Tetrapodal Pentadentate Ligands with NS4 and NP4 Donor Sets: An Elusive Tetrathiol, and a Sterically Encumbered Tetraphosphane
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Christopher Zimmermann
With the intention of preparing tetrapodal pentadentate ligands having NS4 or NP4 donor sets, we investigated reactions of the previously reported tetratosylate 2,6-C5H3N[CMe(CH2OTs)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-C5H3N[CMe(CH2Br)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-C5H3N[CMe(CH2SEt)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 KPPh2 in THF proceeds smoothly (even at -50 "C), to give the tetraphosphane 2,6-C5H3N[CMe(CH2PPh2)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-C5H3N[CMe(CH2P(=O)Ph2)2]2 (6) as a colourless solid in near quantitative yield, with concomitant formation of N2O. All compounds have been characterised by 1H, 13C and 31P NMR spectroscopy (as applicable); IR spectroscopic and elemental analysis data are reported, and the crystal structure of 6 has been determined.
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