5234 Table I. Massbauer Data Temu, OK
AEQ, mmisec 1.oo 1.03
RTd 78
mmlsec 0.33 0.44
r1,6mmisec
rZPmmisec
Areac ratio
Peak heightc ratio
0.311 f 0.010 0.266 f 0.005
0.263 f 0.009 0.264 f 0.005
1.13 1.03
1.34 1.04
The error limits are the standard deviations as calculated from the variance of the line width. a Relative to natural iron foil. R T = room temperature. line with least shift t o line with largest shift from zero velocity.
for a five-coordinate mononuclear complex of iron with The most striking features of the Mossbauer spectrum are the distinct asymmetry exhibited by the two lines split by the quadrupole interaction at room temperature and the decrease in this asymmetry with decreasing temperature. It is apparent from Table I that although the temperature dependence is consistent with the Karyagin12 asymmetry the line-width considerations are not. The line width for each line at 78°K is essentially the same, but at room temperature the line of most positive velocity is distinctly broadened. Thus, it would appear that the Karyagin effect cannot account for the asymmetry in the line widths at room temperature but may be responsible for the small asymmetry observed at 78°K. The observed asymmetry at room temperature could result from spin-spin relaxation effects. l 3 The magnetic susceptibility of the dimer was studied as a function of temperature; in Figure 2 the measured values of perf are compared with values calculated14
N,N’-ethylenebis(salicy1ideneiminate).
c
Ratio of
oxygen2, J = - 100 cm-’;l0 for a complex with planar three-coordinate oxygens, J = - 17 cm-l for the complex reported in this paper; and J = -7 cm-l l o for a complex with pyramidal three-coordinate oxygens.3 Hence, it would appear that the extent of spin-spin exchange can be related directly to the availability of T orbitals on the bridging oxygen. Acknowledgments. This work was supported in part by NSF Grant No. GP-8475. The computational help of the Rich Electronic Computer Center of Georgia Institute of Technology is appreciated. G. J. L. wishes to thank the NSF for Graduate Research Equipment Grant No. GP-8653 and Project Themis Grant No. AFOSR F-44620-69-C-0122 for the purchase of components for the Mossbauer spectrometer. (15) Address correspondence to this author.
J. A. Bertrand,15J. L. Breece, A. R. Kalyanaraman School of Chemistry, Georgia Institute of’Technology Atlanta, Georgia 30332
G. J. Long Department of Chemistry, University of Missouri-Rolla Rolla, Missouri 65401
W. A. Baker, Jr. Department of Chemistry, Syracuse University Syracuse, New York 13210 Received June 6, 1970
401 I
30r
; ‘2
1-
Catalysis of Aromatic Hydrogen-Deuterium Exchange by Metal Hydrides
50
100 T E M P E R AIT5U0 R E “ K ZOO
250
300
Figure 2. Plot of Meff US. temperature ( 0 , experimental points; calculated for J = - 17 cm-1, g = 2.00, N a = 0, and S = 6 / ~ ,6/2).
assuming antiferromagnetic spin-spin exchange between two high-spin iron(II1) ions. The experimental results are best fit with g = 2.00, J = -17.0 f 0.6 cm-l, N ( a ) = 0, and S = 5/2, 5/2; the experimental results are not consistent with the spin-spin interaction between two iron(II1) ions of lower spin multiplicity. For the oxygen-bridged iron(II1) complexes for which magnetic data and X-ray data are available there appears to be a correlation between the extent of spinspin interaction and the coordination of the bridging oxygen: for a complex with a linear two-coordinate (11) (12) (13) (14)
M. Gerloch and F. E. Mabbs, J . Chem. SOC.A , 1598 (1967). S. V. Karyagin, Dokl. Akad. Nauk SSSR, 148, 1102 (1963). M. Blume, Phys. Reu. Leu., 14,96 (1965). A. Earnshaw and J. Lewis,J. Chem. SOC.,396 (1961).
Journal of the American Chemical Society
Sir: The exchange of hydrogen between water and aromatic hydrocarbons is catalyzed by strong acid, by heterogeneous catalysts, and by a homogeneous system based on the PtCL2- ion.2 In contrast, homogeneous catalytic exchange between gaseous hydrogen and a free aromatic hydrocarbon is unknown even though activation of o-H in aromatic phosphorus ligands is well established3 for a number of transition metals. We now wish to report that the hydrides I-IV catalyze exchange between Dzand benzene, probably by oxidative addition of the C-H bond to a coordinatively unsaturated intermediate. ( r-C6H&TaH3
I
11, L
PhPEtz
111, L = EtaP
IV, L = Me3P
Dicyclopentadienyltantalum trihydride (I), like (Ph3P)2ReH7,as reported earlier by Chatt and Coffey,6 (1) J. Hine, “Physical Organic Chemistry,” 2nd ed, McGraw-Hill, New York, N. Y., 1962. (2) R. I. Hodges and J. Garnett, J . Phys. Chem., 73, 1525 (1969), and references cited therein. (3) G. W. Parshall, Accounts Chem. Res., 3, 139 (1970). (4) M. L. H. Green, J. A. McCleverty, L. Pratt, and G. Wilkinson, J . Chem. SOC.,4854 (1961).
/ 92:17 / August 26, 1970
5235 Table I. Catalysis of H D Exchange by Metal Hydrideso Theory* HD
7--
Expt
A B
Complex (mmol)
Ec
I(0.16) I1 (0.11) II(O.11) I1 (0.095) I1 (0.095)
F
(PhPEtz)JIHv
c D
Solvent CeDe Ca6 CeHe CeDe C7Hs CsH6
Gas (mmol) Temp, OC
Hz(0.15) D~(0.08) Dz (0.08) Hz(0.46) Dz(0.49) D~(0.49)
100 25 100 100 100 IO0
(0.093)
HZ
74.1 92.2
24.0 7.7
21.7 4.84 21 .o
49.8 34.3 49.7
-
Gas analysis -FoundDz Hz
1.9(9) 0.15(35)
41.4 72.6 98.5 10
Z8.5(9) 60.8(3) 29.3(9)
62.5 8.8
--
HD
Dz
41.6 24.8 1.5 38 32 38
17.0 2.6 52 5.6 53.2
~~~~~~~~
These experiments were carried out in sealed glass ampoules of 4- or 15-ml capacity to give an H? or Dzpressure of about 1 atm. The Calculated for statistical exchange of D 1with the amount of solvent used in each was 2 ml, and the standard heating period was 24 hr. Mass spectrum of the toluene indicates ca. 1 toluene-dl over the number of ligand hydrogen atoms per molecule indicated in parentheses. natural abundance. a
undergoes exchange of metal-hydrogen with CGDB. The conversion to the new product is low in a sealed tube but is complete under a Nz sweep in refluxing However, the interaction we have observed is more comtoluene. This result suggests that an equilibrium explex than that reported for the rhenium hydride. When ists between the phosphine hydride and the trihydride. a C6D6 solution of I is heated under a hydrogen atThis new complex also catalyzes the exchange of DPwith mosphere, H D and D2 appear in the vapor phase (Table benzene. The thermal evolution of hydrogen from I, experiment A). Under these conditions, deuterium could only come from the solvent. Its presence defthese complexes coupled with the exchange data suginitely indicates catalytic exchange. gest the existence of the equilibria illustrated for the tantalum complex. Similar catalysis of exchange between H2 and C6D6 or between D2 and CeH6 was observed with the iridium hydrides 11,6 and IV.' Exposure of a C6H6solu- H2 +AD (C5H&TaH3 e (C6HdZTaH (CbHdZTa-D tion of 116 to D, for 24 hr at 25" results in statistical 4-H2 -ArD exchange of nine hydrogens per molecule of iridium H ' complex (experiment B), a value which corresponds to +EtsP -?&P reaction of the five Ir-H and four o-C-H bonds. HowH ever at 100" more H2 and H D were present in the gas / phase than could be accounted for even if every hydroGH~)~T~/ (CsH&Ta-D + ArH \ gen in the complex equilibrated with the gas phase PEt3 (experiment C ) . As with (CsH&TaHs, the inverse Similar equilibria are presumed to occur with the experiment in deuteriobenzene with Hz as the exchanging gas gave a substantial amount of H D and Dz after iridium hydrides. 24 hr at 100" (experiment D). Exchange with solvent No evidence is presently available concerning the also occurs with toluene (experiment E). The sixintermediate formed by oxidative addition of an arocoordinate complex n ~ e r - ( P h P E t ~ ) J r H exchanges ~~ matic C-H bond. Aryl hydride complexes have been slightly more than three hydrogens after 24 hr at loo", a reported, lo and a tautomerism between Ru(dmpe)2substantial decline in activity for ligand exchange (exCloH8 and HRu(dmpe)zCloH, (dmpe = bis-1,2-dimethperiment F). ylphosphinoethane) was observed.I0 Garnett and In benzene solution the iridium hydrides 11-IV deHedges, have also postulated an analogous addition compose spontaneously with evolution of hydrogen, product in their exchange mechanism. Such an interbut no metal is deposited.8 When I is heated under N2 mediate could have great importance, for it provides in benzene at 80" for 15 hr, H, is detected in the gas an aromatic ring which is susceptible to electrophilic phase. Heating I in benzene with excess Et3P produces substitution. a red crystalline solid whose nmr spectrumg indicates (10) J. Chatt and J. M. Davidson, J . Chem. Soc., 843 (1965). that it should be formulated as (T - C ~ H ~ ) ~ T ~ ( H ) P E (11) ~ ~ .Address correspondence to this author.
-
41
( 5 ) J. Chatt and R. S. Coffey,J. Chem. SOC.A , 1963 (1969). (6) B. E. Mann, C. Masters, and B. L. Shaw, Chem. Commun., 703 ( 1970).
(7) Prepared by LiAlHa reduction of MesPH+[IrC14(Me3P)z]- in T H F and isolated by sublimation from the residue which remains after evaporation of solvent from the hydrolysate, The 1H nmr spectrum in toluene consists of two triplets in the rato 18 :S.5 at 7 8.26 (JP-cH~= 4 Hz) and 19.73 (JP-I~H = 14 Hz). The alp nmr consists of a symmetrical sextet (0.94 : S : 10) at 192.1 ppm from external trimethyl phosphite. The infrared spectrum shows a strong broad absorption at 1920 cm-1 for (8) A number of new hydride resonances are observed in the nmr spectra of these solutions after decomposition. The compositions of these products are being studied. (9) The proton nmr spectrum in CBDG at 220 MHz shows cyclopentadienyl absorption at 7 5.68 as a doublet of doublets (JPE = 1.7, JHE = 0.5 Hz), a five-line CHz signal (overlapping quartets, JPH= 7, J E H = 7.5 Hz) at 8.91, a five-line CHa signal (overlapping triplets, JPH = 13.5 Hz) at 7 9.16, and TaH absorption at 7 19.48 (JPH = 21 Hz). The relativeintensity ratio CSH~:CZHF,:T~H is 10:15.5:0.9.
+
E. K. Barefield, G. W. Parshal1,ll F. N. Tebbe Contribution No. 1712, Central Research Department Experimental Station, E . I. du Pont de Nemours and Company Wilmington, Delaware 19898 Received June 16, 1970
Electron Spin Resonance Study of Dialkylamino Free Radicals in Solution1
Sir, It is generally accepted that the most stable conformation of simple alkyl free radicals is a planar, sp2 hybridized form with the unpaired electron located in (1) Acknowledgment is made to the Kansas State University Bureau of General Research for support of this work.
Communications to the Editor
