RESUMEN

Molybdenum(III) complexes bearing pincer-type ligands are well-known catalysts for N2-to-NH3 reduction. We investigated herein the impact of an anionic PNP pincer-type ligand in a Mo(III) complex on the (electro)chemical N2 splitting ([LMoCl3]-, 1 -, LH = 2,6-bis((di-tert-butylphosphaneyl)methyl)-pyridin-4-one). The increased electron-donating properties of the anionic ligand should lead to a stronger degree of N2 activation. The catalyst is indeed active in N2-to-NH3 conversion utilizing the proton-coupled electron transfer reagent SmI2/ethylene glycol. The corresponding Mo(V) nitrido complex 2H exhibits similar catalytic activity as 1H and thus could represent a viable intermediate. The Mo(IV) nitrido complex 3 - is also accessible by electrochemical reduction of 1 - under a N2 atmosphere. IR- and UV/vis-SEC measurements suggest that N2 splitting occurs via formation of an "overreduced" but more stable [(L(N2)2Mo0)2µ-N2]2- dimer. In line with this, the yield in the nitrido complex increases with lower applied potentials.

RESUMEN

Irradiation of dibenzyl diselenide BnSeSeBn with X-ray or UV-light cleaves the Se-C and the Se-Se bonds, inducing stable and metastable radical states. They are inevitably important to all natural and life sciences. Structural changes due to X-ray-induced Se-C bond-cleavage could be pin-pointed in various high-resolution X-ray diffraction experiments for the first time. Extended DFT methods were applied to characterize the solid-state structure and support the refinement of the observed residuals as contributions from the BnSeSe⋅ radical species. The X-ray or UV-irradiated crystalline samples of BnSeSeBn were characterized by solid-state EPR. This paper provides insight that in the course of X-ray structure analysis of selenium compounds not only organo-selenide radicals like RSe⋅ may occur, but also organo diselenide BnSeSe⋅ radicals and organic radicals R⋅ are generated, particularly important to know in structural biology.

RESUMEN

The reaction of potassium metal with sulfurtriimide S(Nt Bu)3 (II) gives the long elusive deep blue cage radical [K3 {(Nt Bu)3 S}2 ]. (1_K) that crystallizes at -35 °C from toluene solution. The subsequent physical characterization via X-ray structure analysis, UV/Vis-, and EPR spectroscopy from solution reveals the existence of one unpaired electron delocalized within the whole cage, i.e. coupling with the six nitrogen atoms, as well as the three potassium atoms caped by the two SN3 ligands. The present X-ray structure analysis further supports previous assumptions made on the parent compound 1_Li obtained from [Li4 {(Nt Bu)3 S}2 ] (I) and finally elucidates the structural arrangement of the SN3 caps and alkali metals in such radical cage species.

RESUMEN

Copper-catalyzed electrochemical direct chalcogenations of o-carboranes was established at room temperature. Thereby, a series of cage C-sulfenylated and C-selenylated o-carboranes anchored with valuable functional groups was accessed with high levels of position- and chemo-selectivity control. The cupraelectrocatalysis provided efficient means to activate otherwise inert cage C-H bonds for the late-stage diversification of o-carboranes.

RESUMEN

Herein, we report the synthesis and characterization of the metal free low valent fluoroborylene [(Me-cAAC)2BF] (1) stabilized by cyclic (alkyl)(amino) carbene (cAAC). The fluoroborylene 1 is obtained by the reductive defluorination of Me-cAAC:BF3 with 2.0 equivalents of KC8 in the presence of 1.0 equivalent of Me-cAAC. Due to its highly electron rich nature, 1 underwent one-electron oxidation with 1.0 equivalent of lithium tetrakis(pentafluorophenyl)borate [LiB(C6F5)4] to form the radical cation [(Me-cAAC)2BF]˙+[B(C6F5)4]- (2). DFT studies suggested that the lone pair of electrons is localized on the boron atom in 1, which explains its unprecedented reactivity. Both compounds 1 and 2 were characterized by X-ray crystallography. The radical cation 2 was studied by EPR spectroscopy.

RESUMEN

Direct hydrogenation of CO2 to CO, the reverse water-gas shift reaction, is an attractive route to CO2 utilization. However, the use of molecular catalysts is impeded by the general reactivity of metal hydrides with CO2. Insertion into M-H bonds results in formates (MO(O)CH), whereas the abnormal insertion to the hydroxycarbonyl isomer (MC(O)OH), which is the key intermediate for CO-selective catalysis, has never been directly observed. We here report that the selectivity of CO2 insertion into a Ni-H bond can be inverted from normal to abnormal insertion upon switching from thermal to photochemical conditions. Mechanistic examination for abnormal insertion indicates photochemical N-H reductive elimination as the pivotal step that leads to an umpolung of the hydride ligand. This study conceptually introduces metal-ligand cooperation for selectivity control in photochemical transformations.

RESUMEN

meta-Selective C-H alkylations of bioactive purine derivatives were accomplished by versatile ruthenium catalysis. Thus, the arene-ligand-free complex [Ru(OAc)2 (PPh3 )2 ] enabled remote C-H functionalizations with ample scope and excellent levels of chemo- and positional selectivities. Detailed experimental and computational mechanistic studies provided strong support for a facile C-H activation within a ruthenium(II/III) manifold.

RESUMEN

Silicon difluoride (SiF2 ) 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)2 SiF2 (2), synthesized from the reduction of cAAC-SiF4 (1) by using two equivalents of KC8 in the presence of one equivalent of cAAC. In the solid state, compound 2 is stable at room temperature for a long time under inert conditions. The reduction of compound 1 in the presence of one equivalent KC8 resulted in the first stable silicon trifluoride monoradical (cAAC)SiF3 (3).

RESUMEN

Two cyclic (alkyl)(amino)carbene (cAAC) stabilized mononuclear neutral radicals of aluminum have been synthesized. They contain an ethyl [(cAAC)2AlClEt (1)] and as well a diethyl group [(cAAC)2AlEt2 (2)], and have been prepared from the reduction of EtAlCl2 and Et2AlCl, respectively, with KC8. 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.

RESUMEN

The cyclic alkyl(amino) carbene (cAAC) stabilized biradicals of composition (cAAC)2SiH2 (1), (cAAC)SiMe2-SiMe2(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. Furthermore, the structure and bonding of compounds 1-3 have been investigated by theoretical methods. Compound 1 contains the SiH2 moiety and this is the first instance, where we have isolated 1 without an acceptor molecule.

RESUMEN

The neutral radical (Me2 -cAAC)2 AlCl2 (2) is stabilized by cyclic (alkyl)(amino)carbenes (cAACs). Complex 2 was synthesized by reduction of the Me2 -cAAC:→AlCl3 (1) adduct with KC8 in the presence of another equivalent of Me2 -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. Quantum-chemical calculations from the gas phase give an equilibrium structure with identical Al-C bond lengths. Compound 2 exhibits monoradical character, which was confirmed by EPR measurements. A bonding analysis indicates that the unpaired electron resides mainly at the carbene carbon atoms. Compound 2 is an example for an unusual neutral Al radical.

RESUMEN

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. The crystals of 2 are storable at room temperature for several months and stable up to 215 °C under an inert atmosphere. X-ray single-crystal diffraction revealed that 2 contains a cyclic nonplanar four-membered SiPSiP ring. Magnetic susceptibility measurements confirmed the singlet spin ground state of 2. Cyclic voltammetry of 2 showed a quasi-reversible one-electron reduction indicating the formation of the corresponding radical anion 2(•-), which was further characterized by EPR measurements in solution. The electronic structure and bonding of 2 and 2(•-) were studied by theoretical calculations. The experimentally obtained data are in good agreement with the calculated values.

RESUMEN

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. Radical properties of 2 are confirmed by EPR measurements that suggest the radical electron in 2 couples with (14)N (I = 1), (35/37)Cl (I = 3/2), and (31)P (I = 1/2) nuclei leading to multiple hyperfine lines. Hyperfine coupling parameters computed from DFT calculations are in good agreement with those of experimental values. Electronic distributions obtained from the theoretical calculations suggest that the radical electron mostly resides on the carbene C of 2.

RESUMEN

Cr(i)Cl is a very unstable species. The present work describes the stabilisation of Cr(i)Cl in the low coordinate environment of cyclic alkyl(amino) carbene ligands and its synthetic application to yield an unprecedented cationic complex with a two coordinate Cr(i). One electron reduction of (cAAC)2CrCl2 (1) with equivalent amount of KC8 results in the formation of (cAAC)2CrCl (2), with a distorted trigonal planar configuration at the metal centre. SQUID, EPR and theoretical studies reveal a Cr(i) centre with S = 5/2 spin ground state for 2. It represents the first example of a mononuclear Cr complex showing slow relaxation of magnetisation under an applied magnetic field. The chlorine atom in 2 is expected to be prone to further reactions with appropriate reagents. This qualifies 2 as a promising precursor for the preparation of various interesting complexes with Cr(i) in a low coordinate environment. The first example of this metathesis reaction is observed when 2 is treated with Na[B(C6H3(CF3)2)4] resulting in [(cAAC)2Cr]+[B(C6H3(CF3)2)4]-, a linear cationic complex with two coordinate Cr(i) and an S = 5/2 spin ground state.

RESUMEN

A silicon atom in the zero oxidation state stabilized by two carbene ligands is known as siladicarbene (silylone). There are two pairs of electrons on the silicon atom in silylone. This was recently confirmed by both experimental and theoretical charge density investigations. The silylone is stable up to 195 °C in an inert atmosphere. However, a substoichiometric amount (33 mol%) of potassium metal triggers the activation of the unsaturated C:Si:C backbone, leading to a selective reaction with a tertiary C-H bond in an atom-economical approach to form a six-membered cyclic silylene with three-coordinate silicon atom. Cyclic voltammetry shows that this reaction proceeds via the formation of a silylone radical anion intermediate, which is further confirmed by EPR spectroscopy.

Asunto(s)

RESUMEN

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. This was further confirmed by the EPR spectra, which exhibit multiple hyperfine lines due to the coupling of the unpaired electron with (63,65)Cu isotopes, (14)N, and (1)H nuclei.

Asunto(s)

RESUMEN

Irradiation of rhodium(II) azido complex [Rh(N3){N(CHCHPtBu2)2}] allowed for the spectroscopic characterization of the first reported rhodium complex with a terminal nitrido ligand. DFT computations reveal that the unpaired electron of rhodium(IV) nitride complex [Rh(N){N(CHCHPtBu2)2}] is located in an antibonding Rh-N π* bond involving the nitrido moiety, thus resulting in predominant N-radical character, in turn providing a rationale for its transient nature and observed nitride coupling to dinitrogen.

Asunto(s)

RESUMEN

Two different synthetic methodologies of silicon dihalide bridged biradicals of the general formula (L(n)•)2SiX2 (n = 1, 2) have been developed. First, the metathesis reaction between NHC:SiX2 and L(n): (L(n): = cyclic akyl(amino) carbene in a 1:3 molar ratio leads to the products 2 (n = 1, X = Cl), 4 (n = 2, X = Cl), 6 (n = 1, X = Br), and 7 (n = 2, X = Br). These reactions also produce coupled NHCs (3, 5) under C-C bond formation. The formation of the coupled NHCs (L(m) = cyclic alkyl(amino) carbene substituted N-heterocyclic carbene; m = 3, n = 1 (3) and m = 4, n =2 (5)) is faster during the metathesis reaction between NHC:SiBr2 and L(n): when compared with that of NHC:SiCl2. Second, the reaction of L(1):SiCl4 (8) (L(1): =:C(CH2)(CMe2)2N-2,6-iPr2C6H3) with a non-nucleophilic base LiN(iPr)2 in a 1:1 molar ratio shows an unprecedented methodology for the synthesis of the biradical (L(1)•)2SiCl2 (2). The blue blocks of silicon dichloride bridged biradicals (2, 4) are stable for more than six months under an inert atmosphere and in air for one week. Compounds 2 and 4 melt in the temperature range of 185 to 195 °C. The dibromide (6, 7) analogue is more prone to decomposition in the solution but comparatively more stable in the solid state than in the solution. Decomposition of the products has been observed in the UV-vis spectra. Moreover, compounds 2 and 4 were further converted to stable singlet biradicaloid dicarbene-coordinated (L(n):)2Si(0) (n = 1 (9), 2 (10)) under KC8 reduction. Compounds 2 and 4 were also reduced to dehalogenated products 9 and 10, respectively when treated with RLi (R = Ph, Me, tBu). Cyclic voltametry measurements show that 10 can irreversibly undergo both one electron oxidation and reduction.

Asunto(s)