pH-Dependent competition between κ²N ⁷,O( P) macrochelation and µ-N ¹,N ⁷ oligomer formation for (η⁶-arene)Ru^II complexes of adenosine and guanosine 5′-mono-, -di- and -tri-phosphates

Abstract

The pH-dependent reaction of [Ru(η⁶-C₆H₆)(D₂O) ₃]²⁺ with adenosine and guanosine 5′-mono-, -di- and -tri-phosphates has been studied by ¹H and ³¹P-{¹H} NMR spectroscopy. Diastereomeric µ-1κN¹ : 2κ² N⁶, N⁷ co-ordinated cyclic trimers of the type [{Ru(5′-AMP)(η⁶-C₆H₆) }₃] predominate for adenosine 5′-monophosphate (5′-AMP^2-) in the range pH* 3.30–9.18. An X-ray structural analysis of the Ru_SRu_SRu_S diastereomer [{Ru(5′-AMP)(η⁶-p-MeC₆H ₄Prⁱ)}₃]·7.5H₂O 1b established a pronounced degree of conformational flexibility in the sugar and phosphate residues. In contrast to 5′-AMP^2-, cyclic trimers cannot be observed in more strongly acid solution (pH* ⩽ 3.16) for the equilibrium system 5′-ATP–(η⁶-C₆H₆)Ru ^II (5′-ATP^4- = adenosine 5′-triphosphate) and remain relatively minor species even at neutral or higher pH* values. As confirmed by pronounced low-field ³¹P-{¹H} NMR shifts of up to 7.8 and 8.6 ppm for the β- and γ-phosphorus atoms, κ³N⁷, O(P _β), O(P_γ) macrochelates provide the dominant metal species in acid solution. Time-dependent NMR studies for 5′-ADP–(η⁶-C₆H₆)Ru ^II (5′-ADP^3- = adenosine 5′-diphosphate) indicated that initial macrochelation of this nucleotide is followed by cleavage of the β-phosphate group and formation of cyclic trimers of 5′-AMP^2-. Reaction of guanosine 5′-monophosphate (5′-GMP^2-) with [Ru(η⁶-C₆H₆)(D₂O) ₃]²⁺ afforded κN⁷-co-ordinated 1 : 1 and 2 : 1 complexes in the range pH* 3.69–8.38. In addition to analogous 1 : 1 and 2 : 1 species, κ³N⁷, O(P_β), O(P_γ) macrochelates are observed for the 5′-GTP–(η⁶-C₆H₆)Ru ^II equilibrium system (5′-GTP^4- = guanosine 5′-triphosphate) in acid solution. Initial macrochelation in the 5′-GDP–(η⁶-C₆H₆)Ru ^II system (5′-GDP^3- = guanosine 5′-diphosphate) again leads to rapid cleavage of the terminal β-phosphate function.

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