TEMPO Radical Research Articles

ChemInform Abstract: Copper‐Catalyzed Regio‐ and Stereoselective Intermolecular Three‐Component Oxyarylation of Allenes.

AbstractThe Cu‐catalyzed three‐component reaction of allenes, arylboronic acids, and TEMPO is proposed to proceed through carbocupration of allenes, homolysis of the allylcoper(II) intermediate, and final radical TEMPO trapping.

A theoretical study of the thermodynamic and hydrogen-bond basicity of TEMPO radical and related nitroxides

The use of B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) calculations on TEMPO and four related nitroxide radicals having another oxygen functionality (ketone, hydroxyl and ether) has allowed to determine the relative basicities (thermodynamic and hydrogen-bonded) of the oxygen lone pair relative to the nitroxide radical. The differences are small, especially the B3LYP ones, but in all cases they favor the radical. This is consistent with experimental results in the case of the hydroxyl group but not in the case of the keto group.

Reactivity of organothorium complexes with TEMPO.

Reactions of the 2,2,6,6-tetramethylpiperidin-1-oxyl radical (TEMPO) with thorium metallocenes have been examined to investigate both the radical reaction patterns for organothorium complexes and the coordination chemistry of TEMPO with thorium. (η(5)-C5Me5)2ThMe2 reacts with 2 equiv of TEMPO to generate 1-methoxy-2,2,6,6-tetramethylpiperidine (Me-TEMPO) and (η(5)-C5Me5)2ThMe(η(1)-TEMPO), which contains a TEMPO(-) anion coordinated to thorium through oxygen only. (η(5)-C5Me5)2Th(η(1)-C3H5)(η(3)-C3H5), synthesized from (η(5)-C5Me5)2ThBr2 and (C3H5)MgBr, reacts with 2 equiv of TEMPO to form 1-(2-propen-1-yloxy)-2,2,6,6-tetramethylpiperidine (allyl-TEMPO) and (η(5)-C5Me5)2Th(η(1)-C3H5)(η(1)-TEMPO). Although bis(TEMPO) metallocenes were not obtained in these reactions, the methyl group in (η(5)-C5Me5)2ThMe(η(1)-TEMPO) is reactive with 1 equiv of CuBr to form (η(5)-C5Me5)2ThBr(η(1)-TEMPO). The bis(TEMPO) metallocene (η(5)-C5Me5)2Th(η(1)-TEMPO)2 is accessible in the reaction of [(η(5)-C5Me5)2ThH2]2 with 4 equiv of TEMPO. In contrast, (η(5)-C5Me5)2ThBr2 reacts with 2 equiv of TEMPO by loss of C5Me5 to form (C5Me5)2 and (η(2)-TEMPO)2ThBr2, in which the TEMPO(-) anions bind through oxygen and nitrogen. The bromide ions in (η(2)-TEMPO)2ThBr2 can be replaced by an additional 2 equiv of TEMPO in the presence of 2 equiv of KC8 to form the per(TEMPO) complex Th(η(1)-TEMPO)2(η(2)-TEMPO)2. ThBr4(THF)4 reacts with TEMPO to form ThBr4(THF)2(η(1)-TEMPO), which contains an oxygen-bound TEMPO radical. The Th(3+) complex (η(5)-C5Me4H)3Th is oxidized in the presence of TEMPO, without ligand loss, to afford the Th(4+) species (η(5)-C5Me4H)3Th(η(1)-TEMPO). The reactions show that TEMPO can react with organothorium complexes in several ways including coordination, anion substitution, and cyclopentadienyl replacement.

Chiral tetranuclear and dinuclear copper(ii) complexes for TEMPO-mediated aerobic oxidation of alcohols: are four metal centres better than two?

The one-pot reaction of 3,5-di-tert-butyl-2-hydroxybenzaldehyde, (R)-2-aminoglycinol and Cu(OAc)2·2H2O in a 1 : 1 : 1 ratio in the presence of triethylamine led to the isolation of X-ray quality crystals of the chiral complex (R)- in high yield. The single crystal structure of (R)- reveals a tetranuclear copper(ii) complex that contains a {Cu4(μ-O)2(μ3-O)2N4O4} core. A reaction using (1S,2R)-2-amino-1,2-diphenylethanol as precursor under the same conditions generated the chiral complex (S,R)-; its structure was determined by single crystal X-ray crystallography and was found to contain a {Cu2(μ-O)2N2O2} core. Both (R)- and (S,R)- have been used for catalytic aerobic oxidation of benzylic alcohols in combination with the TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl) radical. (R)- selectively catalyses the conversion of various aromatic primary alcohols to the corresponding aldehydes with high yields (99%) and TONs (770) in the air, while (S,R)- exhibits less promising catalytic performance under the same reaction conditions. The role of the cluster structures in (R)- and (S,R)- in controlling the reactivity towards aerobic oxidation reactions is discussed.

Donor-activated alkali metal dipyridylamides: co-complexation reactions with zinc alkyls and reactivity studies with benzophenone.

Previously it was reported that activation of (t)Bu2Zn by [(TMEDA)Na(μ-dpa)]2 led to tert-butylation of benzophenone at the challenging para-position, where the sodium amide functions as a metalloligand towards (t)Bu2Zn manifested in crystalline [{(TMEDA)Na(dpa)}2Zn(t)Bu2] (TMEDA is N,N,N',N'-tetramethylethylenediamine, dpa is 2,2'-dipyridylamide). Here we find altering the Lewis donor or alkali metal within the metalloligand dictates the reaction outcome, exhibiting a strong influence on alkylation yields and reaction selectivity. Varying the former led to the synthesis of three novel complexes, [(PMDETA)Na(dpa)]2, [(TMDAE)Na(dpa)]2, and [(H6-TREN)Na(dpa)], characterised through combined structural, spectroscopic and theoretical studies [where PMDETA is N,N,N',N'',N''-pentamethyldiethylenetriamine, TMDAE is N,N,N',N'-tetramethyldiaminoethylether and H6-TREN is N',N'-bis(2-aminoethyl)ethane-1,2-diamine]. Each new sodium amide can function as a metalloligand to generate a co-complex with (t)Bu2Zn. Reacting these new co-complexes with benzophenone proved solvent dependent with yields in THF much lower than those in hexane. Most interestingly, sub-stoichiometric amounts of the metalloligands [(TMEDA)Na(dpa)]2 and [(PMEDTA)Na(dpa)]2 with 1 : 1, (t)Bu2Zn-benzophenone mixtures produced good yields of the challenging 1,6-tert-butyl addition product in hexane (52% and 53% respectively). Although exchanging Na for Li gave similar reaction yields, the regioselectivity was significantly compromised; whereas the K system was completely unreactive. Replacing (t)Bu2Zn with (Me3SiCH2)2Zn shut down the alkylation of benzophenone; in contrast, (t)BuLi generates only the reduction product, benzhydrol. Zincation of the parent amine dpa(H) generated the crystalline product [Zn(dpa)2], as structurally elucidated through X-ray crystallography and theoretical calculations. Although the reaction mechanism for the alkylation of benzophenone remains unclear, incorporation of the radical scavenger TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl radical) into the reaction system completely inhibits benzophenone alkylation.

Copper-catalyzed regio- and stereoselective intermolecular three-component oxyarylation of allenes.

A copper(II)-catalyzed intermolecular three-component oxyarylation of allenes using arylboronic acids as a carbon source and TEMPO as an oxygen source is described. The reaction proceeded under mild conditions with high regio- and stereoselectivity and functional group tolerance. A plausible reaction mechanism is proposed, involving carbocupration of allenes, homolysis of the intervening allylcopper(II), and a radical TEMPO trap.

TEMPO and its Derivatives: Synthesis and Applications

The radical TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl radical) and its derivatives are well-established catalysts for oxidation processes, and now used extensively in organic synthesis and industrial applications as a mild, safe, and economical alternative to heavy metal reagents as highly selective oxidation catalysts. In this review, we attempt to fully review the recent advances in the syntheses of TEMPO and its derivatives as well as their applications in recent years. The synthesis of TEMPO and its derivatives are discussed first. The largest section focuses on their application as stoichiometric and catalytic oxidants in organic synthesis, particularly focusing on the formation of C-C, C-O, C=O, C-N, and C=N bond. Simultaneously, the oxidation of alcohols to their corresponding carbonyl compounds also is commented briefly. The last section discusses the application of the TEMPO-catalyzed reactions in the total syntheses of natural products. Keywords: TEMPO, oxidation, radical, synthesis.

Mechanistic Study of the Oxidation of a Methyl Platinum(II) Complex with O2 in Water: PtIIMe-to-PtIVMe and PtIIMe-to-PtIVMe2 Reactivity

The mechanism of oxidation by O2 of (dpms)Pt(II)Me(OH2) (1) and (dpms)Pt(II)Me(OH)(-) (2) [dpms = di(2-pyridyl)methanesulfonate] in water in the pH range of 4-14 at 21 °C was explored using kinetic and isotopic labeling experiments. At pH ≤ 8, the reaction leads to a C1-symmetric monomethyl Pt(IV) complex (dpms)Pt(IV)Me(OH)2 (5) with high selectivity ≥97%; the reaction rate is first-order in [Pt(II)Me] and fastest at pH 8.0. This behavior was accounted for by assuming that (i) the O2 activation at the Pt(II) center to form a Pt(IV) hydroperoxo species 4 is the reaction rate-limiting step and (ii) the anionic complex 2 is more reactive toward O2 than neutral complex 1 (pKa = 8.15 ± 0.02). At pH ≥ 10, the oxidation is inhibited by OH(-) ions; the reaction order in [Pt(II)Me] changes to 2, consistent with a change of the rate-limiting step, which now involves oxidation of complex 2 by Pt(IV) hydroperoxide 4. At pH ≥ 12, formation of a C1-symmetric dimethyl complex 6, (dpms)Pt(IV)Me2(OH), along with [(dpms)Pt(II)(OH)2](-) (7) becomes the dominant reaction pathway (50-70% selectivity). This change in the product distribution is explained by the formation of a Cs-symmetric intermediate (dpms)Pt(IV)Me(OH)2 (8), a good methylating agent. The secondary deuterium kinetic isotope effect in the reaction leading to complex 6 is negligible; kH/kD = 0.98 ± 0.02. This observation and experiments with a radical scavenger TEMPO do not support a homolytic mechanism. A SN2 mechanism was proposed for the formation of complex 6 that involves complex 2 as a nucleophile and intermediate 8 as an electrophile.

A new theoretical approach to simulation of EPR spectra of spin-labels in protein and membrane systems

A certain model of nitroxyl radial movement is used to interpret EPR spectra of spin-labeled products. In the model proposed a fast reorientation of a spin label occurred due to the order parameter, and a slow motion of the carrier of a spin label, already with partially averaged magnetic parameters, is determined by the rotational correlation time of the macromolecule. By the example of different proteins we demonstrate how to get a fairly good coincidence of simulated EPR spectra with experimental ones within that simple model in solution and using an adsorbent. It is minimalistic in sense of involved motion parameters, and thus attempts to minimize the ambiguity of spectra interpretation. We show that TEMPO radical in lipid phase obeys two-motion model, undergoing fast anisotropic reorientation due to being incorporated into axial phase, and, is additionally affected by slow stochastic process characterized by effective nanosecond-scale correlation time, which can be attributed to domain structure of the membrane.

The synthesis and structure of [Zn(TEMPO)2]2 and [Zn(μ-H)(μ(2)-η(1):η(1)-TEMPO)]6.

The commercially available radical TEMPO (2,2,6,6-tetramethylpiperidin-1-yloxy) reacts with [ZnCp*2] (1) to yield the homoleptic compound [Zn(TEMPO)2]2 (2) through coupling of two Cp* radicals. Compound 1 reacts with H2 to afford the hydride complex [Zn(μ-H)(μ(2)-η(1)-η(1)-TEMPO)]6 (3) featuring a planar Zn6H6 ring in the solid state. Preliminary data suggest that the formation of 3 proceeds via a radical mechanism.

Electron Spin Resonance Spectroscopy Studies on Reduction Process of Nitroxyl Radicals used in Molecular Imaging

Electron spin resonance (ESR) spectroscopy studies on the reduction process of nitroxyl radicals were carried out for 1mM concentration of 14N-labeled nitroxyl radicals in 1 mM concentration of ascorbic acid as a function of time. The half life time and decay rate were estimated for 1mM concentration of 14N labeled nitroxyl radicals in 1 mM concentration of ascorbic acid. From the results, the increase in half life time and decrease in decay rate were calculated for TEMPONE compared with TEMPO and TEMPOL radicals, which indicates the higher stability of TEMPONE radical. The observed radical scavenging activity is also higher for TEMPONE radical. The ESR spectrum was also recorded for 1mM concentration of 14N-labeled nitroxyl radicals in pure water and the ESR parameters, line width, hyperfine coupling constant, g-factor, signal intensity ratio and rotational correlation time were obtained. These results indicate that the TEMPONE radical has narrowest line width and fast tumbling motion compared with TEMPO and TEMPOL. Therefore, this study reveals that the TEMPONE radical can act as a good redox sensitive spin probe for molecular imaging.

Synthesis, crystal structures and magnetic behaviour of four coordination compounds constructed with a phosphinic amide-TEMPO radical and [M(hfac)2] (M = Cu(II), Co(II) and Mn(II)).

In the present work we describe the synthesis, crystal structures and magnetic properties of four coordination compounds obtained by assembling a new phosphinic amide containing the TEMPO moiety, 1-piperidinyloxy-4-[(diphenylphosphinyl)amino]-2,2,6,6-tetramethyl radical (dppnTEMPO), with [M(hfac)2] building blocks (M = Cu(II), Co(II), Mn(II)). The crystal structures of the coordination compounds revealed the usefulness of the functionalized radical to provide discrete or extended architectures. In the copper compound () the ligand is coordinated through the oxygen atom of the NP[double bond, length as m-dash]O linkage to the metal, which exists in a square pyramidal or octahedral geometry. For the cobalt and manganese complexes (), both the phosphinic amide and the nitroxide oxygen atoms are involved in the coordination to the metal leading to one dimensional systems. In the cobalt complex () an interesting spin topology in the zig-zag chain was obtained due to the oxygen atom of the phosphinic amide group being μ2 coordinated to two cobalt(ii) ions. The magnetic behaviour of the coordination compounds shows overall antiferromagnetic interactions involving the metal ion and the organic radical. DFT calculations were performed in order to assign the main path for the magnetic interactions.

SYNTHESIS AND ELECTROCHEMICAL PROPERTIES OF POLY-[2, 5-DI-N-(2, 2, 6, 6-TETRAMETHYL-4-PIPERIDINE–N-OXYL) BENZAMIDE] ANILINE AS A CATHODE MATERIAL FOR LITHIUM-ION BATTERIES

Novel TEMPO radical grafted PANI [polymer (3)] was synthesized from 2,5-dicarboxyaniline by two-step process, and investigated with Fourier transform infrared spectroscopy, electron spin resonance spectra, thermogravimetric analysis, scanning electron microscopy, cyclic voltammograms, and galvanostatic charge–discharge techniques. The polymer (3) shows six-electron redox reaction process in the voltage ranges of 2.0–4.0 V versus Li +/ Li . A high specific capacity of 156 mA h g−1 is obtained in the experiment, close to half of the theoretical capacity of the polymer based on six-electron redox reaction (330 mA h g−1. The polymer (3) also exhibits good rate capability and cycle life. The excellent electrochemical properties benefit from fast redox reaction of nitroxy radical and conductive polyaniline nanotubes structure.

Oxidative Addition of Diorgano Disulfides to Distannyne [{2,6‐(Me2NCH2)2C6H3}Sn]2

AbstractReactivity of [{2,6‐(Me2NCH2)2C6H3}Sn]2 (1) towards functionalized diorganodisulfides R2S2 as oxidizing agents was studied. The reactions provided organotin(II) thio derivatives {2,6‐(Me2NCH2)2C6H3}Sn(S2CNMe2) (2), {2,6‐(Me2NCH2)2C6H3}Sn (SC6H4‐2‐NH2) (3) and {2,6‐(Me2NCH2)2C6H3}Sn(SC5H4N) (4). The 119Sn NMR spectroscopic data of 2–4 suggest different electronic saturation of the tin(II) atoms in 2–4, which caused redox stability of 2 and easy oxidation of 3 and 4. The treatment of the latter compounds with diorgano disulfides R2S2 yielded organotin(IV) thio derivatives {2,6‐(Me2NCH2)2C6H3}Sn(SC6H4‐2‐NH2)3 (5) and {2,6‐(Me2NCH2)2C6H3}Sn(SC5H4N)3 (6). In addition, oxidation of 3 and 4 with the radical TEMPO (2,2,6,6‐tetramethylpiperidin‐1‐oxyl) provided organotin(IV) hydroxide [{2,6‐(Me2NCH2)2C6H3}Sn(SC6H4‐2‐NH)(μ‐OH)]2 (7) and organotin(IV) oxide [{2,6‐(Me2NCH2)2C6H3}Sn(SC5H4N)(μ‐O)]3 (8). All prepared compounds 2–8 were characterized by using multinuclear NMR spectroscopy, and the molecular structures of 2, 4, 7 and 8 were determined by using X‐ray diffraction. Compounds 2–4 were also characterized by cyclic voltammetry.

Complexation of a gaseous spin probe with cyclodextrins bound to the silica microspheres: Molecular dynamics of the complexed probes and the effect of aromatic hydrocarbon vapors on it

The formation of guest-host complexes of a gaseous radical TEMPO with cyclodextrins bound to the silica microspheres (MSs) either covalently with a chemical linker or by physical adsorption has been investigated. For the former complexes, two conformational states with strongly different rotational mobilities have been found similar to those described in [9] for aqueous MS suspensions. These states are assigned to the adsorbed (s) and desorbed (w) complexes, the fraction of s-states in the gaseous phase being noticeably higher than that in the suspensions. The binding of TEMPO to the copolymers of carboxymethyl β-CD (PCCD) or sulfated β-CD (PSCD) with epichlorhydrin adsorbed on MS results in dipole-dipole interactions (DDI) between the radicals. The local concentrations of the radicals have been estimated from the spectral parameters of DDI. The concentrations considerably exceed the average ones, indicating the preferred radical location in mesopores. The TEMPO rotational mobility in complexes with PCMCD is satisfactorily described by the model of anisotropic molecular rotation in the liquid-crystal-like orienting potential in the temperature interval of 206–273 K but deviates from this model at T > 290 K. The rotational diffusion coefficients are in the range of 106.8–107.6 with an activation energy of 2.5 ± 0.15 kcal/mole and order parameter D20 in the interval of 0.85-0.55. The dependence of the portion of the s-states and rotational mobility of the w-states on the vapors of the aromatic hydrocarbons and on the CD structure was discovered for complexes with the covalently bound CD. The changes in molecular dynamics in the presence of benzene, toluene, and naphthalene were displayed for both types of sensor layers: covalently or noncovalently bound to MS. The character of these changes is specific for these hydrocarbons, so it may be used for their identification.

Toward the Understanding of Radical Reactions: Experimental and Computational Studies of Titanium(III) Diamine Bis(phenolate) Complexes

Radical reactions of titanium(III) [Ti((tBu2)O2NN')Cl(S)] (S = THF, 1; S = py, 2; (tBu2)O2NN' = Me2N(CH2)2N(CH2-2-O-3,5-(t)Bu2C6H2)2) are described. Reactions with neutral electron acceptors led to metal oxidation to Ti(IV), [Ti((tBu2)O2NN')Cl(TEMPO)] (4) being formed with the TEMPO radical and [Ti((tBu2)O2NN')Cl2] (9) with PhN═NPh. [Ti((tBu2)O2NN')Cl2] was also formed when [Ti((tBu2)O2NN')Cl(S)] was oxidized by [Cp2Fe][BPh4], but the [Cp2Fe][PF6] analogue yielded [Ti((tBu2)O2NN')ClF] (8). The reactions of [Ti((tBu2)O2NN')Cl(S)] with O2 gave [Ti((tBu2)O2NN')Cl]2(μ-O) (3). The DFT calculated Gibbs energy for the above reaction showed it to be exergonic (ΔG298 = -123.6 kcal·mol(-1)). [Ti((tBu2)O2NN')(CH2Ph)(S)] (S = THF, 5; py, 6) are not stable in solution for long periods and in diethyl ether gave 1:1 cocrystals of [Ti((tBu2)O2NN')(CH2Ph)2] (7) and [Ti((tBu2)O2NN')Cl]2(μ-O) (3), most probably resulting from a disproportionation process of titanium(III) followed by oxygen abstraction by the resulting Ti(II) species. The oxidation of [Ti((tBu2)O2NN')(κ(2)-{CH2-2-(NMe2)-C6H4})] (10), which is a Ti(III) benzyl stabilized by the intramolecular coordination of the NMe2 moiety, led to a complex mixture. Recrystallization of this mixture under air led to a 1:1 cocrystal of two coordination isomers of the titanium oxo dimer (3). In one of these isomers, one metal is pentacoordinate and the dimethylamine moiety of the diamine bis(phenolate) ligand is not bonded to the metal, displaying a coordination mode of the ligand never observed before. The other titanium center is distorted octahedral with two cis-phenolate moieties. In the second unit, the coordination of the two ancillary ligands to the titanium centers reveals mutually cis-phenolate groups in one-half of the molecule and trans-coordinated in the other titanium center, keeping a distorted octahedral environment around each titanium.

Synthesis and Structural Characterization of a Dendrimer Model Compound Based on a Cyclotriphosphazene Core with TEMPO Radicals as Substituents

The synthesis of the 3Gc0T zero generation dendrimer with a cyclotriphosphazene core functionalized with nitroxyl radicals in its six branches has been performed. The radical units have been used as probes to determine the orientation of the six branches in solution experimentally by Electron Paramagnetic Resonance (EPR) spectroscopy compared with the structure obtained in the solid state by X-ray diffraction. The orientation of the dendrimer branches is the same in solution as in the solid state.

Functionalization of Phenols Using a Biomimetic Couple of Sodium Nitrite/Hydrogen Peroxide in an Acidic Medium

Sodium nitrite and hydrogen peroxide are ubiquitous in bioorganic and environmental chemistry, and their role has been exceptionally important and under continuous investigation. Sodium nitrite is a potential source of cytotoxic peroxynitrite in biological systems, while in a tandem with hydrogen peroxide it may serve as a precursor of mutagenic aromatic nitro compounds in the environment. A biomimetic couple of sodium nitrite and an aqueous solution of hydrogen peroxide was utilized for the transformation of phenols in an acidic medium. Phenols were regioselectively converted into the corresponding nitro derivatives; the sterically hindered phenols of vitamin E type were oxidized into the quinone derivatives. This is an indication that vitamin E and its derivatives can be good nitrite scavengers. Cresols were transformed to the mixture of mono- and dinitro derivatives; the o-substitution was substantial. Regioselectivity of nitration of 3,4-dialkyl substituted phenols was in accordance with the Mills-Nixon effect; estrone was converted into a mixture of its 2- and 4-nitro analogues. Nitration of phenols obviously encountered a single electron transfer, because the transformation was completely suppressed in the presence of a free radical TEMPO. The most likely nitrating agent was nitronium ion derived from nitrogen dioxide. The NaNO2/H2O2/H2SO4 system differs from the ‘classic’ HNO3/H2SO4 system, since a considerably different selectivity was established.

Copper(II) Complexes with Schiff Bases Containing a Disiloxane Unit: Synthesis, Structure, Bonding Features and Catalytic Activity for Aerobic Oxidation of Benzyl Alcohol

AbstractMononuclear copper(II) salen‐type Schiff base complexes, CuIIL1–5 [H2L1 to H2L5 = tetradentate N,N,O,O ligands derived from 2‐hydroxybenzaldehyde, 2,4‐dihydroxybenzaldehyde, 3,5‐dibromo‐2‐hydroxybenzaldehyde, 2‐hydroxy‐5‐nitrobenzaldehyde, 5‐chloro‐2‐hydroxybenzaldehyde and 1,3‐bis(3‐aminopropyl)tetramethyldisiloxane, respectively] were prepared in situ in the presence of a copper(II) salt or by direct complexation between a copper(II) salt and a presynthesised Schiff base. The compounds {CuL1, CuL1·0.5Py, CuL2·0.375CH2Cl2, (CuL3)[Cu(4‐Me‐Py)4Cl]Cl·2H2O, CuL4, CuL4·CHCl3 and CuL5, as well as the isolated ligand H2L3} were characterised by elemental analysis, spectroscopic methods (IR, UV/Vis, 1H NMR, EPR) and X‐ray crystallography. The formation of a 12‐membered central chelate ring in these complexes is effected by the tetramethyldisiloxane unit, which separates the aliphatic chains, thus significantly reducing the mechanical strain in such a chelate ring. We dub this a “shoulder yoke effect” by analogy with the load‐spreading ability of such an ancient device. The coordination geometry of copper(II) in CuIIL1–5 can be described as tetrahedrally distorted square‐planar. Maximum tetrahedral distortion of the coordination geometry expressed by the parameter τ4 was observed for CuL1 (0.460), while distortion was minimal for the two crystallographically independent molecules of CuL2 (0.219 and 0.284). The Si–O–Si bond angle varies markedly between 169.75(2)° for CuL1 and 154.2(3)° for CuL4·CHCl3. Charge‐density and DFT calculations on CuL1 indicate high ionic character of the Si–O bonds in the tetramethyldisiloxane fragment. The new copper(II) complexes bearing the disiloxane moiety have been shown to act as catalyst precursors for the aerobic oxidation of benzyl alcohol to benzaldehyde mediated by the TEMPO radical, reaching yields and TONs up to 99 % and 990, respectively, under mild and environmentally friendly conditions (50 °C; MeCN/H2O, 1:1).

Effect of glassy modes on electron spin–lattice relaxation in solid ethanol

Electron spin–lattice relaxation (SLR) of TEMPO radical was measured in the crystalline and glassy states of deuterated ethanol in the temperature range 5–80K using X-band electron paramagnetic resonance (EPR). The measured SLR rates are higher in the glassy than in crystalline state and the excess SLR rate in glassy state is much lower than in ethanol. This result suggests that extra modes in glassy state, i.e. glassy modes, produce the excess SLR rate via the electron-nuclear dipolar (END) interaction between the electron spin of radical and the matrix protons or deuterons. Using the soft-potential model and assuming the END interaction between the electron spin and the matrix protons, the contributions to SLR rate of various mechanisms of glassy modes were theoretically analyzed. The evaluations of SLR rates in glassy ethanol indicate two main mechanisms of glassy modes: thermally activated relaxation of double-well systems and phonon-induced relaxation of quasi-harmonic local modes. The SLR rates induced by these mechanisms correlate well with the experimental data.