4362
J. Phys. Chem. 1983, 87, 4362-4363
Nuclear Magnetlc Resonance Spectrum of the Sodium Anlon In Sodium-N,N-Diethylacetamide Solutlons: The Sodlde Ion in the Presence of a Carbonyl Moiety P. P. Edwards,” S. C. Guy, D. M. Holton, UniversnY Chemical Laboratory, Cam-
D. C. Johnson;
CB2 1EW. England
M. J. Slenko,
Baker Laboratow of Chemlstty, Come11 Unlverslty. Itheca, New York 14853
W. McFarlane, and B. Wood Department of C k m k w , CW of London Poktechnk, London EC3N 2EY. England (Received July 29, 1983)
We report the q a NMFt spectrum of the sodium anion, Na-, in fluid solutionsof sodium in N,N-diethylacetamide (DEA);even in the presence of the (amide) carbonyl moiety, the sodide ion exists in this solvent as a stable, gaslike centrosymmetric species with two valence electrons residing in an unperturbed 3s orbital on sodium. and carbon-13 (13C) NMR spectra are consistent with this picture, with solvent nuclei showing Proton (‘H) only minor shifts from the pure DEA situation.
A conventional view of metals is based on the assumption of cation formation in highly polar solvents.’ However, the existence of stable, long-lived metal anions in the gas phase has been recognized for at least 30 year~.~B Although Binge13*first gave a guarded prediction for the existence of a species of stoichiometry Na- in fluid sodium-ammonia solutions, there is no conclusive evidence for a genuine alkali metal anion3bin these systems.” Here the majority of diamagnetic species are well accounted for in terms of dielectron entities (e22-)s,and corresponding metal-based triple-ion complexes,4 e.g., e;K+e;, etc., where the subscript s denotes a solvated species. In contast, by 1973, the accumulated evidence, both direct and indirect, for an alkali metal anion species, M-, in amine and ether solutions was very ~ t r o n g . ~This ~ ? ~culminated in the isolation and characterization of a crystalline compound containing a cryptated sodium cation, NaC+, and a sodium anion, Na-.6 The fingerprint NMR spectrum of Na- in a solution of NaC+.Na- in anhydrous ethylamine has also been reported.’ From thermodynamic considerations, Dye5 and Schindewolf et al.* have since shown that the stabilization of Na- in these three-component systems is largely due to the intrinsic stability of the complexed, or coordinated, sodium cation toward electron reduction by N e . Nonaqueous solvents may also, of
themselues, fulfill many of the requirements generally sought from (added) cryptand and crown-ether complexing agents. These include high metal solubility via cation complexation, intrinsic stability toward electron reduction from either e; or Na-, and relatively weak anion solvating p r o p e r t i e ~ . ~ JA~ recent pulse radiolysis study” of the solvent DEA revealed a long-lived absorption from e;, and in solutions of sodium tetraphenylborate (NaBPh,) in DEA, a transient absorption characteristic of a species of stoichiometry Na- (see also ref 12). These observations are in contrast to the more familiar, facile reduction of the carbonyl group by the solvated electron in metal solut i o n ~ . ’ ~In the Birch reduction, for example, amides are readily reduced by metal-ammonia or metal-amine solutions to give aldehydes in good yield.’, However, the detection of the 2SNaNMR spectrum of the sodide ion in DEA, reported herein, demonstrates unequivocably that Na- can exist as a stable, long-lived species even in the presence of the carbonyl moiety. Scrupously clean glassware and stringent high-vacuum conditions (ca. lo4 torr) are essential for the preparation of stable blue metal solutions in DEA. The solvent was purified by two high-vacuum distillations: first, from calcium hydride and NaK alloy, and secondly, from pure, freshly prepared NaK alloy. Sodium purification was achieved by multiple high-vacuum distillation in the sample preparation cell. NMR spectra were recorded on a Joel-FXSOQ pulsed Fourier transform multinuclear spectrometer operating at frequencies of 23.64,6.42, 22.50, and 89.56 MHz, respectively, for the following nuclei: 23Na(I = 3/2; abundance loo%), 14N(I = 1; 99.6%), I3C (I= 1/2; l . l % ) , and ‘H(I= 1/2; 99.99%). The magnetic field was
w+(%2-)B,
(1) See, for example, (a) Parish, R. V. “The Metallic Elements”; Longman: London, 1977. (b) Barnard, A. K. “Theoretical Basis of Inorganic Chemistry”) McGraw H i k New York, 1965. (2) For a review see Hotop, H.; Lineberger, W. C. J. Phys. Chem. Ref. Data 1975,4, 539. (3) (a) Bingel, W.Ann. Phys. 1953.12, 57. (b) Golden, S.;Guttman, C.; Tuttle, P. R. J. Am. Chem. SOC.1965, 87, 135. (c) Dye, J. L. In “Electrons in Fluids”, Colloque Weyl III; Jortner, J.; Kestner, N. R., Ed.; Springer-Verlag: Berlin, 1973; p 77. (4) (a) For recent calculations on the spin-paired species in metalammonia solutions, see Kestner, N. R.; Rao, B. K.; Finley, C. W. J.Phys. Chem. 1983, 87, 1464. (b) For a recent review, see Edwards, P. P. In “Advancea in Inorganic Chemistry and Radiochemistry”;Emeleus, H. J.; Sharpe, A. G. Ed.; Academic Press: New York; Vol. 25, p 135. (5) (a) See, for example, the hstorical account by Dye, J. L. Sci. Am. 1977,237, 92. (b) Dye, J. L. J. Chem. Educ. 1977,54, 332. (6) Tehan, F. J.; Barnett, B. L.; Dye, J. L. J.Am. Chem. SOC.1974,69, 7203. (7) Dye, J. L.; Andrews, C. W.; Ceraso, J. M. J.Phys. Chem. 1975, 79, 3076. (8) Schindewolf, U.;Le, Long Dink Dye, J. L. J.Phys. Chem. 1982, 86, 2284. 0022-3654/83/2087-4362$01 .SO10
(9) Dye, J. L. In “Progress in Macrocyclic Chemistry”; Izatt, R. M.; Christensen, J. J., Ed.;Wiley-Interscience: New York, 1979; Vol. I, pp 63-113. (10) Edwards, P. P.; Guy, S. C.; Holton, D. M.; McFarlane, W. J. Chem. Soc., Chem. Commun. 1981, 1185. (11) Guy, S.C.; Edwards, P. P.; Salmon, G. A. J . Chem. SOC.,Chem. Commun. 1982, 1257. (12) Young, C. A.;Dewald, R. R. J.Chem. Soc., Chem. Commun. 1977, 188. (13) Birch, A. J.; Cymeman-Craig,J.; Slaytor, M. Aust. J . Chem. 1955, 8,512. (14) Birch, A. J.; Smith, H. Q. Reu. Chem. SOC.1958, 12, 17. Birch, A. J.; Subba Rao, G. Adu. Org. Chem. 1972,8, 1 .
0 1983 Amerlcan Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 22, 1983
Letters n F i
CHT-C-
I
I -62.9
I /
3.322.03 1.06
in CH3-!!-N~zcH31
x -C11
CH2CH3
* R CH3-C-
0
I
I68 8
412 218140
Figure 1. Multinuclear ("Na, 13C, 'H) NMR spectra of a solution of sodium in dlethylacetamlde at 203 K. Resonance shifts for are relative to Na+ in D,O; for ''C and 'H, relative to Me,SI (see text).
stabilized by the deuterium signal from an external D20 sample in the probe. Resonance shifta (uncorrected for bulk susceptibilities) were measured by substitution relative to a solution of 1M NaCl in D20 for 23Na,and relative to MelSi for 'H and resonances. We adopt the accepted convention that NMR shifta (6) have the same sign as frequency shifts, with a negative 6 corresponding to a low-frequency shift and an increase in nuclear shielding; viz. 6 = lo(
-)
VNa - vref vref
where vNa and vref are, respectively, the sample frequency and the reference frequency for a solution of Na+(NaCl) in D20, and ( V N ~- Vre3 a 4 6 - are&* ~ ~ Figure 1shows typical 23Na,13C,and 'H NMR spectra from a Na-DEA solution a t 203 K. We have been so far unable to detect any signal from the quadrupolar 14N, presumably because of ita extreme width; this also appears to be the reason for the absence of any 23Na+resonance in Na-DEA and Na-HMPA'O solutions. Indeed, at 203 K the 23Na+resonance from a solution of the salt NaBPh,
4363
in DEA is some 3500 Hz wide. The 23Nasignal in the metal solution, shifted some 62.9 ppm to low frequency of the external reference has a ) 8 Hz; both feapeak-width at half-height ( A V ~ ,of~ only tures are characteristic of the sodium anion in s o l ~ t i o n . ~ * ~ The virtual absence of any solvent-inducedvariation of the chemical shift for Na- indicates a genuine, spherically symmetric anion existing in the solution (e.g., HMPA, -62.0 ppm at 263 K; CH3NH2, -62.6 ppm at 258 K;799 CH,CH2NH2,-62.8 ppm, 256-274 K'). Thus, the sodium 2p electrons in Na- are completely shielded from specific solvent interactions by the 3s electrons on the metal. Holz and Zeidler15 also suggest that the large radius of Nacompared to Na+ may significantly reduce quadrupolar relaxation in the metal anion. 13C and 'H NMR spectra are very similar to those observed from pure DEA. Chemical shifts in the metal solution (Figure l, referenced to Me,Si) are generally small and negative; for example, relative to the pure solvent resonance, a shift of -0.46 ppm for the l 3 C 4 resonance and -0.26 ppm for the amide -CH3 protons. This direct NMR evidence for the stability of Na- in the presence of the carbonyl group raises some interesting questions concerning electron attachment and solvation in these systems. Clearly, formation of the complex Ntg+Na- effectively further inhibits electron reduction of the (coordinated) solvent molecules. Furthermore, the relatively large electron affinity2of the sodium atom (0.546 eV>provides an effective potential trap for the detached electron originating from the parent atom in the dissolution process; viz. 2Na0
-
Nq+Na-
Given the choice of carbonyl vs. sodium-atom reduction in this system, the excess electron clearly favors sodide ion formation!
Acknowledgment. This work was supported by the SERC (U.K.) and NSF (Grant No. 20324). Funds for the continuing U.K./U.S.A. collaboration were kindly provided by NATO Grant No. 250.81. We also gratefully acknowledge the award of a Trinity College Research Fellowship to S.C.G. Registry No. Na-, 19181-13-6; Na, 7440-23-5. (15) Holz, M.; Zeidler, M. D. In "Nuclear Magnetic Resonance";The Chemical Society: London, 1977; Spec. Period. Rep., Vol. 6, p 92.
