A Ligand-Bridged Heterotetranuclear (Fe2Cu2) Redox System with Fc

Article pubs.acs.org/Organometallics

A Ligand-Bridged Heterotetranuclear (Fe2Cu2) Redox System with Fc/ Fc+ and Radical Ion Intermediates Rajkumar Jana,† Falk Lissner,† Brigitte Schwederski,† Jan Fiedler,‡ and Wolfgang Kaim*,† †

Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejškova 3, CZ-18223 Prague, Czech Republic

‡

S Supporting Information *

ABSTRACT: The redox pair [(μ-abcp){Cu(dppf)}2]2+/+ (abcp = 2,2′-azobis(5chloropyrimidine) and dppf =1,1′-bis(diphenylphosphino)ferrocene) has been structurally characterized to reveal the lengthening of the NN and shortening of the CNazo bonds on reduction, each by about 0.04 Å. These and other charge forms, [(μabcp){Cu(dppf)}2]n+ (n = 0, 3+, 4+), have been investigated spectroelectrochemically (UV−vis−near-IR, EPR) to reveal an abcp-based second reduction and a stepwise ferrocene-centered oxidation of the 2+ precursor. In contrast to the small but detectable comproportionation constant of Kc = 17 for the Fc/Fc+ mixed-valence (3+) charge state, the monocationic radical complex exhibits a very large Kc value of 1016.

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INTRODUCTION Bis-chelating π-electron-deficient azo ligands have found our interest because of charge and electron transfer to the low-lying azo-centered π* molecular orbitals and because of rather short molecule-bridged metal−metal distances of about 5 Å.1−7 Stable radical intermediates, a strong metal−metal interaction, and low-energy charge transfer absorption (MLCT, LMCT) in the visible and near-infrared (near-IR) regions were reported. The N−N bond length is approximately 1.25 Å when the bond order is 2 but increases to about 1.33 Å in the azo anion radical form (bond order 1.5) and to about 1.45 Å in the hydrazido(2−) form.7 Among various substituted ligands in the azo ligand family1 abcp (=2,2′-azobis(5-chloropyrimidine); Scheme 1) has certain advantages. The presence of two electron-withdrawing chlorine atoms at the C5,C5′ positions and of four N atoms in the two pyrimidine rings make it a superb π-acceptor and easily reducible at less negative potentials.3 This ligand can be prepared by chlorinating oxidative coupling of 2-aminopyrimidine.2 The azo ligands are capable of undergoing two one-electron reduction processes, accommodating the added electrons in the π* molecular orbital (Scheme 1). Metal complexes containing the above forms of the ligand are characterized by low-energy metal-to-ligand charge transfer (MLCT) bands. The effect of metal coordination is also found in the corresponding azo bond lengths.7 The favored binding mode (Scheme 2) is the formation of one five-membered chelate ring in the mononuclear complexes and two joint edge-sharing fivemembered chelate rings in the dinuclear complexes. Dinuclear CuI complexes were found to be stabilized by phosphines.2 Although there have been literature reports on the structural and spectroscopic evidence for azo-bridged mononuclear and © 2013 American Chemical Society

dinuclear copper, rhenium, ruthenium, and platinum complexes,1−8 there has not yet been a report on structural evidence for both the neutral azo and the azo anion radical forms for a given complex: i.e., a redox pair. In this work we provide structural information on an abcp-bridged oligonuclear complex in both redox states, using the [1,1′-bis(diphenylphosphino)ferrocene]copper(I) unit as the complex fragment. Structural information on a dinuclear copper complex with abcp in the radical form was reported earlier;2 however, the corresponding oxidized state could not be structurally characterized. Following a general synthetic route for the synthesis of such complexes, it was now possible to structurally characterize a heterotetranuclear Cu2Fe2 complex with nonreduced abcp as a bridging ligand and 1,1′-bis(diphenylphosphino)ferrocene (=dppf) as a coligand on copper. 1,1′-Bis(diorganylphosphino)ferrocenes are exceptional ligands not only for catalysis but also for the construction of functional materials,9 offering electron transfer activity10 and a defined chelate function. The significant difference of almost 1 V between the first and second reduction potentials of this complex allowed us to isolate the radical form of the complex by bulk electrolysis, and an X-ray diffraction study of this form provided its structural details. While the corresponding azo ligand based redox system was investigated using electrochemistry (CV, DPV) and spectroelectrochemistry (UV−vis− near-IR, EPR), the presence of two equivalent ferrocene moieties invited a study of their interaction by applying (spectro)electrochemistry to anodic oxidation. Numerous reports have been directed at probing the ferrocene−ferrocene Special Issue: Ferrocene - Beauty and Function Received: May 24, 2013 Published: August 23, 2013 5879

dx.doi.org/10.1021/om400466u | Organometallics 2013, 32, 5879−5886

Organometallics

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Scheme 1. Two-Step Reduction Showing Electronic Configurations and N−N Distances in Different Redox States of abcp

(Supporting Information). Data of [(μ-abcp){Cu(PPh3)2}2](PF6) from ref 2 are included for comparison in Table 1. In the molecular structure of [1](BF4)2 (Figure 2), the two CuP2N2 coordination spheres of the dication exhibit a similarly distorted tetrahedral geometry around the copper atoms. Two CuP2N2 five-membered ring chelates share a common edge (Figure 2). The copper centers are bridged by abcp at a distance of 4.8013(3) Å. The ferrocene iron atoms were found at a distance of 12.4578(7) Å, and the Fe−Cu distance was found at about 4.06 Å. The bite angles of dppf and abcp around the Cu centers are 109.79(6) and 76.92(18)°, respectively. At 2.014(4) Å the Cu−Nazo bonds were found to be shorter than the Cu−Npy bonds (2.054(5) Å) due to the greater πacceptor effect of azo N donors vs pyrimidine N donors. At 1.308(9) Å, the N−N bond length is still in the expected range for nonreduced abcp.7 The azo bond length is shorter in comparison to the one-electron-reduced azo bond of 1.345(7) Å, reported for [Cu(abcp)(PPh3)2](BF4).2 Relative to the free ligand, the azo bond length has increased by 0.078 Å on 2-fold [Cu(dppf)]+ coordination due to efficient π back-donation from the d10 CuI centers to the empty π* molecular orbital of abcp (σ2π2π*). The metalation effect is also reflected by the C− Nazo bond lengths, which have shortened (on average) by 0.036 Å. Electron density transfer from the metal to the azo-centered π* molecular orbital is responsible for the observed shortening. The Cu−P bond lengths are slightly different at Cu−P1 = 2.246(2) and Cu−P2 = 2.273(2) Å, hinting at a typical16 distortion toward a trigonal-planar arrangement. The torsional angles of 15.0(8)° for N3′−N3−C1−N2 and of 164.9(6)° for N3′−N3−C1−N1 illustrate a certain deviation from planarity for the abcp ligand (“twisting”). In the crystal structure of the radical complex [1](BF4), two crystallographically different molecules were found in each unit cell (Figure 3). The azo bond lengths in the two cations were found at 1.348(11) and 1.351(12) Å. The NN bond length has thus increased by about 0.04 Å in this reduced radical form 1•+

Scheme 2. Favored Binding Modes of the abcp Ligand with Metal Fragments MLn

interaction by determination of the redox potential splitting in coupled dinuclear or oligonuclear systems.11−14

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RESULTS AND DISCUSSION Synthesis and Characterization. The diamagnetic heterotetranuclear complex [(μ-abcp){Cu(dppf)}2](BF4)2 ([1](BF4)2) was synthesized from abcp and the metal precursor [Cu(dppf)(CH3CN)2](BF4) in a 1:2 molar ratio in dichloromethane (Scheme 3). A self-assembly process in the presence of the azo ligand, involving comproportionation of Cu0 and CuII, is generally utilized to obtain radical dinuclear complexes.15 A similar reaction pathway for the heterotetranuclear abcp radical complex was found to be ineffective due to the formation of a mixture of products and instability at higher temperature. To isolate the radical complex, we have thus employed electrochemical reduction under an argon atmosphere. The paramagnetic compound {(μ-abcp)[Cu(dppf)]2}(BF4) ([1](BF4)) was obtained from bulk electrolysis of the diamagnetic [1](BF4)2 at a mercury-pool electrode, using CH2Cl2/ Bu4NBF4 as electrolyte (Scheme 4, Figure 1). After complete electrolysis, a workup of the solution yielded brown [1](BF4); repeated crystallization at −4 °C gave the pure crystalline material. Crystal Structures of [1](BF4)2 and [1](BF4). Crystallographic data and structure parameters obtained for single crystalline solvated forms are given in Tables 1 and S1 Scheme 3. Synthesis of [(μ-abcp){Cu(dppf)}2](BF4)2 ([1](BF4)2)

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Scheme 4. Electrochemical Conversion of [1](BF4)2 to [1](BF4)

complex in Table 3. The abcp ligand can be reduced in two one-electron steps at −1.01 and −1.49 V versus ferrocene/ ferrocenium in acetonitrile.4 The expected two reductions were also observed for the corresponding heterotetranuclear metal complex [1](BF4)2, but at much less negative potentials of −0.24 and −1.20 V (Figure 4, top). The first reduction potential has decreased by 0.77 V and the second reduction potential by 0.29 V in the complex, relative to the values of free abcp. As a result, a 2-fold increase in the difference between the first and the second reduction potentials (ΔE = E1/2(red1-red2)complex − E1/2(red1-red2)abcp) is observed on chelation of two [Cu(dppf)]+ complex fragments, probably due to coordination-induced structural change. The resulting unusual stability of the one-electron-reduced product 1+ is quantitatively illustrated by a very large comproportionation constant (Kc; eq 1) of 1016.

Figure 1. Bulk electrolysis on a mercury-pool electrode monitored by polarography: (1) 1.25 mM solution of [1](BF4)2 in CH2Cl2/0.1 M Bu4NBF4 before electrolysis, (2) solution after reduction by 0.5 F/mol at E = −0.5 V; (3) after reduction by 1.0 F/mol at E = −0.5 V.

Kc =

relative to the diamagnetic dicationic precursor 12+. Such an effect may look small but has been estimated similarly from calculations of dinuclear Re(CO)3Hal complexes with abcp and abcp•−.5 The Cu−N bond lengths have also increased slightly in comparison to the nonreduced form. The NN bond lengths are comparable to the 1.345(7) Å from the reported crystal structure of [(μ-abcp){Cu(PPh3)2}2](PF6).2 The C−Nazo bond lengths have decreased by about 0.04 Å to 1.361(9)−1.368(9) Å, signifying the participation of the accepting chloropyrimidine rings in the charge delocalization. As a consequence of the NN and CN bond lengthening, the metal−metal distances increase slightly on reduction. The twisting of the abcp•− bridge has significantly decreased, with NNCN torsional angles of about 5 and 175°, from 15 and 165° for the precursor; this guarantees a better π spin delocalization which is beneficial for the stability of the π radical ligand bridge. The distortion at the copper centers is smaller than that observed for the oxidized form. The experimental values from Table 2 illustrate the strong structural response of the abcp acceptor ligand to coordination of electron-donating copper(I) complex fragments. The effect of 2-fold CuI binding to neutral abcp (back-donation) is larger than that of an added electron in the existing dicopper complex. As outlined before for the series of abpy (2,2′-azobis(pyridine))-containing complexes,7 there is a structural continuum resulting from internal and/or external charge transfer to the azo function, making an uncritical use of structure data for oxidation state determination difficult. Electrochemistry. Cyclic voltammograms were measured for complexes [1](BF4)2 and [1](BF4) in CH2Cl2/0.1 M Bu4NPF6 at room temperature with 100 mV/s scan rate and compared with the results for the free ligand and a related

[1+ ]2 = 10ΔE /59 mV = 1.9 × 1016 (at 298 K) [1][12 + ] (1)

The ferrocene-based two-electron oxidation wave, corresponding to two dppf units, was found at +0.30 V. The broad nature of the oxidation wave was further investigated by a differential pulse voltammetric experiment in CH2Cl2/0.1 M Bu4NPF6 at room temperature with 70 mV/s scan rate. The differential pulse voltammogram (Figure 4, bottom) showed two one-electron oxidations at slightly different potentials, separated by 72 mV. The intermediate complex [1]3+ thus exhibits a Kc value of 17 for the the Fc/Fc+ (FeIIFeIII) mixedvalence intermediate. UV−Vis−Near-IR Spectroelectrochemistry. UV−vis− near-IR spectroelectrochemical measurements of [1](BF4)2 were carried out in CH2Cl2/0.1 M Bu4NPF6, using an optically transparent thin layer electrode (OTTLE) cell17 for all reversible oxidation and reduction processes (Figures 5 and 6, Table 4). The precursor complex [1](BF4)2 exhibits an MLCT band of the type d10π(Cu) → π*(abcp) at 948 nm with ε = 8270 M−1 cm−1. Two shoulders found at 450 and 337 nm are assigned to intraligand π-to-π* transitions of the nonreduced, metalcoordinated abcp. On the first reversible one-electron reduction, the original MLCT band at 948 nm hypsochromically shifts to 800 nm and the ε value is diminished to 3110 M−1 cm−1. The shift of the MLCT band and its lower intensity result from the single occupation of the π* molecular orbital in abcp. In addition, a band arises at 566 nm (ε = 4210 M−1 cm−1) which can be 5881


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Table 1. Comparison of Structural Parameters (Distances in Å, Angles in deg) for {(μ-abcp)[Cu(dppf)]2}(BF4)2·2CH2Cl2 and [Cu(dppf)]2}(BF4)·CH2Cl2 [1](BF4)·CH2Cl2 bond N−N Cu−N

Cu−P

Cu−Cu Fe−Fe C−N

C−N

C−C

N−Cu−N P−Cu−P C−N−N N−C−N Cu−N−N N−N−C−N

a

[1](BF4)2·2CH2Cl2 N3−N3 1.308(9) Cu−N2 2.054(5) Cu−N3 2.014(4) Cu−P1 2.246(2) Cu−P2 2.273(2) 4.8013(3) 12.4578(7) C1−N3 1.403(7) C1−N1 1.330(7) C1−N2 1.348(7) C2−N1 1.335(8) C4−N2 1.338(7) C2−C3 1.390(9) C3−C4 1.369(8) N2−Cu−N3 76.92(18) P1−Cu−P2 109.79(6) C1−N3−N3 111.8(5) N2−C1−N3 117.5(5) Cu−N3−N3 119.0(5) N3−N3−C1−N1 164.9(6) N3−N3−C1−N2 15.0(8)

[(μ-abcp){Cu(PPh3)2}2](PF6)a N3−N3 1.348(11) Cu1−N2 2.089(6) Cu1−N3 2.017(6) Cu1−P1 2.245(2) Cu1−P2 2.258(2) 4.8223(1) 12.8240(3) C1−N3 1.361(9) C1−N1 1.333(9) C1−N2 1.368(9) C2−N1 1.342(10) C4−N2 1.328(10) C2−C3 1.424(11) C3−C4 1.376(10) N2−Cu1−N3 77.1(2) P1−Cu1−P2 110.16(7) C1−N3−N3 113.5(7) N2−C1−N3 118.8(6) Cu1−N3−N3 118.3(6) N3−N3−C1−N1 176.8(7) N3−N3−C1−N2 3.8(11)

N6−N6 1.351(12) Cu2−N5 2.065(6) Cu2−N6 2.003(6) Cu2−P3 2.234(2) Cu2−P4 2.246(2) 4.8016(1) 12.3462(3) C39−N6 1.368(9) C39−N4 1.361(9) C39−N5 1.368(9) C40−N4 1.323(11) C42−N5 1.331(10) C40−C41 1.397(12) C41−C42 1.374(10) N5−Cu2−N6 77.9(2) P3−Cu2−P4 111.07(8) C39−N6−N6 111.9(8) N5−C39−N6 119.8(6) Cu2−N6−N6 118.6(7) N6−N6−C39−N4 174.0(8) N6−N6−C9−N5 6.2(11)

N2−N2′ 1.345(7) Cu1−N1 2.098(3) Cu1−N2′ 2.045(3) Cu1−P1 2.264(1) Cu1−P2 2.246(1) 4.8656(7) n.a C1−N2 1.363(5) C1−N3 1.344(5) C1−N1 1.360(5) C4−N3 1.322(6) C2−N1 1.377(5) C4−C3 1.402(6) C2−C3 1.379(6) N1−Cu1−N2′ 76.58(14) P1−Cu1−P2 124.10(5) C1−N2−N2′ 113.6(4) N1−C1−N2 119.2(4) Cu1−N2′−N2 117.8(4) N2−N2−C1−N3 174.6(4) N2−N2−C1−N1 6.5(6)

From ref 2.

intense absorption at 900 nm. A near-infrared band which may arise from an intervalence charge transfer (metal-to-metal charge transfer) transition in a mixed-valent situation19 was also not detected in the near-IR region. On the second oxidation, the MLCT band shifts further hypsochromically to 880 nm. Again, the typically weak ferrocenium absorption expected around 700 nm18 could not be detected due to the intense band centered now at 880 nm. All spectroelectrochemical data along with those of related compounds are given in Table 4. EPR Spectroscopy. An EPR spectrum could be obtained for the electrochemically produced {(μ-abcp)[Cu(dppf)]2}•+, using 0.1 M Bu4NPF6 as electrolyte in dichloromethane at 298 K. The in situ generated spectrum was of better quality in comparison to that of the isolated material. The isotropic g value measured at 2.0091 indicates predominantly ligand-

attributed to a metal-to-ligand charge transfer transition. The intraligand charge transfer band is hypsochromically shifted to 411 nm. These features occur also for the isolated and structurally characterized radical intermediate [1](BF4). On further reduction to the neutral species at a potential of −1.20 V versus ferrocene/ferrocenium, the MLCT band diminishes completely due to full occupancy of the π* molecular orbital of abcp. In agreement with the splitting observed on oxidation of [1](BF4)2 in differential pulse voltammetry (Figure 4, bottom), two different trends were observed in the spectra measured over the range of oxidation during the OTTLE spectroelectrochemical experiment. On first oxidation, the long-wavelength MLCT band shifts from 948 to 910 nm. The typical, albeit weak, ferrocenium band at about 700 nm18 could not be observed here because it would be obscured by the much more 5882


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Table 2. Azo Bond Lengths in abcp Complexes compd

N−N distance (Å)

Δ(N−N) (Å)a

1.230(2) 1.308(9)

n.a. 0.078

1.348(11)

0.118

1.351(12) 1.345(7)

0.121 0.115

abcp {(μ-abcp)[Cu(dppf)]2}(BF4)2 ([1](BF4)2) {(μ-abcp)[Cu(dppf)]2}(BF4) ([1] (BF4)) [(μ-abcp){Cu(PPh3)2}2](PF6) a

ref 4 this work this work 2

Δ(N−N) = Change in azo bond length on complexation.

Table 3. Comparison of the Redox Potentialsa Obtained from Cyclic Voltammetry compound abcp [(μ-abcp) {Cu(PPh3)2}2] (PF6)2 [(μ-abcp) {Cu(dppf)}2] (BF4)2 ([1] (BF4)2)

Figure 2. Molecular structure of the complex dication [(μ-abcp){Cu(dppf)}2]2+ in the crystal of [1](BF4)2·2CH2Cl2. The BF4− counteranions and dichloromethane molecules are omitted for clarity.

centered spin. A small shift of g is thus observed in comparison to that of free abcp•− (g = 2.0041) and of {(μ-abcp)[Cu(PPh3)2]2}•+ (g = 2.0071).4 This result confirms that the first reduction corresponds to the addition of an electron to the π* molecular orbital of abcp. Insufficient hyperfine resolution (Figure 7) precluded computer simulations to obtain individual coupling constants from EPR-active nuclei such as 1H, 14N, 31P, and 63,65Cu.4 EPR spectra of the oxidized forms could not be observed, since the ferrocenium EPR signals are known to show very rapid relaxation,10a,20 especially in dinuclear configurations.

Eox

Ered 1

Ered 2

solvent

n.d. +1.42 (i)

−1.01 +0.06

−1.49 (qr) −0.75

CH3CN CH3CN

4 4

+0.30b

−0.24

−1.20

CH2Cl2

this work

ref

a

Potentials (V) vs ferrocenium/ferrocene. Half-wave potentials, unless noted otherwise: (qr): quasi-reversible; (i): irreversible (peak potential given). bBroad two-electron wave for Fc oxidation; for splitting see Figure 4 (bottom).

First, it should be noted that the copper(I) oxidation state remains unchanged in the series of Scheme 5. Oxidation to copper(II) is disfavored in the presence of phosphane and imine-type N donor ligands, and if it occurs, it is usually found to be irreversible. A remarkable stabilization of Kc = 1016 is observed for the radical ion intermediate, which allowed for its isolation and structural analysis. The data illustrate a delocalization of added charge beyond the azo function, confirming the special acceptor properties of the abcp ligand.

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CONCLUSION The above experimental results lead to the assignment of oxidation states given in Scheme 5.

Figure 3. Molecular structure of two crystallographically independent radical cations [(μ-abcp){Cu(dppf)}2]+ in the crystal of [1](BF4)·CH2Cl2. One BF4− counteranion and one dichloromethane molecule have been omitted for clarity. 5883


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Figure 6. (top) UV−vis−near-IR spectroelectrochemical oxidation of {(μ-abcp)[Cu(dppf)]2}2+ to {(μ-abcp)[Cu(dppf)]2}3+ in CH2Cl2/0.1 M Bu4NPF6. (bottom) UV−vis−near-IR spectroelectrochemical oxidation of {(μ-abcp)[Cu(dppf)]2}3+ to {(μ-abcp)[Cu(dppf)]2}4+ in CH2Cl2/0.1 M Bu4NPF6.

Figure 4. (top) Cyclic voltammogram of [1](BF4)2 in CH2Cl2/0.1 M Bu4NPF6 at room temperature with 100 mV/s scan rate. (bottom) Differential pulse voltammogram of [1](BF4)2 in CH2Cl2/0.1 M Bu4NPF6 at room temperature with 70 mV/s scan rate (potentials are not calibrated via Fc+/0).

Table 4. Data from UV−Vis Spectroelectrochemistrya of [1](BF4)2 and Related Compounds λmax/nm (ε/103 M−1 cm−1) abcp abcp•− abcp2− {(μ-abcp) [Cu(PPh3)2]2}2+ {(μ-abcp) [Cu(PPh3)2]2}•+ {(μ-abcp) [Cu(PPh3)2]2} {(μ-abcp) [Cu(dppf)]2}4+ {(μ-abcp) [Cu(dppf)]2}3+ {(μ-abcp) [Cu(dppf)]2}2+ {(μ-abcp) [Cu(dppf)]2}•+ {(μ-abcp) [Cu(dppf)]2} a

Figure 5. (Top) UV−vis−near-IR spectroelectrochemistry for the reduction of {(μ-abcp)[Cu(dppf)]2}2+ to {(μ-abcp)[Cu(dppf)]2}•+ in CH2Cl2/0.1 M Bu4NPF6 in the OTTLE cell. (bottom) UV−vis−nearIR spectroelectrochemistry for the reduction of {(μ-abcp)[Cu(dppf)]2}•+ to {(μ-abcp)[Cu(dppf)]2} in CH2Cl2/0.1 M Bu4NPF6.

ref

455 (0.37), 286 (26.0) 555 (sh), 477 (sh), 404 (44),, 350 (sh) 457 (sh), 352 (45) 93 (1.1), 870 (0.8), 365 (1.54)

4 4 4 4

700 (0.63), 560 (0.9), ,403 (2.6), 373 (2.6) 515 (sh), 363 (2.8)

4

880 (7.25), 435 (sh), 375 (11.63) 910 (8.00), 445 (sh), 367 (10.75) 948 (8.27), 451 (sh), 337 (10.65), 260 (sh) 800 (3.11), 566 (4.21), 411 (11.33), 372 (10.78) 500 (sh), 372 (sh), 270 (sh)

this work this work this work this work this work this work

From spectroelectrochemistry in CH2Cl2/0.1 M Bu4NPF6.

infrared prevented us from directly characterizing the Fc+/Fc mixed-valent intermediates. The example shown here, involving the bridging of two weakly communicating ferrocene units by an azo ligand bridged dicopper platform, confirms the suitability of both the strongly π accepting abcp bis-chelate ligand and the [1,1′-bis(diorganophosphino)ferrocene]copper(I) complex fragment for the construction of larger coordination systems with special electrochemical and spectroscopic properties.

On the other hand, a small but detectable communication is found by way of slightly split oxidations of the two equivalent ferrocene groups, despite their separation by more than 12 Å. Rapid EPR relaxation and obscurance of typical ferrocenium bands by intense charge transfer absorptions in the near5884


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8.24 (s, br, 2H, abcp), 8.86 (s, br, 2H, abcp). 31P NMR (CDCl3, δ (ppm)): −14.13. MS (ESI, Micromass Q-ToF): M − 1 peak at m/z 1490.1 (Figure S1 in the Supporting Information), corresponding to [{(μ-abcp)[Cu(dppf)]2} − H]+ (calculated m/z 1489.94). Synthesis of {(μ-abcp)[Cu(dppf)]2}(BF4). An 18.3 mg portion of {(μ-abcp)[Cu(dppf)]2}(BF4)2 was dissolved in 8 mL of CH2Cl2/ Bu4NBF4 and stepwise electrolyzed at a mercury surface with 0.94 F of dc current. After complete electrolysis, indicated by the absence of the polarographic maxima of the first reduction process, the solution was transferred to a Schlenk flask under an argon atmosphere. The polarographic experiment indicated no over-reduction or decomposition even after 15 min. The solvent was removed under reduced pressure to give a brown solid which was characterized crystallographically as {(μ-abcp)[Cu(dppf)]2}(BF4) after crystallization by diffusion of hexane into a dichloromethane solution of the electrolyzed product at 4 °C. Yield: 14.2 mg (81.8%). MS (ESI, Micromass QToF): peak at m/z 1488.05, corresponding to [1]+ (calculated m/z 1488.05). Crystallography. Green, air-stable single crystals of [1](BF4)2· 2CH2Cl2 were grown at −4 °C by slow diffusion of hexane into a dichloromethane solution of the complex. The complex [1](BF4)· CH2Cl2 was crystallized under an inert atmosphere by liquid-phase diffusion of dry hexane to a dichloromethane solution of the complex. After a few days, brown single crystals had grown inside thin glass tubes. Air-sensitive crystals of [1](BF4) were taken from the capillaries and immediately covered with a layer of paraffin oil. A selected single crystal was mounted in a liquid nitrogen stream for the diffraction measurement. The structures were solved by direct methods using SHELXS-97 and refined by full-matrix least squares with SHELXL-97, refining on F2.22 The positions of all the atoms were obtained by direct methods. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed in geometrically constrained positions and refined with isotropic temperature factors, generally 1.2 times the Ueq values of their parent atoms.

Figure 7. EPR spectrum of {(μ-abcp)[Cu(dppf)]2}•+ in CH2Cl2/0.1 M Bu4NPF6.

Scheme 5. Assignments of Oxidation States in the Electron Transfer Series

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ASSOCIATED CONTENT

S Supporting Information *

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Experimental and simulated isotope combinations for the ESI mass spectrum of compound [1](BF4)2 (Figure S1), crystallographic information (Table S1) and CIF files. This material is available free of charge via the Internet at http://pubs.acs.org.

EXPERIMENTAL SECTION

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Instrumentation. EPR spectra in the X band were recorded with a Bruker System EMX instrument. 1H NMR spectra were taken on a Bruker AC 250 spectrometer. UV−vis−near-IR absorption spectra were recorded on J&M TIDAS and Shimadzu UV 3101 PC spectrophotometers. Cyclic voltammetry was carried out on 0.1 M Bu4NPF6 solutions using a three-electrode configuration (glassycarbon working electrode, Pt counter electrode, Ag/AgCl reference) and a PAR 273 potentiostat and function generator. The ferrocene/ ferrocenium (Fc/Fc+) couple served as internal reference. A standard mercury dropping electrode was used for polarography. The bulk electrolysis and coulometry was performed on the mercury-pool working electrode in the electrolytic cell with the counter electrode compartment separated by sintered glass. Spectroelectrochemistry was performed using an optically transparent thin-layer electrode (OTTLE) cell.17a A two-electrode capillary served to generate intermediates for X-band EPR studies. The abcp ligand2 and the metal precursor complex [Cu(dppf)(CH3CN)2](BF4)21 were synthesized according to literature procedures. Synthesis of abcp-Bridged Heteronuclear Complexes. Synthesis of {(μ-abcp)[Cu(dppf)]2}(BF4)2 ([1](BF4)2). A mixture of abcp (16.21 mg, 0.064 mmol) and [Cu(dppf)(CH3CN)2](BF4) (100 mg, 0.127 mmol) in 30 mL of dry dichloromethane was stirred overnight under argon to give a green solution. After removal of the solvent, the solid was washed with hexane and crystallized from dichloromethane/ hexane (1/6) at 4 °C. Yield: 79.3 mg (75%). Anal. Calcd for C76H60B2Cl2Cu2F8Fe2N6P4·2CH2Cl2 (fw =1834.33): C, 51.07; H, 3.52; N, 4.58. Found: C, 51.03; H, 4.11; N, 5.24. 1H NMR (CDCl3, 250 MHz, δ (ppm)): 4.54 (m, 16H, Cp), 6.97−7.46 (m, br, 40H, Ph),

AUTHOR INFORMATION

Corresponding Author

*E-mail for W.K.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the Land Baden-Württemberg and the COST programme of the EU is gratefully acknowledged. J.F. expresses thanks to the Grant Agency of the Czech Republic (grant 203/ 09/0705) for support.

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REFERENCES

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A Ligand-Bridged Heterotetranuclear (Fe2Cu2) Redox System with Fc

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