Synthesis of Non-Symmetric Ruthenium(II) POCOP Pincer Complexes and Their Bimetallic Derivatives by π-Coordination of Arenophile Fragments

Article abstract of DOI:10.1002/ejic.202000425

A series of neutral 16-electron square pyramidal ruthenium(II) complexes of the general formula [Ru(POCOP)(X)(CO)] (X = Cl, H) have readily been obtained in good yields from [RuCl₂(COD)]_n and different POCOP pincer ligands based on the naphthoresorcinate backbone. The molecular structure of two of the compounds was unequivocally determined by single-crystal X-ray diffraction, showing different stereochemistries upon the coordination of chloride or hydride ligand. The observed discrepancies are explained by DFT calculations. The regiospecific η⁶-coordination of a second 12-electron organometallic fragment on the exo aromatic ring of the POCOP pincer ligand was achieved under mild conditions, and the corresponding [(ML_n)Ru(POCOP)(X)(CO)]PF₆ (ML_n = [RuCp]⁺, [RuCp*]⁺ and [FeCp*]⁺) bimetallic sandwich-like species were obtained as air-stable solids.

Full text of DOI:10.1002/ejic.202000425

European Journal of Inorganic Chemistry
Accepted Article
Title: Synthesis of non-symmetric ruthenium(II) POCOP pincer
complexes and their bimetallic derivatives by π-coordination of
arenophile fragments
Authors: Manuel A. Roque Ramires, Longzhu Shen, and Ronan Le
Lagadec
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01/2020

10.1002/ejic.202000425
European Journal of Inorganic Chemistry
Synthesis of non-symmetric ruthenium(II) POCOP pincer complexes and their
bimetallic derivatives by π-coordination of arenophile fragments
Manuel A. Roque-Ramires,[a] Longzhu Shen,[b] and Ronan Le Lagadec*[a]
[a]
Manuel A. Roque-Ramires, Dr. Ronan Le Lagadec
Instituto de Química, UNAM, Circuito Exterior s/n, Ciudad Universitaria, 04510 Ciudad de
México, México
E-mail: ronan@unam.mx
https://iquimica.unam.mx/dr-ronan-le-lagadec
[b]
Dr. Longzhu Shen
University of Cambridge, CB2 3EJ, United Kingdom
Abstract: Series of neutral 16-electron square pyramidal ruthenium(II) complexes of the general
formula [Ru(POCOP)(X)(CO)] (X = Cl, H) have readily been obtained in good yields from
[RuCl2(COD)]n and different POCOP pincer ligands based on the naphthoresorcinate backbone. The
molecular structure of two of the compounds was unequivocally determined by single-crystal X-ray
diffraction, showing different stereochemistries upon the coordination of chloride or hydride ligand.
DFT calculations allowed to explain those observed discrepancies. The regiospecific η6-coordination
of a second 12-electron organometallic fragment on the exo aromatic ring of the POCOP pincer ligand
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was achieved in mild conditions, and the corresponding [(MLn)Ru(POCOP)(X)(CO)]PF6 (MLn =
[RuCp]+, [RuCp*]+ and [FeCp*]+) bimetallic sandwich-like species were obtained as air-stable solids.
Introduction
Because of their stability and ease of modification, transition metal complexes with tridentate ECE
pincer ligands have been a center of attraction for more than a decade [1]. In particular, they have
displayed important applications as catalysts in organic synthesis [1], as components of dendrimers
[2], in chemical sensors [3] and in medicinal chemistry [4]. In the field of homogenous catalysis,
compounds with phosphorus as donor atoms, i.e. PCP and POCOP (an extra oxygen atom connects the
donor phosphorus atom with the carbon skeleton) complexes, have been of particular interest [5]. The
aforementioned derivatives have been used in a large number of processes such as hydrogen transfer,
C-C couplings or polymerization reactions [6]. Even if the PCP ligands are still the most common class
of pincer ligands bearing phosphorus as donor atoms, complexes with POCOP ligands are of special
interest due to an easier synthetic route based on the deprotonation of a resorcinol derivative and the
subsequent reaction with chlorophosphines, coupled with enhanced thermal stability [7]. However,
most of the studies on the chemistry of POCOP ligands have been performed with group 10 metals (Ni,
Pd, Pt). For instance, nickel and palladium POCOP pincer complexes have widely been used as
efficient catalysts in Suzuki coupling and benzylic halide homocoupling reactions [6e, 8]. On the other
hand, ruthenium PCP pincer complexes have been used with excellent results in the dehydrogenative
coupling of alcohols with amines in order to produce an imine [9]. Nevertheless, despite the wide use
of ruthenium in catalysis, very few ruthenium complexes bearing POCOP ligands have been prepared.
To our knowledge, only three series of ruthenium POCOP derivatives have been reported in the
literature by Bedford, Huang and Koridze (Figure 1) [7a, 7d, 10].
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Figure 1. Ruthenium POCOP pincer complexes obtained by Bedford (left), Huang (center) and
Koridze (right) [7a, 7d, 10].
Besides, the obtention of catalysts with a metal-centered chirality has taken special interest due to the
potential applications in enantioselective processes. For example, Meggers reported high ee values for
the asymmetric Nazarov cyclization, the intramolecular Canizzaro reaction and the conversion of 1,4,2-
dioxazol-5-ones to chiral γ-lactams catalyzed by series of ruthenium and iron complexes exhibiting
helicoidal chirality [11]. Very recently, Gladysz published the use of chiral pseudo-octahedral
ruthenium catalysts in various nucleophilic additions, but the ee values were unsatisfactory [12].
Another example is a planar chiral cyclopentadienyl-ruthenium(II) complex reported by Takahashi
which catalyzed allylic substitutions with ee values up to 74% [13]. Additionally, the use of non-
symmetric pincer ligands has proven to be specially useful to generate chiral compounds without the
use of a ligand containing a chiral element [14]. For instance, our research group has described the
synthesis of heterobimetallic chiral complexes by regiospecific π-coordination of arenophile fragments
to the exo ring of a series of non-symmetric POCOP nickel, palladium and platinum complexes [15].
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The subsequent resolution of the racemic mixture and the separation of the diastereoisomers were
achieved in a preparative manner through the use the chiral Δ-TRISPHAT anion [16]. Nevertheless, it
is noteworthy that most examples of planar chiral complexes are based on square planar complexes or
ferrocene derivatives [15, 17].
Another area of interest is the synthesis of metal hydride complexes. Such compounds have been
reported in many catalytic processes, either as the catalyst or as key intermediates. For example,
hydride complexes of iridium, rhodium, cobalt, nickel and ruthenium have been used in the particularly
relevant reduction of carbon dioxide to different hydrogenated products, where the presence of a
hydride ligand is fundamental in order to have a successful catalytic process [18]. For this reason, the
development of new and easy synthetic routes for metal hydride complexes is of prime importance.
In this paper, we wish to report on the facile synthesis of a new family of stable 16-electron square
pyramidal ruthenium(II) POCOP pincer complexes bearing either a chloride or a hydride ligand and
their bimetallic derivatives by π-coordination of 12-electron arenophile fragments ([RuCp]+, [RuCp*]+
and [FeCp*]+).
Results and Discussion
The synthesis of the non-symmetric POCOP ligands is based on our earlier work [15]. The
deprotonation of naphthoresorcinol by triethylamine, followed by the reaction with the corresponding
chlorophosphine produces the POCOP ligands in excellent yields. This methodology previously
t
i
reported for ligands bearing Bu (a), Pr (b) and Ph (c) substituents was successfully applied to
cyclohexyl (Cy, d) as shown in Figure 2.
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Figure 2. General procedure for the synthesis of the non-symmetric POCOP ligands.
For the coordination to ruthenium, a reaction based on the work of Koridze was first evaluated [10]. A
solution of [RuCl2(COD)]n and ligand a (tert-butyl substituted) in 2-methoxyethanol, which can act as
both the solvent and the carbonyl source, was refluxed overnight in the presence of triethylamine, and
a mixture of two pentacoordinate ruthenium(II) compounds was obtained. The complexes were easily
separated by column chromatography on silica using a dicholoromethane:hexane (50:50) mixture as
the eluent, and [Ru(POCOP)(Cl)(CO)] (1a), and [Ru(POCOP)(H)(CO)] (2a) were isolated in 55% and
27% yield respectively (Figure 3).
Figure 3. Synthesis of Ru(II) non-symmetric pincer complexes and nomenclature used for the
description of the complexes in NMR.
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In the 1H-NMR (CDCl3) spectrum of complex 1a, the expected 4 aromatic protons of the exo ring are
observed at 8.07 ppm (d, H5), 7.72 ppm (d, H2) and 7.38 ppm (m, H3 and H4), while the proton of the
endo ring appears at 7.16 ppm as a singlet and the aliphatic protons of the tert-butyl groups are divided
in two multiplet signals at 1.54 and 1.29 ppm. Similar chemical shifts are observed for 2a at 8.11 ppm
(d, H5), 7.71 (d, H2), 7.33 (m, H3 and H4), 7.08 and 1.31 ppm. Additionally, for 2a a triplet is observed
at -7.54 ppm (JH-P = 21.6 Hz) for the hydride ligand, a value which is in the range previously observed
for similar ruthenium complexes [10]. Interestingly, the complexes displayed a different pattern for the
signals observed in 31P-NMR in CDCl3 (Figure S1). Complex 1a shows only a singlet signal at 188.42
ppm instead of the doublet of doublets signal expected for 2 non-equivalent phosphorus atoms in trans
position. This is probably because the dissymmetry of the molecule is not effective enough to observe
the expected multiplicity. However, for complex 2a, a signal centered at 219.34 ppm is observed with
a multiplicity close to the expected doublet of doublets, corresponding to an unfolding of the signal
due to a better differentiation between the phosphorus atoms. Similar behaviour has previously been
observed. For example, a system consisting of two doublets with a JP-P = 331 Hz has been detected for
square planar Ni and Pd complexes bearing analogous POCOP ligands, while in the case of the related
Pt complex, a singlet was observed in spite of the non-symmetry of the ligands [15]. The FT-IR spectra
showed a strong band at 1930 and 1917 cm-1 corresponding to the CO stretching for 1a and 2a
respectively.
Crystals suitable for X-ray diffraction studies were grown for both compounds by slow diffusion from
dichloromethane:methanol (see Table S1 for details of the structures parameters and experimental
data). The molecular structures of the complexes show the metal center into a slightly distorted square
pyramidal environment where two of the coordination sites are occupied by the phosphorus donor
atoms in a mutually trans conformation (Figures 4 and 5). The P-Ru bond distances are slightly shorter
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in complex 2a than in 1a, and marginally different between the two phosphorus within the same
compound (ΔdP-Ru = 0.0342 Å for 1a, 0.0013 Å for 2a). Interestingly, the CO ligand in complex 1a lies
in the axial position, trans to a vacant site, while in compound 2a, the carbonyl ligand is found in the
equatorial plane, trans to the metalated carbon atom of the pincer ligand. For such reason, the C(O)-
Ru bond is relatively longer in 2a (1.885 Å) than in 1a (1.757 Å).
Figure 4. ORTEP diagram of 1a. Thermal ellipsoids are drawn with 50% of probability level.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Ru(1)-C(2), 2.033(5);
Ru(1)-C(27), 1.757(8); Ru(1)-Cl(1), 2.423(4); Ru(1)-P(2), 2.3364(18); Ru(1)-P(1), 2.3706(19); C(27)-
Ru(1)-C(2), 90.3(3); C(27)-Ru(1)-Cl(1), 106.5(3); C(27)-Ru(1)-P(2), 92.5(3); C(27)-Ru(1)-P(1),
93.8(3); C(2)-Ru(1)-P(2), 79.57(15); P(2)-Ru(1)-Cl(1), 99.00(15); C(2)-Ru(1)-P(1), 78.90(15); P(1)-
Ru(1)-Cl(1), 99.77(11).
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Figure 5. ORTEP diagram of 2a. Thermal ellipsoids are drawn with 50% of probability level.
Hydrogen atoms, except hydride ligand, are omitted for clarity. Selected bond lengths (Å) and angles
(°): Ru(1)-C(2), 2.085(5); Ru(1)-C(27), 1.885(7); Ru(1)-P(2), 2.3081(16); Ru(1)-P(1), 2.3094(16);
C(2)-Ru(1)-P(2), 77.84(16); C(2)-Ru(1)-P(1), 78.14(16); C(27)-Ru(1)-P(2), 102.6(2); C(27)-Ru(1)-
P(1), 101.4(2); C(27)-Ru(1)-C(2), 177.8(3); P(2)-Ru(1)-P(1), 155.98(6).
As mentioned, one important difference between the complexes is that 2a adopts a conformation with
the hydride in the axial position, while in 1a the chloride in the equatorial position is preferred. This
interesting observation motived us to investigate the possible rational behind different configurational
choices between 1a and 2a. We manually constructed the stereo-analogues 1aA (the chloride ligand in
axial position) and 2aE (the hydride ligand in equatorial position) and subjected all four molecules to
quantum mechanical geometrical optimizations at the DFT level (see Experimental Section for details
of the methods used). The optimized isomeric pairs 1a/1aA and 2a/2aE showed great similarities as
illustrated in the overlaid graphs in Figure 6. The primary discrepancies reside in the position of the
axial and equatorial groups, which are summarized in Table 1. For instance, 1aA presents a slightly
more upright axial group (Cl-) than 1a (CO). Similarly, for 2a/2aE, the axial group of 2aE (CO) is
further tilted than that of 2a (H-). Consequently, the equatorial hydride of 2aE bends downward,
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probably to maintain the approximately H-Ru-CO right angle. Energetically, the DFT simulation
showed that 1a and 2a are favored by 18.6 and 10.9 kcal/mol in free energy, respectively, when
compared to their isomeric counterparts. The rational behind this energy difference probably lies in the
possible favored electrostatic interactions between the CO group and peripheral tert-butyl groups. An
axial CO group benefits 1a from this favorable electrostatic interactions at the expense of a small loss
due to more steric conflict compared to 1aA. This is possible because Cl- is only slightly less bulky
than CO. However, the picture changed for 2a, where the hydride is a much smaller functional group.
With this very small H-, 2a cuts much more energy penalty in the axial orientation than its isomer 2aE.
Thus, theoretical calculations confirmed the experimental observations that isomers 1a and 2a are
favored.
Figure 6. Optimized structures overlaid between the isomeric analogues 1a/1aA (left) and 2a/2aE
(right). The 1a and 2a isomers are shown in blue while their constructed isomers are shown in yellow.
Hydrogen atoms are omitted for clarity.
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Table 1. Molecular angle (°) between the aromatic POCOP, Ru atom and the ligand X (H-, Cl- or CO).
Complex
1a
Angle (C-Ru-X, axial)
Angle (C-Ru-X, equatorial)a
97.1
94.8
90.6
106.5
-11.2
177.7
178.6
-10.2
1aA
2a
2aE
a Negative values mean the groups bend downward, below the horizontal plane.
Even though the separation and purification of the complexes were relatively straightforward, the next
step was to prepare each compound selectively. To do so, a possible mechanism based on
decarbonylation reactions reported in the literature for iridium and rhodium complexes was studied
(Scheme 1) [19].
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Scheme 1. Proposed mechanism for the formation of 1a and 2a.
The first step would be the metalation of the pincer moiety through the coordination of the phosphorus
atoms followed by C–H activation. The oxidative addition of 2-methoxyethanol used as a solvent,
followed by a β-elimination would lead to 2-methoxyacetaldehyde and a 16-electron ruthenium(IV)
dihydride intermediate D. It is worth mentioning that even if the oxidative addition of alcohol is not a
common reaction, various examples have been described in the literature, either as to prepare isolated
complexes or to generate intermediates in catalytic processes [20]. A reductive elimination of either
molecular hydrogen or hydrochloric acid, followed by the oxidative addition of the aldehyde previously
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formed, would give an acyl intermediate F. A deinsertion step with a final reductive elimination of
dimethylether generates complexes 1a or 2a.
Alternatively, it is known that the reaction between a strong base and an alcohol can give the
corresponding alkoxide, which is then able to substitute a coordinated halide, allowing a subsequent β-
elimination in order to yield the metallic hydride [5d]. So, this opens the possibility of another
mechanism, consisting first in the formation of complex 1a, and the subsequent conversion to 2a.
Indeed, the formation of 2a was almost quantitative in only 4 hours when 1a was stirred at reflux of
dichloroethane in the presence of KOH and 1-butanol.
With such mechanisms in mind, the modification of the reaction conditions allowed the selective
preparation of the complexes. In order to generate 1a, the formation of the dihydride intermediate D
should be avoided, which was achieved using an aldehyde instead of an alcohol as the carbonyl source.
When the POCOP ligand and [RuCl2(COD)]n were refluxed in 1,2–dichloroethane in the presence of
an excess of benzaldehyde (5 equivalents) and Et3N, 1a could be isolated in 72% yield. No formation
of 2a was observed. The scope of the reaction was successfully extended to a series of POCOP pincer
ligands bearing different substituents on the phosphorus atoms, obtaining good yields in all cases
(Figure 7a). Despite being 16-electron species, complexes 1a-d are stable either in solid state or
solution under inert conditions. However, it is noteworthy that compounds bearing bulkier substituents
(1a and 1d) are much more stable than 1b and 1c under atmospheric conditions and can be stored in
solid state for weeks without noticeable decomposition.
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Figure 7. a) Synthesis of complexes 1a, 1b, 1c and 1d. b) Synthesis of complex 2a. c) Formation of
hydride complex 2a from 1a.
In the case of the formation of complex 2a, the key step is the reductive elimination of HCl from D, so
the use of a strong base should drive the reaction towards the formation of the hydride complex. The
use of an excess of KOH in place of Et3N allowed the exclusive formation of 2a in a 79% yield (Figure
7b). It should be mentioned, however, that when the reaction was performed with POCOP ligands
1
bearing substituents other than tert-butyl, the hydride complexes could be detected by H-NMR, but
they tend to decompose in solution and hence are difficult to purify. Nevertheless, complex 2a is air-
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stable for hours and can be stored under a nitrogen atmosphere for weeks. Therefore, for practical
reasons, in this study we focused on this derivative. As discussed above, compound 2a could also be
prepared in excellent yield (95%) from 1a in the presence of 5 equivalents KOH and 1-butanol (Figure
7c). Importantly, 1H-NMR data confirmed that the stereochemistry of complex 2a prepared either from
[RuCl2(COD)]n (Figure 7b) or from 1a (Figure 7c) is not modified and the hydride ligand remained in
axial position.
As previously mentioned, the main objective of the use of non-symmetric pincer ligands is the design
of bimetallic derivatives which display planar chirality by π-coordination of an electron deficient
fragment. The cationic compounds [Ru(Cp)(CH3CN)3]PF6 and [Ru(Cp*)(CH3CN)3]PF6 are readily
available precursors to cyclopentadienyl- and pentamethylcyclopentadienyl-ruthenium complexes due
to the substitutional lability of the acetonitrile ligands [21]. Additionally, the high affinity of the
[RuCp]+ and [RuCp*]+ fragments for arene rings has also led to applications in the synthesis of
heterobimetallic complexes by π-coordination on aromatic substituents from different ligands [7c, 15]
Thus, the bimetallic complexes 3a and 4a were prepared in a simple manner by reacting
[Ru(Cp)(CH3CN)3]PF6 or [Ru(Cp*)(CH3CN)3]PF6 with a slight excess of the POCOP complex 1a in
dichloromethane at room temperature (Figure 8). Highly reactive [Fe(Cp*)(CH3CN)3]PF6 was also
successfully used to obtain heterobimetallic derivative 6a in acetonitrile. Likewise, the reaction was
applied to the hydride derivative 2a and complex 5a bearing the [RuCp]+ fragment was isolated in 62%
yield with no loss of the hydride ligand. In this series of reactions, tert-butyl substituents were preferred
over phenyl to avoid the formation of side-products by coordination of the arenophile moiety to the
phenyl ring and also because of the higher stability of complexes 1a and 2a. Compounds 3a-6a are air-
stable for days in the solid state, while they slowly decompose in dichloromethane solution. In
particular, complex 6a readily suffers demetallation in halogenated solvents.
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Figure 8. Synthesis of the bimetallic derivatives.
The reaction is regiospecific, with the η6-coordination of the 12-electron fragment taking place only
on the exo ring of the naphtyl backbone, as previously observed for group 10 metals [15]. This
specificity can be clearly seen by 1H-NMR. Because of the coordination of the electron-withdrawing
moiety, the protons corresponding to the exo ring are noticeably more shifted to high field than the
proton of the endo ring. For instance, the signals of the aromatic protons (H2-H5) of the exo ring are
displaced from 7.38 – 8.07 ppm in 1a to 6.13 – 6.99 ppm for 3a and 5.89 – 6.36 ppm in 4a, while the
signal for the proton H1 of the endo ring is only shifted by approximately 0.40 ppm (from 7.16 in 1a
to 6.69 for 3a and 6.65 ppm in 4a). Similar results are observed for complexes 5a and 6a (Table S2).
Another interesting observation is the multiplicity observed for the signal corresponding to the hydride
ligand. In the monometallic complex 1a, a triplet is observed at -7.54 ppm, while in the bimetallic
derivative 5a the hydride signal is observed at -7.62 ppm but with a significant difference in the
multiplicity. The signal splits into a doublet of triplets, probably because the electron-withdrawing
effect of the [RuCp]+ fragment that allows a better differentiation of the two phosphorus atoms (Figure
31
S3). The P-NMR signals are shifted to the downfield region by about 6-11 ppm for all bimetallic
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derivatives when compared to 1a and 2a, probably due to the deprotection effect that the electron-
withdrawing fragment induces on the phosphorus atoms (Table S3). The multiplicity of the signals is
also affected. While 1a displayed a singlet, a doublet is observed for 3a, 4a and 6a. This is consistent
with our previous observations that the coordination of the second organometallic fragment allows a
more important splitting of the signals in 31P-NMR [15]. In the case of 5a, the original multiplet in 2a
transforms into a well-defined doublet. In infrared spectroscopy, the bands for the carbonyl ligand are
displaced by about 50 cm-1 for 3a, 4a and 6a towards a higher wavenumber, as the electron-
withdrawing properties of the arenophile fragment result in a less electron rich ruthenium center, and
subsequently a less favored retrodonation to the carbonyl ligand. Interestingly, this effect is no
observed for the carbonyl band for 5a, which is observed practically at the same wavenumber than for
2a (Table S4).
Conclusions
The synthesis of new non-symmetric pentacoordinate ruthenium(II) POCOP pincer complexes has
readily been carried out in good yields. The modification of the reaction conditions allowed the
selective preparation of either the Ru-Cl or Ru-H complex. Furthermore, the regioselective
coordination of arenophile fragments was successfully performed on one arene ring of the pincer
ligand, obtaining the respective bimetallic derivatives. In particular, the synthetic methods reported
represent an easy way to prepare ruthenium hydride derivatives, which can show a high potential in
catalytic processes like carbon dioxide reduction. Importantly, the reactions can be carried out without
the use of reactive precursors such as metallic hydrides, often used for related reactions. In principle,
all the bimetallic derivatives should display planar chirality, and it is important to mention that such
compounds would be the first examples of planar chiral ruthenium complexes based on a square
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pyramidal geometry. The resolution of the enantiomers is currently underway, as well as the study of
the catalytic properties of the new compounds, and the results will be published elsewhere.
Experimental Section.
Materials and reagents. All reactions were carried out under inert atmosphere (nitrogen or argon)
using conventional Schlenk techniques. All solvents were dried using established procedures and
distilled under nitrogen prior to use. All reagents were purchased from Sigma Aldrich and were used
as received, except for the chlorodiphenylphosphine which was distilled under vacuum at 120 °C prior
to use. The metallic precursors [RuCl2(COD)]n [22], [Ru(Cp)(CH3CN)3]PF6 [23],
[Ru(Cp*)(CH3CN)3]PF6 [24] and [Fe(Cp*)(CH3CN)3]PF6 [25] were synthetized according to the
literature. Chromatography columns were carried out on Macherey-Nagel silica 60 standardized
previously dried and deoxygenated under inert atmosphere.
The NMR spectra were obtained at room temperature on a AC-300 Bruker or a AC-400 Bruker
1
31
spectrometer. H NMR spectra were recorded at 300.13 MHz (AC-300) and 400.13 (AC-400), P
NMR spectra were recorded at 121.5 MHz (AC-300) and 161.95 MHz (AC-400) and 13C NMR spectra
were recorded at 75.56 MHz (AC-300). Chemical shifts (δ) are in ppm downfield of TMS using the
residual solvent as internal standard. Coupling constants (J) are expressed in Hz. Multiplicity: s =
singlet, d = doublet, t = triplet, sept = septuplet, m = multiplet. The infrared spectra were recorded on
an Alpha ATR spectrometer from Bruker Optics and analyzed with OPUS software. Elemental
analyses were carried out with an Exeter Analytical CE-440 instrument. Mass spectra (FAB+) were
obtained using a JEOL JMS-SX102A instrument with m-nitrobenzyl alcohol as a matrix.
Computational details. Complexes 1a, 2a and their constructed stereoisomers (1aA, with the chloride
in axial position and 2aE with the hydride in equatorial position) underwent geometrical optimization
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using Becke’s three parameter hybrid functional (B3) [26, 27] and the Lee−Yang−Parr correlation
functional (LYP) [28] as implemented in the computational suite Gaussian 09, rev. D.01 [29]. Basis
set LanL2DZ [30] was applied to Ru and 6-31G* for the other elements. Dichloromethane as solvent
was simulated using the SMD continuum model [31]. Molecular geometries were obtained upon
numerical convergence at the default criteria.
Crystallography. Crystalline prisms of 1a and 2a were grown from dichloromethane:methanol by
slow diffusion. The X-ray intensity data were measured at 298(2) K for both compounds with a Bruker
Smart Apex CCD diffractometer using standard Mo-K radiation (λ = 0.71073 Å). A multiscan
absorption correction procedure was applied. The data integration for 1a was performed by using a
orthorhombic unit cell to yield a total of 46267 reflections to a maximum 2θ angle of 50.66°, of which
5246 [R(int) = 0.0526] were independent. For compound 2a, the process was done by using a
orthorhombic unit cell to yield a total of 24070 reflections to a maximum 2θ angle of 50.48°, where
5207 [R(int) = 0.1169] reflections were independent. Structure solutions were performed using
SHELXS-2014/7 and refinement (full-matrix least squares) were performed using the SHELXS-2016/6
program [32]. The hydrogen atoms were input at calculated positions and allowed to ride on the atoms
to which they are attached. For 1a, the Ru and Cl atoms are disordered and were modeled and refined
in two major contributors using a variable site occupational factor (SOF). The ratio of SOF for
disordered atoms was 0.87/0.13. Restrains were applied as follows: 12 for SIMU and 12 for DELU
instructions. Crystal structure parameters and experimental data on the structure solution and
refinement are given in Table S1. CCDC 1998131 (1a) and CCDC 1998131 (2a) contain the
supplementary crystallographic data for this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre.
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European Journal of Inorganic Chemistry
Non-symmetric POCOP pincer ligands. The ligands were synthesized by the following general
procedure. A solution of triethylamine (0.1 ml / 0.99 mmol) and naphthoresorcinol (70 mg / 0.40 mmol)
in 15 mL of toluene was stirred for 1 hour at room temperature before being added dropwise to a
solution of the corresponding chlorophosphine (0.9 mmol) in 10 mL of toluene. The reaction mixture
was refluxed for 24 hours. After cooling to room temperature, the solution was filtered by canula and
the solvent was evaporated under reduced pressure to give the ligands as pale yellow oils. The yields
were 87 % (tBu, a), 89 % (iPr, b), 83 % (Ph, c) and 85 % (Cy, d). The spectroscopic data matched with
the previously reported [8b, 15] and the compounds were found to be pure enough to be used without
further purification.
Ligand d: 1H-NMR (CDCl3, δ ppm): 8.10 (d, JH-H = 6.0 Hz, 1H; H5), 7.62 (d, JH-H = 6.0 Hz, 1H; H2),
7.37 (t, JH-H = 5.4 Hz , 1H; H3), 7.27 (t, JH-H = 6.0 Hz, 1H; H4), 7.18 (s, 1H; H1), 7.09 (s, 1H; H´), 1.81
(m, 40H; Cy). 31P{1H}-NMR (CDCl3, δ ppm): 140.97 (s).
Complexes 1a and 2a. A mixture of 243 mg (0.6 mmol) of freshly prepared POCOP ligand a, 152 mg
(0.6 mmol) of [RuCl2(COD)]n and 0.8 mL (0.6 mmol) of triethylamine in 20 mL of 2-methoxyethanol
was refluxed for 6 h. An orange solid precipitated, which was filtered off, washed with methanol (3 x
10 mL) and dried under vacuum. The solid was dissolved in 5 mL of dichloromethane and
chromatographed over a silica column, using a 50:50 dichloromethane:hexane mixture as eluent. A
yellow fraction was first collected and the solvent evaporated to dryness, obtaining 94 mg (0.16 mmol)
of 2a as a yellow solid in 27% yield. Further elution with a 80:20 dichloromethane:hexane mixture
gave a red fraction which was collected and evaporated to dryness, obtaining 205 mg (0.33 mmol) of
1a as a red solid in 55% yield.
Complex 1a: 1H-NMR (CDCl3, δ ppm): 8.07 (d, JH-H = 8.1 Hz, 1H; H5), 7.72 (d, JH-H = 7.2 Hz, 1H,
H2), 7.38 (m, 2H; H3, H4), 7.16 (s, H1), 1.54 (m, 18H; Bu), 1.29 (m, 18H; Bu). 31P{1H}-NMR
t
t
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European Journal of Inorganic Chemistry
(CDCl3, δ ppm): 188.42 (s). 13C-NMR (CDCl3, δ ppm): 205.28 (d, JC-P = 13.6 Hz; CO), 165.09 (t, JC-P
= 5.3 Hz; aromatic C bonded to oxygen) 164.63 (t, JC-P = 5.3 Hz; aromatic C bonded to oxygen),
135.27 (t, JC-P = 5.3 Hz; aromatic C), 134.76 (s; aromatic C), 127.00 (s; aromatic C), 126.21 (s; aromatic
C), 123.68 (s; aromatic C), 121.62 (s; aromatic C), 119.08 (t, JC-P = 5.3 Hz; aromatic C), 101.63 (t, JC-
= 5.3 Hz,; metalated C), 41.96 (m; C(CH3)3), 40.72 (dd, JC-P = 20.4 Hz, JC-P = 9.1 Hz; C(CH3)3),
P
28.35 (dd, JC-P = 7.6 Hz, JC-P = 3.8 Hz; C(CH3)3), 27.70 (t, JC-P = 3.0 Hz; C(CH3)3), 27.47 (t, JC-P = 3.0
Hz; C(CH3)3). FTIR: νCO 1930 cm-1. MS (FAB+, m/z): 613 [M], 576 [M - Cl]. Anal. Calcd. for
C27H41ClO3P2Ru: C, 52.98; H, 6.75. Found: C, 53.31; H, 6.92
Complex 2a: 1H-NMR (CDCl3, δ ppm): 8.11 (d, JH-H = 8.1 Hz, 1H; H5), 7.71 (d, JH-H = 8.1 Hz, 1H;
H2), 7.33 (m, 2H; H3, H4), 7.08 (s, 1H; H1), 1.32 (m, 36H; tBu), -7.54 (t, JH-P = 21.6 Hz, 1H; Ru-H).
13
31P{1H}-NMR (CDCl3, δ ppm): 219.34 (dd, JP-P = 10.89 Hz, JP-P = 3.63 Hz). C-NMR (CDCl3, δ
ppm): 208.75 (d, JC-P = 6.0 Hz; CO), 167.25 (t, JC-P = 8.3 Hz; aromatic C bonded to oxygen) 165.21 (t,
JC-P = 9.1 Hz; aromatic C bonded to oxygen), 135.89 (s; aromatic C), 127.22 (s; aromatic C), 125.90
(s; aromatic C), 123.06 (s; aromatic C), 123.01 (s; aromatic C), 118.80 (dd; JC-P = 7.6 Hz, JC-P = 4.5
Hz; aromatic C), 118.68 (s; aromatic C), 99.60 (t, JC-P = 4.5 Hz; metalated C), 42.14 (m; C(CH3)3),
39.03 (m; C(CH3)3), 28.98 (t, JC-P = 3.8 Hz; C(CH3)3), 28.69 (t, JC-P = 4.5 Hz; C(CH3)3), 28.33 (dd, JC-
P = 7.6 Hz, JC-P = 3.0 Hz; C(CH3)3). FTIR: νCO 1917 cm-1. MS (FAB+, m/z): 578 [M], 577 [M - H].
Anal. Calcd. for C27H42O3P2Ru·0.1C6H14: C, 56.14; H, 7.33. Found: C, 56.55; H, 7.46.
Selective synthesis of complex 1a. A mixture of 243 mg (0.6 mmol) of freshly prepared POCOP
ligand a, 152 mg (0.6 mmol) of [RuCl2(COD)]n, 0.3 mL (3 mmol) of benzaldehyde and 0.8 mL (0.6
mmol) of triethylamine in 10 mL of 1,2-dichloroethane was refluxed for 24 h. The solvent was
evaporated to dryness under vacuum and the residue was washed with methanol (3 x 10 mL) and
20
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European Journal of Inorganic Chemistry
vacuum-dried, giving 260 mg (0.42 mmol, 72% yield) of 1a as a red powder. The analytical data are
identical as for 1a prepared as above.
Selective synthesis of complex 2a. A mixture of 243 mg (0.6 mmol) of freshly prepared POCOP
ligand a, 152 mg (0.6 mmol) of [RuCl2(COD)]n and 342 mg (6 mmol) of potassium hydroxide in 15
mL of 2-methoxyethanol was refluxed for 6 h. A yellow solid precipitated, which was filtered off,
washed with methanol (3 x 10 mL) and dried under vacuum, giving 281 mg (0.48 mmol, 79% yield)
of 2a as a yellow solid. The analytical data are identical as for 2a prepared as above.
Complexes 1b, 1c and 1d. The same method as for 1a was used: a mixture of 0.4 mmol of the freshly
prepared POCOP ligand, 152 mg (0.6 mmol) of [RuCl2(COD)]n, 0.3 mL (3 mmol) of benzaldehyde
and 0.8 mL (0.6 mmol) of triethylamine in 10 mL of 1,2-dichloroethane was refluxed for 24 h. The
solvent was evaporated to dryness under vacuum and the residue was chromatographed over a silica
column using a dichloromethane:hexane mixture (70:30 for 1b, 50:50 for 1c and 1d) as eluent. The
first fraction was collected and vacuum evaporated to dryness.
Complex 1b. From 140 mg (0.4 mmol) of ligand b, compound 1b was obtained as purple solid in 67%
yield (150 mg). 1H-NMR (CDCl3, δ ppm): 7.93 (d, JH-H = 7.2 Hz, 1H; H5), 7.72 (m, 2H; H3, H4), 7.59
(t, JH-H = 7.5 Hz, 2H; H2), 7.04 (s, 1H; H1), 2.81 (m, 4H; CH(CH3)2), 1.15 (m, 24H; CH(CH3)2).
31P{1H}-NMR (CDCl3, δ ppm): 174.61 (d, JP-P = 3.63 Hz). FTIR: νCO 1937 cm-1. MS (FAB+, m/z):
556 [M], 519 [M - Cl]. Anal. Calcd. for C23H33ClO3P2Ru: C, 49.69; H, 5.98. Found: C, 50.09; H, 6.21.
Complex 1c. From 180 mg (0.3 mmol) of ligand c, compound 1c was obtained as a red solid in 62%
yield (130 mg) 1H-NMR (CDCl3, δ ppm): 8.13 (d, JH-H = 8.1 Hz, 1H; H5), 8.01 (d, JH-H = 6.6 Hz, 2H;
31
H4, H3), 7.87–7.43 (m, 22H; H1, H2, Ph). P{1H}-NMR (CDCl3, δ ppm): 149.18 (dd, JP-P = 21.78
21
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European Journal of Inorganic Chemistry
Hz, JP-P = 10.89 Hz). FTIR: νCO 1969 cm-1. MS (FAB+, m/z): 692 [M] 655 [M - Cl]. Anal. Calcd. for
C35H25ClO3P2Ru·0.5CH2Cl2: C, 58.05; H, 3.57. Found: C, 58.18; H, 3.85.
Complex 1d. From 220 mg (0.4 mmol) of ligand d, compound 1d was obtained as a purple solid in
63% yield (135 mg). 1H-NMR (CDCl3, δ ppm): 8.01 (t, JH-H = 6.6 Hz, 1H; H5), 7.63 (dd, JH-H = 14.7
Hz, JH-H = 6.3 Hz, 1H; H4) ,7.43 (t, JH-H = 5.4 Hz; 1H; H3), 7.34 (t, JH-H = 5.7 Hz, 1H; H2), 7.09 (s,
1H; H1), 1.57 (m, 44 H, Cy). 31P{1H}-NMR (CDCl3, δ ppm): 197.34 (t, JP-P = 21.78 Hz). FTIR: νCO
1932 cm-1. Anal. Calcd. for C35H49ClO3P2Ru: C, 58.69; H, 6.90. Found: C, 59.08; H, 7.21.
Conversion of 1a to 2a. A mixture of 50 mg (0.8 mmol) of complex 1a, 220 mg (4 mmol) of potassium
hydroxide and 0.2 mL (4 mmol) of 1-butanol in 10 mL of 1,2-dichloroethane was refluxed for 4 h. The
solvent was evaporated to dryness under vacuum. The yellow residue was dissolved in 10 mL of
dichloromethane and chromatographed over a silica column using a 50:50 dichloromethane:hexane
mixture as the eluent. A yellow fraction was collected and vacuum evaporated to dryness obtaining
complex 2a in 95% yield. The analytical data are identical as for 2a prepared as above.
Bimetallic derivatives 3a, 4a and 5a. In a typical experiment, 0.2 mmol of monometallic complex 1a
or 2a and 0.2 mmol of the arenophile precursor in 10 mL of dichloromethane were stirred for 48 h at
room temperature for 3a, 4a and 5a, and in 10 mL of acetonitrile 24 h for 6a. After evaporation of the
solvent under vacuum, the residue was chromatographed over a silica column first using
dichloromethane as eluent to remove unreacted monometallic, followed by
a
95:5
dichloromethane:acetonitrile mixture (80:20 for 6a). A yellow fraction was collected and evaporated
to dryness under vacuum.
Complex 3a. From 50 mg (0.082 mmol) of 1a and 36 mg (0.080 mmol) of [Ru(Cp)(CH3CN)3]PF6,
1
compound 3a was obtained as a yellow solid in 77% yield (59 mg). H-NMR (CDCl3, δ ppm): 6.99
(dd, JH-H = 4.2 Hz, JH-H = 2.4 Hz, 1H; H5), 6.88 (dd, JH-H = 4.5 Hz, JH-H = 2.4 Hz, 1H; H2), 6.69 (s,
22
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European Journal of Inorganic Chemistry
t
1H; H1), 6.13 (dd, JH-H = 4.2 Hz, JH-H = 2.1 Hz, 2H; H3, H4), 4.20 (s, 5H; Cp), 1.45 (m, 36H; Bu).
31P{1H}-NMR (CDCl3, δ ppm): 199.74 (d, JP-P = 54.45 ppm), -144.44 (sept.; PF6). FTIR: νCO 1983
cm-1, νPF 830 cm-1. MS (FAB+, m/z): 897 [M], 578 [M - (Ru + Cp + PF6)]. Anal. Calcd. for
C32H46ClF6O3P3Ru2: C, 41.63; H, 5.02. Found: C, 41.36; H, 4.78.
Complex 4a. From 130 mg (0.2 mmol) of 1a and 120 mg (0.2 mmol) of [Ru(Cp*)(CH3CN)3]PF6,
compound 4a was obtained as a yellow solid in 53% yield (121 mg). 1H-NMR (CDCl3, δ ppm): 6.65
t
(s, 1H; H1), 6.36 (d, JH-H = 7.6 Hz, 1H; H2), 5.89 (m, 3H; H3, H4, H5), 1.48 (m, 51H; Cp*, Bu).
31P{1H}-NMR (CDCl3, δ ppm): 194.69 (d, JP-P = 19.36 Hz), -144.44 (sept.; PF6). FTIR: νCO 1985 cm-
1, νPF 831 cm-1. Anal. Calcd. for C37H56ClF6O3P3Ru2: C, 44.74; H, 5.68. Found: C, 45.13; H, 5.33
Complex 5a. From 49 mg (0.085 mmol) of 2a and 36 mg (0.080 mmol) of [Ru(Cp)(CH3CN)3],
compound 5a was obtained as a yellow solid in 62% yield (48 mg). 1H-NMR (CDCl3, δ ppm): 7.03 (t,
JH-H = 5.7 Hz, 1H; H1), 6.87 (d, JH-H = 4.8 Hz, 1H; H5), 6.62 (d, JH-H = 7.8 Hz, 1H; H2), 6.14 (m, 2H;
H3, H4), 4.82 (s, 5H; Cp), 1.25 (m, 36H; tBu), -7.62 (dt, JH-P = 19.36 Hz, JH-P = 8.47 Hz, 1H; Ru-H).
31P{1H}-NMR (CDCl3, δ ppm): 226.66 (d, JP-P = 36.30 Hz), -144.29 (sept.; PF6). FTIR: νCO 1915 cm-
1, νPF 832 cm-1. MS (FAB+, m/z) 862 [M], 613 [M - (Ru + Cp + PF6)]. Anal. Calcd. for
C32H47F6O3P3Ru2: C, 43.25; H, 5.33. Found: C, 43.63; H, 5.66
Complex 6a. From 130 mg (0.2 mmol) of 1a and 92 mg (0.2 mmol) of [Fe(Cp*)(CH3CN)3]PF6,
compound 6a was obtained as a yellow solid in 54% yield (82 mg). 1H-NMR (CD3CN, δ (ppm): 7.05
(s, H1), 6.92 (d, JH-H = 8.4 Hz, 1H; H5), 6.71 (d, JH-H = 7.8 Hz, 1H; H2), 6.17 (m, 2H; H3, H4), 1.42
(m, 51H, Cp*, tBu) 31P{1H}-NMR (CD3CN, δ ppm): 194.82 (d, JP-P = 9.68 Hz), -144.08 (sept., PF6).
FTIR: νCO 1981 cm-1, νPF 832 cm-1. Anal. Calcd. for C37H56ClF6FeO3P3Ru·0.5CH2Cl2: C, 45.47; H,
5.80. Found: C, 45.63; H, 5.66
Acknowledgments
23
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10.1002/ejic.202000425
European Journal of Inorganic Chemistry
We thank financial support from Consejo Nacional de Ciencia y Tecnología (Project A1-S-15068 and
grant 818255 to MRR), DGAPA - UNAM (PAPIIT project IN-207419). We also thank M. C. Simon
Hernández Ortega, M. C. Hector Rios and Q. María de la Paz Orta Pérez for analytical data and José
David Vázquez Cuevas for computer services. M. C. Nicolás Sánchez López and Dr. Baldomero
Esquivel Rodríguez are thanked for their help in acquiring NMR data during the COVID-19 lockdown.
Keywords: Bimetallic complexes • Hydride ligands • Non-symmetric POCOP pincer ligands • Planar
chirality • Ruthenium
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