Organotin Compounds: New Chemistry and ... - ACS Publications

10 Structure and Bonding in Organotins by Gamma Resonance Spectroscopy ROLFE H. HERBER and MICHAEL F. LEAHY

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0157.ch010

Rutgers University, New Brunswick, N. J. 08903

The recently developed effective vibrating mass (EVM) model, which provides a semi-theoretical framework within which the molecular weight of covalent organometallic compounds can be determined from Raman spectroscopic and temperature-dependent Mössbauer effect experiments, has been extended to two organotin thiol compounds. In [(CH ) SnS] , although the Mössbauer active atom does not occupy the center of mass of the molecule, the agreement between the trimeric formula weight (542) and the calculated effective vibrating mass (552) demonstrates further the validity of the basic assumptions of the EVM model. In the distorted, nearly tetrahedral molecule Sn(SCH CH S) , the EVM model calculations suggest an appreciable interaction between proximal molecules in the solid through S-atom bridging between adjacent tin atoms. This gives rise to quasi-linear Sn-S-Sn- chains in consonance with the chemical and physical properties of this compound, as well as the results of a recent crystal structure determination. 3

2

2

2

3

2

S

hortly after the first results were published concerning the chemical applications of gamma resonance spectroscopy (Môssbauer Effect) (1,2) making use of the 23.8 keV excitation in S n , it became clear that this technique would be valuable in elucidating the structure and bonding in organotin compounds, and a very large literature has grown up in this field (3,4). The earliest data to be extracted from such spectra focussed attention principally on the isomer shift and quadrupole splitting parameters, since it became evident that these hyperfine interactions could elucidate the formal oxidation state of the metal atom [e.g., Sn(II) or Sn(IV)] and its coordination number more readily than most other available techniques. In addition, it was observed (5,6) that there appeared to be a qualitative relationship between the temperature dependence of the S n recoil-free fraction, f(T), and the coordination number of the metal atom and/or intermolecular bonding. The observation of a resonance effect at room tem119

1 1 9

155

Zuckerman; Organotin Compounds: New Chemistry and Applications Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

156

ORGANOTIN COMPOUNDS: NEW CHEMISTRY AND APPLICATIONS

perature has frequently been interpreted in terms of an extended network-associated or polymeric structure for the organotin compound in question. In fact, the temperature dependence of the recoil-free fraction observed in S n Môssbauer spectroscopy of organotin compounds can be used to provide quantitative information concerning the structure and bonding in such compounds. A model has been developed (7,8,9) which permits the exploitation of the data obtained by f(T) and vibrational spectroscopic investigations to obtain a clearer understanding of the association of organotin compounds in the solid state. This model has been called the effective vibrating mass (EVM) model, and the fundamental assumptions of this approach are reviewed briefly. 119


A Brief Review of the EVM Model The principal parameter of interest in the context of this discussion is the recoil free fraction, / , observed in a Môssbauer experiment. This parameter— which can have values ranging from 0 to 1—is the probability of emitting (f ) or absorbing (f ) a gamma ray without recoil. Although in principle it is possible to evaluate this parameter quantitatively under a given set of experimental conditions, it is more convenient to examine the temperature dependence of the recoil free fraction / ( Γ ) by extracting the temperature dependence of the area under the resonance curve A(T) in such spectra. The definition of / and A are given by s

a

/ = exp(- calculated from low temperature specific heat data. The recoil energy, E R , can be evaluated from momentum conservation considerations

so that

dT

M c kP ef{

2

M

where M f f is the effective recoiling mass which contains the Môssbauer active atom. In general, except for certain solids such as intercalation compounds and ionic lattices, the effective recoiling mass is not known, so Equation 8 contains two unknown experimental parameters, M f f and #Μ· In molecular solids it may be assumed that because of both energetic and symmetry considerations the intraand intermolecular motions do not couple, and hence that as a first approximation the organotin compound condensed phase can be thought of as an array of hard sphere particles which interact within the unit cell by van der Waals intermo lecular forces. The motion corresponding to the intra-unit cell vibrations of these molecules against each other can be probed by Raman spectroscopy, and the relevant frequencies are normally observed in the lattice mode region of the spectrum lying generally below ~200 c m . If one of the lattice region vibra tional modes corresponds to the unique intermolecular intraunit cell vibration of two molecules against each other, then the frequency of this mode, O>L, can be used to calculate an effective Debye temperature by the relationship e

e

- 1



158

Inserting this value of θ into Equation 8 leads to the relationship (7): -d

In A _ dT

SE k

,

2

y

.

~M c h*œl e{{

2

in which the only unknown quantity is the effective vibrating mass M f f . It is thus possible to use the temperature dependence of the S n recoil-free fraction to elucidate the possible association of organotin molecules in the solid state, since it should clearly be feasible to distinguish between monomers, dimers, and other low molecular weight polymeric units by means of the above formalism. A number of such studies (13,14) have previously been published for a variety of molecular geometries and coordination numbers of tin, and the general validity of this approach appears to be reasonably well established. However, because of the predominance of s p , sp d, and sp d hybridization of the tin atom in which there are, respectively, four, five, and six identical or similar ligands bonded to the metal, most molecules which have been subjected to temperature dependent Môssbauer effect studies are those in which the tin atom occupies the center of mass of the molecule. Clearly, if the assumptions of the E V M model are correct, this fact should be unimportant in the applicability of this model to molecular systems, and thus a wider test would be to make certain that the details of the intramolecular geometry do not influence the interpretation of the f(T) data. In the present study we have extended the E V M model to the investigation of two organotin compounds in which the metal atom does not occupy the center of mass of the molecule as a whole and in which molecular association by bridging ligands has either been shown by x-ray diffraction data or is presumed to exist on the basis of other spectroscopic and chemical information. e


1 1 9

3

s

s

2

Experimental The experimental details of the Môssbauer spectroscopic methodology used in these studies has been described (7,8,9). All isomer shifts cited in the present work are with respect to a room temperature (295 ° K ) BaSnC>3 absorber spectrum. Velocity calibration of the spectrometer was effected by measuring the Fe(0) magnetic hyperfine spectrum as reported earlier (16). The organotin compounds were prepared by literature methods (17,18,19), and their purity was ascertained by standard analytical techniques including IR, mass spectrometric, and Môssbauer methods as appropriate. Discussion of Results [(CHafeSnSk. The Môssbauer parameters for this compound are summarized in Table I, and the low frequency Raman spectrum is shown in Figure 1. In the latter, it will be noted that there are six distinct bands in the region below ~ 1 0 0 c m , and these are assumed to arise from either discrete lattice modes of the covalent solid or low energy librational and/or torsional modes of the indi- 1



10.

HERBER

AND

Gamma Resonance Spectroscopy

LEAHY

159

Figure 1. Low frequency (lattice mode region) Raman spectrum of [(CHs) SnS]$ at 300° and 80°K. The mode at ~27.5 cm~ has been identified as the intermolecular, intra-unit cell vibration of two trimeric molecules against each other on the basis of the present work. 2

l

200 100 FREQUENCY, C M

-1

vidual molecules. Replacing θ in Equation 8 by the value calculated using the 27.5 c m " band, and using the temperaturre dependence of the S n recoil-free fraction extracted from the Môssbauer experiments, that is, In A = —0.02464T — 3 5 . 8 4 Γ (multiple regression coefficient 0.999), leads to a calculated value of M ff of 552. This is in good agreement with the trimeric mass of 542 and in consonance with the x-ray diffraction evidence (20) for the six-membered ring structure of this compound in the solid state. The temperature dependence of the area under the resonance curve is summarized graphically in Figure 2. It is thus clear that the E V M model assumption that molecules in covalent (organometallic) solids could be considered as hard sphere entities in the context of understanding the temperature dependence of the recoil-free fraction extracted from Môssbauer data is valid even when the center of mass of the molecule is not coincident in space with the lattice position of the Môssbauer-active metal atom. The E V M model thus provides an important and useful approach to the determination of the extent of association of molecular monomers into low weight polymeric arrays in covalent solids, and the basic assumption that the local m i 1

1 1 9

-1

e



160


I

I 80

I

I 100

I

» 120

ι

I M0

I

I 160

I

I 180

I

I 200

1

1—I 220

TEMPERATURE, *K

Figure 2. Temperature dependence of the area under the S n resonance curve for [(CH^SnS]^ in the temperature range 78 < Τ < 218°K. The temperature dependence is given by In A = -0.02464Γ - 35.847/Γ 1 1 9

crostructure of the molecular units is unimportant in this model is well borne out. The application of this model to molecules which cannot, for one reason or an other, be studied by x-ray diffraction techniques (e.g., they are liquids at room temperature, they do not readily form single crystals, they decompose in the x-ray beam, etc.) is thus limited only by the provisions that an otherwise suitable Môssbauer-active atom can be built somewhere into the molecular structure and that the lattice-mode region of the vibrational spectrum is accessible by appropriate (Raman or far-IR) spectroscopic techniques.

Table I. LS. (78°K) mm/sec

Compound

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Sn

Q.S. (78°K) mm/sec

[Sn(SCH CH S) ]„

1.401 ± 0.010

1.026 ± 0.008

[(CH ) SnS]

1.341 ± 0.008

1.824 ± 0.010

2

3

2

2

3

2


10.

HERBER

Gamma Resonance Spectroscopy

A N D LEAHY

161

[Sn(SCM2CH2S)2] . The Môssbauer parameters for this compound are summarized in Table I and a typical Môssbauer spectrum is shown in Figure 3. The low frequency region of the Raman spectrum of this compound is shown in Figure 4. Among the physicochemical properties of this compound which must be considered in an interpretation of these data are the relatively high melting point (21) (182°-183°C), low solubility in nonpolar solvents, appreciable n

100


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Sn(SCH CH S>

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Organotin Compounds: New Chemistry and ... - ACS Publications

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