Cobaltocene with a Naphthalene Handle: Synthesis and

ARTICLE pubs.acs.org/Organometallics

An ansa-Cobaltocene with a Naphthalene Handle: Synthesis and Spectroscopic and Structural Characterization Nils Pagels, Marc H. Prosenc,* and J€urgen Heck* Fachbereich Chemie, Institut f€ur Anorganische und Angewandte Chemie, Universit€at Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany

bS Supporting Information ABSTRACT: The ansa-cobaltocene 1,10 -(naphthalen-1,8diyl)cobaltocene (2) and the corresponding cation 1,10 -(naphthalen-1,8-diyl)cobaltocenium hexafluoridophosphate (2þ) have been synthesized utilizing two different pathways. The solid-state molecular structure of the two complexes was determined by single-crystal X-ray diffraction. The redox properties were characterized by means of cyclic voltammetry. The electronic structure of the neutral, paramagnetic complex 2 has been studied by EPR, variable-temperature (VT) 1H NMR, and UV-vis spectroscopy, as well as DFT calculations. Due to the bent structure of the ansa-cobaltocene 2, the 2-fold degeneracy of the e1g-orbitals of the archetype cobaltocene is broken, which allows recording a well-resolved EPR spectrum at 100 K. The VT 1H NMR measurements in the temperature range -60 to þ60
C confirm 2 as a pure paramagnet. DFT calculations reveal an energy gap between the SOMO and LUMO of about 263 kJ/mol.

’ INTRODUCTION ansa-Metallocenes have attracted some interest, due to their versatile electronic and chemical properties. Their characteristics can be altered with the bridge between the Cp rings, resulting in a distorted, bent structure.1-6 In addition, bridged metallocenes of Fe and Co can be used as monomers in ring-opening polymerization (ROP),3 yielding polymers with interesting electronic, electrochemical, and solubility properties. The nature of the bridge between the two Cp ligands can be modified in a wide range, dominating the physical properties of the resulting polymer. The synthetic approach to an “ansa” function depends on the nature of the metallocene. For the synthesis of ansa-ferrocenes a 1,10 -dimetalation and subsequent salt metathesis is a very convenient method, while this transformation is not applicable to the electronically and magnetically highly interesting cobaltocenes.7,8 The formation of the ansa-metallocenes of Co was successfully performed via a nucleophilic reaction of the corresponding metal salt and two cyclopentadienyl anions, already connected by the bridging function.9 Here, we report on the synthesis, molecular structure, and electronic and magnetic properties of a new bent ansa-cobaltocene, 1,10 -(naphthalene-1,8-diyl)cobaltocene (2). ’ RESULTS AND DISCUSSION

(12þ),10 in the presence of a considerable excess of sodium amalgam (Scheme 1). Given that in a more careful reduction of 12þ the neutral complex 1,8-bis(cobaltocenyl)naphthalene (1) is formed in good yields,11 the mononuclear complex 2 is supposed to be a decomposition product caused by an overreduction of the dinuclear complex 1. As it is known from the neutral mononuclear cobaltocene, it undergoes a facile oneelectron reduction by means of alkali metals, whereupon one Cp ligand is easily cleaved in the presence of other ligands.12 In the case of the synthesis of 1, the presence of an excess of reducing agent may reduce one cobaltocene subunit in complex 1 (Scheme 2). Presumably, complex 2 results from an intramolecular rearrangement following Cp cleavage (Scheme 2). The driving force in the formation of the ansa-cobaltocene 2 might be the chelating effect of the 1,8-bis(cyclopentadienyl)naphthalene ligand. During the reduction process, the color of the reaction mixture turns from bright yellow to dark green, indicating the formation of the neutral, dinuclear complex 1, which is green in solution. With increasing reaction time, the color changes to intense yellow. It should be noted that the reaction time is significantly shorter (minutes instead of hours) when the starting material, complex 1,8-bis(cobaltocen-10 ylium)naphthalene bis(tetrafluoridoborate) (12þ), contains traces of nitromethane.

Synthesis. The ansa-metallocene 1,10 -(naphthalen-1,8-

diyl)cobaltocene (2) was first isolated by reduction of 1,8bis(cobaltocen-10 -ylium)naphthalene bis(tetrafluoridoborate) r 2011 American Chemical Society

Received: January 8, 2011 Published: March 10, 2011 1968

dx.doi.org/10.1021/om200013t | Organometallics 2011, 30, 1968–1974

Organometallics Scheme 1. Synthesis of 2 from 12þ

Scheme 2. Proposed Mechanism for the Formation of 2

The successful synthesis of the ansa-cobaltocene 2 prompted us to search for an alternative synthesis, which was accomplished by a salt metathesis reaction of cobalt dichloride with disodium1,8-bis(cyclopentadiendi-10 -yl)naphthalene (3) (Scheme 3). First attempts yielded an insoluble compound, presumed to be a polymer, which was not identified. Similar results were obtained by Rosenblum. The reaction of a complex, containing two sodium cyclopentadienide functions, with CoCl2 yielded a polymer as well.13 To avoid polymerization, the volume of the solvent was substantially increased, giving complex 2 in 21% yield. Finally, 1,10 -(naphthalene-1,8-diyl)cobaltocene (2) was oxidized with ferrocenium hexafluoridophosphate, yielding the cationic complex 1,10 -(naphthalen-1,8-diyl)cobaltocenium hexafluoridophosphate (2þ). Crystal Structure. Suitable crystals of complex 2 were obtained from saturated methylcyclohexane solution of the cationic species 2þ by slow evaporation of the solvent of a saturated dichloromethane solution. Both complexes have been analyzed by single-crystal X-ray structure determination. The complexes crystallize in the triclinic space group P1 (2) and the monoclinic space group P21/c (2þ). Figure 1(left) illustrates the molecular structure of 2 and numbering of the atoms; Figure 1(right) shows a projection along the C11-21-axis. Table 1 lists selected interplanar and torsion angles and interatomic distances. Whereas the molecule of 2 is slightly distorted from an ideal C2v symmetry in the crystalline state, the molecular structure of the cation of complex 2þ exhibits a C2v symmetry. Selected geometrical parameters are depicted in Figure 2. The angles of the best planes of the Cp rings to each other R, the angle between the centroids of the Cp rings at the Co atom β, and the ring slippage of the Co atom from the centroids of the Cp rings are parameters characterizing the molecular strain in the cobaltocene unit. To emphasize the influence of the metal center and its oxidation state on the overall structure, a comparison with the Fe congener 1,10 -(naphthalen-1,8-diyl)ferrocene 414 and the ansa-cobaltocene tetramethyldisilacobaltocenophane 51 is made (Table 1). The angle R increases in the order 5 < 2þ < 4 < 2, which can be attributed to the increasing population of antibonding orbitals, favoring longer M-C distances, while the rigid bridge constrains the Cipso-Cipso distance. The reversed order of the size for the angle β is found for the same reasons.

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Cyclic Voltammetry. Complex 2þ was subjected to electro-

chemical measurements. In cyclic voltammetric studies 2þ exhibits two reversible redox waves in the range of the archetype cobaltocene.16 While there is a small anodic shift of the potential of the redox couple 0/þ1 (E1/2 = -1.29 V) of about 50 mV for 2þ in comparison to cobaltocene (E1/2 = -1.352 V) and the [2]silacobaltocenophane 5 (E1/2 = -1.34 V), the values for the redox couple -1/0 are essentially the same (E1/2 = -2.3 V) (Table 2). The differences of the peak potentials ΔEP for the 0/ þ1 couple are in very good agreement for all three cobaltocene derivatives. For the -1/0 couple, the ΔEP value of the ansacobaltocenium 2þ is very similar to ΔEP(5), but distinctly larger than for the archetype cobaltocene. The deviation might be due to uncompensated solution resistance, as several subsequent scanning cycles show exactly the same behavior, thus excluding the possibility of an increased ΔEP due to fading reversibility.17 The reduction of cobaltocene to its anion and possible reactions have been thoroughly investigated with electrochemical methods by El Murr and Geiger.16 However, the isolation of the bis(cyclopentadienyl)cobaltate anion has never been reported so far, indicating a considerable lack of chemical stability. Though the cyclic voltammogram of 1,8-bis(cobaltocen-10 ylium)naphthalene bis(tetrafluoridoborate) (12þ) shows a redox wave at the characteristic potential range (E1/2 = -2.58 mV) for the formation of the anion, the cyclic voltammogram reveals more and more additional irreversible oxidation and reduction waves in subsequent scanning cycles. This redox behavior demonstrates the instability of the formed anion of 1,8-bis(cobaltocenyl)naphthalene (1). Electronic Spectra. Complexes 2 and 2þ have been investigated by UV-vis spectroscopy in the range 200 to 800 nm in acetonitrile at room temperature (see Table 3 and Figure 4).19 In analogy with spectroscopic investigations by Ammeter and Bachmann on cobaltocenes, the spectrum of complexes 2 and 2þ can be separated in three main regions: MfCp chargetransfer (CT) transition; CpfM CT and d-d transitions. We assign the very strong band at 214 nm in complex 2, which is absent in complex 2þ, to a MfCp transition. The other very strong transition observed in both spectra can be assigned to a πfπ* transition of the naphthalene unit. It occurs at almost the same energy for both complexes, indicating that the interaction between the metallocene and the organic moiety is weak. This is a result of the orthogonal geometry of the π-systems of the naphthalene unit with regard to the cobaltocene moiety as displayed in the molecular structure from single-crystal structure determination. The CpfM CT transitions of weaker intensity are observed in the region 250 to 400 nm. In the range above 300 nm d-d transitions also occur. The absorption at 341 nm is allocated to the prototypical e2gfe1g transition, which is not observed for complex 2þ. Additionally, a very weak and broad transition is found at ∼630 nm, which is also assigned to the d-d transitions (a1gfe1g). Electron Spin Resonance. Since the ansa-sandwich complex 2 is a cobaltocene derivative, it bears an unpaired electron and thus represents a spin state of S = 1/2, which may be suitable for EPR measurements. Commonly, unstrained cobaltocene complexes display EPR spectra only at very low temperatures due to the short relaxation time caused by the degenerate ground state.20 Bending the sandwich structure and substituting the cobaltocenes in 1,10 -position break the degeneracy.21 For complex 2, a well-resolved EPR spectrum is already obtained at 100 K (Figure 5). The spectrum displays a hyperfine coupling of the 1969

dx.doi.org/10.1021/om200013t |Organometallics 2011, 30, 1968–1974

Organometallics

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Scheme 3. Synthesis of 2 and 22þ Starting with the Dianion 3

Figure 1. (Left) Molecular structure and numbering scheme of the atoms of 2. The molecular structure of the cation 2þ looks very much the same. The hydrogen atoms are omitted for clarity. (Right) Projection of the molecule of 2 along the C11-C21 axis. Ellipsoids are drawn at the 50% probability level.

unpaired electron with the Co nucleus, which is magnetically active (I(59Co) = 7/2). The experimental EPR spectrum could be simulated with parameters listed in Table 4. The different g values and the hyperfine coupling constants (hfcc) of 2 and the tetramethyldisilacobaltocenophane 5 are very much alike, although g1 and g2 are swapped. The g factor of CoCp2, as a Jahn-Teller system, is highly sensitive to the host material, and measurements in toluene are not reported in the literature. However, a 1,10 -disubstituted cobaltocene reflects complex 2 by means of lifting the degeneracy by breaking the symmetry. Extensive studies on 1,10 -dimethylcobaltocene in different host systems by Ammeter revealed g-tensors in the range 1.71-1.81, 1.95-2.00, and 2.01-2.07 for gz, gy, and gx, respectively. The additional methyl groups erase the degeneracy to some degree, and the g-tensor values are close to our findings, indicating the electronic relation of 2 and Co(meCp)2 and demonstrating the comparatively small bending of the cobaltocene entity in 2.22 Nuclear Magnetic Resonance. Figure 6 shows the 1H NMR of 2 in toluene at room temperature; Figure 7, the σ vs 1/T plot of the VT-1H NMR data (temperature range -60 to 60
C) of 2 in toluene. No 2D experiments have been carried out due to the paramagnetic nature of the molecule. We assign the three signals in the diamagnetic range from 4 to 6 ppm to the naphthalene protons and the signals in the negative range at -32 and -115 ppm to the two sets of protons attached to the Cp rings. Due to line broadening at lower temperature, the signals of two naphthalene protons superimpose each other and the signal of the Cp proton with the lowest chemical shift vanishes. The complex obeys the Curie law, thus being paramagnetic in the temperature range in solution.

Electronic Structure Calculations. Geometry optimization of complex 2 revealed a nearly C2v symmetric structure with structural parameters listed in Table 1. The optimized geometric parameters are in good agreement with the parameters obtained by X-ray structural analysis. The ground state of complex 2 was calculated to be a doublet state. The frontier molecular orbitals are depicted in Figure 8. The SOMO of complex 2 is constructed by a nonbonding interaction of the π -system of the Cp ring and the naphthalene ring and an antibonding interaction between the 3dxz-orbital at the Co-center and the π-system of the Cp ring. In the LUMO the antibonding interaction between the 3dyz-orbital at the Co center and the π-system is increased due to the increased overlap between the fragment orbitals.21 An additional antibonding contribution between the Cp ring and the naphthalene ring destabilizes the LUMO and, thus, increases the splitting between the SOMO and LUMO. The large SOMO-LUMO gap of ΔE = 263 kJ/mol excitation energy prevents a thermal population of the LUMO state at room temperature, which is consistent with the Curie-Weiss behavior of the complex in solution.

’ CONCLUSION The synthesis of the ansa-cobaltocene 2 and its monocation 2þ were described for two different pathways. The complexes have been studied by cyclic voltammetry, VT 1H NMR, and UV-vis spectroscopy, indicating the close electronic relationship with cobaltocene and the cobaltocenium cation, respectively. However, single-crystal structure determination demonstrates a considerable bending of the sandwich entity in 2 and 2þ. This distortion in addition to the 1,10 -disubstitution raises the degeneracy of the eg-orbitals of the unperturbed cobaltocene, which is confirmed by means of DFT calculations. The splitting of the eg-orbitals of unsubstituted cobaltocene in a SOMO and a LUMO of different energy for the ansa-cobaltocene 2 enables EPR spectroscopy already at liquid nitrogen temperature. The g-values and the 59Co hfcc fall in the range of 1,10 -dimethylcobaltocene. ’ EXPERIMENTAL SECTION Unless otherwise noted, the reactions were carried out under dry nitrogen using standard Schlenk techniques. Solvents were saturated with nitrogen. Tetrahydrofuran (THF) was dried with potassium; methylcyclohexane, with sodium. CoCl2 was purchased and dried under vacuum at 180
C for several hours before use. 1,8-Bis(cobaltocen-10 -ylium)naphthalene bis(tetrafluoridoborate)10 and disodium-1,8-bis(cyclopentadiendi-10 -yl)naphthalene9 were prepared as described previously. NMR: Varian Gemini 2000 BB Bruker AVANCE 400. The compounds were measured at room temperature relative to the solvent. The AA0 BB0 spin system of the diamagnetic compound 2þ in acetonitrile was simulated using Bruker Biospin 1970

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Table 1. Selected Interplanar and Torsion Angles (deg) and Interatomic Distances (pm)a 2þb

2

4

2-DFTc

5

Angles CpC(11)-C(15)-CpC(21)-C(25) (R)

16.6

13.6

14.1

5.9

29.9

Cent[C(11)-C(15)]-M-Cent[C(21)-C(25)]d(β)

169.5

170.5

170.8

173.6

169.7

CpC(11)-C(15)-naphthalenee

82.2

80.0

83.5

n.a.

89.6

CpC(21)-C(25)-naphthalenee

81.5

sym. rel.f

83.0

n.a.

89.6

C(11)-C(1)-C(8)-C(21)

6.2

9.1

5.4

0.1 (Si-Si-bridge)

0.5

twist angle Cp ringsg

5.2

9.1

4.5

0.1

0.5

ring slippageh C(1)-C(11)

9.6/9.2 147.8

7.9 148.7

5.7/8.1 147.4

3.7/4.1 n.a

2.1 148.7

C(8)-C(21)

148.3

sym. rel.f

148.3

n.a

148.7

C(1)-C(8)

256.8

254.7

256.3

n.a

256.7

C(11)-C(21)

295.9

293.6

292.8

334.7

299.7

Cent[C(11)-C(15)]-Cent[C(21)-C(25)]d

338.2

323.6

324.3

342.0

348.5

M-C11/21

200.2/200.3

199.3

199.0/197.5

214.4/214.1

201.4/201.4

M-C12/15/22/25

208.1-211.3

202.9/201.2

200.7-2027

207.1-207.7

213.8-217.4

M-C13/14/23/24 M-Centd

210.0-212.8 169.8/169.9

204.8/205.4 162.4

204.0-206.3 162.5/162.8

210.7-210.9 171.4/171.1

213.8-217.4 175.0/175.0

C-C bond precision

0.6

0.3

0.3

0.2

Distances

a

See ref 15. b Molecule displays a mirror plane through the Co atom, the P atom, and naphthalene carbon atoms C9 and C10. c DFT geometry optimized parameters; see Electronic Structure Calculations for discussion. d Cent: centroid of the corresponding Cp ligand. e Best plane of the adjacent sixmembered naphthalene ring. f Symmetry related, therefore identical value with related parameter. g Calculated by torsion angle between C11 and C21 and the corresponding centroids of the Cp rings. h Deviation of the metal center perpendicular to the Cp ring plane from the centroid of the corresponding Cp ring.

Table 2. Cyclic Voltammetric Data of 2þ, [CoCp2]PF6, and the PF6 Salt of 5 in Acetonitrile at rt 2þa

Figure 2. Illustration of R, β, and the ring slippage.

[CoCp2]PF6b,c

51

redox

E1/2

ΔEP

E1/2

ΔEP

E1/2

ΔEP

couples

(V)

(mV)

(V)

(mV)

(V)

(mV)

0/þ1

-1.29

71

-1.352

69

-1.34

74

-1/0

-2.30

104

-2.297

70

-2.29

116

a In acetonitrile at room temperature, [nBu4N]PF6 (0.2 M) as the supporting electrolyte, Pt as the standard electrode referenced vs E1/2(ferrocene/ferrocenium) = 0 V, scan rate 200 mV/s. Potentials E in volts ( (0.005 V). b Acetonitrile, room temperature, platinum electrode vs FcH/ FcHþ, ref Ag/Agþ (0.1 M AgNO3), scan rate 100 mV/s. c See ref 18.

Figure 3. Cyclic voltammogram of 2 in acetonitrile at rt. Software TopSpin version 2.1. The paramagnetic VT NMR spectra were recorded with an almost saturated solution of 2 in toluene-d8. EPR spectra were recorded on a X-band spectrometer (Bruker Elexsys E500 CW) with a microwave frequency of 9.47 GHz, a modulation frequency

of 100 kHz, and a modulation amplitude of 5 G. An almost saturated solution of 2 in toluene in a sealed quartz tube was used as EPR sample. The spectrum was simulated with the program EasySpin.23 MS: Finnigan MAT 311 A (EI). Elemental analysis: CHN-O-Rapid, F. Heraeus, Zentrale Elementanalytik, Fachbereich Chemie, Universit€at Hamburg. Melting points were measured with a B€uchi setup after Dr. Tottoli in sealed glass capillaries and were not corrected. Cyclic voltammograms were measured on a Metrohm Autolab PGSTAT-101 with a glass cell suitable for measurements under inert gas. Pt electrodes were used: disk as working electrode, wire as reference, and rod as counter electrode. UV-vis spectra were recorded on a Cary 5E UV-vis-NIR spectrometer using quartz cuvettes (Helmar) with 0.1 mm radiation beam length.

1,10 -(Naphthalen-1,8-diyl)cobaltocene (2) from 1,8-Bis(cobaltocen-10 -ylium)naphthalene Bis(tetrafluoridoborate) by Reduction. To a suspension of 1,8-bis(cobaltocen-10 -ylium)-

naphthalene bis(tetrafluoridoborate) (containing roughly 0.5 equiv 1971

dx.doi.org/10.1021/om200013t |Organometallics 2011, 30, 1968–1974

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Table 3. Electronic Data of 2 and 2þa

Table 4. EPR Data of 2, 5, and Cobaltocene 2þ

2

compound

214(74.4) 221(74.3)

224(75.2)

265(11.3)

260(14.8)

279(1.9)

284(7.0)

291(11.1)

295(5.3)

host

g1

g2

g3

A1 A2 A3

2

toluene

100 2.008 1.852 2.068

80 174 435

5a

pentane 100 1.914 2.005 2.084

67 145 424

Co(C5H3But)2C2Me4b powder a

T [K]

7 g^ = 1.85

g|| = 2.05

See ref 1. b See ref 5.

303(8.3) 341(2.5) 630(0.5) a

λ (nm), ε in parentheses in (103 L cm-1 mol-1).

Figure 6. 1H NMR spectra of 2 in toluene at 294 K relative to solvent signal. Solvent signals are marked with an asterisk. The sample contains minor impurities marked as #, most likely due to solvent traces from the synthesis.

Figure 4. UV-vis spectra of 2 and 2þ in acetonitrile.

The reaction was repeated with the same conditions and stoichiometry, but a different charge of the dicationic species, without any nitromethane impurities. 1,8-Bis(cobaltocen-10 -ylium)naphthalene bis(tetrafluoridoborate) (1.09 g, 1.61 mmol) with NaHg (1.1%, 1.8 mL, 11 mmol) yielded 1,10 -(1,8-naphthyl)cobaltocene (160 mg, 0.511 mmol, 32%). (A significant difference between the two experiments was the time needed to change the color of the reaction mixture from an intense green to yellow. In contrast to the first experiment, the green color was stable for more than 2 h in the second experiment and turned yellow very slowly.) 1H NMR (200 MHz, toluene-d8, rt): δ 6.24 (2H, H-naph), 5.99 (2H, H-naph), 5.42 (2H, H-naph), -28.71 þ -100.99 (AA0 BB0 , 2  4H, H-Cp). Anal. Calcd for C20H14Co: C, 76.68; H, 4.50. Found: C, 76.05; H, 4.94. Mass spectrometry (FAB) m/z (relativ intensity): 313 (77) [M]þ. (FAB-HR): calcd 313.0416, found 313.0428. Melting point: 238-240
C (dec).

1,10 -(Naphthalen-1,8-diyl)cobaltocene (2) from Disodium-1,8-bis(cyclopentadiendi-10 -yl)naphthalene. To a solu-

Figure 5. EPR spectra of 2 in toluene at 100 K. of nitromethane) (0.820 g, 1.21 mmol) in THF (80 mL) was added sodium amalgam (1.1% Na, 1.2 mL, 7.5 mmol). The yellow suspension instantly started to turn green. After 30 min the intense green mixture was almost black, but in the course of the reaction changed to an intense yellow color after stirring for 17 h. The suspension was filtered through Celite521, and the solvent was removed under vacuum. Recrystallization in methylcyclohexane yielded black crystals (142 mg, 0.453 mmol, 37%).

tion of disodium-1,8-bis(cyclopentadiendi-10 -yl)naphthalene (404 mg, 1.08 mmol) in THF (200 mL) was added CoCl2 (144 mg, 1.11 mmol), and the suspension was stirred for 18 h at room temperature. The resulting intense yellow mixture was evaporated in vacuo, and the residue extracted with boiling methylcyclohexane (∼60 mL). The solvent volume was reduced by 50%, and precipitation at -18
C overnight and filtration yielded the product as a black, crystalline powder (73 mg, 0.23 mmol, 21%).

1,1 0 -(Naphthalen-1,8-diyl)cobaltocenium Hexafluoridophosphate (2þ). Complex 1 (73 mg, 0.23 mmol) was dissolved

in dichloromethane (30 mL), and bis(η5-cyclopentadienyl)iron(III) hexafluoridophosphate (88 mg, 0.27 mmol) was added. The reaction was stirred for 3 days and filtered, and diethyl ether was added, until precipitation took place. The suspension was cooled to -18
C overnight, and the yellow solid filtered off and redissolved in acetonitrile 1972

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’ COMPUTATIONAL DETAILS Geometry optimizations of all complexes were performed at the unrestricted DFT level of theory employing the B3LYP hybrid functional.26 For all atoms the def2-TZVP basis set was used.27 The ground state of complex 2 was calculated to be a doublet state in which the geometry optimization was performed. The geometry of complex 2 was checked by frequency calculations revealing no negative force constant. All calculations were performed using the Gaussian09 program package.28

’ ASSOCIATED CONTENT

bS

Supporting Information. Crystallographic tables for compounds 2 and 2þ and spin system simulation parameters are given. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 7. σ vs 1/T plot of the 1H NMR spectra of complex 2.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by Collaborative Research Center 668 (TPA4) of the Deutsche Forschungsgesellschaft (DFG). ’ DEDICATION Dedicated to Prof. Dr. Wolfgang Kaim on the occasion of his 60th birthday ’ REFERENCES Figure 8. Frontier orbitals of 2: SOMO (left), LUMO (right). (20 mL). Diethyl ether (80 mL) was added, and the mixture was cooled overnight to -18
C. The product precipitated as a bright yellow powder (56 mg, 0.12 mmol, 52%). 1H NMR (200 MHz, CD3NO2, rt): δ 8.18 (dd, 3J = 8.3, 4J = 1.3, 2H, H-2/7 or H-4/5), 7.83 (dd, 3J = 7.1, 4J = 1.4, 2H, H-2/7 or H-4/5), 7.68 (dd, 3J = 8.1, 3J = 7.1, 2H, H-3/6), 5.91 (AA0 BB0 , 1  8H, H-Cp). 1H NMR (400 MHz, CD3CN, rt): δ 8.18 (dd, 3 J = 8.4, 4J = 1.2, 2H, H-4/5), 7.81 (dd, 3J = 7.1, 4J = 1.3, 2H, H-2/7), 7.67 (dd, 3J = 8.2, 3J = 7.0, 2H, H-3/6), 5.85 þ 5.80 (AA0 BB0 , 2  4H, H-Cp). 13 C NMR (100 MHz, CD3CN): δ 136.92 (C-9), 136.21 (C-10), 132.14 (C-4/5), 131.37 (C-2/7), 126.32 (C-3/6), 125.27 (C-1/8), 102.31 (C11/21), 88.28 (C-12,15/22,25 or C-13,14/23,24), 87.86 (C-13,14/ 23,24 or C-12,15/22,25). Anal. Calcd for C20H14CoPF6: C, 52.42; H, 3.08. Found: C, 51.98; H, 3.28. Mass spectrometry (FAB) m/z (relative intensity): 313 (77) [M]þ. X-ray Structure Determination. The data were collected on a Bruker AXS Smart APEX CCD, Mo KR, λ = 0.71073 Å. The structures were solved by direct methods (SHELXS-97),24 and the refinements on F2 were carried out by full-matrix least-squares techniques (SHELXL97).25 All non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms were refined with a fixed isotropic thermal parameter related by a factor of 1.2 to the value of the equivalent isotropic parameter of their carrier atoms. Weights were optimized in the final refinement cycles. CCDC-792167 (1) and -792168 (2) 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.

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