Resolution, structures, and vibrational circular dichroism of helicoidal trinickel and tricobalt paddlewheel complexes.

It has been recently shown that enantiomers of the helicoidal paddlewheel complex [Co (dpa) (CH CN) ] (dpa = the anion of 2,2'-dipyridylamine) can be resolved using the chiral [As (tartrate) ] anion (AsT) and that these complexes demonstrate a strong chiroptical response in the ultraviolet-visible and X-ray energy regions. Here we report that the nickel congener, [Ni (dpa) (CH CN) ] , can likewise be resolved using AsT. Depending on the stereochemistry of the enantiopure AsT anion, one or the other of the trinickel enantiomers crystallize from CH CN and diethyl ether in space group P42 2 as the (NBu ) Ni (dpa) (CH CN) ·[solvent] salt. After resolution, the AsT salts were converted into the PF salts by anion exchange, with retention of the chirality of the trinickel complex. The enantiopure Ni (dpa) (CH CN) ·2CH CN and Co (dpa) (CH CN) ·CH CN·C H O compounds crystallize in space groups C2 and P2 , respectively. Both the Ni(II) and Co(II) complex cations are stable towards racemization in CH CN. Vibrational circular dichroism (VCD) data obtained in CD CN demonstrate the expected mirror image spectra for the enantiomers, the observed peaks arising from the dpa ligand. The VCD response is significant, with Δε values up to 6 Lmol cm and vibrational dissymmetry factors on the order of 10 . Density functional theory calculations well reproduce the experimental spectra, showing little difference between the peak position, sign, and intensity in the VCD for the cobalt and nickel complexes. These results suggest that VCD enhancement of these peaks is unlikely, and their remarkable intensity may be due to their rigid helicoidal structure.