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Mossbauer and infrared studies of triphenyltin carboxylates Ford, Beverly F.E 1971

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MOSSBAUER AND INFRARED STUDIES OF TRIPHENYLTIN CARBOXYLATES by BEVERLY F.E. FORD B.Sc. (Hon.) C h e m i s t r y , A p r i l , 1968. A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE I n t h e Department o f CHEMISTRY We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d . THE UNIVERSITY OF BRITISH COLUMBIA J u n e , 1971 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. The University of British Columbi, Vancouver 8, Canada Depa rtment - i -ABSTRACT A study was undertaken to invest igate the so l i d - s t a te structure of about 25 t r i pheny l t i n carboxylates, 0gSnOCOR. The compounds were synthesized and then analyzed by Mossbauer and inf rared spectroscopy. The compounds were divided into three ser ies on the basis of the nature of the R group. The f i r s t series of compounds had R groups which were " l i n e a r " chain hydrocarbons ranging in length from one carbon atom ( t r i pheny l t i n formate) to eighteen carbon atoms ( t r i pheny l t i n stearate). In the second series several compounds had methyl branches at various posit ions along the hydrocarbon chain, some had longer alky! groups and a few had a methylene group bonded to the a-carbon atom. The t h i r d series contained mono-, d i - , and t r i -subst i tuted haloacetates. These compounds were prepared in order to test the assumption that bulky R groups would prevent (by s t e r i c interact ion) polymer formation in the s o l i d . The polymeric structure which i s commonly found for t r iorganot in carboxylates consists of pentacoordinate Sn atoms. Each carboxylate group bridges between two d i f fe rent Sn atoms and th i s occurs i n d e f i n i t e l y to form a polymer. S te r i c in teract ion of the R group with neighbouring phenyl groups (bonding to Sn) could prevent polymer formation. The re su l t i ng structure would be monomeric and have a tetracoordinate Sn atom and a terminal carboxylate group l i k e that for an organic ester. The majority of compounds were found to be polymeric so l id s . Structura l changes (polymeric to monomeric) were observed for a few compounds and th i s could be at t r ibuted to s t e r i c i n te rac t i on . The Mossbauer and in f ra red data were complimentary and conclusive when used to d i f f e r en t i a t e between the two possible s t ructura l types. In a polymeric structure the Sn atom can be v i sua l i zed as being in a t r i gona l bipyramidal environment in which the oxygen atoms are ax ia l and the phenyl groups equator ia l . Using the above idea l i zed s t ructura l type i t was possible to test a point-charge model which had been used to pred ict quadrupole s p l i t t i n g values, A. The model was tested for the tr iphenyl t i n haloacetates and found to give f a i r l y good agreement with the observed quadrupole s p l i t t i n g values. - i i i TABLE OF CONTENTS Introduction 1 The Mbssbauer Effect Pr inc ip les 5 Chemical Appl ications 6 Isomer Sh i f t 7 Quadrupole S p l i t t i n g 10 Experimental Mossbauer 12 Infrared 17 Preparations 19 Results 20 Discussion Preparations -•> 30 Infrared 32 Mbssbauer 35 Structure and Bonding 39 Theoretical Discussion Point-Charge Predict ions of 41 Quadrupole S p l i t t i n g Conclusions References 45 46 LIST OF FIGURES AND TABLES Figures I Polymeric and Monomeric Structures of Triorganotin Carboxylates II Nuclear Energy Level Diagram Showing Isomer Sh i f t and Quadrupole S p l i t t i n g III Mossbauer Spectrometer IV Mbssbauer Spectra of (a) Tr iphenylt in Methacrylate (b) Tr iphenylt in Acrylate Tables A Mossbauer Trans it ion Properties for 1 1 9 S n I - III Analyt ica l Data and Melting Points IV - VI Infrared Data VII - IX Mbssbauer Data X Point-Charge Predicted Quadrupole S p l i t t i n g Values For Tr iphenylt in Haloacetates ACKNOWLEDGMENT Without my research d i r ec to r , Dr. John Sams, the experimental work would not have continued to i t s present scope. Thanks J.R. fo r encouragement when the going was rough and for the opportunity to share ideas as a "co l league". INTRODUCTION In general, compounds having the formula R S^nOCOR (R = a l ky l or ary l and R' = a l k y l ) are polymeric in the s o l i d s t a t e ^ . The t i n atom i s pentacoordinate with bridging carboxylate groups. In d i l u t e solut ions i n non-polar solvents the compounds are monomeric with the t i n atom being tetracoordinate. Conclusions concerning the structures of these t r io rganot in compounds have been based mainly on in f ra red data and in p a r t i c u l a r on the pos i t ion of the two C-0 absorption bands for the s o l i d and fo r a d i l u t e so lut ion of the compound i n a non-polar sol v e n t . ^ ^ ' ^ . The s o l i d state bands are s im i l a r to those of a s a l t - l i k e or br idging carboxylate while the posit ions of the so lut ion bands are much c loser to those for an organic ester. I t has also been observed that increasing the concen-t r a t i on of the so lut ion increases the degree of polymerization. This i s fur ther support for a s o l i d state st ructure that i s polymeric rather than i o n i c . Under certa in conditions both of the C-0 absorption band pos i t ions for the s o l i d and so lut ion are s i m i l a r and are close to those of an organic ester. This occurs when the R group i s bulky, fo r example when R = isoproxyl or cyclohexyl. Tr i i sopropy ' l t in acetate i s a monomeric l i q u i d ^ while t r i cyclohexyl t i n acetate^ - 1 i s a monomeric s o l i d . Janssen et a'l ^ have suggested that lengthening the R' group or introducing bulky R1 groups may resu l t i n "destabi1 i zat ion of a polymeric structure by s t e r i c e f f e c t " . Several compounds were studied in order to tes t th i s assumption. - 2 -A number of t r i pheny l t i n carboxylates, i^SnOCOR, were pre-pared and t h e i r s t ructura l properties examined by Mossbauer and i n f r a -red spectroscopy. The f i r s t ser ies of compounds (TABLES I, IV and VII) * had R groups which were l i nea r chain hydrocarbons ranging in length from one carbon atom ( t r i pheny l t i n formate) to eighteen carbon atoms ( t r i pheny l t i n s tearate). In the second series (TABLES I I , V, and VIII) several compounds had methyl branches at various postions along the hydrocarbon chain, some had longer a lky l groups and a few had a methylene group bonded to the a-carbon atom. There were several reasons for preparing a t h i r d series (TABLES I I I , VI, and IX) of compounds. The compounds were mono-, d i -and t r i - subst ituted haloacetates and the reasons are as fo l lows. After observing the results for the compounds with more bulky groups on the a-carbon atom we concluded that i t might be poss ible to determine a " c r i t i c a l s i z e " (estimated in terms of a van der Walls radius) for a-carbon atom subst ituents. Substituents which were smaller than the " c r i t i c a l s i z e " would give polymeric so l ids while those which were larger would give monomeric s o l i d s . There was some l im i t a t i o n ( i . e . not conveniently ava i lab le) in our choice of su i tab le halo-subst i tuted acids with which to tes t thoroughly the previous hypothesis, however the inf rared and Mossbauer results could be compared with those for the analogous t r i -(5) methylt in compounds. ' As an added benef i t , since a point-charge model allows us to predict quadrupole s p l i t t i n g values for these compounds (assuming them to be i sos t ructura l with the corresponding t r i -methylt in der ivat ives) our results can be used to tes t the pred ic t i ve value of th i s model. * . Note the change from R' to R to re fer to an organic group attached to the carboxylate part of the molecule. - 3 -FIGURE I shows the polymeric and monomeric structures for t r io rganot in carboxylates. When drawing the structures emphasis was placed on two important s t ructura l de t a i l s . One i s the environment of the Sn atom which i s pentacoordinate in the polymer and tetracoordinate in the monomer. The two corresponding symmetry types are a tr igonal bipyramid and a tetrahedron which represent an approximation to the actual s t ructures. The other s t ructura l de ta i l i s the nature of the carboxylate group. In the polymer the carboxylate group i s bridging while in the monomer the carboxylate group i s terminal. - 4 -FIGURE I Triorganotin carboxylases R , S n O C O R ' Po lymer i c structure R' R R M o n o m e r i c s t ruc ture R R R o - 5 -THE MOSSBAUER EFFECT Pr inc ip le s The Mossbauer e f fect i s the r eco i l l e s s emission and resonant, absorption of gamma radiat ion by nuclei bound in s o l i d s . The e f fec t was discovered in 1957 and named a f te r the discoverer Rudolph L. Mossbauer who received the Nobel Pr ize in Physics i n 1961 fo r his work. The fol lowing discussion w i l l deal b r i e f l y with the p r inc ip le s of the Mossbauer e f fec t and then with the appl icat ions to Chemistry. F i r s t l y consider the case where a source and absorber are i d e n t i c a l , that i s the Mossbauer nuclei have the same chemical environment. Normally i t i s d i f f i c u l t to observe resonance absorption of nuclear gamma rad ia t ion . This i s because the free-atom reco i l energy, E^, l o s t by the photons during emission and absorption i s la rger (-10 ) than the natural l inewidths for emission and absorption. This means that, the energy of the emitted photon doesn't correspond to the energy required for absorption. I f the emitt ing and absorbing atoms are confined to l a t t i c e s i t e s and the reco i l energy i s less than the phonon energy or cha rac te r i s t i c energy for l a t t i c e v ibrat ions (-10 eV) then r eco i l l e s s emission and resonant absorption can be observed when there i s no thermal motion of the atoms in the l a t t i c e . Now the free-atom reco i l energy approaches zero as the reco i l mass gets very large by inc luding many atoms in the l a t t i c e . Since the free-atom reco i l energy i s proportional to the square of the gamma-ray energy, the requirement of a "zero phonon" - 6 -event places a r e s t r i c t i o n on the energy of a Mossbauer t r an s i t i on . At present the p rac t i ca l l i m i t i s <150keV. Mossbauer t ran s i t i on properties are given in TABLE A. The Mossbauer e f fec t i s temperature dependent. A decrease in temperature increases the p robab i l i t y of a "zero phonon" event accompanying the emission and absorption of rad ia t ion . Due to the extremely low number of r e co i l - f r ee processes at room temperature i t was necessary to record Mossbauer spectra with the absorbers at a much lower temperature. I t was convenient and r e l a t i v e l y simple to maintain the absorbers at l i q u i d nitrogen temperatures where a s t a t i s t i c a l l y s i g n i f i c an t Mossbauer e f fect could be determined a f te r about ten to f i f t e e n hours. Chemical Appl icat ions The appl icat ions of the Mossbauer e f fect to chemistry involve a s i tua t i on where the absorber i s not i dent i ca l to the source and several d i f f e ren t absorbers or chemical compounds are compared. In th i s experimental work there are two important Mossbauer parameters, the isomer s h i f t and the quadrupole s p l i t t i n g , which give information about the chemical nature of compounds. Changes i n the isomer s h i f t and the quadrupole s p l i t t i n g are re lated respect ively to changes i n the electron density and d i s t r i b u t i o n of electron density near the Sn nucleus. These changes are due mainly to valence electrons. The chemical appl icat ions of the Mossbauer e f fect depend on - 7 -the extremely narrow linewidths which are cha rac te r i s t i c of th i s type of rad ia t ion . The energy of the gamma ray i s defined to with in one part 12 -12 i n 10 , that i s the width of a spectral l i n e i s 10 of the energy of the gamma ray. The interact ion of electron and nuclear charge causes the t ran s i t i on energy for the source to be d i f fe rent from that fo r the absorber. These energy changes can be observed using a Mossbauer o spectrometer since they are of the order of a l inewidth or -10 eV. Experimentally the energy difference i s compensated for by Doppler s h i f t i n g the source of gamma radiat ion by a few mm s e c ~ \ r e l a t i ve to a stat ionary absorber. Isomer Sh i f t A Mossbauer t ran s i t i on occurs between the ground nuclear energy level and a low ly ing excited nuclear energy l e v e l . Emitting and absorbing nuclei which are in d i f f e ren t chemical environments have d i f f e ren t t r an s i t i on energies. This i s a re su l t of the interact ion of electron charge with nuclear charge and i s ca l l ed isomer s h i f t , I.S. (9) An equation for the isomer s h i f t can be der ived v ' by assuming that the nucleus i s a uniformly charged sphere of radius Independent on the state of exc i tat ion) and that the electron charge density i s uniform over nuclear dimensions. The isomer s h i f t depends on the product of an e lect ron ic factor and a nuclear factor . The e lect ron ic factor i s the electron charge density near the nucleus which i s affected by the valence state of the Sn atom and the nuclear factor i s the d i f ference between the radius of the excited state and that of the ground s tate. - 8 -Experimental measurements^ 0^ have shown that the nuclear charge radius of the excited state i s larger than that of the ground state 119 for Sn which means that the sign of the d i f ference i s pos i t i ve- This holds true for a l l Sn containing compounds. Now i t i s possible to re la te an increase in isomer s h i f t or a s h i f t to higher energies with ah increase in electron density at the Sn nucleus for a pa r t i cu l a r absorber. Since s-electron density has a f i n i t e p robab i l i t y of being at or near the nucleus, changes in isomer s h i f t are related to changes in s -e lectron density. In Sn compounds, changes in isomer s h i f t are due mainly to changes in 5 s -e lectron density. These changes are a resu l t of d i f fe rent d i s t r i bu t ions of valence electron density. A good example i s grey Sn(a-Sn) which absorbs at -2.2 mm sec" ^ +4 higher in energy than stannic oxide, SnO^. In SnC^ there are Sn ions which are octahedral ly surrounded by oxygen anions. The e lect ron ic con-f i gu ra t ion of the Sn atom can be approximated by [Kr]4d'°. In grey Sn each Sn atom i s covalently bonded to four others which are at the corners 3 of a tetrahedron. The bonding can be considered as four sp hybrid bonds to each Sn atom and the e lect ron ic configuration can be approximated by [Kr]4d^°5S^5p^. The increase in isomer s h i f t can be att r ibuted to the increase in 5S-electron charge density. Experimentally the nuclear t r an s i t i on energy of the source i s compared with that for various absorbers. I.S. = ABSORBER " ^SOURCE' I t i s conventional to report I.S. values r e l a t i v e to a reference absorber. I t i s possible to report energy s h i f t s r e l a t i v e to the source i f the N U C L E A R E N E R G Y L E V E L S I S O M E R S H I F T , I . S . I.S. = E X C I T E D S T A T E G R O U N D S T A T E S O U R C E ( S ) A B S O R B E R ( A ) Q U A D R U P O L E S P L I T T I N G . A A = EA( i ) - EA( i ) 3 2 E J _ 2 A A S U B S T A T E S ±1 2 ~ 2 FIGURE II - 10 same compound i s run as an absorber in order to determine a zero reference point on the energy scale. For th i s experimental work the isomer s h i f t s aiereported r e l a t i v e to a popular reference absorber, Sn0 2, which i s a r b i t r a r i l y set as 0 mm sec"^, thus 6 = ^ABSORBER " E Sn0 2 ) m m s e c ' Quadrupole S p l i t t i n g Nuclear charge i s not spher i ca l l y symmetric when the nucleus has a spin quantum number greater than I = 1/2. Under these conditions the 119 nucleus has a quadrupole moment and th i s occurs when the Sn nucleus i s excited (I = 3/2). I f the electron charge i s not uniformly d i s t r ibuted over nuclear dimensions then there w i l l be an e l e c t r i c f i e l d gradient at the nucleus. The in teract ion of nuclear charge with the e l e c t r i c f i e l d gradient s p l i t s the energy of the excited state to give two excited substates and consequently two t rans i t ions i n the Mossbauer spectrum. The energy d i f ference between the two t rans i t ions is ca l led the quadrupole s p l i t t i n g , A. The quadrupole s p l i t t i n g can be represented by: A = EA(3/2) - E A ( l/2) where the +3/2 and +1/2 are the quantum numbers for the two excited nuclear substates (FIGURE I I ). The absence of s p l i t t i n g indicates a cubic or near cubic electron charge d i s t r i bu t i on such as tetrahedral or octahedral. - 11 -No contr ibut ion to the quadrupole s p l i t t i n g comes from non-bonding closed she l l s because they are spher ica l l y symmetric with respect to the nucleus. The same i s true fo r S-electron density. An asymmetric electron charge d i s t r i bu t i on comes mainly from charge density imbalances in valence electrons other than S-electrons. For example tet rapheny l t in , 0^Sn, has no quadrupole s p l i t t i n g (A = 0) but jdgSnCl has a quadrupole s p l i t t i n g of about 2.5 mm sec " ' ^12)^ j n (13) (J^Sn the coordination of the Sn atom i s tetrahedral . Since a l l four bonds to Sn are,, to phenyl C atoms the valence electron charge d i s t r i bu t i on w i l l be uniform and hence A = 0. In 0gSnCl a phenyl group has been replaced by a chlor ine atorii which has d i f f e ren t bonding cha rac te r i s t i c s . This i s s u f f i c i e n t to give a f a i r l y large quadrupole s p l i t t i n g by causing an asymmetry in the valence charge d i s t r i bu t i on and also d i s t o r t i n g the tetrahedral symmetry of the Sn atom. - 12 -EXPERIMENTAL Mbssbauer ( i ) Source The source of r ad i oac t i v i t y was powdered barium stannate BaSnO^, which had been enriched with the Mbssbauer nucleide ^ 9 m S n and encapsulated in an a c r y l i c p l a s t i c d i sc . This was mounted on a larger d isc which had been threaded at one end for easy attachment to the ve loc i t y transducer. Barium stannate has extremely good Mbssbauer propert ies. The emission l i n e i s unsp l i t and exhib i ts a narrow l i n e width. It also shows one of the highest percentage ef fects ever reported i n Sn Mbssbauer spectroscopy. In pa r t i cu la r th i s source had a l i n e width of 1.13 mm sec " 1 and a 25% e f f e c t . The average in tens i t y of the source was about 4 m i l l i c u r i e s (1 cur ie = 3.700 x 1 01 0 d i s integrat ions per s e cond )^ ' ) 119 Further Mossbauer t r an s i t i on properties of Sn are given in TABLE A. ( i i ) Absorber The samples ( t r ipheny l t in carboxylates) were powdered and evenly packed in to a c i r c u l a r brass c e l l with Mylar windows which were 1.25 cm. in diameter. With an absorber thickness of about 0.5 mm. i t was possible to produce Mbssbauer spectra with f a i r l y narrow l i n e widths ( r - l . 0 mm s e c " 1 ) . The c e l l was inserted at t h e top of a copper tube which was immersed i n * Commerical preparation by New England Nuclear Corporation, Boston, Massachusetts. - 13 -TABLE A Mossbauer Trans i t ion Properties For , , y S n Ey gamma-ray energy 23.9 keV t l /2^ Y M^ h a l f - l i f e of Mossbauer 1.84 x 10 " 8 sec. t r an s i t i on r natural width 2.48 x 1 0 " 1 1 keV Wo+ minimum observable width 0.62 mm sec"^ Er reco i l energy 2.57 x 10"^ keV I nuclear spin quantum number fo r : Ground state 1/2 F i r s t excited state 3/2 t-j^2 h a l f - l i f e of Mossbauer 250 days Nucleide —1 -8 Energy Conversion Factor 1 mm sec" = 7.96 .x 10~. eV * A.H. Muir, K.J. Ando s and H.M. Coogan Mossbauer Ef fect Data Index, 1958-1965, Interscience Publ ishers, N.Y. 1966. t Wo Equals Twice the Natural Line Width (2r) Because of Overlap of Emission and Absorption Lines. - 14 -a dewar of l i q u i d nitrogen. After the absorber had been aligned with the source i t was insulated with styrofoam to minimize heat t ransfer between the sample and the room. An automatic l i q u i d nitrogen con t ro l l e r maintained the sample at 80° + 1°K. Mossbauer ( i i i ) Instrumentation The Mossbauer spectrometer which was used for th i s experimental work was of the constant accelerat ion type and experiments were per-formed with transmission geometry. This consists of measuring the i n tens i t y of rad ia t ion passing through a resonance absorber (sample) as a function of the r e l a t i v e ve loc i t y between source and absorber. The basic components of a Mossbauer spectrometer are the electromechanical ve loc i t y transducer, the detector, the multichannel analyser (MCA) and the d i scr iminator or s i ng le -channel analyser. The electromechanical ve loc i t y transducer modulates the incident energy by moving the source back and f o r t h . It i s driven by a wave-form (saw-tooth) generator which varies the ve loc i t y l i n e a r l y with time. The detector i s a g a s - f i l l e d (2 atm. Xe-CH^) proportional counter. Output from the detector i s a ser ies of pulses which are ampl i f ied. Only those pulses which l i e w i th in a given energy range are shaped and stored i n the MCA. This MCA had 400 storage spaces or channels. * See FIGURE III - 15 -M O S S B A U E R S P E C T R O M E T E R SCOPE T Y P E W R I T E R 4 0 0 W O R D M E M O R Y S Y N C . S I G N A L , ' A N A L Y S E R W A V E F O R M G E N E R A T O R v ~ — T R A N S D U C E R T III S O U R C E 2 3 . 9 K E V T E C T O R A B S O R B E R FIGURE III - 16 -The function of the d iscr iminator i s to se lect for storage only the Mbssbauer gamma rad iat ion. Before an absorption spectrum can be run a pulse-height spectrum ( i . e . a spectrum of the rad iat ion trans-mitted through the absorber) i s obtained. A typ ica l one i s shown at the lower r i ght in FIGURE I I I . The horizontal pos i t ion of a peak i s determined by i t s energy and the height by i t s r e l a t i v e i n tens i t y . The controls on the d i scr iminator are adjusted to cut out a l l rad iat ion except the Mbssbauer l i n e as shown by the dotted l ines in the same FIGURE. Only that rad iat ion f a l l i n g wi th in these l i m i t s i s passed to the MCA for storage. Now an absorption spectrum can be run. The Mossbauer spectrum consists of a record of counts versus channel number. Each of the 400 channels i s made to correspond to a ve l o c i t y increment i n the ve loc i t y range being scanned and th i s i s done as fo l lows. At the s ta r t of each ve loc i t y cycle the wave-form generator sends a synchronization signal to the MCA to i n i t i a t e sweep of i t s 400 channels. The rate (200 ysec per channel) of sweep i s set so that equal time i s spent counting in each channel. The ve loc i ty range i s scanned repeatedly in th i s manner un t i l s u f f i c i e n t data have been accumulated in the MCA. Final output i s to an automatic typewriter which pr int s the number of counts in each channel. The channel address i s converted to a ve loc i t y scale (mm sec " 1 ) by ca l i b r a t i n g the instrument using the quadrupole s p l i t t i n g (1.726 + 0.0002 mm sec " 1 ) of an N.B.S. standard sodium nitropruss ide (Na9Fe(CN)rN0.2H..,0) absorber. - 17 -( i v ) Treatment of Data and Errors The spectrum consists of .Mossbauer absorption peaks super-5 imposed on a parabol ic background. A minimum number of 1.0 x 10 counts were accumulated in each channel before a spectral run was terminated. This meant that the s t a t i s t i c a l error did not exceed 0.3%. The data along with estimates fo r the peak posit ions and l i n e -widths ( f u l l width at ha l f the maximum height) were substituted into a computer program which performed a least squares f i t of the data to a one or more Lorent iz ian l i n e shapes. The s t a t i s t i c a l scatter was low enough to enable an estimate of the error to be set on the basis of the standard deviat ion of the computer f i t and the r ep roduc ib i l i t y of the spectrum. At least two spectra were run for each compound and i t was found that the Mossbauer parameters, isomer s h i f t and quadrupole s p l i t t i n g , were reproducible to w i th in +0.03 mm sec 1 . An example of two f i t t e d Mbssbauer spectra i s shown in FIGURE IV. Infrared So l id and so lut ion inf rared spectra were recorded on a Perk in-Elmer grating spectrometer (model 457) and were ca l ib rated with poly-styrene. The so l id s were dispersed i n nujol or halocarbon o i l and each -1 spectrum recorded from about 1800 to 250 cm using cesium iodide windows. * Written by the N.B.S. and subsequently modified by J.C. Scott. Ai Bans id - 8L -- 19 -D i lute (3-5% by weight) carbon tet rach lor ide solutions were run using matched sodium chlor ide so lut ion c e l l s . Infrared data are i n Tables IV - VI. Preparations Tr ipheny l t in formate and t r i pheny l t i n propionate were prepared by shaking t r i pheny l t i n chlor ide with a s l i g h t excess of the sodium s a l t of the acid in an ether/water solvent system. The products were obtained from the ether layer and a i r dr ied. The remaining s t ra ight -cha in carboxylates and the branched-chain carboxylates were prepared by react ing t r i pheny l t i n chlor ide with the potassium s a l t of the appropriate acid in methanol (reagent grade) at room temperature. Evaporation of the solvent l e f t a white p rec ip i ta te which was washed with water and a i r d r ied. Some of the products were pu r i f i ed further by r e c r y s t a l l i z a t i o n from carbon tet rach lo r ide. Tr iphenylt in monoiodoacetate was prepared i n re f lux ing methanol by combining 0gSnCl with the sodium s a l t of the ac id. The monobromo, monochloro s and dichloro der ivat ives were prepared by react ing tr iphenylt inhydroxide with a sto ichoimetr ic amount of acid i n methanol (reagent) at room temperature. The mixture was s t i r r ed from one to two hours then the solvent removed under reduced pressure and the products dried under the same condit ions. When t r i c h l o r oace t i c acid was used the reaction produced 0^000001 ^ -MeOH a n ( j pumping for several hours f a i l e d to remove the adducted methanol. - 20 -The t r i f l u o r o and t r i c h l o r o der ivat ives were produced by react ing sto ichiometr ic quant i t ies of tr iphenylt inhydroxide and acid in ethanol (absolute) at room temperature. The solvent was removed under reduced pressure and the products dr ied i n vacuo for a few days to remove " a l l " traces of ethanol. Results The compounds that were studied can be represented by the fol lowing formulation: (^SnOCOR. The t r i pheny l t i n group, ^ S n i s present i n a l l compounds. Since the carboxylate group, 0C0R, varies as the R group changes, only the chemical formulae for the R group i s l i s t e d in the TABLES. TABLES I - III contain ana ly t i ca l data and melting points, TABLES IV - VI contain infrared data, and TABLES VII - IX contain Mossbauer data. * 0 = phenyl = C.H - 21 -TABLE I Analytical Data and Melting Points %C ra.p. R calcd. found calcd. found °C (1) H 57.72 57.86 4.05 4.01 201-202 (2) C H , " 121-124 (3) CH2CH3 59.57 59.63 4.73 4.91 122-123 (4) (CH 2) 4CH 3 61.93 60.39 5.59 5.46 110-111 (5) (CH 2) 6CH 3 63.28 63.51 6.09 6.13 81-83 C6) (CH 2) 7CH 3 63.90 64.25 6.31 6.29 106-107 (7) (CH2)gCH3 64.49 64.22 6.53 6.53 80-82 (.8) CCH 2) 1 0CH 3 65.57 65.37 6.92 6.93 76-78 (9) (CH 2) 1 4CH 3 67.43 67.48 7.60 7.79 77-80 (10) (CH 2) 1 6CH 3 68.25 68.67 7.90 8.35 71-73 a M & T Chemicals Inc. - 22 -TABLE II A n a l y t i c a l Data and Melting Points %C %R. m.p. R ca l c d . found calcd. found °C CD CCR 2) 3CHMe 2 62.63 62.32 5.85 5.76 93-96 C2) CCH 2) 2CHMe 2 62.06 62.08 5.60 5.89 102-105 C3) CH2CHMeEt 62.06 62.33 5.60 5.76 110-112 C4) CH2CHMe2 61.19 61.27 5.32 5.52 103-105 C5) CHMePr 62.06 61.01 5.60 6.10 110-112 C6) CHMe2 60.41 60.35 5.03 5.28 123-125 C7) CR=CH2 59.86 59.72 4.28 4.36 150-151 C8) CMe=CR2 88-89 C9) CHE+Bua 68-69 CIO) CMe„ 3 61.19 60.94 5.32 5.27 103-105 a M & T Chemicals Inc. TABLE III Analytical Data and Melting Points R %C calcd. found %R calcd. found %X calcd. found m.p. °C CD CH2I 44.91 45.17 3.20 3.10 23.72 23.75 133-137 C2) CH2Br 49.23 49.35 3.51 3.31 16.38 16.10 146-149 C32 CR2C1 54.16 54.38 3.72 3.66 7.99 8.21 154-156 (41 CRC12 50.26 50.08 3.37 3.20 14.83 . 14.60 174-177 C5) c c i 3 46.88 47.09 2.95 2.94 20.76 20.58 86-89 C6) CF3 51.84 51.71 3.24 3.27 12.31 12.08 121-123 C7) CCl3'MeOR 46.29 46.09 3.49 3.40 19.56 19.80 104-107 - 24 -TABLE IV I n f r a r e d Data C-0 Stretching Frequencies R Solid-State cel. 4 Solution (cm) -1 Ccm) 1 CD R 1559 1390 1644 1358 C2) CR3 1548 1420 1640 1370 C3) CH2Ctt3 1535 1412 1632 1381 C4) CCH2)4CR3 1534 1407 1634 1381 C5) CCR216CR3 1525 1415 1631 1382 C6) CCH2)7CR3 1525 1409 1628 1368 C7) CCI^JgQ^ 1532 1404 1630 1382 C8) CCR2)10CR3 1531 1406 1628 1380 C9) CCH2)14CR3 1530 1408 1630 1381 (10) CCH2).16CR3 1532 1410 1629 1380 - 25 -TABLE V Infrared Data C-0 Stretching Frequencies R Solid- State CC1, Solution 4 (cm) -1 (cm) 1 CD CCR2)3CHMe2 1529 1416 1631 1387 C2) CCH2) 2CRMe2 1533 1404 1629 1388 C3) CH2CRMeEt 1524 1407 1628 1382 C4) CR2CEMe2 1523 1408 1643 1380 C5). CRMePr 1536 1416 1638 1379 C61 CHMe2 1533 1422 1632 1391 VI CR=CR? 1528 1423 1619 1335 C8) CMe=CR2 1595 1345 1610 1360 C9) CHE+Bu 1630 1336 1625 1340 CIO) CMe3 1622 1330 1624 1332 - 26 -TABLE VI v Infrared Data - 27 -TABLE V I I Mossbauer Data at 80°K. R (iran, sec ) A 3 Cmm. sec } A l 2 (mm. sec ) CD R 1.37 3.58 0.85 0.87 C2) CR3 1.27 3.40 0.77 0.77 C3) CR2CR3 1.33 3.42 0.99 1.01 C4) CCH2)4CR3 1.32 3.43 0.98 1.00 C5) CCH 2) 6CH 3 1.29 3.35 1.03 1.11 C6) CCR 2) 7CR 3 1.28 3.36 1.03 1.17 C7) CCR2)gCH3 1.27 3.46 0.96 0.98 C8) CCR 2) 1 0CH 3 1.24 3.41 0.97 1.07 C9) CCH 2) 1 4CH 3 1.25 3.44 0.94 0.97 CIO) CCH 2) 1 6CR 3 1.26 3.33 0.98 1.06 +0„03'mm. sec b relative to SnO at 80°K - 28 -TABLE VIII Mossbauer Data at 80°K. R fia,b (mm. sec ) A 3 (mm. sec ) r a r a " 1 2 Cmm. sec ) (1) (CH2)3CHMe2 1.25 3.36 0.98 1.01 (2) (CH2)2CHMe2 1.26 3.38 0.97 0.97 (3) CH2CHMeEt 1.29 3.39 1.02 1.03 (A) CH2CHMe2 1.27 3.39 1.02 0.94 (5) CHMePr 1.26 3.34 0.98 1.05 C6) CHMe2 1.28 3.32 0.95 1.07 C7) CH=CH2 1.28 3.41 0.87 0.95 (8) CMe=CR2 1.21 2.26 0.90 0.93 C9) CHE+BU 1.21 2.26 0.97 1.02 CIO) CMe3 1.21 2.40 0.93 0.99 +0.03 mm. sec. b relative to Sn0„ at 80°K. - 29 -TABLE IX Mossbauer Data at 80°K. R fia,b Cmm. sec ) A a Cmm. sec r a r a 1 2 Cmm. sec. ) CD CH 2I 1.31 3.59 0.71 0.75 C2) CH2Br 1.32 3.51 0.90 0.96 C3) CH2C1 1.32 3.53 1.00 1.00 C4) CHC12 1.35 3.81 0.97 0.92 (5) c c i 3 1.37 3.75 1.08 1.11 (6) CF 3 1.40 4.00 0.92 0.97 C7) CCl^MeOH 1.33 3.50 0.87 0.94 +0.03 mm. sec. b relative to SnO at 8 0 ° K . - 30 -DISCUSSION Preparations Most of the preparations were simple to carry through and products were obtained in high y ie ld s (-80%). D i f f i c u l t i e s arose when the fo l lowing preparative method: Me OH 0 3SnCl + Na s a l t of acid — * was t r i e d fo r the halogen-substituted acids. The reaction was carr ied out at room temperature and at the temperature of re f lux ing methanol but was not successful i n g iv ing a quant i tat ive y i e l d of major product. Either no react ion occurred or there were side reactions which gave a mixture of products. Only 03SnOCOCH2I was prepared using the above method. An a l ternate route was to use t r i pheny l t i n hydroxide and the acid as s t a r t i n g mater ia l s . Methanol was kept as the solvent. Af ter several attempts during which the temperature and amount of s ta r t i ng material was var ied, the monobromo, monochloro and dichloro der ivat ives were made. However s im i l a r reaction conditions (sto ichiometr ic amounts of t r i pheny l t i n hydroxide and t r i c h l o r oace t i c acid or t r i f l u o r o a c e t i c acid i n methanol at room temperature) f a i l e d to produce the corresponding t r i ch lo roacetate and t r i f l uo roace ta te der ivat ives . When t r i ch l o r oace t i c acid was used in the preparation, the white s o l i d which remained a f te r the methanol was pumped o f f turned out to be a | •' | methanol adduct of 03SnOCOCCl3. However a s im i l a r addit ion compound was not i so la ted fo r the t r i f luoroacetate der i va t i ve . A f ter several hours of pumping a viscous - 31 -l i q u i d s t i l l remained i n the reaction vesse l . The reaction solvent was changed from methanol to absolute ethanol. I t seemed reasonable that ethanol would not form an addit ion compound with the t r i c h l o r o a c e t a t e der ivat ive: as e a s i l y as methanol. Both s t a r t i n g materials ( t r i p h e n y l t i n hydroxide and t r i c h l o r o a c e t i c acid or t r i f l u o r o a c e t i c acid) were s t i l l as r e a d i l y soluble i n the new solvent and each reaction was c a r r i e d out at room temperature. The desired products were eventually i s o l a t e d . When an excess of t r i f l u o r o a c e t i c or. t r i c h l o r o a c e t i c acid was used or the reaction was c a r r i e d out under re f lux ing conditions a mixture of products was obtained (the Mossbauer spectrum showed four or more l i n e s ) . Although no attempt was made to separate t h i s mixture i t i s poss ib le to i n f e r what other compounds are l i k e l y to have been produced. Recently a number of t r i p h e n y l t i n carboxylates were prepared T r i p h e n y l t i n acetate, propionate, chloroacetate, and isobutyrate were synthesized by the fo l lowing method: 0,SnOH +. ac id anhydrous benzene o — re f lux Attempts to make t r i p h e n y l t i n t r i f l u o r o a c e t a t e and t r i ch loroacetate gave i n s o l u b l e phenylstannoxane carboxylates (3Sn(0)0C0R (R = CF3 and CClg) which a l l had m.p. >360°. I t i s l i k e l y that our "mixture of products" contained some t r i p h e n y l t i n carboxylate, some of the f i n a l product or phenylstannoxane carboxylate and possibly some of the intermediate product, a phenyl t i n t r i c a r b o x y l a t e which i s subsequently hydrolysed to the f i n a l product. - 32 -Infrared ( i ) Introduction The carboxylate group can be characterized by two absorption bands i n the i n f ra red region. The band posit ions ( cn f 1 ) depend on the chemical nature of th i s group. For example organic esters absorb at about 1740 and 1240 cm - 1 and sa l t s of organic acids absorb between 1610-1550 cm" 1 and 1400-1300 c m " 1 . ^ 1 6 ^ The higher energy absorption i s referred to in the l i t e r a t u r e as a carbonyl band or an asymmetric s t retch ing v ib rat ion and the lower energy absorption i s ca l l ed a carboxyl band or a symmetric stretching v ib ra t ion . In the experimental resu l t s (TABLES IV - VI) the bands are referred to as c-o s t retch ing frequencies. (1-3) Several authors^ ' have observed that the band posit ions for the s o l i d s tate spectra of several t r i a l k y l t i n carboxylates d i f f e r s i g n i f i c a n t l y from the band pos it ions for d i l u t e so lut ion spectra of the same compounds. For example van der Kerk et a l . ^  report s o l i d state bands at about 1570 and 1410 cm - 1 and so lut ion bands at about 1650 and 1300 cm - 1 for a number of t r i a l k y l t i n carboxylates, RgSnOCOR' (R=methyl, e thy l , bu t y l , hexyl and R'=methyl, R=methyl and R'=dodecyl) The s o l i d state bands are s im i l a r to those of a s a l t - l i k e or br idging carboxylate whi le the posit ions of the so lut ion bands are - 33 -much c loser to those f o r an organic ester. From these observations the fo l lowing conclusion was made about the structure of the compounds. The s o l i d i s a polymeric compound having a pentacoordinate Sn atom and i n a d i l u t e so lut ion i t becomes a monomer having a tetracoordinate Sn atom. The absorption energies for t r io rganot in carboxylates d i f f e r s i g n i f i c a n t l y from those previously quoted ( f i r s t paragraph, l a s t sentence of t h i s sect ion) . The author 's suggest that th i s d i f ference can be a t t r ibu ted to the presence of a heavy metal atom which influences the c-o v ibrat ions in the molecule. This seems reasonable however i t would be d i f f i c u l t to separate the mass e f fec t from e lect ron ic effects which could also s h i f t the absorption energy. ( i i ) Experimental Results . Only three compounds have the same (small differences can be a t t r ibu ted to solvent e f fect s ) c-o s t retch ing frequencies for the s o l i d and so lu t i on . These are i n the branched series (TABLE IV, compounds 8,9, and 10) and are: t r i pheny l t i n methacrylate,triphenyl t i n 2-ethylhexanoate and t r i pheny l t i n tr imethylacetate. The band posit ions ind icate that the compounds are monomeric in the s o l i d state and in so lu t ion . This implies a tetracoordinate Sn atom. A l l the other compounds i n the branched series (TABLE V) and a l l the compounds in the l i nea r ser ies (TABLE IV) have c-o stretching frequencies fo r the s o l i d which are at approximately 1530 and 1410 cm" 1 . The c-o s t retch ing frequencies for a d i l u t e CCl^ so lut ion of the same compounds are at 1630 and 1380 cm" 1 . These results ind icate that the - 34 -compounds are polymeric in the s o l i d and monomeric in so lut ion. For several of the pentacoordinate compounds i t was observed that i f the concentration of the so lut ion was increased there appeared two low in tens i ty bands where absorption normally occurs for the same compound i n the s o l i d s tate. This i s evidence for association and lends support to a s o l i d state structure which i s polymeric rather than i on i c . The fact that the so l id s are soluble in non-polar solvents also indicates that they are not i on i c . The haloacetates (TABLE VI) show a larger d i f ference between c-o s t retch ing frequencies for d i f f e r en t compounds with in the ser ies and therefore averaging the band posit ions wouldn't be very meaningful. However a f te r comparing the s o l i d and so lut ion c-o bands for each haloacetate i t i s evident that the compounds are polymeric in the s o l i d and monomeric in so lut ion. The haloacetates have been arranged i n the TABLES.in order of increas ing acid strength (for the parent acid) going down the series and there appears to.be a trend. As the acid strength increases there i s a larger separation between c-o bands fo r both the s o l i d and so lut ion. In other words the band posit ions move c loser to those for an organic ester as the acid strength increases. The pK values for the parent acid range from about 3 for monoiodoacetic acid to <1 for t r i c h l o r o -and t r i f l u o r o a c e t i c ac id. There doesn't appear to be any s im i l a r trends in the c-o stretching frequencies for the pentacoordinate l i nea r and branched non-halogenated - 35 -carboxylates. A l l the parent acids are weak and the pK values f a l l i n a narrow range, pK - 4-5. Mossbauer ( i ) Introduction The usefulness of Mossbauer spectroscopy in studying the structures of t r io rganot in carboxylates depends on a comparison of data for unknown samples with that derived from model compounds. For these studies i t would be necessary to know the isomer s h i f t and quadrupole s p l i t t i n g data for a few t r io rganot in carboxylates with a pentacoordinate and a tetracoordinate Sn atom. I t has been f a i r l y well establ ished from inf rared studies that t r imethyt in acetate and t r i ( n ) b u t y l t i n acetate are polymeric in the s o l i d and therefore have pentacoordinate Sn atoms. Recently an x-ray analysis of the c ry s ta l structure of t r imethy l t i n acetate has confirmed that i t (17) i s polymeric. v 1 We have recorded Mossbauer data for these two compounds: 6(mm sec " 1 ) A(mm sec " 1 ) t r imethy l t i n acetate 1.35 3.68 t r i b u t y l t i n acetate 1.45 3.64 The results are very s im i l a r . The isomer sh i f t s are in the range for Sn(IV) compounds and the quadrupole s p l i t t i n g values are reasonably high (3.6 - 3.7 mm sec 1 ) . Model compounds in which the Sn atom i s very probably t e t r a -- 36 -coordinate are derivat ives of tetraneophylt in. The neophyl group Me i s a bulky substituent and consequently i t I (12V Me — C — CH 2 — has been postulated^ ' that t r ineophy l t in 0 chlor ide and t r ineophy l t in acetate have a tetracoordinate Sn atom. Also, chemical reactions involv ing t e t r a -neophyltin and t r ineophy l t in chlor ide are inh ib i ted and th i s has been (18) at t r ibuted to the e f fect of a s t e r i c a l l y hindered Sn atonr ' . 6(mm sec " 1 ) A(mm sec " 1 ) (neophyl)^Sn 1.34 0 1.21 0 04Sn (neophyl),SnCl 1.39 2.65 • 1.37 2.45 03SnCl '(neophyl ),SnOCOCH, 1.35 2.45 6 6 1.27 3.40 03SnOCOCH3 A l l values are from reference 12 except for t r i pheny l t i n acetate which i s ours. The Mbssbauer parameters fo r the corresponding phenyl compounds are included for comparison. From the previous discussion i t i s now expected that both tetra-and pentacoordinate t r i pheny l t i n carboxylates w i l l show quadrupole s p l i t t i n g , however the magnitude of the s p l i t t i n g w i l l be d i f fe rent for each s t ructura l type and lower (-1.0 mm sec " 1 ) for the tetracoordinate compound which has A - 2 . 5 mm s e c - 1 . The isomer s h i f t values are i nd i ca t i ve of Sn(IV) compounds and they d i f f e r s l i g h t l y (-.1 mm sec " 1 ) as the organic groups and more electronegative groups on Sn are changed. - 37 -( i i ) Experimental Results Three compounds (TABLE VIII, compounds 8,9 and 10) have quadrupole s p l i t t i n g values which are s i g n i f i c an t l y lower (1.0-1.7 mm s e c - 1 ) than for a l l other compounds l i s t e d i n the TABLES. The magnitude of the quadrupole s p l i t t i n g (-2.3 mm sec " 1 ) i s s im i l a r to that for the t r i -neophyltin compounds (A~2.5 mm s e c " 1 ) . From these observations t r i -phenyltin 2-ethylhexanoate, t r i pheny l t i n methacrylate and t r i pheny l t i n tr imethylacetate are postulated to have tetracoordinate Sn atoms. A l l the other compounds have quadrupole s p l i t t i n g values which are i nd ica t i ve of a pentacoordinate Sn atom (A ranges from 3.3-4.0). FIGURE IV, page 18, shows the Mossbauer spectrum for a monomeric compound, (a) t r i pheny l t i n methacrylate, and a polymeric compound, (b) t r i pheny l t i n acry late. The difference in quadrupole s p l i t t i n g i s quite marked. " The larger values for the quadrupole s p l i t t i n g of the polymeric compounds can be explained by an increased asymmetry i n 5p-electron charge, assuming no contr ibution from non-bonding closed she l l s and 5 s-electron charge because of t he i r spherical symmetry. The asymmetry in 5p-electron charge could occur as a resu l t of the change i n valence electron d i s t r i bu t i on as the s t ructura l type changes from a d i s tor ted tetrahedron for the monomeric compounds to a t r igonal bipyramid for the polymeric compounds. The largest quadrupole s p l i t t i n g values are found for the haloacetates, TABLE IX. Since these compounds have more e l ec t ro -negative substituents on the a-carbon atom i t i s l i k e l y that th i s further d i s torts ( i nd i r ec t l y v ia the c-o groups) 5p-electron d i s t r i b u t i on . - 38 -The isomer s h i f t values fo r the compounds studied are the same magnitude as other Sn(IV) compounds (6 - 1.2 - 1.3 mm s e c . 1 ) . Within a given ser ies the 6 values fo r some of the compounds d i f f e r by as much as .1 mm sec " 1 . I t i s speculat ive whether th i s i s s i g n i f i c a n t , however i n some instances there appear to be p laus ib le reasons and some of these w i l l be mentioned. The 6 values are consistently- higher fo r the polymeric so l ids ( a l l compounds i n TABLES VII - IX except 8,9 and 10 in TABLE VI I I ) . This means there i s a s l i gh t increase in s -e lectron density at the Sn nucleus and can be explained by a decrease in sh ie ld ing of 5 s-electron charge by 5 p-electron charge. This "is reasonable s ince a change in the d i s t r i bu t i on of valence e lectron charge (mainly 5s and 5p) i s expected for a change in bonding from a tetracoordinate to a pentacoordinate Sn atom. However i t might have been d i f f i c u l t to predict the r e l a t i v e electron density changes without the Mossbauer resu l t s . The 6 values for the haloacetates are s l i g h t l y higher than for the non-haloacetates which are also polymeric. Apparently there i s further deshielding of s -e lectron charge and i n d i r e c t l y th i s could be due to more electronegative substituents on the a-carbon atom. One might therefore •expect R=CF3 to have a higher isomer s h i f t than R=CHpI and th i s i s observed. There i s also a small increase in isomer s h i f t down the series which i s arranged i n order of increasing acid strength for the parent ac id , H0C0R. - 39 -The Mossbauer resu lts i n TABLE VII show no cor re la t ion be-tween the length of the R group and the isomer s h i f t or quadrupole s p l i t t i n g . S imi lar conclusions can be made for the compounds containing branching groups (excluding the three monomeric compounds, TABLE VIII compounds 8,9.and 10) Structure and Bonding Lengthening the hydrocarbon group, R,. to 18 carbon atoms doesn 't i n h i b i t polymer formation i n t r i pheny l t i n carboxylates. Methyl branches at the 2-5 carbon atom posit ions also do not prevent polymer formation. However an ethyl and a buty l branch or three methyl groups at the a-carbon atom are s u f f i c i e n t l y bulky to give a monomeric s o l i d state s t ructure. One might have expected t r i pheny l t i n methacrylate, which has a methyl and a methylene group on the a-carbon atom, to be polymeric l i k e t r i pheny l t i n isobutyrate which has two methyl groups and a hydrogen atom bonded to the a-carbon atom. The methylene group i s about as bulky as a methyl group (van der Waals radius for both ~ 2.0 A°). A l i k e l y ? explanation i s that for the methacrylate the a-carbon forms sp "-hybridized bonds and therefore the three atoms bonded to i t w i l l be close to a t r igonal planar arrangement in cont rad i s t inc t ion to the isobutyrate in which the a-carbon atom is in a tetrahedral environment. The methacrylate group i s now r e s t r i c t ed in i t s o r ientat ion c apab i l i t i e s and consequently interacts to s t e r i c a l l y hinder polymer formation. - 40 -In view of the previous discussion i t i s informative to compare the experimental results for t r i pheny l t i n acry late with that for t r i pheny l t i n methacrylate. The existence of d i f f e ren t structures for these compounds i s evident from t h e i r Mossbauer spectra shown in FIGURE IV. Tr ipheny l t in acry late i s designated as (b) in the Figure. In t r i pheny l t i n acry late the three groups or atoms bonded to the a-carbon atom w i l l again be nearly coplanar. However the replacement of a methyl group by a H atom makes the carboxylate group less bulky and consequently the compound i s polymeric. Halogen atoms bonded to the a-carbon atom do not prevent polymer formation. Even three Cl atoms, where each Cl atom has a van der Waals radius (1.80 A°) very close to that for a methyl group (2.0 A°), do not produce the same s t ructura l change in the s o l i d as three methyl groups. 41 THEORETICAL DISCUSSION Point-Charge Predict ions of Quadrupole S p l i t t i n g Values A point-charge model can be used to predict quadrupole s p l i t t i n g A values for the t r i pheny l t i n haloacetates from A values for the corresponding t r imethy l t i n der ivat ives which are i sos t ructura l compounds.^ The quadrupole s p l i t t i n g i s given by A = 1/2 eQVzz (1 + n 2 / 3 ) 1 / 2 where eQ i s the nuclear quadrupole moment, n = (V^* - Vyy)/V z z , and the V-ji are the diagonal e l e c t r i c f i e l d gradient, efg, tensor elements. By convention the axes are chosen so that |V Z Z| £ |VyV| ^ |V X X|. The e l e c t r i c f i e l d gradient which gives r i se to the quadrupole s p l i t t i n g i s due both to charges on the ligands and surrounding atoms or ions (^L/^JJIQ^) a n d to imbalances in the d i s t r i bu t i on of the valence-shel l electrons on the Sn atom (QYALBICF.)* ^ A S R E C E N ^ V been shownv ; that P V ^ L E ^ Q E dominates the s p l i t t i n g . In the point-charge model the l igands, L, are considered as l o ca l i z a t i on s of e lect ron ic charge, q^, s i tuated at a distance r^ from the central metal atom. The contr ibut ion [L] of a pa r t i cu l a r l igand to the efg i s given by [L] = <q L r L " 3 > The [L] i s used s t r i c t l y as an empirical parameter in our ca lcu la t ions . An expression for A i n terms of [L] can be obtained for a - 42 -molecule of the type R^SnX^ which has t r igonal bipyramidal symmetry about the Sn atom. This i s an ideal s t ructura l type which i s approximated by the polymeric t r i pheny l t i n carboxylates, FIGURE I. Choosing the z-axis along the O-Sn-0 d i rec t i on gives n = 0 because of ax ia l symmetry i . e . the x and y contr ibutions to the efg are equal. The only contr ibut ion comes from V z z which can be wr i t ten as: V Z 2 = E [ L ] (3 cos 2 e L - 1) Subst i tut ing [R] and [X] fo r [L] gives A = 4[X] - 3[R] where the factor 1/2 eQ has been incorporated into the parameter .[[_]. [L] has been referred to as a pa r t i a l quadrupole s p l i t t i n g , PQS, (19) parameter* Parish and P l a t t ^ ' have estimated PQS values for a number of substituents ( a l l the compounds contain a pentacoordinate Sn atom). Using t he i r values for [Me] and [Ph] and the A values for the t r i -methylt in haloacetates^ ; i t i s possible to predict A values (A 1) for the t r i pheny l t i n haloacetates. A = 4[X] - 3[Me] A' - 4[X] - 3[Ph] A1 = A + 3 ([Me] - [Ph]) The ApRcrrj values agree quite wel l (within .1 mm s e c - 1 ) with the A N R C. values. I t i s important to note that the d i f ference - 43 -[Me] - [Ph] i s required in the computation and not the actual values for [Me] and [Ph]. This theoret ica l model of quadrupole interact ions assumes that equivalents of charge are transferable and i t appears to have a f a i r p red ic t i ve accuracy when applied to homologous ser ies . - 44 -TABLE X Point-Charge Predicted Quadrupole-Splitting Values for Triphenyltin Haloacetates R AOBS. ( m S £ C i } CD CR3 3.40 3.35 C2) CH^l 3.59 3.50 C3) CR2Br 3.51 3.57 C4) CR2C1 3.53 3.56 • C51 CRC12 3.81 3.75 C6) c c i 3 3.75 3.82 C7) CF 3 4.00 3.89 - 45 -CONCLUSION In general t r i pheny l t i n carboxylates are polymeric in the so l i d - s ta te and have bridging carboxylate groups and pentacoordinate Sn atoms. Three of the compounds studied were found to be monomeric so l id s . I t was concluded that the s t ructura l change from a polymeric to a monomeric s o l i d could be att r ibuted to s t e r i c in teract ion between substituents on the a-carbon atom and neighbouring phenyl groups bonded to Sn. An attempt was made to determine the minimum s ize of substituent ( t r i - sub s t i t u ted at the a-carbon atom) which would resu l t in a monomeric compound. Three methyl groups (van der Waals radius ~2.0A°) give a monomeric compound but three chlor ine groups (van der Waals.radius ~1.8A°) give a polymeric compound. Apparently the " c r i t i c a l " s i ze i s between 1.8 and 2.0A°. The most unexpected resu l t was the existence of an addit ion compound of t r i pheny l t i n trichloroacetate,J^SnOCOCCl^'MeOrl. The Mbssbauer and infrared data indicate that the compound i s a polymer with bridging carboxylate groups. As a ru le Sn (organotin) has a tendency to increase i t s coordination number from four to f i ve or s i x ^ ° \ consequently i t i s common to f ind addition compounds formed from electron donor solvents. To my knowledge this i s the f i r s t adduct of a t r i -organotin carboxylate that has been reported. r 46 -REFERENCES 1. M.J. Janssen, J.G.A. Lu i j ten and G.J.M. Van der Kerk, Rec. Trav. Chim. 82 (1963) 90, English Translat ion 2. R. Okawara and M. Ohara, J . Organomet. Chem. 1 (T963) 360. 3. R. Okawara and M. Ohara, B u l l . Chem. S o c Japan 36 (1963) 624. 4. N.W. Alcock and R.E. Timms, J . Chem. Soc. (A) 1876 (1968). 5. C. Poder and J.R. Sams, J . Organomet. Chem. 1_9 (1969) 67. 6. B.W. Fitzsimmons, N.J. Seely and A.W. Smith, J . Chem. Soc. (A) 143 (1969). 7. R.V. Parish and R.H. P i a t t , J . Chem. Soc. (A) 2145 (1969); Inorg. Chim. Acta , 4.(1970) 65. 8. R.E.B. Garrod, R.H. P i a t t and J.R. Sams, Inorg. Chem. 1_0 (1971) 424. 9. G.K. Wertheim,"Mossbauer E f fec t : P r inc ip le s and Appl icat ions " Academic Press, London (1964). 10. J.P. Bocquet, Y.Y. Chu, O.C. K i s tner , : M.L. Perlman, and G.T. Emery, Phys. Rev. Let. j_7 (1966) 809. 11. M. Cordey-Hayes, J . Inorg. Nucl. Chem. 26.(1964) 915. 12. R.H. Herber, H.A. Stb'ckler, and W.T. Reichle, J . Chem. Phys. 42 (1965) 2447. 13. P.C. Chieh and J . Trotter J . Chem. Soc. (A) 911 (1970). 14. G. FriedTander, J.W. Kennedy, J.M. M i l l e r , "Nuclear and Radiochemistry" 2nd Ed., John Wiley and Sons, Ind. N.Y. (1964). 15. R.C. P o l l e r , J.N.R. Ruddick, B. Taylor, and D.L.B. Toley, J . Organomet. Chem. 24 (1970) 341. - 47 -16. L.J. Bellamy "The Infrared Spectra of Complex Molecules", 2nd Ed., Methuen and Co. Ltd. London (1968). 17. B.R. Penfold, personal communication. 18. R.V. Parish and C.E. Johnson, Chem. Phys. Let. 6 (1970) 239. 19. G.M. Bancroft, M.J. Mays and B.E. Prater, J . Chem. Soc. (A) 976 (1970). 20. R.C. P o l l e r , J . Organomet. Chem. 3 (1965) 321. 

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