Interplay of Metallophilic Interactions, π-π Stacking, and Ligand Substituent Effects in the Structures and Luminescence Properties of Neutral PtII and PdII 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 center dot center dot center dot M distances of 3.24-3.25 and 3.34 angstrom, respectively, with nearly isostructural Pt and Pd compounds. Both structure types contain void channels running parallel to the M center dot center dot center dot M chains. The channels are 3-4 angstrom wide and vacant for the phenyl structures, while those in the p-tolyl structures are up to 7.6 angstrom wide and contain water. These channeled structures are stabilized by a combination of metallophilic bonding and aryl pi-pi stacking interactions. The Pt structure with 4-F substituents also features extended Pt center dot center dot center dot Pt chains, but with longer 3.79 angstrom distances alternating with shorter 3.37 angstrom contacts. Structures with 4-CF3 and 4-OMe substituents exhibit mostly isolated dimers of M center dot center dot center dot M contacts. In complexes with 2,6-dimethylphenyl isocyanide, steric hindrance precludes any short M center dot center dot center dot M contacts. The primary effect of aryl substitution is to provide alternative packing motifs, such as CF3 center dot center dot center dot pi and CH3 center dot center dot center dot pi interactions, that either augment or disrupt the combination of metallophilic contacts and pi-pi stacking needed to stabilize extended M center dot center dot center dot 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 center dot center dot center dot Pt chain structures display luminescence assignable to (d sigma*-p sigma) 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 center dot center dot center dot M and pi-pi interactions contract upon cooling. Overall, this study suggests that synergy with pi-pi and other interactions is necessary to stabilize extended M center dot center dot center dot M chain structures. Thus, efforts to design functional materials based on metallophilic bonding must consider the full array of available packing motifs.