Lanthanide complexes of bulky hybrid ligands

Abstract

The synthetic and redox chemistry of lanthanide organometallic complexes has considerably expanded since the discovery of Kagan’s reagent in 1977 and divalent ionic complexes are now known for the entire lanthanide series. The synthesis, solid-state structures and reductive chemistry of trivalent and divalent lanthanide complexes with cyclopentadienyl-type ligands is reviewed with a focus on the impact of the ligand on the reducing power of the metal centre. Trimethylsilyl and more recently, phosphine-borane stabilised carbanions have facilitated the isolation of trivalent and divalent lanthanide complexes of alkyl ligands with Ln—C σ-bonds. The synthesis, structures and known reactivity of these compounds is discussed. In order to probe the impact of alkyl carbanion and cyclopentadienyl coordination on lanthanide complex stability, structure and redox reactivity we have designed a novel set of hybrid ligands that combine these two functional groups into a single dianionic ligand. These ligands are potentially very versatile as the sterics and electronics of both groups in the ligand can be modified. The ligands are viable to support sterically congested trivalent complexes for sterically induced reduction as well as metal based reduction and they are good ligands for heteroleptic complexes because they chelate the metal avoiding ligand redistribution equilibria. The synthesis and characterisation of a range of trimethylsilyl-, phosphine-borane- and phosphine-stabilised alkyl bromo- and chlorosilane precursors is described: (Me3Si)2CHSiMe2Br [4], (Me3Si){PMe2(BH3)}CHSiMe2Cl [9], (Me3Si){PnPr2(BH3)}CHSiMe2Cl [13], {PMe2(BH3)}2CHSiMe2Cl [23], {PMe2(BH3)}{PPh2(BH3)}CHSiMe2Cl [27], {PPh2(BH3)}2CHSiMe2Cl [31] and (nPr2P)2CHSiMe2Cl [32]. iv The results of a computational study using NBO methods to investigate the relative stabilising effect of each of these silyl, phosphine-borane and phosphine carbanion stabilising groups on a model system akin to the alkyl part of the hybrid ligand are described. Reaction of (Me3Si)2CHSiMe2Br with Li/Na/K Cp/Cp’/Cp4Me followed by aqueous work-up gave the hybrid proligands {(CpH)Me2Si}(Me3Si)2CH [5], {(Cp’H)Me2Si}(Me3Si)2CH [6] and {Cp4MeH)Me2Si}(Me3Si)2CH [7] as mixtures of regioisomers [Cp4Me = 1,2,3,4-Tetramethyl-cyclopentadiene]. Reaction of LiCp4Me/KCp4Me with (Me3Si){PMe2(BH3)}CHSiMe2Cl and (Me3Si){PnPr2(BH3)}CHSiMe2Cl gave the hybrid proligands (CpH4MeMe2Si)(Me3Si)CH{PMe2(BH3)} [10] and (CpH4MeMe2Si)(Me3Si)- CH{PnPr2(BH3)} [14]. The compounds {Cp4MeH)Me2Si}(Me3Si)2CH, (CpH4MeMe2Si)- (Me3Si)CH{PMe2(BH3)} and (CpH4MeMe2Si)(Me3Si)CH{PnPr2(BH3)} crystallise as solvent-free monomers with very similar molecular conformations. The hybrid pro-ligands exhibit variable moisture sensitivity. Reaction of (Me3Si){PMe2(BH3)}CHSiMe2Cl with NaCp followed by aqueous work-up and column chromatography of the oily residue obtained gave the siloxane [(Me3Si){PMe2(BH3)}HCMe2Si]2O [19] and dicyclopentadiene. Reaction of (nPr2P)2CHSiMe2Cl with NaCp followed by aqueous work-up using deoxygenated water gave (nPr2P)2CH2 in quantitative yield. The same reaction avoiding the aqueous work-up gave (CpHMe2Si)CH(PnPr2)2 [33]. Hybrid ligands were prepared by metalation of {Cp4MeH)Me2Si}(Me3Si)2CH, {(Cp’H)Me2Si}(Me3Si)2CH and {(CpH)Me2Si}(Me3Si)2CH with MeK to give the hemisolvated dipotassium salts [(CpMe2Si)(Me3Si)2C]K2(Et2O)0.5 [50], [(Cp4MeMe2Si)(Me3Si)2C]K2(Et2O)0.5 [51] and [(Cp’Me2Si)(Me3Si)2C]K2(C6H6)0.5 [52], which decomposed slowly in THF and toluene. [(Cp4MeMe2Si)(Me3Si)2C]K2(Et2O) [51.Et2O] was crystallised as an alternative solvate from diethyl ether and the v extended structure consists of chelated [(hybrid ligand)K(Et2O)]- anionic units linked nose to tail by unsolvated K cations in a non-linear (zig-zag) polymer chain. [(Cp’Me2Si)(Me3Si)2C]K2(C6H6)·C6H6 [52.C6H6] also crystallised as a monosolvate from benzene, but with addition uncoordinated solvent present in the structure. There are two distinct types of polymer chain in the structure of [(Cp’Me2Si)(Me3Si)2C]K2(C6H6)·C6H6, one is similar to the chain in [(Cp4MeMe2Si)(Me3Si)2C]K2(Et2O) and in the other half the potassium cations in this chain are coordinated by only a cyclopentadienyl ring and a benzene ring. The remainder of the K cations are sandwiched between a carbanion and cyclopentadienyl ring of different hybrid ligands, as in [(Cp4MeMe2Si)(Me3Si)2C]K2(Et2O). These chains are cross-linked by a short contact between K and the –SiMe3 substituent of the Cp’ ring. Due to their more acidic alkyl protons metalation of (CpH4MeMe2Si)(Me3Si)CH{PMe2(BH3)}, (CpH4MeMe2Si)(Me3Si)CH{PnPr2(BH3)} and (CpHMe2Si)CH(PnPr2)2 could be achieved with BnK in THF. (CpH4MeMe2Si)(Me3Si)CH{PMe2(BH3)} is isoelectronic and isosteric with {Cp4MeH)Me2Si}(Me3Si)2CH. [(Cp4MeMe2Si)(Me3Si){PMe2(BH3)}C]K2(THF) [53.THF] crystallises as a solvate from benzene/THF with a 3D polymeric network structure through multiple agostic-type B—H····K contacts. Of the two symmetry inequivalent K cations in the structure the alkyl carbanion interacts directly only with the unsolvated cations and the solvated cations are coordinated through the borane-hydrogens. Metathesis reactions of [(Cp4MeMe2Si)(Me3Si)2C]K2(Et2O)0.5 and [(Cp4MeMe2Si)(Me3Si){PMe2(BH3)}C]K2(THF)0.5 with LaI3(THF) and SmI3(THF) in THF gave [(Cp4MeMe2Si)(Me3Si)2C]LaI(THF)2 [56] and [(Cp4MeMe2Si)(Me3Si){PMe2(BH3)}C]-SmI(THF)2 [57] after extraction into diethyl ether and crystallisation from toluene/THF and diethyl ether respectively. The vi hybrid ligand bound successfully to the lanthanide metal cations. [(Cp4MeMe2Si)(Me3Si)2C]LaI(THF)2 is a monomer in the solid-state, chelated by the hybrid ligand forming pseudo-four-membered ring and two coordinating molecules of THF in addition to the iodine anion. A zwitterion structure is adopted by [(Cp4MeMe2Si)(Me3Si){PMe2(BH3)}C]SmI(THF)3 with no contact between the Sm(III) cation and the alkyl carbanion centre. The hybrid ligands have a much larger cone angle and are far more sterically bulky when chelating the lanthanide metal, as in [(Cp4MeMe2Si)(Me3Si)2C]LaI(THF)2. The metathesis reaction of [(Cp4MeMe2Si)(Me3Si)2C]K2(Et2O)0.5 with CaI2 gave [(Cp4MeMe2Si)(Me3Si){PMe2(BH3)}C]Ca(THF)2 [58] after extraction and crystallisation from diethyl ether. In this structure the phosphine-borane stabilised carbanion group of the hybrid ligand coordinates the Ca cation through the carbanion centre. Synthesis of the Yb(II) analogue, [(Cp4MeMe2Si)(Me3Si){PMe2(BH3)}C]Yb(THF)x gave an almost identical NMR spectrum, but could not be crystallised.