title: Isocytosine as a hydrogen-bonding partner and as a ligand in metal complexes.
abstract: Isocytosine (ICH; 1) exists in solution in an equilibrium of tautomers 1a and 1b with the N1 and N3 positions carrying the acidic proton, respectively. In the solid state, both tautomers coexist in a 1:1 ratio. As we show, the N3H tautomer 1b can selectively be crystallized in the presence of the model nucleobase 1-methylcytosine (1-MeC). The complex 1b x (1-MeC)2 x H2O (2) forms pairs through three hydrogen bonds between the components; hydrogen bonds between identical molecules are also formed, leading to an infinite tape structure. On the other hand, the N1H tautomer 1a co-crystallizes with protonated ICH to give [1a x ICH2]NO3 (3), again with three hydrogen bonds between the partners, yet the acidic proton is disordered over the two entities. With M(II)(dien) (M=Pt, Pd; dien=diethylenetriamine) preferential coordination of tautomer 1a through the N3 position is observed. DFT calculations, which were also extended to Pt(II)(tmeda) linkage isomers (tmeda=N,N,N',N'-tetramethylethylenediamine), suggest that intramolecular hydrogen bonding between the ICH tautomers and the co-ligands at M, while adding to the preference for N3 coordination, is not the major determining factor. Rather it is the inherently stronger Pt-N3 bond which favors complexation of 1a. With an excess of M(II)(dien), dinuclear species [M2(dien)2(IC-N1,N3)]3+ (M=Pd(II), 4 and Pt(II), 5) also form and were isolated as their ClO4(-) salts and structurally characterized. In strongly acidic medium 5 is converted to [Pt(dien)(ICH-N1)]2+ (6), that is, to the Pt(II) complex of tautomer 1b.
authors.affiliation: Fachbereich Chemie, Universität Dortmund, Otto-Hahn-Str. 6, 44221 Dortmund, Germany.
authors.forename: Deepali
authors.initials: D
authors.lastname: Gupta
authors.forename: Michael
authors.initials: M
authors.lastname: Roitzsch
authors.forename: Bernhard
authors.initials: B
authors.lastname: Lippert
mesh_terms: D018360:Crystallography, X-Ray; D003596:Cytosine / Q000031:analogs & derivatives* / Q000737:chemistry; D006860:Hydrogen Bonding; D008024:Ligands; D019216:Metals, Heavy / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D010165:Palladium / Q000737:chemistry; D010984:Platinum / Q000737:chemistry; D013237:Stereoisomerism
chemical_list: D008024:Ligands; D019216:Metals, Heavy; D009942:Organometallic Compounds; C093270:isocytosine; D010984:Platinum; D010165:Palladium; D003596:Cytosine