Amido Complexes Research Articles

Influence of 1,2,4-Tri-tert-butylcyclopentadienyl Ligand on the Reactivity of the Thorium Bipyridyl Metallocene [η5-1,2,4-(Me3C)3C5H2]2Th(bipy)

The thorium bipyridyl metallocene (Cp3tBu)2Th(bipy) (1; Cp3tBu = η5-1,2,4-(Me3C)3C5H2) shows a rich reactivity toward a series of small molecules. For example, complex 1 may act as a synthon for the (Cp3tBu)2Th(II) fragment as illustrated by its reactivity toward to CuI, hydrazine derivative (PhNH)2, Ph2E2 (E = S, Se), elemental sulfur (S8) and selenium (Se), organic azides, CS2, and isothiocyanates. Moreover, in the presence of polar multiple bonds, such as those in ketones Ph2CO and (CH2)5CO, aldehydes p-MePhCHO and p-ClPhCHO, seleno-ketone (p-MeOPh)2CSe, nitriles PhCN, Ph2CHCN, C6H11CN, and p-(NC)2Ph, and benzoyl cyanide PhCOCN, C-C coupling occurs to furnish (Cp3tBu)2Th[(bipy)(Ph2CO)] (10), (Cp3tBu)2Th[(bipy)((CH2)5CO)] (11), (Cp3tBu)2Th[(bipy)(p-MePhCHO)] (12), (Cp3tBu)2Th[(bipy)(p-ClPhCHO)] (13), (Cp3tBu)2Th[(bipy){(p-MeOPh)2CSe}] (14), (Cp3tBu)2Th[(bipy)(PhCN)] (16), (Cp3tBu)2Th[(bipy)(Ph2CHCN)] (17), (Cp3tBu)2Th[(bipy)(C6H11CN)] (18), [(Cp3tBu)2Th]2{μ-(bipy)[p-Ph(CN)2](bipy)} (20), and (Cp3tBu)2Th{(bipy)[PhC(CN)O]} (21), respectively. Nevertheless, ketazine (PhCH═N)2 or benzyl nitrile PhCH2CN forms the dimeric complexes [(Cp3tBu)Th]2[μ-NC(Ph)(bipy)]2 (15) and (Cp3tBu)2Th[(bipy){C(═CHPh)NH}] (19), respectively. In contrast, C-N bond cleavage and C-C coupling processes occur upon addition of isonitriles Me3CNC and C6H11NC to 1 to yield the thorium isocyanido amido complexes (Cp3tBu)2Th[4-(Me3C)bipy](NC) (22) and (Cp3tBu)2Th[4-(C6H11)bipy](NC) (23), respectively. Furthermore, a single-electron transfer (SET) process ensues when 1 equiv of CuI is added to 1 to yield the Th(VI) bipyridyl iodide complex (Cp3tBu)2Th(I)(bipy) (3).

Elucidating the Impact of Rare Earth or Transition Metal Identity on the Physical and Electronic Structural Properties of a Series of Redox-Active Tris(amido) Complexes.

The use of redox-active ligands with the f-block elements has been employed to promote unique chemical transformations and explore their unique emergent electronic properties for a myriad of applications. In this study, we report eight new tris(amido) metal complexes: 1-Ln (Ln = Tb3+, Dy3+, Ho3+, Er3+, Tm3+, and Yb3+), 1-La, and 1-Ti (an early transition metal analogue). The one-electron oxidation of the tris(amido) ligand was conducted to generate semi-iminato complexes 2-Ln, 2-La, and 2-Ti, and these complexes were studied using EPR. Tris(amido) complexes 1-Ln, 1-La, and 1-Ti were fully characterized using a range of spectroscopic (NMR and UV-vis/NIR) and physical techniques (X-ray diffraction and cyclic voltammetry, with the exception of 1-La). Computational methods were employed to further elucidate the electronic structures of these complexes. Lastly, complexes 1-Ln, 1-La, and 1-Ti were probed as catalysts for alkyl-alkyl cross-coupling, and the initial rate of the reaction was measured to explore the influence of the metal ion.

Palladium-Catalyzed Amination of Aryl Halides with Aqueous Ammonia and Hydroxide Base Enabled by Ligand Development.

The conversion of aryl halides to primary arylamines with a convenient and inexpensive source of ammonia has been a long-standing synthetic challenge. Aqueous ammonia would be the most convenient and least expensive form of ammonia, but such a palladium-catalyzed amination reaction with a high concentration of water faces challenges concerning catalyst stability and competing hydroxylation, and palladium-catalyzed reactions with this practical reagent are rare. Further, most reactions with ammonia to form primary amines are conducted with tert-butoxide base, but reactions with ammonium hydroxide would contain hydroxide as base. Thus, ammonia surrogates, ammonia in organic solvents, and ammonium salts have been used under anhydrous conditions instead with varying levels of selectivity for the primary amine. We report the palladium-catalyzed amination of aryl and heteroaryl chlorides and bromides with aqueous ammonia and a hydroxide base to form the primary arylamine with high selectivity. The palladium catalyst containing a new dialkyl biheteroaryl phosphine ligand (KPhos) suppresses both the formation of aryl alcohol and diarylamine side products. Mechanistic studies with a soluble hydroxide base revealed turnover-limiting reductive elimination of the arylamine and an equilibrium between arylpalladium amido and hydroxo complexes prior to the turnover-limiting step.

Synthesis and Structure of [η5-1,2,4-(Me3Si)3C5H2]2Th(bipy) and Its Reactivity toward Small Molecules.

Halide exchange of (Cp3tms)2ThCl2 (1; Cp3tms = η5-1,2,4-(Me3Si)3C5H2) with Me3SiI furnishes (Cp3tms)2ThI2 (2), which is then reduced with potassium graphite (KC8) in the presence of 2,2'-bipyridine to give the thorium bipyridyl metallocene (Cp3tms)2Th(bipy) (3) in good yield. Complex 3 was fully characterized and readily reacted with various small molecules. For example, 3 may serve as a synthetic equivalent for the (Cp3tms)2Th(II) fragment when exposed to CuI, Ph2S2, organic azides, and CS2. Moreover, upon the addition of thiobenzophenone Ph2CS, p-methylbenzaldehyde (p-MeC6H4)CHO, benzophenone Ph2CO, amidate PhCONH(p-tolyl), seleno-ketone (p,p'-dimethoxy), selenobenzophenone (p-MeOPh)2CSe, di(p-tolyl)methanimine (p-tolyl)2C═NH, 1,2-di(benzylidene)hydrazine (PhCH═N)2, and nitriles PhCN, PhCH2CN, and Ph2CHCN C-C coupling results to give (Cp3tms)2Th[(bipy)(Ph2CS)] (8), (Cp3tms)2Th[(bipy)(p-MePhCHO)] (9), (Cp3tms)2Th[(bipy)(Ph2CO)] (10), (Cp3tms)2Th[(bipy){(p-tolylNH)(Ph)CO}] (11), (Cp3tms)2Th[(bipy){(p-MeOPh)2CSe}] (12), (Cp3tms)2Th[(bipy){(p-tolyl)2CNH}] (13), (Cp3tms)2Th[(bipy)(PhCHNN═CHPh)] (14), (Cp3tms)2Th[(bipy)(PhCN)] (16), (Cp3tms)2Th[(bipy)(PhCH2CN)] (17), and (Cp3tms)2Th[(bipy)(Ph2CHCN)] (18), respectively. However, when thiazole is added to 3, the dimeric sulfido complex [(Cp3tms)2Th]2[μ-(bipy)CH2NCHCHS]2 (15) can be isolated. Moreover, the addition of isonitriles such as Me3CNC and PhCH2NC to 3 results in C-N bond cleavage and C-C coupling processes to form the thorium isocyanido amido complexes (Cp3tms)2Th[4-(Me3C)bipy](NC) (19) and (Cp3tms)2Th[4-(PhCH2)bipy](NC) (20), respectively. Nevertheless, upon exposure of 3 to (trimethylsilyl)diazomethane Me3SiCHN2, the bis-amido complex (Cp3tms)2Th[5,6-(Me3SiCH)bipy] (21), concomitant with N2 release, is isolated.

A Simple Manganese(I) Catalyst for the Efficient and Selective Hydrophosphination of Olefins with PH3, Primary, and Secondary Phosphanes.

A tridentate ligand L with a P,NH,N donor motif was synthesized in few steps from commercially available precursors. Upon reaction with [MnBr(CO)5], an octahedral 18-electron complex [Mn(CO)3(L)]Br (1) is obtained in which L adopts a facial arrangement. After deprotonation of the NH group in the cationic complex unit, a neutral Mn(I) amido complex [Mn(CO)2(L-H)] (2) is formed under loss of CO. Rearrangement of L-H leads to a trigonal bipyramidal structure in which the P and N donor centers are in trans position. Further deprotonation of 2 results in a dep-blue anionic complex fragment [Mn(CO)2(L-2H)]- (3). DFT calculations and a QTAIM analysis show that the amido complex 2 contains a Mn-N bond with partial double bond character and 3 an aromatic MnN2C2 ring. The anion [Mn(CO)2(L-2H)]- reacts with Ph2PH to give a phosphido complex, which serves as phosphide transfer reagent to activated olefins. But the catalytic activity is low. However, the neutral amido complex 2 is an excellent catalyst and with loadings as low as 0.04 mol %, turn over frequencies of >40'000 h-1 can be achieved. Furthermore, secondary and primary alkyl phosphines as well as PH3 can be added in a catalytic hydrophosphination reaction to a wide range of activated olefins such as α,β-unsaturated aldehydes, ketones, esters, and nitriles. But also, vinyl pyridine and some styrene derivatives are converted into the corresponding phosphanes.

A Linear Two-Coordinate Cr(II) Complex: Synthesis, Characterization, and Reactivity.

The synthesis and characterization of a linear two-coordinate Cr(II) amido complex, Cr{N(t Bu)Dipp}2 (Dipp=2,6-diisopropylphenyl), from the reaction of 1 molar equivalent (equiv) of CrCl2 and 2 equiv. of LiN(t Bu)Dipp is reported. Single-crystal X-ray diffractometry (SC-XRD) analysis revealed that it has a short Cr-N bond distance of 1.8878(9) Å, which could be attributed to the relatively less bulky nature of the amido ligand compared with reported systems. Furthermore, the oxidation reaction of the two-coordinate Cr(II) complex was explored. The oxidation reaction of Cr{N(t Bu)Dipp}2 with the one-electron oxidants AgOTf and [FeCp2 ][BArF 4 ] (BArF 4 - =[B{C6 H3 -3,5-(CF3 )2 }4 ]- ) afforded the trigonal planar three- and bent two-coordinate Cr(III) complexes Cr{N(t Bu)Dipp}2 (OTf) and [Cr{N(t Bu)Dipp}2 ][BArF 4 ], respectively. The reaction of Cr{N(t Bu)Dipp}2 with 1 equiv. of the organic azides AdN3 (Ad=1-adamantyl) and PhN3 afforded the three-coordinate Cr(IV) imido complexes Cr{N(t Bu)Dipp}2 (NAd) and Cr{N(t Bu)Dipp}2 (NPh), respectively. The reaction of Cr{N(t Bu)Dipp}2 and two equiv. of Me3 NO afforded the Cr(VI) dioxo complex Cr{N(t Bu)Dipp}2 (O)2 . The reaction of Cr{N(t Bu)Dipp}2 with 1 equiv. of CyN=C=NCy resulted in the insertion of the carbodiimide into the Cr-N bond, with the formation of a three-coordinate Cr(II) complex. Finally, density functional theory (DFT) calculations were used to elucidate the electronic structure of these complexes.

Polymer‐Derived Ceramic Coatings with Excellent Thermal Cycling Stability

In the present work, transition metal‐containing preceramic silicon polymers were synthesized via chemical modification of a commercially available organopolysilazane with Hf and Ta amido complexes as well as with borane dimethyl sulfide complex. The incorporation of transition metals into the polymer structure, their influence on ceramization and processability were thoroughly investigated. Moreover, the prepared preceramics were coated onto silicon wafers via spin coating and converted into crack‐free, amorphous SiHfTa(B)CN‐based ceramic coatings with excellent adhesion to the substrate. The composition of the ceramic coatings was investigated via X‐ray photoelectron spectroscopy (XPS) and their high‐temperature behavior was studied via oxidation tests performed at 1100 °C. Moreover, a thermal cycling procedure to temperatures above 1250 °C with rapid heating and cooling rates (i.e., in the range of 100–120 K s−1) was applied to the ceramic coating, which showed no damage even after ten thermal cycles, indicating their outstanding performance and their potential for use as environmental barrier coatings at high temperatures.

Silyl‐Substituted ortho‐Terphenoxide Group 3–5 Complexes: Synthesis, Structure and Applications in Catalysis

AbstractThe complexation behavior of 3,3’‐silyl‐substituted ortho‐terphenoxide ligands o‐C6H4(C6H2‐2‐OH‐3‐SiR3‐5‐Me)2 (SiR3= SiPh3, SiMePh2, SiMe3, Sit‐BuMe2) with a range of yttrium, titanium, niobium and tantalum precursors was examined. The complexation of the ligands with [Y(N(SiHMe2)2)3(THF)2] yielded monomeric complexes, as confirmed by NMR spectroscopy and X‐ray crystallography for the methyldiphenylsilyl‐ and tert‐butyldimethylsilyl‐substituted complex. The triphenylsilyl‐substituted yttrium bis(dimethylsilyl)amido complex showed good activity in the intramolecular hydroamination reaction of aminoalkenes. Complexation with Ti(NMe2)4 yielded a mixture of products, depending on the silyl substituents and the solvent used for complexation. A mono‐ligated bimetallic titanium complex and a bisligated titanium complex were analyzed by X‐ray crystallography. Complexation with M(NMe2)5 (M=Nb, Ta) produced the corresponding monomeric ortho‐terphenoxide complexes as confirmed by NMR spectroscopy and X‐ray structure analysis of a niobium complex and 3 tantalum complexes.

Square-planar imido complexes of cobalt: synthesis, reactivity and computational study.

Treatment of [Co(N2)(tBuPNP)] (tBuPNP = anion of 2,5-bis(di-tert-butylphosphinomethyl)pyrrole) with one equivalent of an aryl azide generates the four-coordinate imido complexes [Co(NAr)(tBuPNP)] (Ar = mesityl, phenyl, or 4-tBu-phenyl). X-ray crystallographic analysis of the compounds shows an unusual square-planar geometry about cobalt with nearly linear imido units. In the presence of the hydrogen atom donor, TEMPOH, [Co(NPh)(tBuPNP)] undergoes addition of the H atom to the imido nitrogen to generate the corresponding amido complex, [Co(NHPh)(tBuPNP)], whose structure and composition were verified by independent synthesis. Despite the observation of H atom transfer reactivity with TEMPOH, the imido complexes do not show catalytic activity for C-H amination or aziridination for several substrates examined. In the case of [Co(NPh)(tBuPNP)], addition of excess azide produced the tetrazido complex, [Co(N4Ph2)(tBuPNP)], whose bond metrics were most consistent with an anionic Ph2N4 ligand. Density Functional Theory (DFT) investigations of the imido and tetrazido species suggest that they adopt a ground state best described as possessing a low-spin cobalt(II) ion ferromagnetically coupled to an iminyl radical.

Selective Transesterification to Control Copolymer Microstructure in the Ring-Opening Copolymerization of Lactide and ε-Caprolactone by Lanthanum Complexes.

A series of novel lanthanum amido complexes, supported by ligands designed around the salan framework (salan = N,N'-bis(o-hydroxy, m-di-tert-butylbenzyl)-1,2-diaminoethane) were synthesized and fully characterized in the solid and solution states. The ligands incorporate benzyl or 2-pyridyl substituents at each tertiary amine center. The complexes were investigated as catalysts in the ring-opening homopolymerization of lactide (LA) and ε-caprolactone (ε-CL) and copolymerization of equimolar amounts of LA and ε-CL at ambient temperature. Solvent (THF or toluene) and the number of 2-pyridyl groups in the complex were found to influence the reactivity of the catalysts in copolymerization reactions. In all cases, complete conversion of LA to PLA was observed. The use of THF, a coordinating solvent, suppressed ε-CL polymerization, while the presence of one or more 2-pyridyl groups promoted ε-CL polymerization. Each copolymer gave a monomodal trace in gel permeation chromatography-size-exclusion chromatography (GPC-SEC) experiments, indicative of copolymer formation over homopolymerization. Copolymer microstructure was found to be dependent on catalyst structure and reaction solvent, ranging from blocky to close to alternating. Experiments revealed rapid conversion of LA in the initial stages of the reaction, followed by incorporation of ε-CL into the copolymer by either transesterification or propagation reactions. Significantly, the mode of transesterification (TI or TII) that occurs is determined by the structure of the metal complex and the reaction solvent, leading to the possibility of controlling copolymer microstructure through catalyst design.

Insertion reactions and structural studies of [(NHC)CuH]2 with nitrogen-based substrates

The stoichiometric and catalytic reactions of Cu-H dimers supported by N-heterocyclic carbenes (NHCs) have mainly focused on the insertions of aldehydes, ketones, CO2, and unsaturated hydrocarbons. In this report, we investigated the stoichiometric reactions of dimeric [(NHC)CuH]2 (NHC = IPr*, 6Dipp) with unsaturated nitrogen-based substrates of PhN=NPh, N3Ad, pyrazine, and N2O. The spectroscopic and structural chacterizations of the resulting monomeric, two-coordinate Cu(I) complexes containing diphenyl hydrazido, triazenido, pyrazinyl, and hydroxide ligands are discussed. The hydrazido complex of (IPr*)Cu(NPh-NHPh) and triazenido complex of (IPr*)Cu(HNNNAd) are resistant to NN bond cleavage and loss of N2, respectively, to form the corresponding amido complexes even when heated at 80 °C over several hours.

Activated Mn-MACHO Complexes Form Stable CO2 Adducts.

Manganese(I) carbonyl complexes bearing a MACHO-type ligand (HN(CH2 CH2 PR2 )2 ) readily react in their amido form with CO2 to generate 4-membered {Mn-N-C-O} metallacycles. The stability of the adducts decreases with the steric demand of the R groups at phosphorous (R=isopropyl>adamantyl). The CO2 -adducts display generally a lower reactivity as compared to the parent amido complexes. These adducts can thus be interpretated as masked forms of the active amido catalysts and potentially play important roles as off-loop species or branching points in catalytic transformations of carbon dioxide.

Copper-Catalyzed Enantioconvergent Radical C(sp3)-N Cross-Coupling to Access Chiral α-Amino-β-lactams.

A copper-catalyzed enantioconvergent radical C(sp3)-N cross-coupling of racemic tertiary α-bromo-β-lactams with aromatic amines is developed under mild thermal reaction conditions. The use of a sterically demanded oxazoline-derived sulfonamide N,N,N-ligand is crucial for the reaction initiation and effective enantio-discrimination of the azetidinone-derived cyclic alkyl radicals. The strategy provides an attractive approach to access chiral α-amino-β-lactams, an important structural motif in many biologically active molecules. Preliminary mechanistic studies support the formation of azetidinone-derived alkyl radicals from the L*Cu(I)-amido complex and α-bromo-β-lactams.

Mechanistic Evidence of a Ni(0/II/III) Cycle for Nickel Photoredox Amide Arylation.

This work demonstrates the dominance of a Ni(0/II/III) cycle for Ni-photoredox amide arylation, which contrasts with other Ni-photoredox C-heteroatom couplings that operate via Ni(I/III) self-sustained cycles. The kinetic data gathered when using different Ni precatalysts supports an initial Ni(0)-mediated oxidative addition into the aryl bromide. Using NiCl2 as the precatalyst resulted in an observable induction period, which was found to arise from a photochemical activation event to generate Ni(0) and to be prolonged by unproductive comproportionation between the Ni(II) precatalyst and the in situ generated Ni(0) active species. Ligand exchange after oxidative addition yields a Ni(II) aryl amido complex, which was identified as the catalyst resting state for the reaction. Stoichiometric experiments showed that oxidation of this Ni(II) aryl amido intermediate was required to yield functionalized amide products. The kinetic data presented supports a rate-limiting photochemically-mediated Ni(II/III) oxidation to enable C-N reductive elimination. An alternative Ni(I/III) self-sustained manifold was discarded based on EPR and kinetic measurements. The mechanistic insights uncovered herein will inform the community on how subtle changes in Ni-photoredox reaction conditions may impact the reaction pathway, and have enabled us to include aryl chlorides as coupling partners and to reduce the Ni loading by 20-fold without any reactivity loss.

Mechanistic Evidence of a Ni(0/II/III) Cycle for Nickel Photoredox Amide Arylation

AbstractThis work demonstrates the dominance of a Ni(0/II/III) cycle for Ni‐photoredox amide arylation, which contrasts with other Ni‐photoredox C‐heteroatom couplings that operate via Ni(I/III) self‐sustained cycles. The kinetic data gathered when using different Ni precatalysts supports an initial Ni(0)‐mediated oxidative addition into the aryl bromide. Using NiCl2 as the precatalyst resulted in an observable induction period, which was found to arise from a photochemical activation event to generate Ni(0) and to be prolonged by unproductive comproportionation between the Ni(II) precatalyst and the in situ generated Ni(0) active species. Ligand exchange after oxidative addition yields a Ni(II) aryl amido complex, which was identified as the catalyst resting state for the reaction. Stoichiometric experiments showed that oxidation of this Ni(II) aryl amido intermediate was required to yield functionalized amide products. The kinetic data presented supports a rate‐limiting photochemically‐mediated Ni(II/III) oxidation to enable C−N reductive elimination. An alternative Ni(I/III) self‐sustained manifold was discarded based on EPR and kinetic measurements. The mechanistic insights uncovered herein will inform the community on how subtle changes in Ni‐photoredox reaction conditions may impact the reaction pathway, and have enabled us to include aryl chlorides as coupling partners and to reduce the Ni loading by 20‐fold without any reactivity loss.

Atomistic Chemical Elucidation of the Higher-Rate Reaction Mechanism in Hf-Pyridyl Amido-Catalyzed Copolymerization of Ethene and 1-Octene: Application of Red Moon Simulation with Polymer Propagation Diagrams.

The Hf-pyridyl amido complex ((pyridylamido)Hf(IV)) is a cationic catalyst activated by ion-pairing with auxiliary catalyst B(C6F5)4 to show high activity for α-olefin polymerization. Previously, it was experimentally observed that the consumption rate of 1-octene in the 1-octene/ethene copolymerization is 3-fold compared to the 1-octene homopolymerization in coordinative chain transfer polymerization using the catalyst HfCat+-B(C6F5)4- ion pair (IP) and the chain transfer agent (CTA) ZnEt2. In the present study, we have performed atomistic chemical simulations of the IP-catalyzed homopolymerization of 1-octene and copolymerization of 1-octene and ethene on the basis of the Red Moon (RM) methodology. Using the analysis by polymer propagation diagrams (PPDs), in the 1-octene homopolymerization and the 1-octene/ethene copolymerization with the 1-octene-inserted catalyst (oHfCat), it is theoretically shown that the propagation reactions intermittently pause due to the steric hindrance of two hexyl groups of the oHfCat and the 1-octene inserted adjacent to the Hf atom. On the other hand, in the polymerizations with the ethene-inserted catalyst (eHfCat), it is reasonably recognized that the propagation reactions occur smoothly at a constant rate, and the polymerization continuously proceeds due to the relatively smaller steric hindrance. In conclusion, it was shown, for the first time, that the RM method can be used to reveal the microscopic effects of monomers and substituents in the polymerization reaction processes. Therefore, our current work using PPDs demonstrates the promising potential of the RM methodology in studying catalytic olefin polymerizations and complex chemical reaction systems in general.

Supercritical millifluidic reactor for the synthesis of efficient GaN nanophotocatalysts

Here, we demonstrate the continuous synthesis of gallium nitride (GaN) nanophotocatalysts exhibiting high quantum confinement using a preheated supercritical millireactor. The GaN quantum dots (QDs) are obtained from the direct thermolysis of a single source amido complex precursor (tris(dimethyl)amido gallium(III) dimer), in anhydrous supercritical cyclohexane. The quality of the synthesized QDs is highly dependent on the chemical pathways for the single source precursor thermolysis. Through the use of numerical modeling, we designed a millifluidic reactor allowing a sufficiently high heating rate for a direct precursor thermolysis towards efficient GaN nanoparticles. The as-designed preheated supercritical flow reactor yields highly reproducible GaN QDs (optical properties, crystallite phase and morphology) at high throughput, enabling gram scale production. Eventually, the photocatalytic properties of the GaN QDs were evaluated in a direct photochemical reaction through the degradation of a dye molecule (Methyl Orange). The obtained results demonstrate the higher photocatalytic activity of such photocatalysts compared to the commercially and reference available Degussa P25.

Through the Looking Glass: Using the Lens of [SNS]-Pincer Ligands to Examine First-Row Metal Bifunctional Catalysts.

ConspectusHomogeneous catalysis is at the forefront of global efforts to innovate the synthesis of fine chemicals and achieve carbon-neutrality in energy applications. For decades, the push toward sustainable catalysis has focused on the development of first-row transition metal catalysts to supplant widespread use of precious metals. Metal-ligand cooperativity is an effective strategy to yield high-performing first-row metal molecular catalysts. Despite remarkable progress, state of the art catalysts often employ phosphorus-based ligands which are air-sensitive, potentially toxic, and on occasion offset the cost-savings of the metal. Thus, the development of simple and economical ligands composed of biomimetic donors should be a key focus that cannot be overlooked in the pursuit of sustainable catalyst candidates. This is an Account of our group's efforts to develop first-row transition metal complexes which use [SNS]-pincer ligands for bifunctional catalysis. We have synthesized two potentially tridentate ligands, one bearing an amido and two thioether donors [(SMeNSMe), L1] and one which includes thiolate, imine, and thioether donors [(SNSMe), L2], and used them as platforms upon which to explore the reaction pathways of first-row metals. The [SNS] ligand, L1, leads to formation of high-spin paramagnetic metal complexes of the type M(L1)2 in which the 6-membered ring thioether donor is hemilabile (M = Mn, Fe, Co). This allows Mn(L1)2 to function as a carbonyl hydroboration catalyst that operates by a novel hydride-free, inner-sphere reaction pathway. Exploring the reactivity of L2 with Fe and Ni revealed unique coordination chemistry and a variety of mono-, di-, tri-, and tetranuclear complexes enabled by bridging thiolates. Further studies showed L2 undergoes selective Caryl-S bond cleavage upon coordination to a metal with electron-rich phosphine donors, yielding a new (CNS)2- pincer ligand. The analogous reaction with L1 afforded a new (CNSMe)- pincer ligand via both Caryl-S and benzylic C-H bond cleavage. In an attempt to prepare Fe(L2)2, we obtained instead an Fe(N2S3) complex in which imine C-C bond formation affords a potentially hexadentate redox-active ligand. The Fe(N2S3) complex is a selective catalyst for hydroboration of aldehydes and appears to operate through a complicated mechanism. In contrast, a mechanistic study of Mn(L2)(CO)3-photocatalyzed dihydroboration of nitriles indicated that both the flexibility of the κ3-SNSMe ligand (fac- vs mer-coordination) and ability of Mn to undergo a spin-state change are required to access low energy barriers for this transformation. To effectively compare the reactivity of the thiolate vs amido donor, we prepared two Cu complexes, Cu(L1)(IPr) and Cu(L2)(IPr) [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene], showing that, while both served as carbonyl hydroboration catalysts, only the amido complex was an effective catalyst for carbonyl hydrosilylation. In addition, complexes of the type Zn(L1)2, Zn(L2)2, and Zn(L1)(L2), were also effective for catalytic carbonyl hydroboration. While Zn(L1)(L2) was most active, catalyst speciation studies showed that each undergoes bifunctional catalyst activation to form a Zn bis(alkoxide) catalyst. Overall, our observations using [SNS] ligands with first-row transition metals show how the absence of traditional phosphine donors leads to different fundamental reactivity. Furthermore, this Account demonstrates the gap of knowledge which exists in understanding the reactivity of sulfur-based ligands to promote more widespread adoption of sustainable ligands.

Heteroleptic Bis(amido) Ca(II) and Yb(II) NHC Pincer Complexes: Synthesis, Characterization, and Catalytic Activity in Intermolecular Hydrofunctionalization of C═C Bonds

The treatment of 4,5-dimethyl-1,3-bis(2-pyridylmethyl)-1H-imidazolium chloride with M[N(SiMe3)2]2(THF)2 (M = Ca, Yb) in 1:2 molar ratio results in the formation of a carbene fragment, activation of the picolyl CH bond, and dearomatization of one of the pyridine rings and affords bis(amido) complexes [(NHC)MN(SiMe3)2]2 (M = Ca (2), Yb (3)) coordinated by a μ2-bridging pincer NHC amido ligand. Complexes 2 and 3 demonstrate high catalytic activity and excellent regio- and chemoselectivities in the intermolecular hydrophosphination of styrene with both primary and secondary phosphines. Complexes 2 and 3 enable selective addition of PhPH2, Ph2PH, (CH2)4NH, and (CH2)5NH to para-divinylbenzene with high conversions under mild conditions. Tandem hydrophosphination/hydroamination of para-divinylbenzene was also performed.

Small, Electron-Donating Substituents Give CO2 Activation by Permethylpentalene Zirconium Amido Complexes the Upper Hand: A DFT Study of Distortion and Interaction

An insight into factors controlling CO2 activation is necessary to develop molecular systems that utilize CO2 as a chemical feedstock. Two permethylpentalene zirconium cyclopentadienyl (mono)amido complexes, Pn*ZrCp(NR2), were previously assessed for CO2 activation, and a strong dependence on the amido substituent was observed. The R = Me analogue reacted rapidly and quantitatively at room temperature to form the carbamato complex, while the R = Ph species was inert. Here, we investigate the origin of this reactivity difference using DFT and the distortion-interaction model to characterize steric and electronic contributions to the activation barrier. We find that the barrier for CO2 insertion with R = Me (19.1 kcal/mol) is lower than with R = Ph (36.6 kcal/mol), explaining the inertness of the Ph-substituted analogue. The distortion energy trend follows the steric bulk of the amido substituents, and the bulkier Ph-substituted complex has a consistently higher distortion energy along its potential energy surface than that of the Me-substituted complex. The interaction energy trend follows the electronics, and a more electron-donating Me-substituted complex shows a consistently lower interaction energy. The balance of these effects at the corresponding TS gives a reduced activation barrier. Small, electron-donating substituents therefore facilitate CO2 activation in these complexes.