Synthesis and solution and solid-state studies of cobaloxime complexes containing alkylated 6-aminopurine ligands. Molecular and crystal structure of the complex with N(7)-bound 3-benzyladenine and dimethyl phosphonate as the axial ligands

Article abstract of DOI:10.1021/ic50219a036

In order to ascertain the effect on complex stability of long bonds from a metal to an endocyclic nitrogen center of a nucleic acid base, we have examined the formation of complexes containing good trans-directing ligands. Conditions leading to isolable complexes of formula LCo(DH)₂X (where L = 3- or 9-alkylated adenine; X = alkyl or P(O)(OCH₃)₂; DH = monoanion of dimethylglyoxime) have been found. In Me₂SO-d₆ solutions the complex is in equilibrium with the 6-aminopurine base and the solvato complex. The equilibrium was quantified for L = 3-alkyladenine by measuring the areas of the ¹H NMR signals of the oxime methyl groups. The measured stability constants are about 300-500 times smaller than those found earlier for LCo(acac)₂NO₂ (where acac = monoanion of acetylacetone). A complete structural characterization of (3-benzyladenine)Co(DH)₂(P(O)(OCH₃) ₂)·EtOH·H₂O was undertaken. The complex crystallizes from chloroform (containing 0.75% EtOH and 2 drops of H₂O/100 mL) in the monoclinic system, space group P2₁/c, with cell data a = 14.674 (5) A?, b = 13.607 (3) A?, c = 16.735 (9) A?, β = 111.23 (3)°, Z = 4, and V = 3114.7 A?³. The structure was solved by standard heavy-atom methods and has been refined to a final R value of 0.093. Both O?H?O protons in the equatorial plane appear to be localized on one of the two dimethylglyoxime ligands, yielding a neutral and a dianionic ligand. This less usual arrangement for cobaloximes is probably due to the presence of interligand hydrogen bonding involving the 6-amino group of the axial 3-benzyladenine base and the dianionic equatorial dimethylglyoxime ligand. The complex has a long Co-N(7) distance of 2.101 (7) A? and a relatively short Co-P distance of 2.229 (3) A?. The observed length of the Co-N(7) bond most likely arises from a combination of the σ-donor power of the trans dimethyl phosphonate ligand and of steric interactions between the 6-amino group and the equatorial dimethylglyoxime system. The relative instability of the dimethylglyoxime vs. the acetylacetonate complexes is proposed to result from these same two considerations.