Synthesis of a Hafnocene Disilene Complex

Heavier group 14-transition metal π-complex congeners

Since the dawn of organometallic chemistry, transition metal complexes of unsaturated organic molecules, namely π-complexes, have remained a central focus: our thorough understanding of the electronic nature of such species, and their importance in countless reactive processes continues to drive research in their synthesis and utilisation. Since the late 1900s, research regarding the related chemistry for the heavier group 14 elements has become increasingly more fervent. Today, heavier congeners of a vast array of classical π-complexes have been realised, from alkene to arene systems, involving Si, Ge, Sn, and Pb. This has given deeper insights into the bonding observed for these heavier elements, which typically involves a lessened degree of π-bonding and an increased polarisation. This review aims to summarise this field, identifying these disparities, and highlighting areas which we believe may be exciting for future exploration.

Reductive Retrocyclization of a Mangana(II)cyclopentasilane to Form Manganese(0) Bis(η2-disilene) Complexes

Ligand-exchange reactions on a mangana(II)cyclopentasilane complex that contains two THF ligands with aryl isocyanides led to the formation of manganese(0) bis(η2-disilene) complexes via a retrocyclization. In stark contrast, ligand-exchange reactions with CNtBu, an N-heterocyclic carbene, or pyridine-based ligands furnished manganese(II) complexes wherein the manganacyclopentasilane framework remained intact. The thermolysis of the obtained bis(η2-disilene) complex in the presence of mesityl isocyanide led to the formation of a cyclotetrasilane via the formal dimerization of the two η2-disilene moieties. The insertion of a mesityl isocyanide into the Mn-Siβ bond results in the formation of a manganese(II) complex supported by a [SiCSi]-type tridentate ligand scaffold.

Transition Metal Complexes of Heavier Vinylidenes: Allylic Coordination vs Vinylidene-Alkyne Rearrangement at Nickel

Transition metal π-allyl complexes are key reagents/intermediates of various catalytic and stoichiometric allylation reactions. We now report the first transition metal complex of a heavier allylic π-system. The η³-Si₂Ge allyl nickel complex is formally obtained by the oxidative addition of the Si-Cl bond of the heavier vinylidene [R₂(Cl)Si-(R)Si═(NHC)Ge:] to [Ni(COD)₂] (R = 2,4,6-triisopropylphenyl; NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene; COD = 1,5-cyclooctadiene). Due to geometric constraints, the coordination to the Ni(II) center occurs through the formal Si═Ge double bond instead of the residual lone pair of electrons at germanium. In contrast, the Si-N bond of the analogous vinylidene [R₂(Me₂N)Si-(R)Si═(NHC)Ge:] (obtained by nucleophilic substitution of Cl by NMe₂) does not oxidatively add to Ni(0), and a hydridosilagermene-η²-nickel complex is obtained instead. The formation of this complex necessarily implies the isomerization of the heavier vinylidene to the corresponding heteroalkyne with the Si≡Ge triple bond in the coordination sphere of nickel followed by the activation of a C-H bond of one of the isopropyl groups of an N-heterocyclic carbene (NHC) ligand.

Chemistry of a 1,5-Oligosilanylene Dianion Containing a Disiloxane Unit

Synthesis of a number of disiloxane containing cyclo- and bicyclooligosilanes is described starting from the dipotassium 1,5-oligosiloxanylene diide derived from 1,3-bis[tris(trimethylsilyl)silyl]tetramethyldisiloxane. In addition, the use of this particular fragment as ligand for zinc and group 4 metallocene complexes was studied. Both types of compounds exhibit marked structural differences compared to related compounds containing Si-Si-Si units instead of the Si-O-Si fragment.

A Kinetic Study of the Silyl Substitution in Tantalum Amide Silyl Complex (Me2N)3Ta[Si(SiMe3)3]2

Kinetic studies have been performed for the substitution of the first silyl ligand in (Me(2)N)(3)Ta[Si(SiMe(3))(3)](2) (1) by Li(THF)(3)SiButPh(2) at 233 K (THF = tetrahydrofuran). In the presence of excess Li(THF)(3)SiButPh(2), these studies reveal that the reaction likely follows a dissociative pathway. THF, a polar solvent, is found to promote the substitution, and the order of the reaction with respect to THF is 1.7(0.3).

A Stable Schrock-Type Hafnium−Silylene Complex

The first stable hafnium-silylene complex, (eta-C5H4Et)2(PMe3)Hf=Si(SiMetBu2)2 (6) was obtained in the form of the phosphine adduct as red crystals by the coupling reaction of 1,1-dilithiosilane (1) with 0.9 equiv of (eta-C5H4Et)2HfCl2 in dry toluene at -50 degrees C, followed by treatment with an excess of PMe3 at -50 degrees C. In the 29Si NMR spectrum of 6, the signal from the silylene ligand is shifted greatly downfield at 295.4 ppm, with a JSiP coupling constant of 15.0 Hz. X-ray crystallographic analysis of 6 revealed that the Si-Hf bond length (2.6515(9) A) is about 5% shorter than typical Si-Hf single bonds, obviously indicating the double-bond character between the silicon and hafnium atoms. The compound 6 was found to be a Schrock-type silylene complex, a conclusion that was supported by the natural population analysis (NPA) charge distribution for the model complex, (eta-C5H4Et)2(PMe3)Hf=Si(SiMe3)2 (8), showing a negative charge on the silicon atom (-0.34).

Ligand dependence of π-complex character in disilene–palladium complexes

The synthesis and structures of new 16-electron disilene palladium complexes with 2,6-dimethylphenyl isocyanide and phenyldimethylphosphine ligands [L(1)L(2)Pd{(t-BuMe(2)Si)(2)Si=Si(SiMe(2)Bu-t)(2)}, where L(1) = L(2) = PhMe(2)P; L(1) = (cyclohexyl)(3)P, L(2) = 2,6-dimethylphenyl isocyanide; L(1) = L(2) = 2,6-dimethylphenyl isocyanide] are described. Comparison of the X-ray structural parameters around the disilene moiety among these complexes and related bis(trimethylphosphine)(disilene)palladium and 14-electron (tricyclohexylphosphine)(disilene)palladium revealed that the pi-complex character is sensitive to the residual ligands and increases with decreasing the strength of sigma-donation from the ligands.

Activation of a SiSi Bond by η1-Coordination to a Transition Metal

Reaction of the disilenide 1 with Cp2ZrCl2 yields 2, the first transition metal complex with an eta1-sigma-bonded disilene moiety. The compound 2 exhibits a strongly red-shifted UV-vis absorption, resulting in an intensely green color. The Si=Si bond in 2 is considerably activated as shown by the rearrangement to a silyl-substituted zirconocene 3. X-ray diffraction studies on 2 reveal that the Si=Si bond is significantly longer than that in the precursor 1. Conversely, the Zr-Si distance is considerably smaller than the corresponding distance in 3, suggesting a certain degree of electron donation toward the 16e- zirconium center.