Crucial Role of the Amidine Moiety in Methylenamino Phosphine-Type Ligands for the Synthesis of Tethered η⁶-Arene-η¹-P Ruthenium(II) Complexes: Experimental and Theoretical Studies

Methyleneaminophosphine ligands R′C(Ph)N−PPh₂ (R′ = H (1), Ph (4)) are unable to form tethered η⁶-arene-η¹-P ruthenium(II) complexes 3 and 6 starting from their corresponding η¹-P metallic precursors 2 and 5. In marked contrast, straightforward high-yield synthesis of tethered η⁶-arene-η¹-P ruthenium(II) complexes 9a,b was achieved upon addition of methylenaminophosphine-type ligands i-Pr₂N−C(Ph)N−PR₂ 7 (R = Ph (a), i-Pr (b)) on the ruthenium precursor [(p-cymene)RuCl₂]₂ at 80 °C. We have observed by X-ray crystallographic analyses the unprecedented structural adaptive behavior of the N-phosphino amidine ligands 7a,b upon the untethered η¹-P 8a,b or tethered η⁶-arene-η¹-P 9a,b coordination mode in ruthenium(II) complexes. The imino nitrogen atom of the amidine moiety in 7a,b behaves as a “universal joint”. In order to minimize the steric hindrance in the second coordination sphere of complexes 8a,b, the value of the C1−N1−P1 bond angle of the amidine moiety widened from 119−122° in 7a,b to 133°, which corresponds to a dramatic change in the geometry of the N-phosphino amidine ligands. Moreover, in order to reduce the strain induced by the tethered coordination mode, the value of the C1−N1−P1 bond angle in the amidine moiety in ruthenium(II) complexes 9a,b decreases to 116°. DFT calculations have been carried out in order to gain more insight into the structural and electronic properties of the methylenaminophosphine ligands R′-C(Ph)N-PPh₂ as well as the tethered and untethered ruthenium complexes. Moreover, the reaction feasability has also been theoretically discussed.