Synthesis and structural characterization

The trivalent compounds of the bis(dimethylsilyl)amide ligand,

{Ln[N(SiHMe2]2[μ-N(SiHMe)2]}2 (Ln = La, Y, Lu)32,33 and Ln[N(SiHMe2)3](THF)2 (Ln

= La, Ce, Nd, Sm, Er, Lu), have been prepared from metathesis of LnCl3(THF)2 or by

protonolysis of the bis(trimethylsilyl)amide precursor, Ln[N(SiMe3)2]3.13,34,35 Here, we

synthesized {Ce[N(SiHMe2]2[μ-N(SiHMe)2]}2 (2.1) and Ce[N(SiHMe2)3](THF)2 (2.2) by

protonolysis of Ce[N(SiMe3)2]3 with HN(SiHMe2)2 in pentane or THF, respectively

27

Scheme 2.2.1. Synthesis of compounds 2.1, 2.2, 2.3, and 2.4. Adapted from work to be submitted to Dalton Transactions, 2014.

The crystal structure of 2.1 was isomorphous with the lanthanum analogue,32 and

the structure is shown in Figure 2.2.1. Compound 2.1 crystallized solvent-free in the

triclinic P space group. The structure was dimeric, with two terminal and two bridging silylamide ligands coordinated to each cerium cation. The terminal Ce–N bond distances in the compound ranged from 2.3413(12)–2.3690(13) Å and were similar to the Ce–N bond distances in the reported solid state structure of compound 2.2.36 The bridging Ce–

N bond distances were elongated compared to the terminal interactions, and ranged from 2.5958(12)–2.6480(12) Å.

28

Figure 2.2.1. 30% probability thermal ellipsoid plot of compound 2.1.Hydrogen atoms have been removed for clarity. Selected bond distances: Ce(1)–N(1): 2.3413(12) Å, Ce(1)–N(2): 2.3690(13) Å, Ce(1)–N(3): 2.6318(12) Å, Ce(1)–N(4): 2.6480(12) Å, Ce(2)–N(3): 2.5958(12) Å, Ce(2)–N(4): 2.5961(12) Å, Ce(2)–N(5): 2.3477(13) Å, Ce(2)–N(6): 2.3441(12) Å.

Addition of KN(SiHMe2) to a hexanes solutions of 2.1 or 2.2 led to coordination

of a fourth silylamide equivalent to the cerium(III) ion and induced precipitation of the cerium(III)-containing compound K[Ce[N(SiHMe2)2]4] (2.3) (Scheme 2.2.1). A related

compound, Li(THF)[Ce[N(SiHMe2)2]4, has been previously synthesized by salt

metathesis of CeCl3 with LiN(SiHMe2)2.20,37 The crystal structure of 2.3 is shown in

Figure 2.2.2. Compound 2.3 crystallized as a solvent-free coordination polymer in the

monoclinic C2/c space group. The symmetry about the cerium ion was pseudo- tetrahedral, with a τ4 parameter of 0.88.38 The Ce–N bond distances in the compound

were 2.3820(12) and 2.4379(12) Å and were similar to the Ce–N bond distances in compounds 2.1 and 2.2.36 Two of the eight silyl protons in the molecular unit showed

29 short contacts to the Ce(III) center, with SiH–Ce distances of 2.81156(7) Å and Ce–Si distances of 3.27110(7) Å. The remaining silyl protons were used for coordination to the unsolvated potassium ions in the lattice. A related complex, {Li[Y[N(SiHMe2)2]4]}2, has

been synthesized from methathesis of Y(OTf)3 (OTf = CF3SO3

–

) with 4 equiv LiN(SiHMe2)2 and containsintermolecular SiH–Li interactions in the dimeric structure.39

Figure 2.2.2. 30% probability thermal ellipsoid plot of compound 2.3.Methyl hydrogen atoms have been removed for clarity. Selected bond distances: Ce(1)–N(1): 2.4379(12)

Å, Ce(1)–N(2): 2.3820(12) Å. Reprinted from work to be submitted to Dalton

30

Compound 2.3 was insoluble in non-coordinating solvents including benzene,

toluene, hexanes and pentane, but was soluble in the coordinating solvents Et2O, THF,

and pyridine. In coordinating solvents, the potassium ion was solvated and removed from the complex anion, [Ce[N(SiHMe2)2]4]

–

, evidenced by a broad singlet in the 1H NMR at – 8.12 ppm, which corresponds to eight equivalent silyl protons. THF solutions of

compound 2.3 underwent complete desolvation upon application of vacuum, as

evidenced by a lack of THF resonances the 1H NMR subsequently recorded in pyridine-

d5.

Upon oxidation of 2.3 with Ph3CCl, a color change to dark red was observed. The

reaction product was identified as the previously reported tetravalent compound Ce[N(SiHMe2)2]4 (2.4).13 Notably, oxidation of 2.3 was insensitive to the choice of

coordinating or noncoordinating solvent, in contrast to reported syntheses of 2.4 from

oxidation of 2.1, 2.2,or Li(THF)[Ce[N(SiHMe2)2]4].13,20 Reactions of 2.3 with Ph3CCl in

toluene-d8 and THF-d8 monitored with a ferrocene internal standard showed quantitative conversion to 2.4 in 2 h.

Scheme 2.2.2. Synthesis of compounds 2.5 and 2.6. Adapted from work to be submitted to Dalton Transactions, 2014.

31

Compound 2.4 was previously used as a protonlysis precursor to access

cerium(IV) formamidinate and cerium(IV) alkoxide coordination compounds.20,37

Bis(dimethylsilylamide) complexes of trivalent rare-earth metals were previously shown to readily undergo protonolysis reactions with alcoholic substrates.21-27 To probe the general applicability of this alcoholysis for homoleptic tetravalent compound 2.4, we

examined the reactivity of 2.4 with alcohol and phenol substrates, as shown in Scheme

2.2.2.

Upon reaction of 2.4 with excess tert-butanol in hexanes, a fast color change from

dark red to yellow was observed. A yellow crystalline solid was isolated following exposure to pyridine and crystallization from a concentrated hexanes solution at –35 oC. X-ray crystallographic characterization showed that the reaction product had formed by complete protonolysis to yield the recently reported compound Ce(OtBu)4(py)2 (2.5)

(Figure 2.2.3).40 Isolation of this compound was previously reported from metathesis of (NH4)2Ce(NO3)6 with KOtBu or NaOtBu in THF to yield Ce(OtBu)4(THF)2 followed by

solvent substitution.15,40 Crystals isolated by our method showed distinct crystal morphology compared to previously reported data.40 Compound 2.5 crystallized in the

monoclinic C2/c space group. The Ce–O bond distances were 2.0897(13) and 2.1203(12)

Å and the Ce–N distances were 2.6733(14) Å. Compound 2.5 was isomorphous with the

reported thorium analogue, Th(OtBu)4(py)2.41 In addition to Ce(OtBu)4(THF)2 and

Ce(OtBu)4(py)2, previously reported mononuclear neutral cerium(IV) alkoxide complexes

include Ce(hfip)4(donor) (hfip = (CF3)2CHOH, donor = TMEDA, diglyme),42 and

32

Figure 2.2.3. 30% probability thermal ellipsoid plot of compound 2.5. Hydrogen atoms have been removed for clarity. Selected bond distances: Ce(1)–O(1): 2.0897(13) Å, Ce(1)–O(2): 2.1203(12) Å, Ce(1)–N(1): 2.6733(14) Å. Reprinted from work to be submitted to Dalton Transactions, 2014.

Compound 2.4 likewise underwent protonolysis with 4 equiv of 2,6-

diphenylphenol, resulting in a color change from dark red to dark blue. X-ray diffraction analysis of crystals grown from a concentrated toluene solution revealed the connectivity

of the product, the homoleptic monomeric cerium(IV) aryloxide complex, Ce(OAr)4 (Ar

= C6H3-2,6-(C6H5)2) (2.6) (Figure 2.2.4). Compound 2.6 crystallized in the orthorhombic

Pcba space group with two molecules in the asymmetric unit and contained Ce–O bond distances of 2.101(3) and 2.126(3) Å. Each aryloxide ring was parallelly aligned and displaced from a phenyl substituent of a neighboring ligand, indicating arene-arene interactions. The distances between least square planes formed by the interacting aryl rings ranged from 3.360(6)–3.657(6) Å. Trivalent rare earth metal centers coordinated by four 2,6-diphenylphenolate ligands have been previously observed in the solid-state structures of charge-separated [MI(donor)n]+[LnIII(OAr4]– (donor = diglyme, DME) and

33 [Ln/AeII(OAr)3]+[LnIII(OAr)4]– (Ae = Ca, Sr, Ba)44-47 and aryloxide-bridged

[MILnIII(OAr)4] (M = alkali metal).48,49

Figure 2.2.4. 30% probability thermal ellipsoid plot of compound 2.6 (top). Hydrogen atoms have been removed for clarity. Selected bond distances: Ce(1)–O(1): 2.101(3) Å, Ce(1)–O(2): 2.126(3) Å. A second view of the crystallographically determined structure of 2.6 (bottom). Least-squared planes formed by phenyl and phenoxide groups that participate in arene interactions are colored to show stacking between partners. Reprinted

34