Synthesis of a Heteroleptic Pentamethylcyclopentadienyl Yttrium(II) Complex, [K(2.2.2-Cryptand)]{(C5Me5)2YII[N(SiMe3)2]}, and Its C–H Bond Activated Y(III) Derivative

To determine if a C5Me5 complex of Y(II) could be isolated and to examine the synthetic accessibility of heteroleptic Y(II) complexes, the reduction of (C5Me5)2YIII(NR2) (R = SiMe3) with potassium graphite in THF in the presence of 2.2.2-cryptand (crypt) was examined. An intensely dark colored solution is formed that has EPR spectra indicative of a 4d1 Y(II) complex with giso = 1.975 and A(89Y) = 74.5 G. X-ray diffraction revealed that single crystals of the product contained a 65–75% mixture of an Y(II) complex [K(crypt)][(C5Me5)2YII(NR2)] and 25–35% of an Y(III) cyclometalated derivative, [K(crypt)][(C5Me5)2YIII{N(SiMe3)(SiMe2CH2)-κC,κN}], arising from C–H bond activation of a methyl group of the SiMe3 substituent on the amide ligand. An analogous reduction of the tetramethyl complex, (C5Me4H)2YIII(NR2), generated a dark solution with giso = 1.975 and A(89Y) = 71.2 G. Crystallization of that product also revealed a mixture of Y(II) and Y(III) complexes comprised of 10%/90% [K(crypt)][(C5Me4H)2YII(NR2)]/cyclometalated [K(crypt)][(C5Me4H)2YIII{N(SiMe3)(SiMe2CH2)-κC,κN}], which co-crystallized with [K(crypt)][C5Me4H]. Density functional theory (DFT) studies indicate that the HOMOs of both [(C5Me5)2YII(NR2)]1– and [(C5Me4H)2YII(NR2)]1– are primarily 4dz2 in character with some electron density on the SiMe3 methyl groups that may enhance the C–H bond activation reactivity.