Synthesis, structure and reactivity of iridium complexes containing a bis-cyclometalated tridentate C^N^C ligand

Article abstract of DOI:10.1039/d1dt01269j

In an effort to synthesize cyclometalated iridium complexes containing a tridentate C^N^C ligand, transmetallation of [Hg(HC^N^C)Cl] (1) (H₂C^N^C = 2,6-bis(4-tert-butylphenyl)pyridine) with various organoiridium starting materials has been studied. The treatment of1with [Ir(cod)Cl]₂(cod = 1,5-cyclooctadiene) in acetonitrile at room temperature afforded a hexanuclear Ir₄Hg₂complex, [Cl(κ²C,N-HC^N^C)(cod)IrHgIr(cod)Cl₂]₂(2), which features Ir-Hg-Ir and Ir-Cl-Ir bridges. Refluxing2with sodium acetate in tetrahydrofuran (thf) resulted in cyclometalation of the bidentate HC^N^C ligand and formation of trinuclear [(C^N^C)(cod)IrHgIr(cod)Cl₂] (3). On the other hand, refluxing [Ir(cod)Cl]₂with1and sodium acetate in thf yielded [Ir(C^N^C)(cod)(HgCl)] (4). Chlorination of4with PhICl₂gave [Ir(C^N^C)(cod)Cl]·HgCl₂(5·HgCl₂) that reacted with tricyclohexylphosphine to yield Hg-free [Ir(C^N^C)(cod)Cl] (5). Chloride abstraction of5with silver(i) triflate (AgOTf) gave [Ir(C^N^C)(cod)(H₂O)](OTf) (6) that can catalyze the cyclopropanation of styrene with ethyl diazoacetate. Reaction of1and [Ir(CO)₂Cl(py)] (py = pyridine) with sodium acetate in refluxing thf afforded [Ir(C^N^C)(HgCl)(py)(CO)] (7), in which the carbonyl ligand is coplanar with the C^N^C ligand. On the other hand, refluxing1with (PPh₄)[Ir(CO)₂Cl₂] and sodium acetate in acetonitrile gave [Ir(C^N^C)(κ²C,N-HC^N^C)(CO)] (8), the carbonyl ligand of which istransto the pyridyl ring of the bidentate HC^N^C ligand. Upon irradiation with UV light8in thf was isomerized to8′, in which the carbonyl istransto a phenyl group of the bidentate HC^N^C ligand. The isomer pair8and8′exhibited emission at 548 and 514 nm in EtOH/MeOH at 77 K with lifetime of 84.0 and 64.6 μs, respectively. Protonation of8withp-toluenesulfonic acid (TsOH) afforded the bis(bidentate) tosylate complex [Ir(κ²C,N-HC^N^C)₂(CO)(OTs)] (9) that could be reconverted to8upon treatment with sodium acetate. The electrochemistry of the Ir(C^N^C) complexes has been studied using cyclic voltammetry. Reaction of [Ir(PPh₃)₃Cl] with1and sodium acetate in refluxing thf led to isolation of the previously reported compound [Ir(κ²P,C-C₆H₄PPh₂)₂(PPh₃)Cl] (10). The crystal structures of2-5,8,8′,9and10have been determined.

Full text of DOI:10.1039/d1dt01269j

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Synthesis, structure and reactivity of iridium
complexes containing a bis-cyclometalated
tridentate C^N^C ligand†
Cite this: Dalton Trans., 2021, 50,
8512
Wai-Man Cheung,a Man-Chun Chong,a Herman H.-Y. Sung,a Shun-Cheung Cheng,b
a
Ian D. Williams, *a Chi-Chiu Ko *b and Wa-Hung Leung
*
In an effort to synthesize cyclometalated iridium complexes containing a tridentate C^N^C ligand, transme-
tallation of [Hg(HC^N^C)Cl] (1) (H2C^N^C = 2,6-bis(4-tert-butylphenyl)pyridine) with various organoiridium
starting materials has been studied. The treatment of 1 with [Ir(cod)Cl]2 (cod = 1,5-cyclooctadiene) in aceto-
nitrile at room temperature afforded a hexanuclear Ir4Hg2 complex, [Cl(κ2C,N-HC^N^C)(cod)IrHgIr(cod)
Cl2]2 (2), which features Ir–Hg–Ir and Ir–Cl–Ir bridges. Refluxing 2 with sodium acetate in tetrahydrofuran
(thf) resulted in cyclometalation of the bidentate HC^N^C ligand and formation of trinuclear [(C^N^C)(cod)
IrHgIr(cod)Cl2] (3). On the other hand, refluxing [Ir(cod)Cl]2 with 1 and sodium acetate in thf yielded [Ir
(C^N^C)(cod)(HgCl)] (4). Chlorination of 4 with PhICl2 gave [Ir(C^N^C)(cod)Cl]·HgCl2 (5·HgCl2) that reacted
with tricyclohexylphosphine to yield Hg-free [Ir(C^N^C)(cod)Cl] (5). Chloride abstraction of 5 with silver(I)
triflate (AgOTf) gave [Ir(C^N^C)(cod)(H2O)](OTf) (6) that can catalyze the cyclopropanation of styrene with
ethyl diazoacetate. Reaction of 1 and [Ir(CO)2Cl(py)] (py = pyridine) with sodium acetate in refluxing thf
afforded [Ir(C^N^C)(HgCl)(py)(CO)] (7), in which the carbonyl ligand is coplanar with the C^N^C ligand. On
the other hand, refluxing 1 with (PPh4)[Ir(CO)2Cl2] and sodium acetate in acetonitrile gave [Ir(C^N^C)(κ2C,
N-HC^N^C)(CO)] (8), the carbonyl ligand of which is trans to the pyridyl ring of the bidentate HC^N^C
ligand. Upon irradiation with UV light 8 in thf was isomerized to 8’, in which the carbonyl is trans to a phenyl
group of the bidentate HC^N^C ligand. The isomer pair 8 and 8’ exhibited emission at 548 and 514 nm in
EtOH/MeOH at 77 K with lifetime of 84.0 and 64.6 μs, respectively. Protonation of 8 with p-toluenesulfonic
acid (TsOH) afforded the bis(bidentate) tosylate complex [Ir(κ2C,N-HC^N^C)2(CO)(OTs)] (9) that could be
reconverted to 8 upon treatment with sodium acetate. The electrochemistry of the Ir(C^N^C) complexes
has been studied using cyclic voltammetry. Reaction of [Ir(PPh3)3Cl] with 1 and sodium acetate in refluxing
thf led to isolation of the previously reported compound [Ir(κ2P,C-C6H4PPh2)2(PPh3)Cl] (10). The crystal
structures of 2–5, 8, 8’, 9 and 10 have been determined.
Received 17th April 2021,
Accepted 27th May 2021
DOI: 10.1039/d1dt01269j
rsc.li/dalton
pounds with bis-cyclometalated 2,6-diarylpyridine ligands
have been developed. Octahedral Ir(III) bis(tridentate) C^N^C
Introduction
Metal complexes with tridentate C^N^C, notably bis-cyclome- complexes featuring axial symmetry are also of interest
talated 2,6-diarylpyridines, and related ligands have attracted because they lack geometric isomers that exist for the biden-
much attention due to their interesting luminescent properties tate analogues [Ir(C^N)3] and [Ir(C^N)2(N^N)]+. In addition, the
and biological activity.1,2 A variety of luminescent materials intramolecular charge-transfer property of axially symmetric Ir
and devices based on square planar Pt(II)3,4 and Au(III)5 com- (III) bis(tridentate) complexes can be tuned by the substituents
of the tridentate ligands.1c,6–9
C^N^C complexes have been found to display rich organo-
metallic chemistry. In particular, organogold(III) C^N^C com-
plexes bearing a range of ligands, including hydroxide, carbo-
nyl, hydride, alkene, aryl and alkynyl, have been isolated.10,11
[Au(C^N^C)(OH)] (H2C^N^C = 2,6-bis(4′-tert-butylphenyl)pyri-
dine) is a precursor to Au(III) oxo and peroxo species that can
undergo oxygen atom transfer reactions.12 The Au(III) carbonyl
complex [Au(C^N^C)(CO)]+ is highly susceptible to nucleophi-
lic attack exhibiting reactivity that is relevant to the water–gas
aDepartment of Chemistry, The Hong Kong University of Science and Technology,
Clear Water Bay, Kowloon, Hong Kong, China. E-mail: chwill@ust.hk,
chleung@ust.hk
bDepartment of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon,
Hong Kong, China. E-mail: vinccko@cityu.edu.hk
†Electronic supplementary information (ESI) available: NMR, FT-IR and UV-
visible absorption and emission spectra, cyclic voltammograms and X-ray crystal-
lographic data. CCDC 2072948–2072955. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/d1dt01269j
8512 | Dalton Trans., 2021, 50, 8512–8523
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Scheme 1 Syntheses of Ir(C^N^C)(cod) complexes.
shift reaction.13 Oxidative addition of [Pt(C^N^C)(PnBu3)] catalytic activity. Although Au(III) C^N^C complexes have been
(H2C^N^C = 2,6-bis(4′-fluorophenyl)pyridine) afforded cationic synthesized by transmetalation of [Hg(HC^N^C)Cl] with
Pt(IV) species that can undergo intramolecular C–C coupling of [AuCl4]−,1d,g the use of this method for the synthesis of Ir
the aryl ligands or cyclometalation of the phosphine ligand.14 (C^N^C) complexes has not been explored.
These results highlight the potential applications of C^N^C
complexes in organometallic catalysis and activation of small mercury reagent [Hg(HC^N^C)Cl] (1) (H2C^N^C = 2,6-di(4-tert-
molecules. butylphenyl)pyridine) (Scheme 1) with various organoiridium
In this paper, we describe the transmetalation of the
Compared with the Au(III) and Pt(II) analogues, the reaction starting materials. We found that depending on the experi-
chemistry of Ir(C^N^C) complexes has received less attention. mental conditions, reaction of 1 with [Ir(cod)Cl]2 (cod = 1,5-
This is due, in part, to the difficulty in the synthesis and purifi- cyclooctadiene) and [Ir(CO)2Cl(py)] (py = pyridine) afforded
cation of Ir(C^N^C) complexes. Scandola and coworkers first different types of heterometallic Ir/Hg complexes featuring Ir–
synthesized [Ir(tpy*)(C^N^C)]+ (H2C^N^C = 2,6-diaryl-4-arylpyr- Hg bonds. Hg-Free [Ir(C^N^C)(cod)Cl] has been obtained by
idine; Rtpy = (4′-aryl-2 : 2′,6′:2″-terpyridine) by cyclometalation chlorination of [Ir(C^N^C)(cod)(HgCl)] with PhICl2, followed
of H2C^N^C with [Ir(Rtpy)Cl3] in the presence of AgNO3 in by treatment with
a
phosphine. Also, [Ir(C^N^C)(κ2C,N–
ethylene glycol at 190 °C.1c Related Ir(III) C^N^C complexes C^N^C)(CO)] that contains a tridentate C^N^C and a bidentate
containing substituted terpyridine and C^N^C ligands have HC^N^C has been synthesized from the reaction of 1 with
been synthesized by this method recently, and their reverse (PPh4)[Ir(CO)2Cl2] and sodium acetate. This Ir(C^N^C) carbo-
saturable absorption8 and proton-dependent9 luminescent nyl complex was found to undergo photoisomerization upon
properties have been studied. Williams and coworkers syn- irradiation with UV light. The luminescent and redox pro-
thesized [Ir(N^C^N)(C^N^C)] by heating [Ir(N^C^N)Cl2]2 perties of the Ir(C^N^C) complexes have been studied.
(HN^C^N = 1,3-di(2-pyridyl)benzene) with silver(I) triflate in
molten
2,6-bis(2,4-difluorophenyl)pyridine.7a
Recently,
Esteruelas and coworkers synthesized [Ir(C^N^C)(PiPr3)2(H)]
via C–H activation of 2,6-diphenylpyridine with [Ir(H)5(PiPr3)2]
in refluxing toluene.15
Experimental
General considerations
Apart from [Ir(C^N^C)(PiPr3)2(H)], reported Ir(C^N^C) com- All manipulations were carried out under nitrogen by standard
plexes are mostly bis(tridentate) complexes that are substitu- Schlenk techniques. Solvents were dried, distilled and
tionally inert, rendering the study of their reactivity difficult. degassed before use. NMR spectra were recorded on a Bruker
In this connection, we set out to synthesize [Ir(C^N^C)L2X]- AV 400 Spectrometer operating at 400, 376.5, and 162 MHz for
type complexes and study their organometallic chemistry and 1H, 19F and 31P, respectively. Chemical shifts (δ, ppm) were
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reported with reference to SiMe4 (1H), CF3C6H5 (19F), and 85% (58%). 1H NMR (CDCl3): δ 1.30 (m, 4H, cod), 1.48 (s, 18H, tBu),
H3PO4 (31P). Infrared spectra (KBr) were recorded on a 1.84 (m, 2H, cod), 2.10 (m, 2H, cod), 2.50 (m, 1H, cod), 2.65
PerkinElmer 16 PC FT-IR spectrophotometer and mass spectra (m, 2H, cod), 2.95 (m, 2H, cod), 3.97 (br, 3H, cod), 5.06–5.18
on a Waters Xevo G2-XS Tof with an ESI ionization source. (m, 2H, cod), 7.29 (d, J = 7.6 Hz, 2H, H4), 7.61 (d, J = 8.0 Hz,
GC-FID analyses were performed with a Bruker 430-GC instru- 2H, H2), 7.76 (t, J = 7.6 Hz, H5), 7.81 (d, J = 8.0 Hz, 2H, H3),
ment. Cyclic voltammetry was performed with
a
CH 8.37–8.50 (m, 2H, H1). Some resonances are broad and overlap-
Instruments model 600D potentiostat. The working and refer- ping. Anal. calc. for C41H51Cl2HgIr2N: C, 40.57; H, 4.23; N,
ence electrodes were glassy carbon and Ag–AgNO3 (0.1 M in 1.15. Found: C, 40.23; H, 4.13; N, 1.16.
acetonitrile) electrodes, respectively. Potentials were reported
Synthesis of [Ir(C^N^C)(cod)(HgCl)] (4)
with reference to the ferrocenium–ferrocene (Fc+/0) couple. UV-
visible spectra were recorded on a Dynamica HALO DB-20 UV/ A mixture of [Ir(cod)Cl]2 (60 mg, 0.089 mmol), 1 (103 mg,
visible spectrophotometer. Elemental analyses were performed 0.179 mmol) and sodium acetate (18 mg, 0.219 mmol) in thf
by Medac Ltd, Surrey, U.K.
(10 mL) was heated at reflux overnight. The solvent was
The compounds [Hg(HC^N^C)Cl] (H2C^N^C = 2,6-bis(4- pumped off and the residue was washed with Et2O.
tert-butylphenyl)pyridine) (1),1g [Ir(cod)Cl]2,16 [Ir(CO)2Cl(py)], Recrystallization from CH2Cl2/Et2O/hexane afforded pale
(PPh4)[Ir(CO)2Cl2]17 and [Ir(PPh3)3Cl]18 were prepared accord- yellow crystals. Yield: 94 mg (60%). Alternatively, 4 was syn-
ing to literature methods. Other reagents were purchased from thesized by refluxing 2 (40 mg, 0.016 mmol) with 2 equivalents
Aldrich Ltd and used as received. Hydrogen atom labelling of 1 (18 mg, 0.032 mmol) and sodium acetate (3.3 mg,
schemes of the Ir-bound bidentate HC^N^C and tridentate 0.040 mmol) in thf overnight. Yield: 38 mg (68%). 1H NMR
t
C^N^C ligands are shown below.
(CDCl3): δ 1.47 (overlapping s and m, 20H, Bu and cod), 1.83
(m, 2H, cod), 2.07 (m, 2H, cod), 2.48 (t, J = 3.2 Hz, 2H, cod),
2.95 (m, 2H, cod), 5.05 (t, J = 3.2 Hz, 2H, cod), 7.27 (dd, J = 1.6,
8.0 Hz, 2H, H2), 7.60 (d, J = 8.0 Hz, 2H, H3), 7.73 (dd, J = 7.2,
8.4 Hz, 1H, H5), 7.80 (d, J = 8.4 Hz, 2H, H4), 8.37 (d, J = 1.6 Hz,
2H, H1). Anal. calc. for C33H39ClHgIrN·CH2Cl2: C, 42.41; H,
4.29; N, 1.45. Found: C, 42.65; H, 4.19; N, 1.53.
Synthesis of [Ir(C^N^C)(cod)Cl]·HgCl2 (5·HgCl2)
Caution! Mercury compounds are toxic and should be handled
with caution.
To a solution of 4 (88 mg, 0.10 mmol) in CH2Cl2 (5 mL) was
added 1 equivalent of PhICl2 (28 mg, 0.10 mmol) and the reac-
tion mixture was stirred at room temperature overnight. The
solvent was pumped off and the residue was washed with
hexanes. Recrystallization from CH2Cl2/Et2O/hexane afforded
off-white crystals. Yield: 61 g (64%). 1H NMR (CDCl3): δ 1.50 (s,
Synthesis of [Cl(κ2C,N-HC^N^C)(cod)IrHgIr(cod)Cl2]2 (2)
To a solution of [Ir(cod)Cl]2 (33.6 mg, 0.050 mmol) in MeCN
(5 mL) was added 1 equivalent of 1 (28.9 mg, 0.050 mmol),
and the reaction mixture was stirred at room temperature for
2 h. The orange precipitate was collected, washed with MeCN
and extracted with CH2Cl2. Recrystallization from CH2Cl2/
t
18H, Bu), 1.67 (m, 2H, cod), 1.97 (m, 2H, cod), 2.11 (m, 2H,
cod), 3.00 (t, J = 3.6 Hz, 2H, cod), 3.08 (m, 2H, cod), 4.73 (t, J =
3.6 Hz, 2H, cod), 7.36 (dd, J = 2.0, 8.0 Hz, 2H, H2), 7.53 (d, J =
8.0 Hz, 2H, H3), 7.68 (dd, J = 7.6, 8.0 Hz, 1H, H5), 7.70 (d, J =
8.0 Hz, 2H, H4), 8.48 (d, J = 2.0 Hz, 2H, H1). Anal. calc. for
C33H39Cl3HgIrN: C, 41.77; H, 4.14; N, 1.48. Found: C, 41.40; H,
4.12; N, 1.49.
1
Et2O/hexane afforded orange crystals. Yield: 45 mg (72%). H
t
NMR (C6D6): δ 1.10 (m, 6H, cod), 1.22 (s, 18H, Bu), 1.28 (s,
t
18H, Bu), 1.38 (m, 6H, cod), 1.55 (m, 2H, cod), 1.86 (m, 6H,
cod), 2.57 (m, 4H, cod), 2.87 (m, 4H, cod), 4.37 (m, 8H, cod),
4.63 (m, 2H, cod), 5.17 (m, 2H, cod), 5.24 (m, 2H, cod), 6.63 (d,
J = 6.4 Hz, 2H, Ar), 6.96 (m, 2H, Ar), 7.05 (s, 2H, Ar), 7.08 (m,
2H, Ar), 7.20 (m, 2H, Ar), 7.40 (m, 6H, Ar), 7.53 (d, 2H, J = 8.0
Hz, Ar), 7.97 (m, 2H, Ar), 8.13 (m, 2H, Ar). Some resonances of
Synthesis of [Ir(C^N^C)(cod)Cl] (5)
To a solution of 5·HgCl2 (57 mg, 0.060 mmol) in thf (10 mL)
was added 4 equivalents of tricyclohexylphosphine (67 mg,
0.24 mmol) and the reaction mixture was heated at reflux over-
night. The solvent was pumped off, and the residue was
washed with hexane. Recrystallization from CH2Cl2/Et2O/
MeOH afforded white crystals. Yield: 21 mg (52%). 1H NMR
are
broad
and
overlapping.
Anal.
calc.
for
C83H108Cl6Hg2Ir4N2·CH2Cl2: C, 38.72; H, 4.16; N, 1.34. Found:
C, 38.44; H, 4.19; N, 1.31.
Synthesis of [(C^N^C)(cod)IrHgIr(cod)Cl2] (3)
t
(CDCl3): δ 1.48 (s, 18H, Bu), 1.69 (m, 2H, cod), 1.97 (m, 2H,
A mixture of 2 (50 mg, 0.020 mmol) and 2 equivalents of cod), 2.10 (m, 2H, cod), 2.94 (t, J = 3.6 Hz, 2H, cod), 3.07 (m,
sodium acetate (3.3 mg, 0.040 mmol) in tetrahydrofuran (thf) 2H, cod), 4.54 (t, J = 3.6 Hz, 2H, cod), 7.31 (dd, J = 1.6, 8.0 Hz,
(10 mL) was heated at reflux overnight. The solvent was 2H, H2), 7.32 (d, J = 8.0 Hz, 2H, H3), 7.64 (overlapping d and t,
pumped off, and the residue was washed with hexane and 3H, H4,5), 8.43 (d, J = 1.6 Hz, 2H, H1). Anal. calc. for
extracted with CH2Cl2/Et2O (1 : 1, v/v). Recrystallization from C33H39ClIrN·MeOH: C, 57.57; H, 6.11; N, 1.97. Found: C, 57.38;
CH2Cl2/Et2O/hexane afforded orange crystals. Yield: 29 mg H, 5.80; N, 2.07.
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Synthesis of [Ir(C^N^C)(cod)(H2O)](OTf) (OTf− = triflate) (6)
2H, H7′), 6.44 (dd, J = 1.2, 8.4 Hz, 1H, H6′), 6.79 (d, J = 8.4 Hz,
2H, H8′), 7.09 (dd, J = 2.0, 8.0 Hz, 2H, H2), 7.13 (d, J = 2.0 Hz,
2H, H1), 7.32 (dd, J = 2.0, 8.0 Hz, 1H, H2′), 7.37 (d, J = 8.0 Hz,
2H, H3), 7.56 (overlapping d and t, d = 8.4 and 8.4 Hz, 3H, H4
and H5), 7.63 (t, J = 8.0 Hz, 1H, H5′), 8.01 (d, J = 8.0 Hz, 1H,
H3′), 8.11 (dd, J = 1.2, 8.0 Hz, 1H, H4′), 8.14 (d, J = 2.0 Hz, 1H,
H1′). IR (KBr, cm−1): 2007 [ν(C–O)]. Anal. calc. for C51H55IrN2O:
C, 67.74; H, 6.13; N, 3.10. Found: C, 67.23; H, 6.17; N, 3.12.
To solution of 5 (41 mg, 0.061 mmol) in acetone (10 mL) was
added 1 equivalent of AgOTf (16 mg, 0.062 mmol). The
mixture was heated at reflux for 3 h and filtered. The solvent
was removed in vacuo, and the residue was washed with Et2O.
Recrystallization from CH2Cl2/Et2O/hexane afforded pale
orange needles. Yield: 22 mg (46%). 1H NMR (acetone-d6): δ
t
1.54 (s, 18H, Bu), 1.82 (m, 2H, cod), 2.17 (m, 4H, cod), 3.17
(m, 2H, cod), 3.27 (m, 2H, cod), 4.97 (m, 2H, cod), 7.54 (d, J =
8.4 Hz, 2H, H2), 7.96–8.06 (m, 5H, H3,4,5), 8.69 (s, 2H, H1). 19F
δ −78.84 (s). Anal. calc. for
C34H41F3IrNO4S: C, 50.48; H, 5.11; N, 1.73. Found: C, 50.33; H,
5.02; N, 1.85.
Photoisomerization of 8
A solution of 8 (40 mg) in thf (75 mL) was irradiated with UV
light (Hg lamp, 9 W) for 1 h. The solvent was pumped off, and
the yellow residue was redissolved in Et2O/hexane (v/v, 1 : 1)
and then filtered through a pad of silica gel. Recrystallization
from CH2Cl2/hexane afforded yellow crystals which were identi-
fied as an isomer of 8 (8′), in which the carbonyl is trans to a
phenyl group of the bidentate HC^N^C ligand, by X-ray diffrac-
tion. Yield: 31 mg (78%). H NMR (C6D6): δ 0.85 (s, 9H, Bu),
1.07 (s, 9H, tBu), 1.15 (s, 18H, tBu), 6.34 (d, J = 2.0 Hz, 1H, H1′),
6.83 (dd, J = 2.0, 8.4 Hz, 1H, H2′), 7.02–7.08 (m, 4H, H2, H3′,
H4′), 7.11 (d, J = 8.4 Hz, 2H, H3), 7.15 (d, J = 8.0 Hz, 1H, H5′),
7.37 (d, J = 8.4 Hz, 2H, H7′), 7.42 (d, J = 1.6 Hz, 2H, H1), 7.53
(overlapping t and d, J = 8.4 and 8.0 Hz, 3H, H5 and H4), 7.61
(dd, J = 1.6, 8.0 Hz, 1H, H6′), 7.89 (d, J = 8.4 Hz, 2H, H8′). IR
(KBr, cm−1): 2010 [ν(C–O)]. Anal. calc. for C51H55IrN2O: C,
67.74; H, 6.13; N, 3.10. Found: C, 67.61; H, 6.41; N, 3.03.
{1H} NMR (acetone-d6):
Photolysis of 5 in MeCN
A solution of 5 (100 mg, 0.148 mmol) in MeCN (100 mL) was
irradiated with UV light (Hg lamp, 9 W) under nitrogen for
3 h, during which the colour of solution changed from pale
yellow to orange. Evaporation of the solvent afforded an
orange oil that did not crystallize. ESI-MS: m/z 652.8 [Ir
(C^N^C)(MeCN)2Cl + 1]+.
1
t
Ir-Catalyzed cyclopropanation of styrene
To a mixture of styrene (1 mmol) and Ir catalyst (0.005 mmol)
in CH2Cl2 (10 mL) was added ethyldiazoacetate in CH2Cl2
(0.5 mmol in 10 mL) over a period of 1 h. The resulting
mixture was stirred at room temperature overnight. The yields Reaction of 8 with p-toluenesulfonic acid
of the products were determined by GC-FID using iodobenzene
as internal standard.
To a solution of 8 (40 mg, 0.044 mmol) in CH2Cl2 (10 mL) was
added 1 equivalent of TsOH·H2O (8.4 mg, 0.044 mmol), and
the reaction mixture was stirred at room temperature over-
night. The solvent was pumped off, and the residue was
washed with hexane. Recrystallization from CH2Cl2/Et2O/
hexane afforded yellow crystals that were identified as [Ir(κ2C,
N-HC^N^C)2(OTs)(CO)] (9). Yield: 18 mg (38%). 1H NMR
[Ir(C^N^C)(HgCl)(CO)(py)] (py = pyridine) (7)
A mixture of [Ir(CO)2Cl(py)] (50 mg, 0.14 mmol), 1 (78 mg,
0.14 mmol) and sodium acetate (11 mg, 0.14 mmol) in thf
(10 mL) was heated at reflux overnight. The solvent was
pumped off, and the residue was purified by column chrom-
atography (silica gel, CH2Cl2/hexane 1 : 2). The yellow band
was collected. Recrystallization from CH2Cl2/Et2O/hexane gave
yellow crystals. Yield: 64 mg (52%). 1H NMR (acetone-d6): δ
t
t
t
(CDCl3): δ 0.94 (s, 9H, Bu), 1.34 (s, 9H, Bu), 1.36 (s, 9H, Bu),
t
1.40 (s, 9H, Bu), 2.21 (s, 3H, CH3), 6.08 (br, 1H, Ar), 6.61 (m,
2H, Ar), 6.72 (m, 3H, Ar), 6.94 (m, 5H, Ar), 7.43–7.52 (m, 8H,
Ar), 7.62 (d, J = 8.4 Hz, 2H, Ar), 7.69 (dd, 1H, J = 1.2, 8.4 Hz,
1H, Ar), 7.79 (d, J = 1.6 Hz, 2H, Ar). IR (KBr, cm−1): 2025 [ν(C–
O)]. Anal. calc. for C58H63IrN2O4S: C, 64.72; H, 5.90; N, 2.60.
Found: C, 64.30; H, 5.93; N, 2.63.
t
1.38 (s, 18H, Bu), 7.15 (dd, J = 2.0, 8.0 Hz, 2H, H2), 7.25 (m,
2H, Hm of py), 7.70 (d, J = 7.6 Hz, 2H, H3), 7.76 (d, J = 8.0 Hz,
2H, H4), 7.82 (tt, J = 1.6 Hz, 7.6 Hz, 1H, Hp of py), 7.86 (dd, J =
8.0, 8.4 Hz, 1H, H5), 8.07 (d, J = 2.0 Hz, 2H, H1), 8.33 (m, 2H,
Ho of py). IR (KBr, cm−1): 1982 [ν(C–O)]. Anal. calc. for
C31H32ClHgIrN2O: C, 42.46; H, 3.68; N, 3.19. Found: C, 42.33;
H, 3.60; N, 3.17.
Reaction of 9 with sodium acetate
A mixture of 9 (50 mg, 0.046 mmol) and 1.2 equivalents of
sodium acetate (4.6 mg, 0.056 mmol) in MeCN (10 mL) was
heated at reflux overnight. The solvent was pumped off, and
the yellow residue was extracted with CH2Cl2. Recrystallization
[Ir(C^N^C)(κ2C,N-HC^N^C)(CO)] (8)
A mixture of (PPh4)[Ir(CO)2Cl2] (50 mg, 0.076 mmol), 1 (88 mg, from CH2Cl2/Et2O/hexane gave yellow crystals that were charac-
0.15 mmol) and sodium acetate (13 mg, 0.16 mmol) in MeCN terized as 8 by UV/visible, IR and NMR spectroscopy. Yield:
(10 mL) was heated at reflux overnight. The solvent was 36 mg (86%).
pumped off and the residue was purified by column chromato-
graphy (silica gel, Et2O/hexanes 1 : 2). The yellow band was col-
Oxidation of 8
lected. Recrystallization from CH2Cl2/Et2O/hexane gave yellow To a solution of 8 (40 mg, 0.044 mmol) in CH2Cl2 (10 mL) was
crystals. Yield: 45 mg (65%). 1H NMR (acetone-d6): δ 1.13 (s, added
1
equivalent of [(4-BrC6H4)3N]SbCl6 (36 mg,
18H, tBu), 1.30 (s, 9H, tBu), 1.46 (s, 9H, tBu), 6.20 (d, J = 8.4 Hz, 0.44 mmol). The reaction mixture was stirred at room tempera-
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ture overnight to give a pale yellow solution. The solvent was contain the supplementary crystallography data for complexes
pumped off, and the residue was washed with hexane and re- 2–5, 8, 8′, 9 and 10 respectively.
dissolved in CH2Cl2. Addition of hexane afforded gave a yellow
powder. ESI-MS: m/z 940 [Ir(HC^N^C)2(CO)Cl]+. IR (KBr,
cm−1): 2040 [ν(C–O)].
Results and discussion
Ir(C^N^C) cod complexes
Reaction of 1 with [Ir(PPh3)3Cl]
Transmetalation of [Hg(HC^N^C)Cl] (1) with various Ir chlor-
A mixture of [Ir(PPh3)3Cl] (60 mg, 0.059 mmol), 1 (34 mg,
ide compounds has been studied. No C^N^C complexes were
0.059 mmol) and sodium acetate (10 mg, 0.12 mmol) in thf
formed when 1 was refluxed with IrCl3 in a high boiling
(10 mL) was heated at reflux overnight. The solvent was
pumped off and the residue was washed with Et2O and
solvent, such as ethylene glycol. However, depending on the
experimental conditions, reactions of
1 with [Ir(cod)Cl]2
extracted with CH2Cl2. Recrystallization from CH2Cl2/Et2O/
hexane gave off-white crystals that were identified as the pre-
viously reported bis(orthometalated) complex [Ir(PPh3)(κ2P,C-
afforded different types of heterometallic complexes contain-
ing Ir–Hg bonds (Scheme 1). The treatment of 1 with [Ir(cod)
Cl]2 in acetonitrile at room temperature, followed by recrystalli-
zation from CH2Cl2/hexane produced a hexanuclear Ir4Hg2
complex, [Cl(κ2C,N-HC^N^C)(cod)IrHgIr(cod)Cl2]2 (2), which
1
C6H4PPh2)2Cl] (10).19 Yield: 15 mg (25%). H NMR (CDCl3): δ
5.63 (dd, J = 3.2, 7.6 Hz, 2H, Ph), 6.23 (m, 3H, Ph), 6.63 (m, 1H,
Ph), 6.63 (m, 3H, Ph), 6.86 (m, 5H, Ph), 6.95 (m, 5H, Ph), 7.05
(m, 6H, Ph), 7.14 (m, 5H, Ph), 7.35 (m, 4H, Ph), 7.60 (t, J = 8.8
Hz, 6H, Ph), 7.80 (dd, J = 7.2, 9.6 Hz, 2H, Ph), 7.98 (dd, J = 7.6,
9.6 Hz, 2H, Ph). 31P{1H} NMR (CDCl3): δ −83.75 (dd, J = 13.6,
398 Hz, C6H4PPh2), −72.92 (t, J = 13.7 Hz, C6H4PPh2), −2.42
1
features Ir–Hg–Ir and Ir–Cl–Ir bridges. The H NMR spectrum
of 2 displayed signals that can be attributed to the cod and
HC^N^C ligands. Consistent with the bidentate-N,C binding
mode of the HC^N^C ligand, two singlets at δ 1.22 and
1.28 ppm were found for the tert-butyl groups of the cyclometa-
lated and uncoordinated phenyl rings.
(dd,
J
=
13.9, 398 Hz, PPh3). Anal. calc. for
C54H43ClIrP3·0.5CH2Cl2: C, 62.05; H, 4.20. Found: C, 62.36; H,
4.23.
The bidentate HC^N^C ligand in 2 could be further cyclo-
metalated by treatment with sodium acetate. Thus, heating 2
with sodium acetate in tetrahydrofuran (thf) afforded the tri-
nuclear Ir2Hg complex [(C^N^C)(cod)IrHgIr(cod)Cl2] (3). The
tert-butyl protons in 3 appeared as a singlet at δ 1.48 ppm in
Emission measurements
Steady-state emission at room temperature were recorded on
Horiba Jobin Yvon Fluoromax-4 fluorescence spectrometer.
Time-resolved emission and emission lifetime measurements
were recorded on LP920 laser flash photolysis system using the
third harmonic output (355 nm, 6–8 ns fwhm pulse width) of
a Spectra-Physics Quanta-Ray Q-switched LAB-150 pulsed Nd:
YAG laser as the excitation source. Solutions were rigorously
degassed on a high-vacuum line in a two-compartment cell
with no less than four successive freeze pump–thaw cycles.
The luminescence quantum yields of the solution emission
were determined by using the optical-dilution method using
aerated aqueous solution of [Ru(bpy)3]Cl2 (bpy = 2,2′-bipyri-
dine) (ϕem = 0.04) as reference standard.20
1
the H NMR spectrum, indicating the C^N^C ligand binds to
the Ir center in a symmetric, tridentate-C,N,C mode.
On the other hand, refluxing [Ir(cod)Cl]2 with 1 and sodium
acetate in thf yielded dinuclear [Ir(C^N^C)(cod)(HgCl)] (4).
Alternatively, 4 was isolated in good yield from the reaction of 2
with 2 equivalents of 1 and sodium acetate in refluxing thf. It
may be noted that Ir complexes containing the {HgCl} ligand
have been prepared by oxidative addition of Ir(I) complexes with
HgCl2 23 or chlorination of organomercurry iridium complexes.24
The Ir–Hg bond in 4 can be cleaved by chlorination. Thus,
treatment of 4 with PhICl2 afforded a white solid that was ana-
lyzed as [Ir(C^N^C)(cod)Cl]·HgCl2 (5·HgCl2). Subsequent reac-
tion of 5·HgCl2 with tricyclohexylphosphine led to isolation of a
Hg-free Ir(C^N^C) complex, [Ir(C^N^C)(cod)Cl] (5). The 31P{1H}
X-ray crystallography
Crystallographic data and experimental details for complexes NMR spectrum of the reaction mixture showed a triplet at δ
2–5, 8, 8′, 9 and 10 are summarized in Table S1 (ESI†). 51.85 ppm with J(Hg–P) = 2412 Hz, indicating that a Hg phos-
Intensity data of the complexes were collected with a Rigaku phine species, presumably HgCl2·xPCy3 (Cy = cyclohexyl), was
SuperNova Atlas X-ray diffractometer using graphite-mono- produced from the reaction. We have not been able to isolate
chromated Cu-Kα radiation. The collected frames were pro- the Hg phosphine compound for further characterization.
cessed with the CryAlispRro software. Structures were solved
No reaction was found when 5 was refluxed with charge-
by the direct methods and refined by full-matrix least-squares neutral (e.g. PPh3, pyridine) or anionic (e.g. N3−) ligands in thf.
on F2 using the Olex221 software package embedding However, irradiation of 5 in acetonitrile with UV light resulted
SHELXTL.22 Atomic positions of non-hydrogen atoms were in formation of an orange solution. The electrospray ionization
refined with anisotropic parameters and with suitable (ESI) mass spectrum of the orange species showed a peak m/z
restraints. Disordered atoms were refined isotropically. 652.7 corresponding to [Ir(C^N^C)(MeCN)2Cl], indicating the
Hydrogen atoms were generated geometrically and allowed to cod ligand in 5 was substituted by acetonitrile. Unfortunately,
ride on their respective parent carbon atoms before the final we have not been able to crystallize the orange photoproduct
cycle of least-squares refinement. CCDC 2072948–2072955† for further characterization.
8516 | Dalton Trans., 2021, 50, 8512–8523
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Although 5 is substitutionally inert, the chloride ligand diffraction study revealed that the carbonyl in 7 is coplanar
could be abstracted by Ag(I) salts. For example, refluxing 5 with with the C^N^C ligand, and the {HgCl} group is trans to the
Ag(OTf) in acetone afforded a pale orange complex that was chloride (see ESI†). Owing to the disorder of the {HgCl} and
analyzed as [Ir(C^N^C)(cod)(H2O)](OTf) (6). A preliminary chloride ligands, the bond lengths and angles in 7 have not
study showed that 6 is a catalyst for alkene cyclopropanation been analyzed. Attempts to cleave the Ir–Hg bond in 7 with
(Table 1). For example, treatment of styrene with ethyl diazoa- PhICl2 failed. The IR spectrum of 7 showed the C–O stretch at
cetate in the presence of 1 mol% of 6 at room temperature 1982 cm−1, which is lower than that for the starting Ir(I) carbo-
afforded the cyclopropane products in 61% yield with a trans/ nyl complex [Ir(CO)2Cl(py)] (2053 and 1992 cm−1). The Ir–Hg
cis ratio of ca. 2.1 : 1. Coordinately saturated 5 and the Ir–Hg complex 7 can be described by two valence isomeric forms: (A)
complex 4 are less active catalysts (23 and 12% yield, respect- Ir(II)–Hg(I) with a metal–metal covalent bond and (B) Ir(I) →
ively), suggesting that a vacant coordination site on Ir is essen- Hg(II) with a metal–metal dative bond. In this context, the
tial for the catalytic activity of the Ir(C^N^C) complex. It is gen- observation of a rather low C–O stretching frequency for 7 may
erally believed that the active species of Ir-catalyzed carbene point to the contribution of the isomeric form B containing an
transfer reactions are Ir carbenoid species.25 Therefore, we electron-rich formal Ir(I) center, although the dianionic C^N^C
propose that the reaction of 6 with ethyl diazoacetate gener- ligand can also help increase the electron density of the Ir
ated an Ir(C^N^C) carbenoid species, possibly [Ir{vC(CO2Et) atom. It may be noted that related Pt–Hg complexes featuring
H}(C^N^C)(cod)]+, which is responsible for the Ir-catalyzed Pt(II) → Hg(II) dative bonds are well documented.26
cyclopropanation of styrene.
On the other hand, refluxing (PPh4)[Ir(CO)2Cl2] with 1 equi-
valent of 1 and sodium acetate in acetonitrile afforded yellow
crystals that were identified as [Ir(C^N^C)(κ2C,N-HC^N^C)
(CO)] (8) in 30% yield. The yield of 8 was improved to 65%
when 2 equivalents of 1 was used in the synthesis. An X-ray
diffraction study indicated that contrary to 7, the carbonyl in 8
lies above the plane of the C^N^C ligand and is trans to the
pyridyl group of the bidentate HC^N^C ligand. The C–O
stretching frequency of 8 (2007 cm−1) is higher than that in 7.
No substitution was found when 8 was reacted with phos-
phines such as PPh3 or oxidizing agents such as PhIO in
toluene. Irradiation of 8 in thf/MeCN with UV light resulted in
a color change from yellow to light yellow. Recrystallization
from CH2Cl2/hexane afforded yellow crystals that were identi-
fied as an isomer of 8 (8′), in which the carbonyl is trans to the
phenyl group of the bidentate HC^N^C ligand. The C–O
stretching frequency of 8′ (2010 cm−1) is slightly higher than
that in 8.
Ir(C^N^C) carbonyl complexes
Refluxing [Ir(CO)2Cl(py)] (py = pyridine) with 1 and sodium
acetate in thf afforded a yellow crystalline solid that was identi-
fied as [Ir(C^N^C)(HgCl)(CO)(py)] (7) (Scheme 2). An X-ray
Table 1 Ir(C^N^C)-catalyzed cyclopropanation of styrenea
Catalyst
% yield of I (trans/cis)
% yield of II (trans/cis)
4
5
6
7
12 (2.4)
23 (2.5)
61 (2.1)
11 (2.3)
8 (0.3)
15 (0.4)
9 (3.5)
8 (0.3)
Photoisomerization of cyclometalated iridium complexes is
well precedented. For example, upon irradiation with UV light,
mer-[Ir(C^N)3] (HC^N = 2-(aryl)pyridine) in dimethylsulfoxide is
isomerized to the more stable isomer fac-[Ir(C^N)3].27 It is gener-
ally accepted that the photoisomerziation of mer-[Ir(C^N)3] pro-
ceeds through a triplet metal-centered (3MC) excited state(s)
that can be thermally populated from the emissive excited
state.28 According to different theoretical models,29 it is pre-
dicted that the ligands on the axis with the weakest ligand field
a Experimental conditions: Catalyst/EDA/styrene = 1/100/200, r.t., over-
night. Yields determined by GLC.
3
are more labile. Therefore, we believe that a MC excited state,
in which the Ir–C bond of the κ2C,N-HC^N^C ligand is wea-
kened/broken, is involved in the photoisomerization of 8.
Unlike the [Ir(C^N)3] system, however, the three Ir–C bonds in
both 8 and 8′ are in a meriodinal arrangement.
Owing to the mutually trans geometry, one of the two Ir–C
bonds of the C^N^C ligand in 8 can be easily cleaved by
Brønsted acids. For example, treatment of 8 with p-toluenesul-
fonic acid (TsOH) afforded the bis(bidentate) tosylate complex
[Ir(κ2C,N-HC^N^C)2(OTs)(CO)] (9). As expected, 9 is less elec-
tron-rich than 8 and a higher IR C–O stretching frequency
(2024 cm−1) was observed. The Ir-bound C^N^C ligand can
Scheme 2 Syntheses of Ir(C^N^C)(CO) complexes.
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switch between the tridentate and bidentate binding mode
through protonation and deprotonation. Thus, 8 was isolated
in good yield by heating 9 with sodium acetate in MeCN.
Crystal structures
Complexes 2–5, 8, 8′ and 9 have been characterized by single-
crystal X-ray diffraction and their selected bond lengths and
angles are collected in Table 2. The structure of 2 (Fig. 1) con-
sists of two symmetry-related [Cl(κ2C,N–C^N^C)(cod)Ir–Hg–Ir
(cod)Cl2] units that are connected by two chloride bridges. The
geometry around Ir and Hg is approximately octahedral and
linear, respectively. The Ir–Hg distance of the {Ir(κ2C,N-
HC^N^C)(cod)Cl} moiety [2.5829(3) Å] is shorter than that of
the {Ir(cod)Cl2} moiety [2.6314(3) Å], whereas the Ir–C(cod) dis-
tances in the former [av. 2.257 Å] are longer that in the latter
[av. 2.151 Å]. The Ir–Hg distances in 2 are within the range
found for reported heterometallic Ir/Hg complexes.30 The Ir–C
and Ir–N distances for the bidentate HC^N^C ligand in 2
[2.047(6) and 2.182(4) Å, respectively] are typical for Ir cyclome-
talated complexes.
The structure of 3 is shown in Fig. 2. The geometries
around the two Ir centers are approximately octahedral and
square pyramidal, respectively. Upon cyclometalation, the Ir–C
distance of the bidentate HC^N^C ligand in 2 is lengthened
from 2.047(6) to av. 2.128(4) Å, while the Ir–N distance is shor-
tened from 2.128(4) to 2.045(3) Å. In addition, the Ir–Hg–Ir
angle decreases from 176.587(11) to 169.439(9)°, but there is
no significant change in the Ir–Hg distances. The Ir–N and Ir–
C distance of the C^N^C ligand in 3 compare well with
reported Ir(C^N^C) complexes.1c,15
Fig. 1 Molecular structure of [Cl(κ2C,N-HC^N^C)(cod)IrHgIr(cod)Cl2]2
(2). The ellipsoids are drawn at 30% probability level.
ide in 5. The Ir–C and Ir–N distances in the two complexes are
similar to those in 3. The Ir–Hg distance in 4 [2.5705(3) Å]
compares well with that in [Ir(CO)Cl2(PPh3)(HgCl)] [2.570(1) Å],
but the Hg–Cl distance [2.4255(13)°] is longer than that in the
latter [2.366(5) Å].30a
The structures of the isomer pair 8 and 8′ are shown in
Fig. 5 and 6, respectively. The geometry around Ir in both com-
plexes is pseudo octahedral with the carbonyl located above
the plane of the C^N^C ligand. The major structural difference
between the two isomers is the orientation of the bidentate
HC^N^C ligand. In 8, the pyridyl ring of the bidentate
HC^N^C ligand is trans to CO, whereas in 8′, a phenyl group is
opposite to CO. In addition, in structure 8, π–π stacking
4 and 5 have roughly the same structure (Fig. 3 and 4,
respectively) except for the axial ligand: {HgCl} in 4 and chlor-
Table 2 Selected bond lengths and angles of Ir(C^N^C) complexes
2
3
4
5
8
8′
9
Bond length (Å)
Ir–C(HC^N^C) 2.047(6)
Ir–N(HC^N^C) 2.182(4)
Ir–C(C^N^C)
2.028(3)
2.163(2)
2.060(2)
2.0998(16)
2.032(3) 2.022(3)
2.172(2) 2.274(2)
2.126(4) 2.131(4)
2.045(3)
2.147(6) 2.148(6) 2.140(3) 2.139(3) 2.101(3) 2.113(3) 2.096(2) 2.114(2)
Ir–N(C^N^C)
2.040(5)
2.052(2)
2.083(2)
2.0122(17)
Ir–C(cod)
2.207(6) 2.161(6) 2.211(4) 2.187(4)
2.349(6) 2.310(6) 2.180(4) 2.170(4)
2.162(6) 2.159(5)
2.180(6) 2.194(6) 2.177(3) 2.168(3)
2.202(6) 2.216(6) 2.178(3) 2.181(3)
Ir*–C(cod)
2.127(6) 2.156(6)
2.4694(13)
Ir–Clt
2.3993(6)
Ir*–Clt
Ir*–Clb
2.3950(13)
2.4126(12)
2.3335(12) 2.256(3)
2.133(5) 2.191(11)
2.146(10) 2.101(11)
2.5841(3)
Ir–Hg
Ir*–Hg
Hg–Cl
Ir–CO
C–O
2.5829(3)
2.6314(3)
2.5705(3)
2.6656(3)
2.4255(13)
1.821(3)
1.153(3)
1.933(2)
1.135(3)
1.844(3)
1.138(4)
2.1914(19)
Ir–O
Bond angles (°)
Ir–Hg–Ir*
Ir–Hg–Cl
176.587(11)
169.439(9)
171.64(4)
Ir*: Ir without the HC^N^C ligand.
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Fig. 5 Molecular structure of [Ir(C^N^C)(κ2C,N-HC^N^C)(CO)] (8). The
ellipsoids are drawn at 30% probability level.
Fig. 2 Molecular structure of [(C^N^C)(cod)IrHgIr(cod)Cl2] (3). The
ellipsoids are drawn at 30% probability level.
Fig. 6 Molecular structure of the isomer 8’. The ellipsoids are drawn at
30% probability level.
exists in the solid state only. Owing to the trans influence of
the opposite phenyl group, the Ir–CO distance in 8′ [1.933(2) Å]
is significantly longer than that in 8 [1.821(3) Å]. Nevertheless,
the C–O stretching frequencies of 8 and 8′ differ by 3 cm−1
only.
Fig. 3 Molecular structure of [Ir(C^N^C)(cod)(HgCl)] (4). The ellipsoids
are drawn at 30% probability level.
Complex 9 is chiral and the structure of one of the enantio-
mers is shown in Fig. 7. Unlike reported [Ir(C^N)2(L)(X)] com-
plexes, which usually feature mutually trans pyridyl groups, the
two pyridyl rings in 9 are mutually cis and opposite to the car-
bonyl and one phenyl ring, respectively. The Ir–N bond that is
Fig. 4 Molecular structure of [Ir(C^N^C)(cod)Cl] (5). The ellipsoids are
drawn at 30% probability level.
between the pyridyl ring of the tridentate C^N^C ligand and
the uncoordinated phenyl ring of the bidentate HC^N^C
ligand was found. Sharp signals for the pyridyl and phenyl
protons were found in normal regions in the 1H NMR spec-
Fig. 7 Molecular structure of [Ir(κ2C,N-HC^N^C)2(OTs)(CO)] (9). The
trum of 8 in benzene-d6, suggesting that the π–π stacking ellipsoids are drawn at 30% probability level.
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trans to the phenyl ring [2.274(2) Å] is obviously longer than
that trans to the carbonyl [2.172(2) Å]. The Ir–CO distance in 9
is similar to that in 8.
Absorption and emission spectra of Ir(C^N^C) complexes
Absorption and emission spectral data of the Ir(C^N^C) com-
plexes 4, 5, 7, 8 and 8′ are collected in Table 3. On the basis of
the previous spectral assignment of related Ir(C^N^C)
complexes,1c,15 the absorption bands in the high energy region
(<300 nm) are assigned to the intraligand (C^N^C or HC^N^C)
transitions, whereas those with lower energy (300–420 nm) are
ascribed to the ligand-to-ligand charge-transfer (LLCT) mixed
with metal-to-ligand (C^N^C) charge-transfer (MLCT) tran-
sitions. Complexes 4, 5, 7, 8 and 8′ showed structured emission
in 77 K glassy medium (Fig. 8). This is tentatively ascribed to a
predominant ligand-centered (3LC) phosphorescence with
Fig. 8 Overlaid normalized emission spectra of 4, 5, 7, 8 and 8’ in 77 K
EtOH–MeOH (4 : 1, v/v) glassy medium.
3
some MLCT [dπ → π*(C^N^C)] character. The low sensitivity
of the emission energy to the change of the coordination
sphere is in agreement with the assignment. In solution at
room temperature, only 8 and 8′ showed photoluminescence
with a structureless emission band that is significantly red-
shifted relative to the low-temperature emission (Fig. 9).
Moreover, the emission energy varied with the geometric iso-
merism of the cyclometalating ligand (8′ > 8). In view of these
characteristics, the emission is likely derived from a predomi-
nant 3MLCT excited state mixed with 3LLCT character.
However, the luminescent quantum yields are much lower
than many other related cyclometalated Ir(III) complexes.1c,15,31
The low luminescent quantum yield is due to the presence of
the closely lying non-radiative deactivating metal centered
ligand field excited state (3MC) that resulted in the observed
photo-isomerization. In addition, the mixing of LLCT charac-
ter in the emissive excited state is also reported to result in
lower luminescent quantum yields.7 The non-emissive nature
of the solutions of 4–7 at room temperature can be ascribed to
the quenching of the coordinated mercury or cod ligand.
Fig. 9 Overlaid emission spectra of 8 and 8’ in CH2Cl2 solution at
298 K.
electrochemical studies of related Ir(C^N^C) complexes,1c,15c
the first oxidation wave of 5 is tentatively assigned as a metal-
centered oxidation, while the nature of the second one is
unclear. As expected, complex 8 that contains an electron-
donating HC^N^C ligand is oxidized more easily than is 5. The
c.v. of 8 (Fig. S23, ESI†) showed a reversible Ir(IV/III) couple
with E1/2 = +0.28 V. By comparison, the Ir(IV/III) couples for the
related complexes [Ir(C^N^C)(tpy)]+ and [Ir(C^N^C)(PiPr3)2(H)]
Redox property of Ir(C^N^C) complexes
The electrochemistry of Ir(C^N^C) complexes in CH2Cl2 solu-
tion has been studied using cyclic voltammetry. The cyclic vol-
tammogram (c.v.) of the cod complex 5 showed two irreversible
oxidation waves at +0.64 and +0.83 V versus the ferrocenium–
ferrocene (Fc+/0) couple (Fig. S21, ESI†). According to previous
Table 3 Absorption and emission spectral data for Ir(C^N^C) complexes
Emission data
Complex
Absorption maxima (CH2Cl2) λmax/nm
Solvent (temp.)
λmax/nm (τ/μs)
ϕem/%
4
5
7
262, 290, 337sh, 384
277, 327, 371, 389
267, 337, 356, 421
EtOH/MeOH (77 K)
EtOH/MeOH (77 K)
EtOH/MeOH (77 K)
CH2Cl2 (298 K)
475, 511, 540, 587sh (217.9)
473, 508, 537, 584sh (370.0)
469, 504, 518sh, 541sh (37.9 μs)
Non emissive
8
261, 328, 386, 419
276 sh, 327sh, 379
EtOH/MeOH (77 K)
CH2Cl2 (298 K)
EtOH/MeOH (77 K)
CH2Cl2 (298 K)
482, 518, 548, 595sh (86.0 μs)
565 (4.4 μs)
0.12%
0.24%
8′
476, 514, 544sh (64.6 μs)
520, 558sh (1.2 μs)
8520 | Dalton Trans., 2021, 50, 8512–8523
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were found at +1.08 V versus the standard calomel electrode1c
and +0.02 V versus Fc+/0 15c
respectively. On the other hand, the
,
oxidation of the photoisomer 8′ is irreversible with an Epa
value of +0.46 V. The Ir–Hg complexes 4 and 7 are redox inac-
tive in the potential range from −2.0 to + 2.0 V.
Given the reversible nature of the oxidation of 8, an attempt
has been made to isolate the 1-electron oxidation product of 8.
Treatment of 8 in CH2Cl2 with 1 equivalent of R3NSbCl6 (R =
4-bromophenyl) followed by precipitation with hexane led to
isolation of a yellow powder that showed the C–O band in the
IR spectrum at 2040 cm−1, which is similar to that of 9. The
UV-visible spectrum of the oxidation product in CH2Cl2 featur-
ing absorptions at 340 and 350 sh nm (Fig. S11, ESI†) is also
similar to that of 9. The ESI mass spectrum displayed the
molecular ion peak at m/z 940 corresponding to [Ir
(HC^N^C)2(CO)Cl]. Although we have not been able to obtain a
pure sample for analysis, the spectroscopic data indicated that
the yellow powder contained the bis(cyclometalated) chloride
Fig. 10 Molecular structure of 10. The ellipsoids are drawn at 30%
probability level. Selected bond lengths (Å): Ir1–P1 2.3935(8), Ir1–C2
2.079(3), Ir1–P2 2.3937(8), Ir1–C32 2.044(4), Ir1–P3 2.3300(8), Ir1–Cl1
2.4921(8).
cyclometalated phosphine ligands adopt a cis arrangement.
complex [Ir(κ2C,N-HC^N^C)2(CO)Cl]. It seems likely that the The PPh3 ligand is trans to the phosphorus atom of one of the
oxidation of 8 results in removal an electron from an Ir–C
cyclometalated phosphines, whereas the chloride is opposite
bonding molecular orbital of the Ir(C^N^C) moiety, generating
to the phenyl ring of the other cyclometalated ligand. The Ir–P
a C^N^C radical. H-Atom abstraction of the solvent by the aryl bond distances of the two cyclometalated phosphine ligands
radical gives the bidentate HC^N^C ligand. Finally, nucleophi-
are very similar [2.3935(8) and 2.3937(8) Å], although one
might expect the Ir–P bond trans to a phenyl ring to be longer
than that trans to a phosphine. The Ir–P distance of the PPh3
ligand [2.3300(8) Å] is slightly shorter than those of the cyclo-
metalated phosphine ligands. The Ir–C bond lengths in 10
[2.079(3) and 2.044(4) Å] are shorter than that in the mono-
orthometalated complex [Ir(κ2P,C-C6H4PPh2)(H)(Cl)(PPh3)2]
[2.103(2) Å].32
−
lic attack of Ir(III) by a chloride derived from the SbCl6 anion
affords [Ir(κ2C,N-HC^N^C)2(CO)Cl].
Reaction of 1 with [Ir(PPh3)3Cl]
In an attempt to prepare an Ir(C^N^C) phosphine complex, the
reaction of [Ir(PPh3)3Cl] with 1 has been studied. Refluxing [Ir
(PPh3)3Cl] with 1 and sodium acetate in thf resulting in the
formation of a colourless mixture. Recrystallization from
CH2Cl2/hexane afforded off-white crystals that were identified
as the reported bis(orthometalated) complex [Ir(κ2P,C-
PPh2C6H4)2(PPh3)Cl] (10) (Scheme 3) by NMR spectroscopy
and X-ray diffraction. 10 has been previously synthesized by
reaction of mer-[IrCl3(SEt2)3] with 2-LiC6H4PPh2, presumably
via protonolysis of a tris(orthometalated) intermediate, [Ir(κ2P,
C-PPh2C6H4)3], but its crystal structure has not been
reported.19 Reaction of [Ir(PPh3)3Cl] with sodium acetate in
the absence of 1 did not produce 10, suggesting that the
mercury reagent may play a role in the formation of 10. It
seems likely that transmetalation of 1 with [Ir(PPh3)3Cl]
initially gives an Ir aryl species that undergoes intramolecular
C–H activation to yield a mono and subsequently bis-orthome-
talated complex.
Conclusions
In summary, in an effort to synthesize Ir(C^N^C) complexes,
we have studied the transmetalation of the mercury reagent 1
with various organoiridium complexes. Depending on the reac-
tion conditions, transmetalation of 1 with [Ir(cod)Cl]2 and [Ir
(CO)2Cl(py)] produced different types of heterometallic Ir/Hg
C^N^C complexes containing Ir–Hg bonds. A Hg-free cod
complex, [Ir(C^N^C)(cod)Cl], has been obtained by chlori-
nation [Ir(C^N^C)(cod)(HgCl)] with PhICl2, followed by treat-
ment with tricyclohexylphosphine. In addition, a Hg-free car-
bonyl complex, [Ir(C^N^C)(κ2C,N-HC^N^C)(CO)] (8), in which
the carbonyl is trans to the pyridyl group of the bidentate
HC^N^C ligand, has been synthesized from
1 and [Ir
10 is chiral and the structure of one of the isomers in
shown in Fig. 10. Consistent with the NMR spectrum, the two
(CO)2Cl2]−. Upon irradiation with UV light, 8 is readily isomer-
ized to 8′, in which the carbonyl is trans to a phenyl group of
the bidentate HC^N^C ligand. The isomer pair 8 and 8′ exhibi-
ted LC phosphorescence in 77 K EtOH/MeOH glassy medium
and MLCT phosphorescence in CH2Cl2 solution at room temp-
erature. The Ir-bound C^N^C ligand was found to display flex-
ible coordination behaviour and can switch between the tri-
dentate-C,N,C and bidentate-C,N binding modes by protona-
tion/oxidation and deprotonation. A preliminary study indi-
cated that [Ir(C^N^C)(cod)(H2O)](OTf) can catalyze the cyclo-
Scheme 3 Reaction of [Ir(PPh3)3Cl] with 1 and NaOAc.
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propanation of styrene with ethyldiazoacetate, suggesting that
Ir(C^N^C) complexes can find applications in organometallic
catalysis. The synthesis of coordinately unsaturated Ir(C^N^C)
complexes and the study of their organometallic reactivity are
underway.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Hong Kong Research Grants
Council (project numbers 16300917 and 16301019) and the
Hong Kong University of Science and Technology.
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