Chemistry and catalysis advances in organometallic chemistry and catalysis
18. (a) Fan, P. W.; Zhang, F.; Bolton, J. L.; Chem. Res. Toxicol. 2000
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651 18. (a) Fan, P. W.; Zhang, F.; Bolton, J. L.; Chem. Res. Toxicol. 2000, 13 , 45; (b) Zhang, F.; Fan, P. W.; Liu, X.; Shen, L.; van Breeman, R. B.; Bolton, J. L. Chem. Res. Toxicol. 2000, 13 , 53; (c) Dehal, S. S.; Kupfer, D. Cancer Res. 1995, 55 , 1283; (d) Hardcastle, I. R.; Horton, M. N.; Osborne, M. R.; Hewer, A.; Jarman, M.; Phillips, D. H.; Chem. Res. Toxicol. 1998, 11 , 369; (e) Shibutani, S.; Dsaradhi, L.; Terashima, I.; Banoglu, E.; Duffel, M. W. Cancer Res. 1998, 58 , 647; (f) Dasaradhi, L.; Shibutani, S. Chem. Res.
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28. Tan, Y. L. K.; Pigeon, P.; Top, S.; Labb´e, E.; Buriez, O.; Hillard, E.A.; Vessi`eres, A.; Amatore, C.; Leonge, W. K.; Jaouen, G. Dalton Trans. 2012, 41 , 7537. 29. Ferreira, D. C. M.; Tapsoba, I.; Arbault, S.; Bouret, Y.; Moreira, M. S. A.; Pinto, A. V.; Goulart, M. O. F.; Amatore, C. Chembiochem 2009, 10 , 528. 30. Pigeon, P.; Top, S.; Zekri, O.; Hillard, E. A.; Vessi`eres, A.; Plamont, M.-A.; Buriez, O.; Labb´e, E.; Huch´e, M.; Boutamine, S.; Amatore, C.; Jaouen, G. J. Organomet. Chem. 2009, 694 , 895. 31. Buriez, O.; Labb´e, E.; Pigeon, P.; Jaouen, G.; Amatore, C. J. Electroanal. Chem. 2008, 619–620 ,169. 32. Barbier, B.; Pinson, J.; D´esarmot, G.; Sanchez, M. J. Electrochem. Soc. 1990, 137 , 1757. 33. Holm, A. H.; Vase, K. H.; Winther-Jensen, B.; Pedersen, S. U.; Daasbjerg, K. Electrochim. Acta, 2007, 53 , 1680. 34. Liu, J.; Cheng, L.; Liu, B.; Dong, S. Langmuir 2000, 16 , 7471. 35. Huijzer, J. C.; James, J.; Adams, D.; Yost, G. S. Toxicol. Appl. Pharmacol. 1987, 90 , 60. 36. Nocerini, M. R.; Yost, G. S.; Carlson, J. R.; Liberato, D. J.; Breeze, R. G.; Drug Metab. Dispos. 1985, 13 , 690. 37. Charmantray, F.; Duflos, A.; Lhommea, J.; Demeunynck, M. J. Chem. Soc. Perkin Trans. 1 2001, 2962. 38. Powers, J. C.; Oleksyszyn, J.; Narasimhan, S. L.; Kam, C.-M.; Radhakrishnan, R.; Meyer, J. E. F. Biochemistry 1990, 29 , 3108. 39. Chow, M. M.; Edgar, J.; Meyer, F.; Bode, W.; Kam, C.-M.; Radhakrishnan, R.; Vijayalakshmi, J.; Powers, J. C. J. Am. Chem. Soc. 1990, 112 , 7183. 40. Powers, J. C.; Kam, C.-M.; Narasimhan, L.; Oleksyszyn, J.; Hernandez, M. A.; Ueda, T. J. Cell. Biochem. 2004, 39 , 33. 41. Smith, K. S.; Smith, P. L.; Heady, T. N.; Trugman, J. M.; Harman, W. D.; Macdonald, T. L. Chem. Res. Toxicol. 2003, 16 , 123. 42. Hinson, J. A.; Reid, A. B.; McCullough, S. S.; James, L. P. Drug Metab. Rev. 2004, 36 , 805 and references therein. 43. Bose, R.; Ahmad, A. R.; Dicks, A. P.; Novak, M.; Kayser, K. J.; McClelland, R. A. J. Chem. Soc. Perkin Trans. 1 1999, 2 , 1591. 44. Morao, I. A.; Hillier, I. H.; Tetrahedron Lett. 2001, 42 , 4429. 45. Plazuk, D.; Vessi`eres, A.; Hillard, E. A.; Buriez, O.; Labb´e, E.; Pigeon, P.; Plamont, M.-A.; Amatore, C.; Zakrzewski, J.; Jaouen, G.
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2 . Generally, these compounds are divided into three groups according to the nature of the metal–carbon bond: nucleophilic Schrock carbenes, where the carbon center has a nucleophilic character, M( δ + ) =C(δ
− ); and electrophilic Fischer carbenes with the metal–carbon bond polarized in the opposite sense M( δ − ) =C(δ
+ ) (the fifteenth annual survey describing their chemistry was recently published [1]). The third group includes transition-metal complexes with N-heterocyclic carbene ligands (e.g., the Hoveyda– Grubbs catalyst), in which the multiple character of metal–carbon bond is negligible [2]. This chapter is devoted to carbenes of the Fischer type, that is, to aminocarbene complexes. A typical Fischer carbene molecule is composed of a central metal atom in its low oxidation state, coordinated to π-electron acceptor ligands (CO). The metal–carbene carbon bond (formally a double bond) is polarized M(δ − ) =C(δ + ) giving electrophilic character to the carbon atom. The complex is then stabilized by the presence of a π-donor substituent X = −OR or –NR 2 (Fig. 48.1). M = C(R)–X : · · · ↔ · · · (−)
(+)
(R)–X : · · · ↔ · · · (−)
M–C (R) = X (+) Fischer carbene complexes are useful starting materials in organic synthesis [3–5] and promising catalysts [6, 7]. Their molecules offer high structural as well as substitution variability, therefore a fine modification (“tuning”) of their reduction and/or oxidation abilities, structure, and stability of their redox intermediates (often of radicalic nature) and thus their specific reactivity is of high interest. A detailed insight into the structure–reactivity relationship enables better understanding of mechanism of synthetic or catalytic reactions and facilitates design and aimed syntheses of the next generation of compounds with required specific properties.
Electrochemical approach is a very powerful tool for fundamental chemical characterization of molecules that are able to be oxidized or reduced (organic substances, complexes, as well as organometallic compounds), that is, for the research
First Edition. Edited by Armando J. L. Pombeiro. © 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.
654 ELECTROCHEMISTRY OF FISCHER AMINOCARBENE COMPLEXES M C
R Figure 48.1 Electron shifts in Fischer carbene complexes. of structure–reactivity relationship. The possibility of a precise setting of any required working potential can guarantee a maximal selectivity and reproducibility of the redox process. The continuous change of the working potential, starting in real time up to microsecond scale, its cycling or keeping constant, enables not only determination of the respective oxidation or reduction potentials but also revelation of reversibility of the redox processes, nature, kinetics, and equilibrium constants of follow-up reactions, stability and structure of intermediates, type and yield of products, etc. In addition to this, a combination of electrochemical methods with various in situ spectroscopic measurements [UV–vis, near infrared (NIR), infrared (IR), Raman, electron paramagnetic resonance (EPR), mass spectrometry (MS), nuclear magnetic resonance (NMR)] and separation techniques tandem high performance liquid chromatography (HPLC) represents a substantial broadening of the scientific benefit of the electrochemical approach. Interpretation of electrochemical data is usually based on comparison of experimental values acquired from systematic research of similar molecules. The best situation occurs if the studied compounds form homologous series, where only one substituent is systematically changed. Then the group of substances can be treated using the linear free-energy relationship (LFER) approach [8] to analyze the role of substitution and the influence of structural modifications on redox properties [9, 10]. According to the LFER approach, reduction (oxidation) potentials of compounds belonging to the mentioned series are proportional to the experimentally determined Hammett-type constant σ , which is characteristic for each substituent (special tables are available [8, 9, 11]). Recently, a theoretical treatment of substituent constants was published [12]. For characterization of coordination compounds, special electrochemical parameters were introduced [13]. The σ -constants were originally derived for substituents on a benzene ring and reflect their “pushing” or “withdrawing” electronic influence on the rest of the molecule (for more details consult the references). Nevertheless, this concept has rather general significance and σ -constants often fit even for nonaromatic systems. The most used is σ p , originally for substituents in the para position toward the reaction center on the benzene ring, reflecting both induction and mesomeric effects, and σ i , reflecting the inductive effect (e.g., in saturated, nonaromatic systems). In this work, both the sigma constants were used according to Eq. 48.1 and the better fit was taken into account for evaluation of inductive and/or mesomeric effects. E ox (red) = ρ · σ + c. (48.1) In Eq. 48.1, the value of the reaction constant ρ reflects the extent of interaction (electronic communication) between the substituent and the reaction center, that is, how much the measured potential depends on electron pushing/withdrawing ability of the substituent. In this way, a level of π-delocalization along the molecule or displacement of oxidation and reduction centers, respectively, can be experimentally followed and correlated with quantum chemical calculations. Since the energy (potential) needed for oxidation or reduction of the studied molecule is proportional to the energy of the respective frontier molecular orbitals (and can be determined experimentally as E ox and E red by an appropriate electrochemical method), their difference ( E ox
red ) correlates well with the HOMO–LUMO gap obtained spectrophotometrically or using calculations. When a linear correlation of the measured potentials E red (or E ox ) of all involved derivatives versus the corresponding σ p ( σ i ) values (Eq. 48.1) is observed, it follows that, in the frame of the studied series, the reduction (oxidation) centers remain unchanged, the structure of all starting derivatives as well as intermediates is analogous and thus the same reduction (oxidation) mechanism occurs. Any potential value not fitting the linear dependence points to an anomalous behavior of the respective molecule in the frame of the studied series. The anomaly may be caused by a different electron displacement (i.e., different shape of HOMO (LUMO) and different localization of redox centers), hence by a different reaction mechanism yielding thus different intermediates and products. (An example will be given below.) The ρ-values reflect also the distance between the substitution site and the reaction center of the molecule and simultaneously the electronic “conductivity” of the molecular structure (bridge) connecting the substituent and redox center. Investigating several series of compounds, the obtained ρ-values are compared and every difference in their values requires explanation on the basis of the structure and electron delocalization. FUNDAMENTAL ELECTROCHEMICAL BEHAVIOR OF AMINOCARBENE COMPLEXES 655 The additive constant c in Eq. 48.1 is significant when several analogous substitution series with different reduction (oxidation) centers are studied and compared. Then the constant c (in V) speaks about their energetic difference, that is, it compares the reduction (oxidation) abilities of the redox centers involved. The application of electrochemistry to the investigation of new organometallic molecules is illustrated in this contribution on two groups of recently synthesized Fischer-type aminocarbene complexes of chromium, tungsten, and iron involving about 40 derivatives. All of them represent molecules with two separated redox-active centers where the extent of their interaction (mutual influence) is given by the electron delocalization of the bridging unit, by the distance between the centers and/or by sterical effects.
All synthesized aminocarbene derivatives underwent systematic electrochemical investigation in aprotic media [nonaqueous dimethylformamide (DMF) with 0.1 M tetrabutylammonium tetrafluoroborate (TBATFB) as an electrolyte] in order to determine their potentials of reduction and oxidation, to find out the eventual reversibility of the redox process, to elucidate the fate of intermediates, to reveal products of such electrochemically initiated redox reactions, and to discuss the possible reaction pathway or mechanism. From the experimental point of view, for reductions, DC-polarography at the dropping mercury electrode (DME) or cyclic voltammetry (CV) at the hanging mercury drop electrode (HMDE) were used; and for controlled-potential electrolyses at negative potentials, a mercury pool electrode was employed. For both oxidative and reductive experiments, voltammetry at the platinum rotating disk electrode (RDE) and CV at the stationary platinum electrode were applied. All experiments were performed in a three-electrode system with a platinum counter electrode. For measurements in analytical scale (a standard aminocarbene concentration was 3 ×10− 4
a two-compartment cell of the H-type was employed [14]. The potentials were referred to the saturated calomel electrode (SCE), which was separated from the investigated solution by a double-frit bridge. In addition to the purely electrochemical methods, some on-line as well as off-line combined techniques were used (electrochemistry + UV/vis, IR, NMR, EPR, MS, HPLC), for detection and identification of intermediates and products. The most important data for our study are the first reduction and the first oxidation potentials. The general electrochemical pattern is presented in Fig. 48.2. The first oxidation process is quasi reversible, involving one electron. The quasi-reversibility means that the primary one- electron intermediate is not fully stable and undergoes a follow-up reaction. Therefore, the cathodic counterpeak appears often only at higher scan rates (this is different for individual compounds). The oxidation potential is then determined as a −700 −500
−300 −100
100 300
500 −2500
−2000 −1500
−1000 −500
0 E (mV) 500
1000 i a b Figure 48.2 Cyclic voltammetry of II f; a, first scan; b, second scan. 656 ELECTROCHEMISTRY OF FISCHER AMINOCARBENE COMPLEXES medium value between the anodic peak potential ( E pa ) and cathodic peak potential ( E pc ): E ox = 1 2 (E pa + E pc ). (48.2) The first reduction step is a two-electron irreversible process accompanied by adsorption or a surface film formation partly blocking the electrode, as evident from the difference between the first and second scan on CV. As reduction potentials, polarographic half-wave potentials (E 1/2
) were taken for further correlations as cathodic peak potentials ( E pc ) taken from cyclic voltammetry are always more negative and depend on the scan rate and other factors. It is necessary to point out that the oxidation as well as reduction potentials taken from various methods and measured on different electrodes were consistent. When the whole available potential range is scanned (approximately from +2 to −3 V vs SCE in aprotic media), the first reduction, as well as oxidation step, is followed by one or two other processes at more negative (more positive) potentials representing stepwise reduction (oxidation) of intermediates and products. Since various follow-up reactions occur after the first electron transfer, the interpretation of such voltammograms is very complicated, if at all possible, and would deserve special investigation that is beyond the scope of this study. 48.4 BASIC GROUP OF AMINOCARBENES In the first group of aminocarbene complexes (Table 48.1) p-substituted aryl substituents, methyl, or hydrogen were attached to the carbene carbon. Several structural features were electrochemically followed in order to elucidate and formulate how the structural/substituent changes can influence the oxidation and/or reduction potentials. For this reason, several homologous series of compounds were created (I–VIII) and mutually compared.
N
CH 3
3 C CO CO non-chelates CO CO
R N
CO CO
Chelates CO No. M R No.
M R I a Cr
3 -phenyl
II a Cr
3 -phenyl
I b Cr
3 -phenyl
II b Cr
3 -phenyl
I c Cr
II c Cr
I d Cr
II d Cr
I e Cr
3 -phenyl
II e Cr
3 -phenyl
I f Cr
3 -phenyl
II f Cr
3 -phenyl
III c W
IV c W
V a Fe a p-OCH 3 -phenyl VI a Fe b p-OCH 3 -phenyl Download 11.05 Mb. Do'stlaringiz bilan baham: 
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