Chemistry and catalysis advances in organometallic chemistry and catalysis
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263 Rh I CO I I OC O H 3 C − PPN + Rh CO I OC I − PPN + + CH
3 COI
4 1 2277.0
−0.010 0.00
0.01 0.02
0.03 0.04
0.05 0.06
0.07 0.08
A 0.09
0.10 0.11
0.12 0.13
0.14 0.15
0.16 0.17
0.18 0.186
2200 2100
2000 1950
1900 cm −1 1850 1800
1750 1700
1643.3 Figure 20.4 Infrared monitoring of the reaction of 4 under 15 bar CO at 25 ◦ C in dichloromethane. 190 188
186 184
182 180
178 176
174 172
ppm 5% H
2 O w/w
10% H 2 O w/w 20% H 2 O w/w CO CH 3 COOH CH 3 COOCH 3
1 4 4 Figure 20.5 Representative 13 C-HP-NMR spectra recorded under methanol carbonylation conditions (0.1% Rh, CD 3 COOD/MeOAc/ CH 3
13 CO, 90
◦ C) at different water content. (Figs. 20.9–20.11). Surprisingly, it appears that the obtained energy profiles are dramatically low in energy. First, the intermediate acetate complexes [RhI 2 (OAc)(CH
3 CO)(CO)
2 ] − arising from the two fac,cis- or mer,trans- species should spontaneously be formed. Second, from the three possible pathways A, B, and C, the energy barrier varies from 14 to 23 kcal/mol (Figs. 20.9–20.11). The first of the two isomers, produced from mer,trans-4, presents two iodo ligands in trans and the two CO ligands in cis position owing to the moving of one CO in trans of the acetyl ligand (intermediate at −14 kcal/mol, pathway A, Fig. 20.9). The second isomer in pathway B adopts two CO in trans and the two iodo in cis,
264 ACETATO LIGAND IN THE REDUCTIVE ELIMINATION STEP OF THE RHODIUM-CATALYZED METHANOL CARBONYLATION CO CO
CH 3 Rh I I I CO I I − O Me O I CH 3 Rh I CO CO CO O − CH 3 Rh I I I CO CO I I O − − − Me Rh I CO CO − Rh I I I CO − Rh CO I CO I O Me Rh I I CO 3 3 29 1 0 20 26 8 Figure 20.6 Gibbs free-energy pathway (energies in kcal/mol) computed for the internal rearrangement leading to the reductive elimination of CH 3 COI from mer,trans-[RhI 3 (COMe)(CO) 2 ]
and fac,cis-[RhI 3 (COMe)(CO) 2 ] − . 0 10 20 30 40 50 60 70 0 5 10 15 Water content (wt %) Conductivity (ms/cm)
Conductivity measurements of water/acetic acid mixtures. the acetyl and acetate being trans to the iodo ligands (Fig. 20.10). From fac,cis-4, only the isomer in pathway C exists, in which the two iodo and one CO ligand geometry relative to the acetyl ligand remains unchanged (Fig. 20.11). Thus the stabilizations are respectively of −14, −24, and −31 kcal/mol when one iodo ligand is substituted with an acetate ligand. Correlatively, the transition states to produce 1 equiv of acetic anhydride and regenerate the cis- or trans-[RhI 2 (CO)
2 ] − species display low, although slightly different, energy barriers. If we consider first path A (Fig. 20.9), the transition state, in which the oxygen atom approaches the carbon atom of the acetyl group at a distance of 1.59 ˚ A, the two rhodium–oxygen and rhodium–carbon distances being extended to 2.15 and 2.57 ˚ A, has the same energy as the starting mer,trans-4 complex. The energy barrier of 14 kcal/mol is low, presumably because the back donation of the acetyl group on the CO ligand in trans position is important, giving rise to a rather facile C–O bond formation similar to an internal nucleophilic attack of the oxygen atom on the acetyl carbon atom. Then the reductive elimination reaction is largely exergonic ( −36 kcal/mol) producing trans-[RhI 2 (CO)
2 ] − and acetic anhydride. From the mer,trans-4 complex, path B (Fig. 20.10) produces cis,cis,trans-[RhI 2 (OAc)(CH
3 CO)(CO)
2 ] − , and, because of the polarization of the two Rh–I bonds due to strong σ -effect of the acetyl and acetate ligands, the approach of the two fragments, which can be assimilated to a nucleophilic attack, proceeds less easily than in path A. Thus, the calculated energy NMR AND INFRARED HIGH PRESSURE STUDIES AND DFT CALCULATIONS 265 I CO O CH 3 Rh CO O O − + CH 3 I I CO O O O CH 3 CH 3 Rh CO I I I CO CH 3 CH 3 Rh CO I I O O O I CO CH 3 CH 3 CO Rh I I I − − − − − − O O O + + − 0 39 56 −36
TS Figure 20.8 Gibbs free-energy pathway computed for the direct attack of an OAc − ion on the acetyl carbon atom of mer,trans- [RhI 3 (COMe)(CO) 2 ] − . CO I O O O mer,trans Me + CH 3 Rh I I CO CO I O CH 3 CH 3 Rh I CO O O − − CO I O CH 3 CH 3 Rh I CO O O − − −0.57 −0.25 −0.29 0.62 0.75 2.15
2.57 1.59
+ I − + I − I CO Rh CO I − O O O CH 3 CH 3 + + I − 0 0 −14 −36 A
Reaction path A for the substitution of I − with OAc − in mer,trans-[RhI 3 (COMe)(CO) 2 ] − and reductive elimination of CH 3 COOCOCH 3 (Ac 2 O).
barrier is 23 kcal/mol, that is, 64% higher than in the first path. Here also, the energy of this intermediate is about the same as that of the starting mer,trans-4 complex. The reductive elimination step liberates 35 kcal/mol. In path C (Fig. 20.11), the stabilization of cis,cis,cis-[RhI 2 (OAc)(CH 3 CO)(CO)
2 ] − from fac,cis-[RhI 3 (COMe)(CO) 2 ] − reaches 31 kcal/mol, the energy barrier is 18 kcal/mol, and the cis-[RhI 2 (CO) 2 ] − isomer, which is usually found in the catalytic solutions, is at −43 kcal/mol energy level, which is more favorable than the trans-isomer by 7 kcal/mol. Thus the two pathways A and C appear more favorable than the direct reductive elimination of CH 3 COI from 4 (energy barrier of 26 kcal/mol) or path B (23 kcal/mol). Indeed, the {[RhI
2 (CO)
2 ] − + CH 3 COI } system is at the same level of energy than the starting acetyl complex 4, whereas pathways A and C lead to a {[RhI 2
2 ] − + Ac 2 O } system characterized by a
−36 or −43 kcal/mol energy level. For A and C, the Rh–O and Rh–C(acetate) distances are 2.15 and close to 266 ACETATO LIGAND IN THE REDUCTIVE ELIMINATION STEP OF THE RHODIUM-CATALYZED METHANOL CARBONYLATION − CO
O 0 −36 CH 3 Rh I I CO CO O CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 Rh CO I O O I CO CO Rh I I CO O O O Rh CO I I 0.74 -0.27 -0.49 0.71 2.26
2.73 1.50
O O Me + −
+ I −
+ I − + I − O O O − −24 −1 B Figure 20.10 Reaction path B for the substitution of I − with OAc
− in mer,trans-[RhI 3 (COMe)(CO) 2 ] − and reductive elimination of Ac 2 O. − CO fac,cis CO O CH 3 Rh I I I − CO CO O CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 Rh O O I I CO CO − Rh I I O O −
+ Me CO CO O − Rh O O I I
-0.56 0.74 2.15
2.51 1.50
-0.51 0.75 O O O + + I − + I
− + I
− 3 C −28 −10
−43 Figure 20.11 Reaction path C for the substitution of I − with OAc
− in fac,cis-[RhI 3 (COMe)(CO) 2 ] − and reductive elimination of Ac 2 O. 2.54 ˚ A, respectively, whereas in B the Rh–C distance is 2.73 ˚ A, which is consistent with a less electrophilic carbon atom. In addition, path C is certainly the most feasible way to produce acetic anhydride and regenerate the active species cis-1. It is interesting to note that a mechanistic study carried out on the palladium-catalyzed biphasic carbonylation of iodobenzene has shown that an indirect route to produce benzoic acid involves benzoic anhydride [35]. For instance, isolation of [Pd(
μ-OH)Ph(PPh 3 )] 2 and further reaction with PhI under a CO atmosphere leads to [Pd( μ-I)(COPh)(PPh 3 )] 2 , (PhCO)
2 O
REFERENCES 267 and H
2 O in almost quantitative yields. More particularly, the two [Pd(Ph)(COOPh)(PPh 3 )
] and [Pd(Ph)(COOPh)(PCy 3 ) 2 ] complexes, in which the triphenyl- and tricyclohexylphosphine ligands are in trans-position and the benzoate ligand is bonded in a κ 1 -mode, undergo a CO insertion in the palladium–phenyl bond to provide the [Pd(COPh)(COOPh)(PPh 3 ) 2 ] and [Pd(COPh)(COOPh)(PCy 3 ) 2 ] complexes. Reductive elimination to produce benzoic anhydride occurs very slowly at 70 ◦ C under CO for the latter complex, but rather easily for the PPh 3 containing complex, which leads to the palladium(0) cluster [Pd 3 (CO) 3 (PPh
3 ) 3 ]. In addition, the carbonylation of an aryl iodide into the corresponding anhydride with yields as high as 98% has been achieved using the {Pd(OAc) 2 /bis(diphenylphosphino)propane/NEt 3 /DMF as solvent/0.5 equiv H 2
} catalytic system [36]. The authors suppose that the H 2 O molecule attacks the Pd-acyl intermediate to generate a Pd–H species (NEt 3 reacting to remove [NEt 3 H]I and produce [Pd (0) (DPPP)]) and the aryl carboxylate which reacts with the Pd-acyl species. Finally, the nucleophilic attack of a palladium-acyl species has been extensively proposed in the alcoholysis step for the termination step of the carbon monoxide/ethylene alternating copolymerization process [37–39]. However, the characterization of the (benzoyl)benzoate)palladium complex shows that this classical reductive elimination step, in which even the two groups are in trans position, can occur in competition with the nucleophilic attack of an acyl group [35].
The present experimental and theoretical investigations reveal that in the [RhI 3 (COMe)(CO) 2 ] − complex, resulting from the successive oxidative addition of CH 3 I on [RhI 2 (CO)
2 ] − , migratory CO-insertion, and CO coordination, one iodo ligand can be easily substituted with an acetato OAc − ligand to provide the [RhI 2 (OAc)(COMe)(CO) 2 ]
intermediate. From the two cis,cis,cis- and fac,cis-isomers where the acetyl and acetate groups are in cis-position, a significantly exergonic reductive elimination takes place spontaneously to produce acetic anhydride and regenerate the active [RhI 2 (CO)
2 ] − species in the methanol carbonylation reaction. The energy barriers can be as low as 14 and 18 kcal/mol and be consistent with this mechanism, which competes with the classically admitted reductive elimination of CH 3 COI, requiring 25–26 kcal/mol but being almost thermoneutral [40]. REFERENCES 1. Harpe H. J. Industrial Organic Chemistry, 5th ed.; Wiley-VCH Verlag: Weinheim, Germany, 2010; p 175. 2. Reppe, W.; Friederich, H.; von Kutepow, N.; Morsch, W. (BASF). U.S. Patent 2,729,651, 1956. 3. Reppe, W.; Friederich, H. (BASF). U.S. Patent 2,789,137, 1957. 4. Mullen, A. In New Syntheses with Carbon Monoxide; Falbe, J., Ed.; Springer-Verlag: Berlin, 1980; p 243. 5. Forster, D.; Singleton, T. C. J. Mol. Catal. 1982, 17 , 299. 6. Dekleva, T. W.; Forster, D. Adv. Catal. 1986, 34 , 81. 7. Mizoroki, T.; Nakayama, M. Bull. Chem. Soc. Jap. 1965, 38 , 2876; 1968, 41, 1628. 8. Paulik, F. E.; Roth, J. F. J. Chem. Soc. Chem. Commun. 1968, 1578. 9. Roth, J. F.; Craddock, J. H.; Hershman, A.; Paulik, F. E. Chem. Technology 1971, 600. 10. Forster, D. Adv. Organometal. Chem. 1979, 17 , 255. 11. Howard, M. J.; Jones, M. D.; Roberts, M. S.; Taylor, S. A. Catal. Today 1993, 18 , 325. 12. Maitlis, P. M.; Haynes, A.; Sunley, G. J.; Howard, M. J. J. Chem. Soc. Dalton Trans. 1996, 2187. 13. Cheung, H.; Tanke, R. S.; Torrence, G. P. Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed., Wiley-VCH: Weinheim, 2000. 14. Yoneda, N.; Kusano, S.; Yasui, M.; Pujado, P.; Wilcher, S. Appl. Catal. A 2001, 221 , 253. 15. Gauss, M.; Seidel, A.; Torrence, P.; Heymanns, P. In Applied Homogeneous Catalysis with Organometallic Compounds; Cornils, B.; Herrmann, W. A., Eds.; Vol. 1, Wiley-VCH: Weinheim,1996; Torrence, P. In Applied Homogeneous Catalysis with Organometallic
16. Thomas, C. M.; S¨uss-Fink, G. Coord. Chem. Rev. 2003, 243 , 125. 17. Haynes, A. Top. Organometal. Chem. 2006, 18 , 179. 18. Haynes, A. Adv. Catal. 2010, 53 , 1. 19. Jones, J. H. Platinum Metals Rev. 2000, 44 , 94. 20. McConnell, A. C.; Pogorzelec, P. J.; Slawin, M. Z.; Williams, G. L.; Elliott, P. I. P.; Haynes, A.; Marr, A. C.; Cole-Hamilton, D. J. Dalton Trans. 2006, (1), 91. 268 ACETATO LIGAND IN THE REDUCTIVE ELIMINATION STEP OF THE RHODIUM-CATALYZED METHANOL CARBONYLATION 21. Bassetti, M.; Monti, D.; Haynes, A.; Pearson, J. M.; Stanbridge, I. A.; Maitlis, P. M. Gazz. Chim. Ital. 1992, 122 , 391. 22. Ellis, P. R.; Pearson, J. M.; Haynes, A.; Adams, H.; Bailey, N. A.; Maitlis, P. M. Organometallics 1994, 13 , 3215. 23. Nguyen, D. H.; Lassauque, N.; Vendier, L.; Mallet-Ladeira S.; Le Berre C.; Serp P.; Kalck P. Submitted for publication. 24. Gautron, S.; Giordano, R.; Le Berre, C.; Jaud, J.; Daran, J.-C.; Serp, P.; Kalck, P. Inorg. Chem. 2003, 42 , 5523. 25. Sunley, J.G.; Garland, C.S.; Giles, M. F. (BP Chemicals). European Patent 643 034, 1994. 26. Garland, C. S.; Giles, M. F.; Poole, A. D.; Sunley, J. G. (BP Chemicals). European Patent 728 726, 1994. 27. Ghaffar, T.; Adams, H.; Maitlis, P. M.; Sunley, J. G.; Baker, M. J.; Haynes, A. Chem. Comm. 1998, 1023. 28. Le Berre, C.; Serp, P.; Kalck, P.; Layeillon, L.; Thi´ebaut, D. (Acetex Chimie). French Patent 98.13954, 1998. 29. Gautron, S.; Lassauque, N.; Le Berre, C.; Azam, L.; Giordano, R.; Serp, P.; Laurenczy, G.; Duhayon, C.; Thi´ebaut, D.; Kalck, P.
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21 HALF-SANDWICH RHODIUM(III), IRIDIUM(III), AND RUTHENIUM(II) COMPLEXES WITH ANCILLARY PYRAZOLE-BASED LIGANDS Claudio Pettinari*, Riccardo Pettinari, and Corrado Di Nicola Schools of Pharmacy, Chemistry Section, S. Agostino 1, University of Camerino, Camerino, Italy Fabio Marchetti Schools of Science and Technology, Chemistry Section, S. Agostino 1, University of Camerino, Camerino, Italy 21.1 INTRODUCTION Half-sandwich transition metal complexes are a class of compounds with a three-legged piano-stool pseudo-octahedral geometry at the metal center, where the aromatic five- or six-membered ring occupies three coordinating sites (the seat) with three other ligands (the legs) more or less easily interchangeable. The presence of the aromatic ring stabilizes and protects the metal center, for example, in the case of ruthenium(II), preventing its rapid oxidation to ruthenium(III). Generally, the aromatic ring is relatively inert toward substitution reactions and consequently it is often considered as a spectator ligand. In the three remaining coordination sites opposite the aromatic ligand, it is possible to introduce a wide variety of ligands with C-, N-, O-, S- or P-donor atoms. The resulting derivatives can be neutral, mono- or dicationic, and often these ligands are labile and suitable to be exchanged, a feature in solution that is crucial in self-assemblies, biological, and catalytic processes. Pyrazole is a five-membered aromatic heterocycle [1], displaying different acid–base features and coordination ability with respect to analogous imidazole. Both imidazole and pyrazole have two nitrogen atoms for which the positive charge can be delocalized upon protonation. However, pyrazoles result in much weaker bases than imidazoles, the difference being due to the fact that the positive charge in pyrazolium ion is less delocalized than in the imidazolium ion. Pyrazole rings are present in a huge number of organic, inorganic and organometallic derivatives for several applications, for example, as building blocks of other compounds, as agrochemicals, in catalysis, and in medicine. Pyrazole and its derivatives are very versatile and a series of analog can be synthesized, thus influencing the steric and electronic features of their metal derivatives. Pyrazole has also been found as a pharmacophore in a number of active biological molecules, with antimicrobial, antiviral, antiglycemic, anti-inflammatory, antiallergic, and anticancer applications [2]. Many classes of pyrazole-based ligands have been developed in the second half of the last century, which have been employed in the coordination chemistry of main group, transition, and lanthanide metal ions and, in the last decade, biological or catalytic applications have been reported [3].
First Edition. Edited by Armando J. L. Pombeiro. © 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.
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