Carbene vs Olefin Products of C−H Activation on Ruthenium via Competing α- and β-H Elimination

Bulky pincer complexes of ruthenium are capable of C−H activation and H-elimination from the
pincer ligand backbone to produce mixtures of olefin and carbene products. To characterize the products
and determine the mechanisms of the C−H cleavage, reactions of [RuCl2(p-cymene)]2 with N,N‘-bis(di-tert-butylphosphino)-1,3-diaminopropane (L1) and 1,3-bis(di-tert-butylphosphinomethyl)cyclohexane (L2)
were studied using a combination of X-ray crystallography, NMR spectroscopy, and DFT computational
techniques. The reaction of L1 afforded a mixture of an alkylidene, a Fischer carbene, and two olefin isomers
of the 16-e monohydride RuHCl[tBu2PNHC3H4NHPBut2] (2), whereas the reaction of L2 gave two olefin
and two alkylidene isomers of 16-e RuHCl[2,6-(CH2PBut2)2C6H8] (3), all resulting from dehydrogenations
of the ligand backbone of L1 and L2. The key intermediates implicated in the C−H activation reactions
were identified as 14-electron paramagnetic species RuCl(PCP), where PCP = cyclometalated L1 or L2.
Thus the α- and β-H elimination reactions of RuCl(PCP) involved spin change and were formally spin-forbidden. Hydrogenation of 2 and 3 afforded 16-electron dihydrides RuH2Cl(PCP) distinguished by a large
quantum exchange coupling between the hydrides.