Amido Complexes Research Articles

Coinage metal amido and thiolate SNS complexes: consequences of catalyst speciation in Cu(I)-catalysed carbonyl hydroboration.

Three new IPr-Ag- and -Au-SNS amido and thiolate complexes were synthesized and compared to their previously reported Cu analogues as carbonyl hydroboration catalysts (IPr = bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Although these complexes showed no catalytic activity, treatment of the IPr-Ag-SNS amido complex with pinacolborane released the N-borylated ligand, SMeNBpinSMe, (L1-Bpin). This finding led us to reinvestigate the IPr-Cu-SNS amido precatalyst, revealing that immediate loss of L1-Bpin converts our catalyst system to [CuH(IPr)]2.

Catalytic exploration of NHC-Ag(I)HMDS complexes for the hydroboration and hydrosilylation of carbonyl compounds.

The synthesis and full characterisation of two silver(I) amido complexes, stabilised by ancillary N-heterocyclic carbene (NHC) ligands is presented. The light stable complexes [Ag(IDipp)HMDS] 3 and [Ag(IAd)HMDS] 4 were explored as suitable pre-catalysts in the hydroboration and hydrosilylation of a range of carbonyl substrates, with complex 3 out-performing 4 and our previous phosphine-stabilised lead catalyst [Ag(PCy3)HMDS] 5. This study highlights that changing the stabilising Lewis donor in the silver(I)amide system has an influence on catalytic efficiency. Finally, to shed light on the catalytic differences of pre-catalysts 3-5, we used a suite of programs to examine the influence of steric bulk on the Lewis donor ligand including percent buried volume (%VBur), Solid-G and AtomAccess which revealed the most sterically protected Ag(I) metal centre correlated to the most effective pre-catalyst 3.

Novel dinuclear NHC–gold(i)-amido complexes and their application in energy transfer photocatalysis

The development of efficient and operationally simple synthetic routes to dinuclear gold(i)-amido complexes bearing aromatic/aliphatic-bridges are reported.

Ether/Thioether-Functionalized Dianionic α-Iminopyridine Rare-Earth Metal Amido Complexes and Their Catalytic Activity toward Hydrophosphination of Alkenes

The double deprotonation of 2-aminomethyl pyridine by rare-earth metal triamido complexes was found to be an alternative strategy to anchor dianionic iminopyridine on rare-earth metal centers with the morpholinyl group previously. To verify the generality of the protocol with different appendant donor groups, reactions of [(Me3Si)2N]3RE(μ-Cl)Li(THF)3 with ether/thiotether-substituted 2-aminomethyl pyridine 2-(CH3DCH2CH2NHCH2)-6-R-C5H3N (D = O, R = H (1), Ph (2); D = S, R = H (3)) were performed, leading to the formation of ether/thioether-functionalized dianionic α-iminopyridine rare-earth metal amido complexes {μ-η2:σ1:κ1:κ1-2-[CH3ECH2CH2NCH]-6-R-C5H3N}2RE2[N(SiMe3)2]2 (D = O, R = H, RE = Dy (4a), Y (4b), Er (4c), Yb (4d), Lu (4e), R = Ph, RE = Dy (5a), Y (5b), Er (5c), Lu (5d); D = S, R = H, RE = Y (6a), Er (6b), Lu (6c)). These results supported the ability of oxygen or sulfur coordination in promoting the double deprotonation of 2-aminomethyl pyridine to construct new iminopyridine rare-earth metal complexes in the dianionic form. Further examination of these rare-earth metal amido complexes displayed good catalytic activity for hydrophosphination of alkenes, affording anti-Markovnikov addition products under solvent-free conditions.

Niobium Bis‐ and Mono(Pentafulvene) Complexes: E−H Bond Cleavage and Insertion Reactions (E=C, N, O)

AbstractBis(η5:η1‐(di‐para‐tolyl)pentafulvene)niobium chloride (1) reacts with methyl lithium via salt metathesis to the methylated bis(pentafulvene)niobium complex 2, and with lithium 2,6‐diisopropylanilide addition and subsequent N−H bond activation to the imido mono(pentafulvene)niobium complex 3. Avoiding the competing protonation of the chloride, bis(pentafulvene)niobium complex 2 reacts with primary aromatic and aliphatic amines to form terminal niobocene imido complexes, and with water to form the analog terminal oxo complex. Secondary methyl amines undergo a simultaneous N−H and C−H activation to form niobaaziridines under mild conditions. In contrast to other reported examples, 3 can be employed to investigate the uncontested reactivity of mono(pentafulvene)niobium complexes. Reaction with 4‐tert‐butylphenol selectively yields a niobocene phenolate complex. Unprecedented for mono(pentafulvene)niobium complexes, treating 3 with multiple‐bond‐containing substrates (nitriles, isocyanates) smoothly results the insertion into the Nb‐Cexo σ‐bond, forming the corresponding alkylidene amido and imidato complexes.

Redox-Neutral Transformations of Carbon Dioxide Using Coordinatively Unsaturated Late Metal Silyl Amide Complexes.

Two-coordinate silylamido complexes of nickel and copper rapidly react with CO2 to selectively form a new cyanate ligand along with hexamethyldisiloxane byproducts. Mechanistic insight into these reactions was obtained from the synthesis of proposed intermediates, several silyl- and phenyl- substituted amido analogues, and their subsequent reactivity with CO2. These studies suggest that a unique intramolecular double silyl transfer step facilitates CO2 deoxygenation, which likely contributes to the rapid rates of reaction. The deoxygenation reactions create a platform for a synthetic cycle in which copper amido complexes convert CO2 to organic silylcarbamates.

Synthesis, Structures, and Thermal Stability of Dialkyl and Bis(amido) Zirconium(IV) Acen Complexes

AbstractReaction of one equivalent of H2(acen), H2(cis‐Cyacen) or H2(trans‐Cyacen) with [Zr(CH2SiMe3)4] at room temperature afforded [Zr(acen)(CH2SiMe3)2] (1), [Zr(cis‐Cyacen)(CH2SiMe3)2] (2) or [Zr(trans‐Cyacen)(CH2SiMe3)2] (3), respectively (acen=C2H4(NCMeCHC(O)Me)2; Cyacen=1,2‐C6H10(NCMeCHC(O)Me)2). These alkyl compounds are trigonal prismatic in the solid state, and whereas 1 and 3 decomposed without sublimation above 120 °C (5–10 mTorr), 2 sublimed in >95 % yield at 85 °C (5–10 mTorr). However, heating solid 2 at 88 °C under static argon for 24 hours resulted in extensive decomposition to afford H2(cis‐Cyacen) and SiMe4 as the soluble products. Compound 2 reacted cleanly with two equivalents of tBuOH to afford [Zr(cis‐Cyacen)(OtBu)2] (4), but excess tBuOH caused both SiMe4 and H2(cis‐Cyacen) elimination. The 1 : 1 reaction of H2(acen) with [Zr(NMeEt)4] did not proceed cleanly, and 8‐coordinate [Zr(acen)2] (5) was identified as a by‐product; this complex was isolated from the 2 : 1 reaction. A zirconium amido complex, [Zr(acen)(NMeEt)2] (6) was accessed via the reaction of 1 with two equiv. or excess HNMeEt, but decomposed readily in solution at room temperature. More sterically hindered [Zr(acen){N(SiMe3)2}2] (7) was synthesized via the reaction of [Zr(acen)Cl2] with two equivalents of Li{N(SiMe3)2}, but was also thermally unstable as a solid and in solution at room temperature. Compounds 1–3, 5 and 7 were crystallographically characterized.

Synthesis and Characterization of Rare-Earth Metal Complexes Bearing a 2-N,N-Dimethylamino-Ethylene-Imino-Functionalized Indolyl Ligand and Their Catalytic Activities Toward Hydrosilylation of Imines

The reactions of 2-N,N-dimethylamino-ethylene-imino-functionalized indolyl proligand 2-(Me2NCH2CH2N = CH)C8H5NH (HL) with [(Me3Si)2N]3RE(μ-Cl)Li(THF)3 in toluene afforded a series of rare-earth metal amido complexes [κ3-(N,N,N)-2-(Me2NCH2CH2N = CH)C8H5N]RE[N(SiMe3)2]2 (RE = Y (1a), Nd (1b), Sm (1c), Gd (1d), Dy (1e), Er (1f), and Yb (1g)). While the reaction of HL with [(Me3Si)2N]3Eu(μ-Cl)Li(THF)3 produced the europium(II) complex in a dimeric form [{(η5-μ-η1:η1:η1)-2-(Me2NCH2CH2N═CH)C8H5N}Eu{N(SiMe3)2}]2 (2), which is believed to have happened through the homolysis of the Eu–N bond. Further reaction of complex 1a with PhCH═NPh provided an unexpected rare-earth metal complex [κ2-(N,N)-PhN{PhCH(CH2SiMe2)}N(SiMe3)]YL (7) through the sp3 C–H bond activation of the methyl on the silicon atom to form a Y–C bond followed by the insertion of the C═N bond of the imine PhCH═NPh to form an amido group connected to the central metal. All complexes were characterized by spectroscopic methods, elemental analyses, and single-crystal X-ray diffraction, and complexes 1a–1c and 7 were additionally characterized by NMR spectroscopy. The catalytic performance of complexes 1a–1g for the hydrosilylation of imines was investigated, and the catalytic mechanism was proposed based on the experimental results.

Titanium Complexes Bearing NNO‐Tridentate Ligands: Highly Active Olefin Polymerization Catalysts with Great Control on Molecular Weight and Distribution†

Comprehensive SummaryA series of titanium amido complexes (1‐Ti(NMe2)2 to 4‐Ti(NMe2)2) bearing NNO‐tridentate ligands was synthesized from the reactions of phenoxy‐imine‐amine ligands with one equivalent of Ti(NMe2)4. Subsequently, titanium dichloride complexes (1‐TiCl2 to 4‐TiCl2) were prepared by treatment of titanium amido complexes with excess Me3SiCl in toluene. All metal complexes were characterized by 1H and 13C NMR spectroscopy, and the molecular structures of complexes of 2‐Ti(NMe2)2, 2‐TiCl2, and 4‐TiCl2 in the solid state were determined by single‐crystal X‐ray diffraction. Upon activation with MAO, titanium dichloride complexes as single‐site catalysts were extremely active toward ethylene homopolymerization with great control on molecular weight and distribution. In particular, 3‐TiCl2/MAO exhibited an activity as high as 1.35 × 108 g(PE)·mol−1(Ti)·h−1 at 70°C and produced polyethylene with high molecular weight (41.7 × 104 g∙mol−1) and very narrow distribution (Đ = 1.23). In the presence of 1‐octene as comonomer, linear low‐density polyethylenes (LLDPEs) with ultrahigh molecular weights (Mw = 136—326 × 104 g∙mol−1) and narrow distributions (Đ = 1.20—1.50) were successfully synthesized.

Hydroaminoalkylation of alkenes using transition metals complexes grafted on silica SBA15 as catalysts

The gas-phase hydroaminoalkylation reaction of propene catalyzed by group 4 (M = Ti, Zr and Hf) metal amido complexes [(≡Si–O-)(M(-NMe2)3] was investigated by using PBE0-D3/SVP//TZVP level of theory. The geometrical analysis traced the formation of the metallaaziridines and the azametallacyclopentanes as key intermediates in these reactions. The metallaaziridines were simulated through the activation of α-C-H bonds of the amido groups; while the azametallacyclopentanes were configured by slotting the propene double bond onto the M − C bonds of the metallaaziridines. The latter reaction was considered the rate-determining step. Thermochemical calculations showed that the order of catalytic activity is: Ti ≥ Zr > Hf; while the preference of the azametallacyclopentanes is: Hf > Zr ≥ Ti.

Synthesis and characterization of Bis(β‐diketiminato) rare‐earth amido complexes, their activity for catalytic addition of amines to carbodiimides

Reactions of anhydrous RECl3 (RE = Sm, Nd) with 2 equiv. of NaL2-Me (L2-Me = [N(2-MeC6H4)C(Me)]2CH−) afforded the dimeric chlorides [Sm(L2-Me)2(μ-Cl)]2 (1) and [Nd(L2-Me)2(μ-Cl)]2 (2). Salt metathesis reactions between chloride complexes and NaN(SiMe3)2 in THF yielded a series of bis(β-diketiminato) rare-earth amido complexes RE(L2-Me)2[N(SiMe3)2] (RE = Sm (3), Nd (4), Yb (5), Y (6), Pr (7)). Complexes 1−7 were fully characterized including X-ray single crystal structure analyses. Complexes 3−7 can serve as efficient pre-catalysts for catalytic addition of amines to carbodiimides to afford multi-substituted guanidines. Catalytic activity was influenced by the rare-earth metals with an active sequence of Y (6) ∼ Yb (5) < Pr (7) < Nd (4) < Sm (3). The present catalytic addition reaction using complex 3 showed a wide scope of substrates including primary and secondary aromatic amines and aliphatic cyclic amines. The mechanism investigation by the isolation and characterization of the intermediates of the amido complexes RE(L2-Me)2(NHPh)(THF) (RE = Sm (8), Nd (9), Yb (10)) and guanidinate complexes Sm(L2-Me)2[(C6H5NH)C(NCy)2] (11) and Nd(L2-Me)2[(C6H5N)C(NHCy)(NCy)] (12). Their reactivity revealed that the present catalytic addition proceeded through amine-exchange reaction between the pre-catalyst amido complex and an amine to a novel amido complex, nucleophilic addition of the formed amido species to a carbodiimide to yield a guanidinate species, and protonation reaction of the guanidinate species by an amine to regenerate amido species and release the final product.

Role of Temperature, Pressure, and Surface Oxygen Migration in the Initial Atomic Layer Deposition of HfO2on Anatase TiO2(101)

The atomic layer deposition of HfO2 on a TiO2(101) surface from tetrakis(dimethylamido)hafnium and water is investigated using a combination of in situ vacuum X-ray photoelectron spectroscopy (XPS) and time-resolved ambient pressure XPS. Precursor pressures and surface temperature are tuned as to map the space state of the deposition. In the initial stages of ALD, a reaction mechanism based on dissociative adsorption dominates over a classic ligand exchange mechanism, typically evoked when metal-amido complexes and water are used as the precursors for metal oxide ALD. Surface species, including a dimethyl ammonium ion and an imine, are identified. It is found that they can be formed only if the active role of the TiO2(101) surface is taken into consideration. The temperature of the surface enhances the formation of these species based on an insertion reaction of a hydrogen atom, which then assists the formation of more than the expected monolayer of HfO2. A HfO2 overlayer is produced already during the first half-cycle, enabled by a reduction of the TiO2 support. Dosing water at high pressure allows hydroxyl formation, which marks the transition toward a well-described ligand exchange reaction type. From the experiments performed, we find that the ALD of HfO2 at room temperature, performed at high pressure, is mainly based on dissociation and that no side reaction occurs. These insights into the ALD reaction mechanism highlight how in situ studies can help understand how deposition parameters affect the growth of HfO2 and how the ALD model for transition metal oxide formation from amido complexes and water can be extended.

Spontaneous Ammonia Activation Through Coordination-Induced Bond Weakening in Molybdenum Complexes of a Dianionic Pentadentate Ligand Platform.

Ammonia oxidation catalyzed by molecular compounds is of current interest as a carbon-free source of dihydrogen. Activation of N-H bonds through coordination to transition metal centers is a key reaction in this process. We report the substantial activation of ammonia via reaction with low-valent molybdenum complexes of a diborate pentadentate ligand system. Spontaneous loss of dihydrogen from (B2 Pz4 Py)MoII -NH3 at room temperature to produce the dinuclear μ-nitrido compound (B2 Pz4 Py)Mo-N-Mo(B2 Pz4 Py) is observed due to substantial N-H bond weakening upon coordination to Mo. Mechanistic details are supported through the experimental observation/characterization of terminal amido, imido and nitrido complexes and density functional theory computations. The generally under-appreciated role of bridging nitrido intermediates is revealed and discussed, providing guidance for further catalyst development for this process.

Spontaneous Ammonia Activation Through Coordination‐Induced Bond Weakening in Molybdenum Complexes of a Dianionic Pentadentate Ligand Platform**

AbstractAmmonia oxidation catalyzed by molecular compounds is of current interest as a carbon‐free source of dihydrogen. Activation of N−H bonds through coordination to transition metal centers is a key reaction in this process. We report the substantial activation of ammonia via reaction with low‐valent molybdenum complexes of a diborate pentadentate ligand system. Spontaneous loss of dihydrogen from (B2Pz4Py)MoII−NH3 at room temperature to produce the dinuclear μ‐nitrido compound (B2Pz4Py)Mo‐N‐Mo(B2Pz4Py) is observed due to substantial N−H bond weakening upon coordination to Mo. Mechanistic details are supported through the experimental observation/characterization of terminal amido, imido and nitrido complexes and density functional theory computations. The generally under‐appreciated role of bridging nitrido intermediates is revealed and discussed, providing guidance for further catalyst development for this process.

Formation of Iridium(III) and Rhodium(III) Amine, Imine, and Amido Complexes Based on Pyridine-Amine Ligands: Structural Diversity Arising from Reaction Conditions, Substituent Variation, and Metal Centers.

Herein, we present the different coordination modes of half-sandwich iridium(III) and rhodium(III) complexes based on pyridine-amine ligands. The pyridyl-amine iridium(III) and rhodium(III) complexes, the corresponding oxidation pyridyl-imine products, and 16-electron pyridyl-amido complexes can be obtained through the change in reaction conditions (nitrogen/adventitious oxygen atmosphere, reaction time, and solvents) and structural variations in the metal and ligand. Overall, the reaction of pyridine-amine ligands with [(η5-C5(CH3)5)MCl2]2 (M = Ir or Rh) in the presence of adventitious oxygen afforded the oxidized pyridyl-imine complexes. The possible mechanism for the oxidation of iridium(III) and rhodium(III) amine complexes was confirmed by the detection of the byproduct hydrogen peroxide. Moreover, the formation of pyridyl-amine complexes was favored when nonpolar solvent CH2Cl2 was used instead of CH3OH. The rarely reported complex with [(η5-Cp*)IrCl3] anions can also be obtained without the addition of NH4PF6. The introduction of the sterically bulky i-Bu group on the bridge carbon of the ligand led to the formation of stable 16-electron pyridyl-amido complexes. The pyridyl-amine iridium(III) and rhodium(III) complexes were also synthesized under a N2 atmosphere, and no H2O2 was detected in the whole process. In particular, the aqueous solution stability and in vitro cytotoxicity toward A549 and HeLa human cancer cells of these complexes were also evaluated. No obvious selectivity was observed for cancer cells versus normal cells with these complexes. Notably, the represented complex 5a can promote an increase in the reactive oxygen species level and induce cell death via apoptosis.

Synthesis, Characterization, and Reactivity of Low-Coordinate Titanium(III) Amido Complexes

Synthesis, Characterization, and Reactivity of Low-Coordinate Titanium(III) Amido Complexes

A Parent Iron Amido Complex in Catalysis of Ammonia Oxidation.

Parent amido complexes are crucial intermediates in ammonia-based transformations. We report a well-defined ferric ammine system [Cp*Fe(1,2-Ph2PC6H4NH)(NH3)]+ ([1-NH3]+), which processes electrocatalytic ammonia oxidation to N2 and H2 at a mild potential. Through establishing elementary e-/H+ conversions with the ferric ammine, a formal Fe(IV)-amido species, [1-NH2]+, together with its conjugated Lewis acid, [1-NH3]2+, was isolated and structurally characterized for the first time. Mechanism studies indicated that further oxidation of [1-NH2]+ induces the reaction of the parent amido unit with NH3. The formation of hydrazine is realized by the non-innocent nature of the phenylamido ligand that facilitates the concerted transfer of one proton and two electrons.

Alkali Metal Ions Dictate the Structure and Reactivity of an Iron(II) Imido Complex.

The presence of redox innocent metal ions has been proposed to modulate the reactivity of metal ligand multiple bonds; however, insight from structure/function relationships is limited. Here, alkali metal reduction of the Fe(III) imido complex [Ph2B(tBuIm)2Fe═NDipp] (1) provides the series of structurally characterized Fe(II) imido complexes [Ph2B(tBuIm)2Fe═NDippLi(THF)2] (2), [Ph2B(tBuIm)2Fe═NDippNa(THF)3] (3), and [Ph2B(tBuIm)2Fe═NDippK]2 (4), in which the alkali metal cations coordinate the imido ligand. Structural investigations demonstrate that the alkali metal ions modestly lengthen the Fe═N bond distance from that in the charge separated complex [Ph2B(tBuIm)2Fe═NDipp][K(18-C-6)THF2] (5), with the longest bond observed for the smallest alkali metal ion. In contrast to 5, the imido ligands in 2-4 can be protonated and alkylated to afford Fe(II) amido complexes. Combined experimental and computational studies reveal that the alkali metal polarizes the Fe═N bond, and the basicity of imido ligand increases according to 5 < 4 ≈ 3 < 2. The basicity of the imido ligands influences the relative rates of reaction with 1,4-cyclohexadiene, specifically by gating access to complex 5, which is the species that is active for HAT. All complexes 2-4 react with benzophenone form metastable Fe(II) intermediates that subsequently eliminate the metathesis product Ph2C═NDipp, with relative rates dependent on the alkali metal ion. By contrast, the same reaction with 5 does not lead to the formation of Ph2C═NDipp. These results demonstrate that the coordination of alkali metal ions dictate both the structure and reactivity of the imido ligand and moreover can direct the reactivity of reaction intermediates.

Continuous Flow Synthesis of NHC‐Coinage Metal Amido and Thiolato Complexes: A Mechanism‐based Process Development

AbstractThe first continuous flow syntheses of amido and thiolato families are reported. Our studies focused on [M(NHC)(Cbz)] (Cbz=carbazolyl) and [M(NHC)(SPh)] (NHC=N‐heterocyclic carbene) as general proof‐of‐concepts and has led to a simple protocol with unprecedented short reaction times and high product yields. The scope of supporting ligands and substrates permits to assess the versatility of the process and a one‐pot synthesis starting from the imidazolium salt, metal source and carbazole substrate highlights the simplicity and sustainability of this operating design. The process was made possible through a detailed mechanistic understanding of the underlying reaction chemistry.

Iron Complexes of a Proton-Responsive SCS Pincer Ligand with a Sensitive Electronic Structure.

Sulfur/carbon/sulfur pincer ligands have an interesting combination of strong-field and weak-field donors, a coordination environment that is also present in the nitrogenase active site. Here, we explore the electronic structures of iron(II) and iron(III) complexes with such a pincer ligand, bearing a monodentate phosphine, thiolate S donor, amide N donor, ammonia, or CO. The ligand scaffold features a proton-responsive thioamide site, and the protonation state of the ligand greatly influences the reduction potential of iron in the phosphine complex. The N-H bond dissociation free energy, derived from the Bordwell equation, is 56 ± 2 kcal/mol. Electron paramagnetic resonance (EPR) spectroscopy and superconducting quantum interference device (SQUID) magnetometry measurements show that the iron(III) complexes with S and N as the fourth donors have an intermediate spin (S = 3/2) ground state with a large zero field splitting, and X-ray absorption spectra show a high Fe-S covalency. The Mössbauer spectrum changes drastically with the position of a nearby alkali metal cation in the iron(III) amido complex, and density functional theory calculations explain this phenomenon through a change between having the doubly occupied orbital as dz2 or dyz, as the former is more influenced by the nearby positive charge.