Half-Sandwich Zirconium and Hafnium Amidoborane Complexes: Precursors of Hydride Derivatives

Half-sandwich zirconium(IV) and hafnium(IV) complexes with amidoborane and hydride ligands have been isolated in the stoichiometric reactions of mono(pentamethylcyclopentadienyl)metal alkyl and amido derivatives with the amine-boranes NHR2BH3 (R2 = H2, Me2, HtBu). Treatment of the tris(trimethylsilylmethyl) complexes [M(η5-C5Me5)(CH2SiMe3)3] with NH3BH3 (3 equiv) gives the seven-coordinate species [M(η5-C5Me5)(NH2BH3)3] (M = Zr (1), Hf (2)) with three κ2N,H-NH2BH3 ligands. The tris(neophyl) [M(η5-C5Me5)(CH2CMe2Ph)3] or tris(dimethylamido) [M(η5-C5Me5)(NMe2)3] derivatives react with NHMe2BH3 (≥3 equiv) to afford bis(dimethylamidoborane) hydride complexes [M(η5-C5Me5)H(NMe2BH3)2] (M = Zr (3), Hf (4)) via thermally unstable [M(η5-C5Me5)(NMe2BH3)3] species. The reaction of [M(η5-C5Me5)(NMe2)3] and NH2tBuBH3 (≥4 equiv) affords analogous mixed amidoborane hydride derivatives [M(η5-C5Me5)H(NHtBuBH3)(NMe2BH3)] (M = Zr (5), Hf (6)) with κ2N,H-NHtBuBH3 and κ3N,H,H-NMe2BH3 ligands. The addition of NHR2BH3 (≥1 equiv) on the mono(dimethylamido) complexes [M(η5-C5Me5)Cl2(NMe2)] in hexane leads to the precipitation of the ionic compounds [(NHR2)2BH2][{M(η5-C5Me5)Cl2}2(μ-H)3] (R2 = Me2, M = Zr (7), Hf (8); R2 = HtBu, M = Zr (9), Hf (10)). Molecular hydride species [Cl2(η5-C5Me5)M(μ-Cl)(μ-H)2M(η5-C5Me5)Cl(NH2tBu)] (M = Zr (11), Hf (12)) could be isolated from mixtures of complexes [M(η5-C5Me5)Cl2(NMe2)] and lower ratios of NH2tBuBH3. The zirconium complex 11 decomposes in solution to give the mononuclear tert-butylamido derivative [Zr(η5-C5Me5)Cl2(NHtBu)] (13) along with other byproducts.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Ammonia-Borane Derived BN Fragments Trapped on Bi- and Trimetallic Titanium(III) Systems

Titanium(III) complexes containing unprecedented (NH₂ BH₂ NHBH₃ )^2- and {N(BH₃ )₃ }^3- ligands have been isolated, and their structures elucidated by a combination of experimental and theoretical methods. The treatment of the trimethyl derivative [TiCp*Me₃ ] (Cp*=η⁵ -C₅ Me₅ ) with NH₃ BH₃ (3 equiv) at room temperature gives the paramagnetic dinuclear complex [{TiCp*(NH₂ BH₃ )}₂ (μ-NH₂ BH₂ NHBH₃ )], which at 80 °C leads to the trinuclear hydride derivative [{TiCp*(μ-H)}₃ {μ₃ -N(BH₃ )₃ }]. The bonding modes of the anionic BN fragments in those complexes, as well as the dimethylaminoborane group trapped on the analogous trinuclear [{TiCp*(μ-H)}₃ (μ₃ -H)(μ₃ -NMe₂ BH₂ )], have been studied by X-ray crystallography and density functional theory (DFT) calculations.

Amido Ca(ii) complexes supported by Schiff base ligands for catalytic cross-dehydrogenative coupling of amines with silanes

Heteroleptic Ca(ii) amides coordinated by multidentate phenolato ligands proved to be efficient catalysts for catalytic cross-dehydrogenative coupling of amines with silanes.

Group 4 Half-Sandwich Tris(trimethylsilylmethyl) Complexes: Thermal Decomposition and Reactivity with N,N-Dimethylamine-Borane

The thermal decomposition of group 4 trimethylsilylmethyl derivatives [M(η⁵-C₅Me₅)(CH₂SiMe₃)₃] (M = Ti (1), Zr (2), Hf (3)) in solution and their reactivity with N,N-dimethylamine-borane were investigated. Heating of hydrocarbon solutions of compounds 2 and 3 at 130-200 °C results in the elimination of SiMe₄ and the clean formation of the singular alkylidene-alkylidyne zirconium and hafnium compounds [{M(η⁵-C₅Me₅)}₃{(μ-CH)₃SiMe}(μ₃-CSiMe₃)] (M = Zr (4), Hf (5)). The reaction of 2 and 3 with NHMe₂BH₃ (≥1 equiv) at room temperature affords the dialkyl(dimethylamidoborane) complexes [M(η⁵-C₅Me₅)(CH₂SiMe₃)₂(NMe₂BH₃)] (M = Zr (6), Hf (7)). Compounds 6 and 7 are unstable in solution and decompose with formation of the alkyl(dimethylamino)borane [B(CH₂SiMe₃)H(NMe₂)] (8), SiMe₄, and other minor byproducts, including the tetranuclear zirconium(III) octahydride complex [{Zr(η⁵-C₅Me₅)}₄(μ-H)₈] (9) in the decomposition of 6. Addition of NHMe₂BH₃ to the titanium tris(trimethylsilylmethyl) derivative 1 gives the trinuclear mixed valence Ti(II)/Ti(III) tetrahydride complex [{Ti(η⁵-C₅Me₅)(μ-H)}₃(μ₃-H)(μ₃-NMe₂BH₂)] (10) at 45-65 °C. While the complete conversion of 1 under argon atmosphere requires excess NHMe₂BH₃ (up to 15 equiv), complex 10 is readily prepared with 3 equiv of NHMe₂BH₃ under a hydrogen atmosphere indicating that the formation of 10 involves hydrogenolysis of 1 in the presence of (NMe₂BH₂)₂. In absence of amine-borane, the reaction of 1 with H₂ leads to the tetranuclear titanium(III) octahydride [{Ti(η⁵-C₅Me₅)}₄(μ-H)₈] (11), which upon addition of NHMe₂BH₃ and subsequent heating at 65 °C affords complex 10. The X-ray crystal structures of 2, 4, 5, 10, and 11 were determined.

Calcium, Strontium and Barium Homogeneous Catalysts for Fine Chemicals Synthesis

The large alkaline earths (Ae), calcium, strontium and barium, have in the past 15 years yielded a brand new generation of heteroleptic molecular catalysts for the production of fine chemicals. However, the integrity of these complexes is often plagued by ligand redistribution equilibria in solution. This personal account retraces the paths followed in our research group towards the design of stable heteroleptic alkalino-earth complexes, including the use of intramolecular noncovalent Ae···H-Si and Ae···F-C interactions. Their implementation as homogenous precatalysts for reactions such as the intramolecular and intermolecular hydroamination and hydrophosphination of activated alkenes, the hydrophosphonylation of ketones, and the dehydrogenative coupling of amines and hydrosilanes that enable the efficient and controlled formations of CP, CN, or SiN σ-bonds, is presented in a synthetic perspective that highlights their overall outstanding catalytic performance.

Alkaline earths as main group reagents in molecular catalysis

The past decade has witnessed some remarkable advances in our appreciation of the structural and reaction chemistry of the heavier alkaline earth (Ae = Mg, Ca, Sr, Ba) elements. Derived from complexes of these metals in their immutable +2 oxidation state, a broad and widely applicable catalytic chemistry has also emerged, driven by considerations of cost and inherent low toxicity. The considerable adjustments incurred to ionic radius and resultant cation charge density also provide reactivity with significant mechanistic and kinetic variability as group 2 is descended. In an attempt to place these advances in the broader context of contemporary main group element chemistry, this review focusses on the developing state of the art in both multiple bond heterofunctionalisation and cross coupling catalysis. We review specific advances in alkene and alkyne hydroamination and hydrophosphination catalysis and related extensions of this reactivity that allow the synthesis of a wide variety of acyclic and heterocyclic small molecules. The use of heavier alkaline earth hydride derivatives as pre-catalysts and intermediates in multiple bond hydrogenation, hydrosilylation and hydroboration is also described along with the emergence of these and related reagents in a variety of dehydrocoupling processes that allow that facile catalytic construction of Si-C, Si-N and B-N bonds.

The significance of secondary interactions during alkaline earth-promoted dehydrogenation of dialkylamine-boranes

Reactions of anilidoimine magnesium n-butyl and calcium bis(trimethylsilyl)amide derivatives with Me₂NH·BH₃ at 25 °C resulted in the isolation of complexes containing [NMe₂BH₂NMe₂BH₃]⁻ and [NMe₂BH₃]⁻ anions respectively.

Dehydrocoupling routes to element–element bonds catalysed by main group compounds

This Tutorial Review focuses on recent applications of main group compounds in the catalytic synthesis of heteronuclear element–element bonds within the p-block.

s-Block amidoboranes: syntheses, structures, reactivity and applications

The highly versatile amidoborane compounds of the group 1 and 2 metals are reviewed, with an emphasis on their synthesis, structures and reactivity.

Two alternative approaches to access mixed hydride-amido zinc complexes: synthetic, structural and solution implications

In the presence of a bulky N-heterocyclic carbene, Zn(HMDS)₂can be converted in to either a mononuclear amido/hydride complex or a tetranuclear hydride rich cluster depending on the hydride source and reaction conditions employed.

Reactivity and catalytic activity of tert-butoxy-aluminium hydride reagents

The reactivity and catalytic activity of thetert-butoxy aluminium hydride reagents [(^tBuO)_xAlH_3−x] [x= 1 (1), 2 (2)] and (L)Li[(^tBuO)₂AlH₂] [L = THF (3), 1,4-dioxane (4)] were investigated. These reagents exhibit interesting reactivity and catalyse the dehydrocoupling reaction of the amine–borane Me₂NHBH₃into the ring compound [Me₂NBH₂]₂.

Heteroleptic alkyl and amide iminoanilide alkaline earth and divalent rare earth complexes for the catalysis of hydrophosphination and (cyclo)hydroamination reactions

[{N^N}M(X)(thf)n] alkyl (X=CH(SiMe3)2) and amide (X=N(SiMe3)2) complexes of alkaline earths (M=Ca, Sr, Ba) and divalent rare earths (Yb(II) and Eu(II) ) bearing an iminoanilide ligand ({N^N}(-)) are presented. Remarkably, these complexes proved to be kinetically stable in solution. X-ray diffraction studies allowed us to establish size-structure trends. Except for one case of oxidation with [{N^N}Yb(II){N(SiMe3)2}(thf)], all these complexes are stable under the catalytic conditions and constitute effective precatalysts for the cyclohydroamination of terminal aminoalkenes and the intermolecular hydroamination and intermolecular hydrophosphination of activated alkenes. Metals with equal sizes across alkaline earth and rare earth families display almost identical apparent catalytic activity and selectivity. Hydrocarbyl complexes are much better catalyst precursors than their amido analogues. In the case of cyclohydroamination, the apparent activity decreases with metal size: Ca>Sr>Ba, and the kinetic rate law agrees with R(CHA) =k[precatalyst](1)[aminoalkene](1). The intermolecular hydroamination and hydrophosphination of styrene are anti-Markovnikov regiospecific. In both cases, the apparent activity increases with the ionic radius (Ca<Sr<Ba) but the rate laws are different, and obey R(HA) =k[styrene](1)[amine](1)[precatalyst](1) and R(HP) =k[styrene](1)[HPPh2 ](0)[precatalyst](1), respectively. Mechanisms compatible with the rate laws and kinetic isotopic effects are proposed. [{N^N}Ba{N(SiMe3)2}(thf)2] (3) and [{N^N}Ba{CH(SiMe3)2}(thf)2] (10) are the first efficient Ba-based precatalysts for intermolecular hydroamination and hydrophosphination, and display activity values that are above those reported so far. The potential of the precatalysts for C-N and C-P bond formation is detailed and a rare cyclohydroamination-intermolecular hydroamination "domino" sequence is presented.

Molecular early main group metal hydrides: synthetic challenge, structures and applications

Within the general area of early main group metal chemistry, the controlled synthesis of well-defined metal hydride complexes is a rapidly developing research field. As group 1 and 2 metal complexes are generally highly dynamic and lattice energies for their [MH](∞) and [MH(2)](∞) salts are high, the synthesis of well-defined soluble hydride complexes is an obvious challenge. Access to molecular early main group metal hydrides, however, is rewarding: these hydrocarbon-soluble metal hydrides are highly reactive, have found use in early main group metal catalysis and are potentially also valuable molecular model systems for polar metal hydrides as a hydrogen storage material. The article focusses specifically on alkali and alkaline-earth metal hydride complexes and discusses the synthetic challenge, molecular structures, reactivity and applications.