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Effect of Ligand Architecture on the Structure and Properties of Square‐Planar Nickel(II) Complexes of Amide‐Based Macrocycles
Author(s) -
Sharma Savita K.,
Upreti Shailesh,
Gupta Rajeev
Publication year - 2007
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.200700122
Subject(s) - chemistry , amide , deprotonation , ligand (biochemistry) , crystallography , amine gas treating , nickel , stereochemistry , coordination geometry , coordination complex , bond length , octahedron , crystal structure , center (category theory) , molecule , ion , hydrogen bond , metal , organic chemistry , biochemistry , receptor
Abstract The present work discusses the design and synthesis of four novel macrocyclic ligands and their Ni II complexes. All four ligands (H 2 1 EN , H 2 2 EN , H 2 3 OP , H 2 4 OP ) are designed to provide an identical donor‐environment composed of two deprotonated anionic amide‐ N coordination and two neutral amine‐ N coordination and an identical 12‐membered macrocyclic ring around the nickel center. The Ni II complexes ( 1 , 2 , 3 , and 4 of doubly deprotonated ligands [1 EN ] 2– , [2 EN ] 2– , [3 OP ] 2– , and [4 OP ] 2– , respectively) are prepared and thoroughly characterized. The coordination geometry around the Ni II center is square planar as determined by the single crystal structure analyses. The average Ni–N amide bond lengths are shorter by ca. 0.1 Å than the average Ni–N amine distances while the average N amide –Ni–N amide angles are shorter by ca. 4° than the average N amine –Ni–N amine angles. The Ni II ion is housed quite tightly within the aperture of the macrocycle as seen by the short Ni–N bond lengths and a very small displacement (0.11–0.17 Å) of the Ni center out of the N 4 basal plane. The detailed 1 H and 13 C NMR studies on the diamagnetic Ni–complexes reveal that the solid state conformation of the complexes 1 – 4 is retained in the solution. The Ni III/II potential is fairly positive (>0.6 V vs. SCE) for all four complexes, while the complexes with phenylene as part of the ligand also show the oxidation(s) of the ring(s) (>1.1 V vs. SCE). Coulometric measurements confirm both responses as 1e – process. Electrochemical or chemical oxidation at the 1 st potential results in the generation of an unstable “green species” that show distinct features in the absorption spectrum and exhibit anisotropic EPR spectrum. These findings strongly point out that the unstable green species is a Ni III species. The 2 nd and/or 3 rd oxidation is centered on the phenylene ring(s) and lead to an immediate decomposition. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)