Journal of Nuclear and Radiochemical Sciences, Vol. 8, No.1, pp. 21-23, 2007

Metal Atom Dynamics in Sn(II) Phthalocyanin from Temperature-dependent 119SnMössbauer Spectroscopy
Rolfe H. Herber*
Racah Institute of Physics, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel
Received : March 1, 2007; In Final Form : April 27, 2007
Dedicated to Prof. Hirotoshi Sano on the occasion of his 79th birthday
Temperature-dependent 119Sn Mössbauer spectroscopy has been used to elucidate the root-mean-square-amplitudeof-vibration (rmsav) of the metal atom in Sn(II) phthalocyanin and to examine the (absence of) metal atom
motional anisotropy in the temperature range 90<T<190 K.

1. Introduction

3. Results and Discussion

Divalent tin phthalocyanin (SnPc) is an interesting compound due to its unusual structure and the presence of a nonbonding pair of electrons oriented along the four-fold axis
perpendicular to the ring nitrogens. The crystal structure has
been reported by several groups1,2 and it is well documented
that the tin atom, having C4v symmetry, is located 1.11 Å above
the nitrogen atom plane. The lone pair of electrons is assumed
to occupy the space above the metal atom. Due to this uniquegeometry, SnPc has been subjected to several 119Sn Mössbauer
studies3 and the hyperfine parameters – the isomer shift (IS)and quadrupole splitting (QS) – have been reported2,4 at or nearliquid nitrogen temperature.
The focus of the present study is to relate the temperaturedependence of the recoil-free fraction, f, of the tin atom to thedynamics of the metal atom and to examine in detail themotional anisotropy of the metal center over a significant temperature range.

As noted earlier by other investigators, 3,4 the Mössbauer
Effect (ME) spectra of SnPc consist of a well resolved doublet,and a typical spectrum is summarized graphically in Figure 1.
The observed full line width a half maximum of 0.882±0.010
mm s–1 is in reasonable agreement with the theoretical value of0.642±0.004 mm s –1 calculated from the lifetime of the firstexcited state. As will be noted, this spectrum also shows thepresence of a small (10 –15%) impurity with an IS of0.178±0.012 mm s –1 at 90 K which approaches zero at highertemperatures. This impurity is most likely SnO2 and its contribution to the lattice dynamical data reported below has beencorrected for in the data analysis. The IS and QS values for
SnPc at 90 K are included in Table 1 and are in good agreementwith values reported earlier and are not otherwise remarkable.

The experimental details concerning the present studies havebeen detailed earlier.5
Spectrometer calibration was effected using an α-Feabsorber at room temperature in conjunction with a Rh(57Co)
source. The source used in the Sn experiments was Ca119mSnO3
and all isomer shifts are reported with reference to the centroidof a room temperature BaSnO3 absorption spectrum.
The SnPc absorber used was commercially6 available material (despite the presence of a small impurity, vide infra), andtemperature monitoring was effected using the Daswin program of Glaberson and Brettschneider.7 Temperature constancy
to better than ±0.2 K was achieved over the data collection
times (up to 36 hours per temperature point).

RELATIVE TRANSMISSION

2. Experimental

1.00

0.99

0.98

90.5K0.97-80-448VELOCITY / mm s-1

Figure 1. 119Sn Mössbauer spectrum of SnPc at 90.5 K. The velocity
scale is with respect to the centroid of a room temperature absorberspectrum of BaSnO3. The presence of a small impurity, assumed tobe a Sn(IV) oxide at an IS of ~ 0 mm s–1, has been corrected for in thesubsequent data analysis.

TABLE 1: 119Sn Mössbauer parameters for SnPc
IS(90) /mm s–1
2.875±0.018

QS(90) /mm s–1
1.455±0.018

Γ (90) /mm s–1
0.882±0.010-d (ln A)/dT /K–1

R=(A(+)/A(-))–3

0.985±0.015(19.5±0.30)×10

The 4th and 5th columns pertain to the temperature range 90.5<T<189.9 K.
*Corresponding author. E-mail: herber@vms.huji.ac.il. FAX:
+972 2 6586 347© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/30/2007

22Herber

J. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007

0.0

slope = (19.5±0.30)×10-3 K-1
cc: 0.998 (7)

1.04
1.02

In [A(T )/A(90)]

AREA RATIO [ R = A(+)/A(-)]

1.06

1.00
0.980.960.94-0.5-1.0-1.5-2.00.92
80100120140160
TEMPERATURE / K

18020080100120140160180Figure 2. The area ratio of the two components of the doublet spectrum of SnPc. The difference between the mean value (0.986±0.015)
and unity is (presumed) due to a small residual temperature independent texture effect. The two dashed lines indicate +/- one standard
deviation of the data.

Over the 90 to 190 K temperature range, neither of theseparameters is strongly temperature dependent and it is not possible to extract a meaningful value 8 for M ef f from IS(T ).
In numerous earlier studies9,10 of tin organometallics, it has
been noted that the intensity ratio of the two components of theresonance doublet may be a temperature-dependent quantity,
due to the Gol’danskii-Karyagin effect, and from such data it ispossible to extract the vibrational anisotropy of the metal atom
motion; that is, <x2para >−<x2perp>. Because in SnPc the metal
atom is located 1.11 Å above the nitrogen atom plane and has alone pair of electrons located on the four-fold symmetry axis,11
it was expected that this motional anisotropy would be observable in the ME spectra here recorded. The observed area ratio,
R = [A(+)/A(-)], where + and - refer to the doublet components
at greater and lesser velocity than the spectrum centroid, issummarized graphically in Figure 2. The mean value of R =0.986±0.015 and is considered to be temperature independent
over the indicated range. This observation is similar to thatobserved for a tricoordinate stannylium cation12 (nominally tetravalent Sn) but has apparently not been reported in the literature for a nominally divalent Sn complex. Clearly, the metal
atom motion up to 190 K appears to be equal in the two directions parallel and perpendicular to the C4v axis.
The temperature dependence of the recoil-free fraction, f,
extracted from the area under the resonance curve for an optically “thin” absorber, is summarized in Figure 3 in which thecorrections for the small Sn(IV) impurity noted above, havebeen effected. The data in Figure 3 are well fit by a linear
regression and yield −d(ln A) / dT = (19.5±0.3)×10 –3 K–1 with acorrelation coefficient of 0.998 for seven data points. Thisvalue is significantly larger than the value of 14.0×10 –3 reported

Figure 3. Temperature dependence of the recoil-free fraction for
SnPc.

by Molloy and Quill4 in their 1986 study of Sn(II) and Sn(IV)phthalocyanins, but their reported line widths of 1.28 and 1.26mm s –1 at 78 K make the assumption of an optically “thin”
absorber somewhat uncertain. It is also interesting to note that-d(ln A) / dT for the (presumed) SnO2 impurity is (13.0±0.3)
×10 –3 K–1, indicating, as expected, a somewhat “harder” latticein this case.
As noted earlier in connection with other organometallic
compounds investigated by the ME technique,13,14 making theassumption that the recoil-free fraction extrapolates to a value
close to unity at absolute zero permits the calculation of avibrational amplitude of the metal atom in these structures.
Thus ln f = −k 2 <x 2 ave >, where k is the wave vector of the Sn
23.9 keV radiation, k 2 = 1.464×1018 cm 2 , and <x 2 ave > is themean-square amplitude of vibration of the metal atom averaged
over all directions. Using this simplified approach to the evaluation of the metal atom motion leads to the values of the rootmea n-squa re-a mplitude-of-vibration as a function oftemperature summarized in Table 2 which also includes several
other Sn(II) compounds recently studied by this technique.
4. Summary and Conclusions
Temperature-dependent 119Sn Mössbauer effect spectroscopyof Sn(II) phthalocyanin over the temperature range 90<T<180K has shown that there is no evidence of an anisotropy of themetal atom motion (Gol’danskii-Karyagin effect) parallel andperpendicular to the 4-fold symmetry axis running through theSn atom, despite the fact that this atom is located 1.11 Å above
the plane of the four indole nitrogens as well as the presence ofthe lone pair of electrons on this axis. The root-mean-squareamplitude-of-vibration of the metal atom in SnPc has been
deduced from the recoil-free fraction data and compared toanalogous values of related Sn(II) complexes.

TABLE 2: Root-mean-square-amplitudes of vibration of the Sn atom (in Å) in a number of related Sn(II) complexes
T /K100
125
150
175
200

This work
0.115
0.129
0.141
0.153
0.163

200

TEMPERATURE / K

Ref. 12
0.137

Ref. 5
0.143

Ar’Sn=SnAr’
0.131

Ar*Sn=SnAr* a
0.112

0.168

0.176

0.160

0.137

0.194

0.203

0.185

0.158

The errors are estimated to be ~ ± 10 %.
aG. H. Spikes, J. R. Giuliani, M. P. Augustine, I. Nowik, R. H. Herber, and P. P. Power, Inorg. Chem. (in press).

Metal Atom Dynamics in Sn(II) Phthalocyanin fromReferences(1) W. J. Kroenke and M. E. Kenney, Inorg. Chem. 3, 251; 696(1964).
(2) M. K. Friedel, B. F. Hoskins, R. L. Martin, and S. A.
Mason, J. Chem. Soc. D. (Chem. Comm) 400 (1970).
(3) H. A. Stoeckler, H. Sano, and R. H. Herber, J. Chem. Phys.
45, 1182 (1966).
(4) K. C. Molloy and K. Quill, Polyhedron, 5, 1771 (1986); M.
O’Rourke and C. Curran, J. Amer. Chem. Soc. 92, 1501
(1970).
(5) P. P. Power, C. Stanciu, I. Nowik, and R. H. Herber, Inorg.
Chem. 44, 9461 (2005) and references therein.
(6) Aldrich # 40, 454-3, C.A.No. [15304-57-1].
(7) www.phys.huji.ac.il/~glabersn.
(8) R. H. Herber, Chemical Mössbauer Spectroscopy, ed. R. H.

J. Nucl. Radiochem. Sci., Vol. 8, No. 1, 2007

23

Herber, Ch. VII, Plenum Press, New York, (1984).
(9) R. D. Adams, B. Captain, R. H. Herber, M. Johansson, I.
Nowik, J. L. Smith, Jr., and M. D. Smith, Inorg. Chem. 44,6346 (2005) and references therein.
(10) R. H. Herber and S. Chandra, J. Chem. Phys. 52, 6045(1970); R. H. Herber, S. Chandra, and Y. Hazony, J. Chem.
Phys. 53, 3330 (1970).
(11) D. R. Tackley, G. Dent, and W. E. Smith, Phys. Chem.
Chem. Phys. 3,1419 (2001).
(12) J. B. Lambert, L. Lin, I. Nowik, and R. H. Herber, Inorg.
Chem. 43, 405 (2004).
(13) R. H. Herber, I. Nowik, V. Kahlenberg, H. Kopacka, and H.
Schottenberger, Eur. J. Inorg. Chem. 3255 (2006).
(14) R. H. Herber and I. Nowik, Hyperfine Interactions, 166,351 (2005).