On the Existence of Matere Bonds in Pentacoordinated Manganese Complexes: A Combined Experimental and Theoretical Investigation

Four manganese(III) complexes, [MnL1Cl] (1), [MnL2Cl] (2), [Mn2(L2)2(N3)2] (3), and [Mn2(L1)2(NCS)2] (4){H2L1= N,N′-bis(5-chlorosalicylidene)-1,3-diaminopropane, H2L2= N,N′-bis(5-chlorosalicylidene)-2,2-dimethyl-1,3-diaminopropane} have been synthesized and characterized. The structures of the complexes were confirmed by single-crystal X-ray diffraction analysis. All four complexes showed a tendency to form supramolecular dimers in the solid state. Density functional theory (DFT) calculations have been used to investigate the supramolecular assemblies observed in the solid state of these complexes and to characterize the Mn···O interactions in terms of covalent/noncovalent nature and their importance for the formation of self-assembled dimers in complexes 1–4. QTAIM and NCIPlot calculations have been used to disclose the noncovalent nature of the Mn···O contacts in complexes 1–4, which can be considered as matere bonds (MaBs) instead of coordination bonds. Such a type of matere bond is unprecedented in square pyramidal Mn complexes. Energetically, the MaBs exhibit values ranging from −8 to −11 kcal/mol across these complexes. Therefore, this research highlights the attractive interactions between group 7 Mn(III) derivatives and Lewis bases involving a π-hole formed opposite one of the apical coordination bonds.