A New Water-Soluble Phosphine Derived from 1,3,5-Triaza-7-phosphaadamantane (PTA), † 3,7-Diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane. Structural, Bonding, and Solubility Properties

The phosphine 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane, which we will condense to DAPTA, and its oxide have been fully characterized both in solution and in the solid state. These compounds were prepared by acylation of 1,3,5-triaza-7-phosphaadamantane (PTA) and its oxide with acetic anhydride. The nonionic compounds were found to be soluble in most common organic solvents, in addition to possessing extremely large molar solubilities in water. Indeed, the molar solubility of DAPTA was shown to be 7.4 M, which is 4 time more soluble than the commonly utilized water-soluble phosphine, triply meta-sulfonated triphenylphosphine (TPPTS). In the case of DAPTA this enhanced water solubility is attributed to a strong interaction of water with the amide nitrogen−CO bond dipole as revealed by a large red shift of the νC  O vibration on going from a weakly interacting solvent such as CH2Cl2 to water. This latter observation is supported by the short average amide nitrogen−carbonyl carbon bond distance of 1.375 Å as determined via X-ray crystallography, indicative of a strong Coulombic interaction between the nitrogen and carbon atoms. To assess the metal to phosphorus binding characteristics of DAPTA, several group 10 and group 6 complexes were prepared and their M−P bond distances were shown to be quite similar with those of their PTA analogues. For examples, the W−P bond distance in W(CO)5DAPTA of 2.492(3) Å is comparable to that previously reported for W(CO)5PTA of 2.4976(15) Å and slighter shorter than that found in W(CO)5PMe3 (2.516(2) Å). Accordingly, the PTA ligand has generally been characterized as possessing donor properties similar to that of PMe3. Consistent with these bonding parameters determined in the solid state, all three tungsten pentacarbonyl complexes have nearly identical ν(CO) frequencies in solution.