Axial Coordination and Conformational Heterogeneity of Nickel(II) Tetraphenylporphyrin Complexes with Nitrogenous Bases

Abstract

Axial ligation of nickel(II) 5,10,15,20-tetraphenylporphyrin (NiTPP) with pyrrolidine or piperidine has been investigated using X-ray crystallography, UV-visible spectroscopy, resonance Raman spectroscopy, and molecular mechanics (MM) calculations. By varying the pyrrolidine concentration in dichloromethane, distinct nu(4) Raman lines are found for the four-, five-, and six-coordinate species of NiTPP. The equilibrium constants for addition of the first and second pyrrolidine axial ligands are 1.1 and 3.8 M(-)(1), respectively. The axial ligands and their orientations influence the type and magnitude of the calculated nonplanar distortion. The differences in the calculated energies of the conformers having different ligand rotational angles are small so they may coexist in solution. Because of the similarity in macrocyclic structural parameters of these conformers and the free rotation of the axial ligands, narrow and symmetric nu(2) and nu(8) Raman lines are observed. Nonetheless, the normal-coordinate structural-decomposition analysis of the nonplanar distortions of the calculated structures and the crystal structure of the bis(piperidine) complex reveals a relationship between the orientations of axial ligand(s) and the macrocyclic distortions. For the five-coordinate complex with the plane of the axial ligand bisecting the Ni-N(pyrrole) bonds, a primarily ruffled deformation results. With the ligand plane eclipsing the Ni-N(pyrrole) bonds, a mainly saddled deformation occurs. With the addition of the second axial ligand, the small doming of the five-coordinate complexes disappears, and ruffling or saddling deformations change depending on the relative orientation of the two axial ligands. The crystal structure of the NiTPP bis(piperidine) complex shows a macrocycle distortion composed of wav(x) and wav(y) symmetric deformations, but no ruffling, saddling, or doming. The difference in the calculated and observed distortions results partly from the phenyl group orientation imposed by crystal packing forces. MM calculations predict three stable conformers (ruf, sad, and planar) for four-coordinate NiTPP, and resonance Raman evidence for these conformers was given previously.

Coordination‐Induced Spin Crossover (CISCO) through Axial Bonding of Substituted Pyridines to Nickel–Porphyrins: σ‐Donor versus π‐Acceptor Effects

“…However, assuming that the paramagnetic complexes 1·2 d and 1·A C H T U N G T R E N N U N G (2 d) 2 have similar absorption maxima (l max ) and extinction coefficients (e), K 1S and K 2 can be estimated from the decrease of the Soret band of the uncoordinated porphyrin at 406 nm. [11] By using this approximation the association constants of 2 d with 1 were determined at 305 K in toluene as:…”

“…[11,[24][25][26] A set of two coaxial NMR tubes consisting of an ordinary outer NMR tube and an inner tube with a smaller diameter was used. …”

“…La Mar and Walker derived an equation which describes the ratio between paramagnetic and diamagnetic species [Eq. (5)]: [11] ½para ½dia …”

New insight into metalloporphyrin excited state structures and axial ligand binding from X-ray transient absorption spectroscopic studies

“…One metalloporphyrin studied by XTA was Ni(II) tetramesitylporphyrin (NiTMP), whose analogs were previously investigated by ultrafast TA [29][30][31][32][33][34][35][36], resonance Raman spectroscopy [37][38][39][40][41][42][43][44][45], previous XTA measurements [18,[46][47][48][49] and quantum mechanical calculations [49][50][51][52][53] due to their rich photochemical reaction pathways which can be used to understand similar processes in other metalloporphyrins (Fig. 1).…”

“…Consequently, the Ni(II) radius in the new electronic configuration in the T 1 state is effectively larger than that in the ground state. It has been well documented that various Ni(II) porphyrins have severe out-of-plane distortions in solid state systems [48,55] and in solution [40,43,56] due to a mismatch of a small Ni(II) radius and a large porphyrin macrocycle cavity. Thus, when the effective radius of Ni(II) in the excited state becomes larger, the macrocycle need not distort to accommodate a very short Ni N bond length.…”