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Lajos Barcza and Michael T. Pope
Heteroconjugation of Inorganic Anions in Nonaqueous Solvents. 111. Complexes of Polymolybdates and -tungstates with Chloral Hydrate Lajos Barcza and Michael T. Pope"' Department of Chemistry, Georgetown University, Washington, D.C.20007 (Received June 3, 1974)
Heteroconjugates of l,l-dihydroxy-2,2,2-trichloroethane (chloral hydrate) with a number of polyanions in nitrobenzene solution have been investigated by nmr spectroscopy. The computed formation constants (at 37') of the 1:l complexes are as follows: Mo601&, 1.76; PM0120403-, 3.11; SiMo120404-,24.7; Pw120403-, 1.30; C104-, 2.55 M - l . These figures provide the first direct quantitative measure of hydrogen bonding involving heteropoly anions and demonstrate the very low charge densities on the surface of such anions.
Introduction '
In the first paper of this series,2 we pointed out the importance of investigating the interactions of heteropoly anions with neutral organic molecules in homogeneous solution. The structures of isopoly and heteropoly anions resemble fragments of metal oxide lattices and the behavior of such complexes in solution may be used as a model for metal oxide catalyst-substrate interactions. Since hydrogen-bonded complexes involving heteropoly anions are very weak, simple "chelate"-type donors such as pyrocatechol were initially studied. Only indirect evidence for complex formation with pyrocatechol could be observed with polymolybdates owing to irreversible oxidation-reduction react i o n ~In . ~the present work we have used the nonreducible geminal diol, chloral hydrate (l,l-dihydroxy-2,2,2-trichloroethane) as the H-bond donor. Experimental Section
The preparation and purification of the heteropoly salts and solvent are described e l ~ e w h e r e . Chloral ~,~ hydrate (Fisher) was used without further purification. Solutions for nmr measurements were prepared freshly by direct weighing and contained T M S as an internal standard. Results
The chemical shift of the a-methylene protons of the tetrabutylammonium ion was found to be unaffected by the presence of various concentrations of the anions investigated and indicates that no significant ion pairing of these species occurs in nitrobenzene. Although the chemical shift of the two hydroxy protons of chloral hydrate was measured for the determination of the extent of heteroconjugation, the peak of third hydrogen could also be observed. The ratios of the heights of the integrated signals were always 2:l in the concentration range investigated (15-900 mM). The chemical shift of the two hydroxy protons was independent of concentration in solutions more dilute than 0.1 M , but small downfield shifts could be observed in more concentrated chloral hydrate solutions. Assuming that hydrogen-bonded dimers are formed in such solutions, the dimerization constant (in nitrobenzene a t 37') is calculated to be 0.164 f 0.039 M - l . Because of the limited solubilities of the polyanion salts in nitrobenzene, it was only necessary to take dimerization of the chloral perchlorate into account for the perchlorate solutions. The concentration ranges used were as follows: teThe Journal of Physicai Chemistry, Vol. 79, No. 1, 1975
trabutylammonium perchlorate, [A-] , 10-250 mM; chloral hydrate, [HzB], 15-125 mM; mole ratios [A-]/[HzB], 0.5-5. For these solutions the relative chemical shifts, A, ranged from 0.02 to 0.65 ppm. The corresponding figures for the polyanions were as follows: [An-], 10-60 m M (MosO&), 7-28 m M (12-heteropoly anions); [An-]/[H2B], 0.3-4. The heteroconjugation constants K = [AH,B"]/[A"][H,B] were calculated by the interative procedure described previo~sly.~ In this procedure the relative chemical shift of the fully complexed chloral hydrate, - ~ A H ~ B ~is- , also determined. This parameter could be calculated with reasonable accuracy only for the 12-molybdosilicate complex, and its value, &H@= 1.39 ppm, was used in the calculations for the other polyanions. Using this value, and the heteroconjugation constants listed in Table I, it was possible to reproduce the observed chemical shifts to within the limits of measurement for all the solutions studied.
Discussion The formulation of chloral hydrate as a geminal diol is well established by X-ray a n a l y ~ i s ,c~r,y~o ~ c o p yand , ~ I7O and l H nmr.* The equilibrium constant for C13C CHO H2O + C13C CH(0H)z is 2.8 X lo4 a t 25' in water.g That the situation is similar in nitrobenzene solutions is shown by the constancy of the ratio of the proton nmr signals. The formation constant for the perchlorate complex with chloral hydrate is similar to that observed3 for the corresponding vicinal diol, ethylene glycol, but is much smaller than that for the aromatic diol, pyrocatechol (20.6 A4-l). On the other hand, although the stabilities of chloral hydrate complexes of perchlorate and 12-molybdophosphate are similar, we could detect no complex formation of the heteropoly anion with pyrocatechol.*JOPart of this failure may lie in - , the fully complexed H smaller chemical shifts, A A H ~ B ~for donor. The AAH~B- value for the perchlorate-chloral hydrate complex is about three times larger than that for the perchlorate-catechol complex. However, even allowing for this factor, and for solubility limitations, it is evident that H-bonded pyrocatechol complexes of polyanions are not as stable as those of perchlorate.ll The formation constants listed in Table I provide the first direct quantitative demonstration of the very low surface charge densities of isopoly and heteropoly anions. Unlike the perchlorate ion, the surface oxygen atoms of poly-
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Communications to the Editor TABLE I: Formation Constants of Heteroconjugates of l,l-Dihydroxy-2,2,2-trichloroethane with Some Inorganic Anions in Nitrobenzene at 37" Anion K, M-l c104-
M06019~PM0120403SiMo120404 pwizo403-
2.55 f 0.39 1.76 It 0 . 4 1 3.11 i 0 . 6 3 24.7 f 2 . 7 1.30 f 0.45
anions are of two types, terminal and bridging.l2 It seems likely that protonation or hydrogen bonding would take place a t the bridging oxygen atoms, since the terminal oxygens are strongly polarized toward the interior of the anion by x bonding to the metal atoms.13 The differences in stabilities of the molybdate complexes cannot simply be accounted for in terms of electrostatic (-4 us. -3) arguments, but also show influences of the central atom upon the surface charge density. The central atom has elsewhere been shown to affect reduction potentials and spectroscopic properties of 1:12 heteropoly anions.14 Finally we note the variation between the complexes of PM012O4o3- and PW120403-. We have pointed out elsewhere15 that, other things being equal, heteropoly molybdates are somewhat stronger Br4nsted bases than the corresponding heteropoly tungstates, and the present results are consistent with this.
Acknowledgments. The support of this research by AFOSR, through Grant No. AF70-1833, is gratefully acknowledged. References and Notes (1) (2) (3) (4) (5)
Author to whom correspondence should be addressed.
L. Barcza and M. T. Pope, J. Phys. Chem., 77, 1795 (1973).
L. Barcza and M. T. Pope, manuscript in preparation. L. Barcza and M. T. Pope, J. Phys. Chem., 78, 168 (1974). S. Kondo and I. Nitta, X-Sen, 6, 53 (1950) [Chem. Abstr., 45, 3237 (1951)]. (6) K. Ogawa, Bull, Chem. SOC.Jap., 36, 610 (1963). (7) S. R. Jain-and S. Soundararajan, Tetrahedron, 20, 1589 (1964). (8) P. Greenzaid, A. Luz, and D. Samuel, J. Amer. Chem. SOC., 89, 749 (1967). (9) L. C. Gruen and P. T. McTigue, J. Chem. SOC., 5217 (1963). (10) Upon heating the solutions of motybdates and pyrocatechol, reduction of the polymolybdates occurred. In the case of hexamolybdate a stoichiometric reaction produced 1,2-benzoquinone(ref 3). (1 1) Earlier results with pyrocatechol led us to state that 12-molybdosilicate and -phosphate anions were less subsceptible to hydrogen bonding than was perchlorate anion (ref 2). The present results show that this statement is not necessarily valid for chloral hydrate complexes. (12) In the Keggin (1:12 heteropoly anion) structure there are two types of bridging oxygen atom. (13)'A referee has pointed out that Strandberg interpreted long Mo-0 distances in H e P M o ~ 0 3 ~as ~ - evidence for protonated terminal oxygens (Acta Chem. Scand., Sect. A, 26, 217 (1974)).This seems reasonable. but the molybdenum atoms in question each have two terminal oxygen atoms. In the Keggin and Mo60& structures each metal has a single unshared oxygen. Reduction of charge density on the oxygen by T bonding is expected to be more efficient in the latter case. (14) M. T. Pope, Polym. Prepr., Amer. Chem. Soc., Div. Polym. Chem., 13, 787 (1972). (15) E. Papaconstantinouand M. T. Pope, lnorg. Chem., 6, 1152 (1967).
COMMUNICATIONS TO THE EDITOR
Evaluation of Dielectric Permittivity by Time Domain Spectroscopy Publication costs assisted by the Materials Science Program, Brown University, with support from the National Science Foundation
Sir: We present here simple formulas for evaluating complex permittivity e * ( ; ~a)t frequencies u = w / 2 x from Laplace transforms of voltage pulses V O( t) and R ( t ) incident on and reflected from a dielectric sample in coaxial lines, as observed by time domain spectroscopy (tds).' From transmission line and network theory,2 the input admittance y IN of a dielectric sample in length d of coaxial line terminated by an admittance Y d is Y I N = ( y o + y d ) / ( l Z a d ) , where y o = iwC.d((tanh x ) / x ) and 2 , = i w L cd ((tanh x )/x ) are the open-circuit admittance and short-circuit impedance of the dielectric section, C, and L, are the geometric capacitance and inductance per unit line length, x = i o €*ll2d/c,and c = (LcCc)-1'2.y IN is related to the transforms u o ( i w ) and r ( i w ) of V , ( t ) and R ( t ) by
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= Gc(uO r ) / ( u g - r ) , as the input voltage is u g - r for our sign convention and the input current is G, ( u o r ) , where G, = (Cc/Lc)1/2. For a sample inserted in a matched line, Y d = G,, and combining the preceding equations gives
YIN
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The approximation x coth x = 1 - 3/3( w d / c ) * c * gives an explicit solution for € * valid for ( 1 / 4 5 ) ( ~ d / c ) %
