Titanium-Thiolate-Aluminum-Carbide Complexes by Multiple C-H Bond Activation

Transition Metal Carbide Complexes

Carbide complexes remain a rare class of molecules. Their paucity does not reflect exceptional instability but is rather due to the generally narrow scope of synthetic procedures for constructing carbide complexes. The preparation of carbide complexes typically revolves around generating L_nM-CE_x fragments, followed by cleavage of the C-E bonds of the coordinated carbon-based ligands (the alternative being direct C atom transfer). Prime examples involve deoxygenation of carbonyl ligands and deprotonation of methyl ligands, but several other p-block fragments can be cleaved off to afford carbide ligands. This Review outlines synthetic strategies toward terminal carbide complexes, bridging carbide complexes, as well as carbide-carbonyl cluster complexes. It then surveys the reactivity of carbide complexes, covering stoichiometric reactions where the carbide ligands act as C₁ reagents, engage in cross-coupling reactions, and enact Fischer-Tropsch-like chemistry; in addition, we discuss carbide complexes in the context of catalysis. Finally, we examine spectroscopic features of carbide complexes, which helps to establish the presence of the carbide functionality and address its electronic structure.

Synthesis and stability of homoleptic metal(III) tetramethylaluminates

Whereas a number of homoleptic metal(III) tetramethylaluminates M(AlMe(4))(3) of the rare earth metals have proven accessible, the stability of these compounds varies strongly among the metals, with some even escaping preparation altogether. The differences in stability may seem puzzling given that this class of metals usually is considered to be relatively uniform with respect to properties. On the basis of quantum chemically obtained relative energies and atomic and molecular descriptors of homoleptic tris(tetramethylaluminate) and related compounds of rare earth metals, transition metals, p-block metals, and actinides, multivariate modeling has identified the importance of ionic metal-methylaluminate bonding and small steric repulsion between the methylaluminate ligands for obtaining stable homoleptic compounds. Low electronegativity and a sufficiently large ionic radius are thus essential properties for the central metal atom. Whereas scandium and many transition metals are too small and too electronegative for this task, all lanthanides and actinides covered in this study are predicted to give homoleptic compounds stable toward loss of trimethylaluminum, the expected main decomposition reaction. Three of the predicted lanthanide-based compounds Ln(AlMe(4))(3) (Ln = Ce, Tm, Yb) have been prepared and fully characterized in the present work, in addition to Ln(OCH(2)tBu)(3)(AlMe(3))(3) (Ln = Sc, Nd) and [Eu(AlEt(4))(2)](n). At ambient temperature, donor-free hexane solutions of Ln(AlMe(4))(3) of the Ln(3+)/Ln(2+) redox-active metal centers display enhanced reduction to [Ln(AlMe(4))(2)](n) with decreasing negative redox potential, in the order Eu ≫ Yb ≫ Sm. Whereas Eu(AlMe(4))(3) could not be identified, Yb(AlMe(4))(3) turned out to be isolable in low yield. All attempts to prepare the putative Sc(AlMe(4))(3), featuring the smallest rare earth metal center, failed.

Hafnium–phosphinimide complexes

A series of phosphinimide complexes of Hf are prepared and characterized. Reaction of the phosphinimine t-Bu3PNH with Hf(NEt2)4 gave (t-Bu3PN)Hf(NEt2)3 (1) but this species was not readily converted to the corresponding HfCl3-derivative. The reaction of 2 equiv. of t-Bu3PNH with Hf(NEt2)4, however, gave (t-Bu3PN)2Hf(NEt2)2 (2), which was readily converted to (t-Bu3PN)2HfCl2 (3) and (t-Bu3PN)2HfMe2 (4). Employing t-Bu3PNLi and HfCl4 afforded (t-Bu3PN)3HfCl (5) while reaction with CpHfCl3 gave rise to ligand redistribution reactions affording (t-Bu3PN)2HfCl2 and Cp2HfCl2. However, Cp(t-Bu3PN)2HfCl (7) was prepared by treating (t-Bu3PN)2HfCl2 with CpNa. The related species of Cp2(t-Bu3PN)HfCl (8) was synthesized by the reaction of Cp2HfCl2 and t-Bu3PNLi. Ligand redistribution was avoided in the reaction of Cp*HfCl3 as Cp*(t-Bu3PN)HfCl2 (9) and Cp*(i-Pr3PN)HfCl2 (10) were readily obtained and derivatized as Cp*(t-Bu3PN)Hf(NMe2)2 (11) and Cp*(t-Bu3PN)HfMe2 (12), respectively. Similarly, ((Me3Si)2C5H3)(t-Bu3PN)HfCl2 (13) was converted to ((Me3Si)2C5H3)(t-Bu3PN)HfMe2 (14). Reactions with Lewis acid activators were used to prepare Cp*(t-Bu3PN)HfMe(THF)MeB(C6F5)3 (15), (Cp*(t-Bu3PN)HfMe)(B(C6F5)4) (16), and (t-Bu3PN)2Hf(H2B(C6F5)2)2 (17). Preliminary testing of 3, 9, and 13 in ethylene polymerization is reported. Compounds 3, 4, 7, 9, and 17 are characterized crystallographically.

Multiple C−H Bond Activation in Group 3 Chemistry: Synthesis and Structural Characterization of an Yttrium−Aluminum−Methine Cluster

Complete donor-induced alkylaluminate cleavage of halfmetallocene complex Cp*Y(AlMe4)2, that is, treatment of Cp*Y(AlMe4)2 with 2 equiv of diethyl ether, produces [Cp*Y(mu2-Me)2]3 in high yield (95%). In contrast, the equimolar reaction of Cp*Y(AlMe4)2 with diethyl ether reproducibly formed complex [Cp*4Y4(mu2-CH3)2{(CH3)Al(mu2-CH3)2}4(mu4-CH)2] in low yield (10-30%) via a multiple C-H bond activation. The synthesis of the heterooctametallic yttrium-aluminum-methine cluster was also accomplished in moderate yield (47%) by the equimolar reaction of discrete Cp*Y(AlMe4)2 and [Cp*Y(mu2-Me)2]3 in the absence of any donor solvent and "free" AlMe3. This gives strong evidence that preformed heterometal-bridged Y-CH3-Al moieties are prone to multiple hydrogen abstraction in the presence of a highly basic reagent such as [Cp*Y(mu2-Me)2]3. The monocylopentadienyl complexes [Cp*Y(mu2-Me)2]3 and [Cp*4Y4(mu2-CH3)2{(CH3)Al(mu2-CH3)2}4(mu4-CH)2] were structurally characterized.

Improving the performance of methylalumoxane: a facile and efficient method to trap "free" trimethylaluminum

The presence of "free" trimethylaluminum (TMA) in methylalumoxane (MAO) solutions can be highly detrimental to the performance of metallocene and "post-metallocene" olefin polymerization catalysts. The most used strategy to remove "free" TMA is to evaporate MAO solutions to dryness, until a free-flowing white powder ("solid MAO") is left. This procedure is tedious and potentially hazardous, because in some cases the distillate is a concentrated hydrocarbon solution of TMA. Moreover, "solid MAO" is poorly soluble in common polymerization media, and once in solution it can regenerate TMA to some extent. This communication reports on a facile alternative, which consists in the controlled addition of a sterically hindered phenol, such as 2,6-di-tert-butylphenol, effectively trapping "free" TMA. We show here that 2,6-di-tert-butylphenol/MAO solutions activate equally well the dichloro-precursors of well-known zirconocene and bis(phenoxyimine)Ti catalysts, and that their use in propene polymerization results in a substantially higher productivity, polymer stereoregularity, and/or average molecular mass compared with activation by MAO alone.

Isobutene Polymerization Using a Chelating Diborane Co-Initiator

Lewis acidic diborane 1 (J. Am. Chem. Soc. 1999, 121, 3244-3245) is highly effective for both proton- and cationogen-initiated isobutene polymerization in hydrocarbon media at low temperature. Reactions of diborane 1 with cumyl chloride and cumyl methyl ether were studied by variable-temperature 1H and 19F NMR spectroscopy. At low temperatures stable ion pairs 2a and 2b are formed; at higher temperatures these ion-pairs form phenyl-1,3,3-trimethylindan (3) with concomitant release of HCl to form 1 in the case of 2a or degradation of the anion (2b). Reaction between Ph3C-Cl and diborane 1 resulted in the generation of an ion-pair 4 consisting of the Ph3C cation very weakly associated with the chelated, mu-Cl counteranion as revealed by X-ray crystallography.

Bulky aluminum alkyl scavengers in olefin polymerization with group 4 catalysts

The binding of H2O to MeAl(OAr)2 (1: Ar = 2,6-di-tert-butyl-4-methylphenyl) in THF-d8 at -40 degrees C provides aquo complex 2, the structure of which was determined by X-ray crystallography. Complex 2 is unstable above 0 degrees C in THF-d8 and decomposes to form ArOH (major), CH4 (minor), and a methyl aluminoxane of undetermined structure. Decomposition of 2 follows first-order kinetics with k = 3.0 x 10-4 s-1 at 5 degrees C. The hindered phenol ArOH slowly reacts with [Cp2ZrMe][MeB(C6F5)3] (4) in bromobenzene-d5 solution at 25 degrees C to furnish CH4 and [Cp2ZrOAr][MeB(C6F5)3] (5), the structure of which was confirmed by X-ray crystallography. This reaction follows second-order kinetics for [ArOH] = [4] = 0.045 M and with k = 2.8 x 10-3 M-1 s-1 at 25 degrees C. This corresponds to a rate that is >107 x slower than the apparent rate of ethylene insertion for 4 at 25 degrees C at typical concentrations encountered in olefin polymerization. The kinetic data, as well as control experiments involving the addition of ArOH to active catalyst producing poly(ethylene), demonstrate that ArOH has essentially no effect on polymerization kinetics involving 4.

Counterion effects on propylene polymerization using two-state ansa-metallocene complexes

Propylene polymerization using unsymmetrical, ansa-metallocene complexes Me(2)Y(Ind)CpMMe(2) (Y = Si, C, M = Zr, Y = C, M = Hf) and the co-initiators methyl aluminoxane (PMAO), B(C(6)F(5))(3), and [Ph(3)C][B(C(6)F(5))(4)] was studied at a variety of propylene concentrations. Modeling of the polymer microstructure reveals that the catalysts derived from Me(2)Si(Ind)CpZrMe(2) and each of these co-initiators function under conditions where chain inversion is much faster than propagation (Curtin-Hammett conditions). Surprisingly, the microstructure of the PP formed was essentially unaffected by the nature of the counterion, suggesting similar values for the fundamental parameters inherent to two-state catalysts. The tacticity of PP was sensitive to changes in [C(3)H(6)] in the case of catalysts derived from Me(2)C(Ind)CpHfMe(2) and PMAO, or [Ph(3)C][B(C(6)F(5))(4)], but the average tacticity of the polymer produced at a given [C(3)H(6)] decreased in the order [Ph(3)C][B(C(6)F(5))(4)] > PMAO. With B(C(6)F(5))(3), the polymer formed was more stereoregular, and its microstructure was invariant to changes in monomer concentration. The PP pentad distributions in this case could be modeled by assuming that all three catalyst/cocatalyst combinations function with different values for the relative rates of insertion to inversion (Delta) but otherwise feature essentially invariant, intrinsic stereoselectivity for monomer insertion (alpha, beta), while the relative reactivity/stability (g/K) of the isomeric ion-pairs present seems to be only modestly affected, if at all. Similar conclusions can also be made about the published propylene polymerization behavior of the C(s)-symmetric Me(2)C(Flu)CpZrMe(2) complex with different counterions. For every counterion investigated, the principle difference appears to be the operating regime (Delta) rather than intrinsic differences in insertion stereoselectivity (alpha). Surprisingly, the ordering of the various counterions with respect to Delta does not agree with commonly accepted ideas about their coordinating ability. In particular, catalysts when activated with B(C(6)F(5))(3) appear to function at low values of Delta as compared to those featuring B(C(6)F(5))(4) (less coordinating) and FAl[(o-C(6)F(5))C(6)F(4)](3) (more coordinating) or PMAO (more coordinating) counterions where the ordering in Delta is MeB(C(6)F(5))(3) < B(C(6)F(5))(4) < FAl[(o-C(6)F(5))C(6)F(4)](3) approximately PMAO. Possible reasons for this behavior are discussed.

A novel dinuclear ruthenium complex bridged through a substituted phenazine ligand formed by ruthenium-promoted oxidative assembly of 1,3-diaminobenzene

The reaction of [Ru(acac)3] (acac = acetylacetonate) with molten 1,3-diaminobenzene affords the crystalline monometallic compound [Ru(L1)-(acac)21 (1: L1 = N-(3'-aminophenyl)1,2-(3-amino)benzoquinone diimine) along with an unstable dimetallic compound [Ru2(mu-L2)(acac)4] (2: L2=N-4,6-bis(3'-aminophenyl)imino-3,5-diimino-hex-1-ene). Compound 2 transforms to a stable dimetallic compound [Ru2(mu-L3)(acac)4] (3: L3 = 2-amino-6(3'-aminophenyl)imino-9-imino-phenazine) in boiling 2-methoxyethanol. The above compounds are formed by ruthenium-mediated oxidative di- or trimerization of the diamine with the formation of several new C-N bonds. The products have been thoroughly characterized. FAB mass spectra, along with other physicochemical data, were used for their formulations. The compounds 1, 2, and 3 display intense peaks due to their parent molecular ions at m/z 512, 916, and 914, respectively. Final characterization of complex 3 was made by single-crystal X-ray structure determination. The structure of 3 confirmed the formation of three new C-N bonds and the bridging ligand L3 from 1,3-diaminobenzene. The conversion, 2 --> 3 is an oxidative ring-closure reaction, which is associated with dehydrogenation reactions. The monometallic compound 1, showed a reversible metal-based anodic response at 0.35 V. On the other hand, both the compounds 2 and 3 showed a pair of well-resolved metal-based anodic oxidations, for which the separation between the two successive anodic responses were high (>0.4 V). In addition, all of them showed multiple cathodic responses that were in the range -1.0 to -2.0 V.