Bis(diphenylphosphino)methane-Phosphonate Ligands and Their Pd(II), Ni(II) and Cu(I) Complexes. Catalytic Oligomerization of Ethylene

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(II) and Ni(II) centers. The formation of complexes containing the chelating, deprotonated form of ligand L1 or L2 occurred when basic ligands were present in the precursor complex, such as Me or dmba (on Pd(II)) or acac (on Ni(II)). The Cu(I) complex [CuL1]2(BF4)2 (9) was obtained that is suggested to be dinuclear, each Cu(I) 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 [PdCl2{(Ph2P)2CHP(O)(OMe)2-P,P}] (2a), [PdCl2{(Ph2P)2CHP(O)(OPh)2-P,P}] (2b), [Pd{(Ph2P)2CP(O)(OMe)2-P,P}2] (6a) in 6a·2CH2Cl2, [Pd{(Ph2P)2CP(O)(OPh)2-P,P}2] (6b), [Ni{(Ph2P)2CP(O)(OMe)2-P,P}2] (10a) in 10a·4CHCl3, [Ni{(Ph2P)2CP(O)(OPh)2-P,P}2] (10b) in 10b·4CHCl3, [NiCl2{(Ph2P)2CHP(O)(OMe)2-P,P}] (11a) in 11a·CHCl3 and of the dinuclear complexes [Pd(μ-Cl){(Ph2P)2CP(O)(OMe)2-P,P}]2 (7a) and [Pd(μ-Cl){(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(II) complexes were evaluated for the catalytic oligomerization of ethylene and afforded similar results when AlEtCl2 was used as cocatalyst. Complexes 11a,b were the most active, with a turnover frequency (TOF) of 78 300 mol of C2H4/(mol of Ni)h) for complex 11b in the presence of 10 equiv of AlEtCl2 and showed a high selectivity for C4 and C6 olefins with up to 93% C4 for 10a in the presence of 6 equiv AlEtCl2, 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.