Chemistry and catalysis advances in organometallic chemistry and catalysis
Download 11.05 Mb. Pdf ko'rish
|
- Bu sahifa navigatsiya:
- REFERENCES
|
669 Figure 49.1 Molecular structure of [Os(Q y ) 2 ] (R = o-SMe) [20]. 49.2 DISCUSSION OF RESULTS Using the hemilabile noninnocent ligand 4,6-di-tert-butyl-(2-methylthiophenylimino)-o-benzosemiquinone Q y • − (which was developed during efforts to modify a radical/metal/radical three-spin interaction) [18], we could obtain and investigate a system (Eq. 49.4) to prove a predominantly ligand-based electron transfer during the reversible coordination of a thioether sulfur donor [19]. N O
III N O MeS Ir III t Bu
Bu
Bu
Bu +e
SMe •− (49.4) Electron-transfer-dependent reversible coordination changes are known from systems with mostly metal-based electron transfer, for example, for Cu I /Cu
II couples such as (Eq. 49.5) [21], S Me
N Cu S Me N N S Me N N Cu S Me N N −e − +e − 2 (49.5) however, the electron paramagnetic resonance (EPR) and computational analysis of the system (Eq. 49.4) reveals only a fractional spin amount δ of about 8%, delocalized from the iminosemiquinone ligand to the 5d transition metal in the reactive oxidized form. The potentially H 2 activating metal [7] thus adopts an 18 + δ valence electron configuration, in agreement with previous concepts of organometallic electron transfer activation involving fractional oxidation states [22]. Separately prepared and characterized [Ir(C 5 Me 5 )(Q
y )] and its oxidized form [Ir(C 5 Me
)(Q y )](PF 6 ) were found to be connected via Eq. 49.4 before a fully reversible second oxidation for the couple [Ir(C 5 Me 5 )(Q
y )] 2 +/+ takes place. 670 ELECTRON TRANSFER-INDUCED COORDINATION CHANGES IN ORGANOMETALLIC COMPLEXES Structural evidence, supported by DFT calculations, was available to clarify this reactivity: The neutral [Ir(C 5 Me 5 )(Q
y )] exhibits coordinative unsaturation because the electron deficit at the trivalent metal is mitigated by strong σ - and π- electron donation from an o-amidophenolate ligand with averaged and little variable C–C distances of about 1.39 ˚ A in the aromatic ring. Such coordinative unsaturation for d 6 systems has also been found for related [Ir(C 5 Me
)(Q )] [7] and [Ir(C
5 Me 5 )(RNCHCHNR)] [23] systems as well as for [Rh(C 5 Me 5 )(cat)], [Cr(CO) 3 (cat)]
2 − and [Mn(CO) 3 (cat)]
− , cat
= catecholate [24]. Consequently, there is no bonding interaction between Ir and S (d > 4.0 ˚A), and the further structure parameters of the ligand reveal not only an aromatic ring but also C–O and C–N single bonds. On one-electron oxidation, the structure rearranges characteristically Eq. 49.4: Iridium–sulfur coordination occurs with a standard [25] bond length of 2.35 ˚ A, leading to coordinative saturation, a piano-stool arrangement. The metric parameters of the noninnocent ligand reveal bond alternance compatible with a benzosemiquinone ring as well as shortened C–O and C–N bonds. In addition to UV–vis-NIR spectroelectrochemical analysis [19], electron spin resonance (ESR) spectroscopy shows DFT-supported g tensor components at g 1 = 1.996, g 2 = 1.985, and g 3 = 1.951, A 3 ( 14 N) = 1.7 mT. This observation illustrates nonnegligible metal contributions [about 8% according to density functional theory (DFT)] from a 5d element with a high spin-orbit coupling constant. The cation is essentially an anion radical complex [26], corresponding to a semiquinone/Ir III
formulation [23]. Apparently, the one-electron oxidation of the o-amidophenolate to the o-iminobenzosemiquinonate is sufficient to remove the tolerance of coordinative unsaturation, resulting in binding of either weakly donating thioether-S [19], dihydrogen [7], or halogen species [23]. Square schemes (Eq. 49.6), similar to those derived for electron transfer-induced isomerizations [27], involving calculated structures and energy differences can illustrate and confirm that the indirect oxidative addition [28] at the metal involving one-electron exchange [29] on the
metal oxidation state. [M(
-C
R
)(N,O-Q 2 −
[M( η n -C
R
)(N,O-Q •− )]
[M( η n -C
R
)(N,O,E-Q 2 −
[M( η n -C
R
)(N,O,E-Q •− )]
−e − +e − −e − +e −
1
−1
−2
2
1
2 M = Rh, Ir; n = 5 M = Ru; n = 6 E = O or S (49.6) This variant differs from the classical oxidative addition—reductive elimination sequence, one of the most useful and most frequently applied reaction patterns in transition metal chemistry [28]. Metal-based electron configuration changes such as the d
8 /d 6 two-electron interchange have been employed to provide very different coordination situations for substrate binding, conversion, and release, and the special one-electron variant may also prove useful. The different (electro)chemical reversibility patterns of two-step oxidation processes involving [Ir(C 5 Me 5 )] 2 + are
remarkable: While the [Ir(C 5 Me 5 )(Q )]
n system exhibits two conventional waves in the cyclic voltammogram [7], the [Ir(C 5
5 )(RNCHCHNR)] n redox system with R = 2,6-dimethylphenyl showed a reversible first but irreversible second oxidation in propylene carbonate [23]. In yet another variant, the example [Ir(C 5 Me
)Q y ] 0/ +/2+ described here (Eq. 49.4) is distinguished by a first oxidation process of the N,O-coordinated species involving intramolecular thioether coordination after initial electron transfer. The return wave in the cyclic voltammogram is due to the reduction of N,O,S-coordinated [Ir(C 5
5 )Q
] +
The above example has shown in a prototypical way the relationship between a reactive (here even catalytically active) metal complex redox system and an intramolecular model exhibiting a hysteresis-type structure change as a response to electron transfer. To generalize this behavior, we have recently explored [30] the following modifications: 1. using an O-ether (Q a ) instead of an S-ether function (Q y ) in the ligand; 2. removing n-donor functions from the ortho-positions of the ligand (Q
);
DISCUSSION OF RESULTS 671 Figure 49.2 Molecular structure of [(C 5 Me
)Ir(Q x )].
3. replacing Ir by the lighter homolog Rh; 4. replacing the M( η 5
5 R 5 ) organometallic fragment (M = Rh, Ir) by Ru(η 6 -C
R 6 ), C 6 R 6 = p-cymene = 1-iso-propyl- 4-methyl-benzene = Cym. To start with, all neutral precursor complexes exhibit the previously mentioned coordinative unsaturation at the metal (see Fig. 49.2) [30] caused by the highly electron-rich o-amidophenolate ligands such as Q y 2 − , Q a 2 − , or Q x 2 − . The first oxidation of the iridium complexes [Ir(C 5 Me
)(Q)] 0/ + shows different manifestations of the ECEC process (Eq. 49.6). A large splitting is noted between anodic and cathodic peak potentials, E pa and E pc , at
−0.15 and 0.60 V versus Fc +/0 in CH
2 Cl 2 /0.1 M Bu 4 NPF 6 (250 mV/s) for the structurally characterized thioether example (Q = Q
) [19]. By comparison, the corresponding values at −0.10/−0.26 V (Q = Q a ) and
−0.03/−0.12 V (Q = Q x ) illustrated in Figure 49.3 show much smaller effects [30]. The O-ether donor function is expected to coordinate in a much more labile bond than the thioether analog, and with absent intramolecular donors the E pa /E pc difference becomes rather normal, approaching 60 mV. It has to be noted that the not completely “noncoordinating” anions in the excess of electrolyte can albeit weakly bind to the oxidized compounds in order to remove coordinative unsaturation. In contrast, the second oxidation [Ir(C 5 Me
)(Q)] +/2+
is generally well behaved (E pa − E pc < 0.07 V; Fig. 49.3), reflecting negligible structural change [19, 30]. The replacement of iridium by rhodium in the redox systems [M(C 5 Me
)(Q)] n/n +1 is illustrated in Figure 49.4. The data analysis [30] reveals less pronounced wave distortion for the first oxidations: The peak potentials E pa and E pc , at
−0.26 and 0.40 V versus Fc +/0
in CH 2 Cl 2 /0.1 M Bu 4 NPF
6 (250 mV/s) for Q = Q
and
−0.12/−0.20 V (Q = Q a ) show less splitting than in the iridium system. However, the behavior of the first oxidation wave (Fig. 49.4) still signifies a structural change according to Eq. 49.7. O N
SMe O M M +e − −e − MeS + M = Rh, Ir (49.7)
672 ELECTRON TRANSFER-INDUCED COORDINATION CHANGES IN ORGANOMETALLIC COMPLEXES 0.4 (a)
(b) 0.2
0.0 −0.2
−0.4 −0.6
E (v) versus Fc +/0
5 μA 0.4
0.2 0.0
−0.2 −0.4
−0.6 E (v) versus Fc +/0
5 μA Figure 49.3 Cyclic voltammograms of (a) [(C 5 Me
)Ir(Q a )] and (b) [(C 5 Me
)Ir(Q x )] in CH
2 Cl 2 /0.1 M Bu 4 NPF 6 at different scan rates. (See insert for color representation of the figure.) 0.6
0.4 0.2
0.0 −0.2 −0.4 −0.6 −0.8 −1.0 E (v) versus Fc +/0
5 μ A 0.4 (a) (b)
0.2 0.0
−0.2 −0.4
−0.6 E (v) versus Fc +/0
2.5 μ A
Cyclic voltammograms of (a) [(C 5 Me 5 )Rh(Q
y )] and (b) [(C 5 Me
)Rh(Q a )] in CH
2 Cl 2 /0.1 M Bu 4 NPF 6 at different scan rates. (See insert for color representation of the figure.) A notable feature is the small difference between the potentials for first and second oxidation in the case of Q = Q
or Q x . A careful analysis of the whole set in terms of kinetics and equilibrium constants will be required to rationalize this effect. An important kind of information regarding the electronic structure comes from EPR spectroscopy of the paramagnetic intermediates [19], which are also crucial for the small molecule activation reactivity [7]. Figure 49.5 shows the spectrum for [Rh(C
5 Me 5 )(Q a )] + at 110 K, which, like that of [Ir(C 5 Me
)(Q y )] + [19], illustrates largely semiquinoneimine ligand-based spin and no Rh II or Rh
IV configuration. Employing the correspondence [31, 32] of complexes [( η
-C
R
)M(Q)]
, M
= Ir or Rh and n = 5 with organometallic “half-sandwich” analog involving M = Ru and n = 6, we have recently obtained the ruthenium compounds (Eq. 49.8) with C 6 R 6 = p-cymene = Cym, and characterized them electrochemically. N O Ru S N O Ru O (49.8) CONCLUDING REMARKS 673 335
340 345
350 355
B / mT 110 K
Figure 49.5 X-Band EPR spectrum of [(C 5 Me
)Rh(Q a )] + at 110 K, generated in situ in CH 2 Cl 2 /0.1 M Bu 4 NPF
6 . Areneruthenium compounds are popular organometallic complex fragments in areas such as catalysis [33] and inorganic drug development [34]. Figure 49.6 shows that the systems [Ru(Cym)(Q)] n behave similarly to the M(C 5 Me
) examples described earlier, suggesting an ECEC mechanism (Eq. 49.9) for the first oxidation. Differences between the thioether (Q = Q
y ) and O-ether analogs (Q = Q
) will have to be analyzed quantitatively with the help of digital simulation and DFT-supported spectroelectrochemistry [35, 36]. N O Ru E N O Ru E −e − Ru O N E Ru O N E +e − E: S, O (49.9)
49.3 CONCLUDING REMARKS Using a series of new coordination compounds of some platinum group metals with potentially hemilabile redox-active chelate ligands based on the o-benzosemiquinoneimine intermediate, the extent of noninnocent behavior has been established experimentally, via structure determination and EPR, UV–vis-NIR spectroelectrochemistry [35, 36], and computationally 674 ELECTRON TRANSFER-INDUCED COORDINATION CHANGES IN ORGANOMETALLIC COMPLEXES 0.6 0.4
0.2 0.0
−0.2 −0.4 −0.6 −0.8 5
E (v) versus Fc 0/+
0.6 0.4
0.2 0.0
−0.2 −0.4 −0.6 −0.8 E (v) versus Fc 0/+
100 mV/s 10
100 mV/s
Cyclic voltammograms of (a) [Ru(Cym)(Q y )] and (b) [Ru(Cym)(Q a )] in CH
2 Cl 2 /0.1 M Bu 4 NPF 6 . (DFT). As a consequence of the fractional oxidation-state situation corresponding to an 18 + δ valence electron count, we can expect a potential to adopt unusual metal configurations through electron-transfer-dependent intramolecular coordination, accompanied by intermolecular reactivity enhancement toward small molecule activation. One possible function of complexes with noninnocent hemilabile ligands is their role as intramolecular models for intermolecular reactivity, involving weak donor substrates such as dihydrogen. Like for the spin-shift example [Cu(Q
) 2 ] described previously [18], it is remarkable that a rather small change in metal/ligand interaction can bring about a qualitatively altered behavior, for example, in terms of coordination number and substrate activation.
Support from the Land Baden-W¨urttemberg and from the COST program is gratefully acknowledged. Special thanks are due to Angela Winkelmann for her contributions to the preparation of this article and to Drs. Jan Fiedler and Stanislav Z´aliˇs (J. Heyrosvky Institute, Prague, Czech Republic) for continued cooperation. REFERENCES 1. (a) Chirik, P. J. Inorg. Chem. 2011, 50 , 9737; (b) Special Issue: Cooperative & Redox Non-Innocent Ligands in Directing Organometallic Reactivity, Eur. J. Inorg. Chem. 2012, 340. 2. Chaudhuri, P.; Verdani, C.N.; Bill, E.; Bothe, E.; Weyherm¨uller, T.; Wieghardt, K. J. Am. Chem. Soc. 2001, 123 , 2213. 3. Kaim, W. Inorg. Chem. 2011, 50 , 9752. 4. Haneline, M. R.; Heyduk, A. F. J. Am. Chem. Soc. 2006, 128 , 8410. 5. Boyer, J. L.; Cundari, T. R.; DeYonker, N. J.; Rauchfuss, T. B.; Wilson, S. R. Inorg. Chem. 2009, 48 , 638. 6. (a) Dzik, W. I. ; Zhang, X. P.; de Bruin, B. Inorg. Chem. 2011, 50 , 9896; (b) Hetterscheid, D. G. H.; Gr¨utzmacher, H.; Koekoek, A. J. J.; de Bruin, B. Progr. Inorg. Chem. 2007, 55 , 247. 7. (a) Ringenberg, M. R.; Kokatam, S. L.; Heiden, Z. M.; Rauchfuss, T. B. J. Am. Chem. Soc. 2008, 130 , 788; (b) Ringenberg, M. R.; Nilges, M. J.; Rauchfuss, T. B.; Wilson, S. R. Organometallics 2010, 29 , 1956; (c) Ringenberg, M. R.; Rauchfuss, T. B. Eur. J.
8. (a) Geiger, W. E. Organometallics 2011, 30 , 28; (b) Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2 , 270. 9. Jeffrey, J. C.; Rauchfuss, T. B. Inorg. Chem. 1979, 18 , 2658. 10. Braunstein, P.; Naud, F. Angew. Chem. 2001, 113 , 702; Angew. Chem. Int. Ed. 2001, 40, 680. 11. Steinborn, D. Fundamentals of Organometallic Catalysis; Wiley-VCH Verlag GmBH, Weinheim, 2012; p 155. 12. Kaim, W. Eur. J. Inorg. Chem. 2012, 343. 13. Kaim, W.; Schwederski, B. Coord. Chem. Rev. 2010, 254 , 1580. 14. Praneeth, V. K. K.; Ringenberg, M. R.; Ward, T. R. Angew. Chem. Int. Ed. 2012, DOI: 10.1002/anie.201204100. REFERENCES 675 15. Herebian, D.; Bothe, E.; Neese, F.; Weyherm¨uller, T.; Wieghardt, K. J. Am. Chem. Soc. 2003, 125 , 9116. 16. Poddel’sky, A. I.; Cherkasov, V. K.; Abakumov, G. A. Coord. Chem. Rev. 2009, 253 , 291. 17. Brown, S. N. Inorg. Chem. 2012, 51 , 1251. 18. Ye, S.; Sarkar, B.; Lissner, F.; Schleid, Th.; van Slageren, J.; Fiedler, J.; Kaim, W. Angew. Chem. 2005, 117 , 2140; Angew. Chem.
19. H¨ubner, R.; Weber, S.; Strobel, S.; Sarkar, B.; Z´aliˇs, S.; Kaim, W. Organometallics 2011, 30 , 1414. 20. H¨ubner, R.; Sarkar, B.; Z´aliˇs, S.; Kaim, W. Eur. J. Inorg. Chem. 2012, 3569. 21. Schn¨odt, J.; Manzur, J.; Garcia, A.-M.; Hartenbach, I.; Su, C.-Y.; Fiedler, J.; Kaim, W. Eur. J. Inorg. Chem. 2011, 1436. 22. Klein, A.; Vogler, C.; Kaim, W. Organometallics 1996, 15 , 236. 23. Kaim, W.; Sieger, M.; Greulich, S.; Sarkar, B.; Fiedler, J.; Z´aliˇs, S. J. Organomet. Chem. 2010, 695 , 1052. 24. (a) Espinet, P.; Bailey, P. M.; Maitlis, P. M. J. Chem. Soc. Dalton Trans. 1979, 1542; (b) Darensbourg, D. J.; Klausmeyer, K. K.; Reibenspies, J. H. Inorg. Chem. 1995, 34 , 4676; (c) Hartl, F.; Vlˇcek, Jr., A.; deLearie, L. A.; Pierpont, C. G. Inorg. Chem. 1990, 29 , 1073; (d) Hartl, F.; Stufkens, D. J.; Vlˇcek, A. Inorg. Chem. 1992, 31 , 1687. 25. (a) Albrecht, M.; Scheiring, T.; Sixt, T.; Kaim, W. J. Organomet. Chem. 2000, 596 , 84; (b) Ye, S.; Kaim, W.; Albrecht, M.; Lissner, F.; Schleid, T. Inorg. Chim. Acta 2004, 357 , 3325. 26. Kaim, W. Coord. Chem. Rev. 1987, 76 , 187. 27. Pombeiro, A. J. L.; Guedes da Silva, M. F. C.; Lemos, M. A. N. D. A. Coord. Chem. Rev. 2001, 219–221 , 53. 28. Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 5th ed.; Wiley-VCH Verlag GmBH: Weinheim, 2009, Chapter 6. 29. Tejel, C.; Ciriano, M. A.; L´opez, J. A.; Jim´enez, S.; Bordonaba, M.; Oro, L. A. Chem. Eur. J. 2007, 13 , 2044. 30. H¨ubner, R. Dissertation, Universit¨at Stuttgart, 2011. 31. Kaim, W.; Reinhardt, R.; Sieger, M. Inorg. Chem. 1994, 33 , 4453. 32. Kaim, W. In New Trends in Molecular Electrochemistry; Pombeiro, A. J. L., Ed., Fontis Media: Lausanne, 2004, p 127. 33. Therrien, B. Coord. Chem. Rev. 2009, 253 , 493. 34. Kilpin, K. J.; Clavel, C. M.; Edafe, F.; Dyson, P. J. Organometallics 2012, 31 , 7031. 35. Kaim, W.; Klein, A., Eds. Spectroelectrochemistry; Royal Society of Chemistry: Cambridge, 2008. 36. Kaim, W.; Fiedler, J. Chem. Soc. Rev. 2009, 38 , 3373.
|
