Organometallics
COMMUNICATION
Table 1. Reduction of Rhodium(III) Porphyrin Iodide
II
entry
por
time (h)
yield of [Rh (por)]
2
/%
1
2
3
tpOMep 1b
ttp 1c
7
2b 68%
2a 72%
2c 82%
1.5
0.5
tpp 1d
1
III
Figure 2. H NMR (400 MHz) spectra in benzene-d . (A) Rh (tmp)
6
II
III
III
I 5; (B) reaction mixture of Rh (tmp) 6 and (tmp)Rh -OO-Rh (tmp)
, where pyr and Ph designate the pyrrole and phenyl hydrogens,
7
respectively, in species 5, 6, and 7.
unstable in the presence of a strong ligand via ligand-induced
19
disproportionation.
III
To eliminate the formation of the Rh (por) μ-oxo complex,
III
reduction of the sterically hindered Rh (tmp)I (tmp = tetra-
20
III
mesitylporphyrinato dianion) was examined. Similarly, Rh -
(
1
tmp)I 5 was reduced by KOH (10 equiv) in benzene-d at
6
II
20 °C in 2 h to give Rh (tmp) 6 (δ
= 18.2 ppm) in 52%
pyrrole
III
III
yield, and the reported (tmp)Rh -OO-Rh (tmp) 7 (δ
8
(tmp)Rh -OO-Rh (tmp) likely forms from the secondary
insertion reaction of 6 with O2
disproportionation. Therefore, the formation of (tmp)Rh -
OO-Rh (tmp) supports the generation of H O , which also
undergoes catalyzed disproportionation into O and H O.
2 2
=
0
pyrrole
2
.23 ppm) was also observed in 5% yield (eq 4, Figure 2).
III III
1
II
Figure 1. H NMR (400 MHz) spectra in benzene-d
6
. (A) [Rh (ttp)]
2
20
III
formed by the H O
2 2
III
2
a; (B) Rh (ttp)OH 4 from reaction mixture at rt for 1 h; (C)
II
18
[
Rh (ttp)] 2a from reaction mixture at 120 °C for 1 h, where pyr
2
III
and Ph designate the pyrrole and phenyl hydrogens, respectively, in
2
2
1
8
species 2a and 4.
To gain further understanding of the chemistry of Rh-
(
por)OH, we then attempted to synthesize the highly reactive
III
Rh (por)OH independently by oxidative addition with H O ,
2
2
III
analogous to the reaction of pentacyanocobalt(II) with H O to
2
2
In summary, we have reported the reduction of Rh (por)
II
15
II
ꢀ
give pentacyanocobalt(III) hydroxide. [Rh (ttp)] was thus
2
halides by OH to Rh (por). Further studies are ongoing.
16
treated with a stoichiometric amount of H O
in benzene-
2
2(aq)
d6 at rt in 1 h (eq 3, Figure 1A,B). A new rhodium porphyrin
’
ASSOCIATED CONTENT
species formed and only exhibits porphyrin resonances without
1
any axial ligand signal in its H NMR spectrum. The high-
S
b
Supporting Information. Experimental information,
resolution mass spectrum shows the molecular ion at m/z
compound characterization, and spectra. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
1
7
III
13
7
88.1997 and is assigned to be Rh (ttp)OH. However, the
III
1
absence of a Rh (ttp)OꢀH H NMR signal is likely due to the
III
rapid exchange with residual water in solvent. To demonstrate
’ AUTHOR INFORMATION
that Rh (ttp)OH is indeed an intermediate for the formation of
II
Corresponding Author
*E-mail: ksc@cuhk.edu.hk.
[
1
[
Rh (ttp)] via reduction, this reaction mixture was heated at
20 °C. To our delight, Rh (ttp)OH was reduced to
Rh (ttp)] in 30% yield in 0.5 h (eq 3, Figure 1C), which
2
III
II
2
indirectly supports the formation of H O coproduct. Further-
2
2
’ ACKNOWLEDGMENT
more, eq 3 is reversible, its equilibrium constant is hard to
measure due to the rapid base-catalyzed disproportionation
We thank the Research Grants Council of Hong Kong of the
SAR of China for financial support (No. 400309).
1
8
of H O to O and H O.
2
2
2
2
’
REFERENCES
(
1) (a) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533–3539. (b)
Holland, P. L.; Anderson, R. A.; Bergman, R. G. Comments Inorg. Chem.
999, 21, 115–129. (c) Fulton, J. R.; Holland, A. R.; Fox, D. J.; Bergman,
R. G. Acc. Chem. Res. 2002, 35, 44–56.
2) Woerpel, K. A.; Bergman, R. G. J. Am. Chem. Soc. 1993,
1
The indirect detection of H O coproduct was further con-
2
2
ducted by carrying out the reduction of Rh(ttp)I with KOH in
the presence of Ph P (1 equiv) as a H O trap. To our delight,
(
3
2
2
115, 7888–7889.
1
Ph PO was observed in 43% yield by H NMR spectroscopy.
The [Rh (ttp)] yield, however, could not be determined, as it is
(3) Tenn, W. J., III; Young, K. J. H.; Oxgarrd, J.; Nielsen, R. J.;
Goddard, W. A., III; Periana, R. A. Organometallics 2006, 25, 5173–5175.
3
II
2
2
634
dx.doi.org/10.1021/om200075f |Organometallics 2011, 30, 2633–2635