The Journal of
Physical Chemistry ~
~~
0 Copyright, 1984, by the American Chemical Society
VOLUME 88, NUMBER 4
FEBRUARY 16,1984
LETTERS
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The CUH H Cu 4- H2 Thermal and Cu Phase Reactions
+ H,
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CUH 4- H Photochemical Matrix
Geoffrey A. Ozin* and Catherine Gracie Lash Miller Chemistry Laboratory, University of Toronto, Toronto, Ontario, Canada M5S I A1 (Received: July 21, 1983)
The thermal reaction of CuH with H atoms in Ar, Kr, and Xe matrices at 18-20 K is found to yield significant quantities of Cu atoms. The initial rates of regeneration of Cu atoms display no appreciable matrix dependence in Ar, Kr, or Xe, nor kinetic isotope effect for CuH + H vs. CUD+ D, implying little or no activation barrier for the CuH + H recombination reaction and negligible diffusion control. This reaction can be considered to be the microscopic reverse of the recently discovered photochemical reaction of 2P Cu atoms with molecular H2 in rare gas solid matrices at 10-12 K.' The forward photochemical reaction of Cu(,P) + H2('Z,+) and the backward thermal reaction of CuH('Z+) + H(2S)both proceed with little or no activation barrier and are entirely downhill in energy.
Introduction The photochemical reaction of 3 10-320-nm-excited Cu atoms with H2 in Kr and Xe matrices at 10-12 K1shows some interesting differences from that recently observed for the similar Fe atom reaction2 which yields FeH2. The former process follows first-order kinetics for the photoinduced decay of Cu atoms (Figure 1) and yields significant quantities of trapped CuH and H atoms characterized by H2, D2, H2/D2, and H D isotopic substitution and IR and ESR spectroscopy.' A large 310-nm quantum yield (4 = O S ) , a small kinetic isotope effect (- 1.16) for the chemical quenching of photoexcited Cu atoms by H 2 and D2 (Figure l), and a 2:1 [H]/[D] ratio for the reaction of photoexcited Cu atoms with HD' were observed. Together these are indicative of an early reactantlike transition state with a small or negligible activation barrier and suggestive of a side-on attack3 of the excited Cu atom (1) G. A. Ozin, J. Garcia-Prieto, and S. A. Mitchell, Angew. Chem., I n t . Ed. Suppl., 785 (1982). ( 2 ) G. A. Ozin, D. F. McIntosh, and J . McCaffrey, J . Phys. Chem., following Letter in this issue.
0022-3654/84/2088-0643$01 S O / O
on the H-H bond of H 2 according to the scheme Cu
310 nm + H, E CuH + H
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The end result of 310-nm 2P 'S resonance excitation of Cu atoms in H2/Kr 1/10 matrices at 10-12 K is thus an intimate mixture of immobilized CuH and H reactive intermediates, physically separated yet delicately poised for evaluating the outcome of the thermal reverse reaction CuH
+H
5 CuH, or Cu + H2
(B)
Such a reaction is of considerable theoretical interest as the ground state of, for example, NiH has been predicted not to be stable to attack by H atoms and depending on the side of NiH attack would lead to either NiH2 or Ni + H2.4 No barriers were (3) W. H. Breckenridge and A. M. Renlund, J. Phys. Chem., 83, 1145 (1979); 83,303 (1979); 82, 1484 (1978); A. B. Callear and J. C. McGurk,
J. A m . Chem. Soc., Faraday, Trans. 2, 68, 289 (1972).
0 1984 American Chemical Society
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Letters
The Journal of Physical Chemistry. Vol. 88, No. 4, 1984
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G
i
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t (mm) Figure 1. First-order kinetic plots showing a linear relationship between the natural logarithm of the absorbance due to isolated Cu atoms and the cummulative 310-nm photolysis time for Cu/H2/Kr N 1/10'/104 and Cu/D2/Kr N 1/103/104matrices at 10-12 K. Included for the purpose of comparison are the first-order kinetic data for Cu/CH,/Kr N 1/103/104and second-order kinetic data ( 1 / A vs. 1 ) for Cu/Kr = 1/104 also performed at IC-12 K. found and the reaction goes downhill all the way to products. Also it is interesting to consider the process CUH H CU H2
+
+
-+
as the group 1B analogue of the now classical gas-phase exchange reaction
DH + H
+
D
+ H2
Figure 2. UV-visible absorption spectra showing (A) a freshly deposited Cu/H2/Kr N 1/103/104matrix at 10-12 K (B) following 30 min of 310-nm photolysis at the atomic resonance absorption of Cu and (C) following 30 min of thermal annealing at 18-20 K. Trace amounts of Cuzcoisolated with Cu are indicated with an asterisk (cf. ref 1) and do not change during the low-temperatureannealing process.
which can be considered to be the microscopic reverse of the photochemical reaction of C U ( ~ Patoms ) with molecular H2.I In a series of careful IR and ESR experiments performed at 10-12 K no convincing evidence was obtained for a dihydridocupric(II), CuH2, intermediate. It is therefore not possible to make any unequivocal statements at this time as to whether the above reaction passes through an unstable CuH, species or proceeds by way of a H-atom abstraction step. It is noteworthy, however, that the matrix reaction
(C) AgH
Results and Discussion Experimentally one observes, for the system under study, that careful annealing of CuH H containing matrices at 18-20 K (generated from Cu(2P)/Hz/rare gas 1/103/104 mixtures at 10-12 K) results in the regeneration of significant quantities of Cu atoms as witnessed by the monotonic re-emergence of the ,P atomic resonance absorption centered around 310 nm (Figure 2). A maximum of about 50% of the original Cu atoms can be reenstated with this thermal annealing technique although the extent and rate of the recovery process are found to be very dependent on the choice of Cu/H2/rare gas concentration conditions, the type and temperature of the matrix support, and the temperature at which CuH H are photolytically generated. The initial rates of regeneration of Cu atoms follow fairly well-behaved second-order recombination kinetics with respect to reactants CuH H and appear to display no appreciable matrix dependence on passing from Ar to Kr to Xe, nor kinetic isotope effect on changing from CuH H to CUD D mixtures. This implies little or no activation barrier for the reaction and negligible diffusion control. The lack of a measurable matrix support effect on the recombination rate indicates that neither the size of the substitutional (or interstitial) sites of the rare gas lattice nor the CuH or H rare gas interaction potentials control the kinetics of this chemical reaction. Instead the chemical picture that emerges is one involving favorably oriented CuH and H reactive fragments, most probably located as nearest or next-nearest neighbors in and around the matrix cage that originally staged the C U ( ~ P+ ) H2 photochemical event that led to CuH + H. The regeneration of Cu atoms therefore appears to have its origin in the thermal reaction
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=
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+
+
+
+
CuH
18-20 K +
Ar, Kr, or Xe*
CU+ H2
(4) M. R. A. Blomberg and P. E. M. Siegbahn, J . Chem. Phys., 78, 5682 (1983), and references cited therein.
A
+H
utilizing reactive fragments generated photochemically from Ag(2P)/H2/raregas "= 1/103/104 matrices at 10-12 K, also yields Ag atoms at 18-20 K in amounts comparable to and with rates similar to those observed for the corresponding CuH H matrix reaction. Because the energy separation between the 5 s and 4d orbitals of Ag is considerably larger than the 4s to 3d spacing for Cu, the above observations for CuH H and AgH H in rare gas solids at 18-20 K appear to be more consistent with the occurrence of an H-atom abstraction process:
+
+
M-H
+H
+
[M..*H*..H]
+
-+
M
+ Hz
(where the geometry of the three-center transition state has yet to be established), rather than a reductive-elimination pathway involving a nonlinear metal dihydride:
L
L
J
A
Whatever the mechanistic details it is apparent that the activation energy for the production of Cu + H 2 from CuH + H (and Ag + H2 from AgH + H ) has an upper limit of around 40 cal mol-', part or all of which might be associated with the barrier to reorientation of the CuH and H reagents with respect to one another in and around the matrix cage at 18-20 K. In this context, preliminary ab initio restricted Hartree-Fock molecular orbital calculations for the Cu/H2 system5have revealed CuH H reaction is exothermic by that (i) the C U ( ~ P+) H, about 17.1 kcal mol-', (ii) there is no evidence for any copper dihydride stable reaction intermediate having a symmetrical linear (D..,,) or nonlinear (C2J geometry, but (iii) a H-Cu-H "interaction complex" with a long H-Cu (one electron) bond of 2.238 8, and a short Cu-H (two-electron) bond of 1.543 8, (cf.
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+
( 5 ) R. A. Pokier and I. G. Csizmadia, personal communication.
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
+
+
+
-
+
+
+
+
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).
645
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 Fe H, e FeH, in Low-Temperature Matrices
+
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.
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0 1984 American Chemical Society
