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Transition metal silyl complexes

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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

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Perspective

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

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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 HSiI moiety will add across HSi before SiI.[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

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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-SiR2CH2) + MgCl2 + CH4
Cp2W(Cl)(CH2SiMe2Cl) + Mg  Cp2W(η2-SiR2CH2) + MgCl2

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

Reactions

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 MCOSi or SiCOM, depending on the reagents.[2]:280–283

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Silane complexes

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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-Hterminal = 135 picometer.[6]

Transition metal silane complexes are coordination compounds containing hydrosilane ligands. An early example is (MeC5H4)Mn(CO)22-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)22-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]

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References

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