Laurent Maron

Discrete, solvent-free alkaline-earth metal cations: metal···fluorine interactions and ROP catalytic activity.

Efficient protocols for the syntheses of well-defined, solvent-free cations of the large alkaline-earth (Ae) metals (Ca, Sr, Ba) and their smaller Zn and Mg analogues have been designed. The reaction of 2,4-di-tert-butyl-6-(morpholinomethyl)phenol ({LO(1)}H), 2-{[bis(2-methoxyethyl)amino]methyl}-4,6-di-tert-butylphenol ({LO(2)}H), 2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-4,6-di-tert-butylphenol ({LO(3)}H), and 2-[(1,4,7,10-tetraoxa-13-azacyclo-pentadecan-13-yl)methyl]-1,1,1,3,3,3-hexafluoropropan-2-ol ({RO(3)}H) with [H(OEt(2))(2)](+)[H(2)N{B(C(6)F(5))(3)}(2)](-) readily afforded the doubly acidic pro-ligands [{LO(1)}HH](+)[X](-) (1), [{LO(2)}HH](+)[X](-) (2), [{LO(3)}HH](+)[X](-) (3), and [{RO(3)}HH](+)[X](-) (4) ([X](-) = [H(2)N{B(C(6)F(5))(3)}(2)](-)). The addition of 2 to Ca[N(SiMe(3))(2)](2)(THF)(2) and Sr[N(SiMe(3))(2)](2)(THF)(2) yielded [{LO(2)}Ca(THF)(0.5)](+)[X](-) (5) and [{LO(2)}Sr(THF)](+)[X](-) (6), respectively. Alternatively, 5 could also be prepared upon treatment of {LO(2)}CaN(SiMe(3))(2) (7) with [H(OEt(2))(2)](+)[X](-). Complexes [{LO(3)}M](+)[X](-) (M = Zn, 8; Mg, 9; Ca, 10; Sr, 11; Ba, 12) and [{RO(3)}M](+)[X](-) (M = Zn, 13; Mg, 14; Ca, 15; Sr, 16; Ba, 17) were synthesized in high yields (70-90%) by reaction of 3 or 4 with the neutral precursors M[N(SiMe(3))(2)](2)(THF)(x) (M = Zn, Mg, x = 0; M = Ca, Sr, Ba, x = 2). All compounds were fully characterized by spectroscopic methods, and the solid-sate structures of compounds 1, 3, 7, 8, 13, 14, {15}(4)·3CD(2)Cl(2), {16}(4)·3CD(2)Cl(2), and {{17}(4)·EtOH}·3CD(2)Cl(2) were determined by X-ray diffraction crystallography. Whereas the complexes are monomeric in the case of Zn and Mg, they form bimetallic cations in the case of Ca, Sr and Ba; there is no contact between the metal and the weakly coordinating anion. In all metal complexes, the multidentate ligand is κ(6)-coordinated to the metal. Strong intramolecular M···F secondary interactions between the metal and F atoms from the ancillary ligands are observed in the structures of {15}(4)·3CD(2)Cl(2), {16}(4)·3CD(2)Cl(2), and {{17}(4)·EtOH}·3CD(2)Cl(2). VT (19)F{(1)H} NMR provided no direct evidence that these interactions are maintained in solution; nevertheless, significant Ae···F energies of stabilization of 25-26 (Ca, Ba) and 40 kcal·mol(-1) (Sr) were calculated by NBO analysis on DFT-optimized structures. The identity and integrity of the cationic complexes are preserved in solution in the presence of an excess of alcohol (BnOH, (i)PrOH) or L-lactide (L-LA). Efficient binary catalytic systems for the immortal ring-opening polymerization of L-LA (up to 3,000 equiv) are produced upon addition of an excess (5-50 equiv) of external protic nucleophilic agents (BnOH, (i)PrOH) to 8-12 or 13-17. PLLAs with M(n) up to 35,000 g·mol(-1) were produced in a very controlled fashion (M(w)/M(n) ≈ 1.10-1.20) and without epimerization. In each series of catalysts, the following order of catalytic activity was established: Mg ≪ Zn < Ca < Sr ≈ Ba; also, Ae complexes supported by the aryloxide ligand are more active than their parents supported by the fluorinated alkoxide ancillary, possibly owing to the presence of Ae···F interactions in the latter case. The rate law -d[L-LA]/dt = k(p)·[L-LA](1.0)·[16](1.0)·[BnOH](1.0) was established by NMR kinetic investigations, with the corresponding activation parameters ΔH(++) = 14.8(5) kcal·mol(-1) and ΔS(++) = -7.6(2.0) cal·K(-1)·mol(-1). DFT calculations indicated that the observed order of catalytic activity matches an increase of the L-LA coordination energy onto the cationic metal centers with parallel decrease of the positive metal charge.

Quantum chemistry-based interpretations on the lowest triplet state of luminescent lanthanides complexes. Part 1. Relation between the triplet state energy of hydroxamate complexes and their luminescence properties

In this paper, we evaluate the potential use of theoretical calculations to obtain an energy scale of the lowest ligand-centred triplet excited state in luminescent terbium(III) complexes. In these complexes, non-radiative deactivation of the terbium emitting state via a back-energy transfer process (T1<--Tb(5D4)) is a common quenching process. Consequently the prediction of the energy gap between these two excited states should be useful for programming highly luminescent Tb(III) systems. We report on a strategy based upon experimental and theoretical investigations of the excited state properties of a series of four simple aromatic hydroxamate ligands coordinated to Tb(III) and Gd(III) ions. By using previously reported crystallographic data, the structural and energies properties of these systems were investigated in the ground and first excited triplet states at the density functional theory (DFT) level of calculations. Our theoretical results are consistent with a triplet excited state T1 which is localised on one ligand only and whose the energy level is independent of the lanthanide ion nature (Tb(III), Gd(III)). A good agreement between the calculated adiabatic transition energies and experimental data derived from emission spectra is obtained when a corrective term is considered. These satisfactory results are an indication that this type of modelling can lead to discriminate in terms of the position of the lowest ligand triplet energy level the best antenna among a family of chromophoric compounds. In addition this theoretical approach has provided indications that the difference between the adiabatic transition energies of all the investigated complexes can be mainly explained by metal-ligand electrostatic interactions. The influence of the number of antennae on the quantum yield and the luminescence lifetime is discussed.

Lanthanum Does Form Stable Molecular Compounds in the +2 Oxidation State

Getting down to business: Reduction of the LaIII tricyclopentadienide complex [LaCp′′3] (Cp′′=η5-1,3-(SiMe3)2C5H3) by K and [18]crown-6 or [2,2,2]cryptand produced thermally stable mononuclear crystalline lanthanate(II) salts. The La +2 oxidation state in these complexes was confirmed both in solution (EPR) and the solid state (EPR, SQUID, X-ray diffraction) and was supported by a computational study.

Metallaboratranes Derived from a Triphosphanyl–Borane: Intrinsic C3 Symmetry Supported by a Z‐Type Ligand

Following the pioneering contributions of Knowles, Kagan, and Noyori, C1 and C2 chiral ligands have played a prominent role in asymmetric catalysis with transition-metal complexes. Over the last few years, increasing attention has been devoted to C3-symmetric derivatives, [2] and spectacular achievements have been reported using facially coordinating tripodal ligands assembled around a remote junction point (which does not enter the coordination sphere), an L-type coordination site, or an X-type coordination site. The trisoxazoline, trisamido, and triphosphane complexes A–C are archetypal examples of these three different situations (Scheme 1). Our interest in ambiphilic ligands that combine donor and acceptor moieties prompted us to investigate the ability of s-acceptor (Z-type) coordination sites to also support such a three-fold geometry. Accordingly, an L3Z tetradentate triphosphanyl–borane (TPB) ligand is reported herein to afford gold and platinum metallaboratranes D featuring dative M!B interactions and exhibiting C3 symmetry. [11–14] Such helical geometry has been shown to result from the tendency of the PCCBM metalla-

A two-step uncontracted determinantal effective Hamiltonian-based SO–CI method

We present a new two-step uncontracted spin-orbit configuration interaction (CI) method which automatically accounts for spin-orbit polarization effects on multiconfigurational wave functions by selecting the single excitations having a significant spin-orbit interaction with a chosen determinantal reference space. This approach is in the line of a conventional two-step method, as a sophisticated correlation treatment in a scalar relativistic approximation is carried out in the first step. In the second step, we define a model space which includes a set of reference configurations able to represent all the wanted states along with singly excited configurations selected with the spin-orbit (SO) operator. We then exploit the first-step calculation in order to include correlation effects via an effective Hamiltonian technique and diagonalize the full matrix on the determinantal basis. The method combines the advantages of both one-step and conventional two-step SO–CI methods; it intends to treat efficiently ...

Reducing CO2 to methanol using frustrated Lewis pairs : on the mechanism of phosphine-borane mediated hydroboration of CO2

The full mechanism of the hydroboration of CO2 by the highly active ambiphilic organocatalyst 1-Bcat-2-PPh2-C6H4 (Bcat = catecholboryl) was determined using computational and experimental methods. The intramolecular Lewis pair was shown to be involved in every step of the stepwise reduction. In contrast to traditional frustrated Lewis pair systems, the lack of steric hindrance around the Lewis basic fragment allows activation of the reducing agent while moderate Lewis acidity/basicity at the active centers promotes catalysis by releasing the reduction products. Simultaneous activation of both the reducing agent and carbon dioxide is the key to efficient catalysis in every reduction step.

Siloxides as Supporting Ligands in Uranium(III)‐Mediated Small‐Molecule Activation

Siloxides can support U! in the reduction of small molecules with uranium complexes. The treatment of [UN(SiMe3)23] with HOSi(OtBu)3 (3 equiv) yielded a novel homoleptic uranium(III) siloxide complex 1, which acted as a two-electron reducing agent toward CS 2 and CO2 (see scheme). Complex 1 also reduced toluene to afford a diuranium inverted-sandwich complex. Copyright

Gold–Silane and Gold–Stannane Complexes: Saturated Molecules as σ‐Acceptor Ligands

The discovery that saturated molecules may form s complexes by side-on coordination of a s bond to a transition metal represents a major breakthrough in transition-metal chemistry. Over the years, considerable progress has been made in the understanding of this bonding situation. The coordination and activation of s bonds involving Group 14 elements (E = C, Si, Ge, Sn, Pb) is at the forefront of developments in this area. An increasing variety of complexes A and B 7] (Scheme 1) featuring side-on coordinated s(E H) and s(E E) bonds have been isolated, and the key factors governing the delicate balance between dissociation and oxidative addition have been progressively identified. The common feature and believed prerequisite for the coordination of saturated molecules free of lone pairs to transition metals is the superposition of ligand!metal donation (from a filled s orbital of the ligand to an empty d orbital of the metal) and metal!ligand back-donation (from a filled d orbital of the metal to an empty s* orbital of the ligand). Aiming at identifying new types of metal–ligand interactions, and stimulated by our work on Group 13 Lewis acids as acceptor ligands, we recently became interested in complexes of type C, in which a saturated Group 14 element could behave as an end-on, s-acceptor ligand toward a transition metal. Heavier Group 14 elements such as silicon and tin are known to readily form hypervalent compounds through donor!acceptor interactions with organic Lewis bases. A related situation is envisioned in complexes C, with a transition metal acting as a Lewis base. Such donor! acceptor interactions between transition metals and silanes or stannanes have been invoked in a few highly strained complexes on the basis of relatively short M E distances. Furthermore, the presence of a Pd!Sn dative bond was recently evidenced structurally and theoretically in a palladastannatrane cage complex supported by four methimazolyl groups. Herein, we report the straightforward synthesis and complete characterization of three gold complexes supported by diphosphino silane and stannane ligands. The presence of metal!silane and metal!stannane interactions in these complexes has been substantiated spectroscopically, structurally, and theoretically, thus providing unambiguous evidence for the existence of complexes of type C. From our previous studies on Group 13 Lewis acids, the use of two phosphine buttresses ligated by ortho-phenylene spacers was considered as a good compromise in order to support, but not impose, the coordination of the Group 14 element. We thus targeted the two complexes [o{(iPr2P)C6H4}2E(Ph)FAuCl] 2 (E = Si) and 4 (E = Sn). The Scheme 1. Complexes A–C featuring Group 14 saturated molecules coordinated to transition metals (E= C, Si, Ge, Sn, Pb).

Rare-Earth Metal Alkyl and Hydride Complexes Stabilized by a Cyclen-Derived [NNNN] Macrocyclic Ancillary Ligand

A trinuclear rare-earth metal hydride complex was synthesized from the dialkyl complex supported by a monoanionic [NNNN] macrocycle and shown to catalyze the hydrosilylation of olefins efficiently.

Copper(I) complexes derived from mono- and diphosphino-boranes: Cu-->B interactions supported by arene coordination.

The monophosphino-boranes o-iPr(2)P(C(6)H(4))BR(2) (1: R = Ph and 3: R = Cy) and diphosphino-boranes [o-R(2)P(C(6)H(4))](2)BPh (5: R = Ph and 6: R = iPr) readily react with CuCl to afford the corresponding complexes {[o-iPr(2)P(C(6)H(4))BPh(2)]Cu(mu-Cl)}(2) 2, {[o-iPr(2)P(C(6)H(4))BCy(2)]Cu(mu-Cl)}(2) 4, {[o-Ph(2)P(C(6)H(4))](2)BPh}CuCl 7, and {[o-iPr(2)P(C(6)H(4))](2)BPh}CuCl 8. The presence of Cu-->B interactions supported by arene coordination within complexes 2, 7, and 8 has been unambiguously evidenced by NMR spectroscopy and X-ray diffraction studies. The unique eta(2)-BC coordination mode adopted by complexes 7 and 8 has been thoroughly analyzed by density-functional theory (DFT) calculations.