A rare unsupported iridium(II) dimer, [IrCl2(CO) 2]2

Article abstract of DOI:10.1039/b803632b

Oxidative additions of dichloromethanes to a diiridium(I) core, bridged by 2-ferrocenyl-1,8-naphthyridines (NP-Fc), provide an iridium(II) dimer, [IrCl₂(CO)₂(η¹-NP-Fc)]₂, featuring an unsupported Ir-Ir single bond (2.7121(8) A). The Royal Society of Chemistry.

Full text of DOI:10.1039/b803632b

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Number 22 | 14 June 2008 | Pages 2485–2576
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COMMUNICATION
Sanjib K. Patra, S. M. Wahidur
Rahaman, Moumita Majumdar,
Arup Sinha and Jitendra K. Bera
A rare unsupported iridium(II) dimer,
[IrCl (CO) ]
FEATURE ARTICLE
Stephen J. Connon
Asymmetric catalysis with bifunctional
cinchona alkaloid-based urea and
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A rare unsupported iridium(II) dimer, [IrCl2(CO)2]2w
Sanjib K. Patra, S. M. Wahidur Rahaman, Moumita Majumdar, Arup Sinha and
Jitendra K. Bera*
Received (in Cambridge, UK) 3rd March 2008, Accepted 10th April 2008
First published as an Advance Article on the web 2nd May 2008
DOI: 10.1039/b803632b
Oxidative additions of dichloromethanes to a diiridium(I) core,
bridged by 2-ferrocenyl-1,8-naphthyridines (NP-Fc), provide an
iridium(II) dimer, [IrCl2(CO)2(g1-NP-Fc)]2, featuring an unsup-
˚
ported Ir–Ir single bond (2.7121(8) A).
Unsupported diiridium(II) compounds are distinguished by
their rarity. Only three examples of an IrII dimer possessing
an unsupported Ir–Ir single bond are known.1 This extraor-
dinary set of compounds involves octaethylporphyrin (oep),2
Scheme 1 Synthesis of NP-R bridged diiridium(I) and unsupported
diiridium(II) compounds.
phthalocyanine (pc)3 or dianion of tetracyanobisimidazole
(tcbim),4 hinting at the prerequisite of ligand-elaboration in
stabilizing an unbridged IrII–IrII bond. The wide ranging
applications of dirhodium(II) compounds in organic transfor-
mations5 and the recent utilization of diruthenium(I) com-
pounds in C–H bond activation and C–C bond formation
reactions6 prompted our interest in isoelectronic diiridium(II)
chemistry. An intriguing result of our exploration, the chemi-
cal synthesis of a novel diiridium(II) compound featuring an
unsupported Ir–Ir bond, is reported herein.
proximity by neutral N-donor ligands, as in 4, is an excep-
tional example although diiridium(I) compounds incorporat-
ing anionic ligands are abundant.
Substitution of COD from [Ir(COD)(Z1-NP-Fc)2][BF4] (2)
by CO in dichloromethane affords a purple solid, character-
ized as [Ir2Cl4(CO)4(Z1-NP-Fc)2] (5).z The X-ray structure of 5
reveals a dimer of ‘[IrCl2(CO)2]’ (Fig. 2). The molecule has an
imposed C2 axis across the unsupported Ir–Ir bond. The
geometry about the iridium centres is near-octahedral with
two cis CO and two cis chlorides bonded to each iridium atom.
The remaining sites are satisfied by axial NP-Fc and the
second iridium. The Ir1–Cl1 and Ir1–Cl2 distances are
2.378(2) and 2.386(3) A; Ir1–C1 and Ir1–C2 distances are
1.871(11) and 1.892(10) A respectively. The axial NP-Fc forms
a linear N1–Ir1–Ir1–N1 axis as reflected in the N1–Ir1–Ir1
bond angle 178.8(2)1. The Ir1–N1 distance is 2.183(7) A. The
iridium and the coordinating atoms at equatorial sites around
it are arranged in a plane; the largest deviation from the best
plane is less than 0.01 A. The staggered geometry of the dimer
is shown in the Cl1–Ir1–Ir1–Cl1 twist angle of 48.54(13)1
(Table 1). An assortment of diiridium compounds pioneered
Reaction of [Ir(COD)(MeCN)2][BF4] with 2-R-1,8-
naphthyridine (NP-R) affords [Ir(COD)(Z1-NP-R)2][BF4]
(Scheme 1). X-ray structures of compounds for R = phenyl
(1) and ferrocenyl (2) reveal 8-N coordination of two NP
ligands (see Fig. S1 and S2w). When carbon monoxide is
bubbled through a dichloromethane solution of 1, [Ir2(CO)4-
(m-NP-Ph)2][BF4]2 (3) precipitates out which is isolated and
characterized by IR, mass and elemental analysis. Utilization of
NP-Me provides an analogous compound [Ir2(CO)4(m-NP-Me)2]-
[BF4]2 (4), the structure of which has been established by an X-
ray technique.z The molecular structure of 4 consists of a
diiridium core with two cis NP-Me ligands, arranged in a
head-to-tail fashion, bridging metal centres (Fig. 1). Each
iridium is additionally bonded to two carbonyls. The Irꢀ ꢀ ꢀIr
non-bonding distances 2.8151(7) and 2.8051(7) A, noted for
two independent molecules in the asymmetric unit, are similar
to those found in weakly interacting d8–d8 systems.7,8 The
Ir–N distances are in the range 2.116(7)–2.133(7) A. The
N11–Ir1–N12 and C1–Ir1–C2 angles are 86.1(2)1 and
90.5(4)1, respectively, manifesting a square planar environ-
ment around each metal centre. Two IrI ions held in close
Department of Chemistry, Indian Institute of Technology, Kanpur,
208016, India. E-mail: jbera@iitk.ac.in; Fax: +91-512-2597436; Tel:
+91-512-2597336
w Electronic supplementary information (ESI) available: Full experi-
mental details, analytical data and X-ray details for 1, 2, 3,
[Ir(COD)(Z1-NP-Me)2][BF4], [Ir(CO)2(m-NP-Fc)2][BF4] and [HꢀNP-
Fc][BF4]. Crystallographic information for compounds 4 and 5
(CCDC 667831 and 667832). See DOI: 10.1039/b803632b
Fig. 1 ORTEP view of dicationic [Ir2(CO)4(m-NP-Me)2] unit in
compound 4.
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This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 2511–2513 | 2511

Scheme 2 Proposed pathways leading to unsupported Ir2Cl4(CO)4
core.
Fig. 2 ORTEP view of the molecular structure of [IrCl2(CO)2-
(Z1-NP-Fc)]2 (5).
pound,10 presumably [Ir2(CO)4(m-NP-Fc)2][BF4]2, generated
in situ via displacement of COD in 2 by carbon monoxides.
Isolation of [Ir2(CO)4(m-NP-Me)2][BF4]2 (4) supports the pre-
mise of a dimetal intermediate. Although a mononuclear
product, [Ir(CO)2(Z1-NP-Fc)2][BF4], is crystallized in 1,2-di-
chlorobenzene (see Fig. S3w), spectroscopic data reveal di-
nuclear species in solution.11 Furthermore, the NP-Fc is
shown to bridge a dimetal unit in [cis-Rh2(OAc)2(m-NP-
Fc)2(OH2)](BF4)2, synthesized and structurally characterized
in our laboratory (see Fig. S4w). Reduction of dichloro-
methanes and subsequent addition of chlorides result in
dicationic [Cl–IrII–IrII–Cl]. The by-product of this reaction,
1,2-dichloroethane, has been identified by a GC technique.
It is our assertion that the [Cl–IrII–IrII–Cl]2+ species
undergoes reduction, generating the [Cl–IrIꢀ ꢀ ꢀIrI–Cl] core
(Scheme 2). The NP-Fc, present in excess of two equivalents
per diiridium, is most likely the reducing agent. The cyclic
voltammogram of NP-Fc in dichloromethane exhibits an
irreversible oxidation at Ep,a = +0.85 V (vs. Ag/AgCl) with a
return reduction wave at Ep,c = +0.50 V. The dicationic
[Cl–IrII–IrII–Cl] core, likely to be a stronger oxidant compared
to its neutral diiridium(II) analogues involving anionic
bridges,9 is reduced, yielding neutral [Cl–IrIꢀ ꢀ ꢀIrI–Cl]. The
oxidized product [NP-Fc]+ proved to be elusive for identifica-
tion. Our attempts to identify the oxidized species invariably
led to the isolation of protonated salt [HꢀNP-Fc]+[BF4]ꢂ (see
Fig. S5w). Subsequent activation of dichloromethanes results
in oxidative additions of two more chlorides across the
diiridium(I) core that proceeds with concomitant growth of
an IrII–IrII single bond. Incorporation of four chlorides, in
addition to the four carbonyls present at the diiridium core,
leads to migration of NP-Fc to an axial position, resulting in
unsupported iridium(II) dimer 5.
largely by Oro and Stobart allows scrutiny of the metal–metal
distances.7,9 The unsupported Ir–Ir distance of 2.7121(8) A in
5 is shorter than that found in association dimers of IrI and
longer than the corresponding distances in bridged diiridiu-
m(II) compounds.9 Notably, the NP-bridged IrIꢀ ꢀ ꢀIrI distance
in 4 is 0.1 A longer than the unsupported IrII–IrII distance in 5,
strongly suggesting a single bond between metals in the latter
compound.
Tedious and unconventional methods have been adopted in
the synthesis of unsupported diiridium(II) compounds:
[Ir(oep)]2 is obtained through the photolysis of [Ir(oep)(CH3)]
in C6D6; [Ir2(pc)2(pyridine)2] is synthesized via controlled ther-
mal decomposition of di(acido)phthalocyaninatoiridates; and
[Ir2(tcbim)2(CO)4(CH3CN)2] is prepared from electrolysis of
salts of [IrI(CO)2(tcbim)]ꢂ in acetonitrile.2–4 Compound
[Ir2Cl4(CO)4(Z1-NP-Fc)2] (5) is a unique addition to this list
which has been synthesized via chemical oxidation under mild
conditions with reasonably good yield (68%). The Ir–Ir distance
in 5 (2.7121(8) A) is similar to that observed for [Ir2(pc)2(py)2]
(2.7071(1) A) but significantly smaller than the 2.826(2) A
reported for [Ir2(tcbim)2(CO)2{P(OEt)3}2 (CH3CN)2].
The source of the chlorides in 5 is the solvent, dichloro-
methane. In 1,2-dichlorobenzene, carbon monoxide treatment
of [Ir(COD)(Z1-NP-Fc)2][BF4] (2) provides the substituted
product [Ir(CO)2(Z1-NP-Fc)2][BF4]. We propose a mechanism
detailing the formation of 5, depicted in Scheme 2. Two-
fragment, two-centre oxidative addition of haloalkane to the
diiridium(I) core, spanned by two anionic bridges, afforded a
class of neutral IrII–IrII compounds.9 The NP-Fc offers unu-
sual reactivity, introducing four chlorides to the diiridium(II)
core. Initially, incorporation of two chlorides occurs via
activation of dichloromethanes across a diiridium(I) com-
DFT calculations on [Ir2Cl4(CO)4] provide insight into the
electronic structure of the diiridium(II) core. The X-ray struc-
ture of 5 was used as an initial geometry for optimization in C2
symmetry and was characterized fully via analytical frequency
calculation as the minima on the potential energy surface. The
metrical parameters obtained from the X-ray structure and
DFT calculation are in good agreement (Table 1). The calcu-
lated Ir–Ir distance (2.683 A) is marginally shorter than that
measured experimentally. Truncation of two axially coordi-
nated NP-Fc in the calculated structure possibly contributes to
a shorter metal–metal distance. The staggered disposition of
ligands about the metals suggests the absence of any d
Table 1 Comparison of selected bond lengths (A) and angles (1) of
[Ir2Cl4(CO)4] derived from X-ray crystallography (compound 5) and
DFT calculation
Ir–Ir
Ir–Cl
Ir–C
ja
X-Ray
DFT
a
2.7121(8)
2.378(2)
2.386(3)
2.329
1.892(10)
1.871(11)
1.904
48.54(13)
44.1(6)
53.21
2.683
2.352
1.912
48.50
Torsional angles Cl1–Ir1–Ir1–Cl1 and C2–Ir1–Ir1–C2, respectively.
ꢁc
This journal is The Royal Society of Chemistry 2008
2512 | Chem. Commun., 2008, 2511–2513

purple solution was stirred for 6 h at RT. It was then concentrated
under vacuum and hexane (15 mL) was added with stirring to induce
precipitation. The solid residue obtained was washed with diethyl
ether (3 ꢃ 5 mL) and dried in vacuum. Crystals suitable for X-ray
study were obtained by layering hexane over the dichloromethane
solution of the compound. Yield: 12 mg (68%, per iridium). 1H NMR
(CDCl3, d, ppm): 9.1–8.5 (br, m, 10H), 4.1 (br, 18H). IR (KBr pellet):
n(CO) 2061,1983 cmꢂ1 Anal. Calcd for C40H28N4O4Cl4Fe2Ir2: C,
37.93; H, 2.23; N, 4.42. Found: C, 37.88; H, 2.29; N, 4.51%. UV–Vis
spectra [lmax, nm (e, dm3 molꢂ1 cmꢂ1)] (in CH3CN): 234 (8.21 ꢃ 103),
322 (4.89 ꢃ 103), 472 (3.88 ꢃ 102), 590 (2.42 ꢃ 102).
Crystal data for compound 4: formula C22H16B2F8Ir2N4O4, M =
958.41, orthorhombic, space group Pbcn (No. 60), a = 10.8097(7),
b = 32.184(2), c = 19.7274(14) A, V = 6863.2(8) A3, Z = 8, rc =
1.855 g cmꢂ3, F000 = 3552, T = 100(2) K, 2ymax = 49.41, 5861 unique,
GooF = 0.973, R1 = 0.0394, wR2 = 0.0843.
Crystal data for compound 5: C42H28Cl8Fe2Ir2N4O4, M = 1432.38,
monoclinic, space group C2/c (No. 15), a = 16.473(2), b = 14.541(2),
c = 20.961(3) A, b = 97.097(2)1, V = 4982.5(12) A3, Z = 4, rc =
1.910 g cmꢂ3, F000 = 2728, T = 100(2) K, 2ymax = 52.71, 13 966
reflections collected, 5057 unique, GooF = 0.991, R1 = 0.0554,
wR2 = 0.1187.
Fig. 3 Contour surface of the LUMO (s) in [Ir2Cl4(CO)4].
bonding. Calculated orbital occupancies support the electronic
configuration of the [Ir2]4+ unit as s2p4d2d*2p*4s*0, the
LUMO being a s orbital resulting virtually from the anti-
bonding interaction of Ir dz2 orbitals (Fig. 3), corresponding to a
formal Ir–Ir bond order of 1. Careful analyses of occupied MOs
illustrate two-orbital, four-electron interactions between chlor-
ide and Ir orbitals. This situation is stabilized by the admixture
of CO p* into the anti-bonding Ir–Cl combination. Thus, there
is a transfer of electron density from chloride to carbonyl
through Ir centres. This charge delocalization can be traced as
the origin of the stable unsupported iridium(II) dimer 5.
The 1,8-naphthyridine ligand with a redox-active ferrocene
appendage provides easy access to an unsupported diiridiu-
m(II) compound. The synthesis and structural elucidation of
the title compound 5 underscore the stability of unsupported
IrII dimer with a combination of common p-donor/p-acceptor
ligands chlorides and carbonyls. Furthermore, the chlorides
offer the prospect of derivatization of the diiridium(II) core
which is being pursued actively. It is our expectation that
compound 5 will emerge as a key precursor in the exploration
of diiridium(II) chemistry.
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We thank Dr J. N. Moorthy for insightful discussions. This
work was financially supported by DST, India through the
grant of a Ramanna fellowship. S.M.W.R. and A.S. thank
UGC and CSIR, India, respectively, for fellowships.
Notes and references
8 For examples of mixed-valence diiridium compounds bridged by
neutral P-donor ligands, see: J. L. Dempsey, A. J. Esswein, D. R.
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z Synthesis of 4: TlBF4 (46 mg, 0.15 mmol) was added to an acetoni-
trile solution (10 mL) of [IrCl(COD)]2 (51 mg, 0.075 mmol) and the
yellow mixture was stirred for 30 min. The TlCl was removed by
Schlenk filtration, then NP-Me (45 mg, 0.31 mmol) was added to the
filtrate and the mixture was stirred for 8 h at RT. The resulting orange
solution was concentrated under vacuum and 15 mL of diethyl ether
were added with stirring to induce precipitation. The isolated solid was
dissolved in dichloromethane (10 mL) and carbon monoxide was
bubbled for 5 min. The purple solid [Ir2(CO)4(m-NP-Me)2][BF4]2 (4)
precipitated out slowly from the reaction medium. The solution was
filtered off through a filter paper-stripped cannula and the solid residue
was washed with hexane (3 ꢃ 5 mL) and dried in vacuum. Crystals
were grown by layering a benzene solution of [Ir(COD)(Z1-NP-
Me)2][BF4] over dichloromethane saturated with carbon monoxide.
Yield: 65 mg (90%, based on iridium). Anal. Calcd for C22H16N4O4B2-
F8Ir2: C, 27.57; H, 1.68; N, 5.85. Found: C, 27.49; H, 1.74; N, 5.79%.
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IR (KBr pellet): n(CO) 2058, 2005; n(BF4ꢂ) 1069 cmꢂ1
.
5: Carbon monoxide was bubbled for 5 min through a dichloro-
methane (10 mL) solution of 2 (30 mg, 0.031 mmol) and the resultant
ꢁc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 2511–2513 | 2513