Alkyne reactions with trimethylphosphine complexes of iridium: Lessons for the catalysis of vinyl ester formation and alkyne dimerization

Article abstract of DOI:10.1016/j.poly.2013.12.017

The combination of iridium with trimethylphosphine ligands yields very electron rich iridium compounds that are active for terminal alkyne dimerization chemistry as well as the addition of carboxylic acids to alkynes. The structures, catalytic and stoichi

Full text of DOI:10.1016/j.poly.2013.12.017

Polyhedron 70 (2014) 125–132
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Alkyne reactions with trimethylphosphine complexes of iridium:
Lessons for the catalysis of vinyl ester formation and alkyne dimerization
1
⇑
Joseph S. Merola , Folami T. Ladipo
Department of Chemistry, Virginia Tech, Blacksburg, VA 24061, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The combination of iridium with trimethylphosphine ligands yields very electron rich iridium com-
pounds that are active for terminal alkyne dimerization chemistry as well as the addition of carboxylic
acids to alkynes. The structures, catalytic and stoichiometric chemistry of some iridium tris-trimethly-
phosphine compounds with benzoate ligands are detailed in this paper. The roles that these compounds
may play in the catalytic cycles involved in the dimerization of terminal alkynes and the addition of ben-
zoic acid to terminal alkynes are examined and possible mechanisms for these catalytic reactions are
proposed.
Received 28 August 2013
Accepted 19 December 2013
Available online 7 January 2014
Keywords:
Iridium
Trimethylphosphine
Catalysis
Ó 2013 Elsevier Ltd. All rights reserved.
Alkyne dimerization
Vinyl esters
1. Introduction
systems. One example is the head to head dimerization of terminal
alkynes in which the C–H bond of one alkyne adds across the triple
Metal phosphine complexes have been widely studied and have
played a large role in the development of homogeneous catalysis
[1]. For example, the low pressure hydroformylation reaction with
HRh(CO)(PPh3)3 has been a significant commercial success [2–5].
The production of L-DOPA uses asymmetric phosphine ligands in
combination with rhodium [6–8].
We have found that the combination of iridium with the small-
est alkylphosphine, trimethylphosphine, is a powerful one for the
study of oxidative addition reactions and have demonstrated that
[Ir(COD)(PMe3)3]Cl, 1a, is a versatile reactant capable of the oxida-
tive addition a variety of E–H bonds where E = H [9], B [10], C
[11,12], N [13], and O [14,15] (Eq. (1)) as well as
bond of another [17–23]. Based on our results from the addition of
carboxylic acids to iridium we became interested in the addition of
carboxylic acids across alkynes to yield vinyl carboxylate esters.
This type of addition reaction has been demonstrated by a number
of metal complex systems [24–27]. Even when catalytic activity is
not great, we have found iridium chemistry to be very useful for
shedding light on catalytic systems because of the ability to isolate
reaction intermediates that may not be observable in a more reac-
tive system. This paper discusses the lessons learned in an iridium
trimethylphosphine system that shed some light on the general
catalysis of the reaction between alkynes and carboxylic acids.
2. Experimental
ð1Þ
2.1. General procedures
All reactions were carried out under an atmosphere of purified
nitrogen. Solvents were purchased from Fisher Scientific. Toluene
was distilled from potassium benzophenone; dichloromethane was
distilled from P2O5; ether and pentane were distilled over Na/K alloy.
Water was deionized and distilled. Deuterated solvents were pur-
chased from Cambridge Isotope Laboratories and dried over molec-
ular sieves. Hydroiridic acid was purchased from PGN Chemicals
and was used as received, The 1H and 31P NMR spectra were recorded
on a Bruker WP200 SY spectrometer operating at 200.132 MHz for
protons, 81.015 MHZ for phosphorus, 67.925 MHz for carbon, and
Bruker WP360 SY spectrometer operating at 360.134 MHz for
inducing the ring-opening oxidative addition of thiophenes, thi-
azoles and selenophene [11,16].
Alkynes are versatile building blocks for organic syntheses and
homogeneous transition metal catalysts have been applied to sys-
tems using alkynes in a variety of ways. Addition reactions across
alkynes can be catalyzed by quite a variety of metal complex
⇑
Corresponding author. Tel.: +1 540 231 4510; fax: +1 540 231 3255.
E-mail addresses: jmerola@vt.edu (J.S. Merola), fladipo@uky.edu (F.T. Ladipo).
Present address: Department of Chemistry, University of Kentucky, Lexington, KY
1
40506, USA.
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.12.017

126
J.S. Merola, F.T. Ladipo / Polyhedron 70 (2014) 125–132
protons, 145.785 MHz for phosphorus, and 90.556 MHz for carbon.
Elemental analyses were performed by Atlantic Microlabs Inc., Nor-
cross, Georgia.
GC/MS analysis was performed by the VPI&SU mass spectrome-
try laboratory on a VG Analytical 7070 E-HF high resolution mass
spectrometer using an electron impact ionization (70 eV) mode.
[Ir(COD)(PMe3)3]Cl, 1a, [28], HIr(O2CPh)(PMe3)3Cl, 2, [15],
HIr(OPh)(PMe3)3Cl, 9, [15], and [Ir(COD)(PMe3)3]O2CPh, 1b, [9] were
prepared according to previously published methods.
The chloro dibenzoate complex, (11a) was identified on the ba-
sis of the following data from the nmr of the mixture:
1H NMR (CDCl3): d 1.57 (t, JP–H = 3.9 Hz, 18H, trans PMe3), 1.67
(d, JP–H = 10.6 Hz, cis PMe3), 7.30–7.60 (m, 3H, phenyl ring), 8.01–
8.10 (m, 2H, phenyl ring). The phenyl resonances overlap to some
degree with those of the dichloro benzoate complex (11b).
2.2.3. Reaction between HIr(O2CPh)(PMe3)3Cl (2) and phenylacetylene
at room temperature
A
screw-capped NMR tube was charged with 0.100 g
(0.173 mmol) of HIr(O2CPh)(PMe3)3Cl (2) under N2 in a dry box.
The tube was brought out of the dry box and 19.0
2.2. Syntheses
ll
2.2.1. Synthesis of HIr(O2CPH)2(PMe3)3 (10)
(ꢀ0.173 mmol, 1.0 eq) of phenylacetylene was added by syringe
followed by 0.500 mL of CD2Cl2. The tube was shaken vigorously
and the reaction was monitored at room temperature for 3 weeks
by 1H NMR spectroscopy. After 3 weeks at room temperature,
the reaction mixture was poured into a 10.0 mL one-necked,
side-armed flask and the CD2Cl2 was stripped off under reduced
pressure to give a yellow oil. The oil was dissolved in ꢀ0.200 mL of
CH2Cl2 and diethyl ether was used to recrystallize a thick yellow oil.
The CH2Cl2/ether solution was filtered into a 10.0 mL one-necked,
side-armed flask and stripped to give yellow solids which were
dried under reduced pressure to give the ‘‘orthometallated-vinyl’’
complex 6 identified on the basis of the following data and X-ray
crystallography:
A 100 mL one-necked side-armed flask, equipped with a mag-
netic stirrer and a septum, was charged with 2.00 g (3.08 mmol)
of [Ir(COD)(PMe3)3]O2CPh and 0.414 g benzoic acid (3.39 mmol,
1.1 eq) under N2 in a dry box. The flask was then connected to a
double manifold (vacuum/nitrogen) Schlenk line and 25.0 mL of
mesitylene added by syringe. The reaction mixture was stirred
magnetically and heated at ꢀ60 °C for 24 h. At completion, the sol-
ids were filtered and then re-dissolved in THF (2.00 mL). Diethyl
ether was used to precipitate light brown solids. The solids were
washed with 10.0 mL of ether and dried under reduced pressure
to yield 1.78 g (2.69 mmol) of HIr(O2CPh)2(PMe3)3, 10, (ꢀ87%
based on amount of [Ir(COD)(PMe3)3][O2CPh] and identified on
the basis of the following data:
1H NMR (d6-acetone): d 1.10 (t, JP–H = 3.7 Hz, 18H, trans PMe3),
1.66 (d, JPH = 7.7 Hz, cis PMe3), 6.67–6.72 (m, 1H, phenyl ring),
6.76–6.82 (m, 1H, phenyl ring), 6.85–6.93 (m, 2H, vinyl protons),
7.02–7.08 (m, 1H, phenyl ring), 7.87–7.92 (m, 1H, phenyl ring).
Anal. Calc. for C23H38P3IrO4: C, 41.56; H, 5.77. Found: C, 41.30;
H, 5.64%.
1H NMR (CDCl3): d ꢁ23.66 (dt, JP–H = 23 Hz, 14 Hz, 1H, Ir–H),
1.53 (t, JP–H = 3.8 Hz, 18H, trans PMe3), 1.71 (d, JP–H = 10.3 Hz, cis
PMe3), 7.29–7.40 (m, 6H, phenyl ring), 8.05–8.12 (m, 4H, phenyl
ring).
2.2.4. Reaction between HIr(O2CPh)(PMe3)3Cl (2) and phenylacetylene
at 40 °C
A 10.0 mL one-necked side-armed flask, equipped with a mag-
netic stirrer and a septum, was charged with 0.200 g (0.346 mmol)
of HIr(O2CPh)(PMe3)3(Cl) (2) and under N2 in a dry box. The flask
was then connected to a double manifold (vacuum/nitrogen)
Schlenk line and 3.00 mL of mesitylene was added by syringe fol-
31P NMR (CDCl3): d ꢁ49.15 (t, JP–P = 21 Hz, 1P, cis PMe3), ꢁ28.89
(d, JP–P = 21 Hz, 2P, trans PMe3).
13C NMR (d6-benzene): d 16.78 (t, JC–P = 18.3 Hz, 6C, trans
PMe3), 21.48 (d, JC–P = 39.6 Hz, 3C, cis PMe3), 129.9, 130.2, 130.4,
130.6 (s, 8C, phenyl), 137.2 and 138.3 (s, 2C, phenyl).
lowed by 8.0
ll (ꢀ0.381 mmol, 1.1 eq) of phenylacetylene. The
2.2.2. Reaction between HIr(O2CPH)2(PMe3)3 (10) and Carbon
tetrachloride
reaction mixture was stirred magnetically and heated at 40 °C for
19 h. At completion, a yellow heterogeneous mixture was ob-
served. The mesitylene solution was filtered off the white
precipitate and was stripped under reduced pressure to give
yellow solids. Diethyl ether was used to extract a yellow solution
that was concentrated to give yellow solids. The white precipitate
was recrystallized from CH2Cl2/ether solution. Both solids were
dried under reduced pressure. The white solids were identified as
HIr(O2CPh(PMe3)3ClꢂHO2Ph (11a-benzoic acid) on the basis of
the following data and X-ray crystallography:
A 100 mL one-necked side-armed flask, equipped with a mag-
netic stirrer and a septum, was charged with 1.50 g (2.26 mmol)
of HIr(O2CPh)2(PMe3)3 under N2 in a dry box. The flask was then
connected to a double manifold (vacuum/nitrogen) Schlenk line
and 16.0 mL of carbon tetrachloride added by syringe. The reaction
mixture was stirred magnetically at room temperature for 3 days.
At completion, the carbon tetrachloride was stripped off under re-
duced pressure. The solids obtained were dissolved in CH2Cl2
(2.00 mL) and pentane was used to recrystallize yellow solids.
The CH2Cl2/pentane solution was filtered off and the solids were
dried under reduced pressure to yield 0.727 g of a 1:2 mixture of
Ir(O2CPH)2(PMe3)3Cl (11a) and Ir(O2CPH)(PMe3)3Cl2 (11b). The
mixture was redissolved in CH2Cl2 and diethyl ether used to
recrystallize a pure sample of Ir(O2CPH)(PMe3)3Cl2 (11b). The di-
chloro benzoate complex, (11b) was identified on the basis of the
following data:
1H NMR (d6-acetone): d ꢁ20.54 (dt, JP–H = 22 Hz, 12 Hz, 1H, Ir–
H), 1.53 (t, JP–H = 3.8 Hz, 18H, trans PMe3), 1.72 (d, JP–H = 10.3 Hz,
cis PMe3), 7.29–7.38 (m, 2H, phenyl), 7.47–7.52 (m, 2H, phenyl),
7.58–7.63 (m, 2H, phenyl), 8.01–8.05 (m, 4H, phenyl ring).The yel-
low solids were the ‘‘orthometallated-vinyl’’ complex (6) obtained
above.
2.2.5. Reaction between HIr(O2CPh)(PMe3)3Cl (2), phenylacetylene
and benzoic acid at 120 °C
Anal. Calc. for C16H32Ir1Cl2O2P3: C, 31.38; H, 5.27. Found: C,
31.9; H, 5.66%.
A 100 mL screw-capped, heavy glass-walled pressure tube,
equipped with a magnetic stirrer and a septum, was charged with
0.200 g (ꢀ0.346 mmol) of HIr(O2CPh)(PMe3)3Cl (2) and 0.044 g
(0.363 mmol, 1.05 eq) of benzoic acid under N2 in a dry box. The
tube was then connected to a double manifold (vacuum/nitrogen)
Schlenk line and 3.00 mL of benzene was added by syringe fol-
lowed by 40.0 ll (0.363 mmol, 1.05 eq) of phenylacetylene. The
reaction mixture was heated at 120 °C for 20 h. At completion, a
yellow solution was observed. The mesitylene solution was
1H NMR (CDCl3): d 1.58 (t, JP–H = 3.9 Hz, 18H, trans PMe3), 1.63
(d, JP–H = 10.6 Hz, cis PMe3), 7.26–7.37 (m, 3H, phenyl ring), 8.01–
8.05 (m, 2H, phenyl ring).
31P NMR (CDCl3): d ꢁ52.69 (t, JP–P = 19 Hz, 1P, cis PMe3), ꢁ34.07
(d, JP–P = 19 Hz, 2P, trans PMe3).
13C NMR (CDCl3): d 12.55 (t, JC–P = 21.6 Hz, 6C, trans PMe3),
15.51 (d, JC–P = 42.8 Hz, 3C, cis PMe3), 127.4 (s, 2C, phenyl), 129.7
(s, 2C, phenyl).

J.S. Merola, F.T. Ladipo / Polyhedron 70 (2014) 125–132
127
Table 1
stripped under reduced pressure to give a yellow, oily precipitate.
Crystal data and refinement parameters for 6, 7, and 8.
The precipitate was dissolved in acetone (0.500 mL) and diethyl
ether was used to recrystallize white solids. The acetone/ether
solution was filtered off and stripped to give yellow solids. Both
solids were dried under reduced pressure. The white solids were
identified as ‘‘orthometallated-benzoate’’ complex, 8, on the basis
of the following data and X-ray crystallography:
1H NMR (d6-acetone): d 1.26 (t, JP–H = 3.9 Hz, 18H, trans PMe3),
1.86 (d, JP–H = 10 Hz, cis PMe3), 6.89–6.94 (m, 1H, phenyl), 7.05 (t of
d, JH–H = 7.4 Hz, 1.8 Hz, phenyl), 7.31 (br d, JH–H = 7.6 Hz, phenyl),
7.40 (d of d, JH–H = 7.3 Hz, 1.6 Hz, phenyl).
6
7
8
Formula
Formula weight
T (K)
C
17H33ClIrP3 C23H38ClIrO4P3 C24H31ClIrO2P3
557.99
290
679.10
298
579.97
290
Space group
a (Å)
b (Å)
P21/c
P21/n
Pnma
17.447(7)
11.847(7)
10.653(6)
90
14.725(5)
20.568(8)
15.0233(5)
90
13.772(4)
12.565(3)
18.459(4)
90
c (Å)
a
(°)
b (°)
102.34(3)
90
4445(3)
8
1.668
6.340
7821
111.56(2)
90
2970.7(13)
4
1.518
4.763
90
90
c
(°)
The yellow solids were the ‘‘orthometallated-vinyl’’ complex (6)
described above.
V (Å3)
2202(2)
4
1.737
6.408
2041
Z
qCalc (g/cm3)
l
(Mo K
a
) (mmꢁ1
)
2.3. Identification of organic products
Total data
5217
Unique data
Parameters
R1 (all data)a
wR2 (all data)b
Flack
5731
416
4323
300
0.0437
0.0789
ꢁ0.017(12)
1.034
1752
129
The following procedure details separation and identification of
the catalysis products. A 100 mL screw-capped, glass-walled pres-
sure tube, equipped with a magnetic stirrer and a septum, was
charged with 0.050 g (ꢀ0.086 mmol) of HIr(O2CPh)(PMe3)3Cl (2)
and 0.849 g (6.92 mmol, 80 eq) of benzoic acid under N2 in a dry
box. The tube was brought out onto the schlenk line and 5.00 mL
of benzene was added by syringe followed by 0.760 mL
(0.6.92 mmol, 80 eq) of phenylacetylene. The reaction mixture
was heated at 120 °C for 72 h. The reaction was followed by 1H
NMR spectroscopy (by taking aliquots of the reaction mixture).
After 72 h, the reaction was stopped and the orange-red solution
was poured onto a silica gel column. Hexane (150 mL) was used
to elute a light yellow fraction which was concentrated on a rotary
evaporator to give yellow solids. The yellow solids were dissolved
in boiling methanol (5.00 mL) and the methanol solution was fil-
tered. The filtrate was stripped on the rotary evaporator and the
yellow solids obtained were dried under reduced pressure. The yel-
low solids were a mixture of the E and Z isomers of 1,4-diph-
enylbutenyne, (PhCH@CHAC„CPh) (E = 3a; Z = 3b) identified on
the basis of the following data: 1H NMR (DMSO-d6) (3a): d 7.00
(d, JH–H = 16.3 Hz, 1H, vinyl), 7.47 (d, JH–H = 16.3 Hz, 1H, vinyl),
7.64–7.89 (m, 8H, phenyl), 7.91–7.94 (m, 2H, phenyl).
0.0681
0.0818
N/A
1.041
0.803,
ꢁ1.255
0.0384
0.0645
N/A
1.036
1.73, ꢁ0.75
GOF
Of Maximum, minimum
1.39, ꢁ0.755
(e/Å3
)
a
R1
wR2 = [
=
R
||Fo| ꢁ |Fc||/
R|Fo|.
b
2
R
w(Fo ꢁ Fc2)2/
R .
w(Fo)2]1/2
of isomers of Ph(H)C@C(O2CPh)C(H)@C(H)Ph (5) on the basis of the
following data:
1H NMR (CDCl3):d 6.44 (s, 1H, vinyl), 6.58 (d, JH–H = 15.9 Hz, 1H,
vinyl), 6.88 (d, 1H, JH–H = 15.9 Hz, vinyl), 7.09–7.68 (m, 13H, phe-
nyl), 8.22–8.28 (m, 2H, phenyl).
GC/MS: m/e (C23H18O2)+ = 326
2.4. Crystallography
X-ray diffraction data were collected on a Bruker R3m/V/Sie-
mens P4 diffractometer using SHELXTL-Plus software as supplied by
Siemens. The data was more recently analyzed using SHELX97 [29]
software accessed from within OLEX2 software [30]. Experimental
data for the crystal structures presented in this paper may be ob-
tained according to the information in the supplementary material
section. A summary of the experimental parameters for the struc-
tures of 6, 7, and 8 can be found in Table 1.
GC/MS: m/e (C16H12)+ = 204
1H NMR (DMSO-d6) (3b): d 6.40 (d, JH–H = 12.0 Hz, 1H, vinyl),
7.20 (d, 1H, JH–H = 12.0 Hz, vinyl), 7.64–7.89 (m, 8H, phenyl),
8.27–8.30 (m, 2H, phenyl).
GC/MS: m/e (C16H12)+ = 204
A second fraction was eluted using 50 mL of hexane which upon
work-up, as described above, was found to be a mixture of alkyne
dimer and ester products.
3. Results and discussion
A solution of CH2Cl2 and hexane (1:8, ꢀ100 mL) was used to
elute a third orange fraction which was concentrated to a red–or-
ange oil and worked up from hot methanol, as described above,
and then washed with distilled water to remove residual benzoic
acid. The oil was dried under reduced pressure and identified as
the geminal isomer of the enol ester (H2C@C(Ph)OCOPh, 4) on
the basis of the following data:
Previously, we had reported on the oxidative addition of car-
boxylic acids to [Ir(COD)(PMe3)3]Cl to yield hydridocarboxylatoiri-
dium complexes [15]. Given that such an oxidative addition is a
plausible step in the catalytic cycle for adding carboxylic acids
across alkynes to form vinyl esters, we examined the use of these
hydridocarboxylatoiridium complexes as catalysts for this type of
system. Specifically, we chose to examine the addition of benzoic
acid to phenylacetylene.
1HNMR(benzene-d6):d5.04(d, JH–H = 2.3 Hz, 1H,vinyl),5.32(d, 1H,
JH–H = 2.3 Hz, vinyl), 6.98–7.07 (m, 6H, phenyl), 7.41–7.45 (m, 2H, phe-
nyl), 8.14–8.17 (m, 2H, phenyl). GC/MS: m/e (C15H12O2)+ = 224.
Further, a solution of CH2Cl2 and hexane (ꢀ1:2, ꢀ100 mL) was
used to elute a fourth (yellowish) fraction which was concentrated
to a oil on a rotary evaporator. Upon addition of hot methanol, a red
oily precipitate was formed. The yellow-orange solution was filtered
from the precipitate and concentrated to an orange oil. The orange oil
was washed with 20.0 mL of water and then dissolved in diethyl
ether (5.00 mL). The ether solution was transfered into a separatory
funnel and 25.0 mL of water was introduced to extract residual ben-
zoic acid. The ether layer was seperated and concentrated to an oil.
The oil was dried under reduced pressure and identified as a mixture
3.1. The benzoic acid and phenylacetylene system
The catalytic addition of benzoic acid to the 1-alkyne, phenyl-
acetylene, was studied using HIr(O2CPh)(PMe3)3Cl (2) as catalyst
precursor. The reaction of benzoic acid (80 eq) with phenylacetylene
(80 eq) in the presence of a catalytic amount of HIr(O2CPh)(PMe3)3Cl
(2) (1 eq) at 120 °C in benzene-d6 (sealed tube) was studied by 1H
NMR spectroscopy over a 72 h period (Eq. (2)). Initially, the
formation of a small amount of styrene (<1 eq) was observed
along with the formation of but-1-en-3-yne-1,4-diyldibenzene,

128
J.S. Merola, F.T. Ladipo / Polyhedron 70 (2014) 125–132
PhCH@CHAC„CPh (E = 3a; Z = 3b), enol ester, H2CAC(Ph)OCOPh
(4) and the ester formed from 3a, 1,4-diphenylbuta-1,3-dien-2-yl
benzoate, Ph(H)CAC(O2CPh)C(H)AC(H)Ph (5). The amount of sty-
rene formed remained the same (much less than one equivalent)
over the course of the reaction. After 72 h, ꢀ56 equivalents of
phenylacetylene had been consumed. The relative yields of
but-1-en-3-yne-1,4-diyldibenzene (3a and 3b), enol ester 4 and
dimer enol ester1,4-diphenylbuta-1,3-dien-2-yl benzoate, 5, were
approximately 52%, 16% and 32%, respectively.
ð2Þ
The small amount of styrene observed is significant in that it is
the product of hydrogenation of phenylacetylene that may be
important in the generation of catalytically active species.
Fig. 1. X-ray crystal structure of 6. Only one of the two independent molecules
shown and hydrogen atoms omitted for clarity. Selected bond distances (Å): Ir1–P1,
2.322(2); Ir1–P2, 2.334(2); Ir1–P3, 2,335(2), Ir1–Cl1, 2.502(2); Ir1–C1, 2.024(8);
Ir1–C4, 2.102(7). Selected bond angles(°): P1–Ir1–P2, 94.73(8); P1–Ir1–P3,
168.12(8); P1–Ir1–Cl1, 87.07(7); P2–Ir1–P3, 95.80(8); P2–Ir1–Cl1, 95.20(9); P3–
Ir1–Cl1, 86.51(8); C4–Ir1–P1, 86.24(18); C4–Ir1–Cl1, 94.3(2); C1–Ir1–P1, 92.0(2);
C1–Ir1–P2, 90.8(3); C1–Ir1–P3, 93.4(2); C1–Ir1–Cl1, 174.0(3); C1–Ir1–C4, 79.8(3).
3.2. The possible roles of some iridium complexes in the catalytic
system
In order to probe the alkyne dimer formation in greater depth,
we explored the reaction between HIr(O2CPh)(PMe3)3Cl (2) and
phenylacetylene without the presence of benzoic acid. It was rea-
soned that any iridium species isolated is potentially an intermedi-
ate in the catalytic reaction to produce alkyne dimer and might
shed some light on how the alkyne dimer and ester products were
being formed. The reaction of 2 with phenylacetylene (1 eq) in
dichloromethane-d2 at room temperature was studied (Eq. (3)).
complex, HIr(O2CPh)(PMe3)3Cl (2), undergoes reductive elimina-
tion of benzoic acid in the presence (Fig. 2) of phenylacetylene
and the benzoic acid thus eliminated forms a hydrogen-bond to
2 yielding complex 7. The mechanism for the formation of com-
pound 6 in both of these reactions is more difficult to explain.
Since the catalytic reaction was performed at 120 °C, the reac-
tion of 2 with phenylacetylene (1 eq) and benzoic acid (1 eq) at
120 °C was studied (Eq. (5)). After work-up, 6 was once again
ð3Þ
After work-up, the iridium complex isolated was determined to
be the ‘‘orthometallated-vinyl’’ complex, 6, using 1H NMR spectros-
copy and X-ray crystallography. The NMR spectroscopy details
may be found in the experimental section and the crystallography
details in the supplemental information. The thermal ellipsoid plot
for 6 is shown in Fig. 1.
ð5Þ
obtained as the predominant product. The other product was
the ‘‘orthometallated-benzoate’’ complex, 8, which was character-
ized by 1H NMR spectroscopy and X-ray crystallography. The
resulting thermal ellipsoid plot of 8 is shown in Fig. 3.
Next, the reaction between HIr(O2CPh)(PMe3)3Cl (2) and phen-
ylacetylene (1 eq) at 40 °C was examined (Eq. (4)). After work-up,
two iridium-containing products were isolated. The predominant
product was again the ‘‘orthometallated-vinyl’’ complex, 6. The
other product was the hydrogen-bonded hydrido benzoato com-
plex, HIr(O2CPh)(PMe3)3ClꢂHO2Ph, 7. Compound 7
The results presented above show that, at the catalytic reaction
temperature (120 °C), the isolable species are not necessarily in
the catalytic cycle. Furthermore, 6 was always the predominant
product isolated from the reactions. Whether 6 could act as a cata-
lyst precursor or was a ‘‘deactivation product’’ for the addition of
benzoic acid to phenylacetylene had to be considered. Therefore,
the reaction of benzoic acid (33 eq) with phenylacetylene (33 eq)
in the presence of a catalytic amount of 6 (1 eq) at 120 °C in ben-
zene-d6 was studied by proton NMR spectroscopy over a 72 h period
(Table 2). After 72 h, approximately 18 equivalents of phenylacety-
lene had been consumed. The relative yields of E and Z-but-1-en-3-
yne-1,4-diyldibenzene (3a and 3b), enol ester (4) and (5) were
approximately 61%, 13% and 26%, respectively: similar yields as
when 2 was used as catalyst precursor. So, while 6 may not be an
intermediate in the catalytic pathway but merely a side reaction,
ð4Þ
was only isolated as a few crystals and so was only character-
ized via 1H NMR spectroscopy and X-ray crystallography. A reason-
able explanation for the formation of 7 is that the hydrido benzoato

J.S. Merola, F.T. Ladipo / Polyhedron 70 (2014) 125–132
129
3.3. Catalytic dimerization of phenylacetylene
To study the formation of alkyne dimer exclusively without the
presence of any benzoic acid or benzoate ligand, we examined an-
other iridium complex formed by the oxidative addition of phenol
to 1. Herein we report that hydrido phenoxo iridium(III) com-
plex,HIr(OPH)(PMe3)3Cl (9) [15], also catalyzes the dimerization of
phenylacetylene. Heating an excess (100 eq) of phenylacetylene in
the presence of 9 in benzene-d6 at 120 °C for 70 h led to exclusive
catalytic formation of PhCH@CHAC„CPh (E = 3a; Z = 3b) with an
E/Z ratio of 90/10. Presumably, reductive elimination of phenol from
HIr(OPH)(PMe3)3Cl (9) generates the transient 16 electron Ir(I) spe-
cies, ‘‘Ir(PMe3)3Cl’’. This is the same species that would be generated
by reductive elimination of benzoic acid from HIr(O2CPh)(PMe3)3Cl,
2, or from the loss of COD ligand from 1, a species which is then
trapped by phenylacetylene to yield the ‘‘orthometallated-vinyl’’
complex, 6. Therefore, it is reasonable to postulate that any iridium
complex that readily generates the 16 electron Ir(I) intermediate,
‘‘Ir(PMe3)3Cl’’ in solution could catalyze the dimerization of phenyl-
acetylene. Consequently, the catalytic dimerization of phenylacety-
lene at 120 °C using as catalyst precursor either [Ir(COD)(PMe3)3]Cl
(1a), HIr(O2CPh)(PMe3)3Cl (2) or Hir(OPh)(PMe3)3 (9) was investi-
gated. In each case, the catalytic formation of alkyne dimer was ob-
served. (Table 2). These results differ from those of Crabtree et al.
who reported that the Z isomer was the predominant product. They
accounted for this by invoking isomerization on the metal center
following formation of the metal alkenyl bond. Interestingly, that
isomerization stopped in the presence of PMe3 thus giving predom-
inantly the E product as we observe here.
Fig. 2. X-ray Crystal Structure of 7. Hydrogen atoms (except for O1A–H to show H-
bonding) have been eliminated for clarity. Selected bond distances (Å): Ir1–P1,
2.3266(16); Ir1–P3, 2.3369(17); Ir1–P2, 2.2314(14); Ir1–Cl1, 2.4955(14); Ir1–O1,
2.133(3). Selected bond angles (°): P1–Ir1–P3, 169.55(5); P1–Ir1–Cl1, 87.06(5); P3–
Ir1–Cl1, 87.41(5); P2–Ir1–P1, 94.84(6); P2–Ir1–P3, 94.88(6); P2–Ir1–Cl1, 100.60(6);
O1–Ir1–P1, 85.54(11); O1–Ir1–P3, 85.05(11); O1–Ir1–P2, 175.65(10); O1–Ir1–Cl1,
83.74(10).
These results indicate that the 16 electron iridium(I) fragment,
‘‘Ir(PMe3)3Cl’’, is the most likely candidate to be the primary cata-
lyst for the formation of the but-1-en-3-yne-1,4-diyldibenzene
product, PhCH@CHAC„CPh (E = 3a; Z = 3b). The fact that the al-
kyne dimer product was the predominant of the three catalytic
products from the addition reaction of benzoic acid with phenyl-
acetylene catalyzed by 2 or 6 can now be understood. Both com-
pounds can generate ‘‘Ir(PMe3)3Cl’’ which catalyzes alkyne dimer
formation.
While a detailed study of the mechanism of the catalytic cycle
for the formation of alkyne dimer, PhCH@CHAC„CPh (E + Z), has
not been conducted, a survey of the experimental results presented
show that iridium complexes capable of generating the 16 electron
iridium(I) intermediate, ‘‘Ir(PMe3)3Cl’’, catalyze the formation of al-
kyne dimer. Consequently, one may infer that the transient 16
electron iridium(I) intermediate, ‘‘Ir(PMe3)3Cl’’, is the catalyst pre-
cusor for the formation of alkyne dimer. Several reports of catalytic
dimerization of 1-alkynes using as catalyst precursor either
[Rh(PMe3)2Cl]2 [31] or Rh(PMe3)3Cl [32] appeared in the literature.
The latter complex, Rh(PMe3)3Cl, is the rhodium analog of the irid-
ium(I) transient intermediate, Ir(PMe3)3Cl, generated from the irid-
ium complexes used as catalyst in this work (Table 2). It is likely
that Ir(PMe3)3Cl catalyzes the dimerization of phenylacetylene
through a similar mechanism (Scheme 1). First, the terminal C–H
bond of the alkyne undergoes oxidative addition to the iridium(I)
center. Then, the reaction proceeds by alkyne insertion into the
Fig. 3. X-ray Crystal Structure of 8. Hydrogen atoms have been omitted for clarity.
Selected bond distances (Å): Ir1–P1, 2.346(2); Ir1–P11, 2.346(2); Ir1–P2, 2.258(2);
Ir1–Cl1, 2.477(2); Ir1–O1, 2.116(5); Ir1–C3, 2.033(7). Selected bond angles (°): P11–
Ir1–P1, 168.04(7); P1–Ir1–Cl1, 86.30(4); P11–Ir1–Cl1, 86.30(4); P2–Ir1–P11,
95.03(4); P2–Ir1–P1, 95.03(4); P2–Ir1–Cl1,95.58(7); O1–Ir1–P1, 85.16(4); O1–Ir1–
P11, 85.16(4); O1–Ir1–P2, 176.75(13); O1–Ir1–Cl1, 87.67(14); C3–Ir1–P1, 92.57(4);
C3–Ir1–P11, 92.57(4); C3–Ir1–P2, 97.15(19); C3–Ir1–Cl1, 167.27(19); C3–Ir1–O1,
79.6(2).
nevertheless it can reenter the catalytic cycle and so is not a deacti-
vation product.
Table 2
‘‘Ir(PMe)3Cl’’-catalyzed dimerization of phenylacetylene.
Catalyst
No. of equiv of alkyne
Time (hours)
% alkyne converteda
Relative (%) yield of E isomer a
Relative (%) yield of Z isomer a
(1a)
(2)
(9)
25
40
100
70
70
70
97
100
100
80
80
90
20
20
10
a
Values were determined from integration of the appropriate resonances in the 1H NMR spectrum and based on the amount of alkyne converted and then confirmed by
GC–MS. Reactions were carried out in benzene-d6 in a sealed tube at 120°.

130
J.S. Merola, F.T. Ladipo / Polyhedron 70 (2014) 125–132
Scheme 1. Mechanism postulated for catalytic dimerization of 1-alkynes.
iridium-hydride bond of the intermediate (alkynyl)(hydride)Ir(III)
complexes to give a (vinyl)(alkynyl)Ir(III) complex. Reductive elim-
ination of the alkyne and vinyl group yield product and regenerates
the active iridium(I) species.
complex, 11a and the dichloro benzoato complex, Ir(O2-
CPh)(PMe3)3Cl2 (11b) was obtained in ꢀ2:1 ratio after work-up
(Eq. (7)). A pure sample of the chloro dibenzoate iridium(III) com-
plex, 11a, could not
3.4. Further investigation of the role of iridium(III) benzoate complexes
We then decided to pursue the syntheses of other variants of
benzoate iridium(III) compounds that could be more active or, at
least, selective for the addition of benzoic acid to phenylacetylene.
Therefore, the synthesis of the chloro dibenzoate iridium(III) com-
plex, Ir(O2CPh)2(PMe3)3Cl was attempted.
ð7Þ
Heating 1b (a variant of 1a with chloride replaced by a benzoate
counterion) [Ir(COD)(PMe3)3][O2CPH], with benzoic acid (1.1 eq) in
mesitylene at 60 °C for 24 h resulted in the formation of the hydr-
ido dibenzoato iridium(III) complex, HIr(O2CPh)2(PMe3)3 (10), in
ꢀ87% yield (Eq. (7)). Complex 10 was characterized by 1H, 31P,
and 13C NMR spectroscopy as well as
be separated. However, a pure sample of the dichloro benzoato
complex, Ir(O2CPh)(PMe3)3Cl2 (11b), was obtained by recrystalliza-
tion from CH2Cl2 and diethyl ether and was characterized by 1H,
31P, and 13C NMR spectroscopy as well as IR spectroscopy. In the
IR spectrum, the positions of the asymmetric and symmetric
stretching vibrations of the carboxylate group (m ;
as = 1620 cmꢁ1
m
s = 1343 cmꢁ1
)
and the value of
D
m
(
mas COOꢁ ꢁ
m
sCOOꢁ)
ð6Þ
(Dm = 277) are consistent with a monodentate coordination of the
carboxylate group [34,35].
The chloro dibenzoato iridium(III) complex has been tentatively
assigned a structure in which both benzoate ligands are coordi-
nated monodentate, Ir(O2CPh)2(PMe3)3Cl (11a), because the pat-
tern of the resonances observed for the phenyl protons of the
benzoate ligand in the 1H NMR spectrum look more like the pat-
tern observed previously for benzoate ligands coordinated
monodentate.
IR spectroscopy. In the IR spectrum, the positions of the asym-
metric and symmetric stretching vibrations of the carboxylate
group (
m
m
as = 1612 cmꢁ1
; m Dm(mas
s = 1375 cmꢁ1) and the value of
COOꢁ ꢁ
sCOOꢁ) (
Dm = 237) are consistent with a monodentate
coordination of the carboxylate group and are in line with those re-
ported for the related dibenzoato complex [(C5Me5)Rh(O2CPh)2(H2-
O)] (m ; m
as = 1550 cmꢁ1 s = 1390 cmꢁ1) where both benzoato ligands
are known from the X-ray crystal structure to be coordinated
3.5. Proposed mechanism of the catalytic cycle for enol ester synthesis
monodentate [33].
Upon stirring a carbon tetrachloride solution of 10 at room tem-
perature for 3 days, a mixture of the chloro dibenzoato iridium(III)
Table 3 delineates a series of catalytic runs with various iridium
catalyst precursors and conditions of the addition reaction of

J.S. Merola, F.T. Ladipo / Polyhedron 70 (2014) 125–132
131
Table 3
Catalytic enol ester synthesis.a
#
Cat
Eq of PhCCH (acid to alkyne ratio 1:1)
Time (day)
% conv. alkynea
% yield Enol estera
% yield of 3a/3ba
3a/3b E:Za
% yield of 5a
1
2
3
4
2b
80
20
40
10
5
10
20
20
3
11
3
8
12
5
70
100
54
100
100
53
16
8
13
4
61
25
19
18
52
71
61
69
2:1
2:1
2:1
2:1
32
21
26
27
P99
41
49
2c
6b
11bc
5
11a/11bc
11a/11bc
11a/11bc
11a/11bc,d
6
34
32
34
2:1
2:1
1:2
7e
8f
12
10
84
91
28
a
Values were determined from integration of the appropriate resonances in the 1H NMR spectrum and based on the amount of alkyne converted and then confirmed by
GC–MS.
b
Reactions were carried out in benzene at 120 °C.
c
d
e
f
Reactions were carried out in toluene at 120 °C.
ꢀ20% of an unidentified product is also obtained.
Ratio of equivalents of benzoic acid to alkyne = 2.
Ratio of equivalents of benzoic acid to alkyne = 8.
benzoic acid to phenylacetylene. Entries 1 and 2 give the results
obtained with the hydrido benzoate iridium complex 2 as catalyst
precursor. The reaction for entry 2 was carried out longer and with
fewer equivalents of alkyne and acid so that 100% conversion could
be reached. There was some difference between the results but
nothing clearly significant. Entry 3 delineates the results obtained
with the hydrido phenoxo iridium complex 6 as catalyst precurson.
Very little difference was noted between 2 and 6 as catalyst pre-
cursors. Of the information in Table 3, entry 5 is the most interest-
ing. Using a mixture of 11a and 11b, the chloro dibenzoato iridium
and the dichloro benzoato iridium complexes as catalyst precur-
sors, after 12 days, the reaction of benzoic acid with phenylacety-
lene gave exclusively the ester formed from the alkyne dimer,
Ph(H)C@C(O2CPh)C(H)@C(H)Ph (5). Apparently, the alkyne dimer
is formed initially and then converted into 5. So, if the reaction is
allowed to proceed for a long enough period of time, the alkyne
is first dimerized, an observation in keeping with the time required
for 100% conversion of alkynes shown in Table 2.
alkyne dimer can compete with phenylacetylene for the site
opened at the metal center when the benzoate ligand dissociates.
However, when an eightfold excess of benzoic acid was used, a sig-
nificant amount (ꢀ20%) of a product which has not been identified
was also obtained.
Based on the reactivity observed for the benzoato complexes, 2
and 8, as well as the results presented in the previous section, the
following mechanism was proposed for the catalytic synthesis of
enol ester and is depicted in Scheme 2. (i) The 18 electron irid-
ium(III) complex, Ir(O2CPh)2(PMe3)3Cl (11a) undergoes dissocia-
tion of a benzoate ligand to open up a coordination site, (ii) a
molecule of phenylacetylene
p-bonds to the five-coordinate, 16
electron iridium(III) intermediate thus generated give the 18 elec-
tron complex, (iii) external attack of benzoate on the coordinated
acetylene results in an 18 electron ‘‘vinyl-benzoato’’ iridium(III)
complex, and (iv) direct protonation of the vinyl carbon bound to
iridium by a molecule of benzoic acid leads to elimination of the
enol ester while subsequent coordination of benzoate regenerates
Ir(O2CPh)2(PMe3)3Cl (11a) .The first two steps, dissociation of a
Then, when substantial amounts of 3a or 3b are formed, benzoic
acid addition to the alkyne portion of the ene-yne takes place.-
When an excess of benzoic acid (40 eq) compared to phenylacety-
lene (20 eq) was used, the combined yield of the ester products
was increased only slightly (Table 3, see #7 & #8). This may explain
why the predominant ester product is 5 since the concentration of
alkyne dimer is rapidly increased in solution and consequently, the
benzoate ligand followed by
are consistent with results obtained from the reactions of HIr
(O2CPh)(PMe3)3Cl (2) or HIr( 2-OCPh)(PMe3)3 with acetylenes.
p-coordination of phenylacetylene,
g
Milstein and co-workers have also reported that the carboxylate
moiety was displaced from the iridium metal center by the
acetylenic moiety in the iridium-a,x-alkynoic acid system,
Scheme 2. Proposed mechanism for enol ester synthesis.

132
J.S. Merola, F.T. Ladipo / Polyhedron 70 (2014) 125–132
Ir(
g
1-CO2(CH2)nCCR)H(PEt3)3Cl [36]. While it is possible that the
can be obtained free of charge via http://www.ccdc.cam.ac.uk/con-
ts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336 033; or e-mail: deposit@ccdc.cam.ac.uk.
p-bonded acetylene inserts into the Ir–O bond, followed by
recoordination of the counter-ion benzoato ligand, we favor exter-
nal attack by the benzoate anion on the coordinated acetylene
because insertions into metal–oxygen bonds are rare [37]. The
direct protonation of the vinyl ligand of the 18 electron iridium(III)
intermediate to yield the enol ester rather than protonation of
iridium followed by C–H reductive elimination is proposed because
the later pathway would require the intermediacy of an 18
electron iridium(V) complex. Although the oxidation state of five
is known for iridium, very few examples of such complexes
are known and generally involve hard ligands e.g. CsIrF6 [38].
Furthermore, direct protonations of metal-vinyl (M–C) bonds are
prevalent in the literature [39–41].
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Financial support for this work was provided by ACS-PRF (Grant
#23961-C1) and by the National Science Foundation (CHE-
902244).
Appendix A. Supplementary data
CCDC 897179-897181 contain the supplementary crystallo-
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graphic data for compounds 7, 8 and 9, respectively. These data