Hydride Derivatives Research Articles

Magnesiation and calciation of C[sbnd]H acidic N-alkyl imidazoles

The reactivity of β-diketiminato alkylmagnesium, hydridomagnesium and hydridocalcium bases toward 1-methylimidazole, 1-tert-butylimidazole and 1-methylbenzimidazole enables the C-magnesiation and calciation of this family of N-heterocyclic molecules. Although 1-methylimidazole, 1-tert-butylimidazole, and 1-methylbenzimidazole are deprotonated at their C-2 positions by both the magnesium alkyl and hydride derivatives, these reagents can also leverage a level of regiodivergent kinetic discrimination and resultant C-4 metallation. In contrast to the straightforward C-2-deprotonation of 1-methylbenzimidazole provided by the magnesium hydride, a rapid cascade of apparent deprotonation, imidazole ring opening and twofold CC coupling was induced by the analogous calcium reagent. This process provides a tricalcium species comprising an unprecedented trianionic unit, which may be considered as a trimethylenemethane dianion isostere equipped with a pendent amide donor. While attempted boron functionalisation of the imidazolyl anions of the magnesium derivatives of 1-methylimidazole and 1-methylbenzimidazole with pinacolborane (HBpin) was unsuccessful, the 1-tert-butylimidazolyl analogue provided the C-2-borylated product irrespective of the position of initial C-2 or C-4 deprotonation. Although the calcium congeners resulting from 1-methylimidazole and 1-tert-butylimidazole were deduced to form similarly borylated products, study of their onward reactivity was prevented by their instability toward Schlenk-type redistribution in solution.

Half-Sandwich Zirconium and Hafnium Amidoborane Complexes: Precursors of Hydride Derivatives.

Half-sandwich zirconium(IV) and hafnium(IV) complexes with amidoborane and hydride ligands have been isolated in the stoichiometric reactions of mono(pentamethylcyclopentadienyl)metal alkyl and amido derivatives with the amine-boranes NHR2BH3 (R2 = H2, Me2, HtBu). Treatment of the tris(trimethylsilylmethyl) complexes [M(η5-C5Me5)(CH2SiMe3)3] with NH3BH3 (3 equiv) gives the seven-coordinate species [M(η5-C5Me5)(NH2BH3)3] (M = Zr (1), Hf (2)) with three κ2N,H-NH2BH3 ligands. The tris(neophyl) [M(η5-C5Me5)(CH2CMe2Ph)3] or tris(dimethylamido) [M(η5-C5Me5)(NMe2)3] derivatives react with NHMe2BH3 (≥3 equiv) to afford bis(dimethylamidoborane) hydride complexes [M(η5-C5Me5)H(NMe2BH3)2] (M = Zr (3), Hf (4)) via thermally unstable [M(η5-C5Me5)(NMe2BH3)3] species. The reaction of [M(η5-C5Me5)(NMe2)3] and NH2tBuBH3 (≥4 equiv) affords analogous mixed amidoborane hydride derivatives [M(η5-C5Me5)H(NHtBuBH3)(NMe2BH3)] (M = Zr (5), Hf (6)) with κ2N,H-NHtBuBH3 and κ3N,H,H-NMe2BH3 ligands. The addition of NHR2BH3 (≥1 equiv) on the mono(dimethylamido) complexes [M(η5-C5Me5)Cl2(NMe2)] in hexane leads to the precipitation of the ionic compounds [(NHR2)2BH2][{M(η5-C5Me5)Cl2}2(μ-H)3] (R2 = Me2, M = Zr (7), Hf (8); R2 = HtBu, M = Zr (9), Hf (10)). Molecular hydride species [Cl2(η5-C5Me5)M(μ-Cl)(μ-H)2M(η5-C5Me5)Cl(NH2tBu)] (M = Zr (11), Hf (12)) could be isolated from mixtures of complexes [M(η5-C5Me5)Cl2(NMe2)] and lower ratios of NH2tBuBH3. The zirconium complex 11 decomposes in solution to give the mononuclear tert-butylamido derivative [Zr(η5-C5Me5)Cl2(NHtBu)] (13) along with other byproducts.

CO2 reduction using aluminum hydride: Generation of in-situ frustrated Lewis pairs and small molecule activation therein.

CO2 reduction is appealing for the long-term production of high-value fuels and chemicals. Herein, using density functional theory (DFT) based calculations, we study the CO2 reduction pathway to formic acid using aluminum hydride and phosphine derivatives. Our primary focus is on aluminum hydride derivatives, aimed at improving the efficiency of the CO2 reduction process. Substituents with σ-donating properties at the aluminum center are discovered to lower the activation barriers. We demonstrate how di-tert-butylphosphine oxide (LB-O)/di-tert-butylphosphine sulfide (LB-S)/di-tert-butylphosphanimine (LB-N) work together with aluminum hydride to facilitate CO2 reduction process and generate in-situ frustrated Lewis pairs (FLPs), such as FLP-O, FLP-S, and FLP-N. The activation strain model (ASM) analysis reveals the significance of strain energy in determining activation barriers. EDA-NOCV and PIO analyses elucidate the orbital interactions at the corresponding transition states. Furthermore, the study delves into the activation of various small molecules, such as dihydrogen, acetylene, ethylene, carbon dioxide, nitrous oxide, and acetonitrile, using those in-situ generated FLPs. The study highlights the low activation barriers and emphasizes the potential for small molecule activation in this context.

Synthesis and Crystal Structure of the Europium(II) Hydride Oxide Iodide Eu5H2O2I4 Showing Blue-Green Luminescence.

As the first europium(II) hydride oxide iodide, dark red single crystals of Eu5H2O2I4 could be synthesized from oxygen-contaminated mixtures of EuH2 and EuI2. Its orthorhombic crystal structure (a = 1636.97(9) pm, b = 1369.54(8) pm, c = 604.36(4) pm, Z = 4) was determined via single-crystal X-ray diffraction in the space group Cmcm. Anion-centred tetrahedra [HEu4]7+ and [OEu4]6+ serve as central building blocks interconnected via common edges to infinite ribbons parallel to the c axis. These ribbons consist of four trans-edge connected (Eu2+)4 tetrahedra as repetition unit, two H--centred ones in the inner part, and two O2--centred ones representing the outer sides. They are positively charged, according to ∞1{[Eu5H2O2]4+}, to become interconnected and charge-balanced by iodide anions. Upon excitation with UV light, the compound shows blue-green luminescence with the shortest Eu2+ emission wavelength ever observed for a hydride derivative, peaking at 463 nm. The magnetic susceptibility of Eu5H2O2I4 follows the Curie-Weiss law down to 100 K, and exhibits a ferromagnetic ordering transition at about 10 K.

Stability and C-H Bond Activation Reactions of Palladium(I) and Platinum(I) Metalloradicals: Carbon-to-Metal H-Atom Transfer and an Organometallic Radical Rebound Mechanism.

One-electron oxidation of palladium(0) and platinum(0) bis(phosphine) complexes enables isolation of a homologous series of linear d9 metalloradicals of the form [M(PR3)2]+ (M = Pd, Pt; R = tBu, Ad), which are stable in 1,2-difluorobenzene (DFB) solution for >1 day at room temperature when partnered with the weakly coordinating [BArF4]- (ArF = 3,5-(CF3)2C6H3) counterion. The metalloradicals exhibit reduced stability in THF, decreasing in the order palladium(I) > platinum(I) and PAd3 > PtBu3, especially in the case of [Pt(PtBu3)2]+, which is converted into a 1:1 mixture of the platinum(II) complexes [Pt(PtBu2CMe2CH2)(PtBu3)]+ and [Pt(PtBu3)2H]+ upon dissolution at room temperature. Cyclometalation of [Pt(PtBu3)2]+ can also be induced by reaction with the 2,4,6-tri-tert-butylphenoxyl radical in DFB, and a common radical rebound mechanism involving carbon-to-metal H-atom transfer and formation of an intermediate platinum(III) hydride complex, [Pt(PtBu2CMe2CH2)H(PtBu3)]+, has been substantiated by computational analysis. Radical C-H bond oxidative addition is correlated with the resulting MII-H bond dissociation energy (M = Pt > Pd), and reactions of the metalloradicals with 9,10-dihydroanthracene in DFB at room temperature provide experimental evidence for the proposed C-H bond activation manifold in the case of platinum, although conversion into platinum(II) hydride derivatives is considerably faster for [Pt(PtBu3)2]+ (t1/2 = 1.2 h) than [Pt(PAd3)2]+ (t1/2 ∼ 40 days).

Bis(3-methylthio-1-azulenyl)phenylmethyl Cations and Dications Connected by a 1,4-Phenylene Spacer: Synthesis and Their Electrochemical Properties

The preparation of bis(3-methylthio-1-azulenyl)phenylmethyl cations and 1,4-phenylenebis[bis(3,6-di-tert-butyl-1-azulenyl)methyl] dications was accomplished by the hydride abstraction of the corresponding hydride derivatives, which were synthesized by the acid-catalyzed condensation of 1-azulenyl methyl sulfide with benzaldehyde and terephthalaldehyde with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. The intramolecular charge transfer among the azulene ring and the methylium moieties of these cations and dications was investigated by UV–Vis spectroscopy and electrochemical analyses. The pKR+ values of the cations were examined for their thermodynamic stability spectrophotometrically. The voltammetry experiments of these cations revealed their reversible reduction waves on their cyclic voltammograms. Moreover, a notable spectral change of cations was observed by spectroelectrochemistry during electrochemical reduction conditions.

Unravelling the Mechanistic Pathway of the Hydrogen Evolution Reaction Driven by a Cobalt Catalyst

AbstractA cobalt complex bearing a κ‐N3P2 ligand is presented (1+ or CoI(L), where L is (1E,1′E)‐1,1′‐(pyridine‐2,6‐diyl)bis(N‐(3‐(diphenylphosphanyl)propyl)ethan‐1‐imine). Complex 1+ is stable under air at oxidation state CoI thanks to the π‐acceptor character of the phosphine groups. Electrochemical behavior of 1+ reveals a two‐electron CoI/CoIII oxidation process and an additional one‐electron reduction, which leads to an enhancement in the current due to hydrogen evolution reaction (HER) at Eonset=−1.6 V vs Fc/Fc+. In the presence of 1 equiv of bis(trifluoromethane)sulfonimide, 1+ forms the cobalt hydride derivative CoIII(L)‐H (22+), which has been fully characterized. Further addition of 1 equiv of CoCp*2 (Cp* is pentamethylcyclopentadienyl) affords the reduced CoII(L)‐H (2+) species, which rapidly forms hydrogen and regenerates the initial CoI(L) (1+). The spectroscopic characterization of catalytic intermediates together with DFT calculations support an unusual bimolecular homolytic mechanism in the catalytic HER with 1+.

Unravelling the Mechanistic Pathway of the Hydrogen Evolution Reaction Driven by a Cobalt Catalyst.

A cobalt complex bearing a κ-N3 P2 ligand is presented (1+ or CoI (L), where L is (1E,1'E)-1,1'-(pyridine-2,6-diyl)bis(N-(3-(diphenylphosphanyl)propyl)ethan-1-imine). Complex 1+ is stable under air at oxidation state CoI thanks to the π-acceptor character of the phosphine groups. Electrochemical behavior of 1+ reveals a two-electron CoI /CoIII oxidation process and an additional one-electron reduction, which leads to an enhancement in the current due to hydrogen evolution reaction (HER) at Eonset =-1.6 V vs Fc/Fc+ . In the presence of 1 equiv of bis(trifluoromethane)sulfonimide, 1+ forms the cobalt hydride derivative CoIII (L)-H (22+ ), which has been fully characterized. Further addition of 1 equiv of CoCp*2 (Cp* is pentamethylcyclopentadienyl) affords the reduced CoII (L)-H (2+ ) species, which rapidly forms hydrogen and regenerates the initial CoI (L) (1+ ). The spectroscopic characterization of catalytic intermediates together with DFT calculations support an unusual bimolecular homolytic mechanism in the catalytic HER with 1+ .

Dynamic thin-film microextraction method using cellulose platforms modified with silver nanoparticles for preconcentration of volatile hydride-forming elements prior to inductively-coupled plasma mass spectrometry determination

In this work, a novel preconcentration method was developed for As, Sb and Bi following trapping of their volatile hydride derivatives onto cellulose-based platforms modified with Ag nanoparticles for ultrasensitive determination by inductively-coupled plasma mass spectrometry (ICP-MS). A continuous flow hydride generation system was coupled to a gas-liquid separation unit integrated with the preconcentration platform. AsH3, SbH3 and BiH3 were retained onto the silver nanoparticles (AgNPs) modified cellulose substrate upon catalytic decomposition. After a 10 min preconcentration time, enrichment factors of 16, 9 and 26 for As, Sb and Bi were obtained, respectively. These values can be increased when needed by a factor of three applying a preconcentration time of 30 min. Elution of analytes trapped onto AgNPs with 0.1 M HNO3 (As, Sb) or 1 M HNO3 (Bi) was performed and metal content in the eluates was measured by ICP-MS. Detection limits of 15, 2 and 1 ng/L for As, Sb and Bi were obtained, respectively. Repeatability values expressed as relative standard deviation (%) was in the range of 1.2–2.7%. Other hydrides tried (i.e. SeH2 and TeH2) did not provide suitable performance owing to inefficient desorption from the cellulose substrate. Simulated water matrices mimicking the major composition of freshwater, seawater, wastewater and groundwater were analyzed, recoveries being in the range of 80–106%.

Synthesis and reactivity of iridium complexes of a macrocyclic PNP pincer ligand.

Having recently reported on the synthesis and rhodium complexes of the novel macrocyclic pincer ligand PNP-14, which is derived from lutidine and features terminal phosphine donors trans-substituted with a tetradecamethylene linker (Dalton Trans., 2020, 49, 2077-2086 and Dalton Trans., 2020, 49, 16649-16652), we herein describe our findings critically examining the chemistry of iridium homologues. The five-coordinate iridium(i) and iridium(iii) complexes [Ir(PNP-14)(η2:η2-cyclooctadiene)][BArF4] and [Ir(PNP-14)(2,2'-biphenyl)][BArF4] are readily prepared and shown to be effective precursors for the generation of iridium(iii) dihydride dihydrogen, iridium(i) bis(ethylene), and iridium(i) carbonyl derivatives that highlight important periodic trends by comparison to rhodium counterparts. Reaction of [Ir(PNP-14)H2(H2)][BArF4] with 3,3-dimethylbutene induced triple C-H bond activation of the methylene chain, yielding an iridium(iii) allyl hydride derivative [Ir(PNP-14*)H][BArF4], whilst catalytic homocoupling of 3,3-dimethylbutyne into Z-tBuC[triple bond, length as m-dash]CCHCHtBu could be promoted at RT by [Ir(PNP-14)(η2:η2-cyclooctadiene)][BArF4] (TOFinitial = 28 h-1). The mechanism of the latter is proposed to involve formation and direct reaction of a vinylidene derivative with HC[triple bond, length as m-dash]CtBu outside of the macrocyclic ring and this suggestion is supported experimentally by isolation and crystallographic characterisation of a catalyst deactivation product.

Water-mediated formation of hydride derivates from flexible Pd-salan complexes: A DFT study

The formation of hydride derivatives of sulfonated palladium(II) salan (hydrogenated salen) complexes was studied with DFT methods. The non-hydrolytic property, chirality, and flexibility lead to a significant difference compared to salen derivatives. We made a detailed computational study to understand the relevance of the flexibility in contrast to the rigid salen complex. Two main pathways were investigated: one of them was a direct monohydride formation where the oxygen of the phenolate group was substituted by a hydrogen molecule followed by a proton transfer. Another was an indirect monohydride formation involving the substitution of phenolate arm by a solvent water molecule in the first step and subsequent reaction with H2 in the second. We focused on weak interactions among the Pd-complex and water molecules. Trigonal, square, and diamond motifs of H-bond networks were found around the oxygen atom of the phenolate arm which is crucial during the proton transfer step, however, substitution steps prefer chain type motifs.

A Series of Ultra-Efficient Blue Borane Fluorophores.

We present the first examples of alkylated derivatives of the macropolyhedral boron hydride, anti-B18H22, which is the gain medium in the first borane laser. This new series of ten highly stable and colorless organic-inorganic hybrid clusters are capable of the conversion of UVA irradiation to blue light with fluorescence quantum yields of unity. This study gives a comprehensive description of their synthesis, isolation, and structural characterization together with a delineation of their photophysical properties using a combined theoretical and experimental approach. Treatment of anti-B18H22 1 with RI (where R = Me or Et) in the presence of AlCl3 gives a series of alkylated derivatives, Rx-anti-B18H22-x (where x = 2 to 6), compounds 2-6, in which the 18-vertex octadecaborane cluster architectures are preserved and yet undergo a linear "polyhedral swelling", depending on the number of cluster alkyl substituents. The use of dichloromethane solvent in the synthetic procedure leads to dichlorination of the borane cluster and increased alkylation to give Me11-anti-B18H9Cl2 11, Me12-anti-B18H8Cl2 12, and Me13-anti-B18H7Cl2 13. All new alkyl derivatives are highly stable, extremely efficient (ΦF = 0.76-1.0) blue fluorophores (λems = 423-427 nm) and are soluble in a wide range of organic solvents and also a polystyrene matrix. The Et4-anti-B18H18 derivative 4b crystallizes from pentane solution in two phases with consequent multiabsorption and multiemission photophysical properties. An ultrafast transient UV-vis absorption spectroscopic study of compounds 4a and 4b reveals that an efficient excited-state absorption at the emission wavelength inhibits the laser performance of these otherwise remarkable luminescent molecules. All these new compounds add to the growing portfolio of octadecaborane-based luminescent species, and in an effort to broaden the perspective on their highly emissive photophysical properties, we highlight emerging patterns that successive substitutions have on the molecular size of the 18-vertex borane cluster structure and the distribution of the electron density within.

Unsymmetrical pyrazole-based PCN pincer NiII halides: Reactivity and catalytic activity in ethylene oligomerization

The reactivity of unsymmetrical pyrazole-based PCN pincer Ni(II) halides has been tested in the presence of copper(II) halides as an oxidizing agent as well as with NaBH4 and LiAlH4 as a hydride source. While the reaction with CuX2 resulted into the generation of metastable PCN pincer Ni(III) species, the treatment of (tBuPCN)NiCl with NaBH4 led to the preparation and isolation of the borohydride complex (tBuPCN)Ni(BH4). Attempts to isolate nickel-hydride species upon treatment of (tBuPCN)NiCl with either NaBH4 or LiAlH4 invariably led to the formation of free PCN(H) ligand and metallic Ni(0) nanoparticles after an in situ reductive elimination from the (metastable) hydride derivative (tBuPCN)NiH. Finally, the halide complexes (pre-activated by MMAO) have been tested as homogeneous catalysts in ethylene oligomerization showing moderate activity (∼ 14 × 103 molC2H4⋅molNi−1⋅h−1) with the formation of even-numbered olefins (mainly C4–C10 fractions) as products.

Multimetallic Alkaline-Earth Hydride Cations

Reactions of dimeric β-diketiminato (BDI) magnesium and calcium hydrides with [(BDI)Mg]+[Al{OC(CF3)3}4]− provide ionic multimetallic hydride derivatives, which have been characterized by single-cry...

Comparative study of 1:1 Lewis acid–base adducts between Cp2M(L)H (M = V, Nb, Ta; L = CO, C2H4, P(CH3)3) and BF3/AlF3

Transition metal hydrides play important roles in organometallic catalysis and synthetic reactions. Metallocenes niobium or tantalum hydride derivatives show high reactivities with variety of main group 13 Lewis acids. In this work, the nature of MH···B/Al interaction between Cp2M(L)H(M = V, Nb, and Ta) and BF3/AlF3 in 1:1 Lewis acid–base adducts as well as the influences of different types of ligands on the interactions is investigated. The results show that in Cp2M(L)H···BF3/AlF3, the MH···B interaction belongs to covalent interaction, and MH···Al shows partly covalent character. The covalent character of MH···B/Al bonds decreases with the increasing period of metal atom. The strength of MH···B/Al interaction is related to the type of metal atom (M) and ligand (L) in Cp2M(L)H, and the influences of L are larger than those of M. For the same M, the strength of MH···B/Al bonds is mainly influenced by the type of ligand, and they increase in the sequence of L = CO, C2H4, and P(CH3)3. The formation of Cp2M(L)H···BF3/AlF3 increases the polarity of M–H bond of Cp2M(L)H.

σ-Silane Platinum(II) Complexes as Intermediates in C-Si Bond-Coupling Processes.

Platinum complexes [Pt(NHC')(NHC)][BArF ] (in which NHC' denotes a cyclometalated N-heterocyclic carbene ligand, NHC) react with primary silanes RSiH3 to afford the cyclometalated platinum(II) silyl complexes [Pt(NHC-SiHR')(NHC)][BArF ] through a process that involves the formation of C-Si and Pt-Si bonds with concomitant extrusion of H2 . Low-temperature NMR studies indicate that the process proceeds through initial formation of the σ-SiH complexes [Pt(NHC')(NHC)(HSiH2 R)][BArF ], which are stable at temperatures below -10 °C. At higher temperatures, activation of one Si-H bond followed by a C-Si coupling reaction generates an agostic SiH platinum hydride derivative [Pt(H)(NHC'-SiH2 R)(NHC)][BArF ], which undergoes a second Si-H bond activation to afford the final products. Computational modeling of the reaction mechanism indicates that the stereochemistry of the silyl/hydride ligands after the first Si-H bond cleavage dictates the nature of the products, favoring the formation of a C-Si bond over a C-H bond, in contrast to previous results obtained for tertiary silanes. Furthermore, the process involves a trans-to-cis isomerization of the NHC ligand before the second Si-H bond cleavage.

Asymmetric ruthenium tricarbonyl cyclopentadienone complexes; synthesis and application to asymmetric hydrogenation of ketones

A series of enantiomerically-pure ruthenium tricarbonyl cyclopentadienone complexes were prepared via the cyclisation of C2-symmetric dialkynes with Ru3(CO)12. Four complexes were characterised by X-ray crystallography and a hydride derivative of one of these was characterised. The complexes were tested in the asymmetric hydrogenation and transfer hydrogenation of acetophenone. Whilst the catalysts were active, acetophenone was reduced in low ee, however a reduction product of up to 46% ee could be obtained when a mild base (iPr2NEt or pyridine) was added to the reaction.

Ammonia-Borane Derived BN Fragments Trapped on Bi- and Trimetallic Titanium(III) Systems.

Titanium(III) complexes containing unprecedented (NH2 BH2 NHBH3 )2- and {N(BH3 )3 }3- ligands have been isolated, and their structures elucidated by a combination of experimental and theoretical methods. The treatment of the trimethyl derivative [TiCp*Me3 ] (Cp*=η5 -C5 Me5 ) with NH3 BH3 (3 equiv) at room temperature gives the paramagnetic dinuclear complex [{TiCp*(NH2 BH3 )}2 (μ-NH2 BH2 NHBH3 )], which at 80 °C leads to the trinuclear hydride derivative [{TiCp*(μ-H)}3 {μ3 -N(BH3 )3 }]. The bonding modes of the anionic BN fragments in those complexes, as well as the dimethylaminoborane group trapped on the analogous trinuclear [{TiCp*(μ-H)}3 (μ3 -H)(μ3 -NMe2 BH2 )], have been studied by X-ray crystallography and density functional theory (DFT) calculations.

Synthetic Designs and Structural Investigations of Biomimetic Ni-Fe Thiolates.

Described are the syntheses of several Ni(μ-SR)2Fe complexes, including hydride derivatives, in a search for improved models for the active site of [NiFe]-hydrogenases. The nickel(II) precursors include (i) nickel with tripodal ligands: Ni(PS3)- and Ni(NS3)- (PS33- = tris(phenyl-2-thiolato)phosphine, NS33- = tris(benzyl-2-thiolato)amine), (ii) traditional diphosphine-dithiolates, including chiral diphosphine R,R-DIPAMP, (iii) cationic Ni(phosphine-imine/amine) complexes, and (iv) organonickel precursors Ni( o-tolyl)Cl(tmeda) and Ni(C6F5)2. The following new nickel precursor complexes were characterized: PPh4[Ni(NS3)] and the dimeric imino/amino-phosphine complexes [NiCl2(PCH═NAn)]2 and [NiCl2(PCH2NHAn)]2 (P = Ph2PC6H4-2-). The iron(II) reagents include [CpFe(CO)2(thf)]BF4, [Cp*Fe(CO)(MeCN)2]BF4, FeI2(CO)4, FeCl2(diphos)(CO)2, and Fe(pdt)(CO)2(diphos) (diphos = chelating diphosphines). Reactions of the nickel and iron complexes gave the following new Ni-Fe compounds: Cp*Fe(CO)Ni(NS3), [Cp(CO)Fe(μ-pdt)Ni(dppbz)]BF4, [( R,R-DIPAMP)Ni(μ-pdt)(H)Fe(CO)3]BArF4, [(PCH═NAn)Ni(μ-pdt)(Cl)Fe(dppbz)(CO)]BF4, [(PCH2NHAn)Ni(μ-pdt)(Cl)Fe(dppbz)(CO)]BF4, [(PCH═NAn)Ni(μ-pdt)(H)Fe(dppbz)(CO)]BF4, [(dppv)(CO)Fe(μ-pdt)]2Ni, {H[(dppv)(CO)Fe(μ-pdt)]2Ni]}BF4, and (C6F5)2Ni(μ-pdt)Fe(CO)2(dppv) (DIPAMP = (CH2P(C6H4-2-OMe)2)2; BArF4- = [B(C6H3-3,5-(CF3)2]4-)) Within the context of Ni-(SR)2-Fe complexes, these new complexes feature new microenvironments for the nickel center: tetrahedral Ni, chirality, imine, and amine coligands, and Ni-C bonds. In the case of {H[(dppv)(CO)Fe(μ-pdt)]2Ni}+, four low-energy isomers are separated by ≤3 kcal/mol, one of which features a biomimetic HNi(SR)4 site, as supported by density functional theory calculations.

A Zirconium Photosensitizer with a Long-Lived Excited State: Mechanistic Insight into Photoinduced Single-Electron Transfer

Time-resolved emission spectroscopy for the luminescent zirconium complex Zr(MePDP)2 (MePDP = 2,6-bis(5-methyl-3-phenyl-1 H-pyrrol-2-yl)pyridine) revealed a long-lived excited state with a lifetime τ = 325 ± 10 μs. Computational studies using time-dependent density functional theory were conducted to identify the nature of the luminescent excited state as a mixed triplet intraligand/ligand-to-metal charge-transfer state. Stern-Volmer experiments showed a strong dependence of the quenching rate on the redox potential of the quencher indicating photoinduced single-electron transfer (SET) as the quenching pathway. Mechanistic investigations of the photocatalytic homocoupling of benzyl bromide allowed the detection of organic radical intermediates during turnover and provided further evidence for SET mediated by Zr(MePDP)2. Isolation of the one-electron-reduced form of the photosensitizer, [Zr(MePDP)2]-, enabled studies of its electronic structure by a combination of experimental and computational techniques and confirmed its role as a strong reductant. Additionally, the role of the benzimidazolium hydride derivatives as two-electron sacrificial reductants during photoredox catalysis was investigated. In combination, the results presented in this report establish a detailed mechanistic picture of a photoredox catalytic reaction promoted by an earth-abundant early transition metal photosensitizer.