Synthesis, Characterization, and Structure of [{PtMe₃(9-MeA)}₃] (9-MeAH = 9-Methyladenine): A Cyclic Trimeric Platinum(IV) Complex with a Nucleobase

Reaction of [{PtMe3I}4] with AgOAc in acetone results in formation of trimethylplatinum(IV) acetate (1) that reacts with 9-methyladenine (9-MeAH) in ratio 1:1 with its deprotonation yielding a trimethyl(9-methyladeninato)platinum(IV) complex (2) that was obtained from acetone/diethyl ether as [{PtMe3(9-MeA)}3]·Me2CO (2a) and from chloroform/diethyl ether as [{PtMe3(9-MeA)}3]·1.5Et2O·2H2O (2b). Single-crystal X-ray investigations revealed that 2a and 2b in the solid state form cyclic trimeric molecules in which three PtMe3 moieties are bridged by deprotonated 9-methyladenine ligands in μ-κN1:κ2N6,N7 coordination mode. The methyladeninato ligands include angles with the plane defined by the Pt3 unit between 55.9(3)° and 66.6(3)°. Thus, the structures of the cyclic trimers resemble hollow truncated cones. Crystals of the acetone solvate 2a consist of stacks of unidirectional trimeric molecules; the solvate acetone molecules lie between the trimers. In the crystals of complex 2b, the trimeric molecules are arranged head-to-head, and the cavity between is filled with four water molecules. Complex 2 was also fully characterized by 1H, 13C, and 195Pt NMR spectroscopies. The existence of the cyclic trimeric structure of complex 2 in acetone solution was confirmed by ESI mass spectrometry exhibiting the protonated trimeric complex [{PtMe3(9-MeA)}3H]+ [2 + H]+ (m/z 1165) as base peak. CID experiments of that parent ion showed as main fragmentation processes loss of two methyl ligands (probably as ethane) and cleavage of a methyladenine ligand.