Unsaturated Pd(0), Pd(I), and Pd(II) Complexes of a New Methoxy-Substituted Benzyl Phosphine. Aryl−X (X = Cl, I) Oxidative Addition, C−O Cleavage, and Suzuki−Miyaura Coupling of Aryl Chlorides

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.