B. Bhattacharyya, A.J. Kalita, A.K. Guha et al.
Journal of Organometallic Chemistry 953 (2021) 122067
ester or phosphonic acid functionalized NHC complexes offer sig-
nificant potential in devising efficient catalytic systems.
to be at their local minima with real frequencies while the tran-
sition state was found to have one imaginary value of the fre-
quency. Further, the transition state was verified with intrinsic re-
action coordinate analysis. Energies are zero point and thermally
corrected. Electronic structures were analyzed using natural bond
orbital (NBO) analysis. Calculations were performed using GAUS-
SIAN16 suite of program [22].
During this study, NHC-Pd(II)-PEPPSI and bis(NHC)-Pd(II) type
complexes are prepared by employing a precursor imidazolium lig-
and containing N-pendant phosphonate group and their catalytic
efficacies in Suzuki-Miyaura cross coupling reaction were explored.
Starting from diethy-4-aminobenzylphosphonate, the precursor 4-
imidazol-1-yl-benzyl-phosphonic acid diethyl ester was prepared.
A bis(NHC)-Pd(II) complex and two NHC-Pd(II)-PEPPSI type com-
plexes were prepared by the reaction of the precursor phospho-
nate functionalized imidazolium salt with Pd(OAc)2 and PdCl2 re-
spectively. To the best of our knowledge, this set of complexes
are the only examples of NHC-Pd(II) complexes having N-pendant
phosphonate groups reported so far. The effect of the phospho-
nate group in the catalytic activity was also investigated during
this study. The activities of the catalysts were explored in Suzuki
coupling reaction of aryl chlorides and bromides with boronic acid.
2.3. Synthesis of [Pd(L1)2I2](1)
3-[4-(Diethoxy-phosphorylmethyl)-phenyl]-1-methyl-3H-
imidazol-1-ium iodide [HL1]I (0.5 mmol, 218 mg) was added
to a solution of Pd(OAc)2 (0.25 mmol, 56 mg) in dimethylsulphox-
ide (DMSO) and the resulting mixture was stirred for 6 hours at
ambient temperature. Thereafter, the reaction mixture was heated
at 130 °C under continuous stirring for another 4 hours. The re-
sultant mixture was added drop wise to a mixture of diethyl ether
(Et2O) and dichloromethane (4:1 v/v) under vigorous stirring. The
precipitate thus obtained was filtered and washed with Et2O and
dried at room temperature to isolate the product [Pd(L1)2I2] (1)
as yellow solid. Yield: 145 mg (58%), Elemental analysis % found
(calculated for C30H42I2N4O6P2Pd): C: 36.72 % (36.89 %) H: 4.62 %
(4.33 %) N: 5.66 % (5.74 %), FT-IR (KBr, cm−1): υ 3417 (br), 2983
(m), 2907 (w), 1573 (w), 1520 (m), 1384 (m), 1226 (s), 1052 (vs),
1022 (vs), 962 (s), 856 (m), 570 (m). 1H NMR (400 MHz, DMSO-d6,
δ/ppm): 8.06 (1H, ImH ), 7.69 (1H, ImH), 7.56 (2H, ArH), 7.45 (2H,
ArH), 3.94 (4H, -CH2), 3.78 (3H, -CH3), 3.35-3.32 (2H, -P-CH2), 1.19
(6H, -CH3). 13C NMR (100 MHz, DMSO-d6, δ/ppm): 133.08, 131.72,
130.83, 125.50, 124.07, 62.85, 33.14-31.80, 29.62, 16.85. 31P NMR
(DMSO-d6, δ/ppm): 26.48.
2. Experimental section
2.1. Materials and methods
Starting materials were purchased from commercial sources
and used without further purification. Ligand 4-imidazol-1-
yl-benzyl-phosphonic acid diethyl ester and 3-[4-(diethoxy-
phosphorylmethyl)-phenyl]-1-methyl-3H-imidazol-1-ium
iodide
[HL1]I, used for the synthesis of the three reported NHC-Pd(II)
complexes were synthesised by following an unpublished proce-
dure designed by our group with the help of a reported procedure
[19]. Solvents were purified by conventional techniques and dis-
tilled prior to use. Fourier transformed infrared spectra from 400
to 4000 cm−1 were recorded on Perkin Elmer Frontier MIR-FIR
FT-IR spectrophotometer. Elemental analyses were performed on
a Perkin Elmer Model PR 2400 series II elemental analyzer. NMR
spectra were recorded on a JEOL JNM-ECS 400 NMR spectrometer
operating at 400 MHz and Bruker ASCEND 600 NMR spectrometer
operating at 600 MHz and samples were dissolved in deuter-
ated solvents. Chemical shifts were reported in parts per million
downfield of Me4Si (TMS) as internal standard.
2.4. Synthesis of [Pd(L1)(py)I2] (2)
A mixture of ligand 3-[4-(diethoxy-phosphorylmethyl)-phenyl]-
1-methyl-3H-imidazol-1-ium iodide [HL1]I (218 mg, 0.5 mmol),
PdCl2 (97 mg, 0.55 mmol), KI (1 mmol, 166 mg) and K2CO3 (345
mg, 2.5 mmol) are dissolved in 10 mL of pyridine. The resultant so-
lution was heated to reflux and stirred overnight under N2 atmo-
sphere. After cooling the mixture to room temperature, all volatiles
were evaporated under reduced pressure. The obtained residue
was dissolved in CHCl3 (50 mL) and H2O (50 mL) was added. The
separated organic layer was washed with 5% CuSO4 solution, fol-
lowed by 5% brine solution and then dried over Na2SO4. The ex-
tracted organic layer was further filtered over short pad of silica
and washed thoroughly with hexane/acetone (2:1). The combined
organic solution was evaporated to give the product [Pd(L1)(py)I2]
(2) as orange powder. The complex thus obtained was dissolved
in CHCl3, and upon slow evaporation yellow block shape crys-
tal of the complex was obtained. Yield: 245 mg (65%), Elemen-
tal analysis % found (calculated for C20H26I2N3O3PPd): C: 31.80 %
(32.13 %) H: 3.52% (3.51%) N: 5.20% (5.62%). FT-IR (KBr, cm−1): υ
3436(br), 2924(s), 2853(w), 1603(m), 1512(m), 1450(s), 1387(m),
1237(s), 1237(s), 1017(s), 935(s), 866(w), 759(s), 693(s), 568(s),
482(w). 1H NMR (400 MHz, CDCl3, δ/ppm): 8.90 (1H, PyH ), 8.81
(2H, PyH), 7.96 (2H, ImH), 7.66-7.09 (4H, ArH), 7.50 (2H, PyH), 4.08
(3H, -CH3), 4.02 (4H, m, -CH2), 3.26-3.21 (2H, -P-CH2), 1.24-1.22
(6H, -CH3).13C NMR (150 MHz, DMSO-d6, δ/ppm): 178.57, 158.21,
150.87, 143.73, 137.64, 135.49, 135.44, 130.17, 129.98, 128.51, 66.88,
36.73-35.03, 34.23, 21.53. 31P NMR (CDCl3, δ/ppm): 26.27.
Single crystals of the complexes suitable for diffraction mea-
surements were used directly from the reaction mixtures. The
diffraction data for the compounds were collected on a Bruker
˚
APEX-II CCD Diffractometer using MoK radiation (λ=0.71073 A)
α
using ϕ and ω scans of narrow (0.5°) frames at 296 K. The struc-
tures were solved by direct methods using SHELXL-97 as imple-
mented in the WinGX program system [20]. Anisotropic refine-
ment was executed on non-hydrogen atoms. The aliphatic and aro-
matic hydrogen atoms were placed on calculated positions but
were allowed to ride on their parent atoms during subsequent
cycles of refinements. Positions of O-H hydrogen atoms were lo-
cated on a different Fourier map and allowed to ride on their
parent atoms during subsequent cycles of refinements. Crystallo-
graphic data for the structures in this paper have been deposited
with the Cambridge Crystallographic Data Centre, CCDC, 12 Union
Road, Cambridge CB21EZ, UK. Copies of the data can be obtained
free of charge on quoting the depository numbers CCDC 2096459
2.2. Computational details
2.5. Synthesis of [Pd(L2)(py)I2](3)
The structures were fully optimized without any symmetry con-
straints in the gas phase and at 298 K using M06-2X/def2-TZVP
level of theory [21]. Harmonic frequency calculations were also
performed at the same level of theory to understand the nature
of the stationary states. Intermediates and products were found
To [Pd(L1)(py)I2] (2) (0.24 mmol, 180 mg), 10 mL dry
dichloromethane was added. After complete dissolution of 2 in
dichloromethane, 10 equivalents of (CH3)3SiBr (312 mL, 2.36
mmol) was added and stirred for 4 hours at room temperature
2