Variable pi-bonding in iron(II) porphyrinates with nitrite, CO, and tert-butyl isocyanide: characterization of [Fe(TpivPP)(NO2)(CO)]-

The addition of the strongly pi-bonding ligands CO or tert-butyl isocyanide to the low-spin five-coordinate iron(II) nitrite species [Fe(TpivPP)(NO2)]- (TpivPP = picket fence porphyrin) gives two new six-coordinate species [Fe(TpivPP)(NO2)(CO)]- and [Fe(TpivPP)(NO2)(t-BuNC)]-. These species have been characterized by single-crystal structure determinations and by UV-vis, IR, and M?ssbauer spectroscopies. All evidence shows that in the mixed-ligand iron(II) porphyrin species, [Fe(TpivPP)(NO2)(CO)]-, the two trans, pi-accepting ligands CO and nitrite compete for pi density. The CO ligand however dominates the bonding. The Fe-N(NO2) bond lengths for the two independent anions in the unit cell at 2.006(4) and 2.009(4) A are lengthened compared to other nitrite species with either no trans ligands or non-pi-accepting trans ligands to nitrite. The Fe-C(CO) bond lengths are 1.782(4) A and 1.789(5) A for the two anions. The two Fe-C-O angles at 175.5(4) and 177.5(4) degrees are essentially linear in both anions. The quadrupole splitting for [Fe(TpivPP)(NO2)(CO)]- was determined to be 0.32 mm/s, and the isomer shift was 0.18 mm/s at room temperature in zero applied field. Both of the M?ssbauer parameters are much smaller than those found for six-coordinate low-spin iron(II) porphyrinates with neutral nitrogen-donating ligands as well as iron(II) nitro complexes. However, the M?ssbauer parameters are typical of other six-coordinate CO porphyrinates signifying that CO is the more dominant ligand. The CO stretching frequency of 1974 cm(-1) is shifted only slightly to higher energy compared to six-coordinate CO complexes with neutral nitrogen-donor ligands trans to CO. Crystal data for [K(222)][Fe(TpivPP)(NO2)(CO)].1/2C6H5Cl: monoclinic, space group P2(1)/c, Z = 8, a = 33.548(6) A, b = 18.8172(15) A, c = 27.187(2) A, beta = 95.240(7) degrees, V = 17091(4) A3.     

Structure of the deoxymyoglobin model [Fe(TPP)(2-MeHIm)] reveals unusual porphyrin core distortions

The preparation and characterization of the deoxymyoglobin model (2-methylimidazole)(tetraphenylporphinato)iron(II) is described. The preparation and crystallization from chlorobenzene leads to a new crystalline phase that has been structurally characterized. The complex is the most ordered example of a deoxymyoglobin model yet characterized. The X-ray structure determination reveals a number of distortions both in the iron coordination group and in the porphyrin core that result from the steric bulk of the axial ligand. Some of these distortions have been noted previously in related species; however, the demonstration of porphyrin core distortions and an asymmetry in the Fe-N(p) bond distances are new observations. These may have functional significance for this important type of heme protein coordination group. The new structure emphasizes that high-spin iron(II) porphyrinate derivatives display substantial structural pliability with significant variations in iron atom displacements, porphyrin core hole size, and axial and equatorial Fe-N bond lengths. The new complex has also been characterized by zero-field and applied field magnetic M?ssbauer spectroscopy. M?ssbauer parameters are characteristic for high-spin iron, although they also reveal an extremely rhombic site for iron(II). Crystal data at 130 K for [Fe(TPP)(2-MeHIm)].1.5C(6)H(5)Cl: a = 12.334(3) A, b = 13.515(6) A, c = 14.241(7) A, alpha = 70.62(3) degrees, beta = 88.29(2) degrees, gamma = 88.24(3) degrees, triclinic, space group, P, V = 2238(2) A(3), Z = 2.     

Ligand orientation control in low-spin six-coordinate (porphinato)iron(II) species

The synthesis of a low-spin six-coordinate iron(II) porphyrinate in which the two axial ligands are forced to have a relative perpendicular orientation has been successfully accomplished for the first time. The reaction of four-coordinate (tetramesitylporphinato)iron(II) with 2-methylimidazole leads to the preparation of [Fe(TMP)(2-MeHIm)(2)] which cocrystallizes with five-coordinate [Fe(TMP)(2-MeHIm)]. The six-coordinate complex accommodates the sterically crowded pair of imidazoles with a strongly ruffled core and relative perpendicular orientation. This leads to shortened equatorial bonds of 1.963(6) A and slightly elongated axial Fe-N bond lengths of 2.034(9) A that are about 0.04 A shorter and 0.03 A longer, respectively, in comparison to those of the bis-imidazole-ligated iron(II) species with parallel oriented axial ligands. The Mossbauer spectrum shows a pair of quadrupole doublets that can be assigned to the components of the cocrystallized crystalline solid. High-spin five-coordinate [Fe(TMP)(2-MeHIm)] has DeltaE(Q) = 2.25 mm/s and delta = 0.90 mm/s at 15 K. The quadrupole splitting, DeltaE(Q), for [Fe(TMP)(2-MeHIm)(2)] is 1.71 mm/s, and the isomer shift is 0.43 mm/s at 15 K. The quadrupole splitting value is significantly larger than that found for low-spin iron(II) derivatives with relative parallel orientations for the two axial ligands. Mossbauer spectra thus provide a probe for ligand orientation when structural data are otherwise not available.     

Electronic configuration of five-coordinate high-spin pyrazole-ligated iron(II) porphyrinates

Pyrazole, a neutral nitrogen ligand and an isomer of imidazole, has been used as a fifth ligand to prepare two new species, [Fe(TPP)(Hdmpz)] and [Fe(Tp-OCH(3)PP)(Hdmpz)] (Hdmpz = 3,5-dimethylpyrazole), the first structurally characterized examples of five-coordinate iron(II) porphyrinates with a nonimidazole neutral ligand. Both complexes are characterized by X-ray crystallography, and structures show common features for five-coordinate iron(II) species, such as an expanded porphyrinato core, large equatorial Fe-N(p) bond distances, and a significant out-of-plane displacement of the iron(II) atom. The Fe-N(pyrazole) and Fe-N(p) bond distances are similar to those in imidazole-ligated species. These suggest that the coordination abilities to iron(II) for imidazole and pyrazole are very similar even though pyrazole is less basic than imidazole. Mo?ssbauer studies reveal that [Fe(TPP)(Hdmpz)] has the same behavior as those of imidazole-ligated species, such as negative quadrupole splitting values and relative large asymmetry parameters. Both the structures and the Mo?ssbauer spectra suggest pyrazole-ligated five-coordinate iron(II) porphyrinates have the same electronic configuration as imidazole-ligated species.     

Iron complexes of 5,10,15,20-tetraphenyl-21-oxaporphyrin

The iron complexes of 5,10,15,20-tetraphenyl-21-oxaporphyrin (OTPP)H have been investigated. Insertion of iron(II) followed by one-electron oxidation yielded a high-spin, six-coordinate (OTPP)Fe(III)Cl(2) complex. The reduction of (OTPP)Fe(III)Cl(2) has been accomplished by means of moderate reducing reagents producing high-spin five-coordinate (OTPP)Fe(II)Cl. The molecular structure of (OTPP)Fe(III)Cl(2) has been determined by X-ray diffraction. The iron(III) 21-oxaporphyrin skeleton is essentially planar. The furan ring coordinates in the eta(1) fashion through the oxygen atom, which acquires trigonal geometry. The iron(III) apically coordinates two chloride ligands. Addition of potassium cyanide to a solution of (OTPP)Fe(III)Cl(2) in methanol-d(4) results in its conversion to a six-coordinate, low-spin complex [OTPP)Fe(III)(CN)(2)] which is spontaneously reduced to [OTPP)Fe(II)(CN)(2)](-) by excess cyanide. The spectroscopic features of [OTPP)Fe(III)(CN)(2)] correspond to the common low-spin iron(III) porphyrin (d(xy))(2)(d(xz)d(yz))(3) electronic configuration. Titration of (OTPP)Fe(III)Cl(2) or (OTPP)Fe(II)Cl with n-BuLi (toluene-d(8), 205 K) resulted in the formation of (OTPP)Fe(II)(CH(2)CH(2)CH(2)CH(3)). (OTPP)Fe(II)(n-Bu) decomposes via homolytic cleavage of the iron-carbon bond to produce (OTPP)Fe(I). The EPR spectrum (toluene-d(8), 77 K) is consistent with a (d(xy))(2)(d(xz))(2)(d(yz))(2)(d(z)(2)(1)(d[(x)(2)-(y)(2)])(0) ground electronic state of iron(I) oxaporphyrin (g(1) = 2.234, g(2) = 2.032, g(3) = 1.990). The (1)H NMR spectra of (OTPP)Fe(III)Cl(2), (OTPP)Fe(III)(CN)(2), ([(OTPP)Fe(III))](2)O)(2+), and (OTPP)Fe(II)Cl have been analyzed. There are considerable similarities in (1)H NMR properties within each iron(n) oxaporphyrin-iron(n) regular porphyrin or N-methylporphyrin pair (n = 2, 3). Contrary to this observation, the pattern of downfield positions of pyrrole resonances at 156.2, 126.5, 76.3 ppm and furan resonance at 161.4 ppm (273 K) detected for the two-electron reduction product of (OTPP)Fe(III)Cl(2) is unprecedented in the group of iron(I) porphyrins.     

Spin density distribution in five- and six-coordinate iron(II)-porphyrin NO complexes evidenced by magnetic circular dichroism spectroscopy

Using magnetic circular dichroism (MCD) spectroscopy together with DFT calculations, the spin density distributions in five-coordinate [Fe(TPP)(NO)] (I) and six-coordinate [Fe(TPP)(MI)(NO)] (II, MI = 1-methylimidazole) are defined. In the five-coordinate complex, a strong Fe-NO sigma bond between pi(*)(h) and d(z)(2) is present that leads to a large transfer of spin density from the NO ligand to Fe(II) corresponding to an electronic structure with noticeable Fe(I)-NO(+) character. Consequently, the MCD spectrum is dominated by paramagnetic C-term contributions. On coordination of the sixth ligand, the spin density is pushed back from the iron toward the NO ligand, resulting in an Fe(II)-NO(radical) type of electronic structure. This is reflected by the fact that the MCD spectrum is dominated by diamagnetic contributions.     

Modulation of metal displacements in a saddle distorted macrocycle: synthesis, structure, and properties of high-spin Fe(III) porphyrins and implications for the hemoproteins

A rare family of five and six-coordinated high-spin Fe(III) porphyrins incorporating weak axial ligands are synthesized and structurally characterized which demonstrate, for the first time, stepwise metal displacements in a single distorted macrocyclic environment that has generally been seen in many biological systems. The introduction of four nitro groups into the meso-positions of octaethyl porphyrin severely distorts the porphyrin geometry and provides an interesting modulation of the macrocycle properties which enables the facile isolation of "pure" high-spin Fe(III)(tn-OEP)Cl, Fe(III)(tn-OEP)(MeOH)Cl, and Fe(III)(tn-OEP)(H2O)2(+) in excellent yields in a saddle distorted macrocyclic environment that are known to stabilize intermediate spin states. The stepwise out-of-plane displacements of iron are as follows: 0.47 A for Fe(III)(tn-OEP)Cl; 0.09 A for Fe(III)(tn-OEP)(MeOH)Cl, and 0.01 A for Fe(III)(tn-OEP)(H2O)2(+) from the mean plane of the porphyrins. However, in both five and six-coordinated Fe(III) porphyrins, the Fe-Np distances are quite comparable while the porphyrin cores have expanded significantly, virtually to the same extent for the six-coordinate complexes reported here. The large size of the high-spin iron(III) atom in Fe(III)(tn-OEP)(H2O)2(+) is accommodated perfectly with no displacement of the metal. This expansion is accompanied by a significant decrease of the saddle distortion with a clear increase of the ruffling. Furthermore, the Fe atom in Fe(III)(tn-OEP)(MeOH)Cl is not out of plane because of the larger atom size; however, the displacement of the iron depends on both the relative strength of the axial ligands, as well as the nature and extent of the ring deformation. Our characterization demonstrates that increase in ruffling and/or decrease in macrocycle deformation brings the iron atom more into the plane in a distorted macrocyclic environment. Our observations thus suggest that the displacements of iron in proteins are the consequences of nonequivalent axial coordination, as well as protein induced deformations at the heme. The high-spin nature of the complexes reported here is believed to be due to the larger Fe-Np distances which then reduce substantially the interaction between iron d(x2)-y2 and porphyrin a(2u) orbital. The Fe(III)/Fe(II) reduction potential of Fe(III)(tn-OEP)Cl shows a reversible peak at large positive value (0.20 V), and no ring-centered oxidation was observed within the solvent limit (approximately 1.80 V). It is thus easier to reduce Fe(III)(tn-OEP)Cl by almost 700 mV compared to Fe(III)(OEP)Cl while oxidations are very difficult. Furthermore, the addition of 3-Cl-pyridine to Fe(III)(tn-OEP)Cl in air undergoes spontaneous auto reduction to produce the rare air-stable Fe(II)(tn-OEP)(3-Cl-py)2 that shows Fe(II)/Fe(III) oxidation peaks at high positive potential (0.79 V), which is approximately 600 mV more anodic compared to [Fe(II)(tn-OEP)Cl](-). This large anodic shift illustrates the effective removal of metal-centered electron density by the macrocycle when the metal is constrained to reside in the porphyrin plane.     

Nitrosyliron(III) porphyrinates: porphyrin core conformation and FeNO geometry. Any correlation?

The synthesis, structural, and spectroscopic characterization of (nitrosyl)iron(III) porphyrinate complexes designed to have strongly nonplanar porphyrin core conformations is reported. The species have a nitrogen-donor axial ligand trans to the nitrosyl ligand and display planar as well as highly nonplanar porphyrin core conformations. The systems were designed to test the idea, expressly discussed for the heme protein nitrophorin (Roberts, et al. Biochemistry 2001, 40, 11327), that porphyrin core distortions could lead to an unexpected, bent geometry for the FeNO group. For [Fe(OETPP)(1-MeIm)(NO)]ClO(4).C(6)H(5)Cl (H(2)OETPP = octaethyltetraphenylporphyrin), the porphyrin core is found to be severely saddled. However, this distortion has little or no effect on the geometric parameters of the coordination group: Fe-N(p) = 1.990(9) A, Fe-N(NO) = 1.650(2) A, Fe-N(L) = 1.983(2) A, and Fe-N-O = 177.0(3) degrees. For the complex [Fe(OEP)(2-MeHIm)(NO)]ClO(4).0.5CH(2)Cl(2) (H(2)OEP = octaethylporphyrin), there are two independent molecules in the asymmetric unit. The cation denoted [Fe(OEP)(2-MeHIm)(NO)](+)(pla) has a close-to-planar porphyrin core. For this cation, Fe-N(p) = 2.014(8) A, Fe-N(NO) = 1.649(2) A, Fe-N(L) = 2.053(2) A, and Fe-N-O = 175.6(2) degrees. The second cation, [Fe(OEP)(2-MeHIm)(NO)](+)(ruf), has a ruffled core: Fe-N(p) = 2.003(7) A, Fe-N(NO) = 1.648(2) A, Fe-N(L) = 2.032(2) A, and Fe-N-O = 177.4(2) degrees. Thus, there is no effect on the coordination group geometry caused by either type of nonplanar core deformation; it is unlikely that a protein engendered core deformation would cause FeNO bending either. The solid-state nitrosyl stretching frequencies of 1917 cm(-)(1) for [Fe(OEP)(2-MeHIm)(NO)]ClO(4) and 1871 cm(-)(1) for [Fe(OETPP)(1-MeIm)(NO)]ClO(4) are well within the range seen for linear Fe-N-O groups. M?ssbauer data for [Fe(OEP)(2-MeHIm)(NO)]ClO(4) confirm that the ground state is diamagnetic. In addition, the quadrupole splitting value of 1.88 mm/s and isomer shift (0.05 mm/s) at 4.2 K are similar to other (nitrosyl)iron(III) porphyrin complexes with linear Fe-N-O groups. Crystal data: [Fe(OETPP)(1-MeIm)(NO)]ClO(4).C(6)H(5)Cl, monoclinic, space group P2(1)/c, Z = 4, with a = 12.9829(6) A, b = 36.305(2) A, c = 14.0126(6) A, beta = 108.087(1) degrees; [Fe(OEP)(2-MeHIm)(NO)]ClO(4).0.5CH(2)Cl(2), triclinic, space group Ponemacr;, Z = 4, with a = 14.062(2) A, b = 16.175(3) A, c = 19.948(3) A, alpha = 69.427(3) degrees, beta = 71.504(3) degrees, gamma = 89.054(3) degrees.     

Structural characterization of the picket fence (TpivPP) porphyrins Co(TpivPP), Co(TpivPP)(NO2)(1-MeIm), and Co(TpivPP)(NO2)(1,2-Me2Im)

The compounds Co(TpivPP) (1), Co(TpivPP)(NO2)(1-MeIm) (2), and Co(TpivPP)(NO2)(1,2-Me2Im) (3) have been synthesized (TpivPP = meso-tetrakis(alpha, alpha, alpha, alpha-o-pivalamidophenyl)porphyrinato dianion), and their structures have been determined with single-crystal X-ray diffraction methods. 1: a = 17.578(1) A, b = 17.596(1) A, c = 20.639(1) A, beta = 115.03(1) degrees, P2(1)/c, Z = 4, T = -120 degrees C. 2: a = 18.522(4) A, b = 18.942(4) A, c = 18.177(4) A, beta = 90.68(3) degrees, C2/c, Z = 4, T = -70 degrees C. 3: a = 18.998(4) A, b = 19.187(4) A, c = 18.000(4) A, beta = 90.96(3) degrees, C2/c, Z = 4, T = -120 degrees C. Compounds 2 and 3 have crystallographically imposed 2-fold axes. In 2 and 3, which represent R-state (relaxed) and T-state (tense) models, respectively, for hemoglobin, the NO2 ligand is bound on the "picket" side to the Co atom, and either 1-MeIm (for 2) or 1,2-Me2Im (for 3) is bound to the Co atom at the sixth coordination site on the sterically unhindered side of the molecule. The average deviations of atoms from the 24-atom porphyrin core are 0.031, 0.129, and 0.117 A for 1, 2, and 3, respectively. The Co atom is -0.043(1) A out of the mean 24-atom porphyrin plane toward the 1-MeIm ligand in 2 and -0.089(1) A out of the plane toward the 1,2-Me2Im ligand in 3. The bonds of both axial ligands in the R-state model 2, 1.898(4) A for Co-N(O2) and 1.995(4) A for Co-N(base), are shorter than the corresponding bonds in the T-state model 3, 1.917(4) A for Co-N(O2) and 2.091(4) A for Co-N(base).     

Spectroscopic properties and electronic structure of five- and six-coordinate iron(II) porphyrin NO complexes: Effect of the axial N-donor ligand

In this paper, the differences in the spectroscopic properties and electronic structures of five- and six-coordinate iron(II) porphyrin NO complexes are explored using [Fe(TPP)(NO)] (1; TPP = tetraphenylporphyrin) and [Fe(TPP)(MI)(NO)] (2; MI = 1-methylimidazole) type systems. Binding of N-donor ligands in axial position trans to NO to five-coordinate complexes of type 1 is investigated using UV-vis absorption and 1H NMR spectroscopies. This way, the corresponding binding constants Keq are determined and the 1H NMR spectra of 1 and 2 are assigned for the first time. In addition, 1H NMR allows for the determination of the degree of denitrosylation in solutions of 1 with excess base. The influence of the axial ligand on the properties of the coordinated NO is then investigated. Vibrational spectra (IR and Raman) of 1 and 2 are presented and assigned using isotope substitution and normal-coordinate analysis. Obtained force constants are 12.53 (N-O) and 2.98 mdyn/A (Fe-NO) for 1 compared to 11.55 (N-O) and 2.55 mdyn/A (Fe-NO) for 2. Together with the NMR results, this provides experimental evidence that binding of the trans ligand weakens the Fe-NO bond. The principal bonding schemes of 1 and 2 are very similar. In both cases, the Fe-N-O subunit is strongly bent. Donation from the singly occupied pi* orbital of NO into d(z2) of iron(II) leads to the formation of an Fe-NO sigma bond. In addition, a medium-strong pi back-bond is present in these complexes. The most important difference in the electronic structures of 1 and 2 occurs for the Fe-NO sigma bond, which is distinctively stronger for 1 in agreement with the experimental force constants. The increased sigma donation from NO in 1 also leads to a significant transfer of spin density from NO to iron, as has been shown by magnetic circular dichroism (MCD) spectroscopy in a preceding Communication (Praneeth, V. K. K.; Neese, F.; Lehnert, N. Inorg. Chem. 2005, 44, 2570-2572). This is confirmed by the 1H NMR results presented here. Hence, further experimental and computational evidence is provided that complex 1 has noticeable Fe(I)NO+ character relative to 2, which is an Fe(II)NO(radical) complex. Finally, using MCD theory and quantum chemical calculations, the absorption and MCD C-term spectra of 1 and 2 are assigned for the first time.     

Facile ring opening of iron(III) and iron(II) complexes of meso-amino-octaethylporphyrin by dioxygen

Pyridine solutions of ClFe(III)(meso-NH(2)-OEP) undergo oxidative ring opening when exposed to dioxygen. The high-spin iron(III) complex, ClFe(III)(meso-NH(2)-OEP), has been isolated and characterized by X-ray crystallography. In the solid state, it has a five-coordinate structure typical for high-spin (S = 5/2) iron(III) complex. In chloroform-d solution, ClFe(III)(meso-NH(2)-OEP) displays an (1)H NMR spectrum characteristic of a high-spin, five-coordinate complex and is unreactive toward dioxygen. However, in pyridine-d(5) solution a temperature-dependent equilibrium exists between the high-spin (S = 5/2), six-coordinate complex, [(py)ClFe(III)(meso-NH(2)-OEP)], and the six-coordinate, low spin (S = 1/2 with the less common (d(xz)d(yz))(4)(d(xy))(1) ground state)) complex, [(py)(2)Fe(III)(meso-NH(2)-OEP)](+). Such pyridine solutions are air-sensitive, and the remarkable degradation has been monitored by (1)H NMR spectroscopy. These studies reveal a stepwise conversion of ClFe(III)(meso-NH(2)-OEP) into an open-chain tetrapyrrole complex in which the original amino group and the attached meso carbon atom have been converted into a nitrile group. Additional oxidation at an adjacent meso carbon occurs to produce a ligand that binds iron by three pyrrole nitrogen atoms and the oxygen atom introduced at a meso carbon. This open-chain tetrapyrrole complex itself is sensitive to attack by dioxygen and is converted into a tripyrrole complex that is stable to further oxidation and has been isolated. The process of oxidation of the Fe(III) complex, ClFe(III)(meso-NH(2)-OEP), is compared with that of the iron(II) complex, (py)(2)Fe(II)(meso-NH(2)-OEP); both converge to form identical products.     

Relative axial ligand orientation in bis(imidazole)iron(II) porphyrinates: are "picket fence" derivatives different?

The synthesis of three new bis(imidazole)-ligated iron(II) picket fence porphyrin derivatives, [Fe(TpivPP)(1-RIm) 2] 1-RIm = 1-methyl-, 1-ethyl-, or 1-vinylimidazole) are reported. X-ray structure determinations reveal that the steric requirements of the four alpha,alpha,alpha,alpha-o-pivalamidophenyl groups lead to very restricted rotation of the imidazole ligand on the picket side of the porphyrin plane; the crowding leads to an imidazole plane orientation eclipsing an iron-porphyrin nitrogen bond. An unusual feature for these diamagnetic iron(II) species is that all three derivatives have the two axial ligands with a relative perpendicular orientation; the dihedral angles between the two imidazole planes are 77.2 degrees , 62.4 degrees , and 78.5 degrees . All three derivatives have nearly planar porphyrin cores. M?ssbauer spectroscopic characterization shows that all three derivatives have quadrupole splitting constants around 1.00 mm/s at 100K.     

Laser photolysis studies of the photochemical reactions of chromium(III) octaethylporphyrin and tetramesitylporphyrin complexes in toluene solution

A nanosecond laser photolysis study was carried out for the Cr(III) porphyrin complexes of 2,3,7,8,12,13,17,18-octaethylporphyrin, [Cr(OEP)(Cl)(L)], and of 5,10,15,20-tetramesitylporphyrin, [Cr(TMP)(Cl)(L)], in toluene containing water and an excess amount of L (L = axial ligand). The laser photolysis generates the triplet excited state of the parent complex as well as a five-coordinate complex, [Cr(porphyrin)(Cl)], produced by the photodissociation of the axial ligand L. The yields for the formation of the triplet state and the photodissociation of L are found to markedly depend on the nature of both L and porphyrin ligand. The five-coordinate [Cr(porphyrin)(Cl)] readily reacts with both H(2)O and L in the bulk solution to give [Cr(porphyrin)(Cl)(H(2)O)] and [Cr(porphyrin)(Cl)(L)], respectively. The axial H(2)O ligand in [Cr(porphyrin)(Cl)(H(2)O)] is then substituted by the ligand L to regenerate the original complex [Cr(porphyrin)(Cl)(L)]. In principle, the substitution reaction takes place by the dissociative mechanism: the first step is the dissociation of H(2)O from [Cr(porphyrin)(Cl)(H(2)O)], followed by the reaction of the five-coordinate [Cr(porphyrin)(Cl)] with the ligand L to regenerate [Cr(porphyrin)(Cl)(L)]. The rate constants for this ligand substitution reaction are found to exhibit bell-shaped ligand concentration dependence. The detailed kinetic analysis revealed that both ligands L and H(2)O in toluene make a hydrogen bond with the axial H(2)O ligand in [Cr(porphyrin)(Cl)(H(2)O)] to yield dead-end complexes for the substitution reaction. The reaction mechanisms are discussed on the basis of the substituent effects of the porphyrin peripheral groups and the kinetic parameters determined from the temperature dependence of the rate constants.     

Ligand Recruitment and Spin Transitions in the Solid-State Photochemistry of Fe((III))TPPCl

We report evidence for the formation of long-lived photoproducts following excitation of iron(III) tetraphenylporphyrin chloride (Fe((III))TPPCl) in a 1:1 glass of toluene and CH(2)Cl(2) at 77 K. The formation of these photoproducts is dependent on solvent environment and temperature, appearing only in the presence of toluene. No long-lived product is observed in neat CH(2)Cl(2) solvent. A 2-photon absorption model is proposed to account for the power-dependent photoproduct populations. The products are formed in a mixture of spin states of the central iron(III) metal atom. Metastable six-coordinate high-spin and low-spin complexes and a five-coordinate high-spin complex of iron(III) tetraphenylporphyrin are assigned using structure-sensitive vibrations in the resonance Raman spectrum. These species appear in conjunction with resonantly enhanced toluene solvent vibrations, indicating that the Fe((III)) compound formed following photoexcitation recruits a toluene ligand from the surrounding environment. Low-temperature transient absorption (TA) measurements are used to explain the dependence of product formation on excitation frequency in this photochemical model. The six-coordinate photoproduct is initially formed in the high-spin Fe((III)) state, but population relaxes into both high-spin and low-spin state at 77 K. This is the first demonstration of coupling between the optical and magnetic properties of an iron-centered porphyrin molecule.     

Effect of the sixth axial ligand in CS-ligated iron(II)octaethylporphyrinates: structural and Mössbauer studies

The effect of a sixth ligand in a series of low-spin thiocarbonyl-ligated iron(II)octaethylporphyrinates has been investigated. Six-coordinate complexes have been synthesized and characterized by M?ssbauer and infrared spectroscopy and single-crystal X-ray structure determinations. The results are compared with the five-coordinate parent complex. The crystal structures of [Fe(OEP)(CS)(1-MeIm)] and [Fe(OEP)(CS)(Py)] are reported and discussed. The 1-methylimidazole and pyridine derivatives exhibit Fe-C(CS) bond distances of 1.703(4) and 1.706(2) A that are significantly longer than the 1.662(3) A reported for five-coordinate [Fe(OEP)(CS)] (Scheidt, W. R.; Geiger, D. K. Inorg. Chem. 1982, 21, 1208). The trans Fe-N(ligand) distances of 2.112(3) and 2.1550(15) A observed for the 1-methylimidazole and pyridine complex are approximately 0.13 A longer than those observed for analogous bis-ligated complexes and are consistent with a significant structural trans effect for the CS ligand. M?ssbauer investigations carried out for five- and six-coordinate thiocarbonyl derivatives with several different sixth axial ligands reveal interesting features. All derivatives exhibit very small isomer shift values, consistent with a very strong interaction between iron and CS. The five-coordinate derivative has delta(Fe) = 0.08 mm/s, and the six-coordinate complexes exhibit delta(Fe) = 0.14 to 0.19 mm/s at 4.2 K. The five-coordinate complex shows a large quadrupole splitting (DeltaE(q) = 1.93 mm/s at 4.2 K) which is reduced on coordination of the sixth ligand (DeltaE(q) = 0.42-0.80 mm/s at 4.2 K). Addition of a sixth ligand also leads to a small decrease in the value of nu(CS). Correlations in structural, IR, and M?ssbauer results suggest that the sixth ligand effect is primarily induced by changes in sigma-bonding. The structure of [Fe(OEP)(CS)(CH(3)OH)] is briefly reported. Crystal data: [Fe(OEP)(CS)(1-MeIm)] crystallizes in the monoclinic system, space group P2(1)/n, Z = 4, a = 9.5906(5) A, b = 16.704(4) A, c = 23.1417(6) A, beta = 100.453(7) degrees. [Fe(OEP)(CS)(Py)] crystallizes in the triclinic system, space group P1, Z = 5, a = 13.9073(6) A, b = 16.2624(7) A, c = 22.0709(9) A, alpha = 70.586(1) degrees, beta = 77.242(1) degrees, gamma = 77.959(1) degrees. [Fe(OEP)(CS)(CH(3)OH)] crystallizes in the triclinic system, space group P1, Z = 1, a = 9.0599(5) A, b = 9.4389(5) A, c = 11.0676(6) A, alpha = 90.261(1) degrees, beta = 100.362(1) degrees, gamma = 114.664(1) degrees.     

Hydrogen bonding influence of 1,10-phenanthroline on five-coordinate high-spin imidazole-ligated iron(II) porphyrinates

The influence of a hydrogen bond to the coordinated imidazole on the geometric and electronic structure of iron has been further studied in new complexes of five-coordinate high-spin imidazole-ligated iron(II) porphyrinates. With 1,10-phenanthroline (1,10-phen) as the hydrogen-bond acceptor, several new octaethylporphyrin dianion (OEP) and meso-tetraphenylporphyrin dianion (TPP) derivatives have been synthesized and characterized by X-ray crystallography and M?ssbauer spectroscopy. In all three new structures, the porphyrin molecules and 1,10-phenanthroline molecules have been found with a ratio of 1:1. All the porphyrin derivatives are five-coordinate 2-methylimidazole-ligated iron(II) species. 1,10-Phenanthroline is hydrogen bonded to the coordinated imidazole to form two unequal hydrogen bonds. The Fe-N p and Fe-N Im bond lengths and displacement of the iron atom out of the porphyrin plane are similar to those in imidazole-ligated species. M?ssbauer measurements showed remarkable temperature dependence; the analysis of the data obtained in applied magnetic field for [Fe(OEP)(2-MeHIm)].(1,10-phen) gave a negative quadrupole splitting value and large asymmetry parameters. All the structural and M?ssbauer properties suggest that these new hydrogen-bonded species have the same electronic configuration as imidazole-ligated species.     

Monomeric, trimeric, and tetrameric transition metal complexes (Mn, Fe, Co) containing N,N-bis(2-pyridylmethyl)-2-aminoethanol/-ate: preparation, crystal structure, molecular magnetism and oxidation catalysis

The reaction of N,N-bis(2-pyridylmethyl)-2-aminoethanol (bpaeOH), NaSCN/NaN(3), and metal (M) ions [M = Mn(II), Fe(II/III), Co(II)] in MeOH, leads to the isolation of a series of monomeric, trimeric, and tetrameric metal complexes, namely [Mn(bpaeOH)(NCS)(2)] (1), [Mn(bpaeO)(N(3))(2)] (2), [Fe(bpaeOH)(NCS)(2)] (3), [Fe(4)(bpaeO)(2)(CH(3)O)(2)(N(3))(8)] (4), [Co(bpaeOH)(NCS)(2)] (5), and [Co(3)(bpaeO)(2)(NO(3))(N(3))(4)](NO(3)) (6). These compounds have been investigated by single crystal X-ray diffractometry and magnetochemistry. In complex 1 the Mn(II) is bonded to one bpaeOH and two thiocyanate ions, while in complex 2 it is coordinated to a deprotonated bpaeO(-) and two azide ions. The oxidation states of manganese ions are 2+ for 1 and 3+ for 2, respectively, indicating that the different oxidation states depend on the type of binding anions. The structures of monomeric iron(II) and cobalt(II) complexes 3 and 5 with two thiocyanate ions are isomorphous to that of 1. Compounds 1, 2, 3, and 5 exhibit high-spin states in the temperature range 5 to 300 K. 4 contains two different iron(III) ions in an asymmetric unit, one is coordinated to a deprotonated bpaeO(-), an azide ion, and a methoxy group, and the other is bonded to three azide ions and two oxygens from bpaeO(-) and a methoxy group. Two independent iron(III) ions in 4 form a tetranuclear complex by symmetry. 4 displays both ferromagnetic and antiferromagnetic couplings (J = 9.8 and -14.3 cm(-1)) between the iron(III) ions. 6 is a mixed-valence trinuclear cobalt complex, which is formulated as Co(III)(S = 0)-Co(II)(S = 3/2)-Co(III)(S = 0). The effective magnetic moment at room temperature corresponds to the high-spin cobalt(II) ion (～4.27 μ(B)). Interestingly, 6 showed efficient catalytic activities toward various olefins and alcohols with modest to excellent yields, and it has been proposed that a high-valent Co(V)-oxo species might be responsible for oxygen atom transfer in the olefin epoxidation and alcohol oxidation reactions.     

Structural insights into ligand dynamics: correlated oxygen and picket motion in oxycobalt picket fence porphyrins

Two different oxygen-ligated cobalt porphyrins have been synthesized and the solid-state structures have been determined at several temperatures. The solid-state structures provide insight into the dynamics of Co-O(2) rotation and correlation with protecting group disorder. [Co(TpivPP)(1-EtIm)(O(2))] (TpivPP = picket fence porphyrin) is prepared by oxygenation of [Co(TpivPP)(1-EtIm)(2)] in benzene solution. The structure at room temperature has the oxygen ligand within the ligand binding pocket and disordered over four sites and the trans imidazole is disordered over two sites. The structure at 100 K, after the crystal has been carefully annealed to yield a reversible phase change, is almost completely ordered. The phase change is reversed upon warming the crystal to 200 K, whereupon the oxygen ligand is again disordered but with quite unequal populations. Further warming to 300 K leads to greater disorder of the oxygen ligands with nearly equal O(2) occupancies at all four positions. The disorder of the tert-butyl groups of the protecting pickets is correlated with rotation of the O(2) around the Co-O(O(2)) bond. [Co(TpivPP)(2-MeHIm)(O(2))] is synthesized by a solid-state oxygenation reaction from the five-coordinate precursor [Co(TpivPP)(2-MeHIm)]. Exposure to 1 atm of O(2) leads to incomplete oxygenation, however, exposure at 5 atm yields complete oxygenation. Complete oxygenation leads to picket disorder whereas partial (40%) oxygenation does not. Crystallinity is retained on complete degassing of oxygen in the solid, and complete ordering of the pickets is restored. The results should provide basic information needed to better model M-O(2) dynamics in protein environments.     

mer-[Fe III(bpca)(CN)3]-: a new low-spin iron(III) complex to build heterometallic ladder-like chains

The novel mononuclear complex PPh(4)-mer-[Fe(III)(bpca)(3)(CN)(3)].H(2)O (1) [PPh(4)(+) = tetraphenylphosphonium cation and bpca = bis(2-pyridylcarbonyl)amidate anion] and ladder-like chain compound [[Fe(III)(bpca)(micro-CN)(3)Mn(II)(H(2)O)(3)] [Fe(III)(bpca)(CN)(3)]].3H(2)O (2) have been prepared and characterized by X-ray diffraction analysis. Compound 1 is a low-spin iron(III) compound with three cyanide ligands in mer arrangement and a tridentate N-donor ligand building a distorted octahedral environment around the iron atom. Compound 2 is an ionic salt made up of cationic ladder-like chains [[Fe(III)(bpca)(micro-CN)(3)Mn(II)(H(2)O)(3)]](+) and uncoordinated anions [Fe(III)(bpca)(3)(CN)(3)](-). The magnetic properties of 2 correspond to those of a ferrimagnetic chain with significant intrachain antiferromagnetic coupling between the low-spin iron(III) centers and the high-spin manganese(II) cations. This compound exhibits ferrimagnetic ordering below 2.0 K.     

Coordination of diatomic ligands to heme: simply CO

The synthesis and molecular structures of three iron(II) porphyrinates with only CO as the axial ligand(s) are reported. Two five-coordinate [Fe(OEP)(CO)] derivatives have Fe-C = 1.7077(13) and 1.7140(10) A, much shorter than those of six-coordinate [Fe(OEP)(Im)(CO)], although nu(C-O) is 1944-1948 cm(-1). The six-coordinate species [Fe(OEP)(CO)2] has also been studied. The competition for pi-back-bonding of two CO ligands leads to Fe-C distance of 1.8558(10) A and nu(C-O) being increased to 2021 cm(-1). The M?ssbauer spectrum has a quadrupole splitting constant of 0 mm/s at 4.2 K, indicating high electronic symmetry.