Correlation of infrared spectra of zinc(II) carboxylates with their structures

The correlation of the infrared spectra of zinc(II) carboxylates with their structures was investigated in the paper. The complexes with different modes of the carboxylate binding, from chelating, through bridging (syn–syn, syn–anti, monatomic), ionic to monodentate were used for the study, namely [Zn(C6H5CHCHCOO)2(H2O)2] (I) with chelating carboxylate group (C6H5CHCHCOO = cinnamate), [Zn2(C6H5COO)4(pap)2] (II) with syn–syn bridging carboxylate (C6H5COO = benzoate; pap = papaverine), [Zn(C6H5CHCHCOO)2(mpcm)]n (III) with syn–anti carboxylate bridge (mpcm = methyl-3-pyridylcarbamate), [Zn(C5H4NCOO)2(H2O)4] (IV) with ionic carboxylate group (C5H4NCOO = nicotinate), [Zn(C6H5COO)2(pcb)2]n (V) with monodentate carboxylate coordination (pcb = 3-pyridylcarbinol) and [Zn3(C6H5COO)6(nia)2] (VI) with syn–syn and monatomic carboxylate bridges (nia = nicotinamide). First, the mode of the carboxylate binding was assigned from the infrared spectra using the magnitude of the separation between the carboxylate stretches, Δexp = νas(COO−) − νs(COO−). Then the values Δexp were compared with those calculated from structural data of the carboxylate anion (Δcalc). The conclusions about the carboxylate binding which resulted from the Δ values, were confronted with the crystal structure of the complexes. The limitations and recommendations were formulated to assign the mode of the carboxylate binding from the infrared spectra. The dependence of the Δexp values on the magnitudes of Zn–O–C angles in bidentate carboxylate coordination was observed.