Small “Yaw” Angles, Large “Bite” Angles and an Electron-Rich Metal: Revealing a Stereoelectronic Synergy To Enhance Hydride-Transfer Activity

Article abstract of DOI:10.1002/chem.201702173

Cyclometalated complexes are an important class of (pre)catalysts in many reactions including hydride transfer. The ring size of such complexes could therefore be a relevant aspect to consider while modulating their catalytic activity. However, any correl

Full text of DOI:10.1002/chem.201702173

A Journal of
Accepted Article
Title: Small 'Yaw', Large 'Bite' and Electron-Rich Metal: Revealing a
Stereoelectronic Synergy to Enhance Hydride Transfer Activity
Authors: Joyanta Choudhury, Shrivats Semwal, Indulekha Mukkatt,
and Ranjeesh Thenarukandiyil
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To be cited as: Chem. Eur. J. 10.1002/chem.201702173
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Small ‘Yaw’, Large ‘Bite’ and Electron-Rich Metal: Revealing a
Stereoelectronic Synergy to Enhance Hydride Transfer Activity
Shrivats Semwal,[a] Indulekha Mukkatt,[a] Ranjeesh Thenarukandiyil,[a] and Joyanta Choudhury*[a]
Abstract: Cyclometalated complexes are an important class of
(pre)catalysts in many reactions including hydride transfer. Ring size
of such complexes could therefore be a relevant aspect to consider
while modulating their catalytic activity. However, any correlation
between the cyclometalating ring size and the catalytic activity
should be drawn by careful assessment of the pertinent geometrical
parameters, and overall electronic effects thereof. In this study, we
investigated the vital role of key stereoelectronic functions of two
classes of iridacyclic complexes
— five-membered and six-
membered, in manupulating the catalytic efficiency in a model
hydride transfer reaction. Our investigation revealed that there exists
an interesting multidimensional synergy among all the relevant
stereoelectronic factors — yaw angle, bite angle, ligand electronic,
as well as electronic of the metal center, to govern the hydride donor
ability (hydricity) of the complexes during catalysis. Thus the six-
membered chelate complexes having small yaw and large bite
angles, strong donor ligand, and electron-rich metal, were found to
be better catalysts than their five-membered analogues. A frontier
molecular orbital analysis supported the significant role of the above
stereoelectronic synergistic effect associated with the chelate ring to
control the hydride donor ability of the complexes.
Figure 1. Steric descriptors of ligands. (a) Cone angle is defined as the solid
angle formed with the metal at the vertex and the hydrogen atoms of the
ligand at the perimeter of the cone; (b) Bite angle is defined as the chelation
angle of a bidentate ligand formed at the metal center; (c) %Vburr of an NHC
ligand is defined as the percentage of a sphere of radius r occupied or ‘buried’
by the ligand when coordinated to a metal at the centre of the sphere with a
bond length d; the value of d as 2ꢀÅ is generally considered; (d) Yaw angle is
defined as the in-plane distortion measuring the difference in the two M-C-N
angles, and expressed as θ = ½ (α – β).
developed
a correlation methodology for evaluating the
stereoelectronic property of monodentate NHC ligands based on
13C NMR chemical shifts of the trans NHC (HEP, Huynh
electronic parameter) in square planar palladium(II)
complexes.[3d] Since chelate complexes are more stable and
hinders catalyst deactivation[3e] compared to non-chelated mono-
coordinated ones, we envisioned to delve into an investigation
on the influence of the combined steric and electronic factor of
NHC-based chelated complexes on their catalytic activity.
Hydride transfer reactions play a key role in several important
catalytic processes such as hydrogenation and transfer
hydrogenation, C–H functionalization, olefin isomerization, H2
generation and storage etc.[4] Therefore, an in-depth
understanding of the role of steric and electronic properties of
metal complexes in hydride transfer reactions would certainly be
critical for future improvement. Motivated by this justification,
transfer hydrogenation reaction[5] was selected as model hydride
transfer catalysis in our study employing NHC-based chelated
iridium catalysts. To the best of our knowledge, only electronic
role of monodentate NHC ligands on transfer hydrogenation
catalysis has been demonstrated previously.[6] Steric property of
chelated NHC complex can be related to the size of the chelate
ring, central metal ion and the wingtip groups (N-substituents of
the NHC). On keeping the metal same (iridium), two important
geometrical parameters — bite angle and yaw angle — are
associated with the size of the chelate ring. While bite angle
involves the ML2 plane, yaw distortion angle, as defined by
Crabtree,[7] is the ‘rotation of the NHC about the axis normal to
the azole plane’ (Figure 1d). It has been analyzed that the yaw
distortion has a steric origin contributed from conformational ring
strain within the metallacycle and complicated by additional
‘metal-ligand substituents’ steric interactions, compared to bite
angle which has been reported to influence steric and
electronics of the system.[8] Yaw distortion has been used
Introduction
In the field of metal-mediated molecular catalysis, knowledge
about structure of the metal-ligand complex is essential to
evaluate any influence of it on the catalytic activity and for
further tuning such effect. With regard to this, the steric and
electronic parameters of the ligands are generally considered as
the key to fine tune the property/activity of the resulting
complexes.[1] As simple substitution on ligand backbone can
change the electronic and steric property of a complex, both of
these parameters are intimately related and their influences are
difficult to separate in pure way. However, Tolman cone angle[2a]
(Figure 1a) and Tolman electronic parameter (TEP)[2b] are
generally used for deriving steric/electronic influence of
monodentate ligands in catalysis. Bite angle is used in case of
bidentate (phosphine) ligands (Figure 1b). %Vburr (percentage
burried volume)[2c] (Figure 1c) is specifically used to describe
steric bulk for N-heterocyclic carbene (NHC) ligands. Recently
Crabtree,[3a] Nolan,[3b] and Glorius[3c] correlated steric and
electronic parameters of NHC ligands with the corresponding
phosphine ligands to postulate better σ donacity of NHC,
although this was limited to monodentate NHCs only. Huynh
[a]
S. Semwal, I. Mukkatt, R. Thenarukandiyil, Dr. J. Choudhury
Organometallics & Smart Materials Laboratory
Department of Chemistry
Indian Institute of Science Education and Research Bhopal
Bhopal 462 066, India
E-mail: joyanta@iiserb.ac.in
Supporting information for this article is given via a link at the end of
the document.
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Figure 2. (a) Synthesis of five- (1-5) and six-membered (6-10) iridium(III)-NHC cyclometalated complexes. (b) Single crystal X-ray structures of 1-5 and 7-10.
Hydrogen atoms and solvents were not shown for clarity. Crystal structure of 6 was reported earlier[10]
.
Table 1. Structural, NMR and Redox Parameters of Complexes 1-10.
Complex
Bite angle
(°)[a]
Yaw angle (°)[a]
δ 13C (ppm)
CNHC, Caryl
E1/2, IrIII/IV
[b]
(mV vs Fc0/+1
)
1
2
77.40(14)
76.95(18)
77.62(12)
77.40(3)
77.20(3)
85.91(4)
85.8(3)
9.66
8.42
9.54
10.09
9.82
3.13
3.25
2.52
3.00
2.52
164.0, 146.2
164.5, 146.0
166.1, 151.6
162.6, 157.5
163.4, 144.0
154.7, 141.2
154.7, 147.4
153.9, 147.0
158.0, 154.4
154.4, 147.5
186.5
207.5
316.5
148.5
141.5
166.5
185.5
325.5
175.5
145.5
3
4
5
6
7
8
85.07(4)
85.3(3)
9
10
86.4(2)
[a] Yaw and bite angles were measured from the X-ray crystal structures of the complexes. [b] E1/2
values were measured by differential pulse voltammetry (DPV). See supporting information for
details.
previously to describe steric effect for bis-carbene complexes.[9]
hydrogenation reaction. Thus the six-membered chelate
complexes satisfying the synergistic influence of small yaw and
large bite angles, strong donor ligand, and electron-rich metal,
were found to be more active catalysts than their five-membered
analogues without these criteria fulfilled. A frontier molecular
orbital analysis was performed, validating the significant role of
Using a series of NHC-based cyclometalated iridium complexes,
this study demonstrates that the ring size of the metallachelate
plays a vital role to transmit steric effect (through variable yaw
and bite angles) and electronic effect (through overall geometric
disposition of the ligands around the metal center) in cooperative
manner and thereby to influence the catalytic activity in transfer
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the above stereoelectronic synergy associated with the chelate
ring to control the hydride donor ability of the complexes.
Results and Discussion
Two sets of NHC-cyclometalated iridium complexes were
synthesized from the corresponding ligand precursors via
Cs2CO3 method as depicted in Figure 2a (for details, see
Supporting Information). Complexes 1-5 consisted of five-
membered iridacycle with electronic perturbation at the N-
substituent (1 vs 2) or at the cyclometalated aryl ring (1 vs 3, 4,
5). On the other hand, complexes 6-10 were designed to
represent six-membered iridacyclic congeners of complexes 1-5
respectively. All the complexes 1-10 were characterized fully by
1H and 13C{1H} NMR spectroscopy, electron-spray ionization
mass spectrometry (ESI-MS), and single-crystal X-ray diffraction
analyses[10] (Figure 2b, for details, see Supporting Information).
It was interesting to note that the five-membered cyclometalated
complexes 1-5 exhibited bite angle values in the range of 76-77°
which is correlated to piano-stool geometry. The bite angle of
six-membered congeners 6-10 was found to be larger (~85°)
and more closer to 90° which is related to octahedral geometry
(Table 1). Yaw distortion angle at the NHC ligand was found to
be higher for five-membered metallacycles (9.2±0.8°) in
complexes 1-5 compared to their six-membered counterparts
(2.8°±0.3) in complexes 6-10 (Table 1). These values indicated
that in-plane tilting of NHC-ligand within metallachelate 6-10 was
much less than that in 1-5. The yaw angle at the aryl ligand of
the cyclometalated ring has the similar trend as that of the NHC
ligand. This fact probably signifies relatively less conformational
strain in the larger ring-size complexes, and an increase of yaw
distortion in smaller chelate rings (with smaller bite angle) is a
result of steric necessity. It is noteworthy to mention here that
the yaw distortion was not found to have any significant impact
to the donor property of the ligand as suggested by TEP study
reported previously by Crabtree et al.[7] 13C{1H} NMR chemical
shift of the metal-bound CNHC resonance was about 164±2 ppm
in complexes 1-5 and 156±2 ppm in complexes 6-10 (Table 1),
suggesting relatively more electron-rich CNHC in the six-
membered rings than in the five-membered congeners. This
difference may be ascribed to the variable electronic effect of the
NHC ligands in the two types of complexes, due to the presence
(in the case of complexes 1-5) or absence (in complexes 6-10)
of electronic delocalization of the N-lone pair with the pendant
aryl ring, A comparison of the electronically originated 13C{1H}
NMR chemical shift of CNHC with geometrically originated yaw
and bite angle indicated a ‘similar trend’ and ‘opposite trend’
respectively on moving from five- to six-membered
cyclometalated complexes (Figure 3).
Figure 3. Variation of yaw angle, bite angle, and 13C{1H} NMR chemical shift
of CNHC of complexes 1-10.
(c)
100
(d)
100
10
6
10
6
9
9
80
60
40
20
7
7
75
50
25
5
5
2
4
1
4
8
8
1
2
3
3
2
3
4
5
6
7
8
9 10
75
78
81
84
87
Yaw angle (deg)
Bite angle (deg)
(e)
(f)
100
80
60
40
20
10
6
10
100
80
60
40
20
9
6
9
7
7
8
8
5
2
1
5
4
4
1
2
3
3
153 156 159 162 165
13
0.56 0.60 0.64 0.68 0.72 0.76
(V vs SCE)

CNHC (ppm)
E
1/2
Figure 4. (a) Schematic of a model transfer hydrogenation catalysis. (b) A
representative time course of catalytic transfer hydrogenation catalyzed by 1
and 6. Trend of catalytic activity of the complexes 1-10 as a function of (c) yaw
angle, (d) bite angle, (e) 13C{1H} NMR chemical shift of CNHC, and (d) E1/2
values of Ir3+/4+ redox couple.
Next, the influences of variable yaw angle, bite angle, and
electronic at CNHC within the complexes 1-5 and 6-10 were
examined in transfer hydrogenation activity. The reaction of
acetophenone with 2-propanol in the presence of KOH at 100 °C
was performed using 1 mol% of complexes 1-10 (Figure 4a). A
representative time versus %yield plot of the catalytic reactions
performed with 1 and 6 showed that the reaction rate was much
higher with 6 than that with 1 (Figure 4b).[11] For relative
comparison, all the reactions were run for 90 minutes. The
results were plotted in Figures 4c-e. From Figure 4c, it was
indicated that the six-membered cyclometalated complexes 6-10
having low yaw distortion are catalytically more active than their
five-membered congeners which are more distorted in terms of
yaw angle. However, in spite of experiencing less in-plane yaw
distortion, complex 8 was only poorly active. Since bite angle
was in ‘opposite relationship’ with yaw distortion, it was expected
that complexes 6-10 having larger bite angle should have higher
activity than complexes 1-5 having smaller bite angle, which was
also evidenced from Figure 4d. Here also, complex 8 was found
to deviate by not obeying the correlation. Recent theoretical
studies showed that more flexible or non-linear geometry
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translated into lower barrier for oxidative addition of bonds to
catalysis (Figure 5). For NHC based Cp*Ir catalytic systems,
inner-sphere MPV (Meerwein–Ponndorf–Verley)-type concerted
mechanism has been reported earlier,[14] but this type of
mechanism is possible on generating additional vacant
coordination site at the metal center by decoordination of the
cyclometalated/bidentate NHC wingtip. However, via 1H NMR
spectroscopic analysis, we did not observe any decoordination
(or ring-opening) of the cyclometalated ring of complex 7 when it
was refluxed in isopropanol for 1 h (see Supporting Information).
Moreover, no loss of Cp* ligand to create vacant site near the
metal centre was observed as reported by Crabtree.[15] This fact
suggested that a concerted MPV-type inner-sphere mechanism
is not applicable here, and hence the outer-sphere pathway was
proposed. In the proposed mechanistic cycle, acetophenone is
activated via hydrogen bonding between oxygen atom of ketone
and proton from solvent isopropanol. Activation of carbonyl
group by proton or hydrogen bond donor is well known in
literature.[16] This type of hydrogen bonding interaction might
reduce acetophenone LUMO energy thus rendering hydride
addition easier. The hydride transfer from the in situ generated
iridium-hydride to acetophenone is believed to be the rate
determining step as the complexes having electron donating
group substituent at the NHC ligand (for example, complexes 5
and 10) provided more active than the complexes with non-
substituted (1 and 6) or electron-withdrawing group substituted
NHC ligands (3 and 8). It may be due to fact that the electron-
donating group would stabilize the positive charge at the metal
center generated after hydride donation. Furthermore, to
substantiate the above proposal, Hammett study was performed
on para-substituted acetophenone substrates by varying the
substituent from electron-donating to electron-withdrawing in
nature. The relative order of reactivity for acetophenone was
found to be para-CH3O (kx/kH= 0.68) <para-H (1) <para-Br (1.20)
<para-NO2 (2.36). The dependency of the relative rate of
reaction on the σ value of the para-substitutent of acetophenone
suggested that the hydride transfer from the metal-hydride to the
acetophenone substrate is rate-determining.[17] The Hammett
plot resulted in a ρ (reaction constant) value of +1.25 (Figure 6).
The positive ρ value can be interpreted as building up of
negative charge on the ketone substrate in the transition state
during the rate-determining step, which is better stabilized by the
electron-withdrawing substituents. The magnitude of +1.25
indicated a moderately anionic nature of the transition state,
which might be due to outer-sphere hydride-transfer mechanism.
complexes of type d10-ML2 where
L is bulky phosphine
ligands.[12] In line with this, it is noteworthy that the orientation of
the CNHC with respect to phenyl ring could also affect the activity
of complexes because it could alter the accessibility and the
steric environment around the iridium center. Apart from the
above geometrical factors, we believe that the activity should
also be affected by the electron density at the CNHC of the
metallacycles. It was indeed observed that the six-membered
cyclometalated complexes having electronically rich CNHC were
catalytically more active than their five-membered counterparts,
with an exception of complex 8 (Figure 4e).
The above facts suggested that the small yaw and large bite
angles enhanced the catalytic activity of the complexes.
Additionally, more donating NHC ligand favored. However, this
was not general because complex 8 was proved to be only
poorly active although it had small yaw and large bite angle, and
also more donating NHC. Therefore, at this point, it was
reasonable to argue that these requisite properties/features
played a conjugative role along with some other factor. To
investigate this aspect, we considered the overall electronic
perturbation on the metal center and measured its redox
potential, E1/2, which should be the result of all operating effects
including overall geometry of the complexes (planar five-
membered rings in 1-5 versus bent six-membered rings in 6-10),
spectator Cp* ligand, yaw and bite angles, as well as the nature
of the N-substituents. Thus the Ir3+/4+ redox potentials were
measured for all the complexes by differential pulse voltammetry
(DPV) (Table 1). For the set of five-membered cyclometallated
complexes 1-5, the iridium center was found to be the most
electron-rich in complexes 5 and 4, moderate in 1 and 2, and the
least in 3 (E1/2 values in the order of 5<4<1<2<3). Similar trend
was also observed for the six-membered family (E1/2 values in
the order of 10<9≈6<7<8). This impact of variable electronic
perturbation at the metal center on complexes 1-10 in catalysis
was examined as shown in Figure 4f. It was indicated that the
complexes having electron-rich metal center were more active
than the electron-poor ones resulting the relative activity order of
5>4>1>2>3 and 10>9≈6>7>8. Moreover, catalytic activity
increased vastly when ring size of the complexes increased from
five-membered to six-membered. This result was thus
complimentary to the above findings, and showed that not only
small yaw angle, large bite angle and more donor CNHC were
required, but also for better activity of the complexes, the metal
center should be more electron-rich. Because complex 8 did not
fulfil this last criterion despite having small yaw and large bite, it
was only poorly active. Similarly, although complexes 1, 2, 4,
and 5 had more electron-rich metal center, they were poorly
active catalyst because they could not fulfil the criteria of small
yaw and large bite angle imposed earlier. Therefore, it was
evident that the overall stereoelectronic effect of chelating ring
size of the complexes was synergistically transmitted through all
plausible dimensions, namely yaw distortion, bite angle, ligand
electronic, and effective electron density at the metal center.
This synergistic effect in turn played a profound role in governing
the overall catalytic activity of the complexes. The plausible
reason for enhanced activity of the conformationally less-
strained (with small yaw and large bite angles) and
electronically-rich complexes might be a relatively easy hydride
transfer from the in situ generated iridium-hydride species to the
ketone substrate probably via outer-sphere mechanism[13] during
[17]
As the rate-limiting step of the catalytic transfer
hydrogenation reactions is the hydride transfer from the iridium-
hydride species to the acetophenone substrate, the hydride
donor ability or hydricity of the iridium-hydride species can be
the key. The hydricity of the ‘Cp*Ir(CNHC-Caryl)H’ species, which
may be assumed as the structural analogues of the
corresponding iodide complexes 1-10, can be governed by the
energy of the lowest unoccupied molecular orbital (LUMO) of the
corresponding
hypothetical
16-electron
five-coordinate
‘[Cp*Ir(CNHC-Caryl)]+’ fragment.[18,4] According to the MO theory,
this LUMO will receive electrons from the hydride ligand during
Ir–H bond formation, and therefore the stability of the resulting
hydride complex can be attributed to the energy of this LUMO.
Thus, a lower-energy stabilized LUMO would lead to stronger
metal-hydride bond and less hydride donor ability, while an
increased LUMO energy would result in less stable hydride
complex making it a better hydride donor. A frontier orbital
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Figure 5. Plausible catalytic cycle.
(a)
(b)
0.9
0.6
0.3
0.0
-0.3
20
p-NO2
15
4-OMe
H
4-NO2
10
p-H
p-Br
5
0
 = + 1.25
4-Br
p-OMe
0
3
6
9
12 15
-0.2 0.0 0.2 0.4 0.6 0.8
Time (min)

Figure 6. (a) Rate plots of transfer hydrogenation of para-substituted (p-OMe,
p-H, p-NO2, and p-Br) acetophenones. (b) Hammett plot. For details, see
Supporting Information.
analysis showed that the LUMO of the ‘[Cp*Ir(CNHC-Caryl)]+’
fragment derived from complexes 1-10 consisted of significant
dxz character (Figures 7a-b).[19] However, the geometries of this
LUMO were significantly different in these two cases. Bending of
the Cp* and Ir(CNHC-Caryl) planes resulted in pyramidal distortion
in the case of six-membered complexes 6-10 as compared to
the linear arrangement of these planes in the case of five-
membered analogues 1-5. Large bite and small yaw angles
favored this pyramidal distortion. Eventually, this affected the
energies of the frontier orbitals. Energies of a few closely-lying
HOMOs (HOMO to HOMO−3) and the LUMO of the
‘[Cp*Ir(CNHC-Caryl)]+’ fragments generated from 1 and 6, for
example, are shown in Figure 7c. It is clearly evident that the
LUMO energies in case of 6-10 (except 8 again) are higher than
that for 1-5 (Figure 7d). As a result, the hydricity of the hydride
complexes of 6, 7, 9 and 10 is higher making them more active
in the hydride-transfer reactions. This is in good agreement with
the experimental results discussed earlier. It is to be noted that
hydride transfer mechanism was preferably proposed in these
iridium hydride species over an alternative electron/H-atom
transfer mechanism, as the calculated thermodynamic hydricity
(the free energy required to remove a hydride anion), ΔGH−
values of the third-row transition metal hydrides are generally
found to be the lowest, making them most eager to transfer H−.[4]
(c)
(d)
-0.20
-5.6
9
7
10
LUMO
6
-0.24 LUMO
-0.28
-5.8
-6.0
-6.2
-6.4
4
2
5
8
1
-0.32
-0.36
1
3
6
Complex
Figure 7. Frontier molecular orbital analyses of hypothetical 16-electron five-
coordinate ‘[Cp*Ir(CNHC-Caryl)]+’ fragments related to (a) five-membered chelate
complexes 1-5, and (b) six-membered chelate complexes 6-10 along with the
representative LUMO pictures of 1 and 6. (c) Energies of a few HOMOs
(HOMO to HOMO−3) and the LUMO of the ‘[Cp*Ir(CNHC-Caryl)]+’ fragments
generated from 1 and 6. (d) Plot showing energy of the LUMO of ‘[Cp*Ir(CNHC
Caryl)]+’ fragment derived from 1-10.
-
Conclusions
This investigative study demonstrated a hypothesis that catalytic
activity of metal complexes can be deeply associated with
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e) S. Díez-González, S. P. Nolan, Coord. Chem. Rev., 2007, 251, 874–
883.
exquisite control exerted from both steric and electronic
functions instead of a single dominant factor. The same was
illustrated with a set of five- and six-membered iridacyclic
[2]
[3]
a) C. A. Tolman, J. Am. Chem. Soc., 1970, 92, 2956‒2965; b) C. A.
Tolman, J. Am. Chem. Soc., 1970, 92 , 2953‒2956; c) H. Clavier, S. P.
Nolan. Chem. Commun., 2010, 46, 841‒861.
complexes, [Cp*Ir(CNHC-Caryl)I], applied in
a model hydride
transfer catalysis. The results established that the
multidimensional stereoelectronic synergy among all the
relevant factors — yaw angle, bite angle, ligand electronic, as
well as electronic of the metal center, governed the hydride
donor ability (hydricity) of the complexes during catalysis. In
effect, the six-membered chelate complexes satisfying all the
criteria of having small yaw and large bite angles, strong donor
ligand, and electron-rich metal, were catalytically more active
than their five-membered analogues which lack those properties.
A frontier molecular orbital analysis validated the significant role
of the above stereoelectronic synergistic effect associated with
the chelate ring to increase the LUMO energy of the hypothetical
‘[Cp*Ir(CNHC-Caryl)]+’ fragment derived from the six-membered
complexes, in comparison to their five-membered congeners,
and thereby enhance the hydride donor ability of the former
complexes during catalysis.
a) A. R. Chianese, X. Li, M. C. Janzen, J. W. Faller, R. H. Crabtree,
Organometallics, 2003, 22, 1663‒1667; b) R. Dorta, E. D. Stevens, N.
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Lehmann, F. Glorius, J. Am. Chem. Soc., 2004, 126, 15195‒15201; d)
Q. Teng, H. V. Huynh, Dalton Trans., 2017, 46, 614–627; e) R. H.
Crabtree, Chem. Rev., 2015, 115, 127-150.
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Experimental Section
Electrochemical analysis of the complexes
Cyclic voltammetry of complexes (1-10) was carried out by three
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counter electrode: a Pt wire; reference electrode: saturated calomel
electrode, SCE. Sample was prepared in dry deoxygenated acetonitrile.
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electrolyte. Ferrocene (E1/2, Fc/Fc+ = 0.44 volts vs. SCE) was used as an
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General procedure for transfer hydrogenation of ketone
Acetophenone (1 mmol, 120.2 mg), catalyst (0.01 mmol), potassium
hydroxide (0.2 mmol, 11.2 mg), and 2-propanol (5 mL) were added in a
flask and stirred at 100°C. Aliquots were withdrawn at different time
intervals to calculate the conversion by 1H NMR spectroscopic analysis in
CDCl3. The conversions were based on the average of three runs. The
characteristic doublet peak from the product 1-phenylethanol at 1.5 ppm
and the characteristic singlet peak from acetophenone at 2.6 ppm were
integrated to determine the yield.
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Acknowledgements
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(hydricity) of [HM(diphosphine)2]+ (M= Ni, Pd, Pt) complexes, see: a) M.
R. Nimlos, C. H. Chang, C. J. Curtis, A. Miedaner, H. M. Pilath, D. L.
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Generous financial assistance from IISER Bhopal, doctoral
fellowships from the UGC (to S.S. and R.T.) and BS-MS
INSPIRE fellowship from the DST (to I.M.) are gratefully
acknowledged.
[19] For molecular orbital diagram of similar 16-electron, five-coordinate
[Cp*Ru(NN)]+ (LL = NN or PP donor) fragments, see: C. Gemel, V. N.
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Keywords: Hydride transfer, iridium, N-heterocyclic carbene,
hydricity, yaw angle, bite angle, steric.
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10.1002/chem.201702173
Chemistry - A European Journal
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Entry for the Table of Contents
FULL PAPER
In this study, we disclosed a
Shrivats Semwal, Indulekha Mukkatt,
Ranjeesh Thenarukandiyil, and
Joyanta Choudhury*
multidimensional ‘stereo-electronic
synergy’ in metal complexes that
controlled hydride transfer catalysis.
Page No. – Page No.
Small ‘Yaw’, Large ‘Bite’ and
Electron-Rich Metal: Revealing a
Stereoelectronic Synergy to
Enhance Hydride Transfer Activity
This article is protected by copyright. All rights reserved.