Communication pubs.acs.org/JACS
Transfer Reagent for Bonding Isomers of Iron Complexes Monika Schmidt,†,# Andreas E. Seitz,†,# Maria Eckhardt,† Gábor Balázs,† Eugenia V. Peresypkina,†,‡ Alexander V. Virovets,†,‡ Felix Riedlberger,† Michael Bodensteiner,† Eva M. Zolnhofer,§ Karsten Meyer,§ and Manfred Scheer*,† †
Institute of Inorganic Chemistry, University of Regensburg, Regensburg 93053, Germany Nikolaev Institute of Inorganic Chemistry SB RAS, Novosibirsk 630090, Russia § Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander University Erlangen-Nürnberg, Erlangen 91054, Germany ‡
S Supporting Information *
respectively. Since we recently found the suitability of [Cp″2Zr(η1:1-P4)]8 for the transfer of P1 moieties resulting in heteroatomic benzene derivatives and unprecedented phosphorus−carbon cage compounds,9 it was logical to synthesize the heavier congener [Cp″2Zr(η1:1-As4)] (1). Herein, we report on the surprising results of this synthesis and the use of 1 as a promising transfer reagent for the synthesis of polyarsenic derivatives under very mild conditions.
ABSTRACT: The cothermolysis of As4 and [Cp″2Zr(CO)2] (Cp″ = η5-C5H3tBu2) results in the formation of [Cp″2Zr(η1:1-As4)] (1) in high yields and the arsenic-rich complex [(Cp″2Zr)(Cp″Zr)(μ,η2:2:1-As5)] (2) as a minor product. In contrast to yellow arsenic, 1 is a light-stable, weighable and storable arsenic source for subsequent reactions. The transfer reaction of 1 with [Cp‴Fe(μ-Br)]2 (Cp‴ = η5-C5H2tBu3) yields the unprecedented bond isomeric complexes [(Cp‴Fe)2(μ,η4:4-As4)] (3a) and [(Cp‴Fe)2(μ,η4:4-cyclo-As4)] (3b). In contrast, the analogous reaction with the CpBn derivative [CpBnFe(μ-Br)]2 (CpBn = η5-C5(CH2(C6H5)5) leads exclusively to the triple decker complex [(CpBnFe)2(μ,η4:4-As4)] (4) possessing the tetraarsabutadiene-type ligand analogous to 3a. To elucidate the stability of the bonding isomers 3a and 3b, DFT calculations were performed. The oxidation of 4 with AgBF4 affords [(CpBnFe)2(μ,η5:5-As5)][BF4] (5), which is a product expanded by one arsenic atom, instead of the expected complex [(CpBnFe)2(μ,η4:4-cyclo-As4)]+.
Scheme 1. Synthesis of 1 and 2
The reaction of [Cp″2Zr(CO)2] with As4 leads to [Cp″2Zr(η1:1-As4)] (1) in excellent yields (84%), which can be obtained in gram scales (Scheme 1).10 Beside the formation of 1, the unprecedented complex [(Cp″2Zr)(Cp″Zr)(μ,η2:2:1-As5)] (2) could be isolated in traces, which is in contrast to [Cp″2Zr(η1:1P4)] as the only product of the corresponding P4 reaction.10,11 The molecular structures of 1 and 2 are depicted in Figure 1. The central structural motif of 1 consists of a [Cp″2Zr] fragment inserted into one As−As edge of the As4 tetrahedron to give a tetraarsabicyclo-[1.1.0]-butane framework. The As−As
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he activation of white phosphorus by main group and transition metal units and its transformation to different phosphorus units represents a very active field of research in chemistry with remarkable advances in recent years.1 In contrast, less is known about the reactivity of the heavier homologue As4 toward transition metal moieties2 and main group compounds.3 This can be attributed to the extreme challenges when working with yellow arsenic (As4). Besides its toxicity, its very time-consuming preparation2e,4 as well as its instability as a solid, it is extremely difficult to handle solutions of As4 because of their pronounced light- and air-sensitivity. Moreover, accurate stoichiometric reactions are hardly practicable due to the usually unknown concentration of As4 because of the continuous formation of gray arsenic from these solutions. To overcome this, it has been of general interest to synthesize a storable, weighable and light-stable arsenic source for the preparation of polyarsenic compounds.5 We therefore targeted on the development of an arsenic transfer reagent suitable for subsequent transfer reactions. Over the past years, the Cummins group extended the concept of P-containing transfer reagents highlighted in the synthesis of AsP36 and [P2N3]−7 as the heavier congeners of E4 (E = P, As) and Cp−, © 2017 American Chemical Society
Figure 1. Molecular structures of 1 and 2 in the solid state. H atoms are omitted for clarity. Cp″ ligands are drawn in a wire-frame model. Thermal ellipsoids are drawn at 50% probability level.10 Received: July 14, 2017 Published: September 21, 2017 13981
DOI: 10.1021/jacs.7b07354 J. Am. Chem. Soc. 2017, 139, 13981−13984
Communication
Journal of the American Chemical Society bond lengths of the As42− moiety (2.4154(4)−2.4650(4) Å) are in the range of As−As single bonds.12 In contrast, the molecular structure of 2 contains a folded As5 moiety coordinating to one [Cp″2Zr] and one [Cp″Zr] fragment. The As−As bond lengths within the As5 ligand of 2 are slightly elongated (2.4511(8)− 2.4797(9) Å) compared to As−As single bonds.10 Remarkably, 1 is absolutely light-stable and no decomposition of 1 was observed after the longtime storage in a nitrogen glovebox. Concomitant with the access to 1 in gram scale amounts, we got interested to investigate its suitability for the transfer of As42− units to iron fragments under mild reaction conditions in contrast to the usual cothermolytic approach by using As4 and the dimeric iron carbonyl complexes [CpRFe(CO)2]2 (CpR = Cp, Cp*, Cp+).13 The use of that approach results in the formation of [Cp4Fe4(As2)2]13a or [CpRFe(η5As5)],13b [(CpRFe)3As6]13c and [(CpRFe)3As6{(η3-As3)Fe}]13c (CpR = Cp*, Cp+). Contrary to the corresponding P and the mixed P/As containing complexes resulting from the related cothermolyses, no formation of triple decker complexes such as [(CpRFe)2(μ,η4:4-As4)] has been reported so far.14 Note that the sterically encumbered complexes [CpRFe(CO)2] (CpR = CpBIG, Cp‴) react under mild conditions with As4 in a radical reaction to produce the doubly substituted butterfly complexes [{CpRFe(CO)2}2(μ,η1:1-As4)].2k,l After chromatographic workup, the reaction of 1 with [Cp‴Fe(μ-Br)]2 afforded the unprecedented “bonding isomers”16 [(Cp‴Fe)2(μ,η4:4-As4)] (3a) and [(Cp‴Fe)2(μ,η4:4cyclo-As4)] (3b) in an overall isolated yield of 66% (Scheme 2), crystallizing simultaneously with 3a being the major product.10,15,19
3a exhibits a cisoid acyclic As4 middle deck, stabilized by two [Cp‴Fe] fragments, the triple decker complex 3b shows a planar cyclo-As4 ligand. In addition, 3a and 3b differ regarding the Fe−Fe distances. In 3a, the Fe−Fe bond is elongated (2.6927(6) Å) but in good agreement with the Fe−Fe bond lengths reported, e.g., for the corresponding phosphorus complexes (2.616(1)−2.6430(8) Å).14 In 3b, no Fe−Fe bond (Fe1···Fe2 3.5019(9) Å)) is observed. Moreover, the As1−As2 and As3−As4 bond lengths of 3a are similar, indicating a distinct double bond character,17 whereas the As2−As3 bond (2.6100(5) Å) in the backbone is elongated compared to an As−As single bond.12 Accordingly, the As4 moiety can be best described as a cis-tetraarsa-1,3-diene ligand. In contrast, the cyclo-As4 ligand of 3b is slightly distorted with two shorter (2.4095(9), 2.4135(9) Å) and two slightly longer distances (2.4560(9), 2.4720(9) Å), but still in the range of an As−As single bond.12 In comparison, the cationic isoelectronic complex [(CptBuCo)2(μ,η4:4-As4)]2+ is less distorted and differs only slightly from a square.18 The 1H and 13C{1H} NMR spectra of the crystalline mixture of 319 reveal one set of signals for chemically and magnetically equivalent Cp‴ ligands. These signals are assigned to 3a, since it was shown experimentally to be the unambiguous major compound of the crystalline mixture.10,19 Furthermore, 1H NMR spectroscopy using the Evans method20 showed no paramagnetic species in solution. The molecular ion peak of 3 was detected by mass spectrometry, and elemental analysis proved its elemental composition. Moreover, we performed zero-field 57Fe Mössbauer spectroscopy of a mixture of 3 revealing only one doublet with an isomer shift (δ) of 0.50 mm·s−1 and a quadrupole splitting ΔEQ of 3.66 mm·s−1, indicating an iron(II) low-spin configuration.10 DFT calculations show that the isomer shift and quadrupole splitting for 3a and 3b are very similar (3a: δ = 0.46 mm·s−1, ΔEQ = 2.85 mm·s−1; 3b: δ = 0.47 mm·s−1, ΔEQ = 2.26 mm·s−1) and, hence, the two isomers cannot be distinguished by 57Fe Mössbauer spectroscopy.10 According to DFT calculations, 3a is with 17.51 kJ·mol−1 more stable compared to 3b.10 However, both 3a and 3b are minima on the potential energy surface. A constrained geometry optimization with fixed Fe−Fe distance shows that, for the isomerization of 3b to 3a, an activation energy of roughly 36.6 kJ·mol−1 is needed indicating that 3b is a metastable compound (Figure 3).10,21 Hence, 3b can be considered as the kinetic product, while 3a represents the thermodynamic one. Nevertheless, the formation of 3a and 3b might be explained by a bond fluctuation of the As4 framework as observed for the phosphorus analogue [(Cp″Fe)2(μ,η4:4-P4)] in solution going from a butadiene-like structure at low temperatures to a formal cyclo-P4 unit as a transition state at higher temperatures, determined by VT 31P NMR spectroscopy.14a Therefore, for the first time, both isomers are achievable in the solid state for the As derivatives 3a,b showing the potential of As chemistry in comparison to the related P chemistry. In addition, Walter et al. reported on the complex [(Cp‴Fe)2(μ-P4)], containing a P4 middle deck with a kite-like distortion. Interestingly, heating this compound to 75 °C results in the transformation to the isomer [(Cp‴Fe)2(μ,η4:4-P4)] containing a cis-tetraphosphabutadiene unit, among other products.22 Regardless of several attempts, VT 1H NMR spectroscopic investigations of a crystalline mixture of 3 revealed no dynamic behavior.10
Scheme 2. Synthesis of 3a, 3b and 4, Respectively
Figure 2 shows the molecular structures of 3a and 3b in the solid state. Although the chemical composition of 3a and 3b is identical, their central structural motifs vary significantly. While
Figure 2. Molecular structures of 3a, 3b and 4 in the solid state. H atoms are omitted for clarity. CpR ligands (CpR = Cp‴, CpBn) are drawn in a wire-frame model. Thermal ellipsoids are drawn at 50% probability level.10 13982
DOI: 10.1021/jacs.7b07354 J. Am. Chem. Soc. 2017, 139, 13981−13984
Communication
Journal of the American Chemical Society
contrast to 4, the iron atoms are not bound to each other, since the Fe···Fe distance is considerably longer than the sum of the covalent radii of iron (2.64 Å)25 being in good agreement with the reported bond length in [CpFe(μ,η5:5-As5)FeCp*][PF6].14a In conclusion, we reported on a high yield synthesis of [Cp″2Zr(η1:1-As4)] (1) starting from As4 and [Cp″2Zr(CO)2] as a novel transfer reagent for As4 units. In addition, the unprecedented arsenic-rich complex [(Cp″2Zr)(Cp″Zr)(μ,η2:2:1-As5)] (2) has been isolated and characterized. Moreover, the light-stable and storable arsenic compound 1 has been used as transfer reagent for the synthesis of polyarsenic ligand complexes under mild reaction conditions. Thus, it was possible to obtain simultaneously the unprecedented “bonding isomers” [(Cp‴Fe)2(μ,η4:4-As4)] (3), revealing both a tetraarsabutadiene-like and a cyclo-As4 ligand. According to DFT calculations, 3b represents the kinetic product of the reaction. This isomer represents as an isolated product the long-time proposed transition state in the dynamic behavior of the P congener [(Cp‴Fe)2(μ,η4:4-P4)]. In contrast, the reaction of the CpBn derivative [CpBnFe(μBr)]2 and 1 yielded solely the As4-butadiene containing complex [(CpBnFe)2(μ,η4:4-As4)] (4). The chemical oxidation of 4 with AgBF 4 leads to the unexpected complex [(CpBnFe)2(μ,η5:5-As5)][BF4] (5) possessing a cyclo-As5 ligand stabilized by two [CpBnFe] fragments. Here, an unprecedented ring expansion occurred. These results demonstrate the capability of 1 as a promising arsenic transfer reagent under mild reaction conditions resulting even in metastable products. In further studies, we will target transfer reactions of these As42− units from 1 to yield kinetically controlled products of unprecedented polyarsenic derivatives of transition metals and main group elements.
Figure 3. Potential energy surface scan of 3 along the Fe−Fe distance. Calculated at the BP86/def2-TZVP level of theory.
In contrast to the results with the Cp‴ containing Fe derivative, the corresponding transfer reaction using the CpBn derivative [CpBnFe(μ-Br)]2 under similar conditions leads exclusively to the formation of [(CpBnFe)2(μ,η4:4-As4)] (4) with a butadiene-like As4 middle deck (Scheme 2). This could be caused by the different steric demand and electronic properties of the CpBn ligand.23 The central structural motif of 4 is isostructural to 3a possessing a cisoid As4 unit stabilized by two [CpBnFe] fragments (Figure 2).12,18 In contrast to 3a, the Fe−Fe distance is even shorter (2.6927(6) vs 2.6654(5) Å) and coincides with a usual Fe−Fe single bond.14 The 1H and 13 C{1H} NMR spectra of 4 reveal the expected signal sets for two chemically and magnetically equivalent CpBn ligands.10 Moreover, the redox chemistry of 4 was studied by cyclic voltammetry which revealed only one reversible oxidation of 4 (E1/2 = −0.69 V) and one reversible reduction (E1/2 = −2.15 V; potentials are referenced against [Cp2Fe]/[Cp2Fe]+).10 Here, we became interested in the oxidation of 4 with Ag(I) salts such as AgBF4. Surprisingly, no formation of an oxidized species with a cyclo-As4 middle deck was observed, but the unexpected 30 VE complex [(CpBnFe)2(μ,η5:5-As5)][BF4] (5) was isolated in a 19% yield showing a ring expansion reaction to give a cyclo-As5 middle deck (Scheme 3). The crystal structure of 5 revealed an expansion of the As4 middle deck resulting in a cyclo-As5 ligand stabilized by two [CpBnFe] fragments.10,24 In the [Fe2As5] core of 5, the As−As bond distances are in-between a double and single bond as expected for a cyclo-As5 ligand (av. 2.36 Å).10,12,13a,18,24 In
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b07354. Experimental section, crystallographic and computational details, and spectra (PDF) Data for C26H42As4Zr (CIF) Data for (Cp″2Zr)(Cp″Zr)As5 (CIF) Data for C34H58As4Fe2 (CIF) Data for C34H58As4Fe2 (CIF) Data for C80H70As4Fe2 (CIF) Data for (C80H70As5Fe2), (BF4), (CH2Cl2)0.5 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] Scheme 3. Oxidation of 4 with AgBF4 under Formation of 5
ORCID
Karsten Meyer: 0000-0002-7844-2998 Manfred Scheer: 0000-0003-2182-5020 Author Contributions #
Authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (DFG). A. E. Seitz is grateful for a Ph.D. fellowship of the Fonds der Chemischen Industrie. K.M. 13983
DOI: 10.1021/jacs.7b07354 J. Am. Chem. Soc. 2017, 139, 13981−13984
Communication
Journal of the American Chemical Society acknowledges financial support from the Friedrich-AlexanderUniversity Erlangen-Nürnberg (FAU). Some results are also part of the doctoral theses of Dr. Monika Schmidt, University of Regensburg (Germany), 2016 and Dr. Maria Eckhardt, University of Regensburg (Germany), 2014.
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(14) (a) Scherer, O. J.; Schwarz, G.; Wolmershäuser, G. Z. Anorg. Allg. Chem. 1996, 622, 951. (b) Scherer, O. J.; Hilt, T.; Wolmershäuser, G. Organometallics 1998, 17, 4110. (c) Heinl, S.; Balázs, G.; Scheer, M. Phosphorus, Sulfur Silicon Relat. Elem. 2014, 189, 924. (d) Schwarzmaier, C.; Bodensteiner, M.; Timoshkin, A. Y.; Scheer, M. Angew. Chem., Int. Ed. 2014, 53, 290. (15) The reactions were performed with in situ generated [CpRFe(μBr)]2 (CpR = Cp‴, CpBn). (16) For discussions on “bond-stretch isomers” see: Rodriguez, A.; Olsen, R. A.; Ghaderi, N.; Scheschkewitz, D.; Tham, F. S.; Mueller, L. J.; Bertrand, G. Angew. Chem., Int. Ed. 2004, 43, 4880. as review, cf.: Rohmer, M.-M.; Benard, M. Chem. Soc. Rev. 2001, 30, 340. (17) Cowley, A. H.; Norman, N. C.; Pakulski, M. J. Chem. Soc., Dalton Trans. 1985, 383. (18) von Hänisch, C.; Fenske, D.; Weigend, F.; Ahlrichs, R. Chem. Eur. J. 1997, 3, 1494 ([As7(SiMe3)3] was used as an As source). (19) Note that the complexes 3a and 3b crystallize simultaneously. Despite several attempts, the separation of analytically pure 3a and 3b was not possible, since, visually, both compounds were hardly distinguishable. However, it was found by checking the unit cell parameters of dozens of crystals that 3a is the unequivocal main product and 3b the minor product. (20) Evans, F. D. J. Chem. Soc. 1959, 2003. (21) The calculated energies along the potential energy surface can vary widely depending upon the choice of functional and basis set. For example, see: Plata, R. E.; Singleton, D. A. J. Am. Chem. Soc. 2015, 137, 3811. (22) Walter, M. D.; Grunenberg, J.; White, P. S. Chem. Sci. 2011, 2, 2120. (23) (a) Janiak, C.; Schumann, H. In Advances in Organometallic Chemistry; Stone, F. G. A., Robert, W., Eds.; Academic Press, 1991, Vol. 33, p 291. (b) Glockner, A.; Bauer, H.; Maekawa, M.; Bannenberg, T.; Daniliuc, C. G.; Jones, P. G.; Sun, Y.; Sitzmann, H.; Tamm, M.; Walter, M. D. Dalton Trans. 2012, 41, 6614. (24) Due to serious disorder of 5 and its systematic pseudomerohedry twinning, respectively, the synthesis was also performed with AgOTf (OTf− = CF3SO3−) presumably leading to the isostructural complex [(CpBnFe)2(μ,η5:5-As5)][OTf]. For details, see SI. (25) Fleischmann, M.; Welsch, S.; Krauss, H.; Schmidt, M.; Bodensteiner, M.; Peresypkina, E. V.; Sierka, M.; Gröger, C.; Scheer, M. Chem. - Eur. J. 2014, 20, 3759.
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