The Chemistry of N-Alkylporphyrin Complexes: A Comparison of Reactivities and Structures of Chlorozinc(II) Complexes of N-Benzyl-, N-Methyl-, and N-Phenyltetraphenylporphyrin

Article abstract of DOI:10.1021/ja00247a016

Zinc and copper complexes of N-substituted porphyrins undergo nucleophilic displacement of the N-substituent at rates that are strongly dependent on the nature of the N-substituent.Substituents capable of stabilizing incipient carbocations allow faster reaction.For the reactions of chloro(N-benzyltetraphenylporphynato)- and chloro(N-methyltetraphenylporphynato)zinc(II) complexes with di-n-butylamine, ΔH^<*>=71 kJ mol^-1 K^-1, ΔS^<*>=-92 J mol^-1 K^-1, and ΔG^<*>=100 kJ mol^-1 (318 K) and ΔH^<*>=176 kJ mol^-1, ΔS^<*>=230 J mol^-1 K^-1, and ΔG^<*>=103 kJ mol^-1 (318 K), respectively.The reaction of the chloro(N-phenyltetraphenylporphinato)zinc(II) complex is very slow.The structures of the N-benzyl and N-phenyl zinc(II) complexes have been determined by single-crystal X-ray diffraction (: triclinic, a=10.579(2) Angstroem, b=12.310(2) Angstroem, c=17.957(3) Angstroem, α=85.31(1) deg, β=80.89(1) deg, γ=72.63(1) deg, -130 deg C, P<*>, Z=2. : orthorhombic, a=15.078(5) Angstroem, b=15.272(5) Angstroem, c=17.268(5) Angstroem, 20 deg C, P2₁2₁2₁, Z=4).In each case, the coordination geometry is highly distorted square pyramid, with chloride ion occupying the apical position and a long bond to the substituted nitrogen atom (, Zn-N1=2.477(3) Angstroem; , Zn-N1=2.492(6) Angstroem).The N-benzyl Zn(II) complex resembles the product of the displacement reaction more closely than does the N-methyl or the N-phenyl complex (Zn(II) displacement above the N2, N3, N4 reference plane of 0.67, 0.65, and 0.59 Angstroem, cant of the N-substituted pyrrole ring from the reference plane of 42 deg, 39 deg, and 32 deg, and Zn-Cl bond distances of 2.227(2), 2.232(3), and 2.265(1) Angstroem for N-Ph, N-Me, and N-Bz, respectively).These structural changes are correlated with the relative rates of the nucleophilic displacement reactions for these complexes.