NMRF-F COUPLING CONSTANTS IN FLUOROTITANATE COMPLEXES
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Nuclear Magnetic Resonance Fluorine-Fluorine Coupling Constants in Fluorotitanate Complexes
by Daniel S. Dyer and Ronald 0. Ragsdale Department of ChemistTy, Universdy of Utah, Bait Lake Cdy, Utah 8411% (Received Januury 23,1967)
The factors which determine the FIO spin-spin coupling constants in the TiF4.2(donor) complexes (TiF4.2D and TiFd.D,D’) have been studied. The coupling constants for several complexes are reported. The data are discussed in terms of through-space F-F nuclear interaction.
Introduction
Results
In a fluorine-19 nmr study‘ of a series of titanium tetrafluoride adducts, TiF, (CH3)2NC(0)CH3,4-ZC5H4N0, the chemical shifts were shown to be sensitive to the nature of the para substituent (Z) on pyridine 1-oxide. On the other hand, the spin-spin coupling constants of these complexes are only very slightly affected by electron-density changes produced by the different Z substituents. Since relatively little was known about the factors which determine the couplings between fluorine19 nuclei in complex ions, a study of spin-spin interaction in the cis-TiF4-2D (D = donor) complexes was conducted. The results of this research are reported, and the data are examined in view of related studies of fluorine-substituted organic compounds.
The identical donor molecules in the cis-TiF4.2D octahedron (1) create two electronic environments each containing two FIBn u ~ l e i . ~ JAt sufficiently low
Experimental Section The spectra were obtained at 56.4 Mc/sec with a Varian A-56/60A spectrometer equipped with the V-6057 variable-temperature system. The studies were conducted on samples cooled to temperatures at which the spin-spin multiplets are completely resolved. The mixed adduct (TiF4.D,D’) solutions were prepared by the method reported previously.1 The TiF4.2D samples were prepared either by dissolving titanium tetrafluoride (Allied Chemical Corp.) in an excess of organic donor or, when possible, by isolating the TiF4 diadduct and dissolving it in an appropriate solvent. The solvents chloroform, dichloromethane, acetonitrile, and chloroacetonitrile were purified by standard procedures. The nmr samples were of sufficient purity that no extraneous FIBpeaks were observed.
T
=
D
F Fa 1
temperatures, depending upon the nature of the donor molecules, the nmr spectra of these complexes consist of two triplets of equal intensity (first-order A Z 2 ) . The spin-spin coupling constants ( J ) for several TiF4. 2D complexes were determined and are listed in Table I. Whenever possible the J value of a complex was determined in more than one solvent (chloroform, dichloromethane, acetonitrile, chloroacetonitrile, or excess organic donor). Within experimental error the different solvents did not appear to affect the magnitude of the coupling constants. In the cis-TiF,.D,D’ complexes’ there are two equivalent and two nonequivalent fluorine atoms. The spectra of the mixed adducts considered here are of the AzMX type, consisting of a doublet of doublets and two doublets of triplets. The doublet of doublets has twice the intensity of either of the doublets of triplets. The coupling constants of the TiF4* DMA,4-ZC6H4N0 (1) D. S. Dyer and R. 0. Ragsdale, Inorg. Chem., 6 , 8 (1967). (2) E.L. Muetterties, J . Am. Chem. Soc., 82, 1082 (1960). (3) R. 0.Ragsdale and B. B. Stewart, Inwg. Chem., 2 , 1002 (1963).
Volume 71, Number 7 June 1967
DANIELS. DYERAND RONALD 0. RAGSDALE
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Table I : F10 SpinSpin Coupling Constants“ (Magnitudes) for the TiF4.2D Complexes
three coupling constants of each TiFJ*D,D’complex are quite different from each other.
Discussion It has been shown that H-H spin coupling constants
complexes (2, DMA = N,N-dimethylacetamide) are listed in Table I1 for reference. The J values for three new TiFd.D,D’ complexes, TiF4.TMU14-ZC5H4NO (TMU = tetramethylurea and Z = CHIO, H, and NOz), are also given in Table 11. The peaks in the spectra of these complexes were assigned on the same basis
2
R= CH3 or (CH3)aN
as in the study of TiF4.DMA,4ZC5H4N0,1 where it was shown that the chemical shift of the fluorine trans to 4-ZC5H4N0 is influenced most by Z.
Table II: FluSpinSpin Coupling Constants” (Magnitudes) for the TiFa. D,D‘ Complexes
4-CHa02CCsHdNO 4-ClCsHaO 4-HCsaNO 4-CHsCsHrNO 4-CHaOCsH4NO 4-OzN CsHdN0 4-HCsH4NO 4-CHaOCsH4NO
N,N-Dimethylacetamide N, N-Dimethylacetamide N,N-Dimethylacetamide N,N-Dimethylacetamide N,N-Dimethylacetamide Tetramethylurea Tetramethylurea Tetramethylurea
39 39 39 39 39 41 41 41
34 34 35 35 35 34 35 35
48 48 48 48 48 49 49 49
f l cps.
Two results of this study are especially noteworthy: (1) comparison of the F-F coupling constants of the
TZ4.D,D’ complexes shows that the spin-spin interactions are insensitive to the nature of 2, and (2) the The Journal of Physical Chemistry
can be calculated semiquantitatively by a consideration of the valence-bond theory of the Fermi contact term in the Hamilt~nian.~-’o The deviations from perfect pairing of the binding electrons provide the dominant contributions to the interactions between protons separated by two or more bonds. Physically the magnitude of the coupling constant depends in part on the number of covalent bonds through which the protons interact and also on the geometrical orientation of the interacting nuclei. The theory of coupling between nuclei of atoms other than hydrogen is complicated by the larger contributions of the electron-spin and the electron-orbital terms in the Hamiltonian. Qualitatively, the problem of FIg spin-spin coupling is made more diffcult by the presence of occupied p orbitals in the vicinity of the fluorine nuclei. It might be expected, therefore, that F19nuclear coupling could occur via more than one mechanism. Convincing evidence for the existence of “throughspace” coupling between fluorine nuclei has been This type of nuclear interaction is thought to come about as a result of overlap between the electronic clouds of the coupled fluorine atoms. Through-space interaction can be expected when the distance between the fluorine nuclei becomes less than twice the van der Waals radius for a fluorine atom (-2.72 A). However, an exact expression of throughspace coupling as a function of the F-F internuclear distance has not as yet been reported.ls Isolation of a through-space contribution to a fluorine-19 coupling constant is complicated by the possibility of contributions of the same or opposite sign from other pathways for the transmission of spin information. (4) M.Karplus, D.H. Anderson, T. C . Farrar, and H. S. Gutowsky, J . Chem. Phys., 27, 597 (1957). (5) M.Karplus and D. H. Anderson, ibid., 30, 6 (1959). (6) M.Karplus, ibid., 30, 11 (1959). (7) H.S. Gutowsky, M. Karplus, and D. M. Grant, ibid., 31, 1278 (1959). (8) M.Karplus and D. M. Grant, Proc. Natl. Acad. Sei. U. S., 45, 1269 (1959). (9) M.Karplus, J . Chem. Phys., 33, 1842 (1960). (10) M. Barfield and D. M. Grant, ibid., 36, 2054 (1962); J . Am. Chem. SOC.,83, 4726 (1961); 85, 1899 (1963). (11) L. Petrakis and C. H. Sederholm, J . Chem. Phys., 35, 1243 (1961). (12) 8. Ng and C. H. Sederholm, ibid., 40, 2090 (1964). (13) N. Boden, J. Feeney, and L. H. Sutclifie, J. Chem. Soc., 3482 (1965).
NMRF-F COUPLING CONSTANTS IN FLUOROTITANATE COMPLEXES
All of the F-F couplings in the TiF4.2D and TiF4*D,D' complexes are of the geminal type (interaction between nuclei separated by two bonds). The geminal fluorine coupling constants of the fluoroethanes extend over a wide range and in one series, QFtG CClHF, were found t o be dependent on the electronegativity of the substituent &.'* An nmr study of a variety of trifluorovinyl derivativesI5 (YCF=CFZ) showed that the chemical shifts and the coupling constants of the vinyl fluorines are very sensitive to the mesomeric and inductive nature of the species to which the trifluorovinyl group is bonded. In another study of the trifluorovinyl derivatives, a linear relationship between the geminal coupling constant and the mean of the two chemical shifts of the terminal fluorines was pointed out.'" It was recently suggestedI5 that through-space coupling in the trifluorovinyl compounds does not seem plausible. In contrast to the coupling behavior of geminal fluorines bonded to carbon, the F-F coupling constants in the TiF4* DMA,4-ZCsH4NO and TiF4*TMU, 4-ZCsH4NO complexes were not sensitive to the mesomeric or inductive characteristics of the substituents. In spite of tht: considerable influence which the para substituent exerts upon the r- and ?r-bonding ability of the N-tO o ~ y g e n , ~the * ' ~largest range observed for any of the three couplings was 1 cps. (This can be compared with the range of J,, in the trifluorovinyl derivatives (5 to 101 cps).) The insensitivity of the F19 coupling constants of the TiFd.D,D' complexes to the electron density about titanium can be explained if the F-F nuclear interaction is assumed to occur primarily through space rather than through the bonds. I n throughspace interaction a decrease in the F-F internuclear distance is expected to cause an increase in the magnitude of the coupling constant. Consider model 3 for the TiF4*DMA,4-ZC5H4N0 complexes. Steric in-
/;C-CHS 0
3
teraction between DMA and the adjacent (cis) fluorines should shorten the internuclear distances Fa-F, and F,-Fp by decreasing the angles F,TiF, and FBTiFBt,
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and increasing the magnitudes of J,, and J B p To a first approximation the angle F,TiF,J and JUS' would not be affected by the presence of the DMA molecule. By the same reasoning and as seen from model 4, repulsion between 4-ZCsH4NO and the adjacent fluorines should affect Just and J,,., but not JaB. The angle F,TiF,j is therefore decreased by both donorfluorine interactions and would be expected to be the smallest FTiF angle in the complex. Experimentally, JOB'is found to be the largest of the three coupling constants. In addition Jus, the coupling constant determined by fluorine-DMA repulsion, is equal to J for TiF4*2DMA, and J,,', the coupling constant determined by fluorine-pyridine 1-oxide repulsion, is equal to J for the corresponding TiF4.2(4-ZCsH4NO) complex.
4
The same reasoning explains the relative values of the three coupling constants for each TiF4.TMU,4ZC5H4N0 complex. The largest coupling is again between FB and
[email protected] coupling constants determined by the fluorine-TMU repulsion and the fluorine-pyridine 1-oxide repulsion are equal respectively to the coupling constants for TiF4.2TR/IU and TiF4.2(4ZCsH4NO). It appears, therefore, that spinspin interaction in the TiF4-D,D' complexes can be qualitatively explained on the basis of a through-space coupling mechanism, although other mechanisms might exhibit a similar dependence on F-F internuclear separation. This model may be used to elucidate some of the finer structural features of the TiF4*2Dcomplexes. The relative magnitudes of the J values of TiF4.2(N,N-dimethylformamide) (I), TiF4.2(N,N-diethylfonnamide) (11), TiF4.2(N,N-dimethylylacetamide) (111), and TiF4 -2(N,N-dimethylpropionamide)(IV) decrease in the order JIV > J I I I> J I I = J I . This indicates that the alkyl group attached to the carbonyl (14) J. Dyer, PTOC. C h a . SOC.,275 (1963). (15) C.G. Moreland and W. S. Brey, Jr., J. C h m . Phys., 45, 803 (1966). (16) J. Reuben, Y.Shoo,and A. Demiel, J . Am. Chem. SOC.,87,3995 (1965). (17) H.H.Jaff6, J . Ore. Chem., 23, 1790 (1958).
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DANIELS. DYERAND RONALD 0. RAGSDALE
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rather than the dialkylamino group has the greater steric interaction with the fluorines. The coupling constants for the TiF4.2ROH complexes (R = CHs, CzH6,and i-C3H7) are experimentally the same, suggesting that the donor molecules are oriented such that there is no difference within the series in steric interaction with the fluorines. The F-F coupling constants of the TiF4.2D and TiFd.D,D’ complexes are small when compared with the geminal coupling constants of other inorganic fluorides. (For examples see Table I in ref 13.) The small values could result either from mutual cancellation by couplings of opposite sign or, as was suggested earlier, the through-bond contribution to spin coupling in the TiF4 complexes may be relatively small. Since ionic bonds would be unable to transmit through-bond nuclear interaction, and since the Ti-F bonds in the TiF4.2D complexes might be expected to be appreciably ionic,I* the latter explanation seems to be more plausible. A previous study’ indicated that there is some fluorine-titanium pa-d?r interaction in the TiF, complexes. The existence of both high ionic character in the 5 bonds and ?r donation by the fluorines is compatible with contributions from valence bond structures such as
The Journal of Phy&
Chtktry
D D
I
F
D
F-
D
I
Ff
F
Conclusions Fluorine19 spin-spin interaction in the TiF4.2D and TiF4*D,D’ complexes appears to occur via a through-space mechanism. In contrast to the geminal F-F couplings in fluorinated organic compounds the coupling constants of these complexes are insensitive to electron-density changes produced by electronwithdrawing or -donating substituents. The apparent lack of through-bond nuclear interaction is suggested to be a result of high ionic character in the F-Ti u bonds.
Acknowledgment. Support of this work by the Air Force Materials Laboratory, Research and Technology Division, Wright-Patterson Air Force Base, Ohio, is gratefully acknowledged. (18) R.
F. Fenske, I w g . Chem., 4, 33 (1965).
