Hydrogenation of CO2, carbonyl and imine substrates catalyzed by [IrH3(PhPNHP)] complex

Article abstract of DOI:10.1016/j.jorganchem.2018.12.017

A series of iridium and rhodium complexes [M(COD)(^PhPN^HP)]Cl {M = Ir (1), Rh (2)}, [MH₂Cl(^PhPN^HP)] {M = Ir (3), Rh (4)} and [IrH₃(^PhPN^HP)] (6) supported by pincer ligand H–N(CH₂CH₂PPh₂)₂ {^PhPN^HP} have been synthesized and characterized. All complexes were isolated in good yields. The iridium trihydride complex [IrH₃(^PhPN^HP)] (6) was found to be an active catalyst for the hydrogenation of CO₂ in 1 M aqueous KOH solution. It also acts as a catalyst for the base-free hydrogenation of carbonyl and imine substrates in MeOH. Under similar hydrogenation conditions, 2-cyclohexen-1-one undergoes solvent assisted tandem Michael addition-reduction mediated by bifunctional Lewis-acid-catalyst [IrH₃(^PhPN^HP)] in ROH (R = Me, Et) at room temperature. The complexes 1, 3, 4, and 6 were characterized by X-ray crystallography. Extensive hydrogen bonding interactions N–H?H–Ir (2.15 ?), N–H?Cl (2.370 ?) were noted in the crystal structures of these complexes.

Full text of DOI:10.1016/j.jorganchem.2018.12.017

Accepted Manuscript
Hydrogenation of CO , carbonyl and imine substrates catalyzed by [IrH (PhPNHP)]
2
3
complex
Ayyappan Ramaraj, Munirathinam Nethaji, Balaji R. Jagirdar
PII:
S0022-328X(18)30978-1
DOI:
https://doi.org/10.1016/j.jorganchem.2018.12.017
JOM 20674
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 2 November 2018
Revised Date: 25 December 2018
Accepted Date: 26 December 2018
Please cite this article as: A. Ramaraj, M. Nethaji, B.R. Jagirdar, Hydrogenation of CO , carbonyl and
2
imine substrates catalyzed by [IrH (PhPNHP)] complex, Journal of Organometallic Chemistry (2019),
3
doi: https://doi.org/10.1016/j.jorganchem.2018.12.017.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Hydrogenation of CO2, Carbonyl and Imine Substrates Catalyzed by [IrH3(PhPNHP)]
complex
Ayyappan Ramaraj, Munirathinam Nethaji, and Balaji R Jagirdar * a
Department of Inorganic & Physical Chemistry, Indian Institute of Science, Bangalore 560012,
India. Tel: +91-80-2293 2825; Fax: +91-80-2360 1552; E-mail: jagirdar@iisc.ac.in
Abstract
A series of iridium and rhodium complexes [M(COD)(PhPNHP)]Cl {M = Ir (1), Rh (2)},
[MH2Cl(PhPNHP)] {M = Ir (3), Rh (4)} and [IrH3(PhPNHP)] (6) supported by pincer ligand H-
N(CH2CH2PPh2)2 {PhPNHP} have been synthesized and characterized. All complexes were
isolated in good yields. The iridium trihydride complex [IrH3(PhPNHP)] (6) was found to be an
active catalyst for the hydrogenation of CO2 in 1 M aqueous KOH solution. It also acts as a
catalyst for the base-free hydrogenation of carbonyl and imine substrates in MeOH. Under
similar hydrogenation conditions, 2-cyclohexen-1-one undergoes solvent assisted tandem
Michael addition-reduction mediated by bifunctional Lewis-acid-catalyst [IrH3(PhPNHP)] in ROH
(R = Me, Et) at room temperature. The complexes 1, 3, 4, and 6 were characterized by X-ray
crystallography. Extensive hydrogen bonding interactions N−H···H−Ir (2.15 Å), N−H···Cl
(2.370 Å) were noted in the crystal structures of these complexes.
Keywords
Pincer ligand, Iridium complexes, NMR spectroscopy, CO2 activation, hydrogenation, catalysis
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1. Introduction
Significant research attention has been focused to develop methodologies which could bring
down CO2 levels in the atmosphere such that the hazards involved with global warming process
could be curbed. [1] Currently, the majority of value-added chemicals are produced using CO as
a C1 synthon which in turn is obtained by steam reforming of non-renewable, limited oil or
natural gas resources. So in the last two decades, CO2 has emerged as a front runner for an
alternative, sustainable C1 feed-stock compared to fossil fuel sources. Parallelly, it must be noted
that the effective utilization of CO2 into value-added chemicals would critically depend on the
production of H2 from non-fossil fuel based energy sources like electrolysis of water, solar water
splitting etc.
catalyst
H2 + CO2
(1)
HCOOH, CH3OH
Among the different ways to utilize CO2, approaches based on conversion of CO2 into
value added chemicals such as HCOOH, CH3OH etc. using transition metal catalysts are
being actively pursued (eq 1). [2,3] In this context, transition metal complexes possessing
multidentate pincer scaffold for the activation and functionalization of CO2 has become
increasingly prominent owing to their thermal stability, chelation rigidity and modulation
of their electronic and steric properties. [4] The substituents on the pincer motif could
readily be altered due to ease in synthesis and their catalytic activities could be evaluated.
Thus, these ligands find applications in many areas such as hydrogenation,
dehydrogenation, C−C coupling reactions, CO2 activation and condensation reactions, etc.
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The most commonly employed pincer ligands relevant to the present work are shown in
chart 1.
PR2
PR2
PR2
PR2
PR2
R'
N
N
PR2
A
C
B
RPCP
RPNR'P
RPNP
O
O
PR2
PR2
O
PR2
PR2
E
D
RPOP Xantphos
RPOCOP
R = iPr, tBu, Ph
Chart 1. The most commonly employed pincer ligands relevant to the present work are
shown.
Among these, complexes with type (A) ligand H-N(CH2CH2PR2)2 where R = isopropyl
(iPr) substituent on phosphorous have been used for the activation of CO2. Hazari and co-
workers reported conversion of CO2 into HCOOK using [IrH3(HN(CH2CH2PiPr2)2)]
{HN(CH2CH2PiPr2)2 = iPrPNHP} complex as catalyst at 185 °C and 55 bar of total
pressure (H2 + CO2) in the presence of 1 M aqueous KOH. [5] In addition to CO2
activation, the transition metal complexes of iPrPNHP ligand were used for the
hydrogenation of carbonyl and imine substrates. Gusev et al. reported a series of metal
hydride complexes such as trans-[ReH2(NO)(iPrPNHP)], mer-[IrH3(κ3-PNHN))] {PNHN –
tBu2PC2H4NHC2H4NEt2} and [IrH3(iPrPNHP)] to be excellent catalysts for the
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hydrogenation of aldehydes, ketones, imines and
α,β-unsaturated ketones, etc. [6]
Hydrogenation of polar bonds such as those in carbonyl (R2C=O) and imine (R2C=N-R)
moieties in organic substrates is an important reaction for the production of fine
chemicals and building block reagents such as alcohols and amines respectively in
pharmaceutical, fragrance and agricultural industry. [7] Direct hydrogenation in the
presence of catalyst avoids the use of handling of stoichiometric, hazardous and air-
sensitive reducing agents like NaBH4 and LiAlH4 and the subsequent waste generated
after the workup. [8,9]
R
i
t
Presence of different substituents in PNHP (R = Pr, Bu, Ph) ligand could alter the
electronic and steric profile at the metal center upon complexation. For example,
Heinekey et al. studied the effect of substituents in RPOCOP (R = iPr, tBu) pincer skeleton
with iridium complexes and have shown a dramatic effect on solubility, steric hindrance
of substituents and reactivity upon changing the substituents on the pincer phosphorus.
[10] Though very few early reports were documented on hydrogenation of cyclohexene
using four coordinate Ir and Rh complexes [MCl(L)] {L=HN(CH2CH2PPh2)2 (PhPNHP),
Ph
M = Ir, Rh} in mid 1980s, metal complexes of PNHP ligand remain largely unexplored
and their synthetic routes and characterization, not well established. Indeed, the
[MH2Cl(PhPNHP)] {M = Ir, Rh} complexes have been proposed as intermediates in the
hydrogenation of cyclohexene using [MCl(L)] complexes under H2 atmosphere based on
IR spectroscopy. [11] Recently, during the preparation of this manuscript, Casado et al.
reported the synthesis of an extremely air sensitive four coordinate iridium amido
complex [Ir(COE)(N(CH2CH2PPh2)2)] and demonstrated its utility in hydroamination of
activated alkenes with NH3(g) and C−H activation of alkynes. [12]
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In the present work, we have explored the untapped complexation potential of
HN(CH2CH2PPh2)2 ligand with [M(COD)Cl]2 {M = Ir, Rh} precursor and investigated
the potential of the resulting complexes in catalysis. During the course of study, a series
of iridium and rhodium complexes, [M(COD)(PhPNHP)]Cl {M = Ir (
[MH2Cl(PhPNHP)] {M = Ir ( )} and [IrH3(PhPNHP)] ( ) supported by PhPNHP
1), Rh (2)},
3), Rh (
4
6
pincer ligand has been synthesized and well characterized. Complexes
1, 3, 4, and 6 were
characterized by X-ray crystallography. The iridium trihydride complex [IrH3(PhPNHP)]
(6) was found to be an active catalyst for the hydrogenation of CO2, carbonyl, 2-
cyclohexen-1-one, and imines substrates. The results will be discussed in this paper.
2. Experimental Section
All reactions were carried out under N2 / Ar atmosphere using standard Schlenk techniques or
nitrogen filled glove box unless otherwise stated. [13] All glasswares were dried at 403 K
overnight before use. 1,4-dioxane, n-hexane, and THF were dried over Na-benzophenone.
MeOH was dried over Mg/I2. [14] The solvents were degassed for 10-15 min just before use.
C6D6, 1,4-dioxane-d8, THF-d8, CD2Cl2 and CDCl3 were purchased either from Sigma Aldrich or
Cambridge Isotope Laboratories and used as received. NMR spectra were recorded using Avance
Bruker NMR spectrometer. The spectrometer frequencies for 1H, 31P{1H} and 13C{1H} nuclei are
1
400, 162, and 100 MHz, respectively. The H and 13C NMR spectral chemical shifts were
referenced to the residual proton signal of the deuterated solvents. 31P NMR spectra were
referenced relative to 85% H3PO4 aqueous solution (external reference). FTIR spectra of powder
samples were recorded using a Bruker ALPHA-P spectrometer. Mass spectra were recorded
using Micromass Q-TOF (HRMS) spectrometer in the Department of Organic Chemistry, Indian
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Institute of Science (IISc), Bangalore. Elemental analyses were carried out on ThermoFinnigan
Flash EA 1112 CHNS analyzer, in the Department of Organic Chemistry, IISc, Bangalore. GC-
MS data were collected using Agilent model equipped with GC 7890 A and MS 5975c model in
the division of biological sciences, Indian Institute of Science, Bangalore.
1,5-Cyclooctadiene (COD), cyclooctene (COE), NaBPh4, KOtBu and LiN(SiMe3)2 were
purchased from Sigma Aldrich and used as received. The compounds [IrCl(COD)]2,
F
F
[IrCl(COE)2]2, [RhCl(COD)]2, NaBAr4 (BAr4 = B[3,5-(CF3)2C6H3]4), HN(CH2CH2PPh2)2
(PhPNHP) and Me-N(CH2CH2PPh2)2 (PhPNMeP) were prepared according to literature procedures.
[15,16,17,18] The imine compounds, N-benzylidene aniline and N-benzylidene benzylamine
used for the hydrogenation reactions were synthesized using the Dean-Stark method. [19] High
pressure experiments were carried out either using Parr 4590 series micro-reactor equipped with
4843 series temperature controller or a home-built Swagelok dual mode high pressure / high
vacuum setup. The high purity grade gases (> 99.9999 %) such as H2, CH4, and CO2 were
purchased from Bhuruka Gases Ltd, Bangalore, India. All precautions were strictly adhered to
while carrying out high pressure experiments.
31
13
1H and P NMR spectral data for complexes 1-7 have been summarized in Table 1. C NMR
spectral data for all the compounds have been deposited in the S.M.
2.1 Synthesis of [Ir(COD)(PhPNHP)]Cl (1)
[IrCl(COD)]2 (0.200 g, 0.298 mmol) and HN-(CH2CH2PPh2)2 (0.263 g, 0.596 mmol) were stirred
together in THF (50 mL) for 12 h at 298 K. Then the solution was concentrated to about 5 mL,
addition of n−hexane gave a colorless precipitate which was washed further with n−hexane (3 ×
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10 mL). The compound was dried in vacuo. Complex 1 was isolated as a colorless solid. Yield:
86 % (0.400 g).
Characterization details of [Ir(COD)(PhPNHP)]Cl (1):
IR (ν(NH)): 3370 cm–1. HRMS: m/z = 742.2288 [M]+ (calcd m/z for C36H41IrNP2 = 742.2343).
Elemental analysis for C44H55NO2P2ClIr: Calcd: C, 57.47; H, 6.03; N, 1.52; Found: C, 57.37; H,
5.30; N, 1.64. Two molecules of THF were found by NMR spectroscopy.
2.2. Synthesis of [Rh(COD)(PhPNHP)]Cl (2): The synthetic procedure of [Rh(COD)(PhPNHP]Cl
(2) is similar to that of complex 1 except that [RhCl(COD)]2 (0.114 g, 0.231 mmol) and HN-
(CH2CH2PPh2)2 (0.200 g, 0.462 mmol) were used as starting materials. The reaction was stirred
for 2 h at 273 K under Ar atm. Complex 2 was isolated as a highly air-sensitive yellow solid
upon washing with n-hexane. Yield: 91 % (0.290 g)
Characterization details of [Rh(COD)(PhPNHP)]Cl (2):
IR (ν(NH)): 3370 cm–1. HRMS: m/z = 652.1759 [M]+ (calcd m/z for C36H41NP2Rh = 652.1769).
Elemental analysis for C36H41NP2ClRh: Calcd: C, 62.84; H, 6.01; N, 2.04; Found: C, 59.87; H,
5.77; N, 1.99. The discrepancy could be attributed to oxidation of phosphine while weighing the
sample in air for analysis (calcd for C36H41NP2ClRh·O2: Calc: C, 60.05; H, 5.74; N, 1.95).
2.3 Synthesis of [IrH2Cl(PhPNHP)] (3)
[Ir(COD)(PhPNHP)]Cl (0.486 g, 0.715 mmol) was dissolved in 1,4-dioxane (30 mL) in a Parr
reaction vessel (30 mL). The reaction setup was purged with H2 before pressurizing the reactor
with 10 bar of H2. Subsequently, the reaction was stirred at 70 °C for 16 h. Then the solution was
7

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filtered over Celite, concentrated to about 5 mL, addition of n-hexane gave a colorless precipitate
which was washed further with more n-hexane (3 × 10 mL) and then dried in vacuo. Complex 3
was isolated as a colorless solid. Yield: 70 % (0.336 g).
Characterization details of [IrH2Cl(PhPNHP)] (3):
IR (ν(NH)): 3386 cm–1. HRMS: m/z = 636.1465 [M−Cl]+ (calcd m/z for C28H31NP2Ir =
636.1561).
Elemental analysis for C28H31NP2ClIr: Calcd: C, 50.11; H, 4.66; N, 2.09; Found: C, 50.05; H,
4.749; N, 2.12.
2.4 Synthesis of [RhH2Cl(PhPNHP)] (4): The synthetic procedure of [RhH2Cl(PhPNHP)] 4 is
similar to that of complex 3 except that [Rh(COD)(PhPNHP)]Cl (0.200 g, 0.290 mmol) was used
as the starting material. Complex 4 was isolated as a pale brown colored solid. Yield: 60 %
(0.100 g).
Characterization details of [RhH2Cl(PhPNHP)] (4):
IR (νNH): 3410 cm–1. HRMS: m/z = 544.0830 [M−(Cl+H2)]+ (calcd m/z for C28H29NP2Rh =
544.0830).
Elemental analysis for C28H31NP2ClRh: Calcd: C, 57.80; H, 5.37; N, 2.41; Found: C, 57.53; H,
5.24; N, 2.29.
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2.5 In-situ characterization of [IrH2(PhPNP)] complex (5) {PNP = -N-(CH2CH2PPh2)2}
Inside a glove box, [IrH2Cl(PhPNHP)] complex (0.019 g, 0.028 mmol) and KOtBu / LiN(SiMe3)2
(0.014 g, 0.120 mmol / 0.008 g, 0.048 mmol) were suspended in C6D6 (0.6 mL) in a vial. The
reaction mixture was shaken for few minutes. The colorless solution turned dark yellow. The
clear solution was transferred to a NMR tube and spectra were acquired. Complex 5 was
characterized only in solution due to its instability during isolation.
Characterization details of [IrH2(PhPNP)] (5): 1H (C6D6, 293 K): −20.10 (t, J(H,Pcis) = 9 Hz, 1H,
Ir-H), 2.69 (m, 4H, NCH2), 2.88 (m, 2H, PCH2), 3.49 (m, 2H, PCH2), 7.01-8.09 (m, 20H, Ph).
31P (C6D6, 293 K): 53.89 (s).
2.6 Synthesis of [IrH3(PhPNHP)] (6)
[IrH2Cl(PhPNHP)] complex (0.418 g, 0.623 mmol) and KOtBu (0.278 g, 1.69 mmol) were taken
together in a Parr hydrogenation reaction vessel. Then, THF (10 mL) was added under H2 atm.
The reaction flask was purged with H2 before pressurizing the vessel to 10 bar. Then, the
reaction contents were heated to 50 °C for 12 h. The reaction mixture was cooled to 298 K and
filtered through Celite to remove KCl. Solvent was removed under vacuum. The yellow solid
was washed with n-hexane (3 x 10 mL) and dried under vacuum. Complex 6 was isolated as a
pale yellow solid. Yield: 81 % (322 mg).
From the NMR spectra, it was evident that the mer (6a) and fac (6b) isomers were formed in
50:50 ratio respectively.
Characterization details of [IrH3(PhPNHP)] (6):
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IR (ν, cm-1): 3370 (NH), 2090 (Ir-H). HRMS: 632.1193 {(M+-(H+2H2)
Elemental analysis: for C28H32NP2Ir·KCl: Calcd: C, 47.28; H, 4.53; N, 1.97; Found: C, 46.88; H,
4.78; N, 1.86.
2.7 Synthesis of [IrH2(η1-O-C(O)H)(PhPNHP)] (7)
[IrH3(PhPNHP)] complex (0.020 g, 0.031 mmol) was stirred under CO2 (1 atm) for 12 h at room
temperature in THF (5 mL). The yellow colored solution paled down. The volume of the solvent
was reduced by purging CO2 through the solution before acquiring the NMR spectra. Attempts to
isolate complex 7 were not successful because of its instability in the solid state.
2.8 Catalytic hydrogenation of CO2
A general procedure for the catalytic hydrogenation of CO2 is described here. Catalyst
[IrH3(PhPNHP)] (0.006 g, 9.42 µmol) was weighed inside the glove box and transferred to a 25
mL Parr reactor vessel. Then, a small amount of THF (1 mL) and an aqueous solution of base
were added (2-4 mL, 2-4 mmol). Finally, the reactor was pressurized with CO2 (7 bar) followed
by H2 (7 bar) to a total pressure of 14 bar (1:1). The reaction parameters such as temperature (T
(°C)) and time (t (h)) were set as summarized in Table 3 (S.M). Upon reaction completion, H2O
and THF were removed under vacuum. Then, D2O was added to the reaction mixture and it was
analyzed using 1H and 13C NMR spectroscopy.
2.9 General procedure for hydrogenation of carbonyl and imine substrates
[IrH3(PhPNHP)] (0.005 g, 7.85 µmol) was transferred to a 25 mL Parr reaction vessel inside a
glove box. Then, substrate (7.85 mmol) and 10 mL of MeOH were added. The reaction contents
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Table 1. 1H and 31P NMR spectral data for complexes 1-7
Complex
1H
31P
J (Hz)
CH2
L
N-H
Ph
PhPNH J(Rh-
Ir–H / Rh−H
P
P)
[Ir(PhPNHP)(COD)]Cl (1) a
[Rh(PhPNHP)(COD)]Cl (2) a
[IrH2Cl(PhPNHP)] (3)
-
-
-
2.30-
1.44 (8H) 8.21 (br 7.10-7.58
(m, 20H)
(br s, 4 H)
15.00
-
3.46 (m, and 3.94 s, 1H)
8H)
-
2.48-
1.47 (8H) 8.74 (br 7.20-7.79
36.67
(d)
120
-
3.25 (m, and 4.31 s, 1H)
8H)
(m, 20H)
(br s, 4H)
(H,P) = 16, 2.52-
(H,H) = 7.5 3.51 (m,
-
4.19 (t, 7.36-7.90
26.31
−23.98 (td, 1H),
−19.04 (td, 1H)
1H),
(m, 20H)
Hz
8H)
J(H,H)
= 10 Hz
[RhH2Cl(PhPNHP)] (4)
(Rh,P) = 28, 2.30
(H,P) = 16, 3.27 (m,
–
-
4.30 (t, 7.32-7.87
49.89
(d)
120
−18.74 (m, 1H),
−14.86 (m, 1H)
1H),
(m, 20H)
(H,H)
Hz
=
=
9
9
8H)
J(H,H)
= 10 Hz
[IrH2(PhPNP)] (5)
(H,P)
Hz
2.69-
3.49 (m,
8H)
-
-
-
7.01-8.09
(m, 20H)
53.89
32.18
−20.10 (t, 2H)
mer-[IrH3(PhPNHP)] (6a)
(H,P)
=
2.12-
6.31 (t, 6.75-8.33
1H) (m, 20H)
-
-
−19.30 (m, 1H),
−10.87 (td, 1H),
−10.65 (td, 1H)
18.5, (H,H) 2.63 (m,
= 5 Hz
8H)
fac-[IrH3(PhPNHP)] (6b)
(H,Ptrans
)
=
1.72
–
-
5.26 (m, 6.75-8.33
1H) (m, 20H)
27.89
(s)
−19.47 (m, 1H),
−10.35 (dd, 2H)
126, (H,Pcis) 3.45 (m,
= 21, (H,H) 8H)
= 5 Hz
[IrH2(η1-O-C(O)H)(PhPNHP)]
(7) b
(H,P)
13.5, (H,H) 3.54 (m,
= 7 Hz 8H)
=
2.28
–
9.09 (t)
8.81 (br 7.32-7.95
s, 1H) (m, 20H)
31.63
-
−27.10 (m, 1H),
17.97 (m, 1H)
a
b
L = COD, L = HCOO−: For complexes 1 and 3, NMR spectra were recorded in CDCl3. For
complexes 2 and 7, NMR spectra were acquired in THF-d8. For complex 4, NMR spectrum was
acquired in 1,4-dioxane-d8. For complexes 5, and 6, NMR spectra were collected in C6D6.
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were purged with H2(g) for few minutes. The reaction mixture was stirred at 50 °C for 6-9 h
(Table 4). Upon reaction completion, it was allowed to cool to 298 K and the solvent was
removed under vacuum. The crude contents were passed through silica column to remove the
catalyst and eluted with EtOAc / pet ether. The sample was analyzed by NMR spectroscopy and
GC-MS (S.M.).
2.10 X-ray crystal structure determination of complexes [Ir(COD)(PhPNHP)]Cl (1),
[IrH2Cl(PhPNHP)] (3), [RhH2Cl(PhPNHP)] (4) and [IrH3(PhPNHP)] (6)
Colorless crystals of [Ir(COD)(PhPNHP)]Cl (1) complex was obtained by slow diffusion of
hexane into a concentrated CH2Cl2 solution at 280 K. Colorless crystals of [IrH2Cl(PhPNHP)] (3),
[RhH2Cl(PhPNHP)] (4) and yellow crystals of [IrH3(PhPNHP)] (6) complex were obtained from
their THF solutions upon standing at 298 K. A single crystal suitable for diffraction study was
chosen after careful examination under a microscope. The crystal was coated with paraton oil
and mounted. Data was collected at 100 / 160 K. The unit cell parameters and intensity data were
collected using a Bruker SMART APEX CCD diffractometer equipped with a fine focus Mo-Kα
X-ray source. The SMART software was used for data acquisition and the SAINT program was
used for data reduction. [20] The empirical absorption corrections were made using SADABS
program. [21] The structure was solved using SHELX program. [22] All non-hydrogen atoms
were located by difference Fourier map and refined anisotropically. The Ir−H and N−H hydrogen
atoms were located in the difference Fourier map and refined isotropically. Other relevant data
for structure refinement are summarized in the S.M.
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3. Results and Discussion
3.1 Synthesis and characterization of [IrH3(PhPNHP)] complex (6)
The [IrH3(PhPNHP)] complex (6) was synthesized as shown in Scheme 1. Treatment of
[MCl(COD)]2 {M = Ir, Rh} precursor complex with pincer ligand HN-(CH2CH2PPh2)2 {PhPNHP}
in THF gave [M(COD)(PhPNHP)]Cl {M = Ir (1), Rh (2)} complexes in excellent yields, 86 and
91 %, respectively. The allylic and the vinylic protons of Ir-COD fragment appear at 1.43 (br s)
1
and 3.92 (br s) ppm respectively, in the H NMR spectrum. Further, cooling the solution of
complex 1 below 243 K, resulted in two different of sets signals for the allylic and vinylic the
protons of COD moiety, respectively (S.M.). The N−H hydrogen signal was noted downfield at
8.18 (br s) ppm suggesting that there could be possible N-H···Cl interaction. In the IR spectrum,
a broad band at 3370 cm-1 further supports hydrogen bonding interaction in this complex. Similar
spectral features were noted for the Rh analogue. The PhPNHP ligand appeared at 15.00 (s) and
1
36.71 (d) ppm, respectively for complexes 1 and 2 in the 31P NMR spectra. The J(Rh,P) was
found to be 120 Hz. The [Rh(COD)(PhPNHP)]Cl (2) complex was found to be extremely air
sensitive whereas, the iridium analogue 1, air stable. The N−H in [M(COD)(PhPNHP)]Cl
complexes readily get deprotonated by Et3N to form neutral complexes [M(COD)(PhPNP)] {M =
Ir (1a), Rh (2a); PhPNP = -N(CH2CH2PPh2)2} respectively along with the formation of Et3NHCl.
Previously, Don Tilley et al. reported the synthesis of [Ir(COD)(PNP)]Cl complex via reaction of
[Ir(COD)Cl]2 with PNP ligand having a rigid spacer (PNP = HN(2-PPh2-4-Me-C6H3)2) in
toluene. [23] During the preparation of this manuscript, Casado et al. reported the synthesis and
structural characterization of [Ir(COD)(PhPNHP)]Cl (1) complex. [12]
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Reaction of [M(COD)(PhPNHP)]Cl {M = Ir (1), Rh = (2)} complex with H2 (10 bar) at 70 °C in
THF / 1,4-dioxane solvent resulted in the oxidative addition of H2 to form the cis-dihydride
[MH2Cl(PhPNHP)] {M = Ir (3), Rh (4)] complex. During the course of the reaction, COD gets
hydrogenated into cyclooctane. Complexes 3 and 4 were isolated in moderate yields, 70 and 60
% respectively. The Ir−H hydrogen signals were noted at −23.98 (td) and −19.04 (td) ppm with
1
cis J(H,P) = 16 Hz in the H NMR spectrum of 3. The mutual coupling constant between the
hydrides was found to be 7.5 Hz. Whereas, in the case of [RhH2Cl(PhPNHP)] complex 4, the
Scheme 1. Synthetic route for the preparation of [IrH3(PhPNHP)] complex (6).
PPh2
Cl
PPh2
THF
rt
Cl
Cl
H
N
H
N
+
M
M
M
PPh2
P
Ph2
(
PhPNHP)
M = Ir, Rh
M = Ir (1), Rh (2)
H2 (10 bar)
1,4-dioxane
Cl
H
N
H
H
N
H-H
H
KOtBu
PPh2
H
N
PPh2
H
H
Ph2P
P
Ir
H
M
H
Ir
+
P
P
THF
- KCl, tBuOH
H
Ph2
Ph2
Ph2
H
(6b)
(6a)
mer isomer
M = Ir (3), Rh(4)
fac isomer
hydride signals appeared at −18.74 (m) and −14.86 (m) ppm with J(H,Pcis) of 16 Hz and 2J(H,H)
1
of 9 Hz in the H NMR spectrum. Upon 31P decoupling, the J(Rh,H) was found to be 28 Hz
(S.M.). The origin of the difference in hydride chemical shifts between Ir−H and Rh−H could be
traced to the electronic structure of 5d vs 4d metal center. [24] The N−H hydrogen signal
14

ACCEPTED MANUSCRIPT
appeared at 4.19 (t) and 4.30 (t) ppm for complexes 3 and 4, respectively. In the 31P NMR
spectrum, the PhPNHP ligand appeared at 26.31 (s) and 49.89 (d) ppm, respectively, for
complexes 3 and 4. The J(Rh, P) coupling constant was found to be 120 Hz for complex 4. In the
IR spectrum, a broad band was noted at 3386–3410 cm-1 for these complexes indicating
hydrogen bonding interactions between N−H···Cl (S.M). The rhodium complexes are highly air
sensitive whereas, the iridium analogues are air stable. Complex 3 could also be synthesized by
the reaction of [IrCl(COE)2]2 with the PhPNHP ligand under H2 (10 bar).
The analogous Ir complex [IrH2Cl(iPrPNHP)] {iPrPNHP = HN-(CH2CH2PiPr2)2} was synthesized
i
by reacting [Ir(COE)2Cl]2 with HN(CH2CH2PiPr2)2 in PrOH at 80 °C. [6] Iluc and co-workers
reported the formation of [IrH2Cl(PNP)] {PNP = N-di(2-diisopropylphosphine-4-methylphenyl-
O-tolylamine)} complex via reaction of [Ir(COD)Cl]2 with PNP ligand under H2 atm. The PNP
ligand has a rigid backbone based on arene moiety and the nitrogen is substituted by tolyl (tolyl
=
C6H4CH3) moiety. [25] Similarly, Goldman et al. reported the synthesis of
[(tBufurPOP)RhH2Cl] {tBufurPOP = 2,5-bis((di-tert-butylphosphino)methyl)furan} complex
through reaction of [Rh(NBD)Cl]2 (NBD = norbornadiene) with tBufurPOP ligand in toluene
under 1 atm of H2 at 110 °C. [26]
Reaction of [IrH2Cl(PhPNHP)] (3) complex with excess KOtBu / LiN(SiMe3)2 in C6D6 / THF-d8
resulted in the formation of a highly unstable 16-electron amido dihydride complex
−
t
[IrH2(PhPNP)] (PNP = N(CH2CH2PPh2)2) (5) via elimination of KCl and BuOH / LiCl and
HN(SiMe3)2 respectively. A single hydride signal was noted at −20.10 (t) with a J(H,Pcis) of 10
1
31
Hz in the H NMR spectrum. The P NMR spectrum is comprised of a signal at 53.90 (s) ppm
which is significantly upfield shifted compared to the starting material 3. Based on the NMR
15

ACCEPTED MANUSCRIPT
spectral characteristics, the geometry of complex 5 could be described as a trigonal bipyramid.
The intermediate [IrH2(PNP)] (5) is too unstable to be explored for further reactions. On the
other hand, a related complex [IrH2(iPrPNP)] (iPrPNP = -N-(CH2CH2PiPr2)2) where the phosphine
ligand bears isopropyl substituents has been isolated, suggesting the role of steric and electronic
factors imparted by isopropyl substituents in stabilizing 16-electron species. [6]
The iridium amido dihydride complex [IrH2(PhPNP)] (5) reacts rapidly with H2 (6-10 bar) to
afford the iridium trihydride complex [IrH3(PhPNHP)] (6) wherein H2 is added across the Ir−N
bond via 1,2-addition. [27] Fryzuk and his co-workers found that H2 adds across the Ir−N bond
-
in [IrH2(PNP)] {PNP = N(SiMe2CH2PPh2)2} system to form H−N−Ir−H motif in an iridium
amine trihydride complex [IrH3(PNHP)] {PNP = HN(SiMe2CH2PPh2)2}. Later, 1,2-addition
reaction was observed in case of a few other systems based on Re and Ru pincer complexes. [28]
It is now recognized as an important mode of bond activation in catalysis. [29] In contrast, the
t
reaction of [IrH2(κ3-PNN)] {PNN = Bu2PC2H4NC2H4NEt2} complex in which the pincer arms
are unsymmetrical with H2 did not result in the formation of the anticipated trihydride complex,
[IrH3(κ3-PNHN)]. Instead, the presumed transient trihydride quickly dimerizes into a bridging
hydride complex [IrH2(κ2-PNHN)]2(µ-H)2 suggesting subtle electronic factors in stabilizing the
trihydride systems. [30] During the course of investigation of chiral N-alkyl pincer ligands for
hydrogenation reactions, Bianchini et al. isolated a facial trihydride complex [IrH3(PNRP)] (PNP
=
CH3CH2CH(Me)-N(CH2CH2PPh2)2,
R
=
-CH(Me)CH2CH3) via reaction of
[Ir(COD)(H)(PNP)] with H2 at room temperature. [31]
The iridium trihydride [IrH3(PhPNHP)] (6) complex exists as two isomers: meridional (mer) and
facial (fac), in the latter, the mutual trans arrangement of the strong trans influencing hydride
16

ACCEPTED MANUSCRIPT
1
ligands is avoided. From the H NMR spectrum, these two isomers were found to be in a 50:50
ratio. Eight sets of distinct ‘CH2’ signals in the range of 1.72-3.45 ppm were observed in the 1H
1
NMR spectrum for both mer and fac isomers, respectively. The H NMR spectrum of the mer
isomer 6a shows two distinct hydride signals with identical triplet of doublet (td) pattern at
−10.87 and −10.65 ppm (td) for the trans arrangement Ir−H moiety. The two trans hydrides were
not equivalent. The coupling constants of J(H,Pcis) of 18.5 Hz and J(H,H) of 5 Hz were found.
For the fac-isomer (6b), the hydride signals appear as a doublet of doublet pattern (dd) centered
at −10.35 ppm with J(H,Ptrans) of 120 Hz and J(H,Pcis) of 17 Hz. The signal of the hydride trans
to N appeared as a multiplet at −19.39 and −19.52 ppm for mer and fac isomers, respectively.
The N−H signal was noted at 5.26 (s) and 6.31 (t) respectively for the fac and mer isomers
(S.M.) in the 1H NMR spectrum. The PhPNHP ligand appeared at 31.89 (s) and 27.43 (s) ppm for
fac and mer isomers of [IrH3(PhPNHP)] (6), respectively in the 31P NMR spectrum. A broad band
at 3370 cm-1 in the IR spectrum suggests the presence of hydrogen bonding in complex 6.
3.2 X-ray crystal structures
ORTEP views of crystal structures of complexes 3 and 6 are shown in Fig 1. The crystal
structures of [Ir(COD)(PhPNHP)]Cl (1) and [RhH2Cl(PhPNHP)] (4) complexes and the ball and
stick packing of all the structures have been deposited in the S.M. In all of the structures, we
were able to locate the hydrogen atoms bound to iridium and nitrogen from the difference
Fourier maps and refined freely. The important bond lengths and bond angles of these complexes
have been summarized in Table 2. Complex 1 was crystallized in CH2Cl2 / n-hexane solvent
system. Casado et al. recently reported the structural characterization of complex 1 via
crystallization in acetone / diethyl ether. [12] In the crystal lattice two CH2Cl2 molecules were
17

ACCEPTED MANUSCRIPT
found which facilitates extensive hydrogen bonding network in the solid state. The structural
parameters of complex 1 compare fairly well with reported structure except marginal difference
in the torsion angles of the Ir-COD fragment (C8-C1-C2-C3 and C4-C5-C6-C7). [12]
Fig 1. ORTEP views of [IrH2Cl(PhPNHP] (3) and [IrH3(PhPNHP] (6) complexes shown at 50%
ellipsoid probability level. All hydrogen atoms were omitted for clarity except Ir−H and N−H.
MH2Cl(PhPNHP)] {M = Ir (3), Rh (4)} complexes crystallize in triclinic (P-1) and monoclinic
(P21/c) crystal systems, respectively. These complexes adopt a slightly distorted octahedral
structure. The bond angles P(1)-M(1)-P(2) and N(1)-M(1)-H(1) are in the range of 165.26-
166.97° and 175.40-178.67°, respectively. The ligand rearranges to meridional geometry as
compared to facial arrangement in the starting complexes 1 and 2. One hydrogen atom was
located trans to chloride and the other was located trans to the nitrogen atom. Additionally,
extensive intra- (2.840 Å) and inter-molecular (2.515 Å) hydrogen bonding interactions between
N−H···Cl were found. The metrical parameters of complex [IrH2Cl(PhPNHP)] is comparable to
18

ACCEPTED MANUSCRIPT
the [IrH2Cl(iPrPNHP)] analogue.6 The bond distances and bond angles in [RhH2Cl(PhPNHP)] (4)
are comparable to those of [(tBuXanPOP)RhH2Cl] (tBuXanPOP
tertbutylphosphino)-9,9-dimethyl-9H-xanthene) complex. [26]
=
4,5-bis(di-
[IrH3(PhPNHP] (6) complex crystallizes in monoclinic crystal system in P21/c space group. The
structure shows deviations from an ideal octahedral geometry. The P(1)-Ir(1)-P(2) and N(1)-
Ir(1)-H(2) bond angles are 166.28 (4)° and 178.5 (15)°, respectively. The PhPNHP pincer ligand is
arranged in a meridional geometry and the other three positions are taken up by hydrogen atoms.
Two hydrogen atoms were located mutually trans to one another and the other hydrogen was
located trans to the nitrogen donor. In the crystal packing weak intra- (2.691 Å) and inter-
molecular (2.158 Å) N−H···H−Ir dihydrogen bonding interactions were evident. Dihydrogen
bond is an intriguing type of hydrogen bond where M−H (M = transition metal / main group
element) acts an acceptor and X−H (N, O) acts as a donor to constitute M−H···H−X interaction.
[32] The strength of this interaction was estimated to be in the range 3-7 kcal / mol. Dihydrogen
bonding has been recognized to play an important role in homogeneous catalysis where bi-
functional / metal-ligand cooperative mechanism is involved. [33]
It must be noted that the crystal structure of the catalytically active relevant iridium trihydride
complexes [IrH3(PNHP)] and {PNHP = HN-(CH2CH2PiPr2)2 and HN(SiMe2CH2PPh2)2} has not
been previously reported. [34] However, crystal structure of iridium trihydride [IrH3(PNP)] (PNP
= lutidine based pincer scaffold) complex supported by pyridine derived pincer ligands have
been reported. [35] Compared to complex 6, Ir−N (2.110(7) Å) bond distance is shorter and the
Ir−P (2.252(2) Å) bond distance is longer in [IrH3(PNP)] (PNP = lutidine based pincer scaffold).
The P(1)-Ir-P(2) bond angle of 167.43 (8)° in [IrH3(PNP)] (PNP = lutidine based pincer scaffold)
19

ACCEPTED MANUSCRIPT
is comparable to that in complex 6. In the [IrH3(PNP)] (PNP = lutidine based pincer scaffold)
complex, the pincer motif was found to adopt planar arrangement whereas in complex 6, the
pincer moiety is non-planar. This is attributed to the ‘CH2’ spacer in the PhPNHP ligand.
Table 2. Important bond length (Å) and bond angle (°) parameters for [IrH2Cl(PhPNHP)] (3) and
[IrH3(PhPNHP)] (6)
3
6
2.15 (5)
2.201 (3)
2.245 (10)
2.245 (10)
1.61 (12)
1.54 (4)
M−N(1)
2.248 (17)
2.274 (17)
2.478 (14)
1.60 (4)
M−P(1)
M−P(2)
M− /Cl(1) / H
M−H(1)
1.90 (3)
1.51 (4)
M−H(2)
84.60 (13)
167.25 (6)
165.17 (10)
173.31 (10)
84.36 (9)
166.29 (4)
178.31 (15)
175.64 (2)
N(1)−M−P(1)
P(1)−M−P(2)
N(1)−M− H(1)
H(2)−Ir−Cl / H(1)
3.3 Reaction of [IrH3(PhPNHP)] (6) with CO2
Reaction of the iridium trihydride complex [IrH3(PhPNHP)] (6) with CO2 (1 atm) under ambient
conditions lead to the formation of the formate derivative [IrH2(η1-OC(O)H)(PhPNHP)] (7) via
insertion of CO2 into the Ir−H bond (eq 2). The 1H NMR spectrum of complex 7 is comprised of
two hydride signals at −17.97 (td) and −27.07 (m) ppm with J(H,Pcis) of 13.5 Hz. The coupling
20

ACCEPTED MANUSCRIPT
between the inequivalent hydrides was found to be J(H,H) = 7 Hz. The signal for the formate
moiety HCOO− was found at 9.08 (t) ppm with J(H,H) = 10 Hz. The N−H signal was seen in the
downfield region as a broad singlet at 8.08 ppm. The 31P NMR spectrum shows a signal at 31.63
ppm for the PhPNHP ligand. The carbonyl carbon of the formate group appears at 172.16 ppm in
the 13C NMR spectrum. These spectral characteristics are similar to those of other reported
complexes. For example, reaction of trans-[ReH2(NO)(iPrPNHP)] with CO2 led to its facile
insertion into the Re−H bond to form trans-[ReH(O2CH)(NO)(iPrPNHP)]. [36] Similarly,
Bernskoetter et al. reported the formation of a formate complex [FeH(η1-OC(O)H)(CN-
R’)(iPrPNHP)] {R’ = 2,6-dimethoxyphenyl, 4-methoxyphenyl} via reaction of an unstable trans-
[FeH2(CN-R’)(iPrPNHP)] complex with CO2 (1 atm). The Fe-formate complex decomposed
slowly at room temperature in few hours. In addition to Fe-formate complex, few other
unassigned products were noted in this reaction. [37] Attempts to isolate complex 7 were not
successful because of its instability in the solid state.
H
O
O
H
H
Ir
rt
N
H
H
N
PPh2
H
CO2 (1 atm)
N
PPh2
H
H
Ph2P
P
Ir
+
(2)
Ir
H
P
P
C6D6
H
Ph2
Ph2
Ph2
H
H
(7)
(6b)
(6a)
3.4 Catalytic hydrogenation of CO2
Catalytic hydrogenation of CO2 was carried out at a total pressure of 14 bar of H2 and CO2 (1:1)
in the presence of 1 M base (aq). The performance of the catalyst [IrH3(PhPNHP)] (6) was
evaluated under different conditions and the data are summarized in Table 3. In all cases small
21

ACCEPTED MANUSCRIPT
amount of THF was added to dissolve the catalyst. In the absence of the catalyst, formation of
HCOOK was found to be negligible. An optimum catalyst loading of 0.2–0.4 mol % was used.
The turn over number (TON) was calculated using Me3Si(CD2)2-CO2Na (sodium-3-
1
trimethylsilylpropionate) as an internal standard by H NMR spectroscopy (S.M.). A maximum
TON of 144 was obtained at room temperature using KOH as an aqueous base. The highest TOF
of 47 h-1 was reached at 80 °C. Using KOH (aq) or NaOH (aq) as a base lead to the formation of
KHCO3 and NaHCO3 along with HCOOK / HCOONa, respectively under the reaction
conditions. This was confirmed by analysis of the reaction mixture using 13C NMR spectroscopy
(S.M.). Comparable TONs of 65 and 50 were achieved when LiOH·H2O and NaOH were used as
bases, respectively at 65 °C. Using [IrH2Cl(PhPNHP)] complex 3 as catalyst, a TON of 7 was
achieved at room temperature in a duration of 24 h; however at 100 °C, a TON of 144 was
reached in 14 h.
The relatively low TON, though under mild conditions of complex 6 could be attributed to the
instability of the catalyst in aqueous KOH solution. Also, the relatively high electrophilicity of
the metal complex compared to other reported complexes due to the presence of phenyl group on
the phosphorus would also contribute to the low TON. Nozaki et al. reported the formation of
HCOOK in 1 M KOH (aq) solution from H2 and CO2 promoted by [IrH3(PNP)] (PNP = lutidine
based pincer scaffold) catalyst with the highest TON and TOF of 350000 and 150000 h-1,
respectively. The catalyst operates at 200 °C and 50 bar of total pressure of 1:1 H2 and CO2. [35]
Hazari and co-workers investigated the role of secondary sphere interactions between N−H
fragment of pincer ligand and the oxygen atom of formate moiety in M-O-C(O)H which
facilitates the insertion of CO2 into Ir−H bond in [IrH3(iPrPNHP)] complex. The bifunctional
22

ACCEPTED MANUSCRIPT
behavior of the catalyst due to N−H···O−C(O)H−M interaction makes it a very active catalyst.
This catalyst also operates at 185 °C and 55 bar of total pressure. [5]
Table 3. Optimization of reaction conditions for the catalytic hydrogenation of CO2
cat (0.2 mol %)
(3)
H2 + CO2
HCOOK
1 M KOH(aq)
T °C, t h
Sl No
T (°C)
t (h)
Aq. Base
(1M)
TON
1
2
3
25
25
45
19
30
36
KOH
LiOH
LiOH
144
66
96
4
5
6
7
8
50
65
65
65
80
15
24
24
31
42
KOH
NaOH
LiOH
KOH
KOH
122
65
50
72
101
9 a
100
100
14
15
KOH
KOH
144
141
10
a
In all the cases, 1 mL of THF was added to solubilize the catalyst; [IrH2Cl(PhPNHP)] complex
was used as a catalyst. TON = No of moles of product formed / moles of catalyst. The calculated
TON is the average of two runs.
The proposed mechanism for the hydrogenation of CO2 in 1 M base (aq) is outlined in Scheme 2.
Attack of the nucleophilic trans Ir−H bond onto CO2 in the first step leads to formation of the
23

ACCEPTED MANUSCRIPT
Scheme 2. Proposed catalytic cycle for the formation of HCOOK using [IrH3(PhPNHP)] complex
(6) in 1 M KOH aqueous solution.
H
O
O
H
H
H
N
H
N
PPh2
H
N
PPh2
H
H
H
CO2 (1atm)
THF
Ph2P
P
Ir
H
+
Ir
Ir
P
P
Ph2
Ph2
Ph2
H
H
(7)
(6b)
(6a)
KOH(aq)
HCOOK, H2O
H2
PPh2
H
Ir
N
H
PPh2
(5)
formate complex [IrH2(η1-OC(O)H)(PhPNHP)] (7). [38] The formate complex [IrH2(η1-
OC(O)H)(PhPNHP)] (7) further gets stabilized by the secondary sphere interactions between the
1
oxygen atom of formate group HCOO-Ir and N-H.5 Evidence for this was obtained from H
NMR spectroscopy of a signal for the N−H moiety (δ 8.8 ppm). In the second step, reaction of
the formate (7) complex with KOH results in the elimination of HCOOK with concomitant
formation of a highly reactive amido dihydride complex [IrH2(PhPNP)] (5) (PNP =
−N(CH2CH2PPh2)2) and H2O. Subsequently, H2 adds across the Ir−N bond in the amido
dihydride [IrH2(PNP)] complex (5) to regenerate the catalyst [IrH3(PhPNHP)] (6), thus completing
the catalytic cycle. The mechanism is supported by NMR spectral study of the catalytic cycle
(S.M.). The plausible mechanism proposed here is closely related to the ones reported in the
literature. [39]
24

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3.5 Catalytic hydrogenation of carbonyl and imine substrates
In addition to catalytic hydrogenation of CO2, [IrH3(PhPNHP)] (6) complex acts as a catalyst for
hydrogenation of polar organic substrates containing carbonyl and imine functionalities (Table
4). Reaction conditions were optimized using acetophenone as a model substrate (S.M.). A
catalyst loading of 0.1 mol % was used. Acetophenone and its derivatives were hydrogenated
completely at 50 °C with H2(g) (10 bar) in 5-9 h. In case of p-nitroacetophenone only 10 %
conversion to the corresponding alcohol product was noted under similar conditions.
Benzaldehyde and 2-butanone, were quantitatively reduced to benzyl alcohol and 2-butanol,
respectively in 6 h at 50 °C. Benzophenone and tetralone were reduced to the corresponding
alcohol products, respectively in 75 % conversion in 6 h at 50 °C. However, with increased
reaction time (9 h, 60 °C), complete conversion was achieved. Moderate conversions were noted
for the hydrogenation of N-benzylidene aniline and N-benzylidene benzylamine (49 %) at much
slower rates compared to carbonyl compounds under similar conditions. Styrene, 3-pentanone
and methyl benzoate did not undergo hydrogenation under the reaction conditions (20 bar, 50 °C,
6 h) or even harsh conditions. The lack of hydrogenation of 3-pentanone compared to 2-butanone
is attributed to its symmetrical nature and steric hindrance associated with it.
To date, there have been very few reports for the hydrogenation of carbonyl and imine substrates
using iridium metal complexes as catalysts. [40] Base-free hydrogenation reactions using H2(g)
catalyzed by iridium hydride metal complexes are even more scarce.
25

ACCEPTED MANUSCRIPT
Table 4. Hydrogenation of carbonyl and imine substrates using either [IrH3(PNHP] (6) or
[IrH2Cl(PhPNHP)] (3) / KOtBu as a catalyst.
X
cat 0.1 mol %
XH
R'
R, R' = H, aryl or alkyl
cat = [IrH3(PNHP)]
(4)
R
R'
R
H2 (g)
X = O, N-R
MeOH
26

ACCEPTED MANUSCRIPT
Sl No
substrate
Product
% conversion
> 99
t (h)
5-9
O
OH
CH3
CH3
1a
Y
Y
Y = H, OMe, Cl, NO2
O
OH
2
3
75
6
6
6
6
6
6
6
-
O
H
OH
> 99
> 99
> 99
74
OH
4
O
5
O
OH
6
O
OH
7b
8c
9
Ph
Ph
Ph
N
N
H
31
N
N
O
Ph
H
49
O
CH3
No reaction
No reaction
-
10
-
-
27

ACCEPTED MANUSCRIPT
Reaction conditions: (a) Y = NO2, only 10 % conversion was noted at H2 (20 bar), 50 °C, 6 h in
MeOH. (b) S:C ratio is 400:1 (c) The substrate to catalyst ratio was 1000:1 for all substrates
except imines (400:1). The products were isolated through silica gel column chromatography
(EtOAc / pet ether). All the reactions were repeated twice.
Compared to transfer hydrogenation, direct hydrogenation using H2(g) is considered to be a
green route for industrial scale applications. [41] For example, Abdur-Rashid et al. reported
direct hydrogenation (H2 = 10 bar) of carbonyl substrates in MeOH at room temperature using
[IrH3(iPrPNHP)] complex. [42] The catalytic activity of [IrH3(PhPNHP)] (6) is comparable to that
of [IrH3(iPrPNHP)] complex under the given conditions. The substituents on phosphorus of the
i
RPNHP pincer ligand (R = Pr vs Ph) have no influence on the reduction of carbonyl and imine
compounds. Additionally, complex 6 also hydrogenates imines N-benzylidene aniline and N-
benzylidene benzylamine (entries 7 and 8, Table 3). Claver et al. reported catalytic
hydrogenation of imines using diorthometalated iridium phosphite complex [IrH(COD)P(OC6H2-
2,4-tBu2)2(OC6H2-2,4-tBu2)] at 30 bar H2(g) at 40 °C in MeOH with a substrate to precatalyst
ratio of 100:1. [43] Herrera and co-workers reported hydrogenation of imines at 200 psi pressure
of H2(g) at 60 °C for 18 h with a S:C of 100:1 using [M(COD)(PPh3)2]PF6 {M = Ir, Rh}. [44]
Our attempts to employ [RhH2Cl(PhPNHP)] (4) complex as a catalyst for the hydrogenation of
CO2, carbonyl and imine substrates were not successful. This could be attributed to the lack of
formation of an active catalyst, [RhH3(PhPNHP)] complex under the reaction conditions.
To elucidate the mechanism of the catalytic reaction, hydrogenation of acetophenone was
investigated using NMR spectroscopy. Dissolution of [IrH3(PhPNHP)] (6) in MeOD-d4 resulted in
H−D exchange between MeO-D and Ir−H. Free H−D signal was noted at 4.60 ppm with J(H,D)
28