Addition of [(η5-C5Me5)IrH 4] to a zwitterionic silylene: Stepwise formation of iridium(v)-silyl and iridium(iii)-silylene complexes

Article abstract of DOI:10.1039/c0dt00236d

The first zwitterionic silyl-iridium(v) complex 3 is generated by insertion of silylene 1 into an iridium-hydrogen bond of the iridium(v) hydride 2, [(η⁵-C₅Me₅)IrH₄]. Complex 3 undergoes proton migration from an IrH moiety to the terminal CH₂ group of the silyl ligand to furnish the N-donor stabilised Ir(iii)-silylene complex 5.

Full text of DOI:10.1039/c0dt00236d

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Addition of [(g5-C5Me5)IrH4] to a zwitterionic silylene: stepwise formation of
iridium(V)-silyl and iridium(III)-silylene complexes†
Ann-Katrin Jungton,a Antje Meltzer,b Carsten Pra¨sang,b Thomas Braun,*a Matthias Driess*b and Anna Pennera
Received 10th February 2010, Accepted 19th April 2010
First published as an Advance Article on the web 5th May 2010
DOI: 10.1039/c0dt00236d
The first zwitterionic silyl-iridium(V) complex 3 is generated
by insertion of silylene 1 into an iridium–hydrogen bond of the
5
iridium(V) hydride 2, [(g -C5Me5)IrH4]. Complex 3 undergoes
proton migration from an IrH moiety to the terminal CH2
group of the silyl ligand to furnish the N-donor stabilised
Ir(III)-silylene complex 5.
Transition metal complexes bearing N-heterocyclic silylenes as
ligands exhibit reactivity patterns that are distinctively different
from those of well-known N-heterocyclic carbene complexes.1–3
The latter have been applied successfully as ligands in a variety
of catalytic transformations, but catalytic applications of metal
derivatives of N-heterocyclic silylenes are rare.3 It has been shown
that the donor–acceptor properties of the zwitterionic silylene
14 (Scheme 1) can be tuned while 1 remains coordinated to a
nickel centre by modifying the substitution pattern at 1.5,6 Thus,
the silylene 1 enables heterolytic H–X bond activation (X =
O, S, NR2, NR–NR2) without disruption of the coordinative
Si(II) → metal bond and can, therefore, serve as a non-innocent
ligand in transition metal complexes. The electronic flexibility of
1 represents an opportunity for the development of new types of
catalytic cycles in which a silicon-based ligand plays an important
role for substrate activation.
In this communication we present the stepwise hydrogenation
of 1 at an iridium centre. This unprecedented reaction pathway
Scheme 1 Reactivity of the silylene 1 towards 2.
demonstrates the variability of 1 in the coordination sphere of a
metal. Remarkably, silylene 1 retains its zwitterionic properties
while bound to the metal, which is essential for the observed
reactivity.4
1
of the methyl groups of the Cp* ligand. The H NMR spectrum
shows a singlet at d -15.58 for the three hydrido ligands.8 Cooling
a [D8]toluene solution of 3 led to decoalescence at 198 K and to
the observation of three resonances at d -15.11, d -15.96 and
The reaction of
1 with the iridium hydrido complex
d -17.40 at 188 K.
5
[(h -C5Me5)IrH4] 2 gave initially complex 3 by insertion of 1 into
an iridium–hydrogen bond, i.e. an oxidative addition of the Ir–H
bond at the silicon(II) centre (Scheme 1). As expected, compound
3 exhibits a 29Si resonance signal in the H,29Si-HMBC NMR
The molecular structure of 3 was confirmed by single-crystal
X-ray diffraction analysis (Fig. 1).§ The Ir–Si distance of
2.3293(11) A is within the range of values observed for other silyl
iridium complexes.7 The coordination sphere at the silicon atom
is close to tetrahedral: the sum of the angles at the silicon atom
in the IrSiN2 moiety is 335 . The latter is located 0.566 A above
the plane defined by N(1), N(2) and Ir(1) and 0.616 A above the
˚
1
spectrum at higher field than 1 (d 88.4 versus d -7.7 for 3).‡
Comparable chemical shifts have been observed for other iridium
silyl complexes.7 The 29Si resonance of 3 correlates with signals
◦
˚
˚
1
in the H domain which can be assigned to the hydrido ligands,
one defined by the N2C3 subunit. The N(1)–Si(1)–N(2) angle of
the silicon bound hydrogen atom (JHSi = 215 Hz) and the protons
100.27(18)◦ is comparable to the corresponding value observed
◦
6
in the Ni(0) bound silylene [(h -toluene)Ni(1)] of 99.32(14) , but
is larger than in [Ni{Si(nacnac)(OH)}(CO)3] (nacnac = (2,6-
iPr2C6H3)NC(Me)C(H)C(Me)N(2,6-iPr2C6H3) [(96.30(15)◦].5,6
The initial step for the formation of 3 is presumably a coor-
dination of the silylene 1 at the iridium centre of 2 followed by a
migration of a hydrido ligand from the iridium to the silicon(II) site.
Prior to this pre-coordination a free coordination site at iridium
might be required, which could, for instance, be generated by
a haptotropic shift of the C5Me5 ligand. Note that the silicon(II)
aHumboldt-Universita¨t zu Berlin, Institut fu¨r Chemie, Brook-Taylor-Straße
2, 12489, Berlin. E-mail: thomas.braun@chemie.hu-berlin.de; Fax: 030-
20936966; Tel: 030-20933913
bTechnische Universita¨t Berlin, Institut fu¨r Chemie, Straße des 17. Juni 115,
10623, Berlin. E-mail: matthias.driess@tu-berlin.de; Fax: 030-31429732;
Tel: 030-31429731
† Electronic supplementary information (ESI) available: Experimental
details. CCDC reference numbers 765860 and 765861. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt00236d
5436 | Dalton Trans., 2010, 39, 5436–5438
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N-donor stabilised silylene complexes [Ni{Si(nacnac)(X)}(CO)3]
(X = OH, OSO2CF3) also exhibit deshielded 29Si nuclei with
resonance signals in the 29Si NMR spectrum at d 35.0 and d 38.8,
respectively.6 The signal for the hydrido ligands in 5 appears at
1
1
d -19.24 in the H NMR spectrum. Again, low temperature H
NMR experiments did not lead to decoalescence. The T1 times
reveal a minimum of 297 ms at 233 K (400 MHz), which is
consistent with classical bound hydrido ligands.8
Single crystals suitable for an X-ray structure analysis were
obtained by cooling a solution of 5 in toluene at 243 K (Fig. 2).§
˚
The Ir–Si distance of 2.2328(9) A is, to the best of our knowledge,
the shortest iridium–silicon bond length which has been described
in the literature.7,10-13 The short value suggests Ir–Si multiple bond
character which is most likely caused by p-backbonding from the
Ir(III) centre to the novel N-donor supported silylene ligand in
5 bearing a Si(II)–H moiety. The short Ir(III)–Si(II) bond length
is in accordance with an elongation of the Si–N distances in 5
in comparison to the corresponding values observed in 3. The
N(1)–Si(1)–N(2) angle in 5 is 94.46(13)◦, similar to the corre-
sponding one in the related complex [Ni{Si(nacnac)(OH)}(CO)3]
[(96.30(15)].6
Fig. 1 ORTEP representation of the molecular structure of 3; ellipsoids
are drawn at the 50% probability level. Hydrogen atoms have been omitted
for clarity except those at the C(2), C(4) and C(5) atoms. The hydrogen
atoms which are coordinated at the Si and Ir centres could not be
˚
localized in the difference Fourier map. Selected bond lengths (A) and
angles (◦): Ir(1)–Si(1) 2.3293(11), N(1)–Si(1) 1.785(4), N(2)–Si(1) 1.771(4),
C(1)–C(4) 1.373(6), C(3)–C(5) 1.486(7); N(1)–Si(1)–N(2) 100.27(18),
N(1)–Si(1)–Ir(1) 119.75(13), N(2)–Si(1)–Ir(1) 115.27(12).
centres in silylene complexes can exhibit an inherent Lewis-acidity,
which in the case of 1 most likely persists after coordination at the
iridium centre and which induces the migration of a metal bound
hydride.5,9 Hydrogen migration steps to metal bound silylenes are
very rare, but have been proposed in the iridium-mediated catalytic
hydrosilylation of alkenes.10,11 Typically, the reverse reactions have
been observed.9,12
Remarkably, the exocyclic methylene moiety in 3 exhibits nucle-
ophilic properties. This is expressed by the mesomeric structures 3a
and 3b, which reflect the description of 3 either as a silyl complex or
as an N-donor stabilised silylene derivative (Scheme 1).4,5 The nu-
cleophilicity was demonstrated by the reaction of 3 with B(C6F5)3
to give complex 4.‡ The 1H,29Si-HMBC NMR spectrum of 4 shows
a resonance at d{ Si} 12.7 in the 29Si domain which correlates with
29
Fig. 2 ORTEP view of the molecular structure of 5; ellipsoids are
drawn at the 50% probability level. Hydrogen atoms have been omitted
for clarity except these at the SiN2C3 moiety. Whereas iridium bound
hydrogen atoms could not be localised in the difference Fourier map,
the signal for the silicon bound hydrogen atom (JHSi = 224 Hz).
1
The 11B{ H} NMR spectrum of 4 displays a singlet at d -15.5.
6
The related nickel complex [Ni{Si(L)}(h -toluene)] [L = (2,6-
iPr2C6H3)NC(Me)C(H)C{CH2B(C6F5)3}N(2,6-iPr2C6H3)] shows
˚
the position of H(1) could be refined. Selected bond lengths (A)
a signal at d -15.1.5 Compound 4 exhibits a singlet at d
and angles (◦): Ir(1)–Si(1) 2.2328(9), N(1)–Si(1) 1.850(3), N(2)–Si(1)
1.855(3), C(1)–C(4) 1.495(5), C(3)–C(5) 1.504(5), Si(1)–H(1) 1.45(5);
N(1)–Si(1)–N(2) 94.46(13), N(1)–Si(1)–Ir(1) 116.93(10), N(2)–Si(1)–Ir(1)
120.01(10).
1
-15.65 in the H NMR spectrum, which can be assigned to the
1
iridium bound hydrido ligands. Variable temperature H NMR
experiments did not show decoalescence, but led to a broadening
of the signal at 193 K. Measurement of the longitudinal T1 spin
lattice relaxation times at different temperatures reveal a minimum
at 243 K with a T1 = 231 ms at 400 MHz. The data indicate the
presence of classical bound hydrido ligands.8
In conclusion, we observed an unprecedented reactivity of the
zwitterionic silylene ligand 1 in the coordination sphere of iridium.
Remarkably, the Si(II) atom in 1 undergoes an insertion reaction
into the Ir–H bond at the hydrido precursor complex 2 to give the
new Ir(V) hydrido silyl complex 3. Subsequently, proton migration
from an Ir–H moiety to the basic methylene group of the ylide-like
silyl ligand affords the novel, zwitterionic Ir(III) silylene complex 5
bearing a Si–H moiety. A transfer of electron density from iridium
to the silicon containing ligand could result in multiple bond
character of the Ir–Si bond. Whereas the zwitterionic behaviour
of 1 and of its metal complexes has already been demonstrated,4,6
its ability to act after all as a H- as well as H+ acceptor may open
up new opportunities to gain access to cooperative catalysts for
Stirring the reaction solution of 3 in toluene at ambient
temperature for 24 h led to the generation of the remarkable
zwitterionic silylene iridium(III) compound 5 (Scheme 1).‡ The
reaction is considerably faster in the presence of NH3 (2 h). We
therefore assume that it occurs by a transfer of a proton from the
iridium centre to the methylene group. At least in the presence
of the base the reaction could then also be intermolecular.6 The
1H,29Si-HMBC NMR spectrum of 5 reveals a correlation of a
29
resonance in the 29Si domain at d{ Si} 13.6 with the signal for the
Si bound hydrogen atom at d(1H) 6.45 (JHSi = 170 Hz). Likewise,
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heterolytic H–X (X = C, N, O, H) bond activation. The silylene
ligand 1 could pave the way to new metal-mediated reaction
pathways with extraordinary applications in catalysis.6,14
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Fu¨rstner, H. Krause and C. W. Lehmann, Chem. Commun., 2001, 2372–
2373; M. Zhang, X. Liu, C. Shi, C. Ren, Y. Ding and H. W. Roesky,
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Acknowledgements
We thank the Cluster of Excellence “Unifying Concepts in
Catalysis” funded by the Deutsche Forschungsgemeinschaft and
administered by the TU Berlin for financial support.
4 M. Driess, S. Yao, M. Brym, C. van Wu¨llen and D. Lentz, J. Am. Chem.
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5 A. Meltzer, C. Pra¨sang, C. Milsmann and M. Driess, Angew. Chem.,
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Notes and references
‡ Selected NMR data for 3–5 at 298 K: 3 1H NMR (400 MHz, [D8]toluene):
d 7.20–6.97 (m, 7 H, 2,6-iPr2C6H3 and SiH), 5.37 (s, 1 H, ring-CH), 3.84
(s, 1 H, NCCH2), 3.58 (sept, JHH = 6.8 Hz, 1 H, CH(CH3)2), 3.44 (sept,
JHH = 6.8 Hz, 1 H, CH(CH3)2), 3.22 (s, 1 H, NCCH2), 1.99 (s, 15 H,
C5(CH3)5), 1.34 (s, 3 H, NCCH3), 1.31 (d, JHH = 7.3 Hz, 6 H, CH(CH3)2),
1.30 (d, JHH = 7.0 Hz, 6 H, CH(CH3)2), 1.26 (d, JHH = 7.0 Hz, 6 H
CH(CH3)2), 1.16 (d, JHH = 6.9 Hz, 6 H, CH(CH3)2), -15.58 (s, 3 H, IrH;
at 198 K: d -15.11 (s, br), -15.96 (s, br), -17.40 (s, br); 1H,29Si HMBC
7 M. Okazaki, H. Tobita, Y. Kawano, S. Inomata and H. Ogino,
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NMR (400/79.49 MHz, [D8]toluene): d(29Si) -7.7 (JHSi = 215 Hz). 4 H
1
NMR (400 MHz, [D8]toluene): d 7.23–6.94 (m, 6 H, 2,6-iPr2C6H3), 6.53
and 6.52 (both s, 2 H, SiH, ring-CH), 3.26 (sept, JHH = 6.8 Hz, 1 H,
CH(CH3)2), 3.12 (d, br, JHH = 20 Hz, 1 H, CH2), 2.98 (sept, JHH = 6.8 Hz,
1 H, CH(CH3)2), 2.84 (sept, JHH = 6.8 Hz, 1 H, CH(CH3)2), 2.57 (sept,
JHH = 6.7 Hz, 1 H, CH(CH3)2), 2.41 (d, br, JHH = 20 Hz, 1 H, CH2), 1.72
(d, JHH = 6.7 Hz, 3 H, CH(CH3)2), 1.62 (s, 6 H, NCCH3), 1.45 (s, 15 H,
C5(CH3)5), 1.42 (d, JHH = 6.8 Hz, 3 H, CH(CH3)2), 1.38 (d, JHH = 6.9 Hz,
3 H, CH(CH3)2), 1.25 (d, JHH = 6.8 Hz, 3 H, CH(CH3)2), 1.22 (d, JHH
=
6.9 Hz, 3 H, CH(CH3)2), 1.04 (d, JHH = 6.6 Hz, 3 H, CH(CH3)2), 0.96 (d,
JHH = 6.9 Hz, 3 H, CH(CH3)2), 0.92 (d, JHH = 6.8 Hz, 3 H, CH(CH3)2),
-15.65 (s, 3 H, IrH); 11B NMR (128.37 MHz, [D8]toluene): d -15.5; 1H,29Si-
NMR (400/79.49 MHz, [D8]toluene): d(29Si) 12.7 (JHSi = 224 Hz). 5 H
1
NMR (400 MHz, C6D6): d 7.22–7.10 (m, 6 H, 2,6-iPr2C6H3), 6.45 (s, 1 H,
SiH), 4.95 (s, 1 H, ring-CH), 3.57 (sept, JHH = 6.9 Hz, 2 H, CH(CH3)2),
3.01 (sept, JHH = 6.9 Hz, 2 H, CH(CH3)2), 1.93 (s, 15 H, C5(CH3)5), 1.62
(d, JHH = 6.9 Hz, 6 H, CH(CH3)2), 1.49 (d, JHH = 6.8 Hz, 6 H, CH(CH3)2),
1.48 (s, 6 H, NCCH3), 1.17 (d, JHH = 6.9 Hz, 6 H, CH(CH3)2), 1.07 (d,
JHH = 7.2 Hz, 6 H, CH(CH3)2), -19.24 (s, 2 H, IrH); 1H,29Si HMBC NMR
(400/79.49 MHz, [D8]toluene): d(29Si) 13.6 (JHSi = 170 Hz).
9 R. Waterman, P. G. Hayes and T. Don Tilley, Acc. Chem. Res., 2007,
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§ Crystallographic data for 3: C39H59IrN2Si, M = 776.17, orthorhombic,
10 E. Calimano and T. Don Tilley, J. Am. Chem. Soc., 2009, 131, 11161–
˚
space group Pna21, a = 18.1058(4), b = 11.9735(3), c = 16.7760(5) A, V =
3
-1
11173.
˚
3636.87(16) A , T = 100(2) K, Z = 4, m(Mo-Ka) = 3.732 mm , 42 435
reflections measured, 11 005 unique (Rint = 0.0583), final R1, wR2 values
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on all data: 0.0508, 0.0609, R1, wR2 values for 9327 reflections with Io
>
2s(Io): 0.0376, 0.0586; crystallographic data for 5: C39H59IrN2Si, M =
776.17, monoclinic, space group ◦C2/c, a = 30.6330(10), b = 16.3992(4),
3
˚
˚
c = 16.7702(6) A, b = 118.185(3) , V = 7425.7(4) A , T = 100(2) K, Z =
8, m(Mo-Ka) = 3.656 mm-1, 34 952 reflections measured, 10 001 unique
(Rint = 0.0509), final R1, wR2 values on all data: 0.0470, 0.0825, R1, wR2
values for 8627 reflections with Io > 2s(Io): 0.0382, 0.0795. The structures
were solved by direct methods and refined with the full matrix least square
13 M. A. Esteruelas, F. J. Lahoz, E. On˜ate, L. A. Oro and L. Rodriguez,
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2
15
methods on Fs
.
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J. Zhang, H. Song and C. Cui, Dalton Trans., 2009, 5444–5446;
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¨
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5438 | Dalton Trans., 2010, 39, 5436–5438
This journal is
The Royal Society of Chemistry 2010
©