Enhancements in catalytic reactivity and selectivity of homobimetallic complexes containing heteroditopic ligands

Article abstract of DOI:10.1039/c7dt01294b

Rh(i) and Ir(i) homobimetallic complexes were synthesised using a heteroditopic ligand system on a xanthene scaffold containing a monodentate N-heterocyclic carbene ligand and a bidentate bis(pyrazol-1-yl)methane ligand. The complexes were tested as catalysts for the two-step dihydroalkoxylation and two-step hydroamination/hydrosilylation reactions. This is the first known report of an organometallic group 9 complex, Ir(i) bimetallic complex, 13, to selectively favour the opposite spirocyclisation product from that reported in the literature, 14cvs.14b. The Ir(i) homobimetallic complex catalyses the intramolecular hydroamination reaction of alkynamines efficiently and proved to be a highly active catalyst for promoting the subsequent hydrosilylation of the pyrrolines; completing the hydrosilylation reactions in less than 40 seconds. A chloro-bridged bimetallic species was observed in the solid state, revealing that the COD co-ligands present underwent an oxidation.

Full text of DOI:10.1039/c7dt01294b

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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Enhancements in Catalytic Reactivity and Selectivity of
Homobimetallic Complexes Containing Heteroditopic Ligands
Mark R. D. Gatus,ab Roy T. McBurney,b Mohan Bhadbhadec and Barbara A. Messerleab*
Received 00th January 20xx,
Accepted 00th January 20xx
Rh(I) and Ir(I) homobimetallic complexes were synthesised using a heteroditopic ligand system on a xanthene scaffold
containing a monodentate N-heterocyclic carbene ligand and a bidentate bis(pyrazol-1-yl)methane ligand. The complexes
were tested as catalysts for the two-step dihydroalkoxylation and two-step hydroamination/hydrosilylation reactions. This
is the first known report of an organometallic group 9 complex, Ir(I) bimetallic complex, 13, to selectively favour the
opposite spirocyclisation product from that reported in the literature, 14c vs 14b. The Ir(I) homobimetallic complex
catalyses the intramolecular hydroamination reaction of alkynamines efficiently and proved to be a highly active catalyst
for promoting the subsequent hydrosilylation of the pyrrolines; completing the hydrosilylation reactions in less than 40
seconds. A chloro-bridged bimetallic species was observed in the solid state, revealing that the COD co-ligands present
underwent an oxidation.
DOI: 10.1039/x0xx00000x
www.rsc.org/
between the metal centres via delocalised electrons on the
scaffold.6 All of these factors have been well documented. In
1993, Stanley, et al. demonstrated a correlation between the
Introduction
The formation of C-X bonds via the addition of N-H, O-H or
Si-H bonds across unsaturated C-C or C-N bonds allows us to
build upon small molecules leading to highly sought after
pharmaceuticals, fine chemicals and materials. Transition
metal complexes have been used to catalyse reactions that
efficiently achieve these transformations, in particular
hydroalkoxylation,1 hydroamination2 and hydrosilylation3
reactions provide key routes to heterocycles, amines, and
ethers. Transition metal complexes that are effective for
promoting these reactions include complexes with both early
or late transition metal centres, including mono- and bimetallic
species. Bimetallic complexes that consist of two independent
monometallic sites bound to a single scaffold have been found
to be highly effective catalysts for a range of reactions
compared to their monometallic counterparts for a variety of
organic transformations including hydroformylation reactions
and olefin polymerisations.4
metal-metal distance and the activity of the bimetallic complex
which was used as a hydroformylation catalyst.4b Peris, et al.
have contributed significantly to understanding how the
scaffold and spatial proximity of the metal pairs influences the
degree of cooperativity observed in many catalysed reactions.
Peris’ group have developed scaffolds with a high degree of
rigidity through the use of fused and Y-shaped bis- or tris-N-
heterocyclic carbene (NHC) scaffolds (Fig. 1, 1). The metal-
metal intermetallic distances are well-defined due to the
rigidity of the ligands developed with these NHC systems,
however, this restricts the ability to vary the separation
between the metal centres.5b, 5d
We have previously demonstrated the reactivity of Rh(I)
and Ir(I) homoditopic homobimetallic complexes for promoting
the
two-step
dihydroalkoxylation
and
sequential
2-4
).7 Herein,
hydroamination/hydrosilylation reactions (Fig. 1,
we report the synthesis of a novel heteroditopic ligand
featuring N-heterocyclic carbene (NHC) and bis(pyrazol-1-
yl)methane (bpm) coordination sites on a xanthene scaffold.
Our interest lies in being able to assess the catalytic effect of
The choice of the scaffold used to accommodate bimetallic
complexes significantly influences the catalytic activity, this is
dependent on a number of factors including the spatial
4d
distance between the metal centres,4b, the rigidity of the
scaffold,5 and the degree of electronic communication
Fig. 1 Rh and Ir homobimetallic complexes anchored on various scaffolds.
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having two identical metal ions coordinated in different dicarboxaldehyde was achieved by recrystallising the desired
electronic environments within the same catalyst architecture. compound in 73% yield from hot n-hexane. Bis(pyrazol-1-
7
t
Being able to control metal coordination in a ditopic complex yl)methane was the first ligand to be attached to the Xan
would also open the possibility for heterobimetallic complexes scaffold via a cobalt catalysed condensation reaction of an in
to be synthesized. The coordination chemistry of this situ generated 1,1'-sulfinyldipyrazole with the carboxaldehyde
heteroditopic ligand was investigated with two monovalent present on
ions, Rh(I) and Ir(I). Both bimetallic complexes were tested as via an Ullman-type coupling reaction using a stoichiometric
catalysts in two-step dihydroalkoxylation reactions and amount of copper iodide to yield . Methylation of the
sequential hydroamination-hydrosilylation reactions. The unbound nitrogen of the imidazole ring of generated the
efficiency of the Rh(I) and Ir(I) bimetallic catalysts for the desired imidazolium salt ligand precursor [(LNHC)/(LBpm)tXan][I]
7. Finally, imidazole was attached to the scaffold
9
9
dihydroalkoxylation reactions was compared to our previously
(10). The generation of an imidazolium salt allows direct access
reported Rh(I) and Ir(I) homobimetallic catalysts.7 A chloro- to generating an NHC by simple deprotonation of the 2-
bridged bimetallic dimer was observed in the solid state. The position of the imidazolium ring.
species was found to be unusual given that both COD co-
ligands underwent an oxidation and the rhodium ions spectroscopy, mass spectrometry and elemental analysis. The
The formation of 10 was confirmed by 1H, 13C NMR
coordinated to the bpm are now in a +3 oxidation state.
key observation indicative of imidazolium formation was the
observed downfield shift of the proton resonance of the 2-
position of the imidazole ring from 7.10 to 10.36 ppm in the 1H
NMR spectrum (CDCl3). This observed downfield shift is typical
of the formation of imidazolium salts from imidazole.9
Results and discussion
Preparation of the Heteroditopic Ligand ([(LNHC)/(LBpm)tXan][I] (10)
The initial preparation of the xanthene scaffold 2,7-bis(1,1-
Preparation
of
Homobimetallic
Complexes
(12) and
dimethylethyl)-9,9-dimethylxanthene (tXan,
5
, Scheme 1a) was
inspired by the procedure reported by Rebek et al.8
Dibromination of was realised by an alternate means, a
mixture of and bromine in glacial acetic acid was heated for
[(Rh(CO)2ClLNHC)/(Rh(CO)2LBpm)tXan][BPh4]
[(Ir(CO)2ClLNHC)/(Ir(CO)2LBpm)tXan][BPh4] (13)
5
Preparation of the Rh(I) and Ir(I) homobimetallic complexes
from 10 can be achieved either by deprotonation of the
imidazolium ring or transmetallation of a silver carbene
complex. We chose the transmetallation route as there are
numerous reports on the synthesis of monometallic Rh(I) and
Ir(I) NHCs via transmetallation of Ag(I) NHC complexes
(Scheme 1b).9b, 9c The ligand precursor (10) was reacted with
5
20 h achieving the same yield as that reported by Rebek.8 The
following step was key in allowing us to produce the
asymmetric ligand precursor (10), we were able to selectively
lithiate one bromine position of
of phenyllithium (PhLi), this allowed for the electrophilic
addition of the aldehyde to form the carboxaldehyde ( ).
was controlled by adding PhLi dropwise
6 using one molar equivalent
7
Ag2O
to
furnish
the
silver
carbene
complex
Mono-lithiation of
6
[(AgILNHC)/(LBpm)tXan] (11) quantitatively. 1H and 13C NMR
while maintaining the temperature of the reaction at -78 oC.
Purification to remove any unreacted material and/or the
spectroscopy confirmed that the reaction was successful with
1. 1 eq. PhLi,
-78 oC, 1 h
Br
Br
Br
CHO
a)
2. DMF, -78 oC - r.t,
O
O
O
O
Cl
Br2
FeCl3,
1 h
AcOH, 50 oC,
20 h, 82 %
THF, 73 %
DCM, 0 oC, 4 h
56 %
5
6
7
1. NaH, 0 oC,
30 min
I
N
N
N
N
N
N
N
N
N
N
N
N
H
2. SOCl2, 0 oC - r.t,
N
N
N
N
K2CO3,
imidazole, CuI
N
N
N
Br
N
N
N
MeI
CoCl2
30 min
S
O
O
O
THF, ꢀ, 16 h
85 %
DMF, 180 oC, 44 h
65 %
THF
O
CH3CN , 18 h
93 %
10
8
9
b)
[BPh4]
OC CO
M
I
1. [M(ꢁ-Cl)(COD)]2,
NaBPh4
N
N
N
N
N
N
N
N
N
N
N
N
N
N
OC
N
Ag
M
I
OC
N
N
N
2. CO(g)
0.75 Ag2O
Cl
O
O
O
Acetone, 18h
99 %
DCM
11
10
12: M = Rh, 57 %
13: M = Ir, 31 %
Scheme 1 a) Synthesis of the heteroditopic ligand scaffold (10), and b) Complexation of 10 to form the Rh(I) and Ir(I) homobimetallic complexes 12 and 13.
2 | J. Name., 2012, 00, 1-3
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the absence of the imidazolium proton previously observed at anthracene or benzene scaffolds.7a, 7c The activity of the Rh(I)
10.36 ppm in 10 as well as a downfield shift of the carbenic and Ir(I) homobimetallic catalysts 12 and 13 was tested in the
carbon resonance from 137.8 ppm for 10 to 184.8 ppm for 11 dihydroalkoxylation reaction of the alkynediols 2-(5-
which is characteristic of silver carbene complexes.
Transmetallation of the silver NHC complex
hydroxypent-1-ynyl)benzyl alcohol (14a, Fig. 3a) and 2-(4-
11) by hydroxybut-1-ynyl)benzyl alcohol (15a, Fig. 3b). The general
(
addition of [M(ꢁ-Cl)(COD)]2 (where M = Rh or Ir, COD = 1,5- procedure for the catalysed dihydroalkoxylation reactions
cyclooctadiene) and NaBPh4, followed by carbonylation of the involved dissolving the substrate and catalyst (0.5 mol%) in
COD complexes yielded the two homobimetallic complexes 1,1,2,2-tetrachloroethane-d2 at 100 °C, the reaction was
[(Rh(CO)2ClLNHC)/(Rh(CO)LBpm)tXan][BPh4],
12
[(Ir(CO)2ClLNHC)/(Ir(CO)LBpm)tXan][BPh4], 13, as yellow solids was determined by the relative integral ratios of select 1H
(57% and 31% yield respectively). resonances of the substrate and product(s). Turnover
and monitored by 1H NMR spectroscopy and the conversion (%)
Confirmation that metals had been added to both frequencies (TOF) were calculated at the point of 50%
1
coordination sites was achieved by analysing the H, 13C NMR conversion of substrate to product.
and IR spectra. The 13C NMR spectrum of the Ag(I) complex 11
We initially tested the Rh(I) homobimetallic complex (12
contained a carbene carbon resonance at 185 ppm (Fig. 2a). as a catalyst for the dihydroalkoxylation reaction of 14a
The 13C NMR spectrum of the rhodium bimetallic complex (12
Complex 12 was found to be extremely slow at catalysing the
)
.
)
exhibited five doublet resonances between 174-186 ppm with reaction, achieving 98% conversion in 17 hours with a turnover
1JC-Rh couplings between 44-76 Hz, and these were assigned to frequency (TOF) of 14 h-1-. It is well known that the addition of
the four carbonyl co-ligands of the two Rh(I) centres and the
a
halide abstracting agent (such as NaBArF (BArF
=
4
4
carbene carbon direct attached to Rh(I) (Fig. 2b). The IR tetrakis[3,5-bis(trifluoromethyl)phenyl]borate)) enhances the
spectrum of 12 revealed four distinct sharp signals at 2104, reactivity of transition metal NHC complexes that contain a
2087, 2045 and 2011 cm-1, representative of the carbonyl co- halide co-ligand. Complexes containing the BPh4 counterion
-
ligands. The 13C NMR spectrum of the iridium bimetallic have also been shown to be less reactive than their BArF
4
complex (13) exhibited five singlet resonances between 166- counterparts.7a, 10 To investigate the chloride/anion effect for
182 ppm also assigned to the carbonyl co-ligands of Ir(I) and the bimetallic complexes under investigation, the catalyst (12
)
carbene carbon directly attached to Ir(I) (Fig. 2c). The IR was re-tested for the dihydroalkoxylation reaction of 14a with
spectrum of 13 exhibited four distinct signals at 2092, 2073, the addition of two molar equivalents of NaBArF relative to
4
2027, and 1994 cm-1, representative of the carbonyl co-ligands. the catalyst. In this particular case we observed a significant
Multiple attempts to grow crystals of the two bimetallic enhancement in the reaction rate with the time to complete
complexes (12 and 13) containing BPh4 proved unsuccessful, converison reduced to 0.9 hours compared to 17 hours and
-
resulting in glass like materials containing entrapped the TOF was 43 fold greater (601 h-1). Substitution of the BPh4
microcrystals.
anion and salt methathesis of the chloride co-ligand with the
BArF anion for complex 12 was successful and led to a
-
4
Catalysed Dihydroalkoxylation Reactions of Alkynediols
significant enhancement in the reactivity of the catalyst. In
light of these promising results all other catalysed reactions
We were interested in how the Rh(I) and Ir(I)
homobimetallic complexes 12 and 13 compared as catalysts to
homobimetallic complexes previously reported which
contained a pair of homoditopic bpm or NHC ligands on
were tested with the addition of NaBArF into the reaction
4
mixture.
The heteroditopic Rh(I) bimetallic complex 12 was more
reactive than the Rh(I) bimetallic complex containing two NHC
ligands bound to a benzene scaffold (2
; TOF = 135 h-1, 3.8 h >
98 % conv.).7a The two bimetallic catalysts 12 and 13, did not
catalyse the reaction as efficiently as the previously reported
Rh(I) and Ir(I) homobimetallic complexes attached to
a
homoditopic pair of bpm ligands on an anthracene scaffold (
TOF = 9523 h-1, 4.8 min > 98% conv.,
> 98% conv.).7c
3
;
4
; TOF = 2468 h-1, 16 min
Of greater interest was the difference in the product
selectivity for the catalysed dihydroalkoxylation reaction of
14a using Ir(I) homobimetallic complex 13. In all previously
reported cases the ratio of products 14b
the reaction for Rh(I) and Ir(I) mono- and bimetallic complexes
was typically 3:1. Surprisingly the Ir(I) bimetallic complex (13
had the opposite selectivity yielding 14c as the favoured
product with a ratio of 1:2 (14b 14c). This is the first known
:14c formed during
)
Fig. 2 13C NMR (150.9 MHz) spectra of the carbene/carbonyl region of the a) Ag(I)
complex 11, (CDCl3), b) Rh(I) homobimetallic complex 12 (CD2Cl2), and c) Ir(I)
homobimetallic complex 13 (CD2Cl2).
:
report of a group 9 metal complex preferentially forming this
particular product. There has only been one other report of an
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Fig. 3 Reaction profiles monitored by 1H NMR spectroscopy for the dihydroalkoxylation reactions: a) 2-(5-hydroxypent-1-ynyl)benzyl alcohol (14a) using the Rh(I) and Ir(I)
homobimetallic complexes 12 (without NaBArF4 and with 1.0 mol % NaBArF4) and 13 (with 1.0 mol % NaBArF4); and b) 2-(4-hydroxybut-1-ynyl)benzyl alcohol (15a) using the Rh(I)
and Ir(I) homobimetallic complexes 12 and 13 (with 1.0 mol % NaBArF4) in 1,1,2,2,-tetrachloroethane-d2 at 100 °C. Note: The %conversion represents how much substrate has
converted into the corresponding products.
Table 1 Comparison of Rh(I) and Ir(I) homobimetallic complexes with Rh(I) and Ir(I)
Au(I) complex that was also successful in preferentially
forming 14c 11
.
monometallic complexes for the dihydroalkoxylation reactions of 14a and 15a.
Ir(I) bpm complexes have been shown to be more efficient
at forming 5-membered than 6-membered O-heterocycles
during the hydroalkoxylation reaction (this is reflected in the
slow reactivity of the Ir(I) bimetallic complex for the
dihydroalkoxylation reaction of 14a).12 Rh(I) however, favours
the formation of 6-membered O-heterocycles, hence, it’s
reactivity was greater for the dihydroalkoxylation reaction of
Time/min
(% conv.)
TOF
(h-1)e
Entry
1a
Cat.
Substrate
[BPh4
]
OC CO
Rh
N
14a
54 (98)
601
OC
Rh
N
N
N
OC
N
N
Cl
12
O
2a
3a,b
4a,b
5a
15a
14a
15a
14a
41 (94)
5 (98)
660
9523
>3030
88
14a 7c
homobimetallic complex
synergistically giving rise to the formation of the 5,6-spiroketal
14c) as the major product. Without a detailed mechanistic
.
In this case, the two Ir(I) metal centres of the Ir/Ir
13 appear to be behaving
3
(
)
2 (98)
[BPh4
]
OC CO
Ir
(
N
229 (98)
OC
Ir
N
N
N
N
OC
study, it is hard to comment on the exact nature of the
observed change of reactivity, however it is clear the use of
Ir(I) and the combination of the heteroditopic ligand is key.
The same trend in catalytic activity was observed for the
dihydroalkoxylation reaction of 15a, with Rh(I) homobimetallic
complex 12 catalysing the reaction in a much shorter time
(94% conversion after 0.7 hours, TOF = 660 h-1) compared to
the Ir(I) homobimetallic complex 13 which plateaued at 91%
conversion after 2.5 hours (TOF = 120 h-1). Both complexes 12
and 13 were more efficient at catalysing the cyclisation of this
particular alkynediol compared to 14a. In comparison, the
Rh(I) and Ir(I) homobimetallic complexes on the anthracene
scaffold are significantly more efficient at catalysing this
N
Cl
13
O
6a
15a
14a
15a
150 (91)
16 (98)
8 (98)
120
2468
2754
7a,b
8a,b
4
[BArF
]
9c
14a
15a
14a
15a
35 (98)
19 (98)
13 (98)
5.4 (98)
374
535
4
4
OC CO
Ir
N
N
N
N
10c
11c
12c
16
[BArF
]
OC CO
Rh
N
N
961
N
N
17
1121
N
13d
14a
228 (98)
28
OC
Rh
OC
N
Cl
18
3 4
; < 2 min, TOF = > 3030 h-1, ; < 9 min, TOF = 2754 h-
14d
15a
204 (41)
<1
reaction (
1).7c
aConditions – 0.5 mol % cat, 1 mol % NaBArF4, 2x10-4 mol substrate, 1,1,2,2-
tetrachloroethane-d2, 100 mol
C; bConditions cat, 2x10-4 mol
substrate, 1,1,2,2-tetrachloroethane-d2, 100
cConditions – 1 mol % cat, 1
°
–
1
%
Again, the selectivity of the two catalysts 12 and 13
behaved markedly differently to one another during the
°
C
mol % NaBArF4, 2x10-4 mol substrate, 1,1,2,2-tetrachloroethane-d2, 100
°C
dTOFs (Turn Over Frequency) were calculated at the point of 50% conversion
of substrate to product(s).
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reaction. We have previously reported the formation of a the amine (19a or 20a) at 100 °C in toluene-d8. Upon near
tetracyclic dimer (15c) when using a Rh(I) homobimetallic completion (>95% conversion) of the hydroamination reaction,
complex for the dihydroalkoxylation reaction of 15a 13
The two molar equivalents of Ph2SiH2 was added to the reaction
.
Rh(I) homobimetallic complex 12 also formed a substantial mixture and the mixture heated at 100 °C. The final silylated
amount of the same dimer 15c (30%), whereas the Ir(I) products did not have any traces of the initial amines (19a and
homombimetallic complex 13 solely forms the desired 20a), only the desired products were observed.
spiroketal 15b
cooperativity occuring between the Ir(I) metals centres giving reaction of 4-pentynamine (19a) and 5-phenyl-4-pentynamine
rise to the preferrential formation of 5-membered O- 20a was carried out using both the Rh(I) and Ir(I)
heterocycles compared to 6-membered O-heterocycles homobimetallic complexes 12 and 13 (Fig. 4b). The Ir(I)
favoured by Rh(I). bimetallic catalyst 13 outperformed the Rh(I) bimetallic
A comparison of the homobimetallic complexes 12 and 13 catalyst for the hydroamination of both 19a and 20a 19a; TOF
with the homobimetallic complexes 3 and 13) vs. 173
which contain a = 2913 h-1 13) vs. 2118 h-1 12), 20a; TOF = 432 h-1
pair of bis(pyrazol-1-yl)methane ligands on an anthracene h-1
12)), to form the pyrrolines 19b and 20b. Both complexes
scaffold ( and ) as well as the monometallic half units (16 17 12 and 13 were able to catalyse the hydroamination reaction
.
This again suggests that there is some
The sequential two-step hydroamination/hydrosilylation
(
)
(
4
(
(
(
(
3
4
,
and 18), with the exception of the Ir(I) derivative of the Rh(I) of 4-pentynamine (19a) more efficiently than that of 5-phenyl-
NHC complex (18) is shown in Table 1. It is clear from this table 4-pentynamine (20a). We attribute this to steric factors as the
that the two homobimetallic complexes are less reactive than alkyne of 19a is more easily approached (R = H) than 20a (R =
the Ir(I) and Rh(I) bpm complexes 16 and 17 when used as Ph). This steric difference would allow the exo-cyclisation to
catalysts for the dihydroalkoxylation reaction of 14a and 15a
However, the Rh(I) homobimetallic complex 12 was thermodynamically favoured product.
significantly more reactive than the Rh(I) NHC complex 18 To our surprise, the Ir(I) bimetallic catalyst (13) was also
.
occur more efficiently, considering this is the
,
demonstrating that the presence of the Rh(I) NHC moiety on extremely active for the hydrosilylation reaction of 2-
the homobimetallic complex 12 did not have a negative impact methylpyrroline (19b) and 2-benzylpyrroline (20b) compared
on the reactivity of the complex. The homobimetallic to the Rh(I) bimetallic complex (12). The Ir(I) catalyst (13) was
complexes 12 and 13 are significantly less reactive than the 25-fold more reactive for 19b and 9-fold more reactive for
homobimetallic complexes
3 and 4 taking up to 10 times 20b, compared to the Rh (I) complex (12). The hydrosilylation
longer to catalyse the dihydroalkoxylation reactions. However, reactions for 19b and 20b were both complete before the first
while we did not see any similar synergistic effects as that acquisition on the NMR spectrometer was collected (11 s and
observed in complexes
3 and 4 in terms of the speed of 35 s respectively), compared to the Rh(I) catalyst (12) which
reactivity, we did see a significant change in the selectivity reached 94% conversion of 19b after 47 min and >98%
when catalysing the reaction of 14a using complex 13 which conversion of 20b after 17 min. The calculated TOFs for the
was not observed with
3
or
4.
Ir(I) bimetallic catalyst for the hydrosilylation reactions were
>16200 h-1 and >5100 h-1 for 19b and 20b respectively
Catalysed Sequential Two-step Hydroamination-Hydrosilylation
Reactions of Alkynamines 19a and 20a
The intramolecular hydroamination reaction of alkynes and
alkenes is an atom economical method for producing
N-heterocyclic imines/amines. These motifs are prevalent in
many biologically active molecules and are a highly desirable
synthetic target in the chemical and pharmaceutical
industries.2 The reduction of imines can be carried out by a
number of different methods such as (transfer)
hydrogenation,14 or hydrosilylation.15 The catalysed
hydrosilylation reaction is an effective route for reduction of
alkenes, ketones, imines, and other unsaturated functional
groups as they avoid extreme reaction conditions such as high
temperatures and pressures that are commonly used for
hydrogenation reactions. We were interested in studying how
the Rh(I) and Ir(I) homobimetallic complexes would perform
for the sequential two-step hydroamination reaction of 4-
pentyn-1-amine (19a) and 5-phenyl-4-pentyn-1-amine (20a
)
followed by the hydrosilylation reaction of 2-methylpyrroline
Fig.
4
Reaction profiles monitored by 1H NMR spectroscopy for the two-step
(
19b) and 2-benzylpyrroline (20b) (Fig. 4a).
hydroamination/hydrosilylation reactions of 4-pentyn-1-amine (19a) and 5-phenyl-4-
pentyn-1-amine (20a) using the Rh(I) and Ir(I) homobimetallic complexes 12 and 13
in Toluene-d8 at 100 °C. Two molar equivalents of Ph2SiH2 was added to the reaction
mixture upon >95 % formation of 19b or 20b was achieved.
The reactions were carried out using 1 mol% of Rh(I) or Ir(I)
homobimetallic complexes 12 and 13, 2 mol% of NaBArF and
4
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Table 2 Comparison of Rh(I) and Ir(I) homobimetallic complexes 12 and 13 with Rh(I) and Ir(I) monometallic complexes 16-18 for the sequential two-step
hydroamination-hydrosilylation reactions of 19a and 20a.
Hydroamination Step
Hydrosilylation Stepd
e
)
Entry
1a
Cat.
Substrate
Time/min (% conv.)
TOF (h-1)e
2118
Time/min (% conv.)
TOF (h-1
[BPh4
]
OC CO
Rh
N
14a
OC
Rh
0.4 (>95)
4.6 (>95)
0.1 (>95)
1.5 (>95)
47.0 (94)
17.0 (>98)
0.2 (>98)
0.6 (>98)
642
N
N
N
OC
N
N
Cl
12
O
2a
3a
4a
15a
14a
15a
173
546
[BPh4
]
OC CO
Ir
N
OC
Ir
2913
>16200
>5100
N
N
N
N
OC
N
Cl
13
O
432
[BArF
]
]
5b
6b
7b
8b
14a
15a
14a
15a
4
OC CO
Ir
0.04 (>95)
3.6 (>95)
3983
62
1.0 (>98)
1.6 (>98)
>1365
>1694
N
N
N
N
16
[BArF
4
OC CO
Rh
0.4 (>95)
4.9 (>95)
1726
53
6.0 (>98)
459
199
N
N
N
N
17
18.0 (>98)
N
9c
14a
OC
0.6 (>95)
436
3.6 (>98)
365
Rh
OC
N
Cl
18
10c
15a
5.0 (>95)
140
13.2 (>98)
212
aConditions – 1.0 mol % cat, 2 mol % NaBArF4, 2x10-4 mol substrate, Toluene-d8, 100 °C; bConditions – 2.0 mol % cat, 2x10-4 mol substrate, Toluene-d8, 100 °C;
cConditions – 2.0 mol % cat, 2.0 mol % NaBArF4, 2x10-4 mol substrate, Toluene-d8, 100 °C; d4x10-4 mol of Ph2SiH2 was added to the mixture; eTOFs (Turn Over
Frequency) were calculated at the point of 50% conversion of substrate to product(s).
compared to the TOFs for the Rh(I) bimetallic catalyst (12
which was calculated to be 642 h-1 and 546 h-1 respectively.
)
anthracene scaffold in some cases, we have observed
enhanced selectivity for the dihydroalkoxylation reactions.
While there was no apparent enhancement in reactivity for
the hydroamination reactions of 19a and 20a when either of
the two bimetallic complexes 12 and 13 were tested as
catalysts compared to the monometallic complexes 16, 17 and
18 (Table 2). A significant rate enhancement was observed for
the hydrosilylation reactions of pyrrolines (19b and 20b) in the
reactivity of the Ir(I) homobimetallic complex 13 compared to
Observation of a chloro-bridged bimetallic dimer
Multiple attempts to grow crystals of the two bimetallic
complexes (12 and 13) containing BPh4 proved unsuccessful,
resulting in glass like materials containing entrapped
microcrystals. However, we were able to isolate X-ray quality
crystals of the chloride derivative of 21 this was achieved by
both the Rh(I) homobimetallic complex
(12) and the
complexation of Rh(I) to the silver carbene complex (11
without the addition of NaBPh4 (Scheme 2). Instead of the
expected structure, i.e. discrete Rh(I) homobimetallic
)
monometallic Rh(I) and Ir(I) complexes (16-18), with yielding
TOFs that were 10-fold higher than the other complexes.
a
complex 21, it was found to be a dimer containing two
molecules of 21 bridged through the Rh-chloride precursor
resulting in complex 22 (Scheme 2). The bridging chlorides
remained intact due to the absence of a chloride abstracting
Bimetallic Cooperativity
It is clear that two characteristically different (electronically
and sterically) ligand donors on
a scaffold significantly
1
agent such as NaBPh4. Interestingly, the H NMR spectrum of
the isolated complex 22 contained numerous broad 1H
influence the cooperativity between the two metal centres.
This led to some interesting reactivity and selectivity for the
Rh(I) and Ir(I) homobimetallic complexes 12 and 13 for the
intramolecular dihydroalkoxylation and the sequential
intramolecular hydroamination/hydrosilylation reactions.
Although the reactivity of the complex was not as efficient as
resonances belonging to the bpm moiety compared to the
1
BPh4 salt (12), which did not contain any broad H resonances.
This was a result of the κ1-coordination of the bpm ligand to
the Rh(I) centre, allowing a greater degree of flexibility for the
ligand moiety by undergoing rotation about the bpm-scaffold
bond as well as the bpm-metal bond.
the homobimetallic Rh(I) and Ir(I) complexes (3 and 4) on the
6 | J. Name., 2012, 00, 1-3
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Scheme 2. Synthesis of the Rh(I) homobimetallic complex 21 and the solid-state
structure of the complex (22) obtained by growing crystals of 21 in a mixture of
CH2Cl2/n-pentane.
Table 3. Selected bond lengths (Å) and angles (o) of the inner coordination
sphere of the Rh(I) centres for complex 22.
Atoms
Atoms
(Rh LBpm centre)
N2-Rh1
Distance
(Å)
Distance (Å)
(Rh LNHC centre)
C1a-Rh2
Cl1-Rh2
C20-Rh2
C21-Rh2
C24-Rh2
C25-Rh2
O
Cl
2.023(6)
2.367(2)
2.185(8)
2.214(8)
2.089(7)
2.101(5)
2.154(4)
2.484(1)
2.151(6)
2.087(6)
2.178(6)
2.053(5)
N
Rh
N
N
N
N
Cl2-Rh1
Cl Rh
N
N
N
N
N
N
C30-Rh1
C31-Rh1
C32-Rh1
C35-Rh1
Rh
Atmospheric
O2
HO
Cl
Rh
Cl
[Rh(COD)(ꢁ-Cl)]2
N
11
O
Cl
CH2Cl2, 1.5 h
OH
Rh
CH2Cl2/C5H12
(no NaBPh4 added)
N
N
N
N
N
Rh
Cl
N
Atoms
(Rh LNHC centre)
Atoms
(Rh LBpm centre)
21
Angles (°)
Angles (°)
O
C1a-Rh2-Cl1
Cl1-Rh2-C20
C20-Rh2-C25
C25-Rh2-C1a
90.1(2)
94.0(2)
81.4(3)
88.5(2)
N2-Rh1-C32
Cl2-Rh1-C30
Cl2-Rh1-C35
170.1(2)
166.3(2)
176.0(2)
22
In the solid state structure of complex 22 (Fig. 5), the
rhodium centre (Rh1) was coordinated to only one of the two
pyrazole rings ( 2 position). The expected bidentate chelation
between two previously reported catalysts where bimetallic
cooperativity was shown to be in effect. Clearly 22 is
significantly different from the catalysts, 12 and 13, under
study here, however it provides an insight into what the solid-
state structure of either catalyst may look like and potential
for bimetallic cooperativity leading to enhanced catalysis.
N
of the bpm ligand to the rhodium centre did not occur due to
the strong binding affinity of the bridging chloride co-ligands.
Of particular note is the coordination geometry of the rhodium
centre bound to the pyrazole ring (Rh1), it displayed a
distorted octahedral geometry with an oxidation state of +III.
Additionally, the 1,5-cyclooctadiene co-ligand bound to Rh1
atom had undergone an oxidation and isomerisation reaction
via the addition of molecular oxygen across the rhodium
centre. Similar structures have been reported in the literature
where an oxidised 1,5-cyclooctadiene is still coordinated to the
metal centre proceed through a bimetallic rhodium or iridium
intermediate.16
Experimental
All manipulations of metal complexes and air sensitive
reagents were carried out using standard Schlenk or vacuum
techniques,18 or in a nitrogen-filled or argon-filled Braun glove
box. Reagents were purchased from Aldrich Chemical Co. Inc.
or Alfa Aesar Inc. and were used as received. The metal halide
salts RhCl3.xH2O and IrCl3.xH2O were purchased from Precious
Metals Online PMO P/L and were used without further
purification. The metal precursors, [Pd(PPh3)4],18 [Rh(COD)(
Cl)]2,19 [Ir(COD)( -Cl)]2,20 NaBArF 21 were synthesised according
to literature methods. (2,7-Bis(1,1-dimethylethyl)-9,9-
The distance between the two metal centres (Rh···Rh) for
22 is 6.771(1) Å, which is 1.6 Å shorter than the Rh···Rh
intermetallic distance of the homobimetallic Rh(I) complex
(8.371(1) Å) reported by Choy et al.,7b and 2.6 Å greater than
the analogous distance in the Rh(I) bimetallic complex on the
xanthene scaffold reported previously (4.1937(5) Å).13
Therefore, the intermetallic distance in complex 22 lies
ꢁ-
ꢁ
4,
dimethylxanthene) (5) was synthesised according to the
procedure reported by Rebek et al.8 The substrates 2-(5-
hydroxypent-1-ynyl)benzyl alcohol (14a), 2-(4-hydroxybut-1-
ynyl)benzyl alcohol (15a),22 4-pentyn-1-amine (19a) and 5-
23
phenyl-4-pentyn-1-amine (20a
)
were synthesized following
literature procedures. The products formed during the
13, 22
catalysed dihydroalkoxylation (14b
,
14c,
23a
15b and 15c
)
and
the hydroamination (19b and 20b
)
were confirmed by
comparing the 1H NMR data with literature data. The N-
silylamines (19c and 20c) could not be isolated for complete
characterisation, therefore they were identified by diagnostic
signals in the 1H NMR spectra (see Supporting Information).
For the purposes of air sensitive manipulations and in the
preparation of metal complexes, solvents were dried and
distilled under an atmosphere of nitrogen or argon using
standard procedures24 and stored under nitrogen or argon in
glass ampoules, each fitted with a Young’s Teflon valve prior to
use. Tetrahydrofuran (THF), was distilled from sodium-
benzophenone ketyl. Dichloromethane (DCM), acetonitrile,
hexane, acetone and N,N-dimethylformamide (DMF) were
Fig 5. ORTEP depiction of the solid-state structure of the Rh(I) homobimetallic dimer
(22) with thermal ellipsoids at 50% probability for all non-hydrogen atoms. The Rh⋅⋅⋅Rh
intermetallic distance between the bpm and NHC ligand is 6.771(1) Å.
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obtained dry under nitrogen from
a PuraSolv solvent ethanol upon which a white precipitate formed. The solid was
purification system. Ethanol (EtOH) was distilled from collected by filtration and dried in air yielding a white solid (
diethoxymagnesium. The bulk compressed gases argon Yield: 10.9 g (22.6 mmol), 82%.
(>99.999%) and carbon monoxide (>99.5%) were obtained
6).
2,7-Ditertbutyl-4-bromo-5-CHO-9,9-dimethylxanthene (7)
from Air Liquide and used as received. Nitrogen gas for Schlenk
line operation came from in-house liquid nitrogen boil-off.
1H and 13C NMR spectra were recorded on Bruker Avance
III 300, 400, 500, and 600 spectrometers operating at 300, 400,
500, and 600 MHz (1H), respectively, and 75, 100, 125, and 150
MHz (13C), respectively. Unless otherwise stated, spectra were
recorded at 25 °C and chemical shifts (δ) are quoted in ppm.
Coupling constants (J) are quoted in Hz and have uncertainties
of ±0.05 Hz for 1H and ±0.5 Hz for 13C. 1H and 13C NMR
chemical shifts were referenced internally to residual solvent
resonances. Deuterated solvents were purchased from
Cambridge Stable Isotopes and used as received. Air-sensitive
NMR samples were prepared in an inert-gas glovebox or by
vacuum transfer of deuterated solvents into NMR tubes fitted
with a Young Teflon valve. For air-sensitive NMR samples
CD2Cl2 and CDCl3 were distilled over calcium hydride. Acetone-
d6 was distilled from calcium sulfate.
Compound 6 (5.03g, 10.5 mmol) was added to THF (200
o
mL) and the mixture was cooled to -78 C. Phenyllithium (1.8
M solution in di-n-butyl ether) (6.66 mL, 12.1 mmol) was
o
added dropwise over a period of 15 min and stirred at -78 C
for 1 hour followed by the dropwise addition of DMF at -78 oC.
The solution was allowed to warm to r.t and was stirred for
one hour. Water (100 mL) was added to the mixture and THF
was removed in vacuo leaving a white precipitate. The
precipitate was filtered and recrystallised from hot hexane
yielding the desired product white solid (7). Yield: 3.29 g (7.66
mmol), 73%. ESI-MS (MeOH), m/z (%): 451.41 (100) [M + Na]+
amu. Anal. Found: C, 67.96; H, 7.21. Calcd. for C24H29BrO2.0.25
C6H14 : C, 67.92; H, 7.26%.
2,7-Ditertbutyl-4-bromo-5-bpm-9,9-dimethylxanthene (8)
Pyrazole (3.05 g, 44.9 mmol) was added to a mixture of 60
% NaH in dispersion oil (1.80 g, 44.9 mmol) in THF (150 mL) at
o
0 C and the mixture was stirred for 30 min. SOCl2 (1.63 mL,
Infrared spectra were measured using a Nicolet 380 Avatar
FTIR spectrometer as solutions in dichloromethane.
Microanalyses were carried out at the Campbell
Microanalytical Laboratory, University of Otago, Otago, New
Zealand, or the Microanalytical Unit, Research School of
Chemistry, Australian National University, Canberra, Australia.
22.5 mmol) was added dropwise and the solution was stirred
at r.t for 30 min. CoCl2 (145 mg, 1.12 mmol) and 7 (3.22 g, 7.49
mmol) were added and the mixture was refluxed for 20 h. The
mixture was cooled to r.t; water (100 mL) was added and the
crude product was extracted from the aqueous phase with
DCM (3 x 50 mL). The organic layer was then stirred in a sat
solution of Na2EDTA for two hours and the organic layer was
separated and dried over MgSO4, filtered and the solvent
removed in vacuo. The crude material was dissolved in the
minimum amount of boiling EtOH, filtered and the solvent was
Mass spectra were acquired using
a Micromass ZQ
massspectrometer (ESI-MS) at the University of New South
Wales. M is defined as the molecular weight of the compound
of interest or cationic fragment for cationic metal complexes.
Single crystal X-ray analysis was carried out at the Australian
Synchrotron in Melbourne, Australia. X-ray diffraction
measurements were carried out on beamline 3-BM1 using
Synchrotron radiation (λ = 0.71073 Å). The software to collect
and refine compound 11 used was BLU-ICE,25 XDS,26
SHELXS97,27 OLEX-228 and SHELXL2013.29 CCDC-1497753
contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
removed from the filtrate yielding an off white solid (8). Yield:
3.47g (6.33 mmol), 85%. ESI-MS (MeOH), m/z (%): 569.52 (100)
[M + Na]+ amu. Anal. Found: C, 66.38; H, 5.92; N, 10.23. Calcd.
for C30H35BrN4O: C, 65.82; H, 6.44; N, 10.23%.
2,7-Ditertbutyl-4-imidazole-5-bpm-9,9-dimethylxanthene (9)
Cambridge
Crystallographic
Data
Centre
NMR
via
Compound
8 (2.07 g, 4.94 mmol), K2CO3 (955 mg, 6.92
www.ccdc.cam.ac.uk/data%5Frequest/cif.All
data
mmol), CuI (1.13 g, 5.93 mmol) and imidazole (471 mg, 6.92
mmol) were added to DMF (150 mL) and the mixture was
refluxed for 44 h. The mixture was cooled to r.t, filtered and 28
% NH3 (30 mL), water (50 mL) and DCM (100 mL) were added
to the filtrate and it was stirred for 5 min. The organic layer
was separated and stirred in a solution of 28% NH3 (30 mL) and
water (50 mL) followed by extraction of the organic layer. The
organic extracts were then stirred for two hours in a saturated
solution of Na2EDTA (50 mL). The organic layer was separated
and dried over MgSO4, filtered and the solvent was removed
from the filtrate. The crude oil was recrystallised from hot
hexane (30 mL), cooled and a white solid precipitated. Yield:
1.709 g (3.2 mmol), 65%. ESI-MS (MeOH), m/z (%): 557.40
(100) [M + Na]+ amu. Anal. Found: C, 73.66; H, 7.27; N, 15.35.
Calcd. for C33H38N6O: C, 74.13; H, 7.16; N, 15.72%.
established for new compounds are presented in the
Supporting Information.
2,7-Ditertbutyl-4,5-dibromo-9,9-dimethylxanthene (6)
Bromine (15.6 mL, 304 mmol) was added dropwise over a
period of 10 minutes to a stirring suspension of 2,7-ditertbutyl-
9,9-dimethylxanthene (8.90 g, 27.6 mmol) in glacial acetic acid
(60 mL). The mixture was then heated at 50 oC for 20 h and
then cooled to room temperature and poured into vigorously
stirring water (250 mL). K2CO3 was added to the mixture slowly
until the orange solution became pale yellow/green in colour.
The product was extracted from the aqueous fraction with
DCM (3 x 50 mL) and dried over MgSO4, filtered and the
solvent removed in vacuo. The crude material was
recrystallised by dissolving the material in the minimum
amount of THF followed by the addition of excess absolute
8 | J. Name., 2012, 00, 1-3
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[(LNHC)/(LBpm )tXan][I] (10)
(DCM) vCO = 2092, 2073, 2027, 1994 (s) cm-1. IR (solid) vCO
2092, 2063, 2028, 1973 (s) cm-1.
=
Compound
9 (1.71 g, 3.20 mmol) was dissolved in the
minimum amount of CH3CN (ca. 10mL) in a pressure tube and
CH3I (2.27 g, 16.0 mmol) was added to the mixture. The tube
was sealed to avoid evaporation of CH3I and the mixture was
stirred for 18 h at r.t. A white precipitate formed, the solvent
was removed in vacuo and the crude product was
recrystallised by dissolving in the minimum amount of DCM
followed by the addition of excess pentane to yield an off-
white precipitate. NOTE: The purity of the final product is
determined by the amount of solvent (CH3CN) used in the
Conclusions
We have successfully synthesised a novel heteroditopic ligand
containing monodentate N-heterocyclic carbene donor
a
ligand and a bidentate bis(pyrazol-1-yl)methane donor ligand
bound to a xanthene scaffold (10). Separately, both two Rh(I)
and two Ir(I) ions were introduced to create homobimetallic
complexes 12 and 13 respectively. Spectroscopic evidence
demonstrated that the metals occupied both ditopic
coordination sites. A crystal structure of a Rh(I) homobimetallic
reaction. The starting material
minimum amount of CH3CN (for this scale ca. 10 mL) or the
product ( ) does not precipitate which allows the excess CH3I
8 must be dissolved in the
1
dimer
(19) was obtained demonstrating the successful
to react at the N-positions of the bpm moiety. Yield: 2.02 g
(2.99 mmol), 93%. ESI-MS (MeOH), m/z (%): 549.48 (100) [M –
I]+ amu. Anal. Found: C, 59.68; H, 5.98; N, 12.22. Calcd. for
C34H42IN6O.0.5H2O: C, 59.56; H, 6.17; N, 12.16%.
coordination of two metal centres onto the heteroditopic
ligand (10), allowing an assessment of intermetallic distance.
The Rh(I) and Ir(I) homobimetallic complexes 12 and 13
were tested as catalysts for the two-step dihydroalkoxylation
reactions of 2-(5-hydroxypent-1-ynyl)benzyl alcohol (14a) and
2-(4-hydroxybut-1-ynyl)benzyl alcohol (15a) and the two-step
hydroamination/hydrosilylation of 4-pentynamine (19a) and 5-
phenyl-4-penynamine (20a). Whilst the bimetallic complexes
were less efficient at catalysing the dihydroalkoxylation
reaction compared to our previously reported Rh(I)
homobimetallic complexes, we observed enhanced selectivity
when using Ir(I) homobimetallic complex 13, it is the first
known complex to have produced more of the kinetically
favoured spiroketal product 14c. Additionally, the Ir(I) complex
13 was able to cleanly form the spiroketal 15b, while the Rh(I)
complex 12 formed a mixture of 15b and the tetracyclic dimer
[AgI(LNHC)/(LBpm)tXan] (11)
Compound 10 (416 mg, 615 μmol) and Ag2O (107 mg, 461
μmol) were stirred in acetone (20 mL) for 18 h. The mixture
was filtered through celite and the solvent was removed in
vacuo yielding an off-white solid. Yield: 478 mg (610 μmol),
99%. ESI-MS (MeOH), m/z (%): 549.48 (100) [M – AgI + H+]+,
1205.37 (23) [M2 – Ag – 2I]+ amu. Anal. Found: C, 62.86; H,
6.75; N, 10.40. Calcd. for: C42H52ClN6ORh.0.5 H2O; C, 62.72; H,
6.64; N, 10.45%.
[Rh(CO)2Cl(LNHC)/Rh(CO)2(LBpm)tXan][BPh4] (12
[Rh(COD)( -Cl)]2 (143 mg, 291 mol) was added to the
silver complex 11 (228mg, 291 mol) in DCM (20 mL), the
mixture was stirred for one hour and NaBPh4 (100 mg, 291
mol) was added and the mixture was stirred for an additional
)
15c
. Both complexes behaved quite similarly for the
ꢁ
ꢁ
hydroamination reaction of both 19a and 20a with the Ir(I)
ꢁ
homobimetallic complex catalysing the reactions more
efficiently. To our surprise, the Ir(I) complex
(13) was
ꢁ
extremely efficient at catalysing the hydrosilylation reaction of
both 2-methylpyrroline (19b) and 2-benzylpyrroline (20b) with
complete conversions achieved in less than 40 seconds,
compared to the Rh(I) complex (12) which required between
17-47 minutes.
hour. The mixture was filtered into another schlenk flask, the
filtrate was degassed by freeze, pump, thawing the solution
three times followed by the introduction of a carbon monoxide
atmosphere. The yellow solution was stirred for 30 mins
turning pale yellow. Excess n-pentane was added to the
mixture until precipitation of a pale yellow solid occurred and
the solution was filtered by cannulation yielding the complex
12 as a pale yellow solid. Yield: 203 mg (166 µmol). ESI-MS
(MeOH), m/z (%): 707.26 (65) [M – Rh(CO)2 – BPh4 – Cl]+ amu.
Anal. Found: C, 59.93; H, 5.00; N, 6.85. Calcd for:
C62H60BClN6O5Rh2.H2O: C, 59.89; H, 5.35; N, 6.76%. IR (DCM)
vCO = 2104, 2087, 2045, 2011 (s) cm-1. IR (solid) vCO = 2098,
2078, 2038, 2001 (s) cm-1.
We are currently investigating the potential for the
heteroditopic ligand (10) to accommodate two different metal
centres to generate heterobimetallic complexes and
investigate their reactivity for a range of one-step and two-
step catalytic transformations.
Acknowledgements
The research was supported under the Australian Research
Council’s Discovery Projects funding scheme (project number
DP1101001611). We gratefully acknowledge the University of
New South Wales (UNSW) for financial support and
acknowledge and the Australian Government for the Ph.D.
stipend (M.G). Additionally we thank the Mark Wainwright
Analytical Centre (UNSW) and Macquarie University.
[Ir(CO)2Cl(LNHC)/Ir(CO)2(LBpm)tXan][BPh4] (13)
The Ir(I) bimetallic complex
same procedure as used for the Rh(I) bimetallic complex
yield a pale yellow solid, the following amounts of reagents
were used: 11 (203 mg, 260 µmol), [Ir(COD)( -Cl]2 (175 mg,
3
was synthesised following the
2
to
ꢁ
260 µmol), NaBPh4 (89.0 mg, 260 µmol), DCM (20 mL). Yield:
112 (80 µmol). Anal. Found: C, 51.21; H, 4.64; N, 5.47. Calcd
for: C62H60BClIr2N6O5 + CH2Cl2: C, 50.96; H, 4.21; N, 5.66%. IR
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ARTICLE
Journal Name
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DOI: 10.1039/C7DT01294B