Cationic Organoscandium β-Diketiminato Chemistry:  Arene Exchange Kinetics in Solvent Separated Ion Pairs

Abstract

Abstraction of methide from the beta-diketiminato supported organoscandium complex [L1ScMe2]2 using the trityl borate activator [Ph3C][B(C6F5)4] in arene solvents gives solvent separated ion pairs in which the arene (C6H5Br, 1a; C6H6, 1b; C7H8, 1c; 1,3,5-Me3C6H3, 1d) is coordinated to the cationic scandium center in an eta6 bonding mode. L1 incorporates methyl groups in the 2,4 positions of the ligand backbone and bulky 2,6-diisopropylphenyl groups on the nitrogen atoms. The relative binding strength of the arenes is C6H5Br < C6H6 < 1,3,5-Me3C6H3 < C7H8. Ion pairs 1a and 1c have been characterized crystallographically, and the C6H5Br derivative is notable for its eta6 bonding mode in preference to the more common eta1 bonding mode via the halogen atom. The kinetics of displacement of mesitylene by toluene (1d --> 1c) yield activation parameters of DeltaH = 21.4(6) kcal mol-1 and DeltaS = 6(1) cal mol-1 K-1. In combination with the observed lack of dependence of [toluene] on the rate of displacement, these data suggest a mechanism involving partial dissociation of the coordinated arene, followed by attack of the incoming arene. This chemistry has relevance to the role of these solvent separated ion pairs in olefin polymerization processes and presents a rare opportunity for the detailed study of these ephemeral species.

1,3-Diene Polymerization Mediated by Homoleptic Tetramethylaluminates of the Rare-Earth Metals

“…As a clear identification of the active species was not successful so far, due to the paramagnetic character of most of the lanthanide ions and the low tendency of the active species toward crystallization, active species elucidation remains challenging. A reasonable interpretation of this polymerization behavior seems to be that solvent [4,5,13,37], pre-reaction side products (e.g., Ph 3 CMe (A) and PhNMe 2 (B): Ln(III)-arene coordination) [38][39][40], or even anion coordination (e.g., via Ln(III)-F interactions) [41] come into play. Furthermore, dimerization might take place or a η 2 -to-η 3 coordination switch of the remaining tetramethylaluminate moiety, which tendency seems more pronounced for the larger lanthanides as found for the neutral precatalysts [4,10,12].…”

Tuning steric and electronic effects in transition-metal β-diketiminate complexes

“…As a result, dimeric/polymeric metal complexes are more often formed with less sterically hindered β-diketiminate ligands. For example, comparisons with more hindered monomeric analogues were reported for [LScCl 2 ] n (L tBu,iPr , 16 n=1; L Me,iPr , 17 n=2), [LSc(CH 3 ) 2 ] n (L tBu,iPr , 16 n=1; L Me,iPr , 17 n=2), [LFeCl] n (L tBu,iPr , 18 n=1; L Me,iPr , 19 Me L Me,Me , 20 n=2), [LFeF] n (L tBu,iPr , n=1; L Me,iPr , n=2), 21 [LCoCl] n (L tBu,iPr , 22 n=1; L Me,iPr , 23 n=2), [LNiCl] n (L tBu,iPr , 22 n=1; L Me,iPr , 24 L Me,Me , 25 n=2), [LNi(CO)] n , (L tBu,iPr , 26 L Me,iPr , 27 n=1; L Me,Me , 28 n=2), [L R,iPr CuCl] n (L Me,iPr , 29 Cl L Me,iPr , 29 n=1; Ph L H,iPr , 30 L Me,Cl , 31 n=2), and [LPd(μ-OAc)] n (L Me,iPr , 32 n=1; L Me,H 32 Cl L Me,H , 33 n=2). The angle between the two β-diketiminate ligand planes in dimeric metal complexes is often influenced by the different substituents on the ligand (Table 3.1.1).…”

Rare Earth Arene-Bridged Complexes Obtained by Reduction of Organometallic Precursors

“…The gas-phase reaction of scandium ion with benzene and its derivatives led to products that could only be characterized by mass spectrometry (Huang et al, 1987). Neutral arene scandium(III) complexes supported by -46 diketiminato ligands were isolated and structurally characterized by Piers's group (Chart 21) (Hayes et al, 2003;Hayes et al, 2007). The reports from Piers's group highlighted the strong Lewis acidity of scandium(III).…”