Selective Dialkylation of a Doubly Linked Dicyclopentadiene Ligand and the Ensuing Ruthenium Complexes - Research Paper Example

The authors state the motivation behind such an approach as “While Ru3(CO)12 is an ideal starting material with regard to the cis to trans isomeric ratio formed around the doubly linked dicyclopentadienyl ligand, the CO ligand usually requires high heat or photolytic conditions and laborious chemicalsteps to remove it from the metal center. These chemical steps reduce the synthetic versatility of the resulting ruthenium complexes that contain a CO ligand” (Fier, et al., 2011). The paper concludes with a structure refinement of the obtained diruthenium complex which has the structural representation: Synthesis Route The reaction begins with the hydroxyl bromination of 4,4,8,8-tetramethyl-1,4,7,8-tetrahydro-s-indacene (1a; C2v symmetry) and 4,4,8,8-tetramethyl-1,4,5,8-tetrahydro-s-indacene (1b; C2h symmetry) by N-bromosuccinimide (NBS), NH4OAc and H2O/ Acetone.

The dehydration of the resulting compound was done under anaerobic conditions in the presence of p-TsOH.H2O to yield the compounds 2 a,b. Reaction of 2a,b with MeMgBr in the presence of Ni-(dppf)Br2 (dppf = 1,1?-bis(diphenylphosphino)ferrocene) gave (C5H3Me)2(CMe2)2 (3a,b) in 31% yield. However, it was found that this compound 3 a,b did not react very well with RuCl3.3H20. Adding a bulkier alkyl group could lead to a specific 2,6 bonding making advantage of stearic repulsion with the methyl groups in the 4,8 position.

Thus, the first reaction was modified to yield: Reaction of 4a,b with RuCl3·3H2O in MeOH (140 °C,15 min) gives a green-black heterogeneous mixture, with filtration of the mixture yielding a dark purple-black solid which was found to be diamagnetic. A subsequent X-ray crystallographic study of the new compound showed it to be the half-sandwich complex cis-{(?5-C5H2-(t-Bu))2(CMe2)2}Ru2(?-Cl)2Cl2 (5), containing RuII / RuII metal centers. The preferred way to synthesize 5 is to bypass the filtration and isolation step of the purple intermediate and to just remove the MeOH solvent from the crude reactionmixture, add CHCl3, and heat to 80 °C for 15 min to give 5 in 28% yield (Scheme 2).

The Structure of 5 shows a Ru?Ru single bond with a bond length of 2.7057(3) A, which would explain the diamagnetic nature of 5. Evidence for the RuII/RuII core is provided by the dissolution of the purple solid in MeCN to form cis-[{(?5-C5H2(t-Bu))2(CMe2)2}Ru2(MeCN)4(?-Cl)]+ (6), Complex 6 can also be synthesized by the reduction of 5 using Zn dust in MeCN and isolated in 62% yield (Scheme 3) The authors have successfully alkylated the 2,6-positions of the doubly linked dicyclopentadiene ligand 1a,b with either Me or tertbutylgroups.

When the alkyl groups are tert-butyl groups, the resulting ligand reacts with RuCl3·3H2O to yield 5, the doubly linked analogue of [CpRuCl2]2 and [Cp*RuCl2]2. 5 can be reduced with Zn in MeCN to yield 6, an acetonitrile complex with a bridging chloro ligand. The bridging chloro ligand in 6 can be removed by the addition of AgOTf to yield 7, a ruthenium complex containing only the doubly linked dicyclopentadienyland acetonitrile ligands. Chin, R. M., Simonson, A., Mauldin, J., & Criswell, J. (2010).

Organometallics , 29 (17), 3868?3875. Fier, P.