An 18+δ iridium dimer releasing metalloradicals spontaneously

Article abstract of DOI:10.1039/c0dt00605j

Reductive elimination of a bridged chlorine from a diiridium(iii) core, [(dfpbo)₂Ir(μ-Cl)]₂ (dfpbo = 2-(3.5-difluorophenyl) benzoxazolato-N,C²), afforded an iridium dimer, [(fpbo) ₂Ir]₂(μ-Cl), showing an 18+δ structure with a bent bridge, which can release metalloradicals spontaneously in solution at room temperature.

Full text of DOI:10.1039/c0dt00605j

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An 18+d iridium dimer releasing metalloradicals spontaneously†
Tsun-Ren Chen,*a Hsiu-Pen Lee,a Jhy-Der Chenb and Kelvin H.-C. Chena
Received 4th June 2010, Accepted 11th August 2010
DOI: 10.1039/c0dt00605j
Reductive elimination of
a
bridged chlorine from
a
diiridium(III) core, [(dfpbo)2Ir(l-Cl)]2 (dfpbo
=
2-(3.5-
difluorophenyl)benzoxazolato-N,C2), afforded an iridium
dimer, [(fpbo)2Ir]2(l-Cl), showing an 18+d structure with a
bent bridge, which can release metalloradicals spontaneously
in solution at room temperature.
Iridium metalloradicals are known to play an important role in the
investigation of DNA oxidative chemistry, energy transfer in bi-
ological systems, and organometallic substrate transformations.1
Mostly, iridium metalloradicals are generated by the dissociation
of unstable molecules, for example, via a photochemical, chemical
or electrochemical reaction.2 The concept of 18+d was introduced
in 1988, in which the value of d reflects the degree of electron
residing on the metal centre.3 Since then, very few 18+d complexes
have been described.4 Such complexes are difficult to synthesize
and isolate due to their propensity to undergo further reactions
such as ligand dissociation, and the reduction or oxidation of metal
atoms. Our interest in the iridium radical chemistry has discovered
a novel diiridium compound showing an 18+d structure which
can spontaneously release iridium metalloradicals under ambient
temperature. We report here the isolation and characterization of
this title compound.
Fig. 1 ORTEP plots of 1. Thermal ellipsoids are drawn at the 30%
probability level and hydrogen atoms are omitted for clarity.
readily dissociated to form the relatively stable metalloradical
[(dfpbo)2Ir]∑. Furthermore, two of these metalloradicals could
combine with each other to form a binuclear species: [(dfpbo)2Ir]2
Reaction of [(dfpbo)2Ir(m-Cl)]2 (dfpbo = 2-(3.5-difluorophenyl)-
benzoxazolato-N,C2)5 with potassium carbonate in a solvent
system of toluene and ethoxyethanol afforded an unusual iridium
dimer [(dfpbo)2Ir]2(m-Cl) (1) (Scheme S1, ESI†).‡ A single-crystal
X-ray structure determination confirmed the dimeric nature of
1 (Fig. 1). The Ir1–Cl–Ir2 and Cl–Ir1–Ir2 angles are 76.21(3)
and 51.90(3)◦, respectively. This structure possesses a bent single
bridge connecting the two iridium core. The Ir–Ir bond length of
[
193Ir, 193Ir, m/z 1307 (2.2%) and 193Ir, 191Ir, m/z 1305 (1.1%)].
The frozen-solution (77 K, in toluene) EPR spectrum of com-
pound 1 displays an isotropic signal at g = 2.0009, characteristic of
an organic centered radical (Fig. 2(a)), which, with no resolvable
iridium hyperfine, suggests that only a very small degree of spin-
free electron is delocalized from the dfpbo ligands to the metal.7 At
˚
3.0433(3) A is similar to those of previously reported complexes of
single bridged iridium dimer, such as {[Cp*Ir(CO)H]2(m-H)}BAr¢4
6
˚
˚
(2.928 A) and {[Cp*Ir(PMe3)H]2(m-H)}PF6 (2.983 A).
The (FAB+) (Fast Atom Bombardment) spectrum of 1 (Fig. S1,
ESI†) shows a medium-intensity complex pattern of molecular
peaks: [Ir2(dfpbo)4(Cl)]+: [193Ir, 193Ir, 35Cl and 193Ir, 191Ir, 37Cl,
m/z 1341 (15%)]. A high-intensity pattern of peaks of radical-
cation was also observed: [(dfpbo)2Ir]∑+ 193Ir, m/z 653 (38%)
[
and 191Ir, m/z 651 (32%)], which reveals that compound 1 is
aDepartment of Chemical Biology, National Pingtung University of Educa-
tion, Pingtung, Taiwan, 90003. E-mail: trchen@mail.npue.edu.tw; Fax: 886-
8-7230305; Tel: 886-8-7226141 ext. 33258
bDepartment of Chemistry, Chung-Yuan Christian University, Chung-Li,
Taiwan, 32023
† Electronic supplementary information (ESI) available: Full experimental
details, analytical data and X-ray details for 1, 4 and 5; mass spectral
data for 1; a table of Cartesian coordinates, SOMO and LUMO orbital
compositions for 1. CCDC reference numbers for compounds 1, 4 and
5, 778557–778559. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0dt00605j
Fig. 2 EPR spectra of 1 in toluene after degassing at (a) 77 K, and (b)
298 K. EPR settings: microwave frequency, 9.879 GHz; microwave power,
10.080 mW; number of scans, 1.
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Fig. 3 Electron density of the SOMO (left) and LUMO (right) in [(fpbo)2Ir]2(m-Cl).
◦
˚
Table 1 Comparison of selected bond lengths (A) and angles ( ) of 1
from crystallography and DFT calculation
a
Ir–Ir
Ir–Cl
Ir–N
y
X-Ray
DFT
3.0433(3)
3.042
2.4658(14)
2.465
2.030(4)
2.062(4)
2.029
10.4
168.9
10.36
168.85
2.061
a Torsional angles N1–Ir–Ir–N1a and N2–Ir–Ir–N2a respectively.
room temperature, the solution of compound 1 in toluene shows
a complicated EPR spectrum, Fig. 2(b), implying that there may
have more than one paramagnetic species exist in the solution
system and whose spectra overlap with each other.8
Fig. 4 Ir K-edge XANES of crystalline (a) [(dfpbo)2IrIII(m-Cl)]2, (b) com-
pound 1, and (c) [(dfpbo)2IrII]2. A Si(111) double crystal monochromator
was employed for energy scanning. Fluorescence data were obtained at
room temperature using an Ar-filled ionization chamber detector, each
sample was scanned 3 times for averaging.
DFT (B3LYP/LANL2DZ level) calculation on 1 provide in-
sight into the electronic structure of 1. The X-ray structure of 1 was
used as an initial geometry for optimization. The metrical parame-
ters obtained from the X-ray structure and DFT calculation are in
good agreement (Table 1). The singly occupied molecular orbital
(SOMO) calculated for compound 1 is composed of 4.5% d(Ir)
and 94.3% p (dfpbo) and the lowest unoccupied molecular orbital
(LUMO) is composed of 15% d(Ir) and 84.5% p (dfpbo) (Fig. 3),
which implies that the unpaired electron is mostly delocalized
onto the four ligands dfpbo and gives rise to the characteristic
of an ligand centered radical for compound 1 in frozen solution.7
As the unpaired electron is mostly distributed over the ligands,
there is only a part of the unpaired electron located on the metal
centre; therefore, this complex could be categorized as an 18+d
compound. This delocalization stabilized the structure of 1 and
led to the isolation of this compound.
To understand the formal charge of iridium atoms in compound
1, the normalized X-ray absorption near edge structure (XANES)
spectra at the Ir K-edge of compound 1 was obtained (Fig. 4(b)).
For comparison, the spectra of [(dfpbo)2IrIII(m-Cl)]2 (Fig. 4(a))
and [(dfpbo)2IrII]2 (Fig. 4(c)) were also acquired. The white line
intensity of compound 1 in Fig. 4 shows that the iridium atoms in
1 have a formal oxidation between +3 and +2.9 As the odd electron
is mostly delocalized on the ligands, the metal atoms possess more
positive charge; therefore, the XANES data of complex 1 is similar
to that of [(dfpbo)2IrIII(m-Cl)]2 rather than that of [(dfpbo)2IrII]2.
5
Two iridium dimers, [(dfpbo)2Ir]2 (3)10 and [(dfpbo)2Ir(m-Cl)]2
were obtained when the temperature of the solution 1 was
◦
increased to about 100 C. Complex 3 (40% yield) should come
from the recombination of two iridium radicals [(dfpbo)2Ir]∑ 2a,
and the iridium dimer [(dfpbo)2Ir(m-Cl)]2 (40% yield) should come
from the recombination of two iridium radicals [(dfpbo)2Ir(m-
Cl)]∑ 2b, (Scheme 1). The reactivity of these iridium radicals
have been the subject of two studies, a carbon–hydrogen bond
activation (CHA) and a rare but important carbon–carbon bond
activation (CCA) for substrates.11 An illustration of CHA is that
an unsaturated ketone, chalcone, could be converted to a 1,3-
diketone, dibenzoylmethane, in the presence of trace amount of
water and these iridium radicals via a radical-assisted oxidation.
The dibenzoylmethane formed could act as an ancillary ligand
and coordinate to the iridium radical 2a, [(dfpbo)2Ir]∑, to produce
a stable cyclometalated iridium complex [(dfpbo)2Ir(dbm)] (dbm =
dibenzoylmethanate) (4) (Scheme 1). For the reaction mentioned
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chromatography over silica gel using n-hexane–toluene (3 : 1 ratio) as
eluent to obtain the pure product. Yield: 293 mg (60% based on iridium).
Anal. Calcd for C52H24N4O4ClF8Ir2·CH2Cl2: C, 44.65; H, 1.84; N, 3.93.
Found: C, 44.75; H, 1.67; N, 3.78%. 4: a mixture of chalcone (312 mg,
1.5 mmol) and compound 1 (50 mg, 0.037 mmol) in toluene (30 ml)
was stirred under N2 at 110 ◦C for 2 d. The final solution was taken
to dryness in vacuo. The residue was purified via column chromatography.
Yield: 14 mg (43% based on iridium). (Anal. Calcd for C41H23F4IrN2O4:
C, 56.22; H, 2.65; N, 3.20. Found: C, 56.25 H, 2.66; N, 3.21%. 5: a
mixture of 3,5-dibromobenzaldehyde (396 mg, 1.5 mmol) and compound
1 (50 mg, 0.037 mmol) in toluene (30 ml) was stirred under N2 at 110 ◦
C
for 2 d. The final solution was taken to dryness in vacuo. The residue
was purified via column chromatography. Yield: 16 mg (47% based on
iridium). Anal. Calcd for C33H15N2O3Br2F4Ir: C, 43.29; H, 1.65; N, 3.06.
Found: C, 43.32; H, 1.66; N, 3.07%.; IR (KBr pellet): n(CO) 2360 cm-1.
Crystal data for compound 1: formula C52H24N4O4ClF8Ir2, M = 1340.67,
orthorhombic, space group Aba2, a = 21.8706(4), b = 14.4783(3), c =
3
-3
˚
˚
16.9835(3) A, V = 5377.81(18) A , Z = 4, rc = 2.075 g cm , F(000) = 3228,
T = 100(2) K, 2qmax = 58.5◦, 25 544 reflections collected, 6320 unique,
GooF = 1.192, R1 = 0.0317, wR2 = 0.0637. Crystal data for compound
¯
4: formula C41H23F4IrN2O4, M = 875.85, triclinic, space group P1, a =
˚
12.0381(3), b◦= 14.6705(4), c = 19.5320(5) A, a = 102.358(2), b = 95.205(2),
3
-3
˚
g = 94.203(2) , V = 3340.35(15) A , Z = 4, rc = 1.742 g cm , F(000) = 1712,
T = 120(2) K, 2qmax = 58.7◦, 29 949 reflections collected, 15 412 unique,
GooF = 1.132, R1 = 0.0516, wR2 = 0.1161. Crystal data for compound 5:
formula C33H15N2O3Br2F4Ir, M = 915.52, monoclinic, space group P21/n,
Scheme 1 Formation of iridium metalloradicals from 1, and the reactions.
◦
˚
a = 14.5053(3), b = 12.1682(3), c = 16.3047(3) A, b = 102.514(2) , V =
above, two iridium radicals 2b, [(dfpbo)2Ir(m-Cl)]∑, could combine
with each other to form an iridium dimer [(dfpbo)2Ir(m-Cl)]2. Cy-
clometalated iridium complexes with ancillary ligands have been
widely investigated recently because they can be used as emissive
dopants in organic light emitting devices (OLEDs), photocatalysts
for CO2 reduction, photooxidants, and biological reagents.12 The
metalloradical reaction described herein provides a useful way to
prepare cyclometalated iridium complexes.‡ An example of CCA
is that an aldehyde, 3,5-dibromobenzaldehyde, could react with
iridium radical [(dfpbo)2Ir]∑ to introduce a carbonyl into the metal
centre readily to obtain a carbonyl product [(dfpbo)2Ir(dbp)(CO)]
(dbp = 3,5-dibromophenyl)) (5) by the oxidative addition and
retromigration insertion (Scheme 1), which provides an ap-
proach to prepare iridium carbonyl complexes under mild
condition.‡
3
-3
˚
2809.47(10) A , Z = 4, rc = 2.164 g cm , F(000) = 1736, T = 110(2) K, 2qmax
=
58.4◦, 13 799 reflections collected, 6500 unique, GooF = 1.023, R1 = 0.0246,
wR2 = 0.0484.
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In conclusion, we present an unusual iridium compound
possessing a bent single bridge. The structural study shows that
this compound holds an 18+d electron configuration, in which
the unpaired electron is mostly delocalized onto the four dfpbo
ligands. This compound could spontaneously release iridium
metalloradicals at ambient temperature in solution, and the
metalloradicals could activate many kinds of substrates, such as
alkene, ketones, and aldehydes. The reactivity of this compound
suggests that it could be a key precursor in the explorations for
iridium chemistry and materials science.
This work was supported by the National Science Council of
the Republic of China (Grant No. NSC 99-2113-M-153-001).
We are grateful to Dr Jyh-Fu Lee from National Synchrotron
Radiation Research Center (NSRRC) for the X-ray absorption
measurements.
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Notes and references
‡ Synthesis of 1: a flask was charged with 500 mg of [(fpbo)2Ir(m-Cl)]2
(0.363 mmol) and 55 mg of anhydrous potassium carbonate (0.40 mmol).
Toluene (15 ml) and ethoxyethanol (0.5 ml) were added to give a clear
yellow solution, and the solution was stirred under N2 and warmed
to 100 ◦C for 24 h. After cooling to room temperature, the reaction
mixture was filtered through silica gel. The filtrate was purified via column
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