Synthesis and the Optical and Electrochemical Properties of Indium(III) Bis(arylimino)acenaphthene Complexes

Article abstract of DOI:10.1021/acs.inorgchem.7b00539

Aryl bis(imino)acenaphthenes (Ar-BIANs) are well-established rigid and sterically bulky diimine ligands, which are redox-noninnocent and versatile π-acceptors due to their low-lying π? orbitals and are frequently used to bind transition metals. However, t

Full text of DOI:10.1021/acs.inorgchem.7b00539

Article
pubs.acs.org/IC
Synthesis and the Optical and Electrochemical Properties of
Indium(III) Bis(arylimino)acenaphthene Complexes
Jingyi Wang,† Rakesh Ganguly,† Li Yongxin,† Jesus Díaz,*,‡ Han Sen Soo,*,†,§,∥ and Felipe García*,†
†Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
21 Nanyang Link, 637371, Singapore
‡Departamento de Química Organ
́ ́ ́
ica e Inorganica, Facultad de Veterinaria, Universidad de Extremadura, Caceres 10071, Spain
§Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE), 1 Create Way, 138602 Singapore
∥Solar Fuels Laboratory, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore
S
* Supporting Information
ABSTRACT: Aryl bis(imino)acenaphthenes (Ar-BIANs) are
well-established rigid and sterically bulky diimine ligands,
which are redox-noninnocent and versatile π-acceptors due to
their low-lying π* orbitals and are frequently used to bind
transition metals. However, the coordination chemistry of Ar-
BIAN ligands to main group elements is not as well-developed
as that of their transition metal counterparts. In particular,
there are no comprehensive studies describing the spectro-
scopic and electrochemical properties of main group Ar-BIAN
complexes. Herein, we report the synthesis and full character-
ization of a series of new indium(III) Ar-BIAN complexes,
bearing 2,6-dialkyl (1b and 2b), 4-nitro (3b), and 4-
dimethylamino (4b) groups at the aryl-diimine part of the
ligand. Their optical and electrochemical properties have been revealed by UV−vis spectroscopy and cyclic voltammetry,
respectively. Additionally, DFT calculations were performed to gain insights into the nature of the properties displayed.
ligands for transition metal centers. The extensive π-system of
the acenaphthene ring combined with the sterically modular
aniline provide a broad range of π-acceptor frameworks,
offering precise control over the steric, optical, and electronic
properties.14,15 Consequently, Ar-BIAN ligands have been
employed in the preparation of numerous transition metal
molecular compounds, which could be of use in photo-
sensitization applications due to the characterized MLCT
(metal-to-ligand charge transfer) transition from the d-orbitals
of the transition metal to the π* orbitals of the Ar-BIAN
ligand.9,10,16 The low-lying π* orbitals of Ar-BIAN ligands have
been successfully exploited as “capacitors” for multielectron
reductions in redox noninnocent ligands.17−19 Consequently,
there have been reports of transition metal Ar-BIAN complexes
being used as catalysts for organic reactions such as
cycloaddition of azides and alkynes,7,8 as well as olefin
polymerization, work that had been pioneered by Brookhart
and co-workers using Ni(II) and Pd(II) Ar-BIAN com-
plexes.20−27 More recently, Cu(II) Ar-BIAN complexes have
been investigated as catalysts for the reverse atom transfer
radical polymerization (ATRP) of styrene.28 Moreover,
INTRODUCTION
■
Redox processes and redox-active ligands play important roles
in modern chemical transformations constituting exciting areas
of research in coordination and organometallic chemistry,1 and
catalysis.2 The term “innocent ligand” was first coined by
Jørgensen as a ligand that allows the oxidation states of the
central atoms to be readily determined.3 In contrast, “non-
innocent ligands” refer to ligands in metal complexes where the
oxidation states are ambiguous and need to be experimentally
determined.1 Noninnocent ligands have energetically accessible
electronic levels rendering these species redox active at mild
potentials.4 Aryl bis(imino)acenaphthenes (Ar-BIAN) belong
to one class of noninnocent ligands. Ar-BIAN ligands were first
reported in the 1960s and have been widely investigated as
robust ligands for catalytically active transition metal centers
since the early 1990s.5 Although many coordination complexes
between transition metal and Ar-BIAN ligands have been
described,6−10 their main group and lanthanide counterparts
remain poorly explored, with a limited selection of main group
Ar-BIAN complexes being synthesized and crystallographically
characterized.11,12
Ar-BIAN ligands are highly tunable species that are readily
synthesized from cheap and commercially available anilines and
acenaphthenequinone.13,14 Functionalized Ar-BIAN com-
pounds are acknowledged as oxidatively and thermally stable
Received: March 2, 2017
© XXXX American Chemical Society
A
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a
Scheme 1. General Synthetic Scheme of Ar-BIAN Ligands 1a−5a
a
The symbol for mechanical milling above the arrow of the equation in the lower half has been proposed by Hanusa et al.58
applications of Cu(I) Ar-BIAN complexes as light harvesters
that can absorb to the NIR region have also been reported.6,9,16
In terms of main group chemistry, however, groups 1 and 2
constitute the majority of the Ar-BIAN main group metal
complexes.17−19,29−35 In these examples, each Ar-BIAN ligand
undergoes facile reduction due to its electron accepting nature
to form the corresponding metal complex.11 With respect to
group 14 metals, a small number of complexes have been
previously reported. For instance, N-heterocyclic germylenes
that contain anionic Ar-BIAN ligands include [(dipp-BIAN)-
Ge:], [(dtb-BIAN)Ge:] (dipp = 2,6-diisopropylphenyl, dtb =
2,5-(t-Bu)2Ph), and [(bph-BIAN)Ge:] (bph = (2-PhC6H4)).36
Moreover, complexes such as (dipp-BIAN)GeCl have been
prepared by reacting the neutral Ar-BIAN ligands with GeCl2,
where GeCl2 acts as a reducing agent for the Ar-BIAN ligands
to form their radical anionic counterparts.37,38 On going down
this group, tin complexes supported by neutral Ar-BIAN ligands
have also been described, including [(mes-BIAN)SnCl4] (mes
presenting the related tetrakis(imino)py-racene (TIP) ligand.51
Hence, the synthesis of new, heavy group 13 Ar-BIAN
complexes and the evaluation of their absorption and
electrochemical properties still remain under-explored areas.
Our group is interested in applying mechanochemical
approaches to synthetic main group compounds52 and has
recently reported the preparation of new indium(III) bis-
(arylimino)-acenaphthene complexes by using an essentially
solvent-free mechanochemical approach.50 The indium(III)
complexes were characterized by density functional theory
(DFT) calculations, electrochemistry, and UV−vis−NIR spec-
troscopy. The electrochemical data suggested that the Ar-BIAN
indium(III) complexes could be converted to their indium(I)
congeners by mild reducing agents, offering us the opportunity
to explore the intriguing redox noninnocence of the Ar-BIAN
systems. Upon reduction of the indium(III) center, these
complexes show potential for applications as photosensitizers,
dovetailing with our interest in the use of earth-abundant
elements in artificial photosynthesis.16,53−55
= 2,4,6-trimethylphenyl) and [(dtb-BIAN)SnCl2]).12,38,39
A
Herein, we present a family of new indium(III) Ar-BIAN
complexes with the synthesis of a broad range of Ar-BIAN
ligands comprising electron-donating and -deficient substitu-
ents on the arylimino fragment. All of the complexes reported
here have been structurally characterized and probed with
electrochemical and UV−vis spectroscopic measurements, as
well as DFT and TD-DFT calculations.
series of group 15 and 16 Ar-BIAN metal complexes have been
previously characterized as well. For example, [(dipp-BIAN)-
SbCl3 and [(mes-BIAN)BiCl3])12 supported by neutral Ar-
BIAN ligands, and [(dipp-BIAN)E] [SnCl5·THF] obtained by
reduction of ECl3 (E = P and As) with SnCl2, followed by
treatment with dipp-BIAN to give their dianionic form.40 As for
the chalcogens, the reaction of TeI4 with neutral dipp-BIAN
results in a two-electron reduction of the metal center to form
[(dipp-BIAN)TeI2], in which the ligand is neutral and the
chalcogen has been reduced to the +2 oxidation state.41 More
recently, the (dipp-BIAN)SeCl2 and (dipp-BIAN)SeBr2
counterparts have also been described.42 Clearly, the redox
noninnocence of Ar-BIAN ligands have contributed to some
rich chemistry and oxidation state ambiguity even among main
group coordination compounds.
RESULTS AND DISCUSSION
■
Synthesis of Complexes. The alkyl- and nitro-substituted
Ar-BIAN ligands (1a−3a) were synthesized by condensation of
acenaphthoquinone with the corresponding aniline under acidic
conditions.56,57 The Lewis acidic ZnCl2 was used as the
templating agent, after which demetalation with K2CO3 or
Na2C2O4 was carried out (Scheme 1, top half). However, this
method proved unsuccessful for the synthesis of para-amino-
substituted 4a, and ligand hydrolysis was observed after the
demetalation step in an aqueous Na2C2O4 solution. Alter-
natively, we have previously demonstrated that substituted Ar-
BIAN ligands can be successfully synthesized via mechano-
chemical methodologies, which bypass the ZnCl2 templating
In the case of group 13 complexes, other than a limited
number of boron compounds,39,43 only a few aluminum and
gallium Ar-BIAN complexes have been isolated and structurally
characterized.43−49 In particular, until very recently,50 there was
only one crystallographically characterized example of a
monometallic Ar-BIAN indium(III) complex12 and two reports
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Scheme 2. Synthetic Route to 1b−5b
Figure 1. Selected bond lengths (Å) and angles (deg) of complexes (1b) In1−N1 2.342(5), In1−N2 2.339(5), In1−Cl1 2.3835(16), In1−Cl2
2.3800(17), In1−Cl3 2.3643(17); (2b) In1−N1 2.3196(15), In1−Cl1 2.3897(5), In1−Cl2 2.3769(7); (3b) In1−N1 2.3230(14), In1−N2
2.3114(14), In1−Cl1 2.4013(4), In1−Cl2 2.4722(5), In1−Cl3 2.4032(5), In1−O5 2.2733(13); (4b) In1−N1 2.317(3), In1−N2 2.331(3), In1−Cl1
2.4029(10), In1−Cl2 2.4241(9), In1−Cl3 2.4408(10), In1−O1 2.259(3); (5b) In1−N1 2.3182(14), In1−N2 2.3444(13), In1−Cl1 2.4417(4), In1−
Cl2 2.4301(4), In1−Cl3 2.4058(5), In1−O1 2.2306(13). Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been
omitted for clarity.
and subsequent demetalation step.50 Accordingly, 4a was
prepared via a ball-milling mechanochemical approach in the
presence of a catalytic amount of acetic acid and Na2SO4
(Scheme 1, bottom half). The synthesis of 5a, which we
previously reported, has been included in Scheme 1 for the
completeness of our study.50 In a typical procedure, all
reactants and additives were loaded into a stainless-steel
grinder jar containing a 10 mm stainless steel ball (4 g weight).
The reaction mixture was milled for 5 h in a Retsch MM400
mixer mill operating at 30 Hz.58,59 On the basis of the 1H NMR
spectra, 4a was formed with only trace amounts of unreacted
precursors. Subsequent reaction of the Ar-BIAN ligand (1a−
5a) with InCl3 in tetrahydrofuran (THF) afforded the
respective indium(III) Ar-BIAN complexes (1b−5b). The
syntheses of 1b, 2b, 4b, and 5b60 were conducted at room
temperature, whereas 3b was prepared under reflux conditions
in THF (Scheme 2).
the formation of coordinated, neutral Ar-BIAN ligands on
indium(III). In general, the 1H NMR spectra of the coordinated
Ar-BIAN ligands in complexes 1b−5b each exhibit a downfield
shift compared to the corresponding spectra of ligands 1a−5a.
Compounds 1b and 2b were purified by recrystallization from
1,2-dimethoxyethane (DME) and tetrahydrofuran (THF),
respectively. In the case of 3b, single crystals suitable for
single-crystal X-ray diffraction analysis were obtained by
recrystallization from acetonitrile (ACN); however, batch
purification was carried out by slow diffusion of diethyl ether
(Et2O) into DMF. Therefore, the characteristic DMF signals
1
were present in the H NMR spectrum of 3b recorded in
deuterated acetone (Figure S5). In the case of 4b and 5b, vapor
diffusion of Et2O into DMF has been used for recrystallization
and, hence, one DMF molecule is coordinated to each
indium(III) metal center. Thus, the 1H NMR spectra for
both complexes reveal signals corresponding to DMF (Figures
S6 and S8).
1
Spectroscopic and Crystallographic Studies. The H
and 13C{1H} NMR spectra obtained for 1b−5b concurred with
C
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All indium(III) Ar-BIAN complexes were successfully
recrystallized to provide samples suitable for single-crystal X-
ray structural analyses (Figure 1). We have already previously
reported two structures, [(p-MeOAr-BIAN)2InCl2]+[InCl4]−
(6) and (p-BrAr-BIAN)InCl3 (7), which we compare with
and refer to in this report for the completeness of our studies.50
Compound [(p-MeOAr-BIAN)2InCl2]+[InCl4]− (6) was orig-
inally recrystallized from ACN; however, when 6 is recrystal-
lized from a more strongly coordinating solvent like DMF,
mono Ar-BIAN complex 5b is formed. We attribute the
formation of 5b to the stronger coordinating ability of DMF
compared to that of ACN. Coordinating solvents (H2O for 3b,
DMF for 4b and 5b) are bound to each indium center resulting
in distorted octahedral geometries for the complexes. The
equatorial bond angles range from 72.64(5) to 101.06(2)° in
3b, from 72.85(10) to 97.94(8)° in 4b, and from 72.68(5) to
96.77(4)° in 5b, deviating significantly from the ideal value of
90°. Similarly, the axial bond angles (166.45(4), 164.01(7), and
162.18(4)° in 3b−5b, respectively) also deviate from the
expected 180°. This deviation of axial and equatorial bond
angles in a distorted octahedral geometry was also reported in a
similar mes-BIAN-InCl3 complex.12
complexes at 298 K are depicted in Figure 2. Complexes 1b−
5b typically exhibit two major sets of absorption bands. The
Figure 2. UV−vis spectra for 1b and 2b in THF, 3b and 5b in DMF,
and 4b in DCM solution. All solutions were prepared at
concentrations of 0.10 mM.
bands are assigned to a mixture of the intraligand π−π* charge
transfer transitions between the arylimine and the acenaph-
thene motifs, according to the TD-DFT calculations (vide
infra). The spectrum is significantly red-shifted when the
electron donating −NMe2 group is present on the di-
(arylimino) moiety, inducing a smaller HOMO−LUMO gap
due to a more destabilized HOMO. Although inductively
electron-rich alkyl groups are present in 1b and 2b (i.e., 2,6-
diisopropyl and 2,6-dimethyl), the effect is not as strong as the
mesomerically donating groups in 4b and 5b (i.e., 4-
dimethylamino and 4-methoxy). This is consistent with DFT
theoretical calculations from the Zysman-Colman group, where
they reported that the HOMO of the ligand is destabilized by
1.1 eV when a mesomerically electron donating (−NMe2)
group is present within the ligand backbone, whereas a
destabilization of only 0.29 eV is achieved when an inductively
electron-rich alkyl (2,4,6-trimethyl) group is introduced.14 The
single intense absorption band at ca. 320 nm in 3b may result
from a coincidental convergence of multiple intraligand charge
transfer transitions.
With an initial objective to identify a suitable reducing agent
with an adequate reduction potential to obtain the indium(I)
counterparts of complexes 1b to 4b, their redox properties were
examined by cyclic voltammetry. The redox potentials of
accessible reduction and oxidation waves are summarized in
Table 1. The voltamogram for 5b has not been collected in this
report since its redox potential is expected to be identical to the
previously reported compound 6, which has been recorded in
ACN. Since the molecular structures of 5b and 6 are solvent
dependent, their redox potentials would be affected by the
solvent used.50 All potentials are reported relative to the
ferrocenium/ferrocene (Fc+/Fc) redox couple. A direct
comparison of the voltamograms of 1b−4b cannot be made
due to their disparate solubility properties and the differences in
their coordination spheres. However, it is worth noting that the
electrochemical behavior of 1b, 2b, and 3b consist of four
irreversible reductions and a series of oxidations (Figure 3).
The irreversibility may stem from the following. First,
detachment of a chloride ligand or coordination of a solvent
molecule to the indium species formed during the voltammetric
sweep may occur, since the reduction of the indium(III) center
may involve structural changes arising from the anion
dissociation. Second, the ligand itself may reduce due to its
noninnocent nature. Third, the putative indium(I) intermedi-
In contrast, 1b and 2b adopt distorted square pyramidal
geometries presumably due to the increased steric hindrance
around the metal nucleus introduced by substitution of
isopropyl (in 1b) and methyl (in 2b) groups at the ortho
positions of the aniline moiety. The axial bond angles range
from 95.89(13) to 110.12(6)° in 1b and from 100.86(4) to
107.42(19)° in 2b, similar to those in the related tetrakis-
(imino)pyracene (TIP) complexes that also adopt distorted
square pyramidal geometries around the indium center (94.2−
117.4°).51 Complex 2b exhibits a plane of symmetry bisecting
the acenaphthene fragment and the indium metal center,
making the two diimine N and Cl atoms at the equatorial
positions equivalent. The C−C and C−N bond distances
within the diimine moiety in 1b−5b (ranging from 1.517(3) to
1.535(8) Å and from 1.274(2) to 1.288(4) Å, respectively) are
consistent with typical C−C and CN bond lengths.12
Therefore, complexes 1b−5b can be considered to consist of
neutral Ar-BIAN ligands coordinated to indium(III) via dative
bonds. The average In−N bond distances for 1b and 2b (2.341
and 2.320 Å, respectively) are slightly shorter than those in
12
their mes-BIAN-InCl3 and dipp-TIP-InCl3 counterparts (2.35
and 2.38 Å).51 The introduction of an electron-withdrawing
−NO2 group (in 3b) and electron-donating −NMe2 (in 4b)
and −OMe groups (in 5b) did not affect the average In−N
bond distances greatly. Their average In−N bond distances
(2.317 Å in 3b, 2.324 Å in 4b, and 2.331 Å in 5b) remained
similar to those for 1b and 2b. Notably, complexes 1b−5b all
exhibit In−Cl bond distances significantly longer than that for
InCl3(THF)2 (2.331(3) Å).61 The In−Cl bond distances in 1b
and 2b range from 2.3643(17) to 2.3897(5) Å and are shorter
than those in the related mes-BIAN-InCl3, likely due to the
increased electron donation from the mes-BIAN ligand in the
latter, resulting in a weakening of the In−Cl bonds.12 Although
4b and 5b bear ligands that are more electron-rich than those of
3b, the average In−Cl bond distances in these complexes are
similar (2.425, 2.423, and 2.426 Å in 3b, 4b, and 5b,
respectively).
Absorption and Electrochemical Studies. To under-
stand the absorption profile of the indium(III) complexes,
UV−vis absorption spectroscopic measurements were carried
out for 1b−5b. The absorption spectra obtained for each of the
D
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complex due to the stronger electron donation from 4a to the
metal center, thus preventing dissociation of the Ar-BIAN
ligand upon reduction. This is consistent with our previous
observations on the voltammogram recorded for 6 bearing
electron donating −OMe groups. Compound 6 also showed an
electrochemically reversible reduction wave at −1.46 V, which
was attributed to the stronger electron donation from the
corresponding −OMe substituted ligand to the indium(III)
center.50 Although the assignment of each reduction and
oxidation processes in 1b−4b was hampered by the complex
processes at the electrode surface, the redox behaviors of 1b−
4b indicate that mild reducing agents with potentials between
−1.0 and −2.0 V can be utilized to effect reduction of the
In(III) complexes.62
Table 1. Electrochemical Data for 1b−4b
potential (V vs Fc+/Fc)
cathodic peaks
anodic peaks
−1.61
−1.87
−2.41
−2.72
−2.26
−1.75
−0.54
+0.22
1b
2b
3b
4b
−2.95
−1.14
−1.61
−2.30
−2.57
−2.11
−1.38
−0.98
−0.42
−2.84
In view of this, the reduction of 1b using magnesium
anthracene was attempted in THF. Interestingly, based on the
1H NMR spectra and visual observations of the reaction
mixture, reduction of the Ar-BIAN ligand and the indium(III)
nucleus appeared to occur concurrently. Indium metal
deposition was observed from the reaction mixture, and the
protonated reduced Ar-BIAN ligand was the only isolated
reaction product as indicated by a singlet at 5.06 ppm (Figure
S11). This chemical shift corresponds to the two N−H bonds
of a reduced Ar-BIAN ligand, which matches the spectrum
obtained from an independent reduction of ligand 1a. Similarly,
reduction reactions of 6 using CoCp2 resulted in formation of
the reduced Ar-BIAN ligand MeOAr-BIAN-H2 (8). The
−1.25
−1.97
−2.32
−2.14
−1.64
−1.21
−0.08
+0.51
−2.49
+0.28
−0.06
−1.24
−1.62
−1.85
−1.47
−1.15
−0.72
+0.44
+0.64
63
1
presence of 8 was confirmed by H NMR spectroscopy and
single-crystal X-ray diffraction (Figure S12). The observed
formation of the reduced ligand in the attempted reduction
reactions of 1b and 6 may be attributed to the redox
noninnocence of the Ar-BIAN ligands. As mentioned in the
Introduction, it has been demonstrated that reduction reactions
sometimes occur on the diimine ligand rather than the metal
center among both main group64,65 and transition metal
ates formed at the electrode surface may disproportionate,
which is also supported by the experimental findings (vide
infra). In contrast, the electrochemical behavior of 4b in DCM
showed a chemically reversible reduction wave at −1.20 V on
the time scale of the experiment (Figure 3d). The reversibility
of the first reduction wave observed in 4b suggested an
increased stability of the transient [(NMe2Ar-BIAN)InCl3]−
Figure 3. Cyclic voltammograms of the first reduction wave for 1b (a) in THF, 2b (b) in THF, 3b (c) in DMF, and 4b (d) in DCM solution at 298
K with 0.10 M (nBu4N)PF6 as the supporting electrolyte and glassy carbon (3 mm in diameter) as the working electrode. Data were collected using
scan rates ranging from 50 to 300 mV s−1, and all potentials reported were referenced to Fc+/Fc.
E
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Figure 4. Electron density distributions of the frontier orbitals for 1b−4b.
complexes.1,66 Future studies will involve the use of (i) bulkier
aryl substituents to kinetically stabilize the indium(I) products,
which has been successfully adopted for indium(I) complexes
comprising redox innocent ligands;67 (ii) more electron-rich
aryl substituents to preclude electron transfer from the
indium(I) center to the Ar-BIAN ligand; and (iii) ligands and
solvents of different electronic properties to stabilize the
heteroleptic indium(I) center. Studies involving these ap-
proaches are currently undergoing.
CONCLUSIONS
■
We have conducted a comprehensive investigation on the
synthesis, absorption, and electrochemical properties of a series
of five indium(III) complexes bearing redox noninnocent Ar-
BIAN ligands. All the complexes within this study absorb light
over wavelengths ranging from 380−800 nm, with 4b absorbing
furthest to the red region, which can be attributed to the small
HOMO−LUMO gap due to the −NMe2 substituent. On the
other hand, the absorption spectrum of 3b revealed a single
intense band at 320 nm resulting from enlarged frontier orbital
energy gaps arising from the electron-withdrawing −NO2
group. The CVs showed irreversible reduction waves ranging
from −1.2 to −1.9 V for compounds 1b−3b, whereas this
reduction was observed to be chemically reversible for
compound 4b and our previously reported complex 6.
Although disproportionation and ligand reduction were
observed during our attempted reduction of 1b and 6, we
ascribe it to the noninnocent behavior of the Ar-BIAN ligand,
which can accommodate the extra electrons more readily than
the In metal center. Future work will involve modifying both
the steric and electronic properties of the Ar-BIAN ligands, as
well as using different counteranions and ligands in heteroleptic
systems to prevent electron transfer from the metal to the Ar-
BIAN ligand.
Computational Studies. DFT and TD-DFT calculations
were undertaken to predict the electronic absorption spectra of
the complexes described herein. The geometry of each complex
was optimized with the hybrid functional B3LYP/6-31+G*,
using the pseudo potential LANL2DZ basis set for the indium
and chlorine atoms. A larger basis set 6-31++G* was used for
our previously reported complexes 6 and 7, with almost
identical outcomes as those obtained from the basis set 6-
31+G*. Hence, all geometry optimizations were carried out
with the smaller 6-31+G* basis set in this article since it would
be computationally cheaper. The optimized ground state
structures for all complexes agreed closely with our X-ray
crystallographic studies. Each optimized structure of 1b and 2b
displays a plane of symmetry bisecting the acenaphthene and
indium metal center. The frontier orbitals illustrating the
electron density distribution for 1b−4b are shown in Figure 4.
For 1b, 2b, and 4b, the HOMO is mostly distributed on the
di(arylimine) part of the Ar-BIAN ligand, while the LUMO is
mainly localized on the acenaphthene moiety. However, for 3b,
the HOMO is localized on the acenaphthene bay region,
whereas the LUMO has distributed contributions throughout
the entire ligand. This intriguing reversal in orbital density
when strongly electron-withdrawing groups are installed on the
di(arylimine) motif is consistent with previous DFT calcu-
lations performed on a series of Ar-BIAN ligand by Zysman-
Colman and co-workers.14 From the TD-DFT analysis, the
absorption maxima in 1b and 2b (λmax = 320 nm) are mainly
attributed to electronic transitions from the HOMO−3 →
LUMO+1, while for 3b (λmax = 318 nm), it corresponds largely
to a combination of the HOMO → LUMO+3, HOMO−1 →
LUMO+2, and HOMO−8 → LUMO transition (Tables S32−
S34). In the case of 4b, the calculated predominant
contribution to the lower energy absorption maximum (λmax
= 630 nm) corresponds exclusively to the HOMO → LUMO
transition, whereas the higher energy UV absorption band (λmax
= 267 nm) consists of a combination of contributions from
HOMO−14 → LUMO, HOMO−15 → LUMO, and HOMO
→ LUMO+7 transitions (Table S35).
EXPERIMENTAL DETAILS
■
General Procedures. All reactions with InCl3 and the
recrystallization of products were performed under dry and inert
atmospheres by using a combination of standard Schlenk line
techniques and a Vacuum Atmospheres Company N2 glovebox.
Solvents including tetrahydrofuran (THF), diethyl ether (Et2O), and
pentane were distilled over Na/benzophenone and degassed using
freeze−pump−thaw cycles prior to use. Anhydrous acetonitrile (ACN)
was collected from a PURE SOLV MD-5 solvent purification system
and stored in a glovebox. Anhydrous 1,2-dimethoxyethane (DME) and
dichloromethane (DCM) were distilled over calcium hydride (CaH2)
and stored in a glovebox. Anhydrous N,N-dimethylformamide (DMF)
in a Sure-Seal bottle was purchased from Sigma-Aldrich and used as
received. Deuterated chloroform (CDCl3) and acetonitrile (CD3CN)
were distilled over CaH2 and stored over 4 Å molecular sieves.
Deuterated acetone was distilled over potassium carbonate (K2CO3).
All reagents were purchased from commercial sources and were used
without further purification. Synthetic procedures for 6 and 7 were
reported by our team previously.50 The 1H and 13C{1H} NMR spectra
were recorded at 298 K on Bruker AV400 and BBFO 400
spectrometers. The chemical shift values are reported in parts per
million (ppm) relative to TMS, using residual protonated solvents as
the internal standards (1H: δ = 7.26 for CDCl3 and δ = 1.94 for
CD3CN; 13C: δ = 77.2 for CDCl3 and δ = 118.3 for CD3CN). The
F
DOI: 10.1021/acs.inorgchem.7b00539
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
115In NMR data have not been reported due to the large quadrupolar
moments of the nuclei, which result in broad and essentially
unobservable NMR signals in the solid-state.68−70 This is especially
the case due to the low symmetry of our reported complexes.
Other Instrumentation. High-resolution mass spectra were
obtained by using a Waters Q-Tof Premier, with ESI mode.
Crystallographic data were recorded on a Bruker X8 CCD
diffractometer. UV−vis spectroscopic measurements were performed
using a Shimadzu UV-3600 UV−vis−NIR spectrophotometer. Cyclic
voltammetry (CV) experiments were conducted in a glovebox with N2
atmosphere, using a Biologic SP-300 potentiostat with 1.0 mM
solutions of each sample and 0.10 M of tetrabutylammonium
hexafluorophosphate (n-Bu4NPF6) as the electrolyte. Melting points
were measured on an OptiMelt automated melting pointing system
using sealed glass capillaries under argon and were uncorrected.
Synthesis of Indium(III) Complexes. (dipp-BIAN)InCl3 (1b). A
solution of InCl3 (0.025 g, 0.11 mmol) in THF (5 mL) was added to a
solution of dipp-BIAN (0.056 g, 0.11 mmol) in THF (5 mL) at room
temperature. (See Supporting Information for synthesis of dipp-
BIAN.) The resulting orange solution was stirred overnight, after
which all volatiles were removed under vacuum. The resultant orange
powder was recrystallized by vapor diffusion of pentane into a DME
solution of the crude product, and the isolated yield was 0.061 g
(76%). 1H NMR (400 MHz, CDCl3): δ = 0.87 (d, J = 7.2 Hz, 12 H),
1.38 (d, J = 6.8 Hz, 12 H), 3.07 (sept, J = 6.8 Hz, 4 H), 6.75 (d, J = 7.2
Hz, 2 H), 7.40 (d, J = 7.6 Hz, 4 H), 7.50−7.54 (m, 2 H), 7.60 (t, J =
8.0 Hz, 2 H), 8.17 (d, J = 8.0 Hz, 2 H). 13C{1H} NMR (100 MHz,
CDCl3): δ = 24.4, 24.8, 29.9, 125.1, 126.2, 127.8, 129.1, 129.3, 131.1,
133.2, 140.0, 140.3, 144.3, 162.7. M.p.: decomposed at 280 °C. HRMS
(ESI+, m/z) calcd for C36H40N2InCl3Na [M + Na]+ m/z = 743.1194,
found 743.1179. Anal. Calcd for C36H40N2InCl3: C, 59.90; H, 5.59; N,
3.88. Found: C, 59.56; H, 5.43; N, 3.82.
for synthesis of p-NMe2Ar-BIAN.) The purple solution was stirred
overnight, after which all volatiles were removed under vacuum. The
resultant deep blue solid was washed with Et2O. Purple needle-like
crystals suitable for single-crystal X-ray analysis were obtained from
recrystallization by vapor diffusion of Et2O into a DMF solution of the
crude product, and the isolated yield was 0.13 g (68%). 1H NMR (400
MHz, CD2Cl2, 25 °C, fac-isomer): δ = 3.09 (s, broad, 12 H), 6.88 (d, J
= 9.2, 2 H), 7.04−7.06 (m, 4 H), 7.40 (d, J = 6.8, 2 H), 7.54−7.60 (m,
4 H), 8.17 (d, J = 8.0 Hz, 2 H) . 13C{1H} NMR (100 MHz, CD2Cl2):
δ = 40.6, 40.7, 112.5, 123.0 (broad, may have peaks overlapping),
124.3, 126.1 (broad, may have peaks overlapping), 127.2, 128.8, 129.1,
131.3 (two peaks), 131.7, 132.8 (broad, may have peaks overlapping),
133.5, 143.8, 144.1, 150.9, 151.3 (broad, may have peaks overlapping),
157.7. M.p.: 198−202 °C. HRMS (ESI+, m/z) calcd for
C31H33N5InCl3Na [M + Na]+ m/z = 734.0687, found 734.0652.
Anal. Calcd for C31H33N5OInCl3·H2O: C, 50.95; H, 4.83; N, 9.58.
Found: C, 50.90; H, 4.90; N, 9.32.
(p-MeOAr-BIAN)InCl3·DMF (5b).50 The synthetic procedure was
reported in a recent publication by our team and has been included
here for completeness. Recrystallization by vapor diffusion of Et2O
into a DMF solution of the red solid resulted in samples suitable for
single-crystal X-ray analysis. 1H NMR (400 MHz, CD3CN, fac-
isomer): δ = 3.92 (s, 6 H), 7.16−7.18 (m, 4 H), 7.33−7.36 (m, 4 H),
7.56−7.65 (m, 4 H), 8.31 (d, J = 6.8 Hz, 2 H). 13C{1H}3.33 NMR
(100 MHz, DMF-d7): δ = 56.4, 114.4, 115.8, 116.0, 119.9, 120.4
(broad overlapping peaks), 122.8, 124.3 (broad overlapping peaks),
124.6, 127.2 (broad overlapping peaks), 129.0 (broad overlapping
peaks), 130.1, 130.3, 133.5, 138.7, 158.1, 160.6. HRMS (ESI+, m/z)
calcd for C29H27Cl3InN3O3Na [M + Na]+ m/z = 708.0054, found
708.0081. Anal. Calcd for C29H27N3O3InCl3·H2O: C, 49.43; H, 4.15;
N, 5.96. Found: C, 49.84; H, 4.55; N, 6.12.
Reduction of [(p-MeOAr-BIAN)2InCl2]+[InCl4]− (6). A THF solution
(5 mL) of CoCp2 was added dropwise to a THF suspension (5 mL) of
6 at room temperature. The resulting brown suspension was stirred for
2 h, after which all volatiles were removed under vacuum. The crude
purple solid was washed with Et2O, and the purple filtrate was filtered
(2,6-Me2Ar-BIAN)InCl3 (2b). The procedure was identical to the one
reported above for 1b. The following quantities of reagents were used:
InCl3 (0.023 g, 0.10 mmol) in THF (5 mL), 2,6-Me2Ar-BIAN (0.040
g, 0.10 mmol) in THF (5 mL). (zsee Supporting Information for
synthesis of 2,6-Me2Ar-BIAN.) Orange block-like crystals suitable for
single-crystal X-ray analysis were obtained by recrystallization via vapor
diffusion of pentane into a THF solution of the crude product, and the
1
and concentrated. The H NMR experiments were conducted on the
resultant purple solid. 1H NMR (400 MHz, C6D6): 3.33 (s, 6 H), 4.93
(s, 2 H), 6.72−6.77 (m, 8 H), 7.12−7.42 (m, overlapping with signal
from C6D6), 7.43 (d, J = 8.0 Hz, 2 H).
1
isolated yield was 0.026 g (41%). H NMR (400 MHz, CDCl3): δ =
Theoretical Basis. DFT calculations were used to evaluate both
the geometries and energies of the different indium complexes
possessing one or two coordinated Ar-BIAN ligands. The initial energy
and geometry optimization was carried out at the B3LYP71−73 level
with the 6-31+G* basis set for nonmetal atoms,74 together with the
pseudopotential LANL2DZ75 for the In and Cl atoms. The agreement
between the experimentally determined and the computed structures
supports the method used. Frequency calculations were carried out at
this level to confirm that a minimum had been achieved. To account
for solvent effects, a reaction field calculation with radii and
nonelectrostatic terms and the SMD solvation model was used.76 All
the TD-DFT calculations were carried out using the Gaussian 09
program package.71−73,77
2.30 (s, 12 H), 6.79 (d, J = 7.2 Hz, 2 H), 7.28−7.30 (m, 4 H), 7.33−
7.37 (m, 2 H), 7.62 (t, J = 7.8 Hz, 2 H), 8.21 (d, J = 8.4 Hz, 2 H).
13C{1H} NMR (100 MHz, CDCl3): δ = 19.0, 125.8, 126.7, 128.4,
129.6 (two overlapping signals), 129.9, 131.1, 133.5, 142.1, 144.6,
162.3. M.p.: decomposed at 273 °C. HRMS (ESI+, m/z) calcd for
C28H24N2InCl3Na [M + Na]+ m/z = 630.9942, found 630.9954. Anal.
Calcd for C28H24N2InCl3·THF·H2O: C, 54.92; H, 4.90; N, 4.00.
Found: C, 55.16; H, 4.93; N, 4.26.
(p-NO2Ar-BIAN)InCl3 (3b). The procedure was similar to the one
reported above for complex 1b except that this reaction was carried
out under reflux conditions overnight. The following quantities of
compounds were used: InCl3 (0.044 g, 0.20 mmol) and p-NO2Ar-
BIAN (0.084 g, 0.20 mmol) in THF (7 mL). (See Supporting
Information for synthesis of p-NO2Ar-BIAN.) The resultant yellow
solid was purified by washing with THF, and the isolated yield was
0.083 g (64%). The purity of the product was confirmed by elemental
analysis. Yellow block-like crystals suitable for single-crystal X-ray
analysis were obtained by recrystallization from an ACN solution of
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00539.
1
the crude product. H NMR (400 MHz, acetone-d6): δ = 7.27 (d, J =
6.8 Hz, 2 H), 7.75−7.79 (m, 6 H), 8.44 (d, J = 8.4 Hz, 2 H), 8.55−
8.58 (m, 4 H). The 13C{1H} NMR could not be obtained due to the
poor solubility of 3b in most of the available deuterated solvents. M.p.:
decomposed after 320 °C. HRMS (ESI+, m/z) calcd for
C24H14Cl3InN4O4Na [M + Na]+ m/z = 664.9017, found 664.9005.
Anal. Calcd for C24H14Cl3InN4O4·THF: C, 46.99; H, 3.10; N, 7.83.
Found: C, 46.63; H, 3.45; N, 8.15.
(p-NMe2Ar-BIAN)InCl3 (4b). The procedure was identical to the one
reported above for 1b. The following quantities of reagents were used:
InCl3 (0.066 g, 0.30 mmol) in THF (5 mL) and p-NMe2Ar-BIAN
(0.130 g, 0.30 mmol) in THF (5 mL). (See Supporting Information
General procedures, experimental details, photophysical
1
and electrochemical characterization protocols, H and
13C NMR spectra, crystallographic data, and computa-
tional details (PDF)
Accession Codes
CCDC 1535077−1535081 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
G
DOI: 10.1021/acs.inorgchem.7b00539
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: jdal@unex.es.
*E-mail: hansen@ntu.edu.sg.
*E-mail: fgarcia@ntu.edu.sg.
ORCID
Han Sen Soo: 0000-0001-6502-2313
Felipe García: 0000-0002-9605-3611
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors also gratefully acknowledge the Agency for
Science, Technology and Research (A*STAR), AME IRG
grant A1783c0003 for funding this research. FGG is also
supported by NTU start-up grant (M4080552) and MOE Tier
1 grant (M4011441) for financial support. H.S.S. is supported
by a NTU start-up grant (M4081012), MOE Tier 1 grants
(M4011144 and M4011611), and the Nanyang Assistant
Professorship (M4081154). H.S.S. also thanks the support
from the Solar Fuels Laboratory at NTU and the Singapore-
Berkeley Research Initiative for Sustainable Energy (SinBeR-
́
ISE) CREATE Program. J.D. thanks the Fundacion Compu-
́
́
tacion y Tecnologıas Avanzadas de Extremadura (COMPU-
TAEX) for computing resources.
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