1429
Communications to the Editor
breiter, D. E. J. Am. Cbem. SOC. 1979, 101, 5654. Estimation of Strength of the "Three-Electron Shapiro, R. H.; Lipton, M. F.; Kolonko, K. J.; Buswell, R. L.; Capuano, L. A. u Bond" of a Hexaalkylhydrazine M3nocation Radical TetrahedronLett. 1975, 1181. Dauben, W. G.; Rivers, G. T.; Zimmerman, W. T.; Yang, N. C.; Lim, B.; Yang, J. bid. 1976, 2951. Beam. C. F.; Dyer, M. C. D.; Schwarz. R. A.; Hauser, C. R. J. Org. Chem. Sir: 1970. 35. 1806. Kofron, i .G.; Yeh, M.-K. J. Org. Cbem. 1976, 41, 439. One-electron reduction of tricyclic hexaalkylhydrazine diJung, M. E.; Blair, P. A,; Lowe, J. A. TetrahedronLett. 1978, 1439. Lyle, cations 12+ and Z2+ has been shown to give very long-lived R. E.; Fribush, H. M.; Lyle, G. G.; Saavedra, J. E. J. Org. Chem. 1978, 43, red radical cations which appear to be best described as having 1275. Bellassoued, M.; Dardoize, F.; Frangin, Y.; Gaudermar, M. J. Or@nomet. Cbem. 1979, 165, 1. "three-electron u bonds", i.e., with one electron in an orbital Seebach, D.; Wykypiel, W.; Rubosch, W.; Kalinsowski, H.-0. Helv. Chem. which is largely u* N-N in character. W e report here that the Acta 1978, 61, 3100. less constrained dication 32+ gives a similar but much shorter Beak, P.; McKinnie, G.; Reitz, D. B. Tetrahedron Lett. 1977, 1839. Fraser, R. R.; Grindley, T. B.; Passannati. S. Can. J. Cbem. 1975, 53, lived species, whose lifetime allows estimation of the strength 2473. of the three electron bond for this molecule. Fraser, R. R.; Ng.. L. K. J. Am. Cbem. Sac. 1976, 98, 5895. Owen, N. L.; Sheppard, N. Trans. FaradaySoc. 1964, 60, 634. Epiotis, N. 1976, 98, D.; Yates, R. L.; Bernardi. F.; Schlegel, H. B. J. Am. Chem. SOC. 2385. Bouma, W. J.; Poppinger. D.; Radom, L. bid. 1977, 99, 6443. Jones, G. I. L.; Owen, N. L. J. Mol. Struct. 1973, 18, 1, and references therein. Messe, C. 0.;Walter, W.; Berger. M. J. Am. Chem. SOC.1974, 96, 2259. Lumbroso, H.; Pappalarado. G. C. J. Mol. Struct. 1978, 43, 96. The hydrocarbon of this type, the syn-crotyl anion, becomes relatively m e stable in solution as the solvent is changed from hexane to THF and as the 22+ 32+ 12+ counterion is progressively changed from Li+ to C S + . ~However, ICR measurements imply that the anti-crotyl anion is 0.2 kcal/mol more stable Solvated electrons were produced by pulse radiolysis4 of than the syncrotyl anion in the gas phase,5 while 4-31G calculations N2-saturated water containing 0.1 M tert-butyl alcohol, to suggest the syn anion is 1.5 kcal/mol more stable.6 (a) Epiotis, N. D.; Bjorkquist, D.; Bjorkquist, L.; Sarkanen. S. J. Am. Cbem. scavenge the hydroxyl radicals produced, and 1-2 X MI SOC.1973, 95, 7558. Epiotis, N. D.; Sarkanen, S.;Bjorkquist, D.; Bjorkquist, dication. Decay of optical absorption of eaq- was monitored L.; Yates, R. J. Am. Cbem. Soc. 1974, 96, 4075; Epiotis, N. D.; Cherry, W. R.; Shaik, S.;Yates, R. L.; Bernardi, F. Fortscbr. Chem. Forsch. 1977, 70, a t 600 nm and was accompanied by growth of monocation 1. (b) Larson. J. R.; Epiotis, N. D.; Bernardi. F. J. Am. Cbem. SOC. 1978, radical 1+--3+. absorption. All three dications react rapidly 100, 5713. with eaq- (see Table I), although the rate constant for 12+ reSee, for example: Pople, J. A. Tetrahedron 1974, 30, 1605. Brunck, T. K.; Weinhold, F. J. Am. Cbem. SOC.1979, 101, 1700 and references duction is significantly lower than that for the other two. A therein. larger geometry change upon accepting an electron is suggested See, for example, Eliel, E.; Allinger, N. L.; Angyal, S. J.; Morrison, G. A. "Conformational Analysis"; Wiley-Interscience: New York, 1965, pp for system 1 by the 14.7-G nitrogen splitting constant of 1'460-462 and references therein. (indicating nearly planar nitrogens) than for 2, because a(N) (a) P. Beak proposed such an explanation to account for relative stabilities of 2+* is 34.4 G (indicating nearly tetrahedral nitrogens).lS2 of species closely related to those considered here: Phillips, W. G.; Ratts, K. W. J. Org. Cbem. 1970, 35, 3144. (b) The preference for the syn conInterestingly, these dications are not reduced by isopropyl alformation of methyl formate was carefully analyzed and found to be comcohol radical (.CMe*OH, generated by pulse radiolysis of patible with dominant electrostatic effects: Wennerstrom, H.; Forsen, S.; Roos, B. J. Phys. Cbem. 1972, 26, 2430. N2O-saturated water containing 10-1 M isopropyl alcohol), Hehre. W. J.; Ditchfield. R.; Newton, M. D.; Pople, J. A,, GAUSSIAN 70, O C E despite the fact that the electron transfer is quite exothermics 236. A substantial steric effect on the rate of electron transfer apUnbound HOMOS, a theoretically untenable situation, are frequently encountered. However, all of the anions described here have slightly negative pears to be involved. All three radical cations have very similar HOMO energies at the 4-31G level. Calculations with diffuse s functions absorption spectra (see Table I). Those observed for 1+. and in the basis set are in progress. Radom, L. Aust. J. Cbem. 1976, 29, 1635. 2+- agree well with data obtained by conventional methods, Boerth, D. W.; Streitweiser, A., Jr. J. Am. Cbem. SOC. 1978, 100, 750. although a significantly higher t value was observed for 1+- in Previous ab initio calculations with standard or partially optimized this work. We presume that partial decomposition had ocgeometries predicted the anti imine to be 2.2 kcal/mol more stable than the syn (Radom, L.; Hehre, W. J.; Pople, J. A. J. Am. Chem. SOC. 1971, 93, curred in the samples of 1+ previously prepared.' 289), and the syn 1.7 kcal/mol more stable than the anti (Stolkin, I.;Ha, Although 1+. and 2+-are known to be long lived, the optical T.-K.; anthard, Hs. H. Cbem. Phys. 1971, 21,327). Ex erimentally, antiimines are 22 kcal/mol more stable than the ~ y n . ~ ~ ~ ~ absorptions ~ of these species were found to disappear partially Parry, K. A. W.; Robinson, P. J.; Sauisbwy, P. J.; Waller, M. J. J. Chem. Soc., in a rapid process. This is indicated by the optical spectra be6 1970, 700. Hine, J.; Yeh, C. Y. J. Am. Cbem. SOC. 1967, 89, 2669. fore and after this rapid decay which are identical except for Indirect evidence for the 4 f 2 kcal/mol greater stabilily of I1than I has been cited: Teysseyre, J.; Arriau, J.; Dargelos, A,; Elguero, J.; Katritzky. A. R. intensity. The decay process essentially followed second-order Bull. Cbem. SOC.Belg. 1976, 85, 39, and references therein. kinetics. These facts would indicate that the radical cations Since 4-3 1G calculations overestimate charge ~eparation,'~ most of these calculations have also been Derformed at the STO-3G//4-31G level(1). produced react with another transient produced by the pulse Essentially identical energy differences between isomers are found. which we suggest to be the C H 2 C M e 2 0 H radical. This latter (40) Binkley, J. S.;Pople, J. A. Cbem. Pbys. Lett. 1977, 45, 197. species is formed by the reaction of OH- radicals and H- atoms (41) Although all types of quantum mechanical interactions are based in eiectrostatics, we consider the "electrostatic effects" discussed here as priwith tert-butyl alcohol and in a yield exceeding that of the marily a result of differences in electron-electron repulsion in stereoisoradical cation only by 20-25%. The only partial decay of the meric species. This electrostatic effect is, therefore, a twoelectron effect not included in one-electron, or "orbital interaction", effects. radical cation absorption is explained by the fact that the (42) These calculations were performed with a program kindly furnished to us eCH2CMe2OH 1+. (2+. or 3+.) reaction ( k 2 -2-3 X IO9 by Professor K. Morokuma. See Kitaura, K.; Morokuma, K. lnt. J. Quantum M-' s - l ) has to compete with the bimolecular decay of two Cbem. 1976, 10,325. (43) Calculations using the point charge approximation and Mulliken charges aCH2CMe20H radicals (2k2 = 1.3 X 1O9 M-I s-I lead to the same conclusion. Because 2+. cannot be reacting by addition to the weak three (44) The stabilities of cis-I,2difluoroethylene and related haloalkenes are examples which cannot be explained on the basis of electrostatic efelectron u bond as this would require a nitrogen inversion feCts,28.40.45 which is far too costly in energy to be consistent with the rapid (45) Skancke, A,; Boggs, J. E. J. Am. Cbem. SOC. 1979, 101, 4063.
+
1.
K. N. Houk,* Robert W. Strozier, Nelson G . Rondan Department of Chemistry, Louisiana State University Baton Rouge, Louisiana 70803
Robert R. Fraser,* N. Chuaqui-Offermanns Department of Chemistry, University of Ottawa Ottawa K I N 9B4, Canada Received October IO, 1979
0002-78631801 I502-1429$01 .OO/O
Table I. R a t e Constants for Reaction of Hexaalkylhydrazine Dications w i t h eao-, and Absorption M a x i m a
k z , M-' starting compound
s-l,
for eaq-
t dication
of the Products
cation radical, A,, n m ( E , M-l cm-1)
I*+
1.4 X 1O'O
480 (2600)
22+
5.0 X 10'0 4.9 x 10'0
470 (4500) 470 (4600)
3*+
0 1980 A m e r i c a n Chemical Society
I430
Journal of the American Cheniical Society
reaction observed, an a-hydrogen abstraction is presumed to be occurring in a t least this case. As expected, once the tertbutyl alcohol radical is consumed, the optical signals for 1+. and 2+-are completely stable for over 10 s (nonflowing conditions, the slowest time scale that could be employed using the pulse radiolysis equipment). In contrast, the 3+. optical signal decreases by a relatively slow first-order process after the initial fast reaction with mCH2CMe20H is completed. The observed rate constant for 3+. disappearance is 135 f 15 s-I a t 25 OC, pH independent between 2.7 and 5.3 (the lower limit is imposed by the decreased yield of 3+. when the eaq- H + reaction consumes too great a fraction of the electrons produced, and the upper limit by decomposition of 32+).We suggest that this uncatalyzed first-order decomposition of 3+. is caused by thermal cleavage of the three-electron u bond. This cleavage allows the nitrogens to move apart until they no longer interact significantly, and the acidic amine radical cation6 and basic free amino group produced will undergo very rapid, irreversible net proton transfer a t the p H employed; cleavage of the R3N--NR3+ bond should be irreversible. The 5-ms observed half-life corresponds to AC* of 14.5 kcal/mol using the Erying equation, which we suggest is an experimental measure of the strength of the three-electron r~ bond of 3+-. The tricyclic radical cations 1+. and 2+- are structurally prevented from cleaving the N - N bond and have solution lifetimes of hours and months,
+
References and Notes (1)Alder, R. W.; Sessions, R. 6.: Mellor, J. M.; Rawlins, M. F. J. Chem. Soc., Chem. Commun. 1977,747.
(2)Alder, R. W.; Sessions, R. B. J. Am. Chem. SOC. 1979, 101, 3651. (3) N,NDimethylpiperazine (Snyder, H. R.; Michels, J. G. J. Org. Chem. 1963, 28, 1144)was heated with excess 1,4dibromobutane at 100 OC for 15 min. The resulting monoquaternary salt was treated with AgBF4 in aqueous HBF4 and after filtration with AgBr, 3 (BF4)2, mp 166-167 O C , was precipitated upon addition of ethyl alcohol. Satisfactory analytical and spectroscopic data were obtained. In particular the I3C NMR in D20/D2S04 at 25 O C showed signals at 16.4(double intensity), 49.7,61.6,and 65.6ppm, and the signals at 61.6and 65.6coalesced to a single line at 60 OC,showing that the cis isomer had been obtained. An attempt to prepare the trans isomer by double methylation of 1,6diazabicyclo[4.4.O]decane with MeOS02F led to no more than monoalkylation. (4) The 1.6-and 4.0-MeV van de Graaff generator pulse radiolysis apparatus at the H.M.I. was employed in this work. (5) The half-wave potential of CMe20H has been reported to be -1.3 V vs SCE: Lilie. J.; Beck, G.; Henglein. A. Ber. Bunsenges. Phys. Chem. 1971, 75, 458. (6)Kelly, R. P.; Lindsay, Smith, J. R. J. Chem. SOC., Chem. Commun. 1978,
329.
Stephen F. Nelsen* S . M . McEloain Laboratory of Organic Chemistry Department of Chemistry, Unicersity of Wisconsin Madison, Wisconsin 5.3706 Roger W. Alder, Richard B. Sessions Department of Organic Chemistrji The Unicersity, Bristol BS8 17S, England Klaus-Dieter Asmus, Karl-Otmar Hiller, Manfred Gob1 Bereich Strahlenchemie Hahn-Meitner-Institut fiir Kernforschung Postfach 28 01 39, 0-1000 Berlin 39, West Germany Receiced October 29, I979
Synthesis of a Class of Complexes Containing Tungsten-Tungsten Quadruple Bonds Sir: Despite the large number of complexes of Mo containing quadruple metal-metal bonds, few are known for W . ' This probably is due largely to the fact that a W analogue of M02(0Ac)4 (the major starting material for preparing com0002-7863/80/ 1502- I430$0 I .OO/O
1 102:4 /
February 13. 1980
plexes containing quadruple bonds) has not yet been prepared.2 We have been attempting to systematize the chemistry of tungsten halides and have found that a class of complexes with the formula W2C14L4 ( L = a phosphine ligand) can be prepared readily, either by reducing [WC14], with sodium amalgam in the presence of L or by pyrolyzing monomeric W(I1) complexes. The former demonstrates that, once a metal-metal bond of order greater than one is present, it will quite likely survive upon reducing the metal to give a bond of higher order.3 Phosphines react fairly slowly with WCl4 suspended in T H F to yield WC14L, (x = 2 or 3) complexes. However, when 2 equiv of sodium amalgam (0.4%) is added to a stirred mixture of WC14 and PBu3 in T H F at -20 O C and the mixture is warmed to room temperature, WC14 is consumed and the solution turns blue-green. W ~ C I ~ ( P Bcan U ~be) ~isolated in high yield after filtering the mixture through Celite. Analogous green to blue-green complexes (1) containing PMe3, PMezPh, or PMePh2 can be obtained similarly i n 60-80% yield (eq I ) . W e have also used this method to prepare more simply and directly one member [blue M O ~ C I ~ ( P B U of~the ) ~ well-known ] class of analogous molybdenum c ~ m p l e x e s . ~
+
+
THF
WC14 2Na/Hg 21- +W2C14L4 (1) L = PMe3, PMezPh, PMePh2, or PBu3
(I)
The reaction fails to give W2C14(L-L)2 (2, L-L = dnipe or diphos) directly. These must be prepared by displacing PBu3 from WzC14(PBu3)4 in toluene a t 80 O C . Sparingly soluble, brown W2C14(diphos)2 or green WzCId(dmpe)2 crystallize from the reaction mixture in 60 and 95% yields, respectively. Actually brown W2C14(diphos)2 contains -1 0% green crystalline form. (Two forms of Mo2C14(diphos)2 were also isolated.5) The formulations of 1 and 2 are based on elemental analyses6 and the following data (cf. the analogous Mo comp l e x e ~ ~All ~ ~are ) . air stable in the solid state and only moderately sensitive in solution. A cryoscopic molecular weight determination for W2Cll(PBu3)4 (calcd 13 17; found 1320) and parent peaks in the mass spectra of WzCl4(PMe3)~and W2C14(dmpe)2 show that they are dimers. The 3 1 P and I 1 N M R spectra are virtually identical with those reported for the Mo2C14L4 complexes except for the presence of l S 5 W satellites in the 3 i P spectra.' A Rainan spectrum8 of W ~ C I ~ ( P B revealed U ~ ) ~ an intense peak at 260 f I O em-' which we tentatively assign as the tungsten-tungsten stretch. I f only 1 equiv of sodium amalgam is used, a mixture of WC14 and PMe3 yields a red solution from which red, crystalline W2CI6(PMe3)4 (3a) can be isolated in 75% yield. We believe that 3a has a structure analogous to the known W2Cl6Py4 (3b)9 based on the fact that its ' H N M R spectrum shows two types of phosphine ligands. The reduction of 3a with 1 more equiv of sodium amalgam/W yields W2Cl~(PMe3)4 in 80% yield.
3a, L = PMe, b, L = Py c, L = THF
If phosphine is absent, 1 equiv of sodium amalgam reduces WC14 to give a greenish yellow solution in which greenish yellow crystals form on addition of pentane. Since the ' H N M R spectrum of this complex shows two types of T H F l i 0 1980 American Chemical Societ]
