The Institutional Presidency from a Comparative Perspective: Argentina and Brazil since the 1980s

This paper focuses on the evolution of the institutional presidency - meaning the cluster of agencies that directly support the chief of the executive - in Argentina and Brazil since their redemocratization in the 1980s. It investigates what explains the changes that have come about regarding the size of the institutional presidency and the types of agency that form it. Following the specialized literature, we argue that the growth of the institutional presidency is connected to developments occurring in the larger political system - that is, to the political challenges that the various presidents of the two countries have faced. Presidents adjust the format and mandate of the different agencies under their authority so as to better manage their relations with the political environment. In particular, we argue that the type of government (coalition or single-party) has had consequences for the structure of the presidency or, in other words, that different cabinet structures pose different challenges to presidents. This factor has not played a significant role in presidency-related studies until now, which have hitherto mostly been based on the case of the United States. Our empirical references, the presidencies of Argentina and Brazil, typical cases of coalitional as well as single-party presidentialism respectively allow us to show the impact of the type of government on the number and type of presidential agencies

Ruthenium Complexes Stabilized by Bidentate Enamido-Phosphine Ligands: Aspects of Cooperative H₂ Activation

Four bidentate, hybrid ligands (^R(NP)^R′H) featuring imine-nitrogen and alkyl-phosphine donors linked by a cyclopentyl ring were synthesized. The ortho position of the aryl group attached to nitrogen is varied such that R is Me or Prⁱ; additionally, the groups decorating phosphorus (R′) are varied between Bu^t or Prⁱ. The addition of each ligand to RuHCl­(PPrⁱ₃)₂(CO) in the presence of KOBu^t generates four enamido-phosphine complexes RuH­{^R(NP)^R′}­(PPrⁱ₃)­(CO) that were characterized by NMR spectroscopy, elemental analyses, and, in the case of R = Prⁱ and R′ = Bu^t or Prⁱ, X-ray crystallography. Depending on R′, the reaction of RuH­{^R(NP)^R′}­(PPrⁱ₃)­(CO) with H₂ generates varying amounts of the imine-phosphine complex RuH₂{^R(NP)^R′H}­(PPrⁱ₃)­(CO). Insights into the mechanism of H₂ activation by these enamido derivatives were explored using RuH­{^Pr(NP)^Pri}­(PPrⁱ₃)­(CO), for which an intermediate was identified as the dihydrogen–dihydride complex, RuH₂(H₂)­{^Pri(NP)^PriH}­(PPrⁱ₃)­(CO), on the basis of the <i>T</i>_1,min value of 22 ms for the ¹H NMR resonance at δ −7.2 at 238 K (measured at 400 MHz). The N donor of the enamine tautomeric form of the ligand is protonated by H₂ or D₂ and dissociates from Ru. Tautomerization of the enamine to the imine form of the dissociated arm is involved in formation of the final product

β‑Hydrogen Elimination and Reductive Elimination from a κ³‑PPC Nickel Complex

The incorporation of a PPC donor ligand set onto nickel is described. While C–H activation routes using Ni­(II) precursors and benzyl- and phenethyl-substituted diphosphines Bu^t₂PCH₂CH₂P­(Bu^t)­R failed, success was achieved via oxidative addition of the 2-bromo-benzyl-ligand precursor Bu^t₂PCH₂CH₂P­(Bu^t)­(CH₂-<i>o</i>-C₆H₄Br) with Ni­(COD)₂ to generate [κ³-^BnPPC]­NiBr. Subsequent reaction with KBEt₃H resulted in decomposition unless PPh₃ was present, which allowed isolation of the tricoordinate Ni(0) complex [κ²-^BnPP]­Ni­(PPh₃). Reaction of [κ³-^BnPPC]­NiBr with EtMgCl also resulted in the formation of the Ni(0) ethylene complex, [κ²-^BnPP]­Ni­(η²-C₂H₄), via β-elimination followed by reductive elimination. Deuterium-labeling studies are consistent with a reversible β-elimination process prior to reductive elimination, which scrambles the deuterium isotopes. Reaction of [κ³-^BnPPC]­NiBr with CH₃Li results in the formation of the Ni­(II) methyl complex, [κ³-^BnPPC]­Ni­(CH₃). Heating this species in the presence of PPh₃ results in the formation of the Ni(0) derivative, [κ²-^{Bn‑<i>o</i>‑Me}PP]­Ni­(PPh₃), in which reductive elimination of the methyl and aryl units has occurred. Reaction of [κ³-^BnPPC]­NiBr with heteroatom-containing species such as sodium isopropoxide, sodium 2-propanethiolate, and lithium methylphenylamide resulted in different outcomes depending on the heteroatom; with isopropoxide, the products isolated were geometric isomers of Ni(0) η²-acetone adducts, which presumably arise via a β-elimination–reductive elimination sequence; the analogous sulfur reagent generated isolable Ni­(II) isopropyl thiolate species that was resistant to β-elimination upon thermolysis; with methylphenylamide, the Ni­(II) amido complex could be isolated and structurally characterized; subsequent heating to 80 °C resulted in β-elimination followed by reductive elimination to generate a Ni(0) phenylimine complex. While the κ³-PPC donor set generates stable Ni­(II) derivatives, further functionalization at nickel with hydride, alkyl, and heteroatom-containing moieties can lead to Ni(0) products via a combination of β-elimination and/or reductive elimination

Anionic Tantalum Dihydride Complexes: Heterobimetallic Coupling Reactions and Reactivity toward Small-Molecule Activation

The anionic dihydride complex [Cp₂TaH₂]⁻ was synthesized as a well-defined molecular species by deprotonation of Cp₂TaH₃ while different solubilizing agents, such as [2.2.2]­cryptand and 18-crown-6, were applied to encapsulate the alkali-metal counterion. The ion pairs were characterized by multiple spectroscopic methods as well as X-ray crystallography, revealing varying degrees of interaction between the hydride ligands of the anion and the respective countercation in solution and in the solid state. The [Cp₂TaH₂]⁻ complex anion shows slow exchange of the hydride ligands when kept under a D₂ atmosphere, but a very fast reaction is observed when [Cp₂TaH₂]⁻ is reacted with CO₂, from which Cp₂TaH­(CO) is obtained as the tantalum-containing reaction product, along with inorganic salts. Furthermore, [Cp₂TaH₂]⁻ can act as a synthon in heterobimetallic coupling reactions with transition-metal halide complexes. Thus, the heterobimetallic complexes Cp₂Ta­(μ-H)₂Rh­(dippp) and Cp₂Ta­(μ-H)₂Ru­(H)­(CO)­(PⁱPr₃)₂ were synthesized and characterized by various spectroscopies and via single-crystal X-ray diffraction. The new hydride bridged tantalum–rhodium heterobimetallic complex is cleaved under a CO atmosphere to yield mononuclear species and slowly exchanges protons and hydride ligands when exposed to D₂ gas

Carbon–Carbon Bond Forming Reactions with Tantalum Diamidophosphine Complexes That Incorporate Alkyne Ligands

The incorporation of <i>o-</i>phenylene-linked diamidophosphine ligands onto the readily available alkyne complexes Ta­(alkyne)­Cl₃(DME) (where alkyne = hex-3-yne or 1,2-bis­(trimethylsilylacetylene); DME = 1,2-dimethoxyethane) results in the formation of a versatile set of starting materials of the general formula [^PhNPN*]­Ta­(alkyne)­Cl (where [^PhNPN*] = PhP­(2-(<i>N</i>-mesityl)-5-Me-C₆H₃)₂). Upon reaction with KBEt₃H, the synthesis of the corresponding hydride complexes [^PhNPN*]­Ta­(alkyne)H can be achieved; these complexes feature extremely downfield (δ ∼21 ppm) doublet resonances (²<i>J</i>_HP = ∼35 Hz) in the respective ¹H NMR spectra that are assigned to the newly formed Ta–H moieties. Subsequent reaction of these Ta hydrides with 2,6-dimethylphenylisocyanide and phenylacetylene results in the insertion of these species into the Ta–H bond and the formation of the corresponding iminoformyl and phenylvinyl complexes, respectively. While the former intermediate cannot be detected, the latter was characterized by NMR spectroscopy. Both of these processes result in the further transformation to generate C–C coupled products by a reductive elimination sequence with the coordinated alkyne; in the case of the iminoformyl, an azadiene results, whereas with the phenylvinyl derivative a butadienyl fragment is generated. Single-crystal X-ray diffraction and a suite of NMR spectroscopic techniques were used to characterize these species. A discussion of the bonding of the products in the context of the process by which they form is presented. The rate of formation of the butadienyl moiety from the phenylvinyl intermediate results in the activation parameters of Δ<i>H</i>^⧧ = 22.2 ± 0.3 kcal/mol and Δ<i>S</i>^⧧ = −8.7 ± 0.2 cal/(mol)­(K)

Synthesis of a Dinuclear Ferrocene-Linked Bis(phosphinoamide)scandium Hydride Complex

The ferrocene-based bis­(phosphinoamide) fc­(NHP^<i>i</i>Pr₂)₂ (where fc = 1,1′-ferrocenyl) has been synthesized and coordinated to scandium via alkane elimination. The subsequent reaction of fc­(NP^<i>i</i>Pr₂)₂ScCH₂SiMe₃(THF) with H₂ yields a dinuclear scandium hydride species wherein the scandium centers are bridged by both hydride and phosphinoamide fragments

Synthetic and Computational Studies on the Thermal and Photochemical Reactions of [NPN]TaMe₃ (NPN = PhP(CH₂SiMe₂NPh)₂) and [^MesNPN]TaMe₃ (^MesNPN = PhP(CH₂SiMe₂N(2,4,6-Me₃C₆H₂))₂)

The thermolysis of [PhP­(CH₂SiMe₂NPh)₂]­TaMe₃ leads to the elimination of methane and the formation of cyclometalated derivative [PhP­(CH₂SiMe₂NPh)­(CH₂SiMe₂N-<i>o-</i>C₆H₄)]­TaMe₂, which was characterized by NMR spectroscopy and single crystal X-ray analysis. Computational studies confirm the expected four-membered transition state involving an <i>ortho</i>-<i>N</i>-phenyl-C–H bond and a Ta-methyl unit. The photolysis of [PhP­(CH₂SiMe₂NPh)₂]­TaMe₃ takes a different course; loss of methane also occurs but results in the formation the methylidene complex, [PhP­(CH₂SiMe₂NPh)₂]­TaCH₂(Me), which was characterized by NMR spectroscopy. Attempts to block the cyclometalation process by replacement of the <i>N</i>-phenyl substituent with <i>N</i>-Mesityl (Mesityl = 2,4,6-Me₃C₆H₂) is also reported. With this bulkier ancillary ligand, the reactions are more complicated with multiple products being observed in an overall slow process. The reactions of the trimethyl, the cyclometalated product and the methylidene with H₂ were also investigated and found to exhibit different rates of hydrogenolysis. This has implications for some of the steps in the reaction of [PhP­(CH₂SiMe₂NPh)₂]­TaMe₃ with H₂ to generate dinuclear tetrahydride ([PhP­(CH₂SiMe₂NPh)₂]­Ta)₂(μ-H)₄

Hydrogenolysis of Tantalum Hydrocarbyl Complexes: Intermediates on the Road to a Dinuclear Tantalum Tetrahydride Derivative

The synthesis, characterization, and reactivity with H₂ of a series of tantalum hydrocarbyl complexes are reported. The reaction of [NPN*]­TaMe₃ (<b>4</b>, where NPN* = PhP­(2-(<i>N</i>-mesityl)-5-Me-C₆H₃)₂) with dihydrogen (H₂, 4 atm) results in the formation of the dinuclear tetrahydride ([NPN*]­Ta)₂(μ-H)₄ (<b>5</b>), without the observation of intermediates. The preparations of two alkyne benzyl complexes of the formula [NPN*]­Ta­(alkyne)­(CH₂Ph) (where alkyne = BTA = bis­(trimethylsilyl)­acetylene (<b>6</b>), 3-hexyne (<b>7</b>)) are reported starting from the respective chloroalkyne complexes [NPN*]­Ta­(alkyne)­Cl, by addition of benzylpotassium. Hydrogenation of these two alkyne benzyl complexes ultimately results in the formation of the same dinuclear tetrahydride <b>5</b>; however, using lower pressures of H₂ and shorter reaction times results in the isolation of an intermediate in each case. Hydrogenation of <b>6</b> generates the alkene hydride complex [NPN*]­Ta­(<i>trans</i>-1,2-C₂H₂(SiMe₃)₂)H (<b>8</b>); addition of H₂ to <b>7</b> gives [NPN*]­Ta­(1-hexene)H (<b>9</b>), in which the 3-hexyne moiety has been partially hydrogenated and isomerized to the 1-hexene regioisomer. Both of these alkene hydride complexes can be converted to the dinuclear tetrahydride complex <b>5</b> by addition of H₂. A mechanism is proposed for the formation of the intermediates that involves hydrogenolysis of the alkyne moiety prior to the benzyl ligand; the formation of the <i>trans</i>-alkene units is suggested to be a result of a zwitterionic alkylidene intermediate that allows free rotation of a C–C single bond

Synthetic and Computational Studies on the Thermal and Photochemical Reactions of [NPN]TaMe₃ (NPN = PhP(CH₂SiMe₂NPh)₂) and [^MesNPN]TaMe₃ (^MesNPN = PhP(CH₂SiMe₂N(2,4,6-Me₃C₆H₂))₂)

The thermolysis of [PhP­(CH₂SiMe₂NPh)₂]­TaMe₃ leads to the elimination of methane and the formation of cyclometalated derivative [PhP­(CH₂SiMe₂NPh)­(CH₂SiMe₂N-<i>o-</i>C₆H₄)]­TaMe₂, which was characterized by NMR spectroscopy and single crystal X-ray analysis. Computational studies confirm the expected four-membered transition state involving an <i>ortho</i>-<i>N</i>-phenyl-C–H bond and a Ta-methyl unit. The photolysis of [PhP­(CH₂SiMe₂NPh)₂]­TaMe₃ takes a different course; loss of methane also occurs but results in the formation the methylidene complex, [PhP­(CH₂SiMe₂NPh)₂]­TaCH₂(Me), which was characterized by NMR spectroscopy. Attempts to block the cyclometalation process by replacement of the <i>N</i>-phenyl substituent with <i>N</i>-Mesityl (Mesityl = 2,4,6-Me₃C₆H₂) is also reported. With this bulkier ancillary ligand, the reactions are more complicated with multiple products being observed in an overall slow process. The reactions of the trimethyl, the cyclometalated product and the methylidene with H₂ were also investigated and found to exhibit different rates of hydrogenolysis. This has implications for some of the steps in the reaction of [PhP­(CH₂SiMe₂NPh)₂]­TaMe₃ with H₂ to generate dinuclear tetrahydride ([PhP­(CH₂SiMe₂NPh)₂]­Ta)₂(μ-H)₄

Synthesis and Reactivity of a Low-Coordinate Iron(II) Hydride Complex: Applications in Catalytic Hydrodefluorination

A low-coordinate iron hydride complex bearing an unsymmetrical NpN (enamido–phosphinimine) ligand scaffold was synthesized and fully characterized. Insertion reactivity with azobenzene, 3-hexyne, and 1-azidoadamantane was explored, and the isolated products were analogous to previously reported β-diketiminate iron hydride insertion products. Surprisingly, the NpN iron hydride displays unprecedented reactivity toward hexafluorobenzene, affording an NpN iron fluoride complex and pentafluorobenzene as products. The NpN iron hydride is a precatalyst for catalytic hydro-defluorination of perfluorinated aromatics in the presence of silane. Kinetic studies indicated that the rate-determining step during catalysis involved silane