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1. 14-Electron Rh and Ir silylphosphine complexes and their catalytic activity in alkene functionalization with hydrosilanes

   https://doi.org/10.1039/D1DT00677K  

   Abeynayake, Niroshani S.; Zamora-Moreno, Julio; Gorla, Saidulu; Donnadieu, Bruno; Muñoz-Hernández, Miguel A.; Montiel-Palma, Virginia (August 2021, Dalton Transactions)

   Herein we report an experimental and computational study of a family of four coordinated 14-electron complexes of Rh( iii ) devoid of agostic interactions. The complexes [X–Rh(κ 3 ( P,Si,Si )PhP( o -C 6 H 4 CH 2 Si i Pr 2 ) 2 ], where X = Cl (Rh-1), Br (Rh-2), I (Rh-3), OTf (Rh-4), Cl·GaCl 3 (Rh-5); derive from a bis(silyl)- o -tolylphosphine with isopropyl substituents on the Si atoms. All five complexes display a sawhorse geometry around Rh and exhibit similar spectroscopic and structural properties. The catalytic activity of these complexes and [Cl–Ir(κ 3 ( P,Si,Si )PhP( o -C 6 H 4 CH 2 Si i Pr 2 ) 2 ], Ir-1, in styrene and aliphatic alkene functionalizations with hydrosilanes is disclosed. We show that Rh-1 catalyzes effectively the dehydrogenative silylation of styrene with Et 3 SiH in toluene while it leads to hydrosilylation products in acetonitrile. Rh-1 is an excellent catalyst in the sequential isomerization/hydrosilylation of terminal and remote aliphatic alkenes with Et 3 SiH including hexene isomers, leading efficiently and selectively to the terminal anti-Markonikov hydrosilylation product in all cases. With aliphatic alkenes, no hydrogenation products are observed. Conversely, catalysis of the same hexene isomers by Ir-1 renders allyl silanes, the tandem isomerization/dehydrogenative silylation products. A mechanistic proposal is made to explain the catalysis with these M( iii ) complexes. 

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2. Heterobimetallic Complexes of IrM (M = FeII, CoII, and NiII) Core and Bridging 2-(Diphenylphosphino)pyridine: Electronic Structure and Electrochemical Behavior

   https://doi.org/10.1039/D0DT01801E  

   Cherepakhin, Valeriy; Hellman, Ashley; Lan, Zhenzhuo; Mallikarjun Sharada, Shaama; Williams, Travis J. (January 2020, Dalton transactions)

   Three complexes based on an Ir–M (M = FeII, CoII, and NiII) heterobimetallic core and 2-(diphenylphosphino)pyridine (Ph2PPy) ligand were synthesized via the reaction of trans-[IrCl(CO)(Ph2PPy)2] and the corresponding metal chloride. Their structures were established by single-crystal X-ray diffraction as [Ir(CO)(μ-Cl)(μ-Ph2PPy)2FeCl2]·2CH2Cl2 (2), [IrCl(CO)(μ- Ph2PPy)2CoCl2]·2CH2Cl2 (3), and [Ir(CO)(μ-Cl)(μ-Ph2PPy)2NiCl2]·2CH2Cl2 (4). Time-dependent DFT computations suggest a donor-acceptor interaction between a filled 5dz2 orbital on iridium and an empty orbital on the first-row metal atom, which is supported by UV-vis studies. Magnetic moment measurements show that the first-row metals are in their high- spin electronic configurations. Cyclic voltammetry data show that all the complexes undergo irreversible decomposition upon either reduction or oxidation. Reduction of 4 proceeds through ECE mechanism. While these complexes are not stable to electrocatalysis conditions, the data presented here refine our understanding of the bonding synergies of the first-row and third-row metals. 

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3. Alkane Dehydrogenation and H/D Exchange by a Cationic Pincer-Ir(III) Hydride: A Novel Mechanism of C-H Addition and 𝛽-H Elimination Leads to Unprecedented Selectivity

   https://doi.org/10.26434/chemrxiv-2024-plh2j  

   Parihar, Ashish; Emge, Thomas J; Hasanayn, Faraj; Goldman, Alan S (October 2024, ChemRxiv)

   We report that the cationic iridium complex (iPrPCP)IrH+ undergoes addition of alkane C-H bonds, which is manifested by catalytic alkane transfer-dehydrogenation to give alkenes and by hydrogen isotope (H/D) exchange (HIE). Contrary to established selectivity trends found for C-H activation by transition metal complexes, strained cycloalkanes, including cyclopentane, cycloheptane, and cyclooctane, undergo C-H addition much more readily than n-alkanes which in turn are much more reactive than cyclohexane. Aromatic C-H bonds also undergo H/D exchange much less rapidly than those of the strained cycloalkanes, but much more favorably than cyclohexane. The order of reactivity toward dehydrogenation correlates qualitatively with the reaction thermodynamics, but the magnitude is much greater than can be explained by thermodynamics. Accordingly, the cycloalkenes corresponding to the strained cycloalkanes undergo hydrogenation much more readily than cyclohexene, despite the less favorable thermodynamics of such hydrogenations. Computational (DFT) studies allow rationalization of the origin of reactivity and the unusual selectivity. Specifically, the initial C-H addition is strongly assisted by 𝛽-agostic interactions, which are particularly favorable for the strained cycloalkanes. Subsequent to 𝛼-C-H addition, the H atom of the 𝛽-agostic C-H bond is transferred to the hydride ligand of (iPrPCP)IrH+, to give a dihydrogen ligand. The overall processes, C-H addition and 𝛽-H-transfer to hydride, generally show intermediates on the IRC surface but they are extremely shallow, such that the 1,2-dehydrogenations are presumed to be effectively concerted although asynchronous. 

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4. Electrochemical C–H bond activation via cationic iridium hydride pincer complexes

   https://doi.org/10.1039/C9SC03076J  

   Lindley, Brian M.; Walden, Andrew G.; Brasacchio, Ann Marie; Casuras, Andrea; Lease, Nicholas; Chen, Chun-Hsing; Goldman, Alan S.; Miller, Alexander J. (January 2019, Chemical Science)

   A C–H bond activation strategy based on electrochemical activation of a metal hydride is introduced. Electrochemical oxidation of ( tBu4 PCP)IrH 4 ( tBu4 PCP is [1,3-( t Bu 2 PCH 2 )-C 6 H 3 ] − ) in the presence of pyridine derivatives generates cationic Ir hydride complexes of the type [( tBu4 PCP)IrH(L)] + (where L = pyridine, 2,6-lutidine, or 2-phenylpyridine). Facile deprotonation of [( tBu4 PCP)IrH(2,6-lutidine)] + with the phosphazene base tert -butylimino-tris(pyrrolidino)phosphorane, t BuP 1 (pyrr), results in selective C–H activation of 1,2-difluorobenzene (1,2-DFB) solvent to generate ( tBu4 PCP)Ir(H)(2,3-C 6 F 2 H 3 ). The overall electrochemical C–H activation reaction proceeds at room temperature without need for chemical activation by a sacrificial alkene hydrogen acceptor. This rare example of undirected electrochemical C–H activation holds promise for the development of future catalytic processes. 

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5. A unique series of chromium( iii ) mono-alkynyl complexes supported by tetraazamacrocycles

   https://doi.org/10.1039/d1dt00707f  

   Schuman, Ashley J.; Robey, Sarah F.; Judkins, Eileen C.; Zeller, Matthias; Ren, Tong (April 2021, Dalton Transactions)

   null (Ed.)

   Described herein is the synthesis and characterization of macrocyclic Cr III mono-alkynyl complexes. By using the meso -form of the tetraazamacrocycle HMC (HMC = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane), trans -[Cr(HMC)(C 2 Ph)Cl]OTf ( 1a ), trans -[Cr(HMC)(C 2 Np)Cl]OTf ( 2a ), trans -[Cr(HMC)(C 2 C 6 H 4 t Bu)Cl]OTf ( 3a ), and trans -[Cr(HMC)(C 2 (3,5-Cl 2 C 6 H 3 ))Cl]OTf ( 4a ) complexes have been realized. These complexes were synthesized in high yield through the reaction of trans -[Cr( meso -HMC)(C 2 Ar) 2 ]OTf ( 1b–4b ) with stoichiometric amounts of methanolic HCl. Single crystal X-ray diffraction showed that the trans -stereochemistry and pseudo-octahedral geometry is retained in the desired mono-alkynyl complexes. The absorption spectra of complexes 1a–4a display d–d bands with distinct vibronic progressions that are slightly red shifted from trans -[Cr(HMC)(C 2 Ar) 2 ] + with approximately halved molar extinction coefficients. Time-delayed measurements of the emission spectra for complexes 1a–4a at 77 K revealed phosphorescence with lifetimes ranging between 343 μs ( 4a ) and 397 μs ( 1a ). The phosphorescence spectra of 1a–4a also exhibit more structuring than the bis-alkynyl complexes due to a strengthened vibronic coupling between the Cr III metal center and alkynyl ligands. 

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