Examining the Generality of Metal-Ligand Cooperativity Across a Series of First-Row Transition Metals: Capture, Bond Activation, and Stabilization

We outline the generality and requirements for cooperative N2H4 capture, N-N bond scission, and amido stabilization across a series of first-row transition metal complexes bearing a pyridine(dipyrazole) ligand. This ligand contains a pair of flexibly tethered trialkylborane Lewis acids that enable hydrazine capture and M-NH2 stabilization. While the Lewis acids are required to bind N2H4, the identity of the metal dictates whether N-N bond scission can occur. The redox properties of the M(II) bis(amidoborane) series of complexes were investigated and reveal that ligand-based events prevail; oxidation results in the generation of a transiently formed aminyl radical, while reduction occurs at the redox-active pyridine(dipyrazole) ligand.

Reactive Extrusion and Magnesium (II) N-Heterocyclic Carbene Catalyst in Continuous PLA Production

Reactive extrusion and magnesium (II) N-heterocyclic carbene catalyst are successfully employed in continuous polylactide synthesis. The possibility of using six-membered N-heterocyclic carbene adducts to act as efficient catalysts towards the sustainable synthesis of poly(l-lactide) through ring-opening polymerization of l-lactide (LA) is first investigated in bulk batch reactions. Under optimized solvent-free conditions, polylactide (PLA) of moderate to high molecular weights and excellent optical activities are successfully achieved. These promising results are further applied in the continuous production of PLA in an extruder.

Transmetalation from Magnesium–NHCs—Convenient Synthesis of Chelating π-Acidic NHC Complexes

The synthesis of chelating N-heterocyclic carbene (NHC) complexes with considerable π-acceptor properties can be a challenging task. This is due to the dimerization of free carbene ligands, the moisture sensitivity of reaction intermediates or reagents, and challenges associated with the workup procedure. Herein, we report a general route using transmetalation from magnesium–NHCs. Notably, this route gives access to transition-metal complexes in quantitative conversion without the formation of byproducts. It therefore produces transition-metal complexes outperforming the conventional routes based on free or lithium-coordinated carbene, silver complexes, or in situ metalation in dimethyl sulfoxide (DMSO). We therefore propose transmetalation from magnesium–NHCs as a convenient and general route to obtain NHC complexes.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

Salt metathesis versus protonolysis routes for the synthesis of silylamide Hauser base (R2NMgX; X = halogen) and amido-Grignard (R2NMgR) complexes

The preparation of silylamide Hauser base (R2NMgX; X = halide) and amido-Grignard (R2NMgR) complexes from simple Grignard reagents using [K{N(SiMe2(t)Bu)2}]n, [K{N(SiMe2(t)Bu)(Si(i)Pr3)}]n and [K{N(Si(i)Pr3)2}]n, and their parent silylamines, was explored. Both salt metathesis and protonolysis routes proved ineffective with allylmagnesium chloride as a starting material due to complex Schlenk equilibria, with [Mg(N(RR'))(μ-Cl)(THF)]2 (N(RR') = {N(Si(t)BuMe2)2}(-), 1; {N(Si(t)BuMe2)(Si(i)Pr3)}(-), 2; {N(Si(i)Pr3)2}(-), 3) and [Mg{N(Si(i)Pr3)2}(μ-C3H5)]∞ (4) identified as minor products. In contrast, salt metathesis protocols using potassium silylamides and methylmagnesium iodide gave [Mg(N(RR'))(μ-CH3)]2 (N(RR') = {N(Si(t)BuMe2)2}(-), 7a; {N(Si(t)BuMe2)(Si(i)Pr3)}(-), 8; {N(Si(i)Pr3)2}(-), 9) and [Mg{N(Si(t)BuMe2)2}(CH3)(DME)] (7b), with [Mg{N(Si(t)BuMe2)2}(μ-I)(THF)]2 (10) isolated as a side-product during the preparation of 7a. Unusually, methylmagnesium iodide, di-n-butylmagnesium and 7-9 did not react with HN(RR') under the conditions we employed. The synthesis of [Na{N(Si(t)BuMe2)2}(THF)]2 (5a) and [Na{N(Si(t)BuMe2)2}(DME)2] (5b) from benzyl sodium and HN(Si(t)BuMe2)2, and a solvent-free structure of [K{N(Si(t)BuMe2)2}] (6), are also reported. Complexes 1, 5b, 7a, 7b, 8, 9 and 10 are fully characterised by single crystal XRD, multinuclear NMR and IR spectroscopy and elemental analysis, whereas complexes 2-4, 5a and 6 were identified by XRD only.

Surface modification of polyhedral nanocrystalline MgO with imidazolium carboxylates for dehydration reactions: a new approach

The surface of nanocrystalline MgO was modified with achiral and chiral imidazolium carboxylates which generate MgO–[NHC] complexes. Thus as synthesized complexes were utilized in the dehydration of glucose and also in selective dehydration of (±) nitro alcohol.

Synthesis, structure and reactivity study of magnesium amidinato complexes derived from carbodiimides and N,N'-bis(2,6-diisopropylphenyl)-1,4-diaza-butadiene ligands

We report an amidinato ligand-supported series of magnesium complexes obtained from the insertion of a magnesium-carbon bond into a carbon-nitrogen double bond of different carbodiimides and α-diimine ligands. The magnesium complexes [Mg(CH2Ph){CyN[double bond, length as m-dash]C(CH2Ph)NCy}]2 (), [Mg(CH2Ph){(i)PrN[double bond, length as m-dash]C(CH2Ph)N(i)Pr}]2 () and the homoleptic [Mg{(t)BuN[double bond, length as m-dash]C(CH2Ph)N(t)Bu}2] () (Cy = cyclohexyl, (i)Pr = isopropyl, (t)Bu = tert-butyl) were prepared by the reaction of dibenzyl magnesium [Mg(CH2Ph)2(Et2O)2] with the respective carbodiimides either in 1 : 1 or 1 : 2 molar ratio in toluene. The analogous reaction of [Mg(CH2Ph)2(Et2O)2] with the N,N'-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (Dipp2DAD) ligand afforded the corresponding homoleptic magnesium complex [Mg{DippN[double bond, length as m-dash]C(CH2Ph)CH2NDipp}2] () (Dipp = 2,6 diisopropylphenyl) in good yield. The solid-state structures of magnesium complexes were confirmed by single-crystal X-ray diffraction analysis. It was observed that in each case, a magnesium-carbon bond was inserted into the carbon-nitrogen double bond of either carbodiimides or Dipp2DAD resulting in a monoanionic amido-imino ligand. In a further reaction between and N-aryliminopyrrolyl ligand 2-(2,6-(i)Pr2C6H3N[double bond, length as m-dash]CH)C4H3NH (ImpDipp-H) in 1 : 2 molar ratio, a new magnesium complex [Mg(ImpDipp)2{CyN[double bond, length as m-dash]C(CH2Ph)NHCy}] (), with one amidinato and two aryliminopyrrolyl ligands in the coordination sphere, was obtained in good yield. In contrast, the homoleptic magnesium complex reacted with one equivalent of N-aryliminopyrrolyl ligand (ImpDipp-H) to produce another mixed ligated magnesium complex [Mg{DippN[double bond, length as m-dash]C(CH2Ph)CH2NDipp}(ImpDipp)] (), with a benzylated DAD ligand and aryliminopyrrolyl ligands in the coordination sphere. Further reaction of complex with benzyl alcohol (PhCH2OH) afforded the third mixed ligated magnesium complex [Mg{DippN[double bond, length as m-dash]C(CH2Ph)CH2NDipp}(OCH2Ph)2] () in very good yield. The magnesium complexes were characterised using standard analytical/spectroscopic techniques and their solid-state structures were established by single-crystal X-ray diffraction analysis.

Group 1 and 2 cyclic (alkyl)(amino)carbene complexes

The first examples of cyclic (alkyl)(amino)carbene (CAAC) ligands bound to electropositive metal centres (K, Mg, Sr and Ba) have been isolated and characterised.

An organo-NHC catalyzed domino addition approach for the selective synthesis of 5-butynylisoxazoles and subsequent Sonogashira coupling

An organo-NHC catalysed domino addition approach for the selective synthesis of 5-butynylisoxazoles and the subsequent Sonogashira cross-coupling for the selective synthesis of corresponding internal alkynes is described.

Magnesium and zinc complexes of functionalised, saturated N-heterocyclic carbene ligands: carbene lability and functionalisation, and lactide polymerisation catalysis

The synthesis of magnesium and zinc complexes of bidentate anionic alkoxide ligands with saturated-backbone carbene groups is reported. Mono(ligand) and bis(ligand) complexes [M(L(R))N''](2) and [M(L(R))(2)] (M = Mg, Zn, N'' = N(SiMe(3))(2), L(R) = [OCMe(2)CH(2){CNCH(2)CH(2)NR}] R = (i)Pr, Mes, Dipp) have been isolated, and some structurally characterised and compared with the new unsaturated carbene complex [Mg(L)(2)]. Reactions with silyl halides show either addition across the metal carbene bond, or across the metal alkoxide bond, in accordance with the metals' electronegativity difference: the metal alkoxide bonds are stronger for Mg(II) complexes, for which the carbene is silylated to form zwitterionic [MgI(Me(3)SiL(R))N''] (Me(3)SiL(R) = OCMe(2)CH(2){Me(3)SiCNCH(2)CH(2)NR}) while the metal-bound alkoxide group is silylated in the Zn(II) complexes forming [ZnI(Me(3)SiOL(R))N''] (Me(3)SiOL(R) = Me(3)SiOCMe(2)CH(2){CNCH(2)CH(2)NR}). The proligand [HL(R)] is silylated at the alcohol group, forming the iodide salt [Me(3)SiOCMe(2)CH(2){HCNCH(2)CH(2)NR}]I.Preliminary results on the use of these complexes as initiators for the polymerisation of rac-lactide are reported, and suggest different initiation mechanisms are occurring for the two metals, in agreement with the different silylation reactivity observed. The polymerisation reactions are facile at room temperature even without an initiator, and yield polymers of reasonable molecular weight and heterotacticity and with good PDI. These are the first magnesium NHC complexes demonstrated to effect lactide polymerisation.Also, an adduct instead of the anticipated potassium alkoxycarbene is generated from the reaction of the proligand [HL(R)] with potassium amide KN''; this has been structurally characterised.