Lanthanide Borohydrides Research Articles

Lanthanide Borohydride Complexes Supported by Diaminobis(phenoxide) Ligands for the Polymerization of ε-Caprolactone andl- andrac-Lactide

Reaction of Na2O2NN' [H2O2NN' = (2-C5H4N)CH2N[2-HO-3,5-C6H2(t)Bu2]2] with M(BH4)3(THF)3 afforded the dimeric, rare-earth borohydride compounds [M(O2NN')(mu-BH4)(THF)n]2 [M = Y(III), n = 0.5 (1-Y); M = NdIII, n = 1 (1-Nd); M = SmIII, n = 0 (1-Sm)]. For comparison the chloride analogues [M(O2NN')(mu-Cl)(THF)n]2 (2-M; M = La(III) or Sm(III), n = 0; M = Nd(III), n = 1) and the corresponding pyridine adducts [M(O2NN')(mu-X)(py)]2 [X = BH4 (3-M) or Cl (4-M); M = La(III), Nd(III), or Sm(III)] were prepared and structurally characterized for 4-La. Compounds 1-M initiated the ring-opening polymerization of epsilon-caprolactone. The best molecular weight control (suppression of chain transfer) for all three monomers was found for the samarium system 1-Sm. The most effective heterotactic enrichment (Pr) in the polymerization of rac-lactide was found for 1-Y (P(r) = 87%). Compound 1-Nd catalyzed the block copolymerization of epsilon-caprolactone and L- and rac-lactide provided that epsilon-caprolactone was added first. Attempted block polymerization by the addition of L-lactide first, or random copolymerization of a ca. 1:1 mixture of epsilon-caprolactone and L-lactide, gave only a poly(L-lactide) homopolymer.

Lanthanide mono(borohydride) complexes of diamide-diamine donor ligands: novel single site catalysts for the polymerisation of methyl methacrylate

Samarium chloride and borohydride complexes of the diamide-diamine ligands (2-C5H4N)CH2N(CH2CH2NR)2(R = SiMe3 or mesityl) are described; the borohydride compounds are the first polydentate amide-supported single component lanthanide catalysts for the controlled polymerisation of polar monomers, and also represent the first lanthanide borohydride complex for the polymerisation of methyl methacrylate.

Unprecedented polymerization of epsilon-Caprolactone initiated by a single-site lanthanide borohydride complex, [Sm(eta-C5Me5)2(BH4)(thf)]: mechanistic insights.

The monoborohydride lanthanide complex [Sm(Cp*)2(BH4)(thf)] (1a) (Cp* = eta-C5Me5), has been successfully used for the controlled ring-opening polymerization of epsilon-caprolactone (epsilon-CL). The organometallic samarium(III) initiator 1 a produces, in quantitative yields, alpha,omega-dihydroxytelechelic poly(epsilon-caprolactone) displaying relatively narrow polydispersity indices (<1.3) within a short period of time (30 min). The polymers have been characterized by 1H and 13C NMR, SEC, and MALDI-TOF MS analyses. Use of the single-site initiator 1 a allows a better understanding of the polymerization mechanism, in particular with the identification of the intermediate compound [Sm(Cp*)2(BH4)(epsilon-CL)] (1b). Indeed, one molecule of epsilon-CL initially displaces the coordinated THF in 1 a to give 1 b. Then, epsilon-CL opening (through cleavage of the cyclic ester oxygen-acyl bond) and insertion into the Sm--HBH3 bond followed by reduction of the carbonyl function by the BH3 end-group ligand, leads to the samarium alkoxyborane derivative [Sm(Cp*)2[O(CH2)6O(BH2)]] (2). This compound subsequently initiates the polymerization of epsilon-CL through a coordination-insertion mechanism. Finally, upon hydrolysis, alpha,omega-dihydroxypoly(epsilon-caprolactone), HO(CH2)5C(O)[O(CH2)5C(O)]nO(CH2)6OH (4) is recovered. The stereoelectronic contribution of the two Cp* ligands appears to slow down the polymerization and to limit transesterification reactions.

Lanthanide Borohydrides as Precursors to Organometallic Compounds. Mono(cyclooctatetraenyl) Neodymium Complexes

Reactions of Nd(BH4)3(THF)3 (1) with KCp* (Cp* = η-C5Me5), KC4Me4P, and K2C8H8 gave the organometallic derivatives (Cp*)Nd(BH4)2(THF)2 (2), [K(THF)][(C4Me4P)2Nd(BH4)2] (3), and (COT)Nd(BH4)(THF)2 (4) (COT = η-C8H8), respectively. Treatment of 4 with NEt3HBPh4 afforded the cationic complex [(COT)Nd(THF)4][BPh4] (5). The mixed ring compound (COT)Nd(Cp*)(THF) (6), the alkoxide [(COT)Nd(OEt)(THF)]2 (7), and the thiolates Na[(COT)Nd(StBu)2] (8) and [Na(THF)2][{(COT)Nd}2(StBu)3] (9) were synthesized by treating 4 or 5 with the alkali metal salt of the corresponding anionic reagent. The X-ray crystal structures of 7 and of a toluene solvate of 9 have been determined.

First Chemical Transformations of Lanthanide Borohydride Compounds: Synthesis and Crystal Structures of [(η-C8H8)Nd(BH4)(THF)]2 and [(η-C8H8)Nd(THF)4][BPh4

Reaction of Nd(BH4)3(THF)3 with K2COT (COT = η-C8H8) in tetrahydrofuran (THF) gave the first (cyclooctatetraene)lanthanide borohydride, (COT)Nd(BH4)(THF)2 (1). By treatment with NHEt3BPh4, 1 was transformed into [(COT)Nd(THF)4][BPh4] (3), a unique example of a (cyclooctatetraene)lanthanide cation; both 1 and 3 gave (COT)Nd(Cp*)(THF) (Cp* = η-C5Me5) upon reaction with KCp*.

Sigma-Bond Metathesis Reactions Involving Lanthanide-Silicon and Lanthanide-Hydrogen Bonds

Abstract Neutral lanthanide silyl complexes Cp∗2LnSiH(SiMe3)2 (1, Ln=Sm; 2, Ln=Nd; 3, Ln=Y; Cp∗=η5-C5Me5) are synthesized by reaction of the corresponding alkyls Cp∗2LnCH(SiMe3)2 with neat SiH2(SiMe3)2. These complexes are monomeric in solution, but form dimers in the solid state through intermolecular LnηCH3-Si interactions. Mechanistic studies on this reaction indicate that it proceeds via a second-order autocatalytic process catalyzed by [Cp∗2LnH]2. Reactions of [Cp∗2LnH]2 with Cp2WH2 give the σ-bond metathesis products Cp∗2Ln(μ−η1,η5-C5H4)(μ-H)2WCp (5, Ln=Sm; 6, Ln=Y), via activation of a C-H (and not a W-H) bond of the tungsten hydride. The lanthanide borohydride complexes Cp∗2Ln(η2-H2BMes2) (Mes=mesityl; 7, Ln=Sm; 8, Ln=Y) are obtained by the reaction of [Cp∗2LnH]2 with [HBMes2]2.

Spectroscopic properties of rare earth borohydrides: Er(BH 4) 3·3THF in pure and mixed crystals

The optical spectra of Er(BH 4) 3·3THF neat crystals and La, Gd, Y(BH 4) 3·3THF mixed crystals are reported and analyzed. Lanthanum borohydride is found to have a different room temperature crystal structure (triclinic) from Er, Gd, Y(BH 4) 3·3THF (Pbcn). At low temperature the Pbcn crystals undergo a phase transition to a structure with two crystallographically inequivalent sites in a unit cell. The optical spectra of Er(BH 4) 3·3THF in Er, Y, Gd(BH 4) 3·3THF crystals clearly evidence these two sites. Large vibronic intensity is observed at 1.6 K and 77 K and nine “molecular” vibrations are assigned. These modes are quite similar to those found for U(BH 4) 4. Er (BH 4) 3·3THF spectra are very different: no vibronic transitions are observed but many (often upwards of fifty for a given manifold) weak sharp “satellite” lines are found associated with pure electronic transitions. These data are discussed in terms of structural differences and comments on bonding and covalent character in lanthanide borohydrides are made.