Transition metal silyl complexes

In chemistry, transition metal silyl complexes describe coordination complexes in which a transition metal is bonded to an anionic silyl ligand, forming a metal-silicon sigma bond. This class of complexes are numerous and some are technologically significant as intermediates in hydrosilylation.^{[1][2]: 246} These complexes are a subset of organosilicon compounds.

Synthesis

Silyl halides and hydrides easily add oxidatively to "low-valent, electron-rich complexes". Other reagents for oxidative additions are rare, and typically require a strained bond for the metal to insert into.^[2]: 249

Electron-poor complexes form when a silanide displaces an X-type ligand, and often form oligomeric ring clusters. The complexes are extremely reactive with oxygen, and must be investigated air-free. If they also bear organyl ligands, the complex may rearrange to an organosilane and a hydride ligand.^{[2]: 254–255}

From silyl halides

The first silyl complexes were prepared by treatment of sodium cyclopentadienyliron dicarbonyl with trimethylsilyl chloride:^[3]

(C₅H₅)Fe(CO)₂Na + Me₃SiCl → (C₅H₅)Fe(CO)₂SiMe₃ + NaCl

Although metal carbonyl anions are convenient reaction substrates, polar solvents promote nucleophilic attack on the carbonyl oxygen instead. In such cases, the result is instead a Fischer carbene that usually decomposes to a siloxane.^[2]: 250

From hydrosilanes

Hydrosilane oxidative addition

Hydrosilanes oxidatively add to low-valent metal complexes to give silyl metal hydrides.^[2]: 247 Such species are assumed intermediates in hydrosilylation catalysis.

Compact, electronegative substituents on the silicon favor the addition; hence a H–Si–I moiety will add across H–Si before Si–I.^[2]: 247 The process begins when the intact hydrosilane associates to the unsaturated metal center, affording an agostic σ-silane complex (see § Silane complexes).^{[citation needed]}

Alternatively, the hydrosilane may reduce another ligand.^[2]: 248 Diphenylsilylating the Petasis reagent eliminates methane (Me = CH₃, Ph = C₆H₅):

2 (C₅H₅)₂TiMe₂ + 2 Ph₂SiH₂ → [(C₅H₅)₂TiSiPh₂]₂ + 4 MeH

Likewise, certain early transition metal hydrides react with hydrosilanes at high temperature, eliminating H₂.^[2]: 256

From disilanes

Low valent metals insert into the Si-Si bond of disilanes. The main limitation of this reaction is the paucity of disilanes as reagents.^[2]: 250 Bis(silyl)mercury reagents behave similarly.^[2]: 251

Acid-base metathesis

Acidic metal hydrides can condense with silazanes, but the reverse-polarity reaction between a silane and an amino complex is not possible.^[2]: 252

Silyl complexes with Si–Si bonds

Beyond simple ligands like SiR₃–, silyl ligands with Si-Si bonds are known. (C₅H₅)Fe(CO)₂-SiMe₂SiPh₃ is one example (Me = CH₃, Ph = C₆H₅).^[4] Another example is the metalacycle derived from titanocene dichloride, (C₅H₅)₂Ti(SiPh₂)₅.^[5]

Silene and disilene complexes

Compounds containing a 3-membered metal-silicon-carbon ring are formally η² complexes of silenes, although they are not prepared from such unstable precursors. Instead, silenes are produced from Grignard or Barbier reagents, e.g.:^{[2]: 332–336}

Cp*(PMe₃)IrClMe + ClMgCH₂SiPh₂H → Cp*(PMe₃)Ir(η²-SiR′₂CH₂) + MgCl₂ + CH₄

Cp₂W(Cl)(CH₂SiMe₂Cl) + Mg → Cp₂W(η²-SiR′₂CH₂) + MgCl₂

Disilene complexes are typically produced by dehydrogenation of the corresponding hydrodisilanes.^{[2]: 337–340}

Reactions

See also: hydrosilation

Electron-rich metal complexes undergo nucleophilic attack at silicon, and Brønsted acids usually convert silyl complexes to hydride complexes. The product may eliminate with retention or inversion of stereochemistry at silicon, or may form a 3-membered ring in which silicon is hypercoordinate.^{[2]: 273–274}

Geminal dihalides react with silylmetal anions to give a halide anion, a silyl halide, and a metal carbene complex.^{[2]: 346–347}

Insertions between the metal and silicon are hindered by the partial π bond formed through negative hyperconjugation. The process proceeds with alkenes and alkynes,^{[2]: 276–280} possibly through [2+2] addition to form a metallasilacyclobutane intermediate.^{[citation needed]}

Ketones and aldehydes react extensively with metal silanes, producing either M–C–O–Si or Si–C–O–M, depending on the reagents.^{[2]: 280–283}

Silane complexes

Structure of (MeC₅H₄)Mn(CO)(PMe₃)(η²-H₂SiPh₂), a diphenylsilane σ complex. Selected distances: Si-Mn = 325, H-Fe = 149, Si-H(Mn) = 177, Si-H_terminal = 135 picometer.^[6]

Transition metal silane complexes are coordination compounds containing hydrosilane ligands. An early example is (MeC₅H₄)Mn(CO)₂(η²-HSiPh₃) (Ph = C₆H₅).^[7]

The bonding in silane sigma complexes is similar to that invoked in agostic interactions. The metal center engages the Si-H entity via a 3-center, 2-electron bond. It is widely assumed that these sigma complexes are intermediates in the oxidative addition of hydrosilanes to give metal silyl hydrides. This transformation is invoked in hydrosilylation catalysis.

Evidence for sigma-silane complexes is provided by proton NMR spectroscopy. For (MeC₅H₄)Mn(CO)₂(η²-HSiPh₃), J(²⁹Si,¹H) = 65 Hz compared to 180 Hz in free diphenylsilane. In silyl hydride complexes, the coupling in about 6 Hz. Neutron diffraction studies reveal a Si-H distance of 1.802(5) Å in the corresponding η²-HSiFPh₂ complex vs 1.48 Å in free HSiFPh₂. Elongated Si-H bonds are characteristic of these sigma complexes.^[8]