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

Abstract

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.

Spin-Crossover in Iron(II) Complexes of N,N′-Disubstituted 2,6-Bis(Pyrazol-3-yl)Pyridines: An Effect of a Distal Substituent in the 2,6-Dibromophenyl Group

“…Two separate techniques were used for this purpose: the popular [12] Evans technique [50,51] and the less popular but more powerful [40] analysis of a temperature dependence of chemical shifts in the NMR spectra [40,66] that are collected with the same experimental setup (see experimental section). The former allows for measuring the magnetic susceptibility of a solution as an alternative to a more cumbersome magnetometry [12], while the latter allows for screening of paramagnetic complexes that may exist in mixtures of unknown concentrations [40]. The two approaches rooted in the widely available NMR spectroscopy are helpful tools in molecular design of SCO compounds [12,40] exploiting the spin state behaviour decoupled from crystal packing [23,24] or substrate [25,26] effects that sometimes block a SCO [23,24].…”

“…The former allows for measuring the magnetic susceptibility of a solution as an alternative to a more cumbersome magnetometry [12], while the latter allows for screening of paramagnetic complexes that may exist in mixtures of unknown concentrations [40]. The two approaches rooted in the widely available NMR spectroscopy are helpful tools in molecular design of SCO compounds [12,40] exploiting the spin state behaviour decoupled from crystal packing [23,24] or substrate [25,26] effects that sometimes block a SCO [23,24].…”

“…For [Fe(L R )2](ClO4)2, both methods show the complexes with the para-substituted 2,6dibromophenyl groups to undergo a temperature-induced SCO between 235 and 325 K in acetonitrile solutions, while [Fe(L H )2](ClO4)2 remains HS even in methanol, which provides access to lower temperatures down to 190 K. The latter follows from the χT values Although typical of N,N -disubstituted 3-bpp ligands [27], the observed 'locking' [37] of the complexes [Fe(L R ) 2 ](ClO 4 ) 2 in the HS state is, however, in a stark contrast with their behavior in solutions as probed by variable-temperature NMR spectroscopy. Two separate techniques were used for this purpose: the popular [12] Evans technique [50,51] and the less popular but more powerful [40] analysis of a temperature dependence of chemical shifts in the NMR spectra [40,66] that are collected with the same experimental setup (see experimental section). The former allows for measuring the magnetic susceptibility of a solution as an alternative to a more cumbersome magnetometry [12], while the latter allows for screening of paramagnetic complexes that may exist in mixtures of unknown concentrations [40].…”

“…We have proposed [27] a ligand design to obtain the first SCO-active iron(II) (and cobalt(II) [40]) complexes of N,N -disubstituted 3-bpp by introducing bulky ortho-groups into N-phenyl substituents. Following this research, here we report a series of 3-bpp ligands with 2,6-dibromophenyl groups functionalized in its para-position by another bromine atom and a methyl group with different electronic and steric properties (Scheme 1).…”

Unravelling of a [High Spin—Low Spin] ↔ [Low Spin—High Spin] Equilibrium in Spin‐Crossover Iron(II) Dinuclear Helicates Using Paramagnetic NMR Spectroscopy

“…For a SCO‐active compound, the same linear dependence is observed at high temperatures but a gradual population of the LS state upon cooling causes deviations from the linearity. If the switching between the two spin states is fast in the NMR timescale, which is almost always the case, [29, 30] the chemical shifts become weighted averages of those for the HS and LS species [31, 32] . Below a certain temperature, only the diamagnetic LS state of the ion(II) ion is populated in a solution, as corroborated by temperature‐independent NMR spectra.…”