Ferracarborane Benzene Complexes [(η-9-L-7,8-C2B9H10)Fe(η

Apr 23, 2010 - Fe(η-9-NMe3-7,8-C2B9H10) (6) was obtained by photochemical reaction of3awith1c. The structures of2a,. 3aBPh4, and6were determined by ...
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Organometallics 2010, 29, 2260–2271 DOI: 10.1021/om901085y

Ferracarborane Benzene Complexes [(η-9-L-7,8-C2B9H10)Fe(η-C6H6)]þ (L = SMe2, NMe3): Synthesis, Reactivity, Electrochemistry, M€ ossbauer Effect Studies, and Bonding Alexander R. Kudinov,*,† Piero Zanello,*,‡ Rolfe H. Herber,*,§ Dmitry A. Loginov,† Mikhail M. Vinogradov,† Anna V. Vologzhanina,† Zoya A. Starikova,† Maddalena Corsini,‡ Gianluca Giorgi,‡ and Israel Nowik§ †

Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow, Russian Federation, ‡ Dipartimento di Chimica, Universit a di Siena, 53100 Siena, Italy, and §Racah Institute of Physics, The Hebrew University, 91904 Jerusalem, Israel Received December 17, 2009

The benzene complexes [(η-9-L-7,8-C2B9R2H8)Fe(η-C6H6)]þ (3a: L = SMe2, R = H; 3b: L = SMe2, R = Me; 3c: L = NMe3, R = H) were prepared by photochemical reaction of [(η5-C6H7)Fe(η-C6H6)]þ with carborane anions [9-L-7,8-C2B9R2H8]- (1a-c) followed by treatment of the (η-9-L-7,8-C2B9R2H8)Fe(η5-C6H7) (2a-c) ferracarboranes formed with HCl. Visible light irradiation of 3a with tBuNC or P(OMe)3 in acetonitrile results in replacement of the benzene ligand, giving the tris(ligand) derivatives [(η-9SMe2-7,8-C2B9H10)Fe(ligand)3]þ (4, 5). The unsymmetrical carborane complex (η-9-SMe2-7,8-C2B9H10)Fe(η-9-NMe3-7,8-C2B9H10) (6) was obtained by photochemical reaction of 3a with 1c. The structures of 2a, ossbauer spectroscopy has 3aBPh4, and 6 were determined by X-ray diffraction. Temperature-dependent M€ been used to elucidate the hyperfine parameters and metal atom vibrational amplitudes in a number of these complexes. The redox activity of the ferracarboranes 2a,c and 3a,c has been investigated by electrochemical techniques and compared with that of the related cyclopentadienyl complexes. Electrochemistry gives evidence that the conversion of 2a-c to 3a-c can also be triggered by a two-electron oxidation followed by deprotonation. DFT calculations of the redox potentials and the respective geometrical changes were performed. Data on electrostatic potentials at iron nuclei suggest that anions 1a,c are stronger donors than Cp- in cationic complexes, but weaker donors in the neutral derivatives.

Introduction The dicarbollide anion [C2B9H11]2- is similar to Cp- in coordinating ability toward transition metals.1 However, different charges of these anions lead to significant differences in the properties of their complexes. The usage of monoanionic charge-compensated ligands [L-C2B9H10]made it possible to synthesize closer analogues of cyclopentadienyl complexes. Those with the unsymmetrically substituted anion [9-SMe2-7,8-C2B9H10]- were most widely studied due to the easy accessibility of this anion. In particular, metallacarborane analogues of sandwich compounds *To whom correspondence should be addressed. E-mail: arkudinov@ ineos.ac.ru (A.K.); [email protected] (P.Z.); [email protected] (R.H.). (1) (a) Hawthorne, M. F.; Young, D. C.; Wegner, P. A. J. Am. Chem. Soc. 1965, 87, 1818–1819. (b) Grimes, R. N. In Comprehensive Organometallic Chemistry II, Vol. 1; Abel, E. W.; Stone, F. G. A.; Wilkinson, G., Eds.; Pergamon Press: New York, 1995; p 373. (c) Grimes, R. N. Chem. Rev. 1992, 92, 251–268. (d) Saxena, A. K.; Hosmane, N. S. Chem. Rev. 1993, 92, 1081–1124. (e) Grimes, R. N. Coord. Chem. Rev. 2000, 200, 773–811. (2) (a) Yan, Y.-K.; Mingos, D. M. P.; M€ uller, T. E.; Williams, D. J.; Kurmoo, M. J. Chem. Soc., Dalton Trans. 1995, 2509–2514. (b) Kudinov, A. R.; Meshcheryakov, V. I.; Petrovskii, P. V.; Rybinskaya, M. I. Izv. Akad. Nauk, Ser. Khim. 1999, 177–179. [ Russ. Chem. Bull., 1999, 48, 176-178]. (3) Kudinov, A. R.; Petrovskii, P. V.; Meshcheryakov, V. I.; Rybinskaya, M. I. Izv. Akad. Nauk, Ser. Khim. 1999, 1368–1373. [ Russ. Chem. Bull., 1999, 48, 1356-1361]. pubs.acs.org/Organometallics

Published on Web 04/23/2010

MCp2 (M = Fe,2 Ni,3 Ru,3,4 Rh,5 Ir6), [CpM(C4Me4)]nþ (M = Co,2b,7 Pt8), and [CpM(arene)]nþ (M = Ru,9 Rh, Ir10) were (4) Rosair, G. M.; Welch, A. J.; Weller, A. S. Organometallics 1998, 17, 3227–3235. (5) Kudinov, A. R.; Perekalin, D. S.; Petrovskii, P. V.; Lyssenko, K. A.; Grintselev-Knyazev, G. V.; Starikova, Z. A. J. Organomet. Chem. 2002, 657, 115–122. (6) Loginov, D. A.; Vinogradov, M. M.; Perekalin, D. S.; Starikova, Z. A.; Lyssenko, K. A.; Petrovskii, P. V.; Kudinov, A. R. Izv. Akad. Nauk, Ser. Khim. 2006, 81–84. [ Russ. Chem. Bull. 2006, 55, 84-88 (Engl. Transl.)]. (7) (a) Meshcheryakov, V. I.; Kitaev, P. S.; Lyssenko, K. A.; Starikova, Z. A.; Petrovskii, P. V.; Janousek, Z.; Corsini, M.; Laschi, F.; Zanello, P.; Kudinov, A. R. J. Organomet. Chem. 2005, 690, 4745– 4754. (b) Kudinov, A. R.; Mutseneck, E. V.; Loginov, D. A. Coord. Chem. Rev. 2004, 248, 571–585. (8) Loginov, D. A.; Starikova, Z. A.; Petrovskaya, E. A.; Kudinov, A. R. J. Organomet. Chem. 2009, 691, 157–160. (9) (a) Kudinov, A. R.; Perekalin, D. S.; Petrovskii, P. V.; GrintselevKnyazev, G. V. Izv. Akad. Nauk, Ser. Khim. 2002, 1775–1777. [ Russ. Chem. Bull. 2002, 51, 1928-1930]. (b) Planas, J. G.; Vinas, C.; Teixidor, F.; Hursthouse, M. B.; Light, M. E. J. Chem. Soc., Dalton Trans. 2004, 2059– 2061. (c) Planas, J. G.; Vinas, C.; Teixidor, F.; Light, M. E.; Hursthouse, M. B.; Ogilvie, H. R. Eur. J. Inorg. Chem. 2005, 4193–4205. (10) (a) Loginov, D. A.; Muratov, D. V.; Starikova, Z. A.; Petrovskii, P. V.; Kudinov, A. R. J. Organomet. Chem. 2006, 691, 3646–3651. (b) Corsini, M.; Losi, S.; Grigiotti, E.; Rossi, F.; Zanello, P.; Kudinov, A. R.; Loginov, D. A.; Vinogradov, M. M.; Starikova, Z. A. J. Solid State Electrochem. 2007, 11, 1643–1653. (c) Loginov, D. A.; Muratov, D. V.; Kudinov, A. R. Izv. Akad. Nauk, Ser. Khim. 2008, 1–7. [ Russ. Chem. Bull. 2008, 57, 1-6 (Engl. Transl.)]. r 2010 American Chemical Society

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Organometallics, Vol. 29, No. 10, 2010

described. A few complexes with the symmetrically substituted [10-SMe2-7,8-C2B9H10]- anion were also described.11 The (benzene)iron complex [CpFe(η-C6H6)]þ undergoes benzene replacement by other ligands under visible light irradiation, making it a useful synthon of the CpFe moiety.12 Herein we report the synthesis of the related ferracarborane complexes [(η-9-L-7,8-C2B9H10)Fe(η-C6H6)]þ (L = SMe2, NMe3) and the first examples of photochemical arene replacement in the metallacarborane series.13 The results of structural, electrochemical, and M€ ossbauer effect studies of the complexes prepared are also presented.

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Scheme 1

Results and Discussion Synthesis and Reactivity. Recently, we have shown that visible light irradiation of the cyclohexadienyl complex [(η5C6H7)Fe(η-C6H6)]þ leads to the replacement of benzene by other ligands, such as MeCN, isonitriles, phosphites, and arenes.14 In the present study, we found that photochemical reactions of this cation with charge-compensated carborane anions [9-L-7,8-C2B9R2H8]- (1a-c) give ferracarboranes (η-9-L-7,8-C2B9R2H8)Fe(η5-C6H7) (2a-c) (Scheme 1). Treatment of the latter with hydrochloric or acetic acid affords the benzene complexes [(η-9-L-7,8-C2B9R2H8)Fe(η-C6H6)]þ (3a-c) as a result of hydride elimination from the cyclohexadienyl ligand (iodine and NBS can also be used as an (11) (a) Plesek, J.; Stibr, B.; Cooke, P. A.; Kennedy, J. D.; McGrath, T. D.; Thornton-Pett, M. Acta Crystallogr. 1998, 54C, 36–38. (b) Tutusaus, O.; Vinas, C.; Nunez, R.; Teixidor, F.; Demonceau, A.; Delfosse, S.; Noels, A. F.; Mata, I.; Molins, E. J. Am. Chem. Soc. 2003, 125, 11830–11831. (c) Nunez, R.; Tutusaus, O.; Teixidor, F.; Vinas, C.; Sillanpaa, R.; Kivekas, R. Organometallics 2004, 23, 2273–2280. (d) Tutusaus, O.; Nunez, R.; Vinas, C.; Teixidor, F.; Mata, I.; Molins, E. Inorg. Chem. 2004, 43, 6067–6074. (e) Nunez, R.; Tutusaus, O.; Teixidor, F.; Vinas, C.; Sillanpaa, R.; Kivekas, R. Chem.—Eur. J. 2005, 11, 5637–5647. (12) (a) Gill, T. P.; Mann, K. R. Inorg. Chem. 1980, 19, 3007–3010. (b) Kudinov, A. R.; Rybinskaya, M. I.; Struchkov, Yu. T.; Yanovskii, A. I.; Petrovskii, P. V. J. Organomet. Chem. 1987, 336, 187–197. (c) Scherer, O. J.; Br€ uck, T.; Wolmersh€auser, G. Chem. Ber. 1989, 122, 2049–2054. (d) Kudinov, A. R.; Muratov, D. V.; Rybinskaya, M. I.; Petrovskii, P. V.; Mironov, A. V.; Timofeeva, T. V.; Slovokhotov, Yu. L.; Struchkov, Yu. T. J. Organomet. Chem. 1991, 414, 97–107. (e) Kudinov, A. R.; Fil'chikov, A. A.; Petrovskii, P. V.; Rybinskaya, M. I. Izv. Akad. Nauk, Ser. Khim. 1999, 1364–1367. [ Russ. Chem. Bull. 1999, 48, 1352-1355 (Engl. Transl.)]. (f) Kudinov, A. R.; Loginov, D. A.; Starikova, Z. A.; Petrovskii, P. V.; Corsini, M.; Zanello, P. Eur. J. Inorg. Chem. 2002, 3018–3027. (g) Aranzaes, J. R.; Astruc, D. Inorg. Chim. Acta 2008, 361, 1–4. (13) Preliminary account of this work: Loginov, D. A.; Vinogradov, M. M.; Shul’pina, L. S.; Vologzhanina, A. V.; Petrovskii, P. V.; Kudinov, A. R. Izv. Akad. Nauk, Ser. Khim. 2007, 2046–2048. [ Russ. Chem. Bull. 2007, 56, 2118-2120 (Engl. Transl.)]. (14) (a) Kudinov, A. R.; Loginov, D. A.; Petrovskii, P. V. Izv. Akad. Nauk, Ser. Khim. 2007, 1864–1865. [ Russ. Chem. Bull. 2007, 56, 1930-1931 (Engl. Transl.)]. (b) Loginov, D. A.; Vinogradov, M. M.; Starikova, Z. A.; Petrovskii, P. V.; Kudinov, A. R. Izv. Akad. Nauk, Ser. Khim. 2007, 2088–2091. [ Russ. Chem. Bull. 2007, 56, 2162-2165 (Engl. Transl.)]. (c) Zanello, P.; Herber, R. H.; Kudinov, A. R.; Corsini, M.; Fabrizi de Biani, F.; Nowik, I.; Loginov, D. A.; Vinogradov, M. M.; Shul'pina, L. S.; Ivanov, I. A.; Vologzhanina, A. V. J. Organomet. Chem. 2009, 694, 1161– 1171. (d) Loginov, D. A.; Vinogradov, M. M.; Starikova, Z. A.; Petrovskii, P. V.; Holub, J.; Kudinov, A. R. Izv. Akad. Nauk, Ser. Khim. 2008, 2250– 2252. [ Russ. Chem. Bull. 2008, 57, 2294-2297 (Engl. Transl.)]. (15) For neutral complexes see: (a) Garcia, M. P.; Green, M.; Stone, F. G. A.; Somerville, R. G.; Welch, A. J. J. Chem. Soc., Chem. Commun. 1981, 871–872. (b) Hanusa, T. P.; Huffman, J. C.; Todd, L. J. Polyhedron 1982, 1, 77–82. (c) Swisher, R. G.; Sinn, E.; Grimes, R. N. Organometallics 1983, 2, 506–514. (d) Swisher, R. G.; Sinn, E.; Butcher, R. J.; Grimes, R. N. Organometallics 1985, 4, 882–890. (e) Swisher, R. G.; Sinn, E.; Grimes, R. N. Organometallics 1985, 4, 890–895. (f) Swisher, R. G.; Sinn, E.; Grimes, R. N. Organometallics 1985, 4, 896–901. (g) Garcia, M. P.; Green, M.; Stone, F. G. A.; Somerville, R. G.; Welch, A. J.; Briant, C. E.; Cox, D. N.; Mingos, D. M. P. J. Chem. Soc., Dalton Trans. 1985, 2343–2348. (h) Stibr, B.; Bakardjiev, M.; Holub, J.; Ruzicka, A.; Kvicalova, M. Inorg. Chem. 2009, 48, 10904–10906.

Scheme 2

acceptor of H-). Thus, the two-step procedure presented in Scheme 1 makes it possible to synthesize the ferracarborane benzene complexes from cation [(η5-C6H7)Fe(η-C6H6)]þ. Complexes 3a-c are the first cationic (arene)ferracarboranes.15 Noteworthy, the interaction of 3a-c with NaBH4 regenerates the cyclohexadienyl compounds 2a-c. The benzene complex 3a reacts with tBuNC and P(OMe)3 under visible light irradiation in acetonitrile, giving tris(ligand) derivatives [(η-9-SMe2-7,8-C2B9H10)Fe(L)3]þ (4, 5) (Scheme 2). This reaction demonstrates that cation 3a can be used as a synthon of the cationic ferracarborane species [(η-9-SMe2-7,8-C2B9H10)Fe]þ. Unfortunately, analogous reactions of 3b,c lead to unidentified mixtures of products, probably due to steric hindrance. Photochemical reaction of cation 3a with anion 1c affords a mixture of unsymmetrical and symmetrical carborane complexes (η-9-SMe2-7,8-C2B9H10)Fe(η-9-NMe3-7,8C2B9H10) (6) and Fe(η-9-SMe2-7,8-C2B9H10)2 (7) in a 2:1 ratio (Scheme 3). Complex 7 is formed as a result of a symmetrization side reaction. Unfortunately, we were unable to separate this mixture by recrystallization or by chromatography. X-ray Diffraction Study. The structures of 2a, [3a]BPh4, and [6]0.65[7]0.35 (designated as 6 in further discussion for clarity) were determined by X-ray diffraction (Figures 1-3).16 The folding angle of the C6H7 ring in 2a (dihedral angle between planes C1C2C6 and C2C3C4C5C6) is equal to 44.3. This value is close to that in the phosphite (16) X-ray structure of complex [4]PF6 was published earlier.13

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Kudinov et al.

Scheme 3

complex [(η5-C6H7)Fe(P(OMe)3)3]PF6 (43.0)14c and larger than in the isonitrile derivative [(η5-C6H7)Fe(tBuNC)3]PF6 (33.9)14b in accordance with the strong steric effect of carborane and phosphite ligands. The CH2 group of the C6H7 ligand and SMe2 moiety were found to be in cisoid conformation, which is characterized by the largest S 3 3 3 H intramolecular distance. The Cambridge Structural Database contains crystal structures of compounds such as 717 and (1a)FeCp*,2a as well as (1a)H.18 For all iron complexes the mean planes of the π-ligands are almost parallel. The angle between the C5 and C2B3 planes in 2a is equal to 2.0 and in (1a)FeCp* -6.1, that between the C6 and C2B3 planes in 3a is 4.0, and that between two C2B3 planes in 6 and 7 varies from 0.0 to 7.6. It is interesting to note that methyl groups of the initial ligand (1a)H are situated on the opposite sides of the C2B3 mean plane, while in the structures of iron complexes 2a, 3a, 6, 7, and (1a)FeCp* the SMe2 substituent is oriented so that the iron atom and both methyl groups are situated on the opposite sides of the C2B3 mean plane. Coordination of 1a by iron(II) also causes elongation of the B-S bond from 1.879 A˚ in (1a)H to 1.903-1.937 A˚ in the complexes. The metal-to-ring Fe 3 3 3 C6 distance in cation 3a (1.59 A˚) is longer than that in the cyclopentadienyl complexes [(η-C5R5)Fe(η-C6H6)]þ (1.53-1.54 A˚ for R = H, Me),14c suggesting weaker Fe-C6H6 bonding in the case of 3a. This was confirmed by DFT calculations (vide infra). The Fe 3 3 3 C2B3 distances in 2a (1.48 A˚), 3a (1.50 A˚), and 6 (1.51 A˚) are close to those in the dicarbollide complexes (17) (a) Yan, Y.-K.; Mingos, D. M. P.; Muller, T. E.; Williams, D. J.; Kurmoo, M. J. Chem. Soc., Dalton Trans. 1994, 1735–1741. (b) Yan, Y.-K.; Mingos, D. M. P.; Williams, D. J. J. Organomet. Chem. 1995, 498, 267–274. (18) Cowey, J.; Hamilton, E. J. M.; Laurie, J. C. V.; Welch, A. J. Acta Crystallogr. 1988, 44C, 1648–1650. (19) Herber, R. H.; Kudinov, A. R.; Zanello, P.; Nowik, I.; Perekalin, D. S.; Meshcheryakov, V. I.; Lyssenko, K. A.; Corsini, M.; Fedi, S. Eur. J. Inorg. Chem. 2006, 1786–1795. (20) Lee, S. S.; Knobler, C. B.; Hawthorne, M. F. J. Organomet. Chem. 1990, 394, 29–36. (21) (a) Xinmin, Y.; King, W. A.; Sabat, M.; Marks, T. J. Organometallics 1993, 12, 4254–4258. (b) Kang, H. C.; Lee, S. S.; Knobler, C. B.; Hawthorne, M. F. Inorg. Chem. 1991, 30, 2024–2031.

Figure 1. Structure of complex 2a. Ellipsoids are shown at the 50% level. Selected bond lengths [A˚] and angles [deg]: Fe1-C2 2.1244(16), Fe1-C3 2.0622(16), Fe1-C4 2.0713(15), Fe1-C5 2.0465(15), Fe1-C6 2.0954(15), Fe1-C7 2.0625(14), Fe1-C8 2.0187(14), Fe1-B9 2.0634(15), Fe1-B10 2.1277(16), Fe1-B11 2.1305(17), C1-C2 1.511(2), C2-C3 1.407(2), C3-C4 1.422(2), C4-C5 1.411(2), C5-C6 1.408(2), C1-C6 1.500(2), C7-C8 1.6255(19), C8-B9 1.693(2), C7-B11 1.688(2), B9-B10 1.785(2), B10-B11 1.789(2), B9-S1 1.9127(16), C2-C1-C6 101.01(12), C1-C2-C3 119.09(14), C2-C3-C4 119.18(15), C3-C4-C5 117.69(14), C4-C5-C6 118.83(16), C5-C6-C1 119.62(16), C7-C8-B9 110.63(10), C8-B9-B10 106.14(11), B9-B10-B11 105.82(11), B10-B11-C7 105.51(11), B11C7-C8 111.85(11).

Figure 2. Structure of cation 3a. Ellipsoids are shown at the 50% level. Selected bond lengths [A˚] and angles [deg]: Fe1-C1 2.109(3), Fe1-C2 2.101(3), Fe1-C3 2.120(3), Fe1-C4 2.125(3), Fe1-C5 2.135(3), Fe1-C6 2.116(3), Fe1-C7 2.051(3), Fe1-C8 2.052(3), Fe1-B9 2.099(3), Fe1-B10 2.151(3), Fe1-B11 2.115(3), C1-C2 1.415(5), C2-C3 1.404(5), C3-C4 1.400(4), C4-C5 1.400(4), C5-C6 1.404(4), C1-C6 1.399(4), C7-C8 1.623(4), C8-B9 1.697(4), C7-B11 1.711(4), B9-B10 1.784(4), B10-B11 1.798(5), B9-S1 1.916(3), C7-C8-B9 109.9(2), C8-B9-B10 107.4(2), B9-B10-B11 105.2(2), B10-B11-C7 105.3(2), B11-C7-C8 112.1(2).

[(η-7,8-C 2B9 H11 )FeCp]- (1.44 A˚), 19 (η-7,8-C 2B9 H11)Fe(C6H6) (1.49 A˚),20 and [(η-7,8-C2B9H10)2Fe]2- (1.48 A˚).21 M€ ossbauer Effect Study. Five of the compounds discussed above, 2a, 2c, 3aBPh4, 3cPF6, and 4PF6, have been examined by temperature-dependent M€ ossbauer effect (ME) spectroscopy. The iron atom resonance for all samples is reflected in simple quadrupole-split spectra, and a typical spectral trace is shown in Figure 4. The hyperfine parameters at 90 K, as well as the derived dynamical data derived therefrom, are summarized in Table 1. The isomer shifts (IS) for 2a and 2c are significantly smaller than that observed for 3aBPh4,

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Figure 3. Structure of 6. Ellipsoids are shown at the 50% level. Only one independent molecule is shown. The NMe3 and SMe2 moieties are disordered in both molecules in the ratio 0.65:0.35; the latter group is depicted with hollow lines. Selected bond lengths [A˚] for one of two independent molecules: Fe21-C7C 2.042(10), Fe21-C8C 2.057(12), Fe21-C7D 2.067(11), Fe21C8D 2.050(11), Fe21-B9C 2.142(14), Fe21-B10C 2.158(12), Fe21-B11C 2.108(12), Fe21-B9D 2.104(13), Fe21-B10D 2.167(12), Fe21-B11D 2.138(13), C7C-C8C 1.632(16), C7CB11C 1.695(16), C8C-B9C 1.707(17), B9C-B10C 1.777(18), B10C-B11C 1.786(17), C7D-C8D 1.615(15), C7D-B11D 1.690(16), C8D-B9D 1.690(16), B9D-B10D 1.766(18), B10D-B11D 1.788(18).

Figure 4. 57Fe M€ ossbauer spectrum of 2a at 98 K. The velocity scale is with respect to the centroid of a room-temperature R-Fe absorption spectrum, which is also used to calibrate the spectrometer.

suggesting that the replacement of the cyclohexadienyl ring by a benzene ring in the latter results in a smaller s-electron density at the iron nucleus in 3aBPh4 compared to 2a and 2c. Since 3aBPh4 consists of a cationic metal center together with a balancing BPh4- anion, it might have been expected that the iron atom resonance would show a quadrupole splitting (QS) typical of Fe3þ that is on the order of 0.1 to 0.3 mm s-1. However, as noted in Table 1, the QS of 3aBPh4 at 90 K is similar to that observed in 2a and 2c, and thus there is essentially no electron withdrawal from the metal center in the former.

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Further confirmation of this conclusion is provided by a magnetic in-field experiment at room temperature, of the type described earlier.22 The results of this experiment are summarized graphically in Figure 5, in which the upper trace is the ME spectrum in zero field, while the lower trace is that accumulated in an external field of about 2.4 ( 0.04 kOe. As will be noted, the in-field spectrum shows a triplet absorbance at an isomer shift more negative than the spectrum centroid and a doublet absorbance at a velocity more positive than the spectrum centroid. This clearly shows that the QS parameter is positive, in consonance with results obtained for other diamagnetic [Fe(II)] resonances in related organometallics. Finally, it is also worth noting that the goodness of fit parameter (χ2) in the spectral fitting program is consistently better for QS > 0 than for QS < 0. The unambiguous interpretation of these data is that the formation of the cationic complex does not involve the removal of substantial electron density from the metal center in 3aBPh4. With respect to the dynamical parameters extracted from the ME data it is noteworthy that the temperature dependence of the recoil-free fraction, as reflected in the area under the resonance curve, is much larger for 3aBPh4 than for 2a, and this effect is also noted in a comparison of the lattice temperature, ΘM, for the two compounds. Since singlecrystal X-ray data have been determined for both complexes, it is possible to calculate the root-mean-square amplitude of vibration (rmsav) of the metal atom from the crystallographic Ui,j values and to compare these with the corresponding data extracted from the ME data, using the procedure detailed earlier.23 For 2a, these values at 120 K are Fx,120 = 0.64 ( 0.05 and FM,120 = 0.68 ( 0.06. For 3aBPh4, the corresponding values at 100 K are Fx,100 = 0.86 ( 0.01 and FM,100 = 0.93 ( 0.06. This agreement validates the extraction of vibrational amplitudes from the ME data. Since F = k2Æxav2æ, where k is the wave vector of the M€ ossbauer gamma ray, it is possible to derive the rmsav values from the temperature-dependent ME data, and this has been done at five different temperatures, as summarized in Table 2. From these data it is seen that the rmsav values are consistently larger for 3aBPh4 than for 2a. This is in agreement with the data extracted from the single-crystal X-ray data, which show that for 3aBPh4 the average Fe-C bond length is 2.052 A˚, while for 2a this value is 2.041 A˚. Similarly, the Fe-B bond lengths are 2.122 A˚ in 3aBPh4 and 2.106 A˚ in 2a. Clearly, the metal-carborane bonding is tighter in 2a than in 3aBPh4, and the rmsav of the metal atom is smaller in 2a than in 3aBPh4, as reflected in the ΘM values listed in Table 1. Finally, in this discussion of the dynamical properties of the metal atom in 2a, 2c, and 3aBPh4, it is worth noting that over the temperature interval of the ME measurements (90-300 K) the metal atom motion is essentially isotropic and does not show the motional anisotropy reported earlier in ferrocenoid complexes. This conclusion is based on the temperature independence of the area ratio of the two components of the QS split spectra, which show no evidence of a Gol’danskii-Karyagin effect at the 2% confidence level. This isotropy is also evident from Figure 2 and the X-ray data at 100 K, which shows that the three Ui,j values are identical within 5.2%. (22) Nowik, I.; Herber, R. H. Eur. J. Inorg. Chem. 2006, 5069–5075. (23) Herber, R. H.; Nowik, I. J. Organomet. Chem. 2008, 693, 3007-3010, and references therein.

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Table 1. 57Fe M€ossbauer Parameters of the Compounds Discussed in the Text parameter

2a

IS(90) QS(90) -d IS/d T -d ln A/d T Meff ΘM k2Æxav2æX k2Æxav2æM a

2c

0.380(2) 1.740(2) 4.02(11) 5.70(13) 104 ( 3 115 ( 3 0.64 ( 0.05b 0.68 ( 0.06b

3aBPh4

0.386(1) 1.929(1) 5.73(6)a 5.26(50) 72.6 ( 0.8 143 ( 8

0.434(3) 1.895(3) 2.95(5) 9.30(40) 141 ( 3 77 ( 3 0.86 ( 0.01c 0.93 ( 0.06c

3cPF6

4PF6

0.453(3) 2.022(3)

0.090(3) 1.906(3)

5.95(1)

6.57(10) 0.68 ( 0.01c 0.66 ( 0.02c

units -1

mm s mm s-1  10-4 mm s-1 K-1  10-3 K-1 Da K

T range 237-319 K. b At 120 K. c At 100 K.

Figure 6. Cyclic voltammetric responses recorded at a glassy carbon electrode in CH2Cl2 solution of 2a (1.4  10-3 M): (a) original solution; (b) after exhaustive one-electron oxidation. [NBu4]PF6 (0.2 M) supporting electrolyte. Scan rate = 0.1 V s-1. T = 253 K.

Figure 5. 57Fe M€ ossbauer spectra of 3aBPh4 in zero field (top trace) and in an external magnetic field of 2.36 kOe, both recorded at room temperature. The characteristic signature indicates that QS > 0. Table 2. F Parameters and Root Mean Square Amplitudes of Vibration of the Metal Atom in Complexes 2a, 3aBPh4, and 4PF6 2a

3aBPh4

4PF6

temp/K

k2Æxav2æ

Æxavæ

k2Æxav2æ

Æxavæ

k2Æxav2æ

Æxavæ

100 150 200 250 300

0.567 0.852 1.142 1.424 1.711

0.103 0.126 0.146 0.163 0.179

0.923 1.396 1.856 2.324 2.784

0.132 0.162 0.187 0.209 0.228

0.660 0.985 1.311 1.642 1.971

0.111 0.136 0.157 0.175 0.192

The hyperfine parameters of 3cPF6 and 4PF6 are included in Table 1. For both of these compounds the temperature dependence of the IS shows significant curvature over the range ∼90 < T < 300 K, so that no meaningful values for Meff and ΘM can be extracted from these data. It will be noted that the IS for 4PF6 is very much smaller than those observed for the other compounds in this study, indicating a larger s-electron density at the metal center; however since this is the only compound in which the carborane ligand is accompanied by three tBuNC ligands (rather than a ring system), it is not immediately obvious what the origin of this electron density is. The QS parameter at 90 K is nearly the

same for both cations and similar to that observed for 2c and 3aBPh4 at the same temperature. The observed IS for 4PF6 is characteristic of Fe3þ, and the absorbance data from a room-temperature in-field magnet experiment (as described above for the 3aBPh4 complex) are best accounted for with QS < 0. In the absence of an aromatic ring ligand, the positive charge on the cationic moiety presumably involves significant electron withdrawal from the metal center compared to 3aBPh4. The small IS is then a consequence of smaller s-electron shielding by the 3d electrons of the Fe center, as noted in Table 1. The observed magnetic hyperfine field for this compound is larger than the applied external field, as has been observed earlier in the case of paramagnetic ferrocenoid complexes.22 For 4PF6 the temperature dependence of the logarithm under the resonance curve is well fitted by a linear regression (cc = 0.995 for 11 data points), and the corresponding slope is included in Table 1. The crystal structure for this compound has been reported earlier,7 and thus here, again, it is possible to compare the vibrational amplitude data for the iron atom from both the X-ray and ME results; these are included in Table 1 and show good agreement between the two methodologies. The respective rmsav values for 4PF6 at five temperatures are included in Table 2. Electrochemistry. Figure 6 illustrates the redox behavior of the cyclohexadienyl complex 2a in CH2Cl2 solution before (a) and after exhaustive one-electron oxidation (b), at 253 K. It undergoes an oxidation process, which is accompanied by slow chemical complications.24 In fact, analysis of the cyclic voltammograms at scan rates progressively increasing (24) Zanello, P. Inorganic Electrochemistry. Theory, Practice and Application; Royal Society of Chemistry: Cambridge, U.K., 2003.

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Table 3. Formal Electrode Potentials (V, vs SCE), Current Ratio (ip(reverse)/ip(direct)), and Peak-to-Peak Separation (mV) for the Redox Changes Exhibited by Complexes 2a,c, 3a,c, and 4 in CH2Cl2 Solution at 253 K oxidation 0

complex

E

2a 2c 3a 3c 4

þ0.20 þ0.16

þ1.24

reduction 0

ipc/ipa ΔEp E first reduction ipa/ipca ΔEpa Ep second reduction a

0.8 0.9

1.0

a

74 64

119

-1.10 -1.26b,c -1.78b

0.8

85

-1.96

Measured at 0.5 V s-1. b Peak potential value for irreversible processes. c Coupled to electrodeposition of byproducts (see text). a

Figure 7. Cyclic voltammetric responses recorded at a glassy carbon electrode in CH2Cl2 solution of 3a (1.5  10-3 M). [NBu4]PF6 (0.2 M) supporting electrolyte. Scan rate = 0.1 V s-1. T = 253 K. Scheme 4

Figure 8. Cyclic voltammetric response recorded at a glassy carbon electrode in CH2Cl2 solution of 4 (0.8  10-3 M). [NBu4]PF6 (0.2 M) supporting electrolyte. Scan rate = 0.1 V s-1. T = 253 K.

from 0.02 to 10.24 V s-1 shows that (i) the current ratio ipc/ipa progressively increases from 0.7 to 0.95; (ii) the current function ipav-1/2 is substantially constant; (iii) the peak-to-peak separation, ΔEp, progressively increases from 60 to 140 mV. Controlled potential coulometric tests (Ew = þ0.6 V) consume about 1.9 electrons per molecule. As a consequence of the exhaustive oxidation, the original yellow solution turns pale pink, and as shown in Figure 6b, it displays a first reduction possessing features of (partial) chemical reversibility, followed by a second irreversible reduction. In this connection, Figure 7 illustrates the redox activity of the benzene complex 3a. It is evident that the redox fingerprint is quite coincident with that arising from the exhaustive oxidation of 2a, so that it indicates that the conversion from 2a-c to 3a-c illustrated in Scheme 1 can be triggered also by the electron-transfer process shown in Scheme 4. As far as the analysis of the cyclic voltammetric responses of the first reduction at different scan rates is concerned, also in this case the process is coupled to chemical complications. As a matter of fact, not only does exhaustive one-electron oxidation completely destroy the original voltammetric profile, but no well-defined oxidation process is generated; that is, at variance with the chemical findings, the conversion from cyclohexadienyl to benzene complex is not reversible from the electrochemical viewpoint. A similar redox pattern holds for complexes 2c and 3c, except for the fact that exhaustive electrolysis in correspondence with the first reduction of complex 3c causes electrodeposition of an unidentified pink solid. The formal electrode potentials of the above-described redox changes are compiled in Table 3. Finally, Figure 8 illustrates the cyclic voltammetric response given by complex 4. The complex undergoes either an

irreversible reduction or an oxidation that possesses features of chemical reversibility in the time scale of cyclic voltammetry (or, the current ratio ipc/ipa is constantly equal to 1 also at the lowest scan rates). On the basis of the comparison with the response of equimolar amounts of 1,2-diferrocenylethane (MW = 398.11), which affords an oxidation process (E0 = þ0.28 V) about twice that of the carboranyl monocation 4 (MW = 498.77), it is plausible to assume that the mentioned oxidation process involves a one-electron removal. Nevertheless, in the long times of exhaustive oxidation the process is coupled to chemical complications that likely involve the reorganization of one of the three basal tBuNC ligands from terminal to bridging position in between the dicarboranyl unit and the central Fe(III) ion (see Figures S1-S4 and the accompanying discussion in the Supporting Information). Computational Electrochemistry. The redox potentials were also estimated by DFT calculations. Table 4 compares the experimental and the calculated formal electrode potentials for complexes 3a,c, 2a,c, and CpFe(η-9-L-7,8-C2B9H10) (8a,c, Chart 1) together with those for the related cyclopentadienyl derivatives [CpFe(η-C6H6)]þ, CpFe(η5-C6H7), and FeCp2. Independently from the solvation model (PCM or COSMO), computation satisfactorily predicts the potentials (the maximum deviation from experimental value is 0.26 V for PCM and 0.22 V for COSMO). Noteworthy, only a few examples of such calculations for organometallic compounds are described in the literature.14c,25 (25) (a) Baik, M.-H.; Friesner, R. A. J. Phys. Chem. A 2002, 106, 7407–7412. (b) Cossi, M.; Iozzi, M. F.; Marrani, A. G.; Lavecchia, T.; Galloni, P.; Zanoni, R.; Decker, F. J. Phys. Chem. B 2006, 110, 22961– 22965. (c) Nafady, A.; Costa, P. J.; Calhorda, M. J.; Geiger, W. E. J. Am. Chem. Soc. 2006, 128, 16587–16599. (d) Chong, D.; Laws, D. R.; Nafady, A.; Costa, P. J.; Rheingold, A. L.; Calhorda, M. J.; Geiger, W. E. J. Am. Chem. Soc. 2008, 130, 2692–2703. (e) Siebert, W.; Kudinov, A. R.; Zanello, P.; Antipin, M. Yu.; Scherban, V. V.; Romanov, A. S.; Muratov, D. V.; Starikova, Z. A.; Corsini, M. Organometallics 2009, 28, 2707–2715.

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Table 4. Experimental and Calculated Formal Electrode Potentials (V, vs SCE) for Iron Complexes (optimized at BP86/TZ2P) in CH2Cl2 Solution E(calcd) compound

transition

E(exptl)

COSMOa

PCMb

3a 3c [CpFe(η-C6H6)]þ 2a 2c CpFe(η5-C6H7) 8a 8c FeCp2

þ/0 þ/0 þ/0 0/þ 0/þ 0/þ 0/þ 0/þ 0/þ

-1.10 -1.26 -1.44 þ0.20 þ0.16 þ0.14 c þ0.47 þ0.40 þ0.44

-1.10 -1.11 -1.54 þ0.33 þ0.34 þ0.17 þ0.56 þ0.52 þ0.56

-1.19 -1.19 -1.70 þ0.23 þ0.22 0.00 þ0.48 þ0.43 þ0.38

a

BP86/TZ2P. b BP86/def2-TZVPP//BP86/TZ2P. c The extent of chemical reversibility of the anodic oxidation is lower than that of 2a.

Chart 1

Table 5. Electrostatic Potentials at Nuclei (E, in au) and NBO Charges (q, in au) for (L)Fe(ring) Complexesa complex [3a] [3a]

þ 0

2þ

-115.530 -117.366 -115.710 -117.545 -115.532 -117.368 -115.713 -117.549 -115.513 -117.344 -115.723 -117.556 -115.702 -117.537 -115.524 -117.358 -115.704 -117.541 -115.524 -117.359 -115.713 -117.547 -115.508 -117.339 -115.703 -117.540 -115.516 -117.351 -115.706 -117.545 -115.518 -117.355 -115.715 -117.549 -115.491 -117.322

-14.598 -14.617 -14.750 -14.770 -14.598 -14.618 -14.752 -14.772 -14.586 -14.604 -14.763 -14.782 -14.761 -14.781 -14.607 -14.627 -14.763 -14.784 -14.608 -14.630 -14.773 -14.795 -14.592 -14.611 -14.758 -14.779 -14.606 -14.626 -14.760 -14.782 -14.608 -14.628 -14.771 -14.791 -14.588 -14.606

1þ

[FeCp(C6H6)]þ

2þ

[FeCp(C6H6)]0

1þ

[2a]0

2þ

[2a]þ

3þ

[2c]0

2þ 3þ 5

[CpFe(η -C6H7)]

0

[CpFe(η5-C6H7)]þ [8a]

0

[8c]

3þ

3þ

0

2þ

[8c]þ [FeCp2]

2þ

2þ

[8a]þ

(26) The optimized geometries of complexes 2a and 3a are in a good agreement with X-ray diffraction data (Fe-C and Fe-B max. deviation 0.021 A˚, av deviation 0.008 A˚).

EC(ring) av

2þ

0

[2c]þ

The calculations also allowed to evaluate structural changes accompanying redox processes.26 Reduction of the benzene complexes 3a,c leads to elongation of the Fe 3 3 3 C2B3 (by ca. 0.10 A˚) and Fe 3 3 3 C6H6 (by 0.04 A˚) distances. Reduction practically does not affect the coplanarity of the C2B3 and C6H6 rings (dihedral angles: [3a]þ/[3a]0 5.6/6.8, [3c]þ/[3c]0 10.7/12.5). The C2B3 and C6H6 rings are almost planar in the starting cations, but become considerably folded in the reduced species (folding angles: C2B3 ca. 8.0, C6H6 6.0). A similar pattern was observed earlier for the cyclopentadienyl analogues [FeCp(C6H6)]þ/0.14c Oxidation of the cyclohexadienyl and cyclopentadienyl complexes 2a,c and 8a,c results in elongation of the Fe 3 3 3 C2B3 (by ca. 0.05-0.08 A˚), Fe 3 3 3 C6H7 (by 0.030.05 A˚), and Fe 3 3 3 Cp (by 0.06-0.07 A˚) distances. In this case the C2B3 and C5 bonding faces are almost planar for both the neutral and cationic species. The cyclohexadienyl ligand in 2a,c becomes less folded upon oxidation (folding angles: [2a]0/[2a]þ 46.8/38.1, [2c]0/[2c]þ 47.1/40.5). Comparison of redox potentials of metal complexes is often used to evaluate ligand electronic effects. However, this approach is well applicable only for closely related ligands (e.g., substituted cyclopentadienyls). For ligands of different nature (e.g., Cp- and carborane anions 1a,c) erroneous conclusions can be made owing to considerable differences in HOMO and/or LUMO orbitals of their complexes. For instance, although LUMOs of 3a and [FeCp(C6H6)]þ are similar, their HOMOs are quite different (Figure S5 in the Supporting Information). Reduction potentials become more negative in the order 3a < 3c < [FeCp(C6H6)]þ, suggesting that the donor ability of anions 1a, 1c, and Cpincreases in the same sequence.

EFe

1þ

[3c]þ [3c]

Fe redox state

3þ 0

[FeCp2]þ

2þ 3þ

qFe 0.031 0.131 0.030 0.128 0.172 0.270 -0.035 0.332 -0.044 0.347 0.139 0.413 -0.039 0.395 -0.055 0.392 0.122 0.599

a Values at BP86/def2-TZVPP//BP86/TZ2P are shown in normal type and at BP86/TZ2P in italics.

Recently, electrostatic potentials at carbon nuclei were shown to be useful criteria for evaluation of substituent effects in the benzene ring.27 This parameter can also be helpful in organometallic chemistry.25e The electrochemically suggested greater donor ability of 1c compared with 1a is supported by slightly higher (by absolute magnitude) electrostatic potentials (E) at the Fe and C(C6H6) nuclei in cation 3c than in 3a (Table 5). However, the EFe and EC(ring) values for 3a,c are considerably higher than for [FeCp(C6H6)]þ, indicating stronger donor ability of anions 1a,c compared with Cp-. It may be concluded that the donor ability increases in the sequence Cp- < 1a e 1c. At the same time, for the reduced 19 VE (valence electron) species the EFe and EC(ring) values increase in the order [3a]0 e [3c]0 < [FeCp(C6H6)]0, indicating the weaker donor ability of 1a,c in this case. The same order is also deduced for the series of neutral 18 VE complexes 2a/2c/CpFe(η5-C6H7) and 8a/8c/FeCp2. This conclusion correlates with electrochemical data. However, for the corresponding oxidized 17 VE cationic species anions 1a,c are stronger donors than Cp-. Overall, anions 1a,c are stronger donors than Cp- in cationic complexes (both diamagnetic and paramagnetic), but are weaker donors in the neutral derivatives. The NMe3substituted carborane ligand 1c is a slightly stronger donor than the SMe2 derivative 1a in all cases. Interestingly, in (27) Galabov, B.; Ilieva, S.; Schaefer, H. F., III. J. Org. Chem. 2006, 71, 6382–6387.

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neutral complexes with the acceptor Mn(CO)3 fragment, anion 1a is a stronger donor than Cp-,30 as in the cationic complexes discussed here. The NBO charges can also be used as an indicator of ligand electronic effect. However, the presence of electropositive boron atoms decreases the metal NBO charges (e.g., qFe in complexes 2a,c and 8a,c is even negative), thus reducing their importance. In addition, in the 19 VE neutral species [3a,c]0 qFe is suprisingly higher than in 18 VE cations [3a,c]þ. Iron-Benzene Bonding in [(η-9-L-7,8-C2B9H10)Fe(C6H6)]þ Complexes. To compare the Fe-C6H6 bonding in complexes [(η-9-L-7,8-C2B9H10)Fe(C6H6)]þ with that in the cyclopentadienyl analogue [CpFe(C6H6)]þ, we carried out their energy decomposition analysis (EDA).28 According to the EDA method, the interaction energy between the bonding fragments ΔEint can be divided into three main components:

ΔEint ¼ ΔEelstat þ ΔEPauli þ ΔEorb ΔEelstat is the electrostatic interaction energy between the fragments with a frozen electron density distribution, ΔEPauli represents the repulsive four-electron interactions between occupied orbitals (Pauli repulsion), and ΔEorb refers to the stabilizing orbital interactions. The ratio ΔEelstat/ΔEorb indicates the electrostatic/covalent character of the bond. The bond dissociation energy is

De ¼ - ðΔEint þ ΔEprep Þ where ΔEprep (the fragment preparation energy) is the energy that is necessary to promote the fragments from their equilibrium geometry and electronic ground state to the geometry and electronic state that they have in the optimized structure. This method has already proven its usefulness for the analysis of the nature of metal-ligand bonding in ferrocene and some other sandwich compounds.14c,25e,29 The EDA data for complexes [(L)Fe(C6H6)]þ (3a, 3c, and [CpFe(C6H6)]þ) in terms of interactions between [(L)Fe]þ and C6H6 fragments are given in Table 6 (the complexes are arranged according to the increase of ΔEint). The electro(28) For recent reviews see: (a) Frenking, G.; Fr€ ohlich, N. Chem. Rev. 2000, 100, 717–774. (b) Frenking, G. J. Organomet. Chem. 2001, 635, 9–23. (c) Frenking, G.; Wichmann, K.; Fr€ohlich, N.; Loschen, C.; Lein, M.; Frunzke, J.; Rayon, V. M. Coord. Chem. Rev. 2003, 238, 55–82. (d) Lein, M.; Frenking, G. In Theory and Applications of Computational Chemistry: The First Forty Years; Dykstra, C. E.; Frenking, G.; Kim, K. S.; Scuseria, G. E., Eds.; Elsevier: Amsterdam, 2005; pp 291-372. (e) Frenking, G.; Krapp, A. J. Comput. Chem. 2007, 28, 15–24. (f) Ziegler, T.; Autschbach, J. Chem. Rev. 2005, 105, 2695–2722. (29) (a) Lein, M.; Frunzke, J.; Timoshkin, A.; Frenking, G. Chem.— Eur. J. 2001, 7, 4155–4163. (b) Rayon, V. M.; Frenking, G. Chem.—Eur. J. 2002, 8, 4693–4707. (c) Frunzke, J.; Lein, M.; Frenking, G. Organometallics 2002, 21, 3351–3359. (d) Rayon, V. M.; Frenking, G. Organometallics 2003, 22, 3304–3308. (e) Lein, M.; Frunzke, J.; Frenking, G. Inorg. Chem. 2003, 42, 2504–2511. (f) Velazquez, A.; Fernandez, I.; Frenking, G.; Merino, G. Organometallics 2007, 26, 4731–4736. (g) Lee, V. Ya.; Kato, R.; Sekiguchi, A.; Krapp, A.; Frenking, G. J. Am. Chem. Soc. 2007, 129, 10340–10341. (h) Fernandez, I.; Cerpa, E.; Merino, G.; Frenking, G. Organometallics 2008, 27, 1106–1111. (i) Menconi, G.; Kaltsoyannis, N. Organometallics 2005, 24, 1189–1197. (j) Swart, M. Inorg. Chim. Acta 2007, 360, 179–189. (k) Kan, Y. J. Mol. Struct. (THEOCHEM) 2007, 805, 127–132. (l) Loginov, D. A.; Vinogradov, M. M.; Starikova, Z. A.; Petrovskaya, E. A.; Zanello, P.; Laschi, F.; Rossi, F.; Cinquantini, A.; Kudinov, A. R. J. Organomet. Chem. 2007, 692, 5777–5787. (m) Loginov, D. A.; Pronin, A. A.; Shul'pina, L. S.; Mutseneck, E. V.; Starikova, Z. A.; Petrovskii, P. V.; Kudinov, A. R. Izv. Akad. Nauk, Ser. Khim. 2008, 535–539. [ Russ. Chem. Bull. 2008, 57, 546-551 (Engl. Transl.)]. (n) Erhardt, S.; Frenking, G. J. Organomet. Chem. 2009, 694, 1091–1100.

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Table 6. Results of EDA for [(L)Fe(C6H6)]þ Complexes Using [(L)Fe]þ and C6H6 As Interacting Fragments at BP86/TZ2P (energy values in kcal mol-1) ΔEint ΔEPauli ΔEelstata ΔEorba ΔEprep De Fe 3 3 3 C6H6 (A˚)

3c

3a

[CpFe(C6H6)]þ

-84.1 187.7 -116.2 (42.8%) -155.6 (57.2%) 15.1 69.0 1.606 b

-88.3 185.0 -115.2 (42.1%) -158.3 (57.9%) 10.7 77.5 1.588 b 1.586 c -581.1

-105.8 162.3 -103.7 (38.7%) -164.4 (61.3%) 5.3 100.5 1.549 b 1.543-1.574 c -567.3

ΔEint(L- 3 3 3 Fe2þ) -587.9 in free [(L)Fe]þd

a Values in parentheses give the percentage contribution to the total attractive interactions. b Calculated. c XRD. d Fe2þ valence electron configuration [dz2, dxy, dx2-y2]6.

static attraction (ΔEelstat) for 3a and 3c is higher than that for [CpFe(C6H6)]þ (by 12 and 13 kcal mol-1, respectively). However, the attractive orbital interaction (ΔEorb) is lower by 6 and 9 kcal mol-1, and in addition, the Pauli repulsion is higher by 23 and 25 kcal mol-1. As a result, the Fe-C6H6 interaction energies ΔEint for 3a and 3c are lower (by 17 and 21 kcal mol-1) than that for [CpFe(C6H6)]þ. The ΔEint values correlate well with the Fe 3 3 3 C6H6 distances. Noteworthy, [(L)Fe]þ fragments with stronger L- 3 3 3 Fe2þ interaction (last entry in Table 6) form weaker Fe-C6H6 bonds. The preparation energies for 3a and 3c are higher by 5 and 10 kcal mol-1 than for [CpFe(C6H6)]þ (owing to the presence of bulky substituents in the carborane ligands), facilitating dissociation of the Fe-C6H6 bond. The NMe3-substituted complex 3c proved to be the most labile in this series. Finally, the energy partitioning suggests that the attractive interactions between [(L)Fe]þ and C6H6 fragments are ca. 60% covalent and 40% electrostatic, being slightly more electrostatic for the carborane ligands. Orientation of the SMe2 Substituent. The twist of the SMe2 substituent in anion 1a and its metal complexes has been previously described by Welch et al. by the parameter τ, the torsion angle C(8)-B(9)-S(1)-S(lone pair) calculated as a half of the sum of the C(8)-B(9)-S(1)-C(9) and C(8)B(9)-S(1)-C(10) angles (see Figure 1 or 2).4,30 As follows from DFT calculations, in anion 1a (10.6) and its parent analogue [C2B9H10(SH2)]- (2.8) τ adopts small values. This has been previously ascribed by Welch to a S(lone pair) 3 3 3 H(8)δþ bonding interaction. Let us first consider the parent anion [C2B9H10(SH2)](Figure 9). In conformer A the sulfur lone pair is located nearly opposite the C(8) atom (when viewed along the S-B bond), whereas in conformer B it is opposite the middle of the B(5)-B(10) bond. The different orientation is explained by the higher positive NBO charge at the hydrogen atom of the CH vertex (0.26) than of BH (ca. 0.05), making the electrostatic attraction between the lone pair and CH hydrogen stronger. Both conformers for the parent anion are almost equal in energy. However, in the case of anion 1a the conformer of type A is slightly more favorable than of type B (by 1.77 kcal mol-1). This can be explained by the stronger electrostatic repulsion between positively charged hydrogen atoms of the cage and of the methyl groups in the case of conformer B. (30) Cowie, J.; Hamilton, E. J. M.; Laurie, J. C. V.; Welch, A. J. J. Organomet. Chem. 1990, 394, 1–13.

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by 17-21 kcal mol-1. The orientation of the SMe2 substituent in anion 1a and its complexes is caused by minimization of electrostatic and steric repulsion.

Experimental Section

Figure 9. Conformers of the parent anion [9-SH 2-7,8C2B9H10]-.

In metal complexes the orientation of the SMe2 substituent is additionally influenced by steric demands of auxiliary ligands. Indeed, in uncrowded complexes, e.g., (1a)Mn(CO)3 (-4.1)30 and (1a)Rh(CO)2 (-13.1),31 τ adopts small values, as in the free anion 1a. However, in complexes with bulky ligands, e.g., (1a)Co(C4Me4) (-36.6)7a and (1a)RuCp* (-33.3),4 τ has large absolute values (the sign, minus or plus, depends on the enantiomer). In complexes studied in this work, 3a (calcd -36.8, XRD -38.6), 2a (calcd -29.3, XRD -35.6), and 8a (calcd -25.5), τ also has large absolute values. Conformers with opposite orientation of the SMe2 substituent are less stable by 1.03, 1.70, and 1.67 kcal mol-1, respectively, making their formation unfavorable.

Conclusion Photochemical reaction of the (cyclohexadienyl)iron complex [(η5-C6H7)Fe(η-C6H6)]þ with the carborane anions 1a-c followed by elimination of H- allows one to obtain new (benzene)ferracarboranes 3a-c. The benzene ligand in cation 3a is replaced under visible light irradiation, similar to [CpFe(η-C6H6)]þ, making it a useful synthon of the ferracarborane fragment [(η-9-SMe2-7,8-C2B9H10)Fe]þ. M€ ossbauer spectroscopy in an external magnetic field shows unambiguously that the cation in complex 3aBPh4 does not involve substantial removal of an electron from the metal center, whereas such electron removal in 4PF6 is much more pronounced and leads to a negative quadrupole splitting of the iron resonance. Electrochemical investigation of the electron-transfer ability of the complexes under study gives evidence that the (cyclohexadienyl)ferracarborane complexes 2a-c convert to the corresponding (benzene)ferracarborane complexes 3a-c by two-electron oxidation. Computation satisfactorily predicts the redox potentials for the electron-transfer processes displaying features of complete or partial chemical reversibility. The electrostatic potentials at iron and carbon nuclei of the Fe(ring) unit suggest that anions 1a,c are stronger donors than Cp- in cationic complexes, but weaker donors in the neutral species. The NMe3-substituted carborane ligand 1c is a slightly stronger donor than the SMe2 derivative 1a in all cases. In accordance with the energy decomposition scheme, the attractive interactions between [(L-C2B9H10)Fe]þ and C6H6 fragments in complexes 3a,c are ca. 60% covalent and 40% electrostatic, being slightly more electrostatic than the similar interaction with [CpFe]þ. The benzene bonding with the ferracarborane cations is weaker than that with [CpFe]þ (31) Douek, N. L.; Welch, A. J. J. Chem. Soc., Dalton Trans. 1993, 1917–1925.

General Procedures. The reactions were carried out under an inert atmosphere in dry solvents. The isolation of products was conducted in air. Starting complex [(η5-C6H7)Fe(ηC6H6)]PF614b and sodium salts of the carborane anions 1a-c7a were prepared as described in the literature. 1H, 11B{1H}, and 31 P{1H} NMR spectra were recorded with a Bruker Avance-400 spectrometer operating at 400.13, 128.38, and 161.98 MHz, respectively. Synthesis of (η-9-L-7,8-C2B9H10)Fe(η5-C6H7) (2a,c). A solution of Na[1a,c] (0.88 mmol) in THF was added to a solution of complex [(η5-C6H7)Fe(η-C6H6)]PF6 (300 mg, 0.84 mmol) in acetonitrile (10 mL). The reaction mixture was irradiated in a Schlenck tube using a high-pressure mercury vapor lamp with a phosphor-coated bulb (total power 650 W) for 6 h. Both the tube and the lamp were placed into a vessel of an appropriate volume covered inside with aluminum foil; cooling was accomplished by running water. Then the reaction mixture was filtered and the solvent was removed in vacuo. The residue was washed with methanol (3  1.5 mL) and reprecipitated from CH2Cl2 with petroleum ether. Complexes 2a,c were obtained as red solids. 2a, L = SMe2: yield 195 mg (71%). Anal. Calcd for C10H23B9FeS: C, 36.56; H, 7.06; B, 29.62. Found: C, 36.54; H, 7.19; B, 29.29. 1H NMR (acetone-d6): δ 6.25 (m, 1H, C6H7), 4.81 (m, 1H, C6H7), 4.40 (m, 1H, C6H7), 3.17 (s, 1H, CH), 3.09 (s, 3H, SMe2), 3.03 (s, 1H, CH), 2.85 (s, 3H, SMe2), 2.69 (m, 1H, C6H7), 2.49 (m, 1H, C6H7), 1.57 (d, 1H, C6H7). 11B{1H} NMR (acetone-d6): δ -3.4 (2B), -11.2 (1B), -13.0 (1B), -14.3 (1B), -15.7 (1B), -23.7 (1B), -25.3 (1B), -28.7 (1B). 2c, L = NMe3: yield 188 mg (69%). Anal. Calcd for C11H26B9FeN: C, 40.59; H, 8.05; B, 29.89; N, 4.30. Found: C, 40.06; H, 8.31; B, 29.99; N, 4.17. 1H NMR (acetone-d6): δ 6.19 (m, 1H, C6H7), 5.00 (m, 1H, C6H7), 4.46 (m, 1H, C6H7), 3.53 (s, 9H, NMe3), 3.47 (s, 1H, CH), 2.91 (s, 1H, CH), 2.70 (m, 1H, C6H7), 2.54 (m, 2H, C6H7), 1.50 (d, 1H, C6H7). 11B{1H} NMR (acetone-d6): δ 6.2 (1B), -4.4 (1B), -12.0 (1B), -13.7 (2B), -14.2 (1B), -24.4 (1B), -25.9 (1B), -28.7 (1B). Synthesis of [(η-9-L-7,8-C2B9H10)Fe(η-C6H6)]PF6 ([3a,c]PF6). A mixture of 2a,c (0.27 mmol) and concentrated HCl (0.2 mL) in acetone (3 mL) was stirred in the dark for 1 h. Then an aqueous NH4PF6 solution was added. The orange precipitate that formed was filtered off, washed with dichloromethane (4  1 mL), and reprecipitated from acetone with diethyl ether. Complexes [3a,c]PF6 were obtained as orange solids. [3a]PF6, L = SMe2: yield 101 mg (79%). Anal. Calcd for C10H22B9F6FePS: C, 25.42; H, 4.69; B, 20.59. Found: C, 25.49; H, 4.71; B, 20.62. 1H NMR (acetone-d6): δ 6.92 (s, 6H, C6H6), 5.42 (s, 1H, CH), 4.58 (s, 1H, CH), 2.95 (s, 3H, SMe2), 2.78 (s, 3H, SMe2). 11B{1H} NMR (acetone-d6): δ -1.2 (1B), -1.7 (1B), -4.1 (1B), -7.1 (1B), -8.8 (1B), -11.5 (1B), -19.1 (1B), -21.1 (1B), -24.7 (1B). [3c]PF6, L = NMe3: yield 80 mg (63%). Anal. Calcd for C11H25B9F6FeNP: C, 28.14; H, 5.37; B, 20.73; N, 2.98. Found: C, 27.98; H, 5.36; B, 20.39; N, 2.94. 1H NMR (acetone-d6): δ 7.05 (s, 6H, C6H6), 5.57 (s, 1H, CH), 4.62 (s, 1H, CH), 3.39 (s, 9H, NMe3). 11B{1H} NMR (acetone-d6): δ 5.4 (1B), -1.5 (2B), -8.8 (2B), -10.1 (1B), -20.1 (1B), -21.2 (1B), -25.9 (1B). Using in this reaction iodine or NBS (in methylene chloride or methanol) instead of HCl also results in [3a,c]PF6 with yields of 30-70%. One-Pot Synthesis of [(η-9-SMe 2-7,8-C2 B9Me 2H 8)Fe(η-C6H6)]PF6 ([3b]PF6). A solution of Na[1b] (0.85 mmol) in THF was added to a solution of complex [(η5-C6H7)Fe(η-C6H6)]PF6 (300 mg, 0.84 mmol) in acetonitrile (10 mL). The

Article reaction mixture was irradiated in a Schlenck tube using a highpressure mercury vapor lamp with a phosphor-coated bulb (total power 650 W) for 6 h, with cooling by running water. Then the reaction mixture was filtered and the solvent was removed in vacuo. Concentrated HCl (0.4 mL) in acetone (5 mL) was added to the residue. The reaction mixture was stirred in the dark for 1 h, and an aqueous NH4PF6 solution was added. An orange solid of [3b]PF6 was isolated as described for [3a,c]PF6. Yield: 206 mg (49%). Anal. Calcd for C12H26B9F6FePS: C, 28.80; H, 5.24; B, 19.44. Found: C, 28.81; H, 5.26; B, 19.44. 1H NMR (acetone-d6): δ 6.91 (s, 6H, C6H6), 3.13 (s, 3H, CMe), 2.85 (s, 3H, CMe), 2.81 (s, 3H, SMe2), 2.52 (s, 3H, SMe2). 11B{1H} NMR (acetone-d6): δ -1.2 (3B), -5.0 (1B), -7.5 (1B), -12.4 (1B), -13.4 (1B), -14.4 (1B), -15.9 (1B). Reaction of [3a]PF6 with NaBH4. A mixture of [3a]PF6 (45 mg, 0.09 mmol) and NaBH4 (8 mg, 0.21 mmol) in THF (5 mL) was stirred for 2 h at 0 C. The solvent was removed in vacuo. The residue was dissolved in CH2Cl2 and filtered. Complex 2a was precipitated by petroleum ether. Yield: 26 mg (84%). Synthesis of [(η-9-SMe2-7,8-C2B9H10)Fe(L)3]PF6 ([4,5]PF6). A solution of [3a]PF6 (70 mg, 0.15 mmol) and tBuNC or P(OMe)3 (0.15 mL) in acetonitrile (2 mL) was irradiated with the use of a 150 W incandescent lamp at 0 C for 1.5 h. The solvent was removed in vacuo. The residue was reprecipitated from CH2Cl2 with diethyl ether. Complexes [4,5]PF6 were obtained as yellow solids. [4]PF6, L = tBuNC: yield 91 mg (95%). Anal. Calcd for C19H43B9F6FePN3S: C, 35.45; H, 6.73; B, 15.11; N, 6.53. Found: C, 35.31; H, 6.74; B, 14.94; N, 6.37. 1H NMR (acetone-d6): δ 4.18 (s, 1H, CH), 3.26 (s, 1H, CH), 2.89 (s, 3H, SMe2), 2.73 (s, 3H, SMe2), 1.63 (s, 27H, tBuNC). 11B{1H} NMR (acetone-d6): δ -0.3 (1B), -2.6 (1B), -8.0 (1B), -8.7 (1B), -11.3 (1B), -12.7 (1B), -17.5 (1B), -20.1 (1B), -26.0 (1B). [5]PF6, L = P(OMe)3: yield 73 mg (64%). Anal. Calcd for C13H43B9F6FeP4O9S: C, 20.37; H, 5.65; B, 12.69. Found: C, 20.51; H, 5.64; B, 12.69. 1H NMR (acetone-d6): δ 4.29 (s, 1H, CH), 3.93 (s, 27H, P(OMe)3), 2.64 (s, 1H, CH), 2.50 (s, 6H, SMe2). 11B{1H} NMR (acetone-d6): δ -3.0 (1B), -4.5 (1B), -8.7 (1B), -10.9 (2B), -15.2 (1B), -16.6 (1B), -20.9 (1B), -26.6 (1B). 31P{1H} NMR (acetone-d6): δ 159.9. Synthesis of (η-9-SMe2-7,8-C2B9H10)Fe(η-9-NMe3-7,8C2B9H10) (6). A solution of [3a]PF6 (100 mg, 0.21 mmol) in acetonitrile (3 mL) was cooled to -40 C, and Na[1c] (0.23 mmol) in THF was added. The reaction mixture was irradiated with the use of a 150 W incandescent lamp at -40 C for 1.5 h. The solvent was removed in vacuo, and the residue was eluted through the silica gel column (18  1 cm) with petroleum ether/CH2Cl2 (1:2). The second purple band was collected, and the solvent was removed in vacuo. The mixture of 6 and (η-9-SMe2-7,8C2B9H10)2Fe (7) in 2:1 ratio was obtained as a purple solid, yield 36 mg (51%). 1H NMR (acetone-d6): δ 4.57 (s, 1H, CH), 3.97 (s, 1H, CH), 3.39 (s, 9H, NMe3), 3.15 (s, 2H, CH), 2.89 (s, 3H, SMe2), 2.61 (s, 3H, SMe2). 11B{1H} NMR (acetone-d6): δ 5.0 (1B), -4.4 (2B), -6.3 (1B), -8.9 (3B), -11.0 (1B), -11.7 (2B), -12.4 (1B), -14.3 (2B), -23.0 (2B), -24.4 (1B), -25.7 (1B), -27.3 (1B). M€ossbauer Effect Spectroscopy. ME data were accumulated in normal transmission geometry using a 50 mCi 57Co(Rh) source at room temperature. Sample temperatures were monitored using the DASWIN program detailed earlier.32 Temperature control over the ME data acquisition times (up to 24 h) was estimated at better than (0.2 K. All IS data are with respect to the centroid of an R-Fe room-temperature absorption spectrum, which was also used for spectrometer calibration. The in-field experiments were carried out at room temperature as described previously.22 (32) Glaberson, W.; Brettschneider; M. http://www.phys.huji.ac.il/ ∼glabersn.

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Electrochemistry and Mass Spectrometry. Anhydrous 99.8% dichloromethane was an Aldrich product. [NBu4]PF6 (Fluka, electrochemical grade) was used as supporting electrolyte (0.2 mol dm-3). Cyclic voltammetry was performed in a threeelectrode cell containing the working electrode surrounded by a platinum-spiral counter electrode and the reference electrode mounted with a Luggin capillary. Glassy carbon, platinum, and gold were used as materials of the working electrodes. A BAS 100 W electrochemical analyzer was used as polarizing unit. Controlled potential coulometry was performed in an H-shaped cell with anodic and cathodic compartments separated by a sintered-glass disk. The working macroelectrode was a platinum gauze; a mercury pool was used as the counter electrode. For low-temperature experiments the cell was enclosed by a thermostatic jacket through which a cooled liquid was circulated. An Ag/AgCl electrode, filled with the solution under investigation, was used as reference electrode. Under the present experimental conditions, the one-electron oxidation of ferrocene occurs at þ0.44 V. Electrospray measurements have been carried out on a LCQDECA ion trap (Thermo, Bremen, D). Operating conditions of the ESI source were as follows: spray voltage 4.5 kV; capillary temperature 200 C; sheath gas (nitrogen) flow rate ca. 0.75 L/ min. Solutions of the initial product and after electrolysis were introduced via direct infusion at a flow rate of 5 μL/min. MS/ MS product ion experiments have been carried out by using ultrapure helium as collision gas and collision energy in the range 0.5-1 eV (laboratory frame). Computational Details. The geometries have been optimized without constraints at the gradient-corrected DFT level of theory using the exchange functional of Becke33 and the correlation functional of Perdew34 (BP86). Uncontracted Slater-type orbitals were employed as basis functions for the SCF calculations.35 Scalar relativistic effects were considered using the zero-order regular approximation (ZORA).36 The all-electron ZORA relativistic valence triple-ζ basis set augmented by two polarization functions, TZ2P, was used. The bonding interactions were studied by means of Morokuma-Ziegler energy decomposition analysis.37 The calculations were carried out using the ADF 2006.01 program package.38 Natural charges were obtained with the Gaussian 98 program39 for the BP86/TZ2P optimized structures using the NBO scheme40 with BP86 functional and a basis set of triple-ζ quality (33) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100. (34) Perdew, J. P. Phys. Rev. B 1986, 33, 8822–8824. (35) Snijders, J. G.; Vernooijs, P.; Baerends, E. J. At. Data Nucl. Data Tables 1982, 26, 483–509. (36) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993, 99, 4597–4610. (37) (a) Morokuma, K. Chem. Phys. 1971, 55, 1236–1244. (b) Ziegler, T.; Rauk, A. Theor. Chim. Acta 1977, 46, 1–10. (38) (a) te Velde, G.; Bickelhaupt, F. M.; van Gisbergen, S. J. A.; Fonseca Guerra, C.; Baerends, E. J.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22, 931–967. (b) ADF 2006.01; SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands, http://www. scm.com. (39) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98 (Revision A.6); Gaussian, Inc.: Pittsburgh, PA, 1998. (40) (a) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899–926. (b) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO Version 3.1.

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Table 7. Crystallographic Data and Structure Refinement Parameters for 2a, [3a]BPh4, and [6]0.65[7]0.35 2a empirical formula molecular wt cryst color, habit cryst size (mm) cryst syst space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Dcalcd (g cm-3) diffractometer T, K θ range (deg) μ (cm-3) absorp corr Tmax/Tmin collected reflns indep reflns obsd reflns (I > 2σ(I)) params R1 (on F for obsd reflns) wR2 (on F2 for all reflns) weighting scheme a b F(000) goodness-of-fit largest diff peak and hole (e A˚-3)

[3a]BPh4

C10H23B9FeS C34H42B10FeS 328.48 646.69 red plate yellow prism 0.40  0.35  0.20 0.46  0.25  0.13 orthorhombic monoclinic P21/n P212121 8.8361(4) 12.903(4) 12.1482(6) 15.953(5) 14.5361(7) 17.252(4) 90 90 90 111.018(10) 90 90 1560.34(13) 3314.8(17) 4 4 1.398 1.296 Bruker SMART 1000 CCD Bruker Apex II CCD 120 100 2.70-30.00 1.71-28.00 10.79 5.44 multiscan 0.8131/0.6720 0.934/0.846 10 722 20 833 7978 (Rint = 0.0709) 4538 (Rint = 0.0212) 4307 4963 193 417 0.0235 0.0493 0.0612 0.1075 w-1 = σ2(Fo2) þ (aP)2 þ bP, P = 1/3(Fo2þ 2Fc2) 0.0400 0.0320 0.0000 2.1000 680 1352 1.014 0.998 0.303 and -0.329 0.390 and -0.564

with two polarization functions, def2-TZVPP.41 Electrostatic potentials at nuclei were calculated at the same level of theory and at BP86/TZ2P. The redox potentials relative to SCE (Eredox) were calculated using Eredox = [-(Ered - Eox) - 4.68]/n, where Ered and Eox are energies (in eV) of the reduced and oxidized species including solvation and n is the number of electrons (equal to 1 in our case). The value 4.68 corresponds to the absolute potential of the reference electrode (SCE).42 The solvent (CH2Cl2) effects were included using either the polarizable continuum model (PCM)43 or the conductor-like screening model (COSMO).44 The PCM and COSMO calculations were performed by Gaussian 98 and ADF 2006.01 programs, respectively. The default settings were employed in both cases. The ChemCraft program45 was used for molecular modeling and visualization. X-ray Diffraction Study. Crystals of compounds 2a, [3a]BPh4, and 6 were grown by slow diffusion of ether into solutions of the complexes in CH2Cl2. The principal crystallographic data, procedures for collecting experimental data, and characteristics of structure refinement are listed in Table 7. Single-crystal X-ray diffraction experiments were carried out with Bruker SMART 1000 CCD or Bruker Apex II CCD area detectors, using graphite-monochromated Mo KR radiation (λ = 0.71073 A˚). Absorption corrections were integrated using SADABS or (41) (a) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. (b) http://www.ipc.uni-karlsruhe.de/tch/tch1/TBL/tbl.html. (42) Trasatti, S. Pure Appl. Chem. 1986, 58, 955–966. (43) Mierts, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117– 129. €rmann, G. J. Chem. Soc., Perkin Trans. 2 1993, (44) Klamt, A.; Sch€ uu 799–805. (45) Zhurko, G. A. ChemCraft 1.6, http://www.chemcraftprog.com, 2008. (46) (a) SADABS, Bruker/Siemens Area Detector Absorption Correction Program, v.2.01; Bruker AXS; Madison, WI, 2004. (b) APEX2 Softwarwe Package; Bruker AXS Inc.: Madison, WI, 2005.

[6]0.65[7]0.35 C9.65H35.95B18Cl2FeN0.65S1.35 525.85 red prism 0.45  0.30  0.20 orthorhombic P212121 13.8588(8) 17.5889(10) 21.1295(12) 90 90 90 5150.5(5) 8 1.356 100 2.25-28.94 9.05 0.840/0.686 58 245 12 382 (Rint = 0.0529) 11 408 673 0.0335 0.0789 0.0345 2.3000 2160 0.997 0.802 and -0.568

APEX2 software.46 The structures were solved by the direct method and refined by the full-matrix least-squares against F2hkl in isotropic approximation for non-hydrogen atoms. The 6 crystal contains two crystallographically independent 6 and solvate molecules. Two types of iron(II) complexes in the structure of 6 were found, namely, C9H35B18FeNS and C8H32B18FeS2. The complexes were refined as disorder of NMe3 and SMe2 moieties for both independent molecules and the site occupancy factors of these groups were stated to be 0.65 and 0.35, respectively. The hydrogen atoms of the BH groups were found in the difference Fourier synthesis, and the positions of other hydrogen atoms were calculated. All hydrogen atoms were refined in isotropic approximation in a riding model with the Uiso(H) parameters equal to 1.5Ueq(Ci) for methyl groups and to 1.2Ueq(Cii) and 1.2Ueq(Bi) for other atoms, where Ueq(B) and Ueq(C) are the equivalent thermal parameters of the atoms to which the corresponding H atoms are bound. All calculations were performed on an IBM PC/AT using the SHELXTL PLUS 5 software.47 CCDC-738655 (for 2a), -738656 (for [3a]BPh4), and -738657 (for [6]0.65[7]0.35) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif.

Acknowledgment. This work was supported by the General Chemistry and Material Science Division of the Russian Academy of Sciences. A.V.V. thanks the President of the Russian Federation for the grant (Project MK-966.2008.3). This research has been partially supported by the Israel Science Foundation (ISF, 2004 grant (47) SHELXTL v. 5.10, Structure Determination Software Suite; Bruker AXS: Madison, WI, 1998.

Article

number 618/04) and is herewith gratefully acknowledged. The financial support of the University of Siena (PAR 2007) is gratefully acknowledged. Supporting Information Available: Crystallographic information (cif files) for compounds 2a, 3aBPh4, and [6]0.65[7]0.35;

Organometallics, Vol. 29, No. 10, 2010

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details of DFT calculations for anions [9-SH2-7,8-C2B9H10]-, 1a,c, complexes [2a,c]0/þ, [3a,c]þ/0, [8a,c]0/þ, and their cyclopentadienyl analogues (atomic coordinates for optimized geometry, energy data); Figures S1-S5 and the discussion of the chemical complication following the anodic oxidation of 4. This material is available free of charge via the Internet at http://pubs.acs.org.