Substituents effects in POP pincer complexes of ruthenium

Reactions of [RuCl2(p-cymene)](2) with ((Bu2PCH2CH2)-Bu-t)(2)O (POP-Bu-t) and (Pr2PCH2CH2)(2)O (POP-Pr-i) afforded RuCl2(POP-Bu-t) (1) and [Ru-2(mu-Cl)(3)(POP-Pr-i)(2)]Cl (2 center dot Cl), respectively. The POP ligand is coordinated in a mer fashion in complex 1, whose crystal structure revealed a gamma-agostic C-H center dot center dot center dot Ru interaction of one Bu-t group. Spectroscopic evidence indicated that this agostic interaction is retained in 1 in solution. A related compound, [Ru(N-2)Cl(POP-Bu-t)]BPh4 (4), which also showed agostic bonding of a Bu-t group, was obtained by substitution of N-2 for Cl- in 1, in the presence of NaBPh4. Compound 2 center dot Cl readily underwent ion exchange with LiBPh4 or LiPF6 to give 2 center dot BPh4 or 2 center dot PF6 salts, respectively. A crystallographic analysis of 2 center dot PF6 established a co-facial bioctahedral geometry of the [Ru-2(mu-Cl)(3)(POP-Pr-i)(2)](+) cation containing two POP ligands coordinated in a fac fashion. Reactions of 1 and 2 with H-2 afforded the dihydrogen complexes cis,trans-Ru(H-2)Cl-2(POP-Bu-t) (3) and cis,cis-Ru(H-2)Cl-2(POP-Pr-i) (5), respectively. The H-H bond distances are very similar in both compounds, r(H-H) = 1.0 +/- 0.1 angstrom, based on the T-1min and J(HD) data and results of DFT calculations. Reaction of 2 with N-2 gave the dinitrogen complex cis,cis-Ru(N-2)Cl-2(POP-Pr-i) (6), but solutions of 1 under a nitrogen atmosphere showed no evidence of an analogous compound. The different steric requirements of the phosphorus substituents of the POP ligands were identified as the source of the differences in the coordination properties of the POP-Bu-t and POP-iPr complexes 1-6.