The reaction of trans‐M(N2)2(dppe)2 (M=Mo, 1Mo, M=W, 1W) with B(C6F5)3 (2) provides the adducts (dppe)2M=N=N‐B(C6F5)3 (3) which can be regarded as M/B transition‐metal frustrated Lewis pair (TMFLP) ...templates activating dinitrogen. Easy borylation and silylation of the activated dinitrogen ligands in complexes 3 with a hydroborane and hydrosilane occur by splitting of the B−H and Si−H bonds between the N2 moiety and the perfluoroaryl borane. This reactivity of 3 is reminiscent of conventional frustrated Lewis pair chemistry and constitutes an unprecedented approach for the functionalization of dinitrogen.
Dinitrogen is one of the missing small‐molecule targets of the frustrated Lewis pair (FLP) chemistry. The use of a Group 6 metal (Mo or W)/boron comound allows activation of N2 that is reminiscent of conventional FLPs. This mode of activation allows its mild and easy borylation and silylation, under a mechanism in which coordinated N2 acts as the Lewis base component of an FLP.
Functionalization of CO2 is a challenging goal and precedents exist for the generation of HCOOH, CO, CH3OH, and CH4 in mild conditions. In this series, CH2O, a very reactive molecule, remains an ...elementary C1 building block to be observed. Herein we report the direct observation of free formaldehyde from the borane reduction of CO2 catalyzed by a polyhydride ruthenium complex. Guided by mechanistic studies, we disclose the selective trapping of formaldehyde by in situ condensation with a primary amine into the corresponding imine in very mild conditions. Subsequent hydrolysis into amine and a formalin solution demonstrates for the first time that CO2 can be used as a C1 feedstock to produce formaldehyde.
One and two: The C2 compound pinBOCH2OCHO (see scheme; HBpin=pinacolborane) and several C1 compounds have been obtained from the borane‐mediated reduction of CO2 under mild conditions with the ...catalyst precursor RuH2(H2)2(PCy3)2. Mechanistic investigation highlights the role of a series of new carbonyl ruthenium complexes that were characterized by multinuclear NMR spectroscopy, IR spectroscopy, and X‐ray diffraction studies.
The synthesis and characterization of original NHC ligands based on an imidazo1,5‐apyridin‐3‐ylidene (IPy) scaffold functionalized with a flanking barbituric heterocycle is described as well as their ...use as tunable ligands for efficient gold‐catalyzed C−N, C−O, and C−C bond formations. High activity, regio‐, chemo‐, and stereoselectivities are obtained for hydroelementation and domino processes, underlining the excellent performance (TONs and TOFs) of these IPy‐based ligands in gold catalysis. The gold‐catalyzed domino reactions of 1,6‐enynes give rise to functionalized heterocycles in excellent isolated yields under mild conditions. The efficiency of the NHC gold 5Me complex is remarkable and mostly arises from a combination of steric protection and stabilization of the cationic AuI active species by ligand 1Me.
Broadband ligands: The synthesis and characterization of original NHC ligands based on a imidazo1,5‐apyridin‐3‐ylidene (IPy) scaffold functionalized with a flanking barbituric heterocycle is described as well as their use as tunable ligands. High and unprecedented efficiency for gold‐catalyzed C−N, C−O, and C−C bond formations is disclosed.
The first example of a formal 1,3‐B−H bond addition across the M−N≡N unit of an end‐on dinitrogen complex has been achieved. The use of Piers’ borane HB(C6F5)2 was essential to observe this ...reactivity and it plays a triple role in this transformation: 1) electrophilic N2‐borylation agent, 2) Lewis acid in a frustrated Lewis pair‐type B−H bond activation, and 3) hydride shuttle to the metal center. This chemistry is supported by NMR spectroscopy and solid‐state characterization of products and intermediates. The combination of chelate effect and strong σ donation in the diphosphine ligand 1,2‐bis(diethylphosphino)ethane was mandatory to avoid phosphine dissociation that otherwise led to complexes where borylation of N2 occurred without hydride transfer.
And the B goes on. Application of frustrated Lewis pair (FLP)‐type reactions in dinitrogen coordination chemistry has led to the achievement of 1,3‐B−H bond addition across the M−N≡N unit of a N2 complex. A chelating, strongly σ‐donating phosphine ligand is necessary to observe the title reaction. The use of HB(C6F5)2 is essential as it plays a triple role: N2‐borylation agent, Lewis acid in a FLP‐type B−H bond activation, and hydride shuttle.
One small step for ammonia–borane: The simplest elementary aminoborane compound H2BNH2, which results from dehydrogenation of ammonia–borane, has been trapped by a ruthenium complex fragment leading ...to the isolation of a bis(σ‐BH) aminoborane complex. The analogous H2BNHMe and H2BNMe2 complexes were also prepared. (Picture: ruthenium complex; Ru purple, P orange, N blue, B brown, H white.)
Clioquinol and PBT2, based on a 8‐hydroxyquinoline scaffold, have been proposed as drugs able to treat the disruption of metal homeostasis in neurodegenerative diseases, especially Alzheimer's ...disease (AD). They are believed to reduce metal–amyloid (Aβ) interactions and regulate redox homeostasis in AD brains. Therefore, the understanding of the molecular aspects of metal chelation by 8‐hydroxyquinolines is clearly important for the development of future metal chelators as putative drugs. Despite this, literature data on these aspects are rather limited. We report herein the stoichiometries and structures of the copper(II) and zinc(II) complexes of PBT2. The reported data indicate that the coordination chemistry of PBT2 is characterized by high versatility and a lack of metal selectivity. For each metal, several complexes can co‐exist, and the copper complexes differ from their zinc analogues. The structures of the PBT2 complexes are significantly different to those reported for complexes of clioquinol, the prototype of this series. As PBT2 can be a bi‐ or tridentate ligand, different complexes can be formed with copper, including ternary complexes PBT2–Cu–X. The versatile structures of the metal complexes of PBT2 in vivo, especially in the brains of AD patients, should be dependent on competitive ligands, including amyloids. This feature might be a limitation for an efficient extraction of copper from amyloids.
The coordination of CuII and ZnII by PBT2, an 8‐hydroxyquinoline derivative proposed to regulate copper homeostasis in Alzheimer's disease, is very versatile. The ability of PBT2 to form 2:1 complexes with CuII may lessen the potential of 8‐hydroxyquinolines as future metal‐chelating drugs. Tetradentate chelating ligands should be preferred.
Single crystals of the {FeII(pyrazine)Pt(CN)4} spin crossover complex were synthesized by a slow diffusion method. The crystals exhibit a thermal spin transition around room temperature (298 K), ...which is accompanied by a 14 K wide hysteresis loop. X-ray single-crystal analysis confirms that this compound crystallizes in the tetragonal P4/mmm space group in both spin states. Within the thermal hysteresis region a complete bidirectional photoconversion was induced between the two phases (high spin ⇄ low spin) when a short single laser pulse (4 ns, 532 nm) was shined on the sample.
Research of effective drugs against Alzheimer's disease (AD) is currently one of the most challenging topics in medicinal chemistry. Despite the documented detrimental effect of the disruption of ...copper ion homeostasis in AD, this potential pharmacological target has been weakly explored. The design of chelators as drug candidates for copper regulation in AD brain should meet critical coordination chemistry requirements, in addition to requested biological parameters (membrane crossing, activity, …). Among the various possibilities offered by the diversity of metal ligands, we found that N4‐tetradentate 8‐aminoquinoline ligands able to generate stricly square planar Cu(II) complexes, are the most suitable for the transfer of copper from metal‐amyloids to metal‐carrier proteins, and are able to inhibit the catalytic reduction of dioxygen produced by copper‐loaded amyloids exposed to a biological reductant. In vivo, such tetradentate ligands are able to inhibit the loss of episodic memory in non‐transgenic amyloid‐impaired mice.
To efficiently regulate copper homeostasis in the brain of patients with Alzheimer's disease, the designed chelators should meet some critical coordination requirements. N4‐tetradentate 8‐aminoquinoline ligands, which generate square planar 1:1 Cu(II) complexes, efficiently inhibit the Cu‐amyloid induced oxidative stress, an important pathological hallmark of Alzheimer's disease.
The synthesis and structure of several new early transition metal (Ti, Zr, Hf, and V) NHC complexes supported by one or two IMes ligands is described. Reaction of M(NMe2)4 with IMes.HCl gives ...compounds VCl2(NMe2)(IMes)2 (2), and MCl2(NMe2)2(IMes) (M = V (3), Zr (4), Hf (5)). Treatment of TiCl3(THF)3 with 2 equiv. of IMes and 1 equiv. of LiNMe2 affords the titanium(III) compound TiCl2(NMe2)(IMes)2. Reaction of VCl3(THF)3 with 2 equiv. of IMes in toluene produces the trivalent complex VCl3(IMes)2 (6), while VCl2(Py)4 gives the divalent compound VCl2(IMes)(Py)3 (7), and ZrCl4(THF)2 leads to ZrCl4(IMes)2 (8). Imido compounds M(N-2,6-iPr2-C6H3)Cl2(NHMe2)(IMes) (M = V (9), Zr (10)) are prepared from V(NR)Cl2(NHMe2)2 or ZrCl2(NMe2)2(IMes) synthons. Ionic imido complexes M(N-2,6-iPr2-C6H3)Cl3(IMes)-HIMes+ (M = V (12), Ti (13)) are obtained in modest yields from the treatment of M(NR)Cl2(NHMe2)2 with 2 equiv. of IMes. Treatment of a toluene solution of V(NMe2)4 and an amide RC(O)NH2 (R = tBu, Ph) in the presence of an excess of Me3SiCl produces the terminal oxo complex V(O)Cl2(NHMe2)2 (13), which reacts with two equiv. of IMes to give the bis-NHC oxo-complex V(O)Cl2(IMes)2 (14). All new compounds have been characterised by various spectroscopic techniques, elemental analyses, and magnetism studies for paramagnetic compounds. The solid-state structure of 12 of these complexes and of IMes.HCl have further been confirmed by X-ray diffraction analyses.
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