Enhancement of Stannylene Character in Stannole Dianion

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Enhancement of Stannylene Character in Stannole Dianion Equivalents Evidenced by NMR and Mö ssbauer Spectroscopy and Theoretical Studies of Newly Synthesized Silyl-Substituted Dilithiostannoles Takuya Kuwabara,† Jing-Dong Guo,‡ Shigeru Nagase,‡ Mao Minoura,§ Rolfe H. Herber,∥ and Masaichi Saito*,† †

Department of Chemistry, Graduate School of Science and Engineering, Saitama University, Shimo-okubo, Sakura-ku, Saitama-city, Saitama 338-8570, Japan ‡ Fukui Institute for Fundamental Chemistry, Kyoto University, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan § Department of Chemistry, Graduate School of Science, Rikkyo University, Nishi-ikebukuro, Toshima-ku, Tokyo, Japan ∥ Racah Institute of Physics, Hebrew University of Jerusalem, 91904, Jerusalem, Israel S Supporting Information *

ABSTRACT: Dilithiostannoles, which are aromatic tin-containing ring compounds, were proposed to have stannylene character, as judged from their NMR analysis. We herein report on the synthesis of silyl-substituted dilithiostannoles, which were characterized by NMR spectroscopy and X-ray diffraction analysis. The silylsubstituted derivatives also exhibit features characteristic of aromatic dilithiostannoles such as 7Li NMR signals at high-field area and no C−C bond alternation in the stannole rings. Theoretical calculations and the 119Sn NMR chemical shifts revealed that the stannylene character in the silylsubstituted dilithiostannoles is enhanced due to greater interaction between 5p (Sn) and LUMO (butadiene) in comparison to those in alkyl and aryl derivatives. The 119Sn Mössbauer spectra of dilithiostannoles were measured for the first time, indicating that each of the tin atoms in dilithiostannoles can be characterized as having Sn(II) character. eavier congeners of cyclopentadienyl anions (Cp−) are attractive from the viewpoint of heavy aromatic compounds.1 In contrast to the sp2 hybridization of the five skeletal carbon atoms in Cp−, its heavy analogues (EC4− where E = Si, Ge, Sn, Pb) have highly pyramidarized metal centers, suggesting that they are nonaromatic.2 Notably, group 14 metalloles can form more reduced species, metallole dianions (EC42−, where E = Si,2a,3 Ge,2a,4 Sn,5 Pb2c), and it was concluded that metallole dianions are aromatic. The aromaticity of metallole dianions originates from the delocalization of two electrons in the out-ofplane p orbital of E toward the LUMO of the butadiene moiety, as shown in Chart 1.5a A decrease of the energy gap between these two orbitals could therefore enhance their interaction. In general, silyl groups attached to the π system stabilize the LUMO through σ−π* conjugation.6 In fact, our preliminary calculations predict that silyl groups on the terminal carbon atoms of the butadiene moiety can lower the energy level of its LUMO,7 leading to enhancement of the interaction between 5p(Sn) and the LUMO of butadiene to lower the HOMO (5.25 vs 4.01 eV; see Chart 1). It is therefore expected that silyl-substituted dilithiostannoles have more stannylene character than the corresponding alkyl- and aryl-substituted species. Inspired by these motivations, we report herein the synthesis, structures, NMR, and theoretical studies on silyl-substituted dilithiostannoles. To gain further insight into the oxidation states of the tin

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© XXXX American Chemical Society

Chart 1. Origin of the Aromatic Nature of Metallole Dianions and Comparison of LUMO Energy Levels of Butadienes Bearing Alkyl and Silyl Groups7

atoms in dilithiostannoles that we synthesized, the Mössbauer spectra of the dilithiostannoles are also discussed.

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RESULTS AND DISCUSSION 1,4-Dilithio-1,4-disilyl-1,3-butadienes 1a,b are reasonable precursors for the synthesis of silyl-substituted stannoles. We found that the yield of 1b was improved from 25%8 to 79% by changing the solvent for recrystallization from hexane to diethyl ether. Received: April 8, 2014

A

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Table 1. Comparison of 119Sn NMR Chemical Shifts of Diphenyl- and Dilithiostannolesa

Treatment of 1a,b with Ph2SnCl2 in diethyl ether and hexane provided diphenylstannoles 2a,b in 77% and 57% yields, respectively. Similar to the synthesis of tetraphenyl- and tetraethyldilithiostannoles 3c,d, refluxing a diethyl ether solution of diphenylstannoles 2a,b with excess lithium afforded dilithiostannoles 3a,b, respectively (Scheme 1).

δ(119Sn)

2 3

Scheme 1. Synthesis of Diphenylstannoles 2a,b and Dilithiostannoles 3a,b

a

b

c

d

R1 = TMS R2 = Ph −3.0 446.4

R1 = TBDMS R2 = Ph −6.2 472.6

R1 = Ph R2 = Ph −88 163.3

R1 = Et R2 = Et −83 −27.8

a

These data were recorded in CDCl3 (2a−c), C6D6 (2d, 3a,b), C6D6/ Et2O (5/1) (3c), and C6D6/THF (5/1) (3d).

To gain more insight into the effects of silyl groups on the electronic states in dilithiostannoles, theoretical calculations for the model free dianions (3a′, R1 = SiH3, R2 = CH3; 3d′, R1 = R2 = CH3) were next performed using the Gaussian 03 program.7,9 On the basis of the optimized structure, natural population analyses were performed (Table 2). The tin atom of 3a′ is more electron

The molecular structures of 2a and 3a,b were determined by X-ray diffraction analysis (Figure 1 and Figure S1 (Supporting

Table 2. Comparison of Natural Charges and Selected Bond Lengths for the Model Stannole Dianions 3 and Stannacyclopentadienylidenes 4

3a′ 1

Sn Cα Si or C (R1) Sn−Cα (Å) Cα−Cβ (Å) Cβ−Cβ (Å)

Figure 1. Molecular structures of 2a (left) and 3a (right) (50% probability). All hydrogen atoms and a toluene molecule in 3a were omitted for clarity. Selected bond lengths (Å): 2a, Sn−C1 2.1447(16), C1−C2 1.350(2), C2−C2# 1.517(3); 3a, Sn−C1 2.186(6), C1 C2 1.422(8), C2−C3 1.450(8), C3−C4 1.411(8), Sn C4 2.188(6).

Information)). As expected, the five-membered ring of diphenylstannole 2a has 1,3-diene character with C−C bond alternation, whereas the C−C bond lengths in dilithiostannoles 3a,b are nearly equal, suggesting considerable delocalization of π electrons over the stannole rings in 3. Each of the lithium atoms is coordinated by the stannole ring and a THF molecule in η5 and η1 fashions, respectively. Upon reduction, the 13C NMR signals of the α-carbons of stannoles are shifted to lower field: 145.09 vs 182.38 ppm for the TMS derivatives and 142.36 vs 176.53 ppm for the TBDMS derivatives. The 7Li nuclei of 3a,b resonate at −5.8 and −5.4 ppm, respectively, which are in a region similar to those of tetraphenyldilithiostannole 3c (−4.4 ppm) and tetraethyl derivative 3d (−5.2 ppm), reflecting their aromatic nature. Remarkably, in the 119Sn NMR, the signals of dilithiostannoles 3a,b are largely shifted to lower field (446.4 and 472.6 ppm, respectively), in comparison with those of the starting diphenylstannoles 2a,b (−3.0 and −6.2 ppm, respectively). Although such low-field shifts were also found in the case of 3c,d, it is noted that the degree of the low-field shifts in the silylsubstituted dilithiostannoles is much larger than those in the Et and Ph derivatives, as shown in Table 1. The larger shifts found in the silyl cases indicate that electron density of the tin atoms in 3a,b is lower than those in 3c,d. In other words, the interaction between 5p (Sn) and LUMO of the butadiene is enhanced in the silyl cases, in comparison with those in the alkyl and aryl versions.

R = SiH3 R2 = CH3 +0.10 −1.08 +0.88 2.167 1.420 1.415

3d′ 1

R = CH3 R2 = CH3 −0.43 −0.46 −0.69 2.187 1.392 1.457

4a′ 1

R = SiH3 R2 = CH3 +1.08 −0.99 +0.88 2.197 1.360 1.523

4d′ 1

R = CH3 R2 = CH3 +0.99 −0.50 −0.72 2.189 1.356 1.522

deficient, in comparison with that of 3d′. This tendency is consistent with the remarkable low-field shifts found in the 119Sn NMR of silyl-substituted dilithiostannoles 3a,b. It is noted that the Cβ−Cβ bond in 3a′ is shorter than that in 3d′. Because a π bond between the two β carbons is found in the LUMO of butadiene (Figure S2 (Supporting Information)), the greater interaction between 5p (Sn) and the LUMO in 3a′ in comparison to that of 3d′ resulted in the shorter Cβ−Cβ bond in 3a′. In fact, the HOMOs shown in Figure 2 indicate that

Figure 2. HOMOs of 3a′ (left) and 3d′ (right) with isovalues of 0.05.

delocalization of electrons on the tin atoms toward α carbons is enhanced in 3a′ in comparison to that found in 3d′, suggesting that the electron density of the α carbons of 3a′ is greater than that of 3d′. As expected, the α carbons of 3a′ are charged more negatively than those of 3d′, according to the natural charges for the α carbons in 3a′,d′ of −1.08 and −0.46, respectively. At first we thought that the more negatively charged character of the α carbons in 3a′ was due to the α effect of the silyl group. However, B

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Unfortunately, the situation is even less favorable for compound 3b, for which the temperature dependence of the recoil-free fraction, which is twice as large as for 3c, limited the accessible temperature range to 97 < T < 123 K, and thus no reliable conclusions can be drawn regarding the dynamics of the tin atom in this compound, except that the metal atom bonding to its nearest neighbors is significantly weaker than in 3c.

theoretical calculations for model stannacyclopentadienylidenes 4a′,d′, which are neutral stannylenes, revealed that the α-carbons of the silyl-substituted stannacyclopentadienylidenes are more negatively charged than those of Me derivatives, even though they have no negative charges (Table 2). Moreover, no significant changes were found in the natural charges for α carbons upon reduction from 4 to 3 (−0.09 and +0.04 for a′ and d′, respectively). Therefore, the more negatively charged character of the α carbons in stannole dianions seems to be due to a difference in electronegativity between silicon and carbon atoms. To evaluate the stannylene character in the dilithiostannoles, we performed Mössbauer spectroscopic analysis of 3b,c.10 Samples of 3b,c were placed in sealed ampules and transferred to Perspex sample holders in an inert-atmosphere glovebox and immediately cooled to liquid nitrogen temperature. These samples were examined in standard transmission Mössbauer effect (ME) geometry using a 5 mCi 119mCaSnO3 source at room temperature, as described previously.11 Isomer shifts (IS) are referred to room-temperature spectra of BaSnO 3 , and spectrometer calibration was effected using an α-Fe absorber at room temperature. Line widths were on the order of 0.86 mm s−1, in agreement with expectations. Spectra of both compounds consisted of clean doublets with no evidence of contamination by tin compound impurities (Figure 3 and Figure S3 (Supporting Information) for 3b,c, respectively). The ME parameters at 90 K and the derived values are summarized in Table 3.

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CONCLUSIONS The first silyl-substituted dilithiostannoles were successfully synthesized and characterized by NMR and X-ray diffraction analysis. The large low-field shifts in the 119Sn NMR and the theoretical calculations of the silyl derivatives revealed that silyl groups enhance the interaction between 5p (Sn) and LUMO (butadiene), to increase the stannylene character of dilithiostannole. The Mössbauer spectra of 3b,c clearly showed that the tin atoms in the dilithiostannoles have Sn(II) character, indicating that the contribution of the stannylene character, caused by the decrease of electron density from the tin atoms, is of considerable importance in understanding the electronic properties of dilithiostannoles. Reactions of the silyl-substituted dilithiostannoles thus obtained with transition-metal reagents are currently under investigation.

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Figure 3. Mössbauer spectrum of compound 3b at 112 K.

Table 3. Results of 119Sn Mössbauer Spectroscopy for Compounds 3b,c IS(90), mm s−1 QS(90), mm s−1 −d(ln A)/dT, K−1

3b

3c

2.32(2) 1.93(2) 5.79(1) × 10−2

2.397(3) 1.766(6) 2.67(4) × 10−2

EXPERIMENTAL SECTION

General Considerations. All experiments were performed under an argon atmosphere in a glovebox or using standard Schlenk techniques. Diethyl ether, THF, toluene, and benzene-d6 for NMR mesurements were purified with potassium mirror before being used. 1a13 and 1b8 were prepared according to literature procedures. 1H NMR (400 or 500 MHz), 13C NMR (101 or 127 MHz), 7Li NMR (194 MHz), 29 Si NMR (99 MHz), and 119Sn NMR (187 MHz) were recorded on a Bruker DPX-400 Cryo or a AVANCE-500 instrument. The intensity data for X-ray crystallographic analyses were collected at −173 or −148 °C on a Bruker SMART APEX or APEX II instrument equipped with a CCD area detector with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) and graphite monochromator. The structures were solved by direct methods and refined by full-matrix least-squares methods by the SHELXL-97 program.14 1,4-Dilithio-1,4-bis(tert-butyldimethylsilyl)-2,3-diphenyl-1,3butadiene (1b). Lithium (370 mg, 53.4 mmol) was charged in a Schlenk flask, and 80 mL of Et2O was added. (tert-Butyldimethylsilyl)phenylacetylene (5.45 g, 22.0 mmol) was injected into this suspension, and the mixture was stirred for 6 h at room temperature. Lithium was filtered off, and the filtrate was concentrated to about 20 mL. Recrystallization at −30 °C provided red crystals of 1b coordinated by two Et2O molecules (5.17 g, 79%). 1,1,3,4-Tetraphenyl-2,5-bis(trimethylsilyl)stannole (2a). A diethyl ether (15 mL) solution of diphenyldichlorostannane (313 mg, 0.911 mmol) was transferred to a Et2O (10 mL) solution of dilithiobutadiene 1a (375 mg, 0.913 mmol) at 0 °C. The mixture was stirred for 30 min at 0 °C, warmed to room temperature, and stirred for 3 h at the same temperature. The resulting mixture was filtrated through Celite, and the solution was evaporated to give white crystalline solids of 2a (446 mg, 77%). Data for 2a are as follows. Mp: 179.5−181.5 °C. 1H NMR (500 MHz, CDCl3): δ −0.27 (s, 18H, SiMe3), 6.92 (d, J = 7 Hz, 4H, CPh), 6.99−7.07 (m, 6H, CPh), 7.39−7.48 (m, 6H, SnPh), 7.66− 7.79 (m, 4H, SnPh). 13C NMR (126 MHz, CDCl3): δ 1.50 (SiMe3, JSn−C = 52 Hz), 126.19 (Ph), 127.29 (Ph), 129.02 (Ph, JSn−C = 49 Hz), 129.40 (Ph), 129.56 (Ph), 137.60 (Ph, JSn−C = 39 Hz), 140.27 (4°, SnCipso, JSn−C = 436, 457 Hz), 144.85 (4°, Cipso, JSn−C = 109 Hz), 145.09 (Cα, JSn−C = 232, 243 Hz), 168.80 (Cβ, JSn−C = 59 Hz). 29Si NMR (99 MHz, CDCl3): δ −6.7. 119Sn NMR (187 MHz, CDCl3): δ −3.0 (JSn−Si = 75 Hz). Anal. Calcd for C34H38Si2Sn: C, 65.70; H, 6.16. Found: C, 65.53; H, 6.14. 1,1,3,4-Tetraphenyl-2,5-bis(tert-butyldimethylsilyl)stannole (2b). To diphenyldichlorostannane (1.72 g, 5.00 mmol) and dilithiobutadiene 1b (2.65 g, 5.09 mmol), placed in a Schlenk flask,

The IS values, while at the lower end associated with organotin compounds, clearly indicate that in both cases the tin atom is appropriately characterized as Sn(II). The QS parameter in 3b is significantly smaller than that observed in 3c. While the temperature dependence of the recoil-free fraction (−d(ln A)/ dT) for 3c is well fitted by a linear regression, the temperature range of the ME measurements was limited to the interval 96 < T < 171 K. The Γ parameter12 is summarized graphically in Figure S4 (Supporting Information) and is only modestly in agreement with the value derived from the single-crystal X-ray Uij values at 298 K, taking into account the long extrapolation implied in Figure S4, since no reliable ME data could be acquired in the interval 200 < T < 300 K. C

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was added hexane (50 mL) at 0 °C, and the mixture was stirred for 5 h at room temperature. The resulting mixture was concentrated to half the original volume, resulting in precipitation of a white powder. Materials soluble in hexane were removed by filtration to afford a white powder, which was twice washed with 10 mL of hexane. The white powder was dissolved in 40 mL of dichloromethane, and insoluble materials were removed by filtration. After evaporation of the filtrate, compound 2b was obtained as a white powder (2.03 g, 57%). Data for 2b are as follows. Mp: 221.0−222.0 °C. 1H NMR (500 MHz, CDCl3): δ 0.39 (s, 12H, SiMe2tBu), 0.50 (s, 18H, SiMe2tBu), 6.87−6.92 (m, 4H, CPh), 6.95− 6.99 (m, 6H, CPh), 7.39−7.46 (m, 6H, SnPh), 7.68−7.81 (m, 4H, SnPh). 13C NMR (101 MHz, CDCl3): δ −1.60 (SiMe2tBu), 18.07 (4°, SiMe2tBu), 27.86 (1°, SiMe2tBu), 126.04 (Ph), 126.96 (Ph), 129.00 (Ph, JSn−C = 50 Hz), 129.33 (Ph), 130.38 (Ph), 137.55 (Ph, JSn−C = 37 Hz), 141.09 (4°, SnCipso, JSn−C = 436, 456 Hz), 142.36 (Cα, JSn−C = 228, 238 Hz), 145.33 (4°, Cipso, JSn−C = 110 Hz), 170.09 (Cβ, JSn−C = 58 Hz). 29Si NMR (99 MHz, CDCl3): δ 1.3. 119Sn NMR (187 MHz, CDCl3): δ −6.2. Anal. Calcd for C40H50Si2Sn: C, 68.08; H, 7.14. Found: C, 67.95; H, 7.14. [Li2(thf)2][SnC4(SiMe3)2Ph2] (3a). A diethyl ether (25 mL) solution of 2a (0.737 g, 1.00 mmol) with lithium (89 mg, 13 mmol) was stirred for 2 h at room temperature. After freeze−pump−thaw cycles, the mixture was heated to 80 °C for 36 h. Lithium and materials insoluble in diethyl ether were removed by filtration in a glovebox. After removal of the solvent, the resulting brown powder was washed with 5 mL of hexane to give a crude product. Single crystals suitable for X-ray diffraction analysis were obtained by recrystallization of the crude product from a mixture of THF and toluene (1:5) at −30 °C (340 mg, 47%). Data for 3a are as follows. Mp: 230 °C dec. 1H NMR (500 MHz, C6D6): δ 0.42 (s, 18H, SiMe3), 6.97−7.02 (m, 2H, Ph), 7.09−7.12 (m, 8H, Ph). 13C NMR (126 MHz, C6D6): δ 5.52 (SiMe3), 124.79 (Ph), 127.11 (Ph), 131.33 (Ph), 146.23 (Cβ, JSn−C = 23 Hz), 149.55 (4°, Cipso, JSn−C = 17 Hz), 182.38 (Cα, JSn−C = 394 Hz). 7Li NMR (99 MHz, C6D6): δ −5.8. 29Si NMR (99 MHz, C6D6): δ −9.8. 119Sn NMR (187 MHz, C6D6): δ 446.4. [Li2(thf)2][SnC4(SitBuMe2)2Ph2] (3b). Compound 2b (2.20 g, 3.12 mmol), lithium (370 mg, 53.0 mmol), and diethyl ether (20 mL) were placed in a Schlenk flask. After freeze−pump−thaw cycles, the mixture was heated to 80 °C for 30 h. Lithium and materials insoluble in diethyl ether were removed by filtration in a glovebox. After removal of the solvent, pure 3b was obtained as a brown powder (2.17 g, 98%). Single crystals suitable for X-ray diffraction analysis were obtained by recrystallization of the crude product from a mixture of THF and toluene (1:5) at −30 °C. Data for 3b are as follows. Mp: 221.0−222.0 °C. 1H NMR (500 MHz, C6D6): δ 0.32 (s, 12H, SiMe2tBu), 1.11−1.15 (m, 8H, thf), 1.24 (s, 18H, SiMe2tBu), 3.36−3.39 (m, 8H, thf), 6.94− 6.98 (m, 2H, Ph), 7.03−7.11 (m, 8H, Ph). 13C NMR (101 MHz): δ 2.49 (SiMe2tBu), 17.21 (4°, SiMe2tBu), 25.14 (thf), 29.25 (1°, SiMe2tBu), 69.59 (thf), 124.87 (Ph), 126.70 (Ph), 132.03 (Ph), 146.98 (4°, Cipso), 150.24 (Cβ), 176.53 (Cα, JSn−C = 391 Hz). 7Li NMR (99 MHz, C6D6): δ −5.4. 29Si NMR (99 MHz, C6D6): δ −0.3. 119Sn NMR (187 MHz, C6D6): δ 472.6.

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ACKNOWLEDGMENTS This work was partially supported by Grants-in-Aid for Challenging Exploratory Research (No. 23655029 for M.S.) and by the Nanotechnology Support Project and Specially Promoted Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. M.S. acknowledges a research grant from the Mitsubishi Foundation. T.K. acknowledges a Sasakawa Scientific Research Grant from the Japan Science Society and Research Fellowship for Young Scientists from the JSPS.

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DEDICATION This paper is dedicated to Professor Rudolph Willem on the occasion of his retirement from Vrije Universiteit Brussel. REFERENCES

(1) (a) Saito, M.; Yoshioka, M. Coord. Chem. Rev. 2005, 249, 765. (b) Lee, V. Y.; Sekiguchi, A. Angew. Chem., Int. Ed. 2007, 46, 6596. (c) Saito, M. Coord. Chem. Rev. 2012, 256, 627. (2) (a) Freeman, W. P.; Tilley, T. D.; Liable-Sands, L. M.; Rheingold, A. L. J. Am. Chem. Soc. 1996, 118, 10457. (b) Saito, M.; Kuwabara, T.; Ishimura, K.; Nagase, S. Bull. Chem. Soc. Jpn. 2010, 83, 825. (c) Saito, M.; Sakaguchi, M.; Tajima, T.; Ishimura, K.; Nagase, S.; Hada, M. Science 2010, 328, 339. (3) (a) West, R.; Sohn, H.; Bankwitz, U.; Calabrese, J.; Apeloig, Y.; Mueller, T. J. Am. Chem. Soc. 1995, 117, 11608. (b) Freeman, W. P.; Tilley, T. D.; Yap, G. P. A.; Rheingold, A. L. Angew. Chem., Int. Ed. Engl. 1996, 35, 882. (4) West, R.; Sohn, H.; Powell, D. R.; Müller, T.; Apeloig, Y. Angew. Chem., Int. Ed. Engl. 1996, 35, 1002. (5) (a) Saito, M.; Haga, R.; Yoshioka, M.; Ishimura, K.; Nagase, S. Angew. Chem., Int. Ed. 2005, 44, 6553. (b) Saito, M.; Kuwabara, T.; Kambayashi, C.; Yoshioka, M.; Ishimura, K.; Nagase, S. Chem. Lett. 2010, 39, 700. (6) Giordan, J. C. J. Am. Chem. Soc. 1983, 105, 6544. (7) For details of the theoretical calculations, see the Supporting Information. (8) Saito, M.; Nakamura, M.; Tajima, T.; Yoshioka, M. Angew. Chem., Int. Ed. 2007, 46, 1504. (9) Frisch, M. J., et al. GAUSSIAN 03, Revision E.01; Gaussian, Inc., Wallingford, CT, 2004. (10) (a) Cotton, J. D.; Davidson, P. J.; Lappert, M. F.; Donaldson, J. D.; Silvet, J. J. Chem. Soc., Dalton Trans. 1976, 2286. (b) Bigwood, M. P.; Corvan, P. J.; Zuckerman, J. J. J. Am. Chem. Soc. 1981, 103, 7643. (c) de Lima, G. M.; Pierssens, L. J. M.; Mahieu, B. Hyperfine Interact. 1999, 122, 327. (d) Rivard, E.; Fischer, R.; Wolf, R.; Peng, Y.; Merrill, W.; Schley, N.; Zhu, Z.; Pu, L.; Fettinger, J.; Teat, S.; Nowik, I.; Herber, R.; Takagi, N.; Nagase, S.; Power, P. J. Am. Chem. Soc. 2007, 129, 16197. (e) AlRafia, S. M. I.; Shynkaruk, O.; McDonald, S. M.; Liew, S. K.; Ferguson, M. J.; McDonald, R.; Herber, R. H.; Rivard, E. Inorg. Chem. 2013, 52, 5581. (f) Herber, R. H.; Parisi, G. I. Inorg. Chem. 1966, 5, 769. (g) Herber, R. H.; Leahy, M. F. J. Chem. Phys. 1977, 67, 2718. (h) Power, P.; Stanciu, C.; Nowik, I.; Herber, R. Inorg. Chem. 2005, 44, 9461. (11) Harder, S.; Naglav, D.; Schwerdtfeger, P.; Nowik, I.; Herber, R. H. Inorg. Chem. 2014, 53, 2188. (12) (a) Herber, R. H.; Nowik, I. J. Nucl. Radiochem. Sci. 2008, 9, 33. (b) Herber, R. H.; Nowik, I. J. Organomet. Chem. 2008, 693, 3007. (c) Herber, R. H. J. Organomet. Chem. 2012, 717, 41. (13) (a) Xi, C.; Huo, S.; Afifi, T. H.; Hara, R.; Takahashi, T. Tetrahedron Lett. 1997, 38, 4099. (b) Liu, L.; Zhang, W.-X.; Wang, C.; Wang, C.; Xi, Z. Angew. Chem., Int. Ed. 2009, 48, 8111. (14) Sheldrick, G. M. Acta Crystallogr., Sect. A 2007, 64, 112.

ASSOCIATED CONTENT

S Supporting Information *

Text, figures, and xyz and CIF files giving the full author list of ref 9, details of the theoretical calculations, all computed Cartesian coordinates in a format for convenient visualization, 1H and 13C NMR data for 3a,b, Mössbauer spectra of 3b,c, and crystallographic data for 2a and 3a,b. This material is available free of charge via the Internet at http://pubs.acs.org.

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Note

AUTHOR INFORMATION

Corresponding Author

*E-mail for M.S.: [email protected]. Notes

The authors declare no competing financial interest. D

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