New Systematic Route to Mixed-Valence Triiron Clusters Derived from

Communication pubs.acs.org/Organometallics

New Systematic Route to Mixed-Valence Triiron Clusters Derived from Dinuclear Models of the Active Site of [Fe−Fe]-Hydrogenases Laetitia Beaume,† Martin Clémancey,‡,§ Geneviève Blondin,*,‡,∥ Claudio Greco,*,§ François Y. Pétillon,† Philippe Schollhammer,*,† and Jean Talarmin*,† †

UMR CNRS 6521, Université de Bretagne Occidentale, 6 Avenue Le Gorgeu, C.S. 93837, 29238 Brest, France CEA, iRTSV-LCBM-PMB, 17 rue des Martyrs, 38000 Grenoble, France § Univ. Grenoble Alpes, iRTSV-LCBM-PMB, 17 rue des Martyrs, 38000 Grenoble, France ∥ CNRS, iRTSV-LCBM-PMB, 17 rue des Martyrs, 38000 Grenoble, France § Department of Earth and Environmental Sciences, University of Milano-Bicocca, Piazza della Scienza, 1, 20126 Milan, Italy ‡

S Supporting Information *

ABSTRACT: A novel reaction pathway to synthesize the linear trinuclear clusters [Fe3(CO)5(κ2-diphosphine)(μ-dithiolate)2] via the direct reaction of the dinuclear hexacarbonyl precursor [Fe2(CO)6(μ-dithiolate)] with the mononuclear species [Fe(CO)2(κ2-diphosphine)(κ2-dithiolate)] has been developed with diphosphine (dppe) and dithiolate (pdt = propanedithiolate) (1) or adtBn ({SCH2}2NBn = azadithiolate) (2). A crystallographic study was carried out on 2 and Mössbauer spectroscopy, and DFT calculations have been used to describe the electronic and structural properties of 1. The electrochemical properties of 1 in the absence and in the presence of a weak acid have been the subject of a preliminary investigation.

T

the study of the complexes [Fe3(CO)4L(κ2-diphosphine)(μedt)2] (edt = ethanedithiolate, L = PPh3),12 obtained by using [Fe3(CO)7(μ-edt)2]15 as a precursor and diphosphine, prompted us to report our results concerning the systematic synthesis of species of the general formula [Fe3(CO)5(κ2-dppe)(μ-pdt)(μxdt)] (1, xdt = propanedithiolate (pdt); 2, xdt = azadithiolate (adtBn) with adtBn = {SCH2}2NBn (2)). Preliminary results concerning the electrochemical behavior of [Fe3(CO)5(κ2dppe)(μ-pdt)2] (1), in the absence and the presence of acid, are also presented as well as Mössbauer measurements and calculations on 1. Recently, Rauchfuss and co-workers have reported a simple and powerful synthetic strategy16 to obtain models of the [NiFe]hydrogenase active site by using the mononuclear synthon [Fe(CO)2(κ2-dppe)(κ2-pdt)].17 This procedure was then extended to the design of various heterobimetallic species.18 We decided to use this reaction pathway for the synthesis of the trinuclear species [Fe3(CO)5(κ2-dppe)(μ-pdt)(μ-xdt)] by reacting [Fe(CO)2(κ2-dppe)(κ2-pdt)] with diiron hexacarbonyl derivatives [Fe2(CO)6(μ-xdt)] (xdt = pdt,15 adtBn 19) (Scheme 1). The complex [Fe3(CO)5(κ2-dppe)(μ-pdt)2] (1) was obtained in moderate yields (55%) by reacting [Fe2(CO)6(μ-pdt)] with [Fe(CO)2(κ2-dppe)(κ2-pdt)]. The formation of the wellidentified minor side products [Fe2(CO)4(κ2-dppe)(μ-pdt)] (3%) and [{Fe2(CO)5(μ-pdt)}2(μ-dppe)] (6%), which are often observed in reactions of [Fe2(CO)6(μ-pdt)] with diphosphine,20

he structural determination of [Fe−Fe]-hydrogenases in the late 1990s1 has led during the last 15 years to a deep reinvestigation and development of the pioneering chemistry on iron carbonyl thiolate complexes developed in the 1980s.2 The use of well-known organometallic tools has allowed the reproduction of some chemical and structural precedents of the natural site which have contributed to a better understanding of the chemistry of the H cluster.3,4 During the course of these studies very original side products featuring quasi-linear tri- and tetranuclear arrangements have been reported.5−12 Even if these novel polynuclear species do not model directly the H cluster, their chemistry is related. For example, it has been shown that the reduced form of the mixed-valence tetrairon cluster [Fe4(CO)8{μ3-(SCH2)3CMe}2], featuring a FeIFeIIFeIIFeI core, may be a better catalyst than the related biomimetic hexacarbonyl diiron dithiolate complexes.13 Recently, we reported the unexpected formation, as a side product (6% yield), of the trinuclear mixed-valence {FeIFeIIFeI} species [Fe3(CO)5(PPh2NPh2)(μ-pdt)2] (PPh2NPh2 = (PPhCH2NPhCH2)2; pdt = propanedithiolate) from the reaction between [Fe2(CO)6(μpdt)] and PPh2NPh2 through an unestablished mechanism.9 The similarity of the original structure of this compound with that of mixed-valent FeIFeII systems14 and the possible efficient activity, as electrocatalyst, of such trinuclear compounds led us to explore a general route to elaborate this class of derivatives, which can be considered formally as constituted from the combination of a diiron framework with a monoiron moiety as a metalloligand. The main problem in studying such products, often obtained in low yields by serendipity, is to rationalize their synthesis. Very recent works reported by Hogarth and co-workers concerning © 2014 American Chemical Society

Received: October 21, 2014 Published: November 6, 2014 6290

dx.doi.org/10.1021/om5010594 | Organometallics 2014, 33, 6290−6293

Organometallics

Communication

Scheme 1

indicates that the exchange of dppe between the mononuclear derivative and the diiron hexacarbonyl precursor remains limited. 1 has been previously reported as a side product in the reaction of [Ni(dppe)(CO)2] with [Fe(CO)3(BDA)] (BDA = benzylideneacetone).5 It was fully characterized here by IR, 1H, 31P, and 13 C NMR, elemental analyses, and X-ray diffraction. These data are in accord with previous characterization data and complete these data (see the Supporting Information). The extension of this reaction to azadithiolate derivatives was made in order to generalize this method of synthesis. The reaction of [Fe2(CO)6(μ-adtBn)] with [Fe(CO)2(κ2-dppe)(κ2pdt)] afforded, as a major product, the novel expected linear triiron species [Fe3(CO)5(κ2-dppe)(μ-pdt)(μ-adtBn)] (2) in 53% yield and minor side products in very low yields ([Fe2(CO)4(κ2-dppe)(μ-adtBn)] (3%) and unidentified species). The IR spectrum in CH2Cl2 displays three typical absorptions at 2025 (sharp and strong), 1960 (large and strong with a shoulder) and 1811 cm−1 (weak), the last being characteristic of a semibridging CO. The 1H NMR spectrum is consistent with the structure of 2, with the expected signals for dppe, pdt, and adtBn groups. The 31P{1H} NMR spectrum shows a singlet at 66 ppm, suggesting the presence in solution of only one isomer, with a dibasal coordination of the diphosphine. The 13C{1H} NMR spectrum of 2 in CD2Cl2 (see the Supporting Information) shows, in the carbonyl region, the presence of two triplets at 243.9 (2JPC = 5.2 Hz) and 217.7 ppm (2JPC = 13.9 Hz) attributed to the semibridging CO and the terminal CO group bound to the FeP2 moiety, respectively. A broad and weak resonance at 212.7 ppm is assigned to the three CO ligands of the {Fe(CO)3} group, suggesting their exchange in solution. A crystallographic study was carried out on 2 that confirmed its structure (Figure 1), which is based on a central quasi-linear triiron core (Fe1−Fe2− Fe3 = 157.09(2)°) with three chemically inequivalent iron atoms that are bridged by two dithiolate groups with an anti arrangement. Iron−iron distances (Fe1−Fe2 = 2.5464(6), Fe2−Fe3 = 2.5313(7) Å) suggest two single iron−iron bonds, which are in accord with the diamagnetism of 2 and the electroncounting rule. The diphosphine lies in a dibasal position (Fe1− P1 = 2.2426(10), Fe1−P2 = 2.2619(10) Å) with a bite angle P1− Fe1−P2 of 87.07(4)°. One carbonyl occupies a semibridging position (C2−Fe2 = 1.762(4), C2−Fe1 = 2.361(3) Å; Fe2−C2− O2 = 158.5(3)°). 2 compares well with other similar Fe3 species, in accordance with a formal FeIFeIIFeI arrangement. Figure 2 reproduces the Mössbauer spectra recorded at 4.2 K on a powder sample of 1 using a 60 mT (left part) or a 7 T (right part) external magnetic field applied parallel to the γ-rays. They can be satisfyingly reproduced assuming a diamagnetic species with three different iron nuclei in a 1:1:1 ratio. Several

Figure 1. Ortep view (elliposids at the 30% probability level) of 2. Selected bond lengths (Å) and angles (deg): Fe1−Fe2, 2.5464(6); Fe2− Fe3, 2.5313(7); Fe1−P1, 2.2426(10); Fe1−P2 = 2.2619(10); C2−Fe2, 1.762(4); C2−Fe1, 2.361(3); Fe1−Fe2−Fe3, 157.09(2); P1−Fe1−P2, 87.07(4); Fe2−C2−O2 = 158.5(3).

Figure 2. Experimental Mössbauer spectra (hashed marks) recorded at 4.2 K on a powder sample of 1 with a 60 mT (left) or 7 T (right) external magnetic field applied parallel to the γ-beam. The black solid lines correspond to the simulations obtained with the nuclear parameters given in Table 1. The contributions of sites a−c are reproduced as solid lines drawn in blue, red, and green, respectively.

simulations of equivalent quality were obtained (see the Supporting Information). Table 1 gives the nuclear parameters Table 1. Nuclear Parameters of 1 site

δ (mm/s)

ΔEQ (mm/s)

η

Γfwhm (mm/s)

amt (%)

a b c

0.18(2) 0.05(2) 0.23(2)

−0.62(1) −1.07(1) 0.90(1)

0.0 0.0 1.0

0.26 0.25 0.27

33 33 33

for the simulation that is favored on the basis of the following arguments. Site b presents the lowest isomer shift among the series, and the value is similar to that observed in [Fe2(CO)6(μpdt)] (see the Supporting Information) or similar complexes.21,22 Consequently, site b is tentatively attributed to the terminal Fe3 center. The parameters of site c are close to those obtained for the iron center of [(κ2-dppe)(OC)Fe(μ-pdt)Mo(CO)4] (δ = 0.20 mm/s, ΔEQ = −0.95 mm/s, η = 0.89).23 Note that the sign of the quadrupole splitting is meaningless when η = 1. It is thus tempting to identify site c with the terminal Fe1 center. The remaining site, site a, would therefore correspond to the central Fe2 ion. The 0.18 mm/s isomer shift value is fully consistent with a low-spin Fe(II) site.24 However, the 6291

dx.doi.org/10.1021/om5010594 | Organometallics 2014, 33, 6290−6293

Organometallics

Communication

diamagnetic nature of compound 1 precludes ruling out the intermediate spin S = 1 for the ferrous Fe2 site. Theoretical calculations using density functional theory (level of theory B3LYP/TZVP; see Methods in the Supporting Information for details) were also performed. An analysis of the electronic structure reveals that the nature of the HOMO and the LUMO is similar to that reported previously for similar trinuclear compounds12 (Figure 3). The HOMO can be

Scheme 2

that a second electron is transferred in MeCN−[NBu4][PF6] at slow scan rates (ECE process). The same features were observed for [Fe2(CO)4(κ2-dppe)(μ-pdt)] in these solvents.25 The redox processes of the diiron complex occur at potentials similar to those of 1, and the oxidation of both compounds is a chemically reversible step in CH2Cl2. However, the reduction of the diiron derivative was hardly observed, due to an electron transfer catalyzed isomerization to [Fe2(CO)4(μ-dppe)(μ-pdt)],26 which is not observed for 1. This might suggest that the semibridging carbonyl hinders the migration of one end of the diphosphine ligand from one iron to the other. It should be noted that the redox processes of complex 1 are appreciably more reversible and occur at potentials more negative than those of the edt-bridged analogue [Fe3(CO)5(κ2-dppe)(μ-edt)2].12 Preliminary results on the reductive properties of 1 in the presence of protons reveal that the weak acid CH3SO3H does not protonate 1, as shown by the fact that the oxidation peak current of the complex is almost unaffected by the additions of acid (Figure S7, Supporting Information). A catalytic reduction of CH3SO3H (pKa = 8.4 in MeCN)27 is detected around −2 V and is thus initiated by the one-electron reduction of the triiron complex (Figure 5). Catalytic reduction of the weak lutidinium

Figure 3. (a) HOMO and (b) LUMO of model 1. The isovalue used for contour plots is 0.022. The color code for the atoms is as follows: C, gray; P, violet; O, red; Fe, brown; S, yellow; H, white.

described as a Fe−Fe bonding orbital involving the iron atoms not coordinated to the dppe ligand. On the other hand, the LUMO is more delocalized, with antibonding character at the level of all Fe−Fe bonds, and bonding character at the level of the interaction between the carbon atom of the semibridging carbonyl and the iron atom that coordinates dppe. Notably, atomic charges computed at various levels (natural and Mulliken population analyses, and electrostatic potential mapping) support the attribution of redox state +2 for Fe2, while for Fe1 and Fe3 they are consistent with the +1 state (see the Supporting Information). The electrochemical properties of [Fe3(CO)5(κ2-dppe)(μpdt)2] (1) in the absence and in the presence of a weak acid have been the subject of a preliminary investigation. Cyclic voltammetry of [Fe3(CO)5(κ2-dppe)(μ-pdt)2] in CH2Cl2− [NBu4][PF6] shows that the triiron complex undergoes quasireversible reduction and oxidation steps at E1/2red = −1.98 V and E1/2ox = −0.21 V, respectively (Figure 4, Scheme 2). Further irreversible oxidation processes are noted at 0.63 and 0.78 V. While the scan rate dependence of the peak current for the oxidation is consistent with a diffusion-controlled oneelectron event in dichloromethane at all scan rates, it indicates

Figure 5. Cyclic voltammetry of [Fe3(CO)5(κ2-dppe)(μ-pdt)2] (1) (0.5 mM) in the absence of acid and in the presence of increasing amounts of CH3SO3H in CH2Cl2−[NBu4][PF6] (vitreous carbon electrode, scan rate 0.2 V s−1, potentials in V vs Fc+/Fc).

acid (pKa = 14.13 in MeCN)28 by [Fe4(CO)8{(SCH2)3CMe}2] has been also initiated by reduction, but in the case of the tetrairon complex, the most efficient catalytic process involved the dianion and occurred more efficiently at a less negative potential than for 1.6a,13a In summary, this work opens a new systematic route to the synthesis of trinuclear iron complexes of general formula

Figure 4. Cyclic voltammetry of [Fe3(CO)5(κ2-dppe)(μ-pdt)2] (1 mM) in CH2Cl2−[NBu4][PF6] (vitreous carbon electrode, scan rate 0.2 V s−1, potentials in V vs Fc+/Fc). 6292

dx.doi.org/10.1021/om5010594 | Organometallics 2014, 33, 6290−6293

Organometallics

Communication

(8) Peter, U.-A.; Troegel, D.; Halpin, Y.; Tschierlei, S.; Uhlemann, U.; Görls, H.; Schmitt, M.; Popp, J.; Dunne, P.; Venkatesan, M.; Coey, M.; Rudolph, M.; Vos, J. G.; Tacke, R.; Weigand, W. Inorg. Chem. 2010, 49, 10117−10132. (9) Lounissi, S.; Capon, J.-F.; Gloaguen, F.; Matoussi, F.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J. Chem. Commun. 2011, 47, 878−880. (10) Ghosh, S.; Hogarth, G.; Holt, K. B.; Kabir, S. E.; Rahaman, A.; Unwin, D. G. Chem. Commun. 2011, 47, 11222−11224. (11) Charreteur, K.; Capon, J.-F.; Gloaguen, F.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J. Eur. J. Inorg. Chem. 2011, 1038−1042. (12) Rahaman, A.; Ghosh, S.; Unwin, D. G.; Modi-Basak, S.; Holt, K. B.; Kabir, S. E.; Nordlander, E.; Richmond, M. G.; Hogarth, G. Organometallics 2014, 33, 1356−1366. (13) (a) Cheah, M. H.; Tard, C.; Borg, S. J.; Liu, X.; Ibrahim, S. K.; Pickett, C. J.; Best, S. P. J. Am. Chem. Soc. 2007, 129, 11085−11092. (b) Surawatanawong, P.; Hall, M. B. Inorg. Chem. 2010, 49, 5737−5747. (14) (a) Liu, T.; Darensbourg, M. Y. J. Am. Chem. Soc. 2007, 129, 7008−7009. (b) Justice, A. K.; De Gioia, L.; Nilges, M. J.; Rauchfuss, T. B.; Wilson, S. R.; Zampella, G. Inorg. Chem. 2008, 47, 7405−7414. (c) Singleton, M. L.; Bhuvanesh, N.; Reibenspies, J. H.; Darensbourg, M. Y. Angew. Chem., Int. Ed. 2008, 47, 9492−9495. (d) Justice, A. K.; Rauchfuss, T. B.; Wilson, S. R. Angew. Chem., Int. Ed. 2007, 46, 6152− 6154. (15) Winter, A.; Zsolnai, L.; Huttner, G. Z. Naturforsch., B 1982, 37b, 1430−1436. (16) Barton, B. E.; Whaley, M.; Rauchfuss, T. B.; Gray, D. L. J. Am. Chem. Soc. 2009, 131, 6942−6943. (17) Takács, J.; Markó, L.; Párkányi, L. J. Organomet. Chem. 1989, 361, 109−116. (18) (a) Gao, H.; Huang, J.; Chen, L.; Liu, R.; Chen, J. RSC Adv. 2013, 3, 3557−3565. (b) Caroll, M. E.; Chen, J.; Gray, D. E.; Lansing, J. C.; Rauchfuss, T. B.; Schilter, D.; Volkers, P. I.; Wilson, S. R. Organometallics 2014, 33, 858−867. (19) Laurence, J. D.; Li, H.; Rauchfuss, T. B. Chem. Commun. 2001, 1482−1483. (20) (a) Adam, F. I.; Hogarth, G.; Kabir, S. E.; Richards, I. C. R. Chim. 2008, 11, 890−905. (b) Adam, F. I.; Hogarth, G.; Richards, I. J. Organomet. Chem. 2007, 629, 3957−3968. (c) Gao, W.; Ekström, J.; Liu, J.; Eriksson, L.; Weng, L.; Åkermark, B.; Sun, L. Inorg. Chem. 2007, 46, 1981−1991. (d) Wang, N.; Wang, M.; Liu, T.; Zhang, T.; Darensbourg, M. Y.; Sun, L. Inorg. Chem. 2008, 47, 6948−6955. (21) Schilter, D.; Nilges, M. J.; Chakrabarti, M.; Lindahl, P. A.; Rauchfuss, T. B.; Stein, M. Inorg. Chem. 2012, 51, 2338−2348. (22) Stoian, S. A.; Hseih, C.-H.; Singleton, M. L.; Casuras, A. F.; Darensbourg, M. Y.; McNeely, K.; Sweely, K.; Popescu, C. V. J. Biol. Inorg. Chem. 2013, 18, 609−622. (23) Bouchard, S.; Clémancey, M.; Blondin, G.; Bruschi, M.; Charreteur, K.; De Gioa, L.; Le Roy, C.; Pét illon, F. Y.; Schollhammer, P.; Talarmin, J. Inorg. Chem. 2014, 53, 11345−11347. (24) Gütlich, P.; Bill, E.; Trautwein, A. X. Mössbauer Spectroscopy and Transition Metal Chemistry. Fundamentals and Applications; SpringerVerlag: Berlin, Heidelberg, 2011. (25) Chouffai, D.; Zampella, G.; Capon, J.-F.; De Gioia, L.; Gloaguen, F.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J. Inorg. Chem. 2011, 50, 12575−12585. (26) Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J. Inorg. Chem. 2007, 46, 9863−9872. (27) Fujinaga, T.; Sakamoto, I. J. Electroanal. Chem. 1977, 85, 185− 201. (28) Sooväli, L.; Kaljurand, I.; Kütt, A.; Leito, I. Anal. Chim. Acta 2006, 566, 290−303.

[Fe3(CO)5(chelate)(μ-dithiolate)2]. We are now studying the effects of different combinations of chelating ligands and dithiolate bridges on the electrochemical behavior and activity toward protons.

■

ASSOCIATED CONTENT

S Supporting Information *

Figures, tables, text, and CIF and xyz files giving details on instrumentation, methods, and synthetic procedures, electrochemical data (Figure S7), spectroscopic data, crystallographic data for 1, an alternative analysis of the Mössbauer spectra of 1 and Mössbauer spectra of [Fe2(CO)6(μ-pdt)]. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail for G.B.: [email protected]. *E-mail for C.G.: [email protected]. *E-mail for P.S.: [email protected]. *E-mail for J.T.: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The Centre National de la Recherche Scientifique (CNRS), Université de Bretagne Occidentale (UBO), Université Joseph Fourier, Grenoble-1, the Agence Nationale de la Recherche (ANR-11-LABX-0003-01), and the University of MilanoBicocca are acknowledged for financial support. Région Bretagne is acknowledged for funding (L.B.). We are grateful to the X-ray and NMR departments of UBO for crystallographic measurements (Dr. F. Michaud) and recording of NMR experiments.

■

REFERENCES

(1) (a) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science 1998, 282, 1853−1858. (b) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, C. E.; Fontecilla-Camps, J. C. Structure 1999, 7, 13−23. (2) (a) Seyferth, D.; Song, L.-C.; Henderson, R. S. J. Am. Chem. Soc. 1981, 103, 5103−5107. (b) Seyferth, D.; Henderson, R. S.; Song, L.-C. Organometallics 1982, 1, 125−133. (c) Darensbourg, M. Y.; Lyon, E. J.; Smee, J. J. Coord. Chem. Rev. 2000, 206−207, 533−561. (d) King, R. B.; Bitterwolf, T. E. Coord. Chem. Rev. 2000, 206−207, 563−579. (3) Gordon, J. C.; Kubas, G. J. Organometallics 2010, 29, 4682−4701. (4) (a) Simmons, T. R.; Berggren, G.; Bacchi, M.; Fontecave, M.; Artero, V. Coord. Chem. Rev. 2014, 270−271, 127−150. (b) Lubitz, W.; Ogata, H.; Rüdiger, O.; Reijerse, E. Chem. Rev. 2014, 114, 4081−4148. (c) Capon, J.-F.; Gloaguen, F.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J. Eur. J. Inorg. Chem. 2008, 4671−4681. (d) Capon, J.-F.; Gloaguen, F.; Pétillon, F. Y.; Schollhammer, P.; Talarmin, J. C. R. Chim. 2008, 11, 842−851. (e) Gloaguen, F.; Rauchfuss, T. B. Chem. Soc. Rev. 2009, 38, 100−108. (f) Tard, C.; Pickett, C. J. Chem. Rev. 2009, 109, 2245−2274. (g) Capon, J.-F.; Gloaguen, F.; Pét illon, F. Y.; Schollhammer, P.; Talarmin, J. Coord. Chem. Rev. 2009, 253, 1476− 1494. (h) Darensbourg, M. Y.; Weigand, W. Eur. J. Inorg. Chem. 2011, 994−1004. (i) Tschierlei, S.; Ott, S.; Lomoth, R. Energy Environ. Sci. 2011, 4, 2340−2352. (j) Wang, N.; Wang, M.; Chen, L.; Sun, L. Dalton Trans. 2013, 42, 12059−12071. (5) Zhu, W.; Marr, A. C.; Wang, Q.; Neese, F.; Spencer, D. J. E.; Blake, A. J.; Cooke, P. A.; Wilson, C.; Schröder, M. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 18280−18285. (6) (a) Tard, C.; Liu, X.; Hughes, D. L.; Pickett, C. J. Chem. Commun. 2005, 133−135. (b) Best, S. P.; Borg, S. J.; White, J. M.; Razavet, M.; Pickett, C. J. Chem. Commun. 2007, 4348−4350. (7) Volkers, P. I.; Boyke, C. A.; Chen, J.; Rauchfuss, T.B.; Whaley, C. M.; Wilson, S. R.; Yao, H. Inorg. Chem. 2008, 47, 7002−7008. 6293

dx.doi.org/10.1021/om5010594 | Organometallics 2014, 33, 6290−6293