Photomagnetism of a Series of Dinuclear Iron(II) Complexes

Abstract

Spin crossover: The photomagnetic properties of a series of [{Fe(NCS)(py-X)}(2)(bpypz)(2)] (NCS=thiocyanate, py=pyridine, X=4-Mepy, py, 3-Mepy, 3-Clpy and 3-Brpy, and bpypz=3,5-bis(pyridine-2-yl)pyazolate) binuclear complexes are close to the antiferromagnetic response of [{Fe(NCS)(3,5-dmpy)}(3)(bpypz)(2)] (3,5-dmpy=3,5-dimethylpyrazine), which is characterised by two iron(II) metal ions in a high-spin (HS) electronic configuration (see figure).This paper describes the photomagnetic properties of a series of binuclear iron(II) complexes belonging to the [{Fe(NCS)(py-X)}(2)(bpypz)(2)] family (NCS=thiocyanate; py=pyridine; bpypz=3,5-bis(pyridine-2-yl)pyrazolate; and py-X=4-Mepy (1), py (3), 3-Mepy (4), 3-Clpy (5), and 3-Brpy (6)). All of these complexes display a complete thermal spin transition centred between 100 and 150 K, and undergo the light-induced excited-spin-state trapping (LIESST) effect at low temperatures. The T(LIESST) relaxation temperature of the photoinduced high-spin state for each compound has been determined. For all of the complexes, it has been found that the T(LIESST) curves at low temperature are close to the antiferromagnetic response of the [{Fe(NCS)(3,5-dmpy)}(2)(bpypz)(2)] (3,5-dmpy=3,5-dimethylpyridine) (7) complex that is characterised by two iron(II) metal ions in a HS electronic configuration, giving some evidence of a quantitative low spin-low spin-->high spin-high spin (LS-LS-->HS-HS) photoconversion process. Depending on the nature of the cooperativity, the kinetics have been treated with stretched exponential, simply exponential, or sigmoidal models. Interestingly, this series of dinuclear complexes follows a previously proposed linear relationship between the T(LIESST) and their thermal spin transition temperatures T(1/2): T(LIESST)=T(0)-0.3T(1/2). T(0) for these compounds is equal to 100 K. Based on this, and by using the empirical linear relationship found between the thermal spin transition and the Hammett constant, the HS-HS properties of complex 7 have been understood as a reflection of the physical impossibility that the T(LIESST) was higher than T(1/2). The close vicinity of the thermal spin-crossover phenomenon of 7 has been successfully checked by applying hydrostatic pressure.

The Effect of Ligand Design on Metal Ion Spin State—Lessons from Spin Crossover Complexes

“…The first report concerned a series of di-iron complexes [Fe 2 (NCE) 2 (py R ) 2 (µ-bpypz) 2 ] (E = S or BH 3 ), where py R is a 3-or 4-substituted pyridine terminal ligand, which all exhibit comparably gradual thermal SCO in the solid state involving both iron atoms (Scheme 7). Two groups of complexes with thiocyanato or cyanoborohydride co-ligands, each with six different substituted pyridines, show similar positive linear free energy relationships between the pyridine substituent Hammett parameter [89] and the SCO T ½ [90,91]. That is, more electron-withdrawing pyridine substitutents raise T ½ and stabilize the low-spin states of the complexes.…”

Simultaneous Spin‐Crossover Transition and Conductivity Switching in a Dinuclear Iron(II) Coordination Compound Based on 7,7′,8,8′‐Tetracyano‐p‐quinodimethane

“…The Fe centers of 1 are six-coordinate with ad istorted octahedral geometry.I nt he equatorial coordinatione nvironment around the Fe centers, two bpypz À ligands adopt a m 2 -(N',N 1 ,N 2 ,N")d ouble bridging coordination mode, thus forming ab is(bpypz À )-bridged Fe dimer,[ Fe 2 (bpypz) 2 ], in which the FeÀN pyridine bond lengths are much longerthan those of FeÀN pyrazolato due to the monoanion-ic pyrazolato moiety. [55][56][57][58][59][60][61][62][63][64][65] TCNQ occupies two axial positions on the Fe center and adopts a trans-m 2 -bridging mode, consequently generating ao ne-dimensional (1D) ladder; each 1D ladderi sc omposed of alternating [Fe 2 (m-bpypz) 2 ]u nits and two TCNQ units connected through bridging CN groups of TCNQ. The empirical bond valence sum (BVS) [66] analysis, based on SCXRD data at 140 K, suggests aL S+ 2o xidation state (calculated value = 2.09) for both Fe centers in 1,w hichi sf urther supported by 57 Fe Mçssbauer spectroscopy,magnetic measurements,a nd the charge balance of the TCNQ andb pypz À fragments (see below).…”

“…[53] However,t his compound showed an abrupt SCO but no conductivity,a cting as an insulator (room-temperature conductivity is 5 10 À10 Scm À1 )d ue to fully monoanionic TCNQ radicals. [54] In this study,w er eport an ew multifunctional moleculebased crystalline material, assembled by coordinate covalent bonds and p-p stacking interactions between ad inuclear Fe II SCO fragment [55][56][57][58][59][60][61][62][63][64][65] and partially chargedT CNQ radicals, which exhibits thermally induced simultaneouss witching of magnetism and electrical conductivity.…”

From Mononuclear to Dinuclear: Magnetic Properties of Transition Metal Complexes

“…Spin crossover is not restricted to mononuclear complexes but has also been observed in complexes with more than one transition metal center [68][69][70][71][72][73] and extended systems [74][75][76]. Among polynuclear species, binuclear complexes occupy a special place being the simplest systems to combine magnetic coupling with spin crossover properties.…”

“…) [73,77,78]. Beside the L=bt; X=S member of the above mentioned family, 57 Fe Mössbauer spectroscopy also probes the existence of a two-step SCO in [Fe(phdia)(NCS) 2 ] 2 (phdia) (phdia = 4,7-phenanthroline-5,6-diamine) and [Fe(NCS) 2 (picen)] (picen= bis(2-pyridilmethyl)ethylenediamine) [79,80].…”