Two isomeric salicylic aldehyde nucleobases have been prepared and incorporated into various DNA duplexes. Reaction with ethylenediamine leads to formation of the well-known salen ligand inside the DNA double helix. Addition of transition-metal ions such as Cu(2+), Mn(2+), Ni(2+), Fe(2+), or VO(2+) results in the formation of metal-salen-base-pair complexes, which were studied by using UV and circular dichroism (CD) spectroscopy. HPLC and ESI mass spectrometric measurements reveal an unusually high stability of the DNA-metal system. These metal-salen complexes act as interstrand cross-links and thereby lead to a strong stabilization of the DNA duplexes, as studied by thermal de- and renaturing experiments. Complex formation is strong enough to override sequence information even when the preorganization of the ligand precursors is unfavorable and the DNA duplex is distorted by the metal complexation. Furthermore, melting-point studies show that the salen complex derived from ligand 2 fits better into the DNA duplex, in accordance with results obtained from the crystal structure of the corresponding copper-salen complex 8.

The equimolar reaction of calcium bis[bis(trimethylsilyl)amide] with 2,2,6,6‐tetramethylheptane‐3,5‐dione (Htmhd) in THF yields liquid mononuclear [Ca(tmhd){N(SiMe3)2}(THF)3] (1). A similar reaction in toluene with a stoichiometric ratio of 2:3 gives the dinuclear complex [{(THF)Ca}2(tmhd)2(µ‐tmhd){µ‐N(SiMe3)2}] (2). The calcium atoms of these complexes are in a distorted octahedral environment. In 2 the complex consists of two octahedra connected by a common face of one nitrogen base and two oxygen atoms; the bridging Ca−N bond lengths are extremely large. The metalation of the tetradentate Jacobsen reagent with calcium bis[bis(trimethylsilyl)amide] in 1,2‐dimethoxyethane (DME) gives the corresponding calcium complex 3, nearly quantitatively, as its DME adduct. The calcium atom is in an unusual trigonal prismatic coordination sphere. The metathesis reaction of 3 with tin(II) chloride yields the corresponding yellow tin(II) complex with the metal atom in a distorted square‐pyramidal environment. Complexes 2 and 3 show catalytic reactivity in the ring‐opening polymerization of cyclic esters such as lactones and lactides. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)