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 eta(6)-arene-eta(1)-P ruthenium(II) complexes 3 and 6 starting from their corresponding eta(1)-P metallic precursors 2 and 5. In marked contrast, straightforward high-yield synthesis of tethered eta(6)-arene-eta(1)-P ruthenium(II) complexes 9a,b was achieved upon addition of methylenaminophosphine-type ligands i-Pr2N-C(Ph)=N-PR(2)7 (R = Ph (a), i-Pr (b)) on the ruthenium precursor [(p-cymene)RuCl2](2) at 80 degrees 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 eta(1)-P 8a,b or tethered eta(6)-arene-eta(1)-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 degrees in 7a,b to 133 degrees, 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 degrees. 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.