398
J. HALPERN
Vol. 63
HOMOGENEOUS CATALYTIC ACTIVATION OF MOLECULAR HYDROGEN BY METAL IONS A N D COMPLEXES BY J. HALPERN Department of Chemistry, University of British Columbia, Varncouver,B. C., Canada Received October 4 #Ig68
Certain metal ions and complexes, notably those of copper, silver and mercury, have been found t o react homogeneously with molecular hydrogen in solution, or to catalyze homogeneously the reactions between hydrogen and other dissolved substrates. Such reactions have been studied in a variety of solvents including water, amines, carboxylic acids and hydrocarbons. Because of their greater simplicity, both from the chemical and the kinetic standpoint, it has been possible t o obtain a much clearer understanding of the nature and function of the catalytic species in these systems, than in systems involving heterogeneous hydrogenation catalysts, The path of splitting of hydrogen2 as deduced from kinetic and thermodynamic considerations and from isotopic exchan e studies, may be either heterolytic or homolytic, depending on the nature of the catalytic species, its valence state an! its environment (state of complexing, solvent, etc.). Some metal ions (e.g., Cu+ or Ag+) may activate hydrogen by either path and in such cases the path involving heteroIytic s litting is favored when the metal ion is surrounded by basic ligands or solvent molecules which can act as proton acceptors. batalytic activity in these systems appears to be linked primarily t o electronic, rather than to geometric factors. Particularly pronounced catalytic activity is exhibited by “bi-functional” complexes in which electron-accepting and proton-accepting sites are both present. Some aspects of heterogeneous and enzymic catalysis, and of catalytic poisoning, also may be interpreted in the light of these considerations.
It is now well recognized that even under relatively mild conditions, molecular hydrogen can undergo many reactions homogeneously in solution. This recognition is, however, of fairly recent origin and can be traced to Calvin’s2 discovery in 1938 that a t temperatures of about 100°,hydrogen reduces cupric acetate or benzoquinone homogeneously in quinoline solution, in the presence of dissolved cuprous acetate. The latter functions as a true homogeneous catalyst for these, as well as for reactions of hydrogen, e.g. , parahydrogen conversion. Subsequently, and particularly since 1953, many other systems in which hydrogen undergoes homogeneous reactions, have been discovered and ~ t u d i e d .Most ~ of these involve metal ions or complexes as reactants or catalysts. I n this paper, the mechanisms by which these species “activate” or split hydrogen are examined, with a view t o finding a common basis for their activity and to interpreting the dependence of this activity on electron configuration, ligands and solvent. An attempt also is made to relate the catalytic properties of these relatively simple species to those of the more conventional heterogeneous hydrogenation catalysts. Metal Ions in Aqueous Solution.-Among the species which activate hydrogen are a number of simple metal ions, e.g., Cu++, Ag+, Hg++and Hgz++ and the oxyanion Mii04-. I n aqueous solution these ions are reduced homogeneously by hydrogen a t temperatures below 100” to compounds of lower valence or to the metallic state. Cu++ and Ag+ also catalyze exchange of hydrogen with water and the hydrogenation of dissolved substrates such as oxygen or dichromate which do not react with hydrogen in the absence of a catalyst. These reactions can be divided, on the basis of (1) Support of this project through grants from the Research Corporation, the National Research Council of Canada, and Imperial Oil Limited, is gratefully acknowledged. (2) M. Calvin, Trans. Faraday S O C .34, , 1181 (1938); J . A m . Chem. Soc., 61,2230 (1939). (3) Much of the historical background and experimental work pertinent t o the discussion in this paper is reviewed in (a) S. W. Weller and G. A. Mille, Advances in Catalysis, 8 , 163 (1956); (b) J. Halpern, i b d , 9, 302 (1957); Quart. Revs. (London), 10,463 (1956); (c) A. H. Webster and J. Halpern, THIEJOURNAL, 61, 1239, 1245 (1957).
their kinetics and mechanisms, into two classes, depending on whether one or two metal ions participate, together with a hydrogen molecule, in the rate-determining step. It seems likely that the hydrogen undergoes heterolytic splitting in reactions of the first type and homolytic splitting in those of the second type. This is illustrated by the following equations which are believed to represent the rate-determining steps in some of the homogeneous hydrogenation reactions which have been observed in aqueous solution. I n each case the measured activation energy (kcal.) is also given. CLASSI REACTIONS C U + + Hz +CUH’ H + ( E = 27) Agf H) +AgH H + ( E = 24) Hg++ Hz +HgHf H + ( E = 18) H2 HMn04‘ H+( E = 15) MnO4-
+ + + +
Agf
+
+ + +
+
(1) (2) (3) (4)
CLASSI1 REACTIONS 2Ag+ Hz +2AgH+ ( E = 15) (5) Hgz++ Hz +2HgHf ( E = 20) (6) MnOdHO+ AgH+ HMn04- ( E = 9) (7)
+
+ +
+
The corresponding entropies of activation are all in the “normnl” range, i.e., of the order of -10 e.u. for the bimolecular reactions and -25 e.u. for the two termoleculnr ones. The mechanisms proposed for the splitting of hydrogen by Cu++ and Ag+ have been adequately substantiated. For some of the other reactions there is some uncertainty about the configurations of the intermediates4 and the proposed representations are based on analogy and on indirect kinetic and thermodynamic evidence. Most of these intermediates are unstable and either undergo backreaction to regenerate hydrogen or take part in. some further reactions in which the metal ion itself, or some other dissolved substrate, is reduced. It is noteworthy that Ag+ can split hydrogen either heterolytically or homolytically. The latter (4) For example, HgH +(as) is probably unstable with respect to Hg(aq) and H+(aq) but its formation as the initial produat of the splitting of Hz by both H g + + and Her++, as shown, is favored on mechanistic grounds. Similar considerations apply to the Mn(V) and Mn(V1) intermediates in equations 4 and 7, respectively.
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March, 1959
399
CATALYTIC ACTIVATION OF MOLECULAR HYDROGEN BY METALIONS
path is termolecular but is favored relative to the alternative bimolecular path, particularly a t low temperatures because of its lower activation energy (15 us. 24 kcal.). Similarly the Ag+-catalyzed reduction of Mn04- by Hz, involving a termolecular rate-determining step (eq. 7) competes with the bimolecular uncatalyzed reduction (eq. 4) because of its lower activation energy (9 us. 15. kcal). A number of other metal ions including Ca2+, Mg2+ Zn2+,Mnzf, Co2+, Cd2+,Pb2+, Al3+, Fea+, Cr3+,'Tl3f, Ce4+, UOZ2+,VOs- and Cr02- have been tested, in this Laboratory, for catalytic activity in aqueous solution, in most cases a t temperatures up to 150" and found to be inactive. No autocatalysis due to Cu+ was noted6 during the reduction of aqueous solution of cupric perchlorate or sulfate by hydrogen, indicating that the activity of Cu+, if any, is small compared with that of Cu++. This is of interest in view of the pronounced catalytic activity which cuprous salts display in certain organic media.2JJ Similarly, no autocatalysis due to MnOd was noted in the reduction of MnOabasic solutions.8 Pd ++ also appears to activate hydrogen in aqueous solution, although attempts to study this system quantitatively were unsuccessful because of the tendency for traces of metallic palladium, a powerful heterogeneous catalyst, to form; however, catalytic activity has been demonstratedgfor the complex, PdC14-. Effects of Comp1exing.-The rates of activation of hydrogen by metal ions are greatly influenced by complexing as shown by Table I, in which the reactivities of a number of complexes of copper(II),lo mercury(1I)'l and s i l ~ e r ( 1 are ) ~ ~compared with those of the corresponding aqua-ions. The reactions being considered in this comparison all belong t o Class I and involve the heterolytic splitting of hydrogen in the rate-determining step. A plausible representation of the transition state in such a reaction, for a complex MXnZ+,is Z+ X(n-I)-M-X
I
1
+
where X is a ligand molecule or ion (a water molecule in the case of the aqua ion) originally coordinated to the metal. Formation of the transition state thus involves heterolytic stretching of the M-X and H-H bonds and incipient formation of covalent M-H and X-H bonds. The reactivity of the complex should thus depend directly on the strengths of the M-H and X-H bonds and inversely on the strength of the M-X bond, all of which vary with the nature of X. Most of the (5) E. R. Macgregor and J. Halpern, Trans. M e t . Soc. A . I .M . R . , 212, 244 (1958). (6) ( a ) S. Weller and G. A. Mills, J . A m . Chem. Soc., 7 6 , 769 (1953); (b) L. W. Wright and S. Weller, i b i d . , 76,3345 (1954); ( 0 ) M.Calvin and W. K. Wilmarth, ibid., 78, 1301 (1955); (d) W. K. Wi1mart.h and M. Barsh, i b i d . , 78, 2237 (1953); 7 8 , 1305 (1935). (7) A. J. Chalk and J. Halpern, unpublished. (8) A. H. Webster and J. Halpern, Trans. Faraday Soc., 53, 51 (1957). (9) P. E. P o t t e r and J. Halpern, unpublished. (10) E. Peters and J. Halpern, Can. J . Chem., 84,554 (1956). (11) G. J. Korinek and J. Halpern, ibid., 84, 1372 (1956): THIS JOURNAL,60,285 (1956).
TABLE I EFFECTOF COMPLEXINQ O N ACTIVITY Complexa
"Mean" formation oonstantb
Relative activity
CUBU~ ... 150 CuPr2 ... 150 CUAQ 30 120 CUSOC 100 6.5 CuC14' Nl 2.5 Cu++ ... 1 CuGh 5 x 107
