Stereospecific control of the metal-centred chirality of rhodium(III) and iridium(III) complexes bearing tetradentate CNN′P ligands

Article abstract of DOI:10.1039/c7dt01446e

Ligands LH1-LH3 have been prepared by two successive condensation/reduction steps. These ligands react with MCl₃ (M = Rh, Ir) rendering the trichlorido complexes [MCl₃(κ³N,N′,P-LH)] (M = Rh, LH = LH1 (1), LH2 (2), LH3 (3); M = Ir, LH = LH1, (4)) as racemic mixtures of fac and mer isomers. Only one of the two possible fac isomers was detected. The mer isomer of the rhodium compounds 1-3 quantitatively isomerizes to the more stable fac isomer, whereas the mer isomer of the iridium complex 4 does not. DFT calculations indicate a dissociative pathway for this isomerization. In the presence of acetate or trifluoroacetate, complexes 1-3 or 4, respectively, undergo cyclometallation of their free benzylic arm affording the corresponding dichlorido compounds [MCl₂(κ⁴C,N,N′,P-L)] (M = Rh, L = L1 (5), L2 (6), L3 (7); M = Ir, L = L1 (8)). Only one of the three possible enantiomeric pairs of coordination isomers was detected. The configuration at the stereogenic centres, namely the metal and the iminic nitrogen atom is stereospecifically predetermined. DFT calculations reveal that the cyclometallation follows an acetate-assisted mechanism and indicate that the isolated isomers are the most stable. Complexes 1-8 have been characterized by analytical and spectroscopic means and by the determination of the crystal structures of the complexes 1, 3 and 5-8 by X-ray diffractometry.

Full text of DOI:10.1039/c7dt01446e

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DOI: 10.1039/C7DT01446E.
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DOI: 10.1039/C7DT01446E
Stereospecific control of the metal-centred chirality of rhodium(III)
and iridium(III) complexes bearing tetradentate CNN’P ligands†
a
,a
a
,a,b
María Carmona, Ricardo Rodríguez,* Isabel Méndez, Vincenzo Passarelli,*
a
a
,a
Fernando J. Lahoz, Pilar García-Orduña and Daniel Carmona*
Received ……..
DOI:
Ligands LH1-LH3 have been prepared by two successive condensation/reduction steps.
These ligands react with MCl (M = Rh, Ir) rendering the trichlorido complexes
3
3
[
(
MCl (κ N,N’,P−LH)] (M = Rh, LH = LH1 (1), LH2 (2), LH3 (3); M = Ir, LH = LH1,
3
4)) as racemic mixtures of fac and mer isomers. Only one of the two possible fac
isomers was detected. The mer isomer of the rhodium compounds 1-3 quantitatively
isomerizes to the more stable fac isomer, whereas the mer isomer of the iridium
complex 4 does not. DFT calculations indicate a dissociative pathway for this
isomerization. In the presence of acetate or trifluoroacetate, complexes 1-3 or 4,
respectively, undergo cyclometallation of their free benzylic arm affording the
4
corresponding dichlorido compounds [MCl (κ C,N,N’,P−L)] (M = Rh, L = L1 (5), L2
2
(6), L3 (7); M = Ir, L = L1 (8)). Only one of the three possible enantiomeric pairs of
coordination isomers was detected. The configuration at the stereogenic centres, namely
the metal and the iminic nitrogen atom is stereospecifically predetermined. DFT
a
Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), CSIC - Universidad de Zaragoza,
Departamento de Química Inorgánica, Pedro Cerbuna 12, 50009 Zaragoza, Spain, E-
mail: dcarmona@unizar.es (D. C.), riromar@unizar.e, (R. R.), passarel@unizar.es (V. P.)
b
Centro Universitario de la Defensa, Ctra. Huesca s/n, 50090 Zaragoza, Spain
†
Electronic supplementary information (ESI) available: Preparation and characterization of the imines
I1-I3 and amines A1-A3. Selected NMR spectra of compound 5. X-ray crystallographic information files
containing full details of the structural analysis of complexes 1, 3, 5, 6, 7 and 8 (CIF format): CCDC
Atomic coordinates of calculated structures. Energies from relaxed PSE calculations: CCDC 1545261-
1
545266. For ESI and crystallographic data in CIF or other electronic format see DOI: ………..
1

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DOI: 10.1039/C7DT01446E
calculations reveal that the cyclometallation follows an acetate-assisted mechanism and
indicate that the isolated isomers are the most stable. Complexes 1-8 have been
characterized by analytical and spectroscopic means and by the determination of the
crystal structures of the complexes 1, 3 and 5-8 by X-ray diffractometry.
2

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DOI: 10.1039/C7DT01446E
Introduction
Nowadays, the most efficient and versatile methodology for the preparation of
enantioenriched compounds relies on the employment of transition metal complexes as
1
asymmetric catalysts. Indeed, over the last decades, a wide variety of chemical
transformations has been accomplished under the control of chiral metal-containing
2
catalysts.
By far, the most developed strategy in this area is based on the association of chiral
enantiopure organic ligands with metal ions. Chirality is located in a coordination atom
or, more frequently, in the ligand backbone and the asymmetric environment created
around the metal (where the substrates react) is enough to achieve high enantiomeric
1
,3
excesses. However, higher stereoselectivities would be expected if the metal atom
itself were a stereogenic centre in an enantiopure compound. In fact, in some efficient
catalysts, chirality does not only reside in the ligand but also in the metal. These
catalysts generally form diastereoselectively by chiral induction from enantiopure
ligands and metals usually exhibit octahedral or pseudo-octahedral geometries. Relevant
4
5
6
examples are complexes containing N , N O or N P chiral tetradentate ligands as
4
2
2
2 2
7
n
n+
well as half-sandwich compounds of the type [(η −ring)M(LL*)L] in which LL*
represents a chiral bidentate ligand.
However, examples of only chiral-at-metal catalysts are scarce most probably due
to the limitation that entails the preparation of a single stereoisomer through asymmetric
8
9
synthesis or chiral resolution. The simultaneous requirement for the metal of
configurational stability and free, or potentially available, coordination sites makes this
1
0
preparation a very challenging task. Only a few catalysts of this type have been
9
e
developed. Typically, they consist of octahedral compounds bearing two bidentate
achiral NN or CN ligands which impart “propeller” chirality to the complex.
3

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DOI: 10.1039/C7DT01446E
In 2003, Fontecave et al. reported that chiral-at-metal bis-diimine Ru(II) complexes
catalyse the oxidation of sulfides to sulfoxides by hydrogen peroxide with up to 18 % e.
1
1
e. This result demonstrated that complexes which are chiral only at metal can be used
for asymmetric catalysis. In the very last years, the group of Meggers and Gong has
9
d,12
9b,e,13
developed a family of rhodium(III)
and iridium(III)
complexes that efficiently
9
d,12b
9e,13b
catalyses a variety of organic transformations, such as Michael,
Friedel-Crafts
9
b
13
or conjugate additions, hydrogen transfer reactions, dehydrogenative cross-coupling
3
12c
12a
reactions between two C(sp )−H groups, or alkylynation reactions, achieving high
levels of enantioselectivity.
With all these concerns in mind, we envisaged the possibility of studying the
application as asymmetric catalysts of chiral-at-metal octahedral complexes derived
from chiral but unresolved tripodal tetradentate ligands. As far as we know, no chiral
resolution of octahedral complexes derived from optically not resolved chiral
tetradentate ligands has been reported until now. In order to control the stereochemistry
of the resulting complexes, the stereochemical richness of octahedral coordination
makes very convenient to introduce steric constrains within the ligands. Tripodal
tetradentate ligands would offer relatively rigid and well-defined frameworks and
provide two mutually cis coordination sites. Taking into account these considerations,
we chose the potentially tetradentate ligands depicted in Scheme 1 as ligands to build up
6
+3
+3
octahedral complexes of the d metal ions Rh and Ir .
The four coordination points would be the aminic and pyridinic nitrogen atoms, the
phosphorus and one of the aromatic carbons of the benzyl substituent. The presence of
four distinct coordination atoms in a tripodal tetradentate ligand guarantees the
4
stereogenicity of the metal in complexes with a κ C,N,N’,P coordination mode. The
coordinated aminic nitrogen is also a stereogenic centre and, as we will see in this
4

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1
4
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DOI: 10.1039/C7DT01446E
paper, the chirality of the molecule is predetermined: a determined absolute
configuration at nitrogen corresponds to an established absolute metal configuration.
Furthermore, the coordination features of these ligands suggest a high stability of the
configuration at the metal.
R1
R2
N
N
PR3
2
Scheme 1 Tripodal tetradentate ligands.
Notably, the ligands can be prepared by successive inclusion of the three
substituents at the central nitrogen in a stepwise manner and, therefore, the
stereoelectronic properties of the ligands can be modulated at will within a wide scope.
In the present paper, we disclose a synthetic route to rhodium and iridium
4
dichlorido complexes of formula [MCl (κ C,N,N’,P−L)] (5-8) where LH represents a
2
tetradentate tripodal ligand of the type collected in Scheme 1. Trichlorido intermediates
3
of stoichiometry [MCl (κ N,N’,P−LH)] (1-4) have been isolated and characterized. The
3
kinetic and thermodynamic relative stability of the possible stereoisomers of the new
complexes have been studied by DFT methods. From experimental and theoretical data,
a plausible mechanism has been proposed for the metallation reaction that complexes 1-
4
undergo to afford dichloridos 5-8.
The chiral resolution of the dichlorido complexes and the application of derived
solvate complexes as catalyst precursors for enantioselective organic transformations
will be reported in due course.
5

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Results and discussion
Synthesis of the ligands
Ligands LH1-LH3 could be prepared in a straightforward manner (Scheme 2).
1
5
Condensation of pyridine-2-carboxaldehyde with benzylamines renders imines I1-I3
that are reduced to the corresponding amines A1-A3 by treatment with methanolic
1
6
NaBH solutions (see ESI). Subsequent one-pot condensation of amines A1-A3 with
4
17
2
-(diphenylphosphino)benzaldehyde followed by reduction with NaBH(AcO)3
afforded ligands LH1-LH3 in high overall yield.
N
CH
2
Cl
2
N
NaBH
4
H
N
N
+
NH
2
N
MeOH
R
OHC
R
R
R = H, I1; OMe, I2; CF
3
, I3
3
R = H, A1; OMe, A2; CF , A3
PPh
2
NaBH(AcO)
3
H
N
N
N
N
+
CHO
CH
2
Cl
2
PPh
2
R
R
3
R = H, LH1; OMe, LH2; CF , LH3
Scheme 2 Synthesis of the ligands.
3
Synthesis of the trichlorido compounds [MCl (κ N,N’,P−LH)] (M = Rh, LH = LH1
3
(1), LH2 (2), LH3 (3); M = Ir, LH = LH1 (4))
Trichlorido complexes 1-4 were prepared by treating RhCl ·xH O or IrCl ·xH O with
3
2
3
2
stoichiometric amounts of the corresponding ligand in refluxing ethanol, overnight for
the rhodium compounds 1-3 or during 4 days for the iridium complex 4 (Eq. 1). The
long required reaction times (especially for the iridium compound) may be due to the
low solubility of the reagents under the reaction conditions.
Analytical and spectroscopic data indicate that the isolated solid consists of a ca.
3
1:69 (1), 40:60 (2), 29:71 (3) and 80:20 (4) mixture of two isomers derived from the
6

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EtOH, ∆
3
MCl3 + LH
[MCl (κ N,N',P-LH)]
3
Eq. 1
M = Rh, LH = LH1 (1), LH2 (2), LH3 (3)
M = Ir, LH = LH1 (4)
coordination to the metal of the two nitrogens and the phosphorus atom of the ligand.
3
From a stereochemical point of view, it is important to note that a κ N,N’P coordination
mode strongly hampers the inversion at the aminic nitrogen. Moreover, in an octahedral
MCl NN’P compound the metal may also be stereogenic. All the possible isomers of
3
complexes 1-4 (an enantiomeric pair of mer and two enantiomeric pairs of fac
diastereomers) are depicted in Scheme 3 labelled with their corresponding
1
8
stereochemical descriptors.
fac
R
R
N
M
Cl
N
PPh2
Cl
Ph2P
Cl
mer
N
M
N
Cl
Cl
Cl
R
R
Cl
Cl
M
Cl
(
SNam)-OC-6-43-C
(RNam)-OC-6-43-A
N
N
PPh2
Cl
Ph2P
Cl
M = Rh, R = H (1b), OMe (2b),
CF3 (3b)
M = Ir, R = H (4b)
M = Rh, R = H (1b'), OMe (2b'),
CF3 (3b')
M = Ir, R = H (4b')
M
N
(
N
Cl
SNam)-OC-6-41
(RNam)-OC-6-41
M = Rh, R = H (1a), OMe (2a),
CF3 (3a)
M = Rh, R = H (1a'), OMe (2a'),
CF3 (3a')
R
R
M = Ir, R = H (4a)
M = Ir, R = H (4a')
N
N
PPh2
Ph2P
Cl
N
M
Cl
M
N
Cl
Cl
Cl
Cl
(RNam)-OC-6-43-C
(SNam)-OC-6-43-A
M = Rh, R = H (1c), OMe (2c),
CF3 (3c)
M = Rh, R = H (1c'), OMe (2c'),
CF3 (3c')
M = Ir, R = H (4c)
M = Ir, R = H (4c')
Scheme 3 Trichlorido isomers.
The complexes were characterized by analytical and spectroscopic means (see
Experimental Section). Assignment of the NMR signals was verified by two-
3
1
1
dimensional homonuclear and heteronuclear correlations. The P{ H} NMR spectrum
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of each mixture of rhodium isomers consisted of two doublets, at ca. 30.5 and 27.5
ppm, respectively, exhibiting J(RhP) coupling constants in the 107-116 Hz range.
3
1
1
The P{ H} NMR spectrum of the isolated mixture of the two isomers of the iridium
compound 4 consisted of two singlets one at −20.96 ppm and the other at −22.76 ppm.
In all the complexes, the three pairs of methylene protons of the ligands become
diastereotopic as a consequence of the coordination and resonate as anisochronous AB
or AX systems.
The proton-6 of the pyridine moiety of the minor isomer of the rhodium complexes
1
-3 and that of the major isomer of the iridium complex 4 presents a coupling constant
to the phosphorus of about 5 Hz. This coupling was not detected for the other isomers.
As only in the mer isomers the phosphorus atom is trans to the pyridinic nitrogen,
reasonably the less abundant rhodium and the most abundant iridium isomers are the
pair of mer diastereomers. Assuming this assignment, only one of the two possible pairs
of fac diastereomers is present in the isolated mixture but, at this point, we are not able
to discriminate which of the two possible pairs of fac isomers has been obtained.
The mer isomers 1a-3a of the rhodium compounds completely convert into the
corresponding fac isomers by refluxing overnight suspensions of the obtained mixture
of both isomers, in dichloromethane or in ethanol. However, according to NMR
observations, the isomerization of the mer iridium compound 4a is negligible under
these conditions or even after 24 hours at 135 ºC in 2-ethoxyethanol. The composition
of a mer/fac mixture of complex 4 changes from 80/20 to 70/30 molar ratio by heating
overnight a decahydronaphthalene suspension at 170 °C.
8

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Molecular structures of the compounds [RhCl (κ N,N’,P−LH)] (LH = LH1
3
(1b/1b’), LH3 (3b/3b’))
In order to unequivocally ascertain the structure of the isolated fac isomers, the crystal
structures of complexes 1b and 3b have been determined by X-ray diffractometric
methods. Single crystals have been obtained from dichloromethane solutions of the
compounds, after complete isomerization of the isolated solids to the fac isomer. A view
of the molecular structures of both complexes is depicted in Figure 1 (only one of the
two enantiomers is shown) and relevant characteristics of the metal coordination
spheres are summarized in Table 1. Both complexes share structural features. They
exhibit a distorted-octahedral coordination environment with the rhodium atom bonded
to the phosphorus and the aminic (N , N(1)) and pyridinic (N , N(2)) nitrogen atoms
am
py
of the LH ligand. Three chlorido ligands complete a fac disposition around the metal.
Both complexes crystallize in the P-1 centrosymmetric space group and therefore their
unit cell (Z = 2) contain a pair of fac diastereomeric enantiomers specifically the (SNam)-
1
8
OC-6-43-C (1b, 3b) and (RNam)-OC-6-43-A (1b’, 3b’) isomers.
Fig. 1 Molecular structure of the complexes 1b and 3b, (SNam)-OC-6-43-C enantiomer. For
clarity hydrogen atoms have been omitted and only the ipso carbon of the phenyls of the PPh2
group have been included.
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Table 1 Bond lengths (Å) and angles (°) for complexes 1b/1b’ and 3b/3b’
1
b/1b’
3b/3b’
2.3309(8) Cl(2)-Rh-Cl(3) 90.338(17)
2.3433(8) Cl(2)-Rh-P 86.608(17)
1b/1b’
3b/3b’
Rh-Cl(1)
Rh-Cl(2)
Rh-Cl(3)
Rh-P
2.3406(5)
2.3440(5)
2.4331(5)
2.2805(5)
88.50(3)
86.98(3)
2.4299(9) Cl(2)-Rh-N(1) 93.60(4)
2.2780(9) Cl(2)-Rh-N(2) 174.65(5)
93.63(7)
173.92(8)
Rh-N(1)
Rh-N(2)
2.1488(15) 2.164(3) Cl(3)-Rh-P
176.724(17) 174.57(3)
2.0366(15) 2.052(3) Cl(3)-Rh-N(1) 87.73(4)
89.41(8)
87.39(8)
93.88(8)
97.35(8)
81.85(11)
Cl(1)-Rh-Cl(2) 92.086(17) 92.53(3) Cl(3)-Rh-N(2) 86.54(5)
Cl(1)-Rh-Cl(3) 88.797(17) 89.67(3) P-Rh-N(1)
Cl(1)-Rh-P 90.129(18) 87.52(3) P-Rh-N(2)
93.64(4)
96.59(5)
81.93(6)
Cl(1)-Rh-N(1) 173.35(4)
Cl(1)-Rh-N(2) 92.20(5)
173.74(8) N(1)-Rh-N(2)
91.93(8)
It is noteworthy to point out that the Rh−Cl(1) and Rh−Cl(2) bond lengths are
similar (close to 2.34 Å) but, due to the stronger structural trans influence of
phosphanes compared to amines or pyridines, the Rh−Cl(3) bond length is significantly
longer, about 2.43 Å. Similar disparity between Rh−Cl bond lengths has been reported
19
in related complexes. Thus, while in a tris(2-pyridylmethyl)amine rhodium complex,
Rh−Cl distances of 2.332(1) and 2.362(1) Å have been measured for Rh(III)−Cl bonds
trans to Npy and Nam atoms, respectively, a Rh−Cl distance of 2.425(14) Å has been
reported for the Rh(III)−Cl bond trans to the PPh group in [RhCl (N,N’-1-alkyl-2-
3
3
2
0
(
naphthyl-α-azo)(PPh )]. The Rh−N bond distances of 1b/1b’ and 3b/3b’ lie in the
3
range 2.0366(15)-2.164(3) Å, with the Rh−Nam elongated compared to the Rh−Npy
bond length.
The tridentate coordination of the LH ligand gives rise to the formation of two
2
fused five- and six-membered metallacycles. The former adopts a E envelop
2
conformation in both complexes 1b and 3b, with a distortion towards twisted T in
3
1
0

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3
b. In both complexes, the six-membered ring is strongly puckered as indicated by
DOI: 10.1039/C7DT01446E
2
1d
the high puckering amplitude (0.816(2) Å in 1b and 0.825(3) Å in 3b). The out-of-
2
1d
2,5
plane deformation of the metallacycle could be described as a B boat conformation
in both isomers. As a consequence of crystal symmetry enantiomeric conformations are
observed for 1b’ and 3b’.
The main structural difference between complexes 1b and 3b concerns the
geometrical arrangement of the benzyl substituent of the ligand LH as evidenced by Rh-
N(1)-C(26)-C(27) and N(1)-C(26)-C(27)-C(28) torsion angles of −164.36(13)° and
−142.6(2)° and of −98.7(2)° and −84.2(2)° in 1b and 3b, respectively. Probably,
packing interactions are responsible for this behaviour.
DFT calculations on the trichlorido complexes 1-4
DFT calculations were carried out in order to assess the relative stability of the isomers
3
of the complexes [MCl (κ N,N’,P−LH)] (1-4). Table 2 shows the relative free energy
3
values for the mer (1a-4a) and the fac (1b-4b and 1c-4c) isomers of both rhodium and
iridium complexes. For the fac isomers 1b-4b, two conformations of the six-membered
2
,5
Rh-P-C-C-C-N ring, namely B and B , have been encountered.
2
,5
In agreement with the crystal structures of compounds 1b and 3b, in all the cases,
the pair of fac enantiomers (SNam)-OC-6-43-C and (RNam)-OC-6-43-A (1b-4b) with
2
,5
a
B conformation of the Rh-P-C-C-C-N ring is more stable than the pair of mer
enantiomers (SNam)-OC-6-41 and (RNam)-OC-6-41 (1a-4a). Interestingly, the pair of the
(SNam)-OC-6-43-C and (RNam)-OC-6-43-A enantiomers with a B2,5 conformation of the
2
,5
Rh-P-C-C-C-N ring was found to be slightly less stable than the pair with a
B
conformation. Also, the pair of fac enantiomers (SNam)-OC-6-43-A and (RNam)-OC-6-
1
1

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4
3-C (1c-4c) is significantly less stable and, as a consequence, its formation in the
DOI: 10.1039/C7DT01446E
reaction between MCl and LH (Eq. 1) can be ruled out.
3
Table 2
mer
Nam )-OC-6-41
fac
fac
fac
2,5
(S
(SNam )-OC-6-43-C ( B)
(SNam )-OC-6-43-C (B2,5
)
(SNam )-OC-6-43-A
2
2
2
2
,5
,5
,5
,5
[
[
[
RhCl
RhCl
RhCl
3
3
3
(LH1)]
(LH2)]
(LH3)]
1a
2a
3a
4a
1.5
1.5
1.3
2.0
1b-
2b-
3b-
4b-
B
B
B
B
0.0
0.0
0.0
0.0
1b-B2,5
2b-B2,5
3b-B2,5
4b-B2,5
2.8
1.9
2.5
2.5
1c
2c
3c
4c
21.6
21.6
21.8
24.1
[
3
IrCl (LH1)]
a
For brevity, for each pair of enantiomers, only the structure of the rhodium enantiomer
containing LH1 with an S configuration of the aminic nitrogen is shown, the others being similar
see ESI). For clarity, all hydrogen atoms are omitted.
(
2
,5
The mechanism of the conformational equilibrium BB2,5 has been elucidated
3
only in the case of complex [RhCl (κ N,N’,P−LH1)] (1b). On one hand, the Rh−P
3
2
,5
-1
dissociation in 1b- B is highly endoergonic (∆G = 33.9 kcal·mol , vide infra), thus a
dis
dissociative mechanism could not be operative. On the other hand, a non-dissociative
mechanism with a transition state exhibiting an almost planar six-membered Rh-P-C-C-
2
,5
‡
C-N ring (TS_ B-B , Figure 2) has been found to be kinetically accessible (ΔG =
2,5
–
1
2,5
1
6.9 kcal·mol , 298 K). Also, the low activation barrier for the equilibrium BB2,5
suggests that it should be fast at room temperature thus causing averaged NMR spectra
and preventing the direct observation of the two non-equivalent conformers.
1
2

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2
,5
Fig. 2 View along the N -Rh bond of 1b- B, 1b-B
and of the transition state of the
–1
am
2,5
2,5
equilibrium BB2,5, and its free energy profile (kcal·mol , 298 K, ethanol). For clarity all
hydrogen atoms have been omitted and only the ipso carbon atoms of the PPh moieties are
2
shown.
3
The mechanism of the merfac isomerization of [MCl (κ N,N’,P−LH)] (1-4),
3
namely the transformation of the isomer (SNam)-OC-6-41 into the isomer (SNam)-OC-6-
2
,5
4
3-C ( B) was also elucidated. As mentioned before, in spite of the similar structure
determined for the rhodium and iridium complexes, the isomerization is observed only
for the rhodium complexes 1a-3a but not for the iridium derivative 4a. On this
3
3
background, mer-[RhCl (κ N,N’,P−LH1)] (1a) and mer-[IrCl (κ N,N’,P−LH1)] (4a)
3
3
have been chosen as the reference compounds and three different dissociative pathways
have been considered. Figure 3 shows the intermediates and their relative free energies
1
3

Dalton Transactions
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N
N
-
l
N
N
N
N
-
N
N
-
N
N
C
C
l
l
-
N
N
N
N
+Cl
-
C
C
l
l
N
N
-
C
C
l
+Cl
M
M
M
M
M
M
M
M
C
Cl
P
P
C
C
l
l
P
P
C
C
l
l
P
P
C
C
l
l
P
P
C
C
l
l
C
C
l
l
C
C
l
l
Cl
C
l
-
,
2,5
2 5
=
=
=
1
1
a (M = Rh)
a M Rh
(
)
I
I
I
I
I
I
1
1
4
4
b
b
-
B
B
( )
(M = Rh)
M Rh
,
B
M I
B (M = Ir)
( )
a M Ir
a (M = Ir)
-
2
2
,5
5
=
r
4
4
(
)
b
b
-
N
C
Cl
l
N
N
N
N
N
N
N
C
C
l
l
M
M
M
M
C
C
l
l
P
P
Cl
C
l
P
P
C
l
l
I
I
Cl
C
l
IV
IV
C
II
II
P
P
P
P
N
N
N
N
N
N
C
C
l
l
N
N
M
M
M
M
C
C
l
l
Cl
C
l
Cl
C
l
Cl
C
l
Cl
C
l
V
V
VI
VI
V
V
VI
VI
.
.
+
+
+
3 .5
3
4
4
5
+3
3
3
3
.9
9
=
.
M
M
= R
R
h
h
+
2
4
5
.
+
2
4
.5
+29 0
29.0
+
IV
IV
III
III
.
+
+20.8
2
0 8
_ -
.
+
+1
1
1
1
.7
7
TS_I-II
S
T I II
I
I
-
-
Cl
.
N
.
-
+
-
N
+
.
+
1
1
.5
5
-Cl
N
N
+1
1
.5
5
+
-
+Cl
C
l
0.0
0 0
Rh
Rh
1
1
a
a
I
I
I
I
-
,
C
Cl
P
P
2
2
,5
5
1
1
b
b
-
B
B
Cl
C
l
.
.
+
41
+
+40.3
4
0 3
V
V
+41.7
7
VI
VI
.
.
+
+34 3
+34.3
2
8
8
.4
4
=
r
+
2
M
M
= I
I
r
IV
IV
.
I
I
I
I
I
+26
6_ . 88
-
+
2
.
TS_I-II
TS I II
+
+
1
1
6
6
.2
2
N
N
I
I
N
N
.
-
-
l
+
+4.4
4 4
-
.
C
-
I
I
r
r
+
-C
l
+Cl
-
.
+
2
2
.0
0
C
Cl
P
P
+Cl
0
0
.0
0
C
C
l
l
II
II
4
4
a
a
-
,
2
2 5
,5
4
4
b
b
-
B
B
3
Fig.
[
3
Dissociative pathways for the isomerization mer-[MCl (κ N,N’,P−LH1)]fac-
3
–1
3
MCl (κ N,N’,P−LH1)] (M = Rh, Ir) and the corresponding free energy profiles (kcal·mol , 298
3
K, ethanol).
for the pathways via chlorido dissociation (intermediates I and II), via pyridinic
nitrogen dissociation (intermediates III and IV) and via phosphorus dissociation
(intermediates V and VI). In addition, the transition state for the isomerization III
–
1
(
see Figure 3) has been calculated (TS_I-II: +19.3 (Rh) or +24.8 (Ir) kcal·mol vs. the
2
2
isomer 1a or 4a, respectively).
The data given in Figure 3 indicate that for both rhodium and iridium the most
favourable isomerization pathway starts from the dissociation of one chlorido ligand
from the mer isomer 1a or 4a and is followed by the isomerization of the resulting
1
4

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cationic intermediate I to II through the transition state TS_I-II. Eventually, the
2
,5
pentacoordinated cation II reacts with chloride yielding the fac isomer 1b- B or 4b-
2
,5
B. Not surprisingly, despite the fact that the reaction pathways for rhodium and
iridium are similar, the activation barrier for iridium is significantly higher than for
‡
–1
rhodium (ΔΔG = 5.5 kcal·mol ). This value reasonably accounts for the
3
experimentally not observed isomerization of mer-[IrCl (κ N,N’,P−LH1)] (4a) in the
3
2
3
mixture obtained from the reaction of IrCl ·xH O and LH1 (Eq. 1).
3
2
4
Synthesis of the cyclometallated dichloride compounds [MCl (κ C,N,N’,P−L)] (M
2
=
Rh, L = L1 (5), L2 (6), L3 (7); M = Ir, L = L1 (8))
Refluxing in ethanol for 3 h suspensions of the fac isomers 1b-3b or of mer/fac
3
mixtures of the rhodium trichlorido complexes [RhCl (κ N,N’,P−LH)] in the presence
3
of 3 equivalents of NaOAc afforded the corresponding cyclometallated dichloridos
4
[
[
[
RhCl (κ C,N,N’,P−L)] (L = L1 (5), L2 (6), L3 (7)). The iridium analogue
2
4
IrCl (κ C,N,N’,P−L1)] (8) was prepared by heating a solution of a mer/fac mixture of
2
3
IrCl (κ N,N’,P−LH1)] (4a+4b), at 170 ºC, in decahydronaphthalene, for 48 h, in the
3
presence of 3 equivalents of CF COONa. Under these conditions, complex 8 was
3
−
isolated together with about a 30 % of an uncharacterized CF COO containing iridium
3
complex. Treatment of this mixture with excess of HCl gave pure 8 in 72 % isolated
yield. Taking into account that the mer to fac isomerization of the iridium compound 4
was scant even under harsh conditions (see above), the high yield achieved for the
cyclometallated compound 8 suggests that a direct pathway from the mer iridium
2
4
isomer was operating.
Metallation of the methoxo and trifluoromethyl substituted ligands LH2 and LH3
could take place at the C-2 or at C-4 carbons of the benzyl arm (see Schemes 3 and 4).
1
5

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However, in the formation of complexes 6 and 7 only metallation at C-4 carbon has
been observed in both cases (see DFT calculations below).
R
R
N
N
N
N
M
M
PPh2
Cl
Cl
PPh2
Cl
Cl
(RNam)-OC-6-54-C
(SNam)-OC-6-54-A
M = Rh; R = H (5'), OMe (6'), CF (7')
M = Ir; R = H (8')
M = Rh; R = H (5), OMe (6), CF (7)
M = Ir; R = H (8)
3
3
R
R
N
N
N
N
M
M
Cl
PPh2
Cl
PPh2
Cl
Cl
(RNam)-OC-6-43-C
(SNam)-OC-6-43-A
N
N
PPh2
Ph P
2
N
N
M
M
Cl
Cl
Cl
Cl
R
R
(
RNam)-OC-6-42-A
(SNam)-OC-6-42-C
Scheme 4 Cyclometallated dichlorido isomers.
Scheme 4 shows the three pairs of diastereomeric enantiomers that could form by
metallation of the trichloridos 1-4. However, only the isomers 5/5’-8/8’ were detected.
The new complexes were characterized by analytical and spectroscopic means (see
Experimental Section). The three methylene groups of protons are diastereotopic and, at
a spectrometer frequency of 500 MHz, resonate as AX systems. The value of the J(PH)
coupling constant together with COSY, HSQC and HMBC experiments permit the
1
6

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assignment of the six protons which have been labelled as CH (Py), CH (Ph) and
2
2
CH (P) (see Scheme 5A). The NOE pattern for the six methylene protons (Scheme 5B,
2
see ESI) is only compatible with the pair of enantiomers (RNam)-OC-6-54-C and (SNam)-
OC-6-54-A in which the phosphorus and the pyridinic nitrogen atoms are mutually trans
(see Scheme 4). Accordingly, a J(PH) coupling constant of about 5 Hz was measured in
all cases for the proton-6 of the pyridine moiety. Hence, we propose that the only
obtained compounds are racemic mixtures of the isomers (RNam)-OC-6-54-C and
(SNam)-OC-6-54-A shown in Scheme 4.
A: Methylene descriptors
B: NOE pattern
CH (Ph)
2
Hpro-R
Hpro-S Hpro-R
Hpro-S
Hpro-S
CH (Py)
2
CH (Ph)
2
Hpro-R
Hpro-S
N
H
pro-R
N
CH (Py)
R
2
Hpro-R
Hpro-S
CH (PPh )
PPh2
Hpro-S
Hpro-R
2
3
CH (PPh )
2
3
Scheme 5 Assignment of the methylene descriptors (A) and NOE pattern (B) on (RNam)-OC-6-
4-C isomers.
5
3
1
1
The P{ H} NMR spectra consist of a doublet, around 35 ppm with a J(RhP)
coupling constant of 126-129 Hz for the rhodium complexes 5-7, and a singlet at -10.07
ppm, for the iridium complex 8.
4
Molecular structures of the compounds [MCl (κ C,N,N’,P−L)] (M = Rh, L = L1
2
(5/5’), L2 (6’), L3 (7/7’); M = Ir, L = L1 (8(8’))
The crystal structures of the four cyclometallated complexes 5-8 have been determined
by X-ray diffraction means. Most of the features of the four molecules are similar. A
view of the molecular structure of the four complexes is depicted in Figure 4 and some
structural parameters of the metal coordination are listed in Table 3. Their molecular
1
7

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structure illustrated distorted-octahedral coordination geometries around the central
metal atom. Two chlorido ligands occupy two cis coordination sites and the four
donating atoms of the tetradentate ligand complete the coordination metal sphere. As
indicated by solution NMR data, the phosphorus atom is trans to the pyridinic nitrogen
N(2), while the chlorine atoms are found to be trans to the aminic nitrogen N(1) and to
the aromatic C(28) atoms.
Fig. 4 Molecular structure of the (RNam)-OC-6-54-C (5, 7 and 8) and (SNam)-OC-6-54-A (6’)
enantiomers. For clarity hydrogen atoms have been omitted and only the ipso carbons of the
PPh groups are shown.
2
The aminic nitrogen and the metal are stereogenic centres. The crystal structure of
complexes 5, 7 and 8 contains symmetry planes and, therefore, the enantiomeric pairs of
diastereomers 5/5’, 7/7’ and 8/8’ (see Scheme 4) are present in their unit cells (only the
unprimed isomers 5, 7 and 8 are shown in Figure 4). In contrast, complex 6 crystallizes
as a conglomerate: the observed space group is non-centrosymmetric and the unit cell of
the chosen crystal only contains the 6’ isomer.
1
8

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Due to the strong trans influence of the sp carbon atom, the M−Cl(2) bond length
is significantly longer (by 0.11-0.16 Å) than the M−Cl(1) bond length.
The tetradentate coordination of the ligand L leads to the formation of two five-
membered and one six-membered metallacycles. The M-N(1)-C(20)-C(21)-N(2)
2
metallacycle exhibits and E envelope conformation (enantiomorphic E conformation
2
Table 3 Bond lengths (Å) and angles (°) for complexes 5/5’, 6’, 7/7’
(
M = Rh) and 8/8’ (M = Ir)
5/5’
6’
7/7’
8/8’
M-Cl(1)
M-Cl(2)
M-P
2.3634(13) 2.367(2) 2.3615(7) 2.3714(8)
2.5221(12) 2.528(2) 2.4794(7) 2.4847(9)
2.2728(14) 2.258(3) 2.2620(7) 2.2584(9)
M-N(1)
M-N(2)
2.081(4)
2.108(4)
2.012(5)
90.68(5)
92.58(5)
2.084(8)
2.112(10) 2.114(3)
1.997(9)
89.85(8)
90.50(9)
2.087(2)
2.092(3)
2.110(3)
2.022(3)
90.10(3)
91.87(3)
M-C(28)
Cl(1)-M-Cl(2)
Cl(1)-M-P
Cl(1)-M-N(1)
Cl(1)-M-N(2)
Cl(1)-M-C(28) 94.07(14)
Cl(2)-M-P
Cl(2)-M-N(1)
Cl(2)-M-N(2)
1.993(3)
92.24(2)
91.54(2)
172.63(14) 175.5(2) 172.66(6) 173.83(8)
92.21(14)
95.4(3)
94.6(3)
98.71(9)
91.0(2)
85.5(2)
92.60(7)
92.82(8)
93.40(2)
90.16(6)
88.16(7)
92.96(8)
94.98(10)
95.05(3)
90.55(8)
84.28(8)
91.76(5)
90.60(12)
92.10(11)
Cl(2)-M-C(28) 174.94(15) 172.1(3) 173.75(8) 170.49(10)
P-M-N(1)
94.64(14)
93.7(2)
95.24(6)
94.19(8)
P-M-N(2)
173.82(13) 172.8(3) 175.52(6) 175.12(9)
P-M-C(28)
N(1)-M-N(2)
N(1)-M-C(28)
N(2)-M-C(28)
89.75(13)
80.50(19)
84.46(18)
86.00(17)
87.8(3)
80.3(3)
84.2(4)
87.6(4)
90.12(7)
80.54(9)
84.37(10) 83.55(13)
87.96(10) 87.42(12)
92.81(10)
80.99(11)
in complex 6’) with puckering parameters similar to those found for complexes 1b and
25
3
b. The M-N(1)-C(26)-C(27)-C(28) metallacycle is almost planar, as indicated by the
2
5b
value of the puckering amplitude (a maximal value of 0.045(5) Å is observed in 5).
The six-membered metallacycle in the dichlorido complexes 5-8 is less distorted from
planarity than that of the trichlorido compounds 1b and 3b, as the puckering amplitudes
2
5c
found in 5-8, in the range 0.531(5)-0.695(9) Å, are significantly smaller than the
values reported in complexes 1b and 3b, 0.816(2) and 0.825(3) Å, respectively. The
5
conformation of this six-membered ring in 5, 7 and 8 shows a skew disposition ( S )
6
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9

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,5
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with a slight distortion towards a B conformation. As expected, complex 6’ shows an
6
enantiomeric S (slighted distorted towards B ) conformation.
5
2,5
Taking into account the potential application of these compounds as catalyst
precursors in asymmetric transformations, it is interesting to point out that the phenyl
groups of the phosphano arm of the ligand L shields one of the faces of the coordination
plane containing both chlorido ligands. However, the other face of this plane remains
essentially clear. Hence, if a prochiral planar chelate replaces the chlorido ligands, one
of its enantiofaces would be better shielded than the other and, therefore,
enantioselective attacks would be anticipated.
Fig. 5 Views of isomer 5, (RNam)-OC-6-54-C, along the Cl(1)−Rh and P−Rh directions. Similar
projections are obtained for complexes 6-8.
4
DFT calculations on the dichlorido complexes MCl (κ C,N,N’,P−L)
2
4
Table 4 shows the relative free energies of the isomers of [MCl (κ C,N,N’,P−L)] (M =
2
Rh, L = L1, L2, L3; M = Ir, L = L1) taking into account the configurations of the metal
centre (A, C) and of the aminic nitrogen atom (S, R). Notably, in agreement with the
crystal structure of 5-8, for all the ligands, the pair (RNam)-OC-6-54-C/(SNam)-OC-6-54-
A is the most stable. In addition, as far as L2 and L3 are concerned, the higher stability
of the derivatives in which the C-4 carbon is metallated in comparison with the
corresponding C-2 carbon metallated isomers (Table 4) fairly holds with the observed
2
0

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regioselectivity of the cyclometallation reaction in both [RhCl (κ N,N’,P−LH2)] and
3
3
[
RhCl (κ N,N’,P−LH3)].
3
Table 4
(R
Nam)-OC-6-54-C
(RNam)-OC-6-43-C
(RNam)-OC-6-42-A
RhCl
Rh/L1 Rh/L2
2
(L1)
RhCl
Rh/L1 Rh/L2
2
(L1)
RhCl
Rh/L1 Rh/L2
2
(L1)
c
c
c
c
c
c
Rh/L3
0.0
(13.0)
Ir/L1
0.0
Rh/L3
9.2
(19.6)
Ir/L1
10.3
Rh/L3
8.3
(17.2)
Ir/L1
11.4
0.0
0.0
6.9)
9.2
9.4
(14.0)
8.4
8.9
(12.5)
(
a
b
At 298 K, ethanol for rhodium complexes, cyclohexane for iridium complexes. For brevity, for
each pair of enantiomers only the structure of the rhodium enantiomer containing L1 with an R
configuration of the aminic nitrogen is shown, the others being similar (see ESI). For clarity, all
c
hydrogen atoms are omitted and only ipso carbon atoms of the PPh moieties are shown. The
2
first free energy value given for L2 and L3 corresponds to the regioisomer containing metallated
C-4 and the other (between brackets) corresponds to the regioisomer containing metallated C-
2.
DFT calculations on the metallation mechanism
When dealing with the mechanism of the cyclometallation reaction in the complexes
3
[
MCl (κ N,N’,P−LH)] (1-4), DFT calculations indicate that it follows the acetate-
3
assisted mechanism which has already been described for late transition metal acetato
25
complexes. Figure 6 (A, B) shows the independent pathways for both the mer and fac
3
2,5
isomers of [RhCl (κ N,N’,P−LH1)]. Starting from the fac isomer (1b- B), the
3
2
2
3
+
κ O,O’−acetato derivative [RhCl(κ O,O’−CH COO)(κ N,N’,P−LH1)] (VIIb) forms
3
and eventually converts into VIIIb. Remarkably the acetato ligand of VIIIb exhibits a
monodentate coordination mode and takes part in an intramolecular CH···O interaction
(Figure 6C). In addition, a weak CH···Rh agostic interaction is present as well. Selected
bond lengths and angles of VIIIb are given in Figure 6 and are similar to those already
2
1

Dalton Transactions
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2
5a,d-g
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DOI: 10.1039/C7DT01446E
reported in related systems.
place via a concerted mechanism with the transition state TS_VIII-IXb shown in
Figure 6C. As result of the activation, the cation
In the following step, the metallation reaction takes
a
4
+
[
RhCl(κ C,N,N’,P−L1)(κO−CH COOH)] (IXb) is obtained, which finally reacts with
3
N
N
N
Rh
Cl
N
N
N
P
Cl
Cl
N
P
N
N
N
P
C OH
O
i
O
O
Rh
Rh
H
Rh
H
Rh
P
P
O
O
O
O
Cl
Cl
ii
Cl
VIIIb
Cl
_
-
-2
,
5
5
=
VIIb
TS VIII IXb
IXb
N
1
b
B M Rh
(
)
-2,
N
C
P
= Ir
4
b
B M
Rh
(
)
Cl
ii
Cl
=
=
5
8 M
M
Rh
)
(
I
r
)
N
N
N
Rh
N
N
Rh
O
N
(
N
Cl
N
N
N
C
P
i
O
Rh
Cl
Rh
O
H
P
O
H
Rh
Cl
P
Cl
P
Cl
Cl
P
Cl
O
O
H
O
O
=
_
-
1
4
a M Rh
VIIa
VIIIa
TS VIII IXa
IXa
(
)
=
a M Ir
(
)
-
-
;
-
-
-
+
+
i
CH COO 2 Cl ii Cl CH COOH
3
3
A
(
)
_
-
TS VIII IXa
.
+
1
8 9
VIIIb
VIIIa
+6 6
_
-
1
a
.
TS VIII IXb
.
+
.
1
5
+
4
0
-2
,
1
b
5B
0
i
VIIIb
.
-
IXb
- .
.
0
4 2
VIIa
9
1
-
.
11 9
VIIb
ii
-
.
IXa
12 2
-
.
1
4 7
5
-
.
2
3 5
_
-
TS VIII IXb
B
C
( )
(
)
Fig. 6 (A) Intermediates and transition states of the acetate-assisted cyclometallation of fac-
3
2,5
3
[
RhCl (κ N,N’,P−LH1)] (black, 1b- B) and mer-[RhCl (κ N,N’,P−LH1)] (grey, 1a). (B) Free
3 3
–1
energy profiles (kcal·mol , 298 K, ethanol). (C) Calculated structures of the cation VIIIb and of
TS_VIII-IXb. For clarity, most hydrogen atoms are omitted and only ipso carbon atoms of the
PPh2 moieties are shown. Selected bond lengths (Å) and angles (º) are: VIIIb Rh···C 2.83,
Rh···H 2.44, Rh-H-C 99.4, C-H 1.10, O···HC 2.06, C-H-O 164.6; TS_VIII-IXb: Rh···C 2.24,
Rh···H 2.29, Rh-H-C 71.0, C···H 1.33, O···H 1.30, C-H-O 166.9.
2
2

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4
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DOI: 10.1039/C7DT01446E
chloride releasing acetic acid and yielding the final product [RhCl (κ C,N,N’,P−L1)]
2
(5). It is noteworthy that in the course of the transformation of IXb into 5 a
rearrangement of the metallated ligand should take place (vide infra).
3
The pathway from mer-[RhCl (κ N,N’,P−LH1)] (Figure 6A, B) fairly parallels the
3
pathway described for the fac isomer. Indeed, the acetato derivative VIIa forms and
converts into the intermediate VIIIa containing a CH···O interaction, which gives place
to the concerted transition state TS_VIII-IXa. In the following step, the cationic
4
+
intermediate [RhCl(κ C,N,N’,P−L1)(κO−CH COOH)] (IXa) is obtained which finally
3
undergoes a substitution reaction with chloride, losing the coordinated acetic acid and
–
yielding 5. At variance with the step IXb + Cl  5 + CH COOH, no rearrangement of
3
the metallated ligand takes place in the reaction of IXa with chloride.
On this background, despite the fact that the pathway from mer-
3
[
[
RhCl (κ N,N’,P−LH1)] (1a) is more straightforward than that starting from fac-
3
3
RhCl (κ N,N’,P−LH1)] (1b), the difference between the calculated activation barriers
3
‡
–1
of the CH cleavage (∆∆G = 14.6 kcal·mol ) makes the pathway from the mer isomer
not operative. Thus, on one hand, when a pure sample of 1b is used, 5 should be
obtained straightforwardly through the pathway 1bVIIbVIIIbTS_VIII-
IXbIXb5 depicted in Figure 6A. On the other hand, when the mer/fac mixture
1
a+1b obtained from the reaction between RhCl ·xH O and LH1 (Eq. 1) is used as the
3 2
starting material, the isomerization of the mer isomer 1a to the fac isomer 1b should
take place in the first place and thereafter the resulting fac isomer should undergo the
cyclometallation reaction.
With respect to the rearrangement of the metallated ligand from
4
+
4
[
RhCl(κ C,N,N’,P−L1)(κO−CH COOH)] (IXb) to [RhCl (κ C,N,N’,P−L1)] (5), the
3
2
proposed acetate catalysed mechanism is depicted in Figure 7. The coordinated acetic
2
3

Dalton Transactions
Page 24 of 53
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DOI: 10.1039/C7DT01446E
acid in IXb is deprotonated by free acetate affording the acetato derivative Xb, which
undergoes the stepwise isomerization XbXIbXIIb in which the coordination mode
2
of the acetato ligand goes through the sequence κO (Xb)κ O,O’ (XIb)κO’ (XIIb)
and, in a simultaneous way, the phosphorus atom, initially trans to carbon (Xb),
dissociates and eventually coordinates to rhodium occupying the coordination site trans
to nitrogen (XIIb). In the final step, a chloride ion replaces the acetato ligand of XIIb
yielding 5.
N
N
P
C
Rh
Cl
XIIIb
- .
P
N
N
0
3
N
P
C
O
H
N
C
O
O
Rh
Rh
O
Cl
-
.
ii
7 6
Cl
XIb
N
Rh
Cl
IXb
N
O
C
P
O
i
Xb
-
.
- .
9
1
9 0
N
N
C
O
-
Rh
Cl
XIIb
P
O
N
-
.
15 1
N
C
P
Rh
-
+
-
iii Cl
i
CH COO CH COOH
3
3
Cl
5
.
ii CH COOH
3
-
-
-
+
iii Cl CH COO
3
-
2
3 5
–1
–
Fig. 7 Free energy profile (kcal·mol , 298 K, ethanol) for the reaction IXb + Cl  5 +
CH COOH.
3
It is worth mentioning that a dissociative pathway should be ruled out since the
4
+
formation of the pentacoordinated cation [RhCl(κ C,N,N’,P−L1)] (XIIIb) is
–
1
prohibitively endoergonic (∆GIXb
= 8.8 kcal·mol , Figure 7).
XIIIb

4
With respect to the formation of [IrCl (κ C,N,N’,P−L1)] (8) in
2
decahydronaphthalene at 170 ºC, experimental data indicate that, at variance with 1a, a
direct cyclometallation pathway from the mer isomer 4a should exist (vide supra). In
this regard, Figure 8A reports the calculated energy profiles for the acetato-assisted CH
cleavage for the acetato intermediates XIVa and XIVb coming from the 4a and 4b,
3
respectively. Notably, similar to [RhCl (κ N,N’,P−LH1)], the CH cleavage reaction for
3
2
4

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the iridium fac isomer XIVb presents a lower barrier than for the mer isomer XIVa
‡
–1
(
∆∆G = 13.2 kcal·mol ). Nevertheless, the activation barrier for the CH cleavage in
‡
–1
the iridium mer isomer XIVa (∆G = 25.7 kcal·mol ) is found to be smaller than that
‡
–1
for the rhodium analogue VIIa (∆G = 30.8 kcal·mol ) thus confirming that, in contrast
with 1a, the cyclometallation of the iridium mer isomer 4a could be operating under the
experimental reaction conditions and should take place along with the cyclometallation
of 4b when the mixture 4a+4b converts to 8.
N
N
Ir
H
Cl
P
O
O
=
R
CF3
R
_
-
TS XV XVIa
.
N
Ir
+
2
5 8
N
H
Cl
P
O
O
_
-
R
XVa
TS XV XVIa
N
Ir
.
N
Ir
+
1
2 5
N
P
C
O
OH
N
.
+
11 2
O
R
N
Cl
P
Cl
R
N
O
I
r
XVIb
.
H
.
+
XIVa
+
4
4
P
0 9
.
O
+
O
0
1
0
Cl
R
.
0
N
_
-
- .
TS XV XVIb
3 7
N
Ir
H
N
P
O
O
N
C
N
Ir
Cl
R
XVb
N
P
Cl
P
OH
O
O
_
-
Ir
R
TS XV XVIb
O
Cl
R
XVIa
XIVb
A
B
( )
(
)
–1
Fig. 8 (A) Free energy profiles (kcal·mol , 298 K, cyclohexane) of the acetate-assisted CH
2
3
cleavage in [IrCl(κ O,O’−CF COO)(κ N,N’,P−LH1)]. (B) Calculated structures of the transition
3
states TS_XV-XVIa and TS_XV-XVIb. For clarity, most hydrogen atoms are omitted and only
ipso carbon atoms of the PPh moieties are shown. Selected bond lengths (Å) and angles (º)
2
are: TS_XV-XVIa: Ir···C 2.21, Ir···H 2.06, Ir-H-C 76.1, C···H 1.44, O···H 1.35, C-H-O 147.4;
TS_XV-XVIb: Ir···C 2.22, Ir···H 2.34, Ir-H-C 67.8, C···H 1.34, O···H 1.29, C-H-O 169.1.
For the sake of comparison, Figure 9 shows the free energy profiles for the
4
formation of the rhodium derivatives [RhCl (κ C,N,N’,P−L2)] (6) and
2
4
[
RhCl (κ C,N,N’,P−L3)] (7). In both cases, the intermediates and the transition state of
2
2
5

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the CH bond cleavage are similar to those described for the formation of
4
[
RhCl (κ C,N,N’,P−L1)] (5) starting from the fac isomer (Figure 6).
2
Interestingly, the activation barriers of the metallation of the C-2 carbon and of the
‡
C-4 carbon of the substituted phenyl group are significantly different (∆∆G = 2.8,
–
1
LH2; 3.5 kcal·mol , LH3), which along with the above mentioned different stability of
the regioisomers (Table 4) definitely indicate that the regioselective formation of
isomers 6 and 7 is favoured both kinetically and thermodynamically.
X
X
2
2
4
4
N
N
N
N
N
Rh
Cl
N
Rh
Cl
=
X
X
N
P
N
P
Cl
Cl
N
P
N
N
C
O
O
H
N
C
P
i
O
O
ii
Rh
Cl
5
Rh
Cl
Rh
H
Rh
H
P
P
Cl
O
O
O
O
Cl
Cl
-2
,
_
-
=
2
b
b
B X OMe
XVII
XVIII
TS XVIII XIX
XIX
6 X OMe
(
)
(
)
-2
,
=
5
=
3
B X CF3
7 (X CF
3
)
(
)
-
-
-
-
-
+
+
i
CH COO ; 2 Cl ; ii Cl ; CH COOH
3
3
_
-
TS XVIII XIX
.
_
-
+
=
TS XVIII XIX
8 9
X
OMe LH2
(
=
X
)
CF LH3
3
( )
.
+
4
5
.
+
-2,5B
-
,
5 4
2 5
2
b
3b
B
.
XIX
+
XVIII
1 7
XIX
- .
.
.
-
.
- .
0
0
0 0
5
2
.
5 1
XVIII
7 9
- .
8 5
1 5
-
.
-
ii
5
4
- .
i
i
6 0
XVII
ii
7
XVII
- .
-
.
9
7
-
.
1
0 9
-
.
6
.
1
2 2
1
2 4
-
ii 16 2
ii
-
.
2
3 1
-
-
.
2
5 2
-
carbon 4
carbon 2
Fig. 9 (top) Intermediates and transition state of the cyclometallation reaction of fac-
3
–1
[
RhCl (κ N,N’,P−LH)] (LH = LH2, LH3). (bottom) Free energy profiles (kcal·mol , 298 K,
3
ethanol). In both cases, “carbon-2” and “carbon-4” profiles refer to the cyclometallation at C-2
and C-4 carbons, respectively.
2
6

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CONCLUSIONS
6
+3
+3
Ligands LH1-LH3 give tri and tetracoordinated complexes of d Rh and Ir ions of
3
4
general formulae [MCl (κ N,N’,P−LH)] and [MCl (κ C,N,N’,P−L)], in which the
3
2
aminic nitrogen and the metal atoms are stereogenic centres. Except for
3
[
IrCl (κ N,N’,P−LH)] (4) the metallic compounds could be obtained as racemic
3
mixtures of only one isomer. Notably, the absolute configuration of the centres is
predetermined in the sense that given a configuration at nitrogen (S or R) only one
configuration at the metal (A or C) is observed. In particular, for the trichlorido
3
complexes [RhCl (κ N,N’,P−LH)], the pair of fac enantiomers of configuration (SNam)-
3
OC-6-43-C and (RNam)-OC-6-43-A was obtained and, for the dichloridos
4
[
MCl (κ C,N,N’,P−L)] (M = Rh, Ir) the sole isolated isomers presented (RNam)-OC-6-
2
5
4-C and (SNam)-OC-6-54-A configurations.
DFT calculations clearly indicate that the observed isomers are the most stable, thus
suggesting that thermodynamic factors are responsible for their selective formation and
3
isolation. Further, the cyclometallation of [MCl (κ N,N’,P−LH)] follows an acetate-
3
assisted mechanism in which an intramolecular hydrogen transfer from the phenyl CH
bond to the coordinated acetate takes place through a six-membered transition state.
Inspection of the molecular structure of the cyclometallated compounds reveals that
the asymmetry generated by the centred chiralities results in a strong
stereodifferentiation between the two semi-spaces above and below the virtual triangle
formed by the metal and the two chlorido ligands. This dissymmetry makes the
cyclometallated compounds potentially good starting materials for the preparation of
enantioselective catalysts after optical resolution and exchange of the coordinated
chlorido ligands by more labile ligands. We are currently working in these lines in our
laboratory.
2
7

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EXPERIMENTAL SECTION
General Information
All preparations have been carried out under argon, unless otherwise stated. All solvents
were treated in a PS-400-6 Innovative Technologies Solvent Purification System (SPS)
and degassed prior to use. Carbon, hydrogen and nitrogen analyses were performed
1
13
31
19
using a Perkin-Elmer 240 B microanalyzer. H, C, P and F spectra were recorded
on a Varian UNITY 300, a Bruker AV-300 (300.13 MHz), a Bruker AV-400 (400.16
MHz) or a Bruker AV-500 (500.13 MHz) spectrometers. Chemical shifts are expressed
3
1
19
in ppm up field from SiMe , 85% H PO4 ( P) or CFCl ( F). J values are given in Hz.
4
3
3
1
1
13
31
COSY, NOESY, HSQC, HMQC, and HMBC H-X (X = H, C, P) correlation
spectra were obtained using standard procedures. Mass spectra were obtained with a
Micro Tof-Q Bruker Daltonics spectrometer.
Rhodium and iridium trichlorides were purchased from Johnson Matthey.
Preparation and characterization of the ligands LH1-LH3. To a solution of the
corresponding amine A1-A3 (12.6 mmol) in 40 mL of CH Cl , 3.68 g (12.6 mmol) of
2
2
2
-diphenylphosphinobenzaldehyde and 4.00 g (18.9 mmol) of NaBH(AcO) were
3
added. The resulting solution was stirred for 15 min at room temperature and then, 80
mL of a saturated solution of NaHCO in water were added. After 20 min of additional
3
stirring the organic phase was separated by decantation, washed with water (3 × 75 mL)
and dried (MgSO ). The insoluble materials were removed by filtration and all the
4
volatiles were removed under vacuum. The addition of 15 mL of n-hexane to the oily
residue affords LH1-LH3 as analytical pure solid compounds. Yield: LH1, 5.54 g (93
%); LH2, 5.24 g (83 %); LH3, 5.94 g (87 %).
2
8

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CH2
N
View Article Online
DOI: 10.1039/C7DT01446E
(Py)
CH2
(Ph)
N
6-CH(Py)
PPh2
CH2
(P)
R = H (LH1), OMe (LH2), CF (LH3)
3
1
LH1. H NMR (300.13 MHz, CD Cl , RT, ppm): δ = 8.55 (ddd, J = 4.9, 1.6, 0.9 Hz, 1H,
2
2
6
-CH(Py)), 7.99 (dd, J = 7.6, 4.3 Hz, 1H, H(Ar)), 7.73-6.87 (m, 21H, H(Ar)), 4.06 (d, J
1
3
1
=
2.4 Hz, 2H, CH (P)), 3.87 (s, 2H, CH (Py)), and 3.72 (s, 2H, CH (Ph)). C{ H}
2 2 2
NMR (75.48 MHz, CD Cl , RT, ppm): δ = 160.11 (s, 2-C(Py)), 148.89 (s, 6-CH(Py)),
2
2
1
44.09 (d, J = 22.8 Hz, 2C, C(Ar)), 139.11 (s, C(Ar)), 136.99 (d, J = 10.8 Hz, 2-
C(PhP)), 136.50 (d, J = 13.7 Hz, C(Ar)), 136.41 (s, CH(Ar)), 134.21 (s, 2C, CH(Ar)),
33.94 (s, 2C, CH(Ar)), 133.68 (s, CH(Ar)), 129.05 (s, 3C, CH(Ar)), 129.03 (d, J = 13.7
Hz, CH(Ar)), 128.82 (s, 2C, CH(Ar)), 128.75 (d, J = 11.1 Hz, 4C, CH(Ar)), 128.36 (s,
C, CH(Ar)), 127.27 (s, CH(Ar)), 127.07 (s, CH(Ar)), 122.87 (s, CH(Ar)), 121.91 (s,
CH(Ar)), 59.69 (s, CH (Py)), 58.31 (s, CH (Ph)), and 56.50 (d, J = 22.9 Hz,
1
2
2
2
31
1
CH (P)). P{ H} NMR (121.42 MHz, CD Cl , RT, ppm): δ = −16.04 (s). HRMS (µ-
2
2
2
+
TOF): C H N P, [M+H] : calc. 473.2141, found 473.2126.
3
2
29
2
1
LH2. H NMR (300.13 MHz, CD Cl , RT, ppm): δ = 8.50 (ddd, J = 4.9, 1.7, 0.9 Hz, 1H,
2
2
6
-CH(Py)), 7.89 (dd, J = 7.5, 4.3 Hz, 1H, H(Ar)), 7.69-6.75 (m, 20H, H(Ar)), 3.92 (d, J
2.5 Hz, 2H, CH (P)), 3.82 (s, 3H, OMe), 3.75 (s, 2H, CH (Py)), and 3.60 (s, 2H,
=
2
2
1
3
1
CH (Ph)). C{ H} NMR (75.48 MHz, CD Cl , RT, ppm): δ = 160.08 (s, 2-C(Py)),
2
2
2
1
59.60 (s, C(OMe)), 148.79 (s, 6-CH(Py)), 143.91 (d, J = 21.6 Hz, 2C, C(Ar)), 140.86
s, C(Ar)), 136.78 (d, J = 10.7 Hz, 2-C(PhP)), 136.33 (s, CH(Ar)), 136.30 (d, J = 14.4
Hz, C(Ar)), 134.09 (s, 2C, CH(Ar)), 133.83 (s, 2C, CH(Ar)), 133.41 (s, CH(Ar)),
29.18 (s, CH(Ar)), 128.90 (s, CH(Ar)), 128.84 (d, J = 5.2 Hz, CH(Ar)), 128.68 (s, 2C,
CH(Ar)), 128.57 (d, J = 6.0 Hz, 4C, CH(Ar)), 127.07 (s, CH(Ar)), 122.70 (s, CH(Ar)),
(
1
2
9