Metalloporphyrins M

Hill J R ef a/1995 Vibrational relaxation of oarbon monoxide in model heme oompounds 6-ooordinate metalloporphyrins (M = Fe, Ru, Os) Chem. Phys. Lett. 224 218-23... 

Sanders (14) has exploited the strong and selective coordination of phosphine donor groups to Ru(II) to construct hetero-dimetallic porphyrin dimers (17, Fig. 5). An alkyne-phosphine moiety introduced on the periphery of a free base or metalloporphyrin (M = Zn or Ni) spontaneously coordinates to a Ru(II)(CO) porphyrin when the two porphyrins are mixed in a 1 1 ratio. Coordination is characterized by a downfield shift of the 31P resonance (A<531P = 19 ppm). There is no evidence of self-coordination of the zinc porphyrin at 10 6 m in toluene, there is no shift in the Soret band in the UV-Vis absorption spectrum. The Ni-Ru dimer was observed by MALDI-TOF mass spectrometry. Heating the Ru(II)CO porphyrin with 2 equivalents of the phosphine porphyrins led to quantitative formation of trimeric assemblies. 

Ligand substitution reactions of NO leading to metal-nitrosyl bond formation were first quantitatively studied for metalloporphyrins, (M(Por)), and heme proteins a few decades ago (20), and have been the subject of a recent review (20d). Despite the large volume of work, systematic mechanistic studies have been limited. As with the Rum(salen) complexes discussed above, photoexcitation of met allop or phyr in nitrosyls results in labilization of NO. In such studies, laser flash photolysis is used to labilize NO from a M(Por)(NO) precursor, and subsequent relaxation of the non-steady state system back to equilibrium (Eq. (9)) is monitored spectroscopically. 

Alkaline earth metalloporphyrins, M(Por), are obtained by porphyrin metallation with meta perchlorates in pyridine or metal hydroxides in methanol.175 They dissociate under mild conditions even in acetic acid. 

The rate constants (ket) of electron transfer from Fc to [(TPP)M] agree well with those evaluated in light of the Marcus equations [91] for outer-sphere electron transfer (Eq. 4) [215]. Such agreement clearly demonstrates that electron transfer from Fc to [(TPP)M]+ in Scheme 15 proceeds via an outer-sphere pathway. In contrast to this, the ket value of the acid-catalyzed electron transfer from (TPP)Co to O2 is 10 -fold larger than that expected from an outer-sphere electron transfer [215]. Such huge enhancement of the observed rate relative to that calculated for outer-sphere electron transfer indicates the strong inner-sphere nature of acid-catalyzed electron transfer from (TPP)Co to O2 this should result in formation of the hydroperoxo complex, [(TPP)Co02H]+ (Scheme 15, M = Co). Other metalloporphyrins (M = Fe and Mn) can also act as efficient catalysts of the reduction of... 

Trivalent yttrium and lanthanide metals, except for promethium, have been complexed to octaethylporphyrin by heating at 210 °C in an imidazole melt. The complexes obtained as hydroxides, M "(OEP)(OH), are unstable in acidic media. As the charge radius ratio rule predicts, the early lanthanide metalloporphyrins, M "(OEP)(OH) (M = La, Ce, PR, Nd), are demetallated during purification, and the middle series (M = Sm, Eu, Gd, Tb, Dy) in 1% acetic acid in methanol, while the last five (M = Ho, Er, Tm, Yb, Lu) survive in 2% acetic acid in methanol but are dissociated in dilute hydrochloric acid. The M "(OEP)(OH) appears to coordinate more than one equivalent of pyridine and piperidine, and dimerizes in noncoordinating solvents such as benzene and dichloromethane at 10" M concentration. The dimer is considered to be a di-ju-hydroxo-bridged species, different from the p-oxo dimer, Sc" (OEP) 20 (Scheme 6). 

Most a-bonded metalloporphyrins are obtained from the reaction of chloro and per-chlorato metalloporphyrins M(Por)X (n = 1 or 2) with Grignard reagents or with alkyl (aryl) lithium (Scheme 1). The corresponding mono or dialkyl (aryl) derivatives are generally isolated in good yield but this will depend upon the nature of the metal. 

The oxidation of OH by [Fe(CN)6] in solution has been examined. Application of an electrical potential drives the reaction electrochemically, rather than merely generating a local concentration of OH at the anode, as has been suggested previously, to produce both O and [Fe(CN)6] in the vicinity of the same electrode. With high [OH ] or [Fe(CN)6] /[Fe(CN)6] ratio, the reaction proceeds spontaneously with a second-order rate constant of 2.2 x 10 M s at 25 °C. Under anaerobic conditions, iron(III) porphyrin complexes in dimethyl sulfoxide solution are reduced to the iron(II) state by addition of hydroxide ion or alkoxide ions. Excess hydroxide ion serves to generate the hydroxoiron(II) complex. The oxidation of hydroxide and phenoxide ions in acetonitrile has been characterized electrochemically " in the presence of transition metal complexes [Mn(II)L] [M = Fe,Mn,Co,Ni L = (OPPh2)4,(bipy)3] and metalloporphyrins, M(por) [M = Mn(III), Fe(III), Co(II) por = 5,10,15,20-tetraphenylpor-phinato(2-), 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphinato(2-)]. Shifts to less positive potentials for OH and PhO are suggested to be due to the stabilization of the oxy radical products (OH and PhO ) via a covalent bond. Oxidation is facilitated by an ECE mechanism when OH is in excess. 

The electron self-exchange rate constants for several Fe(II)/Fe(III) porphyrin couples have been measured by H NMR line-broadening techniques in 5 1 acetone/water at -20 The relative rate constants for the [Fe(P)(l-MeIm)2] couples, P = octaethylporphyrin chlorin < isobacteriochlorin, have been attributed to differences in outer-sphere reorganization, related to the steric bulk. The rate-determining step in the metallopophyrin-catalyzed reductions of dioxygen by substituted ferrocenes is the electron transfer between the ferrocene and the metalloporphyrin (M = Fe, Co, and The Marcus relationship provides a... 

Paulat F, Praneeth VK et al (2006) Quantum chemistry-based analysis of the vibrational spectra of five-coordinate metalloporphyrins [M (TPP)C1]. Inorg Chem 45 2835-2856... 

Hoffmann, P., G. Labat, A. Robert, and B. Meunier (1990). Highly selective bromina-tion of tetramesitylporphyrin An easy access to robust metalloporphyrins, M-BrsTMP and M-BrgTMPS. Examples of application in catalytic oxygenation and oxidation reactions. Tetrahedron Lett. 31,1991—1994. 

Figure 12 Schematic representation of the construction of porous porphyrinic MOFs with two kinds of metalloporphyrins [M =2H, Pd, Al(OH) or Fe(Cl), = 2H or Mn(Cl)]...

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