Fabian Ehret

Formation of Trichlorosilyl-Substituted Carbon-Centered Stable Radicals through the Use of π-Accepting Carbenes†

Carbon or silicon radicals with silyl substituents are important intermediates in organometallic and organic chemistry. Already in 1970, Bassindale et al. had reported the persistent tris(trimethylsilyl)methyl radical, (Me3Si)3CC, which has a lifetime of several days at 298 K. The groups of Ingold, Apeloig, Bravo-Zhivotovskii, Lee, Sekiguchi, and others have shown that the lifetime of the radicals largely depends on the steric bulk of the substituents. In 2002, Sekiguchi et al. reported the first stable silicon-centered radical without any p conjugation. Several reports describe the preparation of this class of radicals. The most successful ones proceed through the photolytic or thermal cleavage of a Si Si bond, and R3Si SiHCl2 can be reacted with bulky reagents, such as (tBu)2MeSiLi, according to the Apeloig– Sekiguchi method. Several radical species of main-group elements, such as PNC, P2C , phosphinyl radical cations, HBC, and ketenes with biradical character, were also stabilized by cyclic alkyl(amino) carbenes (cAACs). We have observed that the chemical response of cAACs toward silylenes is very different from that of N-heterocyclic carbenes (NHCs). Recently, we have demonstrated that NHC!SiCl2 1 reacts with Me2-cAAC in a redox reaction to form the biradical (Me2-cAACC)2SiCl2 2 in a two-electron redox step. [8g] Consequently, we were curious to investigate the one-electron redox process. Therefore, we reacted cAAC!SiCl4 3 with KC8 in an equimolar ratio in n-hexane to yield stable radicals 4, of the general formula (cAACC)–SiCl3. The highly reactive trichloromethane radical CCl3C [9a] and its congener SiCl3C [9b] are frequently generated by flash photolysis. These species are active radical intermediates in many photochemical transformations. Recently, the chemical reactivity of the TEMPO radical towards SiCl4 [9c] and metal ions was explored, and the catalytic and fashionable magnetic properties of these adducts were studied. Although handling and controlling the chemical behavior of radicals can be very difficult, the chemistry of radicals has always been captivating. To the best of our knowledge, stable radicals with the SiCl3 group next to the radical center have not been reported so far. The carbene carbon atom of an NHC is bound to two both swithdrawing and p-donating nitrogen atoms. In a cAAC, one nitrogen atom of the NHC is replaced by a s-donating quaternary carbon atom. Theoretical calculations showed that the HOMO–LUMO energy gap is smaller in cAACs. Thus, cAACs are both more nucleophilic and more electrophilic than NHCs. Recently, P NMR analysis of a number of carbene–phenylphosphinidene adducts revealed that cAACs are better p acceptors than NHCs. These inherent differences may play a pivotal role in the replacement of an NHC by a cAAC and their behavior in a reaction. The NHC!SiCl4 adduct was reduced to NHC!SiCl2, (NHC! SiCl)2, and NHC!Si=Si !NHC through the use of KC8. In detail, an equimolar mixture of cAAC!SiCl4 3 (1 mmol) and KC8 (1 mmol) in n-hexane (85 mL) was initially reacted at 78 8C. The resulting suspension was slowly warmed to room temperature to obtain a clear colorless solution and an unreacted deposit of insoluble KC8. Upon stirring for 24 h, the color of the solution changed to a clear light yellow with the black deposit of graphite; after filtration, the solution was concentrated to a volume of 2–3 mL to obtain fluorescent yellow plates/needles of the (cAACC)-SiCl3 radical 4. Herein, we report the synthesis, structural correlation, DFT calculations, and EPR studies of the two carbon-centered radicals 4a and 4b (Scheme 1). The syntheses of 3a and 3b are described in the Supporting Information. The melting points of compounds 3a, 3b, 4a, and 4b range from 105 8C to 118 8C. The color of compounds 4a and 4b is a fluorescent yellow, whereas 3a and 3b are colorless, as expected. Compounds 4a and 4b are soluble in toluene, benzene, and THF, whereas 3a is only soluble in THF, and 3b is only sparingly soluble in this solvent. Compounds 3a, 3b, 4a, and 4b are stable in inert [*] Dr. K. C. Mondal, Prof. Dr. H. W. Roesky, Dr. A. C. St ckl Institut f r Anorganische Chemie, Universit t Gcttingen Tammannstrasse 4, 37077 Gcttingen (Germany) E-mail: hroesky@gwdg.de

Experimental and Theoretical Investigations of the Existence of Cu(II), Cu(III), and Cu(IV) in Copper Corrolato Complexes.

The most common oxidation states of copper in stable complexes are +I and +II. Cu(III) complexes are often considered as intermediates in biological and homogeneous catalysis. More recently, Cu(IV) species have been postulated as possible intermediates in oxidation catalysis. Despite the importance of these higher oxidation states of copper, spectroscopic data for these oxidation states remain scarce, with such information on Cu(IV) complexes being non-existent. We herein present the synthesis and characterization of three copper corrolato complexes. A combination of electrochemistry, UV/Vis/NIR/EPR spectroelectrochemistry, XANES measurements, and DFT calculations points to existence of three distinct redox states in these molecules for which the oxidation states +II, +III, and +IV can be invoked for the copper centers. The present results thus represent the first spectroscopic and theoretical investigation of a Cu(IV) species, and describe a redox series where Cu(II), Cu(III), and Cu(IV) are discussed within the same molecular platform.

Sensitivity of a strained C-C single bond to charge transfer: redox activity in mononuclear and dinuclear ruthenium complexes of bis(arylimino)acenaphthene (BIAN) ligands.

The new compounds [Ru(acac)2(BIAN)], BIAN = bis(arylimino)acenaphthene (aryl = Ph (1a), 4-MeC6H4 (2a), 4-OMeC6H4 (3a), 4-ClC6H4 (4a), 4-NO2C6H4 (5a)), were synthesized and structurally, electrochemically, spectroscopically, and computationally characterized. The α-diimine sections of the compounds exhibit intrachelate ring bond lengths 1.304 Å < d(CN) < 1.334 and 1.425 Å < d(CC) < 1.449 Å, which indicate considerable metal-to-ligand charge transfer in the ground state, approaching a Ru(III)(BIAN(•-)) oxidation state formulation. The particular structural sensitivity of the strained peri-connecting C-C bond in the BIAN ligands toward metal-to-ligand charge transfer is discussed. Oxidation of [Ru(acac)2(BIAN)] produces electron paramagnetic resonance (EPR) and UV-vis-NIR (NIR = near infrared) spectroelectrochemically detectable Ru(III) species, while the reduction yields predominantly BIAN-based spin, in agreement with density functional theory (DFT) spin-density calculations. Variation of the substituents from CH3 to NO2 has little effect on the spin distribution but affects the absorption spectra. The dinuclear compounds {(μ-tppz)[Ru(Cl)(BIAN)]2}(ClO4)2, tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine; aryl (BIAN) = Ph ([1b](ClO4)2), 4-MeC6H4 ([2b](ClO4)2), 4-OMeC6H4 ([3b](ClO4)2), 4-ClC6H4 ([4b](ClO4)2), were also obtained and investigated. The structure determination of [2b](ClO4)2 and [3b](ClO4)2 reveals trans configuration of the chloride ligands and unreduced BIAN ligands. The DFT and spectroelectrochemical results (UV-vis-NIR, EPR) indicate oxidation to a weakly coupled Ru(III)Ru(II) mixed-valent species but reduction to a tppz-centered radical state. The effect of the π electron-accepting BIAN ancillary ligands is to diminish the metal-metal interaction due to competition with the acceptor bridge tppz.

Electronic structure and catalytic aspects of [Ru(tpm)(bqdi)(Cl/H2O)]n, tpm = tris(1-pyrazolyl)methane and bqdi = o-benzoquinonediimine

The diamagnetic complexes [Ru(tpm)(bqdi)(Cl)]ClO(4) ([1]ClO(4)) (tpm = tris(1-pyrazolyl)methane, bqdi = o-benzoquinonediimine) and [Ru(tpm)(bqdi)(H(2)O)](ClO(4))(2) ([2](ClO(4))(2)) have been synthesized. The valence state-sensitive bond distances of coordinated bqdi [C-N: 1.311(5)/1.322(5) Å in [1]ClO(4); 1.316(7)/1.314(7) Å in molecule A and 1.315(6)/1.299(7) Å in molecule B of [2](ClO(4))(2)] imply its fully oxidised quinonediimine (bqdi(0)) character. DFT calculations of 1(+) confirm the {Ru(II)-bqdi(0)} versus the antiferromagnetically coupled {Ru(III)-bqdi˙(-)} alternative. The (1)H NMR spectra of [1]ClO(4) in different solvents show variations in chemical shift positions of the NH (bqdi) and CH (tpm) proton resonances due to their different degrees of acidity in different solvents. In CH(3)CN/0.1 mol dm(-3) Et(4)NClO(4), [1]ClO(4) undergoes one reversible Ru(II)⇌ Ru(III) oxidation and two reductions, the reversible first electron uptake being bqdi based (bqdi(0)/bqdi˙(-)). The electrogenerated paramagnetic species {Ru(III)-bqdi(0)}(1(2+)) and {Ru(II)-Q˙(-)}(1) exhibit Ru(III)-type (1(2+): = 2.211/Δg = 0.580) and radical-type (1: g = 1.988) EPR signals, respectively, as is confirmed by calculated spin densities (Ru: 0.767 in 1(2+), bqdi: 0.857 in 1). The aqua complex [2](ClO(4))(2) exhibits two one-electron oxidations at pH = 7, suggesting the formation of {Ru(IV)[double bond, length as m-dash]O} species. The electronic spectral features of 1(n) (n = charge associated with the different redox states of the chloro complex: 2+, 1+, 0) in CH(3)CN and of 2(2+) in H(2)O have been interpreted based on the TD-DFT calculations. The application potential of the aqua complex 2(2+) as a pre-catalyst towards the epoxidation of olefins has been explored in the presence of the sacrificial oxidant PhI(OAc)(2) in CH(2)Cl(2) at 298 K, showing the desired selectivity with a wide variety of alkenes. DFT calculations based on styrene as the model substrate predict that the epoxidation reaction proceeds through a concerted transition state pathway.

Stabilization of a Two-Coordinate Mononuclear Cobalt(0) Compound

Compound (Me2 -cAAC:)2 Co(0) (2; Me2 -cAAC:=cyclic (alkyl) amino carbene; :C(CH2 )(CMe2 )2 N-2,6-iPr2 C6 H3 ) was synthesized by the reduction of the precursor (Me2 -cAAC:)2 Co(I) Cl (1) with KC8 in THF. The cyclic voltammogram of 1 exhibited one-electron reduction, which suggests that synthesis of a bent 2-metallaallene (2) from 1 should be possible. Compound 2 contains one cobalt atom in the formal oxidation state zero, which is stabilized by two Me2 -cAAC: ligands. Bond lengths from X-ray diffraction are 1.871(2) and 1.877(2) Å with a C-Co-C bond angle of 170.12(8)°. The EPR spectrum of 2 exhibited a broad resonance attributed to the unique quasi-linear structure, which favors near degeneracy and gives rise to very rapid relaxation conditions. The cAACCo bond in 2 can be considered as a typical Dewar-Chatt-Duncanson type of bonding, which in turn retains 2.5 electron pairs on the Co atom as nonbonding electrons.

Discovering More Non‐Innocence: Triazenido versus Triazenyl Radical Ligand Function, and a Comment on [NO2]n as a “Suspect” Ligand

The unusual suspects: Depending on co-ligands L(n) and the effects of substituents (R), the well-known triazenides [N(NR)2](-) may act as EPR detectable coordinated triazenyl ligands, [N(NR)2](·). They are thus new non-innocent ligands and are related to the hitherto unused non-innocent nitrogen dioxide ligand, [NO2](·).

Synthesis and characterization of a triphenyl-substituted radical and an unprecedented formation of a carbene-functionalized quinodimethane.

The trichlorosilylcarbene monoradical (Cy-cAAC ·)SiCl3 (1) was directly converted to (Cy-cAAC ·)SiPh3 (2) by substitution of the three chlorine atoms with phenyl groups without affecting the radical center adjacent to the silicon atom. In addition to the structure determination, compound 2 was studied by EPR spectroscopy and DFT calculations. The three hyperfine lines in the EPR spectrum of 2 are due to the coupling with (14)N nucleus. Functionalized 1,4-quinodimethane Me2-cAAC=C6H4=CPh2 (7) was isolated, whereas carbon analogue of radical 2 was targeted. Cyclic voltammogram of 7 indicated that a stable radical-anion 7  ·-, as well as a radical-cation 7  ·+, can be prepared. Theoretical calculations showed that one-electron ionization energy and electron affinity of 7 are 5.1 and 0.7 eV mol(-1), respectively.

On the energetics of P-P bond dissociation of sterically strained tetraamino-diphosphanes.

The homolytic P-P bond fission in a series of sterically congested tetraaminodiphosphanes (R2N)2P-P(NR2)2 ({4}2-{9}2, two of which were newly synthesized and fully characterized) into diaminophosphanyl radicals (R2N)2P˙ (4-9) was monitored by VT EPR spectroscopy. Determination of the radical concentration from the EPR spectra permitted to calculate free dissociation energies ΔGDiss(295) as well as dissociation enthalpies ΔHDiss and entropies ΔSDiss, respectively. Large positive values of ΔGDiss(295) indicate that the degree of dissociation is in most cases low, and the concentration of persistent radicals--even if they are spectroscopically observable at ambient temperature--remains small. Appreciable dissociation was established only for the sterically highly congested acyclic derivative {9}2. Analysis of the trends in experimental data in connection with DFT studies indicate that radical formation is favoured by large entropy contributions and the energetic effect of structural relaxation (geometrical distortions and conformational changes in acyclic derivatives) in the radicals, and disfavoured by attractive dispersion forces. Comparison of the energetics of formation for CC-saturated N-heterocyclic diphosphanes and the 7π-radical 3c indicates that the effect of energetic stabilization by π-electron delocalization in the latter is visible, but stands back behind those of steric and entropic contributions. Evaluation of spectroscopic and computational data indicates that diaminophosphanyl radicals exhibit, in contrast to aminophosphenium cations, no strong energetic preference for a planar arrangement of the (R2N)2P unit.

Metal‐Induced Thiophene Ring Opening and CC Bond Formation To Produce Unique Hexa‐1,3,5‐trienediyl‐Coupled Non‐Innocent Ligand Chelates

Ring opening of thiophenes containing an azo function in 2-position and subsequent dimerization through C-C coupling were observed on reaction with [Ru(acac)2 (CH3 CN)2 ] (acac=acetylacetonate) to produce two 1,3,5-hexatriene-linked redox-active azothiocarbonyl chelate systems. Interaction of the non-innocent chelate ligands and of the metals at a nanoscale distance of 1.45 nm via the conjugated hexatriene bridge was studied by magnetic and electron spectroscopic measurements in conjunction with DFT calculations, revealing four-center magnetic interactions of this unique setting and weak intervalence coupling after reduction.