Monocationic μ-Diborolyl Triple-Decker Complexes [CpCo(μ-1,3

Article pubs.acs.org/Organometallics

Monocationic μ‑Diborolyl Triple-Decker Complexes [CpCo(μ1,3‑C3B2Me5)M(ring)]+: Synthesis, Structures, and Electrochemistry Dmitry V. Muratov,† Alexander S. Romanov,†,‡ Tatiana V. Timofeeva,‡ Walter Siebert,§ Maddalena Corsini,∥ Serena Fedi,∥ Piero Zanello,*,∥ and Alexander R. Kudinov*,† †

Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow, Russian Federation Department of Natural Sciences, New Mexico Highlands University, Las Vegas, New Mexico 87701, United States § Anorganisch-Chemisches Institut der Universität Heidelberg, 69120 Heidelberg, Germany ∥ Dipartimento di Chimica, Università di Siena, 53100 Siena, Italy ‡

S Supporting Information *

ABSTRACT: Cationic triple-decker complexes with a bridging diborolyl ligand, [CpCo(μ-1,3-C3B2Me5)M(ring)]+ (M(ring) = CoCp (2a), CoCp* (2b), RhCp (3a), RhCp* (3b), IrCp (4a), IrCp* (4b), Ru(C6H6) (5a), Ru(p-MeC6H4Pri) (5b), Ru(C6Me6) (5c), Ru(η6-cycloheptatriene) (6)), were synthesized by reaction of CpCo(μ-1,3-C3B2Me5)Tl with [M(ring)Hal2]2. The structures of 2aBPh4, 2bPF6, 4aPF6, 5aOTf, and 5cPF6 were determined by X-ray diffraction. The electron-transfer ability of the complexes has been ascertained by electrochemical and spectroelectrochemical techniques. In general, they are able to shuttle reversibly in the sequence 2+/ +/0/−, plausibly affording completely delocalized mixed-valence derivatives. DFT calculations revealed structural changes accompanying redox processes and satisfactorily predicted the potentials for the first reduction and first oxidation.

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covered.10 Such complexes have found a wide range of applications in catalysis and materials science.11 Herein we describe a straightforward approach to the 30-VE cationic complexes [CpCo(μ-1,3-C3B2Me5)M(ring)]+,12 on the basis of the reactions of anion 1 (as a thallium derivative) with [M(ring)X2]2.13 The electrochemical behavior and structural features of the complexes synthesized are also described.

INTRODUCTION Electrophilic stacking of sandwich compounds with the cationic fragments [M(ring)]+ (in the form of their labile derivatives) is widely used for the synthesis of triple-decker complexes with bridging Cp,1 C4BH5,2 C5BH6,3 C4H4P,4 and cyclo-P55 ligands. In particular, cationic triple-decker complexes with bridging C4BH52b,6 and C5BH63a,7 heterocycles were obtained by electrophilic stacking of various sandwich compounds with the labile solvates [M(ring)(solv)3]2+. Using the halides [M(ring)X2]2 as synthons of [M(ring)]2+ fragments, Grimes et al. have synthesized neutral complexes with bifacially bonded C2B3H5 ring derivatives.8 We have synthesized neutral 30-VE (valence-electron) triple-decker complexes CpCo(μ-1,3C3B2Me5)M(ring) by reaction of the sandwich anion [CpCo(1,3-C3B2Me5)]− (1; Chart 1) with [M(ring)(MeCN)3]+ cations (M(ring) = Ru(C5R5), Co(C4Me4)).9 Recently, the area of metallacarborane chemistry including the boroncontaining triple-decker complexes was comprehensively

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RESULTS AND DISCUSSION Synthesis. We have established earlier similarity of the bonding properties of anions Cp− and 1.9 The Cp− anion (in the form of the thallium salt) is known to react with [MCp*Cl2]2 (M = Rh, Ir)14 and [M(arene)Cl2]2 (M = Ru, Os)15,16 in acetonitrile, giving [CpMCp*]+ and [CpM(arene)]+ cations, respectively. As shown in the present work, anion 1 reacts with the same reagents with the formation of tripledecker complexes, providing additional evidence of its similarity to Cp−. Recently, we have described the synthesis of a thallium derivative of anion 1, CpCo(μ-1,3-C3B2Me5)Tl (1Tl).17 Similarly to CpTl, it is a milder and more selective reagent than the corresponding alkali-metal derivatives; it can be easily stored and weighed. The thallium derivative, 1Tl, reacts with the halide complexes [M(C5R5)Hal2]2 (M(C5R5) = CoCp,

Chart 1

Received: March 4, 2013 Published: April 18, 2013 © 2013 American Chemical Society

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RhCp, RhCp*, IrCp, IrCp*) in MeCN, giving the cationic triple-decker complexes [CpCo(μ-1,3-C3B2Me5)M(C5R5)]+ (2−4) in yields from moderate to good (50−80%; Scheme 1).18

Scheme 3

Scheme 1

Scheme 4

As an exception, the reaction of 1Tl with [Cp*CoCl2]2 in MeCN gives [CpCo(μ-1,3-C3B2Me5)CoCp*]+ (2b) in low yield (∼10%) along with other products. The use of another CoCp* synthon ([Cp*CoI2]2, [Cp*Co(solv)3]2+) or solvent (THF) also did not allow us to obtain 2b selectively. The best yield (26%) was achieved using [Cp*CoI2]2 in MeCN; however, even in this case 2b was formed as a mixture with 2a and [CpCoCp*]+ (∼1:1:1) (Scheme 2); isolation of 2b in pure form was performed by column chromatography. Scheme 2 Scheme 5

Salts of cations 2−6 with PF6−, OTf−, or BPh4− anions are deeply colored solids, which are air-stable both in the solid state and in solution. The 1H and 11B NMR spectral data for 2−6 are consistent with the expected structures. X-ray Diffraction Study. Structures of the triple-decker complexes 2aBPh4, 2bPF6, 4aPF6, 5aOTf, and 5cPF6 were determined by X-ray diffraction (Figures 1−5, respectively). Selected parameters for these structures and for the previously described13 3bPF6, 4bPF6, and 5bPF6 are given in Table 1 (see also Table S1 in the Supporting Information for selected bond lengths and angles). All of the cations 2a,b, 3b, 4a,b, and 5a−c are formed by three cyclic ligands, between which two metal atoms are located. The ring ligands are almost parallel, and metal atoms are placed nearly over the centroids of the rings. The mutual orientation of C5R5(Co) and C3B2Me5 ligands in 2a,b is staggered (for all molecules), while for 4a it is eclipsed for molecule A and staggered for molecule B. The conformation of Cp(Co) and C3B2Me5 ligands in 5a−c is eclipsed for 5a and staggered for 5b,c. The mutual orientation

Reaction of 1Tl with chlorides [Ru(arene)Cl2]2 (arene = C6H6, p-MeC6H4Pri) affords the cationic triple-decker complexes [CpCo(μ-1,3-C3B2Me5)Ru(arene)]+ (5a,b) in high yields (62−75%; Scheme 3). However, a similar reaction with the hexamethylbenzene derivative [Ru(C6Me6)Cl2]2 gives a mixture of 5c, 2a, and [CpRu(C6Me6)]+ in a ca. 1:1:1 ratio (Scheme 4); pure complexes 5c (17%) and 2a were isolated by column chromatography. The interaction of 1Tl with the cycloheptatriene derivative [Ru(cht)Cl2]2 leads in moderate yield (46%) to the cationic triple-decker complex 6, which is closely related to Ru(arene) complexes 5a−c (Scheme 5). 2714

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Figure 1. Structure of cation 2a. Ellipsoids are shown at the 50% level. Hydrogen atoms are omitted for clarity.

Figure 3. Structure of cation 4a. Ellipsoids are shown at the 50% level. Hydrogen atoms are omitted for clarity.

Figure 2. Structure of cation 2b. Ellipsoids are shown at the 50% level. Hydrogen atoms are omitted for clarity.

Figure 4. Structure of cation 5a. Ellipsoids are shown at the 50% level. Hydrogen atoms are omitted for clarity.

of the Cp(Ir) and C3B2Me5 ligands in 4a is eclipsed for both independent molecules, while in the methylated analogue 4b the Cp*(Ir) and C3B2Me5 ligands are staggered, probably owing to the steric repulsion of the methyl groups. The C3B2 ring in all cations is almost planar, with a negligible folding along the B···B line (ca. 0.8°). It is noteworthy that the crystals 2b−4b are isomorphous. The C−C bond in the C3B2 ring for the cationic complexes discussed here (1.417−1.485 Å) is generally shorter than that for their neutral analogues CpCo(μ-1,3-C3B2Me5)M(ring) (M(ring) = RuCp, RuCp*, CoC4Me4; 1.495−1.517 Å).9 However, the four C−B bonds are slightly longer (1.555− 1.641 Å) for the cationic complexes as compared with the neutral ones (1.550−1.595 Å). As a result, the perimeter of the C3B2 ring is larger for the cationic complexes (see Table 1). The comparison of the two related Co2 structures 2a,b suggests that introduction of five methyl groups into the cyclopentadienyl ring leads to the shortening of the Co···C5 distance (by 0.01 Å) and elongation of Co···C3B2 (by 0.02 Å). An analogous trend to an even greater extent is observed for

the other two related CoIr complexes 4a,b (shortening of Ir···C5 by 0.01 Å and elongation of Ir···C3B2 by 0.03 Å). The strengthening of the M···C5 bond is explained by its greater population due to the donor effect of the methyl groups. It is accompanied by loosening of the bond with the second π ligand in accordance with two-side orientation of metal atom orbitals (trans effect). A similar pattern has been revealed earlier for the pair of triple-decker complexes CpRu(μ-1,3-C3B2Me5)CoCp and Cp*Ru(μ-1,3-C3B2Me5)CoCp.9 Interestingly, as follows from a comparison of the related CoRu complexes 5a,c, introduction of six methyl groups into the C6 ring leads to elongation of both Ru···C6 (by 0.01 Å) and Ru···C3B2 distances (by 0.03 Å). This elongation for 5b is intermediate between those of 5a and 5c. As follows from the Cambridge Crystallographic Database,19 for the complexes [(C5R5)Ru(C6R6)]+ (R = H, Alk) an analogous situation is observed. For example, the shortest Ru···arene and Ru···Cp* distances were observed in [(C5Me5)Ru(C6H6)]+ (1.703 and 2715

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Figure 6. π(C(sp2))−π(C(sp2)) stacking interactions (dashed lines) between pairs of molecules in cation 4a. Symmetry code: (C) 1 + x, y, z.

Figure 5. Structure of cation 5c. Ellipsoids are shown at the 50% level. Hydrogen atoms are omitted for clarity.

1.809 Å, respectively)20 and the longest in [(C5Me5)Ru(η6-5,6dipropyl-2,3-dihydro-1H-indene)]+ (1.742 and 1.841 Å, respectively).21 Analysis of intermolecular contacts indicates that cations 4a are arranged in chains by weak π(C(sp2))−π(C(sp2)) stacking interactions Cp(Co)···Cp(Ir) (ca. 3.3 Å) (Figure 6). The interaction between unequal fragments could have an electrostatic origin.22 In contrast, cations 2b−4b with one Cp* ring form dimers via intermolecular π(C(sp2))−π(C(sp2)) stacking between two Cp(Co) rings (ca. 3.1 Å), apparently owing to steric reasons (Figure 7). Electrochemistry.23 In agreement with the previously reported electrochemical behavior of the neutral dicobalt derivative CpCo(C3B2Et4Me)CoCp,12 the dicobalt cation [2a]+ affords a quite similar profile in cyclic voltammetry (Figure 8), consistent with a single one-electron oxidation and two one-electron reductions possessing features of chemical reversibility on the time scale of cyclic voltammetry. Further reduction (observed for CpCo(C3B2Et4Me)CoCp at Ep = −2.56 V, vs SCE)12 was hardly detected in our case. In fact, such a redox sequence can be assumed as typical of the monocations [CpCoIII(C3B2Me5)MIII(C5R5)]+ ([2a]+, [3a,b]+, and [4a,b]+), even if, depending upon the solvent as

Figure 7. π(C(sp2))−π(C(sp2)) stacking interactions (dashed lines) between pairs of molecules in cation 2b. Symmetry code: (A) 1 − x, −y, 1 − z.

well as the shifts of the potential values imposed by the nature of M or the electronic effects of the substituents R, in some cases the oxidation process and the most cathodic reduction were partially masked by the solvent discharges. As an example, Figure 9 shows that the oxidation process of the CoRh

Table 1. M···Ring Distances, Ring Perimeters, And Selected C3B2 Bond Lengths (Å) for Complexes 2aBPh4, 2bPF6, 3bPF6, 4aPF6, 4bPF6, 5aOTf, 5bPF6, and 5cPF6 2aBPh4a M(ring) Co···Cp Co···C3B2 M···C3B2 M···C5 M···C6 C3B2 perimeter C7−C8 C7−B1, C8−B2 C6−B1, C6−B2 a

2bPF6

3bPF6b

4aPF6a

4bPF6b

CoCp 1.656 1.582 1.582 1.658

CoCp* 1.648 1.572 1.604 1.652

RhCp* 1.649 1.573 1.760 1.804

IrCp 1.655 1.561 1.731 1.822

IrCp* 1.648 1.563 1.763 1.811

7.755(5) 1.462(5) 1.601(6), 1.587(5) 1.546(6), 1.564(5)

7.755(4) 1.464(3) 1.592(4), 1.591(4) 1.570(3), 1.558(4)

7.825(3) 1.478(3) 1.596(3), 1.608(3) 1.570(3), 1.575(3)

7.825(16) 1.417(15) 1.641(16), 1.605(17) 1.551(18), 1.613(15)

7.861(4) 1.484(3) 1.604(4), 1.610(3) 1.580(4), 1.582(4)

5aOTf

5bPF6c

5cPF6

Ru(C6H6) 1.656 1.568 1.737

Ru(p-C6H4MePri) 1.645 1.562 1.756

Ru(C6Me6) 1.644 1.563 1.764

1.695 7.846(6) 1.474(6) 1.613(6), 1.626(7) 1.567(6), 1.566(7)

1.705 7.826(5) 1.463(5) 1.592(5), 1.621(5) 1.566(5), 1.588(5)

1.708 7.815(3) 1.485(3) 1.594(3), 1.590(1) 1.570(3), 1.576(3)

The distances are shown only for the independent molecule A. bFrom ref 13a. cFrom ref 13b. 2716

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(II), whereas the oxidation process is assigned to the passage Co(III)Co(III)/Co(III)Co(IV), the expected oxidation of the second Co(III) likely being masked by the solvent discharge. As previously discussed,12 the large separation of the two reduction processes (ΔE°′ = 1.06 and 1.03 V, respectively, in both solvents) preliminarily suggests that the two metal centers are strongly electronically communicating. In particular, as a qualitative proposal, the neutral Co(III)−Co(II) species [2a]0 could be classified as a completely delocalized Robin−Day class III mixed-valence species (Kcom > 1017).24 The formal electrode potentials of the redox changes exhibited by complexes [2a]+, [3a,b]+, and [4a,b]+ are compiled in Table 2. It is useful to note that, in spite of the lack of marked color changes upon one-electron reduction of [2a]+, the electrogenerated neutral species [2a]0 displays a spectrum in which a new band appears in the near-IR region (Figure 10). As shown in Table 3, the same trend occurs upon one-electron reduction of [3a,b]+ and [4a,b]+. Dealing with dinuclear complexes, we could have in principle assigned such a new band (which can be deconvoluted in a few almost overlapping bands) to an intervalence band. Nevertheless, the fact that its location is independent from the solvent (as on the other hand happens for related triple-decker complexes)9,25,26 precludes the notion that it arises from a d−d charge transfer. Such a hypothesis has been further supported by the fact that an increase of temperature causes a slight shift of such a band toward higher wavelengths27 (see the Supporting Information). The progressive decrease from 958 to 670 to 512 nm of the band under discussion on passing from the neutral species CoCo ([2a]0) to CoRh ([3a]0) to CoIr ([4a]0), likely being due to the higher separation of the d orbitals going down group 9,28 further supports its d−d charge transfer nature. It is consistent with the successive increase of the HOMO/SOMO gap (1.20, 1.35, 1.36 eV, respectively), as it follows from DFT calculations. It is finally noted that the lack of intervalence bands appears to confirm the previously outlined complete charge delocalization between the two metal centers.26 As illustrated in Figure S1 in the Supporting Information, the visible band displayed by [3a] (λmax 670 nm) does not exhibit

Figure 8. Cyclic voltammogram recorded at a glassy-carbon electrode in MeCN solution of [2a]+ (1.5 × 10−3 mol dm−3). Conditions: [NEt4]PF6 (0.1 mol dm−3) supporting electrolyte; scan rate 0.2 V s−1.

monocation [3a]+, which in CH2Cl2 is masked by the anodic discharge, in MeCN solution becomes well-defined. On the other hand, in the same solvent, the oxidation of the cyclopentadienyl-methylated [3b]+ is easier than that of [3a]+. Under the reliable assumption that the two reduction processes are alternatively centered on the two M(III) sites (see below), the fact that upon methylation of one cyclopentadienyl ring both the reduction processes are cathodically shifted suggests that the increase of electronic density brought on by methylation is in reality delocalized over the two metal centers through the interposed borylated ring. Let us discuss the nature of the different redox changes exhibited by the present complexes, making reference to the symmetric [CpCo(C3B2Me5)CoCp]+ ([2a]+). As expected,12 exhaustive one-electron generation of the neutral congener CpCo(C3B2Me5)CoCp ([2a]0) in MeCN solution (Ew = −0.8 V) does not lead to marked color changes with respect to the original green-brown. The analysis of the pertinent cyclic voltammetric responses with scan rates progressively increasing from 0.02 to 2.00 V s−1 also supports the chemical and electrochemical reversibility of the process: the current ratio ipa/ipc is constantly equal to 1; the peak-to-peak separation slightly departs from the theoretical value of about 60 mV only at the highest scan rates; the current function ipcv−1/2 remains constant.24 As a consequence, we assign the reduction processes to the stepwise passages Co(III)Co(III)/Co(II)Co(III)/Co(II)Co-

Figure 9. Cyclic voltammograms recorded at a glassy-carbon electrode: (a) CH2Cl2 solution of [3a]+ (0.9 × 10−3 mol dm−3); (b) CH2Cl2 solution of [3b]+ (0.5 × 10−3 mol dm−3); (c) MeCN solution of [3a]+ (0.7 × 10−3 mol dm−3); (d) MeCN solution of [3b]+ (0.7 × 10−3 mol dm−3). Supporting electrolytes: CH2Cl2 solution, [NBu4]PF6 (0.2 mol dm−3); MeCN solution, [NEt4]PF6 (0.1 mol dm−3). Scan rates: (a) 0.2 V s−1; (b) 5.12 V s−1; (c, d) 1.0 V s−1. 2717

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Table 2. Formal Electrode Potentials (V, vs SCE) and Peak-to-Peak Separations (mV) for the Redox Changes Exhibited by Complexes [CpCoIII(C3B2Me5)MIII(C5R5)]+ in Different Solvents oxidation E°′

+

CoCp

[3a]+

RhCp

[3b]+

RhCp*

b +1.75c b +1.81c,d +1.78d +1.71c b +1.80 +1.81c +1.70

[2a]

[4a]

+

[4b]+

IrCp RhCp*

reduction ΔEpa

M(C5R5)

complex

117

87f 84 133f 98f

E°′

ΔEpa

E°′

ΔEpa

solvent

−0.60 −0.60 −0.73 −0.73 −0.94 −0.94 −0.86 −0.86 −1.00 −0.99

61 72 73 68 78 73 69 73 80 83

−1.66 −1.53 −1.77 −1.60 −1.96 −1.82 −1.96 −1.82 −2.08d −1.96

67 99 82 74 162e 92f 88f 92

CH2Cl2 MeCN CH2Cl2 MeCN CH2Cl2 MeCN CH2Cl2 MeCN CH2Cl2 MeCN

64

Measured at 0.1 V s−1. bMasked by the solvent discharge. cPartial chemical reversibility. dFrom OSWV. eMeasured at 5.12 V s−1. fMeasured at 1.00 V s−1.

a

Information summarizes the shifts of the two maxima as a function of the temperature. Finally, a comparison of the UV−vis spectra of the monocations [3a]+ and [4a]+ with those of the methylated analogues [3b]+ and [4b]+ evidences a shift toward higher wavelengths. Therefore, we assign the pertinent bands as charge transfer bands involving the capping cyclopentadienyl ligands. Interestingly, the fact that the one-electron reduction of such derivatives (see Table 2), as on the other hand happens for the UV−vis bands, depends on methylation but not on the polarity of the solvents suggests the lack of dipolar contributions to their LUMO orbitals. Let us now continue to the Co(III)−Ru(II) arene complexes [5a,b]+ also in comparison with the related neutral CoRu complex CpCoIII(C3B2Me5)RuIICp (7; Chart 2).9 Chart 2

Figure 10. Spectral changes recorded in CH2Cl2 solution of [2a]+ upon progressive one-electron reduction in a OTTLE cell.

significant changes in either energy or intensity in the temperature range from 273 to 303 K. As shown in Figure S2 in the Supporting Information, the Gaussian deconvolution (R = 0.999) proves that, as outlined in the text dealing with the spectrum of [2a], the band is mainly formed by two intense, partially overlapping absorptions. Table S3 in the Supporting Table 3. Spectroscopic Changes Recorded upon One-Electron Reduction of the Monocations [CpCoIII(C3B2Me5)MIII(C5R5)]+ in Different Solvents CH2Cl2

CH3CN

λmax (nm)

a

complex

M(C5R5)

[2a]+ [2a]0 [3a]+ [3a]0 [3b]+ [3b]0 [4a]+ [4a]0 [4b]+ [4b]0

CoCp CoCp RhCp RhCp RhCp* RhCp* IrCp IrCp IrCp* IrCp*

λmax (nm) color

345 345 331 331 342 342 312 312 325 d

404 404 408b 430 430 430b 386 392a,b

958a 670a 510 670c 512a 512

green-brown green-brown red green orange olive green orange dark orange orange

color 341 341 329 329 340

403 403b 408b 408b 430b

958a 670a

green-brown green-brown red green orange

d

313 313 323 e

371 393b

483a

orange brown orange

Average value. bShoulder. cVery flat band. dEasily reoxidized. eUnstable. 2718

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As happens for complexes [2a]+, [3a,b]+, and [4a,b]+, the ruthenium arene derivatives [5a,b]+ give rise to a redox pattern consisting of two sequential reductions and one oxidation. The plausible assignment attributes the oxidation and the first reduction to ruthenium-centered processes, whereas the most cathodic reduction is, as in the previous cases, attributed to the Co(III) site. However, DFT calculations suggest that the oxidation is cobalt-centered, whereas the Co and Ru atoms are also almost equally involved in reduction processes (vide infra). As illustrated in Figure 11, which refers to complex [5b]+, some

monocation [2a]+ (λmax 345 nm), as ruthenium belongs to the second series of transition metals. Finally, as illustrated in Figure 12, the cycloheptatriene complex [6]+ displays a redox path which differs from that of the derivatives discussed above, in that it consists of three oneelectron reductions having features of chemical reversibility (on the cyclic voltammetric time scale) and an irreversible oxidation. Controlled-potential coulometry at the first reduction consumed one electron per molecule, but the pertinent cyclic voltammetric test on the resulting solution indicated destruction of the original molecule. In this case, the nature of the reduction processes can be conveniently explained by taking into account that cycloheptatriene in nonaqueous solvents undergoes one-electron reduction at very negative potential values (for instance, −2.6 V vs SCE in both MeCN and DMF).30 In this light, it does not seem too presumptuous to assign the most cathodic step (E°′ = −2.60 V) just to the one-electron reduction of the cycloheptatriene capping subunit. Less straightforward is the assignment of the anodic process (Ep = +1.61 V) as Ru(II) centered, in that cycloheptatriene also undergoes irreversible oxidation (Ep = +1.43 V in MeCN, vs SCE).31 Computational Electrochemistry. The redox potentials were also estimated by DFT calculations.9,32 Table 6 compares the calculated formal electrode potentials (using the PCM solvation model) for complexes [2a]+, [3a,b]+, [4a,b]+, and [5a]+ with the experimental values. The computation satisfactorily predicts the potentials for the first reduction and first oxidation (average deviation from experimental values 0.23 V, maximum deviation 0.30 V). However, in the case of the second reduction the deviation is greater (average deviation 0.41 V, maximum deviation 0.53 V). The calculations also allowed us to evaluate structural changes accompanying redox processes (Table S4 in the Supporting Information).33 For instance, oxidation of the 30VE dicobalt monocation [2a]+ results in the shortening of the Co···Co distance by ca. 0.06 Å in accordance with the removal of one electron from HOMO, having antibonding character for the Co−C3B2 bonds (Figure S3 in the Supporting Information). Both reduction steps lead to elongation of the Co···Co distance (by ca. 0.11 and 0.13 Å, respectively), in accordance with the addition of electrons to antibonding orbitals (LUMOs of [2a]+ and [2a]0). It may be concluded that the increase in the number of electrons results in progressive elongation of the Co···Co distance owing to the weakening of the Co−C3B2 bonds. It is accompanied by a decrease of the C3B2 ring perimeter. Similar structural changes are also observed for other complexes studied in this work.

Figure 11. Cyclic voltammograms recorded at a gold electrode: (a) CH2Cl2 solution of [5b]+ (0.5 × 10−3 mol dm−3); (b, c) MeCN solution of [5b]+ (0.7 × 10−3 mol dm−3). Supporting electrolytes: (a) [NBu4]PF6 (0.2 mol dm−3); (b, c) [NEt4]PF6 (0.1 mol dm−3). Scan rates: (a, c) 1.0 V s−1; (b) 0.2 V s−1.

complications (which generate a new, at moment unknown, redox-active product) are presently concerned with the oxidation process in MeCN solution. It is noted that, as illustrated in Table 4, the chemically reversible oxidation of the neutral complex [7]0 is quite easier, whereas the first reduction is significantly more difficult with respect to the related arene monocations [5a,b]+. This undoubtedly arises in part from electrostatic effects. The spectral changes accompanying the first reduction of the present complexes are compiled in Table 5. As happens for related triple-decker complexes,29 all the CoRu derivatives display the most intense band in the ultraviolet region (λmax ∼307 nm) at lower wavelengths with respect to the CoCo

Table 4. Formal Electrode Potentials (V, vs SCE) and Peak-to-Peak Separations (mV) for the Redox Changes Exhibited by the CoRu Complexes [5a,b]+, [6]+, and [7]0 in Different Solvents oxidation E°′

complex [5a]

+

[5b]+ [6]+ [7]0

a

b +1.56c,d +1.63 +1.55c,d

reduction ΔEp

a

120e

d

+0.43 +0.43

85 84

E°′ −1.04 −0.95 −1.02 −0.98 −0.90 −1.84c −1.74

ΔEpa

E°′

59 59 72 64

−2.13 −1.90 −2.09c −1.95 −1.78

84 85

ΔEpa c

62

−2.72c

solvent CH2Cl2 MeCN CH2Cl2 MeCN MeCN CH2Cl2 MeCN

Measured at 0.1 V s−1. bMasked by the solvent discharge. cFrom OSWV. dNot assigned (see text). eMeasured at 2.0 V s−1. 2719

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Table 5. Spectroscopic Changes Recorded upon One-Electron Reduction of CoRu Complexes [5a,b]+ and [6]+ and OneElectron Oxidation of [7]0 in Different Solvents CH2Cl2

CH3CN

λmax (nm)

λmax (nm)

complex +

[5a] [5a]0 [5b]+ [5b]0 [6]+ [6]0 [7]+ d [7]0 d a

color 307 290, 320 307 291, 317 306 296, 315 308 308

a

b

354 354a 350a 350 350a 382a 421a 421a

500 530c 500b 554c 514b 530c 548 574

color

orange pink orange brown red brown green-brown blue

307 314c 309 309c 306 e

a

b

350 390 350a 360 350a

500 500b 500 500c 514a

305

420

573

red-orange red-orange red-orange brown brown

gray-blue

Shoulder. bVery flat band. cAverage value. dFrom ref 1. eUnstable.

comparison of the frontier orbitals of the CoRu cation [5a]+ and dication [5a]2+ (Figure S5 in the Supporting Information) suggests that the oxidation of [5a]+ is also cobalt-centered (in spite of the lower oxidation state of Ru). The LUMO of monocation [4a]+ and SOMO of the reduced neutral form [4a]0 are essentially delocalized over cobalt and iridium atoms (Figure S6 in the Supporting Information), suggesting that upon the first reduction the electron density is added almost equally to both metal centers. The LUMO of [4a]0 and SOMO+1 of anion [4a]− are also delocalized (Figure S7 in the Supporting Information),34 suggesting that upon the second reduction both metal atoms are affected as well. Analogously, comparison of the frontier orbitals for the cobalt− ruthenium cation [5a]+ and its reduced forms [5a]0 and [5a]− (Figures S8 and S9 in the Supporting Information) indicates that the Co and Ru atoms are also almost equally involved in reduction processes. A similar pattern is observed for other complexes studied in this work. Bonding Analysis. The covalent bonding in sandwich compounds is usually described in terms of ligand → M π and σ donation and M → ligand δ back-donation. In order to estimate contributions of π, σ, and δ interactions, we analyzed fragment orbital occupancies (determined by Mulliken population analysis) in series of the related mononuclear and triple-decker cobalt complexes [CpCo(ring)]2−n and [CpCo(ring)CoCp]4−n (ring = Cp, C4BH5, C3B2H5, C2B3H5; n is an absolute value of charge of the six-electron ring ligand: 1, 2, 3, and 4, respectively).

Figure 12. Cyclic (a) and differential pulse (b) voltammograms recorded at a glassy-carbon electrode in MeCN solution of [6]+ (0.8 × 10−3 mol dm−3). Conditions: supporting electrolyte [NEt4]PF6 (0.1 mol dm−3); scan rates (a) 0.2 V s−1 and (b) 0.02 V s−1.

It is worth noting that the SOMO of the neutral 31-VE species [2a]0 is delocalized over the two cobalt centers, confirming the electrochemically established conclusion about strong electronic communication between metal atoms. The HOMO of the 30-VE cation [2a]+ is also delocalized, suggesting a strong metal−metal interaction in this case as well. The calculations may be also useful for the assignment of the redox processes. For instance, comparison of the HOMO of the CoIr monocation [4a]+ with the SOMO of the dication [4a]2+ (Figure S4 in the Supporting Information) suggests that upon oxidation the electron density is predominantly removed from the cobalt atom: i.e., the oxidation is cobalt-centered. A similar

Table 6. Calculateda and Experimental Formal Electrode Potentials (V, vs SCE) for the Triple-Decker Complexes [CpCo(C3B2Me5)M(ring)]+ in MeCN Solution +/2+ species

a

M(ring)

calcd

+/0 exptl b

2a

CoCp

+1.52

+1.75

3a

RhCp

+1.62

+1.81b

3b 4a

RhCp* IrCp

+1.45 +1.55

+1.71b +1.80

4b 5a

IrCp* Ru(C6H6)

+1.43 +1.70

+1.70 +1.56b

0/−

calcd

exptl

calcd

exptl

−0.90 −0.75c −0.99 −0.85 −1.19 −1.13 −0.99 −1.26 −1.08

−0.60 −0.60 −0.73 −0.73 −0.94 −0.86 −0.86 −0.99 −0.95

−1.96

−1.53

−2.02

−1.60

−2.25 −2.02

−1.82b −1.82b

−2.42 −2.42

−1.96 −1.90

At the PCM/BP86/LANL2DZ//PBE/L1 level. bPartial chemical reversibility. cValues in italics refer to CH2Cl2 solution. 2720

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As follows from Table 7, upon an increase of number of boron atoms in the five-membered ring of the sandwich

contribution (%) π

σ

δ

MBO

70.3 [73.0]c

19.6 [12.9]c

10.1 [14.1]c

2.83

[C4BH5]2−

76.5 56.9 (55.5) 80.2 58.6 (58.3) 84.4 69.7 (65.6)

15.7 34.6 (34.2) 13.0 35.1 (34.2) 9.0 24.9 (27.3)

7.8 8.6 (10.3) 6.7 6.3 (7.4) 6.5 5.4 (7.1)

2.92 4.98 2.96 5.06 3.11 5.96

ring

[C3B2H5]3− [C2B3H5]4−

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(1,3C3B2Me5)Tl (1Tl),17 [(C6H6)RuCl2]2,35 [(p-MeC6H4Pri)RuCl2]2,36 [(C6Me6)RuCl2]2,36 [(cht)RuCl2]2,37 [CpCoI2]2,38 [CpRhI2]2,39 [CpIrI2]2,7a [Cp*CoI2]2,40 [Cp*RhCl2]2,41 and [Cp*IrCl2]2.30 The 1 H and 11B{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.25 Synthesis of [CpCo(1,3-C3B2Me5)CoCp]PF6 (2aPF6). A 5 mL portion of MeCN was added to a mixture of [CpCoI2]2 (0.131 g, 0.171 mmol) and 1Tl (0.164 g, 0.355 mmol). The reaction mixture was stirred overnight. The solvent was removed in vacuo, and the residue was extracted with water (2 × 4 mL). The combined water extracts were filtered, and the product was precipitated by adding an excess of an aqueous NH4PF6 solution to the filtrate. The brown precipitate was filtered, washed with water, and dried in vacuo. After reprecipitation with diethyl ether from acetone, the complex 2aPF6 was obtained as a brown solid. Yield: 0.143 g (78%). 1H NMR (acetone-d6): δ 5.06 (s, 10H, Cp), 2.68 (s, 6H, 4,5-Me), 2.08 (s, 3H, 2Me), 1.56 (s, 6H, 1,3-Me). 11B{1H} NMR (acetone-d6): δ 17.63 (bs). Anal. Calcd for C18H25B2Co2F6P (525.85): C, 41.11; H, 4.79; B, 4.11. Found: C, 41.17; H, 4.62; B, 4.09. MS (EI): M+ 381.2. 2aBPh4 was obtained in an analogous manner, using NaBPh4 instead of NH4PF6. Synthesis of [CpCo(μ-1,3-C3B2Me5)RhCp]PF6 (3aPF6). Complex 3aPF6 was prepared similarly to 2aPF6 from 1Tl (0.081 g, 0.22 mmol) and [CpRhI2]2 (0.070 g, 0.08 mmol). Red solid. Yield: 0.062 g (67%). 1H NMR (acetone-d6): δ 5.43 (d, 0.5 Hz, 5H, CpRh), 5.21 (s, 5H, CpCo), 2.64 (s, 6H, 4,5-Me), 2.15 (s, 3H, 2-Me), 1.41 (s, 6H, 1,3Me). 11B{1H} NMR (acetone-d6): δ 15.16 (bs). Anal. Calcd for C18H25B2CoF6PRh (569.82): C, 37.94; H, 4.42; B, 3.79. Found: C, 38.05; H, 4.00; B, 4.13. MS (EI): M+ 423.2. Synthesis of [CpCo(μ-1,3-C3B2Me5)IrCp]PF6 (4aPF6). Complex 4aPF6 was prepared similarly to 2aPF6 from 1Tl (0.072 g, 0.16 mmol) and [CpIrI2]2 (0.076 g, 0.07 mmol). Red solid. Yield: 0.050 g (51%). 1 H NMR (acetone-d6): δ 5.48 (s, 5H, CpIr), 5.26 (s, 5H, CpCo), 2.77 (s, 6H, 4,5-Me), 2.30 (s, 3H, 2-Me), 1.54 (s, 6H, 1,3-Me). 11B{1H} NMR (acetone-d6): δ 8.83 (bs). Anal. Calcd for C18H25B2CoF6IrP (659.13): C, 32.80; H, 3.82; B, 3.28. Found: C, 32.84; H, 3.74; B, 3.29. MS (EI): M+ 515.1. Synthesis of [CpCo(1,3-C3B2Me5)CoCp*]PF6 (2bPF6). Complex 2bPF6 was prepared similarly to 2aPF6 from 1Tl (0.115 g, 0.25 mmol) and [Cp*CoI2]2 (0.107 g, 0.12 mmol). After the counterion was changed to PF6− the 1H NMR was measured and showed the ratio 2b:2a:[CpCoCp*]+ to be approximately 1:1:1. Chromatography on an alumina column (0.5 × 20 cm) with CH2Cl2/Et2O (1:2) gave a yellow band of [CpCoCp*]PF6, then with CH2Cl2/Et2O (1:1.3) gave a brown band of 2bPF6, and, finally, with CH2Cl2/Et2O (1.3:1) gave a brown band of 2aPF6. Evaporation gave yellow [CpCoCp*]PF6 (0.031 g); brown 2aPF6 (0.043 g), and brown 2bPF6 (0.038 g, 26%). 1H NMR (acetone-d6): δ 4.96 (s, 5H, Cp), 2.42 (s, 6H, 4,5-Me), 1.86 (s, 3H, 2-Me), 1.60 (s, 15H, Cp*), 1.31 (s, 6H, 1,3-Me). 11B{1H} NMR (acetone-d6): δ 15.79 (bs). Anal. Calcd for C23H35B2Co2F6P (595.98): C, 46.35; H, 5.92; B, 3.63. Found: C, 46.49; H, 5.98; B, 3.55. Synthesis of [CpCo(μ-1,3-C3B2Me5)RhCp*]PF6 (3bPF6). Complex 3bPF6 was prepared similarly to 2aPF6 from 1Tl (0.154 g, 0.33 mmol) and [Cp*RhCl2]2 (0.095 g, 0.15 mmol). Orange solid. Yield: 0.150 g (78%). 1H NMR (acetone-d6): δ 5.12 (s, 5H, Cp), 2.30 (s, 6H, 4,5-Me), 1.85 (s, 3H, 2-Me), 1.61 (s, 15H, Cp*), 1.09 (s, 6H, 1,3-Me). 11 B{ 1H} NMR (acetone-d 6): δ 13.80 (bs). Anal. Calcd for C23H35B2CoF6PRh (639.95): C, 43.17; H, 5.51; B, 3.38. Found: C, 43.38; H, 5.60, B, 3.34. MS (EI): M+ 495.2. Synthesis of [CpCo(μ-1,3-C3B2Me5)IrCp*]PF6 (4bPF6). Complex 4bPF6 was prepared similarly to 2aPF6 from 1Tl (0.165 g, 0.36 mmol) and [Cp*IrCl2]2 (0.142 g, 0.18 mmol). Orange-red solid. Yield: 0.198 g (77%). 1H NMR (acetone-d6): δ 5.21 (s, 5H, Cp), 2.43

Table 7. Percentage Contributions of π, σ, and δ Interactions and Mayer Bond Orders (MBO) for [CpCo(ring)]2−n (Roman Type)a and [CpCo(ring)CoCp]4−n (Italics)b at the BP86/def2TZVPP//BP86/TZ2P Level

Cp−

Article

a

Using [CoCp]2+ and [(ring)]n−. bUsing [CpCo···CoCp]4+ and [(ring)]n− or two [CoCp]2+ and [(ring)]n− (in parentheses). cValues in brackets are given according to EDA at the BP86/TZ2P level.

compounds [CpCo(ring)]2−n the contribution of π donation increases, whereas that of σ donation and δ back-donation decreases. In the triple-decker complexes [CpCo(ring)CoCp]4−n a central ring ligand is bifacially bonded with two metal atoms, forcing it to more effectively use the available orbitals. In the first place, it leads to the strengthening of interactions which were weak in mononuclear complexes. In accordance with this tendency, the percentage contribution of σ donation increases and that of π donation decreases, although the latter is always of primary importance. The role of δ back-donation remains almost unchanged. Therefore, the contribution of σ donation is much greater in the case of the triple-decker complexes in comparison to that for sandwich compounds. Finally, Mayer bond orders (MBO) clearly indicate strenthening of the Co− ring bond on an increase of boron atoms, in both mononuclear and triple-decker complexes.

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CONCLUSION Reactions of the thallium derivative CpCo(μ-1,3-C3B2Me5)Tl with halides [M(ring)Hal2]2 allowed us to synthesize a wide range of cationic μ-diborolyl triple-decker complexes [CpCo(μ1,3-C3B2Me5)M(ring)]+. An electrochemical and spectroelectrochemical study revealed that they undergo reversibly a one-electron oxidation and two separate one-electron reductions. The large separation of the two reduction processes allowed us to classify the neutral Co(III)Co(II) species as completely delocalized Robin−Day class III mixed-valence systems. The computation satisfactorily predicted the potentials for the first reduction and first oxidation. An electrochemically induced increase in the number of electrons results in progressive elongation of the Co···M distance owing to the weakening of the metal−diborolyl bonds. A comparison of frontier orbitals of the redox species suggests that the oxidation is cobalt-centered whereas the reduction processes almost equally involve both metal atoms. According to Mulliken population analysis, the contribution of σ donation is much greater in the case of the triple-decker complexes in comparison to mononuclear compounds. 2721

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are bonded. All calculations were performed using the SHELXTL software.43 CCDC 927002 (for 2aBPh4), 927003 (for 2bPF6), 927004 (for 4aPF6 ), 927005 (5aOTf), and 927006 (for 5cPF6 ) contain 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. Computational Details. For the Computational Electrochemistry part, geometry optimizations were performed without constraints using the PBE exchange-correlation functional,44 the scalar-relativistic Hamiltonian,45 atomic basis sets of generally contracted Gaussian functions,46 and a density-fitting technique47 as implemented in a recent version of Priroda code.48 The all-electron double-ζ basis set L1 augmented by one polarization function49 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).50 The solvent (CH2Cl2) effects were included using the polarizable continuum model (PCM).51 The PCM calculations were performed by the Gaussian 03 program.52 For the Bonding Analysis part, the geometries have been optimized at the gradient corrected DFT level of theory using the exchange functional of Becke53 and the correlation functional of Perdew54 (BP86). Uncontracted Slater-type orbitals were employed as basis functions for the SCF calculations.55 Scalar relativistic effects were considered using the zero-order regular approximation (ZORA).56 An 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.57 The calculations were carried out using the ADF 2006.01 program package.58 Fragment orbital occupations were determined by Mulliken population analysis using the AOMix program.59 The input files were obtained from single-point calculations at the BP86/TZ2P optimized structures with the Gaussian 03 program using the BP86 functional and a basis set of triple-ζ quality with two polarization functions def2-TZVPP.60 Mayer bond orders61 were calculated at the same level of theory with the help of the Gaussian 03 and Chemissian62 programs. The ChemCraft program63 was used for molecular modeling and visualization.

(s, 6H, 4,5-Me), 2.01 (s, 3H, 2-Me), 1.70 (s, 15H, Cp*), 1.24 (s, 6H, 1,3-Me). 11B{1H} NMR (acetone-d6): δ 8.30 (bs). Anal. Calcd for C23H35B2CoF6IrP (729.27): C, 37.88; H, 4.84; B, 2.96. Found: C, 38.03; H, 4.88; B, 3.04. MS (EI): M+ 585.3. Synthesis of [CpCo(1,3-C3B2Me5)Ru(C6H6)]BPh4 (5aBPh4). Complex 5aBPh4 was prepared similarly to 2aPF6 from 1Tl (0.144 g, 0.31 mmol) and [(C6H6)RuCl2]2 (0.077 g, 0.15 mmol), using NaBPh4 instead of NH4PF6. Red solid. Yield: 0.085 g (75%). 1H NMR (acetone-d6): δ 7.33 (m, 8H, m-Ph, BPh4), 6.92 (m, 8H, o-Ph, BPh4), 6.78 (m, 4H, p-Ph, BPh4), 5.58 (s, 6H, C6H6), 5.01 (s, 5H, Cp), 2.53 (s, 6H, 4,5-Me), 2.03 (s, 3H, 2-Me), 1.36 (s, 6H, 1,3-Me). 11B{1H} NMR (acetone-d6): δ 14.62 (bs, 2B, C3B2), −6.51 (bs, 1B, BPh4). Anal. Calcd for C43H46B3CoRu (755.27): C, 39.28; H, 4.51; B, 3.72. Found: C, 39.15; H, 4.32; B, 3.71. MS (EI): M+ 436.7. Synthesis of [CpCo(1,3-C 3 B 2 Me 5 )Ru(p-MeC 6 H 4 Pr i )]PF 6 (5bPF6). Complex 5bPF6 was prepared similarly to 2aPF6 from 1Tl (0.120 g, 0.26 mmol) and [(p-MeC6H4Pri)RuCl2]2 (0.076 g, 0.12 mmol). Red solid. Yield: 0.093 g (62%). 1H NMR (acetone-d6): δ 5.49 (s, 4H, C6H4), 5.01 (s, 5H, Cp), 2.69 (sept, 1H, CHMe2), 2.49 (s, 6H, 4,5-Me), 2.16 (s, 3H, MeC6H4), 2.00 (s, 3H, 2-Me), 1.32 (s, 6H, 1,3Me), 1.23 (d, 6H, CHMe2). 11B{1H} NMR (acetone-d6): δ 14.30 (bs). Anal. Calcd for C23H34B2CoF6PRu (637.11): C, 43.36; H, 5.38; B, 3.39. Found: C, 43.76; H, 5.03; B, 3.39. MS (EI): M+ 492.8. Synthesis of [CpCo(1,3-C3B2Me5)Ru(C6Me6)]PF6 (5cPF6). Complex 5cPF6 was prepared similarly to 2aPF6 from 1Tl (0.142 g, 0.31 mmol) and [(C6Me6)RuCl2]2 (0.100 g, 0.15 mmol). After the counterion was changed to PF6− the 1H NMR was measured and showed the ratio 5c:2a:[CpRu(C6Me6)]+ to be approximately 1:1:1. Chromatography on an alumina column (0.5 × 20 cm) with CH2Cl2/ Et2O (1:2.5) gave [CpRu(C6Me6)]PF6, then with CH2Cl2/Et2O (1:1.5) gave a red band of 5c, and, finally, with CH2Cl2/Et2O (1.3:1) gave a brown band of 2a. Evaporation gave colorless [CpRu(C6Me6)]PF6 (0.041 g), brown 2aPF6 (0.038 g), and red 5cPF6 (0.034 g, 17%). 1 H NMR (acetone-d6): δ 5.01 (s, 5H, Cp), 2.29 (s, 6H, 4,5-Me), 2.04 (s, 18H, C6Me6), 1.79 (s, 3H, 2-Me), 1.15 (s, 6H, 1,3-Me). 11B{1H} NMR (acetone-d6): δ 13.40 (bs). Anal. Calcd for C25H38B2CoF6PRu (665.16): C, 45.04; H, 5.75; B, 3.31. Found: C, 45.37; H, 5.91; B, 3.26. Synthesis of [CpCo(1,3-C3B2Me5)Ru(η6-C7H8)]PF6 (6PF6). Complex 6PF6 was prepared similarly to 2aPF6 from 1Tl (0.220 g, 0.47 mmol) and [(η6-C7H8)RuCl2]2 (0.124 g, 0.23 mmol). Red solid. Yield: 0.127 g (46%). 1H NMR (acetone-d6): δ 6.19 (m, 2H, H3,H4 C7H8), 5.08 (s, 5H, Cp), 4.98 (m, 2H, H2,H5 C7H8)), 3.46 (m, 2H, H1,H6 C7H8)), 2.90 (d of t, 1H, Jvic = 18 Hz, H7(endo) C7H8)), 2.40 (s, 6H, 4,5-Me), 2.08 (s, 3H, 2-Me), 1.26 (s, 6H, 1,3-Me), 0.72 (d of t, 1H, Jvic = 18 Hz, H7(exo) C7H8)). 11B{1H} NMR (acetone-d6): δ 16.04 (bs). Anal. Calcd for C20H28B2CoF6PRu (595.03): C, 40.37; H, 4.74; B, 3.63. Found: C, 40.58; H, 4.91; B, 3.57. MS (EI): M+ 450.7. X-ray Diffraction Study. Crystals of 2aBPh4, 2bPF6, 4aPF6, 5aOTf, and 5cPF6 suitable for X-ray study were grown by slow diffusion of Et2O into their acetone solutions in a desiccator. The principal crystallographic data and refinement parameters are given in Table S2 in the Supporting Information. X-ray diffraction experiments were carried out with a Bruker Apex II CCD area detector, using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 100 K. Absorption correction was applied semiempirically using the APEX2 program.42 The structures were solved by direct methods and refined by full-matrix least squares against F2 in an anisotropic (for nonhydrogen atoms) approximation. 2aBPh4 crystallizes with four independent molecules in the unit cell, while 4aPF6 crystallizes with two independent molecules in the unit cell. The independent molecules of complexes 2aBPh4 and 4aPF6 differ from each other by torsion angles between the ligands. 2aBPh4 and 4aPF6 were grown as twinned crystals and further refined with HKLF 4 (BASF = 0.327) and HKLF 5 (BASF = 0.398), respectively. The counteranion PF6 of molecule A for 4aPF6 was disordered and left in an isotropic model. All hydrogen atom positions were refined in an isotropic approximation in the “riding” model with the Uiso(H) parameters equal to 1.2[Ueq(Ci)] and for methyl groups equal to 1.5[Ueq(Cii)], where U(Ci) and U(Cii) are respectively the equivalent thermal parameters of the carbon atoms to which the corresponding H atoms

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ASSOCIATED CONTENT

* Supporting Information S

CIF files giving crystallographic information for 2aBPh4, 2bPF6, 4aPF6, 5aOTf, and 5cPF6, Tables S1−S3, Figures S1−S9, and details of DFT calculations (atomic coordinates for optimized geometry and energy data) for [CpCo(μ-1,3-C3B2R5)M(ring)]−/0/+/2+, [CpCo(ring)]2−n, and [CpCo(ring)CoCp]4−n ([ring]n− = [Cp]−, [C4BH5]2−, [C3B2H5]3−, and [C2B3H5]4−). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.R.K.); [email protected] (P.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.Z. gratefully acknowledges the financial support of the University of Siena. A.S.R. and T.V.T. were supported by NSF grant DMR-0934212 (PREM). 2722

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DEDICATION This article is dedicated in memoriam to Professor Mikhail Yu. Antipin.

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