Metal compounds of boron-substituted alkoxide (‘boroxide’) ligands

Lithium, Tin(II), and Zinc Amino-Boryloxy Complexes: Synthesis and Characterization

Analogous to the ubiquitous alkoxide ligand, metal boroxide and boryloxy complexes are an underexplored class of hard anionic O- ligand. A new series of amine-stabilized Li, Sn(II), and Zn boryloxy complexes, comprising electron-rich tetrahedral boron centers have been synthesized and characterized. All complexes have been characterized by one-dimensional (1D), two-dimensional (2D), and DOSY NMR, which are consistent with the solid-state structures unambiguously determined via single-crystal X-ray diffraction. Electron-rich μ2- (Sn and Zn) and μ3- (Li) boryloxy binding modes are observed. Compounds 6-9 are the first complexes of this class, with the chelating bis- and tris-phenol ligands providing a scaffold that can be easily functionalized and provides access to the boronic acid pro-ligand, hence allowing facile direct synthesis of the resulting compounds. Computational quantum chemical studies suggest a significant enhancement of the π-donor ability of the amine-stabilized boryloxy ligand because of electron donation from the amine functionality into the p-orbital of the boron atom.

Reactivity Studies on Lanthanum and Cerium Hydrido Metallocenes

The alkyl complexes [5-1,3-(Me3C)2C5H3]2Ln(CH2C6H4-o-NMe2) (Ln = La 1, Ce 2) react with 9-borabicyclo[3.3.1]nonane (9-BBN) in THF to afford the lanthanum boroxide complex [5-1,3-(Me3C)2C5H3]2La(µ-OBC8H14)(THF) (3) and cerium boroxide complex [5-1,3-(Me3C)2C5H3]2Ce(µ-OBC8H14)(THF) (4)....

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.

Applications of boroxide ligands in supporting small molecule activation by U(iii) and U(iv) complexes

The boroxide ligand [OBAr2]- (Ar = Mes, Trip) is shown to be able to support both UIII and UIV centres for the first time. The synthesis and structures of homoleptic and heteroleptic UIII and UIV complexes are reported. The UX3 complex with larger substituents, [U(OBTrip2)3]2, exhibits greater thermal stability compared to less encumbered [U(OBMes2)3]2 but reacts with a smaller range of the small molecules tested to date. Initial studies on their capacity to participate in small molecule chemistry show that dark purple [U(OBMes2)3]2 binds and/or reductively activates a variety of small molecules such as pyridine-oxide, triphenylphosphineoxide, sulfur, and dicyclohexylcarbodiimide. While [U(OBMes2)3]2 shows no reaction with CO or CO2, [U(OBTrip2)3]2 is oxidised by both, in the former case forming [U(OBTrip2)4], and in the latter case forming a small quantity of the structurally characterised μ-carbonate product [(μ-CO3){U(OBTrip2)3}2].

An N-Heterocyclic Boryloxy Ligand Isoelectronic with N-Heterocyclic Imines: Access to an Acyclic Dioxysilylene and its Heavier Congeners

Introduced here is a new type of strongly donating N-heterocyclic boryloxy (NHBO) ligand, [(HCDippN)₂ BO]^- (Dipp=2,6-diisopropylphenyl), which is isoelectronic with the well-known N-heterocyclic iminato (NHI) donor class. This 1,3,2-diazaborole functionalized oxy ligand has been used to stabilize the first acyclic two-coordinate dioxysilylene and its Ge, Sn, and Pb congeners, thereby presenting the first complete series of heavier group 14 dioxycarbene analogues. All four compounds have been characterized by X-ray crystallography and density-functional theory, enabling analysis of periodic trends: the potential for the [(HCDippN)₂ BO]^- ligand to subtly vary its electronic-donor capabilities is revealed by snapshots showing the gradual evolution of arene π coordination on going from Si to Pb.

Diradical Anion of Potassium Aggregate: Reduction of Dimer Boroxide Complex

Crystalline aggregates containing metal cation clusters "wrapped" by reduced hydrocarbon anions have been presented. Initially, a dimeric complex (2) possessing a bimetallic K₂O₂ core was synthesized from the reaction between an anthracene substituted boronic acid (2-(anthracen-9-yl)phenyl)(hydroxy)(mesityl)borane (1-H) and KN(SiMe₃)₂ in THF solution. The B-O bond length (1.281(4) Å) in complex 2 is comparable to those observed in oxoboranes, indicating this may be a double bond, which is supported by its Wiberg bond order (1.9) predicted by density functional theory calculations. Subsequently, potassium aggregate complexes, diradical 3 and dihydro anion 4, were obtained through the reduction reactions of dimeric complex 2 and 1-H, respectively. They exhibit similar K₃O₂B₂ aggregate structures, but differ significantly in the geometry of the anthracene units. Complex 3 features a triplet diradical character with planar anthracene units that carry the unpaired electrons, while the anthracene groups in complex 4 display a puckered structure due to the addition of hydrogen atoms.

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.