Simple Access to the Heaviest Alkaline Earth Metal Hydride: A Strongly Reducing Hydrocarbon-Soluble Barium Hydride Cluster.

Reaction of Ba[N(SiMe3 )2 ]2 with PhSiH3 in toluene gave simple access to the unique Ba hydride cluster Ba7 H7 [N(SiMe3 )2 ]7 that can be described as a square pyramid spanned by five Ba2+ ions with two flanking BaH[N(SiMe3 )2 ] units. This heptanuclear cluster is well soluble in aromatic solvents, and the hydride 1 H NMR signals and coupling pattern suggests that the structure is stable in solution. At 95 °C, no coalescence of hydride signals is observed but the cluster slowly decomposes to undefined barium hydride species. The complex Ba7 H7 [N(SiMe3 )2 ]7 is a very strong reducing agent that already at room temperature reacts with Me3 SiCH=CH2 , norbornadiene, and ethylene. The highly reactive alkyl barium intermediates cannot be observed and deprotonate the (Me3 Si)2 N- ion, as confirmed by the crystal structure of Ba14 H12 [N(SiMe3 )2 ]12 [(Me3 Si)(Me2 SiCH2 )N]4 .

Accessing Stable Magnesium Acyl Compounds: Reductive Cleavage of Esters by Magnesium(I) Dimers.

The first examples of magnesium acyls, [(Nacnac)Mg{µ-C(Ph)O}(µ-OR)Mg(Nacnac)] (R=Me, tBu or Ph; Nacnac=[HC(MeCNAr)2 ]- ; Ar=C6 H2 Me3 -2,4,6 (Mes Nacnac), C6 H3 Et2 -2,6 (Dep Nacnac), C6 H3 iPr2 -2,6 (Dip Nacnac)), have been prepared by reductive cleavage of a series of esters using dimeric magnesium(I) reducing agents, [{(Nacnac)Mg}2 ]. Crystallographic studies reveal the complexes to be dimeric, being bridged by both phenyl-acyl and alkoxide/aryloxide fragments. The crystal structures, combined with results of spectroscopic and computational studies suggest that the nature of the acyl ligands within these complexes should be viewed as lying somewhere between anionic umpolung acyl and oxo-carbene. However, reactions of the acyl complexes with a variety of organic electrophiles did not provide evidence of umpolung acyl reactivity. A number of attempts to prepare alkoxide free magnesium acyls were carried out, and while these were unsuccessful, they did lead to unusual products, the crystallographic and spectroscopic details of which are discussed.

A Simple Route to Calcium and Strontium Hydride Clusters.

The first strontium hydride complex has been obtained by simply treating Sr[N(SiMe3 )2 ]2 with PhSiH3 in the presence of PMDTA. The Sr complex Sr6 H9 [N(SiMe3 )2 ]3 ⋅(PMDTA)3 crystallizes as an "inverse cryptand": an interstitial H- is surrounded by a Sr6 H84+ cage decorated with amide and PMDTA ligands. The analogous Ca complex could also be obtained and both retain their solid-state structures in solution: 1 H NMR spectra in C6 D6 show two doublets and one nonet (4:4:1). Up to 90 °C, no coalescence is observed. The Ca cluster was investigated by DFT calculations and shows atypically low charges on Ca (+1.14) and H (-0.59) which signifies an unexpectedly low ionicity. AIM analysis shows hydride⋅⋅⋅hydride bond paths with considerable electron densities in the bond critical point. The clusters thermally decompose into larger, undefined, metal hydride aggregates.

Self-Assembly of Magnesium Hydride Clusters Driven by Chameleon-Type Ligands.

While magnesium hydride complexes are generally stabilized by hard, bulky N-donor ligands, softer ligands with a broad variety of coordination modes are shown to efficiently adapt themselves to the large variety of Mg2+ centers in a growing magnesium hydride cluster. A P,N-chelating ligand is introduced that displays coordination modes between that of enamide, aza-allyl, and phosphinomethanide. Slight changes in the ligand bite angle have dramatic consequences for the structure type. The hitherto largest neutral magnesium hydride clusters are isolated either in a nonanuclear sheet-structure (brucite-type) or a dodecanuclear ring structure.

Highly Electron-Rich ß-Diketiminato Systems: Synthesis and Coordination Chemistry of Amino-Functionalized "N-nacnac" Ligands.

The synthesis of a class of electron-rich amino-functionalized ß-diketiminato (N-nacnac) ligands is reported, with two synthetic methodologies having been developed for systems bearing backbone NMe2 or NEt2 groups and a range of N-bound aryl substituents. In contrast to their (Nacnac)H counterparts, the structures of the protio-ligands feature the bis(imine) tautomer and a backbone CH2 group. Direct metalation with lithium, magnesium, or aluminium alkyls allows access to the respective metal complexes through deprotonation of the methylene function; in each case X-ray structures are consistent with a delocalized imino-amide ligand description. Transmetalation using lithium N-nacnac complexes is then exploited to access p- and f-block metal complexes, which allow for like-for-like benchmarking of the N-nacnac ligand family against their more familiar Nacnac counterparts. In the case of SnII , the degree of electronic perturbation effected by introduction of the backbone NR2 groups appears to be constrained by the inability of the amino group to achieve effective conjugation with the N2 C3 heterocycle. More obvious divergence from established structural norms is observed for complexes of the harder YbII ion, with azaallyl/imino and even azaallyl/NMe2 coordination modes being demonstrated by X-ray crystallography.

Mononuclear three-coordinate magnesium complexes of a highly sterically encumbered ß-diketiminate ligand.

The highly sterically encumbered chelating ß-diketiminate ligand, [HC{C(Me)N(2,6-CHPh2-4-MeC6H2)}2](-), (Ar)L(-), has been used to prepare a series of heteroleptic three-coordinate magnesium complexes. Both the bis(imine) and imine-enamine tautomers of the ligand precursor, (Ar)LH, as well as the diethyl ether adduct of the bromide complex [(Ar)LMgBr(OEt2)], the monomeric methyl complex [(Ar)LMgMe], the THF-solvated and unsolvated n-butylmagnesium complexes [(Ar)LMg(n)Bu(THF)] and [(Ar)LMg(n)Bu], and the 1-hexynyl analogue [(Ar)LMgC≡C(n)Bu] have been crystallographically characterized. Both n-butylmagnesium complexes showed remarkable stability in air, both in the solid state and in solution. Single crystals of the highly sensitive magnesium hydride, [(Ar)LMgH], underwent partial hydrolysis by solid-state water diffusion to the isostructural hydroxide compound [(Ar)LMgOH].