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

Abstract

It has been recently shown that enantiomers of the helicoidal paddlewheel complex [Co3(dpa)4(CH3CN)2] 2+ (dpa = the anion of 2,2'-dipyridylamine) can be resolved using the chiral [As2(tartrate)2] 2− anion (AsT), and that these complexes demonstrate a strong chiroptical response in the UV-visible and X-ray energy regions. Here we report that the nickel congener, [Ni3(dpa)4(CH3CN)2] 2+ , 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 CH3CN and diethyl ether in space group P4212 as the (NBu4)2[Ni3(dpa)4(CH3CN)2](AsT)2•[solvent] salt. After resolution, the AsT salts were converted into the PF6 − salts by anion exchange, with retention of the chirality of the trinickel complex. The enantiopure [Ni3(dpa)4(CH3CN)2](PF6)2•2CH3CN and [Co3(dpa)4(CH3CN)2](PF6)2•CH3CN•C4H10O compounds crystallize in space groups C2 and P21, respectively. Both the Ni(II) and Co(II) complex cations are stable towards racemization in CH3CN. Vibrational circular dichroism (VCD) data obtained in CD3CN demonstrate the expected mirror image spectra for the enantiomers, the observed peaks arising from the dpa ligand. The VCD response is significant, with  values up to 6 Lmol −1 cm −1 and vibrational dissymmetry factors on the order of 10 −3 . DFT 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.