Facile Oxidative Addition of N-C and N-H Bonds to Monovalent Rhodium and Iridium

Article abstract of DOI:10.1021/ja049659l

An investigation of room-temperature oxidative addition of N-C and N-H bonds to Rh^I and Ir^I in solution and in the solid state is presented. The rigid, product-adapted framework of the pincer bis(ortho-phosphinoaryl)amine (PNP) ligan

Full text of DOI:10.1021/ja049659l

Published on Web 03/26/2004
Facile Oxidative Addition of N-C and N-H Bonds to Monovalent Rhodium
and Iridium
Oleg V. Ozerov,* Chengyun Guo, Vyacheslav A. Papkov, and Bruce M. Foxman
Department of Chemistry, Brandeis UniVersity, MS015, 415 South Street, Waltham Massachusetts 02454
Received January 19, 2004; E-mail: ozerov@brandeis.edu
Scheme 1
Tremendous advances in transition metal-catalyzed aromatic
amination over the past decade have made it a practical, widely
used reaction.1 Formation of C-N bonds via reductive elimination
(RE) is a crucial step of this process that has received substantial
attention.2 Its microscopic reverse, oxidative addition (OA) of a
C-N bond, may be essential for hydrodenitrogenation of petro-
leum.3 The related OA of N-H bonds is of relevance to hydroami-
nation of alkenes.4 However, well characterized examples of OA
of C-N bonds are exceedingly rare5 and those of OA of N-H
bonds are relatively few, as well.6 Investigations of well-defined
elementary steps and their microscopic reVerses provide thermo-
dynamic and kinetic information that is vital to our ability to design
and improve catalytic processes. A recent mechanistically fruitful
example of the microscopic reverse approach is the studies of C-C
and C-H RE on Pt.7
We report here our direct observations of oxidative addition of
N-H and N-C bonds to RhI and to IrI. The C-N OA examples
described here are unique in that they involve unstrained N-C(sp3)
bonds.
We have previously reported the synthesis of ligand 1 (Scheme
1) and its N-H cleavage chemistry with Pd.8,9 The N-Me
derivative 2 (Scheme 1) was prepared in an analogous fashion. Our
anticipation of the OA of N-H and N-C bonds of 1 or 2 to RhI
and IrI was based partly on the apparent stability (toward RE of
C-N) of related compounds prepared by Fryzuk et al.10 (A) and
on the facile C-H and C-C oxidative addition processes described
by Shaw et al. and Milstein et al. for the topologically similar PCP
ligand on Rh and Ir (B).11a-e Activation of C-O bonds by pincer-
ligated Rh, Pd, and Ni complexes has also been reported.11f,g
During the course of the formation of 3a and 3b, the initially
produced 1,5-COD is isomerized into 1,3-COD. It seems likely that
the isomerization of COD by the unsaturated hydrides 3 proceeds
by an insertion/â-H elimination pathway.14 Complexes 3a and 3b
also undergo H/D exchange with the C6D6 solvent at ambient
temperature (50% exchange in 10-15 h).
Mixing 2 and 0.5 equiv of [(COD)RhCl]2 in ether or C6D6 results
in the rapid formation of 4 and 1 equiv of free 1,5-COD. Over
time, yellow 4 slowly evolves into deep-green 5a, a product of
N-C oxidative addition. The most revealing indicators of the
migration of the Me group are its 1H (4: s, 3.72 ppm; 5a: td, 2.33
ppm, JH-Rh ) 3 Hz, JH-P ) 5 Hz) and 13C NMR (4: app q, 62.7
ppm, JCP‚JC-Rh‚2 Hz; 5a: dt, 1.9 ppm, JC-Rh ) 29 Hz, JC-P ) 5
Hz) characteristics. In addition, the similarity of JRh-P in 3a and
5a (106 and 109 Hz, respectively) is consistent with the RhIII
formulation, different from the RhI in 4 (JRh-P ) 154 Hz).
Thermolysis of a mixture of 2 and 0.5 equiv of [(COD)IrCl]2 in
an aromatic solvent (12 h, 85 °C) produces a green-colored mixture
of 5b and a pair of as yet unidentified Ir complexes (31P NMR
1
evidence). The H resonances of the PNP ligand in 5b are very
similar to those of 5a (but not to those of 4). The triplet (2JPH
)
5.5 Hz) signal at 2.33 ppm in the 1H NMR spectrum and the triplet
at -28.2 ppm in the 13C NMR spectrum of the mixture are assigned
to Ir-CH3. Under selective irradiation of the 31P NMR frequency
of 5b, the former peak appears as a singlet. The NMR data for
5a,b are thus in good agreement with those of the structurally
similar A and B.10,11
The transformation of 4 to 5a was investigated by variable-
temperature (25-62 °C) 31P NMR kinetic studies in C6D6. At all
temperatures the reaction followed a first-order rate law (d(ln[4])/
dt ) -k; k298 ) 1.45(3) × 10-5 s-1; t1/2(298 K)‚13 h), consistent
with an intramolecular process. The activation parameters were
determined (∆Hq ) 23.5(9) kcal/mol; ∆Sq ) -2(3) kcal/mol;
∆Gq298 ) 24.0(18) kcal/mol). The near-zero ∆Sq is consistent with
little increase in order in the transition state. This particularly argues
against dissociation of N or P in the transition state and is most
consistent with a least motion 1,2-shift of Me+. It is conceivable
that such a mechanism (coordination of the heteroatom followed
by “slip” of R+) may sometimes be operative for OA of RX (X )
Mixtures of several new PNP complexes are formed upon
allowing 1 to react with 0.5 equiv of [(COD)MCl]2 (M ) Rh, Ir;
COD ) cyclooctadiene) in an aromatic solvent at ambient tem-
perature. Free COD is concomitantly produced. Over time these
mixtures evolve into 3a or 3b nearly quantitatively. Conversion in
excess of 98% (by 31P NMR) is reached after 10 h (5 h) for 3a
(3b) at 22 °C and in less than 1 h for both 3a and 3b at 65 °C. The
distinctive spectroscopic feature of compounds 3a and 3b is the
upfield M-H resonance, found at -29.91 (-45.61) ppm for 3a
(3b). Such strongly upfield shifts are typical for five-coordinate
square pyramidal complexes of d6 metals (e.g., (R3P)2M(H)Cl(CO)
for M ) Ru, Os, or (R3P)2MHCl2 for M ) Rh, Ir)12,13 where the
hydride is trans to an empty site. A related N-H oxidative addition
to RhI to give a saturated product for a similar PNP ligand has
been recently reported.9b
9
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J. AM. CHEM. SOC. 2004, 126, 4792-4793
10.1021/ja049659l CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
functionality to Rh. Remarkably, in the solid state the oxidative
addition of N-C to Rh proceeds in a crystal-to-crystal fashion,
transforming only one of the two independent molecules in the
crystal.
Acknowledgment. We are grateful to Brandeis University for
support of this research. C.G. and B.M.F. are thankful to NSF (Grant
DMR-0089257) for partial support of this work. We thank Wei
Weng for the sample preparation for XRD studies.
Figure 1. ORTEP drawing (30% probability ellipsoids) of 4 (left, I of
D1) and 5a (right, II of D2) showing selected atom labeling. Omitted for
clarity: H atoms and Me groups except for Rh-Me.
Supporting Information Available: Experimental details, char-
acterization data, and CIF files. This material is available free of charge
via the Internet at http://pubs.acs.org.
heteroatom) in general. The kinetic importance of coordination of
aryl halide to Pd during amination reactions has been demonstrated
in some instances.15
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