Organoiridium compounds with bidentate phosphines as highly regioselective catalysts for alkynes cyclotrimerization

Article abstract of DOI:10.1016/j.ica.2004.03.033

The iridium derivatives HIr(cod)(L-L) (cod=1,5-cyclooctadiene; L-L=Ph ₂PCH₂PPh₂ (dppm); Ph₂P(CH ₂)₂PPh₂ (dppe); Ph₂P(CH ₂)₃PPh₂ (dppp); Ph₂P(CH ₂)₄PPh₂ (dppb); o-C₆H ₄(PPh₂)₂; Cy₂P(CH₂) ₂PCy₂ (dcpe); o-Me₂NC₆H ₄PPh₂ (P-NMe₂)) and Ir(OMe)(cod)(dppe-F) (dppe-F=(C₆F₅)₂P(CH₂) ₂P(C₆F₅)₂) are active catalysts for the cyclotrimerization of phenylacetylene and substituted derivatives. The nature of the phosphine ligand has a pronounced effect on catalytic activity and selectivity of the reactions: in some cases (L-L=dcpe, dppe-F) mixtures of oligomerization and polymerization products are obtained, in others only cyclomerization is observed, with regioselectivities as high as 100% of the 1,2,4-trisubstituted benzenes in the reactions catalyzed by HIr(cod)(dppm). The observed regioselectivity is discussed in terms of preferential formation of one of the possible metallacyclic intermediates of the catalytic reaction.

Full text of DOI:10.1016/j.ica.2004.03.033

Inorganica Chimica Acta 357 (2004) 2881–2888
www.elsevier.com/locate/ica
Organoiridium compounds with bidentate phosphines as highly
regioselective catalysts for alkynes cyclotrimerization
*
Matteo Fabbian, Nazario Marsich, Erica Farnetti
Dipartimento di Scienze Chimiche, Universita’ di Trieste, Via L. Giorgieri 1, I-34127 Trieste, Italy
Received 30 December 2003; accepted 20 March 2004
Abstract
The iridium derivatives HIr(cod)(L-L) (cod ¼ 1,5-cyclooctadiene; L-L ¼ Ph2PCH2PPh2 (dppm); Ph2P(CH2)2PPh2 (dppe);
Ph2P(CH2)3PPh2 (dppp); Ph2P(CH2)4PPh2 (dppb); o-C6H4(PPh2)2; Cy2P(CH2)2PCy2 (dcpe); o-Me2NC6H4PPh2 (P-NMe2)) and
Ir(OMe)(cod)(dppe-F) (dppe-F ¼ (C6F5)2P(CH2)2P(C6F5)2) are active catalysts for the cyclotrimerization of phenylacetylene and
substituted derivatives. The nature of the phosphine ligand has a pronounced effect on catalytic activity and selectivity of the re-
actions: in some cases (L-L ¼ dcpe, dppe-F) mixtures of oligomerization and polymerization products are obtained, in others only
cyclomerization is observed, with regioselectivities as high as 100% of the 1,2,4-trisubstituted benzenes in the reactions catalyzed by
HIr(cod)(dppm). The observed regioselectivity is discussed in terms of preferential formation of one of the possible metallacyclic
intermediates of the catalytic reaction.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Iridium; Bidentate phosphines; Alkynes; Cyclotrimerization; Regioselectivity
1. Introduction
On the other hand, preparation of benzene derivatives
via alkynes cyclotrimerization (Fig. 1) has attracted much
attention in the last years, howeverthe low regioselectivity
generally observed hasseverely limited the applicability of
the reaction for synthetic purposes. Nearly all transition
metals have been reported to promote the cyclotrimer-
ization of acetylenes, although examples of iridium-
catalyzed reactions [6–10] are not very common.
During our recent studies on alkynes polymerization
homogeneously catalyzed by rhodium and iridium de-
rivatives, we found that the organoiridium compounds
HIr(cod)(PR3)2 (cod ¼ 1,5-cyclooctadiene; PR3 ¼ PPh3,
P(p-MeOC6H4)3, P(o-MeOC6H4)Ph2, PCyPh2) promote
the highly stereoselective polymerization of phenylacet-
ylene to the trans-polyene (Fig. 2) [11].
We were then interested in investigating the effect of
substitution of the two monodentate phosphines with
one bidentate phosphine: as a matter of fact, we had
previously experienced that a similar substitution in a
series of rhodium derivatives had a beneficial effect on
alkynes polymerization catalysis [12], especially with
regard to the stability of the catalytically active species.
Although the ability of iridium derivatives to pro-
mote C–C bond formation of alkynes has been known
for the last 40 years, the oligomerization of acetylenes
has mainly been studied with other metals such as rho-
dium, ruthenium and palladium. Recently, however,
some papers have appeared in the literature that suggest
a renewed interest towards iridium-based catalysts
which promote this reaction.
Formation of linear oligomers (dimers and trimers)
has been reported in the presence of organoiridium
derivatives, but the reactions often suffer from a low
selectivity, as mixtures of products are usually obtained
[1–4]. One important exception is the catalyst Ir-
(biph)(PMe3)3Cl (biph ¼ biphenyl-2,20-diyl), which pro-
motes the selective formation of the Z-enynes [5].
* Corresponding author. Tel.: +39-40-5583938; fax: +39-40-5583903.
E-mail address: farnetti@univ.trieste.it (E. Farnetti).
0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.03.033

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M. Fabbian et al. / Inorganica Chimica Acta 357 (2004) 2881–2888
and 161.82 MHz, respectively. 1H chemical shifts are
reported relative to tetramethylsilane; 13C chemical
shifts are reported relative to solvent peak (d 77.0 for
CDCl3, 128.0 for C6D6); 31P chemical shifts are reported
relative to external 85% H3PO4, with downfield shift
positive. Infrared spectra were recorded in Nujol mull
on a Perkin–Elmer System 2000 FT-IR spectrometer.
Chemical yields of the catalytic reactions were de-
termined by GC on a HP 6890 instrument, using
naphthalene as internal standard. Molecular weight
distributions of the polymers were determined by SEC in
CHCl3 at 25 °C on a Milton Roy CM4000 instrument
using a UV spectrometer detector operating at 270 nm,
equipped with CHROMPACK Microgel-5 columns.
Fig. 1. Cyclotrimerization products from terminal alkynes.
Fig. 2. Trans-polyphenylacetylene.
2.3. Preparation of HIr(cod)(L-L) (L-L ¼ dppe, dppp,
dppb, o-C6H4(PPh2)2, o-Me2NC6H4PPh2)
However, as we have reported in a preliminary ac-
count [13], the compounds HIr(cod)(Ph2P(CH2)nPPh2)
(n ¼ 1–4) do not behave as polymerization catalysts, at
variance they promote the cyclotrimerization of phe-
nylacetylenes. Interestingly, in some of the catalytic re-
actions 1,2,4-triphenylbenzene is formed with high to
excellent regioselectivity. In this contribution we now
present the results of an extended study on the alkynes
cyclotrimerization reaction catalyzed by iridium-biden-
tate phosphine derivatives: the effect on the catalytic
reaction of the nature of phosphine ligand on one hand,
and of substrate substituents on the other will be
discussed.
A suspension of 100 mg (0.15 mmol) of [Ir(cod)
(OMe)]2 in 15 ml of methanol was treated with 0.30
mmol of the ligand L-L. The resulting mixture was
stirred at room temperature for 3–5 h, yielding a solid
product which was filtered, washed with methanol and
dried under vacuum. Yields 72–78%.
2.4. Spectroscopic data for HIr(cod)(dppe)
31P {1H}NMR (C6D6, 25 °C): d + 43.8 d and +29.7 d
1
(JPP ¼ 2:0 Hz). H NMR (C6D6, 25 °C): d 8.2–7.0 (m,
20H, Ar); 5.15 (bm, 1H, allyl); 5.07 (bm, 1H, allyl); 4.01
(m, 1H, allyl); 2.5 (bm, 2H, dppe); 2.0 (bm, 2H, dppe);
2.2–1.8 (multiplets, 9H, Ir–CH and CH2); )11.55 (dd,
1H, Ir–H, JHP ¼ 20:6 and 14.7 Hz).
2. Experimental
13C {1H}NMR (C6D6, 25 °C): d 138–128 (Ar); 91.3
(s, allyl); 63.2 (d, allyl, JCP ¼ 37:2 Hz); 56.5 (s, CH2);
55.3 (d, CH2, JCP ¼ 13:9 Hz); 35.7 (m, dppe); 33.0 (dd,
Ir–C, JPC ¼ 72:6 and 4.6 Hz); 29.6 (m, dppe); 29.4 (s,
CH2); 27.0 (d, CH2, JPC ¼ 29:3 and 13.9 Hz).
2.1. General
All the reactions and manipulations were routinely
performed under an argon atmosphere using standard
Schlenk tube techniques.
Tetrahydrofuran (THF) was distilled over sodium
benzophenone ketyl just before use; benzene and di-
chloromethane were distilled over CaH2, methanol was
distilled over CaO, and they were stored under an inert
atmosphere. Naphthalene was purified by recrystalli-
zation from ethanol. All the other chemicals were re-
agent grade and were used as received by commercial
suppliers.
The phosphine o-Me2NC6H4PPh2 [14] and the com-
pounds [Ir(cod)(OMe)]2 [15] and HIr(cod)(dppm) [16]
were prepared according to the procedures reported in
the literature. All the iridium derivatives were stored
under inert atmosphere, to avoid decomposition.
2.5. Spectroscopic data for HIr(cod)(dppp)
1
31P {1H}NMR (C6D6, 25 °C): d – 13.1 s. H NMR
(C6D6, 25 °C): d 7.8–7.0 (m, 20H, Ar); 3.77 (bs, 4H,
C@CH); 2.5 (bm, 4H, dppp); 2.0 (bm, 2H, dppp); 2.2–
1.9 (multiplets, 8H, CH2); )13.35 (t, 1H, Ir–H,
JHP ¼ 21:5 Hz).
2.6. Spectroscopic data for HIr(cod)(dppb)
1
31P {1H}NMR (CDCl3, 25 °C): d +0.8 s. H NMR
(C6D6, 25 °C): d 7.8–7.0 (m, 20H, Ar); 3.84 (bs, 4H,
C@CH); 2.1–1.8 (multiplets, 8H, CH2); 2.7, 2.4, 1.6, 1.1
(multiplets, 2H each, dppb); )13.51 (t, 1H, Ir–H,
JHP ¼ 21:6 Hz).
2.2. Instrumental
1H, 13C and 31P NMR spectra were recorded on a
JEOL EX400 spectrometer operating at 399.77, 100.54
13C {1H}NMR (C6D6, 25 °C): d 143–127 (Ar); 39.1
(t, C@CH, JCP ¼ 13:9 Hz); 24.2 (CH2).

M. Fabbian et al. / Inorganica Chimica Acta 357 (2004) 2881–2888
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2.7. Spectroscopic data for HIr(cod)(o-C6H4(PPh2)2
31P {1H}NMR (C6D6, 25 °C): d +39.3 d and +28.4 d
minor isomer d 3.51 (s, OMe); 2.91 (bm, 4H, C@CH);
2.5–2.0 (multiplets, 12H, CH2 of cod and dppe-F). 13C
NMR not obtained due to low solubility.
1
(JPP ¼ 2:0 Hz). H NMR (C6D6, 25 °C): d 8.0–6.7 (m,
24H, Ar); 4.79 (bm, 1H, allyl); 4.73 (bm, 1H, allyl); 4.63
(m, 1H, allyl); 2.5–1.8 (multiplets, 9H, Ir–CH and CH2);
)11.14 (dd, 1H, Ir–H, JHP ¼ 19:5 and 15.6 Hz). 13C
{1H}NMR (C6D6, 25 °C): d 135–127 (Ar); 89.8 (s, allyl);
75.2 (s, allyl); 64.7 (d, allyl, JCP ¼ 37:1 Hz); 56.8 (s,
CH2); 55.4 (d, CH2, JCP ¼ 12:4 Hz); 34.5 (dd, Ir–C);
29.1 (s, CH2); 26.9 (s, CH2).
2.11. Cyclotrimerization reactions
A typical procedure for the catalytic reactions is de-
scribed in the following. A solution of HIr(cod)(L-L)
(0.017 mmol) and of the GC standard naphthalene
(100 mg) in 5.0 ml of solvent (usually THF) was
thermostatted at the appropriate temperature under in-
ert atmosphere. Then 173 mg of phenylacetylene (1.7
mmol, [sub]/[Ir] ¼ 100) were added. Samples were with-
drawn from the reaction mixture at time intervals for
GC analysis. The final reaction mixture, after evapora-
2.8. Spectroscopic data for HIr(cod)(o-Me2NC6H4
PPh2)
1
1
31P {1H}NMR (C6D6, 25 °C): d +25.8 s. H NMR
tion of the solvent, was analyzed by H and 13C NMR.
(C6D6, 25 °C): d 8.2–6.7 (m, 14H, Ar); 5.12 (bm, 1H,
allyl); 4.86 (bm, 1H, allyl); 3.79 (m, 1H, allyl); 2.51 (s,
6H, NMe); 2.2–1.4 (multiplets, 9H, Ir–CH and CH2);
)9.64 (d, 1H, Ir–H, JHP ¼ 21:5 Hz). 13C {1H}NMR
(C6D6, 25 °C): d 141–118 (Ar); 96.3 (s, allyl); 73.6 (s,
allyl); 63.2 (d, allyl, JCP ¼ 39:2 Hz); 54.8 (s, CH2); 54.2
(s, NMe); 51.3 (d, CH2); 29.8 (s, CH2); 26.0 (m, IrCH);
17.5 (d, CH2, JCP ¼ 7:7 Hz).
2.12. Determination of product distribution and stereo-
chemistry
Yields of 1,3,5-triaryllbenzene and 1,2,4-triarylben-
zene were determined by GC analysis, and confirmed by
1H and 13C NMR [13]. Yields of (E)-1,4-diphenyl-1-
butyn-3-ene and (Z)-1,4-diphenyl-1-butyn-3-ene were
1
determined by H NMR (CDCl3) by integration of the
signals of the corresponding vinyl protons [11].
2.9. Preparation of HIr(cod)(dcpe)
The stereochemistry of the polyphenylacetylene ob-
1
A solution of 80 mg (0.12 mmol) of [Ir(cod)(OMe)]2
in 2 ml of THF was treated with 101 mg (0.24 mmol) of
dcpe. The resulting solution was stirred at room tem-
perature for 2 h, after which time it was concentrated to
approx. 1 ml. Addition of pentane caused precipitation
of a yellow solid which was filtered, washed with pen-
tane and dried under vacuum. Yield: 48%.
tained was determined by H and 13C NMR [11]. De-
termination of polyene molecular weights via SEC was
performed on freshly prepared chloroform solutions of
the polymer. The number average molecular weight (Mn)
and polydispersion index (Mw=Mn) of the polymers were
calculated on calibrations using polystyrene standards.
31P {1H}NMR (C6D6, 25 °C): d +47.9 d and +36.3 d
(JPP ¼ 3:0 Hz). 1H NMR (C6D6, 25 °C): d 5.14 (bm, 2H,
allyl); 4.60 (m, 1H, allyl); 2.1–1.8 (multiplets, 9H, Ir–CH
and CH2); )12.44 (t, 1H, Ir–H, JHP ¼ 18:6 Hz).
13C {1H}NMR (C6D6, 25 °C): d 85.8 (s, allyl); 69.9 (s,
allyl); 60.0 (d, allyl, JCP ¼ 36:9 Hz); 57.3 (d, CH2,
JCP ¼ 13:1 Hz); 56.1 (s, CH2); 39.0 (d, Ir–C, JPC ¼ 22:3
Hz); 30–25 (CH2 and Cy).
3. Results and discussion
3.1. Preparation and properties of iridium-diphosphines
compounds
The compounds HIr(cod)(Ph2P(CH2)nPPh2) (n ¼ 1,
bis(diphenylphosphino)methane (dppm); 2, 1,2-bis(dip-
henylphosphino)ethane (dppe); 3, 1,3-bis(diphenylpho-
sphino)propane (dppp); 4, 1,4-bis(diphenylphosphino)-
butane (dppb)) were synthesized according to the
procedure reported by Oro et al. [16] for the deriva-
tive with dppm. A suspension of [Ir(cod)(OMe)]2 in
methanol was treated with one equivalent of the
phosphine at room temperature: initial formation of
the mononuclear methoxo species 1a–d, a red com-
pound partially soluble in the reaction medium, was
followed by hydrogen b-elimination which was ex-
pected to give the off-white or pale yellow hydride
derivative 2a–d (see Scheme 1). In fact the structure of
the product with dppm, reported by Oro, reveals that
with this ligand 3a has actually been obtained, where
2.10. Preparation of Ir(OMe)(cod)(dppe-F)
A suspension of 89 mg (0.14 mmol) of [Ir(cod)
(OMe)]2 in 30 ml of methanol was treated with 207 mg
(0.27 mmol) of dppe-F. The resulting mixture was stir-
red at room temperature for 24 h, yielding a yellow solid
which was filtered, washed with methanol and dried
under vacuum. Yield 61%. IR (Nujol): 1090 and 1037
cmꢀ1 (Ir–OMe).
31P {1H}NMR (CDCl3, 25 °C): d +0.8 s (major iso-
1
mer) and +122.7 s (minor isomer). H NMR (CDCl3,
25 °C): major isomer d 3.77 (bm, 4H, C@CH); 3.48 (s,
OMe); 2.5–2.0 (multiplets, 12H, CH2 of cod and dppe-F);

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M. Fabbian et al. / Inorganica Chimica Acta 357 (2004) 2881–2888
Scheme 1.
the diene is coordinated to iridium in an unusual g1,
g3-binding mode.
1H NMR spectrum after an overnight accumulation for
the 13C spectrum: in the final mixture the signals of both
2c and 3c were detectable in a 1:2 ratio.
We now find that also in the analogue reaction with
dppe the carbocyclic ligand of the product 3b is coor-
dinated in the same fashion, as indicated by the
1H NMR spectrum in C6D6, where the hydride signal is
a doublet of doublets at d )11.55 (JHP ¼ 20:6 and 14.7
Hz) and the allylic protons resonate as multiplets at d
5.15, 5.07 and 4.01 ppm, whereas the 31P NMR spec-
trum shows two inequivalent phosphorous nuclei at d
+43.8 and +29.7 (doublets, JPP ¼ 2:0 Hz). The 13C
NMR data (see Section 2) are in agreement with the
proposed structure.
In the synthesis of the iridium hydride with the ligand
dppb compound 2d was formed, as evidenced by spec-
troscopic data: 31P NMR showed a singlet at d +0.8,
whereas 1H and 13C spectra were consistent with a
carbocyclic ligand coordinated as a typical diolefin (see
Table 1 and Section 2). However, also in this case at
higher temperature the irreversible isomerization
2d ! 3d took place, as evidenced by a variable temper-
ature NMR experiment: compound 3d was detected at
35 °C albeit in traces (31P NMR d +4.7 d and )2.3 d,
1
A more intriguing situation is present for the reaction
of [Ir(cod)(OMe)]2 and dppp, as a mixture of two
products is obtained: compound 2c with ‘‘normally’’
coordinated g2,g2-cod, and compound 3c with an
g1,g3-binding mode of carbocyclic ligand, with an in-
tensity ratio of about 8:1. The 31P NMR spectrum in
C6D6 consists in a singlet at d )13.1 (2c) and two dou-
JPP ¼ 14:8 Hz; H NMR d )11.40 t (Ir–H, JHP ¼ 16:6
Hz); 5.13, 4.24, 3.2 (bm, allyl protons)); the amount of
3d slightly increased at higher temperature up to 8% of
the total mixture.
Besides the ligands of the series Ph2P(CH2)nPPh2,
other bidentate phosphines where employed for the
syntheses of the corresponding iridium-hydrides. The
reaction of [Ir(cod)(OMe)]2 with o-C6H4(PPh2)2 with
the usual procedure gave compound 3e, as evidenced by
31P NMR data (two doublets at d +39.3 and +28.4,
JPP ¼ 2:0 Hz) as well as 1H and 13C spectra (see Table 1
and Section 2).
The corresponding synthesis with dcpe ( ¼ 1,2-bis
(dicyclohexylphosphino)ethane) in methanol only pro-
duced decomposition products; the preparation was
successfully performed in THF solution at r.t. for 2 h,
after which time the mixture was concentrated, and the
product was recovered by addition of pentane. Also in
this case the NMR spectra (see Table 1) indicated that
product 3f with the rearranged carbocyclic ligand was
obtained.
1
blets at d )6.5 and )14.9 (JPP ¼ 23:7 Hz, 3c). The H
NMR resonances of the vinyl protons of cod in 2c at d
3.77 are accompanied by multiplets at d 5.29, 4.35 and
3.35, assignable to iridium-allyl protons of 3c; in the
hydride region a major triplet at d )13.35 (JHP ¼ 21:5
Hz, 2c) and a smaller one at d )11.28 (JHP ¼ 17:7 Hz,
3c) are detected. Interestingly, a variable temperature
NMR study showed that by increasing the temperature
of a C6D6 solution containing 2c and 3c the former
compound was partially converted into the latter:
whereas at r.t. the ratio between 2c and 3c was 8:1, at
35 °C it lowered to 2:1, and at 55 °C it was 1:3, i.e. the
prevalent compound was now 3c; a further increase of
the temperature (up to 75 °C) caused no change in the
product ratio. The conversion of 2c into 3c was irre-
versible, as the final intensity ratio of 1:3 was maintained
when the sample temperature was eventually lowered to
20 °C. Notably, this interconversion takes place also at
room temperature, as observed by recording the
The ligand o-Me2NC6H4PPh2 (P-NMe2), which can
coordinate in a bidentate fashion through P and N at-
oms, was also reacted with [Ir(cod)(OMe)]2: the isom-
erization product 3g was produced also with this hybrid
ligand (see Table 1).

M. Fabbian et al. / Inorganica Chimica Acta 357 (2004) 2881–2888
2885
Table 1
Selected NMR data for HIr(cod)(L-L)
L-L (compound)
31P
1H
H-Ir
cod
dppe (3b)
+43.8 d; +29.7 d
(JPP ¼ 2:0)
)11.55 dd
(JHP ¼ 20:6 and 14.7)
5.15 (1H); 5.07 (1H);
4.01 (1H); 2.2–1.8 (9H)
dppp (2c)
)13.1 s
)13.35 t
(JHP ¼ 21:5)
3.77 (4H);
2.2–1.9 (8H)
dppb (2d)
+0.8 s
)13.51 t
(JHP ¼ 21:6)
3.84 (4H);
2.1–1.8 (8H)
o-C6H4(PPh2)2 (3e)
dcpe (3f)
+39.3 d; +28.4 d
(JPP ¼ 2:0)
+47.9 d; +36.3 d
)11.14 dd
(JHP ¼ 19:5 and 15.6)
4.79 (1H); 4.73 (1H);
4.63 (1H); 2.5–1.8 (9H)
)12.44 t
(JHP ¼ 18:6)
5.14 (2H); 4.60 (1H);
2.1–1.8 (9H)
(JPP ¼ 3:0)
+25.8 s
P-NMe2 (3g)
)9.64 d
(JHP ¼ 21:5)
5.12 (1H); 4.86 (1H);
3.79 (1H); 2.2–1.4 (9H)
Experimental conditions: C6D6 solutions, 25 °C.
Chemical shifts expressed in ppm; coupling constants expressed in Hz.
The results of these syntheses suggest that with
bidentate phosphines which on coordination form
comparatively small chelate rings such as dppm (four-
membered chelate ring) and dppe, o-C6H4(PPh2)2, dcpe,
P-NMe2 (five-membered chelate rings) the rearrange-
ment of cod to the g1,g3-binding mode is favoured. By
increasing the chelate ring size to six- (dppp) and seven-
membered rings (dppb) such isomerization is observed
at high temperature, and in the latter case only to a
small extent. According to the studies reported by Oro
on the dppm derivative [16], the rearrangement occurs
via a succession of olefin insertions into the metal-hy-
dride bond and b-eliminations from the iridium-alkyl. A
similar situation was reported by some of us [17] with
the compound HIr(cod)(PNP) (PNP ¼ C3H7N(CH2
CH2PPh2)2) where the insertion product Ir(PNP)(r; g2-
C8H13) was stabilized by coordination to the metal of
the nitrogen atom of the phosphine ligand, thus pre-
venting the successive steps of the rearrangement. With
regard to the compounds object of the present studies,
we are presently investigating on the factors which de-
termine the rearrangement of cod by photoemission
spectroscopy using Synchrotron Radiation; the results
of such measurements will be compared with DFT cal-
culations on the same iridium compounds.
3.2. Catalytic reactions with iridium/diphosphine com-
plexes
The iridium complexes HIr(cod)(L-L) (L-L ¼ dppm,
3a; dppe, 3b; dppp, 2c; dppb, 2d; o-C6H4(PPh2)2, 3e;
dcpe, 3f; P-NMe2, 3g) and the methoxo compound
Ir-(OMe)(cod)(dppe-F) (1h), synthesized as above de-
scribed, were employed as catalysts for the cyclo-
trimerization of phenylacetylene. In a typical catalytic
reaction, the light yellow solution of the iridium deriv-
ative, thermostatted at the appropriate temperature, was
treated with the alkyne, thus giving an orange–brown
solution. The reaction was followed by GC analysis of
samples withdrawn at time intervals, and the final
1
products were analyzed by GC and H and 13C NMR
and – when the polyene was formed – by SEC (size
exclusion chromatography) to determine its molecular
weight and polydispersion.
The results obtained in the catalytic reactions with
phenylacetylene in THF are reported in Table 2; similar
results were obtained in other solvents such as toluene
and methanol, in spite of the limited solubility of the
iridium derivative in the latter, whereas dichlorome-
thane and chloroform caused partial decomposition of
the catalysts.
Finally, a different behaviour was observed in the
synthesis of the derivative with 1,2-bis(dipentafluorop-
henylphosphino)ethane (dppe-F): reaction of this ligand
with [Ir(cod)(OMe)]2 produced a mixture of two isomers
of the methoxo derivative 1h. Further reaction to give
the hydride species was not observed even at longer re-
action times and higher temperatures: apparently, the
ligand with perfluoro phenyl groups stabilized the
methoxo compound with regard to the b-elimination
reaction.
The series of compounds with the ligands dppm,
dppe, dppp and dppb (entries 1–4) promoted the cy-
clotrimerization of the alkyne, whereas no formation of
either polyphenylacetylene (PPA) or linear dimers was
detected in the corresponding reactions. Notably, for-
mation of triphenylbenzenes occurred with high to ex-
cellent regioselectivity (90–99%) towards the asymmetric
isomer 1,2,4-triphenylbenzene. Both catalytic activity
and regioselectivity decreased in the order dppm >
dppe > dppp > dppb, i.e. with increasing the iridium-

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M. Fabbian et al. / Inorganica Chimica Acta 357 (2004) 2881–2888
Table 2
Cyclotrimerization of phenylacetylene with iridium/diphosphine catalysts
Entry
Catalyst
Conversion (%)
trans-PPA (%)
1,2,4-Ph3C6H4 (%)
1,3,5-Ph3C6H4 (%)
1
2
3
4
5
6
7
8
HIr(cod)(dppm)
HIr(cod)(dppe)
100a
98
99
97
45
28
88
3
1
1
2
3
4
3
6
5
HIr(cod)(dppp)
HIr(cod)(dppb)
47
31
HIr(cod)(o-C6H4(PPh3)2)
HIr(cod)(dcpe)
92
31b
20
18
8
HIr(cod)(P-NMe2)
Ir(OMe)(cod)(dppe-F)
6
32
22
5
Experimental conditions: [Ir] ¼ 3.4 ꢁ 10ꢀ3 mol lꢀ1; [sub] ¼ 0.34 mol lꢀ1; [sub]/[Ir] ¼ 100; solvent ¼ THF; T ¼ 60 °C; reaction time: 6 h.
PPA ¼ polyphenylacetylene.
a Reaction time 15 min.
b Dimerization products also obtained.
diphosphine chelate ring size. Among the other cata-
lysts employed, only the adduct with o-C6H4(PPh2)2
gave comparable results (Table 2 entry 5), as it
only promoted formation of the cyclotrimers with good
regioselectivity.
At variance, in the reactions catalyzed by the iridium
derivatives with dcpe, P-NMe2 and dppe-F (Table 2
entries 6–8) mixture of products were obtained: forma-
tion of triphenylbenzenes was not regioselective, and it
was always accompanied by the polymerization product.
In one case (entry 6) also enynes were formed via phe-
nylacetylene dimerization. With regard to the polymer-
ization reaction, the polyene produced had similar
properties as that obtained in the reactions catalyzed by
HIr(cod)(PR3)2 [11]: high stereoselectivity (virtually
100% of trans sequences), Mn between 3000 and 8000,
polydispersion index of 1.3–1.7.
3.3. Cyclotrimerization of arylacetylenes
In order to study the influence of the nature of
the alkyne on the catalytic reaction, as well as the gen-
erality of the cyclotrimerization, the substituted phe-
nylacetylenes p-MeOC6H4CBCH, p-CF3C6H4CBCH
and o-CF3C6H4CBCH were also employed. For this
investigation we selected, among the series above re-
ported, two catalysts which only promote the cyclotri-
merization reaction, i.e. HIr(cod)(dppm) (3a) and
HIr(cod)(dppb) (2d). The results of the corresponding
catalytic reactions are reported in Tables 3 and 4.
In the presence of the dppm derivative 3a, the cata-
lytic reactions with the para-substituted phenylacetyl-
enes at 60 °C were rather fast (Table 3 entries 1–3),
therefore a lower temperature was employed to appre-
ciate differences in reaction rates. At 40 °C a trend was
Table 3
Cyclotrimerization of arylacetylenes catalyzed by HIr(cod)(dppm)
Entry
Substrate
T (°C)
t
Conversion (%)
1,2,4-Ar3C6H4 (%)
1,3,5-Ar3C6H4 (%)
1
1
2
3
4
5
6
7
PhCBCH
60
60
60
60
40
40
40
50
90
100
87
89
100
86
p-MeOC6H4CBCH
p-CF3C6H4CBCH
o-CF3C6H4CBCH
PhCBCH
p-MeOC6H4CBCH
p-CF3C6H4CBCH
50
50
1
38
3
5 h
1.5 h
1 h
2 h
85a
22
95
98
100
98
97
97
3
1
Experimental conditions: [Ir] ¼ 3.4 ꢁ 10ꢀ3 mol lꢀ1; [sub] ¼ 0.34 mol lꢀ1; [sub]/[Ir] ¼ 100; solvent ¼ THF.
a Other products: trans-PPA and dimers.
Table 4
Cyclotrimerization of arylacetylenes catalyzed by HIr(cod)(dppb)
Entry
Substrate
Conversion (%)
1,2,4-Ar3C6H4 (%)
1,3,5-Ar3C6H4 (%)
1
2
3
4
PhCBCH
31
39
40
26a
28
36
3
p-MeOC6H4CBCH
p-CF3C6H4CBCH
o-CF3C6H4CBCH
3
39
n.d.
1
n.d.
Experimental conditions: see Table 3. T ¼ 60 °C; reaction time 6 h.
a Products: trans-PPA, dimers and cyclotrimers.

M. Fabbian et al. / Inorganica Chimica Acta 357 (2004) 2881–2888
2887
evidenced in the reaction rates, which increase on in-
creasing the electrodonating properties of the sub-
stituent on the phenyl ring: p-CF3C6H4CBCH <
PhCBCH < p-MeOC6H4CBCH. With these substrates
the cyclotrimerization reactions catalyzed by 3a were
highly regioselective, notably with the alkyne bearing
the para-methoxy group the corresponding 1,2,4-tria-
rylbenzene was obtained with 100% yield.
On the other hand, the reactions of the p-substituted
phenylacetylenes catalyzed by the dppb derivative 2d
were as expected much slower (see Table 4 entries 1–3),
however in this case there was no apparent trend of
reaction rate and regioselectivity as a function of elec-
tronic properties of the substituent.
At variance, with both catalysts in the reactions of the
ortho substituted phenylacetylene o-CF3C6H4CBCH a
mixture of products was obtained, including comparable
amounts of the cyclotrimers, trans-polyene and enynes
(see Table 3 entry 4 and Table 4 entry 4). Apparently,
with this more sterically demanding alkyne different
reaction pathways are operative, probably also involv-
ing catalytic species with the phosphine coordinated in a
monodentate fashion.
Finally, the catalysts under investigation do not
promote the cyclotrimerization of disubstituted acety-
lenes, as evidenced by reactions performed with phen-
ylpropyne (PhCBCMe) in the presence of either 3a or
3b, which produced neither oligomeric nor polymeric
products.
Scheme 2.
atriene intermediate or in a concerted fashion, however
in most cases proofs in support of the concerted mech-
anism have been produced.
In the iridium-catalyzed cyclotrimerization here re-
ported, selective formation of the 1,2,4-trisubstituted
benzenes is possibly determined by regioselective for-
mation of the metallacyclopentadiene intermediate. As a
matter of fact, both 2,5 and 3,4-diarylmetallacycles (see
Fig. 3) can only form the asymmetric arene indepen-
dently by the orientation of addition of the third alkyne,
whereas from the 2,4-diarylmetallacycle either isomers
of the trisubstituted arene can be produced, according to
the regiochemistry of the subsequent alkyne addition.
We believe that selective formation of the interme-
diate 2,5-diarylmetallacycle is responsible for the regio-
selective cyclotrimerization to the 1,2,4-triarylbenzene
catalyzed by the iridium-diphosphines complexes. As a
matter of fact, the dependence of both reaction rate and
regioselectivity on the chelate ring size in the series
dppm–dppe–dppp–dppb (vide supra) indicates a dra-
matic effect of the chain length connecting the two
phosphorous atoms on the catalytic reaction. In other
words, an increase of the bite angle of the diphosphine
has a negative effect on the cyclotrimerization reaction.
These results suggest that the iridacyclopentadiene with
3.4. Mechanism of cyclotrimerization
Although the cyclotrimerization of alkynes has been
studied by several research groups with a variety of early
and late transition metal derivatives, regioselective for-
mation of one of the possible isomeric products has
rarely been reported. Recently, Ladipo and coworkers
described supported titanium-arene complexes which
catalyze the cyclotrimerization of phenylacetylene to
1,2,4-triphenylbenzene with 99% yield [18]; the same
reaction occurs with 96% selectivity in the presence of a
cobalt carbonyl cluster [19]. Regioselective cyclotrimer-
ization of p-MeC6H4CBCH has been reported with
palladium complexes, with yields in the asymmetric
isomer up to 95% [20].
Therefore, the regioselectivity observed in some of the
reactions we are reporting here, and particularly those
performed with the catalyst HIr(cod)(dppm), where
yields of 1,2,4-triarylbenzenes up to 100% are obtained,
is unprecedented but for the titanium-based catalysts of
Ladipo [18].
As reported in previous studies [21–24], metal-medi-
ated cyclotrimerization reactions typically proceed
through formation of metallacyclopentadiene interme-
diates (see Scheme 2). The final step of the reaction has
been proposed to occur either via a metallacyclohept-
Fig. 3. Iridacyclopentadienes.

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M. Fabbian et al. / Inorganica Chimica Acta 357 (2004) 2881–2888
the substituents adjacent to iridium is the intermediate
involved in the catalytic reaction, as the stability of this
species is expected to be heavily dependent on the steric
hindrance in the coordination sphere of iridium, and in
the series of ligands considered it would be most stabi-
lized by dppm which has the smallest bite angle. It is to
be noted that steric factors are often invoked as deter-
mining the regioselectivity of alkynes cyclotrimerization
[18,25]. Moreover, the suggestion of selective formation
of the 2,5-diarylmetallacycle is in agreement with the
results of Wakatzuki and coworkers, who isolated a
series of cobaltacyclopentadiene intermediates: in all
cases selective formation of one of the possible metal-
lacycles was observed, i.e. the adduct where the cobalt is
bound to the carbon atoms bearing the bulkier substit-
uents [26].
Acknowledgements
The authors thank the Italian Ministero dell’ Uni-
ꢀ
versita e della Ricerca Scientifica e Tecnologica for
finantial support.
References
[1] T. Ohmura, S. Yorozuya, Y. Yamamoto, N. Miyaura, Organo-
metallics 19 (2000) 365.
[2] C.S.Chin,H.Lee,H.Park,M.Kim,Organometallics21(2002)3889.
[3] C.S. Chin, G. Won, J. Song, Bull. Korean Chem. Soc. 15 (1994) 961.
[4] S.-I. Lee, S.-C. Shim, T.-J. Kim, J. Polym. Sci. Part A: Polym.
Chem. 34 (1996) 2377.
[5] C.-H. Jun, Z. Lu, R.H. Crabtree, Tetrahedron Lett. 33 (1992)
7119.
[6] J.P. Collman, J.W. Kang, W.F. Little, M.F. Sullivan, Inorg.
Chem. 7 (1968) 1298.
The alternative possibility, i.e. formation of 1,2,4-
triarylbenzenes via intermediate 2,4-diarylbenzene fol-
lowed by approach of the third alkyne with controlled
orientation, cannot however be ruled out on the basis of
the results so far obtained.
[7] W.H. Baddley, G.B. Tupper, J. Organomet. Chem. 67 (1974) C16.
[8] C. Bianchini, K.G. Caulton, C. Chardon, O. Eisenstein,
K. Folting, T.J. Johnson, A. Meli, M. Peruzzini, D.J. Rauscher,
W.E. Streib, F. Vizza, J. Am. Chem. Soc. 113 (1991) 5127.
[9] J.M. O’Connor, A. Closson, K. Hiibner, R. Merwin, P. Gantzel,
Organometallics 20 (2001) 3710.
Further investigations are underway to identify the
iridium derivatives involved in the catalytic cycles; by
these means we propose to establish the origin of the
regioselectivity, as well as the reasons of the observed
trend of catalytic activity as a function of the electronic
properties of the alkyne substituents.
[10] R. Takeuchi, S. Tanaka, Y. Nakaya, Tetrahedron Lett. 42 (2001)
2991.
[11] M. Marigo, D. Millos, N. Marsich, E. Farnetti, J. Mol. Catal. A:
Chemical 206 (2003) 319.
[12] M. Falcon, E. Farnetti, N. Marsich, J. Organomet. Chem. 629
(2001) 187.
[13] E. Farnetti, N. Marsich, J. Organomet. Chem. 689 (2004) 14.
[14] H.P. Fritz, I.R. Gordon, K.E. Schwarzhans, L.M. Venanzi,
J. Chem. Soc. (1965) 5210.
[15] M. Marigo, N. Marsich, E. Farnetti, J. Mol. Catal. A: Chemical
187 (2002) 169.
4. Conclusions
[16] M.A. Esteruelas, M. Olivan, L.A. Oro, M. Schulz, E. Sola,
H. Werner, Organometallics 11 (1992) 3659.
A series of organoiridium derivatives with bidentate
phosphines of the type HIr(cod)(L-L) has been synthe-
sized and characterized. In these compounds, according
to the phosphine employed the diene can be coordinated
either in a ‘‘normal’’ diolefin fashion, or it can be rear-
ranged to an g1,g3-binding mode; such rearrangement
appears to be disfavoured by an increase of the iridium-
phosphine chelate ring size.
[17] C. Bianchini, E. Farnetti, M. Graziani, G. Nardin, A. Vacca,
F. Zanobini, J. Am. Chem. Soc. 112 (1990) 9190.
[18] O.V. Ozerov, B.O. Patrick, F.T. Ladipo, J. Am. Chem. Soc. 122
(2000) 6423.
[19] T. Sugihara, A. Wakabayashi, Y. Nagai, H. Takao, H. Imagawa,
M. Nishizawa, Chem. Commun. (2002) 576.
[20] J. Li, H. Jiang, M. Chen, J. Org. Chem. 66 (2001) 3627.
[21] J.P. Collman, J.W. Kang, W.F. Little, M.F. Sullivan, Inorg.
Chem. 7 (1968) 1298.
The iridium derivatives catalyze the regioselective
cyclotrimerization of phenylacetylenes to the corre-
sponding 1,2,4-triarylbenzenes with yields up to 100%.
The results of the catalytic reactions have been dis-
cussed, and the observed regioselectivity has been ra-
tionalized in terms of selective formation of one of the
possible metallacyclopentadienyl intermediates. Studies
concerning the mechanism of the catalytic reaction are
currently in progress.
[22] D.P. Smith, J.R. Strickler, S.D. Gray, M.A. Bruck, R.S. Holmes,
D.E. Wigley, Organometallics 11 (1992) 1275.
[23] R. Diercks, B.E. Eaton, S. Gurtzgen, S. Jalisatgi, A.J. Matzger,
R.H. Radde, K.P.C. Vollhardt, J. Am. Chem. Soc. 120 (1998) 8247.
[24] R.J. Baxter, G.R. Knox, P.L. Pauson, M.D. Spicer, Organomet-
allics 18 (1999) 197.
[25] G.A. Ardizzoia, S. Brenna, G. La Monica, A. Maspero,
N. Masciocchi, J. Organomet. Chem. 649 (2002) 173.
[26] Y. Wakatsuki, O. Nomura, K. Kitaura, K. Morokuma,
H. Yamazaki, J. Am. Chem. Soc. 105 (1983) 1907.