Coordination Chemistry of the PdmBOX Pincer Ligand: Reactivity at the Metal and the Ligand

Article abstract of DOI:10.1002/ejic.201701195

The coordination chemistry of PdmBOX [2,5-bis(2-oxazolinyldimethylmethyl)pyrrole] pincer ligands, in which a dimethylmethylidene linker has been introduced to preclude rearrangements in the backbone, has been studied with 3d and 4d late transition metals. The in situ lithiated PdmBOX ligand was treated with MnCl₂, FeCl₂ and CoCl₂ to give [^iPr(PdmBOX)MCl] (M = Mn: 2; Fe: 3; Co: 4) which were found to adopt a distorted tetrahedral coordination geometry in the crystal. Addition of 2,6-dimethylphenylisocyanide to 3 gave the diamagnetic tris(isocyanide) derivative [^iPr(PdmBOX)Fe(CNxyl)₃]Cl (5) with the chloride as the counteranion. Complexes 2–4 were found to be precatalysts for the enantioselective hydrosilylation and hydroboration of acetophenone. The synthesis of PdmBOX-palladium(II) complexes was achieved by deprotonation of the protioligand with potassium bis(trimethylsilyl)amide (KHMDS) and subsequent reaction with [(cod)PdCl₂] giving [^iPr(PdmBOX)PdCl] (6) or by stirring (S)-^iPr(PdmBOX)H with palladium acetate yielding the palladium acetato complex 7. Treatment of the latter with phenylsilane at 60 °C also led to the slow formation of the metal hydride complex which was the key species in the catalytic hydrosilylation of styrene oxide, albeit with low activity. The coordination of (S)-^iPr(PdmBOX)H to rhodium(I) was accomplished by reaction with (acetylacetonato)dicarbonylrhodium(I) to give the corresponding PdmBOX-Rh complex 8 which reacted with methyl iodide to give the octahedrally coordinated rhodium(III) complex [^iPr(PdmBOX)Rh(CH₃)I(CO)] 9. Reaction of 8 with one equivalent of CsBr₃ led to the full conversion to the dibromorhodium(III) complex [^iPr(PdmBOX)Rh(CO)Br₂] 10. Upon reaction of 10 with N-bromosuccinimide (NBS), the pyrrole backbone in 10 was brominated to give complex 11. In a similar way, a selective conversion of [^iPr(PdmBOX)NiCl] to the backbone-chlorinated [^iPr(^ClPdmBOX)NiCl] (12) was achieved by the treatment with N-chlorosuccinimide.

Full text of DOI:10.1002/ejic.201701195

A Journal of
Accepted Article
Title: Coordination Chemistry of the PdmBOX Pincer Ligand: Reactivity
at the Metal and the Ligand
Authors: Lutz Gade, Jan Wenz, Alexander Kochan, Hubert Wadepohl,
and Vladislav Vasilenko
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201701195
Link to VoR: http://dx.doi.org/10.1002/ejic.201701195

10.1002/ejic.201701195
European Journal of Inorganic Chemistry
FULL PAPER
Coordination Chemistry of the PdmBOX Pincer Ligand: Reactivity
at the Metal and the Ligand
Jan Wenz,[a] Vladislav Vasilenko,[a] Alexander Kochan,[a] Hubert Wadepohl,[a] and Lutz H. Gade[a],*
Dedicated to Professor Mary McPartlin on the occasion of her 85th birthday
Abstract: The coordination chemistry of PdmBOX (2,5-bis(2-oxazolinyldimethylmethyl)pyrrol) pincer ligands, in which
dimethylmethylidene linker has been introduced to preclude rearrangements in the backbone, has been studied with 3d and 4d late
transition metals. The in situ lithiated PdmBox ligand was reacted with MnCl2, FeCl2 and CoCl2 to give [iPr(PdmBox)MCl] (M = Mn: 2;
Fe: 3; Co: 4) which were found to adopt a distorted tetrahedral coordination geometry in the crystal. Addition of 2,6-
dimethylphenylisocyanide to 3 gave the diamagnetic tris(isonitrile) derivative [iPr(PdmBOX)Fe(CNxyl)3]Cl (5) with the chloride as the
counteranion. Complexes 2 - 4 were found to be precatalysts for the enantioselective hydrosilylation and hydroboration of
acetophenone. The synthesis of PdmBOX-palladium(II) complexes was achieved by deprotonation of the protioligand with potassium
bis(trimethylsilyl)amide (KHMDS) and subsequent reaction with [(cod)PdCl2] giving [iPr(PdmBox)PdCl] (6) or by stirring (S)-
iPr(PdmBOX)H with palladium acetate yielding the palladium acetato complex 7. Treatment of the latter with phenylsilane at 60 °C
also led to the slow formation of the metal hydride complex which was the key species in the catalytic hydrosilylation of styrene oxide,
albeit with low activity. The coordination of (S)-iPr(PdmBOX)H to rhodium(I) was accomplished by reaction with
(acetylacetonato)dicarbonylrhodium(I) to give the corresponding PdmBOX-Rh complex 8 which reacted with methyl iodide to give the
octahedrally coordinated rhodium(III) complex [iPr(PdmBOX)Rh(CH3)I(CO)] 9. Reaction of 8 with one equivalent of CsBr3 led to the full
conversion to the dibromorhodium(III) complex [iPr(PdmBOX)Rh(CO)Br2] 10. Upon reaction of 10 with N-bromosuccinimide (NBS), the
pyrrole backbone in 10 was brominated to give complex 11. In a similar way, a selective conversion of [iPr(PdmBox)NiCl] to the
backbone-chlorinated [iPr(ClPdmBox)NiCl] (12) was achieved by the treatment with N-chlorosuccinimide.
a
31 conjugated π-electron system was also found to be essential
1 Introduction
32 for the stability of nickel(I) species, which have for example
33 been used as catalysts for asymmetric hydrodehalogenation
[6d]
2 Tridentate mono-anionic ligands known as “pincers” have
34 reactions.
3 been widely employed as ancillary ligands for metal
35
The
Boxmi
[bis(oxazolinylmethylidene)isoindole]
[1],[2]
4 complexes involved in catalytic processes.
Whereas the
36 system was subsequently developed and can formally be
5 central formally anionic aryl or hetero-aryl unit acts as an
6 “anchor” in the coordination of the metal, their stability and
7 reactivity can be tuned by subtle modifications of substitution
8 patterns or variation of the donor atoms. Pincer ligand design
37 considered as an extended dehydro-PmBOX pincer
[8]
38 (Scheme 1). Here, the conjugated π-system is already
39 present in the protioligand and is not formed after
40 coordination to a metal center. This ligand proved to be
9 for enantioselective catalysis has been based largely on their
41 effective in
a large number of asymmetric catalytic
[3, 8-9]
10 assembly from rigid heterocycles, and chiral ligating units in
42 transformations.
11 the “wingtip” positions.[3] An early example of this approach
43
The propensity of these NNN-pincers to undergo
44 transformations upon coordination to a transition metal to
45 generate the conjugated π-systems mentioned above
46 strongly influences their reactivity. In order to avoid this, we
47 aimed to "decouple" the central anchor unit from the wingtip
48 donors. To this end, the PdmBOX [2,5-bis(2-
12 has been the "PheBox" ligand, first reported in 1997, which
13 has found extensive use in asymmetric catalysis since
14 then.[4]
15
To facilitate the ligation at the central aryl unit, we
16 developed monoanionic N-donor ligands based on the
17 combination of two oxazoline rings and a central pyrrole unit
18 a decade ago.[5] The (2,5-bis(oxazolinylmethyl)pyrrole) ligand
49 oxazolinyldimethylmethyl)pyrrole]
ligands
have
been
50 developed very recently in which a dimethylmethylidene
19 (PmBOX) system is able to stabilize mononuclear chiral C2-
5
1 linker, and thus the presence of a quaternary carbon atom,
52 prevents rearrangements in the backbone. We initially
53 employed these new pincers in the synthesis of nickel
54 compounds, including stable nickel(II) hydride species, for
55 which this structural modification suppressed the previously
20 symmetric complexes with a molecular geometry (Scheme 1).
21 The coordination chemistry of the PmBOX ligands with
22 transition metals such as nickel[6], palladium[5b] and
23 rhodium[7] has been studied in detail, and it has been found
24 that a characteristic feature is their tendency to form a
56 observed tendency to undergo redox-transformations to T-
[10]
25 delocalized π-system. This may either occur via a 1,3-
26 hydrogen shift to give the iso-PmBOX ligands or under
27 oxidative conditions to give the dehydro-PmBOX derivatives
28 (Scheme 1). In both cases, the relatively flexible PmBOX
29 pincers are transformed into rigid tridentate ligands enforcing
30 meridional coordination within the plane. The formation of the
57 shaped nickel(I) complexes.
We have demonstrated that
58 these (PdmBOX)Ni(II)-hydrides cleave strained C-O bonds of
59 epoxides, giving rise to the first nickel-catalyzed,
60 regioselective hydrosilylation of styrene oxide derivatives to
[11]
61 form the corresponding primary alcohols.
This catalytic
62 transformation could be realized exclusively with the newly
63 developed spectator ligand since the competition of one-
64 electron redox steps and the resulting radical transformations
65 were found to be completely suppressed.
[a]
Anorganisch-Chemisches Institut, Universität Heidelberg,
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
E-mail: lutz.gade@uni-heidelberg.de
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/ejic.201701195
European Journal of Inorganic Chemistry
FULL PAPER
27 pentane at −20 °C, producing single crystals suitable for X-
28 ray diffraction with the diethyl ether ligand replaced by a THF
29 molecule. The molecular structure along with the most
30 important structural parameters are depicted in Figure 1. The
31 coordination geometry around the lithium central atom is a
N
O
O
N
R
M
X
N
R
Oxidative Addition
Isomerization
Oxidation
PmBOX
32 consequence of the steric repulsion of the ligand substituents
33 and can be best described as a distorted tetrahedral. While
34 1-THF shows no symmetry elements in the solid-state, an
M =
Rh, Ni, Pd
N
N
N
I
O
O
O
O
O
O
35 effective C2 symmetry on the NMR timescale was found in
N
R
N
R
M
X
N
R
N
R
M
X
N
R
N
Me
Rh
CO
36 solution. All attempts to freeze out the intramolecular
37 dynamics by means of low-temperature NMR spectroscopy
38 remained unsuccessful. In most subsequent applications to
39 transition metal chemistry the ligand transfer reagents 1-
40 Et2O or 1-THF have been generated in situ and then reacted
41 with the metal complex precursors.
R
iso-PmBOX
dehydro-PmBOX
PmBOX
This Work
N
N
O
O
O
O
N
R
M
X
N
R
N
R
M
X
N
R
Boxmi
PdmBOX
1
2
3
Scheme 1. Overview of coordination chemistry involving the PmBox system
and the development of the PdmBOX ligand.
4 Given the interesting reactive behavior of the PdmBox nickel
5 complexes, we were interested in studying the coordination
6 chemistry of these new pincers more systematically and with
7 other transition metals. Additionally, first applications in
8 enantioselective catalytic transformations were investigated.
9 A first account of these studies will be presented in this work
10 along with an assessment of potential future developments.42
43 Figure 1. Molecular structure of complex 1-THF. Hydrogen atoms are omitted
44 for clarity; displacement ellipsoids displayed at 50% probability. Selected bond
45 lengths [Å] and angles [°]: Li-O(3) 1.988(3), Li-N(1) 2.100(3), Li-N(2) 1.952(3),
46 Li-N(3) 2.033(3), N(1)-Li-O(3) 110.64(13), N(2)-Li-O(3) 122.50(15), N(3)-Li-
47 O(3) 100.39(13), N(1)-Li-N(2) 96.39(12), N(1)-Li-N(3) 135.37(16), N(2)-Li-N(3)
48 92.90(12).
11 Results and Discussion
12 Synthesis and Structural Characterization of the
13 PdmBOX -Lithium Complex. The reaction of a diethyl ether
14 solution of the protioligand (S)-iPr(PdmBOX)H with 1.0 equ4iv9 Coordination Chemistry with Late 3d Metals. Previous
50 work in our group has shown the potential of NNN pincers as
15 of lithium diisopropylamide (LDA) at ambient temperature
51 ancillary ligands in earth-abundant 3d metal catalysts for
16 gave a clean conversion to the lithium compound 1-Et2O
17 (Scheme 2), yielding the reaction product as a colorless solid
9c, 9d]
52 enantioselective synthesis.[9a,
Accordingly, we were
1
53 interested in the properties of related catalysts, in which the
54 ligand had enough flexibility to adapt to the favored geometry
55 of the metal center. The close structural resemblance of the
56 Boxmi, PmBOX and PdmBOX ligands facilitates a systematic
57 study of such steric and electronic effects on catalyst activity
58 and some insight towards a systematic catalyst development.
18 in almost quantitative yield. Its elemental analysis and the H
19 NMR spectrum were consistent with the diethyl ether adduct
20 of 1-Et O, whereas lithium incorporation was confirmed by
2
7
21 observation of Li NMR resonance with a chemical shift of
22 1.14 ppm.
59 Synthesis and Structural Characterization of Manganese,
60 Iron, and Cobalt Complexes. We employed the standard
61 transmetallation approach for the synthesis of the 3d metal
62 complexes (Scheme 3): The in situ lithiated PdmBOX ligand
63 was first generated by addition of an LDA solution to a
64 solution of the ligand at room temperature in THF and the
65 resulting mixture was added to suspensions of the metal
66 precursors (MnCl2, FeCl2, CoCl2), producing a colourless,
67 yellow and a purple solution, respectively. Interestingly,
68 whereas the iron and cobalt salts reacted instantly and with
23
24 Scheme 2. Synthesis of PdmBOX lithium complex (1-Et2O).
25
The isolated [iPr(PdmBOX)Li(Et2O)] (1-Et2O) was
26 recrystallized from a mixture of tetrahydrofuran and n-
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10.1002/ejic.201701195
European Journal of Inorganic Chemistry
FULL PAPER
1 quantitative conversion, prolonged refluxing was necessary
2 for manganese to achieve comparable results. The products
3 could be isolated in good to excellent yields after exchanging
4 the solvent to toluene and filtering over a pad of Celite® to
5 remove the lithium salts generated in the process.
6
44
7
8
Scheme 3. Synthetic protocol for the preparation of PdmBOX manganese(II) 2,
iron(II) 3 and cobalt(II) 4 complexes.
9
Elemental analysis of the products showed no
10 indication of the presence of additional solvent molecules in
11 the coordination sphere of the metals. X-ray quality single
12 crystals of the iron and cobalt compounds could be grown by
13 layering toluene solutions with pentane at −40°C, and the
14 molecular structures observed therein support
a
15 tetracoordinated metal center (Figure 2). The geometries in
45
16 both complexes are structurally closely related, and similar to
17 the previously described lithium complex, adopting
a
46 Figure 2. Molecular structure of complexes 3 (top) and 4 (bottom). Hydrogen
47 atoms have been omitted for clarity; displacement ellipsoids displayed at 50%
48 probability. Selected bond lengths [Å] and angles [°] for 3: Fe-Cl 2.2579(12),
18 distorted tetrahedral coordination (for bond angles see
19 caption the in Figure 2). The distortion is also clear4ly9 Fe-N(1) 2.0937(18), Fe-N(2) 2.0022(17), Fe-N(3) 2.0936(18), N(1)-Fe-Cl
50 104.27(5), N(2)-Fe-Cl 126.35(5), N(3)-Fe-Cl 110.84(5), N(1)-Fe-N(2) 92.63(7),
20 manifested by the strong deviation between the directionality
21 of the pyrrole nitrogen metal bond and the plane of the
51 N(1)-Fe-N(3) 132.70(7), N(2)-Fe-N(3) 91.27(8); for 4: Co-Cl 2.2491(6), Co-
22 pyrrole cycle, which is in stark contrast to the previous5ly2 N(1) 2.0248(19), Co-N(2) 1.9548(17), Co-N(3) 2.0437(19), N(1)-Co-Cl
53 103.99(6), N(2)-Co-Cl 131.58(6), N(3)-Co-Cl 112.57(6), N(1)-Co-N(2) 92.61(7),
23 reported nickel complexes.[10-11]
As expected for an NN5N4 N(1)-Co-N(3) 123.05(8), N(2)-Co-N(3) 94.04(8).
24 pincer, the monoanionic pyrrole nitrogen donor features a
25 significantly shorter N-M distance than the corresponding
55
The conversion of paramagnetic high-spin iron(II)
26 neutral oxazoline nitrogens (2.00 Å vs. 2.09 Å and 1.96 Å vs.
27 2.03 Å for Fe (3) and Co (4), respectively), establishing the
28 basis for a robust pincer framework. The bond distances are
56 complexes to the corresponding diamagnetic low-spin
57 compounds by addition of strong π-accepting donors such
58 as carbonyls, phosphines or nitriles has been the focus of
29 all within the expected range for 3d metal complexes of th5is9 recent studies with PNP pincers.[13-14] In the light of this work,
[9a,
30 type and similar to previously reported Boxmi structures. 60 we attempted to synthesize the analogous low-spin NNN
9d]
61 pincer complexes (Scheme 4). The addition of an isonitrile
62 did not afford the neutral bis(isonitrile) chlorido complex, but
63 led to the selective formation of the diamagnetic
64 tris(isonitrile) derivative with a chloride as the counteranion,
65 [iPr(PdmBOX)Fe(CNxyl)3]Cl (5) (xyl = 2,6-dimethylphenyl),
66 regardless of the chosen stoichiometry. This observation
67 confirms that the presence of P donor moieties in the pincer
68 framework is no prerequisite for stable diamagnetic iron(II)
69 complexes.
31
32
Whereas the manganese and cobalt complexes gave
33 rise to very poorly resolved paramagnetic NMR spectra,
34 relatively sharp signals were observed in both proton and
35 carbon NMR measurements of the iron compound . It should
36 be noted that only a single set of resonances was detected
37 for the diastereotopic methyl carbons, suggesting that a
38 dynamic configurational exchange is present in solution.
39 Accordingly, computational modeling of the chemical shifts
40 by means of DFT did not lead to a sufficient correlation
41 between computed and measured data to assign signals to
42 the corresponding positions.[12]
43
70
71 Scheme 4. Synthesis of diamagnetic 5 via coordination of three equivalents of
72 the isonitrile CNxyl (xyl = 2,6-dimethylphenyl).
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10.1002/ejic.201701195
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1 Application in Ketone Hydroelementation Catalysis. In3a3 hydrolementation reactions is currently under investigation in
2 first assessment of the potential of the PdmBOX framewor3k4 our laboratory.
3 as an ancillary ligand in 3d metal catalysis, we investigated
4 the activity of the newly synthesized complexes in standard
5 benchmark reactions, the reduction of ketones with boranes
6 and silanes (Table 1).[14-16]
35 Synthesis and Structural Characterization of Palladium
36 and Rhodium Complexes. Much of pincer complex
37 chemistry has focused on the heavier transition metals and
38 has given rise to varied and interesting patterns of reactivity.
39 In this study of the coordination chemistry of the PdmBOX
40 system we therefore investigated the complexation of second
41 row transition metals to probe some aspects of their
42 reactivity.
7
8
Table 1. Catalytic hydrolementation of acetophenone catalyzed by Mn, Fe and
Co complexes.
43 Synthesis and Structural Characterization of PdmBox -
44 Palladium(II) Complexes. The synthesis of palladium(II)
45 complexes bearing the PdmBOX pincer ligand was achieved
46 by two methods. Deprotonation of the protioligand (S)-
47 iPr(PdmBOX)H with potassium bis(trimethylsilyl)amide
ee% 48 (KHMDS) and subsequent reaction with [(cod)PdCl2] gave
30 (S)
9
[a]
#
1
2
3
4
M
Lig
iPr
iPr
iPr
Ph
Ph
Ph
iPr
iPr
iPr
Ph
Ph
Ph
HE
conv. %[a]
>95
>95
>95
>95
>95
>95
>95
28
Mn
Fe
Co
Mn
Fe
Co
Mn
Fe
Co
Mn
Fe
Co
HBPin
HBPin
HBPin
HBPin
HBPin
HBPin
49 the
corresponding
palladiumchlorido
complex
iPr
30 (S)
50 [ (PdmBOX)PdCl] (6), albeit contaminated with impurities
0
51 which had to be removed by column chromatography. On the
11 (R)
52 other hand, the preparation of the palladium acetato complex
5
6
5 (R)53 7 was achieved simply by stirring (S)-iPr(PdmBOX)H with
22 (S55) 54 pcoarllraedsipuomnding nickel complex is not accessible in the similar
acetate in dichloromethane. While the
7
8
9
10
11
12
(EtO)2SiMeH
(EtO)2SiMeH
(EtO)2SiMeH
(EtO)2SiMeH
(EtO)2SiMeH
(EtO)2SiMeH
9 (R)
0
56 was way, [(PdmBOX)PdOAc] is stable under acetic
40 (R5) 7 conditions and was isolated as a yellow oil in about 75%
19
>95
80
50 (R5) 8 yield (Scheme 5).
56 (R)
60 (S)
90
[a] Determined by HPLC. Reaction conditions: 1 h at room temperature
(hydroboration), 24 h at room temperature (hydrosilylation). For
manganese, Mn(CH2TMS)2 was used as the precursor. For iron and cobalt
the corresponding tmeda adduct (tmeda)M(CH2TMS)2 was employed.
10
Two approaches were chosen to generate an active
11 catalyst, (i) the alkylation of the metal chloride with
59
12 methyltrimethylsilyl lithium or (ii) the direct generation of the
13 alkyl complex starting from a suitable alkyl precursor and the
14 protioligand. We have shown previously, that both strategies
15 produce identical results.[9a] However, the latter approach
16 allows for much more flexibility in screenings and was
17 therefore typically employed in this work. All metal
18 complexes were found to be active catalysts for the
19 hydroboration of acetophenone. Employing pinacolborane
20 (HBPin) as the reducing agent, full conversion was observed
21 for all metals within less than 1 h. Despite the high activity,
22 all catalysts achieved only a poor enantiodiscrimination (up
23 to 30 %ee). Interestingly, an inversion of the favored product
24 configuration is observed for M = Mn, Fe when changing the
25 oxazoline substituent from R = iPr to Ph. This is reminiscent
26 of our results for manganese Boxmi catalysts, where a
27 similar inversion was observed due to a change of the
28 dominant mechanism. As in the latter, the hydrosilylation of
29 acetophenone proceeds much slower (24 h vs. 1 h), albeit
30 with improved enantiodiscrimination (up to 60 %ee). Whether
31 the reduced activity is due to a less efficient catalyst
32 activation or a variation of the operative mechanism in these
60 Scheme 5. Synthesis of PdmBOX Palladium(II) Complexes.
61 To establish the structural details of 6, a single crystal X-ray
62 structure analysis was carried out. Its molecular structure is
63 depicted in Figure 3 along with the principal bond lengths
64 and angles. The coordination geometry at the palladium
65 center is almost ideally square planar (N(1)–Pd–N(2) and
66 N(2)–Pd–Cl approx. 174°). The distances of the three
67 nitrogen donors of the meridionally coordinating ligand to the
68 central atom are determined by the size of the metal center.
69 Compared to nickel, a slight expansion of the chelate
70 structure is observed in case of the larger palladium atom.
71 The Pd–N bonds involving the two oxazoline units (Pd–
72 N(1)/N(3) approx. 2.01 Å), are slightly longer than Pd–N(2) of
73 the central pyrrolato unit (approx. 1.98 Å). The Pd-Cl bond is
[5b, 17]
74 in a typical range for similar palladium chlorides.
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10.1002/ejic.201701195
European Journal of Inorganic Chemistry
FULL PAPER
50
51 Scheme 6. Catalytic hydrosilylation of styrene oxide using [(PdmBOX)PdOAc]
52 7 as precatalyst.
1
2
3
4
5
6
7
8
9
Figure 3. Molecular structure of complex 6 (only one of the two independent
molecules is shown). Hydrogen atoms are omitted for clarity; displaceme5nt3
ellipsoids displayed at 50% probability. Selected bond lengths [Å] and angles
Cl 2.3174(11) [2.3137(12)], Pd–N(1) 2.006(3) [2.007(3)], Pd–N(2) 1.977(
5
[°]. Values in square brackets refer to the second independent molecule: Pd2–)4 Scheme 7. Synthesis of [iPr(PdmBOX)Rh(CO)] 8.
[1.980(3)], Pd–N(3) 2.012(3) [2.016(3)], N(1)–Pd–Cl 93.05(8) [91.70(9)], N(3)–
Pd–Cl 92.63(8) [93.10(8)], N(2)–Pd–N(1) 86.72(11) [87.15(11)], N(2)-Pd-Cl
177.83(7) [175.93(8)], N(3)–Pd–N(1) 173.86(11) [174.51(11)], helix 36.49(52)5 Synthesis and Structural Characterization of PdmBOX -
10 [38.88(6)].
56 Rhodium(I) Carbonyl Complexes. The coordination of the
57 bis(2-oxazolinyldimethylmethyl)pyrrole (S)-iPr(PdmBOX)H to
11
Transition metal hydride complexes often function as
58 rhodium(I) was readily accomplished by stirring the
12 key intermediates in organometallic chemistry, especial5ly9 protioligand with (acetylacetonato)dicarbonylrhodium(I) in
13 related to homogeneous catalysis. The reaction of
resulting
60 chloroform
at
room
temperature.
The
14 [iPr(PdmBOX)PdCl] 6 with commercially available hydride
61 rhodium(I)carbonyl complex (8) could be isolated in almost
15 transfer reagents, such as LiHBEt3, NaHB(OMe)3, LiAlH4 or
62 quantitative yield as a yellow solid (Scheme 7). Complex 8 is
1
16 NaBH , led to the formation of a dark precipitate and a slo6w3 air sensitive in solution and in the solid state and the H and
4
18 monitored by H NMR spectroscopy, the slow formation of6a4 13C NMR spectral patterns are consistent with a C2-
17 decomposition of the ligand. When the reaction was
1
65 symmetric molecular species. The characteristic signal for
19 singlet resonance at δ = −16 ppm was observed. This is in6a6 the carbonyl ligand was observed as a doublet at δ = 191.4
20 typical range for palladium hydrides.[17c,
18]
67 ppm (d, JRh,CO = 70.4 Hz) in the 13C NMR spectrum. The
However, all
1
-1
21 efforts to obtain the desired PdmBOX palladium hydrido
68 carbonyl band appears at 1940 cm in the IR spectrum
22 complex directly from the chloride precursor resulted in 6a9 which is within the range established previously for related
23 very low yield (below 10%) and the formation of large
70 systems. A thermal decomposition was not observed when a
24 amounts undefined byproducts. The treatment of the
71 solution of 8 in toluene heated to 100 °C for several days.
25 palladium acetato complex with phenylsilane at 60 °C led
26 also to the slow formation of the metal hydride complex. This
27 reaction proceeds slowly and allowed only the in situ
28 generation of the desired PdmBox palladium hydrido
29 complex.
30
Nevertheless, the reactivity of the carboxylate complex
31 7 was tested in the hydrosilylation of epoxides using styrene
32 oxide as the substrate. After 16 hours at 60 °C the complete
33 conversion of the starting material had occurred. In contrast
34 to the nickel catalyzed hydrosilylation we have presented
35 recently, this reaction proceeded unselectively. [11] The main
36 product was the silyl-protected primary alcohol, which was
37 formed in about 40%. The use of deuterated phenylsilane led
72
38 to the selective deuterium incorporation in the benzyl7ic3 Figure 4. Molecular structure of complex 8 (only one of the six independent
39 position of the product silyl ether, which resulted after
40 alkaline workup the β-monodeuterated alcohol (Scheme 6).74 molecules is shown). Hydrogen atoms are omitted for clarity; displacement
75 ellipsoids displayed at 50% probability. Selected bond lengths [Å] and angles
1
76 [°]. Minimum and maximum from six independent molecules. Rh-N(1)
41
Upon monitoring the catalytic transformation by
H
77 2.032(6) ... 2.047(7), Rh-N(2) 2.037(7) ... 2.060(6), Rh-N(3) 2.039(7) ...
78 2.056(7), Rh-C(23) 1.802(10) ... 1.827(9), N(1)-Rh-N(2) 85.8(3) ... 87.1(3),
79 N(3)-Rh-N(1) 171.5(3) ... 173.5(2), N(3)-Rh-N(2) 85.3(3) ... 87.3(2), C(23)-Rh-
42 NMR, the palladium hydrido complex was identified as the
43 resting state, while the presence of palladium alkoxido
44 species was not observed. In addition, the unreacte8d0 N(1) 92.4(3) ... 93.8(3), C(23)-Rh-N(2) 176.8(4) ... 179.2(4).
45 carboxylato precatalyst was present throughout entire
81 The details of the molecular structure of the rhodium (I)
46 reaction. Since the latter was transformed very slowly to the
82 complex 8 were established by X-ray diffraction. The
47 catalytically active hydride, high catalyst loadings of about 20
83 structure in the solid state is depicted in Figure 4 along with
48 mol% were necessary for the complete conversion of the
84 selected bond lengths and angles. The gross structural
49 starting material.
85 features of all d8 configurated metal complexes are similar.
86 The four ligating atoms, N(1)-N(3) and C(23) (CO) adopt an
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10.1002/ejic.201701195
European Journal of Inorganic Chemistry
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1 almost ideal square planar arrangement around the metal
2 center. The helical twist [helix = 30.2(6)° (averaged from
3 six independent molecules)] between the coordination plane
4 of the metal center and the plane of the pyrrole ring results
5 from the structural flexibility of the ligand backbone, which
6 allows the donor functions to adapt to the size of the central
39
7 metal atom.
40 Scheme 9. Oxidative Addition of Br2 to Complex [iPr(PdmBox)Rh(CO)] 8.
8 Synthesis of PdmBox Rhodium(III) Complexes by
9 Oxidative Addition. The reaction of 8 with an excess of
10 methyl iodide at ambient temperature gave the octahedral4ly1 The rhodium(III) center is coordinated octahedrally, while the
42 carbonyl ligand is still in trans position to N(2) and spans a
11 coordinated rhodium(III) complex [iPr(PdmBox)Rh(CH3)I(CO)]
43 plane with the three nitrogen atoms of the meridional
12 9 in quantitative yield (Scheme 9). Its elemental analysis 4is4 coordinating ligand. The angles between the trans-donor
13 consistent with a product of an oxidative addition, and 4a5 atoms (N(2)-Rh-CO
14 characteristic shift to higher wavenumber of the ν(CO) band
=
178.42(11)°, N(3)-Rh-N(1)
=
46 175.87(9)° and C(23)-Rh-I(1) = 175.80(8)°) are also close to
47 the ideal value for an octahedral coordination of 180°,
48 respectively. The Rh-N distances of tridentate ligand are
15 in the infrared spectrum is observed due to the weaker
16 π-backbonding of the carbonyl ligand to the metal center in
III
I
-1
17 the rhodium(III) compound [Rh 9: 2048, Rh 8: 1940 cm ]. 49 close to those found for to the rhodium(I) carbonyl complex 8
50 which indicates that the oxidation state of the metal seems to
51 have a less significant impact on this parameter than the
52 structure of the meridionally coordinating pincer ligand. The
53 bond length of the rhodium center to the carbonyl ligand is in
54 the same range compared to the Rhodium(I) complex 8
55 (RhIII-CO = 1.883(3) Å 9, RhI-CO = 1.802(10) ... 1.827(9) Å
56 8).
18
19 Scheme 8. Oxidative Addition of CH3I to Complex [iPr(PdmBox)Rh(CO)] 8.
57 The reaction of complex 8 with one equivalent of CsBr3 as a
58 mild brominating reagent in toluene at room temperature led
20 The presence of two different axial ligands after oxidative
21 addition (I, CH3) removes the effective C2 symmetry and
22 renders all NMR nuclei chemically inequivalent in the product.
23 The resonances of the metal-bonded methyl group and the
24 carbonyl ligand in trans disposition of the central pyrrolato
25 unit are observed at δ (13CH3) = 9.4 ppm (d, 1JRh,C = 19.8 Hz)
59 to the full conversion of the rhodium(I) species 8 to the
iPr
60 dibromo rhodium(III) complex [ (PdmBox)Rh(CO)Br2] 10. In
61 this transformation, the use of an excess of cesium
62 tribromide led to the formation of a by-product, which gave
63 rise to an almost complete set of ligand signals for a C2
1
64 symmetrical species in the H NMR spectrum. However, the
1
26 and δ (13CO) = 189.3 (d, JRh,C = 56.0 Hz), respectively.
65 characteristic pyrrole signal at δ ≈ 6 ppm was not observed.
66 Mass spectroscopic investigations showed that besides the
67 oxidative addition at the metal center bromination of the
27 Which one of the three possible stereoisomers of 9 was
28 formed was not apparent from the NMR data but was
29 established by X-ray diffraction. The molecular structure of698 pyrrole building block had occurred.
30 is depicted in Figure 5.
69
Selective access to the fully brominated product 11
70 could not be achieved by the use of an excess of CsBr3. The
71 oxidative addition of the metal center preferably proceeds
72 and 11 is obtained as a by-product in about 10 % yield. In its
73 formation, the pyrrole moiety is brominated without further
74 degradation. This aspect was investigated in more detail in a
75 final part of this study presented below.
31
76
32 Figure 5. Molecular structure of complex 9. Hydrogen atoms are omitted for
33 clarity; displacement ellipsoids displayed at 50% probability. Selected bond
34 lengths [Å] and angles [°]: Rh-I(1) 2.8570(9), Rh-N(1) 2.055(2), Rh-N(2)
35 2.060(2), Rh-N(3) 2.043(2), Rh-C(23) 2.077(3), Rh-C(24) 1.883(3), O(3)-C(24)
36 1.120(3), N(1)-Rh-I(1) 98.85(6), N(1)-Rh-N(2) 89.97(9), N(2)-Rh-C(24)
37 178.42(11), N(2)-Rh-I(1) 95.56(7), N(2)-Rh-C(23) 87.09(10),
77 Scheme 10. Bromination of [iPr(PdmBox)Rh(CO)Br2] 10 with NBS.
N(3)-Rh-I(71)8 Bromination of the Pyrrole Backbone in the PdmBOX
79 Ligand. All attempts to generate the
38 85.25(6), N(3)-Rh-N(1) 175.87(9), C(23)-Rh-I(1) 175.80(8).
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10.1002/ejic.201701195
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1 [iPr(BrPdmBOX)RhBr2(CO)] (11) by the reaction o3f9 All attempts to halogenate the ligand framework selectively,
2 [iPr(PdmBOX)Rh(CO)] (8) or [iPr(PdmBOX)Rh(CO)Br2] (104)0 while leaving the metal center in [iPr(PdmBox)Rh(CO)] (8)
3 with elemental bromine resulted in
a
nonselectiv4e1 unmodified, remained unsuccessful. The use of other
4 decomposition. However, upon using the milder brominatin4g2 brominating agents resulted in an unselective reaction or the
5 agent N-bromosuccinimide (NBS), the Rh(III) complex 1403 decomposition of the reaction mixture. However, on
6 could be transformed completely to the desired product 1414 performing such halogenations on complexes with metal
7 within minutes (Scheme 10). The reaction product wa4s5 centers which were more difficult to oxidize than rhodium(I)
8 isolated as a red air-stable compound in almost quantitativ4e6 the selective substitution of the pyrrole backbone proved to
9 yield.
47 be possible as shown for the NiII compounds in the following
48 section.
10 The structural details of the Rh(III) product 11 were
11 established by single crystal X-ray structure analysis. Its
12 molecular structure is depicted in Figure 6 along with
13 principal bond lengths and angles. The molecular geometry
14 of complex is almost ideally octahedral and the significant
15 structural parameters are very similar to those found for the
16 already mentioned [iPr(PdmBOX)RhI(CO)Me] complex (9).
17 Thus, the distance of the carbonyl ligand to the central atom
18 is elongated with RhIIICO = 1.915(3) Å, while the bond length
19 to the central pyrrole unit is not strongly influenced (Rh-N(2)
20 = 2.053(2) Å) as compared to the rhodium(I) compound 8.
21
49
50 Scheme 11. Electrophilic Aromatic Substitution of [iPr(PdmBOX)NiCl] with NCS.
22 Similar to complex 9, the carbonyl infrared stretching ban5d1 Chlorination of the PdmBOX Ligand in its Ni(II)
23 was observed at considerably higher wavenumbers for th5e2 complexes. The PdmBOX nickel(II) chlorido complexes we
24 Rh(III) products 10 and 11 than that of the starting material583 reported previously were found to be stable towards air,
25 (ν(CO) = 2083 (10) and 2085 (11) compared to 1935 cm-1 fo5r4 moisture and elevated temperatures. In addition, their metal-
26 the latter). The bromination of the ligand backbone has littl5e5 centered oxidation was thought to be highly unlikely. A
27 influence on the spectroscopic data of the CO ligand. Th5e6 selective
conversion
of
[iPr(PdmBOX)NiCl]
to
28 infrared bands as well as chemical shifts in the 13C NM5R7 [iPr(ClPdmBOX)NiCl] complex (12) was achieved by the
29 spectra are nearly identical δ = 183.2 ppm for 10 an5d8 treatment with two equivalents N-chlorosuccinimide (NCS)
30 δ = 175.8 ppm for 11 and the coupling constants are bot5h9 (Scheme 11). By slowly cooling a solution of 12 in diethyl
1
31 JRh,CO = 48.3 Hz.
60 ether, single crystals were obtained which were suitable for
61 X-ray structure analysis. The molecular structure is depicted
62 in Figure 7 and a comparative listing of the principal bond
63 lengths and angles of [iPr(PdmBOX)NiCl] and 12 is provided
64 in Table 2.
32
65
33 Figure 6. Molecular structure of complex 11. Hydrogen atoms are omitted for
34 clarity; displacement ellipsoids displayed at 50% probability. Selected bond
35 lengths [Å] and angles [°]: Rh-C(23) 1.915(3), Rh-N(3) 2.033(2), Rh-N(1)
36 2.039(3), Rh-N(2) 2.053(2), Rh-Br(1) 2.4718(4), Rh-Br(2) 2.4906(4), Br(3)-C(6)
37 1.888(3), Br(4)-C(7) 1.888(3), N(3)-Rh-N(1) 178.13(10), C(23)-Rh-N(2)
38 179.31(13), Br(1)-Rh-Br(2) 177.903(15).
66 Figure 7. Molecular structure of complex 12. Hydrogen atoms are omitted for
67 clarity; displacement ellipsoids displayed at 50% probability.
68
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10.1002/ejic.201701195
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37 GmbH and dried over Granusic© phosphorus pentoxide granulate)
38 using standard Schlenk techniques or by working in a glove box. The
39 solvents were dried over sodium (toluene), potassium (hexane) or
40 sodium/potassium alloy (pentane, diethyl ether), distilled and
41 degassed prior to their use.[20] Deuterated solvents were purchased
a42 from Deutero GmbH, dried over potassium (C6D6), vacuum distilled,
(PdmBox)NiCl] 43 degassed and stored in Teflon valve ampoules under argon. The
Table 2. Comparison of selected bond lengths [Å] and angles [°] of
compounds 12 and [iPr(PdmBOX)NiCl].
iPr(XPdmBOX)NiCl]
X = Cl (12)
X = H [iPr
44 2,5-bis(oxazolinyldimethylmethyl)pyrrole protioligand (iPrPdmBOX)H
45 as well as the complex [(iPrPdmBox)NiCl] were synthesized
46 according to literature procedures.[10] All other reagents were
47 obtained from commercial sources and were used as received
48 unless explicitly stated otherwise. Air-sensitive samples for NMR
49 spectroscopy were prepared under argon in 5 mm Wilmad tubes
50 equipped with J. Young Teflon valves. 1H- and 13C-NMR spectra
51 were recorded on a Bruker Avance (200 MHz), a Bruker Avance II
52 (400 MHz) and a Bruker Avance III (600 MHz, equipped with a
53 CryoProbeTM) NMR spectrometers and were referenced internally
54 using the residual protio solvent (1H) or solvent (13C).[21] The
55 appearance of the signals was described using the following
56 abbreviations: s (singlet), d (doublet), dd (doublet of doublet), ddd
57 (doublet of doublet of doublet), dt (doublet of triplet), t (triplet), q
58 (quartet), m (multiplet), b (broad signal). Elemental analyses were
59 recorded by the analytical service of the Heidelberg Chemistry
60 Department using the vario EL and vario MIKRO cube analytical
61 devices. Mass spectra were acquired on Bruker ApexQe hybrid 9.4
62 T FT-IVR (HR-ESI, HR-DART) and JEOL JMS-700 magnetic sector
63 (HR-FAB, HR-EI, LIFDI) spectrometers at the mass spectrometry
64 facility of the Organic Department at the University of Heidelberg.
65 Either 3-nitrobenzyl alcohol (NBA) or o-nitrophenyloctyl ether
66 (NPOE) were used as the matrix in the FAB-MS measurements.
67 X-Ray diffraction analysis was performed at the laboratory for
68 structural analysis of the Inorganic Chemistry Department at the
69 University of Heidelberg. An Agilent Technologies Supernova-E
Ni–Cl
Ni–N(1)
Ni–N(2)
2.1849(8)
1.873(2)
1.878(2)
1.896(3)
90.31(7)
92.02(8)
88.30(10)
175.80(11)
2.1968(8)
1.8953(14)
1.8688(13)
1.8962(14)
90.30(4)
Ni–N(3)
N(1)–Ni–Cl
N(3)–Ni–Cl
N(2)–Ni–N(1)
N(3)–Ni–N(1)
92.37(4)
88.64(6)
176.05(4)
[a] Taken from Ref [10]
iPr Cl
1 The structural comparison of [ ( PdmBOX)NiCl] (12) with
2 the unsubstituted PdmBOX nickel(II) analogon shows also
3 that the coordination at the metal center is independent of
4 the substitution pattern of the pyrrole ring.[10] We have shown
5 that the release of the (PdmBOX)H protioligand from the
6 corresponding nickel(II) chlorido complex could be achieved
7 by the addition of an excess of potassium cyanide. Following
8 this protocol the chlorinated protioligand (ClPdmBOX)H (13)
9 was obtained. An analogous functionalization of the
10 uncoordinated PdmBOX ligands was not possible, which is
11 why the functionalization of the nickel template with
12 subsequent cyanide treatment appears to be essential at th7is0 CCD (Cu-Kα or Mo-Kα X-radiation, microfocus tube, multilayer mirror
13 stage.[19]
71 optics) and a Bruker AXS Smart 1000 CCD diffractometer (Mo (Kα)
72 radiation, graphite monochromator, λ = 0.71073 Å) was used for
73 data acquisition. UV/Vis spectra were recorded on a Varian Cary
74 5000 UV/VIS/NIR spectrometer. All other chemicals were purchased
14 Conclusions
75 from commercial sources and used without purification unless
76 otherwise stated.
77
78
(S)-iPr(PdmBOX)Li(Et2O) (1) The protioligand (S)-iPr(PdmBOX)H
15 The PdmBOX pincer ligands introduced recently by us are in
79 (200 mg, 0.53 mmol) was dissolved in 5 ml Diethylether and then
16 many ways complementary to the previously studied PmBO8X0 metallated at room temperature by the slow addition of
17 and Boxmi pincers which have been highly efficient
81 Lithiumdiisopropylamide (58 mg, 0.54 mmol, 1.01 equiv.). The
18 stereodirecting ligands in
a
wide range of catalytic
82 solution turned from colourless to yellow and was stirred for 15
83 minutes at room temperature. The solvents were removed in vacuo.
84 The residue was washed with n-pentane to give the product as
85 colourless solid in 99% yield. Anal. found: C, 68.48; H, 9.50; N, 9.66.
19 transformations. They display greater flexibility in the
20 coordination to transition metals and may stabilize square
21 planar or octahedral coordination geometries on the one
1
86 Calcd. for C26H44LiN3O3: C, 68.85; H, 9.78; N, 9.26. H NMR (C6D6,
22 hand and (distorted) tetrahedral arrangements, on the othe8r.7 600 MHz, 295 K): δ (ppm) = 6.61 (s, 2H, H1), 3.62 (dd, JH,H
=
3
23
The effective "decoupling" of the three ligating
3
88 9.5 Hz, 8.9 Hz, 2H, H6), 3.54 (dd, JH,H = 8.7 Hz, 6.6 Hz, 2H, H6'),
24 heterocycles due to the dimethylmethylene linker effective8ly9 3.25 (q, 3JH,H = 7.0 Hz, 4H, H10), 3.21 (dt, JH,H = 9.8 Hz, 6.0 Hz, 2H,
3
previous9ly0 H7), 1.98 (s, 6H, H4), 1.97 (s, 6H, H4'), 1.25-1.16 (m, 2H, H8), 1.11 (t,
25 suppresses the intra-ligand rearrangements
91 JH,H = 7.0 Hz, 3H, H11), 0.65 (d, 3JH,H = 2.1 Hz, 6H, H9), 0.64 (d, 3JH,H
92 = 1.8 Hz, 6H, H9'). 13C NMR (C6D6, 150 MHz, 295 K): δ (ppm) =
93 182.2 (C5), 142.6 (C2), 103.0 (C1), 70.2 (C10), 69.0 (C6), 65.9 (C7),
94 40.4 (C3), 32.5 (C4), 28.3 (C8), 27.7 (C4'), 18.2 (C9'), 17.6 (C9),
95 15.6(C11).6Li NMR (C6D6, 155 MHz, 295 K): δ (ppm) = 1.14 (bs).
96
3
26 observed for the PmBOX system. The central pyrrole unit,
27 which remains intact, may be readily functionalized via
28 halogenation of the metal-ligated pincer. In which way the
29 reactivity of the corresponding metal complexes is influenced
30 by the halogenation of the pyrrole building block is of
31 particular interest and the focus of our current and future
97
(S)-iPr(PdmBOX)MnCl (2) To
a solution of the protioligand
98 (S)-iPr(PdmBOX)H (800 mg, 2.14 mmol, 1.0 equiv.) in 20 mL THF
99 was added a solution of LDA (229 mg, 2.14 mmol, 1.0 equiv.,
100 10 mL). The mixture was stirred at room temperature for 30 min and
32 research.
101 was transferred to
a
suspension of MnCl2 (269 mg,
33 Experimental Section
102 2.14 mmol,1.0 equiv.) in 20 mL THF. The reaction was heated to
103 reflux overnight, the solvent was evaporated and the residue was
34
Methods and Instrumentation. All manipulations of air- and
104 resuspended in toluene. After filtration over Celite, the eluent was
105 evaporated, affording the pure product as a colorless solid (690 mg,
106 1.49 mmol, 70 %). Anal. found: C, 56.87; H, 7.30; N,8.92. Calcd. for
35 moisture-sensitive materials were carried out under an inert
36 atmosphere of dry argon (Argon 5.0 purchased from Messer Group
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10.1002/ejic.201701195
European Journal of Inorganic Chemistry
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1
2
3
4
5
6
7
8
9
C22H34ClN3O2Mn: C, 57.08; H, 7.40; N, 9.08. No resolved NM7R1 (hydroboration) or 24 h (hydrosilylation). The resulting boronic ester
spectra could be measured. FAB+-MS found (calcd.): m/z 462.171752 was subjected to hydrolysis by addition onto a short silica column.
(462.1735) for C22H34ClN3O2Mn.
73 The residue was eluted with DCM. The resulting silyl ether was
74 hydrolyzed by addition of a saturated K2CO3/MeOH solution. The
75 mixture was stirred for 30 min, filtered through a short pad of silica
a solution of the protioligand
(S)-iPr(PdmBOX)FeCl (3) To
76 and eluted with DCM. Conversion and enantiomeric excess were
77 determined by HPLC on an Agilent 1200 Series machine (Chiralcel
78 OD-H, T = 20 °C, λ = 210 nm, 1 mL/min, tr (R) =15.2 min, tr (S) =
79 19.9 min).
(S)-iPr(PdmBOX)H (800 mg, 2.14 mmol, 1.0 equiv.) in 20 mL THF
was added a solution of LDA (229 mg, 2.14 mmol, 1.0 equiv.,
10 mL). The mixture was stirred at room temperature for 30 min and
was transferred to
a
suspension of FeCl2
(272 m8g,0
10 2.14 mmol,1.0 equiv.) in 20 mL THF. The reaction was stirred
81
(S)-iPr(PdmBOX)PdCl (6) The protioligand (S)-iPr(PdmBOX)H
82 (600 mg, 1.61 mmol) was dissolved in 20 ml acetonitrile and then
83 metallated at room temperature by the slow addition of KHMDS
11 overnight, the volatiles were removed and the residue was
12 redissolved in toluene. After filtration over Celite, the eluent was
13
evaporated, giving the pure product as a yellow solid (615 m8g,4 (321 mg, 1.61 mmol, 1.0 equiv.). The solution turned from colorless
14 1.33 mmol, 62 %). Anal. found: C, 56.71; H, 7.27; N,8.88. Calcd. for
85 to yellow and was canulated to a suspension of [(cod)PdCl2]
15 C22H34ClN3O2Fe: C, 56.97; H, 7.39; N, 9.06. 1H NMR (Toluene-D8,
600 MHz, 295 K): δ (ppm) = 76.2, 6.2, -4.5, -6.9, -16.0, -17.8, -34.85.6 (482 mg, 1.7 mmol, 1.05 equiv.) in 5 ml acetonitrile. After 5 minutes
16
87 the suspension turned to brown. The mixture was stirred for 2 h at
13
17
C NMR (Toluene-D , 150 MHz, 295 K): δ (ppm) = 826.8, 693.85,8 room temperature. The solvents were removed in vacuo. The
8
18 421.3, 147.8, 88.7, 17.8, -5.5, -15.3, -22.0. FAB+-MS found (calcd.):
89 residue was treated with a toluene/pentane (1:2) mixture and the
19 m/z 463.1683 (463.1674) for C22H34ClN3O2Fe.
90 inorganic metal salts were filtered off. After removing of all volatiles
20
91 the product could be isolated by column chromatography (DCM)
(S)-iPr(PdmBOX)CoCl (4) To
a solution of the protioligand
21
92 followed by crystallization from pentane at -20°C to give 6 as a
93 yellow solid (43%). Anal. found: C, 51.28; H, 6.51; N, 8.04. Calcd. for
94 C22H34ClN3O2Pd: C, 51.37; H, 6.66; N, 8.17. 1H-NMR (C6D6, 600
95 MHz, 295 K): δ (ppm) = 6.16 (bs, 2H, H1), 4.72-4.67 (m, 2H, H7),
97 9.6 Hz, 2H, H6'), 2.79-2.71 (m, 2H, H8), 2.08 (s, 6H, H4), 1.60 (s, 6H,
22 (S)-iPr(PdmBOX)H (800 mg, 2.14 mmol, 1.0 equiv.) in 20 mL THF
23 was added a solution of LDA (229 mg, 2.14 mmol, 1.0 equiv.,
24 10 mL). The mixture was stirred at room temperature for 30 min and
25 was transferred to
a
suspension of CoCl2
(278 m9g,6 3.64 (dd, JH,H = 5.7 Hz, 9.1 Hz, 2H, H6), 3.47 (dd, JH,H = 9.6 Hz,
3
3
26 2.14 mmol,1.0 equiv.) in 20 mL THF. The reaction was heated to
27 reflux overnight, the solvent was evaporated and the residue was
28 resuspended in toluene. After filtration over Celite, the eluent was
3
3
98 H4'), 0.60 (d, JH,H = 7.3 Hz, 6H, H9), 0.58 (d, JH,H = 7.0 Hz, 6 H,
evaporated, affording the pure product as a purple solid (900 m9g,9 H9').13C-NMR (C6D6, 150 MHz, 295 K): δ (ppm) = 175.7 (C5), 135.91
29
100 (C2), 102.5 (C1), 69.6 (C6), 68.1 (C7), 39.8 (C3), 34.1 (C4), 30.6 (C8),
30 1.93 mmol, 90 %). Anal. found: C, 56.97; H, 7.32; N, 9.14. Calcd. for
101 22.5 (C4'), 18.2 (C9'), 14.5 (C9). FAB+-MS found (calcd.): m/z
31
C H ClN O Co: C, 56.59; H, 7.34; N, 9.00. No resolved N1M0R2 514.1373 (513.1374) for C22H34ClN3O2Pd.
22 34
3
2
32 spectra could be measured. FAB+-MS found (calcd.): m/z 466.1661
103
33 (466.1666) for C22H34ClN3O2Co.
104
(S)-iPr(PdmBOX)Pd(OAc) (7) The protioligand (S)-iPr(PdmBOX)H
34
105 (320 mg, 0.85 μmol) and [Pd(OAc)2]·(230 mg, 1.03 mmol, 1.2 equiv.)
106 were dissolved in dichlormethane (15 mL). The mixture was stirred
107 for 2 h at room temperature. The solution turned from colourless to
solution of 2,6-
35 [(S)-iPr(PdmBOX)Fe(CNC6H3(CH3)2)3]Cl (5) To a solution of the
36 iron(II) compound (S)-iPr(PdmBOX)FeCl (50 mg, 108 µmol,
37 1.0 equiv.) in 2 mL DCM was added
a
108 yellow. The solvents were removed in vacuo. The residue was
38 Dimethylphenylisonitrile (42.4 mg, 324 µmol, 3.0 equiv.). The
mixture was stirred at rt for 1 h, the solvent was removed un1de0r9 dissolved in n-pentane and filtered through a pad of celite. After
110 removing of all volatiles the product obtained as a yellow oil in 75 %
39
40
41
reduced pressure. The resulting mixture was redissolved in 11m1L1 yield. Anal. found: C, 50.78; H, 6.85; N, 6.75. Calcd. for
toluene, layered with pentane and stored at -40 °C, producing1a1n2 C24H37ClN3O4Pd·0.5CH2Cl2: C, 50.70; H, 6.60; N, 7.24. 1H-NMR
42 orange precipitate, which was dried in vacuo. The pure product was
113 (C6D6, 600 MHz, 295 K): δ (ppm) = 6.16 (s, 2H, H1), 3.99-3.95 (m,
43 isolated as an orange solid in excellent yield (90 mg, 105 µmol,
114 2H, H6), 3.63 (dd, JH,H = 8.9 Hz, 8.9 Hz, 2 H6'), 3.58 (dd, JH,H
=
3
3
44
97 %). Despite several attempts, the microanalysis values for 1th1is5 9.6 Hz, 9.6 Hz, 2H, H7), 2.57 (ds, 3JH,H = 6.3 Hz, 3.8 Hz, 2H, H8), 2.25
45 compound are outside the range associated with analytical purity,
probably due to some unidentified impurities, not visible in 1th1e6 (s, 3H, H10), 2.18 (s, 6H, H4), 1.62 (s, 6H, H4'), 0.58 (d, 3JH,H = 7.0 Hz,
46
spectra. 1H NMR (CD Cl , 600 MHz, 295 K): δ (ppm) = 7.24-7.221(m1,7 12H, H9). 13C-NMR (C6D6, 150 MHz, 295 K): δ (ppm) = 175.0 (C11),
47
1 H, HAr), 7.21-7.17 (m, 4 H, HAr) 7.11-7.05 (m, 4 H, HAr), 6.31 (s, 121H8, 174.6 (C5), 135.1 (C2), 101.5 (C1), 68.5 (C6), 67.0 (C7), 38.7 (C3),
2
2
48
), 3.75-3.67 1(m1,9 32.1 (C4), 30.7 (C8), 29.2 (C10), 22.5 (C4'), 17.1 (C9'), 13.3 (C9).
49 H1), 4.15-4.08 (m, 2 H, H6/7), 3.99-3.90 (m, 2 H, H6/7
50 2 H, H6/7), 2.63 (s, 6 H, CH3eq) 2.17 (s, 12 H, CH3 ), 1.54 (s, 6 H, H ),
ax
4
120
51 1.36 (s, 6 H, H4'), 1.33-1.21 (m, 2 H, H8), 0.54 (d, 3JH,H = 7.1 Hz, 61H2,1
(S)-iPr(PdmBOX)Rh(CO) (8) The protioligand (S)-iPr(PdmBOX)H
52 H9), 0.46 (d, JH,H = 6.7 Hz, 6 H, H9'). 13C NMR (CD2Cl2, 150 M1H2z,2 (500,0 mg, 1.34 mmol) and [Rh(CO)2(acac)] (450 mg, 1.27 mmol,
3
53 295 K): δ (ppm) = 176.44 (2 C, C-5), 140.49 (Cq), 135.64 (C12),3 0.95 equiv.) were dissolved in dichlormethane CHCl3 (3 mL). The
q
54 135.25 (2 C, CAr), 134.67 (Cq), 129.96 (Cq), 129.85 (2 C, CAr), 1291.1264 mixture was stirred for 48 h at room temperature. After removal of all
55 (2 C, CAr), 129.06 (Cq), 128.57 (Cq), 128.25 (4 C, CAr), 107.63 (21C2,5 volatiles under reduced pressure, the crude reaction product was
56 C-1), 75.29 (2 C, C-6), 67.15 (2 C, C5), 38.59 (2 C, C3), 31.19 (21C2,6 recrystallized from a solvent mixture of diethylether : pentane (1:10).
57 C4), 30.97 (2 C, C4'), 28.83 (2 C, C8), 19.51 (2 C, CH3), 18.31 (41C2,7 The product was obtained as a yellow crystalline solud in 84 % yield.
58 C9', CH3), 12.90 (2 C, C9'). ESI+-MS found (calcd.): m/z 821.4118278 Anal. found: C, 54.98; H, 7.10; N, 8.65. Calcd. for C23H34N3O3Rh: C,
1
59 (821.4200) for C49H61N6O2Fe.
129 54.87; H, 6.81; N, 8.35. H NMR (CDCl3, 600 MHz, 295 K): δ (ppm)
60
130 = 5.83 (bs, 2H, H1), 4.37 (dd, 3JH,H = 9.5 Hz, 9.5 Hz, 2H, H6), 4.62
3
3
61
Generation of the Alkyl Precatalyst A solution of the ligand13in1 (dd, JH,H = 9.0 Hz, 6.2 Hz, 2H, H6'), 3.97 (ddd, JH,H = 10.2 Hz,
62 0.75 mL toluene was added to an equimolar solution of 1th3e2 6.3 Hz, 3.8 Hz, 2H, H7), 2.72-2.64 (m, 2H, H8), 1.88 (s, 6H, H4), 1.57
63 corresponding alkyl precursor in 0.75 mL. The mixture was stirred1fo3r3 (s, 6H, H4'), 0.91 (d, JH,H = 7.0 Hz, 6H, H9), 0.68 (d, JH,H = 6.9 Hz,
3
3
64 30 min and used in the catalytic procedure without furt1he3r4 6H, H9'). 13C-NMR (C6D6, 150 MHz, 295 K): δ (ppm) = 191.4 (d,
1
65 purification steps.
135 JRh,CO = 70.4 Hz, CO), 174.9 (C5), 137.7 (C2), 98.8 (C1), 73.2 (d, J
66
General Procedure for the Catalytic Hydroelementation1o3f6 = 1.4 Hz, C7), 67.5 (C6), 38.4 (C3), 33.8 (C4), 30.0 (C8), 21.5 (C4'),
67 Acetophenone: To solution of the alkyl precatalyst (3.75 µm1o3l,7 17.4 (C9'), 13.4 (C9). IR (KBr plates): ν cm−1 = 2923 (m,b) 1940 (s,
68 2.5 mol%) and the substrate (0.15 mmol, 1.0 equiv.) in 1.5 mL w1a3s8 CO), 1630 (m), 1464 (w), 1386 (w), 1225 (w), 1114 (w). DART+-MS
69 added neat HBPin or (EtO)2SiMeH (0.30 mmol, 2.0 equiv.) in o1n3e9 found (calcd.) m/z 504.1725 (503.1655) for C23H34N3O3Rh.
70 portion at roomtemperature. The mixture was stirred for 114h0
This article is protected by copyright. All rights reserved.

10.1002/ejic.201701195
European Journal of Inorganic Chemistry
FULL PAPER
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2
3
4
5
6
7
8
9
(S)-iPr(PdmBOX)Rh(CH3)I(CO) (9) To a solution of complex (S6)-7 diethylether and filtered through a pad of celite. The solvents were
iPr(PdmBOX)RhCO] (8) (125 mg, 250 μmol) in benzene-d6 (0.5 mL6)8 removed in vacuo to give the product as red solid in 85 % yield. Anal.
[
of methyl iodide (0.2 mL) was added. The conversion was monitore6d9 found: C, 49.33; H, 6.23; N, 7.43. Calcd. for C22H34N3O3NiCl3: C,
by NMR spectroscopy. After 3 h all volatiles were removed unde7r0 49.34; H, 6.02; N, 7.85. 1H-NMR (C6D6, 600 MHz, 295 K): δ (ppm) =
reduced pressure and light yellow reaction product was obtained 7in1 4.24-4.16 (m, 2H, H6), 3.41 (dd, 3JH,H = 9.1 Hz, 5.8 Hz, 2H, H6), 3.27
quantitative yield. Anal. found: C, 44.74; H, 5.84; N, 6.80. Calcd. fo7r2 (dd, 3JH,H = 10.0 Hz, 10.0 Hz, 2H, H7), 2.98 (ds, 3JH,H = 7.5 Hz, 3.6 Hz,
3
C24H37N3O3RhI: C, 44.67; H, 5.78; N, 6.51. 1H NMR (CDCl3, 60703 2H, H8), 2.86 (s, 6H, H4), 1.85 (s, 6H, H4'), 0.54 (d, JH,H = 6.9 Hz,
MHz, 295 K): δ (ppm) = 6.14-6.11 (m, 2H, H1, H1'), 4.50-4.46 (m, 1H7,4 12H, H9). 13C-NMR (CDCl3, 150 MHz, 295 K): δ (ppm) = 174.6 (C5),
3
H6), 4.45-4.38 (m, 3H, H6', H6'b, H7'), 4.26 (dd, JH,H = 9.2 Hz, 9.2 H7z,5 128.9 (C2), 105.3 (C1), 68.8 (C6), 66.8 (C7), 40.0 (C3), 33.9 (C5), 30.3
3
10 H6, H6b), 3.84 (dt, JH,H = 9.4 Hz, 2.4 Hz, 1H, H7), 3.01-2.98 (m, 1H7,6 (C4'), 30.2 (C4), 17.8 (C9), 14.3 (C9'). DART+ MS found (calcd.) m/z
11 H8), 2.18-2.07 (m, 1H, H8'), 1.75 (s, 3H, H4), 1.63 (s, 3H, H4), 1.64 (7s,7 534.0995 (534.0986) for C22H33N3O3NiCl3.
12 3H, H4b), 1.63 (s, 3H, H4'), 1.60 (s, 3H, H4'b), 1.09 (d, 2JRh,H = 2.0 Hz,
13 3H, H10), 0.98 (d, JH,H = 7.3 Hz, 3H, H9), 0.94 (d, JH,H = 7.2 Hz, 3H,
3
3
14 H9b), 0.88 (d, JH,H = 6.7 Hz, 3H, H9'), 0.78 (d, JH,H = 7.3 Hz, 3H,
3
3
78
(S)-iPr(ClPdmBOX)H (13) Complex (S)-iPr(PdmBOX)NiCl (50.0 mg,
15 H9'b).13C-NMR (C6D6, 150 MHz, 295 K): δ (ppm) = 189.3 (d, JRh,C
=
1
5
5'
2
2'
56.0 Hz, CO), 175.9 (C ), 175.8 (C ), 142.0 (C ), 139.8 (C ), 1057.99 93.4 μmol) and potassium cyanide (125,0 mg, 1.87 mmol, 20 eq.)
16
1
1'
7
7'
6
6'
3
(C ), 105.1 (C ), 76.2 (C ), 75.9 (C ), 68.2 (C ), 68.1 (C ), 40.0 (C8),0 The mixture was stirred for 24 h at room temperature. After removal
17
3'
4
4'
8
8'
4b
81 of all volatiles under reduced pressure the crude reaction product
18 39.5 (C ), 32.7 (C ), 31.8 (C ), 29.2 (C ), 28.7 (C ), 28.1 (C ),
19 25.3(C4'b), 18.5 (C9), 18.4 (C9'), 14.3 (C9b), 14.0 (C9'b), 9.4 (d, 1JRh,C
20 19.8 Hz, C10). IR (KBr plates): ν cm−1
=
82 was dissolved in dichloromethane and filtered through a pad of celite.
83 The solvents were removed in vacuo to give the product as a
= 2961 (m,b) 2048 (s, CO8),4 yellowish oil in 60 % yield. 1H-NMR (CDCl3, 600 MHz, 295 K): δ
21 1630 (m), 1461 (w), 1369 (w), 1219 (w), 966 (w). DART+ MS found
85 (ppm) = 11.77 (bs, 1H, NH), 4.42-4.18 (m, 2H, H6), 3.96-3.91 (m, 4H,
22 (calcd.) m/z 646.1003 (646.0935) for C24H38N3O3RhI.
3
86 H6', H7), 3.49 (dd, JH,H = 9.4 Hz, 9.4 Hz, 2H, H7), 1.76-1.71 (m, 2H,
23
(S8)-7 H8), 1.62 (s, 6H, H4), 1.60 (s, 6H, H4'), 0.94 (d, 3JH,H = 6.7 Hz, 6H, H9),
88 0.86 (d, 3JH,H = 6.7 Hz, 6H, H9').13C-NMR (CDCl3, 150 MHz, 295 K): δ
(185 m8g,9 (ppm) = 171.0 (C5), 128.1 (C2), 107.2 (C1), 71.8 (C6/7), 69.8 (C6/7),
24
(S)-iPr(PdmBOX)RhBr2(CO)
(10)
Complex
25
[
iPr(PdmBOX)RhCO] (X) (225 mg, 496 μmol) and CsBr3
26 496 μmol, 1 equiv.) were suspended in toluene (5 mL). The mixture
27 was stirred for 24 h at room temperature. After removal of all
28 volatiles under reduced pressure, the crude reaction product was
29 dissolved in a mixture of toluene : n-pentane (1:5) and filtered
30 through a pad of celite. The solvents were removed in vacuo to give
90 37.3 (C3), 32.5 (C5), 25.8 (C4'), 25.7 (C4), 18.8 (C9), 18.1 (C9'). EI+ MS
91 found (calcd.) m/z 441.1995 (441.1950) for C22H33N3O3Cl2.
31 the product as red solid in 72 % yield. Anal. found: C, 41.16; H, 4.86;
92
X-Ray Diffraction Studies. Details of the structure determinations
1
32 N, 5.98. Calcd. for C23H34N3O3RhBr2: C, 41.65; H, 5.17; N, 6.34. H
93 are provided in the supporting information. CCDC 1578292
-
33 NMR (C6D6, 600 MHz, 295 K): δ (ppm) = 6.51 (bs, 2H, H1), 3.97 (dd,
94 1578299 contains the supplementary crystallographic data for this
95 paper. These data can be obtained free of charge from The
34 JH,H = 9.4 Hz, 9.5 Hz, 2H, H6), 3.71-3.65 (m, 2H,H6), 3.59 (dd, JH,H
3
3
35 = 9.0 Hz, 9.0 Hz, 2H, H7), 2.84-2.71 (m, 2H, H8), 1.80 (s, 6H, H4),
96 Cambridge
Crystallographic
Data
Centre
via
36 1.78 (s, 6H, H4'), 0.70 (d, JH,H = 6.5 Hz, 6H, H9), 0.48 (d, JH,H
=
3
3
97 https://www.ccdc.cam.ac.uk/data_request/cif
37 6.9 Hz, 6H, H9'). 13C-NMR (C6D6, 150 MHz, 295 K): δ (ppm) = 183.2
38 (d, JRh,CO = 48.3 Hz, CO), 175.5 (C5), 138.4 (C2), 105.7 (C1), 75.2
1
39 ( C7), 66.5 (C6), 39.1 (C3), 31.4 (C4), 28.5 (C8), 28.9 (C4'), 16.8 (C9'),
40 12.9 (C9). IR (KBr plates): ν cm−1 = 2963 (m), 2083 (s, CO), 2000 (m),
98 Acknowledgements
41 1650 (w), 1634 (m), 1477 (w), 1447 (w), 1214(m), 741 (m).
99 We acknowledge the award of a predoctoral fellowship to
100 V.V. from the Landesgraduiertenförderung of the state of
42
(S)-iPr(BrPdmBOX)RhBr2(CO)
(11)
75.5
Complex
μmol) and
(S)-
101 Baden-Württemberg and generous funding by the University
43 iPr(PdmBOX)RhBr2(CO)
(50 mg,
1N0-2 of
Heidelberg
as
well
as
the
Deutsche
44 Bromosuccinimide (28.0 mg, 158 μmol, 2.1 equiv.) were dissolved in
103 Forschungsgemeinschaft (DFG-Ga488/9-2).
45 tetrahydrofurane (3 mL). The mixture was stirred for 1 h at room
46 temperature. After removal of all volatiles under reduced pressure.
the crude reaction product was dissolved in diethylether and filte1re0d4 Keywords: • Asymmetric catalysis •Iron• Cobalt •Manganese,
47
105 •Nickel, • Palladium, • Rhodium • Homogeneous catalysis
1
48 through a pad of celite. The solvents were removed in vacuo to give
49 the product as red solid in 92 % yield. Anal. found: C, 33.16; H, 4.1003;6 • Hydride ligands • Hydrosilylation • N ligands, • Reduction.
50 N, 5.05. Calcd. for C23H34N3O3RhBr4: C, 33.65; H, 3.93; N, 5.12. H
51 NMR (C6D6, 600 MHz, 295 K): δ (ppm) = 3.87 (dt, 3JH,H
= 9.7 1H0z,7 Literature:
52 3.2 Hz, 2H, H6), 3.58 (dd, JH,H = 8.9 Hz, 4.0 Hz, 2H, H ), 3.49 (dd,
3
6
53 JH,H = 9.4 Hz, 9.4 Hz, 2H, H7), 2.88 (ds, JH,H = 6.7 Hz, 2.8 Hz, 2H,
3
3
108
54 H8), 1.97 (s, 6H, H4), 1.56 (s, 6H, H4'), 0.64 (d, 3JH,H = 6.7 Hz, 6H, H9),
109 [1] Monographs: a) D. Morales-Morales, C. M. Jensen, The
55 0.46 (d, 3JH,H = 7.0 Hz, 6H, H9'). 13
C-NMR (C D , 150 MHz, 295 K1):1δ0
Chemistry of Pincer Compounds, Elsevier , Amsterdam, 2007,
b) G. van Koten, D. Milstein, Topics in Organometallic
Chemistry, Vol 40, Springer, Heidelberg, 2013; G. van Koten, R.
Gossage, Topics in Organometallic Chemistry, Vol 54, Springer,
Heidelberg, 2016.
6
6
56 (ppm) = 175.8 (d, 1JRh,CO = 48.3 Hz, CO), 171.9 (C5), 131.2 (C2), 99.3
111
57 (C1), 76.7 (C7), 66.4 (C6), 41.4 (C3), 28.3 (C4), 28.1 (C8), 26.5 (C4'),
112
58 17.6 (C9'), 13.5 (C9). IR (KBr plates): ν cm−1
= 2964 (m), 20851(1s,3
59 CO), 1606 (s), 1536 (s), 1460 (w), 1434 (w), 1389 (w), 1356 (w),
114
60 1343 (w),1296 (w), 1258 (m), 1232 (s), 1072 (w), 1021 (m), 961 (w),
115 [2] Key early reviews: a) M. Albrecht, G. van Koten, Angew. Chem.
61 756 (w).
116
Int. Ed. 2001, 40, 3750-3781; b) M. E. van der Boom, D.
Milstein, Chem. Rev. 2003, 103, 1759-1792.
117
62
(S)-iPr(ClPdmBOX)NiCl (12) Complex (S)-iPr(PdmBOX)NiCl (1.0 g,
118 [3] a) Q.-H. Deng, R. L. Melen, L. H. Gade, Acc. Chem. Res. 2014,
63 2.14 mmol) and N- Chlorosuccinimide (600.0 mg, 4.28 mm1o1l,9
47, 3162-3173; b) R. L. Melen, L. H. Gade, Topics in
Organometallic Chemistry: Organometallic Pincer Chemistry II,
(Ed. G. van Koten and R. A. Gossage) 2016, 54, 179 -208; see
64 2.1 equiv.) were dissolved in tetrahydrofurane (5 mL). The mixture
120
65 was stirred for 1 h at room temperature. After removal of all volat1ile2s1
66 under reduced pressure the crude reaction product was dissolved in
This article is protected by copyright. All rights reserved.

10.1002/ejic.201701195
European Journal of Inorganic Chemistry
FULL PAPER
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10.1002/ejic.201701195
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10.1002/ejic.201701195
European Journal of Inorganic Chemistry
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FULL PAPER
Pincer Chemistry
Jan Wenz, Vladislav Vasilenko,
Alexander Kochan, Hubert Wadepohl,
and Lutz H. Gade*
Page No. – Page No.
Coordination Chemistry of the
PdmBox Pincer Ligand: Reactivity
at the Metal and the Ligand
The coordination chemistry of the newly developed PdmBox ligand with first and
second row transition metals is presented. Since an isomerization of the ligand
backbone is prevented, the pyrrole building block is also retained in the
corresponding metal complexes.Text for Table of Contents
This article is protected by copyright. All rights reserved.