Rational design and assembly of M2M ' L-3(6) supramolecular clusters with C-3h symmetry by exploiting incommensurate symmetry numbers

A rational approach to heterometallic cluster formation is described that u ses incommensurate symmetry requirements at two different metals to control the stoichiometry of the assembly. Critical to this strategy is the proper design and synthesis of hybrid ligands with coordination sites selective t oward each metal. The phosphino-catechol ligand 4-(diphenylphosphino)benzen e-1,2-diol (H2L) possesses both hard catecholate and soft phosphine donor s ites and serves such a role, using soft (C-2-symmetric) and hard (C-3- symm etric) metal centers. The ML3 catecholate complexes (M = Fe-III, Ga-III, T- IV, Sn-IV) have been prepared and characterized as C-3-symmetry precursors for the stepwise assembly (aufbau) of heterometallic clusters. While the si ngle-crystal X-ray structure of the Cs-2[TiL3] salt shows a C-1 mer-configu ration in the solid -state, room-temperature solution NMR data of this and related complexes are consistent with either exclusive formation of the G-S ac-isomer with all PPh2 donor sites syn to each other or facile fac/mer iso merization. Coordination of these [ML3](2-) (M = Ti-IV, Sn-IV) metallaligan ds via their soft P donor sites to C-2-symmetric PdBr2 units gives exclusiv ely pentametallic [M2Pd3Br6L6](4-) (M = Ti, Sn) clusters. These clusters ha ve been fully characterized by spectral and X-ray structural data as C-3h m esocates with Cs+ or protonated 1,4-diazabicyclo-[2.2.2]octane (DABCO H-.()) cations incorporated into deep molecular clefts. Exclusive formation of this type of supramolecular species is sensitive to the nature of the count erions. Alkali cations such as K+, Rb+, and Cs+ give high-yield formation o f the respective clusters while NEt3H+ and NMe4+ yield none of the desired products. Extension of the aufbau assembly to produce related [M2Pd3Cl6L6]( 4-), [M2Pd3I6L6](4-), and [M2Cr3(CO)(12)L-6](4-) (M = Ti, Sn) clusters has also been realized. In addition to this aufbau approach, self-assembly of s everal of these [M2Pd3Br6L6](4-) clusters from all eleven components (two M -IV, three PdBr2, six H2L) was also accomplished under appropriate reaction conditions.