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 [Rh-2(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), [Rh-2(OAc)(4)(L)] (the dirhodium-core:L ratio of 1:1). The reaction of [Rh-2(HNOCCF3)(4)] or [Rh-2(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 [Rh-2(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 [Rh-2(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, [Rh-2(OAc)(2)(HNOCCF3)(2)(9-ethylguanine)(2)].2MeOH.2H(2)O (2), [Rh-2(HNOCCF3)(4)(L)(2)].3H(2)O (L = guanosine (3), deoxyguanosine (4), or inosine (5)), and [Rh-2(OAc)(2)(HNOCCF3)(2)(guanosine)(2)].3H(2)O (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 0(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 [Rh-2(OOCR)(4)], where the carboxylato ligand could function as hydrogen-bonding acceptor only. Similarly, the formation of interligand NH...O(6) hydrogen-bonding observed in 2-6 suggests that [Rh-2(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 [Rh-2(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. (C) 2002 Elsevier Science B.V. All rights reserved.