Evaluating the kinetic indenyl effect of a π-thiaphentalenyl ancillary ligand

Article abstract of DOI:10.1016/j.jorganchem.2007.12.027

The magnitude of the kinetic indenyl effect for the π-thiapentalenyl ligand, isoelectronic heteroanalogue of the indenyl ligand, has been tested using the reaction between complex, Rh(η⁵-th)(CO)₂ 3 (th = 2-ethyl-5-methyl-cyclopenta[b]thienyl or thiapentalenyl) and PPh₃ as a model process. Only moderate kinetic indenyl effect of the thiapentalenyl ligand was observed. The rational for such behavior was also supported by DFT computational correlation of ground and transition states for the reaction of carbonyl group substitution by a phosphine in the corresponding indenyl and thiapentalenyl rhodium complexes.

Full text of DOI:10.1016/j.jorganchem.2007.12.027

Available online at www.sciencedirect.com
Journal of Organometallic Chemistry 693 (2008) 1058–1064
www.elsevier.com/locate/jorganchem
Evaluating the kinetic indenyl effect of a p-thiaphentalenyl
ancillary ligand
Denis A. Kissounko a,b,*, Maxim V. Zabalov b, Neil M. Boag a, Yurii F. Oprunenko b,
Dmitri A. Lemenovskii b
a Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4WT, UK
b Moscow State University, Chemistry Department, Vorob’evy Gory, Moscow 119899, Russia
Received 14 September 2007; received in revised form 19 December 2007; accepted 20 December 2007
Available online 28 December 2007
Abstract
The magnitude of the ‘‘kinetic indenyl effect” for the p-thiapentalenyl ligand, isoelectronic heteroanalogue of the indenyl ligand, has
been tested using the reaction between complex, Rh(g5-th)(CO)2 3 (th = 2-ethyl-5-methyl-cyclopenta[b]thienyl or thiapentalenyl) and
PPh3 as a model process. Only moderate ‘‘kinetic indenyl effect” of the thiapentalenyl ligand was observed. The rational for such behav-
ior was also supported by DFT computational correlation of ground and transition states for the reaction of carbonyl group substitution
by a phosphine in the corresponding indenyl and thiapentalenyl rhodium complexes.
Ó 2008 Published by Elsevier B.V.
Keywords: Kinetic indenyl effect; Thiapentalenyl ligand; Rhodium
1. Introduction
page from g5- to g3-bonding mode (resonance structures
I and II at Chart 1) with a metal leading to much faster
ancillary ligand substitution via associative mechanism ver-
sus a dissociative pathway for the corresponding cyclopen-
tadienyl analogues [13]. The alternative argument to this is
that the enhanced substitution reactivity of indenyl species
compared to their cyclopentadienyl analogues may arise
from a ground rather than transition state effect (i.e. from
a weaker bonding of indenyl ligand to the metal compared
to cyclopentadienyl analogue), since many indenyl com-
plexes exhibit considerable distortion from an ideal g5-
geometry [14]. It has also been shown that ligand substitu-
tion in indenyl complexes can occur through g1-intermedi-
ates [15].
In a wake of extensive research into organometallic
indenyl species there has been a growing number of reports
of complexes with aza- and thia-heterocyclic ligands,
isolectronic to the p-indenyl ligand, which also have
demonstrated enhanced catalytic activity in a-olefin poly-
merization processes when compared to the related car-
boanalogues [16–24]. However, the paucity of isolated
Organometallic complexes with indenyl ancillary ligand
have received considerable attention over the past two dec-
ades for the reasons of their enhanced catalytic activity in
olefin polymerization processes [1–3] as well as fundamen-
tal structure/properties relationship studies [4]. Part of the
rational for the elevated interest into indenyl ligand-con-
taining species can be attributed to the ‘‘kinetic indenyl
effect” first described by Mawby et al. [5,6] followed by
extensive work by Basolo et al. for a series of indenyl
and cyclopentadienyl rhodium derivatives [7–11], which
defined the significant kinetic rate enhancement for the
substitution of ancillary ligands in indenyl versus cyclopen-
tadienyl organometallic derivatives [12]. Such enhancement
is commonly attributed to much easier indenyl ring slip-
*
Corresponding author. Present address: Center for Composite Mate-
rials, University of Delaware, Newark, DE 19716, United States. Tel.: +1
302 831 0376.
E-mail address: kissounk@udel.edu (D.A. Kissounko).
0022-328X/$ - see front matter Ó 2008 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2007.12.027

D.A. Kissounko et al. / Journal of Organometallic Chemistry 693 (2008) 1058–1064
1059
0.1 g (0.257 mmol) of [Rh(CO)2Cl]2 was added to the solu-
tion of the lithium salt 2 and the reaction mixture was stir-
red at the ambient temperature for another 16 h. The
solvent was then removed in vacuo and the residue
extracted into hexane. A stream of CO was flushed through
the solution for 30 min to convert any traces of Rh2(g5-
th)2(CO)3 into 3, whereupon the solution color gradually
turned from brown to red. The solution was concentrated
in vacuo and passed through a short column of silica gel
using hexane as the eluent. The first band of organic impu-
rities was rejected. The second red-brown band containing
both 3 and the dimer, [Rh2(g5-th)2(CO)3], was collected,
the volume reduced to c.a. 10 ml and a stream of CO was
passed through the solution once again. Then all the vola-
tiles were removed in vacuo and the residue was re-crystal-
lized from a small amount of hexane at ꢀ20 °C and dried in
high vacuum to afford 0.073 mg (61%) of 3 as red solid.
1H NMR (C6D6, numbering according to the heterocy-
Chart 1.
organometallic species with such ligands have precluded
any relevant fundamental reactivity studies of a heterocy-
clic ligand. We have recently synthesized a number of
new organometallic derivatives of r-and p-bonded
2-ethyl-5-methyl-cyclopenta[b]thienyl (or otherwise thia-
pentalenyl) and 4H-cyclopenta[2,1-b:3,4-b0]dithiophene
(or otherwise thiafluorenyl) ligands [25–27]. In continua-
tion of our thiapentalenyl ligand fundamental reactivity
studies we now report the evaluation of its ‘‘kinetic indenyl
effect” (i.e. g5 ? g3-thiapentalenyl ring slippage as illus-
trated by structures I0 and II0 at Chart 1) by means of
kinetic and computational DFT studies following the
carbonyl group substitution reaction by a phosphine in a
rhodium complex, Rh(g5-Th)(CO)2 3, as a convenient
model system, and direct correlation of the data with
related carbocyclic indenyl analogues.
3
clic nomenclature): d 0.95 (t, 3H, J = 7.5 Hz, CH3CH2),
1.78 (s, 3H, Me), 2.35 (q, 2H, 3J = 7.5 Hz, CH3CH2),
4.98 (s, 1H, H4), 5.02 (s, 1H, H6), 6.08 (s, 1H, H3).
13C{1H} NMR (C6D6): d 15.1 (CH3CH2), 15.7 (Me), 24.5
(CH3CH2), 73.9 (C4), 75.02 (C6), 112.1 (C3), 115.2 (C8),
119.0 (C5), 122.5 (C7), 149.1 (C2), 192.2 (d, 1JRhC = 85 Hz,
CO). IR (hexane, cmꢀ1): 2040, 1980 (CO). Elemental Anal.
Calc. for C12H11O2RhS: C, 44.74; H, 3.44. Found: C,
44.27; H, 3.24%.
2. Experimental
2.1. General considerations
All syntheses were performed under an atmosphere of
dinitrogen. Standard Schlenk techniques were used for
the preparation of organometallic compounds. Liquids
were transferred by means of steel catheters through suba
seal fittings or by syringe.
Solvents were freshly distilled from sodium/benzophe-
none (THF) and sodium/potassium alloy (hexane). Iso-
meric 2-ethyl-5-methyl-6H-cyclopenta[b]thiophene and
2.3. Kinetic measurements of the reaction between 3 and
PPh3
A solution of PPh3 of appropriate concentration in tol-
uene was added to a quartz cell adapted for air sensitive
operations filled with nitrogen. The cell was placed in
thermostated cell holder and allowed to equilibrate at
25 °C. An aliquot of 7 ll of 1.7 mM solution of complex
3 in toluene was added to the well-stirred solution of
PPh3. Rate constants were determined by monitoring the
increase in absorption of monosubstituted phosphine
derivative 4 at kmax = 365 nm. The correlation coefficient
of the least-squares line (R2 > 0.998) was very good with
the intercept very close to zero (1.6 ꢁ 10ꢀ6). All the exper-
iments were carried out under pseudo-first-order condi-
tions with a greater than tenfold excess of phosphine,
and went to a 100% conversion of the starting complex 3.
No other byproducts were detected by either IR or NMR
spectroscopy of crude reaction mixture.
2-ethyl-5-methyl-4H-cyclopenta[b]thiophene (as
a
7:3
mixture, respectively) [28,29], and [Rh(CO)2Cl]2 [30], were
prepared according to the literature methods. All other
reagents were used as supplied.
Infrared spectra were recorded on a Perkin–Elmer 1710
Fourier Transform spectrometer using CaF2 solution cells
fitted with Luer lock taps. UV measurements were per-
formed on Agilent 8453 UV–Vis spectrometer. NMR spec-
tra were recorded a Bruker AC-300 spectrometer (1H at
300.13 MHz and 13C{1H} at 75.47 MHz, referenced to
SiMe4). All spectra were recorded at ambient temperatures.
Column chromatography was undertaken using inter-
mediate activity silica (Grade 4).
2.4. Isolation of 4
Microanalysis was carried out by the microanalysis
group at Moscow State University.
Combined solutions left over from kinetic studies of
reaction between 3 and PPh3 were concentrated under vac-
uum to a volume of c.a. 1 ml, whereupon the entire mixture
was transferred to a chromatography column packed with
silica and eluted with a toluene/hexane 3:1 mixture. The
single orange bang was collected and stripped to dryness
yielding to 4 as an orange solid.
2.2. Synthesis of 3
To a mixture of isomeric 1a and 1b (0.16 g, 1 mmol) in
20 ml of THF, 0.4 ml of 1.5 M n-BuLi was added at
ꢀ20 °C. After stirring overnight at ambient temperature,

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D.A. Kissounko et al. / Journal of Organometallic Chemistry 693 (2008) 1058–1064
1H NMR (C6D6): d 1.07 (t, 3H,3J = 7.5 Hz, CH3CH2),
To elucidate the magnitude of ‘‘kinetic indenyl effect” of
the p-thiapentalenyl moiety, system heteroatom on the
reactivity of the metal center, kinetics of CO substitution
in 3 using PPh3 as a nucleophile have been studied. The
conditions were identical to those described by Rerek and
Basolo [9] for the series of related indenyl and cyclopenta-
dienyl rhodium complexes. The reaction was followed by
an increase in intensity of a UV/Vis absorption band of
the monosubstituted phosphine complex 4 (kmax = 365 nm,
toluene). The experiments were carried out under pseudo-
first-order conditions using a ten fold or greater excess of
phosphine. All the reactions were run to completion. Good
first-order plots were obtained with a linear dependence of
the pseudo-first rate constants kobs on the concentration
of phosphine (Fig. 1) giving a second-order rate constant
1.45 (s, 3H, Me), 2.46 (q, 2H, 3J = 7.5 Hz, CH3CH2),
4.94 (s, 1H, H4), 5.18 (d, 1H, JPH = 3 Hz, H6), 6.22 (d,
1H, JPH = 5 Hz, H3), 6.92–7.86 (m, 15H, PPh3). 13C{1H}
NMR (C6D6):
d 15.1 (CH3CH2), 16.2 (Me), 23.8
(CH3CH2), 72.9 (C4), 75.02 (C6), 112.1 (C3), 115.2 (C8),
119.0 (C5), 121.8 (C7), 123–154 (C2, PPh3), 197.2 (dd,
2
1JRhC = 85 Hz, JPC = 20.6 Hz, CO). 31P{1H} NMR
1
(C6D6): d 52.1 (d, JRhP = 198 Hz). IR (hexane, cmꢀ1):
1948 (CO). Elemental Anal. Calc. for C29H26OPRhS: C,
62.59; H, 4.71. Found: C, 62.97; H, 4.84%.
2.5. Computational procedures
Density functional (DFT) study was carried out using
the ab initio generalized gradient approximation and the
mPBE [31] or PBE [32,33] functionals with the TZ2P
basis set and Priroda software [34,35]. Geometry optimi-
zation was carried out for all stable compounds and for
the structures corresponding to the saddle points (in the
case of transition states, TS). The types of the stationary
points were confirmed by the vibrational frequency anal-
ysis. For the saddle points, the reaction coordinates were
also calculated. All computations were performed for the
gas phase.
of 9.8 ꢁ 10ꢀ3 Mꢀ1 sꢀ1
.
Although the rate observed (9.8 ꢁ 10ꢀ3 Mꢀ1 sꢀ1) shows
moderate rate enhancement compared to the correspond-
ing value for Rh(g5-Cp)(CO)2 (1.3 ꢁ 10ꢀ4 Mꢀ1 sꢀ1) [10],
it is nowhere near the magnitude exhibited by Rh(g5-
Ind)(CO)2 (2.8 ꢁ 104 Mꢀ1 sꢀ1) measured at 25 °C [9]. In
fact, it is closer to the moderate rate enhancement afforded
by the dimethylamino group in Rh(g5-Me2NCp)(CO)2
(1.05 ꢁ 10ꢀ3 Mꢀ1 sꢀ1 at 25 °C), which was attributed to
resonance stabilisation of a g3-intermediate by the nitrogen
non-bonding electron pair [10]. Such mechanism is also
plausible for the sulfur heterocycle, and the data suggest
that aromatisation of the thiophene ring (i.e. the indenyl
effect as shown in Chart 1) plays little part in the substitu-
tion chemistry of complexes containing such heterocyclic
system.
3. Results and discussion
A mixture of isomeric 1a and 1b readily reacts with n-
BuLi in THF generating the lithium salt 2, followed by
addition of [Rh(CO)2Cl]2 to afford Rh(g5-th)(CO)2 3 as a
red solid in 61% yield (Scheme 1).
To further confirm the difference in ‘‘kinetic indenyl
effect” between indenyl and thiapentalenyl ligands, DFT
calculations were performed for reaction 2 (Scheme 1)
and related reaction with Rh(g5-Ind)(CO)2 using PMe3 as
a model reactant. The connections between stationary
points in Fig. 2 were established according to IRC calcula-
tions. For comparison the geometries were optimized in
two functional (mPBE and PBE). Difference between two
functionals was not significant.
Isolated complex 3 was identified by IR- and NMR
spectroscopy. Also, a reaction mixture solution shows the
presence of second minor species, which was tentatively
identified as a dimeric complex, [Rh2(g5-th)2(l-CO)(CO)2],
by comparison of its IR-spectrum stretches (1971,
1842 cmꢀ1, THF). Such spectroscopic pattern matches
characteristics of the corresponding indenyl derivative
Rh2(g5-Ind)2(l-CO)(CO)2 [36]. Upon passing CO gas
through the reaction mixture solution such species quanti-
tatively convert into 3; hence no attempts were made to iso-
late this minor impurity.
Thus, two different directions for a phosphine approach,
i.e. endo- and exo-, give rise to two possible transition
states: TS I and TS II for indenyl ligand versus TS I0 and
Scheme 1. Synthesis of complex 3 and CO substitution reaction by a phosphine.

D.A. Kissounko et al. / Journal of Organometallic Chemistry 693 (2008) 1058–1064
1061
Table 1
Calculated bond lengths (A) and angles (°) for intermediates B and C
˚
Atoms
Distance
Atoms
Angle
B
C
B
C
Rh–C(1)
Rh–C(2)
Rh–C(3)
Rh–C(8)
Rh–C(9)
Rh–C(10)
Rh–C(11)
C(10)–O(1)
C(11)–O(2)
2.316
2.150
2.313
3.000
2.998
1.932
1.933
1.160
1.160
3.291
2.510
2.236
3.625
3.116
1.950
1.915
1.159
1.164
Rh–C(9)–O(1)
Rh–C(10)–O(2)
C(1)–Rh–C(2)
C(1)–C(2)–C(3)
C(2)–Rh–C(3)
C(2)–C(1)–C(7)
C(2)–C(3)–C(8)
C(9)–Rh–C(10)
177.7
177.5
37.4
105.3
37.4
107.8
107.8
94.7
169.6
171.8
23.0
109.8
35.3
108.9
105.3
140.7
Table 2
0
0
˚
Calculated bond lengths (A) and angles (°) for intermediates B and C .
Fig. 1. Plot of kobs vs. the concentration of PPh3, 25 °C, toluene.
Atoms
Distance
Atoms
Angle
B0
C0
B0
C0
TS II0 for thiaanalogue (Fig. 2, Table 1). Although the exo-
approach pathway cannot be completely overruled, espe-
cially under thermodynamically controlled reaction, the
endo-attack seems to be much more preferential under
kinetic control, since it requires the lowest activation
energy. The rational for such endo-regioselective attack,
which was also observed recently for cationic rhodium
indenyl derivatives [37], was explained earlier by the rela-
tive trans-effects of indenyl, carbonyl and phosphine
ligands, where the ligand with the highest trans-effect
(CO) is trans-position to the six-membered ring [38] (see
Table 2).
Rh–C(1)
Rh–C(2)
Rh–C(3)
Rh–C(7)
Rh–C(8)
Rh–C(9)
Rh–C(10)
C(9)–O(1)
C(10)–O(2)
2.231
2.742
3.621
3.131
3.828
1.907
1.949
1.161
1.152
3.353
2.527
2.236
3.644
3.122
1.948
1.915
1.159
1.164
Rh–C(10)–O(1)
Rh–C(11)–O(2)
C(1)–Rh–C(2)
C(1)–C(2)–C(3)
C(2)–Rh–C(3)
C(2)–C(1)–C(8)
C(2)–C(3)–C(9)
C(10)–Rh–C(11)
179.4
174.0
32.6
110.4
19.5
103.0
108.5
91.9
170.6
171.8
22.3
109.9
35.1
107.3
105.0
143.2
attack by a phosphine on molecules A and A0 leads to both
indenyl and thiapentalenyl rings slippage. The earlier work
by Ceccon et al. rationalized the g5 ? g3 indenyl ligand
slippage in rhodium complexes upon the attack by a nucle-
ophile in terms of MO interactions and difined the least
motion pathway for a nucleophilic species approach to
the metal center [39]. Thus, of particular interest is the
observation that whereas the substitution in Rh(g5-
Ind)(CO)2 occurs through postulated g5 ? g3 indenyl
ligand slippage leading to the intermediate B, the corre-
Both ground states A and A0 exhibit distorted g5- or
g3 + g2-coordination of indenyl and thiapentalenyl
ligands, respectively, to the rhodium with the correspond-
˚
˚
ing bond distances of 2.270 A [Rh–C(1)], 2.274 A [Rh–
˚
˚
˚
C(2)], 2.270 A [Rh–C(3)], 2.517 A [Rh–C(4)], 2.517 A
[Rh–C(5)] for complex A; and 2.284 A [Rh–C(1)],
2.285 A [Rh–C(2)], 2.278 A [Rh–C(3)], 2.498 A [Rh–
˚
˚
˚
˚
0
˚
C(4)], 2.479 A [Rh–C(5)] for complex A . The following
Fig. 2. Energy profile for CO to phosphine ligand substitution in indenyl and thiapentalenyl complexes derived from DFT/mPBE or DFT/PBE (in
parenthesizes) calculations.

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D.A. Kissounko et al. / Journal of Organometallic Chemistry 693 (2008) 1058–1064
sponding reaction for the thiaanalogue leads to the inter-
mediate B0, where the ancillary ligand is essentially g1-
bound to the rhodium (Fig. 2, Table 1). Accordingly, for
the g1-type intermediate B0 Rh–C(1), Rh–C(2) and Rh–
C(3) bond distances of 2.231, 2.742 and 3.621 A, respec-
˚
tively, differ significantly from those for the analogous
Fig. 3. Structures of transition states (TS) and intermediates for the reaction between indenyl and thiapentalenyl rhodium complexes with phosphine.

D.A. Kissounko et al. / Journal of Organometallic Chemistry 693 (2008) 1058–1064
1063
3
˚
g -type indenyl species B [Rh–C(1) 2.316 A, Rh–C(2)
Acknowledgements
˚
˚
2.150 A and Rh–C(3) 2.313 A], which represent almost
undistorted allylic moiety. Also, in B0 the Rh–C(11)–O(2)
moiety (174.0°) is considerably tilted away from almost lin-
ear geometry as in indenyl analogue B [Rh–C(10)–O(2)
177.5°]. Hence, such g5 ? g1 observed in A0 ? B0 pathway
ring slippage requires more energy to occur, slowing down
the kinetic rate of the process. Also, the diagram unambig-
uously shows that the differences in ‘‘kinetic indenyl effect”
for complex 3 and the indenyl analogue, Rh(g5-Ind)(CO)2,
can be attributed to higher energy of both transition states
for the thiapentalenyl ligand complex [3.87 kcal/mol (TS
I0) versus 2.0 kcal/mol (TS I), and 5.11 kcal/mol (TS III0)
versus 3.29 kcal/mol (TS III)], whereas energies corre-
sponding to the intermediate molecules (B and B0) and
final products (D and D0) lie within a smaller range.
Although the energy differences found for indenyl and
thiapentalenyl intermediates and transition states (Fig. 3)
are not large enough to be solely responsible for the dra-
matic difference of the indenyl effect in corresponding rho-
dium dicarbonyl derivatives, they clearly bear a pivotal
role in reactivity decline for species 3 versus the corre-
sponding indenyl analogue. We also speculate that addi-
tional weak interactions between the sulfur heteroatom
and a metal center in 3 can influence the reactivity of car-
bonyl groups in the later complex. Both experimental and
computational studies have been launched to further inves-
tigate this hypothesis, and the results will be reported
elsewhere.
The financial support from Russian Foundation for Ba-
sic Research (RFBR #98-03-32995) and the University of
Salford is gratefully acknowledged.
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The data reported here significantly advance our
understanding of the role played by a heteroatom in a
p-bound ligand on the reactivity of other ancillary
ligands at the metal center. Thus, it appears that the rate
enhancement for the substitution of other ancillary
ligands in a metal’s coordination sphere caused by the
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indenyl ligand. Moreover, as has been shown by DFT
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involving g1-intermediate B0, whereas the reaction
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necessary to elucidate whether the sulfur heteroatom
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reactivity of other ancillary ligands, the results if this
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organometallic species compared to isoelectronic indenyl-
containing analogues. The moderate rate enhancement
for thiapentalenyl rhodium derivatives was found compa-
rable to the substituted cyclopentadienyl rhodium
analogues.

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