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Two different homogeneous catalyst precursor complexes, i.e. [Rh(COD) 2 ]BF 4 and [Rh(COD)Cl] 2 , were immobilized on phosphotungstic acid-modified alumina to form γ-Al 2 O 3 /PTA/ Rh(COD) 2 BF 4 (1b) and γ-Al 2 O 3 /PTA/[Rh(COD)Cl] 2 (2b), respectively. These immobilized complexes have been modified with (R,R)-MeDuPHOS to form the immobilized chiral catalysts γ-Al 2 O 3 /PTA/Rh((R,R)-MeDuPHOS)(COD)BF 4 (1c) and γ-Al 2 O 3 / PTA/Rh((R,R)-MeDuPHOS)(COD)Cl (2c). It is shown that immobilization and subsequent modification by ligand exchange reactions of general precursor complexes is a powerful method to prepare chiral and achiral anchored rhodium catalysts. Enantioselective hydrogenation reactions show that the activity and selectivity differences between the homogeneous catalysts [((R,R)-MeDuPHOS)Rh(COD)]BF 4 (1a) and the in situ prepared [((R,R)-MeDuPHOS)Rh(COD)]Cl (2a) are larger than the differences between the immobilized analogues (1c, 2c). At elevated temperature and H 2 pressure the activity and selectivity of 1c are comparable to those of its homogeneous analogue. Complex 1b was also used to prepare γ-Al 2 O 3 /PTA/Rh(DiPFc)-(COD)BF 4 (1d) via a ligand exchange reaction with 1,1′-bis-(diisopropylphosphino)ferrocene (DiPFc). This complex was used as a selective and heterogeneous hydrogenation catalyst with special chemoselective hydrogenation properties. The described immobilized rhodium catalysts prepared from the general precursor complexes 1b and 2b display hydrogenation activity and selectivity comparable to those of their homogeneous analogues. Moreover, it is demonstrated that these catalysts can be reused multiple times with neither activity nor selectivity loss and that leaching can be minimized by using optimized reaction parameters.

Reaction of the aryl and alkyl metal derivatives, [Li(C 6

The synthesis and characterization of new, five-coordinate molybdenum bis(imidoaryl) complexes [Mo(NAr) 2 (C-N)X] (Ar) C 6 H 3 i-Pr 2-2,6; C-N) [C 6 H 4 (CH 2 NMe 2)-2]-; X) Cl (1), Me (2), Et (3), Bu (4), CH 2 SiMe 3 (5), (p-tolyl) (6), (C-N) (7)) is reported. The solid-state structure of 2 has been elucidated by single-crystal X-ray analysis. Compounds 2, 3, 4, 5, and 6 react with alkyl-or aryllithium compounds to form lithium molybdate(VI) derivatives, of which [Li(DME)Mo(NAr) 2 (C-N)(Me)(p-tolyl)] (10), formed by the reaction of 2 with [Li-(p-tolyl)], has been structurally (X-ray) characterized. Thermal activation of these lithium molybdates leads to the formation of paramagnetic lithium molybdate(V) compounds instead of the anticipated molybdenum(VI) alkylidenes. The actual temperature (between-10 and 80°C) at which paramagnetic Mo(V) radical anions are formed is dependent on both the type of alkyl or aryl substituent (introduced via LiR′) and the solvent. The synthesis of [LiMo-(NAr) 2 (C-N) 2 ] (11) by reaction of 7 with n-BuLi is described. The initially formed lithium molybdate(VI) compound [Li(DME) n Mo(NAr) 2 (C-N) 2 (n-Bu)] is not stable at room temperature and converts directly to the lithium molybdate(V) derivative 11. The solid-state structure of 11 has been elucidated by single-crystal X-ray analysis. None of the lithium molybdate(VI) nor-(V) derivatives described herein are active catalysts for ROMP, as thermal activation does not lead to the formation of a molybdenum alkylidene complex but to electron transfer and formation of a lithium molybdate(V) instead. However, upon treatment of a solution of any of the molybdate(V) derivatives with dry air, catalytic ROMP is observed.

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