Using Diamagnetic Yttrium and Lanthanum Complexes to Explore Ligand Reduction and C–H Bond Activation in a Tris(aryloxide)mesitylene Ligand System

[Y­(N­(SiMe₃)₂)₃] reacts with (^Ad,MeArOH)₃mes to form the Y³⁺ complex [((^Ad,MeArO)₃mes)­Y], 1-Y. This complex reacts with potassium metal in the presence of 2.2.2-cryptand to give a cocrystallized mixture of [K­(2.2.2-cryptand)]­[((^Ad,MeArO)₃mes)­Y], 2-Y, and [K­(2.2.2-cryptand)]­[((^Ad,MeArO)₃mes)­YH], 3-Y. The electron paramagnetic resonance spectrum of this crystalline mixture exhibits an isotropic signal at 77 K (g_iso = 2.000, W_iso = 1.8 mT), suggesting that 2-Y is best described as a Y³⁺ complex of the tris­(aryloxide)­mesitylene radical ((^Ad,MeArO)₃mes)^4–. Evidence of the hydride ligand in 3-Y was obtained by ⁸⁹Y–¹H heteronuclear multiple quantum coherence NMR spectroscopy, and a coupling constant of J_YH = 93 Hz was observed. A single crystal of 3-Y was also obtained in pure form and structurally characterized for comparison with the crystal data on the mixed component 2-Ln/3-Ln crystals. The origin of the hydride in 3-Ln is unknown, but further studies of the reduction of 1-La, previously found to form 2-La, revealed a possible source. Ligand-based C–H bond activation and loss of hydrogen can occur under reducing conditions to form a tetraanionic ligand derived from ((^Ad,MeArO)₃mes)^3–, as observed in [K­(2.2.2-cryptand)]­[((^Ad,MeArO)₃(C₆Me₃(CH₂)₂CH)­La], 4-La.