J . Phys. Chem. 1984,88, 645-648 CH3-.Cu-H, recently discovered in the Cu/CH4 system6) does exhibit a local minimum and is stabilized with respect to C U ( ~ P ) + H2 by 18.1 kcal mol-' and with respect to the separated H + CuH fragments by 1.0 kcal mol-'. These calculations are interesting in the light of our experimental observations for the C U ( ~ P ) H2photochemical reaction and the CuH H thermal reaction. The lack of theoretical evidence at the R H F level for a barrier for the photochemical step and the predicted nonexistence of a symmetrical copper dihydride, CuH2, intermediate are both in line with our experimental observations. Moreover, the proposed stability of an H . 4 h - H interaction complex, with respect to isolated H CuH fragments, could help explain our experimental inability to regenerate all of the original Cu atoms at 18-20 K. This effect might have its origin (partly or entirely) in competitive recombination reactions involving differently oriented CuH H fragments in the matrix cage leading to either H CU-H H4u-H or CU-H H CU + Hz
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where the chemical outcome depends on the geometry of attack ( 6 ) R. A. Poirier, G. A. Ozin, D. F. McIntosh, I. G. Csizmadia, and R. Daudel, Chem. Phys. Lett., 101, 221 (1983). (7) D. Bhattacharya and J. E. Willard, J . Phys. Chem., 85, 154 (1981).
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of H on CuH (cf. ref 4). Other pathways which might operate simultaneously involve H and Cu atom recombination reactions to yield species such as Cu,, H2, CuH, and Cu,H. However, these may not be of major importance as H and Cu atom diffusion and agglomeration process are negligibly slow at 18-20 K in Kr and Xe matrices.' Although the geometries, energy surfaces for different electronic states, and atom-molecule state correlations for CuH2, CuH H, and C U ( ~ S2D, , 2P) H, have yet to be established and are not expected to be identical with those of the corresponding nickel-hydrogen system: it is clear from the results of the present study that the reaction CuH + H proceeds with little or no barrier and is entirely downhill in energy.
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Note Added in Proof: At the UHF-CI level, linear symmetrical CuH2 shows a local minimum (ref 5 ) . Acknowledgment. The generous financial assistance of the Natural Sciences and Engineering Research Council of Canada is greatly appreciated. Stimulating discussions with Professor W. A. G. Goodard, I11 (Caltech), particularly his suggestion of the AgH H experiment, and with Dr. Raymond Poirier, concerning the (3u/Hz and CuH/H systems, generally proved to be most helpful. Registry No. Copper hydride, 13517-00-5;hydrogen atom, 1238513-6; copper, 7440-50-9;hydrogen, 1333-74-0.
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The Photoreversible Oxidative-Addition, Reductive-Elimination Reactions H, e FeH, in Low-Temperature Matrices Fe
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Geoffrey A. Ozin* and John G. McCaffrey Lash Miller Chemistry Laboratory, University of Toronto, Toronto, Ontario, Canada M5S 1 A1 (Received: July 21, 1983)
307-nm photoexcited Fe atoms in H2/rare gas 12 K matrices undergo an activated and concerted insertion reaction into the H-H bond of H, to form iron dihydride, FeH2, having a nonlinear geometry, with no detectable involvement of H-atom abstraction products FeH + H, higher iron hydrides FeH, ( x 1 3), or molecular dihydrogen complexes, Fe(H2). The microscopic reverse of the photoinsertion reaction can be induced by 440-nm photoexcitation of FeH, at 12 K and proceeds by a nonactivated and concerted reductive-elimination pathway with no observable participation of FeH, H, FeH, ( x 1 3), or Fe(H,) reaction intermediates.
There currently exists an intense scientific interest in ligand-free transition-metal dihydrides, MH2. For instance, the interaction of dihydrogen with a single metal atom to give either a molecular dihydrogen complex or an oxidative-addition product is a fundamental step in organometallic and surface chemistry, playing key roles in several homogeneous and heterogeneous catalytic industrial processes.' The microscopic reverse, reductive-elimination reaction is of equal importance in the understanding of a number of catalytic phenomena involving dihydrogen. The determination of mechanistic detail for both the forward and backward steps in the model M H2 * MH, system presents both an experimental and theoretical challenge which impinges on such questions as symmetry-based electronic state correlations,
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TABLE I: Infrared Spectra of Iron Dihvdride in Solid Xenon
FeH,
FeHD
FeD,
mode assignt
1665 1660 1636 1198 1204 1188 323
280 232
(1) B. Gates, J. R. Katzer, and G. C. A. Schmit, "Chemistry of Catalytic Processes", McGraw-Hill, New York, 1979; "The Nature of the Surface Chemical Bond", T. N. Rhodin and G. Ertl, Ed., North Holland, Amsterdam,
transition states, activation barriers, molecular-electronic structure, and bonding properties.2 Up till now, however, experimental methods for exploring the insertion and elimination steps in ligand-free M H2 e MH2
1979; J. P. Collman and L. S. Hegedus, "Principles and Applications of Organotransition Metal Chemistry", University Science Books, Mill Valley, CA; G. W. Parshall, 'Homogeneous Catalysis", Wiley-Interscience, New York, 1980, and references cited therein.
(2) M. R. A. Blomberg and Per. E. M. Seigbalm, J. Chem. Phys., 78,5682 (1983), and references cited therein.
0022-3654/84/2088-0645$01.50/0
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0 1984 American Chemical Society
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646 The Journal of Physical Chemistry, Vol. 88, No. 4, 1984
H2/Xe than in D,/Xe matrices under identical experimental conditions. The corresponding infrared experiments (Figure 2) clearly substantiate that the observed photobleaching of Fe atoms in H2/rare gas matrices is chemical in origin. This is witnessed by the production of an iron-hydrogen containing species absorbing at 1636/323 cm-’ in H2/Xe and 1188/232 cm-’ in D2/Xe matrices (Figure 2, B and C). The corresponding frequencies in H2/Kr E 1/10 showed small blue matrix shifts to 1650/322 crn-l. The observation of iron-hydrogen stretching and deformational modes having the same growth behavior confirms that the species is not FeH (the latter absorbing at 1658 cm-’ in xenon matrices5). The analogous experiments in HD/Xe 1/10 matrices show the respective stretching and deformational modes at 1665/ 1198/280 cm-’ (Figure 2D). This is the isotopic pattern expected for a FeHD insertion product, rather than for a molecular dihydrogen complex Fe(HD), thereby specifying the photochemical pathway to an iron dihydride (the u(FeH) IR stretching mode of Kr entrapped HFeCl and HFeBr molecules6 occurs at 1738 cm-’). The weak bands labeled a and a’ at 1660 and 1204 cm-l (Figure 2) which are associated with FeH, and FeD,, respectively, and which absorb at slightly higher frequencies than the asymmetric v(FeH2) and u(FeD2) stretching modes are clearly absent in FeHD. They are therefore best ascribed to the symmetrical v(FeH2) and v(FeD,) stretching modes, rather than with a secondary trapping site of iron dihydride and hence evidence for a nonlinear geometry. The low-intensity shoulders denoted b and b’ (Figure 2) do, on the other hand, behave as a multiple trapping site of FeH2 and FeD,, respectively, and have been assigned accordingly. The ratio of the infrared absorbances of the stretching modes of FeH2 (bond 2mH. dipole model) is given by ZB2/ZAl= tanZ 4/2[(mF, sin2 4/2)/(mFe + 2mH cos2 4/2)], where 4 is the apical angle. This yields a value of around 117’ for FeH2 (a GVB-CI molecular orbital calculation for ligand-free FeH2 predicts an angle of 120° for the equilibrium geometry of the molecule).’ The corresponding IR spectra recorded in H2/D2/Xe 1/1/20 matrices display only those absorptions associated with FeH2 and FeD2 with no evidence for scrambling as seen by the spectroscopic absence of FeHD (Figure 2E). No sign of FeH or FeD could be detected in the IR experiments, nor H or D atoms in the analogous ESR experiments. In the H2/D2/Xe matrices, it is noteworthy that FeH2 is found to form preferentially over FeD, by a factor of 5.76 (IR absorbance measurements on the stretching modes (Figure 2E)). As expected this parallels the five times faster bleaching rate observed for 307-nm photoexcited Fe atoms in H2/Xe compared to D2/Xe matrices (vide infra). These observations alert one to the existence of an activation barrier for the insertion step. Note that for a pseudo-first-order competitive insertion reaction
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Figure 1. Optical spectra of (A) a freshly deposited Fe/H2/Xe N 1/ 103/10412 K matrix and (B) following 45 min of 307-nm atomic Fe photolysis. ( C ) Scale expansion of 380480-nm region of (B). (D) Scale expansion of the Fe/D2/Xe u 1/103/104analogue of (C). (E) Optical spectra of the matrix sample shown in (B) following 15 min of 440-nm FeH, photolysis. The FeH2and FeD, product bands are denoted a, b, c and a’, b’, c’, respectively.
=
systems have not been previously reported. A new method for achieving this important goal forms the main thrust of this brief paper. Iron atoms were codeposited with H2/Kr, Hz/Xe, D2/Kr, D2/Xe, HD/Xe, and Hz/D2/Xe mixtures at 12 K under mononuclear conditions with respect to the iron (Fe/dihydrogen/rare gas l/103/104). The optical spectra of these matrices, aside from small shifts and intensity alterations, were essentially the same and display only the atomic resonance lines of isolated Fe atoms! the energies and oscillator strengths of which can be readily correlated with those reported for Fe atoms in the gas phase.4 Narrow-band irradiation into the intense 307-nm (3d64s14p1,’P3 3d64s24p0,’D4)atomic resonance line of Fe in, for example, Hz/Xe N 1/ 10 matrices (Figure 1A) caused rapid annihilation of all Fe atom bands with concurrent production of new weak absorptions around 400, 419, and 441 nm (Figure 1, B and C). In D2/Xe N 1/ 10 matrices similar bleaching effects were observed yielding new bands at 402,420, and 439 nm (Figure 1D). The small shifts upon deuterium substitution indicate that these new absorptions are associated with an iron-hydrogen containing product (the corresponding bands for HJAr N 1/10 occur at 398,420, and 442 nm). The Fe atom photodecay rate at 12 K followed first-order kinetics, being roughly five times faster in
(note that in the above expressions the factor of arises because the total hydrogen concentration [H2] [D,] used in the mixed isotope experiment is retained the same as that used in the isotopically pure counterpart experiments) which leads in one exH2 and M* D2 periment (rather than two separate M* experiments) directly to kH/kD = [MH2]/[MD2],thus mirroring
(3) G. A. Ozin and J. G. McCaffrey, J. Am. Chem. SOC.,104, 7351 (1982). (4) D. H. W. Carstens, W.Brashear, D. R. Eslinger, and D. M. Green, Appl. Spectrosc., 26, 184 (1972); C. Moore, Natl. Bur. Stand. (US.) Circ., Vol. I, 467 (1949); Vol. I1 (1952); Vol. 111 (1958).
( 5 ) A. Dendramis, R. J. Van Zee, and W. Weltner, Jr., Astrophys. J. 231, 632 (1979). ( 6 ) S. F. Parker, C. H. F. Peden, P. H. Barrett, and R. G.Pearson, J. Am. Chem. SOC.,in press. (7) J. Beauchamp, private communication.
M*
+ H2 + D2
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MH2
+ MD2
the time dependence of the concentration of the MH2 and MD2 photoproducts (IR spectroscopy) is given by
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The Journal of Physical Chemistry, Vol. 88, No. 4, 1984 647
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