85 research outputs found
Isomeric separation in donor–acceptor systems of Pd(II) and Pt(II) and a combined structural, electrochemical and spectroelectrochemical study
Compounds of the form [(pap)M(Q2−)] (pap = phenylazopyridine; Q = 3,5-di-tert-
butyl-benzoquinone, M = Pd, 1a and 1b, M = Pt, 2a and 2b; Q = 4-tert-butyl-
benzoquinone, M = Pd, 3a and 3b; M = Pt, 4a and 4b) were synthesized in a one-
pot reaction. The geometrical isomers, which are possible because of the built
in asymmetry of these ligands, have been separated by using different
temperatures and variable solubility. Structural characterization of 1b shows
that the metal centers are in a square planar environment, the pap ligand is
in the unreduced neutral state and the quinones are in the doubly reduced,
Q2−catecholate form. Cyclic voltammetric measurements on the complexes display
two one-electron oxidations and two one-electron reductions. EPR and vis-NIR
spectra of the one-electron oxidized forms of the complexes indicate that the
first oxidation takes place on the Q2− ligands to produce a metal bound
semiquinone (Q˙−) radical. Reduction takes place on the pap ligand, generating
metal bound pap˙− as seen from the 14N (I = 1) coupling in their EPR spectrum.
All the complexes in their [(pap)M(Q2−)] neutral forms show strong absorptions
in the NIR region which are largely LLCT (ligand to ligand charge transfer) in
origin. These NIR bands can be tuned over a wide energy range by varying the
metal center as well as the Q ligand. In addition, the intensity of NIR bands
can be switched on and off by a simple electron transfer at relatively low
potentials. DFT studies were used to corroborate these findings
a combined structural, electrochemical and spectroscopic study
Reactions of [(az-H)Pd(μ-Cl)2Pd(az-H)] (az = azobenzene) with the
zwitterionic, p-benzoquinonemonoimine-type ligands
4-(n-butylamino)-6(n-butylimino)-3-oxocyclohexa-1,4-dien-1-olate (Q1) or
4-(isopropylamino)-6(isopropylimino)-3-oxocyclohexa-1,4-dien-1-olate) (Q2) in
the presence of a base leads to the formation of the mononuclear complexes
[(az-H)Pd(Q1-H)] (1) and [(az-H)Pd(Q2-H)] (2) respectively. Structural
characterization of 2 shows an almost square planar coordination geometry
around the Pd(II) centre, a short Pd–C bond, a slight elongation of the
N[double bond, length as m-dash]N double bond of the az-H ligand and
localization of the double bonds within the Q2-H ligand. Additionally,
intermolecular N–H–O interactions exist between the uncoordinated N–H and O
groups of two different molecules. Cyclic voltammetry of the complexes reveals
an irreversible oxidation and two reversible reduction processes. A
combination of electrochemical and UV-vis-NIR and EPR spectroelectrochemical
studies are used to show that both coordinated ligands participate
successively in the redox processes, thus revealing their non-innocent
character
weak intermolecular interactions and catalytic ethylene oligomerisation
The ligands 1-(cyclohexyl)-4-(2-pyridyl)-1,2,3-triazole (1),
1-(2,6-diisopropylphenyl)-4-(2-pyridyl)-1,2,3-triazole (2),
1-(4-butoxyphenyl)-4-(2-pyridyl)-1,2,3-triazole (3) and
1-(methyl)-4-(2-pyridyl)-1,2,3-triazole (4) were synthesized by the Cu(I)
catalyzed “Click” reaction between 2-pyridylacetylene and the corresponding
azides. The ligands were then reacted with NiBr2·3H2O to generate the
complexes (1)2NiBr2 (1a), (2)2NiBr2 (2a), (3)2NiBr2 (3a) and (4)2NiBr2 (4a).
Structural characterization of 1a confirmed the mononuclear and distorted
octahedral environment around the Ni(II) center, with the pyridyl-triazole
ligands coordinating in a bis-chelating fashion. Bond length analysis inside
the 1,2,3-triazole ring shows a short N[double bond, length as m-dash]N double
bond that is flanked by two longer C–N and N–N bonds pointing to the existence
of “azo” character in the ring. The highly polar five-membered 1,2,3-triazole
ring makes its C–H bond acidic, and these bonds participate in an extended
weak intermolecular C–HBr interactions with the Br-groups of neighboring
molecules, resulting in a 3-D network. The nickel complexes with these “Click”
ligands were tested as pre-catalysts for ethylene oligomerization, and the
complexes showed moderate activity in that reaction with good selectivity
towards C4 oligomers
Influencing the coordination mode of tbta (tbta = tris[(1-benzyl- 1H-1,2,3-triazol-4-yl)methyl]amine) in dicobalt complexes through changes in metal oxidation states
The complexes [(tbta)Co(μ-CA-2H)Co(tbta)(CH3CN)](BF4)21 and
[(tbta)Co(μ-OH)2Co(tbta)](BF4)42 (tbta = tris[(1-benzyl-
1H-1,2,3-triazol-4-yl)methyl]amine and CA = chloranilic acid) were synthesized
and characterized by X-ray crystallography, SQUID magnetometry and NMR
spectroscopy. The reactions to form these complexes deliver 1 as a
paramagnetic species containing two high spin Co(II) centers, and 2 as a
diamagnetic compound with two low spin Co(III) centers. Structural analysis
shows that in 1 the capped-octahedral environment around the Co(II) centers is
highly distorted with rather long bonds between the metal and donor atoms. The
tbta ligand binds to the Co(II) centers through the three triazole nitrogen
donor atoms in a facial form, with the Co–N(amine) distance of 2.494(2) Å
acting as a capping bond to the octahedron. In the crystal an unusual
observation of one acetonitrile molecule statistically occupying the
coordination sites at both Co(II) centers is made. 1 displays a series of
intermolecular C–HCl and π–π interactions leading to extended three-
dimensional structures in the solid state. These interactions lead to the
formation of voids and explain why only one acetonitrile molecule can be bound
to the dinuclear complexes. In contrast to 1, the cobalt centers in 2 display
a more regular octahedral environment with shorter cobalt–donor atom
distances, as would be expected for a low spin Co(III) situation. The tbta
ligand acts as a perfect tetradentate ligand in this case with the
cobalt–N(amine) distance of 2.012(3) Å falling in the range of a normal bond.
Thus, we present the rare instances where the ligand tbta has been observed to
bind in a perfectly tetradentate fashion in its metal complexes. The room
temperature magnetic moment of 6.30 μB for 1 shows values typical of two high
spin Co(II) centers, and this value decreases at temperatures lower than 30 K
indicating a weak antiferromagnetic coupling and zero field splitting. Mass
spectrometric analysis of 2 provided evidence for the formation of an oxo-
bridged dicobalt complex in the gas phase
Influencing the coordination mode of tbta (tbta = tris[(1-benzyl- 1H-1,2,3-triazol-4-yl)methyl]amine) in dicobalt complexes through changes in metal oxidation states
The complexes [(tbta)Co(μ-CA-2H)Co(tbta)(CH3CN)](BF4)21 and
[(tbta)Co(μ-OH)2Co(tbta)](BF4)42 (tbta = tris[(1-benzyl-
1H-1,2,3-triazol-4-yl)methyl]amine and CA = chloranilic acid) were synthesized
and characterized by X-ray crystallography, SQUID magnetometry and NMR
spectroscopy. The reactions to form these complexes deliver 1 as a
paramagnetic species containing two high spin Co(II) centers, and 2 as a
diamagnetic compound with two low spin Co(III) centers. Structural analysis
shows that in 1 the capped-octahedral environment around the Co(II) centers is
highly distorted with rather long bonds between the metal and donor atoms. The
tbta ligand binds to the Co(II) centers through the three triazole nitrogen
donor atoms in a facial form, with the Co–N(amine) distance of 2.494(2) Å
acting as a capping bond to the octahedron. In the crystal an unusual
observation of one acetonitrile molecule statistically occupying the
coordination sites at both Co(II) centers is made. 1 displays a series of
intermolecular C–HCl and π–π interactions leading to extended three-
dimensional structures in the solid state. These interactions lead to the
formation of voids and explain why only one acetonitrile molecule can be bound
to the dinuclear complexes. In contrast to 1, the cobalt centers in 2 display
a more regular octahedral environment with shorter cobalt–donor atom
distances, as would be expected for a low spin Co(III) situation. The tbta
ligand acts as a perfect tetradentate ligand in this case with the
cobalt–N(amine) distance of 2.012(3) Å falling in the range of a normal bond.
Thus, we present the rare instances where the ligand tbta has been observed to
bind in a perfectly tetradentate fashion in its metal complexes. The room
temperature magnetic moment of 6.30 μB for 1 shows values typical of two high
spin Co(II) centers, and this value decreases at temperatures lower than 30 K
indicating a weak antiferromagnetic coupling and zero field splitting. Mass
spectrometric analysis of 2 provided evidence for the formation of an oxo-
bridged dicobalt complex in the gas phase
Sensing external spins with nitrogen-vacancy diamond
A single nitrogen-vacancy (NV) center is used to sense individual, as well as
small ensembles of, electron spins placed outside the diamond lattice.
Applying double electron–electron resonance techniques, we were able to
observe Rabi nutations of these external spins as well as the coupling
strength between the external spins and the NV sensor, via modulations and
accelerated decay of the NV spin echo. Echo modulation frequencies as large as
600 kHz have been observed, being equivalent to a few nanometers distance
between the NV and an unpaired electron spin. Upon surface modification, the
coupling disappears, suggesting the spins to be localized at surface defects.
The present study is important for understanding the properties of diamond
surface spins so that their effects on NV sensors can eventually be mitigated.
This would enable potential applications such as the imaging and tracking of
single atoms and molecules in living cells or the use of NVs on scanning probe
tips to entangle remote spins for scalable room temperature quantum computers
Strong metal–metal coupling in mixed-valent intermediates [Cl(L)Ru(μ-tppz)Ru(L)Cl]+, L = β-diketonato ligands, tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine
Five diruthenium(II) complexes [Cl(L)Ru(μ-tppz)Ru(L)Cl] (1–5) containing
differently substituted β-diketonato derivatives (1: L = 2,4-pentanedionato;
2: L = 3,5-heptanedionato; 3: L = 2,2,6,6-tetramethyl-3,5-heptanedionato; 4: L
= 3-methyl-2,4-pentanedionato; 5: L = 3-ethyl-2,4-pentanedionato) as ancillary
ligands (L) were synthesized and studied by spectroelectrochemistry (UV-Vis-
NIR, electron paramagnetic resonance (EPR)). X-ray structural characterisation
revealed anti (1, 2, 5) or syn (3) configuration as well as non-planarity of
the bis-tridentate tppz bridge and strong dπ(RuII) → π*(pyrazine, tppz) back-
bonding. The widely separated one-electron oxidation steps, RuIIRuII/RuIIRuIII
and RuIIRuIII/RuIIIRuIII, result in large comproportionation constants (Kc) of
≥1010 for the mixed-valent intermediates. The syn-configurated 3n exhibits a
particularly high Kc of 1012 for n = 1+, accompanied by density functional
theory (DFT)-calculated minimum Ru–N bond lengths for this RuIIRuIII
intermediate. The electrogenerated mixed-valent states 1+–5+ exhibit
anisotropic EPR spectra at 110 K with average values of 2.304–2.234 and g
anisotropies Δg = g1–g3 of 0.82–0.99. Metal-to-metal charge transfer (MMCT)
absorptions occur for 1+–5+ in the NIR region at 1660 nm–1750 nm (ε ≈ 2700 dm3
mol−1 cm−1, Δν1/2 ≈ 1800 cm−1). DFT calculations of 1+ and 3+ yield comparable
Mulliken spin densities of about 0.60 for the metal ions, corresponding to
valence-delocalised situations (Ru2.5)2. Rather large spin densities of about
−0.4 were calculated for the tppz bridges in 1+ and 3+. The calculated
electronic interaction values (VAB) for 1+–5+ are about 3000 cm−1, comparable
to that for the Creutz–Taube ion at 3185 cm−1. The DFT calculations predict
that the RuIIIRuIII forms in 12+–52+ prefer a triplet (S = 1) ground state
with ΔE (S = 0 − S = 1) [similar]5000 cm−1. One-electron reduction takes place
at the tppz bridge which results in species [Cl(L)RuII(μ-tppz˙−)RuII(L)Cl]−
(1˙−–3˙−, 5˙−) which exhibit free radical-type EPR signals and NIR transitions
typical of the tppz radical anion. The system 4n is distinguished by lability
of the Ru–Cl bonds
Cymantrene–Triazole "Click" Products: Structural Characterization and Electrochemical Properties
We report the first known examples of triazole-derivatized cymantrene complexes (η5-[4-substituted triazol-1-yl]cyclopentadienyl)tricarbonylmanganese(I), obtained via a “click” chemical synthesis, bearing a phenyl, 3-aminophenyl, or 4-aminophenyl moiety at the 4-position of the triazole ring. Structural characterization data using multinuclear NMR, UV–vis, ATR-IR, and mass spectrometric methods are provided, as well as crystallographic data for (η5-[4-phenyltriazol-1-yl]cyclopentadienyl)tricarbonylmanganese(I) and (η5-[4-(3-aminophenyl)triazol-1-yl]cyclopentadienyl)tricarbonylmanganese(I). Cyclic voltammetric characterization of the redox behavior of each of the three cymantrene–triazole complexes is presented together with digital simulations, in situ infrared spectroelectrochemistry, and DFT calculations to extract the associated kinetic and thermodynamic parameters. The trypanocidal activity of each cymantrene–triazole complex is also examined, and these complexes are found to be more active than cymantrene alone
Valence and spin situations in isomeric [(bpy)Ru(Q′)2]n (Q′ = 3,5-di-tert- butyl-N-aryl-1,2-benzoquinonemonoimine). An experimental and DFT analysis
The article deals with the ruthenium complexes, [(bpy)Ru(Q′)2] (1–3)
incorporating two unsymmetrical redox-noninnocent iminoquinone moieties [bpy =
2,2′-bipyridine; Q′ = 3,5-di-tert-butyl-N-aryl-1,2-benzoquinonemonoimine, aryl
= C6H5 (Q′1), 1; m-Cl2C6H3 (Q′2), 2; m-(OCH3)2C6H3 (Q′3), 3]. 1 and 3 have
been preferentially stabilised in the cc-isomeric form while both the ct- and
cc-isomeric forms of 2 are isolated [ct: cis and trans and cc: cis and cis
with respect to the mutual orientations of O and N donors of two Q′]. The
isomeric identities of 1–3 have been authenticated by their single-crystal
X-ray structures. The collective consideration of crystallographic and DFT
data along with other analytical events reveals that 1–3 exhibit the valence
configuration of [(bpy)RuII(Q′Sq)2]. The magnetization studies reveal a
ferromagnetic response at 300 K and virtual diamagnetic behaviour at 2 K. DFT
calculations on representative 2a and 2b predict that the excited triplet (S =
1) state is lying close to the singlet (S = 0) ground state with
singlet–triplet separation of 0.038 eV and 0.075 eV, respectively. In
corroboration with the paramagnetic features the complexes exhibit free
radical EPR signals with g [similar]2 and 1HNMR spectra with broad aromatic
proton signals associated with the Q′ at 300 K. Experimental results in
conjunction with the DFT (for representative 2a and 2b) reveal iminoquinone
based preferential electron-transfer processes leaving the ruthenium(II) ion
mostly as a redox insensitive entity: [(bpy)RuII(Q′Q)2]2+ (12+–32+)
[leftrightharpoons] [(bpy)RuII(Q′Sq)(Q′Q)]+ (1+–3+) [leftrightharpoons]
[(bpy)RuII(Q′Sq)2] (1–3) [leftrightharpoons]
[(bpy)RuII(Q′Sq)(Q′Cat)]−/[(bpy)RuIII(Q′Cat)2]− (1−–3−). The diamagnetic
doubly oxidised state, [(bpy)RuII(Q′Q)2]2+ in 12+–32+ has been authenticated
further by the crystal structure determination of the representative
[(bpy)RuII(Q′3)2](ClO4)2 [3](ClO4)2 as well as by its sharp 1H NMR spectrum.
The key electronic transitions in each redox state of 1n–3n have been assigned
by TD–DFT calculations on representative 2a and 2b
The redox series [Ru(bpy)2(L)]n, n = +3, +2, +1, 0, with L = bipyridine, “click” derived pyridyl-triazole or bis-triazole: a combined structural, electrochemical, spectroelectrochemical and DFT investigation
The compounds [Ru(bpy)2(L1)](ClO4)2 (1(ClO4)2), [Ru(bpy)2(L2)](ClO4)2
(2(ClO4)2), [Ru(bpy)2(L3)](ClO4)2 (3(ClO4)2), [Ru(bpy)2(L4)](ClO4)2
(4(ClO4)2), [Ru(bpy)2(L5)](ClO4)2 (5(ClO4)2), and
[Ru(bpy)2(L6)](ClO4)26(ClO4)2 (bpy = 2,2′-bipyridine, L1 = 1-(4-isopropyl-
phenyl)-4-(2-pyridyl)-1,2,3-triazole, L2 = 1-(4-butoxy-
phenyl)-4-(2-pyridyl)-1,2,3-triazole, L3 = 1-(2-trifluoromethyl-
phenyl)-4-(2-pyridyl)-1,2,3-triazole, L4 = 4,4′-bis-{1-(2,6-diisopropyl-
phenyl)}-1,2,3-triazole, L5 = 4,4′-bis-{(1-phenyl)}-1,2,3-triazole, L6 =
4,4′-bis-{1-(2-trifluoromethyl-phenyl)}-1,2,3-triazole) were synthesized from
[Ru(bpy)2(EtOH)2](ClO4)2 and the corresponding “click”-derived pyridyl-
triazole or bis-triazole ligands, and characterized by 1H-NMR spectroscopy,
elemental analysis, mass spectrometry and X-ray crystallography. Structural
analysis showed a distorted octahedral coordination environment about the
Ru(II) centers, and shorter Ru–N(triazole) bond distances compared to
Ru–N(pyridine) distances in complexes of mixed-donor ligands. All the
complexes were subjected to cyclic voltammetric studies, and the results were
compared to the well-known [Ru(bpy)3]2+ compound. The oxidation and reduction
potentials were found to be largely uninfluenced by ligand changes, with all
the investigated complexes showing their oxidation and reduction steps at
rather similar potentials. A combined UV-vis-NIR and EPR
spectroelectrochemical investigation, together with DFT calculations, was used
to determine the site of electron transfer in these complexes. These results
provided insights into their electronic structures in the various investigated
redox states, showed subtle differences in the spectroscopic signatures of
these complexes despite their similar electrochemical properties, and provided
clues to the unperturbed redox potentials in these complexes with respect to
ligand substitutions. The reduced forms of the complexes display structured
absorption bands in the NIR region. Additionally, we also present new
synthetic routes for the ligands presented here using Cu-abnormal carbene
catalysts
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