Salt metathesis versus protonolysis routes for the synthesis of silylamide Hauser base (R2NMgX; X = halogen) and amido-Grignard (R2NMgR) complexes

A Blueprint for the Stabilization of Sub-Valent Alkaline Earth Complexes

The study of sub-valent Group 2 chemistry is a relatively new research field, being established in 2007 with the report of the first Mg(I) dimers. These species are stabilized by the formation of a Mg-Mg covalent bond; however, the extension of this chemistry to heavier alkaline earth (AE) metals has been frustrated by significant synthetic challenges, primarily associated with the instability of heavy AE-AE interactions. Here we present a new blueprint for the stabilization of heavy AE(I) complexes, based upon the reduction of AE(II) precursors with planar coordination geometries. We report the synthesis and structural characterisation of homoleptic trigonal planar AE(II) complexes of the monodentate amides {N(SiMe3 )2 }- and {N(Mes)(SiMe3 )}- . DFT calculations showed that the LUMOs of these complexes all show some d-character for AE = Ca-Ba. DFT analysis of the square planar Sr(II) complex [Sr{N(SiMe3 )2 }(dioxane)2 ]∞ revealed analogous frontier orbital d-character. AE(I) complexes that could be accessed by reduction of these AE(II) precursors were modelled computationally, revealing exergonic formation in all cases. Crucially, NBO calculations show that some d-character is preserved in the SOMO of theoretical AE(I) products upon reduction, showing that d-orbitals could play a crucial role in achieving stable heavy AE(I) complexes.

Synthesis, characterisation and reactivity of group 2 complexes with a thiopyridyl scorpionate ligand

Herein we report the reactivity of the proligand tris(2-pyridylthio)methane (HTptm) with various Alkaline Earth (AE) reagents: (1) dialkylmagnesium reagents and (2) AE bis-amides (AE = Mg-Ba). Heteroleptic complexes of general formulae [Mg(Tptm)(R)] (R = Me, ⁿBu; Tptm = {C(S-C₅H₄N)₃}^-) and [AE(Tptm)(N'')] (AE = Mg-Ba; N'' = {N(SiMe₃)₂}^-) were targeted from the reaction of HTptm with R₂Mg or [AE(N'')₂]₂. Reaction of the proligand with dialkylmagnesium reagents led to formation of [{Mg(κ³C,N,N-C{Bu}{S-C₅H₄N}₂)(μ-S-C₅H₄N)}₂] (1) and [{Mg(κ³C,N,N-C{Me}{S-C₅H₄N}₂)(μ-OSiMe₃)}₂] (2) respectively, as a result of a novel transfer of an alkyl group onto the methanide carbon with concomitant C-S bond cleavage. However, reactivity of bis-amide precursors for Mg and Ca did afford the target species [AE(Tptm)(N'')] (3-AE; AE = Mg-Ca), although these proved susceptible to ligand degradation processes. DFT calculations show that alkyl transfer in the putative [Mg(Tptm)(ⁿBu)] (1m') system and amide transfer in 3-Ca is a facile process that induces C-S bond cleavage in the Tptm ligand. 3-Mg and 3-Ca were also tested as catalysts for the hydrophosphination of selected alkenes and alkynes, including the first example of mono-hydrophosphination of 4-ethynylpyridine which was achieved with high conversions and excellent regio- and stereochemical control.

Structural Investigation of Magnesium Complexes Supported by a Thiopyridyl Scorpionate Ligand

Herein, we report the synthesis of a series of heteroleptic magnesium complexes stabilized with the scorpionate ligand tris(2-pyridylthio)methanide (Tptm). The compounds of the general formula [Mg(Tptm)(X)] (1-X; X = Cl, Br, I) were obtained via protonolysis reaction between the proligand and selected Grignard reagents. Attempts to isolate the potassium derivative K(Tptm) lead to decomposition of Tptm and formation of the alkene (C₅H₄N-S)₂C=C(C₅H₄N-S)₂, and this degradation was also modelled using DFT methods. Compound 1-I was treated with K(CH₂Ph), affording the degradation product [Mg(Bptm)₂] (2; Bptm = {CH(S-C₅NH₃)₂}⁻). We analyzed and quantified the steric properties of the Tptm ligand using the structural information of the compounds obtained in this study paired with buried volume calculations, also adding the structural data of HTptm and its CF₃-substituted congener (HTptm^CF3). These studies highlight the highly flexible nature of this ligand scaffold and its ability to stabilize various coordination motifs and geometries, which is a highly desirable feature in the design of novel organometallic reagents and catalysts.

Sodium Hexamethyldisilazide: Using 15N-29Si Scalar Coupling to Determine Aggregation and Solvation States

²⁹Si NMR spectroscopy, the method of continuous variations, and density functional theory computations show that sodium hexamethyldisilazide (NaHMDS) is a disolvated dimer in toluene, a mixture of disolvated dimer and tetrasolvated monomer in THF/toluene, and exclusively monomer in neat THF. The dioxane-solvated NaHMDS only partially deaggregates to monomer even in neat dioxane. ¹⁵N-²⁹Si coupling constants and ²⁹Si chemical shifts show a high and dependable correlation with the aggregation state. Monitoring either chemical shift or coupling constant versus THF concentration even in the high-temperature, rapid-exchange limit affords the solvation numbers consistent with DFT computations. The preparation of ¹⁵N-labeled NaHMDS has been improved.

Reactivity of the carbodiphosphorane, (Ph3P)2C, towards main group metal alkyl compounds: coordination and cyclometalation

The carbodiphosphorane, (Ph3P)2C, reacts with Me3Al and Me3Ga to afford the adducts, [(Ph3P)2C]AlMe3 and [(Ph3P)2C]GaMe3, which have been structurally characterized by X-ray diffraction. (Ph3P)2C also reacts with Me2Zn and Me2Cd to generate an adduct but the formation is reversible on the NMR time scale. At elevated temperatures, however, elimination of methane and cyclometalation occurs to afford [κ2-Ph3PC{PPh2(C6H4)}]ZnMe and [κ2-Ph3PC{PPh2(C6H4)}]CdMe. Analogous cyclometalated products, [κ2-Ph3P{CPPh2(C6H4)}]ZnN(SiMe3)2 and [κ2-Ph3P{CPPh2(C6H4)}]CdN(SiMe3)2, are also obtained upon reaction of (Ph3P)2C with Zn[N(SiMe3)2]2 and Cd[N(SiMe3)2]2. The magnesium compounds, Me2Mg and {Mg[N(SiMe3)2]2}2, likewise react with (Ph3P)2C to afford cyclometalated derivatives, namely [κ2-Ph3PC{PPh2(C6H4)}]MgN(SiMe3)2 and {[κ2-Ph3PC{PPh2(C6H4)}]MgMe}2. While this reactivity is similar to the zinc system, the magnesium methyl complex is a dimer with bridging methyl groups, whereas the zinc complex is a monomer. The greater tendency of the methyl groups to bridge magnesium centers rather than zinc centers is supported by density functional theory calculations.

Structural Characterization of Lithium and Sodium Bulky Bis(silyl)amide Complexes

Alkali metal amides are vital reagents in synthetic chemistry and the bis(silyl)amide {N(SiMe₃)₂} (N′′) is one of the most widely-utilized examples. Given that N′′ has provided landmark complexes, we have investigated synthetic routes to lithium and sodium bis(silyl)amides with increased steric bulk to analyse the effects of R-group substitution on structural features. To perform this study, the bulky bis(silyl)amines {HN(Si^tBuMe₂)(SiMe₃)}, {HN(SiⁱPr₃)(SiMe₃)}, {HN(Si^tBuMe₂)₂}, {HN(SiⁱPr₃)(Si^tBuMe₂)} and {HN(SiⁱPr₃)₂} (1) were prepared by literature procedures as colourless oils; on one occasion crystals of 1 were obtained. These were treated separately with ⁿBuLi to afford the respective lithium bis(silyl)amides [Li{μ-N(Si^tBuMe₂)(SiMe₃)}]₂ (2), [Li{μ-N(SiⁱPr₃)(SiMe₃)}]₂ (3), [Li{N(Si^tBuMe₂)₂}{μ-N(Si^tBuMe₂)₂}Li(THF)] (4), [Li{N(SiⁱPr₃)(Si^tBuMe₂)}(DME)] (6) and [Li{N(SiⁱPr₃)₂}(THF)] (7) following workup and recrystallization. On one occasion during the synthesis of 4 several crystals of the ‘ate’ complex [Li₂{μ-N(Si^tBuMe₂)₂}(μ-ⁿBu)]₂ (5) formed and a trace amount of [Li{N(SiⁱPr₃)₂}(THF)₂] (8) was identified during the recrystallization of 7. The reaction of {HN(Si^tBuMe₂)₂} with NaH in the presence of 2 mol % of NaO^tBu gave crystals of [Na{μ-N(Si^tBuMe₂)₂}(THF)]₂ (9-THF), whilst [Na{N(SiⁱPr₃)₂}(C₇H₈)] (10) was prepared by deprotonation of 1 with ⁿBuNa. The solid-state structures of 1⁻10 were determined by single crystal X-ray crystallography, whilst 2⁻4, 7, 9 and 10 were additionally characterized by NMR and FTIR spectroscopy and elemental microanalysis.

Salt metathesis routes to homoleptic near-linear Mg(ii) and Ca(ii) bulky bis(silyl)amide complexes

A series of bulky bis(silyl)amide ligands have been utilised to stabilise rare examples of formally 2-coordinate linear or near-linear Mg and Ca complexes.

Redox transmetallation approaches to the synthesis of extremely bulky amido-lanthanoid(ii) and -calcium(ii) complexes

Redox transmetallation protolysis and direct redox transmetallation reactions have been employed to access a variety of extremely bulky amido-lanthanoid(ii), and related calcium(ii) complexes which cannot be prepared using classical salt metathesis pathways.