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

Abstract

[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.

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

“…Both heteroleptic reduction products, 2 and 4 , decompose readily at room temperature, and attempts to crystallize the Y­(II) products at low temperature formed crystals that contained [N­(SiMe 3 )­(SiMe 2 CH 2 )] 2– ligands apparently arising from C–H bond activation of a trimethylsilyl group. Cyclometalation of the [N­(SiMe 3 ) 2 ] 1– ligand is a common reaction, − and as described in the Introduction, C–H bond activation has been observed in other Ln­(II) and U­(II) systems. − ,,− , The fact that the precursor (C 5 Me 5 ) 2 Y III (NR 2 ), 1 , had an agostic interaction of a methyl C–H with the yttrium metal center in one complex of the two present in the unit cell suggests that this reaction could occur with little structural change. The DFT calculations on both [(C 5 Me 5 ) 2 Y II (NR 2 )] 1– and [(C 5 Me 4 H) 2 Y II (NR 2 )] 1– show HOMOs with electron density on the C–H bond, which is consistent with the reactivity observed.…”

“…Cyclometalation of the [N(SiMe 3 ) 2 ] 1− ligand is a common reaction, 39−43 and as described in the Introduction, C−H bond activation has been observed in other Ln(II) and U(II) systems. [13][14][15]19,[25][26][27][28][29][30][31][32][33][34]36 The fact that the precursor (C 5 Me 5 ) 2 Y III (NR 2 ), 1, had an agostic interaction of a methyl C−H with the yttrium metal center in one complex of the two present in the unit cell suggests that this reaction could occur with little structural change. The DFT calculations on both [(C 5 Me 5 ) 2 Y II (NR 2 )] 1− and [(C 5 Me 4 H) 2 Y II (NR 2 )] 1− show HOMOs with electron density on the C−H bond, which is consistent with the reactivity observed.…”

“…Homoleptic complexes of the general formula (Ln II A 3 ) 1– for the rare-earth metals beyond Eu, Yb, Sm, Tm, Dy, and Nd have subsequently been characterized by X-ray crystallography for A = Cp′, Cp″, C 5 Me 4 H (Cp tet ), C 5 H 4 Me­(Cp Me ), C 5 H 4 CMe 3 (Cp t ), NR 2 (R = SiMe 3 ), 2,6-Ad 2 -4 -t -Bu-C 6 H 2 O (OAr Ad,Ad, t ‑Bu ), and A 3 = [( Ad,Me ArO) 3 mes]. − One notable exception to the ligand list above is A = C 5 Me 5 because the homoleptic precursor complexes, (C 5 Me 5 ) 3 Ln III , are highly reactive, sterically crowded species that react with THF, which is the common solvent used in Ln III A 3 reductions. , This is unfortunate, since C 5 Me 5 is an excellent ligand for the isolation, stabilization, and generation of crystalline bis­(cyclopentadienyl) rare-earth metal complexes . This study was initiated to see if an Y­(II) complex containing C 5 Me 5 could be isolated from a heteroleptic precursor, namely, (C 5 Me 5 ) 2 Y III (NR 2 ).…”

“…Reported here is the synthesis and X-ray crystal structure of the first Y­(II) complex containing C 5 Me 5 as a ligand along with a C–H bond activated derivative. C–H bond activation has been observed in several Ln­(II) systems including (a) the [(( Ad,Me ArO) 3 mes)­Ln II ] 1– complexes which form [(( Ad,Me ArO) 3 mes)­Ln III H] 1– compounds, which co-crystallize with the Ln­(II) compounds, − (b) the [(C 5 H 2 t Bu 3 ) 2 Nd II (μ-I)­K­(18-crown-6)] complex, which forms [(C 5 H 2 t Bu 3 )­(η 5 -C 3 H 2 t Bu 2 CMe 2 CH 2 - κC )­Nd III (μ-I)­K­(18-crown-6)], (c) the tris­(indenyl) complex, (C 9 H 7 ) 2 Dy III , that upon reduction forms the metalated [K­(crypt)]­[(C 9 H 7 ) 2 Dy III (μ-η 5 :η 1 -C 9 H 6 )] 2 complex, and (d) Cp″ 3 Y III which upon reduction led to the isolation of (Cp″ 2 Y III H) 2 . C–H bond activation has also been observed in uranium chemistry in reactions attempting to make U­(II) complexes by reductions of U­(III) species. − This was observed with the [( Ad,Me ArO) 3 mes] 3– ligand system described above and more recently with the (C 5 Me 5 ) 2 U III (NR 2 ) analogue of the complexes examined in this study …”

The importance of the counter-cation in reductive rare-earth metal chemistry: 18-crown-6 instead of 2,2,2-cryptand allows isolation of [YII(NR2)3]1− and ynediolate and enediolate complexes from CO reactions

“…C-H bond activation in Ln(II) complexes has been previously observed. 17,[42][43][44][45] Clearly, the details of these rare earth reduction reactions are critical to the isolation of crystalline products.…”