Magnesium complexes based on an amido-bis(pyrazolyl) ligand: Synthesis, crystal structures, and use in lactide polymerization

Isoselective Polymerization of rac-Lactide by Magnesium Initiators Bearing Achiral Di(2-pyridyl)methyl Substituted Aminophenolate Ligands

Reactions of achiral di(2-pyridyl)methyl substituted aminophenols L^1-6H (2-{N-R³-N-[di(2-pyridyl)methyl]aminomethyl}-4-R¹-6-R²-C₆H₂OH: R¹ = R² = ^tBu, R³ = ⁿBu (L¹H), R³ = ⁿhexyl (L²H), R³ = cyclohexyl (L³H); R¹ = R² = cumyl, R³ = ⁿBu (L⁴H), R³ = ⁿhexyl (L⁵H), R³ = cyclohexyl (L⁶H)) with {Mg[N(SiMe₃)₂]₂}₂ ([L^1-6H]:[Mg] = 1:1) afforded a series of magnesium silylamido complexes 1-6. In the solid state, the magnesium center of 3, 4, and 6 is penta-coordinated by the tetradentate aminophenloate ligand and one silylamido ligand to form a seriously distorted square-pyramidal geometry as confirmed by X-ray crystallography diffraction analysis. VT ¹H NMR and ROESY experiments further indicate that these magnesium complexes are also five-coordinated in solutions where the coordination of either of the two pyridyl pendants to the magnesium center is maintained. Complexes 1-6 are highly active toward the ring-opening polymerization of rac-lactide (rac-LA) at r.t. both in toluene and in tetrahydrofuran, capable of polymerizing 500 equiv of monomer to high conversions just within minutes. Among them, complex 3 exhibited the highest iso-stereoselectivity, affording moderately isotactic polylactide in toluene (P_m = 0.75). It is found that the isoselectivities and activities of these magnesium complexes toward the polymerization of rac-LA are closely associated with the substituents at the ortho-position of the phenoxide unit and on the skeleton nitrogen atom of the ligand. On the basis of NMR spectroscopic studies, the formation of isotactic PLAs with dominant stereoblock sequences was witnessed by using these magnesium complexes as initiators, and the inequivalent coordination of two pyridyl pendant arms in these magnesium complexes might be the source of exerting isoselective control.

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.

Synthesis and catalytic application of magnesium complexes bearing pendant indolyl ligands

Three novel indole-based ligand precursors [HIndPh(R), R = methoxy, HIndPh(OMe) (); thiomethoxy, HIndPh(SMe) (); and N,N'-dimethylamino, HIndPh(NMe2) ()] have been synthesized via Sonogashira and cyclization reactions with moderate to high yield. Reactions of these ligand precursors with 0.6 equivalent of Mg(n)Bu2 in THF afforded the magnesium bis-indolyl complexes , respectively. All the ligand precursors and related magnesium complexes have been characterized by NMR spectroscopy and elemental analyses. The molecular structures are reported for compounds and . Under optimized conditions, compound demonstrates efficient catalytic activities towards the ring opening polymerization of l-lactide and ε-caprolactone in the presence of BnOH.

Ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate catalyzed by magnesium and zinc complexes derived from binaphthyl-based iminophenolate ligands

Magnesium and zinc complexes derived from binaphthyl-based iminophenolate ligands efficiently initiated the ring-opening polymerization of rac-lactide and α-MeTMC, with the highest heteroselectivity (P_r = 0.84) or regioselectivity (X_reg = 0.98) obtained for the zinc complex.

Coordination of bis-(pyrazol-1-yl)amine to palladium(II): influence of the co-ligands and counter-ions on the mol-ecular and crystal structures

The structures of a series of complexes with general formula n[Pd(pza)X]Y·mH2O (n = 1, 2; X = Cl, Br, I, N3, NCS; Y = NO3, I, N3, [Pd(SCN)4]; m = 0, 0.5, 1) have been determined, where pza is the tridentate ligand bis-[2-(3,5-di-methyl-pyrazol-1-yl)eth-yl]amine, C14H23N5. In all complexes, {bis-[2-(3,5-di-methyl-pyrazol-1-yl-κN (2))eth-yl]amine-κN}chlorido-palladium nitrate, [Pd(pza)Cl]NO3, (1), {bis-[2-(3,5-di-methyl-pyrazol-1-yl-κN (2))eth-yl]amine-κN}bromido-palladium nitrate, [Pd(pza)Br]NO3, (2), {bis-[2-(3,5-di-methyl-pyrazol-1-yl-κN (2))eth-yl]amine-κN}iodido-palladium iodide hemihydrate, [Pd(pza)I]I·0.5H2O, (3), azido{bis-[2-(3,5-di-methyl-pyrazol-1-yl-κN (2))eth-yl]amine-κN}palladium azide monohydrate, [Pd(pza)N3]N3·H2O, (4), and bis-[{bis-[2-(3,5-di-methyl-pyrazol-1-yl-κN (2))eth-yl]amine-κN}(thio-cyanato-κN)palladium] tetra-kis-(thio-cyanato-κS)palladate, [Pd(pza)NCS]2[Pd(SCN)4], (5), the [Pd(pza)X](+) complex cation displays a square-planar coordination geometry, and the pza ligand is twisted, approximating twofold rotation symmetry. Although the pza ligand is found with the same conformation along the series, the dihedral angle between pyrazole rings depends on the co-ligand X. This angle span the range 79.0 (3)-88.6 (1)° for the studied complexes. In (3), two complex cations, two I(-) anions and one water mol-ecule of crystallization are present in the asymmetric unit. In (5), the central amine group of pza is disordered over two positions [occupancy ratio 0.770 (18):0.230 (18)]. The complex [Pd(SCN)4](2-) anion of this compound exhibits inversion symmetry and shows the Pd(2+) transition metal cation likewise in a square-planar coordination environment. Compound (5) is also a rare occurrence of a non-polymeric compound in which the pseudohalide ligand NCS(-) behaves both as thio-cyanate and iso-thio-cyanate, i.e. is coordinating either through the N atom (in the cation) or the S atom (in the anion).

Ring-opening polymerization of lactides catalyzed by natural amino-acid based zinc catalysts

A series of chiral NNO-tridentate Schiff base ligands derived from natural amino acids were reacted with zinc(bis-trimethylsilylamide)(2) to provide metal complexes which have been fully characterized. One of these derivatives was further reacted with p-fluorophenol to yield a phenoxide complex. X-ray crystallographic studies reveal the zinc Schiff base amide complexes to be monomeric, whereas, the p-fluorophenolate complex was shown to be dimeric with bridging phenoxide ligands. All zinc complexes were shown to be very effective catalysts for the ring-opening polymerization (ROP) of lactides at ambient temperature, producing polymers with controlled and narrow molecular weight distributions. These enantiomerically pure zinc complexes did not show selectivity toward either L- or D-lactide, that is, k(D(obsd))/k(L(obsd)) approximately = 1. However, steric substituents on the Schiff base ligands exhibited moderate to excellent stereocontrol for the ROP of rac-lactide. Heterotactic polylactides were produced from rac-lactide with P(r) values ranging from 0.68 to 0.89, depending on the catalyst employed and the reaction temperature. The reactivities of the various catalysts were greatly affected by substituents on the Schiff base ligands, with sterically bulky substituents being rate enhancing.

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.