“…Arai et al have followed a new methodology, the ligand introduction route, and demonstrated the synthesis of chiral cationic palladium(II) complex [(NCN)Pd(OTf)] [ 11 (Chart )], which catalyzes the asymmetric conjugate addition of malononitriles with nitroalkenes to form the addition product with ≤93% enantioselectivity . Liu et al have reported the cationic dinuclear palladium(II) pincer complex, [2,4-(Me 2 NCH 2 ) 2 C 6 H 2 -1-{Pd(H 2 O)(Py)}-5-{Pd(OTf)(Py)}] + [OTf] − [ 12 (Chart )], which has been used for the methanolysis of the pesticide fenitrothion . Another example of cationic palladium(II) NCN pincer complex ( S , S )-[2,6-bis(4-isopropyl-1,4-dimethyl-4,5-dihydro-1 H -imidazol-5-on-2-yl)phenyl]palladium(II) tetrafluoroborate-aqua [ 13 (Chart )] has been reported and has been used for Friedel–Craft alkylation .…”
The
NCN palladium(II) pincer complex [Benzoyl(N∧C∧N)PdBr]
(16) was synthesized by the oxidative addition of Benzoyl(N∧C∧N)Br to Pd(dba)2 in 85%
yield [(N∧C∧N) = 5-tert-butyl-1,3-bis(N-substituted benzimidazol-2′-yl)phenyl)]. Then treatment
of complex 16 with KI yielded the iodopalladium complex
[Benzoyl(N∧C∧N)PdI] (17) in
92% yield. Furthermore, a series of cationic palladium(II) complexes,
including [Benzoyl(N∧C∧N)Pd(MeCN)]+[BF4]− (18), [Benzoyl(N∧C∧N)Pd(MeCN)]+[SbF6]− (19), and [Benzoyl(N∧C∧N)Pd(OTf)]
(20), were prepared in 68–79% yields by the reaction
of the neutral palladium(II) complex (16) with AgBF4, AgSbF6, and AgOTf, respectively. Similarly, previously
synthesized Tosyl(N∧C∧N)PdBr [5-tert-butyl-1,3-bis(N-tosylbenzimidazol-2′-yl)phenyl]palladium
bromide (5b) was treated with AgSO3CF3 and AgSbF6 to afford cationic palladium(II) complexes
[Tosyl(N∧C∧N)Pd(OTf)] (21) and
[Tosyl(N∧C∧N)Pd(MeCN)]+[SbF6]− (22) in 41 and 61% yields,
respectively. 5-tert-Butyl-1,3-bis[{(N-tosylbenzimidazol-2′-yl)phenyl}palladium(II)] triflate (21) exhibited an unsupported metallophillic Pd···Pd
interaction [3.166(8) Å] that is corroborated by X-ray crystallographic
studies. Compared to other cationic palladium complexes, complex 21 was found to be less stable. In Atoms in Molecule (AIM)
analysis, the bond critical point (ρ) between Pd and Pd atoms
is 0.000865 au, supporting the presence of metallophillic interaction
in complex 21. The bond strength of the Pd···Pd
bond was also measured by density functional theory calculations that
indicated that the calculated bond order was approximately one-fourth
of the normal covalent Pd–Pd bond (natural atomic orbital bond
order method). All eight complexes, two neutral and six cationic,
were characterized by common spectroscopic techniques, and six complexes
were corroborated by X-ray diffraction studies.
“…Arai et al have followed a new methodology, the ligand introduction route, and demonstrated the synthesis of chiral cationic palladium(II) complex [(NCN)Pd(OTf)] [ 11 (Chart )], which catalyzes the asymmetric conjugate addition of malononitriles with nitroalkenes to form the addition product with ≤93% enantioselectivity . Liu et al have reported the cationic dinuclear palladium(II) pincer complex, [2,4-(Me 2 NCH 2 ) 2 C 6 H 2 -1-{Pd(H 2 O)(Py)}-5-{Pd(OTf)(Py)}] + [OTf] − [ 12 (Chart )], which has been used for the methanolysis of the pesticide fenitrothion . Another example of cationic palladium(II) NCN pincer complex ( S , S )-[2,6-bis(4-isopropyl-1,4-dimethyl-4,5-dihydro-1 H -imidazol-5-on-2-yl)phenyl]palladium(II) tetrafluoroborate-aqua [ 13 (Chart )] has been reported and has been used for Friedel–Craft alkylation .…”
The
NCN palladium(II) pincer complex [Benzoyl(N∧C∧N)PdBr]
(16) was synthesized by the oxidative addition of Benzoyl(N∧C∧N)Br to Pd(dba)2 in 85%
yield [(N∧C∧N) = 5-tert-butyl-1,3-bis(N-substituted benzimidazol-2′-yl)phenyl)]. Then treatment
of complex 16 with KI yielded the iodopalladium complex
[Benzoyl(N∧C∧N)PdI] (17) in
92% yield. Furthermore, a series of cationic palladium(II) complexes,
including [Benzoyl(N∧C∧N)Pd(MeCN)]+[BF4]− (18), [Benzoyl(N∧C∧N)Pd(MeCN)]+[SbF6]− (19), and [Benzoyl(N∧C∧N)Pd(OTf)]
(20), were prepared in 68–79% yields by the reaction
of the neutral palladium(II) complex (16) with AgBF4, AgSbF6, and AgOTf, respectively. Similarly, previously
synthesized Tosyl(N∧C∧N)PdBr [5-tert-butyl-1,3-bis(N-tosylbenzimidazol-2′-yl)phenyl]palladium
bromide (5b) was treated with AgSO3CF3 and AgSbF6 to afford cationic palladium(II) complexes
[Tosyl(N∧C∧N)Pd(OTf)] (21) and
[Tosyl(N∧C∧N)Pd(MeCN)]+[SbF6]− (22) in 41 and 61% yields,
respectively. 5-tert-Butyl-1,3-bis[{(N-tosylbenzimidazol-2′-yl)phenyl}palladium(II)] triflate (21) exhibited an unsupported metallophillic Pd···Pd
interaction [3.166(8) Å] that is corroborated by X-ray crystallographic
studies. Compared to other cationic palladium complexes, complex 21 was found to be less stable. In Atoms in Molecule (AIM)
analysis, the bond critical point (ρ) between Pd and Pd atoms
is 0.000865 au, supporting the presence of metallophillic interaction
in complex 21. The bond strength of the Pd···Pd
bond was also measured by density functional theory calculations that
indicated that the calculated bond order was approximately one-fourth
of the normal covalent Pd–Pd bond (natural atomic orbital bond
order method). All eight complexes, two neutral and six cationic,
were characterized by common spectroscopic techniques, and six complexes
were corroborated by X-ray diffraction studies.
“…1) in the SM coupling of sterically demanding and electronically deactivated aryl bromides with phenylboronic acid. The synthesis has been reported for: 1 [25][26][27], 2 [28], 7a [29], 7b-f [30] and 8-11 [31].…”
Rationalization of the mechanism of in situ Pd(0) formation for cross-coupling reactions from novel unsymmetrical pincer palladacycles using DFT calculations. Journal of Organometallic Chemistry, 845. pp. 71-81.
“…Recently, we studied the formation reaction pathway of two symmetrical pincer palladacycles I [ 40 ] and II [ 41 ] ( figure 6 ) from their respective ligands and palladium(II) chloride using DFT, and atoms in molecules (AIM) analysis was used to establish the nature of the bonding. Several computational model chemistries were investigated, and suitable candidates determined, one of which is used here [ 27 ].…”
Section: Resultsmentioning
confidence: 99%
“… Figure 6. Symmetrical SCS ( I ) [ 40 ], symmetrical NCN ( II ) [ 41 ], and unsymmetrical SCN ( III ), unsymmetrical pyridine SCN ligand ( 1 ) and unsymmetrical pyridine SCN palladacycle ( 6 ). …”
The SCN ligand 2-{3-[(methylsulfanyl)methyl]phenyl}pyridine, 1, has been synthesized starting from an initial Suzuki–Miyaura (SM) coupling between 3-((hydroxymethyl)phenyl)boronic acid and 2-bromopyridine. The C–H activation of 1 with in situ formed Pd(MeCN)4(BF4)2 has been studied and leads to a mixture of palladacycles, which were characterized by X-ray crystallography. The monomeric palladacycle LPdCl 6, where L-H = 1, has been synthesized, and tested in SM couplings of aryl bromides, where it showed moderate activity. Density functional theory and the atoms in molecules (AIM) method have been used to investigate the formation and bonding of 6, revealing a difference in the nature of the Pd–S and Pd–N bonds. It was found that S-coordination to the metal in the rate determining C–H bond activation step leads to better stabilization of the Pd(II) centre (by 13–28 kJ mol−1) than with N-coordination. This is attributed to the electron donating ability of the donor atoms determined by Bader charges. The AIM analysis also revealed that the Pd–N bonds are stronger than the Pd–S bonds influencing the stability of key intermediates in the palladacycle formation reaction pathway.
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