Pushing the σ-donor strength in iridium pincer complexes: Bis(silylene) and bis(germylene) ligands are stronger donors than bis(phosphorus(III)) ligands

Article abstract of DOI:10.1002/anie.201205570

Breaking the donor limits: The remarkable coordination chemistry of ECHE (E=Si, Ge) ligands with Group-9 metals (Ir, Rh) and their application in catalytic C-H borylation of arenes was investigated. The spectroscopic and structural features of the first [ECE] iridium complexes show the stronger σ-donating properties of the divalent Si and Ge pincer ligands compared to analogous P^III-based ligands. Copyright

Full text of DOI:10.1002/anie.201205570

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DOI: 10.1002/anie.201205570
Pincer Ligands
Pushing the s-Donor Strength in Iridium Pincer Complexes: Bis-
(silylene) and Bis(germylene) Ligands Are Stronger Donors than
Bis(phosphorus(III)) Ligands**
Andreas Brꢀck, Daniel Gallego, Wenyuan Wang, Elisabeth Irran, Matthias Driess,* and
John F. Hartwig*
Dedicated to Professor Cameron Jones on the occasion of his 50th birthday
The development of new ligands to control the reactivity of
transition-metal (TM) centers in complexes is a fundamental
goal in organometallic chemistry. The “pincer” (EDE) motif
describes tridentate, meridonal coordinating ligands that
offer a plethora of opportunities to fine-tune the properties
of TM complexes.[1] The arms (E) usually consist of neutral,
two-electron Lewis basic donor moieties (for example, E =
PR2, NR2, or SR), which are connected over a linker group
(often CH2, N, or O) to the neutral or mono-anionic
anchoring site (D; for example, a pyridyl or phenyl group).
With respect to iridium as the metal center, pincer ligands
have been used for the generation of low-coordinated, highly
din-2-ylidene (SIPr).[6] Therefore, we wanted to combine the
unique properties of the pincer motif with divalent silicon and
germanium donor moieties as a means to develop the
ꢀ
activation of X H bonds by TM complexes.
We recently reported the synthesis of the first bis(silylene)
SiCHSi pincer ligand and its unexpected coordination
chemistry with Pd0.[7] The sole product from the reaction of
SiCHSi with [Pd(PPh3)4] was the mixed silylene(SiII)–silyl-
(SiIV)-pincer complex depicted in Scheme 1. Computational
electron-rich complexes used in the activation of strong bonds
[2]
ꢀ
such as the N H of ammonia or aniline, the selective H–D
exchange of olefinic protons with deuterobenzene,[3] and
alkane dehydrogenation.[4]
Pincer ligands containing N-heterocyclic-stabilized diva-
lent silicon (silylenyl) moieties RSi: (R = amidinate or b-
diketiminate) instead of the common PR2, NR2, or SR donor
arms E appear promising to achieve stronger s-donating
properties, which could facilitate the activation of even less
reactive bonds.[5] In fact, the determination of the CO
stretching vibrations in [LNi(CO)3] complexes with six-
membered diketiminate SiII ligands L revealed a A1 band as
low as 2046 cmꢀ1 versus 2056 cmꢀ1 for L = PtBu3, and
2052 cmꢀ1 for L = 1,3-bis(2,6-di-isopropylphenyl)imidazoli-
Scheme 1. Earlier and present studies with the SiCHSi pincer ligand.
[*] Dr. A. Brꢀck, M. Sc. D. Gallego, Dipl.-Chem. W. Wang,
Prof. Dr. M. Driess
Department of Chemistry: Metalorganics and Inorganic Materials,
Technische Universitꢁt Berlin
Strasse des 17. Juni 135, Sekr. C2, 10623 Berlin (Germany)
E-mail: matthias.driess@tu-berlin.de
studies indicated that this complex forms by a mechanism in
ꢀ
which the ligand underwent a C -H oxidative addition to give
an {(SiCSi)Pd(H)} intermediate, followed by a hydride shift
from Pd onto Si and single coordination of a second SiCHSi
ligand. We now report the different coordination behavior of
the SiCHSi ligand toward the Group 9 metals iridium and
rhodium. Furthermore, we describe the synthesis of the first
GeCHGe pincer ligand and the use of the respective
Prof. Dr. J. F. Hartwig
University of California, Department of Chemistry
718 Latimer Hall, Berkeley, CA 94720 (USA)
E-mail: jhartwig@berkeley.edu
[**] Financial support by the Einstein Foundation Berlin (J.F.H.) and the
Cluster of Excellence UniCat (financed by the Deutsche For-
schungsgemeinschaft and administered by the TU Berlin) is
gratefully acknowledged. A.B. thanks the Deutsche Forschungsge-
meinschaft for an initial postdoctoral fellowship to work with J.F.H.
(BR 4069/1-1). We thank Dr. S. Krackl for help with X-ray crystal
structures.
ꢀ
{(SiCSi)Ir} and {(GeCGe)Ir} complexes for catalytic C H
borylation of arenes. The SiCSi and GeCGe ligands show
a strikingly stronger s-donor ability than analogous PIII pincer
ligands.
Addition of SiCHSi to a solution of [{IrCl(coe)2}2] in C6D6
led to the immediate and quantitative formation of a new
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205570.
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compound corresponding to a hydride signal in the 1H NMR
spectrum at ca. d = ꢀ25.6 ppm. After purification, the 1H and
13C NMR spectroscopic characterization revealed (in addition
to the hydride signal) three sets of singlets for the tert-butyl
groups, one set of signals for the phenyl groups, and one set of
signals corresponding to coordinated cyclooctene (coe).
These data, in combination with one singlet at d = 54.9 ppm
in the 29Si NMR spectrum, indicated that the product was the
complex [(SiCSi)IrHCl(coe)] depicted in Scheme 2.
group, 1.409(9) ꢀ, is one of the longest reported[11] and
resembles the high electron density observed at the Ir center
through SiII!M donation (see below).
With [(SiCSi)IrHCl(coe)] in hand, we sought to determine
whether the homologous bis(germylene) ligand GeCHGe
could afford the corresponding iridium and rhodium com-
plexes [ECE]MHCl(coe), and how the properties of these
ECE ligands compare to those of the isoelectronic PIII-based
compounds.
Single-crystal X-ray diffraction revealed the structural
The first bis(germylene) ligand GeCHGe was synthesized
by deprotonation of 4,6-di-tert-butylresorcinol and a subse-
quent salt metathesis reaction with the respective chloroger-
mylene precursor.[12] The GeCHGe compound was isolated
after recrystallization in hexane in 83% yield and structurally
characterized by single-crystal X-ray analysis (Scheme 2).
Addition of GeCHGe to a solution of [{IrCl(coe)2}2] in C6D6
led to the immediate formation of a new compound corre-
sponding to a hydride signal at d = ꢀ26.8 ppm in the 1H NMR
spectrum. Further spectroscopic characterization of the iso-
lated complex showed the same general features as those of
the Si analogue [(SiCSi)IrHCl(coe)].
features of this complex (Figure 1a).[8] The Ir Si bond lengths
ꢀ
of 2.305(1) and 2.301(1) ꢀ are nearly identical to those in the
silylene(SiII)–silyl(SiIV) pincer palladium complex (see above;
II
IV
ꢀ
ꢀ
Pd Si 2.3038(11) and 2.3271(12) ꢀ, Pd Si 2.3561(12) ꢀ).
As expected, these values fall in between the average IrIII–SiIV
distances of about 2.39 ꢀ for octahedral Ir–silyl complexes
and the distances of about 2.24 ꢀ for non-s-donor-stabilized
IrIII–silylene complexes.[9,10] The C Ccoe bond of the olefinic
ꢀ
To learn more about the coordination and reactivity
characteristics of the new ligands, we tested Vaskaꢁs complex,
[IrCl(CO)(PPh3)2], and [IrH(CO)(PPh3)3] as metal precur-
sors. Reaction of both ECHE ligands (E = Si, Ge) with
[IrCl(CO)(PPh3)2] led to a mixture of unknown products.
However, heating solutions of SiCHSi with [IrH(CO)(PPh3)3]
in C6D6 for 1 h at 1008C led to the clean conversion of the
starting material to a new hydride species, which corre-
sponded to a singlet at d = ꢀ10.2 ppm in the 1H NMR
spectrum. Solely signals for free PPh3 were observed in the
31P NMR spectrum. Along with the hydride signal with
a relative integral of two, one set of aromatic signals and
two sets of signals for the six tert-butyl groups were observed
at d = 1.34 and 1.82 ppm with an integral of 36 to 18,
respectively. The IR spectrum showed one CO stretching
vibration at n = 1968 cmꢀ1 and one weak band for the
hydrides at n = 2251 cmꢀ1. These data indicate that the
product is [(SiCSi)Ir(H)2(CO)] in Scheme 3.
Scheme 2. a) Synthesis of ECE pincer ligands and the reactivity with
[{IrCl(coe)2}2]. b) Molecular structure of GeCHGe.
In contrast, GeCHGe slowly decomposed when heated in
solution with [IrH(CO)(PPh3)3] at 1008C for a prolonged
Figure 1. Molecular structures with selected bond lengths [ꢂ] and angles [8] of a) [(SiCSi)IrHCl(coe)]: Ir1–Si1 2.301(1), Ir1–Cl1 2.514(1), Ir1–Si2
2.305(1), Ir1–C31 2.127(4), Ir1–C45 2.222(4), Ir1–C52 2.232(4); N1-Si1-N2 70.4(1), N2-Si1-Ir1 134.3(1), N3-Si2-Ir1 137.4(1), Ir1-Si2-N4 126.1(1),
N4-Si2-N3 71.3(2), N1-Si1-Ir1 129.7(1); b) [(tBu-PCP)IrHCl]: Ir1–C1 2.014(5), Ir1–Cl1 2.412(2), Ir1–P2 2.293(1), Ir1–P1 2.294(1); Cl1-Ir1-P2
99.44(4), Cl1-Ir1-P1 99.43(4), P1-Ir1-C1 80.4(1), P2-Ir1-C1 80.7(1); and c) [(iPrN-PCP)IrHCl(coe)]: P1–Ir1 2.290(2), Ir1–Cl1 2.506(2), Ir1–P2
2.282(2), C9–Ir1 2.053(6), Ir1–C9 2.053(6), Ir1–C1 2.285(7), Ir1–C2 2.307(7); Ir1-P1-N2 126.9(2), Ir1-P1-N1 121.1(2); Ir1-P2-N4 128.9(2), Ir1-P2-N3
119.3(2), N1-P1-N2 93.2(3), N4-P2-N3 92.9(3). Ellipsoids are set at 50% probability; hydrogen atoms are omitted for clarity.
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Scheme 3. Synthesis of {(SiCSi)IrIII} and {(SiCSi)RhIII} complexes by
ꢀ
substitution of PPh3 and concomitant C H oxidative addition.
Scheme 4. a) Synthesis of five-coordinate IrIII phosphinite and a six-
coordinate IrIII phosphorodiamidate complexes. b) Molecular structure
of tBu-PCHP.
period of time. There was no sign of the formation of a new
hydride species. This outcome might be explained by the
lower electron-donating properties of GeCHGe compared to
SiCHSi (see below). That is, initial substitution of one PPh3
ligand would lead to an intermediate [(k1-E-ECHE)IrH(CO)-
(PPh3)2] complex, which in the case of GeCHGe might not be
2
one new hydride species at d = ꢀ24.6 ppm (triplet, JHP
=
15.1 Hz), along with signals for coordinated coe. Identifica-
1
ꢀ
electron-rich enough to undergo a (probably) irreversible C
tion of the olefinic resonances by H–13C correlation spec-
H oxidative addition reaction to form [(GeCGe)Ir(H)2(CO)].
Obtaining defined complexes from rhodium and ECHE
proved to be more challenging than from iridium. Reactions
conducted with [{RhCl(coe)2}2] or [{RhCl(CO)2}2] as the
metal precursor produced undefined reaction mixtures, but
Wilkinsonꢁs dimer [{RhCl(PPh3)2}2] (generated in situ)
reacted with the SiCHSi ligand to cleanly form a new hydride
species. The 1H NMR spectrum contained a doublet of
doublets (2JHP = 11.0 Hz, 2JHRh = 22.1 Hz, 1H) at d =
ꢀ17.2 ppm, three sets of signals for the tert-butyl groups at
d = 0.90 (18H), 1.34 (18H) and 1.88 (18H) ppm, a doublet at
36.6 ppm (1JPRh = 97.6 Hz) in the 31P NMR spectrum, and
a doublet of doublets at d = 66.4 ppm (2JSiP = 20.6.0 Hz,
1JSiRh = 59.4 Hz) in the 29Si NMR spectrum. The chemical
troscopy revealed the chemical shifts of d = 4.31 and 81.6 ppm
in the 1H and 13C NMR spectra, respectively. X-ray diffraction
analysis of single crystals for this new compound confirmed
the coordination of coe (Figure 1c) and formation of [(iPrN-
PCP)IrHCl(coe)]. The molecular structure shows disordering
in the periphery of the coordinated coe, but the olefinic group
ꢀ
remains unaffected. The C Ccoe bond of the olefinic group is
1.35(1) ꢀ, which is slightly longer than circa 1.34 ꢀ for an
ꢀ
uncoordinated C C double bond and considerably shorter
ꢀ
than the C C bond of 1.409(9) ꢀ in [(SiCSi)IrHCl(coe)].
The NMR chemical shifts of the olefinic groups can serve
as a probe for the electronic properties of the metal,
according to the Chatt–Dewar–Duncanson bonding model:
a more electron-rich metal center leads to stronger p-back-
shift of the hydride and the coupling constants (2JHP and 1JPRh
)
bonding to the olefin, resulting in a lengthening of the C C
ꢀ
are sensitive to the TM coordination environment and can be
used for structural assignment. These data indicated the
structure of [(SiCSi)RhHCl(PPh3)] to be the one depicted in
Scheme 3.[13]
double bond and a higher-field chemical shift of the olefinic
protons and carbons in the NMR spectra.[16] Determination
and comparison of the chemical shift of the olefinic groups for
[(SiCSi)IrHCl(coe)] (1H d = 3.37, 13C d = 55.8 ppm, X-ray
1.409(9) ꢀ), [(GeCGe)IrHCl(coe)] (1H d = 4.06 ppm, 13C d =
65.1 ppm) and [(iPrN-PCP)IrHCl(coe)] (1H d = 4.31, 13C d =
81.6 ppm, X-ray 1.35(1) ꢀ) allows the conclusion to be drawn
that the ligand donor strength decreases in the following
order: SiCSi > GeCGe > iPrN-PCP > tBu-PCP.
To determine differences and similarities of our novel
ECE complexes with well-established PIII ligand systems, we
synthesized the related ligands tBu-PCHP and iPrN-PCHP[14]
(Scheme 4). Reaction of the phosphinite ligand tBu-PCHP
with [{IrCl(coe)2}2] led to the five-coordinate complex [(tBu-
PCP)IrHCl] (see Figure 1b for the crystal structure and the
Supporting Information for further information). This out-
come is not surprising, as many similar resorcinol-based
{[PCP]IrHCl} complexes without the two additional tert-butyl
groups in the phenyl backbone have been reported.[4,15] This
preference for a five- over a six-coordination might be
explained by a less electron-rich metal center, which in case
of ECE requires an additional p-accepting ligand (coe) for
stabilization of the metal center.
Catalytic C H borylation[17] of arenes with pinacolborane
ꢀ
(HBPin) was chosen to probe our novel complexes for activity
ꢀ
in C H functionalization reactions. The [(ECE)IrHCl(coe)]
complexes are coordinatively saturated and Cl–H substitution
to form a [(ECE)Ir(H)2(coe)] species with subsequent hydro-
genation of the coordinated coe was envisioned as a facile
pathway for activation.
In an initial NMR experiment, we added about 20 molar
equiv of HBPin to a solution of [(SiCSi)IrHCl(coe)] in C6D6.
After heating to 1008C for 2 h, we observed the formation of
small amounts of borylated benzene (PhBPin) and hydro-
genated cyclooctene by NMR and GC-MS. Continuing
heating for several hours did not show the growth of the
Reaction of the sterically and electronically more related
iPrN-PCHP ligand with [{IrCl(coe)2}2] at room temperature
led to a mixture of products, but addition at ꢀ788C and slow
warming to room temperature showed the clean conversion to
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ꢀ
Scheme 5. Proposed mechanism for the C H borylation of arenes.
ꢀ
an olefin could coordinate and undergo vinylic C H boryla-
tion or hydroboration. Five-coordinate, phosphinite-based
[(PCP)Ir(H)2] complexes are used as starting materials in
alkane dehydrogenation reactions and are known to release
H2 at elevated temperature,[4] thereby following pathway (a).
In contrast, SiCSi and GeCGe prefer a six-coordinated
environment, and, taking the stronger donor properties into
account, most likely have a higher barrier for the reductive
elimination and release of H2. Therefore, addition of coe as
a H2 acceptor has a beneficial effect on the turnover
frequency and number.
In summary, we synthesized and characterized the first
iridium and rhodium bis(silylene) and bis(germylene) pincer
complexes. A spectroscopic and structural comparison of the
iridium complexes to related PIII compounds revealed the
strikingly stronger s-donating properties of the divalent
silicon and germanium systems, and a significant change in
ꢀ
Figure 2. C H borylation of benzene using HBPin with 5 mol%
precatalyst generated in situ (SiCHSi: black/squares; GeCHGe: gray/
crosses) in the presence (straight lines) and absence (dashed lines) of
additional coe.
product peaks in NMR over time. However, addition of coe to
the same sample led to fast formation of the product in high
yield. Figure 2 shows the reaction profiles using 5 mol% in-
situ generated [(SiCSi)IrHCl(coe)] and [(GeCGe)IrHCl-
(coe)] (from ECE and [{IrCl(coe)2}2]) with and without one
equivalent of added coe. The reaction proceeds significantly
faster with both ligand systems and is higher yielding in the
presence of coe: 90 versus 53%, and 80 versus 46% yield after
24 h for SiCHSi and GeCHGe, respectively. These data
strongly contrast those with the PIII based systems in this
study. The concentration of coe had little effect using tBu-
PCHP as the ligand: 64 versus 60% yield of PhBPin were
determined after 24 h in the presence or absence of coe,
respectively.
ꢀ
reactivity and selectivity in catalytic C H borylation reactions
with arenes. These studies show that N-heterocyclic silylene
and germylene ligands are not just simple substitutes for
isoelectronic PIII compounds, but they are an emerging class
of ligands with unique donor properties.
Received: July 13, 2012
Published online: October 16, 2012
Addition of coe had a dentrimental effect in the case of
iPrN-PCHP. Not only did the yields of PhBPin decrease from
55 to 21% in the presence of additional coe, but the selectivity
changed from C H borylation towards hydroboration of coe
(35%). Toluene as a substrate still showed the formation of
Keywords: germylene · iridium · pincer ligands · rhodium ·
.
silylene
ꢀ
91% (meta/para = 1.6:1) and 39% (meta:para = 1.5:1) yield
ꢀ
of the C H borylation products for SiCHSi and GeCHGe, but
sterically more demanding substrates like ortho- and meta-
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van Koten, Angew. Chem. 2001, 113, 3866; Angew. Chem. Int.
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[4] Review: J. Choi, A. H. R. MacArthur, M. Brookhart, A. S.
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xylene showed only 3–15% of ArBPin, and the main products
came from coe hydroboration and vinylic C H borylation
(see the Supporting Information for further details).
The catalytic performance of the {(ECE)Ir} pincer
systems used in this study is significantly lower compared to
our benchmark systems using bidentate nitrogen ligands,[16]
which is most likely due to the enormous increase of the steric
bulk around the metal center. However, our main goal was to
identify differences and similarities of SiII/GeII to PIII based
systems. Scheme 5 shows a mechanistic hypothesis derived
ꢀ
ꢀ
[5] For reviews on Group 14 metallylenes, see: a) Y. Mizuhata, T.
Sasamori, N. Tokitoh, Chem. Rev. 2009, 109, 3479; b) S. K.
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Chem. 2009, 121, 3216; Angew. Chem. Int. Ed. 2009, 48, 3170;
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from our own results and previous studies in C H borylation
and alkane dehydrogenation.[4,16] The steps involved are
I
ꢀ
oxidative addition of HBPin to Ir , C H activation of the
arene, formation of ArBPin and a {IrIII(H)2} species either via
a oxidative addition/reductive elimination or a s-bond meta-
thesis pathway, and a) direct release of H2, or b) hydro-
genation of coe to close the cycle. In alternative to the arene,
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131, 7232; c) A. Meltzer, S. Inoue, C. Prꢂsang, M. Driess, J. Am.
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[7] W. Wang, S. Inoue, E. Irran, M. Driess, Angew. Chem. 2012, 124,
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[8] CCDC 889254 ([PCP]IrHCl), CCDC 889255 ([SiCSi]IrHCl-
(coe)), CCDC 889256 ([PCP]IrHCl(coe)), 889257 (PCHP), and
CCDC 889258 (GeCHGe) contain 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.
[9] Reviews on TM-silyl compounds: a) J. Y. Corey, J. Braddock-
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[13] For a related phosphine complex, see: S.-Y. Liou, M. Gozin, D.
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ꢀ
=
[10] Ir Si distances in [{PhB(CH2PPh2)3}(H)2Ir SiRR’]: 2.250(3) ꢀ
for R = R’ = mesityl: a) J. C. Peters, J. D. Feldman, T. D. Tilley, J.
Am. Chem. Soc. 1999, 121, 9871; and 2.260(3) ꢀ for R = mesityl,
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[14] R. B. Bedford, M. Betham, J. P. H. Charmant, M. F. Haddow,
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ꢀ
Organometallics 2002, 21, 4065. The Ir Si distance in the five-
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=
[CB11H6Br6] is 2.210(2) ꢀ: c) T. D. Tilley, J. Am. Chem. Soc.
2009, 131, 11161.
[11] Structurally well characterized mono-olefin complexes of IrIII
[16] J. F. Hartwig, Organotransition Metal Chemistry. From Bonding
To Catalysis, University Science Books, Sausalito, 2010, p. 47.
ꢀ
are scarce: The measured C Colefin bond in [(POCOP)Ir(H)-
(C3H6)] [BArF] is 1.141 ꢀ (calculated 1.39 ꢀ): a) M. Findlater,
[17] For reviews on C H borylation, see: a) I. A. I. Mkhalid, J. H.
ꢀ
A. Cartwright-Sykes, P. S. White, C. K. Schauer, M. Brookhart, J.
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