Synthesis and characterization of a bimetallic iridium complex with a ten sp2-carbon chain bridge

Article abstract of DOI:10.1039/b706900f

A special sp²-carbon chain bridged bimetallic iridium complex has been synthesized and characterized; the compound has excellent air-stability, thermo-stability and electrochemical properties. The Royal Society of Chemistry.

Full text of DOI:10.1039/b706900f

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Synthesis and characterization of a bimetallic iridium complex with a ten
sp2-carbon chain bridge†‡
Lei Gong, Liqiong Wu, Yumei Lin, Hong Zhang, Fangzu Yang, Tingbin Wen and Haiping Xia*
Received 8th May 2007, Accepted 25th June 2007
First published as an Advance Article on the web 4th July 2007
DOI: 10.1039/b706900f
A special sp2-carbon chain bridged bimetallic iridium com-
plex has been synthesized and characterized; the compound
has excellent air-stability, thermo-stability and electrochemi-
cal properties.
Organic compounds having conjugated p-systems can act as
electronic bridges between transition metal atoms. Numerous
experimental and theoretical studies on dinuclear organometallic
compounds with two metal centres linked by a conjugated organic
bridge have been reported,1–8 especially those of ruthenium,1
iron,2 rhenium,3 and platinum.4 In contrast, conjugated bimetallic
iridium complexes5 are rare up to now. Additionally, many
conjugated dinuclear complexes with sp-carbon bridges: –Cx–6 or
mixed bridges such as —(C≡C)x–(C6H4)y–(C≡C)x– are known,7
while those with pure sp2-carbon bridges, such as –(CH)x– bridged
complexes, are still few.8
During our investigation of the reactions of HC≡CCH-
(OH)C≡CH9 with transition-metal compounds, we have iso-
lated a novel bimetallic iridium complex from the reaction of
HC≡CCH(OH)C≡CH with cis-IrHCl2(PPh3)3 (1).10 The com-
poundis madeupoftwoiridiumcentres connectedbyaconjugated
organic bridge of ten sp2-carbons. Interestingly, it can also be
Scheme 1 Reagents and conditions: i) HC≡CCH(OH)C≡CH, CH2Cl2,
considered as two planar six-membered rings containing iridium
linked by a –(CH)2– bridge. The compound has excellent air-
RT; ii) excess PBu3, CH2Cl2, O2, RT.
stability, thermo-stability and electrochemical properties. The
building of stable bimetallic systems with unusual structure allow-
ing communication is also important for practical applications.11
and improve the solubility of 2, we have tried to replace the triph-
enylphosphine ligand on the iridium with tri-n-butylphosphine.
Treatment of 2 with PBu3 in dichloromethane under nitrogen
Treatment of cis-IrHCl2(PPh3)3 (1) with HC≡CCH(OH)C≡CH
readily gave rise to a brown solution containing a mixture of 3
in dichloromethane led to the formation of the dark yellow com-
and another very unstable complex which could not be isolated.
A green mixture was produced when the experiment was carried
out under air. We have monitored the reaction and analyzed the
final solution to draw the following conclusion: under air a mixture
pound 2 in high yield (Scheme 1), which has been characterized
by solution NMR spectroscopy and X-ray diffraction.§ The X-ray
structure shown in Fig. 1 indicates that 2 has a distorted octahedral
geometry around iridium. The metal atom and four carbon atoms
was generated after a slow and intricate process which may involve
of the penta-1,4-dien-3-ol ligand constitute a partially unsaturated
oxidative coupling and other reactions. Fortunately, we succeeded
five-membered ring. The metallacycle is almost planar as reflected
in separating the orange product 3 and another interesting green
product 4.
˚
by the small mean RMS deviation (0.1108 A) from the least-
squares plane through the five atoms Ir1, C1, C2, C3 and C4.
Five-membered ringed metallacycles are numerous, many of
them act as intermediates or precursors of transition-metal-
assisted reactions.12 At room temperature, 2 remains nearly
unchanged over 3 h in solution. To examine the reaction activity
3 could be regarded as a product formed by replacement of PPh3
with PBu3 and the dehydrogenation of CH(OH) in compound 2.
The structure of 3 has also been confirmed by X-ray diffraction§
and its ORTEP drawing is reproduced in Fig. 2. Unlike complex
2, the five-membered metallacycle of 3 has perfect co-planarity,
˚
which is reflected by the very small mean RMS deviation (0.022 A)
from the least-squares plane through the eight atoms C1, C2, C3,
C4, C5, O1, P3 and Ir1. Complex 3 also shows better air-stability
and thermal stability. It remains nearly unchanged when heated at
100 ◦C in toluene for several hours.
The most interesting product isolated from the reaction is
complex 4. The structure of 4 has been confirmed by a single
College of Chemistry and Chemical Engineering, State Key Laboratory for
Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen, 361005,
China. E-mail: hpxia@xmu.edu.cn; Fax: +86-592-2186628
† CCDC reference numbers 633228–633230. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b706900f
‡ Electronic supplementary information (ESI) available: Synthesis and
characterization of all compounds. See DOI: 10.1039/b706900f
4122 | Dalton Trans., 2007, 4122–4125
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Fig. 3 An ORTEP plot of complex 4 with thermal ellipsoids at 50%
probability levels. Three CHCl3 molecules and some of the hydrogen atoms
are omitted for clarity. Symmetry transformations used to generate equiv-
◦
˚
alent atoms: _3 −x, −y + 1, −z. Selected bond distances [A] and angles [ ]:
Ir1–C1 1.947(4), Ir1–O2 2.042(3), C1–C2 1.352(6), C2–C3 1.474(6), C3–C4
1.507(6), C4–C5 1.373(6), C3–O1 1.226(5); C5–C5_3 1.436(8); C1–Ir1–O2
91.68(15), C4–O2–Ir1 128.3(3), C2–C1–Ir1 127.3(3), C1–C2–C3 127.2(4),
C2–C3–C4 120.5(4), C3–C4–C5 115.2(4), C2–C3–O1 117.8(4), C4–C3–O1
121.7(4), O2–C4–C5 121.0(4), O2–C4–C3 123.8(4), C4–C5–C5_3 123.4(5).
Fig. 1 An ORTEP plot of complex 2 with thermal ellipsoids at 50%
probability levels. Two CH2Cl2 molecules and some of the hydrogen atoms
◦
˚
are omitted for clarity. Selected bond distances [A] and angles [ ]: Ir1–C1
1.986(11), Ir1–C4 2.026(11), C1–C2 1.326(16), C2–C3 1.482(18), C3–C4
1.502(18), C4–C5 1.281(18); C1–Ir1–C4 81.0(5), C1–C2–C3 115.4(11),
C2–C3–C4 109.4(10), C3–C4–C5 118.3(12), C2–C3–O1 116.9(12),
C4–C3–O1 119.9(13), C5–C4–Ir1 131.4(11), C3–C4–Ir1 110.5(8).
an inversion centre at the ten carbon chain linking the two Ir
atoms. Atom Ir1 is linked across the inversion centre to Ir1_3 at
(−x, −y + 1, −z). Such a structure can also be viewed as two
special six-membered iridacycles linked by a conjugated carbon
bridge. Interestingly, the sixteen atoms of the rings and the bridge
˚
are almost co-planar, which is reflected by a deviation (0.0536 A)
from the RMS planes of the best fit. The exact mechanism for the
formation of this novel complex is unclear at this moment.
The NMR spectroscopic data are consistent with the X-ray
1
structure. The room-temperature 13C{ H} NMR in CDCl3 dis-
played a carbonyl signal at d = 186.8 ppm and an Ir–C signal
at d = 195.2 ppm which is significantly downfield compared with
chemical shifts of typical Ir–C signals (110–140 ppm).13 The signal
of C5 is observed at d = 112.9 ppm, which is comparable to those
of CH groups of normal alkenes. Other signals of the bridging
ligand were observed at 112.4 ppm (C2) and 156.9 ppm (C4). In
the 1H NMR (in CDCl3) spectrum, the signals attributed to Ir–CH
and bridgehead CH were observed at 13.8 ppm and 6.9 ppm . In
addition, the highly symmetric structure could also be confirmed
by the 31P NMR spectrum: only two peaks at 17.5 ppm (t, J (PP) =
3.6 Hz, CPPh3) and −20.4 ppm (d, J(PP) = 3.6 Hz, IrPBu3) were
observed though there are six phosphorus atoms in the structure.
The electron-transfer properties of 4 have been studied in
dichloromethane containing 0.10 M Bu4NClO4 by cyclic voltam-
metry. A representative voltammogram is shown in Fig. 4.
Compound 4 exhibits two redox processes; E1/2 of the first wave
(A) at −0.14 V, DEp = 112 mV, and E1/2 of the second wave (B) at
0.61 V, DEp = 130 mV, with a scan rate of 100 mV s−1. The redox
peak potential differences at 20 mV s−1 are 80 mV (A) and 90 mV
(B). The differential pulse voltammogram also shows two signals,
which have identical integrated areas, consistent with two one-
electron-transfer processes.14 The relative peak heights ipa/ipc≈1,
and peak currents are found to be linear with the square root of
scan rate (t1/2) in the range from 20 to 200 mV s−1, demonstrate
Fig. 2 An ORTEP plot of complex 3 with thermal ellipsoids at 50%
probability levels. One diethyl ether molecule and some of the hydrogen
˚
atoms are omitted for clarity. Selected bond distances [A] and angles
[◦]: Ir1–C1 1.965(2), Ir1–C4 2.064(2), C1–C2 1.350(2), C2–C3 1.451(3),
C3–C4 1.508(3), C4–C5 1.285(4); C1–Ir1–C4 81.8(1), C1–C2–C3 115.6(2),
C2–C3–C4 113.4 (2), C3–C4–C5 118.3(2), C2–C3–O1 124.0(2), C4–C3–O1
122.6(2), C5–C4–Ir1 131.2(2), C3–C4–Ir1 110.5(2).
crystal X-ray diffraction study.§ The ORTEP drawing is shown in
Fig. 3. In this complex, the two iridium atoms are connected by
a bridge consisting of a ten sp2-carbon atom chain and a shorter
bridge namely Ir1–O2–C4–C5–C5_3–C4_3–O2_3–Ir1_3. There is
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independent reflections (Rint = 0.0693); R1 = 0.0837, wR2 = 0.1924 for
619 parameters and 9407 reflections with [I>2r(I)]. Crystal data for
=
=
3: IrCl2(CH C(PPh3)CH(OH)C( CH2))(PBu3)2·Et2O, Mr = 1083.18,
˚
˚
monoclinic, space group Cc, Z = 4, a = 21.421(4) A, b = 14.267(3) A,
◦
3
˚
˚
c = 18.226(3) A, a = 90, b = 106.765(3), c = 90 , V = 5333.5(17) A ;
20742 reflections, 8087 independent reflections (Rint = 0.0294); R1
=
0.0350, wR2 = 0.0897 for 473 parameters and 9151 reflections with
=
[I>2r(I)]. Crystal data for 4: [(–IrCl2(PBu3)2CH C(PPh3)C(O)CO–)
=
CH)–]2·3CHCl3, Mr = 2644.94, monoclinic, space group P2(1)/n, Z =
˚
2, a = 14.197(2), b = 19.240(3), c = 23.412(4) A, a = 90, b = 99.442(3), c =
◦
3
˚
90 , V = 6308.5(18) A . 48381 reflections, 10877 independent reflections
(Rint = 0.0278); R1 = 0.0450, wR2 = 0.1285 for 557 parameters and 12382
reflections with [I>2r(I)].
1 Examples of bridged ruthenium complexes: W.-Z. Chen and T. Ren,
Inorg. Chem., 2006, 45, 9175–9177; G.-L. Xu, G. Zou, Y.-H. Ni, M. C.
DeRosa, R. J. Crutchley and T. Ren, J. Am. Chem. Soc., 2003, 125,
10057–10065; H. P. Xia, W. F. Wu, W. S. Ng, I. D. Williams and G. Jia,
Organometallics, 1997, 16, 2940–2947; H. P. Xia, W. S. Ng, J. S. Ye, X.-Y.
Li, W. T. Wong, Z. Lin, C. Yang and G. Jia, Organometallics, 1999, 18,
4552–4557; G. Jia, H. P. Xia, W. F. Wu and W. S. Ng, Organometallics,
1996, 15, 3634–3636; H. Qi, A. Gupta, B. C. Noll, G. L. Snider, Y.
Lu, C. Lent and T. P. Fehlner, J. Am. Chem. Soc., 2005, 127, 15218–
15227; M. C. DeRosa, C. A. White, C. E. B. Evans and R. J. Crutchley,
J. Am. Chem. Soc., 2001, 123, 1396–1402; P. J. Mosher, G. P. A. Yap and
R. J. Crutchley, Inorg. Chem., 2001, 40, 1189–1195; S. K. Seetharaman,
M.-C. Chung, U. Englich, K. Ruhlandt-Senge and M. B. Sponsler,
Inorg. Chem., 2007, 46, 561–567.
2 Examples of bridged iron complexes: G.-L. Xu, B. Xi, J. B. Updegraff,
J. D. Protasiewicz and T. Ren, Organometallics, 2006, 25, 5213–5215;
R. D. Adams, B. Qu and M. D. Smith, Organometallics, 2002, 21,
3867–3872; R. D. Adams, O.-S. Kwon, B. Qu and M. D. Smith,
Organometallics, 2001, 20, 5225–5232; N. L. Narvor, L. Toupet and
C. Lapinte, J. Am. Chem. Soc., 1995, 117, 7129–7138; M. Guillemot,
L. Toupet and C. Lapinte, Organometallics, 1998, 17, 1928–1930; R. D.
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21, 2970–2978; M.-C. Chung, X. Gu, B. A. Etzenhouser, A. M. Spuches,
P. T. Rye, S. K. Seetharaman, D. J. Rose, J. Zubieta and M. B. Sponsler,
Organometallics, 2003, 22, 3485–3494.
Fig. 4 Cyclic (a) and differential pulse (b) voltammograms of 1 mM
compound 4 (vs. Ag/AgCl). The voltammograms were recorded in CH2Cl2
and Bu4NClO4 (0.1 M) as the supporting electrolyte, scan rate: (1) 20, (2)
50, (3) 100, (4)150, (5) 200 mV s−1
.
3 Examples of bridged rhenium complexes: R. Dembinski, T. Bartik, B.
Bartik, M. Jaeger and J. A. Gladysz, J. Am. Chem. Soc., 2000, 122,
810–822; W. E. Meyer, A. J. Amoroso, C. R. Horn, M. Jaeger and J. A.
Gladysz, Organometallics, 2001, 20, 1115–1127; F. Paul, W. E. Meyer,
L. Toupet, H. Jiao, J. A. Gladysz and C. Lapinte, J. Am. Chem. Soc.,
2000, 122, 9405–9414.
4 Examples of bridged platinum complexes: N. Das, A. Ghosh, A. M.
Arif and P. J. Stang, Inorg. Chem., 2005, 44, 7130–7137; P. S. Mukherjee,
N. Das, Y. K. Kryschenko, A. M. Arif and P. J. Stang, J. Am. Chem.
Soc., 2004, 126, 2464–2473; N. Das, P. S. Mukherjee, A. M. Arif
and P. J. Stang, J. Am. Chem. Soc., 2003, 125, 13950–13951; C. J.
Kuehl, S. D. Huang and P. J. Stang, J. Am. Chem. Soc., 2001, 123,
9634–9641; J. Manna, C. J. Kuehl, J. A. Whiteford and P. J. Stang,
J. Am. Chem. Soc., 1997, 16, 1897–1905; M. C. DeRosa, F. Al-mutlaq
and R. J. Crutchley, Inorg. Chem., 2001, 40, 1406–1407; W. Kaim, B.
Schwederski, A. Dogan, J. Fiedler, C. J. Kuehl and P. J. Stang, Inorg.
Chem., 2002, 41, 4025–4028.
that the two quasi-reversible one-electron processes approach that
of a reversible system on the cyclic voltammetric time scale. These
two oxidation waves may be attributed to the formation of 4+ and
42+, respectively. Observation of two oxidation waves for complex
4 imply that the transient intermediates of 4 are stable, probably
dependent on conjugation and planarity, and there is some degree
of electronic interaction between the two metal centers, due to a
bridge consisting of a 10 sp2 carbon atom chain or Ir1–O2–C4–
C5–C5_3–C4_3–O2_3–Ir1_3 as the coupling medium.
As a result of the highly conjugated and planar structure,
compound 4 is quite stable to air and is also thermally stable (the
sample◦of 4 in toluene remains nearly unchanged when heated
at 100 C in air for several hours) as well as exhibiting excellent
electrochemical properties. Additionally, there are active groups
like chloride on the metal atoms, which enable 4 to be linked
by polydentate ligands to form a polymer. Such a polymer may
be a useful precursor for mixed-valence (MV), low-dimensional
conductive (LDC) and non-linear optical (NLO) materials.15
We acknowledge with thanks the sponsorship of the pro-
gram New Century Excellent Talents in Universities of China
(NCET-04–0603) and the National Science Foundation of China
(20572089).
5 Examples of bridged iridium complexes: R. R. Tykwinski and P. J.
Stang, Organometallics, 1994, 13, 3203–3208; C. S. Chin, M. Kim,
H. Lee, S. Noh and K. M. Ok, Organometallics, 2002, 21, 4785–
4793.
6 Examples of –Cx– bridged complexes: M. I. Bruce, P. J. Low, K.
Costuas, J.-F. Halet, S. P. Best and G. A. Heath, J. Am. Chem. Soc.,
2000, 122, 1949–1962; M. I. Bruce, B. G. Ellis, P. J. Low, B. W. Skelton
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and C. Lapinte, J. Am. Chem. Soc., 2003, 125, 9511–9522; J. Stahl, J. C.
Bohling, E. B. Bauer, T. B. Peters, W. Mohr, J. M. Mart´ın-Alvarez, F.
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1876.
Notes and references
=
=
§ Crystal
data
for
2:
IrCl2(CH C(PPh3)CH(OH)C( CH2))
7 Examples of mixed bridged complexes: S. Fraysse, C. Coudret and J.-
P. Launay, J. Am. Chem. Soc., 2003, 125, 5880–5888; T. Weyland, I.
Ledoux, S. Brasselet, J. Zyss and C. Lapinte, Organometallics, 2000,
19, 5235–5237.
(PPh3)2·2CH2Cl2, Mr = 1300.85, monoclinic, space group P2(1)/n,
˚
˚
˚
Z = 4, a = 12.833(2) A, b = 32.704(5) A, c = 13.692(2) A, a = 90,
◦
3
˚
b = 111.703(2), c = 90 , V = 5339.1(17) A ; 38479 reflections, 8156
4124 | Dalton Trans., 2007, 4122–4125
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