Hysteretic spin crossover in iron(ii) complexes of a new pyridine-triazole-pyrazine ligand is tuned by choice of NCE co-ligand

Article abstract of DOI:10.1039/c3cc43922d

A family of three new mononuclear complexes of the general form [Fe(L ^pz)₂(NCE)₂] has been prepared (L^pz = 4-p-tolyl-3-(2-pyrazinyl)-5-(2-pyridyl)-1,2,4-triazole; E = S, Se, BH ₃). All three exhibit spin crossover, in two cases with hysteresis, with T_1/2 being predictably tuned by varying the coordinated anion. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc43922d

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Hysteretic spin crossover in iron(II) complexes of a
new pyridine–triazole–pyrazine ligand is tuned by
choice of NCE co-ligand†
Cite this: Chem. Commun., 2014,
50, 1435
Received 25th May 2013,
Ross W. Hogue, Reece G. Miller, Nicholas G. White, Humphrey L. C. Feltham,
Guy N. L. Jameson and Sally Brooker*
Accepted 10th December 2013
DOI: 10.1039/c3cc43922d
www.rsc.org/chemcomm
A family of three new mononuclear complexes of the general form
[Fe(Lpz)2(NCE)2] has been prepared (Lpz = 4-p-tolyl-3-(2-pyrazinyl)-
5-(2-pyridyl)-1,2,4-triazole; E = S, Se, BH3). All three exhibit spin
crossover, in two cases with hysteresis, with T1/2 being predictably
tuned by varying the coordinated anion.
The ability of first-row transition metal complexes to undergo spin
crossover (SCO) between the high spin (HS) and low spin (LS) states is
well-documented.1,2 Driven by scientific curiosity and the potential uses
of these complexes as nanotechnological components (either sensors
Fig. 1 Rdpt ligands used to prepare SCO-active complexes of iron(II),
including ptdpt, the dipyridyl analogue of the ligand used in this work.
or displays3–10) we, and many others, have been attempting to coordinated it is expected to stabilise the LS state of iron(II),23,24 i.e.
rationally design SCO complexes with tuneable properties. Rapidly raise the T1/2 of any SCO event. Regardless of whether the pyrazine
growing numbers of SCO-active complexes have been prepared, coordinates or not, the packing interactions, another vitally impor-
particularly those containing iron(II), but the ability to achieve and tant factor in SCO,25–27 will also be modified. We were also keen to
tune SCO deliberately, rather than relying on serendipity is still rare.11,12 probe whether the SCO behaviour of iron(II) complexes of this new
Within those complexes that do display SCO behaviour, the advanta- ligand could be further tuned by varying the coordinated anions.
geous property of hysteresis, i.e. a memory effect, is rarer still.
The novel ligand Lpz is, to the best of our knowledge, the
One class of ligand that has been used to deliberately first example of a pyrazine-containing analogue of an Rdpt
prepare SCO-active iron(II) complexes is the 4-substituted-3,5- ligand,28 and was prepared according to our general method29
di(2-pyridyl)-triazole (Rdpt) family (Fig. 1).13 Compounds of the as shown in Scheme 1. Briefly, the tolyl-substituted thioamide
form [Fe(Rdpt)2X2] (where X is a trans-coordinated anion) give was activated by alkylation and then condensed with pyrazine-
SCO for a number of R-substituents including NH2,14–19 4-tolyl 2-carbohydrazide, giving Lpz in 64% yield over two steps. The
1
(ptdpt, Fig. 1),20 pyrrolyl21 and hexadecyl.22 Notably, when R = ligand was characterised by H and 13C NMR spectroscopy, IR
NH2, tuneable SCO is observed where the T1/2 (temperature at spectroscopy, mass spectrometry, elemental analysis and X-ray
which there is a 1 : 1 HS : LS ratio) can be altered by varying the crystallography (Fig. S3, see ESI† for experimental details and
ligand field strength of the anion X (X = NCS, NCSe, N(CN)2, NMR spectra of the new compounds; CCDC 937363–937365).
and the radical anion of tetracyanoquinodimethane).14–16
Reaction of two equivalents of Lpz with [Fe(py)4(NCS)2] or
We were interested to see if a new ligand incorporating pyrazine [Fe(py)4(NCSe)2] in methanol causes precipitation of the complexes
in place of one of the pyridine rings in Rdpt would lead to enhanced [Fe(Lpz )2(NCS)2] and [Fe(Lpz)2(NCSe)2] as analytically clean powders
SCO properties of the resulting iron(II) complexes. Pyrazine is a with colours ranging from pale pink to red. Two distinct polymorphs
better p-acceptor and poorer s-donor than pyridine,23 so if of [Fe(Lpz)2(NCSe)2] were obtained: pink polymorph (A) showed
hysteretic SCO (see later) whereas pale pink polymorph (B) was HS
Department of Chemistry and MacDiarmid Institute for Advanced Materials and
Nanotechnology, University of Otago, PO Box 56, Dunedin, 9054, New Zealand.
E-mail: sbrooker@chemistry.otago.ac.nz; Fax: +64 3 4797906; Tel: +64 3 4797919
† Electronic supplementary information (ESI) available: Synthetic details, NMR
from 300–4 K (Fig. S8, ESI†). These two polymorphs also gave
distinctly different powder X-ray diffraction patterns (Fig. S5, ESI†).
The reaction of Lpz with [Fe(py)4(NCBH3)2] in methanol did not lead
to precipitation of any solid, and so analytically clean
[Fe(Lpz)2(NCBH3)2] was instead obtained, as a red powder, by con-
ducting the reaction in CHCl3/CH3OH and subjecting the reaction
¨
spectra, instrumentation, plus additional structural, magnetic and Mossbauer
information and diagrams. CCDC 937363–937365. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3cc43922d
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Scheme 1 Synthesis of the novel pyridine–triazole–pyrazine ligand Lpz
.
mixture to diethyl ether vapour diffusion. As well as elemental
analysis, the three complexes were also characterised by IR spectro-
scopy: analysis of the CRN stretching modes of the NCE anions
suggests that all complexes are HS at room temperature20,22,30 and
have trans-coordinated anions.31 High resolution ESI mass spectro-
metry also supported the proposed formulation, with molecular ions
of the form [Fe(Lpz)2(NCE)]+ being observed for all complexes. Diethyl
ether vapour diffusion into a methanol solution of a [Fe(Lpz)2(NCSe)2]
reaction mixture that had been taken to dryness gave a few solventless
single crystals, allowing determination of the solid state structure of
polymorph B of the complex at 100 K32 and 260 K, by single crystal X-
ray crystallography (Fig. 2). Careful analysis (see ESI†) clearly showed
that in polymorph B the iron(II) centre is coordinated to the pyridine,
not pyrazine, ring. The bond lengths and angles are fully consistent
with HS iron(II) at both temperatures, in contrast to polymorph A,
which undergoes SCO (see later), demonstrating once again the
critical effect of the polymorph on the observed SCO behaviour.33
The temperature dependence of the magnetic susceptibility of
each of the four powder samples of the solvent-free complexes was
examined between 4 and 300 K (Fig. 3; for polymorph B see Fig. S8,
ESI†). At 300 K, the effective magnetic moments range from 5.0 to 5.6
BM, consistent with fully HS iron(II) in all four samples. On cooling,
Fig. 3 Plots of meff vs. temperature for [Fe(Lpz)2(NCS)2], polymorph A of
[Fe(Lpz)2(NCSe)2] and [Fe(Lpz)2(NCBH3)2], measuring continuously at the
following scan rates: between 300–50 K at 5 K minÀ1, 50–10 K at 2 K minÀ1
,
10–4 K at 1 K minÀ1
.
the magnetic moment of the three SCO active complexes drops, such
that at 50 K, the moments are 1.1, 1.1 and 2.8 BM for E = BH3, Se
(polymorph A) and S, respectively. Examining the magnetic properties
in both cooling and warming modes revealed that all three complexes
undergo SCO and two of these are hysteretic, albeit with modest loop
widths (of 5 and 7 K at a scan rate of 5 K minÀ1, see ref. 34 and 35).
The complex [Fe(Lpz)2(NCS)2] undergoes an incomplete SCO, with
a meff of 2.8 BM and wT of 0.98 cm3 kmolÀ1 at 50 K indicative of
approximately three quarters of the iron(II) centres undergoing SCO.
The SCO is hysteretic, with T1/2k = 142 K, and T1/2m = 149 K36
(a hysteresis loop width of 7 K). [Fe(Lpz)2(NCSe)2] has a meff of 1.1 BM
and wT of 0.15 cm3 kmolÀ1 at 50 K, consistent with all of the iron(II)
centres undergoing reversible hysteretic SCO (Fig. S5 and S6, ESI†). As
expected, the NCSe anions exert a stronger ligand field than NCS and
so the SCO temperatures are increased, to T1/2k = 172 K and
T
1/2m = 177 K (a 5 K hysteresis loop). Likewise [Fe(Lpz)2(NCBH3)2]
has a meff of just 1.1 BM and wT of 0.15 cm3 kmolÀ1 at 50 K, consistent
¨
with all of the iron(II) centres undergoing SCO (see Mossbauer
spectrum, later). The NCBH3 anions exert a stronger field than NCS
or NCSe and so the SCO temperature increases further, to T1/2 = 247 K.
Variable temperature 57Fe Mossbauer spectroscopy was also used
¨
to probe the oxidation and spin states of the iron complexes (Fig. 4;
¨
Fig. S9 and S10, Tables S2 and S3, ESI†). The Mossbauer parameters
of all three complexes (Tables S2 and S3, Fig. S9 and S10, ESI†) are
nearly identical to related complexes.21,22 There is a general decrease
in the isomer shift of the LS species in the order NCS > NCSe >
NCBH3 and the reverse order for the HS species, as has been seen
previously.30 The Mossbauer spectra confirm that the NCS coordi-
¨
nated complex is fully HS at 250 K and that at B5 K three quarters of
the iron(II) has undergone SCO to the LS state (Fig. 4 and Table S2,
ESI†), similarly, the NCSe complex is mostly HS at 250 K but has
undergone SCO to be almost completely LS at B5 K (Fig. S9 and
Table S3, ESI†). Spectra taken near the SCO event (at 150 K and 170 K
Fig. 2 Solid state structure of polymorph B of [Fe(Lpz)2(NCSe)2] at 100 K.
Hydrogen atoms are omitted for clarity, thermal ellipsoids are drawn at
50% probability.
1436 | Chem. Commun., 2014, 50, 1435--1437
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¨
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´
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at 5.6 K. All results are in good agreement with the magnetic data.
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for all three complexes, with hysteresis observed in two of these cases,
and the crossover temperatures can be predictably tuned by varying
the coordinated anion. Further work investigating the intricacies of
this and related systems is currently underway.
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32 The crystal was cooled at a rate of 2 K minÀ1 to prevent ‘‘flash
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We thank the University of Otago and the Marsden Fund
(RSNZ) for supporting this research, the MacDiarmid Institute
¨
for the Mossbauer spectrometer and magnetometers, Dr S.
´
33 J. Olguın and S. Brooker, in Spin-Crossover Materials: Properties and
Chong (Callaghan Innovation) for his help, and Dr J. L. Tallon
(Callaghan Innovation) for advice.
Applications, ed. M. A. Halcrow, John Wiley & Sons, Ltd, 1st edn, 2013,
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´
34 R. Kulmaczewski, J. Olguın, J. A. Kitchen, H. L. C. Feltham, G. N. L.
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36 Interestingly, this crossover occurs at a significantly lower tempera-
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Notes and references
1 Spin Crossover in Transition Metal Compounds, Volumes I–III of Topics in
Current Chemistry, ed. P. Gu¨tlich and H. A. Goodwin, 2004; P. Gu¨tlich,
A. B. Gaspar and Y. Garcia, Beilstein J. Org. Chem., 2013, 9, 342–391.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 1435--1437 | 1437