Interligand interactions affecting specific metal bonding to nucleic acid bases. A case of [Rh2(OAc)4], [Rh2(HNOCCF3)4], and [Rh2(OAc)2(HNOCCF3)2] toward purine nucleobases and nucleosides

The reaction of [Rh2(OAc)4] with adenine, 9-methyladenine, 9-ethylguanine, adenosine, guanosine, deoxyguanosine, or inosine in methanolic aqueous solution at room temperature gave specifically and quantitatively pink complexes of adenine or its derivatives (=L), [Rh2(OAc)4(L)] (the dirhodium-core:L ratio of 1:1). The reaction of [Rh2(HNOCCF3)4] or [Rh2(OAc)2(HNOCCF3)2] with these nucleobases or nucleosides (=L) gave non-specifically pink or red complexes with dirhodium-core:L=1:1 for adenine derivatives while 1:2 for guanine derivatives. Recrystallization of a 9-methyladenine adduct from a nitric acid solution yielded [Rh2(OAc)2(HNOCCF3)2(9-methyladeninium)2]·(NO3)2 (1), whose crystal structure was determined by X-ray diffraction, showing that two N(1)-protonated 9-methyladeninium molecules coordinate to both the axial-positions of a [Rh2(OAc)2(HNOCCF3)2] nucleus through N(7) with the formation of an intramolecular interligand N(6)H⋯O(acetato) hydrogen bond. Crystal structures of guanine derivatives, [Rh2(OAc)2(HNOCCF3)2(9-ethylguanine)2]·2MeOH·2H2O (2), [Rh2(HNOCCF3)4(L)2]·3H2O (L=guanosine (3), deoxyguanosine (4), or inosine (5)), and [Rh2(OAc)2(HNOCCF3)2(guanosine)2]·3H2O (6) were determined, showing that the dirhodium core is occupied at either axial-side by two guanine or hypoxanthine moieties through N(7) with the formation of an intramolecular interligand N(amidato)H⋯O(6) hydrogen bond. Each of guanosine molecules in 3, 4, and 6 or inosine molecules in 5 adopts the syn, gauche–gauche (or gauche–trans for one molecule in 3), and C(2′)-endo (or C(2′)-endo-C(3′)-exo for one molecule in 4) conformations to form an intramolecular O(5′)H⋯N(3) hydrogen bond. The formation of interligand N(6)H⋯O hydrogen-bonding associated with the sterically constrained octahedral coordination geometry about the rhodium atom bonded to N(7), as observed in 1, and/or to N(1) of adenine may give a structural basis for the adenine-specific reactivity of [Rh2(OOCR)4], where the carboxylato ligand could function as hydrogen-bonding acceptor only. Similarly, the formation of interligand NH⋯O(6) hydrogen-bonding observed in 2–6 suggests that [Rh2(HNHNCR)4] could function as guanine-specific reagents, where the amidinato ligand could function as hydrogen-bonding donor only. The interligand steric constraint due to the octahedral coordination geometry may also explain no reactivity of [Rh2(OOCR)4] toward native B-DNA (right-handed double-stranded structure) but specific bonding to single-stranded and possibly to left-handed double-stranded structures.