and fac isomers of the trihydride species Ir(CO)(PPh3)2(H)37 and to
both propane and propene were visible. In addition small amounts
of the known complex Ir(CO)(PPh3)3(H) were detected.8 In order
for the propane and propene signals to exhibit p-H2 enhancement,
two protons that originate in a single p-H2 molecule must be located
in the product. This requires the transfer of a proton to the allyl
terminus and the formation of a propene hydride complex, a
reaction that has been observed previously.9 The formation of an
iridium propyl complex places a second p-H2 derived proton into
the substrate, and accounts for the observation of enhanced propane
signals. Competitive b-hydride migration is necessary to account
for the propene enhancement.
carbonyl catalysis.10 A related complex, Ir(dppe)(CO)(COEt)(H)2
which mimics more closely the bisphosphine base rhodium
catalysed system has been observed by Eisenberg.11
One striking observation that needs further comment relates to
the observation of a p-H2 enhanced signal at d 3.05 (Fig. 1c) for 8
which arises from a proton that was originally in p-H2 but now
1
corresponds to a metal-bound CH2 proton of an h -allyl ligand.
Such an occurrence again requires the reversible generation of a
1
propene hydride complex in order to obtain an h -allyl group where
one p-H2 derived proton is located on the C1 atom. The spectral
features associated with 8 therefore correspond to the observation
of two distinct forms, one where both hydrides originate from the
same p-H2 molecule, and one where exchange has moved one of
these ligands on to the allyl group. Upon warming such a sample to
318 K, the species mer and fac Ir(PPh3)2(CO)(H)3, and Ir-
(CO)(PPh3)3(H) are again detected and GC-MS analysis reveals the
formation of the hydroformylation products CH3CH2CH2CHO and
CH3CH(CH3)CHO.
When a toluene-d8 based sample of 1 was placed under a CO
atmosphere, four species were formed immediately and fully
3
characterised by NMR spectroscopy; Ir(CO)2(PPh3)(h -C3H5) 4,
1
Ir(CO)3(PPh3)(h -CH2CHNCH2)
5,
Ir(CO)2(PPh3)2(COCH2
CHNCH2) 6 and Ir(CO)3(PPh3)(COCH2 = CHCH2) 7 (Scheme 2).†
Species 6 and 7 correspond to reactive acyl complexes of the type
that feature in proposed hydroformylation mechanisms.2
3
In conclusion we have demonstrated that Ir(CO)(PPh3)2(h -
When the reaction of 1 with a mixture of CO / p-H2 (ratio 2 : 1,
total 3 atm) is studied in toluene-d8 at 295 K by 1H NMR
spectroscopy, neither 2 nor 3, nor propane nor propene, is observed
but p-H2 enhanced hydride signals are visible at d 28.91 and d
28.40 due to species 8 and 9 (Fig. 1c and Scheme 2).† 13C
information, obtained via HMQC methods, showed that the d
28.91 site connected to a single terminal carbonyl resonance at d
173.6 while the d 28.40 site connected to two signals at d 171.6 and
d 209.5 due to terminal carbonyl and acyl ligands respectively. The
mono-phosphine dihydride acyl species 9, Ir(CO)2(PPh3)-
(COC3H5)(H)2, can form from either 6 or 7 if PPh3 or CO are
appropriately lost. When 13C labelling was introduced, the
appearance of the single hydride resonances observed for 8 and 9
matched those expected for dihydrides with square planar cis,cis
Ir(13CO)2(H)2 cores. The geometry of complex 9 matches that
proposed for the analogous key intermediate in modified cobalt
C3H5) 1 is a suitable precursor to study the hydroformylation
reaction by NMR spectroscopy. Reaction of 1 with p-H2 at 273 K
3
revealed for the first time the formation of two isomers of the h -
3
allyl dihydride species Ir(CO)(PPh3)(h -C3H5)(H)2 2 and 3 which
upon warming to 295 K, yield propene, propane via reversible
hydride transfer, and subsequently the fac and mer isomers of
Ir(CO)(PPh3)2(H)3. This confirms that the CO deficient atmosphere
favours hydrogenation over carbonylation. When 1 reacts with CO
alone, equilibria are established between 4, 5, 6 and 7 in which the
latter two products result from CO insertion into an Ir–C bond
(Scheme 2). When a mixture of CO and H2 was added to 1,
hydrogenation is suppressed and the novel dihydride products,
cis,cis Ir(CO)2(PPh3)(COCH2CHCH2)(H)2
8 and cis,cis Ir-
1
(CO)2(PPh3)(h -CH2CHNCH2)(H)2 9, are detected prior to the
corresponding hydroformylation products. Collectively the ob-
servations correspond to the detection of all the key species
required to complete the hydroformylation of a metal alkyl as
shown in Scheme 2.
Notes and references
‡ Crystal data for 1: C40H35IrOP2, FW 785.82, yellow blocks, crystal
dimensions 0.28 3 0.14 3 0.04 mm, monoclinic, P21/n, a = 10.2878(7), b
= 18.4100(12), c = 17.6432(11) Å, b = 91.633(2)°, V = 3340.2(4) Å3, Z
=
4, m(Mo-Ka)
= = 115(2) K; 26828 reflections
4.124 mm21, T
measured, Rint = 0.0330.
1 (a) For example see: G. W. Parshall and S. D. Ittel, Homogeneous
Catalysis, Wiley, New York, 1992; (b) B. Breit, Acc. Chem. Res., 2003,
36, 264.
2 B. Cornils and W. A. Hermann, Applied homogeneous catalysis with
organometallic compounds: a comprehensive handbook in 2 volumes,
VCH, Weinheim, New York, 1996.
3 (a) R. Whyman, J. Organomet.Chem., 1975, 94, 303; (b) G. Wilkinson,
G. Yagupsky and C. K. Brown, J. Chem. Soc. (A), 1970, 1392; (c) P. M.
Maitlis, A. Haynes, G. J. Sunley and M. J. Howard, J. Chem. Soc.,
Dalton Trans., 1996, 2187.
4 (a) C. R. Bowers, D. H. Jones, N. D. Kurur, J. A. Labinger, M. G.
Pravica and D. P. Weitekamp, Adv. Magn. Reson., 1990, 14, 269; (b) J.
Natterer and J. Bargon, Prog. Nucl. Magn. Reson. Spectrosc., 1997, 31,
293; (c) S. B. Duckett and C. J. Sleigh, Prog. Nucl. Magn. Reson.
Spectrosc., 1999, 34, 71.
5 C. K. Brown, W. Mowat, G. Yagupsky and G. Wilkinson, J. Chem. Soc.
(A)., 1971, 850.
6 K. Osakada, M. Kimura and J-C. Choi, J. Organomet. Chem., 2000,
602, 144.
7 S. K. Hasnip, S. A. Colebrooke, C. J. Sleigh, S. B. Duckett, D. R. Taylor,
G. K. Barlow and M. J. Taylor, J. Chem. Soc., Dalton Trans., 2002, 5,
743.
8 S. S. Bath and L. Vaska, J. Am. Chem. Soc., 1963, 85, 3500.
9 (a) D. L. Cedeno and E. Weitz, Organometallics, 2003, 22, 2652; (b) T.
H. Tuip and J. A. Ibers, J. Am. Chem. Soc., 1979, 79, 4202.
10 For example see: S. M. Massick, T. Buttner and P. C. Ford, Inorg.
Chem., 2003, 42, 575.
3
Scheme 2 Reactivity of Ir(CO)(PPh3)2(h -C3H5) 1 in the presence of 1 atm
CO and 2 atm H2 (R = CH2CHNCH2): Note 8 goes on to eliminate
propene.
11 A. B. Permin and R. Eisenberg, J. Am. Chem. Soc., 2002, 124, 12408.
C h e m . C o m m u n . , 2 0 0 4 , 1 8 2 6 – 1 8 2 7
1827