Magnesium Hydride Complexes Research Articles

Deoxygenative Coupling of CO with a Tetrametallic Magnesium Hydride Complex.

Addition of CO to a tetrametallic magnesium hydride cluster results in both carbon-carbon bond formation and deoxygenation to generate an acetaldehyde enolate [C2OH3]- which remains coordinated to the cluster. To the best of our knowledge, this is the first example of formation of an isolable complex containing an [C2OH3]- fragment from reaction of CO with a metal hydride, and the first example of CO homologation and deoxygenation at a main group metal. DFT studies suggest that key steps in the mechanism involve nucleophilic attack of an oxymethylene on a formyl ligand to generate an unstable [C2O2H3]3- fragment, which undergoes subsequent deoxygenation.

Deoxygenative Coupling of CO with a Tetrametallic Magnesium Hydride Complex

AbstractAddition of CO to a tetrametallic magnesium hydride cluster results in both carbon‐carbon bond formation and deoxygenation to generate an acetaldehyde enolate [C2OH3]− which remains coordinated to the cluster. To the best of our knowledge, this is the first example of formation of an isolable complex containing an [C2OH3]− fragment from reaction of CO with a metal hydride, and the first example of CO homologation and deoxygenation at a main group metal. DFT studies suggest that key steps in the mechanism involve nucleophilic attack of an oxymethylene on a formyl ligand to generate an unstable [C2O2H3]3− fragment, which undergoes subsequent deoxygenation.

A Convenient One-Pot Synthesis of a Sterically Demanding Aniline from Aryllithium Using Trimethylsilyl Azide, Conversion to β-Diketimines and Synthesis of a β-Diketiminate Magnesium Hydride Complex.

This work reports the one-pot synthesis of sterically demanding aniline derivatives from aryllithium species utilising trimethylsilyl azide to introduce amine functionalities and conversions to new examples of a common N,N'-chelating ligand system. The reaction of TripLi (Trip = 2,4,6-iPr3-C6H2) with trimethylsilyl azide afforded the silyltriazene TripN2N(SiMe3)2 in situ, which readily reacts with methanol under dinitrogen elimination to the aniline TripNH2 in good yield. The reaction pathways and by-products of the system have been studied. The extension of this reaction to a much more sterically demanding terphenyl system suggested that TerLi (Ter = 2,6-Trip2-C6H3) slowly reacted with trimethylsilyl azide to form a silyl(terphenyl)triazenide lithium complex in situ, predominantly underwent nitrogen loss to TerN(SiMe3)Li in parallel, which afforded TerN(SiMe3)H after workup, and can be deprotected under acidic conditions to form the aniline TerNH2. TripNH2 was furthermore converted to the sterically demanding β-diketimines RTripnacnacH (=HC{RCN(Trip)}2H), with R = Me, Et and iPr, in one-pot procedures from the corresponding 1,3-diketones. The bulkiest proligand was employed to synthesise the magnesium hydride complex [{(iPrTripnacnac)MgH}2], which shows a distorted dimeric structure caused by the substituents of the sterically demanding ligand moieties.

Synthesis and Characterization of Magnesium-Hydride Complexes with Alkylene-Bridged Bis(β-diketiminate) Ligands

Synthesis and Characterization of Magnesium-Hydride Complexes with Alkylene-Bridged Bis(β-diketiminate) Ligands

A tetranuclear magnesium hydride cluster with a four-coordinate hydride in near square-planar geometry.

A tetranuclear magnesium hydride complex based on the β-oxo-δ-diiminate (BODDI) ligand was isolated. In the solid state, the complex features three bridging μ2-hydrides and one unprecedented four-coordinate hydride in a nearly square-planar coordination environment. In the solid state, two different coordination polymers are observed depending on whether the axial or equatorial oxygen lone pairs of 1,4-dioxane are engaged in bonding to magnesium.

Hydroboration of CO2 catalyzed by heteroscorpionate zwitterionic zinc and magnesium hydride complexes.

The heteroscorpionate zinc hydride complex LZnH 2, (L = (MePz)2CP(Ph)2NPh, MePz = 3,5-dimethylpyrazolyl), its formate complex 3, and magnesium hydride complex LMgH 5 with the same ligand were synthesized and detected for the catalytic hydroboration reaction of CO2. With BH3·SMe2 as the reductant, zinc-based hydride complex 2 and formate complex 3 show a similar capability of hydroboration of CO2, featuring excellent reactivity and selectivity. The conversion of BH3·SMe2 reached 84%, the highest TON of 252 compared to other zinc catalysts was achieved at room temperature and borate ester products at reduction levels of CH3OH were obtained. Magnesium-based hydride complex 5 showed inferior activity for the hydroboration reduction of CO2.

Stabilizing Magnesium Hydride Complexes with Neutral Ligands

The reaction of Mg[N(SiMe3)2]2 with PhSiH3 in benzene at room temperature gave a white precipitate of approximate constitution MgH1.5N′′0.5. This undefined hydride rich Mg salt can be dissolved by addition of well‐known neutral donors, such as PMDTA, DABCO or the N‐heterocyclic carbene IMeiPr (N,N′‐diisopropyl‐2,3‐dimethylimidazol‐2‐ylidine). This allowed for crystallization and structural characterization of five new magnesium hydride complexes: Mg2(µ‐H)2N′′2·(PMDTA) (1), Mg4(µ‐H)6N′′2·(PMDTA)2 (2), Mg5(µ‐H)7N′′3·(PMDTA)2 (3), Mg2(µ‐H)2N′′2·(DABCO)2 (4) and Mg2(µ‐H)2N′′2·(IMeiPr)2 (5). The PMDTA complexes 1–3 were only isolated as mixtures that could not be separated but complexes 4 and 5 have been fully characterized. The structures of the Mg hydride complexes 1, 4 and 5 are reproduced well by DFT calculations (B3PW91/6‐311++G**). Bonding between Mg and hydride is largely ionic (circa 80 %) and there is very little charge transfer from the neutral donor to Mg2+. The calculated solvation enthalpies (ΔHsolv) per ligand are –12.4 kcal/mol (PMDTA), –15.9 kcal/mol (DABCO) and –20.8 kcal/mol (IMeiPr). In complexes 4 and 5 Bond‐Critical‐Points are found between the two negatively charged hydride ligands. The very high electron densities of 0.15–0.16 e/Å3 in these unusual BCP's are related to the short H–···H– distances of circa 2.55 Å.

Role of Alkaline-Earth Metal-Catalyst: A Theoretical Study of Pyridines Hydroboration.

Density functional theory (DFT) calculations have been performed to investigate the mechanism of alkaline-earth-metal-catalyzed hydroboration of pyridines with borane. In this reaction, the active catalytic species is considered to be an alkaline earth metal hydride complex when the corresponding alkaline earth metal is used as the catalyst. The theoretical results reveal that initiation of the catalytic cycle is hydride transfer to generate a magnesium hydride complex when β-diimine alkylmagnesium is used as a pre-catalyst. The magnesium hydride complex can undergo coordination of the pyridine reactant followed by hydride transfer to form a dearomatized magnesium pyridine intermediate. Coordination of borane and hydride transfer from borohydride to magnesium then give the hydroboration product and regenerate the active magnesium hydride catalyst. The rate-determining step of the catalytic cycle is hydride transfer to pyridine with a free energy barrier of 29.7 kcal/mol. Other alkaline earth metal complexes, including calcium and strontium complexes, were also considered. The DFT calculations show that the corresponding activation free energies for the rate-determining step of this reaction with calcium and strontium catalysts are much lower than with the magnesium catalyst. Therefore, calcium and strontium complexes can be used as the catalyst for the reaction, which could allow mild reaction conditions.

Low-nuclearity magnesium hydride complexes stabilized by N-heterocyclic carbenes.

Herein we report the synthesis and characterization of dinuclear magnesium-hydride complexes, [(IiPrMe2)Mg(μ-H)(HMDS)]2 [4, IiPrMe2 = N,N'-diisopropyl-2,3-dimethylimidazol-2-ylidine, HMDS = bis(trimethylsilyl)amide] and [(IiPrMe2)Mg(μ-H)(ASCP)]2 (5, ASCP = 2,2,5,5-tetramethyl-2,5-disila-1-azacyclopent-1-yl). Compounds 4 and 5 exhibit the lowest nuclearity of any carbene-magnesium hydride complex to date.

Regioselective Ring Opening Reactions of Pyridine N-Oxide Analogues by Magnesium Hydride Complexes

The stoichiometric reactions of phosphinimino-amino (PIA)-supported magnesium hydride complex 1, [L1MgH]2 (L1 = (2,6-iPr2-C6H3)NC(Me)CHP(Cy2)N(2,6-Me2-C6H3)), with pyridine N-oxide and 2-phenylpyridine N-oxide afforded 2,4-pentadiene-1-oximate complex 2 and 5-phenyl-2,4-pentadiene-1-oximate complex 3, respectively. The reaction of 1 with 2-methylpyridine N-oxide showed a unique regioselectivity to produce 2,4-hexadiene-1-oximate 4a in toluene and 3,5-hexadiene-2-oximate 4b in THF, respectively. Treatment of β-diketiminato (BDI)-supported magnesium hydride complex 5, [L2MgH]2 (L2 = (2,6-iPr2-C6H3)NC(Me)CHC(Me)N(2,6-iPr2-C6H3)), with quinoline N-oxide gave 1,2-dihydroquinoline type product 6, while treatment of complex 5 with 2-methylpyridine N-oxide either in toluene or THF afforded 1-methyl-2,4-pentadiene-1-oximate complex 7 as the only product. All these complexes were fully characterized by NMR spectroscopy and X-ray diffraction analyses, and mechanism researches were conducted to understand the ring-opening reaction of pyridine N-oxide.

Self-Assembly of Magnesium Hydride Clusters Driven by Chameleon-Type Ligands.

While magnesium hydride complexes are generally stabilized by hard, bulky N-donor ligands, softer ligands with a broad variety of coordination modes are shown to efficiently adapt themselves to the large variety of Mg2+ centers in a growing magnesium hydride cluster. A P,N-chelating ligand is introduced that displays coordination modes between that of enamide, aza-allyl, and phosphinomethanide. Slight changes in the ligand bite angle have dramatic consequences for the structure type. The hitherto largest neutral magnesium hydride clusters are isolated either in a nonanuclear sheet-structure (brucite-type) or a dodecanuclear ring structure.

Self‐Assembly of Magnesium Hydride Clusters Driven by Chameleon‐Type Ligands

AbstractWhile magnesium hydride complexes are generally stabilized by hard, bulky N‐donor ligands, softer ligands with a broad variety of coordination modes are shown to efficiently adapt themselves to the large variety of Mg2+ centers in a growing magnesium hydride cluster. A P,N‐chelating ligand is introduced that displays coordination modes between that of enamide, aza‐allyl, and phosphinomethanide. Slight changes in the ligand bite angle have dramatic consequences for the structure type. The hitherto largest neutral magnesium hydride clusters are isolated either in a nonanuclear sheet‐structure (brucite‐type) or a dodecanuclear ring structure.

A Soft Grip: Magnesium Complexes with a Phosphine-Modified Phosphonium Diylidic Lewis Base.

Simple strategies to obtain magnesium complexes with the soft chelating diylidic ligand [Ph2 PCHPPh2 (fluorenylidene)]- (dppmflu- ) were developed to evaluate the influence of the hard acid (cation) and soft base (anion) mismatch on the stability and reactivity of the formed derivatives. Deprotonation of the precursor Ph2 PCH2 PPh2 (flu) (dppmfluH) by an alkylmagnesium derivative or magnesium amide provided access to [{Mg(dppmflu)(μ-nBu)}2 ], [Mg(dppmflu){N(SiMe3 )2 }], and [{Mg(dppmflu)(μ-Me)}2 ], which were used as starting materials for further investigations. The reaction of [{Mg(dppmflu)(μ-nBu)}2 ] with PhSiH3 in the presence of THF allowed isolation of the magnesium hydride complex [{Mg(dppmflu)(μ-H)(thf)}2 ] without a stabilizing nitrogen donor ligand. Prolonged heating enforced ligand redistribution and [{Mg(dppmflu)(μ-H)(thf)}2 ] was converted to [Mg(dppmflu)2 ] and MgH2 . The homoleptic derivative [Mg(dppmflu)2 ], in which the magnesium center is in a very soft ligand environment, can open a THF molecule by frustrated Lewis pair reactivity to give [{Mg(dppmflu)(μ-OC4 H8 dppmflu)}2 ].

Calcium Hydride Catalyzed Highly 1,2-Selective Pyridine Hydrosilylation.

Reaction of the calcium hydride complex (DIPPnacnac-CaH⋅THF)2 with pyridine is much faster and selective than that of the corresponding magnesium hydride complex (DIPPnacnac = [(2,6-iPr2 C6 H3 )NC(Me)]2 CH). With a range of pyridine, picoline and quinoline substrates, exclusive transfer of the hydride ligand to the 2-position is observed and also at higher temperatures no 1,2→1,4 isomerization is found. The heteroleptic product DIPPnacnac-Ca(1,2-dihydropyridide)⋅(pyridine) shows fast ligand exchange into homoleptic calcium complexes and therefore could not be isolated. Calcium hydride reduction of isoquinoline gave well-defined homoleptic products which could be characterized by X-ray diffraction: Ca(1,2-dihydroisoquinolide)2 ⋅(isoquinoline)4 and Ca3 (1,2-dihydroisoquinolide)6 ⋅(isoquinoline)6 . The striking selectivity difference in the dearomatization of pyridines by Mg or Ca complexes could be explained by DFT theory and was utilized in catalysis. Whereas hydroboration of pyridine with pinacol borane with a calcium hydride catalyst gave only minor conversion, the hydrosilylation of pyridine and quinolines with PhSiH3 yields exclusively 1,2-dihydropyridine and 1,2-dihydroquinoline silanes with 80-90 % conversion. Similar results can be achieved with the catalyst Ca[N(SiMe3 )2 ]2 ⋅(THF)2 . These calcium complexes represent the first catalysts for the 1,2-selective hydrosilylation of pyridines.

Phosphinimino-amino supported complex: Synthesis, polymerization of ethylene and dearomatisation of pyridine

Alkane elimination reaction of phosphinimino-amine ligand HL (L = (2,6-iPr2-C6H3)NC(Me)CHP(Cy2)N(2,6-Me2-C6H3)) with MgnBu2 and Sc(CH2SiMe3)3(THF)2, respectively, can afford the corresponding phosphinimino-amine supported magnesium monoalkyl complex LMgnBu(THF) (1) and scandium dialkyl complex LSc(CH2SiMe3)2 (2). 2 can initiate the polymerization of ethylene efficiently under the activation of [Ph3C][B(C6F5)4]. Magnesium hydride complex [LMgH]2 (3) can be easily obtained by the reaction of 1 and phenylsilane. The hydride species in 3 can conduct the dearomatisation of pyridine to give dearomatisation product 1,2-dihydropyridide (4).

Multinuclear Magnesium Hydride Clusters: Selective Reduction and Catalytic Hydroboration of Pyridines

Multinuclear magnesium hydride complexes react with pyridine, forming 1,2- and 1,4-dihydropyridide (DHP) complexes. Reaction of PARA3Mg8H10 with pyridine initially formed 1,2-DHP and 1,4-DHP product mixtures which converted at 60 °C into PARA-[Mg(1,4-DHP)]2·(pyridine)2 (PARA = [(2,6-iPr2C6H3)NC(Me)C(H)C(Me)N]2-(p-C6H4)). Reaction of [NN-(MgH)2]2 with pyridine gave exclusive formation of the 1,2-DHP product NN-[Mg(1,2-DHP)]2·(pyridine)2 (NN = [(2,6-iPr2C6H3)NC(Me)CHC(Me)N−]2). Both products were characterized by crystal structure determinations. The unusual preference for 1,2-addition is likely caused by secondary intramolecular interactions based on mutual communication between the metal coordination geometries: an extended network of C–H···C π-interactions and π-stacking interactions is found. Whereas PARA3Mg8H10 is hardly active in magnesium-catalyzed hydroboration of pyridines with pinacolborane, [NN-(MgH)2]2 shows efficient coupling. However, the regioselectivity of the stoichiometric reaction is not translated to the catalytic regime. This result is taken as an indication for a potential alternative mechanism in which magnesium hydride intermediates do not play a role but the hydride is transferred from an intermediate borate complex.

Synthesis and thermal decomposition of a pyridylene-bridged bis-β-diketiminate magnesium hydride cluster

Reaction of PYR-(MgnBu)2, in which PYR is 2,6-[(DIPP)NC(Me)CHC(Me)N-]2-pyridine and DIPP is 2,6-iPr2-phenyl, with (DIPP)NH2BH3 gave PYR-[MgNH(DIPP)BH3]2 (56%) which was characterized by crystal structure determination. Addition of THF resulted in β-H elimination and formation of PYR-[MgNH(DIPP)BH3](MgH)·THF (57%), likewise characterized by crystal structure determination. Conversion of the second amidoborane anion in H(-) could not be achieved. Reaction of PYR-(MgnBu)2 with PhSiH3 gave PYR-(MgH)2, which crystallized as a dimer. The structure of [PYR-(MgH)2]2 shows an 8-membered ring of Mg(2+) and H(-) ions. Thermal decomposition at 130 °C releases one equivalent of H2, i.e. 50% of the expected value. Nucleophilic attack at the para-position and reduction of the pyridylene bridge might explain reduced H2 release.

β‐Diketiminate‐Stabilized Magnesium(I) Dimers and Magnesium(II) Hydride Complexes: Synthesis, Characterization, Adduct Formation, and Reactivity Studies

The preparation and characterization of a series of magnesium(II) iodide complexes incorporating beta-diketiminate ligands of varying steric bulk and denticity, namely, [(ArNCMe)(2)CH](-) (Ar=phenyl, ((Ph)Nacnac), mesityl ((Mes)Nacnac), or 2,6-diisopropylphenyl (Dipp, (Dipp)Nacnac)), [(DippNCtBu)(2)CH](-) ((tBu)Nacnac), and [(DippNCMe)(Me(2)NCH(2)CH(2)NCMe)CH](-) ((Dmeda)Nacnac) are reported. The complexes [((Ph)Nacnac)MgI(OEt(2))], [((Mes)Nacnac)MgI(OEt(2))], [((Dmeda)Nacnac)MgI(OEt(2))], [((Mes)Nacnac)MgI(thf)], [((Dipp)Nacnac)MgI(thf)], [((tBu)Nacnac)MgI], and [((tBu)Nacnac)MgI(DMAP)] (DMAP=4-dimethylaminopyridine) were shown to be monomeric by X-ray crystallography. In addition, the related beta-diketiminato beryllium and calcium iodide complexes, [((Mes)Nacnac)BeI] and [{((Dipp)Nacnac)CaI(OEt(2))}(2)] were prepared and crystallographically characterized. The reductions of all metal(II) iodide complexes by using various reagents were attempted. In two cases these reactions led to the magnesium(I) dimers, [((Mes)Nacnac)MgMg((Mes)Nacnac)] and [((tBu)Nacnac)MgMg((tBu)Nacnac)]. The reduction of a 1:1 mixture of [((Dipp)Nacnac)MgI(OEt(2))] and [((Mes)Nacnac)MgI(OEt(2))] with potassium gave a low yield of the crystallographically characterized complex [((Dipp)Nacnac)Mg(mu-H)(mu-I)Mg((Mes)Nacnac)]. All attempts to form beryllium(I) or calcium(I) dimers by reductions of [((Mes)Nacnac)BeI], [{((Dipp)Nacnac)CaI(OEt(2))}(2)], or [{((tBu)Nacnac)CaI(thf)}(2)] have so far been unsuccessful. The further reactivity of the magnesium(I) complexes [((Mes)Nacnac)MgMg((Mes)Nacnac)] and [((tBu)Nacnac)MgMg((tBu)Nacnac)] towards a variety of Lewis bases and unsaturated organic substrates was explored. These studies led to the complexes [((Mes)Nacnac)Mg(L)Mg(L)((Mes)Nacnac)] (L=THF or DMAP), [((Mes)Nacnac)Mg(mu-AdN(6)Ad)Mg((Mes)Nacnac)] (Ad=1-adamantyl), [((tBu)Nacnac)Mg(mu-AdN(6)Ad)Mg((tBu)Nacnac)], and [((Mes)Nacnac)Mg(mu-tBu(2)N(2)C(2)O(2))Mg((Mes)Nacnac)] and revealed that, in general, the reactivity of the magnesium(I) dimers is inversely proportional to their steric bulk. The preparation and characterization of [((tBu)Nacnac)Mg(mu-H)(2)Mg((tBu)Nacnac)] has shown the compound to have different structural and physical properties to [((tBu)Nacnac)MgMg((tBu)Nacnac)]. Treatment of the former with DMAP has given [((tBu)Nacnac)Mg(H)(DMAP)], the X-ray crystal structure of which disclosed it to be the first structurally authenticated terminal magnesium hydride complex. Although attempts to prepare [((Mes)Nacnac)Mg(mu-H)(2)Mg((Mes)Nacnac)] were not successful, a neutron diffraction study of the corresponding magnesium(I) complex, [((Mes)Nacnac)MgMg((Mes)Nacnac)] confirmed that the compound is devoid of hydride ligands.

Activation of the (Trimethylsilyl)tetramethylcyclopentadienyl Ligand in Zirconocene Complexes

Reduction of [ZrCl2{η5-C5Me4(SiMe3)}2] (1) with excess magnesium in tetrahydrofuran affords a mixture of the monomeric zirconocene hydride [ZrH{η1:η5-C5Me4(SiMe2CH2)}{η5-C5Me4(SiMe3)}] (2) and the dimeric, tetranuclear Zr(III)− and Zr(IV)−magnesium hydride complexes 3 and 4, respectively. In the presence of bis(trimethylsilyl)ethyne (btmse), a similar reduction yields the η2-alkyne complex [Zr{η5-C5Me4(SiMe3)}2(η2-btmse)] (5) as the major product and compounds 2−4 as minor impurities. Upon thermolysis under vacuum, 5 undergoes a 2-fold hydrogen transfer from trimethylsilyl groups in the zirconocene intermediate to leaving btmse to afford [Zr{η5:η1-C5Me4(SiMe2CH2)}2] (6) and a mixture of alkenes (E)- and (Z)-Me3SiCHCHSiMe3 (E ≫ Z). Crystal structures of compounds 2 and 4−6 were determined by X-ray crystallography.

Formation of a binuclear titanocene hydride–magnesium hydride carbyl-bridged complex in the (C 5Me 4Ph) 2TiCl 2/Mg/THF system

The reduction of (C 5Me 4Ph) 2TiCl 2 by magnesium in tetrahydrofuran affords a mixture of the diamagnetic doubly tucked-in titanocene complex (C 5Me 4Ph)[C 5Me 2(CH 2) 2Ph]Ti ( 1), the paramagnetic trinuclear Ti–Mg–Ti hydride bridged complex [(C 5Me 4Ph) 2Ti( μ-H) 2] 2Mg ( 2) and the paramagnetic binuclear titanocene hydride–magnesium hydride complex (C 5Me 4Ph)[C 5Me 4( o-C 6H 4)]Ti( μ-H) 2Mg(THF) 2 ( 3). The X-ray diffraction analysis of 3 revealed that the magnesium atom forms the σ-bond to the ortho-carbon atom of one of the phenyl rings and binds 2 molecules of THF.