PALLADIUM 1-METHYLCYTOSINE CHEMISTRY - N3 AND N3,N4 METAL-BINDING TO 1-METHYLCYTOSINE AND AN UNEXPECTED TRANS-]CIS ISOMERIZATION OF 2 (NH3)2PD - ENTITIES

A series of complexes of Pd(II) with the model nucleobase 1-methylcytosine (1-MeC) has been prepared and studied by spectroscopic (H-1 NMR, IR, Raman) and X-ray methods: trans-[Pd(NH3)2(1-MeC)2]X2.nH2O (X = NO3- (1a); CIO4- (1b)), [Pd(1-MeC)4]X2.nH2O (X = NO3-, n = 2 (2a)); X = Cl-, n = 7 (2b)), cis-[(NH3)2Pd(1-MeC-)2Pd(NH3)2](NO3)2.2H2O (3), Pd(1-MeC-)2.4H2O (4), and [Pd(NH3)(1-MeC)3](NO3)2 (5). Metal binding in 1, 2, and 5 is through N(3) of 1-MeC, whereas N(3) and the deprotonated N(4) position are involved in metal binding in 3 and 4. While 1, 2, and 5 are obtained in a planned and predictable fashion, formation of 3 and 4 is not readily rationalized. In particular, the reaction of 1a with trans-[Pd(NH3)2(H2O)2]2+, which gives 3 with cis-(NH3)2Pd(III) entities and head-tail arranged 1-methylcytosinato ligands, is unexpected. The behavior of 1, 5 and trans-[Pd(1-MeC)2(D2O)2]2+ in D2O at 40-45-degrees-C is rather complex and points to extensive scrambling of all ligands. The structures of 1a, 2a, and 3 have been determined by X-ray crystallography. 1a: monoclinic system, space group P2(1)/n, a = 6.804 (2) angstrom, b = 10.292 (3) angstrom, c = 12.952 (3) angstrom, beta = 102.05 (2)-degrees, V = 887.0 (4) angstrom-3, Z = 2. 2a: triclinic system, space group P1BAR, a = 9.138 (1) angstrom, b = 12.670 (3) angstrom, c = 14.137 (3) angstrom, alpha = 80.50 (2)-degrees, beta = 74.14 (2)-degrees, gamma = 87.89 (1)-degrees, V = 1552.9 (5) angstrom-3, Z = 2. 3: monoclinic system, space group P2(1)/n, a = 10.015 (2) angstrom, b = 17.094 (3) angstrom, c = 14.216 (2) angstrom, beta = 101.72 (2)-degrees, V = 2383.0 (9) angstrom-3, Z = 4. The structures were refined to R = 0.022 (1a), 0.058 (2a), and 0.059 (3) and R(w) = 0.024 (1a), 0.060 (2a), and 0.061 (3). The Pd...Pd distance in the dinuclear cation of 3 is 2.948 (1) angstrom.