Dinuclear iridium and rhodium complexes with bridging arylimidazolide-N3,C2 ligands: Synthetic, structural, reactivity, electrochemical and spectroscopic studies

Article abstract of DOI:10.1039/c5dt02403j

Deprotonation of 1-arylimidazoles (aryl = mesityl (Mes), 2,6-diisopropylphenyl (Dipp)), with n-butyl lithium afforded the corresponding derivatives (1-aryl-1H-imidazol-2-yl)lithium (1a, Ar = Mes; 1b, Ar = Dipp) in good yield. Reaction of 1a with 0.5 equiv. of [Ir(cod)(μ-Cl)]₂ yielded two geometrical isomers of a doubly C2,N3-bridged dinuclear complex [Ir(cod){μ-C₃H₂N₂(Mes)-κC2,κN3}]₂ (3), 3_H-H, a head-to-head (H-H) isomer of C_S symmetry, and 3_H-T, the thermodynamically preferred head-to-tail (H-T) isomer of C₂ symmetry. The metallated carbon of the 4 electron donor anionic bridging ligands has some carbene character, reminiscent of the situation in N-metallated protic NHC complexes. Displacement of cod ligands from 3_H-H and 3_H-T afforded the tetracarbonyl complexes [Ir(CO)₂{μ-C₃H₂N₂(Mes)-κC2,κN3}]₂4_H-H and 4_H-T, respectively. The reaction with PMe₃, which gave only one complex, [Ir(CO)(PMe₃){μ-C₃H₂N₂(Mes)-κC2,κN3}]₂ (5), demonstrates that the isomerization of the central core Ir[μ-C₃H₂N₂(Mes)-κC2,κN3]₂Ir from H-H to H-T on going from 4_H-H to 5 is readily triggered by phosphine substitution under mild conditions. Oxidative-addition of MeI to 5 afforded the formally metal-metal bonded d⁷-d⁷ complex [Ir₂(CO)₂(PMe₃)₂(Me)I{μ-C₃H₂N₂(Mes)-κC2,κN3}₂] (6). The blue [Ir(C₂H₄)₂{μ-C₃H₂N₂(Mes)-κC2,κN3}]₂ (7) and purple [Rh(C₂H₄)₂{μ-C₃H₂N₂(Dipp)-κC2,κN3}]₂ (9) tetraethylene complexes were also obtained with only a H-T arrangement of the bridging ligands. Although only modestly efficient in alkane dehydrogenation, complex 7 was found to be a more active pre-catalyst than 3_H-T, 4_H-T and 5, probably because of the favorable lability of the ethylene ligands. From cyclic voltammetry, exhaustive coulometry and spectroelectrochemistry studies, it was concluded that 3_H-T undergoes a metal-based one electron oxidation to generate the mixed-valent Ir(i)/Ir(ii) system. The energy of the intervalence band for the orange dirhodium complex [Rh(cod){μ-C₃H₂N₂(Mes)-κC2,κN3}]₂ (8) is shifted toward lower energies in comparison with 3_H-T, reflecting the decrease of the energy with the intermetallic distance. It was concluded from the EPR study that the Ir and Rh centres contribute substantially to the experimental magnetic anisotropy and thus to the singly occupied molecular orbital (SOMO) in the mixed-valent Ir(i)/Ir(ii) and Rh(i)/Rh(ii) systems. The molecular structures of 3_H-H, 3_H-T, 8 and 9 have been determined by X-ray diffraction.

Full text of DOI:10.1039/c5dt02403j

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DOI: 10.1039/C5DT02403J
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ARTICLE
Dinuclear Iridium and Rhodium Complexes with Bridging
3
2
Arylimidazolide-N ,C Ligands: Synthetic, Structural, Reactivity,
Electrochemical and Spectroscopic Studies†
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Dinuclear Iridium and Rhodium Complexes with Bridging
Arylimidazolide-N ,C Ligands: Synthetic, Structural, Reactivity,
Electrochemical and Spectroscopic Studies†
3
2
www.rsc.org/
a
b
b
c
e
Fan He, Laurent Ruhlmann, Jean-Paul Gisselbrecht, Sylvie Choua, Maylis Orio, Marcel
a
a,d
a
Wesolek, Andreas A. Danopoulos,* Pierre Braunstein*
Deprotonation of 1-arylimidazoles (aryl = mesityl (Mes), 2,6-diisopropylphenyl (Dipp)), with n-butyl lithium afforded the
corresponding derivatives (1-aryl-1H-imidazol-2-yl)lithium (1a, Ar = Mes; 1b, Ar = Dipp) in good yield. Reaction of 1a with
0
.5 equiv. of [Ir(cod)(μ-Cl)]
(Mes)- C2, N3}] (3), 3H-H, a head-to-head (H-H) isomer of C
symmetry. The metallated carbon of the 4 electron donor anionic bridging ligands has some
carbene character, reminiscent of the situation in N-metallated protic NHC complexes. Displacement of cod ligands from
H-H and 3H-T afforded the tetracarbonyl complexes [Ir(CO) {µ-C (Mes)- C2, N3}] H-H and 4H-T, respectively. The
reaction with PMe , which gave only one complex, [Ir(CO)(PMe (Mes)- C2, N3}] (5), demonstrates that the
isomerization of the central core Ir[µ-C (Mes)- C2, N3] Ir from H-H to H-T on going from 4H-H to 5 is readily triggered
by phosphine substitution under mild conditions. Oxidative-addition of MeI to 5 afforded the formally metal-metal bonded
2
yielded two geometrical isomers of a doubly C2,N3-bridged dinuclear complex [Ir(cod){µ-
C
3
2
H N
2
κ
κ
2
S
symmetry, and 3H-T, the thermodynamically preferred
head-to-tail (H-T) isomer of C
2
3
2
3
2
H N
2
κ
κ
2
4
3
3
){µ-C
3
H
2
N
2
κ
κ
2
H
3 2
N
2
κ
κ
2
7
7
d -d complex [Ir
2
(CO)
2
(PMe
3
)
2
(Me)I{µ-C (Mes)-
3
H
2
N
2
2 2 4 2 3 2 2 2
κC2,κN3} ] (6). The blue [Ir(C H ) {µ-C H N (Mes)-κC2,κN3}] (7) and
purple [Rh(C {µ-C
2
H
4
)
2
H
3 2
N
2
(Dipp)-
κ
C2, N3}] (9) tetraethylene complexes were also obtained with only a H-T arrangement
κ
2
of the bridging ligands. Although only modestly efficient in alkane dehydrogenation, complex 7 was found to be a more
active pre-catalyst than 3H-T, 4H-T and 5, probably because of the favorable lability of the ethylene ligands. From cyclic
voltammetry, exhaustive coulometry and spectroelectrochemistry studies, it was concluded that 3H-T undergoes a metal-
based one electron oxidation to generate the mixed-valent Ir(I)/Ir(II) system. The energy of the intervalence band for the
3 2 2 2
orange dirhodium complex [Rh(cod){µ-C H N (Mes)-κC2,κN3}] (8) is shifted toward lower energies in comparison with
3
H-T, reflecting the decrease of the energy with the intermetallic distance. It was concluded from the EPR study that the Ir
and Rh centres contribute substantially to the experimental magnetic anisotropy and thus to the singly occupied molecular
orbital (SOMO) in the mixed-valent Ir(I)/Ir(II) and Rh(I)/Rh(II) systems. The molecular structures of 3H-H, 3H-T, 8 and 9 have
a.
Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS),
Université de Strasbourg, 4 rue Blaise Pascal, 67081 Strasbourg Cedex (France). E-
mail: braunstein@unistra.fr
b.
Laboratoire d'Electrochimie et de Chimie Physique du Corps Solide, Institut de
Chimie (UMR 7177 CNRS), Université de Strasbourg, 4 rue Blaise Pascal, 67081
Strasbourg Cedex (France). E-mail: danopoulos@unistra.fr
c.
d.
e.
Institut de Chimie, Université de Strasbourg, 1 rue Blaise Pascal, BP 296 R8, F-
6
7008 Strasbourg, Cedex (France).
Université de Strasbourg, Institute for Advanced Study (USIAS), Strasbourg
France).
(
Institut des Sciences Moléculaires de Marseille, Aix Marseille Université, CNRS,
Centrale Marseille, ISM2 UMR 7313, 13397, Marseille, France
† Electronic Supplementary Information (ESI) available: Table S1 containing the
crystal data for
H-H H-T
3 , 3 , 8 and 9 (CCDC 1052655–1052658), figures showing UV-
1
visible-NIR spectra (S1, S3), H NMR spectra (S2, S4), cyclic voltammograms (S5–
S7), time-resolved UV-visible-NIR absorption spectra (S8, S9), EPR spectra (S10),
the optimized structures (S11, S13), spin population distribution and SOMO (S12,
S14). For ESI and crystallographic data in CIF format see DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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been determined by X-ray diffraction.
dinuclear iridium complexes bearing imidazolides has yet been
1
6
Introduction
carried out. A brief report described in 1983 the synthesis of
dinuclear imidazolide Rh(I) complexes by deprotonation with
The isolation by Arduengo and co-workers of stable N-
heterocyclic carbenes (NHCs) of the imidazole type with bulky
1
N-substituents, has triggered a fast growing interest for this
1
7
MeLi of a mononuclear imidazole complex. As part of our
current investigations on the tautomerism/metallotropism
2
between pNHC and imidazole ligands in iridium complexes
9
Scheme 2), we describe herein the synthesis, structural and
class of ligands, in particular in organometallic chemistry.
(
Protic NHCs (pNHCs) are characterised by the presence of a N-
bound H atom and have been comparatively much less
investigated, despite their strong σ-donor character and the
possibility for the NH group to be involved in secondary
spectroscopic characterisation, reactivity and electrochemical
properties of a series of doubly C,N-bridged dinuclear iridium
and rhodium complexes bearing 1-arylimidazolide ligands.
3
interactions of potential relevance to bifunctional catalysis,
5
4
substrate recognition and biological systems.
N
N
N
N
Different synthetic methodologies allow access to pNHC
metal complexes: building the C-bound heterocycle in the
R
H
R
M
H
6
metal coordination sphere, using suitable N-protecting groups
M
7
that are removed after metal coordination, or facilitating the
Scheme 2 Tautomerism/metallotropism between pNHC and imidazole
kinetic formation of the M–CNHC bond by the oxidative-
8
addition of the C–X bond of halo-imidazoles (X = halide). Most
9
ligands.
recently, we found that N-arylimine-functionalized pNHC Ir(I)
and Ir(III) complexes could be readily obtained from cationic or Results and discussion
neutral Ir(I) imidazole complexes using excess TlPF
6
or
Synthesis and characterisation of the di-iridium complexes.
9
Ir(cod)(µ-Cl)] , respectively. Deprotonation of such a pNHC
[
2
Starting from 1-arylimidazoles (aryl = mesityl (Mes), 2,6-
diisopropylphenyl (Dipp)), the corresponding derivatives (1-
aryl-1H-imidazol-2-yl)lithium (1a, Ar = Mes; 1b, Ar = Dipp) were
prepared in good yield by deprotonation with a stoichiometric
amount of n-butyl lithium in pentane at -30 °C (Scheme 3).
Ir(I) complex was shown to give rise to an equilibrium between
a mononuclear complex containing a C-bound ‘anionic’
imidazolide and its dimer in which this moiety binds in a µ-C,N
9
bridging mode (Scheme 1).
Ar
+
N
n-BuLi
N
NH
N
N
N
N
N
N
N
Ar
Ar
pentane, -30 o
C
N
base
N
Ar
-
1/2
N
Li
1a Ar = Mes, yield 70%
Ar = Dipp, yield 92%
Ir
PF
6
Ir
Ir
Ar
N
Ar = Dipp
N
1
b
N
Ar
Scheme 3 Synthesis of (1-aryl-1H-imidazol-2-yl)lithium (1a,b).
Scheme 1 Deprotonation of a protic NHC (pNHC) Ir(I) complex leading to an
1
, the absence of the H NMR
In their NMR spectra in THF-d
8
equilibrium between a mononuclear complex containing a C-bound ‘anionic’
1
3
1
9
imidazolide and its dimer.
resonance of the proton at C2 and the values of C{ H} NMR
resonance due to the C2 carbon, at 205.8 for (1-mesityl-1H-
imidazol-2-yl)lithium (1a) and  202.3 for (1-(2,6-diisopropyl
δ
δ
Anionic imidazolides possess interesting properties. Lithium phenyl)-1H-imidazol-2-yl)lithium (1b), consistent with the
1
-methyl-(4-t-butyl)imidazolide was found by Boche and co- metallation of this carbon.
13
workers to have carbene character, as supported by the
C
NMR chemical shift of its C2 atom (δ 195.9) and an X-ray
1
0
diffraction study. Kostyuk and co-workers reported a method
to synthesize N-phosphorylated carbenes by the reaction
between lithium imidazolides, bearing a bulky N-bound t-butyl
1
1
or adamantyl group, and di(t-butyl)chlorophosphine.
Furthermore, an imidazolide can act as a N,C-bidentate ligand,
comparable to pyrazolide. While dinuclear bis(μ-
pyrazolido)iridium(I) complexes have been widely investigated
a
1
2
13
in oxidative addition reactions, substitution chemistry,
1
kinetic and theoretical studies, no extensive study on
4
15
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conformation of
3
H-H is similar to that of the analogous bridged
1
2b
pyrazolido complex.
interaction since the separation between the metal atoms is
.1844(9) Å. The iridium(I) centres adopt an approximate
There is no direct iridium-iridium
3
square planar coordination geometry, defined by two olefinic
bonds of the 1,5-cyclooctadiene ligand and two carbon atoms
(or two nitrogen atoms) from the imidazolide bridging ligands.
The latter can be formally considered as 4 electron anionic
donors toward Ir(I) centres. The electronic environment at the
metals is unsymmetrical and Ir(1) is more electron-rich than
Ir(2) since it is bound to two carbanionic donors. This is also
supported by the difference in redox potentials between the
isomers 3H-H and 3H-T (Table 1) although in the case of 3H-H the
redox waves are irreversible. The C1–N1 [1.35(2) Å] and C21–
N3 [1.35(2) Å] bond lengths are not significantly shorter
Scheme 4 Stepwise or direct synthesis of the two isomers of the dinuclear
(within 3σ) than those of C1–N2 [1.38(2) Å] and C21–N4
1.39(2) Å]. This is indicative of electronic delocalization
complexes 3H-H and 3H-T
.
[
Treatment of 1a with 0.5 equiv. of [Ir(cod)(μ-Cl)]
THF led to the formation of the doubly C,N-bridged dinuclear respectively) and of a carbene character for C1 and C21,
complex [Ir(cod){µ-C (Mes)- C2, N3}]
) as a red solid in respectively (Scheme 5).
nearly quantitative yield (Scheme 4). Its H and C{ H} NMR
spectra in C revealed the presence of a ca. 40:60 mixture of
two constitutional isomers, H-H, a head-to-head isomer of C
symmetry, and H-T, a head-to-tail isomer of C symmetry. In
the NMR spectra of the mixture in C , each isomer displays
one set of mesityl, imidazolide and cod signals. The chemical
2
at -78 °C in between the N1, C1 and N2 atoms (N3, C21 and N4,
3
H
2
N
2
κ
κ
2
(3
1
13
1
6 6
D
3
S
3
2
6 6
D
1
3
1
shifts of the C{ H} NMR resonance due to the C2 carbon (
δ
71.3 and 172.0) are considerably upfield-shifted when
1
compared to the value of 205.8 ppm in 1a. It turned out to be Scheme 5 Limiting resonance structures for the diiridium(I) complex 3H-H
emphasising the dipolar nature of this isomer.
difficult to efficiently separate and isolate each isomer pure
out of this mixture because of their similar solubilities. Another
procedure to prepare doubly C,N-bridged dinuclear complexes
was found to consist of the deprotonation of 1-
mesitylimidazolyl(cycloocta-1,5-diene)
iridium(I)
chloride
[
Ir(cod)Cl{C (Mes)-κN3}] ( ) with a stoichiometric amount
of potassium bis(trimethylsilyl)amide (KHMDS) in THF at -78 °C.
3
H
3
N
2
2
According to the NMR spectra, a ca. 40:60 mixture of the same
two isomers
the reaction using 1a was repeated in Et
suspension was found to consist of a ca. 90:10 mixture of
3H-H and 3H-T was again obtained. However, when
1
6b
2
O, the resulting red
3
H-H
1
and
difference in proportions obtained in THF is likely due to the
lower solubility of O. After recrystallization from a
toluene/ Et O solution at -30 °C, deep red crystals of one pure
isomer were obtained in 80% yield. In its H NMR spectrum
), the C4 and C5 imidazolyl protons gave rise to an AX
H-T
3 , after dissolution in toluene and H NMR analysis. This
3H-H in Et
2
2
1
6 6
(C D
3
13
1
pattern at
NMR spectrum, the three resonances at
22.4 are assigned to the C2, C4 and C5 imidazolyl carbons,
δ
7.20 (d) and 6.42 (d, J = 1.4 Hz). In the C{ H}
δ
171.3, 125.5 and
1
respectively. However, a definitive assignment of this isomer
as H-H or H-T was impossible on the exclusive basis of the
spectroscopic data. Fortunately, its structure was elucidated
by X-ray diffraction analysis and established its H-H
Fig. 1 View of the molecular structure of 3H-H, H atoms and methyl groups
are omitted for clarity. Thermal ellipsoids are at the 30% level. Selected
bond lengths (Å) and angles (°): Ir1···Ir2 3.1844(9), C1–N1 1.35(2), C1–N2
arrangement. The molecular structure of
with selected bond lengths and angles. The C
symmetry of the complex in solution is almost retained in the Ir1–C17 2.17(2), Ir1–C18 2.16(2), C13–C14 1.38(2), C17–C18 1.36(3), C21–N3
3H-H is shown in Fig. 1,
s
molecular
1
.38(2), Ir1–C1 2.05(1), Ir1–C21 2.05(1), Ir1–C13 2.14(1), Ir1–C14 2.18(1),
1
.35(2), C21–N4 1.39(2), Ir2–N1 2.07(1), Ir2–N3 2.06(1), Ir2–C33 2.10(1),
solid state, as indicated by NMR spectroscopy. The boat
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Ir2–C34 2.09(1), Ir2–C37 2.11(2), Ir2–C38 2.10(2), C33–C34 1.39(3), C37–C38
.40(3); N1–C1–N2 106(1), N3–C21–N4 105(1), C1–Ir1–C21 87.4(6), C13–Ir1–
C14 37.2(6), C14–Ir1–C17 80.6(7), C17–Ir1–C18 36.5(7), C18–Ir1–C13 81.4(6),
N1–Ir2–N3 86.5(5), C33–Ir2–C34 38.7(7), C34–Ir2–C37 82.8(6), C37–Ir2–C38
A boat conformation is observed for the structure of 3H-T
similar to that of its isomer H-H. The distance between two
iridium atoms (3.1407(2) Å) is shorter than in H-H but still too
,
1
3
3
long to represent a direct metal-metal interaction. Each
iridium atom has a 16 valence electron configuration and
adopts an approximate square planar coordination geometry,
3
8.7(7), C38–Ir2–C33 81.3(7).
The complex
3H-H can be thermally converted to its isomer defined by two olefinic bonds of the 1,5-cyclooctadiene ligand,
3
H-T upon refluxing a THF solution for 24 h (Scheme 4). In the one carbon atom of one imidazolide and one nitrogen atom of
1
H NMR spectrum of
6 6
3H-T in C D , the protons at the the other bridging imidazolide ligand. The C1–N1 and C21–N4
imidazolide backbone carbons C4 and C5 give rise to an AX distances of 1.349(5) Å and 1.335(5) Å, respectively, are almost
3
13
1
pattern at
NMR spectrum, the resonances at
assigned to C4 and C5, respectively. The C{ H} NMR and C21–N3 1.378(5) which compare with the corresponding
resonance of the imidazolide C2 carbon (C1 and C21 in Fig. 2) values of 1.38(2) and 1.39(2) Å in H-H. This, together with the
in  172.0) is downfield shifted compared to that in H-H similarity between the C-N bond lengths in H-T, is consistent
 171.3) (C1 and C21 in Fig. 1). The UV-visible-NIR absorption with the resonance structures shown in Scheme 6.
spectrum of Cl is shown in Fig. S1 (see ESI). Single
crystals of H-T suitable for X-ray diffraction were obtained by
slow diffusion of a layer of Et O into a THF solution of
room temperature under argon. The molecular structure of
is shown in Fig. 2, with selected bond lengths and angles.
δ
7.02 (d) and 6.09 (d, J = 1.6 Hz). In the C{ H} identical to the corresponding distances in
3H-H (1.35(2) Å). A
δ
 125.4 and 121.3 are similar comment applies to the bond lengths C1–N2 1.373(5)
1
3
1
3
3
H-T
(
δ
3
3
(
δ
3H-T in CH
2
2
3
2
3H-T at
3
H-
T
Scheme 6 Limiting resonance structures for 3H-T..
Displacement Reactions of the cod Ligands.
With the original aim to prepare derivatives of 3H-H and 3H-T in
which replacement of the cod ligands with two-electron donor
ligands could modify the redox properties, as monitored by
cyclic voltammetry, the tetracarbonyl derivatives [Ir(CO) {µ-
2
C H N (Mes)-κ
readily synthesized by reaction with carbon monoxide in Et O
C2,
κN3}]2 (4H-H and 4H-T, respectively) were
3
2 2
2
(Scheme 7). While a clear brown Et O solution of 4H-T was
2
obtained, the reaction with 3H-H led to a yellow suspension.
The higher solubility of 4H-T in Et O compared to that of 4H-H is
2
consistent with their different polarity. Indeed, 4H-T has a good
solubility in nonpolar solvents such as pentane or n-hexane.
1
3
1
Fig. 2 View of the molecular structure of 3H-T, H atoms and methyl groups The C{ H} NMR spectroscopic data for
4H-H and 4H-T in C D
6 6
are omitted for clarity. Thermal ellipsoids are at the 30% level. Selected show for each isomer one set of mesitylimidazolide and two
bond lengths (Å) and angles (°): Ir1···Ir2 3.1407(2), C1–N1 1.349(5), C1–N2
CO signals, consistent with either a C or C molecular
s
2
1
2
1
2
2
.373(5), Ir1–C1 2.058(4), Ir1–N4 2.068(3), Ir1–C13 2.121(4), Ir1–C14
.112(4), Ir1–C17 2.166(4), Ir1–C18 2.158(4), C13–C14 1.409(6), C17–C18
.390(6), C21–N3 1.378(5), C21–N4 1.335(5), Ir2–N1 2.067(3), Ir2–C21
symmetry. Compared to the cod precursors (3H-H and 3H-T), the
1
3
1
C{ H} NMR resonances of the imidazolide C2 are downfield
 172.8 in  175.1 in H-T). The IR spectra of
.159(4), C33–C34 1.404(7), C37–C38 1.395(6); N1–C1–N2 106.1(3), N3– 4H-H and 4H-T show three v(CO) bands at 2060, 2037, 1954 and
.043(4), Ir2–C33 2.135(4), Ir2–C34 2.118(4), Ir2–C37 2.168(4), Ir2–C38 shifted (
δ
4H-H and
δ
4
C21–N4 106.7(3), C1–Ir1–N4 86.9(1), C13–Ir1–C14 38.9(2), C14–Ir1–C17
-1
2
059, 2041, 1968 cm , respectively, corresponding to the
8
2.0(2), C17–Ir1–C18 37.5(2), C18–Ir1–C13 81.2(2), N1–Ir2–C21 86.6(1),
pattern for a dinuclear, folded tetracarbonyl framework.
C33–Ir2–C34 38.6(2), C34–Ir2–C37 82.0(2), C37–Ir2–C38 37.6(2), C38–Ir2–
C33 81.2(2).
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ARTICLE
Mes
Mes
Mes
N
Mes
OC
N
N
N
N
CO
OC
N
N
CO PMe3
N
PMe
3
Ir
Ir
Ir
Ir
Ir
Ir
Et O
2
toluene
CO
Me3P
N
CO
OC
N
N
N
Mes
Mes
3H-H
4H-H
5
reflux
THF
e
MeI THF
M
P
Mes
Mes
OC
Mes
N
N
N
N
CO
N
CO
CO
OC
Me
N
PMe3
Ir
Ir
Ir
Ir
Ir
Ir
I
Et O
2
N
OC
N
Me3P
N
CO
N
N
N
Mes
Mes
Mes
3H-T
4H-T
6
3
Scheme 7 Reactions of 3H-H and 3H-T with CO or PMe .
resonances of the Cimidazolide carbons are significantly upfield
3
1
1
The reaction of
4
H-H or
4
H-T with 2.0 equiv. of shifted (
δ
142.8 and 140.8 vs.
δ
177.1 in
5
). The P{ H} NMR
-46.6 and -48.9,
consistent with similarly bonded but inequivalent phosphine
trimethylphosphine in toluene at room temperature afforded spectrum shows two close resonances at
the same red solid [Ir(CO)(PMe ){µ-C (Mes)- C2, N3}]
), which shows two (CO) bands at 2000 and 1911 cm in the ligands. Attempts to crystallize
infra-red spectrum (Scheme 7). According to the NMR spectra spectrum of shows two (CO) bands at 2019 and 1952 cm ,
in C , this complex contains one set of mesitylimidazolide, indicative of weaker back-bonding from the metal to the CO
trimethylphosphine and CO signals. The imidazolide backbone ligands compared to and consistent with the presence of
where a formal metal-metal bond leads
to a 18 electron count to each metal centre. In contrast to
28.3 and 120.1. In the C{ H} NMR spectrum, the resonances an NMR experiment showed that H-T reacted reversibly with
181.4 (d, MeI since placing a solution of the oxidative-addition product,
δ
3
3
H
2
N
2
κ
κ
2
-
1
12a
(
5
ν
6 were unsuccessful. The IR
-
1
6
ν
6 6
D
5
3
protons at C4 and C5 are observed at
δ
6.89 (d) and 6.68 (d, J iridium(II) centres in
6
1
3
1
=
0.9 Hz) and the corresponding C{ H} NMR resonances at
δ
5,
1
3
1
1
3
due to the CO and Cimidazolide carbons are found at
δ
J
C-P = 10.0 Hz) and 177.1 (d, JC-P = 112.0 Hz), respectively. The obtained in the presence of excess MeI, under reduced
1
coordinated trimethylphosphine ligands give rise to a H NMR pressure regenerated
3
H-T. Such a behaviour has been
2
13
1
resonance at
δ 1.13 (d, JH-P = 8.7 Hz), a C{ H} NMR signal at previously observed with diiridium pyrazolido-bridged
3
 17.0 (d, JC-P = 31.6 Hz) and a singlet in P{ H} NMR at
1
1
δ
2
δ - complexes and explained by the steric hindrance caused by the
molecular cod ligands that prevents the iridium centres to get closer to
4.1. All these data are consistent with a C
2
1
2h
symmetry for
5, i.e. a H-T arrangement. The isomerization of each other in the oxidised product.
the central core Ir[µ-C
on going from
substitution under mild conditions. The preference for the H-T
arrangement is consistent with being the
3
H
2
N
2
(Mes)-
2
κC2,κN3] Ir from H-H to H-T
4
H-H to 5 has thus been triggered by phosphine
Synthesis and Characterisation of Diiridium Tetraethylene
Complexes
3
H-T
thermodynamically favoured isomer of 3.
Treatment of 1a with 0.5 equiv. [Ir(C
Et O led to the formation of the doubly C,N-bridged
imidazolide dinuclear complex [Ir(C {µ-C (Mes)-
C2, N3}] ) as a blue solid in 77% yield (Scheme 8).
2 4 2 2
H ) (μ-Cl)] at -78 °C in
2
Oxidative-Addition of MeI.
2
H
4
)
2
3 2 2
H N
κ
κ
2
(7
Treatment of
complex [Ir (CO)
corresponding to a 1:1 addition product (Scheme 7). The
5
with a slight excess of MeI afforded the yellow
2
2
(PMe (Me)I{µ-C (Mes)- C2, N3} ] (
3
)
2
3
H
2
N
2
κ
κ
2
6)
1
observation in C
6 6
D of two sets of H NMR signals for the
mesitylimidazolide and trimethylphosphine protons and of
1
3
1
C{ H} NMR signals for the CO ligands is consistent with a non-
1
symmetric system. In the C{ H} NMR spectrum, the
3
1
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Mes
molecular structure of
8
is shown in Fig. 3, with selected bond
N
lengths and angles. This complex adopts a H-T arrangement
and the same boat conformation as its iridium analog. The
separation between the two Rh atoms is 3.2117(4) Å, which is
too long to represent a direct metal-metal interaction. Each
rhodium atom is in a 16 valence electron configuration and
adopts an approximate square planar geometry, defined by
two olefinic bonds of the 1,5-cyclooctadiene ligand, one
carbon atom of one imidazolide and one nitrogen atom of the
another bridging imidazolide ligand.
1
/2 [Ir(C
2
H
4
)
2
(ꢀ-Cl)]
2
N
Ir
Ir
Et
2
O, -78 oC to RT
N
N
Mes
7
N
N
Mes
Mes
Li
N
1a
N
1
/2 [Rh(cod)(ꢀ-Cl)]
2
Rh
Rh
Et
2
O, -78 oC to RT
N
N
Mes
8
Dipp
N
1/2 [Rh(C
2
H
4
)
2
(ꢀ-Cl)]
2
N
N
N
Rh
Rh
Dipp
Et
2
O, -78 oC to RT
N
Li
N
Dipp
1b
9
Scheme 8 Synthesis of the isolated dinuclear olefinic complexes 7-9.
The colour of
diiridium complexes reported in the literature.
spectra in C only showed one set of mesitylimidazolide
signals. In the C{ H} NMR spectrum, the resonance of the
 171.5. The imidazolide protons at C4
7 is similar to that of other tetraethylene
1
3c
Its NMR
6 6
D
1
3
1
C
imidazolide is observed at δ
3
and C5 gave rise to an AX pattern at
δ 7.09 (d) and 6.49 (d, J =
.5 Hz) and the corresponding C{ H} NMR resonances
1
3
1
1
1
occurred at δ 124.6 and 122.0, respectively. In the H NMR
spectrum, two AA'BB' patterns are anticipated for the
Fig 3 View of the molecular structure of 8, H atoms and methyl groups are
omitted for clarity. Thermal ellipsoids are at the 30% level. Selected bond
The resolution of the spectra did not allow lengths (Å) and angles (°): Rh1···Rh2 3.2117(4), C1–N1 1.376(3), C1–N2
chemically-distinct coordinated ethylene ligands (trans to C
1
3c,18
and trans to N).
1
2
1
2
2
.342(3), Rh1–C1 2.045(2), Rh1–N4 2.079(2), Rh1–C25 2.135(3), Rh1–C26
.117(3), Rh1–C29 2.188(3), Rh1–C30 2.171(3), C25–C26 1.382(4), C29–C30
.372(4), C13–N3 1.378(3), C13–N4 1.345(3), Rh2–N2 2.067(3), Rh2–C13
.043(4), Rh2–C33 2.172(3), Rh2–C34 2.183(3), Rh2–C37 2.118(3), Rh2–C38
.122(3), C33–C34 1.376(4), C37–C38 1.392(4); N1–C1–N2 106.7(2), N3–
to extract all the corresponding coupling constants.
Nevertheless, one AA'BB' pattern with 3.13 and 2.89 and
a second AA'BB' pattern with 2.63 and 2.36 were
observed (Fig. S2). Since the H NMR chemical shifts of the
mesityl and imidazolide groups are similar to those of
arrangement of should also be H-T but attempts to crystallize
were unsuccessful.
δ

δ
A
δ

δ
A
1
9
1
3
H-T, the C13–N4 106.3(2), C1–Rh1–N4 85.73(9), C25–Rh1–C26 37.9(1), C26–Rh1–C29
2.3(1), C29–Rh1–C30 36.7(1), C30–Rh1–C25 81.4(1), N2–Rh2–C13 86.27(9),
8
7
C33–Rh2–C34 36.8(1), C34–Rh2–C37 82.1(1), C37–Rh2–C38 38.3(1), C38–
Rh2–C33 81.5(1).
7
Synthesis and Characterisation of Dirhodium cod and
Tetraethylene Complexes
The reaction of 1b with [Rh(C
afforded [Rh(C {µ-C (Dipp)-
Its NMR spectrum in C only showed one set of Dipp
imidazolide signals and a doublet due to C2 at 175.1 (JC-Rh =
2
H
4
)
2
(μ-Cl)]
2
at -78 °C in Et
2
O
2
H
4
)
2
3
H
2
N
2
κ
C2, N3}]
κ
2
(9) (Scheme 8).
To extend the scope of the reactivity of 1a, it was treated with
.5 equiv. of [Rh(cod)(μ-Cl)] , and the new complex [Rh(cod){µ-
(Mes)- C2, N3}] ) was isolated (Scheme 8) As for
the C{ H} NMR spectrum of in C only showed one set of
6 6
D
0
2
δ
C
3
H
2
1
N
2
κ
κ
2
(
8
.
8,
4
9.0 Hz). The resonances of the H atoms at the backbone
3
1
8
6 6
D
carbons C4 and C5 were observed at δ 7.02 and 6.64,
respectively, as two singlets, and the corresponding C{ H}
mesitylimidazolyl signals and a doublet due to C2 at  176.2 (JC-
13
1
Rh = 50.4 Hz). The protons at C4 and C5 were observed in the
1
1
NMR resonances at
spectrum, like in , two AA'BB' patterns are anticipated for the
chemically distinct coordinated ethylene ligands.
δ 125.9 and 122.8. In the H NMR
H NMR spectrum at δ 6.96 and 6.51 as two broad singlets (the
7
3
13
1
J
H-H coupling was not resolved) and the corresponding C{ H}
resonances were found at 126.8 and 120.3. The UV-visible-
NIR absorption spectrum of in CH Cl is shown in Fig. S3.
Orange single crystals of suitable for X-ray diffraction were
13c,18
However,
δ
the resolution of the spectra did not allow extraction of all the
corresponding coupling constants. Nevertheless one AA'BB'
8
2
2
8
pattern with
δ

3.65 and
A
δ 3.58 and a second AA'BB' pattern
grown from a toluene solution at 0 °C under argon. The
19
2.62 were observed (Fig. S4). Purple single
with 2.86 and
δ

δ
A
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9
suitable for X-ray diffraction were grown from n- or H-H arrangements of the bridging ligands, since they lead to
hexane solution at -30 °C under ethylene atmosphere. A a symmetrical or unsymmetrical electronic environment of the
1
similar reaction using 1a instead of 1b proceeded similarly ( H metal centres, respectively. An electrochemical investigation
monitoring) although a well-characterised solid product could of complexes
not be obtained due to limited solubility in hexane from which and rotating disk voltammetry in CH
could be crystallised. The molecular structure of is shown Cyclic voltammograms of with added ferrocene are
in Fig. 4, with selected bond lengths and angles. The structure presented in Figs. S5 and S6 (see ESI) and show reversible
of adopts a boat conformation and like in , the arrangement processes. In contrast, irreversible oxidation was observed by
3H-T and
8
was carried out by cyclic voltammetry
2
Cl + 0.1 M [n-Bu N]PF .
2
4
6
9
9
3H-T and 8
9
8
of the bridging ligands is of the H-T type. The separation cyclic voltammetry in the case of complex
between two Rh atoms is 3.3146(9) Å, which is again too long ESI)
3H-H (see Fig. S7 in
to represent a direct bonding interaction. Each 16 electron
As shown in Fig. 5 and Table 1, two successive oxidations
H-T. The first oxidation step
rhodium atom adopts an approximately square planar have been detected in the case of
+
geometry, defined by two ethylene ligands, one carbon atom occurs at -0.45 V vs. Fc /Fc and corresponds to a reversible
3
of one imidazolido and one nitrogen atom of the other electron transfer, while the second step presents an
+
H-T, the metrical data irreversible oxidation peak at +0.67 V vs. Fc /Fc.
bridging imidizolido ligand. Like in
3
indicate a more pronounced double bond character for the N-
C bond located in the bridging part of the ligands.
The number of electrons exchanged during the first
oxidation step was determined by exhaustive coulometry.
During the electrolysis, the oxidation current decreased
exponentially with time. When this current reached the
residual value measured in the absence of electroactive
material, typically after 4 h under the above conditions, the
number of electrons transferred was 0.9/molecule of
H-T
3 ,
which strongly suggests that oxidation of only one Ir(I) centre
to Ir(II) has occurred, leading to the mixed-valent Ir(I)/Ir(II)
intermediate. Attempts to chemically oxidize
3H-T using AgOTf
2 2
in CH Cl gave intractable mixture of species.
a
Table 1. Electrochemical data for 3H-T, 3H-H and 8.
Complex
b
3H-T
-0.45 (64)
+0.67
b
b
3
H-H
-1.23
-0.39
b
8
-0.40 (63)
+0.58
Fig. 4 View of the molecular structure of 9, H atoms and isopropyl groups
are omitted for clarity. Thermal ellipsoids are at the 30% level. Selected
bond lengths (Å) and angles (°): Rh1···Rh2 3.3146(9), C1–N1 1.372(3), C1–N2
a
+
All potentials in V vs. Fc /Fc were obtained from cyclic voltammetry in CH
2
Cl
. Scan rate = 0.1 V s . Working electrode: glassy
carbon electrode. The given half-wave potentials in the case of the reversible
couple are equal to E1/2 = (Epa-Epc)/2. In bracket: (ꢁE , peak splitting in mV at a
2
-
1
-1
4 6
containing 0.1 mol L [n-Bu N]PF
1
.344(3), Rh1–C1 2.040(3), Rh1–N4 2.081(2), Rh1–C16 2.141(3), Rh1–C17
.125(3), Rh1–C18 2.208(3), Rh1–C19 2.192(3), C16–C17 1.369(5), C18–C19
.351(5), C20–N3 1.379(3), C20–N4 1.342(3), Rh2–N2 2.080(2), Rh2–C20
.041(3), Rh2–C35 2.202(3), Rh2–C36 2.211(3), Rh2–C37 2.129(3), Rh2–C38
.124(3), C35–C36 1.354(5), C37–C38 1.385(5); N1–C1–N2 106.7(2), N3–
p
2
1
2
2
-
1
b
scan rate of 0.1 Vs ). Irreversible electron transfer.
C20–N4 106.4(2), C1–Rh1–N4 84.6(1), C16–Rh1–C17 37.4(1), C17–Rh1–C19
9.2(2), C18–Rh1–C19 35.8(1), C16–Rh1–C18 86.9(1), N2–Rh2–C20 84.9(1),
8
C35–Rh2–C36 35.7(1), C36–Rh2–C38 87.0(1), C37–Rh2–C38 38.0(1), C35–
Rh2–C37 89.1(2).
Electrochemical investigations.
Since a two-electron oxidation of the dinuclear unit could
formally give an Ir(II)-Ir(II) complex or a mixed-valence
Ir(I)/Ir(III) complex in which only one metal centre would have
been formally oxidised, we became interested in studying the
redox behaviour of representatives of these dinuclear
complexes, in particular, with respect to the two possible H-T
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For
3H-T, time resolved UV-Vis-NIR spectroelectrochemistry
data were recorded during cyclic voltammetry between -0.70
+
V and +0.20 V vs. Fc /Fc (scan rate 20 mVs ). As seen from the
-1
array of spectra depicted in Fig. 6 (and Fig. S8 for the plot
versus
λ (nm)), during the oxidation the generated Ir(I)/Ir(II)
species is characterised by low-energy intervalence charge
transfer (IVCT) bands in the near infrared region at 1024 nm.
On the reverse potential scan, a decrease of the intensity for
this IVCT band was observed and the oxidized Ir(I)/Ir(II) system
reverted quantitatively to the initial Ir(I)/Ir(I) species.
From the results obtained by cyclic voltammetry (reversible
+
oxidation at -0.45 V vs. Fc /Fc), exhaustive coulometry (one-
electron process) and spectroelectrochemistry (new NIR band
at λmax = 1024 nm for the oxidized system), it can be concluded
2 2 4 6
Fig. 5 Cyclic voltammograms of 3H-T (CH Cl + 0.1 M [n-Bu N]PF , glassy that 3H-T undergoes a one electron oxidation on the metal
+
-1
carbon electrode, scan rate 0.1 Vs , vs. Fc /Fc).
generating the mixed-valent Ir(I)/Ir(II) system, in equilibrium
with Ir(II)/Ir(I), and giving rise to an IVCT band in the NIR region.
In-situ spectroelectrochemical studies have been carried out,
under argon atmosphere, to gain further insight into the
nature of the electrogenerated species during this first
reversible oxidation (Fig. 6).
For comparison, we examined a H-T dirhodium complex.
Features similar to those for H-T were observed with and the
3
8
mixed-valent system Rh(I)/Rh(II). The cyclic voltammogram
+
shows two successive oxidations, the first at -0.40 V vs. Fc /Fc
corresponds to a reversible electron transfer, while the second
step is associated to an irreversible oxidation process at +0.58
+
V vs. Fc /Fc (Fig. 7). Again, exhaustive coulometry indicated a
number of electrons transferred of 0.9/molecule for
to the mixed-valent intermediate Rh(I)/Rh(II).
8, leading
Fig. 7 Cyclic voltammograms of 8 (CH
electrode, scan rate 0.1 Vs , vs. Fc /Fc).
2
2 4 6
Cl + 0.1 M [n-Bu N]PF , Glassy carbon
-
1
+
Spectroelectrochemistry measurements at the first
oxidation process evidenced a new low-energy intervalence
charge transfer (IVCT) band around 1073 nm, which is in good
agreement with the formation of a mixed-valent intermediate
Rh(I)/Rh(II) (Fig. 8 and Fig. S9 for the plot versus
new band at 612 nm might correspond to the Rh(II).
For and for 3H-T, we observed one additional band at 612
λ (nm)). The
Fig. 6 A) Time-resolved UV-visible-NIR spectra of 3H-T for the first oxidation
8
2 2 4 6
step (transition Ir(I)/Ir(I) to Ir(I)/Ir(II)) in CH Cl + 0.1 M [n-Bu N]PF (spectra
nm and 781 nm, respectively. In studies on mixed-valent
Ir(I)/Ir(II) or Rh(I)/Rh(II) intermediates, similar bands were also
recorded every 5 s). B) UV-visible spectral evolution for the first oxidation
step.
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observed, but not at the same wavelength, because of the comparison with
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3
H-T, reflecting the decrease of the energy
2
0
presence of different ligands. Such bands may be attributed with the intermetallic distance (dIr-Ir = 3.141 Å, dRh-Rh = 3.212 Å).
to the Ir(II) or Rh(II) component of the complexes.
The transition from charge localized to charge-delocalized
which are also called class II and class III, respectively, in the
(
2
2
Robin−Day terminology) occurs at λ = 2Hab, so mixed-valent
MV) compounds with λ > 2Hab are charge-localized and those
with λ 2Hab are charge-delocalized. In our case:
(
∼
-1
-1
3
H-T: λ (9823 cm ) > 2Hab (2 x 3960 = 7920 cm )
-1
: λ (9320 cm ) is
-1
8
∼ 2Hab (2 x 4746 = 9492 cm )
-
1
For
These data thus suggest that
is a charge-delocalized system under the conditions of our
8, λ (9320 cm ) is ∼ 2Hab
3
H-T is a charge-localized while
8
room temperature measurements.
EPR measurements.
EPR spectroscopy constitutes a highly suitable tool to evaluate
the electronic structure of metal complexes with unpaired
electrons, especially on heavy metal centres. At low
temperature, the average <g> value and the anisotropy (ꢁg)
calculated from the principal values of the g-tensor afford a
qualitative estimate of the extent of electron localization over
2
3
the metal and/or over the ligand. We thus wanted to
examine in more detail the electronic structure of the mixed-
valent Ir(I)/Ir(II) and Rh(I)/Rh(II) species. The oxidized species
were generated at room temperature by electrolysis after
consumption of about 0.9 electron/molecule and the resulting
solutions were transferred into an EPR tube under argon. The
X-band EPR spectrum in frozen CH
oxidized species of H-T) exhibited a signal with an axial
symmetry (Fig. 9) with g = g = 1.987 and g = 2.123 values
see Table 2). Increasing the temperature to ambient caused
the EPR signal to disappear. In the case of the Rh(I)/Rh(II)
system obtained by oxidation of , the EPR spectrum at low
2 2
Cl solution of Ir(I)/Ir(II) (the
3
1
2
3
(
8
temperature displayed a rhombic signal (Fig. S10 in ESI). At
room temperature, a very weak EPR signal is recorded with
only one line centred at around g = 2.00 without hyperfine
structure. Satisfying simulations were obtained with one or
Fig. 8 A) Time-resolved UV-visible-NIR spectra of 8 for the first oxidation
step (transition Rh(I)/Rh(I) to Rh(I)/Rh(II)) in CH
spectra recorded every 5 s). B) Time-resolved UV-visible-NIR differential
spectra for the first oxidation step.
2 2 4 6
Cl + 0.1 M [n-Bu N]PF
(
103
two equivalent Rh nucleus (I = 1/2 isotopic abundance 100%)
and in order to discriminate between the two possibilities DFT
calculations were performed. The structure of the Rh(I)/Rh(II)
complex was subjected to geometry optimization (Fig. S11)
and its electronic structure was investigated (Fig. S12).
For a dinuclear system, the electronic coupling calculated
2
1
from the Hush equation decreases with the distance
between the metal centres. Hush proposed that the electronic
coupling could be extracted from the intervalence band shape
according to eq (1):
Mulliken population analysis indicates an equally distributed
spin density between the two rhodium atoms with positive
spin populations found at Rh(1) (0.49) and Rh(2) (0.49). The
spin density of the Rh atoms accounts for 90% of the total spin
density and the remaining 10% are spread over the ligands.
The Singly Occupied Molecular Orbital (SOMO) of the complex
1
/2
H
ab = 0.0206(εmaxΔν1/2λ) /dab (1)
-1
-1
-1
where εmax (M ⋅cm ) is the maximum intensity, Δν1/2 (cm ) is
the width at half-height, λ (cm ) is the energy maximum band
-1
displays 90% Rh character and features the
2
σ
interaction between the Rh 3d orbitals. The EPR parameters
antibonding
(
E
op), and dab (Å) is the diabatic electron-transfer distance. Eop
-1
-1
z
occurs at 9823 cm (λmax = 1018 nm) and at 9320 cm (λmax
073 nm) for . The measured Δν1/2 values for
are 47393 cm and 40486 cm , respectively. According to eq
1), the corresponding calculated electronic couplings are HIr-Ir
=
1
03
arising from the spectral simulation for two equivalent Rh
nuclei are reported in Table 2. The results of the DFT
calculations for the Ir(I)/Ir(II) system are similar to those
obtained for Rh(I)/Rh(II) and also suggest a delocalized mixed-
valent species (Figs. S13 and S14).
1
3
H-T and
8
3H-T and
-1
-1
8
(
-1
-1
=
3960 cm and HRh-Rh = 4746 cm . Thus, the energy of the
is shifted toward lower energies in
intervalence band for
8
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Table 2. EPR parameters of the oxidized complexes
a
b
g
1
g
2
g
3
<g>
ꢁg


 G
-


 G
-


 G
-
Ir(I)/Ir(II)
1.987
1.993
1.987
2.031
2.123
2.105
2.033
2.043
0.136
0.112
Rh(I)/Rh(II)
33
18
27
a
2
2
+ g
2
1/2
b
<
g> = [( g
1
+ g
2
3
)/3]
;
3 1
ꢁg = g -g
Table 3. Catalytic transfer dehydrogenation of cyclooctane in the
a
presence of t-butylethylene with 3H-T, 4H-T, 5 and 7
b
c
-1
entry
catalyst
TON
4.6
-
TOF /h
0.46
-
1
2
3
4
3
4
H-T
H-T
5
7
5.8
62.4
0.58
6.24
a
2
Reaction conditions: [Ir ] catalyst (0.010 mmol), COA (4.0 mL, 30.3 mmol),
b
TBE (0.40 mL, 3.1 mmol), 200 °C, 10 h; the number of moles of COA that a
c
2
mole of [Ir ] catalyst can convert in 10 h; turnover number per hour.
The results of preliminary experiments on the catalytic
transfer dehydrogenation of COA to COE are summarized in
Fig. 9 EPR spectra of the Ir(I)/Ir(II) system electrochemically generated from
H-T system electrochemically generated in CH Cl at 100 K: a) Experimental
spectrum b) Simulated spectrum.
Table 3. Using complex
7 as precursor gave better results
3
2
2
and 2.06% of the cyclooctane was converted to cis-
-1
cyclooctene at a TOF of 6.24 h . This higher value
compared to the other pre-catalysts is probably due to the
easier de-coordination of the ethylene ligands from the
iridium centres, which facilitates the metal-alkane
interactions.
The oxidized forms are characterised by a g anisotropy
and the average g values are significantly larger than for the
free electron (2.0023). Such features may imply that the Ir
and Rh centres contribute substantially to the experimental
magnetic anisotropy and thus to SOMO.
Conclusions
Attempted catalytic transfer dehydrogenation of cyclooctane
using iridium complexes 3H-T, 4H-T, 5 and 7 as precatalysts.
In this work, we have provided
a more detailed
investigation on dinuclear Ir(I) and Rh(I) complexes
Iridium pincer complexes have shown very promising
2
catalytic properties in alkane dehydrogenation reactions,
containing a bridging 1-arylimidazolide ligand, C2 and N3-
16b
4
bound to the metals.
Initial deprotonation of 1-
due to their high thermal stability and high efficiency. The
synergistic action of two reactive sites in close proximity
could be seen as a key element in the design of powerful
catalysts. This prompted us to explore the catalytic activity
of these dinuclear iridium complexes towards alkane
dehydrogenation. The reaction of cyclooctane (COA) and t-
butylethylene (TBE), as sacrificial olefin, to form
cyclooctene (COE) and t-butylethane (TBA) catalyzed by the
arylimidazoles (aryl = mesityl (Mes), 2,6-diisopropylphenyl
(Dipp)) by n-butyl lithium in pentane at -30 °C was carried
out to afford the corresponding derivatives (1-aryl-1H-
imidazol-2-yl)lithium (1a,b) which were then used to
prepare the doubly C,N-bridged dinuclear Ir(I) complexes
These Ir(I) complexes exist in two isomeric forms, 3H-H
which is the head-to-head isomer of C symmetry, and 3H-T
3.
S
,
the head-to-tail isomer of C2 symmetry, which is
2
5
POCOP iridium pincer complex was employed in the
benchmark reaction.
thermodynamically more stable. The X-ray diffraction data
suggest electron delocalisation within the N⋅⋅⋅
and thus some carbene character for the Ir-bound
imidazolide carbon atom, that is more pronounced in
The metal-bound imidazolide may be viewed as
deprotonated pNHC system. When the tetracarbonyl
derivatives H-T were reacted with PMe , one CO
ligand was displaced from each Ir centre and only one
C⋅⋅⋅N system
3 .
H-H
a
4H-H and
4
3
isomer was formed,
5, in which the imidazolide ligands are
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bound in a H-T manner This ligand arrangement was
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pentane (30 mL) was added dropwise a solution of n-BuLi
(1.6 M in n-hexane, 5.0 mL, 8.0 mmol) at -78 °C over 2 min.
The reaction mixture was stirred for 1 h at -30 °C, then
allowed to warm to room temperature and stirred for
another 2 h. The resulting clear orange solution was
evaporated in vacuo. The residue was washed with pentane
(3 × 5 mL) to yield a white powder which was collected by
7
7
retained in the formally metal-metal bonded, d -d Ir(II)-Ir(II)
complex, , resulting from oxidative-addition of MeI across
. From the results obtained by cyclic voltammetry,
exhaustive coulometry and spectroelectrochemistry, it was
concluded that H-T undergoes a metal-based one electron
6
5
3
oxidation to generate the mixed-valent Ir(I)/Ir(II) system, in
equilibrium with Ir(II)/Ir(I), characterised by an IVCT band in
the NIR region.
1
filtration and dried in vacuo (1.08 g, 70%). H NMR (500
MHz, THF-d
8
): δ 7.04 (s, 1H, NCHCHN(mesityl)), 6.83 (s, 2H,
EPR studies combined with DFT calculations suggested
that the Ir and Rh centres contribute substantially to the
experimental magnetic anisotropy in the oxidized species
and thus to the SOMO. These data evidenced that both
Ir(II)/Ir(I) and Rh(II)/Rh(I) complexes are delocalized mixed-
valent species.
aryl-H), 6.70 (s, 1H, NCHCHN(mesityl)), 2.25 (s, 3H, p-
1
3
1
CH3(mesityl)), 1.95 (s, 6H, o-CH3(mesityl)). C{ H} NMR (125 MHz,
205.8 (NCN(mesityl)), 142.3 (C(mesityl)), 136.5
(C(mesityl)), 135.5 (C(mesityl) 128.3 (CH(mesityl)), 127.9
8
THF-d ): δ
)
(NCHCHN(mesityl)), 116.8 (NCHCHN(mesityl)), 20.9 (p-CH3(mesityl)),
18.2 (o-CH3(mesityl)).
The dinuclear iridium complexes were found to be only
moderately active pre-catalysts in the reaction of
cyclooctane and t-butylethylene to form cyclooctene and t-
butylethane. Whether these results are directly linked to a
difficulty to access coordinatively unsaturated and reactive
species or to an inhibiting effect of the cod ligands cannot
be stated at this stage. In favour of the latter hypothesis is
that slightly better performances were obtained with the
(1-(2,6-Diisopropylphenyl)-1H-imidazol-2-yl)lithium (1b).
To a stirred solution of 1-(2,6-diisopropylphenyl)-imidazole
(1.83 g, 8.0 mmol) in pentane (30 mL) was added dropwise
a solution of n-BuLi (1.6 M in n-hexane, 5.0 mL, 8.0 mmol)
at -78 °C over 2 min. Then the reaction mixture was allowed
to warm to room temperature and stirred for another 2 h.
The precipitate was collected by filtration, washed with
pentane and dried in vacuo to obtain a white powder (1.72
1
3
tetraethylene complex
7, may be due to the favourable
g, 92%). H NMR (500 MHz, THF-d
3
8
):
δ
p-aryl-H), 7.11 (d, J = 7.6 Hz, 2H, m-aryl-H), 7.04 (s, 1H,
7.19 (t, J = 7.6 Hz, 1H,
lability of the ethylene ligands.
3
NCHCHN(Dipp)), 6.81 (s, 1H, NCHCHN(Dipp)), 2.72 (sept, J = 6.9
3
), 1.04 (d, J = 6.9 Hz, 6H, CH(CH
Hz, 2H, CH(CH
3
3
)
2
3
)
2
), 1.03 (d,
):
13
). C{ H} NMR (125 MHz, THF-d
1
Experimental
J = 6.9 Hz, 6H, CH(CH
3
)
2
8
δ
2
02.3 (NCN(Dipp)), 147.4 (C(Dipp)), 142.2 (C(Dipp)), 127.4
General Considerations
(NCHCHN(Dipp)), 127.1 (CH(Dipp)), 123.0 (CH(Dipp)), 119.0
All manipulations involving organometallics were
performed under argon in a Braun glove-box or using
standard Schlenk techniques. Solvents were dried using
standard methods and distilled over sodium/
benzophenone under argon prior use or passed through
columns of activated alumina and subsequently purged
3 2 3 2
(NCHCHN(Dipp)), 28.3 (CH(CH ) ), 25.1 (CH(CH ) ), 24.3
(CH(CH ) ).
3
2
1-Mesitylimidazolyl(cycloocta-1,5-diene)iridium(I)
chloride [Ir(cod)Cl{C H N (Mes)- N3}] (2). A solution of 1-
3
3 2
κ
mesitylimidazole (0.028 g, 0.15 mmol) in THF (2 mL) was
added to a stirred solution of [Ir(cod)(μ-Cl)] (0.050 g, 0.074
2
2
6
with argon. The starting materials [Rh(cod)(μ-Cl)]2,
1
mmol) in THF (2 mL). The mixture was stirred for 1 h at
2h
27
[
Ir(C
2
H
4
)
2
(μ-Cl)]
2
and [Rh(C
2
H
4
)
2
(μ-Cl)]
2
were prepared
is
room temperature and then the volatiles were removed in
vacuo. The residue was washed with pentane (3 × 2 mL)
according to the literature and [Ir(cod)(μ-Cl)]
2
commercially available from Johnson Matthey PLC. NMR
spectra of complexes were recorded on a Bruker 300 MHz,
and dried under vacuum to give a yellow powder (0.074 g,
1
0.14 mmol, 94%). H NMR (500 MHz, CD Cl ):
2
2
δ 8.27
4
3,4
4
00 MHz, 500 MHz or 600 MHz instrument at ambient
(apparent t, J = 1.4 Hz, 1H, NCHN), 7.28 (apparent t, J =
1.4 Hz, 1H, NCHCHN(mesityl)), 6.99 (s, 2H, aryl-H), 6.96
3,4
1
temperature and referenced using the proton ( H) or
1
3
carbon ( C) resonance of the residual solvent, with
downfield shifts reported as positive. Assignments are
1
based on H, H-COSY, H-NOESY, H/ C-HSQC, and H/ C-
(apparent t, J = 1.4 Hz, 1H, NCHCHN(mesityl)), 4.20 (br s, 2H,
CH(cod)), 3.71 (br s, 2H, CH(cod)), 2.33 (s, 3H, p-CH3(mesityl)),
2.27 (m, 4H, CH2(cod)), 1.98 (s, 6H, o-CH3(mesityl)), 1.62 (m, 2H,
1
1
1
13
1
13
3
1
1
13
1
HMBC experiments. P{ H} NMR spectra were recorded on
a Bruker Avance 300 instrument at 121.49 MHz using H PO
85% in D O) as external standard. IR spectra were recorded
CH2(cod)), 1.51 (m, 2H, CH2(cod)). C{ H} NMR (125 MHz,
CD Cl ): 140.4 (NCHN), 140.3 (p-C(mesityl)), 135.3 (o-C(mesityl)),
(ipso-C(mesityl)), 129.5 (m-C(mesityl)), 126.9
3
4
2
2
δ
(
2
132.6
-
1
in the region 4000–100 cm on a Nicolet 6700 FT-IR
spectrometer (ATR mode, diamond crystal). Elemental
analyses were performed by the “Service de microanalyses”,
Université de Strasbourg.
(NCHCHN(mesityl)), 121.5 (NCHCHN(mesityl)), 67.1 (CH(cod)), 58.2
(CH(cod)), 32.3 (CH2(cod)), 31.5 (CH2(cod)), 21.2 (p-CH3(mesityl)),
17.5 (o-CH3(mesityl)). Anal. Calcd for C H ClIrN (%): C, 46.01;
20
26
2
H, 5.02; N, 5.37. Found: C, 45.79; H, 5.11; N, 5.90.
Ir(cod){µ-C (Mes)- C2, N3}] . To a stirred
solution of 1a (0.100 g, 0.52 mmol) in Et O (10 mL) was
[
3
H
2
N
2
κ
κ
2
(3H-H)
Synthetic procedures
2
(1-Mesityl-1H-imidazol-2-yl)lithium (1a). To a stirred
added a solution of [Ir(cod)(μ-Cl)] (0.168 g, 0.25 mmol) in
2
solution of 1-mesitylimidazole (1.49 g, 8.0 mmol) in
Et O (5 mL) at -78 °C. The reaction mixture was allowed to
2
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1
filtration and dried in vacuo (0.070 g, 81%). H NMR (500
warm to room temperature gradually and was stirred for 12
h. After removal of the volatiles under vacuum, the residue
was extracted with toluene and the solution was filtered
through Celite. The filtrate was concentrated to ca. 2 mL,
3
6 6
MHz, C D ): δ 7.11 (d, J = 1.5 Hz, 2H, NCHCHN(mesityl)), 6.67
3
(s, 4H, aryl-H), 6.33 (d, J = 1.5 Hz, 2H, NCHCHN(mesityl)), 2.04
(s, 6H, CH3(Mes)), 1.96 (s, 6H, CH3(Mes)), 1.82 (s, 6H, CH3(Mes)).
1
3
1
Et
2
O (3 mL) was added and this solution was cooled to -
6 6
C{ H} NMR (125 MHz, C D ): δ 181.4 (CO), 175.5 (CO),
3
0 °C to yield a dark red crystalline solid (0.194 g, 80%)
172.8 (NCN(mesityl)), 138.7 (C(mesityl)), 136.8 (C(mesityl)), 136.4
(C(mesityl)), 136.1 (C(mesityl)), 131.4 (NCHCHN(mesityl)), 129.4
(CH(mesityl)), 129.0 (CH(mesityl)), 123.0 ((NCHCHN(mesityl))), 21.0
(CH3(mesityl)), 19.6 (CH3(mesityl)), 18.1 (CH3(mesityl)). IR (pure,
1
which was collected by filtration and dried in vacuo. H
3
NMR (400 MHz, C
6 6
D ): δ 7.20 (d, J = 1.4 Hz, 2H,
NCHCHN(mesityl)), 6.74 (s, 2H, aryl-H), 6.68 (s, 2H, aryl-H),
3
-1
6
.42 (d, J = 1.4 Hz, 2H, NCHCHN(mesityl)), 4.24 (m, 2H, CH(cod)),
.89 (m, 4H, CH(cod)), 3.55 (m, 2H, CH(cod)), 2.65–2.31 (m, 8H,
orbit diamond): vCO = 2060, 2037, 1954 cm . Anal. Calcd for
3
C
28
H26Ir
2
N
4
O
4
(%): C, 38.79; H, 3.02; N, 6.46. Found: C, 38.53;
H, 3.15; N, 6.29.
[Ir(CO) {µ-C
similar to that used for the synthesis of
CH2(cod)), 2.10 (s, 12H, o-CH3(mesityl)), 1.91 (s, 6H, p-CH3(mesityl)),
1
3
1
1
.85 (m, 4H, CH2(cod)), 1.64 (m, 4H, CH2(cod)). C{ H} NMR
100 MHz, C ): 171.3 (NCN(mesityl)), 138.5 (C(mesityl)), 137.3
(mesityl)), 136.9 (C(mesityl)), 135.8 (C(mesityl)), 128.9 (CH(mesityl)),
(CH(mesityl)), (NCHCHN(mesityl)),
NCHCHN(mesityl)), 69.4, 69.0, 65.2 and 59.5 (CH(cod)), 32.9,
2.7, 32.5 and 32.1 (CH2(cod)), 21.0 (p-CH3(mesityl)), 20.2 (o-
CH3(mesityl)), 18.3 (o-CH3(mesityl)). Anal. Calcd for C40 50Ir
2
3
H
2
N
2
(Mes)-
κ
C2,
κ
N3}]
2
(4H-T). A procedure
H-H but starting
(
(
6
D
6
δ
4
1
C
from
3
H-T yielded a brown solid (0.065 g, 75%). H NMR (300
3
128.8
125.5
122.4
MHz, C
6
D
6
):
δ
6.95 (d, J = 1.6 Hz, 2H, NCHCHN(mesityl)), 6.79
3
(
(s, 2H, aryl-H), 6.77 (s, 2H, aryl-H), 6.41 (d, J = 1.6 Hz, 2H,
NCHCHN(mesityl)), 2.09 (s, 6H, p-CH3(mesityl)), 1.95 (s, 12H, o-
3
1
3
1
H
2
N
4
6 6
CH3(mesityl)). C{ H} NMR (75 MHz, C D ): δ 183.7 (CO), 175.7
(
%): C, 49.46; H, 5.19; N, 5.77. Found: C, 48.68; H, 5.00; N,
.72.
From : To a stirred solution of 2 (0.052 g, 0.10 mmol) in
(CO), 175.1 (NCN(mesityl)), 138.7 (C(mesityl)), 136.5 (C(mesityl)),
135.6 (C(mesityl)), 130.5 (NCHCHN(mesityl)), 129.4 (CH(mesityl)),
129.3 (CH(mesityl)), 122.1 (NCHCHN(mesityl)), 21.1 (p-CH3(mesityl)),
18.7 (o-CH3(mesityl)), 18.0 (o-CH3(mesityl)). IR (pure, orbit
5
2
THF (5 mL) was added a solution of KHMDS (0.020 g, 0.10
mmol) in THF (2 mL) at -78 °C. The reaction mixture was
allowed to warm to room temperature gradually and was
stirred for 12 h. After removal of the volatiles under
vacuum, the residue was extracted with toluene and the
solution was filtered through Celite. The filtrate was
evaporated to dryness under reduced pressure to yield a
-1
diamond): vCO = 2059, 2041, 1968 cm . Anal. Calcd for
(%): C, 38.79; H, 3.02; N, 6.46. Found: C, 38.42;
H, 3.22; N, 6.63.
[Ir(CO)(PMe ){µ-C
28 2 4 4
C H26Ir N O
3
3
H
2
N
2
(Mes)-
κC2,κ
N3}]
2
(5).
This
complex can be synthesized by reaction of either
4H-T or 4
H-H
with 2.0 equiv. of trimethylphosphine.
1
red solid (0.044 g, 91%). H NMR analysis of the resulting
From
4H-T: To a solution of
4H-T (0.043 g, 0.05 mmol) in
red solid revealed a ca. 40:60 mixture of
Ir(cod){µ-C (Mes)- C2, N3}] (3H-T). A solution of
H-H (0.048 g, 0.050 mmol) in THF (5 mL) was refluxed for 24
3
H-H and
3
H-T
.
toluene (5 mL) was added a solution of PMe
3
(0.1 M in Et O,
2
[
3
H
2
N
2
κ
κ
2
1.0 mL, 0.1 mmol). The mixture was stirred for 4 h and the
initially yellow solution became red. After removal of the
volatiles under vacuum, the residue was washed with
pentane (2 × 1 mL) to yield a red crystalline solid which was
3
h. After removal of the solvent under vacuum, the residue
was washed with pentane (2 × 1 mL) to yield a purple
1
crystalline solid (0.046 g, 96%) which was collected by
1
filtration and dried in vacuo. H NMR (300 MHz, C
collected by filtration and dried in vacuo (0.045 g, 93%). H
3
6.90 (s, 2H, aryl-H), 6.89 (d, J = 1.0
6
D
6
):
δ
.02 (d, J = 1.1 Hz, 2H, NCHCHN(mesityl)), 6.89 (s, 2H, aryl-H),
NMR (500 MHz, C
6
D
6
):
δ
Hz, 2H, NCHCHN(mesityl)), 6.77 (s, 2H, aryl-H), 6.68 (apparent t,
3
7
6
4
3
.77 (s, 2H, aryl-H), 6.53 (d, J = 1.1 Hz, 2H, NCHCHN(mesityl)),
3
5
J
H-H, JH-P = 1.0 Hz, 2H, NCHCHN(mesityl)), 2.28 (s, 6H, o-
CH3(mesityl)), 2.18 (s, 6H, o-CH3(mesityl)), 2.08 (s, 6H, p-
.32 (m, 2H, CH(cod)), 3.85 (m, 2H, CH(cod)), 3.66 (m, 4H,
2
31
). P{ H} NMR
1
CH(cod)), 2.67–2.27 (m, 8H, CH2(cod)), 2.21 (s, 6H, o-CH3(mesityl)),
CH3(mesityl)), 1.13 (d, JH-P = 8.7 Hz, 18H, PCH
3
1
3
1
2
.18 (s, 6H, o-CH3(mesityl)), 1.97 (s, 6H, p-CH3(mesityl)), 1.81–
(121.5 MHz, C
6
D
6
):
δ
-24.1. C{ H} NMR (125 MHz, C ): δ
6
D
6
1
3
1
.49 (m, 8H, CH2(cod)). C{ H} NMR (75 MHz, C
2
2
1
6
D
6
):
δ
172.0
181.4 (d, JC-P = 10.0 Hz, CO), 177.1 (d, JC-P = 112.0 Hz,
NCN(mesityl)), 138.3 (C(mesityl)), 137.5 (C(mesityl)), 136.7 (C(mesityl)),
135.5 (C(mesityl)), 129.4 (CH(mesityl)), 128.9 (CH(mesityl)), 128.0 (d,
(
NCN(mesityl)), 138.3 (C(mesityl)), 137.2 (C(mesityl)), 136.8 (C(mesityl)),
35.0 (C(mesityl)), 129.3 (CH(mesityl)), 128.1 (CH(mesityl)), 125.4
NCHCHN(mesityl)), 121.3 (NCHCHN(mesityl)), 75.7, 73.8, 59.8
1
4
4
(
JC-P = 5.2 Hz, NCHCHN(mesityl)), 120.1 (d,
JC-P = 2.0 Hz,
and 56.0 (CH(cod)), 35.4, 33.4, 32.4, 29.4 (CH2(cod)), 21.1 (p-
CH3(mesityl)), 19.2 (o-CH3(mesityl)), 18.2 (o-CH3(mesityl)). Anal.
NCHCHN(mesityl)), 21.1 (p-CH3(mesityl)), 18.9 (o-CH3(mesityl)), 18.3
1
(o-CH3(mesityl)), 17.0 (d, JC-P = 31.6 Hz, PCH
3
). IR (pure, orbit
2000, 1911 cm . Anal. Calcd for
(%): C, 39.91; H, 4.60; N, 5.82. Found: C,
39.88; H, 4.70; N, 5.75.
From H-H: A procedure similar to that used with
-1
Calcd for C40
H
50Ir
2
N
4
(%): C, 49.46; H, 5.19; N, 5.77. Found: C,
9.21; H, 4.96; N, 5.60.
Ir(CO) {µ-C (Mes)-
H-H (0.097 g, 0.100 mmol) in Et
diamond): vCO =
4
32 2 4 2 2
C H44Ir N O P
[
2
3
H
2
N
2
κ
C2,
κ
N3}]
2
(4H-H). A suspension
of
3
2
O (5 mL) was stirred
4
4H-T was
under CO (1 bar) at room temperature and the color turned
from red to yellow immediately. The suspension was
further stirred for 30 min. After removal of the volatiles
under vacuum, the residue was washed with pentane (2 × 1
mL) to yield a yellow crystalline solid which was collected by
employed to yield the same red crystalline solid (0.046 g,
96%).
[Ir
2
(CO)
2
(PMe
3
)
2
(Me)I{µ-C
3 2 2 2
H N (Mes)-κC2,κN3} ] (6). To
a stirred solution of
5
(0.030 g, 0.032 mmol) in THF (5 mL)
was added dropwise over 2 min a solution of MeI (0.1 M in
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Et O, 0.4 mL, 0.04 mmol) and the mixture was stirred for 5
Dalton Transactions
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ARTICLE
1
afforded an orange solid (0.168 g, 85%). H NMR (500 MHz,
2
min. Concentration of the solution under vacuum to 2 mL
followed by addition of pentane (5 mL) afforded the
6 6
C D ): δ 6.96 (an overlap of two s, 4H, NCHCHN(mesityl) and
aryl-H), 6.77 (s, 2H, aryl-H), 6.51 (s, 2H, NCHCHN(mesityl)),
4.72 (m, 2H, CH(cod)), 4.48 (m, 2H, CH(cod)), 4.09 (m, 2H,
CH(cod)), 3.76 (m, 2H, CH(cod)), 2.73–2.55 (m, 4H, CH2(cod)),
2.31 (s, 6H, o-CH3(mesityl)), 2.20 (s, 6H, o-CH3(mesityl)), 2.09–
1.96 (m, 8H, CH2(cod)), 1.94 (s, 6H, p-CH3(mesityl)), 1.86–1.77
1
product as a yellow powder (0.032 g, 90%). H NMR (500
3
MHz, C
6
D
6
):
δ
s, 1H, aryl-H), 6.89 (s, 1H, aryl-H), 6.75 (s, 2H, aryl-H), 6.73
7.87 (d, J = 1.2 Hz, 1H, NCHCHN(mesityl)), 6.97
(
3
(
d, J = 1.2 Hz, 1H, NCHCHN(mesityl)), 6.19 (t, JH-H, JH-P = 1.2 Hz,
H, NCHCHN(mesityl)), 6.04 (t, JH-H, JH-P = 1.2 Hz, 1H,
1
3
1
1
6 6
(m, 4H, CH2(cod)). C{ H} NMR (125 MHz, C D ): δ 176.2 (d,
1
NCHCHN(mesityl)), 2.26 (s, 3H, CH3(mesityl)), 2.22 (s, 3H,
JC-Rh = 50.4 Hz, NCN(mesityl)), 138.8 (C(mesityl)), 137.2 (C(mesityl)),
CH3(mesityl)), 2.15 (s, 3H, CH3(mesityl)), 2.12 (s, 3H, CH3(mesityl)),
137.0 (C(mesityl)), 135.1 (C(mesityl)), 129.3 (CH(mesityl)), 128.6
(CH(mesityl)), 126.8 (NCHCHN(mesityl)), 120.3 (NCHCHN(mesityl)),
2
1
.96 (s, 3H, CH3(mesityl)), 1.74 (s, 3H, CH3(mesityl)), 1.51 (d, JH-P
=
2
), 1.02 (d, JH-P = 9.6 Hz, 9H, PCH
1
1
9
.6 Hz, 9H, PCH
3
3
), 0.71 (d,
90.3 (d, JC-Rh = 8.4 Hz, CH(cod)), 89.3 (d, JC-Rh = 7.3 Hz,
3
31
1
). P{ H} NMR (121.5 MHz, C
1
1
J
H-P = 5.7 Hz, 3H, IrCH
3
6
D
6
2
):
δ
-
=
=
CH(cod)), 79.2 (d, JC-Rh = 12.9 Hz, CH(cod)), 72.0 (d, JC-Rh = 11.9
Hz, CH(cod)), 33.4, 33.3, 30.7 and 29.6 (CH2(cod)), 21.1 (p-
CH3(mesityl)), 19.5 (o-CH3(mesityl)), 18.2 (o-CH3(mesityl)). Calcd for
1
6.6, -48.9. C{ H} NMR (125 MHz, C
3
1
4
1
1
1
6
D
6
):
δ 181.5 (d, JC-P
0.5 Hz, CO), 180.0 (d, JC-P = 9.6 Hz, CO), 142.8 (d, JC-P
2
2
2
36.7 Hz, NCN(mesityl)), 140.8 (d, JC-P = 141.1 Hz, NCN(mesityl)),
C
40
H50Rh
2
N
4
(%): C, 60.61; H, 6.36; N, 7.07. Found: C, 60.13;
H, 6.48; N, 7.32.
[Rh(C {µ-C
solution of 1b (0.120 g, 0.51 mmol) in Et
added a solution of [Rh(C (μ-Cl)] (0.085 g, 0.25 mmol)
in Et O (5 mL) at -78 °C. The reaction mixture was allowed
38.2, 138.1, 137.9, 136.8, 136.7, 136.5, 135.2 and 135.0
4
C
(
(mesityl)), 131.9 (d, JC-P = 4.6 Hz, NCHCHN(mesityl)), 129.8,
2
H
4
)
2
3
H
2
N
2
(Dipp)-
κ
C2,κ
N3}]
2
(9). To a stirred
O (10 mL) was
4
129.1, 129.0 and 128.9 (CH(mesityl)), 122.2 (d, JC-P = 3.8 Hz,
2
4
NCHCHN(mesityl)), 121.5 (d,
JC-P = 3.8 Hz, NCHCHN(mesityl)),
2
H
4
)
2
2
4
121.0 (d, JC-P = 3.7 Hz, NCHCHN(mesityl)), 21.4 (CH3(mesityl)),
2
1
21.2 (CH3(mesityl)), 18.7 (CH3(mesityl)), 18.6 (d, JC-P = 34.9 Hz,
to warm to room temperature gradually and was further
stirred for 4 h. After removal of the volatiles under vacuum,
the residue was extracted with n-hexane and the solution
was filtered through Celite. The filtrate was concentrated to
ca. 4 mL under reduced presssure and then was cooled to -
30 °C in an ethylene atmosphere to obtain purple crystals,
PCH
4.8 (d, JC-P = 33.5 Hz, PCH
IR (pure, orbit diamond): vCO = 2019, 1952 cm . Anal. Calcd
for C33 (%): C, 35.87; H, 4.29; N, 5.07. Found: C,
5.33; H, 4.02; N, 5.21.
Ir(C {µ-C (Mes)-
solution of 1a (0.100 g, 0.52 mmol) in Et
added a solution of [Ir(C (μ-Cl)] (0.142 g, 0.25 mmol) in
Et O (5 mL) at -78 °C. The reaction mixture was allowed to
3
), 18.4 (CH3(mesityl)), 17.8 (CH3(mesityl)), 17.3 (CH3(mesityl)),
1
2
1
3 3
), -31.8 (d, JC-P = 2.3 Hz, IrCH ).
-1
2 4 2 2
H47IIr N O P
3
1
[
2
H
4
)
2
3
H
2
N
2
κ
C2,
κ
N3}]
2
(7). To a stirred
O (10 mL) was
which were stored at -30 °C (0.135 g, 70%). H NMR (400
3
3
2
6 6
MHz, C D ): δ 7.28 (d, J = 4.4 Hz, 4H, m-aryl-H), 7.08 (t, J =
2
H
4
)
2
2
4.4 Hz, 2H, p-aryl-H), 7.02 (s, 2H, NCHCHN(Dipp), 6.64 (s, 2H,
NCHCHN(Dipp)), 3.65 (B part of a broad poorly resolved
AA’BB’ spin system, 4H, CH2(ethylene)) and 3.58 (A part of a
2
warm to room temperature gradually and was further
stirred for 4 h. After removal of volatiles in vacuo, the
residue was extracted with toluene and the solution was
filtered through Celite. After evaporation of the toluene,
the solid was washed with pentane (2 × 1 mL) at 0 °C and
dried in vacuo to yield a blue solid, which was stored at -
broad poorly resolved AA’BB’ spin system, 4H, CH2(ethylene)),
3
3.25 (sept, J = 6.8 Hz, 2H, CH(CH
3
)
2
), 2.86 (B part of an
AA’BB’ spin system with |N| ≈ 7.5 Hz and poorly resolved
Rhodium coupling, 4H, CH2(ethylene)) and 2.62 (A part of an
AA’BB’ spin system with |N| ≈ 7.5 Hz and poorly resolved
1
3
3
3
0 °C (0.167 g, 77%). H NMR (600 MHz, C
6
D
6
):
δ
1.5 Hz, 2H, NCHCHN(mesityl)), 6.80 (s, 2H, aryl-H), 6.74 (s, 2H,
7.09 (d, J
Rhodium coupling, 4H, CH2(ethylene)), 2.48 (sept, J = 6.8 Hz,
3
), 1.73 (d, J = 6.8 Hz, 6H, CH(CH
3
=
2H, CH(CH
3
)
2
3
)
2
), 1.16 (d, J
),
3
3
), 1.06 (d, J = 6.8 Hz, 6H, CH(CH
aryl-H), 6.49 (d, J = 1.5 Hz, 2H, NCHCHN(mesityl)), 3.13 (broad
B part of an AA’BB’ spin system, 4H, CH2(ethylene)) and 2.89
= 6.8 Hz, 6H, CH(CH
3
3
)
2
3
)
2
13
1
3 2
0.80 (d, J = 6.8 Hz, 6H, CH(CH ) ). C{ H} NMR (100 MHz,
1
(
broad A part of an AA’BB’ spin system, 4H, CH2(ethylene)),
.63 (B part of an AA’BB’ spin system with |N| ≈ 8.2 Hz, 4H,
CH2(ethylene)) and 2.36 (A part of an AA’BB’ spin system with
N| ≈ 8.2 Hz, 4H, CH2(ethylene)), 2.11 (s, 6H, o-CH3(mesityl)), 2.08
6 6
C D ): δ 175.1 (d, JC-Rh = 49.0 Hz, NCN(Dipp)), 147.1 (C(Dipp)),
2
146.1 (C(Dipp)), 137.8 (C(Dipp)), 128.9 (CH(Dipp)), 125.9
(NCHCHN(Dipp)), 124.1 (CH(Dipp)), 123.8 (CH(Dipp)), 122.8
1
|
(NCHCHN(Dipp)), 78.7 (d, JC-Rh = 6.6 Hz, CH2(ethylene)), 58.0 (d,
1
3
1
1
(
(
(
s, 6H, o-CH3(mesityl)), 1.97 (s, 6H, p-CH3(mesityl)). C{ H} NMR
171.5 (NCN(mesityl)), 137.8 (C(mesityl)), 137.5
(mesityl)), 136.7 (C(mesityl)), 134.8 (C(mesityl)), 129.7 (CH(mesityl)),
3 2 3 2
JC-Rh = 12.1 Hz, CH2(ethylene)), 28.8 (CH(CH ) ), 28.3(CH(CH ) ),
150 MHz, C
6
D
6
):
δ
26.3, 24.9, 24.2 and 23.4 (CH(CH
3
)
2
2 4
). Calcd for C38H54Rh N
C
(%): C, 59.07; H, 7.04; N, 7.25. Found: C, 58.63; H, 6.87; N,
7.42.
and 128.8 (CH(mesityl)), 124.6 (NCHCHN(mesityl)), 122.0
(
NCHCHN(mesityl)), 60.5 (CH2(ethylene)), 42.3 (CH2(ethylene)), 21.5
p-CH3(mesityl)), 19.0 (o-CH3(mesityl)), 18.1 (o-CH3(mesityl)). Calcd
Catalytic transfer dehydrogenation of cyclooctane.
(
for C32
H
42Ir
2
N
4
(%): C, 44.32; H, 4.88; N, 6.46. Found: C,
3.88; H, 4.95; N, 6.73.
Rh(cod){µ-C (Mes)-
similar to that used for the synthesis of
starting from [Rh(cod)(μ-Cl)] . Treatment of 1a (0.100 g,
.52 mmol) with [Rh(cod)(μ-Cl)] (0.123 g, 0.25 mmol)
In a preliminary investigation, a red suspension of complex
4
3H-T (9.7 mg, 0.010 mmol) in a mixture of COA (4.0 mL, 30.3
[
3
H
2
N
2
κ
C2,
κ
N3}]
2
(8). A procedure
H-H was used but
mmol) and TBE (0.40 mL, 3.1 mmol), in a sealed tube under
argon, was heated at 200 ºC for 10 h (Table 3, entry 1). Gas
chromatographic (GC) analysis of the products indicated
0.15% of the cyclooctane was converted to cis-cyclooctene
3
2
0
2
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ARTICLE
-
1
at a turnover frequency (TOF) 0.46 h . The formation of
1
TBA, confirmed by GC and H NMR analysis, also indicated
The number of exchanged electrons for the first
oxidation step was determined by exhaustive electrolysis.
Prior to electrolysis, the corresponding mixtures were
stirred and degassed by bubbling argon through the
solution for 10 min. Then, the desired working potential
was applied. During anodic oxidation, the electrolyzed
solution was continuously stirred and maintained under
argon. Coulometric measurements were performed in a
standard 40 mL cell. The working and the auxiliary
electrodes were a platinum wire (o.d. 0.8 mm) of 15 cm
length. For the controlled-potential electrolysis, the anodic
and cathodic compartments were separated by a fritted-
glass disk to prevent diffusion of the electrogenerated
species.
that the transfer dehydrogenation reaction did occur. When
complex
reaction conditions (Table 3, entry 2), no catalytic activity
was observed. In the catalytic reaction using complex
Table 3, entry 3), in which two CO ligands of complex are
replaced by two molecules of PMe , GC analysis of the
products indicated that 0.19% of the cyclooctane was
4H-T was used as the precatalyst under the same
5
(
4
3
-
1
converted to cis-cyclooctene at a TOF of 0.58 h . Better
results were obtained with complex where 2.06% of the
cyclooctane was converted to cis-cyclooctene at a TOF of
7
-
1
6
.24 h .
X-ray Data Collection, Structure Solution, and Refinement for
All Compounds.
The reference electrode was a saturated calomel
electrode (SCE) that was electrically connected to the
studied solution by a junction bridge filled with the
corresponding solvent-supporting electrolyte solution.
Spectroelectrochemical experiments were carried out as
Suitable crystals for the X-ray analysis of all compounds
H-H
were obtained as described above. Data for 3 , 3H-T and 8
were collected on an APEX-II CCD (graphite-
monochromated Mo-Kα radiation, λ = 0.71073 Å) at 173(2)
3
0
described elsewhere, using a Zeiss MCS 601 UV–vis–NIR
diode array spectrometer.
K and data for
diffractometer (graphite-monochromated Mo-Kα radiation,
0.71073 Å) at 173(2) K. Crystallographic and
9 were collected on a Kappa CCD
UV-visible-NIR spectroscopy
λ
=
-
5
experimental details for these structures are summarized in
Table S1 (see ESI). The structures were solved by direct
UV–Vis absorption spectra have been recorded for 4.41 10
-5
-1
-1
2 2
mol L (3H-T) and 8.07 10 mol L (8) solutions in CH Cl on
2
8
methods (SHELXS-97 ) and refined by full-matrix least-
a Perkin–Elmer Lambda 35 spectrophotometer in quartz
cells (1 cm). The UV-visible-NIR absorption spectroscopic
2
squares procedures (based on F , SHELXL-97) with
anisotropic thermal parameters for all the non-hydrogen
atoms. The hydrogen atoms were introduced into the
geometrically calculated positions (SHELXS-97 procedures).
measurements were performed in CH
absorption spectrum of H-T (Fig. S1 in ESI) was
characterised by a band in the visible domain around 525
2 2
Cl . The optical
3
3
-1
-1
The SQUEEZE instruction in PLATON was applied for
the residual electron density was assigned to half a
molecule of disordered n-hexane. In , one methyl group
C11) was found disordered over two positions.
9
and
nm (ε525 nm = 3648 dm mol cm ). In the case of the
S3 in ESI), the band was observed at 476 nm (ε476 nm = 4585
8 (Fig.
3
-1
-1
dm mol cm ).
9
(
EPR experiments
Electrochemistry
EPR spectra were recorded with an EMX spectrometer
(Bruker) operating at X-band and equipped with a standard
HSN cavity and a variable temperature attachment.
Computer simulations of the EPR spectra were performed
All compounds were studied in CH
2
Cl
2
+ 0.1 mM [n-Bu
4
N]PF
Cl
Merck, UVasol ) were used as received. The
electrochemical measurements were carried out at room
temperature (20 °C) in CH Cl containing 0.1 M [n-Bu N]PF
6
6
.
[n-Bu
4
N]PF
6
(Fluka, electrochemical grade) and CH
2
2
®
(
3
1
with the help of Easy Spin software.
2
2
4
in a classical three-electrode cell. The electrolyte was
degassed by bubbling argon through the solution for at
least 5 min, and an argon flow was kept over the solution
during measurements. The electrochemical cell was
connected to a computerized multipurpose electrochemical
device (Autolab, Eco Chemie BV, The Netherlands)
controlled by a GPES software (v. 4.7) running on a PC
computer. The working electrode was a glassy carbon (GC)
disk electrode (diameter: 3 mm), used either motionless for
DFT calculations
All theoretical calculations were performed with the ORCA
32
program package. Geometry optimization was carried out
3
3
using the GGA functional BP86 and by taking advantage of the
34
resolution of the identity (RI) approximation in the Split-RI-J
35
variant with the appropriate Coulomb fitting sets. Increased
integration grids (Grid4 and GridX4 in ORCA convention) and
tight SCF convergence criteria were used. Solvent effects were
accounted for according to the experimental conditions. For that
-
1
-1
cyclic voltammetry (100 mV s to 10 V s ) or as a rotating
disk electrode. The auxiliary electrode was a Pt wire, and
the pseudo reference electrode a Pt wire. All potentials are
purpose, we used the CH
2 2
Cl (

= 9.08) solvent within the
framework of the conductor like screening (COSMO) dielectric
3
6
+
continuum approach. Electronic structures were obtained from
given vs. Fc /Fc used as internal reference in agreement
9
3
7
2
single-point calculations using the B3LYP functional. Scalar
relativistic effects were included using the scalar relativisitc
with the IUPAC recommendation and are uncorrected
from ohmic drop.
3
8
zero-order regular approximation (ZORA) and the scalar
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DOI: 10.1039/C5DT02403J
ARTICLE
3
9
relativistically recontracted (SARC) version of the def2-TZVP(-f)
basis set together with the decontracted def2-TZVP/J Coulomb
fitting basis sets for all atoms. Spin density and molecular
orbitals were plotted using the orca_plot utility program and
11. A. P. Marchenko, H. N. Koidan, I. I. Pervak, A. N. Huryeva, E.
V. Zarudnitskii, A. A. Tolmachev and A. N. Kostyuk,
Tetrahedron Lett., 2012, 53, 494.
1
2. (a) K. A. Beveridge, G. W. Bushnell, K. R. Dixon, D. T. Eadie, S.
R. Stobart, J. L. Atwood and M. J. Zaworotko, J. Am. Chem.
Soc., 1982, 104, 920; (b) A. W. Coleman, D. T. Eadie, S. R.
Stobart, M. J. Zaworotko and J. L. Atwood, J. Am. Chem. Soc.,
40
visualized with Chemcraft software.
1
982, 104, 922; (c) G. W. Bushnell, D. O. K. Fjeldsted, S. R.
Acknowledgements
Stobart and M. J. Zaworotko, J. Chem. Soc. Chem. Commun.,
1983, 580; (d) J. L. Atwood, K. A. Beveridge, G. W. Bushnell, K.
R. Dixon, D. T. Eadie, S. R. Stobart and M. J. Zaworotko, Inorg.
Chem., 1984, 23, 4050; (e) G. W. Bushnell, M. J. Decker, D. T.
Eadie, S. R. Stobart, R. Vefghi, J. L. Atwood and M. J.
Zaworotko, Organometallics, 1985, 4, 2106; (f) D. O. K.
Fjeldsted, S. R. Stobart and M. J. Zaworotko, J. Am. Chem.
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R. Stobart and J. Wang, Organometallics, 1996, 15, 3785; (h)
M. A. Arthurs, J. Bickerton, S. R. Stobart and J. Wang,
Organometallics, 1998, 17, 2743.
The USIAS, CNRS, UdS, Région Alsace and Communauté
Urbaine de Strasbourg are gratefully acknowledged for the
award of fellowships and a Gutenberg Excellence Chair
(
2010–11) to AAD and support. We also thank the ucFRC
www.icfrc.fr) for support and the China Scholarship Council
(
for
a PhD grant to F. H., and the Service de
Radiocristallographie (Institut de Chimie, Strasbourg) for
the determination of the crystal structures.
1
3. (a) K. A. Beveridge, G. W. Bushnell, S. R. Stobart, J. L. Atwood
and M. J. Zaworotko, Organometallics, 1983, 2, 1447; (b) G.
W. Bushnell, D. O. K. Fjeldsted, S. R. Stobart, M. J. Zaworotko,
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1107; (c) Y. Yuan, M. V. Jiménez, E. Sola, F. J. Lahoz and L. A.
Oro, J. Am. Chem. Soc., 2002, 124, 752.
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4
5
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a
good
approximation as an AA'XX'. See: H. Günther, NMR
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6
.
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7
.
.
2
011, 40, 35.
(a) T. Kösterke, T. Pape and F. E. Hahn, J. Am. Chem. Soc.,
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2
1. N. S. Hush, Electrochim. Acta, 1968, 13, 1005.
8
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1
/2 [Ir(cod)(µ-Cl)]2
Cl
Page 17 of 17
Dalton Transactions
N
N
N
N
Mes
Mes
Ir(cod)
C
C
H
THF, RT
H
KHMDS THF, -78 oC to RT
Mes
Mes
N
N
1
/2 [Ir(cod)(µ-Cl)]2
C
N
C
N
N
N
0.2 (cod)Ir
Ir(cod) + 0.3 (cod)Ir
Ir(cod)
Mes
C
THF
-78 oC to RT
C
N
N C
Li
N
N
Mes
thermally
Mes