[IrCl{N(CHCHPtBu2)2}]-: A versatile source of the IrI(PNP) pincer platform

Article abstract of DOI:10.1039/c3dt53304b

The iridium(ii) complex [IrCl{N(CHCHPtBu₂)₂}] is reduced by KC₈ to give the anionic iridium(i) pincer complex [IrCl{N(CHCHPtBu₂)₂}]^- which was isolated and fully characterized upon stabilization of the counter cation with crown ether as [K(15-cr-5)₂][IrCl{N(CHCHPtBu₂)₂}]. This unprecedented anionic iridium(i) pincer complex completes the unusual, structurally characterized Ir^I/Ir^II/Ir^III redox series [IrCl{N(CHCHPtBu₂)₂}]^-/0/+, all in a square-planar coordination geometry, emphasizing the versatility of this PNP pincer ligand in stabilizing a broad range of oxidation states. The anionic chloro complex is a versatile source of the Ir(PNP) platform. Its reactivity was examined towards chloride ligand substitution against CO and N₂, and oxidative addition of C-electrophiles, C-H bonds and dioxygen, allowing for the isolation of iridium(i) and iridium(iii) (PNP) carbonyl, hydrocarbyl and peroxo complexes which were spectroscopically and crystallographically characterized.

Full text of DOI:10.1039/c3dt53304b

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
[IrCl{N(CHCHPtBu2)2}]−: a versatile source of the
IrI(PNP) pincer platform†
Cite this: Dalton Trans., 2014, 43,
4506
Markus Kinauer,a Markus G. Scheibel,a Josh Abbenseth,a Frank W. Heinemann,b
Peter Stollberg,a Christian Würtelea and Sven Schneider*a
The iridium(II) complex [IrCl{N(CHCHPtBu2)2}] is reduced by KC8 to give the anionic iridium(I) pincer
complex [IrCl{N(CHCHPtBu2)2}]− which was isolated and fully characterized upon stabilization of the
counter cation with crown ether as [K(15-cr-5)2][IrCl{N(CHCHPtBu2)2}]. This unprecedented anionic
iridium(I) pincer complex completes the unusual, structurally characterized IrI/IrII/IrIII redox series [IrCl-
{N(CHCHPtBu2)2}]−/0/+, all in a square-planar coordination geometry, emphasizing the versatility of this
PNP pincer ligand in stabilizing a broad range of oxidation states. The anionic chloro complex is a versatile
source of the Ir(PNP) platform. Its reactivity was examined towards chloride ligand substitution against CO
and N2, and oxidative addition of C-electrophiles, C–H bonds and dioxygen, allowing for the isolation of
iridium(I) and iridium(III) (PNP) carbonyl, hydrocarbyl and peroxo complexes which were spectroscopically
and crystallographically characterized.
Received 22nd November 2013,
Accepted 18th December 2013
DOI: 10.1039/c3dt53304b
www.rsc.org/dalton
Introduction
Pincer ligands have attracted great interest in recent years as
rigid, meridionally chelating ligand platforms.1,2 In many
cases, pincer ligands provide tailorable steric protection
around the metal, which proved highly useful for the stabili-
zation of reactive metal fragments in challenging small mole-
cule activation reactions and catalytic transformations, such as
alkane dehydrogenation.3 Particularly the ‘archetypical’
anionic PCP ligand {C6H3-2,6-(CH2PtBu2)2}− (L1tBu) was exten-
sively employed. However, besides sterics, the electronic pro-
perties have a profound impact on the reactivity, as for
example demonstrated for the thermodynamics of C–H oxi-
dative addition.4 Particularly, the variation of the central atom
of the anionic pincer framework offers strong variation of the
donor properties. Hence, the introduction of related anionic
pincer ligands with potentially π-donating donors, such as
PNP pincers with diarylamide (Fig. 1, L2), disilylamide (L3),
‘dearomatized’ pyridine based (L4) or dialkylamide (L5) frame-
works, is of particular interest.5–8 As for PCP complexes, C–H
activation has been studied with some of these ligands,
emphasizing the prominent role of transient M(PEP) (E = C, N)
Fig. 1 Monoanionic PNP pincer ligands.
with d8 ions (e.g. M = RhI, IrI) in T-shaped coordination geome-
tries as pivotal species for facile C–H oxidative addition.
Typical pathways for the generation of these reactive inter-
mediates comprise the thermal or photochemical dissociation
of small molecules, such as N2, CO, and olefins,9 or the
reductive elimination of hydrocarbons from MIII hydride
hydrocarbyl intermediates, e.g. intramolecularly within a cyclo-
metalated complex10,11 or by reaction of a dihydrido complex
with an alkene as a H2 acceptor.12,13
Recently, we reported the synthesis of the iridium(II)
complex [IrCl(L6tBu)] (1, L6tBu = N(CHCHPtBu2)2 Scheme 1) by
the oxidation of [IrCl(H)R(HL4tBu)] (R = 1-cyclooctenyl) with
benzoquinone as a hydrogen acceptor.14 Spectroscopic and
computational characterization of 1 indicated the formation of
aGeorg-August-Universität, Institut für Anorganische Chemie, Tammannstr. 4,
37077 Göttingen, Germany. E-mail: sven.schneider@chemie.uni-goettingen.de
bDepartment Chemie und Pharmazie, Friedrich-Alexander-Universität Erlangen-
Nürnberg, Egerlandstraße 1, 91058 Erlangen, Germany
†Electronic supplementary information (ESI) available. CCDC 967913, 967914,
967915 and 973313. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt53304b
4506 | Dalton Trans., 2014, 43, 4506–4513
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Scheme 1 Syntheses of the IrI/IrII/IrIII redox series 1–3.
a metal based radical, hence a rare example of a square-planar
iridium(II) complex. This dehydrogenative template ligand
functionalization provides access to the novel PNP pincer
ligand framework L6, which represents an aliphatic analogue
of L2. Notably, in contrast to L6, extremely bulky diarylamido
PNP ligands, e.g. tBu substituted L2tBu, have not been syntheti-
cally accessible to date. Surprisingly, the characterization of 1
by cyclic voltammetry in CH2Cl2 indicated the reversible oxi-
dation to iridium(III) and chemical oxidation with AgPF6
allowed for the isolation of the first iridium(III) complex with a
square-planar coordination geometry, [IrCl(L6tBu)]PF6 (2). The
isolation of such electronically highly unsaturated complexes 1
and 2 enabled the stabilization of extremely rare iridium
Fig. 3 Molecular structure of 3 in the crystal. Selected bond lengths [Å]
and angles [°]: Ir1–Cl1 2.3995(8), Ir1–N1 2.030(2), Ir1–P1 2.2779(9), Ir1–
P2 2.2760(9), N1–C2 1.370(4), N1–C3 1.369(4), C1–C2 1.349(4), C3–C4
1.350(4); N1–Ir1–Cl1 178.72(7), P1–Ir1–P2 164.00(3).
showed the IrI/IrII redox couple to be reversible even at low
scan rates (10–400 mV s−1
) indicating that irreversible
reduction of 1 in CH2Cl2 might be attributed to reoxidation by
the solvent. Accordingly, chemical reduction of 1 with KC8 in
THF and in the presence of crown ether (15-cr-5) affords the
isolation of the anionic reduction product [K(15-cr-5)2][IrCl-
(L6tBu)] (3) in almost 90% yield as an orange compound
(Scheme 1). The high air sensitivity of 3 is in line with the low
oxidation potential E1/2 = −1.8 V obtained by CV (Fig. 2).
Characterization by NMR at room temperature is in agreement
with C2v symmetry on the NMR timescale. The 31P chemical
shift of 3 exhibits a remarkable downfield shift by Δδ =
+45 ppm as compared with the cationic complex 2 indicating
the profound influence of the 2-electron reduction on the para-
magnetic screening tensor.
3 was characterized by single crystal X-ray diffraction
(Fig. 3) completing a unique, structurally characterized IrI/IrII/
IrIII redox series (1–3) in an identical coordination environ-
ment. The molecular structure of 3 confirms the distorted
square-planar ligand arrangement in the solid state. The dis-
tortion arises from the pincer bite angle (P1–Ir1–P2: 164.00(3)°),
which is the smallest within the series (1: 166.22(2)°; 2:
167.56(2)°). This trend seems to be a consequence of the
strong dependence of the Ir–N bond length on the metal oxi-
dation state which is transferred via the rigid pincer ligand
backbone. In fact, the Ir–N bond becomes considerably shorter
on going from IrI (2.030(2) Å) to IrII (1.985(2) Å) and IrIII
(1.922(2) Å), respectively.14 This observation can be most easily
rationalized with a simple Lewis structure formalism (Fig. 4).
nitrido complexes [Ir(N)(L6tBu)]0/+ 15
However, similarly surpris-
.
ing was the observation that electrochemical characterization
of 1 under the same conditions also indicated irreversibility of
the IrI/IrII reduction wave given that the square-planar coordi-
nation geometry is by far predominant for iridium(I).
In this paper we describe the isolation and structural
characterization of the anionic iridium(I) PNP pincer complex
[K(15-cr-5)2][IrCl(L6tBu)] (3) by stabilization of the counter
cation with crown ether. This result allows for the comparison
of the unusual, isolable IrI/IrII/IrIII redox series [IrCl(L6tBu)]−/0/+
.
Complex 3 is a versatile source of the Ir(PNP) platform as
demonstrated by ligand substitution, oxidative addition reac-
tions and C–H and O2 activation.
Results and discussion
The anionic iridium(I) pincer complex [IrCl(L6tBu)]−
The electrochemistry of 1 was reevaluated by cyclic voltamme-
try (CV) in tetrahydrofuran (Fig. 2). The change of solvent
Fig. 2 Cyclic voltammogram of 1 in THF (0.1 M [NnBu4][PF6], room
temperature, Pt working electrode) at different scan rates.
Fig. 4 Selected resonance structures and bond lengths for the redox
series 3 (IrI, blue), 1 (IrII, red), and 2 (IrIII, black).
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 4506–4513 | 4507

View Article Online
Paper
Dalton Transactions
As N → Ir π-donation increases with rising metal oxidation
state, the Lewis structure A (Fig. 4) is weighed stronger result-
ing in a shorter Ir–N distance. Note that the LUMO of 2 has a
predominantly Ir–N π*-antibonding character, therefore exhi-
biting considerable Ir–N double bond character.14 Upon suc-
cessive reduction to 1 and 3 this orbital will be filled with two
electrons resulting in no net Ir–N π-bond in 3, which provides
an MO basis for the observed trend in bond lengths. In turn,
the mesomeric Lewis structure B exhibits a higher weight
upon reduction as reflected in the average pincer backbone N–
C (IrI: 1.37 Å, IrII: 1.39 Å, IrIII: 1.41 Å) and C–C (IrI: 1.35 Å, IrII:
1.34 Å, IrIII: 1.33 Å) bond lengths. However, the effect is much
smaller as compared with the Ir–N bond length. Interestingly,
the Ir–P bond lengths (2: 2.34 Å; 1: 2.32 Å; 3: 2.28 Å) exhibit
the opposite trend as expected on a simple consideration of
ionic radii, therefore pointing to increased Ir → P back
donation upon reduction.
Fig. 5 Molecular structure of 4 in the crystal. Selected bond lengths [Å]
and angles [°]: Ir1–C21 1.829(2), Ir1–N1 2.061(2), Ir1–P1 2.3137(6), Ir1–P2
2.3080(6), N1–C1 1.367(3), N1–C3 1.371(3), C1–C2 1.347(3), C3–C4
1.351(3), O1–C21 1.153(3); N1–Ir1–C21 178.20(9), P1–Ir1–P2 163.37(2).
observation supports an associative Cl vs. CO substitution
mechanism after reduction of 1.
The solution NMR data of 4 are in agreement with C2v sym-
metry. The molecular structure in the solid state was also
derived by single-crystal X-ray diffraction (Fig. 5) confirming
the distorted square-planar coordination geometry around the
metal centre with a P1–Ir1–P2 bite angle (163.37(2)°) similar to
that in 3. Accordingly, the Ir–N distance (2.061(2) Å) is slightly
longer as in the case of parent 3 (2.030(2) Å) as a consequence
of the higher CO vs. Cl trans-influence, again emphasizing the
relationship of the pincer bite angle and the Ir–N distance
within this rigid framework. The CO stretching vibration can
be assigned to a signal at 1937 cm−1 in the IR spectrum. This
value compares well with the CO band of [Ir(CO)Cl(PiPr3)2]
Hence, the structural parameters within the IrI/IrII/IrIII
redox series are in agreement with competitive π-acceptance of
the N-lone-pair by the metal centre and the vinyl substituents,
respectively, subject to the availability of a vacant (2) or half
filled (1) metal d-orbital with a suitable symmetry. This simple
interpretation of the electronic structure explains the stabili-
zation of this unusual redox series by the divinylamido PNP
pincer ligand. To the best of our knowledge, isolation of a
[MCl(PEP)]− anion (M = Rh, Ir; PEP = anionic pincer ligand) is
unprecedented in pincer chemistry. One related compound,
[IrI(dippe)2]+ [IrICl2(dippe)]− (dippe = iPr2P(CH2)2PiPr2), was
previously reported as a coordinatively relatively labile inter-
mediate leading to [Ir(μ-Cl)(dippe)]2 and [Ir(dippe)2]Cl.16 [Ir-
(OTf)2(dfepe)]− (dfepe = (C2F5)2P(CH2)2P(C2F5)2) with a highly
fluorinated phosphine ligand was also reported, pointing to
stabilization by π-acidic ligands.17 Accordingly, the dihalo-
dicarbonyl anions of iridium are known for a long time and
play an important role in catalytic carbonylation reactions.18 In
our case, the combination of steric protection and electronic
flexibility provided by the pincer ligand effectively stabilizes
the unusually electron rich anionic complex and the complete
IrI/IrII/IrIII redox series.
(1939 cm−1 19
(CO)(L3tBu)]: 1930 cm−1 20
1930 cm−1 21
PNP pincer complexes. In contrast, the dialkyl-
)
or with some corresponding disilylamido ([Ir-
and diarylamido ([Ir(CO)(L2iPr)]:
)
)
amido PNP complex [Ir(CO)(L5iPr)] exhibits a CO band at con-
siderably lower wavenumber (1908 cm−1),22 emphasizing
reduced L → M electron donation by the divinylamido ligand.
σ- and π-bonding effects cannot be separated merely from the
CO stretching vibration. However, the conjugation of the
azallylic CvC double bonds with the N-lone pair, which is
indicated by the trends within the pincer ligand backbone
bond lengths of the IrI/IrII/IrIII redox series (see above), indi-
cates that reduced L → M π-donation of the divinyl- vs.
the dialkylamido ligand probably constitutes a significant
contribution to the higher CO stretching vibration in 4.
Carbonyl complex [Ir(CO)(L6tBu)]
The electronic properties of the Ir(PNP) pincer platform were
further probed by the preparation of the iridium(I) carbonyl
complex [Ir(CO)(L6tBu)] (4, Scheme 2). 4 can be isolated from
direct reduction of 1 with KC8 under an atmosphere of CO,
demonstrating the suitability of the chloride as a leaving
group. Interestingly, 3 is stable in solution in the absence of
CO, yet readily reacts with CO as well, to give 4. Hence, this
ð1Þ
Clean reduction of 1 is also accomplished by reaction with
nBuLi. Monitoring this reaction by 31P NMR reveals the quanti-
tative formation of a diamagnetic compound at 56.5 ppm
(THF), which was assigned to Li[IrCl(L6tBu)] by comparison
with 3 (55.5 ppm). These solutions were only stable below
−20 °C, probably owing to LiCl elimination. However, the
Ir(L6tBu) fragment could be trapped by thawing a frozen solution
under a N2 atmosphere (1 bar) giving [Ir(N2)(L6tBu)] (5)15 in
around 60% spectroscopic yield by 31P NMR (eqn (1)). Hence,
Scheme 2 Syntheses of carbonyl complex 4.
4508 | Dalton Trans., 2014, 43, 4506–4513
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
the observation of dinitrogen complex 5 suggests that C–H acti-
vation reactions should be carried out under argon to avoid
inhibition by N2 binding to the Ir(L6tBu) fragment. This obser-
vation was also reported for the Ir(L1) platform.23,24
ð3Þ
Besides nucleophilic C–H activation, attack of C-electro-
philes also provides access to iridium(III) hydrocarbyl com-
plexes. The reaction of 3 with MeOTf yields the iridium(III)
methyl complex [IrCl(CH3)(L6tBu)] (7) in around 90% yield
(eqn (3)). Spectroscopic characterization of 7 is in agreement
with Cs symmetry on the NMR timescale. The methyl ligand
was assigned to signals at 2.08 (1H NMR) and −27.1 ppm (13C
NMR), respectively, both exhibiting triplet multiplicity due to
coupling with the 31P nuclei, unequivocally indicating for-
mation of an iridium(III) methyl complex. In comparison, the
dialkyamido complex [Ir(PMe3)(L5iPr)] is selectively methylated
with MeOTf at the ligand nitrogen atom to form the iridium(I)
complex [Ir(PMe3){MeN(CH2CH2PiPr2)2}]OTf.26 Such a ligand
centered nucleophilic reactivity was similarly observed for
palladium(II) dialkyamido complexes with MeOTf.27,28 Also,
the five-coordinate alkylvinylamido ruthenium(II) complex
[RuH(PMe3){N(CHCHiPr2)(CH2CH2iPr2)}] exhibits exclusive
ligand methylation upon reaction with MeOTf (eqn (4)).29 In
this context, the high selectivity of metal alkylation in the case
of 3 is remarkable and emphasizes the rigid, pincer-type
behaviour of ligand L6.
Oxidative addition to [IrCl(L6tBu)]−
Besides substitution for CO, chloride dissociation can also be
utilized as the source of Ir(L6tBu) which undergoes C–H oxi-
dative addition. Reduction of 1 with Na/Hg in benzene results
in facile formation of the red iridium(III) hydrido phenyl
complex 6 in almost quantitative spectroscopic (31P NMR)
yield (eqn (2)). Isolation of the highly lipophilic compound by
crystallization from pentane resulted in isolated yields just
below 50%. The addition of benzene to Ir(L6tBu) was confirmed
by NMR spectroscopy and elemental analysis. The chemical
shift of the hydride ligand (δ = −46.5 ppm) suggests a vacant
coordination site in trans-position, and hence the occupation
of the apical position within a square-pyramidal coordination
geometry around the metal. The three phenyl 1H and 13C
signals at room temperature, respectively, indicate rapid
rotation around the Ir–C bond on the NMR timescale.
ð2Þ
The structural assignments from solution NMR spec-
troscopy were confirmed by single crystal X-ray diffraction
(Fig. 6). In the solid state, the five-coordinate metal atom exhi-
bits a square pyramidal coordination geometry with the
hydride ligand in apical position and an almost linear N–Ir–
phenyl bond angle (179.8(3)°). The Ir1–N1 bond (2.109(6) Å) is
particularly long due to the strong trans-ligand C6H5 also
resulting in a smaller P1–Ir1–P2 pincer bite angle (162.58(7)°).
The C–H oxidative addition most likely proceeds via three-
coordinate [Ir(L6tBu)] after reduction of 1 and NaCl elimination
in the absence of stabilizing crown ether. Such three-coordi-
nate d8 intermediates are prone to C–H oxidative addition via
an intermediate C–H σ-complex.25 However, to the best of our
knowledge, halides as leaving groups were not previously
reported for their formation.
ð4Þ
O2 activation
The activation of dioxygen with iridium complexes has been
the subject of several studies, e.g. in the context of alkene
oxygenation.30–32 With phosphine pincer ligands, the for-
mation of iridium(III) peroxo complexes, such as [Ir(O2)(L3tBu)]
or [Ir(O2)(L1tBu)], which is in equilibrium with the bisdioxygen
complex [Ir(O2)2(L1tBu)], was reported upon hydrocarbon reduc-
tive elimination from iridium(III) under O2.11,33 However, to
the best of our knowledge, the only crystallographically charac-
terized iridium pincer mono-O2 adduct is the ‘POCOP’-
complex [Ir(O2){C6H3-2,6-(OPRF ) }] (9, RF = C6H2-2,4,6-(CF3)3)
2 2
which was synthesized in the solid state and was in turn not
spectroscopically examined.34 Hence, a full set of spectroscopic
and structural data for this class of compounds is surprisingly
not available.
Tejel, De Bruin and co-workers observed the backbone oxy-
genation of a vinylenediamido ligand instead of cyclooctadiene
upon reaction of the corresponding anionic iridium(I) complex
with dioxygen (eqn (5)).35 Importantly, this reactivity was
attributed to the redox non-innocent behaviour of the NNN
ligand. Hence, the relationship of this iridate(I) with 3 sparked
our interest in examining the reactivity with molecular oxygen.
Fig. 6 Molecular structure of 6 in the crystal. Selected bond lengths [Å]
and angles [°]: Ir1–C21 2.080(7), Ir1–N1 2.109(6), Ir1–P1 2.337(2), Ir1–P2
2.3276(19), N1–C1 1.348(10), N1–C11 1.357(10), C1–C2 1.362(12), C11–
C12 1.352(10); N1–Ir1–C21 179.8(3), P1–Ir1–P2 162.58(7).
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 4506–4513 | 4509

View Article Online
Paper
Dalton Transactions
Ir(η2-O2) complexes (1.43–1.53 Å).38 Furthermore, this bond is
slightly longer than the ones reported for 9 (1.372(15) Å) and
related rhodium dioxygen pincer complexes [Rh(O2)(L3tBu)]
ð5Þ
tBu
(1.363(10) Å) and [Rh(O2)(L1atBu)] (1.365(18) Å; L1a
=
Me2HC6(CH2PtBu2)2).34,39,40 Hence, the structural features are
in agreement with assignment to an iridium(III) peroxo
complex, as also indicated by IR spectroscopy. However, as
Caulton and co-workers carefully stated about the O–O bond
lengths in such η2-O2 complexes, perhaps this parameter is
not truly reliable for establishing the charge state, but rather
only the degree of back bonding.39
A solution of 3 in THF immediately turns red upon stirring
under dioxygen. From this solution, the oxygen adduct [Ir(O2)-
(L6tBu)] (8) was isolated as a stable compound in around 80%
yield (eqn (6)). The NMR spectroscopic characterization reveals
the formation of a diamagnetic compound with C2v symmetry
on the NMR timescale at room temperature. In the IR spec-
trum a strong band at 910 cm−1 was assigned to the O–O
stretching vibration by comparison with parent 3. This value is
at the upper end of the range reported for peroxo ligands and
Experimental section
General considerations
close to the one reported for [Ir(O2)(L1tBu)] (895 cm−1
)
suggesting a formal iridium(III) oxidation state for 8.36 The
apparently slightly weaker reducing activation of the dioxygen
ligand by the Ir(L6tBu) compared with the Ir(L1tBu) fragment is
in line with the CO stretching vibrations of 4 (1937 cm−1) vs.
[Ir(CO)(L1tBu)] (1913 cm−1).37
All experiments were carried out using Schlenk and glove-box
(argon atmosphere) techniques. All solvents were dried by
passing through columns packed with activated alumina.
Deuterated solvents were obtained from Euriso-Top GmbH,
dried over Na/K (C6D6 and d8-THF), distilled by trap-to-trap
transfer in vacuo, and degassed by three freeze–pump–thaw
cycles, respectively. C10H10O5 (Acros), CH3SO3CF3 (Sigma-
Aldrich), O2, H2 and CO (Linde gas) were used as purchased.
Complex 2, Na/Hg and KC8 were prepared according to pub-
lished procedures.14,41,42 Elemental analyses were obtained
from the Analytical Laboratories at Friedrich-Alexander-Univer-
sity and Georg-August-University, respectively. IR spectra were
recorded as nujol mulls between KBr disks using a JASCO FT/
IR 4100 Spectrometer. NMR spectra were recorded on a Bruker
Avance III 300 and a Bruker Avance DRX 500 spectrometer and
were calibrated to the residual proton resonance of the solvent
(C6D6: δH = 7.16 ppm; δC = 128.39; d8-THF: δH = 3.58 ppm,
1.72 ppm; δC = 67.2 ppm, 25.3 ppm). 31P NMR chemical shifts
are reported relative to external phosphoric acid (δ = 0.0 ppm).
Signal multiplicities are abbreviated as: s (singlet), d (doublet),
t (triplet), q (quartet), m (multiplet), br (broad). Cyclic voltam-
mograms were measured on an Autolab PGSTAT101 from
Metrohm in a 0.1 M [nBu4N][PF6]-solution referenced against
the [(C5Me5)2Fe]0/+ couple, which was referenced to a potential
ð6Þ
The spectroscopic interpretations are corroborated by the
molecular structure of 8 (Fig. 7). The crystal exhibited disorder
with two superimposed positions of the molecule (see ESI†).
While for this reason the experimental bond lengths and
angles should be interpreted with care, some important con-
clusions regarding O2 bonding from structural parameters can
be drawn. Complex 8 exhibits a distorted square-planar geome-
try and the side-on η2-O2 ligand occupies the trans-position to
nitrogen with perpendicular orientation to this plane. The
weak trans-influence of the dioxygen ligand, similar to chloride
in 3, is indicated by the Ir–N distance (2.015(5) Å) and the
resulting P–Ir–P bite angle (162.79(4)°). The O–O distance
(1.415(7) Å) suggests considerable activation of the dioxygen
ligand (dOO = 1.21 Å) and is close to the typical range found for
of −0.440 versus [(C5H5)2Fe]0/+ 43
.
Syntheses
[K(C10H10O5)2][Ir(Cl){N(CHCHPtBu2)2}] (3). A mixture of 1
(200.0 mg; 342.4 µmol; 1 eq.) and KC8 (55.5 mg; 410.8 µmol;
1.2 eq.) is dissolved in a solution of C10H10O5 (164.0 µL; 821.7
µmol; 2.4 eq.) in THF (15 mL) at −50 °C. After 5 minutes stir-
ring, all volatiles are removed i. vac. and the residue is washed
with benzene (3 × 5 mL). The crude product is extracted with
THF (3 × 7 mL), layered with pentane and crystallized at
−32 °C overnight. The crystals are collected by filtration,
washed with pentane (2 × 5 mL) and dried in vacuo. 3 is
obtained as an orange microcrystalline compound (yield:
164.0 mg; 154.2 µmol; 45%). Anal. calc. for C40H80ClIrKNO10P2
(1063.69): C, 45.16; H, 7.58; N, 1.32. Found: C, 44.97; H, 7.60;
Fig. 7 Molecular structure of 8 in the crystal (one of two orientations
within the disordered crystal). Selected bond lengths [Å] and angles [°]:
Ir1–O1 1.945(3), Ir1–N1 2.015(5), Ir1–P1 2.3461(13), O1–O1’ 1.415(7), N1–
C1 1.407(4), C1–C2 1.352(5); N1–Ir1–O1 158.66(11), O1–Ir1–O1’ 42.7(2),
P1–Ir1–P1’ 162.79(4).
1
N, 1.29. NMR (d8-THF, [ppm]): H NMR (300 MHz, 20 °C): δ =
4510 | Dalton Trans., 2014, 43, 4506–4513
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
4
3
3
6.93 (ABXX′B′A′, N = |3JHP + JHP′| = 17.2 Hz, JHH = 5.2 Hz, 2H, 5.6 Hz, 2H, NCHCHP), 7.20 (m, 2H, meta-C6H5), 6.97 (t, JHH
=
NCHCHP) 4.04 (ABXX′B′A′, N = |2JHP
+
4JHP′| = 4.3 Hz, JHH
=
7.2 Hz, 1H, para-C6H5), 4.27 (ABXX′B′A′, N = |2JHP + JHP′| = 3.7
3
4
5.1 Hz, 2H, NCHCHP), 3.64 (s, 40H, (CH2CH2O)5) 1.35 (A18XX′- Hz, JHH = 5.5 Hz, 2H, NCHCHP), 1.14 (A9XX′A′9, N = |3JAX
+
+
3
A′18, N = |3JHP
+
5JHP′| = 5.9 Hz, 36H, P(C(CH3)3)2). 13C NMR 5JAX′| = 7.0 Hz, 18H, P(C(CH3)3)2), 1.12 (A9XX′A′9, N = |3JAX
(75 MHz, 20 °C): δ = 122.4 (AXX′A′, N = |2JCP + JCP′| = 5.8 Hz, 5JAX′| = 7.0 Hz, 18H, P(C(CH3)3)2), −46.52 (t, 2JHP = 12.6 Hz, 1H,
3
NCHCHP), 95.0 (AXX′A′, N = |1JCP + 3JCP′| = 22.5 Hz, NCHCHP), IrH). 13C NMR (75.5 MHz, 21 °C): δ = 163.6 (AXX′A′, N = |2JAX
+
68.6 (s, 20C, (CH2CH2O)5), 38.8 (A2XX′A′2, N = |1JCP
11.2 Hz, 4C, PC(CH3)3), 29.9 (A6XX′A′6, N = |2JCP
+
+
3JCP′| = 3JAX′| = 7.3 Hz, NCHCHP), 144.6 (t, JCP = 6.1 Hz, ortho-C6H5),
3
4JCP′| = 128.9 (t, JCP = 0.1 Hz, ipso-C6H5), 127.1–126.6 (m, meta-C6H5),
2
1.7 Hz, PC(CH3)3). 31P NMR (121 MHz, 20 °C): δ = 55.5 (s).
120.5 (s, para-C6H5), 85.8 (AXX′A′, N = |1JAX + JAX′| = 22.6 Hz,
3
[Ir(CO){N(CHCHPtBu2)2}] (4). A mixture of 1 (60.0 mg; 102.7 NCHCHP), 39.7 (AXX′A′,
N
=
|1JAX
+
3JAX′
|
=
12.1 Hz,
µmol; 1 eq.) and KC8 (13.9 mg; 102.7 µmol; 1 eq.) is dissolved P(C(CH3)3)2), 35.3 (AXX′A′, N = |1JAX
+
3JAX′| = 13.4 Hz,
at −50 °C in a THF solution (15 mL) saturated with CO and PC(CH3)3), 29.5 (A3XX′A′3, N = |2JAX + 4JAX′| = 3.0 Hz, PC(CH3)3),
stirred for 15 min. After removal of all volatiles in vacuo, the 29.3 (A3XX′A′3, N = |2JAX + JAX′| = 2.7 Hz, PC(CH3)3). 31P NMR
4
residue is extracted with pentane (3 × 4 mL) and after evapor- (121.5 MHz, 21 °C): δ = 61.3 (s).
ation to dryness, the crude product is purified by column
[Ir(Cl)(CH3){N(CHCHPtBu2)2}] (7). Methyl triflate (3.7 µL,
chromatography (silanized silica gel, 0.3 × 6 mL, pentane). 33.7 µmol) is added to a solution of 3 (35.4 mg, 33.3 µmol) in
After removing the solvent in vacuo, the product is lyophilized toluene (2 mL) at −20 °C and stirred for 5 minutes. After the
out of benzene (5 mL) and 4 is obtained as bright yellow color of the solution changes from orange to violet, the solu-
powder (30.4 mg; 58.9 µmol; 51%). Anal. calcd for tion is filtered off and all volatiles are removed i. vac. The
C21H40IrNOP2 (576.71): C, 43.73; H, 6.99; N, 2.43. Found: C, residue is dissolved in benzene (10 mL), filtered off and lyophi-
43.82; H, 7.01; N, 2.16. NMR(C6D6, [ppm]): 1H NMR (400 MHz, lized i. vac. overnight. 7 is obtained as analytically pure violet
4
3
21 °C): δ = 7.02 (ABXX′B′A′, N = |3JHP + JHP′| = 18.9 Hz, JHH
=
powder (yield: 17.7 mg; 29.5 µmol; 89%). Anal. calc. for
5.8 Hz, 2H, NCHCHP), 4.31 (ABXX′B′A′, N = |2JHP + JHP′| = 3.4 C21H43ClIrNP2 (599.19): C, 42.09; H, 7.23; N, 2.34. Found: C,
4
Hz, JHH = 5.5 Hz, 2H, NCHCHP), 1.36 (A18XX′A′18, N = |3JHP
+
42.43; H, 7.06; N, 1.92. NMR (C6D6, [ppm]): 1H NMR
3
5JHP′| = 7.0 Hz, P(C(CH3)3). 13C NMR (100 MHz, 21 °C): δ = (300 MHz, 20 °C): δ = 6.82 (ABXX′B′A′, N = |3JAX + JAX′| = 17.8
4
190.6 (t, JCP = 7.6, Ir–CO), 163.7 (AXX′A′, N = |2JCP
+ +
3JCP′| = Hz, JAB = 5.8 Hz, 2H, NCHCHP), 4.06 (ABXX′B′A′, N = |2JBX
2
3
9.6 Hz, NCHCHP), 85.8 (AXX′A′, N = |1JCP
+
3JCP′| = 21.5 Hz, 3JBX′| = 3.4 Hz, JBA = 6.2 Hz, 2H, NCHCHP), 2.08 (t, JHP = 5.3
3JCP′| = 13.5 Hz, Hz, 3H, Ir–CH3), 1.32 (A9XX′A′9, N = |3JAX + 5JAX′| = 6.6 Hz, 18H,
4JCP′| = 3.0 Hz, P(C(CH3)3)2), 1.24 (A9XX′A′9, N = |3JAX 5JAX′| = 6.6 Hz, 18H,
3
3
NCHCHP), 36.7 (A2XX′A′2, N = |1JCP
P(C(CH3)3), 29.8 (A6XX′A′6, N = |2JCP
+
+
+
P(C(CH3)3). 31P NMR (162 MHz, 21 °C): δ = 82.0 (s). IR (cm−1): P(C(CH3)3)2). 13C NMR (75 MHz, 20 °C): δ = 163.3 (AXX′A′, N =
1937 vs. (νCO).
|2JAX + 3JAX′| = 7.2 Hz, NCHCHP), 85.3 (AXX′A′, N = |1JAX + 3JAX′| =
Reduction of 1 under a N2 atmosphere. 1 (4.5 mg, 7.7 µmol) 20.4 Hz, NCHCHP), 40.0 (AXX′A′, N = |1JAX + JAX′| = 12.1 Hz,
3
is dissolved in pentane (0.5 mL), cooled to −20 °C and n-butyl- P(C(CH3)3)2), 36.2 (AXX′A′, N = |1JAX
lithium (4.8 µL, 1.6 M in n-hexane, 7.7 µmol) is added. The P(C(CH3)3)2), 30.6 (A3XX′A′3, N = |2JAX
+
3JAX′| = 12.1 Hz,
3JAX′| = 2.2 Hz,
3JAX′| = 2.2 Hz,
+
+
solution immediately turns yellow and is cooled to −80 °C. All P(C(CH3)3)2), 30.0 (A3XX′A′3, N = |2JAX
volatiles are removed in vacuo. The residue is dissolved in P(C(CH3)3)2), −27.1 (t, JCP = 4.0 Hz, Ir–CH3). 31P NMR
3
pentane, and the solution is degassed by three freeze–pump– (121 MHz, 20 °C): δ = 41.9 (s, P(C(CH3)3)2).
thaw cycles. After backfilling the vessel with N2 the mixture is
[Ir(O2){N(CHCHPtBu2)2}] (8). A solution of 3 (60.0 mg, 56.4
warmed to room temperature and examined by NMR spec- µmol) in 20 mL THF is degassed by one pump–freeze–thaw
troscopy. [Ir(N2){N(CHCHPtBu2)2}] (5, δ(31P) = 70.3 ppm) is cycle and the reaction vessel is backfilled with oxygen (1 bar)
identified as the main product (61%).15
and stirred at −60 °C for 1 h. All volatiles are removed i. vac.
[Ir(H)(C6H5){N(CHCHPtBu2)2}] (6). A solution of 1 (30.0 mg; and the residue is washed with pentane (2 × 5 mL) and
51.4 µmol; 1 eq.) in benzene (5 mL) is added to Na/Hg (1 mol extracted with THF (3 × 5 mL) and filtered. The THF solution
L−1; 833.4 mg; 61.6 µmol; 1.2 eq.) and stirred at room tempera- is layered with pentane (45 mL) and the product is crystallized
ture for 16 h. The solution is decanted off, the Hg slurry is at −32 °C. Red crystals of 8 are collected by filtration, washed
extracted with benzene (2 × 5 mL) and the combined organic with pentane and dried in vacuo (26.3 mg, 45.2 µmol, 80%).
fractions are filtered over a pad of celite. After the removal of Anal. calcd for C20H40IrNO2P2 (580.71): C, 41.37; H, 6.94; N,
all volatiles in vacuo, the crude product is extracted with 2.41. Found: C, 41.06; H, 6.67; N, 2.23. NMR (d8-THF, [ppm]):
pentane (2 × 5 mL), concentrated and crystallized at −82 °C. 1H NMR (300 MHz, 20 °C): δ = 6.77 (ABXX′B′A′, N = |3JHP
+
3
The crystals are collected by filtration, washed with cold 4JHP| = 17.9 Hz, JHH = 6.0 Hz, 2H, NCHCHP), 4.76 (ABXX′B′A′,
4
3
pentane (3 mL), dissolved in benzene (7 mL) and lyophilized N = |2JHP + JHP| = 4.4 Hz, JHH = 6.0 Hz, 2H, NCHCHP), 1.39
overnight. 6 is obtained as red powder (13.0 mg; 21.2 µmol; (A18XX′A′18, N = |3JHP + JHP| = 6.2 Hz, 36H, P(C(CH3)3)2). 13C
5
41%). Anal. calcd for C26H46IrNP2 (626.83): C, 49.82; H, 7.40; NMR (75.5 MHz, 20 °C): δ = 166.9 (AXX′A′, N = |2JCP + JCP| =
3
N, 2.23. Found: C, 49.42; H, 7.25; N, 2.15. NMR(C6D6, [ppm]): 7.3 Hz, NCHCHP), 98.7 (AXX′A′, N = |1JCP
+
3JCP| = 18.7 Hz,
1H NMR (300 MHz, 21 °C): δ = 7.73 (d, JHH = 7.5 Hz, 2H, NCHCHP), 36.2 (A2XX′A′2, N = |1JCP
+
3JCP| = 11.5 Hz,
3
ortho-C6H5), 7.37 (ABXX′B′A′, N = |3JHP + 4JHP′| = 16.3 Hz, 3JHH
=
P(C(CH3)3)2), 29.9 (A6XX′A′6, N = |2JAX 4JAX′| = 3.0 Hz,
+
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 4506–4513 | 4511

View Article Online
Paper
Dalton Transactions
P(C(CH3)3)2). 31P NMR (121 MHz, 20 °C): δ = 43.8 (s, substituents, such as PtBu2, which are on the other hand not
P(C(CH3)3)2). IR (cm−1): 910 s (νOO).
available for L2.
The unprecedented anionic iridium(I) chloro pincer
complex 3 is an excellent starting material for the synthesis of
several iridium(I) and iridium(III) complexes by ligand substi-
tution and oxidative addition. In contrast to [Ir(PMe3)(L5iPr)],
exclusively metal-directed oxidative addition of MeOTf is
observed, demonstrating the rigid and chemically inert charac-
ter of the L6 pincer platform. The use of chloride as a leaving
group represents a new route to generate the transient M(PEP)
(M = d8 ion) intermediates, which readily oxidatively add
hydrocarbons, like benzene. In reverse, the stabilization of the
M(PEP) platform by chloride emphasizes that excess halide
salt formation in catalytic transformations could have a detri-
mental effect on C–H activation with such pincer species, as
was observed for N2. Finally, we presented the first fully spec-
troscopically and structurally characterized iridium pincer η2-
O2 complex indicating the formation of an iridium(III) peroxo
compound.
Crystallographic characterization
Suitable crystals for single crystal X-ray diffraction analysis
were selected from the mother liquor under an inert gas
atmosphere and transferred into protective perfluoro polyether
oil on a microscope slide. Crystals of compound 3 were
selected on a microscope slide cooled by a nitrogen gas flow
from an X-Temp2 device.44,45 The selected and mounted crys-
tals were transferred to the cold gas stream of the
diffractometer.
Diffraction data for 3, 6 and 8 were collected at 100 K using
a Bruker D8 three-circle diffractometer equipped with a
SMART APEX II CCD detector and an INCOATEC microfocus
source46 with Quazar mirror optics (λ = 0.56086 Å). The data
were integrated with SAINT47 and a multi-scan absorption cor-
rection with SADABS48 was applied. The structures were solved
by direct methods (SHELXS-2013) and refined against all data
by full-matrix least-squares methods on F2 (SHELXL-2013)49,50
within the SHELXLE GUI.51 The diffraction data for 4 was col-
lected at 150 K using a Bruker-Nonius KappaCCD diffracto-
meter using MoKα radiation (λ = 0.71073 Å) and a graphite
monochromator. The data were integrated with EvalCCD52 and
a semi-empirical absorption correction based on multiple
scans with SADABS53 was performed. The structure was solved
by direct methods and the full-matrix least-squares refinement
was carried out on F2 using SHELXTL NT 6.12.54
Acknowledgements
S.S. is grateful to the Deutsche Forschungsgemeinschaft for
funding (Emmy Noether Program SCHN950/2). The authors
thank Dr Regine Herbst-Irmer for help in solving the mole-
cular structure of complex 8.
All non-hydrogen atoms were refined with anisotropic dis-
placement parameters. The C–H hydrogen atoms were refined
isotropically on calculated positions by using a riding model
with their Uiso values constrained to 1.5 Ueq. of their pivot
atoms for terminal sp3 carbon atoms and 1.2 times for all
other carbon atoms. The Ir–H hydrogen atom of 6 was located
on the electron density map and isotropically refined. The
structure of 3 exhibits disorder of one of the CH3 groups. Two
alternative sites were refined with site occupation factors of
0.75 and 0.25, respectively. The disorder of 8 was refined using
the PART −1 command and some restraints on the distances
and anisotropic displacement parameters (SAME, RIGU).
Notes and references
1 The Chemistry of Pincer Compounds, ed. D. Morales-Morales
and C. Jensen, Elsevier, New York, 2007.
2 G. van Koten, D. Milstein (Eds.), Top. Organomet. Chem.,
2013, 40.
3 J. Choi, A. H. R. MacArthur, M. Brookhart and
A. S. Goldman, Chem. Rev., 2011, 111, 1761.
4 K. Krogh-Jespersen, M. Czerw, K. Zhu, B. Singh,
M. Kanzelberger, N. Darji, P. D. Achord, K. B. Renkema and
A. S. Goldman, J. Am. Chem. Soc., 2002, 124, 10797.
5 L.-C. Liang, Coord. Chem. Rev., 2006, 250, 1152.
6 C. Gunanathan and D. Milstein, Acc. Chem. Res., 2011, 44,
588.
Conclusions
This study emphasizes the versatility of divinylamido pincer
ligand L6 to stabilize a wide range of oxidation states in identi-
cal coordination environments and 1-electron redox-processes
within the platinum metals, as demonstrated by an unusual
IrI/IrII/IrIII redox series. The bond parameters indicate that the
extended π-system within the Ir–N(–CvC)2 moiety is a decisive
7 J. I. Van der Vlugt and J. N. H. Reek, Angew. Chem., Int. Ed.,
2009, 48, 8832.
8 S. Schneider, J. Meiners and B. Askevold, Eur. J. Inorg.
Chem., 2012, 412.
9 J. Meiners, A. Friedrich, E. Herdtweck and S. Schneider,
Organometallics, 2009, 28, 6331.
electronic feature for the stability of 1–3. While this ligand 10 A. Y. Verat, M. Pink, H. Fan, J. Tomaszewski and
platform has similar electronic properties as the popular PNP K. G. Caulton, Organometallics, 2008, 27, 166.
ligands L2 and L3, e.g. expressed in carbonyl stretching 11 N. P. Tvetkov, M. F. Laird, H. Fan, M. Pink and
vibrations of the corresponding [Ir(CO)(PNP)] complexes, it K. G. Caulton, Chem. Commun., 2009, 4578.
combines the hydrolytic stability of L2 (unlike L3) with accessi- 12 X. Zhang, M. Kanzelberger, T. J. Emge and A. S. Goldman,
bility of the sterically highly protecting phosphine
J. Am. Chem. Soc., 2004, 126, 13192.
4512 | Dalton Trans., 2014, 43, 4506–4513
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
13 L. Fan, S. Parkin and O. V. Ozerov, J. Am. Chem. Soc., 2005, 33 D. B. Williams, W. Kaminsky, J. M. Mayer and
127, 16772. K. I. Goldberg, Chem. Commun., 2008, 4195.
14 J. Meiners, M. G. Scheibel, M.-H. Lemée-Cailleau, 34 Z. Huang, P. S. White and M. Brookhart, Nature, 2010, 465,
S. A. Mason, M. B. Boeddinghaus, T. F. Fässler, 598.
E. Herdtweck, M. M. Khusniyarov and S. Schneider, Angew. 35 C. Tejel, M. P. del Río, M. A. Ciriano, E. J. Reijerse, F. Hartl,
Chem., Int. Ed., 2011, 50, 8184.
15 M. G. Scheibel, B. Askevold, F. W. Heinemann,
S. Záliš, D. G. H. Hetterscheid, N. Tsichlis i Spithas and
B. de Bruin, Chem.–Eur. J., 2009, 15, 11878.
E. J. Reijerse, B. de Bruin and S. Schneider, Nat. Chem., 36 J. M. Praetorius, D. P. Allen, R. Wang, J. D. Webb, F. Grein,
2012, 4, 552.
P. Kennepohl and C. M. Crudden, J. Am. Chem. Soc., 2008,
130, 3724.
16 S. S. Oster and W. D. Jones, Polyhedron, 2004, 23, 2959.
17 R. C. Schnabel and D. M. Roddick, Organometallics, 1996, 37 D. W. Lee, C. M. Jensen and D. Morales-Morales, Organo-
15, 3550. metallics, 2003, 22, 4744.
18 M. A. F. Hernandez-Gruel, J. S. J. Pérez-Torrente, 38 H. Lebel, C. Ladjel, F. Bélanger-Gariépy and F. Schaper,
M. A. Ciriano and L. A. Oro, Inorg. Synth., 2004, 34, 127. J. Organomet. Chem., 2008, 693, 2645.
19 R. Meij, D. J. Stufkens, K. Vrieze, W. van Gerresheim and 39 A. Y. Verat, H. Fan, M. Pink, Y.-S. Chen and K. G. Caulton,
C. H. Stam, J. Organomet. Chem., 1979, 164, 353. Chem.–Eur. J., 2008, 14, 7680.
20 M. D. Fryzuk, P. A. McNeil and S. J. Rettig, Organometallics, 40 C. M. Frech, L. J. W. Shimon and D. Milstein, Helv. Chim.
1986, 5, 2469. Acta, 2006, 89, 1730.
21 M. T. Whithead and R. H. Grubbs, J. Am. Chem. Soc., 2008, 41 S. H. Babcock, Inorg. Synth., 1939, 1, 10.
130, 5874.
42 J.-M. Lalancette, G. Rollin and P. Dumas, Can. J. Chem.,
22 A. Friedrich, R. Ghosh, R. Kolb, E. Herdtweck and
S. Schneider, Organometallics, 2009, 28, 708.
23 D. W. Lee, W. C. Kaska and C. M. Jensen, Organometallics,
1998, 17, 1.
1972, 50, 3058.
43 J. R. Aranzaes, M.-C. Daniel and D. Astruc, Can. J. Chem.,
2006, 84, 288.
44 D. Stalke, Chem. Soc. Rev., 1998, 27, 171.
24 R. Ghosh, M. Kanzelberger, T. J. Emge, G. S. Hall and 45 T. Kottke and D. Stalke, J. Appl. Crystallogr., 1993, 26, 615.
A. S. Goldman, Organometallics, 2006, 25, 5668.
25 A. S. Goldman and K. I. Goldberg, in ACS Symposium Series
885: Activation and Functionalization of C-H Bonds, ed.
46 T. Schulz, K. Meindl, D. Leusser, D. Stern, J. Graf,
C. Michaelsen, M. Ruf, G. M. Sheldrick and D. Stalke,
J. Appl. Crystallogr., 2009, 42, 885.
K. I. Goldberg and A. S. Goldman, American Chemical 47 SAINT 7.68A, Bruker AXS Inst. Inc., WI, USA, Madison,
Society, Washington, DC, 2004, pp. 1–43. 2009.
26 B. Askevold, A. Friedrich, M. R. Buchner, B. Lewall, 48 G. M. Sheldrick, SADABS 2012/1, Göttingen, 2012.
A. C. Filippou, E. Herdtweck and S. Schneider, J. Organo- 49 G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystal-
met. Chem., 2013, 744, 35.
27 S. Park, A. L. Rheingold and D. M. Roundhill, Organometal- 50 P. Müller, R. Herbst-Irmer, A. L. Spek, T. R. Schneider and
logr., 2008, 64, 112.
lics, 1991, 10, 615.
28 A. Marziale, E. Herdtweck, J. Eppinger and S. Schneider,
Inorg. Chem., 2009, 48, 3699.
29 A. Friedrich, M. Drees, M. Käss, E. Herdtweck and
S. Schneider, Inorg. Chem., 2010, 49, 5482.
30 B. de Bruin, P. H. M. Budzelaar and A. W. Gal, Angew.
Chem., Int. Ed., 2004, 43, 4142.
31 C. Tejel and M. A. Ciriano, Top. Organomet. Chem., 2007,
22, 97.
M. R. Sawaya, in Crystal Structure Refinement – A Crystallo-
grapher’s Guide to SHELXL, IUCr Texts on Crystallography,
ed. P. Müller, Oxford University Press, Oxford, UK, 2006,
vol. 8.
51 C. B. Hübschle, G. M. Sheldrick and B. Dittrich, J. Appl.
Crystallogr., 2011, 44, 1281.
52 A. J. M. Duisenberg, L. M. J. Kroon-Batenburg and
A. M. M. Schreurs, J. Appl. Crystallogr., 2003, 36, 220.
53 SADABS 2.06, Bruker AXS Inc., Madison, WI, USA, 2002.
32 M. R. Kelley and J.-U. Rohde, Chem. Commun., 2012, 48, 54 SHELXTL NT 6.12, Bruker AXS Inc., Madison, WI, USA,
2876.
2002.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 4506–4513 | 4513