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Resolution, structures, and vibrational circular dichroism of helicoidal trinickel and tricobalt paddlewheel complexes
Author(s) -
Cortijo Miguel,
ValentínPérez Ángela,
Rosa Patrick,
Daugey Nicolas,
Buffeteau Thierry,
Hillard Elizabeth A.
Publication year - 2020
Publication title -
chirality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.43
H-Index - 77
eISSN - 1520-636X
pISSN - 0899-0042
DOI - 10.1002/chir.23211
Subject(s) - chemistry , enantiopure drug , enantiomer , crystallography , vibrational circular dichroism , circular dichroism , chirality (physics) , stereochemistry , racemization , enantioselective synthesis , organic chemistry , nambu–jona lasinio model , chiral symmetry breaking , physics , quantum mechanics , quark , catalysis
It has been recently shown that enantiomers of the helicoidal paddlewheel complex [Co 3 (dpa) 4 (CH 3 CN) 2 ] 2+ (dpa = the anion of 2,2′‐dipyridylamine) can be resolved using the chiral [As 2 (tartrate) 2 ] 2− 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 3 (dpa) 4 (CH 3 CN) 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 CH 3 CN and diethyl ether in space group P 42 1 2 as the (NBu 4 ) 2 [Ni 3 (dpa) 4 (CH 3 CN) 2 ](AsT) 2 ·[solvent] salt. After resolution, the AsT salts were converted into the PF 6 − salts by anion exchange, with retention of the chirality of the trinickel complex. The enantiopure [Ni 3 (dpa) 4 (CH 3 CN) 2 ](PF 6 ) 2 ·2CH 3 CN and [Co 3 (dpa) 4 (CH 3 CN) 2 ](PF 6 ) 2 ·CH 3 CN·C 4 H 10 O compounds crystallize in space groups C 2 and P 2 1 , respectively. Both the Ni(II) and Co(II) complex cations are stable towards racemization in CH 3 CN. Vibrational circular dichroism (VCD) data obtained in CD 3 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 −1 cm −1 and vibrational dissymmetry factors on the order of 10 −3 . 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.