Synthesis of Pd-NNP Phosphoryl Mononuclear and Phosphinous Acid-Phosphoryl-Bridged Dinuclear Complexes and Ambient Light-Promoted Oxygenation of Benzyl Ligands

Abstract

Novel tridentate NNP preligands bearing 2,2′-bipyridine and secondary phosphine oxide (SPO) moieties (NNP(O)H and MeNNP(O)H preligands) were readily synthesized from 6-methyl-2,2′bipyridine or 6,6′-dimethyl-2,2′-bipyridine and ethyl phenylphosphinate, respectively. The reaction of the preligands with [PdMe(Cl)-(cod)] (cod = 1,5-cyclooctadiene) afforded [PdCl(NNP(O))] and [PdCl(MeNNP(O))], in which the NNP(O) ligands coordinated to Pd in a tridentate manner. The phosphorus moiety coordinated as a phosphoryl ligand with the loss of a proton from the SPO moiety in the original preligands. Cationic complexes [Pd(NNP(O))(MeCN)][OTf] and [Pd(MeNNP(O))(MeCN)][OTf] were prepared from the above complexes and AgOTf in MeCN. The reaction of [PdBn(Cl)(cod)] with AgOTf and MeNNP(O)H afforded the phosphinous acidphosphoryl-bridged dinuclear benzyl complex [{PdBn(MeNNP(O))} 2 H][OTf]. Oxygenation of benzyl ligands in the dinuclear complex was investigated under various reaction conditions. Ambient light or blue LED light promoted the reaction to afford oxygenated products, that is, BnOOH, BnOH, and PhCHO. Oxygenation mainly proceeded by the reaction of the excited triplet state(s) of the PdBn complex(es) with 3 O 2 , without involving a benzyl radical. The addition of PPh 3 to the reaction mixture after oxygenation afforded [Pd(MeNNP(O))(PPh 3 )][OTf]. [{PdBn(MeNNP(O))} 2 H][OTf] was converted into [{PdCl(MeNNP-(O))} 2 H][OTf] by exposure to ambient light or blue LED light in chloroform under N 2 via homolysis of a Pd−Bn bond.

Comparative study of CO2insertion into pincer supported palladium alkyl and aryl complexes

“…The benzyl ligand binds in an η 1 -fashion with Pd–C bond distances of 2.260(3) Å in 1-Bn and 2.249(3) Å in 1- OMe Bn , which are longer than almost all other palladium complexes that feature an η 1 -benzyl ligand. 22 Further, the carbon atom bound to palladium (C(1)) is significantly distorted from tetrahedral, with Pd(1)–C(1)–C(2) angles of 127.7(2)° and 123.44(18)° observed for 1-Bn and 1- OMe Bn , respectively. Although this deviation from tetrahedral is typical for Group 10 benzyl complexes, 21 a , c ,23 these are some of the largest angles reported perhaps due to the steric congestion around the palladium.…”

“…The reaction of 1-Cl with (CH 2 C 6 H 5 )MgCl or (4-OMe-CH 2 C 6 H 4 )MgCl resulted in the isolation of 1-Bn or 1- OMe Bn, which are rare examples of pincer supported benzyl complexes, 22 in yields of 45% or 55%, respectively ( eqn (2) ). In both cases, it is important to remove the MgCl 2 by-product from the Grignard reagent or the benzyl complexes slowly convert back to 1-Cl in solution.…”