Neutral and Cationic Bis-Chelate Monoorganosilicon(IV) Complexes of 1-Hydroxy-2-pyridinone

A series of spirocyclic monoorganosilicon compounds of the form RSi(OPO)2Cl [R = phenyl (1); p-tolyl (2); benzyl (3); Me (4); tBu (5); thexyl (6)] (OPO = 1-oxo-2-pyridinone) was synthesized and characterized by 1H , 13C, and 29Si NMR spectroscopy, X-ray crystallography, and elemental analysis. In the solid state, complexes 1, 2, and 3 are neutral and possess cis-OPO ligands in an octahedral arrangement, and complexes 4, 5, and 6 are cationic and possess effectively trans­-OPO ligands in nearly ideal square pyramidal geometries along the Berry-pseudorotation coordinate. In 4-6, chloride dissociation is attributed to the additive effect of multiple intermolecular C—H∙∙∙Cl interactions in their crystals. In DMSO-d6 solution, compounds 1-6 form cationic hexacoordinate DMSO adducts with trans-OPO ligands, all of which undergo dynamic isomerization with energy barriers of ~18-19 kcal/mol. Compounds with better leaving groups, (p-tolyl)Si(OPO)2X [X = I (7); X = triflate (8)], exhibit identical solution NMR spectra as 2, supporting anion dissociation in each. The fluoride derivatives RSi(OPO)2F [R = benzyl (9); Me (10)] exhibit hexacoordinate geometries with cis­-OPO ligands in the solid state and exhibit dynamic isomerization in solution. Overall, these studies indicate, in both the solid and solution states, that the trans-OPO ligand arrangement is favored when anions are dissociated and a cis­-OPO ligand arrangement when anions are coordinated

Chelation and Stereodynamic Equilibria in Neutral Hypercoordinate Organosilicon Complexes of 1-Hydroxy-2-pyridinone

A series of neutral organosilicon compounds, R3Si(OPO) [R = Me (1), Et (2), Ph (3)], cis-R2Si(OPO)2 [R = Me (4), Et (5), iPr (6), tBu (7), Ph (9)], (CH2)3Si(OPO)2 (8), and cis-R2Si(OPO)Cl [R = Me (10), Et (11)] (OPO = 1-oxo-2-pyridinone) have been prepared and fully characterized. X-ray crystallographic analyses show 1 to be tetracoordinate, 3, 7, and 10 to be pentacoordinate, and 4, 5, 6, 8, and 9 to be hexacoordinate. In the hexacoordinate structures, a mixture of diastereomers is observed in the form of C/N site disorder in each OPO ligand. Variable-temperature 13C and 29Si NMR studies indicate reversible Si←OC bond dissociation occurring in all pentacoordinate and hexacoordinate complexes to a varying degree with greater tendency toward dissociation in hydrogen-bonding donor solvents. Significant weakening of the dative Si←OC bond in 3 is observed in the co-crystallized adduct solvate, 3·Ph3SiOH·½C5H12, providing structural evidence for the decrease in coordination number of the OPO ligand by hydrogen-bonding donors. In the hexacoordinate complexes, increasing steric bulk of ancillary ligands also was found to promote dissociation. 1H and 13C VT-NMR studies of 4, 6, 8, and 9 indicate stereoisomerization equilibria concurrent with Si←OC bond dissociation proposed to occur through trigonal bipyramidal intermediates.

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A series of mononuclear nickel­(II) thiolate complexes (Et₄N)­Ni­(X-pyS)₃ (Et₄N = tetraethylammonium; X = 5-H (<b>1a</b>), 5-Cl (<b>1b</b>), 5-CF₃ (<b>1c</b>), 6-CH₃ (<b>1d</b>); pyS = pyridine-2-thiolate), Ni­(pySH)₄(NO₃)₂ (<b>2</b>), (Et₄N)­Ni­(4,6-Y₂-pymS)₃ (Y = H (<b>3a</b>), CH₃ (<b>3b</b>); pymS = pyrimidine-2-thiolate), and Ni­(4,4′-Z-2,2′-bpy)­(pyS)₂ (Z = H (<b>4a</b>), CH₃ (<b>4b</b>), OCH₃ (<b>4c</b>); bpy = bipyridine) have been synthesized in high yield and characterized. X-ray diffraction studies show that <b>2</b> is square planar, while the other complexes possess tris-chelated distorted-octahedral geometries. All of the complexes are active catalysts for both the photocatalytic and electrocatalytic production of hydrogen in 1/1 EtOH/H₂O. When coupled with fluorescein (Fl) as the photosensitizer (PS) and triethylamine (TEA) as the sacrificial electron donor, these complexes exhibit activity for light-driven hydrogen generation that correlates with ligand electron donor ability. Complex <b>4c</b> achieves over 7300 turnovers of H₂ in 30 h, which is among the highest reported for a molecular noble metal-free system. The initial photochemical step is reductive quenching of Fl* by TEA because of the latter’s greater concentration. When system concentrations are modified so that oxidative quenching of Fl* by catalyst becomes more dominant, system durability increases, with a system lifetime of over 60 h. System variations and cyclic voltammetry experiments are consistent with a CECE mechanism that is common to electrocatalytic and photocatalytic hydrogen production. This mechanism involves initial protonation of the catalyst followed by reduction and then additional protonation and reduction steps to give a key Ni–H^–/N–H⁺ intermediate that forms the H–H bond in the turnover-limiting step of the catalytic cycle. A key to the activity of these catalysts is the reversible dechelation and protonation of the pyridine N atoms, which enable an internal heterocoupling of a metal hydride and an N-bound proton to produce H₂

Oxalate Oxidase Model Studies – Substrate Reactivity

The synthesis and structure of [MnLCl]0.5H2O (1·0.5H2O, HL = 1‐benzyl‐4‐acetato‐1,4,7‐triazacyclononane) is reported. Complex 1 exists as a coordination polymer in the solid state, and the MnII center is bonded to three amine nitrogen atoms, one carboxylate oxygen atom, a chlorido ligand, and an adjacent carboxylate group in a chelating fashion to afford a seven‐coordinate center. The dissolution of 1 in acetonitrile containing excess oxalate (ox) ions results in a monomeric species. When mixtures of 1 and oxalate ions are exposed to oxygen under ambient conditions, a dark pink EPR‐silent species is generated. The pink species is believed to be [MnIII(ox)2]–, which results from the displacement of the ligand L– by an oxalate ion. The decomposition of this species ultimately results in the formation of 1 equiv. of CO2 per oxalate ion consumed, a HCO3– ion, and a MnII species. Further reaction of the resulting MnII species with excess oxalate in the presence of oxygen leads to additional oxalate degradation.MnLCl (HL = 1‐benzyl‐4‐acetato‐1,4,7‐triazacyclononane) is investigated as a structural and functional model for oxalate oxidase. MnLCl effects the catalytic degradation of oxalate ions under ambient conditions. MnLCl is converted to a light‐sensitive intermediate during catalysis. Analysis of the reaction mixture indicates that 1 equiv. of CO2 per oxalate ion is produced along with a HCO3– ion.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/110613/1/646_ftp.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/110613/2/ejic_201402835_sm_miscellaneous_information.pd

A second polymorph of [1,2-bis­(di-tert-butyl­phosphino)ethane]dichlorido­platinum(II)

The title complex, [PtCl2(C18H40P2)], contains a PtII center in an approximately square-planar geometry [cis angle range = 88.09 (3)–91.39 (3)°; twist angle = 1.19 (5)°]. The Pt—P bond lengths of 2.2536 (8) and 2.2513 (8) Å and the Pt—Cl bond lengths of 2.3750 (8) and 2.3588 (8) Å are normal. This crystal form is a polymorph of a structure reported previously [Harada, Kai, Yasuoka & Kasai (1976 ▶). Bull. Chem. Soc. Jpn, 49, 3472–3477]

Effective Alkyl‐Alkyl Cross‐Coupling with an Iron‐Xantphos Catalyst: Mechanistic and Structural Insights

While iron‐catalyzed C(sp2)−C(sp3) cross‐couplings have been widely studied and developed in the last decade, alkyl‐alkyl cross‐coupling systems with iron remain underdeveloped despite the importance of C(sp3)−C(sp3) bonds in organic synthesis. A major challenge to the development of these reactions is the current lack of fundamental insight into ligand effects and organoiron intermediates that enable effective alkyl‐alkyl couplings. The current study addresses this longstanding limitation using a combination of 57Fe Mössbauer spectroscopy, SC‐XRD (single‐crystal X‐ray diffraction) and reactivity studies of alkyl‐alkyl coupling with iron‐Xantphos to define the in situ formed iron‐Xantphos intermediates in catalysis. Combined with detailed reactivity studies, the nature of the key mechanistic pathways in catalysis and ligands effects to achieve effective alkyl‐alkyl cross‐coupling over competing β‐H elimination pathways are probed. Overall, these foundational studies provide a platform for future bespoke ligand and pre‐catalyst design for alkyl‐alkyl cross‐coupling methods development with sustainable iron catalysis

[2,2′-Bis(diphenyl­phosphan­yl)-1,1′-binaphthyl-κ2 P,P′]chlorido(4-methyl­phenyl­sulfon­yl-κS)palladium(II) dichloro­methane tris­olvate monohydrate

In the title compound, [Pd(C7H7O2S)Cl(C44H32P2)]·3CH2Cl2·H2O, the geometry around the metal atom is distorted square planar, with a twist angle between the P—Pd—P and S—Pd—Cl planes of 28.11 (2)°. The two Pd—P bond lengths differ by about 0.04 Å and the biphosphane bite angle is slightly obtuse [92.92 (2)°]. There are three dichloro­methane and one water mol­ecule co-crystallized with the palladium mol­ecule, all with atoms in general positions. Alternating water and palladium mol­ecules form four-membered cyclic units through O—H⋯Cl and O—H⋯O hydrogen bonding. One of the dichloromethane solvent molecules is disordered over two positions in a 0.55:0.45 ratio

Effective Alkyl‐Alkyl Cross‐Coupling with an Iron‐Xantphos Catalyst: Mechanistic and Structural Insights

While iron‐catalyzed C(sp2)−C(sp3) cross‐couplings have been widely studied and developed in the last decade, alkyl‐alkyl cross‐coupling systems with iron remain underdeveloped despite the importance of C(sp3)−C(sp3) bonds in organic synthesis. A major challenge to the development of these reactions is the current lack of fundamental insight into ligand effects and organoiron intermediates that enable effective alkyl‐alkyl couplings. The current study addresses this longstanding limitation using a combination of 57Fe Mössbauer spectroscopy, SC‐XRD (single‐crystal X‐ray diffraction) and reactivity studies of alkyl‐alkyl coupling with iron‐Xantphos to define the in situ formed iron‐Xantphos intermediates in catalysis. Combined with detailed reactivity studies, the nature of the key mechanistic pathways in catalysis and ligands effects to achieve effective alkyl‐alkyl cross‐coupling over competing β‐H elimination pathways are probed. Overall, these foundational studies provide a platform for future bespoke ligand and pre‐catalyst design for alkyl‐alkyl cross‐coupling methods development with sustainable iron catalysis

Bis(η5-penta­methyl­cyclo­penta­dien­yl)cobalt(II)

The crystal structure of the title compound, deca­methyl­cobaltocene, [Co(C10H15)2], has been determined. High-quality single crystals were grown from a cold saturated hexa­methyl­disiloxane solution. The structure is related to the manganese and iron analogs. The molecule has D 5d symmetry, with the Co atom in a crystallographic 2/m position. The cobalt–centroid(C5) distance is 1.71Å and the centroid(C5)–Co–centroid(C5) angle is 180°, by symmetry

TMEDA in Iron‐Catalyzed Hydromagnesiation: Formation of Iron(II)‐Alkyl Species for Controlled Reduction to Alkene‐Stabilized Iron(0)

N,N,N’,N’‐Tetramethylethylenediamine (TMEDA) has been one of the most prevalent and successful additives used in iron‐catalysis, finding application in reactions as diverse as cross‐coupling, C‐H activation and borylation. However, the role that TMEDA plays in these reactions remains largely undefined. Herein, studying the iron‐catalyzed hydromagnesiation of styrene derivatives using TMEDA has provided molecular‐level insight into the role of TMEDA in achieving effective catalysis. Key is the initial formation of TMEDA‐iron(II) alkyl species which undergo a controlled reduction to selectively form catalytically active styrene‐stabilized iron(0)‐alkyl complexes. While TMEDA is not bound to the catalytically active species, these active iron(0) complexes cannot be accessed in the absence of TMEDA. This mode of action, allowing for controlled reduction and access to iron(0) species, represents a new paradigm for the role of this important reaction additive in iron catalysis