C-H activation of 2,4,6-triphenylphosphinine: Unprecedented formation of cyclometalated [(PC)Ir(iii)] and [(PC)Rh(iii)] complexes

Article abstract of DOI:10.1039/c0cc04660d

An unprecedented C-H activation of 2,4,6-triphenylphosphinine by Ir(iii) and Rh(iii) has been observed. Time-dependent ³¹P{¹H} NMR spectroscopy gave insight into the cyclometalation reaction and the corresponding coordination compoun

Full text of DOI:10.1039/c0cc04660d

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C–H activation of 2,4,6-triphenylphosphinine: unprecedented formation
of cyclometalated [(P^C)Ir(III)] and [(P^C)Rh(III)] complexeswz
Leen E. E. Broeckx,a Martin Lutz,b Dieter Vogta and Christian Mu
¨
ller*a
Received 28th October 2010, Accepted 3rd December 2010
DOI: 10.1039/c0cc04660d
An unprecedented C–H activation of 2,4,6-triphenylphosphinine
by Ir(III) and Rh(III) has been observed. Time-dependent 31P{1H}
NMR spectroscopy gave insight into the cyclometalation
reaction and the corresponding coordination compounds were
characterized by means of X-ray crystallography. In contrast,
2,4,6-triphenylpyridine does not show any ortho-metalation,
demonstrating a remarkable difference in reactivity between
these two structurally related aromatic heterocycles.
higher oxidation states has virtually been neglected. Earlier
attempts to prepare such compounds have shown that they are
extremely sensitive towards nucleophilic attack, making their
straightforward synthesis rather unattractive.5 In fact,
phosphinine-based coordination compounds containing transition
metal centers in the formal oxidation state +2 are rare, while a
stabilization of even higher oxidation states has not been reported
in the literature to date.6 Yet, the access to such compounds would
open up new perspectives in the field of phosphorus containing
molecular materials as well as homogeneous catalysis.
Cyclometalated transition metal complexes have attracted
considerable interest for a wide variety of applications.1
Especially the intramolecular C–H activation of 2-phenyl-
pyridines by d6-metals leads to functional coordination
compounds, which find use as triplet-emitters in organic light
emitting diodes (OLEDs) or as catalysts for the oxidation of
water (type A, Fig. 1).2,3
We could recently demonstrate that the aryl-groups in
ortho-position of donor functionalized 2,4,6-triarylphosphinines
along with the chelate effect contribute significantly to the
formation and efficient kinetic stabilization of corresponding
Pd(II) and Pt(II) complexes.7 Motivated by these results we
envisaged to use 2,4,6-triphenylphosphinine (1) as a phosphorus-
derivative of 2-phenylpyridine and to study its potential
reactivity towards cyclometalation reactions (type B,
Fig. 1).8,9 The metal precursors [Cp*MCl2]2 (M = Ir, Rh)
were chosen as they readily undergo C–H activation of
2-phenylpyridine in the presence of NaOAc, which assists as
intramolecular base in the ortho-metalation reaction.3,10,11
Reaction of 1 with [Cp*IrCl2]2 and NaOAc in the ratio
2 : 1 : 2 in CD2Cl2 quantitatively leads to species 2 that shows
a broad signal at d = 133.0 ppm in the 31P{1H} NMR spectrum
(Fig. 2, t = 0) and a doublet (3JHꢀP = 19.2 Hz) at d = 8.03 ppm
The replacement of nitrogen by phosphorus in similar
structures causes significantly diverse properties due to the
electronic difference that exists between these heteroatoms.
Phosphinines, for example, the homologs of pyridines, are
particularly suitable for the stabilization of late transition
metal centers in low oxidation states because of their
pronounced p-accepting character.4 In contrast, the prepara-
tion of phosphinine-complexes containing metal centers in
1
in the H NMR spectrum, characteristic for the two peripheral
protons of the phosphinine-core. The use of hydrated NaOAc
was avoided in order to prevent potential reaction of H2O with
the phosphinine–metal complex.5 MALDI-TOF analysis of the
reaction mixture reveals that
2
has the composition
C33H32Cl2IrP, which corresponds to the simple monomeric
Fig. 1 Cyclometalated coordination compounds and phosphinine 1.
a Department of Chemical Engineering and Chemistry,
Laboratory of Homogeneous Catalysis and Coordination Chemistry,
Eindhoven University of Technology, Eindhoven, The Netherlands.
E-mail: c.mueller@tue.nl; Fax: +31 40-2455054;
Tel: +31 40-2474679
b Bijvoet Center for Biomolecular Research, Crystal and Structural
Chemistry, Utrecht University, The Netherlands
w This paper is dedicated in memoriam to Prof. Dr Pascal Le Floch.
z Electronic supplementary information (ESI) available: A detailed list
of all experimental procedures, cif file and crystallographic data for
compounds 3 and 5. CCDC 798976 and 798977. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0cc04660d
Fig. 2 Time-dependent 31P{1H} NMR spectra for the reaction of
1 with [Cp*IrCl2]2 in the presence of NaOAc at T = 80 1C (CD2Cl2).
c
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P-coordinated neutral adduct [Cp*IrCl2(1)] formed upon
reaction of the Ir-dimer with the phosphinine. Any attempt to
isolate and to further characterize 2 failed. X-Ray crystallo-
graphic investigations of the crystals formed after diffusion of
Et2O into the reaction mixture showed only the presence of
regenerated [Cp*IrCl2]2 dimer. This is in line with earlier
observations that monodentate phosphinines generally do not
form stable coordination compounds with transition metal centers
in higher oxidation states due to their weak s-donor properties.4
Heating the reaction mixture to T = 80 1C in a sealed NMR
tube reveals, however, consumption of 2 and the appearance
of a new phosphorus-containing compound (3) with a
chemical shift of d = 170.8 ppm in the 31P{1H} NMR
spectrum. Moreover, free phosphinine 1 (d = 185.5 ppm)
can be detected as an intermediate. Full conversion of 2 into 3
is finally achieved after 6 h at T = 80 1C (Fig. 2). In the
1H NMR spectrum of 3 (CD2Cl2) two doublet of doublets
monitoring it by means of 31P{1H} NMR spectroscopy reveals
consumption of 2 (d = 133.0 ppm) and exclusive formation of
3 (d = 170.8 ppm, see ESIz). The reaction is considerably
slower compared to the cyclometalation process in the
presence of NaOAc and no free 1 is observed. Full conversion
is not achieved and the maximum yield of 3 is 75% after
5 days. Interestingly, the C–H activation reaction is completely
reversible. Cooling the sealed NMR tube to room temperature
leads to the quantitative formation of the starting material 2
within 1 day. Pure 3 can thus be obtained by removing HCl
during the cyclometalation reaction by means of vacuum.
We assume that the formation of 3 proceeds by an acetate-
assisted electrophilic C–H activation in analogy to the
mechanism proposed for the reaction of [Cp*MCl2]2 with
nitrogen-containing substrates.10–12 The free phosphinine 1
observed during the course of the cyclometalation reaction
in the presence of NaOAc might be the result of the reversible
formation of 2 from 1 and [Cp*IrCl2]2 (vide supra).11a In the
case of iridium, this equilibrium lies far to the product side.
Transient Ir-dimer can, however, react reversibly with NaOAc
under formation of Cp*Ir(III)-acetate and diacetate species of
the type C (Scheme 1, lower pathway).10,12 The phosphinine-
containing cationic species D undergoes subsequently the
acetate-assisted C–H activation and could not be detected by
means of 31P NMR spectroscopy. The considerably slower
cyclometalation reaction in the absence of NaOAc presumably
proceeds by electrophilic substitution via a metal arenium
4
(3JHꢀP = 22.4, 21.6 Hz; JHꢀH = 1.6 Hz) at d = 8.35 and
8.01 ppm are observed for the two peripheral protons of the
phosphorus heterocycle. This pattern is characteristic for an
asymmetrically substituted 2,4,6-triarylphosphinine.
Orange crystals suitable for X-ray diffraction were obtained
by slow diffusion of Et2O into a solution of 3 in CH2Cl2. The
Ir-complex crystallizes enantiomerically pure in the non-
centrosymmetric space group P212121 (no. 19) and the
molecular structure along with selected bond lengths and
angles is shown in Fig. 3.
The X-ray crystal structure analysis of 3 unambiguously
confirms the cyclometalation of 2,4,6-triphenylphosphinine by
Ir(III) with elimination of HCl. The molecular structure of 3
shows the characteristic three-legged ‘‘piano-stool’’ arrange-
ment around the iridium atom and represents the first
crystallographic characterization of a phosphinine–M(III)
complex reported in the literature. The chelate effect of the
formally anionic bidentate ligand apparently contributes to
the formation of such a coordination compound.
(Wheland) intermediate after formation of
Ir-complex (Scheme 1, upper pathway).
a cationic
Similarly, the reaction of 1 with [Cp*RhCl2]2 and NaOAc in
the ratio 2 : 1 : 2 in CH2Cl2 at T = 80 1C gives the corres-
ponding cyclometalated Rh-complex 5. The reaction is slower
and less selective than with iridium and 5 is obtained in 81%
spectroscopic yield after 18 h. Moreover, the Rh-complex 4
could not be detected, which is probably due to a fast
exchange process. The pure complex was isolated by slow
diffusion of Et2O into a solution of 5 in CH2Cl2 and shows a
doublet at d = 208.8 ppm in the 31P{1H} NMR spectrum
(JPꢀRh = 187.8 Hz) and two doublets (3JHꢀP = 21.2, 18.8 Hz)
at d = 8.34 and 8.03 ppm for the two peripheral protons of the
phosphorus heterocycle in the 1H NMR spectrum. Red
crystals of 5 suitable for X-ray diffraction were obtained by
slow crystallization from CH2Cl2/Et2O. The Rh-complex
crystallizes in the centrosymmetric space group P21/c
The presence of NaOAc turned out not to be essential for
the C–H activation process. Heating a 2 : 1 mixture of 1 and
[Cp*IrCl2]2 in CH2Cl2 to T = 80 1C in a sealed NMR tube and
Fig. 3 Molecular structure of 3 in the crystal. Displacement ellipsoids
are shown at the 50% probability level. Selected bond lengths (A) and
angles (1): P(1)–Ir(1): 2.2396(12); C(7)–Ir(1): 2.076(5); P(1)–C(1):
1.724(5); P(1)–C(5): 1.724(4); Ir(1)–Cp(cent.): 1.861(2); Ir(1)–Cl(1):
2.3902(12); C(1)–P(1)–C(5): 105.7(2); C(7)–Ir(1)–P(1): 78.72(14).
Scheme 1 Proposed reaction scheme for the cyclometalation of 1.
c
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lengths in pyridine. As a result, the phenyl groups in ortho-
position of the phosphinine framework are bend away from the
phosphorus atom, making the more diffuse and less directional
phosphorus lone-pair even more accessible for s-coordination
to a metal center. The C–H activation can subsequently
proceed. In contrast, the more localized nitrogen lone-pair in
2,4,6-triphenylpyridine is much better shielded by the phenyl
groups in 2- and 6-position, reducing considerably its accessibility.
In summary we have achieved for the first time an unpre-
cedented C–H activation of 2,4,6-triphenylphosphinine by
Ir(III) and Rh(III) precursors. 31P{1H} NMR spectroscopic
investigations gave insight in the cyclometalation reaction
and the corresponding coordination compounds were
characterized by means of X-ray crystallography. These
compounds represent the first examples of isolated and crystallo-
graphically characterized phosphinine–M(III) complexes
reported so far in the literature. The analogous reaction of
2,4,6-triphenylpyridine does not show any ortho-metalation,
demonstrating a remarkable difference in reactivity between
these two related heterocycles. This observed novel reactivity
mode of phosphinines might open up new perspectives for
future applications in phosphorus containing molecular
materials and homogeneous catalysis. Corresponding studies
are currently carried out in our laboratories.
Fig. 4 Molecular structure of 5 in the crystal. Displacement ellipsoids
are shown at the 50% probability level. Selected bond lengths (A) and
angles (1): P(1)–Rh(1): 2.2156(4); C(11)–Rh(1): 2.0687(15); P(1)–C(1):
1.7176(17); P(1)–C(5): 1.7160(15); Rh(1)–Cp(cent.): 1.8584(7); Rh(1)–Cl(1):
2.3931(4); C(1)–P(1)–C(5): 105.76(7); C(11)–Rh(1)–P(1): 78.26(4).
(no. 14) and the result of the X-ray crystal structure analysis
along with selected bond lengths and angles is shown in Fig. 4.
The molecular structure essentially resembles the one
described for compound 3 and is the first structurally
characterized phosphinine–Rh(III) complex.
C.M. thanks The Netherlands Organization for Scientific
Research (NWO-CW) for a personal grant. The COST action
PhoSciNet (CM0802) is gratefully acknowledged.
Most strikingly, the here described results show that 3 and 5
are unexpectedly stable under the applied reaction conditions,
as the P|C double bond generally becomes more reactive with
increasing oxidation state of the metal center.4,5 We assume
that the additional phenyl-group in 2-position of the
phosphorus heterocycle contributes significantly to a kinetic
stabilization of the metal complex, as the P|C double bond is
sterically more shielded for addition reactions in contrast to
less substituted phosphinines.7a
Notes and references
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In order to compare the reactivity of 1 with its pyridine-
counterpart, we performed analogous experiments with 2,4,6-
triphenylpyridine (6).13 Interestingly, the reaction of 6 with either
[Cp*IrCl2]2 or [Cp*RhCl2]2 in the presence of NaOAc or
NaOAcꢁ3H2O at T = 80 1C does not lead to the corresponding
4 For recent reviews see: (a) L. Kollar and G. Keglevich, Chem. Rev.,
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5 B. Schmid, L. M. Venanzi, A. Albinati and F. Mathey, Inorg.
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1
cyclometalated products 7 and 8 (Scheme 2). In the H NMR
spectrum of the reaction mixtures no traces of C–H activation in
the aromatic region can be detected even after 8 days of heating.
Only changes in the aliphatic region can be observed, which
indicates reaction of the metal precursor with sodium acetate.10
Since the reaction of 2-phenylpyridine with [Cp*MCl2]2
dimers is well known,3,10–12 the here observed lack of reactivity
can be attributed purely to steric factors. In fact, the phosphi-
nine heterocycle is best described as distorted hexagon due to
the larger P–C bond distances compared to the N–C bond
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Scheme 2 Attempted cyclometalation of 2,4,6-triphenylpyridine 6.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2003–2005 2005