Configurational Flexibility of a Triaryl-Supported SBS Ligand with Rh and Ir: Structural Investigations and Olefin Isomerization Catalysis

Here, we report Rh and Ir complexes containing a triaryl SBS pincer ligand flanked by neutral thiomethyl donor groups. Oxidative addition of the B–H bond in the diazaborole proligand H­(MeSBSMe) to [Ir­(COD)­Cl]2 or [Rh­(COE)­Cl]2 yielded the chloride-bridged dimers [(MeSBSMe)­MH­(μ-Cl)]2, where M = Ir (1) or Rh (2). Addition of CO to 1 and 2 yielded unstable products that were difficult to isolate, but crystals of monomeric (MeSBSMe)­IrH­(CO)Cl (3) recovered from the reactions with 1 revealed a change in the MeSBSMe coordination mode from mer to fac. Treating 1 and 2 with LiN­(SiMe3)2 yielded mer-(MeSBSMe)­IrH­[N­(SiMe3)2] (4) and the unusual RhI–RhIII dimer (MeSBSMe)­Rh­(μ-H)­[(MeS­(μ-B)­(μ-SMe)]­Rh­[N­(SiMe3)2] (5) with both mer and fac MeSBSMe. 4 and 5 did not exhibit any alkane transfer hydrogenation reactivity when tested with tert-butylethylene and cyclooctane, but they are highly active for alkene isomerization with 1-hexene. Optimized isomerization reactions showed the highest turnover number (TON) with 4 at 60 °C after 16 h (TON = 10 000), and both catalysts are effective even when tested at room temperature with similar loadings (TON = 600). Collectively, these data highlight the reactivity and inherent coordinative flexibility of the MeSBSMe ligand for comparison to more well-established PBP complexes

CNS and CNP Iron(II) Mono-Iron Hydrogenase (Hmd) Mimics: Role of Deprotonated Methylene(acyl) and the <i>trans</i>-Acyl Site in H₂ Heterolysis

We report syntheses and H2 activation involving model complexes of mono-iron hydrogenase (Hmd) derived from acyl-containing pincer ligand precursors bearing thioether (CNSPre) or phosphine (CNPPre) donor sets. Both complexes feature pseudo-octahedral iron­(II) dicarbonyl units. While the CNS pincer adopts the expected mer-CNS (pincer) geometry, the CNP ligand unexpectedly adopts the fac-CNP coordination geometry. Both complexes exhibit surprisingly acidic methylene C–H bond (reversibly de/protonated by a bulky phenolate), which affords a putative dearomatized pyridinate-bound intermediate. Such base treatment of Fe-CNS also results in deligation of the thioether sulfur donor, generating an open coordination site trans from the acyl unit. In contrast, Fe-CNP maintains a CO ligand trans from the acyl site both in the parent and dearomatized complexes (the −PPh2 donor is cis to acyl). The dearomatized mer-Fe-CNS was competent for H2 activation (5 atm D2(g) plus phenolate as base), which is attributed to both the basic site on the ligand framework and the open coordination site trans to the acyl donor. In contrast, the dearomatized fac-Fe-CNP was not competent for H2 activation, which is ascribed to the blocked coordination site trans from acyl (occupied by CO ligand). These results highlight the importance of both (i) the open coordination site trans to the organometallic acyl donor and (ii) a pendant base in the enzyme active site

Iron Hydride Detection and Intramolecular Hydride Transfer in a Synthetic Model of Mono-Iron Hydrogenase with a CNS Chelate

We report the identification and reactivity of an iron hydride species in a synthetic model complex of monoiron hydrogenase. The hydride complex is derived from a phosphine-free CNS chelate that includes a Fe–C^NH(O) bond (carbamoyl) as a mimic of the active site iron acyl. The reaction of [(^OC^HNN^pyS^Me)­Fe­(CO)₂(Br)] (<b>1</b>) with NaHBEt₃ generates the iron hydride intermediate [(^OC^HNN^pyS^Me)­Fe­(H)­(CO)₂] (<b>2</b>; δ_{Fe–<i>H</i>} = −5.08 ppm). Above −40 °C, the hydride species extrudes CH₃S^– via intramolecular hydride transfer, which is stoichiometrically trapped in the structurally characterized dimer μ₂-(CH₃S)₂-[(^OC^HNN^Ph)­Fe­(CO)₂]₂ (<b>3</b>). Alternately, when activated by base (^<i>t</i>BuOK), <b>1</b> undergoes desulfurization to form a cyclometalated species, [(^OC^NHNC^Ph)­Fe­(CO)₂] (<b>5</b>); derivatization of <b>5</b> with PPh₃ affords the structurally characterized species [(^OC^NHNC)­Fe­(CO)­(PPh₃)₂] (<b>6</b>), indicating complex <b>6</b> as the common intermediate along each pathway of desulfurization

Influence of the Substituents on the Electronic and Electrochemical Properties of a New Square-Planar Nickel-Bis(quinoxaline-6,7-dithiolate) System: Synthesis, Spectroscopy, Electrochemistry, Crystallography, and Theoretical Investigation

We describe the synthesis, crystal structures, electronic absorption spectra, and electrochemistry of a series of square-planar nickel-bis­(quinoxaline-6,7-dithiolate) complexes with the general formula [Bu4N]2[Ni­(X26,7-qdt)2], where X = H (1a), Ph (2a), Cl (3), and Me (4). The solution and solid-state electronic absorption spectral behavior and electrochemical properties of these compounds are strongly dependent on the electron donating/accepting nature of the substituent X, attached to the quinoxaline-6,7-dithiolate ring in the system [Bu4N]2[Ni­(X26,7-qdt)2]. Particularly, the charge transfer (CT) transition bands observed in the visible region are greatly affected by the electronic nature of the substituent. A possible explanation for this influence of the substituents on electronic absorption and electrochemistry is described based on highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) gaps, which is further supported by ground-state electronic structure calculations. In addition to this, the observed CT bands in all the complexes are sensitive to the solvent polarity. Interestingly, compounds 1a, 2a, 3, and 4 undergo reversible oxidation at very low oxidation potentials appearing at E1/2 = +0.12 V, 0.033 V, 0.18 V, and 0.044 V vs Ag/AgCl, respectively, in MeOH solutions, corresponding to the respective couples [Ni­(X26,7-qdt)2]−/[Ni­(X26,7-qdt)2]2–. Compounds 1a, 3, and 4 have been characterized unambiguously by single crystal X-ray structural analysis; compound 2a could not be characterized by single crystal X-ray structure determination because of the poor quality of the concerned crystals. Thus, we have synthesized the tetraphenyl phosphonium salt of the complex anion of 2a, [PPh4]2[Ni­(Ph26,7-qdt)2]·3DMF (2b) for its structural characterization

Influence of the Substituents on the Electronic and Electrochemical Properties of a New Square-Planar Nickel-Bis(quinoxaline-6,7-dithiolate) System: Synthesis, Spectroscopy, Electrochemistry, Crystallography, and Theoretical Investigation

We describe the synthesis, crystal structures, electronic absorption spectra, and electrochemistry of a series of square-planar nickel-bis­(quinoxaline-6,7-dithiolate) complexes with the general formula [Bu4N]2[Ni­(X26,7-qdt)2], where X = H (1a), Ph (2a), Cl (3), and Me (4). The solution and solid-state electronic absorption spectral behavior and electrochemical properties of these compounds are strongly dependent on the electron donating/accepting nature of the substituent X, attached to the quinoxaline-6,7-dithiolate ring in the system [Bu4N]2[Ni­(X26,7-qdt)2]. Particularly, the charge transfer (CT) transition bands observed in the visible region are greatly affected by the electronic nature of the substituent. A possible explanation for this influence of the substituents on electronic absorption and electrochemistry is described based on highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) gaps, which is further supported by ground-state electronic structure calculations. In addition to this, the observed CT bands in all the complexes are sensitive to the solvent polarity. Interestingly, compounds 1a, 2a, 3, and 4 undergo reversible oxidation at very low oxidation potentials appearing at E1/2 = +0.12 V, 0.033 V, 0.18 V, and 0.044 V vs Ag/AgCl, respectively, in MeOH solutions, corresponding to the respective couples [Ni­(X26,7-qdt)2]−/[Ni­(X26,7-qdt)2]2–. Compounds 1a, 3, and 4 have been characterized unambiguously by single crystal X-ray structural analysis; compound 2a could not be characterized by single crystal X-ray structure determination because of the poor quality of the concerned crystals. Thus, we have synthesized the tetraphenyl phosphonium salt of the complex anion of 2a, [PPh4]2[Ni­(Ph26,7-qdt)2]·3DMF (2b) for its structural characterization