Ligand metalation in an iridiumtris(diisopropylphosphinomethyl)borato complex: Synthesis, molecular structure and reactivity

Article abstract of DOI:10.1016/j.jorganchem.2008.09.044

The reaction of [IrCl(dmso)₃] with trisphosphinomethylborato ligand Li(THF){PhB(CH₂PⁱPr₂)₃} at room temperature results in intramolecular C-H activation of one of the ⁱPr substituents affor

Full text of DOI:10.1016/j.jorganchem.2008.09.044

Journal of Organometallic Chemistry 693 (2008) 3943–3946
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Journal of Organometallic Chemistry
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Note
Ligand metalation in an iridiumtris(diisopropylphosphinomethyl)borato
complex: Synthesis, molecular structure and reactivity
*
Magnus R. Buchner, Eberhardt Herdtweck, Sven Schneider
Technische Universität München, Department Chemie, Lichtenbergstraße 4, D-85747 Garching b. München, Germany
a r t i c l e i n f o
a b s t r a c t
The reaction of [IrCl(dmso)3] with trisphosphinomethylborato ligand Li(THF){PhB(CH2PiPr2)3} at room
temperature results in intramolecular C–H activation of one of the iPr substituents affording two diaste-
reomers of cyclometalated iridium(III) complex [Ir(H)(dmso){PhB(CH2PiPr2)2(CH2PiPrCHMeCH2)}] (1) in
high yield in approximately equimolar ratio. NMR spectroscopic characterization indicates that only
the diastereomers with the hydride ligand in cis position with respect to the metalacyclic phosphorous
atom are formed as confirmed by single crystal X-ray diffraction. Facile ring opening with H2 at room
temperature gives dihydride [Ir(H)2(dmso){PhB(CH2PiPr2)3}] (2). However, C–H activation of benzene
was not observed.
Article history:
Received 7 August 2008
Received in revised form 23 September
2008
Accepted 24 September 2008
Available online 30 September 2008
Dedicated to Professor Dr. h.c. mult. W. A.
Herrmann on the occasion of his 60th
birthday.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords:
Cyclometalation
C–H activation
Iridium
Trisphosphinomethylborato ligands
1. Introduction
Tp complexes have been observed, as well [6c,7]. In the past years,
{PhBPR3}-type ligands were utilized in late transition metal chem-
The activation of C–H bonds by transition metal complexes is of
major interest owing to its potential for hydrocarbon functionali-
zation [1]. While intramolecular C–H oxidative addition to metal
centers, such as the ortho-metalation of coordinated arylphos-
phines, was discovered more than 40 years ago and turned out to
be quite common, advances in intermolecular C–H activation are
much more recent and the topic remains a challenging goal [2].
Particularly intermolecular alkane and arene C–H activation with
derivatives of cyclopentadienyl (Cp) and trispyrazolylborato (Tp)
iridium(I) complexes was the subject of numerous mechanistic
studies [3,4]. Although both monoanionic 6-electron donor ligands
are generally considered similar, distinct differences, such as the
stronger electron donating ability of the soft cylopentadienyl
derivatives and the much higher propensity of the hard Tp ligands
for g2 coordination, have been pointed out in the past [4,5].
Recently, Tilley, Peters, and Nocera have introduced tripodal,
monoanionic trisphosphinomethylborato ligands of the type
{PhB(CH2PR2)3}À ({PhBPR3}À; R = Ph, iPr). [6] According to CO
stretching vibrations in carbonyl complexes, strong electron dona-
tion renders this ligand type comparable with electron rich Cp li-
istry for various applications, e.g., for the stabilization of terminal
iridium silylene and late 3d metal imido and nitride complexes
[6a,8,9], dinitrogen activation [10], or in single source precursors
for chemical vapor deposition (CVD) [11]. However, while the acti-
vation of H–H and Si–H bonds with M{PhBPR3} (M = Ir, Rh) frag-
ments has been studied before [7], not much emphasis was put
on C–H activation.
In this context we were interested if Ir{PhBPR3} fragments can
be used for intermolecular hydrocarbon activation. Intermolecular,
allylic C–H activation was observed in the reaction of IrCl(olefin)n
i
(olefin = cyclooctene, propene) with Li(THF){PhBPR3} (R = Ph, Pr)
[7,12]. However, for R = Ph olefin reductive elimination affords fac-
ile and irreversible ortho-metalation. Since trialkylphosphine
cyclometalation is much less common and more likely to be
reversible given the tendency for iridium to activate C–H bonds
in the order aryl >1° > 2° ꢀ 3°, we examined if the use of the
{PhBPiPr3} ligand permits intermolecular aromatic C–H activation
[1a,13].
2. Results and discussion
gands. On the other hand, facile changes in hapticity (g2 3) as in
/g
The reaction of [IrCl(dmso)3] with Li(THF){PhBPiPr3} affords two
compounds by NMR spectroscopy in approximately equimolar
ratio in high overall yield (Scheme 1). Repeated fractional crystal-
* Corresponding author. Fax: +49 89 28913473.
E-mail address: sven.schneider@ch.tum.de (S. Schneider).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.09.044

3944
M.R. Buchner et al. / Journal of Organometallic Chemistry 693 (2008) 3943–3946
Ph
B
P
Ir
iPr2
[IrCl(dmso)3]
iPr2P
H
PiPr2
H2
+
1a/b
benzene
benzene
Li(THF)PhB(CH2PiPr2)3
S(O)Me2
H
2
Scheme 1. Synthesis of Ir{PhBPiPr3} complexes.
lization allowed isolation of one complex, albeit in very low yield
due to their close solubility. NMR spectroscopic characterization
and combustion analysis of the product mixture are in agreement
with the formation of two diastereomers of cyclometalated
[Ir(H)(dmso){PhB(CH2PiPr2)2(CH2PiPrCHMeCH2)}] (1). Both iso-
mers feature three signals in the 31P NMR spectrum, respectively,
indicating low symmetry around the metal centers. The distinct
upfield shift of one 31P NMR signal (1a: À42.5 ppm/1b:
À31.0 ppm) is characteristic of four-membered phosphacycles as
in cyclometalated phosphine ligands [14]. In the 1H NMR spectrum
one hydridic signal is observed, respectively, as a doublet of triplets
with one large trans 2JHP (1a: 121.5 Hz/1b: 119.9 Hz) and a small cis
2JHP coupling constant (1a: 12.8 Hz/1b: 11.6 Hz). In the alkyl region
a complicated pattern is observed with multiple superimposed sig-
nals for the iso-propyl groups owing to the low symmetry of the
complex. However, two sharp singlet signals for each complex,
respectively, can be assigned to a sulfur-bound dmso ligand indi-
cating hindered rotation about the Ir–S bond.
With respect to the two stereocenters in complex 1, the metal
center and the metalacyclic methyne carbon atom (C16, Fig. 2),
four diastereomers could be possible products (Fig. 1) if idealized
local C3v symmetry for the {BP3} backbone is assumed [15].
Selectively decoupled 1H-{31P} NMR spectra indicate, that the hy-
dride ligands in the diastereomers obtained are not located trans
to the metalacyclic phosphorous atoms but to the ones with 31P
NMR signals at À10.3 ppm (1a) and À12.7 ppm (1b), respectively.
Therefore, structures C and D can be excluded by NMR spectros-
copy, rendering structures A and B, and their respective enantio-
mers, the isomers observed for 1.
The molecular structure of diastereomer 1a, which is less solu-
ble in hydrocarbon solvents, could be derived by single crystal X-
ray diffraction (Fig. 2). The structure, which was solved in the cen-
trosymmetric space group P21/n, features facial coordination of the
trisphosphinoborato ligand with one iso-propyl substituent cyclo-
Fig. 2. DIAMOND plot of 1a as derived by single-crystal X-ray diffraction with
thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted
for clarity. Selected bond lengths (Å) and angles (deg): Ir–P1 2.426(1), Ir–P2
2.281(1), Ir–P3 2.390(1), Ir–C17 2.168(4), Ir–S 2.306(1); P1–Ir–P2 91.12(4), P1–Ir–
P3 89.55(4), P2–Ir–P3 90.85(4), P1–Ir–C17 94.0(1), P2–Ir–C17 69.9(1), P1–Ir–S
100.27(4), P2–Ir–S 159.41(4), P3–Ir–C17 160.5(1).
metalated to give a four-membered phophametalacycle. Further-
more, an S-bound dmso ligand is located trans to the
metalacyclic phosphorous atom P2. The remaining hydride was
not located on the electron density map but calculated at the
remaining coordination site trans to P1, in agreement with the
large Ir–P1 distance (2.426(1) Å) due to the strong hydride ligand
trans-influence. Therefore, the stereochemistry of 1a is represented
by structure A (Fig. 1). The three P–Ir–P angles are close to 90°, as
typically observed for trisphosphinoborato ligands [12]. Major dis-
tortion from ideal octahedral geometry arises from ring strain
within the four-membered metalacycle (P2–Ir–C17: 69.9(1)°) and
steric repulsion of the dmso ligand with PiPr2 groups resulting in
large S–Ir–P1 (100.27(4)°) and S–Ir–P3 (106.22(4)°) cis angles and
a small S–Ir–P2 (159.41(4)°) trans angle. The metalacycle is almost
perfectly planar with a sum of bond angles within the four mem-
bered ring of 359.3°.
Ph
Ph
A
C
B
D
B
B
PiPr2
Ir
P
Ir
iPr2
iPr
PiPr2
iPr
P
PiPr2
P
S(O)Me2
H
S(O)Me2
H
Ph
Ph
B
B
iPr2
P
P
Ir
iPr2
Hydrogenation of a 1a/1b mixture under H2 (1 atm) in benzene
affords ring opening of the four-membered metallacycle and for-
mation of [Ir(H)2(dmso){PhBPiPr3}] (2) as the only product (Scheme
1). The related complexes [Ir(H)2(L){PhBPiPr3}] (L = PMe3, PH2Cy,
CO) were reported earlier by Tilley and co-workers [7]. 2 features
two signals in the 31P NMR spectrum with a 2:1 ratio as expected
iPr
iPr
P
PiPr2
H
P
PiPr2
H
Ir
Me2(O)S
S(O)Me2
Fig. 1. Possible diastereomers of complex 1.

M.R. Buchner et al. / Journal of Organometallic Chemistry 693 (2008) 3943–3946
3945
from Na/K alloy. Li(THF){PhB(CH2PiPr2)3} and [IrCl(dmso)3] were
prepared as reported in the literature [6b,16].
Table 1
Crystallographic data for 1a.
1a
4.2. Analytical methods
Empirical formula
Formula weight
C29H59BIrOP3S
751.79
Color/habit
Pale yellow/fragment
0.10 Â 0.18 Â 0.48
Monoclinic
P21/n (no. 14)
10.1048(2)
17.1257(5)
19.0007(6)
99.165(3)
3246.13(16)
4
Elemental analyses were obtained from the Microanalytical
Laboratory of Technische Universität München. NMR spectra were
recorded on a Jeol Lambda 400 spectrometer at room temperature
and were calibrated to the residual proton resonance and the nat-
ural abundance 13C resonance of the solvent (C6D6, dH = 7.16 and
dC = 128.06 ppm). 31P NMR chemical shifts are reported relative
to external phosphoric acid (d0.0 ppm).
Crystal dimensions (mm3)
Crystal system
Space Group
a (Å)
b (Å)
c (Å)
b (°)
V (Å3)
Z
4.3. Syntheses
T (K)
153
1.538
4.346
1536
2.98–25.37
59342
5917/0.043
4598
5917/0/338
0.0325/0.0876
0.0450/0.0940
1.074
DCalc (g cmÀ3
)
l
(mmÀ1
F(000)
h Range (°)
)
[Ir(H)(dmso){PhB(CH2PiPr2)2(CH2PiPrCHMeCH2}] (1). A solid mix-
ture of IrCl(dmso)3 (0.108 g; 0.23 mmol) and [Li(THF)PhB(CH2-
PiPr2)3] (0.130 g; 0.23 mmol) is dissolved in benzene (5 mL) and
stirred over night. The orange-yellow solution is evaporated to dry-
ness and the yellow-brown residue is extracted with benzene.
After filtration the solvent is removed i. vac. to give a pale yellow
microcrystalline solid consisting of a mixture of the two diastereo-
mers 1a and 1b (57:43). Yield: 0.127 g (0.319 mmol; 82%). Anal.
Calc. for C29H59BIrOP3S (751.79): C, 46.33; H, 7.91; S, 4.27. Found:
C 47.18; H, 7.76; S, 3.58%.
Reflections collected
Independent reflections/Rint
Observed reflections (I > 2
r(I))
Data/restraints/parameters
R1/wR2 (I > 2r
(I))a
R1/wR2 (all data)a
GOF (on F2)a
Largest difference peak and hole (e ÅÀ3
)
+2.43/À1.98
GOF = {S[w(F2oÀFc2)2]/
a
R1 = S(||Fo|À|Fc||)/S|Fo|; wR2 = {S[w(F2oÀFc2)2]/S[w(Fo2)2]}1/2
;
NMR (C6D6, r.t., [ppm]) data for diastereomer 1a: 1H NMR
(nÀp)}1/2
.
2
2
(399.8 MHz): d-11.54 (dt, 1 H, JHP = 121.5 Hz, JHP = 12.8 Hz, Ir-
H), 0.27 (br, 1H, BCH2P), 0.42 (br, 1H, BCH2P), 0.52 (br, 1H, BCH2P),
0.69 (br, 3H, BCH2P), 0.80–1.60 (m, 36H, 11 Â CH3 + IrCH2 + PCH),
for a Cs symmetric complex. The two hydride ligands were found
by 1H NMR spectroscopy at À12.55 ppm as a multiplet owing to
the complex AA‘MXX’ spin system. Heating of 1a/1b at 80 °C in
C6D6 over 4 days results in minor decomposition products (<5%
by 31P NMR) which could not be further characterized. However,
heating of a mixture that was enriched in one of the two diastereo-
mers did not result in equilibration of the 1a: 1b ratio, suggesting
that the cyclometalation is irreversible under these conditions.
2
3
1.66 (dhept, 1H, JHP = 6.8 Hz, JHH = 7.2 Hz, PCH), 1.94 (br, 1H,
PCH), 2.45 (dhept, 1H, 2JHP = 6.7 Hz, 3JHH = 7.3 Hz, PCH), 2.53 (dhept,
2
3
1 H, JHP = 4.1 Hz, JHH = 11.3 Hz, PCH), 3.09 (br, 1H, PCH), 2.58 (s,
3
3H, SCH3), 3.17 (s, 3H, SCH3), 7.33 (t, 1H, JHH = 7.2 Hz, CHpara),
7.58 (t, 2H, 3JHH = 7.2 Hz, CHmeta), 7.93 (d, 2H, 3JHH = 7.2 Hz, CHortho).
31P-{1H} NMR (161.8 MHz): d-6.9 (dd, 2JPP = 19.9 Hz, 2JPP = 14.4 Hz,
PiPr2), À10.3 (br, PiPr2), À42.5 (t, JPP = 14.4 Hz, PiPrCHMeCH2). 11B
2
i
Furthermore, no H/D exchange of the hydride ligand or on the Pr
NMR (128.3 MHz): d-12.6 (s).
groups was observed by 2H NMR. The same observation was made
for ortho-metalated [Ir(H)(PMe3){PhB(CH2PPh2)2(CH2PPhC6H4)}]
[8]. Feldman et al. have suggested that the catalytic deuteration
of cyclooctene (COE) by [Ir(H)2(COE){PhBPPh3}] in C6D6 proceeds
via an Ir(III)/Ir(V) cycle with C6D6 oxidative addition after COE dis-
sociation [8]. It is reasonable to assume that dmso and PMe3 are
bound to Ir(III) considerably stronger than COE inhibiting benzene
C–H oxidative addition. However, steric considerations have to be
taken into account, as well.
NMR (C6D6, r.t., [ppm]) data for diastereomer 1b: 1H NMR
2
2
(399.8 MHz): d-11.96 (dt, 1H, JHP = 119.9 Hz, JHP = 11.6 Hz, Ir-H),
0.52 (m, 2H, BCH2P), 0.72 (m, 4H, BCH2P), 0.80–1.80 (m, 37H,
11 Â CH3 + IrCH2 + 2 Â PCH),1.87 (br, 1H, PCH), 2.17 (dhept, 1H,
2JHP = 7.3 Hz, JHH = 6.7 Hz, PCH), 2.30 (m, 1H, PCH), 2.75 (br, 1H,
3
PCH), 2.78 (s, 3H, SCH3), 2.93 (br, 1H, PCH), 3.11 (s, 3H, SCH3), 7.33
(t, 1H, 3JHH = 7.2 Hz, CHpara), 7.58 (t, 2H, 3JHH = 7.2 Hz, CHmeta), 7.91
3
(d, 2H, JHH = 7.2 Hz, CHortho). 31P-{1H} NMR (161.8 MHz): d-1.3
2
2
(dd, JPP = 17.8 Hz, JPP = 13.0 Hz, PiPr2), À12.7 (br, PiPr2), À31.0 (t,
2JPP = 13 Hz, PiPrCHMeCH2). 11B NMR (128.3 MHz): d-12.1 (s).
[Ir(H)2(dmso){PhB(CH2PiPr2)3}] (2). 10 mg (13
lmol) of a mixture
3. Conclusions
of 1a/1b are dissolved in 0.5 mL C6D6 in a J-Young NMR tube. The
solution is frozen, evacuated and backfilled with H2 (1 atm). Over
the course of 10 h 1a/1b is slowly converted to [Ir(H)2(dm-
so){PhB(CH2PiPr2)3}] (2). NMR (C6D6, r.t., [ppm]) data of 2: 1H
NMR (399.8 MHz): d-12.55 (m, 2H, Ir-H), 0.80–1.60 (m, 42H,
CH3 + BCH2), 1.67 (m, 2H, PCH), 1.76 (m, 2H, PCH), 2.74 (m, 2H,
The reaction of [IrCl(dmso)3] with Li(THF){PhBPiPr3} affords two
diastereomers of cyclometalated complex 1 in approximately equi-
molar ratio. Diastereomers with the hydride ligand in trans posi-
tion to the metalated phosphine atom are not observed. While
the facile reaction with H2 at room temperature gives dihydride
2 quantitatively, ring opening via hydrocarbon addition was not
observed rendering this system unsuitable for intermolecular aro-
matic C–H activation.
3
PCH), 3.22 (s, 6H, SCH3), 7.33 (t, 1H, JHH = 7.0 Hz, CHpara), 7.59 (t,
3
2H, JHH = 7.3 Hz, CHmeta), 8.0 (br, 2H, CHortho). 31P-{1H} NMR
2
(161.8 MHz): d-15.9 (t, 1P, JPP = 17.8 Hz, PiPr2 trans to dmso),
À1.0 (br, 2P, PiPr2 trans to H). 11B NMR (128.3 MHz): d-12.7 (s).
4.4. X-ray crystal structure determination
4. Experimental
4.1. Materials and methods
A clear pale yellow crystal was mounted on a glass fiber. X-ray
All experiments were carried out under an atmosphere of argon
using Schlenk and glove-box techniques. Benzene was dried over
Na/benzophenone/tetraglyme, distilled under argon, and deoxy-
genated prior to use. Deuterated benzene was dried by distillation
data were collected on an Oxford Xcalibur system, using Mo Ka
radiation. Data was processed and corrected for Lorentz and
polarization effects, and for absorption with the Crysalis suite of
programs [17]. Structural solution and refinement were carried

3946
M.R. Buchner et al. / Journal of Organometallic Chemistry 693 (2008) 3943–3946
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(b) A.A. Barney, A.F. Heyduk, D.G. Nocera, Chem. Commun. (1999) 2379;
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solved by direct methods and subsequent difference Fourier syn-
theses. Hydrogen atoms were placed in calculated positions. Crys-
tal and refinement data are summarized in Table 1.
(e) J.C. Peters, T.J. Christopher, A.J. Dupont, C.G. Riordan, Inorg. Synth. 34
(2004) 8.
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Acknowledgments
The authors thank Prof. W. A. Herrmann for generous support
and Dr. S. D. Hoffmann for recording X-ray diffraction data. S. S.
is grateful to the Deutsche Forschungsgemeinschaft for a grant
within the Emmy-Noether Programm (SCHN950/2-1).
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