Copper-Catalyzed One-Pot Synthesis of Thiazolidin-2-imines.

The synthesis of thiazolines, thiazolidines, and thiazolidinones has been extensively studied, due to their biological activity related to neurodegenerative diseases, such as Parkinson's and Alzheimer's, as well as their antiparasitic and antihypertensive properties. The closely related thiazolidin-2-imines have been studied less, and efficient strategies for synthesizing them, mainly based on the reaction of propargylamines with isothiocyanates, have been explored less. The use of one-pot approaches, providing modular, straightforward, and sustainable access to these compounds, has also received very little attention. Herein, we report a novel, one-pot, multicomponent, copper-catalyzed reaction among primary amines, ketones, terminal alkynes, and isothiocyanates, toward thiazolidin-2-imines bearing quaternary carbon centers on the five-membered ring, in good to excellent yields. Density functional theory calculations, combined with experimental mechanistic findings, suggest that the copper(I)-catalyzed reaction between the in situ-formed propargylamines and isothiocyanates proceeds with a lower energy barrier in the pathway leading to the S-cyclized product, compared to that of the N-cyclized one, toward the chemo- and regioselective formation of 5-exo-dig S-cyclized thiazolidin-2-imines.

New Pyridine Dicarbene Ligands with Ring Expanded NHCs and their Nickel and Chromium Complexes.

The pincer complexes [NiIIBr(CNC)]Br (4), [CrIIIBr3(CNC)] (5a) and [CrIIIBr2.3Cl0.7(CNC)] (5b), where CNC = 3,3'-(pyridine-2,6-diyl)bis(1-mesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene), were obtained from the novel ligand CNC, generated in situ from the precursor (CHNCH)Br2 and [NiIIBr2(PPh3)2] or from [CrII{N(SiMe3)2}2(THF)2] and (CHNCH)Br2 by aminolysis, respectively. The tetrahedrally distorted square planar (τ4 ≅ 0.30) geometry and the singlet ground state of Ni in 4 were attributed to steric constraints of the CNC backbone. Computational methods highlighted the dependence of the coordination geometry and the singlet-triplet energy difference on the size of the N-substituent of the tetrahydropyrimidine wingtips and contrasted it to the situation in 5-membered imidazolin-2-ylidene pincer analogues. The octahedral CrIII metal center in 5a and 5b is presumably formed after one electron oxidation from CH2Cl2. 4/MAO and 5a/MAO were catalysts of moderate activity for the oligomerization and polymerization of ethylene, respectively. The analogous (CH^N^CH)Br2 precursor, where (CH^N^CH) = 3,3'-(pyridine-2,6-diylbis(methylene))bis(1-mesityl-3,4,5,6-tetrahydropyrimidin-1-ium), was also prepared, however its coordination chemistry was not studied due to the inherent instability of the resulting free C^N^C ligand.

T-Shaped Palladium and Platinum {MNO}10 Nitrosyl Complexes.

The synthesis and characterization of a homologous series of T-shaped {MNO}10 nitrosyl complexes of the form [M(PR3)2(NO)]+ (M = Pd, Pt; R = tBu, Ad) are reported. These diamagnetic nitrosyls are obtained from monovalent or zerovalent precursors by treatment with NO and NO+, respectively, and are notable for distinctly bent M-NO angles of ∼123° in the solid state. Adoption of this coordination mode in solution is also supported by the analysis of isotopically enriched samples by 15N NMR spectroscopy. Effective oxidation states of M0/NO+ are calculated, and metal-nitrosyl bonding has been interrogated using DFT-based energy decomposition analysis techniques. While a linear nitrosyl coordination mode was found to be electronically preferred, the M-NO and P-M-P angles are inversely correlated to the extent that binding in this manner is prevented by steric repulsion between the bulky ancillary phosphine ligands.

Secondary Metabolites with Anti-Inflammatory Activity from Laurencia majuscula Collected in the Red Sea.

The chemical investigation of the organic extract of the red alga Laurencia majuscula collected from Hurghada reef in the Red Sea resulted in the isolation of five C15 acetogenins, including four tricyclic ones of the maneonene type (1-4) and a 5-membered one (5), 15 sesquiterpenes, including seven lauranes (6-12), one cuparane (13), one seco-laurane (14), one snyderane (15), two chamigranes (16, 17), two rearranged chamigranes (18, 19) and one aristolane (20), as well as a tricyclic diterpene (21) and a chlorinated fatty acid derivative (22). Among them, compounds 1-3, 5, 7, 8, 10, 11 and 14 are new natural products. The structures and the relative configurations of the isolated natural products have been established based on extensive analysis of their NMR and MS data, while the absolute configuration of maneonenes F (1) and G (2) was determined on the basis of single-crystal X-ray diffraction analysis. The anti-inflammatory activity of compounds 1, 2, 4-8, 10, 12-16, 18 and 20-22 was evaluated by measuring suppression of nitric oxide (NO) release in TLR4-activated RAW 264.7 macrophages in culture. All compounds, except 6, exhibited significant anti-inflammatory activity. Among them, metabolites 1, 4 and 18 did not exhibit any cytostatic activity at the tested concentrations. The most prominent anti-inflammatory activity, accompanied by absence of cytostatic activity at the same concentration, was exerted by compounds 5 and 18, with IC50 values of 3.69 µM and 3.55 µΜ, respectively.

Thorium- and Uranium-Mediated C-H Activation of a Silyl-Substituted Cyclobutadienyl Ligand.

Cyclobutadienyl complexes of the f-elements are a relatively new yet poorly understood class of sandwich and half-sandwich organometallic compounds. We now describe cyclobutadienyl transfer reactions of the magnesium reagent [(η4-Cb'''')Mg(THF)3] (1), where Cb'''' is tetrakis(trimethylsilyl)cyclobutadienyl, toward thorium(IV) and uranium(IV) tetrachlorides. The 1:1 stoichiometric reactions between 1 and AnCl4 proceed with intact transfer of Cb'''' to give the half-sandwich complexes [(η4-Cb'''')AnCl(µ-Cl)3Mg(THF)3] (An = Th, 2; An = U, 3). Using a 2:1 reaction stoichiometry produces [Mg2Cl3(THF)6][(η4-Cb'''')An(η3-C4H(SiMe3)3-κ-(CH2SiMe2)(Cl)] (An = Th, [Mg2Cl3(THF)6][4]; An = U [Mg2Cl3(THF)6][5]), in which one Cb'''' ligand has undergone cyclometalation of a trimethylsilyl group, resulting in the formation of an An-C σ-bond, protonation of the four-membered ring, and an η3-allylic interaction with the actinide. Complex solution-phase dynamics are observed with multinuclear nuclear magnetic resonance spectroscopy for both sandwich complexes. A computational analysis of the reaction mechanism leading to the formation of 4 and 5 indicates that the cyclobutadienyl ligands undergo C-H activation across the actinide center.

Synthesis, bonding properties and ether activation reactivity of cyclobutadienyl-ligated hybrid uranocenes.

A series of hybrid uranocenes consisting of uranium(iv) sandwiched between cyclobutadienyl (Cb) and cyclo-octatetraenyl (COT) ligands has been synthesized, structurally characterized and studied computationally. The dimetallic species [(η4-Cb'''')(η8-COT)U(µ:η2:η8-COT)U(THF)(η4-Cb'''')] (1) forms concomitantly with, and can be separated from, monometallic [(η4-Cb'''')U(THF)(η8-COT)] (2) (Cb'''' = 1,2,3,4-tetrakis(trimethylsilyl)cyclobutadienyl, COT = cyclo-octatetraenyl). In toluene solution at room temperature, 1 dissociates into 2 and the unsolvated uranocene [(η4-Cb'''')U(η8-COT)] (3). By applying a high vacuum, both 1 and 2 can be converted directly into 3. Using bulky silyl substituents on the COT ligand allowed isolation of base-free [(η4-Cb'''')U{η8-1,4-(iPr3Si)2C8H6}] (4), with compounds 3 and 4 being new members of the bis(annulene) family of actinocenes and the first to contain a cyclobutadienyl ligand. Computational studies show that the bonding in the hybrid uranocenes 3 and 4 has non-negligible covalency. New insight into actinocene bonding is provided by the complementary interactions of the different ligands with uranium, whereby the 6d orbitals interact most strongly with the cyclobutadienyl ligand and the 5f orbitals do so with the COT ligands. The redox-neutral activation of diethyl ether by [(η4-Cb'''')U(η8-C8H8)] is also described and represents a uranium-cyclobutadienyl cooperative process, potentially forming the basis of further small-molecule activation chemistry.

Uranium(iv) cyclobutadienyl sandwich compounds: synthesis, structure and chemical bonding.

The 1 : 1 reactions of uranium(iv) tetrakis(borohydride) with the sodium and potassium salts of the cyclobutadienyl anion [C4(SiMe3)4]2- (Cb'''') produce the half-sandwich complexes [Na(12-crown-4)2][U(η4-Cb'''')(BH4)3] and [U(η4-Cb'''')(µ-BH4)3{K(THF)2}]2. In the 1 : 2 reaction of U(BH4)4 with Na2Cb'''', formation of [U(η4-Cb'''')(η3-C4H(SiMe3)3-κ-(CH2SiMe2)(BH4))]- reveals that a Cb'''' ligand undergoes an intramolecular deprotonation, resulting in an allyl/tuck-in bonding mode. A computational study reveals that the uranium-Cb'''' bonding has an appreciable covalent component with contributions from the uranium 5f and 6d orbitals.

Isolation of a Perfectly Linear Uranium(II) Metallocene.

Reduction of the uranium(III) metallocene [(η5 -C5 i Pr5 )2 UI] (1) with potassium graphite produces the "second-generation" uranocene [(η5 -C5 i Pr5 )2 U] (2), which contains uranium in the formal divalent oxidation state. The geometry of 2 is that of a perfectly linear bis(cyclopentadienyl) sandwich complex, with the ground-state valence electron configuration of uranium(II) revealed by electronic spectroscopy and density functional theory to be 5f3 6d1 . Appreciable covalent contributions to the metal-ligand bonds were determined from a computational study of 2, including participation from the uranium 5f and 6d orbitals. Whereas three unpaired electrons in 2 occupy orbitals with essentially pure 5f character, the fourth electron resides in an orbital defined by strong 7s-6d z 2 mixing.

Ethene Activation and Catalytic Hydrogenation by a Low-Valent Uranium Pentalene Complex.

The reaction of the uranium(III) complex [U(η8-Pn††)(η5-Cp*)] (1) (Pn†† = C8H4(1,4-SiiPr3)2, Cp* = C5Me5) with ethene at atmospheric pressure produces the ethene-bridged diuranium complex [{(η8-Pn††)(η5-Cp*)U}2(µ-η2:η2-C2H4)] (2). A computational analysis of 2 revealed that coordination of ethene to uranium reduces the carbon-carbon bond order from 2 to a value consistent with a single bond, with a concomitant change in the formal uranium oxidation state from +3 in 1 to +4 in 2. Furthermore, the uranium-ethene bonding in 2 is of the δ type, with the dominant uranium contribution being from f-d hybrid orbitals. Complex 2 reacts with hydrogen to produce ethane and reform 1, leading to the discovery that complex 1 also catalyzes the hydrogenation of ethene under ambient conditions.

Bis(pentalene)dititanium chemistry: C-H, C-X and H-H bond activation.

The reaction of the bis(pentalene)dititanium complex Ti2(µ:η5,η5-Pn†)2 (Pn† = C8H4(1,4-SiiPr3)2) (1) with the N-heterocyclic carbene 1,3,4,5-tetramethylimidazol-2-ylidene results in intramolecular C-H activation of an isopropyl substituent to form a tucked-in hydride (3). Whilst pyridine will also effect this cyclometallation reaction to form (5), the pyridine analogue of (3), the bases 1,2,4,5-tetramethyl-imidazole, 2,6-lutidine, DABCO or trimethylphosphine are ineffective. The reaction of (1) with 2,6-dichloro-pyridine affords crystallographically characterised (6) which is the product of oxidative addition of one of the C-Cl bonds in 2,6-dichloro-pyridine across the Ti-Ti double bond in (1). The tucked-in hydride (3) reacts with hydrogen to afford a dihydride complex (4) in which the tuck-in process has been reversed; detailed experimental and computational studies on this reaction using D2, HD or H2/D2 support a mechanism for the formation of (4) which does not involve σ-bond metathesis of H2 with the tucked-in C-H bond in (3). The reaction of (3) with tBuCCH yields the corresponding acetylide hydrido complex (7), where deuteration studies show that again the reaction does not proceed via σ-bond metathesis. Finally, treatment of (3) with HCl affords the chloro-derivative (9) [(NHC)Ti(µ-H)Ti{(µ,η5:η5)Pn†}2Cl], whereas protonation with [NEt3H]BPh4 yielded a cationic hydride (10) featuring an agostic interaction between a Ti centre and an iPr Me group.

Preparation and reactivity of rhodium and iridium complexes containing a methylborohydride based unit supported by two 7-azaindolyl heterocycles.

The synthesis and characterisation of a new anionic flexible scorpionate ligand, methyl(bis-7-azaindolyl)borohydride [MeBai]- is reported herein. The ligand was coordinated to a series of group nine transition metal centres forming the complexes, [Ir(MeBai)(COD)] (1), [Rh(MeBai)(COD)] (2), [Rh(MeBai)(CODMe)] (2-Me) and [Rh(MeBai)(NBD)] (3), where COD = 1,5-cyclooctadiene, CODMe = 3-methyl-1,5-cyclooctadiene and NBD = 2,5-norbornadiene. In all cases, the boron based ligand was found to bind to the metal centres via a κ3-N,N,H coordination mode. The ligand and complexes were fully characterised by spectroscopic and analytical methods. The structures of the ligand and three of the complexes were confirmed by X-ray crystallography. The potential for migration of the "hydride" or "methyl" units from boron to the metal centre was also explored. During these studies an unusual transformation, involving the oxidation of the rhodium centre, was observed in complex 2. In this case, the η4-COD unit transformed into a η1,η3-C8H12 unit where the ring was bound via one sigma bond and one allyl unit. This is the first time such a transformation has been observed at a rhodium centre.

Activation of carbon suboxide (C3O2) by U(iii) to form a cyclobutane-1,3-dione ring.

The activation of C3O2 by the U(iii) complex [U(η5-Cp')3] (Cp' = C5H4SiMe3) is described. The reaction results in the reductive coupling of three C3O2 units to form a tetranuclear complex with a central cyclobutane-1,3-dione ring, with concomitant loss of CO. Careful control of reaction conditions has allowed the trapping of an intermediate, a dimeric bridging ketene complex, which undergoes insertion of C3O2 to form the final product.

Trimerisation of carbon suboxide at a di-titanium centre to form a pyrone ring system.

The reaction of the syn-bimetallic bis(pentalene)dititanium complex Ti2(µ:η5,η5-Pn†)2 (Pn† = C8H4(1,4-SiiPr3)2) 1 with carbon suboxide (O[double bond, length as m-dash]C[double bond, length as m-dash]C[double bond, length as m-dash]C[double bond, length as m-dash]O, C3O2) results in trimerisation of the latter and formation of the structurally characterised complex [{Ti2(µ:η5,η5-Pn†)2}{µ-C9O6}]. The trimeric bridging C9O6 unit in the latter contains a 4-pyrone core, a key feature of both the hexamer and octamer of carbon suboxide which are formed in the body from trace amounts of C3O2 and are, for example, potent inhibitors of Na+/K+-ATP-ase. The mechanism of this reaction has been studied in detail by DFT computational studies, which also suggest that the reaction proceeds via the initial formation of a mono-adduct of 1 with C3O2. Indeed, the carefully controlled reaction of 1 with C3O2 affords [Ti2(µ:η5,η5-Pn†)2 (η2-C3O2)], as the first structurally authenticated complex of carbon suboxide.

Correction: C-H and H-H activation at a di-titanium centre.

Correction for 'C-H and H-H activation at a di-titanium centre' by Nikolaos Tsoureas et al., Chem. Commun., 2017, 53, 13117-13120.

C-H and H-H activation at a di-titanium centre.

The reaction of the bis(pentalene)dititanium complex Ti2(µ:η5,η5-Pn†)2 (Pn† = C8H4(1,4-SiiPr3)2) with the N-heterocyclic carbene 1,3,4,5-tetramethylimidazol-2-ylidene results in intramolecular C-H activation of one of the iPr methyl groups of a Pn† ligand and formation of a "tucked-in" bridging hydride complex. The "tuck-in" process is reversed by the addition of hydrogen, which yields a dihydride featuring terminal and bridging hydrides.

Cu(II) Coordination Polymers as Vehicles in the A3 Coupling.

A family of benzotriazole based coordination compounds, obtained in two steps and good yields from commercially available materials, formulated as [CuII(L1)2(MeCN)2]·2ClO4·MeCN (1), [CuII(L1)(NO3)2]·MeCN (2), [ZnII(L1)2(H2O)2]·2ClO4·2MeCN (3), [CuII(L1)2Cl2]2 (4), [CuII5(L1)2Cl10] (5), [CuII2(L1)4Br2]·4MeCN·CuII2Br6 (6), [CuII(L1)2(MeCN)2]·2BF4 (7), [CuII(L1)2(CF3SO3)2] (8), [ZnII(L1)2(MeCN)2]·2CF3SO3 (9), [CuII2(L2)4(H2O)2]·4CF3SO3·4Me2CO (10), and [CuII2(L3)4(CF3SO3)2]·2CF3SO3·Me2CO (11), are reported. These air-stable compounds were tested as homogeneous catalysts for the A3 coupling synthesis of propargylamine derivatives from aldehyde, amine, and alkyne under a noninert atmosphere. Fine tuning of the catalyst resulted in a one-dimensional (1D) coordination polymer (CP) (8) with excellent catalytic activity in a wide range of substrates, avoiding any issues that would inhibit its performance.

Steric control of redox events in organo-uranium chemistry: synthesis and characterisation of U(v) oxo and nitrido complexes.

The synthesis and molecular structures of a U(v) neutral terminal oxo complex and a U(v) sodium uranium nitride contact ion pair are described. The synthesis of the former is achieved by the use of t BuNCO as a mild oxygen transfer reagent, whilst that of the latter is via the reduction of NaN3. Both mono-uranium complexes are stabilised by the presence of bulky silyl substituents on the ligand framework that facilitate a 2e- oxidation of a single U(iii) centre. In contrast, when steric hindrance around the metal centre is reduced by the use of less bulky silyl groups, the products are di-uranium, U(iv) bridging oxo and (anionic) nitride complexes, resulting from 1e- oxidations of two U(iii) centres. SQUID magnetometry supports the formal oxidation states of the reported complexes. Electrochemical studies show that the U(v) terminal oxo complex can be reduced and the [U(iv)O]- anion was accessed via reduction with K/Hg, and structurally characterised. Both the nitride complexes display complex electrochemical behaviour but each exhibits a quasi-reversible oxidation at ca. -1.6 V vs. Fc+/0.

Bis(pentalene)di-titanium: a bent double-sandwich complex with a very short Ti-Ti bond.

The novel bimetallic bis(pentalene) complex Ti2(µ:η(5),η(5)-Pn(†))2 (Pn(†) = C8H4{Si(i)Pr3-1,4}2) has been synthesised and structurally characterised. Structural data show a Ti-Ti distance of 2.399(2) Å, consistent with a strong metal-metal interaction, which DFT calculations best describe as a double bond with σ and π components.

Synthesis and characterisation of group nine transition metal complexes containing new mesityl and naphthyl based azaindole scorpionate ligands.

Two novel boron-based flexible scorpionate ligands based on 7-azaindole, Li[HB(azaindolyl)(2)(1-naphthyl)] and Li[HB(azaindolyl)(2)(mesityl)] {Li[(Naphth)Bai] and Li[(Mes)Bai] respectively}, have been prepared (mesityl = 2,4,6-trimethylphenyl). These salts have been isolated in two forms, either as dimeric structures which contain bridging hydride interactions with the lithium centres or as crystalline material containing mono nuclear bis-acetonitrile solvates. The newly formed ligands have been utilised to prepare a range of group nine transition metal complexes with the general formula [M(COD){κ(3)-NNH-HB (azaindolyl)(2)(Ar)}] (where M = rhodium, iridium; Ar = 1-naphthyl, mesityl; COD = 1,5-cyclooctadiene) and [Rh(NBD){κ(3)-NNH-HB (azaindolyl)(2)(Ar)}] (where NBD = 2,5-norbornadiene; Ar = 1-naphthyl, mesityl). These new complexes have been compared to the previously reported compounds which contain the related scorpionate ligands Li[HB(azaindolyl)(2)(phenyl)] and K[HB(azaindolyl)(3)] {Li[(Ph)Bai] and K[Tai] respectively}. Structural characterisation of the complexes [Rh(COD){κ(3)-NNH-HB (azaindolyl)(2)(mesityl)}], [Ir(COD){κ(3)-NNH-HB (azaindolyl)(2)(mesityl)}] and [Rh(NBD){κ(3)-NNH-HB (azaindolyl)(2)(naphthyl)}] confirm the expected κ(3)-NNH coordination mode for these new ligands. Spectroscopic analysis suggests strong interactions of the B-H functional group with the metal centres in all cases.

Strong agostic-type interactions in ruthenium benzylidene complexes containing 7-azaindole based scorpionate ligands.

The complexes [Ru(Tai)Cl{=C(H)Ph}(PCy(3))] (4) and [Ru((Ph)Bai)Cl{=C(H)Ph}(PCy(3))] (5) [where Tai = HB(7-azaindolyl)(3) and (Ph)Bai = Ph(H)B(7-azaindolyl)(2)] have been prepared and structurally characterised. The borohydride unit is located in the coordination site trans to the chloride ligand in both complexes. The degree of interaction between the borohydride group and the metal centre was found to be significantly large in both cases. Thermolysis reactions involving complex 4 led to a dehydrogenation reaction forming [Ru(Tai)Cl{PCy(2)(η(2)-C(6)H(9))}] (6) where the benzylidene group acts as a hydrogen acceptor.