Bernhard Nuber

The design of a new type of very rigid tetradentate ligand

Molecular mechanics calculations were used to compute the structural properties of a new type of very rigid tetradentate ligand for tetrahedral co-ordination geometries. The calculations indicate that the pendant arms of the disubstituted bispidine (3,7-diazabicyclo[3.3.1]nonane) backbone need to form six-membered chelate rings with the metal to allow a distorted tetrahedral geometry. Smaller rings lead to five- (trigonal bipyramidal) or six-co-ordinate (octahedral) transition-metal compounds. The quality of these predictions is supported by the experimentally determined structure of a cobalt(II) compound of the ligand with co-ordinated pyridine substituents (five-membered chelate rings) and an additional bidentate nitrate ligand. Comparison of the computed structures with the crystal structure of the cobalt(II) compound and with that of a ligand with methyl-protected phenyl substituents supports the rigidity of the bispidine backbone and indicates that rotation of co-ordinating side chains around a C–C single bond is the only flexibility in these ligands.

Synthesis and co-ordination chemistry of the macrocycle 1,4,7-triisopropyl-1,4,7-triazacyclononane

The bulky tridentate macrocycle 1,4,7-triisopropyl-1,4,7-triazacyclononane (L) has been prepared and its co-ordination chemistry investigated. Reaction of L with [M(CO)6] in refluxing mesitylene affords airstable [ML(CO)3](M = Cr 1, Mo 2 or W 3) in excellent yields. Oxidation of complexes 1 and 2 with SOCl2 in CCl4 yields [MLCl3](M = Cr 4 or Mo 5) whereas [MLBr3](M = Cr 6 or Mo 7) were obtained by using bromine as oxidant in CHCl3. Exhaustive oxidative decarbonylation of 2 in semi-concentrated nitric acid affords colourless [MoLO3]·3H2O 8 which reacts with phenyl isocyanate in dry toluene yielding [MoLO3]·(PhNH)2CO 9. In dry MeCN the ligand reacts with VCl3 to give [VLCl3]10 which undergoes ligand substitution in water in the presence of NaSCN to give [VL(NCS)3]11. When this reaction was carried out in the presence of oxygen [VOL(NCS)2]12 was obtained. The molecular structures of 2, 8 and 9 have been established by single-crystal X-ray analysis.