A new tetradentate mixed aza-thioether macrocycle and its complexation behavior towards Fe(II), Ni(II) and Cu(II) ions

Article abstract of DOI:10.3390/molecules25092030

A new tetradentate mixed aza-thioether macrocyclic ligand 2,6-dithia[7](2,9)-1,10-phenanthrolinophane ([13]ane(phenN₂)S₂) was successfully synthesized. Reacting metal precursors [Fe(CH₃CN)₂(OTf)₂], Ni(ClO₄)₂·6H₂O, and Cu(ClO₄)₂·6H₂O with one equivalent of [13]ane(phenN₂)S₂ afforded [Fe([13]ane(phenN₂)S₂)(OTf)₂] (1), [Ni([13]ane(phenN₂)S₂)](ClO₄)₂ (2(ClO₄)₂), and [Cu([13]ane(phenN₂)S₂)(OH₂)](ClO₄)₂ (3(ClO₄)₂), respectively. The structures of [13]ane(phenN₂)S₂ and all of its metal complexes were investigated by X-ray crystallography. The [13]ane(phenN₂)S₂ was found to behave as a tetradentate ligand via its donor atoms N and S.

Full text of DOI:10.3390/molecules25092030

molecules
Article
A New Tetradentate Mixed Aza-Thioether Macrocycle
and Its Complexation Behavior towards Fe(II), Ni(II)
and Cu(II) Ions
Sze-Wing Ng 1,2,†, Siu-Chung Chan 1,†, Chi-Fung Yeung 1,2, Shek-Man Yiu 1 and
Chun-Yuen Wong 1,2,
*
1
Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong
State Key Laboratory of Terahertz and Millimeter Waves, City University of Hong Kong, Tat Chee Avenue,
Kowloon, Hong Kong
2
*
†
Correspondence: acywong@cityu.edu.hk; Tel.: +852-3442-6831
These authors contributed equally to this work.
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: Emilia Furia and Tiziana Marino
Received: 5 April 2020; Accepted: 22 April 2020; Published: 27 April 2020
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: A new tetradentate mixed aza-thioether macrocyclic ligand 2,6-dithia[7](2,9)-1,10-
phenanthrolinophane ([13]ane(phenN2)S2) was successfully synthesized. Reacting metal
precursors [Fe(CH3CN)2(OTf)2], Ni(ClO4)2 6H2O, and Cu(ClO4)2 6H2O with one equivalent of
[13]ane(phenN2)S2 afforded [Fe([13]ane(phenN2)S2)(OTf)2] ( ), [Ni([13]ane(phenN2)S2)](ClO4)2
(ClO4)2), and [Cu([13]ane(phenN2)S2)(OH2)](ClO4)2 ( (ClO4)2), respectively. The structures of
·
·
1
(
2
3
[13]ane(phenN2)S2 and all of its metal complexes were investigated by X-ray crystallography.
The [13]ane(phenN2)S2 was found to behave as a tetradentate ligand via its donor atoms N and S.
Keywords: first-row transition metal ion; macrocyclic ligand; coordination complex
1. Introduction
Coordination chemistry of first-row transition metal ions has attracted considerable attention
for decades due to their high biological relevance. Not only do the coordination complexes serve as
models to study biologically related metallomolecules, but they also provide insights into biomimetic
designs [1–15]. Among a reservoir of ligand designs, macrocycles have been demonstrated to be a
valuable type of auxiliary ligand for these studies because (1) many macrocycles are incorporated
in naturally-occurring metalloproteins and metalloenzymes, such as porphyrin in haemoglobin and
corrin in vitamin B12, and (2) the properties of the derived metal complexes can be modulated via
modification in the ring size and donor atoms on the macrocyclic ligands.
Our group has recently initiated a paradigm in scrutinizing the reactivity between low-valent
transition metal precursors and heteroatom-functionalized alkynes [16–29]. Gratifyingly, several
Ru- and Os-heterocyclic complexes reported by our group exhibit interesting biological applications.
The success of preparing these functional metalated heterocyclic complexes prompted us to expand the
scope of our studies to other biologically relevant first-row transition metals. Moreover, we envision
that employing multidentate macrocycles as auxiliary ligands in these studies can reduce the ambiguity
in mechanistic elucidations because well-designed macrocycles can effectively minimize partial ligand
dissociation. With these considerations in mind, we would like to prepare some new first-row transition
metal complexes supported by macrocycles for analogous reactivity studies. Among a variety of
macrocycles, we are interested in mixed phenanthroline-thioether ligands, not only because their
preparation is straightforward but they are well-known to stabilize various low-valent first-row
transition metal ions (Scheme 1) [30–47]. While most of the reported mixed phenanthroline-thioether
Molecules 2020, 25, 2030; doi:10.3390/molecules25092030
www.mdpi.com/journal/molecules

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macrocycles are a 15-membered ring with 5 donor atoms to behave as pentadentate ligands, analogs
with smaller ring sizes that can increase their conformational rigidity are rare. In this context,
we report the preparation and complexation ability of a new tetradentate macrocyclic ligand 2,6-dithia
[7](2,9)-1,10-phenanthrolinophane (denoted as [13]ane(phenN2)S2). To our delight, this macrocycle
allows the preparation of low-valent transition metal complexes [Fe([13]ane(phenN2)S2)(OTf)2],
[Ni([13]ane(phenN2)S2)](ClO4)2 and [Cu([13]ane(phenN2)S2)(OH2)](ClO4)2 under mild reaction
conditions with high yields. The solid-state structures of [13]ane(phenN2)S2 and its metal complexes
have been determined by X-ray crystallography. The success of preparing these macrocyclic complexes
undoubtedly paves the way for performing further reactivity studies.
Scheme 1. Mixed phenanthroline-thioether macrocycles reported in previous works and this work.
2. Results and Discussion
2.1. Synthesis and Characterization of [13]ane(phenN2)S2
The new mixed aza-thioether macrocyclic ligand [13]ane(phenN2)S2 was prepared from
cyclization between 2,9-bis(chloromethyl)-1,10-phenanthroline and 1,3-propanedithiol in the presence
of Cs2CO3 as a base and source of template cation under high dilution conditions (Scheme 2).
The 2,9-bis(chloromethyl)-1,10-phenanthroline can be synthesized in accordance with previously
reported literature methods from 2,9-dimethyl-1,l0-phenanthroline as depicted in Scheme 2 [48–52].
The formation of this air-stable macrocycle was characterized by mass spectroscopy together with 1H
and 13C-NMR spectroscopy.
Scheme 2. Synthesis of [13]ane(phenN2)S2.
The structure of [13]ane(phenN2)S2 was further confirmed by X-ray crystallography (Figure 1).
While the phenanthroline moiety of this macrocycle is essentially planar, the 13-membered ring
adopts a folded conformation in the solid state. More specifically, the dimethylphenanthroline moiety
is essentially planar whereas the -S(CH2)3S- chain is tilted over the phenanthroline unit, with an
angle of 51.6◦ between the mean planes constructed by C(1), C(10), C(13), C(17) and C(13), C(17),
S(1), S(2). The C-C bonds along the -S(CH2)3S- unit take on anti conformations. All the C-S bond
distances (1.822(2)-1.823(2) Å) are typical for thioethers [47]. Since the lone pairs of the S-donors adopt
exodentate orientation pointing out of the ring cavity, large conformational change is expected for
[13]ane(phenN2)S2 to act as a tetradentate ligand via donor atoms N and S.

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Figure 1. Perspective view of [13]ane(phenN2)S2 as represented by 30% probability ellipsoids. Selected
bond lengths (Å) and angles (◦): N(1)-C(1), 1.326(2); N(1)-C(5), 1.352(2); C(5)-C(6), 1.456(2); N(2)-C(6),
1.349(3); N(2)-C(10), 1.323(2); C(1)-C(17), 1.498(3); C(10)-C(13), 1.501(3); S(1)-C(13), 1.822(2); S(1)-C(14),
1.823(2); S(2)-C(16), 1.819(2); S(2)-C(17), 1.822(2); C(1)-C(17)-S(2), 114.6(1), C(10)-C(13)-S(1), 115.3(1);
C(13)-S(1)-C(14), 103.5(1); C(16)-S(2)-C(17), 103.8(1).
To explore the possible low-energy conformers, the X-ray structure for [13]ane(phenN2)S2 was
optimized at the density functional theory (DFT) level to yield the first conformer, Conformer I.
The search for other low-energy conformers was then performed by exploring the torsional energy
surface of the aliphatic chain, followed by structural optimization. Two other conformers, denoted
as Conformers II and III, were obtained throughout the conformer search. These conformers I–III
are labeled in ascending order of their relative energy (0.0, 1.2 and 4.4 kcal mol−1, respectively).
Their structures are depicted in Figure 2, with selected bond lengths (Å) and angles (◦) summarized in
Table 1. Notably, the structure of Conformer I is in good agreement with the X-ray structure, in which
the two S atoms are tilted over the dimethylphenanthroline moiety in the same direction and the two
C-C bonds on the -S(CH2)3S- unit take on anti conformation. On the other hand, one of the S atoms
in Conformer II is coplanar with the dimethylphenanthroline moiety, while another S atom keeps
tilting over the phenanthroline unit. Although the two S atoms in Conformer III are tilted over the
dimethylphenanthroline moiety in the same direction, the C-C bonds on the -S(CH2)3S- unit take on
a combination of anti and gauche conformations, making it structurally distinct from Conformer I.
As none of these conformers have their donor atoms pointing to a common center, large distortion is
expected for the ligand to act as a tetradentate ligand.
Table 1. Selected bond lengths (Å) and angles (◦) for Conformers I–III.
Conformers
I
II
III
N(1)-C(1)
N(1)-C(5)
N(2)-C(6)
N(2)-C(10)
C(1)-C(17)
C(10)-C(13)
S(1)-C(13)
S(1)-C(14)
S(2)-C(16)
S(2)-C(17)
1.332
1.349
1.349
1.332
1.511
1.511
1.844
1.845
1.846
1.846
114.5
114.4
102.7
102.6
1.332
1.348
1.353
1.327
1.506
1.522
1.825
1.850
1.838
1.851
112.9
116.9
101.7
103.6
1.331
1.348
1.343
1.330
1.512
1.511
1.850
1.846
1.851
1.856
111.6
111.9
101.8
105.1
C(1)-C(17)-S(2)
C(10)-C(13)-S(1)
C(13)-S(1)-C(14)
C(16)-S(2)-C(17)

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Figure 2.
(a) Front views and (b) side views of three possible low-energy conformers of [13]ane
(phenN2)S2.
2.2. Synthesis and Characterization of [13]ane(phenN2)S2-Containing Transition Metal Complexes
The coordination behavior of [13]ane(phenN2)S2 ligand was explored with various
first-row transition metal ions including Fe(II), Ni(II) and Cu(II) (Scheme 3). Reacting metal
precursors [Fe(CH3CN)2(OTf)2], Ni(ClO4)2
[13]ane(phenN2)S2 afforded a six-coordinated Fe complex [Fe([13]ane(phenN2)S2)(OTf)2] (
four-coordinated Ni complex [Ni([13]ane(phenN2)S2)](ClO4)2 ( (ClO4)2), and five-coordinated Cu
complex [Cu([13]ane(phenN2)S2)(OH2)](ClO4)2 ( (ClO4)2), respectively. All of them are isolable in
good yields (70–80%). It is noteworthy that both (ClO4)2 and (ClO4)2 are very stable in CH3CN
as their X-ray structures can be obtained upon recrystallization under ambient conditions. On the
other hand, exhibits a certain degree of instability towards air- and moisture-sensitive conditions.
For example, the color of changed from yellow to deep brown during recrystallization in CH3CN
under ambient conditions, leading to an ill-defined product. Despite this observation, cannot be
·6H2O, and Cu(ClO4)2
·
6H2O with one equivalent of
1),
2
3
2
3
1
1
1
regarded as very air-sensitive since its UV-visible absorption profile in non-degassed CH3CN remained
unchanged for 16 h (Figure S1). The solubility of all metal complexes was also investigated in common
organic solvents (CH3CN, acetone, CH2Cl2, THF and MeOH) and H2O. All of the three complexes
were found to be soluble in CH3CN. In addition,
1 is soluble in acetone and CH2Cl2, whereas 2(ClO4)2
and (ClO4)2 are soluble in H2O. The solution magnetic moments for these complexes were determined
3
1
via H-NMR spectroscopy using the Evans method [53,54]; the Fe(II) and Ni(II) complexes were
determined to possess high-spin electronic states (S = 2 for Fe and S = 1 for Ni), whereas the electronic
spin determined for the Cu(II) complex was the expected S = 1/2. The molecular structures for all these
metal complexes were determined by X-ray crystallography. Their perspective views and selected
structural parameters are depicted in Figure 3, Figures 5 and 6. It is evident that [13]ane(phenN2)S2
possesses structural flexibility to behave as a tetradentate ligand, and the energy required for the
conformational change is expected to be small as complexes
at room temperature.
1, 2(ClO4)2 and 3(ClO4)2 can be prepared

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Scheme 3. Synthesis of [13]ane(phenN2)S2-containing complexes 1, 2(ClO4)2 and 3(ClO4)2.
Figure 3. Perspective view of
1 as represented by 30% probability ellipsoids (hydrogen atoms on
[13]ane(phenN2)S2 are omitted for clarity). Selected bond lengths (Å) and angles (◦): Fe(1)-N(1),
2.156(2); Fe(1)-N(2), 2.209(2); Fe(1)-S(1), 2.486(1); Fe(1)-S(2), 2.531(1); Fe(1)-O(1), 2.134(2); Fe(1)-O(4),
2.126(2); N(1)-Fe(1)-N(2), 72.3(1); N(2)-Fe(1)-S(1), 74.5(1); S(1)-Fe(1)-S(2), 92.10(3); N(1)-Fe(1)-S(2),
75.4(1); O(1)-Fe(1)-N(1), 98.8(1); O(1)-Fe(1)-N(2), 81.9(1); O(4)-Fe(1)-S(1), 97.7(1); O(4)-Fe(1)-S(2), 82.6(1).
In high-spin (S = 2) Fe complex 1, the [13]ane(phenN2)S2 acts as a tetradentate ligand, with two N
and S atoms coordinating to the Fe center (Figure 3). Meanwhile, the Fe center is coordinated with
two O-bound OTf. The coordination of OTf to the Fe center in solution was revealed by 19F-NMR
study, with the observation of a single peak at
−
72.05 ppm indicating the coordinating nature of
OTf [55 57]. The coordination geometry of is best described as trigonal prismatic rather than
–
1
octahedral. As depicted in Figure 4, the angles formed by the lateral faces f1 and f2 (61.5◦), f2 and f3
(59.1◦), f3 and f1 (60.5◦) are all close to 60◦, and the angle between the two triangular basal planes of
the trigonal prism is small (7.9o), indicating that these basal planes are nearly parallel to each other.
The distortion from a perfect trigonal prismatic geometry is characterized by the average twist angle
of 14.9◦. Meanwhile, the Fe-N (2.156(2)-2.209(2) Å) and Fe-S (2.486(1)-2.531(1) Å) bond distances are

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consistent with those of high-spin Fe(II) species bearing diimine moiety [58] and thioether linker [59].
The Fe-O bond distances (2.126(2)-2.134(2) Å) of
bearing coordinated OTf (2.025-2.211 Å) [60–62].
1 are typical of known high-spin Fe(II) complexes
Figure 4. (a) Two different views of the coordination environment surrounding the Fe center of 1 and
) illustrations of the three lateral faces (f1–f3), centroids (Ct1 and Ct2) and twist angles (θ1 θ3) of the
two triangular basal planes on the twisted trigonal prism.
(
b
–
In high-spin (S = 1) 2(ClO4)2, the Ni atom adopts a square planar geometry with the N and
S atoms of [13]ane(phenN2)S2 occupying the equatorial position (Figure 5). The Ni atom only lies
above the mean plane constructed by the four coordinating atoms of [13]ane(phenN2)S2 by 0.073 Å,
and the geometry index
τ
[
63
,
64] calculated for this complex is 0.11 (cf. τ ≈ 0 and τ ≈ 1 for square
planar and tetrahedral geometry, respectively, in four-coordinated compounds). In comparison with
[Ni([15]ane(phenN2)S3(CH3CN)]2+ where the Ni atom adopts a distorted octahedral geometry [46],
the Ni-N and Ni-S bond distances in 2(ClO4)2 (1.849(2)-1.855(2) Å for Ni-N, 2.155(1)-2.156(1) Å for
Ni-S) are shorter than those in [Ni([15]ane(phenN2)S3(CH3CN)]2+ (2.013(2)–2.025(2) Å for Ni-N,
2.434(1)-2.444(1) Å for Ni-S). These observations are probably originated from the restricted flexibility
of the shorter aliphatic linker in [13]ane(phenN2)S2.
In
3
(ClO4)2, the Cu atom adopts a square pyramidal geometry (
τ
= 0.02; cf. τ ≈ 0 and τ ≈ 1 for
square pyramidal and trigonal bipyramidal geometry, respectively, in five-coordinated compounds),
with the N and S atoms of [13]ane(phenN2)S2 occupying the equatorial position, and the O atom of H2O
in the axial position (Figure 6). The Cu atom is found to be located above the mean plane constructed
by the four coordinating atoms of [13]ane(phenN2)S2 by 0.444 Å. The Cu-N bond distances of
3(ClO4)2
(1.950(3)-1.963(3) Å) are comparable to that in [Cu([15]ane(phenN2)S2)(ClO4)]+ (1.956(5) Å) [47].
On the other hand, the Cu-S bond distances in 3(ClO4)2 (2.292(1)-2.296(1) Å) are shorter than those in
[Cu([15]ane(phenN2)S2)(ClO4)]+ (2.347(2) Å). This again reflects the restricted flexibility of the shorter
aliphatic linker in [13]ane(phenN2)S2.

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Figure 5. Perspective view of
2 as represented by 30% probability ellipsoids (hydrogen atoms on
[13]ane(phenN2)S2 are omitted for clarity). Selected bond lengths (Å) and angles (◦): Ni(1)-N(1), 1.849(2);
Ni(1)-N(2), 1.855(2); Ni(1)-S(1), 2.155(1); Ni(1)-S(2), 2.156(1); N(1)-Ni(1)-N(2), 85.3(1); N(2)-Ni(1)-S(1),
87.5(1); S(1)-Ni(1)-S(2), 99.10(2); S(2)-Ni(1)-N(1), 87.8(1).
Figure 6. Perspective view of
[13]ane(phenN2)S2 are omitted for clarity). Only one of the independent cations in [
depicted. Selected bond lengths (Å) and angles (◦) are listed in the order of Cu(1) moiety and
then Cu(2) moiety: Cu(1)-N(1), 1.963(3), 1.950(3); Cu(1) N(2), 1.959(3), 1.955(3); Cu(1)-S(1), 2.293(1),
3
as represented by 30% probability ellipsoids (hydrogen atoms on
3(ClO4)2]2 is
−
2.293(1); Cu(1)-S(2), 2.292(1), 2.296(1); Cu(1)-O(1), 2.160(3), 2.165(3); N(1)-Cu(1)-N(2), 81.6(1), 82.1(2);
N(2)-Cu(1)-S(1), 84.5(1), 84.4(1); S(1)-Cu(1)-S(2), 99.72(4), 100.51(4); S(2)-Cu(1)-N(1), 84.1(1), 84.2(1).
3. Materials and Methods
3.1. General Procedures
All reactions were performed under an argon atmosphere using standard Schlenk
techniques unless otherwise stated. All reagents were used as received, and solvents for
reactions were purified by a PureSolv MD5 solvent purification (Amesbury, MA, USA) system.

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The 2,9-dicarbaldehyde-1,10-phenanthroline [50
,
52], 2,9-bis(hydroxymethyl)-1,10-phenanthroline [51],
2,9-bis(chloromethyl)-1,10-phenanthroline [48
,
49] and Fe(CH3CN)2(OTf)2 [65], were prepared in
accordance with the literature methods. 1H, 13C{1H}, H-1H COSY, 1H-1H-ROESY, 1H–13C-HSQC
and 1H-13C-HMBC-NMR spectra were recorded on a Bruker 600 AVANCE III FT-NMR spectrometer
(Billerica, MA, USA). Peak positions were calibrated with solvent residue peaks as internal standard.
Labeling scheme for H and C atoms in the NMR assignments is shown in Scheme 4. Electrospray mass
spectrometry was performed on a PE-SCIEX API 3200 triple quadrupole mass spectrometer (Tokyo,
Japan). Elemental analyses were done on an Elementar Vario Micro Cube carbon-hydrogen-nitrogen
elemental microanalyzer (Okehampton, UK). UV-visible spectra were recorded on a Shimadzu
UV-1800 spectrophotometer (San Francisco, CA, USA). Neocuproine (98%), nickel(II) perchlorate
1
hexahydrate (Ni(ClO4)2
·
6H2O) and copper(II) perchlorate hexahydrate (Cu(ClO4)2 6H2O) were
·
purchased from J&K Scientific Ltd (Hong Kong). Iron(II) chloride (FeCl2, anhydrous, 98%) and
trimethylsilyl trifluoromethanesulphonate ((CH3)3SiOTf, 98%) were purchased from Fluorochem Ltd.
(Derbyshire, UK)
Scheme 4. Labeling scheme for H and C atoms in [13]ane(phenN2)S2.
3.2. Solution Magnetic Susceptibility Measurements
The solution state molar magnetic susceptibility (χm) of
1, 2(ClO4)2 and 3(ClO4)2 was measured
according to the Evans method [53 54]. This involves the placement of a coaxial insert containing
,
sample solution (0.010 M) into a standard 5 mm NMR tube containing only solvent. CD3CN containing
one drop of benzene as internal reference was used as the solvent throughout. Based on the chemical
shift difference of the benzene internal reference between inner and outer tubes, values of molar
magnetic susceptibility (χm) and magnetic moment (µe f f ) were calculated using the equations shown
below [66,67]:
χm = (3·∆ f)/(1000·f·c)
µe f f = 798 T·χm
where χm = molar magnetic susceptibility of the sample (m3 mol−1);
shift (Hz) between the internal references of inner and outer tubes; f = operating frequency of the NMR
spectrometer (Hz); c = concentration of the metal complex (mol dm−3); T = probe temperature of the
measurement (K).
(1)
p
(2)
4
f = difference of the chemical
3.3. Geometry Index
The geometry index (
coordination center in four- and five-coordinated compounds.
For four-coordinated compounds, the is used to distinguish whether the coordination geometry
of the metal center is square planar (τ ≈ 0) or tetrahedral (τ ≈ 1) [64]. The formula is shown below:
τ) is a parameter ranging from 0 to 1 to describe the geometry of the
τ
τ = (β − α)/(360◦ − θ) + (180◦ − β)/(180◦ − θ) ≈ −0.00399α − 0.01019β + 2.55
(3)
where
α
and
β
are the two largest valence angles of the coordination center (
β
>
α
) and
θ
= cos−1
(−
1/3)
≈ 109.5◦ is a tetrahedral angle.

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For 2(ClO4)2, α(N(2)-Ni(1)-S(2)) = 171.9◦ and β(N(1)-Ni(1)-S(1)) = 172.0◦.
For five-coordinated compounds, the is used to distinguish whether the coordination geometry
τ
of the metal center is square pyramidal (τ ≈ 0) or trigonal bipyramidal (τ ≈ 1) [63]. The formula is
shown below:
τ = (β − α)/60◦ ≈ −0.01667α + 0.01667β
(4)
where α and β are the two largest valence angles of the coordination center (β > α).
For 3(ClO4)2, α(N(1)-Cu(1)-S(1)) = 154.0◦ and β(N(2)-Cu(1)-S(2)) = 155.0◦.
3.4. Synthesis of ([13]ane(phenN2)S2) and [13]ane(phenN2)S2-containing Fe(II), Ni(II), Cu(II) complexes
Synthesis of 2,9-dicarbaldehyde-1,10-phenanthroline. The ligand was synthesized from a modified literature
procedure [52]: To a solution of selenium oxide (2.26 g, 20.41 mmol) in deionized water (5 mL) and
1,4-dioxane (70 mL), 2,9-dimethyl-1,10-phenanthroline (2.00 g, 9.60 mmol) in 1,4-dioxane (20 mL) was
added in a dropwise manner. The reaction mixture was refluxed for 0.5 h, during which the color of the
reaction mixture changed from yellow to reddish brown with black deposits. Upon cooling to room
temperature, the brown solids were collected by suction filtration and washed with Et2O (10 mL × 3).
These solids were further purified by recrystallization with warm CHCl3 and allowed to cool and
stand overnight. Yellowish orange solids were collected by suction filtration and washed with Et2O
1
(10 mL
×
3) to give 2,9-dicarbaldehyde-1,10-phenanthroline. H-NMR signals are consistent with that
of the literature reported [50]. Yield: 1.95 g, 86%.
Synthesis of 2,9-bis(hydroxymethyl)-1,10-phenanthroline. The ligand was synthesized from a modified
literature procedure [51]: To a solution of 2,9-dicarbaldehyde-1,10-phenanthroline (1.60 g, 6.77 mmol)
in absolute EtOH (120 mL), anhydrous NaBH4 (1.02 g, 27.09 mmol) was added. The reaction mixture
was refluxed under argon for 1.5 h, during which the color of the reaction mixture changed from
yellow to brown. Upon cooling to room temperature, the reaction mixture was removed by reduced
pressure. The aqueous layer was extracted with EtOAc (50 mL
×
2). The organic phases were
combined, washed with brine (50 mL
×
2), dried over anhydrous MgSO4, and concentrated to give
1
2,9-bis(hydroxymethyl)-1,10-phenanthroline as pale pink solids. H-NMR signals are consistent with
that of literature reported. Yield: 1.20 g, 74%.
Synthesis of 2,9-bis(chloromethyl)-1,10-phenanthroline. The ligand was synthesized from a modified
literature procedure [49]: To a SOCl2 solution (16 mL) at 0 ◦C, 2,9-bis(hydroxymethyl)-1,10-
phenanthroline (1.60 g, 6.66 mmol) was added. The reaction mixture was stirred for 2 h at 0 ◦C, during
which the color of the reaction mixture changed from colorless to yellow. Upon adding n-hexane
(30 mL) to the reaction mixture, the deep yellow precipitates were collected, dissolved in CHCl3 and
extracted with an aqueous layer containing 5% (w/v) Na2CO3 and 5% (w/v) NaHCO3 for neutralization.
The organic phases were combined, washed with brine (50 mL
concentrated to give 2,9-bis(chloromethyl)-1,10-phenanthroline as yellow solids. H-NMR signals are
consistent with that of literature reported [48]. Yield: 1.52 g, 82%.
×
2), dried over anhydrous MgSO4 and
1
Synthesis of 2,6-dithia[7](2,9)-1,10-phenanthrolinophane ([13]ane(phenN2)S2). To a well-stirred suspension
of dry Cs2CO3 (3.34 g, 10.24 mmol) in anhydrous DMF (224 mL) at 55 ◦C, a mixture of 1,3-propanedithiol
(0.51 mL, 5.12 mmol) and 2,9-bis(chloromethyl)-1,10-phenanthroline (1.43 g, 5.12 mmol) in anhydrous
DMF (112 mL) was added under argon for 42 h at a rate of 2.67 mL/min, during which the color of the
reaction mixture changed from colorless to pale yellow suspension. Upon cooling to room temperature
over a period of 24 h, the reaction mixture was removed by vacuum distillation. After dissolving
the residue in CH2Cl2, the pale orange suspension was filtered to remove Cs2CO3. The filtrate was
concentrated to give a red oil. The [13]ane(phenN2)S2 was obtained as white solids after silica gel
column chromatography (silica gel, gradual elution with CH2Cl2/EtOAc (8:2, v/v); Rf value = 0.70
(silica gel TLC; CH2Cl2/EtOAc (8:2, v/v))). Recrystallization by slow diffusion of Et2O solution into a
CH2Cl2 solution of the white solids gave analytically pure [13]ane(phenN2)S2 as white crystals. Yield:

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0.45 g, 30%. Anal. Calcd. for C17H16N2S2: C, 65.35; H, 5.16; N, 8.97. Found: C, 65.38; H, 5.18; N,
8.99. ESI-MS found (calcd.): m/z 313.30 (313.46) [C17H16N2S2 + H]+. 1H-NMR (600 MHz, CDCl3):
δ 2.83–2.98 (m, 2H, Hf), 3.11–3.22 (m, 4H, He), 4.12–4.29 (m, 4H, Hd), 7.43 (d, J = 6.0 Hz, 2H, Hb), 7.73
(s, 2H, Ha), 8.14 (d, J = 6.0 Hz, 2H, Hc). 13C-NMR (150 MHz, CDCl3):
δ 34.57 (Cf), 36.72 (Ce), 41.81 (Cd),
121.71 (Cb), 125.73 (Ca), 127.37 (CII), 136.35 (Cc), 146.07 (CI), 160.79 (CIII).
Synthesis of [Fe(CH3CN)2(OTf)2]. The metal precursor was synthesized from a modified literature
procedure [65]: To a solution of FeCl2 (0.50 g, 3.94 mmol) in anhydrous CH3CN (20 mL), (CH3)3SiOTf
(4 mL in 10 mL anhydrous CH3CN, 22.10 mmol) was added in a dropwise manner. The reaction
mixture was stirred under argon for 16 h, during which the color of the reaction mixture changed from
pale yellow to colorless. The colorless solution was concentrated to about 5 mL by reduced pressure,
and then added to Et2O (150 mL) to give white precipitates. The solids were collected by suction
filtration, washed with Et2O (10 mL
Yield: 1.20 g, 70%.
×
3) and dried by reduced pressure to give [Fe(CH3CN)2(OTf)2].
Synthesis of 1. A mixture of [Fe(CH3CN)2(OTf)2] (0.40 g, 0.91 mmol) and [13]ane(phenN2)S2 (0.28 g,
0.91 mmol) was stirred in CH3CN (30 mL) under argon for 16 h, during which the color of the reaction
mixture changed from red to deep red with pale yellow precipitates. Upon filtration for removal
of these precipitates, the supernatant was collected and concentrated to give deep red precipitates.
The solids were collected by suction filtration and washed with Et2O (10 mL
by slow diffusion of Et2O solution into a CH3CN solution of the deep red solids under argon gave
×
3). Recrystallization
analytically pure 1 as orange crystals. Yield: 0.43 g, 71%. Anal. Calcd. for C19H16N2S4FeF6O6: C, 34.24;
H, 2.42; N, 4.20. Found: C, 34.28; H, 2.44; N, 4.21. 19F-NMR (400 MHz, CD3CN): δ −72.05. ESI-MS
found (calcd.): m/z 517.30 (517.38) [C18H16N2S3FeF3O3]+. Evans NMR at 298 K: µeff = 4.98 µB.
Synthesis of
2
(ClO4)2. To a solution of Ni(ClO4)2 6H2O (0.04 g, 0.10 mmol) in nitromethane (15 mL) and
·
acetic anhydride (0.62 mL), [13]ane(phenN2)S2 (0.03 g, 0.10 mmol) was added. The reaction mixture
was stirred under argon for 2 h, during which the color of the reaction mixture changed from pale green
to yellow. The reaction mixture was concentrated to about 5 mL by reduced pressure and then added to
Et2O (150 mL) to give orange precipitates. The solids were collected by suction filtration, washed with
deionized water (5 mL
×
2), cold EtOH (5 mL
×
2), and finally Et2O (10 mL
×
3). Recrystallization by
(ClO4)2
slow diffusion of Et2O into a CH3CN solution of the orange precipitates gave analytically pure
2
as brown crystals. Yield: 0.05 g, 80%. Anal. Calcd. for C17H16N2S2NiCl2O8: C, 35.82; H, 2.83; N, 4.91.
Found: C, 35.85; H, 2.86; N, 4.93. ESI-MS found (calcd.): m/z 470.30 (470.60) [C17H16N2S2NiClO4]+.
Evans NMR at 298 K: µeff = 2.77 µB.
Synthesis of
3(ClO4)2. To a solution of a mixture of [13]ane(phenN2)S2 (0.03 g, 0.10 mmol) in EtOH/CH2Cl2
(10 mL, 2:1 (v/v)), Cu(ClO4)2
was stirred under argon for 4 h, during which the color of the reaction mixture changed from pale
blue to pale purple with bluish green precipitates. The solids were collected by suction filtration,
·
6H2O (0.89 g, 2.40 mmol) in EtOH (5 mL) was added. The reaction mixture
washed with cold EtOH (5 mL
Et2O into a CH3CN solution of the bluish green precipitates gave analytically pure
×
2), and finally Et2O (10 mL
×
3). Recrystallization by slow diffusion of
(ClO4)2 as purple
3
crystals. Yield: 1.00 g, 70%. Anal. Calcd. for C17H18N2S2CuCl2O9: C, 34.44; H, 3.06; N, 4.72. Found: C,
34.48; H, 3.08; N, 4.76. ESI-MS found (calcd.): m/z 475.30 (475.45) [C17H16N2S2CuClO4]+. Evans NMR
at 298 K: µeff = 1.69 µB.
3.5. X-ray Crstayllographic Data
CCDC 1994366–1994369 contain the supplementary crystallographic data for this paper. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC,
12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk).
Crystal Data for [13]ane(phenN2)S2 (C17H16N2S2; M = 312.44 g/mol): triclinic, space group P-1 (no. 2),
a = 8.7121(4) Å, b = 10.2500(6) Å, c = 10.4864(5) Å,
α β γ
= 62.718(2)◦, = 68.073(2)◦, = 70.672(2)◦,

Molecules 2020, 25, 2030
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V = 757.41(7) Å3, Z = 2, T = 212.99 K,
µ
(MoK
α
) = 0.346 mm−1, Dcalc = 1.370 g/cm3, 9505 reflections
measured (4.558◦
≤
2
Θ ≤ 52.754◦), 3087 unique (Rint = 0.0502, Rsigma = 0.0514) which were used in all
calculations. The final R1 was 0.0393 (I > 2σ(I)) and wR2 was 0.0973 (all data). (CCDC 1994366)
Crystal Data for
a = 28.3742(8) Å, b = 9.9821(3) Å, c = 18.6161(5) Å,
(MoK
) = 1.048 mm−1, Dcalc = 1.815 g/cm3, 29,372 reflections measured (4.366◦
unique (Rint = 0.0523, Rsigma = 0.0336) which were used in all calculations. The final R1 was 0.0367
(I > 2σ(I)) and wR2 was 0.0898 (all data). (CCDC 1994367)
1 (C19H16F6O6S4N2Fe; M = 666.43 g/mol): monoclinic, space group C2/c (no. 15),
β
= 112.3340(10)◦, V = 4877.2(2) Å3, Z = 8, T = 172.98 K
,
µ
α
≤
2
Θ ≤ 52.764◦), 4989
Crystal Data for
a = 8.8218(7) Å, b = 10.0103(8) Å, c = 12.7982(9) Å,
V = 1010.95(13) Å3, Z = 2, T = 293(2) K, ) = 6.231 mm−1, Dcalc = 1.873 g/cm3, 6468 reflections
(CuKα
2
(ClO4)2 (C17H16N2O8S2Cl2Ni; M = 570.05 g/mol): triclinic, space group P-1 (no. 2),
α
= 108.789(7)◦, = 106.996(7)◦, = 92.621(6)◦,
β
γ
µ
measured (7.72◦
≤
2
Θ ≤ 134.98◦), 3576 unique (Rint = 0.0202, Rsigma = 0.0256) which were used in all
calculations. The final R1 was 0.0295 (I > 2σ(I)) and wR2 was 0.0810 (all data). (CCDC 1994368)
Crystal Data for
(no. 14), a = 26.9718(6) Å, b = 11.0279(3) Å, c = 14.7341(3) Å,
(MoK
3
(ClO4)2 (C17H18N2O9S2Cl2Cu; M = 592.89 g/mol): monoclinic, space group P21/c
β
= 99.779(2)◦, V = 4318.87(18) Å3, Z = 8
,
T = 293(2) K
,
µ
α
) = 1.508 mm−1, Dcalc = 1.824 g/cm3, 20,460 reflections measured (6.62◦
≤ Θ ≤
2
53◦), 8925 unique (Rint = 0.0213, Rsigma = 0.0291) which were used in all calculations. The final R1 was
0.0487 (I >2σ(I)) and wR2 was 0.1291 (all data). (CCDC 1994369)
3.6. Computational Methodology
The structures and relative energies of Conformers I−III were calculated by density functional
theory (DFT) calculations using the ORCA software package (version 4.1.1, Max-Planck-Institut für
Chemische Energiekonversion, Mülheim an der Ruhr, North Rhine-Westphalia, Germany) [68]. All the
calculated structures were optimized in the gas phase at the BP86/def2-SVP level with the Grimme’s
DFTD3 method to account for the dispersion effects, and the Resolution of Identity (RI) approximation
to speed up the calculations. Tight self-consistent field (SCF) convergence criteria were used throughout.
The optimized conformers were confirmed to be local minima via frequency calculations (no negative
frequency).
4. Conclusions
A
new tetradentate mixed aza-thioether macrocyclic ligand 2,6-dithia[7](2,9)-1,10-
phenanthrolinophane ([13]ane(phenN2)S2) was successfully prepared. Three low-valent first-row
transition metal complexes supported by [13]ane(phenN2)S2, namely [Fe([13]ane(phenN2)S2)(OTf)2],
[Ni([13]ane(phenN2)S2)](ClO4)2 and [Cu([13]ane(phenN2)S2)(OH2)](ClO4)2, were synthesized from
reactions between corresponding transition metal precursors and [13]ane(phenN2)S2 under mild
reaction conditions. The X-ray structures of the metal complexes revealed that [13]ane(phenN2)S2 acts
as a tetradentate ligand by coordinating to the metal centers via donor atoms N and S. The success
of preparing these macrocyclic complexes undeniably opens new opportunities for further reactivity
studies and possible biological applications.
Supplementary Materials: The following are available online, Table S1–S3: Cartesian coordinates of calculated
structures of Conformers I–III. Figure S1: UV-visible absorption spectra of 1, 2(ClO4)2 and 3(ClO4)2 in CH3CN at 298
K, Figure S2: 1H-NMR spectrum of [13]ane(phenN2)S2, Figure S3: 13C{1H}-NMR spectrum of [13]ane(phenN2)S2,
Figure S4: 1H-1H COSY of [13]ane(phenN2)S2, Figure S5: 1H-1H ROESY of [13]ane(phenN2)S2, Figure S6: 1H-13
HSQC of [13]ane(phenN2)S2, Figure S7: 1H-13C HMBC of [13]ane(phenN2)S2.
C
Author Contributions: Conceptualization, S.-W.N., S.-C.C., C.-Y.W.; investigation, S.-W.N., S.-C.C., C.-F.Y., S.-M.Y.;
writing—original draft preparation, S.-W.N., S.-C.C.; writing—review and editing, S.-W.N., C.-Y.W.; supervision,
C.-Y.W.; funding acquisition, C.-Y.W. All authors have read and agreed to the published version of the manuscript.
Funding: The work described in this paper was supported by the Research Grants Council of Hong Kong SAR
(CityU 11207117 and T42-103/16-N) and City University of Hong Kong (7005096, 7005257, 9680206).

Molecules 2020, 25, 2030
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Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the newly prepared compounds are available from the authors.
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