Preparation, structure and electrochemical behavior of dinuclear cyclooctadiene-chelated Ir(I) complexes with 2-aminopyridinato bridges

Article abstract of DOI:10.1016/S0020-1693(99)00218-2

Dinuclear iridium(I) complexes with two bridging aminopyridinato ligands, [Ir(μ-L)(COD)]₂ (COD=1,5-cyclooctadiene (1): L=2-aminopyridinato (ap) (2): L=2-anilinopyridinato (anp)), were prepared from [Ir(μ-Cl)(COD)]₂ and Li⁺L^- in a 24-36% yield. These compounds were characterized by X-ray structure analysis and ¹H NMR spectroscopy. Two iridium atoms were bridged by two aminopyridinato ligands and each iridium atom was coordinated by one chelating COD. The coordination sphere of each Ir(I) center, which was formed by two N atoms and two olefinic π bonds, was square planar. IrIr separations in 1 and 2 were 3.0998(6) and 3.0681(3) A, respectively. Cyclic voltammetry of 1 and 2 in n-Bu₄NPF₆/CH₂Cl₂ exhibited a chemically reversible oxidation wave at -0.41 and -0.36 V versus Fc⁺/Fc, respectively. These potentials were lower than those of [Ir(μ-form)(COD)]₂ (form=anion of N,N′-di-p-tolylformamidine), [Ir(μ-hp)(COD)]₂ (hp=anion of 2-hydroxypyridine), [Ir(μ-mhp)(COD)]₂ (mhp=anion of 6-methyl-2-hydroxypyridine) and [Ir(μ-pz)(COD)]₂ (pz=anion of pyrazole). Electrolytic one-electron oxidation of 2 gave its stable cationic radical, [Ir(μ-anp)(COD)]₂⁺, of which frozen solution ESR spectrum was rhombic with g₁=2.43, g₂=2.30 and g₃=2.08 and no hyperfine splitting was resolved.

Full text of DOI:10.1016/S0020-1693(99)00218-2

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Inorganica Chimica Acta 292 (1999) 244–248
Preparation, structure and electrochemical behavior of dinuclear
cyclooctadiene-chelated Ir(I) complexes with 2-aminopyridinato
bridges
Naohiro Kanematsu, Masahiro Ebihara, Takashi Kawamura *
Department of Chemistry, Faculty of Engineering, Gifu Uni6ersity, Yanagido, Gifu 501-1193, Japan
Received 14 January 1999; accepted 23 April 1999
Abstract
Dinuclear iridium(I) complexes with two bridging aminopyridinato ligands, [Ir(m-L)(COD)]2 (COD=1,5-cyclooctadiene (1):
L=2-aminopyridinato (ap) (2): L=2-anilinopyridinato (anp)), were prepared from [Ir(m-Cl)(COD)]2 and Li+L− in a 24–36%
yield. These compounds were characterized by X-ray structure analysis and 1H NMR spectroscopy. Two iridium atoms were
bridged by two aminopyridinato ligands and each iridium atom was coordinated by one chelating COD. The coordination sphere
of each Ir(I) center, which was formed by two N atoms and two olefinic p bonds, was square planar. Ir···Ir separations in 1 and
,
2 were 3.0998(6) and 3.0681(3) A, respectively. Cyclic voltammetry of 1 and 2 in n-Bu4NPF6/CH2Cl2 exhibited a chemically
reversible oxidation wave at −0.41 and −0.36 V versus Fc+/Fc, respectively. These potentials were lower than those of
[Ir(m-form)(COD)]2 (form=anion of N,N%-di-p-tolylformamidine), [Ir(m-hp)(COD)]2 (hp=anion of 2-hydroxypyridine), [Ir(m-
mhp)(COD)]2 (mhp=anion of 6-methyl-2-hydroxypyridine) and [Ir(m-pz)(COD)]2 (pz=anion of pyrazole). Electrolytic one-elec-
tron oxidation of 2 gave its stable cationic radical, [Ir(m-anp)(COD)]2+, of which frozen solution ESR spectrum was rhombic with
g1=2.43, g2=2.30 and g3=2.08 and no hyperfine splitting was resolved. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Electrochemistry; Iridium complexes; Aminopyridinate complexes; Dinuclear complexes
tothiazoline) [8] and mpm (anion of (6-methyl-2-
pyridyl)methyl) [9] and yields were 55–90%. In terms of
oxidation of Ir(I) complexes to Ir(II) species, complexes
with strong donor ligands such as aminopyridinato
ligands are expected to facilitate their oxidation. We
report here on the synthesis, structure and electrochem-
istry of Ir(I) dinuclear complexes with 2-aminopyridi-
nato and 2-anilinopyridinato ligands.
1. Introduction
A variety of lantern type dinuclear complexes have
been synthesized and characterized in the last three
decades [1]. Although the chemistry of lantern type
Rh(II) dinuclear complexes with a variety of ligands
has been extensively studied [1], only a few of the
corresponding dinuclear complexes of Ir(II) have been
reported [2]. Since metal–metal and metal–ligand elec-
tronic interactions are expected to be enhanced in 5d
metal complexes, we considered that development of
synthetic route of Ir(II) dimers is an important field.
We are interested in Ir(I) dimers, [Ir(m-L)(COD)]2, as a
starting material for Ir(II) complexes. [Ir(m-L)(COD)]2
complexes have been synthesized by the substitution
reaction of [Ir(m-Cl)(COD)]2 with ligands such as form
[3], hp [4], mhp [4,5], pz [6,7], mpz (anion of 2-mercap-
2. Experimental
2.1. General
All operations were carried out under an argon at-
mosphere using the standard Schlenk line techniques.
Solvents were distilled from P2O5. 2-Aminopyridine and
2-anilinopyridine were recrystallized from hexane. [Ir(m-
Cl)(COD)]2 was prepared by following a literature
method [10].
* Corresponding author. Tel.: +81-58-239 2567; fax: +81-58-230
1893.
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00218-2

N. Kanematsu et al. / Inorganica Chimica Acta 292 (1999) 244–248
245
Table 1
C38H42Ir2N4: C, 48.60; H, 4.51; N, 5.97. Found: C,
48.62; H, 4.55; N, 5.90%. 1H NMR (CD2Cl2, 20°C,
ppm) l 8.26 (d, 2H), 7.41 (br, 8H), 7.15 (m, 2H), 6.72
(t, 2H), 6.03 (t, 2H), 5.67 (d, 2H), 4.17 (m, 2H), 3.80
(m, 2H), 3.27 (m, 2H), 2.89 (m, 2H), 2.62 (br, 4H), 2.10
(m, 2H), 2.00 (m, 2H), 1.59 (m, 6H), 1.39 (m, 2H).
Crystallographic data for [Ir(m-ap)(COD)]2 (1) and [Ir(m-anp)(COD)]2
(2)
1
2
Empirical formula
Formula weight
Crystal color
C26H34Ir2N4
787.02
dark red
0.30×0.10×0.05 0.65×0.10×0.10
P1 (no. 2)
9.975(2)
14.573(3)
9.108(1)
97.14(2)
110.71(1)
101.57(2)
1185.5(4)
2
C38H42Ir2N4
939.22
dark red
Crystal dimensions (mm3)
Space group
2.4. Crystallography
(
(
P1 (no. 2)
,
a (A)
10.506(1)
18.138(2)
9.1563(8)
96.882(8)
114.034(6)
86.263(9)
1581.8(3)
2
1.972
84.62
−80
7253
Crystallographic data and details of the structure
refinement for 1 and 2 are summarized in Table 1.
Intensity data were collected on a Rigaku AFC7R
diffractometer by the ꢀ–2q scan method in the range
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
,
4B2qB55° with Mo Ka radiation (u=0.71069 A).
3
,
V (A )
Z
All calculations were performed by using a TEXSAN
crystallographic software package [11]. All structures
were solved using the Patterson method. The data were
corrected for absorption by DIFABS [12]. Final full-ma-
trix least-squares cycle included all non-hydrogen atoms
with anisotropic thermal parameters and all hydrogen
atoms at calculated positions using idealized geometries
zcalc (g cm−3
)
2.205
112.67
−80
5427
3848
v(Mo Ka) (cm−1
Temperature (°C)
No. unique data
)
Data with I\2|(I)
No. of variables
Transmission factors, min./
max.
6028
398
0.195/0.429
290
0.332/0.569
,
of 0.95 A of CꢀH distance with isotropic thermal
a
parameters which were 1.2 times the connected atoms.
R
0.039
0.046
1.19
0.033
0.044
1.56
b
Rw
Goodness-of-fit indicator c
2.5. Measurements
a R=ꢀꢁFoꢂ−ꢂFcꢁ/ꢀꢂFoꢂ.
b Rw=[ꢀw(ꢂFoꢂ−ꢂFcꢂ)2/ꢀwꢂFoꢂ2]1/2; w=1/|2(ꢂFoꢂ).
1H NMR spectra were recorded on a JEOL a400
spectrometer. UV–Vis absorption spectra were ob-
tained by using a Hitachi U-3500 spectrophotometer.
ESR spectra were measured on a JEOL JES-TE200
spectrometer at 77 K. Cyclic voltammetry (CV) studies
were carried out by using a Hokuto HB-111 function
generator and an HA-105 potentiostat/galvanostat. The
working, counter and reference electrodes were a glassy
carbon button, a platinum wire and a BAS RE-5
Ag+/Ag electrode, respectively. All the CV were mea-
sured at the potential sweep rate of 100 mV s−1. For
the electrolysis study the working electrode was a plat-
inum wire and the counter was a gold wire. The sup-
porting electrolyte was tetra-n-butylammonium hexa-
fluorophosphate. To correct liquid junction potential,
the oxidation potential of ferrocene (Fc) in the same
electrolytic solution was recorded after each CV mea-
surement and the electrode potentials were converted
into those relative to Fc+/Fc.
c Goodness-of-fit indicator=[ꢀw(ꢂFoꢂ−ꢂFcꢂ)2/(Nobs−Nparameters)]1/2
.
2.2. Preparation of [Ir(v-ap)(COD)]2 (1)
A solution of lithium 2-aminopyridinate which was
prepared by stirring 2-aminopyridine (0.059 g, 0.63
mmol) and t-BuLi (0.45 ml of 1.7 M solution in pen-
tane, 0.68 mmol) in toluene (10 ml) for 1 h at room
temperature (r.t.) was added to [Ir(m-Cl)(COD)]2 (0.20
g, 0.29 mmol). After stirring for 12 h the resulting
dark-red solution was filtered and evaporated to dry-
ness. The dark-red residue was recrystallized from hex-
ane to give 1 as dark-red crystals: yield 0.054 g (24%).
Anal. Calc. for C26H34Ir2N4: C, 39.68; H, 4.35; N, 7.12.
Found: C, 39.43; H, 4.48; N, 6.95%. 1H NMR (CD2Cl2,
−40°C, ppm) l 7.74 (d, 2H), 6.81 (t, 2H), 6.08 (d, 2H),
6.00 (t, 2H), 4.74 (br, 2H), 3.71 (br, 2H), 3.58 (br, 2H),
3.52 (br, 2H), 2.71 (br, 2H), 2.59 (br, 2H), 2.29 (br,
4H), 1.92 (br, 2H), 1.74 (br, 4H), 1.32 (br, 4H). At r.t.
the COD resonances exhibited fluxional behavior.
3. Results and discussion
2.3. Preparation of [Ir(v-anp)(COD)]2 (2)
3.1. Synthesis
The preparation was similar to that of [Ir(m-ap)-
(COD)]2. Starting materials of [Ir(m-Cl)(COD)]2 (0.20 g,
0.30 mmol), Hanp (0.10 g, 0.59 mmol) and t-BuLi (0.35
ml of 1.7 M solution in pentane, 0.60 mmol) gave 2 as
dark-red crystals: yield 0.10 g (36%). Anal. Calc. for
The reaction of [Ir(m-Cl)(COD)]2 with lithium salts of
aminopyridines in toluene gave [Ir(m-L)(COD)]2 (1:
L=ap, 2: L=anp) with yields between 24 and 36%.
These compounds in solution were unstable in air, but

246
N. Kanematsu et al. / Inorganica Chimica Acta 292 (1999) 244–248
Table 2
,
Selected bond distances (A) and angles (°) for [Ir(m-ap)(COD)]2 (1)
and [Ir(m-anp)(COD)]2 (2)
1
2
Bond distances
Ir(1)ꢀN(1)
Ir(1)ꢀN(4)
Ir(1)ꢀC(11)
Ir(1)ꢀC(12)
Ir(1)ꢀC(15)
Ir(1)ꢀC(16)
Ir(2)ꢀN(2)
Ir(2)ꢀN(3)
Ir(2)ꢀC(19)
Ir(2)ꢀC(20)
Ir(2)ꢀC(23)
Ir(2)ꢀC(24)
2.133(8)
2.044(8)
2.08(1)
2.124(9)
2.13(1)
Ir(1)ꢀN(1)
Ir(1)ꢀN(4)
Ir(1)ꢀC(23)
Ir(1)ꢀC(24)
Ir(1)ꢀC(27)
Ir(1)ꢀC(28)
Ir(2)ꢀN(2)
Ir(2)ꢀN(3)
Ir(2)ꢀC(31)
Ir(2)ꢀC(32)
Ir(2)ꢀC(35)
Ir(2)ꢀC(36)
2.125(5)
2.078(5)
2.109(6)
2.121(6)
2.125(6)
2.141(6)
2.098(4)
2.155(5)
2.120(6)
2.110(6)
2.111(6)
2.139(6)
2.13(1)
2.048(8)
2.135(8)
2.124(10)
2.133(10)
2.10(1)
2.11(1)
Bond angles
N(1)ꢀIr(1)ꢀN(4)
N(2)ꢀIr(2)ꢀN(3)
Ir(1)ꢀN(1)ꢀC(1)
Ir(1)ꢀN(1)ꢀC(5)
Ir(1)ꢀN(4)ꢀC(10) 126.9(7)
Ir(2)ꢀN(2)ꢀC(5)
Ir(2)ꢀN(3)ꢀC(6)
Ir(2)ꢀN(3)ꢀC(10) 123.8(6)
N(1)ꢀC(5)ꢀN(2) 117.2(8)
N(3)ꢀC(10)ꢀN(4) 117.8(9)
90.0(3)
89.4(3)
116.4(7)
124.0(6)
N(1)ꢀIr(1)ꢀN(4)
N(2)ꢀIr(2)ꢀN(3)
Ir(1)ꢀN(1)ꢀC(1)
Ir(1)ꢀN(1)ꢀC(5)
Ir(1)ꢀN(4)ꢀC(16)
Ir(1)ꢀN(4)ꢀC(17)
Ir(2)ꢀN(2)ꢀC(5)
Ir(2)ꢀN(2)ꢀC(6)
Ir(2)ꢀN(3)ꢀC(12)
Ir(2)ꢀN(3)ꢀC(16)
N(1)ꢀC(5)ꢀN(2)
N(3)ꢀC(16)ꢀN(4)
88.3(2)
87.6(2)
Fig. 1. ORTEP view of [Ir(m-ap)(COD)]2 (1) with the 50% probability
ellipsoids.
112.7(4)
124.4(4)
120.4(4)
122.6(4)
123.2(4)
120.7(4)
113.5(4)
126.8(4)
117.9(5)
118.2(5)
in crystalline form they were stable in air for at least a
month.
127.3(7)
114.8(7)
3.2. Structure
ORTEP views of 1 and 2 are depicted in Figs. 1 and 2,
respectively. Important distances and angles of 1 and 2
are summarized in Table 2. In complex 1 two iridium
atoms are bridged by two aminopyridinato ligands in a
head-to-tail arrangement. The coordination sphere of
each Ir(I) center is square planar, which is made up of
two p bonds of a chelating COD ligand, one pyridine
nitrogen atom from one bridging aminopyridinato
group and an amido nitrogen atom of the other. Ge-
ometry of the bridging aminopyridinato and COD lig-
ands is normal. The average IrꢀN(pyridine) distance of
,
2.134(8) A which is similar to those in [Ir(m-
mhp)(COD)]2 [4,5] and [Ir(m-pz)(COD)]2 [6,7] is longer
than the IrꢀN(amido) distance of 2.046(8) A. The aver-
age IrꢀC distance of 2.11(1) A is similar to those of
reported [Ir(m-L)(COD)]2 complexes [4–7]. In complex
2 the average IrꢀN(pyridine) and IrꢀN(amido) distances
were 2.140(5) and 2.088(5) A, respectively. Complexes 1
and 2 have a pseudo-two-fold axis bisecting the line
between the two Ir atoms. Dihedral angles between the
two coordination planes of the metal atoms were 54.2
and 55.1° for 1 and 2, respectively. N1ꢀIr1ꢀIr2ꢀN2
torsion angles for 1 and 2 were 29.3(4) and 31.3(2)°,
respectively. These dihedral and torsion angles are sim-
ilar to those of [Ir(m-mhp)(COD)]2 where the dihedral
and torsion angles were 55.5 and 37.4(2)° [4]. The Ir···Ir
,
,
,
,
separations in 1 and 2 are 3.0998(6) and 3.0681(3) A,
respectively (Table 3). These Ir···Ir distances are very
,
similar to each other and are shorter than 3.242(1) A in
,
[Ir(m-mhp)(COD)]2 [4,5] and 3.161(1) A in [Ir(m-
pz)(COD)]2 [6,7]. There is no formal IrꢀIr bond in these
complexes.
3.3. Properties
The UV–Vis absorption spectra of 1 and 2 are
shown in Fig. 3. The longest wavelength absorption
peak was observed at 536 and 562 nm for 1 and 2,
Fig. 2. ORTEP view of [Ir(m-anp)(COD)]2 (2) with the 50% probability
ellipsoids. Hydrogen atoms are omitted for clarity.

N. Kanematsu et al. / Inorganica Chimica Acta 292 (1999) 244–248
247
Table 3
Ir···Ir separation in [Ir(m-L)(COD)]2
,
L
Ir···Ir (A)
Ref.
a
a
ap
anp
pz
mhp
mtz
mpm
3.0998(6)
3.0681(3)
3.216(1)
3.242(1)
3.5434(1)
3.6806(3)
[7]
[4]
[8]
[9]
a Present work.
Fig. 4. Cyclic voltammogram of [Ir(m-ap)(COD)]2 (1) (a and b) and
[Ir(m-anp)(COD)]2 (2) (c) in CH2Cl2 containing 0.1 M n-Bu4NPF6.
Fig. 3. UV–Vis spectra of [Ir(m-ap)(COD)]2 (1) (solid line) and
[Ir(m-anp)(COD)]2 (2) (dotted line) in CH2Cl2 solution.
Table 4
Oxidation potentials of [Ir(m-L)(COD)2]
L
E11/2 (V a)
E2pa (V a,b
)
Ref.
respectively, which is red shifted compared with those
of [Ir(m-mhp)(COD)]2 (484 nm) [13] and [Ir(m-
pz)(COD)]2 (499 nm) [14]. These bands have been as-
signed primarily to the metal-centered ds*–ps
transition which has been diagnostic of the d8–d8 inter-
action [13,14]. The short metal–metal separation in the
present complexes causes strong M···M interaction and
the occupied ds*IrꢀIr orbital would be destabilized and
the unoccupied psIrꢀIr orbital is expected to be stabi-
lized. As a result, the ds*–ps energy gap becomes
smaller and the corresponding transition shifts to a
longer wavelength. Thus, for the present aminopyridi-
nato complexes the red shift of these bands is consistent
with their short Ir···Ir separations.
The cyclic voltammogram of 1 in 0.1 M n-Bu4NPF6/
CH2Cl2 exhibited a partially reversible oxidation wave
at −0.41 V and an irreversible oxidation peak at 0.45
V versus Fc+/Fc as shown in Fig. 4. Complex 2
exhibited a chemically reversible oxidation response at
−0.36 V versus Fc+/Fc followed by an irreversible
oxidation peak at 0.44 V versus Fc+/Fc (Table 4).
These first oxidation potentials are about 0.3–0.5 V
lower than those of [Ir(m-mhp)(COD)]2 [4], [Ir(m-
pz)(COD)]2 [4], [Ir(m-form)(COD)]2 [15] and [Ir(m-
hp)(COD)]2 [15]. These low oxidation potentials should
be due to increased electron donor properties of the
present aminopyridinato ligands in comparison to those
of mhp and pz ligands and also to the upward shift of
c
c
ap
−0.41
−0.36
−0.13
−0.36
0.06
+0.45
anp
mhp
pz
form
hp
+0.44
d
[4]
[7]
[15]
[15]
d
d
d
0.23
a Potential vs. Fc+/Fc.
b Irreversible.
c Present work.
d Not reported.
the energy level of the ds* HOMO (vide infra) induced
by the short Ir···Ir separation. Electrolytic oxidation of
2 was carried out at −0.02 V versus Fc+/Fc in n-
Bu4NPF6/CH2Cl2. The coulometry showed that this
process is a one-electron oxidation. After the electroly-
sis the cyclic voltammogram of the solution showed a
quasi-reversible reduction wave at the same potential as
the oxidation of 2. The ESR spectrum of the elec-
trolyzed solution frozen at 77 K was obtained as shown
in Fig. 5. This spectrum is rhombic with g1=2.43,
g2=2.30 and g3=2.08 and hyperfine coupling was not
resolved. The large shifts of the principal values of the
g tensor from the free spin value (ge=2.002) shows that
the odd electron of the cationic radical, 2+, is dis-
tributed predominantly on the iridium atoms, most
probably resulting in an Ir···Ir bond with a bond order

248
N. Kanematsu et al. / Inorganica Chimica Acta 292 (1999) 244–248
(COD)]2. Copies of the data can be obtained free of
charge on application to The Director, CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (Fax: +44-
1223-336-033 or e-mail: deposit@ccdc.cam.ac.uk).
Acknowledgements
We are grateful for the financial support from the
Ministry of Education, Science and Culture of Japan.
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Crystallographic data (excluding structure factors)
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