Electrospray Ionization Mass Spectrometry of Palladium(II) Quinolinylaminophosphonate Complexes

The mass spectrometric behavior of palladium(II) halide complexes of three types of quinolinylaminophosphonates, diethyl and dibutyl esters of α-anilino-(quinolin-2-yl)methyl]phosphonic (L1, L2), α-anilino-(quinolin-3-yl)methyl]phosphonic (L3, L4), and α-(quinolin-3-ylamino)-N-benzyl]phosphonic acid (L5, L6), was investigated under positive ion electrospray ionization conditions. Each type of ligand forms complexes with different metal–ligand interactions. Mononuclear dihalide adducts cis-Pd(L1/L2)X₂] (1–4) and trans-Pd(L3/L4)₂X₂] (5–8) as well as dinuclear tetrahalide complexes Pd₂(L5/L6)₃X₄] (9–12) (X = Cl, Br) are formed by metal bonding either through the quinoline or both the quinoline and amino nitrogen atoms. The sodiated molecule M + Na]⁺ is observed in the mass spectra of all the complexes, and its abundance as well as the fragmentation pathway depend on the type of the complex. In the cis complexes (1–4) the initial decomposition goes under two fragmentation routes: those in which the sodium molecular adduct sequentially loses halides HX/NaX and those in which this loss is in the competition with the loss of dialkyl phosphite. The predominant pathways for decomposition of trans dihalide (5–8) and tetrahalide (9–12) complexes include three competitive reactions; the loss of halides, dialkyl phosphites and the intact phosphonate ligand molecule and its fragments formed by ester dissociation or complete loss of the phosphonate ester moiety. A series of acetonitrile adducts and cluster ions derived from dimolecular clusters 2M + Na]⁺ were also detected. The most important fragmentation patterns are rationalized and supported by the MS ⁿ studies.