Heteroleptic Iron(II) Spin-Crossover Complexes Based on a 2,6-Bis(pyrazol-1-yl)pyridine-type Ligand Functionalized with a Carboxylic Acid

Abstract

Two new heteroleptic complexes [Fe(1bppCOOH)(3bpp-bph)](ClO 4 ) 2 •solv (1•solv, solv = various solvents; 1bppCOOH = 2,6-bis(1H-pyrazol-1-yl)isonicotinic acid; 3bpp-bph = 2,6-bis(5-([1,1'-biphenyl]-4-yl)-1H-pyrazol-3-yl)pyridine) and [Fe(1bppCOOH)(1bppCOOEt)](ClO 4 ) 2 •0.5Me 2 CO (2•0.5Me 2 CO, 1bppCOOEt = ethyl 2,6bis(1H-pyrazol-1-yl)isonicotinate) have been designed and prepared. The heteroleptic compound 1•solv was obtained by the combination of stoichiometric amounts of Fe(ClO 4 ) 2 , 1bppCOOH and 3bpp-bph, and it was designed to fine tune the spin crossover (SCO) properties with respect to the previously reported homoleptic compound [Fe(1bppCOOH) 2 ](ClO 4 ) 2 . Indeed, the introduction of a new substituted 3bpp ligand induces a weaker ligand field in addition to promoting the formation of π•••π and C−H•••π intermolecular interactions through the biphenyl groups. For the desolvated counterpart 1, this results into a shift of the SCO curve towards room temperature and the observation of a 13 K hysteresis width. Besides, compound 2•0.5Me 2 CO, which represents the first example of a heteroleptic complex containing two 1bpp tridentate ligands, stabilizes the LS state at room temperature confirming the same trend observed for the corresponding homoleptic compounds. Interestingly, both 1 and 2•0.5Me 2 CO heteroleptic complexes exhibit photo-switchable properties when irradiating with a 523 nm laser at 10 K. Preliminary characterization of the deposited complexes on native SiO 2 by X-Ray Absorption (XAS) measurements suggests oxidation and decomposition of the complexes.

Spin-crossover iron(ii) complex showing thermal hysteresis around room temperature with symmetry breaking and an unusually highT(LIESST) of 120 K

“…This strategy yielded homoleptic and heteroleptic complexes, which showed abrupt thermal SCO with hysteresis and LIESST effect, and a polynuclear complex. [11][12][13][14][15] Here, we . Abrupt thermal hysteretic spin transitions with ∆T = 1-4 K are common in bpp iron(II) complexes with a particular packing motif, the so-called "terpyridine embrace" one.…”

Benchmarking magnetic and spectroscopic properties on highly stable 3d metal complexes with tuneable bis(benzoxazol-2-yl)methanide ligands

“…In return, unfortunately, they are often air and moisture sensitive, preventing further investigation through less common experimental methods that currently focus on the analysis of stable compounds, such as advanced X-ray diffraction experiments, 14 polarized neutron diffraction 15 or synchrotron XAS/XMCD spectroscopy of coated surfaces. 16 The complexity of the relaxation processes, 9 responsible for the magnetic performances, highlights the tremendous need for magnetostructural correlations to better direct further experimental investigations. To this aim, we herein present a novel family of stable 3d metal complexes as an easily accessible and tuneable platform suitable for benchmarking magnetic properties.…”

In Situ NMR Search for Spin-Crossover in Heteroleptic Cobalt(II) Complexes

“…Only demanding a correct assignment of at least some of the signals in the NMR spectra, they are potentially applicable to mixtures of different compounds, including those obtained in situ in an NMR tube. , If confirmed, this possibility may provide a huge advantage in high throughput screening of potential SCO compounds to search for an SCO behavior and fine-tune it by proper chemical modifications. The latter can be expanded even further by using heteroleptic complexes ,− that contain distinct ligands, one stabilizing the LS state and another, the HS state of the metal ion …”

“…Unfortunately, our early attempts to obtain such a complex of iron­(II) with two types of popular ligands–from related terpyridine (terpy) and 2,6-bis­(pirazol-3-yl)­pyridine (3-bpp) families (Scheme )–by a statistical approach were unsuccessful: a plausible reason behind it being bulky N -phenyl groups in N , N ′-disubstituted 3-bpp that block a tendency , of other 3-bpp to form heteroleptic iron­(II) complexes. ,− For cobalt­(II) complexes with a very limited choice of the ligands ,, (mostly terpy and its derivatives) , to produce an SCO, it could open new possibilities for their molecular design as SCO compounds, ,,,,,− similar to those exploited for iron­(II) complexes, ,− ,,− , to tailor them for the above applications.…”