A Neutral Borylene and its Conversion to a Radical by Selective Hydrogen Transfer.

A successful selective reduction of XB-Tip (Tip = 1,3,5-Pr-CH, X = I, Br) with KC and Mg metal, respectively, in the presence of a hybrid ligand (CH(PPh)LSi) leads to a stable low-valent five-membered ring as a boryl radical [CH(PPh)LSiBTip][Br] () and neutral borylene [CH(PPh)LSiBTip] (). Compound reacts with 1,4-cyclohexadiene, resulting in hydrogen abstraction to afford the radical [CH(PPh)LSiB(H)Tip] (). Quantum chemical studies reveal that compound is a B-centered radical, and compound is a phosphane and silylene stabilized neutral borylene in a trigonal planar environment, whereas compound is an amidinate-centered radical.

Excellent yield of a variety of silicon-boron radicals and their reactivity.

Herein we report stable silicon-boron radicals of composition LSi(NMe)-B(Br)Tip (1), LSi(NMe)-B(I)Tip (2) LSi(Bu)-B(I)Tip (3) [L = PhC(NBu)]. They were prepared in high yield using a one pot reaction of LSiR, XBTip and KC in a 1 : 1 : 1 molar ratio (R = Bu, NMe; X = Br, I). The reaction of the silicon-boron radical with Br and Se affords the dihalogenated compound LSi(Bu)-B(Br)Tip (4) and oxidative addition product LSi(Bu)Se (5).

Diosmium compounds containing bis(imidazole)--quinone bridging ligands.

The doubly deprotonated bridging ligand L derived from 2,6-bis(2-pyridyl)-1,5-dihydro-1',4'-benzoquinono[2',3'-:5',6'-']diimidazole HL forms coordination compounds with two bis(2,2'-bipyridine)osmium(II) complex fragments in ([1](ClO)) and configurations ([2](ClO)) of {(μ-L)[Os(bpy)]}(ClO), as evident from crystal structure analyses. Exchange of the metal-coordinating 2-pyridyl functions in the bridge through non-coordinating 4-tolyl substituents (L → L) leads to [3](ClO) which involves chelation of the [Os(bpy)] groups through imidazole-N and carbonyl-O atoms of the central -quinone function. In addition to identification, the compounds were subjected to electrochemical (CV, DPV) and spectroelectrochemical (UV-vis-NIR, EPR) analyses of electron transfer, the results being supported by results from TD-DFT calculations.

Selective Route to Stable Silicon-Boron Radicals and Their Corresponding Cations.

Herein, we report on a facile and selective one-pot synthetic route to silicon-boron radicals. Reduction of BrBTip (Tip = 2,4,6-PrCH) with KC in the presence of LSi-R affords LSi(Bu)-B(Br)Tip () and LSi(N(TMS))-B(Br)Tip () [L = PhC(NBu)]. These first examples of silicon-boron isolated radical species feature spin density on the silicon and boron atoms.

Bis-[3]Ferrocenophanes with Central >E-E'< Bonds (E, E'=P, SiH): Preparation, Properties, and Thermal Activation.

A series of bis-[3]ferrocenophanes of the general type Fe(CHE')E-E(E'CH)Fe (E=P, SiH and E'=PBu, NPentyl, NSi(CH)) with an isolobal molecular framework have been prepared and characterized by heteronuclear NMR spectroscopy and X-ray crystallography. The thermal dissociation behavior with respect to homolytic fission of the central bond generating phosphorus centered radicals was investigated using EPR spectroscopy and quantum chemical calculations.

Bis-[3]Ferrocenophanes with Central >E-E'< Bonds (E, E'=P, SiH): Preparation, Properties, and Thermal Activation.

Invited for this month's cover picture are the groups of Professors Rudolf Pietschnig at the University of Kassel, Professor Dietrich Gudat at the University of Stuttgart and Professor László Nyulászi at the Budapest University of Technology and Economics. The cover picture shows the thermally induced homolytic cleavage of the central P-P bond in a phosphorus-rich bis-ferrocenophane furnishing P-centered radicals (as evidenced by the computed spin-density highlighted in blue). The central P unit in the title compound is a structural analog of the connecting unit in Hittorf's violet phosphorus, which links the orthogonally arranged tubular entities.

Isolation of base stabilized fluoroborylene and its radical cation.

Herein, we report the synthesis and characterization of the metal free low valent fluoroborylene [(Me-cAAC)BF] (1) stabilized by cyclic (alkyl)(amino) carbene (cAAC). The fluoroborylene 1 is obtained by the reductive defluorination of Me-cAAC:BF with 2.0 equivalents of KC in the presence of 1.

Synthesis of cAAC stabilized biradical of "MeSi" and "MeSiCl" monoradical from MeSiCl - an important feedstock material.

The cyclic alkyl(amino) carbene (cAAC) coordinated biradical of dimethylsilicon was isolated as (cAAC)2Me2Si (1), (cAAC = C(CH2)(CMe2)2N-2,6-i-Pr2C6H3), synthesized from the reduction of Me2SiCl2 using two equivalents of KC8 in the presence of two equivalents of cAAC. The reduction of Me2SiCl2 by one equivalent of KC8 in the presence of one equivalent of cAAC resulted in the stable dimethylsiliconchloride monoradical (cAAC)Me2SiCl (2).

Isolation of Transient Acyclic Germanium(I) Radicals Stabilized by Cyclic Alkyl(amino) Carbenes.

Despite the notable progress in the stabilization of main group radicals by NHCs and cAACs, no germanium radicals have been isolated so far due to synthetic challenges. Stabilization of neutral [:ER] (E = Si, Ge) radicals is an uphill task, as these reactive transient species are highly susceptible to dimerization. Herein, we report the synthesis of acyclic neutral germanium(I) radicals Cy-cAAC:GeN(SiMe)Dip (1) and Me-cAAC:GeN(SiPh)Mes (2) obtained by the reduction of [Ar(SiR)NGeCl] with KC in the presence of cAAC.

Effect of positional isomerism on the spectroelectrochemical response of 3,6-bis(2-pyridyl)-diketopyrrolopyrrolate bridged bis(carbonylhydridoruthenium) compounds.

Reaction of 3,6-bis(2-pyridyl)-diketopyrrolopyrrole (H2-BPDPP) with two equivalents of [Ru(H)(CO)(Cl)(PPh3)3] in EtOH produced two symmetrical dinuclear isomers, (μ-BPDPP)[Ru(CO)H(PPh3)2]2, green 1 and blue 2, which could be separated chromatographically and characterised spectroscopically (1H and 31P NMR, IR, and UV-VIS). Isomeric forms of 1 and 2 were authenticated using their single crystal X-ray structures. In addition to the essentially planar bis-chelating bridge BPDPP2- and the mutually trans positioned axial PPh3 ligands in both complexes, compound 1 was established with the CO groups trans to the pyrrolate-N atoms, whereas 2 has the π acceptors CO and pyridine-N situated trans to each other.

Diruthenium Complexes of p-Benzoquinone-Imidazole Hybrid Ligands: Innocent or Noninnocent Behavior of the Quinone Moiety.

After double deprotonation, 2,6-diaryl-p-benzoquinonodiimidazoles (aryl=4-tolyl (I) or 2-pyridyl (II)) were shown to bridge two [Ru(bpy) ] (bpy=2,2'-bipyridine) complex fragments through the imidazolate N and p-quinone O (I→1 ) or through the imidazolate N and pyridyl N donor atoms (II→2 ). Characterization by crystal structure analysis, H/ C NMR spectroscopy, cyclic and differential pulse voltammetry, and spectroelectrochemistry (UV/Vis/NIR, IR, EPR) in combination with TD-DFT calculations revealed surprisingly different electronic structures for redox systems 1 and 2 . Whereas 1 is reduced to a radical complex with considerable semiquinone character, the reduction of 2 with its exclusive N coordination exhibits little spin on the now redox-innocent quinone moiety, compared with the electron uptake by the pyridyl-imidazolate chelating site.

At the Borderline between Metal-Metal Mixed Valency and a Radical Bridge Situation: Four Charge States of a Diruthenium Complex with a Redox-Active Bis( mer-tridentate) Ligand.

The complex ions [LRu(μ,η:η-BL)RuL] (1 , L = 4,4',4″-tri- tert-butyl-2,6,2',6″-terpyridine and HBL = 1,2-bis(salicyloyl)hydrazide(2-)) were isolated with PF or ClO counterions ( n = 1) and as bis(hexafluorophosphate) ( n = 2). Structural, electrochemical, and spectroscopic characterization reveals the monocation as intermediate ( K = 10) in the three-step reversible redox system 1. The 1 ion has the molecule-bridged (Ru- - -Ru 4.

A Route to Base Coordinate Silicon Difluoride and the Silicon Trifluoride Radical.

Silicon difluoride (SiF ) is highly unstable at room temperature and condenses at this temperature rapidly to a polymeric material of unknown structure. Therefore, the isolation of a stable monomeric silicon difluoride species is a challenging task. The cyclic alkyl(amino) carbene (cAAC) coordinated silicon difluoride was isolated as (cAAC) SiF (2), synthesized from the reduction of cAAC-SiF (1) by using two equivalents of KC in the presence of one equivalent of cAAC.

Non-innocence and mixed valency in tri- and tetranuclear ruthenium complexes of a heteroquinone bridging ligand.

The redox-active ligand 5,7,12,14-tetraazapentacene-6,13-quinone = L forms structurally characterised compounds with three (1) or four (2) [Ru(acac)] complex fragments in which each of the metals is N,O-chelated. The new tris- and tetrakis-bidentate chelate compounds exhibit ruthenium centres bridged at about 4 Å by quinone O atoms which are then situated across the pentacene π system at about 6-8 Å distance. Several electron transfer processes were observed by voltammetry (CV, DPV) and the intermediates identified by EPR and UV-Vis-NIR spectroelectrochemistry.

Synthesis and characterization of Lewis base stabilized mono- and di-organo aluminum radicals.

Two cyclic (alkyl)(amino)carbene (cAAC) stabilized mononuclear neutral radicals of aluminum have been synthesized. They contain an ethyl [(cAAC)AlClEt (1)] and as well a diethyl group [(cAAC)AlEt (2)], and have been prepared from the reduction of EtAlCl and EtAlCl, respectively, with KC. Compounds 1 and 2 are monoradicals, which were confirmed by EPR measurements to have the spin located on the carbene carbon of one of the cAAC ligands.

Organosilicon Radicals with Si-H and Si-Me Bonds from Commodity Precursors.

The cyclic alkyl(amino) carbene (cAAC) stabilized biradicals of composition (cAAC)SiH (1), (cAAC)SiMe-SiMe(cAAC) (2), and (cAAC)SiMeCl-SiMeCl(cAAC) (3) have been isolated as molecular species. All the compounds are stable at room temperature for more than 6 months under inert conditions in the solid state. All radical species were fully characterized by single-crystal X-ray structure analysis and EPR spectroscopy.

A Stable Neutral Radical in the Coordination Sphere of Aluminum.

The neutral radical (Me -cAAC) AlCl (2) is stabilized by cyclic (alkyl)(amino)carbenes (cAACs). Complex 2 was synthesized by reduction of the Me -cAAC:→AlCl (1) adduct with KC in the presence of another equivalent of Me -cAAC. The crystal structure of 2 shows that the Al-C bond lengths of the two carbenes bound to the Al center are considerably different, which is likely the result of intermolecular interactions.

Hydroxyl radical production by a heterogeneous Fenton reaction supported in insoluble tannin from bark of Pinus radiata.

Fenton reactions driven by dihydroxybenzenes (DHBs) have been used for pollutant removal via advanced oxidation processes (AOPs), but such systems have the disadvantage of DHB release into the aqueous phase. In this work, insoluble tannins from bark can be used to drive Fenton reactions and as a heterogeneous support. This avoids the release of DHBs into the aqueous phase and can be used for AOPs.

Noninnocently Behaving Bridging Anions of the Widely Distributed Antioxidant Ellagic Acid in Diruthenium Complexes.

Dinuclear compounds [L2Ru(μ-E)RuL2](n) where L is acetylacetonate (acac(-), 2,4-pentanedionate), 2,2'-bipyridine (bpy), or 2-phenylazopyridine (pap) and EH4 is ellagic acid, an antioxidative bis-catechol natural product, were studied by voltammetric and spectroelectrochemical techniques (UV-vis-NIR and electron paramagnetic resonance (EPR)). The electronic structures of the isolated forms (NBu4)2[(acac)2Ru(μ-E)Ru(acac)2] ((NBu4)2[1]), [(bpy)2Ru(μ-E)Ru(bpy)2]ClO4 ([2]ClO4), and [(pap)2Ru(μ-E)Ru(pap)2] ([3]) were characterized by density functional theory (DFT) in conjunction with EPR and UV-vis-NIR measurements. The crystal structure of (NBu4)2[1] revealed the meso form and a largely planar Ru(μ-E)Ru center.

Evidence for Bidirectional Noninnocent Behavior of a Formazanate Ligand in Ruthenium Complexes.

Redox series of the complexes [Ru(L)(L')2](n), L = 1,5-diphenyl-3-(4-tolyl)-formazanate and L' = 2,4-pentanedionate (acac(-)), 2,2'-bipyridine (bpy), or 2-phenylazopyridine (pap), were studied by cyclic and differential pulse voltammetry and by TD-DFT-supported spectroelectrochemistry (UV-vis-NIR, EPR). The precursors [Ru(III)(L(-))(acac(-))2], [Ru(II)(L(-))(bpy)2]ClO4, and [Ru(II)(L(-))(pap)2]ClO4 were identified in their indicated oxidation states by X-ray crystal structure determination. The six-membered formazanato-ruthenium chelate rings have an envelope conformation with puckering of the metal.

Varying Electronic Structures of Diosmium Complexes from Noninnocently Behaving Anthraquinone-Derived Bis-chelate Ligands.

The new compounds [(bpy)2Os(II)(μ-L1(2-))Os(II)(bpy)2](ClO4)2 ([1](ClO4)2) and [(pap)2Os(II)(μ-L1(2-))Os(II)(pap)2](ClO4)2 ([2](ClO4)2) (H2L1 = 1,4-dihydroxy-9,10-anthraquinone, bpy = 2,2(/)-bipyridine, and pap = 2-phenylazopyridine) and [(bpy)2Os(II)(μ-L2(•-))Os(II)(bpy)2](ClO4)3 ([3](ClO4)3) and [(pap)2Os(II)(μ-L2(2-))Os(II)(pap)2](ClO4)2 ([4](ClO4)2) (H2L2 = 1,4-diamino-9,10-anthraquinone) have been analytically identified as the meso and rac diastereoisomers, respectively. The paramagnetic [3](ClO4)3 was also characterized by crystal structure determination. In CD3CN solution, [3](ClO4)3 displays rather narrow but widely split (13 > δ > -8 ppm) resonances in the (1)H NMR spectrum, yet no EPR signal was observed down to 120 K.

Carbene supported dimer of heavier ketenimine analogue with p and si atoms.

A cyclic alkyl(amino) carbene (cAAC) stabilized dimer [(cAAC)Si(P-Tip)]2 (2) (Tip = 2,4,6-triisopropylphenyl) is reported. 2 can be considered as a dimer of the heavier ketenimine (R2C═C═N-R) analogue. The dark-red rod-shaped crystals of 2 were synthesized by reduction of the precursor, cAAC-dichlorosilylene-stabilized phosphinidene (cAAC)SiCl2→P-Tip with sodium napthalenide.

Stable radicals from commonly used precursors trichlorosilane and diphenylchlorophosphine.

Intermediate species dichlorosilylene was generated in situ from trichlorosilane and inserted into the P-Cl bond of diphenylchlorophosphine (Ph2P-Cl) to obtain Ph2P-SiCl3 (1). Monodechlorination of 1 by cyclic alkyl(amino) carbenes (cAACs)/KC8 in THF at low temperature led to the formation of stable radicals Ph2P-Si(cAAC·)Cl2 (2a,b). Compounds 2a,b were characterized by X-ray single crystal diffraction, mass spectrometry and studied by cyclic voltammetry and theoretical calculations.

Ancillary ligand control of electronic structure in o-benzoquinonediimine-ruthenium complex redox series: structures, electron paramagnetic resonance (EPR), and ultraviolet-visible-near-infrared (UV-vis-NIR) spectroelectrochemistry.

The compounds Ru(acac)2(Q) (1), [Ru(bpy)2(Q)](ClO4)2 ([2](ClO4)2), and [Ru(pap)2(Q)]PF6 ([3]PF6), containing Q = N,N'-diphenyl-o-benzoquinonediimine and donating 2,4-pentanedionate ligands (acac(-)), π-accepting 2,2(/)-bipyridine (bpy), or strongly π-accepting 2-phenylazopyridine (pap) were prepared and structurally identified. The electronic structures of the complexes and several accessible oxidized and reduced forms were studied experimentally (electrochemistry, magnetic resonance, ultraviolet-visible-near-infrared (UV-vis-NIR) spectroelectrochemistry) and computationally (DFT/TD-DFT) to reveal significantly variable electron transfer behavior and charge distribution. While the redox system 1(+)-1(-) prefers trivalent ruthenium with corresponding oxidation states Q(0)-Q(2-) of the noninnocent ligand, the series 2(2+)-2(0) and 3(2+)-3(-) retain Ru(II).

Isolation of neutral mononuclear copper complexes stabilized by two cyclic (alkyl)(amino)carbenes.

Two (cAAC)2Cu complexes, featuring a two-coordinate copper atom in the formal oxidation state zero, were prepared by reducing (Et2-cAAC)2Cu(+)I(-) with metallic sodium in THF, and by a one-pot synthesis using Me2-cAAC, Cu(II)Cl2, and KC8 in toluene in a molar ratio of 2:1:2, respectively. Both complexes are highly air and moisture sensitive but can be stored in the solid state for a month at room temperature. DFT calculations showed that in these complexes the copper center has a d(10) electronic configuration and the unpaired electron is delocalized over two carbene carbon atoms.