The molecular design of spin‐crossover complexes relies on controlling the spin state of a transition metal ion by proper chemical modifications of the ligands. Herein, the first N,N’‐disubstituted 2,6‐bis(pyrazol‐3‐yl)pyridines (3‐bpp) are reported that, against the common wisdom, induce a spin‐crossover in otherwise high‐spin iron(II) complexes by increasing the steric demand of a bulky substituent, an ortho‐functionalized phenyl group. As N,N’‐disubstituted 3‐bpp complexes have no pendant NH groups that make their spin state extremely sensitive to the environment, the proposed ligand design, which may be applicable to isomeric 1‐bpp or other families of popular bi‐, tri‐ and higher denticity ligands, opens the way for their molecular design as spin‐crossover compounds for future breakthrough applications.
Here
we report the first successful attempt to identify spin-crossover
compounds in solutions of metal complexes produced by mixing different
ligands and an appropriate metal salt by variable-temperature nuclear
magnetic resonance (NMR) spectroscopy. Screening the spin state of
a cobalt(II) ion in a series of thus obtained homoleptic and heteroleptic
compounds of terpyridines (terpy) and 2,6-bis(pyrazol-3-yl)pyridines
(3-bpp) by using this NMR-based approach, which only relies on the
temperature behavior of chemical shifts, revealed the first cobalt(II)
complexes with a 3-bpp ligand to undergo a thermally induced spin-crossover.
A simple analysis of NMR spectra collected from mixtures of different
compounds without their isolation or purification required by the
current method of choice, the Evans technique, thus emerges as a powerful
tool in a search for new spin-crossover compounds and their molecular
design boosted by wide possibilities for chemical modifications in
heteroleptic complexes.
A series of three different solvatomorphs of a new iron(II) complex with N,N′-disubstituted 2,6-bis(pyrazol-3-yl)pyridine, including those with the same lattice solvent, has been identified by X-ray diffraction under the same crystallization conditions with the metal ion trapped in the different spin states. A thermally induced switching between them, however, occurs in a solution, as unambiguously confirmed by the Evans technique and an analysis of paramagnetic chemical shifts, both based on variable-temperature NMR spectroscopy. The observed stabilization of the high-spin state by an electron-donating substituent contributes to the controversial results for the iron(II) complexes of 2,6-bis(pyrazol-3-yl)pyridines, preventing ‘molecular’ design of their spin-crossover activity; the synthesized complex being only the fourth of the spin-crossover (SCO)-active kind with an N,N′-disubstituted ligand.
Here we report two new ligands from a 2,6-bis-(pyrazol-3-yl)pyridine family often used in spin-crossover research and their iron(II) and cobalt(II) complexes with the metal ion trapped in the high-spin state in solids (according to magnetometry and X-ray diffraction data). The iron(II) complexes, however, show a gradual spin-crossover in acetonitrile solution, [a] A.N.
A series of new bis(pyrazol-3-yl)pyridines (LR) N,N′-disubstituted by 4-functionalized 2,6-dibromophenyl groups have been synthesized to study the effect of a distal substituent on the spin-crossover (SCO) behaviour of the iron(II) complexes [Fe(LR)2](ClO4)2 by variable-temperature magnetometry, NMR spectroscopy, and X-ray diffraction. The SCO-assisting tendency of the substituents with different electronic and steric properties (i.e., the bromine atom and the methyl group) in the para-position of the 2,6-dibromophenyl group is discussed. Together with earlier reported SCO-active iron(II) complexes with N,N′-disubstituted bis(pyrazol-3-yl)pyridines, these new complexes open the way for this family of SCO compounds to emerge as an effective ‘tool’ in revealing structure–function relations, a prerequisite for successful molecular design of switchable materials for future breakthrough applications in sensing, switching, and memory devices.
Here, we report a
combined study of the effects of two chemical
modifications to an N,N′-disubstituted
bis(pyrazol-3-yl)pyridine (3-bpp) and of different solvents on the
spin-crossover (SCO) behavior in otherwise high-spin iron(II) complexes
by solution NMR spectroscopy. The observed stabilization of the low-spin
state by electron-withdrawing substituents in the two positions of
the ligand that induce opposite electronic effects in SCO–active
iron(II) complexes of isomeric bis(pyrazol-1-yl)pyridines (1-bpp)
was previously hidden by NH functionalities in 3-bpp precluding the
molecular design of SCO compounds with this family of ligands. With
the recent SCO-assisting substituent design, the uncovered trends
converged toward the first iron(II) complex of N,N′-disubstituted 3-bpp to undergo an almost complete
SCO centered at room temperature in a less polar solvent of a high
hydrogen-bond acceptor ability.
A new
synthetic pathway is devised to selectively produce previously
elusive heteroleptic iron(II) complexes of terpyridine and N,N′-disubstituted bis(pyrazol-3-yl)pyridines
that stabilize the opposite spin states of the metal ion. Such a combination
of the ligands in a series of the heteroleptic complexes induces the
spin-crossover (SCO) not experienced by the homoleptic complexes of
these ligands or shifts it to lower/higher temperatures respective
to the SCO-active homoleptic complex. The midpoint temperatures of
the resulting SCO span from ca. 200 K to the ambient temperature and
beyond the highest temperature accessible by NMR spectroscopy and
SQUID magnetometry. The proposed “one-pot” approach
is applicable to other N-donor ligands to selectively
produce heteroleptic complexesincluding those inaccessible
by alternative synthetic pathwayswith highly tunable SCO behaviors
for practical applications in sensing, switching, and multifunctional
devices.
Here we report new porous carbon materials obtained by 3D printing from photopolymer compositions with zinc- and nickel-based metal–organic frameworks, ZIF-8 and Ni-BTC, followed by high-temperature pyrolysis. The pyrolyzed materials that retain the shapes of complex objects contain pores, which were produced by boiling zinc and magnetic nickel particles. The two thus provided functionalities—large specific surface area and ferromagnetism—that pave the way towards creating heterogenous catalysts that can be easily removed from reaction mixtures in industrial catalytic processes.
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