Lewis Base Complexes of an Enantiomeric Germanium(II) Cation

Aug 5, 2011 - The coordinated THF was exchanged with the Lewis bases ... David M. Ermert , Jesse B. Gordon , Khalil A. Abboud , and Leslie J. Murray...
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Lewis Base Complexes of an Enantiomeric Germanium(II) Cation Bearing a Bis(oxazoline) Ligand Hidekazu Arii, Fumiya Nakadate, Kunio Mochida,* and Takayuki Kawashima Department of Chemistry, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan

bS Supporting Information ABSTRACT: A tetrahydrofuran (THF) complex of an enantiomeric germanium(II) cation functionalized by an anionic bis(oxazoline) ligand was synthesized in a mixture of dimethoxyethane (DME) and THF by abstraction of the chloride ion from the corresponding chlorogermylene. The coordinated THF was exchanged with the Lewis bases phosphine and pyridine at the same coordination site. X-ray crystallographic analysis revealed that the Lewis bases donate to a vacant p orbital of the germanium atom to form a trigonal-pyramidal geometry with a lone pair of electrons.

D

ivalent group 14 element containing compounds (R2E:) have attracted much attention in fundamental main-group chemistry because of their unique structures and reactivities.1,2 Carbenes are known to function as Lewis bases in organic syntheses3 and can activate an H H or N H bond, mainly by strong σ donation to the antibonding orbital of the corresponding single bond.4 Enantiomeric carbene is also employed as a strong donor and/or a supporting ligand for metal complexes to catalyze asymmetric reactions.5 Although heavier R2E: compounds with donor/acceptor abilities toward metal centers have been synthesized for decades, examples of enantiomeric derivatives are limited.6,7 Glorius and co-workers reported the synthesis of chiral germylene and stannylene compounds bearing an o-phenylenediamine derivative with an (R)-1-phenylethyl group on the N atoms.7 Moreover, the heavier divalent group 14 element cations RE:+ are highly reactive species with two potential coordination sites for a Lewis base molecule (Figure 1).8,9 We have previously synthesized and carried out spectroscopic and structural characterizations of the copper germylene complexes [Cu(iPrnacnac){Ge(X)(iPrnacnac)}] (X = H, Me, Cl), where popular β-diketinimate ligands were bound to both copper and germanium centers.10 Various bidentate chelating ligands that are N donors and common ligands for transition-metal complexes are also able to bind to the main-group germanium(II) atom. In this study, we focused on one of the bidentate ligands, 1,1-bis[(4S)-4-phenyl-1,3-oxazolin-2-yl]ethane ({(S)-box-Ph}H), which can participate in asymmetric reactions, and synthesized several Lewis base complexes of an enantiomeric germanium(II) cation by using its THF complex as a key intermediate. The latter is prepared by abstraction of a chloride ion from the corresponding chlorogermylene as described hereafter. The reaction of lithio-(S)-box-Ph with GeCl2 3 (dioxane) in THF at 80 °C under an Ar atmosphere afforded the corresponding enantiomeric chlorogermylene [GeCl((S)-box-Ph)] (1) in 54% yield (eq 1). The crystal structure of 1 (Figure 2) showed that the germanium center has a trigonal-pyramidal r 2011 American Chemical Society

Figure 1. Molecular structure of the cationic germanium(II) compounds. Dipp = 2,6-diisopropylphenyl.

geometry with an N2Cl donor set, which indicates that the pyramidal germanium center has a lone pair of electrons. The similarities between the two N C bond lengths (1.324(3) and 1.324(3) Å) and between the two C C bond lengths (1.391(3) and 1.395(3) Å) in the chelate ring indicate delocalization of the π electrons of the N C C C N backbone. The germanium center in 1 is displaced by 0.550(3) Å from the N C C C N chelate plane. This is consistent with other chlorogermylene compounds bound to monoanionic bidentate ligands, such as diamidinate and β-diketiminate, where the germanium center is out of the plane of the chelate ring.11 The absolute configuration of the chiral carbon at the 4-position of the oxazoline rings retained the S configuration of the starting material. In the 1H NMR spectrum of 1 in THF-d8, the three broad peaks of the oxazoline rings observed at 20 °C changed to five apparent triplets at 4.25, 4.85, 4.91, 5.29, and 5.33 ppm upon lowering the temperature to 60 °C. A dynamic behavior with respect to the liberation/rebinding of the chloride ion can be interpreted to Received: February 17, 2011 Published: August 05, 2011 4471

dx.doi.org/10.1021/om200149a | Organometallics 2011, 30, 4471–4474

Organometallics result in the observation of two unequivalent oxazoline rings diagnostic of a C1-symmetric molecule at 60 °C. The 13C NMR spectrum of 1 at 60 °C also exhibited two nonequivalent phenyl groups and oxazoline rings, except for OCH2, which is consistent with the 1H NMR results. The signal of the carbon nucleus bridging between the two oxazoline rings was shifted downfield from 34.1 ppm to 69.9 ppm for {(S)-box-Ph}H12 because of the change in the charge on the N C C C N backbone.

The addition of Ag(SbF6) to a DME solution of 1 with excess THF gave a THF complex of the germanium(II) cation, [Ge((S)-box-Ph)(thf)](SbF6) (2), which was isolated as colorless crystals in 50% yield (eq 2).13 The crystal structure of 2 (Figure 3) revealed that the germanium center has a trigonalpyramidal geometry with an N2O donor set and a lone pair of electrons, in which the O donor of the THF molecule coordinates at the site where the chloride ion is in 1. The chiral carbons of 2 had an S configuration that was maintained during the salt elimination reaction, as expected. The Ge O bond length of 2.148(2) Å in 2 is ca. 0.1 Å shorter than those reported for the germanium(II) cations [Ge(iPr2ATI)(OTf)] (2.255(2) Å)8b

Figure 2. Crystal structure of 1, showing 50% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity.

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and [Ge(L)(OEt2)] (2.2291(17) Å), which can be explained by the high coordination ability of THF.8f The Ge N bond lengths in 2 (1.924(2) and 1.931(2) Å) are slightly shorter than those of 1 (1.945(3) and 1.940(4) Å), a phenomenon assigned to an increase in the positive charge at the germanium center. The germanium center is 0.147(3) Å from the N C C C N chelate ring plane, indicating that there is a strong interaction between the π electron of the nitrogen atom and the p orbital of the germanium center. The proton resonances of the oxazoline rings of 2 in THF-d8, independent of temperature, were observed as three apparent triplets at 4.62, 5.01, and 5.44 ppm, shifted to magnetic field lower than those of 1. Compound 2 undergoes rapid ligand exchange at the germanium center in the presence of a large excess of THF-d8, even at 80 °C.

Treatment of 2 with excess PMe2Ph or pyridine in DME resulted in the substitution of THF to give the corresponding germanium cation complexes [Ge((S)-box-Ph)(PMe2Ph)](SbF6) (3) and [Ge((S)-box-Ph)(py)](SbF6) (4), respectively, in moderate yields (eq 3). On the basis of the crystal structures of 3 and 4 (Figure 3), the average Ge N bond lengths of the oxazoline rings become longer in the following order: 2 (1.928 Å), 3 (1.933 Å), 4 (1.939 Å). This trend correlates with the donor ability of the Lewis base molecules.

The strong coordination of the Lewis base molecules decreases the π interaction of the N donor bis(oxazoline) ligand with the vacant p orbital on the germanium center, resulting in elongated Ge N bonds. The Ge P bond lengths in 3 (average 2.588 Å) and Ge N(pyridine) bond lengths in 4

Figure 3. Crystal structure of the cationic parts of 2 (A), 3 (B, one of the two independent molecules), and 4 (C, one of the three independent molecules), showing 50% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. 4472

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’ ACKNOWLEDGMENT This work was financially supported by a Grant-in-Aid for Scientific Research on Priority Areas (No. 19027051) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. ’ REFERENCES

Figure 4. Variable-temperature 1H NMR spectra of 4 in THF-d8 in the oxazoline and aromatic region.

(average 2.133 Å) are consistent with Ge P and Ge N coordination bond lengths reported previously.14,15 The phenyl group of PMe2Ph in 3 is perpendicular to the oxazoline ring, due to steric repulsion with the phenyl group of the bis(oxazoline) ligand. For the variable-temperature 1H NMR spectra of 3 in THF-d8, the signals of the oxazoline ring were broadened below 20 °C, indicating a dynamic behavior attributed to rapid liberation/rebinding of PMe2Ph. However, the pyridine proton resonances of 4 in THF-d8 (7.63, 8.18, and 8.10 ppm) were significantly different from those in free pyridine (7.38, 7.79, and 8.54 ppm) at 80 °C (Figure 4). NOE was observed between the methine proton of the oxazoline ring and the ortho protons of pyridine (Figure S4, Supporting Information), indicating that the binding of pyridine to the germanium center is stronger than that of THF. In conclusion, we synthesized enantiomeric the chlorogermylene [GeCl((S)-box-Ph)] (1) using a monoanionic bis(oxazoline) ligand. The dechlorination of 1 with Ag(SbF6) in the presence of THF led to the formation of the enantiomeric germanium(II) cation THF complex [Ge((S)-box-Ph)(thf)](SbF6) (2). We also showed that the coordinated THF at the germanium center of 2 can be replaced by other Lewis base molecules.

’ ASSOCIATED CONTENT

bS

Supporting Information. Text, figures, tables, and CIF files giving details of the syntheses and spectroscopic and crystallographic data of germanium(II) compounds 1 4. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +81-3-3986-0221. Fax: +81-3-5992-1029. E-mail: kunio. [email protected].

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