Synthetic, spectroscopic, structural, and electrochemical investigations of ferricenium derivatives with weakly coordinating anions: ion pairing, substituent, and solvent effects

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Synthetic, spectroscopic, structural, and

electrochemical investigations of ferricenium

derivatives with weakly coordinating anions: ion

pairing, substituent, and solvent eﬀects†

Cite this: DOI: 10.1039/d1dt01192h

Firoz Shah Tuglak Khan, ‡ Amy L. Waldbusser, ‡ Maria C. Carrasco,

Hadi Pourhadi

and Shabnam Hematian

*

A facile and eﬀective strategy for the preparation of a series of ferricenium complexes bearing either elec-

tron-donating or electron-withdrawing substituents with weakly coordinating anions such as [B(C₆F₅)₄]⁻

−

or SbF₆is reported. These systems were thoroughly investigated for their ground state electronic struc-

tures in both solution and solid states using infrared (IR) and nuclear magnetic resonance (NMR) spectro-

scopies as well as single crystal X-ray crystallography and electrochemical measurements. The X-ray

structures of the six electron-deﬁcient ferricenium derivatives are of particular interest as only a handful

(∼5) of such derivatives have been structurally characterized to date. Comparison of the structural data for

both neutral and oxidized derivatives reveals that the nature of the substituents on the cyclopentadienyl

(Cp) ligands displays a more signiﬁcant impact on the metal–ligand separations (Fe⋯Ct) in the oxidized

species than in their neutral analogs. Our ¹H-NMR measurements corroborate that in the neutral ferro-

cene derivatives, electron-donating ring substitutions lead to a greater shielding of the ring protons while

electron-withdrawing groups via induction deshield the nearby ring protons. However, the data for the

paramagnetic ferricenium derivatives reveals that this substitutional behavior is more complex and funda-

mentally reversed, which is further supported by our structural studies. We ascribe this reversal of behavior

in the ferricenium derivatives to the δ back-donation from the iron atom into the Cp rings which can lead

to the overall shielding of the ring protons. Interestingly, our NMR results for the electron-deﬁcient ferri-

cenium derivatives in solution also indicate a direct correlation between the solvent dielectric constant

and the energy barrier for rotation around the metal–ligand bond in these systems, whereas such a corre-

lation is absent or not signiﬁcant in the case of the electron-rich ferricenium species or the corresponding

neutral ferrocene analogs. In this work, we also present the electrochemical behavior of the corres-

ponding ferricenium/ferrocene redox couples including potential values (E_1/2), peak-to-peak separation

(ΔE_1/2), and diﬀusion coeﬃcients (D) of the redox active species in order to provide a concise outline of

these data in one place. Our electrochemical studies involved three diﬀerent solvents and two supporting

electrolytes. Notably, our ﬁndings point to the signiﬁcant eﬀect of ion-pairing in lowering the energy

necessary for reduction of the ferricenium ion and E_1/2in lower-polarity media. This has signiﬁcant impli-

cations in applications of the ferrocene or ferricenium derivatives as redox agents in low-polarity solvents

where an accurate determination of redox potential is critical.

Received 11th April 2021,

Accepted 27th April 2021

DOI: 10.1039/d1dt01192h

rsc.li/dalton

Introduction

Bis(η⁵-cyclopentadienyl) iron(II), more commonly known as

ferrocene (Fc), was discovered in the mid-20^thcentury.¹Ever

since, this fascinating sandwich complex has been widely

studied in terms of its remarkable structure, chemical

bonding and reactivity and it is often considered to be a show-

piece of modern organometallic chemistry.²Ferrocene and its

numerous derivatives have increasingly found use in catalysis,

particularly those involving asymmetric and stereoselective

Department of Chemistry and Biochemistry, University of North Carolina at

Greensboro, Greensboro, NC 27402, USA. E-mail: s_hemati@uncg.edu

†Electronic supplementary information (ESI) available: Details concerning spec-

troscopy including IR and NMR as well as electrochemical data and crystallo-

graphic details. CCDC 2045745–2045755. For ESI and crystallographic data in

CIF or other electronic format see DOI: 10.1039/d1dt01192h

‡These authors contributed equally.

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transformations, as well as in the development of new func-

In the present study, we report a concise and straight-

tional materials such as optical and redox sensors, batteries, and forward strategy for preparation of a series of ferricenium com-

bioconjugates for medicinal and biotechnological applications.³

plexes with electron-donating and electron-withdrawing substi-

−

Ferrocene can undergo a chemically reversible, outer- tuents with either BArF₂₀or SbF₆as a counterion. Nine new

sphere one-electron oxidation to generate bis(η⁵-cyclopentadie- ferricenium derivatives are readily isolated in high yields and

nyl) iron(+1), also known as the ferricenium ion (Fc⁺).⁴This all of them show exceptional solubility in lower-polarity sol-

metal-based outer–sphere electron transfer in the ferricenium vents (e.g., tetrahydrofuran (THF), diethyl ether, toluene and

ion/ferrocene couple is commonly employed as an internal or benzene) as compared to their common PF₆⁻or BF₄⁻salts that

external reference for electrochemical measurements in are only soluble in high-polarity organic solvents such as aceto-

organic solvents.⁵It is worth noting that, recently, the first nitrile, acetone, and alcohols. Scheme 1 shows the structures

examples of the two-electron oxidized as well as the one-elec- of ferricenium complexes described in this study.

tron reduced form of ferrocene derivatives have also been

structurally and spectroscopically characterized.⁶

With the exception of the parent ferricenium BArF₂₀(that

has previously been structurally characterized),¹⁷molecular

Ferricenium salts are generally prepared through the structures of all nine ferricenium derivatives used in this study

electrochemical or chemical oxidation of the ferrocene precur- were determined via single crystal X-ray crystallography. Other

sors. The first isolated ferricenium salt was the dark blue crys- than the structure of 1,1′-diacetylferricenium (^Ac2Fc⁺) that has

talline ferricenium tetrachlorogallate (Fc[GaCl₄]) reported by been previously reported as the N(SO₂CF₃)₂⁻, NTf₂⁻, salt,¹⁸the

Wilkinson in 1952.⁷Subsequently, a combination of theore- structures of five ferriceniums with electron-withdrawing

tical and experimental eﬀorts was focused on understanding substituents (i.e., ^BrFc⁺, ^AcFc⁺, ^BzFc⁺, ^Br2Fc⁺, and ^Bz2Fc⁺) are

the electronic structure and physicochemical properties of reported here for the first time.¹⁹These systems were further

ferrocene and the ferricenium ion.⁸

Ferricenium salts are considered mild one-electron outer-

sphere oxidants. The redox potential of the ferricenium salts

can be tuned by altering the nature of the substituents on the

cyclopentadienyl (Cp) rings. Thus, the ferricenium derivatives

with defined redox potentials are especially useful in both

redox catalysis and redox titrations or stoichiometric reactions

where selective oxidation under mild conditions is desired.⁹

Ferricenium salts are moderately stable in acidic aqueous solu-

tions but they rapidly decompose in many organic solvents

and in air.¹⁰The preparation of ferricenium derivatives, par-

ticularly those bearing electron-withdrawing substituents, is

relatively diﬃcult due their instability towards water, dioxygen,

and nucleophilic reagents.^8b,9,11The ferricenium ions can be

prepared with a variety of counter anions, such as tetra-fluoro-

borate (BF₄⁻), hexafluorophosphate (PF₆⁻), hexafluoroantimo-

nate (SbF₆⁻), or tetra-phenylborate (BPh₄⁻).⁹Ferricenium hexa-

fluorophosphate (Fc[PF₆]) and ferricenium tetra-fluoroborate

(Fc[BF₄]) are the only commercially available and the two most

commonly used ferricenium salts.¹²The nature of the counter

anions not only alters the magnetic moment of ferricenium

complexes, but also dictates their solubility in organic

solvents.^3e,8c,13

In more recent studies, fluoroarylborates such as tetrakis(-

pentafluorophenyl)borate ([B(C₆F₅)₄]⁻or BArF₂₀), or its aryl

CF₃-substituted analog tetrakis[3,5-bis(trifluoromethyl)phenyl]

borate (BArF₂₄), have been used as counter anions for the

preparation of ferricenium salts.¹⁴These bulky counter anions

with lower nucleophilicity exhibit remarkable chemical stabi-

lity and their weaker ion-pairing interactions with the ferrice-

nium ion result in a markedly increased solubility of the salts

in lower-polarity media.¹⁵These systems are particularly very

attractive one-electron chemical redox agents for synthesis, cat-

alysis, and kinetic and mechanistic investigations of a variety

of redox systems in lower-polarity solvents, particularly low-

temperature studies.^9,16

Scheme 1 Synthetic scheme for the preparation of the ferricenium

complexes described in this study. All ferricenium complexes were pre-

pared with BArF₂₀as the counter anion, except for ^Bz2Fc[SbF₆].

See the Experimental section for an alternative synthetic procedure for

^BzFc[B(C₆F₅)₄] and further details.

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studied for their ground state electronic structures using infra- main preparative procedures with diﬀerent oxidizing strengths

red (IR) and nuclear magnetic resonance (NMR) spectrosco- listed here:

pies. The latter revealed an interesting correlation between the

solvent dielectric constant and the energy barrier for rotation [Ag(MeCN)₄][B(C₆F₅)₄] in MeTHF.

1. Mild oxidizing condition (<100 mV vs. Fc^+/0):

around the Fe-Cp axis in the electron-deficient ferricenium

derivatives.

2. Strong oxidizing condition (100 to 400 mV vs. Fc^+/0):

[Ag(MeCN)₄][B(C₆F₅)₄] in DCM.

Herein, we also describe the redox behavior and potential

3. Very strong oxidizing condition (>400 mV vs. Fc^+/0):

values of the corresponding ferricenium/ferrocene (Fc^+/0) redox Ag[SbF₆] in DCM.

couples in a number of organic media in order to provide a

The solvent and counter anion dependencies of the oxidation

concise outline of these data in one place. Our electrochemical potential of silver(^I) were used to provide the mild, strong, and

analyses involved three diﬀerent solvents and two tetra-n-buty- very strong oxidizing environments in MeTHF or DCM

lammonium supporting electrolytes with a more traditional (Scheme 1). The redox potential for the Ag^+/0couple in non-

anion, PF₆⁻, as well as [B(C₆F₅)₄]⁻. The results point to the sig- aqueous solutions is not easily measured and generally estimated

nificant eﬀect of the ion-pairing in reducing the energy necess- values are available in the literature. For example, the formal

ary for reduction of the ferricenium ion and the overall E_1/2redox potential of the Ag^+/0couple in THF (E⁰′ = 410 mV vs. Fc^+/0

)

potential values.

is reported to be about 0.24 V more negative than that in DCM

(E⁰′ = 650 mV vs. Fc^+/0).^9,23Our results for [Ag(MeCN)₄][B(C₆F₅)₄],

already bearing the four coordinated acetonitrile in MeTHF

(168 < E°′ < 244 mV vs. Fc^+/0) and DCM (357 < E°′ < 533 mV vs.

Fc^+/0), are in agreement with the previous reports showing that,

with the increase in the coordination ability of the solvent the

Results and discussion

Synthesis and characterization of ferricenium derivatives

We used two silver(^I) salts as one-electron oxidizing agents for oxidizing strength of silver(^I) salts significantly decreases.

preparation of all of the ferricenium complexes.²⁰One is the

The redox potentials of the ferrocene derivatives measured

BArF₂₀analog of silver(I) which is a mild to strong oxidant in this study (vide infra) were used in the Nernst equation to

depending on the nature of the solvent. The synthesis of silver predict the position of the redox equilibriums in order to

complex, [Ag(MeCN)₄][B(C₆F₅)₄], was performed using a modi- assign the appropriate preparation procedures. The sub-stoi-

fied procedure of Zhang et al.²¹The silver(I) salt was readily chiometric oxidation of all ferrocene derivatives was accom-

prepared by metathesis of silver nitrate, AgNO₃, and the com- plished by limiting the amount of the appropriate silver(^I)

mercially available potassium tetrakis(pentafluorophenyl) agents to assure full consumption of the oxidants. The remain-

borate, K[B(C₆F₅)₄], in acetonitrile (MeCN). Additionally, the ing excess ferrocene complexes were simply removed by several

molecular structure of the silver complex and coordination of dry hexanes rinses.

four acetonitrile ligands were unambiguously confirmed by

¹H-NMR and IR spectroscopies as well as X-ray crystallography, tron-rich derivatives bearing one or more electron-donating

see Experimental section.

substituents, including n-butylferrocene (^nBuFc), 1,1′-dimethyl-

The IR spectrum of [Ag(MeCN)₄]⁺exhibits two bands in the ferrocene ^Me2Fc), and decamethylferrocene ^Me10Fc), was

Mild oxidation of the parent ferrocene and three of its elec-

(

CuN stretching region, at 2322 and 2295 cm⁻¹. The band at achieved with the addition of a sub-stoichiometric amount of

higher energy appears to arise from binary combination of the silver(^I) BArF₂₀salt in MeTHF solution. In turn, the sub-stoi-

symmetric methyl deformation at 1367 cm⁻¹and symmetric chiometric amount of silver(^I) BArF₂₀salt in DCM solution was

C–C stretch at around 950 cm⁻¹(Fig. S1†). The latter is buried employed for the oxidation of all three monosubstituted ferro-

under the counter anion signals and upon deuteration it shifts cene derivatives with electron-withdrawing groups, including

to 840 cm⁻¹. These two vibration modes are both of A₁sym- 1-bromoferrocene (^BrFc), 1-acetylferrocene (^AcFc), and 1-ben-

metry, thus forming a combination mode. In the deuterated zoylferrocene (^BzFc), as well as one 1,1′-disubstituted deriva-

complex, [Ag(CD₃CN)₄]⁺, the combination band is absent tive, 1,1′-dibromoferrocene (^Br2Fc).

which further supports this supposition that only the band at

The corresponding ferricenium BArF₂₀analogs were iso-

2295 cm⁻¹is due to the CuN stretching vibration. This band lated in high yields (>80%). Note: For both ketone-substituted

appears at slightly lower frequency (2287 cm⁻¹) in the deute- ferrocenes, the order of addition of reagents is reversed (i.e.,

rated complex (Fig. S2†). Additionally, as a result of complexa- the solution of substituted ferrocene is gradually added to the

tion, the CuN stretching frequencies are shifted to higher fre- silver(^I) solution), see Experimental section. This is due to the

quencies in [Ag(MeCN)₄]⁺(i.e., Δν_(CuN): +29 cm⁻¹) as com- ability of the acetyl or benzoyl groups on the Cp rings to coor-

pared to free acetonitrile (ν_free(CuN): 2266 cm⁻¹), which is well- dinate to the silver(^I) center which can hinder the electron

known for nitrile adducts.²²

transfer process from ferrocene to Ag(^I). This is consistent with

The other oxidizing agent is the silver(I) salt with the hexa- the considerably lower oxidizing ability of silver(^I) salts in

fluoroantimonate counter anion, Ag[SbF₆], which is commer- acetone (E°′ = 180 mV vs. Fc^+/0).⁹As an alternative procedure

cially available and was used as a very strong oxidizing agent. A for preparation of ^BzFc[B(C₆F₅)₄], a 1 : 1 solution of the 1-ben-

series of ferricenium BArF₂₀complexes with electron-donating zoylferrocene and K[B(C₆F₅)₄] can be added dropwise to a solu-

or electron-withdrawing groups were synthesized using three tion of Ag[SbF₆] in DCM.

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The other two highly electron-deficient derivatives, 1,1′-dia- earlier literature are not the starting ferrocene complexes.²⁵

cetylferrocene (^Ac2Fc) and 1,1′-dibenzoylferrocene (^Bz2Fc), were Carbon and hydrogen analyses of all nine ferricenium BArF₂₀

oxidized using Ag[SbF₆] in DCM. The stronger oxidizing complexes, as well as ^Bz2Fc[SbF₆], indicated that the com-

strength of this silver(^I) salt in DCM was confirmed through pounds were ≥99% pure, see Experimental section. Before we

its reactivity toward the tris (4-bromophenyl)amine (i.e., discuss further characterization of the ferricenium derivatives,

N(C₆H₄Br-4)₃). While [Ag(MeCN)₄][B(C₆F₅)₄] does not react it is helpful to point out the unique structural feature of the

with the amine in DCM, the addition of Ag[SbF₆] instantly oxi- ferricenium ion. Due to a very small rotational energy barrier,

dizes the triarylamine forming the signature blue color of the Fc⁺can adopt an eclipsed (D_5h) or staggered (D_5d) confor-

corresponding radical cation, known as Magic Blue (E°′ = mation or with the rings slightly twisted, it can even conform

700 mV vs. Fc^+/0). We also independently confirmed the redox to an intermediate skewed (D₅) geometry. We will expand on

potential of tris (4-bromophenyl)amine in DCM, in the pres- this point later in the discussion of the X-ray structures.

−

ence of PF₆(i.e., as a model for SbF₆⁻) and BArF₂₀, to be

The signature blue or green color (λ_max∼ 621–780 nm) of

the ferricenium complexes is present in all of our derivatives.

about 705 and 675 mV vs. Fc^+/0, respectively (Fig. S3†).

The two oxygen atoms of the ketone moieties of both ^Ac2Fc This is the ligand-to-metal charge transfer (LMCT) transition

and ^Bz2Fc can also chelate to the silver(I) center forming a red which is from the e_1uorbitals of Cp ligands to the hole in the

complex as observed previously for similar compounds.²⁴essentially non–bonding e_2gorbitals on the ferric center.^8c,d,26

Therefore, the order in which the reagents are added is critical The lowest energy absorption maxima and extinction coeﬃ-

to initiate the redox reaction. The isolated ^Ac2Fc[SbF₆] complex cient values for all ferricenium derivatives are provided in the

was then converted to the very soluble BArF₂₀analog through Experimental section.

metathesis by K[B(C₆F₅)₄] in 1,2-difluorobenzene. The ^Bz2Fc

[SbF₆] on the other hand was used as the [SbF₆]⁻salt due to its Fourier Transform Infrared (FT-IR) spectra of the ferrocene

satisfactory solubility and stability.

derivatives were obtained in the solid state except for ^nBuFc

Infrared spectroscopy. The attenuated total reflection (ATR)

Alternatively, the ^Ac2Fc[B(C₆F₅)₄] complex can be readily which is liquid at room temperature. The strongest fundamen-

obtained by dropwise addition of a 1 : 1 mixture of ^Ac2Fc and tal vibrations for the ferrocene derivatives appear around 815,

K[B(C₆F₅)₄] to a solution of Ag[SbF₆] in DCM. Adding the first 1000, 1410 and 3100 cm⁻¹which can respectively be ascribed

drop of the mixture leads to the development of a light pink to C–H out-of-plane bending, C–H in-plane bending, C–C

color indicating the transient formation of the ferrocene che- stretching and C–H stretching of the cyclopentadienyl

lated silver(^I) complex. This complex is then slowly oxidized by rings.^26c,27One binary combination band containing the C–H

the excess silver(^I) ions present in the solution to the green out-of-plane bending and C–H stretching is also observed at

^Ac2Fc⁺species. After this point, each additional drop of the around 3915 cm⁻¹. As expected, in ^Me10Fc spectrum the three

mixture leads to fast oxidation of the ^Ac2Fc and further appear- signature absorptions associated with the C–H bonds of the

ance of the green color. This suggests that the initially gener- Cp ligands, as well as the binary combination band, are

ated ferricenium species may act as an electron transfer absent. Tables S1, S2, and S3 in ESI† list the vibrational fre-

mediator/relay between the ferrocene-chelated silver(^I) complex quencies for all ferrocene and ferricenium derivatives.

and the excess silver(^I) pool allowing for the faster oxidation

process.

Aside from the counterion peaks (i.e., BArF₂₀or SbF₆⁻), the

infrared spectra of ferricenium derivatives noticeably have

Generally, removal of silver metal as the byproduct of the fewer strong bands than their neutral ferrocene counterparts

oxidation of the ferrocene complexes is straightforward. (Fig. S4–S13†). Due to the one-electron oxidation, the C–H

However, it is important to note that due to the reversibility of stretching frequencies of the Cp rings are shifted to higher

the ferricenium/ferrocene couple, the presence of a slight energies by about 30–40 cm⁻¹in all of the ferricenium

amount of silver metal impurity can result in partial re- derivatives.

reduction of the ferricenium sample upon dissolution in more

The carbonyl stretching modes of the ketone-substituted

coordinating solvents in which the silver salt is a weaker ferrocene species are located in the 1620–1650 cm⁻¹region

oxidant (i.e., the product distribution is governed by the and were assigned based on previous literature reports.^24a,28

Nernst equation). Since the BArF₂₀analogs of all these ferrice- The CvO stretching bands of all ketone-substituted

nium derivatives are highly soluble in either MeTHF or DCM, ferriceniums appear at about 35–48 cm⁻¹higher frequencies

the silver metal is a very finely divided precipitate and eﬀective when compared with the neutral counterparts, indicating a

filtration can be achieved through the use of a filtration aid significant strengthening of the carbonyl bond in the oxidized

such as Celite.

forms, see Table 1 and ESI.† This is in agreement with the

It is also worth mentioning that all our synthetic pro- shorter CvO bond distances (∼1.212 Å) obtained for the oxi-

cedures were performed under rigorous air-free conditions as dized complexes from our X-ray crystallography measurements

many of the erratic results reported in literature for many ferri- as compared to those reported for the neutral ketone-substi-

cenium complexes including their relatively low extinction tuted ferrocenes (∼1.224 Å), vide infra. The first overtone of the

coeﬃcients can be explained by the irreversible decomposition CvO stretching band is also observed in the 3295–3380 cm⁻¹

of ferricenium species in solutions exposed to air. It is con- region in both neutral and oxidized ketone-substituted

firmed that the yellow decomposition products reported in the species.²⁹

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Table 1 Comparison of CvO stretching frequencies and bond lengths

in ketone-substituted derivatives

donation from the iron d_x₂_−y₂and d_xyorbitals to the Cp ring

(i.e., the increase in bonding character of the e_2gmolecular

orbitals) is responsible for this reversal of behavior. In the sub-

stituted ferricenium, the ring protons of the Cp ligand with

the more stabilized orbitals experience more significant shield-

ing due to a better energy match (and overlap) with the iron

and stronger δ back-donation.

ν_(CvO)

1^stovertone ν_(CvO)

CvO

(Å)

Reference for

X-ray structure

Compound

(cm⁻¹

)

(cm⁻¹

)

^AcFc

1650

1698

1650

1697

1624

1659

1630

1665

3297

3378

3296

3376

3242

3308

3252

3315

1.223

1.209

1.224

1.209

1.225

1.215

1.222

1.215

30

This work

31

This work

32

This work

33

This work

^AcFc⁺

^Ac2Fc

^Ac2Fc⁺

^BzFc

The ¹H-NMR signal of the parent paramagnetic Fc⁺

complex (S = 1/2) appears as a single broad peak at 33.2 ppm,

which is shifted ∼29 ppm downfield relative to that of Fc

(Fig. S15†). Interestingly, the introduction of electron-donating

group(s) in the ferricenium species have a net deshielding

eﬀect on the ring protons, whereas electron-withdrawing sub-

stituents tend to shield the substituted ring protons. The

¹H-NMR spectrum of the electron-rich ^Me10Fc, with no Cp ring

protons, only displays a singlet at 1.66 ppm for the protons of

the methyl groups. In the paramagnetic ^Me10Fc⁺counterpart,

the singlet methyl proton signal shifts to a lower frequency

(δ = –37.6 ppm), see Fig. S16 and S17.†

^BzFc⁺

^Bz2Fc

^Bz2Fc⁺

NMR spectroscopy. All ferricenium species, like many other

paramagnetic sandwich complexes, are NMR-active owing to

their very short electron spin relaxation times, which is a con-

sequence of their doubly degenerate electronic ground state

2

(i.e., E_2g).³⁴Therefore, the solution structures of all ten ferri-

cenium derivatives presented in this work were conveniently

confirmed by ¹H- and ¹⁹F-NMR spectroscopies.

Mono- or 1,1′-di-substituted ferrocenes typically exhibit

NMR signals which can be readily assigned except for those

from protons in the 2,5- and 3,4-positions. The assignment of

these ring protons for some substituted ferrocenes have pre-

viously been achieved through specific deuteration, heteronuc-

lear diﬀerential nuclear Overhauser eﬀect diﬀerence (NOE)

spectroscopy, or ¹J(¹³C–¹³C) coupling measurements along

with selective proton decoupling.³⁵By analogy, we can assign

the 2,5- and 3,4-protons of all mono- and 1,1′-di-substituted

ferrocene species described in this study (Table 2). In deute-

rated acetone at room temperature, the resonances represent-

ing the protons in 2,5- and 3,4-positions in nearly all of our

substituted ferrocenes appear as a pair of apparent triplets

with the coupling constant, ³J(¹H–¹H), value of about 2 Hz.

Table S4† lists the coupling constants of the Cp protons for all

the substituted ferrocene species. Table 2 summarizes the

¹H-NMR chemical shifts for the cyclopentadienyl protons of

both ferrocene derivatives and their ferricenium counterparts.

For ^Me2Fc, the protons of two methyl substituents resonate

at 1.95 ppm and the lower symmetry of the substituted ring

system predictably leads to a pair of triplets for the Cp protons

(δ = 3.94 and 3.96 ppm; Fig. S18 and S19†). In ^nBuFc, the 2,5-

protons and, to a greater extent, the 3,4-protons experience the

shielding associated with the electron-donating substituent,^34a

resulting in an upfield shift of these substituted ring protons

relative to that of the unsubstituted Cp ring. The elongated

n-butyl chain gives rise to three separate (–CH₂–) resonances at

2.34 (t, 2H, a-H), 1.48 (m, 2H, b-H), and 1.34 (m, 2H, c-H) ppm

plus a triplet centered around 0.9 ppm for the three protons of

the CH₃group (Fig. S20 and S21†).

The ¹H-NMR spectrum of the parent diamagnetic Fc dis-

plays one sharp resonance at 4.2 ppm for the Cp rings

(Fig. S14†). The position and multiplicity of the signal for Cp

protons are sensitive to ring substitution. In the neutral ferro-

cene derivatives, electron-donating ring substitutions give rise

to a greater shielding of the ring protons (δ = 3.9–4.1 ppm)

while electron-withdrawing groups via induction deshield the

nearby Cp protons (δ = 4.3–4.9 ppm). This substitutional be-

havior is more complex and reversed in ferricenium derivatives

(Fig. 1). We propose that in the oxidized complexes, the δ back-

The presence of electron-withdrawing substituents such as

bromo, acetyl, or benzoyl groups on the cyclopentadienyl rings

of the neutral complexes leads to more pronounced changes in

chemical shifts between the 2,5- and the 3,4-protons of the

substituted Cp rings (Fig. 1 and S22–S29†). In these electron-

deficient systems, both resonances are shifted to a lower field

compared to the Cp resonance of the parent ferrocene. In the

Fig. 1 Part of the ¹H-NMR spectra of (left) the ferrocene derivatives and

(right) the ferricenium analogs in acetone-d₆at room temperature. Due

to reaction with acetone, the spectra of ^Ac2Fc⁺and ^Bz2Fc⁺were recorded

in CD₂Cl₂.

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a

Table 2 ¹H-NMR chemical shifts of Cp protons for the ferrocene and ferricenium derivatives in acetone-d₆or CD₂Cl₂

δ (ppm)

Compound

H_Cp

H_2,5

H_3,4

Compound

H_Cp

H_2,5

H_3,4

^Me2Fc

^nBuFc

Fc

—

4.08

4.20

4.23

4.24

—

3.96

4.06

—

4.45

4.78

4.87

4.47

4.81

4.98

3.94

4.01

—

4.17

4.52

4.65

4.27

4.58

4.68

^Me2Fc⁺

^nBuFc⁺

Fc⁺

—

31.3

33.2

34.0

36.3

35.4

—

34.8

37.4

—

28.8

27.5

27.6

29.6

30.5^b

30.2^b

31.6

33.9

—

32.0

31.8

31.3

34.2

^BrFc

^BrFc⁺

^AcFc

^BzFc

^Br2Fc

^Ac2Fc

^Bz2Fc

^AcFc⁺

^BzFc⁺

^Br2Fc⁺

^Ac2Fc^{+ a}

^Bz2Fc^{+ a}

—

Numbering scheme:

^bOnly one single broad resonance.

case of ^BrFc, the 2,5-proton signal appears at a lower field and

the 3,4-protons resonate at a slightly higher field compared to

the resonance for unsubstituted Cp ring.

For the oxidized species, in most cases, the diﬀerent Cp

protons can still be distinguished in spite of the broadened

resonances. For example, both 1,1′-disubstituted ^Me2Fc⁺and

^Br2Fc⁺complexes show two broad downfield Cp proton reso-

nances. This is in agreement with our X-ray crystallography

data that confirmed their “locked” eclipsed structures in the

solid state (vide infra) and supporting that this conformation is

retained in both solution and solid state.

In deuterated acetone, the 2,5- and 3,4-proton peaks for

^Me2Fc⁺appear at 31.6 and 34.8 ppm and for ^Br2Fc⁺at 29.6 and

34.2 ppm, respectively. Going from deuterated acetone to

DCM, the peak separation for ^Br2Fc⁺diminishes from 4.6 to

1.8 ppm (i.e., for ^Br2Fc⁺δ = 32.7 and 34.5 ppm in CD₂Cl₂;

ΔΔδ = –2.8 ppm) while the peak separation for ^Me2Fc⁺remains

essentially the same (i.e., for ^Me2Fc⁺δ = 32.5 and 35.8 ppm in

CD₂Cl₂; ΔΔδ = 0.1 ppm), see Fig. 2 and Fig. S30–S33.† Here,

the lower dielectric constant of the media results in lowering

the rotational barrier of the substituted rings in ^Br2Fc⁺to some

degree and not in ^Me2Fc⁺. This may be due to the diﬀerent

nature of the bonding and overall spin density delocalization

in these ferricenium complexes. The methyl protons of ^Me2Fc⁺

resonate in the upfield region as a rather sharp singlet, i.e., δ =

–10.5 ppm in acetone-d₆and δ = –9.0 ppm in CD₂Cl₂.

Fig. 2 Part of the ¹H-NMR spectra of two electron-rich ferricenium

derivatives (left; i.e., ^Me2Fc⁺and ^nBuFc⁺) and two electron-deﬁcient

analogs (right; i.e., ^Br2Fc⁺and ^BzFc⁺) recorded in deuterated acetone (ε ≈

21) vs. dichloromethane (ε ≈ 9) at room temperature. The behaviors of

the 2,5- and 3,4-protons of the substituted Cp ligand are compared.

On the other hand, in the case of the 1,1′-diketone-substi-

tuted ferricenium species such as ^Ac2Fc⁺and ^Bz2Fc⁺, only one

broad Cp proton resonance is observed in CD₂Cl₂(Fig. 1, S34

and S35†). Apparently, the ketone-substituted Cp rings in

these complexes have lower rotational barriers and rotation

rates are suﬃciently high, exceeding the NMR time scale and

preventing the observation of separate resonances by the 2,5- barrier in acetone-d₆and the reduced rotation rates lie within

and 3,4-protons. Our X-ray crystallography data for the oxi- the timescale of NMR measurements. For the electron-rich

dized forms of these complexes also support a less restricted ^nBuFc⁺, the unsubstituted Cp ring resonance moves upfield up

rotation around the Fe-Cp axis in these systems, vide infra.

to 1.9 ppm from that of the parent ferrocene Cp protons. In

In all four monosubstituted ferricenium species the 2,5- turn, the 2,5-protons of the n-butyl-substituted ring and, to a

and 3,4-protons of the substituted ring along with the unsub- lesser extent, the 3,4-protons are deshielded relative to that of

stituted Cp protons resolve into three broad peaks. Here, the the parent Fc⁺protons (Table 2, Fig. S36 and S37†). The

Cp ring carrying the substituent faces a larger rotational protons of the n-butyl substituent resonate in the upfield

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region as four separate signals at −6.8 (2H, a-H), −18.3 (2H, the electrolyte and its structure was determined via single

b-H), 1.2 (2H, c-H), and −1.0 (3H, d-H) ppm. crystal X-ray crystallography at 100 K (Table S5†). The electro-

On passing from the electron-rich monosubstituted ferrice- lyte, which was previously reported by Bolte and coworkers at

nium ion to electron-deficient monosubstituted ferricenium 173 K,³⁷crystallizes in the monoclinic space group Cc, with

species bearing a bromo, acetyl, or benzoyl group, the unsub- one tetra-n-butylammonium moiety paired with one BArF₂₀

stituted Cp ring signal shifts to

a

lower field (Δδ

=

anion per asymmetric unit (Fig. S60†).

0.8–3.1 ppm) in reference to the Cp ring signal of the parent

The crystals of [Ag(MeCN)₄][B(C₆F₅)₄] were grown by

ferricenium complex (Fig. S38–S43†). Additionally, the protons keeping a saturated solution of the complex in acetonitrile at

of the 2,5-positions and to a lesser degree, the 3,4-positions of −35 °C. Although the X-ray structure of this complex was

the substituted Cp ring become increasingly shielded with the recently reported,³⁸we were able to obtain higher quality data

increasing electronegativity of the substituent and resonate at (Table S5†). The X-ray structure clearly shows the ligation of

a higher field relative to that of the parent Fc⁺protons. Here four acetonitrile molecules to the silver(^I) center in a pseudo-

again, in CD₂Cl₂, the rotational barrier of the substituted rings tetrahedral fashion (i.e., ∠N–Ag–N is in the range of 91 to

decreases more significantly in the electron-deficient ferrice- 136°) with one slightly bent acetonitrile ligand while BArF₂₀

nium species (i.e., ^BrFc⁺, ^AcFc⁺, and ^BzFc⁺) than in the electron- remains in the crystal lattice as the counter anion, residing

rich system such as ^nBuFc⁺, and the faster ring rotation results near the largest N–Ag–N angle in the silver complex (Fig. S61†).

in much more severe broadening and overlap of the 2,5- and

3,4-protons of the substituted Cp ligand, Fig. 2.

Ferricenium derivatives. Molecular structures of all the ferri-

cenium species used in this study were determined via single

It is also worth noting that the peak separations for the 2,5- crystal X-ray crystallography, except for the parent ferricenium

and the 3,4-protons of the substituted Cp rings in both higher BArF₂₀that has been previously structurally characterized.¹⁷

and lower polarity organic solvents (e.g., acetone-d₆vs. CDCl₃) Suitable crystals for X-ray structure determination were grown

stay the same across the series of neutral ferrocene derivatives in the glovebox, through the slow diﬀusion of hexanes into

discussed in this study. This points to the significant diﬀer- either MeTHF or DCM solutions of the ferricenium derivatives

ence in bonding and electronic structures of the one electron in 5 mm glass tubes at room temperature. Details of the data

oxidized and neutral species.

collection and refinement parameters as well as selected struc-

The ¹⁹F-NMR spectra of the paramagnetic ferricenium salts tural parameters are listed in Tables S6–S8 and S9–S12,†

were also recorded. The BArF₂₀anion of all ferricenium respectively. All the complexes contain [B(C₆F₅)₄]⁻as the coun-

species in deuterated acetone gives rise to three ¹⁹F-NMR reso- terion, except for ^Bz2Fc[SbF₆]. The corresponding molecular

nances at −133.0, −164.4, and −168.4 ppm for the ortho-, para- packing patterns are presented in Fig. S62–S70.†

, and meta-fluorines in a ratio of around 8 : 4 : 8, respectively.

As briefly discussed earlier, both ferrocene and ferricenium

In a lower polarity solvent such as CD₂Cl₂, these ¹⁹F-NMR derivatives possess a high degree of molecular flexibility,

signals are more shielded, and the largest shift is observed for adopting a variety of conformations. This conformation flexi-

the ortho-fluorines by no more than −2.4 ppm, see Fig. S44– bility is typically in response to diﬀerent electronic, steric, or

S58.† This confirms that although the ferricenium derivatives crystal packing forces in diﬀerent structures. In addition to

and BArF₂₀anion are unpaired in acetone, they are likely ion their highly flexible geometries, the ferrocene or ferricenium

paired in CD₂Cl₂. As previously described, the BArF₂₀anion derivatives, even symmetrically substituted ones, can become

lacks specificity in where to ion pair to the cation.^15b

chiral to some extent or exhibit conformational chirality.³⁹In

The ¹⁹F-NMR spectrum of ^Bz2Fc⁺displays one broad reso- the following, we analyze the overall conformational arrange-

−

nance at 133.2 ppm for the SbF₆counterion with a sextet ments of all ferricenium derivatives reported here according to

pattern (¹J(¹⁹F–¹²¹Sb) ∼2 kHz). Given that the two most abun- the four geometrical parameters shown in Fig. 3. We also

dant isotopes of antimony are both quadrupolar, ¹²¹Sb I = 5/2

−

and ¹²³Sb I = 7/2, broadening of the ¹⁹F-NMR signal in SbF₆

due to the quadrupolar relaxation is expected. In the presence

of Ag[SbF₆], the splitting pattern of ¹⁹F resonance disappears

giving a single broadened signal that is shifted to −134.7 ppm

which points to a fast mode of exchange in this system

(Fig. S59†).

X-Ray crystallography

A couple of decades ago, Geiger and coworkers first introduced

tetra-n-butylammonium BArF₂₀, [(nBu)₄N][B(C₆F₅)₄], as an

ideal non-coordinating supporting electrolyte for electro-

Fig. 3 Schematic representation of the geometrical parameters used to

describe the conformations of ferrocene and ferricenium derivatives. Ct

refers to the Cp ring centroid while Cs indicates the substituted C-atom

chemical studies in low-polarity solvents.³⁶Here, we describe

an alternative procedure for preparation of this electrolyte.

Diﬀraction quality crystals of [(nBu)₄N][B(C₆F₅)₄] were obtained

on the ring. In the case of monosubstituted structures, the second Cs is

by slow liquid diﬀusion of hexanes into the DCM solution of the closest C-atom on the unsubstituted ring.

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imposed by the substitution of all Cp protons by methyl

groups.

The dark blue single crystals of ^Me2FcBArF₂₀were obtained

in an orthorhombic system with the Pbcn space group. The

asymmetric unit contains three molecules and one out of the

three ^Me2Fc⁺entities is disordered over two diﬀerent orien-

tations and the occupancy factor of the major component

refines to 0.618(3). The methyl groups are closer to the

eclipsed conformation (ϕ = 13.6°, −14.9°, or −19.2) in the oxi-

dized complex and the torsion angle becomes even smaller on

going to the neutral analog, ^Me2Fc, (ϕ = –3.6°).³¹Here, an

increase of 0.06 Å in Fe–C_avgbond length is observed for

^Me2FcBArF₂₀relative to ^Me2Fc which is the largest elongation of

the Fe–C bonds upon oxidation observed in all the derivatives

discussed in this study. The separation of the Cp rings also

increases by 0.17 Å, in going from ^Me2Fc to ^Me2FcBArF₂₀.

Unsurprisingly, in the oxidized complex, ^Me2FcBArF₂₀, with the

greater Cp ring separation (i.e., 3.463 Å), both methyl groups

tend to come within the Cp plane with an average out-of-plane

displacement of 0.01 Å as compared to 0.06 Å in ^Me2Fc.

The peacock blue ^nBuFcBArF₂₀complex crystallizes in a tri-

Fig. 4 Perspective views of the electron-rich ferricenium derivatives at

100 K: (a) ^Me10FcBArF₂₀, (b) ^Me2FcBArF₂₀, and (c) ^nBuFcBArF₂₀showing

50% thermal contours for all non-hydrogen atoms. Hydrogen atoms

have been omitted for clarity.

ˉ

clinic crystal system with P1 space group. The asymmetric unit

contains two molecules and the n-butyl moieties of both ferri-

cenium entities are disordered over two diﬀerent orientations.

The occupancy factor of the major component in disorder is

0.552(9). The ^nBuFc complex is a brownish orange liquid at

room temperature and no crystal structure is available for this

compare these values with those of the corresponding ferro- neutral counterpart for comparison. Instead, we used the

cene counterparts (Tables S9 and S10†).

structural data reported for the two closely related ferrocene

Fig. 4 depicts the molecular structure of the ferricenium derivatives, n-tetradecylferrocene (^nC14Fc) and 1,8-bis(ferroce-

derivatives bearing electron-donating groups in the solid state. nyl)octane (Fc–(CH₂)₈–Fc).⁴⁰In the ^nBuFcBArF₂₀complex, the

The most electron-rich ferricenium complex described in this substituted and unsubstituted Cp rings are nearly eclipsed

work, ^Me10FcBArF₂₀, crystallizes as dark green single crystals in (ϕ = 7.9° or −1.2°) and the iron center is about 1.702 and

a P2₁/c space group and the crystallographic asymmetric unit 1.703 Å away from the centroids of the substituted and unsub-

contains two ferricenium entities and two BArF₂₀counterions. stituted Cp ligands, respectively. In the neutral analogs, the

The two entities exist in slightly diﬀerent relative confor- rings maintain the eclipsed structure (ϕ = –0.2° in ^nC14Fc and

mations. The Cp rings in one of the ferricenium centers are −7.8° in Fc–(CH₂)₈–Fc) and both metal–ligand separations are

almost perfectly staggered with a torsional angle of 35.9° reduced by about 0.5 Å (Fe⋯Ct _sub.= 1.648 Å and Fe⋯Ct _unsub.

between the two opposing methyl groups while the second fer- 1.650 Å). See Tables S9 and S10† for further structural details.

=

ricenium entity has the rings slightly skewed by an angle of

In moving on to the electron-deficient ferricenium deriva-

16.9°. Interestingly, in the case of the neutral ^Me10Fc, the Cp tives, we obtained molecular structures of three monosubsti-

rings only adopt absolute staggered orientation (ϕ = 36°).³¹tuted systems bearing a bromo, acetyl, or benzoyl substituent

The Fe–C_avgbond distance in ^Me10FcBArF₂₀was found to be on one of the Cp rings (Fig. 5). These are the first examples of

2.101 Å which is about 0.05 Å larger than that of ^Me10Fc X-ray crystal structures of the ferricenium derivatives with

(Table S9†). The distance between the iron center and Cp ring these electron-withdrawing substituents. To date, only a very

centroid (i.e., Fe⋯Ct_avg) is also about 0.06 Å longer in the ferri- limited number of structural data for electron-deficient ferrice-

cenium complex which highlights the small elongation of the nium species (∼5) are deposited in the Cambridge Structural

Fe–C bonds upon oxidation. A very similar trend is observed Database (CSD), which is most likely a reflection of challenges

for the neutral and oxidized states of the other ferrocene in their preparation due to the necessity of meticulous exclu-

derivatives (Fig. S71†). This constancy of Fe–C bond length in sion of potential reactive nucleophiles and reducing reagents

going from the neutral ferrocene state to the ferricenium state (i.e., air, moisture, coordinating and redox-active solvents and

in all derivatives has been ascribed to the very weakly bonding counterions).

nature of the e_2gorbitals.^8c,d,26aThe methyl substituents in

Our first monosubstituted ferricenium derivative of this

^Me10FcBArF₂₀are slightly out of the Cp planes and away from class is ^BrFcBArF₂₀(Fig. 5a) which was obtained from DCM/

the iron center (γ = 1.4°) which is also observed in the neutral hexanes. This complex crystallizes in a monoclinic crystal

analog, ^Me10Fc, (γ = 1.8°) pointing to the steric constraints system with P2₁/n space group and the asymmetric unit con-

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pared to the significantly smaller interplanar angle of 37.7°

(i.e., the benzyl moiety has an out-of-plane bend of 0.33 Å away

from the iron center) in the neutral ^BzFc counterpart.³²

Alternatively, this substantial diﬀerence in the orientation of

benzoyl groups may be due to changes in the molecular

packing of the neutral and oxidized forms.

The molecular structures of the three 1,1′-disubstituted fer-

ricenium derivatives bearing electron-withdrawing groups are

shown in Fig. 6. All these complexes crystallize in centro-

symmetric space groups and contain no more than one inde-

pendent molecule of the compound in the asymmetric unit.

The ^Br2FcBArF₂₀complex crystallizes in a monoclinic crystal

system with the P2₁/n space group.

The Cp rings in ^Br2Fc⁺are approximately eclipsed with a

torsion angle of about 3.5° between the two bromo substitu-

ents which is larger than that of ^Br2Fc (ϕ = 0.6°).⁴²Due to the

smaller torsion angle and shorter Ct⋯Ct distance (∼3.298 Å)

in the neutral analog, the two Br-atoms are forced out of the

Cp planes with out-of-plane displacement of 0.14 Å and 0.08 Å

(i.e., γ = 2.7 and 4° away from the ferrous center) and a Br⋯Br

non-bonding separation of 3.617 Å. By comparison, the

smaller torsion angle restraints and greater Ct⋯Ct distance

(∼3.405 Å) in ^Br2Fc⁺lead to a decrease of the out-of-plane dis-

placement of both Br-atoms to less than 0.02 Å (i.e., γ = 1.1°

and 1.3° toward the ferric center) and the Br⋯Br non-bonding

separation is about 3.773 Å.

Fig. 5 Perspective views of the electron-deﬁcient monosubstituted fer-

ricenium derivatives at 100 K: (a) ^BrFcBArF₂₀, (b) ^AcFcBArF₂₀, and (c)

^BzFcBArF₂₀showing 50% thermal contours for all non-hydrogen atoms.

Hydrogen atoms have been omitted for clarity.

tains one molecule. The Cp rings in ^BrFc⁺adopt an approxi-

Among all the derivatives discussed in this study, the

mately eclipsed conformation with a torsion angle of 9.1° dibromo substituted system, in either oxidized or neutral

which is in the range of those found for the neutral complex, form, exhibits the most significant metal-centered bending (α

^BrFc (ϕ = –2.6° or 28.6°).⁴¹In ^BrFc⁺, the Fe–C_avgand Fe⋯Ct dis-

tances are 2.089 and 1.706 Å for the bromosubstituted Cp ring

while 2.082 and 1.701 Å for the unsubstituted ring, respect-

ively. As expected, the neutral complex, ^BrFc, possesses smaller

Fe–C_avgand Fe⋯Ct distances. Here, the oxidation results in a

larger degree of elongation of the Fe⋯Ct distance for the sub-

stituted ring as compared to unsubstituted ring, while in the

electron-rich ^nBuFc⁺complex both substituted and unsubsti-

tuted rings experienced similar degrees of displacement upon

oxidation (i.e., Δ(Fe⋯Ct) ≈ 0.05 Å), vide supra.

Both electron-deficient mono-ketone-substituted ferrice-

nium complexes, ^AcFcBArF₂₀and ^BzFcBArF₂₀, crystallize in the

ˉ

triclinic crystal system with the P1 space group. Similarly, one

electron oxidation imposes larger separation (i.e., by ∼0.1 Å) of

the iron center from the substituted Cp ring as compared to

the unsubstituted ligand in these complexes. The Cp rings in

both ^AcFc⁺and ^AcFc are nearly eclipsed with the torsion angles

of about −3.4° and 0.6°, respectively.³⁰The acetyl group shows

a rotation of 180° around the C–C bond of the Cp ring and

acetyl group in going from the neutral to oxidized complex

while retaining a similar degree of out-of-plane displacement

in both forms.

Interestingly, the one-electron oxidation of ^BzFc also triggers

a significant rearrangement of the benzoyl substituent. In the

oxidized form, the carbonyl group of the benzoyl moiety bows

toward the iron center (i.e., an out-of-plane bend of 1.8 Å) with

Fig. 6 Perspective views of the electron-deﬁcient disubstituted

ferricenium derivatives at 100 K: (a) ^Br2FcBArF₂₀, (b) ^Ac2FcBArF₂₀, and (c)

^Bz2Fc[SbF₆] showing 50% thermal contours for all non-hydrogen atoms.

the phenyl group having an interplanar angle of 81.8°, as com- Hydrogen atoms have been omitted for clarity.

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= 177.0° in ^Br2Fc⁺and 177.7° in ^Br2Fc⁺). This is consistent with tive position of the Cp rings in a way that the interplanar angle

the overall trend observed in ferrocene derivatives highlighting between the Cp rings changes from 1.4° in ^Ac2Fc to 2.6° in

that the structures with eclipsed conformations reach con- ^Ac2FcBArF₂₀.

siderably larger bending angles.^39,43

As mentioned earlier, only a handful of ferricenium deriva-

The diﬀerence between structures of ^Ac2FcBArF₂₀and its tives with electron-withdrawing groups have been structurally

neutral analog, ^Ac2Fc is even more pronounced. The ^Ac2Fc⁺characterized to date. Among the electron-deficient derivatives

complex crystallizes in the monoclinic space group P2₁/c, with presented in this work, only the structure of ^Ac2Fc⁺as the

the Cp rings in staggered arrangements (ϕ = –26.4°) in contrast N(SO₂CF₃)₂⁻, NTf₂⁻, salt has been previously reported.¹⁸The

to the nearly eclipsed conformation observed for ^Ac2Fc (ϕ = Fe–C_avgand Ct⋯Ct distances in this ^Ac2FcNTf₂complex are

139.6°).³¹Here, upon one–electron oxidation, the two acetyl 2.093 and 3.416 Å, respectively, which are very similar to those

groups on the rings drastically move toward and pass each of ^Ac2FcBArF₂₀reported here. The major structural discrepancy

other (Δϕ = 166°). The average out–of–plane displacement of for these two complexes is found in the relative orientation of

the acetyl moieties in ^Ac2Fc is about 0.098 Å and it decreases to the acetyl substituents on the Cp rings. The torsion angle of

0.042 Å in ^Ac2FcBArF₂₀, which in turn slightly aﬀects the rela- 180.0° in ^Ac2FcNTf₂clearly indicates that acetyl groups lie in

perfectly opposite positions from each other, while the acetyl

groups in ^Ac2FcBArF₂₀are only 26.4° apart. Another interesting

observation is the rotation of the C–C bond between one of the

Cp rings and the attached acetyl group in ^Ac2FcNTf₂and both

in the case of ^Ac2FcBArF₂₀as compared to the structure of

neutral complex (Fig. 7).

The complex ^Bz2Fc[SbF₆] crystallizes in a triclinic crystal

ˉ

system with the P1 space group and the asymmetric unit con-

tains one half of the molecule where the Fe and Sb atoms are

located on the crystallographic inversion center. The torsion

angle between the two substituted Cp rings in ^Bz2Fc[SbF₆] is

180.0° which is significantly larger than that of the neutral

counterpart, ^Bz2Fc (ϕ = 130.4°).³³From the top view of the two

complexes shown in Fig. 8, it is clear that the Cp rings in ^Bz2Fc

Fig. 7 Top view of the capped stick representations of the molecular

structures of ^Ac2Fc,^{31 Ac2}FcNTf₂,¹⁸and ^Ac2FcBArF₂₀. The top and bottom

[SbF₆] are almost perfectly staggered whereas in the neutral

Cp rings are shown in black and grey, respectively. Counterions and

counterpart they adopt a close to eclipsed conformation. The

hydrogen atoms are omitted for clarity. Arrows represent the confor-

Fe–C_avgand Ct⋯Ct distances are about 2.042 and 3.296 Å in

mational rearrangements required for the oxidized complex to adopt a

similar conformation as the neutral complex.

^Bz2Fc while for the oxidized species, ^Bz2Fc[SbF₆], they increase

Fig. 8 Ball and stick representation of the top view of the molecular structures of ferrocene and corresponding ferricenium derivatives depicting

the changes in the ring stagger torsion angle that accompany the oxidation process. The ferric and ferrous centers are shown in green and pink,

respectively. Hydrogen atoms are omitted for clarity.

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to 2.089 and 3.410 Å, respectively. Since in the neutral and oxi- Electrochemical analyses

dized forms, the substituents are far apart from each other,

In order to further understand the redox behavior of the ferro-

cene and ferricenium derivatives, we conducted cyclic voltam-

metry measurements under 5 diﬀerent conditions of solvent

and supporting electrolyte. All electrochemical experiments

were performed in dry and oxygen-free MeCN, DCM,

the Cp rings are highly coplanar in both cases with interplanar

angles of 0.4° and 0.0°, respectively. We will return to this

point later in the electrochemistry discussion.

A comparison of the iron-ring centroid distances for both

ferrocene and ferricenium derivatives is shown in Fig. 9. As

discussed earlier, the removal of one electron from the metal

e_2gorbitals gives rise to a larger separation between the iron

and Cp ligands. This is consistent with the very weakly

bonding character of the e_2gorbitals. Additionally, the largest

Fe⋯Ct elongation (by ∼0.085 Å) is observed for the oxidation

of the dimethyl substituted system.

or MeTHF containing 0.1

M of one of two chosen

supporting electrolytes, i.e., tetra-n-butylammonium tetrakis(-

pentafluorophenyl)-borate, [(nBu)₄N][B(C₆F₅)₄], or a more tra-

ditional electrolyte, tetra-n-butylammonium hexafluoro-

phosphate, [(nBu)₄N][PF₆] (Fig. 10 and S72–S75†). The latter is

not soluble in MeTHF.

Prior to each cyclic voltammetry experiment, the uncom-

pensated solution resistance (R_u) of each solvent/electrolyte

combination was measured using potentiostatic electro-

chemical impedance spectroscopy (PEIS) at frequencies

ranging from 1 MHz to 100 mHz at open circuit potential. Our

results for five diﬀerent media followed the trends reported in

the literature,⁴⁴see ESI† for further details. In DCM, using

[(nBu)₄N][B(C₆F₅)₄] as the supporting electrolyte results in

lower solution resistance compared to [(nBu)₄N][PF₆], and an

opposite trend was observed in MeCN. Additionally, for a

0.1 M solution of tetra-n-butylammonium perchlorate at room

temperature, the specific resistance in MeCN (ρ = 132 Ω cm) is

shown to be significantly smaller than in DCM (ρ = 725 Ω

cm).⁴⁵Such data has not been reported for MeTHF, however,

the fact that the specific resistance value obtained in THF (ρ =

2670 Ω cm) is considerably larger than in DCM, infers that

MeTHF follows a similar trend. To avoid instabilities in the

potentiostat, the iR drop was corrected for only 85% of the

uncompensated solution resistance during the cyclic voltam-

metry measurements through positive feedback using the Bio-

Logic EC-Lab software. The half-wave potential, E_1/2, (V vs. Ag/

AgCl) and peak-to-peak separation, ΔE_1/2, of each ferricenium/

ferrocene couple in various media are listed in Table 3.

The oxidation of ferrocene leads to more significant short-

ening of the C–C bond lengths in the cyclopentadienyl ligands

in the parent ferricenium complex (i.e., ∼0.033 Å) relative to

those of their substituted analogs (i.e.,<0.015 Å). For example,

the C–C bond length stays nearly intact throughout the oxi-

dation of 1-benzoylferrocene, see Table S9† for details on the

average change in C–C bond length across all ferrocene and

ferricenium derivatives.

A closer look at this metal–ligand separation also reveals

that the nature of the substituents has a more significant

eﬀect on the Fe⋯Ct distances in the oxidized species (i.e.,

Δ(Fe⋯Ct) ≈ 0.03 Å) than in their neutral counterparts (i.e.,

Δ(Fe⋯Ct) ≈ 0.01 Å). Another interesting finding is that, in the

monosubstituted ferrocene derivatives bearing an electron-

withdrawing group (e.g., ^BrFc, ^AcFc, and ^BzFc), the distance

between the ferrous center and the unsubstituted ring is

slightly larger than that of the electron-deficient substituted

ring. Although smaller in magnitude, a reversal of behavior is

observed in the oxidized counterparts of these electron-

deficient monosubstituted systems. This reversal of behavior is

in excellent agreement with our NMR results (Fig. 1).

▲

Fig. 9 The separation of the iron and centroid (Fe⋯Ct) of the top ( ) or

○

bottom ( ) cyclopentadienyl ring is plotted for all of ferrocene and ferri-

cenium derivatives discussed in this study. *For the neutral form average

of the Fe⋯Ct distances for two closely related derivatives, ^nC14Fc and

Fc–(CH₂)₈–Fc, was used.

Fig. 10 Normalized cyclic voltammograms of ferrocene and its deriva-

tives in DCM with 100 mM of [(nBu)₄N][PF₆] at 100 mV·s⁻¹scan rate.

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Table 3 E_1/2and ΔE_1/2^avalues (V vs. Ag/AgCl) of various ferrocene derivatives in diﬀerent media

MeCN

DCM

MeTHF

[(nBu)₄N][PF₆]

[(nBu)₄N][B(C₆F₅)₄]

[(nBu)₄N][PF₆]

[(nBu)₄N][B(C₆F₅)₄]

E_1/2

ΔE_1/2

E_1/2

ΔE_1/2

E_1/2

ΔE_1/2

E_1/2

ΔE_1/2

E_1/2

ΔE_1/2

^Me10Fc

^Me2Fc

^nBuFc

Fc

−0.060

0.347

0.396

0.450

0.628

0.700

0.705

0.763

0.925

0.927

0.066

0.080

0.076

0.081

0.094

0.077

0.087

0.093

0.102

−0.068

0.341

0.386

0.451

0.630

0.689

0.697

0.751

0.930

0.903

0.076

0.093

0.100

0.089

0.092

0.086

0.094

0.122

0.129

0.100

0.014

0.462

0.523

0.550

0.725

0.803

0.810

0.887

1.020

1.070

0.186

0.174

0.203

0.217

0.215

0.158

0.185

0.166

0.203

0.157

0.028

0.513

0.538

0.577

0.747

0.861

0.878

0.934

1.110

1.230

0.156

0.164

0.142

0.163

0.175

0.130

0.133

0.150

0.236

0.047

0.460

0.515

0.589

0.757

0.833

0.806

0.900

1.037

1.003

0.134

0.170

0.160

0.183

0.162

0.132

0.150

0.165

0.125

0.124

^BrFc

^AcFc

^BzFc

^Br2Fc

^Ac2Fc

^Bz2Fc

^aThe values were obtained at 100 mV s⁻¹scan rate.

As expected, the incorporation of various electron-donating

or -withdrawing substituents on the Cp rings altered the redox

potential cathodically or anodically. Electron-rich systems with

one or more alkyl substituent(s) such as ^Me10Fc, ^Me2Fc, and

^nBuFc all possess redox potentials lower than that of Fc, while

having electron-withdrawing groups on the rings creates an

electron-deficient system such as ^BrFc, ^AcFc, ^BzFc, ^Br2Fc, ^Ac2Fc,

and ^Bz2Fc, hence increasing the E°′ values.

The cyclic voltammograms of all the derivatives in DCM

with the [(nBu)₄N][PF₆] supporting electrolyte are shown in

Fig. 10. For the cyclic voltammograms collected in the other

solvent/electrolyte combinations, see section 3 of the ESI.† The

neutral ferrocene derivatives are generally very soluble in the

three solvents chosen for this study, i.e., DCM, MeTHF, and

MeCN. Only ^Me10Fc has a limited (ca. 10⁻³M) solubility in

acetonitrile. For that reason, the cyclic voltammograms of all

ferrocene derivatives are plotted using the molar current

density (A M⁻¹cm⁻²) rather than the current alone. This was

done in order to compare the cyclic voltammetry measure-

ments independent of the ferrocene concentration and surface

area of the working electrode.

Fig. 11 The half-wave potential, E_1/2, of all ferrocene derivatives dis-

cussed in this study in MeTHF with 100 mM of [(nBu)₄N][B(C₆F₅)₄]

plotted vs. sum of the Hammett values, Σσ_p,m

.

benzoyl substituted ferrocenes are expected to be less electron-

The correlation between the redox potentials of the substi- deficient than the acetyl substituted analogs. This trend holds

tuted ferrocenes and the sum of Hammett substituent con- true in MeTHF but not in DCM, see Table 3.

stants in the MeTHF solution is shown in Fig. 11. Typically,

The one-electron transfer redox processes showed quasi-

the E_1/2data of substituted ferrocenes correlate linearly with reversible behavior with peak-to-peak separation values greater

the sum of the Hammett values, Σσ_p,m, which is a combination than 57 mV (i.e., ΔE ranging from 66 to 236 mV; see Table 3)

of para- and meta-substituents (i.e., σ_pand σ_m).

and anodic/cathodic peak current ratios between 0.96 and 1.09

For all of our mono- and 1,1′-di-substituted ferrocenes, only (Table S14†), except for the two 1,1′-diketone-substituted ferro-

σ_pwas taken into account. The σ_pvalues for methyl, n-butyl, cenes in MeCN (i_pa/i_pc= 1.04–1.24), vide infra. Our Randles–

bromo, benzoyl and acetyl groups are −0.17, −0.16, +0.23, Sevcik analysis of the peak current vs. the square root of the

+0.43, +0.50, respectively.⁴⁶The impact of the substitution in scan rate confirmed that in all cases the species involved in

3- or 4- (and 3′- or 4′) positions of a Cp ring is included using the redox reactions were freely diﬀusing through the electro-

σ_m.^35b,47For example, in ^Me10Fc, the Σσ_p,mvalue contains con- chemical cell (Fig. 12 and S76–S125†), rather than adsorbed on

tributions of both para and meta methyl substituents (i.e., σ_mthe surface of the working electrode.

for a methyl group is −0.07 and Σσ_p,m= [6 × (−0.17) + 4 ×

The eﬀect of the medium on the redox potential, ΔE_1/2, and

(−0.07)] = –1.3). One apparent discrepancy that merits special diﬀusion of the redox active species is a complicated function

attention at this point is that, based on the σ_pvalues, the of the interactions between solvent and solute and their overall

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Fig. 13 The half-wave potential, E_1/2, and peak-to-peak separation,

ΔE_1/2, of all ferricenium/ferrocene couples discussed in this study in

diﬀerent media: (a) with [(nBu)₄N][PF₆] as the supporting electrolyte in

MeCN (red) and DCM (blue) and (b) with [(nBu)₄N][B(C₆F₅)₄] as the sup-

porting electrolyte in MeCN (red), MeTHF (green), and DCM (blue).

Fig. 12 Representative cyclic voltammograms of the parent Fc⁺/Fc

couple (2 mM) at various scan rates in MeCN with [(nBu)₄N][PF₆] as the

supporting electrolyte (100 mM). The inset shows the Randles–Sevcik

plot of the CV data.

ion-pairing.^44,48Some of the most important properties that

govern these interactions are dielectric constant (ε), dipole

moment (μ), donor number (DN), acceptor number (AN), and

absolute viscosity (η). The relevant parameters for the solvents

chosen for this study are listed in Table 4. Acetonitrile (MeCN),

which has by far the largest dielectric constant (ε = 36.6), is

historically favored for most electrochemical measurements in

organic systems. Here, in addition to MeCN, we employed two

lower polarity solvents with dielectric constants of less than 10

(i.e., DCM and MeTHF). For the latter, using [(nBu)₄N]

[B(C₆F₅)₄] as the supporting electrolyte, rather than the more

traditional analogs such as [(nBu)₄N][PF₆], can remarkedly

enhance the conductivity and decrease the overall ohmic

drop.⁴⁴

As shown in Fig. 13 and Table 3, the redox potentials of the

ferrocene derivatives vary with the nature of the solvent.

Considering solely the solvent polarity for a given electrolyte,

the redox potentials are predicted to be the lowest in MeCN

compared to those in DCM and MeTHF. All of our data follow

the expected trend in acetonitrile. When comparing the two

lower polarity solvents, DCM and MeTHF, the donor and

acceptor numbers of the solvents are taken into consideration

in predicting the redox behavior of the ferrocene derivatives in

solution.

The donor and acceptor numbers of DCM are reported to

be around 0 and 20, respectively, while MeTHF has a donor

number of 18 and an estimated acceptor number in the range

of 3.9 to 8.0.⁵⁰The diﬀerences in the donor and acceptor pro-

perties of these two solvent influence electrolyte dissociation.

Additionally, higher donor number indicates that MeTHF can

act more as a Lewis base stabilizing the oxidized species as

well as improving the thermodynamics of the ion paring

between ferricenium and the anion of the supporting electro-

lyte, in turn lowering the redox potential. On the other hand,

the higher acceptor number of DCM suggests a higher degree

of Lewis acidity, stabilizing the neutral form and in turn

increasing the potential needed to oxidize the ferrocene deriva-

tives. The pattern of higher redox potentials in DCM compared

to MeTHF was observed for most ferrocene derivatives except

for ^Me10Fc, Fc, and ^BrFc. Lay and coworkers previously ascribed

the considerably weaker solvent eﬀects on the redox potential

of ^Me10Fc to the shell of the methyl substituents that can

protect the iron center against close interactions with solvents

and electrolytes.⁵³However, it is not clear as to why Fc and

^BrFc also do not follow the pattern and further understanding

of solvent and solute interactions other than electrostatic

eﬀects is necessary to explain their behavior.

The role of the two electrolyte anions, PF₆and [B(C₆F₅)₄]⁻,

−

Table 4 Relevant solvent parameters

in altering redox potentials and peak-to-peak separations in

both MeCN and DCM was investigated. As described in the

Introduction, the high degree of charge delocalization in a

large weakly coordinating anion such as [B(C₆F₅)₄]⁻makes it a

weak nucleophile and generally well soluble in lower-polarity

solvents. This enhanced solubility can minimize adsorption

problems with cationic electrode products (e.g., the ferrice-

nium derivatives). The [B(C₆F₅)₄]⁻anion is also considered a

weakly ion-pairing anion.^48a

MeCN

DCM

MeTHF

Dielectric constant, ε

Dipole moment, μ (D)

Donor number DN

Acceptor number AN

Absolute viscosity, η (mPa s)

36.6^a

3.92^a

14^a

8.93^a

1.60^a

(0)^a

6.97^b

1.36^b

18^b

19^a

20^a

3.9–8.0^c

0.492^e

0.375^d

0.426^d

^aFrom ref. 48a. ^bFrom ref. 49. ^cThe AN is not reported for MeTHF,

although it can be inferred to be close to values reported for tetra-

hydrofuran (8.0) and diethyl ether (3.9).^{50 d}From ref. 51. ^eFrom ref. 52.

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In MeCN, the nature of the electrolyte anion has limited

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eﬀect on the E_1/2and ΔE_1/2values, as the diﬀerences observed

for all the ferrocene derivatives with the two electrolyte anions,

−

PF₆and [B(C₆F₅)₄]⁻, on average are about 7 mV and 15 mV,

respectively (Table 3). This is likely due to the high polarity of

MeCN minimizing the ion-pairing eﬀects of the diﬀerent elec-

trolytes.⁴⁴As for DCM, there is a more observable trend

wherein ^Me10Fc, ^nBuFc, Fc, and ^BrFc experience the smallest

electrolyte-induced change in E_1/2(i.e., 14–27 mV) while the di-

substituted ferrocenes ^Me2Fc and ^Br2Fc show diﬀerences of 47

and 51 mV, respectively.

The largest anodic shifts in E_1/2(i.e., 58–160 mV) are seen

for mono- and 1,1′-di-ketone-substituted ferrocene derivatives

going from [(nBu)₄N][PF₆] to [(nBu)₄N][B(C₆F₅)₄] in DCM. Quite

curiously, the electrolyte-induced changes of ΔE_1/2follow a

very diﬀerent trend (Table 3). For example, the largest diﬀer-

ence of about 77 mV in ΔE_1/2is observed for the parent Fc and

its most electron-deficient derivative, ^Bz2Fc. However, in the

presence of [B(C₆F₅)₄]⁻as compared to PF₆⁻, the peak-to-peak

separation becomes markedly smaller for the parent com-

Fig. 15 Cyclic voltammograms of ^Bz2Fc (2 mM) in MeCN at 100 mV·s⁻¹

with 100 mM of [(nBu)₄N][B(C₆F₅)₄] as the supporting electrolyte. Arrows

indicate the decay of the current towards the baseline as the scans pro-

gressed (i.e., an E_rC_iprocess).

pound while it significantly increases for the ^Bz2Fc derivative, scanning at 100 mV s⁻¹, the peak current moved closer to the

see Table 3.

baseline (Fig. 15). The voltammograms become more revers-

As shown in Fig. 14, the anodic peak of the parent ferrocene ible at scan rates above 100 mV s⁻¹. It was found that

is barely aﬀected (i.e., only by 8 mV) by the nature of electrolyte 1500 mV s⁻¹was the optimal scan rate, as that is where the

anion in DCM while the cathodic peak shifts by about 67 mV. cathodic/anodic peak current ratio reached the highest value

This again emphasizes the fact that in low-polarity solvents the at 0.81. This is a notable feature for E_rC_ireactions, at higher

ferricenium species can be further stabilized when the electro- scan rates, the chemical reaction following the initial electron

lyte anion is changed from weakly (i.e., [B(C₆F₅)₄]⁻) to relatively transfer step is too slow to compete, leaving more oxidized

strongly (PF₆⁻) ion-pairing, highlighting the often overlooked species to become subsequently reduced in the reverse scan.⁵⁵

eﬀects of counter anions in governing the redox potentials.

As ^Ac2Fc demonstrated a similar behavior, a series of faster

Also, the scan rate has the least influence on the ΔE_1/2values scan rates was also used in MeCN to outrun the chemical reac-

of diﬀerent ferrocene derivatives in acetonitrile (Fig. S126– tion that accompany the electron transfer process for this

S135†) due to negligible incomplete iR compensation.⁵⁴

derivative. Therefore, for the studies of ^Bz2Fc and ^Ac2Fc in

The cyclic voltammetry measurements of ^Bz2Fc in MeCN MeCN with both electrolytes, the scan rates were varied at

with either electrolytes revealed a possible E_rC_iprocess which 1500, 1250, 1000, 500, 250, and 100 mV s⁻¹and the working

means that ^Bz2Fc⁺reacts in a homogenous chemical reaction electrode was cleaned between each scan rate variation. These

upon oxidation, thus, being chemically irreversible. While results are consistent with our direct observation of the reactiv-

ity of the chemically synthesized ketone-substituted ferrice-

nium derivatives toward acetonitrile. The exact mechanism of

this reactivity is outside the scope of this report and will be

discussed elsewhere.

As mentioned earlier, the electron transfer processes were

diﬀusion-controlled for all the ferrocene and ferricenium

derivatives used in this study in five diﬀerent media. The

diﬀusion coeﬃcients (D) are presented in Table 5.

The diﬀusion coeﬃcients of the neutral ferrocene deriva-

tives and their ferricenium counterparts are typically not very

diﬀerent.⁵⁶As expected, the species moved the fastest in

MeCN compared to DCM or MeTHF. This can be attributed to

the high polarity of the solvent, which minimizes the analyte-

electrolyte ion-pairing and subsequently decreases the resis-

tance of the solution.^48aAnother factor that can facilitate the

movement of analytes in the solution is the lower absolute vis-

cosity of acetonitrile compared to DCM and MeTHF

Fig. 14 Representative cyclic voltammograms recorded for Fc (2 mM)

(Table 4).^49,51Among the ferrocene derivatives described here,

in DCM on a glassy carbon disk electrode at 100 mV·s⁻¹in two diﬀerent

electrolytes.

both oxidized and neutral forms of ^Me2Fc, ^nBuFc, and ^Br2Fc

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Table 5 Diﬀusion coeﬃcient values of various ferrocene/ferricenium derivatives in diﬀerent solvent/electrolyte solutions

10⁶× D [(cm²s⁻¹)] in MeCN

[(nBu)₄N][PF₆] [(nBu)₄N][B(C₆F₅)₄]

Anodic Cathodic Anodic Cathodic

10⁶× D [(cm²s⁻¹)] in DCM

[(nBu)₄N][PF₆] [(nBu)₄N][B(C₆F₅)₄]

Anodic Cathodic Anodic Cathodic

10⁶× D [(cm²s⁻¹)] in MeTHF

[(nBu)₄N][B(C₆F₅)₄]

Anodic

Cathodic

^Me10Fc 12.96 0.00 13.86 0.00 12.96 0.00 12.66 0.00 5.62 0.01 6.22 0.01 3.69 0.01 3.36 0.01 4.78 0.01

^Me2Fc 17.83 0.00 18.79 0.00 14.18 0.01 15.59 0.01 9.40 0.04 9.05 0.01 5.03 0.02 2.66 0.02 5.51 0.01

^nBuFc 18.34 0.00 18.60 0.00 14.12 0.01 14.84 0.01 7.92 0.02 7.64 0.01 8.07 0.01 6.93 0.01 4.54 0.01

3.91 0.01

5.92 0.01

4.52 0.01

4.59 0.01

5.17 0.01

6.74 0.01

5.16 0.01

2.66 0.01

3.99 0.01

3.04 0.01

Fc

13.67 0.00 13.28 0.01 8.80 0.01 13.19 0.00 9.27 0.02 9.37 0.02 12.3 0.01 12.2 0.01 4.66 0.01

13.28 0.00 12.74 0.00 12.31 0.01 13.04 0.00 7.81 0.01 7.73 0.01 6.70 0.01 4.55 0.01 5.82 0.00

15.30 0.01 12.35 0.01 13.60 0.00 14.55 0.01 9.81 0.01 9.32 0.01 5.77 0.01 3.96 0.01 7.20 0.01

13.29 0.01 11.00 0.01 13.38 0.01 13.72 0.01 6.61 0.01 6.48 0.01 6.11 0.01 5.69 0.01 5.98 0.01

18.08 0.00 14.98 0.01 11.18 0.01 9.94 0.01 9.15 0.01 8.76 0.01 10.7 0.00 9.71 0.01 4.60 0.01

12.51 0.01 10.49 0.02 12.85 0.00 9.78 0.02 5.60 0.01 5.68 0.01 2.75 0.00 1.30 0.01 6.14 0.00

9.37 0.01 6.72 0.01 7.30 0.00 5.67 0.02 2.95 0.01 2.29 0.02 1.33 0.00 0.21 0.01 4.65 0.00

^BrFc

^AcFc

^BzFc

^Br2Fc

^Ac2Fc

^Bz2Fc

diﬀuse faster than the parent ferrocene and ferricenium by observed for ^Bz2Fc and to a lesser extent for ^Ac2Fc, particularly

about 35% through the acetonitrile media.

in DCM with [B(C₆F₅)₄]⁻as the counter anion (Fig. S72†), can

The diﬀusion coeﬃcients obtained in this study for both be the consequence of the much slower diﬀusion (Table 5)

oxidized and neutral parent ferrocene species in MeCN with and significant rearrangements of the substituents that accom-

[(nBu)₄N][PF₆] are lower (i.e., D_Fc= 1.367 × 10⁻⁵cm²s⁻¹) than pany the electron transfer process. This is in agreement with

most previously reported values (i.e., D_Fc≈ 2 × 10^−5–2.7 × 10⁻⁵our crystallographic data and the one broad Cp proton reso-

cm^{2 −1}),^55,57which we contribute to our rigorous eﬀorts to nance observed for their oxidized forms, ^Ac2Fc⁺and ^Bz2Fc⁺, in

s

minimize the amount of water present in the solutions during our ¹H-NMR studies.

our electrochemical measurements,⁵⁸see ESI† for more

details.

Some of the complexes (i.e., ^nBuFc, Fc, ^BrFc, ^BzFc, and ^Br2Fc)

Conclusion

are found to diﬀuse slowest in the MeTHF solution which is in

line with the higher viscosity and lower dielectric constant of

the solvent compared to DCM. However, the remaining deriva-

tives such as ^Me10Fc, ^Me2Fc, ^AcFc, ^Ac2Fc, and ^Bz2Fc follow a

reverse trend and have larger diﬀusion constants in MeTHF

than DCM. Perhaps this stems from the fact that the substitu-

ents in these derivatives may disrupt the “normal” charge

density distribution throughout the complex, interfering with

ion-pairing interaction in MeTHF, therefore, disrupting the

expected trend of the solvent/analyte interactions prevalent in

solvents of low-polarity.⁵³

Despite the enormous number of publications in the field of

ferrocenes and their redox counterparts, ferriceniums, infor-

mation on the synthesis and characterization of these com-

plexes is relatively sparse and in most cases the reports lack

the relevant experimental details. Moreover, most eﬀorts have

been focused on the electron-rich ferricenium species and very

limited data are available on the electron-deficient systems.

This represents a surprising knowledge gap in the literature.

Herein, we attempted to address some of that gap through a

systematic and thorough evaluation of a library of highly

organic soluble ferricenium derivatives.

Furthermore, the diﬀusion coeﬃcient of a given solute is

inversely proportional to the size (i.e., hydrodynamic radius) of

the solute and, hence, to its molecular weight assuming all

species are hard spheres and have the same density.⁵⁹

Our ¹H-NMR measurements revealed that the substitutional

behavior in the paramagnetic ferricenium derivatives is more

complex and fundamentally reversed as compared to the

neutral ferrocene counterparts. We proposed that the δ back-

donation from the iron atom into the substituted Cp rings

leads to the overall shielding of the ring protons in the

Gonzalez and coworkers described

a linear correlation

between the diﬀusion coeﬃcient and molecular weight.^57aAs

the molecular weight increases, the diﬀusion coeﬃcient

decreases. This was generally observed in our experiments,

although not perfectly, with the highest correlation obtained

in the DCM solution with [(nBu)₄N][PF₆] as the supporting

electrolyte. Slight outliers are labeled in Fig. S136.†

ferricenium derivatives. This shielding through

δ back-

donation is more pronounced in the electron-deficient rings

with lower energy molecular orbitals. Our data for the electron-

deficient ferricenium derivatives in solution also drew a direct

correlation between the solvent dielectric constant and the

rotation of the cyclopentadienyl ligands around the Fe–Cp

bond in these systems.

Here, nine new X-ray structures are also added to the library

of ferricenium derivatives, five of which presented the

first examples of molecular structures of such derivatives.

Structural comparison of the neutral ferrocene derivatives and

As mentioned earlier, all the ferrocene derivatives described

here show a quasi-reversibility of the redox behavior which is

in agreement with our crystallographic data confirming that

the diﬀerence in Fe–C bond length between their oxidized and

neutral form is not larger than 0.05 Å, except for ^Me2Fc which

experiences about 0.06 Å Fe–C bond elongation upon oxidation

(Table S9†). Additionally, the relatively lower current density

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their oxidized counterparts revealed that the oxidation of ferro- Elemental analysis was accomplished at Midwest Microlab

cene results in more substantial shortening of the C–C bond (Indianapolis, IN). Electrochemical data was collected using a

lengths in the Cp rings of the parent ferricenium complex rela- Bio-Logic SP-200 potentiostat. Single-crystal X-ray data were

tive to those of their substituted counterparts. Interestingly, collected using a Gemini R (Agilent Technologies) diﬀract-

our results also show that the nature of the substituents has a ometer at the X-ray diﬀraction facility of the Joint School of

more significant eﬀect on the metal–ligand separations in the Nanoscience and Nanoengineering (JSNN). The temperature of

oxidized species than in their neutral analogs. For the ketone- the data collection was controlled using the system Cryojet

substituted ferricenium derivatives, the increase in the oxi- (manufactured by Oxford Instruments).

dation state was reflected in a significant strengthening of the

carbonyl bond(s) by about 35–48 cm⁻¹when compared with ferrocene (99%) were purchased from Alfa Aesar; 1-benzoylfer-

the neutral ferrocene counterparts. rocene (>98%), 1,1′-dibromoferrocene (>98%), and 1,1′-diben-

The compounds, 1-acetylferrocene (97%), and decamethyl-

Additionally, the redox behavior of the corresponding ferri- zoylferrocene (98%) from Tokyo Chemical Industry (TCI); ferro-

cenium/ferrocene (Fc^+/0) redox couples such as potential cene (98%), 1-bromoferrocene (98.8%), 1,1′-diacetylferrocene

values (E_1/2), peak-to-peak separation (ΔE_1/2), and diﬀusion (97%), and ferrocenecarboxylic acid (99%) from Sigma-Aldrich;

coeﬃcients (D) of the redox active species in three diﬀerent 1,1′-dimethylferrocene (98%), and n-butylferrocene (99%) from

solvents and two supporting electrolytes are reported in this Stream Chemicals. Potassium tetrakis(pentafluorophenyl)

work. The results point to the significant eﬀect of the ion- borate, K[B(C₆F₅)₄], (99.9%) was purchased from Boulder

pairing in lowering the energy necessary for reduction of the Scientific Company; AgNO₃(99.99%) from Alfa Aesar; Ag[SbF₆]

ferricenium species and the overall half-wave potential.

(98%) and tris (4-bromophenyl)amine from Sigma-Aldrich;

In order to explain some of the observed spectroscopic and tetra-n-butylammonium hexafluorophosphate (98%) from

structural features of the ferricenium systems, particularly Oakwood Chemicals. tetra-n-Butylammonium chloride hydrate

those bearing electron withdrawing substituents, further (99.84%) and potassium chloride (99%) were purchased from

investigations are required; as is often the case in bonding and Alfa Aesar.

electronic structures, “the devil is in the details”. Additional Deuterated solvents including acetone-d₆(D, 99.9%) and

experimental and computational eﬀorts are currently under- methylene chloride-d₂(D, 99.8%) were purchased from

way in our laboratory.

Cambridge Isotope Laboratories. Distilled water was further

purified by a PURELAB flex 1 Analytical Ultrapure Water

System (ELGA) to obtain the specific resistance of 18.2 MΩ cm

at 25 °C.

Experimental section

General methods

Synthesis and characterization

All chemicals and solvents were of commercially available

Tetrakis(acetonitrile)silver(^I) BArF₂₀([Ag(MeCN)₄][B(C₆F₅)₄]).

grade, unless otherwise noted. Acetonitrile (MeCN), 2-methyl- The current synthetic procedure is a slightly modified version

tetrahydrofuran (MeTHF) (inhibitor free, 673277), dichloro- of the method reported earlier for the synthesis of

methane (DCM) and hexanes were purchased from Sigma- [Ag(MeCN)₄][B(C₆F₅)₄].²¹In the glove box,

a solution of

Aldrich. 1,2-Difluorobenzene was purchased from Alfa Aesar. K[B(C₆F₅)₄] (863.0 mg, 1.202 mmol) in MeCN (15 mL) was

All solvents were further purified by passing through a 60 or added to a solution of AgNO₃(204.1 mg, 1.202 mmol) in

18 cm-long column of activated alumina under argon using an MeCN (9 mL) and the reaction mixture was stirred for 1 h

Innovative Technologies or Inert PureSolv Micro solvent purifi- under reduced light at RT. The reaction mixture was then

cation system. The solvents were further deoxygenated by either filtered to remove the precipitate, KNO₃. The solvent was

repeated freeze/pump/thaw cycles or bubbling with argon for removed under vacuum. The solid semi-crystalline product

45–60 min followed by storage over 3 or 5 Å molecular sieves for was re-dissolved in a minimum amount of MeCN and was kept

at least 72 hours prior to use. Air- and moisture-sensitive com- in the freezer at −35 °C overnight to obtain white needle

pounds were synthesized and handled under a dry oxygen-free shaped crystals suitable for X-ray structural determination

argon atmosphere using standard Schlenk techniques or in a (Fig. S61†). After drying under vacuum, the crystals of

Vacuum Atmospheres OMNI-Lab inert atmosphere (<0.5 ppm of [Ag(MeCN)₄][B(C₆F₅)₄] weighed 983.2 mg (86.14% yield). Anal.

O₂and H₂O) glovebox filled with nitrogen.

calcd for C₃₂H₁₂AgBF₂₀N₄: C, 40.41; H, 1.27; N, 5.89. Found: C,

Bench-top UV-vis spectra were recorded with a Cary-60 40.25; H, 1.28; N, 5.65. FT-IR (solid; cm⁻¹): ν_(CuN)= 2295

spectrophotometer using a 2 or 10 mm modified Schlenk (Fig. S1†). The deuterated complex, [Ag(CD₃CN)₄][B(C₆F₅)₄],

cuvette. Infrared (IR) spectra were obtained using a Thermo was prepared by the same procedure described above using

Scientific Nicolet iS5 Fourier Transform IR (FT-IR) spectro- deuterated acetonitrile, CD₃CN. FT-IR (solid; cm⁻¹): ν_(CuN)

meter equipped with an iD7 attenuated total reflection (ATR) 2287 (Fig. S2†). H-NMR (acetone-d₆, 500 MHz; δ, ppm): 2.04

accessory. ¹H-NMR spectra were recorded on a JEOL 400 or (s, 12H) (Fig. S137†). ¹⁹F-NMR (acetone-d₆, 470 MHz; δ, ppm):

500 MHz instrument. The chemical shifts were referenced −133.0, −164.3, −168.2 (Fig. S138†).

=

1

against tetramethyl-silane (TMS, δ = 0.00 ppm). The NMR

The BArF₂₀salts of the parent ferricenium complex and a

solvent residual peaks were used as a secondary reference. series of ferricenium derivatives bearing electron-donating

Dalton Trans.

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substituents were prepared following the general procedure C, 47.15; H, 1.26; N, 0.00. UV-vis [λ_max, nm (ε_max, M⁻¹cm⁻¹)]:

described here. The complexes include ^Me10FcBArF₂₀, 469 (141), 536 (181), 621 (441) in MeTHF and 469 (156), 535

^Me2FcBArF₂₀, ^nBuFcBArF₂₀, and FcBArF₂₀. Details are given for (199), 618 (445) in MeCN. FT-IR (solid; cm⁻¹): ν_(Cp–H)= 3128

decamethylferricenium BArF₂₀, as a representative case.

Decamethylferricenium BArF₂₀

^Me10FcBArF₂₀). In the glove 10H) (Fig. S15†). ¹⁹F-NMR (acetone-d₆, 470 MHz; δ, ppm):

box, to solution of decamethylferrocene (249.8 mg, −133.0, −164.4, −168.4 (Fig. S49†).

0.758 mmol) in MeTHF (5 mL) was added the solution of The BArF₂₀salts of a series of ferricenium derivatives

(Fig. S7†). ¹H-NMR (acetone-d₆, 500 MHz; δ, ppm): 33.2 (s, vbr,

(

a

[Ag(MeCN)₄][B(C₆F₅)₄] (648.7 mg, 0.682 mmol) in MeTHF bearing one electron-withdrawing substituents as well as the

(10 mL). The reaction mixture was allowed to stir for 1 h under 1,1′-dibromo substituted ferricenium were prepared following

reduced light at RT. The solution was then filtered through the general procedure described here. Those complexes

Celite to remove the silver metal. The filtrate was evaporated include ^BrFcBArF₂₀, ^AcFcBArF₂₀, ^BzFcBArF₂₀, and ^Br2FcBArF₂₀.

and washed with hexanes to remove the remaining, unreacted Details are given for 1-bromoferricenium BArF₂₀, as a represen-

decamethylferrocene. The compound obtained was then dried tative case. Note: For both mono-ketone-substituted ferrocenes,

and crystallized from MeTHF/hexanes. The crystals were the order of addition of reagents is reversed (i.e., the solution

further washed with hexanes and were again recrystallized of substituted ferrocene is gradually added to the silver(^I)

from MeTHF/hexanes. Dark green crystals were obtained solution). An alternative procedure for the preparation of

which were suitable for X-ray structure determination ^BzFc[B(C₆F₅)₄] is also included.

(Fig. S62†). After vacuum drying, the yield of ^Me10FcBArF₂₀was

88.12% (604.3 mg). Anal. calcd for C₄₄H₃₀BF₂₀Fe: C, 52.57; H, solution of 1-bromoferrocene (252.2 mg, 0.944 mmol) in DCM

3.01; N, 0.00. Found: C, 52.31; H, 2.99; N, 0.00. UV-vis [λ_max(5 mL) was added to the solution of [Ag(MeCN)₄][B(C₆F₅)₄]

1-Bromoferricenium BArF₂₀(^BrFcBArF₂₀). In the glove box, a

,

nm (ε_max, M⁻¹cm⁻¹)]: 652 (215), 719 (320), 780 (580) in MeTHF (808.4 mg, 0.850 mmol) in DCM (20 mL). The mixture was

and 652 (222), 715 (320), 778 (581) in MeCN. FT-IR (solid; stirred for 1 h under reduced light at RT. The solution was fil-

cm⁻¹): ν_(C–H):Me= 2987, 2978, 2925 (Fig. S4†). ¹H-NMR tered through Celite to remove the silver metal. The filtrate

(acetone-d₆, 500 MHz; δ, ppm): −37.6 (s, br, 30H) (Fig. S17†). was evaporated, washed with hexanes to remove the unreacted

¹⁹F-NMR (acetone-d₆, 470 MHz; δ, ppm): −133.0, −164.3, 1-bromoferrocene, and dried before it was crystallized from

−168.3 (Fig. S44†).

1,1′-Dimethylferricenium BArF₂₀

MeTHF/hexanes. After the crystallization, the solvent was dec-

(

^Me2FcBArF₂₀). Prussian anted. Crystals were further washed with hexanes and were

blue crystals were collected (Fig. S63†), 155.4 mg (89.67% recrystallized from MeTHF/hexanes. The crystals were dark

yield). Anal. calcd for C₃₆H₁₄BF₂₀Fe: C, 48.41; H, 1.58; N, 0.00. blue and suitable for X-ray structure determination (Fig. S65†).

Found: C, 48.34; H, 1.59; N, 0.00. UV-vis [λ_max, nm (ε_max

,

After vacuum drying, the crystals weighed 697.2 mg (86.89%

M⁻¹cm⁻¹)]: 472 (175), 572 (227), 654 (358) in MeTHF and 473 yield). Anal. calcd for C₃₄H₉BF₂₀BrFe: C, 43.26; H, 0.96; N,

(148), 569 (203), 654 (325) in MeCN. FT-IR (solid; cm⁻¹): ν_(Cp–H)0.00. Found: C, 42.99; H, 1.07; N, 0.00. UV-vis [λ_max, nm (ε_max

,

1

= 3115; ν_(C–H):Me= 2934, 2919, 2895, 2877 (Fig. S5†). H-NMR M⁻¹cm⁻¹)]: 477 (205), 567 (237), 683 (365) in MeTHF and 476

(acetone-d₆, 500 MHz; δ, ppm): 34.8 (vbr, 4H), 31.6 (vbr, 4H), (207), 563 (234), 676 (362) in MeCN. FT-IR (solid; cm⁻¹): ν_(Cp–H)

−10.5 (br, 6H) (Fig. S40†). ¹⁹F-NMR (acetone-d₆, 470 MHz; δ, = 3124, 3114, 3102 (Fig. S8†). ¹H-NMR (acetone-d₆, 500 MHz; δ,

ppm): −133.1, −164.4, −168.5 (Fig. S45†). ¹H-NMR (CD₂Cl₂, ppm): 34.0 (s, vbr, 5H), 32.0 (s, vbr, 2H), 28.8 (s, vbr, 2H)

500 MHz; δ, ppm): 35.8 (vbr, 4H), 32.5 (vbr, 4H), −9.0 (br, 6H) (Fig. S38†). ¹⁹F-NMR (acetone-d₆, 470 MHz; δ, ppm): −133.0,

(Fig. S41†). ¹⁹F-NMR (CD₂Cl₂, 470 MHz; δ, ppm): −134.9, −164.4, −168.4 (Fig. S50†). ¹H-NMR (CD₂Cl₂, 500 MHz; δ,

−164.3, −168.9 (Fig. S46†).

n-Butylferricenium BArF₂₀

ppm): 35.0 (s, vbr, 5H), 33.1 (s, vbr, 4H) (Fig. S39†). ¹⁹F-NMR

(

^nBuFcBArF₂₀). Peacock blue crys- (CD₂Cl₂, 470 MHz; δ, ppm): −135.2, −164.2, −169.0 (Fig. S51†).

tals were collected (Fig. S64†), 122.5 mg (80.59% yield). Anal.

1-Acetylferricenium BArF₂₀(^AcFcBArF₂₀). In the glove box, a

calcd for C₃₈H₁₈BF₂₀Fe: C, 49.55; H, 1.97; N, 0.00. Found: C, solution of 1-acetylferrocene (175.4 mg, 0.746 mmol) in DCM

49.43; H, 1.97; N, 0.00. UV-vis [λ_max, nm (ε_max, M⁻¹cm⁻¹)]: 471 (3 mL) was added to the solution of [Ag(MeCN)₄][B(C₆F₅)₄]

(188), 556 (236), 630 (364) in MeTHF and 472 (162), 559 (226), (354.7 mg, 0.373 mmol) in DCM (10 mL). The unreacted 1-acet-

628 (380) in MeCN. FT-IR (solid; cm⁻¹): ν_(Cp–H)= 3128; ν_(C–H):ylferrocene was removed by washing the crude product with

1

= 2960, 2934, 2876, 2865. (Fig. S6†). H-NMR (acetone-d₆, hexanes. The crystals were grown in DCM/hexanes. 278.4 mg

nBu

500 MHz; δ, ppm): 37.4 (vbr, 2H), 33.9 (vbr, 2H), 31.3 (vbr, 5H), (82.28% yield), see Fig. S66† for the molecular packing. Anal.

1.2 (s, br, 2H), −1.0 (s, 3H), −6.8 (s, 2H), −18.3 (s, br, 2H) calcd for C₃₆H₁₂BF₂₀FeO: C, 47.67; H, 1.33; N, 0.00. Found: C,

(Fig. S36†). ¹⁹F-NMR (acetone-d₆, 470 MHz; δ, ppm): −133.0, 47.43; H, 1.36; N, 0.00. UV-vis [λ_max, nm (ε_max, M⁻¹cm⁻¹)]: 475

−164.3, −168.3 (Fig. S47†). ¹H-NMR (CD₂Cl₂, 500 MHz; δ, (198), 550 (202), 638 (425) in MeTHF and 481 (188), 555 (216),

ppm): 38.6 (vbr, 2H), 35.5 (vbr, 2H), 32.1 (vbr, 5H), 1.0 (s, br, 638 (463) in DCM. FT-IR (solid; cm⁻¹): ν_(Cp–H)= 3377; ν_(C–H):Ac

=

2H), −1.1 (s, 3H), −7.4 (s, 2H), −16.7 (s, br, 2H) (Fig. S37†). 3140, 3125, 3115, 3092; ν_(CvO):Ac= 1698 (Fig. S9†). ¹H-NMR

¹⁹F-NMR (CD₂Cl₂, 470 MHz; δ, ppm): −135.0, −164.3, −167.0 (acetone-d₆, 500 MHz; δ, ppm): 36.3 (s, vbr, 5H), 31.8 (s, vbr,

(Fig. S48†).

2H), 27.5 (s, vbr, 2H), −15.9 (s, br, 3H) (Fig. S40†). ¹⁹F-NMR

Ferricenium BArF₂₀(FcBArF₂₀). 921.5 mg (88.26% yield). (acetone-d₆, 470 MHz; δ, ppm): −133.0, −164.4, −168.4

1

Anal. calcd for C₃₄H₁₀BF₂₀Fe: C, 47.21; H, 1.17; N, 0.00. Found: (Fig. S52†). H-NMR (CD₂Cl₂, 500 MHz; δ, ppm): 37.2 (s, vbr,

Dalton Trans.

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Dalton Transactions

5H), 32.4 (s, vbr, 2H), 30.4 (s, vbr, 2H), −13.0 (s, br, 3H) were further washed with hexanes. Dark green crystals, suit-

(Fig. S41†). ¹⁹F-NMR (CD₂Cl₂, 470 MHz; δ, ppm): −135.2, able for X-ray structure determination, were obtained with

−164.1, −169.0 (Fig. S53†).

1-Benzoylferricenium BArF₂₀

further recrystallization with DCM/hexanes (Fig. S69†).

(

^BzFcBArF₂₀). An additional fil- 279.3 mg (81.96% yield). Anal. calcd for C₃₈H₁₄BF₂₀FeO₂: C,

tration step was performed to remove any silver(^I) salt impur- 48.09; H, 1.49; N, 0.00. Found: C, 48.23; H, 1.48; N, 0.00. UV-

ity. As an alternative procedure, in the glove box, a solution of vis [λ_max, nm (ε_max, M⁻¹cm⁻¹)]: 476 (283), 602 (278), 652 (422)

benzoylferrocene (111.0 mg, 0.375 mmol) and K[B(C₆F₅)₄] in 1,2-difluorobenzene and 476 (295), 602 (298), 654 (439) in

(269.3 mg, 0.375 mmol) in DCM (10 mL) was slowly added to DCM. FT-IR (solid; cm⁻¹): ν_(Cp–H)= 3377; ν_(Cp–H):Ac= 3117,

1

the solution of Ag[SbF₆] (131.5 mg, 0.375 mmol) in DCM 3104; ν_(CvO):Ac= 1697 (Fig. S12†). H-NMR (CD₂Cl₂, 500 MHz;

(3 mL). The reaction mixture was allowed to stir for 15 min δ, ppm): 30.69 (vbr, 8H), −9.43 (s, br, 6H) (Fig. S34†). ¹⁹F-NMR

under reduced light at RT. The solution was then filtered (CD₂Cl₂, 470 MHz; δ, ppm): −134.7, −163.8, −168.5 (Fig. S58†).

through Celite to remove the silver metal and insoluble K

1,1′-Dibenzoylferricenium SbF₆

(

^Bz2Fc[SbF₆]). In the glove

[SbF₆]. The solvent was removed under vacuum and the result- box,

a

solution of 1,1′-dibenzoylferrocene (499.7 mg,

ing solid product was washed with hexanes to remove the 1.268 mmol) in DCM (10 mL) was added to a solution of

remaining, unreacted 1-benzoylferrocene. Moss green crystals Ag[SbF₆] (444.6 mg, 1.268 mmol) in DCM (12 mL). The mixture

were grown from DCM/hexanes (296.1 mg, 81.47% yield), see was stirred for 1 h under reduced light at RT. The reaction

Fig. S67† for the molecular packing. Anal. calcd for mixture was then filtered through Celite to remove the silver

C₄₁H₁₄BF₂₀FeO: C, 50.81; H, 1.46; N, 0.00. Found: C, 50.15; H, metal. The filtrate was dried under vacuum, and the complex

1.45; N, 0.00. UV-vis [λ_max, nm (ε_max, M⁻¹cm⁻¹)]: 473 (239), 555 was crystallized from DCM/hexanes. After the crystallization,

(231), 640 (448) in MeTHF and 476 (230), 572 (249), 638 (439) the solvent was decanted, and crystals were further washed

in DCM. FT-IR (solid; cm⁻¹): ν_{(Cp–H)/(C–H):Bz}= 3298, 3103, 3082, with hexanes. Dark green crystals, suitable for X-ray structure

2981, 2965; ν_(CvO):Bz= 1658 (Fig. S10†). ¹H-NMR (acetone-d₆, determination, were grown through the slow diﬀusion of

500 MHz; δ, ppm): 35.4 (vbr, 5H), 31.3 (vbr, 2H), 27.6 (vbr, 2H), hexanes into the concentrated solution of the ^Bz2Fc[SbF₆] in

7.5 (s, br, 2H), 6.3 (s, 2H), 6.2 (s, H) (Fig. S42†). ¹⁹F-NMR DCM (Fig. S70†). After vacuum drying, the crystals weighed

(acetone-d₆, 470 MHz; δ, ppm): −133.0, −164.3, −168.3 679.6 mg (85.07% yield). Anal. calcd for C₂₄H₁₈F₆FeO₂Sb: C,

(Fig. S54†). ¹H-NMR (CD₂Cl₂, 500 MHz; δ, ppm): 36.0 (vbr, 5H), 45.76; H, 2.88; N, 0.00. Found: C, 45.53; H, 2.79; N, 0.00. UV-

30.5 (vbr, 4H), 12.4 (s, br, 2H), 7.3 (s, 2H), 6.7 (s, H) (Fig. S43†). vis [λ_max, nm (ε_max, M⁻¹cm⁻¹)]: 492 (650), 650 (545) in 1,2-

¹⁹F-NMR (CD₂Cl₂, 470 MHz; δ, ppm): −134.9, −164.4, −168.5 difluorobenzene and 493 (588), 587 (474), 653 (564) in

(Fig. S55†).

1,1′-Dibromoferricenium BArF₂₀

DCM. FT-IR (solid; cm⁻¹): ν_(Cp–H)= 3308; ν_(C–H):Bz= 3123, 3112,

1665; ν_(SbF6)651 (Fig. S13†).

(

^Br2FcBArF₂₀). 542.1 mg 3100, 3068; ν_(CvO):Bz

=

(88.62% yield). See Fig. S68† for the molecular packing. Anal. ¹H-NMR (CD₂Cl₂, 500 MHz; δ, ppm): 30.2 (s, vbr, 8H), 10.3 (s,

calcd for C₃₄H₈BF₂₀Br₂Fe: C, 39.92; H, 0.79; N, 0.00. Found: C, br, 4H), δ 7.0 (s, 4H), δ 6.7 (s, 2H) (Fig. S35†). ¹⁹F-NMR

39.94; H, 0.86; N, 0.00. UV-vis [λ_max, nm (ε_max, M⁻¹cm⁻¹)]: 487 (CD₂Cl₂, 470 MHz; δ, ppm): −133.2 (¹J(¹⁹F–¹²¹Sb) ≈ 2 kHz)

(280), 594 (282), 716 (411) in MeTHF and 486 (257), 599 (256), (Fig. S59†).

708 (389) in MeCN. FT-IR (solid; cm⁻¹): ν_(Cp–H)= 3133, 3124,

Tetra-n-Butylammonium BArF₂₀([(nBu)₄N][B(C₆F₅)₄]).

A

1

3101 (Fig. S11†). H-NMR (acetone-d₆, 400 MHz; δ, ppm): 34.2 solutions of [(nBu)₄N]Cl (4.50 g, 16.2 mmol) in dry MeCN

(s, br, 4H), 29.6 (s, br, 4H) (Fig. S32†). ¹⁹F-NMR (acetone-d₆, (50 mL) was added into a solution of K[B(C₆F₅)₄] (11.66 g,

376 MHz; δ, ppm): −133.0, −164.2, −168.4 (Fig. S56†). ¹H-NMR 16.2 mmol) in dry MeCN (900 mL), in a 3-neck 1-liter round

(CD₂Cl₂, 500 MHz; δ, ppm): 34.5 (s, br, 4H), 32.7 (s, br, 4H) bottom flask under dry argon. Upon mixing, a white precipi-

(Fig. S33†). ¹⁹F-NMR (CD₂Cl₂, 470 MHz; δ, ppm): −135.4, tate was observed, and the mixture was stirred for 2 h to

−164.3, −169.1 (Fig. S57†).

1,1′-Diacetylferricenium BArF₂₀

ensure complete precipitation of KCl, followed by filtration.

(

^Ac2FcBArF₂₀). ^Ac2FcBArF₂₀The solvent was removed under a vacuum. The resulting clear

can be prepared following a similar procedure as ^Bz2Fc[SbF₆] oil was further dried under vacuum overnight to yield a white

(vide infra), followed by an additional metathesis step using solid. This solid was then dissolved in DCM and layered with

K[B(C₆F₅)₄] in 1,2-difluorobenzene. Alternatively, a more facile hexanes for crystallization. Clear crystals were collected, dried

procedure is also described here. In the glove box, the solution and recrystallized using the same conditions. After drying the

of 1,1′-diacetylferrocene (100.0 mg, 0.359 mmol) and crystals, these were ground up to produce a fine white powder

K[B(C₆F₅)₄] (257.8 mg, 0.359 mmol) in DCM (20 mL) was that was dried for an additional day under vacuum. The

slowly added to the solution of Ag[SbF₆] (125.9 mg, [(nBu)₄N][B(C₆F₅)₄] powder was then stored under dry argon or

0.359 mmol) in DCM (3 mL). The reaction mixture was allowed in the glove box (12.89 g, 86.34% yield). Anal. calcd for

to stir for 5 min under reduced light at RT. The solution was C₄₀H₃₆NBF₂₀: C, 52.14; H, 3.94; N, 1.52. Found: C, 52.28; H,

then filtered through Celite to remove the silver metal and in- 3.80; N, 1.51. FT–IR (solid; cm⁻¹): ν_(C–H):nBu= 2980, 2969, 2944,

soluble K[SbF₆]. The filtrate was evaporated and washed with 2880 (Fig. S139†). ¹H-NMR (acetone-d₆, 500 MHz; δ, ppm): 0.98

benzene to remove the remaining, unreacted 1,1′-diacetylferro- (t, 12H), 1.44 (sextet, 8H), 1.84 (q, 8H), 3.46 (m, 8H)

cene. Then it was dried and crystallized from DCM/hexanes. (Fig. S140†). ¹⁹F-NMR (acetone-d₆, 470 MHz; δ, ppm): −133.0,

After the crystallization, the solvent was decanted, and crystals −164.3, −168.2 (Fig. S141†).

Dalton Trans.

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Crystallographic studies

diﬀraction facility. We are also indebted to Professor Harry

B. Gray and Dr. Maxime A. Siegler for helpful discussions.

Suitable X-ray quality single crystals were grown in the glove-

box by layering hexanes onto the solution of the ferricenium

derivatives in either MeTHF or DCM in 5 mm glass tubes at

RT. The complexes ^Me10FcBArF₂₀, ^Me2FcBArF₂₀, ^nBuFcBArF₂₀References

and ^Br2FcBArF₂₀were crystallized from MeTHF/hexanes while

1 (a) T. J. Kealy and P. L. Pauson, A New Type of Organo-Iron

Compound, Nature, 1951, 168(4285), 1039–1040;

(b) G. Wilkinson, M. Rosenblum, M. C. Whiting and

R. B. Woodward, The Structure of Iron Bis-

Cyclopentadienyl, J. Am. Chem. Soc., 1952, 74(8), 2125–

2126.

all other ferricenium derivatives were crystallized using DCM/

hexanes. All reflection intensities were measured at 100(2) K

using a Gemini R diﬀractometer (equipped with Atlas detector)

with Mo Kα radiation (λ = 0.71073 Å) under the program

CrysAlisPro (Version CrysAlisPro 1.171.38.43f, Rigaku OD,

2015). The same program (but

a diﬀerent version viz.

2 (a) A. Haaland, Molecular Structure and Bonding in the 3d

Metallocenes, Acc. Chem. Res., 1979, 12(11), 415–422;

(b) S. Cantrill, An Iron-Clad Structure, Nature, 2014,

511(7509), 11–11; (c) J. I. Seeman and S. Cantrill, Wrong

But Seminal, Nat. Chem., 2016, 8(3), 193–200.

CrysAlisPro 1.171.40.53, Rigaku OD, 2019) was used to refine

the cell dimensions and for data reduction. The structures

were solved with the program SHELXT-2018/2 and were refined

on F2 by full-matrix least-squares technique using the

SHELXL-2018/3 program package.⁶⁰Numerical absorption cor-

rection based on Gaussian integration was applied using a

multifaceted crystal model by CrysAlisPro. Non-hydrogen

atoms were refined anisotropically. In the refinement, hydro-

gen was treated as riding atoms using SHELXL default

parameters.

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Electrochemical measurements

A three-electrode setup was used for all voltammetry experi-

ments with a 3.0 mm glassy carbon disk working electrode, a

carbon rod counter electrode, and a leak-free Ag/AgCl reference

electrode inside an inert atmosphere box. Three separate

2.0 mm diameter reference electrodes were filled with 3.4 M

KCl aqueous solutions (i.e., LF2) from Innovative Instruments,

Inc. The electrodes were stored in either a 0.05 M H₂SO₄

aqueous solution or a saturated KCl aqueous solution between

experiments. All potentials were referenced to the leak-free Ag/

AgCl electrode by measuring the ferrocene/ferricenium couple

under identical conditions. The electrodes were cleaned with

acetone and ultrapure water. The solvents used were MeCN,

DCM, and MeTHF. The electrolytes, [(nBu)₄N][B(C₆F₅)₄] and

[(nBu)₄N][PF₆], were further purified by recrystallization from

MeCN and ethanol, respectively. The solutions (1 or 2 mM

analyte, 100 mM electrolyte) were scanned anodically then

cathodically within a 1.5–2 V potential range at varying scan

rates. (For further details, see ESI†).

4 Soon after the discovery of the bis(η⁵-cyclopentadienyl) iron

(^II) complex in 1951, the term “ferrocene” was coined by

Woodward and Whiting. They established the aromatic

character of the cyclopentadienyl rings of ferrocene,

coining its now ingrained name having the suﬃx “-ene” in

analogy to organic aromatic compounds such as benzene,

naphthalene, etc. Additionally, the prefix “ferro-” was used

to specify the iron(^II) oxidation state in ferrocene while the

prefix “ferri-” was assigned to the one-electron oxidized

counterpart with the iron(^III) center and thus named the

ferricenium ion. A common alternative spelling for the

latter is ferricinium. Our review of the past seven decades

of literature revealed that with a dramatic increase in appli-

cations of ferrocene and ferricenium ion in a variety of

fields including materials, medicine and energy, the orig-

inal name of the oxidized complex gradually fell out of

common usage and was superseded by the name ferroce-

nium. The ratio of the usage of the names ferricenium/fer-

ricinium and ferrocenium has reversed over the past

several decades. Today, the oxidized ferrocene is commer-

Conﬂicts of interest

The authors declare no competing financial interest.

Acknowledgements

The authors are most thankful to the University of North

Carolina at Greensboro for the financial support provided in

the form of startup funds and the New Faculty Research

Award. The Joint School of Nanoscience and Nanoengineering

is gratefully acknowledged for providing access to the X-ray

Dalton Trans.

View Article Online

Paper

Dalton Transactions

cially marketed as “ferrocenium” and either names are

found in the current literature. In 2005, IUPAC strongly rec-

Methodology Composed of Cyclic Voltammetry and

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−

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Article Doi

Synthetic, spectroscopic, structural, and electrochemical investigations of ferricenium derivatives with weakly coordinating anions: ion pairing, substituent, and solvent effects

DOI: 10.1039/d1dt01192h

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Full text of DOI:10.1039/d1dt01192h

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