μ-Carbido Diporphyrinates and Diphthalocyaninates of Iron and Ruthenium

Abstract

μ-Carbido diporphyrinates and diphthalocyaninates of general formula [{ Mp 2−}2(μ- C )] ( p 2− = tpp ( M = Fe ), oep ( Fe ), pc ( Fe , Ru ); H 2 tpp : 21H,23H-5,10,15,20-tetraphenylporphine; H 2 oep : 21H,23H-2,3,4,8,12,13,17,18-octaethylporphine; H 2 pc : 29H,31H-phthalocyanine) of formally Fe IV and Ru IV are prepared by a new and improved ‘one-pot’ synthesis. The corresponding chloro complexes of the tervalent metal ions react successively with potassium hydroxide in boiling 2-propanol and then with trichloromethane. Potassium hydroxide is proven to be a very versatile and powerful reductant in tetrapyrrolic chemistry. As evidenced from electron spin resonance and UV vis spectral measurements, the precursor is reduced primarily to an ate-complex of type [ M I p 2−]− of a formally monovalent metal ion. This active species is assumed to react with trichloromethane via a dichlorocarbene complex of type [ M II( CCl 2) p 2−] to yield the actual -carbido complex. [{ Feoep 2−}2(μ- C )] is crystallographically characterized. It is monoclinic, space group C12/c1 (15), with a = 18.279(3) Å, b = 15.005(2) Å, c = 23.392(7) Å, β = 107.12(2)°, Z = 4, R1 = 0.0773. The iron atom is displaced by 0.192(3) Å out of the centre ( Ct ) of the ( N p )4 plane toward the (μ- C ) atom. Average d ( Fe - N p ) is 1.986(5) Å; d ( Fe -(μ- C )) is 1.6638(9) Å. The Fe - C - Fe skeleton is linear (179.5(3)°). The two slightly waving porphyrinato cores are in a staggered conformation, the ( N p - Fe - Fe ″- N p ″) torsion angle being 21.0(3)°. Solutions of each μ-carbido complex in pyridine/dichloromethane show four distinct quasi-reversible redox processes in their differential-pulse voltammograms and these are assigned to the successive one-electron reduction and oxidation of the macrocyclic ligands. 13 C CP MAS NMR spectra indicate effective four-fold symmetry within the series of the μ-carbido complexes with isotropic shifts occurring at similar fields to those of the corresponding macrocyclic complex of a closed-shell metal ion. Resonances of the bridging carbon atom are not detected. A characteristic increase of line broadening within the series tpp 2− > oep 2− > pc 2− may be due to Fermi contact interactions with the strongly coupled low-spin M IV centres. The magnetic susceptibility studies show that the complexes all display non-zero μ values at 295 K increasing from pc 2− to tpp 2−. Mössbauer spectra confirm the low-spin Fe IV oxidation state for the iron centres. Isomer shift, δ, and quadrupole splitting, ΔE Q , for [{ Fepc 2−}2(μ- C )] and [{ Fetpp 2−}2(μ- C )] are identical to those previously reported. Data for [{ Feoep 2−}2(μ- C )] are essentially the same as for the pc and tpp complex. Thus the order of δ is tpp ≈ oep > pc whilst that of Δ E Q is pc >> oep > tpp . Small impurity lines are observed which help explain the magnetic data. UV vis/NIR spectra of the μ-carbido complexes show the characteristic π-π* transitions. These are shifted with respect to the corresponding mononuclear complexes to higher energy because of excitonic interactions. Vibrational spectra are discussed in detail ν as ( M - C - M ) (in cm−1) is at 937 ( M = Fe ; tpp ) < 976 ( Fe / oep ) < 997 ( Fe / pc ) < 1050 ( Ru / pc ), ν s ( M - C - M ) (in cm−1 at 433 ( Fe / tpp ) < 460 ( Fe / oep ) < 477 ( Fe / pc ). Hence, valence force constants increase significantly in the order tpp < oep < pc . ν s ( Fe - C - Fe ) of [{ Fepc 2−}2(μ- C )] is selectively resonance Raman enhanced. As evidenced from the excitation profile a C → Fe charge transfer, not detected in the vis spectrum, is assumed to be present at 22 000 > ν > 25 000 cm −1.