Metallointercalators: syntheses, structures, and photochemical characterizations of phenanthrenequinone diimine complexes of rhodium(III)

Abstract

Short and very efficient syntheses of a series of d^6 rhodium(II1) complexes, [Rh(X_4)phi]^(3+) (X_4 = (NH_3)_4, (en)_2, tren, [12]aneN_4, [12]aneS_4), containing the 9,10-phenanthrenequinone diimine (phi) ligand and mono-, bi-, and tetradentate amine- or sulfur-containing ligands have been carried out. The synthetic strategies include two-step reaction sequences, in which the coordinated chlorides of [Rh(X_4)Cl_2]^+ are replaced by 9,10-diaminophenanthrene, and a single-step method, in which RhCl_3 is first reacted with 9,10-diaminophenanthrene and then the ancillary ligands are added, followed by oxidation with dioxygen. The complexes have been investigated with respect to their solid-state structure, spectroscopy, and photochemical reactivity. This detailed characterization provides the basis to explore systematically the contributions of hydrogen bonding and van der Waals interaction to sequence-specific recognition of DNA. Crystal structures of [Rh[(NH_3)_4phi]Cl_3•3H_2O (triclinic crystal system, space group P1 (No. 2), Z = 2, a = 7.605(2) Å, b = 9.081(2) Å, c = 16.729(4) Å, α = 87.95(2)°, β = 76.92(2)°, γ = 84.42(2)°, V= 1119.9(4) Å^3), and [Rh([12]aneN_4)phi](SCN)_3•2H_2O triclinic crystal system, space group P1 (No. 2), Z = 2, a = 15.479(5) Å, b = 12.312(2) Å, c = 8.679(2) Å, α = 72.10(2)°, β = 83.98(2)°, γ = 71.52(2)°, V = 1492.7(7) Å^3), and Rh([12]aneS_4)phi]Br_3•3H_2O (triclinic crystal system, space group P1 (No. 2), Z = 2, a = 8.921(2) Å, b = 12.846(4) Å, c = 13.325(4) Å, α = 70.50(2)°, β = 86.43(2)°, γ = 86.68(2)°, V= 1435.5(7) Å^3) are reported. Two isomers, which are also seen in solution in a ratio of 60:40 (ΔG = 0.99 kJ/mol), of C_(2ν) (14% population) and C_S (86% population) symmetry have been found in the same crystal of [Rh([12]aneN_4)phi](SCN)_3•2H_2O, but only one isomer (point group C_S) has been found for the sulfur analogue [Rh([12]aneS_4)phi]Br_3. The substantial distortion of the octahedral geometry in the complexes [Rh([12]aneN_4)phi]^(3+) and [Rh([12]aneS_4)phi]^(3+) is associated with the small hole size of the coordinating macrocycles. Angles N(4)-Rh-N(6) between axial nitrogen atoms are as small as 160° in [Rh([12]aneN_4)phil]_3^+ or 168.5° for the angle S(1)-Rh-S(3) between the axial sulfur atoms of [Rh( [12]aneS_4)phi]^(3+). The effect on the overall topology of different conformations of the independently coordinating bidentate ligands in [Rh(en)_2phi]^(3+) is compared to the largely restricted conformations of tetradentate macrocycles in [Rh([12]aneN_4)phi]^(3+) and [Rh([12]aneS_4)phi]^(3+). ^(13)C and ^1H NMR spectroscopic data show that the structures seen in the solid state are also retained in solution. The chemical shift of the C1 or C8 hydrogen atom of the phi ligand is systematically influenced by the relative configuration at the heteroatom which is coordinated to the rhodium in the equatorial position. Electronic absorption spectra of the phi complexes of rhodium are found to be quite similar, dominated by strong bands between 250 and 280 nm and a broad transition between 360 and 410 nm. A reversible blue shift and hypochromicity (13-32%) are observed in the UV spectrum upon changing the pH from 2 to 10. The optical pK_a values for the phi complexes are correlated with the pK_b values of the ancillary ligands. [Rh(X_4)phi]^(3+) complexes undergo photodecomposition upon irradiation with UV light at 325 nm with the preferential loss of the phi ligand. The pH dependence of the quantum yields for photoanation supports a mechanism in which light-induced charge transfer from the aromatic phi ligand to the rhodium(II1) center is involved.

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