Molybdenum imido alkylidene complex

Molybdenum imido alkylidene complexes are among the most successful and representative types of Schrock catalysts. They are pivotal organometallic compounds consisting of a molybdenum center bonded to both an imido and an alkylidene ligand. These complexes have garnered significant attention due to their role as catalysts in olefin metathesis reactions, which involves the redistribution of carbon-carbon double bonds in alkenes. Richard R. Schrock was awarded the Nobel Prize in Chemistry of 2005 along with Yves Chauvin and Robert H.Grubbs for their contributions to the field of olefin metathesis.

Structure Molybdenum(VI) imido alkylidenes have 14 valence electrons and d0 metal centers. The complexes are generally characterized by their high oxidation state, imido ligands, and bulky alkoxide ligands. The Mo(VI) electron-deficient metal centers are electron-poor and electrophilic with vacant orbitals that strongly attract and bind olefin substrates via π-coordination, thus facilitating olefin coordination when catalyzing olefin metathesis. The imido ligands (M=NR, [NR]2-) are dianionic and can be sterically protected by a large R group to avoid bimolecular decomposition reactions. They have moderate π-donation capability and are weaker electron donors compared to traditional carbenes, thus contributing to higher electrophilicity of the metal center. Finally, the bulky alkoxide ligands can also sterically hinder unwanted bimolecular decomposition pathways involving the intermediate [Mo(NAr)(CHR′)(OR)2] species and stabilize reactive intermediates during catalysis; a highly electron-withdrawing group like OCMe(CF3)2 further contributes to the electrophilicity of the metal center and increasing its affinity for metathesis reactions. Additionally, Mo(VI) imido alkylidene complexes can be categorized as Schrock carbenes, which have considerable π-backbonding and thus support a strong Mo=C double bond that is nucleophilic. The electron-withdrawing alkoxide ligands also enhances this metal-carbon double bond, contributing to the metal-center being oxyphilic. The carbene is thus polarized with Mo having partial positive and C having partial negative charge, δ+Mo=Cδ-. Molybdenum imido alkylidene complexes exhibit a slightly distorted tetrahedral geometry around the metal center, with d orbital splitting resembling a tetrahedral transition metal complex as described by Crystal Field Theory. As the molybdenum(VI) center is highly oxidized and without any valence electrons in d orbitals, its LUMO is the lowest-energy d-orbital dz2. dz2 is stabilized by σ-donation from the nitrogen lone pair (with sp-hybridized orbital) of the imido ligand as well as the sp2-hybridized orbital from the carbon of the alkylidene ligand. The dxy, dyz, dxz orbitals receive π-donation from the π-systems of the respective imido and alkylidene ligands to become more stabilized. Furthermore, the metal center donates electron density to the α-carbon of the alkylidene to allow for π-backbonding; The polarization of electron density in bonding orbitals towards the α-carbon (and the opposite for the corresponding anti-bonding orbitals) indicates the polarized bond between the ligand and the metal center. In addition, the entirely vacant d orbitals of the complex strongly encourage the binding of olefins to form metallacyclobutane during catalysis, further validating the potential for highly efficient and reactive catalysis during olefin metathesis.

Two distinct rotamers—syn and anti—exist due to the orientation of the alkylidene ligand relative to the imido ligand plane and the limited number of accessible π orbitals that the highly oxidized metal center can use to form more than one metal-alkylidene π bond, especially when a strong π-bonding ligands like the imido ligand is present. These rotamers exhibit dramatically different reactivities as a result of variations in orbital overlap and steric interactions. The syn form typically shows stronger CH α-agostic interactions (interaction between metal center and adjacent C–H bonds), stabilizing reactive intermediates and influencing catalytic performance.

History

Pioneering work in transition metal carbenes

In the early 1970s, Schrock, then at DuPont, attempted to synthesize tantalum(V) complexes such as [Ta(CH2CMe3)5]. Instead, he isolated the first stable metal-alkylidene complex, [Ta(CH2CMe3)3(=CHCMe3)], characterized by a tantalum-carbon double bond. This accidental discovery marked the birth of well-defined metal-alkylidene chemistry. Unlike Fischer carbenes that were characterized previously, which relied on π-backbonding from low-valent metals, Schrock’s high-valent tantalum alkylidenes exhibited σ-bonding character, making them more reactive toward olefins. However, these early tantalum complexes failed to catalyze olefin metathesis. Later, Schrock recognized that alkoxide ligands could modulate the electrophilicity of the metal center. By 1980, his group at MIT reported a tantalum-alkylidene complex, [Ta(=CHC(CH3)3)Cl(PMe3)(OC(CH3)3)2], which successfully catalyzed the metathesis of cis-2-pentene. The alkoxide ligands stabilized the electrophilic metal center while allowing substrate binding—a critical design principle for future catalysts.

The shift to group 6 metals

By the mid-1980s, Schrock turned to molybdenum and tungsten, recognizing their superior activity in traditional metathesis systems. But as bonds between molybdenum and ligands are generally weaker than those between tungsten and ligands, Schrock theorized that a molybdacyclobutane complex might lose an olefin more readily than a tungsten one. In 1986, his group synthesized the first well-defined molybdenum imido alkylidene complexes of the general formula [Mo(=CHR)(=N-Ar)(OR')2], where Ar is an aryl group and R' is a bulky alkoxide (e.g., OCMe(CF3)2). These 14-electron, d⁰ complexes exhibited remarkable activity, enabling living ring-opening metathesis polymerization (ROMP) of strained olefins like norbornene.

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