The 14e- Pd(0)L2 complex 2 was prepared by reduction of [Pd(2-methylallyl)Cl]2 in the presence of the new, electron-rich, bulky methoxy benzyl phosphine (dmobp) ligand 1. Structural characterization of this complex indicates that the methoxy groups are not coordinated to the metal center. Complex 2 undergoes oxidative addition of iodo- and chlorobenzene at room temperature to yield the monophosphine complexes LPd(Ph)X (4, X = I; 5, X = Cl) in which the methoxy group is coordinated to the Pd(II) center in the solid state, as indicated by the X-ray structure of 4. In solution there is no evidence for methoxy coordination, indicating the availability of a Pd(II) 14e- complex. The Me-O bond in 4 is longer than the corresponding bond in 2, indicating that coordination of the methoxy group weakens the C-O bond. Reaction of complex 4 or 5 with the free ligand 1 results in nucleophilic attack and C-O cleavage, leading to the dimeric phenoxy-bridged complex 7, which was structurally characterized. Partial reduction of [Pd(2-methylallyl)Cl] 2 in the presence of the ligand 1 leads to the Pd(I) dimer 3, which can be converted to the Pd(0) complex 2 by addition of ligand 1 and a base. This complex, which bears only one phosphine for each Pd atom, is a suitable precursor to a presumed catalytically active 12e- Pd(0) catalyst, Complexes 2 and 3 catalyze the Suzuki-Miyaura cross-coupling of chlorobenzene with PhB(OH)2 even at room temperature, albeit slowly, while the C-O cleaved phenoxy-bridged complex 7 is not catalytically active at 40°C, indicating that it is not an intermediate in the catalysis. The dmobp ligand 1 is more effective in Suzuki-Miyaura coupling than an analogous benzyl ligand lacking methoxy substituents.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry