The development of N2S2 metal complexes as bidentate ligands for organometallic chemistry



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Texas A&M University


Electronic and steric parameters for square planar NiN2S2 complexes as bidentate, S-donor ligands have been established. According to the (CO) stretching frequencies and associated computed Cotton-Kraihanzel force constants of (NiN2S2)W(CO)4 adducts, a ranking of donor abilities and a comparison with classical bidentate ligands are as follows: Ni(ema)= > { [NiN2S2]0 } > bipy phen > Ph2PCH2CH2PPh2 > Ph2PCH2PPh2. In addition, we have demonstrated that the NiN2S2 ligands are hemilabile as evidenced from CO addition to (NiN2S2)W(CO)4, which is in equilibrium with the resulting (NiN2S2)W(CO)5 species (Keq = 2.8 M-1, G = -1.4 kJ/mole at 50C). Complete NiN2S2 ligand displacement by CO-cleavage of the remaining W-S bond to form W(CO)6 was not observed, indicating that the remaining W-S bond is considerably strengthened upon ring-opening. Several new cluster compounds based on the NiN2S2 ligands bound to CuI, RhI, PdII and W0 are reported. Structural analysis of (NiN2S2)MLn complexes show a unique structural feature defined by the dihedral angle formed by the intersection of NiN2S2/WS2C2 planes; placing the NiN2S2 ligand in closer proximity to one side of the reactive metal center. This unique orientational feature of the NiN2S2 ligands in the series of bimetallic compounds contrasts with classical diphosphine or diimine ligands. The "hinge angle" ranges in value from 136 as in the (Ni-1*)W(CO)4 to 101 in the (Ni-1)Pd(CH3)(Cl) complexes. The rigidity of the SR hinge of the nickeldithiolate ligands suggests that they might be suitable for stereochemical and regioselective substrate addition to catalytically active metals such as RhI and PdII.. The structural as well as functional similarities of the acetyl CoA synthase enzyme (ACS) and a palladium-metal based industrial type catalyst led to the preparation of a [(Ni-1)Pd(CH3)]+ bimetallic complex. This complex facilitates CO and ethylene copolymerization to produce polyketone similar to conventional (diphosphine)Pd(X)2 catalysts. However, the diphosphine ligands produce more efficient catalysts as the electron-rich character of the NiN2S2 ligand favors the resting state of the catalyst, [(Ni-1)Pd(C(O)CH3)(CO)]+, over the reactive form (Ni-1)Pd(C(O)CH3)(2-C2H4)]+. An exploratory investigation with the Ni-Pd heterobimetallic showed that this complex also facilitated the C-S coupling reaction to form a thioester similar to the ACS enzyme.