Organometallics 1996, 14,3349-3356
3349
Tetraphenylborate Anion as a Phenylating Agent: Chemical and Electrochemical Reactivity of BPb--Rh Complexes toward Mono- and Dienes and Carbon Dioxide Michele Aresta,* Eugenio Quaranta, and Immacolata Tommasi Dipartimento di Chimica and Centro CNR MISO, Campus Universitario, 70126 Bari, Italy
Sylvie D6rien Laboratoire de Chimie de Coordination Organique, CNRS, U R 415, UniversitL Rennes-1, Campus de Beaulieu, 35042 Rennes, France
Elizabeth Duiiach Laboratoire de Chimie MolCculaire, CNRS, URA 426, UniversitL de Nice “SophiaAntipolis: 06108 Nice, France Received October 17, 1994@
A few aspects of the chemical and electrochemical reactivity of mono-, di-, and trinuclear ( 11,{ [(C2H4)2Rh(y6-Ph)12BPh2}03y6-tetraphenylborate-Rh complexes [(C2H4)2Rh(y6-PhBPh3) SCF3 (21, and { [(C2H4)2Rh(y6-Ph)13BPh}(O~SCF3)2 (311are described. The factors governing the phenyl transfer reaction from coordinated BPb- anion to monoenes and dienes are highlighted. Phenyl transfer to ethylene affords, among other products, styrene and ethylbenzene; styrene is converted into trans-stilbene. Complexes 1 and 2 react with isoprene affording 1,l-dimethylindene and 1,2-dihydro-2-methyl-and 1,2-dihydro-3-methyl-naphthalene, species formally involving phenyl transfer from boron to coordinated isoprene and intramolecular cyclization of the resulting intermediate. The reactivity of 1-3 toward carbon dioxide has been investigated and compared with that of (diphos)Rh(y6-PhBPh3). Using complexes 1-3, coordinated ethylene competes with C02 toward coupling with phenyl. The electrochemical behavior of the dinuclear complex, 2, has been investigated. It undergoes two consecutive irreversible one-electron reductions at -1.53 and -1.74 V us SCE, respectively. Analogous experiments carried out in the presence of C02 have clearly shown that the reduced species are both able to react catalytically with the heterocumulene under controlled-potential conditions (-1.5 and -1.8 V). C02 was coupled with both ethylene and phenyl affording propionic and benzoic acid. The former species was the only product under an ethylenelcarbon dioxide atmosphere. Introduction The coordination chemistry of the tetraphenylborate anion binding to metal centers’ represents an area of -s Abstract
published in Advance ACS Abstracts, April 15, 1995. (1)ia) N o h , M. J.;Gafner, G.; Haines, L. M. J . Chem. Soc., Chem. Commun. 1969,1406. tb) Schrock, R.R.; Osborn, J. A. Inorg. Chem. 1970,9, 2339. icJ Gosser, L.W.; Parshall, G. W. Inorg. Chem. 1974, 13,1947. (d)Kruger, G. J.; du Preez, A. L.; Haines, R. J. J . Chem. SOC.,Dalton Trans. 1974, 1302. ie) Ashworth, T. V.; Nolte, M. J.; Reimann, R. H.; Singleton, E. J . Chem. SOC.,Chem. Commun. 1977, 937. (fl Hossain Bilayet, M.; van der Helm, D. Inorg. Chem. 1978,17, 2893. IgJ Albano, P.; Aresta, M.; Manassero, M. Inorg. Chem. 1980, 19,1069.ih) Pasquali, M.; Floriani, C.; Gaetani-Manfredotti, A. Inorg. Chem. 1980, 19, 1191. ii) Dartinguenave, M.; Dartinguenave, I.; Beauchamp, A. L. J . A m . Chem. Soc. 1984,106,6849. ij)Fachinetti, G.;Funaioli, T.; Zanazzi, E’. F. J . Chem. Soc., Chem. Commun. 1988. 1100. ik) Fagan, P. J.; Ward, M. D.; Calabrese, J. C. J . A m . Chem. SOC.1989,111,1698. i1) Bochmann, M.; Karger, G.; Jaggar, A. G. J . Chem. Soc., Chem. Commun. 1990, 1038. ( m ) Horton, A. D.; Frijns, J. H. G. Angew. Chem., Int. Ed. Engl. 1991,30,1152. ( n )Thomas, B. J.; Noh, S. K.; Schulte, G. K.; Sendlinger, S. C.; Theopold, K. H. J . A m . Chem. SOC.1991,113,893. ( 0 )Longato, B.; Pilloni, G.; Graziani, R.; Casellato, U. J . Organomet. Chem. 1991,407,369.ip) Calderazzo, F.; Englert, U.; Pampaloni, G.; Rocchi, L. Angew. Chem., Int. Ed. Engl. 1992,31,1235. (q)Bochmann, M. Angew. Chem., Int. Ed. Engl. 1992, 31,1181. ir) Albinati, A.; Aresta, M.; Quaranta, E. Organometallics 1993,12, 2032. ( s ) Aresta, M.; Quaranta, E. J . Organomet. Chem. 1993,463,215. 0276-7333/95/2314-3349$09.QQIQ 0
growing interest due t o the structural aspects related to the variety of binding modes shown by this anion when coordinated to a transition metal ion, the fluxional character shown by several BPh--metal complexes,1iJ~mJ,s,2 and the prospects that such a chemistry opens in the fields of molecular engineering,lk cat a l y s i ~ , land ~ - ~both organometalliclg~~-~ and ~ r g a n i & ~ - ~ ~ synthesis. i2)Aresta, M.; Quaranta, E.; Tommasi, I. Gazz. Chim. Ital. 1993, 123,271. i3J ia) Albano, P.; Aresta, M. J . Organomet. Chem. 1980,190,243. ib)Aresta, M.; Quaranta, E.; Ciccarese, A. C1 Mol. Chem. 1985,1,283. !a)Clark, H. C.; Manzer, L. E. Inorg. Chem. 1971,10,2669. tb) (4) Siegmann, K.;Pregosin, P. S.; Venanzi, L. M. Organometallics 1989, 8,2659. (5)Coates, G. E. Organometallic Compounds, J. Wiley: New York, 1960. (6)Haines, R.J.;Du Preez, A. L. J. Chem. Soc., Dalton Trans. 1972, 944. (7)Sacconi, L.;Dapporto, P.; Stoppioni, P. Inorg. Chem. 1976,15, 325. ( 8 ) Feng Chin, P. K.; Hartley, F. R. Inorg. Chem. 1976,15,982. (9,Crociani, B.; Nicolini, M.; Richards, R. L. J . Organomet. Chem. 1976,104,259. ! 10)Taylor, S. H.; Maitlis, P. M. J . A m . Chem. SOC.1978,100,4700. (11)Crociani, B.; Di Bianca, F.; Uguagliati, P.; Canovese. L.; Berton, A. J . Chem. SOC.,Dalton Trans. 1991,71.
1995 American Chemical Society
Aresta et al.
3350 Organometallics, Vol. 14, No. 7, 1995
Recently, we have reported the isolation from (C2H4)2Rh(v6-PhBPh3)12( 1) of { [(CzH4)2Rh(v6-Ph)12BPh2}03SCF3 (2) and {[(C2H4)2Rh(v6-Ph)13BPh}(03SCF3)2(31, examples of a tetraphenylborate anion bridging two and three metal centers, respectively. Compounds 2 and 3 have been characterized in the solid state and so1ution.l’ As an extension of our studies on the use of coordinated BPh4- as a phenylating agent,2 we wish to describe in this paper a few aspects of the chemical reactivity of complexes 1-3 toward monoenes (ethylene, styrene), dienes (isoprene) and the use of 2 as an electrochemical catalyst for ethylene-CO2 coupling.
berment around the boron atom that increases in the order 1 < 2 < 3 < ([(C2H4)2Rh(v6-Ph)14B}(03SCF3)3. Such a view is supported by the fact that substitution of ethylene with dipy or diphos as ancillary ligands allows the coordination of only one Rh atom to BPh4-. Indeed, when (diphos)Rh(y6-PhBPh3)is reacted with Rh(C2&)2(03SCF3) (41, the dinuclear complex {[(CzH&Rh(y6-Ph)l[(diphos)Rh(~6-Ph)lBPh2}03SCF3 is formed14 but not isolated, as it converts quickly into 1 and Rh(diphos)(O3SCFs)(eq 3).
-
(diphos)Rh($-PhBPh,) + Rh(C2H4),(0,SCF,)
-
{ [(C2H4),Rh~g6-Ph)l[(diphos)Rh(~6-Ph)lBPh2)0,SCF,
Results and Discussion Stability of Coordinated Tetraphenylborate Anion. (C2H4)2Rh(v6-PhBPh3) (11, synthesized from [(C2H4)2RhC112and NaBPb, can be conveniently used as the starting material for the synthesis of di-, tri-, and tetranuclear $-BPb-Rh complexes according to eq 1.
+
Rh(diphos)(O,SCF,) (C,H,),Rh($-PhBPh,) (3) This reaction represents the first example of coordinated tetraphenylborate transfer between two metal centers. Reactivity of Complexes 1-3. (A) Phenyl Transfer from Boron to Monoolefins. When THF or CH2C12 solutions of one of the complexes 1-3 are left at room temperature (293 K) in uacuo or under an inert gas atmosphere (N2, Ar), a black precipitate (probably containing metal Rh) slowly separates within a few hours. The GC-MS analysis of these solutions reveals the formation of diphenyl, benzene, ethylbenzene, and styrene (eq 4). The formation of styrene and ethylben(CzH4)zRh(q6-PhBPh3) or
In the solid state, compounds 1-3 are stable for a long time, even in the presence of oxygen or moisture. In solution, their behavior strongly depends on the nature of the solvent and the experimental conditions. Coordinating solvents (CH3CN, DMF) easily displace the tetraphenylborate anion from the metal center to afford the ionic species [(C2H4)2Rh(S)n103SCF3and [(C2H4)2Rh(S),IBPh4 (S is solvent), the formation of which has been monitored by means of NMR and conductometric techniques.1$2However, loss of the BPh4- anion can be observed also in less coordinating solvents (THF, dichloromethane) when more than two Rh(C2H& moieties are coordinated to the BPh4- anion and according to the temperature. As a matter of fact, we have ascertained by lH NMR (see the Experimental Section)that complex 3 is partially dissociated (38% at 293 K) in CD2C12 according to equilibrium 2, which is shifted to right upon
increasing temperature (43% at 301 K). No evidence for a dissociative equilibrium analogous to (2)has been found for complexes 1 and 2 at temperatures lower than 310 K, both in CH2Cl2 and THF.13 In our opinion such a different behavior can be related t o the steric encum(12)This complex was first prepared by Osborn and Schrock (see ref 1bJ. In ref I r we have described a new synthetic procedure and reported the complete IR and NMR characterization. (13)Studies at temperatures markedly higher than 310 K were prevented from the reactivity shown by 1 and 2 (see later, in the text).
PhCH-CHz PhCHzCH3 I[(CzH4)zRh(q6-Ph)]~BPh~~O~SCF3 -------> PhH (4) or Ph-Ph I [(CZH~)ZR~(?~-P~)]~BP~}(O~SCF~)~ c-cc coupling
products (see also Table 1)
zene is particularly interesting as it requires a two-step mechanism based on (a) phenyl transfer from boron to coordinated ethylene with formation of a new C-C bond and (b) hydrogen transfer. Owing to the potentiality of using the tetraphenylborate anion as a stable, nontoxic, commercially available phenylating agent, we have further investigated this reaction at temperatures higher than 293 K focusing our attention on compounds 1 and 2,as 3 shows a high tendency to give 2 upon dissociation. In Table 1 we summarize the findings on the reactivity of 1 and 2 under C2H4 (0.1 MPa) at 338 K. After heating of THF solutions of 1 or 2 under the conditions given in Table 1 (entries 1 and 2) for about 12 h or less, benzene, diphenyl, styrene, and ethylbenzene were formed, together with minor amounts of other products. The phenyl group and ethylene itself can compete as hydrogen acceptors to afford benzene and ethane, respectively.’j The total balance of phenylated products shows that at least three phenyl groups per (14)The formation of {[(C~~)~Rh(~6-Ph)l[(diphos)Rh(~6-Ph)IBPh2}0~SCFs is justified considering that idiphos)Rh(tf-PhBPh3) in solution does not generate free BPh4-, as demonstrated by conductivity and NMR (IlB, ‘HI measurements.lg.2 The addition of Rh(C2H4)2(03SCF3) to the solution of Idiphos)Rh(@-PhBPh3)causes the change of the ‘H spectrum in the region of the co-ordinated and free phenyl groups accounting for an intermediate with two Rh atoms co-ordinated to tetraphenylborate. This species finally evolves to afford the limit isolated complexes (diphos)Rh(OsSCFs)and 1, with the elimination from the tetraphenylborate co-ordination sphere of the most encumbered moiety “(diphos)Rh”. I 15)The ‘H NMR (200 MHz) spectrum of a THF-de solution of 1 heated at 338 K in Vacuo or under ethylene shows a signahs) at 0.85 ppm indicative of the presence of ethane.
BH*--Rh Complexes
Organometallics, Vol. 14, No. 7, 1995 3351
Table 1. Reactiviw of 1.2, or NaBPL under Ethylene (0.1 m a ) at 338
Ka16
~~
productsC entry
compd
solvent
PhH
Ph-Ph
WCH-CHZ
PhEt
othersd
1 2 3 4 5
l(0.098 mmol) 2 (0.072 mmol) 2 (0.061 mmol) N a B P b (1.42 mmol) 1(0.042 mmol) + BPh3 (0.080 mmolp
THF ( 3 mL) THF ( 3 mL) CH3CN (3 mL) THF ( 3 mL) THF ( 3 mL)
0.90 1.15 0.68 traces 2.81
0.47 0.23 0.35 0.03 0.60
0.71 0.63 0.32
0.20 0.14 0.04
e e
0.47
f
g
0.36
The reaction time was equal to 12 h in all runs. The reacting system was very sensitive to oxygen: some phenol has been detected among the products when air was left to enter the reaction vessel. In the latter case, 2-OH-THF and/or :I-butyrolactone were also formed as side products when the reaction solvent was THF. Yields are expressed in moles of product per mole of BPh4- anion. These products have been identified by GC-MS analysis on the basis of their fragmentation pattern and by comparison with the mass spectra of pure (mlz 1321, and trans-stilbene were also formed compounds. e Butylbenzenes (m/z134), phenylbutenes (mlz 132), l-methyl-2,3-dihydroindene in yields lower than 1-28. fTraces of PhCH2CH2CN were found. The formation of this compound was not observed when a CH3CN solution of NaBPh4 was heated at 338 K under an ethylene atmosphere for 12 h. 8 Traces of terphenyls were detected in the reaction mixture. The GC-MS analysis of the reaction mixture, once the reaction was stopped, did not show the presence of residual amounts of BPh3. (I
Table 2. Reactivity of 1,2, (diphos)Rh(rf-PhBPL),or NaBPL toward Styrene at 343 KaJ’rL entry 1 2 3 4
Rh complex l(0.034 mmol) 2 (0.035 mmol)
(diphos)Rh(@-PhBPh3)(0.036 mmol) NaBPh4 (0.139 mmol)
PhH
Ph-Ph
PhEt
trans-PhCHICHPh
otherse
styrene conv (%)
0.86 1.19 1.27 traces
0.49 0.17 0.19 traces
0.66 0.38 0.77
0.50 0.21 1.53
f g
20 89 19
h
a The free styrene/BPh*- molar ratio was approximately equal to 20 in all runs, except run 4 where it was equal to 60. The reaction time was equal to 6.5 h in all runs. All runs were performed in THF ( 3 mL) under a dinitrogen atmosphere (0.1 MPa). Yields are expressed in moles of product per mole of BPh4- anion. e These products have been identified by GC-MS analysis on the basis of their fragmentation pattern and by comparison with the mass spectra of pure compounds. fThe GC-MS analysis of the reaction mixture showed the formation of small amounts of isomeric products with a masdcharge ratio of mlz 132 (formally involving styrene-ethylene coupling), Ph2CHCH3, Ph2C-CH2, and PhCH2CHzPh. Dimers of styrene (mlz 208), a few of which partially hydrogenated (mlz 210), were also formed in variable yields: among them we have identified 1,3-diphenyl-l-butene, 1,4-diphenyl-l-butene, and l,4-diphenylbutane. g The GC-MS analysis of the reaction mixture showed the formation of small amounts of isomeric products with a masdcharge ratio of mlz 132 (see footnote f, this Table), PhzCHCH3, Ph2C-CH2, and PhCHZCHzPh. Dimers, trimers, and higher oligomers of styrene were also found in the reaction mixture. The GC-MS analysis of the reaction mixture showed the formation of PhzCHCH3, PhZC-CHz, and PhCH2CHZPh. styrene oligomers [mlz 208 and 210 ( 1.3-diphenyl-1-butene and 1,4-diphenylbutane),m/z 3121, and other products formally - involving phenylation of styrene dimers (mlz 286).
BPh4- are converted into organic products. This result matches our previous findings of four phenyls per BPbused in the phenylation of aldehydes., Interestingly, under an ethylene atmosphere styrene is preferentially produced with respect to ethylbenzene with an overall reaction given in eq 5. However, the analysis of the data in Table 1 shows that benzene can be formed by other independent routes. Rh + C2H4 Ph-H + PhCH-CH, + boron species (“BPh,”) ( 5 )
BPh4-
The formation of all above mentioned organic compounds is, without a doubt, promoted by the Rh center. In fact, no formation of styrene, ethylbenzene, stilbene, and other substituted benzenes was observed when a THF solution of NaBPb was heated under ethylene a t 338 K (entry 4, Table 1): only trace amounts of benzene and diphenyl were formed in this case. In runs 1-3 (Table 1) BPh3 was never found at the end of the reaction. More interestingly, when free BPh3 was added t o the reaction mixture (entry 5 , Table 11, it was involved into the phenylation reaction and influenced the styrene/ethylbenzene molar ratio (the overall phenyl-ethylene coupling yield was practically unchanged). This result is quite intriguing. It was reported that BPh3 can coordinate to an electron-rich metal center through boron.16 The Rh species that are generated in solution do not appear t o fully meet this requirement. In fact, we have demonstrated that Rh(C2H4)2(03SCFd, (16)Dahlenburg, L.; Hock, N.; Berke. H. Chem. Ber. 1988,121,2083.
a possible daughter species, reacts with B P h , which can coordinate to Rh through the phenyl n-electron system.17 This might explain the BPh3 reactivity, although at the present state we cannot exclude a boron coordination. In CH3CN (entry 3, Table l), which is able to promote the decoordination of B P b - anion from the rhodium center and afford ionic species, 2 slowly converts into [WC2H4)2(C&CN)21BPh4and [Rh(CzH4)dCH3CN)z103SCF3. Under these conditions, the formation of styrene results to be repressed and only traces of ethylbenzene were detected in the reaction mixture. Moreover, stilbene and other benzene derivatives were not formed. The lower yield of phenylated products in CH3CN with respect to THF supports the idea that coordination of BPb- to the metal center promotes the phenyl transfer. A coordinating solvent can, thus, affect markedly both the distribution of products and the yield of the olefinphenyl coupling reaction. The formation of minor amounts of trans-stilbene and phenylbutenes can be explained as a further conversion of the styrene generated by PWCzH4 coupling. Table 2 (entries 1 and 2) shows the results obtained when complexes 1 and 2 were reacted with pure styrene. trans-Stilbene was formed in this case, and the yield was significantly affected by suitably changing the ancillary ligands at rhodium (entry 3, Table 2). The phenyl transfer reaction from boron to styrene is both regio- and stereoselective as no formation of cis-stilbene has been observed, whereas 1,l-diphenyl-ethylene was (
17)Aresta, M.; Quaranta, E. Forthcoming paper.
3352 Organometallics, Vol. 14,No. 7, 1995 formed only in traces. The phenylation of styrene is accompanied by an oligomerization reaction. The formation of styrene oligomers was particularly important when 2 was used as catalyst (see below). (B)Phenyl Transfer from Boron to Isoprene. The reaction of 1 or 2 with isoprene (dieneiBPh4- molar ratio = 30-50) in THF at 343 K (17 h) affords 1,ldimethylindene, the major product, and a mixture of 1,2dihydro-3-methyl- and 1,2-dihydro-2-methylnaphthalene (product distribution: 1:0.7:0.4, respectively). All these species formally involve phenyl transfer from boron to isoprene followed by the intramolecular cyclization of the resulting intermediate (eq 6). Other
Aresta et al. N2. The partial involvement of the solvent is clearly demonstrated by the fact that when (diphos)Rh(@ PhBPh3) was heated in CsD6 at 348 K (under Nz), diphenyldo, diphenylds, and diphenyldlo, besides small amounts of terphenyl-d, (n = 0,4,5),were found in the reaction mixture. Diphenyl can be formed by the coupling of two phenyl groups coming from the tetraphenylborate anion or generated from the benzened6 present in the reaction mixture. The formation of diphenyl-dlo from C6D6 suggests that, upon heating, (diphos)Rh(y6-PhBPh3)is able to promote the activation of aromatic C-D bonds. Most probably, the reaction involves Rh-Ph’g species according to eq 7a-d. The formation of (diphos)RhPh has been clearly documented and reported in a previous paper.‘g (diphos)Rh($-PhBPh,)
(diphos)RhPh isoprene phenylation products (mlz 146) were formed in very low yield. One mole of isoprene was phenylated per mole of phenyl transferred. The formation of both styrene and ethylbenzene is completely repressed under these conditions (see below paragraph D). It may be of interest to note that complexes 1 and 2 show a different behavior toward isoprene. The mononuclear complex 1 shows a negligible ability to oligomerize isoprene, while the dinuclear complex afforded dimers (mlz 1361, trimers (mlz 202 and 204), and tetramers (mlz 272) as well species involving the coupling of some of these oligomers with ethylene. The most abundant of all these species is 1,5,9-trimethyl-1,5,9-cyclododecatriene. Moreover, benzene was formed only in traces in both cases, and the dinuclear complex 2 produced diphenyl in smaller amounts (practically traces) than the mononuclear homologue 1. In order to get further information about the role of the metal center in these phenyl transfer processes, we have carried out the same reaction using NaBPh4. When NaBPh4 was reacted with in THF, coupling products were not isoprene (5.0“01) formed nor were isoprene oligomers.’* (C) Role of Solvents as the Source of Hydrogen Atoms. In the reactions discussed above, part of the BPh4- phenyls gave rise to benzene and diphenyl. In order to throw light on the reaction mechanism and the source of hydrogen demanded by benzene, we have investigated the thermal behavior of 1 in deuterated solvents. The GC-MS of a THF-d8 solution of 1 heated at 348 K shows the presence of benzene-d, [n = 0 and 1 (less abundant)] and diphenyl-&.lg The relative amounts of these products demonstrates that the hydrogen atom source is essentially ethylene itself or a phenyl group bound to boron but less probably the solvent. The generation of benzene and diphenyl seems to be a general feature of the reactivity of the tetraphenylborate anion $%oordinated to Rh centerse2 In fact, we have observed the formation of benzened,, diphenyld,, and terphenyls-d, (n = 0, 1) when a THF-da solution of (diphos)Rh($-PhBPh3) was heated at 348 K under i 181The GC-MS analysis of the reaction mixture showed the presence of diphenyl and traces of benzene. (19)Diphenyl-dl could not be unequivocally confirmed despite a number of attempts changing the reaction conditions.
(diphos)RhPh
2. (diphos)RhPh
+ BPh,
(7a)
(diphos)Rh(C,D,)
+ PhD
(7b)
+ C6D6 -
+ C&
-
C,H,-C,D, (diphos)Rh(C,D,)
+ C6D6-
+ “(diphos)RhD” (7c) +
C,D5-C6D5 “(diphos)RhD” (7d)
(D) Mechanistic Considerations. The phenyl transfer t o Rh from BPh4- (eq 7a) is expected to take place much more easily with complexes 1-3 with respect to Rh(diphos)($-PhBPhs), because of the higher electrophilicity of the Rh centers resulting from both the different donor-acceptor properties of ethylene with respect to diphos and, in the case of complexes 1 and 2, the net positive charge on the Rh atoms.20 Noteworthy, a species of type (C2H412RhPh (5) could not be isolated or detected spectroscopically, probably because of a higher kinetic lability with respect to the isolatedlg (diphos)RhPh. Starting from complexes 2 and 3, besides (C2H4)2RhPh, (C2H4)2Rh(03SCF3)(4) and BPh3 may be generated. In an ad hoc study we have found evidence that triphenylboron can react with 4 affording a new adduct in which BPh3 is q-coordinated to rhodium.” This finding can, in part, explain the reactivity of BPh3 as a source of phenyl groups reported above. The intermediate (C2H&RhPh can be generated from any of the complexes 1-3. Insertion of ethylene in the Rh-Ph bond of 5 can produce a “RhCH2CHzPh”species that can evolve to give styrene (eq 8).21The formation
-
“RhCH2CH2Ph “Rh-H”
+ CH2=CHPh
(8)
of PhEt or also ethane can be explained on the basis of (20) In THF-de solution, indeed, tdiphos)Rh(q6-PhBPh31 shows a much higher thermal stability than compounds 1-3. ( 2 1I The above described reactions are all Rh-mediated and do not appear to be initiated by radical species. As a matter of fact, when benzoyl peroxide was heated in THF in the presence of an excess of styrene or isoprene, in conditions suitable for the production of phenyl radicals 1373 K),”2we have not observed the formation of any coupling product such as ethylbenzene, stilbene, 1,l-dimethyl-indene, and 12dihydro-2-methyl- and 1,2-dihydro-3-methylnaphthalene. i 22 I Detar, D. F.; Long, R. A. J.: Rendceman, J.:Bradley, J.; Duncan, P. J . Am. Chem. SOC.1967,89. 4051.
BH4- -Rh Complexes
Organometallics, Vol. 14, No. 7, 1995 3353
Scheme 1
Scheme 2
A
Ph
Ph
1
/ \
L
Ph
J
n
-
phHd HPh
H
I
Rh I
I
L"
r
"Rh-H"
+ H
Ph
a hydrogen transfer to styrene or ethylene. As far as PhEt formation is concerned, it may be worth t o emphasize the following: significant amounts of PhEt were formed when 1 or 2 were reacted with styrene (Table 2); ethylbenzene was preferentially formed with respect to styrene when complexes 1 or 2 were heated in THF in vacuo or in an inert gas atmosphere, whereas the opposite took place in the presence of an excess of ethylene. Very little hydrogen comes from the solvent as confirmed by the fact that minor amounts of PhCHDCH3 and PhCHDCHzD were formed when 1 was heated in THF-ds (both under ethylene and i n These results agree with the above described role of solvent as source of hydrogen. BPh3 produced seems to be not involved in further production of styrene: more likely, it generates diphenyl and benzene. In fact, no increase of phenyl-ethylene coupling product has been observed when 1 was heated under ethylene in the presence of an excess of BPh3, whereas both diphenyl and, more markedly, benzene formation resulted in being favored. The decomposition of triarylboron compounds to symmetric biaryls in the presence of Pd(0) has been reported by Chiusoli and cow o r k e r ~who, , ~ ~ however, have not described the formation of benzene. The mechanism through which these transformations can take place is still unclear; further investigation is needed also for clarifymg the final fate of boron. When isoprene or styrene are added to 1 or 2, the ethylene substitution is a fast process that precedes the phenyl migration from boron to rhodium. This is supported by the fact that, in the reaction of 1 or 2 with isoprene, both styrene and ethylbenzene formation are almost totally repressed. If styrene is the substrate, the ligand substitution step results to be thermodynamically or kinetically less favored for 2 than for 1,most probably because of the cationic character of the catalyst, as demonstrated by the significantly different yield of stilbene and PhEt in the two cases (entries 1 and 2, Table 2). Schemes 1 and 2 summarily show the pathways for the formation of stilbene from styrene and 1,l-dimethylindene and 1,2-dihydro-2-methyl-and 1,2-dihydro-2methylnaphthalene from isoprene. The fact that 2 behaves as a good catalyst for the formation of styrene or isoprene oligomers is explained 123)The experiments in deuterated solvents also reveal the formation of low amounts of styrenedl in addition to the more abundant styrene-&. 124)Catellani, M.; Chiusoli, G. P.; Fornasari, V. Gazz. Chim. Ital. 1990,120,779.
/
M \e
1
-
"Rh-H"
on the basis of the fact that 2 easily generates the complex L2Rh03SCF3 (L = styrene, isoprene) that has been isolated. (C2H4)2RhO3SCF3lrwas also prepared by an independent route and reacted with styrene or isoprene in conditions analogous t o those used for their reaction with 125and shown to afford the same products discussed here. (E)Complexes 1-3 us (diphos)Rh(rf-PhBPb) Reactivity toward Carbon Dioxide. Our interest in carbon dioxide activation by metal centers and in carboxylation reactions of organic substrates using C0226pushed us to investigate the reactivity of complexes 1-3 toward C02. Differently from (diphos)Rh($-PhBPh3), which has been shown to be able to coordinate C02 (CH2C12,293K, 1MPa C 0 2 pressure),'g complexes 1-3 are not able to afford stable adducts with the heterocumulene. Complexes (diphos)Rh($-PhBPh) and 1-3 are all 18 e- systems and, as judged by their NMR spectra," the r6 q4 v2 bound phenyl fluxional process, which would make available coordination sites on rhodium, is absent at room temperature. The interaction of LzRh($-Ph) moieties [Lz = (CzHd2 or diphosl with carbon dioxide should involve, as in the a full orbital of the metal and an case of Rh(diars)~Cl,~' empty antibonding n-orbital of C02. The different behavior of compounds 1-3 toward C 0 2 with respect to (diphos)Rh(@-PhBPh3) emphasizes the role and importance of the ancillary ligands in stabilizing the coordination of carbon dioxide to a metal center: the weaker a-donor and better n-acceptor C&, with respect to diphos, does not favor the formation of a stable COS-
- -
125)Aresta, M.; Quaranta, E. Manuscript in preparation. 126)Aresta, M.; Quaranta, E.; Tommasi, I. New J.Chem. 1994.18, 133 and references therein. i27)Calabrese, J. C.; Herskovitz, T.; Kinney, J. B. J. Am. Chem. SOC.198% 105, 5914.
Aresta et al.
3354 Organometallics, Vol. 14,No. 7, 1995
-2
/
-1
A
/ under C02 (0.1 MPa) curve ( b )
Figure 1. Cyclic voltammograms obtained with a freshly polished gold microelectrode (3 mm2,Tacussel), at 293 K at a scan rate of 200 mV/s, for a solution of 2 (1.25mM) in THF (20 mLJ containing 0.15 M tetrabutylammoniumtetrafluoroborate as supporting electrolyte: (a) solution under argon; (b) solution saturated with COZ. Rh adduct. Moreover, the stability of a M-);I'(C)-CO~ adduct is strongly affected by the coulombic interaction between the net charge on the metal and the C02 carbon atom:28the more positive the charge on the metal, the more destabilizing the contribution of this energetic term. Consequently, the existence of a net positive charge on the Rh atoms of 1 ( f V 2 e ) and 2 (f2/3 e ) represents a further action which contributes to determine the scarce reactivity of 1 and 2 toward carbon dioxide. Heating (diphos)Rh(g6-PhBPh3)under a carbon dioxide atmosphere produces in good yield the benzoato complex (diphos)RhO(O)CPh.lg The formation of this species has been rationalized as a result of the insertion of the heterocumulene in the Rh-Ph bond of (diphodRhPh, the product obtained upon transfer of a phenyl from boron to rhodium. No significant formation of carboxylation products has been observed when complexes 1-3 were reacted with C02 or C2WCO2 mixtures a t 353 K, but benzene, diphenyl, and the phenylethylene coupling products were observed. This suggests that the ethylene-CO2 competition for the phenyl moiety is resolved in favor of the olefin, most likely because ethylene is already in the coordination sphere of the metal. This result is a further demonstration of the fact that all the chemistry described above is not radical chemistry but takes place around the metal center. The higher n-acceptor character of C2H4 with respect to diphos is another point in favor of the ethylene-phenyl preferential coupling.29 Electrochemical Behavior of {[(C2&)2Rh(qsPh)hBPh2)0sSCF3. The poor tendency of complexes 1-3 to undergo a chemical interaction with C02 suggested us to investigate if the olefin-CO2 coupling could be better achieved using other strategies. We have explored the electrochemical way, focusing our attention on complex 2. The cyclic voltammetry curves of complex 2 in THF containing tetrabutylammonium tetrafluoroborate as (28)Sasaki, S.;Dedieu, A. Inorg. Chem. 1987, 26, 3278.
supporting electrolyte are presented in Figure 1, curve a. An irreversible one-electron reduction peak at -1.35 V us SCE is followed by a second irreversible oneelectron reduction peak a t -1.74 V. Only low-intensity reoxidation peaks appear around -0.3 and +0.1 V. The addition of an internal alkyne such as 4-octyne to a solution of complex 2 in THF (alkynd2molar ratio equal t o 1) did not modify the electrochemical behavior of 2. Thus, it appears that the alkyne does not coordinate to the rhodium complex, and ethylene ligands in 2 are not exchanged with the alkyne. Conversely, the addition of DMF (2 mL, 5% v/v) to the initial THF solution of 2 results in the shift of the reduction peaks to -1.30 and -1.60 V, respectively: the rhodium complex is now modified. Most probably, cationic complexes are formed with the highly coordinating DMF cosolvent (see above), which are more easily reducible. This behavior is in very good agreement with the chemical results. The further addition of DMF does not modify the reduction potentials. Where the electron is located, if it is at the Rh atom or it is delocalized on the extended n-electron system involving the ethylene molecules and the coordinated phenyl, is difficult to say. We have in progress ESR studies to elucidate this point. Bubbling C02 into a solution of 2 results in the modified cyclic voltammogram as illustrated in Figure 1, curve b. The first nonreversible reduction peak is shifted to -1.45 V, and its intensity is increased. The second peak appears at -1.80 V, also with a higher intensity. The increase of the peak intensities corresponds t o a slight irreversible catalytic current both at -1.45 and -1.80 V.30 The effect of C 0 2 addition is reversible and does not decompose the rhodium complex. As a matter of fact, when the CO2-containing solution (29)A study by Darensbourg [Darensbourg, D. J.; Grotsch, C. G.; Wiegreffe, P.; Rheingold, A. L.Inorg. Chem. 1987,26,38271has shown that COz insertion in Rh(I)-Ph bond is favored by strong cr-donor ligands in the coordination sphere of Rh. (30)As a comparison,the electrochemical reduction of [Rh(CzH&Clln in THF shows a single irreversible reduction peak at -1.65 V us SCE, which is not modified in the presence of CO:! (0.1 MPa).
Organometallics, Vol. 14, No. 7, 1995 3355
BHd--Rh Complexes Scheme 3 [Rh]*
+
CO,
+ e--
-1.45 V
[Rh]
of curve b is degassed with argon, the initial electrochemical behavior of complex 2 is found ( e g . curve a). It is worth noting that the direct reduction of C02 occurs a t potentials more negative than -2.2 V us SCE, in agreement with literature data.31 Earlier work has shown that the catalytic process observed is characteristic of the reduction of coordinated COZto its radical anion (Scheme 3).32 The potential shift of 2 in the presence of CO2 indicates an interaction between the rhodium and COz, by C02 coordination to the reduced species formed at -1.35 and -1.74 V. The reversibility of the CO2 coordination supposes that the radical anion of CO:! evolves to form species without modification of the Rh complex. The reduced CO2 can then yield oxalate, by dimerization, or CO and carbonate, by disproportionThe reduced complex 2 (both at 1 and 2e-) ation.31132,33 is, thus, a catalyst for the COz reduction. However, the relatively small increase of the peak intensities under COz indicates a slow C02 reduction process at the time scale of the cyclic voltammetry. The reactivity of the electrogenerated species at the potentials shown in Figure 1 was examined in electrolyses carried out at room temperature in a singlecompartment cell fitted with a consumable magnesium anode. Recent advances in catalyzed electrochemical reactions involving COZactivation have been associated with the use of sacrificial anodes.34 The electrolysis of 2 in THF, at -1.5 V, under a Cod Ar atmosphere (0.05 MPa C02 pressure, total pressure = 0.1 MPa) proceeded up to the consumption of 4 F/mol of 2. The solution, initially sharp yellow, turned quickly to orange and, then, to dark-brown. The electrolysis was stopped, the solution was stirred for 2 h, and the organic products were analyzed by GC and GC-MS. The reactivity of 2 and COZa t -1.50 V is shown in eq 9. Diphenyl is formed in high yield together with several 2+
co,
~ ( - 1 . 5 V i Mganode ,
THF, 293 K
0.05 MPa Ph-Ph (50%) PhEt (7%) PhCH=CH, (2%) + Ph(CH2),Ph (2%) CH3CH2C(0)OH(6%)+ PhC(0)OH (1%) (9)
+
+
+
phenyl-ethylene coupling products, which is reminiscent of their chemical behavior. Interestingly, ethylene could b e carboxylated u n d e r the very mild reaction conditions, leading to propionic acid in 6% yield. COz (31) Amatore, C.;Saveant, J.M. J . Am. Chem. SOC.1981,103,5021 and references therein. (32)( a ) Daniele, S.;Ugo, P.; Bontempelli, G.; Fiorani, M. J . Electroanal. Chem. Interfacial Electrochem. 1987,219,259. ( b )Gamier, L.;Rollin, Y., Perichon, J. New J . Chem. 1989,13,53. (33)Dubois, D. L.;Miedanes, A. J . A m . Chem. Soc. 1987,109,113. (a)Derien, S.;DuAach, E.; Perichon, J. J.A m . Chem. Soc. 1991, (34) 113,8447. ( b ) Derien, S.;Clinet, J. C.; DuAach, E.;Perichon, J. J . Org. Chem. 1993,58,2578.
was also partially incorporated into phenyl groups yielding benzoic acid. The electricity consumption of 4 F/mol of 2 indicates a rhodium-catalyzed COZreduction in agreement with the cyclic voltammetry experiments. The higher electron density a t the metal center or the localized electron density on ethylene promote the interaction of the metal-ethylene moiety with the heterocumulene that results in its fwation. In order to investigate if ethylene could be carboxylated to a higher extent, we have investigated the electrochemical behavior of a solution of 2 in THF under a 1:l mixture of ethylene and carbon dioxide (0.1 MPa overall pressure) at -1.80 V up to the passage of 4 F/mol of 2, when the intensity slowly started to decrease to zero. The COZ incorporation into ethylene afforded propionic acid (eq 10) in 16%yield. Benzoic acid was not formed, and the formation of benzene (4%) and diphenyl (3%) was repressed. Styrene is formed in higher yield than in absence of ethylene.
+ +
+e (-1.8V), Mg anode
C02 C2H4 THF. 293 K 0.05 MPa 0.05 MPa Ph-Ph (3%) PhEt (4%) PhCH-CHZ (10%) PhH (4%) CH,CH,C(O)OH (16%) (10)
2+
+
+
+
These results are in good agreement with the chemical findings and show that carbon dioxide interaction with the metal system takes place when an excess negative charge is present. Noteworthy, the ethylene carboxylation takes place easily and in the presence of free ethylene the yield is increased. The essential role of the metal is confirmed. Studies are in progress aimed at identificating the species obtained upon 1 or 2etransfer to 2.
Experimental Section G e n e r a l C o m m e n t s . Unless otherwise stated, all reactions and manipulations were carried out under dinitrogen, argon, or carbon dioxide atmosphere (as specified in the text) with rigorous exclusion of both air and atmospheric moisture, by using vacuum line techniques. All solvents were dried as described in the l i t e r a t ~ r eand ~ ~ stored under dinitrogen. Complexes 1-3 and (diphos)Rh(q6-PhBPh3)were prepared as previously described.’gJ Styrene and isoprene (from Fluka) were distilled before using. Tetrabutylammonium tetrafluoroborate (Fluka) was dried by heating overnight a t 343 “C in vacuo. Magnesium used as the anode was of high purity (magnesium rod, 99.8%, 1 cm diameter, Prolabo). Ethylene (99.7% minimum) and COn (99.99% pure) were from Matheson and SI0 SPA, respectively. Infrared spectra were obtained with a Perkin-Elmer 883 spectrophotometer. NMR spectra were recorded with a Varian XL 200 or a Bruker AM 500. GC-MS analyses were carried out with a H P 5890 gas chromatograph linked to a H P 5970 selective mass detector (capillary column: SE-30, 30 m x 0.000 32 m, 0.25 Jim film thickness). GC analyses were obtained with a DAN1 HR 3800 gas chromatograph equipped with a Carbopack C 0.1% packed column or a Durawax-DX1 capillary column (30 m x 0.000 329 m, 0.25 pm). HPLC analyses were performed with a Perkin-Elmer Series 4 LC (column: Erbasil C18/M, 10 Jim, 250 x 4.6 mm) connected with a LC 290 UV/vis spectrophotometer detector. HPLC separation of the products was performed using a Perkin-Elmer LC Series 2 (column: Partisil M9 10/25 ODS). (35)Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals; Pergamon Press: Oxford, England, 1986.
3356
Organometallics, Vol. 14, No. 7, 1995
The electrochemical one-compartment cell is a cylidrical glass vessel of 40 mL volume such as that described in ref 36, equipped with a gold grid cathode (20 em2)),a magnesium-rod (immersed 3 cm) anode, and a SCE electrode. Cyclic voltammetric experiments and controlled-potential electrolyses were performed with the aid of Solea-Tacussel conventional equipment and were carried out a t 293 K by utilizing a gold microelectrode (Tacussel). All potentials were quoted with respect to a saturated calomel electrode (SCE). Behavior of 3 in Solution. The behavior of 3 in solution was studied by NMR. An analytically pure sample of 3 was dissolved in CD2C12, and the 'H NMR spectrum of the resulting solution was recorded a t both 293 and 301 K. The aromatic region showed, besides the signals of the trinuclear complex 3 L6.56 (t, Hm.rls.-'phl:i, 3 J ~ , - =~ ,3 J ~ , - = ~ ,5.9 HZ), 6.69 (d, Ho.r,6.,phl.,),7.34 (m, Hm.BPh), 7.39 (m, Hp.r/s.-'Phi,,), 7.40 (m, Hm.BPhr), 7.69 (d, Ho.Bph, ' J H ~=- 7.16 H ~Hz)], also the resonances due to the dinuclear species 2 [6.36 (t, Hm,,,%ph12, 3 J ~ , -= ~ ,3, J ~ , -=~ p 6.38 Hz), 6.52 (d, Ho,,,%ph12), 7.13 (m, Hp.BPhr),7.24 (m, Hm.BPhZ), 7.26 (t, Hp.p.,phig,7.40 (d, Ho.Bph2,3 J ~ - =~ 6.84 , Hz)]. The relative amounts of 2 and 3, under equilibrium conditions a t the given temperature, could be calculated from the integrals ofthe protons. Hm.,,S.{phr2 (6.36 ppm, t ) and Ho.,,K-iphi:I (6.69 ppm, d), which do not overlap with the signals of other protons, are the most reliable. Reaction of 1-3 with Ethylene: General Procedure. A THF ( 3 mL) solution of the complex (see Table 1 ) was prepared under ethylene (0.1 MPa) in a 10 mL glass tube. After the tube was sealed, the solution was stirred a t 338 K for 12 h and then cooled to room temperature and analyzed by means of chromatographic techniques. The isolation of the products was performed by HPLC. (36)Chaussard, J.; Folest, J. C.;Nedelec, J. Y.; Perichon, J.; Sibille,
S.; Troupel, M. Synthesis 1990, 5, 369.
Aresta et al. Reaction of 1-3 with Styrene or Isoprene: General Procedure. A THF ( 3 mL) solution of the complex and the substrate [reaction with styrene, see Table 2; reaction with isoprene, NaBPh4 (0.051 05 g, 0.15 mmol), isoprene (0.5 mL, 5 mmol); l ( 0 . 0 4 9 05 g, 0.10 mmol), isoprene ( 0 . 3mL, 3 mmol); 2 (0.023 g, 0.029 mmol), isoprene (0.15 mL, 1.5 mmol)] was prepared under dinitrogen in a 10 mL glass tube. After the tube was sealed, the solution was stirred a t 343 K for 6.5 h (styrene)or 17 h (isoprene) and then cooled to room temperature and analyzed by means of chromatographic techniques. The isolation of pure products was performed by HPLC. Electrocarboxylation: General Procedure. A THF (30 mL) solution containing 2 (0.097 g, 0.12 mmol) and tetrabutylammonium tetrafluoroborate ( 1 g, 3 mmol) was placed in the cell under COz (or COz/CzH4) and stirred a t 393 K. The desired potential was applied between the electrodes and the electrolysis continued until the intensity was negligible. Without the anode, the reaction mixture was stirred for 1-2 h under CO:! (or COdC2H4). The carboxylic acids of the mixture were directly esterified by adding anhydrous K2C03 ( 0 . 5 mmol) and benzyl bromide ( 1 mmol) in THF/acetone ( 1 : l v/v, 60 mL) and stirring a t 323 K for 10 h. The solution was hydrolyzed with 70 mL of 0.01 M HC1 solution and extracted with diethyl ether. The organic layer was washed with water, dried over MgS04, and evaporated. The products were analyzed by GC and GC-MS, the carboxylic acids being analyzed as benzyl esters.
Acknowledgment. This work was supported by the Minister0 dell'universita e della Ricerca Scientifica e Tecnologica (MURST, 40% and 60%funds) and the CNR (Progetto Finalizzato Chimica Fine 11). OM940795G
