Organometallics 2009, 28, 3319–3326
3319
Mixed Valence Properties in Ferrocenyl-Based Bimetallic FeCp-Indenyl-MLn Complexes: Effect of the MLn Group Saverio Santi,* Christian Durante, Alessandro Donoli, Annalisa Bisello, Laura Orian, and Alberto Ceccon* Dipartimento di Scienze Chimiche, UniVersita` degli Studi di PadoVa, Via Marzolo 1, 35131 PadoVa, Italy
Laura Crociani and Franco Benetollo CNR, Istituto di Chimica Inorganica e delle Superfici, C.so Stati Uniti 4, 35127 PadoVa, Italy ReceiVed October 3, 2008
A series of ferrocenyl-based complexes of general structure [η5-(2-ferrocenyl)indenyl]MLn [MLn ) RuCp*, FeCp, IrCOD, Mn(CO)3, and Cr(CO)2NO] were synthesized with the aim of tuning the effect of the nature of the second metal group MLn on the magnitude of the metal-metal electronic coupling in their mixed valence ions generated by electrochemical oxidation. The electronic interaction was probed by determining different and independent physical properties, the potential splitting in the cyclic voltammograms, and the IT bands in the near-IR spectra, which were rationalized in the framework of Marcus-Hush theory and at the quantum chemistry level by the density functional theory and TD density functional theory methods. On the basis of the obtained results, we were able to establish a trend based on the magnitude of the Fe-M electron transfer parameters Hab and R ranging from weakly to moderately coupled mixed valence ions. Introduction One of the reasons for the interest in mixed valence systems is the possibility to measure the rate constants and the activation parameters for electron transfer. They often show typical physical properties such as conductivity, absorption in the nearIR region, an increase or quenching of intensity, and a shift of luminescence whose characteristics can be correlated with the extent of the metal-to-metal electronic coupling. The electronic coupling occurs by the mixing of the metalbased donor (D) and acceptor (A) orbitals with those of appropriate symmetry of the bridging ligands. Thus, delocalization or vectorial transfer of an unpaired electron in mixed valence complexes made by the D-spacer-A unit can be varied by changing the metal, its ancillary ligands, the geometric and electronic structure of the bridge, and finally the medium (solvent, solid matrix, etc.). The structural, electrochemical, and spectroscopic results on mixed valence compounds are usually rationalized by means of classical and semiclassical models.1 The studies of the electronic interaction between two metal centers deal mostly with complexes where the two active sites are equivalent (homobimetallic complexes). Conversely, there is less information on the heterobimetallic systems, which indeed allow the investigation of the dependence of the cooperative effects on the redox asymmetry and, in particular, of the variations of the properties of one site in the presence of a (1) (a) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391. (b) Allen, C. C.; Hush, N. S. Prog. Inorg. Chem. 1967, 8, 357. (c) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247. (d) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1. (e) Crutchley, R. J. AdV. Inorg. Chem. 1994, 41, 273. (f) Demadis, K. D.; Haertshorn, C. M.; Meyer, T. J. Chem. ReV. 2001, 101, 2655. (g) Brunschwig, B. S.; Creutz, C.; Sutin, N. Chem. Soc. ReV. 2002, 31, 168. (h) D’Alessandro, D. M.; Keene, F. R. Chem. Soc. ReV. 2006, 35, 424.
different redox center in the same molecule.2 For instance, in a heterobimetallic compound one metal group which undergoes rapid chemically reversible oxidation or reduction may behave as a redox switch that can be turned off and on by reversible electron transfer. For these purposes, ferrocene is one of the most widely employed organometallic components.3 Its stability, excellent redox properties, specific electron donor character, and well-developed functionalization chemistry make it a primary candidate for testing the communication properties in novel π-conjugated carbon chains. Compounds containing at least one ferrocenyl (Fc) unit4 were thoroughly investigated as far as ground-state electronic coupling through space or through bonds is concerned. The magnitude of electronic coupling is largely dependent on the structure of the bridge. In this regard, we have recently reported that the electronic coupling between iron and rhodium in the mixed valence ions of the two isomers [η5-(2-ferroce(2) (a) Ceccon, A.; Santi, S.; Orian, L.; Bisello, A. Coord. Chem. ReV. 2004, 248, 683. (b) Kaim, W.; Lahiri, G. K. Angew. Chem. 2007, 119, 1808; Angew. Chem., Int. Ed. 2007, 46, 1778. (c) ComprehensiVe Organometallic Chemistry III; Mingos, D. M. P., Crabtree, R. H., Hiyama, T., Eds.; Elsevier: Oxford, U.K, 2007; Vol. 12, Chapter 4. (3) Ferrocenes; Stepnicka, P., Ed.; Wiley-VCH: Weinheim, Germany, 2008. (4) (a) Santi, S.; Orian, L.; Donoli, A.; Bisello, A.; Scapinello, M.; Benetollo, F.; Ganis, P.; Ceccon, A. Angew. Chem. 2008, 120 (29), 5411; Angew. Chem., Int. Ed. 2008, 47, 5331. (b) Venkatasubbaiah, K.; Doshi, A.; Nowik, I.; Herber, R. H.; Reingold, A. L.; Ja¨kle, F. Chem.sEur. J. 2008, 14, 444. (c) Santi, S.; Orian, L.; Durante, C.; Bencze, E. Z.; Bisello, A.; Donoli, A.; Ceccon, A.; Benetollo, F.; Crociani, L. Chem.sEur. J. 2007, 13, 7933. (d) Wagner, M. Angew. Chem. 2006, 118, 6060; Angew. Chem., Int. Ed. 2006, 45, 5916. (e) Nishihara, H. Bull. Chem. Soc. Jpn. 2001, 74, 19. (f) Debroy, P.; Roy, S. Coord. Chem. ReV. 2007, 251, 203. (g) Metallocenes: Synthesis, ReactiVity, Applications; Togni, A., Halterman, R. L., Eds.; Wiley-VCH: Weinheim, Germany, 2006. (h) A special issue was dedicated to ferrocene and its derivatives: J. Organomet. Chem. 2001, 637-639, 1. (i) Barlow, S.; O’Hare, D. Chem. ReV. 1997, 97, 637. (j) Ferrocenes; Togni, A., Hayashi, T., Eds.; Wiley-VCH: Weinheim, Germany, 1995.
10.1021/om800954b CCC: $40.75 2009 American Chemical Society Publication on Web 05/22/2009
3320 Organometallics, Vol. 28, No. 12, 2009 Scheme 1
nyl)indenyl]RhL2 and [η5-(1-ferrocenyl)indenyl]RhL2 (L ) CO; L2 ) COD, NBD) is much more pronounced in the former due to the almost coplanarity of the indenyl and cyclopentadienyl moieties in the bridging ligand whereas planarity is lost in the 1-ferrocenyl isomer.5a,b In the near-IR region intervalence electron transfer occurs in mixed valence compounds in which two or more redox sites exist in different oxidation states.5c This evidence prompted us to prepare a series of (2-ferrocenyl)indenyl complexes, [η5-(2-ferrocenyl)indenyl]RuCp* (1), [η5-(2-ferrocenyl)indenyl]FeCp (2) [η5-(2-ferrocenyl)indenyl]IrCOD (3), [η5-(2-ferrocenyl)indenyl]Mn(CO)3 (4), and [η5(2-ferrocenyl)indenyl]Cr(CO)2NO (5) (Scheme 1), and to study the phenomenon of the electron transfer in their mixed valence cations. Cyclic voltammetry, IR, and near-IR results on 1+-7+ are interpreted on the basis of the different electronic structures of the various metals. The characteristics of the new compounds are compared with the data recently obtained for [η5-(2ferrocenyl)indenyl]RhCOD (6) and [η5-(2-ferrocenyl)indenyl]RhNBD (7).5a Density functional theory (DFT) calculations carried out on both neutral and charged species quantify and rationalize the electron factors ruling the extent of electronic delocalization on varying the nature of the metals in bimetallic complexes. This specific series of compounds has been designed with the aim to obtain unsymmetrical cations with wide-ranging “redox asymmetry” that rules the metal-metal coupling. To accomplish this requisite, the second metal group of the binuclear complexes, all of which containing the (2-ferrocenyl)indenyl skeleton, must be necessarily very different in nature, i.e., electron-releasing, RuCp*, FeCp, and IrCOD, and electron-withdrawing, Mn(CO)3 and Cr(CO)2NO. (5) (a) Santi, S.; Orian, L.; Durante, C.; Bisello, A.; Benetollo, F.; Crociani, L.; Ganis, P.; Ceccon, A. Chem.sEur. J. 2007, 13, 1955. (b) Santi, S.; Ceccon, A.; Bisello, A.; Durante, C.; Ganis, P.; Orian, L.; Benetollo, F.; Crociani, L. Organometallics 2005, 24, 4691. (c) The mixed valency concept may be extended to heteronuclear systems by interpreting that each metal exists in different oxidation states. Strictly speaking, a mixed valence species exists in two valence isomers between which a vectorial ET occurs and an IT band appears in the near-IR. Examples of heterobimetallic species featuring unsaturated bridges in which two different metals aresignificantlycoupledarequiterare.Amongthese,thespecies[(Cp)Re(NO)(PPh3C4-(dppe)Fe(Cp)]+ reported by Lapinte and Gladysz was defined as a mixed valence radical ion: Paul, F.; Meyer, W. E.; Toupet, L.; Jiao, H.; Gladysz, J. A.; Lapinte, C. J. Am. Chem. Soc. 2000, 122, 9405.
Santi et al.
Results and Discussion Synthesis and Crystal Structure. The synthetic approach to the new complexes 1, 3, and 4 is to attach a suitable metalated (2-ferrocenyl)indenyl of potassium (1, 3) or trimethylstannate (4) to the corresponding metal center [Ru(µ-Cl)(Cp*)]n, [Ir(µCl)(COD)]2, and Mn(CO)5Br, respectively. Differently, complex 5 was obtained by treating with Diazald the η5-Cr(CO)3 anion formed first by metalation of the heterobimetallic complex [η6(2-ferrocenyl)indene]Cr(CO)35b with potassium hydride at low temperature and then by raising the temperature to produce the rapid η6 f η5 haptotropic rearrangement.6 Finally, the previously reported synthesis of complex 27 was modified by adopting a reaction scheme8a,b which proceeds through an initial thermal step providing the intermediate η1-[(2-ferrocenyl)indenyl]Fe(CO)2Cp followed by a photochemical η1 f η5 conversion with concurrent elimination of two molecules of CO.8c,d The crystal structure obtained for complex 4 is shown in Figure 1. The crystallographic data and the most significant interatomic distances and angles are collected in Tables 1 and 2. As expected on the basis of previously reported structures of the related complexes 6 and [η5-(2-ferrocenyl)indenyl]Rh(CO)2,5a the value of the torsion angle C1-C2-C11-C12 is quite low (11.1(5)°). The Cp-indenyl bridge is almost planar, a conformation which favors the π-electron resonance in the conjugated fulvalenyl-like structure. As found for complexes 6 and [η5-(2-ferrocenyl)indenyl]Rh(CO)2, the structure of 4 assumes a transoid conformation in which the two metal groups are coordinated on the opposite sides of the bridging ligand. The role of the steric hindrance against a hypothetical cisoid configuration, in which the Mn(CO)3 group would face the ferrocenyl group at prohibitive contact distances, is easily conjecturable.5a Similarly, in the crystal structure of 2, the two iron atoms are in a transoid arrangement7 but the torsion angle between the cyclopentadienyl and indenyl rings is much higher (24.2°) than in 4. Cyclic Voltammetry. The oxidation potential and the reversibility of compounds 1-5 were determined by cyclic voltammetry (CV) in CH2Cl2 solution containing 0.1 M n-Bu4NPF6. All the compounds display two oxidation waves in the range of 0-1.5 V vs SCE (Figure 2), and the potentials are reported in Table 3. The first oxidation waves are fully reversible (chemically and electrochemically) and are assigned to the redox couple FeII/FeIII. They occur at potentials which differ from the value of ferrocene depending on the donoracceptor features of the second metal groups. In particular, the FeCp-indenyl moiety of the homobimetallic complex 2 is oxidized at a far lower potential than ferrocene due the higher donor capability of indenyl with respect to Cp.7 Among the heterobimetallic complexes, the first wave of 3-5 was assigned with good confidence to the oxidation of the ferrocenyl group on the basis of the E1/2 values found for the corresponding monometallic complexes Fc and 11-13. Thus, in the heterobimetallic complexes, the Fc group is first oxidized and the values of E1/2 (Table 3) are in the order 1 ≈ (6) Ceccon, A.; Gambaro, A.; Gottardi, A.; Santi, S.; Venzo, A.; Lucchini, V. J. Organomet. Chem. 1989, 412, 85. (7) Lee, S. G.; Lee, S. S.; Chung, Y. K. Inorg. Chim. Acta 1999, 286, 215. (8) (a) Stradiotto, M.; Hughes, D. W.; Bain, A. D.; Brook, M. A.; McGlinchey, M. J. Organometallics 1997, 16, 5563. (b) Kerber, R. C.; Garcia, R.; Nobre, A. L. Organometallics 1996, 15, 5563. (c) Belmont, J. A.; Wrighton, M. S. Organometallics 1986, 5, 1421. (d) Mitchell, R. H.; Chen, Y.; Khalifa, N.; Zhou, P. J. Am. Chem. Soc. 1998, 120, 1785. (9) Richardson, D. E.; Taube, H. Coord. Chem. ReV. 1984, 60, 107. (10) Gassman, P. G.; Sowa, J. R., Jr.; Hill, M. G.; Mann, K. R. Organometallic 1995, 14, 4879.
Mixed Valence Properties in FeCp-Indenyl-MLn Complexes
Organometallics, Vol. 28, No. 12, 2009 3321
Figure 1. DFT-optimized molecular structures of 1, 2, and 5 and crystallographic structure of 4. In 4 hydrogen atoms are omitted for clarity. Table 1. Crystallographic Data for Complex 4 empirical formula mol wt cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcalcd (g cm-3) µ(Mo KR) (mm-1) F(000) no. of reflns collected no. of reflns used [I g 2σ(I)] goodness-of-fit on F2 R ) ∑(|Fo| - |Fc|)/∑|Fo| Rw ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2
C22H15O3FeMn 438.13 monoclinic P21/c 10.192(3) 8.945(2) 20.796(3) 100.87(3) 1861.9(7) 4 1.563 1.48 888 4335 3979 1.270 0.042 0.091
3 , 5 ≈ 4, indicating clear-cut electronic effects of the hetero metal groups. The second oxidation waves are fully reversible as well, except that of compound 5, which displays a chemically irreversible profile but becomes partially chemically reversible at a high scan rate (V g 100 V s-1), and the oxidation potential depends strongly on the nature of the second metal group too. Thus, the electrochemical characteristics allow determination for 1-5 of the values of the peak separation between the two waves (∆E1/2) and the related values of the equilibrium constant (Table 3) for the comproportionation, Kc (eq 1). The estimated values of Kc are indicative of the thermodynamic stability9 of the cationic species 1+-5+, suggesting that the characterization the mixed valence cations 1+-5+ is viable. Interestingly, the ∆E1/2 values found for 1-5, especially that of 1, are larger than the difference of the oxidation potentials between the corresponding monometallic compounds, i.e., ferrocene and (indenyl)RuCp* 10 (9), (indenyl)FeCp7 (10), (indenyl)IrCOD (11),
(indenyl)Mn(CO)3 (12), or (indenyl)Cr(CO)2NO (13) (Table 3), suggesting that metal-metal interaction might exist in the cations 1+-5+. However, the assessment of the degree of metal-metal electronic communication by means of the ∆E1/2 values must be handled cautiously. In fact, the energetics of the mixed valence systems ∆Gc related to Kc may strongly depend on the magnetic exchange between the unpaired electrons in the doubly oxidized reagent,11 in our case the species FeIIIsMn+1 of eq 1, especially in weakly interacting mixed valence systems. Moreover, it was pointed out that ∆E1/2 is influenced by ion-pairing and the supporting electrolyte, which can modify the electrostatic interaction in the dications.5a,12 More information can be obtained by the analysis of near-IR spectra.
[FeII-Mn] + [FeIII-Mn+1]2+ a 2[FeIII-Mn]+
(1)
Near-IR Spectroscopy. The near-IR is a spectral region diagnostic for donor-acceptor charge transfer processes, and the analysis of the IT bands by using the classical two-state electron transfer model (Hush theory) represents a powerful probe for evaluating the degree of the metal-metal interaction in mixed valence systems.1 According to Robin and Day classification,1c three classes of mixed valence systems can be distinguished on the basis of the magnitude of the metal-metal electronic coupling.. The strength of electronic interaction between the oxidized and reduced sites ranges from essentially zero (class I) to moderate (11) (a) Evans, C. E.; Naklicki, M. L.; Rezvani, A. R.; White, C. A.; Kondratiev, V. V.; Crutchley, R. J. J. Am. Chem. Soc. 1998, 120, 13096. (b) Lapinte, C. J. Organomet. Chem. 2008, 693, 793. (12) (a) Barrie´re, F.; Geiger, W. E. J. Am. Chem. Soc. 2006, 128, 3980. (b) Barrie´re, F.; Camire, N.; Geiger, W. E.; Mueller-Westerhoff, U. T.; Sanders, R. J. Am. Chem. Soc. 2002, 124, 7262. (c) D’Alessandro, D. M.; Keene, F. R. Dalton Trans. 2004, 3950.
3322 Organometallics, Vol. 28, No. 12, 2009
Santi et al.
Table 2. Significant Calculated Interatomic Distances (Å), Angles (deg), and Structural Parametersa 1
1+
2
2+
Fe-Ru
5.35
5.37
Fe-Fe′
5.18
5.19
Fe-Mn
Fe-Q1b
1.65
1.68
Fe-Q1b
1.65
1.68
Fe-Q1b
Fe-Q2b
1.66
1.70
Fe-Q2b
1.66
1.69
Fe-Q2b
Ru-q1c
1.89
1.89
Fe′-q1c
1.67
1.70
Mn-q1c
Ru-q2c
1.82
1.82
Fe′-q2c
1.66
1.70
Mn-COd
q2-Ru-q1c
0.5
0.5
q2-Fe′-q1c
1.3
2.6
C-Od
5
5+
Fe-Cr
5.36
5.37
1.70
Fe-Q1b
1.65
1.70
1.71
Fe-Q2b
1.66
1.71
1.82
Cr-q1c
1.90
1.89
1.80
Cr-COd
1.85
1.87
1.17 1.142
1.16
Cr-NO C-Od
1.70 1.17
1.70 1.16
Mn-C-Od
178.5 178.0
178.3
N-O Cr-C-Od
1.19 178.5
1.18 178.9
0.5 1.8 0.1 1.2 8.6 3.8 0.07 0.06 0.002 0.007 2.0 0.5 177.7 169.5
2.8
Cr-N-O Q1-Fe-Q2b
4.5 0.8
4.1 2.2
0.5
Q1-MP-Q1b-e
0.6
2.3
6.3
6.5
Q1-Fe-Q2b
0.3
0.7
Q1-Fe-Q2b
0.4
1.7
Q1-Fe-Q2b
q1-MP-Q1b-e
2.5
3.3
q1-MP-Q1b-e
0.9
0.6
q1-MP-Q1b-e
f Ind
f Ind
f Ind
FA
8.1
6.0
FA
4.7
6.6
FA
∆Rug
0.07
0.08
∆Feg
0.07
0.1
∆Mng
g
g
∆Fe
0.003
0.03
∆Fe′
FACpf
2.1
2.6
FACpf
h
γ
178.1
178.1
h
γ
g
0.002
0.02
∆Fe
1.6
2.1
FACpf
179.1
180.0
h
γ
4
4+
5.29 5.153 1.65 1.637 1.66 1.641 1.83 1.735 1.78 1.795
5.31
f Ind
5.8
FA
0.1
∆Crg
0.06
0.09
0.02
g
∆Fe
0.0
0.03
4.3
FACpf
1.5
4.0
178.0
178.3
178.2
h
γ
a Crystallographic data are reported in italics. b Q1 denotes the centroid of the Cp ring linked to the indenyl moiety; Q2 denotes the centroid of the free Cp ring. c q1 denotes the centroid of the Cp ring of the indenyl moiety; q2 denotes the centroid of the free Cp* ring. d Average value. e MP denotes the middle point of the σ bond linking the Cp and the indenyl moieties. f FA is the folding angle, i.e., the dihedral angle between the planes containing C1-C2-C3 and C1-C3-C4-C5 (FACp) or between the planes containing C11-C12-C19 and C12-C13-C18-C19 (FAInd); the numbering scheme is shown in Figure1. g ∆ is the slippage parameter defined as 0.5[(Fe-C1 + Fe-C3) - (Fe-C4 + Fe-C5)] (∆Fe) or 0.5[(M-C12 + M-C19) (M-C13 + M-C18)] (∆M); the numbering scheme is shown in Figure1. h γ is the dihedral angle M′-q1-Q1-Fe.
Figure 2. CVs of complexes 1-5 in CH2Cl2/0.1 M n-Bu4NPF6. Scan rate V ) 0.5 V s-1 (1-4) and 100 V s-1 (5).
(class II) to very strong electronic coupling (class III). Recently, Meyer and co-workers13 have defined borderline class II-class III systems which exhibit both delocalized and localized behavior. The Hush theory applies in the case of the class II regime. Spectroelectrochemistry in the vis-near-IR region of all complexes 1-5 in CH2Cl2 and 0.1 mol dm-3 Bu4NPF6 (Figure 3, Table 4) showed the appearance of intense and Gaussianshaped absorption bands. At a glance, the absorption bands of the mixed valence cations 1+-5+ are spread in two clear-cut optical regions, visible and near-IR, depending on the electronic properties of the second metal groups. Gaussian deconvolution of the spectra of 1+, 2+, 4+, and 5+ evidenced that multiple (13) Chen, P.; Meyer, T. J. Chem. ReV. 1998, 98, 1439.
transitions are present in the range of 6000-14000, while for complex 3+ only one intense and low-energy band is observed (Table 4). According to the first oxidation potential relative to the Fc/ Fc+ couple, the cations 1+ and 3+, in which the electron-rich RuCp* and IrCOD groups are coordinated to the Cp moiety of indenyl, display their maximum absorption at 7485 and 6950 cm-1, respectively. Conversely, 4+ and 5+, containing the electron-poor Mn(CO)3 and Cr(CO)2NO groups, show the most intense band in the visible region at 13560 and 13620 cm-1 (εmax ) 720 mol-1 dm3 cm-1) and very weak bands in the nearIR (Table 4). Interestingly, in the spectrum of 2+ two absorption bands at different energies are clear. At difference with the other complexes, complex 2 is first oxidized at FeCp-indenyl and not at the ferrocenyl site at a far lower potential than ferrocene, suggesting the sense of the vectorial electron transfer of its mixed valence state is different with respect to the Fc-oxidized cations 1+ and 3+-5+. The low-energy bands in the near-IR are generally assigned to charge transfer IT transitions. In particular, we found that IT bands of mixed valence heterobimetallic complexes of the Cp-indene and Cp-indenyl ligands lie at wavenumbers lower than 10000 cm-1. The classical electron transfer model of Hush relates the absorption maximum (ν˜ max) of the IT transition to the difference in the electrochemical potentials between the donor and acceptor metal groups by
ν˜ max ) E0 + λ
(2)
in which E0 is the redox asymmetry, i.e., the energy difference between the two states FeIII-spacer-Mn and FeII-spacer-Mn+1,
Mixed Valence Properties in FeCp-Indenyl-MLn Complexes
Organometallics, Vol. 28, No. 12, 2009 3323
Table 3. CV Dataa 1 2 3 4 5 Fc 9c 10d 11e 12e 13e
E1p
E2p
1 E1/2
2 E1/2
0.37 0.28 0.39 0.59 0.58 0.51 0.51 0.35 0.67 1.24f 1.04f
0.82 0.66 0.72 1.33 1.08f
0.34 0.25 0.36 0.54 0.54 0.48 0.48 0.32 0.64 1.21 1.00
0.77 0.62 0.66 1.28 1.04f
1 E1p - Ep/2
0.061 0.062 0.063 0.072 0.083 0.063
2 E2p - Ep/2
∆E1/2b
0.065 0.059 0.055 0.082 0.080
0.44 0.37 0.30 0.74 0.50
Kc 3.7 2.3 1.4 5.2 3.9
× × × × ×
107 106 105 1012 108
0.059 0.059 0.062 0.091
a 2 All potentials are in volts vs SCE. The solvent was CH2Cl2, the supporting electrolyte 0.1 M n-Bu4NPF6, and the scan rate 0.5 V s-1. b ∆E1/2 ) E1/2 1 - E1/2 . c Reference 10. d Reference 7. e See the Supporting Information. f Estimated at a high scan rate (100 V s-1).
Figure 3. Near-IR spectroelectrochemistry in CH2Cl2/0.1 mol dm-3 NBu4PF6 at an applied potential from 0.2 to 0.4 V (1-3) and from 0.2 to 0.6 V (3-5) (solution concentration 3.0 mol dm-3, T ) -20 °C, scan rate V ) 5 mV s-1).
and λ is the nuclear reorganization energy. E0 can be estimated IndMLn from the E1/2 values (Table 3) by using the formula [(E1/2 1 IndMLn 0.08) - E1/2 ], where E1/2 is the formal potential of the oxidation wave of monometallic indenyl-MLn complexes and 0.08 V represents the substituent effect of ferrocene.14 Provided that λ is constant, a plot of ν˜ max vs E0 is linear, and this occurs for the unsymmetrical mixed valence cations 1+-7+ (Figure 4). The least-squares fit gives a slope of 0.89 ( 0.19 in good agreement with the theoretical value of unity, suggesting that ν˜ max can be predicted for the redox asymmetry E0. The resulting linear plot has been employed to assign the low-energy bands of the cations 4+and 5+ to IT transitions at 9795 and 10580 cm-1, respectively. The oscillator strength of a Gaussian-shaped band can be experimentally determined by means of
f ) (4.6 × 10-9)εmax∆V˜ 1/2
(3)
Linear correlation of the calculated values of f as a function of E0 is found within the series of unsymmetrical mixed valence cations 1+-7+ (except 2+), clearly indicating that the decrease of the energy gap between the donor and the acceptor, the redox asymmetry E0, results in greater oscillator strength in the charge transfer transition. The noncorrelation of 2+ resides in the fact that the first oxidation potential occurs at the FeCp-indenyl site with the result that the sense of the vectorial electron transfer is inverted with respect to the other mixed valence compounds. Together with the linear correlation of V˜ max with E0, these results indicate that a more efficient charge transfer corresponds (14) Yeung, L. K.; Kim, J. E.; Chung, Y. K.; Rieger, P. H.; Sweigart, D. A. Organometallics 1996, 15, 3891.
to a smaller E0. The explanation resides in the Hush relationships1 between the intensity and shape of the IT band and the degree of charge delocalization (see Table 4). By using in the calculation the geometrical metal-metal separation, which can be considerably longer than d, we estimate a lower limit of R and Hab (Table 4). For this purpose we used the mean value, 5.3 Å, of the DFT-calculated metal-metal distances of 1+, 2+, 4+-6+, and [η5-(2-ferrocenyl)indenyl]Rh(CO)2.5a In particular, the bimetallic cations 1+-3+ containing electron-rich MLn groups are characterized by lower redox asymmetry E0 and higher electron transfer parameters (f, Hab, and R, Table 4) with respect to 4+ and 5+, which have electron-poor MLn groups. Two distinct zones of ν˜ max and f are sharply defined in the plots of Figures 4 and 5. Accordingly, the calculated thermal barrier to the electron transfer ∆Gq 15 (Table 4) increases in the order 1+ ≈ 2+ < 3+ , 5+ < 4+ with the values of 4+ and 5+ much higher than those of 1+-3+. The classical Hush analysis is known to fail when applied to systems with strong electronic coupling, and thus, it is not suitable to describe borderline class II-class III compounds. Nevertheless, it is traditionally employed, and a disagreement between the calculated and the experimental values of ∆V˜ 1/2 is ascribed to the presence of a rather strong interaction. This situation was previously found by us for the correlated mixed valence cations 6+ and 7+, which have borderline characteristics. Nevertheless, the oscillator strengths of 6+ and 7+ calculated in the class II limit nicely correlate in the linear plot of Figures 4 and 5. This evidence prompted us to verify whether 1+ and 3+, which display thermodynamic and optical behavior similar to that of 6+ and 7+, might be assigned to the class of borderline mixed valence species. In particular, the near-IR band of 1+ is quite narrow, and its experimental ∆V˜ 1/2 value is much lower than that expected by using the Hush relationship (∆ν˜ 1/2)Hush (cm-1) ) [16RT(ln 2)(ν˜ max - E0)]1/2. To this purpose, we studied the solvent effect on the IT band comparing the ν˜ max values obtained in three solvents (Table 5), namely, CH2Cl2, CH3CN, and DMF,16 with noticeably different dielectric constants and donor numbers (DNs). For 1+, 2+, and 3+ a significant blue (15) Brunschwig, B. S.; Sutin, N. Coord. Chem. ReV. 1999, 187, 233. (16) Abboud, J.-L. M.; Rotario, R. Pure Appl. Chem. 1999, 71, 645. (17) (a) Santi, S.; Orian, L.; Donoli, A.; Durante, C.; Bisello, A.; Ganis, P.; Ceccon, A.; Crociani, L.; Benetollo, F. Organometallics 2007, 26, 5867. (b) Merola, J. S.; Kackmarcik, R. T. Organometallics 1989, 8, 778. (c) King, R. B.; Efraty, A. J. Organomet. Chem. 1970, 23, 527. (d) Atwood, J. L.; Shakir, R.; Malito, J. T. J. Organomet. Chem. 1979, 165, 65. (18) (a) North, A. T. C.; Philips, D. C.; Mathews, F. S. Acta Crystallogr., A 1968, 24, 351. (b) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115. (c) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (d) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
3324 Organometallics, Vol. 28, No. 12, 2009
Santi et al.
Table 4. Near-IR Data in CH2Cl2/n-Bu4NPF6, T ) -20 °C ν˜ max (cm-1) (εmax (M-1 cm-1)) 1+ 2+ 3+ 4+ 5+ 6+ 7+
7485 (1975) 12400 (170) 6010 (475) 12320 (1110) 6950 (2200) 6170 (30) 10580 (230) 13560 (1220) 7060 (108) 9868 (72) 13620 (722) 6080 (2620) 6630 (2480)
E0 (cm-1)
(∆ν˜ 1/2)obsd (cm-1)
(∆ν˜ 1/2)calcda (cm-1)
Habb (cm-1)
Rc
∆Gq d (kJ mol-1)
3654
899
0.12
15.0
3560
375
0.06
15.3
1613
2484 2760 3020 1330 3014
3227
860
0.13
18.2
4678
4556
3390
351
0.03
53.0
3065
1520
3643
197
0.02
40.5
1210 1050
2240 2250
3082 3300
3040 3315
0.5 0.5
0 0
645 -484
a ∆V˜ 1/2 is the half-bandwidth, d is the adiabatic electron transfer distance, and (∆ν˜ 1/2)Hush (cm-1) ) [16RT(ln 2)(ν˜ max - E0)]1/2. b Hab is the electronic coupling,Hab ) (0.0205(εmaxV˜ max∆V˜ 1/2)1/2)/d. c R is the delocalization coefficient, i.e., the fraction of valence electronic charge transferred from the donor to the acceptor metal centers, R ) Hab/V˜ max. d ∆Gq is the calculated thermal barrier to the electron transfer, ∆Gq ) λ/4 + E0/2 + E02/(4(λ - 2Hab)) Hab + Hab2/(λ + E0).
Table 5. Solvent Effect on the IT Band for 1+ and 3+ εra DNb 1/n2 - 1/εrc ν˜ max(1+)d ν˜ max(3+)d
CH2Cl2
DMF
CH3CN
9.1 1 0.382 7485 6950
37.06 26.6 0.462 9995 7850
35.9 14.6 0.526 9980 -e
a Relative permittivity (dielectric constant), ref 16. b Donor number, ref 16. c n is the refractive index, ref 16, and λ is predicted to be linear with 1/n2 - 1/εr by the classic two-state Hush theory, refs 1a and 1b. d ν˜ max is in cm-1 ((4 cm-1), T ) -20 °C. e 3+ decomposes in CH3CN.
Figure 4. Optical energy (ν˜ max) of the near-IR transition vs E0 for 1+-5+. The data of 6+and 7+ are reported for comparison.
Figure 5. Oscillator strength (f) of the near-IR transition vs E0 for 1+-5+. The data of 6+ and 7+ are reported for comparison.
shift of ν˜ max was observed with an increase of the dielectric parameter 1/n2 -1/εr1a,b as expected for a class II mixed valence system and in agreement with the dielectric continuum approximation.1a,b On the basis of these results together with the relatively low value of the thermal barrier ∆Gq (Table 4), we can definitely assign 1+ and 3+ to the moderately coupled class II of mixed valence systems. DFT Analysis. Model geometries of 1, 2, and 5 have been fully optimized as described in the Computational Details; 4 was optimized starting from the crystallographic structure. The relevant interatomic distances and angles are reported in Table 2. In 1 the Cp rings of the ferrocenyl pendant are parallel and eclipsed; the Cp* moiety is still parallel to the indenyl plane, (19) (a) Bax, A.; Subramanian, S. J. Magn. Reson. 1986, 67, 565. (b) Summers, M. F.; Marzilli, L. G.; Bax, A. J. Am. Chem. Soc. 1986, 108, 4285. (c) Bax, A.; Summers, M. F. J. Am. Chem. Soc. 1986, 108, 2093.
but slightly staggered. The Cp-indenyl bridging ligand is essentially planar. The slippage of Ru (0.07 Å) is accompanied by a folding angle of 8.1°; in contrast the coordination of Fe is almost perfectly η5 (∆ ) 0.003 Å and FA ) 2.1°). The largest Fe d orbital contribution (Fe, 66.8%; Ru, 12.8%) to the HOMO justifies the oxidation of this metal center at first. In 4 (HOMO, 76.6% Fe d orbitals) and 5 (HOMO, 75.7% Fe d orbitals) analogous geometric features are predicted for the ferrocenyl unit. Interestingly, no Mn and Cr contribution to the HOMO of 4 and 5 is computed. In contrast, in 2 both the HOMO and the LUMO are localized on the indenyl-Fe′Cp fragment (HOMO, 75.6% Fe′ d orbitals; LUMO, 32.9% Fe′ d orbitals); this is in agreement with the experimental evidence of oxidation of the iron coordinated to indenyl at first. The LUMOs of 1, 4, and 5 are localized on the Ru-indenyl (16.8% Ru d orbitals), Cr-indenyl (18.6% Cr d orbitals), and Mn-indenyl (28.7% Mn d orbitals) fragments, respectively, and have no Fe contribution. Upon oxidation, significant changes occur in the ferrocenyl pendant and involve the distance between Fe and the centroid of the Cp rings (Table 2). In 1+ Kohn-Sham frontier molecular R spin-orbitals are localized on Ru (HOMOR, 47.2%) and Fe (LUMOR, 58.3%); in contrast in β spin-orbital contributions of both the metals are present (HOMOβ, 34.9% Fe and 34.4% Ru; LUMOβ, 40.6% Fe and 30.3% Ru). On the basis of the composition of these frontier levels, the lowest electronic absorption detected in the near-IR region can be assigned to metal-to-metal, i.e., rutheniumto-iron, transitions. Similarly, in 4+ and 5+ the frontier R spin-orbitals are localized on Mn (HOMOR, 38.4%) or Cr (HOMOR, 23.2%) and on Fe (LUMOR, 63.4% and 71.3%, respectively); in contrast in the β levels contributions of both the metals are present (4+, HOMOβ, Mn 35.8%, Fe 9.1%, and LUMOβ, Mn 11.6%, Fe 60.5%; 5+, HOMOβ, Cr 33.5%, Fe 18.7%, and LUMOβ, Cr 15.1%, Fe 57.3%). This allows interpretation of the near-IR
Mixed Valence Properties in FeCp-Indenyl-MLn Complexes
Organometallics, Vol. 28, No. 12, 2009 3325
Table 6. Voronoi Charges (Vq) and Mulliken Spin Densities (Sd) 1+ Fe Ru
2+
Vq
Sd
-0.005 0.23
0.57 0.42
Fe Fe′
4+
Vq
Sd
-0.014 0.014
0.46 0.67
spectrum as evidence of manganese-to-iron and chromium-toiron charge transfers. Finally, an analysis of the metal composition of the frontier Kohn-Sham molecular spin-orbitals of 2+ reveals that HOMOR is centered on Fe (83.2%) and LUMOR on Fe′ (52.1%), while in the β levels contributions of both Fe and Fe′ d orbitals are found (HOMOβ, Fe 46.1%, Fe′ 31.9%, and LUMOβ, Fe 32.0%, Fe′ 47.0%). It is worth noticing that as in the other mixed valence ions the percentage values indicate that the lowest transition can be confidently assigned to an iron-to-iron charge transfer, but the donor in this case is the iron nucleus of the ferrocenyl pendant. The adiabatic ionization potentials nicely match the trend of the E1/2 values of the first oxidation wave (Table 3): 1 (5.91 eV) ≈ 2 (5.95 eV) < 4 (6.59 eV) ≈ 5 (6.51). The computed charges and spin densities (Table 6) allow quantification of the delocalization of the unpaired electron in the mixed valence species. In particular, the spin density is prominently localized on Fe, with the Fe/M ratio ranging from 4.4 (4+) to 2.8 (5+) and 1.4 (1+) in the compound of the series in which the most efficient metal-metal interaction is detected. In 2+ the spin density ratio Fe′/Fe is 1.5.
Conclusions The evaluation of the structural parameters combined with the electrochemical data and the analysis of the IT transition in the near-IR of a series of bimetallic η5-[(2-ferrocenyl)indenyl]MLn complexes has allowed us to estimate the degree of the electronic coupling between two different metal sites as a function of the nature of the second metal M and of the ancillary ligand(s) L. A clear-cut difference of the electrochemical and optical properties has been found depending on the electronreleasing (RuCp*, IrCOD, and FeCp) and electron-withdrawing (Mn(CO)3 and Cr(CO)2NO) properties of the MLn. In fact, the results obtained for the series of unsymmetrical bimetallic cations 1+-5+ compared with those already reported and structurally correlated 6+ and 7+ provide rare experimental evidence of a straightforward correlation between metal-to-metal electronic coupling and the redox asymmetry E0 of the donor and acceptor sites predicted by the Hush theory: the “redox matching” of donor and acceptor oxidation potentials results in a higher oscillator strength and lower optical and thermal barriers to the electron transfer. Furthermore, we have demonstrated that the unsymmetrical mixed valence cations of general formula [(2-ferrocenyl)indene-MLn]+ (1+-7+) are electrochromic systems in which ν˜ max can be predicted for their redox asymmetry and the absorption band in the near-IR can be switched on and off by the Fc/Fc+ redox couple. DFT calculations indicate that the nature of the near-IR absorption can be clearly assigned to an MLn-to-Fc charge transfer band in the case of 1+, 4+, and 5+, but to an Fc-toMLn transition in the case of 2+ as indicated by the oxidation potentials and by the nonlinear correlation of its oscillator strength value with E0. Finally, Hush analysis has allowed estimation of the values of the thermal barrier of the electron transfer, evidencing that the rate of the metal-to-metal electron transfer strongly depends
Fe Mn
5+
Vq
Sd
0.025 -0.026
0.87 0.20
Fe Cr
Vq
Sd
0.022 0.37
0.82 0.29
on the electronic effect of MLn and increases in the order 1+ ≈ 3+ ≈ 2+ , 5+ < 4+.
Experimental Section General Procedure. All reactions and complex manipulations were performed in an oxygen- and moisture-free atmosphere utilizing standard Schlenk techniques or in a Mecaplex glovebox. THF (Acros) was purified by distillation from Na/benzophenone under an argon atmosphere and then oxygen freed with vacuum line techniques just before use. Ferrocene (Acros) was purified by crystallization before use. (2-Ferrocenyl)indene (10),17a (indenyl)IrCOD (11),17b (indenyl)Mn(CO)3 (12),17c and (indenyl)Cr(CO)2NO (13)17d were prepared according to the published procedure. Physical Measurements. The X-ray structures were obtained by collecting the intensity data at room temperature using a Philips PW1100 single-crystal diffractometer (FEBO system) using graphitemonochromated (Mo KR) radiation, following the standard procedures. All intensities were corrected for Lorentz polarization and absorption.18a The structure was solved by direct methods using SIR-97.18b Refinement was carried out by full-matrix least-squares procedures (based on Fo2) using anisotropic temperature factors for all non-hydrogen atoms. The H-atoms were placed in calculated positions with fixed, isotropic thermal parameters (1.2Uequiv of the parent carbon atom). The calculations were performed with the SHELXL-97 program18c implemented in the WinGX package.18d IR spectra were recorded on a Bruker Equinox 55 FT-IR spectrometer. 1H and 13C NMR spectra were obtained on a Bruker Avance DRX spectrometer (T ) 298 K) operating at 400.13 and 100.61 MHz, respectively. The assignments of the proton resonances were performed by standard chemical shift correlation and 2D homonuclear correlation experiments (NOESY and COSY). The 13 C resonances were attributed through 2D heteronuclear correlation experiments (HMQC19a,b for the H-bonded carbon atoms, and HMBC19b,c for the quaternary ones). All 2D experiments were acquired by using a pulsed field gradient for coherence selection.20 CV experiments were performed in an airtight three-electrode cell connected to a vacuum/argon line. The reference electrode was an SCE (Tacussel ECS C10) separated from the solution by a bridge compartment filled with the same solvent/supporting electrolyte solution used in the cell. The counter electrode was a platinum spiral with ca. 1 cm2 apparent surface area. The working electrodes were disks obtained from cross sections of gold wires of different diameters (0.5, 0.125, and 0.025 mm) sealed in glass. Between successive CV scans, the working electrodes were polished on alumina according to standard procedures and sonicated before use. An EG&G PAR-175 signal generator was used. The currents and potentials were recorded on a Lecroy 9310 L oscilloscope. The potentiostat was home-built with a positive feedback loop for compensation of the ohmic drop.21 Mid-IR, near-IR, and visible spectroelectrochemistry experiments at variable temperatures were carried out with a cryostated (low-T) optically transparent thinlayer electrochemical (OTTLE) cell (IDEAS!UvA B.V., University (20) (a) Parella, T. Magn. Reson. Chem. 1998, 36, 467. (b) Ruiz-Cabello, J.; Vuister, G. W.; Moonen, C. T. W.; van Gelderen, P.; Cohen, J. S.; van Zijl, P. J. Magn. Reson. 1992, 100, 282. (c) Willker, W.; Leibfritz, D.; Kerrsebaum, R.; Bermel, W. Magn. Reson. Chem. 1993, 31, 287. (21) Amatore, C.; Lefron, C.; Pflu¨ger, F. J. Elecroanal. Chem. 1989, 270, 43.
3326 Organometallics, Vol. 28, No. 12, 2009 of Amsterdam, The Netherlands)22 equipped with CaF2 windows. Pt working (80% transmittance) and Pt auxiliary minigrid electrodes and a pseudoreference Ag wire were melt-sealed in the insulating polyethylene spacer with an optical path of 0.024 cm. General Procedure for the Synthesis of 1, 3, 4, and 5. All the complexes were prepared by reacting (2-ferrocenyl)indenyl anion with the appropriate metalating agent. In the case of the synthesis of complexes 1, 3, and 5 the deprotonation was achieved by adding (2-ferrocenyl)indene (250 mg, 0.83 mmol) to a THF (50 mL) solution at -20 °C containing an excess of KH and stirring for 30 min (solution A). For the synthesis of complex 4 the deprotonation was achieved by adding dropwise an equimolar amount of t-BuLi (1.6 M solution in pentane) to a solution of (2-ferrocenyl)indene (300 mg, 1 mmol) in THF (20 mL) at -40 °C and stirring for 30 min (solution B). Synthesis of [η5-(2-Ferrocenyl)indenyl]RuCp* (1). The filtered solution A was added dropwise to a THF solution (15 mL) of [RuClCp*] (256 mg, 0.83 mmol) cooled at -20 °C. The reaction mixture was stirred for 45 min and then allowed to warm to room temperature. The solvent was removed under vacuum and the residue washed with cold pentane (5 × 15 mL). The crude product was further purified by MPLC eluting with petroleum ether. The final complex 1 (220 mg, 0.41 mmol, 50%) was obtained as a red powder. 1H NMR (400.13 MHz, C3D6O, δ (ppm), 298 K, TMS): δ 7.11 (m, 2H, H4, H7), 7.79 (m, 2H, H5, H6), 5.05 (s, 2H, H1, H3), 4.39 (t, 2H, J(H,H) ) 1.8 Hz, HR, HR′), 4.17 (t, 2H, J(H,H) ) 1.8 Hz, Hβ, Hβ′), 3.95 (s, 5H, C5H5), 1.4 (s, 15H, RuCp methyl protons). 13C NMR (100.61 MHz, C3D6O, δ (ppm), 298 K, TMS): δ 125.61 (C4, C7), 121.33 (C5, C6), 93.37 (C3a, C7a), 92.06 (C2), 83.50 (Cγ), 82.76 (RuCp carbon atoms), 69.72 (C5H5), 68.14 (Cβ, Cβ′), 67.05 (C1, C3), 65.97 (CR, CR′), 10.30 (RuCp methyl carbon atoms). Anal. Calcd for C29H30RuFe: C, 65.04; H, 5.65. Found: C, 64.86; H, 5.79. Synthesis of [η5-(2-Ferrocenyl)indenyl]IrCOD (3). The filtered solution A was added dropwise to a THF solution (15 mL) of [Ir(µCl)(COD)]2 (279 mg, 0.42 mmol) cooled at -20 °C. The reaction mixture was stirred for 45 min and then allowed to warm to room temperature. The solvent was removed under vacuum and the residue washed with cold pentane (5 × 15 mL). The crude product was extracted with CH2Cl2 (3 × 10 mL), the solution was filtered, and the solvent was removed under vacuum to yield 3 (348 mg, 0.58 mmol, 70%) as a red powder. 1H NMR (400.13 MHz, C3D6O, δ (ppm), 298 K, TMS): δ 7.29 (m, 2H, H4, H7), 7.05 (m, 2H, H5, H6), 5.71 (s, 2H, H1, H3), 4.56 (t, 2H, J(H,H) ) 1.8 Hz, HR, HR′), 4.22 (t, 2H, J(H,H) ) 1.8 Hz, Hβ, Hβ′), 4.05 (s, 5H, C5H5), 3.83 (m, 4H, COD olefin protons), 1.50 ppm (m, 8H, COD methylene protons). 13C NMR (100.61 MHz, C3D6O, δ (ppm), 298 K, TMS): δ 123.93 (C5, C6), 120.70 (C4, C7), 110.70 (C3a, C7a), 103.55 (C2), 82.35 (Cγ), 71.33 (C1, C3), 69.66 (C5H5), 68.49 (Cβ, Cβ′), 66.61 (CR, CR′), 51.29 (COD olefin carbon atoms), 33.08 ppm (COD methylene carbon atoms). Anal. Calcd for C27H27FeIr: C, 54.10; H, 4.54. Found: C, 54.03; H, 4.63. Synthesis [η5-(2-Ferrocenyl)indenyl]Cr(CO)2NO (5). Solution A was filtered and then allowed slowly to warm to 0 °C. Diazald (152 mg, 0.71 mmol) was subsequently added, and the resulting solution was refluxed for 2 h. The solvent was removed under vacuum, and the residue was extracted with Et2O (3 × 10 mL). The crude product was purified by MPLC eluting with petroleum ether. The final complex 5 (158 mg, 0.36 mmol, 43%) was obtained as an orange oil. 1H NMR (400.13 MHz, C3D6O, δ (ppm), 298 K, TMS): δ 7.56 (m, 2H, H4, H7), 7.12 (m, 2H, H5, H6), 6.03 (s, 2H, H1, H3), 4.63 (t, 2H, J(H,H) ) 1.7 Hz, HR, HR′), 4.29 (t, 2H, J(H,H) (22) (a) Hartl, F.; Luyten, H.; Nieuwenhuis, H. A.; Schoemaker, G. C. Appl. Spectrosc. 1994, 48, 1522. (b) Mahabiersing, T.; Luyten, H.; Nieuwendam, R. C.; Hartl, F. Collect. Czech. Chem. Commun. 2003, 68, 1687.
Santi et al. ) 1.7 Hz, Hβ, Hβ′), 4.07 (s, 5H, C5H5). 13C NMR (100.61 MHz, C3D6O, δ (ppm), 298 K, TMS): δ 226.05 (CO), 126.31 (C5, C6), 125.38 (C4, C7), 116.55 (C2), 110.34 (C3a, C7a), 78.30 (Cγ), 77.49 (C1, C3), 70.13 (C5H5), 69.54 (Cβ, Cβ′), 67.13 (CR, CR′). Anal. Calcd for C21H15NFeCrO3: C, 57.69; H, 3.46; N, 3.20. Found: C, 57.95; H, 3.53, N, 3.33. Synthesis of [η5-(2-Ferrocenyl)indenyl]Mn(CO)3 (4). Me3SnCl (199 mg, 1 mmol) was added to solution B at -40 °C. After the resulting solution was stirred for 45 min, Mn(CO)5Br (275 mg, 1 mmol) was added and the resulting solution was allowed to warm slowly to room temperature. The solvent was removed under vacuum and the residue washed with cold pentane (5 × 15 mL). The crude product was further purified by MPLC eluting with petroleum ether/diethyl ether (9:1). The final complex 4 (123 mg, 0.28 mmol, 28%) was obtained as an orange powder. 1H NMR (400.13 MHz, C3D6O, δ (ppm), 298 K, TMS): δ 7.61 (m, 2H, H4, H7), 7.15 (m, 2H, H5, H6), 5.77 (s, 2H, H1, H3), 4.65 (s, 2H, HR, HR′), 4.31 (s, 2H, Hβ, Hβ′), 4.05 (s, 5H, Cp). 13C NMR (100.61 MHz, C3D6O, δ (ppm), 298 K, TMS): δ 226.13 (CO), 127.03 (C5, C6), 126.24 (C4, C7), 110.28 (C2), 103.95 (C3a, C7a), 78.08 (Cγ), 70.34 (C1, C3, C5H5), 69.90 (Cβ, Cβ′), 67.54 (CR, CR′). Anal. Calcd for C22H15FeMnO3: C, 60.31; H, 3.45. Found: C, 60.12; H, 3.58. Computational Details. DFT calculations were carried out using the Amsterdam Density Functional (ADF) program.23 Electron correlation was treated within the local density approximation (LDA) in the Vosko-Wilk-Nusair parametrization,24 and the nonlocal corrections of Becke25 and Perdew26 were added to the exchange and correlation energies. The basis sets used for the atoms in the optimization procedures were TZP (triple-ζ Slater-type orbital (STO) basis, extended with a single-ζ polarization function) for H, frozen core up to 2p for Fe, Cr, and Mn, and frozen core up to 1s for N, C, and O. Single-point calculations were subsequently carried out on the optimized geometries employing the TZ2P basis set (triple-STO basis, extended with a double-ζ polarization function) for H, frozen core up to 2p for Fe, Cr, and Mn, and frozen core up to 1s for N, C, and O, and the results obtained with this basis set are reported in the tables and discussed in the text. Spin contamination in the open shell calculations was carefully monitored to assess the reliability of the wave function.
Acknowledgment. This work was supported by the Ministero dell’Istruzione, dell’Universita` e della Ricerca (MIUR), within PRAT 2006. CINECA (Consorzio di calcolo del Nord-Est, Casalecchio di Reno) is gratefully acknowledged for the access to the computational facilities, i.e., IBM SP5. We gratefully acknowledge Dr. Roberta Cardena for her contribution to the electrochemical and spectroelectrochemical measures. Supporting Information Available: Cyclic voltammetry of 11-13, xyz coordinates of the calculated structures, selected Kohn-Sham MOs, and CIF files giving crystallographic data sets. This material is available free of charge via the Internet at http://pubs.acs.org. OM800954B (23) (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. (b) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor. Chem. Acc. 1998, 99, 391. (c) ADF 2007.01, SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands, http://www.scm.com. (24) Vosko, S. D.; Wilk, L.; Nusair, M. Can. J. Chem. 1990, 58, 1200. (25) (a) Becke, A. D. J. Chem. Phys. 1986, 84, 4524. (b) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (26) (a) Perdew, J. P. Phys. ReV. B 1986, 33, 8822. (b) Perdew, J. P. Phys. ReV. B 1986, 34, 7406.
