POCOP is a type of pincer ligand. Pincer type ligands are tridentate ligands that bind three sites on one plane of a metal complex. POCOP forms complexes with one M-C(aryl) bond and two phosphinite ligands. The term POCOP is used both for the ligand, with formula C6H4(OPPh2)2, and its complexes, with formula C6H3(OPPh2)2]− (Ph = C6H5)
Interest in POCOP arises from its easy synthesis, which accommodates many substituents on phosphorus and results in higher yields than other PCP analogues.[1] The parent POCOP ligand is prepared by treating resorcinol with chlorodiphenylphosphine:[2]
C6H4(OH)2 + 2 ClPPh2 → C6H4(OPPh2)2 + 2 HCl
Related ligands can be prepared from chlorodiisopropylphosphine.
Representative complexes are of the type Ni(POCOP)X (X = halide, alkyl, thiolate). The halides arise by direct reaction of the ligand and a nickel halide and offer a relatively cheap, nontoxic and readily available option of the ligand for various applications. There can also be variable conformations of POCOP pertaining to the various R groups branching off of the donor atoms.
In most cases the MPOCOP center features a planar MC3O2P2 core. The rigid conformation and square planar geometry of the POCOP ligand allow for systematic changes over the steric and electronic environment at the metal center.[1] The geometry of metal compounds formed with POCOP ligands can be seen through the P-Ni-P (164.01 Å) and C-Ni-Cl (178.31Å) bond angles found for (POCOP)NiCl. The bond angles of this compound are representative of angles found for several other pocop metal compounds. Because of relatively small P-Ni-P bond angle the otherwise Ni complexes exhibit a slight tetrahedral distortion.
Pincer ligands participate in a variety of organic transformation, such as the hydrosilylation, and ethylene oligomerizations, and homocoupling of benzyl halides.[3] Ni PCP pincer complexes promote sulfur-carbon forming reactions.[1]