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Nitrosyl-a,/3,7,5-tetraphenylporphinatocobalt(II), ONCoTPP, crystallizes preferentially from solutions in which the six-coordinate ONCoNbTPP [Nb = piperidine) is presumably the most plentiful complexed species. With a square-pyramidal coordination group, but a strongly kinked Co-N-O linkage, the ONCoTPP molecule observes statistically required Co, symmetry in an eightfold-disordered variant of a well-known structural type based on the tetragonal space group, 14/m. Cell data are: a = 13.434, c = 9.754 A; Z = 2; /joaiod = 1.32, readily indicated if we first specify the cobalt porphyrins for which we have obtained definitive stereochemical data. X-Ray analyses of crystalline structure for five cobalt porphyrins derived from the free base, ,ß, , -

The six-coordinate nitrosyl sigma-bonded aryl(iron) and -(ruthenium) porphyrin complexes (OEP)Fe(NO)(p-C(6)H(4)F) and (OEP)Ru(NO)(p-C(6)H(4)F) (OEP = octaethylporphyrinato dianion) have been synthesized and characterized. Single-crystal X-ray structure determinations reveal an unprecedented bending and tilting of the MNO group for both [MNO](6) species as well as significant lengthening of trans axial bond distances. In (OEP)Fe(NO)(p-C(6)H(4)F) the Fe-N-O angle is 157.4(2) degrees, the nitrosyl nitrogen atom is tilted off of the normal to the heme plane by 9.2 degrees, Fe-N(NO) = 1.728(2) A, and Fe-C(aryl) = 2.040(3) A. In (OEP)Ru(NO)(p-C(6)H(4)F) the Ru-N-O angle is 154.9(3) degrees, the nitrosyl nitrogen atom is tilted off of the heme normal by 10.8 degrees, Ru-N(NO) = 1.807(3) A, and Ru-C(aryl) = 2.111(3) A. We show that these structural features are intrinsic to the molecules and are imposed by the strongly sigma-donating aryl ligand trans to the nitrosyl. Density functional-based calculations reproduce the structural distortions observed in the parent (OEP)Fe(NO)(p-C(6)H(4)F) and, combined with the results of extended Hückel calculations, show that the observed bending and tilting of the FeNO group indeed represent a low-energy conformation. We have identified specific orbital interactions that favor the unexpected bending and tilting of the FeNO group. The aryl ligand also affects the Fe-NO pi-bonding as measured by infrared and (57)Fe Mössbauer spectroscopies. The solid-state nitrosyl stretching frequencies for the iron complex (1791 cm(-)(1)) and the ruthenium complex (1773 cm(-)(1)) are significantly reduced compared to their respective [MNO](6) counterparts. The Mössbauer data for (OEP)Fe(NO)(p-C(6)H(4)F) yield the quadrupole splitting parameter +0.57 mm/s and the isomer shift 0.14 mm/s at 4.2 K. The results of our study show, for the first time, that bent Fe-N-O linkages are possible in formally ferric nitrosyl porphyrins.

An intermediate-spin state very close to the mid-spin state (S = 3/2) can be stabilized in a ferric porphyrin by an integrated approach which combines the favorable effects of a weak axial field strength and of a small macrocycle hole. Axial ligand exchange by reaction of chloroiron(III)tetramethylchiroporphyrin [(TMCP)FeCl] with silver perchlorate in ethanol-chloroform leads to ethanol-ligated ferric chiroporphyrins. Two distinct crystalline products containing a bisethanol complex [[(TMCP)FeIII(EtOH)2]ClO4] and three variants of a mixed ethanol-water complex [[(TMCP)FeIII(EtOH)(H2O)]ClO4] have been structurally characterized in the solid state. The small hole of the ruffled chiroporphyrin and the weak axial oxygen ligation result in strongly tetragonally distorted complexes. The six-coordinate species exhibit long axial Fe-O bond distances (2.173(5)-2.272(4) A) and the shortest equatorial Fe-N(av) distances (1.950(5)-1.978(7) A) found as yet in a ferric porphyrin, reflecting a singly occupied dz2 orbital and a largely depopulated dx2-y2 orbital. An intriguing case of bond-stretch isomerism is seen for the axial Fe-O bonds in two crystallographically independent mixed ethanol-water species, and it is accounted for by their distinct intra- and intermolecular hydrogen-bond arrays. The Mössbauer spectrum (delta = 0.35(1) mm s-1 and delta EQ = 3.79(1) mm s-1 at 77 K) indicates a strong tetragonal distortion around the ferric ion, in agreement with the structural data. The value of the magnetic moment (mu eff = 3.8 mu B in the range 50-300 K) strongly supports a mid-spin state (S = 3/2). The EPR spectrum at 80 K (g perpendicular approximately 4.0, g parallel approximately 2.00) is consistent with a nearly pure mid-spin state (4A2) with little rhombic distortion. The 1H NMR spectra in CDCl3-EtOH exhibit upfield-shifted resonances for the pyrrole protons (delta approximately -30 ppm) which are consistent with the depopulated iron dx2-y2 orbital. Solution equilibria with water and various alcohols, and the spin state of the corresponding species, are discussed on the basis of the NMR data. The bisethanol and ethanol-water species are potential models of unknown hemoprotein ligation states such as Tyr(OH)/Tyr(OH) or Tyr(OH)/H2O that could be obtained by site-directed mutagenesis.

The synthesis and characterization of four low-spin (carbonyl)iron(II) tetraphenylporphyrinates, [Fe (TPP)(CO)(L)], where L = 1-methylimidazole, 2-methylimidazole, 1,2-dimethylimidazole (unsolvated) and 1,2-dimethylimidazole (toluene solvate) are reported. The complexes show nearly the same value of ν C-O in toluene solution (1969-72 cm −1 ) but a large range of CO stretching frequencies in the solid-state cm −1 ). The large solid-state variation results from CO interactions in the solid-state as shown by an examination of the crystal structures of the four complexes. The high precision of the four structures obtained allows us to make a number of structural and spectroscopic correlations that describe the Fe-C-O and N Im -Fe-CO units. The values of ν C-O and the Fe-C and C-O bond distances are strongly correlated and provide a structural as well as a spectroscopic correlation of the π back-bonding model. The interactions of CO described are closely related to the large range of CO stretching frequencies observed in heme proteins and specific interactions observed in carbonylmyoglobin (MbCO).

The reaction of BF(3).OEt(2) with the bis(nitro) complex of iron(III) picket-fence porphyrin, [K(18C6)(OH(2))][Fe(TpivPP)(NO(2))(2)], leads to the formation of a transient porphyrin intermediate, assigned on the basis of its rhombic low-spin EPR spectrum as the five-coordinate N-bound mono(nitro) iron(III) derivative, [Fe(TpivPP)(NO(2))]. This species is reactive and readily undergoes oxygen atom transfer to form [Fe(III)(TpivPP)(NO(3))] and [Fe(II)(TpivPP)(NO)]. The reactions have been followed by EPR and IR spectroscopy. [Fe(TpivPP)(NO(2))] has a rhombic EPR spectrum (g = 2.60, 2.35, and 1.75) in chlorobenzene and CH(2)Cl(2) and is spectroscopically distinct from the bis(nitro) starting material (g = 2.70, 2.50, and 1.57). Oxidation of the nitrosyl species to [Fe(TpivPP)(NO(3))] proceeds via an intermediate assigned as [Fe(TpivPP)(NO(2))] on the basis of its EPR spectrum. The crystal structure of one of the reaction products, [Fe(TpivPP)(NO(3))], has been determined. The nitrate ion of [Fe(TpivPP)(NO(3))] is bound to the iron(III) ion in a "symmetric" bidentate fashion within the ligand-binding pocket of the porphyrin pickets. Individual Fe-O distances are 2.123(3) and 2.226(3) Å. The dihedral angle between the plane of the nitrate ion and the closest N(p)-Fe-N(p) plane is 10.0 degrees. The Fe-N(p) bonds (and trans N(p)-Fe-N(p) angles) perpendicular and parallel to the plane of the axial ligand average to 2.060(5) Å (154.84(9) degrees ) and 2.083(3) Å (146.14(9) degrees ), respectively. Crystal data for [Fe(TpivPP)(NO(3))]: a = 23.530(2) Å, b = 10.0822(5) Å, c = 48.748(3) Å, beta = 92.145(5) degrees, monoclinic, space group I2/a, V = 11556.4(14) Å(3), Z = 8, FeN(9)O(7)C(64)H(64), 8798 observed data, R(1) = 0.0606, wR(2) = 0.1313, all observations at 127(2) K.

Single-crystal EPR measurements have been performed on the triclinic form of [Fe(OEP)(NO)] (Ellison, M. K.; Scheidt, W. R. J. Am. Chem. Soc. 1997, 119, 7404) and on the isomorphous cobalt derivative [Co(OEP)(NO)] (Ellison, M. K.; Scheidt, W. R. Inorg. Chem. 1998, 37, 382) which has been doped with [Fe(OEP)(NO)]. Principal values of the g tensor determined at room temperature are gmax = 2.106, gmid = 2.057, and gmin = 2.015. The principal direction associated with the minimum g value lies 8 degrees from the Fe-N(NO) direction, 2 degrees from the normal to the heme plane, and 42 degrees from the N-O direction. The direction associated with the maximum g value lies 9 degrees from the normal to the Fe-N-O plane. The fact that the direction of gmin is near the Fe-N(NO) direction is consistent with the dominant role of spin-orbit coupling at the iron atom in determining the g tensor and with the picture of the electronic structure of the compound from restricted calculations, which makes the half-filled orbital mostly dz2 on the iron atom. The hyperfine tensor is nearly isotropic and was only resolved in the doped samples. Principal values of the A tensor determined at room temperature are 40.9, 49.7, and 42.7 MHz. Principal values of the g tensor determined from the doped samples at 77 K are gmax = 2.110, gmid = 2.040, and gmin = 2.012. Principal values of the A tensor are 42.5, 52.8, and 44.1 MHz at 77 K. The small change in g values with temperature is in contrast to the large temperature dependence on g values observed in samples of MbNO (Hori et al. J. Biol. Chem. 1981, 256, 7849).

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