Charge-induced facial-selectivity in the formation of new cationic planar chiral iridacycles derived from aniline

Article abstract of DOI:10.1039/c0cc05711h

The reaction of chiral chlorido-iridacyclic 2-(4-N,N-dimethylaminophenyl) pyridines with solvato-type [Cp*M(S)₃]^q+ (M = Ru, S = MeCN, q = 1; M = Ir, S = MeC(O)Me, q = 2) complexes produces new cationic racemic planar chiral iridacycl

Full text of DOI:10.1039/c0cc05711h

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COMMUNICATION
Charge-induced facial-selectivity in the formation of new cationic planar
chiral iridacycles derived from anilinew
a
a
a
b
´ ´
Jean-Pierre Djukic,* Wissam Iali, Michel Pfeffer and Xavier-Frederic Le Goff
Received 21st December 2010, Accepted 25th January 2011
DOI: 10.1039/c0cc05711h
The reaction of chiral chlorido-iridacyclic 2-(4-N,N-dimethyl-
aminophenyl)pyridines with solvato-type [Cp*M(S)3]q+ (M = Ru,
S = MeCN, q = 1; M = Ir, S = MeC(O)Me, q = 2) complexes
produces new cationic racemic planar chiral iridacycles in an
efficient and diastereospecific way.
confirms the essential role of the –NMe2 group11 in the con-
solidation of the p–metal bonding, particularly in bis-cationic
complexes.
The starting metallacycles used in this study were synthesized
by the reaction of ligands 1a11 and b with [Cp*IrCl2]2
12
in CH2Cl2 in the presence of hydrated NaOAc at room
temperature.13 The resulting racemic Ir(III) complexes 2a
and b were used in subsequent reactions with solutions
In spite of the growing interest1 in the use of planar chiral
metallacycles2 as catalysts, the majority of reports deal exclusively
with complexes of square-planar (SP-4) Pd or Pt centres.3,4 By
expanding the variety of coordination geometries at the chelated
metal centre to octahedral (OC-6) and pseudo-tetrahedral
(T-4) one would not only open up new horizons but also
introduce the major issues of stereospecificity and conforma-
tional instability at the stereogenic centres.4 Planar-chiral
metallacycles can be synthesized mainly by two methods;4
i.e. the cyclometalation of a planar pro-chiral ligand and the
p-coordination of an unsaturated metal moiety to a preformed
metallacycle.5 The predominant role of Coulomb repulsion
between ligands rationalized the high stereospecificity of the
cyclometalation reaction of p-Cr(CO)3 complexes used as
ligands by pseudo-tetrahedral (T-4) and octahedral (OC-6)
centres.6,7 Charged (cationic or anionic) planar chiral analogues
constitute a class of organometallic compounds of high value
because their resolution into enantio-enriched compounds
should be readily achievable in principle.8 Surprisingly, only
complexes of SP-4 chelates have been reported in the literature
so far.5,9–11 In this communication, we disclose the efficient
and stereospecific syntheses of unprecedented cationic and
binuclear planar chiral iridacycles from the face-selective
p-coordination of an electron-rich aniline-derived ligand with
cationic metal centres. Dynamic variable temperature 1H NMR
experiments, backed with thorough DFT investigations, further
of [Cp*Ru(MeCN)3][PF6]14 and {Cp*Ir[Me2C(O)]3}[PF6]2
15
(Scheme 1). In all cases, according to X-ray diffraction analysis,
the products of the reaction with these cationic reagents
possess endo stereochemistry (Scheme 1), i.e the geometry
wherein the Ir-bound chlorido ligand and the p-bonded metal
moiety are in a syn relationship with respect to the mean plane
of the chelate.
The structuresz of complexes 2a, [endo-3a][PF6]2 and
[endo-4a][PF6] are displayed in Fig. 1. Notably, the synthesis
of the corresponding neutral, Cr(CO)3-containing analogs
yielded, as expected,7 the exo stereoisomers [exo-5a and b].
This was the case regardless of the synthetic method chosen,
i.e. either the direct ligand exchange reaction of 2b with
(Z6-naphthalene)Cr(CO)316 (quantitative) or the cycloiridation7
a
`
Laboratoire de Syntheses Metallo-induites, Institut de Chimie,
Universite de Strasbourg, 4 rue Blaise Pascal, F-67000 Strasbourg,
´
´
France. E-mail: djukic@unistra.fr; Tel: +33 (0)368 85 15 23
´ ´ ´ ´
Laboratoire Heteroelements et Coordination, Ecole Polytechnique,
Route de Saclay, 91128 Palaiseau Cedex, France
b
w Electronic supplementary information (ESI) available: (1) full
experimental procedures and analytical data, (2) extensive crystallo-
graphic information, (3) dynamic 1H NMR data, (4) full computa-
tional details with references and (5) cartesian coordinates for
optimized geometries of II–V, [endo-3a]2+ and their associated
rotational transition states TS1-2. CCDC 805113–805116. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0cc05711h
Scheme 1 The syntheses of compounds 2a and b, [endo-3a][PF6]2,
[endo-4a][PF6] and [exo-5a and b].
c
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Chem. Commun., 2011, 47, 3631–3633 3631

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effects.17 The model of [exo-5b], i.e. V, was also simplified by
replacing the tBu group by H. The set of gas phase model
geometries II–V reproduces the trends observed in the
experimental structures; the distance dM increases in the order
[endo-4a]+ B [exo-5b] o [endo-3a]2+ and dN decreases in the
order 2a B [endo-4a]+ B [exo-5b] 4 [endo-3a]2+
.
These observations suggest that the increase in the double
bond character for the Me2N–Cipso bond in [endo-3a]2+ is a
response to the major electron density withdrawal operated by
the Cp*Ir moiety; the Wiberg bond indices (wbis) for the
Me2N–Cipso bond of the models clearly support this assump-
tion. Compared to that of II, the wbis for IV and V are only
slightly larger by 0.03 units. For III the wbi indicates a net
shift to some double bond character with a value of 1.35.
Alternatively, analysis of the natural molecular orbitals (NMOs)
indicates that there is no strong p-type donor–acceptor inter-
action between the nitrogen’s lone pair and the arene’s
p-system. Natural population at the lone pair is evaluated to
be ca. 1.66 eꢀ for II, IV and V, whereas for III it is 1.54 eꢀ.
The Me2N–Cipso bond possesses a dominant s character. The
extent of the electron population ‘‘transfer’’ from the lone pair
of NMe2 to the rest of the molecule was qualitatively evaluated
by using the rotamers resulting from the 901 rotation of
the Me2N group around the Cipso–N axis as reference struc-
tures. These structures are conformational transition states
(Fig. 2, TS1 and TS2): the natural electron population at the
lone pair rises to ca. 1.86 eꢀ for TS1-2II and similar values
are obtained for TS1-2III–V. Consequently, upon relaxation of
TS1-2 to the ground state (GS, Fig. 2), ca. 0.2 eꢀ in IV and V
and ca. 0.3 eꢀ in III are being transferred to the rest of the
molecule.
Fig. 1 ORTEP-type drawings of the structures of (a) 2a, (b) [endo-3a]2+
,
(c) [endo-4a]+, (d) [exo-5b] with partial atom numbering. Ellipsoids
were drawn at a 30% probability level. Hydrogen atoms and molecules
of solvent were omitted for the sake of clarity.
of ligands 1c and d (76 and 72% isolated yield, respectively) in
the conditions used for the synthesis of 2a and b. Table 1
provides
a selection of geometric parameters for 2a,
[endo-3a][PF6]2, [endo-4a][PF6] and [exo-5b]. The slight variation
of dCl is worthy of note, as it varies in response to the electron-
withdrawing effect of the p-bonded metal, i.e. by shortening
the Ir–Cl distance following the order 2a 4 exo-5b B endo-4a 4
endo-3a. The distance separating the ipso position bearing
the amino group and the p-bound metal, i.e. dM, formally
increases as the charge of the complex increases. As expected,
longer dM and shorter dN distances are observed for the
bis-iridium complex [endo-3a]2+. Table 1 lists geometrical
parameters selected from the relaxed gas-phase geometries of
models of the compounds addressed herein, i.e. II–V. For
practical reasons, the Cp* ligand was replaced by Cp, which does
not greatly affect the overall analysis of the stereo-electronic
The most remarkable feature of [endo-3a][PF6]2 is the
diastereotopicity of the methyl groups of the NMe2 substituent,
which is expressed in the corresponding room temperature
1H NMR spectra (CD2Cl2) by two distinct singlets appearing
at d3.59 and 3.46 ppm (Dd(CD2Cl2) = 0.15 ppm). This feature
is not observed with the other complexes, which all display a
single signal for the two methyls of the –NMe2 group, from
+20 1C down to ꢀ70 1C. It is worth noting that the inter-peak
distance Dd separating the two methyl singlets depends on the
nature of the anions present in the solution; the same applies
to the aromatic region. For [endo-3a][ClO4]2 Dd(CD2Cl2) =
0.21 ppm. The barriers to rotation, i.e. DGrot (T = 298.15 K),
associated with the CAr–NMe2 rotor, were computed by
considering the two possible orientations of the amino group
depicted in TS1 and TS2 (Fig. 2). The DGrot values com-
puted for IV and V are comparable to that computed for II
(Table 2). One can infer that the electronic effect of the
p-bonded Cr(CO)3 and CpRu+ (and by extension Cp*Ru+)
Table 1 A list of selected experimentalz (arabic numbers) and
theoretical (roman numerals) interatomic distances (A) and intra-
annular angles a (1). Wiberg bond indices (wbi) are provided for the
computed model geometries (cf. ESIw for full computational details)
wbib dN
wbib dCl
wbib aa
a
dM
a
a
Cmpd
2a
IIa
—
—
1.379(3)
1.387
1.326(7)
2.4122(6) 118.0(2)
—
1.15 2.401
2.373(2)
1.35 2.388
2.387(4)
1.19 2.398
2.402(1)
1.18 2.407
0.52 118.9
114.6(5)
0.50 112.2
117.5(3)
0.51 116.1
117.2(2)
0.51 117.3
[endo-3a]2+ 2.444(3)
IIIa
2.524
0.09 1.335
1.378(4)
0.18 1.371
1.366(6)
0.14 1.375
[endo-4a]+ 2.325(3)
IVa
2.387
2.359(5)
2.391
[exo-5b]
Va
a
b
Calculated from relaxed gas-phase geometries. Computed from the
natural atomic orbital basis (NAO) for relaxed gas-phase geometries.
Fig. 2 Newman-type views of the singlet ground state (GS) and
rotational transition states (TS1 and TS2).
c
3632 Chem. Commun., 2011, 47, 3631–3633
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Table 2 Theoretical and experimental Gibbs rotational barriers
(DGrot in kcal molꢀ1) at 298.15 K of the –NMe2 group (see ESIw for
full computational details)
supplemented by largely negative CO oxygens, disqualifies the
endo arrangement, which is in full agreement with earlier
observations.17
In summary, this study shows that cationic and bis-cationic
(T-4) iridacycles derived from 2-anilinylpyridines can be
synthesized stereospecifically in high yield by reaction with
the labile solvato complexes of ‘‘Cp*Ru+’’ and ‘‘Cp*Ir2+’’.
Theoretical investigation points to the central role of Coulomb
interactions in the preference given to the endo isomer of
the cationic binuclear products. The key role of the NMe2
group has also been outlined with the support of DFT. The
rotational properties disclosed here indicate that the high
rotational barrier results exclusively from electronic effects.
This work was funded by the CNRS and the University of
Strasbourg. Dr Lydia Brelot is thanked for the resolution of
the structure of 2a.
Theory
Experiment
a
a
a
DGrot1
DGrot2
DGrot
Model/compound
IIb
6.6
21.3
11.8
10.6
17.9
6.6
20.9
11.5
12.1
15.3
IIIb
IVb
Vb
[endo-3a]2+c
16.7(3)d
a
b
Calculated for T = 298.15 K. Gas-phase relaxed structure. COSMO
c
d
(acetone) relaxed structure. [endo-3a][PF6]2 in d6.acetone, 1H NMR
(500 MHz, 298 K o T o 343 K), line shape analysis.
fragments on the rotational barrier of NMe2 is rather limited,
which explainsꢀ why the diastereotopic Me groups in 2a,
[endo-4a]+,PF6 and [exo-5b] resonate as a single (coalesced)
singlet within both H and 13C NMR time scale. DGrot values
1
Notes and references
for III are about three times larger than those for II. Note here
that the values of DGrot1 and DGrot2 are relatively similar in
most cases. VT H NMR experiments were carried out with
z See ESIw for detailed crystallography.
1
y Ionic strength I was adjusted by adding [N(nBu)4][PF6] in three
different VT NMR experiments, with I = 27 (no added salt), 37
(1 eq. of salt) and 121 mM (10 eq. of salt).
9.3 mM solutions of [endo-3a][PF6]2 in d6-acetone and sub-
sequent line-shape analysis of the dynamics of mutual exchange
involving the methyls of the –NMe2 substituent yielded the
associated rotational barrier.18 Worthy to note, the exchange
was found to be insensitive to variations of ionic strength (I) in
acetone.y Linear fitting (R2 = 0.99) of the Eyring plot, i.e.
ln(krot/T) vs. 1/T. (krot: rotational exchange rate constant),
yielded a value for DGrot (298.15 K) of +16.7(3) kcal molꢀ1
(DHrot = +12.9(4) kcal molꢀ1, DSrot = ꢀ12(1) cal molꢀ1 Kꢀ1).
This value of DGrot was corroborated by further DFT calcula-
tions (including solvation) using the actual structure of the
bis-cation [endo-3a]2+ and its associated transition rotamers
TS1-23a (Table 2). Information on the relative rehybridization
of the nitrogen atom of the NMe2 group upon rotation can be
inferred from the geometries of TS1-23a (Fig. S1, ESIw).
It is clear that the coordination of ‘‘Cp*Ir2+’’ to metallacycle
2a draws the electronic structure of the anilinyl fragment
slightly closer to that of a Z5-cyclohexadienyliminium limit
form. The decrease of the wbi associated with the increase
of dM is consistent with a net shift to a quasi Z5-coordination
mode in III.19 The largest variation of natural (negative)
charge at the nitrogen atom of the amino group relative
to II, i.e Dq(i)N = qN(i) ꢀ qN(II) (qN(II) = ꢀ0.398), is found
for III (DqN(III) = 0.06), whereas only minor variations are
computed for IV and V (0.008 and 0.004, respectively).
Analysis of the natural charges borne by the p-bound metals
provides a rationale for the observed facial selectivity in the
formation of [endo-3a]2+ and [endo-4a]+. In both models III
(qp-Ir = +0.58) and IV (qp-Ru = +0.29) the natural charge at
the p-bound metal is significantly positive, which favors
stabilizing interactions with the vicinal Ir-bound chlorido
ligand (qCl B ꢀ0.44). The strong charge density surrounding
the latter atom is somewhat encapsulated within a ‘‘pocket’’ of
low charge density, i.e. the rest of the molecule (cf. ESIw).
For V, a large negative charge at the Cr atom (qp-Cr B ꢀ0.89),
1 M. E. Gunay and C. J. Richards, Organometallics, 2009, 28,
5833–5836.
2 M. Albrecht, Chem. Rev., 2010, 110, 576–623.
3 S. Jautze, S. Diethelm, W. Frey and R. Peters, Organometallics,
2009, 28, 2001–2004.
4 J.-P. Djukic, A. Hijazi, H. D. Flack and G. Bernardinelli, Chem.
Soc. Rev., 2008, 37, 406–425.
5 S. Bonnet, M. Lutz, A. L. Spek, G. van Koten and R. J. M. Klein
Gebbink, Organometallics, 2008, 27, 159–162.
6 A. Hijazi, J.-P. Djukic, L. Allouche, A. de Cian, M. Pfeffer,
X.-F. Le Goff and L. Ricard, Organometallics, 2007, 26,
4180–4196.
7 C. Scheeren, F. Maasarani, A. Hijazi, J.-P. Djukic, M. Pfeffer,
S. D. Zaric, X.-F. Le Goff and L. Ricard, Organometallics, 2007,
26, 3336–3345.
8 S. Bonnet, J. Li, M. A. Siegler, L. S. von Chrzanowski, A. L. Spek,
G. van Koten and R. J. M. Klein Gebbink, Chem.–Eur. J., 2009,
15, 3340–3343.
9 S. Bonnet, J. H. van Lenthe, M. A. Siegler, A. L. Spek, G. van
Koten and R. J. M. Klein Gebbink, Organometallics, 2009, 28,
2325–2333.
10 S. Bonnet, M. Lutz, A. L. Spek, G. van Koten and R. J. M. Klein
Gebbink, Organometallics, 2010, 29, 1157–1167.
11 J.-P. Djukic, L. Fetzer, A. Czysz, W. Iali, C. Sirlin and M. Pfeffer,
Organometallics, 2010, 29, 1675–1679.
12 J. W. Kang, K. Moseley and P. M. Maitlis, J. Am. Chem. Soc.,
1969, 91, 5970–5977.
13 D. L. Davies, O. Al-Duaij, J. Fawcett, M. Giardiello, S. T. Hilton
and D. R. Russell, Dalton Trans., 2003, 4132–4138.
14 M. D. Mbaye, B. Demerseman, J. L. Renaud, L. Toupet and
C. Bruneau, Adv. Synth. Catal., 2004, 346, 835–841.
15 I. I. R. Salas, M. A. Paz-Sandoval and H. Noth, Organometallics,
2002, 21, 4696–4710.
¨
16 J. A. S. Howell, N. F. Ashford, D. T. Dixon, J. C. Kola,
T. A. Albright and S. K. Kang, Organometallics, 1991, 10,
1852–1864.
17 J.-P. Djukic, C. Boulho, D. Sredojevic, C. Scheeren, S. Zaric,
L. Ricard and M. Pfeffer, Chem.–Eur. J., 2009, 15, 10830–10842.
18 J. Sandstrom, Dynamic NMR spectroscopy, Academic Press,
¨
London, 1982.
19 A. D. Hunter, L. Shilliday, W. S. Furey and M. J. Zaworotko,
Organometallics, 1992, 11, 1550–1560.
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This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 3631–3633 3633