Synthesis and Characterization of Palladium Complexes Supported by an NPN-Phosphido Ancillary Ligand

The PdII coordination chemistry of the phosphido
ligand
[(2-Me2NC6H4)2P]− (NPN) is reported, along with the results of studies documenting
the utility of such complexes as catalyst precursors for the Heck
arylation of olefins. Examples of κ1-, κ2-, and κ3-NPN coordination to Pd are described.
The secondary phosphine (NPN)H (1) was reacted with 0.5
equiv of [(C3H5)­PdCl]2 to afford
the dimeric species [(κ2-NPN)­PdCl]2 (2) in 96% yield. Similarly, the reaction of 1 with 0.33 equiv of [Pd­(OAc)2]3 led
to the formation of dimeric [(κ2-NPN)­PdOAc]2 (3) in 98% yield. Complexes 2 and 3 were both crystallographically characterized
to reveal dipalladium species that feature bridging phosphido groups,
as well as terminal chloride and acetate ligands, respectively. Treatment
of 3 with Me3SiOTf resulted in the formation
of the analogous triflate complex [(κ2-NPN)­PdOTf]2 (4; 78% yield). Treatment of 2 with (C3H5)­MgCl afforded the thermally
sensitive, phosphido-bridged complex [(κ1-NPN)­Pd­(η3-C3H5)]2 (5) in 87% yield. The solid-state structure
of 5 was confirmed by X-ray crystallography. Precoordination
of BPh3 to the NPN phosphido group was attempted in order
to discourage the formation of phosphido-bridged (NPN)Pd species.
Thus, the reaction of 1 with 1 equiv each of KCH2Ph and BPh3 led to the formation of [N­(P·BPh3)­N]K (6), which was isolated in 88% yield. The
related complex [N­(P·BH3)­N]H (7) was
prepared by treatment of 1 with BH3·THF,
and crystallographic characterization confirmed the preferential coordination
of the NPN ligand phosphorus to BH3. The reaction of 6 with 0.5 equiv of [Pd­(C3H5)­Cl]2 led to the formation of [κ3-N(P·BPh3)N]­Pd­(η1-C3H5), which was isolated in 45% yield.
Complexes 2–4 were established as
precatalysts for the Heck coupling of ethyl acrylate and haloarenes.