Direct observation of fine structure transitions in a paramagnetic nickel(II) complex using far-infrared magnetic spectroscopy: A new method for studying high-spin transition metal complexes

Article abstract of DOI:10.1021/ic030013o

Novel far-infrared (FIR) absorption spectroscopy in conjunction with multiple, fixed external magnetic fields (FIR magnetic spectroscopy, FIRMS) has been used to investigate pseudotetrahedral complexes with the formula M(PPh₃)₂Cl₂ (M = Ni, Zn; Ph = C₆H₅). Crystal structures have been reported for the Ni complex; we report the structure of the Zn complex. Transmission spectra at 5 K of Ni-(PPh₃)₂Cl₂(S = 1) at zero magnetic field exhibit absorption bands at 11.41, 15.28, and 23.0 cm^-1. The two lower frequency bands show great sensitivity to external magnetic field, and their field dependence is as expected for electron spin transitions allowing precise determination of the following parameters: |D| = 13.35(1) cm^-1, |E| = 1.93(1) cm^-1, g_x,y = 2.20(1), g_z = 2.00(1). Corresponding spectra of Zn(PPh₃)₂Cl₂ (S = O) exhibit bands only at >20 cm^-1, which show no field dependence. FIRMS is a promising technique for direct investigation of the electronic structure of high-spin transition metal complexes.

Full text of DOI:10.1021/ic030013o

Inorg. Chem. 2003, 42, 1788−1790
Direct Observation of Fine Structure Transitions in a Paramagnetic
Nickel(II) Complex Using Far-Infrared Magnetic Spectroscopy: A New
Method for Studying High-Spin Transition Metal Complexes
†
†,‡
†
§
|
Suriyakan Vongtragool, Boris Gorshunov, Martin Dressel, J. Krzystek, David M. Eichhorn, and
Joshua Telser*
,⊥
1. Physikalisches Institut, UniVersit a¨ t Stuttgart, Pfaffenwaldring 57, Stuttgart, D-70550 Germany,
National High Magnetic Field Laboratory, Florida State UniVersity, Tallahassee, Florida 32310,
Department of Chemistry, Wichita State UniVersity, Wichita, Kansas 67260, and Chemistry
Program, RooseVelt UniVersity, Chicago, Illinois 60605
Received January 13, 2003
Novel far-infrared (FIR) absorption spectroscopy in conjunction with
multiple, fixed external magnetic fields (FIR magnetic spectroscopy,
FIRMS) has been used to investigate pseudotetrahedral complexes
swept HFEPR spectra and analysis have recently been
reported for many integer (high) spin transition metal ions
for which conventional EPR is unsuitable. These include
3
4
5-7
8
9-11
V(III), Cr(II), Mn(III), Fe(II), and Ni(II).
A comple-
with the formula M(PPh
3
)
2
Cl
2
6 5
(M ) Ni, Zn; Ph ) C H ). Crystal
mentary method to study such complexes depends on
sweeping the frequency while keeping the magnetic field at
zero or some fixed value. This technique was originated by
Richards and co-workers using a far-IR Michelson interfer-
structures have been reported for the Ni complex; we report the
structure of the Zn complex. Transmission spectra at 5 K of Ni-
(
3 2 2
PPh ) Cl (S ) 1) at zero magnetic field exhibit absorption bands
-
1
at 11.41, 15.28, and 23.0 cm . The two lower frequency bands
show great sensitivity to external magnetic field, and their field
dependence is as expected for electron spin transitions allowing
precise determination of the following parameters: |D| )
1
2,13
ometer
ers.
and also employed by Champion and co-work-
Despite the initial success of these studies, which
included studies of metalloproteins such as deoxy-hemoglo-
14,15
6
15
bin (Fe(II), 3d , S ) 2) in frozen solution, far-IR magnetic
spectroscopy (FIRMS) has not been subsequently pursued,
presumably due to low sensitivity and generally inadequate
technology. However, with the recent development of
spectroscopy based on coherent, monochromatic, frequency-
-
1
-1
1
3.35(1) cm , |E| ) 1.93(1) cm , gx,y ) 2.20(1), g
z
) 2.00(1).
Corresponding spectra of Zn(PPh ) Cl (S ) 0) exhibit bands only
3 2 2
-
1
at >20 cm , which show no field dependence. FIRMS is a
promising technique for direct investigation of the electronic
structure of high-spin transition metal complexes.
(
(
(
3) Tregenna-Piggott, P. L. W.; Weihe, H.; Bendix, J.; Barra, A.-L.; G u¨ del,
H.-U. Inorg. Chem. 1999, 38, 5928-5929.
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37, 5769-5775.
Electron paramagnetic resonance (EPR) is widely em-
ployed in the study of transition metal ion complexes,
particularly those with Kramers (half-integer) spin ground
5) Krzystek, J.; Pardi, L. A.; Brunel, L.-C.; Goldberg, D. P.; Hoffman,
B. M.; Licoccia, S.; Telser, J. Spectrochim. Acta, Part A 2002, 58,
1113-1127.
1
(6) Limburg, J.; Vrettos, J. S.; Crabtree, R. H.; Brudvig, G. W.; de Paula,
J. C.; Hassan, A.; Barra, A.-L.; Duboc-Toia, C.; Collomb, M.-N. Inorg.
Chem. 2001, 40, 1698-1703.
(7) Barra, A.-L.; Gatteschi, D.; Sessoli, R.; Abbati, G. L.; Cornia, A.;
Fabretti, A. C.; Uytterhoeven, M. G. Angew. Chem., Int. Ed. Engl.
states. Complexes with non-Kramers (integer) spin states,
however, are often difficult to study by conventional EPR,
due to the effects of zero-field splitting (zfs). Within the past
few years, high-frequency and -field EPR (HFEPR) has
1997, 36, 2329-2331.
2
overcome some of these difficulties, by the use of frequen-
(8) Knapp, M. J.; Krzystek, J.; Brunel, L.-C.; Hendrickson, D. N. Inorg.
Chem. 2000, 39, 281-288.
cies venturing into the far-infrared (FIR) region (up to 700
GHz) combined with high magnetic fields (up to 25 T). Field-
(
9) Pardi, L. A.; Hassan, A. K.; Hulsbergen, F. B.; Reedijk, J.; Spek, A.
L.; Brunel, L.-C. Inorg. Chem. 2000, 39, 159-164.
(
10) van Dam, P. J.; Klaassen, A. A. K.; Reijerse, E. J.; Hagen, W. R. J.
*
Corresponding author. E-mail: jtelser@roosevelt.edu.
Universit a¨ t Stuttgart.
Permanent address: General Physics Institute, Russian Academy of
Magn. Reson. 1998, 130, 140-144.
†
(11) Krzystek, J.; Park, J.-H.; Meisel, M. W.; Hitchman, M. A.; Stratemeier,
H.; Brunel, L.-C.; Telser, J. Inorg. Chem. 2002, 41, 4478-4487.
(12) Joyce, R. R.; Richards, P. L. Phys. ReV. 1969, 179, 375-380.
(13) Brackett, G. C.; Richards, P. L.; Caughey, W. S. J. Chem. Phys. 1971,
54, 4383-4401.
‡
Sciences, Moscow, Russia.
§
Florida State University.
Wichita State University.
Roosevelt University.
|
⊥
(14) Champion, P. M.; Sievers, A. J. J. Chem. Phys. 1977, 66, 1819-
1825.
(
1) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of
Transition Ions; Dover Publications: New York, 1986.
2) Hagen, W. Coord. Chem. ReV. 1999, 190, 209-229.
(15) Champion, P. M.; Sievers, A. J. J. Chem. Phys. 1980, 72, 1569-
(
1582.
1788 Inorganic Chemistry, Vol. 42, No. 6, 2003
10.1021/ic030013o CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/19/2003

COMMUNICATION
tunable radiation sources (backward wave oscillators) operat-
16
ing in the millimeter/submillimeter wavelength region, there
now exists a powerful method to study paramagnetic
transition metal complexes.1 This modern FIRMS allows
recording transmission (or reflection, etc.) spectra at 1.5-
6,17
3
1
7
00 K using an optical configuration over the range 40-
,200 GHz (λ ) 7.5-0.25 mm) and at fields currently up to
18
T.
We introduce here an application of FIRMS in the area
of coordination chemistry, specifically to study the complex
Ni(PPh Cl (Ph ) phenyl), which contains pseudotetrahe-
)
3 2
2
8
dral Ni(II) (3d , S ) 1) and is totally “EPR-silent” using
conventional fields and frequencies. We have recently
reported a detailed HFEPR study on this compound in the
Figure 1. Transmission coefficient spectra of plane parallel pellets of
M(PPh3)2Cl2 (M ) Ni, b; M ) Zn, 4) at 5 K and zero magnetic field. The
11
19
solid state, which, combined with earlier magnetic,
-
1
inset magnifies the bands seen at 11.41 and 15.28 cm . Shallow periodic
2
0
21
optical, and structural studies, allowed a complete de-
scription of the electronic structure of the Ni(II) ion. Of
particular importance are the spin Hamiltonian parameters
26
oscillations are due to interference within the pellets; absorption bands
are deeper minima and are fit by Lorentzian terms.1
8,25
using well-known optical principles.25,26 Absorption bands,
of whatever origin (vibrational, electronic, ferromagnetic
resonance, etc.), are easily seen as deeper minima.
-
1
determined primarily by HFEPR: D ) +13.20 cm , |E| )
.85 cm , isotropic g ) 2.20.
To facilitate spectral interpretation of the paramagnetic Ni-
II) complex, we also applied FIRMS to a diamagnetic
analogue, Zn(PPh Cl , as a control. This Zn(II) complex
had not, despite having been reported many years ago, been
-
1
11
1
Ni(PPh
3 2 2
) Cl at 5.5 K exhibited a strong, relatively broad
(
-
1
band at 23.0(1) cm , and two narrow, weaker bands at
1.41(2) and 15.28(2) cm (Figure 1). At higher tempera-
3
)
2
2
-
1
1
22
-
1
tures, the band at 23 cm broadened and shifted to slightly
higher frequencies, (see Figure S2) but was not affected by
application of an external magnetic field. In contrast, the
bands at 11.41 and 15.28 cm showed very little temperature
dependence of frequency, but very strong field dependence
3 2 2
structurally characterized. Crystals of Zn(PPh ) Cl were
grown from 1-butanol solution,23 and the structure was
determined by X-ray crystallography.24 As expected, Zn-
-1
(
PPh
3
)
2
Cl
2
is a pseudotetrahedral complex (C2V symmetry
Cl (see Figure
about Zn), structurally similar to Ni(PPh
3
)
2
2
(
see later). Comparison with the zfs parameters determined
S1, Supporting Information). A detailed comparison of the
two structures is beyond the scope of this paper; however,
we note that the P-Ni-P and Cl-Ni-Cl bond angles are
by HFEPR for Ni(PPh Cl allows assignment of these two
3
)
2
2
bands to the transitions with energies, |D| - |E| and |D| +
1
1
-1
|
E| in zero-field, yielding |D| ) 13.35(1) cm , |E| )
21
very different (111.5° and 128.0°, respectively ), while the
-
1
1
.93(1) cm . While the agreement between HFEPR and
P-Zn-P and Cl-Zn-Cl bond angles are very similar (115.0
FIRMS techniques is excellent, the latter is more accurate,
given that it is a true zero-field measurement.
1
0
(
0.2°), due to the absence of ligand-field effects in 3d
Zn(II).
The Zn(II) complex at 5.5 K exhibited two absorption
Transmission coefficient spectra were recorded on pressed
-1
-1
bands, 24.0(1) cm (Figure 1) and 34.0(1) cm (not shown),
but none at lower frequency. The temperature dependence
plane-parallel pellets (∼100 mg; diameter, 10 mm; thickness,
3 2 2
1.5-1.8 mm) of M(PPh ) Cl (M ) Ni, Zn) at zero external
-
1
of the band at 24.0 cm was essentially the same as that of
the corresponding band for the Ni(II) complex (see Figure
S2). Lacking knowledge of the crystal and molecular
structure of these two complexes over the 5-300 K range,
we dare not propose a reason for the modest shift to higher
frequency with increasing temperature. The similarity in
behavior, however, suggests a common origin for the band.
A simple molecular mechanics calculation suggested that
torsional modes involving primarily the multiple phenyl rings
occur at the following frequencies: 10, 24, and 33-40
magnetic field, and at fixed fields up to 7 T, over the
temperature range 5-300 K.18 The spectra contain shallow
oscillations due to constructive and destructive interference
of the radiation inside the plane pellet, which acts as a
Fabry-Perot resonator. These oscillations are easily modeled
(
16) Kozlov, G.; Volkov, A. In Topics in Applied Physics: Millimeter and
Submillimeter WaVe Spectroscopy of Solids; Gr u¨ ner, G., Ed.;
Springer: Berlin, 1998; Vol. 74, pp 51-109.
2
7
(
17) Mukhin, A.; Gorshunov, B.; Dressel, M.; Sangregorio, C.; Gatteschi,
D. Phys. ReV. B 2001, 63, 214411.
-
1 28
(18) Vongtragool, S.; Gorshunov, B.; Dressel, M. In preparation.
19) Davies, J. E.; Gerloch, M.; Phillips, D. J. J. Chem. Soc., Dalton Trans.
cm . Few studies have been done on very low frequency
(
-1
(<100 cm ) modes in molecular complexes; acetanilide has
1
979, 1836-1842.
20) Fereday, R. J.; Hathaway, B. J.; Dudley, R. J. J. Chem. Soc. A 1970,
71-574.
(
5
(25) Dressel, M.; Gr u¨ ner, G. Electrodynamics of Solids; Cambridge
University Press: Cambridge, U.K., 2002.
(26) Born, M.; Wolf, E. Principles of Optics; Pergamon Press: Oxford,
U.K., 1986.
(27) PC-SPARTAN, version 2.02; Wavefunction, Inc.: Irvine, CA, 2002.
(28) Vibrations primarily due to M-Cl or M-P stretching modes occur at
(
(
(
21) Brammer, L.; Stevens, E. D. Acta Crystallogr. 1989, C45, 400-403.
22) Coates, G.; Ridley, D. J. Chem. Soc. 1964, 166-173.
23) Microcrystalline material needed for FIRMS (∼100 mg) was conve-
niently prepared by addition of anhydrous ZnCl2 in diethyl ether to
PPh3 in benzene (1:2 mol ratio).
-1
(
24) Data collected at 25 °C; colorless crystal; monoclinic P21/c; a )
much higher frequencies, >150 cm , see: Shobatake, K.; Nakamoto,
K. Inorg. Chem. 1970, 9, 3332-3335. Deacon, G. B.; Green, J. H. S.
J. Chem. Soc., Chem. Commun. 1966, 629-630.
1
3
1.816(7) Å, b ) 17.205(7) Å, c )16.940 Å, â ) 104.55(4)°, V )
3
333(3) Å ; Z ) 4; R ) 0.085; Rw ) 0.101; GOF ) 2.80.
Inorganic Chemistry, Vol. 42, No. 6, 2003 1789

COMMUNICATION
Figure 3. Magnetic field dependence of far-IR bands for Ni(PPh3)2Cl2 at
.5 K. Experimental resonances are given by circles (b) (z transitions),
5
Figure 2. Magnetic field dependence of magnetic absorption bands in
the transmission spectra of a Ni(PPh3)2Cl2 pellet at 5 K; spectra for Zn-
squares (9) (x), and triangles (2) (y); error bars are within these. The lines
represent magnetic resonance transitions calculated for S ) 1 with D )
(PPh3)2Cl2 showed no effect of magnetic field. Absorption lines are
-1
-1
+
13.347 cm , E ) 1.932 cm , gx,y ) 2.20, gz ) 2.00. The solid lines are
highlighted by arrows and are labeled with the field value. Experimental
points are given by open circles (O) for B ) 0 T, by solid circles (b) (3 T),
and by triangles (4) (4 T). The lines show fits using Lorentzians for the
for the magnetic field along the molecular z axis, the long dashed lines
along x, and the short dashed lines along y. The transitions are also identified
by the standard nomenclature for triplet systems with rhombic symmetry.
32
-
1
absorption bands. At higher fields, the band at ∼11 cm broadens and
-
1
splits while that at ∼15 cm moves to higher frequencies.
taken directly from the zero-field FIRMS spectrum; the gx,y
1
1
value was taken from HFEPR and gives good agreement
been studied somewhat, as a model for polypeptides,29,30 but
a detailed neutron scattering study on acetanilide assigned a
with the observed field-dependent transitions. Use of g )
z
2.00 ( 0.01 fit the FIRMS B transition better than did use
3z
-
1
band at 27 cm to a torsional mode of the single phenyl
of g ) 2.20 from HFEPR. The g value in integer-spin
z
z
3
0,31
-1
ring.
in M(PPh
phenyl rings.
We thus assign the bands observed at 23-24 cm
systems can sometimes be difficult to determine by HFEPR
3
)
2
Cl to vibrational modes, possibly involving the
2
as z transitions are much weaker than x,y transitions in
5
powder pattern spectra.
Application of external magnetic fields, using a Voigt
configuration, confirmed the zero-field assignment. Fields
up to 7 T at a given temperature had no effect on the
We have used FIRMS for the first time to record resonant
absorptions in the millimeter/sub-millimeter wavelength
range in a high-spin non-Kramers molecular paramagnet.
Excellent agreement of the field and temperature behavior
of the observed transitions with calculations allowed us to
differentiate them into magnetic transitions within the Ni-
(II) triplet spin system, and molecular vibrational transitions
within the ligands. FIRMS thus represents a technique for
the direct measurement of electronic parameters in high-spin
transition metal complexes, particularly in determining zfs
parameters to high precision, but also g values. One should
therefore consider adding FIRMS to the armamentarium of
physical methods in inorganic chemistry, along with well
established techniques such as (HF)EPR and magnetic
measurements including MCD, as well as new methods such
-
1
spectrum of Zn(PPh
Ni(PPh Cl affected. However, as shown in Figure 2, the
bands for Ni(PPh
3 2 2
) Cl , nor was the band at 23 cm in
3
)
2
2
-
1
3 2 2
) Cl at 11.41 and 15.28 cm were
strongly affected: shifting and splitting and/or broadening
significantly with field. The field dependence of the bands
is nonlinear, which is as expected for a rhombic, non-
Kramers spin system.11 This can be seen more easily in
Figure 3, which presents the electronic spin transition
energies as a function of field. The transitions are identified
by the formalism for spin triplets.32 Both the |D| - |E| and
|
D| + |E| branches are shown; not shown is the branch
-1
originating at 2|E| ) 3.8(1) cm . No transitions within this
branch were observed here, nor previously by HFEPR,
1
1
33
as inelastic neutron scattering (INS).
probably due to their low intensities. For D > 0, as
determined by HFEPR, the 2|E| zero-field transition origi-
nates from an excited state with a Boltzmann population at
Acknowledgment. We thank the Deutsche Forschungs-
gemeinschaft (S.V., B.G., M.D.), NHMFL (J.K., J.T.), and
Roosevelt University (J.T.) for financial support.
5
.5 K that is ∼20× smaller than the ground state.
The calculated lines in Figure 2 used |D| ) 13.347 cm ,
E| ) 1.932 cm , gx,y ) 2.20, g ) 2.00. The zfs values are
-
1
Supporting Information Available: Crystallographic informa-
-
1
|
z
tion file and tables of bond distances and angles for Zn(PPh
Figure showing the molecular structures of Ni(PPh Cl
Zn(PPh Cl . Figure showing temperature dependent FIRMS
3
)
2
Cl
2
.
)
3 2
2
and
(
(
(
29) Spire, A.; Barthes, M.; Kellouai, H.; De Nunzio, G. Physica D 2000,
)
2
3
2
137, 392-401.
spectra. This material is available free of charge via the Internet at
http://pubs.acs.org.
30) Hayward, R. L.; Middendorf, H. D.; Wanderlingh, U.; Smith, J. C. J.
Chem. Phys. 1995, 102, 5525-5541.
2
9
31) This study and a far-IR study also identified bands in this region as
arising from translational or librational modes, which might be the
case here as well, and crystal lattice changes might be the source of
the temperature dependence of these bands.
IC030013O
(33) Basler, R.; Tregenna-Piggott, P. L. W.; Andres, H.; Dobe, C.; G u¨ del,
H.-U.; Janssen, S.; McIntyre, G. J. J. Am. Chem. Soc. 2001, 123,
3377-3378.
(
32) Wasserman, E.; Snyder, L. C.; Yager, W. A. J. Chem. Phys. 1964,
41, 1763-1772.
1790 Inorganic Chemistry, Vol. 42, No. 6, 2003