Hydrogenation of alkenes and alkynes catalysed by N?O (imino)phenol palladium(II) complexes: Structural, kinetics and chemoselectivity studies

Article abstract of DOI:10.1016/j.jorganchem.2018.07.034

Reactions of 2-(2-methoxyethylimino)ethyl)phenol (HL1), 2-(2-hydroxyethylimino)ethyl)phenol (HL2), 2-(2-aminoethylimino)ethyl)phenol (HL3) and 2-(2-hydroxyethylimino)methyl)phenol (HL4) with [PdCl₂(COD)] afforded the neutral palladium complexes [PdCl₂(HL1)] (1), [PdCl₂(HL2)] (2), [PdCl₂(HL3)] (3), [PdCl₂(HL4)] (4) respectively. Treatment of complex 1 with PPh₃ gave the cationic complex [Pd(HL1)ClPPh₃]⁺ (5), while reactions of 4 with Pd(OAc)₂, in the presence of PPh₃ and p-TsOH produced the corresponding palladium complex, [Pd(L1) (OTs) (PPh₃)] (6). The molecular structure of 4a (derivative of 4) contained two bidentate anionic ligands (L4). Complexes 1–6 formed active catalysts in hydrogenation of alkenes and alkynes, in which the catalytic activities were largely dependent on the pendant donor atom of the ligand motif. Isomerization reactions were dominant in terminal alkenes hydrogenation reactions, while hydrogenation of alkynes to alkanes occurred in two steps via alkene intermediates. Kinetics data were consistent with homogeneous active species. Density functional theory studies supported the hemi-labile nature of the ligands, and offered insights into the catalytic activity trends observed.

Full text of DOI:10.1016/j.jorganchem.2018.07.034

Accepted Manuscript
Hydrogenation of alkenes and alkynes catalysed by N^O (imino)phenol palladium(II)
complexes: Structural, kinetics and chemoselectivity studies
Thandeka A. Tshabalala, Stephen O. Ojwach
PII:
S0022-328X(18)30366-8
10.1016/j.jorganchem.2018.07.034
JOM 20521
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 19 May 2018
Revised Date: 20 July 2018
Accepted Date: 29 July 2018
Please cite this article as: T.A. Tshabalala, S.O. Ojwach, Hydrogenation of alkenes and alkynes
catalysed by N^O (imino)phenol palladium(II) complexes: Structural, kinetics and chemoselectivity
studies, Journal of Organometallic Chemistry (2018), doi: 10.1016/j.jorganchem.2018.07.034.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Hydrogenation of alkenes and alkynes catalysed by N^O (imino)phenol palladium(II)
complexes: structural, kinetics and chemoselectivity studies
Thandeka A. Tshabalala and Stephen O. Ojwach*
School of Chemistry and Physics, Pietermaritzburg Campus, University of KwaZulu-Natal,
Private Bag X01, Scottsville 3209, South Africa
Abstract
Reactions
of
2-(2-methoxyethylimino)ethyl)phenol
(HL1),
2-(2-
hydroxyethylimino)ethyl)phenol (HL2), 2-(2-aminoethylimino)ethyl)phenol (HL3) and 2-(2-
hydroxyethylimino)methyl)phenol (HL4) with [PdCl2(COD)] afforded the neutral palladium
complexes [PdCl2(HL1)] (1), [PdCl2(HL2)] (2), [PdCl2(HL3)] (3), [PdCl2(HL4)] (4)
respectively. Treatment of complex 1 with PPh3 gave the cationic complex
[Pd(HL1)ClPPh3]+ (5), while reactions of 4 with Pd(OAc)2, in the presence of PPh3 and p-
TsOH produced the corresponding palladium complex, [Pd(L1)(OTs)(PPh3)] (6). The
molecular structure of 4a (derivative of 4) contained two bidentate anionic ligands (L4).
Complexes 1-6 formed active catalysts in hydrogenation of alkenes and alkynes, in which the
catalytic activities were largely dependent on the pendant donor atom of the ligand motif.
Isomerization reactions were dominant in terminal alkenes hydrogenation reactions, while
hydrogenation of alkynes to alkanes occurred in two steps via alkene intermediates. Kinetics
data were consistent with homogeneous active species. Density functional theory studies
supported the hemi-labile nature of the ligands, and offered insights into the catalytic activity
trends observed.
Keywords: Hydrogenation, palladium, syntheses, alkene, kinetics
*Corresponding author: Tel.: +27 (33) 260 5239; Fax: +27 (33) 260 5009
1

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Email: ojwach@ukzn.ac.za (SO Ojwach)
Introduction
Transition metal catalysed hydrogenation reactions are among the most valuable
organic, widely used in the conversion of unsaturated systems to a range of useful domestic
and industrial feedstock’s.1,2 Catalysts in these reactions have been derived mainly from
platinum3, ruthenium4, chromium5, rhodium6 and iron7 complexes. Among these systems,
palladium catalysts are currently receiving much attention due to their superior catalytic
activities and selectivity.8 To date, a number of palladium catalysts containing varied ligand
architectures have been reported to show variable catalytic activities and selectivity in
homogenous hydrogenation of alkenes and alkynes. Established examples include
palladium(II) complexes derived from bidentate phosphine9 and nitrogen-donor10 catalysts.
Even though palladium(II) catalysts of N^N and P^P-donor ligands show appreciable
catalytic activities, improved catalyst robustness have been witnessed with complexes
containing hybrid ligands such as P^N11, N^S12 and N^O donors.13
Another approach that is currently gaining momentum in the design of active and
stable catalyst systems are the hemilabile ligands, first introduced by Jeffrey and
Rauchfuss.14 The hemi-labile nature of the ligands in these catalysts have been demonstrated
in a number of reports of palladium(II) catalysts bearing P^N^O15, P^N^S16, N^N^S17;
S^O^S.18 In one such study by Bacchi et al.,16 palladium(II) complexes of hemilabile
potential P^N^S ligands were found to be inactive due to the strong coordination ability of
the pendant S-donor atom, thus limiting substrate coordination, i.e adopt tridentate
coordination upon activation. On the other hand, Yilmas et al.18 employed palladium(II)
complexes of S^O^S and S^O^O donor ligands, which display improved catalytic activities.
The higher catalytic activities of S^O^S and S^O^O systems has been assigned to the weakly
coordinating ability of the O donor atom compared to the S atom.18
2

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We recently reported the use of palladium(II) complexes supported by
(pyridyl)benzoazole19 and (pyrazolylmethyl)pyridine20 ligands to give active catalysts in the
molecular hydrogenation of alkenes and alkynes. However, these systems suffered from
catalyst decomposition to form palladium nanoparticles, and partly behave like
heterogeneous systems. In attempt to design relatively stable and more active palladium(II)
catalysts, we have designed potential hemilabile ligand systems based on hybrid N^O^O,
N^N^O donor ligands. Indeed, the complexes showed significant improvement in terms of
stability and activity in molecular hydrogenation of alkenes and alkynes. In this contribution,
we report the synthesis of palladium(II) complexes of (imino)phenol ligands containing
pendant arms and their applications as catalysts in molecular hydrogenation of alkenes and
alkynes. Detailed kinetics, chemo-selectivity and theoretical studies have been performed and
are herein discussed.
1.
Experimental section
1.1.
Material, instrumentation and methods
All moisture and air sensitive reactions were performed using standard Schlenk line
techniques. Methanol (ACS reagent, ≥99.8%), toluene (ACS reagent, ≥99.5%), diethyl ether
(ACS reagent, ≥98%), dichloromethane (ACS reagent, ≥99.8%), absolute ethanol (ACS
reagent, ≥98%), DMSO-d6 (99.8%), were purchased from Merck. Chloroform-CDCl3 (98%),
p-TsOH (ACS reagent, ≥98.5%), and PPh3 (Reagent Plus®, 99%) were obtained from
Sigma–Aldrich. Solvents were dried and distilled under nitrogen atmosphere in the presence
of suitable drying agents: diethyl ether, toluene and acetone were dried over sodium
wire/benzophenone, methanol and absolute ethanol over calcium oxide, and dichloromethane
over phosphorus pentoxide. The chemicals, 1-(2-hydroxyphenyl)ethanone (99%), 2-
methoxyethanamine (98%), 2-aminoethanol (≥98%), ethane-1,2-diamine(Reagent Plus®,
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≥99%), 2-hydroxybenzaldehyde (reagent grade, 98%) were purchased from Sigma–Aldrich
and were used without further purification. The starting material [PdCl2(COD)] was
synthesized following a literature method.21 Nuclear magnetic resonance spectra were
1
13
31
acquired at 400 MHz for H, 100 MHz for C and 162 MHz for P on a Bruker Avance
spectrometer equipped with Bruker magnet (9.395 T). All coupling constants (J) are
measured in Hertz, Hz. Mass spectral analyses were acquired on a micromass LC premier
micro-mass spectrometer and elemental analyses were performed on a Thermal Scientific
Flash 2000. The infrared spectra were recorded on a Perkin-Elmer spectrum 100 in the 4000-
650 cm−1 range. X-ray data were recorded on a Bruker Apex Duo diffractometer equipped
with an Oxford Instruments.
1.2.
Synthesis of (ethylimino)ethyl)phenol amine ligands and palladium(II) complexes
1.2.1. 2-(2-methoxyethylimino)ethyl)phenol (HL1)
To a solution of 1-(2-hydroxyphenyl)ethanone (1.50 g, 11.00 mmol) in ethanol (50
mL) was added 2-methoxyethanamine (0.83 g, 11.00 mmol), and catalytic amount of p-
TsOH (85 mg). The reaction mixture was refluxed for 24 h at 60 °C and ethanol was removed
after the reaction period under reduced pressure to give L1 as a brown oil. Yield = 1.93 g
1
(91%). H NMR (400 MHz, CDCl3): δH (ppm): 2.41 (s, 3H, CH3); 3.45 (s, 3H, OCH3); 3.78
(t, 3JHH = 4.0 Hz, 2H, CH2-N); 3.82 (t, 3JHH = 4.0 Hz, 2H, CH2-O); 6.81 (t, 3JHH = 8.0 Hz, 1H,
3
H-Ph); 6.99 (d, 3JHH = 8.0 Hz, 1H, H-Ph); 7.34 (t, 3JHH = 12.0 Hz, 1H, H-Ph); 7.55 (d, JHH
=
8.0 Hz, 1H, H-Ph). 13C NMR (CDCl3): δC(ppm): 14.74 (CH3); 26.62 (OCH3); 51.42 (CH2-
N); 62.23 (CH2-O); 116.98 (Ph); 119.03 (Ph); 128.17 (Ph); 130.71 (Ph); 132.82 (Ph); 161.24
(Ph-OH); 164.47 (C=N). MS (ESI) m/z (%) 194 (M+, 100). HRMS-ESI ([M++H+]): m/z calc:
193.1236; found: 194.0478. FT-IR (cm-1): υ(OH) = 3157; υ(C=N) =1613.
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Compounds HL2 − HL4 were prepared following the same procedure described for
compound HL1.
1.2.2. 2-(2-hydroxyethylimino)ethyl)phenol (HL2)
Compound HL2 was synthesized from 1-(2-hydroxyphenyl)ethanone (1.50 g, 11.00
1
mmol) and 2-aminoethanol (0.67 g, 11.00 mmol). Brown solid. Yield = 1.80 g (92%). H
3
NMR (400 MHz, CDCl3): δH (ppm): 2.39 (s, 3H, CH3); 3.76 (t, JHH = 5.4 Hz, 2H, CH2-N);
3
3
3
4.02 (t, JHH = 5.3 Hz, 2H, CH2-OH); 6.80 (t, JHH = 16.0 Hz, 1H, H-Ph); 6.94 (d, JHH = 8.2
3
3
13
Hz, 1H, H-Ph); 7.32 (t, JHH = 15.5Hz, 1H, H-Ph); 7.54 (d, JHH = 8.0 Hz, 1H, H-Ph). C
NMR (CDCl3): δC (ppm): 18.04 (CH3); 40.99 (CH2-N); 62.59 (CH2-N); 75.37 (Ph); 93.25
(Ph); 128.06 (Ph); 149.60 (Ph); 151.02 (Ph); 162.35 (Ph-OH); 164.45 (C=N). MS (ESI) m/z
(%) 180 (M+, 100). HRMS-ESI ([M++H+]): m/z calc: 179.1102; found: 180.0145. FT-IR (cm-
1): υ(OHph) = 3158; υ(C=N) =1603.
1.2.3. 2-(2-aminoethylimino)ethyl)phenol (HL3)
Ethane-1,2-diamine (0.66 g, 11.00 mmol) and1-(2-hydroxyphenyl)ethanone (1.50 g,
11.00 mmol) . Yellow solid. Yield =1.77 g (90%). 1H NMR (400 MHz, CDCl3): δH (ppm):
2.41 (s, 3H, CH3); 4.02 (dd, 3JHH = 5.9 Hz, 4H, CH2-CH2); 6.82 (t, 3JHH = 16.3 Hz, 1H, H-Ph);
6.93 (d, 3JHH = 8.3 Hz, 1H, H-Ph); 7.32 (t, 3JHH = 17.0 Hz, 1H, H-Ph); 7.55 (d, 3JHH = 9.4 Hz,
1H, Hg). 13C NMR (CDCl3): δC (ppm): 16.70 (CH3); 43.61 (CH2-N); 51.05 (CH2-O); 116.04
(Ph); 121.54 (Ph); 124.91 (Ph); 130.69 (Ph); 132.57 (Ph); 161.14 (Ph-OH); 164.68 (C=N).
MS (ESI) m/z (%) 179 (M+, 100). HRMS-ESI ([M++H+]): m/z calc: 178.10427; found:
179.0047. FT-IR (cm-1): υ(OHph) = 3055; υ(C=N) =1607.
2.2.4 2-(2-hydroxyethylimino)methyl)phenol (HL4)
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Ligand HL4 was prepared using 2-hydroxybenzaldehyde (1.50 g, 12.30 mmol) and 2-
1
aminoethanol (0.75 g, 12.30mmol). Brown oil. Yield = 1.81 g (88%). H NMR (400 MHz,
3
CDCl3): δH (ppm): 3.73 (t, JHH = 4.0 Hz, 2H, CH2-N); 3.89 (t, 3JHH = 4.0 Hz, 2H, CH2-OH);
6.84 (t, 3JHH = 12.0 Hz, 1H, H-Ph); 6.95 (d, 3JHH = 8.0 Hz, 1H, H-Ph); 7.25 (d, 3JHH = 8.0 Hz,
3
13
1H, H-Ph); 7.32 (t, JHH = 12.0 Hz, 1H, H-Ph); 8.36 (s, 1H, H-C=N). C NMR (CDCl3): δC
(ppm): 43.55 (CH2-N); 61.90 (CH2-O); 117.15 (Ph); 118.60 (Ph); 121.25 (Ph); 131.49 (Ph);
132.52 (Ph); 161.47 (Ph-OH); 166.83 (C=N). MS (ESI) m/z (%) 166 (M+, 100). HRMS-ESI
([M++H+]): m/z calc: 165.2013; found: 166.0102. FT-IR (cm-1): υ(OHph) = 3321; υ(C=N) =1631.
2.2.5 [2-(2-methoxyethylimino)ethyl)phenol PdCl2] (1)
To a solution of HL1 (0.06 g, 0.31 mmol) in CH2Cl2 (10 mL) was added a solution of
Pd(COD)Cl2 (0.10 g, 0.31 mmol) in CH2Cl2 (10 mL) and the mixture was stirred for 24 h to
give a yellow precipitate. The crude product was filtered and recrystallized from CH2Cl2-
hexane mixture to afford complex 1 as an analytically pure light yellow solid. Yield = 0.12 g
(68%). 1H NMR (400 MHz, DMSO): δH (ppm): 2.63 (s, 3H, CH3); 3.31 (s, 3H, OCH3); 5.46
(d, 3JHH = 6.2 Hz, 2H, CH2-N); 5.51 (d, 3JHH = 5.3 Hz, 2H, CH2-OCH3); 6.94 (t, 3JHH = 7.9 Hz,
1H, H-Ph); 7.38 (t, 3JHH = 7.8 Hz, 1H, H-Ph); 7.47 (t, 3JHH = 8.1 Hz, 1H, H-Ph); 7.71 (d, 3JHH
13
= 8.1 Hz, 1H, H-Ph). C NMR (DMSO): δC (ppm): 12.01 (CH3); 16.75 (CH2-N); 59.03
(CH2-O); 73.34 (Ph); 116.00 (Ph); 121.55 (Ph); 124.92 (Ph); 130.66 (Ph); 132.64 (Ph);
161.11 (Ph-OH); 164.04 (C=N). MS (ESI) m/z (%) 299 (M+ - Cl2, 55%). FT-IR (cm-1): υ(OH)
=3398; υ(C=N) =1628. Anal. Calc. for C11H15Cl2NO2Pd:0.25CH2Cl2: C, 34.49; H, 3.99; N,
3.57. Found: C, 34.41; H, 3.90; N, 3.39.
2.2.6 [2-(2-hydroxyethylimino)ethyl)phenol)PdCl2] (2)
Complex 2 was synthesized following the procedure described for complex 1 using
HL2 (0.06 g, 0.31 mmol) and Pd(COD)Cl2 (0.10 g, 0.31 mmol) in CH2Cl2 (10 mL). Yield =
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1
3
0.09 g (78%). H NMR (400 MHz, DMSO): δH (ppm): 3.73 (s, 3H, CH3); 5.52 (dd, JHH
=
11.4 Hz, 4H, CH2-CH2); 6.87 (t, 3JHH = 8.4 Hz, 2H, H-Ph); 7.38 (t, 3JHH = 8.4 Hz, 1H, H-Ph);
3
7.68 (d, JHH = 7.8 Hz, 1H, H-Ph). 13C NMR (DMSO): δC (ppm): 17.70 (CH3); 31.01 (CH2-
N); 58.25(CH2-O); 116.04 (Ph); 121.54 (Ph); 124.96 (Ph); 130.65 (Ph); 132.53 (Ph); 157.88
(Ph-OH); 164.61 (C=N). MS (ESI) m/z (%) 322 (M+- Cl, 71%). FT-IR (cm-1): υ(OH) = 3381;
υ(C=N) =1625. Anal. Calc. for C10H13Cl2NO2Pd: C, 33.69; H, 3.68; N, 3.93. Found: C, 33.65;
H, 3.62; N, 3.63.
2.2.7 [2-(2-aminoethylimino)ethyl)phenol )PdCl2] (3)
Complex 3 was synthesized following the procedure described for complex 1 using
HL3 (0.06 g, 0.31 mmol) and Pd(COD)Cl2 (0.10 g, 0.31 mmol) in CH2Cl2 (20 mL). Yield =
1
3
0.10 g (91%). H NMR (400 MHz, DMSO): δH (ppm): 2.29 (s, 3H, CH3); 4.68 (dd, JHH
=
3
3
10.8 Hz, 4H, CH2-CH2); 6.58 (t, JHH = 14.0 Hz, 1H, H-Ph); 6.86 (d, JHH = 8.4 Hz, 1H, H-
3
3
Ph); 7.24 (t, JHH = 14.0 Hz, 1H, H-Ph); 7.66 (d, JHH = 9.7 Hz, 1H, H-Ph). 13C NMR
(DMSO): δC (ppm):17.72 (CH3); 32.01 (CH2-N); 43.03 (CH2-O); 116.08 (Ph); 121.55 (Ph);
124.98 (Ph); 130.64 (Ph); 132.53 (Ph); 157.60 (Ph-OH); 164.61 (C=N). MS (ESI) m/z (%)
283 (M+- Cl2, 40%).FT-IR (cm-1): υ(OH) = 3014; υ(C=N) =1639.Anal. Calc. for
C10H14Cl2N2OPd: C, 31.68; H, 3.80; N, 7.04. Found: C, 31.66; H, 3.79; N, 6.98.
2.2.8 [2-(2-hydroxyethylimino)methyl)phenol)PdCl2] (4)
Complex 4 was synthesized following the procedure described for complex 1 using
HL4 (0.05 g, 0.31 mmol) and Pd(COD)Cl2 (0.10 g, 0.31 mmol) in CH2Cl2 (20 mL).
Recrystallization of a solution of 4 in CH2Cl2 solution at room temperature afforded single
1
crystals suitable for X-ray analyses. Yield =0.10 g (91%). H NMR (400 MHz, DMSO): δH
(ppm): 5.15 (t, 3JHH = 5.5 Hz, 2H, CH2-N); 5.51 (t, 3JHH = 5.6 Hz, 2H, CH2-OH); 6.59 (t, 3JHH
= 8.3 Hz, 1H, H-Ph); 6.77 (d, 3JHH = 7.8 Hz, 1H, Hd); 7.27 (t, 3JHH = 8.6 Hz, 1H, H-Ph); 7.86
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3
(d, JHH = 7.8 Hz, 1H, H-Ph); 8.36 (s, 1H, H-C=N). 13C NMR (DMSO): δC (ppm): 61.24
(CH2-N); 65.34 (CH2-O); 115.22 (Ph); 120.25 (Ph); 120.51 (Ph); 128.98 (Ph); 139.32 (Ph);
163.82 (Ph-OH); 164.01 (C=N). MS (ESI) m/z (%) 307 (M+- Cl, 15%).FT-IR (cm-1): υ(OH)
=
3399; υ(C=N) =1626. Anal. Calc. for C9H11Cl2NO2Pd: C, 31.56; H, 3.24; N, 4.09 Found: C,
31.29; H, 3.18; N, 3.73.
2.2.9 [2-(2-methoxyethylimino)ethyl)phenol PdClPPh3] (5)
To a suspension of complex 1 (0.05 g, 0.11 mmol) in CH2Cl2 (10 mL), PPh3 (0.13 g,
0.11 mmol) was added and stirred for 24 h. The mixture was filtered and the filtrate
concentrated to approximately 3 mL and recrystallized from hexane/CH2Cl2 solvent mixture
to obtain complex 5 as a yellow crystalline solid. Yield = 0.06 g (74%). 1H NMR (CDCl3): δH
(ppm): 2.51 (s, 3H, CH3); 3.35 (s, 3H, OCH3); 5.77 (dd, JHH = 3.60 Hz, 4H, CH2-CH2); 7.30
(m, 1H, Ph-Phenol); 7.43 (m, 1H, Ph-Phenol); 7.55 (m, 7H, PPh3); 7.61 (m, 2H, Ph-Phenol);
7.64 (m, 8H, PPh3).13C NMR (CDCl3): δC (ppm):18.56; 39.00; 59.01; 74.15; 116.03; 121.53;
124.91; 128.81; 128.96; 130.66; 132.52; 137.33; 137.47; 157.88; 164.62. 31P NMR (CDCl3):
δ (ppm): 32.96; 23.78. MS (ESI) m/z (%) 561 (M+ - Cl, 19%). Anal. Calc. for
C29H30Cl2NO2PPd:0.5CH2Cl2: C, 52.48; H, 4.63; N, 2.07. Found: C, 52.41; H, 4.59; N, 2.00.
2.2.10 [{2-(2-methoxyethylimino)ethyl)phenol}Pd(OTs)(PPh3)] (6)
To a solution of HL1 (0.08 g, 0.44 mmol) in chloroform (5 mL), was added drop-wise
a solution of Pd(OAc)2 (0.10 g, 0.44 mmol) in chloroform (10 mL) followed by a solution of
PPh3 (0.23 g, 0.89 mmol) and p-TsOH (0.15 g, 0.89 mmol) in chloroform (10 mL) and stirred
at room temperature for 24 h. The organic volatiles were removed in vacuo, and
recrystallization from CH2Cl2-hexane gave complex 6 as a light yellow solid. Yield = 0.29 g
1
(91%). H NMR (CDCl3): δH (ppm): 2.33 (s, 3H, CH3-N). 2.50 (s, 3H, CH3-OTs); 3.34 (s,
3
3H, OCH3); 4.28 (dd, JHH = 12.0 Hz, 4H, CH2-CH2); 7.28 (m, 2H, Ph-ethanone); 7.36 (m,
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2H, Ph-ethanone); 7.48 (m, 2H, Ph-OTs); 7.51 (m, 9H, PPh3); 7.52 (m, 6H, PPh3); 7.67 (m,
2H, Ph-OTs). 31P NMR (CDCl3): δ (ppm): 29.55; 22.22. 13C NMR (CDCl3):δC (ppm): 19.30;
24.30; 32.00; 57.81; 59.10; 74.92; 116.02; 121.55; 124.93; 128.00; 128.81; 128.93; 130.44;
130.65; 132.54; 137.36; 137.48; 142.22; 146.89; 164.63. MS (ESI) m/z (%) 732 (M+, 22%).
Anal. Calc. for C36H36NO5PPdS: C, 59.06; H, 4.96; N, 1.91; O, 10.93. Found: C, 59.36; H,
4.98; N, 2.02; O, 10.95.
2.3 General procedure for the hydrogenation reactions of alkenes and alkynes
The catalytic hydrogenation reactions were performed in a stainless steel autoclave
equipped with temperature control unit and a sample valve. In a typical experiment, styrene
(0.5 mL, 4.36 mmol) and complex 1 (16 mg, 0.004 mmol, S/C 1000) were dissolved in
toluene (50 mL). The reactor was evacuated and the catalytic solution was introduced to the
reactor via a cannula. The reactor was purged three times with hydrogen, and then set at the
equipped pressure, heated to the desired temperature and the reaction stirred at 500 rpm. At
the end of the reaction time, the reaction was cooled, excess hydrogen was vented off and the
samples drawn, filtered using 0.45 µm micro filters and analysed by Gas Chromatography to
determine the percentage conversion of styrene to ethylbenzene. Standard authentic samples
were used to confirm the presence and composition of the hydrogenation products.
2.4 General procedure for kinetics
Kinetics for the hydrogenation reactions were investigated for complexes 1-5 by
monitoring the reactions using Gas Chromatography. Sampling was done by cooling the
reaction mixture, venting off the excess hydrogen and withdrawing aliquots of the reaction
mixture at regular time intervals (15 min – 1.5 h). The samples were analysed by Gas
Chromatography to determine the percentage conversions of styrene to ethylbenzene. The
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observed rate constants, kobs, were extracted from the slopes of the lines of best-fit of the plots
of In[Sty]0/[Sty]t vs time (Fig. S7 – S10).
2.5 Density Functional Theoretical (DFT) studies
DFT calculations were performed in a gas phase to identify the energy-minimized
structures based on B3LYP/LANL2DZ (Los Almos National Laboratory 2 double ζ) level
theory.22 Split bases set, LANL2DZ for palladium and 6-311G for all other atoms was used to
optimize the geometries and energies of the complexes. The Gaussian09 suite of programs
was used for all the computations.
3
Results and discussion
3.1. Synthesis and characterization of ligands and their palladium complexes
The (imino)phenol ligands HL1 – HL4 were obtained in good yields (75-92%) from
condensation reactions between appropriate aldehydes or ketones and the corresponding
amines (Scheme 1). Reactions of HL1 – HL4 with [PdCl2(COD)] afforded the corresponding
palladium(II) complexes 1-4 (Scheme 2) in good yields. Treatment of the neutral complex 1
with PPh3 resulted in the formation of the corresponding cationic complex, while reactions of
HL1 with Pd(OAc)2 and equivalence of p-TsOH in the presence of PPh3 afforded the neutral
complex 6 in moderate yields (Scheme 2).
Y
R
R
H2N
EtOH, 60 °C, p-TsOH
OH
N
OH
O
Y= OCH3, R =CH3 (L1), Y= OH, R= CH3 (L2)
Y= NH2, R = CH3 (L3), Y= OH, R = H (L4)
Y
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Scheme 1: Synthesis of (imino)phenol ligands HL1 – HL4 bearing pendant arms
+
Cl-
R
CH2Cl2
PPh3
CH3
R
CH3
Pd(AOc)2
Pd(COD)Cl2
CH2Cl2
PPh3/p-TsOH
HO
N
HO
N
OH
N
O
N
Pd
Pd
Pd
Cl
Cl
TsO
Cl
PPh3
PPh3
Y
Y
Y
Y
Y = OCH3 (5)
Y = OCH3 (6)
Y = OCH3, R = CH3 (1), Y = OH, R = CH3 (2)
Y = NH2, R = CH3 (3), Y= OH, R = H (4)
Scheme 2: Synthesis of neutral and cationic palladium (II) complexes 1-6
1
All the new compounds were characterized by H{13C}{31P}NMR and IR
spectroscopy, mass spectrometry, elemental analysis to establish their identity and purity. For
1
example, the H NMR spectra of HL1 and the corresponding complex 1 (Fig. S1) showed a
downfield shift of the ethylene linker protons from 3.78 ppm and 3.82 ppm to 5.46 ppm and
13
1
5.51 ppm respectively. The C NMR spectral data was consistent with the H NMR spectra
31
(Fig. S2). Complexes 5 and 6 were also analysed using P NMR spectroscopy. In both
cases, two peaks between 22.78 ppm and 29.55 ppm were recorded (Fig. S3), in good
agreement with coordinated PPh3 ligand as observed in related palladium(II) complexes.23,24
FT-IR spectrum of complex 4 in relation to its ligand HL4 also established the
successful formation of the complexes. For instance, the phenolic OH IR frequencies were
observed as broad and sharp peaks at 3321 cm-1 and 3399 cm-1 in HL4 and complex 4
respectively (Fig. S4), indicating coordination of the OH group to palladium (II) atom.
Similarly, an up field shift of the imine signal from 1631 cm-1 in HL4 to 1626 cm-1 in
complex 4 was consistent with the proposed structures in Scheme 2. Mass spectra of all the
complexes showed typical fragmentation patterns consistent with a loss of a halide group. For
instance, complex 3, showed an m/z peak at 318.9 amu corresponding to the [Pd(L3)Cl]+
11

ACCEPTED MANUSCRIPT
fragment (Fig. S5). Elemental analyses data of all the complexes were in tandem with the
proposed structures in Scheme 2 and also confirmed the purity of the bulk materials.
In order to confirm the proposed binding modes of HL1 - HL4 and coordination
chemistry of the formed palladium complexes, attempts were made to grow single crystals
suitable for X-ray analysis of the complexes. In one such instance, single crystals of
compound 4 suitable for X-ray analysis were isolated. Crystal data collection and structural
refinement parameters of complex 4a are given in Table S1. However, the structure obtained
(4a) contained two anionic bidentate L4 units (Fig. 1), indicating in situ deprotonation of
HL4 during crystallisation as shown in Eq. 1. We recently observed a similar transformation
for nickel(II) complexes of HL4.25 Separately, Boltina et. al. previously reported the in-situ
deprotonation of O^N^O (pyrazolyl/imadazolyl)imine ligands to form pincer-type
palladium(II)
complexes.26
+
(1)
2HCl
[Pd(HL4)Cl2]
[Pd(L4)2]
12

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Fig.1: Thermal ellipsoid plot (50% probability) of 4a illustrating the square planar
coordination geometry of the Pd(II) ion. Hydrogen atoms and the atoms of the minor
conformer have been rendered as spheres of arbitrary radius. Selected bond lengths [Å] and
angles [°]: Pd(2)-N(2), 2.019(15); Pd(2)-O(4), 1.982(13); Pd(2)-O(4), 1.982(13); O(4)-Pd(2)-
O(4), 180.00; O(4)-Pd(2)-N(2), 92.00(6); N(2)-Pd(2)-N(2), 180.00.
The bond angles around the palladium(II) atom for O(4)-Pd(2)-O(4) of 180.00o and
O(4)-Pd(2)-N(2) of 92.00o(6) show little deviation from the expected 180o and 90o, an
indication of minimal distortion from a square planar geometry. This is not surprising due to
the limited steric encumbrance around the metal center. The bond length for Pd(2)-N(2) of
2.019(15) Å is comparable to the average for Pd-Nimine bond distances of 2.017 Å reported for
105 similar palladium(II) complexes.27 Similarly, the bond distance for Pd-O(4) of 1.982(13)
Å compares favourably to the average for Pd-O bond lengths of 1.988 Å observed in 302
related palladium complexes.27 There exists intermolecular hydrogen bonding between the
OH molecules.
13

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3.2.
Hydrogenation reactions of alkenes and alkynes catalysed by complexes 1-6
3.2.1. Preliminary screening of palladium complexes 1-6 in molecular hydrogenation of
styrene
Preliminary evaluation of complexes 1-6 in the molecular hydrogenation reactions of
styrene were performed at 5 bar, 30 °C and [styrene]:[Pd] ratio of 1000:1. Under these
conditions, all the complexes (1-6) formed active catalysts giving conversions ranging from
50% - 97% within 1.5 h (Table 1). The identities and compositions of the products were
determined by GC and GC-MS (Fig. S6). Control experiments conducted without the use of a
palladium (II) complex gave only 2% conversions (Table 1, entry 8), clearly demonstrating
that higher catalytic activities observed were due to the complexes 1-6. Upon establishing
that complexes 1-6 form active hydrogenation catalysts, we further performed detailed
kinetics, selectivity and theoretical studies of the hydrogenation reactions of a range
substrates including higher alkenes and alkynes.
3.2.2. Influence of complex structure on the kinetics of hydrogenation reactions of styrene
The influence of complex structure on the molecular hydrogenation of styrene was
investigated by determining the rate constants for complexes 1-6 (Table 1). A linear plot of
In[Sty]0/[Sty]t vs time (Fig. S7) established that the hydrogenations reactions were pseudo
-
first order with respect to styrene according to equation 2.
Rate =
k
[styrene]1
(2)
14

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Table 1: Effect of catalyst structure on the hydrogenation of styrene by complexes 1-6a
Entry
Catalysts
Conversion b(mol%)
k
obs (h-1)
TOF c(h-1)
646
1
1
97
89
78
88
63
51
86
2
2.15 (±0.07)
1.36 (±0.13)
1.03 (±0.07)
1.32 (±0.05)
0.67 (± 0.05)
0.55 (± 0.06)
nd
2
2
592
3
3
519
4
4
586
5
5
420
6
6
339
7d
8e
1
526
__
nd
__
aConditions: styrene, substrate/catalyst = 1000; substrate, 4.36 mmol; catalyst; 0.01 mmol;
solvent, toluene; pressure, 5 bar; temperature, 30 °C; time, 1.5 h. bDetermined by GC. cTOF
in molsubstratemolcatalyst-1 h-1 (h-1). dMercury drop test (5 drops of mercury were added to the
reaction mixture). eControl experiment, no catalyst used, time 8 h.
It is clear that the catalytic activity of the complexes was influenced by the pendant
arm of the ligands. For example, replacing the pendant OCH3 group in 1 by an ammine group
(3) was followed by a drop in k
obs from 2.15 h-1 (TOF = 646 h-) to 1.03 h- (TOF = 519 h-).
This is likely to result from a stronger coordination of the nitrogen atom to palladium(II)28
resulting in competition with the styrene substrate for the active site (Scheme 3). The
observed catalytic activities of complexes 1-6, in comparison to the inactive palladium(II)
complexes of P^N^S ligands16 point to the hemi-lability of ligands HL1 - HL4. On the other
hand, there was no discernible effect of the imine carbon substituent on the catalytic activities
15

ACCEPTED MANUSCRIPT
of the complexes as shown by comparable catalytic activities of complexes 2 and 4 (Table 1,
entries 2 and 4).
+ Cl-
+ Cl-
R
R
R
O
H
N
HCl
H2
H
Pd
Y
O
N
O
N
H
Pd
+
H
Pd
Cl
Cl
H
Y
Y
Scheme 3: Hemi-labile nature of the ligands (Y = OH, NH2, OCH3); a strongly coordinating
Y group is likely to reduce the catalytic activity by limiting substrate coordination to the
metal centre.
Consistent with our previous reports, a comparison of the catalytic activities of the
neutral and cationic systems, revealed that the neutral complexes were more active.19,20 For
example, rate constants of k k
obs = 2.15 h-1 and obs = 0.67 h-1 were reported for catalysts 1 and
5 respectively (Table 2, entries 1, 5-6). This behaviour may be assigned to the poor lability of
-TsOH groups, in addition to increased steric bulk in catalyst 5.29
the PPh3 and
p
3.2.3 Influence of catalyst concentration on the kinetics of hydrogenation reactions of styrene
The effect of catalyst concentration and order of reaction with respect to complex 1
was investigated by varying the substrate/catalyst ratios from 400 to 1800 at fixed substrate
concentration. A plot of In[Sty]0/[Sty]t vs time (Fig. S8) gave a straight line from which the
kobs at each ratio was determined (Table 3). It is significant to note that, while a decrease in
catalyst loading resulted in increased TOFs, there was an appreciable decrease in obs. For
k
example, kobs of 2.15 h-1 and 0.96 h-1 were observed at substrate/catalyst ratios of 1000 and
1400 corresponding to TOFs of 646 h-1 and 718 h-1 respectively (Table 3, entries 3 and 5).
16

ACCEPTED MANUSCRIPT
This behaviour showed that increasing catalyst loading did not increase the catalytic activity
by a similar magnitude. Thus it would be beneficial (economically viable) to use lower
catalyst loading due to higher turn-over frequencies reported.
A plot of –In(kobs) vs -In[1] allowed us to determine the order of the reaction with
respect to catalyst 1 as 2.6 ± 0.2 h-1 (Fig. 2). The higher reaction order with respect to catalyst
1, is indicative that the reaction is highly sensitive to a change in catalyst concentration.
Fractional orders of reaction with respect to catalyst concentrations is known to arise from the
possible existence of different active sites or catalyst aggregation.30,31 In order to understand
the nature of the active species and possible formation of palladium nanoparticles, we carried
out a mercury drop test by adding a few drops of mercury to the reaction mixture of styrene
and catalyst 1. The minimal reduction in catalytic activity from 97% (TOF = 646 h-1) to 86%
(TOF = 526 h-1) pointed to lack of formation of palladium(0) nanoparticles as active species,
hence homogenous nature of the reactions.32 This contrasts our previous reports using
(pyridyl)benzoazole palladium(II)13 complexes which recorded significant reduction in
catalytic activity from 98% to 55%; a feature we can allude to enhanced stabilization of the
active species by the pendant arms of the ligand L1 in this this current study.
17

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Equation
W ei ght
y = a + b*x
No Weighting
Residual Su 0.00775
m of Squares
0.8
0.6
0.4
Pearson's r
0.99568
Adj. R-S quar 0.98708
Value
Intercept 19.0696
Standard Err
1.24911
B
B
Slope
2.64903
0.17459
0.2
0.0
-0.2
-0.4
-0.6
-7.4
-7.3
-7.2
-7.1
-7.0
-6.9
In[1]
Fig. 2: Plot of In(kbos) vs In[1] for the determination of the order of reaction with respect to
catalyst 1.
The effect of H2 concentration was also investigated by varying the pressure from 2.5
to 12.5 bar at S/C of 1000. From the plot of In[Sty]0/[Sty]t vs time the rate of hydrogenation
reaction is seen to linearly dependent on the H2 pressure (Fig. S9). From the plot of kobs vs
H2 pressure, the order of reaction with respect to [H2] was derived as 0.39 ± 0.04 (Fig. 3).
This partial and low reaction orders with respect to H2 concentration has been associated with
disfavoured activation of the H2 molecule to form the dihydride species.33 The observed
dependency of the rate of hydrogenation reactions on styrene and hydrogen pressure is
consistent
of
with either
following
the
+
R
Cl-
O
O
O
N
N
N
H2
(3)
equations:
Pd
Pd
Pd
Cl
Cl
R
Cl
H
H
HCl
R
+
O
Cl-
O
O
N
N
N
H2
(4)
Pd
Pd
Pd
Cl
Cl
R
H
H
H
2Cl-
HCl
18

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Table 3: Effect of reaction conditions on the hydrogenation of styrene using catalyst 1a
Entry
1d
2d
3
Sub/Cat
400
P
H2 (bar)
T (°C)
Conversionb
>99
>99
97
k
obs (h-1)
TOFc (h-1)
800
5
30
__
800
5
30
30
30
30
30
30
30
30
30
20
40
50
60
__
1066
646
1000
1200
1400
1800
1000
1000
1000
1000
1000
1000
1000
1000
5
2.15
1.34
0.96
0.59
0.63
2.99
4.36
__
4
5
86
688
5
5
77
718
6
5
54
648
7
2.5
7.5
10
12.5
5
62
412
8
>99
>99
>99
77
666
9e
799
10d
11
12
13f
14d
1998
542
1.01
3.15
4.93
__
5
>99
>99
>99
659
5
999
5
1998
19

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aConditions: styrene, 4.36 mmol; solvent, toluene; pressure, 5 bar; temperature, 30 °C; time,
c
-1
1.5 h. bDetermined by GC. TOF in molsubstratemolcatalyst h-1.dTime, 0.5 h, time too short to
get enough for points. eTime, 1.25 h.f time, 1.0 h.
The lower order of reaction with respect to [H2] of 0.39 ± 0.04 in addition to the
absence of colour change during hydrogenation reactions support the formation of
monohydride species. This points to the first mechanism (3) as the most probable pathway,34
consistent with Scheme 3. From these kinetics result, the overall experimental rate law for the
hydrogenation of styrene catalysed by catalyst 1 can thus be formulated as given in equation
5. The major import of this equation is that the formation of the hydride species appears to be
rate limiting step.35
0.39
Rate =
k
[styrene]1[1]2.6[PH2
]
(5)
Fig. 3: Plot of observed rate constant (kobs) vs hydrogen pressure to determine the order of
reaction with respect to H2 concentration for catalyst 1.
To determine the activation parameters of the hydrogenation reactions, we varied the
temperature from 20 to 60 °C using catalyst 1 at [sty]/[1] ratio of 100 and H2 pressure of 5
20

ACCEPTED MANUSCRIPT
bar. Expectedly, an increase in temperature from 20 °C to 40 °C, was followed by a
significant increase in kobs from 1.01 h-1 (TOF = 542 h-1) to 3.15 h- (TOF = 659 h-1)
respectively (Table 3, entries 11 and 12).36,37 It is important to note that, we did not witness
the formation of palladium(0) black deposits even at elevated temperatures, depicting the
thermal stability of catalyst 1. The activation energy, standard enthalpy (∆H≠) and entropy
(
∆S≠) of activation were obtained from the Arrhenius and Eyring plots (Figure 4). The large
S≠ of -1184.91 ± 1.6 J/mol K indicates a highly ordered transition
negative entropy value for
∆
state consistent with the formation of Pd-hydride intermediate as the active species.35 In
addition, the parameters: Ea= 40.17 ± 1.6 kJ/mol and ∆H≠=37.60 ± 1.6 kJ/mol are comparable
to those reported in literature for homogenous catalysts.38
Fig. 4: Arrhenius plot (a) and Eyring plot (b) for the determination of the Ea= 40.17 ± 1.6
kJ/mol, ∆H≠ 37.60 ± 1.6 kJ/mol, ∆S≠ = -1184.91 ± 1.6 J/mol K and ∆G≠ = 420.8 ± 1.6 kJ/mol.
3.2.3 Substrate scope and chemo-selectivity studies using complex
Complex 1 was further used to expand the scope of the alkene and alkyne substrates
which included; 1-hexene, 1-octene, 1-hexyne, 1-octyne and phenyl acetylene (Table 3, Fig.
21

ACCEPTED MANUSCRIPT
S10). The trends in both the reactivity and product distribution obtained (Table 3 and Fig.
S11) were similar to our recent reports,19,20 hence does not require further elaborations. As
illustration, alkynes were more reactive than the corresponding alkenes while the reactivities
of the substrates diminished with increase in chain length. In terms of production distribution,
hydrogenation of alkenes were accompanied by isomerization reactions, while alkyne
hydrogenation reactions occurred in two steps to produce the respective alkenes and alkanes
(Fig. S11).
Table 3: Effect of substrates on the catalytic activity of catalyst 1a
Entry
Substrate
(kobs)(h-1)
TOFb (h-1)
646
%Alkanec
1
2
3
4
5
6
7
8
Styrene
2.15
97
78
72
100
66
55
53
44
1-Hexene
1.19(±0.07)
1.12 (±0.09)
1.92 (±0.03)
2.75 (±0.07)
2.34 (±0.11)
0.68 (±0.05)
0.62 (±0.06)
540
1-Octene
527
Phenyl-acetylene
1-Hexyne
640
440
1-Octyne
367
trans-2-hexene
trans-2-octene
488
460
aConditions: substrate, substrate/catalyst = 1000; substrate, 4.36 mmol; catalyst; 0.01 mmol;
b
solvent, toluene; pressure, 5 bar; solvent, toluene; temperature, 30 °C; time, 1.5 h. TOF in
molsubstratemolcatalyst-1 h-1.cselectivity towards alkane hydrogenation products after 1.5 h.
22

ACCEPTED MANUSCRIPT
To gain more insight in the isomerization reactions and incomplete conversions of
internal alkenes to alkanes, we used internal alkenes trans-2-hexene and trans-2-octene
substrates (Table 3, entries 7 and 8). A significant drop in catalytic activity observed from
72% to 44% observed for 1-octene and trans-2-octene respectively, may thus explain the
incomplete hydrogenations of the internal alkenes to their corresponding alkanes (Fig. S11a).
Additionally, it was observed that like in terminal alkenes, there occurred tandem
hydrogenation and isomerization reactions for internal alkenes (Fig. S12). For example, final
compositions of 66% for n-octane and 34% for internal octenes (trans-3-octene = 27%, 2/4-
octene = 7%) was observed for hydrogenation of trans-2-octene (Fig. 11b).
3.2.3 Evidence of hemi-lability from DFT studies
The structure/property-activity relationships and trends observed in the hydrogenation
reactions using complexes 1-6 pointed to potential hemi-lability of ligands L1-L4. We
therefore ventured to establish this phenomenon using Density Functional Theory (DFT)
calculations. The geometry-optimized structures for the palladium (II) complexes are shown
in Fig. S13. The relationship between the Pd-Y bond length (Y = pendant arm) and the
catalytic activity showed significant correlation (Fig S14). For example, while complex 1b
(derived from 1) with a Pd-OCH3 bond length of 2.08 Å displayed kobs of 2.15 h-1, complex
3b, with Pd-NH2 bond length of 1.95Å had a k
obs of 1.03 h-1 (Table 4). From these results, it
is evident that coordination of styrene substrate to the vacant palladium atom is preceded by
breaking of the Pd-Y bond, thus essential in controlling the overall reactivity (Scheme 3). The
HOMO-LUMO energy gaps were also observed to influence the catalytic activities of the
complexes 1b-4b, where a larger HOMO-LUMO energy gap resulted in lower catalytic
activities. For example, catalysts 1b and 3b having a HOMO-LUMO energy gaps of 142.61
23

ACCEPTED MANUSCRIPT
kcal/mol and 149.47 kcal/mol exhibited
k
obs of 2.15 h-1 and 1.03 h-1 respectively. It has been
reported that a smaller HOMO and LUMO gap promotes coordination (due to a low lying
LUMO) of the substrate to the metal centre39, consistent with the observed catalytic trends.
Another parameter that appeared to control the catalytic activities of the complexes was the
charge on the metal atom. For instance, catalyst 1b carrying a positive charge of 0.420 was
more active than catalyst 3b, with a charge of 0.314 on the palladium(II) atom, in agreement
with enhanced substrate coordination to an electrophilic metal centre.40
Table 4: DFT-calculated data for the bifunctional palladium (II) complexes.
+ Cl-
+ Cl-
R
R
R
H2
-HCl
Styrene
Toluene, 5 bar, 30 °C, 500 rpm
O
N
O
N
O
H
N
+
H
Pd
H
H
Pd
Pd
Y
H
Cl
Cl
Y
Y
Y = OCH3, R = CH3 (1b), Y= OH, R= CH3 (2b)
Y= NH2, R = CH3 (3b), Y = OH, R= H (4b)
Y = OCH3, R = CH3 (1), Y= OH, R= CH3 (2)
Y= NH2, R = CH3 (3), Y = OH, R= H (4)
Y = OCH3, R = CH3 (1c), Y= OH, R= CH3 (2c)
Y= NH2, R = CH3 (3c), Y = OH, R= H (4c)
LUMO (eV)
HOMO (eV)
-4.28
-3.82
-10.12
6.30
-3.76
-10.24
6.48
-3.90
-10.35
6.45
-10.47
6.18
∆EL-H (eV)
∆ɛ[kcal mol-1]
142.61
145.26
0.408
2.02
149.47
0.314
1.95
148.66
0.407
2.00
NBO charges (Pd) 0.420
Pd-Y (Å)
2.08
646
TOF (h-1) a
592
519
586
k
obs(h-1)
2.15
1.36
1.03
1.32
aTOF in molsubstratemolcatalyst-1 h-1 (h-1).
24

ACCEPTED MANUSCRIPT
3.3.
Conclusions
In summary, palladium (II) complexes anchored on (imino)phenol ligands with
pendant donor groups have been synthesized and structural characterized. Single crystal X-
ray diffraction studies of complex 4a showed that the complex contained two bidentate
anionic ligands. The complexes formed active catalysts in the hydrogenation of alkenes and
alkynes under mild conditions. Hydrogenation reactions of terminal alkenes were
accompanied by isomerization reactions, while hydrogenation of alkynes occurred in two
steps via alkenes. Kinetic studies on the hydrogenation reactions were established to be
pseudo-first order with respect to alkene substrate, but partial orders with respect to H2 and
catalyst concentrations. The activation parameters indicate a highly ordered transition state
and homogenous nature of the active species. DFT studies point to the hemi-labile nature of
the ligands, in which breaking the Pd-pendant donor atom bonds prior to substrate
coordination controls the reactivity of the complexes.
4
1
2
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