Amide-Linked Heterobi- and Heterotermetallocenes with Very Low

Article pubs.acs.org/Organometallics

Amide-Linked Heterobi- and Heterotermetallocenes with Very Low HOMO−LUMO Gaps Hannah Huesmann,† Christoph Förster,† Daniel Siebler,† Teuta Gasi,‡ and Katja Heinze*,† †

Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg-University of Mainz, Duesbergweg 10-14, 55128 Mainz, Germany ‡ Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg-University of Mainz, Staudingerweg 9, 55128 Mainz, Germany S Supporting Information *

ABSTRACT: The novel heterobi- and heterotermetallocenes [Fc-NHCO-Cc]+ (1) and [Fc-NHCO-Cc-CONH-Fc]+ (2) have been prepared by amide coupling of aminoferrocene and cobaltocenium carboxylic acid or 1,1′-cobaltocenium dicarboxylic acid. Very small differences of the redox potentials, ΔG0 ≈ 1 V, were observed due to the electron-donating and electronwithdrawing nature of the substituents at the ferrocene and cobaltocenium units, respectively. In accordance with this small zero-level energy difference, ΔG0 is the low-energy MM’CT absorption that extends into the near-infrared spectral region. Large solvatochromic shifts were observed for 1 and 2 due to the pronounced difference in ground-state and excited-state dipole moments. The low-energy optical transitions were assigned as MM’CT transitions by DFT-PCM and TDDFT-PCM approaches, and the experimental negative solvatochromic effect has been reproduced by the TDDFT-PCM calculations.

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were pioneered by Astruc.11,12 Recently, Manners et al. have prepared heterobimetallic block copolymers (Scheme 1) based on a ferrocene block and a cobaltocenium block that display oxidation and reduction waves separated by ca. 1.5 V.13 Our research efforts are directed toward directional oligonuclear mixed-valent/mixed-metal systems with amides as connecting units for potential applications as molecular wires and switches (Scheme 2).14−19 The amide linkages give rise to electronic interactions of adjacent metal complexes (e.g., ferrocenes or ruthenium(II) complexes14−21) as well as to well-defined secondary structures based on hydrogen bonding similar to natural peptides composed of α-amino acids.16−18 In addition to the chemical robustness of the amide linkage and the possibility of using solid-phase peptide synthesis techniques for the construction of larger systems,15,18,21,22 amides provide an intrinsic directionality to the oligomers (N-terminus → Cterminus). We have combined these unique electronic and structural effects in oligoferrocene amides into a switching process, namely, the unfolding of the secondary zigzag structure to an extended stretched conformation by oxidation of ferrocene units to ferrocenium sites.17,18 However, one drawback of 17 valence electron ferrocenium units is their paramagnetic nature precluding a reliable NMR-based conformational analysis of charged oligoferrocenium amides. Thus, one particularly intriguing aspect of a mixed-metal peptide based on cobaltocenium and ferrocene would be the possibility of introducing a positive charge at specific sites by a formal “FeII → CoIII mutagenesis” while retaining a closed-shell diamagnetic

INTRODUCTION Electron transfer is one of the fundamental and most important phenomena in chemistry and biology. Mixed-valence (MV) compounds are valuable systems for studying electron-transfer reactions. Analysis of their charge-transfer spectra (intervalence transition, IVCT) in the context of Marcus theory was first outlined by Hush in 1967.1 Linked metallocenes2−4 in the form of the prototypical biferrocene Fc-Fc and its MV congener [FcFc]+ as well as mixed-metal oligometallocenes, for example, the stable closed-shell d6−d6 bimetallocene [Fc-Cc]+ play a major role in this research area (Scheme 1; Fc = (C5H5)Fe(C5H4), Cc = [(C5H5)Co(C5H4)]+).5 Thermally and optically driven electron transfer can be observed in mixed-valent systems (IVCT), whereas in mixed-metal systems, usually metal-tometal charge-transfer bands are observed (MM’CT). The MM’CT bands of the heterobimetallocenes [Fc-Cc]+, [FcCHCH-Cc] + , and [Fc-CC-Cc]+ 6,7 as well as the pseudocentrosymmetric heterotermetallocenes [Fc-Cc-Fc]+, [Fc-CH2-Cc-CH2−Fc]+, and [Fc-CHCH-Cc-CHCHFc]+7,8 (Scheme 1) found in the visible spectral region are distinctly solvatochromic. Beer et al. studied the interaction of a cationic mixed ferrocene/cobaltocenium conjugate (Scheme 1) with different anions by electrochemical means.9 The difference in redox potentials of the cobaltocenium and the ferrocenium site has been determined as 1.05 V. A glucose sensor has been devised by the groups of Cuadrado and Losada using a dendritic core with ferrocene and cobaltocenium units attached in a statistical manner (Scheme 1). These dendrimers also feature separated oxidation and reduction waves (ca. 1.3 V).10 Giant dendrimers with more than 104 peripheral cobaltocenium or ferrocene units © 2011 American Chemical Society

Received: October 19, 2011 Published: December 22, 2011 413

dx.doi.org/10.1021/om201007g | Organometallics 2012, 31, 413−427

Organometallics

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hexafluorophosphate salts of cobaltocenium carboxylic acid and 1,1′-cobaltocenium dicarboxylic acid were activated as acid chlorides by thionyl chloride and then treated with an excess amount of aminoferrocene in the presence of triethylamine as a HCl scavenger (Scheme 3). The dark blue products were precipitated by an ethanolic ammonium hexafluorophosphate solution as hexafluorophosphate salts. Spectroscopic Characterization. ESI mass spectra of conjugates 1 and 2 confirm the molecular composition of the respective molecular cations, and IR spectra are compatible with the presence of amide functional groups (amide I and amide II vibrations: ν̃ = 1674/1571 cm−1 (1) and ν̃ = 1671/ 1555 cm−1 (2)) and hexafluorophosphate counterions (ν̃PF = 836 cm−1). In the 1H NMR spectra (CD3CN), the amide protons H5 resonate at comparably high frequency at δ = 8.43 ppm (1) and δ = 8.90 ppm (2), respectively. The 13C{1H} NMR spectra also substantiate the presence of CONH units by the carbon resonances of the C5 nuclei at δ = 160.1 ppm (1) and δ = 159.4 ppm (2), respectively (see the Experimental Section). The cobaltocenium and ferrocene Cp proton and carbon resonances of 1 and 2 (H2−4, H2′−4′, C1−4, C1′−4′) are assigned straightforwardly by comparison with known individual compounds and 2D NMR experiments, such as 1H−1HCOSY, 1H−13C-HMBC, 1H−13C-HSQC, and 1H−1H-NOESY (see the Experimental Section). The ferrocene proton resonances are found between δ = 4.1 and 4.8 ppm while the cobaltocenium proton resonances appear at higher frequency between δ = 5.7 and 6.2 ppm. Characteristic for the amide linkage in 1 and 2, the amide proton H5 features NOE cross peaks to H2 and H2′ of the adjacent cobaltocenium and ferrocene moieties, respectively (see the Supporting Information). The D−A conjugate 1 is quite sensitive to oxygen, and paramagnetically shifted and broadened NMR spectra are observed when 1 is exposed to oxygen. Treating the oxidized solution with ascorbic acid restores the original 1H NMR spectrum of 1, substantiating a reversible oxidation to 1-ox (see below for electrochemical results). Indeed, after oxidation of 1 with oxygen and counterion exchange with tetrafluoroborate, crystals of doubly charged 1-ox suitable for X-ray crystallographic analysis were obtained. 1-ox crystallized in the tetragonal space group P42bc as a racemic twin with the dinuclear molecule being 4-fold disordered over the crystallographic 42 axis. Despite the severe disorder, it is possible to

Scheme 1. Some Conjugates of Ferrocene and Cobaltocenium Cations Relevant to This Work5,9,10,13

material amenable to detailed NMR investigations (Scheme 2). A second issue pertaining to mixed-metal ferrocene/cobaltocenium amides is the possible interaction across the amide bridge, especially in the situation with an electron-rich ferrocenyl unit (NH substitution) and an electron-poor cobaltocenium unit (CO substitution). This pattern is realized in the heterobimetallocene [Fc-NHCO-Cc]+ (1) and in the heterotermetallocene [Fc-NHCO-Cc-CONH-Fc]+ (2). In this study, we present our results on the synthesis and the electronic properties of the novel push−pull chromophores 1 and 2.

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RESULTS AND DISCUSSION Synthesis of Heterobi- and Heterotermetallic D−A and D−A−D Conjugates. For preparation of the donor− acceptor dyad 1 and donor−acceptor−donor triad 2, the

Scheme 2. Formal “FeII → CoIII Mutagenesis” in Ferrocene Peptides

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Scheme 3. Synthesis of D−A Conjugate 1 and D−A−D Conjugate 2 and Atom Numbering Used for NMR Assignments

0.451 mm s −1 and δ = 0.438 mm s −1 with quadrupole splittings ΔEQ = 2.391 mm s −1 and ΔEQ = 2.377 mm s −1 for 1 and 2,

extract the supramolecular connectivity. As found for many systems with amide units,14,16−19 intermolecular hydrogen bonds NH···OC are formed (N···O 2.50−2.61 Å), giving hydrogen bonded chains of dinuclear molecules along the c axis in the case of 1-ox (see the Supporting Information for a pictorial description). The tetrafluoroborate counterions of 1ox are not involved in the hydrogen bond network but fill holes of the cation sublattice. The fact that 1 is easily oxidized is also apparent in its cyclic voltammogram. Figure 1 depicts cyclic voltammograms of 1 and 2. Dyad 1 features two reversible redox waves, namely, the Fc/Fc+ couple at E1/2 = −15 mV and the Cc/Cc+ couple at E1/2

Figure 2. 57Fe Mössbauer spectra of 1 and 2 at 295 K.

respectively, as expected for genuine iron(II) centers (c.f. FcNHCO-Fc gives δ = 0.453 mm s −1 and ΔEQ = 2.383 mm s −1 for both Fe(II) sites17). The positive charge of the adjacent cobaltocenium moiety obviously affects the Mö ssbauer parameters of the Fe(II) site only marginally. Thus, both 1 and 2 possess a localized FeII/CoIII ground state and no chargeaveraging process FeII/CoIII ↔ FeIIICoII is operative in the ground state, which had been invoked24,25 and refuted6 for the doubly bridged ferrocenylene cobaltocenylenium mixed-metal system [Fe-μ-(C5H4-C5H4)2Co]+. However, the positively charged cobaltocenium site renders 1 and 2 distinct donor−acceptor and donor−acceptor−donor systems, respectively. It is noted that the color of 1 strongly depends on the solvent; for example, a solution of 1 appears

Figure 1. Cyclic voltammograms of 1 (top) and 2 (bottom) in CH3CN/(nBu4N)(PF6). Scan rate, 100 mV s−1; E vs FcH/FcH+.

= −1150 mV. For the trimetallic conjugate 2, the Fc/Fc+ wave is apparently unsplit and at an essentially identical potential as observed for 1 (E1/2 = −20 mV). The Cc/Cc+ couple, however, is shifted by 180 mV to E1/2 = −970 mV due to the second electron-withdrawing CO substituent.23 Thus, the electrochemically derived HOMO−LUMO difference drops from 1.135 eV (1) to 0.950 eV (2). Mössbauer spectra of 1 and 2 were collected at 295 K and are shown in Figure 2. Quadrupole doublets are observed at δ = 415


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Figure 3. Normalized (to the low-energy absorption band) UV/vis spectra of 1 (left) and 2 (right) in selected solvents (ACN, methanol, tetrahydrofuran, 1,4-dioxane, pyridine, DCM, 1,2-dichlorobenzene).

Table 1. Solvatochromic Effects and Electrochemical Data of Bi- and Termetallocenes Composed of Ferrocene and Cobaltocenium Units [Fc-Cc]+a,e [Fc-CHCH-Cc]+a [Fc-CC-Cc]+a [FcNHCO-Cc]+ (1)f [Fc-Cc-Fc]+a [Fc-CH2-Cc-CH2-Fc]+b [Fc-CHCH-Cc-CHCH-Fc]+a [Fc-C6H4-NHCO-Cc-CONH-C6H4-Fc]+d [Fc-NHCO-Cc-CONH-Fc]+ (2)f,g a

λmax (CH3CN) (nm)

λmax (CH2Cl2) (nm)

Δν̃solv (cm−1)

ΔG0 (CH3CN) (V)

549/552 548 524 534 546 482 563 not given 575/694/764

582/580 595 568 620 577 508 608 not given 587/735/951

1035/875 1440 1480 2598 985 1060 1315 not given 356/794/2574

1.580c 1.410 not given 1.135 not given 1.480 not given 1.050d 0.950

From ref 7. bFrom ref 8. cFrom ref 5. dFrom ref 9. eFrom ref 6. fThis work. gFit from Gaussian line-shape analysis.

Table 2. Results of the Hush Analysis of the MM’CT Bands of Several Bi- and Termetallocenes Composed of Ferrocene and Cobaltocenium Moieties in ACN and DCM ν̃max (cm−1) (ε (M−1 cm−1)) [Fc-Cc]

+a

[Fc-NHCO-Cc]+ (1)b [Fc-CH2-Cc-CH2-Fc]+c,d [Fc-NHCO-Cc-CONH-Fc]+ (2)b,d

ACN DCM ACN DCM ACN DCM ACN

DCM

18115 17241 18736 16138 17480 15700 17397 14406 13097 17045 13612 10517

(2480) (2440) (347) (294) (704) (710) (135) (232) (51) (108) (119) (23)

Δν̃1/2 (cm−1)

ΔG0 (cm−1)

r (Å)

HAB (cm−1)

α

λ (cm−1)

4670 4550 5584 5327 6900 6600

12 751 12 751 9154 9154 8830 8310

5.1 5.1 7.2e 7.2e

1301 1243 384 320

0.0719 0.0721 0.0205 0.0198

5364 4490 9582 6984 8650 7390 9727 6736 5409 9375 5942 2847

7662

7662

a

From refs 5 and 6. bThis work. The bands were fitted by deconvolution into Gaussian band shapes. cFrom ref 8. dThe MM’CT absorptions are split into several discernible bands of different intensities and widths; they were fitted by three Gaussian functions. eThe distance between the redox sites Co···Fe has been estimated on the basis of DFT calculations.

For the [Fc-CHCH-Cc]+ solvatochromic system, no simple straightforward correlation to a single parameter, such as solvent polarity, had been observed,7 and a similar situation is encountered for 1 and 2. With fitting to the Kamlet−Taft solvent parameters (polarizability, hydrogen-donor and hydrogen-acceptor ability),26,27 reasonable correlations were obtained for chromophores 1 and 2 (see the Supporting Information for correlation plots). The largest impact seems to arise from the polarity and the hydrogen-acceptor property of the solvent.

purple in acetonitrile (ACN), blue in pyridine, and green in dichloromethane (DCM) (Figure 3). The electronic absorption spectra of 1 and 2 in different solvents are depicted in Figure 3. The absorption bands of 1 and 2 reach into the near-infrared spectral region. Termetallocence 2 features two discernible lowenergy bands in some solvents, for example, in ACN or DCM (Figure 3). In other solvents, for example, dioxane, these two bands are not clearly resolved but can be inferred from the width of the absorption bands. 416


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Figure 4. Hush diagrams for 1 (top), 2 (middle), and [Fc-Cc]+ in ACN and DCM along the electron-transfer reaction coordinate. The MM’CT transitions are depicted by arrows. The relative position of the excited-state minimum is highlighted and indicated by a red arrow.

The latter fact can be ascribed to the NH functional groups acting as hydrogen donors. In comparison to the known heterobimetallocenes [Fc-Cc]+, [Fc-CHCH-Cc]+, and [FcCC-Cc]+,6,7 the present amide bridged system 1 exhibits the largest (negative) solvatochromic effect, Δν̃solv, ever reported (see Table 1 for a comparison of the solvents ACN and DCM). A comparable trend is observed for the heterotermetallocenes [Fc-Cc-Fc]+, [Fc-CH2-Cc-CH2−Fc]+, [Fc-CHCH-Cc-CH CH−Fc]+, and 2 (Table 1).7,8 One issue is doubtlessly the length of the bridge connecting the Cp rings (zero, one, or two atoms) and thus the donor−acceptor distance that influences the magnitude of the ground- and excited-state dipole moments. A second aspect is the stronger electronic asymmetry of dyes 1 and 2 due to the directional amide linkage that additionally regulates ground- and excited-state dipole moments. The large negative solvatochromic effects observed for 1 and 2 point to ground states with large dipole moments.

The low-energy bands of mixed cobaltocenium/ferrocene conjugates have been assigned to metal-to-metal charge transfer transitions.6−8 Recently, Tuczek et al. thoroughly investigated the MM’CT transitions in the mixed-metal bimetallocene [FcCc]+.6 Theoretical considerations led to a modified Hush expression1 (by a factor of 0.51/2) for the electronic coupling matrix element, HAB = 2.05 × 10−2 (0.5εmaxΔν̃1/2ν̃max)1/2/r (eq 1), for the mixed-metal bimetallocenes as compared to openshell FeII/FeIII MV dimers.6 Table 2 compiles the results of the Hush analyses. The orbital delocalization parameter is calculated as usual by α = HAB/ν̃max (eq 2). The low-energy MM’CT band of the termetallocenes is composed of several overlapping transitions originating from the two FeII centers to the central CoIII site. The experimental band shape can be fitted by Gaussian deconvolution into one, two, or three bands, depending on the solvent (see the Supporting Information). However, even more transitions to excited states of different 417


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Table 3. Pertinent Geometric Data of Models of 1 in Different Environments (Solvents and Counterions) and Conformations, Ground-State Dipole Moments μGS, Data of the Vertically Excited MM’CT States (Energy, Oscillator Strength f, Orbital Contributions, μES) d(Fe···Co) (Å)

O−C5−C1−C2 (deg)

μGS (D)

λmax ( f) (nm) [excited state #]

relevant orbital contributions

μES (D)

1 (gas)

7.2

35.3

13.2

1726 (0.0211) [#2]

4.5

1 (DCM)

7.2

15.2

17.3

800 (0.0262) [#1]

95 → 97 (0.13) 96 → 97 (0.69) 95 → 100 (0.20) 96 → 97 (0.61) 96 → 99 (0.14) 96 → 101 (0.25) 95 → 100 (0.45) 96 → 97 (−0.34) 96 → 99 (0.18) 96 → 101 (0.37) 95 → 100 (0.30) 96 → 97 (0.50) 96 → 99 (0.20) 96 → 101 (0.34) 95 → 100 (−0.39) 96 → 97 (0.48) 96 → 99 (−0.14) 96 → 101 (−0.28) 106 → 111 (−0.31) 107 → 108 (0.48) 107 → 110 (0.22) 107 → 112 (−0.34) 106 → 111 (0.38) 107 → 108 (0.50) 107 → 110 (−0.14) 107 → 112 (0.26) 125 → 130 (0.22) 126 → 127 (0.59) 126 → 129 (0.16) 126 → 131 (0.27) 125 → 130 (0.43) 126 → 127 (−0.37) 126 → 128 (−0.13) 126 → 129 (0.18) 126 → 131 (0.34) 125 → 130 (0.30) 126 → 127 (0.50) 126 → 129 (0.20) 126 → 131 (0.33) 125 → 130 (−0.39) 126 → 127 (0.47) 126 → 129 (−0.14) 126 → 131 (−0.27)

678 (0.0057) [#4]

1 (ACN)

7.2

14.0

17.9

758 (0.0200) [#1]

657 (0.0139) [#3]

1···ACN (ACN)

7.2

16.1

18.9

757 (0.0206) [#1]

656 (0.0169) [#3]

1···(PF6)− (DCM)

7.1

−8.0

18.5

785 (0.0232) [#1]

674 (0.0073) [#4]

1···(PF6)− (ACN)

7.2

7.4

23.9

758 (0.0216) [#1]

658 (0.0137) [#3]

6.7

10.2

6.5

5.7

10.3

9.5

13.4

13.3

14.9

14.7

has been ascribed to the more delocalized nature of the LUMO of the cobaltocenium part as compared with the more metalcentered SOMO of the ferrocenium ion (δ-bonding orbital composed of dx2−y2/dxy iron orbitals with only marginal Cp contribution).8 Indeed, in 1 and 2, the acceptor orbitals of the MM’CT are π* antibonding molecular orbitals composed of dxz, dyz(Co), and Cp orbitals, which promote electronic interaction of the individual sites similar to the [Fc-Cc]+ case6 (see below). The reorganization energy,29,30 λ = ν̃max − ΔG0 = λinner + λouter (eq 3), of an asymmetric charge-transfer system depends on the zero-level energy difference ΔG0 of the ground and excited states, which can be estimated from the difference of the redox potentials. The pertinent values are compiled in Table 2.

MM’CT character might be involved (at least three transitions with appreciable oscillator strength; see below) so that a reliable Hush analysis for individual excited states is impossible. Compared with the open-shell mixed-valent ferrocene/ ferrocenium derivative [Fc-NHCO-Fc]+ (HAB = 140 ± 10 cm−1; r = 6.5 Å in THF17), the electronic coupling matrix element of 1 (HAB = 372 cm−1; r = 7.2 Å, in THF) is significantly larger, although this comparison is not strictly valid due to the inverse ground-state charge distribution in [FcNHCO-Fc]+ with the positive charge localized at the NHsubstituted ferrocene.17 The larger interaction in 1 is consistent with the corresponding values of the [Fc-Cc]+/[Fc-Fc]+ pair with HAB = 1243 cm−16 and HAB = 513 cm−1,28 respectively (r = 5.1 Å, in CH2Cl2). The enhanced coupling between Cc+ and Fc 418


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Table 4. Pertinent Geometric Data of Models of 2 in Different Environments (Solvents and Counterions) and Conformations, Ground-State Dipole Moments μGS, Data of the Vertically Excited MM’CT States (Energy, Oscillator Strength f, Orbital Contributions, μES) d(Fe...Co) (Å) 2-open (gas)

7.2/7.2

O−C5−C1−C2 (deg) 34.1/−33.7

μGS (D)

λmax ( f) (nm) [excited state #]

5.2

1699 (0.0160) [#1]

1691 (0.0299) [#2]

1477 (0.0028) [#5] 1429 (0.0041) [#6] 2-open (DCM)

7.2/7.2

25.0/−21.3

7.8

877 (0.0481) [#1]

862 (0.0121) [#2]

689 (0.0042) [#9]

688 (0.001) [#10]

2-open (ACN)

7.2/7.2

18.3/−16.6

6.8

827 (0.0604) [#1]

818 (0.0043) [#2]

679 (0.0110) [#9]

678 (0.0013) [#10]

419

relevant orbital contributions 147 148 149 147 148 149 148 149 148 149 147 149 149 146 148 148 146 147 148 149 149 149 149 149 149 146 147 148 148 148 148 148 149 147 148 149 149 146 148 148 149 146 146 147 148 148 148 148 149 149 149 146 147 147 148 148 148 149 149

→ → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → →

150 150 150 150 150 150 151 151 151 151 155 150 157 154 150 156 154 155 156 157 150 151 152 153 157 154 155 150 151 152 153 156 157 155 157 150 156 154 150 151 157 154 155 155 151 152 156 157 150 153 156 154 154 155 150 153 156 151 152

(0.56) (−0.12) (−0.41) (0.42) (0.21) (0.52) (−0.36) (0.61) (0.60) (0.36) (−0.10) (0.66) (−0.14) (−0.11) (0.66) (0.15) (−0.17) (0.43) (0.14) (0.11) (0.19) (0.12) (−0.14) (0.12) (0.36) (0.43) (0.17) (0.20) (−0.14) (0.10) (0.15) (−0.37) (0.16) (−0.12) (−0.14) (0.64) (0.14) (0.13) (0.63) (0.16) (−0.14) (0.33) (0.13) (−0.22) (0.33) (0.13) (0.11) (−0.24) (−0.20) (0.11) (0.23) (0.21) (−0.13) (0.32) (−0.21) (0.11) (0.22) (0.35) (0.13)

μES (D) 16.3

10.4

17.2 20.1 16.2

21.9

6.2

8.4

4.8

10.0

7.4

4.3


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Table 4. continued d(Fe...Co) (Å) 2-ring (gas)

7.2/6.9

O−C5−C1−C2 (deg) 38.7/26.1

μGS (D) 13.7

λmax ( f) (nm) [excited state #] 2315 (0.0236) [#3]

2063 (0.0011) (#4)

2-ring (DCM)

7.2/6.9

38.6/34.0

1284 (0.0128) (#5) 1228 (0.0048) (#7) 905 (0.0102) [#1]

19.9

872 (0.0057) [#2] 803 (0.0206) [#5]

684 (0.0026) [#11]

2-ring (ACN)

7.2/6.9

37.9/34.8

20.8

810 (0.0126) [#1]

778 (0.0027) [#2]

766 (0.0186) [#4]

670 (0.0031) [#10]

669 (0.0042) [#11]

2···(PF6)− (DCM)

7.0/7.1

−13.8/−10.7

12.9

863 (0.0350) [#1]

850 (0.0166) [#2]

420

relevant orbital contributions 149 148 149 149 148 148 147 147 149 149 149 149 146 147 147 147 147 146 147 147 147 147 148 149 149 149 149 148 148 149 149 146 147 147 149 149 149 146 146 146 147 147 147 147 147 146 146 146 147 147 147 147 147 177 178 179 179 176 178 178 179

→ → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → →

157 150 150 151 150 151 150 151 150 151 150 151 154 150 151 152 155 154 150 151 152 155 156 150 151 153 157 150 151 150 151 154 150 151 152 155 156 150 151 154 150 151 152 154 155 150 151 154 150 151 152 154 155 185 180 180 187 184 180 186 180

(−0.25) (−0.10) (0.50) (0.49) (−0.29) (0.64) (0.70) (0.70) (0.66) (0.20) (−0.22) (0.66) (−0.18) (0.61) (−0.12) (−0.14) (−0.22) (0.45) (0.30) (−0.13) (0.21) (0.38) (0.12) (0.60) (0.28) (−0.12) (0.16) (−0.19) (−0.13) (−0.32) (0.55) (−0.27) (0.53) (−0.12) (−0.19) (−0.27) (−0.13) (0.28) (0.39) (0.28) (0.27) (−0.16) (0.11) (−0.15) (0.19) (0.27) (0.36) (−0.30) (−0.28) (0.17) (−0.12) (−0.13) (−0.20) (0.10) (0.18) (0.64) (0.13) (0.11) (0.63) (0.14) (−0.18)

μES (D) 6.7

6.6 22.8 23.9 6.0 6.5 23.2

18.3

3.6

5.0

21.0

22.8

22.3

13.3

14.4


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Table 4. continued d(Fe...Co) (Å)

O−C5−C1−C2 (deg)

μGS (D)

λmax ( f) (nm) [excited state #] 688 (0.0049) [#9]

687 (0.0013) [#10]

2···(PF6)− (ACN)

7.0/7.1

−14.6/−11.6

13.6

813 (0.0354) [#1]

804 (0.0159) [#2]

677 (0.0099) [#9]

676 (0.0033) [#10]

relevant orbital contributions 176 177 178 178 178 178 178 178 179 179 178 176 177 178 178 178 179 179 179 179 179 177 178 178 179 179 179 176 178 178 179 179 176 177 178 178 178 178 179 179 179 179 176 177 178 178 178 178 179 179 179 179

→ → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → → →

184 185 180 181 182 183 186 187 180 186 187 184 185 180 183 186 180 181 182 186 187 185 180 187 180 186 187 184 180 186 180 187 184 185 181 182 186 187 180 181 186 187 184 185 180 181 183 186 180 181 182 187

(0.38) (0.25) (−0.15) (0.18) (0.13) (0.12) (0.29) (−0.17) (−0.13) (0.14) (0.18) (−0.25) (0.38) (0.14) (−0.10) (−0.22) (−0.14) (−0.17) (−0.15) (0.11) (0.32) (0.14) (0.17) (−0.11) (0.60) (0.13) (0.11) (0.15) (0.60) (0.16) (−0.18) (−0.13) (0.29) (0.27) (0.36) (0.11) (0.14) (−0.19) (−0.17) (−0.15) (0.19) (0.13) (0.27) (−0.30) (−0.20) (0.17) (0.11) (0.21) (0.11) (0.33) (0.13) (−0.22)

μES (D) 10.9

10.5

7.4

8.5

8.0

7.9

solvents is not obvious (see above). Indeed, a plot of ν̃max versus 1/η2 − 1/DS (η = refractive index, DS = dielectric constant of the solvent) based on a dielectric continuum model displays a poor correlation (see the Supporting Information), whereas for [Fc-Cc]+, a reasonable correlation had been achieved.6 The complex solvent dependence of the reorganization energies of 1 and 2 precludes a reliable partitioning of λ

A dramatic difference between ACN and DCM is noted in all reported cases with the larger reorganization energy observed in the more polar ACN. As ΔG0 is quite insensitive to the solvent,6 the reorganization energy λ primarily depends on ν̃max. This implies that, the larger the solvatochromic effect (Table 1), the larger the difference in reorganization energies in the respective solvents. However, a simple correlation for all 421


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Figure 5. Relevant Kohn−Sham molecular orbitals of 1 in ACN; contour value, 0.1 au. The arrows indicate low-energy transitions with MM’CT character.

into an inner sphere λinner and outer sphere fraction λouter for 1 and 2. From the comparison of λ of 1 and 2 with known [Fc-bridgeCc]+ systems (Table 2), it is apparent that the reorganization energies of the amide systems 1 and 2 are larger than in [FcCc]+ and [Fc-CH2-Cc-CH2-Fc]+. If we assume similar inner reorganization energies for all [Fc-bridge-Cc]+ conjugates, this suggests that the changes of the dipole moments of 1 and 2 during electron transfer are larger than those of [Fc-Cc]+ and [Fc-CH2-Cc-CH2−Fc]+, respectively, resulting in a larger solvent reorganization energy. From the IVCT band17 and the redox potentials 16 of the mixed-valent ferrocene/ ferrocenium derivative [Fc-NHCO-Fc]+, a reorganization energy of λ = 6563 cm−1 (in THF) can be estimated, which is also lower than that obtained for 1 (λ = 8564 cm−1, in THF), thus pointing to a larger change of dipole moment of 1 during electron transfer as compared with [Fc-NHCO-Fc]+ (again, assuming similar inner reorganization energies). From the experimental data, we have concluded that 1 and 2 have larger ground-state dipole moments than systems with symmetrical bridges [Fc-bridge-Cc]+, [Fc-bridge-Cc-bridgeFc]+, and the amide-bridged all-iron mixed-valent system [FcNHCO-Fc]+. The negative solvatochromic behavior of 1 and 2 suggests that the respective excited-state dipole moments are lower than the ground-state ones.

On the basis of a two-state Hush model and the parameters ν̃max and ΔG0 from Table 2, potential energy diagrams have been constructed for 1, 2, and [Fc-Cc]+ in ACN and DCM, respectively (Figure 4). The diagrams suggest that the MM’CT states of 1 and 2 are borderline cases between metastable states with double-well potentials and conventional excited states. The MM’CT state of the prototypical [Fc-Cc]+ corresponds to an excited state in both solvents. The difference between 1 and [Fc-Cc]+ is primarily based on the much lower ΔG0 of 1 (Table 2). To further stabilize the MM’CT state with respect to relaxation to the ground state and thus to obtain potentially switchable systems with (more pronounced) double-well potentials, such as, for example, iron(II) high-spin/low-spin isomers32−34 or cobalt(III) catecholato/cobalt(II) semiquinonato valence isomers,35,36 the zero-level energy difference ΔG0 has to be lowered even further. To substantiate the interpretations given above, we employed spin-restricted DFT calculations (B3LYP, LANL2DZ6,14−21,31) on models of 1 and 2. As the experimental properties of 1 and 2, namely, the electronic spectra, strongly depend on the solvent, all optimizations were performed both in the gas phase and with solvent modeling based on the integral equation formalism polarizable continuum model (IEFPCM, ACN and DCM). The termetallocene 2 was additionally considered in two conformations, namely, in an 422


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extended one (2-open) and in a hydrogen-bonded ring structure (2-ring). Furthermore, attempts to include the effects of one acetonitrile explicitly hydrogen bonded to NH groups and one counterion (PF6)− hydrogen bonded to NH groups are presented. For assignment of the observed solvatochromic electronic transitions, we performed time-dependent DFT calculations (TDDFT). It has been noted earlier6 and is also observed in our cases that the results of the gas-phase TDDFT calculations agree rather poorly with the experiment (e.g., MM’CT absorption bands are calculated at 789,6 1726, and 1695 nm for [Fc-Cc]+, 1, and 2 in the gas phase, respectively, Tables 3 and 4). Thus, we will focus our discussion on the structures and properties obtained from calculations with inclusion of solvent effects. Especially for 1, the gas-phase calculations yield geometries with CO groups strongly bent toward the CoIII center (see Tables 3 and 4 for O−C5−C1−C2 torsion angles), which is probably a result of intramolecularly stabilizing the positive charge in the gas phase. In the solvent cage (ACN or DCM), this bending seems to be obsolete and the amide unit is less twisted relative to the Cp ring of cobalt, giving rise to a better π overlap between amide and Cp. As already inferred from the experimental data, the groundstate dipole moment of 1 is calculated to be quite large by DFT (μGS > 17 D). TDDFT calculations on vertically excited states of 1 suggest that the dipole moments of the MM’ excited states are considerably lower (μES ≤ 10 D). This fits to the observed negative solvatochromic effect, which suggests a more polar ground state (Table 1). A partial molecular orbital diagram of 1 in ACN is obtained from the DFT calculations, and it is given in Figure 5. Changing the solvent to DCM qualitatively retains the molecular orbital scheme. As expected, the quasidegenerate HOMOs (#95, #96) essentially are δ-bonding orbitals of the ferrocene part (dx2−y2/ dxy), whereas the quasidegenerate LUMOs (#97, #98) are the π-antibonding orbitals of the cobaltocenium unit (dxz, dyz) with significant contribution from the cyclopentadienyl ring. The HOMO (#96) features a small contribution from the Co center and the LUMO (#97), a small contribution from the Fe center (see the Supporting Information for a visualization of the frontier orbitals with a contour value of 0.03 au; cf. ref 6). The HOMO-1 (#95), however, is essentially localized on iron, whereas the LUMO+1 (#98) is essentially localized on cobalt, which results in a very low transition dipole matrix element. The next-highest unoccupied molecular orbital (#99) features a large contribution from the carbonyl group in addition to a cobalt-centered orbital. The π-antibonding orbitals of ferrocene (dxz, dyz) are located at higher energy (#100, #101). These results fully conform with the evaluation of Tuzcek et al. on [Fc-Cc]+.6 Allowed electronic transitions with significant MM’CT character involve the HOMO (#96), the LUMO (#97), and the orbital #99 with carbonyl/cobalt contributions. Their ± combination yields two transitions at 758/657 nm (ACN) and 800/678 nm (DCM), which qualitatively reproduces the experimental bathochromic shift from ACN to DCM (534 nm → 620 nm) (Figure 6). Explicit inclusion of a hydrogenbonded ACN molecule (NH···NC−CH3; d(H···N) = 1.92 Å in ACN) does not significantly change the calculated transition energies (Table 3). Although we do not expect strong coordination of the (PF6)− counterion to the amide NH, this situation was also modeled (NH···F−PF5; d(H···F) = 1.90 Å in ACN and in DCM), but no significant effect was observed, suggesting only marginal influence of the weakly coordinating

Figure 6. UV/vis spectrum (black) of 1 in ACN (top) and DCM (bottom); deconvoluted bands (red) and TDDFT calculated spectra (blue, bands with fwhm = 80 cm−1).

counterion. Thus, we believe that the description of the cation of 1 in a solvent cage is a reasonable model for the calculations. The termetallocene has been considered in two conformations (Figure 7), namely, a nearly symmetric open conformation (2-open) and a cyclic one with an intramolecular NH···OC hydrogen bond (2-ring; NH···OC; d(H···O) = 1.81 Å in ACN and DCM). Both conformations are essentially isoenergetic, independent of the environment (ΔG < 5 kJ mol−1). In the open (almost symmetric) conformation of 2, the local dipole moments partially annihilate and the total dipole moment is quite low (μGS < 8 D), which contradicts the experimental interpretation with large ground-state dipole moments. Thus, it is expected that most transitions originate from less symmetric conformations. Indeed, the ring structure 2-ring (as one possible conformation of lower symmetry) features large ground-state dipole moments (μGS > 18 D) and lower excited-state dipole moments for the two lowest excited states (μES < 7 D). The lower symmetry and the hydrogen bond energetically differentiate the two ferrocene units. The molecular orbitals #146 and #147 are δ orbitals of the ferrocenyl unit with the free NH group, whereas the molecular orbitals #148 and #149 are δ orbitals of the ferrocenyl moiety with the hydrogen-bonded NH group (Figure 8). The LUMOs #150 and #151 consist of cobalt-centered orbitals with π* character (dxz and dyz) with #150 pointing to the free NH group and #151 pointing to the hydrogen-bonded NH group. Combinations of the occupied ferrocenyl-based orbitals #146, #147, and #149 and the empty cobalt centered orbitals #150 and #151 give rise to several transitions with MM’CT character. All these transitions experience a bathochromic shift from ACN to DCM, in accordance with the experiment (Figure 423


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CONCLUSIONS

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EXPERIMENTAL SECTION

Article

Metallocene amides were prepared from aminoferrocene and cobaltocenium carboxylic acid or 1,1′-cobaltocenium dicarboxylic acid (1, 2). Very small differences of the redox potentials ΔG0 ≈ 1 V were observed due to the electron-donating and electron-withdrawing nature of the substituents at the ferrocene and cobaltocenium units, respectively. In accordance with the small zero-level energy differences ΔG0 are the low-energy FeII → CoIII charge-transfer absorptions (MM’CT) that extend into the near-infrared spectral region. Furthermore, large solvatochromic shifts were observed with these amide-linked bis- and termetallocenes composed of ferrocene and cobaltocenium units due to the large difference in ground-state and excitedstate dipole moments. In the polar solvent acetonitrile, the experimental data suggest that the MM’CT states are stabilized to such an extent that the systems lie on the borderline of a conventional MM’CT excited state and a double-minimum potential with a metastable MM’CT state. The low-energy optical transitions were assigned by DFTPCM and TDDFT-PCM approaches as MM’CT transitions involving δ orbitals of the ferrocene (dx2−y2/dxy) and π* orbitals of the cobaltocenium moieties (dxz/dyz) in combination with Cp orbitals. The experimental negative solvatochromic effect has been reproduced by the TDDFT-PCM calculations. Future work will concentrate on oligometallocenes with even lower ΔG0 values and thus the potential of developing into real switchable two-state systems with a long-lived intramolecular charge-transfer state. A further focus will be on the “inverse complexes” [Cc-NHCO-Fc]+ or [Cc-NHCO-Fc-CONH-Cc]2+ as well as on peptides involving cobaltocenium building blocks in the main chain. One specific synthetic challenge is the acylation of the strongly deactivated amino group of aminocobaltocenium ions (pKB = 15.637) due to the positive charge of the cobaltocenium site. Work into these directions is currently in progress.

Figure 7. DFT models of 2 in ACN.

Experimental General Considerations. All reactions were performed under an argon atmosphere unless otherwise noted. Solvents were dried by standard methods. Aminoferrocene,14,38 cobaltocenium-carboxylic acid-hexafluorophosphate,37 and 1,1′-cobaltocenium-dicarboxylic acid-hexafluorophosphate37 were prepared according to literature procedures. All other reagents were used as received from commercial suppliers (Acros, Sigma-Aldrich). NMR spectra were recorded on a Bruker Avance DRX 400 spectrometer at 400.31 MHz (1H) and 100.05 MHz (13C{1H}). All resonances are reported in parts per million versus the solvent signal as an internal standard: CD3CN (1H: δ = 1.93 ppm; 13C: δ = 1.3, 117.7 ppm); (s) = singlet, (pt) = pseudo triplet (unresolved doublet of doublets), (br s) = broad singlet. IR spectra were recorded with a BioRad Excalibur FTS 3100 spectrometer as CsI disks: (s) = strong, (m) = medium, (w) = weak. Electrochemical experiments were carried out on a BioLogic SP50 voltammetric analyzer using a glassy carbon working electrode, a platinum wire as the counter electrode, and a 0.01 M Ag/AgNO3 electrode as the reference electrode. The measurements were carried out at a scan rate of 100 mV s−1 for cyclic voltammetry experiments and for square-wave voltammetry experiments using 0.1 M (nBu4N)(PF6) as the supporting electrolyte in CH3CN. Potentials are given relative to the ferrocene/ferrocenium couple. UV/vis/NIR spectra were recorded on a Varian Cary 5000 spectrometer using 1.0 cm cells (Hellma, suprasil). ESI mass spectra were recorded on a Micromass QTOF-Ultima spectrometer. 57Fe Mössbauer measurements of powder samples were performed in transmission geometry using a constantacceleration spectrometer and the source 57Co(Rh). The Recoil 1.03 Mössbauer Analysis Software was used to fit the experimental spectra

Figure 8. Relevant Kohn−Sham molecular orbitals of 2-ring in ACN; contour value, 0.1 au. The arrows indicate low-energy transitions with MM’CT character.

9). The two lowest excited states (excited states #1 and #2, Table 4) both contribute to the observed low-energy absorption band of 2. The calculated transition energies for the hydrogen-bonded system 2-ring match the experimental values somewhat better than the ones for 2-open, suggesting a larger contribution of unsymmetric conformers (see above). Explicit inclusion of a counterion hydrogen bonded to both NH groups in a chelating fashion (Figure 7) again modifies the pseudosymmetry and also, to some extent, the calculated transition energies (Table 4). However, the conclusion that the solvatochromic low-energy transitions are of mainly MM’CT character is still valid. 424


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Figure 9. UV/vis spectrum (black) of 2 in ACN (top) and DCM (bottom); deconvoluted bands (red) and TDDFT calculated spectra for 2-open (left) and 2-ring (right) (blue, bands with fwhm = 80 cm−1). with Lorentzian peaks.39 Isomer shift values are quoted relative to α-Fe at 295 K. Melting points were determined using a Gallenkamp capillary melting point apparatus MFB 595 010 and were not corrected. Elemental analyses were performed by the microanalytical laboratory of the chemical institutes of the University of Mainz. Density functional calculations were carried out with the Gaussian 09/DFT series40 of programs. The B3LYP formulation of density functional theory was used employing the LANL2DZ basis set and dtype polarization function for the phosphorus ((PF6)− counterion; P (ζ = 0.340)).41 No symmetry constraints were imposed on the molecules. The presence of energetic minima of the ground states was checked by analytical frequency calculations. Solvent modeling was done employing the integral equation formalism polarizable continuum model (IEFPCM, acetonitrile and dichloromethane). Crystal Structure Determinations. Intensity data were collected with a Bruker AXS Smart1000 CCD diffractometer with an APEX II detector and an Oxford cooling system and corrected for absorption and other effects using Mo Kα radiation (λ = 0.71073 Å). The diffraction frames were integrated using the SAINT package, and most were corrected for absorption with MULABS.42,43 The structures were solved by direct methods and refined by the full-matrix method based on F2 using the SHELXTL software package.44,45 All non-hydrogen atoms were refined anisotropically while the positions of all hydrogen atoms were generated with appropriate geometric constraints and allowed to ride on their respective parent carbon atoms with fixed isotropic thermal parameters. Crystal data: C21H19B2CoF8FeNO (589.77); T = 193 K; black needle; 0.30 × 0.12 × 0.03 mm; tetragonal; P42bc; a = b = 15.140(2) Å; c = 9.6200(19) Å; V = 2205.1(6) Å3; Z = 4; F(000) = 1180.0; ζ = 1.776 g cm−3; μ = 1.493 mm−1; 2θ range = 6.84−57.98°; index ranges, −20 ≤ h ≤ 20, −20 ≤ k ≤ 20, −12 ≤ l ≤ 13; reflections collected, 39 478; 2906 independent reflections; 525 parameters; max/min transmission = 0.9566/0.6629; goodness-of-fit on F2 = 1.007; largest difference peak and hole = 0.277/−0.245 e Å−3; R1(I > 2σ) = 0.0397; R1(all data) = 0.0839; Rw(I > 2σ) = 0.1021; Rw(all data) = 0.1285; absolute structure parameter, 0.50(8). CCDC-848982 (1-ox) contains the supplementary crystallo-

graphic 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. Synthesis of 1. Cobaltocenium-carboxylic acid-hexafluorophosphate (208 mg, 0.55 mmol) was heated under reflux in thionyl chloride (5 mL) for 6 h. After removal of the thionyl chloride under reduced pressure, the acid chloride was washed with diethyl ether (2 × 5 mL) and suspended in THF (5 mL). In a separate Schlenk tube, aminoferrocene (181 mg, 0.90 mmol) was dissolved in THF (5 mL) and triethylamine (0.1 mL) was added by syringe. This solution was added to the suspension of the acid chloride. The mixture turned from yellow to deep blue. After stirring for 2 h at room temperature, the solvent was removed under reduced pressure. The residue was dissolved in a saturated ethanolic ammonium hexafluorophosphate solution (10 mL). Covering the solution with diethyl ether (10 mL) precipitated a dark powder that could be recrystallized from acetonitrile/diethyl ether. Yield: 86% (265 mg, 0.47 mmol). mp 161−163 °C. Anal. Calcd for C21H19NOFeCoPF6 (561.13) × 1 /4CH3CN × 3/2H2O: C, 43.15; H, 3.83; N, 2.93. Found: C, 43.31; H, 4.34; N, 3.11. 1H NMR (400 MHz, CD3CN): δ = 4.09 (pt, 2H, 3 JHH = 1.9 Hz, H3′), 4.20 (s, 5H, H4′), 4.75 (pt, 2H, 3JHH = 1.9 Hz, H2′), 5.71 (s, 5H, H4), 5.77 (pt, 2H, 3JHH = 2.1 Hz, H3), 6.16 (pt, 2H, 3 JHH = 2.1 Hz, H2), 8.43 (br s, 1H, H5) ppm. 13C{1H} NMR (100 MHz, CD3CN): δ = 62.7 (s, C2′), 65.7 (s, C3′), 70.2 (s, C4′), 84.8 (s, C2), 86.8 (s, C3), 87.1 (s, C4), 95.0 (s, C1′), 95.8 (s, C1), 160.1 (s, C5) ppm. MS (ESI): m/z = 416 (100) [M − PF6]+, 977 (5) [2M − PF6]+. UV/vis (CH3CN): λ (ε) = 534 nm (355 M−1 cm−1). IR (KBr): ν̃ = 3410−2535 (m, NH), 1674 (s, amide I), 1571 (w, amide II), 1390 (w), 1322 (w), 1276 (w), 1140 (m), 1027 (m), 990 (w), 836 (s, PF), 747 (w), 558 (w) cm−1. CV (CH3CN): E1/2 = −0.015 V (1e, Fc0/+), −1.150 V (1e, Cc0/+) vs FcH/FcH+. Synthesis of 2. 1,1′-Cobaltocenium-dicarboxylic acid-hexafluorophosphate (60 mg, 0.131 mmol) was heated under reflux in thionyl chloride (5 mL) for 6 h. After removal of the thionyl chloride under reduced pressure, the acid chloride was washed with diethyl ether (2 × 425


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5 mL) and suspended in THF (5 mL). In a separate Schlenk tube, aminoferrocene (60 mg, 0.30 mmol) was dissolved in THF (5 mL) and triethylamine (0.05 mL) was added by syringe. This solution was added to the suspension of the acid chloride. The mixture turned from yellow to deep blue. After stirring for 2 h at room temperature, the solvent was removed under reduced pressure. The residue was dissolved in a saturated ethanolic ammonium hexafluorophosphate solution (5 mL). Covering the solution with diethyl ether (10 mL) precipitated a dark powder that could be recrystallized from acetonitrile/diethyl ether. Yield: 98% (101 mg, 0.128 mmol). mp > 280 °C. Anal. Calcd for C32H28N2O2Fe2CoPF6 (788.18) × 1/2CH3CN × H2O: C, 47.94; H, 3.84; N, 4.24. Found: C, 47.39; H, 3.03; N, 4.47. 1 H NMR (400 MHz, CD3CN): δ = 4.11 (br pt, 4H, H3′), 4.24 (s, 10H, H4′), 4.78 (br pt, 4H, H2′), 5.85 (br pt, 4H, H3), 6.08 (br pt, 4H, H2), 8.90 (br s, 2H, H5) ppm. 13C{1H} NMR (100 MHz, CD3CN): δ = 62.0 (s, C2′), 64.9 (s, C3′), 69.3 (s, C4′), 85.6 (s, C2), 86.6 (s, C3), 93.9 (s, C1′), 96.0 (s, C1), 159.4 (s, C5) ppm. MS (ESI): m/z = 643 (100) [M − PF6]+. UV/vis (CH3CN): λ (ε) = 565 nm (385 M−1 cm−1). IR (KBr): ν̃ = 3410−2490 (m, NH), 1671, 1647 (m, amide I), 1555 (m, amide II), 1491 (m), 1478 (m), 1462 (m), 1400 (s), 1278 (m), 1161 (w), 1142 (w), 1107 (w), 1064 (w), 1036 (w), 836 (s, PF), 559 (m) cm−1. CV (CH3CN): E1/2 = −0.020 V (2e, Fc0/+), −0.970 V (1e, Cc0/+) vs FcH/FcH+.

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ASSOCIATED CONTENT

S Supporting Information *

NOESY spectra of 1 and 2, graphical description of the molecular structure of 1-ox in the crystal showing the supramolecular arrangement, Kamlet−Taft correlations for 1 and 2, Gaussian deconvolution of the MM’CT bands of 1 and 2 in different solvents, plot of absorption maxima of 1 and 2 versus solvent function 1/η2 − 1/DS, graphical representation of HOMO and LUMO of 1 with a contour value 0.03 au, and Cartesian coordinates of the DFT calculated models of 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: + 49-6131-3927277. E-mail: [email protected].

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ACKNOWLEDGMENTS We thank the Deutsche Forschungsgemeinschaft for financial support. We are grateful to Dr. Dieter Schollmeyer (X-ray data collection) and Dr. Vadim Ksenofontov and Prof. Dr. Claudia Felser (Mössbauer measurements).

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REFERENCES

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Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (41) Huzinaga, S.; Andzelm, J.; Klobukowski, M.; Radzio-Andzelm, E.; Sakai, Y.; Tatewaki, H. Gaussian Basis Sets for Molecular Orbital Calculations; Elsevier: Amsterdam, 1984. (42) SMART Data Collection and SAINT-Plus Data Processing Software for the SMART System (various versions); Bruker Analytical X-Ray Instruments, Inc.: Madison, WI, 2000. (43) Blessing, B. Acta Crystallogr. 1995, A51, 33. (44) Sheldrick, G. M. SHELXTL, version 5.1; Bruker AXS: Madison, WI, 1998. (45) Sheldrick, G. M. SHELXL-97; University of Gö ttingen: Göttingen, Germany, 1997.

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Amide-Linked Heterobi- and Heterotermetallocenes with Very Low

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