Synthesis and reactivity of ruthenium arene complexes incorporating novel Ph2PCH2CH2BR2 ligands.: Easy access to the four-membered ruthenacycle [(p-cymene)RuCl(kC,P-CH2CH2PPh2)]

The ambiphilic ligands Ph2PCH2CH2BR2 (1a,b: BR2 = BCy2 (a), 9-BBN (b)) were readily prepared by hydroboration of vinyldiphenylphosphine. Reaction of 1a,b with [(p-cymene)RuCl2](2) afforded the corresponding complexes [(p-cymene)RuCl2(Ph2PCH2CH2BR2)] (2a,b), in which the borane moiety remains pendant, as confirmed by an X-ray diffraction analysis of 2b. Reaction of 2a,b with AgBF4 in the presence of acetonitrile leads to the formation of the corresponding cationic complexes [(p-cymene)RuCl(Ph2PCH2CH2BR2)(CH3CN)][BF4] (3a,b) without alteration of the pendant borane moiety. In contrast, treatment of 2a,b with AgOAc induces CH2-B bond cleavage and affords the four-membered ruthenacycle [(p-cymene)RuCl(K-C,K-P-CH2CH2PPh2)] (4), characterized by X-ray diffraction. By reaction with chlorodicyclohexylborane, 4 gives back 2a via ring-opening sigma-bond metathesis, whereas 4 reacts with chlorodiethylalane via alkylation at ruthenium with retention of the four-membered metallacycle to afford the ethyl complex [(p-cymene)RuEt(K-C,K-P-CH2CH2PPh2)] (5).