Mononuclear (O, O′ or N, N′) and heterodinuclear (O, O′ and N, N′) transition-metal complexes of ortho -quinoid bis(pyrazol-1-yl)methane ligands

Article abstract of DOI:10.1021/om3000746

The ortho-hydroquinone-substituted bis(3,5-dimethylpyrazol-1-yl)methane ligand (HO)₂C₆H₃-C(H)(pz^Me,Me) ₂ (7) has been synthesized and fully characterized. Together with its bis(3-tert-butylpyrazol-1-yl)methane congener (HO)₂C ₆H₃-C(H)(pz^tBu)₂ (6), 7 was employed in oxidation and complexation studies. 6 has been oxidized with 2,3-dichloro-5,6-dicyano-para-benzoquinone to the corresponding ortho-benzoquinone form 9 on a preparative scale. Pure samples of 9 are stable for several hours in solution and for approximately one day in the solid state. Attempts at a N,N′ coordination of [PdCl₂] to 6 led to decomposition of the -C(H)(pz^tBu)₂ moiety, whereas an introduction of [PdCl₂] exclusively at the N,N′ site was possible in high yield for ligand 7. A selectively O,O′-chelated [(p-cym)Ru]²⁺ complex was accessible for ligand 6, but not for 7. In contrast, [(ppy)₂Ir]⁺ and [(Cp*)Ir]²⁺ gave O,O′ complexes with both donors 6 and 7 (Hppy = 2-phenylpyridine; HCp* = pentamethylcyclopentadiene). The heterodinuclear complex [(Cp*)Ir(O)₂C₆H₃-C(H)(pz ^Me,Me)₂PdCl₂] (16) was obtained from 14 and [(MeCN)₂PdCl₂].

Full text of DOI:10.1021/om3000746

Article
pubs.acs.org/Organometallics
Mononuclear (O,O′ or N,N′) and Heterodinuclear (O,O′ and N,N′)
Transition-Metal Complexes of ortho-Quinoid Bis(pyrazol-1-
yl)methane Ligands
Florian Blasberg, Michael Bolte, Hans-Wolfram Lerner, and Matthias Wagner*
Institut fur Anorganische und Analytische Chemie, Goethe-Universitat Frankfurt, Max-von-Laue-Straße 7, D-60438 Frankfurt (Main),
̈
̈
Germany
S
* Supporting Information
ABSTRACT: The ortho-hydroquinone-substituted bis(3,5-
dimethylpyrazol-1-yl)methane ligand (HO)2C6H3−C(H)-
(pzMe,Me)2 (7) has been synthesized and fully characterized.
Together with its bis(3-tert-butylpyrazol-1-yl)methane con-
gener (HO)2C6H3−C(H)(pztBu
) (6), 7 was employed in
oxidation and complexation studies. 6 has been oxidized with
2,3-dichloro-5,6-dicyano-para-benzoquinone to the corre-
2
sponding ortho-benzoquinone form 9 on a preparative scale. Pure samples of 9 are stable for several hours in solution and
for approximately one day in the solid state. Attempts at a N,N′ coordination of [PdCl2] to 6 led to decomposition of the
−C(H)(pztBu)2 moiety, whereas an introduction of [PdCl2] exclusively at the N,N′ site was possible in high yield for ligand 7. A
selectively O,O′-chelated [(p-cym)Ru]2+ complex was accessible for ligand 6, but not for 7. In contrast, [(ppy)2Ir]+ and
[(Cp*)Ir]2+ gave O,O′ complexes with both donors 6 and 7 (Hppy = 2-phenylpyridine; HCp* = pentamethylcyclopentadiene).
The heterodinuclear complex [(Cp*)Ir(O)2C6H3−C(H)(pzMe,Me)2PdCl2] (16) was obtained from 14 and [(MeCN)2PdCl2].
benzoquinone moiety is obviously possible through space,
without direct coordination of the metal atom to the organic
redox-active fragment.
INTRODUCTION
■
Cooperative catalysis, a phenomenon frequently observed in
enzymatic systems,1−3 has evolved into a guiding principle of
catalyst design.4−6 Two different variants of cooperative
substrate activation are commonly observed: (i) cooperation
of a transition metal and a redox-active ligand and (ii)
cooperation of two or more (different) transition metals.7
Case (i) is well represented by the oxidative addition of alkyl
halides to a square-planar Co(III) complex with two redox
noninnocent amidophenolate ligands, which deliver the
electrons for substrate activation. The metal does not change
its oxidation state during this process.8 Case (ii) is illustrated by
the asymmetric ring-opening (ARO) of meso epoxides with
Me3SiN3: An ARO catalyst consisting of two covalently
tethered chiral [(salen)Cr] complexes is 1−2 orders of
magnitude more reactive than the corresponding monomeric
complexes.9
Our group has a long-standing interest in the properties of
complexes showing electronic communication between redox-
active ligands and metal ions.10 Special emphasis has been put
on ferrocene- or quinone-containing oligotopic poly(pyrazol-1-
yl)borate11,12 and bis(pyrazol-1-yl)methane ligands13−17 (note:
in this paper, the general term “quinone” or “quinoid” will be
used whenever the oxidation state of any derivative of ortho-
hydroquinone/ortho-benzoquinone does not need to be
specified). Using the para-naphthoquinone-substituted bis-
(pyrazol-1-yl)methane Pd(II) complex 1 (Scheme 1), we
were able to prove the formation of a para-naphthosemiquin-
onate anion (3), following selective Pd(II) → Pd(0) reduction
(1 → 2).15 Thus, electron transfer from Pd(0) to a para-
Considering that electron transfer can proceed in both
directions, from the metal to the ligand and vice versa, we
recently set out to develop synthetic routes to (hetero)-
dimetallic complexes featuring redox-active quinoid bis-
(pyrazol-1-yl)methane linkers for potential applications in
combined redox-/transition metal-mediated catalysis (cf. 4−7;
Scheme 1). To this end, the para-quinone redox reservoirs were
replaced by ortho-quinones. The latter provide a chelating
coordination site of their own, which (i) greatly differs from the
bis(pyrazol-1-yl)methane donor set (i.e., O,O′ vs N,N′), (ii)
improves metal ↔ ligand electron transfer due to direct
conjugation, and (iii) decouples electron from proton transfer
in the O,O′ complexes.
The purpose of this paper is to report on selected
mononuclear and heterodinuclear transition-metal complexes
of ortho-quinoid bis(pyrazol-1-yl)methane ligands with sub-
stituents of varying steric demand about the N,N′ coordination
site. The ligand design was guided by the observation that an
increasing bulk of the pyrazolyl substituents leads to improved
solubility and to kinetic stabilization of both the N,N′-
complexed metal ion and the ortho-benzoquinone state of the
ligand. As a drawback, it renders the bis(pyrazol-1-yl)methane
moieties more prone to Lewis acid-induced degradation so that
the steric requirements of the ligand have to be properly
Received: January 31, 2012
Published: March 20, 2012
© 2012 American Chemical Society
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Scheme 1. para-Naphthoquinone Bis(pyrazol-1-yl)methane
Complex 1, Its Selective Pd(II) → Pd(0) Reduction with
NEt3, and the Spectroscopically Observed para-
Naphthosemiquinonate Pd(I) Complex 3, as Well as the
ortho-Quinoid Bis(pyrazol-1-yl)methane Ligands 4−7
Figure 1. Molecular structure of ligand 7 (50% displacement
ellipsoids; H atoms, except for OH groups, omitted for clarity).
Selected bond lengths [Å], bond angles [deg], torsion angles [deg],
and dihedral angles [deg]: C1−N1 1.466(3), C1−N11 1.453(2), C1−
C21 1.527(2), C23−O23 1.372(2), C24−O24 1.365(2); N1−C1−
N11 112.2(2), N1−C1−C21 111.4(2), N11−C1−C21 114.7(2);
C21−C1−N1−N2 116.5(2), C21−C1−N11−N12 47.2(2); HQ//
pz(N1) 78.4, HQ//pz(N11) 53.2, pz(N1)//pz(N11) 69.4.
Table 1. Selected Crystallographic Data and Structure
Refinement Details for 7 and 10
7
10
formula
fw
C17H20N4O2
312.37
C17H20Cl2N4O2Pd·EtOH
535.74
balanced. The coordination behavior was first studied with a
special emphasis on the selective introduction of one given
metal complex fragment at either the O,O′ or the N,N′
coordination site. In the second step, we used the knowledge
gained by these experiments for the targeted preparation of a
heterodinuclear species.
color, shape
colorless
parallelogram
yellow-brown plate
temp (K)
radiation
cryst syst
space group
a (Å)
173(2)
173(2)
Mo Kα, 0.71073 Å Mo Kα, 0.71073 Å
monoclinic
P2/n
monoclinic
P21
9.8973(9)
7.4957(4)
21.100(2)
90.00
9.1607(4)
14.1667(8)
9.6108(4)
90.00
RESULTS AND DISCUSSION
■
b (Å)
Synthesis, Characterization, and Chemical Oxidation
of Substituted ortho-Quinoid Bis(pyrazol-1-yl)methane
Ligands. Compound 7 was synthesized starting from 3,4-
dihydroxybenzaldehyde and (pzMe,Me)2SO in a similar way to
the ortho-quinoid bis(pyrazol-1-yl)methane ligands 4, 5, and
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
93.407(7)
90.00
114.902(3)
90.00
1562.6(2)
4
1131.30(9)
2
617 (Scheme 1; pzMe,Me = 3,5-dimethylpyrazol-1-yl). The H
1
Z
and 13C NMR spectra (d6-DMSO) of 7 show no peculiarities
compared to the related species 4−6 and therefore do not
merit further discussion.
Dcalcd. (g cm−3
)
1.328
1.573
F(000)
μ (mm−1
664
544
)
0.090
1.083
To further corroborate the proposed ligand structure, crystals
of 7, grown from a saturated MeCN/C6H6 solution, were
investigated by X-ray diffraction (Figure 1, Table 1). Key
metrical parameters, namely, C−O single bonds (av 1.369(2)
Å; cf. ortho-hydroquinone: av 1.371 Å18 vs ortho-benzoquinone:
1.220 Å19) and a small spread of C−C bond lengths
(1.380(3)−1.402(3) Å) indicate the presence of an ortho-
hydroquinone moiety. Thus, as already evidenced by the NMR
spectroscopic results, no air oxidation had taken place.
Since the redox properties of the ligands are particularly
relevant in the context of this paper, we next studied the
electrochemical behavior of ligand 6, which possesses the
highest solubility of the four derivatives in all common organic
solvents.
cryst size (mm)
0.25 × 0.18 × 0.17 0.50 × 0.35 × 0.12
no of rflns coll
11 340
15 962
no of indep rflns (Rint)
data/restr/params
GOOF on F2
2914 (0.0925)
2914/0/221
0.904
4518 (0.0598)
4518/1/269
1.022
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
largest diff peak and hole
0.0449, 0.0930
0.0787, 0.1026
0.190, −0.232
0.0330, 0.0843
0.0332, 0.0845
0.686, −0.920
(e Å−3
)
regime, an irreversible reduction wave appears at a peak
potential of −0.54 V.
The parent ortho-benzoquinone readily decomposes under
ambient conditions.19,20 It therefore had to be clarified by
chemical oxidation and analysis of the product(s) whether the
irreversibility of the redox transitions of 6 is simply due to loss
of OH protons or whether a more severe structural
reorganization takes place.
In cyclic voltammetric experiments (DMF, 0.1 M [Bu4N]-
[PF6]), 6 is irreversibly oxidized at a peak potential of 0.63 V
(vs FcH/FcH+, scan rate = 200 mV s−1; cf. the Supporting
Information (SI) for a plot of the cyclic voltammogram).
When, after oxidation, the sweep is continued into the cathodic
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Under the reaction conditions typically applied for oxidizing
hydroquinones (i.e., [Ce(NH4)2(NO3)6] = CAN; MeCN/
H2O), only product mixtures were obtained; the same was true
for other strong oxidants such as Pb(IV) and Ag(I). However,
after 6 was treated with CAN in the presence of excess 4-tert-
butylpyridine, the pyridinium salt 8 could be isolated in single-
crystalline form (Scheme 2; cf. the SI for more information
decomposition17 that we had been encountering with 5 and
6. For this reason, ligand 7 was employed in most of the
complexation experiments throughout this study.
Selective N,N′ coordination of a [PdCl2] moiety was
achieved in high yield under base-free conditions using 7 and
[(MeCN)2PdCl2] (cf. 10; Scheme 3). The 1H NMR
a
Scheme 3. Preparation of Palladium Complex 10
a
Scheme 2. Oxidation of 6 to 9 and Trapping of 9
a
Conditions: (i) 1 equiv of [(MeCN)2PdCl2], MeCN, rt, 2.5 h.
spectroscopic properties (d3-MeCN) of 10 are unexceptional,
the only remarkable feature being the presence of two OH
resonances at 6.86 and 7.15 ppm, consistent with the proposed
N,N′ binding mode. The X-ray crystal structure determination
of 10 (Figure 2, Table 1) reveals a cis-N2PdCl2 fragment with a
a
Conditions: (i) 3 equiv of CAN, 5 equiv of 4-tBu-pyridine, MeCN/
H2O 2:1, rt, 75 min; (ii) 1 equiv of DDQ, C6H6, rt, 2 min. CAN =
[Ce(NH4)2(NO3)6], DDQ = 2,3-dichloro-5,6-dicyano-para-benzoqui-
none.
including an X-ray crystal structure determination). This
trapping experiment, which was inspired by the synthesis of
the alkaloid punicin (Scheme 2) from para-benzoquinone and
pyridine,21−23 indirectly proves the intermediate formation of
the ortho-benzoquinone 9 and shows that it is sufficiently stable
to be used for further derivatization.
Figure 2. Molecular structure of palladium complex 10 (50%
displacement ellipsoids; H atoms, except for OH groups, omitted for
clarity). Selected bond lengths [Å], bond angles [deg], torsion angles
[deg], and dihedral angles [deg]: Pd1−Cl1 2.291(1), Pd1−Cl2
2.290(1), Pd1−N2 2.013(2), Pd1−N12 2.047(3), Pd1···COG(HQ)
3.76, C1−N1 1.464(4), C1−N11 1.449(4), C1−C21 1.529(4), C23−
O23 1.360(5), C24−O24 1.371(4); Cl1−Pd1−Cl2 90.2(1), Cl1−
Pd1−N2 90.6(2), Cl2−Pd1−N12 92.8(1), N2−Pd1−N12 86.3(2),
N1−C1−N11 109.0(2), N1−C1−C21 112.4(2), N11−C1−C21
111.5(2); Cl1−Pd1−N2−N1 131.5(2), Cl2−Pd1−N12−N11
−136.7(2), C21−C1−N1−N2 −66.1(4), C21−C1−N11−N12
64.0(3); HQ//pz(N1) 87.9, HQ//pz(N11) 75.9, pz(N1)//pz(N11)
65.5. COG = ring centroid.
Finally, we found that DDQ (2,3-dichloro-5,6-dicyano-para-
benzoquinone) cleanly and quantitatively oxidizes 6 to 9. The
key to success was the application of C6H6 as the solvent, which
allowed for facile separation of the insoluble reduced side
product DDQH2. Following this protocol, 9 can be prepared in
high purity and either used immediately as C6H6 solution or
isolated as a finely divided powder after freeze-drying.24
The NMR spectroscopic characteristics of 9 are in accord
with a bis(3-tert-butylpyrazol-1-yl)methane-substituted ortho-
benzoquinone. Especially the 13C NMR shift values of the
carbonyl groups (178.8, 179.2 ppm; C6D6) serve as clear
indicators of a successful oxidation (cf. 6: HQ-C3,4 = 145.7,
146.1 ppm; C6D6; cf. SI). In the solid state, 9 decomposes
within one day, even under an argon atmosphere; in C6H6
solution the decomposition is complete after a few hours
(NMR spectroscopic control).
square-planar configuration. The Pd(II) ion is located in close
proximity to the hydroquinone π face (Pd1···COG(HQ) = 3.76
Å; COG = ring centroid). The Pd−Cl (av 2.291(1) Å) and
Pd−N (av 2.030(3) Å) bond lengths compare well with other
published [(R)(H)C(pzMe,Me)PdCl2] complexes.25
Having established selective N,N′ complexation, we next
turned our attention to the O,O′ coordination site. Ruthenium
and iridium are both catalytically active metals,26 of which
ortho-quinone complexes are known27 and which were
therefore chosen for further investigations. Since metal binding
at the O,O′ position requires a base to capture the OH protons,
several Brønsted bases were screened with the focus on easily
Selective Introduction of Transition Metals at Either
Coordination Site of ortho-Quinoid Bis(pyrazol-1-yl)-
methane Ligands 6 and 7. Exploratory screening experi-
ments with the ligands 4−7 and various soluble Pd(II)Cl2
sources revealed that 7 provides far better solubility than 4 and
does not show the problems of bis(pyrazol-1-yl)methane
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separable conjugated acids. The presence of thallium cations in
combination with metal chloride precursors proved beneficial
for M−O bond formation, because TlCl precipitation provides
a strong thermodynamic driving force. Thus, basic thallium
species appeared to be the reagents of choice. In order to
circumvent the highly toxic, volatile TlOEt, the crystalline base
TlOtBu28 was used instead.
165.0 ppm; cf. the SI for the result of an X-ray crystal structure
determination of 11).
Even though the successful synthesis of 11 can be regarded
as an important proof of concept, the compound itself is only of
limited practical use in the present context, because we have
previously shown that ligand 6 decomposes upon treatment
with, for example, [(MeCN)2PdCl2].17 We therefore looked for
alternative complex fragments allowing for exclusive O,O′
coordination, not only in the case of the sterically congested
ligand 6 but also in the case of 7. For our purposes,
[(ppy)2IrCl]2 (Hppy = 2-phenylpyridine) appeared to be a
convenient starting material, easily prepared in gram quanti-
ties.29 The reactions of 7 and 6 first with TlOtBu and then with
[(ppy)2IrCl]2 yield, after workup, the desired ortho-hydro-
quinonate iridium(III) complexes 12 and 13 (Scheme 4) in
excellent yields as their thallium salts. The octahedral iridium
complexes are chiral and have a fixed configuration (cis-C,C).30
Thus, two isomers form after addition to the nonsymmetric
ligands 7 and 6, as is evident from the fact that two signal sets
As a versatile precursor of ruthenium complex fragments,
[(p-cym)RuCl2]2 (p-cym = para-cymene = 1-isopropyl-4-
methylbenzene) was selected for complexation with 7. The
purple solid isolated from the reaction mixture gave only broad
resonances in the NMR spectra (d8-THF), which therefore did
not provide sufficient information for a definite assignment of
the product as [(p-cym)Ru(7)]. Numerous crystallization
attempts also remained unsuccessful. Two different explan-
ations for the special NMR features observed can be
envisioned. One would be the formation of paramagnetic
compounds. If this was the case, the amount of these impurities
would have to be small, because the broad resonances appear in
the same shift regions as the signals of the [(p-cym)Ru]2+
complex 11 discussed below. Alternatively, the [(p-cym)Ru]2+
fragment in hypothetical [(p-cym)Ru(7)] might undergo a
dynamic association−dissociation equilibrium, possibly even
with scrambling between the O,O′ and N,N′ coordination sites.
As a test for this assumption, we repeated the experiment
with the sterically more congested tBu ligand 6, which should
favor O,O′ over N,N′ binding (Scheme 4). Now, the reaction
1
of equal intensity are visible both in the H and in the 13C
NMR spectra of analytically pure samples. The NMR spectra
are further characterized by broad line widths and unusually
highfield-shifted ortho-hydroquinonate proton resonances. We
tentatively attribute these features to the presence of dynamic
Tl(I)-bridged aggregates (cf. the solid-state structure of (12)2
below).
Compound 12 crystallizes from an EtCN solution in the
form of doubly Tl(I)-bridged dimers (12)2 (Figure 3; Table 2).
As planned, octahedrally coordinated [(ppy)2Ir]+ ions reside
exclusively at O,O′ sites (av Ir−O = 2.145(8) Å; cf. 2.113(3) Å
for a [(Cp*)Ir(III)]2+ biphenolate complex31). Tl1 is
Scheme 4. Synthesis of Ruthenium and Iridium Complexes
11−15
a
a
Conditions: (i) 2 equiv of TlOtBu, 0.5 equiv of [(p-cym)RuCl2]2,
THF, rt, 12 h; (ii) 2 equiv of TlOtBu, 0.5 equiv of [(ppy)2IrCl]2, THF,
rt, 12 h; (iii) 2 equiv of TlOtBu, 0.5 equiv of [(Cp*)IrCl2]2, THF, rt,
12 h. p-cym = 1-isopropyl-4-methylbenzene, Hppy = 2-phenylpyridine,
HCp* = pentamethylcyclopentadiene.
Figure 3. POV-ray representation of the dimer (12)2 in the solid state
(H atoms omitted for clarity). Selected bond lengths [Å] and bond
angles [deg] are given for both monomers of (12)2: Ir1−O23
2.143(8), Ir1−O24 2.143(8), Ir1−N31 2.034(11), Ir1−N51
2.015(11), Ir1−C41 1.976(13), Ir1−C61 2.009(13), Ir1A−O23A
2.136(8), Ir1A−O24A 2.157(8), Ir1A−N31A 2.050(11), Ir1A−N51A
2.047(11), Ir1A−C41A 2.000(12), Ir1A−C61A 2.019(12), Tl1−O24
2.505(8), Tl1−O24A 2.488(8), Tl1···COG(C61) 3.471, Tl1···COG-
(C41A) 3.478, Tl2−N12 2.837(12), Tl2−N2A 2.823(10), Tl2···COG-
(C21) 3.171, Tl2···COG(C21A) 3.012; O23−Ir1−O24 79.5(3),
O23A−Ir1A−O24A 79.0(3). COG = ring centroid.
proceeded smoothly to provide analytically pure 11 in
essentially quantitative yield. The 1H NMR spectrum (d8-
THF) of 11 is characterized by the absence of OH resonances
and the presence of signals assignable to [(p-cym)Ru]2+ as well
as deprotonated 6 in a 1:1 ratio. In the 13C NMR spectrum (d8-
THF) we find, compared to free 6, a notable 20 ppm downfield
shift of the oxygen-carrying carbon atoms HQ-C3,4 (164.3,
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Table 2. Selected Crystallographic Data and Structure Refinement Details for (12)2, 14, and 16
(12)2
14
16
formula
fw
C78H68Ir2N12O4Tl2·3EtCN
2195.82
C27H33IrN4O2·3THF
854.09
C27H33Cl2IrN4O2Pd·THF·0.5H2O
896.19
color, shape
temp (K)
radiation
cryst syst
space group
a (Å)
red plate
red plate
red block
173(2)
173(2)
173(2)
Mo Kα, 0.71073 Å
triclinic
Mo Kα, 0.71073 Å
triclinic
Mo Kα, 0.71073 Å
monoclinic
C2/c
P1
̅
P1
̅
14.2553(6)
17.3216(7)
17.9782(8)
101.431(4)
102.004(3)
91.638(3)
4244.8(3)
2
8.7625(6)
12.9304(10)
17.2848(13)
90.590(6)
101.634(6)
95.570(6)
1908.2(2)
2
21.5132(8)
14.3154(3)
22.5580(8)
90.00
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
102.434(3)
90.00
6784.2(4)
8
Z
Dcalcd. (g cm−3
)
1.718
1.486
1.755
F(000)
μ (mm−1
2116
872
3528
)
6.967
3.545
4.645
cryst size (mm)
0.17 × 0.05 × 0.04
52 244
0.27 × 0.18 × 0.08
23 748
0.35 × 0.20 × 0.15
32 784
no of rflns coll
no of indep rflns (Rint)
data/restr/params
GOOF on F2
15871 (0.1132)
15871/66/986
0.754
6735 (0.1098)
6735/15/451
0.926
6348 (0.0661)
6348/1/407
1.024
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
largest diff peak and hole (e Å−3
0.0531, 0.1156
0.0976, 0.1255
2.405, −3.277
0.0527, 0.1116
0.0760, 0.1179
2.270, −2.487
0.0344, 0.0722
0.0452, 0.0753
0.610, −1.498
)
surrounded in a distorted tetrahedral fashion by two oxygen
atoms of two different ortho-hydroquinonate moieties (i.e.,
O24, O24A) and by the centroids of the C6H4 rings containing
C61 and C41A. The average Tl1−O distance of 2.496(8) Å is
comparable to those of other mixed thallium/metal μ-
alkoxides;32 in contrast, the average Tl1···COG distance of
3.475 Å is longer than the average Tl···COG contact found in
[Tl(toluene)2]+ (3.04 Å).33 Tl2 establishes the four closest
contacts to two nitrogen atoms of two different bis(pyrazol-1-
yl)methane ligands (i.e., N12, N2A) and to the centroids of
both ortho-hydroquinonate rings. In this case, the ligand
environment comes close to a square-planar arrangement. The
Tl2−N bonds (av value: 2.830(12) Å) are somewhat longer
than in a related scorpionate ligand containing a Tl···Ph (2.959
Å) and a Tl−N(pz) contact (2.579(5) Å).34
coordination site scrambling of the metal ion, as had likely
taken place in [(p-cym)Ru(7)], appears not to be an issue here.
Downfield-shifted signals of the HQ carbon atoms C3 and C4
(165.0, 165.2 ppm for 14; 165.0, 165.6 ppm for 15) are present
in the 13C NMR spectra following Ir(III) coordination.
Complex 14, which crystallizes from THF together with
three equivalents of THF, was structurally characterized by X-
ray crystallography (Figure 4, Table 2; cf. the SI for the result of
an X-ray crystal structure determination of 15). The Ir(III)
center is chelated by the two oxygen atoms of the ortho-
hydroquinonate moiety and adopts a two-legged piano stool
configuration; no close contacts of cocrystallized THF
The [(ppy)2Ir]+ fragment is compatible with ligand 7 (rather
than with only 6, as was [(p-cym)Ru]2+), and, consequently,
further coordination of [PdCl2] at the N,N′ site of the resulting
compound 12 should be achievable. Unfortunately, the use of
[(ppy)2Ir]+ brings about the new disadvantage of isomer
formation, which arises from its chirality and complicates its
NMR spectra but does not rule out its potential use as, for
example, a light-harvesting group. At the present stage,
however, we found it more important to be able to readily
characterize new compounds by NMR spectroscopy rather than
to establish certain functionalities and therefore turned our
attention to [(Cp*)Ir]2+ (HCp* = pentamethylcyclopenta-
diene). The 16 VE iridium complexes 14/15 (Scheme 4) were
readily accessible from [(Cp*)IrCl2]2 and 7/6 in the presence
of 2 equiv of TlOtBu.
Figure 4. Molecular structure of the iridium complex 14 (50%
displacement ellipsoids; H atoms omitted for clarity). Selected bond
lengths [Å], bond angle [deg], and dihedral angles [deg] for 14: Ir1−
O23 1.979(6), Ir1−O24 1.991(6), Ir1···COG(Cp*) 1.78, C23−O23
1.361(10), C24−O24 1.358(9); O23−Ir1−O24 81.3(2); HQ//
pz(N1) 80.2, HQ//pz(N11) 79.3, pz(N1)//pz(N11) 78.8. COG =
ring centroid.
The aimed-for O,O′ coordination mode is once again evident
from missing OH resonances in the 1H NMR spectra (d8-THF)
of 14 and 15. Since all signals are narrow and well resolved,
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molecules are found in the solid state. The average Ir−O bond
length amounts to 1.985(6) Å; the distance between the
iridium ion and the centroid of the Cp* ligand is 1.78 Å (cf.
1.75 Å for a [(Cp*)Ir(III)]2+ biphenolate complex31). Apart
from slightly contracted C−O bonds, the O,O′/N,N′ ligand
framework of 14 shows only minor deviations in bond lengths
and bond angles compared to the free ligand 7 (Figure 1).
Complex 14 was also investigated by cyclic voltammetry
(DMF, 0.1 M [Bu4N][PF6]). A reduction wave appears at E1/2
= −2.10 V, which reveals features of electrochemical
reversibility; that is, the peak-to-peak separation (ΔE = 78
mV) does not deviate from the value found for the internal
ferrocene standard (ΔE(FcH) = 78 mV), and the current ratio
ipc/ipa is close to 1, at least at high scan rates (cf. the SI for a
plot of the cyclic voltammogram). A comparable reduction
event at a potential value of E1/2 = −2.11 V (vs FcH/FcH+) has
been reported for the iridium dithiolene complex [(Cp*)Ir-
(S2C2(COOMe)2)].35 For 14, a second reversible electron
transition takes place at E1/2 = −0.18 V (ΔE = 97 mV), which
we tentatively assign to the ortho-hydroquinonate → ortho-
semiquinonate oxidation.
In order to gain conclusive evidence for the proposed N,N′-
Pd/O,O′-Ir coordination mode, the molecular structure of 16
was determined by X-ray diffraction on a single crystal grown
from a THF solution (Figure 5, Table 2). The Pd−N (av
Figure 5. Molecular structure of the palladium/iridium complex 16
(50% displacement ellipsoids; H atoms omitted for clarity). Selected
bond lengths [Å], bond angles [deg], and dihedral angles [deg]: Ir1−
O23 1.993(4), Ir1−O24 1.997(3), Ir1···COG(Cp*) 1.76, Pd1−Cl1
2.287(1), Pd1−Cl2 2.283(1), Pd1−N2 2.053(4), Pd1−N12 2.030(4),
Pd1···COG(HQ) 3.95; O23−Ir1−O24 80.9(1), Cl1−Pd1−Cl2
89.53(5), Cl1−Pd1−N12 91.9(1), Cl2−Pd1−N2 91.9(1), N2−
Pd1−N12 86.8(2); HQ//pz(N1) 78.3, HQ//pz(N11) 84.1, pz-
(N1)//pz(N11) 61.8. COG = ring centroid.
Preparation of a Heterodimetallic Complex. After the
selective synthesis of either N,N′- or O,O′-coordinated
mononuclear complexes (i.e., 10 and 14, respectively), the
targeted preparation of a heterodinuclear species combining
both structural motifs was attempted next.
Starting from palladium complex 10, it was not possible to
coordinate [(Cp*)Ir]2+ to the ortho-hydroquinonate donor,
because unwanted side reactions prevailed (Scheme 5).36 The
2.042(4) Å) and Pd−Cl bond lengths (av 2.285(1) Å) are
similar to those in 10, and the same holds true for the Ir−O
bonds (av 1.995(4) Å) when compared to 14. As was the case
for 10, the bis(pyrazol-1-yl)methane donor brings the Pd(II)
ion in proximity to the π face of the ortho-hydroquinonate
moiety.
a
Scheme 5. Synthesis of the Pd(II)/Ir(III) Complex 16
The cyclic voltammogram of 16 (DMF, 0.1 M [Bu4N][PF6])
shows essentially the same two redox events as the
mononuclear Ir(III) complex 14. In addition, a very broad
irreversible feature with an onset at E = −1.30 V appears, which
is likely due to Pd(II) reduction (vs FcH/FcH+; cf. the SI for a
plot of the cyclic voltammogram).
CONCLUSION
■
The ortho-hydroquinone-substituted bis(3,5-dimethylpyrazol-1-
yl)methane (HO)2C6H3−C(H)(pzMe,Me
)
(7) provides a
2
versatile ligand platform for the synthesis of O,O′- or N,N′-
coordinated mononuclear complexes as well as O,O′/N,N′-
bonded heterodinuclear aggregates. A [PdCl2] fragment
attached to the N,N′ site of 7 resides directly above the
ortho-hydroquinone π face (at least in the solid state) and is
therefore able to electronically interact with the redox-active
linker through space. In contrast, [(p-cym)Ru]2+, [(ppy)2Ir]+,
and [(Cp*)Ir]2+ complexes chelated by the O,O′ site are
capable of metal ↔ ligand d(π)−p(π) interaction. The redox
properties of the new ligand class were studied using the tert-
a
Conditions: (i) 1 equiv of [(MeCN)2PdCl2], d3-MeCN, 150 °C, 2
min; (ii) 2 equiv of LiOtBu, 0.5 equiv of [(Cp*)IrCl2]2, THF, rt, 12 h.
alternative approach, i.e., reaction of [(MeCN)2PdCl2] with the
[(Cp*)Ir]2+ complex 14 (MeCN, 150 °C; sealed tube), led to
full consumption of the starting materials within minutes under
formation of the Pd(II)/Ir(III) complex 16 in ca. 90% purity
(NMR spectroscopic control; Scheme 5). Crystallization from
THF afforded analytically pure 16 in good yield.
butyl derivative (HO)2C6H3−C(H)(pztBu
) (6), because it
2
possesses a higher solubility and kinetic stability of its ortho-
benzoquinone form than 7. The free ligand 6 undergoes an
irreversible two-electron oxidation at a peak potential of 0.63 V
(vs FcH/FcH+). Since it was possible to synthesize the ortho-
benzoquinone form 9 of 6 by chemical oxidation on a
preparative scale, we conclude that the irreversibility of the
oxidation event under the conditions of cyclic voltammetry is
solely due to the loss of OH protons. After O,O′ coordination
1
The H and 13C NMR spectroscopic data of 16 gave no
indication of coordination site scrambling. All key changes in
the NMR spectroscopic parameters that are observed upon
going from the free ligand 7 to the Pd(II) complex 10 or the
Ir(III) complex 14 are well reproduced in the NMR spectra of
the Pd(II)/Ir(III) complex 16.
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6.67 (d, 3JHH = 8.3 Hz, 1 H; HQ-H5), 7.50 (n.r., 1 H; CH), 8.99 (s, 2
H; OH; D2O-exchangable). 13C NMR (62.9 MHz, d6-DMSO): δ 11.2,
13.5 (pz-CH3), 72.6 (Cpz2), 106.1 (pz-C4), 114.6 (HQ-C2), 115.1
(HQ-C5), 118.0 (HQ-C6), 127.3 (HQ-C1), 140.1 (pz-C5), 145.0,
145.4 (HQ-C3,4), 146.5 (pz-C3). UV−vis (CH2Cl2): λmax/nm (ε) 281
(2700). ESI-MS: m/z (%) = 215 [M − Me2pz − 2H]− (95), 311 [M −
H]− (100). Anal. Calcd for C17H20N4O2 [312.37]: C 65.37, H 6.45, N
17.94. Found: C 65.54, H 6.24, N 18.09.
Synthesis of 9. A mixture of 6 (50 mg, 0.136 mmol) and DDQ (31
mg, 0.136 mmol) was prepared in C6H6 (1 mL) at room temperature
and stirred for 2 min. The resulting suspension was filtered, and the
residue (DDQH2) was washed with C6H6 (1 × 1 mL). Freeze-drying
of the combined C6H6 phases gave 9 as a red-brown powder. Yield: 46
mg (93%). 1H NMR (300.0 MHz, C6D6): δ 1.30 (s, 18 H; CH3), 5.59
(m, 1 H; BQ-H2), 5.62 (m, 1 H; BQ-H5), 5.87 (ddd, 3JHH = 10.3 Hz,
4JHH = 2.0 Hz, 4JHH = 0.4 Hz, 1 H; BQ-H6), 5.95 (d, 3JHH = 2.5 Hz, 2
H; pz-H4), 6.61 (d, 4JHH = 2.0 Hz, 1 H; CH), 7.02 (d, 3JHH = 2.5 Hz, 2
H; pz-H5). 13C NMR (75.4 MHz, C6D6): δ 30.6 (CH3), 32.5 (CCH3),
75.9 (Cpz2), 104.0 (pz-C4), 128.2 (BQ-C2), 129.6 (BQ-C5), 130.0
(pz-C5), 137.0 (BQ-C6), 147.7 (BQ-C1), 164.0 (pz-C3), 178.8, 179.2
(BQ-C3,4). ESI-MS: m/z (%) = 368 [M + H]+ (100). Anal. Calcd for
C21H26N4O2 [366.46]·0.25H2O [18.02]: C 67.99, H 7.20, N 15.10.
Found: C 67.58, H 6.82, N 14.76.
of [(Cp*)Ir]2+, an oxidation event can be observed already at a
potential value of E1/2 = −0.18 V. In this case, electron transfer
is no longer coupled to proton transfer and the electron
transition becomes reversible. The heterodinuclear complex
[(Cp*)Ir(O)2C6H3−C(H)(pzMe,Me)2PdCl2] (16) meets impor-
tant structural requirements not only for ligand ↔ metal but
also for metal ↔ metal electron transfer: (i) the close proximity
of one metal (Pd) to the ortho-hydroquinone π face and (ii) the
direct conjugation of the other metal (Ir) with the ortho-
hydroquinone via the oxygen donors.
In the future, we will employ the O,O′-bonded metal center
as a handle to fine-tune the redox behavior of the ligand and
thereby to influence the (catalytic) activity of the metal center
residing at the N,N′ site. In this context, special emphasis will be
put on [(ppy)2Ir]+ and related species, which can act as light-
harvesting antennas.
EXPERIMENTAL PART
■
General Considerations. If not explicitly mentioned otherwise, all
reactions and manipulations of air-sensitive compounds were carried
out under dry, oxygen-free nitrogen by using standard Schlenk ware or
in an argon-filled glovebox. THF, d8-THF, C6H6, C6D6, and pentane
were freshly distilled under argon from Na/benzophenone. DMF,
MeCN, d3-MeCN, and EtCN were dried over molecular sieves (4 Å)
for at least two days, distilled, degassed by three freeze−pump−thaw
cycles, and stored in Young’s valve ampules. NaH was used as a 60%
per weight suspension in mineral oil (commercially available);
however, throughout the Experimental Part, all weighed-out quantities
Synthesis of 10. 7 (200 mg, 0.640 mmol) and [(MeCN)2PdCl2]
(166 mg, 0.640 mmol) were suspended in MeCN (5 mL), and the
suspension was stirred at room temperature for 2.5 h (note: this
reaction requires neither dry MeCN nor exclusion of air and
moisture). After filtration, the filtrate was evaporated to dryness
using a rotary evaporator and the residue was briefly heated to 150 °C
on a vacuum line (10−3 mbar; heat gun) to obtain 10·H2O as an
orange foam. Yield of 10·H2O: 312 mg (96%). Single crystals of
10·EtOH suitable for X-ray diffraction were grown by slow
refer to neat NaH. 6,17 TlOtBu,28 and [(ppy)2IrCl]2 were prepared
29
according to literature procedures.
1
evaporation of an EtOH solution of 10·H2O. H NMR (300.0 MHz,
1D and 2D NMR spectra were recorded on Bruker AM-250, Bruker
Avance-300, and Bruker Avance-400 spectrometers. Chemical shifts
are given in ppm and are referenced to residual solvent signals
(1H/13C{1H}: C6D6: 7.16/128.06; d8-THF: 1.72, 3.58/25.31, 67.21;
d6-DMSO: 2.50/39.52; d3-MeCN: 1.94/1.32, 118.26). Abbreviations: s
= singlet; d = doublet; dd = doublet of doublets; ddd = doublet of
doublets of doublets; m = multiplet; sept = septet; bs = broad singlet;
n.r. = multiplet not fully resolved; n.o. = signal expected in the NMR
spectrum, but not observed; pz = (substituted) pyrazol-1-yl; HQ =
ortho-hydroquinone core; BQ = ortho-benzoquinone core.
d3-MeCN): δ 2.46 (d, nJHH = 0.4 Hz, 6 H; pz-CH3), 2.48 (s, 6 H; pz-
CH3), 6.10 (n.r., 2 H; pz-H4), 6.38 (ddd, 3JHH = 8.3 Hz, 4JHH = 2.4 Hz,
4JHH = 0.9 Hz, 1 H; HQ-H6), 6.44 (dd, 4JHH = 2.4 Hz, 4JHH = 0.9 Hz, 1
H; HQ-H2), 6.86 (s, 1 H; OH; D2O-exchangable), 6.96 (d, 3JHH = 8.3
Hz, 1 H; HQ-H5), 7.15 (s, 1 H; OH; D2O-exchangable), 7.52 (n.r., 1
H; CH). 13C NMR (75.4 MHz, d3-MeCN): δ 11.7, 15.2 (pz-CH3),
70.4 (Cpz2), 108.9 (pz-C4), 115.1 (HQ-C2), 116.6 (HQ-C5), 120.2
(HQ-C6), 128.3 (HQ-C1), 145.3 (pz-C5), 146.2, 146.6 (HQ-C3,4),
155.0 (pz-C3). UV−vis (CH2Cl2): λmax/nm (ε) 281 (3800), 384
(260). Anal. Calcd for C17H20Cl2N4O2Pd [489.69]·H2O [18.02]: C
40.22, H 4.37, N 11.04. Found: C 39.89, H 4.60, N 10.90.
Elemental analyses were performed by the Microanalytical
Laboratory of the University of Frankfurt or the Mikroanalytisches
Labor Pascher, Remagen, Germany. Mass spectra of MeCN solutions
were recorded with a VG PLATFORM II mass spectrometer. UV−vis
spectra were recorded on a Varian Cary 50 UV−vis spectropho-
tometer. Cyclic voltammograms were recorded using an EG&G
Princeton Applied Research 263A potentiostat with a glassy carbon
working electrode. Carefully dried and degassed DMF was used as the
solvent and [Bu4N][PF6] as the supporting electrolyte (0.1 M). All
potential values are referenced against the FcH/FcH+ couple.
Synthesis of 7. Neat 3,5-dimethylpyrazole (2.50 g, 26.01 mmol)
was slowly added to a stirred suspension of NaH (0.62 g, 25.85 mmol)
in THF (100 mL). After 30 min, H2 evolution had ceased and neat
SOCl2 (0.95 mL, 1.55 g, 13.00 mmol) was added in one portion via
syringe. After addition of neat 3,4-dihydroxybenzaldehyde (1.80 g,
13.00 mmol) and neat pyridine (10.49 mL, 10.28 g, 130.00 mmol), the
reaction mixture was kept at reflux temperature for 12 h. H2O (50 mL)
was added, and the mixture was extracted with CH2Cl2 (3 × 50 mL).
The combined CH2Cl2 extracts were dried over MgSO4 and filtered.
The filtrate was evaporated using a rotary evaporator, and the highly
viscous oily residue briefly heated to 150 °C on a vacuum line (10−3
mbar; heat gun) to remove residual pyridine. EtOAc (100 mL) was
added to transform the oil into a colorless solid. Crude 7 was
subsequently recrystallized from MeCN/C6H6 (5:1) to obtain single
Synthesis of 11. In a representative procedure, 6 (100 mg, 0.271
mmol) was treated with TlOtBu (151 mg, 0.542 mmol) in THF (2.5
mL) at room temperature. After a clear orange solution had formed,
[(p-cym)RuCl2]2 (83 mg, 0.136 mmol) was added, and the resulting
purple suspension was stirred at room temperature overnight. After
centrifugation, the supernatant was separated and evaporated to
dryness to yield a purple-brown foam. Yield: 163 mg (quant.). Single
crystals were grown by gas-phase diffusion of pentane into a THF
solution of 11. 1H NMR (400.1 MHz, d8-THF): δ 1.24 (s, 18 H; tBu-
CH3), 1.39 (d, 3JHH = 6.9 Hz, 6 H; iPr-CH3), 2.29 (s, 3 H; cym-CH3),
2.76 (sept, 3JHH = 6.9 Hz, 1 H; iPr-CH), 5.80, 5.84 (2 × d, 2 × 3JHH
=
3
6.0 Hz, 2 × 2 H; cym-H), 6.04 (d, JHH = 2.4 Hz, 2 H; pz-H4), 6.25
3
4
4
(dd, JHH = 8.3 Hz, JHH = 1.9 Hz, 1 H; HQ-H6), 6.55 (d, JHH = 1.9
Hz, 1 H; HQ-H2), 6.80 (d, 3JHH = 8.3 Hz, 1 H; HQ-H5), 7.26 (d, 3JHH
= 2.4 Hz, 2 H; pz-H5), 7.41 (s, 1 H; CH). 13C NMR (100.6 MHz, d8-
THF): δ 19.4 (cym-CH3), 23.3 (iPr-CH3), 31.0 (tBu-CH3), 32.8 (iPr-
CH), 33.1 (tBu-CCH3), 75.1, 77.2 (cym-C2,3,5,6), 79.2 (Cpz2), 90.6,
99.8 (cym-C1,4), 102.2 (pz-C4), 115.4 (HQ-C2), 116.0 (HQ-C5),
117.7 (HQ-C6), 128.3 (HQ-C1), 130.1 (pz-C5), 162.2 (pz-C3),
164.3, 165.0 (HQ-C3,4). UV−vis (CH2Cl2): λmax/nm (ε): 287 (45
100), 323 (27 700), 490 (46 300). ESI-MS: m/z (%) = 602 [M + H]+
(100). Anal. Calcd for C31H40N4O2Ru [601.74]: C 61.88, H 6.70, N
9.31. Found: C 61.87, H 6.74, N 9.18.
1
crystals. Yield: 1.36 g (33%). H NMR (300.0 MHz, d6-DMSO): δ
2.08 (s, 6 H; pz-CH3), 2.10 (d, 5JHH = 0.6 Hz, 6 H; pz-CH3), 5.87 (s, 2
Synthesis of 12. Following the general procedure outlined for the
synthesis of 11, 7 (30 mg, 0.096 mmol) was treated with TlOtBu (53
mg, 0.191 mmol) and [(ppy)2IrCl]2 (52 mg, 0.048 mmol) in THF
3
4
H; pz-H4), 6.19 (ddd, JHH = 8.3 Hz, 4JHH = 2.2 Hz, JHH = 0.6 Hz, 1
4
4
H; HQ-H6), 6.41 (dd, JHH = 2.2 Hz, JHH = 0.6 Hz, 1 H; HQ-H2),
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(2.0 mL) to obtain a red solid. Yield: 98 mg (quant.). Single crystals
suitable for X-ray diffraction were grown from a saturated EtCN
solution of 12. Some 1H NMR signals are poorly resolved, due to the
presence of two isomers with very similar chemical shift values. An
unambiguous assignment of the signals to the individual isomers is not
possible. 1H NMR (300.0 MHz, d8-THF): δ 1.96, 2.06 (2 × bs, 2 × 6
H; 2 × pz-CH3), 2.18, 2.22 (2 × s, 2 × 6 H; 2 × pz-CH3), 5.03 (n.r., 2
H; HQ-H), 5.71, 5.80 (2 × s, 2 × 2 H; 2 × pz-H4), 5.94 (n.r., 2 H;
HQ-H), 6.08, 6.28 (2 × m, 2 × 2 H; 2 × ppy-H), 6.45, 6.67 (2 × m, 2
× 4 H; ppy-H), 6.91, 6.99 (2 × n.r., 2 × 2 H; 2 × ppy-H), 7.25 (bs, 2
H; CH), 7.44, 7.54, 7.75 (3 × m, 3 × 4 H; ppy-H), 8.79, 8.90 (2 × n.r.,
2 × 2 H; 2 × ppy-H), n.o. HQ-H. The two different isomers lead to
two sets of 13C NMR signals with very similar chemical shift values; in
some cases, not all signals are resolved, so that the number of
resonances listed does not equal twice the number of C atoms. 13C
NMR (100.6 MHz, d8-THF): δ 11.3, 11.5 (pz-CH3), 13.8, 14.0 (pz-
CH3), 71.8 (Cpz2), 105.8, 106.3 (pz-C4), 113.3 (HQ-C), 115.2 (HQ-
C), 118.6, 120.3, 120.6, 121.8, 124.3, 124.6 (4 × ppy-C), 127.5 (HQ-
C1), 129.2, 134.1, 135.3, 136.6, 136.8 (3 × ppy-C), 140.9, 141.5 (pz-
C3,5), 148.2, 149.1, 149.9, 153.0 (4 × ppy-C), 169.1, 169.6 (HQ-
C3,4), n.o. HQ-C, ppy-C1. UV−vis (CH2Cl2): λmax/nm (ε): 370 (14
900), 451 (8200), 524 (4200). ESI-MS: m/z (%) = 811 [M − Tl]−
(100). Anal. Calcd for C39H34IrN6O2Tl [1015.33]: C 46.13, H 3.38, N
8.28. Found: C 46.14, H 3.21, N 8.49.
16) or with graphite-monochromated Cu Kα radiation (8). In all
cases, repeatedly measured reflections remained stable. Empirical
absorption corrections were performed for 10, 11, 12, 14, 15, and 16
using the MULABS37 option in PLATON.38 Equivalent reflections
were averaged. The structures were solved by direct methods using the
program SHELXS39 and refined with full-matrix least-squares on F2
using the program SHELXL-97.40 If not mentioned otherwise,
hydrogen atoms were placed on ideal positions and refined with
fixed isotropic displacement parameters using a riding model.
In 7, the H atoms bonded to O were freely refined. For 10, the
absolute structure was determined by refining the Flack parameter to a
value of −0.02(2); the C−C−O−H torsion angles were refined. In 12,
the displacement ellipsoids of the atoms N12A and C16A and of the
three EtCN molecules were restrained to an isotropic behavior. In 14,
one of the THF molecules was refined with the O−C distances
restrained to 1.40(1) Å and the C−C distances restrained to 1.50(1)
Å; angle distances in both THF molecules were restrained to 2.3(1) Å.
In 15, one of the tBu groups is disordered over two positions with a
site occupation factor of 0.57(1) for the major occupied site. In 16,
one C atom of the THF molecule is disordered over two positions
with a site occupation factor of 0.58(6) for the major occupied site; the
H atom of cocrystallized water was isotropically refined with the O−H
distance restrained to 0.82(1) Å.
CCDC reference numbers: 864576 (7), 864577 (8), 864578 (10),
864579 (11), 864580 (12), 864581 (14), 864582 (15), 864583 (16).
Synthesis of 14. Following the representative procedure outlined
for the synthesis of 11, 7 (100 mg, 0.320 mmol) was treated with
TlOtBu (178 mg, 0.640 mmol) and [(Cp*)IrCl2]2 (128 mg, 0.160
mmol) in THF (2.0 mL) to obtain a brick-red solid. Yield: 179 mg
(88%). Structure determination was performed on a single crystal
grown by slow evaporation of a THF solution of 14. 1H NMR (300.0
MHz, d8-THF): δ 1.88 (s, 15 H; Cp*-CH3), 2.10 (s, 6 H; pz-CH3),
ASSOCIATED CONTENT
■
S
* Supporting Information
NMR spectroscopic data of 6 in C6D6, synthesis and analytical
data of 8, 13, and 15; X-ray crystal structure analysis of 8, 11,
and 15; cyclic voltammetry on 6, 14, and 16. This material is
available free of charge via the Internet at http://pubs.acs.org.
n
2.13 (d, JHH = 0.7 Hz, 6 H; pz-CH3), 5.73 (n.r., 2 H; pz-H4), 6.17
(ddd, 3JHH = 8.3 Hz, 4JHH = 2.0 Hz, 4JHH = 0.7 Hz, 1 H; HQ-H6), 6.36
4
4
3
(dd, JHH = 2.0 Hz, JHH = 0.7 Hz, 1 H; HQ-H2), 6.77 (d, JHH = 8.3
Hz, 1 H; HQ-H5), 7.58 (n.r., 1 H; CH). 13C NMR (75.4 MHz, d8-
THF): δ 10.2 (Cp*-CH3), 12.1, 13.8 (pz-CH3), 75.2 (Cpz2), 85.2
(Cp*), 106.4 (pz-C4), 113.6 (HQ-C2), 114.1 (HQ-C5), 117.0 (HQ-
C6), 127.2 (HQ-C1), 141.2 (pz-C5), 147.5 (pz-C3), 165.0, 165.2
(HQ-C3,4). UV−vis (CH2Cl2): λmax/nm (ε) 287 (13 100), 429 (22
800). ESI-MS: m/z (%) = 678 [M + MeCN + H]+ (100). Anal. Calcd
for C27H33IrN4O2 [637.79]: C 50.85, H 5.22, N 8.78. Found: C 50.79,
H 5.33, N 8.58.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: Matthias.Wagner@chemie.uni-frankfurt.de.
Notes
The authors declare no competing financial interest.
Synthesis of 16. 14 (13 mg, 0.020 mmol) and [(MeCN)2PdCl2] (5
mg, 0.020 mmol) were placed into a flame-dried NMR tube, and d3-
MeCN (0.75 mL) was added. The tube was flame-sealed under
vacuum and heated to 150 °C for 2 min with shaking (heat gun). After
ACKNOWLEDGMENTS
■
This research was supported by the Deutsche Forschungsge-
meinschaft (SFB/TR 49). F.B. is grateful to the Fonds der
Chemischen Industrie (FCI) for a Ph.D. grant. The authors
wish to thank Prof. Dr. A. Schmidt (Technische Universitat
Clausthal, Germany) for helpful discussions on ortho-
benzoquinone trapping reactions and Dr. Dieter Schollmeyer
(Johannes Gutenberg-Universitat Mainz, Germany) for acquir-
ing the X-ray data set of compound 8.
1
that time, H NMR spectroscopy indicated full consumption of the
̈
starting materials with formation of 16 in ca. 90% purity. The NMR
tube was centrifuged and opened, and the solution was evaporated to
dryness under vacuum. The residue was placed into a new NMR tube
and dissolved in THF (1 mL), and the solution was stored at room
temperature overnight without exclusion of air and moisture to obtain
red X-ray quality crystals. Yield of single crystals: 13 mg (73%). To
prepare a sample for the combustion analysis, some crystals were
washed with Et2O and then stored at room temperature under vacuum
̈
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