Synthesis of Superbulky Amide Ligands by Addition of Polar Reagents to Sila-Imine.

The chemistry of extremely bulky amide ligands is troubled by difficulties in deprotonation of the parent amine. As an alternative route to superbulky amide reagents, the addition of polar reagents to a sila-imine has been investigated. Attempts to synthesize the superbulky amide anion (tBu3Si)2N- by addition of tBuLi to tBu2Si=N(SitBu3) failed and gave tBu3Si(tBu2HSi)NLi and isobutene. Reaction of the sila-imine with KOtBu successfully led to tBu3Si[tBu2(tBuO)Si]NK which crystallized as a separated ion-pair. Reaction with the slightly bulkier KOAd (Ad=1-adamantyl) led in presence of THF to ether ring-opening. Reaction with tBuOH gave tBu3Si[tBu2(tBuO)Si]NH but this amine cannot be easily deprotonated. Reaction with (BDI*)MgnBu in presence of THF gave (BDI*)Mg+ ⋅ (THF)2 and the non-coordinating anion tBu3Si[tBu2(nBu)Si]N-; BDI*=ß-diketiminate ligand HC[C(tBu)N-DIPP]2, DIPP=2,6-diisopropylphenyl. Reaction of Mg(nBu)2 with tBu2Si=N(SitBu3) led to a Mg complex with one amide ligand: tBu3Si[tBu2(nBu)Si]N-. The other superbulky amide anion isomerized by internal deprotonation of a tBu-substituent to give a primary carbanion that is also coordinated to Mg. Although the amide-to-carbanion isomerization is highly contrathermodynamic, it allows for coordination of both anions to a single Mg center. The new bulky amides are rare cases of halogen-free weakly coordinating anions.

Metal Vapour Synthesis of an Organometallic Barium(0) Synthon.

A hydrocarbon-soluble barium anthracene complex was prepared by means of metal vapour synthesis. Reaction of 9,10-bis(trimethylsilyl)anthracene (Anth'') with barium vapour gave deep purple Ba(Anth'') which after extraction with diethyl ether crystallised as the cyclic octamer [Ba(Anth'')⋅Et2 O]8 . Dissolution in benzene or toluene led to replacement of the Et2 O ligand with a softer arene ligand and isolation of Ba(Anth'')⋅arene. Diffusion ordered spectroscopy (DOSY NMR ) measurements in benzene-d6 indicate solution species with a molecular weight that equals a trimeric constitution. Natural population analysis (NPA) assigned charges of +1.70 and -1.70 to Ba and Anth'', respectively, relating to highly ionic Ba2+ /Anth''2- bonding. Preliminary reactivity studies with air, Ph2 C=NPh, or H2 show that the complex reacts as a Ba0 synthon by release of neutral Anth''. This soluble molecular Ba0 /BaII redox synthon provides new routes for the syntheses of barium complexes under mild conditions.

Formation, Structure and Reactivity of a Beryllium(0) Complex with Mgδ+-Beδ- Bond Polarization.

Attempts to create a novel Mg-Be bond by reaction of [(DIPePBDI*)MgNa]2 with Be[N(SiMe3)2]2 failed; DIPePBDI*=HC[(tBu)C=N(DIPeP)]2, DIPeP=2,6-Et2C-phenyl. Even at elevated temperatures, no conversion was observed. This is likely caused by strong steric shielding of the Be center. A similar reaction with the more open Cp*BeCl gave in quantitative yield (DIPePBDI*)MgBeCp* (1). The crystal structure shows a Mg-Be bond of 2.469(4) Å. Homolytic cleavage of the Mg-Be bond requires ΔH=69.6 kcal mol-1 (cf. CpBe-BeCp 69.0 kcal mol-1 and (DIPPBDI)Mg-Mg(DIPPBDI) 55.8 kcal mol-1). Natural-Population-Analysis (NPA) shows fragment charges: (DIPePBDI*)Mg +0.27/BeCp* -0.27. The very low NPA charge on Be (+0.62) compared to Mg (+1.21) and the strongly upfield 9Be NMR signal at -23.7 ppm are in line with considerable electron density on Be and the formal oxidation state assignment of MgII-Be0. Despite this Mgδ+-Beδ- polarity, 1 is extremely thermally stable and unreactive towards H2, CO, N2, cyclohexene and carbodiimide. It reacted with benzophenone, azobenzene, phenyl acetylene, CO2 and CS2. Reaction with 1-adamantyl azide led to reductive coupling and formation of an N6-chain. The azide reagent also inserted in the Cp*-Be bond. The inertness of 1 is likely due to bulky ligands protecting the Mg-Be unit.

Similarities and Differences in Benzene Reduction with Ca, Sr, Yb and Sm: Strong Evidence for Tetra-Anionic Benzene.

Inverse sandwich complexes of Yb and Sm stabilized by a bulky ß-diketiminate (BDI) ligand have been prepared: (BDI)Ln(η6,η6-C6H6)Ln(BDI); Ln=lanthanide. Coordinated benzene ligands can be neutral, di-anionic or, often controversially discussed, even tetra-anionic. The formal charge on benzene is correlated to assignment of the metal oxidation state which generally poses a problem. Herein, we take advantage of the structural similarities found when comparing CaII with YbII, and SrII with SmII complexes. In this work, we found an excellent overlap of the Ca/Yb inverse sandwich structures but a striking difference for the Sr/Sm pair. The much shorter Sm-N and Sm-C6H6 distances are strong evidence for a SmIII-benzene-4-SmIII assignment. This was further supported by NMR spectroscopy, magnetic susceptibility, reactivity and comprehensive computational investigation.

Silver cation tagged on 5,7,12,14-tetraphenyl-6,13-diazapentacene and its dihydro-form.

The attachment of silver(I) cations to 5,7,12,14-tetraphenyl-6,13-diazapentacene and its reduced dihydro-form has been studied by electrospray ionization mass spectrometry (ESI-MS). The structure elucidation of the Ag+ complexes has been accomplished in gas-phase collision experiments in conjunction with density functional theory (DFT) calculations. The oxidized form provides a favourable cavity for the Ag+ ion, leading to the [1 : 1] complex with the highest resilience towards dissociation and severely hindering the attainment of a second molecular ligand. When the nitrogen is hydrogenated in the reduced dihydro-form, the cavity is partly blocked. This leads to a less strongly bound [1 : 1] complex ion but facilitates the attachment of a second molecular ligand to the Ag+. The resulting complex is the most stable among the [2 : 1] complexes. DFT calculations provide valuable insight into the geometries of the complex ions. Adding silver(I) to the reduced dihydro-form for cationization also induces its oxidation in solution. The oxidative dehydrogenation reaction, for which a mechanism is proposed, proceeds by first order kinetics and is markedly accelerated by day light.

Lithium Aluminium Hydride and Metallic Iron: A Powerful Team in Alkene and Arene Hydrogenation Catalysis.

Alkenes that normally do not react with LiAlH4 (3-hexene, cyclohexene, 1-Me-cyclohexene), can be reduced to the corresponding alkanes by a mixture of LiAlH4 and Fe0 (the iron was activated by Metal-Vapour-Synthesis). This alkene-to-alkane conversion with a stoichiometric quantity of LiAlH4 /Fe0 does not need quenching with water or acids, implying that both H's originate from LiAlH4 . The LiAlH4 /Fe0 combination is also a remarkably potent cooperative catalyst for hydrogenation of multi-substituted alkenes and benzene or toluene. An induction period of circa two hours and the minimally required temperature of 120 °C, suggests that the actual catalyst is a combination of Fe0 and the decomposition product of LiAlH4 (LiH and Al0 ). A thermally pre-activated LiAlH4 /Fe0 catalyst did not need an induction time and is also active at room temperature and 1 bar H2 . A combination of AliBu3 and Fe0 is an even more active hydrogenation catalyst. Without pre-activation, tetra-substituted alkenes like Me2 C=CMe2 and toluene could be fully hydrogenated.

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.

Access to a Labile Monomeric Magnesium Radical by Ball-Milling.

In order to isolate a monometallic Mg radical, the precursor (Am)MgI⋅(CAAC) (1) was prepared (Am=tBuC(N-DIPP)2 , DIPP=2,6-diisopropylphenyl, CAAC=cyclic (alkyl)(amino)carbene). Reduction of a solution of 1 in toluene with the reducing agent K/KI led to formation of a deep purple complex that rapidly decomposed. Ball-milling of 1 with K/KI gave the low-valent MgI complex (Am)Mg⋅(CAAC) (2) which after rapid extraction with pentane and crystallization was isolated in 15 % yield. Although a benzene solution of 2 decomposes rapidly to give Mg(Am)2 (3) and unidentified products, the radical is stable in the solid state. Its crystal structure shows planar trigonal coordination at Mg. The extremely short Mg-C distance of 2.056(2) Šindicates strong Mg-CAAC bonding. Calculations and EPR measurements show that most of the spin density is in a π* orbital located at the C-N bond in CAAC, leading to significant C-N bond elongation. This is supported by calculated NPA charges in 2: Mg +1.73, CAAC -0.82. Similar metal-to-CAAC charge transfer was calculated for M0 (CAAC)2 and [MI (CAAC)2 + ] (M=Be, Mg, Ca) complexes in which the metal charges range from +1.50 to +1.70. Although the spin density of the radical is mainly located at the CAAC ligand, complex 2 reacts as a low-valent MgI complex: reaction with a I2 solution in toluene gave (Am)MgI⋅(CAAC) (1) as the major product.

Insights into LiAlH4 Catalyzed Imine Hydrogenation.

Commercial LiAlH4 can be used in catalytic quantities in the hydrogenation of imines to amines with H2 . Combined experimental and theoretical investigations give deeper insight in the mechanism and identifies the most likely catalytic cycle. Activity is lost when Li in LiAlH4 is exchanged for Na or K. Exchanging Al for B or Ga also led to dramatically reduced activities. This indicates a heterobimetallic mechanism in which cooperation between Li and Al is crucial. Potential intermediates on the catalytic pathway have been isolated from reactions of MAlH4 (M=Li, Na, K) and different imines. Depending on the imine, double, triple or quadruple imine insertion has been observed. Prolonged reaction of LiAlH4 with PhC(H)=NtBu led to a side-reaction and gave the double insertion product LiAlH2 [N]2 ([N]=N(tBu)CH2 Ph) which at higher temperature reacts further by ortho-metallation of the Ph ring. A DFT study led to a number of conclusions. The most likely catalyst for hydrogenation of PhC(H)=NtBu with LiAlH4 is LiAlH2 [N]2 . Insertion of a third imine via a heterobimetallic transition state has a barrier of +23.2 kcal mol-1 (ΔH). The rate-determining step is hydrogenolysis of LiAlH[N]3 with H2 with a barrier of +29.2 kcal mol-1 . In agreement with experiment, replacing Li for Na (or K) and Al for B (or Ga) led to higher calculated barriers. Also, the AlH4 - anion showed very high barriers. Calculations support the experimentally observed effects of the imine substituents at C and N: the lowest barriers are calculated for imines with aryl-substituents at C and alkyl-substituents at N.

Unsupported Mg-Alkene Bonding.

The first intermolecular early main group metal-alkene complexes were isolated. This was enabled by using highly Lewis acidic Mg centers in the Lewis base-free cations (Me BDI)Mg+ and (tBu BDI)Mg+ with B(C6 F5 )4 - counterions (Me BDI=CH[C(CH3 )N(DIPP)]2 , tBu BDI=CH[C(tBu)N(DIPP)]2 , DIPP=2,6-diisopropylphenyl). Coordination complexes with various mono- and bis-alkene ligands, typically used in transition metal chemistry, were structurally characterized for 1,3-divinyltetramethyldisiloxane, 1,5-cyclooctadiene, cyclooctene, 1,3,5-cycloheptatriene, 2,3-dimethylbuta-1,3-diene, and 2-ethyl-1-butene. In all cases, asymmetric Mg-alkene bonding with a short and a long Mg-C bond is observed. This asymmetry is most extreme for Mg-(H2 C=CEt2 ) bonding. In bromobenzene solution, the Mg-alkene complexes are either dissociated or in a dissociation equilibrium. A DFT study and AIM analysis showed that the C=C bonds hardly change on coordination and there is very little alkene→Mg electron transfer. The Mg-alkene bonds are mainly electrostatic and should be described as Mg2+ ion-induced dipole interactions.

Cationic Aluminium Complexes as Catalysts for Imine Hydrogenation.

Strongly Lewis acidic cationic aluminium complexes, stabilized by ß-diketiminate (BDI) ligands and free of Lewis bases, have been prepared as their B(C6 F5 )4 - salts and were investigated for catalytic activity in imine hydrogenation. The backbone (R1) and N (R2) substituents on the R1,R2 BDI ligand (R1,R2 BDI=HC[C(R1)N(R2)]2 ) influence sterics and Lewis acidity. Ligand bulk increases along the row Me,DIPP BDI

Cationic Heterobimetallic Mg(Zn)/Al(Ga) Combinations for Cooperative C-F Bond Cleavage.

Low-valent (Me BDI)Al and (Me BDI)Ga and highly Lewis acidic cations in [(tBu BDI)M+ ⋅C6 H6 ][(B(C6 F5 )4 - ] (M=Mg or Zn, Me BDI=HC[C(Me)N-DIPP]2 , tBu BDI=HC[C(tBu)N-DIPP]2 , DIPP=2,6-diisopropylphenyl) react to heterobimetallic cations [(tBu BDI)Mg-Al(Me BDI)+ ], [(tBu BDI)Mg-Ga(Me BDI)+ ] and [(tBu BDI)Zn-Ga(Me BDI)+ ]. These cations feature long Mg-Al (or Ga) bonds while the Zn-Ga bond is short. The [(tBu BDI)Zn-Al(Me BDI)+ ] cation was not formed. Combined AIM and charge calculations suggest that the metal-metal bonds to Zn are considerably more covalent, whereas those to Mg should be described as weak AlI (or GaI )→Mg2+ donor bonds. Failure to isolate the Zn-Al combination originates from cleavage of the C-F bond in the solvent fluorobenzene to give (tBu BDI)ZnPh and (Me BDI)AlF+ which is extremely Lewis acidic and was not observed, but (Me BDI)Al(F)-(µ-F)-(F)Al(Me BDI)+ was verified by X-ray diffraction. DFT calculations show that the remarkably facile C-F bond cleavage follows a dearomatization/rearomatization route.

Metallic Barium: A Versatile and Efficient Hydrogenation Catalyst.

Ba metal was activated by evaporation and cocondensation with heptane. This black powder is a highly active hydrogenation catalyst for the reduction of a variety of unactivated (non-conjugated) mono-, di- and tri-substituted alkenes, tetraphenylethylene, benzene, a number of polycyclic aromatic hydrocarbons, aldimines, ketimines and various pyridines. The performance of metallic Ba in hydrogenation catalysis tops that of the hitherto most active molecular group 2 metal catalysts. Depending on the substrate, two different catalytic cycles are proposed. A: a classical metal hydride cycle and B: the Ba metal cycle. The latter is proposed for substrates that are easily reduced by Ba0 , that is, conjugated alkenes, alkynes, annulated rings, imines and pyridines. In addition, a mechanism in which Ba0 and BaH2 are both essential is discussed. DFT calculations on benzene hydrogenation with a simple model system (Ba/BaH2 ) confirm that the presence of metallic Ba has an accelerating effect.

Boosting Low-Valent Aluminum(I) Reactivity with a Potassium Reagent.

The reagent RK [R=CH(SiMe3 )2 or N(SiMe3 )2 ] was expected to react with the low-valent (DIPP BDI)Al (DIPP BDI=HC[C(Me)N(DIPP)]2 , DIPP=2,6-iPr-phenyl) to give [(DIPP BDI)AlR]- K+ . However, deprotonation of the Me group in the ligand backbone was observed and [H2 C=C(N-DIPP)-C(H)=C(Me)-N-DIPP]Al- K+ (1) crystallized as a bright-yellow product (73 %). Like most anionic AlI complexes, 1 forms a dimer in which formally negatively charged Al centers are bridged by K+ ions, showing strong K+ ⋅⋅⋅DIPP interactions. The rather short Al-K bonds [3.499(1)-3.588(1) Å] indicate tight bonding of the dimer. According to DOSY NMR analysis, 1 is dimeric in C6 H6 and monomeric in THF, but slowly reacts with both solvents. In reaction with C6 H6 , two C-H bond activations are observed and a product with a para-phenylene moiety was exclusively isolated. DFT calculations confirm that the Al center in 1 is more reactive than that in (DIPP BDI)Al. Calculations show that both AlI and K+ work in concert and determines the reactivity of 1.

Highly Active Superbulky Alkaline Earth Metal Amide Catalysts for Hydrogenation of Challenging Alkenes and Aromatic Rings.

Two series of bulky alkaline earth (Ae) metal amide complexes have been prepared: Ae[N(TRIP)2 ]2 (1-Ae) and Ae[N(TRIP)(DIPP)]2 (2-Ae) (Ae=Mg, Ca, Sr, Ba; TRIP=SiiPr3 , DIPP=2,6-diisopropylphenyl). While monomeric 1-Ca was already known, the new complexes have been structurally characterized. Monomers 1-Ae are highly linear while the monomers 2-Ae are slightly bent. The bulkier amide complexes 1-Ae are by far the most active catalysts in alkene hydrogenation with activities increasing from Mg to Ba. Catalyst 1-Ba can reduce internal alkenes like cyclohexene or 3-hexene and highly challenging substrates like 1-Me-cyclohexene or tetraphenylethylene. It is also active in arene hydrogenation reducing anthracene and naphthalene (even when substituted with an alkyl) as well as biphenyl. Benzene could be reduced to cyclohexane but full conversion was not reached. The first step in catalytic hydrogenation is formation of an (amide)AeH species, which can form larger aggregates. Increasing the bulk of the amide ligand decreases aggregate size but it is unclear what the true catalyst(s) is (are). DFT calculations suggest that amide bulk also has a noticeable influence on the thermodynamics for formation of the (amide)AeH species. Complex 1-Ba is currently the most powerful Ae metal hydrogenation catalyst. Due to tremendously increased activities in comparison to those of previously reported catalysts, the substrate scope in hydrogenation catalysis could be extended to challenging multi-substituted unactivated alkenes and even to arenes among which benzene.

d-d Dative Bonding Between Iron and the Alkaline-Earth Metals Calcium, Strontium, and Barium.

Double deprotonation of the diamine 1,1'-(tBuCH2 NH)-ferrocene (1-H2 ) by alkaline-earth (Ae) or EuII metal reagents gave the complexes 1-Ae (Ae=Mg, Ca, Sr, Ba) and 1-Eu. 1-Mg crystallized as a monomer while the heavier complexes crystallized as dimers. The Fe⋅⋅⋅Mg distance in 1-Mg is too long for a bonding interaction, but short Fe⋅⋅⋅Ae distances in 1-Ca, 1-Sr, and 1-Ba clearly support intramolecular Fe⋅⋅⋅Ae bonding. Further evidence for interactions is provided by a tilting of the Cp rings and the related 1 H NMR chemical-shift difference between the Cp α and ß protons. While electrochemical studies are complicated by complex decomposition, UV/Vis spectral features of the complexes support Fe→Ae dative bonding. A comprehensive bonding analysis of all 1-Ae complexes shows that the heavier species 1-Ca, 1-Sr, and 1-Ba possess genuine Fe→Ae bonds which involve vacant d-orbitals of the alkaline-earth atoms and partially filled d-orbitals on Fe. In 1-Mg, a weak Fe→Mg donation into vacant p-orbitals of the Mg atom is observed.

Early Main Group Metal Catalysts for Imine Hydrosilylation.

The efficient catalytic reduction of imines with phenylsilane is achieved by using the potassium, calcium and strontium based catalysts [(DMAT)K (THF)]∞ , (DMAT)2 Ca⋅(THF)2 and (DMAT)2 Sr⋅(THF)2 (DMAT=2-dimethylamino-α-trimethylsilylbenzyl). Eight different aldimines and the ketimine Ph2 C=NPh could be successfully reduced by PhSiH3 at temperatures between 25-60 °C with catalyst loadings down to 2.5 mol %. Also, simple amides like KN(SiMe3 )2 or Ae[N(SiMe3 )2 ]2 (Ae=Ca, Sr, Ba) catalyze this reaction. Activities increase with metal size. For most substrates the activity increases along the row K

Complexation and Versatile Reactivity of a Highly Lewis Acidic Cationic Mg Complex with Alkynes and Phosphines.

[(BDI)Mg+ ][B(C6 F5 )4 - ] (1; BDI=CH[C(CH3 )NDipp]2 ; Dipp=2,6-diisopropylphenyl) was prepared by reaction of (BDI)MgnPr with [Ph3 C+ ][B(C6 F5 )4 - ]. Addition of 3-hexyne gave [(BDI)Mg+ ⋅(EtC≡CEt)][B(C6 F5 )4 - ]. Single-crystal X-ray analysis, NMR investigations, Raman spectra, and DFT calculations indicate a significant Mg-alkyne interaction. Addition of the terminal alkynes PhC≡CH or Me3 SiC≡CH led to alkyne deprotonation by the BDI ligand to give [(BDI-H)Mg+ (C≡CPh)]2 ⋅2 [B(C6 F5 )4 - ] (2, 70 %) and [(BDI-H)Mg+ (C≡CSiMe3 )]2 ⋅2 [B(C6 F5 )4 - ] (3, 63 %). Addition of internal alkynes PhC≡CPh or PhC≡CMe led to [4+2] cycloadditions with the BDI ligand to give {Mg+ C(Ph)=C(Ph)C[C(Me)=NDipp]2 }2 ⋅ 2 [B(C6 F5 )4 - ] (4, 53 %) and {Mg+ C(Ph)=C(Me)C[C(Me)=NDipp]2 }2 ⋅2 [B(C6 F5 )4 - ] (5, 73 %), in which the Mg center is N,N,C-chelated. The (BDI)Mg+ cation can be viewed as an intramolecular frustrated Lewis pair (FLP) with a Lewis acidic site (Mg) and a Lewis (or Brønsted) basic site (BDI). Reaction of [(BDI)Mg+ ][B(C6 F5 )4 - ] (1) with a range of phosphines varying in bulk and donor strength generated [(BDI)Mg+ ⋅PPh3 ][B(C6 F5 )4 - ] (6), [(BDI)Mg+ ⋅PCy3 ][B(C6 F5 )4 - ] (7), and [(BDI)Mg+ ⋅ PtBu3 ][B(C6 F5 )4 - ] (8). The bulkier phosphine PMes3 (Mes=mesityl) did not show any interaction. Combinations of [(BDI)Mg+ ][B(C6 F5 )4 - ] and phosphines did not result in addition to the triple bond in 3-hexyne, but during the screening process it was discovered that the cationic magnesium complex catalyzes the hydrophosphination of PhC≡CH with HPPh2 , for which an FLP-type mechanism is tentatively proposed.

Magnesium Cyanide or Isocyanide?

Preference for the binding mode of the CN- ligand to Mg (Mg-CN vs. Mg-NC) is investigated. A monomeric Mg complex with a terminal CN ligand was prepared using the dipyrromethene ligand Mes DPM which successfully blocks dimerization. While reaction of (Mes DPM)MgN(SiMe3 )2 with Me3 SiCN gave the coordination complex (Mes DPM)MgN(SiMe3 )2 ⋅NCSiMe3 , reaction with (Mes DPM)Mg(nBu) led to (Mes DPM)MgNC⋅(THF)2 . A Mg-NC/Mg-CN ratio of ≈95:5 was established by crystal-structure determination and DFT calculations. IR studies show absorbances for CN stretching at 2085 cm-1 (Mg-NC) and 2162 cm-1 (Mg-CN) as confirmed by 13 C labeling. In solution and in the solid state, the CN ligand rotates within the pocket. The calculated isomerization barrier is only 12.0 kcal mol-1 and the 13 C NMR signal for CN decoalesces at -85 °C (Mg-NC: 175.9 ppm, Mg-CN: 144.3 ppm). Experiment and theory both indicate that Mg complexes with the CN- ligand should not be named cyanides but are more properly defined as isocyanides.

Calcium-Catalyzed Arene C-H Bond Activation by Low-Valent AlI.

The low-valent ß-diketiminate complex (DIPP BDI)Al is stable in benzene but addition of catalytic quantities of [(DIPP BDI)CaH]2 at 20 °C led to (DIPP BDI)Al(Ph)H (DIPP BDI=CH[C(CH3 )N-DIPP]2 , DIPP=2,6-diisopropylphenyl). Similar Ca-catalyzed C-H bond activation is demonstrated for toluene or p-xylene. For toluene a remarkable selectivity for meta-functionalization has been observed. Reaction of (DIPP BDI)Al(m-tolyl)H with I2 gave m-tolyl iodide, H2 and (DIPP BDI)AlI2 which was recycled to (DIPP BDI)Al. Attempts to catalyze this reaction with Mg or Zn hydride catalysts failed. Instead, the highly stable complexes (DIPP BDI)Al(H)M(DIPP BDI) (M=Mg, Zn) were formed. DFT calculations on the Ca hydride catalyzed arene alumination suggest that a similar but more loosely bound complex is formed: (DIPP BDI)Al(H)Ca(DIPP BDI). This is in equilibrium with the hydride bridged complex (DIPP BDI)Al(µ-H)Ca(DIPP BDI) which shows strongly increased electron density at Al. The combination of Ca-arene bonding and a highly nucleophilic Al center are key to facile C-H bond activation.