Synthesis, Characterization, and Spectroelectrochemistry of Cobalt Porphyrins Containing Axially Bound Nitric Oxide

Abstract

Several cobalt nitrosyl porphyrins of the form (T(p/m-X)PP)Co(NO) (p/m-X = p-OCH(3) (1), p-CH(3) (2), m-CH(3) (3), p-H (4), m-OCH(3) (5), p-OCF(3) (6), p-CF(3) (7), p-CN (8)) have been synthesized in 30-85% yields by reaction of the precursor cobalt porphyrin with nitric oxide. Compounds 1-7 were also prepared by reaction of the precursor cobalt porphyrin with nitrosonium tetrafluoroborate followed by reduction with cobaltocene. Compounds 1-8 have been characterized by elemental analysis, IR and (1)H NMR spectroscopy, mass spectrometry, and UV-vis spectrophotometry. They are diamagnetic and display nu(NO) bands in CH(2)Cl(2) between 1681 and 1695 cm(-)(1). The molecular structure of 1, determined by a single-crystal X-ray crystallographic analysis, reveals a Co-N-O angle of 119.6(4) degrees. Crystals of 1 are monoclinic, P2/c, with a = 15.052(1) Å, b = 9.390(1) Å, c = 16.274(2) Å, beta = 111.04(1) degrees, V = 2146.8(4) Å(3), Z = 2, T = 228(2) K, D(calcd) = 1.271 g cm(-)(3), and final R1 = 0.0599 (wR2 = 0.1567, GOF = 1.054) for 3330 "observed" reflections with I >/= 2sigma(I). Cyclic voltammetry studies in CH(2)Cl(2) reveal that compounds 1-7 undergo two reversible oxidations and two reversible reductions at low temperature. This is not the case for compound 8, which undergoes two reversible reductions but an irreversible oxidation due to adsorption of the oxidized product onto the electrode surface. Combined electrochemistry-infrared studies demonstrate that each of the compounds 1-7 undergoes a first oxidation at the porphyrin pi ring system and a first reduction at either the metal center or the nitrosyl axial ligand. The formulation for the singly oxidized products of compounds 1-7 as porphyrin pi-cation radicals was confirmed by the presence of bands in the 1289-1294 cm(-)(1) region (for compounds 1-5), which are diagnostic IR bands for generation of tetraarylporphyrin pi-cation radicals.

Nitroxylcob(III)alamin: Synthesis and X‐ray Structural Characterization

“…Evidence for the presence of a NO À group as opposed to the oxidized form NO 2 À comes from comparison of the N À O bond lengths of NOCbl with those of other structures. The NÀO bond lengths for the three orientations of NOCbl range from 1.14 to 1.20 and are entirely consistent with a NÀO double bond, as reported for the nitroxyl complexes of tetraphenylporphyrin (tpp), [21,22] octylethylporphyrin (oep), [23] and two tropocoronand ligands (tc-3,3 and tc-4,4; Table 1). [24] Nitro ligands, on the other hand, typically show two distinct NÀO bond lengths (NÀO1 and NÀO2, Table 2).…”

Dynamics of NO Motion in Solid-State [Co(tetraphenylporphinato)(NO)]

“…Perhaps surprisingly, the quality and resolution of the tetragonal structures does not improve with decreasing temperature. The apparent Co–N–O angle in the tetragonal structures, 133(2)°, is also larger than has been seen for more ordered nitrosyl structures, 123(2)° 10, 32, 33. Other results of the I 4 /m symmetry are that the Co–N NO bond is normal to the porphyrin plane and the dihedral angle between the unique phenyl group and the porphyrin core is 90°.…”

“…The oxygen electron density is shared between the eight positions, which leads to shorter than expected N–O bond lengths in the tetragonal system. Table 2 shows that all of the structures with eightfold disorder of the nitrosyl have average nitrosyl bond lengths of ~1.00 Å, which is shorter than the typical 1.167(1) Å N–O bond lengths in comparable porphyrin systems that have no disorder 10, 32, 33. Perhaps surprisingly, the quality and resolution of the tetragonal structures does not improve with decreasing temperature.…”

“…The out-of-plane location of the Co was assumed to be along the ${\text{N}}_{\text{NO}}-{\text{N}}_{\text{NO}}^{\prime }$ vector because of the slight off-axis tilt of the Co–N NO bond in the triclinic systems. The Co–N NO distance was set equal to the average of the literature values of the Co–N NO bond lengths, 1.842 Å 10, 18, 32, 33. Based on this assumption, idealized fractional coordinates for the Co position were obtained using a locally written program for geometry calculations.…”

“…In iron(II) and cobalt(II) metalloporphyrins, however, NO is coordinated in a bent fashion. Such complexes are sometimes ordered,9, 10, 11, 12 sometimes have twofold disorder,13, 14, 15, 16, 17 and sometimes have eightfold disorder 18, 19, 20. It is the last of these possibilities that is examined in this study.…”