Variable temperature far and mid FT-IR as a valuable tool to determine the spin transition temperature of iron(II) spin-crossover coordination compounds

Variable temperature far and mid FT-IR as a valuable tool todetermine the spin transition temperature of iron(II)

spin-crossover coordination compounds

Peter Weinberger*, Matthias GrunertInstitute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9/163–AC,

A–1060 Vienna, Austria

Received 14 April 2003; received in revised form 30 June 2003; accepted 8 July 2003

Abstract

The thermally induced spin transition behaviour of selected iron(II) coordination compounds with substituted tetrazole ligands has been

monitored using variable temperature FT-IR spectroscopy. The reliability of these results is discussed and compared with independent

analytical techniques such as SQUID measurements and 57Fe Mossbauer spectroscopy proofing variable temperature IR spectroscopy to be a

valuable tool in the determination of the spin transition temperature T1/2.

# 2003 Elsevier B.V. All rights reserved.

Keywords: Variable temperature far FT-IR; Variable temperature mid FT-IR; Spin crossover; Iron(II)

1. Introduction

Since the first spin-crossover compounds based on iron(II)

coordination complexes have been reported in the early

1960s [1] the field has been expanded to an immensely

extent by various research groups investigating the thermally

induced spin-transition behaviour both in solution [2] as well

as in solid state [3]. As reported exhaustively by Gutlich et al.

[4] the discovery of new effects in the mid-1980s such as the

light induced excited spin state trapping (LIESST), the

nuclear decay induced excited spin state trapping (NIESST)

as well as the investigations on the pressure dependence of

the spin-transition behaviour gave the research in this field a

further boost. With the first applications of spin-crossover

compounds like molecular-based information storage

devices and display devices within reach [5] or already

patented [6] the main focus of interest shifted towards the

design of the spin-transition features via sophisticated ligand

systems [7].

To characterise the spin-transition behaviour several

analytical tools are at disposal. The most obvious one is

the variable temperature measurement of the magnetic

properties, which change upon spin transition between

the diamagnetic low-spin state (S ¼ 0) and the paramag-

netic high-spin state (S ¼ 2) of iron(II) spin-crossover

complexes. However, magnetic susceptibility measure-

ments are only a macroscopic method observing a bulk

phenomenon thus not able to distinguish between a—at

least feasible—gradual change of the magnetic moment of

the molecules and a change of the ratio between two

magnetically different species. Therefore, at least one

additional microscopic method needs to be employed.

The most accurate method of choice is the variable tem-

perature 57Fe Mossbauer spectroscopy yielding direct

insight into the spin state of the electrons surrounding

the iron(II) coordination centre. Unfortunately, the weak

signal-to-noise ratio in very soft materials can expand the

time to accomplish reasonable good results from a few days

to several weeks at a given temperature. To overcome this

dilemma it seems only logical to observe the effects of the

spin transition of iron(II) spin-crossover compounds on the

binding properties of the molecule directly. Thus variable

temperature FT-IR spectroscopy has been employed to

determine the spin-transition temperature T1/2, i.e. the

temperature representing equal amounts of low-spin (LS)

and high-spin (HS) species.

In principle, the spin transition affects mostly the metal-

to-ligand bond due to the population of the non-populated

eg-orbitals upon the LS $ high-spin HS transition. As the

eg-orbitals feature anti-bonding character this yields a sig-

nificant weakening of the bond strength, which manifests

Vibrational Spectroscopy 34 (2004) 175–186

* Corresponding author. Fax: þ43-1-58-801-16299.

E-mail address: [email protected] (P. Weinberger).

0924-2031/$ – see front matter # 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.vibspec.2003.07.006

itself in a bond length change of approximately 10 rel.%, i.e.

�0.2 A. In the case of those spin-crossover compounds

reported in this article this means a bond length change

of the iron–nitrogen bond, which has been already proved

for some of them by variable temperature single crystal X-

ray diffraction [7]. Depending on the specific ligand arrange-

ment within the crystal lattice, i.e. the formation of mono-

nuclear complexes, 1-D chain-type or 3D network building

coordination polymers, there will be a corresponding change

in the Fe–N bond resulting in a rather significant shift of IR

absorption bands assigned to vibrational modes associated

with iron–nitrogen stretching vibrations, which are detected

in the far IR region around �300–400 cm�1 [8]. Unfortu-

nately, these absorption bands are sometimes overlaid by

intra-ligand vibrational modes resulting in partial or com-

plete obscuring of unambiguously assignable peaks. In such

cases ill-defined baselines severely hamper the peak area

determination being essential for a quantification of the

respective species. But as the bond strength change of the

iron towards the first coordinating atom of the ligand (in the

reported class of compounds this is always nitrogen) is

somewhat influencing the bond strength of the first-near-

est-neighbouring bonds as well, it is well known from

literature to monitor bond strength changes of neighbouring

co-ligands such as CO or CN groups, which are easily

detected in the mid-range IR region [8,9]. A further devel-

opment of this idea focussing on the single aromatic CH

stretching vibration of the tetrazole moiety will be presented

in this paper.

2. Synthesis

A series of substituted tetrazole ligands with the general

composition (tetrazole-1-yl)-alkane ranging from

R ¼ methyl to dodecyl was synthesized (see Scheme 1)

and their structure determined via single crystal X-ray

diffraction according to Grunert [10]. For the purpose of

using a systematic abbreviation this class of tetrazoles 1-

substituted with n-alkane moieties is called ntz with n being

the number of carbons of the alkyl side-chain.

Additionally, two coordination polymers formed by a-

bis(tetrazole-1-yl)-alkanes (with n ¼ 2, 4; abbreviated nditz;

see Scheme 2) are investigated.

Using the ligands ntz described above a series of com-

plexes of the general composition hexakis(1-alkyltetrazole)

iron(II) bis-tetrafluoroborate have been obtained according

to Scheme 3.

The different chain length made it indispensable to

develop for each complex an own and mostly slightly

different synthetic pathway described in detail in [10]. Their

molecular structure is determined using single crystal X-ray

diffraction. In a similar way coordination polymers based on

bridging nditz ligands are obtained.

3. Experimental

Variable temperature vibrational spectra have been

recorded using a Perkin–Elmer System 2000 far FT-IR

spectrometer within the range of 700–30 cm�1, and using

a Perkin–Elmer 16PC FT-IR spectrometer within the range

of 4400–450 cm�1. The undiluted solid state samples have

been pressed using an Aldrich press dye applying a pressure

of 10 tons yielding standard sample pellets of an average

thickness of <0.5 mm. For both spectral ranges a Graseby-

Specac thermostatable sample holder equipped with poly-

ethylene and silicium windows, respectively, was used. To

accomplish a reasonably signal-to-noise ratio 1000 scans

had to be summed up in the far IR region, whereas 64 scans

have been summed up in the mid IR range. For thermal

equilibration each temperature was hold 10 min before the

measurement started. To avoid moisture influencing the

measurement the sample compartments have been con-

stantly purged with nitrogen.

4. Results and discussion

4.1. Vibrational characterisation of the ntz ligands

As a first set of experiments the whole series of ligands

was measured. The comparison among the uncoordinated 1-

alkyltetrazoles facilitates the distinguishing of vibrations of

the tetrazole moiety and vibrations of the alkyl chain. With

longer chain length, the C–H and C–C bands should become

more intensive compared to the tetrazole bands. In Fig. 1 the

measured spectra are baseline corrected to 100% and the

spectra are stretched or compressed in such a way, that the

well known nC–H vibration [11–13] is normalized to a

transmittance of 75%. The comparison to the neighbouring

aliphatic C–H stretching vibrations [14,15] already shows

that the aliphatic bands around 2950 cm�1 increases with

longer chain length.

NN

N

N

R

Scheme 1. General composition of the ntz-ligands.

NN

N

N

R N

N N

N

Scheme 2. General composition of the nditz-ligands.

NN

N

N

(CH2)CH3

Fe(BF4)2. 6H2O -6H2O

[Fe(ntz)6](BF4)2n-16 +

Scheme 3. General complexation pathway with n ¼ 1–10, 12, 16, 18.

176 P. Weinberger, M. Grunert / Vibrational Spectroscopy 34 (2004) 175–186

This also enables the assignment of bands like the nN2¼N3

vibration [16,17] being close to the neighbouring aliphatic

deformations bands [14,15] (dCH2and dCH3

). At about

1100 cm�1 the in-plane C–H deformation [13] (dC–H) of

the tetrazole hydrogen as well as C–C stretching vibrations

[14,15] (nC–C) are expected. A closer look in this region

shows that nC–C is missing in the 1tz spectrum, whereas it

appears as a shoulder of the tetrazole band for the 2tz and

3tz, and becomes a clear distinguishable band for longer

alkyl chains. Another aliphatic C–H deformation band

appears about 750 cm�1. They overlap with an out-of-plane

ring band of the tetrazole. In this case, it seems that the bands

grow with increasing chain length, but it is still an individual

band on the top of a background originating from the

aliphatic vibrations.

In a further step, ab initio calculations were performed for

one of the ligands—the 1-propyltetrazole. Using the HF

method the 6–31G** basis set implemented in the program

HyperChemTM [18] was used for the optimisation of the

geometry as well as for the vibrational analysis. The calcu-

lations were performed for a single isolated molecule in

vacuo. The optimised geometry (see Fig. 2), energy, inten-

sity and an animation of vibrational modes of the molecule

have been obtained. To obtain absorption bands from the line

spectrum the calculated intensities were multiplied by the

following Lorentzian function:

f ðxÞ ¼ a

1 þ ððx � x0Þ=bÞ2

with a being the intensity, x0 representing the position of the

band and b ¼ 10 for a tentative half width of the bands. Fig. 3

depicts the calculated values with an observed spectrum of

3tz in a KBr matrix.

The comparison reveals a qualitative similar calculated

spectrum shifted slightly to higher energies, which is due to

the rather small basis set used [19]. Nevertheless, with some

Fig. 1. Overview of the mid-IR spectra normalised to a baseline with 100% transmittance and a nC–H (tetrazole) of 75%. In the detailed range fragments it

can be seen, that aliphatic bands arise with a longer chain while the tetrazole bands are unchanged. The range fragment around 750 cm�1 shows an example

where aliphatic and tetrazole bands are overlapping.

Fig. 2. Ab initio optimised geometry of 3tz.

P. Weinberger, M. Grunert / Vibrational Spectroscopy 34 (2004) 175–186 177

of the well known bands it is possible to fit the data for a

reasonably good match. Therefore, the calculated spec-

trum is shifted and compressed in a way that, beside the

point of 0 cm�1 three empirically known bands coincide

between theoretical and experimental data. The calculated

data were transformed to represent a transmittance spec-

trum via

f ðxÞ ¼ y0 �a

1 þ ððx � x0Þ=bÞ2

with y ¼ 100and for a, b, and x0 see above. The measured

spectrum was baseline corrected to 100% transmittance as

well.

After these procedures, in the region of the C–H stretching

bonds a good agreement has been achieved for the frequency

of the bands and their expected interpretation (Fig. 4). The

highest energy corresponds to the aromatic tetrazole hydro-

gen, followed by stretching vibration of the >CH2 groups.

The lowest C–H stretching band at 2865 cm�1 is assigned to

a symmetrical motion of the –CH3 group. The tetrazole

bands in the region between 1500 and 990 cm�1 represent

several stretching vibrations which lead to in-plane motions

of the tetrazole ring. This is in an agreement with the

literature [12,13], as well as with the theoretically predicted

out-of-plane vibrations of the tetrazole ring at 722 cm�1 and

at lower energy. According to the literature [14,15] the main

bands for the alkyl chain are expected around 1440 cm�1

(dCH2), between 1000 and 1100 cm�1 (nC–C) and deforma-

tions of the >CH2 groups at 750 cm�1. In Fig. 5 the finger-

print region of the observed and calculated spectra of 3tz are

presented.

Figs. 4 and 5 reflect a good agreement between the

calculated and the observed spectrum with sometimes large

0

25

50

75

0500100015002000250030003500

wave number [cm-1]

trans

mitt

ance

[%]

0

25

50

75

rel.

inte

nsity

[%]

3tz - measurement in T [%]calculated - rel. intensity [%]calculated with Lorentzian peaks

Fig. 3. Comparison of the calculated vibrations of 3tz in vacuo and an observed solid state spectrum recorded in a KBr matrix.

25

50

75

100

2700290031003300

wave number [cm-1]

trans

mitt

ance

[%]

3tz - measured

calculated

calculated - withLorentzian peaks

Fig. 4. Comparison of the region of C–H stretching bands of 3tz compared

with a measured sample.

30

50

70

90

500700900110013001500

wave number [cm-1]

tra

nsm

itta

nce

[%

]

3tz - measured

calculated

calculated - with Lorentzian peaks

Fig. 5. Comparison of the calculated and corrected finger-print region of the mid-IR spectrum of 3tz with the measured sample.


discrepancies in the intensities. Because calculated IR inten-

sities depend strongly on the size of the basis set, this is

expected due to the much to small basis set employed (see

Rauhut and Pulay [19] and references there in). As the

intensities are not essential for the qualitative interpretations

they will not be further discussed. According to the vibra-

tional analysis performed most of the assignments given

describe only the main contribution to the total energy of the

respective vibrational mode. Table 1 summarises this ana-

lysis of the band assignments based on an ab initio calcula-

tion in conjunction with a comparison to similar tetrazoles

known from the literature (mainly the unsubstituted 1H-

tetrazole [12,13]) and the comparison of 1-alkyltetrazoles

with different chain length.

The Table 2 in the appendix contains data of all important

mid infrared bands of all synthesised 1-alkyltetrazoles. The

changes in energy of a band by varying the chain length are

mostly quite small as shown in Fig. 1. Large changes are

only observed at the C–H out-of-plane deformation (gC–H)

and g>CH2and g�CH3

bands.

The calculation of the far infrared region exhibits three

vibrational modes involving motions of the whole mole-

cule’s backbone at 407, 376 and 331 cm�1, which are

deformations of the whole molecule, and at lower energies

(below 272 cm�1) several bands of rotational motions of

parts of the molecule (e.g. C–CH3).

Fig. 6 shows the calculated far-IR spectrum of 3tz. Due to

experimental problem with the liquid 3tz the solid 12tz was

taken for comparison. The agreements in energy of the

calculated out-of-plain bands of the tetrazole ring at about

680 cm�1, the backbone vibration of the whole molecule at

350 cm�1 and the rotational modes of parts of the molecule

in the region of 100 cm�1 are very good. Off course, the

Table 1

Assignment of IR vibrations based on an ab-initio calculation combined with the interpretation of analogue tetrazole derivatives

Calculated frequencies (cm�1) Observed bands and their assignment

n(C–H) 3132 3132vs n(C–H)

n(C–H) of aliphatic >CH2 and –CH3 groups 2967, 2942 2970vs n(C–H) of aliphatic >CH2 and –CH3 groups

2933, 2915 2940m

2911, 2882 2880m

2865

n(C5–N1) þn(Calkyl–N1) þn(N2¼N3) 1487 1487s n(N2¼N3) {þn(C5–N1) þn(Calkyl–N1)}

1439

d(C–H2) {and d(–CH3)} 1434, 1424 1461m d(C–H2) {and d(–CH3)}

1421, 1415 1443m

1425w

n(N2¼N3) þ n(N1–Calkyl) þ n(C–C) 1368, 1361 1387w n(N1–Calkyl)

1350

d(ring1) 1298, 1278 1249m d(ring1)

d(C–H2) 1255 1280 vw d(C–H2)

n(C¼N4), n(N1–N2) 1194, 1180 1169vs n(C¼N4), n(N1–N2)

n(C–N), d(C–H) 1112 1111vs n(C–N), d(C–H)

n(C–C) 1089 1090sh n(C–C)

n(N3–N4) þ n(N1–N2) 1075 1037sh n(N3–N4) þ n(N1–N2)

1031 1021w

d(ring2) in-plane bending {d(N–N¼C), n(CH2–CH3)} 994 966m d(ring2) in-plane bending {d(NCN), n(N3–N4) þ n(CH2–CH3)}

991

g(C–H) 911 901w g(C–H)

d(C–C–C) þ d(H2C–CH2) 873, 858 875m, 867m d(C–C–C) þ d(H2C–CH2)

d(C–H2) {þ d(C–H3)} 735 751sh d(C–H2) {þ d(C–H3)}

741m

g(ring3) out-of-plane 755 721m g(ring3) out-of-plane

g(ring4) out-of-plane 739 675m g(ring4) out-of-plane

681 663m

646m

Frequencies in cm�1. (n) Stretching; (d) deformation or in-plane vibration of the ring; (g) out-of-plane; (vs) very strong; (s) strong; (m) medium; (w) weak;

(vw) very weak; (sh) shoulder; (b) broad.

0

5

10

15

0100200300400500600700

wave number [cm-1]

inte

nsity

[%]

calculated 3tz

calculated 3tz - withLorentzian peaksmeasured 12tz

Fig. 6. Calculated far-IR spectrum of 3tz and the measured spectra of 12tz.


much longer alkyl chain of the 12tz yields more vibrational

modes of the whole molecule and it was not possible to

obtain reliable data below 60 cm�1, because of the poor

signal-to-noise ratio in this region.

4.2. Variable temperature IR spectroscopy of selected

[Fe(ntz)6](BF4)2 complexes

A series of complexes of the general composition

[Fe(ntz)6](BF4)2 is synthesised as described above. Their

spin transition behaviour is investigated using variable

temperature IR spectroscopy. The T1/2 value derived from

IR data is compared with data obtained by completely

independent techniques such as magnetic measurements

with a super-conducting interferometer device (SQUID)

and 57Fe Mossbauer spectroscopy.

As described above IR spectroscopy directly shows

changes of the binding properties of the molecule under

investigation thus it appears obvious to follow the changes of

the bond strength of the iron–ligand bond upon spin transi-

tion. The power of this method in deriving spin transition

parameters like T1/2 is presented e.g. in variable temperature

mid and far IR studies of [Fe(ntz)6](BF4)2 with n ¼ 3.

The [Fe(3tz)6](BF4)2 is well known as a substance with a

complete and abrupt spin transition at T"

1=2¼ 135 K,

T#

1=2¼ 128 K and a hysteresis of about 7 K [20,21]. In

the literature the substance is dealt with as an example

for its cooperative interactions [22,23].

The comparison of ligand and complex spectra (see Fig. 7)

shows the slight shift of energies of some of the vibrations of

the free ligand compared to the complex. The BF4� counter

ion is responsible for the broad band with the maximum at

1055 cm�1. As Fig. 7 also shows, the frequency shift of

vibrations between LS and HS spectra are quite small. A

closer and quantitative inspection (see Table 3 in the appen-

dix) reveals that the tetrazole bands are slightly shifting with

lower temperatures to higher energies of about 5–7 cm�1.

The >CH2 deformation bands of the alkyl chain (d>CH2) are

shifting only very little to higher energies, while their

stretching vibrations (nCH2) are not influenced. An opposite

trend is found for the N1–Calkyl stretching vibrations

(nN1�Calkyl). Of particular interest is the C–H stretching

vibration of the tetrazole hydrogen, which is close to the

iron coordination sphere and therefore geometrically

strongly influenced by the spin transition.

From Table 3 it can be seen that the frequency of this band

is 3146 cm�1 for the HS and 3152 cm�1 for the LS species.

Measurements were performed in the heating as well as in

the cooling mode to detect a possible hysteresis (see Fig. 8).

It is emphasised that the HS band does not shift during the

spin crossover to higher frequencies, but a new independent

LS band appears. These two bands just change intensities

Table 2

Prominent mid-IR bands of 1-alkyltetrazole ligands

Compound 1tz 2tz 3tz 4tz 5tz 6tz 7tz 8tz 9tz 10tz 12tz 16tz 18tz

n(C–H)99–103 3137vs 3133vs 3132vs 3130vs 3130m 3131m 3129m 3129m 3130m 3131m 3115s 3148w 3119m

3121w

n(C–H)95 of aliphatic C–H2

and C–H3 groups

2963m 2989m 2970vs 2963s 2959vs 2957vs 2956s 2956s 2955s 2854s 2955s 2855s 2956m

2944w 2940m 2937m 2933x 2932vs 2932vs 2928vs 2825vs 2925vs 2919vs 2916vs 2922vs

2880m 2876m 2877m 2871sh 2871sh 2871sh 2870sh 2870sh 2870w 2949vs 2847s

2862m 2860m 2858m 2857s 2856s 2856s 2850s

Aromatic overtone band 1640bm 1711bw 1680bw 1680bw 1676bm 1676bm 1676bm 1667bm 1673bm 1723w 1701bw 1673w 1793vw

n(N2¼N3)99–103 {þn(C5–N1)

þn(Calkyl–N1)}

1496s 1488s 1487s 1486s 1486s 1486s 1485s 1485s 1486m 1485m 1483m 1491m 1491m

d(C–H2)95 {and d(C–H3)} 1445w 1461w 1461m 1467m 1467m 1467m 1466m 1467s 1470s 1467s 1470vs 1468vs 1460vs

1420w 1445m 1443m 1442m 1444m 1444m 1444m 1443m 1455s 1444m 1424w 1434m 1448vw

1427w 1425w 1426w 1426w 1426w 1427sh 1427sh 1426w 1428w 1429w

n(N1–Calkyl)95 1380vw 1386w 1387w 1382w 1381w 1380w 1380w 1380w 1378w 1377w 1380w 1378w 1374m

d(ring1)99–101 1278m 1257m 1249m 1248w 1246vw 1249w 1249w 1251w 1248w 1253w 1256w 1253w 1249w

n(C¼N4), n(N1–N2)101 1175vs 1170vs 1169vs 1169vs 1168vs 1168vs 1167vs 1167vs 1167vs 1167vs 1165vs 1166vs 1176vs

n(C–N), d(C–H)101 1109vs 1111vs 1111vs 1110s 1111s 1112s 1113s 1114s 1114s 1114s 1110vs 1110vs 1126m

n(C–C) 1075sh 1090sh 1089w 1092m 1094m 1095s 1097s 1098s 1099s 1096m 1093s 1096m

n(N3–N4)99–101 1017w 1024w 1021w 1021w 1022w 1022w 1020w 1019w 1020w 1020w 1026w 1025w 1018w

d(ring2)100,101 in-plane bending

{d(NCN), n(N1–N2)}

964m 971m 966m 966m 964m 965m 965m 965w 965m 964m 968m 968m 969m

g(C–H)100,100 925vw 901m 942w 932w 932vw 932 vw 917w 918vw 908m 907w 923w

d(C–C–C) þ d(H2C–CH2) 877s 875m 875m 877m 877bw 876bm 878bw 879bw 880bm 877bw 909m 876bw 896m

d(C–H2)75 {þ d(C–H3)} 799w 769vw 741m 753w 734m 733m 735bsh 739bvw 737bw 737sh 745m 743m 744m

g(ring3)101 out-of-plane 722m 723m 721m 720w 720w 720w 722m 722m 721m 720m 720s 723s 723s

g(ring4)100,101 out-of-plane 680s 678s 675m 676m 675m 675m 675m 676w 676m 676w 673m 665m 663s

657s 648s 663m 664m 664m 664m 664m 664w 665m 664m 667w 653w

646m 647w 647w 647w 646w 648w 647m 647vw

Frequencies in cm�1; (n) stretching; (d) deformation or in-plane vibration of the ring; (g) out-of-plane; (vs) very strong; (s) strong; (m) medium; (w) weak;



upon spin transition. The spin transition occurs between 100

and 170 K for the cooling experiment and in the temperature

range of 130–190 K in the heating experiment, respectively.

The T1/2# is found at 142 K for cooling and T1=2 " ¼ 164 K

for heating the sample resulting in a hysteresisDThyst ¼ 22 K.

These are significant differences to the data obtained by

variable temperature magnetic measurements, which will

be discussed below.

To compare the mid-IR bands of the whole series of

[Fe(ntz)6](BF4)2 complexes Fig. 9 gives an overview on

some prominent mid-IR bands.

Some of the presented types of vibrations contain more

than only one band, e.g. stretching or deformation vibrations

of >CH2 bonds. To keep the figure lucid, in these cases only

the vibration with the highest frequency of the multiplet is

given. Sometimes it was not possible to identify the band of

an vibration, because of different reasons: [Fe(1tz)6](BF4)2

obviously contains no C–C bond. Bands can be hidden under

neighbouring absorption features thus obscuring the correct

position of one of two overlapping bands, e.g. nC–N at about

1130 cm�1 is located close to the broad band of the B–F

stretching vibrations. The relative intensity of bands like

dring1 decreases with longer chain length, so that it was not

possible to reliably detect this band for complexes with

n > 5 due to poor signal-to-noise ratio especially in the

region close to the far-IR. Nevertheless, Fig. 9 still allows

Fig. 7. The mid-IR spectra of 3tz compared with [Fe(3tz)6](BF4)2. The complex is shown at various temperatures between 103 and 303 K.

Table 3

Selection of some important mid-IR bands of various [Fe(ntz)6](BF4)2 complexes at about 300 and 100 K

Compound T [K] 1tz 2tz 3tz 4tz 5tz 6tz 7tz 8tz 9tz 10tz 12tz 16tz 18tz

n(C–H)99–103 300 3156vs 3168vs 3136s 3142w 3144vs 3145s 3146s 3144s 3145m 3144s 3145m 3147m 3148s

100 3146vs 3153vs 3142s 3152w 3146vs 3145s 3152s 3151s 3145m 3147s 3145m 3149m 3155s

n(C–H)95 of 300 3020w 2994m 2968s 2965w 2959vs 2958s 2971s 2960m 2956w 2958s 2955m 2956s 2956s

aliphatic C–H2 100 3027w 2990m 2965s 2965w 2953s 2954s 2971s 2960m 2951w 2951m 2955m 2954s 2953s

n(N2¼N3)99,103 300 1518s 1508s 1507s 1507m 1508s 1507s 1507vs 1505s 1506m 1501s 1507s 1507s 1507s

{þn(C5–N1)} 100 1516s 1512s 1514m 1513m 1514s 1512s 1514vs 1510s 1508m 1510s 1507m 1507s 1507s

d(C–H2)95 300 1444w 1449m 1464w 1460w 1459m 1464s 1459m 1458ms s 1470s 1465w 1472vs

100 1445w 1443m 1468w 1463w 1467m 1466m 1468vs 1467m 1467s s 1471s 1473s 1476m

n(N1–Calkyl)95 300 1389w 1389w 1371w 1380 1379w 1390m 1378m 1377w 1377w 1377w 1374w 1378m

100 1389w 1388m 1373w 1379s 1378w 1388s 1377m 1377w 1376s 1375w 1374w 1376w

d(ring1)99–101 300 1300m 1286w 1267w 1247w 1251w

100 1299m 1287w 1269w 1248w 1255m 1248w 1236w 1260vw

n(C¼N4), n(N1–N2)101 300 1188w 1181m 1280m 1181s 1183s 1181m 1180s 1181m 1181? 1181s 1181m 1182vs

100 1190w 1184m 1286m 1188s 1189s 1186m 1186s 1158w 1186m 1187? 1186s 1188m

n(C–N), d(C–H)101 300 1112w 1117s 1120s 1120w 1124s 1123m 1120m 1122w 1123m 1123m 1124s 1107vs 1126m

100 1115w 1119s 1125s 1124w 1124s 1127m 1125m 1127w 1126s 1126m 1126s 1109vs 1111w

n(C–C) 300 1086w w 1092m 1095m 1098s 1098s 1100w 1102m

100 1088w w 1092m 1095m 1088m 1098s 1101s 1096w 1105m

g(ring3)101 out-of-plane 300 738m 738m 722m 721m 721m 721w 721m 721s 720m 720m 720s 721w 719vs

100 738m 738m 723m 725m 725w 726w 726m 723s 720m 723vs 720s 721w 732vw

g(ring4)101–102 300 683s 679s 673m 673m 670s 670w 673s 677w 678m 678m 688w 661w

out-of-plane 100 682s 680s 680m 679m 677s 674w 680s 671m 675w 674s 681w 688w

Frequencies in cm�1; (n) stretching; (d) deformation or in-plane vibration of the ring; (g) out-of-plane; (vs) very strong; (s) strong; (m) medium; (w) weak;



following several trends in changes in IR bands from the free

ligand to the ligand coordinated to the iron centre at room

temperature, where all the complexes are in the HS state and

the changes of the complex to the compound at 100 K, where

most of the compounds reached the LS state. Differences in

absorption frequencies between the free ligand and the

ligand in an iron(II) tetrafluoroborate complex are signifi-

cant for the bands of the tetrazole ring. The nC–H stretching

vibration of the tetrazole is shifted in the complex about

15 cm�1 to higher frequencies. Shifts with the same trend of

about 20 cm�1 are observed for nN2¼N3. Other influenced

tetrazole bands are nC¼N and dC–H. Shifts in frequencies of

out-of-plane vibrations are not significant. Also no signifi-

cant changes are observed in the C–H stretching or defor-

mation vibrations of the aliphatic bands in the chain, e.g. nC–

H(alkyl) and dC�H2ðalkylÞ. The inspection of changes in band

frequencies of compounds with different alkyl-chain lengths

show parallel trends for the ligands compared to their

complexes. In contrast to the tetrazole bands the vibrational

features associated with the alkyl chain are more influenced

by its elongation, but these changes are only significant

between the 1tz and 5tz, respective their complexes. Longer

chains do not further influence the location of the bands. The

C–H alkyl stretching vibration (nC–H) decreases with longer

chains from 3137 cm�1 for the ligand 1tz to 2959 cm�1 for

5tz; this difference is more than 60 cm�1. The same is found

for the complexes. For the deformation vibration (d>CH2), an

opposite trend can be observed, as well as for the C–C

stretching bond (nC–C). The only affected tetrazole band is

the in-plane ring vibration. This band decreases from 1tz

(1278 cm�1) to 5tz (1246 cm�1). The influence of the chain

length within the coordinated ligands is larger than for the

free ligands.

The effects of frequency changes in the coordinated

ligand between high and low temperatures are rather small

compared to the previous discussed changes between the

free ligand and the ligand in the complex. Significant

changes can be found for vibration in the tetrazole ring

(nN2¼N3, nC¼N4

and dC–H). The disadvantage of using these

bands for the determination of T1/2 upon spin transition is

that they are mostly affected by band overlapping with

neighbouring absorption features, which is not the case

for the C–H stretching vibration of the tetrazole. Unfortu-

nately in these cases, the frequency shifts upon the thermal

induced spin transition are small and only significant for

short chain length (see Table 3 in the appendix).

4.3. Vibrational characterisation of the nditz ligands

Despite of the mononuclear iron(II) complexes with

ntz ligands iron(II) coordination polymers based on a,

Fig. 8. Transmittance of the HS and LS C–Htz band vs. temperature of

[Fe(3tz)6](BF4)2 showing a gradual spin transition with a hysteresis of

about 22 K.

2940

3040

3140

1050

1150

1250

1350

1450

wav

e nu

mbe

r [c

m-1

]

660

680

700

720

0 5 10 15 20n

ν(C-H)

ν(C-H) ofaliphatic C-H2, CH3

ν(N2=N3){+ (C5-N1)

δ(C-H2)

ν(N1-Calkyl)

δ(ring1)

(C=N4),

ν(N1-N2)

ν(C-N),δ(C-H)

ν(C-C)

γ (ring3) out ofplain

f-

(ring3) out-of-plane

ν

γ

ν

Fig. 9. Comparison in energy of the some important bands of ntz ligands

and their iron(II) tetrafluoroborate complexes. () Marks the ligand

frequency, (–) the HS band in the complex at about 300 K, and (-) the LS

band at approximately 100 K.


Fig. 10. Comparison of the ligand (4ditz) above, with its iron(II) hexafluorophosphate complex, measured at different temperatures between 303 and 103 K

(middle) and the focus on the bands nC–H (bottom, left) and nðN2¼N3Þ and dCH2.

Table 4

Prominent mid- and far-IR bands of the ligand 4ditz and its iron(II) hexafluorophosphate complex

Compound 4ditz [Fe(4ditz)3](PF6)2 HS at 303 K [Fe(4ditz)3](PF6)2 LS at 103 K

n(C–H) 3317s 3165vs 3181vs

n(C–H) 2962m 2985m 2986m

2945w 2963m 2965m

2925w 2938m 2889w

2879vw 2876w 2871w

n(N2¼N3) {þn(C5–N1) þ n(Calkyl–N1)} 1492s 1506vs 1485s

d(C–H2) 1450s 1468s 1464s

1422m 1441m 1441m

n(N1–Calkyl) 1359m 1381m 1378m

d(ring1) 1243m 1243w 1253m

n(C¼N4), n(N1–N2) 1173vs 1178vs 1180vs

n(C–N), d(C–H) 1107s 1096s 1099s

n(N3–N4) 1033w 1020m 1020m

n(N1–N2) 973sh 971s 970s

d(ring2) in–plane bending {d(NCN)} 970s 997s 1005m

d(ring3) in-plane bending {d(NNN)} 876m 891vs 885vs

n(P–F) �841s �836s

d(C–H2) 723m 718m 723w

g(ring4) out-of-plane 670s, 647s 662s 668s

d(P–F) 557vs 558vs

d(ligand1) 432vs 453s 460s

n(LS1) (415vw) 412s

n(LS2) (397vw) 392w

n(HS1), n(LS3) 359m 346s

d(ligand2), 349m

Frequencies in cm�1. (n) Stretching; (d) deformation or in-plane vibration of the ring; (g) out-of-plane.


o-bis(tetrazol-1-yl) alkanes (nditz) with n ¼ 2 and 4 have

been synthesised and characterised. Such bridging ligands

allow the building of 1D chain-type coordination polymers,

2D catenanes or a 3D network. 1,2-bis(tetrazol-1-yl)ethane

was synthesised according to Gaponik et al. [24] and

Schweifer et al. [7,25], whereas 1,4-bis(tetrazol-1-yl)butane

was synthesised according to Koningsbruggen et al. [26] and

Schweifer et al. [25]. The assignment of the IR vibrations is

based on those given for the ntz ligands above. The changes

of nC–H at the tetrazole as well as for the in-plane vibrations

of the ring are very small. Especially, the out-of-plane

vibration appears always at the same energy.

4.4. Variable temperature IR spectroscopy of selected

[Fe(nditz)3]X2 complexes

Mid-IR spectra of [Fe(2ditz)3](BF4)2 and [Fe(4ditz)3]-

(PF6)2�0.7MeOH were presented in Schweifer et al. [25],

temperature [K]

80 100 120 140 160 180 200

abso

rban

ce [a

.u.]

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

temperature [K]

80 100 120 140 160 180 2000,0

0,2

0,4

0,6

0,8

1,0

Fig. 11. (Left) absorbance vs. temperature of the nC–H HS (empty dots) and the LS species (filled dots); (right) ratio (g) for the spin state with g :¼ gHS (empty

dots) and g :¼ gLS (filled dots)—the lines are meant to guide the eyes.

Fig. 12. Far-IR spectra of the ligand (4ditz) at room temperature (above), with its iron(II) hexafluorophosphate complex, measured at different temperatures

between 298 and 103 K (middle) and the focus on several bands of the complex, which are changing due to the spin conversion.


but no detailed assignment of the bands were investigated and

no temperature dependence was examined. A band assign-

ment will be discussed exemplary for [Fe(4ditz)3](PF6)2 as

well as its temperature dependent behaviour.

Variable temperature FT-IR spectra were measured

between 4400 and 450 cm�1 for the ligand at room tem-

perature and complex between room temperature and 103 K.

Several characteristic bands for the tetrazole ring as well as

for the alkyl chain have been found. Fig. 10 compares these

spectra. For a detailed description of vibrations see a recent

work by A. Stassen et al. [27] and Table 4 in the appendix.

It is striking, that due to the rigid 3-D network of the

[Fe(4ditz)3](PF6)2 the aromatic hydrogen shifts about

16 cm�1 upon changing from HS to the LS state. On the

other hand the effect of spin conversion on individual

stretching vibrations in the tetrazole ring itself is marginal

(see Fig. 10, bottom). Only an aromatic breathing mode

(dring1) and a more or less broad aromatic overtone band are

significantly affected, as well. Therefore, the focus of inter-

est in our case is the single aromatic C–H bond of the

tetrazole ring, which does not couple with any other vibra-

tional modes. Upon the spin transition of the iron(II) the

bond strengths of the nearest neighbour bonds and—to a

lesser extent but still detectable—of the next-nearest neigh-

bour bonds change yielding a shift of the very sharp and

strong band assigned to this nC–H stretching vibration.

Fig. 11 represents the peak intensity of the nC–H bands for

the HS and LS species, respectively, and the calculated ratio

g of HS and LS species. T1/2 is determined to be 167 K,

which slightly lower than the value determined via magnetic

measurements and 57Fe Mossbauer spectroscopy. This dif-

ference is mainly due to the somewhat arbitrary definition of

the baseline for the peak intensity determination and by band

overlapping. Thus variable temperature mid-range FT-IR

spectroscopy supports the data obtained by magnetic mea-

surements qualitatively and yields an independent proof of

the coexistence of high- and low-spin species.

An exemplary study of far-IR spectroscopy with the

[Fe(4ditz)3](PF6)2 complex is presented in Fig. 12. The

spectra show besides a broadening absorption at

562 cm�1 attributable to vibrations within the PF�6 anion,

intra-ligand deformation bands (e.g. dligand1), which shift

upon coordination (see Table 4). These bands are slightly

affected by the thermally induced spin conversion of the

iron(II) and, therefore, feature a smooth and gradual shift of

about 7 cm�1. In contrast, those bands attributed to metal-to-

ligand vibrations have vanished and occurring according to

the HS $ LS ratio {e.g. nFe–N(HS) and nFe–N(LS)}. Thus,

the parallel existence of the bands for the nFe–N(HS) and nFe–

N(LS) around the T1/2 are an independent proof for the

coexistence of these species. The region around 230 cm�1

yields a multitude of overlapping bands owing to deforma-

tions of the whole ligand and the octahedral surrounded

iron(II). These bands are influenced by the spin transition as

well, but the strong overlapping prevents a more detailed

interpretation.

5. Conclusions

The application of variable temperature mid-IR spectro-

scopy to analyse the spin transition behaviour is known from

literature mainly for monitoring changes of –CO or –CN co-

ligands [8,9]. But in our case we concentrate on the C–H

stretching vibration (nC–H) of the tetrazole ring, which

turned out to be most convenient to observe the spin cross-

over. Because of their close location to the iron coordination

centre the influencing of the bond strength change of the

iron–ligand bonds upon population or depopulation of the

anti-bonding eg-orbitals of the iron(II) is very strong.

Furthermore, the band is not affected by band overlapping,

which facilitates the interpretation not only qualitatively but

also quantitatively. For the complex [Fe(3tz)6](BF4)2, which

is known for its abrupt spin transition with a hysteresis it is

possible to follow the spin crossover and prove the hysteresis

via variable temperature mid-IR spectroscopy. Compared to

the magnetic data from literature the hysteresis is larger and

appears at higher temperatures, which might be due to

baseline effects because of band overlapping of nC–H(HS)

and nC–H(LS). Furthermore, for the calculation of the ratio g(i.e. the amount of species in a respective spin state) the T

(%)-values instead of the integrated band areas were com-

pared, because due to band overlapping a band deconvolu-

tion would have been required in advance. Beside these

improvements, which might influence the results slightly

quantitative, the strong qualitative differences in the results

compared to published magnetic data might go back to a

different sample preparation. To obtain a pellet of

[Fe(3tz)6](BF4)2 the powdered sample was treated with a

pressure of 10,000 kg/cm2. Because it is known, that the spin

transition can be influenced by pressure [26], temperature

dependent measurement in diffuse reflection should be able

to solve this problem.

Thus, with variable temperature IR spectroscopy a tool for

the proof of the coexistence of two spin states of the complex

around the spin-transition temperature T1/2 is available. In

case of a reasonably big band shift of the C–H band, which is

dependent on the packing of the complex, a quantification of

the respective peak intensities allows the independent deter-

mination of T1/2. This opens additional perspectives for the

use of variable temperature IR spectroscopy as a tool in the

investigation of spin-crossover compounds.

Acknowledgements

Thanks for financial support are due to the European

Union funding the TMR project TOSS (‘‘Thermal and

Optical Spin Switching’’) under the contract number

ERB-FMRX-CT98-0199, the European Science Foundation

funding the project ‘‘Molecular Magnets’’, the Austrian

Science Fund (FWF) project P-15874 as well as the

Hochschuljubilaumsfond der Stadt Wien under the project

H-65/2000.


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Documents

Variable temperature far and mid FT-IR as a valuable tool to determine the spin transition temperature of iron(II) spin-crossover coordination compounds