Interplay of metallophilic interactions, π-π stacking, and ligand substituent effects in the structures and luminescence properties of neutral Pt(II) and Pd(II) aryl isocyanide complexes.

Packing interactions in the crystal structures of a series of cis-M(CNAr)(2)Cl(2) complexes (M = Pt, Pd; Ar = substituted phenyl) were examined and correlated with the luminescence properties of the Pt complexes. The structures of the PhNC and p-tolyl isocyanide complexes exhibit extended chains of metallophilic interactions with M···M distances of 3.24-3.25 and 3.34 Å, respectively, with nearly isostructural Pt and Pd compounds. Both structure types contain void channels running parallel to the M···M chains. The channels are 3-4 Å wide and vacant for the phenyl structures, while those in the p-tolyl structures are up to 7.6 Å wide and contain water. These channeled structures are stabilized by a combination of metallophilic bonding and aryl π-π stacking interactions. The Pt structure with 4-F substituents also features extended Pt···Pt chains, but with longer 3.79 Å distances alternating with shorter 3.37 Å contacts. Structures with 4-CF(3) and 4-OMe substituents exhibit mostly isolated dimers of M···M contacts. In complexes with 2,6-dimethylphenyl isocyanide, steric hindrance precludes any short M···M contacts. The primary effect of aryl substitution is to provide alternative packing motifs, such as CF(3)···π and CH(3)···π interactions, that either augment or disrupt the combination of metallophilic contacts and π-π stacking needed to stabilize extended M···M chains. Differences in the Pt and Pd structures containing 4-F and 4-OMe substituents are consistent with a higher driving force for metallophilic interactions for Pt versus Pd. The M-C and M-Cl bond distances indicate a slightly higher trans influence for aryl isocyanides bound to Pt versus Pd. The three extended Pt···Pt chain structures display luminescence assignable to (dσ*→pσ) excited states, demonstrating the existence of substantial orbital communication along the metal-metal chains. Face-indexing shows that the preferred crystal growth axis is along the metal-metal chains for the luminescent structures. Variable temperature structural studies showed that both M···M and π-π interactions contract upon cooling. Overall, this study suggests that synergy with π-π and other interactions is necessary to stabilize extended M···M chain structures. Thus, efforts to design functional materials based on metallophilic bonding must consider the full array of available packing motifs.