R. B. JOHANNESEN, F. E. BRINCKMAN, AND T. D. COYLE
660
Nuclear Magnetic Resonance Studies of Inorganic Fluorides.
V. Fluorosilaneslv2 by R. B. Johannesen, F. E. Brinckman, and T. D. Coyle Inorganic Chemistry Section, National Burmu of Standards, Washington, D. C. $0684 (Received August 4, 1967)
Nuclear magnetic resonance (nmr) chemical shifts of ‘9F nuclei and 29Si-lsF coupling constants have been measured for a number of compounds containing silicon-fluorine bonds. For most of these, 29Sichemical shifts were also determined by (heteronuclear) (19F-(29Si]) double resonance. With few exceptions, 19Fand 29Siresonances in substituted fluorosilanes are on the low-field side of the corresponding resonances in SiFe. The 29Sishift in the SiFe2- ion is about 75 ppm to high field of SiF4 and is well outside the range of previously reported silicon shift values. Magnitudes of 2sSi-1sFcoupling constants range from 167.6 Hz in Si20Feto 384.9 HZ in SizCl5F. The variation of these parameters with the nature and extent of substitution on silicon is discussed. A significant solvent dependence of the chemical shifts in SiFe and similar compounds has been observed.
Introduction The sensitivity of 19Fnmr parameters to the nature and extent of substitution in fluorinated molecules makes the nmr technique extremely useful for structural characterization of these materials. Although most of the 19Fnmr studies previously reported deal with fluoroorganic compounds, considerable attention has been given to selected classes of inorganic fluorides, notably derivatives of boron, nitrogen, sulfur, and phosphorus. a Few data, and, in particular, few coupling constants, have been reported heretofore for the fluorides of silicon. Chemical shifts and spin-coupling constants have been measured for the simplest fluorosilanes, SiF,H4-, (n = 1-4),416 and 19Fshifts have been reported6J for a number of organofluorosilanes. The analysis of the 2% satellites in the l9F spectra of some fluoropolysilanes has been discussed in previous publications from this laboratory.sz9 I n the course of studies in silicon fluoride chemistry, we have measured 19Fchemical shifts and spin-coupling constants for a number of fluorosilane derivatives. For several of these compounds, 29Si chemical shifts have also been determined by heteronuclear double resonance. The objective of this work has been to investigate the range of variability of the nmr parameters and to correlate shifts and coupling constants, particularly the latter, with structure wherever possible. We have been particularly interested in establishing the general trends in the nmr spectra of fluorinated monosilanes as guides for the use of nmr for analysis and characterization of fluoropolysilanes. It is to be hoped that the existence of data such as The Journal of Physical Chemistry
these will also serve t o stimulate efforts directed toward detailed theoretical interpretation of the nmr parameters.
Experimental Section Sample Preparation. All manipulations were carried out in an efficient recirculating drybox or in a conventional chemical high-vacuum apparatus. Solvents were commercial materials of the highest nominal purity available and were used as received except for drying, either with sodium metal or with lithium tetrahydridoaluminate or by vacuum-line fractionation. Organofluorosilanes were prepared from the corresponding chlorides by treatment with SbFs, as described previously for s i ~ F 6 and , ~ were purified by fractional condensation in the vacuum line. No (1) Presented in part at the 150th and 153rd National Meetings of the American Chemical Society, Atlantic City, N. J., Sept 1965, and Miami Beach, Fla., April 1967. (2) Part IV: R . B. Johannesen, J . Chem. Phys., 47, 3088 (1967). (3) For a recent review of 19F and other nmr studies, see J. W. Emsley, J. Feeney, and L. H. Sutcliffe, “High Resolution Nuclear Magnetic Resonance Spectroscopy,” Pergamon Press, Oxford, 1966. (4) E. A. V. Ebsworth and J. J. Turner, J . Chem. Phys., 36, 2628 (1962). (5) E. A. V. Ebsworth and J. J. Turner, J. Phys. Chem., 67, 805 (1963). (6) E. Schnell and E. G. Rochow, J . Inorg. Nucl. Chem., 6, 303 (1958). (7) V. A. Pestunovich, A. N. Egoroohkin, M. G. Voronkov, V. F. Mironov, and Y. I. Skorik, Zh. Strukt. Khim., 6,915 (1965);J . Struct. Chem. (USSR), 6, 878 (1965). (8) R. B. Johannesen, T. C. Farrar, F. E. Brinckman, and T. D. Coyle, J . Chem. Phys., 44, 962 (1966). (9) R. B. Johannesen, ibid., 47, 955 (1967).
SMR STUDIES OF INORGANIC FLUORIDES significant impurities were detectable by infrared or 19Fnmr spectroscopy. Bromo- and chlorofluorosilanes were prepared by heating mixtures of SiF4 with SiBr, or SiC1, in sealed bulbs for several days at 150" in the presence of aluminum chloride. This method has also been applied to the preparation of the analogous halogenated disilanes. lo Samples for nmr measurements contained approximately 15 mol % SiF4 (which provided a sharp line for locking the field and served as an internal reference material) in the fluorosilane or solvent of interest and were sealed under vacuum in 5-mm (0.d.) borosilicateglass sample tubes. In the case of the chlorofluorosilanes, the individual compounds were not isolated. LMixtures of SiF4 and SiC1, of several C1:F ratios were equilibrated as described above, and the entire samples, containing all five possible compounds, were distilled into the nmr tubes. N m r Measurements. Fluorine nmr spectra were recorded at 56.44 MHz, using the field-frequency lock, frequency-sweep spectrometer described previously.s Positions of spectral lines relative to the resonance peak of internal 28SIF4were determined by measuring the pertinent side-band frequency with an electronic frequency counter. Chemical shifts of the 19Fnuclei in the same samples were also determined with reference to an external standard. For convenience, we commonly used a reference tube consisting of 15 mol % SiF, in CFC13. For these measurements, an audio side band was displayed on the oscilloscope, and the reference and sample tubes were inserted, alternately, into the spectrometer. The audio frequency was adjusted until the position on the oscilloscope screen of the upper side band of the (lower field) fluorine resonance in one tube coincided with that of the lower side band of the (higher field) fluorine resonance in the other tube. The audio frequency was then taken as one-half the chemical shift. This method was satisfactory for chemical shifts of 120 to 2500 Hz or greater. For smaller chemical shifts, the upper (or lower) side bands of the two resonance peaks were observed, the oscillator frequency being altered to keep the side bands in coincidence when the tubes were interchanged. Silicon spectra were determined by "spin-tickling" experiments. The apparatus and method have been described el~ewhere.~I n these experiments, both the 19F(56.442 MHa) and the 29Si(11.918 MHz) frequencies for the compounds of interest were counted to the nearest 0.1 Hz with an electronic frequency counter, the field being locked to an internal reference line. Using the 19Fchemical shifts of the compounds relative to the external SiF4-CFCI3 reference and the magnetogyric ratios of IgF and 29Si, the Z9Si resonance frequencies were corrected to the values for the field at which the fluorine atoms in 2gSiF4in the external ref-
661 erence sample are in resonance at 56.4428371 IMHz. The choice of this figure is arbitrary; it was in fact the average of several measurements. It was necessary to count both the 29Si and I9F frequencies, since a drift of 0.1 ppm in the 19Fresonance frequency causes an equal change in the 2gSiresonance frequency and thus in the apparent 29Sichemical shift.
Results and Discussion The 19F and 29Si chemical shifts and the 29Si-lgF spin-coupling constants between directly bonded atoms, as measured in this laboratory for a series of silicon fluorides, are listed in Table I. As noted in the table, internally referenced 19F chemical shifts are reported relative to the resonance in 2*SiF4. Externally referenced I9Fshifts are given relative to the resonance in 2sSiF4in a sample containing SiFe gas at 30 atm (assuming ideal gas behavior), and 29Sishifts are relative to the 29Siresonance in the same gaseous sample. The values reported are the averages of three or more independent measurements. The average deviations for the internally referenced chemical shift values listed in the table ranged from 0.002 to 0.016 ppm. The precision of the externally referenced shift values is somewhat less, the average deviations lying in the range 0.04-0.06 ppm. The average deviations in the coupling-constant values were in the range 0.05-0.26 Ha. For C2HsSiF8, the satellite spectra are complex, and the over-all uncertainty in the reported value is estimated to be k0.7 Hz. In Table 11, we have listed 19Fchemical shifts and z9Si-1gFcoupling constants obtained in this laboratory for certain compounds for which the measurements are considered to be less precise than those given in Table I, together with a number of values taken from the literature. The shift values have been converted somewhat arbitrarily to the SiF, scale by use of the conversions indicated in the table, and the number of significant figures retained is, in general, that given in the original report. These measurements were obtained under various solution conditions and by several referencing procedures. Consequently, the chemicalshift values from different sources are not precisely comparable with each other or with the values in Table I, and shift differences of a few ppm may possibly have no significance. The data in Table I1 are included in order to indicate as completely as possible the variation of nmr parameters in this class of compound. Tables I and I1 together contain substantially all the coupling-constant data available for the fluorosilanes. The 18Fchemical shifts and 2gSi-1gFcoupling constants for compounds for which both parameters are known are summarized graphically in Figure 1. It is (10) R. B. Johannesen, F. E. Brinckman, and T. D. Coyle, Abstracts, 153rd National Meeting of American Chemical Society, Miami Beach, Fla., April 1967, No. L-113. Volume 72, Number 2 February 1068
R. B. JOHANNESEN, F. E. BRINCKMAN, AND T. D. COYLE
662 Table I : Nmr Parameters for Silicon Fluorides
ZQSichemical shift, ppm Externald
'QFchemical shift, ppm Internal6 ExterndC
Compounda
... ...
SiF4 (gas, 30 atm) SiF4 [15 mol yo in (CH&Si] SiF4 [15 mol % in (CzH&O] SiF4 (15 mol % in CClIF) SiF4 (15 mol % in cc14) (SiFa)zO SiFsCle SiF2C1," SiFClt SiFaBr SiFzBrz SiFBrs CH3SiF3 (CH&SiF2 (CH3)aSiF C2HjSiFs CHz=CHSiF3 S ~ F , C O ( C O(neat ) ~ liq)' SizFe SiFClzSiCla(neat liq)
... ...
... -6.13 -- 28.67 -52.66 -71.05 -39.39 -68.72 -86.68 -29.31 -32.30 -6.27 -23.65 -21.74
... -42.25 -48.54
- 39.57 ( SiFs) - 27.10 (SiFz) ...
SiaFsg (NH4)zSiFe(satd aq)
0.0
0.0 -8.6 -9.1 -9.2 -11.9 -10.6 -36.6 -60.6 -78.9 -44.0 -78.4 -99.3 -34.9 -40.3 -14.8 -29.7
...
...
-1.1
... -0.6 -27.3 -54.0 -76.9 -26.6 -41.6 -42.0 -57.2 -117.8 -144.4
...
-82.8 -48.3 -60.1
-84.3 -35.5 -90.3 - 106.5 - 33.5 -95.7 f74.3
-46.6 (SiF3)
- 34.1 (SiFz) -45.4
169.0 174.8 173.7 175.0 176.8 167 6 228.0 273.6 311.5 252.7 318.8 368.7 267.2 287.8 274.5 280.5 259.8 370.0 321.8 384.9 I
...
...
J(zQSi-lQF),HZ
(SiFCl2) (Sicla) (SiFa) (SiFz)
344.4 (SiFa) 356.6 (SiFz) 108.1
'
a Except as noted, samples contained 15 mol percent SiF4 as internal reference. Values referred to internal %iF4. Negative sign indicates resonance to low field of %iF4 peak. ' Values referred to external reference, Z8SiF4resonance in SiF4 gas (30 atm). Values referred to external reference, l9Si resonance in natural abundance in SiF4 gas (30 atm). e In an equilibrated mixture, originally equimolar in SiF4 and SiC14. Sample kindly provided by Professor A. G. MacDimmid, University of Pennsylvania. p Sample kindly provided by Professor J. L. Margrave, Rice University.
'
Table I1 : Additional Nmr Data for Fluorosilanes
Compound
Chemical shift,a ppm
SiFsH SiFZHz SiFHa (CH&SiOSiFa ( CH&NSiFa (CH8)sSiNHSiFa (CH&SiNCNSiF3 C&SiFs CH3SiHzF SiF3SiFzBF~ SiF3SiFzSiFzBFz
-54.5 - 13 +53 -5.5 -7.5 -18.6 -13.7 -30.6 f27.8 -37.5 -39.5
J (zQSi-1QF), He
274.8 298 281 184.2 201.4 202 187.6 249.4 279.8 351 355
Nmr refb
ConditionsC
CClsF, I CClsF, I CClaF, I CClsF, I CClsF, I CaFe, I CClsF, 1 CBFG, I CClsF, I CClsF, E CClsF, E
Dil soln in CClaF Dil soln in CClaF Dil soln in CC13F 20y0 soln in CClaF 20% soln in CCLF Soln in CeF6 20Y0 soln in CClaF Soln in C6F6 Soln in CClsF Neat liq Neat liq
Notes
e d, e d, e d, f dl
d, f
B d, f Q, h d, i d, j d, j
Reference compound used in original manuscript; I indicates internal reference, E indicates a Values given corrected to SiF4 scale. Corrected to SiF4 scale, assuming G(CC1sF) = - 164.01 external reference. Indicates experimental conditions of measurement. J. J. Moscony and A. G. Macppm (value measured in this work for a 15 mol % solution of SiFl in CClaF). References 4 and 5. Diarmid, Chem. Commun., 307 (1965). ' Corrected to SiFl scale, assuming ~(C~FP,)= 1.37 ppm (value measured in this work for a 15 mol % solution of SiF4 in C~P,). Sample kindly provided by Professor J. L. Margrave, Rice University. ' Reference 11. P. L. Timms, T. C. Ehlert, J. L. Margrave, F. E. Brinckman, T. C. Farrar, and T. D. Coyle, J. Am. Chem. Soc., 87, 3819 (1965).
-
'
'
apparent that there is no simple correlation between these parameters which is generally applicable to all the fluorosilanes studied. For the halofluorides and certain trifluorosilyl derivatives, there is a rough proThe Journal of Physical Chemistry
portionality between the shifts and the coupling constants. For these compounds, the largest values of J(29Si-1gF)are found in molecules showing the greatest downfield shifts.
NMMR STUDIESOF INORGANIC FLUORIDES
663
V
t
350
3001
'"t 200
i.Ld
I
&-'o
150
0
-10
,
-20
-30
,
'
'
-40
-50
-60
CHEMICAL SHIF1
I
L
-70
served for compounds having substituents containing a electrons and were attributed t o magnetic aniinteraction^.^ The chemical-shift sotropy or d,-p, sequences C2H5SiF3 > CHaSiF3 and (C2H&SiF > (CH3)aSiF are similarly consistent with the relative inductive effects of the alkyl groups. It is possible that, in such closely similar compounds as the dialkyldifluorosilanes, the rather small chemical) ~ be determined by shift differences (10-15 ~ p m may the relative inductive effects of different alkyl substituents. Thus, the presence of electron-releasing substituents may increase the ionic character of the Si-F bond, thereby reducing the paramagnetic contribution to the chemical shift. On the other hand, the chemical-shift sequence CISiF3 > BrSiF3 and the varying effects of sequential alkyl substitution indicate that simple inductive considerations are inadequate to explain gross chemical shifts between chemically dissimilar fluorosilanes. It is interesting that the SiF3 resonance in SiF3Co(CO)4 occurs at extremely low field, ca. 30 ppm below the closest SiF3 resonance (in SizFa). This large paramagnetic shift parallels the shifts observed for a-fluorine atoms in perfluoroalkyl transition metal compounds [the 19Fresonance of CF3Co(C0)4is about 77 ppm below CF415]. This paramagnetic shift has been attributed16 to the availability of low-lying excited states in the transition metal compounds. A similar explanation may account for the paramagnetic shifts in the halofluorosilanes, although interpretations based on a bonding have also been proposed for the similar 19Fshifts in the chlorofluoromethanes, l7 chlorofluoroboranes,'* and phosphorus(V) chlorofluorides.l 9 2gSi-'9F S p i n Coupling Constants. The magnitudes of the directly-bonded 2gSi-1sFcoupling constants in the fluorosilanes lie in the range from 168 t o 385 Hz. The value for the SiFe2-anion (108 HZ)~O is considerably smaller than the values for the four-coordinate silicon compounds and may reflect the decrease in fractional s character in the hybrid orbitals on silicon. As described previously,2l J(29Si-19F) in SiFr is solvent dependent, observed values ranging from 169.0 Hz in
-80
-90
Figure 1. ZDSi-W coupling constants us. 1*Fchemical shifts (Sif internal reference) for silicon fluorides: 0, SiFaX; 0 , SiFsXz; and , VSiFXa. Connecting lines show effect of sequential substitution: - - -, SiF,Cla-.; - - - -, SiF,Bra- ; , SiF,(SiFa)4-z; and SiF, (CH&-
-
-*-*-e,
=e
lgFChemical Shifts. With three exceptions, SiH3F,S (C2H5)3SiF,B and CH3SiH2F,l1 the fluorine resonances of all substituted fluorosilanes so far investigated occur at lower field than that of the parent compound, SiF4. Replacing a single fluorine in SiF4 with a different group to form the trifluorosilyl derivative invariably results in a shift to lower field. Thus, in SiF3X compounds containing first-row atoms bonded to silicon, chemical shifts decrease (i.e., resonances shift to lower field) in the sequence: SiF3-F > SiF30- > SiF3N= > SiF3C=, and, for halogen substituents, in the sequence SiF3-F > SiFaCl > SiF3Br. The latter sequence parallels chemical shifts in the corresponding CF,X derivatives. 12,13 I n general, the l9F resonance of a given silicon fluoride lies at a higher field than that of the corresponding carbon fluoride. This may reflect the greater polarity of the Si-F bond as compared to the C-F linkage, a s the paramagnetic term should be smaller for more ionic fl~0rides.l~ The effect of introducing a second or third substituent on silicon is considerably less uniform than the effect of the first substitution. I n the halofluorosilanes, chemical shifts decrease almost uniformly in the sequence SiF4, SiF& SiFzX2, SiFXs. With alkyl, hydride, or trifluorosilyl substituents, the downfield shift is reversed with the second or third substitution. A similar variation, differing in detail, occurs in CF,X4-, compounds. l2 It has been suggesteda that the nonuniform trends in chemical shift with increasing alkyl substitution are attributable to competing inductive and mesomeric (d,-p,) effects. For a few diorganodifluorosilanes, RIR2SiFz,the lBFchemical shift has been found to be approximately linearly dependent on the sum of the Taft polar substituent constants, (r*, for R1 and R2, with the l9F resonance at higher field for the more electron-releasing substituents. Deviations were ob-
(11) E. A. V. Ebsworth and 8. G. Frznkiss, Trans. Faraday SOC., 59, 1518 (1963). (12) N.Muller and D. T. Carr, J. Phys. Chem., 67, 112 (1963). (13) R. K.Harris, J . MOL Spectry., 10, 309 (1963). (14) A. Saika and C. P. Slichter, J . Chem. Phys., 22, 26 (1954). (15) W.R. McClellan, J . Am. Chem. Soc., 83, 1598 (1961). (16) E. Pitcher, A. D. Buckingham, and F. G. A. Stone, J . Chem. Phys., 36, 124 (1962). (17) L. H. Meyer and H. 9. Gutowsky, J . Phys. Chem., 57, 481 (1953). (18) T. D.Coyle and F. G. A. Stone, J . Am. Chem. SOC.,82, 6223 (1960). (19) R. R.Holmes and W. P. Gallagher, Inorg. Chem., 2, 433 (1963). (20) G. V. D.Tiers, J . Inorg. Nucl. Chem., 16, 363 (1961). (21) T. D. Coyle, R. B. Johannesen, F. E. Brinckman, and T. C. Farrar, J . Phys. Chem., 70, 1682 (1966). Volume 72,Number 2 February 1968
664
R. B. JOHANNESEN, F. E. BRINCKMAN, AND T. D. COYLE
Table I11 : Additivity Relationships for J(29Si-lgF) in Fluorosilanesa
Compound
SiFaCl SiF2C12 SiFC13 SiF3Br SiFtBrz SiFBra SiFaCH3 SiFz(CHd2 SiF(CHs)3 SiF3Hi SiF2H< SiFHs’ SiF3SiF3 SiF2(SiF&
-“Direct” additivityb-J(add),a Ha
J(obsd),
t*,d
Hz
Ha
225.9 273.6 312.6 252.7 318.8 368.7 267.2 287.8 274.5 274.8 298. 281. 321.8 356.6
112.5 108.5 104.2 139.3 131.1 122.9 153.8 115.6 91.5 161.4 105.7 93.7 208.4 150.0
Pairwise interactionC
J’(ca1od),I
HZ
Hz
Ha
lOOA‘/ J(obsd)
...
...
...
281.7 337.5
8.1 24.9
3.0 8.0
225.9 273.4
0.0 -0.2
0.0 -0.1
...
...
...
335.3 417.9
16.5 49.2
5.2 13.3
...
...
...
364.3 461.4
76.5 186.9
26.6 68.1
...
...
...
349.5 484 2
51.5 203.2
17.3 72.3
I
...
473.5
’
’J(29Si-1gF)in SiF4 taken as
7
lOOA/ J(obsd)
A,f
...
...
116.9
32.8
...
252.7 318.9
A’,h
... 0.0 0.1
...
0.0 0.03
...
...
...
261.1 295.9
-6.1 8.1
-2.3 2.8
...
...
...
267.9 304.9
-6.9 6.9
-2.5 2.3
...
... ...
...
...
...
...
...
Cf. ref 22. Cj. ref 28. 170.0 Hz. rxvalues calculated assuming constant additivity ( r p = 56.7 HE). e Calculated using = 56.7 HZ and value of Ix from SiFaX. A = J(add) - J(obsd). ’Parameters used for calculating J‘: ’IFF = 56.7 HZ, VClCl = 104.2 HZ, VBrBr = 122.9 HZ, ~ C H ~ C=H 91.5 ~ HZ, VHH = 93.7 HZ, vFCl 84.6 HZ , V F B ~= 98.0 Hz, ~ P C H=~ 102.2 Hz, W H = 105.6 Hz. A’ = J’(ca1cd) - J(obsd). ” Cf. ref 4 and 5.
r~
the gas at 30 atm to 178.6 Hz for a 15 mol % solution in SiBrr. Except for (SiF3)*0 (J = 167.6 He), the coupling constants for all the substituted fluorosilanes studied are larger than the maximum value found for SiF4. For the SiF3X derivatives, J(29Si-19F)decreases as the atom bonded t o silicon is changed from C to N to 0. Coupling constants increase in the sequence SiF3C1 < SiF3Br and are largest for SiF, groups bonded to electropositive atoms (Si and Go). The trends parallel trends in J(I3C-l9F) for trifluoromethyl corn pound^.'^ The data in Table I show that the effect on J(29Si”F) of successive introduction of substituent groups is variable. Since this work provides coupling-constant data for several series of compounds of the type SiF,X4-%,it is of interest to examine the applicability of additivity rules that have been used to correlate spincoupling constants in similar series involving other nuclei. It has been foundz2that 13C-H coupling constants in substituted methanes frequently obey a “direct” additivity relationship Jxyz(CH) = sk
+ + Cy
Cz
where JxYz(CH)is the 13C-H coupling constant in CHXYZ, and CX, tu, and f Z are parameters characteristic of the substituents. The { values may be evaluated from J(CH) for CH3X
lx = J(CH) [in CHSX] - 2/3J(CH) [in CHI] The theoretical basis of this relationship has been discussed.2a Several investigators have found that large deviations from “direct” additivity may occur when the substituents are highly electronegative g r o ~ p s .Deviations ~ ~ ~ ~ ~were also found when this rule The Journal of Physical Chemistry
’
was applied to 29Si-H23J4 or 13C-19F13,26 coupling constants. The results of applying the “direct” additivity rule are shown in Table 111. The failure of this relationship is calapparent in two ways. (1) The values of lx culated on the assumption of constant additivity ({F = 56.7 Hz) decrease progressively as the degree of substitution increases. (2) Consequently, the magnitudes of coupling constants J(add), calculated for SiFzXz and SiFX3, using rx calculated from SiF3X derivatives, are larger than the observed coupling constants. The deviation, both absolutely and as a percentage of J(obsd), is greater for SiFX3 than for SiF2X2. These observations agree with those of Harris13 for corresponding carbon fluorides. As the reduced coupling constants, ~ T J X F I ~ ~ Y have X Ythe F , same sign for X = C and Si, the deviations from additivity in these parameters are in the same direction for both the carbon and silicon fluorides. In W-H and 2QSi-H coupling constants, px increases with increasing X substitution and the magnitudes of the calculated J values are lower than the experimental. Inasmuch as M-H and M-F coupling constants are of opposite sign for both 13Cand 29Si,however, it appears that the absolute sign of the deviations in the reduced coupling constants is the same in all four cases, the experimental
(22) E. R. Malinowski, J . Am. Chem. Soc., 83, 4479 (1961). (23) C. Juan and H. 8.Gutowsky, J . Chem. Phys., 37, 2198 (1962). (24) H. J. Campbell-Ferguson, E. A. V. Ebsworth, A. G. MacDiarmid, and T. Yoshioka, J . Phus. Chem., 71, 723 (1967). (25) S. G. Frankiss, ibid., 67, 752 (1963). (26) G. P. van der Kelen and Z. Eeokhaut, J. Mol. Spectry., 10, 141 (1963) I
NMRSTUDIESOF INORGANIC FLUORIDES values being more positive than those calculated on the assumption of additivity. Douglasz7has shown that the introduction of “pairwise interaction” terms allows “prediction” of I3C-H coupling constants in cases where the direct additivity rule fails. Malinovvski and Vladimiroff28 have used a similar approach t o correlate 13C-19F, z9Si-H, Sn-H, and Sn-C-H coupling constants. For the fluorosilanes, Malinowski and Vladimiroff’s relation takes the form J,,,(SiF)
+ +
= txy
qyz
qxO
where J,,,(SiF) is the coupling constant in SiFXYZ and the q’s are parameters characteristic of each distinct pair of substituents. For a given series SiF,X4-, this rule will of course introduce three parameters and might thus be expected to give a somewhat better correlation that the “direct” additivity rule. Values of coupling constants, J’(calcd), calculated for SiFaX and $iF2X2 compounds in four series by use of the qxy values given, are shown in Table 111. Values F qxx taken are simply J(obsd)/3 for SiF4and of ~ F and SiFX3,respectively. The qFX values taken are averages of values derived from the observed coupling constants for SiFzX2and SiF& and all of the deviation between observed and calculated coupling constants thus appears in the parameters for these members of the series. The differences amount to less than 3% of the observed J values in all cases and would presumably be smaller if a more elaborate fitting procedure which also adjusts qFF and qxx were used. I n view of the demonstrated solvent dependence of J(Si-F) (vide infra), which makes the selection of a best value of ~ F someF what arbitrary,, the agreement is as satisfactory as that obtained by this correlation for other spin-coupling const ants . If correlations of this type prove generally applicable, the parameters may be of some theoretical significance. From an experimental viewpoint, it would be extremely useful to have available a reliable means of estimating spin coupling constants for a series of molecules on the basis of values for related compounds. Unfortunately, in this investigation (and in most similar studies), the number of compounds available is very limited and scarcely exceeds the number of parameters needed to establish the relationship. Thus, nine parameters are required to correlate coupling constants in SiF, and the first twelve compounds listed in Table 111. For the four substituents involved, there are, in fact, 35 compounds containing at least one fluorine atom and one or more other substituent atoms, and only 15 interaction parameters. Examination of coupling constants in a significant number of these compounds would clearly provide a more demanding test of the validity of the “pairwise interaction” approach. For the compounds studied here, and for some others to which the (6 pairwise interaction” rule has been applied, it appears that, by use of all available data, adjusted parameters
665
that reproduce the observed coupling constants to within 3% or less can be obtained. If, however, n parameters are evaluated from only n coupling constants chosen from a larger set of data, the agreement between observed and calculated J values for the remaining compounds may be much poorer. The application of the pairwise interaction approach to a large number of z9Si-H coupling constants has recently been reported.z9 Compounds of the type SiHXYZ, where X, Y, Z are “all possible combinations of hydrogen, methyl, phenyl, and chlorine” were investigated, and pairwise interaction parameters were reported. This report does not indicate, however, the agreement of observed and calculated coupling constants and does not consider the predictive usefulness of the parameters in the sense discussed here. 29Si Chemical Shifts. The only previous reports of z9Si chemical shifts are due to Lauterbur and coworkers, 30 who observed the z95i resonances directly. Only one fluorosilane, (CH&SiF, was included in their compilation. As was described in the Experimental Section, we have obtained z9Sishifts for a number of the compounds included in this study by heteronuclear double resonance. The results are given in Table I. The observed chemical shifts lie within the range of values observed by Lauterbur, except for SiFsz-, which has a large high-field shift, approximately 78 ppm above that of any other silicon compound. The fluorosilicate ion is the only six-coordinate species for which a chemical shift has so far been reported. The changes in the z9Sichemical shift with sequential substitution are summarized in Figure 2, which demonstrates the variability of substituent effects. It has been suggested31 that the z9Si shifts for the methylethoxysilanes reflect combined inductive and n-bonding effects on electron density at the silicon atom. The sequence of 29Sishifts in the symmetrical Six4 series (F > Br > O C ~ H > S C1 > CH3) is markedly different from the pattern’ of 13C shifts in the analogous CX4 compounds (Br > CH3 > C1> OCzHs). It is now recognized that the second-order paramagnetic term usually makes the major contribution to nuclear shielding for all but the lightest nuclei.’3 I n a recent theoretical treatmenta2 of alp chemical shifts in several PX3 derivatives, the isotropic para(27) A. W.Douglas, J . Chem. Phys., 40, 2413 (1964). (28) (a) E. R. Malinowski and T. Vladimiroff, J . Am. Chem. SOC., 86, 3575 (1964); (b) T.Vladimiroff and E. R. Malinowski, J. Chem. Phys., 42, 440 (1965). (29) E.0.Bishop and M. A. Jensen, Chem. Commun., 922 (1966). (30) (a) G. R. Holrman, P. C. Lauterbur, J. H. Anderson, and W. Koth, J . Chem. Phys., 25, 172 (1956); (b) P.C. Lauterbur, “Determination of Organic Structures by Physical Methods,” Vol. 2, F. C. Nachod and W. D. Phillips, Ed., Academic Press, New York, N. Y.,1962,Chapter 7. (31) T.Ostdick and P. A. McCusker, Znorg. Chem., 6 , 98 (1967). (32) H.8. Gutowsky and J. Larmann, J. Am. Chem. Soc., 87, 3815 (1965). Volume 72, Number B February 1968
R. B. JOHANNESEN, F. E. BRINCKMAN, AND T. D. COYLE
666
&(si)
(porn)
0
- 20 -40
- 60 -80 -100
SiMe,Fq.,
-120
-740
0
1
2
n
3
4
Figure 2. 2*Sichemical shifts in related compounds (data for Si(CH&(OCaHs)+, from Lauterbur, ref 30).
magnetic shielding of the P nucleus was evaluated in terms of localized bond parameters (hybridization and ionicity). The calculated values of the paramagnetic term were in agreement with the order of chemical shifts (PH3 > P(CH3)3> PF3 > PII > PCls > PBr3). It was pointed out that although a single bond parameter will vary monotonically in, e.g., the sequence F, C1, Br, I, the effects of the various parameters on the chemical shift may not be uniform and may in fact be opposed. These considerations dictate caution in interpreting the observed 29Sichemical shifts in terms of “chemically reasonable” changes in bonding parameters without a quantitative treatment. Solution Efects. The samples used in this study were essentially dilute solutions of SiF4 in the fluorosilanes of interest. As we have described, fluorine chemical shifts were measured both with respect to the internal SiF4 and an external SiFd reference. With one exception, the ordering of the l9Fshifts is identical with both referencing systems, although the magnitudes of the chemical shifts for a particular compound may be quite different for the two referencing procedures. Ideally, chemical shifts should represent differences in the shielding constants for nuclei in isolated molecules. I n liquid samples, intermolecular interactions will usually affect the observed chemical shifts, and it is well known that large solvent effects may be found in 19Fspectra.33 The contributions to solvent shifts and the optimum referencing procedure to minimize them have been d i s c u ~ s e d . ~ ~ ~ ~ ~ For sequences of related compounds in the fluorosilane series, the effects on 19Fshifts of gross changes in molecular structure are clearly much more significant than the intermolecular effects. However, since one obvious application of data such as the results of this The Journal of Phyaical Chemistry
work is the identification of particular species in illdefined mixtures, it is of interest to know the extent to which the apparent shift for a given compound may vary. This is, of course, particularly true when the material of interest gives a structureless main-resonance line, and the concentration is too low to permit examination of the satellite spectra or the use of double irradiation, both of which may provide additional structural information. lo While we were not concerned in this work with a comprehensive examination of solvent effects, some data were obtained which are relevant to this problem. The externally referenced chemical shifts in Table I include contributions from the differences in bulk diamagnetic susceptibility between the sample and the reference. Although the susceptibility data are not available for many of the compounds, we estimate (using Pascal’s constants) that the correction will be 0.5 ppm or less in most cases. For the internally referenced shifts, the shielding of the SiF, fluorine atoms may be significantly influenced by the fluorosilane or other solvent. The differences between the internally referenced shifts for the compounds studied and the shifts with respect to gaseous SiF4 are a measure of the gas-to-solution shifts for SiFl as solute, the fluorine nuclei being deshielded in the solutions. We have previously notedz1 that the zgSi-19Fcoupling constant in SiF4 can differ by as much as 8 Hz from solvent to solvent. There is a corresponding effect on the 19F chemical shift, the resonances generally occurring at higher field in those solvents in which the magnitude of the coupling constant is smaller. Hutton, Bock, and S ~ h a e f e rhave ~~ shown that plotting our measured J’s vs. the heat of vaporization, H,, of the corresponding solvent at its boiling point gives a reasonably good straight line. This was interpreted as indicating that the variation in J can be attributed mainly to dispersion interactions. A similar plot of the gas-to-solution shifts vs. H, suggests a corresponding dependence of the shifts on intermolecular van der Waals interactions. The ‘9F chemical shifts and 29Si-lgF coupling constants of the constituents in mixtures of the chlorofluorosilanes are shown by the data in Table IV. For individual molecules, chemical shifts may differ by 4 ppm or more, relative to external SiF4,in mixtures of different composition. However, since the medium effects on the shifts are in the same direction and roughly comparable for all the fluorosilanes, the shifts of the chlorofluorides relative to internal SiF4 show an
(33) D.F. Evans, J . Chem. SOC.,877 (1960). (34) A. D. Buckingham, T. Schaefer, and W. G. Schneider, J . Chem. Phya., 3 2 , 1227 (1960). (35) J. W.Emsley and L. Phillips, Mol. Phys., 11, 437 (1966). (36) H. M. Hutton, E. Bock, and T. Schaefer, Can. J. Chent., 44, 2772 (1966).
NMRSTUDIES OF INORGANIC F'LUORIDES
667
Table IV : Solvent Effects in Chlorofluorosilane Spectra C1:F 1:3
C1:F 1 : l
Cl:F3:1
6F3 PPm4
SiFa SiFaCl SiF2Cl2 BiFCla
+4.11 -24.81 -49.04 -67.68
+1.31 -27.36 -51.36 -69.74
CI:F1:3 -
I
-0.27 -28.79 -52.62 -70.87
171.89f0.04 225.91f0.26 271.80 f 0 . 3 4
*..
C1:Fl:l JSIF, Hab
173.73 f 0 . 0 8 227.96f0.06 273.60f0.16 311.53 f 0 . 3 4
CI:F3:1
... 229.15f0.25 274.60f0.16 312.62 f 0 . 1 4
Chemical shift measured relative to external reference (15% SiFd in CClsF). Each value is the average of at least three measurements, with average deviation f 0 . 0 4 ppm or less. The values given are averages of three measurements; indicated uncertainties are average deviations.
extreme difference of 1.2 ppm or less. As anticipated, the difference is greatest for SiFCla. Miscellaneous. In the course of this work, the leF spectrum of CsFsSiFa was examined in detail. Chemical shifts were measured with respect to internal Cd?6; the values obtained were: G(SiF3) = -29.2 ppm, G(ortho) = -38.3 ppm, 8(meta) = -4.1 ppm, G(para) = -19.1 ppm. The chemical shift of SiFc at 15 mol % in C6Fa was found to be 1.367 ppm, the average deviation of three measurements being 0.008 ppm. Coupling constants involving ring fluorines were found by firstorder analysis t o be approximately: J(SiF8-ortho) =
8 Hz, J(SiFa-meta, para) = -0, J(orlho-para) = 6.5 Hz, and J(metapara) = 18 Hz. The magnitude of the ortho-para coupling constant and the low field shift of the para fluorine are of interest in connection with the suggestiona7that these are correlated with the extent of Ir-electron withdrawal from the ring. The present results, considered in terms of this correlation, are consistent with the view that the SiFa group functions as a strong ~racceptor. (37) M. G. Hogben, R. 8. Gay, and W. A. G . Graham, J . Am. Chem. Sac., 88, 3457 (1966).
Vokkme 7.9, Number B February 1.968
