Xavier-Frédéric Le Goff

Why Platinum Catalysts Involving Ligands with Large Bite Angle Are so Efficient in the Allylation of Amines: Design of a Highly Active Catalyst and Comprehensive Experimental and DFT Study

The platinum-catalyzed allylation of amines with allyl alcohols was studied experimentally and theoretically. The complexes [Pt(eta(3)-allyl)(dppe)]OTf (2) and [Pt(eta(3)-allyl)(DPP-Xantphos)]PF(6) (5) were synthesized and structurally characterized, and their reactivity toward amines was explored. The bicyclic aminopropyl complex [Pt(CH(2)CH(2)CH(2)NHBn-kappa-C,N)(dppe)]OTf (3) was obtained from the reaction of complex 2 with an excess of benzylamine, and this complex was shown to be a deactivated form of catalyst 2. On the other hand, reaction of complex 5 with benzylamine and allyl alcohol led to formation of the 16-VE platinum(0) complex [Pt(eta(2)-C(3)H(5)OH)(DPP-Xantphos)] (7), which was structurally characterized and appears to be a catalytic intermediate. A DFT study showed that the mechanism of the platinum-catalyzed allylation of amines with allyl alcohols differs from the palladium-catalyzed process, since it involves an associative ligand-exchange step involving formation of a tetracoordinate 18-VE complex. This DFT study also revealed that ligands with large bite angles disfavor the formation of platinum hydride complexes and therefore the formation of a bicyclic aminopropyl complex, which is a thermodynamic sink. Finally, a combination of 5 and a proton source was shown to efficiently catalyze the allylation of a broad variety of amines with allyl alcohols under mild conditions.

Revisiting the molecular roots of a ubiquitously successful synthesis: nickel(0) nanoparticles by reduction of [Ni(acetylacetonate)2].

The widely used preparation of Ni(0) nanoparticles from [Ni(acac)(2)] (acac=acetylacetonate) and oleylamine, often considered to be a thermolysis or a radical reaction, was analyzed anew by using a combination of DFT modeling and designed mechanistic experiments. Firstly, the reaction was followed up by using TGA to evaluate the energy barrier of the limiting step. Secondly, all the byproducts were identified using NMR spectroscopy, mass spectrometry, FTIR, and X-ray crystallography. These methods allowed us to depict both main and side-reaction pathways. Lastly, DFT modeling was utilized to assess the validity of this new scheme by identifying the limiting steps and evaluating the corresponding energy barriers. The oleylamine was shown to reduce the [Ni(acac)(2)] complex not through a one-electron radical mechanism, as often stated, but as an hydride donor through a two-electron chemical reduction route. This finding has strong consequences not only for the design of further nanoparticles syntheses that use long-chain amine as a reactant, but also for advanced understanding of catalytic reactions for which these nanoparticles can be employed.

Hemichelation, a Way To Stabilize Electron-Unsaturated Complexes: The Case of T-Shaped Pd and Pt Metallacycles.

A rational method of synthesis of stable neutral T-shaped 14 electron Pd and Pt complexes is proposed. It takes advantage of the ambiphilic character of the tricarbonyl(η(6)-indenyl)chromium anion, of which the main property is to behave as a hemichelating ligand, that is a nonconventional heteroditopic ligand capable of chelating a metal center by way of covalent and noncovalent bonding, thus preserving its unsaturated valence shell. The reaction of the in situ formed tricarbonyl(η(6)-2-methylindenyl)chromium anion with a series of Pd and Pt metallacycles afforded new air-stable and persistent synfacial heterobimetallic complexes in which the metallacycle binds the indenyl fragment via its metal in an η(1) fashion, leaving the fourth coordination site at the chelated metal virtually vacant. The structures of eight of these novel complexes are disclosed, and their bonding features are investigated by an array of theoretical methods based on the density functional theory (NBO, EDA, ETS-NOCV, AIM, NCI region analysis). Theory shows that the formation of these unusual structures of bimetallic synfacial η(1)-indenyl-Pd/Pt complexes is driven thermodynamically by attractive Coulombic occlusion of the fourth vacant coordination site at Pd/Pt centers by the Cr(CO)3 moiety.

Room-Temperature Palladium-Catalyzed Negishi-Type Coupling: A Combined Experimental and Theoretical Study

An air-stable, bulky electron-accepting phosphine ligand (phosphabarrelene) allows the easy reduction of a Pd(II) precursor to a Pd(0) complex, highly active in room-temperature Negishi-type cross-coupling. DFT calculations show that the use of the electron-accepting ligand favors both transmetalation (TM) and reductive-elimination (RE) processes (see scheme; OA = oxidative addition).

Synthesis of Planar Chiral Iridacycles by Cationic Metal π‐Coordination: Facial Selectivity, and Conformational and Stereochemical Consequences

Facial selectivity during the π-coordination of pseudo-tetrahedral iridacycles by neutral (Cr(CO)(3)), monocationic (Cp*Ru(+)), and biscationic (Cp*Ir(2+)) metal centers was directly influenced by the coulombic imbalance in the coordination sphere of the chelated Ir center. We also showed by using theoretical calculations that the feasibility of the related metallacycles that displayed metallocenic planar chirality was dependent to the presence of an electron-donating group, such as NMe(2), which contributed to the overall stability of the complexes. When the π-bonded moiety was the strongly electron-withdrawing Cp*Ir(2+) group, the electron donation from NMe(2) resulted in major conformational changes, with a barrier to rotation of about 17 kcal  mol(-1) for this group that became spectroscopically diastereotopic (high-field (1)H NMR spectroscopy). This peculiar property is proposed as a means to introduce a new type of constitutional chirality at the nitrogen center: planar chirality at tertiary aromatic amines.

First Stabilization of 14‐Electron Rhodium(I) Complexes by Hemichelation

Hemichelation is emerging as a new mode of coordination where non-covalent interactions crucially contribute to the cohesion of electron-unsaturated organometallic complexes. This study discloses an unprecedented demonstration of this concept to a Group 9 metal, that is, Rh(I). The syntheses of new 14-electron Rh(I) complexes were achieved by choosing the anti-[(η(6):η(6)-fluorenyl){Cr(CO)3}2] anion as the ambiphilic hemichelating ligand, which was treated with [{Rh(nbd)Cl}2] (nbd=norbornadiene) and [{Rh(CO)2Cl}2]. The new T-shaped Rh(I) hemichelates were characterized by analytical and structural methods. Investigations using the methods of the DFT and electron-density topology analysis (NCI region analysis, QTAIM theory) confirmed the closed-shell, non-covalent and attractive characters of the interaction between the Rh(I) center and the proximal Cr(CO)3 moiety. This study shows that, by appropriate tuning of the electronic properties of the ambiphilic ligand, truly coordination-unsaturated Rh(I) complexes can be synthesized in a manageable form.

P4 Activation with Pt0 Metal Centers: Selective Formation of a Dinuclear {Pt2(μ,η2:2‐P2)} Complex

White phosphorus (P4) is the common basis for organophosphorus compounds, by means of a multistep sequence, involving oxidation with chlorine gas as the first step. A more environmentally friendly derivatization of P4 is highly desirable, and has been sought for the past decades. In fact, the coordination chemistry of P4 has been studied in depth since the seminal report of its reactivity toward Wilkinson s catalyst in 1971. However, almost all the early studies after this date used harsh conditions (thermal or photochemical) resulting in unpredictable P4 fragmentation. It was later recognized that carefully designed transition-metal precursors, which provide a single coordination site under “soft conditions” could lead to h coordination of P4, which in this case acts as a “typical” phosphine ligand. [4] In particular; Peruzzini et al. have studied extensively the reactivity of Ru complexes of such h derivatives, and shown possible functionalization. On the other hand, when two coordination sites are made available, insertion of the metal center in a P P bond is typically observed, resulting in the formal oxidation of the metal center, and concomitant reduction of P4. Yet again, the outcome of the reaction is not very predictable (Scheme 1). First investigations of the reaction of P4 with electron-rich Pt precursors, such as [Pt ACHTUNGTRENNUNG(PPh3)2ACHTUNGTRENNUNG(CH2CH2)] or [Pt ACHTUNGTRENNUNG(PPh3)4], were done by Scheer et al., who showed that the use of a stabilizing {Cr(CO)5} fragment was required to control the reactivity and to isolate the trimetallic complex [PtACHTUNGTRENNUNG(PPh3)2(m3,h2:1:1-P4){Cr(CO)5}2] (A). Sch fer and Binder synthesized a {Pt2(m,h -P2)} complex B from a Pt II precursor and LiP ACHTUNGTRENNUNG(SiMe3)2, showing that the “P2” fragment could be stabilized by two Pt centers. Much more recently, Cummins et al. have reported on the generation of a related {(P2)W(CO)5} transient molecule from a diphosphaazide niobium complex. This fragment was efficiently trapped by two {Pt ACHTUNGTRENNUNG(PPh3)2} fragments from the [Pt ACHTUNGTRENNUNG(PPh3)2 ACHTUNGTRENNUNG(CH2CH2)] complex to form [{Pt ACHTUNGTRENNUNG(PPh3)2}2(m3,h2:2:1-P2){W(CO)5}] (C). Most importantly, these “P2” fragments have been trapped by cyclohexadiene to form phosphorus containing tetracyclic species, rare examples of functionalization under very mild conditions. 12] We present here the use of 14-electron complexes of Pt that simultaneously provide two sites of coordination and a d configuration, which result in a strong activation of P4. We show that the use of a bidentate diphosphine ligand (vs. two phosphines) allows the isolation of a novel dinuclear [{Pt ACHTUNGTRENNUNG(dppp)}2(m,h2:2-P2)] complex (dppp= 1,3-bis(diphenylphosphino)propane). A DFT study which rationalizes the formation of this complex as well as its precise electronic structure is also presented. In a first approach, the known [Pt ACHTUNGTRENNUNG(PCy3)2] complex was selected as the Pt precursor. The slow addition of stoichiometric amounts of P4 at 78 8C resulted in a very fast and uncontrollable reaction, leading to a black solution. The P NMR spectrum of the crude mixture showed the absence of the signal of P4 and the starting complex and the sole presence of free PCy3. Based on our previous report that a stoichiometric amount of P4 reacted with [NiACHTUNGTRENNUNG(cod)2] (cod= 1,5-cyclooctadiene) to form the corresponding nickel phosphide species, we reasoned that a platinum phosphide spe[a] M. Demange, Dr. X.-F. Le Goff, Prof. Dr. P. Le Floch, Dr. N. M zailles Laboratoire “H t ro l ments et Coordination” Ecole Polytechnique—CNRS, 91128 Palaiseau cedex (France) Fax: (+33) 169-33-44-40 E-mail : nicolas.mezailles@polytechnique.edu

Neutral ansa-bis(fluorenyl)silane neodymium borohydrides: synthesis, structural study and behaviour as catalysts in butadiene–ethylene copolymerisation

The reaction of silylene-bridged bis(fluorenyl)dipotassium salts with neodymium tris(borohydride) afforded new neutral ansa-bis(fluorenyl)silane neodymium borohydrides: (Flu2SiR2)Nd(BH4)(THF) [R2 = Me2, Et2, (CH2)3, Me(Ph), Flu = C13H8] that were better characterised and more soluble than the previously described anionic [(Flu2SiMe2)Nd(BH4)2]−. The X-ray structures of three of these complexes were determined, and their solid-state geometrical parameters are very similar, despite the ring strain introduced by the silacyclobutane bridge in [Flu2Si(CH2)3]Nd(BH4)(THF). The main geometrical features were satisfactorily reproduced by DFT calculations. The catalytic activity of the title complexes in ethylene–butadiene copolymerisation reactions was assessed and compared to that of the reported activity of [(Flu2SiMe2)Nd(BH4)2]− under similar conditions. From these results it can be concluded that the cyclo-copolymerisation of ethylene with butadiene is characteristic of a catalyst featuring silylene-bridged bis(fluorenyl) ligands around neodymium, and appears to be independent of the substituents at the silicon atom.