Directing iridium-catalyzed C-C bond formation by selection of the ancillary ligands: Polymerization and cyclotrimerization of alkynes

Article abstract of DOI:10.1016/j.ica.2009.03.030

The ability of organoiridium derivatives of catalyzing oligomerization and polymerization of terminal alkynes is markedly influenced by the nature of non-participative ligands coordinated to the metal. The dimeric species [Ir(cod)Cl]₂ and [Ir(c

Full text of DOI:10.1016/j.ica.2009.03.030

Inorganica Chimica Acta 363 (2010) 467–473
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Review
Directing iridium-catalyzed C–C bond formation by selection of the ancillary
ligands: Polymerization and cyclotrimerization of alkynes
1
*
Erica Farnetti , Serena Filipuzzi
Dipartimento di Scienze Chimiche, Universita’ di Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The ability of organoiridium derivatives of catalyzing oligomerization and polymerization of terminal
alkynes is markedly influenced by the nature of non-participative ligands coordinated to the metal.
The dimeric species [Ir(cod)Cl]2 and [Ir(cod)(OMe)]2 (cod = 1,5-cyclooctadiene) as well as the phosphine
complexes HIr(cod)(PR3)2 (PR3 = PPh3, P(p-MeOC6H4)3, P(o-MeOC6H4)Ph2, PCyPh2) catalyze the polymer-
ization reaction, whereas the diphosphine derivatives HIr(cod)(P–P) (P–P = Ph2P(CH2)nPPh2 (n = 1–4),
o-C6H4(PPh2)2) promote the regioselective formation of 1,2,4-trisubstituted benzenes. On the other hand,
the iridium complexes with nitrogen chelating ligands Ir(cod)(N–N)X and Ir(hd)(N–N)X (hd = 1,5-hexadi-
ene; N–N = 1,10-phenanthroline and substituted derivatives; X = halogen) catalyze alkynes polymeriza-
tion. In most cases one catalytic reaction predominates over the other possible routes, so that
polymerization often takes place in the absence of oligomerization side reactions, and conversely cyclo-
trimerization is rarely accompanied by formation of either polyene or dimers.
Received 15 January 2009
Received in revised form 16 March 2009
Accepted 22 March 2009
Available online 28 March 2009
Dedicated to Professor Paul Pregosin for his
contribution to chemistry.
Keywords:
Iridium
Phosphines
Nitrogen ligands
Homogeneous catalysis
Alkynes polymerization
Cyclotrimerization
Ó 2009 Elsevier B.V. All rights reserved.
Erica Farnetti studied chemistry at the University of Trieste – her birthplace – where she got her degree with a thesis in homogeneous catalysis,
awarded with the Bracco-Salata prize. After a postgraduate fellowship at the University of North Wales (Bangor, UK), she went back to the
}
University of Trieste where in 1989 she got a Ph.D. in Chemical Sciences, followed by a postdoc position at ETH Zurich (CH) with Professor L.G.M.
Venanzi. She is currently assistant professor of inorganic chemistry at the University of Trieste. Her present research interests are focussed on the
development of transition metal catalysts for green chemistry applications.
Serena Filipuzzi graduated in chemistry at the University of Trieste with a thesis in homogeneous catalysis under the supervision of Dr. E. Farnetti.
}
She joined Prof. P.S. Pregosin group at ETH Zurich (CH) as graduate student, working on NMR studies of catalytically relevant organometallic
compounds. After completing her Ph.D. in 2008 she started working as environmental chemist at Rockwool International A/S in Denmark.
* Corresponding author.
E-mail address: farnetti@units.it (E. Farnetti).
Present address: Rockwool International A/S, Hovedgaden 584, DK-2640 Hedehusene, Denmark.
1
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.03.030

468
E. Farnetti, S. Filipuzzi / Inorganica Chimica Acta 363 (2010) 467–473
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 468
2. Iridium-diene catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 468
3. Iridium-phosphine catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469
4. Iridium-bidentate phosphine catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470
5. Iridium catalysts with nitrogen chelating ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471
6. Final remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473
1. Introduction
chemistry and it can produce different polyenes according to
their C@C bond configuration (cis or trans) as well as the C–C bond
conformation (cisoidal or transoidal) (Fig. 1). Another interesting
reaction of alkynes is cyclotrimerization (see Scheme 2), which
The study of catalytic reactions has been gaining new impulse
in recent years, as a consequence of greater awareness towards
environmental issues. As stated by Anastas and Kirchhoff [1], catal-
ysis is to be considered as a fundamental pillar of green chemistry:
catalyzed reactions produce reduced amounts of waste in compar-
ison to non-catalyzed processes, thus optimizing atom economy
and E factors [2]. Among the most atom-economic reactions are
polymerization and oligomerization, which generally form no
side-products and therefore in most favourable cases afford E fac-
tor values close to zero.
On the other hand, the development of new polymeric materials
for well defined applications is considered one of the major targets
of current chemical research. Polyacetylenes produced by alkynes
polymerization have attracted attention as promising materials
due to their physico-chemical properties such as photoconductivi-
ty, photoluminescence, oxygen permeability, ferromagnetism and
non-linear optical properties [3–6]. Polymerization of terminal al-
kynes (see Scheme 1) generally occurs with head-to-tail regio-
represents
a potentially useful method for the synthesis of
substituted organic compounds: unfortunately, the low selectivity
usually observed for this reaction is presently limiting its applica-
bility in organic synthesis [7].
Rhodium-catalyzed alkynes polymerization has been exten-
sively studied in the last 15 years, leading to the development of
several catalytic systems, some of which behave in a living fashion
[8–14]. During our investigations on rhodium–diphosphine cata-
lysts for phenylacetylene polymerization [15] we realized that no
iridium-based catalysis had been reported in this reaction, with
the exception of two papers in which small amounts of polyacety-
lene formation were mentioned [16,17]; in fact, some iridium
derivatives were reported to catalyze acetylenes oligomerization,
generally with low selectivity [18–20].
Thus we started a research project focussed on the determina-
tion of the catalytic properties of organoiridium compounds in oli-
go and polymerization of alkynes. Initially the simple dimeric
compounds [Ir(cod)Cl]2 and [Ir(cod)(OMe)]2 were tested [21], fol-
lowed by studies on complexes with monodentate [22] and biden-
tate phosphines [23,24]; finally, derivatives with nitrogen
chelating ligands were investigated [25].
H
H
H
[cat]
Ar
H
Ar
Ar
Ar
Ar
Ar
Here we report a brief account of the results obtained in our stud-
ies on iridium-catalyzed polymerization and cyclotrimerization of
substituted acetylenes: as will be evidenced in the following sec-
tions, iridium proves to be a suitable metal for such transformations,
the nature of non-participative ligands playing a major role in
favouring one specific reaction.
Scheme 1.
Ar
Ar
Ar
Ar
Ar
cis-transoidal
Ar
2. Iridium-diene catalysts
Ar
cis-cisoidal
As a first approach to alkynes polymerization we tested the cat-
alytic properties of the simple dimeric organometallic compounds
[Ir(cod)Cl]2 and [Ir(cod)(OMe)]2; the analogous rhodium deriva-
tives are effective catalysts for this reaction [15,26]. Polymerization
of phenylacetylene catalyzed by [Ir(cod)Cl]2 in tetrahydrofuran
solution at 60 °C produced polyphenylacetylene together with
small amounts of the cyclotrimerizatrion products 1,3,5 and
1,2,4-triphenylbenzene. The catalytic reaction suffered for a deac-
tivation process which limited the overall conversion to 45%; sim-
ilar results were obtained with the methoxo dimer [Ir(cod)(OMe)]2
in THF (see Table 1, entries 2 and 4) as well as in other solvents
such as chloroform, benzene and methanol at the same tempera-
ture. Substitution of the cod precursor with a monoene iridium di-
mer such as [Ir(coe)2Cl]2 (coe = cyclooctene) resulted in no
polymerization catalysis, with formation of only cyclotrimerization
products (Table 1, entry 1). The polyene produced with the cod-di-
mers had mainly trans configuration, with Mn values of about 3000.
Interestingly, when the catalytic reactions were performed at 25 °C
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
trans-cisoidal
trans-transoidal
Fig. 1. Stereoisomers of polyacetylenes.
Ar
Ar
Ar
Ar
+
Ar
H
Ar
Ar
Scheme 2.

E. Farnetti, S. Filipuzzi / Inorganica Chimica Acta 363 (2010) 467–473
469
Table 1
Polymerization of phenylacetylene catalyzed by organoiridium derivatives.
Entry
Catalyst
Solvent
T (°C)
Conv. (%)
cis-PPAa (%)
trans-PPAa (%)
Oligomersa (%)
1
2
3
4
5
6
7
8
[Ir(coe)2Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)(OMe)]2
[Ir(cod)(OMe)]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
THF
THF
THF
THF
THF
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
60
60
25
60
25
60
25
0
10
45
20
51
36
57
44
46
61
45
48
0
0
71
5
75
2
75
32
26
76
46
25
100
2
–
4
5
–
–
–
–
27
95
21
93
25
68
74
24
64
75
9
10
11
[Ir(cod)(OMe)]2
[Ir(cod)(OMe)]2
[Ir(cod)(OMe)]2
60
25
0
–
–
Experimental conditions: [Ir] = 8.0 ꢁ 10ꢂ3 mol Lꢂ1; [sub]/[Ir] = 50; reaction time: 5 h.
a
Product distribution (%); PPA, polyphenylacetylene. Oligomers: 1,3,5-triphenylbenzene and 1,2,4-triphenylbenzene.
1/2[Ir(cod)(OMe)]2 + PR3 ! Ir(OMe)(cod)(PR3)2 ! HIr(cod)(PR3)2 + CH2O
a significant change in polyene stereoselectivity was observed (see
Table 1, entries 3 and 5), with prevalent formation of the cis prod-
uct and slightly higher molecular weights (ꢀ4000). Also at 25 °C
the catalytic reactions slowed down and finally stopped at conver-
sions not exceeding 40%: the presence of free cod detected by GC
analysis of the final reaction mixtures suggested that diene loss
from iridium might be responsible for catalyst deactivation. This
hypothesis was supported by 1H NMR spectra of [Ir(cod)Cl]2 solu-
tions in CDCl3 which were heated at 60 °C in the presence of added
phenylacetylene, where the signals of free cod were clearly visible.
Support to this deactivation pathway was obtained by performing
a catalytic reaction with [Ir(cod)Cl]2 in the presence of added cod:
after 5 h in THF at r.t., with five equivalents of added diene a con-
version of 53% was obtained, to be compared to 20% in the absence
of excess cod.
Further investigations on the iridium dimeric catalysts were
based on the assumption that the catalytically active species was
formed via splitting of the chloro (or methoxo) bridge by a solvent
molecule: thus, use of a more coordinating solvent such as NEt3
was expected to produce higher concentrations of the polymeriza-
tion initiator. Actually, reactions performed in triethylamine at
60 °C gave conversions up to 61% of polyphenylacetylene with pre-
valent trans geometry, without formation of oligomeric products
(see Table 1, entries 6 and 9). Moreover, when the catalytic reac-
tions were repeated at lower temperatures (25 and 0 °C) the con-
versions observed were still higher than 40% (Table 1, entries 7,8
and 10,11). As previously observed in other solvents, by decreasing
the reaction temperature both the polyene stereochemistry and
molecular weights were markedly affected, the former changing
from mainly trans to mainly cis, the latter showing an increase of
Mn value (ꢀ3500 and ꢀ7000 at 60 and 0 °C, respectively), whereas
the polydispersion index Mw/Mn was maintained within the range
1.4–1.7.
The methoxy mononuclear intermediate undergoes b-elimination
and upon loss of formaldehyde yields the desired hydrido com-
pound. Interestingly, reactions with the very bulky phosphines
PCy3 and P(o-MeOC6H4)3 only gave Ir(OMe)(cod)(PCy3) and Ir(O-
Me)(cod)(P(o-MeOC6H4)3), respectively, without undergoing subse-
quent b-elimination. These methoxy compounds showed poor
catalytic properties towards alkynes oligo and polymerization and
were not further investigated.
In contrast, the hydrido complexes HIr(cod)(PR3)2 proved to be
effective catalyst precursors in phenylacetylene polymerization.
Catalytic reactions performed at 60 °C in various solvents (metha-
nol, tetrahydrofuran, chloroform, toluene) produced polyphenyl-
acetylene as the main product, together with variable amounts of
the dimerization products (E)-1,4-diphenylbut-1-yn-3-ene and
(Z)-1,4-diphenylbut-1-yn-3-ene (see Table 2, entries 1–9); traces
of cyclotrimers were also detected in some cases. The reaction
was highly selective with regard to the polyene stereochemistry,
as only trans-polyphenylacetylene was formed. Molecular weights
(Mn) of the polyene determined by GPC were within the range
3000–5400, with polydispersion index (Mw/Mn) varying from 1.4
to 1.6. In all these reactions the overall conversion never exceeded
70%, even when longer reaction times were employed.
Further experiments were performed at higher temperatures
using toluene or 2-propanol as solvents: typical results are re-
ported in Table 2, entries 10–12. By raising the temperature to
80 and 100 °C an increased (up to 100%) substrate consumption
was obtained, however the higher conversion was mainly due to
alkyne dimerization, which became the major catalytic reaction
at 100 °C.
On the whole, the data reported in Table 2 suggest that starting
from the compounds HIr(cod)(PR3)2 two different catalytically ac-
tive species were formed, one of which promoted polymerization,
the other one dimerization of the alkyne. Formation of the latter
was apparently favoured by higher reaction temperatures, there-
fore in order to favour the polymerization reaction moderate tem-
peratures must be selected. Actually, at 60 °C the dimerization
reaction became the minor process, on the other hand deactivation
of the polymerization catalyst was apparently limiting the polyene
yields; further tests performed at temperatures lower than 60 °C
gave low conversions. At first, the catalyst deactivation was
thought to occur via loss of cod from the coordination sphere of
iridium, however such hypothesis was soon discarded on the basis
of the following results: (i) no significant amount of free cod was
detected in the final reaction mixtures; (ii) addition of excess cod
to the catalytic reactions resulted in a minor – although positive-
effect on the conversion.
The main indications emerging from these data can be summa-
rized as follows: (i) the organoiridium derivatives under investiga-
tion behave as catalysts for the polymerization of acetylenes, (ii)
the coordinated diene has a crucial effect on the catalytic proper-
ties, and (iii) the actual catalyst is a monomeric species. The last
point was confirmed by NMR studies in CDCl3 solution of the reac-
tion between [Ir(cod)Cl]2 and NEt3, resulting in the formation of
monomeric species of the type Ir(cod)(NEt3)Cl.
3. Iridium-phosphine catalysts
Further studies on iridium catalyzed alkynes polymerization
were devoted to phosphine-modified iridium-cyclooctadiene
derivatives. Thus, the series of compounds HIr(cod)(PR3)2
(PR3 = PPh3, P(p-MeOC6H4)3, P(o-MeOC6H4)Ph2, PCyPh2) were syn-
thesized by reacting [Ir(cod)(OMe)]2 with 2 equiv. of the phos-
phine, according to the following reaction:
A clue to the deactivation pathway – as well as to the nature of
the catalytically active species – was provided by a closer analysis

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E. Farnetti, S. Filipuzzi / Inorganica Chimica Acta 363 (2010) 467–473
Table 2
Polymerization of phenylacetylene catalyzed by HIr(cod)(PR3)2.
Entry
PR3
Solv.
T (°C)
Conv. (%)
Dimersa (%)
PPAa (%)
1
2
3
4
5
6
7
8
PCyPh2
PCyPh2
THF
MeOH
THF
MeOH
THF
MeOH
THF
MeOH
toluene
i-PrOH
toluene
toluene
60
60
60
60
60
60
60
60
60
80
80
100
68
66
44
38
39
45
47
39
44
51
79
100
10
10
8
5
15
5
9
0
25
30
45
61
90
90
90
95
75
95
91
100
75
70
55
39
P(p-MeOC6H4)3
P(p-MeOC6H4)3
P(o-MeOC6H4)Ph2
P(o-MeOC6H4)Ph2
PPh3
PPh3
PPh3
PPh3
PPh3
9
10
11
12
PPh3
Experimental conditions: [Ir] = 3.4 ꢁ 10ꢂ3 mol Lꢂ1; [sub]/[Ir] = 100; reaction time: 5 h.
a
Selectivity (%); PPA, trans-polyphenylacetylene; dimers: (E)-1,4-diphenylbut-1-yn-3-ene and (Z)-1,4-diphenylbut-1-yn-3-ene. Other products: 1,3,5 and 1,2,4-
triphenylbenzene.
of the product isolated at the end of the reactions catalyzed by
HIr(cod)(PPh3)2: the red-brown polyene contained a new irid-
ium/phosphine species, which was identified as the bis-alkynyl
trisphosphine compound HIr(C„CPh)2(PPh3)3. Such compound
was itself tested as catalyst precursor, but it proved to be thor-
oughly inactive. Analogous bis-alkynyl species were isolated from
the catalytic reactions performed with the PCyPh2, P(p-MeOC6H4)3
and P(o-MeOC6H4)Ph2 derivatives: all of them were catalytically
inactive.
Spectroscopic studies were performed to shed light on the fate of
iridium hydride catalysts, in experimental conditions similar to
those of the catalytic reactions, although at higher iridium concen-
tration and lower [alkyne]/[Ir] ratio. 1H, 13C and 31P NMR spectra
sided by infrared measurements revealed that the evolution of
HIr (cod)(PPh3)2 followed the path described in Scheme 3. Initial
substitution of a coordinated phosphine by phenylacetylene leads
to an intermediate hydrido-alkyne species, which undergoes inser-
tion to form the vinyl compound Ir(cod)(CH@CHPh)(PPh3). The
phosphine released in this step reacts with a second molecule of
HIr(cod)(PPh3)2 via loss of cod: the subsequent reactions leading
to the bis-alkynyl compound HIr(C„CPh)2(PPh3)3 probably include
coordination of a first molecule of alkyne, CH oxidative addition fol-
lowed by H2 reductive elimination, and finally oxidative addition of
a second molecule of phenylacetylene. The catalytically active spe-
cies is likely to be the vinyl derivative Ir(cod) (CH@CHPh)(PPh3):
unfortunately all attempts to isolate such compound from the reac-
tion mixture were unsuccessful.
phosphines with bidentate chelating ligands, which might offer in-
creased stability to the catalytically active species, preventing reac-
tions of the type shown in Scheme 3. Previous successful examples
of this approach we had experienced with rhodium-diphosphine
catalysts, for which the bidentate phosphines proved to have a
beneficial effect on catalyst stability in comparison to the corre-
sponding derivatives with monodentate phosphines [15].
In this paragraph we report on the catalytic properties of irid-
ium catalysts with bidentate phosphines, whereas in the next par-
agraph the results of studies on iridium catalysts with nitrogen
chelating ligands will be discussed.
Initial catalytic tests performed on phenylacetylene with HIr(-
cod)(dppe) (dppe = Ph2PCH2CH2PPh2) evidenced that clean and
efficient C–C bond formation was taking place, yielding the cyclo-
trimerization products 1,2,4 and 1,3,5-triphenylbenzene: no al-
kyne polymerization or dimerization were observed. The same
behaviour was found when using the analogous derivatives with
dppm, dppp and dppb (dppm = Ph2PCH2PPh2; dppp = Ph2P(CH2)3-
PPh2; dppb = Ph2P(CH2)4PPh2): from a perusal of typical results
reported in Table 3 (entries 1–4) one can observe that only cyclo-
trimers were formed, with a remarkable regioselectivity towards
formation of 1,2,4-triphenylbenzene. The catalytic activity de-
creased from dppm to dppb, i.e. with increasing the length of the
carbon chain connecting the phosphorous atoms; the regioselec-
tivity followed the same trend. In particular, the catalyst HIr
(cod)(dppm) promoted the fast and highly selective formation of
the asymmetric cyclotrimer: such reaction represents an unusual
example of highly regioselective cyclotrimerization, together with
few other reported reactions [27–29].
4. Iridium-bidentate phosphine catalysts
With regard to the complexes HIr(cod)(P–P) (P–P = dppm, dppe,
dppp, dppb), some considerations are due to the coordination
mode of the diene, which presents uncommon features. As previ-
ously evidenced by Oro and coworkers [30], the formula HIr
(cod)(dppm) does not fully describe the compound (which was
The iridium catalysts so far discussed share as a common weak
point a lack of stability, which causes deactivation processes
severely limiting the final conversion. A possible strategy to avoid
such problem was to consider the substitution of monodentate
H
H
Ir
H
Ir
H
Ph
PPh3
Ph
PPh3
H
PPh3
PhCCH
Ir
PPh3
PPh3
H
H
H
PhC
PhC
- cod
PPh3
PPh3
PPh3
PPh3
PhCCH
-H2
Ir
Ir
Ph
PPh3
PPh3
Scheme 3.

E. Farnetti, S. Filipuzzi / Inorganica Chimica Acta 363 (2010) 467–473
471
Table 3
Cyclotrimerization of phenylacetylene catalyzed by iridium/chelating phosphine complexes.
Entry
Catalyst
Conv. (%)
1,2,4-Ph3C6H3 (%)
1,3,5-Ph3C6H3 (%)
PPA (%)
1
2
3
4
5
6
7
HIr(cod)(dppm)
HIr(cod)(dppe)
HIr(cod)(dppp)
HIr(cod)(dppb)
HIr(cod)(o-C6H4(PPh2)2)
HIr(cod)(dcpe)
100a
98
47
31
92
99
97
45
28
88
3
1
1
2
3
4
3
6
–
–
–
–
–
18
8
31
20
HIr(cod)(P-NMe2)
6
Experimental conditions: [Ir] = 3.4ꢁ10ꢂ3 mol Lꢂ1; [sub]/[Ir] = 100; solvent: THF; T = 60 °C; reaction time: 6 h. PPA= trans-polyphenylacetylene.
a
Reaction time 15 min.
CH3
CH3
O
H
Ir
H
Ir
O
Ir
P
P
P
P
P
P-P
-CH2O
Ir
Ir
P
O
CH3
Scheme 4.
structurally characterized), as actually in this derivative the diene
is coordinated in g1 g3 fashion (see Scheme 4). We found that a
similar diene isomerization takes place for the dppe derivative,
selectivity. Formation of 1,2,4-triphenylbenzenes requires that, of
the two possible diarylbenzene intermediates, only the asymmetric
2,5-disubstituted species is obtained (see Scheme 5). Such metalla-
cycle becomes disfavoured by increasing the chain length between
phosphorous atoms, which translates into increased steric hin-
drance of the phosphine. Selective formation of the metallacycle
,
whereas the compound with dppp is present at r.t. as a mixture
of the g2 2 and g1 3 isomers. Variable-temperature NMR exper-
,g ,g
iments evidenced that isomerization of g2 2 isomer into g1 3 is
,g ,g
favoured at higher temperatures and it is an irreversible reaction.
with the alkyne substituents in a-position to the metal has been
Also compound HIr(cod)(dppb), which is present at r.t. as only
reported by other authors and in a study on cobalt catalysts such
intermediates have been isolated [31].
g2 g2 isomer, at higher temperatures is partially converted to
,
the g1
other bidentate phosphines evidenced that a crucial factor in
determining the extent of g2 3 isomerization is the chelate
Ir-diphosphine ring size: with four or five-membered chelate rings
(e.g. with dppm and dppe) the rearrangement of cod to g1 3 coor-
,
g3 product. Further spectroscopic studies performed with
An extension of the studies to iridium derivatives with other
bidentate phosphines evidenced different catalytic properties:
apart from HIr(cod)(o-C6H4(PPh3)2) which only promotes cyclotri-
merization (Table 3, entry 5), the complexes HIr(cod)(dcpe)
(dcpe = Cy2P(CH2)2PCy2) and HIr(cod)(P-NMe2) (P-NMe2 = o-Me2-
NC6H4PPh2) resulted to be both less active and less selective, as
mixtures of polyphenylacetylene and cyclotrimers were formed
(see Table 3, entries 6 and 7).
,
g2 g1
– ,g
,g
dination mode appears to be favoured, whereas with larger chelate
rings (e.g. with dppp and dppb) significative isomerization only oc-
curs at temperatures higher than r.t.
With regard to the dependence of regioselectivity on the nature
of the bidentate phosphine observed in the series HIr(cod)(P–P)
(P–P = dppm, dppe, dppp, dppb), a possible explanation relates to
the regioselective formation of the metallacyclopentadiene inter-
mediate, which is likely to be the step determining the product
The effect of substituents on the alkyne was also investigated,
by performing a series of catalytic reactions with p-MeOC6-
H4C„CH, p-CF3C6H4C„CH and o-CF3C6H4C„CH. Selected results
of reactions promoted by HIr(cod)(dppm) are reported in Table
4: analysis of entries 1–3 evidences similar results of the alkynes
substituted in para position, as in all cases total conversion was at-
tained within few minutes, with a regioselectivity close to 100%.
The catalytic reaction with o-CF3C6H4C„CH (entry 4) required
longer reaction times (5 h) and produced mixtures of oligo and
polymerization products. It was convenient to repeat the reactions
at lower temperature (40 °C) in order to appreciate the differences
as a function of the nature of substrate: the data reported in entries
5–7 of Table 4 evidence that the conversion of the catalytic reac-
tions increased in the order p-CF3C6H4C„CH < PhC„CH < p-
MeOC6H4C„CH, in other words the reaction was favoured by the
presence of electron-releasing substituents on the alkyne phenyl
ring. Finally, the catalytic reaction was examined using as sub-
strates internal alkynes such as phenylpropyne (PhC„CMe): in
all cases no reaction (neither oligomerization nor polymerization)
was observed.
Ph
H
Ph
Ph
Ir
Ph
H
Ph
H
Ph
Ph
Ir
Ph
Ph
Ph
5. Iridium catalysts with nitrogen chelating ligands
Ir
Ph
H
Iridium derivatives with nitrogen chelating ligands such as 1,10-
phenanthroline and substituted derivatives, which are known to
promote transfer hydrogenation of ketones [32], were studied as
Ph
H
Scheme 5.

472
E. Farnetti, S. Filipuzzi / Inorganica Chimica Acta 363 (2010) 467–473
Table 4
Cyclotrimerization of arylacetylenes catalyzed by HIr(cod)(dppm).
Entry
Substrate
T (°C)
t (min)
conv. (%)
1,2,4-Ph3C6H3 (%)
1,3,5-Ph3C6H3 (%)
1
2
3
4
5
6
7
PhC„CH
60
60
60
60
40
40
40
5
5
5
300
90
60
120
90
100
87
85
98
89
100
86
22
95
97
97
1
–
1
38
3
p-MeOC6H4C„CH
p-CF3C6H4C„CH
o-CF3C6H4C„CH
PhC„CH
p-MeOC6H4C„CH
p-CF3C6H4C„CH
100
98
3
1
Experimental conditions: [Ir] = 3.4ꢁ10ꢂ3 mol Lꢂ1; [sub]/[Ir] = 100; solvent: THF.
Table 5
Polymerization of phenylacetylene catalyzed by Ir(diene)(N–N)X in toluene.
Entry
Catalyst
[NaOH]/[Ir]
Conv. (%) (t = 1 h)
Conv. (%) (t = 5 h)
1
2
3
4
5
6
7
Ir(cod)(phen)Cl
Ir(cod)(phen)Cl
Ir(hd)(phen)Cl
Ir(hd)(phen)Cl
Ir(hd)(phen)I
2
–
2
–
–
–
–
22
0
23
0
20
15
28
28
38
25
31
59
32
58
Ir(hd)(Me4phen)Cl
Ir(hd)(Me4phen)I
Experimental conditions: [Ir] = 6.8 ꢁ 10ꢂ3 mol Lꢂ1; [sub]/[Ir] = 50; T = 110 °C.
catalysts precursors for alkynes polymerization. The cod-derivatives
Ir(cod)(N–N)Cl (N–N = 1,10-phenanthroline (phen), 4,7-dimethyl-
1,10-phenanthroline (Me2phen), 3,4,7,8-tetramethyl-1,10-phenan-
throline (Me4phen), 4,7-diphenyl-1,10-phenanthroline (Ph2phen))
were initially tested as catalysts for oligo and polymerization of
phenylacetylene. The first part of the studies on these catalysts
was useful to clarify some features of the reactions, namely: (i) the
compounds showed low to moderate catalytic activity, (ii) in all
cases only polyphenylacetylene was formed, oligomers being totally
absent in the reaction mixtures, and (iii) the presence of a basic
cocatalyst such as NaOH had a beneficial effect on the reaction.
Choice of the solvents was limited by poor solubility of the iridium
derivatives, therefore at temperatures lower than 100 °C only alco-
hols could be employed, whereas toluene was a suitable solvent at
higher temperatures. Entries 1 and 2 of Table 5 report typical results
obtained in toluene at 110 °C: in this case no reaction was observed
in the absence of base, whereas in other reactions performed in
methanol or 2-propanol conversions around 5% were obtained with-
out added base.
the cocatalyst was absent the decrease of conversion after 1 h
was not as pronounced.
Further increase of conversion could be obtained by performing
the catalytic reactions in mesitylene at 160 °C (see Table 6). Also in
these experimental conditions, without basic cocatalyst the hexadi-
ene complexes were superior to cod derivatives (entries 2 and 4)
and the halogen effect was confirmed (Cl < Br < I, entries 4–6). The
effect of substituents on the nitrogen ligand was also clearly ob-
served at longer reaction times (see entries 7–10), indicating that
more electron-releasing phenanthrolines give rise to more active
catalysts [33]. Spectroscopic studies regarding the reaction between
phenylacetylene and either Ir(cod)(phen)Cl or Ir(hd)(phen)Cl indi-
cated that in the first case the diene remained coordinated to irid-
ium, whereas in the latter hexadiene left the coordination sphere,
apparently giving rise to a different catalytic species.
With regard to the stereochemistry of the polyene formed,
in all cases the reactions promoted by Ir(diene)(N–N)X catalysts
yielded trans-polyphenylacetylene; Mn values of the polyene were
in the range 7000–10,000 and polydispersion index between 1.5
and 1.8.
Substitution of cyclooctadiene in the catalyst precursor with a
more labile diolefin such as 1,5-hexadiene (hd) could in principle
lead to more active species, as previously evidenced in hydrogen
transfer reactions: therefore the catalytic activity of Ir(hd)(N–N)X
(X = Cl, Br, I) was tested and compared to that of the corresponding
cod derivatives. The results concerning the phen complexes are re-
ported in Table 5: if on one hand the reactions with basic cocatalyst
seemed to be little influenced by the nature of diene (entries 1 and
3), interestingly with the hexadiene precursor the polymerization
took place also without added base (entry 4). In fact, comparison
between the reactions with and without cocatalyst is slightly in
favour of the latter with regard to final conversion, in spite of the
lower initial polyene yield. The effect of nature of the nitrogen
ligand can be evidenced by comparison of entries 4–6 and 5–7
(Table 5), which shows a higher catalytic activity of the derivatives
with Me4phen with respect to those with phen. Also the halogen
appears to play a significant role in the catalytic reaction, according
to comparison between chloro and iodo derivatives (Table 5, en-
tries 4, 5 and 6, 7) which favours the latter compounds.
6. Final remarks
Organoiridium catalysts promote various C–C bond formation
reactions of substituted acetylenes: the reactions can be governed
Table 6
Polymerization of phenylacetylene catalyzed by Ir(diene)(N–N)X in mesitylene.
Entry
Catalyst
[NaOH]/[Ir]
Time (h)
Conv. (%)
1
2
3
4
5
6
7
8
9
10
Ir(cod)(phen)Cl
Ir(cod)(phen)Cl
Ir(hd)(phen)Cl
Ir(hd)(phen)Cl
Ir(hd)(phen)Br
Ir(hd)(phen)I
Ir(hd)(Ph2phen)Cl
Ir(hd)(phen)Cl
Ir(hd)(Me2phen)Cl
Ir(hd)(Me4phen)Cl
2
–
2
–
–
–
–
–
–
–
5
5
5
5
5
2
10
10
10
10
58
46
46
73
97
98
58
81
86
94
Throughout Table 5 the conversions reported for two different
reaction times (1 h and 5 h) suggest that catalyst deactivation
was important in the presence of basic cocatalyst, whereas when
Experimental conditions: [Ir] = 6.8 ꢁ 10ꢂ3 mol Lꢂ1; [sub]/[Ir] = 50; T = 160 °C.

E. Farnetti, S. Filipuzzi / Inorganica Chimica Acta 363 (2010) 467–473
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