Alkaline Earth-Centered CO Homologation, Reduction, and Amine Carbonylation.

Reactions of ß-diketiminato magnesium and calcium hydrides with 1 atm of CO result in a reductive coupling process to produce the corresponding derivatives of the cis-ethenediolate dianion. Computational (DFT) analysis of this process mediated by Ca, Sr, and Ba highlights a common mechanism and a facility for the reaction that is enhanced by increasing alkaline earth atomic weight. Reaction of CO with PhSiH3 in the presence of the magnesium or calcium hydrides results in catalytic reduction to methylsilane and methylene silyl ether products, respectively. These reactions are proposed to ensue via the interception of initially formed group 2 formyl intermediates, an inference which is confirmed by a DFT analysis of the magnesium-centered reaction. The computational results identify the rate-determining process, requiring traversal of a 33.9 kcal mol-1 barrier, as a Mg-H/C-O σ-bond metathesis reaction, associated with the ultimate cleavage of the C-O bond. The carbonylation reactivity is extended to a variety of magnesium and calcium amides. With primary amido complexes, which for calcium include a derivative of the parent [NH2]- anion, CO insertion is facile and ensues with subsequent nitrogen-to-carbon migration of hydrogen to yield a variety of dinuclear and, in one case, trinuclear formamidate species. The generation of initial carbenic carbamoyl intermediates is strongly implicated through the isolation of the CO insertion product of a magnesium N-methylanilide derivative. These observations are reinforced by a DFT analysis of the calcium-centered reaction with aniline, which confirms the exothermicity of the formamidate formation (ΔH = -67.7 kcal mol-1). Stoichiometric reduction of the resultant magnesium and calcium formamidates with pinacolborane results in the synthesis of the corresponding N-borylated methylamines. This takes place via a sequence of reactions initiated through the generation of amidatohydridoborate intermediates and a cascade of reactivity that is analogous to that previously reported for the deoxygenative hydroboration of organic isocyanates catalyzed by the same magnesium hydride precatalyst. Although a sequence of amine formylation and deoxygenation may be readily envisaged for the catalytic utilization of CO as a C1 source in the production of methylamines, our observations demonstrate that competitive amine-borane dehydrocoupling is too facile under the conditions of 1 atm of CO employed.

Terminal Uranium(V/VI) Nitride Activation of Carbon Dioxide and Carbon Disulfide: Factors Governing Diverse and Well-Defined Cleavage and Redox Reactions.

The reactivity of terminal uranium(V/VI) nitrides with CE2 (E=O, S) is presented. Well-defined C=E cleavage followed by zero-, one-, and two-electron redox events is observed. The uranium(V) nitride [U(TrenTIPS )(N)][K(B15C5)2 ] (1, TrenTIPS =N(CH2 CH2 NSiiPr3 )3 ; B15C5=benzo-15-crown-5) reacts with CO2 to give [U(TrenTIPS )(O)(NCO)][K(B15C5)2 ] (3), whereas the uranium(VI) nitride [U(TrenTIPS )(N)] (2) reacts with CO2 to give isolable [U(TrenTIPS )(O)(NCO)] (4); complex 4 rapidly decomposes to known [U(TrenTIPS )(O)] (5) with concomitant formation of N2 and CO proposed, with the latter trapped as a vanadocene adduct. In contrast, 1 reacts with CS2 to give [U(TrenTIPS )(κ2 -CS3 )][K(B15C5)2 ] (6), 2, and [K(B15C5)2 ][NCS] (7), whereas 2 reacts with CS2 to give [U(TrenTIPS )(NCS)] (8) and "S", with the latter trapped as Ph3 PS. Calculated reaction profiles reveal outer-sphere reactivity for uranium(V) but inner-sphere mechanisms for uranium(VI); despite the wide divergence of products the initial activation of CE2 follows mechanistically related pathways, providing insight into the factors of uranium oxidation state, chalcogen, and NCE groups that govern the subsequent divergent redox reactions that include common one-electron reactions and a less-common two-electron redox event. Caution, we suggest, is warranted when utilising CS2 as a reactivity surrogate for CO2 .

New perspectives in organolanthanide chemistry from redox to bond metathesis: insights from theory.

A fifteen year contribution of computational studies carried out in close synergy with experiments is summarized. This interplay has allowed some important breakthroughs in the field of organolanthanide chemistry. The variety of different reaction mechanisms in lanthanide chemistry appear to be broader than the simple bond metathesis.

Actinide-Catalyzed Intermolecular Addition of Alcohols to Carbodiimides.

The unprecedented actinide-catalyzed addition of alcohols to carbodiimides is presented. This represents a rare example of thorium-catalyzed transformations of an alcoholic substrate and the first example of uranium complexes showing catalytic reactivity with alcohols. Using the uranium and thorium amides U[N(SiMe3)2]3 and [(Me3Si)2N]2An[κ(2)-(N,C)-CH2Si(CH3)2N(SiMe3)] (An = Th or U), alcohol additions to unsaturated carbon-nitrogen bonds are achieved in short reaction times with excellent selectivities and high to excellent yields. Computational studies, supported by experimental thermodynamic data, suggest plausible models of the profile of the reaction which allow the system to overcome the high barrier of scission of the actinide-oxygen bond. Accompanied by experimentally determined kinetic parameters, a plausible mechanism is proposed for the catalytic cycle.

Insights into the Cascade Reaction of CO and Heteroallenes Mediated by Dinitrogen Hafnocene Complexes: The Indirect Effect of Nitride's Nucleophilicity.

A DFT mechanistic exploration of the reactivity of the dinitrogen hafnocene complex, [{(η(5)-C5 H2 -1,2,4-Me3)2 Hf}2 (µ2-N2)], towards mixtures of CO/CO2 and CO/OCNtBu is reported. The crucial role of the nitride intermediate is highlighted, as well as the importance of the bridging mode of the cyanate ligand between the two Hf metal atoms throughout the process. Interestingly, the CO2 addition to the nitride intermediate occurs through an outer-sphere transition state, whereas the addition of the heteroallene is governed by the steric congestion imposed by cyclopentadienyl ligands.

Facile CO Cleavage by a Multimetallic CsU2 Nitride Complex.

Uranium nitrides are important materials with potential for application as fuels for nuclear power generation, and as highly active catalysts. Molecular nitride compounds could provide important insight into the nature of the uranium-nitride bond, but currently little is known about their reactivity. In this study, we found that a complex containing a nitride bridging two uranium centers and a cesium cation readily cleaved the C≡O bond (one of the strongest bonds in nature) under ambient conditions. The product formed has a [CsU2 (µ-CN)(µ-O)] core, thus indicating that the three cations cooperate to cleave CO. Moreover, the addition of MeOTf to the nitride complex led to an exceptional valence disproportionation of the CsU(IV) -N-U(IV) core to yield CsU(III) (OTf) and [MeN=U(V) ] fragments. The important role of multimetallic cooperativity in both reactions is illustrated by the computed reaction mechanisms.

Activation of CO by Hydrogenated Magnesium(I) Dimers: Sterically Controlled Formation of Ethenediolate and Cyclopropanetriolate Complexes.

This study details the formal hydrogenation of two magnesium(I) dimers {(Nacnac)Mg}2 (Nacnac = [{(C6H3R2-2,6)NCMe}2CH](-); R = Pr(i) ((Dip)Nacnac), Et ((Dep)Nacnac)) using 1,3-cyclohexadiene. These reactions afford the magnesium(II) hydride complexes, {(Nacnac)Mg(µ-H)}2. Their reactions with excess CO are sterically controlled and lead cleanly to different C-C coupled products, viz. the ethenediolate complex, ((Dip)Nacnac)Mg{κ(1)-O-[((Dip)Nacnac)Mg(κ(2)-O,O-O2C2H2)]}, and the first cyclopropanetriolate complex of any metal, cis-{((Dep)Nacnac)Mg}3{µ-C3(H3)O3}. Computational studies imply the CO activation processes proceed via very similar mechanisms to those previously reported for related reactions involving f-block metal hydride compounds. This work highlights the potential magnesium compounds hold for use in the "Fischer-Tropsch-like" transformation of CO/H2 mixtures to value added oxygenate products.

A Mixed-Valence Tri-Zinc Complex, [LZnZnZnL] (L=Bulky Amide), Bearing a Linear Chain of Two-Coordinate Zinc Atoms.

Reduction of a variety of extremely bulky amido Group 12 metal halide complexes, [LMX(THF)(0,1)] (L=amide; M=Zn, Cd, or Hg; X=halide) with a magnesium(I) dimer gave a homologous series of two-coordinate metal(I) dimers, [L'MML'] (L'=N(Ar(†))(SiMe3), Ar(†)=C6H2{C(H)Ph2}2Pr(i)-2,6,4); and the formally zinc(0) complex, [L*ZnMg((Mes)Nacnac)] (L*=N(Ar*)(SiPr(i)3); Ar*=C6H2{C(H)Ph2}2Me-2,6,4; (Mes)Nacnac=[(MesNCMe)2CH](-), Mes=mesityl), which contains the first unsupported Zn-Mg bond. Two equivalents of [L*ZnMg((Mes)Nacnac)] react with ZnBr2 or ZnBr2(tmeda) to give the mixed valence, two-coordinate, linear tri-zinc complex, [L*Zn(I)Zn(0)Zn(I)L*], and the first zinc(I) halide complex, [L*ZnZnBr(tmeda)], respectively. The analogues [L*ZnMZnL*] (M=Cd or Hg), were also prepared, the Cd species contains the first Zn-Cd bond in a molecular compound. Metal-metal bonding was studied by DFT calculations.

A Dimetalloxycarbene Bonding Mode and Reductive Coupling Mechanism for Oxalate Formation from CO2.

We describe the stable and isolable dimetalloxycarbene [(TiX3 )2 (µ2 -CO2 -κ(2) C,O:κO')] 5, where X=N-(tert-butyl)-3,5-dimethylanilide, which is stabilized by fluctuating µ2 -κ(2) C,O:κ(1) O' coordination of the carbene carbon to both titanium centers of the dinuclear complex 5, as shown by variable-temperature NMR studies. Quantum chemical calculations on the unmodified molecule indicated a higher energy of only +10.5 kJ mol(-1) for the µ2 -κ(1) O:κ(1) O' bonding mode of the free dimetalloxycarbene compared to the µ2 -κ(2) C,O:κ(1) O' bonding mode of the masked dimetalloxycarbene. The parent cationic bridging formate complex [(TiX3 )2 (µ2 -OCHO-κO:κO')][B(C6 F5)4], 4[B(C6 F5)4], was simply deprotonated with the strong base K(N(SiMe3 )2 ) to give 5. Complex 5 reacts smoothly with CO2 to generate the bridging oxalate complex [(TiX3 )2 (µ2 -C2 O4 -κO:κO'')], 6, in a C-C bond formation reaction commonly anticipated for oxalate formation by reductive coupling of CO2 on low-valent transition-metal complexes.

Multimetallic cooperativity in uranium-mediated CO2 activation.

The metal-mediated redox transformation of CO2 in mild conditions is an area of great current interest. The role of cooperativity between a reduced metal center and a Lewis acid center in small-molecule activation is increasingly recognized, but has not so far been investigated for f-elements. Here we show that the presence of potassium at a U, K site supported by sterically demanding tris(tert-butoxy)siloxide ligands induces a large cooperative effect in the reduction of CO2. Specifically, the ion pair complex [K(18c6)][U(OSi(O(t)Bu)3)4], 1, promotes the selective reductive disproportionation of CO2 to yield CO and the mononuclear uranium(IV) carbonate complex [U(OSi(O(t)Bu)3)4(µ-κ(2):κ(1)-CO3)K2(18c6)], 4. In contrast, the heterobimetallic complex [U(OSi(O(t)Bu)3)4K], 2, promotes the potassium-assisted two-electron reductive cleavage of CO2, yielding CO and the U(V) terminal oxo complex [UO(OSi(O(t)Bu)3)4K], 3, thus providing a remarkable example of two-electron transfer in U(III) chemistry. DFT studies support the presence of a cooperative effect of the two metal centers in the transformation of CO2.

Lewis acid triggered reactivity of a Lewis base stabilized scandium-terminal imido complex: C-H bond activation, cycloaddition, and dehydrofluorination.

A stable scandium-terminal imido complex is activated by borane to form an unsaturated terminal imido complex by removing the coordinated Lewis base, 4-(dimethylamino)pyridine, from the metal center. The ensuing terminal imido intermediate can exist as a THF adduct and/or undergo cycloaddition reaction with an internal alkyne, C-H activation of a terminal alkene, and dehydrofluorination of fluoro-substituted benzenes or alkanes at room temperature. DFT investigations further highlight the ease of C-H activation for terminal alkene and fluoroarene. They also shed light on the mechanistic aspects of these two reactions.

Activation of SO2 and CO2 by trivalent uranium leading to sulfite/dithionite and carbonate/oxalate complexes.

The first sulfite [{(((nP,Me) ArO)3 tacn)U(IV) }2 (µ-κ(1) :κ(2) -SO3 )] (tacn=triazacyclononane) and dithionite [{(((nP,Me) ArO)3 tacn)U(IV) }2 (µ-κ(2) :κ(2) -S2 O4 )] complexes of uranium from reaction with gaseous SO2 have been prepared. Additionally, the reductive activation of CO2 was investigated with respect to the rare oxalate [{(((nP,Me) ArO)3 tacn)U(IV) }2 (µ-κ(2) :κ(2) -C2 O4 )] formation. This ultimately provides the unique S2 O4 (2-) /C2 O4 (2-) and SO3 (2-) /CO3 (2-) complex pairs. All new complexes were characterized by a combination of single-crystal X-ray diffraction, elemental analysis, UV/Vis/NIR electronic absorption, IR vibrational, and (1) H NMR spectroscopy, as well as magnetization (VT SQUID) studies. Moreover, density functional theory (DFT) calculations were carried out to gain further insight into the reaction mechanisms. All observations, together with DFT, support the assumption that SO2 and CO2 show similar (dithionite/oxalate) to analogous (sulfite/carbonate) activation behavior with uranium complexes.

Mechanistic study of the selectivity of olefin versus cyclobutene formation by palladium(0)-catalyzed intramolecular C(sp3)-H activation.

This study describes the mechanism and selectivity pattern of the Pd(0)-catalyzed C(sp(3))-H activation of a prototypical substrate bearing two linear alkyl groups. Experimentally, the use of the Pd/P(t-Bu)3 catalytic system leads to a ca. 7:3 mixture of olefin and benzocyclobutene (BCB) products. The C-H activation step was computed to be favored for the secondary position α to the benzylic carbon over the primary position ß to the benzylic carbon by more than 4 kcal mol(-1), in line with previous selectivity trends on analogous substrates. The five-membered palladacycle obtained through this activation step may then follow two different pathways, which were computationally characterized: (1) decoordination of the protonated base and reductive elimination to give the BCB product and (2) proton transfer to the aryl ligand and base-mediated ß-H elimination to give the olefin product. Experiments conducted with deuterated substrates were in accordance with this mechanism. The difference between the highest activation barriers in the two pathways was computed to be 1.2 kcal mol(-1) in favor of BCB formation. However, the use of a kinetic model revealed the critical influence of the kinetics of dissociation of HCO3(-) formed after the C-H activation step in actually directing the reaction toward either of the two pathways.

Qualitative estimation of the single-electron transfer step energetics mediated by samarium(II) complexes: a "SOMO-LUMO gap" approach.

Lanthanide II organometallic complexes usually initiate reactions via a single-electron transfer (SET) from the metal to a bonded substrate. Extensive mechanistic studies were carried out for lanthanide III complexes in which no change of oxidation state is involved. Some case-dependent strategies were reported by our group in order to account for a SET event in organometallic computed studies. In the present study, we show that analysis of DFT orbital spectra allows differentiating between exothermic and endothermic electron transfer. This methodology appears to be general; it allows differentiating between lanthanide centers and substituent effects on metallocenes. For that purpose, we considered mainly various samarocene adducts as well as a SmI2 complex explicitly solvated by THF. Comparison between DFT methods and ab initio (CAS-SCF and HF) computational level revealed that the SOMO-LUMO gap computed at the DFT B3PW91 level, in combination with small-core RECPs and standard basis sets, offers a qualitative estimation of the energetics of the SET that is in line with both CAS-SCF calculations and experimental results when available. This orbital-based approach, based on DFT calculation, affords a fast and efficient methodology for pioneer exploration of the reactivity of lanthanide(II) mediated by SET.

Direct evidence for intermolecular oxidative addition of σ(Si-Si) bonds to gold.

Oxidative addition plays a major role in transition-metal catalysis, but this elementary step remains very elusive in gold chemistry. It is now revealed that in the presence of GaCl3, phosphine gold chlorides promote the oxidative addition of disilanes at low temperature. The ensuing bis(silyl) gold(III) complexes were characterized by quantitative (31)P and (29)Si NMR spectroscopy. Their structures (distorted Y shape) and the reaction profile of σ(Si-Si) bond activation were analyzed by DFT calculations. These results provide evidence for the intermolecular oxidative addition of σ(Si-Si) bonds to gold and open promising perspectives for the development of new gold-catalyzed redox transformations.

Two-electron reductive carbonylation of terminal uranium(V) and uranium(VI) nitrides to cyanate by carbon monoxide.

Two-electron reductive carbonylation of the uranium(VI) nitride [U(Tren(TIPS))(N)] (2, Tren(TIPS)=N(CH2CH2NSiiPr3)3) with CO gave the uranium(IV) cyanate [U(Tren(TIPS))(NCO)] (3). KC8 reduction of 3 resulted in cyanate dissociation to give [U(Tren(TIPS))] (4) and KNCO, or cyanate retention in [U(Tren(TIPS))(NCO)][K(B15C5)2] (5, B15C5=benzo-15-crown-5 ether) with B15C5. Complexes 5 and 4 and KNCO were also prepared from CO and the uranium(V) nitride [{U(Tren(TIPS))(N)K}2] (6), with or without B15C5, respectively. Complex 5 can be prepared directly from CO and [U(Tren(TIPS))(N)][K(B15C5)2] (7). Notably, 7 reacts with CO much faster than 2. This unprecedented f-block reactivity was modeled theoretically, revealing nucleophilic attack of the π* orbital of CO by the nitride with activation energy barriers of 24.7 and 11.3 kcal mol(-1) for uranium(VI) and uranium(V), respectively. A remarkably simple two-step, two-electron cycle for the conversion of azide to nitride to cyanate using 4, NaN3 and CO is presented.

Chameleon behavior of a newly synthesized scandium nitrilimine derivative.

The synthesis, structural characterization, and reactivity of the first example of a scandium-substituted nitrilimine are presented. This unique complex exhibits high thermal stability but shows a rich reactivity toward a variety of unsaturated substrates, including aldehyde, ketone, nitrile, and allene derivatives. The versatility of the complex was further highlighted by density functional theory mechanistic studies.

Versatile reactivity of a four-coordinate scandium phosphinidene complex: reduction, addition, and CO activation reactions.

The four-coordinate scandium phosphinidene complex, [LSc(µ-PAr)]2 (L = (MeC(NDIPP)CHC(Me)(NCH2CH2N((i)Pr)2)), DIPP = 2,6-((i)Pr)2C6H3; Ar = 2,6-Me2C6H3) (1), has been synthesized in good yield, and its reactivity has been investigated. Although 1 has a bis(µ-phosphinidene)discandium structural unit, this coordinatively unsaturated complex shows high and versatile reactivity toward a variety of substrates. First, two-electron reduction occurs when substrates as 2,2'-bipyridine, elemental selenium, elemental tellurium, Me3P═S, or Ph3P═E (E = S, Se) is used, resulting in the oxidative coupling of two phosphinidene ligands 2[PAr](2-) into a diphosphene ligand [ArP-PAr](2-). Complex 1 easily undergoes nucleophilic addition reactions with unsaturated substrates, such as benzylallene, benzonitrile, tert-butyl isocyanide, and CS2. This complex also shows a peculiar reactivity to CO and Mo(CO)6, that includes C-P bond formation, C-C coupling and C-O bond cleavage of CO, to afford novel phosphorus-containing products. In the last two types of reactivity, reaction profiles have been computed (for the insertion of (t)BuNC and the CO activation by 1) at the DFT level. The unexpected/surprising sequence of steps in the latter case is also revealed.

Heteroleptic tin(II) initiators for the ring-opening (co)polymerization of lactide and trimethylene carbonate: mechanistic insights from experiments and computations.

The tin(II) complexes {LO(x)}Sn(X) ({LO(x)}(-) =aminophenolate ancillary) containing amido (1-4), chloro (5), or lactyl (6) coligands (X) promote the ring-opening polymerization (ROP) of cyclic esters. Complex 6, which models the first insertion of L-lactide, initiates the living ROP of L-LA on its own, but the amido derivatives 1-4 require the addition of alcohol to do so. Upon addition of one to ten equivalents of iPrOH, precatalysts 1-4 promote the ROP of trimethylene carbonate (TMC); yet, hardly any activity is observed if tert-butyl (R)-lactate is used instead of iPrOH. Strong inhibition of the reactivity of TMC is also detected for the simultaneous copolymerization of L-LA and TMC, or for the block copolymerization of TMC after that of L-LA. Experimental and computational data for the {LO(x)}Sn(OR)complexes (OR=lactyl or lactidyl) replicating the active species during the tin(II)-mediated ROP of L-LA demonstrate that the formation of a five-membered chelate is largely favored over that of an eight-membered one, and that it constitutes the resting state of the catalyst during this (co)polymerization. Comprehensive DFT calculations show that, out of the four possible monomer insertion sequences during simultaneous copolymerization of L-LA and TMC: 1) TMC then TMC, 2) TMC then L-LA, 3) L-LA then L-LA, and 4) L-LA then TMC, the first three are possible. By contrast, insertion of L-LA followed by that of TMC (i.e., insertion sequence 4) is endothermic by +1.1 kcal mol(-1), which compares unfavorably with consecutive insertions of two L-LA units (i.e., insertion sequence 3) (-10.2 kcal mol(-1)). The copolymerization of L-LA and TMC thus proceeds under thermodynamic control.

Click-like reactions with the inert HCB11Cl11(-) anion lead to carborane-fused heterocycles with unusual aromatic character.

The chlorinated carba-closo-dodecaborate anion HCB11Cl11(-) is an exceptionally stable molecule and has previously been reported to be substitutionally inert at the B-Cl vertices. We present here the discovery of base induced cycloaddition reactions between this carborane anion and organic azides that leads to selective C and B functionalization of the cluster. A single crystal X-ray diffraction study reveals bond lengths in the heterocyclic portion of the ring that are shortened, which suggests electronic delocalization. Molecular orbital analysis of the ensuing heterocycles reveals that two of the bonding orbitals of these systems resemble two of the doubly occupied π-MOs of a simple 5-membered Hückel-aromatic, even though they are entangled in the carborane skeleton. Nucleus independent chemical shift analysis indicates that both the carborane cluster portion of the molecule and the carborane fused heterocyclic region display aromatic character. Computational methods indicate that the reaction likely follows a stepwise addition cyclization pathway.