C-H Activation and Ligand Exchange of CpRe(PPh3)2H2
Organometallics, Vol. 18, No. 9, 1999 1759
Sch em e 4
photosubstitution or H/D exchange catalysis, upon ir-
radiation.11 Intermediate C is ruled out, as 1 does not
undergo exchange of hydride ligands for deuteride
ligands upon irradiation under D2.
As intermediates A-C are inconsistent with our
observations, intermediates D and E were also consid-
ered. The first intermediate (D) arises by way of
migration of a hydride ligand to the C5H5 ring. Revers-
ible cyclometalation of this species would allow for H/D
exchange between the hydride ligand and the ortho-
phenyl of the PPh3 ligand. This cyclometalated inter-
mediate is similar to the known complex (η4-C4H6)Re-
(PPh3)2H3, but with a cyclometalated phenyl ring
replacing one of the hydride ligands.7 Intermediate D
is apparently not capable of activating C-H bonds
intermolecularly; otherwise, H/D exchange of the hy-
dride ligand would be expected to occur. Intermediate
D could also account for the associative phosphine
substitution observed in 1. This 16-electron intermedi-
ate has a choice of either undergoing associative sub-
stitution by PMe3 or undergoing migration of the endo
C5H6 hydrogen back to the metal center. This type of
branching from a photogenerated intermediate is pre-
cisely what the kinetic scheme in eqs 2-4 requires.
To account for the lack of exchange of the hydride
ligands upon catalysis of H/D exchange between sub-
strates, migration of both hydride ligands to the η4-C5H6
group is postulated. This migration would generate the
14-electron π-allyl intermediate E, which is responsible
for the observed H/D exchange catalysis of 1 by oxida-
tive-addition/reductive-elimination pathways involving
ReIII and ReV intermediates. Two possible pathways for
the formation of the allyl species are (1) a concerted
double-hydride migration and (2) a single photochemical
hydride migration to form D, which can then form the
allyl species E by the rapid migration of the remaining
hydride ligand. The rapid and reversible migration of a
hydride ligand to an η4-C5H6 ligand to generate an η3-
allyl ligand has been seen before in the closely related
complex (η4-C5H6)ReH3(PPh3)2;7 therefore, it is plausible
that it could occur in D also. In addition, earlier H/D
exchange catalysis using Re(PPh3)2H7 indicated that the
formation of a 14-electron rhenium complex was re-
quired for alkane activation.12 Other 14 e- complexes
have also been invoked in alkane activation reactions.13
Intermediate D can also be responsible for the formation
of CpRe(PPh3)H2D2 upon irradiation of 1 in the presence
of D2. (η4-C5H6)ReH3(PPh3)2 is known to generate CpRe-
(PPh3)H4 upon photolysis.7
phosphine. Alternatively, phosphine might interfere
with H/D exchange by tying up the vacant site on one
of the coordinatively unsaturated intermediates.
Con clu sion s
In summary, it is apparent that 1 and 2 photochemi-
cally undergo a single metal to ring hydride migration
but that the resulting 16-electron intermediate [(η4-
C5H6)ReH(PPh3)2] is not capable of catalyzing H/D
exchange between a solvent and substrate. It can,
however, reversibly cyclometalate one of the PPh3
ligands present (which is responsible for the removal
of deuterium from the metal center of 2) or undergo a
second (rapid and reversible) metal to ring hydride
migration, as seen previously in the analogous (η4-
C5H6)ReH3(PPh3)2, to form the 14-electron intermediate
[(η3-C5H7)Re(PPh3)2]. It is this 14-electron intermediate
that is proposed to be the species responsible for the
observed H/D exchange catalysis of 1 and 2.
Exp er im en ta l Section
Gen er a l P r oced u r es. Compound 1 is only slightly air
sensitive in the solid state but is unstable in solution toward
prolonged exposure to oxygen and moisture. All reactions were
performed under vacuum or under a nitrogen atmosphere in
a Vacuum Atmospheres Dry-Lab glovebox. All reagents were
obtained commercially and put through three freeze-pump-
thaw cycles for degassing before use. All deuterated solvents
were purchased from MSD Isotopes Merck Chemical Division,
distilled under vacuum from dark purple solutions of ben-
zophenone ketyl, and stored in ampules with Teflon-sealed
vacuum line adapters. All other solvents were dried similarly
and stored in the glovebox.
Proton, carbon, and phosphorus NMR spectra were recorded
1
on a Bruker AMX-400 NMR spectrometer. H NMR chemical
shifts were measured in ppm (δ) relative to tetramethylsilane,
using the residual 1H resonances in the deuterated solvents
as an internal reference: C6D6 (δ 7.15), toluene-d8 (δ 2.09),
and THF-d8 (δ 3.58). 31P NMR chemical shifts were measured
relative to 30% H3PO4. Photolyses were carried out by using
an Oriel 200 W high-pressure Hg focused-beam lamp fitted
with an infrared-absorbing water filter. Additional filters were
used to limit irradiation energies as shown in Figure 1 (BP )
365 nm band-pass; LP ) 345 nm long pass). Experiments
involving more than one sample to be photolyzed under
identical conditions were carried out in a merry-go-round
apparatus. GC-MS spectra were taken using a Hewlett-
Packard 5890 Series II gas chromatograph. Kinetic fits were
performed using Microsoft Excel. All errors are quoted as 95%
confidence limits (((error) ) tσ; σ ) standard deviation, t from
Student’s t distribution).
Two pathways by which intermediate E can catalyze
H/D exchange between hydrocarbons are shown in
Scheme 4. In one of these, E oxidatively adds first one
substrate and then the other, with ReIII and ReV
intermediates involved. The second pathway is similar
but shows that phosphine might be lost prior to the
second oxidative addition. This variation allows for the
inhibition of intermolecular H/D exchange by added
(11) Rosini, G. P.; J ones, W. D. J . Am. Chem. Soc. 1993, 115, 965-
974.
(12) Baudry, D.; Ephritikhine, M.; Felkin, H. J . Chem. Soc., Chem.
Commun. 1980, 1243-1244. Baudry, D.; Ephritikhine, M.; Felkin, H.
J . Chem. Soc., Chem. Commun. 1982, 606-607. Baudry, D.; Ephri-
tikhine, M.; Felkin, H.; Holmes-Smith, R. J . Chem. Soc., Chem.
Commun. 1983, 788-789.
(13) Maguire, J . A.; Boese, W. T.; Goldman, A. S. J . Am. Chem. Soc.
1989, 111, 7088-7093.
The synthesis of CpRe(PPh3)2H2 was as previously reported.7
The synthesis of CpRe(PPh3)2HD was as for CpRe(PPh3)2H2,
using Re(PPh3)2D7 in place of Re(PPh3)2H7.