Lawrence M. Henling

Bimetallic coordination insertion polymerization of unprotected polar monomers: copolymerization of amino olefins and ethylene by dinickel bisphenoxyiminato catalysts.

Dinickel bisphenoxyiminato complexes based on highly substituted p- and m-terphenyl backbones were synthesized, and the corresponding atropisomers were isolated. In the presence of a phosphine scavenger, Ni(COD)2, the phosphine-ligated syn-dinickel complexes copolymerized α-olefins and ethylene in the presence of amines to afford 0.2-1.3% α-olefin incorporation and copolymerized amino olefins and ethylene with a similar range of incorporation (0.1-0.8%). The present rigid catalysts provide a bimetallic strategy for insertion polymerization of polar monomers without masking of the heteroatom group. The effects of the catalyst structure on the reactivity were studied by comparisons of the syn and anti atropisomers and the p- and m-terphenyl systems.

Bis(α-diimine)iron Complexes: Electronic Structure Determination by Spectroscopy and Broken Symmetry Density Functional Theoretical Calculations

The electronic structure of a family comprising tetrahedral (alpha-diimine)iron dichloride, and tetrahedral bis(alpha-diimine)iron compounds has been investigated by Mossbauer spectroscopy, magnetic susceptibility measurements, and X-ray crystallography. In addition, broken-symmetry density functional theoretical (B3LYP) calculations have been performed. A detailed understanding of the electronic structure of these complexes has been obtained. A paramagnetic (St=2), tetrahedral complex [FeII(4L)2], where (4L)1- represents the diamagnetic monoanion N-tert-butylquinolinylamide, has been synthesized and characterized to serve as a benchmark for a Werner-type complex containing a tetrahedral FeIIN4 geometry and a single high-spin ferrous ion. In contrast to the most commonly used description of the electronic structure of bis(alpha-diimine)iron(0) complexes as low-valent iron(0) species with two neutral alpha-diimine ligands, it is established here that they are, in fact, complexes containing two (alpha-diiminato)1-* pi radical monoanions and a high-spin ferrous ion (in tetrahedral N4 geometry) (SFe=2). Intramolecular antiferromagnetic coupling between the pi radical ligands (Srad=1/2) and the ferrous ion (SFe=2) yields the observed St=1 ground state. The study confirms that alpha-diimines are redox noninnocent ligands with an energetically low-lying antibonding pi* lowest unoccupied molecular orbital which can accept one or two electrons from a transition metal ion. The (alpha-diimine)FeCl2 complexes (St=2) are shown to contain a neutral alpha-diimine ligand, a high spin ferrous ion, and two chloride ligands.

Intramolecular and intermolecular C-H activation at a cationic PtII center

Abstract Syntheses and C-H bond activation reactions of the novel electrophilic PtII complexes [(tmeda)Pt(CH3)(OEt2)][BAr1], [(tmeda)Pt(CH3)(THF)][BArf], and [(tmeda)Pt(CH3)(NC5F5)][BArf] are described {[BArf]− = [(3,5-C6H3(CF3)2)4B]−} (tmeda is N,N,N′,N′-tetramethylethylenediamine), [(tmeda)Pt(CH3)(OEt2)][BArf] and [(tmeda)Pt(CH3)(THF)][BArf] are unstable at room temperature, yielding methane and the Fischer carbene PtII hydrides, [(tmeda)Pt(=C(CH3)(OCH2CH3))(H)][BArf] and P t ( = C C H 2 C H 2 C H 2 ⎴ O ) ( H ) ] [ B A r 1 ] . The methane liberated from [(tmeda)Pt(CH3)(OEt2-d10)][BArf] consists of an isotopomeric mixture, (CH4, CH3D, CH2D2 and CHD3), indicating a multiple H/D exchange reaction following the C-D activation and prior to methane loss. [(tmeda)Pt(CH3)(THF-d8)][BAr] liberates CH4 and CH3D. Methane-13C, cyclohexane, toluene, and benzene react with [(tmeda)Pt(CH3)(NC5F5)][BArf] to yield methane and new organoplatinum complexes. Deuterated alkanes and arenes react with [(tmeda)Pt(CH3)(NC5F5] [BArf] to give a mixture of methane isotopomers. The relevance of these results to the oxidation of alkanes by aqueous platinum complexes is discussed.

Origins of Initiation Rate Differences in Ruthenium Olefin Metathesis Catalysts Containing Chelating Benzylidenes

A series of second-generation ruthenium olefin metathesis catalysts was investigated using a combination of reaction kinetics, X-ray crystallography, NMR spectroscopy, and DFT calculations in order to determine the relationship between the structure of the chelating o-alkoxybenzylidene and the observed initiation rate. Included in this series were previously reported catalysts containing a variety of benzylidene modifications as well as four new catalysts containing cyclopropoxy, neopentyloxy, 1-adamantyloxy, and 2-adamantyloxy groups. The initiation rates of this series of catalysts were determined using a UV/vis assay. All four new catalysts were observed to be faster-initiating than the corresponding isopropoxy control, and the 2-adamantyloxy catalyst was found to be among the fastest-initiating Hoveyda-type catalysts reported to date. Analysis of the X-ray crystal structures and computed energy-minimized structures of these catalysts revealed no correlation between the Ru-O bond length and Ru-O bond strength. On the other hand, the initiation rate was found to correlate strongly with the computed Ru-O bond strength. This latter finding enables both the rationalization and prediction of catalyst initiation through the calculation of a single thermodynamic parameter in which no assumptions about the mechanism of the initiation step are made.

Bespoke Photoreductants: Tungsten Arylisocyanides

Modular syntheses of oligoarylisocyanide ligands that are derivatives of 2,6-diisopropylphenyl isocyanide (CNdipp) have been developed; tungsten complexes incorporating these oligoarylisocyanide ligands exhibit intense metal-to-ligand charge-transfer visible absorptions that are red-shifted and more intense than those of the parent W(CNdipp)6 complex. Additionally, these W(CNAr)6 complexes have enhanced excited-state properties, including longer lifetimes and very high quantum yields. The decay kinetics of electronically excited W(CNAr)6 complexes (*W(CNAr)6) show solvent dependences; faster decay is observed in higher dielectric solvents. *W(CNAr)6 lifetimes are temperature dependent, suggestive of a strong coupling nonradiative decay mechanism that promotes repopulation of the ground state. Notably, *W(CNAr)6 complexes are exceptionally strong reductants: [W(CNAr)6](+)/*W(CNAr)6 potentials are more negative than -2.7 V vs [Cp2Fe](+)/Cp2Fe.

Cationic alkylaluminum-complexed zirconocene hydrides: NMR-spectroscopic identification, crystallographic structure determination, and interconversion with other zirconocene cations.

The ansa-zirconocene complex rac-Me(2)Si(1-indenyl)(2)ZrCl(2) ((SBI)ZrCl(2)) reacts with diisobutylaluminum hydride and trityl tetrakis(perfluorophenyl)borate in hydrocarbon solutions to give the cation [(SBI)Zr(μ-H)(3)(Al(i)Bu(2))(2)](+), the identity of which is derived from NMR data and supported by a crystallographic structure determination. Analogous reactions proceed with many other zirconocene dichloride complexes. [(SBI)Zr(μ-H)(3)(Al(i)Bu(2))(2)](+) reacts reversibly with ClAl(i)Bu(2) to give the dichloro-bridged cation [(SBI)Zr(μ-Cl)(2)Al(i)Bu(2)](+). Reaction with AlMe(3) first leads to mixed-alkyl species [(SBI)Zr(μ-H)(3)(AlMe(x)(i)Bu(2-x))(2)](+) by exchange of alkyl groups between aluminum centers. At higher AlMe(3)/Zr ratios, [(SBI)Zr(μ-Me)(2)AlMe(2)](+), a constituent of methylalumoxane-activated catalyst systems, is formed in an equilibrium, in which the hydride cation [(SBI)Zr(μ-H)(3)(AlR(2))(2)](+) strongly predominates at comparable HAl(i)Bu(2) and AlMe(3) concentrations, thus implicating the presence of this hydride cation in olefin polymerization catalyst systems.

Intramolecular CH activation by dicationic Pt(II) complexes

Abstract The dicationic complexes [(ArNC(Me)C(Me)NAr)Pt(solv)2]X2, (Ar=2,6-(CH3)2C6H3; 5a: solv=CH3CN, X=CF3SO3−, BF4−, SbF6−; 5b: solv=(CH3)2CO, X=BF4−, SbF6−) and [(CyNC(H)C(H)NCy)Pt(CH3CN)2]X2, (Cy=C6H11, 6: X=OTf−, BF4−, PF6−, SbF6−) were synthesized from the corresponding Pt dichlorides with two equivalents of AgX. The reactions of 5a with 1-phenylpyrazole, 2-phenylpyridine, 2-vinylpyridine, and 2-(2-thienyl)pyridine afford the cyclometalated products 11–14 via intramolecular CH activation of an sp2 CH bond of the unsaturated sidegroup. Pyridines with saturated groups at the 2-position do not undergo a similar cyclometalation reaction. In trifluoroethanol-d3 solution, 6 undergoes cyclometalation of one of the cyclohexyl groups, an example of sp3 CH bond activation. The latter reaction proceeds only partway to completion, implying that an equilibrium has been reached; in the case where X=OTf−the equilibrium favors the starting dication.

C2-symmetric ansa metallocenes of titanium and zirconium with a ligand system that yields pure rac isomer: preparation and crystal structures of rac-η5-C5H22-SiMe34-CMe3)2SiMe3MCl2(M = Ti or Zr)

Abstract The preparation and structures of rac -[bis(2-trimethylsilyl-4-tert-butyl- η 5 -cyclopentadienyl)dimethylsilane]dichlorotitanium and rac -[bis(2-trimethylsilyl-4-tert-butyl- η 5 -cyclopentadienyl)dimethylsilane]dichlorozirconium, rac -BpMCl 2 (Bp = η 5 -C 5 H 2 2-SiMe 3 4-CMe 3 ) 2 SiMe 2 ; M=Ti or Zr)described. The structures are analogous, although as expected, longer Zr atom distances result in less steric crowding. Repulsions of ring substituents on opposing rings can be seen by the differing angles about the α trimethylsilyl substituents. The [MCl 2 ] and SiC 2 planes of the [SiMe 2 ] bridge are twisted by 11° owing to intramolecular steric repulsions.

Addition of a phosphine ligand switches an N-heterocyclic carbene-zirconium catalyst from oligomerization to polymerization of 1-hexene

A catalyst for the oligomerization of 1-hexene, generated by the activation of a benzimidazolylidene zirconium dibenzyl complex, switches to a polymerization catalyst on addition of a trialkylphosphine.

PENTAMETHYLCYCLOPENTADIENYL-AMINOBOROLE DERIVATIVES OF ZIRCONIUM AND HAFNIUM WITH ALKYL AND ALLYL LIGANDS

The preparations of new alkyl, iodo, and allyl derivatives of zirconium and hafnium with pentamethylcyclopentadienyl and aminoborole ancillary ligands are described. The dialkyl complexes Cp * {~5-CaH4BN(CHM%)2}ZrR2Li (R = Me, C~-C-p-C6H4CH 3, C-=CMe3, CH2Ph) are prepared from Cp*{rIS-C4H4BN(CHM%)z}ZrCI - LiCl and two equivalents of RLi. Cp*ZrI 3 is prepared from