Characterization of a rhodium(III)-imine-orthometalated imine complex: Reversible C-H activation of a coordinated imine

Article abstract of DOI:10.1021/om050240t

Solutions of cis-[Rh(PPh₃)₂(solv)₂]PF ₆ (solv = acetone or MeOH) react at room temperature under Ar with benzophenone imine to yield cis-[Rh(PPh₃)₂(η ¹-NH=CPh₂)₂]PF₆ (3), but depending on reaction conditions, one of the imine ligands undergoes orthometalation to give [RhH{NH=C(Ph)(o-C₆H₄)}(HN=CPh₂)(PPh ₃)₂]PF₆ (2) with trans-phosphines; this is the first recorded complex containing an imine in its η¹-form and as an η²-moiety formed via orthometalation. Complexes 2 and 3 are characterized by elemental analysis, ¹H and ³¹P-{ ¹H} NMR spectroscopy, and, in the case of 2, an X-ray crystal structure.

Full text of DOI:10.1021/om050240t

Organometallics 2005, 24, 3753-3757
3753
Characterization of a
Rhodium(III)-Imine-Orthometalated Imine Complex:
Reversible C-H Activation of a Coordinated Imine
Maria B. Ezhova, Brian O. Patrick, and Brian R. James*
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver,
British Columbia V6T 1Z1, Canada
Received March 30, 2005
Solutions of cis-[Rh(PPh3)2(solv)2]PF6 (solv ) acetone or MeOH) react at room temperature
under Ar with benzophenone imine to yield cis-[Rh(PPh3)2(η1-NHdCPh2)2]PF6 (3), but
depending on reaction conditions, one of the imine ligands undergoes orthometalation to
give [RhH{NHdC(Ph)(o-C6H4)}(HNdCPh2)(PPh3)2]PF6 (2) with trans-phosphines; this is the
first recorded complex containing an imine in its η1-form and as an η2-moiety formed via
1
orthometalation. Complexes 2 and 3 are characterized by elemental analysis, H and 31P-
{1H} NMR spectroscopy, and, in the case of 2, an X-ray crystal structure.
Introduction
More generally, the orthometalation reactions
activation of a C-H bond in the ortho-position of an
aromatic ring by transition metalsscontinues to be an
important class of reactions,3,4 because of potential
applications in diverse areas such as catalysis,4,5 organic
synthesis,6 and material science.7
As part of our ongoing studies on hydrogenation of
imines catalyzed by cationic, Rh(I)-phosphine species as
precursors,1 we reported recently on the room-temper-
ature reaction of imines of the type RNdCR′(R′′), where
R and R′′ are alkyl or aryl groups, and R′ ) H or Me,
with cis-[Rh(PPh3)2(solv)2]PF6 (1) and cis,trans,cis-[Rh-
(H)2(PPh3)2(solv)2]PF6 complexes (solv ) acetone or
MeOH; see Scheme S1 in the Supporting Information).2
In acetone solution, when R′′ ) Ph (or p-OMe-C6H4), the
products are species exemplified by [RhH{RNdCR′(o-
C6H4)}(PPh3)2(acetone)]PF6, in which the imine coordi-
nates with orthometalation in an η2 fashion via the
imine-N and the ortho-C atom of the phenyl group; in
MeOH solution, when R′ ) H, nonorthometalated spe-
cies, η1-species of the type [Rh(PPh3)2(RNdCHR′′)-
(MeOH)]PF6 can be formed and these provide one
pathway for subsequent hydrogenation of the imine by
reaction with H2.2 In an extension of this work, we have
now used benzophenone imine, HNdCPh2, as the reac-
tant imine with the Rh complexes, and from the reaction
in MeOH or acetone, the initially formed product is the
bis(imine) complex cis-[Rh(PPh3)2(η1-NHdCPh2)2]PF6
(3), a rare type of species, but this converts partially to
RhH{HNdC(Ph)(o-C6H4)}(HNdCPh2)(PPh3)2]PF6 (2),
containing the imine in its η1-form and as an η2-moiety
formed via orthometalation; this unique type of complex
is readily isolated. The corresponding reaction in CH2-
Cl2 gives solely complex 3. The interconversion between
2 and 3 is solvent-dependent and represents a further
example of reversible C-H activation within these
imine systems.2 The findings are summarized in Scheme
1.
Experimental Section
General experimental procedures and instrumentation used
have been described in a recent publication.2 The imine, HNd
CPh2 (an Aldrich product, pure according to GC and 1H NMR),
was used as received. The [Rh(COD)(PPh3)2]PF6 precursor, and
in situ [Rh(PPh3)2(solv)2]PF6 (1), [Rh(H)2(PPh3)2(solv)2]PF6 (solv
) acetone, MeOH), and [Rh(PPh3)2]2(PF6)2 species, were
prepared according to literature procedures.8,9
Reaction of cis-[Rh(PPh3)2(solv)2]PF6 (1) with 2 Equiv
of HNdCPh2. An acetone solution (4 mL) of [Rh(COD)(PPh3)2]-
PF6 (42 mg, 0.0475 mmol) was reacted with 1 atm H2 for 2 h
to form [Rh(H)2(PPh3)2(acetone)2]PF6 in situ. The H2 and the
volatile materials (solvent, cyclooctane) were removed under
vacuum at ∼40 °C overnight, and the resulting dark red
9
residue of [Rh(PPh3)2]2(PF6)2 was then redissolved in MeOH
(∼2 mL) under Ar to form [Rh(H)2(PPh3)2(MeOH)2]PF6. Imine
(17.3 mg, 0.095 mmol) was then added, and the reaction was
monitored by 1H and 31P{1H} NMR. After 1 h, the two species
[RhH{NHdC(Ph)(o-C6H4)}(η1-HNdCPh2)(PPh3)2]PF6 (2) and
cis-[Rh(PPh3)2(η1-NHdCPh2)2]PF6 (3) (see below) were ob-
(3) Shilov, A. E.; Shul’pin, G. B. Chem. Rev. 1997, 97, 2879.
(4) Ryabov, A. D. Chem. Rev. 1990, 90, 403.
(5) Trzeciak, A. M.; Zio´łkowski, J. J. Organomet. Chem. 2000, 597,
69.
(6) Ryabov, A. D. Synthesis 1985, 233.
(7) For example: (a) Maestri, M.; Sandrini, D.; Balzani, V.; Maeder,
U.; von Zelewsky, A. Inorg. Chem. 1987, 26, 1323. (b) Didier, P.;
Ortmans, I.; Kirsch-De Mesmaeker, A.; Watts, R. J. Inorg. Chem. 1993,
32, 5239. (c) Di Bella, S.; Fragali, I.; Ledoux, I.; Diaz-Garcia, M. A.;
Marks, T. J. J. Am. Chem. Soc. 1997, 119, 9550. (d) Buey, J.; Coco, S.;
Diez, L.; Espinet, P.; Martin-Alvarez, J. M.; Miguel, J. A.; Garcia-
Granda, S.; Tesouro, A.; Ledoux, I.; Zyss, J. Organometallics 1998, 17,
1750. (e) Aiello, I.; Crispini, A.; Ghedini, M.; La Deda, M.; Barigelletti,
F. Inorg. Chim. Acta 2000, 308, 121.
* To whom correspondence should be addressed. Tel: +1-(604)-822-
6645. Fax: +1-(604)-822-2847. E-mail: brj@chem.ubc.ca.
(1) (a) Marcazzan, P.; Patrick, B. O.; James, B. R. Organometallics
2003, 22, 1177. (b) Marcazzan, P.; Abu-Gnim, C.; Seneviratne, K. P.;
James, B. R. Inorg. Chem. 2004, 43, 4820. (c) Marcazzan, P.; Patrick,
B. O.; James, B. R. Inorg. Chem. 2004, 6838.
(8) Schrock, R. R.; Osborn, J. A. J. Am. Chem. Soc. 1971, 93, 2397.
Haines, L. M.; Singleton, E. J. Chem. Soc., Dalton Trans. 1972, 1891.
(9) Marcazzan, P.; Ezhova, M. B.; Patrick, B. O.; James, B. R. C. R.
Chim. 2002, 5 (J. A. Osborn Memorial Volume).
(2) Marcazzan, P.; Patrick, B. O.; James, B. R. Organometallics
2005, 24, 1445.
10.1021/om050240t CCC: $30.25 © 2005 American Chemical Society
Publication on Web 06/17/2005

3754 Organometallics, Vol. 24, No. 15, 2005
Ezhova et al.
Scheme 1. Formation of Benzophenone-Imine Complexes and Derived Orthometalated Species 2-5a
a
P ) PPh3; S ) solvent (acetone or MeOH).
served in a ratio of ∼1:25, but after 24 h, the colorless,
In CD3OD after 1 h, four Rh-containing species are seen,
1-3, and a new one (4), in an approximate ratio of 6:4:2:1, as
judged by intensities of the 31P{1H} signals; for 4, δP 43.0 (d,
JRhP ) 116), -143.2 (septet, PF6-). In acetone-d6, with 0.5 equiv
of HNdCPh2, just one product (4) was observed after 1 h (4:1
crystalline solid 2 precipitated from the solution. The solid was
filtered off, washed with 1:15 MeOH/hexanes (1 mL), and dried
in a vacuum. Yield: 38 mg (72%). 31P{1H} NMR (CD3OD): δ
45.2 (d, JRhP ) 117), -143.2 (septet, PF6-). 1H NMR (CD3OD):
2
1
δ -11.98 (dt, 1H, RhH, JHP ) 9.6, JRhH ) 11.6), 6.28 (d, 1H,
≈ 1), and there were trace H signals of the free imine. 31P-
m-(o-C6H4), 3JHH ) 7.1), 6.52 (dd, 1H, m-(o-C6H4), 3JHH ) 7.1),
{1H} NMR: δ 43.6 (d, JRhP ) 116), -143.2 (septet, PF6-). H
1
3
2
6.70 (pseudo t, 1H, p-(o-C6H4), JHH ) 7.1), 6.84 (d, 1H, o-(o-
NMR: δ -11.78 (dt, 1H, RhH, JHP ) 9.5, JRhH ) 10.6), 6.41
3
C6H4), JHH ) 7.1), 7.1-7.8 (m, ∼47H, Harom.+ 2NH). MS: 809
(br d, 2H, o-C6H4, JHH ≈ 6.9), 6.67 (pseudo t, 1H, o-C6H4, JHH
≈ 6.6), 6.78 (pseudo t, 1H, JHH ≈ 7.7), 6.88 (pseudo t, 1H,
p-HPhC, JHH ≈ 7.7), 7.0-8.0 (m, 33H, Harom + 1NH), 8.3 (d, 2H,
([Rh(PPh3)2(HNCPh2)]+, 40%), 627 ([Rh(PPh3)2]+, 100%), 287
([RhPPh2-H]+, 45%), 217 (89%), 91 (55%). IR: 3056 (νN-H),
2134 (νRh-H), 1570 (νCdN). Anal. Calcd for C62H52N2P3F6Rh: C,
65.61; H, 4.62; N, 2.47. Found: C, 65.67; H, 4.73; N, 2.59.
o-HPhC,
2JHH ) 6.8). Species 4 is formulated as [RhH{HNd
CPh(o-C6H4)}(PPh3)2(solv)]PF6 containing trans-phosphines.
Various mixtures of 1-4 (in amounts that varied with reaction
time) were observed using 0.5 equiv of imine in CD3OD, or
1.0 equiv of imine in acetone-d6.
If the [Rh(PPh3)2]2(PF6)2 was dissolved in CH2Cl2 (2 mL)
rather than MeOH, prior to reaction with the imine, only the
bis(imine) complex (3) is formed; addition of hexanes (∼1 mL)
gave a dark red solid, which was filtered off, washed with
hexanes, and dried overnight in a vacuum. Yield: 29 mg
(55%).31P{1H} NMR (CD2Cl2): δ 46.8 (d, JRhP ) 167), -143.2
In Situ Reaction of cis,trans,cis-[Rh(H)2(PPh3)2(solv)2]-
PF6 with 2 Equiv of HNdCPh2. [Rh(COD)(PPh3)2]PF6 (15
mg, 0.017 mmol) was used with 1 atm H2 for the in situ
preparation of the dihydride in acetone-d6 or in CD3OD (0.6
mL). The imine (6.17 mg, 0.034 mmol) was added to the
dihydride solution in a glovebox, and the reaction mixture was
then exposed to H2 again at room temperature. The reaction
1
3
(septet, PF6-). H NMR (CD2Cl2): δ 6.12 (d, 4H, o-HPhP, JHH
) 7.6), 7.05-7.20 (m, 15H, Harom.), 7.23 (t, 4H, m-HPhP, 3JHH
)
7.1), 7.35 (t, 2H, p-HPhC, 3JHH ) 7.5), 7.45 (m, 15H, Harom.), 7.65
3
(t, 4H, m-HPhC, JHH ) 7.4), 7.75 (br s, 2H, NH), 7.85 (t, 2H,
p-HPhC, 3JHH ) 7.3), 8.20 (d, 4H, o-HPhC, 3JHH ) 7.1). IR: 3275
(νN-H), 1590 (νCdN). Anal. Calcd for C62H52N2P3F6Rh: C, 65.61;
H, 4.62; N, 2.47. Found: C, 64.95; H, 4.90; N, 2.40. The low C
analysis results from the presence of CH2Cl2 that could not
be removed even on prolonged drying under vacuum (CH2Cl2
was detected at δH 5.60 in an acetone-d6 solution of 3);
inclusion of 0.15-0.20 mol of CH2Cl2 per mole of complex
allows for analytical data in excellent agreement with the
experimental values.
progress was followed by H and 31P{1H} NMR spectroscopy.
1
In the acetone-d6 system after 1 h, 2 and 3 were present in a
ratio of 1:2.3, this changing to 1:3 after 24 h. In CD3OD after
0.5 h, only 3 was evident, but after 24 h, complex 2 had
precipitated out. Neither system showed the presence of H2-
NC(H)Ph2, the imine hydrogenation product.
Catalytic Hydrogenation of Benzophenone Imine. [Rh-
(COD)(PPh3)2]PF6 (5 mg, 0.0056 mmol) and the imine (20.6
mg, 0.114 mmol) were dissolved in MeOH (10 mL), and the
solution was stirred under 1 atm H2 for 24 h; GC analysis (HP-
5890 instrument with an HP-17 column) revealed ∼5% con-
version to H2NC(H)Ph2. Analysis by NMR of an acetone-d6 or
CD3OD solution of the residue, obtained after removing the
MeOH under vacuum, showed the presence of the amine (δCH
5.20), unreacted imine, and initially just species 3, although
this slowly partially converted to 2.
Crystallographic Analysis of Complex 2. Colorless,
needle crystals of [RhH{NHdC(Ph)(o-C6H4)}(HNdCPh2)-
(PPh3)2]PF6 (2) were grown by layering hexanes over a MeOH
solution of the reacting mixture of 1 and 2 equiv of imine (see
above). X-ray measurements were made at 173(1) K on a
Rigaku/ADSC CCD area detector with graphite-monochro-
mated Mo KR radiation (0.71069 Å). Some crystallographic
If the [Rh(PPh3)2]2(PF6)2 was dissolved in acetone-d6 to
generate cis-[Rh(PPh3)2(acetone)2]PF6 and this was similarly
reacted with 2 equiv of imine, close to complete formation of 3
over 1 h was followed by partial conversion to 2 (after 2 days,
3:2 ≈ 2.5).
In Situ Reactions of cis-[Rh(PPh3)2(solv)2]PF6 (1) with
1 or 0.5 Equiv of HNdCPh2. [Rh(PPh3)2]2(PF6)2, obtained
from [Rh(COD)(PPh3)2]PF6 (20 mg, 0.023 mmol) as described
above, was dissolved in 0.6 mL of CD3OD, and HNdCPh2 (4.12
mg, 0.023 mmol) was added to the solution under Ar in a
glovebox; the reaction was monitored at room temperature by
1H and 31P{1H} NMR. For the corresponding reaction in
acetone-d6, the imine addition was done in two stages (2 ×
2.06 mg), each stage being monitored.

Rh(III)-Imine-Orthometalated Imine Complex
Organometallics, Vol. 24, No. 15, 2005 3755
Table 1. Crystallographic Data for 2
converted slowly and partially to the orthometalated-
imine(imine) species 2, with a K3/2 equilibrium constant
of ∼2.5 at ∼20 °C in acetone or MeOH (see Scheme 1).
We have noted recently reversible orthometalation of
the imine PhCH2NdC(H)Ph at a corresponding Rh
center in MeOH, but this system involved a monoimine-
(methanol) species.2 In this current work, in CH2Cl2,
only 3 is formed, and this suggests that the weak
intermolecular interactions involving the hydrido ligand
of 2 with H atoms of the phenyl groups of the trans-
PPh3 ligands (see below) perhaps assist in stabilizing
the orthometalated form in the polar solvents. Complex
3 was readily isolated in 55% yield from the CH2Cl2
solution.
Complexes 2 and 3 are well characterized by elemen-
tal analyses, NMR data, and, in the case of 2, X-ray
crystallography. The ORTEP for the cation of 2 is shown
in Figure 1, and selected bond lengths and angles are
given in Table 2. The structure shows an η2-N-imine
moiety coordinated via the imine-N atom and ortho-
metalated-C atom (Cortho), which thus exists as a five-
membered metallocycle, this being essentially coplanar
with the hydride and the η1-N atom of a second imine.
The hydride is trans to the orthometalated imine-N
atom, and the Cortho atom is trans to the η1-imine-N
atom. The N-Rh-C angle (79.3°) of the metallocycle
and the Cortho-Rh-H angle (102°) are similar to those
of other related cyclometalated Rh-imine systems2,16,17
and to the corresponding angles of the same orthometa-
lated benzophenone in several Os(II)-PiPr3 species.18
The distorted octahedral structure shows trans-PPh3
ligands that are bent slightly toward the hydride and
orthometalated-C atom, as indicated by the C-Rh-P
(87.6° and 88.1°), H-Rh-P (92° and 83°), and P1-Rh-
P2 (172°) angles, and two weak interactions between
the hydride on Rh and protons of the phenyl group on
the P atom (H1-H11 ) 2.264 Å, H1-H26 ) 2.094 Å).
Similar bending of the PPh3 ligands and weak interac-
tions with a Rh-hydride have been noted in related
complexes containing cyclometalated imine16 and cy-
clometalated azobenzene moieties.17
formula
fw
cryst color, habit
cryst size (mm)
cryst syst
space group
a (Å)
C62H52N2F6P3Rh
1134.88
colorless, needle
0.15 × 0.10 × 0.05
orthorhombic
Pna21 (No. 33)
27.571(1)
b (Å)
18.5892(7)
12.1020(5)
6202.6(4)
c (Å)
V (Å3)
Z
4
Dcalcd (g cm-3
)
1.215
µ (cm-1
)
4.07
total no. of reflns
no. of unique reflns
Rint (Friedels not merged)
no. of variables
R1
48 827
10 723
0.135
597
0.0.062 (I > 2.00 σ(I),
6888 obs reflns)
0.155 (all data)
0.96 (all data)
wR2
GOF
data are shown in Table 1. Data were processed using the
d*TREK program10 and integrated using the CrystalClear11
software package. The structure was solved by direct meth-
ods12 and expanded using Fourier techniques.13 The material
crystallized with lattice solvent (MeOH and/or hexanes) that
could not be modeled reasonably, so the SQUEEZE function
found in PLATON14 was used. All calculations were performed
using the teXsan crystallographic software.15 Further details
on the collection of data and the refinement of the structure
are available in the Supporting Information.
Results and Discussion
An equilibrium between the bis(imine) species, cis-
[Rh(PPh3)2(η1-NHdCPh2)2]PF6 (3), and mono(imine)-
orthometalated imine species, [RhH{NHdC(Ph)(o-C6H4)}-
(η1-HNdCPh2)(PPh3)2]PF6 (2), is evident from the
methods used for their syntheses. First, it is worth
noting that the hydrogenation of the [Rh(COD)(PPh3)2]-
PF6 precursor to give [Rh(H)2(PPh3)2(solv)2]PF6 is much
more rapidly achieved in acetone rather than in MeOH
solution, and the subsquent dark red residue isolated
is [Rh(PPh3)2]2(PF6)2, the cation being in fact [(PPh3)-
Rh(µ-PhPPh2)2Rh(PPh3)]2+;9 this dimer exists as such
in CH2Cl2, but in MeOH or acetone is converted to cis-
[Rh(PPh3)2(solv)2]PF6 (1).8,9
Reaction of 1 with 2 equiv of benzophenone imine in
MeOH or in acetone at room temperature gives rapid,
in situ formation of a 25:1 mixture of 3 and 2; in MeOH,
2 eventually precipitates out as a colorless, crystalline
solid in 72% yield, while in acetone the system slowly
equilibriates to a 5:2 mixture of 3:2. Similarly, if 2 is
dissolved in acetone or MeOH, it converts over 2 days
to the 5:2 ratio of 3:2 (after 1 h, the ratio was 1:2). Thus,
the bis(imine) complex 3 is first formed and then is
The bond lengths at the Rh are unexceptional,16,17 and
the CdN bond length in the η2-imine (1.314 Å) is close
to others reported for benzophenone imine orthometa-
lated at Os(II) centers (1.305, 1.291, 1.26 Å).18 The Cd
N bond length of the η1-imine (1.288 Å) is the same as
that found in [Rh{PPh2(CH2)2PPh2}(η1-imine)2]+ for a
cyclic imine,19 the only other structurally characterized
Rh(I)-η1-imine complex of which we are aware, although
such moieties are known with Rh(I)-tridentate, pincer-
type imine systems.20 Complex 2 is the first reported to
contain an imine in its η1-form and as an η2-moiety
formed via orthometalation. Further, we are unaware
of any X-ray structural data on other metal-monohy-
(10) d*TREK: Area Detector Software, version 4.13; Molecular
Structure Corporation: The Woodlands, TX, 1996-1998.
(11) CrystalClear 1.3.5 SP2; Molecular Structure Corporation: The
Woodlands, TX, 2003.
(12) SIR97: Altomare, A.; Burla, M. C.; Cammalli, G.; Cascarano,
M.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Poidori, G.;
Spagna, A. J. Appl. Cryst. 1999, 32, 115.
(16) (a) Ezhova, M. B.; Patrick, B. O.; James, B. R.; Ford, M. E.;
Waller, F. J. Russ. Chem. Bull. Int. Ed. 2003, 52, 2707. (b) Ezhova, M.
B.; Sereviratne, K.; Patrick, B. O.; James, B. R.; Ford, M. E.; Waller,
F. J. Inorg. Chem. 2005, 44, 1482.
(17) Huang, L.-Y.; Aulwurm, U. R.; Heinemann, F. W.; Knoch, F.;
Kisch, H. Chem. Eur. J. 1998, 4, 1641.
(18) (a) Esteruelas, M. A.; Lahoz, F. J.; Lo´pez, A. M.; On˜ate, E.; Oro,
L. A. Organometallics 1995, 14, 2496. (b) Barea, G.; Esteruelas, M.
A.; Lledo´s, A.; Lo´pez, A.; On˜ate, E.; Tolosa, J. I. Organometallics 1998,
17, 4065. (c) Esteruelas, M. A.; Gutie´rrez-Puebla, E.; Lo´pez, A. M.;
On˜ate, E.; Tolosa, J. I. Organometallics 2000, 19, 275.
(19) Becalski, A. G.; Cullen, W. R.; Fryzuk, M. D.; James, B. R.;
Kang, G. J.; Rettig, S. J. Inorg. Chem. 1991, 30, 5002.
(13) DIRDIF94: Beurskens, P. T.; Admiraal, G.; Beurskens, G.;
Bosman, W. P.; de Gelder, R.; Israel, R.; Smits J. M. M. The DIRDIF-
94 program system. Technical Report of the Crystallography Labora-
tory; University of Nijmegen: The Netherlands, 1994.
(14) PLATON: Spek, A. L. A Multipurpose Crystallographic Tool;
Utrecht University: Utrecht, The Netherlands, 2001.
(15) teXsan: Crystal Structure Analysis Package; Molecular Struc-
ture Corporation: The Woodlands, TX, 1985 and 1992.

3756 Organometallics, Vol. 24, No. 15, 2005
Ezhova et al.
Figure 1. ORTEP diagram for the cation of [RhH{NHdC(Ph)(o-C6H4)}(HNdCPh2)(PPh3)2]PF6 (2), with 50% probability
ellipsoids.
Table 2. Selected Bond Angles (deg) and Distances
(Å) for the Cation of Complex 2
highest mass fragment in the LSIMS mass spectrum
corresponded to [Rh(PPh3)2(HNCPh2)]+. The room-tem-
perature 31P{1H} NMR spectrum of 2 (in CD3OD)
consists of a doublet (δ 45.2, JRhP ) 117 Hz), the J value
being indicative of mutually trans-phosphines.2,8,9,16 The
high-field hydride 1H NMR resonance (a doublet of
angle
bond
P1-Rh1-P2
172.09(8)
102(2)
176.8(3)
79.3(3)
81(2)
Rh1-P1
Rh1-P2
Rh1-N1
Rh1-N2
Rh1-H1
C37-Rh1
N1-C43
N2-C50
2.3509(19)
2.3387(19)
2.178(7)
2.131(6)
1.46(6)
2.020(7)
1.314(9)
1.288(9)
C37-Rh1-H1
N2-Rh1-C37
N1-Rh1-C37
N2-Rh1-H1
N1-Rh1-H1
N2-Rh1-N1
C37-Rh1-P1
C37-Rh1-P2
H1-Rh1-P1
H1-Rh1-P2
2
triplets at δ -11.98, JRhH ) 11.6, JHP ) 9.6) is
consistent with a trans-Rh(PPh3)2-hydrido moiety.2 The
1H signals of the orthometalated-Ph ring (seen as two
doublets, a doublet of doublets, and a pseudo-triplet) are
shifted upfield compared with the broad signal at δ 7.1-
7.8 for the other 45 aromatic protons, and such shifts
have been noted previously in related Rh and Ir imine
systems.2,16,22 The imine-NH proton signals must be
buried under the broad signal of the 45 aromatic
protons, where it is impossible to quantify the integra-
tion for the 2 NH protons (for 3, δNH ) 7.75, see below).
176(3)
97.6(2)
87.62(19)
88.1(2)
92(3)
83(3)
drido-η1-imine complexes, which in any case are quite
rare.21
The H atoms attached to the imine N atoms were not
located, but there was crystallographic evidence for their
presence (see Supporting Information), and the geom-
etries of the coordinated η1- and η2-imines (as well as
1H NMR data, see below) unequivocally establish their
presence. The nonlocation of such H atoms by X-ray
analysis is not unusual.18a,c
The NMR spectroscopic data for complex 3 are
consistent with the formulation cis-[Rh(PPh3)2(η1-NHd
CPh2)2]PF6, this being present as a single isomer with
equivalent phosphines and equivalent imines. The JRhP
value for the 31P{1H} NMR doublet in CD2Cl2 is 167 Hz,
the same as that reported for a crystallographically
characterized [Rh{PPh2(CH2)2PPh2}(η1-imine)2]+ com-
plex that necessarily has cis-phosphorus atoms.19
The solid state IR spectrum of 2 shows bands assign-
able to νCdN, νRh-H, and νN-H (but see below), while the
1
(20) (a) Haarman, H. F.; Ernsting, J. M.; Kranenburg, M.; Kooijman,
H.; Veldman, N.; Spek, A. L.; van Leeuwen, P. W. N. M.; Vrieze, K.
Organometallics 1997, 16, 887. (b) Haarman, H. F.; Kaagman, J.-W.
F.; Smeets, W. J. J.; Spek, A. L.; Vrieze, K. Inorg. Chim. Acta 1998,
270, 34. (c) Gaunt, J. A.; Gibson, V. C.; Haynes, A.; Spitzmesser, S.
K.; White, A. J. P.; Williams, D. J. Organometallics 2004, 23, 1015.
(21) (a) Bohanna, C.; Esteruelas, M. A.; Lo´pez, A. M.; Oro, L. A. J.
Organomet. Chem. 1996, 526, 73. (b) Albe´niz, M. J.; Buil, M. L.;
Esteruelas, M. A.; Lo´pez, A. M. J. Organomet. Chem. 1997, 545-546,
495. (c) Barea, G.; Esteruelas, M. A.; Lledo´s, A.; Lo´pez, A. M.; Tolosa,
J. I. Inorg. Chem. 1998, 37, 5033. (d) Aime, S.; Diana, E.; Gobetto, R.;
Milanesio, M.; Valls, E.; Viterbo, D. Organometallics 2002, 21, 50.
Assignment of the H NMR signals was aided by an
1H-1H COSY experiment that revealed correlations
between (a) the doublet at δ 6.12 with the triplets at δ
7.23 and 7.35, and (b) the doublet at δ 8.20 and the
triplets at δ 7.65 and 7.85; each of these correlation sets
corresponds to 10 protons and is thus assigned to phenyl
(22) Marcazzan, P.; Patrick, B. O.; James, B. R. Russ. Chem. Bull.
Int, Ed. 2003, 52, 2715.

Rh(III)-Imine-Orthometalated Imine Complex
Organometallics, Vol. 24, No. 15, 2005 3757
groups labeled as either exo and endo on the imine C
atoms (see Scheme 1). The phenyl groups of coordinated
NHdCPh2 are mutually orthogonal, as seen in the
structures of 2 (Figure 1) and an Os complex;21d in 3,
as defined, the two exo phenyls will be closer to a PPh3
Rh systems, although the processes are solvent-depend-
ent. More generally, formation of five-membered cyclo-
metalated Rh(III)-hydrido complexes from a range of
Rh(I) precursors is usually thermodynamically fa-
vored.3,4,23 The Rh(I)-bis(imine) complex (3) may be
stabilized somewhat by the presence of the second
strongly π-accepting HNdCPh2 ligand.
1
ligand of the square planar complex, and the set of H
signals at the higher field is tentatively attributed to
the exo-phenyl protons. The noncorrelated broad 1H
signal at δ 7.75 is assigned to the imine-NH protons;
this value (and that for 2) is some 1.0-3.7 ppm at higher
field than those reported for η1-NHdCPh2 complexes (or
an orthometalated derivative) of Os(II) and Ru(II).18,21
This observation, together with the νN-H value for 2
(3056 vs 3120-3386 cm-1 for the Os and Ru com-
plexes18,21), suggests there might be interactions be-
tween the NH protons and those of the phenyl groups
The in situ reaction of cis,trans,cis-[Rh(H)2(PPh3)2-
(solv)2]PF6 (solv ) CD3OD or acetone-d6) with 2 equiv
of HNdCPh2 under H2 gave findings qualitatively
similar to those found for the corresponding reaction
with cis-[Rh(PPh3)2(solv)2]PF6 (1), in terms of formation
of the equilibrium mixture of 2 and 3. No H2NC(H)Ph2,
the hydrogenation product, was seen, and neither 2 or
3 reacts with H2; for the latter system presumably the
strongly electron-deficient imine impedes possible oxi-
dative addition to Rh(I). Under catalytic conditions in
MeOH (imine:Rh ) 20) at ambient temperature and 1
atm H2, only ∼5% conversion to the amine was seen
after 24 h. Under corresponding catalytic conditions, we
have shown previously that the imine PhCH2NdC(H)-
Ph can be hydrogenated rapidly via a cis-[Rh(PPh3)2-
(PhCH2NdCHPh)(L)]PF6 intermediate, where L is MeOH
(a species akin to 5) or PhCH2NH2 (formed by hydrolysis
of the imine by adventitious water).1,2 There was no
evidence in the current work for such hydrolysis of the
imine (to give NH3 and acetone), and the slow catalytic
hydrogenation likely results from the bulky phenyl
substituents. Of note, benzophenone imine has been
hydrogenated using [Rh(COD)(PPh3)2]PF6 as catalyst
precursor with 14 atm H2 at 100 °C, but which specific
solvent was used from a list of THF, MeOH, or CH2Cl2
cannot be ascertained from the report.24 Formation of
a species such as 2 must now be considered as a possible
intermediate in catalyzed reactions of any imine that
contains a phenyl substituent on the imine-carbon atom.
at the P atom, although the νN-H value for 3 (3275 cm-1
)
is normal; unfortunately, the NH protons of 2 were not
located crystallographically, and so there is no evidence
for such interactions in the solid state.
On reacting cis-[Rh(PPh3)2(solv)2]PF6 (1) with 1 equiv
of benzophenone imine in MeOH or acetone at room
temperature under Ar, a new species believed to be
[RhH{NHdC(Ph)(o-C6H4)}(PPh3)2(solv)]PF6 (4) was ob-
served along with 2 and 3 and unreacted 1 (see Scheme
1). With use of just 0.5 equiv of the imine in acetone
solution, 4 was the only product observed after 1 h and
was formed in situ in ∼50% yield, the maximum
possible: the 31P{1H} doublet with JRhP ) 116 Hz (again
1
revealing trans-PPh3 ligands), combined with the H
signals (high-field doublet of triplets and upfield-shifted
resonances of aromatic protons attributable to a cyclo-
metalated imine), is consistent with the above formula-
tion, which is a possible precursor to 2 by displacement
of the acetone by an η1-imine (Scheme 1). The NMR data
are very similar to those for the analogous, isolated
[MH{RNdCR′(o-C6H4)}(PPh3)2(acetone)]PF6 species (M
) Rh or Ir; R and R′ ) alkyl or aryl).2,22
Scheme 1 summarizes the essential chemistry incor-
porating production of 2-4 from 1, although 2 was
isolated using [Rh(PPh3)2]2(PF6)2 as precursor. The
mono-η1-imine(solvento) species (5) was not observed,
but these have been detected in MeOH solutions of a
corresponding Rh system with the imine, PhCH2NdCH-
(Ph), and were formed from the corresponding hydrido-
(orthometalated imine) species.2 In the present HNd
CPh2 system, 5 in acetone must be relatively rapidly
converted into the hydrido-orthometalated species 4.
Our earlier paper recognized the reversible orthometa-
lation process within 1:1 imine/Rh systems,2 while the
findings here establish similar behavior in 2:1 imine/
Acknowledgment. We thank the Natural Sciences
and Engineering Research Council of Canada (NSERC)
for funding.
Supporting Information Available: Scheme S1 and
some details on the crystallographic refinement data for the
structure of 2; the X-ray data are also available in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM050240T
(23) Omae, I. Organometallic Intramolecular Coordination Com-
pounds; Elsevier Science Pub. Co.: New York, 1986.
(24) Herrera, V.; Munoz, B.; Landaeta, V.; Canudas, N. J. Mol.
Catal. (A) 2001, 174, 141.