Synthesis and Isolation of an Anionic Bis(dipyrido-annulated) N-Heterocyclic Carbene CCC-Pincer Iridium(III) Complex by Facile C-H Bond Activation

Meridional tridentate N-heterocyclic carbene (NHC)-based pincer ligands contribute to a substantial growth in modern organometallic chemistry in both homogeneous catalysis and luminescence materials. Among all NHC-based pincer ligands, the dianionic LX₂-type CCC-pincer ones constitute the smallest subcategory owing to their limited ligand frameworks suitable for complexation. This work reports a one-pot, high-yield synthesis of a homoleptic anionic all-carbon bis-pincer iridium(III) complex (4) directly from a bis(aryl)-substituted dipyrido-annulated (dpa^Ar2) imidazolium salt and [Ir(COD)Cl]₂ via a cascade of deprotonation/C-H activation processes. Both experimental complexation chemistry and computational mechanistic investigation suggest that the large bite angle and π-rich character of the dpa^Ar2 NHC are responsible for its facile complexation as a dianionic LX₂-type CCC-pincer ligand precursor. The all-carbon ligated iridium(III) complex (4) bearing a π-conjugated ligand scaffold showed remarkably low oxidation potentials, which allows future investigations in its redox chemistry and photophysical properties.

Sustainable Option for Hydrogen Production: Mechanistic Study of the Interaction between Cobalt Pincer Complexes and Ammonia Borane

The mechanism of the solvolysis/hydrolysis of ammonia borane by iridium (Ir), cobalt (Co), iron (Fe) and ruthenium (Ru) complexes with various PNP ligands has been revisited using density functional theory (DFT). The approach of ammonia borane (NH3BH3) to the metal center has been tested on three different possible mechanisms, namely, the stepwise, concerted and proton transfer mechanism. It was found that the theoretical analyses correlate with the experimental results very well, with the activities of the iridium complexes with different PNP ligands following the order: (tBu)2P > (iPr)2P > (Ph)2P through the concerted mechanism. The reaction barriers of the rate-determining steps for the dehydrogenation of ammonia borane catalyzed by the active species [(tBu)2PNP-IrH] (Complex I-8), are found to be 19.3 kcal/mol (stepwise), 15.2 kcal/mol (concerted) and 26.8 kcal/mol (proton transfer), respectively. Thus, the concerted mechanism is the more kinetically favorable pathway. It is interesting to find that stable (tBu)2PNP Co-H2O and (tBu)2PNP Co-NH3 chelation products exist, which could stabilize the active I-8 species during the hydrolysis reaction cycle. The use of more sterically hindered and electron-donating PNP ligands such as (adamantyl)2P- provides similar activity as the t-butyl analogue. This research provides insights into the design of efficient cobalt catalysts instead of using precious and noble metal, which could benefit the development of a more sustainable hydrogen economy.

Solution, Solid-State, and Computational Analysis of Agostic Interactions in a Coherent Set of Low-Coordinate Rhodium(III) and Iridium(III) Complexes

A homologous family of low-coordinate complexes of the formulation trans-[M(2,2'-biphenyl)(PR3 )2 ][BArF4 ] (M=Rh, Ir; R=Ph, Cy, iPr, iBu) has been prepared and extensively structurally characterised. Enabled through a comprehensive set of solution phase (VT 1 H and 31 P NMR spectroscopy) and solid-state (single crystal X-ray diffraction) data, and analysis in silico (DFT-based NBO and QTAIM analysis), the structural features of the constituent agostic interactions have been systematically interrogated. The combined data substantiates the adoption of stronger agostic interactions for the IrIII compared to RhIII complexes and, with respect to the phosphine ligands, in the order PiBu3 >PCy3 >PiPr3 >PPh3 . In addition to these structure-property relationships, the effect of crystal packing on the agostic interactions was investigated in the tricyclohexylphosphine complexes. Compression of the associated cations, through inclusion of a more bulky solvent molecule (1,2-difluorobenzene vs. CH2 Cl2 ) in the lattice or collection of data at very low temperature (25 vs. 150 K), lead to small but statistically significant shortening of the M-H-C distances.

Activation of C-H bonds by rare-earth metallocene-butyl complexes

The stable metallocene-butyl complexes [(Cp^Me)₂M(ⁿBu)]₂ (M = Y, Dy) were synthesized and their reactivity towards to ferrocene and bulky N-heterocyclic carbenes investigated. Selective mono-deprotonation of ferrocene and a benzylic methyl group of IMes were observed, whereas a control reaction of (Cp^Me)₃M with IMes resulted in a normal-to-abnormal NHC rearrangement.

Catalytic dehydrocoupling of amine-boranes and amines into diaminoboranes: isolation of a Pt(ii), Shimoi-type, η(1)-BH complex

The platinum complex [Pt(I(t)Bu')(I(t)Bu)][BAr(F)] is a very efficient catalyst in the synthesis of diaminoboranes through dehydrocoupling of amine-boranes and amines. Shimoi-type, η(1)-BH complexes are key intermediates in the process.

1,2,3-Triazolylidene ruthenium(ii)-cyclometalated complexes and olefin selective hydrogenation catalysis

Silver(i) 1,2,3-triazol-5-ylidenes [(RCH₂C₂N₂(NMe)Ph)₂Ag][AgCl₂] (R = Ph 3a, C₆H₂iPr₃3b, C₆H₂Me₃3c) and [(PhCH₂C₂N₂(NMe)R)₂Ag][AgCl₂] (R = C₆H₄Me 3d, C₆H₄CF₃3e) were synthesized and subsequently treated with RuHCl(PPh₃)₃ and RuHCl(H₂)(PCy₃)₂ to give cyclometallated and Ru-hydride species as the major and minor products, respectively.

1,2,3-Triazolylidene ruthenium(ii)(η6-arene) complexes: synthesis, metallation and reactivity

Three bis(1,2,3-triazolylidene) silver(i) complexes were reacted with [RuCl₂(p-cymene)]₂ and the ruthenium complexes [RCH₂N₂(NMe)C₂Ph]RuCl₂(p-cymene) (R = C₆H₂Me₃4a1, C₆H₂iPr₃4b1, Ph 4c1) were isolated as major products with the minor C(sp²)–H activated products [RCH₂N₂(NMe)C₂C₆H₄]RuCl(p-cymene) (R = C₆H₂Me₃4a2, C₆H₂iPr₃4b2, Ph 4c2) and [(C₆H₄)CH₂N₂(NMe)C₂Ph]RuCl(p-cymene) (4c3).

Modulating reactivity in iridium bis(N-heterocyclic carbene) complexes: influence of ring size on E–H bond activation chemistry

Critical influence of NHC ring size: highly labile [Ir(5-Mes)₂(H)₂]⁺can only be studied by trapping experiments, while [Ir(6-Mes)₂(H)₂]⁺and [Ir(6-Xyl)₂(H)₂]⁺are air/moisture stable.