Abstract
With the intention of preparing tetrapodal pentadentate ligands having NS 4 or NP 4 donor sets, we investigated reactions of the previously reported tetratosylate 2,6-C 5 H 3 N[CMe(CH 2 OTs) 2 ] 2 (2) with thiourea or diphenylphosphide, but found them not to proceed cleanly, and to give mixtures of products. A derivative of 2 better suited to nucleophilic substitution is the corresponding tetrabromide 2,6-C 5 H 3 N[CMe(CH 2 Br) 2 ] 2 (2,6-bis-(2-bromo-1-bromomethyl-1- methyl-ethyl)-pyridine, 3), which is obtained in excellent yield from 2 by treatment with LiBr in dimethylsulfoxide. The reaction of 3 with 4 eq of thiourea in refluxing ethanol gives a single product. Substitution is not quantitative, however, and the product likely is a bis(thiouronium) bis(bromide) salt. Similarly, the reaction of 3 with 4 eq of potassium O-ethyl xanthogenate displaces only two out of the four bromo substituents under the chosen conditions; workup then leads to what is formulated as a bis(thietane) derivative formed by intramolecular cyclisation. By contrast, nucleophilic substitution with NaSEt in ethanol is quantitative, and the thioether 2,6-C 5 H 3 N[CMe(CH 2 SEt) 2 ] 2 (2,6-bis-(2-ethylsulfanyl-1-ethylsulfanylmethyl- 1-methyl-ethyl)-pyridine, 4) has been isolated in close to 60% yield. Likewise, and in spite of the considerable steric bulk amassed in the molecule, the reaction of 3 with an excess of KPPh 2 in THF proceeds smoothly (even at -50 "C), to give the tetraphosphane 2,6-C 5 H 3 N[CMe(CH 2 PPh 2 ) 2 ] 2 (2,6-bis-{2-(diphenyl-phosphanyl)-1-[(diphenyl-phosphanyl)- methyl]-1-methyl-ethylg-pyridine, 5) in 65% yield. In order to assess possible pathways of oxidative degradation relevant to the coordination chemistry of this ligand, 5 was treated with NO in CH2Cl2 or ether at different temperatures. In two cases, reaction was observed to produce the oxide 2,6-C 5 H 3 N[CMe(CH 2 P(=O)Ph 2 ) 2 ] 2 (6) as a colourless solid in near quantitative yield, with concomitant formation of N 2 O. All compounds have been characterised by 1 H, 13 C and 31 P NMR spectroscopy (as applicable); IR spectroscopic and elemental analysis data are reported, and the crystal structure of 6 has been determined.