Stable iridium(iv) complexes supported by tetradentate salen ligands. Synthesis, structures and reactivity

Article abstract of DOI:10.1039/c8cc10220a

A series of trans-dichloroiridium(iv)-salen complexes were synthesized and structurally characterized by spectroscopic means and X-ray crystal structures. These Ir(iv) complexes are able to catalyze intramolecular C-H amination of aryl azides. The catalytic amination was drastically accelerated under microwave-assisted conditions, and possibly involves Ir-imido intermediates as supported by high-resolution ESI-MS analysis.

Full text of DOI:10.1039/c8cc10220a

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Stable iridium(IV) complexes supported by
tetradentate salen ligands. Synthesis, structures
Cite this:DOI: 10.1039/c8cc10220a
and reactivity†
Received 26th December 2018,
Accepted 25th February 2019
a
a
ab
Chi Lun Lee,
Liangliang Wu,
Jie-Sheng Huang *a and Chi-Ming Che
*
DOI: 10.1039/c8cc10220a
rsc.li/chemcomm
A series of trans-dichloroiridium(IV)–salen complexes were synthe-
sized and structurally characterized by spectroscopic means and X-
ray crystal structures. These Ir(IV) complexes are able to catalyze
intramolecular C–H amination of aryl azides. The catalytic amina-
tion was drastically accelerated under microwave-assisted condi-
tions, and possibly involves Ir-imido intermediates as supported by
high-resolution ESI-MS analysis.
Iridium-catalyzed organic transformations have been receiving a
great deal of attention.1 Among the important intermediates
proposed to be involved in these processes are Ir(IV) species.1c,2
Ir(IV) species represent one class of the key intermediates proposed
in iridium-catalyzed water oxidation.3 Nonetheless, isolated Ir(IV)
complexes are relatively sparse, all of which are supported by
monodentate ligands only,4 or bidentate S^S,5 N^N,5,6 B^C,7
N^O,8 O^O ligands,9 or tridentate O^N^O,10 O^O^O,11 P^N^P,12
N^N^O13 ligands, or polyoxometalates.14 Indeed, the chemistry and
Fig. 1 X-band EPR spectra of [IrIV(salen)Cl2] (salen = L1: 1, L2: 2, L3: 3) in
frozen CH2Cl2 at 10 K. H2L1 = bis(3,5-di-tert-butylsalicylidene)-1,2-cyclo-
hexanediamine; H2L2 = bis(3-tert-butyl-5-chlorosalicylidene)-1,2-cyclo-
hexanediamine; H2L3 = bis(3,5-di-tert-butylsalicylidene)-1,10-binaphthyl-
2,20-diamine.
catalytic activity of Ir(IV) complexes in organic transformations several stable mononuclear Ir(IV)–salen complexes [IrIV(salen)Cl2]
remain underdeveloped. We are interested to explore the chemistry (salen = L1: 1, L2: 2, L3: 3; Fig. 1). These Ir(IV)–salen complexes were
and catalytic properties of mononuclear iridium complexes in high found to catalyze intramolecular amination of C(sp3)–H bonds of
oxidation states, which are supported by tetradentate O^N^N^O aryl azides, possibly via reactive Ir-imido species,12,17 with up to 96%
Schiff base ligands, namely, N,N0-bis(salicylidene)-ethylenediamine product yields obtained under microwave-assisted conditions.
derivatives (salens). This type of ligands has been employed to
Complexes 1 and 2 were prepared by reactions of [IrI(Cl)(COD)]2
support efficient Ir(III) catalysts for carbene transfer/insertion with (COD = 1,5-cyclooctadiene) with H2L1 and H2L2, respectively, in
diazo compounds15a,b and nitrene C–H insertion with sulfonyl 1,2,4-trichlorobenzene in open atmosphere at 185 1C for 40 min
azides.15c However, mononuclear Ir(IV)–salen complexes, to the best (1: red solid, 28% yield; 2: orange solid, 20% yield). However, similar
of our knowledge, are unprecedented. The catalytic reaction exam- reaction of [IrI(Cl)(COD)]2 with H2L3 gave [IrIII(L3)(Cl)(CO)]. Treat-
ined in this work is nitrene C–H insertion,15d,16 a catalytic reaction ment of [IrIII(L3)(Cl)(CO)] in refluxing CCl4 under argon atmosphere
previously not documented for Ir(IV) chemistry.1,2,15d,16 Herein we led to the isolation of complex 3 (brown solid, 46% yield).
describe the isolation, characterization, and catalytic behavior of
Complexes 1–3 are paramagnetic, with magnetic moment (meff)
of 1.82 mB for 1, 1.63 mB for 2, and 1.73 mB for 3 (determined by the
Evans method in solution at room temperature), corresponding to
the presence of one unpaired electron in each molecule and
indicating an S = 1/2 spin state.11 DFT calculations using 1 as
example (details in the ESI†) revealed a spin density distribution of
56% on Ir and 43% on phenoxide O atoms, comparable to that
reported for the Ir(IV)–N^O complexes (48–57% on Ir8 and 47–48%
on alkoxide O atoms,8a which indicate high covalency and
a State Key Laboratory of Synthetic Chemistry and Department of Chemistry,
The University of Hong Kong, Pokfulam Road, Hong Kong, China.
E-mail: cmche@hku.hk, jshuang@hku.hk
b HKU Shenzhen Institute of Research & Innovation, Shenzhen, China
† Electronic supplementary information (ESI) available: Experimental proce-
dures, computational details, Fig. S1–S9, Tables S1–S7, Schemes S1–S3. CCDC
1884103, 1884105, 1883928 and 1883931. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c8cc10220a
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delocalization in the coordination bonds8). We measured the X-band
EPR spectra of 1–3 in CH2Cl2 at 10 K (Fig. 1); all the spectra are
anisotropic and are assigned with the help of the empirical simula-
tions, each being simulated as an S = 1/2 system with significant g
anisotropy. The EPR spectra of 1 and 2 both show a rhombic signal
with simulated g values of gx = 2.11, gy = 2.10, gz = 1.55 for 1 (Fig. S5,
ESI†) and gx = 2.14, gy = 2.12, gz = 1.51 for 2 (Fig. S6, ESI†). Complex 3
gives an axial signal with simulated gx B gy = 2.17 and gz = 1.52
(Fig. S7, ESI†). These g values for 1–3 are comparable to those of
previously reported Ir(IV) complexes (gx = 2.15–2.48, gy = 2.15–2.42,
gz = 1.56–1.96).6,7,8b In contrast, [IrIII(L3)(Cl)(CO)] is diamagnetic,
1
showing a well-resolved H NMR spectrum which is not sensitive
to temperature (from ꢀ40 to 50 1C, Fig. S8, ESI†).
The crystal structures of 1–3 and [IrIII(L3)(Cl)(CO)] have been
determined by X-ray crystallographic studies (Fig. 2). Com-
plexes 1 and 2 adopt planar ONNO arrangement as usually
found for metal complexes with this type of salen ligands.15 The
Ir(IV) ions of 1 and 2 are each situated in an axially elongated
octahedral environment with two trans Ir–Cl axial bonds
(cf. Ir–Cl 2.328(2)–2.347(3) Å vs. Ir–O 1.997(8)–2.003(7) Å and
Ir–N 1.964(4)–1.983(9) Å). The trans-Cl–Ir–Cl moiety is basically
linear (bond angles: 177.89(12)1 for 1, 178.15(6)1 for 2) and
nearly perpendicular to the ONNO plane (e.g. Cl–Ir–O angles:
87.8(2)1–91.5(2)1 in 1 and 88.65(10)1–90.03(10)1 in 2).
For the binaphthyl salen complex [IrIII(L3)(Cl)(CO)], its crystal
structure reveals a distorted octahedral coordination geometry with
a cis-b configuration (non-planar ONNO; Ir–Cl 2.3486(9) Å), like
previously reported metal complexes with tetradentate binaphthyl
salen ligands.18 Intriguingly, upon converting this cis-b Ir(III)–L3
complex to the Ir(IV)–L3 complex 3, 3 adopts a trans configuration
(planar ONNO, Fig. 2) with a distorted IrO2N2Cl2 octahedron (Ir–Cl
2.3325(13)–2.3372(12) Å, Ir–O 2.000(4)–2.007(4) Å, Ir–N 2.001(4)–
2.011(4) Å, Cl–Ir–Cl 176.23(5)1, Cl–Ir–O 87.32(11)1–91.22(11)1) similar
to that in 1 and 2. Possibly, a cis-IrIVCl2 configuration of 3 is
destabilized by the imine/phenolate groups trans to Clꢀ and/or by
the repulsion between the cis Clꢀ ligands. The Ir–Cl distances in 1–3
(2.328(2)–2.347(3) Å) compare well with those in trans-[IrIV(N^O)2Cl2]
(2.3195(9)–2.3473(15) Å, N^O = 2-(pyridin-2-yl)propan-2-oate),8b and
Fig. 2 ORTEP drawings of 1–3 (for each of 1 and 2, only one of the two
crystallographically independent molecules is shown) and [IrIII(L3)(Cl)(CO)].
Hydrogen atoms are not shown. Thermal ellipsoid probability: 30%.
slightly longer than those in [IrIV(O^O^O)Cl3] (2.2781(9)–2.2895(9) Å;
11
¨
all Ir–Cl bonds are cis to each other; O^O^O = Klaui tripodal ligand).
The cyclic voltammograms of 1–3 in CH2Cl2 each display two 0.1 M Ag/AgNO3, Fig. 3) are attributable to reduction of IrIV to IrIII
redox couples (reversible or quasi-reversible): an oxidation couple at (note the absence of significant reduction wave for [IrIII(L3)(Cl)(CO)]
E1/2 = 0.75–0.99 V and a reduction couple at E1/2 = 0.08 to ꢀ0.17 V in Fig. 3); these potentials are comparable to the reported
(vs. 0.1 M Ag/AgNO3 in MeCN, Fig. 3). The potentials of both the E1/2(IrIV/IrIII) values for fac-[IrIV(N^O)3]+ 8a and trans-[IrIV(N^O)2Cl2]8b
oxidation and reduction couples are affected by the substituents on in CH2Cl2 (ꢀ0.176 and ꢀ0.387 V vs. Cp2Fe+/0, respectively).
the salen ligand, with the E1/2 values showing an anodic shift from
Complexes 1–3 exhibit similar UV-vis absorption spectra in
0.75 and ꢀ0.17 V for 1 to 0.99 and 0.08 V for 2, respectively, upon CH2Cl2 (Fig. S1, S2 and S4, ESI†). The high energy bands at
replacing two electron-donating tBu groups of 1 with two electron- B350 nm and below 300 nm are dominated by intra-ligand
withdrawing Cl groups to give 2. Complexes 1 and 3, which differ in charge-transfer (ILCT) transitions of the coordinated salen
their salen backbones (cyclohexyl vs. binaphthyl), exhibit the redox ligands. The bands with lmax at 413–575 nm can be assigned
couples at similar potentials, indicating that the backbone of the to ligand-to-metal charge-transfer (LMCT) transitions, similar to
salen ligand has little effect on the E1/2 values. The oxidation couple the assignment of the visible absorption bands for Ir(IV)–N^O
for 1–3 is tentatively assigned to a ligand-based process, as an complexes.8 There are also bands in the near-IR region, analo-
oxidation couple similar to that for the Ir(IV)–L3 complex 3 has also gous to the case of Ir(IV)–N^O complexes,8b with lmax being, for
been observed for [IrIII(L3)(Cl)(CO)] (Fig. 3). The reduction couples example 775 and 931 nm for 1. Such low energy bands for 2 and
for 1–3 at E1/2 = ꢀ0.06 to ꢀ0.31 V vs. Cp2Fe+/0 (0.08 to ꢀ0.17 V vs. 3 appear at lmax of 747–944 nm.
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Table 1 Microwave-accelerated dihydroquinazolinone or quinazolinone
formationa
Entry
Cat.
Substrate
5 (yieldb)
6 (yieldb)
1
2
1
2
Fig. 3 Cyclic voltammograms of 1–3 and [IrIII(L3)(Cl)(CO)] in CH2Cl2
(0.1 M [nBu4N]PF6). Scan rate: 100 mV sꢀ1. Redox potentials and peak
separations are shown in Table S5 (ESI†).
3
4
5
6
7
1
1
1
1
The catalytic properties of the Ir(IV)–salen complexes were
examined with ortho-azidobenzamides (4), a type of aryl azides
previously reported to undergo intramolecular C(sp3)–H amina-
tion to give dihydroquinazolinones (5) and quinazolinones (6)
in the presence of iron porphyrin catalyst [FeIII(F20TPP)Cl]
(H2F20TPP = meso-tetrakis(pentafluorophenyl)porphyrin) with
5 being the dominant products (up to 83% yield; yields of 6:
up to 17%).19 Interestingly, treatment of 4a with 1 (5 mol%) in
refluxing benzene for 18 h afforded quinazolinone 6a as the
major product in 89% yield whereas dihydroquinazolinone 5a
was obtained in only 9% yield (entry 1 in Table S6, ESI†).
Changing the solvent to 1,2-dichloroethane (DCE) increased
the yield of 6a to 95% (no 5a was obtained, entry 4 in Table S6,
ESI†), and the same yield of 6a was obtained when 4a was
treated with 1 (5 mol%) in DCE under irradiation with an
incandescent lamp (150 W) for 12 h (entry 1 in Table S7, ESI†).
Notably, under microwave-assisted conditions, the reaction
time for the 1-catalyzed transformation of 4a to 6a was drama-
tically shortened to 1 h, affording 6a in 96% yield (entry 1,
Table 1; yield of 6a using catalyst 2: 92%, entry 2, Table 1).
Similar reactions of ortho-azidobenzamides 4b–4e gave quina-
zolinones 6b–6e in 57–93% yields (entries 3–6, Table 1). For all
of 4a–4e, their reactions catalyzed by 1 under microwave-
assisted conditions for 1 h gave product yields comparable to
those obtained under irradiation of incandescent lamp (150 W)
for 12 h (6a–6e: 61–95% yields, Table S7, ESI†). Unexpectedly,
upon changing the catalyst to 3 (5 mol%), no 6a was obtained
from the reaction of 4a under microwave-assisted conditions
3
f
8
9g
a
Reactions were performed with substrate 4a–4e (0.1 mmol) and catalyst
b
c
d
(5 mol%) in DCE under N2. Isolated yield. Not obtained. Catalyst 1.
e
f
g
Catalyst 2. [IrIII(L3)(Cl)(CO)]. Control experiment without catalyst.
for 1 h; instead, dihydroquinazolinone 5a was isolated in 40% sealed Schlenk tube revealed the presence of H2. By treating 1
yield (entry 7, Table 1), albeit lower than the 68% yield of 5a with mesityl azide (MesN3, 3 equiv.) in degassed DCE under
isolated from the reaction catalyzed by [IrIII(L3)(Cl)(CO)] under microwave-assisted conditions for 10 min, high-resolution
the same conditions (entry 8, Table 1). In control experiment ESI-MS spectra showed a cluster peak at m/z 928.4160 which,
using 4a in the absence of the iridium catalysts, neither 5a nor together with the isotope distribution pattern, matches the
6a was obtained (entry 9, Table 1).
formulation of an Ir-imido species [Ir(L1)(NMes)Cl] + Na+
To gain insight into the mechanism of the catalytic reac- (Fig. 4). Based on these experiments and previous studies in
tions, we examined a reaction mixture of 1 with 4c (1 equiv.) in [FeIII(F20TPP)Cl]-catalyzed analogues,19 the following mecha-
refluxing DCE under N2 by ESI-MS, which showed a cluster nism for the Ir(IV)-catalyzed intramolecular C(sp3)–H bond
peak at m/z 962.4 attributable to [Ir(L1)(Cl)(5c)] (Fig. S9, ESI†). amination is proposed (Scheme 1): the 1-catalyzed system
GC analysis of the head space of the reaction mixture in a initially generated a mono(chloro) species (from 1) with a
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Conflicts of interest
There are no conflicts to declare.
Notes and references
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tandem catalytic system for the formation of quinazolinones.
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