Convenient synthesis and molecular structure of the cyclometallated complex [IrCl(H)(C6H4PPh2)(PPh3)2]

Article abstract of DOI:10.5560/ZNB.2014-4160

Dedicated to Professor Hubert Schmidbaur on the occasion of his 80th birthday The reaction of [{Ir(μ-Cl)(coe)₂}₂] (coe=cis-cyclooctene) with triphenylphosphane (molar ratio of Ir to P=1 : 3) in dichloromethane at room temperature afforded after a short reaction time the cyclometallated complex [IrCl(H)(C₆H₄PPh₂)(PPh₃)₂] (1) in almost quantitative yield. The molecular structure of the title compound 1 was determined by an X-ray diffraction study.

Full text of DOI:10.5560/ZNB.2014-4160

Convenient Synthesis and Molecular Structure of the Cyclometallated
Complex [IrCl(H)(C6H4PPh2)(PPh3)2]
Hans-Christian Böttcher and Peter Mayer
Department Chemie, Ludwig-Maximilians-Universität, Butenandtstraße 5 – 13, D-81377
München, Germany
Reprint requests to Prof. Dr. Hans-Christian Böttcher. Fax: (+49) (0)89 / 2180 77407.
E-mail: hans.boettcher@cup.uni-muenchen.de
Z. Naturforsch. 2014, 69b, 1237 – 1240 / DOI: 10.5560/ZNB.2014-4160
Received July 21, 2014
Dedicated to Professor Hubert Schmidbaur on the occasion of his 80th birthday
The reaction of [{Ir(µ-Cl)(coe)2}2] (coe = cis-cyclooctene) with triphenylphosphane (molar ratio
of Ir to P = 1 : 3) in dichloromethane at room temperature afforded after a short reaction time the cy-
clometallated complex [IrCl(H)(C6H4PPh2)(PPh3)2] (1) in almost quantitative yield. The molecular
structure of the title compound 1 was determined by an X-ray diffraction study.
Key words: Iridium, Oxidative Addition, Cyclometallated Phosphanes, Crystal Structure
Introduction
complexes bearing cyclometallated phenylpyridinato
ligands beside phosphane ligands [5]. The synthesis
The chemistry of the well-known homogeneous of 1 has been performed starting from [IrCl(PPh3)3]
catalyst complexes [MCl(PPh3)3] (M = Rh, Ir) differs and using lithium or magnesium organometallic
quite significantly. For example, whereas Wilkinson’s compounds [6]. Moreover, the crystal structure of the
catalyst [RhCl(PPh3)3] can easily be prepared from bromido derivative [IrBr(H)(C6H4PPh2)(PPh3)2] has
hydrated rhodium(III) chloride and triphenylphos- been reported [7]. From reaction mixtures containing
phane in refluxing ethanol [1], the same synthetic [{Ir(µ-Cl)(coe)2}2] (coe = cis-cyclooctene) and PPh3
protocol is not applicable for the synthesis of the (molar ratio of Ir to P = 1 : 3) in dichloromethane at
related iridium complex [IrCl(PPh3)3]. The reason is room temperature we obtained the cyclometallated
that under these rough conditions the latter undergoes title complex 1 in high yield. However, during the
a fast intramolecular oxidative addition of one of characterization of 1 by 31P NMR spectroscopy we
the ortho C-H bonds resulting in the cyclometal- noticed some incompatible data reported in the litera-
lated title compound [IrCl(H)(C6H4PPh2)(PPh3)2] ture [3, 5] which were in contradiction with the ones
(1) [2, 3]. The corresponding Rh compound observed by us. For this reason we developed a con-
[RhCl(H)(C6H4PPh2)(PPh3)2] has been mentioned in venient high-yield synthesis for 1 and examined the
only one paper and was obtained by UV irradiation of identity of this compound by X-ray crystallography.
[RhCl(PPh3)3] in the presence of a cyclic organotin
compound [4]. However, a comparison of the 31P Results and Discussion
NMR data of this cyclometallated rhodium species
with those of the corresponding iridium compound Synthesis and characterization
described herein puts the identity of the rhodium
species strongly in question. For the cyclometallated
Treatment of [{Ir(µ-Cl)(coe)2}2] with triphenyl-
rhodium compound no direct synthesis has been phosphane in a molar ratio of Ir : P = 1 : 3 in
reported in the literature to date. Recently the iridium dichloromethane at room temperature is accompa-
complex 1 has been described as an intermediate in nied by a fast color change of the solution from
a one-pot synthesis affording luminescent iridium(III) yellow-orange to deep-red within a few minutes. From
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H.-C. Böttcher – P. Mayer · [IrCl(H)(C6H4PPh2)(PPh3)2]
this solution the title compound [IrCl(H)(C6H4PPh2) lution by 31P NMR spectroscopy after 1 h at room
(PPh3)2] (1) was obtained in almost quantitative yield temperature showed the presence of only two Rh-
as a pale-yellow powder after the addition of n-hexane containing species, namely [RhCl(PPh3)3] and the
(Eq. 1).
dimeric complex [{Rh(µ-Cl)(PPh3)2}2]. Both com-
plexes were unambiguously identified by their char-
acteristic 31P NMR data (see Experimental Section).
It is well established that the chlorido-bridged di-
nuclear type of complex is easily formed in solu-
[{Ir(µ-Cl)(coe)2}2]+6PPh3 →
2[IrCl(H)(C6H4PPh2)(PPh3)2]+4coe
(1)
Obviously, the reaction proceeds via the formation tion by dissociation of one phosphane ligand from
of [IrCl(PPh3)3], and it is known from the literature [RhCl(PPh3)3], affording a reactive 14 valence elec-
that the latter undergoes a fast intramolecular oxida- tron species, see e. g. ref. [9]. Subsequently the latter
tive addition reaction under mild conditions resulting intermediate is stabilized by dimerization resulting in
in the cyclometallation of one phosphane ligand. How- a 16 valence electron configuration at both rhodium
ever, this literature procedure requires a two-step syn- atoms. Interestingly, for the analogous iridium com-
thetic protocol [2, 3]. The procedure described herein pounds [IrCl(PR3)3] such dissociation processes have
is clearly favored over the protocol described in the lit- not been described as yet. Obviously these complexes
erature in which in a first step [IrCl(PPh3)3] has to be exhibit a higher kinetical inertness toward ligand ex-
prepared from [{Ir(µ-Cl)(coe)2}2] and triphenylphos- change reactions, see e. g. ref. [10]. On the other
phane (89 – 95%); in a second step compound 1 is ob- hand, cyclometallation processes under similar condi-
tained by refluxing [IrCl(PPh3)3] in cyclohexane for tions have not been observed for the related rhodium
2 h (70%) [3].
compounds. For [RhCl(H)(C6H4PPh2)(PPh3)2] only
Compound 1 was characterized by elemental anal- two instead of the expected three 31P NMR chemical
1
ysis and H NMR and 31P NMR spectroscopy (see shifts have been reported: δ = 63.8 and 50.6 ppm [4].
Experimental Section). However, our 31P NMR data Furthermore these data do not show the characteris-
of 1 were found to be inconsistent with the ones tic upfield shifts usually found for phosphorus atoms
from the literature [3, 5]: δA = 3.1, δM = −2.5 and in cyclometallated four-membered ring systems [8].
δX = −69.6 ppm (AMX spin system: 2JAM = 10.7 Hz, Details of the isolation and further analytical data
2JMX = 17.3 Hz, 2JAX = 376.0 Hz, C6D6) [3]. The re- of [RhCl(H)(C6H4PPh2)(PPh3)2] have not been re-
markable upfield shift of PX is characteristic of a phos- ported [4].
phorus atom in a cyclometallated four-membered ring
We assume that the formation of the very stable Ir-
system [8]. We were able to confirm the former data of C bond is the driving force of the title reaction (Eq.
1 with the only exception of the signal in the upfield 1). Interestingly, for both complexes [MCl(PPh3)3]
region: δA = 2.3, δM = −3.6 and δX = −92.6 ppm (M = Rh, Ir) agostic interactions between one H atom
(AMX spin system: 2JAM = 10.7 Hz, 2JMX = 17.4 Hz, of a phenyl group and the metal atom were found in
2JAX = 373.9 Hz, CD2Cl2). By contrast, the recently the crystal [10, 11]. This could facilitate the subse-
reported 31P NMR data of the title compound were quent cyclometallation process. However, in the case
completely incorrect (δ = −2.41, −7.54, −9.5 ppm, of rhodium this reaction does not seem to be favored
unresolved multiplets) [5]. To bring more insight into in solution. A possible reason could be the facile dis-
this matter we prepared single crystals of 1 to ensure sociation of one phosphane ligand, thereby preventing
the identity of this compound (vide infra).
the cyclometallation which obviously needs an intact
In this context we were interested in the question coordination sphere.
whether the related cyclometallated rhodium species
can be prepared by an analogous synthetic pathway. The molecular structure of 1
Thus we studied the reaction of [{Rh(µ-Cl)(coe)2}2]
with triphenylphosphane in a molar ratio of Rh to
Compound 1 crystallized as pale-yellow plates from
P = 1 : 3 in dichloromethane at room tempera- dichloromethane-ethanol by the slow diffusion method
ture. The reaction was accompanied by a fast color at room temperature overnight. Crystals of 1 belong
change of the solution from orange to deep red-brown to the triclinic space group P1 with two molecules in
within a few minutes. The investigation of the so- the unit cell. A view of the molecular structure in the
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H.-C. Böttcher – P. Mayer · [IrCl(H)(C6H4PPh2)(PPh3)2]
1239
[IrCl(H)(C6H4PPh2)(PPh3)2] (1) prepared by the re-
action of [{Ir(µ-Cl)(coe)2}2] (coe = cis-cyclooctene)
with triphenylphosphane in dichloromethane at room
temperature. In contrast to earlier reported erroneous
31P NMR data for 1 we were able to provide a full
characterization of the title compound by NMR spec-
troscopy and an X-ray diffraction study. Moreover we
showed that the analogeous reaction of the correspond-
ing rhodium species did not lead to a cyclometallation
product under similar conditions.
Experimental Section
All manipulations were carried out under a dry nitro-
gen atmosphere using standard Schlenk techniques. Sol-
vents were dried according to standard procedures and stored
under argon. All chemicals were purchased from ABCR
and used as received. The complexes [{M(µ-Cl)(coe)2}2]
(M = Rh, Ir) were prepared from the corresponding hydrated
metal(III) chloride and cis-cyclooctene [12]. NMR spectra
were recorded using a Jeol Eclipse 270 instrument operating
at 270 MHz (1H) and 109 MHz (31P), respectively. Elemental
analyses (C, H, Cl) were performed by the Microanalytical
Laboratory of the Department of Chemistry, LMU Munich,
using a Heraeus Elementar Vario EL instrument.
Fig. 1. Molecular structure of 1 in the crystal. Displacement
ellipsoids are drawn at the 50% probability level. Selected
bond lengths (Å) and angles (deg): Ir1–Cl1 2.498(1), Ir1–H1
1.44(3), Ir1–P1 2.345(1), Ir1–P2 2.365(1), Ir1–P3 2.312(1),
Ir1–C1 2.103(2), P1–C2 1.793(2), C1–C2 1.398(3), C1–
C6 1.400 (3); P1–Ir1–Cl1 93.7(1), P1–Ir1–C1 67.3(1), P1–
Ir1–P2 100.6(1), P1–Ir1–P3 150.2(1), P1–Ir1–H1 82.1(10),
Cl1–Ir1–H1 175.8(10), P3–Ir1–Cl1 106.5(1), P3–Ir1–C1
92.7(1), P3–Ir1–P2 101.7(1), P1–C2–C1 100.8(2), C1–Ir1–
Cl1 84.4(1).
Synthesis of [IrCl(H)(C6H4PPh2)(PPh3)2] (1)
To a slurry of [{Ir(µ-Cl)(coe)2}2] (224 mg, 0.25 mmol)
in dichloromethane (20 mL) triphenylphosphane (394 mg,
1.50 mmol) was added and the mixture stirred at room
temperature for 1 h. The solvent was reduced to 2 mL in
vacuo, and 1 was precipitated by the addition of n-hexane
(30 mL) as a pale-yellow powder. Compound 1 was filtered
off, washed twice with 10 mL of n-hexane, and dried in
vacuo. Yield 493 mg (97%); m. p. 185 – 187◦C (decomp.).
crystal is shown in Fig. 1, selected bond lengths and
angles are given in the caption.
The crystal structure determination revealed that
compound 1 is isostructural to the corresponding
bromido complex [IrBr(H)(C6H4PPh2)(PPh3)2] [7],
which also crystallized in the triclinic space group P1
with Z = 2 and similar cell metrics. The coordination
sphere around the iridium atom in 1 can be best de-
scribed as distorted octahedral with the phosphane lig-
ands occupying the meridional sites, and the chlorido
ligand is in trans position to the hydrido ligand. The
main structural feature of 1 is the four-membered met-
allacylic ring system (Ir-P1-C2-C1) resulting from an
intramolecular oxidative addition of one of the origi-
nally two mutally trans-arranged triphenylphosphane
ligands. All bond lengths and angles of the title com-
plex are comparable to those of the bromido analog,
including a high degree of bond angle distortion in the
four-membered metallacycle [7].
–
31P{1H} NMR (CD2Cl2): δA = 2.3 (dd, 2JAM = 10.7 Hz,
2JAX = 373.9 Hz), δM = −3.6 (dd, 2JAM = 10.7 Hz, 2JMX
=
=
2
2
17.4 Hz), δX = −92.6 ppm (dd, JMX = 17.4 Hz, JAX
373.9 Hz). – 1H NMR (CD2Cl2): δ = 7.70 – 6.53 (m, 44
2
H, C6H5 and C6H4), δ = −19.32 ppm (ddd, 1H, JPH
=
10.8 Hz, 2JPH = 15.2 Hz, 2JPH = 18.8 Hz). – C54H45ClIrP3
(1014.53): calcd. C 63.93, H 4.47, Cl 3.49; found C 63.82, H
4.58, Cl 3.74.
Reaction of [{Rh(µ-Cl)(coe)2}2] with triphenylphosphane
To a slurry of [{Rh(µ-Cl)(coe)2}2] (180 mg, 0.25 mmol)
in dichloromethane (20 mL) triphenylphosphane (394 mg,
1.50 mmol) was added and the mixture stirred at room
temperature for 1 h. The solvent was reduced to 5 mL in
vacuo and the solution immediately investigated by 31P{1H}
NMR spectroscopy. As the only phosphorus-containing com-
In conclusion, we reported a convenient high-
yield synthesis of the cyclometallated complex ponents [RhCl(PPh3)3] (δ = 48.7, dt, JRhP = 191.4 Hz,
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1240
H.-C. Böttcher – P. Mayer · [IrCl(H)(C6H4PPh2)(PPh3)2]
2JPP = 38.7 Hz; δ = 32.0 ppm, dd, JRhP = 143.2 Hz, Table 1. Crystal data, data collection and structure refinement
2JPP = 38.7 Hz) and [{Rh(µ-Cl)(PPh3)2}2] (δ = 52.1 ppm,
details for 1.
d, JRhP = 196.1 Hz) were detected. These data correspond
very well with the reported ones [13, 14]. Moreover some
amounts of triphenylphosphane oxide (δ = 28.0, s) were de-
tected in the reaction solution.
Empirical formula
C54H45ClIrP3
1014.526
0.12 × 0.10 × 0.07
173(2)
Mr
Crystal size, mm3
Temperature, K
Crystal system
triclinic
Space group
a, Å
P1
Crystal structure determination
10.9568(4)
12.2532(5)
19.4130(8)
89.0097(12)
76.8461(12)
66.3206(10)
2316.21(16)
2
b, Å
c, Å
Single crystals of 1 suitable for X-ray diffraction anal-
ysis were obtained as described above. A crystal was se-
lected by means of a polarization microscope, mounted on
the tip of a glass fiber, and investigated on a Bruker D8
Venture TXS diffractometer using MoKα radiation (λ =
0.71073 Å). The structure was solved by Direct Methods
(SIR97) [15] and refined by full-matrix least-squares cal-
culations on F2 (SHELXL-97) [16]. Anisotropic displace-
ment parameters were refined for all non-hydrogen atoms.
Crystals of 1 contained heavily disordered solvent molecules
(ethanol) from the crystallization process. This was addi-
tionally confirmed by 1H NMR measurements. Because it
was not possible to model and refine the solvent molecule
properly, electron density in voids was eliminated using
the routine SQUEEZE as incorporated in PLATON [17,
18] (27 electrons fit quite well for ethanol) followed by
the final least-squares refinement of the structural back-
bone. Details of crystal data, data collection, structure so-
lution, and refinement parameters of 1 are summarized in
Table 1.
α, deg
β, deg
γ, deg
V, Å3
Z
Dcalcd., g cm−3
1.46
3.1
1016
µ (MoKα ), mm−1
F(000), e
θ range for data collection, deg
hkl range
Reflections collected/independent
Rint
R1/wR2 [I > 2 σ(I)]
R1/wR2 (all data)
S
∆ρfin (max/min), e Å−3
3.24 – 27.54
−13 → +14, ±15, +25
10 532/9447
0.04
0.0242/0.0310
0.0531/0.0545
1.085
0.92/−0.41
Acknowledgement
The authors are grateful to the Department of Chemistry,
Ludwig-Maximilians-Universität München, for support of
this work. Johnson Matthey plc, Reading, UK, is gratefully
acknowledged for a generous loan of hydrated iridium(III)
chloride.
CCDC 1014828 (1) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
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