Unusual resistance of metallacarborane O-(4-nitrophenyl)- phosphorothioate ester towards alkali hydrolysis

Metallacarboranes, metallocene-type complexes containing carborane ligand(s) and one or more metal ions, have applications in such diverse areas as catalysis, nuclear waste disposition, electrochemistry, medicinal chemistry or material science[1]. Recently we have shown that phosphoric acid bridged 8,8’-dihydroxy-cobalt bis(dicarbollide) ion is a highly efficient catalyst for the organocatalytic hydrogenation process[2]. In a search for analogous thiophosphoric acid bridged catalysts, synthesis of sulfur containing counterpart was undertaken. To our surprise the O,O-[cobalt bis(dicarbollide)ion] O-(4-nitrophenyl)phosphorothioate ester (HNEt3[1]) designed as a precursor of the target compound, though easy to obtain appeared “unhydrolizable” even under harsh alkali conditions. To examine in more details the observed phenomenon alkali hydrolysis of [1]- was studied and its kinetic and thermodynamic parameters were measured and compared with that of analogous phosphate 4-nitrophenyl ester bearing metallacarborane (HNEt3[2]) and two other esters, derivatives of organic alcohols instead of inorganic metallacarborane component, parathion and paraoxon. The susceptibility to hydrolysis was found to decrease in the following order: parathion >>[2]-> [1]-. In the present communication the extreme differences between hydrolytic properties of parathion and [1]-are discussed. An electronic and steric effects of the metallacarborane complex (Cluster-effect) and to the less extent, the presence of sulfur atom instead of oxygen at nonbridging position on phosphorous (Thio-effect)[4] are proposed as major reasons of esters type [1]- hydrolytic stability. Acknowledgments: This research was financially supported by The National Science Centre in Poland, Grant number 2015/16/W/ST5/00413 for years 2015–2021. References.[1] R.N. Grimes, Carboranes, 3th ed., Academic Press, 2016; [2] Synlett, 2014, 795-798; [3] Eur. J. Inorg. Chem. 2010, 2385–2392; [4] M. Ora., T. Lönnberg., H. Lönnberg., in: Erdmann V., Barciszewski J. (eds) From Nucleic Acids Sequences to Molecular Medicine. RNA Technologies. Springer, Berlin, Heidelberg, 2012, pp 47-65.