Symposium on Catalysis and Organometallic Chemistry
Catalytic Reactions of Sterically Unhindered ~ydridobis(phosphine)platinum Complexes Wllllam C. Trogler University of California at San Diego, La Jolla, CA 92093 Transition metal complexes find wide application in homogeneous catalysis ( I ) because they form covalent metal substrate bonds and effect chemical changes of the substrate through reactions such as insertion (eq 1) or reductive elimination (eq 2).
M
R'
I
-+
M
+
R'?
(2)
I~-P,
One requirement for a catalyst is that of high reactivity to obtain rapid turnover of suhstrates. Coordinatively unsaturated 16-electron metal complexes, which do not satisfy the 18-electron rule, (e.g. Ni(II), Pd(II), Rh(I)), often exhibit high reactivity. To take advantage of the susceptibility of such complexes toward associative reactions (where the 18electron rule is satisfied in the transition state), the steric hulk of bound ligands must be minimized. Trivalent phos~ h o r u donor s ligand size is measured by a parameter called the cone angle, which defines the apexangle of the cone of space occupied by the ligand when bound to the metal center (2). Among the smallest such ligands are triethyl- and trimethylphosphine. This article summarizes our studies of the reactivity and catalytic properties of sterically unhindered his(phosphine)platinum complexes. Generailon of Sterically Unhindered Dlhydride COmplexeS When we began our research, sterically unhindered dihydridohis(phosphine) complexes of platinum were thought unstable (3).Initially we generated the reactive 14-electron fragment Pt[P(C2H&I2 by photochemical reductive elimination of COz from a hound oxalato ligand (4,5).On irradiation of the oxalato complex under a hydrogen atmosphere, a reactive dihydride complex forms (eq 3) initially.
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hu
2c02 + L-Pt-H
~2
CHsCN
L
====
I H
u
I
I
M-R
-
L
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Although 16-electron Pt(I1) complexes usually resist addition of another ligand to achieve an 18-electron count, these sterically unhindered compounds exhibit a latent degree of coordinative unsaturation. Thus. addition of P(CIHR)B to .~ P ~ H Z [ P ( C ~ Hyields ~ ) ~ ]t h~ e 1 8 - e l e c t r o n complex PtH2[P(C2H& (6). In contrast to bulky his(phosphine) complexes t h a t a d o p t a t r a n s s t r u c t u r e , such a s PtH2[P(C6H11)3]z,these were the first bis(phosphine)dihydride complexes to exhibit cis-trans isomerization. Once these dihydride complexes were shown to exist it was possible to develop efficient thermal methods of synthesis (eq 4) and to characterize these complexes structurally by X-ray diffraction methods (7).
tr~m-PtH2(P(CH,)JrC10H8 -0
OC
Besides the extreme susceptibility of the hydrido complexes to associative reactions with nucleophiles (e.g., acetylenes, olefins, phosphines) and oxidative addition substrates (e.g., silanes, alkyl halides, germanes) (6, 8 ) cis-to-trans isomerization occurs rapidly (7,s).
Hydrogen Addition to Platinum A characteristic reaction of Ni, I'd, and Pt metals is dissociative chemisor~tionof H,to generate surface hydride species (10). This reactivity isiheikey to the use of these metals in catalytic reactions, such as H r D s exchange (eq 5) and olefin hydrogenation (eq 6).
Therefore, there is fundamental interest in the oxidative addition of Hz to a platinum center and the reverse process, reductive elimination. Several theoretical studies (11) have focused on reductive elimination of Hz from cis-PtHz(PHa)z. These calculations suggest that the transition state for reductive elimination of Hz from a P t center approximates a structure where the H-H bond forms nearly completely (i.e., a late transition state). By the principle of microscopic reversihility oxidative addition of hydrogen to a platinum center must proceed through an early transition state. These theoretical predictions were borne out by a kinetics study (9, 12) of reductive elimination of Hz from the L = PMe3 complex (eq 7).
The inverse isotope effect ( k ~ l k o 0.5) observed requires nearly complete H-H hond formation in the transition state, or else a molecular H1 comnlex as an intermediate. An inverse rather than normal isotope effect arises because a PtH bond with a low IR-stretchine freauencv ( v = 2000 cm-') breaks, while a high f r e q u e n c y ~ - ~ - h o n( du 4400 cm-') forms in the transition state or intermediate. In the familiar examples of normal isotope effects from organic chemistry (13), a high-frequency C-H bond in a reactant proceeds to a more loosely hound (low-frequency IR vibration) hydrogen in the transition state. Catalytic Decomposltlon of Formlc Acid A complex equilibrium that involves the dihydride complex, a formatohydrido complex, and a dinuclear condensation product yields a catalyst for the reaction of eq 8 (14). The proposed mechanism of catalysis is outlined in Figure 1. Reversible insertion of COZinto the P G H bond is the key step in the catalytic cycle. Even a t room temperature the reaction proceeds readily, as shown by labeling experiments
Flgure 1. Mechanism proposedfnthe decompmltion of formic acid catalyzed by IP~&-H),H[P(C~HI)~I~I[~~CHI.
with '3C01. The ease of decarhoxvlation of Pt-0-C-(O)H can he conirasted with the stabilit; of sodium f o r m a t e . ~ h i s difference ~robablvresults from the stahle covalent Pt-H hond that forms when COz is eliminated from the former compound. An important experimental tool for structural characterization of the species present in the catalytic mixtures is NMR spectroscopy. Because hydrogen, phosphorus, carbon (1.1% natural abundance), and platinum (33.7%) all are spin-% nuclei, the analysis of chemical shifts and couplings in the spectra definitively characterizes the coordination environment in these complexes. One must not assume, however, that the dominant species in solution is the catalyst. The inactive dimer of Figure 1is the only species detectable in a catalytically active solution! Independent experiments that establish the ability of nucleophiles to cleave the dimer into its monomeric fragments, and the dependence of the catalytic rate on HCOO- concentration, point toward the mechanism given. Hydrosllation with lmmoblllzed Catalysts Addition of a Si-H hond to an olefin (eq 9) is an important reaction for the synthesis of organosilanes, as well as for cross-linking reactions used to cure silicone rubbers (15). Hexachloroplatinic acid is the homogeneous catalyst of industrial choice, and most soluble platinum complexes catalyze olefin hydrosilation. The catalytically active species in platinum-catalyzed hydrosilation is not well defined and questions remain whether the catalyst is homogeneous or colloidal (16). Therefore, we immobilized (17) a his(phosphine) platinum complex by preparing the complex with a fuuctionalized phosphine ligand that could be attached to a silica suonort throueh the reaction shown in Fieure 2. One can define the coor&nation environment of thFsurface at?'1'. and 29Si NMR meatached com~lexbv solid state ITC. surements, k h i c h correlate well with solution spectra of analogous soluble complexes. For example, the information derived from the 13CNMR spectra (Fig. 3) allows a comparison of both the surfaceconfined and solution oxalate complexes. The surface species shows five different carbon resonances. Comparison with the '3C NMFt spectra of the free complex, and the free ligand, allows for an unambiguous assignment. The resonance a t 6 165, attributed to the oxalate carbons, shows that the oxalate ligand remains intact after attachment of the speciesto the support. The carbon of the OMe group appears at 6 51, and the carbons adjacent to phosphorus appear a t 6 16.8. For the heavier loaded sample the terminal carbon of the PEtz moiety resonates at 6 8.3, along with the carbons adjacent to silicon a t 6 2.3. For the lighter loaded sample
Flgure2.Reaction usedto prepare asilica-bomdplatinum(ll)oxalate complex.
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i&
0
PPM
chemical shifts. For the heavily loaded sample all resonances from the metal complex dominate the spectrum. In thelightly loaded sample the most intense peak (Fig. 3) results from carbons of the OSi(CH,13~ r o u p sused to cap unrearred hvdroxyl groups on the surface. Irradiation of the surface-supported oxalate complex with W light in the presence of HSiC12(CH$)yields aspecies that catalyzes the addition of this silane to l-heptene a t room temperature. The immobilized catalyst differs from other platinum catalyst systems because the reaction shows no induction period. Pyrolysis of the silica-attached oxalate complex produces colloidal platinum on silica (by electron microscopy). This heterogeneous catalyst, in contrast to the photochemically prepared species, shows prolonged induction periods and can be poisoned with metallic mercury. Mercury has been shown (18) to passivate the surface of platinum metal in heterogeneous catalytic reactions by surface amalgam formation. Calalyllc Hydration of Nltrlles While examining the reaction between P ~ H ~ [ P ( C ~ H ~ ) ~ ] Z and excess water in acetonitrile solvent, i t became apparent that catalytic hydration of solvent to acetamide occurred (14). Spectral studies show a mixture of platinum complexes present and a more active catalyst (eq 10) can be obtained from a 1:l molar ratio of PtHCILz and NaOH, where L = P(C2Hs)s or P(CH&, dissolved in a water-nitrile mixture (19). 0
The reaction mechanism of the more stable PEt3 complex was amenable to detailed study. Previous investigations of related bis(phospbine)platinum(II) complexes had also shown catalytic nitrile hydration (20,21). At a high mole fraction of water the rate of catalysis diminished, and a t a high concentration of acetonitrile the rate also decreases. The catalytic rate is independent of [OH-] above pH 9. Through a combination of NMR spectral studies, kinetics experiments, and independent synthesis of several of the presumed intermediates the mechanism of Figure 4 can be deduced (19). In this mechanism the role of the metal is to bind the nitrile group as a ligand in a cationic complex, and thereby polarize it so hydroxide nucleophile attacks at the nitrile carbon to form an N-bound carboxamido ligand. A particularly important experiment uses catalyst containing a Pt-D bond. Deuterium remains bound to
200
150
100
50
b
PPM
Figure 3. (A) Liquidstate I3C NMR spectrum of P1(C20,)[(0Me)3SI(CH2)2PEt2]2 in C.0. solvent. The asterisk denotes the solvent C.H.1C.D. resonance. (6) Solid-state CP-MAS '% NMR spectrum of Hw silica-supported species 5.4 X 10Pmol of Rig SO*.(C) Solid-state CP-MAS l3C [SiO2]L2Fi(C20,)at NMR spechum of the sillca-supportedcomplexat 1.0 X 10-4mol of Fllg SO2.
both resonances shift slightly upfield to 6 6.7 and 0.23, res~ectivelv.The observation of OMe carbons shows that complete tr&s esterification of the ligand Si-OMe bonds to surface hydroxyls does not occur during the synthetic procedure, and the relative intensity of the other carbon signals shows that a large fraction of the Si-OMe bonds remain intact in the [SiO2]L2(C2Oa)Ptcomplex. Apparently, the methylene groups attached to silicon as part of the ligand, L, and the Si-Me groups from the trimethylsilyl cap of surface Si-OH groups overlap a t -6 2.3, because of their similar 296
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H-P,l-NH-c-CH, PR,
0
Figure 4. Mechanism proposed fwthe catalytic hydration of nitriies by hydridcbis(phosphlne)platinum(ll)complexes.
platinum during catalysis, as monitored by deuterium NMR spectroscopy. Thus hydride ligand only serves to labilize the trans position, where catalysis occurs. The strong trans effect of the H- ligand in square planar Pt(I1) complexes is well known (22). Catalytic Olefln Hydration With acrylonitrile substrate both nitrile hydration to acetamide and olefin hydration to 2-cyanoethauol occurs (eq 11).
The pH and substrate concentration dependencies for formation of the two products differs (19).Two plausible mechanisms had been proposed for the Pt(I1)-catalyzed hydration of acrylonitrile to alcohol product (20,21). One mechanism involves nucleophilic attack of hydroxide on the daneline olefin moun of nitrile bound acrvlonitrile, in analogy to ~Ychaeli d i t i o n s to conjugated organic systems. The other mechanism involves nucleo~hilicattack on hound olefin as shown in Figure 5. Notice that in the latter mechanism Pt(I1) . . .nolarizes the olefin to nucleo~hilicattack. Then, by trans-cis isomerization and reductive elimination of the cishvdridoalkvl comvlex. product forms. In this mechanism the h;dmgen aiom on I't i&orporates inro product, which contrasts with the nitrile hydration mechanism. When deuterium labeling experiments were performed, the Pt-D bond did not remain intact, and the deuterium was transferred to form HOCH-CHDCN. Thus we orefer the merhanism of Figure 5. o n e reason Pt(I1) works well as a catalyst for these base-catalyzed hydrations may derive from its soft Lewis acid character. This permits soft nitrile and olefin Lewis bases to coordinate. even in the presence of the good, but hard, hydroxide ligkd. Further ~ " ~ ~for o this r t michanism mav be found in the following section on the insertion of acr;lonitrile into a metal-am& bond. Olefin hydration to prepare terminal alcohols is a reaction of potential technological importance (23). Although the PtHCl[P(CHs)nlz/NaOH catalyst was observed to convert 1hexene.to l-heianol in a micellar catalyst system, the reaction has proved erratic in its reproducibility (24). Olefln Amination Addition of ammonia to ethvlene to form ethyl amine is as therm(1dynamica1lyfavored a; the hydration df ethylene to ethanol. While simde catalvsts exist for the latter reartion, olefin amination remains an unsolved problem, since the
only known catalysts operate at extreme conditions (25). Given the mechanism thought to operate for hydration of acrylonitrile, we wondered whether addition of an amine could be effected hy u similar proredure. Since the neressary mononuclear hydridoamidoromplexes were unknown it was not clear whether they were stable. Addition of NaNH(CsH5) to trans-PtH(N03)[P(C2H5)3]2 cleanly gives PtH[NH(CsH5)][P(C2H5)3j2, as characterized by 15N, lH, and 31P NMR spectroscopy (26) and by lowtemperature X-ray crystallography (27). When allowed to react with acrylonitrile in benzene solvent, insertion occurs as shown in eq 12.
The intermediate hydridoalkyl complex can be isolated and analyzed. Heating this complex a t 70 OC induces reductive elimination of the C-H bond (eq 12);however, the reactionis not catalytic because aniline will not react further to complete the catalytic cycle. Comparative equilibrium experiments suggest the hydridoamido complexes are thermodynamically unstable to reductive elimination of amiue. Current research efforts focus on solving this problem. Acknowledgment Support of research in my laboratory by the U.S. National Srienre Foundation and by the US.Army Research Officeis gratefully acknowledged. Literature Cited
4. Psonessa,R.S.:Trgler.W.C.Olg~wmrfolli~~ 1982,1,76S.Paonessa,R.S.:Prigmo, A. L.;nogier,W . C. Orgawmotollics 1985,4,647. 5. Tmgler, W. C.ACSSymp.Ser.1986,307, 177. 6. Paonesa,R.S.:Trogler,W.C. J.Am.Chem.Soe.1382,104,1138. 7. Paekett,D.L.;Jen~n,C.M.;Cowan,R. L.;S~ouse,C.E.:Troglu.W.C.Iwrg.Chem. 191.24.3578. 6. Paekefh D. L.:Syed, A.;Trogler,W.C. Orgonom@tallics1988,7,159. 9. Psck~tt,D.L.;Thglsr,W.C. J.Am. Chrm.Soe. 1386,108,5036. 10. G r e e n d , N . N.;Eamshaw,A. Chemistry oltheElempnts;Pergaman:Oxford.1984, pp 1333-1336. 11. Low,J.J.;Goddad,W.A., I1I.J. Am. Chem. Sm. 1984.106.6926. Low,J.J.;Goddard. W.A,, 111. Orgonomefallica 1986,5,6W.O h m , S.: Kitaura, K.; Morokuma, K. J. Am Chrm. So?. 19116. IOb. 7482. Belazs. A. C.: Johnson. K. H: Whitesidas. G. M. York.
1980.
14. Paonessa, R. S.; Tmgler, W. C. J. Am. Cham. Soe. 1982,104,3529. Paon-, Tmglor. W. C. Inorg.Chrm. 1983.22.1038. 15. Soeier.1. LAdu. Omonompf. Chem. 1979.17.407.
R. S.:
1819
19. Jcmen C.M l b g l r r . W C .I Am Chem S o c 1986, INI. 7x3 20 R m t t t l t . h1.A.. Yorh~ds.T.JAm Chmz Sor 197l.~.:WJO.J Am Chrm Sor. 1378. I M . I750 Amold D P..Bmnetr.M A J Ordnncnwr i'wm 1980. 199. 119 21. I'nshtda T Mahuaa:l Okam T . Kirstu T . OadLa. S J Am C h e m Sor 1979, 101. 2027. 22. Basolo.F.;ChsthJ.:Grav,H.B.;Pemon,R.G.:Shaw.B.L.J Chem.Sor. 1961.2107. Falk, C. D.: Halporn, J. J.Am Chom. Sm. 1965.87.3W3. 23. wiseman, P. An Introduction to Indwtrial Organic Chemisfry; Applied Science: London, 1979. 24. Jensen, C. M.;Tmglet, W.C. Science 1986,233,1069. 25. Pel, G.P.; Dslle, 3. E.Pur. Appl. Cham. 1985.57,1917. 26. Cowan, R. L.; Trogler, W . C. Orgonometollies 1987,6,1025 27. Cowan, R. L.; Tmgier, W . C., unpublished rpaults.
.
.CH,OH R,P-P!-C? PR,
Figure
CN
-
PR, .CH,OH H-P;-C? PR,
CN
Medmnism proposed far catalytic hydration of acrylonitrile hydridobis(phosphlne)platinum(ll)wmplexes. 5.
by
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