Naphthyridine-derived bimetallic phosphinite and metallophosphorane complexes

Abstract

The studies presented here concern the synthesis and reactivity of bimetallic phosphinite and metallophosphorane complexes. Much of the work presented is on the synthesis of bimetallic complexes using new dinucleating ligands, sometimes using a dehydrohalogenation methodology that has not yet seen much use in the synthesis of bimetallic complexes. The synthesis of Group 10 and 11 metal complexes are presented, with a major focus on the reactivity and catalytic potential of bimetallic complexes of nickel and palladium. Dinucleating "expanded pincer" ligands have previously been used to isolate bimetallic complexes. To add to this body of literature, the first publication presents the synthesis of bimetallic gold(I), silver(I) and copper(I) complexes of the tBuPONNOP ligand, a new expanded pincer ligand based on a 2,7-diphosphinite-substituted-1,8-naphthyridine backbone. A range of coordination modes were observed, with the geometric preference of the transition metal determining if it binds through P,N-, P,P- or P-only coordination modes. However, the low-yielding synthesis of the tBuPONNOP ligand hampered further research into this ligand. The second publication studies the tendency of the tBuPONNOP ligand to rearrange in the presence of NiBr2, which was observed previously for iPrPONNOP. However, a new rearrangement product was accessible with tBuPONNOP; the bimetallic metallophosphorane complex Ni2Br2(tBuPONNOPONNO), which featured formation of a new P-O bond to give a diphosphoranide ligand. Phosphoranides have not previously been incorporated into dinucleating ligand frameworks. New precursors, PONNHO, were developed to access PONNOPONNO products more easily, but the tBuPONNHO and iPrPONNHO precursors were found to give different product distributions. The dinickel(II) complexes Ni2Br2(PONNOPONNO) could be reduced to form highly planar dinickel(I) species, Ni2(PONNOPONNO), which have a direct Ni-Ni bond. While elementary reaction steps are well-understood for monometallic systems, there have been less studies on how bimetallic complexes perform these transformations. The third publication focuses on the reactivity of Ni2(tBuPONNOPONNO) with alkyl halides in order to explore elementary steps. Oxidative addition occurs across the Ni-Ni bond to form species of the type Ni2(R)(X)(tBuPONNOPONNO). However, the ethyl-bridged species are unstable in solution, undergoing beta-hydride elimination to form ethene and Ni2(H)(X)(tBuPONNOPONNO). DFT studies were used to examine the mechanism of beta-hydride elimination, representing one of few mechanistic studies into this step for dinuclear complexes. The macrocyclic structure of the ligand was found to have a major impact on the rate of beta-hydride elimination, with the bending of the backbone in the absence of a coordinating anion reducing the rate by relieving steric pressure on the bridging ethyl ligand. Unpublished reactivity studies are discussed to show the difficulty in understanding the mechanism of oxidative addition to Ni2(tBuPONNOPONNO). The fourth publication explores the use of PONNHO ligands to access bimetallic palladium and platinum complexes via dehydrohalogenation. In this case, monometallic MCl2(PONNHO) complexes could be isolated, which were not observed during studies with nickel. Addition of a base to MCl2(tBuPONNHO) gave bimetallic species of the type M2Cl2(tBuPONNO)2, and these M(II)2 complexes could be reduced to give species with Pd(I)-Pd(I) and Pt(I)-Pt(I) bonds of the form M2(tBuPONNO)2. Interestingly, the macrocyclic diphosphoranide PONNOPONNO ligand does not form when using Pd or Pt precursors. Instead, a weak P-O interaction, of which there have been few reports for transition metal complexes, is apparent from solid-state and solution-state data of M2(tBuPONNO)2. Finally, the catalytic potential of Ni2Br2(PONNOPONNO) and Pd2Cl2(tBuPONNO)2 in cross-coupling was examined. These studies are ongoing. Show more