Photochemical C-H activation and subsequent aerobic oxidation reactions of benzene with Cp*Ir(CO)2

Article abstract of DOI:10.1016/j.jorganchem.2013.03.014

A benzene solution of Cp*Ir(CO)₂ was irradiated for 10 h at room temperature, and then the reaction solution was exposed to air overnight, affording two cis-trans isomeric diiridium(Ir-Ir) complexes [Cp*Ir(μ-CO)(Ph)]₂ (1 and 4), a mononuclear iridium complex Cp*Ir(CO)(Ph)₂ (2), a diiridium(Ir-Ir) complex Cp*₂(CO)₂Ir₂(μ₂-C ₆H₄) (3), a novel hexanuclear iridium complex [Cp*₂(μ₂-Ph)(μ₂-H)Ir ₂](μ₃-CO₂)[Ir₄(CO)₁₁] (5), and trace biphenyl. The molecular structures of complexes 1-5 have been determined by single-crystal X-ray diffraction analysis. Moreover, we studied the reactivities and possible formation paths of some of these complexes.

Full text of DOI:10.1016/j.jorganchem.2013.03.014

Journal of Organometallic Chemistry 735 (2013) 72e79
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Photochemical CeH activation and subsequent aerobic oxidation reactions
of benzene with Cp*Ir(CO)2
*
Xing Tan, Bin Li, Shansheng Xu, Haibin Song, Baiquan Wang
State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Weijin Road 94#, Tianjin 300071, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A benzene solution of Cp*Ir(CO)2 was irradiated for 10 h at room temperature, and then the reaction
solution was exposed to air overnight, affording two cisetrans isomeric diiridium(IreIr) complexes
Received 11 December 2012
Received in revised form
27 February 2013
[Cp*Ir(
m-CO)(Ph)]2 (1 and 4), a mononuclear iridium complex Cp*Ir(CO)(Ph)2 (2), a diiridium(IreIr)
complex Cp*2(CO)2Ir2(m2-C6H4) (3), a novel hexanuclear iridium complex [Cp*2(m2-Ph)(m2-H)Ir2](m3
-
Accepted 12 March 2013
CO2)[Ir4(CO)11] (5), and trace biphenyl. The molecular structures of complexes 1e5 have been deter-
mined by single-crystal X-ray diffraction analysis. Moreover, we studied the reactivities and possible
formation paths of some of these complexes.
Keywords:
Iridium
Photochemical
CeH activation
Aerobic oxidation
X-ray diffraction
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
Cp*Ir(CO)(Ph)2, and trace biphenyl. Moreover, the possible forma-
tion paths of some products have also been investigated. These
The activation of unreactive hydrocarbon CeH bonds by
photolysis of (
5-C5R5)M(L)(H)2 and ( 5-C5R5)M(L)2 complexes
results would help us to understand the reactivities of iridiume
hydrogen complexes toward oxygen.
h
h
(R ¼ H, Me; L ¼ CO, phosphine, etc.; M ¼ Ir, Rh) has been explored
over the past several decades. These reactions have been postulated
to proceed through 16-electron intermediates
2. Results and discussion
(h
5-C5R5)M(L),
which are generated by photoextrusion of H2 [1e8], CO [9e17], or
C2H4 [18e21] from the corresponding dihydride, carbonyl, or
ethylene complexes. The resulting complexes (h5-C5R5)M(L)(H)(R0)
(R0 ¼ C6H5, CH2C(CH3)3, C6H11, etc.) are usually thermally unstable
and air-sensitive, thereby hard to be isolated. To apply this photo-
chemical CeH activation methodology both in stoichiometric re-
actions and in catalysis, it is of great interest to study the reactivities
of the complexes (h5-C5R5)M(L)(H)(R0).
Molecular oxygen is a kind of inexpensive and environmentally
friendly oxidant, and the activation of oxygen by iridium(I) com-
plexes has been demonstrated [22]. Despite this fact, the reactions
of oxygen with IrIIIeH complexes have been scarcely explored [23e
26]. In this paper, after photolysis of Cp*Ir(CO)2 with benzene we
exposed the resulting solution to air, affording three dinuclear
2.1. Synthesis and characterization of complexes 1e5
A benzene solution of Cp*Ir(CO)2 was irradiated for 10 h, and
then the reaction solution was exposed to air overnight. Chro-
matographic separation of the product mixture gave, in order of
elution, trace biphenyl, unreacted Cp*Ir(CO)2 (2%), red-brown
trans-[Cp*Ir(
CO)(Ph)2 (2, 6%) and yellow Cp*2(CO)2Ir2(m2-C6H4) (3, 2%), red-
brown cis-[Cp*Ir( -CO)(Ph)]2 (4, 28%), and red [Cp*2(m2-Ph)(m2-H)
m-CO)(Ph)]2 (1, 9%), a mixture of colorless Cp*Ir(-
m
Ir2](m3-CO2)[Ir4(CO)11] (5, 1%) (Scheme 1).
Single-crystal X-ray diffraction analysis shows that complexes 1
and 4 are cisetrans isomers (Figs. 1 and 2). Both of them contain two
Cp*Ir(CO)(Ph) units, while the two iridium atoms are bridged by two
ꢀ
carbonyl groups. The IreIr distance of 1 is 2.7113(5) A, while that of 4
iridium complexes, a novel hexanuclear iridium complex [Cp*2(m2
-
ꢀ
is 2.722(1) A. The most obvious difference between 1 and 4 is the
Ph)(m2-H)Ir2](m3-CO2)[Ir4(CO)11], a mononuclear iridium complex
position of the two phenyl groups. In complex 1, the two phenyl
groups are far from each other, with an antiparallel arrangement.
Differently, in complex 4 the two phenyl groups are close to each
other with a face to face arrangement, and the dihedral angle be-
tween the two phenyl planes is 20.0ꢀ. The spectroscopic data of 1 and
* Corresponding author. Fax: þ86 22 23504781.
E-mail address: bqwang@nankai.edu.cn (B. Wang).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.03.014

X. Tan et al. / Journal of Organometallic Chemistry 735 (2013) 72e79
73
benzene
hv 10 h
air exposure
Cp*Ir(CO)2
CO
Ir CO
Ir
Ir
Cp*Ir(CO)(Ph)2
Ph-Ph
+
+
+
OC
OC Ir
trace
1 (9%)
2 (6%)
3
(2%)
(CO)2
Ir
OC
Ir
Ir
O
O
Ir
OC CO
Ir
(CO)
H
Ir
(CO)3
+
+
C
Ir
CO
Ir
OC
(CO)2
4 (28%)
5 (1%)
Scheme 1.
4 are consistent with the proposed structure. The IR spectrum of 1
shows one bridging carbonyl absorption at 1732 cmꢁ1, while the IR
spectrum of 4 shows two bridging carbonyl absorptions at 1785 and
1733 cmꢁ1 [27]. Their 1H NMR spectra show a singlet for the methyl
of phenyl groups appear at
d
¼ 7.27e6.91 ppm. Its 13C NMR spec-
trum shows one resonance attributable to the terminal carbonyl at
172.3 ppm. These spectroscopic data are close to those of the
known literature [30,31].
protons of Cp* (
the phenyl groups of complex 4 are present at
which are significantly shifted to higher frequency in comparison
with that of complex 1 (
d
¼ 1.32 ppm for 1,
d
¼ 1.75 ppm for 4). The signals of
Complex 3 is a diiridium-o-phenylene complex, which is
confirmed by single-crystal X-ray diffraction analysis (Fig. 4). The
molecule possesses a C2 axis, bisecting the IreIr bond and the
phenylene ring. The planar phenyl ring builds a plane with the two
iridium atoms having a small dihedral angle of 3ꢀ. The average CeC
d
¼ 6.76e6.34 ppm,
d
¼ 7.14e6.72 ppm). Their 13C NMR spectra
show one resonance attributable to the bridging carbonyl group
ꢀ
(
d
¼ 212.3 ppm for 1,
d
¼ 210.2 ppm for 4). The rhodium analogues
bond distance of the phenylene ring is 1.380 A with individual
ꢀ
ꢀ
[Cp*Rh(
[Cp*Rh(
m
-CO)R]2 (R ¼ CH3, CH2CH3) were prepared by the reaction of
values ranging from 1.373(8) A to 1.386(8) A. The distance of the Ire
ꢀ
m-CO)]2 with Na/K alloy, followed by treatment with halo-
Ir bond is 2.736(1) A, which is slightly longer than that of
Cp2(CO)2Ir2(m2-C6H4) (2.7166(2) A) [9]. The spectroscopic data of 3
ꢀ
genated hydrocarbons [28]. The cobalt analogues [CpCo(
m-CO)R]2
(R ¼ CH3, CH2CH3, CH2CF3) were also prepared bya similar procedure
are consistent with the proposed structure. The IR spectrum shows
[29].
terminal carbonyl absorption at 1941 cmꢁ1. In its 1H NMR spectrum,
Complex 2 is a mononuclear iridium complex Cp*Ir(CO)(Ph)2,
which is confirmed by single-crystal X-ray diffraction analysis
(Fig. 3). Its IR spectrum shows terminal carbonyl absorption at
1977 cmꢁ1. In the 1H NMR spectrum of 2, the signals of methyl
the signals of methyl protons of Cp* appear at
singlet, while the signals of phenyl groups appear at
6.67 ppm as two doublet of doublets.
d
¼ 2.08 ppm as a
d
¼ 7.20e
Single-crystal X-ray diffraction analysis shows that complex 5 is
protons of Cp* appear at
d
¼ 1.80 ppm as a singlet, while the signals
a hexanuclear iridium cluster (Fig. 5). The bridging hydride is
Fig. 2. ORTEP view of cis-[Cp*Ir(
m
-CO)(Ph)]2 (4) showing 30% ellipsoids. Hydrogen
ꢀ
atoms have been omitted for clarity. Selected bond distances (A) and angles (deg):
Ir(1)eIr(2) 2.7219(11), Ir(1)eC(1) 2.021(4), Ir(1)eC(2) 2.023(4), Ir(1)eC(3) 2.071(4),
Ir(2)eC(1) 2.020(4), Ir(2)eC(2) 2.005(4), Ir(2)eC(19) 2.079(4), C(19)eIr(2)eIr(1)
97.03(11), C(3)eIr(1)eIr(2) 95.89(11), C(1)eIr(1)eC(2) 92.65(16), C(2)eIr(2)eC(1)
93.23(16), Ir(2)eC(1)eIr(1) 84.70(15), Ir(2)eC(2)eIr(1) 85.03(16).
Fig. 1. ORTEP view of trans-[Cp*Ir(
m
-CO)(Ph)]2 (1) showing 30% ellipsoids. Hydrogen
ꢀ
atoms have been omitted for clarity. Selected bond distances (A) and angles (deg):
Ir(1)eIr(1A) 2.7113(5), Ir(1)eC(1) 2.006(6), Ir(1)eC(1A) 2.000(6), Ir(1)eC(12) 2.071(6),
Ir(1A)eIr(1)-eC(12) 91.3(2), C(1A)eIr(1)eC(1) 94.8(2), Ir(1A)eC(1)eIr(1) 85.2(2).

74
X. Tan et al. / Journal of Organometallic Chemistry 735 (2013) 72e79
Fig. 3. ORTEP view of Cp*Ir(CO)(Ph)2 (2) showing 30% ellipsoids. Hydrogen atoms have
ꢀ
been omitted for clarity. Selected bond distances (A) and angles (deg): Ir(1)eC(1)
1.839(6), Ir(1)eC(12) 2.072(5), Ir(1)eC(18) 2.088(5), C(12)eIr(1)eC(18) 91.3(2).
Fig. 5. ORTEP view of [Cp*2(m2-Ph)(m2-H)Ir2](m3-CO2)[Ir4(CO)11] (5) showing 30% el-
lipsoids. Hydrogen atoms except for the m2-H have been omitted for clarity. Selected
bond distances (A) and angles (deg): Ir(1)eIr(2) 2.7302(5), Ir(1)eIr(3) 2.7554(7), Ir(1)e
located and refined crystallographically. The molecule has
approximate Cs symmetry, with the symmetric plane passing
through the phenyl ring, C(12), Ir(1), and Ir(3). The tetranuclear
iridium cluster moiety [Ir4(CO)11] is connected to the dinuclear
iridium moiety [Cp*2(m2-Ph)(m2-H)Ir2] through a carboxyl group,
which donates a total of four electrons to this complex. The mole-
cule of complex 5 has a total of 92 valence electrons, so metale
metal bonds should donate 16 electrons according to the EAN
rule. Because there are six IreIr single bonds in tetranuclear iridium
cluster moiety [Ir4(CO)11], it should be an IreIr double bond be-
ꢀ
Ir(4) 2.7191(6), Ir(2)eIr(3) 2.7230(5), Ir(2)eIr(4) 2.7389(7), Ir(3)eIr(4) 2.7310(5), Ir(5)e
Ir(6) 2.7049(7), Ir(1)eC(12) 2.078(6), Ir(5)eO(13) 2.108(4), Ir(5)eC(23) 2.258(6), Ir(5)e
H 1.84(7), Ir(6)eO(12) 2.110(4), Ir(6)eC(23) 2.224(6), Ir(6)-eH 1.66(7), C(23)eC(24)
1.415(8), C(24)eC(25) 1.391(9), C(25)eC(26) 1.383(10), C(26)eC(27) 1.376(9), C(27)e
C(28) 1.383(8), C(23)eC(28) 1.399(9), O(13)eIr(5)eC(23) 90.7(2), O(13)eIr(5)er(6)
83.78(12), C(23)eIr(5)eIr(6) 52.30(16), O(12)eIr(6)eC(23) 91.1(2), O(12)eIr(6)eIr(5)
83.30(11), C(23)eIr(6)eIr(5) 53.47(17), C(12)eO(12)eIr(6) 124.2(4), C(12)-eO(13)e
Ir(5) 123.1(4), O(12)eC(12)eO(13) 124.7(6), O(12)eC(12)eIr(1) 118.7(4), O(13)eC(12)e
Ir(1) 116.6(5), C(28)eC(23)eC(24) 116.8(6), Ir(6)eC(23)eIr(5) 74.24(19).
ꢀ
tween the Ir(5) and Ir(6). The length of Ir(5)eIr(6) (2.7049(7) A) is
found in Ir4(CO)12, which has a disordered structure in the solid
state [36]. It is noteworthy that the phenyl group coordinates with
the two metal centers in a three-center two-electron bond mode.
This bridging coordination mode of phenyl-carbon has been re-
ported for several other metal centers [37e44]. However, only two
examples of rhodium [37] and iridium [38] were known, with the
phenyl group involved in an NCN or PCP pincer ligand. To the best of
our knowledge, complex 5 is the first stable complex with the two
iridium atoms coordinated with an unfunctional phenyl group in
this bridging mode. On the phenyl ring, there are two longer CeC
ꢀ
similar to a normal IreIr double bond [2.71e2.73 A] [32e35]. The
ꢀ
other six IreIr bond distances range from 2.7191(6) to 2.7554(7) A.
ꢀ
These values are slightly longer than the IreIr distance of 2.692 A
ꢀ
ꢀ
bonds [C(23)eC(24) ¼ 1.415(8) A, C(28)eC(23) ¼ 1.399(9) A], and
ꢀ
ꢀ
four shorter CeC bonds in the range from 1.376(9) A to 1.391(9) A.
The length of Ir(1)eC(12) (2.078(6) A) is significant longer than the
ꢀ
lengths of IreC (terminal carbonyl), which are ranging from
ꢀ
1.856(7) to 1.921(8) A. The spectroscopic data of 5 are consistent
with the proposed structure. The IR spectrum shows terminal
carbonyl absorptions at 2067e1826 cmꢁ1. In its 1H NMR spectrum,
the signal of methyl protons of Cp* appears at
singlet, while the signals of the bridging hydride appear at
d
¼ 1.70 ppm as a
d
¼ ꢁ11.6 ppm as a singlet. The signals of five protons of the
bridging phenyl group appear at
blets and three triplets.
d
¼ 7.99e5.90 ppm as two dou-
Complex 5 possesses an Ir52þ core in [Cp*2(m2-Ph)(m2-H)Ir2]
moiety, and an Irþ4 core in [Ir4(CO)11] moiety. Cotton and coworkers
reported the only examples for iridium complexes with Ir52þ cores,
which have a diiridium paddlewheel structure with a mixed-valent
IrIIeIrIII system [45]. We suggest two possible resonance structures
5a and 5b for 5, with a similar IrIIeIrIII system (Scheme 2). The 1H
NMR spectrum shows only a singlet for the methyl protons on the
Fig. 4. ORTEP view of Cp*2(CO)2Ir2(m2-C6H4) (3) showing 30% ellipsoids. Hydrogen
atoms have been omitted for clarity. Selected bond distances (A) and angles (deg):
Ir(1)eIr(1A) 2.7364(12), Ir(1)eC(1) 1.826(6), Ir(1)eC(2) 2.096(6), C(2)eC(2A) 1.382(10),
C(2)eC(3) 1.373(8), C(3)eC(4) 1.386(8), C(4)eC(4A) 1.377(10), C(2)eIr(1)eIr(1A)
71.11(14), Ir(1)eC(2)eC(2A) 108.8(2), C(1)eIr(1)eC(2) 85.0(2).
ꢀ

X. Tan et al. / Journal of Organometallic Chemistry 735 (2013) 72e79
75
(CO)2
Ir
OC
+2.5 Ir
O
O
+1
(CO)
H
Ir
C
Ir
(CO)3
CO
Ir
+2.5
Ir
OC
(CO)2
5
(CO)2
Ir
(CO)2
Ir
OC
OC
+2
Ir
+3
Ir
O
O
O
H
O
+1
+1
(CO)
(CO)
H
Ir
(CO)3
C
Ir
Ir
(CO)3
C
Ir
CO
Ir
CO
+3Ir
Ir
+2
OC
OC
Ir
(CO)2
(CO)2
5b
5a
Scheme 2.
two Cp* rings, indicating that the delocalized structure of 5 is more
reasonable than the resonance structures 5aeb.
affording CoI complex CpCo(CO)2 and CoIII complexes CpCo(CO)R2
[29]. We found that the disproportionation of 4 is cleaner in the
presence of CO. Heating 4 with CO in THF at 100 ꢀC for 3 h afforded
complexes 1 (14%), 2 (36%), Cp*Ir(CO)2 (31%), with complete disap-
pearance of 4 (Scheme 4). Differently, heating 1 in THF at 100 ꢀC for
3 h afforded no product, andheating 1 with CO inTHFat 100 ꢀC for 3 h
afforded 2 (3%), Cp*Ir(CO)2 (3%), and unreacted 1 (84%) (Scheme 4).
As showed in Scheme 3, small amounts of biphenyl could also be
obtained by thermolysis of 4. We firstly considered that biphenyl
may be formed from 2 by reductive elimination. However, ther-
molysis of 2 or heating 2 with CO didn’t afford any biphenyl product.
According to these results, biphenyl should be formed directly from
complex 4 rather than complex 2.
2.2. Reactivity and possible mechanism explanations
According to the known literature [9e17], photolysis of
Cp*Ir(CO)2 with benzene should afforded compound Cp*Ir(-
CO)(H)(Ph). Unfortunately, we could not isolate this compound
because it was very air-sensitive. So we carried out two further
experiments to confirm it (see the Experimental section). In the
first experiment, Cp*Ir(CO)2 was irradiated in benzene under ni-
trogen, after removal of solvent the 1H NMR spectrum of the re-
action mixture in C6D6 showed the signal of the IreH at
d
¼ ꢁ15.07 ppm as a broad singlet. In the second experiment,
According to the above results, a plausible mechanism for
thermolysis of 4 was showed in Scheme 5. Firstly, the complex 4
isomerizes to an intermediate 4a, which possesses a bridging
phenyl group connected to two iridium atoms. Subsequently, 4a
isomerizes to an intermediate 4b, which easily reacts with CO to
afford 2 and Cp*Ir(CO)2. Alternatively, 4a could also be converted to
[Cp*Ir(CO)]2 by reductive elimination of biphenyl. Sutton and co-
workers postulated a diiridium(IreIr) intermediate with a bridging
phenyl group to explain the spectroscopic data of [Cp*2Ir2(CO)2(-
C6H4OMe)][BF4] in solution [55]. However, they haven’t provided
more evidence to confirm this intermediate. In this paper, the
structure of complex 5 supports the existence of these bridging
phenyl intermediates.
Cp*Ir(CO)2 was irradiated in benzene under nitrogen, and then the
reaction mixture was treated with CHBr3, affording a stable com-
pound Cp*Ir(CO)(Br)(Ph) (6). These results strongly suggested the
formation of compound Cp*Ir(CO)(H)(Ph) after the photolysis of
Cp*Ir(CO)2 with benzene in the absence of oxygen.
Sutton and coworkers reported that [Cp*Ir(H)(CO)2][BF4] was
converted to the diiridium(IreIr) complex [Cp*2Ir2(CO)4][[BF4]2 on
exposure to air [23]. This reaction is similar to the formation of
complexes 1 and 4, in which the IrIIIeH complex reacts with oxygen
affording the diiridium(IreIr) complex. Although the precise
mechanism of the formation of these diiridium(IreIr) complexes
has remained unclear, previously reported results [23e26,46e54]
imply that
a
hydroperoxide intermediate Cp*Ir(CO)(OOH)(Ph)
The mechanisms involved in the formation of complexes 3 and 5
are still unclear at present. No experimental evidence shows that
complex 3 could be produced from complexes 1, 2, or 4. Rausch and
coworkers suggested a plausible reaction pathway for the forma-
tion of the analogue Cp2(CO)2Ir2(m2-C6H4) [9], and complex 3 may
be formed by a similar pathway. The formation of complex 5 is very
complicated. Although the ligand induced metal cleavage from Cp
ring was unambiguously established [56,57], no similar chemistry
of decoordination of iridium atoms from the Cp* ligand to form the
Ir4 cluster species was reported. Recently, Finke and coworkers
reported a hydrogenation reaction of [Cp*RhCl2]2, affording the Rh4
subnanometer clusters [58]. Our investigations relating to the
mechanisms involved in the formation of complexes 3 and 5 are
therefore continuing.
may be formed by the reaction of Cp*Ir(CO)(H)(Ph) with O2, pro-
ceeding through O2 insertion into the IreH bond. Wayland and
coworkers reported the decay of the hydroperoxide rhodium
complex (TMP)Rh(OOH) afforded the radical (TMP)RhIIꢂ, H2O, and
O2 [50]. We suggest that a 17-electron radical species Cp*(CO)(Ph)
IrIIꢂ may be formed by a similar process, followed by dimerization to
gave the diiridium(IreIr) complexes.
The two isomers 1 and 4 show absolutely different reactivity.
Heating 4 inTHFat 100 ꢀC for 3 h afforded 1 (28%), 2 (11%), Cp*Ir(CO)2
(10%), [Cp*Ir(CO)]2 (8%), small amounts of biphenyl, and unreacted 4
(14%), while heating 1 in the same reaction condition afforded no
product (Scheme 3). These results indicate that trans-isomer 1 is
more stable product in thermodynamic. In this reaction, IrII complex
4 disproportions to the IrI complex Cp*Ir(CO)2 and IrIII complex 2.
Bergman and coworkers reported similar disproportionation re-
In summary, we have studied the reaction of sequential
photolysis of Cp*Ir(CO)2 with benzene and exposure to air, and
iridium complexes 1e5 and trace of biphenyl were obtained.
actions of CoII complexes [CpCo(
m-CO)R]2 (R ¼ CH3, CH2CH3, CH2CF3),

76
X. Tan et al. / Journal of Organometallic Chemistry 735 (2013) 72e79
Ir
Ir
Ir
CO
Ir
THF
100 oC
3 h
+ Cp*Ir(CO)(Ph)2 + Cp*Ir(CO)2 + [Cp*Ir(CO)]2 + Ph-Ph
CO
OC
OC
4
1
2
8%
3%
28%
11%
10%
(14% recovery)
Ir
CO
Ir
THF
100 oC 3 h
no product
OC
1
THF
100 oC 3 h
no product
Cp*Ir(CO)(Ph)2
Scheme 3.
Thermolysis of complex 4 afforded complexes 1, 2, Cp*Ir(CO)2,
[Cp*Ir(CO)]2, and biphenyl. We suggest disproportionation of 4 to 2
and Cp*Ir(CO)2 may undergo an intermediate 4a, with a bridging
phenyl group connected to two iridium atoms. Compared to com-
plex 4, thermolysis of complexes 1 or 2 afforded no product. These
results indicate that products 2 and biphenyl should both be
formed from complex 4. Complex Cp*2(CO)2Ir2(m2-C6H4) (3) pos-
FT-IR spectrometer. Elemental analyses were performed on a
PerkineElmer 240C analyzer. Cp*Ir(CO)2 [59] and [Cp*Ir(CO)]2 [55]
were prepared by literature methods.
3.2. Synthesis of iridium complexes 1e5 from sequential photolysis
of Cp*Ir(CO)2 with benzene and exposure to air
sesses a similar structure to the known Cp analogue Cp2(CO)2Ir2(m2
-
A solution of Cp*Ir(CO)2 (250 mg, 0.652 mmol) in 20 mL benzene
was added to a quartz Schlenk tube under nitrogen. The Schlenk
tube was placed in a large water bath, and the reaction solution was
irradiated with a Hanovia lamp (450 W, Vycor filter) for 10 h. The
resulting solution was on exposure to air overnight with magnetic
stirring. Then the solvent was removed in vacuo under reduced
pressure, and the residue was absorbed onto a small amount of
alumina. Flash column chromatography on alumina yielded a
colorless band of biphenyl (7 mg), using hexane as eluant, followed
by a yellow band of Cp*Ir(CO)2 (4 mg, 2%), eluted with 1:15 CH2Cl2/
hexane. Increasing the polarity of the solvent allowed the succes-
sive elution of red-brown (1:8 CH2Cl2/hexane, 1, 25 mg, 9%), yellow
(1:4 CH2Cl2/hexane, 2 þ 3, 26 mg), red-brown (1:2 CH2Cl2/hexane,
4, 79 mg, 28%), and red band (1:1 CH2Cl2/hexane, 5, 2 mg,1%). If this
column chromatography process is too slow, yield of 4 will be
dramatically decreased. Yields of products 2 and 3 were estimated
by the 1H NMR spectrum of the mixture of 2 and 3 (6% for 2, and 2%
for 3). Analytically pure samples of 2 (colorless solid) and 3 (yellow
solid) were obtained by recrystallization of this mixture with ether/
hexane.
C6H4). Complex 5 [Cp*2(m2-Ph)(m2-H)Ir2](m3-CO2)[Ir4(CO)11] is a
novel hexanuclear iridium cluster, which represents a rare example
of complexes with Ir52þ cores. Moreover, the bridging phenyl
structure of complex 5 supports the existence of intermediate 4a.
These results may help us gain more insights into the reactivity of
iridiumehydrogen complexes toward oxygen and promote their
application in catalysis.
3. Experimental
3.1. General considerations
Schlenk- and vacuum-line techniques were employed for all
manipulations. All solvents were distilled from appropriate drying
agents under argon before use. 1H and 13C NMR spectra were
recorded using a Bruker AV400 spectrometer. ESI mass spectra
were measured on a Finnigan LCQ Advantage instrument, while
MALDI mass spectra were measured on an Autoflex III TOF/TOF200
instrument. IR spectra were recorded as KBr disks on a Nicolet 380
Ir
Ir
Ir
CO, THF
100 oC, 3 h
CO
+ Cp*Ir(CO)(Ph)2 + Cp*Ir(CO)2
CO
OC
OC Ir
4
1
2
14%
36%
31%
Ir
CO
CO, THF
100 oC, 3 h
Cp*Ir(CO)(Ph)2 + Cp*Ir(CO)2
OC Ir
1
2
(84% recovery)
3%
3%
Scheme 4.

X. Tan et al. / Journal of Organometallic Chemistry 735 (2013) 72e79
77
Ir
Ir
Ir
Ir
OC
OC
CO
OC
Ir
Ir
CO
4b
CO
4
4a
CO
Ph-Ph + [Cp*Ir(CO)]2
Cp*Ir(CO)2 + Cp*Ir(CO)(Ph)2
2
Scheme 5.
3.2.1. Compound 1
M.p. 254e256 ꢀC (dec.). Anal. Calc. for C34H40Ir2O2: C, 47.20; H,
4.66. Found: C, 46.93; H, 4.73%. 1H NMR (CDCl3, 400 MHz)
7.14 (d,
J ¼ 7.1 Hz, 4H, Ph), 6.85 (t, J ¼ 7.3 Hz, 4H, Ph), 6.72 (t, J ¼ 7.1 Hz, 2H,
Ph), 1.32 (s, 30H, C5Me5); 13C{1H} NMR (CDCl3, 100 MHz):
212.3
A solution of Cp*Ir(CO)2 (250 mg, 0.652 mmol) in 20 mL benzene
was added to a quartz Schlenk tube under nitrogen. The Schlenk
tube was placed in a large water bath, and the reaction solution was
irradiated with a Hanovia lamp (450 W, Vycor filter) for 5 h. Then
CHBr3 (200 mg, 0.791 mmol) was added, and the mixture was
stirred overnight. The solvent was removed under reduced pres-
sure, and the residue was absorbed onto a small amount of alumina.
The purification was performed by flash column chromatography
on neutral alumina gel with CH2Cl2/hexane (1:2) to give a yellow
solid 6 (151 mg, 45%).
d
d
(CO), 139.5, 127.2, 125.2, 122.8, 101.2 (C5Me5), 7.9 (C5Me5); ESI-MS,
m/z: 866 [M]þ (based on 193Ir); IR (nCO, cmꢁ1): 1732 (s).
3.2.2. Compound 2
M.p. 150e152 ꢀC. Anal. Calc. for C23H25IrO: C, 54.20; H, 4.94.
Found: C, 54.14; H, 5.09%. 1H NMR (CDCl3, 400 MHz)
d
7.27e7.23 (m,
4H, Ph), 6.94e6.89 (m, 6H, Ph), 1.80 (s, 15H, C5Me5); 13C{1H} NMR
3.3.1. Compound 6
(CDCl3, 100 MHz):
d
172.3 (CO), 140.1, 130.0, 128.0, 122.6, 100.2
M.p. 168e169 ꢀC. Anal. Calc. for C17H20BrIrO: C, 39.84; H, 3.93.
(C5Me5), 9.0 (C5Me5); ESI-MS, 510 [M]þ, 433 [M-Ph]þ, 405 [M-Ph-
CO]þ (based on 193Ir); IR (nCO, cmꢁ1): 1977 (s).
Found: C, 39.96; H, 4.03%. 1H NMR (CDCl3, 400 MHz)
d
7.41 (d,
J ¼ 7.1 Hz, 2H, Ph), 7.02 (t, J ¼ 7.3 Hz, 2H, Ph), 6.96 (t, J ¼ 7.1 Hz, 1H,
Ph), 1.82 (s, 15H, C5Me5); 13C{1H} NMR (CDCl3, 100 MHz):
169.7
d
3.2.3. Compound 3
(CO), 140.8, 129.6, 128.7, 124.0, 100.4 (C5Me5), 9.1 (C5Me5); ESI-MS,
M.p. 173e175 ꢀC. Anal. Calc. for C28H34Ir2O2: C, 42.73; H, 4.35.
m/z: 433 [M ꢁ Br]þ (based on 193Ir); IR (nCO, cmꢁ1): 1997 (s).
Found: C, 42.64; H, 4.58%. 1H NMR (CDCl3, 400 MHz)
d 7.20 (dd,
J ¼ 5.0, 3.3 Hz, 2H, C6H4), 6.67 (dd, J ¼ 5.0, 3.3 Hz, 2H, C6H4), 2.08 (s,
30H, C5Me5); ESI-MS, m/z: 788 [M]þ (based on 193Ir); IR (nCO, cmꢁ1):
1941 (s).
3.4. Thermolysis of 4 in THF
A solution of 4 (20.0 mg, 0.0231 mmol) in THF (5 mL) was heated
at 100 ꢀC for 3 h to give a brown solution. Then the solution was
concentrated and transferred into an NMR tube, and the internal
standard reagent (dioxane) was added. Approximate NMR yields
are: 14% of unreacted 4, 28% of 1, 11% of 2, 10% of Cp*Ir(CO)2, 8% of
[Cp*Ir(CO)]2, and 3% of biphenyl (based on 4). Column chroma-
tography of the resulting mixture gave analytically pure samples of
1, 2, 4, Cp*Ir(CO)2, and biphenyl, which are further confirmed by the
IR spectrum and other spectroscopic data. Unfortunately, product
[Cp*Ir(CO)]2 could not be isolated from the mixture owing to
adsorption in column chromatography. However, we could identify
this complex from the spectroscopic data of the mixture. The IR
spectrum of the mixture shows a bridging carbonyl absorption at
1683 cmꢁ1 (KBr disk) for the bridging carbonyl of complex
[Cp*Ir(CO)]2. Its 1H NMR spectrum shows signals of methyl protons
3.2.4. Compound 4
M.p. 189e191 ꢀC (dec.). Anal. Calc. for C34H40Ir2O2: C, 47.20; H,
4.66. Found: C, 47.14; H, 4.69%. 1H NMR (CDCl3, 400 MHz)
d 6.76 (d,
J ¼ 6.8 Hz, 4H, Ph), 6.38e6.32 (m, 6H, Ph), 1.75 (s, 30H, C5Me5); 13
C
{1H} NMR (CDCl3, 100 MHz):
d 210.2 (CO), 137.7, 126.2, 123.4, 121.7,
100.6 (C5Me5), 8.8 (C5Me5); ESI-MS, m/z : 866 [M]þ, 889 [M þ Na]þ
(based on 193Ir); IR (nCO, cmꢁ1): 1785 (s), 1733(s).
3.2.5. Compound 5
M.p. 169e170 ꢀC (dec.). Anal. Calc. for C38H36Ir6O13: C, 24.62; H,
1.96. Found: C, 24.80; H, 2.07%. 1H NMR (CDCl3, 400 MHz)
d 7.99 (d,
J ¼ 7.6 Hz, 1H, Ph), 7.83 (t, J ¼ 7.3 Hz, 1H, Ph), 6.61 (t, J ¼ 7.2 Hz, 1H,
Ph), 6.46 (t, J ¼ 7.5 Hz, 1H, Ph), 5.90 (d, J ¼ 7.3 Hz, 1H, Ph), 1.70 (s,
15H, C5Me5), ꢁ11.62 (s, 1H, m2-H); IR (nCO, cmꢁ1): 2067 (w), 2031 (s),
2007 (s), 1983 (s), 1962 (s), 1826 (s).
of Cp* at
d
¼ 1.77 ppm as a singlet, while its MALDI mass spectrum
shows the ions at m/z 711.13 due to [M þ H]þ, 681.13 due to [Me
COeH]þ, and 463.19 due to [Cp*2Ir]þ. This spectroscopic data is
very closed to literature results [55,60].
3.3. Evidence experiments for the formation of compound
Cp*Ir(CO)(H)(Ph)
3.5. Attempted thermolysis of 1 and 2
A solution of Cp*Ir(CO)2 (10 mg, 0.026 mmol) in 0.5 mL benzene
was added to a quartz NMR tube under nitrogen. The NMR tube was
placed in a large water bath, and the reaction solution was irradi-
ated with a Hanovia lamp (450 W, Vycor filter) for 5 h. Then the
solvent was removed under reduced pressure, and 0.5 mL C6D6 was
added. The 1H NMR spectrum of the mixture indicated only one
new product Cp*Ir(CO)(H)(Ph) was formed. 1H NMR (C6D6,
Using a procedure similar to that described for the thermolysis
of 4, thermolysis of 1 and 2 gave no product.
3.6. Heating 4 with CO in THF
A solution of 4 (20.0 mg, 0.0231 mmol) in THF (5 mL) was
bubbled by CO for 10 min. Then the reaction tube was heating at
100 ꢀC for 3 h to give a pale-yellow solution. Then the solution was
400 MHz)
d
7.67 (m, 2H, Ph), 7.09 (t, J ¼ 3.2 Hz, 3H, Ph), 1.61 (s, 15H,
C5Me5), ꢁ15.07 (br s, 1H, IreH).

78
X. Tan et al. / Journal of Organometallic Chemistry 735 (2013) 72e79
Table 1
Crystal data and summary of X-ray data collection.
1$2CHCl3
36H42Cl6Ir2O2
1103.80
113(2)
0.71073
Monoclinic
P21/n
9.0546(14)
19.519(3)
11.1913(16)
90
109.830(5)
90
1860.6(5)
2
1.970
7.607
2
3
4
5
Formula
fw
C
C23H25IrO
509.63
113(2)
0.71073
Monoclinic
P21/c
16.605(15)
8.863(8)
13.932(13)
90
111.946(11)
90
1902(3)
4
1.780
7.028
C28H34Ir2O2
786.95
113(2)
0.71073
Monoclinic
C2/c
12.547(5)
11.663(5)
17.982(8)
90
106.744(7)
90
2519.9(18)
4
2.074
10.573
C34H40Ir2O2
865.06
113(2)
0.71073
Monoclinic
P21/c
10.715(4)
27.545(9)
11.288(4)
90
118.032(3)
90
2940.8(18)
4
1.954
9.070
C38H36Ir6O13
1853.87
113(2)
0.71073
Monoclinic
P21/n
14.575(3)
16.556(4)
17.956(4)
90
100.170(3)
90
4264.8(17)
4
2.887
T, K
ꢀ
l
, A
Cryst syst
Space group
ꢀ
a, A
ꢀ
b, A
ꢀ
c, A
a
, deg
, deg
b
g
, deg
3
ꢀ
V, A
Z
Dcalc, g cmꢁ3
m
, mmꢁ1
18.710
F(000)
1060
992
1488
1656
3320
Cryst size, mm
0.20 ꢃ 0.18 ꢃ 0.12
2.09e27.93
17,151
4414/0.0476
213
0.20 ꢃ 0.18 ꢃ 0.10
1.32e28.15
18,067
4542/0.0695
231
0.20 ꢃ 0.18 ꢃ 0.12
2.37e25.02
11,114
2228/0.0777
150
0.20 ꢃ 0.18 ꢃ 0.12
2.15e27.89
23,045
6880/0.0453
353
0.20 ꢃ 0.18 ꢃ 0.10
1.66e27.90
54,228
10,177/0.0851
528
q
range, deg
No. of reflns collected
No. of indep reflns/Rint
No. of params
Goodness-of-fit on F2
1.089
0.990
0.925
1.066
0.884
R1, wR2 [I > 2
R1, wR2 (all data)
Largest diff. peak and hole (e A
s
(I)]
0.0359, 0.1001
0.0436, 0.1035
8.642, ꢁ1.416
0.0366, 0.0756
0.0474, 0.0799
1.983, ꢁ2.727
0.0264, 0.0524
0.0315, 0.0535
1.988, ꢁ1.535
0.0262, 0.0478
0.0333, 0.0492
1.788, ꢁ1.971
0.0297, 0.0538
0.0381, 0.0554
1.568, ꢁ2.837
ꢀꢁ3
)
concentrated and transferred into an NMR tube, and the internal
standard reagent (dioxane) was added. Approximate NMR yields
are: 14% of 1, 36% of 2, and 31% of Cp*Ir(CO)2. Column chromatog-
raphy of the mixture gave analytically pure samples of 1, 2, and
Cp*Ir(CO)2, which are further confirmed by the IR spectrum and
other spectroscopic data.
carbonyl, and Cp* groups of the model (Ir10). So only the structure
of the model (Ir1) was showed in Fig. 1.
Acknowledgments
We thank the National Natural Science Foundation of
China (Nos. 21072101 and 21072097) and the Research Fund
for the Doctoral Program of Higher Education of China (No.
20110031110009) for financial support.
3.7. Heating 1 with CO in THF
Using a procedure similar to that described for the reaction of 4
with CO, this reaction afforded 1, 2, and Cp*Ir(CO)2. The approxi-
mate NMR yields are: 84% of unreacted 1, 3% of 2, and 3% of
Cp*Ir(CO)2.
Appendix A. Supplementary material
CCDC-893378 (for 1), -893379 (for 2), -893380 (for 3), -893381
(for 4), and -893382 (for 5) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
3.8. Heating 2 with CO in THF
Using a procedure similar to that described for the reaction of 4
with CO, this reaction afforded no product.
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