Preparation, Characterization, and Reactivity of Azido Complex Containing a Tp(tBuNC)(PPh3)Ru Fragment and Ruthenium-Catalyzed Cycloaddition of Organic Azides with Alkynes in Organic and Aqueous Media

Ligand-Dominated Activation of CO2 and CS2 by the Putative Nickel Phosphiniminato Intermediates

Room-temperature photoactivation of the first- and second-generation PN3P-pincer nickel azido complexes 1a and 1b in the presence of CO2 or CS2 afforded N-bound carbamates, dithiocarbamates, and isothiocyanates, providing insights into CO2 and CS2 activation and demonstrating how a seemingly small difference in the ligand structure significantly influences the reactivity. Theoretical calculations disclosed that the charge of the phosphorus atom plays a critical role in determining the nitrogen atom transfer to form a plausible nickel phosphiniminato intermediate.

Photochemical C(sp)-C(sp2) Bond Activation in Phosphaalkynes: A New Route to Reactive Terminal Cyaphido Complexes LnM-C≡P

The photochemical activation of the C(sp)-C(sp²) bond in Pt(0)-η²-aryl-phosphaalkyne complexes leads selectively to coordination compounds of the type L_nPt(aryl)(C≡P). The oxidative addition reaction is a novel, clean, and atom-economic route for the synthesis of reactive terminal Pt(II)-cyaphido complexes, which can undergo [3 + 2] cycloaddition reactions with organic azides, yielding the corresponding Pt(II)-triazaphospholato complexes. The C-C bond cleavage reaction is thermodynamically uphill. Upon heating, the reverse and quantitative reductive elimination toward the Pt(0)-phosphaalkyne-π-complex is observed.

[3 + 2] cycloaddition of azido-bridged molybdenum(ii) complex with nitriles and alkynes

The azido-bridged molybdenum complex [N(CH₃)₄][(μ_1,1-N₃)₃{Mo(η³-C₃H₅)(CO)₂}₂], 1, was synthesized and its reactions with unsaturated nitriles and alkynes were investigated. The isolated [3 + 2] cycloaddition products were the N(2), N(3) bound tetrazolate complexes [N(CH₃)₄][(μ_1,1-N₃)₂(μ-N₄C{R}-κ²N²:N³){Mo(η³-C₃H₅)(CO)₂}₂] (R = C(CN)C(CN)₂ (2), C₆H₄NO₂, (3)) and [N(CH₃)₄][(μ-N₄C{R}-κ²N²:N³)₂(μ_1,1-N₃){Mo(η³-C₃H₅)(CO)₂}₂] (R = C(CN)C(CN)₂ (4), C₆H₄NO₂ (5)), and the N(1), N(2) bound triazolate complexes [N(CH₃)₄][(μ-N₃C₂{R}₂-κ²N¹:N²)(μ_1,1-N₃)₂{Mo(η³-C₃H₅)(CO)₂}₂] (R = CO₂CH₃ (6) and R = CO₂CH₂CH₃ (7). The reactivity of these cycloaddition reactions could be determined by the electronic properties of both metal azide and dipolarophile. In the reaction of 1 with nitriles, at most two bridging azido groups can participate in the cycloaddition reactions and elevated temperature is required for the preparations of 3 and 5. In the case of alkynes, only one azido group is active for the reaction. These complexes are fluxional in solution, and isomers were found in 3 and 5. The molecular structures of the above complexes were determined by single-crystal X-ray diffraction analysis, which reveals a distorted octahedral geometry around each molybdenum atom, and the two metal atoms are connected through three bridging ligands. The formation of these heterocycles demonstrated the [3 + 2] cycloaddition reaction could also be applied to the less electron-rich azido-bridged molybdenum complex.

Metallo-supramolecular assembly of protic pincer-type complexes: encapsulation of dinitrogen and carbon disulfide into a multiproton-responsive diruthenium cage

A protic pincer complex and rigid diphosphine linker formed a cage, which incorporated N₂ and CS₂ into the multiproton-responsive cavity.

Catalyst-free room-temperature iClick reaction of molybdenum(ii) and tungsten(ii) azide complexes with electron-poor alkynes: structural preferences and kinetic studies

Two isostructural and isoelectronic group VI azide complexes of the general formula [M(η³-allyl)(N₃)(bpy)(CO)₂] with M = Mo, W and bpy = 2,2'-bipyridine were prepared and fully characterized, including X-ray structure analysis. Both reacted smoothly with electron-poor alkynes such as dimethyl acetylenedicarboxylate (DMAD) and 4,4,4-trifluoro-2-butynoic acid ethyl ester in a catalyst-free room-temperature iClick [3 + 2] cycloaddition reaction. Reaction with phenyl(trifluoromethyl)acetylene, on the other hand, did not lead to any product formation. X-ray structures of the four triazolate complexes isolated showed the monodentate ligand to be N2-coordinated in all cases, which requires a 1,2-shift of the nitrogen from the terminal azide to the triazolate cycloaddition product. On the other hand, a ¹⁹F NMR spectroscopic study of the reaction of the fluorinated alkyne with the tungsten azide complex at 27 °C allowed detection of the N1-coordinated intermediate. With this method, the second-order rate constant was determined as (7.3 ± 0.1) × 10^-2 M^-1 s^-1, which compares favorably with that of first-generation compounds such as difluorocyclooctyne (DIFO) used in the strain-promoted azide-alkyne cycloaddition (SPAAC). In contrast, the reaction of the molybdenum analogue was too fast to be studied with NMR methods. Alternatively, solution IR studies revealed pseudo-first order rate constants of 0.4 to 6.5 × 10^-3 s^-1, which increased in the order of Mo > W and F₃C-C[triple bond, length as m-dash]C-COOEt > DMAD.

Synthesis of heterogeneous Ru(ii)-1,2,3-triazole catalyst supported over SBA-15: application to the hydrogen transfer reaction and unusual highly selective 1,4-disubstituted triazole formation via multicomponent click reaction

Remarkable reactivity of solid catalyst in H₂O medium for the regioselective 1,4-disubstituted 1,2,3-triazole with excellent yield in one pot.

Probing the factors that influence the conformation of a guanidinato ligand in [(η5-C5Me5)M(NN)X] (NN = chelating N,N′,N′′-tri(o-substituted aryl)guanidinate(1−); X = chloro, azido and triazolato)

The conformational difference illustrated is ascribed to a subtle repulsive interaction between the o-Cl substituent of two proximal aryl rings in the guanidinate ligand.

Redox-Active-Ligand-Mediated Formation of an Acyclic Trinuclear Ruthenium Complex with Bridging Nitrido Ligands

Coordination of a redox-active pyridine aminophenol ligand to Ru(II) followed by aerobic oxidation generates two diamagnetic Ru(III) species [1 a (cis) and 1 b (trans)] with ligand-centered radicals. The reaction of 1 a/1 b with excess NaN3 under inert atmosphere resulted in the formation of a rare bis(nitrido)-bridged trinuclear ruthenium complex with two nonlinear asymmetrical Ru-N-Ru fragments. The spontaneous reduction of the ligand centered radical in the parent 1 a/1 b supports the oxidation of a nitride (N(3-) ) to half an equivalent of N2 . The trinuclear omplex is reactive toward TEMPO-H, tin hydrides, thiols, and dihydrogen.

Bis(acetylacetonato)bis(pyrazolato)ruthenate(iii) as a redox-active scorpionate ligand

The potential of a new anionic octahedral metal complex [Ru(III)(acac)2(pz)2](-) ((-)) (pzH = pyrazole) as a ligand with a scorpionate coordination behaviour like tris(pyrazolyl)borate (tp) and reversible redox activity is presented. Trinuclear metal complexes, [Ru(III)2Zn(II)(acac)4(pz)4] () and [Ru(II)Ru(III)2(acac)4(pz)4] (), were each synthesized by the reaction of ZnCl2 or Ru3(CO)12 with [Ru(III)(acac)2(pz)(pzH)] (H) that is in situ deprotonated and acts as a precursor of (-). Single-crystal X-ray diffraction studies clarified that (-) acts as a scorpionate ligand; two (-) units in and one unit in function as bidentate ligands with two pyrazolates as pincers, while another (-) unit in functions as a tridentate ligand with one oxygen atom as a tail in addition to the two pyrazolate pincers. Moreover, and showed reversible multi-stage redox behaviours based on the Ru(II)/Ru(III) and Ru(III)/Ru(IV) couples of the (-) units in the cyclic voltammetry (CV) measurements. Based on the X-ray, IR, and CV measurements and the comparison with other Ru(ii) complexes with tp derivatives, the (-) unit was found to act as a redox-active scorpionate with electron withdrawing properties compared to the tp.

Preparation of pyranylidene complexes of ruthenium

Dimerization of alkylpropiolate on the half-sandwich fragment [Ru(η⁵-C₅H₅)(PPh₃){P(OMe)₃}]⁺ affords pyranylidene derivatives.

Amino acid bioconjugation via iClick reaction of an oxanorbornadiene-masked alkyne with a Mn(I)(bpy)(CO)3-coordinated azide

The catalyst-free room temperature iClick reaction of an unsymmetrically 2,3-disubstituted oxanorbornadiene (OND) as a "masked" alkyne equivalent with [Mn(N3)(bpy(CH3,CH3))(CO)3] leads to isolation of a phenylalanine ester bioconjugate, in which the model amino acid is linked to the metal moiety via a N-2-coordinated triazolate formed in a cycloaddition-retro-Diels-Alder (crDA) reaction sequence, in a novel approach to bioorthogonal coupling reactions based on metal-centered reactivity.

A DFT study on structures, frontier molecular orbitals and UV-vis spectra of RuX(PPh3)(NHCPh2)L (X=Tp and Cp; L=Cl and N3)

Geometry optimization for RuX(PPh3)(NHCPh2)(L) (X=hydridotris(pyrazolyl)borate (Tp) and cyclopentadiene (Cp); L=Cl and N3) are investigated by using density functional theory (DFT) with DZVP2/DZVP all-electron mixed basis sets and compared with available experimental values, and the calculated structures are in very good agreement with experimental data. The frontier molecular orbitals (FMOs) and electronic transitions have been investigated as well. Our calculations show that the π electron-rich ligand (N3) may increase the energies of occupied orbitals and reduce the energy gap of the HOMO-LUMO (ΔEL-H) in these ruthenium based complexes. The simulated UV-vis spectra of these complexes in methanol have been studied with time-dependent density functional theory (TD-DFT), and conductor-like polarizable continuum model (CPCM) was employed to account for the solvent effects. Our results show that a number of absorption peaks are found in the visible region (400-800 nm) with non-zero oscillator strengths. The strongest adsorption feature is associated to a transition from HOMO-2 to LUMO, which is assigned to metal-to-ligand charge transfer (MLCT) or metal/ligand-to-ligand charge transfer (MLCT/LLCT) depending on co-ligands. In addition, the Cp group increases electron-accept ability and results in red shift due to its π electron-rich and π donor characters. According to our results, these ruthenium based complexes are good candidates for dye-sensitized solar cell owing to their absorption intensities and rich absorption bands in the visible region.

Strain-promoted azide-alkyne cycloaddition with ruthenium(II)-azido complexes

The reactivity of an exemplary ruthenium(II)-azido complex towards non-activated, electron-deficient, and towards strain-activated alkynes at room temperature and low millimolar azide and alkyne concentrations has been investigated. Non-activated terminal and internal alkynes failed to react under such conditions, even under copper(I) catalysis conditions. In contrast, as expected, rapid cycloaddition was observed with electron-deficient dimethyl acetylenedicarboxylate (DMAD) as the dipolarophile. Since DMAD and related propargylic esters are excellent Michael acceptors and thus unsuitable for biological applications, we investigated the reactivity of the azido complex towards cycloaddition with derivatives of cyclooctyne (OCT), bicyclo[6.1.0]non-4-yne (BCN), and azadibenzocyclooctyne (ADIBO). While no reaction could be observed in the case of the less strained cyclooctyne OCT, the highly strained cyclooctynes BCN and ADIBO readily reacted with the azido complex, providing the corresponding stable triazolato complexes, which were amenable to purification by conventional silica gel column chromatography. An X-ray crystal structure of an ADIBO cycloadduct was obtained and verified that the formed 1,2,3-triazolato ligand coordinates the metal center through the central N2 atom. Importantly, the determined second-order rate constant for the ADIBO cycloaddition with the azido complex (k2=6.9 × 10(-2) M(-1) s(-1)) is comparable to the rate determined for the ADIBO cycloaddition with organic benzyl azide (k2=4.0 × 10(-1) M(-1) s(-1)). Our results demonstrate that it is possible to transfer the concept of strain-promoted azide-alkyne cycloaddition (SPAAC) from purely organic azides to metal-coordinated azido ligands. The favorable reaction kinetics for the ADIBO-azido-ligand cycloaddition and the well-proven bioorthogonality of strain-activated alkynes should pave the way for applications in living biological systems.

Rhodium and iridium complexes with a new scorpionate phosphane ligand

A straightforward synthesis of a new hybrid scorpionate ligand [(allyl)2B(CH2PPh2)(Pz)](-) ([A2BPN](-)) is reported. Coordination to rhodium resulted in square-planar complexes [Rh(κ(2)-A2BPN)(L)(L')] [L = L' = (1)/2cod (1,5-cyclooctadiene), CN(t)Bu, CO (6); L = CO, L' = NH3, pyridine, PPh3, PMe3] for which spectroscopic data and the molecular structure of [Rh(κ(2)-A2BPN)(CO)PPh3] (11) indicate the ligand to be κN,κP-bound to rhodium with two dangling free allyl groups. Studies in solution point out that the six-membered Rh-N-N-B-C- P metallacycle undergoes a fast inversion in all of them. The bis(carbonyl) complex 6 easily loses a CO group to give [{Rh(A2BPN)(CO)}2], a dinuclear compound in which two mononuclear subunits are brought together by two bridging allyl groups. Coordination to iridium is dominated by a tripodal κN,κP,η(2)-C═C binding mode in the TBPY-5 complexes [Ir(κ(3)-A2BPN)(L)(L')] [L = L' = (1)/2cod (3), CN(t)Bu (5), CO (7); L = CO, L' = PPh3 (13), PMe3 (14), H2C═CH2, (17), MeO2CC≡CCO2Me (dmad, 18)], as confirmed by the single-crystal structure determination of complexes 3 and 18. A fast exchange between the two allyl arms is observed for complexes having L = L' (3, 5, and 7), while those having CO and L ligands (14, 17, and 18) were found to be nonfluxional species. An exception is complex 13, which establishes an equilibrium with the SP-4 configuration. Protonation reactions on complexes 13 and 14 with HCl yielded the hydride complex [Ir(κ(2)-A2BPN)(CO)(Cl)(H)PPh3] (15) and the C-alkyl compound [ Ir{κ(3)-(allyl)B(CH2 CHCH3)(CH2PPh2)(Pz)}(Cl)(CO)PMe3] (16), respectively. The bis(isocyanide) complex 5 reacts with dmad to form [Ir(κ(2)-A2BPN)(CN(t)Bu)2(dmad)]. On the whole, the electronic density provided to the metal by the [A2BPN](-) ligand is very sensitive to the coordination mode. The basicity of the new ligand is similar to that of the Tp(Me2) ligand in the κN,κP mode but comparable to Tp if coordinated in the κN,κP,η(2)-C═C mode.