Seven-co-ordination in metal complexes of quinquedentate macrocyclic ligands. Part 5. Synthesis and properties of pentagonal-bipyramidal and pentagonal-pyramidal manganese(II) complexes and crystal and molecular structure of {2,15-dimethyl-3,7,10,14,20-penta-azabicyclo[14.3.1]eicosa-1(20),2,14,16,18-pentaene}bis(isothiocyanato)manganese(II)

Abstract

The Mn²⁺ ion acts as a template for the synthesis of complexes of the 15-. 16-, and 17-membered N₅ macrocycles (L¹, L², and L³) formed from 2,6-diacetylpyridine and, respectively, 3,6-diazaoctane-1,8 diamine, 3,7-diazanonane-1,9- diamine, and 4,7-diazadecane-1,10-diamine. The complexes are of the type [MnLX₂]·xH₂O or [MnLX-(ClO₄)]·xH₂O (X = Cl, NCS, or BPh₄: x= 0, 0.5, 2, or 6). All the complexes are high spin with S=5//₂ ground states. From spectroscopic, magnetic, and other evidence it is concluded that the complexes of L¹ and L² have pentagonal-bipyramidal structures with the macrocycle defining the equatorial plane and the axial positions occupied by Cl^–, NCS^–, or H₂O. All the complexes of L¹ and L² are mononuclear except [{MnL(NCS)}n][ClO₄]_n which in the solid state are polymeric with the metal ions linked via NCS bridges which also transmit weak antiferromagnetic exchange. An X-ray structure determination of [MnL³(NCS)₂] shows that the complexes of the largest macrocycle have somewhat different structures from those of L¹ and L². Crystals of [MnL³(NCS)₂] are monoclinic with a= 12.007(8), b= 14.300(9), c= 13.227(9)Å, β= 100.51(6)°, Z= 4, space group P2₁/n. The structure has been solved by Patterson and Fourier methods and refined by full-matrix least squares to R 0.058 for 2 023 independent reflections above background measured by diffractometer. While the geometry of the co-ordination sphere is best considered as a distorted pentagonal bipyramid with the thiocyanate groups in axial positions, the macrocycle is distorted from planarity in that, while four nitrogens are coplanar with the metal ion, the pyridine nitrogen atom is 0.92 Å from this plane.Metal–nitrogen(macrocycle) distances are 2.385(6), 2.311(5), 2.354(6), 2.429(6), and 2.385(6)Å. The Mn–N bonds [2.174(7) and 2.294(7)Å] and the Mn–N–C angles [150.9(6) and 135.0(5)°] to the thiocyanates are significantly different and reflect the different environments of the two ligands. A further consequence of the folding of this macrocycle is that the more sterically crowded axial ligand is readily displaced, as in [MnL³(NCS)][ClO₄], to yield six-co-ordinate, presumably pentagonal-pyramidal, complexes.