Mechanism of the alternating copolymerization of epoxides and CO2 using beta-diiminate zinc catalysts: evidence for a bimetallic epoxide enchainment

A series of zinc beta-diiminate (BDI) complexes and their solid-state structures, solution dynamics, and copolymerization behavior with CO(2) and cyclohexene oxide (CHO) are reported. Stoichiometric reactions of the copolymerization initiation steps show that zinc alkoxide and bis(trimethylsilyl)amido complexes insert CO(2), whereas zinc acetates react with CHO. [(BDI-2)ZnOMe](2) [(BDI-2) = 2-((2,6-diethylphenyl)amido)-4-((2,6-diethylphenyl)imino)-2-pentene] and (BDI-1)ZnO(i)Pr [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene] react with CO(2) to form [(BDI-2)Zn(mu-OMe)(mu,eta(2)-O(2)COMe)Zn(BDI-2)] and [(BDI-1)Zn(mu,eta(2)-O(2)CO(i)Pr)](2), respectively. (BDI-2)ZnN(SiMe(3))(2) inserts CO(2) and eliminates trimethylsilyl isocyanate to give [(BDI-2)Zn(mu-OSiMe(3))](2). [(BDI-7)Zn(mu-OAc)](2) [(BDI-7) = 3-cyano-2-((2,6-diethylphenyl)amido)-4-((2,6-diethylphenyl)imino)-2-pentene] reacts with 1.0 equiv of CHO to yield [(BDI-7)Zn(mu,eta(2)-OAc)(mu,eta(1)-OCyOAc)Zn(BDI-7)]. Under typical polymerization conditions, rate studies on the copolymerization exhibit no dependence in [CO(2)], a first-order dependence in [CHO], and orders in [Zn](tot) ranging from 1.0 to 1.8 for [(BDI)ZnOAc] complexes. The copolymerizations of CHO (1.98 M in toluene) and 300 psi CO(2) at 50 degrees C using [(BDI-1)ZnOAc] and [(BDI-2)ZnOAc] show orders in [Zn](tot) of 1.73 +/- 0.06 and 1.02 +/- 0.03, respectively. We propose that two zinc complexes are involved in the transition state of the epoxide ring-opening event.     

Single-site beta-diiminate zinc catalysts for the alternating copolymerization of CO2 and epoxides: catalyst synthesis and unprecedented polymerization activity

Synthetic routes to zinc beta-diiminate complexes are reported. The synthesis of 11 beta-diimine [(BDI)-H] ligands, with varying N-aryl substituents and bridging structures, is described. These ligands are converted to (BDI)ZnX complexes (X = OAc, Et, N(SiMe3)2, Br, Cl, OH, OMe, O(i)Pr). X-ray structural data revealed that all zinc complexes examined exist as micro-X-bridged dimers in the solid state, with the exception of the zinc ethyl and amido complexes which were monomeric. Complexes of the form (BDI)ZnOR (R = alkyl, acyl) and (BDI)ZnN(SiMe3)2 are highly active catalysts for the alternating copolymerization of epoxides and CO2. Copolymerizations of cyclohexene oxide (CHO) and CO2 with (BDI-1)ZnX [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene)] and (BDI-2)ZnX [(BDI-2) = 2-((2,6-diethylphenyl)amido)-4-((2,6-diethylphenyl)imino)-2-pentene)], where X = OAc, Et, N(SiMe3)2, Br, Cl, OH, OMe, O(i)Pr, were attempted at 50 degrees C and 100 psi CO2. Complexes with X = OAc, N(SiMe3)2, OMe, O(i)Pr all produced polycarbonate by the alternated insertion of CHO and CO2 with similar catalytic activities, comparable molecular weights, and narrow molecular weight distributions (MWD approximately 1.1), indicating the copolymerizations are living. Furthermore, ligand effects were shown to dramatically influence the polymerization activity as minor steric changes accelerated or terminated the polymerization activity.     

Polymerization of lactide with zinc and magnesium beta-diiminate complexes: stereocontrol and mechanism.

A series of zinc(II) and magnesium(II) alkoxides based upon a beta-diiminate ligand framework has been prepared. [(BDI-1)ZnO(i)Pr](2) [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene] exhibited the highest activity and stereoselectivity of the zinc complexes studied for the polymerization of rac- and meso-lactide to poly(lactic acid) (PLA). [(BDI-1)ZnO(i)()Pr](2) polymerized (S,S)-lactide to isotactic PLA without epimerization of the monomer, rac-lactide to heterotactic PLA (P(r) = 0.94 at 0 degrees C), and meso-lactide to syndiotactic PLA (P(r) = 0.76 at 0 degrees C). The polymerizations are living, as evidenced by the narrow polydispersities of the isolated polymers in addition to the linear nature of number average molecular weight versus conversion plots and monomer-to-catalyst ratios. The substituents on the beta-diiminate ligand exert a significant influence upon the course of the polymerizations, affecting both the degree of stereoselectivity and the rate of polymerization. Kinetic studies with [(BDI-1)ZnO(i)Pr](2) indicate that the polymerizations are first order with respect to monomer (rac-lactide) and 1.56 order in catalyst. Polymerization experiments with [(BDI-1)MgO(i)Pr](2) revealed that this complex is extremely fast for the polymerization of rac-lactide, polymerizing 500 equiv in 96% yield in less than 5 min at 20 degrees C.     

Structure and reactivity of mono- and dinuclear diiminate zinc alkyl complexes

The synthesis, structure and reactivity of several diiminate ligands are presented. The syntheses of five representative β-diiminate (BDI) zinc alkyl complexes and one β-oxo-δ-diiminate (BODDI) zinc alkyl are described. BDI ligands with varying backbone and N-aryl substituents display different solid state structures. [(BDI)ZnR] are synthesized by the reaction of (BDI)H with ZnR₂ in quantitative yield. Previously reported (BDI-1)ZnEt is a three-coordinate monomer in the solid state whereas [(BDI-3)ZnEt]_∞ [(BDI-3)=2-((2,6-diisopropylphenyl)amido)-3-cyano-4-((2,6-diisopropylphenyl)imino-2-pentene] and [(BDI-4)ZnEt]_∞ [(BDI-4)=2-((2,6-diethylphenyl)amido)-3-cyano-4-((2,6-diethylphenyl)imino-2-pentene] form one dimensional coordination polymers. The bimetallic complex [(BODDI-1)(ZnEt)₂] [(BODDI-1)=2,6-bis((2,6-diisopropylphenyl)amido)-2,5-heptadien-4-one] is prepared through the reaction of (BODDI-1)H₂ with two equivalents ZnEt₂. Both [(BDI)ZnEt] and [(BODDI)ZnEt] complexes react with acetic acid to give the acetate complexes in moderate to high yields, offering a superior synthetic route to these complexes. [(BDI)ZnR] [BDI=(BDI-3) or 1,1,1-trifluoro-2-((2,6-diisopropylphenyl)amido)-4-((2,6-diethylphenyl)imino-2-pentene), (BDI-5)] complexes react with MeOH to produce [{(BDI)Zn(μ-OMe)}₂Zn(μ-OMe)₂] in moderate yields. The molecular structures of [(BDI-3)ZnEt], [(BDI-4)ZnEt], [(BODDI-1)(ZnEt)₂], [(BODDI-1)Zn₂(μ-OAc)₂], [{(BDI-3)Zn(μ-OMe)}₂Zn(μ-OMe)₂] and [{(BDI-5)Zn(μ-OMe)}₂Zn(μ-OMe)₂] have been determined by X-ray diffraction.     

Insertion of Ga(DDP) into the Au-Cl bond of (PPh3)AuCl: a first structurally characterized Au-Ga bond

The reaction of (PPh(3))AuCl with the low valent gallium compound Ga(DDP) (DDP = 2-((2,6-diisopropylphenyl)amino-4-((2,6-diiso-propylphenyl)imino)-2-pentene) yields the insertion products [{Ga(DDP)}Au{Ga(DDP)Cl}] (1) and [(PPh(3))Au{Ga(DDP)Cl}] (2), the first examples of molecular compounds with Au-Ga bonds.     

The Al(I) bisimidinate Al(DDP) as a metalloid NHC type ligand for Pd(0) complexes and clusters

Two clusters with a Pd2 core and Al(DDP) (DDP = 2-((2,6-diisopropylphenyl)amino)-4-((2,6-diisopropylphenyl)imino)-2-pentene) as a bridging ligand have been synthesized and characterized by single crystal structural analysis; the results suggest a strong similarity of the coordination properties of NHC's and the group 13 analogue Al(DDP).     

Comparative study of the coordination chemistry and lactide polymerization of alkoxide and amide complexes of zinc and magnesium with a beta-diiminato ligand bearing ether substituents

A series of beta-diiminato complexes of the form (BDI-3)MX where (BDI-3) = [CH(CMeNC(6)H(4)-2-OMe)(2)]; M = Zn, Mg; X = O(i)()Pr, O(t)()Bu, or N(SiMe(3))(2) has been synthesized. The (BDI-3) ligand is bidentate in (BDI-3)ZnN(SiMe(3))(2) and tetradentate in (BDI-3)MgN(SiMe(3))(2). The alkoxide complexes are shown to be active for lactide polymerization. Polymerization of rac-lactide with (BDI-3)ZnO(i)Pr gives a moderate preference for heterotactic PLA. Polymerization of rac-lactide with [(BDI-3)MgO(t)Bu](2) shows a slight preference for heterotactic PLA in CH(2)Cl(2) but is highly stereoselective in THF in the production of heterotactic PLA.     

Bis(imino)pyridine ligand deprotonation promoted by a transient iron amide

Addition of 2 equiv of LiNMe(2) to the bis(imino)pyridine ferrous dichloride, ((i)(Pr)PDI)FeCl(2) ((i)(Pr)PDI = (2,6-(i)()Pr(2)-C(6)H(3)N=CMe)(2)C(5)H(3)N), resulted in deprotonation of the chelate methyl groups, yielding the bis(enamide)pyridine iron dimethylamine adduct, ((i)(Pr)PDEA)Fe(NHMe(2)) ((i)(Pr)PDEA = (2,6-(i)Pr(2)-C(6)H(3)NC=CH(2))(2)C(5)H(3)N). Performing a similar procedure with KN(SiMe(3))(2) in THF solution afforded the corresponding bis(THF) adduct, ((i)(Pr)PDEA)Fe(THF)(2). ((i)(Pr)PDEA)Fe(NHMe(2)) has also been prepared by addition of the free amine to the iron dialkyl complex, ((i)(Pr)PDI)Fe(CH(2)SiMe(3))(2), implicating formation of a transient iron amide that is sufficiently basic to deprotonate the bis(imino)pyridine methyl groups. Deprotonation of the amine ligand in ((i)(Pr)PDEA)Fe(NHMe(2)) has been accomplished by addition of amide bases to afford the ferrous amide-ate complexes, [((i)(Pr)PDEA)Fe(mu-NMe(2))M] (M = Li, K).     

Insertion of the Ga(I) bis-imidinate Ga(DDP) into the metal halogen bonds of Rh(I) complexes. How electrophilic are coordinated Ga(DDP) fragments?

The reactivity of Ga(DDP) (DDP = 2-((2,6-diisopropylphenyl)amino-4-((2,6-diisopropylphenyl)imino)-2-pentene) towards the rhodium-chloride bonds of [RhCl(PPh3)3] and [RhCl(COE)2]2 (COE = cyclooctene) is investigated. Reaction of the first complex leads to [(Ph3P)2Rh{Ga(DDP)}(mu-Cl)] (1), exhibiting a chloride bridging the gallium and the rhodium atoms, whereas the second complex leads to a full insertion of the Ga(DDP) ligand into the Rh-Cl bond giving [(COE)(benzene)Rh{(DDP)GaCl}] (2) on coordination of the solvent C6H6. Compounds 1 und 2 readily react with the halide abstracting reagent Tl[BArF] (BArF = B[3,5-(CF3)2C6H3]4), yet the products could not be isolated and characterized because of their lability. The Au(I) complex [{(DDP)Ga}Au{Ga(DDP)}Cl] reacts with Na[BArF] giving the linear, symmetric cationic complex [{(DDP)Ga.THF}2Au][BArF] (3.2THF), exhibiting two THF molecules coordinated to the Ga(DDP) moieties.     

Pseudo-tetrablock copolymers with ethylene and a functionalized comonomer

Pseudo-tetrablock copolymers comprised of ethylene and 5-norbomen-2-yl acetate (1), were synthesized using the initiator system (L(i)Pr2)Ni(eta1-CH2Ph)(PMe3)(2)[(L(i)Pr2) = N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)propanamide] and 2.5 equivalents of Ni(COD)2 [bis(1,5-cyclooctadiene) nickel.     

Synthesis and characterization of new unsymmetrical β-diketiminate tris(dimethylamido)hafnium(IV) complexes as potential precursors for the MOCVD of HfO2

The synthesis and characterization of four new unsymmetrical β-diketiminate tris(dimethylamido)hafnium(IV) complexes, [2-(2,6-diisopropylphenyl)amino-4-(phenyl)imino-2-pentene]tris(dimethylamido)hafnium(IV) (5a), [2-(2,6-diisopropylphenyl)amino-4-(4-methylphenyl)imino-2-pentene]tris(dimethylamido)hafnium(IV) (5b), [2-(2,6-diisopropyl-phenyl)amino-4-(4-methoxyphenyl)imino-2-pentene]tris(dimethylamido)hafnium(IV) (5c), and [2-(2,6-diisopropylphenyl)amino-4-(4-chlorophenyl)imino-2-pentene]tris(dimethylamido)hafnium(IV) (5d), are described. Amine elimination reactions work well for introducing unsymmetrical β-diketiminates2-(2,6-diisopropylphenyl)amino-4-(phenyl)imino-2-pentene (4a), 2-(2,6-diisopropylphenyl)amino-4-(4-methylphenyl)imino-2-pentene (4b), 2-(2,6-diisopropylphenyl)amino-4-(4-methoxyphenyl)imino-2-pentene (4c), and 2-(2,6-diisopropylphenyl)amino-4-(4-chlorophenyl)imino-2-pentene (4d) to the tetrakis(dimethylamino)hafnium centre. We discuss the synthetic procedures and characterization using ¹H NMR, ¹³C NMR, IR, mass spectroscopy, and elemental analysis. According to the IR and NMR spectra, unsymmetrical β-diketiminate ligands are bidentate, coordinating through two nitrogens to hafnium.     

Magnesium and zinc complexes of a potentially tridentate beta-diketiminate ligand

The synthesis of the unsymmetrically substituted beta-diketimine, 2-(2-methoxyphenylimino)-4-(2,6-diisopropylphenylamido)pent-2-ene, (BDI-2)H, is described and its complexation chemistry with magnesium and zinc is explored. Emphasis is placed on the preparation of alkoxide and amide derivatives for the ring-opening polymerisation of lactide; their behaviour as polymerisation initiators is compared to analogous compounds supported by the N,N'-bis(2,6-diisopropylphenyl) beta-diketiminate ligand, BDI-1. (BDI-2)H reacts with Me2Mg to give the bis(chelate) complex, (BDI-2)2Mg, 3. Magnesium alkyls supported by BDI-2 may be prepared by increasing the size of the alkyl group. Hence, lithiation of (BDI-2)H affords [(BDI-2)Li]2, 4; its subsequent treatment with iPrMgCl produces (BDI-2)MgiPr, 5. Aminolysis of complex using iPr2NH yields the amide complex, (BDI-2)MgNiPr2, 6. Zn(NTMS2)2 and ZnEt2 react with (BDI-2)H to give (BDI-2)Zn(NTMS2), 7, and (BDI-2)ZnEt, 8, respectively. The former is converted into the siloxide complex, (BDI-2)Zn(OSiPh3), 9, upon reaction with Ph3SiOH. The chloride derivative, (BDI-2)ZnCl, 10, has also been prepared via the reaction of ZnCl2 with 4. Crystallographic analysis of compounds, and reveals that the potential for (BDI-2) to bind in a tridentate manner is only realised with the more electrophilic metals Li and Mg. Compared to their (BDI-1) counterparts, complexes 6, 7 and 9 are more active, but less well-controlled, initiators for the ring-opening polymerisation of rac-lactide, a consequence of the diminished steric protection afforded by (BDI-2) relative to (BDI-1).     

Synthesis and reactivity of the complexes [(dpp-bian)SiCl2] and [(dpp-bian)Si{FeCp(CO)}2(μ-CO)] (dpp-bian is 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene)

Russian Chemical Bulletin - The complex [(dpp-bian)SiCl2] (1) was synthesized by the reaction of the free 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-bian) ligand with Si2Cl6. Despite...     

Stannylenes based on neutral,anionic, and dianionic 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene

Russian Chemical Bulletin - The reaction of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-bian (1)) with one equivalent of SnCl2 gave the complex [(dpp-bian)0SnCl2] (2) containing the...     

Oxidation and reduction of bis(imino)pyridine iron dicarbonyl complexes

The oxidation and reduction of a redox-active aryl-substituted bis(imino)pyridine iron dicarbonyl has been explored to determine whether electron-transfer events are ligand- or metal-based or a combination of both. A series of bis(imino)pyridine iron dicarbonyl compounds, [((iPr)PDI)Fe(CO)(2)](-), ((iPr)PDI)Fe(CO)(2), and [((iPr)PDI)Fe(CO)(2)](+) [(iPr)PDI = 2,6-(2,6-(i)Pr(2)C(6)H(3)N═CMe)(2)C(5)H(3)N], which differ by three oxidation states, were prepared and the electronic structures evaluated using a combination of spectroscopic techniques and, in two cases, [((iPr)PDI)Fe(CO)(2)](+) and [((iPr)PDI)Fe(CO)(2)], metrical parameters from X-ray diffraction. The data establish that the cationic iron dicarbonyl complex is best described as a low-spin iron(I) compound (S(Fe) = ?) with a neutral bis(imino)pyridine chelate. The anionic iron dicarbonyl, [((iPr)PDI)Fe(CO)(2)](-), is also best described as an iron(I) compound but with a two-electron-reduced bis(imino)pyridine. The covalency of the neutral compound, ((iPr)PDI)Fe(CO)(2), suggests that both the oxidative and reductive events are not ligand- or metal-localized but a result of the cooperativity of both entities.     

Synthesis and structure of new compounds with Zn-Ga bonds: insertion of the gallium(I) bisimidinate Ga(DDP) into Zn-X (X = CH3, Cl) and the homoleptic complex cation [Zn(GaCp*)4]2+

Insertion reactions of the low-valent group 13 bisimidinate ligand Ga(DDP) {DDP = 2-[(2,6-diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]-2-pentene} into Zn-Me and Zn-Cl bonds are reported. The reaction of ZnMe2 with 2 equiv of Ga(DDP) yields the double-insertion product [{(DDP)GaMe}2Zn] (1), whereas the insertion of Ga(DDP) into the Zn-Cl bond of ZnCl2 in tetrahydrofuran (THF) leads to the monoinsertion product [{(DDP)GaCl}ZnCl(THF)2] (2). Treatment of 2 with Na[BArF] results in the salt [{THF.Ga(DDP)}Zn(THF)(mu-Cl)]2[BArF]2 (3), with two Cl atoms bridging the Zn centers. The structural features of the Zn-Ga-bonded compounds 1-3 were compared with related complexes and in particular with the compound [Zn(GaCp*)4][BArF]2 (4), which was synthesized by the reaction of ZnMe2, [H(OEt2)2][BArF], and GaCp* in fluorobenzene. The complex cation [Zn(GaCp*)4]2+ of 4 relates to previously reported d10 analogues [M(GaCp*)4] (M = Ni, Pd, Pt). All new compounds were fully characterized by elemental analysis, NMR spectroscopy, and single-crystal X-ray diffraction analysis.     

Reactivity of an aluminum hydride complex with a redox-active diimine ligand

Russian Chemical Bulletin - The reaction of the hydride [(dpp-bian)Al(H)Cl] (1) containing the 1,2-bis[(2,6-diisopropylphenyl) imino]acenaphthene (dpp-bian) radical anion with MeLi affords the...     

Synthesis and Crystal Structures of the Reaction Products of the Magnesium Acenaphthenediimine Complex with Tosyl Azide

Russian Journal of Coordination Chemistry - The reaction of [(Dpp-Bian)Mg(THF)3] (I) (Dpp-Bian is 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) with tosyl azide (TosN3) affords the binuclear...     

Iron-catalyzed intermolecular [2π+2π] cycloaddition

The bis(imino)pyridine iron dinitrogen compounds, ((iPr)PDI)Fe(N(2))(2) and [((Me)PDI)Fe(N(2))](2)(μ(2)-N(2)) ((R)PDI = 2,6-(2,6-R(2)-C(6)H(3)N═CMe)(2)C(5)H(3)N; R = (i)Pr, Me), promote the catalytic intermolecular [2π + 2π] cycloaddition of ethylene and butadiene to form vinylcyclobutane. Stoichiometric experiments resulted in isolation of a catalytically competent iron metallocycle intermediate, which was shown to undergo diene-induced C-C reductive elimination. Deuterium labeling experiments establish competitive cyclometalation of the bis(imino)pyridine aryl substituents during catalytic turnover.