Geometrical and Electronic Structure of Fluorinated and Non-Fluorinated Platinum(II) Tetraphenylporphyrin Complexes

Abstract

The composition of the saturated vapors of two platinum complexes with the macrocyclic ligands 5,10,15,20-tetraphenylporphyrin (PtTPP) and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (PtTF₅PP) and their structures were determined by synchronous gas-phase electron diffraction/mass spectrometry (GED/MS). These porphyrin complexes are those with the heaviest metal atom in the coordination cavity that have been structurally investigated in the gas phase. The mass spectra confirm the presence of a single molecular form of each, PtTPP (T=629 K) and PtTF₅PP (T=597 K). Their structures can serve as references for related complexes in the crystalline state or solutions. Differences between the geometries of PtTPP and PtTF₅PP in the crystalline and gaseous states include a significant deformation of the tetrapyrrole macrocycle in solid PtTPP. The experimental Pt-N bond lengths of both complexes are in agreement with quantum chemical calculations (DFT/B97D/ECP(Pt)) taking into account relativistic effects. The effect of lanthanide contraction is evident from the similarity of the Pd-N and Pt-N internuclear distances of analogous compounds. The strong electron density transfer from the porphyrin backbone to the metal ion and the resulting low effective positive charge on the platinum atom, studied by NBO and QTAIM methods, helps to rationalize the high catalytic activity of such platinum compounds.

Keywords: Gas-phase electron diffraction; Macrocyclic ligand; Metal net charge; Molecular structure; Platinum.

Figure 1

Model of the PtTPP and PtTF₅PP molecules (Y=H, F) with atom labeling. The position of the phenyl fragments relative to the macrocycle is described by the torsion angle α(C₂−C₁−C_m−C_a); imaginary atom X₁ (above the plane of the macrocycle by 1 Å), X₂ – dummy atoms that were used to maintain the molecular symmetry while varying the parameters in the least squares procedure of GED data analysis.

Figure 2

Considered conformations of PtTPP and PtTF₅PP regarding the orientation of the phenyl groups and macrocycle distortion types.

Figure 3

Experimental (circles) and theoretical (solid line) radial distribution functions f(r) for PtTPP and PtTF₅PP and residual curves Δf(r).

Figure 4

The ratio of disagreement factors R _f as a function of torsion angle θ(C2−C1−C_m−C_a), responsible for the rotation of C₆H₅‐ and C₆F₅‐groups in PtTPP, and PtTF₅PP. R _threshold – uncertainty according to Hamilton's statistical criterion at 0.05 significance level.

Figure 5

Frontier orbitals for PtP, PtTPP, PtTF₅PP.