Characterisation and mechanistic study of the oxidative addition reactions of [Ir(cod)(sacac)]

Article abstract of DOI:10.1016/j.jorganchem.2015.10.012

[Ir(cod)(sacac)] (sacac = thioacetyl acetonato) complexes were successfully synthesized and characterised using ¹H and ¹³C NMR, IR and micro elemental analyses. The formation of isosbestic points in different solvents clearly indicate the formation of only one product, while the kinetic results for all of the complexes (except for chlorobenzene) showed simple second order kinetics with a zero intercept (within experimental error). The rate of oxidative addition showed little or no change with a solvent variation even with a large difference in polarity for the selected solvents, varying between 1.78 × 10^-3 and 5.2(3) M^-1 s^-1 for chlorobenzene and acetonitrile respectively. Activation volumes were determined in four of the solvents, which varied between +10.2(9) for chloroform to -18(1) cm³ mol^-1 for acetonitrile. This solvent variation obeyed the Kirkwood equation with an intrinsic volume of activation of -21(3) cm³. A DFT analysis of the oxidative addition reaction shows that the trans addition is energetically favoured and that the trans [Ir(cod) (sacac) (CH₃) (I)]-alkyl product is more stable than the four possible cis [Ir(cod) (sacac) (CH₃) (I)]-alkyl products.

Full text of DOI:10.1016/j.jorganchem.2015.10.012

Accepted Manuscript
Characterisation and mechanistic study of the oxidative addition reactions of [Ir(cod)
(sacac)]
Walter Purcell, Jeanet Conradie, S. Kumar, Johan A. Venter
PII:
S0022-328X(15)30181-9
10.1016/j.jorganchem.2015.10.012
JOM 19269
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 22 July 2015
Revised Date: 1 October 2015
Accepted Date: 6 October 2015
Please cite this article as: W. Purcell, J. Conradie, S. Kumar, J.A. Venter, Characterisation and
mechanistic study of the oxidative addition reactions of [Ir(cod)(sacac)], Journal of Organometallic
Chemistry (2015), doi: 10.1016/j.jorganchem.2015.10.012.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Characterisation and mechanistic study of the oxidative addition reactions of
[
Ir(cod)(sacac)]
Walter Purcell*, Jeanet Conradie, S. Kumar and Johan A. Venter
Department of Chemistry, PO Box 339, University of the Free State, Bloemfontein, 9300,
South Africa.
Abstract
[
Ir(cod)(sacac)] (sacac = thioacetyl acetonato) complexes were successfully synthesized and
1
13
characterised using H and C NMR, IR and micro elemental analyses. The formation of
isosbestic points in different solvents clearly indicate the formation of only one product,
while the kinetic results for all of the complexes (except for chlorobenzene) showed simple
second order kinetics with a zero intercept (within experimental error). The rate of oxidative
addition showed little or no change with a solvent variation even with a large difference in
-3
-1 -1
polarity for the selected solvents, varying between 1.78 x 10 and 5.2(3) M s for
chlorobenzene and acetonitrile respectively. Activation volumes were determined in four of
3
-1
the solvents, which varied between +10.2(9) for chloroform to -18(1) cm mol for
acetonitrile. This solvent variation obeyed the Kirkwood equation with an intrinsic volume of
3
activation of -21(3) cm . A DFT analysis of the oxidative addition reaction shows that the
trans addition is energetically favoured and that the trans [Ir(cod)(sacac)(CH )(I)]-alkyl
3
product is more stable than the four possible cis [Ir(cod)(sacac)(CH )(I)]-alkyl products.
3
*
Corresponding author, Fax: +27-(0)865255382, e-mail:purcellw@ufs.ac.za.
Key words: Iridium (I), thioacetyl acetonato ligand, cyclooctadiene, oxidative addition, high
pressure kinetics, DFT
1

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1
. Introduction
Cyclooctadiene (cod) complexes of iridium(I) are mainly used as i) homogeneous catalysts in
synthetic organic chemistry and ii) as starting material in the syntheses of other iridium
organo-metallic complexes. One of the most well-known iridium (I) cod complexes is the
1
Crabtree catalyst , [Ir(cod)(PCy )(py)]PF , (py = pyridine), which showed a 100 times increase
3
6
in hydrogenation activity compared to the Wilkinson catalyst ([Rh(PPh ) Cl]) and even
3
3
allowed for the hydrogenated of tri and tetra substituted alkene
. Consequently, the success
of [Ir(cod)(PCy )(py)]PF as a catalyst stimulated much research. This complex was used as
3
6
the basis for the development of new catalysts in an attempt to modify its catalytic
2
,3
properties by selectively changing the bonded ligands to metal centre. The reaction of the
[
Ir(OMe)(cod) ] complex, in the presence of dipy catalyses the reaction between 1,2
2
dimethylbenzene and 1,2-di-tert-butyl-1,1,2,2-tetrafluorodisilane to yield 4-silyl 1,2-
4
dimethylbenzene in yields of 99%. It was also found that different arenes and heteroarenes
react with B pin and HBpin in the presence of [Ir(cod)Cl ] and bipyridine to produce the
2
2
2 2
5
resulting aryl and heteroarylboron compounds. Other iridium cod complexes that exhibit
6
+
7,8
catalytic properties are [Ir(cod)(PPh ) ]OTf , [Ir(cod)(bzn)(PR )] (bzn = benzonitrile) ,
3
2
3
+
9
[
[
Ir(cod)(PHOX)]
(PHOX
=
phosphinooxazolidine,
Pfaltz
catalyst)
and
i
10
Ir(cod)( Pr PCH CH OMe)]PF
2
2
2
4.
The lability of the cod ligand renders the iridium cod compounds as ideal starting complexes
for the synthesis of a variety of other iridium(I) complexes.
For example,
1
1
-
[
Ir(biph)(N^N) ](OTf) (N^N = 2,2’ dipyridyl or 4,4’-dimethyl-2,2’ bipyridyl and OFt = CF SO )
2
3
3
was synthesized from the [Ir(cod)(bipy)Cl] dimeric complex, [Ir(Cl)(CO)(TPPTS) ].xH O
2
2
2
(TPPTS = m-trisulfonated triphenylphosphine) was prepared by substituting the cod ligand
-
12
13
with CO in the presence of Cl , as well as [Ir(cod)(py)(P(C H -4-OMe )]PF
6
and
6
4
3
1
4
[
Ir(cod)(ppol)] (ppol = Ph PCH CH CH P(Ph)CH CH CH=CH ) from [Ir(Cl)(cod)] .
2
2
2
2
2
2
2
2
Our interest in the cod complexes of iridium(I) stems from the ability of these complexes to
undergo oxidative addition reactions with CH I. The catalytic production of acetic acid from
3
methanol using both rhodium(I) and iridium(I) complexes involve the key step of oxidative
addition, which is then followed by methyl migration, reductive elimination and finally
substitution to produce acetic acid and the original metal complex. Research has shown
2

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1
5
that the iridium complex is by far the superior catalyst (faster reactions, milder
experimental condition, increased robustness and selectivity) and in order to understand
the chemistry of these complexes, different ligands are bonded to the metal centre to
1
6,17
improve reactivity and selectivity.
Our research has shown that the methyl migration step (or CO insertion) spontaneously
1
8
follows the oxidative addition reaction. These consecutive or sometimes simultaneous
reactions (depending on the rate of the two steps) complicate the kinetics and overall
mechanism, which include different isomers and equilibria, solvent assisted reaction
pathways and non-zero intercepts. High-pressure studies in these cases involve a complete
ligand variation at different temperatures to determine the activation volumes of these
reactions. The absence of the carbonyl ligand from the cod complexes simplify the overall
reaction since no CO insertion can take place, one isomer is normally produced due to the
steric influence of the cod on the final [Ir(cod)(LL’)(CH )I] complex and simplified
3
mechanisms with zero intercept are normally obtained. This allows for the isolated influence
of the different ligands on the oxidative addition step, as well as simplified high-pressure
studies.
[
Ir(cod)(sacac)] (sacac = thioacetyl acetonato) was synthesized in this study to determine the
rate and the mechanism of the oxidative addition step in different solvents, as well as to
determine the intrinsic activation volume for this complex. Experimental results are
complimented by theoretical density functional theory (DFT) results on the geometries and
energies of the reactants, transition state and possible products of the [Ir(sacac)(cod)] and
CH I reaction.
3
2
. Experimental
2
.1. General considerations
Unless otherwise stated, all chemicals were reagent grade and used without further
purification and all preparations carried out in air. Sacac was prepared with slight variations
of previously reported methods. IR spectra were recorded with a Hitachi 270-50
spectrophotometer while the NMR spectra (TMS as internal standard) were obtained at 293
K on a Bruker 600 MHz spectrometer. The methyl iodide was stabilized by silver foil to
prevent decomposition and used in a well-ventilated fume cupboard. The CHN analysis was
3

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performed on a LECO Truspec micro-analyser while the iridium analysis was performed on a
Shimadzu ICPS-7510 ICP-OES sequential plasma spectrometer. MALDI-TOF spectra were
collected by a Bruker Microflex LRF20 in the positive mode with the minimum laser power
required to observe signals.
The UV/visible spectra and kinetic runs were performed on a Hitachi 150-20 UV/visible
spectrophotometer in a thermostatically controlled holder (0.1°C), which has a capacity of 6
cells. The solvents used for the kinetic runs were all purified and dried using prescribed
1
9
methods. All the complexes were tested for stability in the different solvents prior to the
kinetic runs. High-pressure kinetics was performed on a GBC (model 916) UV/VIS
spectrophotometer equipped with a temperature-controlled high-pressure cell. Activation
volumes were calculated from kinetic runs at elevated pressure using the mathematical
≠
-
relationship lnk = -(∆V /RT)p. Typical experimental conditions were [M(cod)(LL’)] = 5.0 x 10
4
M, and [CH I] varied between 0.075 and 0.75 M, which ensured good pseudo-first-order
3
kinetics. The observed first-order rate constants (k ) were calculated for at least two half-
obs
(-kobs*t)
lives from the above plots using the equation A = A + (A - A )e
with A ,A and A the
0 t ∞
t
∞
0
∞
absorbances at time 0, t and infinity respectively, using a non-linear least squares
2
0
program.
2
.2.Synthesis
Thioacetyl acetonato ligand (sacac) was synthesized by two different methods as proposed
2
1
by Mayer.
2
.2.1. Acid catalysis
Dry H S gas was bubbled through a solution containing 15 mL acetyl acetone in 225 mL
2
acetonitrile at -44°C for 90 min. Subsequently dry HCl (from NH Cl and H SO and bubbled
4
2
4
through dry H SO ) was bubbled through the mixture for 120 min during which time the
2
4
colour of the solution changed from colourless to light yellow and then dry H S gas for
2
another 180 min at -40°. The mixture was then added to 370 mL ice water, 225 mL n-
pentane and 75 mL diethyl ether while continuously stirring the mixture. The aqueous layer
was extracted twice with 200 mL n-pentane/ether mixture (3:1). The organic layer was
4

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washed with two 50 mL portions of water (pH ∼ 5.5) and dried over anhydrous Na SO . The
2
4
solution was filtered and concentrated with reduced pressure. The red-brown oil was
fractionally distilled under reduced pressure (5 mmHg) to yield bright yellow oil. Yield: 24%.
2
.2.2. Base catalysis
Acetyl acetone (50 mL, 0.5 mol) and morpholine (4.3 g, 0.05 mol) was mixed in a long
cylindrical container. Small H S gas bubbles were introduced for 180 min at the bottom of
2
the container with the aid of a long glass pipe and a sinter glass frit to ensure effective gas
to solution mass transfer. The un-reacted H S gas was removed by leading the spent gas
2
through a concentrated NaOH solution. The container was sealed and left at room
temperature overnight. The mixture was subsequently washed with 100 mL of petroleum
ether (b.p. 80 -100°C) and then with 3M HCl. The crude product was dried over anhydrous
Na SO and concentrated with reduced pressure. The final gold-yellow product was
2
4
obtained by fractional distillation at 40°C and 5 mmHg. Yield: 71%.
4
2
.2.3. (η -1,5-Cyclooctadiene)(thioacetylacetonato)iridium(I), [Ir(cod)(sacac)]
0
.1 g [IrCl(cod)] was dissolved in a minimum (∼ 10 mL) of dimethylformamide (DMF). 0.35
2
mL thioacetyl acetone dissolved in 3 mL DMF, was added to the first solution while stirring.
Water was added drop wise to the reaction mixture, which afforded the precipitation of a
yellow product. The precipitate was separated from the mother solution with centrifugation
and decantation. The product was subsequently dissolved in a minimum of acetone and re-
crystallized with the drop wise addition of water and separated from the mother solution
with centrifugation and decantation. The yellow product was washed with water,
centrifuged and dried over P O .
2
5
-
1
Yield: 62 %. IR: ν(Ir-S), 360, ν(C-S), 663, 738, ν(C-H), 2920, ν(C=C), 1230 cm . Elemental
analysis (calculated values in brackets): C, 37.10 (37.58), H,4.49 (4.60), S, 6.93 (7.72), Ir, 46.0
1
(
46.26)%.; H NMR (600 MHz, CDCl ): δ 1.79-1.86 (m, 4H), 2.09 (s, 3H), 2.15-2.25 (m, 4H),
3
1
3
2
1
4
.35 (s, 3H), 3.81 (dd, 2H), 4.35 (dd, 2H), 6.57 (s, 1H); C NMR (600 MHz, CDCl ): δ 187.3,
3
+.
79.9, 119.9, 74.1, 53.4,33.1 31.9, 30.2, 29.3 ppm. m/z(M-H) :413.05(29.81), 414.06(5.53),
15.03(48.83), 416.05(11.36), 417.04(3.81) 418.09(4.61%)
5

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4
2
.2.4. (
η
-1,5-Cyclooctadiene)(iodo)(methyl)(thioacetylacetonato)iridium(III),
[
Ir(cod)(sacac)(CH )I]
3
An excess of CH I and [Ir(cod)(sacac)] (0.2 g, 0.47 mmol) was mixed in 2 mL acetone. The
3
solution was sealed for 60 min. The seal was removed and the solution was concentrated in
a vacuum desiccator. The concentrated solution was kept at -10 °C and orange needles,
2
2
suitable for an X-ray crystal structure determination was obtained from the solution.
-
1
IR: ν(Ir-S), 360, ν(C-S), 663, 738, ν(C-H), 2920, ν(C=C), 1230 cm . Elemental analysis
(calculated values in brackets): C, 30.65 (30.16), O, 5.14 (5.85), H, 4.04 (3.98), S, 2.66 (2.87),
1
I 22.10, (22.76), Ir, 35.5 (34.48) %.; H NMR (600 MHz, CDCl ): δ 1.87-1.91 (m, 1H), 1.92 (s,
3
3
2
4
1
H), 2.10 (s, 3H), 2.23-2.30 (m, 1H), 2.23-2.30 (m, 1H), 2.36 (s, 3H), 2.52-2.66 (m, 1H), 2.52-
.66 (m, 1H) 2.52-2.66 (m, 1H) 2.98-3.05 (m,1H), 3.20-3.29 (m, 1H), 3.98-4.02(m, 1H), 4.68-
1
3
.73 (m, 1H), 4.91-4.97 (m, 1H), 5.60-5.64 (m, 1H), 6.37 (s, 1H); C NMR (600 MHz, CDCl ): δ
3
87.6, 179.2, 118.5, 95.4, 94.6, 72.0, 71.1, 34.9, 31.7, 31.3, 30.4, 30.3, 26.0, 11.1 ppm.
+.
m/z(M-I) : 429.10(30.15), 430.10(4.76), 431.10(52.91), 432.11(8.46), 433.10(3.17)
34.10(0.52%)
4
2
.3.Kinetics
The UV/visible spectrum of the oxidative addition reaction between [Ir(cod)(sacac)] and
methyl iodide in acetone showed one reaction with the formation of three isosbestic points
(see Figure 1). The formation of isosbestic points were also observed in different solvents as
well as for the other complexes. A summary of the observed isosbestic points for the
different complexes is given in Table 1. Initial variations of methyl iodide concentrations
indicated a linear relationship with intercept close to zero as illustrated by the oxidative
addition of CH I to Ir(cod)(sacac)] in chloroform in Figure 2.
3
6

ACCEPTED MANUSCRIPT
Table 1. Summary of the kinetic data and activation parameters for the oxidative addition
reactions between [Ir(cod)(sacac)] and CH I in different solvents
3
3
4
≠
≠
≠
Isosbestic
qp
k (10 )
k (10 )
∆H
∆S
∆V
1
-1
Solvent
ε0
6
-1 -1
(M s )
-1
(s )
-1
-1
-1
3
-1
points (nm)
(10 )
(kJ mol ) (J K mol ) (cm mol )
Chloroform
363;402;446 4.64 16.7
364;405;455 5.44 10.2
2.5(2)
1.78(5)
4.5(4)
3.4(2)
5.2(3)
-
61(10)
54(6)
45(7)
25.5
-88(35)
+10.2(9)
-
Chlorobenzene
9(2)
-115(20)
-139(23)
-207(1)
Dichloromethane 361;401;440 8.54 11.5
-
-
-
-3.7(2)
-5.9(4)
-19(1)
Acetone
358;400;450 19.7 6.4
354;408;437 36.1 1.5
Acetonitrile
35(4)
-170(12)
Figure 1. Spectrum change with the addition of CH I to [Ir(cod)(sacac)] in acetone at 298 K
3
and 10 min. intervals, [CH I] = 0.75 M.
3
7

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6
5
4
3
2
1
0
3
5.0 °C
0.1 °C
3
2
5.1 °C
2
0.0 °C
0
0.1
0.2
0.3
0.4
CH I]/mol L
0.5
1
0.6
0.7
0.8
-
[
3
Figure 2. kobs vs [CH I] in chloroform at different temperatures, [Ir(cod)(sacac)] =
3
-
4
-1
5
.0 x 10 mol L
These results led to a very simple reaction mechanism Scheme 1.
Scheme 1: Oxidative addition between [Ir(sacac)(cod)] and CH3I
The rate law for this potential reversible reaction is given by Equation 1
R = k Ir(cod)(sacac)][CH I] – k [Ir(cod)(sacac)(CH )I]
...1
1
3
-1
3
The observed rate constant, after integration and using pseudo-first-order conditions are
given by Equation 2
8

ACCEPTED MANUSCRIPT
kobs = k [CH I] + k
-1
...2
1
3
A summary of the calculated rate constants as well as activation parameters are given in
Table 1. These values indicated that the k values for the different complexes, with the
-
1
exception of the reaction in chlorobenzene are all zero within experimental error. The high-
pressure kinetics between [Ir(cod)(sacac)(cod)] and CH I were performed in four different
3
solvents, namely chloroform, dichloromethane, acetone and acetonitrile in order to
determine the intrinsic volume of activation for this reaction. The results are presented in
Figure. 3.
Table 2. Summary of the kinetic data for the oxidative addition reactions between different
[
Ir(cod)(LL)] and CH I in different solvents and bidentate ligands with different ring sizes at
3
2
5 °C.
2
Donor
atoms
O,O
O,O
S,O
k (x10 )
1
Complex
Ring size
Solvent
Ref
-
1 -1
(M s )
1.69(2)
0.133(2)
0.52(3)
0.34(2)
2.69(6)
2.2(2)
[Ir(acac)(cod)]
[Ir(tfaa)(cod)]
[Ir(cod)(sacac)]
6
6
6
6
5
5
6
5
5
5
6
Acetone
23
Acetone
23
Acetonitrile
Acetone
This study
S,O
This study
[
[
[
[
[
[
[
Ir(cod)(AnMetha)]
S,O
Nitromethane
Nitromethane
Dichloromethane
Acetone
24
24
25
24
24
25
25
Ir(cod)(hpt)]
S,O
Ir(cod)(bpt-NH)]
Ir(cod)(AnMetha)]
Ir(cod)(hpt)]
N,N
S,O
1.44(7)
0.943(10)
0.693(17)
0.35(1)
0.0919(4)
S,O
Acetone
Ir(cod)(bpt)]
N,N
N,N
Acetone
Ir(cod)(bpt-NH)]
Benzene
9

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1
5
0
5
0
5
Chloroform
1
qp x 106
0
2
4
6
8
10
12
14
16
18
-
Dichloromethane
Acetone
-10
-15
-20
-25
Acetonitrile
Figure 3. Plot of the q value of the different solvents vs volume of activation for
p
[
Ir(cod)(sacac)]
2
.4.DFT study
The DFT calculations were carried out with the ADF (Amsterdam Density Functional) 2013
2
6
27
program system employing the PW91 (Perdew-Wang 1991) GGA (Generalized Gradient
Approximation) functional, a Slater-type TZP (triple-ζ plus polarization) basis set, a fine
mesh for numerical integration, and full geometry optimizations with tight convergence
criteria. All structures were calculated as singlet states (S = 0) and the geometry
optimizations were all symmetry-unconstrained. All transition states were confirmed with
frequency analyses and exhibited a single imaginary frequency. Zero-point energy and
thermal corrections (vibrational, rotational and translational) were made to the electronic
energies in the calculation of the thermodynamic parameters. Enthalpies (H) and Gibbs free
energies (G) were calculated from:
U = E + E
...3
el
ZPE
1
0

ACCEPTED MANUSCRIPT
H = U + RT (for gas phase); or H = U (in solution)
…4
G = H – TS
…5
where U = the total energy, E = total electronic energy, and E = internal energy (sum of
el
IE
vibrational, rotational, translational energies including the zero-point energy correction); R =
ideal gas constant, T = temperature and S = entropy. The entropy (S) was calculated from
the temperature dependent partition function in ADF at 298.15 K. The calculated results
assume an ideal gas unless where indicated that solvent effects were taken into account.
2
8
Where indicated, the COSMO (Conductor like Screening Model) model of solvation was
2
9
used as implemented in ADF. The type of cavity used was Esurf and the solvents used
were acetone and acetonitrile ( 0 = 20.7 and 37.5 respectively). Scalar relativistic effects
were used with the ZORA (Zero Order Regular Approximation) formalism.
ε
3
0
3
. Discussion of results
3
.1.Synthesis
[
Ir(cod)(sacac)] as well as [Ir(sacac)(cod)](CH )(I)] were successfully isolated and
3
1
13
characterised with H and C NMR, IR metal analysis, as well as with infrared
2
6
spectroscopy. The protons (eight) of the four methylene groups of cyclobutadiene in the
starting material are observed as two broad peaks at δ 1.79-1.86 and δ 2.15-2.25 ppm in the
1
H NMR spectrum. The same protons in the oxidative addition product are assigned as four
broad multiplets at δ 1.87-1.91, 2.23-2.30, 2.52-2.66 and 2.98-3.29 ppm due to the splitting
of these protons (axial and equatorial position) as a result to the different chemical
environment induced by the methyl and the iodide in the product. The axial protons inside
the ring are more deshielded than those outside the ring and therefore appear downfield
compared to the equatorial protons. The double bond protons (four) of the cyclobutadiene
appear as two peaks at δ 3.81 and 4.35 ppm in the starting material which are then split into
four multiplets at δ 3.98-4.02, 4.68-4.73, 4.91-4.97 and 5.60-5.64 ppm in the oxidative
addition product due to different chemical environment above and below the plane.
The proton at the alpha position (methine) in the sacac ligand, adjacent to the oxygen and
sulphur display a distinct downfield shift due to the conjugation in the ring structure. The
1
1

ACCEPTED MANUSCRIPT
proton in the starting material and product appears as singlets at δ 6.57 and 6.37 ppm
respectively and their downfield shift is also attributed to the electron withdrawing effects
imposed by the sacac moiety. The two methyl groups in [Ir(cod)(sacac)] is assigned as
singlets at δ 2.09 and 2.35 ppm while an additional methyl peak at δ 1.92 ppm is observed in
[
Ir(sacac)(cod)](CH )(I)], confirming the coordination of the methyl iodide to the iridium
3
1
3
centre. The C NMR showed the presence of fourteen carbons in the product (as expected
due to chemical difference above and below the plane) while the APT experiment showed
eight carbons in the negative mode (even number of protons) and six carbons in the positive
mode (odd or uneven number of protons) for the methylene groups and quaternary carbons
respectively.
The cod protons in the region of 4.35 to 1.5 ppm compare favourably with the 3.80 to 1.80
ppm for [Ir(triazolato)(cod)] (triazolato = 3,5-bis(pyridine-2-yl)-1,2,4 triazolate (bpt-NH) and
3
1
4
-amino-3,5-bis(pyridine-2-yl)-1,2,4 triazolate (bpt)) and 4.40 to 1.70 ppm for [Ir(S,O)(cod)]
2
4
(S,O = N-methyl-p-methoxybenzathiohydroxaminato and 1-hydroxi-2-pyridinato ligands)
3
.2 Kinetics
The formation of isosbestic points with the addition of methyl iodide to [Ir(sacac)(cod)]
solutions in different solvents clearly indicate the formation of only one product, namely the
2
6,32
Ir(III) alkyl product
[
Ir(sacac)(cod)]+ CH I→[Ir(sacac)(cod)](CH )(I)]
...(5)
3
3
The kinetic results of the oxidative addition of CH I to [Ir(cod)(sacac)] for all the solvent
3
studied showed simple second order kinetics with almost no reversible reaction as indicated
by the zero intercept (within experimental error). Surprisingly, the rate of oxidative addition
with a solvent variation showed little or no change (factor 2 difference) with a large
-
6
difference in polarity for the selected solvents (qp between 1.5 x 10 and
-
6
1
6.7 x 10 ). An increase of a factor 9 and 4 was observed for the [Ir(sacac)(cod)] complexes
2
5
while an increase of 20 to 40 was observed for the [Rh(sacac)(CO)(PPh )] complex with a
3
solvent variation. A comparison of the rate of oxidative addition for the different iridium
cod complexes in Table 3 show a relatively small variation in rate constant with a variation
1
2

ACCEPTED MANUSCRIPT
-4
of bidentate ligands, ring size and solvent and varies between 9.19 (4) x 10 and
-
2
-1 -1
2
.69(6) x 10 M s (factor 30) for [Ir(cod)(bpt-NH)] and [Ir(cod)(AnMetha)] respectively.
−1
Table 3. PW91/TZP DFT calculated electronic (E) and Gibbs free (G) energies (kJ mol ) of
the possible transition states (TS) and possible products of the [Ir(sacac)(cod)] + CH3I
oxidative addition reaction, relative to the energy of the reactants as 0.
Gas phase
Acetone
∆E
Acetonitrile
2
98K
298K
298K
∆
E
∆G
0.0
∆G
∆E
∆G
0.0
Ir(I) + CH3I
0.0
0.0
0.0
0.0
TS cis 1
147.8 189.8
147.7 192.0
158.6 201.3
157.9 205.4
158.7
157.9
201.5
205.5
80.7 (86 exp)
20.7
TS cis 2
TS trans
35.8
-33.8 22.9
-8.9 44.1
85.2
30.3
-38.8
-7.2
81.1 (87 exp) 29.9
Ir(III)-alkyl cis 1
Ir(III)-alkyl cis 2
Ir(III)-alkyl cis 3
Ir(III)-alkyl cis 4
21.3
45.8
29.7
43.4
13.7
-39.4
-7.2
45.8
-26.0 36.8
-11.9 38.7
-31.0
-11.5
-47.4
-31.4
-11.7
-48.1
29.3
43.2
Ir(III)-alkyl trans -40.5 19.6
13.0
The negative activation entropy values for all the reactions are indicative of associative
activation during the transition state. The high-pressure study, which was performed on
[
Rh(sacac)(cod)] complex with a variation of solvents is clearly illustrated in Figure 3. The
linear relationship between the experimentally obtained activation volume and q values
p
3
3,34
are indicative for reactions that obey the Kirkwood equation
, which predicts a linear
≠
≠
≠
relationship between activation volumes and solvent polarity (∆V
= ∆V
+∆V
(or
solv
exp
intr
2
3
≠
N∆u /r )q )). An extrapolation of this graph yields an intrinsic volume of activation, ∆V intr,
p
3
-1
of -21(3) cm mol . This result indicates that the same transition state exists between
[
Ir(sacac)(cod)] and CH I for the solvents used in this study and this value corresponds
3
3
-1
35
favourably with the -17 and -14.7 cm mol obtained for [IrCl(CO)(PPh ) ] and
3
2
2
5
[
Rh(sacac)(cod)]. The negative value for the intrinsic volume of activation is indicative of
associative activation during the transition state. The largest negative activation volumes
1
3

ACCEPTED MANUSCRIPT
were obtained for the more polar solvents, which suggest charge separation (formation of a
linear transition state) during the oxidative addition of the CH I to the metal complex.
3
3
.3 DFT study
The oxidative addition reaction of CH I to [Ir(cod)(sacac)] may theoretically lead to five
3
possible reaction products: one trans and four cis [Ir(cod)(sacac)(CH )(I)]-alkyl products. The
3
density functional theory (DFT) calculated optimized geometry and relative energies of the
five possible [Ir(cod)(sacac)(CH )(I)]-alkyl products are presented in Figure 4 and Table 3
3
respectively. The gas phase DFT results, in acetone or acetonitrile as solvent, all show that
trans [Ir(cod)(sacac)(CH )(I)]-alkyl has the lowest energy. This result is in agreement with the
3
experimental crystal structure obtained for [Ir(cod)(sacac)(CH )(I)] with methyl and iodide in
3
2
6
the trans position. Although the energy of [Ir(cod)(sacac)(CH )(I)]-alkyl cis 4 is within 10 kJ
3
−1
mol of that of trans [Ir(cod)(sacac)(CH )(I)]-alkyl, the energy needed for cis addition to
3
occur is too high (Table 3).Therefore trans addition is favoured and trans
[
Ir(cod)(sacac)(CH )(I)]-alkyl is the only expected reaction product, in agreement with
3
2
6
experimental observation.
Figure 4. DFT (PW91/TZP gas phase) optimized geometries of the five possible reaction
products of the [Ir(cod)(sacac)] + CH I reaction. The H-atoms on the cod ligand are omitted
3
for clarity.
During the oxidative addition step, the CH I can approach [Ir(cod)(sacac)] with the CH -I
3
3
bond approximately perpendicular (trans addition) or parallel (cis addition) to the plane
1
4

ACCEPTED MANUSCRIPT
through the sacac-backbone, Ir and cod. During cis addition, the CH I can be orientated with
3
the CH group on the side of the oxygen of sacac (cis 1 addition) or on the side nearer to
3
sulphur of sacac (cis 2 addition). In Figure 5 the optimized structures of the trans and two
possible cis transition states (TS) of CH I to [Ir(cod)(sacac)] are given with the activation
3
energies tabulated in Table 3. For all three possible TS the vibrational analysis showed only
one imaginary frequency (279i, 295i and 278i for TS cis 1, TS cis 2 and TS trans respectively,
obtained for PW91/TZP gas phase), which characterized unambiguously the stationary
points as true TS structures. The trans addition is energetically favoured by a large margin of
energy. The DFT calculated activation free energy given in Table 3 are in good agreement
-1
with the experimental activation free energy obtained, 86 − 88 kJ mol , depending on the
reaction solvent.
Figure 5. DFT (PW91/TZP gas phase) optimized geometries of the three possible transtion
states for the [Ir(cod)(sacac)] + CH I reaction. Selected bond lengths (Å) and angles (°) are
3
indicated. The H-atoms on the cod ligand are omitted for clarity.
The HOMO of the transtransition state geometry in Figure 6 show that trans addition begins
with the attack of two non-bonding electrons of the Ir d at the empty orbital of p
z2
z
1
5

ACCEPTED MANUSCRIPT
character on the CH group of CH I. A cationic five-coordinated trigonal pyramidal
3
3
intermediate is formed and the oxidation state of iridium changes from Ir(I) to Ir(III). This is
-
-
followed by the fast barrier less addition of a I ion to the vacant 2e site on Ir(III) leading to
-
-
-
-
an increase by 2e in the electron count (I is a 2e reagent) from the 16e complex to an
-
1
8e complex.
Figure 6. DFT calculated HOMO and LUMO of the optimized geometry of the linear
transition state of the [[Ir(cod)(sacac)] + CH I reaction oxidative addition reaction. The
3
attack of two non-bonding electrons of the Ir d of the HOMO at the empty LUMO of p
z2
z
character on the CH group of CH I is shown by an arrow. The MO plots use a contour of 60
3
3
3
e/nm . Colour code of atoms (online version): Ir (purple), I (magenta), C (black), N (yellow),
O (red), H (white).
4
.
Conclusion
Experimental and theoretical DFT results on the oxidative addition reaction of CH I to
3
[
[
Ir(cod)(sacac)] both showed that the reaction occurs in one step resulting in trans
Ir(cod)(sacac)(CH )(I)]-alkyl. A DFT analysis of the oxidative addition reaction shows that
3
the trans addition involves the attack of two non-bonding d .electrons on Ir at the empty p
z2
z
1
6

ACCEPTED MANUSCRIPT
orbital of the CH group of CH I with the formation of a cationic five-coordinated trigonal
3
3
-
-
pyramidal intermediate, followed by the fast addition of an I ion to the vacant 2e site on
Ir(III). The experimental negative activation entropy obtained, supports the associative
activation while the difference in volume of activation points to charge separation during
the transition state.
Acknowledgements
This work has received support from the Norwegian Supercomputing Program (NOTUR)
through a grant of computer time (Grant No. NN4654K) (JC), the South African National
Research Foundation and the Central Research Fund of the University of the Free State,
Bloemfontein, South Africa.
Supporting Information
The optimized coordinates of the DFT calculations.
1
7

ACCEPTED MANUSCRIPT
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Research highlights
•
•
Rate of oxidative addition showed little or no change with a solvent variation
Activation volumes were determined in four of the solvents, which varied between
3
-1
+
10.2(9) for chloroform to -18(1) cm mol for acetonitrile.
•
•
Solvent variation obeyed the Kirkwood equation with an intrinsic volume of activation of
3
-
21(3) cm .
DFT analysis of the oxidative addition reaction confirmed that the trans addition product
is energetically and more stable than the four possible cis products.