Coordination of neutral, methylene bridged bis-guanidyls at palladium

Abstract

The bis-guanidyl compound H₂C{hpp}₂ (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) coordinates to palladium(II) as a neutral, chelating N,N′-bidentate ligand. Structural analysis of PdMeCl(H₂C{hpp}₂) shows a non-planar metallacycle that is twisted relative to the square plane of the metal and an “anagostic” interaction with a C–H from the bridging methylene group. Generation of the cationic palladium complex, [PdCl(H₂C{hpp}₂)][OTf] was achieved by halide abstraction from the dichloride PdCl₂(H₂C{hpp}₂) using NaOTf. The product was identified as a mixture of different species in solution; in the solid-state, the molecular structure is dimeric, consisting of the μ,μ′-dichlorobridged dication. The new bis-guanidyl compound H₂C{tbo}₂ (Htbo = 1,4,6-triaza-bicyclo[3.3.0]oct-4-ene) was synthesized, and structurally characterized. Coordination of this compound at palladium dichloride was accompanied by ionization of the Pd–Cl bond, and formation of the dication [Pd(H₂C{tbo}₂)₂][Cl]₂. Structural analysis shows a significant reduction in the twisting of this ligand.