Reactions of Nickel(II) 2-Aza-5,10,15,20-tetraphenyl-21- carbaporphyrin with Methyl Iodide. The First Structural Characterization of a Paramagnetic Organometallic Nickel(II) Complex^†

The methylation of a nickel(II) complex (CTPP)Ni^II of 2-aza-5,10,15,20-tetraphenyl-21-carbaporphyrin (CTPPH₂) with the mild methylating agent methyl iodide yields novel stable organonickel(II) complexes:  diamagnetic (21-CH₃TPP)Ni^II and two scarce paramagnetic species (2-NH-21-CH₃CTPP)Ni^IIX and (2-NCH₃-21-CH₃CTPP)Ni^IIX (X = Cl, I). The demetalation procedure resulted in isolation of two 21-carbaporphyrin derivatives, methylated at the internal C(21) carbon atom, i.e., 2-aza-5,10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (21-CH₃-CTPPH₂) and 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (2-NCH₃-21-CH₃CTPPH). The methylation mechanism involves the oxidative addition of the methyl cation to the carbaporphyrin C(21) activated due to Ni−C coordination. These C-methylated macrocycles preserve their coordinating properties as the remetalation processes have been carried out. The reversible concerted addition of HX converts the diamagnetic (21-CH₃TPP)Ni^II into the paramagnetic (2-NH-21-CH₃CTPP)Ni^IIX. The axial coordination step is accompanied by a protonation of the peripheral nitrogen. The structures of (21-CH₃CTPP)NI^II (monoclinic, P2₁/c; a = 15.055(3) Å, b = 15.795(3) Å, c = 17.069(3) Å, β = 115.00(3)°, Z = 4, least-square refinement of 476 parameters using 3622 reflections, R = 0.069) and (2-NCH₃-21-CH₃CTPP)Ni^III (monoclinic, P2₁/n; a = 12.946(3) Å, b = 23.519(5) Å, c = 13.945 Å; β = 93.2(3)°, Z = 4, the least-square refinement of 502 parameters using 3257 reflections, R = 0.0559) have been determined by X-ray diffraction. In the first case the nickel(II) is four-coordinate with bonds to three pyrrole nitrogen atoms (Ni−N distances 1.948(5); 1.955(5); 1.938(5) Å) and the pyrrole carbon (Ni−C 2.005(6) Å). The nickel lies in the plane of the three nitrogens, while the methylated pyrrole is sharply tilted out of the plane with dihedral angle between the plane of three nitrogens and the methylated pyrrole plane being −42.2°. The methylated pyrrole is bound to nickel via a pyramidal carbon in the η¹-fashion. The first structurally characterized paramagnetic organonickel(II) complex (2-NCH₃-21-CH₃CTPP)Ni^III presents unique features related to its electronic structure. The nickel(II) is five-coordinate with bonds to three pyrrole nitrogen atoms (Ni−N distances 2.032(8); 2.057(8); 1.979(8) Å) and to the pyrrole C(21) carbon (Ni−C 2.406(9) Å). The nickel is moved out from the plane of the three nitrogen plane toward the iodide ligand. The 21-CH₃ group and iodide are located on the opposite sides of the macrocycle. The substituted pyrrole is sharply tilted out of the plane with the dihedral angle between the three nitrogen plane and the methylated pyrrole plane being −55.3°. The methylated pyrrole is bound to nickel in the η¹-fashion but the coordinating C(21) atom preserves features related to trigonal sp² hybridization. The angle between the inverted pyrrole plane and the Ni−C bond increases from 42.8° to 70.1° on moving from the diamagnetic to paramagnetic species. The ¹H NMR and ²H NMR spectra of paramagnetic (2-NH-21-CH₃CTPP)Ni^IIX and (2-NCH₃-21-CH₃CTPP)Ni^IIX complexes have been examined. Functional group assignments have been made with use of selective deuteration and 2D COSY experiments. The characteristic patterns of six downfield shifted pyrrole resonances accompanied by upfield shifted 3-H and 2-NH resonances are diagnostic of C-methylation. The 21-CH₃ resonance appears in a characteristic 90−110 ppm region.