New Studies in SOME RUSSIAN PLATINUM METALS RESEARCH By Viatcheslav I. Sokolov and Vasily V. Bashilov Institute of Organoelement Compounds, Russian Acadrmy of Sc*irnc*rs.Moscow. Russia

Studies on fullerene chemistry carried out in the Laboratory of Organometallic ai ZNEOS,Moscow, are briefly reported. These include work with platinum metals complexes, in particular, on novel methods of prepar- ing fifdlerene (Gand C,) complexes ofplatinum, palladium, and . A new appmach is the use of mercury-platinum bimetallic compounds, R-Hg-PtLyX, as a source of the PtL, moiety to be transferred onto a (65) double bond in fullerenes. Bis(aryl)platinum( 11) complexes can react similarly Other products of this reaction are discussed. Thefirst optically active organometallic fullerenes of the type C.M(+) DZOE where n = 60 or 70, have ulso been prepared and the circular dichroism spectra investigated. In addirion, the molecular structuresfor CN,Pd(PPh,)? and C~.tPt(+)DIOPhave been solved. Higher catal-ytic activity for the hydrogenation ofu triple to u double bond has been observed with C,#,Pd(PPh,)l adsorbpd on porous carbon than with palladiumlporous carbon.

The stability of the giant carbon cluster C,,,, such they have attracted a lot of attention in was first predicted by quantum chemical cal- terms of their general reactivity - in particular culations in the 1970s (1,2), but the actual exis- as for transition metals. tence of both C,,, and C7,,in carbon vapour was This latter field of study was initiated by the not discovered until 1985, when it was detected work of Fagan, Calabrese and Malone who by (3), and resulted in the described the first platinum q2-bonded C,,, com- award of the Nobel Prize for Chemistry in 1996 plex (5). Shortly afterwards we reported that to Curl, Kroto and Smalley. the structures of the fullerenes permit them, However, for chemical study, fullerenes only in principle, to act as ligands of different hap- became available in multigram quantities five ticity, continuously from q' to 11"(6). One aspect years later, after the development of the carbon that has been of great interest to us is that chi- arc burning method (4). The study of fullerenes rality may occur in doubly metallated exohedral is interdisciplinary, as they have relevance, for complexes when the two metal groups are iden- example, to: solid state physics, , tical: in if,q"-C,,,,and in q",q"-C7,,,i1",q'-C7,t and chemistry, , astrophysics, life q',~i'-C~,~(7). Both pentahapto- and hexahapto- sciences and geology. metal complexes are well-known in platinum As chemical entities fullerenes are different metals chemistry (as shown for instance, by met- from other forms of carbon, such as diamond, allocenes and arene-metal complexes). However, graphite, or linear carbon, in that they consti- in fullerene chemistry, only q'-ligated metal tute a whole family of isolated individual mol- complexes had been reported until 1996, when ecules rather than a continuous polymeric sys- the synthesis of qi metal derivatives of X,C,.,) tem. From a chemical point of view, fullerenes with a partly broken fullerene structure was can be regarded as highly unsaturated, strained reported (8). , with an unusual three-dimensional Recently, this Journal published a survey of structure, made exclusively of carbon, and as some current results in the area of fullerenes

Pl~tiirirtiiMetuls Rev., 1998, 42, (l), 18-24 18 The of

Compound Reductive wave potentials, - E,,z,,,d,,volts I 0.34 I I 0.78 I I 1.30 I 1.78 I 2.30 CmPd( PPh3)z 0.34 0.57 0.78 0.99 1.27 1.47 CsoPd(PPh3)z + Pd(PPh3)a 0.57 0.81 0.99 1.25 1.48

bonded to the platinum group metals (9) in This suggests that essential electron transfer which one paper of the present authors was occurs from the metal to the fullerene core. The referred to. However, there is other work on this X-ray structure of a related, optically active, topic published by our group, which has largely platinum fullerene complex has also been solved gone unreported in the review literature (10). (vide infra). Therefore, we would like to take this opportu- An electrochemical study of q2-Cb0Pd(PPh& nity of briefly outlining our results. in acetonitrile in the presence of an excess of Topics in fullerene chemistry in which we are either Pd(PPh,), or PPh, revealed the disap- interested include: pearance of some reductive waves common to (i) the synthesis, molecular structure and reac- free CbO, see the Table, above (1 2). This was tivity of C,, and C,, co-ordinated to the plat- interpreted to mean the existence of the inum group metals; following equilibrium: (ii) the formation and reactivity of Z-fullerenyl 2 CJ'd(PPhi)2 ft Cou[Pd(PPh,)i]2 + Cm free radicals, where group Z is linked to the fullerene through the magnetic nucleus (such without phosphine dissociation. as "P, 'OB-l'B, Iq5Pt,etc.) for study by ESR (elec- We have found a novel, alternative method for tron spin resonance) spectroscopy; the preparation of platinum fullerene complexes (iii) optically active organometallic fullerene using mercury-platinum compounds of general derivatives. formula R-Hg-PtL2-X as a source of the PtL2 moiety (1 3). The synthesis of this group of mer- Synthesis of Fullerenes with cury-platinum organobimetallic compounds was Platinum and Palladium previously developed by us; it involves the inser- The synthesis and molecular structures of tion of Pt(0) carbenoids into the mercury-ele- palladium fullerene complexes have been ment bond (Hg-X), where the element X may reported by us (1 1). X-ray investigations have be halogen, carbon or a metal and R is an organyl revealed that distortions of the fullerene cores group (14). The important structural feature of in both platinum and palladium q2-C60M(PPh,)2 these organobimetallics is a ck-arrangement complexes are very similar (1 1). The most inter- of two phosphine ligands (L) around the plat- esting feature of these complexes is their square inum . The stability of these mercury-plat- planar - rather than tetrahedral - arrangement inum compounds has been known to depend of four ligated around the metal. This is strongly on the nature of groups R and X. It different from Pt(0) and Pd(0) complexes with was found that their reactivity, versus fdlerenes, normal alkenes, such as ethylene, but similar to may be different as a consequence. Thus, if only those with electron-withdrawing alkenes, such common groups, such as halogen, aryl or alkyl as tetrafluoroethylene or tetracyanoethylene. are present in the , then Pt(PPh3)z is

Platinum Metals Rev., 1998, 42, (1) 19 hv Ph2C=CH-Pt(PPh,)z-Hg-CF(CF,)z + Csa +

CH=CPhi ESR spectrum g = 2 0013 I hfs constants (gauss). 'Cso - Pt - PPh, 30.5 I ar2 35 PPh, Scheme I A olatinum hllerenvl transferred smoothly onto a fullerenyl [6:6] with bis-organylplatinum complexes, Ar2PtLz, double bond. where Ar = ptolyl, again to give q'-C60Pt(PPh,)Z However, when R is the electron-withdraw- and biaryls (1 6). The reaction appeared to pro- ing perfluoroalkyl group, homolysis of the metal- ceed as a concerted process with initial co-ordi- metal bond occurs selectively, leaving the plat- nation between the platinum atom and the inum-carbon bond intact. Initially a platinum- double bond. centred free radical is formed which immediately attaches to the fullerene to give a relatively sta- Formation and Reactivity of ble platinum fullerenyl radical, observable by Z-Fullerenyl Free Radicals ESR at room temperature (1 3), see Scheme I. Fullerenes themselves are known readily to Its structure is explained by the ESR spectrum, add many kinds of free radicals (1 7). We have where hyperfine splitting (hfs) is observed due studied the behaviour of q'-CouM(PPh,)2dur- to one platinum ('"'Pt) and two non-equiva- ing the reaction with free dialkoxyphosphoryl lent ("P) nuclei. This shows that radicals, (AlkO),P(O)*. Several low intensity the cis-arrangement of the two phosphine lig- ESR signals were observed in addition to the ands is preserved in this novel radical species. parent *C,J'(0)(OAlk)z signal, which indicates Strangely enough, no stable ql-platinum that there is a preferred attack on the metal fol- fullerenyl derivative has been reported previ- lowed by immediate demetallation to give the ously, and the above-mentioned free radical is latter. the only representative of monohapto-platinum- An indirect approach, based on the metalla- fullerenyl compounds. tion of the radical dimer, was used to prepare The above-mentioned approach was success- q'-platinated fullerenyl radicals, see Scheme 111. fully used for the synthesis of qz-C-ioPt(PPh3)z, Because the fullerene-fullerene bond in this see Scheme 11, with the formation of two iso- dimer is very weak (ca. 10 kcal mol I) it can be mers being identified by the "P NMR spectrum. broken by irradiation with red, visible light The major isomer exhibited an AB system (a (630-680 nm). In fact, five distinguishable ESR pair of doublets) that corresponds to addition signals have been observed, each with differ- on the most strained a-b bond near the pole of ent g-factors and hfs constants, a, (due to cou- the ellipsoid. The minor isomer exhibited one pling to phosphorus) which can be assigned to singlet that pointed unambiguously to the equiv- the regioisomers of rf-platinated phosphoryl- alence of both phosphorus atoms. This can hap- fullerenyl radicals (1 8). pen if the addition occurs on the c-c bond (the The reactions of C,,, and C,,, with the com- c-c bond is more removed from the pole and is plexes, HM(CO)(PPh,),, M = rhodium or bisected by the plane of symmetry) (15). indium, occur with high regio- and stereo-selec- Recently it was found that C,, can even react tivity by the replacement of one PPhl ligand to

Platinum Metals Rev., 1998, 42, (1) 20 C7oPt( PPh3)r

+ RHgPt(PPh&Br - Pt(PPhi)z -RHgBr R = PhzCHCHz

Yield: 85%

Selected properties of C70Pt(PPh3)z Analytical data: found, %: C. 80.99; H, 2.24; P. 3.72 C1mH3cP~Pt.C7H8,calculated, %: C, 80.93; H, 2.34; P, 3.79 3'P NMR (, 6 ppm): AB-system 6A= 25.5, 88 = 22.9; I J(PA,Pd = 24.5 Hz, J(Pn, Pt) = 4051 Hz, J(Ps,Pt) = 3875 HZ I ~f-CmPt[( +)DIOP] 1l'-CiaPt(PPh2)2 + (+)DIOP -~l~-CmPt[(+)Dl0P] + 2PPh3 Selected properties of qz-CloPt[(+)DIOP] CD spectrum (toluene, Cotton effect, he,,,nm): 312 (+),335 (-), 386 (+),496 (+) Optical rotation (toluene): [a]36s= -3.92 (c = 0.0255) Analytical data: found, %: C, 78.83; H, 1.96 C,clH3202PZPt,calculated, %: C. 79.06; H, 2.10

3'P NMR (THF, 6 pprn): AB-system, 6* = 8.28, 6e = 6.20; r J(PA,PB) = 29 Hz, J(PA,Pt) = 3849 Hz. J(PB. Pt) = 3683 HZ Scheme I1 Phosphine platinum complexes of C,O give q2metal derivatives on the [6:6] bonds. hydride complex, HIr(COD)(PPh,)2,reacts in With C70ythe double bond located on the sharp a similar way (19). end of its ellipsoid is involved. A related diene- The interaction of photolytically generated

Photolytic generation of R-fullerenyl free radicals R-Hg-R + Cm- R-Csd. R = -P(O)(OR)z,-BBSHSCZRZ

q*-Platinated phosphoryl fullerenyl radicals prepared via dimers 2R-Csa'- R-Cba-Csa-R dimer The 5 Isomers (ESR): k + Pt(PPh3h hv 2.0017 2R-Csb -R&c Go-R 2.0023 I I1 2.0025 qz-Pt(PPh3)z (Ph3P)zPt Pt(PPh3)z 2.0028 2.0032 68.0 Scheme I11 Svnthesis of n'-elathated fullerenyl radicals via a radical dmer

Platinum Metals Rev., 1998, 42, (1) 21 Fig. 1 Molecular structure of the enantiomeric platinum fullerene complex

Bond , A 2.09 2.12 2.28 2.26 1.51 1.44

Angle Angle, 1 deg =)-Pt(l)-C(2) 42.0 C( 1 )-Pt(1 )-P( 1 ) I 104.2 C(2)-Pt(1)-P(2) 104.4 P( 1)-Pt( 1 )-P(2) 109.5

(vide supra) dialkoxyphosphoryl radicals with metric catalysts for use in various organic reac- the iridium fullerene complexes, q2-Cb,JrH(Y)- tions. Additionally, as some platinum complexes PPh,, where Y = (CO)(PPh3) or (COD), at are known to possess biological activity, this can room temperature, was found to result in par- be also expected for the platinum fullerenes. tial demetallation (attack on the metal) and addi- Thus, optically active compounds could be of tion (attack on the fullerene core) to give three interest in different fields, in part they have isomeric phosphoryl(iridium)fullerenyl radicals, potential as materials for non-linear optics. which have: Added to this, the circular dichroism of these compounds can provide information about the g = 2.0026, UP= 66.7 gauss; 1 electronic transitions in the fullerene core. = a,. = gauss; 2 g 2.0037, 64.25 The first optically active organic derivatives of g = 2.0027, up = 59.25 gauss 3 fullerene have been reported by Vasella, with the species 1 being strongly predominant. Diederich and colleagues (21). These are sugar- The parent phosphorylfullerenyl radical, fullerene derivatives whose circular dichroism *C,J'(0)(OAlk)Z, with g = 2.0023, ap= 63.5 spectra were recorded, and exhibited many gauss, formed more slowly from the iridium Cotton effects due to the electronic transitions complex compared with that from the platinum in C,,, itself. analogue (20). We took advantage of the opportunity to intro- duce optically active ligands at the metal atom Optically Active Organometallic in q2-fullerene metal complexes and thus syn- Fullerene Derivatives thesise the palladium (22) and platinum (23) One of our main objectives has been the syn- Gocomplexes bearing the (+)DIOP ligand: [(+)- thesis of optically active organometallic fullerene 2,3-isopropylidene-2,3-trans-dihydroxy-1,4- derivatives. These are interesting both in them- bis(diphenylphosphino)butane] at the metal. selves and as the possible precursors for asym- These were the very first optically active

Platinum Metals Rev., 1998, 42, (1) 22 ELLIPTICIT) MOLECULAR ELLIPTICIT) MOLECULAR

WAVELENGTH, nm Fig. 2 Circular dichroism spectra of the enantiomeric platinum and palladium C, derivatives: 1 (q'-C,)Pt[(+)DIOP] in toluene 2 ($-C,)Pd[(+)DIOP] in toluene 3 ($-G)Pd[(+)DIOP] in dimethylformamide

organometallic fullerenes to be reported. They Figure 2. In some cases the Cotton effects may were prepared either by exchanging the be shifted due to interaction with the metal. triphenylphosphine ligand for (+)DIOP, see In general, the wavelengths of the Cotton effects Scheme 11, or by direct synthesis from the are close to those reported in (21). fullerene, the diphosphine ligand, and the Finally, we have observed the catalytic effect dibenzylideneacetone complex, Pd, (dba) ,: of C,,Pd(PPh,), adsorbed on a porous carbon (sibunite) for the selective hydrogenation of a C,,,+ Pd2(dba),+ (+)DIOP 4 (rf-Cw)Pd[(+)DIOP] triple bond in 3,7-dimethyloctaene-6-yne-1-01- The molecular structure of the enantiomeric 3 to a double bond in 3,7-dimethyloctadiene- platinum fullerene complex was determined by 1,6-01-3 (linalool). This occurred more effi- X-ray study of the solvate with one molecule of ciently than with metallic palladium adsorbed cis-cyclooctene, see Figure 1, isolated after on the same carrier under the same conditions recrystallisation of the compound from a czs- (24). There is no possibility of asymmetric catal- cyclooctene- mixture. The inclusion of ysis in this case, nonetheless there is a chance cis-cyclooctene appeared to be absolutely nec- that the optically active fullerene palladium ana- essary for the growth of good quality single crys- logue would be a useful asymmetric catalyst for tals, but in spite of this, the solvate molecule other hydrogenations which might result in had no close contacts with the fullerene mole- chiral products. cule (23). Similar (+)DIOP complexes have also been prepared &om Go. Acknowledgement Circular dichroism spectra were measured in This work has been supported by grants from the toluene and in dimethylformamide, and several Russian Foundation for Fundamental Research Cotton effects were found (unpublished results) (RFFR), Scientific Programme "Fullerenes and Atomic Clusters", International Science Foundation which could be unambiguously attributed to the (ISF MNR 000 and MNR 300), and the International electronic transitions in the fullerene core, see Science & Technology Centre (Project 079).

Platinum Metals Rev., 1998, 42, (1) 23 References 1 E. Osawa, Kagaku (Kyoto), 1970,25,854 14 V. I. Sokolov, V. V. Bashilov and 0. A. Reutov,J. 2 D. A. Bochvar and E. G. Gal’pern, Dokl. Akad. Organoniet. Chem., 1976, 111, C13 Nauk SSSR, 1973,209,610 15 V. V. Bashilov, V. I. Sokolov, K. P. Butin and T. 3 H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. V. Magdesieva, Inorg. Chini. Acta, 1998, in press Curl and R. E. Smalley, Nature, 1985, 318, 162 16 V. I. Sokolov, Pure Appl. Chem., 1998, in press 4 W. Kraetschmer, L. D. Lamb, K. Fostiropoulos 17 For a survey of closely related ESR spectroscopy and D. R. Hufhan, Nature, 1990, 347, 354 studies of fullerenyl radicals see: B. L. Tumanskii, Izv. RAN, Ser. Khinr., 1996, 2396; [Russ. Cheni. 5 P. J. Fagan, J. C. Calabrese and B. Malone, (i) Bidl., 1996, 45, 2271, (Engl. Transl.)] Science, 1991, 252, 1160; (ii) Acc. Chem. Res., 1992,25, 134 18 B. L. Tumanskii, V. V. Bashilov, N. N. Bubnov, S. P. Solodovnikov and V. I. Sokolov, Izv. RAN, 6 V. I. Sokolov, (i) 18 1st Meedng of Electrochemical Se,: Khini., 1994, 938; [Rim. Chem. Bull., 1994, Society, St. Louis, 1992, Abstracts, FWL 662; (ii) 43, 884, (Engl. Transl.)] Dokl. Akad. Nauk SSSR, 1992,326, 647 19 A. V. Usatov, K. N. Kudin, E. V. Vorontsov, L. E. 7 V. I. Sokolov, Mol. Muter., 1996, 7, 23 Vinogradova and Yu. N. Novikov, 3. Organomet. 8 M. Sawamura, H. Iikura and E. Nakamura, 3. Chenr., 1996, 522, 147 Am. Chem. SOC.,1996,118, 12850 20 B. L. Tumanskii, A. V. Usatov, V. V. Bashilov, S. 9 D. T. Thompson, Platinum Metals Rev., 1996,40, P. Solodovnikov,N. N. Bubnov, Yu. N. Novikov and V. I. Sokolov, Izv. RAN, Ser. Khim., 1997, (11, 23 870 [Russ. Chem. Bull., 1997, 46, 816 (Eng. 10 A. Hirsch, “The Chemistry of Fullerenes”, Transl.)] Thieme, Stuttgart, 1994 21 A. Vasella, P. Uhlmann, C. A. Waldraff, F. 11 V. V. Bashilov, P. V. Peaovskii, V. I. Sokolov, S. V. Diederich and C.Thilgen, Aiigew. Chem., bit. Ed. Lindeman, I. A. Guzey and Yu. T. Struchkov, Engl., 1992, 31, 1388 Organometallics, 1993, 12, 991 22 V. V. Bashilov, P. V. Petrovslui and V. I. Sokolov, 12 T. V. Magdesieva, V. V. Bashilov, S. I. Gorelski, Izv. RAN, Ser. Khini., 1993, 428; [Russ. Chem. V. I. Sokolov and K. P. Butin, Izv. RAN,Ser. Bull., 1993, 42, 392, (Engl. Transl.)] Khim., 1994, 2153; [Russ.Chem. Bull., 1994, 23 V. V. Bashilov, P. V. Petrovskii, V. I. Sokolov, F. 43, 2034, (Engl.Transl.)] M. Dolgushin, A. I. Yanovsky and Yu. T. 13 V. V. Bashilov, B. L. Tumanskii, P. V. Petrovskii Struchkov, Izv. RAN, Ser. Khim., 1996, 1268; and V. I. Sokolov, Izv. RAN, Ser. Khini., 1994, [Russ.Chem. Bull., 1996, 45, 1207, (Engl. Transl.)] 1131; [Russ. Chem. Bull., 1994, 43, 1070, 24 E. M. Sul’man, V. G. Matveeva, V. V. Bashilov (Engl.Transl.)] and V. I. Sokolov, Kiuct. Katal., 1997,38, (2), 274 Dissociative Adsorption on Platinum Surfaces The chemical and physical processes occur- peratures were lowered, the pairs formed clus- ring between gases and solids during surface ters, which grew-into quasi-one dimensional are under continuous investigation and chains, 10 to 50 A long, with branches at 120” revision. Since platinum metals find wide use angles. The chains formed an irregular network, as catalysts, and readily chemisorb followed by the appearance of triangular 0 clus- and dissociate bonds, the interaction between ters at the points where three chains met. The platinum and oxygen was selected for detailed clusters had inhomogeneous distribution along- study. Surface kinetics of a heterogeneous reac- side large areas of bare platinum. Finally, tion are usually described by the Langmuir triangular islands protruded from the surface. model, which assumes that free adsorption sites It is assumed that there is a mobile adsorbed on a surface are randomly occupied. molecular precursor state, which is trapped and Now, however, scientists at the Fritz-Haber- dissociated by already adsorbed 0 atoms - the Institut der Max-Planck-Gesellschaft in Berlin active sites. At lower temperatures the precur- have observed that oxygen dissociation is affected sor lifetime and its mean free path increase, giv- by nearby chemisorbed species (T. Zambelli, ing it a higher probability of reaching an 0 atom. J. V. Barth, J. Wintterlin and G. Ertl, Nature, It is suggested that dissociation of the precur- 1997, 390, (6659), 495-497). Using a plat- sors is highest at the ends of the chains. inum( 11 1) surface, the distribution of chemi- This apparent increased local reactivity and mod- sorbed oxygen was recorded by scanning tun- ification of the electronic properties near chemi- nelling microscopy, at intervals from 160 down sorbed particles is an important finding for the to 50 K, after exposure to 10 ’torr s oxygen. At general description of catalytic reaction kinetics, 160 K the 0 atom coverage first formed ran- and has relevance for most high temperature and domly distributed adatom pairs, then as tem- pressure industrial heterogeneous catalyses.

Platinum Metals Rev., 1998,42, (1) 24