Iridium(III) complexes of the new tridentate bis(8-quinolyl)silyl ('NSiN') ligand

Article abstract of DOI:10.1039/b102821a

Bis(8-quinolyl)methylsilane reacts cleanly with [Ir(coe)₂Cl]₂ (coe = cyclooctene) to give a complex containing an NSiN tridentate ligand, [Ir(NSiN)(coe)(H)(Cl)]; preliminary reactivity studies indicate that the (NSiN)Ir fragment is chemically robust.

Full text of DOI:10.1039/b102821a

Iridium(III) complexes of the new tridentate bis(8-quinolyl)silyl
(‘NSiN’) ligand†
Mark Stradiotto, Kyle L. Fujdala and T. Don Tilley*
Department of Chemistry, University of California at Berkeley, Berkeley, California 94720-1460, USA.
E-mail: tdtilley@socrates.berkeley.edu
Received (in Columbia, MO, USA) 22nd March 2001, Accepted 16th May 2001
First published as an Advance Article on the web 13th June 2001
Bis(8-quinolyl)methylsilane reacts cleanly with [Ir(coe)2Cl]2
(coe = cyclooctene) to give a complex containing an NSiN
tridentate ligand, [Ir(NSiN)(coe)(H)(Cl)]; preliminary re-
activity studies indicate that the (NSiN)Ir fragment is
chemically robust.
Multidentate ligands have featured prominently in the recent
development of new transition metal chemistry. Such ligands
provide stability to a metal complex, and offer diverse
possibilities for the manipulation of steric and electronic
properties in controlling reactivity at the metal center. Notwith-
standing the significant advances in transition metal–silicon
chemistry over the past several years,1,2 multidentate ligands
containing silicon as part of the chelate architecture have
received little attention. The incorporation of silicon into such
structures may offer certain advantages, given (for example) the
high strength of metal–silicon bonds and the strong trans-effect
for silicon ligands.1,3 Stobart and coworkers4 have synthesized
a number of metal complexes supported by a PSiP ligand
framework; tridentate connectivity is attained in these systems
via oxidative addition of an Si–H functionality. This general
strategy should provide synthetic routes to various silicon-
containing multidentate ligands.
With the goal of developing reactive metal fragments
supported by new tridentate ancillary ligands, we identified
nitrogen-based NSiN chelates as potential synthetic targets. A
Scheme 1
tridentate system based on the bis(8-quinolyl)silyl framework
appeared particularly attractive, since the resulting five-
membered metallacyclic rings are expected to stabilize a
complex towards reductive elimination of the silyl unit, while
providing a rather rigid ligand binding arrangement. Support for
the viability of such a ligation strategy comes from the work of
The crystallographically determined structure of compound 2
is shown in Fig. 1.†‡ The non-equivalence of the quinoline
groups noted in the 1H and 13C NMR spectra is consistent with
the observed facial binding mode of the NSiN fragment.
Constraints imposed by this ligand give rise to an Ir(III) center
which is distorted considerably from ideal octahedral geometry.
The chelate ligand ‘bite’ angles [N–Ir–N 87.5(3), N–Ir–Si
83.2(3) and 83.2(2)°] are all significantly compressed, under-
scoring the ‘pinned-back’ nature of the NSiN chelating
framework. Geometric distortions are also observed at the
silicon center. For example, the C(Me)–Si–Ir angle [131.2(4)°]
is expanded well beyond the normal tetrahedral value. The Ir–Si
distance [2.275(3) Å] in 2 can be compared to those in fac-
tris[(8-quinolyl)dimethylsilyl]iridium (av. 2.30 Å),5 and falls
within the previously observed range (2.235–2.454 Å) for such
Watts and coworkers,5 who have prepared Rh(III) and Ir(III
)
complexes of the bidentate (8-quinolyl)dimethylsilyl ligand. In
this contribution, synthesis of the tridentate ligand precursor
bis(8-quinolyl)methylsilane 1 and Ir(III) complexes supported
by this NSiN fragment are reported.
Addition of 0.5 equivalents of methyldichlorosilane to
8-lithioquinoline in THF, followed by purification, provided 1
as pale yellow crystals in moderate yield.†‡ The character-
ization of 1 is based on IR and NMR spectroscopic data.
Notably, the CS symmetry of this compound results in
1
observation of only one set of quinoline H and 13C NMR
resonances.
Treatment of 1 with 0.5 equivalents of [Ir(coe)2Cl]2 in
CH2Cl2 resulted in rapid liberation of cyclooctene (coe) and
near quantitative formation (by NMR spectroscopy) of the new
coordination complex 2 as a single diastereomer (Scheme 1).
Vapor diffusion of diethyl ether into the reaction mixture
permitted isolation of 2 as analytically pure, yellow crystals in
87% yield. As expected, two sets of NMR resonances are
observed for the quinoline groups of compound 2. Evidence for
the presence of the hydride ligand in 2 was obtained from IR
(2172 cm21) and 1H NMR (215.6 ppm) spectroscopic data.
† Electronic supplementary information (ESI) available: synthetic and
crystallographic details for 1–5. See http://www.rsc.org/suppdata/cc/b1/
b102821a/
Fig. 1 Crystallographically determined structure of 2, depicted with 50%
thermal ellipsoids; all hydrogen atoms have been omitted for clarity.
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Chem. Commun., 2001, 1200–1201
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b102821a

Fig. 3 Crystallographically determined structure of 5, depicted with 50%
thermal ellipsoids; the anion, solvates and all hydrogen atoms have been
omitted for clarity. Only one component of the disordered coe fragment is
shown.
Fig. 2 Crystallographically determined structure of 3, depicted with 50%
thermal ellipsoids; all hydrogen atoms have been omitted for clarity.
bonds.2 In contrast, the trans-labilizing nature1–3 of the silyl
group is manifested in a long Ir–Cl bond [2.572(3) Å].6 The
trans-influence of the hydride ligand is also indicated by
elongation of an Ir–N bond [2.191(9) Å], relative to the other
Ir–N bond [2.120(8) Å] in this complex.7
formation (by NMR spectroscopy) of a new product, which we
assume to be the desired mononuclear complex. Attempts are
currently underway to isolate this species in pure form.
In conclusion, Ir(III) complexes containing the new bis(8-
quinolyl)methylsilyl ligand have been prepared as single
diastereomers in excellent yield. The diastereoselectivity asso-
ciated with the clean formation of 2 and its derivatives indicates
that the NSiN chelating ligand may have a directing effect on
transformations at the Ir(III) center.4 The synthesis and re-
activity of various metal complexes supported by NSiN hybrid
ligands will be the subject of future reports.
Acknowledgment is made to the National Science Founda-
tion for their generous support of this work. M. S. thanks the
Natural Sciences and Engineering Research Council of Canada
for financial support in the form of an NSERC Postdoctoral
Fellowship.
Preliminary reactivity studies reveal that the NSiN moiety is
a robust ancillary ligand. Treatment of 2 with either PPh3 or CO
resulted in substitution of the coe ligand, yielding compounds 3
and 4, respectively. Compound 3 gives rise to a singlet (17.0
ppm) in the 31P{1H} NMR spectrum and a phosphorus-coupled
1
doublet (219.1 ppm) in the hydride region of the H NMR
spectrum. The solid-state structure of compound 3 (Fig. 2)
confirmed that the conversion of 2 to 3 occurs with retention of
configuration at the Ir(III) center. In general, the geometric
features associated with 3 parallel those described for 2 (vide
supra). However, the Ir–P distance in 3 [2.234(3) Å] is
noteworthy, since it appears to be contracted relative to Ir–P
bonds found in related Ir(III) compounds (2.264–2.381 Å).4,6,7
The coordination of a terminal carbonyl ligand to the Ir(III
)
center in 4 was confirmed by observation of a strong IR
absorption attributed to this fragment (2010 cm21) and by the
presence of a resonance at 169.9 ppm in the 13C NMR spectrum.
In contrast to the aforementioned transformations, no reaction
was observed when a degassed CD2Cl2 solution of 2 was
exposed to an atmosphere of H2—even after prolonged heating
(72 h at 90 °C). Collectively, these observations indicate that the
Ir–Si linkage in 2 is relatively stable toward reductive
elimination of Si–H.
Notes and references
‡ CCDC 164004–164006. See http://www.rsc.org/suppdata/cc/b1/
b102821a/ for crystallographic data in CIF or other electronic format.
1 M. S. Eisen, The Chemistry of Organic Silicon Compounds, ed. Z.
Rappoport and Y. Apeloig, Wiley, New York, 1998, vol. 2, ch. 35, p.
2037; T. D. Tilley, The Silicon–Heteroatom Bond, ed. S. Patai and Z.
Rappoport, Wiley, New York, 1991, ch. 9, 10, pp. 245, 309; T. D. Tilley,
The Chemistry of Organic Silicon Compounds, ed. S. Patai and Z.
Rappoport, Wiley, New York, 1989, ch. 24, p. 1415.
In an attempt to prepare a cationic NSiN complex of iridium,
CH2Cl2 was added to an equimolar mixture of 2 and
LiB(C6F5)4·2.5Et2O. Surprisingly, the major product obtained
from this reaction was not that resulting from simple anion
exchange, but rather the dinuclear complex 5, which formally
results from in situ trapping of the anticipated cationic species
by an additional molecule of 2. Complex 5 is formed
exclusively as the racemic (C2-symmetric) diastereomer, as
confirmed by X-ray crystallography (Fig. 3).†‡ The overall
connectivity pattern in 5 mirrors that found in 2, with the
exception that both of the Ir–Cl distances in the former
[2.638(2) and 2.645(2) Å] are significantly longer than the Ir–Cl
distance in the latter. Subsequently, compound 5 was prepared
in 93% yield via treatment of 2 with 0.5 equiv. of the lithium
borate. However, slow addition of 2 to an equiv. of LiB-
(C6F5)4·2.5Et2O under more dilute conditions led to clean
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20, 601, and references therein; R. D. Brost, G. C. Bruce, F. L. Joslin and
S. R. Stobart, Organometallics, 1997, 16, 5669; R. A. Gossage, G. D.
McLennan and S. R. Stobart, Inorg. Chem., 1996, 35, 1729; M. J.
Auburn, R. D. Holmes-Smith, S. R. Stobart, P. K. Bakshi and T. S.
Cameron, Organometallics, 1996, 15, 3032; M. J. Auburn and S. R.
Stobart, Inorg. Chem., 1985, 24, 318.
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1992, 31, 3195; P. I. Djurovich, A. Safir, N. Keder and R. J. Watts,
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28, 3084; R. H. Crabtree, E. M. Holt, M. Lavin and S. M. Morehouse,
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