Multinuclear (1H, 13C, 15N, 195Pt) NMR has been used to study the reactions of Pt(II)-nitrite complexes with various acids.
Reaction of K2[Pt(NO2)4] with sulfamic acid in aqueous solution yielded [Pt(NO2)3(H2O)]- and cis-[Pt(NO2)2(H2O)2]2-. The aqua ligands of these complexes could be displaced easily by anions X (X = OH-, CH3CO2-, SO42-, C1-, Br-, NH3, NH2SO3-, SCN-) to give the corresponding complexes [Pt(NO2)3X]n- and cis-/trans- [Pt(NO2)2X2]n-. Thiourea also displaced water from these complexes whilst iodide reduced them to platinum metal. To form [PtI(NO2)3]2- and trans-[PtI2(NO2)2]2-, iodide was added to aqueous solutions of K2[Pt(NO2)4]. NMR parameters (δpt, δN and 1J(Pt-N)) were measured for all these complexes.
The pKa, of [Pt(NO2)3(H2O)]- (5.32 ± 0.01 at 25 °C) was determined by using 15N NMR spectroscopy to monitor the effect of pH on δN. Such a value for cis- [Pt(NO2)2(H2O)2] could not be obtained by this method due to the formation of the oliomers [Pt(NO2)2(OH)]nn- (n = 2,3) at approximately pH = 3.
The reaction of [Pt(NO2)4]2- with aqueous 1 M CF3SO3H and HC1O4 under air or argon yielded successive removal of nitrite ligands, the complexes [Pt(NO2)4]2-, [Pt(NO2)3(H2O)]-, cis-[Pt(NO2)2(H2O)2], [Pt(NO2)(H2O)3]+ and [Pt(H2O)4]2+ being observed by 195Pt spectroscopy. Oxidation also occurred with two Pt(IV) species being formed, [Pt(OH)6]2- and an unidentified complex.
[Pt(NO2)6]2- was formed when [Pt(NO2)4]2- was heated in 1 M HNO3.
When [Pt(NO2)4]2- was heated in 0.5 M H2SO4, several 15N NMR signals were observed, mostly due to Pt(IV) complexes in solution. [Pt(NO2)3(H2O)]- and cis- [Pt(NO2)2(H2O)2] were also present.
Aqueous acetic acid reacted with [Pt(NO2)4]2-, under aerobic and anaerobic conditions, to form a variety of Pt(II), Pt(III) and Pt(IV) complexes. [Pt(NO2)3(OAc)]2- and cis-[Pt(NO2)2(OAc)2]2- were the initial products. These complexes were in equilibrium with the dimeric Pt(II) complexes [Pt2(µ-CH3CO2-O, O’)(NO2)6]3- and [Pt2(µ-CH3CO2-O,O')2(NO2)4]2-. The X-ray crystal structure of the latter complex was obtained.
Subsequent reaction in aqueous acetic acid caused the oxidation of the Pt(II) dimers to PtfLII) complexes containing two or three acetate bridging ligands. The oxidant under anaerobic conditions was shown to be, primarily, nitric acid - a by-product of the reaction of coordinated nitrite with acid. Under aerobic conditions, a variety of oxidants were formed (O2, HNO3 and NO2). All of these can oxidize Pt(II) complexes to Pt(III). No Pt(III) complex in which all the coordinated nitrite had been removed from the metal centres was observed from these reactions.
The Pt(IV) complexes observed, more prevalent under aerobic conditions, were mer-[Pt(NO2)3(OAc)3]2-, trans-[Pt(NO2)4(OAc)2]2- and cis-[Pt(NO2)2(OAc)4]2-.
Reacting [Pt(NO2)4]2- with a 2:1 CH3CO2H / HX (X = CF3SO3-, C1O4-, NO3-) mixture yielded various Pt(III) complexes in solution. The ultimate product of these reactions, however, were the complexes [Pt2(µ-CH3CO2-O,O’)4(H2O)2]X2 which precipitated from solution. X-ray crystal structures were determined for the trifluoromethanesulfonate and perchlorate salts.
The cation, in fresh solvent, dissociated rapidly. In fact, dissociation was rapid in all solvents in which the salts were soluble except glacial acetic acid and dmf. In the former solvent, the neutral complex [Pt2(µ-CH3CO2-O,O')4(OAc)2] was observed by 195Pt NMR spectroscopy. Within five hours, however, decomposition had occurred. The trifluoromethanesulfonate salt was very soluble in dmf and dissociation slow enough to enable the measurement of the 195Pt NMR spectra of the diaqua, bis(dmf) and aqua-dmf complexes. Addition of NaC1, NaBr, NaOAc, pyridine or diethylsulfide caused displacement of the axial water molecules. The solid adducts [Pt2(µ-CH3CO2-O,O’)4X2] (X = C1-, Br-) were isolated.
13C CP-MAS NMR spectra were measured for the complexes [Pt2(µ-CH3CO2-O,O’)4(H2O)2](CF3SO3)2] and [Pt2(µ-CH3CO2-O,O')4C12].
[Pt2(µ-CH3CO2-O,O')4(H2O)2](C1O4)2 reacted in 2 M H2SO4 to yield a solid containing, as shown by IR spectroscopy, both bridging acetate and sulfate ligands. The composition of these solids is uncertain as reproducible microanalyses were not obtained.