Theoretical studies on the structure and protonation of Cu(II) complexes of a series of tripodal aliphatic tetraamines: Good correlations with the experimental data

DFT(B3LYP) studies on first protonation step of a series of Cu(II) complexes of some tripodal tetraamines with general formula N[(CH(2))(n)NH(2)][(CH(2))(m)NH(2)][(CH(2))(p)NH(2)] (n = m = p = 2, tren; n = 3, m = p = 2, pee; n = m = 3, p = 2, ppe; n = m = 3, tpt; n = 2, m = 3, p = 4, epb; and n = m = 3, p = 4; ppb) are reported. First, the gas-phase proton macroaffinity of all latter complexes was calculated with considering following simple reaction: [Cu(L)](2+)(g) + H(+)(g) --> [Cu(HL)](3+)(g). The results showed that there is a good correlation between the calculated proton macroaffinities of all complexes with their stability constants in solution. Then, we tried to determine the possible reliable structures for microspecies involved in protonation process of above complexes. The results showed that, similar to the solid state, the [Cu(L)(H(2)O)](2+) and [Cu(HL)(H(2)O)(2)](3+) are most stable species for latter complexes and their protonated form, respectively, at gas phase. We found that there are acceptable correlations between the formation constants of above complexes with both the -Delta E and -Delta G of following reaction: [Cu(L)(H(2)O)](2+)(g) + H(+)(g) + H(2)O(g) --> [Cu(HL)(H(2)O)(2)](3+)(g). The -Delta E of the latter reaction can be defined as a theoretically solvent-proton macroaffinity of reactant complexes because they have gained one proton and one molecule of the solvent. The unknown formation constant of [Cu(epb)](2+) complex was also predicted from the observed correlations. In addition, the first proton affinity of all complexes was studied in solution using DPCM and CPCM methods. It was shown that there is an acceptable correlation between the solvent-proton affinities of [Cu(L)(H(2)O)](2+) complexes with formation constants of [Cu(HL)(H(2)O)(2)](3+) complexes in solution.