The doubly oxidized, antiaromatic tetraphenylporphyrin complex [Li(TPP)][BF4

[reaction: see text] Oxidation of dilithium tetraphenylporphyrin, Li(2)(TPP), yields the doubly oxidized porphyrin complex [Li(TPP)][BF(4)]. Free TPP appears to be unstable. The crystal structure of [Li(TPP)][BF(4)] reveals that the 16-atom C-N inner ring of the porphyrin consists of alternating single and double bonds. NMR spectroscopy and nucleus-independent chemical shift (NICS) calculations, in addition to the distortion from planarity observed in the crystal structure, indicate that the 16 pi-electron inner C-N ring of the porphyrin is antiaromatic.     

An antiaromatic porphyrin complex: tetraphenylporphyrinato(silicon)(L)2 (L=THF or pyridine)

Treatment of Si(TPP)Cl2 (TPP = tetraphenylporphyrinato) with 2 equiv of Na/Hg in THF yields the reduced porphyrin complex, Si(TPP)(THF)2, in which the porphyrin ring system has an oxidation state of 4- and the complex is antiaromatic. Single-crystal X-ray diffraction reveals that Si(TPP)(THF)2 is highly ruffled and exhibits a unique C-C bond length alternation around its periphery. In addition, experimental 1H and 29Si NMR chemical shifts and NICS (nucleus-independent chemical shift) calculations on a model compound indicate a strong paratropic ring current in Si(TPP).     

Reactions of nitrogen oxides with heme models. Characterization of NO and NO2 dissociation from Fe(TPP)(NO2)(NO) by flash photolysis and rapid dilution techniques: Fe(TPP)(NO2) as an unstable intermediate

Described are studies directed toward elucidating the controversial chemistry relating to the solution phase reactions of nitric oxide with the iron(II) porphyrin complex Fe(TPP)(NO) (1, TPP = meso-tetraphenylporphinato2-). The only reaction observable with clean NO is the formation of the diamagnetic dinitrosyl species Fe(TPP)(NO)2 (2), and this is seen only at low temperatures (K(1) < 3 M(-1) at ambient temperature). However, 1 does readily react reversibly with N2O3 in the presence of excess NO to give the nitro nitrosyl complex Fe(TPP)(NO2)(NO) (3), suggesting that previous claims that 1 promotes NO disproportionation to give 3 may have been compromised by traces of air in the nitric oxide sources. It is also noted that 3 undergoes reversible loss of NO to give the elusive nitro species Fe(TPP)(NO2) (4), which has been implicated as a powerful oxygen atom transfer agent in reactions with various substrates. Furthermore, in the presence of excess NO2, the latter undergoes oxidation to the stable nitrato analogue Fe(TPP)(NO3) (5). Owing to such reactivity of Fe(TPP)(NO2), flash photolysis and stopped-flow kinetics rather than static techniques were necessary for the accurate measurement of dissociation equilibria characteristic of Fe(TPP)(NO2)(NO) in 298 K toluene solution. Flash photolysis of 3 resulted in competitive NO2 and NO dissociation to give Fe(TPP)(NO) and Fe(TPP)(NO2), respectively. The rate constant for the reaction of 1 with N2O3 to generate Fe(TPP)(NO2)(NO) was determined to be 1.8 x 10(6) M(-1) s(-1), and that for the NO reaction with 4 was similarly determined to be 4.2 x 10(5) M(-1) s(-1). Stopped-flow rapid dilution techniques were used to determine the rate constant for NO dissociation from 3 as 2.6 s(-1). The rapid dilution experiments also demonstrated that Fe(TPP)(NO2) readily undergoes further oxidation to give Fe(TPP)(NO3). The mechanistic implications of these observations are discussed, and it is suggested that NO2 liberated spontaneously from Fe(P)(NO2) may play a role in an important oxidative process involving this elusive species.     

Photophysics of reduced silicon tetraphenylporphyrin

The contrasting photophysical properties of two silicon (IV) tetraphenylporphyrins, Si(TPP)(py)2 and Si(TPP)Cl2, have been investigated using static absorption and fluorescence spectroscopy and ultrafast transient absorption measurements. The parent Si(TPP)Cl2, in which the porphyrin macrocycle has its normal 2- oxidation state, has a fluorescence yield of 0.027, and a lifetime of 1.8 ns for the lowest excited singlet state. In marked contrast, the reduced, anti-aromatic complex Si(TPP)(py)2, with the macrocycle in the 4- oxidation state, has an extremely low fluorescence yield (< or =0.0004) and a 750-fold shorter excited-state lifetime (2.4 ps) in the same solvent (pyridine). The rapid deactivation of photoexcited Si(TPP)(py)2 to the ground state is likely associated with its ruffled structure and the presence of low-energy excited states in its electronic manifold.     

Electrochemistry and spectroelectrochemistry of heterobimetallic porphyrin-corrole dyads. Influence of the spacer, metal ion, and oxidation state on the pyridine binding ability

Combined electrochemical and UV-visible spectroelectrochemical methods were utilized to elucidate the prevailing mechanisms for electroreduction of previously synthesized porphyrin-corrole dyads of the form (PCY)H2Co and (PCY)MClCoCl where M = Fe(III) or Mn(III), PC = porphyrin-corrole, and Y is a bridging group, either biphenylenyl (B), 9,9-dimethylxanthenyl (X), anthracenyl (A), or dibenzofuranyl (O). These studies were carried out in pyridine, conditions under which the cobalt(IV) corrole in (PCY)MClCoCl is immediately reduced to its Co(III) form, thus enabling direct comparisons with the free-base porphyrin dyad, (PCY)H2Co(III) under the same solution conditions. The compounds are all reduced in multiple one-electron-transfer steps, the first of which involves the M(III)/M(II) process of the porphyrin in the case of (PCY)MClCoCl and the Co(III)/Co(II) process of the corrole in the case of (PCY)H2Co. Each metal-centered redox reaction may be accompanied by the gain or loss of pyridine axial ligands, with the exact stoichiometry of the exchange process depending upon the specific combination of metal ions in the dyad, their oxidation states, and the particular spacer in the complex. Before this study was started, it was expected that the porphyrin-corrole dyads with the largest spacers, namely, O and A, would readily accommodate the formation of cobalt(III) bis-pyridine adducts because of the larger size of the cavity while dyads with the smallest B spacer would seem to have insufficient room to add even a single pyridine within the cavity, as was structurally seen in the case of (PCB)H2Co(py). This is clearly not the case, as shown in the present study. A reversible Co(III)/Co(II) reaction is seen for (PCB)MnClCoCl at -0.62 V, which when combined with spectroscopic data, leads to the assignment of (PCB)Mn(III)(py)2Co(III)(py) as the species in pyridine. The reduction of (PCB)Mn(III)(py)2Co(III)(py) to (PCB)Mn(II)(py)Co(III)(py) is accompanied on the slower spectroelectrochemical time scale by the appearance of a 603 nm band in the UV-vis spectra and is consistent with the addition of a second pyridine ligand to the Co(III)(py) unit of the dyad as one ligand is lost from the electrogenerated manganese(II) porphyrin, thus maintaining one pyridine ligand within the cavity. A different change in the coordination number is observed in the case of (PCB)FeClCoCl. Here the initial Fe(III) complex can be assigned as (PCB)Fe(III)ClCo(III)(py), which has no pyridine molecule within the cavity and the singly reduced form is characterized as (PCB)Fe(II)(py)2Co(III)(py)2, which contains two pyridine ligands inside the cavity. A following one-electron reduction of the Fe(II)/Co(III) complex then gives [(PCB)Fe(II)(py)2Co(II)]-.     

Synthesis, Characterization, and Electrochemistry of Ruthenium Porphyrins Containing a Nitrosyl Axial Ligand

Two ruthenium nitrosyl porphyrins have been synthesized and characterized by spectroscopic and electrochemical methods. The investigated compounds are represented as [(TPP)Ru(NO)(H(2)O)]BF(4) and (TPP)Ru(NO)(ONO) where TPP is the dianion of 5,10,15,20-tetraphenylporphyrin. (TPP)Ru(NO)(ONO) crystallizes in the tetragonal space group I4, with a = 13.660(1) ?, c = 9.747(1) ?, V = 1818.7(3) ?(3), and Z = 2, 233 K. The most chemically interesting feature of the structure is that the nitrosyl and O-bound nitrito groups are located axial and trans to one another. Both complexes undergo an irreversible reduction at the metal center which is accompanied by dissociation of the axial ligand trans to NO. The addition of 1-10 equiv of pyridine to [(TPP)Ru(NO)(H(2)O)]BF(4) in CH(2)Cl(2) containing 0.1 M TBAP leads to the formation of [(TPP)Ru(NO)(py)](+), a species which is reversibly reduced at E(1/2) = -0.29 V. The electrochemical data indicate that (TPP)Ru(NO)(ONO) can also be converted to [(TPP)Ru(NO)(py)](+) in CH(2)Cl(2) solutions containing pyridine but only under specific experimental conditions. This reaction does not involve a simple displacement of the ONO(-) axial ligand from (TPP)Ru(NO)(ONO) but occurs after reduction of (TPP)Ru(NO)(ONO) to (TPP)Ru(NO)(py) followed by reoxidation to [(TPP)Ru(NO)(py)](+).     

Interaction of nitrogen oxides with sublimed layers of (meso-tetraphenylporphyrinato)cobalt(II); IR evidence of oxo-transfer from (nitro)porphyrinatocobalt(III) to free nitric oxide

Interaction of a low-pressure NO2 with sublimed layers of (meso-tetraphenylporphyrinato)cobalt(II) (Co(TPP)) leads to formation of 5-coordinate nitro complex Co(III)(TPP)(NO2). Upon exposure of these layers to pyridine vapors, the fast reaction with formation of 6-coordinate nitro-pyridine porphyrins (Py)Co(III)(TPP)(NO2) occurs. By means of IR spectroscopy and use of nitrogen oxide isotopomers, it is shown that an oxo-transfer reaction occurs from 5-coordinate species to free nitric oxide (NO) while the 6-coordinate complex is rather inert. It is also demonstrated that the stepwise addition of low-pressure NO2 to nitrosyl complex Co(TPP)(NO) leads to formation of the nitro complex most likely by an exchange reaction.     

Photochemical reaction of Rh2(CO)4(μ-TPP)(TPP=tetraphenyl-porphyrinato) with pyridine (Py) in benzene. Generation of a mononuclear rhodium(II) porphyrin radical complex Rh(TPP)(Py)

Photolytic decarbonylation of Rh₂(CO)₄(TPP) (TPP=tetraphenylporphyrinato) with pyridine (Py) in benzene gives an unusual monomeric Rh^II radical complex Rh(TPP)(Py), which has been characterised by thermal and spectroscopic measurements and by vapour pressure osmosis.     

Reactions of nitrogen oxides with heme models. Spectral and kinetic study of nitric oxide reactions with solid and solute Fe(III)(TPP)(NO3)

The reaction(s) of nitric oxide (nitrogen monoxide) gas with sublimed layers containing the nitrato iron(III) complex Fe(III)(TPP)(eta(2)-O(2)NO) (1, TPP = meso-tetraphenyl porphyrinate(2)(-)) leads to formation of several iron porphyrin species that are ligated by various nitrogen oxides. The eventual products of these low-temperature solid-state reactions are the nitrosyl complex Fe(TPP)(NO), the nitro-nitrosyl complex Fe(TPP)(NO(2))(NO), and 1 itself, and the relative final quantities of these were functions of the NO partial pressure. It is particularly notable that isotope labeling experiments show that the nitrato product is not simply unreacted 1 but is the result of a series of transformations taking place in the layered material. Thus, the nitrato complex formed from solid Fe(TPP)(eta(2)-O(2)NO) maintained under a (15)NO atmosphere was found to be the labeled analogue Fe(TPP)(eta(2)-O(2)(15)NO). The reactivities of the layered solids are compared to the behaviors of the same species in ambient temperature solutions. To interpret the reactions of the labeled nitrogen oxides, the potential exchange reactions between N(2)O(3) and (15)NO were examined, and complete isotope scrambling was observed between these species under the reaction conditions (T = 140 K). Overall it was concluded from isotope labeling experiments that the sequence of reactions is initiated by reaction of 1 with NO to give the nitrato nitrosyl complex Fe(TPP)(eta(1)-ONO(2))(NO) (2) as an intermediate. This is followed by a reaction in the presence of excess NO that is equivalent to the loss of the nitrate radical NO(3)(*)( )()to give Fe(TPP)(NO) as another transient species. A plausible pathway involving NO attack on the coordinated nitrate of 2 resulting in the release of N(2)O(4) concerted with electron transfer to the metal center is proposed.     

Electron transfer. 144. Reductions with germanium(II)

Solutions 0.2-0.4 M in Ge(II) and 6 M in HCl, generated by reaction of Ge(IV) with H3PO2, are stable for more than 3 weeks and can be diluted 200-fold with dilute HCl to give GeCl3- preparations to be used in redox studies. Kinetic profiles for the reduction of Fe(III) by Ge(II), as catalyzed by Cu(II), implicate the odd-electron intermediate, Ge(III), which is formed from Cu(II) and Ge(II) (k = 30 M-1 s-1 in 0.5 M HCl at 24 degrees C) and which is consumed by reaction with Fe(III) (k = 6 x 10(2) M-1 s-1). A slower direct reaction between Ge(II) and Fe(III) (k = 0.66 M-1 s-1) can be detected in 1.0 M HCl. The reaction of Ge(II) with I3- in 0.01-0.50 M iodide is zero order in oxidant and appears to proceed via a rate-determining heterolysis of a Ge(II)-OH2 species (k = 0.045 s-1) which is subject to H(+)-catalysis. Reductions of IrCl6(2-) and PtCl6(2-) by Ge(II) are strongly Cl(-)-catalyzed. The Ir(IV) reaction proceeds through a pair of 1e- changes, of which the initial conversion to Ge(III) is rate-determining, whereas the Pt(IV) oxidant probably utilizes (at least in part) an inner-sphere PtIV-Cl-GeII bridge in which chlorine is transferred (as Cl+) from oxidant to reductant. The 2e- reagent, Ge(II), like its 5s2 counterpart, In(I), can partake in 1e- transactions, but requires more severe constraints: the coreagent must be more powerfully oxidizing and the reaction medium more halide-rich.     

Metallonitrosyl fragment as electron acceptor: intramolecular charge transfer, long range electronic coupling, and electrophilic reactivity in the trans-[NCRu(py)(4)(CN)Ru(py)(4)NO](3+) ion

The new complex trans-[NCRu(py)(4)(CN)Ru(py)(4)NO](PF(6))(3) (I) was synthesized. In acetonitrile solution, I shows an intense visible band (555 nm, epsilon = 5800 M(-1) cm(-1)) and other absorptions below 350 nm, associated with d(pi) --> pi(py) and pi(py) --> pi(py) transitions. The visible band is presently assigned as a donor-acceptor charge transfer (DACT) transition from the remote Ru(II) to the delocalized [Ru(II)-NO(+)] moiety. Photoinduced release of NO is observed upon irradiation at the DACT band. Application of the Hush model reveals strong electronic coupling, with H(DA) = approximately 2000 cm(-1). The difference between the optical absorption energy and redox potentials for the donor and acceptor sites (Ru(III,II), 1.40 V, and NO(+)/NO, 0.50 V, vs Ag/AgCl, 3 M KCl, respectively) (hnu - DeltaE(red)) is 1.33 eV, a large value which probably relates to the significant changes in distances and angles for the Ru-N-O moiety upon reduction. UV-vis absorptions, IR frequencies, and redox potentials are solvent-dependent. Controlled potential reduction (of NO(+)) and oxidation (of Ru(II) associated with the dicyano-chromophore) of I afford stable species, [NCRu(II)(py)(4)(CN)Ru(py)(4)NO](2+) (I(red)) and [NCRu(III)(py)(4)(CN)Ru(py)(4)NO](4+) (I(ox)), respectively, which are characterized by UV-vis and IR spectroscopies. I(red) shows an EPR spectrum characteristic of [Ru(II)-NO(*)] complexes. Compound I is electrophilically reactive in aqueous solution above pH 5: values of the equilibrium constant for the reaction [NCRu(py)(4)(CN)Ru(py)(4)NO](3+)+ 2 OH(-) <--> [NCRu(py)(4)(CN)Ru(py)(4)NO(2)](+) + H(2)O, K = 3.2 +/- 1.4 x 10(15) M(-2), and of the rate constant for the nucleophilic addition of OH(-), k = 9.2 +/- 0.2 x 10(3) M(-1) s(-1)(25 degrees C, I = 1 M), are obtained, with DeltaH = 90.7 +/- 3.8 kJ mol(-1) and DeltaS = 135 +/- 13 J K(-1) mol(-1). The oxidized complex, I(ox), shows an enhanced electrophilic reactivity toward OH(-). This addition reaction is followed by irreversible processes, which most probably lead to disproportionation of bound nitrite and other products.     

Quantum chemistry-based analysis of the vibrational spectra of five-coordinate metalloporphyrins [M(TPP)Cl

Vibrational properties of the five-coordinate porphyrin complexes [M(TPP)(Cl)] (M = Fe, Mn, Co) are analyzed in detail. For [Fe(TPP)(Cl)] (1), a complete vibrational data set is obtained, including nonresonance (NR) Raman, and resonance Raman (RR) spectra at multiple excitation wavelengths as well as IR spectra. These data are completely assigned using density functional (DFT) calculations and polarization measurements. Compared to earlier works, a number of bands are reassigned in this one. These include the important, structure-sensitive band at 390 cm(-1), which is reassigned here to the totally symmetric nu(breathing)(Fe-N) vibration for complex 1. This is in agreement with the assignments for [Ni(TPP)]. In general, the assignments are on the basis of an idealized [M(TPP)]+ core with D(4h) symmetry. In this Work, small deviations from D(4h) are observed in the vibrational spectra and analyzed in detail. On the basis of the assignments of the vibrational spectra of 1, [Mn(TPP)(Cl)] (2), and diamagnetic [Co(TPP)(Cl)] (3), eight metal-sensitive bands are identified. Two of them correspond to the nu(M-N) stretching modes with B(1g) and Eu symmetries and are assigned here for the first time. The shifts of the metal sensitive modes are interpreted on the basis of differences in the porphyrin C-C, C-N, and M-N distances. Besides the porphyrin core vibrations, the M-Cl stretching modes also show strong metal sensitivity. The strength of the M-Cl bond in 1-3 is further investigated. From normal coordinate analysis (NCA), force constants of 1.796 (Fe), 0.932 (Mn), and 1.717 (Co) mdyn/A are obtained for 1-3, respectively. The weakness of the Mn-Cl bond is attributed to the fact that it only corresponds to half a sigma bond. Finally, RR spectroscopy is used to gain detailed insight into the nature of the electronically excited states. This relates to the mechanism of resonance enhancement and the actual nature of the enhanced vibrations. It is of importance that anomalous polarized bands (A(2g) vibrations), which are diagnostic for vibronic mixing, are especially useful for this purpose.     

Redox properties of ruthenium nitrosyl porphyrin complexes with different axial ligation: structural, spectroelectrochemical (IR, UV-visible, and EPR), and theoretical studies

Experimental and computational results for different ruthenium nitrosyl porphyrin complexes [(Por)Ru(NO)(X)] ( n+ ) (where Por (2-) = tetraphenylporphyrin dianion (TPP (2 (-) )) or octaethylporphyrin dianion (OEP (2-)) and X = H 2O ( n = 1, 2, 3) or pyridine, 4-cyanopyridine, or 4- N,N-dimethylaminopyridine ( n = 1, 0)) are reported with respect to their electron-transfer behavior. The structure of [(TPP)Ru(NO)(H 2O)]BF 4 is established as an {MNO} species with an almost-linear RuNO arrangement at 178.1(3) degrees . The compound [(Por)Ru(NO)(H 2O)]BF 4 undergoes two reversible one-electron oxidation processes. Spectroelectrochemical measurements (IR, UV-vis-NIR, and EPR) indicate that the first oxidation occurs on the porphyrin ring, as evident from the appearance of diagnostic porphyrin radical-anion vibrational bands (1530 cm (-1) for OEP (*-) and 1290 cm (-1) for TPP (*-)), from the small shift of approximately 20 cm (-1) for nu NO and from the EPR signal at g iso approximately 2.00. The second oxidation, which was found to be electrochemically reversible for the OEP compound, shows a 55 cm (-1) shift in nu NO, suggesting a partially metal-centered process. The compounds [(Por)Ru(NO)(X)]BF 4, where X = pyridines, undergo a reversible one-electron reduction. The site of the reduction was determined by spectroelectrochemical studies to be NO-centered with a ca. -300 cm (-1) shift in nu NO. The EPR response of the NO (*) complexes was essentially unaffected by the variation in the substituted pyridines X. DFT calculations support the interpretation of the experimental results because the HOMO of [(TPP)Ru(NO)(X)] (+), where X = H 2O or pyridines, was calculated to be centered at the porphyrin pi system, whereas the LUMO of [(TPP)Ru(NO)(X)] (+) has about 50% pi*(NO) character. This confirms that the (first) oxidation of [(Por)Ru(NO)(H 2O)] (+) occurs on the porphyrin ring wheras the reduction of [(Por)Ru(NO)(X)] (+) is largely NO-centered with the metal remaining in the low-spin ruthenium(II) state throughout. The 4% pyridine contribution to the LUMO of [(TPP)Ru(NO)(py)] (+) is correlated with the stability of the reduced form as opposed to that of the aqua complex.     

Structural, NMR, and EPR studies of S = (1)/(2) and S = (3)/(2) Fe(III) bis(4-cyanopyridine) complexes of dodecasubstituted porphyrins

The NMR and EPR spectra for three complexes, iron(III) octamethyltetraphenylporphyrin bis(4-cyanopyridine) perchlorate, [FeOMTPP(4-CNPy)(2)]ClO(4), and its octaethyl- and tetra-beta,beta'-tetramethylenetetraphenylporphyrin analogues, [FeOETPP(4-CNPy)(2)]ClO(4) and [FeTC(6)TPP(4-CNPy)(2)]ClO(4), are presented. The crystal structures of two different forms of [FeOETPP(4-CNPy)(2)]ClO(4) and one form of [FeOMTPP(4-CNPy)(2)]ClO(4) are also reported. Attempts to crystallize [FeTC(6)TPP(4-CNPy)(2)]ClO(4) were not successful. The crystal structure of [FeOMTPP(4-CNPy)(2)]ClO(4) reveals a saddled porphyrin core, a small dihedral angle between the axial ligand planes, 64.3 degrees, and an unusually large tilt angle (24.4 degrees ) of one of the axial 4-cyanopyridine ligands with respect to the normal to the porphyrin mean plane. There are 4 and 2 independent molecules in the asymmetric units of [FeOETPP(4-CNPy)(2)]ClO(4) crystallized from CD(2)Cl(2)/dodecane (1-4) and CDCl(3)/cyclohexane (5-6), respectively. The geometries of the porphyrin cores in 1-6 vary from purely saddled to saddled with 15% ruffling admixture. In all structures, the Fe-N(p) distances (1.958-1.976 A) are very short due to strong nonplanar distortion of the porphyrin cores, while the Fe-N(ax) distances are relatively long ( approximately 2.2 A) compared to the same distances in S = (1)/(2) bis(pyridine)iron(III) porphyrin complexes. An axial EPR signal is observed (g( perpendicular ) = 2.49, g( parallel ) = 1.6) in frozen solutions of both [FeOMTPP(4-CNPy)(2)]ClO(4) and [FeTC(6)TPP(4-CNPy)(2)]ClO(4) at 4.2 K, indicative of the low spin (LS, S = (1)/(2)), (d(yz)d(xz))(4)(d(xy))(1) electronic ground state for these two complexes. In agreement with a recent publication (Ikeue, T.; Ohgo, Y.; Ongayi, O.; Vicente, M. G. H.; Nakamura, M. Inorg. Chem. 2003, 42, 5560-5571), the EPR spectra of [FeOETPP(4-CNPy)(2)]ClO(4) are typical of the S = (3)/(2) state, with g values of 5.21, 4.25, and 2.07. A small amount of LS species with g = 3.03 is also present. However, distinct from previous conclusions, large negative phenyl-H shift differences delta(m) - delta(o) and delta(m) - delta(p) in the (1)H NMR spectra indicate significant negative spin density at the meso-carbons, and the larger than expected positive average CH(2) shifts are also consistent with a significant population of the S = 2 Fe(II), S = (1)/(2) porphyrin pi-cation radical state, with antiferromagnetic coupling between the metal and porphyrin unpaired electrons. This is the first example of this type of porphyrin-to-metal electron transfer to produce a partial or complete porphyrinate radical state, with antiferromagnetic coupling between metal and macrocycle unpaired electrons in an iron porphyrinate. The kinetics of ring inversion were studied for the [FeOETPP(4-CNPy)(2)]ClO(4) complex using NOESY/EXSY techniques and for the [FeTC(6)TPP(4-CNPy)(2)]ClO(4) complex using DNMR techniques. For the former, the free energy of activation, deltaG, and rate of ring inversion in CD(2)Cl(2) extrapolated to 298 K are 63(2) kJ mol(-)(1) and 59 s(-)(1), respectively, while for the latter the rate of ring inversion at 298 K is at least 4.4 x 10(7) s(-)(1), which attests to the much greater flexibility of the TC(6)TPP ring. The NMR and EPR data are consistent with solution magnetic susceptibility measurements that show S = (3)/(2) in the temperature range from 320 to 180 K for [FeOETPP(4-CNPy)(2)](+), while both [FeOMTPP(4-CNPy)(2)](+) and [FeTC(6)TPP(4-CNPy)(2)](+) change their spin state from S = (3)/(2) at room temperature to mainly LS (S = (1)/(2)) upon cooling to 180 K.     

Solid-state synthesis of molybdenum and tungsten porphyrins and aerial oxidation of coordinated benzenethiolate to benzenesulfonate

A new route is developed for the synthesis of molybdenum and tungsten porphyrins using [M(NO)(2)py(2)Cl(2)] (M = Mo, W) as the metal source and TPP (dianion of 5,10,15,20-meso-tetraphenylporphyrin) in the benzoic acid melt. Complexes [Mo(V)O(TPP)(OOCPh)] (1) and [W(V)O(TPP)(OOCPh)] (2) are isolated in almost quantitative yield. These are characterized by single-crystal X-ray structure analysis, electron paramagnetic resonance, electronic and IR spectroscopy, and magnetic moment measurements. Benzenethiol substitutes for PhCOO(-) in 1, forming an intermediate thiolato complex that responds to the intramolecular redox reaction across the Mo(V)-SPh bond to yield [Mo(IV)O(TPP)] (3). Under an excess of benzenethiol, PhS(-) is coordinated to the vacant site in 3, which under aerial oxidation is oxidized to benzenesulfonate to form [Mo(V)O(TPP)(O(3)SPh)] (4). 2 undergoes similar aerial oxidation chemistry albeit slowly.     

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.     

Zirconium and hafnium (1-pyridinio)imido complexes: functionalized terminal hydrazinediido analogues

Reaction of the diamidozirconium complex [Zr(N2(TBS)Npy)(NMe2)2] (1) (N2(TBS)Npy = CH3C(C5H4N)(CH2NSiMe2tBu)2) or the diamidohafnium complex [Hf(N2(TBS)Npy)(NMe2)2] (2) with one molar equiv. of 1-aminopyridinium triflate in the presence of one equiv. of pyridine gave the corresponding (1-pyridinio)imido complexes [Zr(N2(TBS)Npy)(=N-NC5H5)(OTf)(py)] (3) and [Hf(N2(TBS)Npy)(=N-NC5H5)(OTf)(py)] (4). These were converted to the acetylide complexes [Zr(N2(TBS)Npy)(=N-NC5H5)(CCPh)(py)] (5) and [Hf(N2(TBS)Npy)(=N-NC5H5)(CCPh)(py)] (6) by reaction with lithium phenylacetylide and substitution of the triflato ligand. Upon reaction of 3 and 4 with one molar equivalent of R-NC (R = tBu, Cy, 2,6-xyl), N-N bond cleavage in the (1-pyridinio)imido unit took place and the respective carbodiimido complexes [M(N2(TBS)Npy](N=C=NR)(OTf)(py)] (7-12) were formed instantaneously. A similar type of reaction with CO gave the isocyanato complex [Zr(N2(TBS)Npy](NCO)(OTf)(py)] (13). Finally, the abstraction of the pyridine ligand in compounds 3 and 4 with B(C6F5)3 led to the formation of the triflato-bridged dinuclear complexes [Zr(N2(TBS)Npy)(=N-NC5H5)(OTf)]2 (14) and [Hf(N2(TBS)Npy)(=N-NC5H5)(OTf)]2 (15).     

Time-resolved electron spin resonance of gallium and germanium porphyrins in the excited triplet state

Gallium and germanium porphyrin complexes in the lowest excited triplet (T1) state have been studied by time-resolved electron spin resonance (TRESR). It is found that for Ge(TPP)(OH)2 (TPP = dianion of tetraphenylporphyrin) intersystem crossing (ISC) from the lowest excited singlet (S1) state to the T1x and T1y sublevels is faster than that to the T1z sublevel (T1x, T1y, and T1z are sublevels of the T1 state), while the ISC of ZnTPP and Ga(TPP)(OH) is selective to the T1z sublevel. This is interpreted by a weak interaction between the dpi orbital of germanium and LUMO (eg) of the porphyrin ligand, resulting in small spin-orbit coupling (SOC). The interpretation is supported by molecular orbital calculations. The ISC of Ge(OEP)(OH)2 (OEP = dianion of octaethylporphyrin) and Ge(Pc)(OH)2 (Pc = dianion of tetra-tert-butylphthalocyanine) is found to be selective to the T1z sublevel in contrast to Ge(TPP)(OH)2. This dependence on the porphyrin ligand is reasonably explained by a difference between the 3(a(1u)eg) (the OEP and Pc complexes) and 3(a(2u)eg) (the TPP complex) configurations. This is the first observation of a difference in selective ISC between the 3(a(1u)eg) and 3(a(2u)eg) configurations. The TRESR spectrum of Ge(TPP)Br2 is different from those of Ge(TPP)Cl2 and Ge(TPP)(OH)2, and is interpreted by SOC between the T1 and T2 states. From ESR parameters the square of the coefficient of the eg orbital on bromine is evaluated as 0.018 in the T1 state.     

Cooperative ligation,back-bonding,and possible pyridine-pyridine interactions in tetrapyridine-vanadium(II): a visible and X-ray spectroscopic study

The binding of pyridine by V(II) in aqueous solution shows evidence for the late onset of cooperativity. The K(1) governing formation of [V(py)](2+) (lambda(max) = 404 nm, epsilon(max) = 1.43 +/- 0.3 M(-1) cm(-1)) was determined spectrophotometrically to be 11.0 +/- 0.3 M(-)(1), while K(1) for isonicotinamide was found to be 5.0 +/- 0.1 M(-1). These values are in the low range for 3d M(2+) ions and indicate that V(II).py back-bonding is not significant in the formation of the 1:1 complex. Titration of 10.5 mM V(II) with pyridine in aqueous solution showed an absorption plateau at about 1 M added pyridine, indicating a reaction terminus. Vanadium K-edge EXAFS analysis of 63 mM V(II) in 2 M pyridine solution revealed six first-shell N/O ligands at 2.14 A and 4 +/- 1 pyridine ligands per V(II). UV/vis absorption spectroscopy indicated that the same terminal V(II) species was present in both experiments. Model calculations showed that in the absence of back-bonding only 2.0 +/- 0.2 and 2.4 +/- 0.2 pyridine ligands would be present, respectively. Cooperativity in multistage binding of pyridine by [V(aq)](2+) is thus indicated. XAS K-edge spectroscopy of crystalline [V(O(3)SCF(3))(2)(py)(4)] and of V(II) in 2 M pyridine solution each exhibited the analogous 1s --> (5)E(g) and 1s --> (5)T(2g) transitions, at 5465.5 and 5467.5 eV, and 5465.2 and 5467.4 eV, respectively, consistent with the EXAFS analysis. In contrast, [V(py)(6)](PF(6))(2) and [V(H(2)O)(6)]SO(4) show four 1s --> 3d XAS transitions suggestive of a Jahn-Teller distorted excited state. Comparison of the M(II)[bond]N(py) bond lengths in V(II) and Fe(II) tetrapyridines shows that the V(II)[bond]N(py) distances are about 0.06 A shorter than predicted from ionic radii. For [VX(2)(R-py)(4)] (X = Cl(-), CF(3)SO(3)(-); R = 4-Et, H, 3-EtOOC), the E(1/2) values of the V(II)/V(III) couples correlate linearly with the Hammett sigma values of the R group. These findings indicate that pi back-bonding is important in [V(py)(4)](2+) even though absent in [V(py)](2+). The paramagnetism of [V(O(3)SCF(3))(2)(py)(4)] in CHCl(3), 3.8 +/- 0.2 mu(B), revealed that the onset of back-bonding is not accompanied by a spin change. Analysis of the geometries of V(II) and Fe(II) tetrapyridines indicates that the ubiquitous propeller motif accompanying tetrapyridine ligation may be due to eight dipole interactions arising from the juxtaposed C-H edges and pi clouds of adjoining ligands, worth about -6 kJ each. However, this is not the source of the cooperativity in the binding of multiple pyridines by V(II) because the same interactions are present in the Fe(II)-tetrapyridines, which do not show cooperative ligand binding. Cooperativity in the binding of pyridine by V(II) is then assigned by default to V(II)-pyridine back-bonding, which emerges only after the first pyridine is bound.     

Metal- and ligand-centered monoelectronic oxidation of mu-nitrido[((tetraphenylporphyrinato)manganese)phthalocyaninatoiron)], [(TPP)Mn-N-FePc]. X-ray crystal structure of the Fe(IV)-containing species [(THF)(TPP)Mn-N-FePc(H(2)O)](I(5))02THF

The reaction of mu-nitrido[((tetraphenylporphyrinato)manganese)(phthalocyaninatoiron)], [(TPP)Mn-N-FePc], with I(2) in THF develops with the formation of two different species, i.e., [(THF)(TPP)Mn-N-FePc(H(2)O)](I(5)).2THF (I) and [(TPP)Mn(IV)-N-Fe(III)Pc](I(3)) (II). On the basis of single-crystal X-ray work and M?ssbauer, EPR, Raman, and magnetic susceptibility data, I, found to be isostructural with the corresponding Fe-Fe complex, is shown to contain a low-spin triatomic Mn(IV)=N=Fe(IV) system (metal-centered oxidation). Data at hand for II (M?ssbauer, EPR, Raman) show, instead, that oxidation takes place at one of the two macrocycles, very likely TPP (ligand-centered oxidation). The same cationic fragment present in I, and containing the Mn(IV)=N=Fe(IV) bond system, is also obtained when (TPP)Mn-N-FePc is allowed to react in THF with (phen)SbCl(6) (molar ratio 1:1). There are indications that the use of (phen)SbCl(6) in excess (2:1 molar ratio), in benzene, probably determines further oxidation with the formation of a species showing the combined presence of the Mn(IV)-Fe(IV) couple and of a pi-cation radical.