Synthesis, spectroscopy, electrochemistry and DFT of electron-rich ferrocenylsubphthalocyanines

Article abstract of DOI:10.3390/molecules25112575

A series of novel ferrocenylsubphthalocyanine dyads Y-BSubPc(H)₁₂ with ferrocenylcarboxylic acids Y-H = (FcCH₂CO₂-H), (Fc(CH₂)₃CO₂-H) or (FcCO(CH₂)₂CO₂-H) in the axial position were synthesized from the parent Cl-BSubPc(H)₁₂ via an activated triflate-SubPc intermediate. UV/Vis data revealed that the axial ferrocenyl-containing ligand did not influence the Q-band maxima compared to Cl-BSubPc(H)₁₂. A combined electrochemical and density functional theory (DFT) study showed that Fe group of the ferrocenyl-containing axial ligand is involved in the first reversible oxidation process, followed by a second oxidation localized on the macrocycle of the subphthalocyanine. Both observed reductions were ring-based. It was found that the novel Fc(CH₂)₃CO₂BSubPc(H)₁₂ exhibited the lowest first macrocycle-based reduction potential (-1.871Vvs. Fc/Fc⁺) reported for SubPcs till date. The oxidation and reduction values of Fc(CH₂)nCO₂BSubPc(H)₁₂ (n = 0-3), FcCO(CH₂)₂CO₂BSubPc(H)₁₂, and Cl-BSubPc(H)₁₂ illustrated the electronic influence of the carboxyl group, the different alkyl chains and the ferrocenyl group in the axial ligand on the ring-based oxidation and reduction values of the SubPcs.

Full text of DOI:10.3390/molecules25112575

molecules
Article
Synthesis, Spectroscopy, Electrochemistry and DFT
of Electron-Rich Ferrocenylsubphthalocyanines
Pieter J. Swarts 1 and Jeanet Conradie 1,2,
*
1
Department of Chemistry, University of the Free State, Bloemfontein 9300, South Africa;
swarts.pieter@gmail.com
Department of Chemistry, UiT—The Arctic University of Norway, N-9037 Tromsø, Norway
Correspondence: conradj@ufs.ac.za; Tel.: +27-(51)-4012194; Fax: +27-4017295
2
*
Academic Editors: Burgert Blom, Erika Ferrari, Vassilis Tangoulis, C
and Constantinos C. Stoumpos
Received: 14 May 2020; Accepted: 29 May 2020; Published: 1 June 2020
édric R. Mayer, Axel Klein
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
Abstract: A series of novel ferrocenylsubphthalocyanine dyads Y-BSubPc(H)12 with ferrocenyl-
carboxylic acids Y-H = (FcCH CO -H), (Fc(CH ) CO -H) or (FcCO(CH ) CO -H) in the axial position
2
2
2 3
2
2 2
2
were synthesized from the parent Cl-BSubPc(H)12 via an activated triflate-SubPc intermediate.
UV/Vis data revealed that the axial ferrocenyl-containing ligand did not influence the Q-band
maxima compared to Cl-BSubPc(H) . A combined electrochemical and density functional theory
12
(DFT) study showed that Fe group of the ferrocenyl-containing axial ligand is involved in the
first reversible oxidation process, followed by a second oxidation localized on the macrocycle of
the subphthalocyanine. Both observed reductions were ring-based. It was found that the novel
Fc(CH ) CO BSubPc(H) exhibited the lowest first macrocycle-based reduction potential (
−
1.871 V vs.
2
3
2
12
+
Fc/Fc ) reported for SubPcs till date. The oxidation and reduction values of Fc(CH )
n
CO BSubPc(H)
2
2
12
(
n = 0–3), FcCO(CH ) CO BSubPc(H) , and Cl-BSubPc(H) illustrated the electronic influence of
2 2 2 12 12
the carboxyl group, the different alkyl chains and the ferrocenyl group in the axial ligand on the
ring-based oxidation and reduction values of the SubPcs.
Keywords: subphthalocyanines; ferrocene; redox potentials; DFT; electron rich
1
. Introduction
The discovery of ferrocene in 1951 [
ferrocene has been extensively studied and well-reviewed in organic and inorganic chemistry [
The research of ferrocene-containing compounds thrives, and new research keeps on growing due to
their varying successful applications. This includes asymmetric catalysis [ ], non-linear optics [ ],
1] unlocked an entirely new research field and over the years
2–5
].
6,
7
8
antineoplastic properties [
ideal redox behavior of the Fe
Fc(CH2) CO H with n = 0 (
chain separating the ferrocene moiety and electron-withdrawing carboxy group decreases, the formal
reduction potential of Fe of the ferrocenyl group also decreases [13 14]. The electron-withdrawing
carboxyl group directly bound to ferrocene in and FcCO(CH ) CO H ( ), led to an increase in the
formal reduction potential of the ferrocene moiety compared to free ferrocene [13,14].
Subphthalocyanines such as ClBSubPc(H) , , have been used in research for almost 50 years [15
9
,
10], antimalarial activity [11] and especially electrochemistry due to the
II/III
couple [
6
,
9
,
12]. For a series of four ferrocenyl carboxylic acid dyads
) (Figure 1), it was found that as the length of the alkyl
n
2
1
), 1 (
2
), 2 ( ) or 3 (4
3
,
1
5
2
2
2
6
12
]
due to their diverse applications including light-emitting diodes [16], dye-sensitized solar cells [17],
sensors [18] and photodynamic therapy [19]. The uses and reactivity of SubPcs can be modified by
substituting the axial ligand as well as by functionalizing the ring substituents [18]. Redox data of the
ring-based oxidation and reduction processes of SubPcs showed that axial or peripheral substitution
has a smaller influence on the shift of the oxidation and reduction potential of the SubPc (ca 0.1 V
Molecules 2020, 25, 2575; doi:10.3390/molecules25112575
www.mdpi.com/journal/molecules

Molecules 2020, 25, 2575
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shift) than non-peripheral substitution (ca 0.3 V shift) [20]. Ferrocenylsubphthalocyanine dyads with a
direct ferrocene-boron or substituted ferrocene-boron bond have been investigated by Nemykin and
co-workers [21,22]. They found that the first oxidation process in these ferrocenylsubphthalocyanine
dyads is ferrocene based, while second oxidation and the two oberverved reduction processes are
centered at the macrocyclic ligand of the SubPc [21,22].
In this paper, we report the synthesis, characterisation and electrochemical studies of three novel
ferrocenylsubphthalocyanine dyads
8
,
10 and 11. The appropriate ferrocenyl carboxylic acids
, to form the three
, Fc(CH ) CO BSubPc(H) , 10,
2, 4 and
5
was directly bound to the boron atom in the axial position of Cl-BSubPc(H) ,
6
2
1
new ferrocenylsubphthalocyanine dyads, FcCH CO BSubPc(H) ,
8
2
2
12
2 3
2
12
and FcCO(CH ) CO BSubPc(H) , 11 (see Scheme 1 [23] and Figure 1). For comparative purposes,
2
2
2
12
the two known ferrocenylsubphthalocyanine dyads
systematically evaluate (i) the effect of the electron rich macrocycle of SubPcs
reduction potential of Fe of the ferrocenyl group of the ferrocenylcarboxylic acid moieties of
and (ii) the influence of the axially ferrocenylcarboxylic acid on the UV/vis maxima and the
ring-based oxidations and reductions of the ferrocenylsubphthalocyanine dyads 7–11.
7
[
21] and
9
[
24] were included in this study to
11 on the formal
–11,
7
–
7
1–5
Figure 1. Structures of ClBSubPc(H) ,
6, FcCO BSubPc(H) , 7 [21], FcCH CO BSubPc(H) ,
2 12 2 2 12
1
2
8
(novel), Fc(CH ) CO BSubPc(H)12, 9 [24], Fc(CH ) CO BSubPc(H)12, 10 (novel) and FcCO(CH ) CO
2 2 2 2 3 2 2 2 2
BSubPc(H)12
,
11 (novel), containing Cl or the ferrocenylcarboxylic acids FcCO H (
1
), FcCH CO H (
2
),
Fc(CH ) CO H (3), Fc(CH ) CO H (4) and FcCO(CH ) CO H (5) in the axial position. n = number of
2 3 2 2 2 2
2
2
2
2
2
2
alkyl groups in the axial ligand.
2
. Results and Discussion
2
.1. Synthesis
The free ferrocenylcarboxylic acids
1–5, were synthesized using slightly modified methods
than previously published [13], as described in our previous publication [14]. The synthesis of
ferrocenylsubphthalocyanine dyads
reactions. In the first step any of the two well-known halophiles, such as Me Si groups or Ag ions
7
,
8
,
10 and 11 was complex due to the moisture sensitivity of the
+
3
are used to irreversibly substitute the axial chloride in the axial position [23]. We found that using
Me SiOTf did not give desirable yields. Using AgOTf, however, enabled us to increase our yields by
3
more than 30% compared to previous studies [21]. Once the activated triflate species was formed the
activated SubPc showed considerable reactivity toward the different ferrocenyl acids
Scheme 1. The success of the synthesis is drastically affected when not working under strict Schlenk
conditions. SubPcs 10 and 11 were soluble in common organic solvents such as dichloromethane
DCM), chloroform and tetrahydrofuran (THF).
1, 2, 4 and 5 see
7, 8,
(

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Scheme 1. Reaction scheme for the synthesis of FcCO BSubPc(H) , 7, FcCH CO BSubPc(H) , 8,
2
12
2
2
12
Fc(CH ) CO BSubPc(H) ,
9 [24], and Fc(CH ) CO BSubPc(H) , 10, (similar for FcCO(CH ) CO
2 3 2 12 2 2 2
2
2
2
12
BSubPc(H)12
,
11) from ClBSubPc(H)12
,
6
. Note: AgOTf = silver trifluoromethanesulfonate, Me SiOTf
3
=
Trimethylsilyl trifluoromethanesulfonate and DIPEA = N,N-Diisopropylethylamine.
1
2
.2. H NMR
1
The H-NMR results showed that signals of the SubPc ring protons (H) shifted upfield by ca.
0
.04–0.07 ppm for SubPcs
7
–
11 relative to the signals of the parent ring protons ClBSubPc(H) ,
6
.
12
1
The most significant effect on H-NMR was observed for the signals of the ferrocenyl axial ligands peaks.
Ferrocenyl of SubPc 7 (signals of the protons of substituted-Cp = 3.96 and 3.95, unsubstituted-Cp = 3.63)
is the closest to the electron rich macrocycle of SubPc and as a result the signals of the protons of the
substituted and unsubstituted-Cp rings shift the furthest upfield with ppm shifts between 0.50 and
0.88 ppm compared to ferrocenyl acid 1 (substituted-Cp = 4.84 and 4.45, unsubstituted-Cp = 4.24).
With the increase of (CH ) linker groups the distance between the ferrocenyl moiety and SubPc
2
increases and as a result the ferrocene peaks shift less upfield as the chain lengths increase.
2
.3. UV/vis
As usually found for SubPcs, the UV/Vis spectra of SubPcs
7–11 exhibited the two main transitions,
the Soret band between 250 and 350 nm, and the Q-band between 450 and 620 nm, see Figure 2.
The substitution of the parent macrocycles axial chloride with the different ferrocenylcarboxylic acid
groups, had a negligible effect on the Q-bands position, see Table 1. In a similar fashion there was
no shift when comparing the Q-bands of SubPcs
and SubPcs 11 indicates, in agreement with density functional theory (DFT) and time-dependent
density-functional theory (TDDFT) calculations (see discussion in Section 2.5 below), that the Soret and
7–11 to 6. The similar Soret and Q-bands for 6
7
–
Q-bands involve
π
-
π
* transitions. SubPcs
7
to 11 followed the Beer-Lambert law and no aggregation in
−
3
the concentration range of 0.01–0.10 (× 10 M) was observed, see Figure 2.
Table 1. UV/vis data of SubPcs 6–11 in tetrahydrofuran (THF).
Soret Band
Max
Q Band
2nd Shoulder
1st Shoulder
Max
ClBSubPc(H) , 6, [20]
308
299
300
302
328
327
525
515
518
505
521
523
532
530
534
544
539
542
564
563
563
563
563
563
12
FcCO BSubPc(H) , 7
2
12
FcCH CO BSubPc(H) , 8
2
2
12
Fc(CH ) CO BSubPc(H) , 9, [24]
2
2
2
12
Fc(CH ) CO BSubPc(H) , 10
2
3
2
12
FcCO(CH ) CO BSubPc(H) , 11
2
2
2
12

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Figure 2.
.05 and 0.10 (
solvent. UV/vis spectra of SubPc
between the absorbance A and concentration of FcCO BSubPc(H) ,
(
a
)–(
d
×
) The UV/vis spectra of SubPc (
7
,
8
,
10 and 11) at concentrations 0.01, 0.02, 0.03, 0.04,
10 M), obtained with a 1 cm pathlength cuvette with tetrahydrofuran (THF) as
can be found in reference [24]. Insert: The Beer-Lambert correlation
−3
0
9
3
−1
−1
7
12
(
ε
= 176547 dm mol cm );
2
−1
3
−1
3
FcCH CO BSubPc(H)12,
8
(
ε
= 166317 dm mol cm ), Fc(CH ) CO BSubPc(H)12,
9
(ε
= 153630 dm
2
1
2
−1
2 2
3
2
−
−1
−1
mol cm
[24]), Fc(CH ) CO BSubPc(H) , 10
(ε
= 145673 dm mol cm ) and FcCO(CH ) CO
2
3
2
12
−1
2 2
2
3
−1
BSubPc(H) , 11 (ε = 131772 dm mol cm ) at indicated wavelength in nm.
12
2
.4. Cyclic Voltammetry
The redox properties of the ferrocenylsubphthalocyanine dyads, SubPc
7,
8,
10 and 11 were
examined utilizing cyclic voltammetry (CVs) and linear sweep voltammetry (LSV). The CVs and
◦
the LSVs of SubPc
0
7,
8,
10 and 11, performed at 25 C in dichloromethane (DCM) at a scan rate of
−
1
.100 Vs are shown in Figure 3 with the relevant electrochemical data summarised in Table 2. Data
of the free ferrocenylcarboxylic acids 14] and SubPcs 20] and 24] are added for comparative
reasons in Table 2. The CVs of SubPc
1–5
[
6
[
9 [
7,
8,
10 and 11 (Figure 3) showed two oxidation and two
II/III
reduction peaks in the experimental solvent window of DCM. The oxidation of the Fe group (Fe
the ferrocenyl moiety on the axial ligand is the first observed redox process for all five SubPc
) of
11
7
–
.
The assignment that the Fe group is oxidised first, is supported by DFT calculations (see computational
analysis below) and is in agreement with literature [21 22 24]. Both oxidation and the first reduction
p
peaks are chemically reversible with peak ratios approaching 1 and peak current separations, ∆E ,
,
,

Molecules 2020, 25, 2575
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of 0.074–0.076 (ferrocenyl oxidation), 0.080–0.084 V (wave I in Figure 3) and 0.082–0.086 V (wave II in
Figure 3). The second ring-based reduction (wave III in Figure 3) were irreversible and did not show
−
any re-oxidation peaks. The linear sweep voltammetry showed, as expected, 1 e redox couples for Fc
and waves I to III. The reaction scheme for the redox signals of SubPc
Scheme 2. The electrochemical band gaps estimated from the SubPc-based redox potentials (peaks
I and II) become wider in the order of compound 11 and 10. However, the first ring-based
7–10 (similar for 11) is given in
, 7, 8, 9
oxidation process (peak I) originates from a cationic SubPc and not from the neutral SubPc as is the
case of the Q-band maximum, that is constant for SubPcs 7–11.
Figure 3. Cyclic voltammetry (CVs) and linear sweep voltammetry (LSVs) of SubPc,
7—(purple),
8
—(blue), 10—(red) and 11—(grey) in dichloromethane (DCM). Concentration of 7, 8, 10 and
−
3
−1
−1
1
1
= 0.0005 mol dm . Scan rate for CVs is 0.100 V s and LSVs at 0.001 V s . Scan directions are
◦
indicated at starting point of each scan. DmFc was used as internal reference with E ’ (DmFc) =
vs. free Fc/Fc at 0 V.
−
0.610
+

Molecules 2020, 25, 2575
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−3
Table 2. Cyclic voltammetry data of ligands
1
–
5
and SubPcs
6–11 in DCM containing 0.1 mol dm
n
◦
[
N( Bu) ][B(C F ) ] as supporting electrolyte at a scan rate of 0.100 V/s at 25 C.
4
6 5 4
Ep a
E ’ (V), ∆Ep (V)
◦
ip (µA) b, Current Ratio c
Description
FcCOOH d
1
Fc
Fc
Fc
Fc
Fc
0.321
0.047
0.020
0.011
0.330
0.284, 0.074
0.014, 0.066
−0.015, 0.070
−0.024, 0.070
0.295, 0.070
3.60, 0.99
3.79, 0.99
3.87, 0.99
3.98, 0.99
3.66, 0.99
d
FcCH COOH
2
2
d
Fc(CH ) COOH
3
2
2
d
Fc(CH ) COOH
4
2
3
d
FcCO(CH ) COOH
5
2
2
SubPc 6
ClBSubPc(H)12
DmFc
Wave I
Wave II
Wave III
−0.647
−0.610, 0.076
0.628, 0.086
-, -
3.89, 0.99
3.08, 0.99
3.63, -
e
0.674
−1.519
−2.050
-, -
-, -
SubPc 7
DmFc
Fc
Wave I
Wave II
Wave III
−0.647
−0.610, 0.074
0.224, 0.076
0.670, 0.082
−1.601, 0.084
-, -
3.91, 0.99
3.61, 0.99
3.34, 0.99
3.47, 0.99
-, -
FcCO SubPc(H)
0.262
2
12
(
n = 0)
0.711
−1.643
−2.124
SubPc 8
DmFc
Fc
Wave I
Wave II
Wave III
−0.647
−0.005
0.710
−0.610, 0.075
−0.043, 0.076
0.670, 0.080
−1.674, 0.082
-, -
3.84, 0.99
3.61, 0.99
3.38, 0.99
3.49,0.99
-, -
FcCH CO SubPc(H)
2
2
12
(
n = 1)
−1.715
−2.154
SubPc 9
DmFc
Fc
Wave I
Wave II
Wave III
−0.647
−0.021
0.712
−0.610,0.074
−0.058, 0.074
0.670, 0.084
−1.741, 0.084
-, -
3.89, 0.99
3.66, 0.99
3.31, 0.99
3.42, 0.99
-, -
f
Fc(CH ) CO SubPc(H)
2
2
2
12
(n = 2)
−1.783
−2.264
SubPc 10
DmFc
Fc
Wave I
Wave II
Wave III
−0.647
−0.024
0.711
−0.610, 0.075
−0.062, 0.076
0.670, 0.082
−1.829, 0.084
-, -
3.97, 0.99
3.58, 0.99
3.27, 0.99
3.39, 0.99
-, -
Fc(CH ) CO SubPc(H)
2
3
2
12
(n = 3)
−1.871
−2.344
SubPc 11
FcCO(CH ) CO SubPc(H)
DmFc
Fc
Wave I
Wave II
Wave III
−0.647
0.300
0.711
−0.610, 0.074
0.262, 0.076
0.670, 0.082
−1.482, 0.086
-, -
3.84, 0.99
3.57, 0.99
3.35, 0.99
3.41, 0.99
-, -
2
2
2
12
−1.525
−2.004
a
is the peak anodic peak for oxidation (Eox) and peak cathodic peak for reduction (Ered). b
E
p
i
p
is the peak anodic
c
peak for oxidation (ipa) and peak cathodic peak for reduction (ipc). peak current ratio = ipc/ipa for oxidation and
ipa/ipc for reduction.d Data from reference [14]. Data from reference [20]. f Data from reference [24].
e
Scheme 2. The reaction scheme of the redox signals of SubPcs
solvent, see Figure 3. Reaction scheme for 11 is similar.
7–10 in dichloromethane (DCM) as
2
.4.1. Effect of SubPc on Ferrocenyl Moiety Oxidation
The oxidation potentials, E ’, of the Fe group (FeII/III) of the ferrocenyl moiety on the axial ligand
◦
of SubPcs
7
–
11 range between
−
0.062 and 0.262 V and with
∆
E
p
between 0.074 and 0.076 V (Figure 3
◦
◦
and Table 2). Comparing E ’ (of the first process) of the Fc moiety of SubPcs
7–11 with the E ’ of the
free ferrocenyl acids
1–5 (obtained under the experimental same conditions [14] as the SubPcs 7–11) it
is clear that the aromatic SubPc ring acts as an electron-donating specie in the complex, decreasing
◦ ◦
lower oxidation potential) E ’ of the Fe group of SubPcs 7–11 with 0.060–0.038 V relative to E ’ of
(

Molecules 2020, 25, 2575
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the Fe group of the free ferrocenylcarboxylic acids
1–
5. The largest electron-donating effect was,
as expected, on the ferrocenyl moiety closest to the ring, with a decrease of 0.060 V between
7 (0.224 V)
and (0.284 V). As the chain length increases with (CH ) groups, the effect of the SubPc on ferrocenyl
1
2
oxidation becomes less. The Fe group in SubPc 10 with three (CH ) groups separating the ferrocenyl
2
moiety from the SubPc, is more shielded from the electron-donating effect of the SubPc and as a result
◦
E ’ of Fe only shifts with only 0.038 V between 10
(
−
0.062 V) and
4
( 0.024 V). With an additional
−
◦
carbonyl group as well as the (CH ) groups in SubPc 11, E ’ of the Fe group of SubPc 11 shifted the
2
least in the range namely, with only a 0.033 V between 11 (0.262 V) and 5 (0.295 V).
2
.4.2. Effect of Chain Length on Ferrocenyl Moiety Oxidation
◦
The formal reduction potential E ’ of Fe in the axial ligand of SubPc
7
is at 0.224 V compared
◦
to free ferrocene 0 V. With one additional CH spacer group between ferrocene and the SubPc, E ’
2
of Fe is significantly shifted by 0.267 V to
−
0.043 V in SubPc
8
. SubPcs
9
and 10 had two and three
0.058 V and
◦
additional CH spacer groups respectively, with E ’ of the ferrocene group decreasing to
−
2
−
0.062 V respectively. There was an exponential decrease to lower oxidation potential with an increase
in (CH ) chain lengths for SubPc 10, see Figure 4a. This is because Fe of the ferrocene group is
increasingly shielded from the electron donating effect of the SubPc and electron-withdrawing effect of
the carboxy group [14] as the (CH2) chain length increases. This trend was similar as observed for the
free ferrocenylcarboxylic acids 1–4 [14], see Figure 4a.
n
7–
2
n
◦
(a) Relationship between E ’ of the Fe group and the number n of CH groups in the
2
Figure 4.
side chain of Fc–(CH2)
dyads 10. Data of
n
–CO for ferrocenyl carboxylic acids
2
1
–
4
and ferrocenylsubphthalocyanine
7–
1
–
4
from reference [14], data of
9
from reference [24]. ( ) Relationship between
b
◦
E ’(wave II) and Epc (wave III) and n, of the first and second ring-based reductions respectively,
of ferrocenylsubphthalocyanine dyads 7–10.
2
.4.3. Effect of Carboxyl and Carbonyl Group on Ferrocenyl Moiety Oxidation
◦
The formal reduction potential E ’ of the Fe group of SubPc
7
, separated by a carboxyl group from
the electron donating SubPc, is at 0.224 V compared to free ferrocene 0 V, implying that the electron
withdrawing effect of the carboxyl group is larger than the electron donating effect of the SubPc on the
◦
E ’ of the Fe group. The additional electron withdrawing carbonyl group bound next to the ferrocenyl
◦
moiety in SubPc 11 resulted in the highest oxidation potential of the Fe group with E ’ at 0.262 V,
Figure 3. With the two electron withdrawing CO groups in SubPc 11 the oxidation potential was
◦
0
.038 V higher than E ’ of Fe in SubPc
7
(0.224 V) containing only one electron withdrawing CO group.

Molecules 2020, 25, 2575
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2
.4.4. Ring Based Reductions
The first (wave II) and second (wave III) ring-based reductions of SubPcs
7–11 are lower than that of
SubPc
effect on the aromatic ring electrons of SubPcs
effect of the neutral axial ferrocenylcarboxylic ligand is a combination of the electron-withdrawing
effect of the carboxyl group [14], the electron donating effect of the alkyl chain [10 14 25] and the
electron donating effect of ferrocenyl [26]. Both ring-based reductions of SubPcs 11, redox waves
6. The neutral axial ferrocenylcarboxylic ligands of SubPcs
7–11 thus have a net electron donating
7–
11 compared to Cl in SubPc
6. The net electron donating
,
,
7–
II and III in Figure 3 and Table 2, followed the same trend, namely the reduction value decreases
near linear as the number of (CH ) groups, n, in the different axially bonded ferrocenyl carboxylic
2
acid moieties (Fc-(CH2)
than that of SubPc , see Figure 4b. The aromatic ring electron density of the SubPcs systematically
increased as n of the alkyl group in –OCO(CH2) -Fc increases. SubPc 10 exhibited the lowest first
ring-based reduction potential ( 1.872 V), reported to date [18 21 22 27], due to the (OOC(CH ) )Fc
n
2
-CO ) increases in SubPcs 7–10, with the reduction values of SubPc 11 higher
7
n
−
, , ,
2
3
group’s net electron-donating effect. The donating effect is the most prominent on SubPc 10 containing
the longest alkyl chain. Propyl (n = 3) is more electron donating than ethyl (n = 2) that is more electron
donating then methyl (n = 1) [10,14,25].
2
.4.5. Ring Based Oxidation
◦
E ’ of the first ring-based oxidation of SubPcs
7
–
11 (wave I in Figure 3) is exactly the same, with
◦
E ’ = 0.670 V for all five complexes
group (Fc ) in the different oxidised ferrocenyl carboxylic acid moieties (Fc -(CH )
Fc -CO(CH ) -CO ) for SubPcs
7–
11. This is because the charge located on the ferrocenium
+
+
n
-CO ) and
2
2
+
+
(
7–11, is largely isolated from the rest of the molecule. The Fc group
2
2
2
is highly electronegative [26
bonded to it. The first ring-based oxidation of SubPcs
electron withdrawing effect of the oxidized axial ferrocenylcarboxylic ligands of SubPcs
combination of the electron-withdrawing effect of the carboxyl group [14], the electron donating effect
of the alkyl chain [10 14 25] and the highly electron withdrawing effect of ferrocenium [26 28], shifting
11 with exactly the same value namely 0.042 V more positive
,
28], withdrawing any available electron density from the alkyl groups
11 is consequently influenced by the net
11 that is a
7
–
7
–
,
,
,
the first ring-based oxidation of SubPcs
than E ’ of the first ring-based oxidation of SubPcs
7
–
◦
6
7
at 0.628 V. The net electron withdrawing effect of
–11 is thus larger than the electron withdrawing
the oxidized axial ferrocenylcarboxylic ligands in
effect of Cl in
reversible ring-based oxidation with peak current ratios approaching 1 and peak current separation
Ep between 0.080 and 0.084 V, see Table 2.
6, on the aromatic ring electron density of the SubPcs. It was possible to get chemically
∆
2
.5. Computational Analysis
The structure of the ferrocenylcarboxylic-containing SubPc dyads 7–11, were optimised using
density functional theory (DFT) methods to solve the Schrödinger equations in order to gain further
insight into the redox properties of the ferrocene dyads. In agreement with previous studies on related
SubPcs, the top three high-lying occupied molecular orbitals (MOs) of the neutral species are of iron-d
character while the lowest unoccupied MO (LUMO) and the LUMO+1 have
This confirm Fe(II) to Fe(III) oxidation and ring-based reduction respectively. However, since the top
three highest occupied MOs (HOMOs) of 11 are all iron-d based, and Fe(III) to Fe(IV) oxidation is not
expected, it was essential to also optimise the cation (oxidised) species, to locate the locus of the second
experimentally observed oxidation. It is known that orbitals can rearrange upon oxidation [29 31].
The DFT results of oxidised SubPcs 11 all showed that the LUMO is of iron-d character (the first
π-ring character [21,24].
7–
–
7–
oxidation, see Figure 5) and the HOMO is on the SubPc ring (the second oxidation, see Figure 5).
HOMO-1, also on the SubPc ring, will be the second ring oxidation, however, it is out of the solvent
window in cyclic voltammetry scans and not experimentally observed.

Molecules 2020, 25, 2575
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LUMO
HOMO
Neutral
HOMO-1
Cation
Figure 5. Selected PBE1PBE/6-311G(d,p) frontier molecular orbitals (MOs) for cation SubPc,
of 0.03 e/Å was used for the orbital plots. Colour code of atoms (online version): Fe (purple), B (yellow), C
8
. A contour
3
(grey), O (red), H (white). HOMO = highest occupied molecular orbital and LUMO = lowest unoccupied
molecular orbital.
For all neutral SubPcs
7–
11 it was found that the HOMO-3 is located on the macrocycle of the
11, in agreement with TDDFT
calculations, originates thus from HOMO-3 to LUMO excitation of the neutral SubPc. The DFT
calculated energy difference between HOMO-3 and LUMO is the same for SubPcs 11 within 0.003 eV,
explaining why the experimental Q-band maxima of SubPcs 11 is exactly the same, namely 563 nm.
SubPc. The π-π* transition of the Q-band excitation [32] of SubPcs 7–
7–
7
–
3
. Materials and Methods
Solid reagents (Sigma-Aldrich, Johannesburg, South Africa, STREM, Newburyport, MA, USA
and Merck, Johannesburg, South Africa) were used as received. Liquid reagents (Sigma-Aldrich
and Merck) were used without any further purification unless specified otherwise. Solvents were
distilled, and water was double distilled. Organic solvents used in this study were dried according to
published methods [33]. Melting points are uncorrected and were determined with a BX 51 microscope
(Olympus, Johannesburg, South Africa) equipped with a THMS 600 hot stage (Linkam, Johannesburg,
South Africa).
3
.1. Spectroscopy Measurements
1H-, 11B- and 13C-NMR analysis were performed for all compounds in the study. H and 13C
1
◦
spectra were recorded at 25 C on a 600 MHz AVANCE II NMR spectrometer (Bruker, Johannesburg,
South Africa) at 600.28 MHz and 150.95 MHz respectively. 11B-NMR spectra were recorded at 25 C
◦

Molecules 2020, 25, 2575
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on a 400 MHz AVANCE III NMR spectrometer (Bruker, Johannesburg, South Africa) at 128.38 MHz.
Hydrogen and carbon chemical shifts are relative to hydrogen and carbon in CDCl at 7.24 ppm and
3
7
7.16 ppm, respectively. The following abbreviations are used to describe peak patterns: s = singlet,
d = doublet, t = triplet, q = quartet and m = multiplet. UV/vis spectra were recorded on a Cary 5000
UV-Vis-NIR Spectrophotometer (Varian, Johannesburg, South Africa).
3
.2. Cyclic Voltammetry
All the electrochemical experiments were performed in an Lab Master SP glove box (M Bruan
Munich, Germany) under a high purity argon atmosphere (H O and O < 10 ppm). Cyclic voltammetry
2
2
(CV) measurements were performed utilising a PARSTAT 2273 potentiostat (Princeton Applied
Research, Oak Ridge, TN, USA) running the Powersuite software (Version 2.58). A three-electrode cell
−
6
2
was used. A glassy carbon electrode with a surface area 3.14
×
10 m was chosen as working electrode,
platinum wires were chosen as auxiliary and reference electrodes. The glassy carbon working electrode
was polished and prepared before every experiment on a Buhler polishing mat first with 1-micron and
1
4
then with -micron diamond paste, rinsed with H O, acetone and dichloromethane (DCM), and dried
2
before each experiment. Electrochemical analysis of the complexes was performed in DCM (anhydrous,
≥
99.8%, contains 40–150 ppm amylene as a stabiliser) at room temperature. Solutions were made in
3
0
.001 dm spectrochemical grade anhydrous DCM containing ca. 0.0005 M of an analyte, 0.0005 mol
−3
−3
dm of internal reference (decamethylferrocene, DmFc) and 0.1 mol dm of supporting electrolyte
tetrabutylammonium tetrakispentafluorophenylborate, [N( Bu) ][B(C F ) ] in DCM. Experimental
n
4
6 5 4
potential data was collected vs. the Pt wire reference electrode but is reported vs. the redox couple of
+
◦
+
n
ferrocene, Fc/Fc at 0 V. E ’ (DmFc) = 0.610 V vs. Fc/Fc at 0 V in DCM/[N( Bu) ][B(C F ) ]. Scan
−
4 6 5 4
−1
rates were between 0.05 and 5.00 Vs . Electrochemical reversibility (or Nernstian behaviour) of redox
processes is indicated by a peak current ratio (ipc/ipa for oxidation and ipa/ipc for reduction) of 1 [34 35
,
]
and peak current separation ∆E = |Epa − Epc| = 0.059 V for a one-electron transfer process [36]. In this
experiment, due to experimental cell imperfections and ohmic drop effects,
∆
E
p
slightly larger than
-
+
0
.059 V was obtained, even for the known 1 e transfer processes of decamethylferrocene, DmFc /DmFc,
◦
namely 0.074–0.076 V [37
–39]. The formal reduction potential is determined by E ’ = (Epa – Epc)/2 for
an electrochemically reversible (and quasi reversible) process where Epa (Epc) = anodic (cathodic) peak
potential and ipa (ipc) = anodic (cathodic) peak current.
3
.3. DFT Calculations
Density functional theory (DFT) optimisations were performed on the neutral and oxidised
molecules in the gas phase using the hybrid PBE1PBE [40
triple- basis set 6-311G(d,p) basis set, as implemented in the Gaussian 16 package [43]. Single point
calculations using pure BP86 [44 46] exchange-correlation were performed in DCM as the solvent, using
–42] exchange-correlation functional and the
ζ
–
the IEF-PCM model (integral equation formalism - polarisable continuum model) [47] which solved
the non-homogeneous Poisson equation by applying the integral equation formalism (IEF) variant [48].
Time-dependent density-functional theory (TDDFT) calculations were done on the same level of
theory. Both the gas phase PBE1PBE/6-311G(d,p) and solvent phase BP86/6-311G(d,p) results gave the
same molecular orbital (MO) insight into the observed experimental redox processes. The PBE1PBE
exchange-correlation functional previously showed to give good agreement between calculated and
experimentally determined bond distances and angles in ferrocene-containing compounds [22,49,50].
3
.4. Preparationof SubPcs 7, 8, 10 and 11
FcCO H, , FcCH CO H, , Fc(CH ) CO H,
1
2
4
, FcCO(CH ) CO H,
5, and ClBSubPc(H)12,
6, were
2
2
2
2 3
2
2 2
2
synthesised using slight modifications to literature methods (see the Supplementary Materials) [12,13,24]
.
Compounds 7, 8, 10 and 11 were characterised by NMR, UV/Vis, elemental analysis and melting
point (m.p.).

Molecules 2020, 25, 2575
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3
.4.1. Preparation of FcCO SubPc(H) , 7
2 12
To a solution of chlorosubphthalocyanine,
6, (200 mg; 0.46 mmol) and dry toluene (3 mLmL),
silver trifluoromethanesulfonate (150 mg, 0.58 mmol; 1.25 equivalent (eq.)) was added and the
◦
mixture stirred at 45 C, under argon atmosphere for 4 h. Once the (OTf)SubPc(H)12 was generated,
ferrocenylcarboxylic acid, 1, (212 mg, 0.92 mmol, 2 eq.) and N,N-diisopropylethylamine (0.10 mLmL,
◦
7
5 mg, 0.58 mmol, 1.25 eq.) was added. The mixture was stirred at 50 C for 12 h. The solvent
was removed by evaporation under reduced pressure and the product was directly purified by flash
◦
chromatography using hexane: DCM (1:1) (R : 0.82) as eluent to give 184 mg (63%). m.p.: 172–182 C,
f
−1
3
−1
1
◦
UV/vis: λmax = 563 nm,
ε
= 176547 dm mol cm in THF. H-NMR:
CH: Substituted-Cp), 3.95 (2H, pt, 2
CH: Substituted-Cp), 3.63 (5H, s, Unsubstituted-Cp). B-NMR:
(128.38 MHz, CDCl3): δ −16.82
δ
H (600.28 MHz, CDCl3, 25 C):
δ
8.88 (6H, dd, SubPc), 7.90 (6H, dd, SubPc), 3.96 (2H, pt, 2
×
×
11
δ
B
◦
(
1B). 13C- NMR:
δ
C
(150.95 MHz, CDCl , 25 C):
δ
172.43 (1C, Fc-
CO ), 151.67 (6C, SubPc: N-C=N),
2
3
131.20 (6C, SubPc: C=C), 130.02 (6C, SubPc: non-peripheral), 122.47 (6C, SubPc: peripheral), 88.08
(1C, Substituted-Cp-ring), 68.51 (5C, Unsubstituted-Cp-ring), 68.04 (2C, Substituted-Cp-ring), 67.11
(
2C, Substituted-Cp-ring). Elemental analysis calculated for C H BFeN O (element, %): C, 67.34; H,
35
21
6
2
3
.39; N, 13.46. Obtained: C, 67.73; H, 3.47; N, 13.58.
.4.2. Preparation of FcCH CO SubPc(H) , 8
3
2
2
12
To a solution of chlorosubphthalocyanine,
trifluoromethanesulfonate (150 mg, 0.58 mmol; 1.25 eq.) was added and the mixture stirred at 45 C,
under argon atmosphere for 4 h. Once the (OTf)SubPc(H)12 was generated, ferrocenylmethanoic acid,
6
, (200 mg; 0.46 mmol) and dry toluene (3 mL), silver
◦
2
, (225 mg, 0.92 mmol, 2 eq.) and N,N-diisopropylethylamine (0.10 mL, 75 mg, 0.58 mmol, 1.25 eq.)
◦
was added. The mixture was stirred at 50 C for 12 h. The solvent was removed by evaporation under
reduced pressure and the product was directly purified by flash chromatography using hexane: DCM
◦
(
1:1) (R : 0.78) as eluent to give 140 mg (47%). m.p.: 175–183 C, UV/vis: λmax = 563 nm,
ε
= 166317
f
3
−1
−1
1
◦
dm mol cm in THF. H-NMR:
δ
H (600.28 MHz, CDCl3, 25 C):
δ
8.86 (6H, dd, SubPc), 7.89 (6H,
dd, SubPc), 3.79 (2H, pt, 2
×
CH: Substituted-Cp), 3.67 (5H, s, Unsubstituted-Cp), 3.53 (2H, pt, 2
×
CH:
δ
C
CH2). 11B-NMR:
δ
(128.38 MHz, CDCl3): δ −16.76 (1B). C-NMR:
13
Substituted-Cp), 2.28 (2H, s, 1
×
B
◦
(
150.95 MHz, CDCl , 25 C):
δ
169.82 (1C,
C
O), 149.05 (6C, SubPc: N-
C
=N), 128.59 (6C, SubPc: C=C),
3
1
27.41 (6C, SubPc: non-peripheral), 119.85 (6C, SubPc: peripheral), 85.47 (1C, Substituted-Cp-ring),
5.90 (5C, Unsubstituted-Cp-ring), 65.43 (2C, Substituted-Cp-ring), 64.49 (2C, Substituted-Cp-ring),
2.81 (1C, Fc-CH -CO ). Elemental analysis calculated for C H BFeN O (element, %): C, 67.74; H,
6
2
3
2
2
36 23
6
2
.63; N, 13.17. Obtained: C, 67.90; H, 3.82; N, 13.72.
3
.4.3. Preparation of Fc(CH ) CO SubPc(H) , 10
2
3
2
12
To a solution of chlorosubphthalocyanine,
trifluoromethanesulfonate (150 mg, 0.58 mmol; 1.25 eq.) was added and the mixture stirred at 45 C,
6
, (200 mg; 0.46 mmol) and dry toluene (3 mL), silver
◦
under argon atmosphere for 4 h. Once the (OTf)SubPc(H)12 was generated, ferrocenylethanoic acid,
4,
(
250 mg, 0.92 mmol, 2 eq.) and N,N-diisopropylethylamine (0.10 mL, 75 mg, 0.58 mmol, 1.25 eq.) was
◦
added. The mixture was stirred at 50 C for 12 h. The solvent was removed by evaporation under
reduced pressure and the product was directly purified by flash chromatography using hexane: DCM
◦
(
1:1) (R : 0.73) as eluent to give 104 mg (43%). m.p.: 182–190 C, UV/vis: λmax = 563 nm,
ε
= 145673
8.86 (6H, dd, SubPc), 7.88 (6H,
CH: Substituted-Cp), 3.70 (2H, pt, 2
CH ), 1.06 (2H, s, 1
CH2). 11B-NMR:
172.40 (1C, O),
f
3
−1
−1
1
◦
dm mol cm in THF. H-NMR:
dd, SubPc), 3.89 (5H, s, Unsubstituted-Cp), 3.84 (2H, pt, 2
CH: Substituted-Cp), 1.76 (2H, s, 1
δ
δ
H (600.28 MHz, CDCl3, 25 C):
δ
×
×
×
CH ), 1.26 (2H, s, 1
×
×
2
2
(128.38 MHz, CDCl3): δ −16.32 (1B). 1 C-NMR:
3
δ
C
(150.95 MHz, CDCl , 25 C):
51.64 (6C, SubPc: N-C=N), 131.17 (6C, SubPc: C=C), 129.99 (6C, SubPc: non-peripheral), 122.44
◦
δ
C
B
3
1
(
(
6C, SubPc: peripheral), 88.05 (1C, Substituted-Cp-ring), 68.48 (5C, Unsubstituted-Cp-ring), 68.01
2C, Substituted-Cp-ring), 67.08 (2C, Substituted-Cp-ring), 28.52 (1C, Fc-CH -CH -CH2-CO ), 25.40
2
2
2

Molecules 2020, 25, 2575
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(
1C, Fc-CH2-CH2-CH -CO ), 18.45 (1C, Fc-CH2-CH -CH -CO ). Elemental analysis calculated for
2 2 2 2 2
C H BFeN O (element, %): C, 68.50; H, 4.08; N, 12.61. Obtained: C, 68.61; H, 4.16; N, 12.74.
38
27
6
2
3
.4.4. Preparation of FcCO(CH ) CO SubPc(H) , 11
2 2 2 12
To a solution of chlorosubphthalocyanine,
trifluoromethanesulfonate (150 mg, 0.58 mmol; 1.25 eq.) was added and the mixture stirred at 45 C,
under argon atmosphere for 4 h. Once the (OTf)SubPc(H)12 was generated, ferrocenyloxobutanoic acid,
6
, (200 mg; 0.46 mmol) and dry toluene (3 mL), silver
◦
4
, (263 mg, 0.92 mmol, 2 eq.) and N,N-diisopropylethylamine (0.10 mL, 75 mg, 0.58 mmol, 1.25 eq.)
◦
was added. The mixture was stirred at 50 C for 12 h. The solvent was removed by evaporation
under reduced pressure and the product was directly purified by flash chromatography using hexane:
◦
DCM (1:1) (R : 0.62) as eluent to give 114 mg (36%). m.p.: 201–207 C, UV/vis: λmax = 563 nm,
f
3
−1
−1
1
◦
ε = 113729 dm mol cm in THF. H-NMR:
δ
H (600.28 MHz, CDCl3, 25 C):
δ
8.85 (6H, dd, SubPc),
7
3
.88 (6H, dd, SubPc), 4.49 (2H, pt, 2
×
CH: Substituted-Cp), 4.31 (2H, pt, 2
×
CH: Substituted-Cp),
11
.98 (5H, s, Unsubstituted-Cp), 2.33 (2H, s, 1
×
CH ), 1.66 (2H, s, 1
×
CH ). B-NMR:
δ
B
(128.38
2
2
MHz, CDCl3): δ −16.79 (1B). 13C-NMR:
δ
(150.95 MHz, CDCl , 25 C):
◦
δ
176.47 (1C, Fc-CO),
C
3
173.83 (1C, Fc-CO-CH -CH -CO ), 155.71 (6C, SubPc: N-C=N), 135.24 (6C, SubPc: C=C), 134.06
2 2 2
(
(
(
6C, SubPc: non-peripheral), 126.51 (6C, SubPc: peripheral), 92.12 (1C, Substituted-Cp-ring), 72.55
5C, Unsubstituted-Cp-ring), 72.08 (2C, Substituted-Cp-ring), 71.15 (2C, Substituted-Cp-ring), 32.59
1C, Fc-CO-CH2-CH2-CO ), 29.47 (1C, Fc-CO-CH2-CH -CO ). Elemental analysis calculated for
2
2
2
C H BFeN O (element, %): C, 67.09; H, 3.70; N, 12.35. Obtained: C, 67.09; H, 3.82; N, 12.35.
38
25
6
3
4
. Conclusions
Subphthalocyanines with ferrocenylcarboxylic acids in the axial position can be synthesized in
3% yields when reactions are performed under strict Schlenk conditions, in this case in a glovebox.
6
The axial ferrocenylcarboxylic moiety did not influence the UV/Vis wavelength maxima of the Soret
or Q-bands when comparing ferrocenylcarboxylic containing SubPcs 11 with parent macrocycle, 6.
7–
The cyclic voltammetry data revealed that the first reversible oxidation process is ferrocene-centered,
with the second oxidation and all observed reduction processes are SubPc ring-based. DFT optimization
of the oxidized (cation) SubPc was necessary to confirm the locus of the second observed SubPc
ring-based oxidation.
The oxidation potential of Fe of the axial ferrocenyl moiety was affected by the electron-rich
II/III
subphthalocyanine, shifting Fe
potential compared to Fe
oxidation potentials with 0.038–0.060 V to a lower oxidation
II/III
oxidation potentials of the free ferrocenylcarboxylic acids. The electron
withdrawing ferrocenium moiety in the oxidized axial ligand, withdraws charge from the alkyl chains
bonded to it, and consequently the first ring-based oxidation of all ferrocenylsubphthalocyanine dyads
Y-BSubPc(H)12
7
–
11 that is influenced by the net electron withdrawing effect of the oxidized axial
◦
ferrocenylcarboxylic ligand Y, is exactly the same, namely 0.042 V more positive than E ’ of the first
ring-based oxidation of Cl-BSubPc(H)12
6 at 0.628 V. The net electron donating effect of the neutral
axial ferrocenylcarboxylic ligand Y influenced the ring electron density of the neutral SubPc, leading to
a systematic decrease in the two observed ring-based reductions of the ferrocenylsubphthalocyanine
dyads as the number of alkyl groups n in the different axially bonded ferrocenyl carboxylic acid moieties
(
Fc-(CH2)
potential reported to date, due to the strong net electron donating effect of the axial ferrocenylcarboxylic
ligand (Fc(CH ) COO).
n
2 2 3 2 12
-CO ) increases. Fc(CH ) CO BSubPc(H) , 10, has the lowest first ring-based reduction
2
3
Supplementary Materials: The following are available online, Synthesis and characterisation (Section S1,
compounds
1
,
2
,
4
–
6
: FcCO H, FcCH CO H, Fc(CH ) CO H, FcCO(CH ) CO H, ClSubPc(H)12), Figures S1–S3:
2
2
2
2 3
2
2 2
2
1
1
13
11
H, C and B NMR of compound
7
; Figure S4–S6: 1H, 13C and 11B NMR of compound
8; Figures S7–S9:
13
11
1
13
11
H, C and B NMR of compound 10; Figures S10–S12: H, C and B NMR of compound 11; In Section S3
Figures S13–S15: HOMO, LUMO and optimized coordinates of the compounds 7, 8, 10 and 11 (respectively).

Molecules 2020, 25, 2575
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Author Contributions: P.J.S.: Data curation, Investigation, Formal analysis, Writing—original draft, Writing—review
&
editing. J.C.: Supervision, Project administration, Writing—review & editing. All authors have read and agreed to
the published version of the manuscript.
Funding: This research was funded by South African National Research Foundation (Grant numbers 113327
and 96111) and the Central Research Fund of the University of the Free State, Bloemfontein. The High-Performance
Computing facility of the UFS, the CHPC of South Africa and the Norwegian Supercomputing Program (UNINETT
Sigma2, Grant No. NN9684K) are acknowledged for computer time.
Conflicts of Interest: The authors declare no competing financial interest.
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Sample Availability: Samples of the compounds are not available from the authors.
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