Preparation and Degradation of Rhodium and Iridium Diolefin Catalysts for the Acceptorless and Base-Free Dehydrogenation of Secondary Alcohols

Article abstract of DOI:10.1021/acs.organomet.1c00068

Rhodium and iridium diolefin catalysts for the acceptorless and base-free dehydrogenation of secondary alcohols have been prepared, and their degradation has been investigated, during the study of the reactivity of the dimers [M(μ-Cl)(I4-C8H12)]2 (M = Rh (1), Ir (2)) and [M(μ-OH)(I4-C8H12)]2 (M = Rh (3), Ir (4)) with 1,3-bis(6′-methyl-2′-pyridylimino)isoindoline (HBMePHI). Complex 1 reacts with HBMePHI, in dichloromethane, to afford equilibrium mixtures of 1, the mononuclear derivative RhCl(I4-C8H12){κ1-Npy-(HBMePHI)} (5), and the binuclear species [RhCl(I4-C8H12)]2{μ-Npy,Npy-(HBMePHI)} (6). Under the same conditions, complex 2 affords the iridium counterparts IrCl(I4-C8H12){κ1-Npy-(HBMePHI)} (7) and [IrCl(I4-C8H12)]2{μ-Npy,Npy-(HBMePHI)} (8). In contrast to chloride, one of the hydroxide groups of 3 and 4 promotes the deprotonation of HBMePHI to give [M(I4-C8H12)]2(μ-OH){μ-Npy,Niso-(BMePHI)} (M = Rh (9), Ir (10)), which are efficient precatalysts for the acceptorless and base-free dehydrogenation of secondary alcohols. In the presence of KOtBu, the [BMePHI]- ligand undergoes three different degradations: Alcoholysis of an exocyclic isoindoline-N double bond, alcoholysis of a pyridyl-N bond, and opening of the five-membered ring of the isoindoline core.

Full text of DOI:10.1021/acs.organomet.1c00068

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Preparation and Degradation of Rhodium and Iridium Diolefin
Catalysts for the Acceptorless and Base-Free Dehydrogenation of
Secondary Alcohols
María L. Buil, Alba Collado, Miguel A. Esteruelas,* Mar Gómez-Gallego, Susana Izquierdo,
Antonio I. Nicasio, Enrique Onate, and Miguel A. Sierra*
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ABSTRACT: Rhodium and iridium diolefin catalysts for the
acceptorless and base-free dehydrogenation of secondary alcohols
have been prepared, and their degradation has been investigated,
during the study of the reactivity of the dimers [M(μ-Cl)(η4-
C8H12)]2 (M = Rh (1), Ir (2)) and [M(μ-OH)(η4-C8H12)]2 (M =
Rh (3), Ir (4)) with 1,3-bis(6′-methyl-2′-pyridylimino)isoindoline
(HBMePHI). Complex 1 reacts with HBMePHI, in dichloro-
methane, to afford equilibrium mixtures of 1, the mononuclear
derivative RhCl(η4-C8H12){κ1-Npy-(HBMePHI)} (5), and the
binuclear species [RhCl(η4-C8H12)]2{μ-Npy,Npy-(HBMePHI)}
(6). Under the same conditions, complex 2 affords the iridium
counterparts IrCl(η4-C8H12){κ1-Npy-(HBMePHI)} (7) and [IrCl-
(η4-C8H12)]2{μ-Npy,Npy-(HBMePHI)} (8). In contrast to chloride,
one of the hydroxide groups of 3 and 4 promotes the deprotonation of HBMePHI to give [M(η4-C8H12)]2(μ-OH){μ-Npy,Niso-
(BMePHI)} (M = Rh (9), Ir (10)), which are efficient precatalysts for the acceptorless and base-free dehydrogenation of secondary
alcohols. In the presence of KOtBu, the [BMePHI]− ligand undergoes three different degradations: alcoholysis of an exocyclic
isoindoline-N double bond, alcoholysis of a pyridyl-N bond, and opening of the five-membered ring of the isoindoline core.
is generally endothermic at room temperature but can be
performed under mild conditions, for instance refluxing
toluene in open systems, since the hydrogen elimination acts
as a driving force of the reaction.4
INTRODUCTION
■
Ketones are a pivotal class of compounds, which can be easily
transformed to diverse building blocks including (among
others) imines, oximes, amines, and alkenes, the oxidation of
alcohols being one of the most representative methods for
their preparation.1 Traditionally, stoichiometric amounts of
chromium- and manganese-based reagents have been used for
this purpose.1a As a consequence of the large amounts of
noxious waste generated, these methods have been gradually
replaced by transition-metal catalysis operating under more
environmentally friendly oxidants such as O2 and H2O2.2 In
the last few years, a further step was taken with the transition-
metal-catalyzed acceptorless alcohol dehydrogenation, which
does not need the use of oxidants (eq 1). The procedure
Strongly basic media have generally been necessary for the
operation of many catalysts, in particular with cationic
compounds or precursors bearing halide ligands. The base
cocatalyzes the dehydrogenation to generate an alkoxide,
which binds to the metal and evolves into the carbonyl
compound by β-hydrogen elimination.5 To prevent the waste
generated by the base, the development of precursors
operating under base-free conditions is receiving great
attention.6 They coordinate ligands, being engaged in the
deprotonation step. The basic center usually resides in the first
metal coordination sphere7 and sometimes in a remote
position.8
Received: February 4, 2021
Published: March 31, 2021
displays three environmental advantages: it offers an oxidation
procedure for the synthesis of carbonyl compounds, minimiz-
ing waste formation, it is a promising approach to the
production of hydrogen from biomass, and it provides a direct
connection with the research on hydrogen storage and
transport in organic liquids.3 The dehydrogenation of alcohols
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Scheme 1. Sequential N−H and C−H Activations of the Isoindoline Core of HBMePHI
1
■
●
Figure 1. H NMR spectra as a function of the temperature of the equilibrium shown in Scheme 2: blue , 5; yellow ☆, L; and black , 1 (in
CD2Cl2).
We are interested in developing catalysts for the
dehydrogenation of hydrogen carriers,9 in particular those
based on organic liquids.9f−h Thus, in the search for new
precursors, some years ago we initiated a research program
based on platinum-group-metal complexes and the polynitro-
genated organic molecule 1,3-bis(6′-methyl-2′-pyridylimino)-
isoindoline (HBMePHI).10 Previously, with a few excep-
tions,11 the anion of this isoindoline had been used as a pincer
ligand, which modulates the electron density of the metal
center and the steric hindrance around it.12 However, it is
much more than that. We have recently reported that
platinum-group-metal polyhydride complexes promote the
sequential activations of bonds N−H and C−H of the
isoindoline core, to afford homobinuclear and heterobinuclear
compounds via mononuclear intermediates (Scheme 1). The
bonding of the second metal fragment modifies the electronic
structure of the polydentate ligand, which produces a
noticeable perturbation of the electron density around the
initial center. As a consequence of the mutual electronic
influence between the metals, catalytic synergism is observed in
the acceptorless and base-free dehydrogenation of secondary
alcohols. The bridging ligand displays a noninnocent character,
participating in the formation of the metal−alkoxide bond and
in the release of molecular hydrogen.10b
These unusual findings in the chemistry of pyridylimino-
isoindolines prompted us to study the behavior of HBMePHI
toward the dimers [M(μ-Cl)(η4-C8H12)]2 (M = Rh (1), Ir
(2)) and [M(μ-OH)(η4-C8H12)]2 (M = Rh (3), Ir (4)), which
are cornerstones in the development of rhodium13 and
iridium14 organometallic chemistry. This paper reports the
results of this study, including the formation of novel eight-
membered heterodimetallacycles and C−N bond activations in
the isoindoline core, some degradation pathways of the
polydentate ligand in basic medium, and the catalytic ability
of some of the new complexes in the acceptorless and base-free
dehydrogenation of secondary alcohols.
RESULTS AND DISCUSSION
■
Reactions with 1 and 2. The addition of 2.0 mol of
HBMePHI to dichloromethane-d2 solutions of 1 (1.0 equiv per
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Scheme 2. Formation of 5
rhodium), contained in an NMR tube, produces a change in
the solution color from yellow to orange. The 1H NMR
spectrum of the mixture at room temperature shows the
resonances of 1 and HBMePHI (L), which appear slightly
broadened, along with markedly broad signals corresponding
to a new species. When the sample temperature is lowered,
narrowing of all the signals is observed. At the same time, a
decrease in the concentrations of both 1 and the isoindoline
and an increase in the amount of a new species is clearly
evident (Figure 1). Characteristic features of the new
compound are 4 resonances between 4.6 and 3.3 ppm due
to olefinic hydrogen atoms, which are all inequivalent, and 10
aromatic signals between 9.1 and 6.5 ppm corresponding to
the CH hydrogen atoms of the coordinated ligand, which are
Figure 2. van’t Hoff plot for the equilibrium constant K1.
1
also inequivalent. In agreement with the H NMR spectrum,
the 13C{1H} NMR spectrum at 213 K of the new complex
displays 4 doublets (1JC−Rh = 11−13 Hz) between 82 and 74
ppm for the olefinic carbon atoms and 10 aromatic signals for
the coordinated isoindoline. These observations can be
rationalized according to the equilibrium shown in Scheme
2, which involves the formation of the mononuclear square-
planar complex RhCl(η4-C8H12){κ1-Npy-(HBMePHI)} (5), as
a result of the rupture of the chloride bridges of 1 and the
coordination of the polydentate molecule to the metal center
by one of the pyridyl groups. The equilibrium was studied as a
function of the temperature between 293 and 223 K by
integration of the olefinic resonances and the higher field
aromatic signal of the free ligand. Table 1 collects the values of
ing to the free ligand and the presence of signals due to a new
compound. The latter is formed by the reaction of 1 with 5,
and its concentration increases as the sample temperature is
decreased. 5 it has four inequivalent olefinic hydrogen atoms.
1
Thus, its H NMR spectra contain four resonances between
4.6 and 3.4 ppm. Nevertheless, these spectra only show three
complex aromatic signals in the 8.2−6.9 ppm range. These
observations are consistent with the formation of an
equilibrium mixture among 1, 5, and the dimer [RhCl(η4-
C8H12)]2{μ-Npy,Npy-(HBMePHI)} (6 in Scheme 3). The
13C{1H} NMR spectra of the mixture are strong additional
evidence in favor of this equilibrium. Figure 3 shows the
13C{1H}-APT spectrum in the olefinic region, at 183 K. The
equilibrium shown in Scheme 3 was also studied as a function
of the temperature between 283 and 183 K. The thermody-
namic parameters obtained from the values of the equilibrium
constant K2 (Table 1) are ΔH° = −5.8 0.2 kcal mol−1 and
ΔS°= −28.0 0.7 cal mol−1 K−1 (Figure 4).
The iridium dimer 2 also reacts with 1.0 and 0.5 equiv of
HBMePHI. The reactions lead to the iridium counterparts of 5
and 6. These complexes, IrCl(η4-C8H12){κ1-Npy-(HBMePHI)}
(7) and [IrCl(η4-C8H12)]2{μ-Npy,Npy-(HBMePHI)} (8), are
significantly more stable than their rhodium analogues and can
be isolated as pure red (7) and yellow (8) solids in 54% and
80% yields, respectively. The formation of the four compounds
might take place via the intermediates (η4-C8H12)ClM(μ-
Cl)M{κ1-Npy-(HBMePHI)}(η4-C8H12) (M = Rh (A), Ir (B)),
according to Scheme 4.
Complexes 7 and 8 were characterized by X-ray diffraction
analyses. Figure 5 shows the structure of 7, whereas Figure 6
gives a view of 8. They confirm the selective coordination of
the pyridyl groups of the polydentate HBMePHI molecule and
the square-planar environment of the metal centers in these
compounds. The coordination gives rise to Ir−N bonds of
2.124(4) Å (Ir−N(1); 7) and 2.111(6) Å (Ir−N(1); 8). These
bond lengths compare well with those previously reported for
other square-planar iridium(I) pyridine derivatives.15 The 1,5-
cyclooctadiene ligand takes its customary “tub” conformation.
The coordinated bonds display distances of 1.403(7) Å
(C(21)−C(22)) and 1.428(7) Å (C(25)−C(26)) in 7 and
Table 1. Formation Constants K1 and K2 (L mol−1) for 5
and 6
temp (K)
K1 (Scheme 2)
K2 (Scheme 3)
293
283
273
263
253
243
233
223
213
203
193
183
1.897
4.077
7.043
12.883
27.692
41.793
89.937
178.606
0.022
0.028
0.04
0.068
0.136
0.242
0.465
0.945
1.597
1.871
5.160
the equilibrium K1 constants at each temperature. A linear
least-squares analysis of ln K1 versus 1/T (Figure 2) provides
values for ΔH° and ΔS° of −8.2 0.3 kcal mol−1 and −26.4
1.0 cal mol−1 K−1, respectively.
The 1H and 13C{1H} NMR spectra of the solutions resulting
from the addition of 1.0 mol of HBMePHI per dimer to 1 in
dichloromethane-d2 show significant differences with regard to
the previously mentioned spectra. Two noticeable features
should be pointed out: the absence of resonances correspond-
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Scheme 3. Formation of 6
13
1
■
●
▲
Figure 3. Olefinic resonances in the C{ H} NMR spectrum for the equilibrium shown in Scheme 3: blue , 5; green , 1; red , 6 (183 K,
100.6 MHz, in CD2Cl2).
center is bonded to the N atom of the isoindolinate core. This
coordination fashion and the remaining hydroxide group give
rise to a mixed double bridge, which generates an eight-
membered heterodimetallacycle. Under the same conditions,
complex 4 leads to the iridium counterpart [Ir(η4-C8H12)]2(μ-
OH){μ-Niso,Npy-(BMePHI)} (10). The formation of 9 and 10
should take place via the intermediates (η4-C8H12)(OH)M(μ-
OH)M{κ1-Npy-(HBMePHI)}(η4-C8H12) (M = Rh (C), Ir
(D)), the hydroxo counterparts of A and B, according to
Scheme 5. Similarly to 1 and 2, dimers 3 and 4 should initially
undergo the rupture of a bridge, by coordination of a pyridyl
group of HBMePHI to one of the metal centers. Thus, the
subsequent heterolytic N−H activation of the isoindoline core
by the other metal center, using the terminal hydroxide group
as an internal base, would afford the mixed double bridge.
Complexes 9 and 10 were isolated as orange solids in 80% and
47% yields, respectively.
Figure 4. van’t Hoff plot for the equilibrium constant K2.
The rhodium complex 9 was characterized by an X-ray
diffraction analysis. The structure (Figure 7) proves the
formation of the eight-membered heterodimetallacycle, which
displays a boat−boat conformation17 with the metals separated
by 3.423 Å. The environment around each metal is square-
planar, as expected for rhodium(I) centers. The Rh(1)−
pyridine distance of 2.1495(14) Å (Rh(1)−N(1)) is about
0.05 Å longer than the Rh(2)−isoindoline bond length of
2.1047(14) Å (Rh(2)−N(3)), suggesting a higher nucleophil-
icity for the isoindoline N(3) atom than for the pyridine N(1)
atom. As a consequence of this, the Rh(1)−hydroxide bond of
2.0709(12) Å (Rh(1)−O(1)) is about 0.02 Å shorter than the
Rh(2)−hydroxide bond of 2.0912(12) Å (Rh(2)−O(1)). The
1.413(11) Å (C(11)−C(12)) and 1.410(12) Å (C(15)−
C(16)) in 8, which are longer than the C−C double bonds in
the free diolefin (1.34 Å) in agreement with the usual Chatt−
Dewar−Duncanson model.16
Reactions with 3 and 4. In contrast to the chloride
bridging ligand, one of the hydroxide groups of the rhodium
dimer 3 is able to abstract the N−H hydrogen atom of
HBMePHI. Thus, the treatment of yellow suspensions of this
complex, in propan-2-ol, with 1.0 mol of the polydentate
molecule for 2 h affords [Rh(η4-C8H12)]2(μ-OH){μ-Niso,Npy-
(BMePHI)} (9), as a consequence of the asymmetrical
coordination of the resulting anion; one pyridyl group
coordinates to a rhodium atom, whereas the other metal
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Scheme 4. Formation of Complexes 5−8
conformations of similar energy are possible for an eight-
membered cycle. As a consequence, complexes 9 and 10 are
fluxional in toluene-d8 solution, showing a rigid structure at
temperatures lower than 213 K. In agreement with Figure 7,
their 1H NMR spectra display eight olefinic resonances
between 5.5 and 3.0 ppm, whereas the 13C{1H} NMR spectra
contain eight olefinic signals in the 85−52 ppm range.
The chelate κ2-(Niso,Npy) coordination is known for 1,3-
bis(2′-pyridylimino)isoindolate (BPHI) anions.11 However, as
far as we know, the bridge μ-(Niso,Npy) coordination is
unprecedented. Compounds bearing bridging [BPHI]− ligands
are very scarce. Baird and co-workers have observed that
HBPHI displaces an acetate group from Mo2(OAc)4 to give
Mo2(OAc)3(BPHI), with the [BPHI]− ligand bound to one
molybdenum by an imino nitrogen and to the other
molybdenum by the isoindoline nitrogen and a pyridyl
Figure 5. Molecular diagram of complex 7 (50% probability
ellipsoids). Hydrogen atoms (except H(3)) are omitted for clarity.
Selected bond lengths (Å): Ir−Cl = 2.3608(13), Ir−N(1) = 2.124(4),
Ir−C(21) = 2.099(4), Ir−C(22) = 2.112(5), Ir−C(25) = 2.109(5),
Ir−C(26) = 2.096(5), N(1)−C(1) = 1.356(6), N(1)−C(6) =
1.368(5), N(2)−C(6) = 1.381(6), N(3)−C(7) = 1.377(6), N(3)−
C(14) = 1.400(6), C(21)−C(22) = 1.403(7), C(25)−C(26) =
1.428(7).
nitrogen.18 Broring and co-workers have reported that one of
̈
the pyridyl groups of HBMePHI undergoes a palladium-
promoted 1,3-hydrogen shift, from C to N, to afford Pd(κ3-
Npy,Niso,CHpy)-pincer derivatives, which add a second palla-
dium to the free pyridyl-imine moiety.19 We have described
the preparation of homoleptic and heteroleptic bis(osmium)
complexes containing a [μ-(κ2-Npy,Nimine)2-BMePHI]− ligan-
d,10a whereas Li, Yang, Zhang, and co-workers have observed
the same coordination fashion in an intermediate species
formed in the reaction of Lu(CH2SiMe3)3(thf)2 with HBPHI
to give Lu{κ3-mer-(BPHI)}(CH2SiMe3)2.20
Degradation of the [BMePHI]− ligand in Basic
Medium. Alcohol dehydrogenation catalysts combined with
bases promote borrowing-hydrogen reactions, including α-
alkylation of arylacetonitriles and methyl ketones.21 The
carbonyl compound resulting from the dehydrogenation
process undergoes a base-catalyzed condensation with an
alkyl substrate to afford an α,β-unsaturated intermediate,22
which is subsequently reduced to the final product with the
hydrogen generated in the dehydrogenation.21 In order to
explore the ability of the Rh- and Ir(BMePHI)(diolefin)
systems to work in this class of catalysis, we studied the
formation of 9 and 10 in the presence of a strong base.
Treatment of a suspension of 3 in propan-2-ol with 2.0 mol
of HBMePHI and 3.0 equiv of KOtBu at room temperature for
Figure 6. Molecular diagram of complex 8 (50% probability
ellipsoids). Hydrogen atoms (except H(3)) are omitted for clarity.
Selected bond lengths (Å): Ir−Cl = 2.360(2), Ir−N(1) = 2.111(6),
Ir−C(11) = 2.120(7), Ir−C(12) = 2.089(9), Ir−C(15) = 2.137(8),
Ir−C(16) = 2.116(8), N(1)−C(6) = 1.338(9), N(2)−C(6) =
1.438(9), N(2)−C(7) = 1.276(9), N(3)−C(7) = 1.386(8),
C(11)−C(12) = 1.413(11), C(15)−C(16) = 1.410(12).
lengths of the coordinated C−C double bonds to both metal
centers are similar, between 1.393(3) and 1.403(3) Å, and
compare well with the distances found in 7 and 8. Several
2 h leads to a mixture of 9 and [Rh(η4-C8H12)]2{μ-Niso,Nimine
-
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Scheme 5. Formation of 9 and 10
Figure 7. (a) Molecular diagram of complex 9 (50% probability ellipsoids). All hydrogen atoms (except that of the hydroxide ligand) are omitted
for clarity. Selected bond lengths (Å) and angles (deg): Rh(1)−O(1) = 2.0709(12), Rh(2)−O(1) = 2.0912(12), Rh(1)−N(1) = 2.1495(14),
Rh(2)−N(3) = 2.1047(14), Rh(1)−C(21) = 2.1016(18), Rh(1)−C(22) = 2.1155(18), Rh(1)−C(25) = 2.1075(17), Rh(1)−C(26) = 2.1096(17),
N(1)−C(6) = 1.357(2), N(2)−C(6) = 1.382(2), N(2)−C(7) = 1.288(2), N(3)−C(7) = 1.388(2), N(3)−C(14) = 1.391(2), N(4)−C(14) =
1.287(2), N(4)−C(15) = 1.391(2), Rh(1)−O(1)−Rh(2) = 110.63(6), O(1)−Rh(1)−N(1) = 86.48(5), O(1)−Rh(2)−N(3) = 87.63(5). (b)
Molecular core.
(HNC8H4NO)}(μ-NC8H4NOiPr) (11), according to
azavinylidene bridge ([NC8H4NOiPr]−) arises from a
similar process involving two alcoholyses on the imine
functions of a second [BMePHI]− ligand. The main difference
between the generation processes of both bridges is the
number of molecules of propan-2-ol attacking the imine−
isoindoline bond. When only a molecule of propan-2-ol
attacks, the isopropoxide group remains as a substituent at the
2-position of the isoindoline core. Its steric hindrance prevents
the coordination of the isoindoline-N atom, whereas the
electronic difference with the carbonyl oxygen atom seems to
favor the deprotonation of the imine resulting from the
alcoholysis of the imine−pyridyl bond.
Scheme 6.
Scheme 6. Formation of 11
The rhodium atoms display square-planar coordination
environments with a separation between the metals of
3.0672(5) Å, which is long for a single Rh−Rh bond. In
Complexes 9 and 11 were separated by using their different
solubilities in propan-2-ol. Thus, complex 11 was obtained
pure in 20% yield with regard to 3 as red crystals suitable for X-
ray diffraction analysis. Its structure (Figure 8) reveals the
formation of a surprising mixed double bridge. One of the
halves of the bridge is the anion resulting from the
deprotonation of the NH-isoindoline group of 3-iminoisoin-
dolin-1-one (HNC8H4NHO), which coordinates different
metal centers in an Niso,Nimine fashion, whereas the other half is
the azavinylidene resulting from the deprotonation of the NH-
imine unit of 3-isopropoxy-1H-isoindol-1-imine (HN
C8H4NOiPr). The formation of the [HNC8H4NO]− anion
involves two different alcoholysis processes in the imine
moieties of a [BMePHI]− ligand. The CO double bond
could be the result of the substitution of a pyridylimine moiety
by two isopropoxide groups. Then, the resulting diisopropy-
lacetal intermediate23 should lose diisopropyl ether to afford
the carbonyl group.24 In contrast, the other imino group
undergoes alcoholysis of the imine−pyridyl bond. The
2
agreement with this, overlap between their dz orbitals has not
been found by means of DFT calculations (M06/6-311G(d,p)
&SDD(f)). Although the isoindoline coordination of the anion
[HNC8H4NO]− to Rh(1) of 2.090(3) Å (Rh(1)−N(3)) is
about 0.01 Å shorter than the imine coordination to Rh(2) of
2.106(4) Å (Rh(2)−N(4)), the azavinylidene−rhodium bond
lengths of 2.048(3) Å (Rh(1)−N(1)) and 2.049(3) Å
(Rh(2)−N(1)) are statistically identical and similar to those
reported for the complex [Rh(μ-NCPh2)(TFB)]2 (TFB =
tetrafluorobenzobarrelene; 2.046(7), 2.052(7), 2.054(6) and
2.054(7) Å).25 The M−azavinylidene−M angle, Rh(1)−
N(1)−Rh(2), of 96.93(14)° and the distance N(1)−C(1) of
1.260(5) Å compare well with those found in other transition-
metal compounds bearing azavinylidene bridges.26 The lengths
of the coordinated CC double bonds, between 1.353(8) and
1.387(7) Å, are slightly shorter than those found in 7 and 8.
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a
Table 2. Oxidation Potentials of Complexes 11 and 12
complex
Epa1
Epa2
Epa3
b
11
12
0.49 (Epc1 0.39) (ΔE 95 mV)
1.10
0.74
0.56
1.37
a
Data obtained from dichloromethane solutions of 11 and 12
(10−3M), containing (NBu4)PF6 (10−1 M) as the supporting
electrolyte at 20 °C: counter electrode, Pt; working electrode, glassy
carbon; reference electrode, Ag/AgCl; scan rate, 100 mV/s. Values
b
are given in V and referenced vs Ag/AgCl. Quasi-reversible wave.
9). These data suggest that the oxidation events are compatible
with two sequential processes of two electrons: from RhIL2RhI
Figure 8. Molecular diagram of complex 11 (50% probability
ellipsoids). All hydrogen atoms (except the imine hydrogen atom H4)
are omitted for clarity. Selected bond lengths (Å) and angles (deg):
Rh(1)−N(1) = 2.048(3), Rh(1)−N(3) = 2.090(3), Rh(2)−N(1) =
2.049(3), Rh(2)−N(4) = 2.106(4), Rh(1)−C(20) = 2.134(5),
Rh(1)−C(21) = 2.125(5), Rh(1)−C(24) = 2.147(5), Rh(1)−
C(25) = 2.125(5), Rh(2)−C(28) = 2.155(4), Rh(2)−C(29) =
2.115(5), Rh(2)−C(32) = 2.160(5), Rh(2)−C(33) = 2.106(5),
N(1)−C(1) = 1.260(5), N(2)−C(1) = 1.449(5), N(2)−C(8) =
1.286(5), N(3)−C(12) = 1.364(5), N(3)−C(19) = 1.371(5), N(4)−
C(12) = 1.292(5), O(1)−C(8) = 1.334(5), O(1)−C(9) = 1.471(5),
O(2)−C(19) = 1.236(5). Rh(1)−N(1)−Rh(2) = 96.93(14).
1
The H and 13C{1H} NMR spectra of 11 in benzene-d6 at
room temperature are consistent with the structure shown in
1
Figure 8. The H NMR spectrum displays a broad signal at
5.78 ppm corresponding to the imine-NH hydrogen atom and
2
eight olefinic resonances due to the inequivalent Csp -H
hydrogen atoms of the diene, between 6.5 and 3.3 ppm,
whereas the 13C{1H} NMR spectrum contains eight doublets
(1JC−Rh = 10−13 Hz) between 87 and 74 ppm, assigned to the
coordinated carbon atoms.
Figure 9. Computed (DFT/M06/6-311G(d,p)&SDD(f)) HOMO
and LUMO orbitals for complex 11 and for the oxidation products
dication [11]2+ and tetracation [11]4+. Hydrogen atoms are omitted
for clarity. The isosurface value is 0.043.
The study of the electrochemistry of binuclear complexes is
always attractive due to the possible interaction between the
two metals. Unfortunately, complexes 9 and 10 were unstable
and decomposed in the electrode, but the cyclic voltammetry
of 11 was conducted under an argon atmosphere in dry,
oxygen-free dichloromethane (10−3 M analyte concentration)
containing [NnBu4]PF6 as the supporting electrolyte (10−1 M)
and using a Ag/AgCl reference electrode (3 M, KCl). Under
these conditions complex 11 displays two oxidation events,
one of them quasi-reversible at 0.49 V and the second one
irreversible at 1.10 V (Table 2 and Figures S1−S3). The DFT
(M06/6-311G(d,p)&SDD(f)) calculations reveal that the
HOMO of the complex is equally distributed between the
metal centers, whereas the LUMO is located in the
isoindolinate [HNC8H4NO]− ligand. The loss of one
electron by each metal (two electrons per molecule) leads to
the dication [11]2+, which also has the HOMO mainly
centered on the metals, although some participation of the
coordinated isoindolinate anion is observed. The subsequent
loss of two electrons affords the tetracation [11]4+, having the
HOMO mainly centered on the isoindolinate ligand (Figure
to RhIIL2RhII and from RhIIL2RhII to RhIIIL2RhIII. The
successive oxidations give rise to the approach of the metal
centers to 2.780 Å in the dication and to 2.747 Å in the
tetracation (Figure S6). Although these distances lie within the
range of distances assumed for a Rh−Rh single bond (2.62−
2.84 Å),27 overlapping between the d orbitals of the metals is
not observed.
The Ir(BMePHI)(diolefin) systems are also unstable in
basic medium. As in the rhodium case, the instability is
associated with the reactivity of the [BMePHI]− ligand in basic
medium, which is strongly directed by the metal center. Under
the same conditions as those giving rise to the mixture of 9 and
11, dimer 4 affords a mixture of the isopropoxide dimer [Ir(μ-
OiPr)(η4-C8H12)]2 and the trinuclear derivative Ir3(η4-
C8H12)2(κ1-C,η2-C8H13)(μ-OH)(L) (12 in Scheme 7). Using
complex 10 as a reference, the L ligand of 12 can be described
as the result of the oxidative addition of the C−N bond,
substituted with the free pyridyl-imine group, of the five-
membered ring of the isoindoline core to the pyridyl-
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Scheme 7. Formation of 12
coordinated metal center. The addition of a hydride to one of
the C−C double bonds of the diene coordinated to the
generated iridium(III) center and the addition of an [Ir(η4-
C8H12)]+ fragment to the free pyridyl-imine group give rise to
this novel molecule. The metal-promoted degradation of the
five-membered heterocycle of an isoindoline is certainly
notable. In this context, it should be highlighted that the
isoindoline skeleton is a part of a large variety of biologically
active synthetic compounds, which have a wide range of
applications in medicine.28
[BMePHI]− ligand and the trinuclear nature of the complex,
which is formed by an octahedral iridium(III) center (Ir(1))
and two square-planar iridium(I) centers (Ir(2) and Ir(3)).
The octahedron around Ir(1) is defined by two chelates and a
hydroxide-azavinylidene double bridge. The chelate C8H13-
carbocycle coordinates with three different Ir−C distances, as
expected for the κ1-C,η2-coordination, which compare well
with those reported for other complexes bearing C8H12R rings
similarly linked to iridium(III).29 The σ-Ir(1)−C(21) single
bond of 2.102(5) Å is about 0.06 and 0.07 Å shorter than the
metal−olefin bonds Ir(1)−C(25) and Ir(1)−C(26) of
2.163(4) and 2.172(5) Å, respectively. The Ir(1)−C(21)
bond is disposed trans to the pyridyl group of a (C(7),N(1))-
iminyl-pyridine moiety (C(21)−Ir(1)−N(1) = 169.13(15)°),
which has a N(1)−Ir(1)−C(7) bite angle of 74.85(15)°. The
iridium−pyridine distance of 2.236(4) Å (Ir(1)−N(1)) is
slightly longer than those found in 7 and 8, whereas the Ir(1)−
C(7) bond length of 1.982(4) Å is about 0.02 Å shorter than
the Ir(1)−C(21) single bond and even shorter than those
reported for other iridium-iminyl derivatives.30 This suggests
that, for an adequate description of the Ir(1)−C(7) bonding
situation, the resonance form a shown in Chart 1 should also
Complex 12 was separated from the mixture by extraction in
toluene and crystallized pure in 22% yield with regard to 4 as
orange crystals suitable for X-ray diffraction analysis. Its
structure (Figure 10) proves the degradation of the
Chart 1. Resonance for the Ir(1)−C(7) Bond
be taken into account. Atom C(7) is disposed trans to the
hydroxide group with a C(7)−Ir(1)−O(1) angle of
151.58(14)°, while the other part of the double bridge, the
azavinylidene ligand, lies trans to the C(25)−C(26) double
bond. The double bridge and the Ir(2) atom form a
metalloligand, which coordinates to Ir(1) with an O(1)−
Ir(1)−N(3) bite angle of 73.32(12)°. The iridium−azavinyli-
dene distances of 2.047(3) Å (Ir(1)−N(3)) and 2.035(3) Å
(Ir(2)−N(3)) are statistically identical. However, the Ir(1)−
O(1) distance of 2.253(3) Å is about 0.17 Å longer than the
Ir(2)−O(1) bond length of 2.072(3) Å. The Nimine,Npy chelate
coordinates to Ir(3) with a N(4)−Ir(3)−N(5) bite angle of
63.61(13)°, which compares well with the reported angles for
the κ2-Npy,Nimine-coordination of the [BMePHI]− anion.10 The
coordinated CC double bonds display distances between
1.299(6) and 1.429(6) Å. The asymmetry of 12 is also evident
in the 1H and 13C{1H} NMR spectra. Thus, the former shows
10 olefinic resonances between 5.6 and 3.0 ppm, due to the
inequivalent Csp2H-hydrogen atoms of the carbocycles, in
addition to the signals corresponding to the Ir(1)C(21)H- and
OH-hydrogen atoms, which appear at 0.65 and 0.24 ppm,
Figure 10. Molecular diagram of complex 12 (50% probability
ellipsoids). All hydrogen atoms (except that of the hydroxide ligand)
are omitted for clarity. Selected bond lengths (Å) and angles (deg):
Ir(1)−C(7) = 1.982(4), Ir(1)−N(3) = 2.047(3), Ir(1)−N(1) =
2.236(4), Ir(1)−O(1) = 2.253(3), Ir(1)−C(21) = 2.102(5), Ir(1)−
C(25) = 2.163(4), Ir(1)−C(26) = 2.172(5), Ir(2)−N(3) = 2.035(3),
Ir(2)−O(1) = 2.072(3), Ir(2)−C(29) = 2.071(4), Ir(2)−C(30) =
2.120(4), Ir(2)−C(33) = 2.101(4), Ir(2)−C(34) = 2.130(4), Ir(3)−
N(4) = 2.072(3), Ir(3)−C(37) = 2.096(4), Ir(3)−C(38) = 2.111(5),
Ir(3)−C(41) = 2.114(5), Ir(3)−C(42) = 2.092(5), N(1)−C(1) =
1.367(5), N(1)−C(6) = 1.370(5), N(2)−C(6) = 1.396(5), N(2)−
C(7) = 1.301(5), N(3)−C(14) = 1.296(5), N(4)−C(14) = 1.403(5),
N(4)−C(15) = 1.379(5), N(5)−C(15) = 1.357(5), N(5)−C(19) =
1.357(5), C(21)−Ir(1)−N(1) = 169.13(15), N(1)−Ir(1)−C(7) =
74.85(15), C(7)−Ir(1)−O(1) = 151.58(14), O(1)−Ir(1)−N(3) =
73.32(12), N(4)−Ir(3)−N(5) = 63.61(13).
1
respectively. The 13C{1H} NMR spectrum agrees with the H
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NMR spectrum. Thus, it contains 10 olefinic resonances
between 86.5 and 32.3 ppm. The signal corresponding to the
carbocyclic C(21) atom is observed at 32.1 ppm, whereas that
due to the iminyl C(7) atom appears at 198.9 ppm. This
chemical shift, at an unusually low field, is more evidence for a
significant contribution of the resonance form a to the Ir(1)−
C(7) bond.
Table 3. Metal-Mediated Acceptorless and Base-Free
Dehydrogenation of a Secondary Alcohol
a
The electrochemical behavior of binuclear complex 12 is
summarized in Table 2. As shown in Figures S4 and S5, it
undergoes three irreversible oxidations at 0.56, 0.74, and 1.37
V. The DFT (M06/6-311G(d,p)&SDD(f)) calculations reveal
that the HOMO of the molecule is mainly located on the
iridium(I) center Ir(2), whereas the LUMO is distributed
along the octahedral iridium(III) center Ir(1) and its
associated ligands (Figures S7−S9). After the loss of one
electron, the spin density of the molecule is still localized on
Ir(2) (computed spin density 0.71 e−). Thus, it is reasonable
to think that the second oxidation also takes place on this
center. Thus, the peaks at 0.56 and 0.74 V can be assigned to
the sequential oxidations of Ir(2), from Ir(I) to Ir(II) and from
Ir(II) to Ir(III). It is likely that the third oxidation at 1.37 V
could correspond to the other iridium(I) center, Ir(3). Overall,
the oxidation of 12 is mainly determined by the oxidation
states of the metal centers, which behave independently from
each other.
Aceptorless and Base-Free Dehydrogenation of
Secondary Alcohols Catalyzed by 9 and 10. The reactions
were performed in toluene at 100 °C, using a substrate
concentration of 0.255 M and a catalyst concentration of 9.0 ×
10−3 M (i.e., 7 mol % of the metal). Table 3 collects the
alcohols studied and the yield of carbonyl compounds formed
as a function of the catalyst after 24 h.
Ketones are obtained in moderate to high yields after 24 h.
Both catalysts are more efficient for the dehydrogenation of
aliphatic alcohols in comparison to that for benzylic or
benzhydrylic alcohols. Thus, while ketones resulting from the
dehydrogenation of substrates such as 1-phenylethanol, 3-
pyridylethanol, 1-(2-furyl)ethanol, and diphenylmethanol are
obtained in 20−60% yield, 2-octanol and 1-cyclohexylethanol
are dehydrogenated in about 70% yield. Complexes 9 and 10
are even more efficient than the binuclear polyhydrides shown
in Scheme 1, (PiPr3)2H2Ir{μ-(κ2-Npy,Nimine-BMePI-κ2-
a
Conditions: complex 9 or 10 (0.009 mmol); substrate (0.255
mmol); toluene (1 mL); heated at 100 °C for 24 h. Conversions were
calculated from the relative peak area integrations of the reactant and
product in the GC spectra.
Npy-(HBMePHI)}(η4-C8H12) (F), the alkoxide counterparts
of A−D. Then, the subsequent β-hydrogen elimination on the
terminal alkoxide group could afford the ketone and the
hydride species (η4-C8H12)HM(μ-OCHR′R)M{κ1-Npy-
(HBMePHI)}(η4-C8H12) (G), which would evolve into (η4-
C8H12)(η2-H2)M(μ-OCHR′R)M{κ1-Npy-(BMePHI)}(η4-
C8H12) (H) via heterolytic H−H formation.31 The subsequent
substitution of the dihydrogen ligand by the isoindoline-N
atom should release molecular hydrogen, regenerating the
active species E.
The cycle shown in Scheme 8 is consistent with those
proposed for the dehydrogenations promoted by the
polyhydrides of Scheme 1. The noninnocent character of the
bridging ligand is shown by the addition of the O−H bond of
the alcohol to the M−Niso bond of E and in the formation of
H. However, in this case, only a metal center would have a
direct participation in the catalysis. The function of the other
should be to keep the isoindoline N−H bond in the proximity
of the active center.
N
imine,C4iso)}IrH2(PiPr3)2 and (PiPr3)2H2Ir{μ-(κ2-Npy,Nimine
-
BMePI-κ2-Nimine,C4iso)}OsH3(PiPr3)2, for the dehydrogenation
of aliphatic alcohols.10b This ability is in contrast to the
generally observed trend. Aromatic groups stabilize the ketone
and appear to increase the dehydrogenation rate of the alcohol.
The rhodium complex 9 is significantly more efficient than the
iridium derivative 10 for the dehydrogenation of aromatic
substrates, in particular for 3-pyridylethanol, 1-(2-furyl)-
ethanol, and diphenylmethanol, whereas the oxidation of
aliphatic alcohols occurs with similar efficiency in the presence
of both complexes.
The catalysis can be rationalized according to Scheme 8.
The alcohol, which is in great excess with regard to the metal
complexes, should initially displace the bridging hydroxide
ligand to afford the related alkoxide derivatives [M(η4-
C8H12)]2(μ-OCHRR′){μ-Niso,Npy-(BMePHI)} (E), which
would be the catalytically active species of the dehydrogenation
process. As in the reactions catalyzed by the polyhydrides
shown in Scheme 1,10 the addition of the O−H bond of the
alcohols to the bond M−Niso of E could generate the
intermediates (η4-C8H12)(R′RCHO)M(μ-OCHR′R)M{κ1-
CONCLUDING REMARKS
■
This study has revealed that a pyridyl group of 1,3-bis(6′-
methyl-2′-pyridylimino)isoindoline (HBMePHI) coordinates
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instrument. Chemical shifts (expressed in ppm) are referenced to
residual solvent peaks (1H, 13C{1H}). Coupling constants J are given
in hertz. C, H, and N analyses were carried out with a PerkinElmer
2400 CHNS/O analyzer or with a Thermo FlashEA 1112 CHNS/O
analyzer. High-resolution electrospray mass spectra (HRMS) were
acquired using a MicroTOF-Q hybrid quadrupole time-of-flight
spectrometer (Bruker Daltonics, Bremen, Germany). [Rh(μ-Cl)(η4-
C8H12)]2 (1),32 [Ir(μ-Cl)(η4-C8H12)]2 (2),33 [Rh(μ-OH)(η4-
C8H12)]2, (3),34 [Ir(μ-OH)(η4-C8H12)]2 (4),35 and 1,3-bis(6′-
methyl-2′-pyridylimino)isoindoline (HBMePHI)36 were prepared
according to the published methods.
Scheme 8. Catalytic Cycle for the Acceptorless and Base-
Free Dehydrogenation of Secondary Alcohols Promoted by
Complexes 9 and 10
General Procedure for the Rh- and Ir-Catalyzed Dehydro-
genation Reactions of Alcohols. A solution of the catalyst (9 or
10, 0.009 mmol) and the corresponding substrate (0.255 mmol) in
toluene (1 mL) was placed in a Schlenk flask equipped with a
condenser under an argon atmosphere. The mixture was stirred at 100
°C for 24 h. After this time the solution was cooled to room
temperature, and the progress of the reaction was monitored by GC
(Agilent 6890N gas chromatograph with a flame ionization detector,
using an Agilent 19091N-133 polyethylene glycol column (30 m ×
250 μm × 0.25 μm thickness)). The oven conditions used are as
follows: 80 °C (hold 5 min) to 200 °C at 15 °C/min (hold 7 min),
except for diphenylmethanol, 150 °C (hold 5 min) to 240 °C at 15
°C/min (hold 13 min). The obtained values of the yield are the
average of two runs. The identity of the compound was confirmed by
comparison of the retention time of the product.
Addition of 2.0 mol of HBMePHI to [Rh(μ-Cl)(η4-C8H12)]2 (1).
At room temperature, 19 mg (0.06 mmol) of HBMePHI was added to
0.5 mL of a dichloromethane-d2 solution of 1 (15 mg, 0.03 mmol) in
an NMR tube. After 10 min, 1H NMR spectra between 293 and 223 K
(Figure 1) and the 13C{1H} NMR spectrum at 213 K (Figure S12)
were recorded. HRMS (electrospray, m/z): calcd for
C28H29RhN5ClNa [M + Na]+ 596.1059, found 596.1038. Selected
spectroscopic data for RhCl(η4-C8H12){κ1-Npy-(HBMePHI)} (5) are
1
as follows. H NMR (400 MHz, CD2Cl2, 223 K): δ 12.70 (s, 1H,
NH), 8.91 (d, 3JH−H = 7.4, 1H, CHarom), 8.04 (dt, 3JH−H = 7.4, 3JH−H
=
to a metal center of the dimers [M(μ-X)(η4-C8H12)]2 (M =
Rh, Ir; X = Cl, OH) to afford square-planar species, splitting at
least one of the bridges. The subsequent deprotonation of the
N−H bond of the isoindoline core by a hydroxo group leads to
the complexes [M(η4-C8H12)]2(μ-OH){μ-Niso,Npy-(BMe-
PHI)} (M = Rh, Ir), which are efficient catalyst precursors
for the acceptorless and base-free dehydrogenation of
secondary alcohols. These compounds cannot be used in
catalytic processes needing a basic medium, because the
[BMePHI]− ligand undergoes degradation. Depending upon
the metal center of the complex, three different deteriorations
have been observed: (i) alcoholysis of an exocyclic isoindoline-
N double bond, (ii) alcoholysis of a N-pyridyl bond, and (iii)
opening of the five-membered ring of the isoindoline core by
oxidative addition of one of the C−N bonds to a metal center
of the catalyst precursor.
In summary, 1,3-bis(2′-pyridylimino)isoindolinates are
interesting organic anions, which can act as noninnocent
bridging ligands in diverse catalysts for the acceptorless and
base-free dehydrogenation of secondary alcohols. However,
they should be not employed to stabilize catalysts of processes
which take place in basic media, since they undergo
degradation.
3
3
1.0, 1H, CHarom), 7.86 (td, JH−H = 7.5, JH−H = 1.2, 1H, CHarom),
3
3
3
7.80 (td, JH−H = 7.5, JH−H = 1.2, 1H, CHarom), 7.74 (t, JH−H = 7.8,
1H, CHarom), 7.63 (t, 3JH−H = 7.6, 1H, CHarom), 7.25 (d, 3JH−H = 7.9,
1H, CHarom), 7.21 (d, 3JH−H = 7.9, 1H, CHarom), 7.08 (d, 3JH−H = 7.6,
3
1H, CHarom), 6.92 (d, JH−H = 7.5, 1H, CHarom), 4.46 (m, 1H, CH
COD), 4.38 (m, 1H, CH COD), 3.55 (m, 1H, CH COD), 3.50 (m,
1H, CH COD), 3.17 (s, 3H, CH3), 2.36 (m, 3H, CH2 COD), 2.24 (s,
3H, CH3), 1.98 (m, 1H, CH2 COD), 1.70 (m, 2H, CH2 COD), 1.66
(m, 1H, CH2 COD), 1.46 (m, 1H, CH2 COD). 13C-APT NMR
(100.6 MHz, CD2Cl2, 213 K): δ 159.5, 159.4, 159.0, 156.0, 153.0,
152.9 (all s, Carom), 139.2, 138.7 (both s, CHarom), 135.8, 133.3 (both
s, Carom), 132.6, 132.0, 123.6, 122.2, 121.0, 120.3, 120.2, 113.7 (all s,
2
2
CHarom), 81.7 (d, JC−Rh = 12.2, CH COD), 80.4 (d, JC−Rh = 11.5,
CH COD), 75.6 (d, 2JC−Rh = 13.6, CH COD), 74.6 (d, 2JC−Rh = 13.4,
CH COD), 31.4, 30.3, 30.0, 29.9 (all s, CH2 COD), 25.5, 24.2 (both
s, py-CH3).
Addition of 1.0 mol of HBMePHI to [Rh(μ-Cl)(η4-C8H12)]2. At
room temperature, 10 mg (0.03 mmol) of HBMePHI was added to
0.5 mL of a dichloromethane-d2 solution of 1 (15 mg, 0.03 mmol) in
an NMR tube. After 10 min, 1H NMR spectra between 283 and 183 K
(Figure S14) and the 13C{1H} NMR spectrum at 183 K (Figure S15)
were recorded. HRMS (electrospray, m/z): calcd for C36H41Rh2N5Cl
[M − Cl]+ 784.1155, found 784.1190. Selected spectroscopic data for
RhCl(η4-C8H12){μ-Npy,Npy-(HBMePHI)}(6): are as follows. 1H
NMR (400 MHz, CD2Cl2, 183 K): δ 8.1−8.0 (3H, CHarom), 7.8−
7.7 (4H, CHarom), 7.1−7.0 (3H, CHarom), 4.50 (br, 1H, CH COD),
4.44 (br, 1H, CH COD), 3.48 (br, 1H, CH COD), 3.42 (br, 1H, CH
COD), 2.80 (s, 6H, py-CH3), 2.40 (m, 4H, CH2 COD), 2.33 (m, 4H,
CH2 COD), 1.66 (m, 8H, CH2 COD). 13C{1H}-APT NMR (100.6
MHz, CD2Cl2, 193 K): δ 158.6, 158.4, 157.5 (all s, Carom), 139.1 (s,
2C, CHarom), 133.7 (s, Carom), 132.7, 121.1 (s, 2C, CHarom), 81.8,
80.8, 76.1, 75.7 (all br, CH COD), 30.8, 30.2, 29.9, 29.6 (all s, CH2
COD), 24.9 (s, 2C, py−CH3).
EXPERIMENTAL SECTION
■
All reactions were carried out with rigorous exclusion of air using
Schlenk-tube techniques. Solvents were dried using standard
procedures and distilled under an argon atmosphere or obtained
dry from an MBraun solvent purification apparatus. H and 13C{1H}
1
NMR spectra (Figures S10−S30) were recorded on a Bruker Avance
300 MHz, Bruker Avance 400 MHz, or Bruker Avance 500 MHz
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Preparation of IrCl(η4-C8H12){κ1-Npy-(HBMePHI)} (7). Complex
2 (0.300 g, 0.446 mmol) was dissolved in toluene (6 mL) and treated
with 2.0 mol of HBMePHI (0.320 g, 0.981 mmol). The resulting
suspension turned red and was stirred for 2 h. Then, the volatiles were
removed under vacuum. The residue was washed with pentane (3 × 3
mL) to afford a red solid, which was dried in vacuo. Yield: 323 mg
(54%). Red crystals suitable for X-ray diffraction analysis were
obtained from slow diffusion of pentane in a concentrated solution of
7 in toluene. Anal. Calcd for C28H29ClIrN5: C, 50.71; H, 4.41; N,
10.56. Found: C, 50.37; H, 4.59; N, 10.55. HRMS (electrospray, m/
z): calcd for for C28H29IrN5 [M − Cl]+ 628.2046, found 628.2047. 1H
NMR (400 MHz, CD2Cl2, 243 K): δ 12.72 (s, 1 H, NH), 8.61 (dd,
3H, py−CH3), 2.44 (m, 2H, CH2 COD), 2.40 (s, 3H, py-CH3), 2.22
(s, 1H, Rh−OH−Rh), 1.86 (m, 2H, CH2 COD), 1.74 (m, 2H, CH2
COD), 1.56 (m, 2H, CH2 COD), 1.31 (m, 2H, CH2 COD), 1.26 (m,
2H, CH2 COD), 1.12 (m, 2H, CH2 COD), 1.01 (m, 2H, CH2 COD).
13C{1H}-APT NMR (100.6 MHz, toluene-d8, 193 K): δ 166.5. 165.1,
164.2, 161.0, 158.6, 156.6, 141.9, 140.9 (all s, Carom), 138.7, 138.4,
131.0, 130.6, 122.3, 121.6, 118.0, 117.9, 117.4, 116.7 (all s, CHarom),
83.7, 81.8, 81.0, 79.3, 74.7, 72.9, 71.5, 71.3 (all s, CH COD), 34.2 (s,
CH2 COD), 31.7 (s, 2C, CH2 COD), 31.2, 30.1, 29.9, 29.3, 28.9 (all
s, CH2 COD), 26.5 (py-CH3), 25.3 (py−CH3).
Preparation of [Ir(η4-C8H12)]2(μ-OH){μ-Niso,Npy-(BMePHI)}
(10). The substrate HBMePHI (0.051 g, 0.157 mmol) was added
to 3 mL of a propan-2-ol suspension of 4 (0.1 g, 0.157 mmol). After 2
h an orange solid was formed. The liquors were separated, and the
orange solid was washed with 2 mL of propan-2-ol at 0 °C. The solid
was solved in toluene. The solution was concentrated in vacuo. The
addition of pentane gives rise to the precipitation of an orange solid,
which was washed with pentane (2 × 3 mL) at 0 °C. Yield: 70 mg
(47%). Orange crystals of 10 were obtained from slow diffusion of
pentane in toluene. Anal. Calcd for C36H41Ir2N5O: C, 45.80; H, 4.38;
N, 7.42. Found: C, 46.10; H, 4.26, N, 7.52. HRMS (electrospray, m/
z): calcd for C36H40Ir2N5 [M − OH]+ 928.2537, found 928.2585. 1H
NMR (400 MHz, toluene-d8, 193 K): δ 8.28 (br, 1H, CHarom), 8.20
(br, 1H, CHarom), 7.13 (br, 3H, CHarom), 7.08 (br, 1H, CHarom), 6.90
(br, 1H, CHarom), 6.77 (br, 1H, CHarom), 6.32 (br, 1H, CHarom), 6.09
(br, 1H, CHarom), 5.07 (br, 1H, CH COD), 4.24 (br, 1H, Ir−OH−Ir),
4.05 (br, 1H, CH COD), 3.86 (br, 1H, CH COD), 3.44 (br, 1H, CH
COD), 3.20 (br, 3H, CH COD), 2.99 (br, 1H, CH COD), 2.87 (s,
3H, py−CH3), 2.33 (s, 3H, py-CH3), 2.32 (br, 2H, CH2 COD), 1.86
(br, 2H, CH2 COD), 1.68 (br, 2H, CH2 COD), 1.44 (br, 2H, CH2
COD), 1.36 (br, 2H, CH2 COD), 1.09 (br, 2H, CH2 COD), 0.98 (br,
2H, CH2 COD), 0.82 (br, 2H, CH2 COD). 13C{1H}-APT NMR
(plus HSQC and HMBC) (100.6 MHz, toluene-d8, 188 K): δ 165.4,
164.9, 163.8, 159.2, 157.0, 155.7, 140.8, 139.9 (all s, Carom), 138.1,
138.0, 130.7, 130.3, 122.0, 121.4, 118.2, 118.0, 116.9, 116.3 (all s,
CHarom), 67.4, 66.5, 65.6, 62.4, 56.3, 55.2, 53.0, 52.2 (all s, CH COD),
34.6, 32.3, 32.2, 31.7, 30.7, 30.2, 30.1, 29.0 (all s, CH2 COD), 25.6,
24.7 (both s, py-CH3).
3
3
3
3JH−H = 6.2, JH−H = 1.6, CHiso), 8.03 (dd, JH−H = 6.2, JH−H = 1.6,
1H, CHiso), 7.81−7.80 (m, 2H, CHarom), 7.76 (dd, 3JH−H = 7.6, 3JH−H
3
= 7.6, 1H, CHpy), 7.66 (m, 1H, CHpy), 7.22 (d, 1H, JH−H = 8.3,
3
3
CHpy) 7.20 (d, 1H, JH−H = 8.3, CHpy), 7.13 (d, 1H, JH−H = 7.6,
CHpy), 6.94 (d, 1 H, 3JH−H = 7.6, CHpy), 4.23 (m, 1H, = CH COD),
4.10 (m, 1H, = CH COD), 3.22 (m, 1 H, = CH COD), 3.12 (m, 1 H,
= CH COD), 3.03 (s, 3 H, CH3), 2.27 (s, 3 H, CH3), 2.15 (m, 3 H,
CH2 COD), 1.99 (m, 1 H, CH2 COD), 1.72 (m, 1 H, CH2 COD),
1.38 (m, 4 H, COD), 1.12 (m, 1 H, COD). 13C{1H}-APT NMR (plus
HSQC and HMBC) (100.6 MHz, CD2Cl2, 243 K): δ 159.6, 159.5,
159.4, 156.3 (all s, Cpy), 153.2, 152.5 (both s, Ciso) 139.2, 139.0 (both
s, CHpy) 136.1, 133.5 (both s, Ciso) 132.7, 132.3, 124.0, 122.5 (all s,
CHiso), 121.8, 120.6, 120.4, 114.6 (all s, CHpy) 66.3, 64.3, 58.1, 57.4
(all s, CH COD) 32.2, 31.3 (both s, CH2 COD) 31.0 (s, 2 C, CH2
COD), 25.3, 24.4 (both s, py-CH3).
Preparation of {IrCl(η4-C8H12)}2(μ-Npy,Npy-HBMePHI) (8).
Complex 2 (0.300 g, 0.446 mmol) was dissolved in toluene (8 mL)
and treated with 1.0 mol of HBMePHI (0.145 g, 0.446 mmol). The
orange suspension turned red, and a yellow precipitate was formed.
After 4 h, at room temperature, the solid was separated from the
liquors. The yellow solid was washed with pentane (3 × 3 mL, 273 K)
and was dried in vacuo. Yield: 357 mg (80%). Yellow crystals suitable
for X-ray diffraction analysis were obtained from slow diffusion of
diethyl ether in a concentrated solution of 8 in dichloromethane.
Anal. Calcd for C36H41Cl2Ir2N5: C, 43.28; H, 4.14; N, 7.01. Found: C,
42.88; H, 3.79; N, 7.34. HRMS (electrospray, m/z): calcd for
Preparation of [Rh(η4-C8H12)]2{μ-Niso,Nimine-(HNC8H4NO)}-
(μ-NC8H4NOiPr) (11). The substrate HBMePHI (173.2 mg, 0.531
mmol) and KOtBu (89 mg, 0.796 mmol) were added to 3 mL of a
propan-2-ol suspension of 3 (121 mg, 0.265 mmol). The mixture was
stirred for 2 h at room temperature, and an orange solid was formed,
which was filtered off. The filtrate was cooled at 4 °C for 3 days, and
red crystals were obtained. Yield: 40 mg (20%). Anal. Calcd for
C35H40N4O2Rh2: C, 55.71; H, 5.34; N, 7.43. Found: C, 55.87; H,
5.42; N, 7.38. HRMS (electrospray, m/z): calcd for C35H41N4O2Rh2
[M + H]+ 755.1334, found 755.1308. 1H NMR (400 MHz, C6D6, 298
1
C36H41Ir2N5Cl [M −Cl]+ 964.2304, found 964.2286. H NMR (400
MHz, CD2Cl2, 253 K): δ 12.70 (s, 1H, NH), 8.83 (m, 2H, CHarom),
3
7.92 (m, 2H, CHarom), 7.62 (m, 2H, CHarom), 7.33 (d, 2H, JH−H
=
3
8.1, CHarom), 7.02 (d, 2H, JH−H = 8.1, CHarom), 4.25 (m, 2H, CH
COD), 4.05 (m, 2H, CH COD), 3.58 (m, 2H, CH COD), 3.25 (m,
2H, CH COD), 2.85 (s, 6H, py-CH3), 2.19 (m, 2H, CH2 COD), 1.99
(m, 2H, CH2 COD), 1.51 (m, 2H, CH2 COD), 1.41 (m, 2H, CH2
COD). 13C{1H}-APT NMR (100.6 MHz, CD2Cl2, 253 K): δ 159.6,
158.2, 152.6 (all s, Carom) 139.4 (s, CHarom), 134.6 (s, Carom), 133.0,
124.6, 121.9, 116.4 (all s, CHarom), 66.1, 65.8, 59.1, 58.6 (all s, CH
COD), 32.4, 31.9, 31.3, 30.8 (all s, CH2 COD), 25.2 (s, py−CH3).
Preparation of [Rh(η4-C8H12)]2(μ-OH){μ-Niso,Npy-(BMePHI)}
(9). The substrate HBMePHI (0.071 g, 0.219 mmol) was added to
3 mL of a propan-2-ol suspension of 3 (0.1 g, 0.219 mmol). After 2 h
an orange solid was formed. The liquors were separated, and the
orange solid was washed with 2 mL of propan-2-ol at 0 °C. The solid
was solved in toluene. The solution was concentrated in vacuo. The
addition of pentane gives rises to the precipitation of an orange solid
which was washed with pentane (2 × 3 mL) at 0 °C. Yield: 132 mg
(80%). Orange crystals of 9 were obtained from slow diffusion of
pentane in toluene. Anal. Calcd for C36H41N5ORh2: C, 56.48; H,
5.40; N, 9.15. Found: C, 56.20; H, 5.35; N, 9.28. HRMS
(electrospray, m/z): calcd for for C36H40N5Rh2 [M − OH]+
748.1388, found 748.1392. 1H NMR (400 MHz, toluene-d8, 183
3
3
K): δ 10.61 (d, JH−H = 7.5, 1 H, CHarom), 7.65 (t, JH−H = 7.5, 1 H,
CHarom), 7.39 (d, 3JH−H = 7.3, 1 H, CHarom), 7.27 (d, 3JH−H = 7.3, 1 H,
CHarom), 7.02 (dd, JH−H = 7.4, 7.2, 1 H, CHarom), 6.74 (dd, JH−H
7.4, 7.2, 1 H, CHarom), 6.64 (dd, JH−H = 7.4, 7.2, 1 H, CHarom), 6.44
(m, 1 H, CH COD), 6.25 (d, 3JH−H = 7.5, 1 H, CHa3rom), 6.21 (m, 1 H,
CH COD), 5.78 (br, 1 H, NH), 5.36 (sept, JH−H = 6.4, 1H,
OCH(CH3)2), 5.15 (m, 1 H, CH COD), 4.98 (dd, 3JH−H = 7.2, 6.9, 1
3
3
=
3
3
H, CH COD), 4.54 (dd, JH−H = 7.2, 6.9, 1 H, CH COD), 3.96 (dd,
3JH−H = 7.2, 6.9, 1 H, CH COD), 3.55 (m, 1 H, CH COD), 3.44 (m,
1 H, CH COD), 3.23 (m, 2 H, CH2 COD), 3.11 (m, 2 H, CH2
COD), 2.36 (m, 6 H, CH2 COD), 2.16 (m, 2 H, CH2 COD), 1.67
3
(m, 4 H, CH2 COD), 1.26 and 1.15 (both d, JH−H = 6.4, 3 H each,
OCH(CH3)2). 13C{1H}-APT NMR (plus HSQC and HMBC) (100.6
MHz, CD2Cl2, 298 K): δ 176.8, 175.3, 174.5, 163.8, 136.5, 136.0,
134.9, 134.6 (all s, Carom), 130.8, 130.6, 129.8, 128.8, 125.9, 121.9,
3
3
K): δ 8.43 (dd, JH−H = 4.1, 3.0, 1H, CHarom), 8.31 (dd, JH−H = 4.1,
3
2
3.0, 1H, CHarom), 7.30 (d, JH−H = 7.8, 1H, CHarom), 7.22 (m, 1H,
119.1, 118.4 (all s, CHarom), 86.9 (d, JC−Rh = 12.9, CH COD), 84.2
CHarom), 7.14 (m, 1H, CHarom), 7.07 (m, 1H, CHarom), 6.98 (d, 3JH−H
= 7.8, 1H, CHarom), 6.88 (dd, 3JH−H = 7.8, 7.8, 1H, CHarom), 6.41 (d,
3JH−H = 7.5, 1H, CHarom), 6.22 (d, 3JH−H = 7.5, 1H, CHarom), 5.34 (m,
1H, CH COD), 4.31 (m, 1H, CH COD), 4.02 (m, 1H, CH COD),
3.64 (m, 1H, CH COD), 3.48 (m, 1H, CH COD), 3.36 (m, 1H, CH
COD), 3.27 (m, 1H, CH COD), 3.21 (m, 1H, CH COD), 3.18 (s,
2
2
(d, JC−Rh = 8.9, CH COD), 81.9 (d, JC−Rh = 11.7, CH COD), 81.6
(d, 2JC−Rh = 10.5, CH COD) 81.2 (d, 2JC−Rh = 10.5, CH COD), 79.0
(d, 2JC−Rh = 10.5, CH COD), 78.2 (d, 2JC−Rh = 12.9, CH COD), 74.5
(d, 2JC−Rh = 11.7, CH COD), 72.2 (s, OCH(CH3)2), 35.2, 35.2, 35.0,
34.3, 29.5, 29.0, 28.6, 28.5 (all s, CH2 COD), 22.0 and 21.9 (both s,
OCHCH3).
999
https://doi.org/10.1021/acs.organomet.1c00068
Organometallics 2021, 40, 989−1003

Organometallics
pubs.acs.org/Organometallics
Article
Preparation of Ir3(η4-C8H12)2(κ1-C,η2-C8H13)(μ-OH)(L) (12).
The substrate HBMePHI (147 mg, 0.450 mmol) and KOtBu (75
mg, 0.675 mmol) were added to 4 mL of a propan-2-ol suspension of
4 (142 mg, 0.225 mmol). The mixture was stirred for 2 h at room
temperature, and an orange solid was formed, which was filtered off
and subsequently was treated with toluene (2 × 4 mL) to afford a
yellow solid and a red solution. The solution was separated by
filtration, and the volatiles were removed under vacuum. The
treatment of the residue with pentane gave an orange solid. Orange
crystals suitable for X-ray diffraction analysis were obtained from a
saturated solution in benzene. Yield: 42 mg (22%). Anal. Calcd for
C44H54Ir3N5O: C, 42.43; H, 4.37; N, 5.62. Found: C, 42.18; H, 4.38;
N, 5.39. HRMS (electrospray, m/z): calculated for C44H53Ir3N5 [M −
Madrid, 28040 Madrid, Spain; orcid.org/0000-0002-
3360-7795; Email: sierraor@ucm.es
Authors
María L. Buil − Departamento de Química Inorgánica,
Instituto de Síntesis Química y Catálisis Homogénea
(ISQCH), Centro de Innovación en Química Avanzada
(ORFEO-CINQA), Universidad de Zaragoza-CSIC, 50009
Zaragoza, Spain; orcid.org/0000-0002-3284-1053
Alba Collado − Departamento de Química Orgánica I,
Facultad de CC. Químicas, Centro de Innovación en Química
Avanzada (ORFEO-CINQA), Universidad Complutense de
Madrid, 28040 Madrid, Spain; orcid.org/0000-0001-
6215-1822
Mar Gómez-Gallego − Departamento de Química Orgánica I,
Facultad de CC. Químicas, Centro de Innovación en Química
Avanzada (ORFEO-CINQA), Universidad Complutense de
Madrid, 28040 Madrid, Spain; orcid.org/0000-0002-
8961-7685
Susana Izquierdo − Departamento de Química Inorgánica,
Instituto de Síntesis Química y Catálisis Homogénea
(ISQCH), Centro de Innovación en Química Avanzada
(ORFEO-CINQA), Universidad de Zaragoza-CSIC, 50009
Zaragoza, Spain
Antonio I. Nicasio − Departamento de Química Inorgánica,
Instituto de Síntesis Química y Catálisis Homogénea
(ISQCH), Centro de Innovación en Química Avanzada
(ORFEO-CINQA), Universidad de Zaragoza-CSIC, 50009
Zaragoza, Spain; orcid.org/0000-0002-2268-4920
1
OH]+ 1230.3183, found 1230.3152. H NMR (500.13 MHz, C6D6,
298 K): δ 8.53 (dd, 3JH−H = 7.8, 1.0, 1H, H12), 8.07 (dd, 3JH−H = 7.3,
3
1.4, 1H, H9), 7.64 (dd, JH−H = 7.9, 1.0, 1H, CHpy), 7.29 (m, 2H,
3
CHarom), 7.25 (dd, JH−H = 7.4, 1.4, 1H, H11), 7.14 (m, 1H, CHpy),
3
7.13 (m, 1H, CHpy), 6.69 (dd, JH−H = 7.3, 1.0, 1H, CHpy), 5.94 (d,
3JH−H = 7.2, 1H, CHpy), 5.55 (m, 1H, CH COD), 4.54 (m, 1H, CH
COD), 4.12 (m, 1H, CH COD), 4.04 (m, 1H, CH COD), 3.88 (m,
1H, CH COD), 3.82 (m, 1H, CH COD), 3.69 (m, 1H, CH COD),
3.46 (m, 1H, CH COD), 3.37 (s, 3H, py-CH3), 3.11 (m, 2H, CH
COD), 2.58 (m, 1H, CH2 COD), 2.50 (m, 1H, CH2 COD), 2.32 (m,
1H, CH2 COD), 2.29 (m, 1H, CH2 COD), 2,13 (m, 3H, CH2 COD),
2.04 (m, 3H, CH2 COD), 1.94 (m, 4H, CH2 COD), 1.85 (s, 3H, py-
CH3), 1.79 (m, 1H, CH2 COD), 1.75 (m, 1H, CH2 COD), 1.20 (m,
3H, CH2 COD), 1.11 (m, 2H, CH2 COD), 0.65 (m, 1H, κ1-CH
COD), 0.24 (s, 1H, Ir−OH−Ir). 13C{1H}-APT NMR (plus HSQC
and HMBC) (75 MHz, C6D6, 298 K): δ 198.9 (s, C7), 179.3 (s, Cpy),
166.7 (s, Cpy), 159.4 (s, C14), 157.9 (s, Cpy), 153.1 (s, Cpy), 145.8 (s,
C13), 137.7 (s, Cpy), 137.6 (s, Cpy), 135.4 (s, C8), 129.1 (s, C12), 128.8
(s, C11), 127.9 (s, C10), 122.3 (s, C9), 119.6 (s, Cpy), 117.6 (s, Cpy),
111.1 (s, Cpy), 110.3 (s, Cpy), 86.4, 71.4, 67.7, 67.0, 63.0, 60.1, 59.7,
59.0, 59.0, 52.6 (all s, = CH COD), 42.7, 36.9, 35.8, 34.5, 32.6, 32.5
(all s, CH2 COD), 32.1 (κ1-CH COD), 31.1, 30.7, 30.3, 29.5, 28.5,
28.2 (all s, CH2 COD), 25.0 (s, py-CH3), 23.4 (s, CH2 COD), 21.0
(s, py-CH3).
Enrique Onate − Departamento de Química Inorgánica,
̃
Instituto de Síntesis Química y Catálisis Homogénea
(ISQCH), Centro de Innovación en Química Avanzada
(ORFEO-CINQA), Universidad de Zaragoza-CSIC, 50009
Zaragoza, Spain; orcid.org/0000-0003-2094-719X
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00068
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00068.
■
sı
Notes
The authors declare no competing financial interest.
Computational and electrochemical data, structural
analysis of complexes 7−9, 11, and 12, and NMR
spectra (PDF)
ACKNOWLEDGMENTS
■
Financial support from the MCI, projects CTQ2017-82935-P,
PID2019-108429RB-I00, and RED2018-102387-T, the Go-
bierno de Aragón (Group E06_17R), Fundación Ramón
Areces (XVIII Concurso Nacional de Ayudas a la Investigación
en Ciencias de la Vida y de la Materia CIVP18A3938), Fondo
Europeo de Desarrollo Regional (FEDER), and the European
Social Fund (FSE) is acknowledged. A.I.N. and A.C. thank the
MINECO (Spain) for a predoctoral fellowship and Juan de la
Cierva-Incorporación Fellowship, respectively.
Cartesian coordinates of computed complexes (XYZ)
Accession Codes
CCDC 1913552−1913556 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
REFERENCES
■
AUTHOR INFORMATION
Corresponding Authors
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■
Miguel A. Esteruelas − Departamento de Química Inorgánica,
Instituto de Síntesis Química y Catálisis Homogénea
(ISQCH), Centro de Innovación en Química Avanzada
(ORFEO-CINQA), Universidad de Zaragoza-CSIC, 50009
Zaragoza, Spain; orcid.org/0000-0002-4829-7590;
Email: maester@unizar.es
Miguel A. Sierra − Departamento de Química Orgánica I,
Facultad de CC. Químicas, Centro de Innovación en Química
Avanzada (ORFEO-CINQA), Universidad Complutense de
1000
https://doi.org/10.1021/acs.organomet.1c00068
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