Chemistry and catalysis advances in organometallic chemistry and catalysis
Download 11.05 Mb. Pdf ko'rish
|
|
Figure 21.25 Coordination chemistry of (arene)ruthenium(II) 4-acyl-5-pyrazolonate complexes. Ru N
O O Ru N N O O Cl Cl (a) Ru Cl N N O F 3 C O N (b)
Figure 21.26 (a) [
{(p-Cymene)RuCl} 2 Q4Q]; (b) [ {(p-cymene)RuCl(Q)] (HQ = 4-(2,2,2-trifluoroacetyl)-1,2-dihydro-5-methyl-2-(pyridin- 2-yl)pyrazol-3-one). ionic mononuclear [(p-cymene)Ru(Q)L]X complexes, and ionic dinuclear complexes of the formula [ {(p-cymene)Ru(Q)} 2 L-
2 being obtained, respectively (Fig. 21.25). X-ray studies show that all of the crystalline forms are racemates, that is, the complexes exist as two enantiomers (S M ) and (R M ) differing only in the metal chirality. A dinuclear compound [ {(p-cymene)RuCl} 2 Q4Q] (H 2 Q4Q
= bis(4-(1-phenyl- 3-methyl-5-pyrazolone)dioxohexane), existing in the RRuSRu (meso form), has been prepared similarly (Fig. 21.26a). With a particular Q ligand, containing a pyridine ring bonded to the pyrazole, a completely different coordination mode has been observed, the ligand acting as N,N -chelating to ruthenium (Fig. 21.26b). The redox properties of these complexes have been investigated by cyclic voltammetry and controlled potential electrolysis, which allowed the ordering of the bidentate acylpyrazolonate ligands according to their electron-donor character and are indicative of a small dependence of the HOMO energy upon the change of the monodentate ligand. This was accounted for by DFT calculations, which showed a relevant contribution of acylpyrazolonate ligand orbitals to the HOMOs, whereas that from the monodentate ligand is minor. Rao et al. have expanded this field reporting the [3 + 2] cycloaddition reaction of selective azido complexes [(arene)Ru(Q)N 3 ] with the activated alkynes dimethyl and diethyl acetylenedicarboxylates, which produced the arene triazolato complexes [(arene)Ru(Q)(triazolato)] (Fig. 21.27), where triazolato ligand is always bonded through N(2) [60]. A copper(I) complex [Cu(Q)(PPh 3 ) 2 ], containing the Janus Q ligand coordinated through the N,N -chelating moiety, reacts with [(p-cymene)RuCl 2 ] 2 in the presence of AgPF 6 affording the ionic heterobimetallic adduct [(p-cymene)Ru(Cl)( μ 4 - O 2 ,N 2 –Q)Cu(PPh 3 )
]PF 6 ·3H 2 O (Fig. 21.28) [61]. 282 HALF-SANDWICH RHODIUM(III), IRIDIUM(III), AND RUTHENIUM(II) COMPLEXES Ru N
O O N N O OR O RO R = Me, Et Ru N N N O O N N COOR
COOR Figure 21.27 [3 + 2] Cycloaddition reaction between [(p-cymene)Ru(Q)N 3 ] and acetylenedicarboxylates. Ru Cl
O O F 3 C N Cu Ph 3 P Ph 3 P PF 6 − Figure 21.28 [(p-Cymene)Ru(Cl)( μ 4
2 ,N 2 –Q)Cu(PPh 3 ) 2 ]PF
6 .
CONCLUSIONS AND PERSPECTIVES In conclusion, many developments in the last decades on the coordination chemistry of half-sandwich Ru(II), Rh(III), and Ir(III) derivatives with pyrazole-based ligands were proposed, and also interesting applications in catalysis and biochemistry reported. However, there is a vast area of coordination chemistry that is waiting to be further explored and developed, and many of the complexes yet prepared can be further tested to check their catalytic or biological properties.
1. (a) Trofimenko, S. Chem. Rev. 1972, 72 , 497; (b) Bonati, F. Gazz. Chim. Ital. 1989, 119 , 291. 2. Keter, F. K.; Darkwa, J. Biometals, 2012, 25 , 9. 3. (a) Mukherjee, R. Coord. Chem. Rev. 2000, 203 , 151; (b) Pettinari, C.; Pettinari, R. Coord. Chem. Rev. 2005, 249 , 525; (c) Pettinari, C.; Pettinari, R. Coord. Chem. Rev. 2005, 249 , 663; (d) Viciano-Chumillas, M.; Tanase, S.; de Jongh, J. L.; Reedijk, J. Eur. J, Inorg.
4. (a) O’Sullivan, D. J.; Lalor, F. J. J. Organomet. Chem. 1973, 57 , C58; (b) Restivo, R. J.; Ferguson, G. J. Chem. Soc. Chem. Commun. 1973, 847; (c) Restivo, R. J.; Ferguson, G.; O’Sullivan, D. J.; Lalor, F. J. Inorg. Chem. 1975, 14 , 3046. 5. Jones, C. J.; McCleverty, J. A.; Rothin, A. S. J. Chem. Soc. Dalton Trans. 1986, 109. 6. Oro, L. A.; Garcia, M. P.; Carmona, D. Inorg. Chim. Acta 1985, 96 , L21. 7. Oro, L. A.; Carmona, D.; Garcia, M. P.; Lahoz, F. J.; Reyes, J.; Foces-Foces, C.; Cano, F. H. J. Organomet. Chem. 1985, 296 , C43-C46. 8. Carmona, D.; Ferrer, J.; Oro, L. A., Apreda, M. C.; Foces-Foces, C.; Cano, F. H.; Elguero, J.; Jimeno, M. L. J. Chem. Soc. Dalton Trans. 1990, 1463. 9. (a) Govindaswamy, P.; Mozharivskyj, Y. A. ; Kollipara, M. R. J. Organomet. Chem. 2004, 689 , 3265; (b) Waheed, A.; Jones, R. A.; Agapiou, K.; Yang, X.; Moore, J. A.; Ekerdt, J. G. Organometallics 2007, 26 , 6778; (c) Albertin, G.; Antoniutti, S.; Castro, J.; Garc´ıa-Font´an, S. Eur. J. Inorg. Chem. 2011, 510. 10. (a) Trib´o, R.; Pons, J.; Y´anez, R.; Piniella, J. F.; Alvarez-Larena, ´ A.; Ros, J. Inorg. Chem. Commun. 2000, 3 , 545; (b) Trib´o, R.; Munoz, S.; Pons, J.; Y´anez, R.; Alvarez-Larena, ´ A.; Piniella, J. F.; Ros, J. J. Organomet. Chem. 2005, 690 , 4072. 11. Carmona, D.; Ferrer, J.; Marzal, I. M.; Oro, L. A. Gazz. Chim. Ital. 1994, 124 , 35. 12. Carmona, D.; Ferrer, J.; Reyes, J.; Oro, L. A. Organometallics 1993, 12 , 4241. 13. Carmona, D.; Ferrer, J.; Atencio, R.; Lahoz, F. J.; Oro, L. A.; Lamata, M. P. Organometallics 1995, 14 , 2057. 14. Carmona, D.; Ferrer, J.; Arilla, J. M.; Reyes, J.; Lahoz, F. J.; Elipe, S.; Modrego, F. J.; Oro, L. A. Organometallics 2000, 19 , 798. REFERENCES 283 15. Carmona, D.; Ferrer, J.; Atencio, R.; Edwards, A. J.; Oro, L. A.; Lamata, M. P.; Esteban, M.; Trofimenko, S. Inorg. Chem. 1996, 35 , 2549. 16. Contreras, R.; Valderrama, M.; Orellana, E. M.; Boys, D.; Carmona, D.; Oro, L. A.; Lamata, M. P.; Ferrer, J. J. Organomet. Chem. 2000, 606 , 197. 17. (a) Bhambri, S.; Tocher, D. A. J. Organomet. Chem. 1996, 507 , 291; (b) Bhambri, S.; Tocher, D. A. Polyhedron 1996, 15 , 2763; (c) Bhambri, S.; Bishop, A.; Kaltsoyannis, N.; Tocher, D. A. J. Chem. Soc. Dalton Trans. 1998, 3379. 18. Bhambri, S.; Tocher, D. A. J. Chem. Soc. Dalton Trans. 1997, 3367. 19. Volcheck, W. M.; Tocher, D. A.; Geiger, W. E. J. Organomet. Chem. 2007, 692 , 3300. 20. Gemel, C.; John, R.; Slugovc, C.; Mereiter, K.; Schmid, R.; Karl, K. J. Chem. Soc. Dalton Trans. 2000, 2607. 21. Bailey, P. J.; Pinho, P.; Parsons, S. Inorg. Chem. 2003, 42 , 8872. 22. Bailey, P. J.; Budd, L.; Cavaco, F. A.; Parsons, S.; Rudolphi, F.; Sanchez-Perucha, A.; White, F. J. Chem. Eur. J. 2010, 16 , 2819. 23. Seino, H.; Yoshikawa, T.; Hidai, M.; Mizobe, Y. Dalton Trans. 2004, 3593. 24. Nagao, S.; Seino, H.; Hidai, M.; Mizobe, Y. Dalton Trans. 2005, 3166. 25. Mutseneck, E. V.; Reus, C.; Sch¨odel, F.; Bolte, M.; Lerner, H.-W.; Wagner, M. Organometallics 2010, 29 , 966. 26. Blasberg, F.; Bolte, M.; Lerner, H.-W.; Wagner, M. Organometallics 2012, 31 , 3213. 27. Marchetti, F.; Pettinari, C.; Pettinari, R.; Cerquetella, A.; Martins, L. M. D. R. S.; Guedes da Silva, M. F.; Silva, T. F. S.; Pombeiro, A. J. L. Organometallics 2011, 30 , 6180. 28. Marchetti, F.; Pettinari, C.; Cerquetella, A.; Cingolani, A.; Pettinari, R.; Monari, M.; Wanke, R.; Kuznetsov, M. L.; Pombeiro, A. J. L. Inorg. Chem. 2009, 48 , 6096. 29. Pettinari, C.; Marchetti, F.; Cerquetella, A.; Pettinari, R.; Monari, M.; Mac Leod, T. C. O.; Martins, L. M. D. R. S.; Pombeiro, A. J. L. Organometallics 2011, 30 , 1616. 30. Prades, A.; Viciano, M.; Sana´u, M.; Peris, E. Organometallics 2008, 27 , 4254. 31. Bellarosa, L.; D´ıez, J.; Gimeno, J.; Lled´os, A.; Su´arez, F. J.; Ujaque, G.; Vicent, C. Chem. Eur. J. 2012, 18 , 7749. 32. Tan, W.; Zhao, X.; Yu, Z. J. Organomet. Chem. 2007, 692 , 5395. 33. Marchetti, F.; Pettinari, C.; Pettinari, R.; Cerquetella, A.; Di Nicola, C.; Macchioni, A.; Zuccaccia, D.; Monari, M.; Piccinelli, F. Inorg. Chem. 2008, 47 , 11593. 34. Reger, D. L.; Semeniuc, R. F.; Pettinari, C.; Luna-Giles, F.; Smith, M. D. Cryst. Growth Des. 2006, 6 , 1068. 35. Di Nicola, C.; Garau, F.; Marchetti, F.; Monari, M.; Pandolfo, L.; Pettinari, C.; Venzo, A. Dalton Trans. 2011, 40 , 4941. 36. Carri´on, M. C.; Jal´on, F. A.; Manzano, B. R.; Rodr´ıguez, A. M.; Sep´ulveda, F.; Maestro, M. Eur. J. Inorg. Chem. 2007, 3961. 37. Carri´on, M. C.; Sep´ulveda, F.; Jal´on, F. A.; Manzano, B. R. Organometallics 2009, 28 , 3822. 38. Soriano, L.; Jal´on, F. A.; Manzano, B. R.; Maestro, M. Inorg. Chim. Acta 2009, 362 , 4486. 39. (a) Catalano, V. J.; Craig, T. J. Polyhedron 2000, 19 , 475; (b)Beach, N. J.; Spivak, G. J. Inorg. Chim. Acta 2003, 343 , 244; (c) Mishra, H.; Mukherjee, R. J. Organomet. Chem. 2006, 691 , 3545; (d) Gupta, G.; Yap, G. P. A.; Therrien, B.; Rao, K. M. Polyhedron 2009, 28 , 844; (e) Gupta, G.; Prasad, K. T.; Das, B.; Yap, G. P. A.; Rao, K. M. J. Organomet. Chem. 2009, 694 , 2618; (f) Prasad, K. T.; Therrien, B.; Geib, S. J.; Rao, K. M. J. Organomet. Chem. 2010, 695 , 495; (g) Prasad, K. T.; Therrien, B.; Rao, K. M. J. Organomet. Chem. 2010, 695 , 1375; (h) Gupta, G.; Prasad, K. T.; Rao, A. V.; Geib, S. J.; Das, B.; Rao, K. M. Inorg. Chim. Acta 2010, 363 , 2287; (i) Gupta, G.; Gloria, S.; Das, B.; Rao, K. M. J. Mol. Struct. 2010, 979 , 205; (j) Gupta, G.; Zheng, C.; Wang, P.; Rao, K. M. Z. Anorg. Allg. Chem. 2010, 636 , 758; (k) Rao, A. V.; Prasad, K. T.; Wang, P.; Rao, K. M. Z. J. Chem. Sci. 2012, 124 , 565. 40. Dougan, S. J.; Melchart, M.; Habtemariam, A.; Parsons, S.; Sadler, P. J. Inorg. Chem. 2006, 45 , 10882. 41. Schuecker, R.; John, R. O.; Jakupec, M. A.; Arion, V. B.; Keppler, B. K. Organometallics 2008, 27 , 6587. 42. Stepanenko, I. N.; Novak, M. S.; M¨uhlgassner, G.; Roller, A.; Hejl, M.; Arion, V. B.; Jakupec, M. A.; Keppler, B. K. Inorg. Chem. 2011, 50 , 11715. 43. Stepanenko, I. N.; Casini, A.; Edafe, F.; Novak, M. S.; Arion, V. B.; Dyson, P. J.; Jakupec, M. A.; Keppler, B. K. Inorg. Chem. 2011, 50 , 12669. 44. Carmona, D.; Oro, L. A.; Lamata, M. P.; Puebla, M. P.; Ruiz, J.; Maitlis, P. M. J. Chem. Soc. Dalton Trans. 1987, 639. 45. (a) Bailey, J. A.; Grundy, S. L.; Stobart, S. R. Organometallics 1990, 9 , 536; (b) Bailey, J. A.; Grundy, S. L.; Stobart, S. R. Inorg.
46. Boutadla, Y.; Davies, D. L.; Jones, R. C.; Singh, K. Chem. Eur. J. 2011, 17 , 3438. 47. Boutadla, Y.; Davies, D. L.; Al-Duaij, O.; Fawcett, J.; Jones, R. C.; Singh, K. Dalton. Trans. 2010, 39 , 10447. 48. (a) Ikariya, T.; Murata, K.; Noyori, R. Org. Biomol. Chem. 2006, 4 , 393; (b) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40 , 1300. 49. (a) Kashiwame, Y.; Kuwata, S.; Ikariya, T. Chem. Eur. J. 2010, 16 , 766–770; (b) Tobisch, S. Chem. Eur. J. 2012, 18 , 7248. 284 HALF-SANDWICH RHODIUM(III), IRIDIUM(III), AND RUTHENIUM(II) COMPLEXES 50. Maenaka, Y.; Suenobu, T.; Fukuzumi, S. J. Am. Chem. Soc. 2012, 134 , 367. 51. Maenaka, Y.; Suenobu, T.; Fukuzumi, S. J. Am. Chem. Soc. 2012, 134 , 9417. 52. (a) Albers, M. O.; Robinson, D. J.; Shaver, A.; Singleton, E. Organometallics 1986, 5 , 2199; (b) McNair, A. M.; Boyd, D. C.; Mann, K. R. Organometallics 1986, 5 , 303. 53. Carmona, E.; Cingolani, A.; Marchetti, F.; Pettinari, C.; Pettinari, R.; Skelton, B. W.; White, A. H. Organometallics 2003, 22 , 2820. 54. Pettinari, C.; Pettinari, R.; Marchetti, F.; Macchioni, A.; Zuccaccia, D.; Skelton, B. W.; White, A. H. Inorg. Chem. 2007, 46 , 896. 55. Kennedy, D. F.; Messerle, B. A.; Smith, M. K. Eur. J. Inorg. Chem. 2007, 80. 56. Ebrahimi, D.; Kennedy, D. F.; Messerle, B. A.; Hibbert, D. B. Analyst 2008, 133 , 817. 57. Marchetti, F.; Pettinari, C.; Pettinari, R. Coord. Chem. Rev. 2005, 249 , 2909. 58. Pettinari, C.; Pettinari, R.; Fianchini, M.; Marchetti, F.; Skelton, B. W.; White, A. H. Inorg. Chem. 2005, 44 , 7933. 59. Marchetti, F.; Pettinari, C.; Pettinari, R.; Cerquetella, A.; Cingolani, A.; Chan, E. J.; Kozawa, K.; Skelton, B. W.; White, A. H.; Wanke, R.; Kuznetsov, M. L.; Martins, L. M. D. R. S.; Pombeiro, A. J. L. Inorg. Chem. 2007, 46 , 8245. 60. Nongbri, S. L.; Das, B.; Rao, K. M. J. Organomet. Chem. 2009, 694 , 3881. 61. Cingolani, A.; Marchetti, F.; Pettinari, C.; Pettinari, R.; Skelton, B. W.; Somers, N.; White, A. H. Polyhedron 2006, 25 , 124. 22 CARBON–SCORPIONATE COMPLEXES IN OXIDATION CATALYSIS Lu´ısa M. D. R. S. Martins* Chemical Engineering Department, ISEL, Lisbon, Portugal; Centro de Qu´ımica Estrutural, Instituto Superior T´ecnico, Universidade de Lisboa, Lisboa, Portugal Armando J. L. Pombeiro Centro de Qu´ımica Estrutural, Instituto Superior T´ecnico, Universidade de Lisboa, Lisboa, Portugal 22.1 INTRODUCTION The development of new transition metal catalysts and single-pot methods for the selective and sustainable oxidative functionalization of alkanes to more valuable products is a topic of high interest in the fields of homogeneous catalysis, organic chemistry, and green chemistry, which remains rather unexplored owing to the high inertness of such species in spite of their constituting huge potential carbon stocks on earth [1]. The present contribution exemplifies a strategy for efficient and highly selective oxidation reactions of industrial interest directed toward organic synthesis, using catalytic systems based on transition metal complexes bearing tris(pyrazol-1- yl)methane-type scorpionates, RC(R pz) 3 (pz
= pyrazol-1-yl; R = H or substituent at the methine carbon; R = H or substituent at the pz ring; Fig. 22.1). The conversion of light hydrocarbons into value-added functionalized products, under mild conditions, is still a serious challenge, but tris(pyrazol-1-yl)methane complexes of V, Fe, Cu, and Re have already been successfully applied as catalysts or catalyst precursors for relevant alkane oxidation reactions, namely, peroxidative oxygenations (to give alcohols and ketones) and carboxylations (to produce carboxylic acids). All these types of alkane reactions are promising toward the eventual exploration of alkanes as unconventional starting materials for synthesis. The chapter mainly concerns homogeneous catalytic systems, but supported catalysts are also included in view of their advantageous separation and recycling. In fact, the immobilization of a catalyst or a catalyst precursor complex on a support is a common and suitable procedure that combines the advantages of homogeneous and heterogeneous catalyses. Transition-metal-catalyzed Baeyer– Villiger (BV) oxidations, namely, the transformation of cyclic and acyclic ketones into lactones and esters, respectively, has also become an important research topic in the past years owing to the wide applications of the products [2]. Owing to economic and environmental reasons, a growing attention has been paid to the replacement of organic peroxy acids, traditionally used as stoichiometric oxidants in the BV oxidation, by more atom-efficient and environmentally friendly oxidants such as molecular oxygen [3] or hydrogen peroxide [4]. The catalytic potential of Re complexes bearing tris(pyrazol-1-yl)methane ligands and with the metal in a wide range of oxidation states, for the BV peroxidative oxidation of ketones, is presented as an extension of the above oxidation studies to other substrates and catalytic transformations. Advances in Organometallic Chemistry and Catalysis: The Silver/Gold Jubilee International Conference on Organometallic Chemistry Celebratory Book, First Edition. Edited by Armando J. L. Pombeiro. © 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.
286 CARBON–SCORPIONATE COMPLEXES IN OXIDATION CATALYSIS (a) (b)
C N N N N N N R R ′ R ′ R ′
(a) Schematic structure of tris(pyrazol-1-yl)methanes. (b) Comparison between a coordination mode of a tris(pyrazol-1- yl)methane-type scorpionate and a scorpion. 22.2 PEROXIDATIVE OXYGENATIONS OF ALKANES 22.2.1 In Liquid Systems Several tris(pyrazol-1-yl)methane complexes of V(III, IV, or V), Fe(II), Cu(II), and Re(III or VII) (Fig. 22.2) have been found to be catalyst precursors for the peroxidative oxidation of cyclohexane (and cyclopentane in the cases tested) to give, in a single pot, the corresponding alcohols and ketones (Scheme 22.1). The reactions are usually carried out in acetonitrile, with aqueous H 2 O 2 as the oxidizing agent, in acidic medium, at room temperature. They proceed via radical mechanisms with possible involvement of both C-centered and O-centered radicals as indicated by radical trap experiments. The C-scorpionate ligands, bearing pyrazolyl moieties, are expected to be able to easily change their denticity during the reaction as they can undergo partial decoordination upon protonation in the used acidic medium (with generation of unsaturated metal centers). They can also assist proton-transfer steps, thus promoting the observed catalytic behavior of their complexes in water/NCMe medium. Moreover, in some cases, they form hydrosoluble complexes, favoring the use of an aqueous reaction medium. The following compounds reveal considerable catalytic activity for the above single-pot oxidation reaction: dioxo- vanadium(V) complexes [VO 2 {RX(R pz) 3 }]
(X = C, R = R = H, n = +1 (1), or R = SO 3 , R
= H, n = 0 (2); X = B, R = H, R
= Me, n = 0 (3) and [VO 2 (3,5-Me 2 Hpz)
3 ][BF
4 ] (4) [5a], the oxo-vanadium(IV) complexes [VOCl 2 {HOCH
2 C(pz)
3 }] (5), [VO {HB(pz) 3 }{H 2 B(pz)
2 }] (6), and [VO(acac) 2 (Hpz)]
•HC(pz) 3 (acac = acetylacetonate) (7) [5a,b], bearing scorpionate- or pyrazole-type ligands; hydrotris(pyrazol-1-yl)methane Fe(II), Cu(II), Re(III), or V(III) chlorocomplexes [MCl n {HC(pz)
3 }] (M = Fe (8) or Cu (9), n = 2; M = V (10), Re (11), n = 3) or [ReCl 2 {HC(pz) 3 }(PPh
3 )][BF
4 ] (12) [5c,f]; compounds bearing C-functionalized tris(pyrazol-1-yl)methanes such as Fe(II), Cu(II), or V(III) chlorocomplexes [MCl
{SO
3 C(pz)
3 }]
(M = Cu, n = 1, x = 0 (13); M = Fe, n = 2, x = −1 (14); M = V, n = 3, x = 0 (15)) [5c], or the oxo-rhenium compounds [ReO 3 {SO 3 C(pz)
3 }] (16), [ReOCl{SO 3 C(pz)
3 }(PPh
3 )]Cl (17) [5f], bearing the tris(pyrazol-1-yl)methanesulfonate ligand; 2,2,2-tris(pyrazol-1-yl)ethanol Fe(II) and Cu(II) complexes [Fe {HOCH
2 C(pz)
3 } 2 ][FeCl 4 ]Cl (18), [Fe {HOCH 2 C(pz) 3 } 2 ]Cl 2 (19), [Fe {HOCH 2 C(pz) 3 } 2 ][FeCl {HOCH
2 C(pz)
3 }(H
2 O) 2 ] 2 (Cl) 4 (20), [Fe {HOCH 2
3 } 2 ] 2 [Fe 2 OCl
6 ](Cl)
2 ·4H
2 O (21) [5d], and [CuCl 2 {HOCH 2 C(pz)
3 }] (22) [5e]; the 2,2,2-tris(pyrazol-1-yl)ethyl methanesulfonate Cu(II) complex [CuCl 2 {CH
3 SO 2 OCH 2 C(pz) 3 }] 2 (23) [5e]; and the related pyrazole complexes of Re(III) [ReClX {N 2 C(O)Ph }(Hpz)
n (PPh
3 )
] (X =
= 2, m =1 (24); X = Cl, n = 1, m = 2 (25) or n = 2, m = 1 (26)) [5f]. Among the V complexes, the dioxo-vanadium(V) lead to the highest yields (up to 24% for 4, Table 22.1 [5a]), while [VOCl 2
2 C(pz)
3 }] (5) allows to reach turnover numbers (TONs, moles of products per mol of catalyst) up to 405 with hydrogen peroxide or up to 1.1 × 10
3 by using m-chloroperoxybenzoic acid (mCPBA) as oxidant [5b]. The dichloro-complexes 8 and 14 are the most active iron(II) complexes (Table 22.1) [5c,d]. Their activity is promoted by acid, reaching remarkable TON values up to 690 for 8 or yields (based on the alkane) up to 25% for the complex 14 bearing the tris(pyrazol-1-yl)methanesulfonate ligand. The use of H 2 O 2 as oxidant and of acetonitrile–water as the solvent medium leads to the highest catalytic activity, but the hydrosoluble Fe complex [FeCl 2 {HC(pz) 3 }] (8) can operate effectively in water without requiring the presence of any organic solvent. This feature that allows the uncommon use of water as the only solvent is particularly significant in terms of developing a “green” catalytic process for alkane oxidations [5c]. Similar yields, under the same conditions, were obtained for the water-soluble Fe(II) complexes 19–21 bearing the C-functionalized tris(pyrazolyl)methane HOCH 2 C(pz)
3 (Table 22.1, [5e]). The hydroxo group of the scorpionate ligand imparts hydrosolubility that allows them to operate also in pure aqueous media (without any organic solvent, although less effectively). [CuCl 2
2 C(pz)
3 }] (22) leads to overall yields up to 23% (Table 22.1, [5e]) and to a remarkably high selectivity toward the formation of cyclohexanol, while cyclohexanone is usually obtained in a much lower quantity. The above yields are considerably higher than those presented [5c] by the related half-sandwich complexes [CuCl 2 {HC(pz)
3 }] (9) or |
