Electrochemical Evidence for Hemilabile Coordination of 1,3

Article pubs.acs.org/Organometallics

Electrochemical Evidence for Hemilabile Coordination of 1,3-Dimethyllumazine to [1,1′-Bis(diorganophosphino)ferrocene]copper(I) Rajkumar Jana,† Biprajit Sarkar,‡ Sabine Strobel,† Shaikh M. Mobin,§ Wolfgang Kaim,*,† and Jan Fiedler*,∥ †

Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany Institut für Chemie und Biochemie, Freie Universität Berlin, Fabeckstraße 34-36, D-14195 Berlin, Germany § Discipline of Chemistry, School of Basic Sciences, Indian Institute of Technology Indore, Indore 452017, India ∥ 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 complex cations [Cu(dippf)(DML)] + ([1]+) and [Cu(dppf)(DML)]+ ([2]+), where dippf = 1,1′bis(diisopropylphosphino)ferrocene, dppf = 1,1′-bis(diphenylphosphino)ferrocene, and DML = 1,3-dimethyllumazine, were prepared and crystallized as BF4− or PF6− salts. Structure determinations of the tetrafluoroborates revealed asymmetric O4,N5 chelation of DML to copper(I) with longer Cu−O bonds of about 2.25 Å. Reversible oxidation to [1]2+ and [2]2+ proceeds at the ferrocene units, while reduction leads to the neutral radical complexes [1] and [2] with the unpaired electron localized on the DML ligand. The occurrence of two voltammetric steps for the one-electronreduction process is attributed to a two-species equilibrium caused by the hemilabile coordination of DML. Electrochemical and spectroelectrochemical measurements (UV−vis, IR) reveal increased coordination lability of the reduced complexes and their slow fragmentation.

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INTRODUCTION Lumazines and other heterocycles (pterins, flavins) containing the pteridine structure (Chart 1) are biochemically important

In vitro, an O,N chelation of transition metals is often found in pertinent biorelevant molecules containing the α-iminoketo function.7,8 Soft metal centers are found to be strongly N5 bonded (pyrazine-N) with comparatively weak O4 coordination in a five-membered chelate arrangement, whereas hard metal centers bind more strongly to O4 and more weakly to N5.8 Although reports on crystal structures of CuI−lumazine complexes can be found in the literature,8 the electrochemical and spectroelectrochemical properties of these and other metal compounds of lumazines have not been discussed in detail.9 The limited biochemical relevance of lumazines is partially due to their less reversible electron transfer behavior. Electrochemical studies of substituted lumazine complexes with various metals showed that reduction of these complexes proceeds mostly in an irreversible fashion,8,9 although a few reversibly formed radical complexes have been described.8,10 The irreversible character of charge transfer diminishes the catalytic and biocatalytic potential of such complexes, and the

Chart 1. Pteridine and Related Compounds

redox-active molecules which can serve as cofactors in biocatalytic reactions.1−4 Cooperation with metals such as Mo, W, Fe, and Cu often occurs in biologically functional electron transfer systems.5 While the flavins and pterins have great biological significance in a wide range of functions associated with electron transfer,1−5 the presence of lumazine in natural products and its enzymatic synthesis within flavin biosynthesis were also reported.6 Studying the metal coordination potential of substituted pteridine heterocycles may thus become significant for understanding the in vivo functions of corresponding electron transfer enzymes. © 2014 American Chemical Society

Special Issue: Organometallic Electrochemistry Received: January 15, 2014 Published: April 30, 2014 4784

dx.doi.org/10.1021/om500039a | Organometallics 2014, 33, 4784−4791

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Scheme 1. Synthesis of Complexes

search for new, reversibly behaving lumazine complexes is therefore desired. Here we report coordination compounds of 1,3-dimethyllumazine with the moderately π-donating organometallic fragments [Cu(dippf)] + (dippf = 1,1′-bis(diisopropylphosphino)ferrocene) and [Cu(dppf)]+ (dppf = 1,1′-bis(diphenylphosphino)ferrocene). The 1,1′-bis(diorganophosphino)ferrocene (dopf) ligands have been widely used in catalysis and for functional molecular materials;11 these ligands can also be considered as redox noninnocent,12 due to the reversible oxidation of the ferrocene framework.13 The heterodinuclear (Fe, Cu) fragments [Cu(dopf)]+ were found to stabilize complexes with strong π acceptors such as o-quinones in the nonreduced state.14 The complexes synthesized were [Cu(dippf)(DML)]PF 6 ([1]PF6 ), [Cu(dippf)(DML)]BF 4 ([1]BF4), and [Cu(dppf)(DML)]BF4 ([2]BF4), characterized by structure determinations (the tetrafluoroborates) and by spectroscopic, electrochemical, and spectroelectrochemical methods (IR, UV−vis, EPR). IR spectroelectrochemistry was thus used to investigate the carbonyl stretching frequencies in the reduced and nonreduced states.

Table 1. Selected Bond Distances (Å) and Angles (deg) of [Cu(dippf)(DML)]+ in [1]BF4·CH2Cl2 and of [Cu(dppf)(DML)]+ in Crystals of [2]BF4 [1]+ a Cu−N Cu−O C4−O C2−O Cu−P O−Cu−N P−Cu−P Cu−N5−C6 C6−N5−C4 N5−C4−C4 O4−C4−C4 a

[2]+ b 2.041(2) 2.261(2) 1.228(3) 1.214(3) 2.243(1) 2.247(1) 78.44(8) 117.61(3) 129.9(2) 113.9(2) 116.2(2) 117.1(2) 122.2(2)

Cu1−N1 Cu1−O1 C7−O1 C5−O2 Cu1−P1 Cu1−P2 O1−Cu1−N1 P1−Cu1−P2 Cu1−N1−C1 Cu1−N1−C8 C1−N1−C8 N1−C8−C7 O1−C7−C8

Heterocyclic numbering (Chart 1). (Figures 1 and 2).

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RESULTS AND DISCUSSION Synthesis and Characterization. 1,3-Dimethyllumazine was synthesized according to the literature procedure.15 The precursor complex [Cu(dppf)(CH3CN)2]BF4 was also reported in the literature;16 [Cu(dippf)(CH3CN)2]PF6 and [Cu(dippf)(CH3CN)2]BF4 were synthesized analogously. The compounds [Cu(DML)(dippf)]BF4 ([1]BF4), [Cu(DML)(dippf)]PF6 ([1]PF6), and [Cu(DML)(dppf)]BF4 ([2]BF4) were obtained by combining the reactants in dichloromethane solution (Scheme 1). The coordination of DML is indicated by a color change (yellow → red) and by downfield shifts of the lumazine ring proton NMR signals. Both 1H and 31P NMR spectroscopy in CDCl3 solution revealed only one set of signals. A detailed characterization of the complexes is provided in the Experimental Section. Crystal Stuctures of [1]BF4 and [2]BF4. Red single crystals of complexes [1]BF4 and [2]BF4 were obtained by slow crystallization from saturated dichloromethane solutions at low temperatures. Crystal and X-ray diffraction refinement data are summarized in Tables S1 and S2 (Supporting Information), and selected bond parameters are given in Table 1. The molecular structures reveal O4,N5 chelation of DML to the CuI center, forming a five-membered planar ring system (Figures 1 and 2). The copper(I) ions of the metal fragments [Cu(dopf)]+ (dopf = dippf, dppf) are bonded strongly to N5 (N1) (2.041(2) and 2.027(4) Å) and more weakly to O4 (O1) (2.261(2) and 2.236(3) Å) for [1]BF4 and [2]BF4, respectively. The negative

b

2.027(4) 2.236(3) 1.226(6) 1.211(6) 2.220(2) 2.250(2) 78.77(14) 114.79(6) 130.0(3) 113.1(3) 116.0(4) 116.7(4) 122.2 (4)

Crystallographic numbering

Figure 1. Molecular structure of the complex cation [1]+ in crystals of [Cu(dippf)(DML)](BF4)·CH2Cl2. The BF4− counteranion and the CH2Cl2 molecule (solvent of crystallization) are omitted for clarity.

Δ values (dCu−N − dCu−O),8 calculated at −0.220 and −0.209 Å for the structures of [1]BF4 and [2]BF4, quantify the stronger binding to the more basic N5 center while confirming effective chelation by the lumazine derivative and thus tetracoordination 4785


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by coupling reactions, including dimerization of the oxidized forms.17−19 Reduction of the complex cations [1]+ or [2]+ appears more complicated. Instead of a simple ligand-centered reduction as might be expected for a CuI complex of the reducible ligand DML, the cyclic voltammetry experiments revealed an intriguing behavior. The reduction peak of [1]+ (Figure 3) is

Figure 2. Molecular structure of the complex cation [2]+ in crystals of [Cu(dppf)(DML)](BF4). The BF4− counteranion is omitted for clarity.

Figure 3. Cyclic voltammetry of [1]PF6 in CH2Cl2/0.1 M Bu4NPF6 at a glassy-carbon electrode, with a scan rate of 200 mV/s.

at copper(I). The implications of this asymmetry will become obvious in the section Electrochemistry. A small bite angle of the lumazine ring (O1−Cu1−N1 = 78.44(8) and 78.77(14)°) and a large bite angle at the dopf unit (P1−Cu−P2 = 117.61(3) and 114.79(6)°) illustrate the distorted-tetrahedral geometry around the CuI center. The carbonyl bond lengths, 1.228(3) and 1.214(3) Å in [1]BF4 and 1.226(6) and 1.211(6) Å in complex [2]BF4, confirm the presence of nonreduced carbonyl groups in the complexes. Electrochemistry. Different anions, BF4− and PF6−, of the complex cation [1]+ did not affect the oxidation and reduction behavior in different ways. Likewise, the use of Bu4NBF4 or Bu4NPF6 as supporting electrolyte did not significantly influence the electrochemical response. The heterobimetallic complexes [1]BF4, [1]PF6, and [2]BF4 exhibit a reversible oneelectron-oxidation wave in dichloromethane, tetrahydrofuran, or acetone, attributed to the phosphino-substituted ferrocene unit. The oxidation potentials (Table 2) are slightly more positive in comparison to those of the free ligands. However, the cyclic voltammetric response of free dopf ligands is affected

slightly smaller in comparison to that of the ferrocene (“internal standard”) oxidation step, and an additional smaller wave appears at a more negative potential. The second wave thus supplements the electron consumption to an overall 1e per molecule (about 0.9e for the first reduction peak and 0.1e for the second). A larger decrease of the first reduction peak and a proportionally higher second wave have been observed with the compound [2]BF4 (Figure S1, Supporting Information). The ratio of heights of the first and second reduction peaks has been found to be approximately constant at different scan rates and at various temperatures. Figure S2 (Supporting Information) shows the nearly linear dependence of voltammetric peak intensities on the square root of the scan rate (ranging from 0.05 to 100 V/s) for the anodic oxidation and for the two cathodic reduction peaks. This test proves the diffusion-controlled character of the electrode processes and indicates that the diffusional transport from the solution to the electrode involves two species, both having the same oxidizable centers, viz., the phosphino−ferrocene unit, but partially different reducible centers which give rise to two voltammetric reduction peaks. We tentatively ascribe the formation of two solution species from one pure crystalline compound to the hemilability of the DML ligand, caused by a weaker and more easily dissociating CuI−O coordinative bond (cf. the bond distances in Table 1 and other evidence below). Hemilabile chelate ligands21,22 with at least one inert and one labile metal− donor bond are being increasingly acknowledged as valuable components of functional coordination compounds. The relative heights of the first and second voltammetric reduction peaks of [1]+ do not change significantly with decreasing temperature, as could be expected when the electrochemical processes are not controlled by diffusion but by kinetics. In Figure S3 (Supporting Information) the peak heights are plotted as ratios relative to the ferrocene (dippf) oxidation peak as “internal standard”, and the vertical scale thus shows relative charge consumptions during the first and second reductions.

Table 2. Redox Potentialsa compd

E(ox)

dippf dppf [1]+

[2]+

E(red 1)

E(red 2)

−1.90b −1.98b

DML 0.01c 0.23c 0.30 (r) 0.25 (r) 0.26 (r) 0.37 (r)

−1.39 −1.38 −1.34 −1.34

(r) (r) (r) (r)

−1.83 −1.90 −1.79 −1.84

(qr) (qr) (qr) (qr)

solvent CH2Cl2 THF CH2Cl2 CH2Cl2 CH2Cl2 THF acetone CH2Cl2

ref this 20 17 18 this this this this

work

work work work work

a Conditions and definitions: potentials (V) vs ferrocenium/ferrocene; half-wave potentials unless noted otherwise; r, reversible step; qr, quasi-reversible step (incompletely Nernstian). bVoltammetric Epc value for irreversible reduction. cElectrochemical response affected by a coupled reaction (see text).

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The tests from Figures S2 and S3 (Supporting Information) prove that the response during the voltammetric reduction scan is not influenced by adsorption or kinetic current. In addition, compound [1]PF6 was also investigated by alternating current (ac) voltammetry in order to exclude the possible influence of an electrode adsorption which could shift the reduction of the adsorbed part of the sample to negative potentials. Figure 4

Figure 5. Polarography of 0.99 mM [1]PF6 in CH2Cl2/0.1 M Bu4NPF6 before electrolysis (blue) and after reduction by 0.5 F/mol (green) or 0.9 F/mol (red). The purple trace is for electrolyte only.

curves obtained in the course of controlled-potential coulometry of the bulk of the solution, with the electrolysis potential set on the top of the first reduction wave; the irregular shape of the second smaller wave is obviously caused by a polarographic maximum. A reversible anodic−cathodic wave due to a mixture of starting complex and its one-electronreduced form can be observed during the middle stage of the electrolysis (0.5 F/mol). However, the primary reduced form disappears during the longer times needed to complete the bulk electrolysis. Secondary products formed by a chemical reaction are polarographically active at potentials outside the original reduction wave (see anodic and cathodic waves registered, after 0.9 F/mol has passed). The product responsible for the negative reduction wave can be identified as the free DML ligand, according to the polarographic potentials. This identification is also supported by cyclic voltammetry measurements. Figure 6 compares the

Figure 4. Phase-sensitive alternating current (ac) cyclic voltammetry of [1]PF6 in CH2Cl2/0.1 M Bu4NPF6 at a glassy-carbon electrode (scan rate 100 mV/s, frequency 80 Hz): (blue) in-phase component; (red) quadrature component of electrode admittance; (solid lines) forward scan; (dashed lines) reverse scan.

(solid lines) shows the phase-resolved ac response during a dc scan in the reduction region. In-phase and quadrature components at the first reduction step are equivalent, confirming the electrochemical reversibility of the charge transfer. No additional increase of the quadrature (capacitive) component of the electrode admittance is observed, and the influence of electrode adsorption with a corresponding change of electrode capacity can thus be excluded. Hence, the elimination of electrochemical effects which could cause the separation of a one-electron reduction process into two reduction peaks provides further evidence for the assumption that two species are formed in the solution due to hemilabile character of the DML−Cu bonding. Analysis of both the cathodic and the reverse anodic parts of the cyclic voltammograms revealed further details. A fully reversible cyclic voltammetric response of the first reduction step has been obtained only at high scan rates (25 V/s, Figure S4 (Supporting Information)). At lower scan rates the ratio of cathodic peak to anodic counter peak deviates from 1.0 (ipc/ipa > 1). The decreased intensity of the anodic counter peak indicates a chemical (electro-inactivating) reaction associated with the reduced form of the complex: i.e., a reaction different from the two-species equilibrium discussed above. The presence of a chemical reaction following the primary electron transfer is also documented by ac cyclic voltammetry. It can be clearly seen from Figure 4 that the ac response in the reverse (dc) scan is significantly lower in comparison to the response from the forward scan. The effect results from a decrease of the “dc” concentration of electroactive species by a follow-up reaction which is equivalent to a decrease of the anodic counter peak in the normal (dc) voltammetry. The responsible chemical reaction is relatively slow, only partially lowering the reverse voltammetric response. Therefore, an exhaustive electrolysis has been performed to clarify the mechanism in more detail. Figure 5 depicts polarographic

Figure 6. Cyclic voltammograms of 0.99 mM [1]PF6 in CH2Cl2/0.1 M Bu4NPF6 at a glassy-carbon electrode with a scan rate of 200 mV/s before (blue) and after (red) bulk reduction by 1.1 F/mol.

cyclic voltammograms recorded at the glassy-carbon electrode before and after complete electrolysis. A final reduction product can be distinguished by its irreversible reduction peak at −1.92 V, a value very close to the position of the irreversible reduction peak of free DML (−1.90 V, Table 2), checked by a separate measurement under similar experimental conditions. These findings imply that the one-electron reduction of [Cu(dippf)(DML)]+ to [Cu(dippf)(DML)] increases the lability of coordinated DML ligand which is finally released 4787


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disappearance of the MLCT band. The final spectrum with a structured band around 330 nm points to the presence of the free ligand DML (cf. Table 3). Although the reduced solution

from the reduced complex. Surprisingly, the dissociated DML has been detected in solution in its native (unreduced) form, although a radical form, DML•−,20 occurs in the reduced form of the complex, [CuI(dippf)(DML•−)] (see evidence in the section EPR Spectroscopy). Dissociation of DML must therefore involve an electron transfer from DML•− to the heterodimetallic fragment, either intramolecularly, which can be understood as an effect of the potential noninnocence of the DML, or intermolecularly, i.e. in a redox exchange between an initially dissociated DML•− and the dimetallic fragment. The remaining unstable [Cu(dippf)] can further decompose, as indicated by several irreversible anodic waves of products in the electrochemical response of the electrolyzed solution. The free dippf ligand can be identified in the solution after electrolysis by its anodic polarographic wave at −0.4 V, which agrees with the response of an authentic sample of dippf under the same conditions. An additional irreversible reduction step can be observed in the cyclic voltammograms of [1]+ and [2]+ in solvents with a wider negative potential range, such as THF and acetone (Figure S5 (Supporting Information)). Due to the instability of the compounds already after the first reduction, this process was not further investigated. UV−Vis−NIR Spectroelectrochemistry. Figure 7 (top) shows the spectral changes during the reversible oxidation of

Table 3. Absorption Maxima from UV−Vis Spectroelectrochemistry in CH2Cl2 λmax/nm (ε/103 M−1 cm−1)

compd DML [1]2+ [1]+ [1] [2]2+ [2]+ [2]

345 815 465 290 770 455 285

(sh), 332 (3.75), 320 (3.73), 308 (sh) (0.22), 410 (sh), 347 (4.62), 284 (7.11) (2.46), 343 (3.23) (sh)a (0.31), 415 (sh), 349 (7.48) (3.22), 343 (5.66), (sh)

a

Other observed maxima (320, 331, 345 (sh)) are attributed to free DML formed by decomposition.

exhibits an anodic counter peak, the reoxidation does not restore the original spectrum; the features of free DML remain in the spectrum on reoxidation. Similar behavior has been observed also with complex [2]+ (Figure S6 (Supporting Information)). The spectroscopic results thus confirm the electrochemically detected dissociation of the complexes on reduction, releasing free DML on the time scale needed for the spectroelectrochemical experiments. The relevant spectral data are summarized in Table 3. IR Spectra and Spectroelectrochemistry. Two CO stretching bands at 1725 and 1685 cm−1 of the free DML ligand in CH2Cl2 solution have been assigned to ν(C2O) and ν(C4O), respectively,20 C4O being the coordination position in metal chelates (cf. Chart 1). The corresponding bands in CH2Cl2 solutions of the investigated complexes are located at 1726 and 1655 cm−1 or at 1728 and 1652 cm−1 for [1]+ and [2]+, respectively (Table 4). The position of the CO Table 4. Carbonyl Stretching Frequencies from IR Spectra compd

ν(C2O)/cm−1

ν(C4O)/cm−1

solvent

DML [1]PF6

1725 1721 1722 1715 1717 1720 1730 1726 1663 1733 1728 1670

1685 1647 1651 1647 1644 1647 1650 1655 1595 1646 1652 1592

CH2Cl2 solida solidb solida solida solidb CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2

[1]BF4 [2]BF4 [1]2+ c [1]+ c [1]c [2]2+ c [2]+ c [2]c a

Crystallized compound. bSolid obtained by fast evaporation of CH2Cl2 solution. cData from spectroelectrochemistry in CH2Cl2/0.1 M Bu4NPF6.

Figure 7. UV−vis spectroelectrochemical change during oxidation (top) and reduction (bottom) of [1]PF6 in CH2Cl2/0.1 M Bu4NPF6.

stretching bands of the complexes changes slightly in the solid state. This effect (Table 4) presumably arises from the hemilabile coordination of the DML ligand involving the weak Cu−O4(DML) bond. IR spectroelectrochemistry of [1]+ and [2]+ in CH2Cl2 shows a reversible behavior on oxidation of the phosphino−ferrocene ligand dopf, which causes an expected small shift (≤6 cm−1) of ν(CO(DML)) (Figure 8, Figure S7 (Supporting Informa-

[1]PF6. In addition to the expected hypsochromic shift of the MLCT (d(Cu) → π*(DML)) band in the visible region, a weak broad band can be detected at about 800 nm which is attributed to the 2E1u transitions in the oxidized ferrocene unit.13,23 Spectroelectrochemical monitoring of the reduction of [1]+ at the first reduction wave (Figure 7, bottom) shows the 4788


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Figure 9. X-band EPR spectra of electrogenerated [1] (top) and [2] (bottom) at 298 K in CH2Cl2/0.1 M Bu4NPF6 solutions.

(Figure 9) suggest obvious differences in the spin distribution between the two systems investigated. The EPR measurements thus confirm the expectation that the reduction of the complexes involves the addition of an electron to the π* molecular orbital of the heterocycle,8,26 forming the organicradical complexes [CuI(dippf)(DML•−)] and [CuI(dppf)(DML•−)].

Figure 8. IR spectroelectrochemical change during oxidation (top) and reduction (bottom) of [1]PF6 in CH2Cl2/0.1 M Bu4NPF6.

tion), Table 4). Spectroelectrochemical reduction of the complexes results in strongly shifted (≥56 cm−1) ν(CO) bands, confirming a DML-located electron uptake (Figure 8 and Figure S7). Apart from the occurrence of small bands from decomposition products, the bands at 1663 and 1595 cm−1 for [CuI(dippf)(DML•−)] and at 1670 and 1592 cm−1 of [CuI(dppf)(DML•−)] could be clearly identified. The spectroelectrochemical reoxidation scan restores reversibly the original spectra by 80−85%. The better reversibility in comparison to UV−vis−NIR spectroelectrochemistry can be ascribed to the higher concentration used for the IR measurements and therefore partially suppressed dissociation of the DML ligand. EPR Spectroscopy. The complexes are diamagnetic in their native monocationic states [1]+ and [2]+. Ferrocene-based oxidation is expected to result in very rapid EPR relaxation and broad lines;24 corresponding signals were not observed at 110 K. As the complexes undergo partially reversible one-electron reduction, the EPR-active radical species could be generated by in situ reduction. Figure 9 shows the EPR spectra of the electrogenerated forms. The g values of 2.0042 and 2.0049 determined for the reduced species [1] and [2], respectively, are close to the free-electron value of 2.0023, indicating a largely ligand-centered spin.8 While lumazine radicals should have the bulk of the spin concentrated on the nitrogen centers of the pyrazine ring,25 the delocalization of the unpaired electron in the unsymmetrical π system and the additional hyperfine interaction14c,26 with coordinated 63,65Cu (I = 3/2) and with two 31P nuclei (I = 1/2) create a total number of 22 × 35 × 43 = 62208 theoretical lines from 10 different coupling constants which are impossible to resolve, let alone assign to individual positions. The two partially resolved EPR spectra

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CONCLUSION Interaction of the potentially hemilabile chelate ligand 1,3dimethyllumazine with two [Cu(dopf)]+ fragments has been investigated by several methods. The structure determination of crystalline [Cu(dopf)(DML)](BF4) established a somewhat asymmetric chelation of copper(I) via the O4,N5 donors of DML. Electrochemical and spectroelectrochemical investigations revealed a complicated behavior of the complexes in solution and in the course of electrode-based charge-transfer processes. The weak Cu−O coordination and thus hemilabile bonding mode of the DML ligand obviously lead to equilibria in solution, as represented in Scheme 2. The plausibility of the Scheme 2. Redox Mechanism

proposed redox mechanism is supported by a digital simulation of experimental cyclic voltammograms. Although the large number of thermodynamic and kinetic parameters (Table S3 (Supporting Information)) did not allow us to optimize all values by an automatic routine, a good agreement of the 4789


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acetonitrile solutions containing 1/1 mixtures of [Cu(CH3CN)4]BF4 or [Cu(CH3CN)4]PF6 and dopf under argon resulted in the formation of the complexes. [Cu(dippf)(CH3CN)2]BF4: yield: 80%. 1H NMR (250 MHz, CDCl3, δ (ppm)): 1.12−1.18 (m, 24H, PCHCH3), 2.07 (s, 6H, CH3CN), 3.61 (s, 3H, NMe), 2.13 (m, 4H, PCHCH3), 4.16 (s, br, 4H, Cp), 4.37 (t, br, 4H, Cp). 31P{1H} NMR (CDCl3, δ (ppm)): 2.60. [Cu(dippf)(CH 3 CN) 2 ]PF 6 : yield: 96%. Anal. Calcd for C26H42CuF6FeN2P3 (fw 708.95): C, 44.05; H, 5.97; N, 3.95. Found: C, 43.89; H, 6.02; N, 3.85. 1H NMR (250 MHz, CD2Cl2, δ (ppm)): 1.27 (m, 24H, PCHCH3), 2.31 (m, br, 4H, PCHCH3), 2.26 (s, 6H, CH3CN), 4.31 (s, br, 4H, Cp), 4.54 (t, br, 4H, Cp). 31P{1H} NMR (CD2Cl2, δ (ppm)): 5.33 (s, dippf), −144.47 (sept, 1JP−F = 710.2 Hz, PF6). Synthesis of 1,3-Dimethyllumazine Complexes. A 1/1 mixture of DML and of the CuI precursor complex were dissolved in dry and deoxygenated dichloromethane under argon. The solution turned deep red, and the mixture was stirred overnight. The solvent was removed under reduced pressure and the obtained solid was washed several times with dry hexane. The red solid was crystallized from a dichloromethane/hexane (2/1) mixture at 4 °C. [Cu(dippf)(DML)]BF 4 : yield: 95%. Anal. Calcd for C30H44BCuF4FeN4O2P2·CH2Cl2 (fw 845.76): C, 44.02; H, 5.48; N, 6.62. Found: C, 44.18; H, 5.44; N, 6.58. 1H NMR (250 MHz, CDCl3, δ (ppm)): 1.07 (dd, 3JH−H = 7.1 Hz, 3JP−H = 15.5, 12H, PCHCH3), 1.22 (dd, 3JH−H = 7.1 Hz, 3JP−H = 15.3, 12H, PCHCH3), 2.15 (m, 4H, PCHCH3), 3.61 (s, 3H, NMe), 3.78 (s, 3H, NMe), 4.37 (s, br, 4H, Cp), 4.47 (t, br, 4H, Cp), 8.85 (d, 2JH−H = 2.5 Hz, 1H, DML), 9.12 (s, br, 1H, DML). 31P{1H} NMR (CDCl3, δ (ppm)): 4.41 (s, dippf). [Cu(dppf)(DML)]BF 4 : yield: 92%. Anal. Calcd for C42H36BCuF4FeN4O2P2 (fw 896.89): C, 56.24; H, 4.05; N, 6.25. Found: C, 55.15; H, 4.02; N, 6.09. 1H NMR (250 MHz, CDCl3, δ (ppm)): 3.42 (s, 3H, NCH3), 3.71 (s, 3H, NCH3), 4.34 (s, 4H, Cp), 4.42 (s, 4H, Cp), 7.36 (m, br, 20H, Ph), 8.82 (s, br, 1H, DML), 9.09 (s, br, 1H, DML). 31P{1H} NMR (CDCl3, δ (ppm)): −9.88 (s, dppf). MS (ESI, Micromass Q-ToF): molecular ion peak centered at m/z 809.10, corresponding to [Cu(dppf)(DML)]+. Crystallography. Single-crystal X-ray diffraction data of [Cu(dippf)(DML)](BF4)·CH2Cl2 were collected on a NONIUS KappaCCD four-circle diffractometer with Mo Kα radiation (0.71073 Å, graphite monochromated) at 100 K. Single-crystal X-ray diffraction data collection for [Cu(dppf)(DML)]BF4 was performed on a CCD Oxford Diffraction XCALIBUR diffractometer at 150 K. The strategy for the data collection was evaluated by using the CrysAlisPro CCD software. The data were collected by the standard ψ−ω scan techniques and were scaled and reduced using the CrysAlisPro RED software.29 The structures were solved via direct methods using the program SHELXS-97.30 Refinement was carried out by the full-matrix leastsquares method employing the program SHELXL-97. All nonhydrogen atoms were refined anisotropically; hydrogen atoms were introduced in proper positions with coupled isotropic factors using the riding model. Absorption corrections were performed empirically using the program HABITUS/Scalepack.31 The program DIAMOND 2.132 was used for structure drawing. The following crystallographic parameters were used: R = (∑(||Fo| − |Fc||)/∑|Fo|; Rw = {∑[w(|Fo|2 − |Fc|2]/∑[w(Fo4)]}1/2; GOF = {∑w(|Fo|2 − |Fc|2)2/(n − m)},where n = number of data and m = number of variables.

simulated and experimental voltammograms could be achieved (Figures S8 and S9 (Supporting Information)). Two waves for a one-electron-reduction step indicate the presence of two species in the native redox states of the complexes under the conditions of cyclic voltammetry: i.e., with an excess of the electrolyte present. 1H and 31P NMR measurements did not reveal the existence of two species in CDCl3 solutions; different experimental conditions for NMR (concentration, absence of electrolyte), dynamic exchange phenomena, and little distinguished signals for the minority species could be responsible. EPR spectroelectrochemistry unambiguously determined the DML ligand as the center of reduction, whereas the oxidation is clearly ferrocene-based. The asymmetry of the O4,N5 donor chelation by the potentially hemilabile chelate ligand DML and similar such ligands is enhanced by the tendency of copper(I) to adopt the coordination number 3 or 3 + 1 instead of 4.26,27 On ligandbased reduction the larger spin and charge concentration at the pyrazine nitrogen center N5 relative to the carboxamide-type oxygen O4 favors hemilabile behavior and eventual dissociation. Further investigations with related biorelevant chelate ligands will have to show whether the hemilability can be diminished or enhanced relative to the present example.

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EXPERIMENTAL SECTION

Dichloromethane was refluxed over LiAlH4 under argon and freshly distilled before an experiment. Predried tetrahydrofuran (THF) was heated under argon with benzophenone and sodium until a deep violet color appeared; it was then distilled. Acetone (Merck GC grade) was used without further purification. The supporting electrolytes nBu4NPF6 and n-Bu4NBF4 (Fluka/Aldrich electrochemical grade) were dried in the vacuum oven at 110 °C. Instrumentation. Cyclic voltammetry was carried out in 0.1 M Bu4NPF6 or Bu4NBF4 solutions using a three-electrode configuration (glassy-carbon working electrode, Pt counter electrode, SCE reference or Ag-wire pseudoreference electrode) and a PAR 273 or PAR263A potentiostat and function generator. A standard mercury dropping electrode was used for polarography. The ac measurements were performed with a PAR263A potentiostat connected to a PAR 5210 lock-in amplifier; the amplitude of the superimposed alternating voltage was set to 10 mV and the components (in-phase and quadrature) of the alternating current were registered. Potentials were determined by electrochemistry through addition of the FeCp*2 standard and recalculation versus FeCp2+/0; the potential difference between FeCp*2+/0 and FeCp2+/0 was determined from the individual measurements under the same experimental conditions. Bulk electrolysis and coulometry were performed using a mercury-pool working electrode in the electrolysis cell with the counter electrode compartment separated by sintered glass. The DigiSim 3.03b program (M. Rudolph and S. W. Feldberg; distributed by BASi) was utilized for cyclic voltammetry simulation and experimental curve fitting. Spectroelectrochemistry was performed using a versatile optically transparent thin-layer electrode (OTTLE) cell.28 UV−vis−NIR absorption spectra were recorded on J&M TIDAS and Agilent 8453 spectrophotometers and IR spectra on Philips PU9800 FTIR and Nicolet 6700 FTIR instruments; solid-state IR measurements were performed with an ATR unit (smart orbit with diamond crystal). 1H NMR spectra were taken on a Bruker AC 250 spectrometer. EPR spectra in the X-band were recorded with a Bruker System EMX instrument. A two-electrode capillary served to generate intermediates for X-band EPR studies. Syntheses. The DML ligand and the metal precursor complex [Cu(dppf)(CH3CN)2]BF4 were synthesized according to the literature procedure.15,16 The precursor complexes [Cu(dippf)(CH3CN)2]BF4 and [Cu(dippf)(CH3CN)2]PF6 were synthesized following procedures similar to those for [Cu(dppf)(CH3CN)2]BF4.16 Overnight stirring of

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ASSOCIATED CONTENT

S Supporting Information *

CIF files and tables giving crystallographic data and refinement parameters for [Cu(dippf)(DML)]BF4·CH2Cl2 (Table S1) and [Cu(dppf)(DML)]BF4 (Table S2), cyclic voltammograms of [2]BF4 (Figure S1), dependence of peak intensities on the scan rate from cyclic voltammetry of [1]PF6 (Figure S2), temperature dependence of the reduction peak heights relative to the height of the anodic oxidation peak from cyclic voltammetry of 4790


Organometallics

Article

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[1]PF6 (Figure S3), cyclic voltammograms of [1]PF6 (Figure S4), cyclic voltammograms of [1]PF6 in in acetone/0.1 M Bu4NPF6 (Figure S5), UV−vis spectroelectrochemical change during oxidation an reduction of [2]PF6 (Figure S6), and IR spectroelectrochemical change during oxidation and reduction of [2]PF6 (Figure S7). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for W.K.: [email protected]. *E-mail for J.F.: jan.fi[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the Land Baden-Württemberg. Support from the European Union (COST program, CM1202) is gratefully ackowledged.

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REFERENCES

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Electrochemical Evidence for Hemilabile Coordination of 1,3

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