CADMIUM AND PROTON NMR STUDY ON ( Cd 2 (BIS(2-PYRIDYL)FORMAMINE) 3 )

The structure of a bi - Cd organic complex [ Cd 2 (bis(2-pyridyl)formamine) 3 or 1], is analyzed thoroughly by 1 H and 113/111 Cd NMR in solution. Only one representative set of pyridyl proton resonance of a total of six pyridines in 1 was observed. The resonance of protons in pyridine ring can be assigned by both 2D COSY and 2D NOESY. Similarly, only one Cd (either 113 Cd or 111 Cd NMR signal was observed too. This indicates that the two Cd atoms in 1 are chemically equivalent. The Cd signal is split by its three neighboring equivalent protons on the bridged carbon atoms ( C a H ) 3 , resulting in a quartet. The 3 J 113 Cd –1 H and 3 J 111 Cd –1 H are 44 and 42 Hz, respectively, measured directly from the spectral pattern. The proton NMR signals of C a H composed from three sources: (1) superimposed doublets arising from Cd – H – 113 Cd and Cd – H – 111 Cd ; (2) a pseudo triplet from 113 Cd – H – 113 Cd , 111 Cd – H – 111 Cd and 113 Cd – H – 111 Cd and (3) a singlet from those H's surrounded by all non-magnetic Cd atoms. They superimpose and appear as a quintet with intensity ratio of 1:12:38:12:1, explainable by the above constituents and their associated Cd – H splitting analyses. We deduced that the three bis(2-pyridyl)formamidine groups have a three-fold symmetry with respect to the Cd – Cd axis, i.e., the three bis(2-pyridyl)formamidine units are co-planar and spaced 120 degrees from each other based on above observations. This implies the key C a atom exists as a hybrid of two canonical forms (– N = C a H – N – ↔ – N – C a H = N –). As a consequence, 1 does not exhibit chirality. We exemplified that Cd NMR study is well suited for investigating those metalloproteins if the calcium and/or zinc ions contained can be replaced by 113 Cd / 111 Cd ions of similar size.