Variation on the π-Acceptor Ligand within a RhI?N-Heterocyclic Carbene Framework: Divergent Catalytic Outcomes for Phenylacetylene-Methanol Transformations

Article abstract of DOI:10.1002/ejic.202100399

A series of neutral and cationic rhodium complexes bearing IPr {IPr=1,3-bis-(2,6-diisopropylphenyl)imidazolin-2-carbene} and π-acceptor ligands are reported. Cationic species [Rh(η⁴-cod)(IPr)(NCCH₃)]⁺ and [Rh(CO)(IPr)(L)₂]⁺ (L=pyridine, CH₃CN) were obtained by chlorido abstraction in suitable complexes, whereas the cod-CO derivative [Rh(η⁴-cod)(IPr)(CO)]⁺ was formed by the carbonylation of [Rh(η⁴-cod)(IPr)(NCCH₃)]⁺. Alternatively, neutral derivatives of type RhCl(IPr)(L)₂ {L=^tBuNC or P(OMe)₃} can be accessed from [Rh(μ-Cl)(η²-coe)(IPr)]₂. In addition, the mononuclear species Rh(CN)(η⁴-cod)(IPr) was prepared by cyanide-chlorido anion exchange, which after carbonylation afforded the unusual trinuclear compound [Rh{1κC,2κN-(CN)}(CO)(IPr)]₃. Divergent catalytic outcomes in the phenylacetylene-methanol transformations have been observed. Thus, enol ethers, arisen from hydroalkoxylation of the alkyne, were obtained with neutral Rh?CO catalyst precursors whereas dienol ethers were formed with cationic catalysts. Variable amounts of alkyne dimerization, cyclotrimerization or polymerization products were obtained in the absence of a strong π-acceptor ligand on the catalyst.

Full text of DOI:10.1002/ejic.202100399

European Journal of Inorganic Chemistry
Accepted Article
Title: Variation on the π-Acceptor Ligand within a RhI-N-Heterocyclic
Carbene Framework: Divergent Catalytic Outcomes for
Phenylacetylene-Methanol Transformations
Authors: Maria Galiana-Cameo, Vincenzo Passarelli, Jesús J. Pérez-
Torrente, Andrea Di Giuseppe, and Ricardo Castarlenas
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100399
Link to VoR: https://doi.org/10.1002/ejic.202100399
01/2020

10.1002/ejic.202100399
European Journal of Inorganic Chemistry
FULL PAPER
Variation on the π-Acceptor Ligand within a RhI-N-Heterocyclic
Carbene Framework: Divergent Catalytic Outcomes for
Phenylacetylene-Methanol Transformations
María Galiana-Cameo,[a] Vincenzo Passarelli,[a] Jesús J. Pérez-Torrente,[a] Andrea Di Giuseppe,*[a,b]
and Ricardo Castarlenas*[a]
[a]
M. Galiana-Cameo, Dr. Vincenzo Passarelli, Prof. J. J. Pérez-Torrente, Dr. A. Di Giuseppe, Dr. R. Castarlenas
Departamento de Química Inorgánica-Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), Universidad de Zaragoza-CSIC
C/Pedro Cerbuna 12, CP. 50009, Zaragoza (Spain)
E-mail: rcastar@unizar.es, andrea.digiuseppe@univaq.it
http://www.isqch.unizar-csic.es/ISQCHportal/grupos.do?id=29
Dr. A. Di Giuseppe
[b]
Dipartimento di Scienze Fisiche e Chimiche, Università dell’Aquila
via Vetoio, I-67100 Coppito (AQ), Italy
Abstract: A series of neutral and cationic rhodium complexes
bearing IPr {IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazolin-2-
carbene} and π-acceptor ligands are reported. Cationic species
[Rh(η4-cod)(IPr)(NCCH3)]+ and [Rh(CO)(IPr)(L)2]+ (L = pyridine,
CH3CN) were obtained by chlorido abstraction in suitable
routine method for the analysis of the electronic properties of
almost any newly synthesized NHC-type architecture.[5]
Regarding catalytic applications, hydroformylation is prevalent for
the Rh-NHC-CO systems, certainly prompted by the involvement
of carbon monoxide as reagent.[6] Nevertheless, interesting
carbon-carbon and carbon-heteroatom coupling reactions have
also been described (Figure 1).[7] Particularly remarkable are the
contributions by the research groups led by Profs. Messerle[7f] and
Bera[7h] dealing with the preparation of enol ethers by alkyne
hydroalkoxylation. Other π-acceptor ligands alternative to CO
include the neutral isocyanide[8] or phosphite[9] moieties or the
cyanide[10] anion.
complexes,
whereas
the
cod-CO
derivative
[Rh(η4-
cod)(IPr)(CO)]+ was formed by the carbonylation of [Rh(η4-
cod)(IPr)(NCCH3)]+. Alternatively, neutral derivatives of type
RhCl(IPr)(L)2 {L = tBuNC or P(OMe)3} can be accesed from [Rh(μ-
Cl)(η2-coe)(IPr)]2. In addition, the mononuclear species
Rh(CN)(η4-cod)(IPr) was prepared by cyanide-chlorido anion
exchange, which after carbonylation afforded the unusual
trinuclear compound [Rh{1C,2N-(CN)}(CO)(IPr)]3. Divergent
catalytic
outcomes
in
the
phenylacetylene-methanol
transformations have been observed. Thus, enol ethers, arisen
from hydroalkoxylation of the alkyne, were obtained with neutral
Rh-CO catalyst precursors whereas dienol ethers were formed
with cationic catalysts. Variable amounts of alkyne dimerization,
cyclotrimerization or polymerization products were obtained in the
absence of a strong π-acceptor ligand on the catalyst.
Introduction
The detailed study of the coordination properties of
organometallic catalysts stands out at the central core of synthetic
efficiency. A complex interplay between the stereoelectronic
influence exerted by the ligands and the availability of vacant sites
in the metal coordination sphere accounts for a successful
catalytic performance. Commonly, each elementary step of a
catalytic cycle (oxidative addition, reductive elimination…) can be
enhanced by opposite electronic effects. Hence, the concurrence
of stronger σ-donor with powerful π-acceptor ligands could play a
synergic role over the whole catalytic process. Moreover, strongly
bonded ligands may prevent undesired reactivity of highly
unsaturated catalytic active species. In this context, N-
Heterocyclic carbenes (NHCs)[1] and carbon monoxide[2] are a
typical pair of donor-acceptor collaborative ligands. The benefits
of this association redound not only to the stability of metal
complexes,[3] but also to the enhancement of the catalytic
activity.[4] Particularly for rhodium-based examples, the
preparation of RhCl(NHC)(CO)2 species is now established as a
Figure 1. Representative catalytic applications of Rh-NHC-CO complexes.
1
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10.1002/ejic.202100399
European Journal of Inorganic Chemistry
FULL PAPER
In the last decade, our research interests have been focused on
the study of catalytic applications of Rh-NHC complexes.[11] We
have developed efficient promoters for an array of transformations
ranging from β-selective H/D exchange of α-olefins, alkyne
hydrofunctionalizations (hydrothiolation, hydrophosphination or
head-to-tail selective dimerization), or carbon-carbon and carbon-
nitrogen couplings via C-H activation. The detailed study of the
coordination chemistry of Rh-NHC-based complexes has enabled
the evolution of the parent catalytic systems.[12] Particularly, the
combined stereoelectronic effects of the tandem NHC-CO have
been fundamental for the selective formation of gem-vinyl sulfides
in alkyne hydrothiolation processes due to the stabilization of RhI
catalytic active species.[12b] Moreover, systematic studies on the
coordination of small molecules to neutral and cationic RhI-NHC
scaffolds have been carried out by James,[13] Crudden,[14] and our
research group.[15] Now, herein we disclose the preparation of a
set of RhI complexes bearing a powerful σ-electron-donor NHC
The crystal structure of 4 and 5 (Figure 2) shows a distorted
square planar environment at the metal centre with a cis
disposition of the NHC and CO ligands [4, C1-Rh-C42 87.98(12)º;
5, C1-Rh1-C36 90.96(16)º], pyridine or acetonitrile occupying the
remaining coordination sites [4, N30-Rh1-N36 84.22(9)º; 5, N30-
Rh1-N33 85.66(13)º]. Remarkably, the rhodium-nitrogen bond
lengths are indicative of a similar trans influence of the CO and
the NHC ligands [4, Rh1-N30 2.110(2) Å, Rh1-N36 2.123(2) Å; 5,
Rh1-N30 2.055(4) Å, Rh1-N33 2.072(3) Å]. Also, it is worth a
mention that the steric hindrance of the NHC ligand forces the cis
pyridine (4) or acetonitrile (5) out of the ideal arrangement with
respect to the rhodium-nitrogen bond. As a matter of fact, as for 4
the pitch angle of the pyridine ligand N36-C37-C38-C39-C40-C41
cis to C1 ( 12.0º) is bigger than that calculated for the pyridine
ligand N30-C31-C32-C33-C34-C35 trans to C1 ( 5.0º). By the
same token, as for 5 the angle Rh1-N33-C34 [170.7(3)º] is smaller
than the angle Rh1-N30-C31 [176.4(5)º]. Finally, it should also be
noted that the NHC ring C1-N2-C3-C4-N5 of 5 adopts the
perpendicular least hindered position with respect to the
coordination plane Rh1-C1-C36-N30-N33 (89.9º) whereas the
NHC ring C1-N2-C3-C4-N5 of 4 significantly deviates from the
perpendicular arrangement (64.4º). In this regard, a thorough
inspection of the structure of 4 revealed that a CH···π interaction
involves the C41-H41 bond and the C18-C19-C20-C21-C22-C23
aromatic ring, reasonably forcing the NHC out of the
perpendicular arrangement with respect to the coordination plane
(Figure 3). In this connection, it is worth noting that the hydrogen-
carbon distances H41-C18 (2.80 Å), H41-C19 (2.92 Å), H41-C20
(2.99 Å) H41-C21 (2.95 Å), H41-C22 (2.87 Å), H41-C23 (2.82 Å)
are smaller than the sum of the Van der Waals radii of carbon and
hydrogen (3.05 Å).
and
a variety of π-acceptor ligands, and its impact on
phenylacetylene-methanol catalytic transformations.
Results and Discussion
Synthesis of cationic complexes [Rh(IPr)(L)2L’]+ and [Rh(4-
cod)(IPr)L]+
The new cationic complexes bearing π-acceptor ligands have
been prepared from the organometallic precursors RhCl(4-
cod)(IPr)[13a] (1), RhCl(CO)2(IPr)[13c] (2), and [Rh(-Cl)(2-
coe)(IPr)]2[13b] (3) IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazolin-
2-carbene; cod = 1,5-cyclooctadiene, coe = cis-cyclooctene.
Treatment of 2 with TlPF6 in coordinating solvents afforded
cationic complexes of type [Rh(CO)(IPr)(L)2][PF6] (4, L = py; 5, L
= CH3CN), as a result of the removal of the chlorido ligand and
the replacement of the carbonyl moiety located trans to the
carbene with a solvent molecule (Scheme 1). The use of AgPF6
resulted in the formation of other unidentified complexes as
impurities. The bis-CO complex [Rh(CO)2(IPr)(NCCH3)][PF6] (6)
was obtained by bubbling carbon monoxide through a CH2Cl2
solution of 5 at room temperature. The carbonylation of 5 is
reversible, thus, addition of CH3CN to a solution of 6 in CH2Cl2
(1:1) resulted in the recovery of bis-acetonitrile complex 5. The
complexes were obtained in 65-80 % yield as yellow solids which
are stable in the open air for months. Decoordination of a CO
molecule located trans to a powerful electron-donor NHC ligand
is not unexpected.[15b,16] In fact, this behavior has been recently
exploited in the biomedical applications of CO-release molecules
(CORMs).[17] Moreover, although pyridine is an ubiquitous ligand
in coordination chemistry, a mutually pyridine-NHC cis disposition
is uncommon in square-planar d8 structures[15b,18] and the
concomitant presence of two molecules of pyridine is
unprecedented in RhI-NHC chemistry.[19]
Figure 2. ORTEP view of [Rh(CO)(IPr)(py)2]+ in
4
(left) and
[Rh(CO)(IPr)(CH3CN)2]+ in 5 (right) with ellipsoids at 50% probability. For clarity
hydrogen atoms are omitted and a wireframe style is adopted for the 2,5-
(iPr)2C6H3 moiety of the IPr ligand. Selected bond lengths (Å) and angles (º) are:
4, Rh1-C42 1.815(3), Rh1-C1 2.010(3), Rh1-N30 2.110(2), Rh1-N36 2.123(2),
O43-C42 1.142(4), C1-Rh1-C42 87.98(12), C1-Rh1-N30 177.36(11), C1-Rh1-
N36 98.34(10), N30-Rh1-N36 84.22(9); 5, C1-Rh1 1.997(3), C36-Rh1 1.814(5),
N30-Rh1 2.055(4), N33-Rh1 2.072(3), C36-O37 1.143(5), C1-Rh1-C36
90.96(16), C1-Rh1-N33 91.96(11), C1-Rh1-N30 177.28(14), N30-Rh1-N33
85.66(13), Rh1-C31-N30 176.4(5), Rh1-C34-N33 170.7(3).
Scheme 1. Preparation of cationic solvento-complexes 4-6.
2
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10.1002/ejic.202100399
European Journal of Inorganic Chemistry
FULL PAPER
Figure 3. Brandl-Weiss view[20] of the C41-H41··· π interaction in 4: C41-X 3.25
Å, C41-H41-X 132.6º, Hp-X 0.20 Å. Hp is the projection of H41 on the plane
C18-C19-C20-C21-C22-C23, and X is the centroid of the C18-C19-C20-C21-
C22-C23 ring.
Figure 4. Selected regions of the 1H-1H NOESY, and 1H-15N HMQC NMR
spectra of 4 in CD2Cl2.
The NMR spectroscopic data confirm that the solid state
structure of complexes is maintained in solution. Typical signals
for pyridine, acetonitrile and IPr are observed in the 1H NMR
spectra of CD2Cl2 solutions of 4-6, with no significant changes at
low temperature, indicating fast rotation of the IPr ligand.[15b]
Discrimination between the different disposition of the solvent
molecules with regard to IPr can be achieved by H-1H-NOESY
1
Scheme 2. Preparation of cationic complexes 7 and 8.
NMR experiments (Figure 4). Rather surprisingly, no exchange
peaks were observed between both nitrogenated ligands in 4 or
5, indicating a tight coordination. Besides, the 13C{1H}-APT NMR
spectra are meaningful for detection of the carbon-coordinated
ligands to rhodium. Thus, two doublets ascribed to IPr and CO
were observed for 4 and 5 at δ > 170 ppm, whereas the spectrum
of the bis-carbonyl-IPr complex 6 displays three doublets in the
same region. Moreover, 1H-13C HMBC correlation peaks between
imidazolinyl protons and carbene-carbon atoms within IPr
moieties facilitates the assignment of the Rh-CIPr doublets.
Resonances corresponding to carbonyl ligands (δ ≈ 185 ppm) are
deshielded around 10 ppm with respect to those of IPr, which in
turn appear slightly shielded with respect to the typical range
observed for neutral Rh-IPr-CO[13c,7k,15b,16c,21] derivatives (δ 171-
177 vs 182-186 ppm), likely as a consequence of a decrease of
the electron density of the metallic center. The presence of two
The crystal structure of 7 and 8 shows a distorted square planar
environment at the metal centre presenting a cis disposition of the
NHC and the CH3CN and CO ligands, respectively, with the
[Rh(η4-cod)(IPr)] moieties of
7
and
8
being virtually
superimposable (Figure 5). Remarkably in both compounds the
NHC C1-N2-C3-C4-N5 ring deviates from the perpendicular
arrangement with respect to the related coordination plane Rh1-
C1-CT01-CT02-N38 or Rh1-C1-CT01-CT02-C38 (7, 50.4º; 8,
50.3º), which may be the consequence of the short contacts
observed within the olefinic moiety C34-H34 and one aromatic
wingtip of the NHC ligand (7, H34···C18 2.53 Å, H34···C23 2.63
Å; 8, H34···C6, 2.62 Å; H34···C11, 2.61 Å). In agreement with the
stronger trans influence of carbon monoxide vs acetonitrile the
C34-C35 bond length in 7 [1.398(2) Å] is longer than in 8 [1.367(4)
Å] whereas the rhodium-centroid distance is shorter in 7
[2.02291(13) Å] than in 8 [2.1710(2) Å]. On the other hand, the
C30-C31 bond lengths as well as the Rh-CT01 distances in 7 and
8 are virtually identical (7: C30-C31, 1.380(3) Å, Rh-CT01
2.09156(14) Å; 8: C30-C31, 1.380(4) Å, Rh-CT01 2.11478(19) Å].
The NMR spectra of 7 and 8 display the typical signals for IPr
and cod. Particularly, a singlet at δ 2.28 ppm, corresponding to
the acetonitrile ligand, appears in the 1H NMR spectrum of 7,
whereas a doublet at δ 180.4 ppm (JC-Rh = 77.2 Hz), ascribed to
the CO ligand, is observed in 13C{1H}-APT NMR spectrum of 8.
According to the solid structure, the very different trans-influence
of acetonitrile and CO ligands is reflected in the chemical shift of
the =CH resonances of the coordinated olefin (Figure 6). The
13C{1H}-APT NMR spectrum of the acetonitrile derivative 7
displays a doublet at δ 78.2 ppm, which is downfield shifted
around 36 ppm for the CO counterpart 8. Moreover, the JC-Rh
decreases from 13.2 to 5.6 Hz as a consequence of longer
separation between rhodium and η2-olefin. In addition, a strong
CO stretching band at 2037 cm-1 was observed in the IR spectra
of 8.
1
pyridine ligands in 4 was further confirmed by a H-15N HMQC
NMR experiment. Two correlation peaks appear at δ 255.0 and
248.8 ppm, in the expected range for coordinated pyridines.[11c]
The IR spectra of 4 and 5 show one strong CO stretching band at
1975 and 1988 cm-1, respectively, whereas two bands at 2102
and 2031 cm-1 were observed for 6, in agreement with a cis
disposition of the CO ligands.
An alternative access to cationic derivatives entails the
extraction of chlorido ligand of the cod-derivative 1. The
acetonitrile-solvento complex [Rh(4-cod)(IPr)(NCCH3)][PF6] (7)
was cleanly prepared by treatment of 1 with AgPF6 and isolated
as a yellow microcrystalline solid in 72 % yield (Scheme 2).
Bubbling CO through a CH2Cl2 solution of 7 did not result in the
expected substitution of the cod ligand, but instead the diolefin-
CO complex [Rh(CO)(4-cod)(IPr)][PF6] (8) was obtained. This
unusual behavior is unprecedented for rhodium(I) chemistry,[22,23]
although some examples has been reported for iridium.[24] As a
curiosity, 8 is a nice example of all metal-carbon bonds complex.
3
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10.1002/ejic.202100399
European Journal of Inorganic Chemistry
FULL PAPER
Scheme 3. Preparation of neutral complexes 9 and 10.
The crystal structure of 9 and 10 reveals a distorted square
planar geometry for the metal centre with a cis disposition of the
NHC and the chlorido ligands [9, C1-Rh-Cl 85.47(5)º; 10, C1-Rh1-
Cl1 91.54(6)º] (Figure 6). The two remaining coordination sites
are occupied by tert-butyl isocyanido [9, C36-Rh-C30 86.65(7)º]
Figure 5. ORTEP view of [Rh(η4-cod)(IPr)(CH3CN)]+ in 7 (left) and [Rh(η4-
cod)(IPr)(CO)]+ in 8 (right) with ellipsoids at 50% probability. For clarity most
hydrogen atoms are omitted and a wireframe style is adopted for the 2,5-
(iPr)2C6H3 moiety of the IPr ligand. Selected bond lengths (Å) and angles (º) are:
7, C1-Rh1 2.0682(14), N38-Rh1 2.0536(14), Rh1-CT01 2.09156(14), C30-C31
1.380(3), Rh1-CT02 2.02291(13), C34-C35 1.398(2), N38-Rh1-C1 92.91(6),
CT02-Rh1-CT01 86.628(6); 8, Rh1-C1 2.067(2), Rh1-C38 1.870(3), C38-O39
1.137(3), Rh1-CT01 2.11478(19), C30-C31 1.380(4), Rh1-CT02 2.1710(2),
C34-C35 1.367(4), C38-Rh1-C1 92.73(10), CT01-Rh1-CT02 84.271(8). CT01
and CT02 are the centroid of C30 and C31, and of C34 and C35, respectively.
or trimethyl phosphite [10, P2-Rh1-P1 90.58(2)º]. As
a
consequence of the higher trans influence of the NHC ligand
when compared with the chlorido ligand, the Rh-C30 and Rh-C36
bond lengths in 9 as well as the Rh-P1 and Rh-P2 bond lengths
in 10 are different. Indeed, Rh-P1 and Rh–C30 (trans to C1) are
longer than Rh-P2 and Rh-C36 (trans to chlorido), respectively. It
is worth mentioning that both in 9 and in 10 the NHC ring slightly
deviates from the perpendicular arrangement with respect to
coordination plane (9, 79.3º; 10, 73.6º). A thorough examination
of the structure did not reveal any intramolecular contacts that
could be responsible for this deviation, so it can be argued that it
is reasonably the consequence of the crystal packing, or possibly
of subtle electronic effects. Finally, the bond angles within the tert-
butyl isocyanido ligand cis to the NHC moiety are remarkable. As
a matter of fact, the Rh-C36-N37 [172.40(16)º] and the C36-N37-
C38 [156.13(18)º] angles deviate from the ideal value of 180º
probably as a consequence of the steric repulsion between the
NHC wingtips and the tert-butyl group.
Figure 6. Selected region of the 13C{1H}-APT NMR spectra showing the Rh-
cod resonances for 7 and 8.
Synthesis of neutral Rh-IPr complexes with π-acceptor
ligands
We sought to study the influence of the coordination of π-
acceptor ligands different to CO, namely isocyanides and
phosphites, on the structure and properties of Rh-IPr complexes.
Isocyanides (R-N≡C) have an isoelectronic structure with carbon
monoxide, being better σ-donors and poorer π-acceptors.[8] In
other way, the π-acceptor character of phosphites relies on their
alkoxy substituents. The introduction of this conically structured
ligand allows to create a more complex sterical interplay between
NHC and the π-acid moiety, rather limited with linear ligands such
as CO.[9] Thus, treatment of 3 with 2 equivalents of tert-butyl
isocyanide (tBuNC) or trimethyl phosphite gives the bis-
substituted neutral complexes [RhCl(CNtBu)2(IPr)] (9, 61 % yield)
or [RhCl(IPr){P(OMe)3}2] (10, 68% yield), respectively (Scheme 3).
It is noteworthy that Rh-NHC complexes bearing isocianyde[25] or
phosphite[26] ligands are scarce.
Figure 7. ORTEP view of [RhCl(CNtBu)2(IPr)] (9) (left) and
[RhCl(IPr){P(OMe)3}2] (10) (right) with ellipsoids at 50% probability. For clarity
hydrogen atoms are omitted and a wireframe style is adopted for the 2,5-
(iPr)2C6H3 moiety of the IPr ligand. Selected bond lengths (Å) and angles (º) are:
9, C1-Rh 2.0512(16), C30-Rh 1.9389(18), C30-N31 1.158(2), C36-N37 1.173(2),
C36-Rh 1.8674(18), C1-Rh-Cl 85.47(5), C36-Rh-C30 86.65(7), C30-Rh-C1
172.74(7), C36-Rh-C1 100.54(7), C30-N31-C32 175.62(19), C36-N37-C38
156.13(18), N37-C36-Rh 172.40(16), N31-C30-Rh 176.85(17); 10, C1-Rh1
2.070(2), P1-Rh1 2.2131(6), P2-Rh1 2.1461(7), C1-Rh1-P2 92.19(6), C1-Rh1-
P1 172.63(6), P2-Rh1-P1 90.58(2), C1-Rh1-Cl1 91.54(6).
The more noticeable feature of the NMR data of 9 was the
appearance of three doublets in the 13C{1H}-APT spectrum at δ
193.6 (JC-Rh = 45.0 Hz), 161.2 (JC-Rh = 71.8.0 Hz), and 150.4 ppm
4
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10.1002/ejic.202100399
European Journal of Inorganic Chemistry
FULL PAPER
(JC-Rh = 52.6 Hz), which are ascribed to IPr and each isocyanido
ligands, respectively. Regarding to 10, the signal of the carbene
carbon atom is observed at δ 192.1 ppm as a doublet of doublets
of doublets in the 13C{1H}-APT spectrum, as a result of C-Rh (45.7
Hz) and C-P (167.4 and 12.6 Hz) couplings. In agreement with
the proposed structure, the 31P{1H} spectrum of 10 displays two
doublets of doublets at δ 143.7 and 141.6 ppm with JP-P = 63.0 Hz
and JP-Rh = 159.9 and 263.3 Hz, respectively. It is interesting to
note, that the carbene resonance of 9 and 10 is downfield shifted
by around 15-20 ppm related to Rh-CO derivatives, indicating that
CO is a much stronger π-acceptor than isocyanido or phosphito
ligands.
The crystal structure of 12 reveals a trinuclear motif supported
by bridging 1C,2N-cyanido ligands in which the [Rh(CN)]3 core
slightly deviates from planarity (max deviation 0.23 Å; mean
absolute deviation 0.13 Å). The square planar environment at
each rhodium centre is completed by one NHC and one carbon
monoxide ligand (CCO-Rh-CNHC
,
95.0º, av.). The three
[Rh(IPr)(CO)(C-CN)(N-NC)] fragments are crystallographically
independent, nonetheless similar bond lengths and angles are
observed. Remarkably, the Rh-C-N-Rh fragments deviate from
linearity reasonably in order for the square planar metal moieties
to fit in the trinuclear core [Rh-CCN-NCN, 169.2º, av.; Rh-NCN-CCN
,
161.6º, av.]. Accordingly, the angles CCN-Rh-NCN (84.4º, av.)
deviate from the expected value for a square planar geometry.
A second synthetic strategy to introduce a π-acid moiety in the
Rh-NHC framework is the replacement of the chlorido ligand in 1
by an anionic π-acceptor ligand, such as cyanide. The cyanide
anion is isoelectronic and isosteric with CO, however, the small
variance in electronegativity between C and N atoms makes the
energy on the lone pair at the C atom similar to that of N. For this
reason, and in contrast to the CO ligand, N≡C- can act as a
1C,2N bridging ligand between two metals.[27] In addition, the
negative formal charge on the C atom makes N≡C- a stronger σ-
donor. The total charge is also responsible for the higher π*-
orbitals energy compared to CO, making cyanide a weaker π-
acceptor than CO. NHC-cyanido transition metal complexes have
recently received an increasing attention for their interesting
photochemical properties.[28] Thus, treatment of complex 1 with
AgCN gave the neutral complex Rh(CN)(η4-cod)(IPr) (11) which
was isolated as a yellow solid in 73% yield (Scheme 4). Similarly
to 8, compound 11 has exclusively carbon donor ligands.
Decoordination of chelate cod could be carried out by bubbling
CO(g) through a CH2Cl2 solution of 11 resulting in the formation
of the cyano-bridged trinuclear complex [Rh{1C,2N-
(CN)}(CO)(IPr)]3 (12) (see below).
Scheme 4. Preparation of neutral complexes 11 and 12.
Figure 8. ORTEP view of [Rh(CN)(η4-cod)(IPr)] (11) (top) and [Rh(1C,2N-
CN)(CO)(IPr)]3 (12) (bottom) with ellipsoids at 50% probability. For clarity
hydrogen atoms are omitted and a wireframe style is adopted for the 2,5-
(iPr)2C6H3 moiety of the IPr ligand. Selected bond lengths (Å) and angles (º) are:
11, C1-Rh 2.060(2), C38-Rh 2.034(2), C38-N39 1.123(3), Rh-CT01 2.0925(4),
C30-C31 1.376(3), Rh-CT02 2.0657(4), C34-C35 1.391(3), C38-Rh-C1
88.56(8), CT02-Rh-CT01 85.83(2), CT01 and CT02 are the centroid of C30 and
C31, and of C34 and C35, respectively; 12, C1-Rh1 2.029(7), C32-Rh1 1.785(8),
C30-Rh1 2.014(8), C30-N31 1.160(8), N64-Rh1 2.036(6), C34-Rh2 2.020(7),
C65-Rh2 1.778(8), C63-Rh2 1.990(8), C63-N64 1.158(8), N97-Rh2 2.073(6),
C67-Rh3 2.024(7), C98-Rh3 1.768(7), C96-Rh3 1.981(8), C96-N97 1.157(8),
N31-Rh3 2.079(6), C32-Rh1-C1 95.0(3), C30-Rh1-C1 168.9(3), C30-Rh1-N64
84.9(2), C65-Rh2-C34 95.6(3), C34-Rh2-N97 89.2(2), C63-Rh2-N97 81.2(2),
C98-Rh3-C67 94.2(3), C96-Rh3-C67 174.9(3), C67-Rh3-N31 91.5(2), C96-
Rh3-N31 87.1(2), N31-C30-Rh1 166.6(6), C30-N31-Rh3 158.0(6).
The structure of the cyanido complexes 11 and 12 was
elucidated by X-ray structural analysis (Figure 8). The
coordination environment of rhodium in the crystal structure of 11
is distorted square planar, the NHC and cyanide ligands
occupying two cis positions [C38-Rh-C1 88.56(8)º]. The olefinic
carbon-carbon bond lengths [C30-C31 1.376(3) Å, C34-C35
1.391(3) Å] as well as the rhodium-centroids distances are similar
[Rh-CT01 2.0925(4) Å, Rh-CT02 2.0657(4) Å], suggesting that the
trans influence of the NHC and cyanide ligands in 11 is similar.
Finally, when compared with the cod derivatives 7 and 8, the NHC
core C1-N2-C3-C4-N5 deviates from the perpendicular
arrangement with respect to the coordination plane Rh-C1-CT01-
CT02-C38 to a lesser extent (71.6º) and the examination of the
crystal structure did not reveal any intermolecular contacts.
5
This article is protected by copyright. All rights reserved.

10.1002/ejic.202100399
European Journal of Inorganic Chemistry
FULL PAPER
The formation of a multinuclear architecture supported by
cyanido-bridged ligands is remarkable but not unprecedented.[27]
Heteronuclear tetrameric pseudoplanar structures involving Rh-
Co,[27b] Rh-Ru[27c] or Co-Ni[27d] pairs or even cubane-type
clusters[27b-c] have been described previously. Particularly,
Hawthorne et al described a rhodium trinuclear complex related
to 12 bearing phosphines and carboranes as ancillary ligands.[27a]
The NMR spectra of 12 show that the cyanido-bridged structure
is maintained in solution. Thus, while the carbon atom of the
6, and 8). The remaining catalysts promoted the transformation of
phenylacetylene without involvement of methanol. Thus, the cod
complexes 1, 7, and 11, lacking the CO moiety, polymerized
phenylacetylene, although with formation of variable amounts of
cyclotrimerization or dimerization products (entries 1, 7 and 11).
The bis-pyridine complex 4 is inefficient as catalyst only
converting 15% of phenylacetylene to unidentified products (entry
4). The presence of methanol did not significantly affect the
catalytic performance of coe dimer 3, which has been previously
found to promote the cyclotrimerization and dimerization of
phenylacetylene (entry 3).[11c] The phosphite complex 10 is the
most efficient among the catalysts presented in this study. Almost
full conversion of phenylacetylene was attained with 98 %
selectivity to the gem-enyne 15a (entry 10), although it is not
cyanide ligand in 11 appears as a doublet at δ 141.0 ppm (JC-Rh
=
45.0 Hz) in the 13C{1H}-APT spectrum, the equivalent cyanide
ligands of 12 are observed as a doublet of doublets at 156.5 ppm
as a result of a direct 1JC-Rh of 45.4 Hz and a 2JC-Rh of 5.4 Hz arising
within the Rh-C≡N-Rh moiety (Figure 9). Although the actual
nuclearity of 12 in solution cannot be undoubtedly determined, we
assume the trinuclear structure to be the most likely, in
accordance to that observed in the solid state. In addition, a
strong CO stretching band at 1954 cm-1 was observed in the IR
spectra of 12.
competitive with other
Rh-NHC
alkyne dimerization
catalysts.[11h,12c] Finally, the cyanido trimer 12 also promotes
alkyne dimerization but less selectively (entry 12).
Scheme 5. Reaction products in the catalytic transformation of phenylacetylene
in MeOH/DMA
Table 1. Phenylacetylene transformations in MeOH/DMA with catalysts 1-12.[a]
Figure 9. Selected region of the 13C{1H}-APT NMR spectra of 12 showing the
Rh-C≡N-Rh resonance.
Enol
ether
16a/b
Dienol
ether
17a/b[b]
Conv.
(%)
Trimer
14a/b
Dimer
15a/b
Entry
Cat.
Polymer
Catalytic
transformations
activity
for
phenylacetylene-methanol
1
2
1
2
81
75
38
---
---
---
---
---
34
---
---
---
87
---
42/20
---
---
---
---
60/38
---
---
2
Hydroalkoxylation of alkynes is a straightforward method for the
preparation of enol ethers. The groups of Messerle[7f] and Bera[7h]
have shown that Rh-NHC complexes bearing CO ligands are
efficient catalysts for these transformations. Thus, we have
studied the catalytic activity of complexes 1-12 for the addition of
methanol to phenylacetylene. After a preliminary screening of
different solvents and temperatures, a mixture of MeOH/DMA (1:1,
2 mL) (DMA = dimethylacetamide) using 1 mmol of alkyne and 2
mol % catalyst loading at 70 ºC for 24 h were established as
optimal conditions[7h] (Scheme 5, Table 1). The catalytic outcome
is strongly dependent on the catalyst. In addition to the expected
alkyne hydroalkoxylation to enol ethers (16), alkyne
polymerization (13),[29] cyclotrimerization (14)[30] or dimerization
(15)[12c] were also observed. The new dienol ethers (17)[31]
resulting from methanol addition to enynes 15 were also formed
in some cases. The best catalyst for hydroalkoxylation of
phenylacetylene to form 16 was the previously prepared neutral
dicarbonyl complex 2. A conversion of 75% with 58:39 E:Z
selectivity was obtained (entry 2). Among the new catalysts, only
the isocyanido complex 9 promotes the formation of enol ethers
with 17:50 E:Z selectivity, although 31% of enynes 15 arisen from
dimerization were also observed (entry 9). Participation of
methanol as substrate took place also for the cationic carbonyl
derivatives 5, 6, and 8, but to yield the dienol ethers 17 (entries 5,
3
3
24
15/55
---
28/2
---
---
4
4
15
---
---
5
5
55
---
---
---
39/61
40/60
---
6
6
52
---
---
---
7
7
>99
87
8/9
---
36/13
---
---
8
8
---
39/61
2
9
9
74
---
13/18
98/2
4/4
17/50
---
10
11
12
10
11
12
>99
>99
>99
---
---
2/3
---
---
---
74/26
---
---
[a] Reaction conditions: 1 mmol of phenylacetylene, 1 mL of methanol, 0.02
mmol of catalyst, 1 mL of DMA (dimethylacetamide), 70 °C, 24 h. [b]
Determinated by GC as isomers mixture.
The results presented in Table 1 show divergent catalytic
outcomes depending on the π-acceptor ligand and the charge of
6
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10.1002/ejic.202100399
European Journal of Inorganic Chemistry
FULL PAPER
the catalyst. It has been previously proposed that
hydroalkoxylation of alkynes promoted by rhodium catalysts
proceeds via vinylidene and alkoxycarbene intermediates, which
were stabilized by a π-acceptor ligand (Scheme 6).[7h,32] In
accordance to that, the neutral Rh-IPr-CO complex 2 is the most
efficient catalyst for the formation of enol ethers. The introduction
of a less π-acidic ligand such as isocyanido in 9, resulted in a
slightly lower selectivity and the concomitant formation of enyne
dimers. In fact, catalysts bearing ligands of lower π-acceptor
ability such as phosphito (10) and cyanido (12) selectively
afforded dimerization products. Regarding catalyst charge,
cationic Rh-CO complexes 5, 6, and 8 promote the formation of
dienol ethers. A rational explanation could arise from the
presence of an extra vacant site with regard to neutral
counterparts which allows the coordination of a second molecule
of alkyne to yield a [2+2] coupling with the alkoxycarbene
intermediate resulting in the formation of dienol ethers.[33] As
previously described,[29] Rh-cod complexes 1, 7, and 11, lacking
displacement of the cod ligand and the formation of the unusual
trinuclear carbonyl compound [Rh{1C,2N-(CN)}(CO)(IPr)]3
supported by bridging cyanido ligands.
The catalytic performance of the new Rh-IPr complexes for the
addition of methanol to phenylacetylene has been studied. It has
been found that the catalytic outcome is strongly dependent on
the Rh-IPr-π-acceptor catalyst. Thus, the neutral bis-CO or bis-
isocyanido catalysts promote the hydroalkoxylation reaction to
yield enol ethers, whereas the bis-phosphito and bis-cyanido
precursors favors alkyne dimerization. In contrast, cationic CO-
derivatives afford dienol ether derivatives resulting from the triple
coupling of two alkynes and methanol. Finally, Rh-cod complexes
promote the polymerization of phenylacetylene. The results
reported herein highlight how a slight fine-tuninig of the
stereoelectronic properties of the catalyst results in divergent
catalytic outcomes.
a
π-acceptor ligand, promoted the polymerization of
Experimental Section
phenylacetylene.
General Considerations. All reactions were carried out with rigorous
exclusion of air and moisture using Schlenk-tube techniques and dry box
when necessary. The organometallic precursors RhCl(4-cod)(IPr)[13a] (1),
RhCl(CO)2(IPr)[13c] (2), and [Rh(µ-Cl)(IPr)(η2-coe)]2[13b] (3), were prepared
as previously described in the literature. Chemical shifts (expressed in
parts per million) are referenced to residual solvent peaks (1H and 13C{1H}),
NH3 (15N), H3PO4 (31P), or CFCl3 (19F). Coupling constants, J, are given in
Hz. Spectral assignments were achieved by combination of 1H-1H COSY,
13C{1H}-APT and 1H-13C HSQC/HMBC experiments.
Preparation of [Rh(CO)(IPr)(py)2][PF6] (4): A yellow solution of 2 (200
mg, 0.345 mmol) in 10 mL of pyridine protected from light by an aluminium
foil was treated with thallium(I) hexafluorophosphate (130 mg, 0.372 mmol)
and it was stirred at room temperature for 30 min. Then, the suspension
was filtered through celite and the solvent was evaporated to dryness. The
resulting solid was dissolved in 10 mL of dichloromethane, filtered again
and concentrated to ca. 1 mL. Addition of diethyl ether induced the
precipitation of a light yellow solid, which was washed with diethyl ether (3
× 5 mL) and dried in vacuo. Yield: 187 mg (65%). Anal. Calcd. for
C38H46N4F6OPRh: C, 55.48; H, 5.64; N, 6.81. Found: C, 55.27; H, 5.44; N,
7.01. IR (cm-1, ATR): 1975 ν(CO). HRMS (ESI+) m/z Calc for RhC33H41N3O
(M+-py): 598.2299 Exp: 598.2298. 1H NMR (400.2 MHz, CD2Cl2, 298 K): δ
8.12 (m, 2H, H2-py-a), 7.59 (m, 4H, Hp-Ph, H4-py-b, H4-py-a), 7.48 (m, 2H, H2-py-
b), 7.38 (d, JH-H = 7.8, 4H, Hm-Ph), 7.29 (s, 2H, =CHN), 7.14 (m, 2H, H3-py-a),
Scheme 6. Tentative mechanistic proposal for the hydroalkoxylation reactions.
7.06 (m, 2H, H3-py-b), 2.98 (sept, JH-H = 6.8, 4H, CHMeIPr), 1.28 (d, JH-H
6.8, 12H, MeIPr-down), 1.18 (d, JH-H = 6.8, 12H, MeIPr-up). 13C{1H}-APT NMR
(100 MHz, CD2Cl2, 298 K) δ 189.5 (d, JC-Rh = 78.7, CO), 176.9 (d, JC-Rh
=
Conclusion
=
56.0, Rh-CIPr), 151.0 (s, C2-py-a), 150.3 (s, C2-py-b), 146.5 (s, Cq-IPr), 139.6,
(s, C3-py-b), 139.1, (s, C3-py-a), 136.2 (s, CqN), 131.1, (s, CHp-Ph), 126.4 (s,
C4-py-b), 126.3 (s, C4-py-a), 125.7 (s, =CHN), 125.2 (s, CHm-Ph), 29.6 (s,
CHMeIPr), 26.3 (s, MeIPr-up), 22.9 (s, MeIPr-down). 19F NMR (282 MHz, CD2Cl2,
298 K): δ -73.1 (d, JF-P = 713.5, PF6). 31P NMR (121 MHz, CD2Cl2, 298 K):
δ -144.6 (sept, JP-F = 713.5, PF6). 1H-15N HMQC NMR (40.5 MHz, CD2Cl2,
298 K): δ 255.0 (Npy-b), 248.8 (Npy-a),194.5 (NIPr).
A series of neutral and cationic Rh-IPr complexes bearing
different π-acceptor ligands have been prepared. Chlorido
abstraction in [RhCl(4-cod)(IPr)] afforded the cationic solvento
complex [Rh(4-cod)(IPr)(NCCH3)]+, whereas the cationic bis-
solvento derivatives [Rh(CO)(IPr)(L)2]+ (L = py, CH3CN) were
obtained from RhCl(CO)2(IPr) by replacement of the trans-IPr CO
ligand by pyridine or acetonitrile. The bis-acetonitrile derivative
reversibly reacted with CO to afford the bis-carbonyl compound
Rh(CO)2(IPr)(NCCH3)]+. In sharp contrast, the carbonylation of
[Rh(4-cod)(IPr)(NCCH3)]+ did not result in the decoordination of
the diolefin but the unexpected cod-carbonyl compound [Rh(4-
cod)(IPr)(CO)]+ was obtained. Alternatively, neutral derivatives of
formulation RhCl(IPr)(L)2 {L = tBuNC or P(OMe)3} were prepared
from [Rh(-Cl)(2-coe)(IPr)]2 by reaction with tBuNC or P(OMe)3.
Anion exchange of the chlorido ligand in RhCl(4-cod)(IPr) by
cyanide afforded the mononuclear complex Rh(CN)(η4-cod)(IPr).
However, carbonylation of this compound resulted in the
Preparation of [Rh(CO)(IPr)(NCCH3)2][PF6] (5): This complex was
prepared as described for 1, starting from 2 (700 mg, 1.208 mmol) and
thallium(I) hexafluorophosphate (464 mg, 1.328 mmol) in 20 mL of
acetonitrile. Yield: 736 mg (82%). Anal. Calcd. for (M
+ H2O),
C32H42F6N4OPRh.H2O: C, 50.27; H, 5.80; N, 7.33. Found: C, 50.40; H,
5.86; N, 6.98. IR (cm-1, ATR): 1988 ν(CO). HRMS (ESI+): m/z Calc for
RhC30H39N3O: (M+-CH3CN): 560.2143 Exp: 560.2201. 1H NMR (400 MHz,
CD2Cl2, 213 K): δ 7.64 (t, JH-H = 7.8, 2H, Hp-Ph), 7.44 (d, JH-H = 7.8, 4H, Hm-
Ph), 7.32 (s, 2H, =CHN), 2.51 (sept, JH-H = 6.7, 4H, CHMeIPr), 2.31 (s, 3H,
NCCH3-cis-IPr), 2.16 (s, 3H, NCCH3-trans-IPr), 1.34 (d, JH-H = 6.7, 12H, MeIPr-
down), 1.15 (d, JH-H = 6.7, 12H, MeIPr-up). 13C{1H}-APT NMR (100 MHz,
7
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10.1002/ejic.202100399
European Journal of Inorganic Chemistry
FULL PAPER
CD2Cl2, 213K): δ 185.0 (d, JC-Rh = 80.0, CO), 171.8 (d, JC-Rh = 52.3, Rh-
CIPr), 145.8 (s, Cq-IPr), 134.7 (s, CqN), 130.9 (s, CHp-Ph), 126.0 (s, =CHN),
124.5 (s, CHm-Ph), 122.6 (d, JC-Rh = 7.6, NCCH3-cis-IPr), 121.6 (d, JC-Rh = 7.1,
NCCH3-trans-IPr), 28.9 (s, CHMeIPr), 26.2 (s, MeIPr-up), 22.3 (s, MeIPr-down), 3.9
(s, NCCH3-cis-IPr), 3.8 (s, NCCH3-trans-IPr). 19F NMR (282 MHz, CD2Cl2, 298
K): δ -73.1 (d, JF-P = 713.5, PF6). 31P NMR (121 MHz, CD2Cl2, 298 K): δ -
144.6 (sept, JP-F = 713.5, PF6).
celite and the solvent was concentrated to ca. 1 mL. Addition of n-hexane
induced the precipitation of a yellow solid, which was washed with n-
hexane (3 × 5 mL) and dried in vacuo. Yield: 67 mg (61%). Anal. Calcd.
for C37H54ClN4Rh: C, 64.11; H, 7.85; N, 8.08. Found: C, 64.02; H, 7.72; N,
8.15. IR (cm-1, ATR): 2142 νa(CN), 2016 νs(CN). HRMS (ESI+) m/z Calc
1
for RhC37H55N4 (M+-Cl+H): 658.3476 Exp: 658.3452. H NMR (400 MHz,
C6D6, 298 K): δ 7.2 – 7.0 (6H, HPh), 6.70 (s, 2H, =CHN), 3.54 (sept, JH-H
=
6.8, 4H, CHMeIPr), 1.64 (d, JH-H = 6.8, 12H, MeIPr-down), 1.11 (d, JH-H = 6.8,
12H, MeIPr-up), 1.05 (s, 9H, tbu-cis-IPr), 0.79 (s, 9H, tbu-trans-IPr). 13C{1H}-APT
NMR (100 MHz, C6D6, 298 K): δ 193.6 (d, JC-Rh = 45.0, Rh-CIPr), 161.2 (d,
JC-Rh = 71.8, Rh-CNCcis-IPr), 150.4 (d, JC-Rh = 52.6, Rh-CNCtrans-IPr), 146.7
(s, Cq-IPr), 137.6 (s, CqN), 129.6 (s, Cp-Ph), 124.0 (s, CHm-Ph), 123.8 (s,
=CHN), 55.9 (s, Rh-CNCcis-IPr), 55.1 (s, Rh-CNCtrans-IPr), 31.3 (s, C(CH3)3-
cis-IPr), 30.2 (s, C(CH3)3-trans-IPr), 28.9 (s, CHMeIPr), 26.3 (s, MeIPr-down), 23.9
(s, MeIPr-up).
Preparation of [Rh(CO)2(IPr)(NCCH3)][PF6] (6): Carbon monoxide was
bubbled through a yellow solution of 5 (100 mg, 0.134 mmol) in 20 mL of
dichloromethane at room temperature for 10 min. The solution was
concentrated to ca. 1 mL and diethyl ether was added to induce the
precipitation of a light yellow solid, which was washed with diethyl ether (3
× 5 mL) and dried in vacuo. Yield: 74 mg (71%). Anal. Calcd. for
C31H39F6N3O2PRh: C, 50.76; H, 5.36; N, 5.73. Found: C, 50.72; H, 5.39;
N, 5.82. IR (cm-1, ATR): 2102, νa(CO), 2031 νs(CO). HRMS (ESI+) m/z Calc
for RhC30H39N3O: (M+-CO): 560.2143 Exp: 560.2156. 1H NMR (400 MHz,
CD2Cl2, 233 K): δ 7.62 (t, JH-H = 7.8, 2H, Hp-Ph), 7.43 (d, JH-H = 7.8, 4H, Hm-
Ph), 7.41 (s, 2H, =CHN), 2.53 (sept, JH-H = 6.8, 4H, CHMeIPr), 2.36 (s, 3H,
NCCH3), 1.35 and 1.16 (both d, JH-H = 6.8, 24H, MeIPr). 13C{1H}-APT NMR
(100 MHz, CD2Cl2, 233 K): δ 182.2 (d, JC-Rh = 75.6, COcis-IPr), 181.1 (d, JC-
Rh = 53.3, COtrans-IPr), 171.0 (d, JC-Rh = 44.6, Rh-CIPr), 145.4 (s, Cq-IPr), 133.7
(s, CqN), 131.3 (s, CHm-Ph), 126.6 (s, =CHN), 126.1 (br, NCCH3), 124.7 (s,
CHp-Ph), 29.0 (s, CHMeIPr), 26.2 and 22.4 (both s, MeIPr), 4.3 (s, NCCH3).
19F NMR (282 MHz, CD2Cl2, 298 K): δ -73.1 (d, JF-P = 713.5, PF6). 31P NMR
(121 MHz, CD2Cl2, 298 K): δ -144.6 (sept, JP-F = 713.5, PF6).
Preparation of RhCl(IPr){P(OCH3)3}2 (10): This complex was prepared
as described for 9 starting from 3 (100 mg, 0.079 mmol) and trimethyl
phosphite (39 mg, 0.318 mmol) and obtained as a yellow solid. Yield: 84
mg (68%). Satisfactory elemental analysis could not be obtained. HRMS
(ESI+) m/z Calc for RhC33H54N2O6P2ClNa (M++Na): 797.2093 Exp:
797.2080. 1H NMR (400 MHz, C6D6, 298 K): δ 7.2 - 7.0 (6H, HPh), 6.60 (s,
2H, =CHN), 3.64 (sept, JH-H = 6.8, 2H, CHMeIPr-cis-Cl), 3.48 (sept, JH-H = 6.7,
2H, CHMeIPr-cis-P), 3.44 (d, JH-H = 11.3, 9H, OCH3-trans-IPr), 3.12 (d, JH-H
=
11.7, 9H, OCH3-cis-IPr), 1.59 (d, JH-H = 6.8, 6H, MeIPr-cis-Cl-down), 1.34 (d, JH-H
= 6.8, 6H, MeIPr-cis-P-down), 1.00 (d, JH-H = 6.8, 6H, MeIPr-cis-P-up), 0.97 (d, JH-
= 6.8, 6H, MeIPr-cis-Cl-up). 13C{1H}-APT NMR (100 MHz, C6D6, 298 K): δ
H
Preparation of [Rh(4-cod)(IPr)(NCCH3)][PF6] (7): The complex was
prepared as described for 5, starting from 1 (200 mg, 0.315 mmol) and
silver(I) hexafluorophosphate (80 mg, 0.316 mmol). A microcrystalline
yellow solid was obtained. Yield: 176 mg (72%). Anal. Calcd. for
C37H51F6N3PRh: C, 56.56; H, 6.54; N, 5.35; Found: C, 56.24; H, 6.52; N,
5.52. HRMS (ESI+) m/z Calc for RhC35H48N2 (M+-CH3CN): 599.2867 Exp:
599.2894. 1H NMR (300 MHz, CD2Cl2, 298 K): δ 7.65 (t, JH-H = 7.8, 2H, Hp-
Ph), 7.47 (d, JH-H = 7.8, 4H, Hm-Ph), 7.25 (s, 2H, =CHN), 4.34 (m, 2H, =CHcod-
trans-IPr), 3.71 (m, 2H, =CHcod-cis-IPr), 2.70 (sept, JH-H = 6.8, 4H, CHMeIPr),
192.1 (ddd, JC-Rh = 45.7, JC-P = 167.4 and 12.6, Rh-CIPr), 148.7 (s, Cq-IPr-cis-
Cl), 145.3 (s, Cq-IPr-cis-P), 137.6 (s, CqN), 129.1 (s, CHp-Ph), 123.9 (s, CHm-Ph-
cis-Cl), 123.7 (s, =CHN), 122.8 (s, CHm-IPr-cis-P), 51.5 (s, OCH3-cis-IPr), 51.4 (s,
OCH3-trans-IPr), 29.0 (s, CHMeIPr-cis-Cl), 27.7 (s, CHMeIPr-cis-P), 26.6 (s, MeIPr-
cis-Cl-up), 26.4 (s, MeIPr-cis-P-up), 23.4 (s, MeIPr-cis-Cl-down), 22.9 (s, MeIPr-cis-P-
down).31P{1H} NMR (121 MHz, C6D6, 298 K): δ 143.7 (dd, JP-Rh = 159.9, JP-
P = 63.0, P(OCH3)3-trans-IPr), 141.6 (dd, JP-Rh = 263.3, JP-P = 63.0, P(OCH3)3-
cis-IPr).
2.28 (s, 3H, NCCH3), 2.0 – 1.7 (8H, CH2-cod), 1.45 and 1.20 (both d, JH-H
=
Preparation of Rh(CN)(4-cod)(IPr) (11): An orange solution of 1 (350
mg, 0.551 mmol) in 20 mL of acetonitrile was treated with silver cyanide
(74 mg, 0.553 mmol) and it was stirred at room temperature for 30 min in
the absence of light. Then, the suspension was filtered through celite and
the solvent was evaporated to dryness. The resulting solid was dissolved
in 10 mL of dichloromethane, the mixture was filtered again and
concentrated to ca. 1 mL. Addition of hexane induced the precipitation of
a yellow solid, which was washed with hexane (3 × 5 mL) and dried in
vacuo. Yield: 255 mg (73%). Anal. Calcd. for C36H48N3Rh: C, 69.11; H,
7.73; N, 6.72. Found: C, 68.98; H, 7.60; N, 6.82. IR (cm-1, ATR): 2101
ν(CN). HRMS (ESI+) m/z Calc for RhC36H49N3 (M++H): 626.2981 Exp:
6.8, 24H, MeIPr). 13C{1H}-APT NMR (75 MHz, CD2Cl2, 298 K): δ 178.9 (d,
JC-Rh = 52.1, Rh-CIPr), 145.7 (s, Cq-IPr), 135.0 (s, CqN), 130.7 (s, CHp-Ph),
125.8 (s, =CHN), 124.6 (br, NCCH3), 124.3 (s, CHm-Ph), 96.2 (d, JC-Rh = 7.9,
CHcod-trans-IPr), 78.2 (d, JC-Rh = 13.2, CHcod-cis-IPr), 31.7 and 28.4 (both s, CH2-
cod), 29.0 (s, CHMeIPr), 26.1 and 22.5 (both s, MeIPr), 4.0 (s, NCCH3). 19
NMR (282 MHz, CD2Cl2, 298 K): δ -73.1 (d, JF-P = 713.5, PF6). 31P NMR
(121 MHz, CD2Cl2, 298 K): δ -144.6 (sept, JP-F = 713.5, PF6).
F
Preparation of [Rh(CO)(4-cod)(IPr)][PF6] (8): The complex was
prepared as described for 6 starting from 7 (100 mg, 0.127 mmol) and
obtained as a yellow solid. Yield: 57 mg (57%). Anal. Calcd. for
C36H48F6N2OPRh: C, 55.96; H, 6.26; N, 3.63; Found: C, 55.85; H, 6.15; N,
3.49. IR (cm-1, ATR): 2037 ν(CO). HRMS (ESI+) m/z Calc for RhC35H48N2:
(M+-CO): 599.2867 Exp: 599.2884. 1H NMR (300 MHz, CD2Cl2, 298 K): δ
7.67 (t, JH-H = 7.8, 2H, Hp-Ph), 7.48 (s, 2H, =CHN), 7.46 (d, JH-H = 7.8, 4H,
Hm-Ph), 5.16 (m, 2H, CHcod-cis-IPr), 4.75 (m, 2H, CHcod-trans-IPr), 2.66 (sept, JH-
H = 6.8, 4H, CHMeIPr), 2.40, 2.27, 2.05 and 1.86 (all m, 8H, CH2-cod), 1.42
and 1.23 (both d, JH-H = 6.8, 24H, MeIPr). 13C{1H}-APT NMR (75 MHz,
CD2Cl2, 298 K): δ 180.4 (d, JC-Rh = 77.2, CO), 173.5 (d, JC-Rh = 52.0, Rh-
CIPr), 145.7 (s, Cq-IPr), 134.2 (s, CqN), 131.5 (s, CHp-Ph), 126.7 (s, =CHN),
124.7 (s, CHm-Ph), 114.1 (d, JC-Rh = 5.6, CHcod-cis-IPr), 95.2 (d, JC-Rh = 6.7,
CHcod-trans-IPr), 30.0 and 29.8 (both s, CH2-cod), 29.2 (s, CHMeIPr), 26.2 and
1
626.2987. RhC35H48N2 (M+-CN): 599.2872 Exp: 599.2866. H NMR (300
MHz, toluene-d8, 213 K): δ 7.4 – 6.9 (6H, HPh), 6.65 (s, 2H, =CHN), 4.91
(br, 2H, CHcod-trans-IPr), 4.02 (br, 2H, CHcod-cis-IPr), 3.97 and 2.33 (both sept,
JH-H = 6.7, 4H, CHMeIPr), 1.86, 1.20, 1.14, and 0.96 (all d, JH-H = 6.7, 24H,
MeIPr), 1.7 – 1.4 (8H, CH2-cod). 13C{1H}-APT NMR (75 MHz, toluene-d8, 213
K): δ 187.9 (d, JC-Rh = 52.6, Rh-CIPr), 147.7 and 145.0 (both s, Cq-IPr), 138.8
(d, JC-Rh = 53.3, CN), 136.0 (s, CqN), 130.0 (s, CHp-Ph), 125.0 (s, =CHN),
122.6 (s, CHm-Ph), 87.6 (d, JC-Rh = 7.3, CHcod-trans-IPr), 84.0 (d, JC-Rh = 8.0,
CHcod-cis-IPr), 32.1, 30.9, 30.0, and 23.2 (all s, CH2-cod), 28.8 and 28.7 (both
s, CHMeIPr), 26.9, 26.5, 24.0, and 22.0 (all s, MeIPr).
Preparation of [Rh{1C,2N-(CN)}(CO)(IPr)]3 (12): Carbon monoxide
was bubbled through a yellow solution of 11 (100 mg, 0.159 mmol) in 20
mL of dichloromethane at room temperature for 10 min. The solution was
concentrated to ca. 1 mL and hexane was added to induce the precipitation
of a yellow solid, which was washed with hexane (3 × 5 mL) and dried in
vacuo. Yield: 39 mg (40%). IR (cm-1, ATR): 2128 ν(CN), 1954 ν(CO).
Satisfactory elemental analysis could not be obtained. HRMS (ESI+) m/z
22.3 (both s, MeIPr). 19F NMR (282 MHz, CD2Cl2, 298 K): δ -73.1 (d, JF-P
713.5, PF6). 31P NMR (121 MHz, CD2Cl2, 298 K): δ -144.6 (sept, JP-F
713.5, PF6).
=
=
Preparation of RhCl(CNtBu)2(IPr) (9): A yellow solution of 3 (100 mg,
0.079 mmol) in 20 mL of tetrahydrofuran was treated with tert-butyl
isocyanide (36 μl, 0.318 mmol) and it was stirred at room temperature for
30 min. Then, the solvent was evaporated to dryness, the solid product
was dissolved in 10 mL of toluene and the solution was filtered through
1
Calc for RhC29H37N3O (M+/3+H): 546.2005 Exp: 546.1986. H NMR (300
MHz, CD2Cl2, 298 K): δ 7.44 (t, JH-H = 7.7, 2H, Hp-IPr), 7.25 (d, JH-H = 7.7,
4H, Hm-IPr), 7.03 (s, 2H, =CHN), 2.75 (sept, JH-H = 6.7, 4H, CHMeIPr), 1.19
8
This article is protected by copyright. All rights reserved.

10.1002/ejic.202100399
European Journal of Inorganic Chemistry
FULL PAPER
(d, JH-H = 6.7, 12H, MeIPr-down), 0.99 (d, JH-H = 6.7, 12H, MeIPr-up). 13C{1H}-
APT NMR (75 MHz, CD2Cl2, 298 K): δ 191.1 (d, JC-Rh = 75.0, CO), 187.9
(d, JC-Rh = 44.3, Rh-CIPr), 156.5 (dd, JC-Rh = 45.4, 5.4, CN), 146.8 (s, Cq-IPr),
136.7 (s, CqN), 129.9 (s, CHp-IPr), 124.6 (s, =CHN), 124.2 (s, CHm-IPr), 28.8
(s, CHMeIPr), 26.4 (s, MeIPr-up) and 23.4 (s, MeIPr-down).
27≤l≤27, reflections collected/independent 30432/8392 [R(int) = 0.0374],
Tmax/Tmin 0.8813/0.7687, data/restraints/parameters 8392/0/432, GooF(F2)
= 1.032, R1 = 0.0357 [I>2·σ(I)], wR2 = 0.0915 (all data), largest diff.
peak/hole 1.149/–0.612 e·Å–3. CCDC deposit number 2079424.
Crystal data and structure refinement for 9. C37H54ClN4Rh, 693.20
g·mol–1, monoclinic, P21/n, a = 12.2685(9) Å, b = 16.1969(11) Å, c =
18.9743(13) Å, β = 103.0180(10)°, V = 3673.5(4) Å3, Z = 4, Dcalc = 1.253
g·cm–3,  = 0.567 mm–1, F(000) = 1464, 0.330 x 0.220 x 0.170 mm3, yellow
prism, min/max 2.118/28.702°, index ranges –16≤h≤16, –21≤k≤20, –
25≤l≤24, reflections collected/independent 56203/9035 [R(int) = 0.0409],
Tmax/Tmin 0.8644/0.7618, data/restraints/parameters 9035/30/433,
GooF(F2) = 1.072, R1 = 0.0289 [I>2·σ(I)], wR2 = 0.0664 (all data), largest
diff. peak/hole 0.560/–0.492 e·Å–3. CCDC deposit number 2079427.
Standard Conditions for the Catalytic reactions. In a Schlenk flask,
0.02 mmol of catalyst was dissolved in 1 mL of DMA and 1 mL of methanol
under argon. Then, 1 mmol of alkyne was added and the Schlenk flask
was heated at 70 ºC for 24 h with magnetic stirring. The resulting mixture
was diluted with 10 mL of ethyl acetate and 20 mL of water were added.
The organic layer was collected and the aqueous layer was extracted three
times with ethyl acetate. The combined organic fractions were washed with
brine, dried over Na2SO4, filtered, and carefully concentrated to remove
volatile materials. The obtained crude material was analyzed by NMR.
Conversion and selectivity was determined by integration of key
resonances of phenylacetylene and the reaction products.
Crystal data and structure refinement for 10. C66H108Cl2N4O12P4Rh2,
1550.16 g·mol–1, monoclinic, P21/c, a = 18.4448(19) Å, b = 20.611(2) Å, c
= 20.722(2) Å, β = 106.3440(10)°, V = 7559.6(13) Å3, Z = 4, Dcalc = 1.362
g·cm–3,  = 0.649 mm–1, F(000) = 3248, 0.220 x 0.175 x 0.140 mm3, yellow
prism, min/max 1.150/28.665°, index ranges –24≤h≤24, –27≤k≤27, –
27≤l≤27, reflections collected/independent 120578/18435 [R(int) = 0.0563],
Tmax/Tmin 0.8813/0.7314, data/restraints/parameters 18435/0/839,
GooF(F2) = 1.072, R1 = 0.0330 [I>2·σ(I)], wR2 = 0.0829 (all data), largest
diff. peak/hole 0.890/–0.990 e·Å–3. CCDC deposit number 2079425.
Crystal Structure Determination. Single crystals suitable for the X-ray
diffraction studies were grown by slow diffusion of diethyl ether into
saturated CH2Cl2 solutions (4, 5, 7, and 8), or alternatively slow diffusion
of hexane over saturated toluene solutions (9, 10, 11 and 12). X-ray
diffraction data were collected at 100(2) K on a Bruker APEX SMART CCD
diffractometer (4, 5, 7-11) or on a Bruker APEX-DUO SMART CCD
diffractometer at 120(2)
K (12), in both cases with graphite-
monochromated Mo−Kα radiation (λ = 0.71073 Å) using <1° ω rotations.
Intensities were integrated and corrected for absorption effects with
SAINT–PLUS[34] and SADABS[35] programs, both included in the APEX2
or APEX3 packages. The structures were solved by the Patterson method
with SHELXS-97[36] and refined by full matrix least-squares on F2 with
SHELXL-2014,[37] under WinGX.[38]
Crystal data and structure refinement for 11. C36H48N3Rh, 625.68
g·mol–1, monoclinic, P21/n, a = 10.815(3) Å, b = 18.845(5) Å, c = 15.801(4)
Å, β = 91.878(3)°, V = 3218.7(15) Å3, Z = 4, Dcalc = 1.291 g·cm–3,  = 0.558
mm–1, F(000) = 1320, 0.270 x 0.180 x 0.070 mm3, yellow prism, min/max
1.682/28.310°, index ranges –14≤h≤14, –25≤k≤24, –21≤l≤20, reflections
collected/independent 44439/7792 [R(int)
=
0.0550], Tmax/Tmin
0.9143/0.7867, data/restraints/parameters 7792/0/369, GooF(F2) = 1.049,
R1 = 0.0331 [I>2·σ(I)], wR2 = 0.0703 (all data), largest diff. peak/hole
0.614/–0.690 e·Å–3. CCDC deposit number 2079429.
Crystal data and structure refinement for 4. C38H46F6N4OPRh, 822.67
g·mol–1, orthorhombic, P212121, a = 11.8846(7) Å, b = 15.2171(9) Å, c =
21.0976(13) Å, V = 3815.5(4) Å3, Z = 4, Dcalc = 1.432 g/cm3,  = 0.554 mm–
1, F(000) = 1696, 0.190 x 0.180 x 0.120 mm3, yellow prism, min/max
1.931/28.750°, index ranges –15≤h≤16, –19≤k≤20, –28≤l≤27, reflections
Crystal data and structure refinement for 12. C96H136N9O3Rh3, 1722.72
g·mol–1, triclinic, P–1, a = 15.246(4) Å, b = 16.376(4) Å, c = 19.993(5) Å, α
= 106.751(3)°, β = 101.342(3)°, γ = 98.562(4)°, V = 4572.9(19) Å3, Z = 2,
Dcalc = 1.251 g·cm–3,  = 0.586 mm–1, F(000) = 1804, 0.150 x 0.120 x 0.030
mm3, yellow prism, min/max 1.099/25.028°, index ranges –18≤h≤18, –
18≤k≤19, –23≤l≤23, reflections collected/independent 36188/16140 [R(int)
collected/independent 48011/9333 [R(int)
=
0.0402], Tmin/Tmax
0.9064/0.8058, data/restraints/parameters 9333/0/ 468, GooF(F2) = 1.016,
R1 = 0.0260 [I>2·σ(I)], wR2 = 0.0567 (all data), absolute structure
parameter –0.025(8), largest diff. peak/hole 0.681/–0.346 e·Å–3. CCDC
deposit number 2079423.
=
0.1008],
Tmax/Tmin
0.9583/0.7674,
data/restraints/parameters
16140/0/943, GooF(F2) = 0.961, R1 = 0.0530 [I>2·σ(I)], wR2 = 0.1473 (all
data), largest diff. peak/hole 1.638/–1.192 e·Å–3, CCDC deposit number
2079428.
Crystal data and structure refinement for 5. C35H49Cl2F6N4O1.50PRh,
868.56 g·mol–1, monoclinic, P21/c, a = 12.7138(9) Å, b = 13.4129(10) Å, c
= 23.8836(17) Å, β = 90.2720(10)°, V = 4072.8(5) Å3, Z = 4, Dcalc = 1.416
g·cm–3,  = 0.651 mm–1, F(000) = 1788, 0.370 x 0.150 x 0.090 mm3, yellow
prism, min/max 1.602/26.372°, index ranges –15≤h≤15, –16≤k≤16, –
29≤l≤29, reflections collected/independent 42583/8307 [R(int) = 0.0344],
Tmax/Tmin 0.8839/0.7946, data/restraints/parameters 8307/28/577,
GooF(F2) = 1.031, R1 = 0.0469 [I>2·σ(I)], wR2 = 0.1176 (all data), largest
diff. peak/hole 1.049/–0.969 e·Å–3. CCDC deposit number 2079448.
Acknowledgements
Financial support from the Spanish Ministerio de Ciencia e
Innovación (MICINN/FEDER) under the Project PID2019-
103965GB-I00 and the Departamento de Ciencia, Universidad y
Sociedad del Conocimiento del Gobierno de Aragón (group
E42_20R) are gratefully acknowledged. A.D.G. thanks the
Spanish Ministerio de Economía y Competitividad (MINECO) for
the postdoctoral grant Juan de la Cierva - Incorporación 2015
(IJCI-2015-27029).
Crystal data and structure refinement for 7. C37H51F6N3PRh, 785.68
g·mol–1, monoclinic, P21/n, a = 16.0083(8) Å, b = 12.2949(6) Å, c =
20.2009(10) Å, β = 111.7170(10)°, V = 3693.7(3) Å3, Z = 4, Dcalc = 1.413
g·cm–3,  = 0.566 mm–1, F(000) = 1632, 0.300 x 0.200 x 0.160 mm3, yellow
prism, min/max 1.980/28.691°, index ranges –21≤h≤21, –16≤k≤16, –
26≤l≤26, reflections collected/independent 69650/9067 [R(int) = 0.0296],
Tmax/Tmin 0.8749/0.7931, data/restraints/parameters 9067/0/442, GooF(F2)
= 1.035, R1 = 0.0265 [I>2·σ(I)], wR2 = 0.0663 (all data), largest diff.
peak/hole 0.601/–0.477 e·Å–3. CCDC deposit number 2079426.
Keywords: N-Heterocyclic carbene • Rhodium • π-acceptor
ligands • Hydroalkoxylation • Alkyne dimerization
Crystal data and structure refinement for 8. C36H48F6N2OPRh, 772.64
g·mol–1, monoclinic, P21/n, a = 16.2096(10) Å, b = 11.6899(7) Å, c =
20.2146(13) Å, β = 111.6700(10)°, V = 3559.7(4) Å3, Z = 4, Dcalc = 1.442
g·cm–3,  = 0.587 mm–1, F(000) = 1600, 0.300 x 0.240 x 0.100 mm3, yellow
prism, min/max 2.021/28.621°, index ranges –21≤h≤21, –14≤k≤15, –
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Entry for the Table of Contents
Puzzling Catalysis: A variated set of cationic or neutral square-planar rhodium complexes bearing a N-heterocyclic carbene and π-
acceptor ligands have been synthesized. Divergent catalytic outcomes for phenylacetylene-methanol transformations resulted in
function on each particular combination of ligands, including alkyne hydroalkoxylation, dimerization or polymerization reactions.
12
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