Phalguni Chaudhuri

Homo- and hetero-polymetallic exchange coupled metal-oximates

The strategy of using ‘metal oximates’ building blocks as ligands to synthesize various homo- and hetero-metal paramagnetic complexes is reviewed. The modular approach with ‘metal-complexes’, in this case ‘metal-oximates’, as ligands enables the synthesis of symmetrical, asymmetrical, and linear complexes, viz. MAMB, MAMBMA, MAMBMBMA (MA, MB being two different metal ions). Even the synthesis of asymmetric heterotrinuclear complexes with MAMBMC cores and of MA(μ3-O)2MB butterfly cores has been achieved. This article is focused on such exchange coupled polymetallic, both homo- and hetero-metallic, systems containing the bridging core MNOM′ with an aim to delineate the exchange mechanism through the bridging oximates. The uniqueness of oximates providing diatomic NO-bridging is demonstrated by several series of isostructural complexes with different terminal metal ions like Fe(III), Mn(III), Mn(IV) and Cr(III). Such isostructural series are not available for any other bridging ligands. A qualitative rationale for the trend and nature of exchange interactions between the spin carriers has been provided. This article confirms the essentially σ nature of the exchange interaction transmitted through the diatomic NO-bridging ligand and the applicability of Goodenough–Kanamori rules, in general, to predict the nature of exchange interactions for different heterometal compounds. The π-conjugated system of the oxime ligand, delocalized over the whole bridging groups and perpendicular to the plane of the oxime ligand, appears to play an important role in the antiferromagnetic interaction between the terminal metal ions, separated by as much as ∼7 A. A comparison of the complexes MAMBMA containing a diamagnetic central ion (MB) clearly indicates the participation of the central metal ion in the transmission of the antiferromagnetic exchange interaction between the terminal paramagnetic MA ions. Competing exchange interactions, or spin frustration in the general sense of the term, in polynuclear complexes may lead to ground-state variability. This type of variable ground-state has been observed for several complexes. The strategy of irregular spin-state structure resulting from a particular spin topology has been found to be more effective, as expected, in obtaining ‘high spin’ molecules than the common strategy of obtaining ferromagnetically coupled systems through involvement of symmetry-related strict orthogonality of the magnetic orbitals of the interacting metal centers. Of particular interest for molecular magnetism is the small but significant effect of bridging ligands like carboxylato anions for cooperation with the ancillary ligand, viz. the oxime ligands, to build up high-nuclearity metal clusters.

Aerobic Oxidation of Primary Alcohols by a New Mononuclear CuII-Radical Catalyst

Primary alcohols such as ethanol or benzyl alcohol are selectively and catalytically oxidized by the mononuclear copper(II) radical complex 1-a functional model of the metalloenzyme galactose oxidase-with oxygen from air at 20°C to give the corresponding aldehydes and H2 O2 in about 60 % yield.

From Structural Models of Galactose Oxidase to Homogeneous Catalysis: Efficient Aerobic Oxidation of Alcohols

Two reaction pathways are catalyzed by the dinuclear copper(II)-phenoxyl complex 1-a functional model for the metalloenzyme galactose oxidase-by the oxidation of primary and secondary alcohols with dioxygen (air) in homogeneous solution to their corresponding aldehydes or ketones and/or 1,2-glycols (oxidative C-C coupling). The reduction product formed is H2 O2 , not water.

Mössbauer and electron paramagnetic resonance study of the double‐exchange and Heisenberg‐exchange interactions in a novel binuclear Fe(II/III) delocalized‐valence compound

In this paper we present the characterization by UV‐VIS, Mossbauer, and EPR spectroscopy of [L2Fe2(μ‐OH)3](ClO4)2⋅2CH3OH⋅2H2O, with L=N,N’,N‘‐trimethyl‐1,4,7‐triazacyclononane, a novel dimeric iron compound, which is shown to possess a central exchange‐coupled delocalized‐valence Fe(II/III) pair. Complete delocalization of the excess electron in the dimeric iron center is concluded from the indistinguishability of the two iron sites in Mossbauer spectroscopy. Mossbauer, EPR, and magnetic susceptibility data imply a system spin St =9/2 for the ground state. This finding is explained as being a consequence of the double‐exchange interaction which is generated by the delocalized electron. Experimental values obtained from UV‐VIS, Mossbauer, and EPR spectroscopy are for the double‐exchange parameter B=1300 cm−1, the g factors gx,y =2.04 and gz =2.3, the parameters for zero‐field splitting D=4 cm−1 and E≊0 cm−1, and for the hyperfine parameters ΔEQ =−2.14 mm s−1, Ax,y =−21.2 T, Az =−27 T, and δ=0.74 mm s−1. Fr...

Biomimetic metal-radical reactivity: aerial oxidation of alcohols, amines, aminophenols and catechols catalyzed by transition metal complexes.

Abstract The contributions of the authors to the research program ‘Radicals in Enzymatic Catalysis’ over the last ca. 5 years are summarized. Significant efforts were directed towards the design and testing of phenol-containing ligands for synthesizing radical-containing transition metal complexes as potential candidates for catalysis of organic substrates like alcohols, amines, aminophenols and catechols. Functional models for different copper oxidases, such as galactose oxidase, amine oxidases, phenoxazinone synthase and catechol oxidase, are reported. The copper complexes synthesized can mimic the function of the metalloenzymes galactose oxidase and amine oxidases by catalyzing the aerial oxidation of alcohols and amines. Even methanol could be oxidized, albeit with a low conversion, by a biradical-copper(II) compound. The presence of a primary kinetic isotope effect, similar to that for galactose oxidase, provides compelling evidence that H-atom abstraction from the α-C-atom of the substrates is the rate-limiting step. Although catechol oxidase and phenoxazinone synthase contain copper, manganese(IV) complexes containing radicals have been found to be useful to study synthetic systems and to understand the naturally occurring processes. An ‘on-off’ mechanism of the radicals without redox participation from the metal centers seems to be operative in the catalysis involving such metal-radical complexes.

An effectively diamagnetic oximato-bridged asymmetric dinuclear copper(II) complex with a Cu(II)I bond

Abstract A new bis(oximato)-bridged dinuclear copper(II) complex containing an axial Cu(II)I bond with the formula [LCu(PyA) 2 Cu(I)]ClO 4 ·CH 3 CN ( 1 ) (where L = 1,4,7-tritrimethyl-1,4,7-triazacyclononane; PyA = monoanion of pyridine-2-aldoxine) has been characterized by IR, UV-Vis, variable temperature (81–283 K) magnetic susceptibility measurements. 1 crystallizes in the monoclinic system space group P2 1 n , with cell constants a= 8.304(2), b = 27.410(7), c = 13.975(5) A β = 93.48(2)° V = 3175(2) A and Z = 4 . The structure consists of oximato-bridged CuIICuII(I) monocations and non-coordinated perchlorate anions. Both copper ions have a distorted square-pyramidal geometry with a Cu ⋯ Cu separation of 3.45 A and Cu(II)I bond length of 2.74 A. The copper centres are very strongly antiferromagnetically coupled, the singlet-triplet splitting being greater than 1000 cm−1.

Synthesis, structure and magnetism of a tetranuclear Fe(III) complex containing an [Fe4(μ3-O)2]8+ core

Abstract The tetranuclear Fe(III) complex [L2Fe2(μ3-O)2(μ2-CH3COO)3(Sao)2Fe2]X (X=ClO4, (1), PF6 (2)), where L is the cyclic amine 1,4,7-trimethyl-1,4,7-triazacyclononane and Sao is the dianion of salicylaldoxime (H2Sao), has been synthesized and characterized on the basis of elemental analysis, IR, electronic, Mossbauer spectroscopy and variable-temperature (4–285 K) magnetic susceptibility measurements. The molecular structure has been established by X-ray diffraction of 2. The structure consists of an [Fe2wing(μ3-O)2Fe2body]8+-butterfly core. Each Fe(III) ion is in a distorted octahedral environment. The two Fe(Sao)+ units function as bridges between two terminal ‘wing’ iron ions through their deprotonated oximate oxygen and represent the ‘body’ iron ions in the tetranuclear core. Three acetate groups are singly bridging between the iron ions. Mossbauer spectra display two quadrupole doublets, consistent with two different high-spin ferric sites. Magnetic susceptibility measurements reveal a diamagnetic ground state with antiferromagnetic exchange interactions among the four high-spin ferric centers. The exchange coupling constant J12 (‘body-body’ interaction) could not be determined due to prevailing spin frustration, but the ‘wing-body’ antiferromagnetic interaction, J, was evaluated to be −46 cm−1 [H=−2J(S1·S3+S2·S3+S1·S4+S2·S4)−2J12S1·S2].

Isolation of a spin-frustrated imidazolate-bridged trinuclear copper(II) complex potentially relevant to the multicopper oxidases

The reaction of Cu(MeCO2)2·H2O with 1,4,7-trimethyl-1,4,7-triazacyclononane (L) and imidazole (ImH) in methanol affords, in the presence of a small amount of triethylamine and NaClO4, the blue trinuclear complex [L3Cu3(Im)3](ClO4)3 which has been characterized by X-ray crystallography, EPR and magnetic susceptibility measurements to be a spin-frustrated triangular imidazolate-bridged copper(II) complex without any µ3-X ligand.

Linkage isomerism in exchange coupled dinuclear Ni(II) complexes with three azide bridges; tris(μ-1,1-azido) versus tris(μ-1,3-azido) binding. Crystal structures of [L2Ni2(μ-1,1-N3)3]ClO4 and [L2Ni2(N3)2-(μ-1,3-N3)2] (L = 1,4,7-trimethyl-1,4,7-triazacyclononane)

Abstract Two linkage isomers of Ni(II), [L2Ni2(N3)3]ClO4, containing either three end-to-end (μ-1,3-azido) 1, or three end-on (μ-1,1-azido) 2 bridging azide ligands and 1,4,7-trimethyl-1,4,7-triazacyclononane (L) as the capping ligands have been reported. The compounds have been characterized on the basis of IR, UV-Vis spectroscopy and variable-temperature (2–290 K) magnetic susceptibility measurements. The crystal structure of 1 has been reported earlier (J. Chem. Soc., Chem. Commun., (1985) 1618). 2 crystallizes in the monoclinic space group P21/c with cell constants a=14.989(1), b=14.610(2), c=14.756(2) A , β=111.12(1)°, V=3014.5 A 3 and Z=4 . The structure of 2 consists of a six-coordinated dinickel(II) center with three azide ions linking two Ni(II) ions adopting the end-on coordination mode. Analysis of the susceptibility data yields a moderately strong antiferromagnetic interaction between nickel centers in 1, but a ferromagnetic interaction in 2. The crystal structure of [L2Ni2(N3)2(μ-1,3-N3)2] (3) has been determined at 193 K. Crystals of 3·CH3OH have the following cell parameters: monoclinic, C2/c; a=24.302(2), b=11.276(1), c=11.437(1) A ; β=109.70(1)°; V=2950.8; Z=4 . The structure of 3 consists of a six-coordinated dinickel(II) center with two end-to-end azide ions linking two Ni(II) ions. Magnetic susceptibility measurements have shown the existence of antiferromagnetic exchange interaction between the nickel centers (2J = −72.8 cm−1). The magnetic behavior of 1,2 and 3 together with other related complexes is discussed to show the difficulty of rationalizing the magnetostructural trends.

Oxidation of an o-Iminobenzosemiquinone Radical Ligand by Molecular Bromine: Structural, Spectroscopic, and Reactivity Studies of a Copper(II) o-Iminobenzoquinone Complex

The bis(o-iminobenzosemiquinonato)copper(II) complex 1, containing the radical form [L(*)SQ](1-) arising from the aerial oxidation of the noninnocent ligand 2-anilino-4,6-di- tert-butylphenol, H2L, is readily oxidized by molecular bromine to a bis(o-iminobenzoquinone)copper(II) complex, 2. Thus, a ligand-based oxidative addition is reported for complex 1 containing an electron-rich Cu(II) d(9) metal ion. The crystal structure of the synthesized hexacoordinated complex [Cu(II)(LBQ)2Br2] (2) has been determined by X-ray crystallography at 100 K. Variable-temperature (2-290 K) magnetic susceptibility measurements and an X-band electron paramagnetic resonance spectrum establish the spin state to be St = 1/2 because of localized spin moments mainly in the (d(x(2)-y(2)))(1) orbital of a Cu(II) d(9) ion, indicating clearly the presence of a neutral iminobenzoquinone form, [LBQ](0), of the ligand in 2, as is found also in the X-ray structure. Electrochemical measurements (cyclic voltammograms and coulometry) indicate two successive one-electron reductions of the ligand. The reactivity of complex 2 as an oxidizing agent toward ethanol and triethylamine has been investigated.