Jianfeng Li

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. Mössbauer spectroscopic characterization shows that all three derivatives have quadrupole splitting constants around 1.00 mm/s at 100K.

Cyanide — a Strong Field Ligand for Ferrohemes and Hemoproteins?

Cyanide ion, a versatile diatomic ligand, has been extensively investigated as both a classic inhibitor and as a ligand for exploring properties of hemes and hemoproteins. Unlike CO and O2 which bind only to iron(II) species, CN− can bind to both iron(II) and -(III) hemo-proteins. Stable low-spin (LS) iron(III) proteins can be straightforwardly prepared.[1–3] In contrast, (cyano)iron(II) hemoproteins are usually indirectly formed by reduction of (cyano)iron(III) proteins. Cyanide bound, iron(II) forms of myoglobin,[4] hemoglobin,[5] horseradish peroxidase[6] and a number of cytochrome oxidase derivatives[7] are known. Many, but not all, of the iron(II) species, have lower binding constants than the iron(III) analogues. The equilibrium constant for cyanide binding for iron(III) hemoproteins is often ≥105 M−1 compared to ≤102 M−1 for iron(II) species.[3] Since the first reported isolation of a (cyano)heme was reported by us in 1980,[8] a number of electronic and geometric structure issues have been brought forward.[9] All of the known species are LS iron(III) derivatives, either bis(cyano) [FeIII(Por)(CN)2]− or mixed-ligand [FeIII(Por)(CN)(L)] complexes.[9] However, there are currently no (cyano)iron(II) porphyrinate derivatives reported, presumably because of its known lower stability/affinity compared to iron(III). It might be thought that (cyano)iron(II) species would be preferred since a filled d6 shell should strongly π-bond to the π-accepting cyanide ligand. We now report the first (cyano)iron(II) porphyrinate species, five-coordinate [K(222)][Fe(TPP)(CN)] (Figure 1). The average equatorial Fe–Np bond distance (1.986 (7) A) and the axial Fe–C distance (1.8783 (10) A) are consistent with a LS state.[10] However, T-dependent Mossbauer spectra reveal a more complicated picture of the iron spin state. A single quadrupole doublet is observed, whose value decreases from 1.827 mm/s at 25 K to 0.85 mm/s at 300 K; the isomer shift varies between 0.37 to 0.47 mm/s. The most probable explanation is that a thermally induced spin crossover is occurring, whose interconversion is rapid on the Mossbauer time scale (< 10−8 s).[11]a This interpretation has been confirmed by both DFT calculations and magnetic susceptibility measurements. Figure 1 100 K ORTEP diagram of [Fe(TPP)(CN)]−. Thermal ellipsoids are contoured at the 50% probability level. Hydrogens omitted for clarity. The magnetic susceptibility of [K(222)][Fe(TPP)(CN)] was investigated over the temperature range of 2–400 K. Figure 2 shows the product of the molar susceptibility (χm) (corrected for paramagnetism (TIP)) and temperature (T) in an external magnetic field of 2 T, which provides direct evidence for an S = 0 (LS) ↔ S = 2 (HS) spin crossover. AT 400 K, the value of χmT (2.96 cm3 K mol−1) is close to that expected for the HS state, but the lack of significant plateau suggests that the transition is not quite complete at this temperature. The spin-state transition occurs over a large temperature range (~175–400 K) and is reversible; both ascending and descending temperature measurements are shown in Figure 2 and no hysteresis was observed. The transition temperature T1/2 (defined as temperature at which complexes shows a population of 50% in the HS state) of this gradually proceeding spin transition is about 265 K. Figure 2 also plots the observed time-averaged quadrupole splitting value against temperature; the strong correlation between the quadrupole splitting and the susceptibility is clear. Figure 2 χmT versus T plot for [K(222)][Fe(TPP)(CN)] at 2T applied field. The Mossbauer quadrupole splitting values are also presented for comparison. To gain a better understanding of the thermodynamics regarding the spin-states, density functional theory was employed (see Supporting Information).[12] At low temperature only the low-spin S = 0 state was thermodynamically accessible. With increasing temperature the S = 2 state became significant, and a spin-crossover event is predicted to occur near 325 K (Figure S1) in good agreement with the value of 265 K from experiment. The intermediate-spin S = 1 state was disfavored over the entire temperature range explored. We have also investigated the T-dependent structures of the iron complex, since changes in metal–donor atom distances, along with changes in magnetic properties, are the two hallmarks of spin-state transitions. Structures have been determined at 100 K (two crystals) and 296, 325 and 400 K.[13] A change from a LS to a HS state in the five-coordinate complex is expected to lead to increases in the axial Fe–C distance, the equatorial Fe–Np bond distances, and the displacement of the iron atom from the mean porphinato plane. The results are summarized in the ORTEP drawings given in Figure 3, for simplicity only the cyanide group and FeN4 porphyrin core are shown. The Fe–C distance elongates by 0.23 A (Figure 3), which is amongst the largest changes in bond lengths that have been observed for iron(II) spin crossover compounds.[11b]b This is in part because the axial and equatorial bond distance increases must be asymmetric owing to the macrocyclic constraints of the porphyrin ring; note that Fe–Np has increased by 0.103 A over the same temperature range. The 100 K Fe–Np average bond length of 1.986 (7) A is that for a pure LS state whereas the 400 K value of 2.089 (8) A is slightly less than expected for anionic HS iron(II) complex, consistent with the idea that the spin state transition is not quite complete. Also completely consonant with expectation are the increases in the displacement of the iron from the mean plane of the four nitrogen plane. Figure 3 Four ORTEP diagrams of [K(222)][Fe(TPP)(CN)] displaying the cyanide groups and the core atoms of porphyrin (Fe and four pyrrole N atoms). Values of axial ligand and average equatorial bond distances are given as well as the iron displacement from the ... The anisotropic thermal parameters also show evidence of the spin crossover. As expected, the magnitude of all atomic anisotropic displacement parameters increase upon increasing temperature. However, the cyanide carbon atom shows different behavior over the temperature range. The thermal parameters at 100 and 400 K are close to isotropic, consistent with a single carbon atom site, whereas at intermediate temperatures with substantial populations of two spin states and differing carbon sites, the thermal parameters are much more prolate with elongation along the Fe–C bond direction. Importantly, the C–N bond distance in all structures remains nearly constant, as expected if only CN− atoms occupy two sites. Additional evidence for the spin crossover comes from T-dependent infrared spectra, which has the advantage of a shorter time scale (10−13 s) and thus can detect both spin isomers. Measurements at 296 K, as either Nujol mulls or KBr pellets, show two distinct ν(C–N) frequencies at 2070 and 2105 cm−1, with the first being the stronger. (S.I.) On cooling, the 2105 cm−1 peak gradually decreases while the 2070 cm−1 peak increases. At 150 to 160 K, the stretch at 2105 cm−1 disappears and thus corresponds to the HS stretch. A similar pattern of T-dependent azide stretches was observed in a 5/2, 3/2 spin crossover complex.[14] In coordination chemistry, cyanide and CO have been deeply entrenched as strong field ligands.[15, 16] Recently, Miller et al. showed that [(NEt4)3][Cr(II)(CN)5][17] is a distorted trigonal bipyramidal complex that was not low spin. Two different theoretical calculations[18] have suggested that the HS state results from the buildup of electrostatic (ligand–ligand) repulsions and not the ligand field of cyanide per se; the cyanide ligand is behaving as a strong field ligand in this Cr complex. However, [K(222)][Fe(TPP)(CN)] represents a case where the CN− should unequivocally lead to LS species. That it does not, strongly demonstrates the weaker field nature of cyanide, even in a case where π-back bonding should be maximized. In summary, the synthesis and characterization of the first cyanoiron(II) porphyrinate, [K(222)][Fe(TPP)(CN)], is presented. It forms a LS to HS crossover complex; coordination of a single axial cyanide ligand does not generate a sufficiently strong ligand field to ensure a low-spin complex under all conditions.[19] This is in distinct contrast to the five-coordinate CO complex, that is low spin under all known conditions.

Oxygenation of Cobalt Porphyrinates: Coordination or Oxidation?

The X-ray characterization of the five-coordinate picket-fence porphyrin complex, [Co(TpivPP)(2-MeHIm)], is reported. The complex has the displacement of cobalt from the porphyrin plane = 0.15 A, and Co-N(Im) = 2.145(3) and (Co-N(p))(av) = 1.979(3) A. This five-coordinate complex, in the presence of dioxygen and excess 2-methylimidazole, undergoes an unanticipated, photoinitiated atropisomerization of the porphyrin ligand, oxidation of cobalt(II), and the formation of the neutral cobalt(III) complex [Co(alpha,alpha,beta,beta-TpivPP)(2-MeHIm)(2-MeIm(-)]. Two distinct examples of this complex have been structurally characterized, and both have structural parameters consistent with cobalt(III). The two new Co(III) porphyrin complexes have axial Co-N(Im) distances ranging from 1.952 to 1.972 A, but which allow for the distinction between imidazole and imidazolate. An interesting intermolecular hydrogen bonding network is observed that leads to infinite helical chains. UV-vis spectroscopic study suggests that [Co(TpivPP)(2-MeHIm)(O(2))] is an intermediate state for the oxidation reaction and that the atropisomerization process is photocatalyzed. A reaction route is proposed based on the spectroscopic studies.

Correlated Ligand Dynamics in Oxyiron Picket Fence Porphyrins: Structural and Mössbauer Investigations

Disorder in the position of the dioxygen ligand is a well-known problem in dioxygen complexes and, in particular, those of picket fence porphyrin species. The dynamics of Fe-O2 rotation and tert-butyl motion in three different picket fence porphyrin derivatives has been studied by a combination of multitemperature X-ray structural studies and Mössbauer spectroscopy. Structural studies show that the motions of the dioxygen ligand also require motions of the protecting pickets of the ligand binding pocket. The two motions appear to be correlated, and the temperature-dependent change in the O2 occupancies cannot be governed by a simple Boltzmann distribution. The three [Fe(TpivPP)(RIm)(O2)] derivatives studied have RIm = 1-methyl-, 1-ethyl-, or 2-methylimidazole. In all three species there is a preferred orientation of the Fe-O2 moiety with respect to the trans imidazole ligand and the population of this orientation increases with decreasing temperature. In the 1-MeIm and 1-EtIm species the Fe-O2 unit is approximately perpendicular to the imidazole plane, whereas in the 2-MeHIm species the Fe-O2 unit is approximately parallel. This reflects the low energy required for rotation of the Fe-O2 unit and the small energy differences in populating the possible pocket quadrants. All dioxygen complexes have a crystallographically required 2-fold axis of symmetry that limits the accuracy of the determined Fe-O2 geometry. However, the 80 K structure of the 2-MeHIm derivative allowed for resolution of the two bonded oxygen atom positions and provided the best geometric description for the Fe-O2 unit. The values determined are Fe-O = 1.811(5) Å, Fe-O-O = 118.2(9)°, O-O = 1.281(12) Å, and an off-axis tilt of 6.2°. Demonstration of the off-axis tilt is a first. We present detailed temperature-dependent simulations of the Mössbauer spectra that model the changing value of the quadrupole splitting and line widths. Residuals to fits are poorer at higher temperature. We believe that this is consistent with the idea that population of the two conformers is related to the concomitant motions of both Fe-O2 rotations and motions of the protecting tert-butyl pickets.

Comparison of Cyanide and Carbon Monoxide as Ligands in Iron(II) Porphyrinates

Spot the difference: The five-coordinate iron(II) cyanoporphyrinates, which are spin-crossover compounds, can be used to synthesize previously unknown six-coordinate complexes. Bis(cyano) and (cyano)imidazole complexes are presented, and the five- and six-coordinate (cyano)iron(II) derivatives are compared with analogous CO complexes.

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.

New Perspectives on Iron–Ligand Vibrations of Oxyheme Complexes

We report our studies of the vibrational dynamics of iron for three imidazole-ligated oxyheme derivatives that mimic the active sites of histidine-ligated heme proteins complexed with dioxygen. The experimental vibrational data are obtained from nuclear resonance vibrational spectroscopy (NRVS) measurements conducted on both powder samples and oriented single crystals, and which includes several in-plane (ip) and out-of-plane (oop) measurements. Vibrational spectral assignments have been made through a combination of the oriented sample spectra and predictions based on density functional theory (DFT) calculations. The two Fe-O(2) modes that have been previously observed by resonance Raman spectroscopy in heme proteins are clearly shown to be very strongly mixed and are not simply either a bending or stretching mode. In addition, a third Fe-O(2) mode, not previously reported, has been identified. The long-sought Fe-Im stretch, not observed in resonance Raman spectra, has been identified and compared with the frequencies observed for the analogous CO and NO species. The studies also suggest that the in-plane iron motion is anisotropic and is controlled by the orientation of the Fe-O(2) group and not sensitive to the in-plane Fe-N(p) bonds and/or imidazole orientations.

Comprehensive Fe-ligand vibration identification in {FeNO}6 hemes.

Oriented single-crystal nuclear resonance vibrational spectroscopy (NRVS) has been used to obtain all iron vibrations in two {FeNO}6 porphyrinate complexes, five-coordinate [Fe(OEP)(NO)]ClO4 and six-coordinate [Fe(OEP)(2-MeHIm)(NO)]ClO4. A new crystal structure was required for measurements of [Fe(OEP)(2-MeHIm)(NO)]ClO4, and the new structure is reported herein. Single crystals of both complexes were oriented to be either parallel or perpendicular to the porphyrin plane and/or axial imidazole ligand plane. Thus, the FeNO bending and stretching modes can now be unambiguously assigned; the pattern of shifts in frequency as a function of coordination number can also be determined. The pattern is quite distinct from those found for CO or {FeNO}7 heme species. This is the result of unchanging Fe–NNO bonding interactions in the {FeNO}6 species, in distinct contrast to the other diatomic ligand species. DFT calculations were also used to obtain detailed predictions of vibrational modes. Predictions were consistent with the intensity and character found in the experimental spectra. The NRVS data allow the assignment and observation of the challenging to obtain Fe–Im stretch in six-coordinate heme derivatives. NRVS data for this and related six-coordinate hemes with the diatomic ligands CO, NO, and O2 reveal a strong correlation between the Fe–Im stretch and Fe–NIm bond distance that is detailed for the first time.

Vibrational Probes and Determinants of the S = 0 ⇌ S = 2 Spin Crossover in Five-Coordinate [Fe(TPP)(CN)]−

The low-frequency vibrational characterization of the spin-crossover complex, five-coordinate cyano(tetraphenylporphyrinato)iron(II), [Fe(TPP)(CN)](-), is reported. Nuclear resonance vibrational spectroscopy has been used to measure all low-frequency vibrations involving iron at several temperatures; this yields vibrational spectra of both the low- (S = 0) and high-spin (S = 2) states. Multitemperature oriented single-crystal measurements facilitate assignments of the vibrational character of all modes and are consistent with the DFT-predicted spectra. The availability of the entire iron vibrational spectrum allows for the complete correlation of the modes between the two spin states. These data demonstrate that not only do the frequencies of the vibrations shift to lower values for the high-spin species as would be expected owing to the weaker bonds in the high-spin state, but also the mixing of iron modes with ligand modes changes substantially. Diagrams illustrating the changing character of the modes and their correlation are given. The reduced iron-ligand frequencies are the primary factor in the entropic stabilization of the high-spin state responsible for the spin crossover.

Electronic Configuration and Ligand Nature of Five-Coordinate Iron Porphyrin Carbene Complexes: An Experimental Study

The five-coordinate iron porphyrin carbene complexes [Fe(TPP) (CCl2)] (TPP = tetraphenylporphyrin), [Fe(TTP) (CCl2)] (TTP = tetratolylporphyrin) and [Fe(TFPP) (CPh2)] (TFPP = tetra(pentafluorophenyl)porphyrin), utilizing two types of carbene ligands (CCl2 and CPh2), have been investigated by single crystal X-ray, XANES (X-ray absorption near edge spectroscopy), Mössbauer, NMR and UV-vis spectroscopies. The XANES suggested the iron(II) oxidation state of the complexes. The multitemperature and high magnetic field Mössbauer experiments, which show very large quadrupole splittings (QS, ΔEQ), determined the S = 0 electronic configuration. More importantly, combined structural and Mössbauer studies, especially the comparison with the low spin iron(II) porphyrin complexes with strong diatomic ligands (CS, CO and CN-) revealed the covalent bond nature of the carbene ligands. A correlation between the iron isomer shifts (IS, δ) and the axial bond distances is established for the first time for these donor carbon ligands (:C-R).