On the existence of low-valent magnesium-calcium complexes.

DFT-Calculations predict that a low-valent complex (BDI)Mg-Ca(BDI) with bulky ß-diketiminate (BDI) ligands is thermodynamically stable. It was attempted to isolate such a complex by salt-metathesis between [(DIPePBDI*)Mg-Na+]2 and [(DIPePBDI)CaI]2 (DIPePBDI = HC[C(Me)N-DIPeP]2; DIPePBDI* = HC[C(tBu)N-DIPeP]2; DIPeP = 2,6-CH(Et)2-phenyl). Whereas in alkane solvents no reaction was observed, salt-metathesis in C6H6 led to immediate C-H activation of benzene to give (DIPePBDI*)MgPh and (DIPePBDI)CaH, the latter crystallizing as a THF-solvated dimer [(DIPePBDI)CaH·THF]2. Calculations suggest reduction and insertion of benzene in the Mg-Ca bond. The activation enthalpy for the subsequent decomposition of C6H62- into Ph- and H- is only 14.4 kcal mol-1. Repeating this reaction in the presence of naphthalene or anthracene led to heterobimetallic complexes in which naphthalene2- or anthracene2- anions are sandwiched between (DIPePBDI*)Mg+ and (DIPePBDI)Ca+ cations. These complexes slowly decompose to their homometallic counterparts and further decomposition products. Complexes in which naphthalene2- or anthracene2- anions are sandwiched between two (DIPePBDI)Ca+ cations were isolated. The low-valent complex (DIPePBDI*)Mg-Ca(DIPePBDI) could not be isolated due to its high reactivity. There is, however, strong evidence that this heterobimetallic compound is a fleeting intermediate.

Alkaline-Earth Metal Mediated Benzene-to-Biphenyl Coupling.

Complex [(DIPeP BDI)Ca]2 (C6 H6 ), with a C6 H6 2- dianion bridging two Ca2+ ions, reacts with benzene to yield [(DIPeP BDI)Ca]2 (biphenyl) with a bridging biphenyl2- dianion (DIPeP BDI=HC[C(Me)N-DIPeP]2 ; DIPeP=2,6-CH(Et)2 -phenyl). The biphenyl complex was also prepared by reacting [(DIPeP BDI)Ca]2 (C6 H6 ) with biphenyl or by reduction of [(DIPeP BDI)CaI]2 with KC8 in presence of biphenyl. Benzene-benzene coupling was also observed when the deep purple product of ball-milling [(DIPP BDI)CaI(THF)]2 with K/KI was extracted with benzene (DIPP=2,6-CH(Me)2 -phenyl) giving crystalline [(DIPP BDI)Ca(THF)]2 (biphenyl) (52 % yield). Reduction of [(DIPeP BDI)SrI]2 with KC8 gave highly labile [(DIPeP BDI)Sr]2 (C6 H6 ) as a black powder (61 % yield) which reacts rapidly and selectively with benzene to [(DIPeP BDI)Sr]2 (biphenyl). DFT calculations show that the most likely route for biphenyl formation is a pathway in which the C6 H6 2- dianion attacks neutral benzene. This is facilitated by metal-benzene coordination.

New horizons in low oxidation state group 2 metal chemistry.

Since the seminal report on Mg in the +I oxidation state in 2007, low-valent complexes featuring a MgI-MgI bond developed from trophy molecules to state-of-the-art reducing agents. Despite increasing interest in low-valency of the other group 2 metals, this area was restricted for a long time to a rare example of a CaI(arene)CaI inverse sandwich. This feature article focuses on the most recent developments in the field, highlighting recent breakthroughs for Be, Mg and Ca. The more exotic metal Be was the first to be isolated as a zero-valent complex which could be oxidized to a BeI species. There also has been interest in breaking the MgI-MgI bond with superbulky ß-diketiminate ligands (BDI) that suppress (BDI)Mg-Mg(BDI) bond formation. This led to Mg-Mg bond elongation or Mg-N bond cleavage. Several reports on attempts to isolate (BDI)Mg˙ radicals by combinations of ligand bulk, addition of neutral ligands or UV(vis) irradiation led to reduction of the aromatic solvents, underscoring the high reactivity of these open shell species. Only recently, zero-valent complexes of Mg were introduced. Double reduction of a (BDI)MgI complex with Na gave [(BDI)Mg-]Na+. This Mg0 complex crystallized as a dimer in which the Na+ cations bridge the two (BDI)Mg- anions which react as Mg nucleophiles. Thermal decomposition led to spontaneous formation of Na0 and a trinuclear (BDI)MgMgMg(BDI) complex. This mixed-valence Mg3-complex is a prime example of the fleeting multinuclear Mgn intermediates discussed on the way from Mg metal to Grignard reagent. Attempts to prepare low-valent CaI compounds by reduction of (BDI)CaI led to dearomatization of the arene solvents: (BDI)Ca(arene)Ca(BDI). Reduction in alkanes prevented this decomposition pathway but led to N2 reduction and isolation of (BDI)Ca(N2)Ca(BDI), representing the first example of molecular nitrogen fixation with an early main group metal. As the N22- anion reacts in most cases as a very strong two-electron reductant, LCa(N2)CaL could be seen as a synthon for hitherto elusive CaI-CaI complexes. Theoretical calculations suggest that participation of Ca d-orbitals is relevant for N2 activation. These most recent developments in low-valent group 2 metal chemistry will revive this area and undoubtly lead to new reactivities and applications.

Mg-Mg bond polarization induced by a superbulky ß-diketiminate ligand.

The bulk of a recently reported superbulky ß-diketiminate ligand was further increased by introducing tBu substituents in the ligand backbone. Attempts to isolate free Mg radicals with this extremely bulky ligand failed. Instead, a dinuclear Mg(i) complex with one chelating and one monodentate ß-diketiminate ligand was isolated. Asymmetry in metal coordination results in a polarized Mg-Mg bond.

Nucleophilic Aromatic Substitution at Benzene with Powerful Strontium Hydride and Alkyl Complexes.

Key to the isolation of the first alkyl strontium complex was the synthesis of a strontium hydride complex that is stable towards ligand exchange reactions. This goal was achieved by using the super bulky ß-diketiminate ligand DIPeP BDI (CH[C(Me)N-DIPeP]2 , DIPeP=2,6-diisopentylphenyl). Reaction of DIPeP BDI-H with Sr[N(SiMe3 )2 ]2 gave (DIPeP BDI)SrN(SiMe3 )2 , which was converted with PhSiH3 into [(DIPeP BDI)SrH]2 . Dissolved in C6 D6 , the strontium hydride complex is stable up to 70 °C. At 60 °C, H-D isotope exchange gave full conversion into [(DIPeP BDI)SrD]2 and C6 D5 H. Since H-D exchange with D2 is facile, the strontium hydride complex served as a catalyst for the deuteration of C6 H6 by D2 . Reaction of [(DIPeP BDI)SrH]2 with ethylene gave [(DIPeP BDI)SrEt]2 . The high reactivity of this alkyl strontium complex is demonstrated by facile ethylene polymerization and nucleophilic aromatic substitution with C6 D6 , giving alkylated aromatic products and [(DIPeP BDI)SrD]2 .

Low Valent Magnesium Chemistry with a Super Bulky ß-Diketiminate Ligand.

The steric bulk of the well-known DIPP BDI ligand (CH[C(CH3 )N-DIPP]2 , DIPP=2,6-diisopropylphenyl) was increased by replacing isopropyl for isopentyl groups. This very bulky DIPeP BDI ligand could not stabilize the radical species (DIPeP BDI)Mg. : reduction of (DIPeP BDI)MgI with Na gave (DIPeP BDI)2 Mg2 with a rather long Mg-Mg bond of 3.0513(8) Å. Addition of TMEDA prior to reduction gave complex (DIPeP BDI)2 Mg2 (C6 H6 ), which could also be obtained as its THF adduct. It is speculated that combination of a bulky spectator ligand and TMEDA prevents dimerization of the intermediate MgI radical, which then reacts with the benzene solvent. Complex (DIPeP BDI)2 Mg2 (C6 H6 ), which formally contains the anti-aromatic anion C6 H6 2- , reacted with tBuOH as a Brønsted base to 1,3- and 1,4-cyclohexadiene and with H2 as a two electron donor to (DIPeP BDI)2 Mg2 H2 and C6 H6 . It also reductively cleaved the C-F bond in fluorobenzene and gave (DIPeP BDI)MgPh, (DIPeP BDI)MgF, and C6 H6 .

Simple Alkaline-Earth Metal Catalysts for Effective Alkene Hydrogenation.

Alkaline earth metal amides (AeN''2 : Ae=Ca, Sr, Ba, N''=N(SiMe3 )2 ) catalyze alkene hydrogenation (80-120 °C, 1-6 bar H2 , 1-10 mol % cat.), with the activity increasing with metal size. Various activated C=C bonds (styrene, p-MeO-styrene, α-Me-styrene, Ph2 C=CH2 , trans-stilbene, cyclohexadiene, 1-Ph-cyclohexene), semi-activated C=C bonds (Me3 SiCH=CH2 , norbornadiene), or non-activated (isolated) C=C bonds (norbornene, 4-vinylcyclohexene, 1-hexene) could be reduced. The results show that neutral Ca or Ba catalysts are active in the challenging hydrogenation of isolated double bonds. For activated alkenes (e.g. styrene), polymerization is fully suppressed due to fast protonation of the highly reactive benzyl intermediate by N''H (formed in the catalyst initiation). Using cyclohexadiene as the H source, the first Ae metal catalyzed H-transfer hydrogenation is reported. DFT calculations on styrene hydrogenation using CaN''2 show that styrene oligomerization competes with styrene hydrogenation. Calculations also show that protonation of the benzylcalcium intermediate with N''H is a low-energy escape route, thus avoiding oligomerization.