Alexei V. Khvostov

Synthesis and structures of crystalline bis(trimethylsilyl)methanido complexes of potassium, calcium and ytterbium

Abstract Crystalline [K(μ-R)(thf)] ∞ ( 1 ) was obtained from equivalent portions of n -butyllithium in hexane, bis(trimethylsilyl)methane (RH) and potassium t -butoxide in thf, removal of volatiles and extraction with hexane. Desolvation of 1 in a vacuum led to KR. The first three-coordinate metallate(II) alkyls [K(MR 3 )] ∞ [M=Ca ( 2 ), M=Yb ( 3 )] of calcium and ytterbium(II) were prepared from the appropriate metal(II) iodide and three equivalents of KR in benzene. Mixing LiR, YbI 2 and two equivalents of KR in a mixture of diethyl ether and small amount of thf yielded the red (like 3 ) [Li(thf) 4 ][YbR 3 ] ( 4 ). Each of 1 – 4 was obtained in good yield and was characterised by multinuclear NMR spectra in C 6 D 6 and single crystal X-ray diffraction. The central metal is in a trigonal planar 1 or pyramidal 2 – 4 environment and the average MC bond lengths are 2.98 ( 1 ), 2.50 ( 2 ), 2.52 ( 3 and 4 ) A. Crystalline 2 and 3 are isomorphous and consist of double chains of [MR 3 ] − anions linked by K + cations along the a axis, whereas complex 4 has an ionic structure with isolated [Li(thf) 4 ] + cation and [YbR 3 ] − anion.

Aspects of non-classical organolanthanide chemistry

This paper provides a survey of our studies on: (i) subvalent compounds of lanthanum and early 4f elements; (ii) cationic mononuclear samarium(II) and ytterbium(II) organic complexes; (iii) low valent samarium and ytterbium -diketiminates; and (iv) mononuclear cerium(IV) amides and a mixed valence trinuclear Ce(IV)/Ce(III) cluster. A brief introductory section points to our earlier (1973–1995) studies on organo-4f-element chemistry.

Unusual crystalline heterobimetallic trinuclear β-diketiminates [Yb{L(µ-Li(thf)}2] and [Yb{L′(µ-Li(thf)}2]·thf [L, L′ = {N(SiMe3)C(R)}2CH, R = Ph, C6H4Ph-4]Electonic supplementary information (ESI) available: NOE data for 1 and NMR data for 2. See http://www.rsc.org/suppdata/cc/b2/b203321f/

YbCl3 with successively 2KL or 2KL′ and then >2Li in thf yields crystalline [Yb{L(μ-Li(thf)}2] 1 or [Yb{L′(μ-Li(thf)}2].thf 2, characterised by X-ray, multinuclear NMR spectral and VT magnetic susceptibility data; 1 and 2 are best formulated as Yb(II) complexes having dianionic L2− or L′2− ligands, L or L′ = {N(SiMe3)C(R)}2CH (R = Ph or C6H4Ph-4).

Ytterbium(II) amides and crown ethers: addition versus amide substitution

Abstract New Yb(II) amides [Yb(NPh2)2(THF)4] (1) and Yb{NPh(SiMe3)}2(THF)x (x=1 or 3) (2) were obtained by the salt elimination method and the structure of the dimer [Yb{NPh(SiMe3)}{μ-NPh(SiMe3)}(THF)]2 (2a) was determined. Reactions of the amides 1, 2 and [Yb{N(SiMe3)2}2(THF)2] (3) with [18]crown-6 gave the crystalline structurally characterised molecular complex [Yb(NPh2)2([18]crown-6)] (4) in the case of the smaller NPh2− ligand and the salt [Yb{N(SiMe3)2}([18]crown-6)][Yb{N(SiMe3)2}3] (5) with the bulky bis(trimethylsilyl)amido ligand. Crystalline 4 has a “threaded” structure with the NPh2 groups on the opposite sides of the [18]crown-6 ligand and the N−Yb−N′ angle of 176.6°. An X-ray diffraction study of the related potassium amide [K([18]crown-6)(NPh2)] (6) reveals that the K ion coordinates to one of the Ph-rings rather than to the amido nitrogen atom. There is significant delocalisation of negative charge in the amido ligand of crystalline 6, unlike in 4.

An electrochemical and DFT study on selected β-diketiminato metal complexes

Selected homoleptic metal beta-diketiminates M(I)L and M(II)L2 [M(I) = Li or K, M(II) = Mg, Ca or Yb; L: L(Ph) = [N(SiMe3)C(Ph)]2CH, L(Bu(t)) = N(SiMe3)C(Ph)C(H)C(Bu(t))N(SiMe3), L* = [N(C6H3Pr(i)2-2,6)C(Me)]2CH] have been studied by cyclic voltammetry (CV). The primary reduction (E(p)red, the peak reduction potential measured vs. SCE in thf containing 0.2 M [NBu4][PF6] with a scan rate 100 mV s(-1) at a vitreous carbon electrode at ambient temperature) is essentially ligand-centred: E(p)red being ca. -2.2 V (LiL(Ph) and KL(Ph)) and -2.4 V [Mg(L(Ph))2, LiL(Bu(t)) and Ca(L(Ph))2], while LiL* is significantly more resistant to reduction (E(p)red = -3.1 V). These observations are consistent with the view that the two (L(Ph)) or single (L(Bu(t))) C-phenyl substituent(s), respectively, are available for -electron-delocalisation of the reduced species, whereas the N-aryl substituents of L* are unable to participate in such conjugation for steric reasons. The primary reduction process was reversible on the CV-time scale only for LiL(Bu(t)), Ca(L(Ph))2 and Yb(L(Ph))2. For the latter this occurs at a potential ca. 500 mV positive of Ca(L(Ph))2, consistent with the notion that the LUMO of Yb(L(Ph))2 has substantial metal character. The successive reversible steps, each separated by ca. 500 mV, indicate that there is strong electronic communication between the two ligands of Yb(L(Ph))2. The overall three-electron transfer sequence shows that the final reduction level corresponds to [Yb(II)(L(Ph))2-(L(Ph))3-]. DFT calculations on complexes Li(L(Ph))(OMe2)2 and Li2(L(Ph))(OMe2)3 showed that both HOMO and LUMO orbitals are only based on the ligand with a HOMO-LUMO gap of 4.21 eV. Similar calculations on a doubly reduced complex Yb[(mu-L(Ph))Li(OMe2)]2 demonstrated that there is a considerable Yb atomic orbital contribution to the HOMO and LUMO of the complex.

Reactions between a sodium amide Na[N(SiMe3)R1] (R1 = SiMe3, SiMe2Ph or But) and a cyanoalkane RCN (R = Ad or But)

Reactions between sodium amides Na[N(SiMe3)R1] [R1 = SiMe3 (1), SiMe2Ph (2) or But (3)] and cyanoalkanes RCN (R = Ad or But) were investigated. In each case the nitrile adduct [Na{mu-N(SiMe3)2}(NCR)]2 [R = Ad (1a) or But (1b)], trans-[Na{mu-N(SiMe3)(SiMe2Ph)}(NCR)]2 [R = Ad (2a) or But (2b)], [(Na{mu-N(SiMe3)But})3(NCAd)3] (3a) or [(Na{mu-N(SiMe3)But})3(NCBut)n] [n = 3 (3b) or 2 (3c)] was isolated. The reaction of complexes 3a or 3b with benzene afforded the ketimido complex [Na{mu-N=C(Ad)(Ph)}]6.2C6H6 (4a) or [Na{mu-N=C(But)(Ph)}]6 (4b); the former was also prepared in more conventional fashion from NaPh and AdCN. The synthesis and structure of an analogue of complex 1a, [Li{mu-N(SiMe3)2}(NCAd)]2 (5a), is also presented. The compounds 1a, 1b, 2a, 2b, 3, 3b, 4a, 4b and 5a were characterised by X-ray diffraction.

Synthesis and structures of the ytterbium(II) β-diketiminates [Yb{N(SiMe3)C(R2)C(H)C(R4)N(SiMe3)}2] (R2 = R4 = Ph, C6H4Me-4, or C6H4Ph-4; or R2 = C6H4Me-4, R4 = 1-adamantyl)

Crystalline, homoleptic mononuclear ytterbium(II) β-diketiminates [Yb{N(SiMe3)C(R2)C(H)C(R4)N(SiMe3)}2] (R2 = R4 = Ph 1, R2 = R4 = Tol 2, R2 = R4 = Dph 3, or R2 = Tol and R4 = Ad 4) (Tol = C6H4Me-4, Dph = C6H4Ph-4, Ad = 1-adamantyl) have been prepared. They have a characteristic 171Yb{1H} chemical shift in the region δ 2650 ± 200 relative to [Yb(η5-C5Me5)2(thf)], although for 2 and 3 this was only observed at low temperatures, indicative of a fast fluxional process at ambient temperature; data recorded ealier for 1 are shown to have been in error. The 1H NMR spectra of 4 showed that two isomers were present in toluene solution, in a ratio of ca. 3 ∶ 2, which interconverted on the spin saturation transfer scale of ca. 1 s−1. NOE data are presented for each of 1–4; these led to (i) assignments of the two types of SiMe3 groups (adjacent to Tol or Ad) and (ii) the conclusion that the two isomers are conformers, one of which probably corresponds to that found in the crystal. The molecular structures of each of 1, 3 and 4 have the ytterbium in a distorted tetrahedral environment, the two ligand planes (1 and 3) or boats (including the Yb atom, 4) approximate to being either orthogonal (1 and 3) or parallel (4). The ligand-to-metal bonding is close to κ2 (1, 3) or η5 (4) and the ligands are both π-delocalised (1, 3) or only one of them in 4.