Cooperative double deprotonation of bis(2-picolyl)amine leading to unexpected bimetallic mixed valence (M-I, MI) rhodium and iridium complexes

Article abstract of DOI:10.1021/ic200395m

Cooperative reductive double deprotonation of the complex [Rh ^I(bpa)(cod)]⁺ ([4]⁺, bpa = PyCH ₂NHCH₂Py) with one molar equivalent of base produces the bimetallic species [(cod)Rh(bpa-2H)Rh(cod)] (7), which displays a large Rh ^-I,Rh^I contribution to its electronic structure. The doubly deprotonated ligand in 7 hosts the two "Rh(cod)" fragments in two distinct compartments: a "square planar compartment" consisting of one of the Py donors and the central nitrogen donor and a "tetrahedral π-imine compartment" consisting of the other pyridine and an "imine C=N" donor. The formation of an "imine donor" in this process is the result of substantial electron transfer from the {bpa-2H}^2- ligand to one of the rhodium centers to form the neutral imine ligand bpi (bpi = PyCH₂N=CHPy). Hence, deprotonation of [Rh^I(bpa)(cod)] ⁺ represents a reductive process, effectively leading to a reduction of the metal oxidation state from Rh^I to Rh^-I. The dinuclear iridium counterpart, complex 8, can also be prepared, but it is unstable in the presence of 1 mol equiv of the free bpa ligand, leading to quantitative formation of the neutral amido mononuclear compound [Ir ^I(bpa-H)(cod)] (2). All attempts to prepare the rhodium analog of 2 failed and led to the spontaneous formation of 7. The thermodynamic differences are readily explained by a lower stability of the M^-I oxidation state for iridium as compared to rhodium. The observed reductive double deprotonation leads to the formation of unusual structures and unexpected reactivity, which underlines the general importance of "redox noninnocent ligands" and their substantial effect on the electronic structure of transition metals.

Full text of DOI:10.1021/ic200395m

ARTICLE
pubs.acs.org/IC
Cooperative Double Deprotonation of Bis(2-picolyl)amine Leading to
ꢀ
I
I
Unexpected Bimetallic Mixed Valence (M , M ) Rhodium and Iridium
Complexes
,
†
†
†
†
†
Cristina Tejel,* M. Pilar del Río, Laura Asensio, Fieke J. van den Bruele, Miguel A. Ciriano,
‡
‡
,‡
Nearchos Tsichlis i Spithas, Dennis G. H. Hetterscheid, and Bas de Bruin*
†
Instituto de Síntesis Química y Cat ꢀa lisis Homog ꢀe nea (ISQCH), CSIC - Universidad de Zaragoza,
Departamento de Química Inorg ꢀa nica, Pedro Cerbuna 12, 50009-Zaragoza, Spain
‡
Van‘t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands
S Supporting Information
b
I
ABSTRACT: Cooperative reductive double deprotonation of the complex [Rh (bpa)-
+
+
(
cod)] ([4] , bpa = PyCH NHCH Py) with one molar equivalent of base produces the
2 2
I I
ꢀ
bimetallic species [(cod)Rh(bpaꢀ2H)Rh(cod)] (7), which displays a large Rh ,Rh
contribution to its electronic structure. The doubly deprotonated ligand in 7 hosts the
two “Rh(cod)” fragments in two distinct compartments: a “square planar compartment”
consisting of one of the Py donors and the central nitrogen donor and a “tetrahedral π-imine
compartment” consisting of the other pyridine and an “imine CdN” donor. The formation
of an “imine donor” in this process is the result of substantial electron transfer from the
2
ꢀ
{
bpaꢀ2H} ligand to one of the rhodium centers to form the neutral imine ligand
I
+
bpi (bpi = PyCH NdCHPy). Hence, deprotonation of [Rh (bpa)(cod)] represents a
2
I
reductive process, effectively leading to a reduction of the metal oxidation state from Rh to
Rh . The dinuclear iridium counterpart, complex 8, can also be prepared, but it is unstable
ꢀ
I
in the presence of 1 mol equiv of the free bpa ligand, leading to quantitative formation of the
I
neutral amido mononuclear compound [Ir (bpaꢀH)(cod)] (2). All attempts to prepare the rhodium analog of 2 failed and led to
ꢀ
I
the spontaneous formation of 7. The thermodynamic differences are readily explained by a lower stability of the M oxidation state
for iridium as compared to rhodium. The observed reductive double deprotonation leads to the formation of unusual structures and
unexpected reactivity, which underlines the general importance of “redox noninnocent ligands” and their substantial effect on the
electronic structure of transition metals.
’
INTRODUCTION
still in its infancy, and therefore the outcome of electron
transfer reactions is quite unpredictable. Nonetheless, a rich
ꢀ
Amido ligands (R N ) are an interesting class of cooperating
2
11
chemistry can be envisaged from both metal-centered and
ligands allowing activation of substrates coordinated to transi-
tion metals. Cooperative substrate activation by rutheniumꢀ
amido complexes in Noyori hydrogenations provides a seminal
1
2,13
ligand-centered oxidations.
redox noninnocence” of the amido ligand, and the resulting
aminyl radical ligands show interesting bond activations by
This revealed among others the
“
1
example. Rhodiumꢀamido complexes have received consider-
1
4
2
radical hydrogen abstraction.
ably less attention in this field. Nonetheless, some recent exam-
We have previously investigated in detail the effect of one-
ples reveal that amido complexes of the type [Rh(Ntrop )-
2
15
16
electron oxidation of ethylene and 1,5-cyclooctadiene (cod)
iridium complexes, and some of these investigations clearly
revealed the “redox noninnocence” of the bis(2-picolyl)amine
(PR )] (Ntrop = bis(tropylidenyl)amide, R = Ph) show inter-
3 2
esting properties, such as cooperative substrate activation in
dehydrogenative coupling reactions of alcohols. Closely related
cooperative reduction of dioxygen can be effectively catalyzed by
3
(
bpa) ligand. Furthermore, bpa deprotonation proved to have a
4
profound effect on the properties and reactivity of [Ir(bpa)-
(
dinuclear amidoꢀrhodium complexes. Additionally, these types
+
17
+
+
cod)] . Sequential deprotonation of [Ir(bpa)(cod)] ([1] )
of systems are able to promote interesting reactions such as
5
at the central amine donor, and subsequently the adjacent
methylene moiety, allows the isolation of the neutral amido
complex [Ir(bpaꢀH)(cod)] (2) and the “dearomatized” anionic
amido transfer to alkenes and vinylarenes, cycloaddition reac-
6
7
tions associated with CꢀN bond formation, and CꢀCl and
8
CꢀH bond activation reactions. In other instances, related
9
dinuclear complexes have been used as precursors for low-
1
0
valent late transition metal imido clusters. Our understanding
of the redox-properties of the “RhꢀNR ” framework is however
Received: February 24, 2011
Published: July 12, 2011
2
r 2011 American Chemical Society
7524
dx.doi.org/10.1021/ic200395m
|Inorg. Chem. 2011, 50, 7524–7534

Inorganic Chemistry
ARTICLE
ꢀ
Scheme 1. Formation of Neutral 2 and Anionic [3] Iridium
Scheme 3. Typical Acid/Base Behavior for Iridium and Co-
operative Reductive Double Deprotonation for Rhodium
+
Complexes upon Sequential Deprotonation of Cationic [1]
and the Double Deprotonation of the Rhodium Complex [4]
+
ꢀ
to [6]
+
Scheme 2. Cooperative Double Deprotonation of [4]
Leading to Formation of the Dinuclear Complex
[
(cod)Rh(bpaꢀ2H)Rh(cod)] (7) and Free bpa
quantitative way the doubly deprotonated compound K[Rh-
(
bpaꢀ2H)(cod)] (K[6], Scheme 1), which contains a “dear-
18a
+
omatized” pyridine moiety. Moreover, the addition of [4] to
thf-d solutions of K[6] at room temperature did not produce
8
the expected acid/base comproportionation of the ligand to
[Rh(bpaꢀH)(cod)] (as the iridium counterpart does). On the
contrary, complex 7 and free bpa were the sole products from the
ꢀ
ꢀ
complex [Ir(bpaꢀ2H)(cod)] ([3] ), respectively (see
1
8
Scheme 1). Related Ru, Rh, and Ir systems have been reported
+
1
9
20
reaction (Scheme 3). Furthermore, monitoring the reaction of [4]
by Schneider et al. and Milstein et al., some of which are
excellent catalysts for transfer hydrogenation and dehydrogena-
tive coupling reactions and offer interesting opportunities for
water splitting.
Inspired by the deprotonation results of the iridium com-
plexes, and motivated by the interesting literature precedents
concerning amido ligand cooperativity in rhodium-mediated
hydrogenation catalysis and the redox noninnocence of rho-
diumꢀamido complexes revealing intriguing atom abstraction
reactions, we decided to investigate the deprotonation of the
cationic [Rh(bpa)(cod)] complex. We aimed at preparing the
neutral rhodium amido complex [Rh(bpaꢀH)(cod)] (5 in
Scheme 1) to investigate its reactivity. However, quite unexpect-
edly, it turned out that a cooperative reductive double deprotonation
process rendering a mixed-valence dinuclear complex takes place.
The reason behind this unusual behavior and the unexpected
results are the topics of this paper.
t
and KO Bu (1:1 molar ratio) at ꢀ70 °C in thf-d allowed the ob-
8
ꢀ
servation of [6] in the reaction medium (along with the starting
4] and small amounts of the products: 7and free bpa). No evidence
+
[
for [Rh(bpaꢀH)(cod)] was found in any experiment.
It thus seems that deprotonation with only one equivalent of
base leads to a cooperative double deprotonation to the anionic
ꢀ
ꢀ
[
Rh(bpaꢀ2H)(cod)] ([6] ), which abstracts a “Rh(cod)”
+
moiety from the remaining cationic complex [Rh(bpa)(cod)]
[4] ) to give 7 along with liberation of an equivalent of the
+
(
+
21
free bpa ligand. This is a thermodynamically driven reaction
(
vide infra).
Sequential acid deprotonation and base protonation generally
becomes increasingly difficult after each step as a result of charge
accumulation (K > K ), and the reverse behavior (K < K )
a1
a2
a1
a2
associated with cooperative (de)protonation is rare. Some
examples of increased amine basicity of macrocyclic proton
receptors after the first protonation have been explained by
conformational rearrangements enforcing hydrogen bonding
2
2
and/or by the encapsulation of mediating water molecules.
’
RESULTS AND DISCUSSION
Apart from these clear examples, a few less-defined examples of
(mononuclear) metal-ion-induced cooperative deprotonation
and coordination of peptide-based ligands in water have been
+
Cooperative Double Deprotonation of [Rh(bpa)(cod)] . In
an initial attempt to prepare the neutral rhodiumꢀamido com-
23
plex [Rh(bpaꢀH)(cod)], the compound [Rh(bpa)(cod)]PF
reported, but the cooperativity was not explained. Chelation
(i.e., bringing the fragments closer to the metal) and solvation
6
t
(
[4]PF ) was treated with 1 mol equiv of KO Bu in thf at room
6
2
4
temperature. This did not produce the expected neutral square
planar rhodium complex (5, Scheme 1). Only 1 equivalent of the
base was enough to achieve the full conversion of [4] into the
must have played an important role in these observations. We
are not aware of any examples of cooperative deprotonation
sequences of coordinated ligands (mononuclear complexes).
+
unsymmetric dinuclear complex [(cod)Rh(bpaꢀ2H)Rh(cod)]
In a similar way, reaction of [{Rh(cod)(μ-OMe)} ] with bpa
2
(
7) along with free bpa in a 1:1 molar ratio (Scheme 2).
(in 1:2 molar ratio) again produces the neutral [(cod)Rh-
(bpaꢀ2H)Rh(cod)] (7) and free bpa without evidence of
mononuclear [Rh(bpaꢀH)(cod)]. According to the stoichiom-
etry of the reaction, complex 7 was cleanly prepared by reacting
In several other experiments, using e1 equivalent of the base,
+
we always obtained the same result. Treatment of [4] with g2
equivalents of the base, however, produced readily and in a
7
525
dx.doi.org/10.1021/ic200395m |Inorg. Chem. 2011, 50, 7524–7534

Inorganic Chemistry
{Rh(cod)(μ-OMe)} ] and bpa (1:1 molar ratio) in diethyl
ether. From these solutions, complex 7 was isolated as dark red
microcrystals suitable for X-ray diffraction studies.
Molecular structure of [(cod)Rh(bpaꢀ2H)Rh(cod)] (7). The
molecular structure of 7 is shown in Figure 1 along with the
labeling scheme used. Selected bond distances and angles are
collected in Table 1.
ARTICLE
0
[
close to 120°, and their sum (∑ = 356°) is close to the expected
2
2
value for an sp hybridized nitrogen (Rh1ꢀN2ꢀC7, 126.7(2)°;
Rh1ꢀN2ꢀC6, 113.7(2)°; and C6ꢀN2ꢀC7, 115.2(2)°). A
similar geometry is observed for C6 with values of 117.7(19),
116.1(19), and 116.0(2)° for the angles N2ꢀC6ꢀH6, C5ꢀC6ꢀ
0
H6, and C5ꢀC6ꢀN2, respectively, although their sum (∑ =
3
50°) suggests some pyramidalization as a consequence of the
The {bpaꢀ2H} ligand in 7 acts as a heterodicompartmental
ligand, hosting the two “Rh(cod)” fragments in two distinct
compartments. The first compartment functions as a hetero-
π-coordination of Rh2. In addition, the maximum deviation from
the best plane defined by the five-membered rhodacycle Rh1ꢀ
N1ꢀC5ꢀC6ꢀN2 is only 0.092 Å. Therefore, N2 and C6 can be
0
bidentate N,N -ligand consisting of one of the Py donors and the
2
described as sp -hybridized atoms, pointing to a relatively strong
central amido nitrogen (N2), thus hosting a Rh(cod) fragment
imine character of the C6ꢀN2 bond. In fact, a relatively short
C6ꢀN2 bond length (1.415(4) Å) is observed. Although the
CdN bond distance in aromatic free imines lies in the range
.25ꢀ1.33 Å, this distance should be enlarged by the π-coordination
(Rh1). The coordination geometry around Rh1 is clearly square
planar, with only a small twist (5.7(2)°) of the planes defined by
N1ꢀRh1ꢀN2 and Ct1ꢀRh1ꢀCt2 (Figure 1). The other Py-
donor and the C6ꢀN2 π bond are coordinated to the second
Rh atom. The angles around N2 (if Rh2 is not considered) are
1
to rhodium.
In this perspective, the “imine” is π-coordinated to Rh2, and as
such, the geometry around this “four-coordinate” Rh2 center is
perhaps best described as distorted tetrahedral. The dihedral
angle between the Ct5ꢀRh2ꢀN3 and Ct3ꢀRh2ꢀCt4 planes
(
56.5(2)°) lies intermediate between tetrahedral (90°) and
square planar (0°), although clearly greatly distorted from the
latter (Figure 1). These structural data suggest that the bpaꢀ2H
bridging ligand in 7 is, in fact, largely transformed into the imine
PyCHdNꢀCH Py (bpi) ligand by transfer of almost two
2
Scheme 4. Contributing Resonance Structures of Complex 7
Figure 1. Molecular structure (ORTEP at 50% level) of complex 7.
Numbers in purple correspond to the middle points (centroids) of the
coordinated double bonds.
ꢀ
I
I
Table 1. Selected Bond Distances (Å) and Angles (deg) for Complexes [(cod)M (bpaꢀ2H)M (cod)] (M = Rh, 7; Ir, 8)
7
8
7
8
M1ꢀN1
M1ꢀN2
M1ꢀCt1
M1ꢀCt2
M2ꢀN2
2.094(2)
2.032(2)
2.010(3)
1.997(3)
2.254(2)
1.394(4)
1.401(4)
1.355(4)
1.401(5)
1.407(4)
1.434(4)
1.460(4)
1.341(4)
1.374(4)
1.375(4)
173.6(1)
175.7(1)
80.4(1)
2.081(5)
2.028(5)
2.005(6)
1.981(6)
2.247(5)
1.408(9)
1.418(9)
1.366(7)
1.399(9)
1.395(8)
1.440(8)
1.467(7)
1.346(7)
1.376(9)
1.378(8)
173.1(2)
174.9(2)
79.8(2)
M2ꢀN3
M2ꢀCt5
M2ꢀCt3
M2ꢀCt4
M2ꢀC6
2.216(2)
2.076(3)
2.043(3)
1.964(3)
2.130(3)
1.396(4)
1.427(4)
1.369(4)
1.374(4)
1.373(4)
1.415(4)
1.509(4)
1.400(4)
1.388(5)
1.354(4)
98.7(1)
2.166(5)
2.078(6)
2.036(7)
1.953(6)
2.145(6)
1.401(9)
1.458(8)
1.360(8)
1.371(9)
1.374(7)
1.427(7)
1.495(8)
1.391(8)
1.401(9)
1.356(8)
98.0(2)
a
a
a
a
a
C13ꢀC14
C17ꢀC18
N1ꢀC1
C21ꢀC22
C25ꢀC26
C1ꢀC2
C2ꢀC3
C3ꢀC4
C4ꢀC5
C5ꢀN1
C5ꢀC6
C6ꢀN2
N2ꢀC7
C7ꢀC8
C8ꢀN3
C8ꢀC9
C9ꢀC10
C10ꢀC11
C12ꢀN3
C11ꢀC12
N1ꢀM1ꢀCt1
N2ꢀM2ꢀCt2
N1ꢀM1ꢀN2
Ct1ꢀM1ꢀCt2
N3ꢀM2ꢀCt3
Ct4ꢀM2ꢀCt5
N3ꢀM2ꢀCt5
Ct3ꢀM2ꢀCt4
120.1(1)
78.2(1)
118.1(2)
78.4(2)
87.7(1)
87.0(2)
87.1(1)
86.8(2)
a
Ct1, Ct2, Ct3, Ct4, and Ct5 are the middle points between C13 and C14, C17 and C18, C21 and C22, C25 and C26, and C6 and N2, respectively.
7
526
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Inorganic Chemistry
ARTICLE
Figure 2. DFT optimized geometry (left) and HOMO (right) of 7.
2
ꢀ
electrons from the initially dianionic {bpaꢀ2H} ligand to
It should be indicated that late transition metal complexes with
2
rhodium with concomitant reduction of this rhodium center to
an η -CdN bonded moiety are very scarce and that they are
ꢀI
B
Rh . Accordingly, the Rh2ꢀN distances (Rh2ꢀN2, 2.254(2) Å;
Rh2ꢀN3, 2.216(2) Å) are longer than the Rh1ꢀN distances
systematically described as metalla-aza-cyclopropanes (7 , Scheme4)
2
7
28
in complexes of palladium and iridium. The single related
precedent in rhodium chemistry corresponds to the com-
plex [Rh(CH Ntrop )(PR )] (HNtrop = bis(tropylidenyl)-
(
Rh1ꢀN1, 2.094(2); Rh1ꢀN2, 2.032(2) Å) as expected for a
reduced rhodium(ꢀI) center. In good agreement, Rh2 seems to
2
2
3
2
10
29
adopt an almost tetrahedral geometry, typical for d -ML
amine, R = Ph) described by Gr €u tzmacher et al., of which
4
complexes. All of these features are similar to the complex
the structural parameters point to a rhoda-aza-cyclopropane
25
2
[
(nbd)Rh(bpaꢀ2H)Rh(ndb)] previously communicated.
rather than an η -iminium ion bound to a reduced metal. In
A
30
The structural data suggest that resonance structure 7
addition, related two-electron mixed-valence complexes M(0,II)
(
Scheme 4) contributes substantially to the electronic structure of
have been synthesized using special ligands capable of stabilizing
redox unsymmetric environments, as developed by Nocera et al.
2
6
I
31
7
.
Nevertheless, a second resonance structure with a Rh ion
B
being part of a rhoda(I)-aza-cyclopropane ring (7 , Scheme 4)
Some of them show interesting new mechanisms in hydrogenation
3
2
33
cannot be completely neglected.
reactions and CꢀH activation reactions and are photoactive
A
I
34
For the main resonance form (7 ), the square-planar Rh can
in hydrogen production. Very recently, an unusual Rh(ꢀI,III)
8
35
be considered as a d cationic 16 VE (VE = valence electron)
complex was reported.
center to which the bpi ligand contributes with four electrons.
The tetrahedral Rh is a d anionic 18 VE center to which the
bpi also contributes with four electrons. Consequently, complex
NMR Spectra of [(cod)Rh(bpaꢀ2H)Rh(cod)] (7). The unu-
ꢀ
I
10
sual structure of 7is maintained in solution according to multinuclear
1
NMR spectra. Figure 3 shows the H NMR spectrum of 7 in C
D
,
6
6
13
1
7
can be considered, in part, as a zwitterionic complex. Reso-
while a selected region of the C{ H} NMR spectrum is included in
B
nance structure 7 leads to an identical electron count for both
rhodium atoms, in which case the bridging {bpa-2H} ligand is
a 10 electron donor ligand (4e to Rh1 and 6e to Rh2).
the inset. The methylene protons from the intact CH group give rise
2
2
ꢀ
to an AB spin system centered at δ 4.18 ppm, while the proton from
the imine (HCdN) produces a singlet at δ 4.32 ppm.
DFT Modeling of [(cod)Rh(bpaꢀ2H)Rh(cod)] (7). The
DFT-optimized geometry of 7 (Figure 2) is very close to the
X-ray structure (Figure 1). The species is clearly diamagnetic.
A closed-shell ground state is expected for second row metals, but
nonetheless we checked for a possible singlet biradical ground
state with two antiferromagnetically coupled electrons with
broken-symmetry UꢀDFT calculations. These, however, in all
possible attempts, converged to the same closed-shell singlet
configuration as in the restricted closed-shell DFT calculations.
Metalꢀligand biradical descriptions can thus be excluded. The
DFT calculations indicate strong Rh d-orbital mixing, which
complicates a direct and simple interpretation of its electronic
structure. A L €o wdin orbital occupancy analysis (b3-lyp, TZVP)
indicates that both Rh atoms in 7 have a total d-orbital occupancy
of almost exactly eight electrons (Rh1, 8.0 electrons; Rh2, 8.1
electrons). These values do not allow us to unambiguously
discriminate between the resonance structures in Scheme 4,
The quite different nature of the corresponding carbon atoms
13
1
is detected in the C{ H} NMR spectrum, where the CH
2
group resonates at the usual chemical shift (δ 60.53 ppm, JC,Rh
=
1.5 Hz), while the signal for the HCdN carbon is shifted to low-
field (δ 82.23 ppm, JC,Rh = 7.6 Hz). The two inequivalent
pyridine rings give rise to two well-defined sets of four signals
1
1
1
1
each, easily identified from the H, H-COSY and H, H-NOESY
spectra. Thus, the two cross-peaks due to NOE effect between
4
4
the CH and A protons and between the HCdN and B , respec-
2
tively, unequivocally correspond to the connectivity shown in
Figure 3. The signals of the nearby pyridine moiety of 7 (labeled
in red) are substantially upfield shifted (up to δ 5.8 ppm),
suggesting some delocalization of electronic density of the
bridging ligand into this pyridine ring.
The cod ligands also behave in a distinct way; one of them
[cod(2)] appears to have an averaged C symmetry on the NMR
2
1
time scale, thus producing two olefinic resonances in the H
NMR spectrum and two in the C{ H}-apt spectrum (see inset
I
B
ꢀI
13
1
since for the Rh based resonance structure 7 and for the Rh
A
based resonance structure 7 , with substantial π-back-donation
in Figure 3). The other, [cod(1)], is observed as a typical cod
2
1
to the “imine” ligand π* orbitals, one expects to find such values.
ligand lacking elements of symmetry. The olefinic protons H
ꢀ
I
22
It thus seems that the Rh ꢀimine π bond is associated with a
strong covalency, and hence the electronic structure of 7 is best
described being somewhere in between the resonance structures
and H of the fluxional cod(2) give two relevant NOE cross-
1 4
peaks with the pyridine A and B protons. Therefore, cod(2) is
coordinated to the “tetrahedral” Rh2 atom. Accordingly, the
olefinic protons of the unsymmetrical cod(1) ligand give rise to
A
B
A
7
and 7 , with a rather large relative contribution of 7 .
7
527
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Inorganic Chemistry
ARTICLE
1
13
1
Figure 3. H NMR spectrum in C
6
D
6
of 7. A selected region of the C{ H}-apt NMR spectrum is shown in the inset.
Scheme 5. Spontaneous Proton Transfer Induced by Chloride
Coordination Producing 7
dinuclear complex [(cod)Rh(bpaꢀ2H)Rh(cod)] (7) differs in
+
only a proton from the cationic [(cod)Rh(bpaꢀH)Rh(cod)] ,
I
37
which contains two Rh centers. Therefore, we decided to
investigate the relationships between these compounds. Indeed, the
+
addition of 1 mol of base to [(cod)Rh(bpaꢀH)Rh(cod)] gives 7
cleanly and quantitatively. The abstraction of a proton from cationic
+
[
(cod)Rh(bpaꢀH)Rh(cod)] is thus accomplished with a strong
I
electronic reorganization, best described as a reduction of Rh
ꢀI
2ꢀ
to Rh with oxidation of the dianionic {bpaꢀ2H} ligand to a
neutral imine ligand PyCH NdCHPy. To the best of our knowl-
2
edge, this ligand-to-metal electron transfer upon deprotonation of
+
[
(cod)Rh(bpaꢀH)Rh(cod)] to produce the redox unsymmetric
38
dinuclear rhodium complex 7 is an unprecedented reaction.
Moreover, under specific conditions, the dinuclear cationic complex
evolves to 7 even in the absence of an external base (see Scheme 5).
While orange solutions of [(cod)Rh(bpaꢀH)Rh(cod)]PF in
6
1
acetone-d show a sharp H NMR spectrum, the addition of one
6
1
the corresponding NOE cross-peaks with the B proton, and with
molar equivalent of [PPN]Cl as a chloride donor ([PNN]Cl d
4
A with smaller intensity, so that cod(1) is coordinated to the
bis(triphenylphosphine)iminium chloride) leads to a color change
1
square-planar Rh1 atom.
of the solution from orange to red. The H NMR signals of this
The fluxional behavior undergone by cod(2) requires some
comments. Apparently, the exchange can be described as the
rotation of this cod ligand around the rhodium atom. Moreover,
the observation of this fluxional process, which cannot be frozen
mixture are substantially broadened. Chloride coordination to the
organometallic cation must be the origin of the enhanced fluxion-
ality. The disappearance of the AB pattern signals of the methylenic
protons clearly reveals the cleavage of the amido bridge and points
to the involvement of the neutral species A (Scheme 5). The
even on cooling to ꢀ80 °C in toluene-d , indicates that it pos-
8
sesses a low energy barrier. The fast rotation of cod(2) around the
addition of toluene-d to the same NMR tube leads to a further color
8
Rh2 center is in good agreement with the “tetrahedral” geometry of
change to dark-red. At this point, lowering the polarity of the
reaction medium by faster evaporation of acetone than toluene
produces spectra showing only complex 7, while a yellow solid
8
I
this metal. For most square-planar d Rh complexes, this is a much
3
6
ꢀI
B
higher-energy process. In the Rh (π-imine) description of 7
(
10
Scheme 4), the d configuration of the “tetrahedral” Rh2 leads to a
precipitates. The yellow compound was isolated and further charac-
+
ligand field stabilization energy of zero (LFSE = 0), which may
terized as the complex double salt [Rh(bpa)(cod)] [RhCl -
2
ꢀ
further contribute to the easy rotation of cod(2).
(cod)] ([1][RhCl (cod)]; Scheme 5).
2
ꢀ
I
I
Synthesis of [(cod)Rh (bpaꢀ2H)Rh (cod)] (7) from
Species A is, in fact, a derivative of the hypothetical amido
compound [Rh(bpaꢀH)(cod)] with a “RhCl(cod)” fragment
I
I
+
[
(cod)Rh (bpaꢀH)Rh (cod)] . The two-electron mixed-valence
7
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Inorganic Chemistry
ARTICLE
Scheme 6. Disproportionation/Comproportionation Equi-
libria between [M(cod)(bpaꢀH)] and
a
[
(cod)M(bpaꢀ2H)M(cod)] + bpa
Figure 4. DFT calculated HOMO of the hypothetical Rh compound 5
and a simplified MO scheme explaining the net π-bonding between Rh
and the amido fragment. The HOMO of 2 is very similar.
a
18a
Relative energies (ΔE) of the equilibria were obtained with DFT
calculations.
coordinated to the pendant pyridyl ring. Consequently, it under-
goes the same atypical acid/base behavior giving the {bpaꢀ2H}
ligand contained in 7 and bpa coordinated in the cation/anion
complex double salt. This result represents a second example for
the intrinsic instability of the “Rh(bpaꢀH)(cod)” framework
and its tendency toward acid/base disproportionation.
pounds, we still wondered why the rhodium amido complex [Rh-
(bpaꢀH)(cod)] (5) spontaneously disproportionates to free
bpa and the dinuclear complex [(cod)Rh(bpaꢀ2H)Rh(cod)]
(7). Are species 2 and 5 both intrinsically unstable with respect to
disproportionation to 8/7 and free bpa with a kinetic barrier
preventing this reaction from occurring for the iridium species?
Or is formation of rhodium species 7 from 5 thermodynamically
favored while formation of 8 from 2 is unfavorable? We tried to
answer these questions through a set of additional experiments
and DFT calculations.
We first calculated the properties of the elusive compound
[Rh(bpaꢀH)(cod)] (5) with DFT, in order to compare its
structure and its frontier orbitals with those of the iridium
analogue 2. These calculations do not reveal any unusual
geometrical differences between 2 and 5, which are almost
completely isostructural, and also the frontier orbitals of these
species are nearly identical. Since the filled p-type amido lone pair
ꢀ
I
I
Synthesis of Homodinuclear Ir ,Ir and Heterodinuclear
ꢀ
I
I
ꢀ
Rh ,Ir Complexes. Considering that [Rh(bpaꢀ2H)(cod)]
ꢀ
(
[6] ) is a likely intermediate in the reaction giving [(cod)Rh-
bpaꢀ2H)Rh(cod)] (7), we thought that the iridium counter-
(
ꢀ
ꢀ
part [Ir(bpaꢀ2H)(cod)] ([3] ) could be a useful precursor for
the generation of Ir analogs of this new class of two-electron
mixed-valence complexes. Consistent with these ideas, complex
ꢀ
ꢀ
[
Ir(bpaꢀ2H)(cod)] ([3] ) reacts with [{Ir(μ-Cl)(cod)} ] to
2
give the dinuclear neutral complex [(cod)Ir(bpaꢀ2H)Ir(cod)]
ꢀ
(
8). A similar reaction of anionic [3] with [{Rh(μ-Cl)(cod)} ]
2
produces the heterometallic [(cod)Rh(bpaꢀ2H)Ir(cod)] (9).
Both complexes were isolated as air-sensitive red-brown solids.
The homobimetallic complex 8 is more easily obtained, in a pure
of the ligand must interact with the filled metal d
orbitals of the
π
8
d
transition metals, this unfavorable interaction could be
19a,39
form and in good yields, by reacting [{Ir(cod)(μ-OMe)} ] with
considered as a π conflict,
which could in principle destabilize
2
bpa in a 1:1 molar ratio in diethyl ether, while reaction of the
isolated iridium-amido complex 2 with [{Rh(cod)(μ-OMe)}2]
in a 2:1 molar ratio produces the heterodinuclear Rh,Ir complex 9
in a more selective manner. Detailed NMR spectroscopic
information for these complexes is given in the Experimental
Section. The X-ray geometry and the bond lengths of 7 and 8 are
very similar (Table 1).
rhodium complex 5 stronger than iridium complex 2. However,
for both species this repulsive interaction is counterbalanced by
an increased π back donation from the metal into the π* orbital
of the cod double bond trans to the amido fragment, thus
resulting in some bonding character of the resulting HOMO.
The overall effect is then a net π-bonding between rhodium or
iridium and the amido fragment (Figure 4).
Back-donation is generally stronger for iridium than for
rhodium, which might in part explain the increased stability of
iridium complex 2 compared with that of rhodium complex 5.
This is confirmed by the large upfield shifts of the olefinic carbons
trans to the amido fragment in 2. The DFT calculations show that
the dominating differences between 2 and 5 are mainly reflecting
the stronger σ interactions of the MꢀN bonds in 2 (Ir)
compared to 5 (Rh), which could contribute to the relative
stability of 2.
13
103
For the Rh,Ir compound 9, Cꢀ Rh couplings in combina-
1
1
tion with Hꢀ H NOESY experiments unequivocally establish
that the “Rh(cod)” fragment is coordinated in the “tetrahedral
π-imine compartment”, while the Ir(cod) fragment is coordi-
nated in the “square planar compartment”. The NMR data of 9 are
clean and clearly confirm the proposed structure. However, complex
9
reveals quite unusual chemical shifts of the “imine” (and some of
the pyridine signals) in the NMR spectra as compared to those of 7
13
and 8. The C signal of the bridging CdN moiety shifts upfield
from δ 82 ppm in the Rh,Rh compound 7 to 72 ppm in the Ir,Ir
compound 8, thus reflecting the stronger covalency of the IrꢀN
bond. For the Rh,Ir compound 9, one would expect an intermediate
chemical shift; however, it is actually observed downfield relative to
While the above data hint to a better stabilization of the
ꢀ
{bpaꢀH} ligand by iridium, it still does not provide a straight-
forward explanation of why Rh complex 5 is not an isolable
compound. Hence, to shed some more light on this problem, we
performed some additional experiments and DFT calculations to
investigate the equilibrium associated with the reactions shown
in Scheme 6. Quite remarkably, reaction of the dinuclear iridium
complex [(cod)Ir(bpaꢀ2H)Ir(cod)] (8) with the free bpa
ligand in toluene quantitatively produces the mononuclear
complex [Ir(bpaꢀH)(cod)] (2, Scheme 6). However, no reac-
tion of the dinuclear Rh complex [(cod)Rh(bpaꢀ2H)Rh(cod)]
7
(δ = 93 ppm). This behavior seems to be temperature dependent,
but presently we do not entirely understand it. It seems that
compound 9 is involved in a fluxional behavior, which we are
currently investigating.
Electronic Structures and Thermodynamic Stability of
Amido Complexes [M(bpaꢀH)(cod)] (M = Rh, Ir). Since the
dinuclear compounds 7ꢀ9 are all isolable and stable com-
7
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Inorganic Chemistry
ARTICLE
Scheme 7. Replacing Rh by Ir in the “Square Planar” Com-
partment and Replacing Ir by Rh in the “Tetrahedral
Compartment”
Scheme 9. Oxygenation of 7 and 8 to Form the Carboxamido
Complexes 10 and 11
Scheme 8. Comproportionation of 7 and 8 to Form Two
a
Molecules of 9
a
The relative energy (ΔE) for the comproportionation equilibrium was
obtained with DFT calculations.
Figure 5. Molecular structure (ORTEP at 50% level) of complex 10.
(
7) with bpa occurs at all, even in a 1:5 ratio molar ratio at 60 °C
in C D for 2 h. This means that the equilibrium shown in
6
6
Scheme 6 lies almost entirely to the right for iridium (in black),
while the corresponding equilibrium for the analogous Rh
complexes lies almost entirely to the left (in red).
Further experiments and DFT calculations clearly reveal that
Rh feels most comfortable in the “tetrahedral π-imine compart-
ment”, while Ir is most comfortable in the “square planar”
compartment. Reaction of [(cod)Rh(bpaꢀ2H)Rh(cod)] (7)
Because the metal in the imine compartment is best described
as being in the ꢀI oxidation state (vide supra), the reaction in
Scheme 6 is in fact an oxidative process in which one of the
with [IrCl(cod)PPh ] in C D at 60 °C produces almost
3 6 6
quantitatively [(cod)Rh(bpaꢀ2H)Ir(cod)] (9) and [RhCl-
ꢀ
I
I
iridium atoms is being oxidized from Ir to Ir . Since it is well
know that the tendency to undergo oxidative processes is
generally higher for the third row transition metals as compared
to the second row transition metals, it is understandable that the
equilibrium in Scheme 6 lies to the right for iridium while the
corresponding equilibrium for the analogous rhodium complexes
lies to the left. This provides a straightforward explanation for the
fact that the amido rhodium complex 5 cannot be prepared, while
(cod)PPh ] (see also the Supporting Information). Thus, ir-
3
idium replaces rhodium in the “square-planar compartment” in
this reaction. Similarly, the reaction of [(cod)Ir(bpaꢀ2H)-
Ir(cod)] (8) with [RhCl(cod)PPh ] under the same conditions
3
produces almost quantitatively 9 (and [IrCl(cod)PPh ]). In this
3
case, rhodium replaces iridium in the “tetrahedral π-imine com-
partment” (Scheme 7).
Moreover, reaction of the dinuclear Rh complex [(cod)Rh-
(bpaꢀ2H)Rh(cod)] (7) with the dinuclear Ir complex [(cod)-
40
the iridium analogue 2 is a stable compound. It is clear that the
amido rhodium complex 5 is thermodynamically unstable toward
disproportionation and readily forms the dinuclear complex 7
and the free bpa ligand. The thermodynamic differences are
Ir(bpaꢀ2H)Ir(cod)] (8) in C
D at 60 °C produces almost
6 6
quantitatively the heterodinuclear complex 9 (Scheme 8) instead
of a statistical mixture of the complexes in a 1:2:1 ratio (see
Supporting Information).
ꢀ
I
readily explained by a higher stability of the M oxidation state
in the dinuclear complexes for rhodium as compared to iridium.
DFT calculations in the gas phase are in good qualitative agree-
ment with these experimental observations (see Scheme 6).
Therefore, the two main reactions commented upon above,
DFT calculations in the gas phase are in good qualitative
agreement with these experiments. The comproportionation of 7
ꢀ1
and 8 to 9 is exothermic by ∼5 kcal mol according to these
calculations (Scheme 8). The calculations further reveal that the
[(cod)Rh(bpaꢀ2H)Ir(cod)] complex 9 with Rh in the “tetra-
hedral π-imine compartment” and Ir in the “square planar”
+
t
[
M(bpa)(cod)] with KO Bu (1 molar equivalent) and [(cod)-
M(bpaꢀ2H)M(cod)] + bpa, can be regarded as proton coupled
electron transfer reactions, with an acid/base component and a
hidden redox process involving the reduction of one of the metals
in the dinuclear entity. In this perspective, the expected acid/base
behavior prevails over the redox part for the iridium complexes,
while the redox process dominates over the acid/base contribu-
tion for the rhodium ones, and consequently, they undergo a
cooperative reductive double deprotonation to the mixed-va-
ꢀ
1
compartment is ∼4 kcal mol more stable than the hypothetical
ꢀ
I
I
0
[(cod)Ir (bpaꢀ2H)Rh (cod)] isomer 9 with Ir in the “tetra-
hedral π-imine compartment” and Rh in the “square planar”
compartment.
Oxygenation of Complexes 7 and 8. In line with the electron-
rich nature of their metalate(ꢀI) centers, the complexes 7 and 8 are
very air-sensitive and react rapidly with O in benzene (Scheme 9).
2
ꢀ
I
I
lence Rh ,Rh compound.
The reactions proceed with the formation of the carboxamido
7
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Inorganic Chemistry
ARTICLE
Table 2. Selected Bond Distances (Å) for Complex 10
iridium, the spontaneity of the process ([8] + bpa f2 [2]) is
reversed from that occurring for rhodium (2 [5] f [7] + bpa).
These thermodynamic differences are readily explained by a
RhꢀN1
RhꢀN2
RhꢀCt1
RhꢀCt2
2.095(3) [2.144(3)]
2.059(2) [2.051(2)]
2.013(3) [2.012(3)]
2.015(3) [2.016(3)]
1.403(4) [1.389(3)]
1.383(4) [1.395(3)]
1.509(4) [1.498(4)]
1.340(4) [1.336(4)]
N1ꢀC1
C1ꢀC2
C2ꢀC3
C3ꢀC4
C4ꢀC5
C5ꢀN1
C6ꢀO
1.342(4) [1.342(4)]
1.387(4) [1.380(4)]
1.379(5) [1.384(4)]
1.389(5) [1.384(4)]
1.382(4) [1.382(4)]
1.348(4) [1.348(4)]
1.243(4) [1.246(3)]
1.457(4) [1.456(4)]
ꢀI
lower stability of the M oxidation state for iridium as compared
a
a
to rhodium in the dinuclear complexes 8 and 7. The formation of
I
two equivalents of [Ir (bpaꢀH)(cod)] from 8 and free bpa is an
C13ꢀC14
C17ꢀC18
C5ꢀC6
oxidative process, which is favorable for iridium. Disproportiona-
I
tion of [Rh (bpaꢀH)(cod)] into 7 and free bpa is a reductive
process, which is favorable for rhodium. The observed reductive
deprotonation process leads to highly unusual structures and
unexpected reactivities, which underlines the general importance
of the substantial effect of “redox noninnocent ligands” on the
C6ꢀN2
N2ꢀC7
a
Ct1 and Ct2 are the middle points between C13 and C14 and C17 and
C18, respectively. In brackets, the data for the second independent
molecule are given.
4
2
electronic structure and catalytic activity of transition metals.
complexes [M(bpamꢀH)(cod)] (M = Rh (10); Ir (11); bpam =
’
EXPERIMENTAL SECTION
N-(2-picolyl)picolinamide). Monitoring the reactions by NMR
further revealed the formation of [{M(cod)(μꢀOH)} ] (M =
2
General methods. All procedures were performed under an argon
or N atmosphere, using standard Schlenk techniques. Solvents were
Rh, Ir) in roughly equimolar amounts. These reactions are similar
2
36
to those previously communicated for the Rh(nbd) analogs.
Complex 11 is also the result of the reaction of the anionic complex
43
dried and distilled under argon before use by standard methods. NMR
experiments were carried out on Bruker AV 500, AV 400, and DPX 200
ꢀ
18a
1
[
3] with oxygen.
Complex 10 was identified by comparing its spectroscopic
data with those of pure samples obtained from the reaction of
{M(cod)(μ-OMe)} ] with bpam in diethyl ether (see the
spectrometers operating at 500, 400, and 200 MHz for H, respectively.
Chemical shifts are reported in parts per million and referenced to
SiMe , using the internal signal of the deuterated solvent as a reference.
4
[
The bpa numbering for the complexes correspond to that in Figure 3
2
4
4
Experimental Section). From these solutions, complex 10 was
obtained as orange single crystals whose molecular structure is
shown in Figure 5. Selected bond distances and angles are
collected in Table 2.
2
for 7. The complexes [{M(cod)(μ-OMe)} ], [Ir(bpa)(cod)]PF6
+
17
+ 21a
([1] ), and [Rh(bpa)(cod)]PF ([4] ) were prepared according
6
to the literature descriptions. All other chemicals are commercially
available and were used without further purification.
Coordination of the bpamꢀH ligand to rhodium in 10 occurs
through the amido nitrogen (N2) and the nitrogen (N1) of the
carbonyl-bound pyridyl ring. The square planar coordination
geometry around rhodium is completed by the chelating cod
ligand. The amido N2 and the carbon C6 are strictly planar [sum
[(cod)Rh(bpaꢀ2H)Rh(cod)] (7). Liquid bis(2-picolyl)amine
(dpa, 97%) (55.7 μL, 0.31 mmol) was added to a yellow suspension
2
of [{Rh(cod)(μ-OMe)} ] (150.0 mg, 0.31 mmol) in 10 mL of diethyl
ether. After stirring for 15 min, the resulting dark-red/brown solution
was concentrated to ca. 7 mL, layered with pentane (10 mL), and left in
the fridge (4 °C) overnight. The mother liquor was decanted, and the
solid was washed with pentane (2 ꢁ 5 mL) and vacuum-dried. Yield:
0
of the angles around the nitrogen ∑ = 359.8° (359.7° for the
0
second independent molecule) and ∑ = 360.0° (360.0°) for the
1
47.1 mg (77%). Crystals suitable for X-ray diffraction resulted by
carbon]. The five-membered metallacycle was found to be
almost planar [the maximum deviation of the plane defined by
Rh, N1, C5, C6, N2 is 0.019 Å (0.028 Å)]. These data clearly
layering with hexane the ethereal red-brown solution mentioned above.
1
A1
H NMR (500 MHz, C
6 6
D , 25 °C): δ 8.76 (br d, J = 4.2 Hz, 1H, H ),
A4 A3
2
7.15 (d, J = 7.6 Hz, 1H, H ), 7.01 (td, J = 7.6, 1.7 Hz, 1H, H ), 6.80
d, J = 6.5 Hz, 1H, H ), 6.58 (m, 2H, H and H ), 6.20 (d, J = 8.5 Hz,
H, H ), 5.78 (td, J = 6.5, 1.2 Hz, 1H, H ), 4.75 (m, 2H, H ), 4.33 (s,
H, HCdN), 4.22 (δ , 1H), 4.12 (δ , J = 17.0 Hz, 1H, CH ), 4.04
m, 1H, H ), 3.70 (m, 3H, H , H , and H ), 3.41 (m, 2H, H ), 2.71
m, 1H, H ), 2.53 (m, 2H, H ), 2.47 (m, 1H, H ), 2.40 (m, 1H,
), 2.31 (m, 1H, H ), 2.16 (m, 2H, H ), 2.12 (m, 2H, H ), 2.05
m, 1H, H ), 2.01 (m, 1H, H ), 1.81 (m, 2H, H ), 1.77 (m, 1H,
reflect an sp hybridization for the N2 and C6 atoms. In addition,
B1
B3
A2
(
1
1
(
(
H
(
H
the N1ꢀC5 and N2ꢀC6 distances are ca. 0.15 Å shorter than
C5ꢀC6, pointing to a single bond between the carbon atoms and
some multiple bond character for both CꢀN bonds.
B4
B2
21
A
B
A,B
2
22
1
1
12
15
16
The formation and isolation of the rhodium carboxamido
compound 10 is in sharp contrast with the thermodynamic
instability of the (structurally and electronically similar) amido
complex 5. Delocalization of the lone pair of N2 to the adjacent
carbonyl (partly removing its π interactions with the filled metal
1
4a
24a
18a
1
3a
17a
24b
23a
14b
18b
23b
17b
13b 13
1
), 1.71 (m, 1H, H ). C{ H} NMR (125 MHz, C D , 25 °C):
6 6
A5
A1
B5
B1
A3
δ 162.2 (C ), 149.2 (C ), 148.5 (C ), 143.0 (C ), 135.6 (C ),
d orbitals) and the inability of 10 to host another metal atom are
B3
A2
A4
B4
B2
π
1
(
1
7
3
30.6 (C ), 121.4 (C ), 121.0 (C ), 115.5 (C ), 110.1 (C ), 82.2
41
the most likely explanations for this different behavior.
11
d, JC,Rh = 7 Hz, HCdN), 79.1 (d, J
= 14 Hz, C ), 76.2 (d, J
= 12 Hz, C ), 73.1 (d, J
=
C,Rh
12
C,Rh
16
1
5
3 Hz, C ), 73.9 (d, J
= 12 Hz, C ),
C,Rh
2
C,Rh
21
2
2
2.7 (d, JC,Rh = 14 Hz, C ), 67.1 (d, JC,Rh = 14 Hz, C ), 60.5 (CH ),
’
SUMMARY AND CONCLUSIONS
I
2
4
18
14
23
17
13
3.6 (C ), 32.7 (C ), 31.7 (C ), 31.2 (C ), 30.5 (C ), 29.6 (C ).
Rh (619.4): C, 54.29 (54.18); H,
+
Deprotonation of the [Ir (bpa)(cod)] complex with 1 mol
Anal. Calcd. (Found) for C28
.69 (5.60); N, 6.78 (6.84).
Alternatively, complex 7 can be prepared as follows in thf: Solid
H
35
N
3
2
equiv of a strong base leads to quantitative formation of the
5
I
neutral monodeprotonated and mononuclear [Ir (bpaꢀH)-
I
t
(
(
cod)] (2) species. All attempts to prepare the [Rh (bpaꢀH)-
KO Bu (62.0 mg, 0.55 mmol) was added to a yellow solution of [4]PF
278.0 mg, 0.50 mmol) in 10 mL of thf. An immediate color change to
6
cod)] analog led to spontaneous cooperative reductive double
deprotonation of the [Rh (bpa)(cod)] complex, thus producing
(
I
+
purple-brown was observed as a 1:1 mixture of bpa, and 7 was obtained.
ꢀ
I
I
ꢀI
I
the Rh ,Rh mixed-valence [(cod)Rh (bpaꢀ2H)Rh (cod)]
The resulting mixture (7 contaminated with KPF ) was evaporated,
6
(
7) species. The dinuclear iridium analog 8 can be prepared
washed with pentane, dried under vacuum conditions, and directly
1
but is unstable in the presence of 1 mol equiv of the free bpa
ligand, leading to the quantitative formation of 2. Hence, for
analyzed by NMR. H NMR (200 MHz, thf-d , 25 °C): δ 8.81 (d, J = 5
8
A1
A3
A4
Hz, 1H, H ), 7.63 (m, 1H, H ), 7.33 (d, J = 8 Hz, 1H, H ), 7.11 (m,
7
531
dx.doi.org/10.1021/ic200395m |Inorg. Chem. 2011, 50, 7524–7534

Inorganic Chemistry
ARTICLE
A2
B1
B3
B4
1
(
1
(
H, H ), 6.96 (m, 2H, H and H ), 6.48 (d, J = 8 Hz, 1H, H ), 6.11
(δ
A
, 1H), 4.58 (δ
B
, JA,B = 16.5 Hz, 1H) (CH
2
bpa), 4.08 (br, 2H, dCH,
B2
m, 1H, H ), 4.48 (m, 2H, cod-CH), 4.19 (s, 1H, CHdN), 4.16 (δ ,
Rh(cod)), 3.91 (m, 1H, dCH, Ir(cod)), 3.54 (br, 2H, dCH, Rh(cod)),
3.40 (m, 2H, dCH, Ir(cod)), 3.25 (m, 1H, dCH, Ir(cod)), 2.52 (m, 1H,
CH
Rh(cod)), 1.91 (m, 2H, CH , Rh(cod)), 1.83 (m, 1H, CH , Ir(cod)),
1.74 (m, 5H, 4 from CH , Rh(cod) and 1 from CH , Ir(cod)). C{ H}
2 2
NMR (125 MHz, C D , 25 °C): δ 161.9 (C ), 149.1 (C ), 141.5 (C ),
136.4 (C ), 134.6 (C ), 128.1 (C ), 121.6 (C ), 121.1 (C ), 116.5
(C ), 110.3 (C ), 92.7 (d; J
A
H), 3.99 (δ
m, 2H, cod-CHd), 2.64 (m, 2H, cod-CH
cod-CHd), 1.95 (m, 8H, cod-CH ), 1.57 (m, 2H, cod-CH ). C{ H}
NMR (50 MHz, thf-d , 25 °C): δ 162.91 (C ), 150.99 (C ), 144.08
C ), 137.13 (C ), 132.17 (C ), 122.90 (C ), 122.18 (C ), 116.69
C ), 111.48 (C ), 81.88 (d, JC,Rh = 8 Hz, CHdN), 82.29 (d, JC,Rh
3 Hz, cod-CHd), 79.12 (d, JC,Rh = 12 Hz, cod-CHd), 76.51 (d, JC,Rh
2 Hz, cod-CHd), 75.54 (d, JC,Rh = 14 Hz, cod-CHd), 73.01 (d, JC,Rh
4 Hz, cod-CHd), 66.98 (d, JC,Rh = 14 Hz, cod-CHd), 61.23 (CH
4.58 (cod-CH ), 33.47 (cod-CH ), 32.40 (cod-CH ), 31.81 (cod-CH
1.54 (cod-CH ), 30.57 (cod-CH ).
(cod)Ir(bpaꢀ2H)Ir(cod)] (8). [(cod)Ir(bpaꢀ2H)Ir(cod)] (8)
can be prepared as described for 7 starting from bis(2-picolyl)amine
bpa, 97%) (45.1 μL, 0.23 mmol) and [{Ir(cod)(μ-OMe)} ] (150.0 mg,
.23 mmol). Yield: 153.5 mg (85%). Crystals suitable for X-ray diffrac-
tion resulted by layering with hexane the ethereal red-brown solution
mentioned above. H NMR (500 MHz, C
B
, JA,B = 17 Hz, 1H, CH
2
), 3.68 (br s, 3H, cod-CHd), 3.01
2
), 2.44 (m, 5H, cod-CH and
2
2 2 2
, Ir(cod)), 2.33 (m, 5H, CH , Ir(cod)), 2.01 (m, 2H, CH ,
1
3
1
2
2
A1
2
2
A5
13
1
8
B1
A3
B3
A2
A4
A5
A1
B1
(
(
6 6
B4
B2
A3
B5 B3 A2 A4
=
=
=
),
),
B4
B2
1
1
1
3
3
= 5.1 Hz; HCdN), 73.7 (d, JRh,C = 14.7
Rh,C
Hz), 72.3 (d, JRh,C = 13.7 Hz; dCH, Rh(cod)), 62.6, 58.7, 56.9, 56.4, (dCH,
Ir(cod)), 60.7 (CH bpa), 34.6, 33.4, 31.1, and 30.4 (CH , Ir(cod)), 32.0
and 31.6 (CH , Rh(cod)). Anal. Calcd. (Found) for C28 IrRh (708.7):
C, 47.45 (47.37); H, 4.98 (4.87); N, 5.93 (5.76).
Alternatively, complex 9 can also be prepared in a similar way as
described for the synthesis of 8 in thf, using [{Rh(μ-Cl)(cod)} ] instead
, 25 °C): δ 8.49 (d,
J = 5 Hz, 1H, H ), 7.62 (t, J = 7 Hz, 1H, H ), 7.42 (d, J = 8 Hz, 1H,
2
2
2
2
2
2
2
2
35 3
H N
2
2
[
2
1
(
0
of [{Ir(μ-Cl)(cod)} ]. H NMR (200 MHz, thf-d
2 8
2
A1
A3
A4
A2
B1
B4
B2
H ), 7.10 (m, 2H, H and H ), 6.92 (m, 2H, H and H ), 6.18 (m,
1
B3
D
6
, 25 °C): δ 9.10 (br d, J =
1H, H ), 6.02 (s, 1H, CHdN), 4.36 (m, 4H, cod-CHd and CH
3.71 (m, 2H, cod-CHd), 3.56 (m, 2H, cod-CHd), 3.29 (m, 2H, cod-
CHd), 2.02 (m, 16H, cod-CH ). C{ H} NMR (50 MHz, thf-d ,
2 8
25 °C): δ 162.1 (C ), 149.6 (C ), 141.9 (C ), 137.0 (C ), 136.3
(C ), 129.1 (C ), 122.2 (C ), 121.6 (C ), 117.3 (C ), 111.1 (C ),
93.1 (d, JC,Rh = 5 Hz, CHdN), 72.6 (d, JC,Rh = 14 Hz, cod-CHd), 70.8 (d,
2
),
6
A1
B1
5
1
1
.3 Hz, 1H, H ), 7.52 (br d, J = 5.6 Hz, 1H, H ), 6.73 (td, J = 8.4,
B3
A3
13
1
.3 Hz, 1H, H ), 6.71 (td, J = 7.7, 1.4 Hz, 1H, H ), 6.44 (d, J = 8.4 Hz,
B4
A4
A5
A1
B1
A3
H, H ), 6.37 (d, J = 7.7 Hz, 1H, H ); 6.21 (td, J = 7.7, 1.1 Hz, 1H,
A2
B2
16
B5 B3 A4 A2 B4 B2
H ), 5.79 (td, J = 8.4, 1.2 Hz, 1H, H ), 4.20 (m, 1H, H ), 4.07
2
1
(
(
δ
A
, 1H), 3.98 (δ
B
, JA,B = 17.2 Hz, 1H, CH
2
), 4.04 (m, 2H, H ), 3.95
15
12
11
m, 1H, H ), 3.54 (m, 1H, H ), 3.42 (m, 1H, H ), 3.30 (s, 1H,
HCdN), 3.09 (m, 2H, H ), 2.78 (m, 2H, H ), 2.71 (m, 1H, H ),
.67 (m, 1H, H ), 2.55 (m, 1H, H ), 2.49 (m, 1H, H ), 2.16 (m, 2H,
), 2.11 (m, 2H, H ), 1.96 (m, 1H, H ), 1.94 (m, 1H, H ), 1.91
m, 1H, H ), 1.81 (m, 1H, H ), 1.38 (m, 2H, H ). C{ H} NMR
125 MHz, C D , 25 °C): δ 175.4 (C ), 163.9 (C ), 153.1 (C ),
43.3 (C ), 133.7 (C ), 132.9 (C ), 122.0 (C ), 121.7 (C ), 117.4
J
C,Rh = 14 Hz, cod-CH=), 62.6 (cod-CHd), 60.9 (CH
2
), 58.7 (cod-CHd),
), 33.6 (cod-CH ),
2.4 (cod-CH ), 31.9 (cod-CH ), 31.5 (cod-CH ), 30.8 (cod-CH ).
24a
24b
17a
56.8 (cod-CHd), 56.4 (cod-CHd), 34.8 (cod-CH
3
2
2
18a
14a
13a
2
H
(
(
1
(
5
3
2
2
2
2
2
3a
22
14b
18b
[
Rh(bpamꢀH)(cod)] (10). Solid N-(2-picolyl)picolinamide (bpam;
34.3 mg, 0.63 mmol) was added to a solution of [{Rh(cod)(μ-OMe)}
150.0 mg, 0.31 mmol) in toluene (5 mL). An immediate orange solution
was formed after mixing the reagents. The solution was evaporated to ca.
mL, layered with hexane (10 mL), and kept undisturbed in the freezer at
30 °C overnight to render orange crystals, which were washed with cold
13b
17b
23b 13
1
1
(
2
]
B5
A5
A1
6 6
B1
A3 B3 A2 A4
B4
B2
15
12
2
ꢀ
2
C ), 114.4 (C ), 71.9 (HCdN), 65.2 (C ), 60.8 (CH ), 59.6 (C ),
1
1
16
22
21
24
14
5.2 (C ), 55.1 (C ), 53.2 (C ), 48.5 (C ), 40.0 (C ), 33.0 (C ),
1
1
3
17
18
23
hexane (3 ꢁ 3 mL) and vacuum-dried. Yield: 223.7 mg (85%). H NMR
2.5 (C ), 32.0 (C ), 31.4 (C ), 30.3 (C ). Anal. Calcd. (Found) for
A1
(
(
(
1
5
400 MHz, C D , 25 °C): δ 8.53 (ddd, J = 4.8, 1.8, 0.9 Hz, 1H, H ), 8.09
6 6
C H N Ir (798.0): C, 42.14 (42.25); H, 4.42 (4.31); N, 5.26 (5.54).
2
8 35 3 2
B4 A4
ddd, J = 7.8, 1.5, 0.7 Hz, 1H, H ), 7.92 (d, J = 7.9 Hz, 1H, H ), 7.15
Alternatively, complex 8 can be prepared as follows: Two equivalents
A3
B1
t
td, J = 7.7, 1.8 Hz, 1H, H ), 6.87 (d, J = 5.3 Hz, 1H, H ), 6.83 (td, J = 7.7,
.5 Hz, 1H, H ), 6.62 (ddd J= 7.4, 4.8, 1.1 Hz, 1H, H ), 6.24(ddd, J=7.3,
.3, 1.5 Hz, 1H, H ), 4.92 (s, 2H, CH
of KO Bu (0.123 g, 1.1 mmol) were added to the light yellow solution
of [1]PF (0.323 g, 0.50 mmol) in 10 mL of thf. [{Ir(μ-Cl)(cod)}
0.099 g, 0.2 mmol) was added to the resulting red-brown solution. The
resulting mixture (8 still contaminated with KPF and KCl salts) was
evaporated, washed with pentane, dried in vacuo, and directly analyzed
B3
A2
6
2
]
B2
bpam
2
), 4.60 (m, 2H), 3.50 (m, 2H),
(
exo
2
endo
2
HC= (cod), 2.19 (m, 4H, CH
(cod)), 1.65 (m, 4H, CH
(cod)).
6
1
3
1
C{ H} NMR (100 MHz, C D , 25 °C): δ 173.2 (d, JC,Rh = 1.5 Hz, CO),
6 6
A1 B1 B3
1
A5
B5
by NMR. H NMR (200 MHz, thf-d
H ), 7.57 (m, 2H, H and H ), 7.27 (m, 2H, H and H ), 7.01 (t,
J = 6 Hz, 1H, H ), 6.73 (d, J = 8 Hz, 1H, H ), 6.31 (m, 1H, H ), 4.27
8
, 25 °C): δ 9.17 (d, J = 5 Hz, 1H,
163.2 (C ), 157.3 (C ), 148.9 (C ), 144.3 (C ), 138.8 (C ), 135.9
A1
A3
B1
A4
B3
A3
B2
B4
A4
A2
(C ), 125.3 (C ), 125.2 (C ), 123.0 (C ), 121.1 (C ), 84.1 (d, JC,Rh
12.9 Hz) and 77.0 (d, JC,Rh = 12.1 Hz; HCd (cod), 51.1 (d, JC,Rh = 1.5 Hz,
=
A2
B4
B2
bpam
2
(
(
m, 2H, CH
m, 2H, cod-CHd), 3.30 (s, 1H, CHdN), 3.20 (m, 2H, cod-CHd),
.72 (m, 4H, cod-CH ), 2.0ꢀ1.5 (m,
), 2.43ꢀ2.24 (m, 2H, cod-CH
, 25 °C): δ
2
), 4.01 (m, 2H, cod-CHd), 3.84 (m, 2H, cod-CHd), 3.36
CH
(423.3): C, 56.74 (56.81); H, 5.24 (5.11); N, 9.93 (9.91).
[Rh(bpa)(cod)][RhCl (cod)] was independently prepared as fol-
lows: bis(2-picolyl)amine (bpa, 97%) (37.5 μL, 0.21 mmol) was added to a
solution of [{Rh(μ-Cl)(cod)} ] (100.0 mg, 0.21 mmol) in toluene (5 mL).
2 22 3
), 31.3, 30.4 CH (cod). Anal. Calcd. (Found) for C20H N ORh
2
1
1
1
7
(
4
3
2
2
2
13
1
2 8
0H, cod-CH ). C{ H} NMR (50 MHz, thf-d
B5
A5
A1
B1
A3
76.2 (C ), 164.9 (C ), 153.7 (C ), 144.0 (C ), 135.2 (C ),
2
B3
A2
A4
B4
B2
33.7 (C ), 123.1 (C ), 121.06 (C ), 118.1 (C ), 114.9 (C )
1.9 (CHdN), 65.4 (cod-CHd), 61.2 (CH ), 59.7 (cod-CHd), 55.1
cod-CHd), 54.9 (cod-CHd), 53.0 (cod-CHd), 48.1 (cod-CHd),
0.2 (cod-CH ), 33.1 (cod-CH ), 32.8 (cod-CH ), 32.2 (cod-CH ),
1.6 (cod-CH ), 30.2 (cod-CH ).
The initial yellow solution turned pale green while a yellow solid started to
precipitate. After stirring for 30 min, the mother liquor was decanted, and
2
the solid was washed with diethyl ether (2 ꢁ 5 mL) and vacuum-dried.
1
Yield: 123.5 mg (88%). H NMR (400 MHz, CD
2 2
Cl , 25°C): δ8.63 (d, J=
2
2
2
2
A1
A3
4.5 Hz, 2H, H ), 7.59 (t, J = 7.4 Hz, 2H, H ), 7.21 (d, J = 6.6 Hz, 2H,
2
2
A4
A2
[
(cod)Rh(bpaꢀ2H)Ir(cod)] (9). To a solution of [Ir(bpaꢀH)-
H ), 7.16 (t, J = 7.4 Hz, 2H, H ), 6.53 (br t, J = 4.2 Hz, 1H, NH), 4.54 (s,
bpa
exo
2
(
[
cod)] (50.0 mg, 0.10 mmol) in toluene (3 mL) was added solid
4H, CH
4H, CH
1.86 (br s, 4H, CH
2
), 4.18 (br s, 4H, HCd), 2.39 (br s, 4H, CH
; (cod ), 3.85 (br s, 4H, HCd), 2.57 (br s, 4H, CH
) and 1.59 (br s,
exo
2
A
exo
{Rh(cod)(μ-OMe)} ] (24.3 mg, 0.05 mmol). After stirring for 15 min,
hexane (8 mL) was added. The dark-brown solid that precipitated was
2
) and
2
exo
2
B
A
B
; (cod ) [cod is that bound to the anion while cod
filtered off, washed with hexane, and vacuum-dried. Yield: 55.1 mg
is that bound to the cation]. Anal. Calcd. (Found) for C28
(692.3): C, 48.58 (48.38); H, 5.39 (5.55); N, 6.07 (6.20).
37 3 2 2
H N Cl Rh
1
(
77.5%). H NMR (500 MHz, C
.7, 0.8 Hz, 1H, H ), 7.43 (d, J = 7.9 Hz, 1H, H ), 7.06 (td, J = 7.7,
6
D
6
, 25 °C): δ 8.51 (ddd, J = 4.9,
A1
A4
1
1
1
1
Reaction of [(cod)Rh(bpaꢀ2H)Rh(cod)] (7) with [(cod)-
Ir(bpaꢀ2H)Ir(cod)] (8). Solid [(cod)Rh(bpaꢀ2H)Rh(cod)] (7;
A3
B1
.8 Hz, 1H, H ), 7.01 (d, J = 6.6 Hz, 1H, H ), 6.60 (dd, J = 6.7, 5.0 Hz,
A2
B3
ꢀ3
H, H ), 6.50 (ddd, J = 8.6, 6.5, 1.1 Hz, 1H, H ), 6.36 (d, J = 8.7 Hz,
H, H ), 5.78 (td, J = 6.6, 1.4 Hz, 1H, H ), 5.62 (s, 1H, HCdN), 4.72
4.8 mg, 7.8 ꢁ 10 mmol) and [(cod)Ir(bpaꢀ2H)Ir(cod)] (8; 6.2
B4
B2
ꢀ3
mg, 7.8 ꢁ 10 mmol) were introduced in a NMR tube, and C D
6
6
7
532
dx.doi.org/10.1021/ic200395m |Inorg. Chem. 2011, 50, 7524–7534

Inorganic Chemistry
0.5 mL) was added. Evolution of the mixture was monitored by H
NMR at 60 °C. NMR data indicate the clean and almost quantitative
formation of complex [(cod)Rh(bpaꢀ2H)Ir(cod)] (9) after 2 h of
heating (see Supporting Information).
ARTICLE
1
(
Research (NWOꢀCW VIDI project 700.55.426), the European
Research Council (ERC, EU seventh framework program, grant
agreement 202886-CatCIR), and the University of Amsterdam.
M.P.d.R. and L.A. thank GA and MICINN/FEDER, respectively,
for a fellowship.
Reaction of [(cod)Rh(bpaꢀ2H)Rh(cod)] (7) with [IrCl-
(
9
cod)(PPh )]. Solid [(cod)Rh(bpaꢀ2H)Rh(cod)] (7) (6.0 mg,
3
ꢀ
3
ꢀ3
.7 ꢁ 10 mmol) and [IrCl(cod)(PPh )] (5.8 mg, 9.7 ꢁ 10 mmol)
3
6 6
were introduced in an NMR tube, and C D (0.5 mL) was added.
Evolution of the mixture was monitored by H NMR at 60 °C. The
’
REFERENCES
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(
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NMR data indicate the clean and almost quantitative formation of
(
[
(cod)Rh(bpaꢀ2H)Ir(cod)] (9) and [RhCl(cod)(PPh )] after 50 min
3
(
of heating (see Supporting Information).
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Reaction of [(cod)Ir(bpaꢀ2H)Ir(cod)] (8) with [RhCl(cod)-
ꢀ
3
(
PPh )]. Solid [(cod)Ir(bpaꢀ2H)Ir(cod)] (8; 6.0 mg, 7.5 ꢁ 10
ꢀ3
3
mmol) and [RhCl(cod)(PPh )] (3.8 mg, 7.5 ꢁ 10 mmol) were
3
6 6
introduced in a NMR tube, and C D (0.5 mL) was added. Evolution of
1
the mixture was monitored by H NMR at 60 °C. The NMR data
indicate the clean and almost quantitative formation of [(cod)Rh-
(
bpaꢀ2H)Ir(cod)] (9) and [IrCl(cod)(PPh )] after 50 min of heating.
3
DFT Geometry Optimizations. The geometry optimizations
45a,b
were carried out with the Turbomole program
Baker optimizer. Geometries were fully optimized as minima at the
coupled to the PQS
46
47
45c,d
ri-DFT BP86 level using the Turbomole SV(P) basis set
on all
on Rh and Ir). Relativistic
effects were included implicitly through the use of ECP's for Rh and
45c,e
atoms (small-core pseudopotentials
(
4) Ishiwata, K.; Kuwata, S.; Ikariya, T. J. Am. Chem. Soc. 2009,
4
8
131, 5001–5009.
Ir. Broken symmetry calculations were performed at the DFT b3-lyp
and def-TZVP
4
5c,f
(5) (a) Tye, J. W.; Hartwig, J. F. J. Am. Chem. Soc. 2009,
131, 14703–14712. (b) Zhao, P.; Krug, C.; Hartwig, J. F. J. Am. Chem.
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4
9levels of theory. Orbitals were visualized with the
Molden program.
X-Ray Diffraction Studies on 7 0.5(C H ), 8 0.5(C H ),
and 10. Selected crystallographic data for the three complexes can be
found in Table S1 (Supporting Information). Intensity measurements
were collected with a Smart Apex diffractometer, with graphite-mono-
chromated Mo KR radiation. A semiempirical absorption correction was
3
6
14
3
6 14
(
(
M.; Oro, L. A. Chem.—Eur. J. 2007, 13, 2044–2053.
50
applied to each data set, with the multiscan methods. All non-hydrogen
atoms were refined with anisotropic temperature factors except those
corresponding to the disordered hexane solvent (complexes 7 and 8),
which were refined with fixed isotropic thermal parameters. The hydrogen
atoms were placed at calculated positions, with the exception of the olefinic
cod protons and the methine HCdN proton, which were found on the
Fourier map. They were refined isotropically in the riding mode. The
structures were solved by the Patterson method and refined by full-matrix
(8) Retboll, M.; Ishii, Y.; Hidai, M. Chem. Lett. 1998, 1217–1218.
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Chem. 2002, 41, 2348–2355.
(
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(
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least-squares with the program SHELX97 in the WINGX package. Two
independent molecules were found for complex 10.
’
ASSOCIATED CONTENT
(12) (a) Sharp, P. R. J. Chem. Soc., Dalton Trans. 2000, 2647–2657. (b)
Ge, Y.-W.; Ye, Y.; Sharp, P. R. J. Am. Chem. Soc. 1994, 116, 8384–8385.
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Bruin, B. J. Am. Chem. Soc. 2005, 127, 1895–1905. (b) Hetterscheid,
D. G. H.; Bens, M.; de Bruin, B. Dalton Trans. 2005, 979–984.
S
Supporting Information. ORTEP of complex 8; selected
b
NMR spectra of complexes 8ꢀ10; monitoring of selected reactions
1
31
1
by H and P{ H} NMR; selected crystal, measurement, and
refinement data for compounds 7, 8, and 10; and CIF files for
these compounds. This material is free of charge via the Internet at
http://pubs.acs.org.
’
AUTHOR INFORMATION
(
16) Hetterscheid, D. G. H.; Klop, M.; Kicken, R. J. N. A. M.; Smits,
J. M. M.; Reijerse, E. J.; de Bruin, B. Chem.—Eur. J. 2007, 13, 3386–3405.
17) Hetterscheid, D. G. H.; de Bruin, B.; Smits, J. M. M.; Gal, A. W.
Organometallics 2003, 22, 3022–3024.
18) (a) Tejel, C.; del Río, M. P.; Ciriano, M. A.; Reijerse, E. J.; Hartl,
Corresponding Author
Fax: (+) 34 976 761187 (C.T.), (+31) 20 5255604 (B.d.B.).
E-mail: ctejel@unizar.es, b.debruin@uva.nl.
(
*
(
F.; Z ꢀa li ꢁs , S.; Hetterscheid, D. G. H.; Tsichlis i Spithas, N.; de Bruin, B.
Chem.—Eur. J. 2009, 15, 11878–11889. (b) Tejel, C.; Ciriano, M. A.; del
Rio, M. P.; Hetterscheid, D. G. H.; Tsichilis i Spitas, N.; Smits, J. M. M.;
de Bruin, B. Chem.—Eur. J. 2008, 14, 10932–10936.
(19) (a) Friedrich, A.; Drees, M.;K €a ss, M.; Herdtweck, E.; Schneider, S.
Inorg. Chem. 2010, 49, 5482–5494. (b) Friedrich, A.; Ghosh, R.; Kolb, R.;
’
ACKNOWLEDGMENT
This research was supported by the MICINN/FEDER (Project
CTQ2008-03860, Spain) and Gobierno de Arag ꢀo n (GA, Project
PI55/08, Spain), The Netherlands Organization for Scientific
7
533
dx.doi.org/10.1021/ic200395m |Inorg. Chem. 2011, 50, 7524–7534

Inorganic Chemistry
ARTICLE
Herdtweck, E.; Schneider, S. Organometallics 2009, 28, 708–718. (c)
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(41) Delocalization to the carbonyl moiety renders the lone pair at
N2 unavailable for binding another metal. According to DFT, com-
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