Anionic Amidinourea–Metal(II) Tautomers: MN2O2 or MN4?

Structures of amidinourea–metal complexes have been obscured by subtle tautomerism and crystal waters. To clarify the most stable form of the tautomers, bis(amidinourea)copper(II) anhydride and the alkyl derivatives were synthesized under basic conditions, and the stability of the CuN2O2 chromophore was suggested. From substituent effects of chelations, UV–vis spectra, IR/Raman spectra, and DFT calculations, it was suggested that the bis(amidinourea)copper(II) has a CuN2O2 chromophore, not CuN4 chromophore. The mutual exclusion rule of IR/Raman spectra supported the trans-CuN2O2 chromophore. Systematic calculations on bis(amidinourea)–Cu(II), –Ni(II), –Pd(II), –Zn(II), and –Cd(II) chelates revealed that the robust stability of CuN2O2 chromophores originates from quasi aromaticity in the six-membered chelate rings. cis- or trans-CuN4 stability reported in the early references is suspicious.