Structure and vibrational spectroscopic study of phthalimido-functionalized N-heterocyclic palladium complexes. Correlations between structure and catalytic activity

Abstract

New complexes of Pd(II) with cationic phthalimido-functionalized N-heterocyclic carbene (NHC), the cis-Diacetonitrile(chloro)[1-(2′-phthalamidoethyl)-3-methylimidazolin-ylidene]palladium(II) Hexafluorophosphate (and its CD3CN derivative) had been successfully used as catalyst in the Suzuki–Miyaura cross-coupling reaction. This complex have been studied by structural (XRD), spectroscopic (infrared, far-infrared and Raman) and theoretical (DFT and normal coordinate calculations) methods. Palladium is bound to two acetonitrile ligands and one-one chloride ion and phthalimido-functionalized imidazolium salt, [3-methyl-1-(2′-phthalimidoethyl) imidazolium] (L = ligand) in a square–planar cis arrangement. The molecular structure of the [PdCl(NCCH3)2L]+ was obtained by XRD and DFT geometry optimization. The two acetonitrile ligands exhibited two different Pd-N bonds a shorter one 2.080 Å (opposite to Cl ion) and a longer one 2.120 Å (trans to heterocyclic ligand). The cationic [PdCl(NCCH3)2L+] complex has as much as 132 fundamental modes and practically all of them have been assigned. The complex has no symmetry elements, but interpretation has been performed on the basis of local symmetries of its fragments. Tans-conformation of the bridging –CH2-CH2– group has been established from vibrational spectra, and this local structure avoided interaction of phthalimido moiety with the palladium center. Special attention has been paid for understanding and interpretation of the characteristic imidazolium ring vibrations connected to the strong electron donor phthalimido ring at 1553, 1397, 1347, 1218, 1149, 1118, 845, 748, 666, 629, 471, 245, 212, 124 and 104 cm−1 which bands are up shifted comparing to those of 2,3-dialkyl substituted ring modes. Clear differences were established between the fundamentals and force constants of the two differently coordinated acetonitrile. The more strongly coordinated CH3CN(1) has Raman bands at 2336, 958 and 279 cm−1 while the other CH3CN(2) has at 2321, 936 and 266 cm−1 like antisymmetric CN, CC and Pd-N stretching modes, respectively. The Pd-N stretching modes were assigned with the help of spectral and calculated data of CD3CN complex. For acetonitrile (1) the light and heavy stretching modes were 279 and 275 cm−1, respectively exhibiting 4 cm−1 isotope shift whyle those for acetonitrile (2) were as 266 and 264 cm−1 with less isotope shift. Rather small but different 1.630 and 1.493 Ncm−1 Pd-N stretching force constants were fitted for stronger and weaker bonded acetonitrile as compared to those of 3.931 Ncm−1 for [Pd(NCCH3)2]2+, and 4.050 Ncm−1 for cis-PtCl2(NCCH3)2. In contrast, the strongly weakened Pd-N bonds the Pd-Cl bond is enhanced and exhibiting relative high 2.03 Ncm−1 force constants in comparison with 1.53 Ncm−1 of [PdCl4]2- anion. The enhanced Pd-Cl bond strength is escorted by strong weakening of Pd-acetonitrile coordination. Surprisingly, from DFT calculation and spectroscopic data higher Pd-N force constant was obtained for the Pd-N bond with longer bond length and opposite, lower force constant for shorter Pd-N bond. These unusual discrepancies between bond lengths and bond force constants can be explained with the presence of electron-rich σ-donating ligand opposite to acetonitrile (1) which lead to enhancement of Pd-N bond. These specific properties of coordination sphere of [PdCl(NCCH3)2L]+ complex providing possibilities for easily releasing acetonitrile ligands during catalytic processes and give reasonable structural explanation of its high catalytic activity.