A monomeric three-coordinate magnesium bis(amide)

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.

Lithium, Magnesium, and Zinc Centers N,N'-Chelated by an Amine-Amide Hybrid Ligand

The synthesis and structure of lithium, magnesium, and zinc complexes N,N'-chelated by a hybrid amine-amido ligand ([2-(Me₂NCH₂)C₆H₄NR]^-, abbreviated as L^NR, where R = H, SiMe₃, or Bn) are reported. The reaction of the least sterically demanding L^NH with various magnesium sources gives the hexameric imide [L^NMg]₆ (4) by the elimination of n-butane from L^NHMgⁿBu (2) or by the reaction of L^NHLi (1) with MeMgBr. [L^NH]₂Mg (3) is obtained through the addition of 0.5 equiv of ⁿBu₂Mg or Mg[N(SiMe₃)₂]₂ to L^NH₂ and with 1 equiv of ⁿBu₂Mg reacting to 2. Both L^NHMgN(SiMe₃)₂ (6) and isostructural L^NHZnN(SiMe₃)₂ (16) have been prepared using two different approaches: monodeprotonation of L^NH₂ by Zn/Mg[N(SiMe₃)₂]₂ in a 1:1 ratio or ligand substitution of 2 or L^NHZnEt (12) by 0.5 equiv of Sn[N(SiMe₃)₂]₂. The reactions of 2 or 3 with 1 provide the heterotrimetallic complex [L^NH]₄Li₂Mg (5). Benzyl- or trimethylsilyl-substituted anilines [L^N(SiMe₃)H (7) and L^N(Bn)H (8)] with 0.5 equiv of ⁿBu₂Mg allow the formation of homoleptic bis(amides) of the [L^N(R)]₂Mg type (10 and 11). Nevertheless, only the silylated secondary amine 7 is able to provide the heteroleptic n-butylmagnesium amide L^N(SiMe₃)MgⁿBu (9) upon reaction with an equimolar amount of ⁿBu₂Mg. Similarly, 12, [L^NH]₂Zn (13), L^N(R)ZnEt (17 and 18), and [L^N(R)]₂Zn [R = SiMe₃ (19) and Bn (20)] were prepared by the monodeprotonation of L^NH₂ or L^N(R)H using Et₂Zn in the corresponding stoichiometric ratio. L^NHZnI was prepared by the nucleophilic substitution of an ethyl chain in 12 by molecular iodine. A heterometallic complex, [L^NH]₄Li₂Zn (14), analogous to 5 was prepared from 12 or 13 with 1 or 2 equiv of 1, respectively.

Sterically shielded primary anilides of the alkaline-earth metals of the type (thf)nAe(NH-Ar*)2 (Ae = Mg, Ca, Sr, and Ba; Ar* = bulky aryl)

Metalation of 2,4,6-triphenylphenylamine (H₂N-C₆H₂-2,4,6-Ph₃, 1a) and 4-methyl-2,6-bis(diphenylmethyl)aniline (2,6-bis(diphenylmethyl)-p-toluidine, H₂N-C₆H₂-4-Me-2,6-(CHPh₂)₂, 2a) with dibutylmagnesium and Ae[N(SiMe₃)₂]₂ with a stoichiometric 1 : 2 ratio in THF at room temperature yields the corresponding primary anilides [(thf)_nAe{N(H)-C₆H₂-2,4,6-Ph₃}₂] (Ae/n = Mg/2 (1b), Ca/2 (1c), Sr/3 (1d), and Ba/3 (1e)) and [(thf)_nAe{N(H)-C₆H₂-4-Me-2,6-(CHPh₂)₂}₂] (Ae/n = Mg/2 (2b), Ca/3 (2c) and Sr/2 (2d)), respectively. The 1 : 1 reaction of Mg(n/sBu)₂ and MgPh₂ with 2a leads to the formation of heteroleptic [(thf)₂Mg(R){N(H)-C₆H₂-4-Me-2,6-(CHPh₂)₂}] (R = n/sBu (2bBu), Ph (2bPh)). At 50 °C, the strontium complex 2d liberates one equivalent of 2avia intramolecular deprotonation of the triarylmethyl functionality yielding dinuclear [(thf)₂Sr{N(H)-C₆H₂-4-Me-2-(CPh₂)-6-(CHPh₂)₂}]₂ (2d'). The barium compound is significantly more reactive and regardless of applied stoichiometry the isotypic barium congener [(thf)₂Ba{N(H)-C₆H₂-4-Me-2-(CPh₂)-6-(CHPh₂)₂}]₂ (2e') forms. The molecular structures of 1c, 2d, 2d', and 2e' are stabilized by metal-phenyl π-interactions.

Insight into the local environment of magnesium and calcium in low-coordination-number organo-complexes using 25Mg and 43Ca solid-state NMR: a DFT study

With the increasing number of organocalcium and organomagnesium complexes under development, there is a real need to be able to characterize in detail their local environment in order to fully rationalize their reactivity. For crystalline structures, in cases when diffraction techniques are insufficient, additional local spectroscopies like ²⁵Mg and ⁴³Ca solid-state NMR may provide valuable information to help fully establish the local environment of the metal ions. In this current work, a prospective DFT investigation on crystalline magnesium and calcium complexes involving low-coordination numbers and N-bearing organic ligands was carried out, in which the ²⁵Mg and ⁴³Ca NMR parameters [isotropic chemical shift, chemical shift anisotropy (CSA) and quadrupolar parameters] were calculated for each structure. The analysis of the calculated parameters in relation to the local environment of the metal ions revealed that they are highly sensitive to very small changes in geometry/distances, and hence that they could be used to assist in the refinement of crystal structures. Moreover, such calculations provide a guideline as to how the NMR measurements will need to be performed, revealing that these will be very challenging.

Structural Diversity in Alkali Metal and Alkali Metal Magnesiate Chemistry of the Bulky 2,6-Diisopropyl-N-(trimethylsilyl)anilino Ligand

Bulky amido ligands are precious in s-block chemistry, since they can implant complementary strong basic and weak nucleophilic properties within compounds. Recent work has shown the pivotal importance of the base structure with enhancement of basicity and extraordinary regioselectivities possible for cyclic alkali metal magnesiates containing mixed n-butyl/amido ligand sets. This work advances alkali metal and alkali metal magnesiate chemistry of the bulky arylsilyl amido ligand [N(SiMe3 )(Dipp)]- (Dipp=2,6-iPr2 -C6 H3 ). Infinite chain structures of the parent sodium and potassium amides are disclosed, adding to the few known crystallographically characterised unsolvated s-block metal amides. Solvation by N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA) or N,N,N',N'-tetramethylethylenediamine (TMEDA) gives molecular variants of the lithium and sodium amides; whereas for potassium, PMDETA gives a molecular structure, TMEDA affords a novel, hemi-solvated infinite chain. Crystal structures of the first magnesiate examples of this amide in [MMg{N(SiMe3 )(Dipp)}2 (μ-nBu)]∞ (M=Na or K) are also revealed, though these breakdown to their homometallic components in donor solvents as revealed through NMR and DOSY studies.

Assembly of Triangular Trimetallic Complexes by Triamidophosphine Ligands: Spin-Frustrated Mn2+ Plaquettes and Diamagnetic Mg2+ Analogues with a Combined Through-Space, Through-Bond Pathway for 31P-31P Spin−Spin Coupling

The reactions of 2 equiv of the ligand precursor P(CH2NHPh)3 or P[CH2NH-3,5-(CF3)2C6H3]3 with 3 equiv of Mn[N(SiMe3)2]2 provide high-yielding routes to the triangular trinuclear Mn(II) complexes [P(CH2NPh)3]2Mn3(THF)3.1.5THF and [P(CH2N-3,5-(CF3)2C6H3)3]2Mn3(THF)3. The solid-state structures of these paramagnetic complexes have approximate C3 symmetry. The magnetic moments from 300 to 1.8 K could be fit as a magnetic Jahn-Teller distorted isosceles triangle. These complexes exhibit spin frustration and possess an S = 1/2 ground state, as revealed by a plot of magnetization versus field at 1.8 K; at fields above 3.8 T, the occupation of an excited state with S = 3/2 becomes significant. The diamagnetic magnesium analogues were prepared by the reaction of the ligand precursor P(CH2NHPh)3, P[CH2NH-3,5-(CF3)2C6H3]3, or P(CH2NH-3,5-Me2C6H3)3 with nBu2Mg. The solid-state structures of [P(CH2NPh)3]2Mg3(THF)3.1.5THF and [P(CH2N-3,5-(CF3)2C6H3)3]2Mg3(THF)3 were determined. Solution 1H NMR spectroscopy was used to demonstrate that the solid-state structures are maintained in solution. The aryl group of the terminal amido donor exhibits slow rotation on the NMR time scale, and this was found to be an electronic effect. Solution 31P{1H} NMR spectroscopy revealed an unexpected 15 Hz coupling between phosphorus nuclei in these complexes. Calculations on a model complex using density functional theory demonstrates that this coupling occurs via a combined through-space, through-bond pathway.