Substituents Effects in POP Pincer Complexes of Ruthenium

Reactions of [RuCl2(p-cymene)]2 with (tBu2PCH2CH2)2O (POP-tBu) and (iPr2PCH2CH2)2O
(POP-iPr) afforded RuCl2(POP-tBu) (1) and [Ru2(μ-Cl)3(POP-iPr)2]Cl (2·Cl), respectively. The
POP ligand is coordinated in a mer fashion in complex 1, whose crystal structure revealed
a γ-agostic C−H···Ru interaction of one tBu group. Spectroscopic evidence indicated that
this agostic interaction is retained in 1 in solution. A related compound, [Ru(N2)Cl(POP-tBu)]BPh4 (4), which also showed agostic bonding of a tBu group, was obtained by substitution
of N2 for Cl- in 1, in the presence of NaBPh4. Compound 2·Cl readily underwent ion exchange
with LiBPh4 or LiPF6 to give 2·BPh4 or 2·PF6 salts, respectively. A crystallographic analysis
of 2·PF6 established a co-facial bioctahedral geometry of the [Ru2(μ-Cl)3(POP-iPr)2]+ cation
containing two POP ligands coordinated in a fac fashion. Reactions of 1 and 2 with H2 afforded
the dihydrogen complexes cis,trans-Ru(H2)Cl2(POP-tBu) (3) and cis,cis-Ru(H2)Cl2(POP-iPr)
(5), respectively. The H−H bond distances are very similar in both compounds, r(H−H) =
1.0 ± 0.1 Å, based on the T1min and JHD data and results of DFT calculations. Reaction of 2
with N2 gave the dinitrogen complex cis,cis-Ru(N2)Cl2(POP-iPr) (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-tBu and POP-iPr complexes 1−6.