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Synthetic, structural, and mechanistic studies of palladium complexes supported by sterically encumbering m- terphenyl isocyanides


The two-coordinate binary Pd(0) isocyanide monomer Pd(CNArDipp²)₂ was prepared using the sterically encumbering m-terphenyl isocyanide ligand CNArDipp² (ArDipp² = 2,6-(2,6-(iPr)₂C₆H₂)₂C₆H₃). Building upon the isolobal relationship between [Pi]-acidic isocyanides and C[Omicron], Pd(CNArDipp²)₂ was targeted as a stable analogue to the highly reactive binary carbonyl species Pd(C[Omicron])₂. The electron-rich nature of Pd(CNArDipp²)₂ was demonstrated by its reactivity towards elecrophilic and Lewis acidic substrates such as I₂, Me[Omicron]Tf, and Tl[Omicron]Tf. In addition, the ability of Pd(CNArDipp²)₂ to oxidatively add across C--Br bonds of aryl bromides made it a promising catalyst precursor for organic cross-coupling reactions. Pd(CNArDipp²)₂ mediated the Suzuki-Miyaura cross-coupling of unactivated aryl bromides and aryl boronic acids with 1 mol % catalyst loading at room temperature. Moderate activity was observed with Pd(CNArDipp²)₂, and was attributed to the presence of an unnecessary equivalent of ligand on the complex. Kinetic studies on the oxidative addition step provided evidence for a dissociative mechanism, which thereby implied the involvement of a monoligated [Pd(0)L] species. Synthetic approaches to complexes containing a 1: 1 L/Pd ratio culminated in the isolation and structural characterization of [Pd([eta]²-Dipp-[mu]-CNArDipp)]₃. This complex is a trinuclear aggregate of [Pd(0)L] and is catalytically competent in Suzuki-Miyaura cross-coupling reactions. Most notably, [Pd([eta]²-Dipp-[mu]-CNArDipp)]₃ exemplifies the ability of sterically encumbering [Pi]- acidic isocyanide ligands to stabilize the highly reactive and electron-rich monoligated [Pd(0)L] species. Further, Pd(CNArDipp²)₂ was employed to probe the coordination chemistry of redox non-innocent ArN[Omicron] ligands. Remarkably, addition of 2 equiv of PhN[Omicron] to Pd(CNArDipp²)₂ generated the complex Pd([kappa]¹-N- PhN[Omicron])₂(CNArDipp₂)₂, which is the first metal complex containing ([eta]¹-N-PhN[Omicron])·¹⁻ units to be structurally characterized. Magnetic susceptibility measurements and broken symmetry calculations confirmed the singlet diradical electronic structure of this complex in the solid state. However, spectroscopic studies indicated that Pd([kappa]¹-N-PhN[Omicron])₂(CNArDipp₂)₂ can only be observed at low temperature in solution. Complexation of two ArN[Omicron] units to Pd(CNArDipp²)₂ in solution at room temperature was achieved using p- [Omicron]CH-C₆H₄N[Omicron]. The resulting complex Pd([kappa]¹-N-p-[Omicron]CH-C₆H₄N[Omicron])₂(CNArDipp²)₂ also exhibited paramagnetism in solution, unlike its PhN[Omicron] derivative. Additional ArN[Omicron] derivatives of Pd(CNArDipp²)₂ were isolated and characterized, and the chemistry pertaining to these compounds is discussed

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