Sterically-Directed Consecutive and Size-Selective Self-Assembly of Palladium Diphosphane Complexes with an Ar-BIAN Ligand: Unexpected Formation of Pentameric and Hexameric Aggregates

The coordination properties of N,N'-bis[4-(4-pyridyl)phenyl]acenaphthenequinonediimine (C) and N,N'-bis[4-(2-pyridyl)phenyl]acenaphthenequinonediimine (L-2) were investigated in self-assembly with palladium diphosphane complexes [Pd(P boolean AND P)(H2O)(2)](OTf)(2) (OTf=triflate) by using various analytical techniques, including multinuclear (H-1, N-15, and P-31) NMR spectroscopy and mass spectrometry (P boolean AND P=dppp, dppf, dppe; dppp=bis(diphenylphosphanyl)propane, dppf=bis(diphenylphosphanyl)ferrocene, and dppe=bis(diphenylphosphanyl)ethane). Beside the expected trimeric and tetrameric species, the interaction of an equimolar mixture of [Pd-(dppp)](2+) ions and L-1 also generates pentameric aggregates. Due to the E/Z isomerism of L-1, a dimeric product was also observed. In all of these species, which correspond to the general formula [Pd(dppp)L-1](n)(OTf)(2n) (n=2-5), the L-1 ligand is coordinated to the Pd center only through the terminal pyridyl groups. Introduction of a second equivalent of the [Pd(dppp)](2+) tecton results in coordination to the internal, sterically more encumbered chelating site and induces enhancement of the higher nuclearity components. The presence of higher-order aggregates (n=5, 6), which were unexpected for the interaction of cis-protected palladium corners with linear ditopic bridging ligands, has been demonstrated both by mass-spectrometric and DOSY NMR spectroscopic analysis. The sequential coordination of the [Pd(dppp)](2+) ion is attributed to the dissimilar steric properties of the two coordination sites. In the self-assembled species formed in a 1:1:1 mixture of [Pd(dppp)](2+)/[Pd(dppe)](2+)/L-1, the sterically more demanding [Pd(dppp)](2+) tectons are attached selectively to the pyridyl groups, whereas the more hindered imino nitrogen atoms coordinate the less bulky dppe complexes, thus resulting in a sterically directed, size-selective sorting of the metal tectons. The propensity of the new ligands to incorporate hydrogen-bonded solvent molecules at the chelating site was confirmed by X-ray diffraction studies.