| Palladium and platinum complexes are of great importance to chemists for their unique ability to assume a wide variety of chemical roles. They are of interest to synthetic chemists chiefly due to their well-known ability to catalyze coupling reactions, greatly increasing yield and efficiency. Since its introduction to the literature in 1974 by Bonnett and Ibers, tris(dibenzylideneacetone)-dipalladium(chloroform) (Pd2(dba)3˙CHCl3) has been reported hundreds of times as a source of zero-valent palladium and is widely used as a conveniently soluble homogeneous palladium catalyst.;A palladium complex reported by Itoh et al. spurred interest in pursuing ligand studies on compounds that have a benzoquinone motif for stabilizing zero-valent palladium and platinum compounds. Itoh's complex, Pd 2(bq)2(nbe)2, resulted in two palladium atoms each coordinated to a double bond of the benzoquinone (bq) and coordinated to a single norbornene (nbe) ligand. Itoh et al. also showed that a single palladium atom could be stabilized by using bq and cyclooctadiene (cod) ligands. This complex, Pd(bq)(cod) further showed that palladium was amenable to eta-bonding interactions with the double bonds of bq, and therefore ligand studies included organic compounds with the bq motif. Previous work by Nate Fisher and Dominick Babbini has included the synthesis and characterization of tripalladium sandwich complexes. This unique system contains three zero-valent palladium centers stabilized by eta-bonding between electron density in the double bonds of the tropylium ion and the palladium atoms.;An issue with zero-valent metal centers is that they bring with them an intrinsic instability, making preparation and shelf-life potential issues for complexes of this nature. Both palladium and platinum are able to form eta, eta, bonds with pi, pi,-conjugated systems like those in dibenzylideneacetone (dba) analogues and tropylium (C 7H7)+. With the massive amount of unsaturated pi-conjugated systems available to chemists, there are plenty of compounds that can potentially be used to stabilize zero-valent metal centers. It is an interest of chemists to develop organometallic compounds that stabilize zero-valent metal centers because these compounds are often readily soluble in organic solvents, but more importantly, the ligands can release the zero-valent metal which can participate in subsequent reactions, such as catalytic processes. The eta-bonding motif often associated with zero-valent palladium complexes is a weaker bond than a direct sigma-bond to a ligand.;With the previous success of the tripalladium [Pd3Tr 2X3]-- motif, it was desirable to synthesize a series of compounds that palladium could theoretically interact with through eta-type bonding. The inclusion of pi-bonds in various organic compounds should, in theory, be amenable to palladium coordination. By designing uncharged ligands, one can isolate zero-valent palladium compounds that could be used as Pd2(dba)3˙CHCl3 analogues. It is for this reason that a series of 7-aryl-6H-dibenzo[b,h]xanthene-5,6,8,13(7H)-tetraone (BNQ), 14-phenyl-14H-dibenzo[a,j]xanthene (DBX), 3,4,5,6,7,9-Hexahydro-3,3,6,6- tetramethyl-9-phenyl-1H-xanthene-1,8(2H)-dione (TMXD), and 4'-Aryl-2,2':6',2''-terpyridines (terpy) compounds were synthesized and characterized. |
