“…This suggests that the diimine−diamine macrocyclic ligand moieties are more rigid, and/or there is a competing contribution from π − π stacking that makes the intercalation of the perchlorate anion less favorable than that in the tetraamine analogue. The Ni−N(amine) distances in the equatorial macrocyclic ring average 194 pm in both complexes, as is characteristic of low-spin Ni II in macrocyclic rings of this size, and this is consistent with their observed diamagnetism. 7 Molecular structure of [L (1) Ni 2 ] 2+ .…”
Section: Resultssupporting
confidence: 76%
“…General Structural Features of the Dinickel Complexes. Nickel(II) complexes with aliphatic, 14-membered tetraazamacrocyclic ligands characteristically have equatorial Ni−N bond lengths that are significantly longer for the six-coordinate complexes in which the Ni II centers have triplet spin multiplicity (typically 206 pm) than those for the planar, four-coordinate complexes in which the Ni II centers have singlet multiplicities (typically 194 pm); see Table . This standard correlation of the Ni II electronic structure with coordination geometry fails with the dinickel complexes discussed here since the large electrostatic attractions favor the intercalation of an anion between the co-facial Ni II - macrocyclic ligand moieties with the ion-pair association constants in excess of 10 4 M -1 and because the Ni−X axial distances vary over a substantial range.…”
Dramatic differences are found between the ambient and 100 K X-ray structures of [L(2)Ni2Br2](ClO4)2 (L(2) = alpha,alpha'-bis{(5,7-dimethyl-1,4,8,11-tetraazacyclotetradeca-6-yl)-o-xylene), in which the bromide-bridged, bimetallic, macrocyclic ligand complexes of nickel(II) are held face-to-face and in which each bimetallic complex has a net triplet spin multiplicity. The ambient structure of this complex consists of very highly ordered, infinite chains of alternating R and S isomers in which the identical Ni(II) coordination spheres are near to the average expected for the high- and low-spin Ni(II) coordination sites, and there is appreciable stereochemical strain in the linkage of the macrocyclic ligands to the phenyl ring. In contrast, every other dinickel complex of the 100 K structure is displaced about 40 pm along the infinite chains to form tetrameric repeat units (pairs of dinickel complexes), in which each dinickel complex has well-defined high-spin and low-spin Ni(II) coordination sites; the high-spin sites are adjacent in the tetramers, and the stereochemical strain in the linkage to the phenyl spacer is relaxed. The molecular magnetic moments and structural contrasts are similar for the 100 K structure and the previously reported ambient structure of [L(2)Ni2Br3](ClO4) complex for which the molecular magnetic moments also correspond to a single triplet state per complex. The halide-bridged, monochloro- and monobromo dinickel complexes also have triplet spin multiplicity, and they crystallize with a coordinated perchlorate completing the axial coordination of the high-spin Ni(II) site, while the other Ni(II) site of these halide-bridged complexes has equatorial Ni-N bond lengths typical of low-spin Ni(II) coordination. The bridging halide is sandwiched between the face-to-face macrocyclic ligand Ni(II) moieties and slightly off the Ni-Ni axis in all of the complexes. The temperature dependence of the magnetic moments of the series of complexes indicates that their singlet-triplet energy gaps are small, with zero point energy differences that are generally less than 10(3) cm(-1). The very weak metal-metal electronic coupling, the triplet state spin multiplicity of each dinickel complex, and the averaged high-spin/low-spin coordination environments of the ambient structure implicate a vibronic mechanism for the electronic configurational exchange in the dibromo and tribromo complexes. The single molecular vibrational mode that correlates with the configurational exchange in these complexes includes the concerted motion of the bridging bromide between the Ni(II) centers. Activation of this vibrational mode is sufficient to effect the configurational exchange. These complexes present especially clear examples of the effects of the coupling of nuclear vibrational motions to the interchange of electronic configuration between two different centers.
“…This suggests that the diimine−diamine macrocyclic ligand moieties are more rigid, and/or there is a competing contribution from π − π stacking that makes the intercalation of the perchlorate anion less favorable than that in the tetraamine analogue. The Ni−N(amine) distances in the equatorial macrocyclic ring average 194 pm in both complexes, as is characteristic of low-spin Ni II in macrocyclic rings of this size, and this is consistent with their observed diamagnetism. 7 Molecular structure of [L (1) Ni 2 ] 2+ .…”
Section: Resultssupporting
confidence: 76%
“…General Structural Features of the Dinickel Complexes. Nickel(II) complexes with aliphatic, 14-membered tetraazamacrocyclic ligands characteristically have equatorial Ni−N bond lengths that are significantly longer for the six-coordinate complexes in which the Ni II centers have triplet spin multiplicity (typically 206 pm) than those for the planar, four-coordinate complexes in which the Ni II centers have singlet multiplicities (typically 194 pm); see Table . This standard correlation of the Ni II electronic structure with coordination geometry fails with the dinickel complexes discussed here since the large electrostatic attractions favor the intercalation of an anion between the co-facial Ni II - macrocyclic ligand moieties with the ion-pair association constants in excess of 10 4 M -1 and because the Ni−X axial distances vary over a substantial range.…”
Dramatic differences are found between the ambient and 100 K X-ray structures of [L(2)Ni2Br2](ClO4)2 (L(2) = alpha,alpha'-bis{(5,7-dimethyl-1,4,8,11-tetraazacyclotetradeca-6-yl)-o-xylene), in which the bromide-bridged, bimetallic, macrocyclic ligand complexes of nickel(II) are held face-to-face and in which each bimetallic complex has a net triplet spin multiplicity. The ambient structure of this complex consists of very highly ordered, infinite chains of alternating R and S isomers in which the identical Ni(II) coordination spheres are near to the average expected for the high- and low-spin Ni(II) coordination sites, and there is appreciable stereochemical strain in the linkage of the macrocyclic ligands to the phenyl ring. In contrast, every other dinickel complex of the 100 K structure is displaced about 40 pm along the infinite chains to form tetrameric repeat units (pairs of dinickel complexes), in which each dinickel complex has well-defined high-spin and low-spin Ni(II) coordination sites; the high-spin sites are adjacent in the tetramers, and the stereochemical strain in the linkage to the phenyl spacer is relaxed. The molecular magnetic moments and structural contrasts are similar for the 100 K structure and the previously reported ambient structure of [L(2)Ni2Br3](ClO4) complex for which the molecular magnetic moments also correspond to a single triplet state per complex. The halide-bridged, monochloro- and monobromo dinickel complexes also have triplet spin multiplicity, and they crystallize with a coordinated perchlorate completing the axial coordination of the high-spin Ni(II) site, while the other Ni(II) site of these halide-bridged complexes has equatorial Ni-N bond lengths typical of low-spin Ni(II) coordination. The bridging halide is sandwiched between the face-to-face macrocyclic ligand Ni(II) moieties and slightly off the Ni-Ni axis in all of the complexes. The temperature dependence of the magnetic moments of the series of complexes indicates that their singlet-triplet energy gaps are small, with zero point energy differences that are generally less than 10(3) cm(-1). The very weak metal-metal electronic coupling, the triplet state spin multiplicity of each dinickel complex, and the averaged high-spin/low-spin coordination environments of the ambient structure implicate a vibronic mechanism for the electronic configurational exchange in the dibromo and tribromo complexes. The single molecular vibrational mode that correlates with the configurational exchange in these complexes includes the concerted motion of the bridging bromide between the Ni(II) centers. Activation of this vibrational mode is sufficient to effect the configurational exchange. These complexes present especially clear examples of the effects of the coupling of nuclear vibrational motions to the interchange of electronic configuration between two different centers.
“…28−31 Depending on the reaction conditions, Scheme 1. Synthesis of Cr III (HMC) Alkynyl Complexes coordinating ligands, and counterions, it is possible for rac-HMC complexes to rearrange from folded to planar 29,32 (never from rac-HMC to meso-HMC, however), 31 which is not the case here.…”
Section: ■ Results and Discussionmentioning
confidence: 80%
“…It is well-known that the nitrogen plane of rac -HMC (also known as tet-b ) ligand is most commonly folded, whereas the nitrogen plane of meso -HMC (also known as tet-a ) is rarely anything but planar (Scheme ). − Depending on the reaction conditions, coordinating ligands, and counterions, it is possible for rac -HMC complexes to rearrange from folded to planar , (never from rac -HMC to meso -HMC, however), which is not the case here.…”
Presented here is the chemistry of Cr(III) alkynyl complexes based on the rac-HMC and meso-HMC ligands (HMC = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). Thus far, two pairs of cis/trans-[Cr(rac/meso-HMC)(C2R)2]Cl (R = Ph, C2H/C2SiMe3) complexes have been synthesized from reactions between cis/trans-[Cr(rac/meso-HMC)Cl2]Cl and LiC2R. These complexes were characterized using single crystal X-ray diffraction, UV-vis spectroscopy, FT-IR spectroscopy, and fluorimetry. Single crystal X-ray diffraction studies revealed that these complexes adopt a pseudo-octahedral geometry. The electronic spectra of both the cis- and trans-[Cr(rac/meso-HMC)(C4R')2]Cl (R' = H or SiMe3) complexes exhibit d-d bands with pronounced vibronic progression associated with the asymmetric stretch of the Cr-bound C≡C bonds. All of these complexes are phosphorescent and show structured emissions originating from the ligand field excited states.
“…As expected, macrocyclic nickel complexes behave differently and yield values of A (28.1 for Ni(cyclam) 2+ and 31.3 for Ni(Me 6 -cyclam) 2+ ), Table , that are much greater than those obtained for reagents believed to react by OSET. Ni(cyclam) 2+ and derivatives exist as mixtures of four-, five-, and six- coordinated complexes , having unoccupied and/or labile sites that provide easy access to redox partners utilizing inner-sphere pathways. Clearly, Fe IV aq O 2+ can, and apparently does, react by such a pathway via an oxo or hydroxo bridge.…”
The kinetics of oxidation of organic and inorganic reductants by aqueous iron(IV) ions, Fe(IV)(H2O)5O(2+) (hereafter Fe(IV)aqO(2+)), are reported. The substrates examined include several water-soluble ferrocenes, hexachloroiridate(III), polypyridyl complexes M(NN)3(2+) (M = Os, Fe and Ru; NN = phenanthroline, bipyridine and derivatives), HABTS(-)/ABTS(2-), phenothiazines, Co(II)(dmgBF2)2, macrocyclic nickel(II) complexes, and aqueous cerium(III). Most of the reductants were oxidized cleanly to the corresponding one-electron oxidation products, with the exception of phenothiazines which produced the corresponding oxides in a single-step reaction, and polypyridyl complexes of Fe(II) and Ru(II) that generated ligand-modified products. Fe(IV)aqO(2+) oxidizes even Ce(III) (E(0) in 1 M HClO4 = 1.7 V) with a rate constant greater than 10(4) M(-1) s(-1). In 0.10 M aqueous HClO4 at 25 °C, the reactions of Os(phen)3(2+) (k = 2.5 × 10(5) M(-1) s(-1)), IrCl6(3-) (1.6 × 10(6)), ABTS(2-) (4.7 × 10(7)), and Fe(cp)(C5H4CH2OH) (6.4 × 10(7)) appear to take place by outer sphere electron transfer (OSET). The rate constants for the oxidation of Os(phen)3(2+) and of ferrocenes remained unchanged in the acidity range 0.05 < [H(+)] < 0.10 M, ruling out prior protonation of Fe(IV)aqO(2+) and further supporting the OSET assignment. A fit to Marcus cross-relation yielded a composite parameter (log k22 + E(0)Fe/0.059) = 17.2 ± 0.8, where k22 and E(0)Fe are the self-exchange rate constant and reduction potential, respectively, for the Fe(IV)aqO(2+)/Fe(III)aqO(+) couple. Comparison with literature work suggests k22 < 10(-5) M(-1) s(-1) and thus E(0)(Fe(IV)aqO(2+)/Fe(III)aqO(+)) > 1.3 V. For proton-coupled electron transfer, the reduction potential is estimated at E(0) (Fe(IV)aqO(2+), H(+)/Fe(III)aqOH(2+)) ≥ 1.95 V.
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