Isolation and Structural Characterization of a Lewis Base-Free

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Isolation and Structural Characterization of a Lewis Base-Free Monolithioferrocene Takahiro Sasamori,* Yuko Suzuki, and Norihiro Tokitoh Institute for Chemical Research, Kyoto University, Gokasho Uji, Kyoto 611-0011, Japan S Supporting Information *

ABSTRACT: A sterically hindered ferrocenyl lithium derivative was successfully isolated as a stable, crystalline compound without any stabilization by Lewis bases. On the grounds of X-ray crystallography and multinuclear NMR spectroscopy, a dimeric structure was assigned to this monolithioferrocene in the crystalline state and in solution.

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Scheme 1. (1) Dismutation of Monolithioferrocene and (2) Lithiation of Ferrocene

errocene is, due to its extraordinary stability and its unique redox behavior, a very attractive organometallic compound.1 One of the most remarkable features of ferrocene is its versatility arising from facile modifications through a variety of well-established synthetic methods, which has found ferrocene applications in many research areas. Especially for the introducing of organic groups or heteroatom moieties, the straightforward metalation of ferrocene is highly interesting. Therefore, this branch of ferrocene chemistry has been extensively investigated and seen the generation of several kinds of ferrocenyl-substituted derivatives.1−3 Among the wealth of synthetic methods reported for the formation of monosubstituted ferrocenyls, the lithiation is probably the most convenient preparative strategy. It is, however, of great importance to isolate the resulting ferrocenyl lithium derivatives and characterize them accurately, as their structural and chemical features play a crucial role in the ensuing chemical reactions. Unfortunately, the controlled generation of monolithioferrocenes is relatively difficult compared to the corresponding 1,1′-dilithioferrocenes, due to the facile disproportionation into ferrocene and the corresponding 1,1′dilithioferrocene (Scheme 1).4 So far, monolithioferrocenes could only be generated by stabilization of the lithium cation via intra- or intermolecular coordination from Lewis bases with N- or O-moieties; and even then, only very few of these examples have been isolated and structurally characterized (Scheme 2).5,6 In this respect, the chemistry of lithiated ferrocenyls is significantly inferior to the chemistry of aryl lithium derivatives, for which many examples, including Lewis base-free species, have been isolated and structurally characterized.7−9 In order to close this gap, we were interested in Lewis base-free monolithioferrocenes, and we based our design principle on the assumption that steric congestion around the lithiated carbon atom of the monolithioferrocene should suppress both the coordination of Lewis bases as well as the © 2014 American Chemical Society

Scheme 2. Examples of Isolated Monolithioferrocene Derivatives5

dismutation into 1,1′-dilithioferrocene and ferrocene. Previously, we have reported the synthesis of the sterically demanding 2,5-bis(3,5-di-t-butylphenyl)-1-bromoferrocene 1 (Fc*Br),10,11 which should be an appropriate precursor for such Lewis base-free monolithioferrocenes. Herein, we would like to report the successful isolation and structural characterization of a sterically hindered monolithioferrocene without any stabilization from N/O-Lewis bases. Received: September 1, 2014 Published: October 27, 2014 6696

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imposed C2 symmetry exhibited almost identical structural parameters (C−Li, 2.125, 2.151 Å; Li−Li, 2.345 Å; Li−Fe, 2.863 Å, stable relative to the C1-structure by only 0.1 kcal/mol in SCF energy).15 In the theoretically optimized structure of 2 with C2 symmetry, the C2Li2 plane was, similarly to the experimentally observed structure, tilted toward the Cp planes (∼40°).16 In solution, the parameters obtained for 2 from multinuclear NMR spectroscopy measurements were also consistent with the assignment of a dimeric structure. For example, the 1H NMR spectrum of 2 in C6D6 showed one singlet signal for the unsubstituted Cp moieties at 3.78 ppm, whereas the β-protons of the substituted Cp moieties resonated as two doublet signals at 5.00 and 5.10 ppm (3JHH = 2.1 Hz). For the 3,5-di-tbutylphenyl substituents, two independent sets of signals were observed in the 1H NMR spectrum, indicative of two magnetically inequivalent aryl groups. This observation was corroborated by the 13C NMR spectrum of 2, which also suggested an unsymmetrical structure. The C2 symmetry of 2, with a dihedral angle between the C2Li2 and the Cp planes observed in the crystalline state, is also reflected in the features of the NMR spectra. In the 13C NMR spectrum of 2, a broadened signal was observed at 87.8 ppm, which was assigned to that of the carbon atoms attached to the Li atoms. The 7Li NMR spectrum of 2 showed a characteristic signal at 4.30 ppm, which is low-field shifted compared to those of unsolvated aryllithium derivatives (e.g., δLi [TipLi]4 = −5.16 ppm and δLi [Li(2,6-Mes2-C6H3)2] = 2.58 ppm).8 The 13C and 7Li NMR chemical shifts for the ipso carbon and lithium atoms in the C(Cp)−Li moiety of 2 with C2 symmetry bearing the tilted C2Li2 moiety were estimated by GIAO-calculations at the B3PW91/6-311+G(2d)//B3PW91/6-31G(d) level of theory to be δC = 87.96 ppm and δLi = 5.07 ppm, respectively. It can thus be concluded that the geometry 2 exhibits in C6D6 solution is similar to that in the crystalline state, probably due to steric reasons. Heating a C6D6 solution of 2 to 65 °C for 1 h did not, according to the 1H NMR spectrum, result in any apparent change, manifesting an impressively high thermal stability of 2 in solution. Addition of H2O or D2O to a C6D6 solution of 2 at room temperature quantitatively afforded Fc*H (3a) or Fc*D (3b), demonstrating that 2 reacts as a monolithioferrocene monomer and that inter- and intramolecular Li−H exchange should be negligible in solution (Scheme 4).14 Moreover, the treatment of 2 with PCl3 in C6D6 resulted in the quantitative formation of Fc*PCl2 (4), making 2 a convenient precursor for a sterically demanding d-electron unit, which can be easily introduced to halogenated main group element moieties. The

Scheme 3. Synthesis of Stable and Lewis Base-Free Monolithioferrocene-Dimer 2

Sterically hindered bromoferrocene 1 was prepared according to literature procedures.10 Treatment of 1 with an excess of n-BuLi (2 equiv) in hexane at 50 °C resulted in the formation of an orange solid, which was filtered and washed with hexane (Scheme 3). Compound 2 is insoluble in hexane, but soluble in benzene and toluene, which allowed a subsequent reprecipitation from toluene to afford orange crystals of monolithioferrocene 2 in 52% yield. Recrystallization of 2 from benzene at room temperature (r.t.) afforded single crystals suitable for Xray crystallographic analysis,12 which exhibited a dimeric structure for the Fc*Li moieties in 2 with pseudo C2 symmetry (Figure 1).13 In each Fc*Li monomer, one carbon atom of the substituted Cp rings is attached to two Li atoms. The Cp rings attached to the Li atoms still exhibit the planar structure typical for ferrocene. The C−Li bond lengths were observed to be ∼2.11−2.13 Å, which is shorter compared to those of the previously reported monolithioferrocenes with inter/intra molecular coordination of the lithium atoms by N/O Lewis bases (∼2.20−2.4 Å).5 Each Li atom in 2 was furthermore coordinated by one Fe atom, with Fe−Li bond lengths of ∼2.83−2.84 Å. It should be noted that the stable ferrocenyl lithium 2 contains Li atoms, which are coordinated only by three other atoms (C1, C2, and Fe), without any other apparent stabilizing coordination.7c Although the detailed properties of the Fe−Li bonds have remained unclear until now, the coordination between the Fe and Li atoms is expected to contribute to the considerable thermal stability of 2 and the observed absence of any disproportionation. The C−Li moiety in 2 is almost perfectly surrounded by the aryl groups in positions 2 and 5 of the substituted Cp rings. The C2Li2 plane was found to be inclined toward the Cp rings by ∼40°, rendering the two Dtp moieties unequal. The experimentally observed structural features could be reproduced in good agreement from theoretical calculations at the B3PW91/631G(d) level of theory (Li−C, ∼2.12−2.15 Å; Li−Li, 2.344 Å; Li−Fe, ∼2.86−2.87 Å).14 The structural optimization for 2 with

Scheme 4. Reactions of Dimeric Monolithioferrocene 2 with H2O, D2O, and PCl3

Figure 1. Molecular structure of monolithioferrocene dimer 2. (a) ORTEP drawing of the entire molecule at 50% probability under omittance of hydrogen atoms. (b) ORTEP drawing of the core of 2 under omittance of the Dtp groups and hydrogen atoms. Selected structural parameters (Å): C1−Li1, 2.126(4); C1−Li2, 2.111(4); C2− Li1, 2.120(4); C2−Li2, 2.118(4); Fe1−Li1, 2.831(4); Fe2−Li2, 2.848(4). 6697

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extraordinary stability of the resulting dichlorophosphine Fc*PCl2, induced by the steric bulk of the Fc* unit, has already been reported.11a In summary, we successfully synthesized and isolated the Lewis base-free monolithioferrocene Fc*Li (2), effectively protected by bulky aryl substituents. This monolithioferrocene was found to exhibit a dimeric structure with intramolecular Fe−Li coordination, both in the crystalline state and in C6D6 solution. The most striking features of this monolithioferrocene dimer are its extraordinary thermal stability, especially in C6D6 solution, and the observed reactivity to act as a monomeric ferrocenyl lithium derivative.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental and computational details as well as and X-ray crystallographic data for 2 and 3. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Grants-in-Aid for Scientific Research (B) (Grant No. 22350017), Young Scientist (A) (Grant No. 23685010), Scientific Research on Innovative Areas, “New Polymeric Materials Based on Element-Blocks” [No. 2401] (Grant No. 25102519), Scientific Research on Innovative Areas, “Stimuli-responsive Chemical Species for the Creation of Functional Molecules” [No. 2408] (Grant No. 24109013), and the MEXT Project of Integrated Research on Chemical Synthesis from Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, as well as by the “Molecular Systems Research” Project of the RIKEN Advanced Science Institute. We would also like to express our appreciation for a kind donation of 1-bromo-3,5-di-tertbutylbenzene from MANAC Inc. The manuscript was written at the Rheinische Friedrich-Whilhelms-Universität Bonn during the tenure of a Friedrich Wilhelm Bessel Research Award for one of the authors (T.S.), who is grateful to the Alexander von Humboldt Stiftung for their generosity and to Prof. R. Streubel and his group members for their warm hospitality.

EXPERIMENTAL SECTION

General Remarks. All manipulations were carried out under an argon atmosphere using either Schlenk line techniques or glove boxes. For details, see the Supporting Information. Synthesis of Monolithioferrocene Dimer (2). To a solution of bromoferrocene 1 (259 mg, 0.400 mmol) in n-hexane (2.0 mL), nBuLi (0.50 mL, 1.60 M in n-hexane, 0.80 mmol) was added at room temperature. The reaction mixture was heated to 50 °C for 12 h, which resulted in the formation of an orange precipitate. The orange powder was removed by filtration, washed with n-hexane, and dried in vacuo, prior to being reprecipitated from toluene at room temperature to give pure dimeric monolithioferrocene 2 (118 mg, 0.104 mmol, 52%). 2: orange crystals; mp 175−178 °C (decomp.); 1H NMR (300 MHz, C6D6) δ 1.22 (s, 36H), 1.38 (s, 36H), 3.78 (s, 10H), 5.00 (d, J = 2.1 Hz, 2H), 5.10 (d, J = 2.1 Hz, 2H), 7.35 (dd, J = 1.8 Hz, 2H), 7.38 (dd, J = 1.5 Hz, 2H), 7.65 (d, J = 1.8 Hz, 4H), 8.07 (d, J = 1.5 Hz, 4H); 13C NMR (150 MHz, C6D6) δ 31.9 (q), 31.9 (q), 35.0 (s), 35.3 (s), 69.3 (d), 73.0 (d), 77.3 (d), 87.8 (br), 93.8 (s), 103.6 (s), 120.0 (d), 121.2 (d), 121.5 (d), 123.4 (d), 143.8 (s), 145.0 (s), 151.7 (s), 152.8 (s); 7Li NMR (120 MHz, C6D6) δ 4.30. Anal. Calcd for C76H98Fe2Li2: C, 80.27%; H, 8.69%. Found: C, 80.07%; H, 8.96%. Reaction of Monolithioferrocene 2 with H2O. H2O (0.1 mL) was added at room temperature to a C6D6 solution (0.7 mL) of 2 (58 mg, 0.10 mmol) in a 5 mm o.d. NMR tube. The NMR spectra showed the quantitative formation of 3 (Fc*H). The reaction mixture was dried over MgSO4, filtered, and all volatiles were evaporated to give Fc*H (3, 54 mg, 0.096 mmol, 96%) as an orange solid; mp 141−143 °C. 1H NMR (300 MHz, C6D6) δ 1.36 (s, 36H), 3.97 (s, 5H), 4.78 (d, J = 1.5 Hz, 2H), 5.37 (t, J = 1.5 Hz, 1H), 7.47 (t, J = 1.7 Hz, 2H), 7.66 (d, J = 1.7 Hz, 4H); 13C NMR (150 MHz, C6D6) δ 31.7 (q), 34.9 (s), 66.7 (d), 68.3 (d), 71.6 (d), 89.0 (s), 120.6 (d), 121.8 (d), 138.9 (s), 150.8 (s). HRMS (ESI), m/z found, 563.3271 ([M + H]+); calcd for C38H51Fe2, 563.3335. Anal. Calcd for C38H50Fe: C, 81.12%; H, 8.96%. Found: C, 81.47%; H, 9.02%. Reaction of Monolithioferrocene 2 with D2O. D2O (0.1 mL) was added to a C6D6 solution (0.7 mL) of 2 (56 mg, 0.099 mmol) in a 5 mm o.d. NMR tube at room temperature. The NMR spectra showed the quantitative formation of 3-D (Fc*D). The reaction mixture was dried over MgSO4, filtered, and all volatiles were evaporated to give Fc*D (3-D, 39.5 mg, 0.070 mmol, 71%) as an orange solid. Reaction of Monolithioferrocene 2 with PCl3. PCl3 (0.1 mL) was added at room temperature to a C6D6 solution (0.7 mL) of 2 (58 mg, 0.10 mmol) in a 5 mm o.d. NMR tube. The 1H NMR spectrum showed the quantitative formation of 4 (Fc*PCl2). The solvent was removed under reduced pressure, and the residues were dissolved in toluene and filtered through Celite. The solvent of the filtrate was removed in vacuo to give Fc*PCl2 (62 mg, 0.093 mmol, 93%) as an orange solid. Spectral and analytical data of 4 were identical to those reported previously.11a

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