Dissociation of 2,4-bis(2,4,6-tri-tert-butylphenyl)-cyclo-1,3-dipnicta-2,4-diazanes (pnict = P, As, Sb) imposed by substituent steric strain: a cyclobutane/olefin analogy

Three 2,4-bis(Mes)-cyclo-1,3-dipnicta-2,4-diazanes, 2PnX, have been synthesized as 1,3-dichloro- (Pn = As; X = Cl) or 1,3-bis(triflato)- (Pn = P, Sb; X = OTf) derivatives (Mes = 2,4,6-tri-tert-butylphenyl; OTf = triflato = trifluoromethanesulfonoxy). The compounds have been structurally characterized as cyclodimers, but spectroscopic characterization of melts and solutions indicates facile dissociation of the diphosphadiazane 2POTf derivative to give the corresponding iminophosphine 1POTf and of the diarsadiazane 2AsCl derivative to give the corresponding iminoarsine 1AsCl. 1,3-Bis(triflato)-2,4-bis(Mes)-cyclo-1,3-distiba-2,4-diazane (2SbOTf) does not dissociate in the melt or in solution. The presence of the sterically bulky Mes substituent does not preclude association of N-Pn olefin analogues to give single bonded cyclobutane analogues. The facile dissociation of 2POTf and isolation of 1POTf implicates a relatively high degree of substituent steric strain in the dimer. In comparison, dissociation of 2AsCl is only apparent in the melt and in solution, likely the result of the lower substituent strain in the larger N(2)As(2) framework. The largest N(2)Pn(2) framework in 2SbOTf provides sufficient space for the Mes substituents, and the monomer is not observed under the conditions examined.