“…Similar IR bands near 1300 cm -1 , assigned to the phenolate ν(C−O) mode, have been observed in the spectra of related μ-phenoxodicopper(II) complexes and are present also in the spectra of compounds 8 and 9 . In fact, the 1304-cm -1 band undergoes a shift to 1294 cm -1 when H 2 18 O 2 is used in the reaction with 1 , with a shift comparable to that reported for the formation of [Cu 2 (XYL−O)(OH)] 2+ . , 1 IR spectra recorded in acetonitrile solution of (A) [Cu 2 ( L )][ClO 4 ] 4 (20 mM); (B) [Cu 2 ( L )][ClO 4 ] 4 (20 mM) + H 2 O 2 (3-fold excess); and (C) [Cu 2 ( L )][ClO 4 ] 4 (20mM) + H 2 18 O 2 (3-fold excess). Spectra B and C were recorded after a 2-h reaction time.…”
The dicopper(II) complex [Cu(2)(L)](4+) (L = alpha,alpha'-bis[bis[2-(1'-methyl-2'-benzimidazolyl)ethyl]amino]-m-xylene) reacts with hydrogen peroxide to give the dicopper(II)-hydroquinone complex in which the xylyl ring of the ligand has undergone a double hydroxylation reaction at ring positions 2 and 5. The dihydroxylated ligand 2,6-bis([bis[2-(3-methyl-1H-benzimidazol-2-yl)ethyl]amino]methyl)benzene-1,4-diol was isolated by decomposition of the product complex. The incorporation of two oxygen atoms from H(2)O(2) into the ligand was confirmed by isotope labeling studies using H(2)(18)O(2). The pathway of the unusual double hydroxylation was investigated by preparing the two isomeric phenolic derivatives of L, namely 3,5-bis([bis[2-(1-methyl-1H-benzimidazol-2-yl)ethyl]amino]methyl)phenol (6) and 2,6-bis([bis[2-(1-methyl-1H-benzimidazol-2-yl)ethyl]amino]methyl)phenol (7), carrying the hydroxyl group in one of the two positions where L is hydroxylated. The dicopper(II) complexes prepared with the new ligands 6 and 7 and containing bridging micro-phenoxo moieties are inactive in the hydroxylation. Though, the dicopper(II) complex 3 derived from 6 and containing a protonated phenol is rapidly hydroxylated by H(2)O(2) and represents the first product formed in the hydroxylation of [Cu(2)(L)](4+). Kinetic studies performed on the reactions of [Cu(2)(L)](4+) and 3 with H(2)O(2) show that the second hydroxylation is faster than the first one at room temperature (0.13 +/- 0.05 s(-1) vs 5.0(+/-0.1) x 10(-3) s(-1)) and both are intramolecular processes. However, the two reactions exhibit different activation parameters (Delta H++ = 39.1 +/- 0.9 kJ mol(-1) and Delta S++ = -115.7 +/- 2.4 J K(-1) mol(-1) for the first hydroxylation; Delta H++ = 77.8 +/- 1.6 kJ mol(-1) and Delta S++ = -14.0 +/- 0.4 J K(-1) mol(-1) for the second hydroxylation). By studying the reaction between [Cu(2)(L)](4+) and H(2)O(2) at low temperature, we were able to characterize the intermediate eta(1):eta(1)-hydroperoxodicopper(II) adduct active in the first hydroxylation step, [Cu(2)(L)(OOH)](3+) [lambda(max) = 342 (epsilon 12,000), 444 (epsilon 1200), and 610 nm (epsilon 800 M(-1)cm(-1)); broad EPR signal in frozen solution indicative of magnetically coupled Cu(II) centers].
“…Similar IR bands near 1300 cm -1 , assigned to the phenolate ν(C−O) mode, have been observed in the spectra of related μ-phenoxodicopper(II) complexes and are present also in the spectra of compounds 8 and 9 . In fact, the 1304-cm -1 band undergoes a shift to 1294 cm -1 when H 2 18 O 2 is used in the reaction with 1 , with a shift comparable to that reported for the formation of [Cu 2 (XYL−O)(OH)] 2+ . , 1 IR spectra recorded in acetonitrile solution of (A) [Cu 2 ( L )][ClO 4 ] 4 (20 mM); (B) [Cu 2 ( L )][ClO 4 ] 4 (20 mM) + H 2 O 2 (3-fold excess); and (C) [Cu 2 ( L )][ClO 4 ] 4 (20mM) + H 2 18 O 2 (3-fold excess). Spectra B and C were recorded after a 2-h reaction time.…”
The dicopper(II) complex [Cu(2)(L)](4+) (L = alpha,alpha'-bis[bis[2-(1'-methyl-2'-benzimidazolyl)ethyl]amino]-m-xylene) reacts with hydrogen peroxide to give the dicopper(II)-hydroquinone complex in which the xylyl ring of the ligand has undergone a double hydroxylation reaction at ring positions 2 and 5. The dihydroxylated ligand 2,6-bis([bis[2-(3-methyl-1H-benzimidazol-2-yl)ethyl]amino]methyl)benzene-1,4-diol was isolated by decomposition of the product complex. The incorporation of two oxygen atoms from H(2)O(2) into the ligand was confirmed by isotope labeling studies using H(2)(18)O(2). The pathway of the unusual double hydroxylation was investigated by preparing the two isomeric phenolic derivatives of L, namely 3,5-bis([bis[2-(1-methyl-1H-benzimidazol-2-yl)ethyl]amino]methyl)phenol (6) and 2,6-bis([bis[2-(1-methyl-1H-benzimidazol-2-yl)ethyl]amino]methyl)phenol (7), carrying the hydroxyl group in one of the two positions where L is hydroxylated. The dicopper(II) complexes prepared with the new ligands 6 and 7 and containing bridging micro-phenoxo moieties are inactive in the hydroxylation. Though, the dicopper(II) complex 3 derived from 6 and containing a protonated phenol is rapidly hydroxylated by H(2)O(2) and represents the first product formed in the hydroxylation of [Cu(2)(L)](4+). Kinetic studies performed on the reactions of [Cu(2)(L)](4+) and 3 with H(2)O(2) show that the second hydroxylation is faster than the first one at room temperature (0.13 +/- 0.05 s(-1) vs 5.0(+/-0.1) x 10(-3) s(-1)) and both are intramolecular processes. However, the two reactions exhibit different activation parameters (Delta H++ = 39.1 +/- 0.9 kJ mol(-1) and Delta S++ = -115.7 +/- 2.4 J K(-1) mol(-1) for the first hydroxylation; Delta H++ = 77.8 +/- 1.6 kJ mol(-1) and Delta S++ = -14.0 +/- 0.4 J K(-1) mol(-1) for the second hydroxylation). By studying the reaction between [Cu(2)(L)](4+) and H(2)O(2) at low temperature, we were able to characterize the intermediate eta(1):eta(1)-hydroperoxodicopper(II) adduct active in the first hydroxylation step, [Cu(2)(L)(OOH)](3+) [lambda(max) = 342 (epsilon 12,000), 444 (epsilon 1200), and 610 nm (epsilon 800 M(-1)cm(-1)); broad EPR signal in frozen solution indicative of magnetically coupled Cu(II) centers].
“…Due to the approximate 2:1 intensity ratio of the 1292 and 1257 cm -1 peaks, the 1290 cm -1 vibration has probably shifted to roughly a 2:1 weighted average of these frequencies (∼1280 cm -1 ; Δ( 18 OPh) ∼ −10 cm -1 ). The frequencies of the phenolate bands are similar for the hydroxo-bridged analogue, [Cu 2 (XYL−O−)(OH)] 2+ , for which three Raman peaks are observed at 1310 (Δ( 18 OPh) = −8 cm -1 ), 1473 and 1596 cm -1 , assigned to phenolate ν(C−O), and two phenolate ν(C−C) vibrations, respectively. , …”
Section: Discussionmentioning
confidence: 76%
“…The 643 cm -1 peak does not shift with 18 O 2 H isotope substitution, consistent with a lack of vibrational coupling between the two bridges. For a closely analogous hydroxo-bridged complex, [Cu 2 (XYL−O−)(OH)] 2+ , two of the Cu 2 O 2 ring modes have been assigned as ν as (Cu−O(Ph)−Cu) at 603 cm -1 (shifted by Δ = −11 cm -1 for the 18 OPh isotope) and ν s (Cu−O(H)−Cu) at 465 cm -1 (shifted by Δ = −12 cm -1 for the 18 OH isotope) . These ν(Cu−O−Cu) peaks in [Cu 2 (XYL−O−)(OH)] 2+ do not shift with isotope substitution of the second bridging group.…”
Spectroscopic studies of a &mgr;-1,1-hydroperoxo-bridged copper dimer are combined with SCF-Xalpha-SW molecular orbital calculations to describe the vibrational and electronic structure of the hydroperoxo-copper complex and compare it to that of previously studied peroxo-copper species. Four vibrational modes of the Cu(2)OOH unit in the resonance Raman and infrared spectra are assigned on the basis of isotope shifts: nu(O-O) = 892 cm(-)(1), nu(as)(Cu-O) = 506 cm(-)(1), nu(s)(Cu-O) = 322 cm(-)(1), and nu(O-H) = 3495 cm(-)(1). The 892 cm(-)(1) O-O stretch of the &mgr;-1,1-hydroperoxo-bridged copper dimer is 89 cm(-)(1) higher than that of the unprotonated complex. Resonance Raman profiles of the 892 cm(-)(1) O-O stretch are used to assign an electronic absorption band at 25 200 cm(-)(1) (epsilon = 6700 M(-)(1) cm(-)(1)) to a hydroperoxide pi-to-Cu charge transfer (CT) transition. This band is approximately 5000 cm(-)(1) higher in energy than the corresponding transition in the unprotonated complex. The pi-to-Cu CT transition intensity defines the degree of hydroperoxide-to-copper charge donation, which is lower than in the unprotonated complex due to the increased electronegativity of the peroxide with protonation. The lower Cu-O covalency of this hydroperoxo-copper complex shows that the high O-O stretching frequency is not due to increased pi-to-Cu charge donation but rather reflects the direct effect of protonation on intra-peroxide bonding. Density functional calculations are used to describe changes in intra-peroxide and Cu-O bonding upon protonation of the peroxo-copper complex and to relate these changes to changes in reactivity.
“…500 cm -1 may be due to its coupling with the Fe−O−Fe chromophore. A Cu−(OH)−Cu stretching vibration was observed at 465 cm -1 by Sanders-Loehr and co-workers, attributed to the coupling of the Cu−(OH)−Cu vibration to a phenolate-to-copper(II) charge-transfer band. Due to the near orthogonality of the Fe−μ-OH bonds (∠Fe−(OH)−Fe = 91.1(2)°), the Fe−(OH)−Fe vibration may be viewed as an isolated ν Fe - OH mode.…”
We have synthesized the first complexes with bis(μ-oxo)diiron(III) and (μ-oxo)(μ-hydroxo)diiron(III) cores (1 and 2, L = TPA (a), 5-Et3-TPA (b), 6-Me3-TPA (c), 4,6-Me6-TPA (d), BQPA (e), BPEEN (f),
and BPMEN (g)) and found them to have novel structural properties. In particular, the presence of two single-atom bridges in these complexes constrains the Fe−Fe distances to 2.7−3.0 Å and the Fe−μ-O−Fe angles to
100° or smaller. The significantly acute Fe−O−Fe angles (e.g., 92.5(2)° for 1c and 100.2(2)° for 2f) enforced
by the Fe2O2(H) core endow these complexes with UV−vis, Raman, and magnetic properties quite distinct
from those of other (μ-oxo)diiron(III) complexes. Complex 1c exhibits visible absorption bands at 470 (ε =
560 M-1 cm-1) and 760 nm (ε = 80 M-1 cm-1), while complexes 2 show features at ca. 550 (ε ≈ 800 M-1
cm-1) and ca. 800 nm (ε ≈ 70 M-1 cm-1), all of which are red shifted compared to those of other (μ-oxo)diiron(III) complexes. These complexes also exhibit distinct νFe-O
-
Fe vibrations at ca. 600 and ca. 670 cm-1
assigned to the νsym and the νasym of the Fe−O−Fe units, respectively. The relative intensities of the νsym and
νasym bands are affected by the symmetry of the Fe−O−Fe units; an unsymmetric core enhances the intensity
of the νasym. Complexes 2 exhibit another band at ca. 500 cm-1, which is assigned to the Fe−(OH)−Fe stretching
mode due to its sensitivity to both H2
18O and 2H2O. Magnetic susceptibility studies reveal J = 54 cm-1 for 1c
and ca. 110 cm-1 for 2 (H = J
S
1·S
2), values smaller than those for the antiferromagnetic interactions found
in (μ-oxo)diiron(III) complexes. This weakening arises from the longer Fe−μ-O bonds and the smaller Fe−μ-O−Fe angles in the Fe2O2(H) diamond core structure. These spectroscopic signatures can thus serve as
useful tools to ascertain the presence of such core structures in metalloenzyme active sites. These two core
structures, Fe2(μ-O)2 (1) and Fe2(μ-O)(μ-OH) (2), can also be interconverted by protonation equilibria with
pK
a's of 16−18 in CH3CN. Furthermore, the Fe2(μ-O)2 core (1) isomerizes to the Fe3(μ2-O)3 core (7), while
the Fe2(μ-O)(μ-OH) core (2) exhibits aquation equilibria to the Fe2(μ-O)(μ-H3O2) core (5), except for L =
6-Me3-TPA and 4,6-Me6-TPA. It is clear from these studies that electronic and steric properties of the ligands
significantly affect the various equilibria, demonstrating a rich chemistry involving water-derived ligands alone.
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