Anionic Iron Complexes with a Bond between an Ate-Type

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Anionic Iron Complexes with a Bond between an Ate-Type Pentacoordinated Germanium and an Iron Atom Naokazu Kano,*,† Naohito Yoshinari,† Yusuke Shibata,† Mariko Miyachi,† Takayuki Kawashima,*,†,∥ Masaya Enomoto,‡ Atsushi Okazawa,§ Norimichi Kojima,§ Jing-Dong Guo,¶ and Shigeru Nagase¶ †

Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Department of Chemistry, Faculty of Science Division I, Tokyo University of Science, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan § Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan ¶ Fukui Institute for Fundamental Chemistry, Kyoto University, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan ‡

S Supporting Information *

ABSTRACT: The first stable anionic iron(0) complexes bearing an atetype pentacoordinated germanium(IV) ligand were synthesized. The X-ray crystallographic analysis shows trigonal-bipyramidal and piano-stool geometries of germanium and iron, respectively. The complexes have moderately electron-rich iron centers and polar Ge−Fe bonds which can be cleaved by oxidation.

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reports on neutral TM complexes bearing a pentacoordinated or hexacoordinated group 14 element ligand,5 the TM was frequently found to undergo σ-donation from a filled d(metal) orbital to the vacant σ*(Si or Sn) orbital, and the group 14 element had a two-electron-accepting character (types IV and VI).6 The σ donation was also found in the anionic iron complex with an ate-type aluminum ligand, (Et4N)+[Ph3AlFe(Cp)(CO)2]−, in which the iron moiety plays a role of a Lewis base.7 We envisaged that an ate-type pentacoordinated group 14 element ligand in an anionic complex, [R4E−MLn]− (type V), would be sterically bulky and electron rich, both of which are important properties for TM complexes to act as efficient catalysts for cross-coupling reactions.8 However, such an anionic TM complex has not been reported to date, as far as we know, except for a few examples incorporated in a closododecaborane framework.9 Given the success in stabilizing anionic pentacoordinated silicon10 and germanium species11 by using a bidentate ligand derived from hexafluorocumyl alcohol,12 a spirogermane bearing the ligand is expected to be a good precursor for [R4Ge−MLn]− complexes. This paper focuses on the structure and electronic properties of the anionic iron complexes with a bond between an ate-type pentacoordinated germanium(IV) and an iron(0) atom. Reaction of spirogermane 111 bearing two sets of the bidentate ligand (−C6H3-4-CH3-2-C(CF3)2O−, denoted as Rf hereafter) with Na+[Fe(Cp)(CO)2]− (2) in THF gave Na+[Rf2GeFe(Cp)(CO)2]−·3THF (3a) in 58% yield (Scheme 1) (the Fe(Cp)(CO)2 fragment is denoted as Fp hereafter). A

here have been many synthetic challenges with transitionmetal (TM) complexes bearing a ligand containing a heavier group 14 element in an unusual coordination state this past quarter century.1 In usual group 14 element−TM complexes, the group 14 element is in a tetracoordinated state (Chart 1, type I)1c and acts as a mild electron-donating Chart 1

ligand.2 Several TM complexes with a low-coordinated group 14 element ligand have been isolated to date, and some of them feature multiple bonds to the metal: e.g., R2EMLn and RE MLn, where E is Si, Ge, Sn, or Pb (types II and III).1b,d,e In these complexes, the heavier group 14 element species is used as a ligand to mimic conventional carbon ligands such as alkyl, alkylidene, and alkylidyne ligands. However, to create TM complexes with novel electronic properties, it is desirable to synthesize complexes bearing a ligand in the coordination state which is unavailable to carbon. Heavier group 14 elements can also adopt a high-coordinated state such as neutral and anionic pentacoordinated structures.3 A few phosphine-bridged nickel and palladium complexes exhibit weak intramolecular electronic interaction between the TM and the silicon atom.4 In recent © 2012 American Chemical Society

Received: September 27, 2012 Published: November 29, 2012 8059

dx.doi.org/10.1021/om300915y | Organometallics 2012, 31, 8059−8062

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are both narrower than those of the butylgermanate bearing two sets of a similar bidentate ligand (173.8(2) and 119.2(3)°, respectively),11 a property reflecting the steric repulsion between the germanium ligand and the Fp moiety. Meanwhile, the Ge1−Fe1 bond (2.4065(7) Å) in 3b shows no meaningful elongation. In the 1H and 13C NMR spectra in THF-d8, the ortho proton signal of the benzene ring of the Rf ligand was observed in a low-field region (δH 8.26 ppm), as a result of the electric dipole of the polar apical Ge−O bonds.14 The NMR data prove the persistence of the TBP structure with dissociation of neither Ge−O nor Ge−Fe bonds in the solution state. Complex 3b was diamagnetic, with the SQUID measurement showing a very low magnetic susceptibility (4.02 × 10−4 cm3 mol−1), which eliminates the monovalence or trivalence of iron. XPS of 3b revealed the binding energy of Fe 2p3/2 of 3b as 708.2 eV (Figure 2A), and it is situated between that of bare

cation-exchange reaction of 3a with benzyltrimethylammonium chloride gave (PhCH2NMe3)+(Rf2GeFp)− (3b) in 89% yield. Scheme 1

The crystal structures of 3a,b were determined by X-ray crystallographic analysis (Figure 1A,B). Both of the complexes

Figure 2. Photoelectron spectra of (A) Fe 2p and (B) Ge 3d of 3b.

iron with zero valence (707.4 eV) and that of [FeCp(CO)2]2 with monovalence (708.5 eV).15 The valence of iron in 3b is concluded to be zero in view of the result of the SQUID measurement. In the case of Ge 3d (Figure 2B), the binding energy of 3b (31.4 eV) is situated between that of Ph4Ge (31.0 eV) and that of GeO2 (32.5 eV), indicating tetravalence of germanium.16 The s-electron density of the iron of 3b was evaluated by 57 Fe Mössbauer spectroscopy at 78 K, and the spectrum showed a quadrupole doublet with an isomer shift of 0.084(2) mm s−1 and a quadrupole splitting of 1.744(4) mm s−1 relative to α-Fe (Figure 3A). In the case of the 57Fe Mössbauer

Figure 1. Thermal ellipsoid plots of complexes 3a (A) and 3b (B). (C) CPK model view of the anion of 3b. Selected bond lengths (Å) and angles (deg) of 3a: Ge1−Fe1, 2.404(2); Ge1−C1, 1.977(5); Ge1−C11, 1.981(5); Ge1−O1, 2.148(4); Ge1−O2, 1.995(4); Fe− C21, 1.752(6); Fe−C22, 1.763(6); C1−Ge1−Fe1, 117.44(15); C11− Ge1−Fe1, 123.70(14); O1−Ge1−O2, 166.12(13); O1−Ge1−Fe1, 97.04(12); C21−Fe1−C22, 91.1(2); C21−Fe1−Ge1, 90.59(18); C22−Fe1−Ge1, 89.57(18). Selected bond lengths (Å) and angles (deg) of 3b: Ge1−Fe1, 2.4065(7); Ge1−C1, 1.969(4); Ge1−C11, 1.966(3); Ge1−O1, 2.043(2); Ge1−O2, 2.047(2); Fe−C21, 1.744(4); Fe−C22, 1.748(4); C1−Ge1−Fe1, 122.96(10); C11−Ge1−Fe1, 123.31(10); O1−Ge1−O2, 169.37(10); O1−Ge1−Fe1, 97.65(7); C21−Fe1−C22, 92.43(17); C21−Fe1−Ge1, 93.02(12); C22−Fe1− Ge1, 85.91(12).

feature a germanium−iron bond, in which germanium and iron show trigonal-bipyramidal (TBP) and piano-stool geometries, respectively. On the one hand, the anionic part of 3a interacts with the sodium ion, Na1, to form an ion pair. As a result, the Ge1−O1 bond of 3a (2.148(4) Å) is significantly longer than the Ge1−O2 bond (1.995(4) Å), although it is within the sum of the van der Waals radii. On the other hand, the anionic part in 3b shows no contact with the benzyltrimethylammonium ion. The Ge1 atom is bonded to one iron, two carbon, and two oxygen atoms. This is the first example, as far as we know, of an anionic iron complex bearing an ate-type pentacoordinated germanium ligand. The germanium ligand is bulky enough to cover one side of the anion part of 3b (Figure 1C). This pentacoordinated germanium ligand should be bulkier than the trimethylgermyl group, which is known to be a bulky group with a high A value (A = 2.07 kcal mol −1 ). 13 The O1−Ge1−O2 angle (169.37(10)°) and the C1−Ge1−C11 angle (113.62(14)°)

Figure 3. (A) 57Fe Mössbauer spectrum of 3b. (B) IR spectrum of 3b in KBr.

spectrum, the isomer shift decreases with increasing s-electron density at the iron nucleus and decreasing d-electron density responsible for the shielding effect. The isomer shift of 3b is almost the same as those of alkyl-substituted Fp complexes (0.076−0.105 mm s−1),17 whose small isomer shifts suggest a large s-electron density in the iron of 3b and small d-electron density because of the π back-donation. The π back-donation from iron to the carbonyl ligands is reflected in the IR spectrum. The IR spectrum of 3b showed two carbonyl stretching bands at 1943 and 1995 cm−1 (Figure 8060


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4C,D). In the isoelectronic neutral complexes with a hypervalent phosphorus ligand, π back-donation from the iron to the antibonding orbital of the apical three-centered four-electron bond was advocated.21 In 3b, a π-type orbital was found between the p orbital of Ge on the equatorial plane and the d orbital of Fe (HOMO-14, −8.00 eV) (Figure 4E). The calculated Ge−Fe homolytic bond dissociation energy (38.9 kcal mol−1) in 3−, the anion part of 3b, is a little bit smaller than that (42.1 kcal mol−1) of 4. This shows the slightly weak orbital interactions between germanium and iron. The NBO analysis of the optimized structure of 3− shows that the σ bonding orbital forms from an sp1.93 hybrid on germanium interacting with an sp1.24d3.56 hybrid on iron. The polarity of the bonding σ(Ge−Fe) NBO shows 68.3% polarization to iron. The Wiberg bond index of the Ge−Fe bond is 0.601, which is similar to the value of 0.609 in 4. Therefore, the germanium−iron bond of 3− is a polar bond, as represented by Geδ+−Feδ−. The polarization level of the Ge−Fe bond is almost the same as that of 4 (Ge(sp2.68)−Fe(sp1.27d3.43) with 67.3% polarization to iron), although the sp2 and sp3 hybridization states of germanium in 3− and 4, respectively, are different from each other. The natural population analysis of charge distribution of selected atoms of 1, 2−, 3−, and 4 was carried out. In 3−, the germanium atom is positively charged (qGe, = +1.76), being more positive than that in neutral tetracoordinated germanium complex 4 (qGe = +1.54) and slightly less positive than that in 1 (qGe = +1.93). All of the atoms bonding to the germanium in 3− are negatively charged (qFe = −1.48; qC = −0.46; qO = −0.87). The negative charge of the iron atom of 3− is almost the same as that of 4 (qFe = −1.50) and slightly more negative than that in 2− (qFe = −1.19). The total anion charge (q = −1) of 3− is shared by the Rf2Ge and Fp moieties, their charges being −0.69 and −0.31, respectively. Supposing the formation process of 3− from 1 and 2−, the negative charges of both Cp and CO ligands of 2− transfer to 1 and the iron atom, resulting in the negative charges in 3− as mentioned above. The charge of the Fp moiety of 3− is intermediate between the values of −0.26 in 4 and −0.34 in Me3GeFp. Thus, the Fp moiety of 3− is in a moderately electron rich state. The electronic properties are reflected in the IR and Mössbauer spectra. Reaction of 3b with methyllithium caused nucleophilic attack on the germanium and resulted in Ge−Fe bond fission to give methylgermanate 5 (75%). Upon treatment with water in THF at 50 °C for 14 days, 3b underwent hydrolysis to give hydroxygermanate 6 together with [Fe(Cp)(CO)2]2, which should form from the intermediate Fe(Cp)(CO)2H.22 Despite the modest resistance to hydrolysis and thermal stability under an inert gas atmosphere, 3b is air sensitive, suggesting its oxidation. CV of 3b exhibited irreversible oxidation waves at the peak potential of the anodic current of +0.31 V versus Ag+/ Ag in THF. This was much lower than that of LFe(Cp)(CO)2 (ranging from +1.335 to +1.421 V), where L is Me, SiMe3, and GeMe3, representing the easily oxidizable characteristics of the anionic complex.23 In fact, oxidation of 3b with silver perchlorate gave 1 quantitatively. The oxidation corresponds to withdrawal of an electron from the σ-bonding orbital, and thus it leads to Ge−Fe bond fission. In summary, we have synthesized the first anionic iron complexes bearing an ate-type pentacoordinated germanium ligand. This is important as the construction of a new type of bond between a transition metal and a pentacoordinated heavier group 14 element in the ate-type structure which

3B). The values are smaller than those of the cationic [LFe(CO)2(Cp)]+ complexes, where L is a phosphine or carbene ligand (νas, 2001−2004 cm−1; νs, 2048−2050 cm−1),18 while they are almost the same as those of Ph3GeFp (4) (νas, 1940 cm−1; νs, 1995 cm−1)19 and other alkyl derivatives (νas, 1935−1947 cm−1; νs, 1996−2005 cm−1).20 The π backdonation from iron to the π*(CO) orbital in 3b is strong and almost the same as that in the tetracoordinated germyl and alkyl derivatives. A possible reason for the strong π backdonation is the high electron density at iron, because 3b is an anionic complex. Chart 2

Despite the substituent effects similar to electron-donating ligands on the Fp moiety, the germanium ligand in 3b may be recognized as an electron acceptor, because the synthetic method for 3b shows that electrons are provided from the Fp fragment to form the Ge−Fe bond. We performed DFT calculations on 3b at the B3PW91/6-311+G(d) level for all atoms to obtain information on orbital interactions and the charge distribution. The optimized structure reproduced the crystal structure well (Ge−Fe, 2.410 Å). The HOMO (energy level, −5.64 eV) of 3b is mainly the σ-bonding orbital of the Ge−Fe bond (Figure 4A). In LUMO+3 (−0.74 eV), a contribution of the corresponding σ*-nonbonding orbital is observed (Figure 4B). The HOMO-10 (−7.39 eV) and HOMO-13 (−7.58 eV) represent σ-type and skewed π-type bonding orbitals, respectively, in the apical O−Ge−O bond linkage (Figure

Figure 4. Selected molecular orbitals of 3b obtained by DFT calculations (isovalue 0.03): (A) HOMO; (B) LUMO+3; (C) HOMO-10; (D) HOMO-13; (E) HOMO-14. 8061


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carbon cannot form. The unprecedented complexes feature a moderately electron rich iron fragment in line with Z-type interactions with the germanium ligand. The π back-donation from iron to the π*(CO) orbital was strong and almost the same as that in the electron-donating germyl and alkyl derivatives. The germanium ligand was found to be bulky. Such properties will provide a new choice of ligands for TM catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

Text, tables, figures, and CIF files giving experimental and computational details and crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (N.K.); Takayuki. [email protected] (T.K.). Present Address ∥

Faculty of Science, Gakushuin University, Mejiro, Toshima-ku, Tokyo 171-8588, Japan. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Research Hub for Advanced Nano Characterization, The University of Tokyo, for the spectral measurements. This work was supported by the Global COE program (Chemistry Innovation), Coordination Programming, and Specially Promoted Research from MEXT, Inamori foundation, Noguchi foundation, Itoh science foundation, and Mitsubishi foundation.

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REFERENCES

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Anionic Iron Complexes with a Bond between an Ate-Type

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