Synthesis of μ-Diborolyl Triple-Decker Complexes by Electrophilic

Apr 17, 2009 - Alexander R. Kudinov , Piero Zanello , Rolfe H. Herber , Dmitry A. Loginov , Mikhail M. Vinogradov , Anna V. Vologzhanina , Zoya A. Sta...
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Organometallics 2009, 28, 2707–2715

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Synthesis of µ-Diborolyl Triple-Decker Complexes by Electrophilic Stacking. Similar Bonding Properties of Anions [CpCo(1,3-C3B2H5)]- and Cp- toward Transition Metals† Walter Siebert,*,‡ Alexander R. Kudinov,*,§ Piero Zanello,*,| Mikhail Yu. Antipin,§,⊥ Vyacheslav V. Scherban,‡,§ Alexander S. Romanov,§ Dmitry V. Muratov,§ Zoya A. Starikova,§ and Maddalena Corsini| Anorganisch-Chemisches Institut der UniVersita¨t Heidelberg, 69120 Heidelberg, Germany, Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow, Russian Federation, Dipartimento di Chimica, UniVersita` di Siena, 53100 Siena, Italy, and Department of Natural Sciences, New Mexico Highlands UniVersity, Las Vegas, New Mexico 87701 ReceiVed January 14, 2009

Triple-decker complexes with a bridging diborolyl ligand CpCo(µ-1,3-C3B2Me5)M(ring) (M(ring) ) RuCp, 4; RuCp*, 5; Co(C4Me4), 6) were synthesized by electrophilic stacking of the sandwich anion [CpCo(1,3-C3B2Me5)]- with the [(ring)M(MeCN)3]+ cations. Structures of 4-6 were confirmed by X-ray diffraction. The electrochemical and spectroelectrochemical behavior of the complexes prepared was studied. DFT calculations of the redox potentials were also performed. Similar bonding properties of anions [CpCo(1,3-C3B2R5)]- and [C5R5]- (R ) H, Me) toward [M(ring)]+ cations were established both experimentally (synthesis, electrochemistry, and X-ray diffraction) and theoretically (energy decomposition and Mulliken population analysis). Introduction Electrophilic stacking of sandwich compounds with cationic fragments [M(ring)]n+ (n ) 1, 2) is widely used for the synthesis of triple-decker complexes.1 In particular, this method proved to be very effective for the preparation of complexes with bridging monoboron heterocycles, borole2 and boratabenzene.3 Grimes et al. have synthesized a variety of complexes with triboron C2B3 rings using a similar approach.4 The radical of the diboraheterocycle 2,3-dihydro-1,3-diborolyl, C3B2H5 (1), is a 3e donor and 3e acceptor. Its unique balance of donor and acceptor abilities favors a bifacial bonding with metal atoms, as documented by using this ligand in triple-, tetra-, penta-, and even hexa-decker complexes.5 µ-Diborolyl tripledecker complexes are generally synthesized by nucleophilic stacking of diborolyl and diborole sandwich compounds CpNi(1,3C3B2R5) and CpCo(1,3-C3B2R5H) (2) with neutral fragments (FeCp, CoCp, NiCp). For instance, stacking of 2 with CpFe(cod) (cod ) 1,5-cyclooctadiene) yields the diamagnetic 30 valence electron (VE) iron-cobalt triple-decker complexes CpCo(µ-1,3C3B2R5)FeCp, and the reaction of CpNi(1,3-C3B2R5) with CpCoL2 (L ) CO, C2H4) gives paramagnetic 32 VE cobalt-nickel analogues CpCo(µ-1,3-C3B2R5)NiCp.6 The anionic sandwich complexes 3 (obtained by deprotonation of 2)6,7 react with metal †

µ-Diborolyl Triple-Decker Complexes. Part 1. * To whom correspondence should be addressed. E-mail: walter.siebert@ urz.uni-heidelberg.de (W.S.); [email protected] (A.K.); [email protected] (P.Z.). ‡ Universita¨t Heidelberg. § Russian Academy of Sciences. | Universita` di Siena. ⊥ New Mexico Highlands University. (1) (a) Kudinov, A. R.; Rybinskaya, M. I.; Struchkov, Yu. T.; Yanovskii, A. I.; Petrovskii, P. V. J. Organomet. Chem. 1987, 336, 187–197. (b) Herberich, G. E.; Englert, U.; Marken, F.; Hofmann, P. Organometallics 1993, 12, 4039–4045. (c) Kudinov, A. R.; Rybinskaya, M. I. IzV. Akad. Nauk, Ser. Khim. 1999, 1636–1642. [Russ. Chem. Bull. 1999, 48, 16151621 (Engl. Transl.)].

halides, leading to tetra-decker complexes CpCo(µ-1,3C3B2R5)M(µ-1,3-C3B2R5)CoCp (M ) Cr, Mn, Fe, Co, Ni, Zn, and Sn).7 Three-component reaction of anion 3, dianion [7,8C2B9H11]2-, and CoCl2 gives the metallacarborane triple-decker complex CpCo(µ-1,3-C3B2R5)Co(C2B9H11).8 A number of analogous compounds containing different heteroborane ligands have been also obtained.9 However, despite the rich chemistry of anions 3, their electrophilic stacking with [M(ring)]n+ fragments was not studied. Herein we describe the first examples of such reactions, which proved to be very effective for the preparation of triple-decker complexes. In addition, we examined the (2) (a) Herberich, G. E.; Hessner, B.; Saive, R.; Ko¨ffer, D. P. J.; Howard, J. A. K. Angew. Chem. 1986, 98, 177–178. [Angew. Chem., Int. Ed. Engl. 1986, 25, 165 (Engl. Transl.)]. (b) Herberich, G. E.; Hessner, B; Saive, R. J. Organomet. Chem. 1987, 319, 9–27. (c) Herberich, G. E.; Hausmann, I.; Klaff, N. Angew. Chem. 1989, 101, 328–329. [Angew. Chem., Int. Ed. Engl. 1989, 28, 319-320 (Engl. Transl.)]. (d) Herberich, G. E.; Dunne, B. J.; Hessner, B. Angew. Chem. 1989, 101, 798–800. [Angew. Chem., Int. Ed. Engl. 1989, 28, 737-738 (Engl. Transl.)]. (e) Herberich, G. E.; Hessner, B.; Saive, R. J. Organomet. Chem. 1989, 362, 243–257. (f) Herberich, G. E.; Bu¨schges, U.; Dunne, B. J.; Hessner, B.; Klaff, N.; Ko¨ffer, D. P. J.; Peters, K. M. J. Organomet. Chem. 1989, 372, 53–60. (g) Herberich, G. E.; Ko¨ffer, D. P. J.; Peters, K. M. Chem. Ber. 1991, 124, 1947–1952. (h) Kudinov, A. R.; Loginov, D. A.; Muratov, D. V.; Petrovskii, P. V. IzV. Akad. Nauk, Ser. Khim. 2001, 1267–1268. [Russ. Chem. Bull. 2001, 50, 1332-1333 (Engl. Transl.)]. (i) Loginov, D. A.; Muratov, D. V.; Petrovskii, P. V.; Starikova, Z. A.; Corsini, M.; Laschi, F.; de Biani Fabrizi, F.; Zanello, P.; Kudinov, A. R. Eur. J. Inorg. Chem. 2005, 1737–1746. (j) Muratov, D. V.; Petrovskii, P. V.; Starikova, Z. A.; Herberich, G. E.; Kudinov, A. R. J. Organomet. Chem. 2006, 691, 3251–3259. (k) Loginov, D. A.; Muratov, D. V.; Starikova, Z. A.; Petrovskii, P. V.; Kudinov, A. R. J. Organomet. Chem. 2006, 691, 3646–3651. (l) Loginov, D. A.; Muratov, D. V.; Perekalin, D. S.; Starikova, Z. A.; Petrovskaya, E. A.; Gutsul, E. I.; Kudinov, A. R. Inorg. Chim. Acta 2008, 361, 1715–1721. (m) 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.)]. (3) (a) Herberich, G. E.; Englert, U.; Pubanz, D. J. Organomet. Chem. 1993, 459, 1–9. (b) Herberich, G. E.; Englert, U.; Ganter, B.; Lamertz, C. Organometallics 1996, 15, 5236–5241. (c) Kudinov, A. R.; Loginov, D. A.; Starikova, Z. A.; Petrovskii, P. V. J. Organomet. Chem. 2002, 649, 136– 140. (d) Loginov, D. A.; Starikova, Z. A.; Petrovskaya, E. A.; Kudinov, A. R. J. Organomet. Chem. 2009, 694, 157-160.

10.1021/om900032z CCC: $40.75  2009 American Chemical Society Publication on Web 04/17/2009

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Chart 1

Scheme 1

Figure 1. Molecular structure of compound 4. Ellipsoids are shown at the 50% level. Hydrogen atoms are omitted for clarity.

Scheme 2

electrochemical behavior of the complexes synthesized, as well as their structural and bonding features.

Results and Discussion Synthesis. The [Ru(C5R5)]+ fragments are widely used for the synthesis of triple-decker complexes.1,10 We found that electrophilic stacking of the anionic pentamethyl derivative 3b with [(C5R5)Ru(MeCN)3]+ cations affords the CoRu tripledecker complexes CpCo(µ-1,3-C3B2Me5)Ru(C5R5) (4 and 5) in (4) (a) Davis, J. H., Jr.; Sinn, E.; Grimes, R. N. J. Am. Chem. Soc. 1989, 111, 4776–4784. (b) Fessenbecker, A.; Atwood, M. D.; Bryan, R. F.; Grimes, R. N.; Woode, M. K.; Stephan, M.; Zenneck, U.; Siebert, W. Inorg. Chem. 1990, 29, 5157–5163. (c) Chase, K. J.; Bryan, R. F.; Woode, M. K.; Grimes, R. N. Organometallics 1991, 10, 2631. (d) Attwood, M. A.; Fonda, K. K.; Grimes, R. N.; Brodt, G.; Hu, D.; Zenneck, U.; Siebert, W. Organometallics 1989, 8, 1300. (e) Grimes, R. N. Chem. ReV. 1992, 92, 251–268. (f) Grimes, R. N. Coord. Chem. ReV. 1995, 143, 71–96. (5) For reviews see:(a) Siebert, W. AdV. Organomet. Chem. 1980, 18, 301–340. (b) Siebert, W. Angew. Chem. 1985, 97, 924–939. [Angew. Chem., Int. Ed. Engl. 1985, 24, 943-958 (Engl. Transl.)]. (c) Siebert, W. Usp. Khim. 1991, 60, 1553–1569. [Russ. Chem. ReV. 1991, 60, 784-791 (Engl. Transl.)]. (d) Siebert, W. AdV. Organomet. Chem. 1993, 35, 187–210. (6) Edwin, J.; Bochmann, M.; Bo¨hm, M. C.; Brennan, D. E.; Geiger, W. E.; Kru¨ger, C.; Pebler, J.; Pritzkow, H.; Siebert, W.; Swiridoff, W.; Wadepohl, H.; Weiss, J.; Zenneck, U. J. Am. Chem. Soc. 1983, 105, 2582– 2598. (7) (a) Edwin, J.; Bo¨hm, M. C.; Chester, N.; Hoffman, D. M.; Hoffmann, R.; Pritzkow, H.; Siebert, W.; Stumpf, K.; Wadepohl, H. Organometallics 1983, 2, 1666–1674. (b) Siebert, W.; Edwin, J.; Wadepohl, H.; Pritzkow, H. Angew. Chem. 1982, 94, 148. [Angew. Chem., Int. Ed. Engl. 1982, 21, 149-150 (Engl. Transl.)]. (c) Enders, M.; Gangnus, B.; Hettrich, R.; MagosMartin, Z.; Stephan, M.; Pritzkow, H.; Siebert, W.; Zenneck, U. Chem. Ber. 1993, 126, 2197–2203. (8) Forward, J. M.; Mingos, D. M. P.; Siebert, W.; Hauss, J.; Powell, H. R. J. Chem. Soc., Dalton Trans. 1993, 1783–1788. (9) (a) Weinmann, W.; Wolf, A.; Pritzkow, H.; Siebert, W.; Barnum, B. A.; Carroll, P. J.; Sneddon, L. G. Organometallics 1995, 14, 1911– 1919. (b) Weinmann, W.; Metzner, F.; Pritzkow, H.; Siebert, W.; Sneddon, L. G. Chem. Ber. 1996, 129, 213–217. (c) Weinmann, W.; Pritzkow, H.; Siebert, W.; Sneddon, L. G. Chem. Ber./Requeil 1997, 130, 329–333.

high yields (Scheme 1). Alternatively, complex 5 was prepared by reaction of 3b with [Cp*RuCl]4. Recently, very convenient synthons of the [Co(C4Me4)]+ fragment have been developed11 and successfully used for the synthesis of triple-decker complexes.10e,f,12 The reaction of anion 3b with [(C4Me4)Co(MeCN)3]+ affords the dicobalt complex CpCo(µ-1,3-C3B2Me5)Co(C4Me4) (6) in 40% yield (Scheme 2). The 30 VE complexes 4-6 are air-stable both in the solid state and in solution. Noteworthy, the reactions of cations [(ring)M(MeCN)3]+ with anion 3b are similar to those with cyclopentadienide anions, and moreover, the conditions of these reactions are almost the same, suggesting similar bonding properties of these anions. X-ray Diffraction Study. X-ray diffraction confirmed the triple-decker structure of complexes 4-6 (Figures 1, 2, and 3). Selected bond lengths and angles are given in Tables 1 and 2. The planes of the ring ligands are almost coplanar, the dihedral angles being C3B2/Cp(Co) 0.7° and C3B2/Cp(Ru) 2.5° for 4; C3B2/Cp 0.7° and C3B2/Cp* 0.1° for 5; and C3B2/Cp 0.3° and C3B2/C4Me4 2.3° for 6. In compounds 4 and 5 the C5R5(Ru) and C3B2 rings are eclipsed, whereas the Cp(Co) ring is staggered; in 6 the cross-orientation of two five-membered rings is perfectly eclipsed. The C3B2 ring in 4-6 is almost planar with a slight folding along the B · · · B line (0.4°, 0.6°, and 0.8° for 4, 5, and 6, respectively); the C4 atom is deviated toward the Co1 atom. The distances Co1 · · · C3B2 in 4 (1.583 Å), 5 (10) (a) Kudinov, A. R.; Petrovskii, P. V.; Struchkov, Yu. T.; Yanovsky, A. I.; Rybinskaya, M. I. J. Organomet. Chem. 1991, 421, 91–115. (b) Kudinov, A. R.; Loginov, D. A.; Petrovskii, P. V.; Rybinskaya, M. I. IzV. Akad. Nauk, Ser. Khim. 1998, 1625–1626. [Russ. Chem. Bull. 1998, 47, 1583-1584 (Engl. Transl.)]. (c) Kudinov, A. R.; Loginov, D. A.; Starikova, Z. A.; Petrovskii, P. V.; Corsini, M.; Zanello, P. Eur. J. Inorg. Chem. 2002, 3018–3027. (d) Mutseneck, E. V.; Petrovskii, P. V.; Kudinov, A. R. IzV. Akad. Nauk, Ser. Khim. 2004, 2003–2004. [Russ. Chem. Bull. 2004, 53, 2090-2091 (Engl. Transl.)]. (e) Mutseneck, E. V.; Starikova, Z. A.; Lyssenko, K. A.; Petrovskii, P. V.; Zanello, P.; Corsini, M.; Kudinov, A. R. Eur. J. Inorg. Chem. 2006, 4519–4527. (f) Mutseneck, E. V.; Wadepohl, H.; Kudinov, A. R.; Siebert, W. Eur. J. Inorg. Chem. 2008, 3320–3329. (11) (a) Kudinov, A. R.; Mutseneck, E. V.; Loginov, D. A. Coord. Chem. ReV. 2004, 248, 571–585. (b) Butovskii, M. V.; Englert, U.; Fil’chikov, A. A.; Herberich, G. E.; Koelle, U.; Kudinov, A. R. Eur. J. Inorg. Chem. 2002, 2656–2663. (c) Mutseneck, E. V.; Loginov, D. A.; Perekalin, D. S.; Starikova, Z. A.; Golovanov, D. G.; Petrovskii, P. V.; Zanello, P.; Corsini, M.; Laschi, F.; Kudinov, A. R. Organometallics 2004, 23, 5944–5957. (12) (a) 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.)]. (b) Kudinov, A. R.; Loginov, D. A.; Ashikhmin, S. N.; Fil’chikov, A. A.; Shul’pina, L. S.; Petrovskii, P. V. IzV. Akad. Nauk, Ser. Khim. 2000, 1647-1649 [Russ. Chem. Bull. 2000, 49, 1637-1639 (Engl. Transl.)].

µ-Diborolyl Triple-Decker Complexes

Organometallics, Vol. 28, No. 9, 2009 2709 Table 2. Selected Bond Lengths [Å] and Angles [deg] for Complex 6 Co(1)-B(1) Co(1)-C(4) Co(1)-C(5) Co(1)-C(3) Co(1)-C(2) Co(1)-C(1) Co(2)-B(1) Co(2)-C(4) Co(2)-C(5) Co(2)-C(9) Co(2)-C(10)

Figure 2. Molecular structure of compound 5. Ellipsoids are shown at the 50% level. Hydrogen atoms are omitted for clarity.

Figure 3. Molecular structure of compound 6. Ellipsoids are shown at the 50% level. Hydrogen atoms are omitted for clarity. Table 1. Selected Bond Lengths [Å] and Angles [deg] for Complexes 4 and 5 Co(1)-B(1) Co(1)-C(1) Co(1)-C(2) Co(1)-C(3) Co(1)-C(4) Co(1)-C(5) Ru(1)-B(1) Ru(1)-C(4) Ru(1)-C(5) Ru(1)-C(9) Ru(1)-C(10)

4

5

2.084(3) 2.057(4) 2.048(3) 2.037(3) 2.044(4) 2.059(3) 2.218(3) 2.183(4) 2.207(3) 2.157(4) 2.166(3)

2.092(5) 2.032(5) 2.040(5) 2.018(7) 2.055(5) 2.061(4) 2.234(5) 2.215(6) 2.203(4) 2.162(5) 2.163(4)

Ru(1)-C(11) B(1)-C(4) B(1)-C(5) C(5)-C(5A) B(1)-C(4)-B(1A) C(4)-B(1)-C(5) B(1)-C(5)-C(5A) Co · · · Cp Co · · · C3B2 Ru · · · C5R5 Ru · · · C3B2

4

5

2.171(3) 1.550(4) 1.595(4) 1.516(5) 111.8(3) 104.9(2) 109.23(15) 1.655 1.583 1.795 1.762

2.167(4) 1.562(6) 1.598(6) 1.490(8) 111.2(5) 104.6(3) 109.9(2) 1.654 1.590 1.788 1.776

(1.590 Å), and 6 (1.585 Å) are close to the corresponding distance in the related CoFe complex CpCo(µ-1,3C3B2Me2Et2H)FeCp (1.580 Å).6 Interestingly, the Co2 · · · C3B2 distance in 6 (1.647 Å) is considerably longer than Co1 · · · C3B2, which may be connected with a stronger covalent bonding of the C3B2 ring with CoCp (formally a 2e donor and 4e acceptor) as compared to Co(C4Me4) (a 1e donor and 5e acceptor). The bonds M-B (M ) Co, Ru) are longer than the bonds M-C(C3B2), in accordance with a general tendency for π-complexes with boron heterocycles.3c The Ru · · · Cp distance in 4 (1.795 Å) is close to the corresponding distance in RuCp2 (1.815 Å)13 and CpRuCp* (1.830 Å),14 whereas the Ru · · · Cp* distance in 5 (1.788 Å) is

2.079(4) 2.028(5) 2.054(4) 2.037(6) 2.047(5) 2.046(4) 2.125(4) 2.096(5) 2.095(3) 1.988(4) 1.978(3)

Co(2)-C(11) B(1)-C(4) B(1)-C(5) C(5)-C(5A) B(1)-C(4)-B(1A) C(4)-B(1)-C(5) B(1)-C(5)-C(5A) Co(1) · · · Cp Co(1) · · · C3B2Me5 Co(2) · · · C4Me4 Co(2) · · · C3B2Me5

1.978(4) 1.527(6) 1.574(5) 1.521(8) 112.7(5) 104.7(3) 108.9(2) 1.655 1.585 1.691 1.647

close to that in CpRuCp* (1.796 Å) and RuCp*2 (1.809 Å).15 Similarly, the Co · · · C4Me4 distance in 6 (1.691 Å) is close to that in the acetylcyclopentadienyl complex (C4Me4)Co(C5H4COMe) (1.683 Å).11c These facts suggest similar donoracceptor abilities of anions 3b and Cp-, which was further confirmed by DFT calculations (vide infra). Comparison of the two related structures 4 and 5 suggests that introduction of five methyl groups into the cyclopentadienyl ring leads to the strengthening of its bond with the ruthenium atom (metal-ring distances 1.795 and 1.788 Å, respectively) and weakening of the Ru-C3B2 bond (1.762 and 1.776 Å), analogously to the borole triple-decker complexes (C4H4BPh)Rh(µ-η5:η5-C4H4BPh)Ru(C5R5).2j The strengthening of the Ru-Cp bond is explained by its greater population due to donor effect of the methyl groups. It is accompanied by weakening of the bond with the second π-ligand, in accordance with twoside orientation of metal atom orbitals (trans effect). A similar pattern was revealed earlier for CpMCp* (M ) Fe, Ru).16 Electrochemistry. Figure 4, which refers to 4 and 6, exemplifies the overall electron-transfer ability of the present complexes. All the complexes substantially display a coulometrically measured one-electron oxidation (into 29 VE cations) possessing features of chemical reversibility, followed by a subsequent irreversible anodic step, which is affected by problems of absorption at the electrode surface, and a partially reversible, one-electron reduction (into 31 VE anions) very close to the solvent discharge. Representatively, analysis of the cyclic voltammetric responses of the first oxidation process of 4 with scan rates progressively increasing from 0.02 to 2.0 V s-1 shows that it involves a simple one-electron process. In fact, (i) the current ratio ipc/ipa is constantly equal to 1; (ii) the current function ipaV-1/2 stays substantially constant; and (iii) the peak-to-peak separation ranges from 62 to 120 mV.17 Confirming that the one-electron-oxidized product [4]+ is indefinitely stable, cyclic voltammetry on the exhaustively oxidized solution (Ew ) +0.6 V) displays a voltammetric profile

Figure 4. Cyclic voltammetric responses recorded at a platinum electrode in CH2Cl2 solutions of (a) 4 (0.9 × 10-3 mol dm-3); (b) 6 (1.0 × 10-3 mol dm-3). [NBu4][PF6] (0.2 mol dm-3) supporting electrolyte. Scan rate 0.2 V s-1.

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Figure 5. Spectral changes recorded in an OTTLE cell upon progressive oxidation of 4 at Ew ) +0.4 V (vs Ag pseudoreference electrode) in CH2Cl2 solution. [NBu4][PF6] (0.2 mol dm-3) supporting electrolyte.

quite complementary to the original one. As a consequence, the original blue solution turns green-maroon. In this connection, Figure 5 shows the UV-vis spectrophotometric trend recorded upon stepwise oxidation. As seen, the oxidation is accompanied by the progressive attenuation of the blue band (λmax ) 573 nm) and the progressive appearance of a charge-transfer band in the NIR region (λmax ) 889 nm). The concomitant appearance of the isosbestic point following the oxidation (λ ) 700 nm) further confirms the chemical reversibility of the process. Similar patterns have been observed for the 5/[5]+ and 6/[6]+ processes. Table 3 compiles the electrochemical characteristics of the mentioned electron-transfer processes together with the relative redox changes. Also reported are the electrochemical data pertinent to the related species CpCo(µ-1,3-C3B2Et4Me)FeCp (7),6,7c as well as the color changes observed upon exhaustive one-electron oxidations (spectroscopic details will be reported in Table 5). As seen, even though all the complexes bear the CpCo(1,3-C3B2R5) subunit, there is no common electron-transfer process, a finding that could preliminarily foresee charge delocalization inside such molecules, eventually reinforced by the constant HOMO-LUMO separation of about 2.2 eV (as resulting from the difference between the potentials of the first oxidation and the reduction).17 We can compare the oxidation potentials of 4-6 with the corresponding values for Cp*M(ring) complexes (Table 4) since the parent anion [CpCo(1,3-C3B2H5)]- is similar to the Cp- (vide infra). Replacement of Cp* with CpCo(1,3-C3B2Me5) seems to stabilize monocations, probably because of the charge delocalization over the two metal fragments. Table 4 also contains the DFT-calculated values. Independently from the solvation model (PCM or COSMO), computation satisfactory predicts the redox potentials (the maximum deviation from experimental value is 0.33 V). Noteworthy, only a few examples of such calculations for organometallic compounds are described in the literature.18 (13) Seiler, P.; Dunitz, J. D. Acta Crystallogr., Sect. B 1980, B36, 2946– 2950. (14) Zanin, I. E; Antipin, M.Yu.; Struchkov, Yu. T. Kristallografiya 1991, 36, 420–428. (15) Albers, M. O.; Liles, D. C.; Robinson, D. J.; Shaver, A.; Singleton, E.; Wiege, M. B.; Boeyens, J. C. A.; Levendis, D. C. Organometallics 1986, 5, 2321–2327. (16) Herberich, G. E.; Englert, U.; Marken, F.; Hofmann, P. Organometallics 1993, 12, 4039–4045. (17) Zanello, P. Inorganic Electrochemistry. Theory, Practice and Application; Royal Society of Chemistry: Cambridge, 2003.

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In order to support the possible electronic interaction between the two metal centers, further UV-vis and IR measurements (in OTTLE cells) have been carried out in different solvents. Figure 6 shows the UV-vis spectra exhibited by 5 in CH2Cl2, MeCN, and THF solutions, respectively. Table 5 compiles the spectral data for complexes 4-6. Since the location of the visible bands for each species is independent from the solvent (in particular, 4 and 5 absorb at the same wavelengths), we assign them to d-d transitions. In addition, given the different wavelengths of the UV bands for the two complexes, we assign them to ligand-to-metal charge transfers (in particular the blue shift recorded on passing from 5 to 4 is reminiscent of the same transitions recorded from MCp*2 to MCp2).19 A similar assignment has been made for 6. As illustrated in Figure 5, upon one-electron oxidation in CH2Cl2 solution, complex 4 exhibits a NIR band at λmax ) 889 nm. In MeCN as well as in THF solution such adsorption occurs at the same wavelengths, thus ruling out its IT nature.20 Such a finding supports that [4]+ might be considered as a completely delocalized mixed-valent species. The one-electron oxidation of complexes 4-6 has been also followed by IR spectroelectrochemistry. Figure 7 exemplifies the spectrum recorded on the [4]/[4]+ passage in the only region in which spectral changes have been detected. As seen, upon oxidation, the original band at 1305 cm-1, which is assigned to the ν(B-Me) stretching vibration,21 progressively shifts to 1322 cm-1 (the intermediate band at 1317 cm-1 is present in the original solution). It is likely that the weakening of the metal-ligand bonds caused by the electron removal makes the pertinent frequencies shift to higher energies. It is noted that, upon oxidation, complexes 5 and 6 afford quite similar changes in their IR patterns, thus suggesting that such a vibration can be exploited as a diagnostic tool to distinguish the neutral from the monocation species inside the present triple-decker series. In this connection, all the complexes, either in the neutral or in the oxidized form, display a band in the region from 1000 to 1100 cm-1, which, based on the ν(C-C) stretching frequencies of the MCp2 or MCp*2 sandwiches,22 is assigned to such vibrations. Bonding Analysis. In the triple-decker complexes CpCo(µ1,3-C3B2R5)M(ring) the anions [CpCo(1,3-C3B2R5)]- (R ) H, 3a; R ) Me, 3b) act as six-electron π-ligands toward the [M(ring)]+ cations, similar to cyclopentadienyl anions C5R5in sandwich compounds (C5R5)M(ring). To compare bonding properties of these ligands, we carried out the energy decomposition analysis (EDA)23 of the (L)M(ring) complexes (L ) 3a,b and C5R5-; M(ring) ) RuCp, CoC4H4). According to the EDA method, the interaction energy between the bonding fragments ∆Eint can be divided into three main components: (18) (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) 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.; VologzhaninaA. V. J. Organomet. Chem. 2009, 694, 1161–1171. (19) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier Science B.V.: Amsterdam, 1984. (20) Hush, N. S. Coord. Chem. ReV. 1985, 64, 135–157. (21) (a) Rao, C. N. R. Chemical Applications of Infrared Spectroscopy; Academic Press: New York, 1963. (b) An Infrared Spectroscopy Atlas for the Coatings Industry; Federation of Societies for Coatings Technology: Philadelphia, 1980.

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Organometallics, Vol. 28, No. 9, 2009 2711

Table 3. Formal Electrode Potentials (V, vs SCE) and Peak-to-Peak Separations (mV) for the Redox Changes Exhibited by the Triple-Decker Complexes under Study in CH2Cl2 Solution oxidations

a

complex

Ep(2nd)

E°′(1st)

4 (RuCo) 5 (RuCo) 6 (Co2) 7 (FeCo) FeCp2

+1.59 +1.53b +1.45

+0.43 +0.25 +0.09 -0.06c +0.39

a

color of solution ∆Ep

original

a

85 89 96 64 72

blue emerald-green green

reduction

one-electron oxidized

E°′Red

∆Epa

green-maroon green-yellow maroon

-1.84 -1.99b -2.13b -1.77c

84

b

65

Measured at 0.1 V s-1. b From Osteryoung square wave voltammetry (0.1 V s-1). c In MeCN solution; from ref 6.

Table 4. Experimental and Calculated Formal Electrode Potentials E°′ (V, vs SCE) for the One-Electron Oxidation of Isolobal Analogues of the Present Triple-Deckers in CH2Cl2 Solution calculated PCM redox change

experimental

e

f

g

COSMOh

[4]0/+ [Cp*RuCp]0/+ [5]0/+ [Cp*RuCp*]0/+ [6]0/+ [Cp*Co(C4Me4)]0/+

+0.43a +0.62b +0.25a +0.55c +0.09a +0.26d

+0.13 +0.56 -0.02 +0.34 -0.06 +0.23

+0.08 +0.55 -0.05 +0.35 -0.10 +0.22

+0.08

+0.25 +0.66 +0.04 +0.38 -0.03 +0.25

-0.08 -0.13

a Present work. b From ref 54. c From ref 55. d From ref 7. BP86/6-311G(d,p) for C, H, and B and LANL2DZ for Co and Ru. f BP86/6-311G(d,p) for C, H, and B and def2-TZVP for Co and Ru. g BP86/def-TZVP. h BP86/TZ2P. e

Table 5. UV-Vis Spectral Data for Complexes 4-6 and Their Monocations in CH2Cl2 (MeCN, THF) Solutions λ (nm) complex

neutral

+

4/[4]

5/[5]+

6/[6]+

574 (573, 573) 421b (420, 420) 308 (305, 307) 580 (576, 576) 420b (418, 418) 328 (326, 326) 270 (269, 269) 603 (600, 600) 438b (447, 447) 382 (382, 382) 289 (289, 289)

a

oxidized 889a 548 421b 308 881a 511 413b 328 270 888a 564 438b 378 289

Figure 7. Spectral IR changes recorded in a OTTLE cell upon progressive oxidation of 4 (Ew ) +0.4 V, vs Ag pseudoreference electrode) in CH2Cl2 solution. [NBu4][PF6] (0.2 mol dm-3) supporting electrolyte.

(Pauli repulsion), and ∆Eorb is the stabilizing orbital interactions. The ratio ∆Eelstat/∆Eorb indicates the electrostatic/covalent character of the bond. The bond dissociation energy is

De ) -(∆Eint + ∆Eprep) where ∆Eprep (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.10f,24 The EDA data for the (L)M(ring) complexes in terms of interactions between L- and [M(ring)]+ fragments are given in

By deconvolution. b Shoulder.

Figure 6. OTTLE-cell UV-vis spectra exhibited by 5 in (a) CH2Cl2, (b) MeCN, and (c) THF solutions. [NBu4][PF6] (0.2 mol dm-3) supporting electrolyte.

∆Eint ) ∆Eelstat + ∆EPauli + ∆Eorb ∆Eelstat is the electrostatic interaction energy between the fragments with a frozen electron density distribution, ∆EPauli is the repulsive four-electron interactions between occupied orbitals

(22) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed.; Wiley-Interscience: New York, 1997; Part B. (23) For recent reviews see:(a) Frenking, G.; Fro¨hlich, N. Chem. ReV. 2000, 100, 717–774. (b) Frenking, G. J. Organomet. Chem. 2001, 635, 9– 23. (c) Frenking, G.; Wichmann, K.; Fro¨hlich, 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. (24) (a) Lein, M.; Frunzke, J.; Timoshkin, A.; Frenking, G. Chem.sEur. J. 2001, 7, 4155–4163. (b) Rayon, V. M.; Frenking, G. Chem.sEur. 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.; Ferna´ndez, 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) Ferna´ndez, 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.

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Table 6. Results of EDA (energy values in kcal mol-1) for (L)M(ring) Complexes Using [M(ring)]+ and L- as Interacting Fragments at BP86/TZ2P (L)Co(C4H4) L

3a

Cp

(L)RuCp 3b

Cp*

3a

Cp

3b

Cp*

∆Eint -209.1 -228.5 -212.5 -235.9 -207.4 -228.5 -211.9 -238.2 ∆EPauli 170.6 157.6 187.3 173.7 237.5 222.4 254.2 233.4 ∆Eelstata -230.2 (60.6%) -231.9 (60.1%) -238.7 (59.7%) -235.4 (57.5%) -266.2 (59.9%) -264.1 (58.6%) -275.7 (59.1%) -265.8 (56.4%) ∆Eorba -149.5 (39.4%) -154.2 (39.9%) -161.1 (40.3%) -174.2 (42.5%) -178.6 (40.1%) -186.9 (41.4%) -190.4 (40.9%) -205.8 (43.6%) ∆Eprep 3.5 1.5 6.9 2.5 6.8 4.9 11.2 8.2 De 205.6 227.0 205.6 233.4 200.5 223.6 200.7 230.0 a

The values in parentheses give the percentage contribution to the total attractive interactions.

Table 7. Percentage Contributions of π, σ, and δ Interactions for (L)M(ring) Complexes at BP86/def2-TZVPP//BP86/TZ2P contribution (%) [M(ring)]+ +

[Co(C4H4)]

L 3a Cp-

[RuCp]+

3a Cp-

π

σ

δ

comment

49.0 46.2 61.9 60.4 50.8 47.8 62.3 60.1 66.2

38.4 43.2 20.0 23.6 33.5 38.4 17.5 20.5 13.8

12.6 10.6 18.2 16.0 15.7 13.8 20.2 19.4 20.0

a b a b a b a b c

a From the [M(ring)]+ FO contributions to occupied MOs. b From the [ring]- FO contributions to occupied MOs. c According to EDA at BP86/TZ2P.

Figure 9. σ-Type orbitals of the sandwich anion 3a (isodensity 0.03 for HOMO-3; 0.05 for HOMO-9 and HOMO-27).

Figure 8. π-Type orbitals of the sandwich anion 3a at BP86/def2TZVPP//BP86/TZ2P (MO isodensity surface 0.05).

Table 6 (Figures S1 and S2 in the Supporting Information). For the parent anions 3a and Cp- the electrostatic attraction (∆Eelstat) is nearly the same. However, the attractive orbital interaction (∆Eorb) for 3a is lower (by 5-8 kcal mol-1) and the Pauli repulsion is higher (by 13-15 kcal mol-1) than for Cp-. As a result, the total bonding energies (∆Eint) for the parent anion 3a are approximately 19-21 kcal mol-1 lower than for Cp-. The dissociation energies for 3a are 17-19 kcal mol-1 lower than for Cp- since the preparation energy (∆Eprep) for 3a is ca. 2 kcal mol-1 higher than for Cp- (in accordance with the folding of the C3B2 ring in the free anion 3a along the B · · · B line (6°) owing to the larger boron covalent radius vs carbon). The attractive interactions in both cases are ca. 40% covalent and 60% electrostatic. Comparison of the methylated anions 3b and Cp*- revealed similar trends. The increase of ∆Eint upon methylation is much lower for the sandwich anion (3-4.5 kcal mol-1) than for cyclopentadienyl anion (7-10 kcal mol-1). Interestingly, the dissociation energies for 3a and 3b are practically the same, owing to the parallel increase of ∆Eprep by 3.4-4.4 kcal mol-1.

The covalent bonding in (L)M(ring) complexes is usually described in terms of π and σ donation L- f [M(ring)]+ and δ back-donation [M(ring)]+ f L-. In order to estimate contributions of π, σ, and δ interactions, we analyzed fragment orbital (FO) occupancies in the complexes determined by Mulliken population analysis. For instance, in (C4H4)CoCp the unoccupied e1 (LUMO, LUMO+1) and a1 (LUMO+2) FOs of [Co(C4H4)]+ (Figure S3 in the Supporting Information) have 0.207, 0.207, and 0.133 occupancies, respectively. Two occupied e2 (HOMO, HOMO-1) FOs have 0.945 and 0.934 occupancies in the complex, suggesting that their contributions to the bonding are equal to 0.055 and 0.066. Thus, the contributions of π, σ, and δ interactions are estimated to be 61.9, 20.0, and 18.2%, respectively. Similar data for the parent (L)M(ring) (L ) Cp-, 3a) complexes, determined both from [M(ring)]+ and L- FO contributions to occupied MOs, are given in Table 7 (Figure S4 in the Supporting Information). The main role in π donation in the triple-decker complexes CpCo(µ-1,3-C3B2H5)M(ring) belongs to π-type frontier orbitals HOMO and HOMO-4 of the sandwich anion 3a (Figure 8). These orbitals are much lower lying (-0.3 and -1.2 eV) than e1 (HOMO and HOMO-1) orbitals of Cp- (1.0 eV),10f resulting in considerably smaller contribution of π bonding for the sandwich anion. The main contribution to σ donation in the case of Cp- is from the σ(π) orbital (-2.6 eV).10f Anion 3a has two similar

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Organometallics, Vol. 28, No. 9, 2009 2713

Table 8. NBO Charges (q, in a.u.)a and Electrostatic Potentials at Nuclei (E, in a.u.)b for (L)M(ring) Complexes (L)Co(C4H4) L

3a

c

0.40 0.47 124.179 14.789

q(M)

E(M) E(Cring) av a

Cp 0.41 124.173 14.788

(L)RuCp 3b

0.38 0.46 124.186 14.796

Cp*

3a

0.40 124.186 14.800

0.05 0.46 250.754 14.789

Cp 0.07 250.754 14.790

3b 0.01 0.46 250.760 14.793

Cp* 0.06 250.767 14.800

At BP86/def2-TZVPP//BP86/TZ2P. b At BP86/TZ2P. c Values in italics correspond to the Co atom in 3a,b moieties.

orbitals, HOMO-3 and HOMO-9 (Figure 9); the former is much higher lying (-0.8 eV) and the latter is only slightly lower lying (-3.0 eV) than that of Cp-, resulting in an increase of donation. In addition, the σ(C-C)-type orbital (HOMO-27) of 3a is also higher in energy (-14.1 eV) than the corresponding orbital of Cp- (-14.9 eV). Therefore the contribution of σ donation is much greater in the case of the sandwich anion 3a compared to Cp-. δ-Type orbitals of anion 3a (LUMO+8, LUMO+10, LUMO+16, and LUMO+17) are much higher lying than e2 (LUMO and LUMO+1) orbitals of Cp-, resulting in considerably lower contribution of δ back-donation. As follows from Table 7, the total contribution of π and σ donation for 3a is higher than for Cp-, suggesting stronger donor ability of the sandwich anion. In accordance with this conclusion, the NBO charges at the metal atoms of the M(ring) unit are slightly lower for the complexes with anions 3a,b compared to C5R5- (Table 8). Noteworthy, in both the (ring)M(3a,b) and (ring)M(C5R5) series methylation causes a slight decrease of the NBO charge due to the donor effect of the methyl groups. In the dicobalt triple-decker complexes (C4H4)Co(3a,b) the Co charge in the Co(C4H4) unit is lower than that in CoCp, suggesting that the oxidation and reduction are centered at the Co(C4H4) and CoCp cobalt atoms, respectively. This is also supported by electron density distribution in the HOMO and LUMO of complex 6 (Figure S5 in the Supporting Information). It should be noted, however, that the presence of electropositive boron atoms decreases the metal NBO charges, reducing their importance as an indicator of ligand electronic effect. Recently, electrostatic potentials at carbon nuclei were shown to be useful criteria for evaluation of substituent effects in the benzene ring.25 This parameter can also be helpful in organometallic chemistry. Indeed, as seen from Table 8, introduction of five methyl groups in the Cp ring of CpM(ring) sandwich compounds considerably increases the electrostatic potential at the metal and carbon nuclei (E(M) and E(Cring), respectively) of the M(ring) unit. The same pattern is observed for the complexes with the sandwich anions 3a,b. In the (L)Co(C4H4) series, the E(Co) and E(CC4H4) values are higher for 3a compared to Cp-, confirming stronger donor ability of the sandwich anion. At the same time, in the (L)RuCp series, E(Ru) and E(CCp) are almost equal for 3a and Cp-, indicating very close donor properties of these anions toward the [RuCp]+ fragment. Overall, the E(M) and E(Cring) data revealed that the donor ability increases in the following order: Cp- e 3a < 3b e Cp*-.

Conclusion Electrophilic stacking of the sandwich anion 3b with [M(ring)]+ fragments was shown to be very effective for the preparation of triple-decker complexes with a bifacially bonded diborolyl ligand. DFT calculations revealed similarities and differences in bonding properties of anions 3a,b and [C5R5](25) Galabov, B.; Ilieva, S.; Schaefer, H. F., III J. Org. Chem. 2006, 71, 6382–6387.

toward transition metals. In accordance with the energy decomposition scheme, the attractive interactions of the parent anions of both types with [RuCp]+ and [Co(C4H4)]+ cations are ca. 40% covalent and 60% electrostatic; however the total bonding energies for 3a are approximately 19-21 kcal mol-1 lower than for Cp-. Mulliken population analysis suggests lower contributions of π donation and δ back-donation to the total covalent bonding for 3a compared with Cp-; however π donation is always of primary importance. At the same time, the contribution of σ donation is much greater in the case of the sandwich anion 3a compared to Cp-. The metal NBO charges and electrostatic potentials at nuclei of the M(ring) unit suggest close donor ability of anions 3a,b and [C5R5]- toward transition metals. This conclusion is also supported by electrochemical study.

Experimental Section General Procedures. All reactions were carried out under argon in anhydrous solvents, which were purified and dried using standard procedures. The isolation of products was conducted in air. Starting materials were prepared as described in the literature: CpCo(1,3C3B2Me5H),26 [CpRu(MeCN)3]PF6,27 [Cp*Ru(MeCN)3]PF6,28 [Cp*RuCl]4,29 and [(C4Me4)Co(MeCN)3]PF6.11c The 1H and 11 B{1H} NMR spectra were recorded with a Bruker AMX 400 spectrometer operating at 400.13 and 128.38 MHz, respectively. Materials and apparatus for electrochemistry have been described elsewhere.30 Synthesis of CpCo(µ-1,3-C3B2Me5)RuCp (4). A solution of CpCo(1,3-C3B2Me5H) (0.101 g, 0.39 mmol) in THF (5 mL) was stirred with 0.2 mL of Na/K3.0 alloy for 1 h. A slight evolution of hydrogen occurred. The resulting greenish solution was taken off with syringe and filtered through a G4 frit into the frozen (-196 °C) suspension of [CpRu(MeCN)3]PF6 (0.170 g, 0.39 mmol) in THF (10 mL). The mixture was stirred for 1 h at -78 °C and for 2 h at -30 °C, then warmed to room temperature and stirred overnight. The solvent was removed in vacuo. The residue was dissolved in petroleum ether, giving a dark blue solution. Chromatography on an alumina column (2 × 25 cm) with petroleum ether gave blue and green bands. Evaporation of the first gave blue complex 4 (0.123 g, 75%). Analytically pure product was obtained by recrystallization from EtOH, 0.104 g (65%). Anal. Calcd for C18H25B2CoRu: C 51.12, H 5.96, B 5.11. Found: C 51.46, H 6.11, B 4.89. 1H NMR (CDCl3): δ 4.23 (s, 5H, CoCp), 3.63 (s, 5H, RuCp), 2.39 (s, 6H, 4,5-Me), 1.88 (s, 3H, 2-Me), 1.25 (s, 6H, 1,3Me). 11B{1H} NMR (CDCl3): δ 14.3 (bs). The second band gave 0.01 g of a greenish oil, which was not characterized. Synthesis of CpCo(µ-1,3-C3B2Me5)RuCp* (5). Complex 5 was prepared similarly to 4 from CpCo(1,3-C3B2Me5H) (0.247 g, 0.96 (26) Kno¨rzer, G.; Siebert, W. Z. Naturforsch. 1990, 45b, 15–18. (27) Gill, T. P.; Mann, K. R. Organometallics 1982, 1, 485–488. (28) Schrenk, J. L.; McNair, A. M.; McCormick, F. B.; Mann, K. R. Inorg. Chem. 1986, 25, 3501–3504. (29) Fagan, P. J.; Ward, M. D.; Calabrese, J. C. J. Am. Chem. Soc. 1989, 111, 1698–1719. (30) Fabrizi de Biani, F.; Corsini, M.; Zanello, P.; Yao, H.; Bluhm, M. E.; Grimes, R. N. J. Am. Chem. Soc. 2004, 126, 11360–11369.

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Table 9. Summary of Crystallographic Data and Structure Refinement for 4-6 4 empirical formula molecular weight cryst syst space group cryst color, habit cryst size (mm) a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g cm-3) 2θmax (deg) abs coeff, µ(Mo KR) (cm-1) absorp corr Tmax/Tmin no. of collected reflns no. of indep reflns no. of obsd reflns (I > 2σ(I)) Rint no. of params R1 (on F for obsd reflns) wR2 (on F2 for all reflns) weighting scheme A B F(000) goodness-of-fit ∆Fmax/∆Fmax (e Å-3)

5

6

C18H25B2CoRu 423.02 monoclinic P21/m dark blue, plate 0.60 × 0.40 × 0.15 8.0993(7) 10.6819(9) 10.3270(9) 90 98.456(2) 90 883.7(1) 2 1.590 54 17.85 0.770/0.420 5597 2021 1886 0.0279 109 0.0300 0.0716 0.0137 2.9308 428 0.997 1.164/-1.126

C23H35B2CoRu C21H32B2Co2 493.15 423.97 orthorhombic monoclinic Pnma P21/m dark blue, prism dark blue, plate 0.60 × 0.40 × 0.30 0.35 × 0.15 × 0.05 17.167(2) 8.537(2) 13.890(2) 12.233(3) 9.630(1) 10.101(2) 90 90 90 100.322(5) 90 90 2296.3(4) 1037.8(4) 4 2 1.426 1.357 60 55 13.85 15.98 SADABS 0.862/0.692 0.492/0.346 18 345 10 602 3474 2628 2179 1598 0.0523 0.0537 133 127 0.0601 0.0492 0.1443 0.1345 w-1 ) σ2(Fo2) + (aP)2 + bP, where P ) 1/3(Fo2 + 2Fc2) 0.927 0.0800 0.0 0.0 1016 444 0.950 0.898 3.822/0.198 1.164/-0.433

mmol) and [Cp*RuCl]4 (0.260 g, 0.24 mmol) or [Cp*Ru(MeCN)3]PF6 (0.484 g, 0.96 mmol) in 15 mL of THF. Yield: 0.369 g (78%) from [Cp*RuCl]4; 0.308 g (65%) from [Cp*Ru(MeCN)3]PF6. Blue solid. Anal. Calcd for C23H35B2CoRu: C 56.03, H 7.15, B 4.38. Found: C 56.40, H 6.97, B 4.50. 1H NMR (CDCl3): δ 4.22 (s, 5H, CoCp), 2.13 (s, 6H, 4,5-Me), 1.61 (s, 3H, 2-Me), 1.46 (s, 15H, RuCp*), 1.02 (s, 6H, 1,3-Me). 11B{1H} NMR (CDCl3): δ 14.2 (bs). The second minor band gave 0.025 g of a yellowbrown oil, which was not characterized. Synthesis of CpCo(µ-1,3-C3B2Me5)Co(C4Me4) (6). Complex 6 was prepared similarly to 4 from CpCo(1,3-C3B2Me5H) (0.141 g, 0.5 mmol) and [(C4Me4)Co(MeCN)3]PF6 (0.196 g, 0.5 mmol) in 15 mL of THF. The solvent was removed in vacuo, and the residue was sublimed at 0.01 mmHg. A small amount of unidentified orange oil, sublimed up to 120 °C, was discarded. Complex 6 was sublimed at 130-150 °C as a dark green solid. Yield: 0.089 g (42%). Anal. Calcd for C21H32B2Co2: C 59.41, H 7.60, B 5.19. Found: C 59.58, H 7.86, B 5.01. 1H NMR (CDCl3): δ 4.14 (s, 5H, CoCp), 2.06 (s, 6H, 4,5-Me), 1.65 (s, 3H, 2-Me), 0.99 (s, 6H, 1,3Me), 0.88 (s, 12H, CoC4Me4). 11B{1H} NMR (CDCl3): δ 15.7 (bs). Computational Details. The geometries have been optimized with Cs symmetry restriction at the gradient-corrected DFT level of theory using the exchange functional of Becke31 and the correlation functional of Perdew32 (BP86). Uncontracted Slatertype orbitals were employed as basis functions for the SCF calculations.33 Scalar relativistic effects were considered using the zero-order regular approximation (ZORA).34 An all-electron ZORA relativistic valence triple-ζ basis set augmented by two polarization functions, TZ2P, was used. The bonding interactions were studied (31) Becke, A. D. Phys. ReV. A 1988, 38, 3098–3100. (32) Perdew, J. P. Phys. ReV. B 1986, 33, 8822–8824. (33) Snijders, J. G.; Vernooijs, P.; Baerends, E. J. At. Data Nucl. Data Tables 1982, 26, 483–509. (34) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993, 99, 4597–4610.

by means of Morokuma-Ziegler energy decomposition analysis.35 The calculations were carried out using the ADF 2006.01 program package.36 Fragment orbital occupations were determined by Mulliken population analysis using the AOMix program.37 The input files were obtained from single-point calculations at the BP86/TZ2Poptimized structures with the Gaussian 98 program38 using BP86 functional and a basis set of triple-ζ quality with two polarization functions, def2-TZVPP.39 Natural charges were obtained using the NBO scheme40 at the same level of theory. For the calculation of the redox potentials, geometry optimizations were performed without constraints using the PBE ex(35) (a) Morokuma, K. Chem. Phys. 1971, 55, 1236–1244. (b) Ziegler, T.; Rauk, A. Theor. Chim. Acta 1977, 46, 1–10. (36) (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. (37) (a) Gorelsky, S. I.; Lever, A. B. P. J. Organomet. Chem. 2001, 635, 187–196. (b) Gorelsky, S. I. AOMix: Program for Molecular Orbital Analysis; http://www.sg-chem.net/ University of Ottawa, 2007. (38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; ScuseriaG. 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.; HeadGordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98 (ReVision A.6); Gaussian, Inc.: Pittsburgh, PA, 1998. (39) (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. (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.

µ-Diborolyl Triple-Decker Complexes change-correlation functional,41 the scalar-relativistic Hamiltonian,42 atomic basis sets of generally contracted Gaussian functions,43 and a density-fitting technique44 as implemented in a recent version of Priroda code.45 The all-electron double-ζ basis set L1 augmented by one polarization function46 was used. The redox potentials relative to SCE (E°redox) were calculated using E°redox ) [-(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).47 The solvent (CH2Cl2) effects were included using either the polarizable continuum model (PCM)48 or the conductor-like screening model (COSMO).49 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 program50 was used for molecular modeling and visualization. X-ray Diffraction Study. The crystals of 4-6, suitable for X-ray study, were grown by slow evaporation of their hexane solutions in air. The principal crystallographic data and refinement parameters are listed in Table 9. Single-crystal X-ray diffraction experiments were carried out with a Bruker SMART 1000 CCD area detector, with graphite-monochromated Mo KR radiation (λ ) 0.71073 Å, ω-scans with a 0.3° step in ω and 10 s per frame exposure, 2θ < 60°) at 110 K (5 and 6) and 120 K (4). The low temperature of the crystals was maintained with a Cryostream (Oxford Cryosystems) open-flow N2 gas cryostat. Reflection intensities were integrated by using the SAINT software51 and the semiempirical method SADABS.52 (41) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865–3868. (42) Dyall, K. G. J. Chem. Phys. 1994, 100, 2118–2127. (43) Laikov, D. N. Chem. Phys. Lett. 2005, 416, 116–120. (44) Laikov, D. N. Chem. Phys. Lett. 1997, 281, 151–156. (45) Laikov, D. N.; Ustynyuk, Yu. A. IzV. Akad. Nauk, Ser. Khim. 2005, 804–810. [Russ. Chem. Bull. 2005, 54, 820-826 (Engl. Transl.)]. (46) Misochko, E. Ya; Akimov, A. V.; Belov, V. A.; Tyurin, D. A.; Laikov, D. N. J. Chem. Phys. 2007, 127, 084301. (47) Trasatti, S. Pure Appl. Chem. 1986, 58, 955–966. (48) Miertsˇ, S; Scrocco, E; Tomasi, J. Chem. Phys. 1981, 55, 117–129. (49) Klamt, A.; Schu¨u¨rmann, G. J. Chem. Soc., Perkin Trans. 2 1993, 799, 805. (50) Zhurko, G. A. ChemCraft 1.6; http://www.chemcraftprog.com, 2008.

Organometallics, Vol. 28, No. 9, 2009 2715 The structures were solved by direct methods and refined by full-matrix least-squares against F2 in an anisotropic (for nonhydrogen atoms) approximation. All hydrogen atom positions were refined in isotropic approximation in a “riding” model with the Uiso(H) parameters equal to 1.2Ueq(Ci), for methyl groups equal to 1.5Ueq(Cii), where U(Ci) and U(Cii) are respectively the equivalent thermal parameters of the carbon atoms to which the corresponding H atoms are bonded. All calculations were performed on an IBM PC/AT using the SHELXTL software.53 CCDC-631833 (for 4), -631834 (for 5), and -631832 (for 6) 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 and Deutsche Forschungsgemeinschaft (DFG; SFB 247). P.Z. gratefully acknowledges the financial support of the University of Siena (PAR 2007). Supporting Information Available: Crystallographic information (cif files) for compounds 4, 5, and 6; details of DFT calculations for complexes CpCo(µ-1,3-C3B2R5)M(ring) (M(ring) ) RuCp, Co(C4H4); R ) H, Me) and (C5R5)M(ring) (M(ring) ) RuCp, Co(C4H4); R ) H, Me), ligands [CpCo(µ-1,3-C3B2R5)]- (R ) H, Me) and [C5R5]- (R ) H, Me), as well as cations [RuCp]+ and [Co(C4H4)]+ (atomic coordinates for optimized geometry, energy data, fragment orbital contributions to occupied MOs); Figures S1-S5. This material is available free of charge via the Internet at http://pubs.acs.org. OM900032Z

(51) SMART v. 5.051 and SAINT v. 5.00, Area detector control and integration software; Bruker AXS Inc.: Madison, WI, 1998. (52) Sheldrick, G. M. SADABS; Bruker AXS Inc.: Madison, WI, 1997. (53) Sheldrick, G. M. SHELXTL-97; AXS Inc.: Madison, WI, 1997. (54) Gassman, P. G.; Winter, C. H. J. Am. Chem. Soc. 1988, 110, 6130– 6135. (55) (a) Koelle, U.; Salzer, A. J. Organomet. Chem. 1983, 243, C27C30. (b) Koelle, U.; Grub, J. J. Organomet. Chem. 1985, 289, 133–139.