2-thia-1,3,5-triaza-7-phosphaadamantane 2,2-dioxide (PASO(2)). Comparative structural and reactivity investigation with the water-soluble phosphine ligand 1,3,5-triaza-7-phosphaadamantane (PTA)
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abstract
The derivative 2-thia-1,3,5-triaza-phosphaadamantane 2,2-dioxide (PASO2) has been characterized by spectroscopy and by X-ray crystallography. Unlike its PTA (1,3,5-triaza-7-phosphaadamantane) analogue, replacement of a -CH2-unit in PTA with -SO2- renders PASO2 largely insoluble in water at ambient temperature (<0.70 g/L vs 236 g/L for PTA). This latter property makes it of no use as a water-solubilizing ligand for organometallic complexes. Interestingly, alkylation of PASO2 with methyl iodide occurs quantitatively at the phosphorus center to afford the iodide salt. This derivative has also been characterized by X-ray crystallography and 1H/31P NMR spectroscopy, where the 31P resonance was observed to be shifted downfield relative to PASO2 (-39.2 vs -115.9 ppm). Although PTA has long been thought to react with methyl iodide to provide exclusively a monoalkylated product at one of the nitrogen centers, a close examination of this process via 31P NMR spectroscopy has shown monomethylation to take place at both nitrogen and phosphorus, with the latter being the minor product. Computational studies (DFT and HF) were performed in an effort to explain this difference in reactivity; however, no definitive conclusions could be reached from these calculations. Comparative studies (based on CO stretching frequencies and force constants) of group 6 metal carbonyl derivatives of PTA and PASO2 showed these ligands to be electronically extremely similar. This conclusion was supported by the bonding parameters determined in W(CO)5PTA and W(CO)5PASO2 complexes by X-ray crystallography.
