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A cobalt silylene (Co=Si) linkage enables a distinct metal/ligand cooperative activation of an organic azide, where nitrene transfer occurs to and from the Co⋅⋅⋅Si linkage without ligand dissociation from the 18‐electron cobalt center. This process utilizes the orthogonal binding affinities of the silicon and cobalt sites to avoid CO poisoning that would otherwise inhibit reactivity, leading to significantly improved catalytic isocyanate generation compared with related systems. The dual‐site approach demonstrates the potential of metal/main‐group bonds to access new and efficient catalytic pathways.

 

A metal/ligand cooperative approach to the reduction of small molecules by metal silylene complexes (R₂Si=M) is demonstrated, whereby silicon activates the incoming substrate and mediates net two‐electron transformations by one‐electron redox processes at two metal centers. An appropriately tuned cationic pincer cobalt(I) complex, featuring a central silylene donor, reacts with CO₂to afford a bimetallic siloxane, featuring two Co^IIcenters, with liberation of CO; reaction of the silylene complex with ethylene yields a similar bimetallic product with an ethylene bridge. Experimental and computational studies suggest a plausible mechanism proceeding by [2+2] cycloaddition to the silylene complex, which is quite sensitive to the steric environment. The Co^II/Co^IIproducts are reactive to oxidation and reduction. Taken together, these findings demonstrate a strategy for metal/ligand cooperative small‐molecule activation that is well‐suited to 3dmetals.

 

The title compounds, [Mo(C 5 H 5 )(COCH 3 )(C 6 H 12 N 3 P)(CO) 2 ], (1), and [Mo(C 5 H 5 )(COCH 3 )(C 9 H 16 N 3 O 2 P)(C 6 H 5 ) 2 ))(CO) 2 ], (2), have been prepared by phosphine-induced migratory insertion from [Mo(C 5 H 5 )(CO) 3 (CH 3 )]. The molecular structures of these complexes are quite similar, exhibiting a four-legged piano-stool geometry with trans -disposed carbonyl ligands. The extended structures of complexes (1) and (2) differ substantially. For complex (1), the molybdenum acetyl unit plays a dominant role in the organization of the extended structure, joining the molecules into centrosymmetrical dimers through C—H...O interactions with a cyclopentadienyl ligand of a neighboring molecule, and these dimers are linked into layers parallel to (100) by C—H...O interactions between the molybdenum acetyl and the cyclopentadienyl ligand of another neighbor. The extended structure of (2) is dominated by C—H...O interactions involving the carbonyl groups of the acetamide groups of the DAPTA ligand, which join the molecules into centrosymmetrical dimers and link them into chains along [010]. Additional C—H...O interactions between the molybdenum acetyl oxygen atom and an acetamide methyl group join the chains into layers parallel to (101). 

The title compound, [Mo(C 5 H 5 )(C 2 H 3 O)(C 24 H 27 P)(CO) 2 ], was prepared by reaction of [Mo(C 5 H 5 )(CO) 3 (CH 3 )] with tris(3,5-dimethylphenyl)phosphane. The complex exhibits a four-legged piano-stool geometry with trans -disposed acetyl and phosphane ligands. The molecular geometry is nearly identical to that of the triphenylphosphane derivative, but introduction of methyl groups on the aromatic phosphane substituents significantly impacts supramolecular organization. In the crystal, non-classical C—H...O interactions involving the acetyl carbonyl group lead to a chain motif along [010], and another set of C—H...O close contacts join inversion-related molecules. 

Electron-rich late metals and electropositive main-group elements (metals and metalloids) can be combined to provide an ambiphilic façade for exploring metal–ligand cooperation, yet the instability of the metal/main-group bond frequently limits the study and application of such units. Incorporating main-group donors into pincer frameworks, where they are stabilized and held in proximity to the transition-metal partner, can allow discovery of new modes of reactivity and incorporation into catalytic processes. This Perspective summarizes common modes of cooperativity that have been demonstrated for pincer frameworks featuring metal/main-group bonds, highlighting similarities among boron, aluminium, and silicon donors and identifying directions for further development. 

Three cyclopentadienylmolybdenum(II) propionyl complexes featuring triarylphosphine ligands with different para substituents, namely, dicarbonyl(η 5 -cyclopentadienyl)propionyl(triphenylphosphane-κ P )molybdenum(II), [Mo(C 5 H 5 )(C 3 H 5 O)(C 18 H 15 P)(CO) 2 ], ( 1 ), dicarbonyl(η 5 -cyclopentadienyl)propionyl[tris(4-fluorophenyl)phosphane-κ P ]molybdenum(II), [Mo(C 5 H 5 )(C 3 H 5 O)(C 18 H 12 F 3 P)(CO) 2 ], ( 2 ), and dicarbonyl(η 5 -cyclopentadienyl)propionyl[tris(4-methoxyphenyl)phosphane-κ P ]molybdenum(II) dichloromethane solvate, [Mo(C 5 H 5 )(C 3 H 5 O)(C 21 H 21 O 3 P)(CO) 2 ]·CH 2 Cl 2 , ( 3 ), have been prepared from the corresponding ethyl complexes via phosphine-induced migratory insertion. These complexes exhibit four-legged piano-stool geometries with molecular structures quite similar to each other and to related acetyl complexes. The extended structures of the three complexes differ somewhat, with the para substituent of the triarylphosphine of ( 2 ) (fluoro) or ( 3 ) (methoxy) engaging in non-classical C—H...F or C—H...O hydrogen-bonding interactions. The structure of ( 3 ) exhibits modest disorder in the position of one Cl atom of the dichloromethane solvent, which was modeled with two sites showing approximately equivalent occupancies [0.532 (15) and 0.478 (15)].