Rhodium and iridium amido complexes supported by silyl pincer ligation: Ammonia N-H bond activation by a [PSiP]lr complex

Article abstract of DOI:10.1021/ja906646v

(Chemical Equation Presented) Ir silyl pincer complexes insert into the N-H bonds of anilines and ammonia under mild conditions to form isolable [Cy-PSiP]Ir(H)(NHR) complexes that are resistant to N-H bond reductive elimination at room temperature, even in the presence of arenes, alkenes, and phosphines.

Full text of DOI:10.1021/ja906646v

Published on Web 09/23/2009
Rhodium and Iridium Amido Complexes Supported by Silyl Pincer Ligation:
Ammonia N-H Bond Activation by a [PSiP]Ir Complex
Erin Morgan,† Darren F. MacLean,† Robert McDonald,‡ and Laura Turculet*,†
Department of Chemistry, Dalhousie UniVersity, Halifax, NoVa Scotia, Canada B3H 4J3, and the X-ray
Crystallography Laboratory, Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G2
Received August 5, 2009; E-mail: laura.turculet@dal.ca; laura.turculet@dal.ca
Scheme 1. Synthesis and Reactivity of [Cy-PSiP]M(H)(NHR) and
[Cy-PSiP]ML Complexes (M ) Rh and Ir)
Due to the prevalence of nitrogen-containing functional groups
in pharmaceuticals and other fine chemicals, significant interest
exists in the development of new pathways for the functionalization
of amines and, most notably, ammonia.1 Indeed, transformations
such as ammonia-arene dehydrogenative coupling and the hy-
droamination of olefins with ammonia have been noted among the
ten greatest current challenges for catalysis.2 Given that E-H (E
) H, B, C, Si) bond oxidative addition to late metal centers
underpins a number of prominent catalytic transformations,3 the
identification of late metal fragments that can insert into the N-H
bonds of ammonia under mild conditions is likely to figure
importantly in the emergence of new catalytic processes that utilize
ammonia as a substrate. However, well-documented examples of
ammonia N-H bond oxidative addition are rare.4-6 In particular,
examples of N-H activation of ammonia to form an isolable,
terminal, late metal LnM(H)(NH2) complex are limited to a single
report by Zhao, Goldman, and Hartwig involving a (PCP)Ir pincer
system.4a In the context of the reactivity challenges cited above,
the identification of alternative late metal complexes capable of
ammonia N-H bond activation, yet possessing divergent reactivity
profiles in the ensuing LnM(H)(NH2) species especially in the
presence of alkenes or arenes, represents an important goal in the
field of ammonia functionalization.
anilide Li(NHR) (Scheme 1). These reactions occur upon mixing
to generate isolable anilido hydride complexes (80-89%). In each
case, the solution 1H, 13C, and 31P NMR spectra indicate the
formation of a Cs-symmetric species and, along with 29Si and 15N
NMR data, support the structural formulation depicted in Scheme
1. For 1, 3, and 4 the connectivity was confirmed by use of single
crystal X-ray diffraction techniques (Figure 1 for 1 and 4). The
geometry at the metal center in each complex can be described as
distorted square-based pyramidal, with Si occupying the apical
coordination site.9 These structures differ from that of the related
complex [C6H3-2,6-(CH2PtBu2)2]Ir(H)(NHPh),7b which features
square pyramidal coordination geometry with the hydride occupying
the apical position and, thus, oriented cis to the anilide ligand. The
Ir-N distances in 3 (2.056(2) Å) and 4 (2.077(3) Å) are comparable
to that of [C6H3-2,6-(CH2PtBu2)2]Ir(H)(NHPh) (Ir-N ) 2.082(2)
Å),7b while the Rh-N distance of 2.123(5) Å in 1 is slightly longer.
In an attempt to prepare parent amido complexes of the type
[Cy-PSiP]M(H)(NH2), [Cy-PSiP]Ir(H)Cl was treated with 5 equiv
In this contribution we report that Ir complexes supported by
silyl pincer ligation undergo N-H bond oxidative addition of both
ammonia and anilines7 to form isolable complexes of the type [Cy-
PSiP]Ir(H)(NHR) (R
)
H, aryl; [Cy-PSiP]
)
[κ3-(2-
Cy2PC6H4)2SiMe]-). Under similar reaction conditions related Rh
species form simple adducts of the type [Cy-PSiP]Rh(NH2R). Our
reactivity studies reveal that, in comparison to previously reported
(PCP)Ir systems, [Cy-PSiP]Ir(H)(NHR) species are significantly
more resistant to N-H bond reductive elimination, even in the
presence of alkene and arene substrates.
We have previously reported that complexes of the type [Cy-
PSiP]Ir(H)(alkyl) rapidly eliminate alkane and in the presence of
arenes undergo sp2-C-H bond activation to generate [Cy-
PSiP]Ir(H)(aryl).8 Given that C-H and N-H bonds feature similar
homolytic bond strengths, we viewed [Cy-PSiP]MLn (M ) Rh, Ir)
species as attractive candidates for the study of N-H bond cleavage
reactions. As complexes of the type [Cy-PSiP]M(H)(NHR) (R )
H, aryl) were unknown prior to this work, we first sought to
establish the viability of putative N-H bond activation products
by rationally preparing and characterizing such compounds.
The synthesis of the anilido hydride complexes [Cy-PSiP]M-
(H)(NHR) (M ) Rh: R ) Ph, 1; R ) 2,6-Me2C6H3, 2; M ) Ir: R
) Ph, 3; R ) 2,6-Me2C6H3, 4) was readily achieved by the reaction
of [Cy-PSiP]M(H)Cl with 1 equiv of the corresponding lithium
Figure 1. Crystallographically determined structures of 1, 4, and 10 shown
with 50% ellipsoids. Selected interatomic distances (Å) and angles (deg)
for: 1, Rh-Si 2.243(1), Rh-N 2.123(5), Si-Rh-N 118.3(2); 4, Ir-Si
2.2791(8), Ir-N 2.077(3), Si-Ir-N 129.7(1); 10, Rh-Si 2.2872(5), Rh-P3
2.3597(5), Si-Rh-P3 147.61(2).
† Dalhousie University.
‡ University of Alberta.
9
14234 J. AM. CHEM. SOC. 2009, 131, 14234–14236
10.1021/ja906646v CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
of LiNH2, which led to clean formation of [Cy-PSiP]Ir(H)(NH2)
(5) upon heating (65 °C, 12 h). Complex 5 was isolated in 92%
yield as a yellow benzene-soluble solid. Isolated 5 features NMR
characteristics that are similar to those of 3 and 4. The 31P and 29Si
NMR spectra of 5 each contain a single resonance at 55.5 and 14.6
ppm, respectively. The 1H NMR spectrum of 5 (benzene-d6) features
a hydride resonance at -20.13 ppm (t, 2JHP ) 15 Hz) as well as a
slightly broad triplet at 5.03 ppm (3JHP ) 6 Hz) corresponding to
the NH2 protons; the latter correlates to a 15N NMR resonance at
an atmosphere of anhydrous ammonia led to the quantitative
generation of new Rh complexes that are tentatively assigned as
adducts of the type [Cy-PSiP]Rh(NH2R) (R ) Ph, 6; R ) H, 7;
1H, 31P, 29Si NMR). For 6, heating of the reaction mixture (65 °C,
72 h) led to 25% conversion to 1 (1H and 31P NMR); additional
heating generated multiple unidentified products and afforded no
further conversion to 1. No reaction was observed upon heating of
7 (65 °C, 48 h) under an atmosphere of ammonia. Complexes 6
and 7 were stable in solution in the presence of an excess of aniline
or ammonia; however the coordinated amine was readily displaced
upon exposure to vacuum, precluding the isolation of these
compounds.
1
-309.8 ppm (referenced to MeNO2) in a H-15N HMQC experi-
ment. Attempts to prepare a Rh variant of 5 by a similar route led
to complex reaction mixtures from which no pure materials could
be isolated.
In a preliminary investigation into the mechanism of Ir-mediated
ammonia activation, we studied the reactivity of in situ generated
[Cy-PSiP]Ir(H)(alkyl) (which undergoes rapid loss of alkane, Vide
supra) with an atmosphere of ammonia-d3. In cyclohexane-d12, the
formation of 5-d3 was observed (1H and 31P NMR) after heating
Having shown that complexes of the type [Cy-PSiP]M(H)(NHR)
are synthetically accessible and readily isolable, we sought to
demonstrate that such compounds could be prepared via N-H bond
activation starting from in situ generated [Cy-PSiP]M(H)(alkyl) and
H2NR (Scheme 1). Treatment of [Cy-PSiP]Ir(H)Cl with 1 equiv of
Me3SiCH2Li in cyclohexane-d12 led to an immediate reaction in
which [Cy-PSiP]Ir(H)Cl was consumed and Me4Si was generated
(1H and 31P NMR).10 The reaction mixture was subsequently treated
2
(65 °C, 20 h; eq 1). H NMR analysis of the reaction mixture (in
C6H6) indicated deuterium incorporation solely at the hydride and
amide positions, with no evidence for deuteration of the ligand PCy2
groups. This observation is in accord with the possible intermediacy
of a [Cy-PSiP]IrI 14 e- species,13 or a reactive equivalent (e.g., an
agostic species), en route to N-H bond oxidative addition and is
in keeping with our previous studies of room temperature sp2-C-H
bond activation by [Cy-PSiP]Ir, where reaction with benzene-d6
cleanly afforded [Cy-PSiP]Ir(D)(Ph-d5).8
1
with 1 equiv of H2NPh, and upon heating (65 °C, 16 h), H and
31P NMR analysis indicated the quantitative formation of 3. In a
preparative-scale reaction 3 was obtained in 96% isolated yield by
this N-H bond activation pathway. For the sterically hindered
H2N(2,6-Me2C6H3), quantitative conversion (1H and 31P NMR) to
4 was attained under similar reaction conditions (65 °C, 72 h)
utilizing 20 equiv of H2N(2,6-Me2C6H3). Under analogous condi-
tions in benzene solution, C-H bond activation of the solvent is
competitive with aniline N-H bond activation. Thus, treatment of
[Cy-PSiP]Ir(H)Cl with Me3SiCH2Li in benzene-d6 solution led to
the formation of [Cy-PSiP]Ir(D)(Ph-d5) and Me4Si (1H and 31P
NMR).8 Subsequent reaction with 1 equiv of H2NPh led to 35%
conversion (1H and 31P NMR) to 3 following heating for 72 h at
65 °C; after 168 h at 65 °C complete conversion to 3 was not
observed. However, quantitative conversion to 3 was obtained when
20 equiv of H2NPh were reacted with [Cy-PSiP]Ir(D)(Ph-d5) in
benzene-d6 solution (65 °C, 70 h).
We also undertook trapping experiments to gain further support
for the intermediacy of [Cy-PSiP]MI species in the observed E-H
(E ) C, N) bond activation chemistry. For M ) Rh, [Cy-
PSiP]Rh(NH2R) complexes (6, 7) were directly observed when [Cy-
PSiP]Rh was generated in the presence of excess NH2R. However,
these complexes were not isolable (Vide supra), and for M ) Ir
intermediates of this type could not be unequivocally identified en
route to 3 and 5. Trapping experiments conducted with donor
ligands such as PMe3 and ethylene did, however, lead to the
formation of isolable [Cy-PSiP]ML complexes (M ) Ir: L ) C2H4,
8; L ) PMe3, 9; M ) Rh: L ) PMe3, 10), confirming for the first
time the viability of isolable RhI and IrI species supported by [Cy-
PSiP] ligation (Scheme 1). Surprisingly, the geometry at Rh in the
crystal structure of 10 (Figure 1) is significantly distorted from
square planarity, as indicated by the P3-Rh-Si bond angle of
147.61(2)°.
The reaction of 5 with an atmosphere of ammonia-d3 was carried
out to assess if 5 undergoes exchange with free ammonia. Upon
standing for 14 h in cyclohexane-d12 (25 °C), evidence for 2H
incorporation was observed only at the IrNH2 position (1H and 2H
NMR). The observed 2H incorporation in 5 suggests that this
transformation does not proceed via simple reductive elimination
of NH3 followed by oxidative addition of ND3; mechanistic
examinations of this transformation are ongoing. Treatment of 5
with 1 equiv of H2NPh at room temperature in cyclohexane-d12
solution resulted in an immediate and quantitative reaction to form
3 as well as free ammonia (1H and 31P NMR; eq 2). In light of the
greater acidity of aniline relative to ammonia, the latter result is
not surprising.14 However, this result stands in contrast to the
observation that [CH(CH2CH2PtBu2)2]Ir(H)(NH{3,5-Me2C6H3})
reacts with ammonia to generate [CH(CH2CH2PtBu2)2]Ir(H)(NH2)
and the free aniline.4a
Remarkably, the Ir parent amido complex 5 was readily prepared
via N-H bond activation of ammonia. Treatment of [Cy-
PSiP]Ir(H)Cl with 1 equiv of Me3SiCH2Li in cyclohexane-d12
resulted in the consumption of [Cy-PSiP]Ir(H)Cl and formation of
1 equiv of Me4Si (1H and 31P NMR). The reaction mixture was
subsequently degassed, and an excess of anhydrous gaseous
ammonia (ca. 1 atm) was introduced. While no reaction was
observed at room temperature, heating at 65 °C for 14 h led to
72% conversion to 5 (1H and 31P NMR). In a preparative scale
experiment, 5 was obtained in 69% isolated yield by this ammonia
N-H bond activation pathway. Under analogous conditions in
benzene-d6 solution, heating at 65 °C over the course of 144 h led
to 45% conversion to 5 (1H and 31P NMR), in which substantial
deuterium incorporation into the Ir-H and PCy2 fragments of 5
was observed (1H and 2H NMR).11 Additional heating at this
temperature led to the formation of multiple unidentified products
and provided no further conversion to 5. The analogous oxidative
addition of ammonia in benzene solution to form (PCP)Ir(H)(NH2)
species has yet to be demonstrated.
Attempts to form amido hydride complexes such as 1 via N-H
bond activation led to the generation of RhI amine adducts.
Treatment of [Cy-PSiP]Rh(H)Cl with 1 equiv of Me3SiCH2Li in
cyclohexane-d12 led to an immediate reaction in which [Cy-
PSiP]Rh(H)Cl was consumed and Me4Si was generated (1H and
31P NMR).12 Subsequent addition of either 20 equiv of H2NPh or
9
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C O M M U N I C A T I O N S
Acknowledgment. We are grateful to the Natural Sciences and
Engineering Research Council (NSERC) of Canada and Dalhousie
University for their support of this work. We also thank Dr. M.
Lumsden (NMR3, Dalhousie) for assistance in the acquisition of
NMR data.
The observation that in the [Cy-PSiP]Ir system both aniline and
ammonia N-H bond activation products are formed in benzene
solution suggests that the thermodynamics of N-H bond activation
are more favorable than those of arene C-H bond activation.
Interestingly, unlike related (PCP)Ir(H)(NHR′) (R′ ) aryl) com-
plexes that at room temperature in arene solvents exhibit an
equilibrium between N-H and arene sp2-C-H bond activation
products (the balance of which depends on the Ir fragment and the
amine involved),7b,c the [Cy-PSiP]M(H)(NHR′) complexes 1-4
reported herein are stable at room temperature in benzene or toluene,
with no spectroscopic evidence of an equilibrium between N-H
and C-H bond activation products. Similarly, no evidence for N-H
bond reductive elimination was observed at room temperature or
at 65 °C (72 h) when [Cy-PSiP]Ir(H)(NH2) (5) was dissolved in
arene solvents; only under forcing conditions (100 °C, 48 h,
benzene-d6) was NH3 reductive elimination observed to cleanly
generate [Cy-PSiP]Ir(D)(Ph-d5). Notably, only two comparator
systems of the type (PCP)Ir(H)(NH2) exist: [C6H3-2,6-
(CH2PtBu2)2]Ir(H)(NH2)7b and [CH(CH2CH2PtBu2)2]Ir(H)(NH2).4a
While the former was generated in situ via dehydrohalogenation
of [C6H3-2,6-(CH2PtBu2)2]Ir(H)(Cl)(NH3) and was observed to
undergo N-H reductive elimination above -10 °C in THF solution,
the latter isolable complex is stable in alkane and ethereal solvents
Supporting Information Available: Experimental details and
characterization data, including crystallographic data for 1, 3·OEt2, 4,
and 10 (CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
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at
room
temperature.
In
benzene-d6
solution
[CH(CH2CH2PtBu2)2]Ir(H)(NH2) undergoes deuterium incorporation
into the backbone methine position of the pincer ligand, possibly
via a mechanism involving N-H reductive elimination.15 Further-
more, while a mixture of [CH(CH2CH2PtBu2)2]Ir(H)(NH2) and
[CH(CH2CH2PtBu2)2]Ir(1-pentene) was formed upon treatment of
the latter with excess 1-pentene (Et2O-d10 or benzene-d6),4a,15 no
reaction was observed at room temperature upon exposure of a
benzene-d6 solution of 5 to an atmosphere of ethylene.16 As well,
whereas treatment of 5 with 1 equiv of PMe3 led to the quantitative
formation of 5·PMe3 (1H, 13C, 31P, and 29Si NMR), upon exposure
to vacuum the loss of PMe3 to reform 5 was observed in the absence
of N-H reductive elimination (Scheme 1).16 Collectively, these
observations confirm that, in addition to supporting reactive Ir
species that are able to undergo N-H bond activation reactions,
[Cy-PSiP] ligation provides a means of stabilizing amido hydride
complexes from N-H reductive elimination in a manner that has
not been demonstrated in previously reported (PCP)Ir systems. Such
ancillary ligand effects may prove important in the development
of novel catalytic chemistry involving amine N-H bond activation.
In conclusion, we have demonstrated that Ir complexes supported
by [Cy-PSiP] ligation undergo N-H bond oxidative addition of
anilines and ammonia under mild conditions to form isolable [Cy-
PSiP]Ir(H)(NHR) complexes. In comparison to previously reported
(PCP)Ir systems, [Cy-PSiP]Ir(H)(NHR) species are significantly
more resistant to N-H bond reductive elimination, even in the
presence of alkene and arene substrates. Such an example of
ammonia N-H bond activation is exceedingly rare and may provide
inroads to new atom-economical chemical transformations that
incorporate N-H bond oxidative addition steps in the functional-
ization of this abundant feedstock.
(8) MacLean, D. F.; McDonald, R.; Ferguson, M. J.; Caddell, A. J.; Turculet,
L. Chem. Commun. 2008, 5146.
(9) Alternatively, given that these complexes feature acute Si-M-H1 angles
(1, 67.8°; 4, 68.8°) that are similar to those previously observed for [Cy-
PSiP]M(H)Cl (M ) Rh, 65.8°; M ) Ir, 68.7°), they can also be described
as “Y-shaped”.8
(10) The Ir product of this reaction, which gives rise to a very broad 31P NMR
resonance at 56.2 ppm, has previously been observed and shown to react
with arenes to form [Cy-PSiP]Ir(H)(aryl).8
(11) Control experiments in which [Cy-PSiP]Ir(D)(Ph-d5) was heated at 65 °C
for 48 h also revealed substantial deuterium incorporation into the PCy2
fragments.
(12) The Rh-containing product of this reaction, which gives rise to a broad 31
P
NMR resonance at 62.9 ppm (1JRhP ) 162 Hz), has previously been
documented.8
(13) Related 14 e- (PCP)MI and (PNP)MI species (M ) Rh, Ir) have been
proposed as reactive intermediates in E-H bond activation reactions:(a)
Kanzelberger, M.; Singh, B.; Czerw, M.; Krogh-Jespersen, K.; Goldman,
A. S. J. Am. Chem. Soc. 2000, 122, 11017. (b) Go¨ttker-Schnetmann, I.;
White, P. S.; Brookhart, M. Organometallics 2004, 23, 1766. (c) Gatard,
S.; Celenligil-Cetin, R.; Guo, C.; Foxman, B. M.; Ozerov, O. V. J. Am.
Chem. Soc. 2006, 128, 2808. (d) Verat, A. Y.; Pink, M.; Fan, H.;
Tomaszewski, J.; Caulton, K. G. Organometalics 2008, 27, 166.
(14) Holland, P. L.; Andersen, R. A.; Bergman, R. G.; Huang, J. K; Nolan,
S. P. J. Am. Chem. Soc. 1997, 119, 12800.
(15) Zhou, J.; Hartwig, J. F. Angew. Chem., Int. Ed. 2008, 47, 5783.
(16) The viability of the ethylene and PMe3 Ir(I) adducts 8 and 9 that could
result from reductive elimination of ammonia was confirmed via indepen-
dent synthesis.
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