Syntheses, Structures, and Properties of Substituted Co(C5Me5)(2,7

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Syntheses, Structures, and Properties of Substituted Co(C5Me5)(2,7di(OCH3)fluorenido Co(II) and Co(III) Complexes Bernhard E. C. Bugenhagen,† Lisa Brinn,† and Marc H. Prosenc*,†,‡ †

Fachbereich Chemie, Universität Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany Institut für Physikalische Chemie, TU Kaiserslautern, Erwin-Schrödinger-Strasse 52, D-67663 Kaiserslautern, Germany

‡

S Supporting Information *

ABSTRACT: The title compound, η5-2,7-dimethoxyfluorenido-η5-pentamethylcyclopentadienylcobalt(II) (1), was synthesized from Cp*Co(acac) and sodium 2,7-dimethoxyfluorenide. 1 was oxidized with AgBF4 to yield the cobaltocenium congener 1BF4. The structural comparison of both compounds reveals structural changes in the ligand scaffold. In the electronic spectra of 1, an MLCT transition was found that could be assigned by TD-DFT calculations. In this transition a significant quantity of spin-density is transferred to the fluorenido ligand, allowing for possible spin-filter or spin-injection applications.

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ray structure analysis. The oxidized Co(III) congener, 1BF4, was synthesized by treating a THF solution of 1 with AgBF4, subsequent extraction with methylene chloride, and crystallization from CH2Cl2/n-hexane. The red crystalline product (1BF4) was obtained in 56% yield. Crystal Structure Analysis. Both substances 1 and 1BF4 crystallize in the monoclinic space group P21 /n with comparable lattice constants (see Table S1). Selected bond distances and angles are listed in Table S2. The structure of complex 1 exhibits a nearly ideally eclipsed configuration of the Cp* ring and the opposing fluorenido ligand, indicated by the torsion angle C20−Cnt1−Cnt2−C12 of 4.5° (Figure 1, top). In the structure of the oxidized species 1BF4 a close to staggered configuration is found (torsion angle C20−Cnt1− Cnt2−C12 = 43.7°, Figure 1, bottom). In complex 1 distances of the Cp* ligand and the cobalt atom (dCnt1‑Co, Figure 1, top) of 169.4 pm and between the Co atom and fluorenido ligand (dCnt2‑Co Figure 1, top) of 174.5 pm are found. In complex 1BF4 distances of dCnt1‑Co = 164.2 pm and dCnt2‑Co = 167.8 pm were obtained. The decreases in distance of Δ(dCnt1‑Co) = −5.2 pm and Δ(dCnt2‑Co) = −6.7 pm upon oxidation of 1 to 1BF4 are expected for the transition from the 19-valence-electron to an 18-valence-electron cobaltocene complex.18,19 In addition, the distances of the carbon atoms in the five-membered ring of the fluorenido ligand vary from dCo−C12 = 204.3(2) pm to dCo−C5 = 217.5(2) pm in 1 and dCo−C12 = 204.3(2) pm to dCo−C11 = 210.7(2) pm. Within the 2,7-dimethoxyfluorenido ligand the most significant change in bond lengths is the distance C5−C6, which is found to be 147.0 pm in complex 1 and 144.6 pm in compound 1BF4. Electronic Spectra. To shed light on the electronic properties of 1 and 1BF4, UV/vis spectra of both compounds

obaltocenes have recently raised considerable interest due to their versatile redox, photophysical, and magnetic properties.1−4 In the emerging field of spintronics, paramagnetic organometallic complexes deposited on surfaces offer new possibilities to design magnetic devices. Currently, advances have been made in the surface deposition of metallocenes among other paramagnetic complexes and their investigation via (spin-resolved) scanning-tunneling microscopy (STM) and atomic-force microscopy (AFM).5−11 From these results some necessary properties of future materials consisting of magnetic molecules on surfaces can be derived. The conservation of the spin within the molecule is one requirement as well as a defined binding interaction between the molecule and the surface. Recent examples for applications of magnetic molecules on surfaces include spin-filter devices.12−14 In this application an exchange of spininformation between the substrate and the molecule is deemed important. To produce a defined substrate−molecule interaction, the paramagnetic compound needs to have a preferred binding site for the surface.4,15 This can be achieved through either specific substitution of the ligands or an extended aromatic system to facilitate an interaction on, for example, graphene or carbon nanotubes. We present the synthesis and investigation of the structural and electronic properties of a paramagnetic cobaltocene derivative with a methoxy-substituted fluorene ligand and its diamagnetic cobaltocenium congener.

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RESULTS AND DISCUSSION

For the synthesis of η 5 -2,7-dimethoxyfluorenido-η 5 pentamethylcyclopentadienylcobalt(II) (1) (Scheme 1), 2,7dimethoxyfluorene16,17 was deprotonated with NaHMDS in THF and treated in situ with a solution of CoCp*(acac) in THF. The product complex 1 was obtained after filtration and recrystallization from THF/n-pentane in 35% yield. Crystallization from cold hexane provided single crystals suitable for X© XXXX American Chemical Society

Received: September 10, 2014

A

dx.doi.org/10.1021/om500928w | Organometallics XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of 1 and 1BF4a

a

NaHMDS: sodium hexamethylsilazide; CoCp*acac: pentamethylcyclopentadienido-acetylacetonatocobalt(II).

compound 1BF4 transitions at 508, 416, and 364 nm were found. The most significant differences between the two spectra are the absorption band at 637 nm in the spectrum of 1, which is absent in the spectrum of the cation, and the band at 319 nm, which is red-shifted to 364 nm in the spectrum of 1BF4. Excitations were assigned based on TD-DFT calculations and subsequent natural transition orbital (NTO) analysis,20,21 which has been successfully employed on related complexes.22 The band at 637 nm in complex 1 was found to be a transition from the metal-localized SOMO to the fluorene-localized LUMO, which is further supported by its absence in the spectra of the oxidized species 1BF4. The excitation at 319 nm in 1 consists of a HOMO−2 to LUMO+1 transition (3 in Figure 4).

Figure 1. Crystal structures of 1 (top) and 1BF4 (bottom). Selected bond lengths [pm] and angles [deg] in 1: Cnt1−Co 169.4, Cnt2−Co 174.5, C5−C6 147.0, torsion C20−Cnt1−Cnt2−C1 4.5 and in 1BF4: Cnt1−Co 164.2, Cnt2−Co 167.8, C5−C6 144.6, C20−Cnt1−Cnt2− C1 43.7. Disordered BF4− anion and hydrogen atoms except for H12 are omitted for clarity; thermal ellipsoids are at 50% probability. Cnt1, Cnt2: Least-squares centroids of corresponding five-membered rings.

were recorded (see Table 1 and Figure 2). In the spectra of complex 1 transitions at 637, 508, 416, and 319 nm and for Figure 3. Experimental and simulated X-band EPR spectrum of 1. The spectrum was recorded in toluene at 77 K.

Table 1. Experimental and Calculated Electronic Transitions (nm) of Compounds 1 and 1BF4 with Extinction Coefficients (103 cm2 mol−1) in Parentheses 637 508 416 319

1

1-DFT

1BF4

1BF4-DFT

(3.1) (2.6) (5.1) (14.3)

572 507/489 409 317

508 (1.3) 416 (3.7) 364 (20.6)

510 419 365

Figure 2. UV/vis spectra of 1 (green) and 1BF4 (red) in THF at room temperature.

Figure 4. NTOs for the electronic transitions in compound 1. Absorption 1 is the MLCT that relocates the spin-density onto the fluorenido ligand. B


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derivatives of reduced symmetry such as ansa-cobaltocenes18,19 and a cobaltocene with a benzannulated fluorene ligand3 (see Table 1). This result is also supported by a VT 1H NMR spectroscopic study (Figure S5), which revealed a Curie−Weiss behavior between 228 and 337 K. DFT Calculations. To gain insight into the electronic structure of the cobaltocene complexes 1 and the cation in 1BF4, DFT calculations30−32 have been performed on both complexes. Selected bond parameters are listed in the Supporting Information (Table S2) and agree with data obtained from X-ray structure analyses. To reveal information on electron distribution and bonding in complexes 1 and 1BF4, an orbital analysis was performed. The 2E1g ground state of a symmetric cobaltocene is split significantly due to the different antibonding overlaps of the dyz and the dxz atomic orbitals at the Co atom with the apparent C5 ring π-orbitals at the Cp* and fluorenido ligands. The resulting SOMO in complex 1 consists of a delocalized orbital between the Cp* and fluorenido ligand and a dxz orbital at the metal center with a little less antibonding character between the metal and ligands. The SOMO in complex 1 becomes the LUMO in the cation of 1BF4, which is in accord with a shortening of the Co−ligand bonds and with only minor electronic reorganization upon oxidation. Within the fluorenido ligand in complex 1 the coefficients at the bridgehead carbon atoms of each C6 ring are opposite in sign and thus antibonding with respect to the C5− C6 bond. Upon oxidation of 1, this orbital becomes unoccupied, and thus the distance of the C−C bond within the fluorenido ligand decreases. This is in agreement with a shortening of the C5−C6 bond in the solid-state structure as well as the calculated data of 1BF4. Calculations of electronic excited states were performed on optimized ground-state structures. Electronic transitions were assigned by the corresponding natural transition orbitals (see Figures 4 and S2). The spin-density calculated for complex 1 reveals the unpaired electron to be mainly localized on the metal center with small contributions of the ligands. This result is supported by the VT 1H NMR spectroscopic spectra (see Figure S5), which are in accord with one defined ground state in the temperature region. It could be determined that only H12 of the fluorenido ligand and the Cp* protons experience a temperature-dependent paramagnetic shift where the signals of the aromatic protons in the fluorenido ligand can be found close to the same positions as in the spectrum of the diamagnetic compound 1BF4. Also the chemical shifts of the latter are temperature-independent. Upon electronic excitation at about 637 nm, the electron is transferred to the fluorenido ligand, resulting in a shift of the spin density to the fluorenido ligand (see Figure S4). If the cobaltocene complex is deposited on a metal or carbon surface presumably by the larger fluorenido ligand, this spin-density would be transferred to the surface. This is a prerequisite for a spintronic device based on cobaltocene complexes. Further studies on deposition and spintransfer are currently being performed.

In the HOMO−2 the orbital interaction between the carbon atoms C5 and C6 of the fluorenido ligand is antibonding, while the interaction in the LUMO+1 is bonding. Upon oxidation, this bond shortens (see above) and thus the LUMO+1 becomes stabilized and the HOMO−2 becomes destabilized, resulting in a smaller gap between these orbitals and thus a redshift of the band. Redox Properties. In the cyclic voltammogram of cation 1BF4 (Figure S4) one redox couple at negative potentials E1/2 of −1.540 V was observed. According to standard criteria,23,24 this redox couple is reversible (see Supporting Information). The potential E1/2 of 1+/1 is anodically shifted by 80 mV in comparison to the pentamethylcobaltocene complex (−1.65 V, propionitrile),25 indicating a weaker electronic donor capability by the bismethoxyfluorenyl ligand compared to the cyclopentadienyl ligand. A stronger cathodic shift of 177 mV compared to a benzannulated fluorene ligand (E1/2 = −1.363 V, CH2Cl2)3 results from the increased mesomeric donor capability of the two methoxy substituents compared to the annulated benzene rings. EPR Spectra. An X-band EPR spectrum of complex 1 was recorded in frozen solution in toluene at 77 K, resulting in a rhombic signal pattern (Figure 3). The spectrum was fitted using the Easyspin program package.26 A model consisting of a spin-1/2 system and a hyperfine coupling to 59Co (I = 7/2) was used. The parameters extracted from the simulation are g1 = 2.1158, g2 = 2.0332, g3 = 1.8617 and A1 = 381.6 MHz, A2 = 65.5 MHz, A3 = 141.0 MHz. These values are indicative of a high ganisotropy, comparable with other cobaltocene compounds of low symmetry (see Table 2). This was thoroughly investigated Table 2. EPR Parameters for 1 and Selected Cp2-Co(II) Compounds According to the Literaturea 1b Cp2Co28c Cp*2Co29 c C10H6(Cp2) Co18 b Me4Si2(Cp2) Co19 d Dbf-Co-Cp*3 e

g1

g2

g3

A1

A2

A3

2.116 1.689 1.140 1.676 1.935 1.852

2.033 1.863 1.219 1.737 1.992 2.008

1.862

65.5 411 361 405 430 80

141.0

1.585 1.721 1.772 2.068

381.6 262 76 36 84 174

1.914

2.005

2.084

424

145

67

1.880

2.010

2.102

179

98

372

309 252 222 435

a

Hyperfine couplings are given in MHz. bSpectrum recorded in toluene at 77 K. cSpectra recorded in 5% doped matrices of the Fe (top) and Ru (bottom) congeners at 4 K. dSpectrum recorded in npentane at 100 K. eSpectrum recorded in CH2Cl2 at 110 K.

by Ammeter et al. on cobaltocene or decamethylcobaltocene, i.e., highly symmetric (D5d) metallocene compounds with an electronic configuration of d7 and a 2E1g ground state.27,28 The degeneracy of the ground state results in Jahn−Teller distortion, and short relaxation times occur; thus, an EPR signal of these compounds can be observed only at very low temperatures. The lower symmetry of 1 (approximately Cs) however is sufficient to lift the degeneracy of the ground state and leads to the observable rhombic EPR signal even at temperatures of liquid nitrogen. As already mentioned above, the Co−C bond lengths vary significantly. This further supports a nondegenerate ground state due to a low local symmetry of the ligand scaffold around the cobalt atom. Thus, the electronic configuration of 1 is comparable to similar cobaltocene

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CONCLUSION The synthesis of 1 and consecutive oxidation to 1BF4 are described. To our knowledge, for the first time a fluorenidocobaltocene was structurally characterized in both its neutral and cationic form. In accord with related studies on cobaltocenes the metal ligand bond distance decreases upon oxidation. Excitation spectra exhibit a spin-active metal to ligand charge transfer band at 637 nm. This charge transfer C


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(19) Braunschweig, H.; Breher, F.; Kaupp, M.; Gross, M.; Kupfer, T.; Nied, D.; Radacki, K.; Schinzel, S. Organometallics 2008, 27, 6427− 6433. (20) Burke, K.; Werschnik, J.; Gross, E. K. U. J. Chem. Phys. 2005, 123, 62206. (21) Martin, R. L. J. Chem. Phys. 2003, 118, 4775. (22) Dabek, S.; Prosenc, M. H.; Heck, J. Organometallics 2012, 31, 6911−6925. (23) Olmstead, M. L.; Nicholson, R. S. Anal. Chem. 1966, 38, 150− 151. (24) Nicholson, R. S. Anal. Chem. 1966, 38, 1406−1406. (25) Hudeczek, P.; Köhler, F. H.; Bergerat, P.; Kahn, O. Chem.Eur. J. 1999, 5, 70−78. (26) Stoll, S.; Schweiger, A. J. Magn. Reson. 2006, 178, 42−55. (27) Ammeter, J. H.; Swalen, J. D. J. Chem. Phys. 1972, 57, 678−698. (28) Ammeter, J. H.; Oswald, N.; Bucher, R. Helv. Chim. Acta 1975, 58, 671−682. (29) Zoller, L.; Moser, E.; Ammeter, J. H. J. Phys. Chem. 1986, 90, 6632−6638. (30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian Inc.: Wallingford, CT, 2009, . (31) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (32) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297. (33) Zhang, Y.-Q.; Kepčija, N.; Kleinschrodt, M.; Diller, K.; Fischer, S.; Papageorgiou, A. C.; Allegretti, F.; Björk, J.; Klyatskaya, S.; Klappenberger, F.; et al. Nat. Commun. 2012, 3, 1286. (34) Abel, M.; Clair, S.; Ourdjini, O.; Mossoyan, M.; Porte, L. J. Am. Chem. Soc. 2011, 133, 1203−1205.

results in a shift of spin-density from the metal to the ligand in the excited state. The methoxy substituents at the fluorenido ligand will allow fixation of the complex on surfaces as well as coupling to larger molecular assemblies6,33,34 to form future spintronic devices.

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ASSOCIATED CONTENT

* Supporting Information S

Additional structural data, synthesis procedures, and theoretical calculations are available free of charge via the Internet at http://pubs.acs.org. Crystal structure data have been deposited at the CCDC with the numbers 1022993 and 1022999.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Collaborative Reasearch Center SFB 668 TP A4. For generous computational support the Regionales Rechenzentrum of the University of Hamburg and the Computing Cluster of the Inorganic and Applied Chemistry Institute are gratefully acknowledged. Further acknowledgements are extended to Prof. Peter Burger for the allocation of lab space.

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REFERENCES

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Syntheses, Structures, and Properties of Substituted Co(C5Me5)(2,7

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