Cyclopalladated Compounds with Bulky Phosphine (dppm): Synthesis, Characterization, and X-ray Diffraction †

: The reaction between a cholro-bridged dinuclear compound (a, b) and a diphosphine (dppm) ligand in a molar ratio of 1:2 yielded a mononuclear compound [{Pd[R-C 6 H 3 C(H)=NCy] {Ph 2 PCH 2 PPh 2 -P,P}][PF 6 ] {R = 3-CHO (1a), 4-CHO (1b)}. The compounds were characterized using IR, 1 H, and 31 P–{ 1 H} NMR spectroscopy, and compound 1b was identiﬁed using X-ray diffraction.


Introduction
One of the classic ways to activate C-H bonds in hetero-substituted organic compounds is through the cyclometallation reaction, which is a well-known procedure [1]. The first cyclometallated compounds were discovered in the mid-1960s [2], and since then, this reaction has received a lot of attention because of the many applications of metallacycles, such as organic synthesis, catalysis, metallomesogen design, asymmetric synthesis, racemic ligand resolution, C-H bond activation, the synthesis and reactivity of organometallic compounds with biologically active ligands, and medical chemistry. In recent years, phosphine ligands have received a lot of attention [3][4][5], such as bis[diphenylphosphino]methane (dppm) ligands, which are widely employed in transition metal chemistry as chelating and bridging coordination modes ligands [6]. However, in square planar metal complexes with a d 8 configuration, the tendency for chelation of diphosphine ligands is very strong [7][8][9], hence several mononuclear dppm-type compounds have shown interest in homogeneous catalysis [10][11][12][13]. These bidentate diphosphine ligands are useful in metalcatalyzed processes. Over the last 30 years, metal-catalyzed cross-coupling reactions have grown in prominence, particularly as convenient procedures for forming C-C bonds [14,15]. Palladium-catalyzed reactions have piqued curiosity [16,17]. The Suzuki-Miyaura reaction, which is catalyzed by palladium, is one of the most important ways for the formation of C-C bonds under very mild experimental conditions and is particularly useful for the creation of biaryls [18]. In the Suzuki cross-coupling reaction, both nitrogen-based ligands (amines or imines) and bulky phosphines (phosphorus ylides) have been successfully described ( Figure 1) [19]. New catalysts are needed for current technologies that are low-cost, easily available, moisture-and air-stable, and most critically, extremely effective at low catalyst loading [20,21].

Result and Discussion
The mononuclear compounds were obtained by treating the halide-bridged dinuclear compound a and b with bis(diphenylphosphino)methane (dppm) in the existence of NH4PF6 in a 1:2 molar ratio (Scheme 1). The IR spectra revealed a shift in the C=N stretch's direction. Compared to the free Schiff base, the lower wavenumber nitrogen coordination of the C=N ligand group in the 1 H NMR spectra, the HC=O resonance shows a singlet signal at δ 9.87 for 1a and δ 9.53 for 1b, and the HC=N resonance appears as a doublet ca. δ 8.43 by connecting to only the 31 P nucleus trans to nitrogen for both compounds. The proton H5, coupled to both phosphorus nuclei, was assigned a doublet at δ 6.90 for 1a [ 4 J(H5P) = 7.9 Hz]and δ 6.80 for 1b [ 4 J(H5P) = 6.4 Hz]. In the 31 P-{ 1 H} NMR spectra, two doublets were seen for the two non-equivalent phosphoruses. The doublets were assigned based on the idea that a ligand with more trans influence causes the resonance of the phosphorus atoms trans to it to shift to a lower frequency [22].
The crystal structure of 1b ( Figure 2) includes a mononuclear molecule and a hexafluorophosphate anion. A (N1) from the imine group, (C1) an ortho carbon atom from the phenyl ring, and (P1,P2) two phosphorus atoms from a chelating dppm form the coordination sphere surrounding the palladium atom. At a palladium, the sums of angles are nearly 360°, with the distortions being more visible at the slightly reduced "bite" angles C1-Pd1-N1 [81.02]°, resulting from chelation. The bond angles P(1)-Pd(1)-P(2) are forced to 70.15 by the demands of the four-membered chelate ring of phosphine. The Pd1-N1 bond length is 2.097 Å, and the Pd1-C1 bond length is 2.025 Å. The Pd-P distance trans to carbon, Pd(1)-P(2), and trans to nitrogen, Pd(1)-P(1), [2.463(13) Å versus 2.248(11) Å] clearly indicate the contrasting influence of the phenyl carbon and imine nitrogen atoms (Table 1).

Result and Discussion
The mononuclear compounds were obtained by treating the halide-bridged dinuclear compound a and b with bis(diphenylphosphino)methane (dppm) in the existence of NH 4 PF 6 in a 1:2 molar ratio (Scheme 1). The IR spectra revealed a shift in the C=N stretch's direction. Compared to the free Schiff base, the lower wavenumber nitrogen coordination of the C=N ligand group in the 1 H NMR spectra, the HC=O resonance shows a singlet signal at δ 9.87 for 1a and δ 9.53 for 1b, and the HC=N resonance appears as a doublet ca. δ 8.43 by connecting to only the 31 P nucleus trans to nitrogen for both compounds. The proton H5, coupled to both phosphorus nuclei, was assigned a doublet at δ 6.90 for 1a [ 4 J(H5P) = 7.9 Hz]and δ 6.80 for 1b [ 4 J(H5P) = 6.4 Hz]. In the 31 P-{ 1 H} NMR spectra, two doublets were seen for the two non-equivalent phosphoruses. The doublets were assigned based on the idea that a ligand with more trans influence causes the resonance of the phosphorus atoms trans to it to shift to a lower frequency [22].

Result and Discussion
The mononuclear compounds were obtained by treating the halide-bridged dinuclear compound a and b with bis(diphenylphosphino)methane (dppm) in the existence of NH4PF6 in a 1:2 molar ratio (Scheme 1). The IR spectra revealed a shift in the C=N stretch's direction. Compared to the free Schiff base, the lower wavenumber nitrogen coordination of the C=N ligand group in the 1 H NMR spectra, the HC=O resonance shows a singlet signal at δ 9.87 for 1a and δ 9.53 for 1b, and the HC=N resonance appears as a doublet ca. δ 8.43 by connecting to only the 31 P nucleus trans to nitrogen for both compounds. The proton H5, coupled to both phosphorus nuclei, was assigned a doublet at δ 6.90 for 1a [ 4 J(H5P) = 7.9 Hz]and δ 6.80 for 1b [ 4 J(H5P) = 6.4 Hz]. In the 31 P-{ 1 H} NMR spectra, two doublets were seen for the two non-equivalent phosphoruses. The doublets were assigned based on the idea that a ligand with more trans influence causes the resonance of the phosphorus atoms trans to it to shift to a lower frequency [22].
The crystal structure of 1b ( Figure 2) includes a mononuclear molecule and a hexafluorophosphate anion. A (N1) from the imine group, (C1) an ortho carbon atom from the phenyl ring, and (P1,P2) two phosphorus atoms from a chelating dppm form the coordination sphere surrounding the palladium atom. At a palladium, the sums of angles are nearly 360°, with the distortions being more visible at the slightly reduced "bite" angles C1-Pd1-N1 [81.02]°, resulting from chelation. The bond angles P(1)-Pd(1)-P(2) are forced to 70.15 by the demands of the four-membered chelate ring of phosphine. The Pd1-N1 bond length is 2.097 Å, and the Pd1-C1 bond length is 2.025 Å. The Pd-P distance trans to carbon, Pd(1)-P(2), and trans to nitrogen, Pd(1)-P(1), [2.463(13) Å versus 2.248(11) Å] clearly indicate the contrasting influence of the phenyl carbon and imine nitrogen atoms (Table 1).
The crystal structure of 1b ( Figure 2) includes a mononuclear molecule and a hexafluorophosphate anion. A (N1) from the imine group, (C1) an ortho carbon atom from the phenyl ring, and (P1,P2) two phosphorus atoms from a chelating dppm form the coordination sphere surrounding the palladium atom. At a palladium, the sums of angles are nearly 360 • , with the distortions being more visible at the slightly reduced "bite" angles C1-Pd1-N1 [81.02] • , resulting from chelation. The bond angles P(1)-Pd(1)-P(2) are forced to 70.15 by the demands of the four-membered chelate ring of phosphine. The Pd1-N1 bond length is 2.097 Å, and the Pd1-C1 bond length is 2.025 Å. The Pd-P distance trans to carbon, Pd(1)-P(2), and trans to nitrogen, Pd(1)-P(1), [2.463(13) Å versus 2.248(11) Å] clearly indicate the contrasting influence of the phenyl carbon and imine nitrogen atoms ( Table 1).