Organometallic Ruthenium(II) Diamine Anticancer Complexes: Arene-Nucleobase Stacking and Stereospecific Hydrogen-Bonding in Guanine Adducts

Organometallic ruthenium(II) arene anticancer complexes of the type [(η6-arene)Ru(II)(en)Cl][PF6] (en = ethylenediamine) specifically target guanine bases of DNA oligomers and form monofunctional
adducts (Morris, R., et al. J. Med. Chem. 2001). We have determined the structures of monofunctional
adducts of the “piano-stool” complexes [(η6-Bip)Ru(II)(en)Cl][PF6] (1, Bip = biphenyl), [(η6-THA)Ru(II)(en)Cl][PF6] (2, THA = 5,8,9,10-tetrahydroanthracene), and [(η6-DHA)Ru(II)(en)Cl][PF6] (3, DHA = 9,10-dihydroanthracene) with guanine derivatives, in the solid state by X-ray crystallography, and in solution
using 2D [1H,1H] NOESY and [1H,15N] HSQC NMR methods. Strong π−π arene-nucleobase stacking is
present in the crystal structures of [(η6-C14H14)Ru(en)(9EtG-N7)][PF6]2·(MeOH) (6) and [(η6-C14H12)Ru(en)(9EtG-N7)][PF6]2·2(MeOH) (7) (9EtG = 9-ethylguanine). The anthracene outer ring (C) stacks over the
purine base at distances of 3.45 Å for 6 and 3.31 Å for 7, with dihedral angles of 3.3° and 3.1°, respectively.
In the crystal structure of [(η6-biphenyl)Ru(en)(9EtG-N7)][PF6]2·(MeOH) (4), there is intermolecular stacking
between the pendant phenyl ring and the purine six-membered ring at a distance of 4.0 Å (dihedral angle
4.5°). This stacking stabilizes a cyclic tetramer structure in the unit cell. The guanosine (Guo) adduct [(η6-biphenyl)Ru(en)(Guo-N7)][PF6]2·3.75(H2O) (5) exhibits intramolecular stacking of the pendant phenyl ring
with the purine five-membered ring (3.8 Å, 23.8°) and intermolecular stacking of the purine six-membered
ring with an adjacent pendant phenyl ring (4.2 Å, 23.0°). These occur alternately giving a columnar-type
structure. A syn orientation of arene and purine is present in the crystal structures 5, 6, and 7, while the
orientation is anti for 4. However, in solution, a syn orientation predominates for all the biphenyl adducts
4, 5, and the guanosine 5‘-monophosphate (5‘-GMP) adduct 8 [(η6-biphenyl)Ru(II)(en)(5‘-GMP-N7)], as
revealed by NMR NOE studies. The predominance of the syn orientation both in the solid state and in
solution can be attributed to hydrophobic interactions between the arene and purine rings. There are
significant reorientations and conformational changes of the arene ligands in [(η6-arene)Ru(II)(en)(G-N7)]
complexes in the solid state, with respect to those of the parent chloro-complexes [(η6-arene)Ru(II)(en)Cl]+. The arene ligands have flexibility through rotation around the arene-Ru π-bonds, propeller twisting
for Bip, and hinge-bending for THA and DHA. Thus propeller twisting of Bip decreases by ca. 10° so as to
maximize intra- or intermolecular stacking with the purine ring, and stacking of THA and DHA with the
purine is optimized when their tricyclic ring systems are bent by ca. 30°, which involves increased bending
of THA and a flattening of DHA. This flexibility makes simultaneous arene-base stacking and N7-covalent
binding compatible. Strong stereospecific intramolecular H-bonding between an en NH proton oriented
away from the arene (en NH(d)) and the C6 carbonyl of G (G O6) is present in the crystal structures of 4,
5, 6, and 7 (average N···O distance 2.8 Å, N−H···O angle 163°). NMR studies of the 5‘-GMP adduct 8
provided evidence that en NH(d) protons are involved in strong H-bonding with the 5‘-phosphate and O6
of 5‘-GMP. The strong H-bonding from G O6 to en NH(d) protons partly accounts for the high preference
for binding of {(η6-arene)Ru(II)en}2+ to G versus A (adenine). These studies suggest that simultaneous
covalent coordination, intercalation, and stereospecific H-bonding can be incorporated into Ru(II) arene
complexes to optimize their DNA recognition behavior, and as potential drug design features.