Dinuclear Pentacoordinated Organoyttrium Biphenolates as Initiators for the Synthesis of High-Molecular Weight Isotactic Poly(2-vinylpyridine).

Dinuclear organoyttrium biphenolato complexes have been synthesized for the first time starting from Y(CH2SiMe3)3(THF)2 and readily available biphenols with bulky substituents at the 3,3'-positions. XRD analysis revealed that these complexes are formed as homochiral dimers even though racemic biphenols were employed. Such dinuclear penta-coordinated yttrium complexes were evaluated as initiators in the polymerization of 2-vinylpyridine (2-VP). High activity and excellent isotacticity (mm > 99%) were observed. DSC analysis of poly(2-VP) showed a Tm = 221 °C, which is the highest value reported to date. Attempts to improve the low initiation efficiency of the present system were made by co-use with several organo-main-group metal reagents. Specifically, good molecular weight control within the high Mn region was achieved by the addition of Et3Al (Al/Y = 10).

Selective Incorporation of Primary Amines into a Trizirconium Imido System and Catalyic Cyclization of Aminoalkynes.

The trinuclear zirconium imido complex [{LZr(NMe2)}3] (2, L = C5Me4CH2CH2N) was synthesized by amine elimination between Zr(NMe2)4 and endo-olefinic isomers of (tetramethylcyclopentadienyl)ethanamine (LH3) (1). To study the fundamental reactivity of the trizirconium system, reactions of 2 with primary amines were examined. Selective incorporations of the primary amines were observed, depending on steric and electronic natures of the amine substrates. The amine-incorporated complexes [(LZrNHR)(LHZrNHR)(LHZr)(µ-NHR)(µ3-NR)] (3, R = Pr, Et), [(LZrNHR)2(LHZr)(µ-NR)] (4, R = Pr, i-Bu), [(LZr)2(LZrNMe2)(µ-NR)] (5, R = neo-Pen), and [(LZr)(LZrNHAr)(LH2Zr)(µ-NAr)2] (6, Ar = Ph, C6H4-4-Br, C6H4-4-OMe) were structurally characterized by NMR and XRD analysis and showed several coordination modes of the substrate nitrogen ligands: i.e., terminal amides, bridging amides, and bridging imides but not terminal imides. Thermolysis of a mixture of 3 and 4 led to C-H bond activation, giving rise to the zirconaaziridines [{LZr(η2-NCHR)}(LZr)(LHZr)(µ-NHCH2R)] (12, R = Et, Me). Complex 2 proved to be a competent precatalyst in the hydroamination of the aminoalkynes (H2NCH2CR12CH2C≡CR2) (13, R1 = H, R2 = Bu, Ph, t-Bu; 14, R1 = Me, R2 = Et, Ph). Stoichiometric or semicatalytic reactions of 2 and the aminoalkynes were studied to explore the reactivity of in situ formed Zr3 species.

A study on Zr-Ir multiple bonding active for C-H bond cleavage.

Zr-Ir hydrido complexes with ansa-(cyclopentadienyl)(amide) as the supporting ligand in the zirconium fragment, e.g., (L(1)ZrR)(Cp*Ir)(µ-H)3 [L(1) = Me2Si(η(5)-C5Me4)(N(t)Bu), R = Cl (5), Ph (7), Me (10), alkyl, and aryl] were designed, synthesized, and isolated as tractable early-late heterodinuclear complexes. Despite the presence of the three supporting hydride ligands, Zr-Ir distances in the crystal structures of 5, alkyl, and aryl complexes [2.74-2.76 Å] were slightly longer than the sum of the element radii of Zr and Ir [2.719 Å]. These hydrocarbyl complexes displayed the thermolytic C-H activation of a variety of aromatic compounds and several organometallic compounds. Also, the substrate scope and limitation in the Zr-Ir system were studied. The regiochemical outcomes during the C-H activation of pyridine derivatives and methoxyarenes suggested the in situ generation of a Lewis acidic active intermediate, i.e., (L(1)Zr)(Cp*IrH2) (III). The existence of III and relevant σ-complex intermediates {L(1)Zr(η(2)-R-H)}(Cp*IrH2) (IIR) (R = Me, Ph) in the ligand exchange was demonstrated by the direct isolation of a Et3PO-adduct of III (39b) from 7 and kinetic studies. The structure of the direct Zr-Ir bonds in IIPh, IIMe, III, and 39b were probed using computational studies. The unprecedented strong M-M' interactions in the early-late heterobimetallic (ELHB) complexes have been proposed herein.

Aluminum-stabilized low-spin iron(II) hydrido complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane.

We investigated herein the reactions of (Me3tacn)FeCln (1a: n = 3, 1b: n = 2) with common aluminum hydride reagents and a bulky dihydridoaluminate {Li(ether)2}{Al(OC6H3-2,6-(t)Bu2)}(µ-H)2, which yielded the diamagnetic hydrido complexes 2-4 containing Fe(II) and Al(III). In particular, the use of divalent 1b afforded excellent isolated yields. The structures of 2-4 were determined using spectroscopic and crystallographic analyses. The crystal structures showed distorted octahedral Fe centers and fairly short Fe-Al distances [2.19-2.24 Å]. The structures of cation moiety 2 and neutral complex 4 were further probed using DFT calculations, which indicated a stable low-spin Fe(II) state and strongly electron-donating nature of the (Me3tacn)FeH3 fragment toward the Al(III) center.

Zirconocene-iridium hydrido complexes: arene carbon-hydrogen bond activation and formation of a planar square Zr2Ir2 complex.

New early-late heterobimetallic hydrides (L(2)ZrCl)(Cp*Ir)(mu-H)(3) (1; L = Cp derivative, Cp* = eta(5)-C(5)Me(5)) were synthesized from zirconocene derivatives (L(2)ZrCl(2)) and LiCp*IrH(3) via a salt elimination reaction and structurally characterized by NMR and X-ray analyses. Upon treatment of 1 with an alkyllithium reagent, hydride abstraction complex 4 underwent thermolytic ligand elimination at the Zr-Ir system to yield a novel planar square complex (L(2)Zr)(2)(Cp*Ir)(2)(mu(3)-H)(4) (2). When a labeling study of the reaction was conducted, it was found that the conversion of 1 to 2 involves rapid aromatic and benzylic C-H activation by a coordinatively unsaturated dinuclear complex (L(2)Zr)(Cp*Ir)(H)(2) (3).