Organomolybdenum chemistry

Organomolybdenum chemistry is the chemistry of chemical compounds with Mo-C bonds. The heavier group 6 elements molybdenum and tungsten form organometallic compounds similar to those in organochromium chemistry but higher oxidation states tend to be more common.^{[2][better source needed]}

Structure of Mo(CH₃)₅, a simple organomolybdenum compound.^[1]

Mo(0) and more reduced states

Molybdenum hexacarbonyl is the precursor to many substituted derivatives. It reacts with organolithium reagents to give anionic acyls which can be O-alkylated to give Fischer carbenes.

Structure of (mesitylene)molybdenum tricarbonyl.

Mo(CO)₆ reacts with arenes to give piano-stool complexes such as (mesitylene)molybdenum tricarbonyl. Cycloheptatrienemolybdenum tricarbonyl, which is related to (arene)Mo(CO)₃, reacts with trityl salts to give the cycloheptatrienyl complex:^[3]

(C₇H₈)Mo(CO)₃ + (C₆H₅)₃C⁺ → [(C₇H₇)Mo(CO)₃]⁺ + (C₆H₅)₃CH

Structure of Cycloheptatrienemolybdenum tricarbonyl.

Reduction of Mo(CO)₆ gives [Mo(CO)₅]²⁻ which is formally Mo(-II).^[4]

CO-free Mo(0) compounds tend to be more reducing and kinetically labile than the carbonyl complexes.^[5] Examples include bis(benzene)molybdenum (Mo(C₆H₆)₂) and tris(butadiene)molybdenum. Such compounds can be prepared by metal vapor synthesis and reductive routes from molybdenum(V) chloride.^[6]

Mo(II)

Halogenation of Mo(CO)₆ gives Mo(II) carbonyl halides, which are also versatile precursors.^[7] One large collection of compounds have the formula (C₅R₅)Mo(CO)₃X, derived from cyclopentadienylmolybdenum tricarbonyl dimer (X = halide, hydride, alkyl).^[8]

Treating molybdenum(II) acetate with methyllithium gives Li₄[Mo₂(CH₃)₈].

Mo(IV)

With the formula of the type Cp₂MoX₂ molybdocene dichloride (X = Cl) and molybdocene dihydride (X = H) are both known as are ansa metallocene analogues.

Molybdocene dihydride.

Mo(V) and Mo(VI)

Mo(CH₃)₅, Mo(CH₃)₆, and salts of [Mo(CH₃)₇]⁻ are known.^[5]

Oxo and imide (RN=) ligands are found in several high oxidation state organomolybdenum compounds. The complexes (C₅R₅)MoO₂X are illustrative.^[9] Schrock's Mo-based olefin metathesis catalysts feature molybdenum(VI) centers supported by alkoxide, alkylidene, and imido ligands.^[10]

Molybdenum neopentylidyne complexes endowed with sterically demanding phenolates or branched fluorinated alkoxides catalyze alkyne metathesis.^[11] However, preparation of these catalysts is problematic by the standard Schrock procedure. The trisalkoxide species 17 is active at room temperature.^[12]

Treating these Mo(III) complexes with dichloromethane gives methylidyne complex and a monochloride.^[13] The alkylidene complex tolerates basic amines and sulfides, which deactivate the more Lewis acidic complex such as Schrock complex. Higher gem-dichlorides RCHCl₂ give longer-lived catalyst.^[14] To reconvert the chloride byproduct, they added magnesium in reaction. The p-nitrophenolate is a very active catalyst.^[15] On the other hand, alcoholysis of 21 with a tridentate ligand leading to still longer lifetime and better substrate scope.^[16]

Molybdenum nitride complexes with siloxide ligands are precatalysts for alkyne metathesis.^[17][18]

Potential applications

Mo-based catalysts are active for olefin metathesis.^[10]

Some commercially available Schrock catalysts.

Trisamidomolybdenum(VI) alkylidyne complexes catalyze alkyne metathesis.^[19]

In the Kauffmann olefination, molybdenum(III) chloride and methyllithium form an organometallic complex capable of carbonyl olefination.^[20]