Divergent Pathways of C−H Bond Activation:  Reactions of (t-Bu₃PN)₂TiMe₂ with Trimethylaluminum

Abstract

The reaction of AlMe(3) with (t-Bu(3)PN)(2)TiMe(2) 1 proceeds via competitive reactions of metathesis and C-H activation leading ultimately to two Ti complexes: [(mu(2)-t-Bu(3)PN)Ti(mu-Me)(mu(4)-C)(AlMe(2))(2)](2) 2, [(t-Bu(3)PN)Ti(mu(2)-t-Bu(3)PN)(mu(3)-CH(2))(2)(AlMe(2))(2)(AlMe(3))] 3, and the byproduct (Me(2)Al)(2)(mu-CH(3))(mu-NP(t-Bu(3))) 4. X-ray structural data for 2 and 3 are reported. Compound 3 undergoes thermolysis to generate a new species [Ti(mu(2)-t-Bu(3)PN)(2)(mu(3)-CH(2))(mu(3)-CH)(AlMe(2))(3)] 5. Monitoring of the reaction of 1 with AlMe(3) by (31)P[(1)H] NMR spectroscopy revealed intermediates including (t-Bu(3)PN)TiMe(3) 6. Compound 6 was shown to react with AlMe(3) to give 2 exclusively. Kinetic studies revealed that the sequence of reactions from 6 to 2 involves an initial C-H activation that is a second-order reaction, dependent on the concentration of Ti and Al. The second-order rate constant k(1) was 3.9(5) x 10(-4) M(-1) s(-1) (DeltaH(#) = 63(2) kJ/mol, DeltaS(#) = -80(6) J/mol x K). The rate constants for the subsequent C-H activations leading to 2 were determined to be k(2) = 1.4(2) x 10(-3) s(-1) and k(3) = 7(1) x 10(-3) s(-1). Returning to the more complex reaction of 1, the rate constant for the ligand metathesis affording 4 and 6 was k(met) = 6.1(5) x 10(-5) s(-1) (DeltaH(#) = 37(3) kJ/mol, DeltaS(#) = -203(9) J/mol x K). The concurrent reaction of 1 leading to 3 was found to proceed with a rate constant of k(obs) of 6(1) x 10(-5) s(-1) (DeltaH(#) = 62(5) kJ/mol, DeltaS(#)= -118(17) J/mol x K). Using these kinetic data for these reactions, a stochastic kinetic model was used to compute the concentration profiles of the products and several intermediates with time for reactions using between 10 and 27 equivalents of AlMe(3). These models support the view that equilibrium between 1 x AlMe(3) and 1 x (AlMe(3))(2) accounts for varying product ratios with the concentration of AlMe(3). In a similar vein, similar equilibria account for the transient concentrations of 6 and an intermediate en route to 3. The implications of these reactions and kinetic and thermodynamic data for both C-H bond activation and deactivation pathways for Ti-phosphinimide olefin polymerization catalysts are considered and discussed.

Eine Seltenerdmetallvariante des Tebbe‐Reagens

“…[9, 10] Vorangegangene Untersuchungen unserer Arbeitsgruppe befassten sich mit Seltenerdmetall(III)-TetramethylaluminatKomplexen [L x Ln{Al(CH 3 ) 4 } y ] (y = 1, 2, 3; x + y = 3, L = einzähniger Hilfsligand, Ln = Seltenerdmetall und Sc, Y, La) als Polymerisationskatalysatoren [11,12] und führten zur Isolation von Ln III -Clustern mit Methylen-, [13,14] Methin- [15] und Carbidfunktionalitäten.[ …”

Subtle Size Effects in C–H Activation Reactions of Lanthanum and Praseodymium Tetramethylaluminates by Neutral Trinitrogen Bases

“…In contrast to above reaction, that of Pr(AlMe 4 ) 3 with TtBuTAC (R = tBu) results in the formation of a sole product, the double methylidene complex 3 (Scheme 2). This product was characterised by elemental analysis and crystal structure determination.…”

“…The small ligand TMTAC (R = Me) reacts with La(AlMe 4 ) 3 to afford [(TMTAC)-La(AlMe 4 )({μ 3 -CH 2 }{AlMe 2 (μ 2 -Me)} 2 )] containing a bis-(aluminate) dianion (one C-H activation step). [15] In contrast, the analogous reaction with Sm(AlMe 4 ) 3 . Closer investigation of this reaction with variation of R showed that the isomeric tris(aluminate) ion with two methylidene units had formed in C-H activation reactions besides BIR, namely [(TiPrTAC)La(AlMe 4 )({μ 3 -CH 2 }{AlMe 2 (μ 2 -Me)} 2 )] in the case of R = iPr.…”

“…[2] Other examples studied in the context of deactivation pathways of olefin polymerization catalyst systems are Ti/ Al carbide clusters formed from titanium phosphinimides such as Cp(R 3 PN)TiMe 2 with Al 2 Me 6 , [3] which are the products of multiple C-H activation and were investigated by Stephan et al The same authors described related reactions leading to [CpTi(μ-SR)(μ-NPiPr 3 )(μ 4 -C)(AlMe 2 ) 2 (μ-SR)AlMe] (R = Ph, Bn), [4] [CpTi(μ-Me)(μ-NPiPr 3 )(μ 4 -C)(μ-AlMe 2 ) 2 (AlMe 2 )] [5] and [CpTi(μ 2 -Me)(μ 2 -NPPh 3 )(μ 5 -C)(AlMe 2 ) 3 (μ 2 -MeAlMe 2 )]. [6] Highly coordinate methylidyne species were also found in the anion [(Me 2 Al) 2 - [7] and in [(Cp*Zr) 4 (μ-Cl) 5 (Cl)(μ-CH) 2 ] and [(Cp*Zr) 5 (μ-Cl) 6 (μ-CH) 3 ], [8] as well as in some carbaalanes [9] and Zr-Al-C aggregates. [10] In a recent review on homoleptic rare earth metal complexes containing Ln-C σ-bonds, [11] Anwander et al included the description of a series of methyl group degradation reactions, where C-H activation reactions are initiated in the coordination sphere of rare earth elements by the action of different bases in Ln(AlMe 4 ) 3 reagents.…”

Catalysis and Organometallic Chemistry of Monometallic Species

“…75 A kinetic study of β-methyl elimination in [Hf(η 5 -C 5 Me 5 ) 2 (µ-MeB(C 6 F 5 ) 2 )R] (where R = Me and CH 2 CMe 3 ) 76 and C-H activation of the aromatic substituents of Cp ligands coordinated to a {Ti(CH 2 Ph) 3 } moiety, 77 and competitive metathesis and C-H activation from reaction between AlMe 3 and [TiMe 2 (NP t Bu 3 ) 2 ] have been reported. 78 Intermolecular C-H and C-F activation by group 4 complexes include computational evidence for the role of alkane adducts in selective C-H activation by titanium amido complexes 79 and activation of arene C-H bonds by cationic hafnium() silyl complexes. 80 The mechanism of C-F activation of nonperfluorinated alkenes 81 ] and C 6 F 6 have also been described.…”