ROBERT E. CONNICK AND RICHARD E. POULSON
568
Vol. 63
F19 NUCLEAR, MAGNETIC RESONANCE OF VARIOUS METAL-FLUORIDE COMPLEXES I N AQUEOUS SOLUTION BY ROBERT E. CONNICK AND RICHARD E. POULSON Contribution from the Department of Chemistry and the Radiation Laboratory, University of California, Berkeley, Cal. Received Bsptember 8, 1968
The chemical shifts of FIBhave been observed for a number of metal fluoride complexes in aqueous solution. No simple correlation was found with the electronegativity of the metal ion or the stability of the complex formed. It appears that metal cations of high atomic number produce a stron decrease in the magnetic shielding and a rough correlation was obtained with A / d , i.e., ratio of the atomic number of t i e metal ion to the interatomic distance in the complex. In several cases separate resonances were observed, and assuming that they represent species which exchange fluorines only slowly, lower limits to the lifetime for exchange were calculated.
The position of the F19 nuclear magnetic resonance in a number of metal fluoride complexes in aqueous solution was investigated to determine whether there was a correlation of the chemical shifts with such properties as the stability of the complex, electronegativity of the metal atom, its charge, radius, etc. The results are shown in Table I. Included are several measurements from the literature and values for the reference compounds used in the present work. The data are reported in terms of the chemical shift u, defined as
where H , and Hr are the field st,rengthn a t resonance for the sample and the reference solutions, respectively. The reference compound was fluorine gas. Where possible, the formulas of the principal species were deduced from known complexing constants from the references cited. In a few cases more than one resonance was observed and these have been interpreted as arising from different fluorine-containing species which do not exchange fluorines rapidly. From the separation of the lines a lower limit can be set to the exchange lifetime1V2from the equation IO'
1 r
e
'
G
=
G
where r e is the exchange lifetime, Au is the difference in angular precession frequencies, Y is the resonance frequency, and ACTis the difference in chemical shifts of the two species. These lower limits to the exchange lifetimes are given in the last column of Table I. In the case of aluminum the two species are known to be AIFz+ and AIF+2, respecti~ely.~The u values of 550 and 552 for titanium and zinc, respectively, almost certainly arise from fluoride ion. The 574 value for the second silicon solution is likely an average of F-, H F and HF2-. The species corresponding to the two tin resonances are unidentified. The observed line widths a t half intensity were approximately the same as for the two aluminum fluoride complexes3, i . e . , 0.G p.P.m., except in the caPe of the silver solutions where they were approximately 5.5 p.p.m. a t lo4 gauss. This broadening with silver ioii might be caused by an ap( I ) H. S. Gutowsky and .4. Saika. J . Chem. Phvs., 21, 10&& (1953). (2) P. W. Anderson, J. Phys. SOC.Japan. 9, 317 (1954). . 5153 (8) R. E. Connirk and R. E. Poulson J. A m . Chem. S O C .79, (1057).
propriate rate of exchange between F- and AgF, although other explanations are possible. Saika and Slichter4 have proposed that the chemical shift in fluorine compounds is due primarily to the second-order paramagnetic term of the electrons on the fluorine. Using this hypothesis they explain the empirical correlation of Gutowsky and Hoffman6 of the FI9 shifts with electronegativity. The present work gives n o evidence for such a correlation.6 Our experiments differ from those of Gutowsky and Hoffman in that aqueous solutions rather than pure liquids were used and the compounds were more ionic than theirs. An examination of the data reveals no obvious Correlation with stability of the complexes. For example, ThF+3 and AIF++ are much more stable complexes than ZnF+ although the resonance of the latter lies between those of the first two. Neither does there appear to be a simple correlation with the charge or radius of the metal ions, or combinations of these two. Perhaps the most striking effect is the large decrease in shielding observed with thorium and uranyl ions. It seems probable that the large number of electrons in these atoms is somehow producing a strong decrease in the shielding of the F19 nucleus. An even l a v e r decrease in shielding has been found for Figin UF,.' Van Dyke Tiers* has reported somewhat analogous effects for iodine, etc., nlterinq the Figresonance in substituted nlkvl halides, although he interpreted the results in terms of steric interactions. A plot of u us. the ratio of the atomic number of the metal ion and the approximate interatomic separation of the metal and fluorine nuclei measured in Angstrom units, i e . , A / d is shown in Fig. 1 for the complex ions in aqueous solution. There is a rough correlation but its physical significance is not apparent. Superimposed on t h e A / d dependence, there may be an electronegativity dependence. The silicon point deviates from that of its more electropositive neighbors in the direction of smaller shielding, as would be expected from (4) A. Saika and C. P. Slichter. J . Cham. Phvs., 22, 26 (1954). (5) H. S. Gutowsky and C. J. Hoffman, ibid., 19, 1259 (1951). (0) For a discussion of this point see R. E. Poulson. Thesis, Uniywsity of California. Nov. 16, 1956: printed as University of California Radiation Laboratory Report UCRL-3567. These various electronegativity values are piven b y Haissinsky ( J . Phys. Rad., [81 7 , 7 (1946); Si, 1.8; Ap, 1.8; Sh(II1). 1.8; Sn(II), 1.65; Ti(IV). 1.6; Be. 1.5; AI, 1.5; Zn,1 . 5 : U(IV), 1.3, Th,1.1. (7)J. N. Shoolery and H. E. Weaver, Ann. Rev. Phya. Chem., 6 , 440 (1955). ( 8 ) G.VRn Dyke Tiers, J . A m Chem. Snc.. 7 8 , 2914 (195n).
April, 1959
FLUORIDE COMPLEXES
F L u O R I N E - ~ NUCLEAR ~ MAGNETIC RESONANCE OF METAL
569
TABLE I CHEMICAL SHIFTSOF F1@ IN METALFLUORIDE COMPLEXES IN AQUEOUS SOLUTIONS Principal fluoride species
Solution
Lower limit for exchange time, r (mcc.)
Uf
ThF+3", 1 .O M Th(NO&, 0 . 5 M NaF 336 UOzF +b 2 . 0 M UOz(NOa)z, 2 . 0 M NaF 395 UOS, ? 1 . 3 M U02(NOa)2, 2 . 6 M NaF 401 CFsCOzH Pure CFsCO2H (507.6)' SnF+, SnF2 ?' 0 . 8 M SnC12, 1 M NaF, 0 . 3 M H F 627,588 0.07 FDilute NaF or KF 547.7 F13.3 M K F (549.2) AgF, F-d 1 M AgN03,l M NaF 550 (very broad) 2 M AgNOa, 1 M NaF 550 (very broad) AgF, F-d SiFe+ (NH&Biie (satd.) (557. 3)h 0 . 1 M (NHi)zSiFe, 0 . 4 M HF, 0 . 4 M NaF 557.3, 574 0.3 SiFe-', HF, F-, HFaTiF,+4-", F- ? K2TiFs (satd.) 573, 550 0.2 ZnF +,FZnFz (satd.) 579,663 0.2 BFa Dissolved in ether (583. 2)' 587.0, 687.6 8 AlFo+, AlF+p 1 M Al(NO&, 1 M NaF 588 .O, 688.8 5 A w n + , AlF+r 2 M Al(NOa)s, 2 M NaF HF 48-51% aq. H F (596.9)' R. A. Day, Jr., and R. M. Powers, ibid., 0 H. W. Dodgen and G. K. Rollefson J . Am. Chem. SOC.,71, 2600 (1949). I. Leden and L. 76, 3895 (1954). a R. E. Connick and Armine D. Paul, un ublished. Some Sn(1V) also present. Marthen, Acta Chem. Scund. 6 , 1125 (1952). e Reference 3. YSee "Experimental" for accuracy. The more intense line is italicized. 0 Reference solution, H. S. Gutowsky D. W. McCall, B. R. McGarvey and L. H. Meyer, J . Am. Chem. Soc., 74, Reference solution, H. S. Gutowsky and C. J. Hoffman, Phys. Rev., 80,110 (1950). 4809 (1952).
Gutowsky and Hoffman'ss correlation. The remainder of the data, except perhaps for silver, are consistent with such an hypothesis but provide little substantiation for it.6
34
I--
i
30 Experimental 26 The ap aratus used is described elsewhere.' The value of 507.6 8 r c of CFaCOzHO was used to refer all values to 21 Fz. Values of u for the reference solutions Sic', BFa(ether), A/& and HF(aq.) were obtained in the following way. The poI8 -
sition of CFaCOOH on Gutowsky and Hoffman's BeFp scale10 was established as 102.6 p.p.m. by comparing the 0'8 of the organic fluorides reported in references 9 and 10, making allowance for the correction reported in footnote 23 of reference 5 . The positions of SiFa', BFs(ether), and HF(aq.) were then known from reference 10. The consistency of the assignment was checked by recording on the same chart, using a 1.5 gauss sawtooth Ewee , the resonances of SiFe', BFs(ether), HF(aq) and CFaC&H, the solutions being contained in 2 mm. 0.d. glass tubes simultaneously inserted in a sample tube. Taking the BFs(ether) and CFsCOzH values as fix points, the SiFs' and HF(a .) values were 0.6 p.p.m. less and 0.5'p.p.m. greater than dutowsky and Hoffman's values, res ectively. The values for the latter two solutions given in%able I are those measured here. The above procedure also calibrated the 1.5 gauss sweep field. Where the sample resonance was within 10 p.m. of a reference solution, the chemical shift between t%e two was determined as described in reference 3. I n other cases, %.e., ThF+3, U02F+ and SnF+, the calibrated 1.5 gauss sweep was used to refer the sample to trifluoroacetic. acid. The 13.3 M K F solution also was measured by the latter procedure. This solution and UOzF+ were subsequently used as secondary reference solutions. The shifts relative to the nearest primary or secondary reference solutions are believed to be correct to about f l p.p.m., except for aluminum and dilute NaF and KF solutions where the are probably within k 0 . 3 p.p.m. and for ThF+aand U d F + where the uncertainty is f 1 0 and f 5 p.p.m., respectively. (9) Footnote u, Table I. (10) Footnote h, Table I.
-
14.10
-
TiF+.-.
0
1
6-
L
I
I
I
I 400
I
I
I
I
I
, , ,
4 50
I
I 500
, , ,
,
I .~:F.**;', 5 SO
qi 600
b.
Fig. 1.-Correlation of u of metal fluoride complexes with ratio of atomic number A of metal ion to interatomic distance d in Ilngstrom units. The beryllium and antimony points are from reference 5. The relative accuracy of the reference solutions depends on Gutowsky and Hoffman's work10 to which they assigned a probable error of f 1 . 5 p.p.m. The accuracy of all results on the F2 scale depends on the accuracy of the value for trifluoroacetic acid.Q The authors gave no statement of accuracy but it may be =kl% as given in some earlier work.' Some earlier chemical shift values given for the present work6 are in error because of oversight of the correction given in footnote 23 of reference 5. I n some cases the Sam les were contained in 15 mm. i.d. Lusteroid test-tubes ratter than in Pyrex tubes. These tubes gave a 30% increase in sample volume over the glass tubes. They also fitted the Varian rf. probe insert snugly. Positioning of the sample inside the receiver coil has a marked effect on the balancing of the nuclear induction circuit when large tubes are used and the circuit can be balanced more readily if the tubes fit the probe insert well.
