Synthesis, Characterization, and Electrochemistry of Diferrocenyl β-Diketones, -Diketonates, and Pyrazoles †

The synthesis of FcC(O)CH(R)C(O)Fc (Fc = Fe(η5-C5H4)(η5-C5H5); R = H, 5; nBu, 7; CH2CH2(OCH2CH2)2OMe, 9), [M(κ2O,O′-FcC(O)CHC(O)Fc)n] (M = Ti, n = 3, 10; M = Fe, n = 3, 11; M = BF2, n = 1, 12), and 1-R′-3,5-Fc2-cC3HN2 (R′ = H, 13; Me, 14; Ph, 15) is discussed. The solid-state structures of 5, 7, 9, 12, 13, 15, and 16 ([TiCl2(κ2O,O′-PhC(O)CHC(O)Ph)2]) show that 7 and 9 exist in their β-diketo form. Compound 13 crystallizes as a tetramer based on a hydrogen bond pattern, including one central water molecule. The electrochemical behavior of 5–7 and 9–16 was studied by cyclic and square-wave voltammetry, showing that the ferrocenyls can separately be oxidized reversibly between −50 and 750 mV (5–7, 9, 12–15: two Fc-related events; 10, 11: six events, being partially superimposed). For complex 10, Ti-centered reversible redox processes appear at −985 (TiII/TiIII) and −520 mV (TiIII/TiIV). Spectro-electrochemical UV-Vis/NIR measurements were carried out on 5, 6, and 12, whereby only 12 showed an IVCT (intervalence charge-transfer) band of considerable strength (νmax = 6250 cm−1, Δν½ = 4725 cm−1, εmax = 240 L·mol−1·cm−1), due to the rigid C3O2B cycle, enlarging the coupling strength between the Fc groups.

In addition, diferrocenyl diketone 5 was applied as a starting material for the synthesis of the titanium, iron, and boron β-diketonato coordination complexes 10-12 (pathway ii), Scheme 2. Therefore, following two synthetic methodologies were used: either ligand exchange [41] or lithiumhalide metathesis. In this respect, coordination complex 10 was accessible by the addition of TiCl4 to a tetrahydrofuran solution containing [Li(κ 2 [41] at −80 °C, and 11 by refluxing [Fe(acac)3] (acac = acetylacetonate) with a 3-fold excess of 5 in acetonitrile (Scheme 2). For the preparation of the purple dioxaborine complex 12, diisopropylamine and [BF3•Et2O] were subsequently reacted with 5 at ambient temperature. The low yields of 10-12 are similar to the recently synthesized Al complex and can be explained with the steric hindrance of the ferrocenylfunctionalized ligands [41].

O,O'-FcC(O)CHC(O)Fc)]
Nonetheless, metalation of 7 and 9 by using different reagents such as KO t Bu, LDA, or n BuLi between −80 °C to 40 °C in different solvents (THF, hexane) was not successful, which most probably is attributed to the low acidity of the α-hydrogen atom of the β-diketone caused by the electron-rich Fc and alkyl groups. In all of these studies, solely the starting materials were recovered in virtually quantitative yield. This was proven for compound 8 by addition of electrophiles (MeI and Me2SO4) to the reaction mixture containing a potentially lithiated species of 8, whereby a methylated compound was not detected. It should be noted that deprotonation of the herein less-acidic αhydrogen could compete with a metalation of the C5H4 group, due to the ortho-directing properties of the adjacent carbonyl X=O (X = C, P, S) functionalities [89][90][91][92][93][94][95]. However, such species were also not observed upon treatment with the mentioned electrophiles at −80, −40, 0, and 40 °C, which might have two reasons. First, the lithiation rate is insufficient at low temperatures (−80 to −40 °C) and nonpolar solvents (hexane) [91,[93][94][95]. Second, it is known that lithiated ferrocenes can be re-protonated Thus, a different approach was applied to synthesize an ethylene glycol functionalized diferrocenyl β-diketone. Hence, compound 5 was treated with 1-iodo-2-[2-(2-methoxyethoxy)ethoxy]ethane and KO t Bu at 50 • C (pathway i), Scheme 2 whereby ICH 2 CH 2 (OCH 2 CH 2 ) 2 OMe was prepared by applying the Finkelstein reaction (treatment of BrCH 2 CH 2 (OCH 2 CH 2 ) 2 OMe with NaI in acetone [85,86]. The respective BrCH 2 CH 2 (OCH 2 CH 2 ) 2 OMe educt was obtained from an Appel reaction starting from 2-[2-(2-methoxyethoxy)ethoxy]ethanol with CBr 4 and PPh 3 , respectively [87,88].) It should be noted that alkyl-substituted 7 and 9 were obtained in their β-diketo form, whereas 5 contained~66% of the enol isomer. The value is similar to those reported in literature [84] and was evidenced by the presence of the CH resonance at~5.9 ppm and a broad signal at~16.5 ppm of the de-shielded OH functionality.
In addition, diferrocenyl diketone 5 was applied as a starting material for the synthesis of the titanium, iron, and boron β-diketonato coordination complexes 10-12 (pathway ii), Scheme 2. Therefore, following two synthetic methodologies were used: either ligand exchange [41] or lithium-halide metathesis. In this respect, coordination complex 10 was accessible by the addition of TiCl 4 to a tetrahydrofuran solution containing [Li(κ 2 O,O -FcC(O)CHC(O)Fc)] [41] at −80 • C, and 11 by refluxing [Fe(acac) 3 ] (acac = acetylacetonate) with a 3-fold excess of 5 in acetonitrile (Scheme 2). For the preparation of the purple dioxaborine complex 12, diisopropylamine and [BF 3 ·Et 2 O] were subsequently reacted with 5 at ambient temperature. The low yields of 10-12 are similar to the recently synthesized Al complex and can be explained with the steric hindrance of the ferrocenyl-functionalized ligands [41].
Nonetheless, metalation of 7 and 9 by using different reagents such as KO t Bu, LDA, or n BuLi between −80 • C to 40 • C in different solvents (THF, hexane) was not successful, which most probably is attributed to the low acidity of the α-hydrogen atom of the β-diketone caused by the electron-rich Fc and alkyl groups. In all of these studies, solely the starting materials were recovered in virtually quantitative yield. This was proven for compound 8 by addition of electrophiles (MeI and Me 2 SO 4 ) to the reaction mixture containing a potentially lithiated species of 8, whereby a methylated compound was not detected. It should be noted that deprotonation of the herein less-acidic α-hydrogen could compete with a metalation of the C 5 H 4 group, due to the ortho-directing properties of the adjacent carbonyl X=O (X = C, P, S) functionalities [89][90][91][92][93][94][95]. However, such species were also not observed upon treatment with the mentioned electrophiles at −80, −40, 0, and 40 • C, which might have two reasons. First, the lithiation rate is insufficient at low temperatures (−80 to −40 • C) and non-polar solvents (hexane) [91,[93][94][95]. Second, it is known that lithiated ferrocenes can be re-protonated via  [96][97][98].
A straightforward synthesis procedure for diferrocenyl-functionalized pyrazoles 13-15 is given by the reaction of 5 with an excess of hydrazines NH 2 -NHR (R = H, Ph), as outlined in Scheme 2 (pathway iii)) (Section 4).
In case of 15, however, the higher electron density of the N-phenyl group reduced the ability of the intermediate hydrazone to undergo a successful ring-closure. Methyl hydrazine failed to react to give 14 and most of 5 was recovered. Thus, formation of methyl derivative 14, possessing an even higher electron density, had to be achieved via methylation of the H-analogue 13.
via ether cleavage of THF or Et2O. The rate constant for this reaction accordingly increases at higher temperatures (40 °C), giving the starting material back [96][97][98].
A straightforward synthesis procedure for diferrocenyl-functionalized pyrazoles 13-15 is given by the reaction of 5 with an excess of hydrazines NH2-NHR (R = H, Ph), as outlined in Scheme 2 (pathway iii)) (Section 4).
In case of 15, however, the higher electron density of the N-phenyl group reduced the ability of the intermediate hydrazone to undergo a successful ring-closure. Methyl hydrazine failed to react to give 14 and most of 5 was recovered. Thus, formation of methyl derivative 14, possessing an even higher electron density, had to be achieved via methylation of the H-analogue 13.
For comparable purposes, regarding the discussion of the electrochemical behavior of 10, we attempted to prepare the isostructural complex [Ti( [99]. However, it appeared that within the reaction of 1,3-diphenyl-1,3-propanedione with TiCl4 solely the corresponding Ti(IV) coordination complex 16 was produced (Scheme 3) [100,101]. After appropriate work-up, coordination compounds 7 and 9-15 could be isolated as red (7,9), green (10), purple (12), or orange (11, 13-15) solids, which dissolve, for example, in dichloromethane and tetrahydrofuran, while in non-polar solvents, they are insoluble. Ti(III) and Fe(III) complexes 10 and 11 were obtained as poorly soluble, paramagnetic compounds. Attempts to enhance their solubility by introducing alkyl or alkyloxy chains failed, due to the described difficulties by attaching such groups to the β-diketonato backbone.
The newly prepared complexes are stable towards air, light, and moisture both in the solid state and in solution. They were characterized by elemental analysis, IR and NMR ( 1 H, 13 C{ 1 H}, if possible) spectroscopy, and high resolution ESI-TOF mass spectrometry. The spectroscopic and spectrometric data are consistent with their formulation as diferrocenyl β-diketones, β-ketonates, or pyrazoles. In addition, the molecular structures of 5, 7, 12, 13, 15, and 16 in the solid state were determined by single crystal X-ray structure analysis. Electrochemical studies (cyclic and square-wave voltammetry) were carried out on 5-7 and 9-16.
A distinctive signature of the 1 H NMR spectra is the appearance of the cyclopentadienyl proton signals between 4.5-5.0 ppm with multiplets or pseudo-triplets for the C5H4 units with JHH = 1.9 Hz and a singlet at ca. 4.1 ppm for the C5H5 protons. The α-hydrogen in β-diketonato complex 12 and pyrazoles 13-15 resonates at 6.1-6.6 ppm, while in 7, it is observed at 4.26 ppm (Experimental). The OH functionality in 5 was observed at 16.4 ppm as a broad signal, due to rapid 1,5-tautomerism, in addition to the CH resonance at 5.9 ppm. The enol form equilibrates with its β-diketo form in a 2:1 ratio. The kinetics of this equilibrium have been studied recently [84].
In the ESI-TOF, mass spectrometric studies the protonated molecular ion peak [M + H] + is found (Section 4.9). This confirms the formation of 10 as a neutral Ti(III) compound, which requires an additional charge to be detected, instead of an already positively charged Ti(IV) species. As shown After appropriate work-up, coordination compounds 7 and 9-15 could be isolated as red (7,9), green (10), purple (12), or orange (11, 13-15) solids, which dissolve, for example, in dichloromethane and tetrahydrofuran, while in non-polar solvents, they are insoluble. Ti(III) and Fe(III) complexes 10 and 11 were obtained as poorly soluble, paramagnetic compounds. Attempts to enhance their solubility by introducing alkyl or alkyloxy chains failed, due to the described difficulties by attaching such groups to the β-diketonato backbone.
The newly prepared complexes are stable towards air, light, and moisture both in the solid state and in solution. They were characterized by elemental analysis, IR and NMR ( 1 H, 13 C{ 1 H}, if possible) spectroscopy, and high resolution ESI-TOF mass spectrometry. The spectroscopic and spectrometric data are consistent with their formulation as diferrocenyl β-diketones, β-ketonates, or pyrazoles. In addition, the molecular structures of 5, 7, 12, 13, 15, and 16 in the solid state were determined by single crystal X-ray structure analysis. Electrochemical studies (cyclic and square-wave voltammetry) were carried out on 5-7 and 9-16.
A distinctive signature of the 1 H NMR spectra is the appearance of the cyclopentadienyl proton signals between 4.5-5.0 ppm with multiplets or pseudo-triplets for the C 5 H 4 units with J HH = 1.9 Hz and a singlet at ca. 4.1 ppm for the C 5 H 5 protons. The α-hydrogen in β-diketonato complex 12 and pyrazoles 13-15 resonates at 6.1-6.6 ppm, while in 7, it is observed at 4.26 ppm (Experimental). The OH functionality in 5 was observed at 16.4 ppm as a broad signal, due to rapid 1,5-tautomerism, in addition to the CH resonance at 5.9 ppm. The enol form equilibrates with its β-diketo form in a 2:1 ratio. The kinetics of this equilibrium have been studied recently [84].
In the ESI-TOF, mass spectrometric studies the protonated molecular ion peak [M + H] + is found (Section 4.9). This confirms the formation of 10 as a neutral Ti(III) compound, which requires an additional charge to be detected, instead of an already positively charged Ti(IV) species. As shown within the formation of 16, containing the respective diphenyl backbone, oxidation towards the Ti(IV) state is common.

Molecular Solid-State Structure
The molecular structures of 5, 7, 9, 12, 13, 15, and 16 in the solid state have been determined by single-crystal X-ray diffraction analysis (Figures 1-7). Crystal and structure refinement data, and crystallization conditions are displayed in Section 4. Selected bond lengths (Å), angles ( • ), and torsion angles ( • ), as well as plane intersections are listed in Tables S1 and S2 (see the ESI).
within the formation of 16, containing the respective diphenyl backbone, oxidation towards the Ti(IV) state is common.

Molecular Solid-State Structure
The molecular structures of 5, 7, 9, 12, 13, 15, and 16 in the solid state have been determined by single-crystal X-ray diffraction analysis (Figures 1-7). Crystal and structure refinement data, and crystallization conditions are displayed in Section 4. Selected bond lengths (Å), angles (°), and torsion angles (°), as well as plane intersections are listed in Tables S1 and S2 (see the ESI).

Molecular Solid-State Structure
The molecular structures of 5, 7, 9, 12, 13, 15, and 16 in the solid state have been determined by single-crystal X-ray diffraction analysis (Figures 1-7). Crystal and structure refinement data, and crystallization conditions are displayed in Section 4. Selected bond lengths (Å), angles (°), and torsion angles (°), as well as plane intersections are listed in Tables S1 and S2 (see the ESI).
The β-diketonato motif in bora-and titana-cyclic structures 12 and 16, formed upon coordination towards BF2 and TiCl2, underwent distortions in order to comply with the requirements for tetrahedral (12) and octahedral (16) coordination environments, which requires a smaller cavity In 7, 9, 10, and 12, identical C−O distances for both CO groups are found, whereby their lengths increase from non-complexed alkyl-(7) and alkoxy-substituted (9) derivatives (1.197(7)-1.224(5) Å) to BF 2 (12) and Ti (16) (1.277(5)-1.314(3) Å). Consequently, the negative charge in the latter two species is delocalized through the β-diketonato system, which also results in equivalent C−C bond lengths (1.379(6)-1.393(6) Å). In contrast, enol 5 shows an alternating sequence of single and double bonds, which also leads to the assignment of O1 as the hydroxy (C1−O1 1.452(3) Å) and O2 as the keto-functionality (C3=O2 1.235(10) Å). A crystal structure of the enol form of 5 has previously been reported [106], showing that both ferrocenyls are in an anti-orientation towards each other. Herein, crystallization from a chloroform solution resulted in a syn-arrangement. The crystallization of the β-diketo form for α-substituted species 7 and 9 is in accordance to literature, where this behavior is exclusively discussed [107,108]. The keto functionalities can either direct in the same (9) or opposite directions (7), due to the rotational freedom around the an sp 3 -hybridized carbon. For the latter species, this is accompanied with an intramolecular T-shaped π-interaction between a C 5 H 4 and a C 5 H 5 ring ( Figure 2). [93][94][95] For the theoretical calculations regarding the properties of parallel-displaced and various types of T-shaped π-interactions, see references [109,110]; for examples involving ferrocenes, see [111][112][113][114].
In contrast to 7 and 9, compounds 5, 12, and 16 adopted a co-planar alignment of the central C 3 O 2 entity. In case of 5, a rather weak hydrogen bond restricted the O1−C1· · · C3−O2 torsion angle to 28.6(7) • , which is further reduced to 1. , due to the stronger trans-influence of the chloride substituent. In contrast, a recently published solid-state structure of 16, which crystallized in the space group Pbca (measured at 193 K) [115], did not show this trans-influence of the chloro substituents, due to a lower C−C bond precision. Nevertheless, a similar trans-influence can be observed for literature-known bis-or tris(β-diketonato) complexes [116][117][118][119]. It should be noted that all bis(β-diketonato)TiX 2 complexes (X = Cl, O, alkoxy, aryloxy) exclusively crystallize as their cis-derivatives [115].
The β-diketonato motif in bora-and titana-cyclic structures 12 and 16, formed upon coordination towards BF 2 and TiCl 2 , underwent distortions in order to comply with the requirements for tetrahedral (12) and octahedral (16) 3) • is observed, which is comparable to other heterocyclic structures where similar out-of-plane shifts prevent involvement of the heteroatom into the π-conjugation [120][121][122].
The ferrocenyl C 5 H 4 -and the adjacent carbonyl functionalities intersect rather co-planar in 5, 7, 9, and 12 with a maximum of 10.6(10) • , whereas the phenyls in 16 and one ferrocenyl group in 5 are slightly rotated out of planarity by up to 23 The pyrazolyl entity in 15 intersects with the adjacent ferrocenyls by 45.65(13) and 36.22 (11) • , which is slightly larger than for the recently published N-benzyl derivative (26.5 and 29.4 • ) [60]. For the N-phenyl (15) and N-benzyl [60] pyrazoles, single and double bonds within the aromatic heterocycle could be distinguished ( Figure 6), contrary to the 1H-derivative 13. Therein, tautomerism causes a similar occupations for hydrogens to be placed at either of the pyrazole s nitrogen atoms. This affects the whole hydrogen bond network within the (13) 4 ·H 2 O arrangement, which is established between four molecules of 13 surrounding one central molecule of water in a tetrahedral geometry (Table 1). This cluster is further stabilized by T-shaped π-interactions between the Fc groups, which are overlapping each other in the upper graphic in Figure 5 ( Figure S1). In order to avoid refinement of the hydrogen bond network over all possible sets of sites, the positioning of the hydrogen atoms followed the highest residual electron density signal (Q-peak) and was extended over the rest of the fragment accordingly. However, the small differences between both possible isomers explains the absence of clear C−C and N−C single and double bonds within the pyrazolyl cores of (13) 4 ·H 2 O. Table 1. Hydrogen bond and T-shaped π-interaction properties (Å/ • ) of 13.
In a simplified representation of the hydrogen bridge-bond pattern in Figure 5 and the corresponding geometric properties (Table 1), it can also be seen that two rather co-planar (15.24(5) and 14.89(15) • ) and two quite perpendicular plane intersections (68.01(12) and 69.17(13) • ) of central heterocyclic cores towards each other are present. The ferrocenyl units between two coplanar moieties are always directed away from the adjacent fragment, whereas a syn-fashion for N1-and N7-based building blocks and an anti-rotation for N3 and N5 pyrazoles is perceived.

Electrochemistry
The redox behavior of 5-7 and 9-16 has been determined by cyclic voltammetry (=CV) and square-wave voltammetry (=SWV) (Figures 8 and 9). The electrochemical measurements were carried out in anhydrous dichloromethane solutions containing [NBu 4 ][B(C 6 F 5 ) 4 ] (0.1 mol·L −1 ) as a supporting electrolyte under inert conditions at 25 • C (Section 4.4) [123,124]. In contrast to smaller counter ions including [PF 6 ] − or [Cl] − , the [B(C 6 F 5 ) 4 ] − anion stabilizes highly charged species in solution, minimizing ion pairing effects. The shielding of the electrostatic interactions between the redox-active groups is realized by ion pairing with the electrolyte's counter-ion. Hence, minimization of this effect leads to an increase of the observed redox potentially splitting [68,125,126].
The redox behavior of 5-7 and 9-16 has been determined by cyclic voltammetry (=CV) and square-wave voltammetry (=SWV) (Figures 8 and 9). The electrochemical measurements were carried out in anhydrous dichloromethane solutions containing [NBu4][B(C6F5)4] (0.1 mol•L −1 ) as a supporting electrolyte under inert conditions at 25 °C (Section 4.4) [123,124]. In contrast to smaller counter ions including [PF6] − or [Cl] − , the [B(C6F5)4] − anion stabilizes highly charged species in solution, minimizing ion pairing effects. The shielding of the electrostatic interactions between the redox-active groups is realized by ion pairing with the electrolyte's counter-ion. Hence, minimization of this effect leads to an increase of the observed redox potentially splitting [68,125,126].
All potentials are referenced to the FcH/FcH + (FcH = Fe(η 5 -C5H5)2) redox couple [127]. The CV data at a scan rate of 100 mV s −1 are summarized in Table 2.      All potentials are referenced to the FcH/FcH + (FcH = Fe(η 5 -C 5 H 5 ) 2 ) redox couple [127]. The CV data at a scan rate of 100 mV s −1 are summarized in Table 2.    [68,125,126]. As it can be seen from Figure 8, compound 5 shows its second redox event to be broadened, most likely due to keto-enol tautomerism in the mixed-valent state [37,38,41]. This differs from 6, since this compound only features one β keto-group. The butyl or glycol groups in α-position of the β-diketones 7 and 9 result in a shift of the first redox potential to more positive values (7, E 1 • = 240 mV; 9, E 1 • = 235 mV) in comparison to 5 (E 1 • = 110 mV). This leads to the assumption that the Fc nearby the enol group possesses a higher electron density, and thus, most likely it will be oxidized first. As it can be seen from Table 2, the redox separations of all analyzed β-diketones are with ∆E • ca. 200 mV similar ( Table 2). Most of these redox-separations, however, are likely to be caused by electrostatic repulsion of the ferrocenium entities in close spatial proximity. In addition, the redox separation for 6 and 13-15 is slightly higher, since the two Fc units are not in chemically equivalent positions. The redox behavior of the 3,5-diferrocenyl-functionalized pyrazoles 13-15 is very similar to each other (E 1 • = 30 mV, ∆E • = ca. 210 mV; Table 2), proving negligible influence of the N-bonded hydrogen (13) methyl (14) or phenyl (15) group on the charge transfer process or the electrostatics in the mono-oxidized mixed-valent compounds [13][14][15] + . β-Diketone 5 was used as ligand in the synthesis of titanium, iron, and boron β-diketonates 10-12 (Scheme 2) in order to study their electrochemical behavior and, hence, the influence of the metal ion on the charge transfer between the ferrocene/ferrocenium groups in the mixed-valent species.
Complex 12 exhibits three reversible one-electron processes. While at a potential of 300 and 600 mV the two Fc/Fc + processes are observed, the reversible redox process at −1845 mV (∆E p = 64 mV) represents a one-electron reduction of the π-system of the six membered C 3 O 2 B cycle. The delocalized character of these π electrons in addition lead to a much better electronic coupling of the ferrocenyl units, and hence, the redox separation is increased to 300 mV, when compared to the parent diketonate that the wave at −520 mV can be assigned to the reversible oxidation of Ti III to Ti IV (∆E p = 64 mV), while the redox event at −985 mV (∆E p = 60 mV) corresponds to the reduction of Ti III to Ti II (Figure 9), which is in agreement with the one found for [Ti 2 L 3 ] (L = 1,3-bis(3-phenyl-3-oxopropanoyl)benzene) at ca. −1100 mV [131]. However, for 16 no reduction process could be observed in the appropriate measured frame (1000 to -1500 mV). In addition to the Ti-related processes in 10, the six ferrocenyl-related redox events are poorly resolved occurring in the potential range between 125 and 670 mV, due to the low solubility of the complex. Square-wave voltammetry allows us to assign five individual processes at 140 mV, 260 mV (2 e − ), 385 mV, 485 mV, and 670 mV ( Figure 9). In contrast, the ferrocenyl-related oxidation processes for iron(III) complex 11 are much better resolved, and hence, the SWV of 11 allows us to identify the respective formal potentials at 100, 230, 305, 430, 560, and 730 mV ( In order to get a deeper insight into the spectroscopic details of the mixed-valent species [5] + , [6] + and [12-15] + in situ UV-Vis/NIR, spectro-electrochemical measurements have been carried out. However, the low solubility of complexes 10 and 11 did not allow for spectro-electrochemical measurements to be carried out. The spectro-electrochemical studies were performed by stepwise increase of the potential from −400 to 1200 mV (step heights: 25, 50 or 100 mV) vs. Ag/AgCl in an optically transparent thin layer electrochemistry cell (=OTTLE) [132]. Dichloromethane solutions of 5, 6, and 12-15 (0.02 M) containing [NBu 4 ][B(C 6 F 5 ) 4 ] (0.1 M) as the supporting electrolyte were used [123,124]. Thereby, the stepwise generation of mixed-valent [5] Figures S4 and S5, see the ESI).
During the oxidation of 5 and 6 an IVCT absorption of negligible strength can be seen. The extinction coefficient of this band, however, is lower than 50 L·mol −1 ·cm −1 , therefore the electronic coupling between the Fc/Fc + is very weak (Figures S4 and S5). The introduction of the BF 2 unit in 12 led to an increase in the extinction of the IVCT band (ν max = 6250 cm −1 , ∆ν 1 2 = 4725 cm −1 , ε max = 240 L·mol −1 ·cm −1 ) (Figure 10, Figure S6), corresponding to a weakly coupled class II system according to the classification of Robin and Day [133]. The formation of the six-membered C 3 O 2 B ring introduced some rigidity in the π-bridge, and hence, the electronic coupling of the ferrocenyl termini became stronger. measurements to be carried out. The spectro-electrochemical studies were performed by stepwise increase of the potential from −400 to 1200 mV (step heights: 25, 50 or 100 mV) vs. Ag/AgCl in an optically transparent thin layer electrochemistry cell (=OTTLE) [132]. Dichloromethane solutions of 5, 6, and 12-15 (0.02 M) containing [NBu4][B(C6F5)4] (0.1 M) as the supporting electrolyte were used [123,124]. Thereby, the stepwise generation of mixed-valent [5] Figures S4 and S5, see the ESI).
During the oxidation of 5 and 6 an IVCT absorption of negligible strength can be seen. The extinction coefficient of this band, however, is lower than 50 L•mol −1 •cm −1 , therefore the electronic coupling between the Fc/Fc + is very weak (Figures S4 and S5). The introduction of the BF2 unit in 12 led to an increase in the extinction of the IVCT band (νmax = 6250 cm −1 , Δν½ = 4725 cm −1 , εmax = 240 L•mol −1 •cm −1 ) (Figure 10, Figure S6), corresponding to a weakly coupled class II system according to the classification of Robin and Day [133]. The formation of the six-membered C3O2B ring introduced some rigidity in the π-bridge, and hence, the electronic coupling of the ferrocenyl termini became stronger. For pyrazoles 13-15 no IVCT band of considerable strength could be found in any oxidation state, demonstrating that the 240 mV redox separation is mainly caused by electrostatic interactions.

Conclusions
The synthesis and characterization of diferrocenyl-substituted β-diketones, 1R-pyrazoles, and β- For pyrazoles 13-15 no IVCT band of considerable strength could be found in any oxidation state, demonstrating that the 240 mV redox separation is mainly caused by electrostatic interactions.

Conclusions
The synthesis and characterization of diferrocenyl-substituted β-diketones, 1R-pyrazoles, and β-diketonato metal complexes of general type FcC(O)C(R)C(OH)Fc (Fc = Fe(η 5 -C 5 H 4 )(η 5 -C 5 H 5 ); R = H, 5; n Bu, 7; CH 2 CH 2 (OCH 2 CH 2 ) 2 OMe, 9), 1-R-3,5-  5, 7, 9,  12, 13, 15, and 16 were determined by single-crystal X-ray diffraction studies, verifying the predicted structures. Alkyl substitution of the β-diketone led to an increased electron density at C α and, hence, resulted in the formation of the diketo form in solution and solid state, in contrast to the H-substituted derivative 5, where the enol form was moreover present, resulting in an intramolecular hydrogen bond in the solid state. In diferrocenyl 1Ph-pyrazole 15, a differentiation between single and double bonds in the heterocyclic core could be observed. In contrast, the NH-derivative 13 crystallized as a tetrameric structure surrounding one central molecule of water, where all five molecules were connected via hydrogen bonds.
Electrochemical measurements confirmed that the ferrocenyls in 5-7, 9, and 12-15 could be oxidized separately; however, the redox separation of 200-250 mV is mainly caused by electrostatic interactions. In addition, for complexes 6 and 13-15, the ferrocenyl units are in chemically non-equivalent positions, and hence, the observed redox separation is increased. The Ti and Fe complexes 10 and 11 showed a convoluted redox behavior, since the six ferrocenyl units are oxidized in a close potential range; however, square-wave voltammetry allowed us to estimate the formal potential of each ferrocenyl oxidation process. In addition, complex 10 showed two titanium-centered redox processes at −985 and −520 mV, corresponding to Ti II /Ti III and Ti III /Ti IV redox couples, respectively.
Compound 12 showed an increased redox separation between the Fc units upon introduction of BF 2 , due to a more rigid backbone, which allows for a better conjugation through the C 3 -π-bridge. Spectro-electrochemical UV/Vis-NIR measurements confirmed a stronger electronic coupling in mixed-valent [12] + between Fc/Fc + than for non-coordinated diketone [5] + .

General Procedures
All reactions were carried out under an atmosphere of argon using standard Schlenk techniques. Tetrahydrofuran was purified by distillation from sodium/benzophenone ketyl. Hexane was purified with a MBRAUN SBS-800 purification system. Dichloromethane was purified by distillation from CaH 2 . For column chromatography, alumina with a particle size of 90 µm (standard, Merck KgaA) or silica with a particle size of 40-60 µm (230-400 mesh (ASTM), Fa. Macherey-Nagel) was used. As filtration support Zeolithe (Riedel de Häen) was applied.

Instruments
Infrared spectra were recorded at ambient conditions with a FT-Nicolet IR 200 equipment or as ATR-FTIR spectra by using a Biorad FTS-165 or a Nicolet iS 10 spectrometer from Thermo Scientific. NMR spectra (500.3 MHz for 1 H, 125.7 MHz for 13 C, 160.5 MHz for 11 B) were recorded using a Bruker Avance III 500 FT-NMR spectrometer at ambient temperature. Chemical shifts are reported in ppm downfield from tetramethylsilane with the solvent as reference signal ( 1 H NMR: δ (CDCl 3 ) = 7.26 ppm; 13 C{ 1 H} NMR: δ (CDCl 3 ) = 77.16 ppm). The melting points were determined with a Gallenkamp MFB 595 010 M melting point apparatus. Elemental analyses were performed with a Thermo FlashEA 1112 Series instrument (ThermoFisher). High-resolution mass spectra were recorded using a micrOTOF QII Bruker Daltonite workstation.

Crystallography
Data were collected with an Oxford Gemini S diffractometer at ≤120 K using Mo K α (λ = 0.71073 Å) radiation. The structures were solved by direct methods and refined by full-matrix least square procedures on F 2 with SHELXL-2013 [134,135]. All non-hydrogen atoms were refined anisotropically, and a riding model was employed in the treatment of the hydrogen atom positions. Graphics of the molecular structures have been created using ORTEP [136].
The acidic hydrogen atom in 5 has been refined as an idealized OH group with the torsion angle derived from electron density (AIFX 147). In 13, idealized aromatic hydrogens (AFIX 43) were used for the calculation of the N−H functionalities, whereas the positions of the water hydrogens were derived from residual density and fixed by using DFIX and DANG instructions. The titanium(III) complex 16 was crystallized from methanol and contained disordered solvent molecules in the asymmetric unit. However, attempts to refine them over several sets of sites have not been successful, and thus, they have been omitted by applying the SQUEEZE [105] procedure of the PLATON [137,138] program package. Solvent-accessible voids of 760 Å 3 per unit cell were found, and 113 electrons have been omitted, which corresponds to slightly less than two molecules of methanol within the asymmetric unit of 16.

Electrochemistry
Electrochemical measurements on 1.0 mmol·L −1 solutions of the analytes in anhydrous, air free dichloromethane containing 0.1 mol·L −1 of [NBu 4 ][B(C 6 F 5 ) 4 ] as a supporting electrolyte were conducted under a blanket of purified argon at 25 • C utilizing a Radiometer Voltalab PGZ 100 electrochemical workstation combined with a personal computer [128,139,140]. A three-electrode cell, which utilized a Pt auxiliary electrode, a glassy carbon working electrode (surface area 0.031 cm 2 ), and an Ag/Ag + (0.01 mol·L −1 AgNO 3 ) reference electrode mounted on a Luggin capillary were used. The working electrode was pretreated by polishing on a Buehler microcloth first with a 1 µm and then with a 1/4 µm diamond paste. The reference electrode was constructed from a silver wire inserted into a solution of 0.01 mol·L Successive experiments under the same experimental conditions showed that all formal reduction and oxidation potentials were reproducible within ±5 mV. Experimentally, potentials were referenced against an Ag/Ag + reference electrode, but results are presented referenced against ferrocene as an internal standard as required by IUPAC [125,126]. When decamethylferrocene was used as an internal standard, the experimentally measured potentials were converted into E vs. FcH/FcH + by addition of −614 mV [141,142]. Data were then manipulated on a Microsoft Excel worksheet to set the formal reduction potentials of the FcH/FcH + couple to ∆E • = 0.0 V. Ferrocene itself showed a redox potential of 220 mV vs. Ag/Ag + (∆E p = 61 mV) within the measurements [143,144]. The cyclic voltammograms were taken after typical three scans and are considered to be steady-state cyclic voltammograms in which the signal pattern differs not from the initial sweep.
UV/Vis-NIR measurements were carried out in an OTTLE (=optically thin-layer electrochemistry) cell with quartz windows similar to that described previously [132] in anhydrous dichloromethane solutions containing 2.0 mmol·L −1 analyte and 0.1 mol·L −1 of [NBu 4 ][B(C 6 F 5 ) 4 ] as a supporting electrolyte using a Varian Cary 5000 spectrophotometer at 25 • C. The working electrode Pt-mesh, the AgCl-coated Ag wire for reference, and the Pt-mesh auxiliary electrode are melt-sealed into a polyethylene spacer. The values obtained by deconvolution could be reproduced within ε max = 100 L·mol −1 ·cm −1 , ν max = 50 cm −1 , and ∆ν 1/2 = 50 cm −1 . Between the spectroscopic measurements, the applied potentials have been increased step-wisely using step heights of 25, 50, or 100 mV. At the end of the measurements, the analyte was reduced at −400 mV for 30 min, and an additional spectrum was recorded to prove the reversibility of the oxidations.

Reagents
[NBu 4 ][B(C 6 F 5 ) 4 ] was prepared by metathesis of lithium tetrakis(pentafluorophenyl)borate etherate (Boulder Scientific) with tetra-n-butylammonium bromide according to reference [129]. All other chemicals were purchased from commercial suppliers and were used without further purification. Ethyl ferrocenecarboxylate 1 was synthesized by mono lithiation of ferrocene with t BuLi and subsequent addition of ethyl chloroformate [79]. Acetylferrocene (2) was synthesized by acylation of ferrocene with acetic anhydride and boron trifluoride etherate [80]. Ferrocenyl ketone 3 was formed by a Friedel-Crafts acylation of ferrocene with hexanoyl chloride and anhydrous aluminum chloride as a catalyst [81]. Hexanoyl chloride used for the synthesis of 3 as well as 2-[2-(2-methoxyethoxy)ethoxy]acetyl chloride, applied in the preparation of 4, were synthesized by refluxing the appropriate carboxylic acids in thionyl chloride for 6 h and subsequent distillation [82,83]. The Claisen condensation of ferrocenyl ester 1 and ketone 2 to give diketone 5 (and as side product 6) were performed similar (ethyl ferrocenoate instead of methyl ferrocenoate) to a literature-known procedure [84]. The analytical data of 5 agree well with the ones in references [36,84]. 1,3-Diferrocenylpropane-1,3-dionato lithium (200 mg, 0.45 mmol) in 200 mL of anhydrous tetrahydrofuran was dropwise treated with TiCl 4 (16 µL, 0.14 mmol) via a syringe at −80 • C. A green colored precipitate was formed immediately, and the reaction solution was stirred until it reached ambient temperature. The green precipitate was filtered off, washed thrice with 10 mL (each) of diethyl ether, and crystallization from toluene afforded a dark green solid of 10. Yield: 29 mg (0.021 mmol, 14% based on 1,3-diferrocenylpropane-1,3-dionato lithium(I)).     Diferrocenyl diketone 5 (700 mg, 1.59 mmol) and 6 equiv. of 64% hydrazine hydrate (0.5 mL) in 10 mL of acetic acid were stirred for 12 h at 70 • C. After cooling the reaction mixture to ambient temperature, it was poured into water and neutralized with a 1 M solution of NaOH. The obtained solution was extracted four times with 30 mL (each) of dichloromethane, the combined organic phases were dried over MgSO 4 , and then, all volatiles were removed in vacuum. The crude product was adsorbed on silica and purified by column chromatography (column size: 20 × 3 cm, silica) using dichloromethane as eluent. The 3rd fraction contained 13. After removing all volatiles under reduced pressure, compound 13 was obtained as an orange-red solid. Yield: 560 mg (1.28 mmol, 80% based on 5).