Switchable Polymerization Triggered by Fast and Quantitative Insertion of Carbon Monoxide into Cobalt–Oxygen Bonds

A strategy that uses carbon monoxide (CO) as a molecular trigger to switch the polymerization mechanism of a cobalt Salen complex [salen=(R,R)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine] from ring-opening copolymerization (ROCOP) of epoxides/anhydrides to organometallic mediated controlled radical polymerization (OMRP) of acrylates is described. The key phenomenon is a rapid and quantitative insertion of CO into the Co−O bond, allowing for in situ transformation of the ROCOP active species (Salen)Co^III-OR into the OMRP photoinitiator (Salen)Co^III-CO₂R. The proposed mechanism, which involves CO coordination to (Salen)Co^III-OR and subsequent intramolecular rearrangement via migratory insertion has been rationalized by DFT calculations. Regulated by both CO and visible light, on-demand sequence control can be achieved for the one-pot synthesis of polyester-b-polyacrylate diblock copolymers (Đ<1.15).     

Oxygen‐Triggered Switchable Polymerization for the One‐Pot Synthesis of CO2‐Based Block Copolymers from Monomer Mixtures

Switchable polymerization provides the opportunity to regulate polymer sequence and structure in a one‐pot process from mixtures of monomers. Herein we report the use of O₂ as an external stimulus to switch the polymerization mechanism from the radical polymerization of vinyl monomers mediated by (Salen)Co^III?R [Salen=N,N′‐bis(3,5‐di‐tert‐butylsalicylidene)‐1,2‐cyclohexanediamine; R=alkyl] to the ring‐opening copolymerization (ROCOP) of CO₂/epoxides. Critical to this process is unprecedented monooxygen insertion into the Co?C bond, as rationalized by DFT calculations, leading to the formation of (Salen)Co^III?O?R as an active species to initiate ROCOP. Diblock poly(vinyl acetate)‐b‐polycarbonate could be obtained by ROCOP of CO₂/epoxides with preactivation of (Salen)Co end‐capped poly(vinyl acetate). Furthermore, a poly(vinyl acetate)‐b‐poly(methyl acrylate)‐b‐polycarbonate triblock copolymer was successfully synthesized by a (Salen)cobalt‐mediated sequential polymerization with an O₂‐triggered switch in a one‐pot process.     

Organometallic mediated radical polymerization of vinyl acetate with Fe(acac)2

The radical polymerization of vinyl acetate (VAc) is moderated by iron(II) acetylacetonate (Fe(acac)₂) by the organometallic route (OMRP), as well as by degenerative transfer polymerization (DTP) when in the presence of excess radicals, through the formation of thermally labile organometallic Fe^III dormant species. The poly(vinyl acetate) (PVAc)‐Fe^III(acac)₂ dormant species has been isolated in the form of an oligomer and characterized by ¹H NMR, EPR, and IR methods, and then used as a single‐component initiator for the OMRP of VAc. The degree of polymerization of this isolated oligomeric species demonstrates the limited ability of Fe(acac)₂, relative to the Co(acac)₂ congener, to rapidly trap the growing PVAc radical chain. Control under OMRP conditions is improved by the presence of Lewis bases, especially PMe₂Ph. On the other hand, iron(II) phthalocyanine inhibits the radical polymerization of VAc completely. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3494–3504     

Coordination Behavior of a D‐Penicillaminato Aurate(I) Metalloligand Toward Coblat(III) Centers

Treatment of (NH₄)[Au(D‐Hpen‐S)₂](D‐H₂pen = D‐penicillamine) with CoCl₂·6H₂O in an acetate buffer solution, followed by air oxidation, gave neutral Au^ICo^III and anionic Au^I₃Co^III₂ polynuclear complexes, [Au₃Co₃(D‐pen‐N,O,S)₆]([ 1 ]) and [Au₃Co₂(D‐pen‐N,S)₆]^3? ([ 2 ]^3?), which were separated by anion‐exchange column chromatography. Complexes [ 1 ] and [ 2 ]^3? each formed a single isomer, and their structures were determined by single‐crystal X‐ray crystallography. In [ 1 ], each of three [Au(D‐pen‐S)₂]^3?metalloligands coordinates to two Co^III ions in a bis‐tridentate‐N,O,S mode to form a cyclic Au^I₃Co^III₃ hexanuclear structure, in which three [Co(D‐pen‐N,O,S)₂]^? octahedral units and six bridging S atoms adopt trans(O) geometrical and R chiral configurations, respectively. In [ 2 ]^3?, each of three [Au(D‐pen‐S)₂]^3? metalloligands coordinates to two Co^III ions in a bis‐bidentate‐N,S mode to form a Au^I₃Co^III₂ pentanuclear structure, in which two [Co(D‐pen‐N,S)₃]^3? units and six bridging S atoms adopt ∧ and R chiral configurations, respectively.     

ATRP/OMRP/CCT Interplay in Styrene Polymerization Mediated by Iron(II) Complexes: A DFT Study of the α‐Diimine System

A DFT study of various model systems has addressed the interference of catalytic chain transfer (CCT) as a function of the R² substituent in the atom‐transfer radical polymerization (ATRP) of styrene catalyzed by [FeCl₂(R¹N?C(R²)?C(R²)?NR¹)] complexes. All model systems used R¹=CH₃ in place of the experimental Cy and tBu substituents and 1‐phenylethyl in place of the polystyrene (PS) chain. A mechanistic investigation of 1) ATRP activation, 2) radical trapping in organometallic‐mediated radical polymerization (OMRP), and 3) pathways to the hydride CCT intermediate was conducted with a simplified system with R²=H. This study suggests that CCT could occur by direct hydrogen‐atom transfer without any activation barrier. Further analysis of more realistic models with R²=p‐C₆H₄F or p‐C₆H₄NMe₂ suggests that the electronic effect of the aryl para substituents significantly alters the ATRP activation barrier. Conversely, the hydrogen‐atom‐transfer barrier is essentially unaffected. Thus, the greater ATRP catalytic activity of the p‐NMe₂ system makes the background CCT process less significant. The DFT study also compares the [FeCl₂(R¹N?C(R²)?C(R²)?NR¹)] systems with a diaminobis(phenolato) derivative for which the CCT process shows even greater accessibility but has less incidence because of faster ATRP chain growth and interplay with a more efficient OMRP trapping. The difference between the two systems is attributed to destabilization of the Fe^II catalyst by the geometric constraints of the tetradentate diaminobis(phenolato) ligand.     

Widely Controllable Syngas Production by a Dye‐Sensitized TiO2 Hybrid System with ReI and CoIII Catalysts under Visible‐Light Irradiation

Visible‐light irradiation of a ternary hybrid catalyst prepared by grafting a dye, an H₂ evolving Co^III catalyst and a CO‐producing Re^I catalyst on TiO₂ have been found to produce both H₂ and CO (syngas) in CO₂‐saturated N ,N ‐dimethyl formamide (DMF)/water solution containing a 0.1 m sacrificial electron donor. The H₂/CO ratios are effectively controlled by changing either the water content of the solvent or the molar ratio of the Re^I and Co^III catalysts ranging from 1:2 to 15:1. The controlled syngas formation is discussed in terms of competitive electron flow from TiO₂ to each of the CO₂‐reduction and hydrogen‐evolving sites depending on the efficiencies of the two catalytic reaction cycles under given reaction conditions.     

Radical Polymerization of Vinyl Acetate with Bis(tetramethylheptadionato)cobalt(II): Coexistence of Three Different Mechanisms

Poly(vinyl acetate) by OMRP : Increasing the steric encumbrance of the β‐diketonate R substituents in vinyl acetate (VAc) polymerization mediator [Co{OC(R)CHC(R)O}₂] from Me to tBu sufficiently weakens the Co^III? PVAc bond of the polymer chain to allow it to operate by both associative (degenerative transfer) and dissociative (organometallic radical polymerization, OMRP) mechanisms (see scheme). The Co^III? PVAc species also acts as a transfer agent in the absence of Lewis bases, whereas the Co^II complex shows catalytic chain transfer (CCT) activity.

     

Yttrium-Mediated Ring-Opening Copolymerization of Oppositely Configurated 4-Alkoxymethylene-β-Propiolactones: Effective Access to Highly Alternated Isotactic Functional PHAs

The ring-opening copolymerization (ROCOP) of functional 4-alkoxymethylene-β-propiolactones (BPL^ORs) by yttrium-bisphenolate complexes was investigated. The ROCOP of equimolar mixtures of BPL^ORs of opposite configurations, namely (R)-BPL^OR1/(S)-BPL^OR2 [R¹, R²=OMe, OAllyl, OCH₂Ph (=OBn), OSiMe₂tBu (=OTBDMS)], by the syndioselective Y{ONOO^cum}/iPrOH catalyst/initiator system affords P(HB^OR1-alt-HB^OR2) copolymers with high alternation degrees (altern.=89–94 %), as determined by comprehensive kinetic, ¹³C{¹H} NMR spectroscopy, MALDI-ToF MS and ESI MS/MS fragmentation studies. The ROCOP of the (R)-BPL^OMe/(S)-BPL^OTBDMS combination, featuring a large difference in the substituents’ bulkiness, constitutes the only observed exception to this trend (altern.=64 %). On the other hand, the isoselectivity of the Y{ONNO^Cl}/iPrOH catalyst/initiator system has been exploited to generate, in a one-pot/one-step procedure, original mixtures of isotactic poly(hydroxyalkanoate)s (PHAs). This system efficiently transforms equimolar mixtures of (R)-BPL^OAll/(S)-BPL^OMe into a 1:1 mixture of the corresponding isotactic iso-(R)-PHB^OAll and iso-(S)-PHB^OMe homopolymers; almost no copolymerization defects are observed. This new approach has been extended successfully to the ROCOP of equimolar mixtures of racemic monomers, rac-BPL^OAll/rac-BPL^OMe.     

Ring Opening and Bidentate Coordination of Amidinate Germylenes and Silylenes on Carbonyl Dicobalt Complexes: The Importance of a Slight Difference in Ligand Volume

The reactions of [Co₂(CO)₈] with one equiv of the benzamidinate (R₂bzam) group‐14 tetrylenes [M(R₂bzam)(HMDS)] (HMDS=N(SiMe₃)₂; 1 : M=Ge, R=iPr; 2 : M=Si, R=tBu; 3 : M=Ge, R=tBu) at 20 °C led to the monosubstituted complexes [Co₂{κ¹M?M(R₂bzam)(HMDS)}(CO)₇] ( 4 : M=Ge, R=iPr; 5 : M=Si, R=tBu; 6 : M=Ge, R=tBu), which contain a terminal κ¹M–tetrylene ligand. Whereas the Co₂Si and Co₂Ge tert‐butyl derivatives 5 and 6 are stable at 20 °C, the Co₂Ge isopropyl derivative 4 evolved to the ligand‐bridged derivative [Co₂{μ‐κ²Ge,N‐Ge(iPr₂bzam)(HMDS)}(μ‐CO)(CO)₅] ( 7 ), in which the Ge atom spans the Co?Co bond and one arm of the amidinate fragment is attached to a Co atom. The mechanism of this reaction has been modeled with the help of DFT calculations, which have also demonstrated that the transformation of amidinate‐tetrylene ligands on the dicobalt framework is negligibly influenced by the nature of the group‐14 metal atom (Si or Ge) but is strongly dependent upon the volume of the amidinate N?R groups. The disubstituted derivatives [Co₂{κ¹M?M(R₂bzam)(HMDS)}₂(CO)₆] ( 8 : M=Ge, R=iPr; 9 : M=Si, R=tBu; 10 : M=Ge, R=tBu), which contain two terminal κ¹M–tetrylene ligands, have been prepared by treating [Co₂(CO)₈] with two equiv of 1 – 3 at 20 °C. The IR spectra of 8 – 10 have shown that the basicity of germylenes 1 and 3 is very high (comparable to that of trialkylphosphanes and 1,3‐diarylimidazol‐2‐ylidenes), whereas that of silylene 2 is even higher.     

UV-sensitized nanomaterial semiconductor catalytic reduction of Co<Superscript>III</Superscript>(N–N)<Subscript>3</Subscript><Superscript>3+</Superscript>/nm-TiO<Subscript>2</Subscript> and Co:TiO<Subscript>2</Subscript> formation: SEM-EDX and HRTEM analyses

Interfacial electron transfer induced by 254 nm light at nanomaterial (nm) titanium dioxide/Co^III(N–N)₃ ³⁺ interface in binary mixed solvent media such as water/methanol (or 1,4-dioxane) has been probed. The distinct photo reduction of cobalt(III) complexes, Co^III(N–N)₃ ³⁺; (N–N)=(NH₃)₂, en (1,2-diamino ethane), pn (1,2-diamino propane), tn (1,3-diamino propane), and bn (1,4-diamino butane), by excited nm-TiO₂ particles: Co^III + nm-TiO₂ + hν → TiO₂ (h⁺;e⁻) + Co^III → nm-TiO₂ (h) + Co^II is solvent controlled. The electron transfer from the conduction band of TiO₂ (e⁻, CB) onto the metal centre of the complex consists of (i) electron transport from CB into surface-adsorbed species A: Co^III(N–N)₃ ³⁺ (ii) solution phase species B: Co^III(N–N)₃ ³⁺ _(sol.), accumulated at the surface of the nanoparticle. In addition, UV irradiation of Co^III(N–N)₃ ³⁺ stimulates generation of \textCo_\textaq^\textII {\text{Co}}_{\text{aq}}^{\text{II}} ion, due to charge transfer transition, in solution phase. After UV irradiation, cobalt-implanted nm-TiO₂ separated as gray ultrafine particles, which were isolated. Photo efficiency of the formation of Co^II ion was estimated and the cobalt implanted nanomaterial crystals isolated from the photolyte solutions were subjected to SEM-EDX, X-ray mapping, and HRTEM-SAED analyses. Solvent medium was found to contribute in both the formation of Co^II ion and interstitial insertion of cobalt into the lattice of nm-TiO₂.     

SN2 or Electron Transfer?? A new technique discriminates the mechanisms of oxidative addition of alkyl halides to corrinato- and porphyrinatocobalt(I)

A fast electrochemical technique for the discrimination of one- and two-electron mechanisms in the oxidative addition of alkylating agents (RX) to corrinato- and porphyrinatocobalt(I) ([Co^IL]) is described. It is based on single-scan voltammograms of [Co^IIL] in the presence of RX and variable amounts of the radical trap acrylonitrile. In the first part of the voltammogram, [Co^IIL] is reduced, and fast oxidative addition of RX to [Co^IL] is triggered. If the reaction proceeds via a two-electron mechanism, [R Co^IIIL] is formed independently of acrylonitrile concentration, but if a transient free radical R is involved, R is competitively trapped by acrylonitrile and [Co^IIL] to yield, at high enough acrylonitrile concentration, exclusively the olefin-inserted [R_CN Co^IIIL]. [R_CN Co^IIIL] is reducible in the intermediate potential range, [R Co^IIIL] at the negative end of the single-scan voltammogram. Hence, from the appearance of the reduction waves due to [R_CN Co^IIIL] and [R Co^IIIL], the mechanism of oxidative addition of RX to [Co^IL] is easily deduced. The method is applied to the study of the mechanistic borderline of oxidative addition using a series of 15 RX and 4 [CoL]'s, i.e. cobalamin (Cbl), heptamethyl cobyrinate (‘Cby’), (tetraphenylporphyrinato) cobalt ([Co(tpp])), and (octaethylporphyrinato) cobalt ([Co(oep)]). All non-activated primary alkyl iodides and bromides exhibit, at room temperature, pure two-electron mechanisms with all [Co^IL]'s, except neopentyliodide with Cb1^I and ‘Cby’^I. All secondary alkyl iodides involve free radicals with Cb1^I and ‘Cby’^I, but a pure two-electron mechanism or a mixed one-electron two-electron mechanism with [Co^I(tpp)] and [Co^I(oep)]. The mechanistic switch from a two-electron to a one-electron mechanism for increasingly sterically demanding RX's occurs earlier with the supernucleophilic Cb1^I and ‘Cby’^I than with [Co^I(tapp)] and [Co^I(oep)].     

White Light Emissive DyIII Single‐Molecule Magnets Sensitized by Diamagnetic [CoIII(CN)6]3− Linkers

The self‐assembly of Dy^III–3‐hydroxypyridine (3‐OHpy) complexes with hexacyanidocobaltate(III) anions in water produces cyanido‐bridged {[Dy^III(3‐OHpy)₂(H₂O)₄] [Co^III(CN)₆]}?H₂O ( 1 ) chains. They reveal a single‐molecule magnet (SMM) behavior with a large zero direct current (dc) field energy barrier, ΔE=266(12) cm^?1 (≈385 K), originating from the single‐ion property of eight‐coordinated Dy^III of an elongated dodecahedral geometry, which are embedded with diamagnetic [Co^III(CN)₆]^3? ions into zig‐zag coordination chains. The SMM character is enhanced by the external dc magnetic field, which results in the ΔE of 320(23) cm^?1 (≈460 K) at H_dc=1 kOe, and the opening of a butterfly hysteresis loop below 6 K. Complex 1 exhibits white Dy^III‐based emission realized by energy transfer from Co^III and 3‐OHpy to Dy^III. Low temperature emission spectra were correlated with SMM property giving the estimation of the zero field ΔE. 1 is a unique example of bifunctional magneto‐luminescent material combining white emission and slow magnetic relaxation with a large energy barrier, both controlled by rich structural and electronic interplay between Dy^III, 3‐OHpy, and [Co^III(CN)₆]^3?.     

Ionic Polymer Microspheres Bearing a CoIII–Salen Moiety as a Bifunctional Heterogeneous Catalyst for the Efficient Cycloaddition of CO2 and Epoxides

We report a unique strategy to obtain the bifunctional heterogeneous catalyst TBB‐Bpy@Salen‐Co (TBB=1,2,4,5‐tetrakis(bromomethyl)benzene, Bpy=4,4’‐bipyridine, Salen‐Co=N,N’‐bis({4‐dimethylamino}salicylidene)ethylenediamino cobalt(III) acetate) by combining a cross‐linked ionic polymer with a Co^III–salen Schiff base. The catalyst showed extra high activity for CO₂ fixation under mild, solvent‐free reaction conditions with no requirement for a co‐catalyst. The synthesized catalyst possessed distinctive spherical structural features, abundant halogen Br^? anions with good leaving group ability, and accessible Lewis acidic Co metal centers. These unique features, together with the synergistic role of the Co and Br^? functional sites, allowed TBB‐Bpy@Salen‐Co to exhibit enhanced catalytic conversion of CO₂ into cyclic carbonates relative to the corresponding monofunctional analogues. This catalyst can be easily recovered and recycled five times without significant leaching of Co or loss of activity. Moreover, based on our experimental results and previous work, a synergistic cycloaddition reaction mechanism was proposed.     

Kinetics and mechanism of reaction of trans-(diaqua)(N,N′-ethylene-bis-salicylidenimine) chromium(iii) with sulfur(iv): The labilizing effect of coordinated sulfite

The reaction of trans-[Cr(Salen)(OH₂)₂]⁺ with aqueous sulfite yields trans-[Cr(Salen)(OH₂)(OSO₂(SINGLEBOND)O)]⁻ (O-bonded isomer). The rate and activation parameter data for the formation of the sulfito complex are consistent with a mechanism involving rate-limiting addition of SO₂ to the Cr^III(SINGLEBOND)OH bond. The complex ions, trans-[(OH₂)Cr(Salen)(OSO₂(SINGLEBOND)O)]⁻, and trans-[(OH)Cr(Salen)(OSO₂(SINGLEBOND)O)]²⁻, undergo reversible anation by NCS⁻, N₃⁻, imidazole, and pyridine resulting in the formation of trans-[XCr(Salen)(OSO₂(SINGLEBOND)O)]^(N+1)−(n=1 for X=N₃⁻,NCS⁻, and 0 for X=imidazole and pyridine) predominantly via dissociative interchange mechanism. The labilizing action of the coordinated sulfite on the trans-Cr^III-X bond in trans-[XCr(Salen)(OSO₂)]⁽ⁿ⁺¹⁾⁻ follows the sequence: NCS⁻pyridine ca. N₃⁻ ca. imidazole. Data analysis indicated that the coordinated sulfite has little trans activating influence. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 373–384, 1998     

Alternating copolymerization of propylene oxide with carbon monoxide catalyzed by Co complex and Co/Ru complexes

Co₂(CO)₈ catalyzes the ring‐opening copolymerization of propylene oxide with CO to afford the polyester in the presence of various amine cocatalysts. The ¹H and ¹³C{¹H} NMR spectra of the polyester, obtained by the Co₂(CO)₈–3‐hydroxypyridine catalyst, show the following structure ? [CH₂? CH(CH₃)? O? CO]_n? . The Co₂(CO)₈–phenol catalyst gives the polyester, which contains the partial structural unit formed through the ring‐opening copolymerization of tetrahydrofuran with CO. The bidentate amines, such as bipyridine and N,N,N′,N′‐tetramethylethylenediamine, enhance the Co complex‐catalyzed copolymerization, which produces the polyester with a regulated structure. Acylcobalt complexes, (RCO)Co(CO)_n (R = Me or CH₂Ph), prepared in situ, do not catalyze the copolymerization even in the presence of pyridine. This suggests that the chain growth involves the intermolecular nucleophilic addition of the OH group of the intermediate complex to the acyl–cobalt bond, forming an ester bond rather than the insertion of propylene oxide into the acyl–cobalt bond. Co₂(CO)₈? Ru₃(CO)₁₂ mixtures also bring about the copolymerization of propylene oxide with CO. The molar ratio of Ru to Co affects the yield, molecular weight, and structure of the produced copolymer. The catalysis is ascribed to the Ru? Co mixed‐metal cluster formed in the reaction mixture. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4530–4537, 2002     

A Paramagnetic Copper(III) Complex Containing an Octahedral CuIIIS6 Coordination Polyhedron

Only the second octahedral, paramagnetic copper(III ) complex (S=1) has now been synthesized and characterized. Six thiolato bridging ligands in the heterotrinuclear species [LCo^IIICu^IIICo^IIIL](ClO₄)₃⋅2 Me₂CO (L=1,4,7‐tris(4‐tert‐butyl‐2‐sulfidobenzyl)‐1,4,7‐triazacyclononane) stabilize this rare electron configuration. A section of the structure of the reduced form (Cu^II, S=½) is shown. XAS, EXAFS, and EPR spectroscopy prove unambiguously that the one‐electron oxidation to the copper(III ) is metal‐ rather than ligand‐centered.     

Kinetic study of RuIII‐catalyzed polymerization of methylmethacrylate initiated with n‐butylamine in the presence of carbon tetrachloride by a charge‐transfer mechanism

Ruthenium trichloride (RuCl₃ or Ru^III) catalyzed polymerization of methylmethacrylate (MMA) initiated with n‐butylamine (BA) in the presence of carbon tetrachloride (CCl₄) by a charge‐transfer mechanism has been investigated in a dimethylsulfoxide (DMSO) medium by employing a dilatometric technique at 60°C. The rate of polymerization (R_p) has been obtained under the conditions [CCl₄]/[BA] ? 1 and [CCl₄]/[BA] ? 1. The kinetic data indicate the possible participation of the charge‐transfer complex formed between the amine–Ru^III complex and CCl₄ in the polymerization of MMA. In the absence of either CCl₄ or BA, no polymerization of MMA is observed under the present experimental conditions. The rate of polymerization is inhibited by hydroquinone, suggesting a free‐radical initiation. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 43: 70–77, 2011     

Entrapment of a Pseudo‐Tetrahedral CoII Center by Thioether Sulfur Bound {Co2(μ‐L)} Fragments: Synthesis,Field‐Induced Single‐Ion Magnetism and Catechol Oxidase Mimicking Activity

Simultaneous incorporation of both Co^II and Co^III ions within a new thioether S‐bearing phenol‐based ligand system, H₃L (2,6‐bis‐[{2‐(2‐hydroxyethylthio)ethylimino}methyl]‐4‐methylphenol) formed [Co₅] aggregates [Co^IICo^III₄L₂(μ‐OH)₂(μ_1,3‐O₂CCH₃)₂](ClO₄)₄?H₂O ( 1 ) and [Co^IICo^III₄L₂(μ‐OH)₂(μ_1,3‐O₂CC₂H₅)₂](ClO₄)₄?H₂O ( 2 ). The magnetic studies revealed axial zero‐field splitting (ZFS) parameter, D/hc=?23.6 and ?24.3 cm^?1, and E/D=0.03 and 0.00, respectively for 1 and 2 . Dynamic magnetic data confirmed the complexes as SIMs with U_eff/k_B=30 K ( 1 ) and 33 K ( 2 ), and τ₀=9.1×10^?8 s ( 1 ), and 4.3×10^?8 s ( 2 ). The larger atomic radius of S compared to N gave rise to less variation in the distortion of tetrahedral geometry around central Co^II centers, thus affecting the D and U_eff/k_B values. Theoretical studies also support the experimental findings and reveal the origin of the anisotropy parameters. In solutions, both 1 and 2 which produce {Co^III₂(μ‐L)} units, display solvent‐dependent catechol oxidation behavior toward 3,5‐di‐tert‐butylcatechol in air. The presence of an adjacent Co^III ion tends to assist the electron transfer from the substrate to the metal ion center, enhancing the catalytic oxidation rate.     

3‐Rhoda‐1,2‐diazacyclopentanes: A Series of Novel Metallacycle Complexes Derived From CN Functionalization of Ethylene

Rh‐containing metallacycles, [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NR)₂‐]Cl; TPA=N,N,N,N‐tris(2‐pyridylmethyl)amine have been accessed through treatment of the Rh^I ethylene complex, [(TPA)Rh(η²‐CH₂CH₂)]Cl ([ 1 ]Cl) with substituted diazenes. We show this methodology to be tolerant of electron‐deficient azo compounds including azo diesters (RCO₂N?NCO₂R; R=Et [ 3 ]Cl, R=iPr [ 4 ]Cl, R=tBu [ 5 ]Cl, and R=Bn [ 6 ]Cl) and a cyclic azo diamide: 4‐phenyl‐1,2,4‐triazole‐3,5‐dione (PTAD), [ 7 ]Cl. The latter complex features two ortho‐fused ring systems and constitutes the first 3‐rhoda‐1,2‐diazabicyclo[3.3.0]octane. Preliminary evidence suggests that these complexes result from N–N coordination followed by insertion of ethylene into a [Rh]?N bond. In terms of reactivity, [ 3 ]Cl and [ 4 ]Cl successfully undergo ring‐opening using p‐toluenesulfonic acid, affording the Rh chlorides, [(TPA)Rh^III(Cl)(κ¹‐(C)‐CH₂CH₂(NCO₂R)(NHCO₂R)]OTs; [ 13 ]OTs and [ 14 ]OTs. Deprotection of [ 5 ]Cl using trifluoroacetic acid was also found to give an ethyl substituted, end‐on coordinated diazene [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NH)₂‐]⁺ [ 16 ]Cl, a hitherto unreported motif. Treatment of [ 16 ]Cl with acetyl chloride resulted in the bisacetylated adduct [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NAc)₂‐]⁺, [ 17 ]Cl. Treatment of [ 1 ]Cl with AcN?NAc did not give the Rh?N insertion product, but instead the N,O‐chelated complex [(TPA)Rh^I ( κ²‐(O,N)‐CH₃(CO)(NH)(N?C(CH₃)(OCH?CH₂))]Cl [ 23 ]Cl, presumably through insertion of ethylene into a [Rh]?O bond.     

Trinuclear [CoIII2–LnIII] (Ln=Tb,Dy) Single‐Ion Magnets with Mixed 6‐Chloro‐2‐Hydroxypyridine and Schiff Base Ligands

The Schiff base ligand N1,N3‐bis(3‐methoxysalicylidene)diethylenetriamine (H₂valdien) and the co‐ligand 6‐chloro‐2‐hydroxypyridine (Hchp) were used to construct two 3d–4f heterometallic single‐ion magnets [Co₂Dy(valdien)₂(OCH₃)₂(chp)₂] ? ClO₄ ? 5 H₂O ( 1 ) and [Co₂Tb(valdien)₂(OCH₃)₂(chp)₂] ? ClO₄ ? 2 H₂O ? CH₃OH ( 2 ). The two trinuclear [Co^III₂Ln^III] complexes behave as a mononuclear Ln^III magnetic system because of the presence of two diamagnetic cobalt(III) ions. Complex 1 has a molecular symmetry center, and it crystallizes in the C2/c space group, whereas complex 2 shows a lower molecular symmetry and crystallizes in the P2₁/c space group. Magnetic investigations indicated that both complexes are field‐induced single‐ion magnets, and the Co^III₂–Dy^III complex possesses a larger energy barrier [74.1(4.2) K] than the Co^III₂–Tb^III complex [32.3(2.6) K].