A theoretical study of the substituent effects on some spectroscopic and bonding parameters of some metal β-diketonates and other macrocyclic complexes.

Theoretical and structural analysis through the PM3 and DFT procedures were used to investigate the geometries, energies, vibrational frequencies and NMR spectra of derivatives of metal b-diketonate complexes, [M(b-dike)3] and their mixed complexes. The b-diketones studied are,Hacac, Htfac, Hhfac, Htbd, Htftbd, Hfbd, Htffbd, Hbzac, Htfbzac, Hfpa, Htpd, Hdbm and Hdhfpd; and their metal complexes with M = Al, Cr, Mn, Fe, and Co. Among the complexes studied, the symmetry point group D3 was observed for acetylacetonate and hexafluoroacetylacetonate chelates and C1 was observed for all other complexes. The region 3500 – 300cm-1 were studied for the IR vibrations. The carbonyl band assigned to asymmetric C = O (υasC = O) stretching mode appears at the 1869-1764cm-1 region for all the complexes except for the chromium complex which band fell below 1700cm-1. The M – O frequencies for all the chelates are weak and fall below 600cm-1.Qualitatively, the calculated frequencies agree with the expterimental results. However, on the quantitative aspects, the calculated frequencies differ by about 160 – 30cm-1 from the expterimental results. The NMR spectral analysis using the B3LYP/6-31G* basis set reveals that an increase in fluorine substitution deshields the attached protons and carbons thereby causing them to be shifted downfield. These findings are consistent with experimental results. The UV spectra of the cobalt derivatives of the complexes examined in the gaseous phase exhibited two broad bands with different λmaxvalues which are higher in energy than are found in the corresponding Aluminium compounds. The general trend of the complexes in terms of stability with respect to the heat of formation irrespective of the central metal is M(hfac)3> M(tfac)3> M(Htffbd)3> M(Htfbzac)3 > M(Htftbd)3> M(acac)3> M(Hfbd)3> M(Hbzac)3> M(Hfpa)3> M(Htbd)3> M(Hftpd)3> M(Hdbm)3> M(Htpd)3> M(Htbzac)3> M(Dhfpd)3. The order gives an indication of the electron withdrawing ability of CF3 groups relative to the positive mesomeric effects of the aromatic substituents. It is also observed that increasing CF3 substitution opens up the O…C…C bond angle and closes up that of C…C…C, C…O…M and O…M…O. PM3 studies on β–diketonate, β-ketoiminate, β-diiminate, β-ketothiolate, β- dithiolate and β- imithiolate complexes of 4,4,4-trifluoro-1-(2-furyl)-1,3-butanedione (htffbd) reveals that the M-S bonds (2.306 Å average) are significantly longer than the M-O bonds (1.895 Å average) which are also longer than the M-N bonds (1.758 Å average).The C = C bonds on the keto (O) side of the ring is longer than the C = C bonds on the imine (N) side with the C = C bonds on the thio (S) side of the ring having the shortest bond distance (ie C = C(O) > C = C(N) > C = C(S) in bond length). Analysis of the HOMO and LUMO energy gap of these complexes gives the order; β–Diketonate< β–Ketothiolate< β–Ketoiminate< β– Diiminate< β–Thioiminate< β–Dithiolate. Considering the calculated bond lengths for the N6- tetradentatemacrocyclic complexes for cobalt, the two Co–Cl bonds were on the average 2.2245Å for trifluoro substituents, 2.2265Å for phenyl substituents and 2.2365Å for methyl substituents. The Co–N bond lengths on the average for the CF3, phenyl and CH3 substituents are respectively 2.040Å, 2.041Å and 1.989Å.

Source: KNUST