Electronic Properties of Oxidized Cyclometalated Diiridium Complexes: Spin Delocalization Controlled by the Mutual Position of the Iridium Centers

Article abstract of DOI:10.1002/chem.201904894

Four cyclometalated diiridium complexes, with IrCp*Cl (Cp=η⁵-C₅Me₅ ^?) termini bridged by 1,4- and 1,3-bis(p-tolyliminoethyl)benzene (1, 2), or 1,4- and 1,3-bis(2-pyridyl)benzene (3, 4), were prepared and characterized by nuclear magnetic resonance (NMR) spectroscopy and single-crystal X-ray diffraction (complexes 1, 2, and 4). The two iridium centers in complexes 1 and 3 are thus bound at the central benzene ring in the para-position (trans-Ir2), whereas those in complexes 2 and 4 are in the meta-position (cis-Ir2). Cyclic voltammograms of all four complexes show two consecutive one-electron oxidations. The potential difference between the two anodic steps in 1 and 3 is distinctly larger than that for 2 and 4. The visible–near-infrared (NIR)–short-wave infrared (SWIR) absorption spectra of trans-Ir2 monocations 1⁺ and 3⁺ are markedly different from those of cis-Ir2 monocations 2⁺ and 4⁺. Notably, strong near-infrared electronic absorption appears only in the spectra of 1⁺ and 3⁺ whereas 2⁺ and 4⁺ absorb only weakly in the NIR-SWIR region. Combined DFT and TD-DFT calculations have revealed that (a) 1⁺ and 3⁺ (the diiridium-benzene trans-isomers) display the highest occupied spin-orbitals (HOSO) and the lowest unoccupied spin-orbital (LUSO) evenly delocalized over both molecule halves, and (b) their electronic absorptions in the NIR-SWIR region are attributed to mixed metal-to-ligand and ligand-to-ligand charge transfers (MLCT and LLCT). In contrast, cis-isomers 2⁺ and 4⁺ do not feature this stabilizing π-delocalization but a localized mixed-valence state showing a weak intervalence charge-transfer (IVCT) absorption in the SWIR region.

Full text of DOI:10.1002/chem.201904894

A Journal of
Accepted Article
Title: Electronic properties of oxidized cyclometalated diiridium
complexes: Spin delocalization controlled by the mutual position
of the iridium centres
Authors: Frantisek Hartl, Yuxuan Hu, Jing Zhang, Fangfang Zhang,
Xiaoyan Wang, Jun Yin, and Shenghua Liu
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To be cited as: Chem. Eur. J. 10.1002/chem.201904894
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Electronic properties of oxidized cyclometalated diiridium
complexes: Spin delocalization controlled by the mutual position
of the iridium centres
Yu Xuan Hu,‖[a] Jing Zhang,‖[a] Fangfang Zhang,[a] Xiaoyan Wang,[a] Jun Yin,[a] František Hartl*[b] and
Sheng Hua Liu*[a]
Abstract: Four cyclometalated diiridium complexes, with IrCp*Cl
molecule electronics has attracted attention of scientists due to
the great potential for further drop in device dimensions. Multiple
redox-active compounds have widely been explored for
applications in molecular electronics technology1-5 and
information storage6-8, and as molecular wires in a mixed-
valence state9-14. A wide range of bridged diruthenium(II)
complexes belonging to the latter family have been thoroughly
explored due to their facile accessibility and high stability. The
ruthenium centres in a major part of the these Ru(BL)Ru
complexes are linked to the conjugated bridging ligands (BL) via
ethynyl or vinyl groups.15-21 The related cyclometalated
ruthenium(II) complexes are less frequently reported, despite of
embedding the metal centre into the π-conjugated system,
which is conducive to increased conjugation between the metal
centres and the carbometalated ligands22-30. The synthesis of
cyclometalated diruthenium complexes involves some
challenges, such as high reaction temperature and tedious
purification. Isoelectronic iridium(III) benefits from high
hydrocarbon activation ability facilitating the formation of
carbon‒iridium bonds31-33 and the preparation of cyclometalated
oligonuclear iridium complexes in high yields under mild
conditions.34-35 In this work, we have synthesized four
cyclometalated diiridium complexes (Chart 1) featuring 1,3- and
1,4-bis(p-tolyliminoethyl)benzene (1, 2) and 1,3- and 1,4-bis(2-
pyridyl)benzene (3, 4) bridges, and fully characterized them by
NMR spectroscopy, elemental analyses and single-crystal X-ray
diffraction (1, 2 and 4). The two iridium centres occupy para-
positions at the core benzene ring in complexes 1 and 3, and
meta-positions in complexes 2 and 4 (Chart 1). The redox and
electronic absorption properties of 1–4 and their stable
monocations were investigated by cyclic voltammetry combined
with UV-Vis-NIR and SWIR-IR spectroelectrochemistry and
complemented by density functional theory calculations.
‒
(Cp* = η5-C5Me5 ) termini bridged by 1,4- and 1,3-bis(p-tolylimino-
ethyl)benzene (1, 2), or 1,4- and 1,3-bis(2-pyridyl)benzene (3, 4),
were prepared and characterized by nuclear magnetic resonance
(NMR) spectroscopy and single-crystal X-ray diffraction (complexes
1, 2 and 4). The two iridium centres in complexes 1 and 3 are thus
bound at the central benzene ring in the para-position (trans-Ir2),
while those in complexes 2 and 4 in the meta-position (cis-Ir2).
Cyclic voltammograms of all four complexes show two consecutive
one-electron oxidations. The potential difference between the two
anodic steps in 1 and 3 is distinctly larger compared to 2 and 4. The
visible - near-infrared (NIR) - short-wave infrared (SWIR) absorption
spectra of trans-Ir2 monocations 1+ and 3+ are markedly different
from those of cis-Ir2 monocations 2+ and 4+. Notably, strong near-
infrared electronic absorption appears only in the spectra of 1+ and
3+ while 2+ and 4+ absorb only weakly in the NIR-SWIR region.
Combined DFT and TD-DFT calculations have revealed that (a) 1+
and 3+ (the diiridium-benzene trans-isomers) display HOSO and
LUSO evenly delocalized over both molecule halves, and (b) their
electronic absorptions in the NIR-SWIR region are attributed to
mixed metal-to-ligand and ligand-to-ligand charge transfers (MLCT
and LLCT). In contrast, cis-isomers 2+ and 4+ do not feature this
stabilizing π-delocalization but a localized mixed-valence state
showing a weak IVCT absorption in the SWIR region.
Introduction
Over the past few decades, the advent of silicon-based
electronic devices brought about a significant development in
the field of electronic technology. The size limitation of silicon-
based solid-state electronic devices has presented one of the
biggest challenges for their miniaturization and the capacity
multiplication of electronic chips. In recent years, the field of
[a]
Y. X. Hu,‖ Dr. J. Zhang,‖ F. Zhang, X. Wang, Prof. Dr. J. Yin, and
Prof. Dr. S. H. Liu*
Key Laboratory of Pesticide and Chemical Biology, Ministry of
Education
College of Chemistry, Central China Normal University
Wuhan 430079, P.R. ChinaDepartment
E-mail: chshliu@mail.ccnu.edu.cn
Prof. Dr.F. Hartl*
[b]
Department of Chemistry
University of Reading
Whiteknights, Reading RG6 6AD, United Kingdom
E-mail: f.hartl@reading.ac.uk
‖These authors contributed equally to this work.
Chart 1. Schematic molecular structures of studied diiridium complexes 1–4.
Supporting information for this article is given via a link at the end of
the document.
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Results and Discussion
and 4 are shown in Figure 1. Details of data collection and
refinement are given in Table S1 (Supporting Information).
Selected parameters (bond lengths (Å) and angles (deg)) from
the crystal structures of 1, 2 and 4 are presented in Tables S2–
S4 (Supporting Information), respectively. In 4, the diiridium-
fused tricyclic core displays nearly perfect planarity; the Ir‒Cl
bonds are anti-parallel and almost perpendicular to the iridacycle.
Molecular structures of 1 and 2 are similar to that of 4, with the
unique difference that the p-tolyl rings are highly rotated out of
the plane of the conjugated core. Differently from 2 and 4, the
structure of 1 is centrosymmetric. The Ir‒C and Ir‒N bond
Syntheses and Characterization
The precursor complex [Cp*IrCl2]2 was prepared from IrCl3·xH2O
according to a literature procedure.36 The general synthetic route
for the preparation of cyclometalated diiridium complexes 1–4 is
outlined in Scheme 1. All complexes were synthesized in high
(>70 %) yields. 1,4-bis(p-tolyliminoethyl)benzene (a) was
obtained by the condensation reaction of p-toluidine and 1,4-
diacetylbenzene in the presence of an excess of sodium
bicarbonate, while 1,3-bis(p-tolyliminoethyl)benzene (b) was
synthesized by a literature method.37 Ligand precursors 1,4- and
1,3-bis(2-pyridyl)benzene (c and d) were obtained in moderate-
to-high yields by palladium(II)/copper(0)-catalyzed cross-
coupling reactions of 2-bromopyridine with 1,4- and 1,3-
benzenediboronic acid, respectively. All these compounds were
characterized by 1H and 13C NMR spectra (Supporting
lengths (see Figure
1 and Tables S2-S4 in Supporting
Information) are similar to those reported in the literature for
related cyclometalated iridium(III) complexes.31-32, 34-35, 38-40 The
sum of the internal angles in the five-membered iridacycles in
complexes 1, 2 and 4 is 540.0°/540.0°, 540.0°/539.6° and
539.2°/539.9°, respectively. This is an additional evidence for
their planarity considered as a fundamental precondition for a
strong conjugation and electronic interaction through thre bridge
core. As shown in Tables S2-S4 (Supporting Information), the
selected bond lengths and angles from DFT-optimized model
structures of 1, 2 and 4 are very close to the experimental
crystal structure data. Complex 1 is centrosymmetric while the
molecules of 2 and 4 have a C2 axis of symmetry along the
direction of C1‒C4. After one-electron oxidation, the structure of
1+ remains centrosymmetric, with slightly changed bond lengths
and angles compared to the neutral precursor, this suggest an
electron-delocalized structure of cation 1+. However, the bonding
situation in singly oxidized 2+ and 4+ is strongly different
compared to 1+. For example in 2+, the Ir1—C6, Ir1—N1, Ir1—
Cl1 and C7—N1 bond lengths are 2.001, 2.113, 2.459 and 1.306
Å, respectively, while Ir2—C2, Ir2—N2, Ir2—Cl2 and C26—N2
deviate from their counterparts, being 2.029, 2.074, 2.376 and
1.323 Å, respectively. This comparison signals a loss of C2
symmetry in the structure of 2+ due to more localized charge
distribution and spin density residing at one side of the cation.
The same asymmetric bonding situation is encountered in 4+.
1
Information). In H NMR spectra, the methyl groups at the Cp*
ligands resonate as sharp and strong singlets at 1.4-1.7 ppm. In
the 13C NMR spectra of complexes 1–4, the Cp* signals are
observed near 90 ppm and 9 ppm. The crystal structures of 1, 2
and 4 have been elucidated by X-ray diffraction.
Scheme 1. Synthetic routes toward ligands a-d and complexes 1–4.
Crystal Structures
Single crystals of complexes 1, 2 and 4 suitable for X-ray
diffraction analysis were grown by slow diffusion of n-hexane
into the dichloromethane solution of the corresponding complex.
The molecular structures (including top and side views) of 1, 2
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Figure 1. X-ray structures of complexes 1 (a and b), 2 (c and d) and 4 (e and
f) with thermal ellipsoids at the 50% probability level; a, c and e - top views, b,
d and f - side views. Atom colour code: carbon - gray; nitrogen - blue; chlorine
- green; iridium - yellow. All hydrogen atoms have been omitted for clarity.
Selected bond lengths [Å] for 1: Ir1A‒C3A 2.030, Ir1A‒N1A 2.082; Selected
bond lengths [Å] for 2, Ir1‒N1 2.083, Ir1‒C6 2.015, Ir2‒N2 2.081, Ir2‒C2
2.024; Selected bond lengths [Å] for 3: Ir1‒N1 2.083, Ir1‒C6 2.024, Ir2‒C2
2.029, Ir2‒N2 2.077.
4
0.33
0.58
250
a
Electrode potentials are reported vs the standard ferrocene/ferrocenium
(Fc/Fc+) couple. bΔE1/2 = E1/2(2) − E1/2
(1)
.
Electrochemical Oxidation
Different degrees of electronic interaction between the two metal
centres in the studied cyclometalated diiridium complexes was
first revealed by cyclic voltammetry (CV) and square-wave
1
2
voltammetry
(SWV).
The
cyclic
and
square-wave
voltammograms of complexes 1–4 and the corresponding
bridging ligands a–d, which are redox-silent in the relevant
potential range, are shown in Figure 2 and Figures S1–S3
(Supporting Information). All four diiridium complexes display
two consecutive one-electron oxidation processes. The detailed
electrochemical data are summarized in Table 1. They feature a
large comproportionation constant Kc for the formally metal-
based process IrIII−IrIII + IrIV−IrIV → 2(IrIII−IrIV) , indicating a high
thermodynamic stability of the mixed-valence state. The reality is
however different. De facto, the bridging ligands are also
strongly involved in the oxidation (see the theoretical DFT
section). The standard electrode potential splitting ΔE1/2 for the
one-electron oxidations involving the two bridged Ir(III) centres is
distinctly larger (by 70 mV) for trans-Ir2 isomers 1 and 3
compared to cis-Ir2 isomers 2 and 4 (Table 1). This observation
points to stronger electronic coupling between the iridacycle
halves in 1 and 3 compared to 2 and 4. The role of the bridging
core in mediating the intermetallic interaction will be discussed in
the theoretical section. Importantly, the different iminoethyl and
pyridyl components of the bridge in the couples 1/2 and 3/4,
respectively, have only a limited impact on the potential values
of the formal iridacycle oxidations. Notably, both anodic steps
are fully reversible and well-defined for complex 2 (Figure 2).
Stable monocations are also observed for 1, 3 and 4 (Figure S2,
Supporting Information). On the other hand, the subsequent
oxidation of 1+, 3+ and 4+ under ambient conditions produces
unstable dications as testified by the corresponding cyclic
0.0
0.2
0.4
0.6
0.8
Potential/V
Figure 2. Cyclic voltammograms (top) and anodic square-wave
voltammograms (bottom) of complexes 1 and 2 in CH2Cl2/n-Bu4NPF6 at v =
100 mV s-1. The potential scale corresponds to the standard ferrocene/ferro-
cenium redox couple.
Anodic UV-vis-NIR and SWIR-IR Spectroelectrochemistry
Spectroelectrochemical studies of 1–4 were carried out to
measure electronic absorption of the stable monooxidized forms
and unravel differences in bonding properties between the two
isomer pairs. The UV-vis-NIR spectral changes accompanying
the one-electron oxidation of 1–4 within an OTTLE cell are
shown in Figure 3. Yellow dichloromethane solutions of neutral
parent cis-Ir2 complexes 2 and 4 reveal strong near-UV
absorptions at 360-370 nm tailing to the visible region, with a
shoulder at 450 nm for 2; no visible absorbance is detected
below 450 nm. On the other hand, the dichloromethane solution
of trans-Ir2 complex 1 is red, which is attributed to two
transitions at 555 nm (H→L) and 434 nm (H-1→L) having a
partial MLCT character – see Table S5 (Supporting Information)
and the following theoretical section. Orange trans-Ir2 complex 3
exhibits the same lowest MLCT (H→L) absorption at 500 nm,
while the more intense one at 403 nm arises from H→L+2 and
H→L+3 excitations.
Apart from the isosbestic spectral changes in the near-UV
region, which are characteristic for each complex 1-4, the most
important information from the spectroscopic monitoring of the
reversible first oxidation is obtained in the visible-NIR (Figure 3)
and SWIR (Figure 4) region. Starting with trans-Ir2 complex 1,
the two medium-intensity MLCT bands between 400-600 nm are
replaced by a new set at 446 and 572 nm. As revaled by TD
DFT calculations presented in the next section, the optical
excitation at 572 nm has hardly any charge transfer character,
originating mainly in the delocalized π-system of the iridacycles
and the central benzene ring. Differently from parent 1, the
monocationic product exhbitis two distinct electronic absorptions
voltammograms in Figure
2 and Figure S3 (Supporting
Information). Also the stability of 22+ is only confined to the short
time scale of cyclic voltammetry, as revealed by the follow up
spectroelectrochemical studies described in the next section.
Table 1. Electrochemical Data for Complexes 1-4 in Dichloromethane/n-Bu-
4NPF6.a
Complex
E1/2(1) (V)
0.29
E1/2(2) (V)
0.63
ΔE1/2 (mV)b
340
1
2
3
0.32
0.59
270
0.28
0.60
320
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in the NIR and SWIR region. The higher one at 995 nm
combines electron transfer from the Cp*-iridacycles and their p-
tolyl substituents to the central benzene ring of the bridge. A
similar charge transfer, Cp*Ir to the benzene core, is affiliated
with the lowest two excitations seen as an absorption band of
medium intensity with a maximum at 2270 nm. The same
electron donor and acceptor moieties, Cp*IrCl and the benzene
core, respectively, participate in the characteristic charge-
transfer electronic excitation of trans-Ir2 complex 3+ in the NIR-
SWIR region, which is shifted to a slightly higher energy
compared to 1+. The weak electronic absorption of 3+ between
400 – 600 nm involves more the conjugated π* system of the
planar pyridylbenzene moiety. The minor differences seen in the
visible NIR-SWIR spectral region refllect similar chromophores
involved in both trans-Ir2 cationic complexes despite the
different 1,4-substituents at the benzene core of the bridging
ligands, and the same kind of spin-delocalized π-bonding with a
strong participation of the whole metalated bridging ligand and
both Ir centres (Chart 1).
readily by distinguished from each other, although a detailed
analysis of the vibrational modes in this region was not
performed. All four stable cationic complexes show a new weak
IR absorption band between 1520 and 1560 cm-1.
0.30
0.15
0.25
0.20
0.10
0.15


0.10
0.05
0.05
0
0
300 400 500 600 700 800 900 1000
Wavelength / nm
300 400 500 600 700 800 900 1000
Wavelength / nm
0.02
0.01
0
0.20
0.15
0.10
0.05
0
0.25
0.20
0.15
0.10
0.05
0
0.01
0
500
500
1000
1500
2000
1000
1500
2000


The electronic absorption of oxidized cis-Ir2 isomers 2+
and 4+ in the NIR-SWIR region is weak and lacking the
pronounced well-defined charge-transfer bands encountered for
trans-Ir2 isomers 1+ and 3+ (Figure 3). The poorly resolved NIR
absorption of 2+ and 4+ is tailing down to 2500 and 4000 cm-1
(4000 and 2500 nm), respectively (Figure 4). The electronic
absorption in the SWIR region of both cis-Ir2 cations most likely
belongs to IVCT transitions between the Cp*Ir centres due to the
highly asymmetric frontier β-spin orbitals involved in the optical
excitation, as revealed by the calculated data presented in the
following section. The 1,3-disubstituted benzene core of the
bridge also strongly participates in these electron transfers.
Cation 4+ further exhibits two weak electronic absorptions with
maxima at 1300 and 699 nm (Figure 3). The former one involves
at least three different excitations that originate from different
asymmetric spin-orbitals localized at the same Cp*-iridacycle or
involving one of the delocalized planar pyridylbenzene-
iridacycle-Cp* chromophores (see the next section). The latter
visible absorption of 4+ is ascribed to two electronic excitations
from less asymmetric spin-orbitals encompassing the whole
planar bis(pyridyl)benzene backbone and one dominant Cp*Ir
centre. This visible absorption has its equivalent in the electronic
absorption of 2+ at 753 nm, where the N-p-tolyl substituents
participate instead of the pyridyl rings in 4+. The assignment of
the weak NIR absorption of 2+ is even less straightforward than
in the case of 4+ due to the participation of diverse, variously
delocalized, asymmetric spin orbitals.
250 500 750 1000 1250 1500 1750 2000 2250
250 500 750 1000 1250 1500 1750 2000 2250
Wavelength / nm
Wavelength / nm
0.25
0.005
0.15
0.20
0.15
0.01
0.00
0.10
0.05
0
0.10
0.05
0
600
800
1000
500
1000
1500


300 400 500 600 700 800 900 1000
Wavelength / nm
300 400 500 600 700 800 900 1000
Wavelength / nm
Figure 3. Electronic absorption changes in the UV-vis-NIR-SWIR region
recorded during the reversible oxidation in an OTTLE cell: 1 → 1+ (a, b), 2 →
2+ (c), 3 → 3+ (d, e), 4 → 4+ (f). Conditions: dichloromethane/n-Bu4NPF6, T =
293 K.
0.02
0.02
0.01
0.01


0
0
7000
6000
5000
4000
3000
2000
-1
7000 6000 5000
4000 3000 2000
-1
Wavenumber / cm
Wavenumber / cm
0.02
0.01
0
0.02
0.01
Complexes 1 and 3 were also oxidized chemically with 1
equiv of tris(4-bromophenyl)aminium hexachloridoantimonate


‒
(TBA+SbCl6 ): E1/2(TBA+/TBA) = 0.70 V vs Fc/Fc+ (Figure S4,
0
Supporting Information). The electronic absorption of reduced
TBA is seen as a rising strong band at 305 nm (Figure S5,
Supporting Information). Apart from that, the observed spectral
changes in the UV-vis-NIR region are in full accordance with the
aforementioned spectroelectrochemical experiments. The one-
electron oxidation of 1-4 was also monitored in the fingerprint IR
spectral region of 1600-1500 cm-1 (Figures S6-S9, Supporting
Information). The pairs of the neutral cis- and trans-isomers can
7000
6000
5000
4000
3000
2000
-1
7000 6000 5000
4000 3000 2000
-1
Wavenumber / cm
Wavenumber / cm
Figure 4. SWIR-IR spectral changes during the reversible oxidation in an
OTTLE cell: 1 → 1+ (a), 2 → 2+ (b), 3 → 3+ (c), 4 → 4+ (d). Black spectrum –
before the oxidation; blue spectrum – during the oxidation; red spectrum –
after the oxidation; green spectrum – after the back reduction. Conditions:
dichloromethane/n-Bu4NPF6, T = 293 K.
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DFT and TD-DFT calculations
β-HOSO-4 → β-LUSO (76%)
β-HOSO-6 → β-LUSO (20%)
0.017
727
According to DFT calculations carried out on the neutral parent
complexes, the symmetric HOMOs of trans-Ir2 isomers 1 and 3
are mainly distributed over the iridium atoms and the central
benzene ring (Figure S10, Supporting Information) while the
HOMOs of cis-Ir2 isomers 2 and 4 tend to delocalize over the
whole π-system of the molecule (Figure S11, Supporting
Information). Red-colored 1 and 3 display a smaller HOMO-
LUMO gap compared to orange/yellow 2 and 4 (Table S5,
Supporting Information). The lowest visible electronic absorption
β-HOSO-12 → β-LUSO (47%) 0.003
572
585 (0.005)
unresolved
β-HOSO → β-LUSO (95%)
β-HOSO-5 → β-LUSO (28%)
β-HOSO-2 → β-LUSO (17%)
β-HOSO-1 → β-LUSO (25%)
β-HOSO-4 → β-LUSO (27%)
β-HOSO-2 → β-LUSO (42%)
β-HOSO-6 → β-LUSO (23%)
β-HOSO-4 → β-LUSO (40%)
β-HOSO-2 → β-LUSO (19%)
β-HOSO-6 → β-LUSO (38%)
β-HOSO-4 → β-LUSO (20%)
β-HOSO → β-LUSO (83%)
β-HOSO-2 → β-LUSO (82%)
β-HOSO-5 → β-LUSO (92%)
α-HOSO → α-LUSO (77%)
β-HOSO → β-LUSO (95%)
β-HOSO-2 → β-LUSO (59%)
β-HOSO-4 → β-LUSO (25%)
β-HOSO-3 → β-LUSO (40%)
β-HOSO-2 → β-LUSO (21%)
β-HOSO-4 → β-LUSO (27%)
β-HOSO-3 → β-LUSO (53%)
β-HOSO-6 → β-LUSO (54%)
β-HOSO-4 → β-LUSO (24%)
β-HOSO-5 → β-LUSO (70%)
0.016
2271
0.013
1212
853
735
699
unresolved
unresolved
0.026
0.048
0.019
2+
of
1
and
3
indeed originates exclusively from the
HOMO→LUMO excitation having an MLCT/XLCT (X = Cl)
character, whilst a multicomponent optical excitation is behind
the hypsochromic shift observed for 2 and 4 (Table S5,
Supporting Information).
753 (0.009)
Comparative Figure
5 and Figure S12 (Supporting
Information) document that there are conspicuous differences in
the distribution of spin density between the isomeric pairs 1+/3+
and 2+/4+. In particular, trans-Ir2 isomers 1+ and 3+ feature highly
delocalized electronic structures with the spin density distributed
symmetrically over the molecular backbone, including both
iridium centres (contributions of 0.256 for 1+, and 0.254 for 3+)
and the benzene bridge. In contrast, cis-Ir2 isomers 2+ and 4+
exhibit highly asymmetric spin density distributions, as testified
by the very different values at the two iridium centres, viz.
0.473/0.008 for 2+ and 0.501/0.002 for 4+ . The smaller ΔE1/2
values for the two-step oxidations of 2 and 4 compared to 1 and
3 (Table 1) may then reflect the effect of the stabilizing highly
symmetric delocalization of the spin density in the singly
oxidized trans-isomers. A weaker electronic interaction between
the formally Ir(III) and Ir(IV) centres in both monocationic cis-
isomers makes the removal of the second electron more
thermodynamically feasible.
0.137
0.168
0.010
0.033
0.015
0.021
1535
1085
761
1923 (0.012)
3+
940 (0.005)
564 (0.004)
unresolved
500
2804
1604
0.023
0.020
896
825
1330 (0.002)
4+
0.053
0.050
744
646
699 (0.005)
The electronic absorption spectra of 1+–4+ provide important
information about their bonding properties. To understand the
different visible-NIR and SWIR absorption spectra of trans-
isomers 1+/3+ and cis-isomers 2+/4+, TD-DFT calculations were
also performed on optimized models of the four monocations.
The corresponding isosurface plots of the molecular orbitals
involved in the major electronic excitations are displayed in
Figures 6 and 7. The electronic absorptions obtained from the
TD-DFT calculations (Table 2) on 1+ and 3+ comply well with the
experimental visible-NIR-SWIR spectra. The broad absorption
band of 1+ between 7000 – 2500 cm-1 (1400 – 4000 nm) can be
assigned to the electronic absorptions calculated at 1105 and
1864 nm, which correspond to the β-HOSO-2→β-LUSO (87%)
π-excitation between the iridacycles and the central benzene
ring (MLCT), and the π-delocalized β-HOSO→β-LUSO (88%)
transition. The experimental absorption of 1+ between 700 –
1250 nm, with a maximum at 995 nm, is attributed to the
calculated mixed ILET and MLCT electronic transitions at 821
and 727 nm (β-HOSO-4/β-HOSO-6→β-LUSO). Finally, the weak
visible absorption of 1+ at 585 nm has its calculated counterpart
at 572 nm, with a dominant contribution from β-HOSO-12→β-
Figure 5. The delocalized and localized spin density distribution in 1+ (left) and
2+ (right), respectively, with the corresponding contributions of different parts.
Contour values: ±0.04 (e/bohr3)1/2
.
Table 2. TD-DFT calculated and experimental electronic absorption of 1+–4+.
λ
max(exp)/nm
Complex
Excitation (percentage)
f
λmax(calc)/nm
(ε/105 M-1 cm-1)
β-HOSO → β-LUSO (88%)
β-HOSO-2 → β-LUSO (87%)
β-HOSO-4 → β-LUSO (19%)
β-HOSO-6 → β-LUSO (73%)
0.132
0.156
1865
1105
2270 (0.017)
1+
0.017
821
995 (0.006)
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LUSO (47%) localized at the π-orbital system of the molecular
backbone. The situation is similar for 3+ but the corresponding
electronic absorptions shift to a higher energy. The calculated
optical excitations of 3+ at 1535 and 1085 nm are responsible for
the broad intense absorption between 8000 – 2500 cm-1 (1200 –
4000 nm). The calculated absorption at 761 nm then belongs to
the experimental absorption between 650 – 1200 nm, which is
attributed to the β-HOSO-5 → β-LUSO (92%) MLCT process.
The calculated absorption at 500 nm can be attributed to the α-
HOSO → α-LUSO (77%) transition that corresponds to the
experimental absorption at 565 nm having a partial MLCT
character. Remarkably, frontier spin orbitals of 1+ and 3+ are all
distributed symmetrically. The absorption bands of these trans-
Ir2 monocations in the NIR and SWIR region are thus attributed
to combined MLCT/ILET/IL processes instead of IVCT.
with another one at 853 nm most likely correspond to the
experimental bands near 600 and 750 nm. The weak NIR-SWIR
electronic absorptions calculated at 1212 and 2271 nm are also
preceptible in the experimental spectrum. Similarly, for 4+, the
calculated moderately strong absorptions at 646 and 744 nm
most likely correspond to the experimental band at 699 nm. The
calculated weaker absorptions at 825, 895 and 1604 nm
correspond well with the recorded broad NIR absorption
showing a maximum at 1330 nm (Figure 3f, inset).
Conclusions
To summarize, we report successful syntheses and full
characterization of four redox-responsive cyclometalated
diiridium complexes, 1–4. Crystal structures of 1, 2 and 4 were
resolved by single-crystal X-ray diffraction. The diiridium-fused
polycycles in these complexes are co-planar, which provides a
fundamental condition for strong π-conjugation and
intramolecular charge transfer. All complexes display two
consecutive one-electron oxidation processes but only the first
one is fully reversible. The smaller ΔE1/2 values for the cis-
isomers (2, 4) compared to the trans-isomers (1, 3) reflect
weaker electronic coupling between the two iridium centres in
the
electronically
asymmetric
(mixed-valence)
cis-Ir2
monocations. For 1+ and 3+, the distribution of the spin density is
symmetric and strongly delocalized over the conjugated π-
system of the molecular backbone. TD-DFT calculations of the
latter monocations have revealed intramolecular charge transfer
(MLCT and ILET) excitations matching well with the reasonably
strong electronic absorption in the NIR and SWIR region. In
contrast, mixed-valence cis-isomers 2+ and 4+ show much
weaker IVCT electronic transitions in the SWIR region. The
different mutual orientation of the two iridacycles at the central
benzene ring has thus pronounced consequences for the anodic
behavior, spin density distribution and the nature of low-energy
electronic transitions. This work therefore provides a strong
stimulus for continued investigations of newly designed
cyclometalated diiridium complexes in order to achieve deeper
understanding of their electronic properties, especially the
consequences of cyclometallation on electronic communication
between metal centres, and for further exploration of their
applicability in molecular electronic devices.
Figure 6. Isosurface plots of molecular orbitals involved in the major electronic
excitations in 1+ (left) and 3+ (right).
Experimental Section
General Materials
All manipulations were carried out under an atmosphere of dry argon (or
nitrogen) gas by using standard Schlenk techniques, unless stated
otherwise. Dichloromethane for reactions was distilled from calcium
hydride under N2 prior to use. Toluene and ethanol used for Suzuki-
Coupling reactions was not distilled but degassed by bubbling with argon
before use. Ultradry benzene was purchased commercially. The starting
materials p-methylaniline, 1,4-diacetylbenzene, 1,3-diacetylbenzene, 1,4-
phenylenediboronic acid, 1,3-phenylenediboronic acid and 2-
Figure 7. Isosurface plots of molecular orbitals involved in the major electronic
excitations calculated for 2+ (left) and 4+ (right).
In contrast to the singly oxidized trans-Ir2 isomers, the
poorly resolved β-HOSO → β-LUSO transitions of 2+ and 4+ in
the SWIR region (Table 2) exhibit an IVCT character. For 2+, the
calculated strongest visible absorption at 735 nm supplemented
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bromopyridine were commercially available and used without further
purification. The catalyst, tetrakis(triphenylphosphine)palladium(0), was
prepared according to the literature41.
General procedure for the preparation of cyclometalated complexes
1–4:
A mixture of [Cp*IrCl2]2 (Cp* = 1,2,3,4,5-pentamethylcyclo-
pentadiene) (1.05 equiv.), the imine or pyridine-based ligand (1 equiv.),
NaOAc (10 equivs) and dichloromethane were stirred at room
temperature for 1 d under nitrogen. The reaction mixture was then filtered
through Celite and dried over MgSO4. The solvent was removed under
vacuum and the resulting solid was washed with diethyl ether/hexane to
afford pure products. Single crystals suitable for X-ray analysis were
obtained by diffusion of diethyl ether into a CH2Cl2 solution of the
respective complex.
1H and 13C NMR spectra of compounds a-d and complexes 1-4 are
presented in Figures S13-S27 (Supporting Information).
Synthesis
of
1,4-Bis{1-[(4-methylphenyl)imino]-ethyl}benzene
(compound a):
A
mixture of NaHCO3 (2.28 g, 27.1 mmol), p-
methylaniline (0.581 g, 5.42 mmol), 1,4-diacetylbenzene (0.440 g, 2.71
mmol), activated molecular sieves (7 g; 4 Å), and benzene (15 mL) was
added to a Schlenk-tube under a nitrogen atmosphere and heated to
reflux overnight. The reaction mixture was filtered through Celite that was
then washed with CH2Cl2. The filtrate was collected, the solvent
evaporated in vacuo and methanol added to afford a brilliant yellow solid
that was washed with methanol and dried in vacuo. Yield: 443 mg, 48%.
1H NMR (400 MHz, CDCl3): δ 8.03 (s, 4H), 7.18 (d, J = 7.6 Hz, 4H), 6.72
(d, J = 8.0 Hz, 4H), 2.36 (s, 6H), 2.27 (s, 6H) ppm. 13C NMR (101 MHz,
CDCl3): δ 165.1, 149.0, 141.3, 132.9, 129.7, 127.3, 119.5, 21.0, 17.6
ppm.
1
Complex 1, dark red solid. Yield: 80%. H NMR (400 MHz, CD2Cl2): δ
7.93 (s, 2H), 7.69 (s, 2H), 7.28 (s, 4H), 6.82 (s, 4H), 2.51 (s, 6H), 2.42 (s,
6H) 1.41 (s, 30H) ppm. 13C NMR (101 MHz, CD2Cl2): δ 181.4, 157.6,
152.0, 148.9, 136.4, 134.9, 123.1, 89.3, 21.1, 17.5, 8.7 ppm. Elemental
analysis: Calcd. for C44H52Cl2Ir2N2 (%): C 49.66, H 4.92, N 2.63. Found
(%): C 49.85, H 4.63, N 2.84.
Complex 2, orange solid. Yield: 74%. 1H NMR (400 MHz, CD2Cl2): δ
8.33 (s, 1H), 7.62 (s, 1H), 7.25 (d, J = 7.2 Hz, 4H), 2.40 (s, 12H), 1.45 (s,
30H) ppm. 13C NMR (101 MHz, CD2Cl2): δ 180.8, 176.7, 148.8, 143.3,
142.6, 136.1, 128.5, 89.8, 21.1, 16.5, 8.7 ppm. Elemental analysis: Calcd.
for C44H52Cl2Ir2N2 (%): C 49.66, H 4.92, N 2.63. Found (%): C 49.47, H
5.27, N 2.75.
Complex 3, red solid. Yield: 92%. 1H NMR (400 MHz, CDCl3): δ 8.71 (d,
J = 5.6 Hz, 2H), 8.13 (s, 2H), 7.92 (d, J = 8.0 Hz, 2H), 7.68 (d, J = 7.2 Hz,
2H), 7.07 (t, J = 6.0, 6.8 Hz, 2H), 1.68 (s, 30H) ppm. 13C NMR (101 MHz,
CDCl3): δ 167.4, 154.0, 151.7, 147.6, 136.9, 130.8, 122.2, 119.5, 88.3,
9.2 ppm. Elemental analysis: Calcd. for C36H40Cl2Ir2N2 (%): C 45.23, H
4.22, N 2.93. Found (%): C 45.68, H 4.69, N 2.70.
Complex 4, yellow solid. Yield: 88%. 1H NMR (400 MHz, CDCl3): δ
8.67 (d, J = 4.8 Hz, 2H), 8.35 (s, 1H), 7.92 (1, 1H), 7.82 (d, J = 7.6 Hz,
2H), 7.60 (t, J = 7.2 Hz, 2H) 6.99 (d, J = 6.0 Hz, 2H), 1.71 (s, 30H) ppm.
13C NMR (101 MHz, CDCl3): δ 170.9, 167.8, 151.3, 144.6, 138.6, 136.7,
121.1, 118.8, 117.8, 88.9, 9.1 ppm. Elemental analysis: Calcd. for
C36H40Cl2Ir2N2 (%): C 45.23, H 4.22, N 2.93. Found (%): C 45.57, H 4.64,
N 2.66.
Synthesis
of
1,3-Bis{1-[(4-methylphenyl)imino]-ethyl}benzene
(compound b): This compound was prepared by the procedure
described in the corresponding literature.37 A solution of p-methylaniline
(1.32 g, 12.4 mmol) and 1,3-diacetylbenzene (1.00 g, 6.20 mmol) in
toluene (20 mL) was treated with p-toluenesulfonic acid (0.2 mg) and
refluxed for 5 h. During this time, water in the solution was removed
using
a DeanStark apparatus. The solution was cooled to room
temperature and concentrated in vacuo. Methanol was then added to
afford a faint yellow solid that was washed with methanol and dried in
vacuo. Yield: 45%. 1H NMR (400 MHz, CDCl3): δ 8.54 (s, 1H), 8.05 (d, J
= 8.0 Hz, 2H), 7.52 (t, J = 7.6, 7.6 Hz, 1H), 7.16 (d, J = 8.0 Hz, 4H), 6.71
(d, J = 8.0 Hz, 4H), 2.35 (s, 6H), 2.29 (s, 6H) ppm. No further
characterization was conducted as the 1H NMR spectrum perfectly
matched the literature data.
Synthesis of 1,4-di(pyridin-2-yl)benzene (compound c): To a stirred
solution of 1,4-phenylenediboronic acid (0.524 g, 3.16 mmol), Na2CO3
(2.00 g, 18.9 mmol), and 2-bromopyridine (1.00 g, 6.33 mmol) in toluene
(20 mL)/ethanol (10 mL)/H2O (10 mL), [Pd(PPh3)4] (0.074 g, 0.064 mmol)
was added under argon. The reaction mixture was refluxed for 22 h and
then cooled to room temperature. The mixture was extracted with CH2Cl2,
dried over Na2SO4 and evaporated in vacuo. Subsequent purification by
column chromatography (silica gel, PE:EA = 5:1, v/v) afforded a white
Crystallographic Details
Single crystals of 1·CH2Cl2, 2 and 4 were grown by slow diffusion of n-
hexane into a dichloromethane solution of the complexes. Crystals with
approximate dimensions of 0.20 × 0.20 × 0.10 mm3 for 1, 0.40 × 0.30 ×
0.20 mm3 for 2 and 0.20 × 0.20 × 0.15 mm3 for 4 were mounted on glass
fibers for diffraction experiments. Intensity data were collected by Bruker
APEX-II CCD diffractometer with Mo Kα radiation (0.71073 Å) at room
temperature. The molecular structures were solved by a combination of
intrinsic phasing methods (SHELXT-97)42 and Fourier difference
techniques and refined with Olex243. All non-H atoms were refined
anisotropically; the hydrogen atoms were placed in ideal positions and
refined as riding atoms. Further crystal data and details of the data
collection are summarized in Table S1 (Supporting Information). Selected
bond lengths and angles are presented in Table S2-S4 (Supporting
Information). The CCDC numbers for 1, 2, and 4 are 1921852, 1921854
and 1921855, respectively.
1
solid. Yield: 0.570 g, 78%. H NMR (400 MHz, CDCl3): δ 8.73 (d, J = 4.4
Hz, 2H), 8.13 (s, 4H), 7.80 (m, 4H), 7.25 (d, J = 10 Hz, 2H) ppm. 13C
NMR (101 MHz, CDCl3): δ 156.9, 149.8, 139.8, 136.8, 127.2, 122.3,
120.6 ppm. EI-MS: m/z calcd for C16H12N2 [M]+: 232.10, found 232.30.
Synthesis of 1,3-di(pyridin-2-yl)benzene (compound d): This white,
solid compound was prepared according to the same method as for the
synthesis of 1,4-di(pyridin-2-yl)benzene, using 1,3-phenylenediboronic
1
acid instead.. Yield: 75%. H NMR (400 MHz, CDCl3): δ 8.71 (d, J = 4.4
Hz, 2H), 8.64 (s, 1H), 8.05 (d, J = 7.6 Hz, 2H), 7.81 (d, J =7.5 Hz, 2H),
7.72 (t, J = 7.6, 7.6 Hz, 2H), 7.57 (t, J = 7.6, 8.0 Hz, 1H), 7.21 (t, J = 5.6,
5.6, 2H) ppm. 13C NMR (101 MHz, CDCl3): δ 157.1, 149.7, 139.9, 136.8,
129.2, 127.5, 125.5, 122.3, 120.7 ppm. EI-MS: m/z calcd for C16H12N2
[M]+: 232.10, found 232.27.
Physical Measurements
1H and 13C NMR spectra (see Supporting Information) were recorded on
Varian MERCURY Plus 400 MHz or Bruker AVANCE III HD-400
spectrometers. 1H and 13C NMR chemical shifts are relative to TMS.
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Elemental analyses (C, H, N) were performed with a Vario ElIII CHNSO
instrument. The controlled-potential electrochemical measurements were
conducted with a CHI 660C potentiostat (CHI USA). An air-tight three-
electrode single-compartment cell was used for the solution of complexes
and supporting electrolyte in dry CH2Cl2. The solution was deaerated by
bubbling with dry argon on a frit for about 1 min before the measurement.
The analyte (complex, ligand) and electrolyte (n-Bu4NPF6) concentrations
were typically 10−3 and 10−1 mol dm−3, respectively. A pre-polished 500-
μm diameter platinum disk working electrode, a platinum wire counter
electrode, and an Ag coil pseudoreference electrode were used.
Spectroelectrochemical experiments at room temperature were
performed with an airtight optically transparent thin-layer electrochemical
(OTTLE) cell (optical path length of ca. 200 μm) equipped with a Pt
minigrid working electrode and CaF2 windows.44 The cell was positioned
work at the University of Reading was funded by
Spectroelectrochemistry Reading, a spinout company.
Keywords: Diiridium Complex • Cyclometalated Isomers •
Cyclic Voltammetry • Spectroelectrochemistry • Mixed-valence
State
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Two isomer pairs of cyclometalated
diiridium complexes, with IrCp*Cl
Yu Xuan Hu, Jing Zhang, Fangfang
Zhang, Xiaoyan Wang, Jun Yin,
František Hartl* and Sheng Hua Liu*
0.02
0.02
0.01
0.01
termini bridged by 1,4- and 1,3-
bis(p-tolylimino-ethyl)benzene, or
1,4- and 1,3-bis(2-pyridyl)benzene,
were prepared and fully
characterized. Their different bon-
ding and electronic properties in the
neutral and monocationic states
were studied by experimental


0
0
7000 6000 5000
4000 3000 2000
-1
7000
6000
5000
4000
3000
2000
-1
Wavenumber / cm
Wavenumber
/
cm
1e- oxidation
Electronic properties of oxidized
cyclometalated diiridium comple-
xes: Spin delocalization controlled
by the mutual position of the
iridium centres
0.02
0.02
0.01
0.01


0
0
7000
6000
5000
4000
3000
2000
-1
7000 6000 5000
4000 3000 2000
-1
Wavenumber / cm
Wavenumber
/
cm
spectro-electrochemical
methods
and DFT/TD-DFT calculations.
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