An Experimental and Theoretical Evaluation of the Reactions of Silver Hyponitrite with Phosphorus Halides. In Search of the Elusive Phosphorus-Containing Hyponitrites

Abstract

In the reaction of F2PBr, F2P(O)Br, (C6F5)2PBr, (CH3)2P(S)Br, and (CH3)2P(O)Cl with silver hyponitrite (AgON=NOAg), nitrous oxide (N2O) and mu-oxo phosphorus species were obtained in all cases rather than the plausible hyponitrite alternative. Theoretical calculations of the geometries and expected decomposition pathways of the phosphorus-containing hypothetical hyponitrites were carried out at the B3LYP/6-311+G(2df)//B3LYP/6-31+G(d) level. The cis-hyponitrite, XON=NOX (X=PF2, OPF2), is predicted to concertedly decompose to N2 plus phosphorus-containing radicals (OPF2, O2PF2) or to N2O plus the mu-oxo phosphorus species, X-O-X, (X=PF2, OPF2) with the former pathway having a smaller activation barrier (4.6 kcal/mol, X=PF2; 10.5 kcal/mol, X=OPF2). On the other hand, trans-hyponitrite can only decompose to N2 plus the phosphorus-containing radicals, because there is a very high barrier for rearrangement to cis-hyponitrite. These results are in disagreement with experiment, because only the mu-oxo phosphorus species are observed. Reconciliation between experiment and theory is made for X=OPF2 when a silver cation is included in the calculations. In THF (as a model for neat F2P(O)Br), the silver cation is predicted to reverse the order of the two transition states through stronger interactions with the oxygen atoms in the transition state of the N2O-producing pathway. Thus, Ag(I) is predicted to be not only catalytic for X=OPF2 but also product-specific toward the mu-oxo products.