Rare-Earth Metalloligands for Low-Valent Cobalt Complexes: Fine Electronic Tuning via Co→RE Dative Interactions.

Rare-earth metalloligand supported low-valent cobalt complexes were synthesized by utilizing a small-sized heptadentate phosphinomethylamine LsNH3 and a large-sized arene-anchored hexadentate phosphinomethylamine LlArH3 ligand precursors. The RE(III)-Co(-I)-N2 (RE = Sc, Lu, Y, Gd, La) complexes containing rare-earth metals including the smallest Sc and largest La were characterized by multinuclear NMR spectroscopy, X-ray diffraction analysis, electrochemistry, and computational studies. The Co(-I)→RE(III) dative interactions were all polarized with major contributions from the 3dz2 orbital of the cobalt center, which was slightly affected by the identity of rare-earth metalloligands. The IR spectroscopic data and redox potentials obtained from cyclic voltammetry revealed that the electronic property of the Co(-I) center was finely tuned by the rare-earth metalloligand, which was revealed by variation of the ligand systems containing LsN, LmN, and LlAr. Unlike the direct alteration of the electronic property of metal center via an ancillary ligand, such a series of rare-earth metalloligand represents a smooth strategy to tune the electronic property of transition metals.

A scandium metalloligand supported Ni(0) complex with a heterobimetallocycle: versatile reactivity with unsaturated bonds.

A N2-bridged tetranuclear Sc(III)-Ni(0) complex featuring a Ni → Sc interaction and a 4-membered [Sc-N-C-Ni] ring was synthesized and characterized. Bimetallic reactivity was demonstrated via reactions with a series of unsaturated compounds containing NC, CN, CC, CO and NN bonds.

Alkoxycarbonyl Groups in Metalloesters Showing Oxocarbenium-like Structure and Alkylating Reactivity.

In contrast to the well-documented acylating reactivity, the alkylating reactivity of the alkoxycarbonyl group, as signified by its oxocarbenium-like resonance structure, remains almost unexplored. Herein, the first series of Co/Ni dinuclear metalloesters exhibiting the novel oxocarbenium-like alkoxycarbonyl groups were synthesized and characterized. In these deformed alkoxycarbonyl groups, the Ccarbonyl-Oalkoxyl bonds were contracted to 1.177(11)~1.191(9) Šwith the elongations of the Ccarbonyl=Ocarbonyl bonds to 1.368(13)~1.441(9) Å. Meanwhile, the O-Calkyl bonds were also elongated to 1.522(11) ~1.607(15) Å, and were by far the longest O-Calkyl bonds reported for alkoxycarbonyl groups. As triggered by the long O-Calkyl distances, the alkylating reactivity of the oxocarbenium-like methoxycarbonyl group towards a series of C/N/O-nucleophiles via the rare BAL2 mechanism at ambient conditions was examined. Furthermore, the homo-etherifications of alcohols mediated by the Co/Ni dinuclear metalloesters were investigated. The yields followed the trend ethanol≫n-propanol≫n-butanol ≈n-pentanol, that closely related to the structure features of the alkoxycarbonyl groups in corresponding metalloesters: while the ethoxycarbonyl group showed the reactive oxocarbenium-like framework, the n-propoxycarbonyl group displayed the dioxocarbenium-like skeleton with a shorter O-Calkyl bond; In comparison, the classical frameworks with unactivated alkyl moieties were observed for n-butoxycarbonyl and n-pentoxycarbonyl groups.

Direct synthesis of oxalic acid via oxidative CO coupling mediated by a dinuclear hydroxycarbonylcobalt(III) complex.

Oxidative coupling of CO is a straightforward and economic benign synthetic route for value-added α-diketone moiety containing C2 or higher carbon compounds in both laboratory and industry, but is still undeveloped to date. In this work, a rare coplanar dinuclear hydroxycarbonylcobalt(III) complex, bearing a Schiff-base macrocyclic equatorial ligand and a µ-κ1(O):κ1(O')-acetate bridging axial ligand, is synthesized and characterized. The Co(III)-COOH bonds in this complex can be feasibly photocleaved, leading to the formation of oxalic acid. Moreover, the light-promoted catalytic direct production of oxalic acid from CO and H2O using O2 as the oxidant with good selectivity (> 95%) and atom economy at ambient temperature and gas pressure based on this dicobalt(III) complex have been achieved, with a turnover number of 38.5. The 13C-labelling and 18O-labelling experiments confirm that CO and H2O act as the sources of the -COOH groups in the dinuclear hydroxycarbonylcobalt(III) complex and the oxalic acid product.

Structural, Spectroscopic, and Bonding Analyses of La(III)/Ce(III)-Tetrel Ate-Complexes.

In comparison with the research of transition-metal-tetrel complexes, the chemistry of lanthanide tetrel complexes, especially for these bearing heavier tetrel element ligands, is still relatively underexplored. In this research, K[Cp3Ln(III)CH2Ph], [(DME)3Li][Cp3Ln(III)GePh3], and [(DME)3Li][Cp3Ln(III)SnPh3] [Ln(III) = La(III), Ce(III)] have been synthesized by reacting [(DME)3Na][Cp3La(µ-Cl)LaCp3] or Cp3Ce(THF) with alkali metal alkyl, germyl, and stannyl reagents. Additionally, [(DME)3Li][Cp3Ce(III)SnPh3] is the first example of Ce(III)-Sn bond containing complex. All the obtained early Ln(III) tetrel ate-complexes were structurally analyzed by single-crystal X-ray diffraction. The formal shortness ratios of the Ln(III)-C, Ln(III)-Ge, and Ln(III)-Sn bonds are in the range of 1.03-1.11. Together with the previously reported [(DME)3Li][Cp3Ln(III)SiPh3], a group of tetrel (up to Sn) lanthanocene ate-complexes with an analogous coordination pattern are presented. Computational studies suggest the strongly polarized nature of the Ln(III)-E (E = C, Si, Ge, Sn) bonds in these complexes, with 77-85% atomic orbital contribution from tetrel elements and 15-23% atomic orbital contribution from Ln(III). The UV-vis measurements of this series of complexes show that the characteristic absorptions are hypsochromically shifted for Ln(III) heavier tetrel complexes in comparison to their lighter congeners. Moreover, the HOMOs, in which the Ln(III)-E σ-bonding orbitals are the dominant components, of these series complexes act as donor orbitals of the major electron transitions, as being disclosed by the time-dependent density functional theory analysis.

Dinitrogen Complexes of Cobalt(-I) Supported by Rare-Earth Metal-Based Metalloligands.

Sequential reactions of heptadentate phosphinoamine LH3 with rare-earth metal tris-alkyl precursor (Me3SiCH2)3Ln(THF)2 (Ln = Sc, Lu, Yb, Y, Gd) and a low-valent cobalt complex (Ph3P)3CoI afforded rare-earth metal-supported cobalt iodide complexes. Reduction of these iodide complexes under N2 allowed the isolation of the first series of dinitrogen complexes of Co(-I) featuring dative Co(-I) → Ln (Ln = Sc, Lu, Yb, Y, Gd) bonding interactions. These compounds were characterized by multinuclear NMR spectroscopy, X-ray diffraction analysis, electrochemistry, and computational studies. The correlation of N-N vibrational frequencies with the pKa of [Ln(H2O)6]3+ showed that strongest activation of N2 was achieved with the least Lewis acidic Gd(III) ion. Interestingly, these Ln-Co-N2 complexes catalyzed silylation of N2 in the presence of KC8 and Me3SiCl with turnover numbers (TONs) up to 16, where the lutetium-supported Co(-I) complex showed the highest activity within the series. The role of the Lewis acidic Ln(III) was crucial to achieve catalytic turnovers and tunable reactivity toward N2 functionalization.

C-H bond activations by the HO˙/(Salophent-Bu)Co(II) radical pair generated via homolysis of a terminal Co(III)-OH bond.

The reactive HO˙/(Salophent-Bu)Co(II) radical pair was observed to be generated via homolysis of the terminal Co(III)-OH bond in transient (Salophent-Bu)(L)Co(III)(OH) (L = Py, MeOH) complexes as indicated by UV-Vis and EPR measurements. Based on this elementary process, C-H bond activations in acetone, 2-butanone, acetonitrile and benzene were achieved under ambient conditions. For the reactions of the first three substrates, the alkylcobalt(III) complexes were formed as the products.

Early Lanthanide(III) Ate Complexes Featuring Ln-Si Bonds (Ln = La, Ce): Synthesis, Structural Characterization, and Bonding Analysis.

While research on lanthanide (Ln) complexes with silyl ligands is receiving growing attention, significantly unbalanced efforts have been devoted to different Ln elements. In comparison with the intense investigations on Ln elements such as Sm and Yb, the chemistry of silyl lanthanum and cerium complexes is much slower to develop, and no solid-state structure of a silyl lanthanum complex has been reported so far. In this research, four types of ate complexes, including [(DME)3Li][Cp3LnSi(H)Mes2], [(18-crown-6)K][Cp3LnSi(CH3)Ph2], [(DME)3Li][Cp3LnSiPh3], and [(12-crown-4)2Na] [Cp3LnSi(Ph)2Si(H)Ph2] (Ln = La, Ce), were synthesized by reacting [(DME)3Na][Cp3La(µ-Cl)LaCp3] or Cp3Ce(THF) with alkali metal silanides. All of the synthesized silyl Ln ate complexes were structurally characterized. La-Si bond lengths are in a range of 3.1733(4)-3.1897(10) Å, and the calculated formal shortness ratios of the La-Si bonds (1.071.08) are comparable to those in the reported silyl complexes having other Ln metal centers. The Ce-Si bond lengths (3.1415(6)-3.1705(9) Å) are within the typical range of reported silyl cerium ate complexes. 29Si solid-state NMR measurements on the diamagnetic silyl lanthanum complexes were conducted, and large one-bond hyperfine splitting constants arising from = 7/2) were resolved. Computational studies on these silyl lanthanum and cerium complexes suggested the polarized covalent feature of the Ln-Si bonds, which is in line with the measured large 1J139La-Si splitting constants.

Phosphinoamido ligand supported heterobimetallic rare-earth metal-palladium complexes: versatile structures and redox reactivities.

Heterobimetallic Ln(III)-Pd(0) complexes (Ln = Y, Sm, Gd, Yb) featuring tetranuclear structures with COD as bridges were obtained via the metallation of tris(phosphinoamido) rare-earth metal complexes [Ph2PNAd]3Ln (Ad = adamantyl) with (COD)Pd(CH2SiMe3)2. Notably, the Sc(III)-Pd(0) complex possesses a C3-symmetry with a very short Sc-Pd bond length of 2.432(2) Å, while the tetranuclear complexes exhibited versatile structures both in solution and in the solid state. Reduction of the trivalent complex (Ph2PNAd)3Yb with one equivalent of KC8 in the presence of 18-c-6 afforded the divalent complex [(Ph2PNAd)3Yb][(18-c-6)K(THF)2], which was further reacted with (Ph3P)4Pd to form the first Yb(II)-Pd(0) complex. The Pd(0) → Yb dative interaction weakened significantly from Yb(III) to Yb(II) based on computational studies, which was attributed to the attenuated Lewis acidity of the Yb(II) center. Reactions of Ln(III)-Pd(0) complexes (Ln = Sc and Yb) with the disulfide PhSSPh showed that the Pd(0) center served as a two-electron donor while the reaction apparently occurred on the Ln(III) centers to form Ln(III)-Pd(0) bis-sulfide complexes.

Synthesis of arylamines and N-heterocycles by direct catalytic nitrogenation using N2.

Ammonia and nitric acid are two key platform chemicals to introduce nitrogen atoms into organic molecules in chemical industry. Indeed, nitric acid is mostly produced through the oxidation of ammonia. The ideal nitrogenation would involve direct use of dinitrogen (N2) as a N source to construct N-containing organic molecules. Herein, we report an example of direct catalytic nitrogenation to afford valuable diarylamines, triarylamines, and N-heterocycles from easily available organohalides using dinitrogen (N2) as the nitrogen source in a one-pot/two-step protocol. With this method, 15N atoms are easily incorporated into organic molecules. Structurally diversified polyanilines are also generated in one pot, showing great potential for materials chemistry. In this protocol, lithium nitride, generated in situ with the use of lithium as a reductant, is confirmed as a key intermediate. This chemistry provides an alternative pathway for catalytic nitrogenation to synthesize highly valuable N-containing chemicals from dinitrogen.

Silylamido supported dinitrogen heterobimetallic complexes: syntheses and their catalytic ability.

Molybdenum dinitrogen complexes supported by monodentate arylsilylamido ligand, [Ar(Me3Si)N]3MoN2Mg(THF)2[N(SiMe3)Ar] (5) and [Ar(Me3Si)N]3MoN2SiMe3 (6) (Ar = 3,5-Me2C6H3) were synthesized and structurally characterized, and proved to be effective catalysts for the disproportionation of cyclohexadienes and isomerization of terminal alkenes. The 1H NMR spectrum suggested that the bridging nitrogen ligand remains intact during the catalytic reaction, indicating possible catalytic ability of the Mo-N=N motif.

3-(Methoxycarbonyl)Cyclobutenone as a Reactive Dienophile in Enantioselective Diels-Alder Reactions Catalyzed by Chiral Oxazaborolidinium Ions.

Cyclobutenone has been used as a highly reactive dienophile in Diels-Alder reactions, however, no enantioselective example has been reported. We disclose herein a chiral oxazaborolidine-aluminum bromide catalyzed enantioselective Diels-Alder reaction of 3-alkoxycarbonyl cyclobutenone with a variety of dienes. Furthermore, a total synthesis of (-)-kingianin F was completed for the first time via enantioenriched cycloadduct bicyclo[4.2.0]octane derivative.

Deep-blue organic light-emitting diodes based on a doublet d-f transition cerium(III) complex with 100% exciton utilization efficiency.

Compared to red and green organic light-emitting diodes (OLEDs), blue OLEDs are still the bottleneck due to the lack of efficient emitters with simultaneous high exciton utilization efficiency (EUE) and short excited-state lifetime. Different from the fluorescence, phosphorescence, thermally activated delayed fluorescence (TADF), and organic radical materials traditionally used in OLEDs, we demonstrate herein a new type of emitter, cerium(III) complex Ce-1 with spin-allowed and parity-allowed d-f transition of the centre Ce3+ ion. The compound exhibits a high EUE up to 100% in OLEDs and a short excited-state lifetime of 42 ns, which is considerably faster than that achieved in efficient phosphorescence and TADF emitters. The optimized OLEDs show an average maximum external quantum efficiency (EQE) of 12.4% and Commission Internationale de L'Eclairage (CIE) coordinates of (0.146, 0.078).

Heterobimetallic Pd(0) complexes with Pd→Ln (Ln = Sc, Y, Yb, Lu) dative bonds: rare-earth metal-dominated frustrated Lewis pair-like reactivity.

A series of heterobimetallic Pd-Ln complexes with Pd→Ln (Ln = Sc, Y, Yb, Lu) dative bonds were synthesized via sequential reactions of phosphinoamine Ph2PNHAd with (Me3SiCH2)3Ln(THF)2 and (Ph3P)4Pd or (COD)Pd(CH2SiMe3)2. These complexes were characterized by NMR spectroscopy, X-ray diffractions, and computational as well as electrochemical studies, which revealed Pd→Ln dative interactions that vary according to the ionic radii of Ln3+. Furthermore, the notable dynamic structural features of the Pd-Ln complexes in solution and their unexpected frustrated Lewis pair-like reactivity toward aryl halides and ketene were also studied.

Heterobimetallic scandium-group 10 metal complexes with LM → Sc (LM = Ni, Pd, Pt) dative bonds.

Heterobimetallic scandium complexes with whole group 10 metals were synthesized. All Sc-LM complexes were characterized by NMR spectroscopy, X-ray diffraction analysis and computational studies, which revealed notable LM → Sc (LM = Ni, Pd, Pt) dative bonding interactions in these heterobimetallic systems. Versatile coordination modes toward apical donors were observed in these heterobimetallic Sc-LM complexes, among which the Sc-Ni complexes 2 and 3 were reversibly bound to N2 and the Sc-Pt complex 5 was coordinated to an additional PPh3 ligand, while in the Sc-Pd complex 4 no apical donor was ligated.

A Series of Iron Nitrosyl Complexes {Fe-NO}6-9 and a Fleeting {Fe-NO}10 Intermediate en Route to a Metalacyclic Iron Nitrosoalkane.

Iron-nitrosyls have fascinated chemists for a long time due to the noninnocent nature of the NO ligand that can exist in up to five different oxidation and spin states. Coordination to an open-shell iron center leads to complex electronic structures, which is the reason Enemark-Feltham introduced the {Fe-NO}n notation. In this work, we succeeded in characterizing a series of {Fe-NO}6-9 complexes, including a reactive {Fe-NO}10 intermediate. All complexes were synthesized with the tris-N-heterocyclic carbene ligand tris[2-(3-mesitylimidazol-2-ylidene)ethyl]amine (TIMENMes), which is known to support iron in high and low oxidation states. Reaction of NOBF4 with [(TIMENMes)Fe]2+ resulted in formation of the {Fe-NO}6 compound [(TIMENMes)Fe(NO)(CH3CN)](BF4)3 (1). Stepwise chemical reduction with Zn, Mg, and Na/Hg leads to the isostructural series of high-spin iron nitrosyl complexes {Fe-NO}7,8,9 (2-4). Reduction of {Fe-NO}9 with Cs electride finally yields the highly reduced {Fe-NO}10 intermediate, key to formation of [Cs(crypt-222)][(TIMENMes)Fe(NO)], (5) featuring a metalacyclic [Fe-(NO-NHC)3-] nitrosoalkane unit. All complexes were characterized by single-crystal XRD analyses, temperature and field-dependent SQUID magnetization methods, as well as 57Fe Mössbauer, IR, UV/vis, multinuclear NMR, and dual-mode EPR spectroscopy. Spectroscopy-based DFT analyses provide insight into the electronic structures of all compounds and allowed assignments of oxidation states to iron and NO ligands. An alternative synthesis to the {Fe-NO}8 complex was found via oxygenation of the nitride complex [(TIMENMes)Fe(N)](BF4). Surprisingly, the resulting {Fe-NO}8 species is electronically and structural similar to the [(TIMENMes)Fe(N)]+ precursor. Based on the structural and electronic similarities between this nitrosyl/nitride complex couple, we adopted the strategy, developed by Wieghardt et al., of extending the Enemark-Feltham nomenclature to nitrido complexes, rendering [(TIMENMes)Fe(N)]+ as a {Fe-N}8 species.

A Scandium Metalloligand-Based Heterobimetallic Pd-Sc Complex: Electronic Tuning Through a Very Short Pd→Sc Dative Bond.

The first heterobimetallic Pd-Sc complex featuring a very short Pd→Sc dative bond has been synthesized and characterized by multinuclear NMR spectroscopy, X-ray diffraction analysis, and electrochemistry. Computational studies elucidated the nature of the Pd→Sc bond as a donor-acceptor interaction, which generates a more electron-deficient Pd0 metal center as compared to that in the mono Pd0 complex in their reactions with isonitrile and carbon monoxide. Cooperative reactivity has been demonstrated in the reaction with MeI.

Electron Paramagnetic Resonance Signature of Tetragonal Low Spin Iron(V)-Nitrido and -Oxo Complexes Derived from the Electronic Structure Analysis of Heme and Non-Heme Archetypes.

Iron(V)-nitrido and -oxo complexes have been proposed as key intermediates in a diverse array of chemical transformations. Herein we present a detailed electronic-structure analysis of [FeV(N)(TPP)] (1, TPP2- = tetraphenylporphyrinato), and [FeV(N)(cyclam-ac)]+ (2, cyclam-ac = 1,4,8,11-tetraazacyclotetradecane-1-acetato) using electron paramagnetic resonance (EPR) and 57Fe Mössbauer spectroscopy coupled with wave function based complete active-space self-consistent field (CASSCF) calculations. The findings were compared with all other well-characterized genuine iron(V)-nitrido and -oxo complexes, [FeV(N)(MePy2tacn)](PF6)2 (3, MePy2tacn = methyl- N', N″-bis(2-picolyl)-1,4,7-triazacyclononane), [FeV(N){PhB( t-BuIm)3}]+ (4, PhB(tBuIm)3- = phenyltris(3- tert-butylimidazol-2-ylidene)borate), and [FeV(O)(TAML)]- (5, TAML4- = tetraamido macrocyclic ligand). Our results revealed that complex 1 is an authenticated iron(V)-nitrido species and contrasts with its oxo congener, compound I, which contains a ferryl unit interacting with a porphyrin radical. More importantly, tetragonal iron(V)-nitrido and -oxo complexes 1-3 and 5 all possess an orbitally nearly doubly degenerate S = 1/2 ground state. Consequently, analogous near-axial EPR spectra with g|| < g⊥ ≤ 2 were measured for them, and their g|| and g⊥ values were found to obey a simple relation of g⊥2 + (2 - g∥)2 = 4. However, the bonding situation for trigonal iron(V)-nitrido complex 4 is completely different as evidenced by its distinct EPR spectrum with g|| < 2 < g⊥. Further in-depth analyses suggested that tetragonal low spin iron(V)-nitrido and -oxo complexes feature electronic structures akin to those found for complexes 1-3 and 5. Therefore, the characteristic EPR signals determined for 1-3 and 5 can be used as a spectroscopic marker to identify such highly reactive intermediates in catalytic processes.

Ni-Catalyzed Cross-Coupling of Dimethyl Aryl Amines with Arylboronic Esters under Reductive Conditions.

Herein, we reported a successful Suzuki-Miyaura coupling of dimethyl aryl amines to forge biaryl skeleton via Ni catalysis in the absence of directing groups and preactivation. This transformation proceeded with high efficiency in the presence of magnesium. Preliminary mechanism studies demonstrated dual roles of magnesium: (i) a reductant that reduced Ni(II) species to active Ni(I) catalyst; (ii) a unique promoter that facilitated the Ni(I)/Ni(III) catalytic cycle.

Production of Formamides from CO and Amines Induced by Porphyrin Rhodium(II) Metalloradical.

It is of fundamental importance to transform carbon monoxide (CO) to petrochemical feedstocks and fine chemicals. Many strategies built on the activation of C≡O bond by π-back bonding from the transition metal center were developed during the past decades. Herein, a new CO activation method, in which the CO was converted to the active acyl-like metalloradical, [(por)Rh(CO)]• (por = porphyrin), was reported. The reactivity of [(por)Rh(CO)]• and other rhodium porphyrin compounds, such as (por)RhCHO and (por)RhC(O)NH nPr, and corresponding mechanism studies were conducted experimentally and computationally and inspired the design of a new conversion system featuring 100% atom economy that promotes carbonylation of amines to formamides using porphyrin rhodium(II) metalloradical. Following this radical based pathway, the carbonylations of a series of primary and secondary aliphatic amines were examined, and turnover numbers up to 224 were obtained.