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46716-B3
Synthesis, Coordination Chemistry, and Catalytic Studies of a Library of Water-Soluble Polydentate Phosphines Derived from 1,3,5-Triaza-7-phosphaadamantane (PTA)

Donald A. Krogstad, Concordia College

The greening of industry has caused recent research to center around the development of water-soluble phosphines and their transition metal catalysts.� One ligand that has been extensively studied is 1,3,5-triaza-7-phosphaadamantane (PTA) because it is easy to synthesize, highly water-soluble (235 mg/mL), resistant to oxidation, and air-stable. These factors have allowed PTA to be utilized as a ligand in an array of catalytic reactions such as hydroformylations, hydrogenations, and hydroaminations.� Due to the fact that the metal catalysts in many organic transformations are ligated with chelating phosphines, PTA has also recently been used as a synthon for the preparation of potentially bidentate ligands. In all of these studies, however, the chelating compounds utilized only one PTA unit.�� To date, no ligands that employ 2 PTA moieties have been reported in the literature.� In an effort to expand upon the existing library of potentially bidentate, water-soluble phosphines, our group attempted to prepare the first ligands that utilize two PTA moieties.

The bis-phosphines, 1,1'-[1,2-phenylenebis(methylene)]bis-3,5-diaza-1-azonia-7-phosphatricyclo[3.3.1.1]decane dibromide (1), 1,1'-[1,3-phenylenebis(methylene)]bis-3,5-diaza-1-azonia-7-phosphatricyclo[3.3.1.1]decane dibromide (2), 1,1'-[1,3-tolylenebis(methylene)]bis-3,5-diaza-1-azonia-7-phosphatricyclo[3.3.1.1]decane dibromide (3), 1,1'-[1,3-anisolylenebis(methylene)]bis-3,5-diaza-1-azonia-7-phosphatricyclo[3.3.1.1]decane dibromide (4) and 1,1'-[1,4-phenylenebis(methylene)]bis-3,5-diaza-1-azonia-7-phosphatricyclo[3.3.1.1]decane dibromide (5)� have been easily prepared in high yields by reacting two equivalents of PTA with 1,2-bis(bromomethyl)benzene, 1,3-bis(bromomethyl)benzene, 1,3-bis(bromomethyl)-5-methyl-benzene, 1,3-bis(bromomethyl)-5-methoxy-benzene, �and 1,4-bis(bromomethyl)benzene in acetone or chloroform (Scheme 1).� The compounds each precipitated from solution as air-stable solids that needed little purification.

Scheme 1.� Synthesis of the Bis-PTA xylenes.

The bis-phosphines were characterized by elemental analysis, ESI-MS, and multinuclear NMR.� Alkylation of the N atoms caused the molecular symmetry of the PTA moieties to be reduced from that of the parent compound.� Consequently, the methylene protons of the PTA units within the phosphines were inequivalent, and in some cases, diastereotopic.� This caused the ¹H NMR of these compounds to be more complicated than that of PTA itself.� The ¹³C NMR spectra of the compounds were well resolved and collectively indicated that the PTA moieties of the bis-phosphines were equivalent on the NMR timescale. This equivalence was also observed in the ³¹P NMR as each bis-phosphine exhibited a single phosphorous resonance.

Compounds 3 and 4 were additionally characterized via single crystal X-ray diffraction.� The geometry about the P atoms of both compounds was compressed pyramidal, as evidenced by their small C-P-C bond angles.� The average tertiary nitrogen, N-C bond distance within 3 (1.460(6) �) and 4 (1.458(8) �) was quite comparable, and slightly shorter than those involving the quaternary nitrogen (1.530(5) and 1.533(7) �).� The PTA moieties of 3 and 4 were orthogonal to the plane of the phenyl rings as evidenced by the average C-N-C-C(ring) torsions angles of 91.5^o (5) and 89.9^o (6) respectively.

Figure 1.� Thermal ellipsoid representations of 3 and 4.� The bromide anions and� solvate molecules have been omitted for clarity.

The primary purpose of this study was to prepare bis-phosphines that could possibly function as water-soluble ligands. Therefore, the solubility of compounds 1-5 was studied in aqueous media.� The substitution pattern of the aromatic ring had a pronounced effect on the water-solubility of the phosphines with the ortho compound 1 being very water soluble (2000 mg/mL) and the para analog (5) far less (12.5 mg/mL). The meta phosphine 2 and its tolyl analog (3) were found to have water-solubilities (810 mg/mL) that were triple that of PTA (235 mg/mL). The difference in water solubility between the ortho (1), meta (2), and para (5) substituted compounds is quite remarkable.� It is hypothesized that this is a consequence of the varied lattice enthalpies of the related systems. The anisolyl compound (4) was determined to be much less soluble (121 mg/mL) than the other meta derivatives, thus indicating that, in addition to the substitution pattern of the ring, the substituents on the ring greatly impact the water-solubility of the compounds. It is assumed that the low solubility of 4 was a consequence of the OCH₃ moiety hydrogen bonding with water to form a polymeric network.

In addition to adding five new phosphines to the library of water-soluble ligands, this study also had great impact on my research program.� First, it allowed four undergraduates to experience the thrill of preparing and analyzing molecules that had never been synthesized.� Two of the students are continuing to work with me this year, and a third went to graduate school to earn her Ph.D. in synthetic chemistry.� Second, this project was important in that it created a new collaboration between my lab and that of Dr. Paola Bergamini of the University of Ferrara in Ferrara, Italy.� The initial preparative results have been quite promising and these synthetic routes will likely lead to new green systems and a long lasting collaboration.

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