Rational design and syntheses of 4-phosphino-catechol hybrid ligands for mixed -metal clusters
Date of Award
Program or Major
Doctor of Philosophy
Edward H Wong
Based on the concepts of Incommensurate Symmetry-Driven Cluster Formation, an approach to mixed-metal cluster formation has been proposed and realized by using the incommensurate symmetry requirements at two different metal centers bridged by a hybrid ligand. Critical to this strategy is the proper design and synthesis of hybrid ligands with coordination sites selective towards each metal. 4-Phosphino-Catechol hybrid ligands with both hard catecholate and soft phosphine donor sites have been shown to serve such a role. Three new ligands; 4-PPh2-Catechol (H2L), 4-PH2-Catechol (H2 2) and 4-P(CH2OH)2-Catechol (H23) as well as their precursors 4-PPh2-Veratrole and 4-PH2-Veratrole have been synthesized and characterized. Several phosphine-coordinated soft-metal [Pd(II), Pt(II), Ru(II), Cr(0)] complexes of 4-PPh2-Veratrole and 4-PH2-Veratrole have been synthesized and spectrally characterized to have 2-fold, Mold or 4-fold symmetry. The 3:1 catecholate complexes of H2L, H22 and H2 3 with several trivalent and tetravalent metal cations [M(III) = Fe, Ga; M(IV) = Ti, Sn] have also been prepared and characterized as potential precursors with C3 symmetry for the stepwise aufbau of heterometallic clusters. While the X-ray structure of the Cs2TiL3 salt revealed a C 1-mer-configuration in the solid-state, room temperature and variable-temperature solution NMR studies of this and other complexes are consistent with either exclusive formation of the C 3-fac-isomer with all phosphine donor sites syn to each other, or facile fac/mer isomerization. While attempts to synthesize mixed-metal clusters from ligand 4-PH2-Catechol (H22) and 4-P(CH2OH)2-Catechol (H23) via either aufbau or self-assembly processes failed because of side reactions and/or the poor solubility of intermediates (or products), aufbau of pentametallic [M2 L6Pd3Br6]4- (M = Ti, Sn) clusters have been successful through coordination of the [M IVL3]2- (M = Ti, Sn) metalla-ligands via their soft P donor sites to C2-symmetric PdBr2 units. These clusters have been fully characterized by spectral and X-ray structural data to be C3h mesocates with Cs+ or protonated 1,4-diazabicyclo[2.2.2]octane (DABCO-H +) cations incorporated into deep molecular clefts. In addition to this aufbau approach, self-assembly of several of these [M 2L6Pd3Br6]4- (M = Ti, Sn) clusters from their eleven components (two M(IV), three PdBr2, six L) can also be accomplished under appropriate reaction conditions. Exclusive formation of this type of supramolecular species is sensitive to the nature of counterions. Alkali cations like Li+ , K+, Rb+, and Cs+ led to high-yield formation of the respective clusters while other countercations such as tetramethylammonium (TMA+), tetra-phenylphosphonium (Ph4P+), and bis(triphenylphosphoranylidene)ammonium (PPN+) gave either a mixture of products or insoluble precipitates. Extension of the aufbau assembly to produce related [M 2L6Pd3Cl6]4- , [M2L6Pd3I6] 4-, CsSn2L6Ag3 and [M2L6Cr3(CO)12] 4- clusters has also been realized.
Sun, Xiankai, "Rational design and syntheses of 4-phosphino-catechol hybrid ligands for mixed -metal clusters" (2000). Doctoral Dissertations. 2130.