Bioorganometallic Chemistry. 27. Synthetic, X‑ray Crystallographic, and Competitive Binding Studies in the Reactions of Nucleobases, Nucleosides, and Nucleotides with [Cp*Rh(H₂O)₃](OTf)_2, as a Function of pH, and the Utilization of Several Cp*Rh–DNA Base Complexes in Host–Guest Chemistry

The reactions of the air- and water-stable tris­(aqua) complex [Cp*Rh­(H₂O)₃]­(OTf)₂ (<b>1</b>; OTf = trifluoromethanesulfonate) with nucleobases and nucleosides that included 9-methyladenine (9-MA), 9-ethylguanine (9-EG), 9-methylhypoxanthine (9-MH), 9-ethylhypoxanthine (9-EH), 1-methylcytosine (1-MC), 1-methylthymine (1-MT), adenosine (Ado), and guanosine (Guo) provided new bonding modes, all as a function of pH. The 9-MA nucleobase provided a novel cyclic trimer, at pH 6, characteristic for all Ado complexes: [Cp*Rh­(μ₂-η¹(<i>N1</i>):η²(<i>N6</i>,<i>N7</i>)-9-MA/Ado)]₃(OTf)₃. The Cp*Rh­(9-EG) and Cp*Rh­(Guo) complexes showed N7 and 6-CO binding modes in water, [Cp*Rh­((η²(<i>N7</i>,<i>O6</i>)-9-EG/Guo)­(OH)]­(OTf), and no cyclic trimer products, due to a pronounced steric effect of the 2-amino group. This was shown convincingly by the results with 9-MH and 9-EH, which did form cyclic trimers at pH 6.1, [Cp*Rh­(μ₂-η¹(<i>N1</i>):η²(<i>N7</i>,<i>O6</i>)-9-MH/9-EH)]₃(OTf)₃, with a structure similar to that of 9-EG, but with no 2-amino group available. At pH 10.2, the p<i>K</i>_a of the 9-MH’s NH1 hydrogen dictated the structure, providing a μ-hydroxy dimer, <i>trans</i>-[Cp*Rh­(η¹(<i>N1</i>)-9-MH)­(μ-OH)]₂(OTf)₂, while in methanol the same reaction provided a mononuclear complex, [Cp*Rh­(η¹(<i>N7</i>)-9-MH)­(MeOH)₂]­(OTf)₂. The reaction of <b>1</b> and 1-MC, at pH 5.4, provided another μ-hydroxy dimer with intramolecular H bonding of the O and H atoms of the μ-OH groups (H-acceptor and H-donor, respectively), <i>trans</i>-[Cp*Rh­(η¹(<i>N3</i>)-1-MC)­(μ-OH)]₂(OTf)₂, while in acetone, the product was a monomeric complex, [(η⁵-Cp*Rh)­(η¹(<i>N3</i>)-1-MC)­(η²(<i>O2</i>,<i>N3</i>)-1-MC)]­(OTf)₂. The reaction of <b>1</b> and 1-MT at pH 10 showed the initial complex <b>1</b> being converted to its equilibrium complex, [(Cp*Rh)₂(μ-OH)₃]⁺, and this led to two components being formed. The anionic component was a linear [(η¹(<i>N3</i>)-MT)–Rh^I–(η¹(<i>N3</i>)-MT)]^<b>–</b> (12e Rh^I center) assembly, formed via a presumed reductive elimination of Cp*OH, and included an orthogonal array of two thymine planes. The cationic component was [(Cp*Rh)₂(μ-OH)₃]⁺, with its Cp* moiety being π–π stacked with thymine rings, as well as the π–π interactions of two thymine rings: {[Rh^I(η¹(<i>N3</i>)-1-MT)₂]₂[(Cp*Rh)₂(μ-OH)₃]₃}­OH. The competitive order of nucleoside reactivity was Ado ≫ Guo, while for the nucleotides it was GMP > AMP ≫ CMP ≈ TMP. Finally, we also discuss several examples of the utilization of these unique Cp*Rh–DNA base complexes, as aqueous hosts for molecular recognition of aromatic amino acids and as NMR shift reagents for many organic compounds