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1. Diverse Coordination Geometries Derived from Trisaminocyclohexane Ligands with Appended Outer‐Sphere Hydrogen Bond Donors

   https://doi.org/10.1002/ejic.202300434  

   Ekanayake, Danushka M. ; Sheridan, Patrick E. ; Lindeman, Sergey V. ; Fiedler, Adam T. ( September 2023 , European Journal of Inorganic Chemistry)

   With the aim of constructing hydrogen‐bonding networks in synthetic complexes, two new ligands derived fromcis,cis‐1,3,5‐triaminocyclohexane (TACH) have been prepared that feature pendant pyrrole or indole rings as outer‐sphere H‐bond donors. The TACH framework offers a facial arrangement of threeN‐donors, thereby mimicking common coordination motifs in the active sites of nonheme Fe and Cu enzymes. X‐ray structural characterization of a series of Cu^I‐X complexes (X=F, Cl, Br, NCS) revealed that these neutral ligands (H₃L^R, R=pyrrole or indole) coordinate in the intended facialN₃manner, yielding four‐coordinate complexes with idealizedC₃symmetry. The N−H units of the outer‐sphere heterocycles form a hydrogen‐bonding cavity around the axial (pseudo)halide ligand, as verified by crystallographic, spectroscopic, and computational analyses. Treatment of H₃L^pyrroleand H₃L^indolewith divalent transition metal chlorides (M^IICl₂, M=Fe, Cu, Zn) causes one heterocycle to deprotonate and coordinate to the M(II) center, giving rise to tetradentate ligands with two remaining outer‐sphere H‐bond donors. Further ligand deprotonation is observed upon reaction with Ni(II) and Cu(II) salts with weakly coordinating counteranions. The reported complexes highlight the versatility of TACH‐based ligands with pendant H‐bond donors, as the resulting scaffolds can support multiple protonation states, coordination geometries, and H‐bonding interactions.

    

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2. Novel Sulfonated CNN Pincer Ligands for Facile C−H Activation at a Pt(II) Center

   https://doi.org/10.1002/ijch.201700063  

   Watts, David ; Wang, Daoyong ; Zavalij, Peter Y. ; Vedernikov, Andrei N. ( October 2017 , Israel Journal of Chemistry)

   Two novel sulfonated CNN‐pincer ligands1band1cand the corresponding chloro and aqua complexes K[^CNNLPtCl] and^CNNLPt(H₂O),3b–3cand2b–2c, were prepared and fully characterized including single crystal X‐ray diffraction. Along with the previously described complexes2aand3a, the derivatives of a CNN pincer ligand1a, these complexes form a family of structurally similar compounds where the pincer core rigidity increases in the series2a (3a)<2b (3b)<2c(3c), as deduced from their XRD data. The increased ligand rigidity affects the aqua ligand dissociation energy of the^CNNLPt(H₂O) complexes, as it follows from DFT calculations and as is reflected in the increased reactivity of the aqua complexes2a,2band2cin processes that involve aqua ligand loss. Among these processes the formation of the presumed dinuclear complexes^CNNL₂Pt₂and, importantly, catalytic C−D bond cleavage in C₆D₆were studied in 2,2,2‐trifluoroethanol solutions. The C−D bond cleavage reactivity was quantified as the rate of H/D exchange between C₆D₆and CF₃CH₂OH at 80 °C.

    

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3. Pincer-ligated Iridium Complexes with Low-Field Ancillary Ligands: Complexes of (iPrPCP)IrCl2 and Comparison with (iPrPCP)IrHCl

   https://doi.org/10.26434/chemrxiv-2023-5l8mq  

   Parihar, Ashish ; Emge, Thomas J ; Goldman, Alan S ( December 2023 , ChemRxiv)

   Pincer-ligated iridium complexes have been widely developed, and (pincer)Ir(III) complexes, particularly five-coordinate, are central to their chemistry. Such complexes typically bear two formally anionic ligands in addition to the pincer ligand itself. Yet despite the prevalence of halides as anionic ligands in transition metal chemistry there are relatively few examples in which both of these ancillary anionic ligands are halides or even other monodentate low-field anions. We report a study of the fragment (iPrPCP)IrCl2 (iPrPCP = 3-2,6-C6H3(CH2PiPr2)), and adducts thereof. These species are found to be thermodynamically disfavored relative to the corresponding hydridohalides. For example, DFT calculations and experiment indicate that one Ir-Cl bond of (iPrPCP)IrCl2 complexes will undergo reaction with H2 to give the (iPrPCP)IrHCl or an adduct thereof. In the presence of aqueous HCl, (iPrPCP)IrCl2 adds a chloride ion to give an unusual example of an anionic transition metal complex ((iPrPCP)IrCl3–) with a Zundel cation (H5O2+). (iPrPCP)IrCl2 is not stable as a monomer at room temperature but exists in solution as a mixture of clusters which can add various small molecules. DFT calculations indicate that dimerization of (iPrPCP)IrCl2 is more favorable than dimerization of (iPrPCP)IrHCl, in accord with its observed tendency to form clusters.

    

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4. Iron(II) Complexes Featuring a Redox‐Active Dihydrazonopyrrole Ligand

   https://doi.org/10.1002/zaac.202100097  

   Jesse, Kate A. ; Chang, Mu‐Chieh ; Filatov, Alexander S. ; Anderson, John S. ( June 2021 , Zeitschrift für anorganische und allgemeine Chemie)

   Nature uses control of the secondary coordination sphere to facilitate an astounding variety of transformations. Similarly, synthetic chemists have found metal‐ligand cooperativity to be a powerful strategy for designing complexes that can mediate challenging reactivity. In particular, this strategy has been used to facilitate two electron reactions with first row transition metals that more typically engage in one electron redox processes. While NNN pincer ligands feature prominently in this area, examples which can potentially engage in both proton and electron transfer are less common. Dihydrazonopyrrole (DHP) ligands have been isolated in a variety of redox and protonation states when complexed to Ni. However, the redox‐state of this ligand scaffold is less obvious when complexed to metal centers with more accessible redox couples. Here, we synthesize a new series of Fe‐DHP complexes in two distinct oxidation states. Detailed characterization supports that the redox‐chemistry in this set is still primarily ligand based. Finally, these complexes exist as 5‐coordinate species with an open coordination site offering the possibility of enhanced reactivity.

    

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5. Rapid reversible borane to boryl hydride exchange by metal shuttling on the carborane cluster surface

   https://doi.org/10.1039/C7SC01846K  

   Eleazer, Bennett J. ; Smith, Mark D. ; Popov, Alexey A. ; Peryshkov, Dmitry V. ( January 2017 , Chemical Science)

   In this work, we introduce a novel concept of a borane group vicinal to a metal boryl bond acting as a supporting hemilabile ligand in exohedrally metalated three-dimensional carborane clusters. The (POBOP)Ru(Cl)(PPh 3 ) pincer complex (POBOP = 1,7-OP( i -Pr) 2 - m -2-carboranyl) features extreme distortion of the two-center-two-electron Ru–B bond due to the presence of a strong three-center-two-electron B–H⋯Ru vicinal interaction. Replacement of the chloride ligand with a hydride afforded the (POBOP)Ru(H)(PPh 3 ) pincer complex, which possesses B–Ru, B–H⋯Ru, and Ru–H bonds. This complex was found to exhibit a rapid exchange between hydrogen atoms of the borane and the terminal hydride through metal center shuttling between two boron atoms of the carborane cage. This exchange process, which involves sequential cleavage and formation of strong covalent metal–boron and metal–hydrogen bonds, is unexpectedly facile at temperatures above −50 °C corresponding to an activation barrier of 12.2 kcal mol −1 . Theoretical calculations suggested two equally probable pathways for the exchange process through formally Ru(0) or Ru( iv ) intermediates, respectively. The presence of this hemilabile vicinal B–H⋯Ru interaction in (POBOP)Ru(H)(PPh 3 ) was found to stabilize a latent coordination site at the metal center promoting efficient catalytic transfer dehydrogenation of cyclooctane under nitrogen and air at 170 °C. 

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