Barium phosphidoboranes and related calcium complexes

The attempted synthesis of [{Carb}BaPPh2] (1) showed this barium-phosphide and its thf adducts, 1·thf and 1·(thf)2, to be unstable in solution. Our strategy to circumvent the fragility of these compounds involved the use of phosphinoboranes HPPh2·BH3 and HPPh2·B(C6F5)3 instead of HPPh2. This allowed for the synthesis of [{Carb}Ae{PPh2·BH3}] (Ae = Ba, 2; Ca, 3), [{Carb}Ca{(H3B)2PPh2}·(thf)] (4), [{Carb}Ba{PPh2·B(C6F5)3}] (5), [{Carb}Ba{O(B(C6F5)3)CH2CH2CH2CH2PPh2}·thf] (6), [Ba{O(B(C6F5)3)CH2CH2CH2CH2PPh2}2·(thf)1.5] (7) and [Ba{PPh2·B(C6F5)3}2·(thp)2] (8) that were characterised by multinuclear NMR spectroscopy (thp = tetrahydropyran). The molecular structures of 4, 6 and 8 were validated by X-ray diffraction crystallography, which revealed the presence of Ba⋯F stabilizing interactions (ca. 9 kcal mol-1) in the fluorine-containing compounds. Compounds 6 and 7 were obtained upon ring-opening of thf by their respective precursors, 5 and the in situ prepared [Ba{PPh2·B(C6F5)3}2]n. By contrast, thp does not undergo ring-opening under the same conditions but affords clean formation of 8. DFT analysis did not highlight any specific weakness of the Ba-P bond in 1·(thf)2. The instability of this compound is instead thought to stem from the high energy of its HOMO, which contains the non-conjugated P lone pair and features significant nucleophilic reactivity.

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.

A Highly Lewis Acidic Strontium ansa-Arene Complex for Lewis Acid Catalysis and Isobutylene Polymerization

The potential of a dicationic strontium ansa‐arene complex for Lewis acid catalysis has been explored. The key to its synthesis was a simple salt metathesis from SrI₂ and 2 Ag[Al(OR^F)₄], giving the base‐free strontium‐perfluoroalkoxyaluminate Sr[Al(OR^F)₄]₂ (OR^F=OC(CF₃)₃). Addition of an ansa‐arene yielded the highly Lewis acidic, dicationic strontium ansa‐arene complex. In preliminary experiments, the complex was successfully applied as a catalyst in CO₂‐reduction to CH₄ and a surprisingly controlled isobutylene polymerization reaction.