Some novel manganese(III) porphyrins with catalytic properties

Abstract

A series of manganese(III) porphyrins with 4-methylimidazole have been prepared. These are high-spin complexes having general formula [Mn^III(THMPP)X(4-MeIm)], where THMP?=?5,10,15,20-tetra(4-hydroxy-3-methoxyphenyl)porphine ligand, X?=?Cl^?, Br^?, NCS^?, or N₃^? and 4-MeIm?=?4-methylimidazole. All the complexes have been characterized by UV-visible, FT-IR, ESI-MS spectra, elemental analyses and magnetic susceptibility measurements. These manganese(III) porphyrins oxidize aromatic alcohols to aldehydes. The oxidation reactions have been carried out at room temperature in the presence of oxidants such as NaIO₄, H₂O₂, and NaOCl. The comparative studies proved that NaIO₄ behaves as the most efficient oxidant in these oxidative transformation reactions.     

Low-temperature magnetisation study on manganese(III) porphyrins

Average magnetisation between 2 and 20 K and 10 and 50 kOe of two typical high-spin manganese(III) porphyrins, namely TPPMnCI and TPPMnCl(py), is reported for the first time. The results have been analysed in terms of spin hamiltonian formalism including both the crystal field and magnetic exchange.     

Magnetic behaviour of spin-mixed iron(III) porphyrins

Detailed average magnetic susceptibility (295-4.2 K) and average magnetisation (20-2 K, 50-10 kOe) are reported for two novel spin-mixed iron(III) porphyrins, namely Fe(TPP)ClO4 and Fe(OEP)ClO4. The results confirm an S = 3/2 ground state substantially spin-mixed with a low-lying S = 5/2 state, but do not agree in detail with the crystal-field model of Maltempo.     

Crystal structure and electronic and magnetic properties of hexacyanoosmate(III)

The [Os(III)(CN)6]3- anion is prepared by chemical oxidation in aqueous solution and isolated as yellow prisms of [Ph4P]3[Os(III)(CN)6].6H2O (1). This species crystallizes in the triclinic space group P with cell parameters a = 13.7609(11) A, b = 16.2275(13) A, c = 17.0895(14) A, alpha = 91.4040(10) degrees , beta = 109.3600(10) degrees , gamma = 102.3970(10) degrees , V = 3497.4(5) A(3), and Z = 2. The slightly distorted octahedral moiety displays Os-C and C-N bond lengths that average 2.058 and 1.146 A, respectively. Spin-orbit-coupling splitting of the ground-state term dominates the NIR region of the electronic spectrum and the magnetic behavior of 1. The experimental information points to higher spin delocalization over the coordinated cyanides than in [Fe(III)(CN)6]3-.     

Studies on iron(III) and manganese(III) derivatives of new meso-aryl substituted and brominated porphyrins

A series of Fe^III and Mn^III porphyrins with various tolyl and naphthyl substituents at the meso positions, and their perbromoderivatives with Br substituents at the -pyrrole positions, have been synthesised and investigated. As seen in the case of the free-base porphyrins, both Fe^III and Mn^III derivatives of the Br-substituted porphyrins also exhibit pronounced red-shifts in both B and Q bands compared to their nonbrominated analogues. This is attributed to the electron-withdrawing ability of eight Br substituents at -pyrrole positions and is also due to distortion brought about in the -framework by the bulky substituents including those at the meso positions. The naphthyl groups seem to be making mesomeric contributions for both nonbrominated and brominated porphyrins of these metal ions as is evident from the higher wavelength absorption of the B band as compared to the tolyl derivatives. While the meso-substituent do not exhibit any isomer dependent change on the electronic properties of Fe^III porphyrins, they show a noticeable effect in the Mn^III derivatives. During the metallation of meso-tetratolylporphyrins by Fe^III ions -oxo dimeric compounds are formed, while the naphthyl porphyrins and the bromoderivatives do not form such dimeric species. The presence of bulky groups at the meso positions and heavy bromines on the -pyrrole positions can be considered to prevent the formation of catalytically inert -oxo dimers.     

The linear dichroism spectra of free base and manganese(III) porphyrins

Linear dichroism of tetraphenylporphyrin and mesoporphyrin IX dimethyl ester are reported together with their manganese (III) derivatives, using both stretched polythene films and liquid crystals as the orienting solvents. All observed electronic transitions in the range 350–900 nm are found to be polarised in the plane of the porphyrin ring.     

Magnetic properties of a manganese(III) Chain with monoatomic bridges: catena-MnF(salen)

In the solid state, MnF(salen) forms chains wherein fairly linear fluoride bridges between high-spin Mn(III) centers are observed. We interpret the magnetic properties of these chains by use of the classical Fisher model and by use of the high-temperature expansion approach, as well as by exact matrix diagonalization of the spin Hamiltonian, of model rings. In solution, electron paramagnetic resonance shows the chains to be symmetrically cleaved to monomeric MnF(salen).     

Sc(III) porphyrins. The molecular structure of two Sc(III) porphyrins and a re-evaluation of the parameters for the molecular mechanics modelling of Sc(III) porphyrins

The crystal structures of two new Sc(III) porphyrins, [Sc(TPP)Cl]·2.5(1-chloronaphthalene), (5,10,15,20-tetraphenylporphyrin)-chloro-scandium(III)·2.5(1-chloronaphthalene) solvate, (Mo Kα, 0.71073 Å, triclinic system  = 9.9530(2) Å, b = 15.4040(3) Å, c = 17.7770(3) Å, α = 86.5190(10)°, β = 89.7680(10)°, γ = 86.9720(10)°, 13101 independent reflections, R₁ = 0.0712) and the dimeric [μ₂-(OH)₂(Sc(TPP))₂], bis-(μ-hydroxo)-(5,10,15,20-tetraphenylporphyrin) scandium(III) (Mo Kα, 0.71073 Å, monoclinic system C2, a = 24.2555(16) Å, b = 11.1598(7) Å, c = 25.6468(17) Å, β = 91.980(2)°, 13084 independent reflections, R₁ = 0.0485) are reported. In [Sc(TPP)Cl] the metal is five-coordinate and the porphyrin is domed with the metal displaced by 0.63 Å from the mean porphyrin towards the axial Cl⁻ ligand. The average Sc-N bond length is 2.143(3) Å, which is shorter than the average bond length of previously reported structures. Two of the phenyl rings are nearly orthogonal to the porphyrin core and the other two are significantly tilted because of contacts with 1-chloronaphthalene solvent molecules, and the phenyl rings of neighbouring porphyrins. In [μ₂-(OH)₂(Sc(TPP))₂] both porphyrins are domed, with the metal displaced from the mean porphyrin plane towards the bridging hydroxo ligands. The average Sc-N bond length is 2.197(12) Å, which is in the upper range of Sc-N bond lengths in known Sc(III) porphyrins but not dissimilar to the average Sc-N bond lengths in another other bis-μ₂-hydroxo Sc(III) porphyrin, [μ₂-(OH)₂(Sc(OEP))₂]. One porphyrin is rotated relative to the upper porphyrin by 25° due to steric contacts between the phenyl substituents. We have used these new structures to re-evaluated our previously reported molecular mechanics force field parameters for modelling Sc(III) porphyrins using the MM2 force field; the training set was augmented from two to seven structures by using all available Sc(III) porphyrin structures and the two new structures. The modelling reproduces the porphyrin core very accurately; bond lengths are reproduced to within 0.01 Å, bond angles to within 0.5° and torsional angles to within 2°. The optimum parameters for modelling the Sc(III)-N bond lengths, determined by finding the minimum difference between the crystallographic and modelling mean bond lengths with the aid of artificial neural network architectures, were found to be 0.90 ± 0.03 mdyn Å⁻¹ for the bond force constant and2.005 ± 0.005 Å for the strain-free bond length. Modelling the seven Sc(III) porphyrins with the new parameters gives an average Sc-N bond length of 2.182 ± 0.018 Å, indistinguishable from the crystallographic mean of 2.181 ± 0.024 Å.     

Magnetic resonance spectroscopic investigations of the electronic ground and excited states in strongly nonplanar iron(III) dodecasubstituted porphyrins

A series of axially ligated complexes of iron(III) octamethyltetraphenylporphyrin, (OMTPP)Fe(III), octaethyltetraphenylporphyrin, (OETPP)Fe(III), its perfluorinated phenyl analogue, (F(20)OETPP)Fe(III), and tetra-(beta,beta'-tetramethylene)tetraphenylporphyrin, (TC(6)TPP)Fe(III), have been prepared and characterized by (1)H NMR spectroscopy: chloride, perchlorate, bis-4-(dimethylamino)pyridine, bis-1-methylimidazole, and bis-cyanide. Complete spectral assignments have been made using 1D and 2D techniques. The temperature dependences of the proton resonances of the complexes show significant deviations from simple Curie behavior and evidence of ligand exchange, ligand rotation, and porphyrin ring inversion at ambient temperatures. At temperatures below the point where dynamics effects contribute, the temperature dependences of the proton chemical shifts of the complexes could be fit to an expanded version of the Curie law using a temperature-dependent fitting program developed in our laboratory that includes consideration of a thermally accessible excited state. The results show that, although the ground state differs for various axial ligand complexes and is usually fully consistent with that observed by EPR spectroscopy at 4.2 K, the excited state often has S = (3)/(2) (or S = (5)/(2) in the cases where the ground state has S = (3)/(2)). The EPR spectra (4.2 K) of bis-4-(dimethylamino)pyridine and bis-1-methylimidazole complexes show "large-g(max)" signals with g(max) = 3.20 and 3.12, respectively, and the latter also shows a normal rhombic EPR signal, indicating the presence of low-spin (LS) (d(xy))(2)(d(xz),d(yz))(3) ground states for both. The bis-cyanide complex also yields a large-g(max) EPR spectrum with g = 3.49 and other features that could suggest that some molecules have the (d(xz),d(yz))(4)(d(xy))(1) ground state. The EPR spectra of all five-coordinate chloride complexes have characteristic features of predominantly S = (5)/(2) ground-state systems with admixture of 1-10% of S = (3)/(2) character.     

Synthesis, structure, and properties of low-spin manganese(III)-poly(pyrazolyl)borate complexes

The manganese(III)-bis[poly(pyrazolyl)borate] complexes, Mn(pzb)2SbF6, where pzb- = tetrakis(pyrazolyl)borate (pzTp) (1), hydrotris(pyrazolyl)borate (Tp) (2), or hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) (3), have been synthesized by oxidation of the corresponding Mn(pzb)2 compounds with NOSbF6. The Mn(III) complexes are low-spin in solution and the solid state (microeff = 2.9-3.8 microB). X-ray crystallography confirms their uncommon low-spin character. The close conformity of mean Mn-N distances of 1.974(4), 1.984(5), and 1.996(4) A in 1, 2, and 3, respectively, indicates absence of the characteristic Jahn-Teller distortion of a high-spin d4 center. N-Mn-N bite angles of slightly less than 90 degrees within the facially coordinated pzb- ligands produce a small trigonal distortion and effective D3d symmetry in 1 and 2. These angles increase to 90.0(4)degrees in 3, yielding an almost perfectly octahedral disposition of N donors in Mn(Tp*)2+. Examination of structural data from 23 metal-bis(pzb) complexes reveals systematic changes within the metal-(pyrazolyl)borate framework as the ligand is changed from pzTp to Tp to Tp*. These deformations consist of significant increases in M-N-N, N-B-N, and N-N-B angles and a minimal increase in Mn-N distance as a consequence of the steric demands of the 3-methyl groups. Less effective overlap of pyrazole lone pairs with metal atom orbitals resulting from the M-N-N angular displacement is suggested to contribute to the lower ligand field strength of Tp* complexes. Mn(pzb)2+ complexes undergo electrochemical reduction and oxidation in CH3CN. The electrochemical rate constant (ks,h) for reduction of t2g4 Mn(pzb)2+ to t2g3eg2 Mn(pzb)2 (a coupled electron-transfer and spin-crossover reaction) is 1-2 orders of magnitude smaller than that for oxidation of t2g4 Mn(pzb)2+ to t2g3 Mn(pzb)22+. ks,h values decrease as Tp* > pzTp > Tp for the Mn(pzb)2+/0 electrode reactions, which contrasts with the behavior of the comparable Fe(pzb)2+/0 and Co(pzb)2+/0 couples.     

Kinetics and mechanism of decomposition of hydrogen peroxide in the presence of manganese(III) porphyrins

The kinetics of homogeneous decomposition of hydrogen peroxide in the presence of manganese complexes with anionic ligands and various aromatic macrocycles were studied by the volumetric method. Ionmolecular mechanism was proposed on the basis of spectrophotometric data for catalytic decomposition of hydrogen peroxide with participation of manganese(III) porphyrins. The catalytic activity of the porphyrin complexes was higher by a factor of 1.5–3 than the activity of the corresponding solvate complexes with anionic ligands. The catalytic activity of porphyrin manganese complexes can be controlled by variation of the electronic structure of the macroring and the nature of anionic ligand coordinated at the apical position.     

Spectral and electrochemical properties of deformed short-chain basket-handle iron (III) porphyrins

The spectral and electrochemical properties of series of deformed iron(III) derivatives of short-chain basket-handle porphyrins are reported. 1HNMR and optical studies demonstrated that deformation arises because of the bridging of opposite phenyl groups by short chains. Axial ligation studies indicate that the straps present over both sides of the porphyrin ring do not prevent the entry of small ligands. Electrochemistry in four different solvents suggests the stabilisation of Fe(II) and Fe(I) over Fe(III) in strong coordinating solvents.     

Synthesis, characterization, and physicochemical properties of manganese(III) and manganese(V)-oxo corrolazines

The structural and physicochemical properties of the manganese-corrolazine (Cz) complexes (TBP8Cz)Mn(V)O (1) and (TBP8Cz)Mn(III) (2) (TBP = p-tert-butylphenyl) have been determined. Recrystallization of 2 from toluene/MeOH resulted in the crystal structure of (TBP8Cz)Mn(III)(CH3OH) (2 x MeOH). The packing diagram of 2 x MeOH reveals hydrogen bonds between MeOH axial ligands and meso N atoms of adjacent molecules. Solution binding studies of 2 with different axial ligands (Cl-, Et3PO, and Ph3PO) reveal strong binding, corroborating the preference of the Mn(III) ion for a five-coordinate environment. High-frequency and field electron paramagnetic resonance (HFEPR) spectroscopy of solid 2 x MeOH shows that 2 x MeOH is best described as a high-spin (S = 2) Mn(III) complex with zero-field splitting parameters typical of corroles. Structural information on 1 was obtained through an X-ray absorption near-edge structure (XANES)/extended X-ray absorption fine structure (EXAFS) study and compared to XANES/EXAFS data for 2 x MeOH. The XANES data for 1 shows an intense pre-edge transition characteristic of a high-valent metal-oxo species, and a best fit of the EXAFS data gives a short Mn-O bond distance of 1.56 A, confirming the structure of the metal-oxo unit in 1. Detailed spectroelectrochemical studies of 1 and 2 were performed revealing multiple reversible redox processes for both complexes, including a relatively low potential for the Mn(V) --> Mn(IV) process in 1 (near 0.0 V vs saturated calomel reference electrode). Chemical reduction of 1 results in the formation of a Mn(III)Mn(IV)(mu-O) dimer as characterized by electron paramagnetic resonance spectroscopy.     

Synthesis,crystal structure and magnetic properties of a dinuclear manganese(III) tetradentate Schiff-base complex

A dinuclear manganese(III) tetradentate Schiff-base complex, [Mn₂(salophen)₂(4,4′-bipy)₃](BPh₄)₂ (1) (salophen = N,N′-o-phenylene-bis(salicylideneaminato)), has been synthesized and structurally characterized. Compound 1 crystallized in the triclinic, P 1 space group, a = 13.431(4), b = 13.791(4), c = 13.886(4) Å, α = 73.599(5)°, β = 80.410(6)°, γ = 71.241(5)°, V = 2328.3(12) ų. Complex 1 contains two Mn(salophen) moieties bridged by 4,4′-bipy to form a dinuclear unit, with two terminal 4,4′-bipy ligands. Variable temperature magnetic susceptibility (2–300 K) shows very weak ferromagnetic interactions between the Mn(III) ions.     

Theoretical investigations of the electronic states of porphyrins. III. Low-lying electronic states of porphinatoiron(II)

Ab initio configuration interaction calculations are reported on the lowest quintet, triplet, and singlet states of Fe^II(P). Due to the large number of states found, a catalog of the low-lying states is presented. Novel triplet and quintet charge-transfer states are reported as low as 1.3 eV. These states are d⁵ (S = 5/2) on the iron low-spin-coupled to the radical anion excited porphyrin ring (S = 1/2 or 3/2). Oscillator strengths originating from each of three low-energy triplet states are reported.     

Monolayers as models for supported catalysts: zirconium phosphonate films containing manganese(III) porphyrins

Organized monolayer films of a manganese tetraphenylporphyrin have been prepared and used as supported oxidation catalysts. Manganese 5,10,15,20-tetrakis(tetrafluorophenyl-4'-octadecyloxyphosphonic acid) porphyrin (1) has been immobilized as a monolayer film by a combination of Langmuir-Blodgett (LB) and self-assembled monolayer techniques that use zirconium phosphonate linkages to bind the molecule to the surface. Analysis by FTIR, XPS, UV-vis and polarized optical spectroscopy show that the films consist of noninteracting molecules effectively anchored and oriented nearly parallel to the surface. The monolayer films are stable to the solvent and temperature conditions needed to explore organic oxidations. The activity of films of 1 toward the epoxidation of cyclooctene using iodosylbenzene as the oxidant was compared to that of Manganese 5,10,15,20-tetrakis(pentafluorophenyl) porphyrin (2) and 1 under equivalent homogeneous conditions. The immobilized porphyrin 1 shows an enhanced activity relative to either homogeneous reaction. The main difference between 1 and 2 is the four alkyl phosphonate arms in 1 designed to incorporate the porphyrin within the films. The increased activity of immobilized 1 is a combination of the porphyrin structure, which prohibits the formation of mu-oxo dimers even in solution, and a change in conformation when anchored to the surface. The study demonstrates that careful monolayer studies can provide useful models for the design and study of supported molecular catalyst systems.     

Structure and magnetic properties of (meso-tetraphenylporphinato)manganese(III) bis(dithiolato)nickelates

The crystal structures of [MnTPP]{Ni[S2C2H(CN)]2} [MnTPP = (meso-tetraphenylporphinato)manganese(III)] and [MnTPP]{Ni[S2C2(CN)2]2} have been determined. These salts possess trans-mu-coordination of S = 1/2 {Ni[S2C2H(CN)]2}*- and {Ni[S(2)C(2)(CN)(2)](2)}*- to Mn(III) and form parallel 1-D coordination polymer chains exhibiting nu(CN) at 2210 and 2200 and 2220 and 2212 cm(-1), respectively. The bis(dithiolato) monoanions are planar and bridge two cations with MnN distances of 2.339(16), and 2.394(3) A, respectively, which are comparable to related MnN distances observed for [MnTPP][TCNE].x(solvates). In addition, [MnTP'P]{Ni[S2C2(CN)2]2} {H2TP'P = meso-tetrakis[3,5-di-tert-butyl-4-hydroxyphenyl)porphyrin] and [MnTP'P(OH2)]{Ni[S2C2(CN)2]2} were prepared. The latter forms isolated paramagnetic ions. The room-temperature values of chiT for 1-D [MnTPP]{Ni[S2C2H(CN)]2}, [MnTPP]{Ni[S2C2(CN)2]2}, and [MnTP'P]{Ni[S2C2(CN)2]2} are 2.55, 3.28, and 2.86 emu K/mol, respectively. Susceptibility (chi) measurements between 2 and 300 K reveal weak antiferromagnetic interactions with theta= -5.9 and -0.2 K for [MnTPP]{Ni[S(2)C(2)H(CN)](2)} and [MnTPP]{Ni[S2C2(CN)2]2}, respectively, and stronger antiferromagnetic coupling of -50 K for [MnTP'P]{Ni[S2C2(CN)2]2} from fits of chi(T) to the Curie-Weiss law. The 1-D intrachain coupling, J(intra), of [MnTPP]{Ni[S2C2H(CN)]2} and [MnTPP]{Ni[S2C2(CN)2]2} was determined from modeling chiT(T) by the Seiden expression (H = -2JSi.Sj) with J/kB = -8.00 K (-5.55 cm(-1); -0.65 meV) for [MnTPP]{Ni[S2C2H(CN)]2}, J/kB = -3.00 K (-2.08 cm(-1); -0.25 meV) for [MnTP'P]{Ni[S2C2(CN)2]2}, and J/kB = -122 K (-85 cm(-1)) for [MnTP'P]{Ni[S2C2(CN)2]2}. These observed negative J(intra)/kB values are indicative of antiferromagnetic coupling. These materials order as ferrimagnets at 5.5, 2.3, and 8.0 K, for [MnTPP]{Ni[S2C2H(CN)]2}, [MnTPP]{Ni[S2C2(CN)2]2}, and [MnTP'P]{Ni[S2C2(CN)2]2}, respectively, based upon the temperature at which maximum in the 10 Hz chi'(T) data occurs. [MnTP'P]{Ni[S2C2(CN)2]2} has a coercivity of 17,700 Oe and remanent magnetizations of 7250 emu Oe/mol at 2 K and 17 Oe and 850 emu Oe/mol at 5 K; hence, upon cooling it goes from being a soft magnet to being a very hard magnet.