Investigations of a MnIV-oxo adduct supported by an N5 ligand with mixed pyridyl and benzimidazolyl ligation uncovers distinct reactivity trends for MnIV-oxo and FeIV-oxo adducts at parity of coordination sphere.

A series of iron(II) benzilate complexes (1-7) with general formula [(L)Fe(benzilate)] have been isolated and characterized to study the effect of supporting ligand (L) on the reactivity of metal-based oxidant generated in the reaction with dioxygen. Five tripodal N ligands (tris(2-pyridylmethyl)amine (TPA in 1), tris(6-methyl-2-pyridylmethyl)amine (6-Me-TPA in 2), N,N-dimethyl-N,N-bis(2-pyridylmethyl)ethane-1,2-diamine (iso-BPMEN in 3), N,N-dimethyl-N,N-bis(6-methyl-2-pyridylmethyl)ethane-1,2-diamine (6-Me-iso-BPMEN in 4), and tris(2-benzimidazolylmethyl)amine (TBimA in 7)) along with two linear tetradentate amine ligands (N,N-dimethyl-N,N-bis(2-pyridylmethyl)ethane-1,2-diamine (BPMEN in 5) and N,N-dimethyl-N,N-bis(6-methyl-2-pyridylmethyl)ethane-1,2-diamine (6-Me-BPMEN in 6)) were employed in the study. Single-crystal X-ray structural studies reveal that each of the complex cations of 1-3 and 5 contains a mononuclear six-coordinate iron(II) center coordinated by a monoanionic benzilate, whereas complex 7 contains a mononuclear five-coordinate iron(II) center. Benzilate binds to the iron center in a monodentate fashion via one of the carboxylate oxygens in 1 and 7, but it coordinates in a bidentate chelating mode through carboxylate oxygen and neutral hydroxy oxygen in 2, 3, and 5. All of the iron(II) complexes react with dioxygen to exhibit quantitative decarboxylation of benzilic acid to benzophenone. In the decarboxylation pathway, dioxygen becomes reduced on the iron center and the resulting iron-oxygen oxidant shows versatile reactivity. The oxidants are nucleophilic in nature and oxidize sulfide to sulfoxide and sulfone. Furthermore, complexes 2 and 4-6 react with alkenes to produce cis-diols in moderate yields with the incorporation of both the oxygen atoms of dioxygen. The oxygen atoms of the nucleophilic oxidants do not exchange with water. On the basis of interception studies, nucleophilic iron(II) hydroperoxides are proposed to generate in situ in the reaction pathways. The difference in reactivity of the complexes toward external substrates could be attributed to the geometry of the O-derived iron-oxygen oxidant. DFT calculations suggest that, among all possible geometries and spin states, high-spin side-on iron(II) hydroperoxides are energetically favorable for the complexes of 6-Me-TPA, 6-Me-iso-BPMEN, BPMEN, and 6-Me-BPMEN ligands, while high spin end-on iron(II) hydroperoxides are favorable for the complexes of TPA, iso-BPMEN, and TBimA ligands.

A m o n o n u c l e a r i r o n ( I I ) c o m p l e x [Fe II (N4Py Me2 )(OTf)](OTf)(1), supported by a new pentadentate ligand, bis(6-methylpyridin-2-yl)-N,N-bis((pyridin-2yl)methyl)methanamine (N4Py Me2 ), has been isolated and characterized. Introduction of methyl groups in the 6-position of two pyridine rings makes the N4Py Me2 a weaker field ligand compared to the parent N4Py ligand. Complex 1 is high-spin in the solid state and converts to [Fe II (N4Py Me2 )(CH 3 CN)]-(OTf) 2 (1a) in acetonitrile solution. The iron(II) complex in acetonitrile displays temperature-dependent spin-crossover behavior over a wide range of temperature. In its reaction with m-CPBA or oxone in acetonitrile at −10 °C, the iron(II) complex converts to an iron(IV)-oxo species, [Fe IV (O)(N4Py Me2 )] 2+ (2). Complex 2 exhibits the Mossbauer parameters δ = 0.05 mm/s and ΔE Q = 0.62 mm/s, typical of N-ligated S = 1 iron(IV)-oxo species. The iron(IV)-oxo complex has a half-life of only 14 min at 25 °C and is reactive toward oxygen-atom-transfer and hydrogen-atom-transfer (HAT) reactions. Compared to the parent complex [Fe IV (O)(N4Py)] 2+ , 2 is more reactive in oxidizing thioanisole and oxygenates the C−H bonds of aliphatic substrates including that of cyclohexane. The enhanced reactivity of 2 toward cyclohexane results from the involvement of the S = 2 transition state in the HAT pathway and a lower triplet-quintet splitting compared to [Fe IV (O)(N4Py)] 2+ , as supported by DFT calculations.The second-order rate constants for HAT by 2 is well correlated with the C−H bond dissociation energies of aliphatic substrates. Surprisingly, the slope of this correlation is different from that of [Fe IV (O)(N4Py)] 2+ , and 2 is more reactive only in the case of strong C−H bonds (>86 kcal/mol), but less reactive in the case of weaker C−H bonds. Using oxone as the oxidant, the iron(II) complex displays catalytic oxidations of substrates with low activity but with good selectivity.

Analysis of extendedX -ray absorption fine structure (EXAFS) data for the Mn IV -oxo complexes [Mn IV (O)( DMM N4py)] 2+ + ,[ Mn IV (O)(2pyN2B)] 2+ + ,a nd [Mn IV (O)(2pyN2Q)] 2+ + ( DMM N4py = N,N-bis(4-methoxy-3,5-dimethyl-2-pyridylmethyl)-N-bis(2-pyridyl)methylamine; 2pyN2B = (N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2pyridylmethyl)amine, and 2pyN2Q = N,N-bis(2-pyridyl)-N,Nbis(2-quinolylmethyl)methanamine) afforded Mn=Oa nd MnÀNb ond lengths.T he Mn=Od istances for [Mn IV (O)( DMM N4py)] 2+ + and [Mn IV (O)(2pyN2B)] 2+ + are 1.72 and 1.70 ,r espectively.I nc ontrast, the Mn=Od istance for [Mn IV (O)(2pyN2Q)] 2+ + was significantly longer( 1.76 ). We attribute this long distance to sample heterogeneity, which is reasonable given the reduced stability of [Mn IV (O)(2pyN2Q)] 2+ + .T he Mn=Od istances for [Mn IV (O)( DMM N4py)] 2+ + and [Mn IV (O)(2pyN2B)] 2+ + could only be well-reproduced using DFT-derived modelst hat included strong hydrogen-bonds between second-sphere solvent 2,2,2-trifluoroethanol molecules and the oxo ligand. These results suggesta ni mportantr olef or the 2,2,2-trifluoroethanol solvent in stabilizing Mn IV -oxo adducts. The DFT methods were extended to investigate the structure of the putative [Mn IV (O)(N4py)] 2+ + ·(HOTf) 2 adduct. These computations suggest that aM n IV -hydroxos pecies is most consistentw ith the availablee xperimental data.[a] Dr.

The heterodinuclear mixed-valence complex [FeMn(ICIMP)-(OAc) 2 Cl] (1) {H 2 ICIMP = 2unsymmetrical N 4 O 2 donor ligand} has been synthesized and fully characterized by several spectroscopic techniques as well as by X-ray crystallography. The crystal structure of the complex reveals that both metal centers in 1 are six-coordinate with the chloride ion occupying the sixth coordination site of the Mn II ion. The phenoxide moiety of the ICIMP ligand and both acetate ligands bridge the two metal ions of the complex. Mössbauer spectroscopy shows that the iron ion in 1 is high-spin Fe III . Two quasi-reversible redox reactions for the complex, attributed to the Fe III Mn II / [a]2204 Fe II Mn II (at -0.67 V versus Fc/Fc + ) and Fe III Mn II /Fe III Mn III (at 0.84 V), were observed by means of cyclic voltammetry. Complex 1, with an Fe III -Mn II distance of 3.58 Å, may serve as a model for the mixed-valence oxidation state of purple acid phosphatase from sweet potato. The capability of the complex to effect organophosphate hydrolysis (phosphatase activity) has been investigated at different pH levels (5.5-11) by using bis(2,4-dinitrophenyl)phosphate (BDNPP) as the substrate. Density functional theory calculations indicate that the substrate coordinates to the Mn II ion. In the transition state, a hydroxide ion that bridges the two metal ions becomes terminally coordinated to the Fe III ion and acts as a nucleophile, attacking the phosphorus center of BDNPP with the concomitant dissociation of the leaving group.

Copper coordination complexes have emerged as a group of transition metal complexes that play important roles in solar energy conversion, utilization and storage, and have the potential to replace the quintessential commonly used transition metals, like Co, Pt, Ir and Ru as light sensitizers, redox mediators, electron donors and catalytic centers. The applications of copper coordination compounds in chemistry and energy related technologies are many and demonstrate their rightful place as sustainable, low toxicity and Earth-abundant alternative materials. In this perspective we show the most recent impact made by copper coordination complexes in dye-sensitized solar cells and other energy relevant applications.

The oxomanganese­(IV) complex [(dpaq)­MnIV(O)]+-M n+ (1-M n+ , M n+ = redox-inactive metal ion, H-dpaq = 2-[bis­(pyridin-2-ylmethyl)]­amino-N-quinolin-8-ylacetamide), generated in the reaction of the precursor hydroxomanganese­(III) complex 1 with iodosylbenzene (PhIO) in the presence of redox-inactive metal triflates, has recently been reported. Herein the generation of the same oxomanganese­(IV) species from 1 using various combinations of protic acids and oxidants at 293 K is reported. The reaction of 1 with triflic acid and the one-electron-oxidizing agent [RuIII(bpy)3]3+ leads to the formation of the oxomanganese­(IV) complex. The putative species has been identified as a mononuclear high-spin (S = 3/2) nonheme oxomanganese­(IV) complex (1-O) on the basis of mass spectrometry, Raman spectroscopy, EPR spectroscopy, and DFT studies. The optical absorption spectrum is well reproduced by theoretical calculations on an S = 3/2 ground spin state of the complex. Isotope labeling studies confirm that the oxygen atom in the oxomanganese­(IV) complex originates from the MnIII–OH precursor and not from water. A mechanistic investigation reveals an initial protonation step forming the MnIII–OH2 complex, which then undergoes one-electron oxidation and subsequent deprotonations to form the oxomanganese­(IV) transient, avoiding the requirements of either oxo-transfer agents or redox-inactive metal ions. The MnIV–oxo complex cleaves the C–H bonds of xanthene (k 2 = 5.5 M–1 s–1), 9,10-DHA (k 2 = 3.9 M–1 s–1), 1,4-CHD (k 2 = 0.25 M–1 s–1), and fluorene (k 2 = 0.11 M–1 s–1) at 293 K. The electrophilic character of the nonheme MnIV–oxo complex is demonstrated by a large negative ρ value of 2.5 in the oxidation of para-substituted thioanisoles. The complex emerges as the “most reactive” among the existing MnIV/V–oxo complexes bearing anionic ligands.

Two binucleating heptadentate ligands 2,6‐bis[{[(2‐hydroxybenzyl)(N,N‐(dimethylamino)ethyl]amino}methyl]‐4‐methylphenol (H3L1) and 2,6‐bis[{(2‐hydroxybenzyl)(N‐(2‐pyridylmethyl)amino}methyl]‐4‐methylphenol (H3L2) were used to synthesize the two new copper(II) complexes [Cu2(L1)(N3)]·2H2O (1·2H2O) and [Cu2(HL2)(O2CPh)(H2O)]PhCO2·H2O (2·H2O). X‐ray diffraction studies disclose that 1 is made up from bridging phenoxido and azido group in an equatorial fashion, whereas 2 is bridged axially–equatorially through a central cresolato and syn–syn benzoate moiety. The geometry around the copper(II) centers is distorted square pyramid in both cases. Variable‐temperature magnetic susceptibility data reveals that 1 is moderately antiferromagnetically coupled (J = –119 cm–1) and 2 is very weakly antiferromagnetic (J = –1.0 cm–1). The structural features, as well as the presence of orbital countercomplementary effects, are associated with the magnetic behavior. Theoretical calculations with the use of broken symmetry density functional theory also establish the experimental values of the exchange coupling constants (J). In our case, only 2 exhibits catalytic activity in the oxidation of 3,5‐di‐tert‐butylcatechol (3,5‐DTBC) at pH 9.5 and hydrolytic cleavage of plasmid DNA in the absence of any added cofactor, whereas complex 1 cannot display catecholase activity or DNA interaction as a result of strong CuII–azido binding.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)