On the appearance of nitrite anion in [PdX(OAc)L2] and [Pd(X)(C∧N)L] syntheses (X = OAc or NO2): photocrystallographic identification of metastable Pd(η1-ONO)(C∧N)PPh3

Pd-3(OAc)(5)NO2, an impurity in Pd(OAc)(2) {formally Pd-3(OAc)(6)}, emerges as a serious issue in the synthesis of pure Pd-II complexes derived from Pd(OAc)(2), for example in our C-H activation precatalyst, Pd(OAc)(2)(pip)(2) (pip piperidine). A previous proposal that nitrite anion can be formed by oxidation of CH3CN by metallic Pd and air, leading to cyclo(ortho)palladated complexes containing nitrite anion, e.g. Pd(NO2)(C boolean AND N) L (C boolean AND N = papaverine; L = CH3CN or DMSO) can be explained by Pd-3(OAc)(5)NO2 acting as the nitrite source. Finally, photocrystallographic metastable linkage isomerisation and complete conversion to an oxygen-bound nitrito complex Pd(eta(1)-ONO)(C boolean AND N)PPh3 has been observed.