Tw oseries of bulkyalkaline earth (Ae) metal amide complexes have been prepared:A e[N(TRIP) 2 ] 2 (1-Ae) and Ae[N(TRIP)(DIPP)] 2 (2-Ae) (Ae = Mg, Ca, Sr,B a; TRIP = SiiPr 3 ,D IPP = 2,6-diisopropylphenyl). While monomeric 1-Ca was already known, the new complexes have been structurally characterized.M onomers 1-Ae are highly linear while the monomers 2-Ae are slightly bent. The bulkier amide complexes 1-Ae are by far the most active catalysts in alkene hydrogenation with activities increasing from Mg to Ba. Catalyst 1-Ba can reduce internal alkenes like cyclohexene or 3-hexene and highly challenging substrates like 1-Me-cyclohexene or tetraphenylethylene.I ti sa lso active in arene hydrogenation reducing anthracene and naphthalene (even when substituted with an alkyl) as well as biphenyl. Benzene could be reduced to cyclohexane but full conversion was not reached. The first step in catalytic hydrogenation is formation of an (amide)AeH species,w hich can form larger aggregates. Increasing the bulk of the amide ligand decreases aggregate sizeb ut it is unclear what the true catalyst(s) is (are). DFT calculations suggest that amide bulk also has an oticeable influence on the thermodynamics for formation of the (amide)AeH species.C omplex 1-Ba is currently the most powerful Ae metal hydrogenation catalyst. Due to tremendously increased activities in comparison to those of previously reported catalysts,t he substrate scope in hydrogenation catalysis could be extended to challenging multi-substituted unactivated alkenes and even to arenes among which benzene. Scheme 4. Energy profiles (DH in kcal mol À1 )for a) the hydrogenation of ethylene by catalysts 1-Ca(orange), 1-Ba (black) and CaN'' 2 (red), and b) benzene hydrogenation by 1-Ba;B3PW91/def2tzvpp including correction for dispersion (GD3BJ) and solvent (PCM = benzene).
The alkene transfer hydrogenation (TH) of a variety of alkenes has been achieved with simple AeN′′2 catalysts [Ae=Ca, Sr, Ba; N′′=N(SiMe3)2] using 1,4‐cyclohexadiene (1,4‐CHD) as a H source. Reaction of 1,4‐CHD with AeN′′2 gave benzene, N′′H, and the metal hydride species N′′AeH (or aggregates thereof), which is a catalyst for alkene hydrogenation. BaN′′2 is by far the most active catalyst. Hydrogenation of activated C=C bonds (e.g. styrene) proceeded at room temperature without polymer formation. Unactivated (isolated) C=C bonds (e.g. 1‐hexene) needed a higher temperature (120 °C) but proceeded without double‐bond isomerization. The ligands fully control the course of the catalytic reaction, which can be: 1) alkene TH, 2) 1,4‐CHD dehydrogenation, or 3) alkene polymerization. DFT calculations support formation of a metal hydride species by deprotonation of 1,4‐CHD followed by H transfer. Convenient access to larger quantities of BaN′′2, its high activity and selectivity, and the many advantages of TH make this a simple but attractive procedure for alkene hydrogenation.
Heteroleptic alkaline earth metal (Ae = Ca, Sr, Ba) amide complexes with the superbulky β-diketiminate ligand DIPePBDI (CH[C(Me)N-DIPeP]2, DIPeP = 2,6-di-iso-pentylphenyl) have been prepared by direct deprotonation of DIPePBDI-H with either AeN′′2 or AeN′′2·(THF)2 (N′′ = N(SiMe3)2). Despite long reaction times of 5–14 days, this convenient one-step synthetic method has the major advantage that metal-pure products are obtained in generally quantitative yields. All (DIPePBDI)AeN′′ and (DIPePBDI)AeN′′·THF complexes are monomeric and stabilized by agostic metal···Me3Si and metal···iso-pentyl interactions. They are highly soluble in toluene and indefinitely stable toward ligand scrambling, even after 2 weeks at 140 °C. The same series with the smaller DIPPBDI ligand (CH[C(Me)N-DIPP]2, DIPP = 2,6-di-iso-propylphenyl) could, except for Ca, also be prepared by direct ligand deprotonation. The (DIPPBDI)CaN′′ and (DIPPBDI)CaN′′·THF complexes are stable toward ligand exchange up to 110 °C. Whereas THF-free (DIPPBDI)SrN′′ and (DIPPBDI)BaN′′ decompose at 50 and 20 °C, respectively, their THF adducts were found to be stable up to 60 °C. This is, however, strongly dependent on complex purity. Slight hydrolysis or contamination with KN′′ accelerates ligand scrambling. Therefore, partial hydrolysis and salt metathesis routes that involve KN′′ should be avoided when synthesizing heteroleptic complexes of the heavier Ae metals.
One-pot reaction of 2,6-iPr 2 -aniline (DIPP-NH 2 ) with (Me 2 SiO) 3 and Sr[N(SiMe 3 ) 2 ] 2 (SrN′′ 2 ) gave a tetranuclear cluster consisting of four dianions [OSiMe 2 N-DIPP] 2and four Sr 2+ ions solvated each by one THF ligand. The general applicability of this method was investigated by variation of amine and metal. Anilines with smaller substituents led to insoluble uncharacterized coordination polymers, whereas bulkier anilines gave soluble product mixtures that could not be purified. Primary alkylamines neither led to isolable products. Introduction of a tBu group in para-position of DIPP-NH 2 , however, gave an isostruc- [a]
The first intermolecular early main group metal–alkene complexes were isolated. This was enabled by using highly Lewis acidic Mg centers in the Lewis base‐free cations (MeBDI)Mg+ and (tBuBDI)Mg+ with B(C6F5)4− counterions (MeBDI=CH[C(CH3)N(DIPP)]2, tBuBDI=CH[C(tBu)N(DIPP)]2, DIPP=2,6‐diisopropylphenyl). Coordination complexes with various mono‐ and bis‐alkene ligands, typically used in transition metal chemistry, were structurally characterized for 1,3‐divinyltetramethyldisiloxane, 1,5‐cyclooctadiene, cyclooctene, 1,3,5‐cycloheptatriene, 2,3‐dimethylbuta‐1,3‐diene, and 2‐ethyl‐1‐butene. In all cases, asymmetric Mg–alkene bonding with a short and a long Mg−C bond is observed. This asymmetry is most extreme for Mg–(H2C=CEt2) bonding. In bromobenzene solution, the Mg–alkene complexes are either dissociated or in a dissociation equilibrium. A DFT study and AIM analysis showed that the C=C bonds hardly change on coordination and there is very little alkene→Mg electron transfer. The Mg–alkene bonds are mainly electrostatic and should be described as Mg2+ ion‐induced dipole interactions.
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