Iridium Catalyzed Dehydrogenative Diboration of Alkyne, Nickel and Silver Complexes of An Alane/Trisphospphine Ligand Build Around A Strongly Lewis Acidic Tris(N-pyrrolyl)aluminum and Exploration On Reactivity of PAlP-ligated Rhodium and Iridium Complexes

Abstract

Phosphination of the ortho C-H bond of the pyrrole was developed to form 2-(diisopropylphosphino)pyrrole. The protolysis of AlMe3 with 2-(diisopropylphosphino)pyrrole leads to a new tripodal alane/tris(phosphine) ligand (AlP3). The synthesis of AlP3-supported Ni complex was reported. The central tris(pyrrolyl)aluminum moiety acts as a stronger Lewis acid towards Ni than other related group 13 element-centered tripodal ligands, as demonstrated by the binding of H2 to Ni and ease of reduction. The Z-type tripodal alane ligand (AlP3) also reacts with AgOTf by coordination of three phosphines to the Ag center and transfer of triflate to the tris(pyrrolyl) Al site. Reaction with Ag[HCB11Cl11] results in the coordination of two phosphines to Ag and one to Al, indicating, with little to no Ag-Al interaction in either structure. The synthesis of PAlP and PBP pincer complexes of Rh possessing a central bis(N-pyrrolyl)aluminyl or -boryl unit was developed. Complex (PAlpyP)Rh(CO)2 possesses an aluminyl site stabilized by coordination of pyridine, resulting in a four-coordinate Al. Attempts to the three-coordinate aluminyl by abstraction of pyridine with BF3Et2O unexpectedly led to the B/Al metathesis with the preservation of the pincer structure in the product (PBP)Rh(CO)2. Abstraction of pyridine was carried out using B(C6F5)3, but the desired (PAlP)Rh(CO)2 underwent dimerization via isocarbonyl bridging, reflecting the elevated Lewis acidity of the N-pyrrolyl-substituted aluminyl. The direct cyclometalation of PCP ligand with Fe2(CO)9 under UV irradiation leads to a C-H activation product (PCP)FeH(CO)2. 1 e- oxidation of (PCP)FeH(CO)2 in C6H5F affords C-H reductive elimination product with [(PCHP)Fe (CO)2] cation with agostic Caryl-H-Fe interaction. While 1 e- oxidation of (PCP)FeH(CO)2 in CH3CN gives rise to H2 and [(PCP)Fe(CO)2NCCH3] cation. Catalytic dehydrogenative diboration (DHDB) of alkyne with HBpin was achieved using [Ir(COD)Cl]2 and other related Ir precursors under CO atmosphere. The selectivity for DHDB over hydroboration is higher in less polar solvent and under increasing CO pressure. It was further improved when catalytic amount of tBuNC was added to the reaction. It was possible to achieve DHDB of both terminal and internal alkynes with selectivity for DHDB of up to 9:1 under the best conditions. Some DHDB products were isolated on the preparative scale.