Synthesis and solid-state structures of (triphos)iridacyclopentadiene complexes as models for vinylidene intermediates in the [2 + 2 + 1] cyclotrimerization of alkynes

Article abstract of DOI:10.1016/j.ica.2010.07.052

The iridium 1,1,1-tris(diphenylphosphinomethyl)ethane (triphos) complexes [{κ²(C¹,C⁴)-CRCRCRCR}{CH ₃C(CH₂PPh₂)₃}Ir(NCMe)]BF ₄ (2-NCMe, R = CO₂Me) and [{κ

Full text of DOI:10.1016/j.ica.2010.07.052

Inorganica Chimica Acta 364 (2010) 220–225
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Synthesis and solid-state structures of (triphos)iridacyclopentadiene complexes
as models for vinylidene intermediates in the [2 + 2 + 1] cyclotrimerization of
alkynes
*
Joseph M. O’Connor , Adam P. Closson, Ryan L. Holland, Stephen K. Cope, Carmen L. Vélez, Curtis E. Moore,
**
Arnold L. Rheingold
Department of Chemistry and Biochemistry (0358), University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0358, USA
a r t i c l e i n f o
a b s t r a c t
The iridium 1,1,1-tris(diphenylphosphinomethyl)ethane (triphos) complexes [{j
2(C1,C4)-CR@CRCR@CR}-
{CH3C(CH2PPh2)3}Ir(NCMe)]BF4 (2-NCMe, R = CO2Me) and [{
Article history:
Available online 24 July 2010
j
2(C1,C4)-CR@CRCR@CR}{CH3C(CH2PPh2)3}
Ir(CO)]BF4 (2-CO, R = CO2Me) serve as models for proposed iridium–vinylidene intermediates of relevance
to the [2 + 2 + 1] cyclotrimerization of alkynes. The solid-state structures of 2-NCMe, 2-CO, and [
CR@CRCR@CR]{CH3C(CH2PPh2)3}Ir(Cl) (2-Cl), were determined by X-ray crystallography.
His coauthors dedicate this manuscript to
Prof. Arnold L. Rheingold: friend, colleague
and scholar
j
2(C1,C4)-
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Iridium
Metallacyclopentadiene
Metallacycle
Iridacyclopentadiene
Triphos
Vinylidene
1. Introduction
In order to better understand the structural parameters in
intermediates of type II, we have now structurally characterized
We previously reported that the reactions of iridacyclopentadi-
ene complexes with terminal alkynes depend in a dramatic way on
the nature of the ancillary ligands (Scheme 1). For example, in the
both 2-NCMe and the carbon monoxide analogue, [{j
2(C1,C4)-
CR@CRCR@CR}{CH3C(CH2PPh2)3}Ir(CO)]BF4 (2-CO, R = CO2Me),
both of which may be viewed as structural models for the presumed
vinylidene intermediate II. In addition, we report the solid-state
bis(triphenylphosphine)iridacyclopentadiene complex [{
CR@CRCR@CR}(PPh3)2Ir(NCMe)(CO)]BF4 (1), the acetonitrile and
2-butadiendiyl ligands occupy meridional coordination sites
[1,2]; whereas, in triphos complex {
2(C1,C4)-CR@CRCR@CR}(tri-
phos)Ir(NCMe)]BF4 (2-NCMe) the acetonitrile and
2-butadiendiyl
j
2(C1,C4)-
structure of [{j
2(C1,C4)-CR@CRCR@CR}{CH3C(CH2PPh2)3}Ir(Cl)]BF4
j
(2-Cl, R = CO2Me).
j
j
ligands occupy facial coordination sites (Scheme 1) [3]. Metallacy-
cle 1 undergoes reaction with phenylacetylene to give the bicyclic
metallalactone complex 3 [1]; whereas, 2-NCMe undergoes reac-
tion with phenylacetylene to give fulvene complex 4 [3,4]. In both
cases, the reaction mechanism appears to involve the formation of
a vinylidene intermediate (I and II). In the case of I, the vinylidene
ligand is trapped by the metallacycle ester substituent. For II, the
vinylidene ligand undergoes a reductive coupling with the but-
adiendiyl ligand.
2. Results and discussion
The reaction of
[j
2(C1,C4)-CR@CRCR@CR](PPh3)2Ir(Cl) (5,
R = CO2Me) [5] with 1,1,1-tris(diphenylphosphinomethyl)ethane
(triphos) has previously been demonstrated to give the neutral
iridacycle–chlorido complex
[j
2(C1,C4)-CR@CRCR@CR](triphos)
Ir(Cl) (2-Cl, R = CO2Me) [3] and upon subsequent reaction with
AgBF4 in acetonitrile, the cationic acetonitrile complex 2-NCMe
(Scheme 2) [6]. Alternatively, treatment of 5 with bis(2-diphenyl-
phosphinoethyl)phenylphosphine generated the facial tris(phos-
phine) complex
Ir(Cl) (6-Cl, R = CO2Me) [7]. Subsequent halide abstraction and
exposure to CO gave the cationic meridional complex [{
2(C1,C4)-
CR@CRCR@CR}{PhP(CH2CH2PPh2)2}Ir(CO)]BF4 (7-CO, R = CO2Me).
[j
2(C1,C4)-CR@CRCR@CR]{PhP(CH2CH2PPh2)2}
* Corresponding author.
** Corresponding author.
E-mail address: jmoconno@ucsd.edu (J.M. O’Connor).
j
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.07.052

J.M. O’Connor et al. / Inorganica Chimica Acta 364 (2010) 220–225
221
Scheme 1. The effect of ancillary ligands on the reactions of iridacyclopentadiene complexes.
When CO is bubbled through a dichloromethane solution of 2-
ligand in intermediate II should be stabilized by
p-backbonding to
NCMe, followed by stirring under an atmosphere of CO for 24 h,
the cationic carbonyl complex 2-CO is formed in 86% isolated yield.
In the IRÀ1spectrum (KBr) of 2-CO the CO stretching vibration
(2094 cm ) is at significantly higher wavenumber than for 7
(2069 cmÀ1, KBr), which indicates that the trans phosphine ligand
a greater extent than the vinylidene ligand in I (Scheme 1).
In the 1H NMR spectrum (CDCl3) of 2-CO, the hydrogen reso-
nances for the methoxycarbonyl groups are observed as singlets
at d 3.86 (6H) and 2.64 (6H). These values are very similar to those
observed for the triphos complexes 2-NCMe (d 3.76 and 2.67) and
2-Cl (d 3.70 and 2.52). In contrast, the bis(phosphine) complexes,
1 (d 3.50 and 3.49), 5 (d 3.46 and 3.26), and 8 (d 3.52 and 3.09) ex-
hibit no methyl hydrogen resonances at higher field than d 3.0. The
is a better donor ligand than the trans
j
2-butadiendiyl ligand. In
[{j
2(C1,C4)-CR@CRCR@CR}(PPh3)2Ir(CO)2]BF4 (8, R = CO2Me), the
CO stretching vibrations are also observed at higher wavenumber
(2080, 2115 cmÀ1; CH2Cl2) than for 2-CO [2c]. Thus, the vinylidene
tris(phosphine) complex 6 also exhibits relatively downfield
Scheme 2. Synthesis of iridacyclopentadiene complexes with tridentate phosphine ligands.

222
J.M. O’Connor et al. / Inorganica Chimica Acta 364 (2010) 220–225
À
Fig. 1. Structural drawing of 2-NCMe (most hydrogen atoms and BF4 counter ion omitted for clarity).
Table 1
Table 2
Selected bond distances (Å) for 2-NCMe, 2-CO, and 2-Cl.
Selected bond angles (°) for 2-NCMe and 2-CO, and 2-Cl.
A–B
2-NCMe
2-CO
2-Cl
A–B–C
2-NCMe
2-CO
2-Cl
Ir–C(1)
Ir–C(4)
Ir–N(1)
Ir–C(18)
Ir–Cl
Ir–P(1)
Ir–P(2)
Ir–P(3)
N(1)–C(18)
C(18)–C(19)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(1)–C(8)
C(2)–C(7)
C(3)–C(6)
C(4)–C(5)
C(8)–O(1)
C(8)–O(2)
C(7)–O(3)
C(7)–O(4)
C(6)–O(5)
C(6)–O(6)
C(5)–O(7)
C(5)–O(8)
C(18)–O(9)
2.117(11)
2.092(12)
2.053(12)
2.076(6)
2.109(7)
2.117(4)
2.116(5)
C(1)–Ir–C(4)
C(1)–Ir–N(1)
C(1)–Ir–C(18)
C(1)–Ir–Cl
C(1)–Ir–P(1)
C(1)–Ir–P(2)
C(1)–Ir–P(3)
C(4)–Ir–N(1)
C(4)–Ir–C(18)
C(4)–Ir–Cl
C(4)–Ir–P(1)
C(4)–Ir–P(2)
C(4)–Ir–P(3)
N(1)–Ir–P(1)
N(1)–Ir–P(2)
N(1)–Ir–P(3)
C(18)–Ir–P(1)
C(18)–Ir–P(2)
C(18)–Ir–P(3)
Cl–Ir–P(1)
78.5(4)
84.8(4)
77.0(2)
77.8(2)
81.2(3)
1.891(7)
83.3(1)
97.1(1)
97.7(1)
172.5(1)
2.411(1)
2.403(1)
2.309(1)
2.386(1)
98.0(3)
93.9(3)
176.0(3)
82.4(4)
97.7(2)
95.6(2)
173.5(2)
2.410(3)
2.326(3)
2.426(3)
2.414(2)
2.416(2)
2.413(2)
1.155(18)
1.432(20)
1.357(17)
1.473(12)
1.376(18)
1.477(13)
1.459(16)
1.532(19)
1.512(14)
1.200(14)
1.348(15)
1.229(14)
1.339(16)
1.202(20)
1.343(17)
1.221(14)
1.327(16)
81.6(3)
81.4(1)
174.9(1)
91.9(1)
99.5(1)
1.348(9)
1.446(7)
1.340(10)
1.514(6)
1.516(10)
1.512(9)
1.509(8)
1.203(9)
1.329(11)
1.205(9)
1.316(10)
1.201(10)
1.326(10)
1.188(11)
1.329(7)
1.143(9)
1.340(6)
1.451(7)
1.361(6)
1.469(7)
1.496(6)
1.482(7)
1.481(7)
1.210(5)
1.348(5)
1.206(5)
1.331(6)
1.211(6)
1.343(6)
1.210(5)
1.327(6)
172.2(3)
96.8(3)
97.7(3)
90.4(3)
178.5(3)
93.5(3)
174.0(1)
95.7(2)
99.0(2)
95.0(2)
176.2(2)
93.3(2)
97.1(1)
97.7(1)
172.5(1)
88.9(1)
85.6(1)
Cl–Ir–P(2)
Cl–Ir–P(3)
P(1)–Ir–P(2)
P(1)–Ir–P(3)
P(2)–Ir–P(3)
Ir–C(1)–C(2)
Ir–C(1)–C(8)
Ir–C(4)–C(3)
Ir–C(4)–C(5)
C(1)–C(2)–C(3)
C(2)–C(3)–C(4)
90.3(1)
85.6(1)
89.8(1)
114.7(6)
128.8(10)
114.8(7)
130.8(9)
115.8(10)
115.8(10)
87.4(1)
86.2(1)
89.8(1)
116.7(4)
124.5(5)
115.1(4)
126.8(6)
114.8(6)
116.2(6)
89.4(1)
114.8(4)
129.4(3)
114.4(4)
129.1(3)
116.8(4)
116.1(4)
methyl hydrogen chemical shifts at d 3.86, 3.54, 3.48, and 3.32.
These chemical shift comparisons suggest that the triphos ligand
adopts a favored conformation in which one or more of the phenyl
groups shields the a-methoxycarbonyl hydrogens.
In order to understand the steric parameters in intermediate II,
X-ray crystallographic analyses were performed on the model
complexes 2-NCMe and 2-CO (Figs. 1 and 2, Tables 1 and 2). Both
structures exhibit a distorted octahedral geometry about iridium
with C(1)–Ir–C(4) angles constrained by the ring structure to
77.0(2)–78.4(5)°. The P(1)–Ir–P(3) angles are significantly larger
at 85.6(1)–86.2(1)°. The axial ligands are bent away from PhA
and PhE with Ir–N(1)–C(18) and Ir–C(18)–O(1) angles in 2-NCMe
and 2-CO of 175.9(10)° and 172.9(6)°, respectively. The largest
deviation of the metallacycle ring atoms from the Ir–C(1)–C(2)–
C(3)–C(4) mean plane is 0.04(1) Å at C(4) in 2-NCMe and
0.04(1) Å at C(1) in 2-CO. Within the metallacycle ring, the C(1)–
C(2) and C(3)–C(4) bond distances are consistent with localized
carbon–carbon double bonds. There is a relatively large displace-
ment of the two ester substituents on the a-carbons of the metal-
lacycle from the mean plane of the metallacycle ring, in a direction
that is away from the P(2) phenyls which extend under the metal-
lacycle ring. Thus, in 2-NCMe the carbonyl carbons exhibit the fol-
lowing displacements from the mean plane of the metallacycle
ring: C(5) – 0.49(1), C(6) – 0.04(2), C(7) – 0.04(1), and C(8) –
0.24(1) Å. For 2-CO the carbonyl carbon displacements from the
mean plane of the metallacycle ring are C(5) – 0.276(8), C(6) –
0.097(8), C(7) – 0.094(8), and C(8) – 0.547(8) Å. The a-methoxycar-

J.M. O’Connor et al. / Inorganica Chimica Acta 364 (2010) 220–225
223
À
Fig. 2. Sructural drawings of 2-CO (most hydrogen atoms and BF4 counter ion omitted for clarity).
bonyl metallacycle-substituents are positioned in the shielding
cone of PhB and PhF, which is consistent with the relatively upfield
chemical shift observed for the ester methyl hydrogens in the 1H
NMR spectra.
The PhA and PhE phenyl rings in 2-CO and 2-NCMe extend above
the metallacycle plane and adopt conformations in which the dihe-
dral angle between the two six-member rings is 32.1(2)° and
35.5(2)°, respectively (see the second structure in Figs. 1 and 2).
These angles generate a slight wedge shape with the open end
directed toward the axial CO/NCMe ligand and result in relatively
short non-bonded distances between the axial CO/NCMe ligands
and an ortho-carbon on each of the phenyls. For 2-NCMe, the
C(6A)Á Á ÁN and C(6E)Á Á ÁN distances are 3.053(11) and 3.148(12) Å,
respectively. With hydrogen atoms placed in idealized positions,
the H(6A)Á Á ÁN(1) and H(6E)Á Á ÁN(1) non-covalent bond distances
in 2-NCMe are 2.82 and 2.64 Å. Similar interactions are observed
for the ortho-carbons of PhA and PhE in 2-CO. The C(6A)Á Á ÁC(18)
and C(6E)Á Á ÁC(18) distances are 3.083(9) and 3.140(10) Å, and the
H(6A)Á Á ÁC(18) and H(6E)Á Á ÁC(18) non-covalent bond distances are
2.61 and 2.81 Å, respectively. Thus, the hydrogens are directed
toward the
p-system of the C–N and C–O triple bonds. There are
also relatively short distances between the same ortho-carbons
(hydrogens) of PhA/PhE and an oxygen atom in each of the two
a-methoxycarbonyl ring-substituents. In 2-NCMe, the two a-meth-
Fig. 3. Angles between the axial ligands and the centroids of the C(1)–Ir–C(4) and
oxycarbonyls adopt a conformation that places O(1) at a 2.30 Å dis-
tance from H(6A) and O(8) at a 2.48 Å distance from H(6E). The
C(6A)–H(6A)–O(1)/C(6E)–H(6E)–O(8) angles are 159.1°/146.6° for
2-NCMe and the C(6A)–H(6A)–O(1)/C(6E)–H(6E)–O(7) angles are
147.4°/160.9° for 2-CO. In 2-CO, the O(1)Á Á ÁH(6A) and
O(7)Á Á ÁH(6E) distances are very short at 2.51 and 2.20 Å, respec-
tively. These OÁ Á ÁH distances are well within the sum of the van
der Waals radii for hydrogen and oxygen (2.62 Å) and taken to-
gether with the C–H–O angles indicate that these are moderate
to weak hydrogen-bonding interactions [8].
P(1)–Ir–P(3) mean-planes.
of the metallacycle ring in 2-NCMe and 2-CO (Fig. 3). For 2-CO,
these small angles result in a C(1)Á Á ÁC(18) and C(4)Á Á ÁC(18) non-
bonded distances of 2.586(10) and 2.619(8) Å.
Whereas 2-NCMe and 2-CO are similar in terms of bond and tor-
sion angles, the structure of 2-Cl exhibits much different conforma-
tional preferences with respect to the iridium–phosphorus and
phosphorus–carbon bonds. The C(1A)–P(1)–Ir–N dihedral angles
are 9.6(3)° (2-NCMe), 30.8(3)° (2-CO) and 86.8(2)° (2-Cl), and the
C(1E)–P(3)–Ir–L (L@N, C(18), Cl) dihedral angles are À28.4(4)° (2-
NCMe), À12.0(3)° (2-CO), and À56.7 (10)° (2-Cl). Most apparent
from viewing the right hand structure in Fig. 4 is the dihedral angle
between the mean planes of the PhA and PhE rings, which at
47.5(1)°, is much larger than in the case of 2-NCMe (32.1(2)°)
and 2-CO (35.5(2)°). Along with these changes in torsion angles,
the non-covalent bond distances between Cl and the ortho-carbons
The triphos PhC and PhD rings extend under the metallacycle
with the following closest non-bonded distances: For 2-NCMe;
H(2C)Á Á ÁC(3) 2.72, H(2C)Á Á ÁC(4) 2.61, H(2D)Á Á ÁC(1) 2.71,
H(2D)Á Á ÁC(2) 2.45. For 2-CO; H(2C)Á Á ÁC(3) 2.56, H(2C)Á Á ÁC(4) 2.51,
H(2D)Á Á ÁC(1) 2.75, H(2D)Á Á ÁC(2) 2.85 Å. As shown in Fig. 3, these
non-bonded interactions between the triphos ligand and the but-
adiendiyl and axial CO ligands results in relatively small 83° and
80° angles between the axial ligand (NCMe/CO) and the centroid

224
J.M. O’Connor et al. / Inorganica Chimica Acta 364 (2010) 220–225
Fig. 4. Structural drawings of 2-Cl (most hydrogen atoms omitted for clarity).
of PhA and PhE are significantly longer than the corresponding dis-
tances to the axial ligand in 2-NCMe and 2-CO: C(6A)Á Á ÁCl 3.391(5),
C(6E)Á Á ÁCl 3.296(4) Å. The corresponding ortho-hydrogen – chlori-
do distances however are similar to the hydrogen-axial ligand (N
or C(18)) interactions in 2-NCMe and 2-CO: H(6A)Á Á ÁCl 2.783 and
H(6E)Á Á ÁCl 2.57 Å, which are within the 2.85 Å sum of the van der
Waals radii for hydrogen and chlorine. The chlorido ligand of M–
Cl bonds is highly polar and X–HÁ Á ÁCl–M hydrogen bonds in crys-
tals typically have an angular range of 80–140° [8]. In the case of
2-Cl, the Ir–Cl–H(6A) and Ir–Cl–H(6E) angles are 89° and 87°,
respectively. There is also a short hydrogen-bonding interaction
between H(6A) and O(2) of 2.28 Å in 2-Cl.
yses were performed by NuMega Resonance Labs Inc. of San Diego,
California.
Preparation of [{j
2(C,C)-CR@CRCR@CR}{CH3C(CH2PPh2)3}Ir-
(CO)]BF4 (2-CO, R = CO2Me).
A 100-mL flask equipped with a magnetic stir bar was charged
with [{j
2(C,C)-CR@CRCR@CR}{CH3C(CH2PPh2)3}Ir(NCMe)]BF4 (2-
NCMe, R = CO2CH3; 199.7 mg, 0.16 mmol) and wet CH2Cl2
(30 mL). Carbon monoxide gas was bubbled through the solution
for 30 min, and the solution was allowed to stir at rt under an
atmosphere of CO for 24 h. The volatiles were removed under vac-
uum and the residue was recrystallized from CH2Cl2/Et2O to give 2-
CO as an off-white powder in 86% yield (171.3 mg). IR (KBr): 3066
(w), 2952 (w), 2094 (s), 1724 (s), 1703 (s), 1437 (s), 1333 (w), 1236
(s) cmÀ1 1H NMR (CDCl3, 400 MHz): d 8.06 (t, 4H, J = 4.8 Hz), 7.68
;
3. Conclusions
(s, 4H), 7.56 (s, 7H), 7.32 (s, 5H), 7.17 (t, 2H, J = 7.2 Hz), 6.84 (t, 4H,
J = 7.2 Hz), 6.15 (dd, 4H, J = 11.1, 8.1 Hz), 3.8 (s, 6H), 3.13 (m, 4H),
2.64 (s, 6H), 2.12 (dd, 2H, J = 13.2, 3.6 Hz), 1.83 (d, 3H, J = 1.5 Hz);
13C NMR (CDCl3, 100 MHz): d 170.68 (m), 165.61, 164.40, 152.44,
141.03 (t, J = 8.2 Hz), 140.29 (t, J = 7.7 Hz), 132.36 (t, J = 2.2 Hz),
132.13, 132.02 (m), 131.81, 131.57, 131.51 (m), 130.58 (d,
J = 54.6 Hz), 130.44, 129.81 (d, J = 10.9 Hz), 129.36 (t, J = 5.4 Hz),
128.67 (t, J = 5.5 Hz), 127.80 (m), 127.23 (m), 52.43, 51.51, 38.59,
38.43 (m), 33.61 (m), 26.54 (d, J = 32.3 Hz). Anal. Calc. for
The most important structural feature of these triphos com-
plexes, with respect to the [2 + 2 + 1] cyclotrimerization of alkynes,
is summarized in Fig. 3 where it is demonstrated that the
a-car-
bons of the butadiendiyl ligand and the axial ligand (CO, NCMe,
Cl), which occupies the site of the vinylidene ligand in II (Scheme
1), are brought into close proximity as a result of the bulky triphos
ancillary ligand. Such close proximity may facilitate the reductive
coupling of the butadiendiyl and vinylidene ligands to give the ob-
served fulvene products. This suggests that a less bulky triphos
analogue would lead to slower reductive coupling and perhaps al-
low for the observation of the vinylidene intermediate by IR or
NMR spectroscopy. Efforts to obtain X-ray crystallographic analy-
ses of related metallacyclopentadiene complexes with alternative,
sterically less-demanding tridentate phosphine ligands, as well as
the trispyrazolylborate (Tp) ligand, have yet to be successful.
C54H51O9IrP3BF4: C, 53.34; H, 4.22. Found: C, 52.97; H, 3.92%.
Acknowledgements
The support of the National Science Foundation is gratefully
acknowledged (Grants CHE-0518707 and CHE-0911765; and
Instrumentation Grants CHE-0741968 and CHE-9709183).
Appendix A. Supplementary material
4. Experimental section
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.07.052.
All reactions and reaction workups were performed in the air
unless otherwise noted. Organic solvents were obtained commer-
cially and dried over either calcium hydride or sodium metal. IR
spectra were recorded on a Nicolet Magna-IR 550 FTIR spectrome-
ter. 1H and 13C NMR spectra were taken on Varian Hg 300
(76 MHz), Hg 400 (101 MHz), or UN 500 (126 MHz) spectrometers.
1H and 13C chemical shifts were referenced to residual protio-sol-
vent signals. FAB mass spectra were determined at the University
of California, Riverside Mass Spectrometry Facility. Elemental anal-
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