Bonding in Barium Boryloxides, Siloxides, Phenoxides and Silazides: A Comparison with the Lighter Alkaline Earths.

Barium complexes ligated by bulky boryloxides [OBR2 ]- (where R=CH(SiMe3 )2 , 2,4,6-i Pr3 -C6 H2 or 2,4,6-(CF3 )3 -C6 H2 ), siloxide [OSi(SiMe3 )3 ]- , and/or phenoxide [O-2,6-Ph2 -C6 H3 ]- , have been prepared. A diversity of coordination patterns is observed in the solid state for both homoleptic and heteroleptic complexes, with coordination numbers ranging between 2 and 4. The identity of the bridging ligand in heteroleptic dimers [Ba(µ2 -X1 )(X2 )]2 depends largely on the given pair of ligands X1 and X2 . Experimentally, the propensity to fill the bridging position increases according to [OB{CH(SiMe3 )2 }2 )]- <[N(SiMe3 )2 ]- <[OSi(SiMe3 )3 ]- <[O(2,6-Ph2 -C6 H3 )]- <[OB(2,4,6-i Pr3 -C6 H2 )2 ]- . This trend is the overall expression of 3 properties: steric constraints, electronic density and σ- and π-donating capability of the negatively charged atom, and ability to generate Ba ⋅ ⋅ ⋅ F, Ba ⋅ ⋅ ⋅ C(π) or Ba ⋅ ⋅ ⋅ H-C secondary interactions. The comparison of the structural motifs in the complexes [Ae{µ2 -N(SiMe3 )2 }(OB{CH(SiMe3 )2 }2 )]2 (Ae = Mg, Ca, Sr and Ba) suggest that these observations may be extended to all alkaline earths. DFT calculations highlight the largely prevailing ionic character of ligand-Ae bonding in all compounds. The ionic character of the Ae-ligand bond encourages bridging coordination, whereas the number of bridging ligands is controlled by steric factors. DFT computations also indicate that in [Ba(µ2 -X1 )(X2 )]2 heteroleptic dimers, ligand predilection for bridging vs. terminal positions is dictated by the ability to establish secondary interactions between the metals and the ligands.

Barium-Catalysed Dehydrocoupling of Hydrosilanes and Borinic Acids: A Mechanistic Insight.

Invited for the cover of this issue are the groups of Yann Sarazin at the University of Rennes and Laura Falivene at the King Abdullah University of Science and Technology. The image depicts two barium figures working in tandem to mimic the dinuclear nature of the active species in the catalytic cycle described in the manuscript. Read the full text of the article at 10.1002/chem.201904933.

Barium-Catalysed Dehydrocoupling of Hydrosilanes and Borinic Acids: A Mechanistic Insight.

Two very rare cases of barium boryloxides, the homoleptic [Ba(OB{CH(SiMe3 )2 }2 )2 ⋅C7 H8 ] and the heteroleptic [{LONO4 }BaOB{CH(SiMe3 )2 }2 ] stabilised by the multidentate aminoetherphenolate {LONO4 }- , are presented, and their structural properties are discussed. The electron-deficient [Ba(OB{CH(SiMe3 )2 }2 )2 ⋅C7 H8 ] shows, in particular, resilient η6 -coordination of the toluene molecule. Together with its amido parents [Ba{N(SiMe3 )2 }2 ⋅thf2 ] and [Ba{N(SiMe3 )2 }2 ]2 , this complex catalyses the fast and chemoselective dehydrocoupling of borinic acids R2 BOH and hydrosilanes HSiR'3 , yielding borasiloxanes R2 BOSiR'3 in a controlled fashion. The assessment of substrate scope indicates that, for now, the reaction is limited to bulky borinic acids. Kinetic analysis shows that the rate-limiting step of the catalytic manifold traverses a dinuclear transition state. A detailed mechanistic scenario is proposed on the basis of DFT computations, the results of which are fully consistent with experimental data. It consists of a stepwise process with rate-determining nucleophilic attack of a metal-bound O-atom onto the incoming hydrosilane, involving throughout dinuclear catalytically active species.

Lead(II) Siloxides.

Although PbII -siloxides have been known until now to decompose to oxoclusters, here [{ROF }PbOSi(SiMe3 )3 ] (ROF =fluoroalkoxide) and, most importantly, [Pb2 {OSi(SiMe3 )3 }{µ2 -OSi(SiMe3 )3 }3 ], are presented. The unusual bonding pattern in the latter thermally stable dinuclear complex was investigated by crystallographic and computational methods. Its stability is much higher than that of the simple, yet elusive "[Pb(OSiPh3 )2 ]", for which decomposition pathways towards [Pb4 (µ4 -O)(µ2 -OSiPh3 )6 ] have been clearly established.

Barium Siloxides and Catalysed Formation of Si-O-Si' Motifs.

The first unsupported barium siloxide, the homoleptic dimer [Ba2 {µ2 -OSi(SiMe3 )3 }3 {OSi(SiMe3 )3 }], is presented, and its structural features are discussed in the light of DFT computations. This complex, together with the related [Ba{µ2 -OSi(SiMe3 )3 }{N(SiMe3 )2 }]2 and their parent [Ba{N(SiMe3 )2 }2 ]2 , mediates the formation of asymmetric siloxanes R3 Si-O-SiR'3 through the first case of main group metal-mediated dehydrocoupling of silanols and hydrosilanes. Early kinetic analysis highlights an unusual catalytic manifold.

Barium complexes with crown-ether-functionalised amidinate and iminoanilide ligands for the hydrophosphination of vinylarenes.

The detailed multistep syntheses of two nitrogen-based sterically congested iminoanilidine and amidine proligands bearing a tethered 15-member aza-ether-crown macrocycle, namely {I^Acrown}H and {Amcrown}H, are reported. These proligands react with [Ba{N(SiMe2H)2}2·(thf)n] to generate the heteroleptic barium complexes [{I^Acrown}BaN(SiMe2H)2] (5) and [{Amcrown}BaN(SiMe2H)2] (6) in high yields. These complexes exhibit high coordination numbers (resp. eight and seven) and are in addition stabilised by mild BaH-Si interactions. Unusually for oxophilic elements such as barium, the amidinate ligand in 6 is only η1-coordinated. Complexes 5 and 6 mediate the intermolecular hydrophosphination of styrene with primary (PhPH2) and secondary (HPPh2) phosphines. Their catalytic performance compares favourably with those of other barium precatalysts for these reactions. During the course of the hydrophosphination of styrene with HPPh2 catalysed by 5, the phosphide complex [{I^Acrown}BaPPh2] (7) could be intercepted and crystallographically characterised.

Heterobimetallic Ba/Li and Ca/Li amides and diphenylmethanide.

The structures of the heterobimetallic Ca/Li and Ba/Li amides [CaLi2{µ2-N(SiMe2H)2}4]∞ and [Ba2Li2{µ2-N(SiMe2H)2}6]∞, stabilised by metalH-Si interactions, and that of a diphenylmethanide calcium salt, [Li(tmeda)2+·Ca(CHPh2)3(thf)-], are presented. These well-defined, storable compounds are inert towards Et2O and thf.

Low-Coordinate Barium Boryloxides: Synthesis and Dehydrocoupling Catalysis for the Production of Borasiloxanes.

The first soluble barium boryloxides [Ba]- OB{CH(SiMe3 )2 } are presented. These mono- or dinuclear complexes feature low coordination numbers, as low as two for [Ba(OB{CH(SiMe3 )2 }2 )2 ], which is further stabilized by intra- and intermolecular Ba⋅⋅⋅H3 C agostic interactions. Barium boryloxides and the parent [Ba{N(SiMe3 )2 }2 ⋅(thf)2 ] catalyze the dehydrocoupling of borinic acids with hydrosilanes, providing borasiloxanes under mild conditions.

Stable lead(ii) boroxides.

The first examples of lead(ii) boroxides, [Pb(OB{CH(SiMe3)2}2)2] (1) and [{N^C}PbOB{CH(SiMe3)2}2] (5; N^C = 2-Me2NCH2C6H4), were prepared via simple protocols. These structurally characterised compounds are stable and, unlike lead(ii) alkoxides and siloxides, do not trigger uncontrolled formation of lead(ii) oxoclusters.

Reactivity of Functionalized Ynamides with Tetracyanoethylene: Scope, Limitations and Optoelectronic Properties of the Adducts.

The reactivity of functionalized ynamides and arylynamines with tetracyanoethylene at room temperature was evaluated. In most cases, the corresponding 1,1,4,4-tetracyanobutadienes (TCBDs) were obtained in good to excellent yields through a [2+2]-cycloaddition/[2+2]-retro-electrocyclization sequence. The influence of diverse functional groups on the yield of the reaction was investigated, in particular concerning multiple ynamides. These TCBDs were characterized by various spectroscopic techniques and electrochemistry and X-ray diffraction in some cases.