Tris(pyrazolyl)borate Complexes of the Alkaline-Earth Metals: Alkylaluminate Precursors and Schlenk-Type Rearrangements

A Preference for Heterolepticity - Schlenk Type Equilibria in Organometallic Beryllium Systems

The reaction of homoleptic beryllium halide with diphenyl beryllium complexes leads to the clean formation of heteroleptic beryllium Grignard compounds [(L)1-2 BePhX]1-2 (X=Cl, Br, I; L=C-, N-, O-donor ligand). The influence of ligands and solvent on these compounds, their formation and exchange equilibria in solution were investigated, together with the factors determining the complex constitution.

Synthesis and reactivity of alkali metal aluminates bearing bis(organoamido)phosphane ligand

In this study, we report a group of alkali metal aluminates bearing bis(organoamido)phosphane ligand. The starting complex {[PhP(NtBu)2]AlMe2}Li·OEt2 (1) was prepared by stepwise deprotonation of the parent PhP(NHtBu)2 by nBuLi and AlMe3. Further derivatization of aluminate 1 was performed by the virtual substitution of lithium -{[PhP(NtBu)2]AlMe2}K (2), methyl substituents - {[PhP(NtBu)2]AlH2}Li·THF (3), modification of steric bulk and induction effects on the phosphorus atom - {[tBuP(N-2,6-iPr2C6H3)2]AlMe2}Li·(OEt2)2 (4), and phosphorus atom oxidation state {[Ph(Y)P(NtBu)2]AlMe2}Li (Y = O (5), S (6), Se (7), Te (8)). The structure causing non-covalent interactions in 1-4 were evaluated with the help of theoretical calculations and topological analysis ranging from π-electron system-metal to agostic interactions of various types. The further reactions of 1 with various nucleophiles were found to be a versatile tool for the preparation of iminophosphonamides via the formation of P-E bond (E = Si, Ge, Sn, Pb, P, and C) and followed by P(III) → P(V) tautomeric shift.

Site-Isolated Main-Group Tris(2-pyridyl)borate Complexes by Pyridine Substitution and Their Ring-Opening Polymerization Catalysis

Tris(2-pyridyl)borates are an emerging class of scorpionate ligands, distinguished as exceptionally robust and electron-donating. However, the rapid formation of inert homoleptic complexes with divalent metals has so far limited their catalytic utility. We report site-isolating tris(2-pyridyl)borate ligands, bearing isopropyl, tert-butyl, and mesityl substituents at the pyridine 6-position to suppress the formation of inert homoleptic complexes. These ligands form the first 1:1 complexes between tris(2-pyridyl)borates and Mg²⁺, Zn²⁺, or Ca²⁺, with isopropyl-substituted Tpy^iPrH showing the most generality. Single-crystal X-ray diffraction analysis of the resulting complexes and comparison to density functional theory (DFT) models showed geometric distortions driven by steric repulsion between the pyridine 6-substituents and the hexamethyldisilazide (HMDS^-, ^-N(SiMe₃)₂) anion. We show that this steric profile is a feature of the six-membered pyridine ring and contrasts with more established tris(pyrazolyl)borate and tris(imidazoline)borate scorpionate complexes. Tpy^iPrMg(HMDS) (1) and its zinc analogue are moderately active for the controlled polymerization of l-lactide, ε-caprolactone, and trimethylene carbonate. Furthermore, 1 gives controlled polymerization under more demanding melt-phase polymerization conditions at 100 °C, and block copolymerization of ε-caprolactone and trimethylene carbonate. These results will enable useful catalysis and coordination chemistry studies with tris(2-pyridyl)borates, and characterizes their structural complementarity to more familiar scorpionate ligands.

Cyclopentadienyl Complexes of the Alkaline Earths in Light of the Periodic Trends

The electron-rich cyclopentadienyl and the analogous indenyl and fluorenyl ligands (collectively denoted here as Cp') have been impactful in stabilizing electron-deficient metal centers including the highly electropositive alkaline earths. Being in the s-block, the group 2 metals follow a major periodic variation in their atomic and ionic properties which is reflected in those Cp' compounds. This article presents an overview of this class of compounds for all the five metals from beryllium to barium (radium is excluded for its radioactivity), highlighting their systematic variation.

The Alkylaluminate/Gallate Trap: Metalation of Benzene by Heterobimetallic Yttrocene Complexes [Cp*2Y(MMe3R)] (M = Al, Ga)

Yttrocene derivatives [Cp*₂Y(MMe₄)] (Cp* = C₅Me₅; M = Al, Ga) and Cp*₂Y[Me₃Al{B(NDippCH)₂}] (Dipp = C₆H₃iPr₂-2,6) deprotonate benzene at elevated temperatures via the release of methane. The formation of [Cp*₂Y(Me₂MPh₂)] (M = Al, Ga), Cp*₂Y(MPh₄) (M = Al, Ga), Cp*₂Y[Me₂AlPh{B(NDippCH)₂}], and Cp*₂Y[AlPh₃{B(NDippCH)₂}] can be controlled via the temperature applied. The activation temperature and formation of the coordinatively unsaturated "reactive" [Cp*₂YMe] strongly depend on the coordination strength of the displaceable Lewis acids [AlMe₃]₂, GaMe₃, and [Me₂Al{B(NDippCH)₂}]₂. Hence, [Cp*₂Y(AlMe₄)] requires temperatures above 100 °C to metalate benzene, while Cp*₂Y[AlMe₃{B(NDippCH)₂}] undergoes C-H-bond activation even at ambient temperatures. A kinetic deuterium isotope effect was observed for the reactions in C₆D₆ solutions. Distinct differences in the stabilities of the bulky Group 13 anions ([Me₂MPh₂]^-, [MPh₄]^-, [Me₃Al{B(NDippCH)₂}]^-, [Me₂AlPh{B(NDippCH)₂}]^-, and [AlPh₃{B(NDippCH)₂}]^-) are assessed by detailed studies of the coordination chemistry with tetrahydrofuran (THF) and by variable-temperature ¹H NMR spectroscopy. Thus, increased steric bulk or a reduced Lewis acidity of the Group 13 metal center promote temperature-sensitive dissociation of trivalent Group 13 alkyl entities. Consequently, compound Cp*₂Y[AlPh₃{B(NDippCH)₂}] was found to engage in a dissociation equilibrium with [Cp*₂YPh] and AlPh₂{B(NDippCH)₂} in a C₆D₆ solution at ambient temperature. The reaction of Cp*₂Y[AlPh₃{B(NDippCH)₂}] with THF results in the concomitant formation of monometallic Cp*₂YPh(THF) and the solvent-separated ion pair [Cp*₂Y(THF)₂][AlPh₃{B(NDippCH)₂}].

Calcium and Magnesium Bis(β-diketiminate) Complexes: Impact of the Alkylene Bridge on Schlenk-Type Rearrangements

Dinuclear heteroleptic alkaline-earth-metal complexes are interesting synthetic targets because the close proximity of two metals allows for cooperative effects. However, these complexes are also prone to undergoing Schlenk-type rearrangements, affording less-active homoleptic complexes. Here we present the metalation of bis(β-diketimine) ligands possessing flexible bridging groups, i.e., 1,2-ethylene, 1,3-propylene, and trans-1,2-cyclohexylene, using calcium and magnesium precursors. Four mononuclear homoleptic calcium complexes were obtained, highlighting the pronounced tendency of calcium to undergo Schlenk-like redistributions. In the case of magnesium, however, the bridging group plays a crucial role, yielding seven dinuclear heteroleptic complexes but also one mononuclear and one dinuclear homoleptic complexes. In addition, a trinuclear mixed heteroleptic-homoleptic magnesium complex, which is a rare example of an intermediate of the Schlenk equilibrium, was isolated.

Barium tetraalkylaluminate complexes supported by the super-bulky hydrotris(pyrazolyl)borate ligand

A heteroleptic barium aminobenzyl complex [(Tp^Ad,iPr)Ba(o-CH₂C₆H₄-NMe₂)] (1) was obtained in excellent yield from a simple one-pot reaction. Treatment of [(Tp^Ad,iPr)Ba(o-CH₂C₆H₄-NMe₂)] (1) with two equivalents of AlR₃ (R = Me, Et) led to the formation of barium tetraalkylaluminate complexes [(Tp^Ad,iPr)Ba(AlR₄)]_n (R = Me, n = 2, 2; R = Et, n = 1, 3) as dimers or monomers in the solid state. The Tp^Ad,iPr ligand-free peralkylated barium complex [Ba(AlEt₄)₂]_n was isolated by the addition of ten equivalents of AlEt₃ under the same conditions. The donor-induced aluminate cleavage is not applicable when donor solvents are added to complexes 2 and 3. In the solution of complexes 2 and 3, the alkylaluminate moieties and Tp^Ad,iPr ligands show a rapid fluxional behavior in [D₈]toluene solution over the temperature range of -70 to 25 °C, without any significant decoalescence of the corresponding proton signals.

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.

Magnesium, calcium and zinc [N2N'] heteroscorpionate complexes

A new family of sterically demanding N2N' heteroscorpionate pro-ligands (HC(tBu2pz)2SiMe2N(H)R (R = iPr, tBu, Ph, Xyl)) has been prepared via a straightforward modular synthetic route. An extensive study into the synthesis and characterisation of lithium, magnesium, calcium and zinc complexes supported by both 3,5-tBu and 3,5-Me substituted N2N' ligand families has been conducted. Attempted deprotonation of the pro-ligands with nBuLi afforded the corresponding lithium salts Li{HC(tBu2pz)2SiMe2NR} (R = iPr (1), tBu (2), Ph (3) and Xyl (4)) but air- and thermal-sensitivity limited the yields of these potentially useful precursors; only the sterically encumbered ligand system allowed clean reactivity. Magnesium methyl complexes Mg{HC(tBu2pz)2SiMe2NR}Me (R = iPr (5) and R = Ph (6)) were prepared using an excess of the Grignard reagent MeMgCl. Magnesium butyl complexes were synthesised in good yields using the dialkyl precursor MgnBu2 to afford Mg{HC(R'2pz)2SiMe2NR}nBu (R' = Me; R = iPr (7), tBu (8), Ad (9), Ph (10). R' = tBu; R = iPr (11), Ph (12)). Protonolylsis reactions were used to synthesise magnesium and calcium amide complexes Mg{HC(R'2pz)2SiMe2NR}{N(SiHMe2)2} (R' = Me; R = iPr (13), tBu (14), Ph (15). R' = tBu; R = Ph (16)) or Mg{HC(R'2pz)2SiMe2NR}{N(SiMe3)2} (R' = Me; R = iPr (17), tBu (18), Ph (19). R' = tBu; R = Ph (20)), and Ca{HC(R'2pz)2SiMe2NR}{N(SiMe2)2} (L) (R' = Me; L = thf; R = iPr (21), tBu (22), Ph (23). R' = tBu; L = none; R = Ph (24). Zinc methyl complexes Zn{HC(R'2pz)2SiMe2NR}Me (R' = Me; R = iPr (25), tBu (26), Ph (27). R' = tBu; R = Ph (28)) were prepared by reaction of the N2N' heteroscorpionate pro-ligands with ZnMe2. In preliminary studies, magnesium amide complexes 16 and 20 were evaluated as initiators for the ring-opening polymerisation (ROP) of ε-caprolactone (ε-CL) and rac-lactide (rac-LA). Although the overall polymerisation control was poor, 16 and 20 were found to be active initiators.

Reactivity of a β-diketiminate-supported magnesium alkyl complex toward small molecules

The reactivity of the magnesium alkyl {[HC(C(Me)N-2,6-iPr2C6H3)2]Mg(nBu)}2 (1) toward various small molecules provides access to a variety of magnesium derivatives. For example, the insertion of elemental chalcogens (S8 and Se8) into the Mg-C bond of complex 1 gives the dimeric magnesium thiolate {[HC(C(Me)N-2,6-iPr2C6H3)2]Mg(μ-SnBu)}2 (2), magnesium selenolate [HC(C(Me)N-2,6-iPr2C6H3)2]Mg(SenBu)(THF) (3), and magnesium diselenolate [HC(C(Me)N-2,6-iPr2C6H3)2]Mg(Se2nBu)(THF) (4). Meanwhile, compound 4 can be readily obtained by further insertion of one selenium atom into complex 3. Moreover, the reactions of complex 1 with diphenyl dichalcogenides (PhSSPh and PhSeSePh) by σ bond metathesis afford the corresponding magnesium phenyl chalcogenolates [HC(C(Me)N-2,6-iPr2C6H3)2]Mg(EPh)(THF) (E = S 5, Se 6) concomitant with PhEnBu release. Furthermore, the treatment of complex 1 with benzonitrile and phenyl isothiocyanate produces the serendipitous magnesium-1-azaallyl complex [HC(C(Me)N-2,6-iPr2C6H3)2]Mg(N(H)C(Ph)[double bond, length as m-dash]CHC3H7)(DME) (7) and the diimino-thioamidato magnesium compound {κ3-N,N',N''-(ArNCMe)2[N(Ph)CS]CH}Mg[(Ph)NC(nBu)S] (8) (Ar = 2,6-iPr2C6H3). In addition, deprotonation occurs between compound 1 and 1-methylimidazole to generate the imidazolyl complex {[HC(C(Me)N-2,6-iPr2C6H3)2]Mg(μ-Im)}2 (9) (Im = 2-N-methylimidazolyl). These results indicated that the butylmagnesium complex 1 possesses high activity toward small molecules and revealed several unusual transformations. All the new compounds were characterized by various spectroscopic methods, and their solid-state structures were further confirmed by single-crystal X-ray diffraction analyses.

Magnesium Stung by Nonclassical Scorpionate Ligands: Synthesis and Cone-Angle Calculations

A series of tris(pyrazolyl)alkane (RCTp) scorpionate ligands of the type RCTp^3-R' (R=Me, nBu, SiMe₃ ; R'=H, Me, Ph, iPr, tBu) were synthesized and their ability to coordinate methylmagnesium moieties was examined. The reaction of Mg(AlMe₄ )₂ with neutral proligands HCTp^3-Ph or Me₃ SiCTp^3-Me , containing a non-innocent backbone methine moiety, led to deprotonation/rearrangement and SiMe₃ /AlMe₃ exchange to afford [(Me₃ AlCTp^3-Ph )₂ Mg] and [(Me₃ AlCTp^3-Me )Mg(AlMe₄ )], respectively, with monoanionic tripodal ligands. Treatment of sterically less demanding RCTp^3-R' with Mg(AlMe₄ )₂ produced isostructural dicationic "metal-in-a-box" complexes of the type [(RCTp^3-R' )₂ Mg][AlMe₄ ]₂ (R=Me, nBu; R'=H, Me). Utilization of the superbulky ligands MeCTp^3-Ph and MeCTp^3-tBu gave monocationic complexes [(MeCTp^3-Ph )MgMe][AlMe₄ ] and [(MeCTp^3-tBu )MgMe][Al₂ Me₇ ] as separated ion pairs. The reaction of Mg(AlMe₄ )₂ with nBuCTp^3-Ph led to the formation of the dimagnesium complex [{(nBuCTp^3-Ph )Mg(AlMe₄ )}₂ (μ-CH₃ )], which features a bridging methyl moiety and terminal η¹ -coordinated tetramethylaluminato ligands. Isopropyl-substituted ligand MeCTp^3-iPr emerged from further fine-tuning of the steric and electronic parameters and, upon reaction with Mg(AlMe₄ )₂ , gave (MeCTp^3-iPr )Mg(AlMe₄ )₂ ; this represents the first example of a magnesium bis(alkyl) complex with an intact RCTp^3-R' ligand. The exact ligand cone angles Θ^° of all magnesium complexes were determined according to the mathematical analysis developed by Allen et al. [J. Comput. Chem. 2013, 34, 1189-1197].

Donor-stabilised molecular Mg/Al-bimetallic hydrides

Low-molecular organometallic hydride complexes of the type [(do)_xMg{(μ₂-H)(AlMe₃)}₂] form in ethereal solvents and display inherent reactivity of existing magnesium hydrido bonding.

Synthesis, structure and reactivity of a terminal magnesium fluoride compound, [TpBut,Me]MgF: hydrogen bonding, halogen bonding and C-F bond formation

The bulky tris(3-tert-butyl-5-pyrazolyl)hydroborato ligand, [TpBut,Me], has been employed to obtain the first structurally characterized example of a molecular magnesium compound that features a terminal fluoride ligand, namely [TpBut,Me]MgF, via the reaction of [TpBut,Me]MgMe with Me3SnF. The chloride, bromide and iodide complexes, [TpBut,Me]MgX (X = Cl, Br, I), can also be obtained by an analogous method using Me3SnX. The molecular structures of the complete series of halide derivatives, [TpBut,Me]MgX (X = F, Cl, Br, I) have been determined by X-ray diffraction. In each case, the Mg-X bond lengths are shorter than the sum of the covalent radii, thereby indicating that there is a significant ionic component to the bonding, in agreement with density functional theory calculations. The fluoride ligand of [TpBut,Me]MgF undergoes halide exchange with Me3SiX (X = Cl, Br, I) to afford [TpBut,Me]MgX and Me3SiF. The other halide derivatives [TpBut,Me]MgX undergo similar exchange reactions, but the thermodynamic driving forces are much smaller than those involving fluoride transfer, a manifestation of the often discussed silaphilicity of fluorine. In accord with the highly polarized Mg-F bond, the fluoride ligand of [TpBut,Me]MgF is capable of serving as a hydrogen bond and halogen bond acceptor, such that it forms adducts with indole and C6F5I. [TpBut,Me]MgF also reacts with Ph3CCl to afford Ph3CF, thereby demonstrating that [TpBut,Me]MgF may be used to form C-F bonds.