Preparation and structure of bis(μ-acetato)dichlorodicarbonyldiiridium(II) complexes with Group 15 ligands and ESR and density functional theory studies of electronic structure of their cationic radicals

Article abstract of DOI:10.1016/S0020-1693(01)00615-6

bis(μ-Acetato)dichlorodicarbonyldiiridium(II) complexes with Group 15 compounds as axial ligands, [Ir₂(μ-O₂CMe)₂Cl₂(CO) ₂L₂] (L=PPh₃: 1, PCy₃: 2, P(OPh)₃: 3, AsPh₃: 4, SbPh₃: 5) were synthesized. The Ir-Ir distances of complexes 2-5 (2.6200(9)-2.6936(7) A?) were longer than those of the complexes with MeCN, py or dmso ligands at the axial sites. Cyclic voltammograms of 1-5 show a chemically reversible one-electron oxidation wave of which E_1/2 (vs. Fc⁺-Fc) values were between 0.22 for 2 and 0.75 V for 3 depending on the axial ligands. Oxidation by electrolysis and/or radiolysis of 1, 2 and 4 gave their cationic radicals. The ESR spectra of 1^+·, 2^+· and 4^+· at 77 K were pseudo-axially symmetric with g tensors of g_⊥=2.15 and g_∥=1.96, 2.18 and 1.95, and 2.20 and 1.96, respectively. Their hyperfine coupling indicates that their odd electron is delocalized equivalently onto the two axial phosphorous or arsenic atoms. The odd electron densities were estimated from the hyperfine coupling tensors as ρ≈0.1 on the P atoms of 1^+· and 2^+· and ρ≈0.15 on the As atoms of 4^+·. These results indicate that their singly occupied molecular orbital (SOMO) is the σ_IrIr orbital with σ_IrP* or σ_IrAs* character. DFT calculations for model complexes, [Ir₂(μ-O₂CH)₂Cl₂(CO) ₂(PH₃)₂]^+· (6^+·) and [Ir₂(μ-O₂CH)₂Cl₂(CO) ₂(AsH₃)₂]^+· (7^+·), gave electronic structures consistent with the ESR results.

Full text of DOI:10.1016/S0020-1693(01)00615-6

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Inorganica Chimica Acta 323 (2001) 96–104
Preparation and structure of
bis(m-acetato)dichlorodicarbonyldiiridium(II) complexes with
Group 15 ligands and ESR and density functional theory studies
of electronic structure of their cationic radicals
Naohiro Kanematsu, Masahiro Ebihara, Takashi Kawamura *
Department of Chemistry, Faculty of Engineering, Gifu Uni6ersity, Yanagido, Gifu 501-1193, Japan
Received 27 April 2001; accepted 6 August 2001
Abstract
bis(m-Acetato)dichlorodicarbonyldiiridium(II) complexes with Group 15 compounds as axial ligands, [Ir2(m-
O2CMe)2Cl2(CO)2L2] (L=PPh3: 1, PCy3: 2, P(OPh)3: 3, AsPh3: 4, SbPh3: 5) were synthesized. The Ir–Ir distances of complexes
,
2–5 (2.6200(9)–2.6936(7) A) were longer than those of the complexes with MeCN, py or dmso ligands at the axial sites. Cyclic
voltammograms of 1–5 show a chemically reversible one-electron oxidation wave of which E1/2 (vs. Fc+–Fc) values were between
0.22 for 2 and 0.75 V for 3 depending on the axial ligands. Oxidation by electrolysis and/or radiolysis of 1, 2 and 4 gave their
+
+
cationic radicals. The ESR spectra of 1+ , 2 and 4 at 77 K were pseudo-axially symmetric with g tensors of gÞ=2.15 and
gꢀꢀ=1.96, 2.18 and 1.95, and 2.20 and 1.96, respectively. Their hyperfine coupling indicates that their odd electron is delocalized
equivalently onto the two axial phosphorous or arsenic atoms. The odd electron densities were estimated from the hyperfine
’
’
’
+
+
coupling tensors as z:0.1 on the P atoms of 1+ and 2 and z:0.15 on the As atoms of 4 . These results indicate that their
’
’
’
singly occupied molecular orbital (SOMO) is the sIrIr orbital with s*IrP or s*IrAs character. DFT calculations for model complexes,
+
+
+
[Ir2(m-O2CH)2Cl2(CO)2(PH3)2]+ (6 ) and [Ir2(m-O2CH)2Cl2(CO)2(AsH3)2] (7 ), gave electronic structures consistent with the
’
’
’
’
ESR results. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Iridium complexes; Metal–metal bonds; Group 15 ligands; Electronic structures; Electron spin resonance; Density functional theory
calculations
1. Introduction
also prepared and characterized. We have also reported
that cationic radicals derived from 11 and 12 are mod-
erately stable and that their odd electron is accommo-
dated in the d*IrIr orbital. The electron configuration for
the Rh–Rh bond in paddlewheel-type Rh25+ complexes
depends on both their bridging and axial ligands.
Strong s-donor ligands such as phosphines at the axial
sites make the sRhRh orbital the SOMO with its en-
hanced delocalization onto the lone-pair orbitals of the
axial ligands [4–7]. To examine whether the electron
configuration of the Ir–Ir bond in Ir25+ complexes also
depends on the ligands, we synthesize Ir24+ complexes
with ligands with group 15 elements at the axial sites,
1–5 (Scheme 1), and examine their redox behavior by
CV. The electronic structures of these complexes are
explored by ESR of cationic radicals of 1, 2 and 4 and
by density functional theory (DFT) calculations.
Dimetal complexes with metal–metal bond have
been a subject of wide interest in these three decades [1].
Enhanced metal–metal and metal–ligand interactions
are expected for 5d metal cluster compounds. However,
rather limited examples of diiridium(II) complexes have
been explored [2]. We reported a one-step preparation
of an Ir24+ complex, [Ir2(m-O2CMe)2Cl2(CO)2] (8), in a
previous paper [3]. Axial adducts of 8 with solvent
molecules [Ir2(m-O2CMe)2Cl2(CO)2L2] (L=MeCN: 9,
dmso: 10, py: 11, 4-isopropylpyridine: 12) have been
* Corresponding author. Tel.: +81-58-293-2567; fax: +81-58-230-
1893.
E-mail address: kawamura@apchem.gifu-u.ac.jp (T. Kawamura).
0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00615-6

N. Kanematsu et al. / Inorganica Chimica Acta 323 (2001) 96–104
97
residue obtained by evaporation of the solvent was
dissolved into CH2Cl2 (10 ml). Pale yellow crystals were
grown by slow diffusion of hexane (10 ml) to the
solution. Yield 98 mg (72%). Anal. Calc. for
C21H18ClIrO6P: C, 40.36; H, 2.90; P, 4.96; Cl, 5.67.
1
Found: C, 40.39; H, 2.86; P, 4.70; Cl, 5.75%. H NMR
(CD2Cl2, l in ppm): 7.29–7.21 (m, 24H), 7.22 (d, 6H),
1.51 (s, 6H, O2CMe). 31P NMR (CDCl3, l in ppm):
57.90. IR (KBr disc, in cm−1): 2056s, 2039s, 1590m,
1560m, 1490s, 1450m, 1448m, 1212m, 1186s, 1072w,
1025m, 1007w, 920s, 759s, 719w, 688m, 617w, 593w.
UV–VIS (CH2Cl2, umax in nm, and m in parentheses in
M−1 cm−1): 301(20800), 340 sh, 378 sh.
Scheme 1.
2. Experimental
2.1. Syntheses
2.1.1. [Ir2(v-O2CMe)2Cl2(CO)2(PPh3)2] (1)
Complex 8 (98 mg, 0.16 mmol) and PPh3 (87 mg,
0.33 mmol) in toluene (10 ml) were refluxed for 2 h
under Ar to give a yellow solution. The yellow crude
product given by evaporation of the solvent was eluted
through a silica gel column with CH2Cl2/hexane (1:1
v/v). The yellow eluent was dried to give yellow crystals
of 1. Yield 96 mg (53%). Anal. Calc. for
C21H18ClIrO3P: C, 43.71; H, 3.14; P, 5.37. Found: C,
2.1.4. [Ir2(v-O2CMe)2Cl2(CO)2(AsPh3)2] (4)
This complex was prepared by a similar method to
that for complex 2 from complex 8 (86 mg, 0.14 mmol)
and AsPh3 (86 mg, 0.28 mmol) in toluene (10 ml) to
give 4 as yellow crystals. Yield 72 mg (42%). Anal. Calc.
for C21H18AsClIrO3: C, 40.62; H, 2.92; Cl, 5.71. Found:
1
C, 40.35; H, 2.81; Cl, 5.93%. H NMR (CD2Cl2, l in
ppm): 7.60–7.57 (m, 12H), 7.51–7.40 (m, 18H), 1.89 (s,
6H, O2CMe). IR (KBr disc, in cm−1): 2032s, 2015s,
1560s, 1543m, 1508m, 1438m, 1186w, 1077w, 1023w,
995w, 736m, 693m. UV–VIS (CH2Cl2, umax in nm, and
m in parentheses in M−1 cm−1): 328 (39300), 346 sh,
416 (10900).
1
43.73; H, 3.15; P, 5.22%. H NMR (CD2Cl2, l in ppm):
7.60 (m, 12H), 7.48–7.40 (m, 18H), 1.77 (s, 6H,
O2CMe). 31P NMR (CDCl3, l in ppm): −14.42. IR
(KBr disc, in cm−1): 2031s, 2013s, 1560s, 1542m,
1508m, 1458m, 1438m, 1093m, 744m, 693m, 518m.
UV–Vis (CH2Cl2, umax in nm, and m in parentheses in
M−1 cm−1): 333 (27900), 354 (33200), 395 sh.
2.1.5. [Ir2(v-O2CMe)2Cl2(CO)2(SbPh3)2] (5)
This complex was synthesized by a similar method to
that for complex 2 from complex 8 (57 mg, 0.091
mmol) and SbPh3 (70 mg, 0.19 mmol) in toluene (10
ml) to give 5 as orange–yellow crystals. Yield 12 mg
(10%). Anal. Calc. for C21H18ClIrO3Sb: C, 37.77; H,
2.1.2. [Ir2(v-O2CMe)2Cl2(CO)2(PCy3)2]·CH2Cl2
(2·CH2Cl2)
To a toluene (20 ml) suspension of complex 8 (92 mg,
0.15 mmol), PCy3 (1 ml of 25% solution in toluene, 0.76
mmol) was added under Ar. The solution was refluxed
for 2 h to give a yellow solution and evaporated to
dryness. The yellow crude product was purified by silica
gel column chromatography eluting with CH2Cl2/hex-
ane (1:1 v/v). Slow evaporation of solvent from a
CH2Cl2 solution of the purified product gave yellow
crystals of 2·CH2Cl2. Yield 32 mg (19%). Anal. Calc.
for C43H74Cl4Ir2O6P2: C, 40.05;H, 5.85; P, 4.86. Found:
1
2.72; Cl, 5.31. Found: C, 37.49; H, 2.76; Cl, 5.42%. H
NMR (CD2Cl2, l in ppm): 7.65 (d, 12H), 7.51–7.42 (m,
18H), 2.03 (s, 6H, O2CMe). IR (KBr disc, in cm−1):
2031s, 2012s, 1560s, 1543m, 1508m, 1432m, 1068w,
730w, 694w. UV–Vis (CH2Cl2, umax in nm, and m in
parentheses in M−1 cm−1): 296 (13700), 346 (35600),
363 (37500), 433 (16400).
1
C, 39.79; H, 5.75; P, 4.62%. H NMR (CD2Cl2, l in
2.2. Measurements
ppm): 2.40 (br, m, 6H), 2.03 (s, 6H, O2CMe), 2.05–1.98
(m, 12H), 1.85–1.62 (m, 30H), 1.29 (br, 18H). 31P
NMR (CDCl3, l in ppm): −1.93. IR (KBr disc, in
cm−1): 2928s, 2850s, 2025s, 2006s, 1560s, 1542m,
1508m, 1458m, 1439s, 1268w, 1006w, 848w, 708w,
538w. UV–Vis (CH2Cl2, umax in nm, and m in parenthe-
ses in M−1 cm−1): 267 (16100), 322 (29000), 337
(28200), 401 sh.
1H and 31P NMR spectra were recorded on a JEOL
a400 or a Varian UNITY INOVA 400 spectrometer,
UV–Vis absorption spectra on a Shimadzu UV-
3100PC spectrophotometer and IR spectra by using a
Perkin–Elmer FT IR-1640 spectrophotometer. CV
studies were performed with a BAS 50W electrochemi-
cal analyzer employing a conventional three-electrode
cell with a platinum disk working electrode, a platinum
wire counter electrode, and a BAS RE-5 Ag+ ꢀ Ag
reference electrode. Electrolytic solutions were CH2Cl2
2.1.3. [Ir2(v-O2CMe)2Cl2(CO)2{P(OPh)3}2] (3)
Complex 8 (89 mg, 0.14 mmol) and P(OPh)3 (118 mg,
0.71 mmol) in toluene (25 ml) were refluxed for 2 h
under Ar to give a yellow solution. The oily yellow
n
or MeCN containing 0.1 M Bu4NPF6. After each CV
measurement, the oxidation potential of ferrocene in

98
N. Kanematsu et al. / Inorganica Chimica Acta 323 (2001) 96–104
Table 1
Crystallographic data for [Ir2(m-O2CMe)2Cl2(CO)2(PCy3)2]·CH2Cl2 (2·CH2Cl2), [Ir2(m-O2CMe)2Cl2(CO)2{P(OPh)3}2] (3), [Ir2(m-
O2CMe)2Cl2(CO)2(AsPh3)2] (4) and [Ir2(m-O2CMe)2Cl2(CO)2(SbPh3)2] (5)
2·CH2Cl2
3
4
5
Empirical formula
Formula weight
Crystal system
Space group
C43H74Cl4Ir2O6P2
1275.25
tetragonal
C
42H36Cl2Ir2O12P2
C42H36As2Cl2Ir2O6
1241.93
monoclinic
P21/a (no. 14)
16.103(2)
C42H36Cl2Ir2O6Sb2
1335.59
triclinic
1250.03
monoclinic
P21/c (no. 14)
13.626(3)
21.015(4)
15.328(2)
(
(
P421c (no. 114)
P1 (no. 2)
,
a (A)
14.797(1)
14.797(1)
23.881(2)
13.6363(9)
15.301(1)
11.506(1)
97.136(8)
92.839(8)
67.163(5)
2195.3(3)
2
,
b (A)
16.879(2)
16.890(2)
,
c (A)
h (°)
i (°)
k (°)
94.46(1)
115.621(9)
3
,
V (A )
5228.5(6)
4
4376(1)
4
4139.4(9)
4
Z
T (°C)
23
54.05
6622
2580
0.029
0.035, 0.034
−40
−80
82.03
14984
4026
0.032
23
74.40
14355
5340
0.031
0.045, 0.042
v(Mo Ka) (cm−1
Data, measured
Data, used a
Rint
)
63.49
14265
5439
0.047
0.069, 0.060
R, R%
0.057, 0.054
a I\2.0|(I) for 2·CH2Cl2, 4 and 5, and I\1.3|(I) for 3.
the same electrolytic solution was measured and the
electrode potential was converted into that relative to
the Fc+–Fc couple. For coulometry, the working elec-
trode was a platinum coil and the counter a gold coil in
a two-compartment cell connected by a sintered glass
disk. Cationic radicals for ESR study were generated by
electrochemical oxidation in the coulometry cell. Radi-
olysis was also adopted for their generation: a degassed
Freon mixture (CH2Cl2–CFCl3–CF2BrCF2Br: 1:5:5 by
vol) saturated with a complex was sealed in an ESR
sample tube [5,8,9]. The frozen solution was exposed to
X-ray (Mo Ka, 14.5 kW) at 77 K for 4 h to generate
the cationic radical of the solute. The frozen solution
was annealed carefully to decay radicals derived from
the solvent. ESR spectra were measured on a JEOL
JES-TE200 spectrometer. Field sweep was monitored
value of 55°. All calculations were performed by using
the TEXSAN program package [10]. Scattering factors
for neutral atoms were from Cromer and Waber [11]
and anomalous dispersion [12] was included. Crystallo-
graphic data and structure refinement parameters are
listed in Table 1. The structures of 2·CH2Cl2, 4 and 5
were solved by direct methods, SHELXS-86 [13] and that
of 1 by SIR 92 [14]. An analytical absorption correction
[15] was applied for 2·CH2Cl2, 4 and 5 and DIFABS for
3 [16]. In the crystal of 4 four of the six phenyl groups
(C(13)–C(18), C(25)–C(30), C(31)–C(36) and C(37)–
C(42)) on the As atoms were disordered. The phenyl
groups were refined in two different conformations (A
and B) by using a rigid model. The C(13) and C(31)
atoms were shared by both of the disordered conforma-
tions. In all the analyses, all non-hydrogen atoms that
are not involved in the disorder were refined with
anisotropic thermal parameters and all hydrogen atoms
at calculated positions using idealized geometries of
1
with an Echo Electronics EFM-200 H NMR gaussme-
ter. The gaussmeter probe was attached beside the ESR
cavity; the field difference between ESR and NMR
sample positions was calibrated by measuring the field
intensity at the resonance of perylene cationic radical in
concentrated H2SO4 (g=2.002583).
,
0.95 A of C–H distance with isotropic thermal parame-
ters which were 1.2 times those of the connected atoms.
2.4. DFT calculations
2.3. Crystallographic study
Geometry optimization was performed with the DFT
method with the B3LYP functional [17–19] by using
the Gaussian 98 program [20]. The ECPs (effective core
potentials) and the basis functions for the Ir, P, Cl and
As atoms were those of LANL2DZ (split valence type)
[21] supplemented with a single-primitive polarization
function for Cl, P and As atoms [22]. The 6-31G set [23]
was used for the H, C, N and O atoms. The d functions
were of the manually orthogonal five-function type.
Slow evaporation of CH2Cl2–nPrCN (10:1 v/v) solu-
tion of 2·CH2Cl2 gave its yellow single crystals. Pris-
matic yellow crystals of 3, 4 and 5 were prepared by
slow diffusion of n-hexane into their CH2Cl2 solutions.
All measurements were carried out on a Rigaku
AFC7R diffractometer with graphite monochromated
,
Mo–Ka radiation (u=0.71069 A). The data were col-
lected using the ꢀ–2q scan technique to a maximum 2q

N. Kanematsu et al. / Inorganica Chimica Acta 323 (2001) 96–104
0/+
99
Geometries of model complexes, 60/+ and 7
’
’
, were
optimized in C2 geometries to the default criteria of
GAUSSIAN-98 [20]. ESR hyperfine coupling tensors of
the nuclei at the axial sites (P, As) were performed also
by the GAUSSIAN-98 program package based on the
DFT-B3LYP method at the above-mentioned opti-
mized geometries but the basis functions of the P and
As atoms were replaced by the 6-31G* set.
Fig. 4. ORTEP diagram of [Ir2(m-O2CMe)2Cl2(CO)2(SbPh3)2] (5) with
the 30% probability ellipsoids. Hydrogen atoms are omitted for
clarity.
Table 2
,
Selected bond distances (A), bond angles (°) and torsion angles (°) for
[Ir2(m-O2CMe)2Cl2(CO)2(PCy3)2]·CH2Cl2 (2·CH2Cl2) a
Bond lengths
Ir(1)–Ir(1%)
Ir(1)–P(1)
Ir(1)–O(2%)
2.6936(7)
2.466(3)
2.062(7)
Ir(1)–Cl(1)
Ir(1)–O(1)
Ir(1)–C(3)
2.344(3)
2.098(7)
1.86(1)
Fig. 1. ORTEP diagram of [Ir2(m-O2CMe)2Cl2(CO)2(PCy3)2] (2) in
2·CH2Cl2 with the 50% probability ellipsoids. Hydrogen atoms are
omitted for clarity. A crystallographic C2 axis bisects the Ir–Ir bond
and the line between C(1) and C(C1%).
Bond angles
Ir(1%)–Ir(1)–P(1)
O(1)–Ir(1)–C(3)
170.55(6)
175.1(4)
Cl(1)–Ir(1)–O(2’) 173.6(2)
Torsion angles
O(1)–Ir(1)–Ir(1%)–O(2) 9.9(3)
a %: 1−x, −y, z.
3. Results and discussion
3.1. Structure
Figs. 1–4 show ORTEP [24] drawings of complexes
2–5, respectively. Selected bond distances and angles of
2 are summarized in Table 2, and those of 3–5 in Table
3. The geometries of the core unit, Ir2(m-O2CMe)2-
Cl2(CO)2, in these complexes resemble each other. The
difference in Ir–Cl, Ir–O and Ir–CO distances are
within experimental errors (Ir–Cl: 2.337(3)–2.350(6),
Fig. 2. ORTEP diagram of [Ir2(m-O2CMe)2Cl2(CO)2{P(OPh)3}2] (3)
with the 30% probability ellipsoids. Hydrogen atoms are omitted for
clarity.
,
Ir–O: 2.06(1)–2.098(7) and Ir–C: 1.82(2)–1.89(2) A).
,
The Ir–P distance in 3 (2.356(4) and 2.376 (4) A) is
,
shorter than that in 2 (2.466(3) A). Similar difference in
M–Paxial distances are observed between [Rh2(m-
O2CR)4(PPh3)2] and [Rh2(m-O2CR)4{P(OPh)3}2] (R=
Me [25], CF3 [26]). The Ir–Ir distance depends on the
axial ligands (Table 4). The Ir–Ir distance of 2
,
(2.6939(7) A) is the longest in the current [Ir2(m-
,
O2CMe)2Cl2(CO)2L2] complexes and 0.12 A longer than
that of complex 9. The other three current complexes
also have longer Ir–Ir distances than those of the
solvent adducts, 9–11 [3]. This trend indicates that the
enhanced s donation from the axial ligands into the
Ir–Ir bond elongates the metal–metal bond.
Fig. 3. ORTEP diagram of [Ir2(m-O2CMe)2Cl2(CO)2(AsPh3)2] (4) with
the 30% probability ellipsoids. Only one (designated by adding A to
the atom numbers) of each pair of the disordered phenyl groups are
displayed. Hydrogen atoms are omitted for clarity.

100
N. Kanematsu et al. / Inorganica Chimica Acta 323 (2001) 96–104
3.2. Electrochemistry
Fig. 5 shows the CVs of a triad, 1, 4 and 5. Their first
oxidation responses are chemically reversible. The scans
to more positive potentials gave second responses,
which are irreversible. Table 5 summarizes the redox
potentials of [Ir2(m-O2CMe)2Cl2(CO)2L2] complexes. All
the complexes in this table did not show a reduction
response down to −1.5 V. Table 5 shows that the
stronger s donor ligands such as a phosphine makes
the oxidation potential lower. On the other hand, com-
plex 9 which has a weak donor ligand, MeCN, has the
highest oxidation potential. The dmso adduct, 10, did
not show any redox response in the potential window
of the dmso electrolytic solution [3].
Fig. 5. Cyclic voltammograms of complexes (a) 1, (b) 4 and (c) 5 in
CH2Cl2. The dotted lines represent the CV responses scanned beyond
the first oxidation response.
Table 3
,
Selected bond distances (A), bond angles (°) and torsion angles (°) for
Table 5
[Ir2(m-O2CMe)2Cl2(CO)2{P(OPh)3}2] (3), [Ir2(m-O2CMe)2Cl2(CO)2-
(AsPh3)2] (4) and [Ir2(m-O2CMe)2Cl2(CO)2(SbPh3)2] (5)
Oxidation potentials (V vs. Fc+–Fc) of [Ir2(m-O2CMe)2Cl2(CO)2L2]
in CH2Cl2 containing 0.1 M nBu4NPF6
3
4
5
ox1
1/2
ox2 a
Complex
E
E
pa
Bond lengths
Ir(1)–Ir(2)
Ir(1)–Ax(1) a
Ir(1)–Cl(1)
Ir(1)–O(1)
Ir(1)–O(3)
Ir(1)–C(5)
Ir(2)–Ax(2) a
Ir(2)–Cl(2)
Ir(2)–O(2)
Ir(2)–O(4)
Ir(2)–C(6)
[Ir2(m-O2CMe)2Cl2(CO)2(MeCN)2] b,c (9)
[Ir2(m-O2CMe)2Cl2(CO)2(py)2] b (11)
[Ir2(m-O2CMe)2Cl2(CO)2(Pripy)2]] b (12)
[Ir2(m-O2CMe)2Cl2(CO)2{P(OPh)3}2] (3)
[Ir2(m-O2CMe)2Cl2(CO)2(AsPh3)2] (4)
[Ir2(m-O2CMe)2Cl2(CO)2(SbPh3)2] (5)
[Ir2(m-O2CMe)2Cl2(CO)2(PPh3)2] (1)
[Ir2(m-O2CMe)2Cl2(CO)2(PCy3)2] (2)
1.30
0.97
0.94
0.75
0.66
0.62
0.47
0.21
2.6458(8)
2.356(4)
2.344(4)
2.062(9)
2.071(10)
1.82(2)
2.376(4)
2.340(4)
2.062(10)
2.067(9)
1.82(2)
2.6207(9)
2.532(2)
2.350(6)
2.06(1)
2.09(1)
1.89(2)
2.550(2)
2.349(6)
2.09(1)
2.07(1)
1.86(2)
2.6200(9)
2.678(1)
2.345(3)
2.064(8)
2.096(8)
1.82(1)
2.680(1)
2.337(3)
2.091(8)
2.065(8)
1.82(1)
1.13
1.21
0.85
1.11
a Irreversible, anodic peak potential.
b Ref. [3].
c In MeCN.
Bond angles
Ir(2)–Ir(1)–Ax(1) a
Ir(1)–Ir(2)–Ax(2) a
Cl(1)–Ir(1)–O(1)
O(3)–Ir(1)–C(5)
Cl(2)–Ir(2)–O(4)
O(2)–Ir(2)–C(6)
169.2(1)
168.07(10)
174.5(3)
175.6(6)
174.4(3)
174.9(5)
169.74(5)
169.10(5)
173.6(4)
178.8(8)
173.0(4)
177.2(8)
168.03(3)
168.29(3)
175.1(2)
177.3(5)
173.1(2)
178.4(5)
A bulk electrolysis of 1 at 0.60 V in CH2Cl2 contain-
ing 0.1 M nBu4NPF6 at −15 °C gave a dark-red
solution. Similar electrochemical oxidation of 2 at 0.35
V and 4 at 0.80 V gave dark-brown solutions. These
colors of the cationic radicals faded out in several
minutes when the solutions were warmed to room
temperature and finally changed to yellow. This behav-
ior is similar to that of 11 [3] and shows limited stability
of the cationic species derived from 1, 2 and 4. Cationic
radicals of 3 and 5 were unstable even at −15 °C.
Torsion angles
O(1)–Ir(1)–Ir(2)–O(2)
O(3)–Ir(1)–Ir(2)–O(4)
12.3(4)
12.4(4)
17.8(5)
18.0(5)
17.3(3)
17.9(3)
a Ax denotes the atom at the axial site: P for 3, As for 4, and Sb for
5.
Table 4
3.3. ESR
,
Ir–Ir distances (A) in [Ir2(m-O2CMe)2Cl2(CO)2L2]
Solutions of cationic radicals of 1, 2 and 4 formed by
Complex
Distance
electrolysis were quickly frozen at 77 K and their ESR
spectra were measured. The spectrum of 1+ at 77 K is
’
[Ir2(m-O2CMe)2Cl2(CO)2(MeCN)2] (9) a
[Ir2(m-O2CMe)2Cl2(CO)2(py)2] (11) a
[Ir2(m-O2CMe)2Cl2(CO)2(dmso)2] (10) a
[Ir2(m-O2CMe)2Cl2(CO)2(SbPh3)2] (5)
[Ir2(m-O2CMe)2Cl2(CO)2(AsPh3)2] (4)
[Ir2(m-O2CMe)2Cl2(CO)2{P(OPh)3}2] (3)
[Ir2(m-O2CMe)2Cl2(CO)2(PCy3)2] (2)
2.569(1)
2.5918(5)
2.5980(5)
2.6200(9)
2.6207(9)
2.6458(8)
2.6936(7)
shown in Fig. 6(a). This is a pseudo-axially symmetric
one with gÞ=2.15 and gꢀꢀ=1.96 with hyperfine split-
ting of a 1:2:1 triplet. The triplet splitting is due to a
pair of equivalent 31P nuclei. The principal values of the
hyperfine tensor are AÞ=11.9×10−3 and Aꢀꢀ=15.4×
10−3 cm−1 (Table 6). The same spectrum was observed
for a frozen Freon mixture solution of 1 at 77 K which
a Ref. [3].

N. Kanematsu et al. / Inorganica Chimica Acta 323 (2001) 96–104
101
Scheme 2.
(24.4×10−3)=0.10, respectively [5,27]. In this
estimation, the spin polarization of inner-shell s and p
electrons on the P atoms has been neglected. The large
odd electron density on the two P atoms (total 26%)
leads to a conclusion that the odd electron is delocal-
ized into the phosphine lone-pair orbitals. Conse-
quently, the SOMO has the s symmetry with respect to
the P–Ir–Ir–P axis. The lone-pair orbitals on the P
atoms can interact with the local sIrIr and sI*rIr orbitals
as shown in Scheme 2. The SOMO can be assigned to
Fig. 6. X-Band ESR spectra of complex 1+ (a) in frozen solution at
77 K and (b) in fluid solution at 273 K.
’
had been irradiated with X-ray and annealed appropri-
ately to decay the solvent radicals. This confirms that
the spectrum in Fig. 6(a) arises from 1+ , because the
’
frozen Freon mixture is known to be an appropriate
matrix for trapping cationic species formed from a
solute upon radiolysis [8,9]. A fluid solution spectrum
the sIrIr/s*IrP orbital
.
of 1+ at 273 K is shown in Fig. 6(b). This isotropic
’
Cationic radicals of 2 and 4 were generated also by
electrolysis. The ESR spectra of 2+ in frozen and fluid
’
spectrum is a broad 1:2:1 triplet with gsltn=2.11 and
A
sltn=12.4×10−3 cm−1. The large shifts of the princi-
solutions were similar to those of 1+ and were ana-
’
pal values of the g tensor from the free spin value
(ge=2.00) show that the odd electron of this cationic
radical is distributed predominantly on the iridium
atoms. The Asltn value is consistent only with the as-
signment of the same sign to both of Aꢀꢀ and AÞ values.
The physically meaningful choice of the signs for these
principal values is positive. The isotropic and an-
isotropic parts of the 31P hyperfine coupling tensor are
lyzed as listed in Table 6. The frozen (77 K) and fluid
solution (223 K) spectra of 4+ are shown in Fig. 7(a)
’
and (b), respectively. This frozen solution spectrum was
analyzed also as pseudo-axially symmetric one. The
1:2:3:4:3:2:1 septet splitting patterns in both of the
parallel and perpendicular components show that there
are two magnetically equivalent 75As nuclei (I=3/2,
100% natural abundance). The principal values of the g
A
A
iso=(2AÞ+Aꢀꢀ)/3=13.1×10−3
iso=2.3×10−3 cm−1, respectively. Odd electron den-
and
2b=Aꢀꢀ−
tensor and of the As hyperfine coupling tensor of 4+
’
are summarized also in Table 6. Odd electron densities
sities on the 3s and 3p orbitals on each P atom can be
on the P or As atoms are estimated as z(P 3s)=0.02
and z(P 3p)=0.11 for 2+ and z(As 4s)=0.03 and
’
estimated by dividing the Aiso and 2b values by the
corresponding atomic values as z(P 3s)=13.1×10−3
(443.8×10−3)=0.03 and z(P 3p)=2.3×10−3
/
/
+
+
z(As 4p)=0.15 for 4+ . Complexes 2 and 4 have
’
’
’
also large odd electron densities on the two P atoms
Table 6
ESR spin Hamiltonian parameters of [Ir2(m-O2CMe)2Cl2(CO)2(L)2]+
’
a
b
a,c
c
c
b,c
Complex
gsltn
gÞ
gꢀꢀ
giso
ꢀAsltn
ꢀ
ꢀAÞꢀ
ꢀAꢀꢀꢀ
ꢀAisoꢀ
1+
2+
4+
2.11
2.10
2.18
2.15
2.18
2.20
1.96
1.95
1.96
2.09
2.10
2.12
12.4
10.8
12.7
11.9
9.3
11.9
15.4
13.2
16.0
13.1
10.6
13.3
’
’
’
a From fluid solution spectrum.
b From frozen solution spectrum as iso=(ꢀꢀ+2Þ)/3.
c Hyperfine coupling energy in unit of 10−3 cm−1
.

102
N. Kanematsu et al. / Inorganica Chimica Acta 323 (2001) 96–104
consistent with the present assignment of their SOMO
to the sIrIr/s*IrP or the sIrIr/sI*rAs orbital. The relatively
+
large deviations of the gꢀꢀ values of 1+ , 2 and 4+
’
’
’
from the ge value would originate partly from the
current approximation to analyze their spectra based
on the co-axially symmetric spin Hamiltonians and
partly from the ground states of these radicals to be
described with multiconfigurational wave-functions
[28].
3.4. DFT calculations
The DFT calculations of geometry optimization of
0/+
the model complexes, 60/+ and 7
, and of hyperfine
’
’
Fig. 7. X-Band ESR spectra of complex 4+ (a) in frozen solution at
77 K and (b) in fluid solution at 223 K.
’
coupling tensors of the axial heavy nuclei of these
cationic radicals were performed in the C2 symmetry
where the C2 axis bisects the Ir–Ir bond. The results of
geometry optimization and hyperfine coupling tensors
(total 26%) and on the two As atoms (total 36%),
respectively. The SOMO of 2+ and 4+ are also the
are summarized in Tables 7 and 8, respectively.
’
’
2
The calculated ground state of 6+ is A with the
’
s
IrIr/sI*rP and the sIrIr/sI*rAs orbital, respectively. As
discussed in the analysis of the ESR of [Rh2(m-
SOMO of the sIrIr/sI*rP orbital which is also delocalized
onto the equatorial Cl atoms with p*IrCl character. The
atomic spin populations on each of the Ir, P and Cl
O2CR)4(PR3)2]+ complexes [5,6], the observed g ten-
’
’
+
sors of 1+ , 2 and 4+ with gÞ\ge and gꢀꢀ:ge are
’
’
Table 7
Optimized geometries and relative energies of Ir24+ and Ir25+ model complexes a,b
c
d
d
State
SOMO
rel E
Ir–Ir
Ir–Cl d
Ir–Ax
[Ir2(O2CH)2(CO)2Cl2(PH3)2]0/+ (60/+
)
Ir25+ 2
A
A
s
IrIr, sI*rP, p*IrCl
0
16.4
−172.2
(0.1127)
(−0.0047)
2.6753
(−0.0556)
(−0.0557)
2.3968
(−0.0861)
(0.0610)
2.4745
Ir52+ 2
Ir24+ 1
B
n(Ir) e, pI*rCl
[Ir2(O2CH)2(CO)2Cl2(AsH3)2]0/+ (70/+
)
Ir25+ 2
Ir25+ 2
Ir24+ 1
A
B
s
IrIr, sI*rP, p*IrCl
0
15.0
−173.3
(0.1018)
(0.0012)
2.6670
(−0.0543)
(−0.0553)
2.3948
(−0.0891)
(0.0524)
2.5870
n(Ir) e, pI*rCl
A
a Geometries of the Ir25+ complexes were optimized for each of the low-lying doublet states.
b B3LYP, LANL2DZ for Ir, As, Cl, and P supplemented with d polarization functions for the heavy typical elements, and 6-31G for O, N, C,
and H.
c Energies relative to the lowest doublet state of the Ir25+ complex in kcal mol−1
.
d
4+
5+
,
,
Bond lengths (A) for the Ir2 complexes and bond length changes (A) in parentheses arising from one-electron oxidation for the Ir2
complexes.
e An Ir non-bonding MO.
Table 8
Calculated isotropic and anisotropic hyperfine coupling constants a (and directions b in parentheses) of the 31P and 75As nuclei in model complexes
c
c
c
State
SOMO
Aiso
BXX
BYY
BZZ
+
[Ir2(v-O2CH)2Cl2(CO)2(PH3)2]+ (6
’
’
)
2A
2B
s
IrIr, sI*rP, p*IrCl
13.06
−0.56
−1.31 (84)
−0.05 (27)
−1.24 (92)
0.01 (97)
2.55 (6)
0.05 (115)
n(Ir), pI*rCl
+
[Ir2(v-O2CH)2Cl2(CO)2(AsH3)2]+ (7
’
’
)
2A
2B
s
IrIr, sI*rAs, pI*rCl
23.27
−1.02
−1.12 (85)
−0.06 (17)
−1.06 (92)
0.02 (92)
2.19 (6)
0.04 (106)
n(Ir), pI*rCl
a In unit of 10−3 cm−1
b From the Ir–Ir axis in unit of degree.
c X, Y and Z are the local principal axes for anisotropic hyperfine coupling tensor, B.
.

N. Kanematsu et al. / Inorganica Chimica Acta 323 (2001) 96–104
103
atoms in this state are 0.21, 0.16 and 0.12, respectively.
The odd electron is considerably delocalized onto the
Cl atoms. The hyperfine anisotropic parameter for the
axial P nucleus was calculated to be BZZ=2.6×10−3
cm−1 (Z is the local unique axis of the calculated
anisotropic hyperfine tensor of a P nucleus and deviates
slightly (6°) from the Ir–P bond axis). This result is
consistent with the corresponding experimental values
which is also delocalized onto the equatorial Cl atoms
with pI*rCl character.
5. Supplementary material
Crystallographic data (excluding structure factors)
for the structural analysis have been deposited with the
Cambridge Crystallographic Data Center, CCDC Nos.
167648, 167649, 167650, and 167651 for 2·CH2Cl2, 3, 4
and 5, respectively. Copies of this information may be
obtained free of charge from The Director, CCDC, 12
Union Road, Cambridge CB2 1EZ, UK [fax: +44-
1223-336033; e-mail: deposit@aadc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk].
of (Aꢀꢀ−Aiso)=2.3×10−3 for 1+ and 2.6×10−3
’
cm−1 for 2+ . The calculated second-lowest doublet
’
2
state was B, which is located 16.5 kcal mol−1 above
2
the A ground state in energy. The SOMO for this state
can be best described as the b-symmetry combination
of the pair of 5dxz atomic orbitals on both of the Ir
atoms, where the local z and x axes are the Ir–Ir and
Ir–Cl bond axes, respectively. This orbital is also exten-
sively delocalized onto the equatorial Cl atoms in pI*rCl
phase. The calculated hyperfine coupling tensor of the
axial P nucleus for this excited state listed in Table 8 is
Acknowledgements
inconsistent with the experimental ones for 1+ and
’
This work was supported by grants-in-aid for Scien-
tific Research (11440194 and 10149105) from the Min-
istry of Education, Science, Sports and Culture, Japan.
The DFT calculations were performed on the IBM SP2
workstation cluster and on the NEC SX5 computer at
the computer center of the Institute for Molecular
Science (Okazaki).
2
2+ , supporting that the B state does not correspond
’
the experimental ground state.
The calculated ground state of model-complex 7+ is
’
2A with the sIrIr/sI*rAs SOMO which is also delocalized
extensively onto the equatorial Cl atoms in p*IrCl phase.
2
The second-lowest doublet-state is B where the SOMO
is similar to the SOMO of the B state for 6+ . The
2
’
experimental 75As hyperfine coupling tensor of 4+.
(Table 6) is consistent only with that for the calculated
2A ground-state for the model complex (Table 8).
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