Bis(diphenylphosphino)methane-phosphonate ligands and their PD(II), Ni(II) and Cu(I) complexes. Catalytic oligomerization of ethylene

Article abstract of DOI:10.1021/ic8020436

The bis(diphenylphosphino)methane-phosphonate-based (dppm-phosphonate) ligands (Ph₂P)2CHP(O)(OR)₂ (R = Me, L¹; R = Ph, L²) form metal complexes by selective coordination of the diphosphine moiety to Pd(ll) and Ni(ll) centers. The formation of complexes containing the chelating, deprotonated form of ligand L¹ or L ² occurred when basic ligands were present in the precursor complex, such as Me or dmba (on Pd(ll)) or acac (on Ni(ll)). The Cu(l) complex [CuL ¹]₂(BF₄)₂ (9) was obtained that is suggested to be dinuclear, each Cu(l) being chelated by a phosphine and a P=O donor and further bound to the phosphine group of another ligand. The crystal structures of the mononuclear complexes [PdCI₂{(Ph₂P) ₂CHP(O)(OMe)₂-P,P}] (2a), [PdCI₂{(Ph ₂P)₂CHP(0)(OPh)₂-P,P}] (2b), [Pd{(Ph ₂P)₂CP(O)(OMe)₂-P,P}₂] (6a) in 6a-2CH₂CI₂, [Pd{(Ph₂P)₂CP(O)(OPh) ₂-P,P}₂] (6b), [Ni{(Ph₂P)₂CP(O)(OMe) ₂-P,P}₂] (10a) in 10a-4CHCI3, [Ni{(Ph₂P) ₂CP(O)(OPh)₂-P,P}₂] (10b) in 10b-4CHCI ₂, [NiCI₂{(Ph₂P)₂CHP(O)(OMe) ₂-P,P}] (11a) in 11a. CHCI₃ and of the dinuclear complexes [Pdμ-CI){(Ph₂P)₂CP(O)(OMe)₂-P,P}] ₂ (7a) and [Pd(μ-CI){(Ph₂P)₂CP(O)(OPh) ₂-P,P}]₂ (7b) have been determined by X-ray diffraction. The complexes [Ni{(Ph₂P)₂CC(O)NPh₂-P,P} ₂] (13) and [Ni{Ph₂PC-(Ph)P(O)(OEt)₂-P,O} ₂] (15) were prepared by reaction of [Ni(acac)₂] with the known, neutral ligands (Ph₂P)₂CHC(O)NPh₂ and Ph₂PCH(Ph)P(O)(OEt)₂, respectively. The Ni(ll) complexes were evaluated for the catalytic oligomerization of ethylene and afforded similar results when AIEtCI₂ was used as cocatalyst. Complexes 11a,b were the most active, with a turnover frequency (TOF) of 78 300 mol of C ₂H₄/(mol of Ni)h) for complex 11b in the presence of 10 equiv of AIEtCI₂ and showed a high selectivity for C₂ and C₂ olefins with up to 93% C₂ for 10a in the presence of 6 equiv AIEtCI₂, whereas when only 3 equiv cocatalyst was used with 10a, 64% trimers were observed. In the presence of methylalumoxane (MAO) as cocatalyst, precatalysts 10a,b, 11a,b, and 15 gave moderate TOFs, up to 11 660 mol of C₂H₄/(mol of Ni)h) for complex 11a with 400 equiv of MAO. Mostly ethylene dimers were formed but up to 39% trimers were observed with 10a in the presence of 400 equiv MAO.