K. Wieghardt

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...

Bioinorganic model complexes for the active site in manganese containing catalases. The crystal structures of [L2MnII2(μ-OH)(μ-O2CCH3)2] (PF6)·CH3OH and [L′2MnIII2(μ-O)(μ-O2CCH3)2] (I3)I·H2O

Abstract The crystal structures of [L2MnII(μ-OH)(μ- O2CCH3)2](PF6)·CH3OH (1) and [L′2MnIII2(μ-O)- (μ-O2CCH3)2](I3)I·H2O (2) have been determined by X-ray crystallography (L = N,N′,N″-trimethyl- 1,4,7-triazacyclononane, C9H21N3; L′ = 1,4,7-triazacyclononane, C6H15N3). 1 contains the μ-hydroxobis (μ-carboxylato)dimanganese(II) core whereas in 2 the μ-oxo-bis(μ-carboxylato)dimanganese(III) entity has been identified. 1 and 2 serve as model compounds for the active site of the manganese containing catalases isolated from Lactobacillus plantarum and Thermus thermophilus.

Synthesis, magnetism and crystal structure of [V2O2(μ-OH)2(tpen)]I2·4H2O; a binuclear complex containing the syn-{VO(μ-OH)2VO}2+ core (tpen = tetrakis(2-pyridylmethyl)ethylenediamine)

Abstract The reaction of N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine (tpen) with VCl3 in CH3CN yields Cl3V(tpen)VCl3 which was hydrolyzed in water in the presence of oxygen affording [V2O2(μ-OH)2(tpen)]I2·2H2O, the crystal structure of which has been determined. Asyn-{OV(μ-OH)2VO}2+ core has been identified where the V(IV) centers are antiferromagnetically coupled (J = −150 cm−1;g = 1.80).

Pendent arm macrocyclic complexes: crystal structures of AI(TCTA) and In(TS-TACN)

Abstract Complexes of Al3+ and In3+ containing the pendent arm macrocycles 1,4,7-tris(acetato)-1,4,7-triazacyclononane (TCTA) and 1,4,7-tris(2-mercaptoethyl)-1,4,7-triazacyclononane (TS-TACN), respectively, have been synthesized and their crystal structures determined. Al(TCTA) contains an octahedral fac-N3O3 donor set, whereas In(TS-TACN) has an octahedral fac-N3S3 donor set. Average Al—O and In—S distances are 1.846 and 2.397 A, respectively.

The electrochemistry of a dimeric and two monomeric cis-trioxomolybdenum(VI) complexes containing cyclic triamine ligands in protic and aprotic media: model compounds for the active site informate dehydrogenase

Abstract The electrochemistry of two monomeric cis -trioxomolybdenum(VI) complexes containing cyclic triamine ligands N , N ′, N ″-trimethyl-1,4,7-triazacyclononane (L) ( 1 ) or 1,4,7-triazacyclononane (L′)( 2 ) has been studied in protic (CH 3 SO 3 H) and aprotic media (DMF) by cyclic voltammetry at an HMD electrode. 1 and 2 are quasi-reversibly reduced by a 2e − , 4H + step in acidic solutions to give monomeric [LMo(IV)O(OH 2 ) 2 ] 2+ and [L′MoO(OH 2 ) 2 ] 2+ , which in the case of 1 is further reduced to a monomeric Mo(III) species which then undergoes a reversible chemical reaction (dimerizationi?). In contrast, reduced 2 undergoes chemical reactions (dimerization or trimerization of [L′MoO(OH 2 ) 2 ] 2+ or comproportionation reactions) due to the fact that the unmethylated cyclic amine in 2 is sterically less demanding. 1 and 2 are redox-inactive in DMF (+1.0 to −1.7 V vs AgAgCl at a Pt or a glassy-carbon electrode). Dimeric [L 2 Mo 2 O 5 ](PF 6 ) 2 , 3 is reversibly reduced to a Mo(V) 2 dimer (2e − , 2H + process) and, finally, to a Mo(III) 2 dimer(4e − , 4H + ) in 0.1 M CH 3 SO 3 H at an HMD electrode, contrasting with the behavior in aprotic medium (DMF, Pt electrode), where two reversible one-electron transfer processes generating the blue, mixed-valence species Mo(VI)/(V) and an Mo(V) 2 dimer have been observed.

The Crystal Structure and Magnetic Behavior of [Cu2(dien)2CI2](CIO4)2

Abstract The compound [Cu2(dien)2Cl2](C1O4)2 crystallizes in the orthorhombic system, space group Pbn21, with unit cell dimensions: a = 7.637(1) A, b = 13.549(3) A, and c = 20.866(4) A, with four molecules in the unit cell (Z = 4). The structure consists of dimeric [Cu2(dien)2C12]2+ units in which the coordination about copper(II) is highly distorted square-pyramidal. Magnetic susceptibility data shows a pair of exchanged coupled S = 1/2 ions and yields J = +0.4(3)cm−1 when the EPR g-factor value of = 2.104 is used.

The reactivity of cationic [trioxo-(1,4,7-triazacyclononane) rhenium(VII)] and oxorhenium(V) complexes containing triazamacrocycles

Abstract The reduction of colourless [LRe VII O 3 ]Br in an acetone-water mixture (6: 1) with zinc amalgam affords green, air-sensitive [LRe V O 2 Br] which forms a violet complex [LReO(μ-O) 2 ReOBr 2 ]in aqueous solution (L = 1,4,7-triazacyclononane; C 6 H 15 N 3 ). From a similar reduction of [LReO 3 ]ReO 4 the violet neutral complex [LReO(μ-O) 2 ReO(ReO 4 ) 2 ] was obtained. [LReO 3 ] + is deprotonated in alkaline solution (p K a = 10.3 + 0.2, 25°C) and [(C 6 H 14 N 3 )ReO 3 ] was isolated as a yellow solid. The latter amido rhenium(VII) compound reacted in dimethylformamide with RX (R = CH 3 , benzyl; X = Cl), affording at the cyclic amine, N,N′,N″-trisalkylated complexes of the type [L′ReO 3 ]X. The monomeric rhenium(V) complexes [LReOX 2 ]X (X = Cl, Br, I) were obtained from the reaction of [ n -Butyl 4 N]ReOX 4 and L in acetonitrile. IR, UV-vis, 17 O NMR spectra of these compounds are reported.

Nucleophilic behaviour of monomeric cis-trioxotungsten(VI) complexes LWO3 (L = tridentate N-macrocycle)

This paper reviews recent work from our laboratory on the nucleophilic behaviour of monomeric complexes of the type LWO3 and L′WO3 (L = 1,4,7-triazacyclononane and L′ = N,N′,N″-trimethyl-1,4,7-triazacyclononane) which contain a cis-trioxometal(VI) unit. The chemistry of oxotungsten(VI) species has received far less attention in the past than the corresponding chemistry of oxomolybdenum(VI) complexes. Crystal structures of LWO3·3H2O, [L′WO2(OH)]Br, [LWO2Br]Br, [L2W2O5](S2O6)·4H2O, [(LWO3)2BPh2][BPh4], [LWO2(μ-O)W(O2)2O(H2O)], [(L′MoO3)4Co][BPh4]2·C2H5OH and [(L′MoO3)4FeIII][BPh4]3·Na[BPh4] have been determined and the results are presented here. From this study it is clearly established that the terminal oxo-groups in cis-trioxometal(VI) complexes display nucleophilic character. They react readily with electrophiles (H+, Co2+, Fe3+) or oxophiles, such as the BPh4− anion, forming a variety of novel complexes.

Coordination of the macrocyclic thioether 1,4,7-trithiacyclononane (TTCN) to chromium(3+) and to dimolybdenum(2+) di-acetato bridged molecules

Abstract Although chromium(III) is a hard acid and the macrocyclic thioether 1,4,7-trithiacyclonone (TTCN) a soft base, the following compounds could be synthesized: [Cr(TTCN)Cl3], [Cr(TTCN)(SO3CF3)3] and [Cr(TTCN)2]3+ (a rare example of a CrIII (thioether)6 coordination sphere). Comparison with the analogous 1,4,7-triazacyclononane (TACN) complexes shows that the bonding is relatively weak and the ligand-field splitting small. This is in contrast to the bonding properties of these ligands in “late” transition metals and results from the lack of π-donor capability of the Cr3+ centre. TTCN forms several dimeric di-acetato bridged complexes with the (Mo2)4+ unit. The unique interaction of steric and electronic effects of TTCN causes the unusual coordination of a tridentate ligand to the [MO2(μ-OAc)2]2+ unit.

Exchange interactions in a trigonal chromium (III) dimer. Optical spectroscopy of tris-(μ-hydroxo)-bis-[(1,4,7-trimethyl-1, 4,7-triazacyclononane) chromium(III)] triperchlorate

Abstract High-resolution absorption, luminescence, Zeeman and MCD experiments were performed on single crystals of the title compound. The singlet-triplet and triplet-quintet exchange splittings in the ground state are 138 and 267 cm −1 , respectively. The corresponding exchange parameters are 2 J = −128 cm −1 and j = 1.6 cm −1 . The zero-field splitting in the ground level is −2.25 cm −1 . The lowest-energy emitting state is a 3 E state, split into four spinor components with a total spread of 8 cm −1 .