Imine hydrolysis and role of a rhodium(I)-imine-amine complex in homogeneous H2-hydrogenation of the imine and a rare example of inequivalent NH2 protons

Article abstract of DOI:10.1021/ic040037a

A Rh-catalyzed, homogeneous hydrogenation of the imine, PhCH ₂N=CHPh, is shown to involve a Rh-imine-amine species that subsequently activates H₂, the amine (benzylamine) being formed via a Rh-catalyzed hydrolysis of the imine by adventitious water. The imine-amine complex, cis-{Rh[P(p-tolyl)₃]₂(PhCH₂N=CHPh) (NH₂CH₂Ph)}-PF₆ (2b), is structurally characterized, and the solution ¹H NMR data reveal inequivalent NH₂ protons.

Full text of DOI:10.1021/ic040037a

Inorg. Chem. 2004, 43, 4820−4824
Imine Hydrolysis and Role of a Rhodium(I)−Imine−Amine Complex in
Homogeneous H -Hydrogenation of the Imine and a Rare Example of
2
Inequivalent NH Protons
2
Paolo Marcazzan, Chalil Abu-Gnim, Kapila N. Seneviratne, and Brian R. James*
Department of Chemistry, The UniVersity of British Columbia,
VancouVer, British Columbia, Canada V6T 1Z1
Received March 5, 2004
A Rh-catalyzed, homogeneous hydrogenation of the imine, PhCH NdCHPh, is shown to involve a Rh−imine−
2
amine species that subsequently activates H , the amine (benzylamine) being formed via a Rh-catalyzed hydrolysis
2
of the imine by adventitious water. The imine−amine complex, cis-{Rh[P(p-tolyl)3]2(PhCH NdCHPh)(NH CH Ph)}-
2
2
2
PF (2b), is structurally characterized, and the solution 1H NMR data reveal inequivalent NH protons.
6
2
PhCH2NdCHPh and PhCH2NH2 were purchased from Aldrich and
purified by distillation prior to use, while the solid imines PhCH2Nd
C(Me)Ph and PhCH2NdC(Ph)2 were synthesized by Dr. D. Fogg6
of this laboratory. The [Rh(COD)(PR3)2]PF6 and corresponding cis-
[Rh(PR3)2(MeOH)2]PF6 precursors (R ) Ph, p-tolyl) were prepared
according to literature procedures.4,7,8 Other reagents were purchased
from commercial sources and used as supplied. Solvents were dried
over the appropriate agents and distilled under N2 prior to use. NMR
Introduction
Mechanisms of catalytic, homogeneous H2-hydrogenation
of imines are poorly understood, relative to those of
corresponding reductions of CdC and CdO functionalities,
partly because the more forcing conditions that are required
complicate experimental investigations and, further, the
amine product can poison the catalyst.1,2
During studies with the catalyst precursor [Rh(COD)-
(PPh3)2]PF6 that homogeneously hydrogenates aldimines
under mild conditions,3,4 we have discovered that an ac-
companying metal-promoted hydrolysis of the imine to amine
(and aldehyde) plays a key mechanistic role in that a resulting
mixed Rh(I)-imine-amine complex activates H2 for sub-
sequent hydrogenation of the imine. The findings are
potentially significant regarding applications of imine hy-
drogenations.5
1
spectra were recorded on Bruker AV 300 (300 MHz for H, 121
MHz for 31P{1H}, 75 MHz for 13C) and AV 400 (400 MHz for 1H,
162 MHz for 31P{1H}, 100 MHz for 13C) spectrometers at 300 K.
Residual solvent proton (1H, relative to external SiMe4 δ 0.00) and
external P(OMe)3 (31P{1H}, δ 141.00 vs external 85% aq H3PO4)
were used as references. All J values are given in hertz. Infrared
spectra were recorded on an ATLI Mattson Genesis Series FTIR
spectrophotometer (range 500-4000 cm-1), and all IR (KBr pellet)
bands are reported in cm-1. GC analyses were performed on a
Hewlett-Packard 5890A instrument equipped with an HP 17
capillary column (He carrier gas) and an FID detector. Elemental
analyses were performed by Mr. P. Borda of this department using
a Carlo Erba 1108 analyzer.
Experimental Section
General. Unless otherwise stated, synthetic procedures were
performed at room temperature (rt, ∼20 °C) using standard Schlenk
techniques under an atmosphere of dry Ar. The liquid imine
Syntheses. cis-[Rh(PR3)2(PhCH2NdCHPh)(PhCH2NH2)]PF6
(R ) Ph, 2a; p-Tolyl, 2b). A red solution of cis-[Rh(PR3)2-
(MeOH)2]PF6 (0.08 mmol) in MeOH (3 mL) was treated with
excess PhCH2NdCHPh (57.0 µL, 0.304 mmol) under Ar, and the
mixture stirred at rt for 24 h; spontaneous precipitation of a light-
orange solid occurred. This was collected, washed with Et2O (3 ×
4 mL), and dried in vacuo.
* To whom correspondence should be addressed. E-mail: brj@
chem.ubc.ca.
(1) (a) James, B. R. Catal. Today 1997, 37, 209. (b) Obora, Y.; Ohta, T.;
Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1997, 119, 3745. (c)
Kobayashi, S.; Ishitani, H. Chem. ReV. 1999, 99, 1069. (d) Abdur-
Rashid, K.; Lough, A. J.; Morris, R. H. Organometallics 2000, 19,
2655. (e) Samec, J. S. M.; Ba¨ckvall, J.-E. Chem. Eur. J. 2002, 8, 2955.
(2) Marcazzan, P.; Patrick, B. O.; James, B. R. Organometallics 2003,
22, 1177.
R ) Ph (2a). Yield: 0.044 g (45%). Anal. Calcd for
C57H52N2P3F6Rh: C, 63.70; H, 4.88; N, 2.61. Found: C, 63.67; H,
(3) Longley, C. J.; Goodwin, T. J.; Wilkinson, G. Polyhedron 1986, 5,
1625.
(4) Marcazzan, P.; Ezhova, M. B.; Patrick, B. O.; James, B. R. C. R.
Chim. 2002, 5, 373 (J. A. Osborn Memorial Volume).
(5) Blaser, H.-U.; Malan, C.; Pugin, B.; Spindler, F.; Steiner, H.; Studer,
M. AdV. Synth. Catal. 2003, 345, 103.
(6) Fogg, D. E. Ph.D. Dissertation, The University of British Columbia,
Vancouver, BC, 1994.
(7) Schrock, R. R.; Osborn, J. A. J. Am. Chem. Soc. 1971, 93, 2397.
(8) Haines, L. M.; Singleton, E. J. Chem. Soc., Dalton Trans. 1972, 1891.
4820 Inorganic Chemistry, Vol. 43, No. 16, 2004
10.1021/ic040037a CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/12/2004

Imine Hydrolysis and Rh(I)-Imine-Amine Complex
(0.0478P)2 + 7.755P], where P ) (Fo2 + 2Fc2)/3. All non-H-atoms
were refined anisotropically, and H-atoms were included in fixed
positions.
Table 1. Crystallographic Data for 2b
2b
formula
fw
cryst color, habit
cryst size (mm3)
space group
a (Å)
C63H64F6N2P3Rh
1158.98
orange, plate
0.52 × 0.32 × 0.12
P21/c
Results and Discussion
The Imine-Amine Complex. The room temperature
reaction of either cis,trans,cis-[Rh(H)2(PR3)2(MeOH)2]PF6 (R
) Ph, 1a; p-tolyl, 1b) or cis-[Rh(PR3)2(MeOH)2]PF6 (R )
Ph or p-tolyl)4,7,8 with an excess of the liquid imine
PhCH2NdCHPh (either with imine/Rh ) 4, as in the
synthetic procedure, or with imine/Rh ) 100, as used in
catalytic conditions under 1 atm H2, see below) in MeOH
under Ar generates the Rh(I) complexes cis-[Rh(PR3)2-
(PhCH2NdCHPh)(NH2CH2Ph)]PF6 (R ) Ph, 2a; p-tolyl, 2b),
containing the η1-imine and η1-benzylamine, a hydrolysis
product of the imine. In situ yields of 2a,b are quantitative,
while isolated yields are ∼50%. Benzaldehyde, the coproduct
11.5104(14)
19.986(2)
b (Å)
c (Å)
25.219(4)
99.299(9)
5725.3(13)
0.442
41237
10051
0.0482
683
â (deg)
V (Å3)
µ (mm-1
)
total reflns
unique reflns
Rint
no. variables
R1 (I > 2σ(I))
wR2a
0.0415 (7604 obsd reflns)
0.1096 (all data)
1.016 (all data)
GOF
a w ) 1/[σ2(Fo ) + (0.0478P)2 + 7.755P], where P ) (Fo + 2Fc )/3.
2
2
2
1
of hydrolysis, was detected by GC and H NMR analyses
1
4.69; N, 2.63. H NMR (300 MHz, CD2Cl2): δ 1.13 (t, 1H, NH2,
of the filtrate from the synthetic procedure; H2 was also
detected by 1H NMR (δ 4.15). The hydrolysis is Rh-
promoted, as the neat imines are stable in MeOH; we have
noted elsewhere Ru-promoted11 and Ir-promoted12 hydrolytic
cleavage of such imines.
2
2JHH ) 12), 1.29 (t, 1H, NH2, JHH ) 12), 2.52 (2 td, 2 H, CH2,
3
2
3
2JHH ) 13, JHH ) 3), 2.63 (2 td, 2H, CH2, JHH ) 13, JHH ) 3),
4.40 (d, 1H, CH2, 2JHH ) 13), 5.12 (d, 1H, CH2, 2JHH ) 13), 5.98
(d, 2H, o-C6H5, 3JHH ) 7), 6.95-7.90 (m, 41H, Ar), 8.01 (bs, 1H,
NdCH), 9.70 (d, 2H, o-C6H5, 3JHH ) 7). 31P{1H} NMR (121 MHz,
The structure of 2b (Figure 1a) reveals essentially square-
planar geometry at Rh, while selected structural parameters
are listed in Table 2; the CdN imine bond is some 0.2 Å
shorter than the CsN bond of the amine. There is close
contact between the phenyl o-H atoms of the amine and an
imine Ph (e.g., C(53)‚‚‚H(63) ) 2.65 Å), which causes
restricted rotation of these two moieties in the solid state
structure and also in solution, see below. Furthermore, one
of the o-protons of the imine Ph ring (H(49)) “docks” over,
and is well within van der Waals distance (2.43 Å) with,
the Rh; the implications of this close interaction are also
evident in the solution structure, see below. The solid state
IR spectrum shows νCdN and νNsH of the coordinated imine
and amine, respectively.
2
CD2Cl2): δ 46.94 (dd, JRhP ) 166, JPP ) 49), 50.07 (dd, JRhP
)
180, 2JPP ) 49). IR (KBr pellet): ν ) 1616 (CdN), 3313 (NsH).
R ) p-Tolyl (2b). Yield: 0.047 g (50%). Anal. Calcd for
C63H64N2P3F6Rh: C, 65.29; H, 5.57; N, 2.42. Found: C, 65.26; H,
1
5.61; N, 2.43. H NMR (300 MHz, CD2Cl2): δ 1.13 (t, 1H, NH2,
2
2JHH ) 12), 1.23 (t, 1H, NH2, JHH ) 12), 2.31 (s, 9H, p-CH3),
2.33 (s, 9H, p-CH3), 2.50 (td, 1H, CH2, 2JHH ) 13, 3JHH ) 3), 2.58
(td, 1H, CH2, 2JHH ) 13, 3JHH ) 3), 4.38 (d, 1H, CH2, 2JHH ) 12),
2
3
5.15 (d, 1H, CH2, JHH ) 12), 5.95 (d, 2H, o-C6H5, JHH ) 7),
6.90-7.85 (m, 35H, Ar), 8.03 (bs, 1H, NdCH), 9.76 (d, 2H,
3
o-C6H5, JHH ) 7). 31P{1H} NMR (121 MHz, CD2Cl2): δ 45.15
(dd, JRhP ) 166, 2JPP ) 49), 48.20 (dd, JRhP ) 180, 2JPP ) 49). IR
(KBr pellet): ν ) 1601 (CdN), 3307 (NsH). An X-ray quality
crystal of 2b was obtained by slow evaporation at rt of a CD3OD
solution of the complex.
The rt 31P{1H} NMR spectrum of 2b in CD2Cl2 shows
the expected AMX, 8-line pattern due to inequivalence of
the two phosphine ligands, and on the basis of JRhP values,13
the upfield (δ 45.15 dd, JRhP ) 166, 2JPP ) 49) and downfield
(δ 48.20 dd, JRhP ) 180, 2JPP ) 49) resonances are assigned
to the P-atom trans to the imine and amine, respectively.
The 1H NMR data are unusual in that both the benzylic and
NH2 protons of the amine, and also the benzylic protons of
H2-Hydrogenation Studies. These experiments were performed
in a three-neck flask, with one neck connected to the H2-inlet, a
second neck fitted with a rubber septum that served as the sampling
as well as the injection port for the imine, and the third neck used
to charge the flask with solvent (MeOH, 10 mL) and catalyst
precursor [Rh(COD)(PR3)2]PF6 (up to 1.0 mM). After this mixture
was reacted with H2 (1 atm) at 30 °C for 1 h to form [Rh(H)2-
(PR3)2(MeOH)2]PF6 (R ) Ph, 1a; p-tolyl, 1b),4,7,8 excess imine (up
to 100 mM) was injected via a microsyringe, and the stirred reaction
mixture was sampled (5 µL) periodically, with conversions being
monitored by GC analysis. Systems were also monitored by H2-
absorption at 30 °C in a constant-pressure gas-uptake apparatus.9
the imine, are inequivalent. The amine benzylic protons
2
appear as two triplets of doublets (δ 2.50 and 2.58, JHH
)
3
13, JHH ) 3), due to coupling to the NH2 protons and to
each other, and similarly the NH2 resonances, although less
well resolved, appear as two somewhat broadened triplets
X-ray Crystallographic Analysis. Measurements were made at
173(2) K on a Siemens SMART CCD diffractometer with graphite
monochromated Mo KR radiation (0.71073 Å). Some crystal-
lographic data for 2b are shown in Table 1. The final unit-cell
parameters were based on 354 reflections with 1.31° < θ < 24.99°.
The structure was solved by direct methods and refined by full-
2
(δ 1.13 and 1.23, JHH ) 12). The NH2 protons are
inequivalent because of restricted rotation of the amine;
similar behavior is seen in a Ru system involving again
2
matrix least-squares (SHELXL-97),10 using w ) 1/[σ2(Fo ) +
(11) Fogg, D. E.; James, B. R. Inorg. Chem. 1995, 34, 2557.
(12) Marcazzan, P.; Patrick, B. O.; James, B. R. Russ. Chem. Bull., Int.
Ed. 2003, 52, 2715 (Mark Vol’pin Memorial Volume).
(13) Marcazzan, P. Ph.D. Dissertation, The University of British Columbia,
Vancouver, BC, 2002.
(9) James, B. R.; Rempel, G. L. Can. J. Chem. 1966, 44, 233.
(10) Sheldrick, G. M. SHELXL-97; Bruker-AXS: Madison, WI, 1997.
Inorganic Chemistry, Vol. 43, No. 16, 2004 4821

Marcazzan et al.
Figure 1. (a) ORTEP diagram of cis-{Rh[P(p-tolyl)3]2(PhCH2NdCHPh)(NH2CH2Ph)}PF6 (2b) with 50% probability thermal ellipsoids. (b) Schematic
diagram of the imine-amine interactions within complex 2b.
Table 2. Selected Bond Distances and Angles for
cis-{Rh[P(p-tolyl)3]2(PhCH2NdCHPh)(NH2CH2Ph)}PF6 (2b) with
Estimated Standard Deviations in Parentheses
bond
length (Å)
bond
angle (deg)
Rh(1)-P(1)
Rh(1)-P(2)
Rh(1)-N(1)
Rh(1)-N(2)
N(1)-C(50)
N(1)-C(43)
N(2)-C(57)
C(43)-C(44)
C(57)-C(58)
C(50)-C(51)
2.2538(12)
2.2503(12)
2.128(4)
2.209(3)
1.495(6)
1.288(6)
1.488(6)
1.459(7)
1.522(6)
1.507(7)
P(1)-Rh(1)-N(1)
P(2)-Rh(1)-N(2)
P(1)-Rh(1)-P(2)
P(1)-Rh(1)-N(2)
P(2)-Rh(1)-N(1)
C(43)-N(1)-C(50)
C(50)-N(1)-Rh(1)
C(43)-N(1)-Rh(1)
C(57)-N(2)-Rh(1)
N(2)-C(57)-C(58)
172.46(10)
176.16(10)
96.95(4)
86.41(10)
90.45(10)
116.1(4)
113.5(3)
130.3(3)
122.2(3)
111.8(4)
Figure 2. Plots of H2 absorbed (mM) vs time for hydrogenation of
PhCH2dNHPh ([ ) 50 mM; 9 ) 10 mM) catalyzed by [Rh(COD)(PPh3)2]-
PF6 (0.50 mM) in MeOH under 1 atm H2 at 30 °C.
benzylamine that was also generated via Ru-promoted
hydrolysis of PhCH2NdCHPh.11 The corresponding 1H-1H
COSY NMR experiment shows strong correlation between
the amine CH2 and NH2 signals, which are not affected by
31P-decoupling. The diastereotopically inequivalent benzylic
protons of the imine are seen as two doublets at δ 4.38 and
5.15 (2JHH ) 12). The singlet resonance for the CHdN proton
at δ 8.03 is upfield to that of the free imine (δ 8.50). Sharp
doublets at δ 5.95 and 9.76 (3JHH ) 7), each integrating for
two protons, are assigned to o-protons of phenyl rings, each
induction period is observed during the first few minutes,
followed by a linear region showing up to ∼90% conversion,
which corresponds to zero-order in [imine]. The linear rate
for the upper curve is 3.5 × 10-5 M s-1. The kinetics13 (see
Figures S1-S3 in the Supporting Information) were deter-
mined from such maximum, linear rates, where the kinetic
order in imine decreases from first-order at lower [imine]
(<20 mM) to zero-order at higher [imine] (> 60 mM);
indeed, at the lower [imine], the conversion plots gave (after
the induction period) standard, logarithmic first-order plots
for the imine dependence (Figure 2, lower curve). The linear
rates are first-order in [Rh], and are first-order in [H2] at
both lower and higher [imine]. Addition of the dibenzylamine
product does not inhibit the catalysis, but addition of
benzylamine (the hydrolysis product) does decrease the
hydrogenation rates, especially at lower [imine]. Of key
importance, the only Rh species detected by NMR in the
linear region of the conversion plots is 2a, the imine-amine
complex. Indeed, use of 2a instead of 1a, at the same [Rh],
gives the same maximum rates, without any observed
induction period. Similarly, there was no induction period
when using an in situ 2a catalyst made by addition of excess
imine to a solution of the benzylamine complex cis-[Rh-
(PPh3)2(NH2CH2Ph)(L)]PF6 (Figure 3, L ) MeOH, 3; L )
PhCH2NH2, 4).14
1
coupling to a neighboring m-proton, because an rt H-13C
HETCOR NMR spectrum reveals correlation of these
resonances with aromatic C-atoms. Consideration of the solid
state structure (Figure 1a,b) suggests that these o-phenyl
protons are those of the amine and imine, and not the
phosphine, with the downfield resonance being attributed to
the imine o-proton (H(49)) involved in a pseudoagostic
interaction with the Rh. The rt NMR spectral data in CD3-
OD, measured because the hydrogenation catalysis is carried
out in MeOH, generally correspond closely to those in CD2-
Cl2, except that the NH2 resonances are not seen because of
H-exchange with the deuterated solvent, and thus, no
coupling is seen to the NH2 protons; the amine CH2 protons
appear as doublets instead of as a triplet of doublets. Also,
the CHdN proton is now seen at δ 8.21, 0.2 ppm downfield
of the value in CD2Cl2. The NMR data for 2a in CD2Cl2
and in CD3OD replicate those of 2b, except for the absence
of the Me proton resonances of the p-tolyl phosphines.
Catalytic Hydrogenation. In a typical catalytic hydro-
genation experiment, PhCH2NdCHPh undergoes complete
reduction in ∼30 min to dibenzylamine (Figure 2); an
These findings show unambiguously that the induction
period is related to the imine hydrolysis and formation of
2a. The proposed overall mechanism (Figure 3) shows the
4822 Inorganic Chemistry, Vol. 43, No. 16, 2004

Imine Hydrolysis and Rh(I)-Imine-Amine Complex
Figure 3. Suggested reaction steps for the hydrogenation of PhCH2NdCHPh.
initial hydrolysis step that must involve loss of H2 from 1a
and coordination of imine, but whether there is trans-attack
by external H2O/OH-, or cis-attack by coordinated H2O/
OH-, is unknown. Once the benzylamine-MeOH species 3
and the accompanying bis(amine) species 413,14 are formed,
rapid pre-equilibria with imine give 2a. A subsequent rate-
determining (k) oxidative addition of H2 gives a dihydrido-
imine-amine species, and the catalytic cycle is completed
by rapid hydrogen transfer with elimination of dibenzylamine
product. This mechanism is consistent with the observed
Michaelis-Menten type kinetics, the dependence on [imine]
going from first-order at low [imine] to zero-order at higher
[imine] when 2a is fully formed; the mechanism also satisfies
the first-order dependences on both [Rh] and [H2], and the
limiting rate at 30 °C gives k ∼ 45 M-1 s-1. The noted
inverse dependence on added L (PhCH2NH2) results from
increased formation of the bis(benzylamine) species 4 (Figure
3, L ) PhCH2NH2).14
offer no information on whether an η2-imine intermediate is
necessary for hydrogenation; such a species, usually followed
by stepwise transfer of two H-atoms via an alkylamido
intermediate, has been invoked invariably in Rh-cata-
lyzed,1,3,15 Ir-catalyzed,16 or Ti-catalyzed17 imine hydrogena-
tions, although a shift to an η2-imine mode may be
unnecessary.18 Concerted transfer of a coordinated hydride
and a proton from an amine-NH219 could also be envisaged
for our Rh system.
An “unsaturated” route, in which a rapidly formed Rh-
imine complex reacts with H2 in a slow step (cf. Figure 3),
has been proposed earlier by our group (on the basis of NMR
and minimal kinetic data) for high pressure imine hydrogen-
ations catalyzed by [Rh(P-P)]+ systems, where P-P is a
chelating diphosphine;1,15 whether a “prehydrolysis” step to
generate a mixed imine-amine species was involved in these
systems is unclear.
Liquid imines may be the source of H2O, because of the
lack of hydrolysis evident when using in synthetic procedures
The hydrolytic generation of 1 mol of benzylamine per
Rh is clearly critical for the successful hydrogenation of
PhCH2NdCHPh. The addition of dibenzylamine (the hy-
drogenation product) does not inhibit the catalysis as this
forms only a labile mono(dibenzylamine) species.2
(15) Ball, G. E.; Cullen, W. R.; Fryzuk, M. D.; Henderson, W. J.; James,
B. R.; MacFarlane, K. S. Inorg. Chem. 1994, 33, 1464 and references
therein.
(16) Herrera, V.; Munoz, B.; Landaeta, V.; Canudas, N. J. Mol. Catal.
2001, 174, 141.
(17) Willoughby, C. A.; Buchwald, S. L. J. Am. Chem. Soc. 1994, 116,
11703.
Attempts to detect a dihydrido intermediate (I, Figure 3)
by low temperature NMR were unsuccessful, and we can
(18) Arndsten, B. A.; Lafrance, D.; Davis, J. L.; Dhawan, R. Organome-
tallics 2001, 20, 1128.
(19) (a) Ohkuma, T.; Ishii, D.; Takeno, H.; Noyori, R. J. Am. Chem. Soc.
2000, 122, 6510. (b) Casey, C. P.; Singer, S. W.; Powell, D. R.;
Hayashi, R. K.; Kavana, M. J. Am. Chem. Soc. 2001, 123, 1090. (c)
Abdur-Rashid, K.; Faatz, M.; Clapham, S.; Hadzovic, A.; Harvey, J.
N.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2002, 124, 15104.
(14) Species 3 and 4 are formed in situ from a 1:1 mixture of cis-[Rh-
(PPh3)2(CD3OD)2]PF6 and PhCH2NH2 in CD3OD under Ar; 3, like
the corresponding dibenzylamine complex,2 is fluxional at rt but is
characterized below 283 K by δP 47.69 dd (JRhP ) 169) and 60.06 dd
(JRhP ) 214) with 2JPP ) 54. The synthesis and structural characteriza-
tion of 413 will be described elsewhere.
Inorganic Chemistry, Vol. 43, No. 16, 2004 4823

Marcazzan et al.
(a) a PhCH2NdCHPh/Rh ratio of 1:1, or (b) the solid imines
PhCH2NdC(Ph)(R) (R ) Ph or Me) at imine/Rh ratios of
2-100. Both systems a and b in MeOH generate the Rh-
(III)-hydrido-orthometalated complexes, [Rh(H){PhCH2Nd
C(R)(o-C6H4)}(MeOH)]PF6 (R ) H, Ph, or Me),13 chemistry
that will be described in detail elsewhere. Of note, neither
of the solid imines was catalytically hydrogenated, even upon
addition of H2O (H2O/imine ) 5) to the system, while
addition of benzylamine (amine/Rh ) 1) to these systems
also did not promote hydrogenation; again, in these attempted
catalytic reactions, the Rh(III)-hydrido-orthometalated com-
plexes noted above were generated. The orthometalated
species (R ) H) on treatment with water can be hydrolyzed
(with loss of PhCHO) to the catalytically important amine-
imine species, but whether the orthometalated species plays
a direct role in the catalysis is unclear.20
The chemistry of the corresponding Ir complexes is
markedly different, presumably because of the stronger
metal-hydride and metal-carbon bonds in the third row
transition metal system.21 Thus, on treatment of cis,trans,cis-
[[Ir(H)2(PPh3)2(MeOH)2]PF6, the Ir analogue of 1a, with
excess PhCH2NdCHPh in MeOH under Ar, hydrolysis of 1
mol equiv of imine is again observed, but the product
generated is now the Ir(III) complex [Ir(H)(PPh3)2{PhCH2Nd
CH(o-C6H4)}(PhCH2NH2)]PF6 containing the amine ligand
and a chelated N,C-bound orthometalated imine;12 this
species does not catalyze hydrogenation of the imine under
ambient conditions, and indeed, it reacts with H2 to give the
catalytically inactive, neutral, dihydrido complex Ir(H)2-
(PPh3)2{PhCH2NdCH(o-C6H4)}.13
Conclusions
We have shown that an important feature of the Rh-
catalyzed H2-hydrogenation of one imine (PhCH2NdCHPh)
in MeOH solution is hydrolysis of the imine and generation
of a Rh-imine-amine species that subsequently activates
H2 to effect the catalysis. A key question remaining, and
being studied, is whether such hydrolysis is important more
generally to effect efficient imine hydrogenation.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada for financial
support, and Dr. Victor G. Young, Jr., and Dr. Maren Pink
(X-ray Crystallographic Laboratory at the University of
Minnesota) for solving the structure of 2b.
Supporting Information Available: X-ray crystallographic data
for the structure of the 2b in CIF format. Kinetic dependence on
[imine] (Figure S1), [Rh] (Figure S2), and [H2] (Figure S3) for
hydrogenation of PhCH2NdCHPh in MeOH at 30 °C. This material
is available free of charge via the Internet at http://pubs.acs.org.
(20) A reviewer suggested that a bis(imine) complex might be involved in
generation of the benzylamine; this is certainly possible, but while
Rh(I)-bis(imine) species are known,15 we have no evidence for such
species within the PhCH2NdCHPh system.
(21) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry; University
Science Books: Mill Valley, CA, 1987; pp 82, 286.
IC040037A
4824 Inorganic Chemistry, Vol. 43, No. 16, 2004