Dihydroperimidine-derived N-heterocyclic pincer carbene complexes via double C-H activation

Article abstract of DOI:10.1021/om300897w

The reactions of 1,8-diaminonaphthalene with paraformaldehyde and secondary phosphines (HPR₂, R = Ph, Cy) directly afford N,N'- bis(phoshinomethyl)dihydroperimidines H₂C(NCH₂PR ₂)₂C₁₀H

Full text of DOI:10.1021/om300897w

Communication
pubs.acs.org/Organometallics
Dihydroperimidine-Derived N‑Heterocyclic Pincer Carbene
Complexes via Double C−H Activation
Anthony F. Hill* and Caitlin M. A. McQueen
Research School of Chemistry, The Australian National University, Canberra, Australian Capital Territory 0200, Australia
S
* Supporting Information
ABSTRACT: The reactions of 1,8-diaminonaphthalene with paraformaldehyde and
secondary phosphines (HPR2, R = Ph, Cy) directly afford N,N′-bis(phoshinomethyl)-
dihydroperimidines H2C(NCH2PR2)2C10H6 (R = Ph (1a), Cy (1b)), the methylene
group of which undergoes chelate-assisted double C−H activation with [RhCl-
(PPh3)3] to afford dihydroperimidine-derived N-heterocyclic pincer carbene (per-
NHC) complexes [RhCl{C(NCH2PR2)2C10H6}] (R = Ph (2a), Cy (2b)). Insight
into the mechanism of these C−H activation processes is provided by the reaction of
1b with [IrCl(CO)(PPh3)2] to provide the dihydroperimidinyl-hydrido complex
[IrHCl(CO){CH(NCH2PR2)2C10H6}] (3b), which in turn reacts with silver salts
Ag[Y] to afford, via hydride abstraction and subsequent C−H activation, the per-NHC
ligated salts [IrHCl(CO){C(NCH2PR2)2C10H6}] ([4b]Y, Y = PF6, SbF6, PO2F2).
-heterocyclic carbene (NHC)1 and pincer2 ligands have
in recent times provided fertile avenues for the design of
and, to date, have required a hydrogen acceptor.12 With this in
mind, we have devised a remarkably convenient one-pot,
solvent-free synthesis of 2,3-dihydriperimidine per-NHC-H2
pincer proligands H2C(NCH2PR2)2C10H6-1,8 (R = Ph (1a),
Cy (1b)) from 1,8-diaminonaphthalene, paraformaldehyde, and
secondary phosphines HPR2 (Scheme 1). The ligands are
obtained analytically pure in high yields after a single
recrystallization.
N
numerous effective and robust metal catalysts. The high
stability and modular variability offered by pincer systems has
led to applications in an extensive range of fields, while strongly
electron-donating NHC ligands have become well-established
as supporting ligands in catalytic systems, emulating the donor
properties of phosphines, but less prone to dissociation. It is
therefore unsurprising that the inclusion of NHC donors within
pincer systems has attracted an increasing level of interest, with
a particular emphasis on their catalytic potential.3 While the
most popular NHC scaffolds remain those based on five-
Scheme 1. Synthesis of 2,3-Dihydroperimidine PCP Pincer
Proligands
membered heterocyclic rings (imidazolinylidenes, dihydroimi-
6
dazolinylidenes, etc.), Fehlhammer,4 Richeson,5 Ozdemir,
̈
Herrmann,7 Dotz,8 and Mashima9 have each described
̈
dihydroperimidine-based NHC ligands (per-NHC) for which
both experimental5−9 and computationally derived data10
indicate enhanced σ basicity. Though per-NHCs have been
comparatively little studied, their catalytic potential has already
been demonstrated,6,9 as has their inclusion as axial donors
within meridional pincer frameworks.9 However, a pincer
system incorporating a per-NHC as the central equatorial group
has not been reported, despite the relaxed geometric
constraints that should allow appended axial donors to more
readily occupy trans sites with reduced metallabicycle strain.
Herein we describe the first examples of such ligands,
coordinated to rhodium(I) and iridium(III) centers, arising
from the double geminal C−H bond activation of the
methylene group of readily accessible neutral precursors.
NHC installation usually requires activation of a cationic
azolium precursor using either a basic coligand (e.g., OMe,
OAc), an external base, or the intermediacy of silver or mercury
reagents.1 Atom-efficient instances of carbene ligand installation
via geminal double C−H bond activation have become
increasingly frequent11 but for NHC synthesis remain rare
Both proligands were structurally characterized, and the
results obtained for 1a are summarized in Figure 1,13 which
suggest that the molecular geometry appears well disposed to
subsequent chelate-assisted C−H activation of the methylenic
unit upon reaction with appropriate metal centers. This was
indeed found to be the case: both 1a and 1b were observed to
undergo double dehydrogenation of the central methylene
group upon direct reaction with [RhCl(PPh3)3] to afford the
per-NHC pincer complexes [RhCl{κ3-P,C,P′C-
(NCH2PR2)2C10H6}] (R = Ph (2a), Cy (2b); Scheme 2).
These reactions proceed at ambient temperature to give the
Received: September 20, 2012
Published: November 28, 2012
© 2012 American Chemical Society
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Figure 2. Molecular structure of 2a (aryl hydrogen atoms omitted,
50% displacement ellipsoids), including a view along the coordination
plane. Selected bond lengths (Å) and angles (deg): Rh1−Cl1 =
2.401(1), Rh1−P1 = 2.247(1), Rh1−P2 = 2.2545(1), Rh1−C1 =
1.948(4), N1−C1 = 1.388(4), N2−C1 = 1.372(5); P1−Rh1−P2 =
165.01(4), N1−C1−N2 = 114.3(3), N1−C1−Rh1 = 122.2(3), N2−
C1−Rh1 = 123.4(2), ∑(N1) = 359.4, ∑(N2) = 359.6.
Figure 1. Molecular structure of 1a (50% displacement ellipsoids).
Selected bond lengths (Å) and angles (deg): C1−N1 = 1.469(3), C1−
N2 = 1.451(3), C2−N1 = 1.402(3), C10−N2 = 1.433(3); ∑(N1) =
350.1, ∑(N2) = 336.0.
Scheme 2. Synthesis of per-NHC Pincer Complexes via
Double Geminal C−H Activation with Associated
Mechanistic Conjecture (in Gray)
rhodium(I) sits comfortably within the per-NHC pincer
embrace.
The facility of the reactions leading to 2, without recourse to
a sacrificial hydrogen acceptor, may be attributed to chelation
assistance. While it might be argued from a Thorpe−Ingold
perspective that steric factors play a role in the rapidity of the
metallacyclization of 2b with regard to 2a, we are inclined to
suspect that it is rather the potent σ basicity of the PCy2 groups
that facilitates C−H oxidative addition. A possible mechanism
for how this chelate-assisted bond activation might proceed is
suggested in Scheme 2,15 in which a putative σ-2-perimidinyl
complex plays a role, arising from the first C−H activation
process. Subsequent loss of dihydrogen could then occur either
via an oxidative addition/reductive elimination sequence or,
perhaps less likely, via a concerted σ-CAM process.16 It should
be noted that, at this stage, we have no evidence that would
indicate whether the first C−H activation precedes or follows
coordination of the second phosphine donor (vide infra). The
inclusion of a π-acidic coligand such as CO might be expected
to retard such processes, while replacement of rhodium by
iridium would be expected to decelerate the kinetics. Given that
σ-2-perimidinyl ligands are as yet without precedent, we
therefore investigated the reaction of 1b with Vaska’s complex
[IrCl(CO)(PPh3)2] and were indeed able to isolate a model
complex in support of the proposed mechanism. A remarkably
facile reaction ensues which is complete within 2 h at room
temperature to afford the complex [IrHCl(CO){κ3-P,C,P′-
CH(NCH2PR2)2C10H6}] (3b) in 93% isolated yield. Although
3b is a complex of iridium(III), infrared data (νIrH, νCO 2012,
1940 cm−1) suggest a particularly π-basic metal center.17 The
hydrido ligand gives rise to a triplet resonance in the 1H NMR
spectrum (δH −17.9, 2JPH = 11 Hz). The spectrum also includes
a singlet resonance (δH 5.75) which correlates with that at δC
87.7 (1H13C HSQC), these being attributed to the methine of
the perimindinyl heterocycle. The approximate Cs symmetry of
3b (cf. C2v for 2) renders the PCH2 protons diastereotopic, as
products in high isolated yields (2a, 88%; 2b, 92%). The
31P{1H} NMR spectra of the reaction mixtures (C6D6) showed
spectroscopically quantitative conversion to the final products,
within 21 h (R = Ph) or 10 min (R = Cy), with the evolution of
1
dihydrogen being confirmed by H NMR spectroscopy.
The formulations of 2 are based on spectroscopic data and
substantiated by a crystal structure determination of 2a (Figure
2).13 Notable among the spectroscopic data for 2b is the 13C
carbene resonance (δC 206.7; cf. 200.4 ppm for [RhCl(cod){
C(NMe)2C10H6}]7). The molecular structure of 2a (Figure 2)
reveals a conventional d8 square-planar geometry about
rhodium, for which the primary distortion involves contraction
of the P1−Rh1−P2 angle to 165.01(4)° to accommodate the
geometric constraints of meridional pincer coordination. The
coordination plane about rhodium is slightly rotated (19.8°)
relative to that of the dihydriperimidine backbone, while the
Rh1−C1 bond length of 1.948(4) Å falls within the range
typical of the copious structural data available for NHC
complexes of rhodium(1).14 We may therefore surmise that
13
1
reflected in the multiplicity of the associated H resonances.
The complex 3b was structurally characterized, thereby
confirming the novel coordination mode for σ-perimidinyl
ligand.13 As depicted in Figure 3, 4-coordination at C1 is also
accompanied by a pyramidalization of the amino nitrogen
centers N1 and N2 (angle sums 342.8, 343.0°). It is interesting
to note that the methine- and iridium-bound hydrogen atoms
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2251 cm−1; NHC = :C(NMeCH)2,20 2068, 2308 cm−1),
though it should be noted that these complexes have the NHC
plane orthogonal to the P−P vector.
The salt [4b][PO2F2] was structurally characterized (Figure
4),13 thereby confirming the formation of the Ir−NHC linkage
Figure 3. Molecular structure of 3b (cyclohexyl and naphthyl
hydrogen atoms omitted, 50% displacement ellipsoids) including a
space-filling representation and a simplified view along the N1−N2
vector. Selected bond lengths (Å) and angles (deg): Ir1−Cl1 =
2.4923(14), Ir1−P1 = 2.2935(15), Ir1−P2 = 2.2998(15), Ir1−C1 =
2.141(5), Ir1−C70 = 1.904(6), Ir1−H1 = 1.539, N1−C1 = 1.493(6),
N2−C1 = 1.477(7); P1−Ir1−P2 = 162.76(5), P1−Ir1−C1 =
83.35(14), P2−Ir1−C1 = 82.31(14), Ir1−C1−N1 = 113.2(4), Ir1−
C1−N2 = 113.0(3), N1−C1−N2 = 107.5(4), Ir1−C1−H11 = 107.7,
∑(N1) = 342.8, ∑(N2) = 343.0.
Figure 4. Crystal structure of the salt [4b][PO2F2] (cyclohexyl and
naphthyl hydrogen atoms omitted, 50% displacement ellipsoids).
Selected bond lengths (Å) and angles (deg): Ir1−Cl1 = 2.4670(12),
Ir1−P1 = 2.3107(11), Ir1−P2 = 2.3184(11), Ir1−C1 = 2.078(4), Ir1−
H1 = 1.56(5), N1−C1 = 1.356(6) N2−C1 = 1.345(5), O3...H212 =
2.243(6); P1−Ir1−P2 = 165.74(4), Ir1−C1−N1 = 121.0(3), Ir1−
C1−N2 = 121.4(3), N1−C1−N2 = 117.6(4), ∑(N1) = 359.9,
∑(N2) = 359.8.
adopt an antiperiplanar disposition, from which it might be
inferred that the mechanism of metallacyclization is distinct
from that leading to 2: i.e., C−H activation must precede
coordination of the second phosphine arm. This geometry also
precludes spontaneous σ-metathesis elimination of dihydrogen
as well as migratory insertion involving the carbonyl ligand.
The ultimate step en route to per-NHC installation requires
transfer of the remaining methine proton to the metal center,
which is precluded in the case of coordinatively saturated 3b. In
an attempt to provide a suitably receptive vacant coordination
site, 3b was treated with various silver salts (Ag[Y]; Y = PF6,
SbF6, PO2F2) with the expectation that halide abstraction
would occur. Surprisingly, these salts resulted in hydride rather
than halide abstraction, but nevertheless the desired α-Ir−H
elimination could be demonstrated with the formation of the
per-NHC salts [IrHCl(CO){C(NCH2PR2)2C10H6}] ([4b]Y,
Y = PF6, SbF6, PO2F2; Scheme 3). Most notable among the
(Ir1−C1 = 2.078(4) Å), which is contracted relative to that in
the precursor 3b (Ir1−C1 = 2.141(5) Å). The difluorophos-
phate counteranion is weakly associated with the complex
cation via hydrogen bonding (O3···H212 = 2.243(6) Å) to one
PCH2 hydrogen, the acidity of which is enhanced by
coordination of the phosphine to a cationic iridium(III) center.
Although mechanistically distinct, it should be noted that
Hahn has recently demonstrated the synthesis of (κ2-P,C)
phosphine-NHC chelates, presumably via chelate-assisted C−H
activation of phosphinomethylbenzimidazoles.21 This type of
metal-mediated benzimidazole/benzimidazolylidene tautomer-
ism, however, recalls the landmark discovery of a similar (acid-
catalyzed) process by Taube some 40 years earlier, in which the
formation of [Ru{C(NH)2C6H4}(NH3)5]2+ from benzimi-
dazole and [Ru(OH2)(NH3)5]2+ proceeds without chelate
asistance.22
Scheme 3. Synthesis of a per-NHC Pincer Complex via
Stepwise Double Geminal C−H Activation
In conclusion, novel N-heterocyclic carbene pincer com-
plexes of rhodium have been generated via chelate-assisted
double C−H activation of substituted 2,3-dihydroperimidine
proligands. This attractively straightforward method of per-
NHC installation, the application of which has so far been
limited, proved to be exceptionally facile for these proligands,
proceeding at ambient temperature without the need for a
hydrogen acceptor. Investigations into the broader applicability
of this method are underway, though as noted the reaction of
1b with [IrCl(CO)(PPh3)2] resulted in only single C−H
activation to form an iridium dihydroperimindinyl hydrido
complex. However, it was shown that per-NHC formation
could be induced via hydride abstraction. The ease of
preparation of the proligands 1 bodes well for the wider
inclusion of highly σ-basic 2,3-dihydroperimidine-based NHC
groups within a variety of pincer ligand scaffolds.
spectroscopic data for [4b]+ is the replacement of the methine
resonance for 3b (δC 87.7) with one at δC 190.1 in a region
typical of NHC complexes of iridium.1 The cationic nature of
the metal center is reflected in an increase in the frequency of
the νCO and νIrH absorptions17 to 2067 and 2189 cm−1. Similar
values are observed for the cationic NHC complexes [IrHCl-
(CO)(PPh3)2(NHC)]+ (NHC = :CNMeCMeCHS,19 2050,
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(16) Perutz, R. N.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2007, 46,
2578.
ASSOCIATED CONTENT
* Supporting Information
■
S
(17) The relative intensities of these two bands in solution and in the
solid state are reversed. Caution is recommended when interpreting
infrared data for metal hydrido carbonyl complexes due to coupling of
the νMH and νCO modes. For 3b and [4b]+ this might have been
expected to be minimal, given the crystallographically confirmed cis-
IrH(CO) geometry: Vaska, L. J. Am. Chem. Soc. 1966, 88, 4100.
(18) Hydride abstraction was also achieved, albeit less cleanly, using
either [Ph3C]PF6 or HPF6.
CIF files giving crystallographic data for 1a (CCDC 898434),
2a (CCDC 898436), 3b (CCDC 898439), and [4b][PO2F2]
(CCDC 898440) and text giving synthetic procedures and
spectroscopic and analytical data for the compounds described.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(19) Fraser, P. J.; Roper, W. R.; Stone, F. G. A. J. Chem. Soc., Dalton
Trans. 1974, 760.
AUTHOR INFORMATION
Corresponding Author
*E-mail: a.hill@anu.edu.au.
Notes
■
(20) Harlow, K. J.; Hill, A. F. Unpublished results.
(21) (a) Hahn, F. E.; Naziruddin, A. R.; Hepp, A.; Pape, T.
Organometallics 2010, 29, 5283. (b) Naziruddin, A. K.; Hepp, A.; Pape,
T.; Hahn, F. E. Organometallics 2011, 30, 5859.
(22) Sundberg, R. J.; Shepherd, R. E.; Taube, H. J. Am. Chem. Soc.
1972, 94, 6558.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Australian Research Council
(DP1093516 and DP110101611). The assistance of Dr.
Anthony C. Willis in the acquisition and interpretation of
crystallographic data is gratefully acknowledged.
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