO) Fe(II) dinuclear complexes of formula [Fe2(tmpa)2(μ2-tcpd)2]·0.8(CH3OH) (1·MeOH) and [Fe2(andmpa)2(μ2-tcpd)2]·2CH3OH (2·MeOH) (tmpa = tris(2-pyridylmethyl)amine, andmpa = bis(2-pyridylmethyl)aminomethyl)aniline, (tcpd)(2-) = 2-dicyanomethylene-1,1,3,3-tetracyanopropanediide) have been synthesized and characterized by infrared spectroscopy, X-ray diffraction, and magnetic measurements. The crystal structure determinations of the two complexes (1·MeOH and 2·MeOH) and the desolvated complex 1 (from 1·MeOH) revealed a neutral centrosymmetrical dinuclear structure in which the (tcpd)(2-) cyanocarbanion acts as a double μ2-bridging ligand between two [FeL](2+) (L = tmpa (1), andmpa (2)) units involving two free coordination sites in the cis configuration. Examination of the shortest intermolecular contacts in 1·MeOH and 1 reveals no significant hydrogen bonding between the dinuclear units, while in 2·MeOH these units are held together by significant hydrogen bonds between one of the uncoordinated nitrile groups and the anilate function, giving rise to 1D supramolecular structure. The three dinuclear complexes 1, 2·MeOH, and 2 exhibit SCO behaviors which have been evidenced by the thermal evolutions of the χmT product and by the average values of the six Fe-N distances for 1 and 2·MeOH, that reveal a gradual conversion with transition temperatures (T1/2) at ca. 352 K (1), 196 K (2), and 180 K (2·MeOH). For the solvated 1·MeOH, the sharp SCO transition observed around 365 K was induced by the desolvatation process above 330 K during the magnetic measurements.

Spin crossover in discrete polynuclear iron(ii) complexes

“…105 In the second case, cyanocarbanions (A1, Chart 7) doubly bridge the two Fe(II) centres, with the N 6 coordination spheres completed by L35 and L36 capping ligands (Chart 8), giving a neutral complex. 106 In the third case, NCS anions doubly bridge the Fe(II) centres with N 5 S coordination environments and L37 capping ligands (Chart 8). 107 The fourth example features a diiminoquinonoid-bridges (A2-A5, Chart 7) with L35 capping ligands (Chart 8).…”

“…After desolvation of this pair of complexes, the T 1/2 values were slightly different, 352 and 196 K, respectively. 106 Harris and co-workers also employed the tetradentate capping ligand L35, but utilised a series of diiminoquinonoid anion bridges, A2-A5 (Chart 7) to generate dinuclear [Fe 2 (L35) 2 (A)]-(B(PhCF 3 ) 4 ) 2 , with A2-A5, complexes in order to systematically study the effect of varying the electronic effects of substituents on the bridge A on the SCO behaviour ( Fig. 26a).…”

Iron(II) Spin Crossover (SCO) Materials Based on Dipyridyl-N-Alkylamine

“…However, iron(II)-based SCO complexes, for which the transition takes place between the paramagnetic high-spin (HS) state (t 4 2g e 2 g , 5 T 2g , S = 2) and the diamagnetic low-spin (LS) state (t 6 2g e 0 g , 1 A 1g , S = 0) are, by far, the most studied switchable molecular materials [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. From the synthetic point of view, one of the relevant strategies to design original SCO systems is based on the use of appropriate polydentate rigid nitrogen-based ligands and simple anionic entities acting as terminal ligands, such as NCX (X = S, Se, BH 3 ) anions [16][17][18][19][20][21][22] or the more sophisticated ones such as cyanocarbanions exhibiting terminal or poly-bridging coordination modes [4,7,[23][24][25][26][27]. The latter are able to tune the ligand field energy and some SCO characteristics such as the transition temperature.…”

“…In the large families of polydentate molecules, the use of the polypyridine-based ligands of different denticities, such as 2,2'-dipyridylamine (dpa) [18,19], tris(2-pyridyl)methane (tpc) [20,28,29], and tris(2-pyridylmethyl)amine (tpma [21,23,[30][31][32][33], has allowed the preparation of discrete and extended coordination compounds exhibiting original SCO transitions, allowing to understand more on the SCO phenomenon, such as the origin of cooperativity, the presence of complete or incomplete transitions, and the occurrence of one-step or multi-step behaviors and photo-induced effects. In this context, we have reported, in the last few years, a new series of dinuclear Fe(II) complexes based on the tetradentate tmpa ligand [23] and, more recently, a dinuclear complex and a one-dimensional coordination polymer, both based on the functionalized tris(2-pyridyl)methane (tpc) tripodal ligands and displaying unusual FeN 5 S coordination spheres. By experimental and theoretical magnetostructural studies, we have shown in both systems the crucial role of the linearity of the N-bound terminal thiocyanato ligand in the presence of the SCO transition.…”

“…modes [4,7,[23][24][25][26][27]. The latter are able to tune the ligand field energy and some SCO characteristics such as the transition temperature.…”