STRONG METAL METAL BONDS BETWEEN TRANS-(AMINE)2PT-II AND TRANS-(AMINE)2-PD-II IN HETERONUCLEAR COMPLEXES OF CYTOSINE NUCLEOBASES - PREPARATION, X-RAY STRUCTURES, AND NMR-SPECTROSCOPY

trans-[a2Pt(I-MeC-N3)2](NO3)2 (a = NH3 (1a), CH3NH2 (1b); I-MeC = 1-methylcytosine, C5H7N3O) reacts with K2PdCl4 and trans-[(NH3)2Pd(H2O)2](NO3)2 to give trans-[a2Pt(1-MeC--N3,N4)PdCl]+, PtPdCl (a = NH3 (2a); a = CH3NH2 (2b)), and trans-[a2Pt(1-MeC--N3,N4)2Pd(NH3)]2+, PtPd(NH3) (a = NH3 (4a, 4c); a = CH3NH2 (4b)), respectively, with two 1-methylcytosinato ligands (deprotonated at N4) binding to Pd. The coordination planes of the two d8 metals are perpendicular to each other, with Pt representing a ligand of Pd and Pd being a fifth ligand of Pt. Displacement of the Cl ligand in 2a and 2b by reaction with a stoichiometric amount of AgNO3 yields the corresponding aqua complexes, trans-[a2Pt(1-MeC--N3,N4)2Pd(H2O)]2+, PtPd(H2O) (a = NH3(3a); a = CH3NH2 (3b)). PtPd (H2O) has been used as a starting material for the preparation of a large variety of PtPdY compounds with Y = nucleobase such as 1-methylcytosine (5a, 5b), 1-methyluracilate (6a, 6b), 1-methylthyminate (7a, 7b), 9-ethylguanine (8b), 9-ethylguaninate (8a, 8c, 8d), and 2-thiouracil (14b), as well as Y =mu-pyrazine (9b), 4,4'-bipyridine (10b), hexamethylenetetramine (11b), SCN- (12b), (CH3)2SO (13b), F- (15b), Br- (16b), and I- (17b). Of all ligands Y applied, only Y = thiourea causes instantaneous decomposition of PtPd andre-formation of trans-[a2Pt(1-MeC)2]2+. Apart from X-ray crystallography, PtPdY compounds are characterized primarily by H-1 and Pt-195 NMR spectroscopy. The H-1 resonances of the bridging 1-MeC-ligands undergo characteristic upfield shifts as compared to 1a and 1b upon Pd binding and deprotonation at the exocyclic amino groups. The Y ligand has a relatively minor effect on the chemical shifts of the 1-MeC- resonances. Exchange of the axially bound Y and free Y as well as between different coordination sites of Y (linkage isomers of Y = 9-ethylguanine) is slow on the H-1 NMR time scale with the nucleobases 1-MeC, 1-MeU, 1-MeT, and 9-EtGH/9-EtG but is fast with 2-thiouracil, pyrazine, 4,4'-bipyridine, hexamethylenetetramine, SCN-, and Me2SO. On the Pt-195 NMR time scale, exchange processes in all cases are slow, with individual resonances observable. Pt-195 NMR chemical shifts of PtPdY compounds cover a range of 500 ppm, depending on Y, with extremes being OH- (-2496 ppm) and Me2SO (-1963ppm). X-ray structures of PtPdY with Y = Cl-(2a), NH3(4a), 1-MeU-(6a), mu-pyz(9b), and SCN- (12b) have been performed. Pt-Pd bond lengths are between 2.492(3) angstrom (9b) and 2.521(1) angstrom (12b). The metal-metal bond in these compounds is interpreted in terms of a Pt --> Pd donor bond.