Synthesis, characterization, and electrochemistry of diferrocenyl β-diketones, -diketonates, and pyrazoles

molecules

Article

Synthesis, Characterization, and Electrochemistry of

Diferrocenyl β-Diketones, -Diketonates, and Pyrazoles ^†

Steve W. Lehrich ¹, Julia Mahrholdt ¹, Marcus Korb ², Alexander Hildebrandt ¹,

Jannie C. Swarts ³and Heinrich Lang ^1,

*

1

Anorganische Chemie, Institut für Chemie, Fakultät für Naturwissenschaften,

Technische Universität Chemnitz, D-09107 Chemnitz, Germany; steve-lehrich@web.de (S.W.L.);

julia.mahrholdt@chemie.tu-chemnitz.de (J.M.); alexander.hildebrandt@chemie.tu-chemnitz.de (A.H.)

School of Molecular Sciences, The University of Western Australia, 35 Stirling Highway, Crawley,

Perth, WA 6009, Australia; marcus.korb@uwa.edu.au

2

3

Department of Chemistry, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa;

swartsjc@ufs.ac.za

*

†

Correspondence: heinrich.lang@chemie.tu-chemnitz.de; Tel./Fax: +49-(0)371-531-21210

Dedicated to Prof. Dr. Wolfgang Kaim on the occasion of his 70th birthday.

ꢀꢁꢂꢀꢃꢄꢅꢆꢇ

ꢀꢁꢂꢃꢄꢅꢆ

Academic Editors: Tiziana Benincori, Patrizia Romana Mussini and Serena Arnaboldi

Received: 17 August 2020; Accepted: 22 September 2020; Published: 29 September 2020

Abstract: The synthesis of FcC(O)CH(R)C(O)Fc (Fc = Fe(

CH₂CH₂(OCH₂CH₂)₂OMe, ), [M(

²O,O⁰-FcC(O)CHC(O)Fc)_n] (M = Ti, n = 3, 10; M = Fe, n = 3,

11; M = BF₂, n = 1, 12), and 1-R⁰-3,5-Fc₂-^cC₃HN₂(R⁰= H, 13; Me, 14; Ph, 15) is discussed. The

solid-state structures of 12 13 15, and 16 ([TiCl₂(

²O,O⁰-PhC(O)CHC(O)Ph)₂]) show that

and exist in their -diketo form. Compound 13 crystallizes as a tetramer based on a hydrogen

bond pattern, including one central water molecule. The electrochemical behavior of and 16

was studied by cyclic and square-wave voltammetry, showing that the ferrocenyls can separately be

oxidized reversibly between 50 and 750 mV ( 12 15: two Fc-related events; 10 11: six events,

being partially superimposed). For complex 10, Ti-centered reversible redox processes appear at

985 (Ti^II/Ti^III) and 520 mV (Ti^III/Ti^IV). Spectro-electrochemical UV-Vis/NIR measurements were

carried out on

considerable strength (

η η 5; 7;

⁵-C₅H₄)( ⁵-C₅H₅); R = H, ⁿBu,

9

κ

5,

7,

9

,

κ

7

9

β

5–

7

9–

−

5

–7,

9,

–

,

−

5, 6, and 12, whereby only 12 showed an IVCT (intervalence charge-transfer) band of

ν

_max= 6250 cm⁻¹

,

∆ν₁= 4725 cm⁻¹

,

ε

_max= 240 L

·

mol⁻¹cm⁻¹), due to the rigid

·

2

C₃O₂B cycle, enlarging the coupling strength between the Fc groups.

Keywords: ferrocenyl; pyrazole;

β-diketonate; solid-state structure; spectro(electrochemistry);

electron transfer

1. Introduction

Main-group element and transition metal

belong to one of the most investigated metal-organic complexes [

can be prepared by straightforward synthetic methodologies [ 10], and that many of them are

β-diketonates, including Lewis-base adducts thereof,

1–7]. One reason is that they

8–

commercially available. Hence, they attained great interest in, for example, the deposition of metal

or metal oxide thin layers by applying diverse deposition techniques, including CVD (=chemical

vapor deposition) [

addition, they received great interest in the synthesis of (hetero)multimetallic complexes [23

as catalysts in organic synthesis [30 32] and as anticancer drugs (especially copper and rhodium

-diketonates) [33 35]. Recently, they gained topical attention in the study of electron transfer

processes, when, for example, redox-active groups such as ferrocenyls are present, i.e., in zirconium,

1

,11

–

14], ALD (=atomic layer deposition) [15

–

19], and spin-coating [20

–

22]. In

–

29]

–

β

–

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Molecules 2020, 25, 4476

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hafnium, copper(I), copper(II), manganese, ruthenium, rhodium(I), and aluminum

complexes [36–41].

β-diketonate

Pyrazoles are a major class of heterocyclic compounds and have been thoroughly studied [42

They show, for example, biological activities possessing antimicrobial, anti-inﬂammatory, or antitumor

properties [48 54]. In this respect, it was found that ferrocenyl-functionalization of pyrazoles enhances

their biological activities [55 59]. Facile synthesis methods of ferrocenyl-substituted pyrazoles were

–47].

–

reported [60–66], with only a brief comment about the parent diferrocenyl NH-pyrazole in 1963 [66].

Against these backgrounds, we recently became interested in the synthesis of ferrocenyl

β-diketones and on their complexation behavior towards transition metal and main-group element

building blocks as well as in the preparation of diferrocenyl pyrazoles to study their electrochemical

and spectro-electrochemical properties. It is a continuation of recently published work from our group

in the ﬁeld of ferrocenyl-substituted carbon-rich π-conjugated systems [67–78].

We herein enrich the families of diferrocenyl

the synthesis of FcC(O)CH(R)C(O)Fc, [M(

²O,O⁰-FcC(O)CHC(O)Fc)_n], and 1-R⁰-3,5-Fc₂-^cC₃HN₂,

respectively, (Fc = Fe(

⁵-C₅H₄)( ⁵-C₅H₅); R = H, CH₂CH₂(OCH₂CH₂)₂OMe; R⁰= H, Me, Ph; M = Ti,

β-diketones, β-diketonates, and pyrazoles by

κ

η

n = 3; M = Fe, n = 3; M = BF₂, n = 1). Their molecular solid-state structures and (spectro)electrochemical

behavior are reported as well.

2. Results and Discussion

2.1. Synthesis and Characterization

The synthetic methodologies to prepare the ferrocenyl-functionalized

in Scheme 1. The therefore necessary starting compounds were synthesized accordingly

to references [79 81]. Ketone was prepared by a Friedel-Crafts acylation of ferrocene with

2-[2-(2-methyoxyethoxy)ethoxy]acetyl chloride, which is accessible by reﬂuxing the appropriate

β-diketones 5–8 is shown

1–3

–

4

carboxylic acid with thionyl chloride in analogy to references [82

Lewis acid catalyst AlCl₃(Section 4.6).

,83], with ferrocene in presence of the

Claisen condensation of

product could be isolated (Scheme 1 ) [84], which was produced by self-aldol condensation of

acetylferrocene (

). Using KO^tBu (DMF, 50 ^◦C) as the base instead of LDA (=lithium diisopropylamide)

minimized the yield of side product 6 to 7%, while slightly increasing the amount of 5 (58%).

To improve the solubility of the respective diferrocenyl -diketones and the appropriate metal

complexes (Schemes 1 and 2), a butyl and ethylene glycol chain, respectively, was introduced in

-position of the -diketone. Thus, Claisen condensation of with yielded , whereas treatment of

with 4 to give 8 was not successful.

1

with

2

resulted in the formation of diferrocenyl

β

-diketone . Side

5

6

2

β

1

3

7

1

Scheme 1. Synthesis of 5–8 1).

(LDA = LiNⁱPr₂; THF = tetrahydrofuran; yields are based on ferrocene

Molecules 2020, 25, 4476

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Scheme 2. Synthesis of

ethoxy]ethane, DMSO, 50 ^◦C, 14 h; (ii) 10: TiCl₄, THF,

[BF₃

Et₂O], HNⁱPr₂, ambient temperature, 1 h; (iii) NH₂-NHR, AcOH, 70 ^◦C, 12 h; (iv) NEtⁱPr₂, PhNO₂,

Me₂SO₄, 70 ^◦C, 12 h (DMSO=dimethyl sulfoxide).

9

(i), 10

–

12 (ii), and 13

–

15 (iii). (i) KO^tBu, 1-iodo-2-[2-(2-methoxyethoxy)

80 ^◦C to 25 ^◦C; 11: [Fe(acac)₃], reﬂux, 3 h; 12

−

:

·

Thus, a diﬀerent approach was applied to synthesize an ethylene glycol functionalized diferrocenyl

-diketone. Hence, compound was treated with 1-iodo-2-[2-(2-methoxyethoxy)ethoxy]ethane and

KO^tBu at 50 ^◦C (pathway i), Scheme 2 whereby ICH₂CH₂(OCH₂CH₂)₂OMe was prepared by applying

the Finkelstein reaction (treatment of BrCH₂CH₂(OCH₂CH₂)₂OMe with NaI in acetone [85 86]. The

respective BrCH₂CH₂(OCH₂CH₂)₂OMe educt was obtained from an Appel reaction starting from

2-[2-(2-methoxyethoxy)ethoxy]ethanol with CBr₄and PPh₃, respectively [87 88].) It should be noted

β

5

,

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

12 (pathway ii), Scheme 2. Therefore,

titanium, iron, and boron -diketonato coordination complexes 10

β

–

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 TiCl₄to a

tetrahydrofuran solution containing [Li(

[Fe(acac)₃] (acac = acetylacetonate) with a 3-fold excess of

preparation of the purple dioxaborine complex 12, diisopropylamine and [BF₃

reacted with 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 and

by using diﬀerent reagents such as KO^tBu, LDA, or ⁿBuLi

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 -hydrogen atom of the -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 by addition of electrophiles (MeI and Me₂SO₄) to

the reaction mixture containing a potentially lithiated species of , whereby a methylated compound

was not detected. It should be noted that deprotonation of the herein less-acidic -hydrogen could

compete with a metalation of the C₅H₄group, due to the ortho-directing properties of the adjacent

carbonyl X=O (X = C, P, S) functionalities [89 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 95]. Second, it is known that lithiated ferrocenes can be re-protonated via

κ

²O,O⁰-FcC(O)CHC(O)Fc)] [41] at

−

80 ^◦C, and 11 by reﬂuxing

5

in acetonitrile (Scheme 2). For the

Et₂O] were subsequently

·

5

–

7

9

−

α

β

8

α

–

−

,

–

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ether cleavage of THF or Et₂O. The rate constant for this reaction accordingly increases at higher

temperatures (40 ^◦C), giving the starting material back [96–98].

A straightforward synthesis procedure for diferrocenyl-functionalized pyrazoles 13

by the reaction of with an excess of hydrazines NH₂-NHR (R = H, Ph), as outlined in Scheme 2

(pathway iii)) (Section 4).

–15 is given

5

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(κ

²O,O⁰-PhC(O)CHC(O)Ph)₃] [99]. However, it

appeared that within the reaction of 1,3-diphenyl-1,3-propanedione with TiCl₄solely the corresponding

Ti(IV) coordination complex 16 was produced (Scheme 3) [100,101].

Scheme 3. Synthesis of 16 (i) THF, −80 ^◦C to 25 ^◦C, 2 h.

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 (¹H, ¹³C{¹H}, 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 12 13 15, and 16 in the solid state were determined by single

5

,

7,

,

crystal X-ray structure analysis. Electrochemical studies (cyclic and square-wave voltammetry) were

carried out on 5–7 and 9–16.

The IR spectra are characterized by the appearance of CO vibrations typical for

β-diketones (5–7,

9

) or metal -diketonates (10 39 40 102 103]. For the respective

β

–

12) in the range of 1500–1680 cm⁻¹

[

,

diferrocenyl pyrazoles 13–

15, representative vibrations could be observed for the N-H (3392 cm⁻¹),

N-C (ca. 1105 cm⁻¹) and N=C units (ca. 1600 cm⁻¹) [60].

A distinctive signature of the ¹H NMR spectra is the appearance of the cyclopentadienyl proton

signals between 4.5–5.0 ppm with multiplets or pseudo-triplets for the C₅H₄units with J_HH= 1.9 Hz

and a singlet at ca. 4.1 ppm for the C₅H₅protons. The

pyrazoles 13 15 resonates at 6.1–6.6 ppm, while in , it is observed at 4.26 ppm (Experimental). The

OH functionality in was observed at 16.4 ppm as a broad signal, due to rapid 1,5-tautomerism, in

α-hydrogen in β-diketonato complex 12 and

–

7

5

addition to the CH resonance at 5.9 ppm. The enol form equilibrates with its

ratio. The kinetics of this equilibrium have been studied recently [84].

β-diketo form in a 2:1

In the ¹³C{¹H} NMR spectra, the

β-diketonate CO groups give rise to a distinctive resonance

signal at ca. 200 ppm. For the ferrocenyl groups, in total, four signals are observed between 65–80 ppm,

which is typical for the C₅H₄and C₅H₅perimeters (Section 4) [36–41].

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

Molecules 2020, 25, 4476

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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.

2.2. 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 metal-organic complexes crystallize in triclinic (P

16), and non-centrosymmetric orthorhombic (P2₁2₁2₁,

space groups, with one molecule in the asymmetric unit, except for

−

1, 13), monoclinic (P2₁/c,

; abs. struct. param.: 0.000(16) [104])

containing two. Compound 16

9, 12; P2₁/n,

7

,

15

,

5

9

was obtained as a methanol solvate, whereby disorder required removal of the packing solvent with

the SQUEEZE procedure implemented in the PLATON program package (see Section 4) [105]. The

asymmetric unit of 13 is described by a (13)₄·H₂O arrangement (see below).

Figure 1. ORTEP (30% probability level) of the molecular structure of

scheme. C-bonded hydrogen atoms have been omitted for clarity. Selected bond distances (Å), angles,

and torsion angles (^◦): C1

O1 1.452(3), C3=O2 1.235(10), C1=C2 1.369(12), C2 C3 1.477(12), C1 C2 C3

124.4(7), O1 C1 C3 O2 10.6(10), O1 C1 C2 C3 2.4(13), C1 C2 C3 O2 31.8(12). Hydrogen bridge

5 with the atom numbering

−

bond properties: O1···O2 2.975(12) Å, O1−H1···O2 119^◦.

Figure 2. ORTEP (50% probability level) of the molecular structure of

scheme and an intramolecular T-shaped CH -interaction (green) between two centroids (Ct). All

hydrogen atoms have been omitted for clarity. Selected bond distances (Å), angles, and torsion angles (^◦):

C1=O1 1.222(6), C3=O2 1.224(5), C1 C2 1.480(7), C2 C3 1.457(7), C1 C2 C3 111.6(4), O1 C1 C2 C3

121.1(5), O2 C3 C2 C24 16.1(7); intramolecular T-shaped

-interaction: Ct···Ct 4.520(3) Å;

75.1(3)^◦

(Ct = centroid of the cyclopentadienyl groups).

7 with the atom numbering

π

−

π

α

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Figure 3. ORTEP (30% probability level) of the molecular structure of

9 with its atom numbering

scheme. Hydrogen atoms and a second crystallographically independent molecule of the asymmetric

unit have been omitted for clarity. Selected bond distances (Å), angles, and torsion angles (^◦):

C=O 1.197(7)

38.9(6)/34.9(7), C1−C2−C3−O2 104.9(6)/103.1(8).

−

1.213(8), C1

−

C2/C2

−

C3 1.469(9)

−

1.478(9), C1

−C2

−C3 108.2(4)/108.4(6), O1

−

C1−

C2 C3

−

Figure 4. ORTEP (50% probability level) of the molecular structure of 12 with the atom numbering

scheme. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å), angles, and torsion

angles (^◦): C1

−

O1 1.314(3), C3

−

O2 1.307(3), C1

−

C2 1.386(3), C2

−

C3 1.384(3), O B1 1.488(3)/1.480(3),

−

C1−C2−C3 119.6(2), O1−C1−C3−O2 2.40(18), O1−C1−C2−C3 3.7(4), C1−C2−C3−O2 6.5(4).

Molecules 2020, 25, 4476

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Figure 5. ORTEP (50% probability level) of the molecular structure of 13 (top) with the atom numbering

scheme. All C-bonded hydrogen atoms and have been omitted for clarity, as well as the ferrocenyls

(pale grey) in the lower graphic. Selected bond distances (Å): C1

1.375(4)−1.396(4), N−N 1.362(4)−1.367(4).

−

N 1.343(4)

−

1.357(4), C_PyC_Py

−

Figure 6. ORTEP (50% probability level) of the molecular structure of 15 with the atom numbering

scheme. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å): N1−C1 1.373(4),

C1=C2 1.373(4), C2−C3 1.404(4), C3=N2 1.332(4), N1−N2 1.375(3).

Molecules 2020, 25, 4476

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Figure 7. ORTEP (30% probability level) of the molecular structure of 16 with the atom numbering

scheme. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å), angles, and

torsion angles (^◦): C

O 1.277(5) 1.291(5), C1 C2/C2 C3 1.379(5) 1.393(6), Ti O1/O3 1.973(3)/1.970(3),

Ti O2/O4 1.928(3)/1.924(3), Ti Cl1 2.2867(13), Ti Cl2 (2.3033(12), C1 C2 C3 123.0(4)/123.6(6),

O1−C1−C2−C3 6.6(6)/1.3(6), C1−C2−C3−O2 3.1(7)/0.9(6).

−

In 7, 9, 10, and 12, identical C−O distances for both CO groups are found, whereby their lengths

increase from non-complexed alkyl- ( ) and alkoxy-substituted ( ) derivatives (1.197(7)–1.224(5) Å)

to BF₂(12) and Ti (16) (1.277(5)–1.314(3) Å). Consequently, the negative charge in the latter two

species is delocalized through the -diketonato system, which also results in equivalent C C bond

lengths (1.379(6)–1.393(6) Å). In contrast, enol 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 has previously been

7

9

β

−

5

−

5

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

exclusively discussed [107

directions (

), due to the rotational freedom around the an sp³-hybridized carbon. For the latter species,

this is accompanied with an intramolecular T-shaped -interaction between a C₅H₄and a C₅H₅ring

(Figure 2). [93 95] For the theoretical calculations regarding the properties of parallel-displaced and

α

-substituted species

7

and

9

is in accordance to literature, where this behavior is

,108]. The keto functionalities can either direct in the same (

9

) or opposite

7

π

–

various types of T-shaped

see [111–114].

π-interactions, see references [109,110]; for examples involving ferrocenes,

In contrast to

C₃O₂entity. In case of

to 28.6(7)^◦, which is further reduced to 1.9(3)–4.5(3)^◦for 12 and 16, containing stronger O

M = Ti, B). The distribution of the negative charge in tetrahedral BF₂-containing 12 is accompanied by

equal B O (1.488(3) and 1.480(3) Å) and B F bond lengths (1.488(3) and 1.480(3) Å). In contrast, the

cis-dichloro substituents in the octahedral coordinated titanium complex 16 caused an elongation of the

Ti O bonds from O Ti O (Ti O: 1.928(3) and 1.924(3) Å) to O Ti Cl bonds (Ti O: 1.973(3) and 1.970(3)

7

and

9

, compounds

5

,

12, and 16 adopted a co-planar alignment of the central

C1· · · C3 O2 torsion angle

M bonds

5, a rather weak hydrogen bond restricted the O1

−

(

−

Å), 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

Nevertheless, a similar trans-inﬂuence can be observed for literature-known bis- or tris(

complexes [116 119]. It should be noted that all bis( -diketonato)TiX₂complexes (X = Cl, O, alkoxy,

aryloxy) exclusively crystallize as their cis-derivatives [115].

−

C bond precision.

β-diketonato)

–

β

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The

towards BF₂and TiCl₂, underwent distortions in order to comply with the requirements for tetrahedral

12) and octahedral (16) coordination environments, which requires a smaller cavity between both

oxygen donors. The comparably long Ti O bond distances in 16 compensate the unfavorable small

Ti -diketonato

O angles of 83.38(12) and 83.96(12)^◦and keep the Ti atom in the almost planar

β-diketonato motif in bora- and titana-cyclic structures 12 and 16, formed upon coordination

(

−

O

−

β

motif (out-of-plane shift of only 0.216(5) and 0.065(5) Å). A distortion of the C₃O₂-planes results in

high rms deviations of 0.0244 and 0.107, resulting in apparently higher out-of-plane shifts of the Ti

atom. Instead, the by ~0.5 Å shorter B

required shortening of the O···O distances by ~0.15 Å from 2.595(4)/2.605(4) (16) to 2.450(2) Å in 12

Although the tetrahedral coordination sphere allows for a larger O

O angle of 111.2(2)^◦, the BF₂

fragment is shifted out of the planar -diketonato entity by 0.368(4) Å (vide supra). If expressed by

the C2

B1 torsion angels, a bend of up to 15.7(3)^◦is observed, which is comparable to other

−

O bond lengths in 12 squeeze the backbone to achieve the

.

−B−

β

−

C

−O−

heterocyclic structures where similar out-of-plane shifts prevent involvement of the heteroatom into

the π-conjugation [120–122].

The ferrocenyl C₅H₄- 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 23.1(8)^◦.

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 pyrazole⁰s nitrogen atoms. This

aﬀects the whole hydrogen bond network within the (13

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 -interactions between the Fc groups, which are

)

₄·H₂O arrangement, which is established

π

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·H₂O.

Table 1. Hydrogen bond and T-shaped π-interaction properties (Å/^◦) of 13.

Hydrogen Bridge-Bonds

D–H· · · A

D–H· · · A/^◦

173(4)

169(4)

165

D–A/Å

O1–H1O· · · N3

O1–H2O· · · N7

N2–H2A· · · O1

N5–H5A· · · O1

N4–H4· · · N6

2.714(3)

2.723(4)

2.801(4)

2.732(3)

3.088(4)

3.084(4)

167

150

164

N8–H8A· · · N1

T-shaped π-interactions

Ct(C₅H₄)· · · Ct(C₅H₅)

α/^◦

D/Å

C27–31· · · C14–18

4.741(3)

4.651(3)

87.7(2)

85.7(2)

C50–54· · · C73–77

The angle α is described by the intersection of the involved C₅H₄/C₅H₅planes.

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

Molecules 2020, 25, 4476

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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.

2.3. 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 [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].

Figure 8. Cyclic voltammograms (solid lines: scan rate 100 mV s⁻¹) and square-wave voltammograms

(dotted lines: step-height 25 mV, pulse-width 5 s, amplitude 5 mV) of

5

-

7

,

9

, and 13

-

15 in dichloromethane

◦

solutions (1.0 mmol

·

L

⁻¹) at 25 C measured with a glassy carbon working electrode. Supporting

electrolyte 0.1 mol·L⁻¹of [NBu₄][B(C₆F₅)₄]).

Molecules 2020, 25, 4476

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Figure 9. Cyclic voltammograms (solid lines: scan rate 100 mV s⁻¹) and square-wave voltammograms

(dotted lines: step-height 25 mV, pulse-width 5 s, amplitude 5 mV) of 10 (left, top), 11 (right), and 12

(left, bottom) in dichloromethane solutions (1.0 mmol L

⁻¹) at 25 ^◦C measured with a glassy carbon

·

working electrode. Supporting electrolyte 0.1 mol·L⁻¹of [NBu₄][B(C₆F₅)₄]).

All potentials are referenced to the FcH/FcH⁺(FcH = Fe( ⁵-C₅H₅)₂) redox couple [127]. The CV

η

data at a scan rate of 100 mV s⁻¹are summarized in Table 2.

Table 2. Cyclic voltammetry data of 5-7 and 9-15. All Fc potentials are given in mV. ^a.

◦0

c

◦0

c

◦0

c

◦0

c

◦0

c

◦0

E₆

c

E₁

E₂

E₃

E₄

E₅

e

Compd.

∆E^◦0

(∆E_p) ^d

5

6

7

110 (60)

240 (60)

235 (60)

140 ^f

310 (88)

330 (60)

460 (74)

430 (70)

260 ^f,g

200

220

195

9

10^b

11

Al^h

12

13

14

15

385 ^f

430 ^f

432 (56)

485 ^f

560 ^f

583 (62)

670 ^f

730 ^f

741 (58)

120/125/100/185

130/75/125/130/170

90/181/128/151/158

100 ^f

230 ^f

305 ^f

304 (72)

33 (62)

300 (62)

−30 (60)

−30 (62)

−30 (65)

123 (62)

600 (64)

210 (64)

205 (66)

2145/300

240

235

Potentials vs. FcH/FcH⁺(FcH = Fe(

η

⁵-C₅H₅)₂), scan rate 100 mV s⁻¹at a glassy-carbon electrode of 1.0 mmol

·

L⁻¹

solutions in anhydrous dichloromethane containing 0.1 mol

25 ^◦C. (^a) Reduction processes for 10 and 12 are:

985 mV (

complex 11 did not show any reduction until

found at 520 mV (

·

L⁻¹of [NBu₄][B(C₆F₅)₄] as supporting electrolyte at

−

∆E_p= 60 mV, 10) and –1845 mV (∆E_p= 64 mV, 12);

−

2200 mV. (^b) An additional reversible redox process for Ti^IV/Ti^IIIis

◦0

∆

−

∆

E_p= 64 mV) (Figure 9). (^c) E_n= Formal potential of n Fc redox process. (^d)

∆E

_p= Difference

between oxidation and reduction potentials. (^e)

E^◦0= Potential difference between the ferrocenyl-related redox

processes. (^f) Potentials from square-wave voltammogram (dotted line: step-height 25 mV, pulse-width 5 s, amplitude

5 mV) (Figure 9 ). (^g) Two electron process. (^h) Al = [Al(κ²O,O⁰-FcC(O)CHC(O)Fc)₃], data taken from Ref. [41 ].

Compounds 5–7, 9, and 13-15 show two reversible one-electron redox processes, conﬁrming

separate oxidation of the Fc groups. The diferroc_◦e₀nyl

β

-diketone

5

was ﬁrst electrochemically studied

◦0

in 1999 using [NBu₄][PF₆] as an electrolyte (E₁= 188 mV and E₂= 297 mV) in order to obtain

group electro-negativities by means of cyclic voltammetry [126

our data, we repeated the CV measurements in the presence of the weak coordinating electrolyte

[NBu₄][B(C₆F₅)₄] [68 125 126]. As it can be seen from Figure 8 , compound 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 , since this compound only features one keto-group. The butyl or glycol groups in

,128–130]. For better comparison with

,

5

,

6

β

Molecules 2020, 25, 4476

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α

-position of the

, E₁= 240 mV;

β

-diketones

, E₁= 235 mV) in comparison to

7

and

9

result in a shift of the ﬁrst redox potential to more positive values

◦0

(7

9

5

(E₁= 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 -diketones are with

E^◦0ca. 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 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 (E₁^◦0= 30 mV, E^◦0= ca. 210 mV; Table 2), proving negligible inﬂuence of the N-bonded

β

∆

6

–

-

∆

hydrogen (13) methyl (14) or phenyl (15) group on the charge transfer process or the electrostatics in

the mono-oxidized mixed-valent compounds [13-15]⁺.

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

represents a one-electron reduction of the

character of these electrons in addition lead to a much better electronic coupling of the ferrocenyl

−

1845 mV (

∆E_p= 64 mV)

π

-system of the six membered C₃O₂B cycle. The delocalized

π

units, and hence, the redox separation is increased to 300 mV, when compared to the parent diketonate

5 (∆E^◦0= 200 mV).

As a result of the poor solubility of ferrocenyl-functionalized metal β-diketonates in, for example,

dichloromethane or acetonitrile, only a few examples of this family of compounds have been

characterized electrochemically in the literature, including [Cu(

κ

²O,O⁰-FcC(O)CHC(O)Fc)₂] [40

]

and [Al(κ²O,O⁰-FcC(O)CHC(O)Fc)₃] [41], respectively.

◦0

Complex 10 exhibits two titanium-related reversible redox processes at E₁

= 985 mV and

−

E₂^◦0= −520 mV, respectively. Comparison with 16 (Figure S2, see the Supporting Information) conﬁrms

that the wave at

the redox event at

is in agreement with the one found for [Ti₂L₃] (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

−

520 mV can be assigned to the reversible oxidation of Ti^IIIto Ti^IV

(∆E_p= 64 mV), while

−

985 mV (∆

E_p= 60 mV) corresponds to the reduction of Ti^IIIto Ti^II(Figure 9), which

−

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 (Figure 9). For the analog [Al(κ

²O,O⁰-FcC(O)CHC(O)Fc)₃] complex, a

comparable redox behavior was observed under similar measurement conditions (E₁^◦0= 33, E₂^◦0= 123

,

◦0

E₃^◦0= 304, E₄= 432, E₅= 583, E₆= 741 mV) [41]. In comparison to [Al(FcC(O)CHC(O)Fc)₃], the

ﬁrst oxidation in 11 takes place at a higher potential (E₁^◦0= 100), due to lower electron density at the

Fc⁰s. Reduction processes for Fe^IIIin 11 could, however, not be found up to

2200 mV (Figure S3,

−

see the ESI).

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

transparent thin layer electrochemistry cell (=OTTLE) [132]. Dichloromethane solutions of

12–15 (0.02 M) containing [NBu₄][B(C₆F₅)₄] (0.1 M) as the supporting electrolyte were used [123 ,124 ].

−

400 to 1200 mV (step heights: 25, 50 or 100 mV) vs. Ag/AgCl in an optically

5

,

6, and

+

2+

Thereby, the stepwise generation of mixed-valent [5 6 –15] and homo-valent [5] ,

]⁺, [ ]⁺, and [12

Molecules 2020, 25, 4476

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[

6

]

²⁺, and [12

(Figures S4 and S5, see the ESI).

During the oxidation of

extinction coeﬃcient of this band, however, is lower than 50 L

-

15

]

²⁺occurred. The spectra of [

5

]⁺and [

6

]⁺are shown in the Supporting Information

5

and

6

an IVCT absorption of negligible strength can be seen. The

mol

⁻¹cm⁻¹, therefore the electronic

·

coupling between the Fc/Fc⁺is very weak (Figures S4 and S5). The introduction of the BF₂unit

in 12 led to an increase in the extinction of the IVCT band (ν_max= 6250 cm⁻¹

,

∆ν₁= 4725 cm⁻¹

,

2

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 C₃O₂B ring

introduced some rigidity in the

became stronger.

π-bridge, and hence, the electronic coupling of the ferrocenyl termini

400

350

300

250

200

150



100

50

0

500

1000

1500

2000

2500

3000

 /nm

Figure 10. UV-Vis/NIR spectra of 12 in a dichloromethane solution (2.0 mmol

·

L

⁻¹) at rising potentials

L⁻¹of [NBu₄][B(C₆F₅)₄]

(

−

200 mV to 1200 mV vs. Ag/AgCl) at 25 ^◦C; supporting electrolyte 0.1 mol

·

black −200 mV (12); blue 750 mV ([12]⁺); red 1200 mV ([12]²⁺).

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.

3. 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-C₅H₄)(

η

⁵-C₅H₅);

n

R = H,

[M(

[TiCl₂(

12 13 15, and 16 were determined by single-crystal X-ray diﬀraction studies, verifying the predicted

structures. Alkyl substitution of the -diketone led to an increased electron density at C_αand, hence,

5

; Bu,

7

; CH₂CH₂(OCH₂CH₂)₂OMe,

9

), 1-R-3,5-Fc₂-^cC₃HN₂(R = H, 13; Me, 14; Ph, 15),

κ

²O,O⁰-FcC(O)CHC(O)Fc)_n] (M = Ti, n = 3, 10; M = Fe, n = 3, 11; M = BF₂, n = 1, 12), and

κ

²O,O⁰-PhC(O)CHC(O)P)₂] (16) is discussed. The molecular solid-state structures of

5, 7, 9,

,

β

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

separately; however, the redox separation of 200–250 mV is mainly caused by electrostatic interactions.

In addition, for complexes and 13 15, the ferrocenyl units are in chemically non-equivalent positions,

5–7, 9, and 12–15 could be oxidized

6

–

Molecules 2020, 25, 4476

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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

−520 mV, corresponding to Ti^II/Ti^IIIand Ti^III/Ti^IVredox couples, respectively.

−

985 and

Compound 12 showed an increased redox separation between the Fc units upon introduction

of BF₂, due to a more rigid backbone, which allows for a better conjugation through the C₃-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]⁺.

4. Materials and Methods

4.1. 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

CaH₂. For column chromatography, alumina with a particle size of 90

µm (standard, Merck KgaA)

or silica with a particle size of 40–60 m (230–400 mesh (ASTM), Fa. Macherey-Nagel) was used. As

µ

ﬁltration support Zeolithe (Riedel de Häen) was applied.

4.2. 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 ¹H, 125.7 MHz for ¹³C, 160.5 MHz for ¹¹B) 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 (¹H NMR:

¹³C{¹H} NMR:

(CDCl₃) = 77.16 ppm). The melting points were determined with a Gallenkamp MFB

δ (CDCl₃) = 7.26 ppm;

δ

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.

4.3. Crystallography

Data were collected with an Oxford Gemini S diﬀractometer at

radiation. The structures were solved by direct methods and reﬁned by full-matrix least square

procedures on F²with SHELXL-2013 [134

135]. All non-hydrogen atoms were reﬁned anisotropically,

≤

120 K using Mo K_α(λ = 0.71073 Å)

,

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

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

5 has been reﬁned as an idealized OH group with the torsion angle

−

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 Å³per 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.

Crystal data for

a = 8.7957(8) Å, b = 9.6795(9) Å, c = 21.569(3) Å, V = 1836.3(3) Å³, Z = 4, ρ_calcd= 1.592 Mg m⁻³

5 λ = 0.71073 Å,

: C₂₃H₂₀Fe₂O₂, M = 440.09 g mol⁻¹, orthorhombic, P2₁2₁2₁,

,

Molecules 2020, 25, 4476

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µ

= 1.595 mm⁻¹, T = 120.00(10) K,

θ

range 3.530–24.990^◦, 5616 reﬂections collected, 3005 independent

reﬂections (R_int= 0.0330), R1 = 0.0614, wR2 = 0.1496 (I > 2

Crystal data for 16: C₃₀H₂₂Cl₂O₄Ti, M = 565.27 g mol⁻¹, monoclinic, P2₁/n,

Å, b =15.3425(8) Å, c = 14.6145(7) Å,

= 97.054(5)^◦, V = 3190.4(3) Å³, Z = 4, ρ_calcd= 1.177 Mg m⁻³

µ = 0.464 mm⁻¹, T = 120.00(14) K, range 3.253–24.999^◦, 13796 reﬂections collected, 5554 independent

σ(I)), absolute structure parameter 0.000(16).

λ

= 0.71073 Å, a = 14.3372(7)

β

,

θ

reﬂections (R_int= 0.0541), R1 = 0.0651, wR2 = 0.1556 (I > 2σ(I)).

4.4. Electrochemistry

Electrochemical measurements on 1.0 mmol

dichloromethane containing 0.1 mol

⁻¹of [NBu₄][B(C₆F₅)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 cm²), and an Ag/Ag⁺

(0.01 mol

L⁻¹AgNO₃) reference electrode mounted on a Luggin capillary were used. The working

electrode was pretreated by polishing on a Buehler microcloth ﬁrst with a 1 m and then with a 1/4

diamond paste. The reference electrode was constructed from a silver wire inserted into a solution of

0.01 mol

⁻¹[AgNO₃] and 0.1 mol ⁻¹[NBu₄][B(C₆F₅)₄] in acetonitrile in a Luggin capillary with a

CoralPor tip. This Luggin capillary was inserted into a second Luggin capillary with a CoralPor tip

ﬁlled with a 0.1 mol

L⁻¹[NBu₄][B(C₆F₅)₄] solution in dichloromethane. 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= 0.0 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

·

L

⁻¹solutions of the analytes in anhydrous, air free

·L

,

·

µ

µm

·L

·

±

,

−

,

∆

(

∆

,

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⁻¹analyte and 0.1 mol

·

L⁻¹o_◦f [NBu₄][B(C₆F₅)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.

4.5. Reagents

[NBu₄][B(C₆F₅)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

^tBuLi and subsequent addition of ethyl chloroformate [79]. Acetylferrocene (

by acylation of ferrocene with acetic anhydride and boron triﬂuoride etherate [80]. Ferrocenyl

ketone 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

1

was synthesized by mono lithiation of ferrocene with

2

) was synthesized

3

Molecules 2020, 25, 4476

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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 and ketone to give diketone

(and as side product ) were performed similar (ethyl ferrocenoate instead of methyl ferrocenoate)

to a literature-known procedure [84]. The analytical data of agree well with the ones in

references [36 84]. The compound 1,3-diferrocenylpropane-1,3-dionato lithium(I) was obtained as an

intermediate in the synthesis of , when the reaction solution was ﬁltered oﬀ before quenching

,

1

2

5

6

5

,

5

with aqueous hydrochloric acid. The compound 1-iodo-2-[2-(2-methoxyethoxy)ethoxy]ethane

was synthesized by reﬂuxing 1-bromo-2-[2-(2-methoxyethoxy)ethoxy]et_◦hane in the presence of

sodium iodide in acetone for 48 h and subsequent distillation at 160 C (37 mbar) [86]. The

compound 1-bromo-2-[2-(2-methoxyethoxy)ethoxy]ethane was prepared by an Appel reaction of

2-[2-(2-methoxyethoxy)ethoxy]ethanol, tetrabromomethane, and triphenylphosphane [88].

4.6. Synthesis of 1-Ferrocenyl-2-[2-(2-methoxyethoxy)ethoxy]ethanone (4)

Ferrocene (6.62 g, 35.6 mmol) and AlCl₃(4.75 g, 35.6 mmol) were dissolved in 70 mL of anhydrous

dichloromethane and the thus-obtained reaction solution was then cooled to 0 ^◦C. The compound

2-[2-(2-methoxyethoxy)ethoxy]acetyl chloride (7.0 g, 35.6 mmol) in 20 mL of dichloromethane was

added dropwise via a dropping funnel within 30 min. The reaction solution was warmed up to ambient

temperature for 2 h and then added in a single portion to an aqueous NaHCO₃solution. The organic

phase was separated. The aqueous solution was extracted twice with 40 mL (each) of dichloromethane.

The organic phases were combined, dried over MgSO₄, and all volatiles were removed in vacuum.

The crude product was puriﬁed by column chromatography (column size: 20

a hexane–dichloromethane mixture of ratio 1:1 (v/v) as eluent. The 4th fraction contained the title

compound , which after removal of all volatiles gave a brownish oily liquid. Yield: 11.3 g (32.6 mmol,

×

3 cm, alumina) using

4

92% based on ferrocene).

Anal. Calcd. for C₁₇H₂₂FeO₄(M = 346.20 g

mol⁻¹): C 58.98, H 6.41; found: C 58.91, H 6.57. ATIR (ATR;

·

v

˜

in cm⁻¹): 3095 (m, ν_C–H), 2875 (m), 1721 (w), 1680 (s), 1556 (m), 1455 (m), 1378 (m), 1361 (m), 1253

1

(m), 1198 (w), 1105 (s), 1064 (s), 1027 (s), 823 (s, π_C–H, C₅H₅), 775 (w). H NMR (CDCl₃,

δ in ppm): 3.38

(s, 3H, CH₃), 3.56 (m, 2H, CH₂), 3.67 (m, 2H, CH₂), 3.73 (m, 2H, CH₂), 3.78 (m, 2H, C(O)CH₂), 4.21 (s,

5H, C₅H₅), 4.51 (pt, ³J_H,H= 1.9 Hz, 2H, C₅H₄), 4.56 (s, 2H, CH₂), 4.83 (pt, ³J_H,H= 1.9 Hz, 2H, C₅H₄).

¹³C{¹H} NMR (CDCl₃,

δ in ppm): 59.2 (CH₃), 69.2 (C₅H₄), 70.1 (C₅H₅), 70.7 (CH₂), 71.0 (CH₂), 71.1

(CH₂), 72.1 (CH₂), 72.5 (C₅H₄), 74.6 (CH₂), 76.3 (ⁱC-C₅H₄), 201 (C=O). HRMS (ESI-TOF, m/z): calcd. for

C₁₇H₂₂FeO₄+ H: 347.0940; found 347.0954 [M + H]⁺.

4.7. Synthesis of 2-Butyl-1,3-diferrocenyl-1,3-propandione (7)

Ferrocenyl ketone

and this solution was treated with 3.0 mL (2 M in tetrahydrofuran) of lithium diisopropylamide at

0 ^◦C. The reaction solution was stirred for 2 h at 50 ^◦C and a solution of ethyl ferrocenoate

(1.72 g,

3 (1.72 g, 6.05 mmol) was dissolved in 20 mL of anhydrous tetrahydrofuran,

1

6.05 mmol) in 10 mL of tetrahydrofuran was added via a syringe. After 10 h of stirring, the reaction

solution was shaken with hydrochloric acid and extracted four times with 60 mL (each) of diethyl

ether. The combined organic phases were dried over MgSO₄, and all volatiles were removed. The

crude product was puriﬁed by column chromatography (column size: 20

hexane–dichloromethane mixture of ratio 2:1 (v/v) as eluent. The 2nd fraction contained

removal of all volatiles, gave 7 as a red solid. Yield: 362 mg (0.73 mmol, 40% based on 3).

×

3 cm, silica) using a

7, which, after

Anal. Calcd. for C₂₇H₂₈Fe₂O₂(M = 496.20 g

mol⁻¹): C 65.35, H 5.69; found: C 65.42, H 5.97. Mp.:

142 ^◦C. IR (KBr, v in cm⁻¹): 3094 (m, ν_C–H), 2950 (m), 2923 (m), 2870 (m), 1671 (m), 1642 (s, ν_C=O),

·

˜

1441 (m),1411 (w), 1374 (m), 1285 (m), 1257 (m), 1107 (m), 1049 (m), 1027 (m), 1003 (m), 852 (m), 827

1

(m, π_C–H, C₅H₅), 734 (m), 545 (m), 521 (m), 508 (m, C₅H₄ring tilt). H NMR (CDCl₃,

δ

in ppm): 0.93

3

(t, J_H,H= 7.0 Hz, 3H, CH₃), 1.38 (m, 4H, CH₂CH₂), 2.13 (m, 2H, CH₂), 4.08 (s, 10H, C₅H₅), 4.26 (t,

Molecules 2020, 25, 4476

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³J_H,H= 7.2 Hz, 1H, CH), 4.51 (ptd, J_H,H= 2.6, 1.4 Hz, 2H, C₅H₄), 4.54 (ptd, J_H,H= 2.5, 1.3 Hz, 2H,

C₅H₄), 4.93 (dpt, J_H,H= 2.5, 1.3 Hz, 2H, C₅H₄), 4.94 (dpt, J_H,H= 2.6, 1.3 Hz, 2H, C₅H₄). ¹³C{¹H} NMR

(CDCl₃,

δ in ppm): 14.1 (CH₃), 22.8 (CH₂), 30.1 (CH₂), 30.8 (CH₂), 65.2 (CH), 70.0 (C₅H₄), 70.2 (C₅H₅),

70.5 (C₅H₄), 72.6 (C₅H₄), 72.7 (C₅H₄), 79.0 (ⁱC-C₅H₄), 199.9 (C=O). HRMS (ESI-TOF, m/z): calcd. for

C₂₇H₂₈Fe₂O₂+ H: 497.0788; found 497.0862 [M + H]⁺.

Crystal data for

Å, b = 10.7155(7) Å, c = 12.6538(10) Å,

µ = 1.337 mm⁻¹, T = 119.95(10) K, range 3.458–24.999^◦, 9367 reﬂections collected, 3849 independent

7

: C₂₇H₂₈Fe₂O₂, M = 496.19 g mol⁻¹, monoclinic, P2₁/c,

= 93.817(8)^◦, V = 2205.2(3) Å³, Z = 4,

λ

= 0.71073 Å, a = 16.2997(15)

β

ρ

_calcd= 1.495 Mg m⁻³

,

θ

reﬂections (R_int= 0.1002), R1 = 0.0601, wR2 = 0.0763 (I > 2σ(I)).

4.8. Synthesis of 2-(2-[2-(2-Methoxyethoxy)ethoxy]ethyl)-1,3-diferrocenyl-1,3-propandione (9)

Diferrocenyl diketone 5 (360 mg, 0.82 mmol), 1-iodo-2-[2-(2-methoxyethoxy)ethoxy]ethane (225 mg,

0.82 mmol) and KO^tBu (92 mg, 0.82 mmol) were dissolved in 20 mL of dimethyl sulfoxide and the

thus-obtained reaction solution was stirred for 14 h at 50 ^◦C. Afterwards, it was poured into 100 mL of

water and extracted ﬁve times with 50 mL (each) of diethyl ether. The combined organic phases were

dried over MgSO₄, and all volatiles were removed in vacuum. The crude product was puriﬁed by

column chromatography (column size: 20

of ratio 1:1 (v/v) as eluent. The 2nd fraction contained

Crystallization from a hexane/dichloromethane mixture of ratio 1:20 (v/v) at

crystals. Yield: 310 mg (0.53 mmol, 64% based on 5).

×

3 cm, silica) using a hexane–dichloromethane mixture

, which, after removal of all volatiles, gave 9.

9

−

20 ^◦C aﬀorded

9 as red

Anal. Calcd. for C₃₀H₃₄Fe₂O₅(M = 586.28 g

mol⁻¹): C 61.46, H 5.85; found: C 61.25, H 6.04. Mp.:

·

74 ^◦C. IR (KBr, in cm⁻¹): 3098 (m, ν_C–H), 3089 (m, ν_C–H), 2884 (m), 1677 (s, ν_C=O), 1656 (s, ν_C=O), 1449

(s), 1410 (m), 1374 (s), 1336 (m), 1295 (m), 1265 (m), 1235 (m), 1132 (m), 1107 (s), 1047 (m), 1026 (m),

1

991 (m), 898 (m), 833 (s, π_C–H, C₅H₅), 658 (m), 532 (m, C₅H₄). H NMR (CDCl₃,

δ in ppm): 2.42 (q,

³J_H,H= 6.1 Hz, 2H, CH₂), 3.37 (s, 3H, CH₃), 3.55 (m, 4H, CH₂), 3.65 (m, 6H, CH₂), 4.11 (s, 10H, C₅H₅),

4.50 (ptd, J_H,H= 2.6, 1.4 Hz, 2H, C₅H₄), 4.53 (ptd, J_H,H= 2.5, 1.3 Hz, 2H, C₅H₄), 4.57 (t, ³J_H,H= 6.9 Hz,

1H, CH), 4.92 (dpt, J_H,H= 2.5, 1.2 Hz, 2H, C₅H₄), 4.94 (dpt, J_H,H= 2.6, 1.3 Hz, 2H, C₅H₄). ¹³C{¹H} NMR

(CDCl₃,

δ in ppm): 30.4 (CH₂), 59.2 (CH₃), 60.2 (CH), 69.1 (CH₂), 70.1 (C₅H₄), 70.2 (C₅H₅), 70.3 (CH₂),

70.4 (C₅H₄), 70.7 (CH₂), 70.8 (CH₂), 72.1 (CH₂), 72.6 (C₅H₄), 72.7 (C₅H₄), 79.0 (ⁱC-C₅H₄), 199.8 (CO).

HRMS (ESI-TOF, m/z): calcd. for C₃₀H₃₄Fe₂O₅: 586.1105; found 586.1070 [M + H]⁺.

Crystal data for

Å, b = 9.6717(7) Å, c = 19.1333(11) Å,

µ = 1.138 mm⁻¹, T = 114.0(6) K, range 3.004–25.000^◦, 24,744 reﬂections collected, 9280 independent

9

: C₃₀H₃₄Fe₂O₅, M = 586.27 g mol⁻¹, monoclinic, P2₁/c,

= 102.630(6)^◦, V = 5280.7(6) Å³, Z = 8, ρ_calcd=1.475 Mg m⁻³

λ = 0.71073 Å, a = 29.244(2)

β

,

θ

reﬂections (R_int= 0.0447), R1 = 0.0838, wR2 = 0.2179 (I > 2σ(I)).

4.9. Synthesis of tris-(1,3-Diferrocenylpropane-1,3-dionato-κ²-O,O⁰)titanium(III) (10)

1,3-Diferrocenylpropane-1,3-dionato lithium (200 mg, 0.45 mmol) in 200 mL of anhydrous

tetrahydrofuran was dropwise treated with TiCl₄(16

µ

L, 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)).

C₆₉H₅₇Fe₆O₆Ti (M = 1365.12 g

·

mol⁻¹). Mp.:

≥

240 ^◦C decomposition. IR (KBr, in cm⁻¹): 1618 (m),

1504 (s, ν_C=O), 1501 (s, ν_C=O), 1445 (m), 1381 (m), 1344 (m), 1331 (m), 1295 (m), 1250 (m), 1213 (w),

1107 (m), 1028 (m), 943 (m), 823 (m, π_C–H, C₅H₅), 731 (m), 675 (m). HRMS (ESI-TOF, m/z): calcd. for

C₆₉H₅₇Fe₆O₆Ti: 1364.9735; found 1364.9675 [M + H]⁺.

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4.10. Synthesis of tris-(1,3-Diferrocenylpropane-1,3-dionato-κ²-O,O⁰)iron(III) (11)

A solution of [Fe(acac)₃] (45 mg, 0.127 mmol) and 5 (167 mg, 0.38 mmol) in 15 mL of acetonitrile

was reﬂuxed for 3 h. The solvent was reduced to 4 mL, and the dark brown precipitate was ﬁltered oﬀ

and washed thrice with 10 mL (each) of diethyl ether to give 11. Yield: 22 mg (0.016 mmol, 12% based

on [Fe(acac)₃]).

Anal. Calcd. for C₆₉H₅₇Fe₇O₆(M = 1373.10 g

·

mol⁻¹): C 60.36, H 4.18; found: C 60.17, H 4.60. Mp.:

≥

230 ^◦C decomposition. IR (KBr, in cm⁻¹): 3094 (m, ν_C–H), 1536 (s, ν_C=O), 1519 (s, ν_C=O), 1462 (m),

1439 (m), 1404 (m), 1370 (m), 1350 (m), 1327 (m), 1300 (m), 1242 (m), 1210 (w), 1106 (m), 1024 (m), 947

(m), 820 (m, π_C–H, C₅H₅), 790 (m), 688 (m), 565 (m, C₅H₄ring tilt).

4.11. Synthesis of bis-(1,3-Diferrocenylpropane-1,3-dionato-κ²-O,O⁰)diﬂuoro-borane(III) (12)

To a solution of

5

(300 mg, 0.68 mmol) and 0.12 mL of diisopropyl amine (0.82 mmol) in 10 mL

.

of dichloromethane was dropwise added 0.1 mL of [BF₃OEt₂] (0.82 mmol) via a syringe at ambient

temperature. The color changed immediately to purple. After 1 h of stirring at this temperature,

all volatiles were removed under reduced pressure, and the crude product was puriﬁed by column

chromatography (column size: 20

contained 12. Crystallization of 12 from dichloromethane at ambient temperature gave dark purple

crystals. Yield: 36 mg (0.074 mmol, 11% based on 5).

×

3 cm, silica) using dichloromethane as eluent. The 1st fraction

Anal. Calcd. for C₂₃H₁₉Fe₂O₂BF₂(M = 487.89 g

·

mol⁻¹): C 56.62, H 3.93; found: C 56.98, H 4.12. Mp.:

≥

280 ^◦C decomposition. IR (KBr, in cm⁻¹): 3110 (w, ν_C–H), 2925 (w), 2852 (w), 1541 (s, ν_C=O), 1497 (s,

ν_C=O), 1456 (m), 1385 (m), 1369 (m), 1352 (m), 1322 (m), 1261 (m), 1218 (w), 1185 (w), 1146 (m), 1026 (m),

1107 (m), 1085 (m), 1062 (m), 1030 (m), 1019 (m), 916 (w), 823 (m, π_C–H, C₅H₅), 807 (m), 739 (m), 685 (m),

1

510 (m, C₅H₄ring tilt). H NMR (CDCl₃,

δ

in ppm): 4.29 (s, 10H, C₅H₅), 4.76 (pt, ³J_H,H= 1.9 Hz, 4H,

C₅H₄), 4.99 (pt, ³J_H,H= 1.9 Hz, 4H, C₅H₄), 6.10 (s, 1H, COCHCO). ¹³C{¹H} NMR (CDCl₃,

δ

in ppm):

in

69.8 (C₅H₄), 71.5 (C₅H₅), 74.5 (C₅H₄), 74.6 (ⁱC₅H₄), 93.3 (CH), 185.5 (C=O). ¹¹B{¹H} NMR (CDCl₃,

δ

ppm): 0.94 ppm.

Crystal data for 12: C₂₃H₁₉BF₂Fe₂O₂, M = 487.89 g mol⁻¹, monoclinic, P2₁/c,

a = 12.0629(6) Å, b = 13.4134(5) Å, c = 12.3500(5) Å,

= 104.741(4)^◦, V = 1932.51(15) Å³, Z = 4,

ρ_calcd= 1.677 Mg m⁻³= 1.538 mm⁻¹, T = 112.8(5) K, range 3.037–24.998^◦, 7728 reﬂections collected,

λ = 0.71073 Å,

β

,

µ

θ

3393 independent reﬂections (R_int= 0.0344), R1 = 0.0334, wR2 = 0.0779 (I > 2σ(I)).

4.12. Synthesis of 3,5-Diferrocenyl-1H-pyrazole (13)

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 MgSO₄, and then, all volatiles were removed in vacuum. The crude product was adsorbed on silica

and purified by column chromatography (column size: 20

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).

×

3 cm, silica) using dichloromethane as eluent.

Anal. Calcd. for C₂₃H₂₀Fe₂N₂(M = 436.11 g

mol⁻¹): C 63.34, H 4.62, N 6.42; found: C 63.98, H 4.12, N

·

6.42. Mp.:

≥

270 ^◦C decomposition. IR (KBr, in cm⁻¹): 3392 (m, ν_N−H), 3213 (m, ν_C–H), 3262 (m, ν_C–H),

3093 (s, ν_C–H), 3042 (m, ν_C–H), 2926 (s), 2869 (m), 2760 (m), 1735 (w), 1637 (w), 1600 (s, ν_C=N), 1550 (m),

1454 (w), 1414 (s, δ_C-H), 1370 (w), 1280 (m), 1237 (w), 1168 (m), 1105 (s, ν_C-N), 1085 (w), 1024 (m), 1001

(m), 980 (m), 879 (m), 813 (s, π_C–H, C₅H₅), 713 (w), 538 (w), 510 (s, C₅H₄ring tilt). H NMR (CDCl₃,

1

δ

in ppm): 4.11 (s, 10H, C₅H₅), 4.31 (broad signal, 4H, C₅H₄), 4.66 (broad signal, 4H, C₅H₄), 6.33 (s,

1H, CH). ¹³C{¹H} NMR (CDCl₃,

in ppm): 66.6 (C₅H₄), 68.9 (C₅H₄), 69.7 (C₅H₅), 100.2 (CH). HRMS

(ESI-TOF, m/z): calcd. for C₂₃H₂₀Fe₂N₂: 436.0320; found 436.0344 [M]⁺.

δ

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Crystal data for 13: C₉₂H₈₂Fe₈N₈O, M = 1762.45 g mol⁻¹, triclinic, P

−

1,

λ

= 0.71073 Å, a = 12.7706(4)

Å, b = 13.1931(5) Å, c = 25.2510(8) Å,

α β γ

= 94.195(3)^◦, = 90.511(2)^◦, = 117.837(3)^◦, V = 3747.5(2) Å³,

Z = 2 θ

,

ρ_calcd= 1.562 Mg m⁻³= 1.559 mm⁻¹, T = 114.8(3) K, range 3.339–24.996^◦, 28,847 reﬂections

,

µ

collected, 13,135 independent reﬂections (R_int= 0.0392), R1 = 0.0401, wR2 = 0.0852 (I > 2σ(I)).

4.13. Synthesis of 1-Methyl-3,5-diferrocenyl-1H-pyrazole (14)

3,5-Diferrocenyl-1H-pyrazole (13) (64 mg, 0.15 mmol) dissolved in 5 mL of NⁱPr₂Et was stirred

with two drops of nitrobenzene and 35 equiv. of dimethyl sulfate (0.5 mL) for 12 h at 70 ^◦C. After

cooling the reaction solution to ambient temperature and removing all volatile materials, the crude

product was adsorbed on silica and puriﬁed by column chromatography (column size: 20

×

3 cm,

silica) using a hexane–dichloromethane mixture of ratio 1:1 (v/v) as eluent. The 1st fraction contained

the title complex. After removing all volatiles under reduced pressure, compound 14 was obtained as

a yellow solid. Yield: 51 mg (0.11 mmol, 76% based on 13).

C₂₄H₂₂Fe₂N₂(M = 450.13 g

3176 (s, ν_C–H), 3262 (s, ν_C–H), 3094 (s, ν_C–H), 2927 (s), 2855 (m), 1735 (w), 1654 (w), 1595 (m, ν_C=N),

1550 (m), 1457 (w), 1412 (m, δ_C-H), 1368 (w), 1274 (m), 1238 (w), 1171 (w), 1104 (s, ν_C-N), 1085 (w), 1039

·

mol⁻¹). Mp.:

≥

230 ^◦C decomposition. IR (KBr, in cm⁻¹): 3223 (s, ν_C–H),

(m), 1030 (s), 1004 (m), 975 (w), 879 (s), 828 (m), 814 (s, π_C–H, C₅H₅), 701 (w), 537 (w), 512 (s, C₅H₄

1

ring tilt). H NMR (CDCl₃,

δ in ppm): 3.97 (s, 3H, CH₃), 4.10 (s, 5H, C₅H₅), 4.18 (s, 5H, C₅H₅), 4.27

(pt, ³J_H,H= 1.8 Hz, 2H, C₅H₄), 4.36 (pt, ³J_H,H= 1.9 Hz, 2H, C₅H₄), 4.53 (pt, ³J_H,H= 1.9 Hz, 2H, C₅H₄),

4.68 (pt, ³J_H,H= 1.8 Hz, 2H, C₅H₄), 6.32 (s, 1H, CH). HRMS (ESI-TOF, m/z): calcd. for C₂₄H₂₂Fe₂N₂:

450.0477; found 450.0478 [M]⁺.

4.14. Synthesis of 1-Phenyl-3,5-diferrocenyl-1H-pyrazole (15)

A mixture of compound

5 (200 mg, 0.454 mmol) and 4.4 equiv. of phenylhydrazine (0.2 mL)

in 10 mL of acetic acid was stirred at 70 ^◦C for 24 h. After cooling the reaction solution to ambient

temperature, it was poured into water and neutralized with a 1 M solution of NaOH. The solution was

extracted thrice with 40 mL of dichloromethane (each), the combined organic phases were dried over

MgSO₄, and then all volatiles were removed in vacuum. The crude product was adsorbed on silica and

puriﬁed by column chromatography (column size: 20

The 2nd fraction contained the title compound. After removing all volatiles under reduced pressure,

×

3 cm, silica) using dichloromethane as eluent.

compound 15 was obtained as an orange solid. Orange crystals of 15 were formed by crystallization of

15 from a hexane–dichloromethane mixture of ratio 1:10 (v/v) at

based on 5).

−

20 ^◦C. Yield: 63 mg (0.123 mmol, 27%

Anal. Calcd. for C₂₉H₂₄Fe₂N₂(M = 512.20 g

mol⁻¹): C 68.00, H 4.72, N 5.47; found: C 67.60, H 4.69,

·

5.37. Mp.: 158^◦. IR (KBr, in cm⁻¹): 3103 (m, ν_C–H), 3095 (m, ν_C–H), 3085 (s, ν_C–H), 1593 (m, ν_C=N), 1573

(m), 1543 (m), 1498 (s), 1449 (m), 1417 (s, δ_C-H), 1393 (m), 1364 (m), 1352 (m), 1323 (m), 1226 (w), 1210

(w), 1180 (m), 1106 (s, ν_C-N), 1095 (w), 1074 (m), 1028 (m), 999 (m), 980 (m), 969 (m), 911 (m), 882 (m),

873 (m), 829 (s), 816 (s, π_C–H, C₅H₅), 799 (s), 759 (s), 697 (m), 684 (m), 650 (m), 579 (m), 528 (m), 513 (s,

1

C₅H₄ring tilt). H NMR (CDCl₃,

δ

in ppm): 4.09 (s, 5H, C₅H₅), 4.14 (s, 5H, C₅H₅), 4.18 (broad signal,

2H, C₅H₄), 4.20 (broad signal, 2H, C₅H₄), 4.30 (broad signal, 2H, C₅H₄), 4.77 (broad signal, 2H, C₅H₄),

6.55 (s, 1H, CH), 7.40 (m, 5H, C₆H₅). ¹³C{¹H} NMR (CDCl₃,

δ in ppm): 66.9 (C₅H₄), 68.7 (C₅H₄), 68.7

(C₅H₄), 69.7 (C₅H₄), 70.0 (C₅H₅), 75.1 (ⁱC-C₅H₄), 78.5 (ⁱC-C₅H₄), 104.3 (CH-C₃N₂), 126.6 (C₆H₅), 128.1

(C₆H₅), 129.0 (C₆H₅), 140.7 (ⁱC C₆H₅), 142.5 (ⁱC-C₃N₂), 151.2 (ⁱC-C₃N₂). HRMS (ESI-TOF, m/z): calcd.

for C₂₉H₂₄Fe₂N₂: 512.0634; found 512.0628 [M]⁺.

Crystal data for 15: C₂₉H₂₄Fe₂N₂, M = 512.20 g mol⁻¹, monoclinic, P2₁/n,

Å, b = 9.8440(3) Å, c = 17.3856(6) Å,

= 90.168(3)^◦, V = 2187.43(12) Å³, Z = 4,

µ = 1.347 mm⁻¹, T = 113.2(4) K, range 3.126–24.998^◦, 14328 reﬂections collected, 3843 independent

reﬂections (R_int= 0.0365), R1 = 0.0444, wR2 = 0.1175 (I > 2σ(I)).

λ

= 0.71073 Å, a = 12.7813(4)

_calcd= 1.555 Mg m⁻³

,

β

ρ

θ

Molecules 2020, 25, 4476

20 of 27

Supplementary Materials: The Supplementary Materials are available online. Figures, Tables, and CIF ﬁles

giving further (spectro)electrochemical spectra, NMR spectra, and crystallographic data. Crystallographic data of

5

,

7

,

9

,

12

,

13

, 15, and 16 are also available from the Cambridge Crystallographic Database as ﬁle numbers CCDC

1548123 (

5

), 1548124 (

7), 1548125 (

9

), 1548126 (12), 1548127 (13₄· · · H₂O), 1548128 (15), 1548129 (16). Figure S1:

Ball-and-Stick model of the molecular structure of 13 showing the intramolecular T-shaped

π

interactions (green).

Geometric properties (Å/^◦): Ct_C4–C8· · · Ct_C55–C59, d = 4.637(8),

α

= 85.6(6); Ct_C37–C41· · · Ct_C78–C82, d = 4.736(7),

α = 87.8(6). Figure S2: Cyclic voltammogram (solid line: scan rate 100 mV s⁻¹) and square-wave voltammograms

(dotted line: step-height 25 mV, pulse-width 5 s, amplitude 5 mV) of 16 in dichloromethane solution (1.0 mmol

at 25 ^◦C measured with a glassy carbon working electrode. Supporting electrolyte 0.1 mol ⁻¹of [NBu₄][B(C₆F₅)₄]).

Figure S3: Cyclic voltammogram (scan rate 100 mV s⁻¹) of 11 in dichloromethane solution (1.0 mmol ⁻¹) at

25 ^◦C measured with a glassy carbon working electrode. Supporting electrolyte 0.1 mol L⁻¹of [NBu₄][B(C₆F₅)₄]).

* Impurities in the electrolyte. Figure S4: UV-Vis/NIR spectra of in a dichloromethane solution (2.0 mmol

⁻¹) at

rising potentials vs. Ag/AgCl at 25 ^◦C; supporting electrolyte 0.1 mol L⁻¹of [NBu₄][B(C₆F₅)₄]. Arrows indicate

in a dichloromethane solution

·

L⁻¹

)

·

L

·

L

·

5

·

L

·

an increase, decrease or shift of absorptions. Figure S5: UV-Vis/NIR spectra of

6

(2.0 mmol·L⁻¹) at rising potentials vs. Ag/AgCl at 25 ^◦C; supporting electrolyte 0.1 mol·L⁻¹of [NBu₄][B(C₆F₅)₄].

Arrows indicate an increase, decrease or shift of absorptions. Figure S6: Deconvolution of NIR absorption of [12]+

using Gaussian shaped bands. Table S1: Geometric properties (Å/^◦) of the ferrocenyls in

5,

7,

9,

12, 13 and 15.

Table S2: Plane Intersections (^◦) of C₅H₄, C₃and phenyl motifs in β-diketones, pyrazols and phenyls.

Author Contributions: Conceptualization, S.W.L., A.H., J.C.S. and H.L.; Synthetic work, S.W.L.; X-Ray

measurements, M.K.; Formal Analysis and Investigation, S.W.L., A.H., M.K.; Writing—Original Draft Preparation,

S.W.L., M.K., J.M., A.H., and H.L.; Writing—Review & Editing, M.K., J.M. and H.L.; Supervision, H.L.; Funding

Acquisition, H.L. All authors have read and agreed to the published version of the manuscript.

Funding: This research received no external funding.

Conﬂicts of Interest: The authors declare no conﬂict of interest.

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Sample Availability: Samples of the compounds are not available from the authors.

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Synthesis, characterization, and electrochemistry of diferrocenyl β-diketones, -diketonates, and pyrazoles

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