Effect of axial coordination on the electronic structure and biological activity of dirhodium(II,II) complexes

The reactivities toward biomolecules of a series of three dirhodium(II,II) complexes that possess an increasing number of accessible axial coordination sites are compared. In cis-[Rh2(OAc)2(np)2]2+ (1; np=1,8-naphthyridine) both axial sites are available for coordination, whereas for cis-[Rh2(OAc)2(np)(pynp)]2+ (2; pynp=2-(2-pyridyl)1,8-naphthyridine) and cis-[Rh2(OAc)2(pynp)2]2+ (3) the bridging pynp ligand blocks one and two of the axial coordination sites in the complexes, respectively. The electronic absorption spectra of the complexes are consistent with strong metal-to-ligand charge transfer transitions at low energy and ligand-centered peaks localized on the np and/or pynp ligands in the UV and near-UV regions. Time-dependent density functional theory calculations were used to aid in the assignments. The three complexes exhibit metal-centered oxidations and reductions, localized on the aromatic ligands. The ability of the complexes to stabilize duplex DNA and to inhibit transcription in vitro is greatly affected by the availability of an open axial coordination site. The present work shows that open axial coordination sites on the dirhodium complexes are necessary for biological activity.

Effect of equatorial ligands of dirhodium(II,II) complexes on the efficiency and mechanism of transcription inhibition in vitro

The nature of the equatorial ligands spanning the dirhodium core was shown to affect the ability and mechanism of various lantern-type complexes to inhibit transcription in vitro. The inhibition of transcription by Rh(2)(mu-O(2)CCF(3))(4), Rh(2)(mu-HNCOCF(3))(4), and [Rh(2)(mu-O(2)CCH(3))(2)(CH(3)CN)(6)](2+) appears to proceed predominantly via binding of the complexes to T7-RNA polymerase (T7-RNAP) and is dependent on the concentration of enzyme and Mg(2+) ions in solution. The concentrations of the aforementioned complexes required to inhibit 50% of the transcription, C(inh)(50), are similar to that measured for activated cisplatin, whereas a significantly higher concentration of Rh(2)(mu-HNCOCH(3))(4) is required to effect similar inhibition; the inhibition induced by Rh(2)(mu-HNCOCH(3))(4) does not involve binding to T7-RNAP. The spectral changes observed for each complex upon addition of enzyme are consistent with Rh(2)(mu-O(2)CCF(3))(4), Rh(2)(mu-HNCOCF(3))(4), and [Rh(2)(mu-O(2)CCH(3))(2)(CH(3)CN)(6)](2+) binding to the enzyme and may involve partial displacement of the equatorial (eq) groups by the Lewis basic sites of T7-RNAP. In contrast, addition of enzyme to solutions of Rh(2)(mu-HNCOCH(3))(4) does not result in significant spectral changes, a finding consistent with lack of enzyme dependence in the transcription inhibition. These differences in reactivity and transcription inhibition mechanism among complexes with different bridging ligands are explained by variations of the Lewis acidity of the axial (ax) sites in the series of complexes Rh(2)(mu-O(2)CCF(3))(4), Rh(2)(mu-HNCOCF(3))(4), and Rh(2)(mu-HNCOCH(3))(4). The Lewis acidity of the ax sites is expected to affect the initial interaction of the complexes with the biomolecules, followed by their rearrangement to eq positions if the bridging ligands are labile.