ACS Catalysis
Letter
suggests a CCA process with IrII(ttp) metalloradical, formed
from the thermal dissociation of the weak (ttp)IrII−IrII(ttp)
bond (∼20 kcal/mol).15 The moderate reaction yield was likely
(eq 9). The initial rates of IrIII(ttp)Bn consumption and toluene
formation were 3 times faster than that of IrIII(ttp)Bn
hydrolysis, estimated from the initial 3 h of the two reaction
the promoting effect of IrIII(ttp)H on faster hydrogenation.
Hence, higher water loading for the hydrolysis of 2 and 3 is not
a must, which agrees with the results depicted in Table 1, eq 1,
entry 6.
attributed to the decomposition of IrII (ttp)2 via reaction with
2
solvent residual water, which will be discussed in detail (eq 10).
Unexpectedly, IrIII(ttp)H reacted with PCP at 150 °C in 35 h
to give mono-Ir 3 as the major CCA product in 21% yield, and
di-Ir 2 as the minor CCA product in 12% yield, with 37%
The proposed bimolecular reductive elimination with
IrIII(ttp)H is consistent with the catalytic hydrogenation of
PCP because IrIII(ttp)H can be generated continuously in the
catalysis. IrIII(ttp)iPr undergoes β-H elimination to give
IrIII(ttp)H. Alternatively, IrII (ttp)2 reacts with H2O to give
2
S2). Interestingly, 2 became the major product upon further
heating to 69 h, yielding 50% of 2 and 33% of 3 as the minor
product, with complete consumption of IrIII(ttp)H (eq 7). The
formation of 2 can be well accounted by the in situ formation of
IrII(ttp) from IrIII(ttp)H for the CCA of PCP. However, the
preferential formation of 3 at the initial stage of reaction was
unexpected. Owing to the slow hydrolysis of IrIII(ttp)Bn at 200
°C (Table 2, eq 4, entry 2), which served as a simplified
structural model of 2, the hydrolysis of 2 to 3 by the solvent
residual water present at 150 °C was not compatible with the
fast formation of 3. We suspect that the excess IrIII(ttp)H
present is promoting other reaction channels to form 3 in a
much faster manner. Based on the precedented reactivity of
RhII(por) and RhIII(por)H (por = porphyrin ligand), the
following two channels were proposed: (1) IrII(ttp)-catalyzed
1,2-addition of IrIII(ttp)H across the benzylic C−C bond of
PCP to form 3;5c or (2) bimolecular reductive elimination from
IrIII(ttp)H and IrIII(ttp)OH. Independent study showed that
IrII (ttp)2 reacted readily with excess H2O in C6D6 at mild
2
temperatures of 27 to 50 °C to give 55% yield of IrIII(ttp)H (eq
moderate as we observed gradual formation of deep brown
precipitates in the sealed NMR tube, presumably due to the
oxidative decomposition of the iridium porphyrin species.
On the basis of the previous understandings of rhodium
porphyrin catalyzed hydrogenation of PCP using H2O and
current findings, a catalytic cycle with three key steps is
proposed in Scheme 2 using IrIII(ttp)H precatalyst as example:
(1) C−C bond activation and (2) hydrogenation.
IrIII(ttp)H and 2 to give 3 and IrII (ttp)2.16a These two
2
possibilities were then investigated.
Scheme 2. Proposed Catalytic Cycle
IrIII(ttp)H with 3 mol% of IrII (ttp)2 reacted with PCP at 150
2
°C in C6D6 to yield 49% of 2 and 39% of 3 in 75 h (eq 8). No
rate enhancement was observed for the formation of 3. (Table
addition of IrIII(ttp)H pathway does not operate.17
We then examined whether IrIII(ttp)H promoted the
bimolecular reductive elimination using IrIII(ttp)Bn as the
model to give toluene as an alternative hydrogenation pathway.
Wayland has reported the bimolecular reductive elimination of
n-propylbenzene from RhIII(oep)H and RhIII(oep)(CH(Ph)-
CH2CH3) (oep = octaethylporphyrinato dianion).16a Elimi-
nation of MeOH from RhIII(oep)H and RhIII(oep)CH2OH has
also been observed.16b In the reversible C−H bond activation
of alkanes and toluene (R-H) with rhodium(II) porphyrin
complexes RhII(por) to give RhIII(por)R and RhIII(por)H, the
bimolecular reductive elimination of R-H from RhIII(por)H and
RhIII(por)R is more favorable at elevated temperature.16c,d We
propose that IrIII(ttp)H can react similarly with 2 and 3 to
afford 1 as the final product in the catalytic system to speed up
the hydrogenation process. To our delight, IrIII(ttp)H reacted
with IrIII(ttp)Bn at 200 °C to give 78% yield of toluene in 43 h
(1). Precatalyst activation. IrIII(ttp)H and IrII (ttp)2 can
2
be interconverted via dehydrogenation8c or oxidative addition
with H2O,18 respectively. IrII (ttp)2 equilibrates with IrII(ttp)
2
metalloradical upon the thermolysis of (ttp)Ir−Ir(ttp) bond.15
In a productive process, IrII(ttp) cleaves the benzylic C−C
bond of PCP to give di-Ir 2.
(2). Hydrogenation. With continuous generation of
IrIII(ttp)H in the catalysis, the CCA products 2 and 3 undergo
faster bimolecular reductive elimination with IrIII(ttp)H to yield
1 as the final hydrogenation product (step 2a). The slow
hydrolysis of 2 and 3 acts as a minor hydrogenation process
(step 2b).
4335
ACS Catal. 2015, 5, 4333−4336