Amido Ln(II) Complexes Coordinated by Bi- and Tridentate Amidinate Ligands: Nonconventional Coordination Modes of Amidinate Ligands and Catalytic Activity in Intermolecular Hydrophosphination of Styrenes and Tolane

Heteroleptic Ln­(II) and Ca­(II) amides
[tBuC­(NC6H3-iPr2-2,6)2]­MN­(SiMe3)2(THF)
(M = Yb (1Yb),
Ca (1Ca)), [2-MeOC6H4NC­(tBu)­N­(C6H3-iPr2-2,6)]­LnN­(SiMe3)2(THF) (Ln = Sm (2Sm), Yb (2Yb)), and [2-Ph2P­(O)­C6H4NC­(tBu)­N­(C6H3-Me2-2,6)]­YbN­(SiMe3)2(THF)
(3Yb) coordinated by bi- and tridentate amidinate ligands
were obtained by the amine elimination reactions of M­[N­(SiMe3)2]­(THF)2 (M = Yb, Sm, Ca) with parent amidines
in good yields. Complex [tBuC­(NC6H3-iPr2-2,6)2]­SmN­(SiMe3)2 can be obtained only by a salt metathesis reaction
of [tBuC­(NC6H3-2,6-iPr2)2]­SmI­(THF)2 with NaN­(SiMe3)2. Unlike 1Yb and 1Ca in 1Sm the amidinate ligand is coordinated to metal
ion in κ1-amido:η6-arene fashion
preventing THF coordination. The derivatives of tridentate amidinate
ligands bearing pendant donor 2-MeOC6H4 or 2-Ph2P­(O)­C6H4N groups feature nonconventional
κ1-N,κ2-O,η6-arene
coordination mode. Complexes 1Ca, 1Sm, 1Yb, 2Sm, 2Yb, and 3Yb proved to be efficient catalysts for styrene hydrophosphination
with PhPH2 and Ph2PH. In styrene hydrophosphination
with PhPH2 all the catalysts perform excellent chemoselectivity
and afford a monoaddition productsecondary phosphine (PhCH2CH2)­PhPH. Moreover, all the catalysts perform hydrophosphination
reactions regioselectively with exclusive formation of the anti-Markovnikov addition product. Within the series of
complexes coordinated by the same amidinate ligand catalytic activity
decreases in the following order 1Ca ≥ 1Sm>1Yb. The turnover frequencies were in the range
of
TOF ≈ 0.3–0.7 h–1. However, application
of tridentate amidinate ligand allowed one to increase catalytic activity
significantly: for 2Sm TOF was found to be 8.3 h–1. For the addition of PhPH2 to para-substituted
styrenes catalyzed by 2Sm it was found that electron-withdrawing
substituents (Cl, F) do not affect the reaction rate while electron-donating
groups (tBu, OMe) noticeably slow down the reaction.