Zirconocene bis(trimethylsilyl)acetylene pyridine

Zirconocene bis(trimethylsilyl)acetylene pyridine is an organozirconium metallocene compound that appears as a dark purple solid with a melting point of 125-126°C. Also referred to as Rosenthal’s reagent, this complex features two cyclopentadienyl ligands, a bis(trimethylsilyl)acetylene ligand and a coordinating pyridine molecule. Named after the work of Uwe Rosenthal, this compound with various coordinating solvents replacing the pyridine and the solvent free titanium analogue all share the title of Rosenthal’s reagent. The general reactivity for this version of Rosenthal’s reagent differs from its titanium counterpart and is most often reacted with alkynes to replace the zirconocyclopropene with a larger zirconacyclopentadiene rings.

Synthesis
Zirconocene bis(trimethylsilyl)acetylene pyridine was originally synthesized by Rosenthal’s group in 1994 after they exchanged the coordinating solvent from tetrahydrofuran (THF) to pyridine. The exchange of the THF for the pyridine ligand provides extra stability in organic solvents preventing dimerization. The original synthesis involved the reduction of zirconocene dichloride and the addition of bis(trimethylsilyl)acetylene in THF before transferring to pyridine as mentioned above. More recently, Tilley and coworkers demonstrated a simpler synthesis with a higher yield bypassing the isolation of the less stable THF adduct. This newer method reacts zirconocene dichloride with 2 equivalents of n-Butyllithium in THF to form a metallacyclopropane which is subsequently substituted by bis(trimethylsilyl)acetylene and pyridine.

Bonding
The two main resonance structures of Zirconocene bis(trimethylsilyl)acetylene pyridine include a variation where the C-C triple bond binds side on to the metal and another with the 1-metallacyclopropene configuration. Density functional theory (DFT) calculations showed metal-carbon sigma bonds in addition to an out of plane pi bond corresponding to the 1-metallacyclopropene depiction being a major resonance form. However, a subsequent series of calculations by Leites and colleagues using a higher level of theory showed molecular orbitals more consistent with the triple bond description.

Reactivity
The original creation of zirconocene bis(trimethylsilyl)acetylene pyridine was accompanied by reactivity studies of the complex with common small molecules in the form of carbon dioxide and water. Both reactions involved the loss of the pyridine ligand and creation of bimetallic complexes containing bridging-oxo substituents, with the carbon dioxide inserting to create a series of fused metallacycles and the water’s hydrogen atoms breaking up the metallacyclopropenes.

More generally, the main reactivity for this version of Rosenthal’s reagent is its reaction with alkynes to replace the zirconacyclopropene with a larger zirconacyclopentadiene rings. T. Don Tilley and colleagues have extensively utilized this functionality to create zirconocene based macrocycles with considerable tunability based on the alkyne used. These large macrocycles can subsequently be reacted with hydrochloric acid to lose the zirconocene dichloride leaving behind new carbon-carbon bonds. Following these macrocycles, the Tilley group also showed that the zirconocene bis(trimethylsilyl)acetylene pyridine could aid in the creation of various polycyclic aromatic hydrocarbons via [2+2+n] cycloaddition reactions. As seen with the acidic conditions above, the zirconocene fragment is easily displaced and Tilley demonstrated the ability to insert selenium into the framework. Rivard's group also showed an analogous transmetalation process allowing for the replacement of zirconium with tellurium. Rosenthal also continued exploring the reactivity of the zirconocene bis(trimethylsilyl)acetylene pyridine showing the ability to functionalize the zirconacyclopentadienes in addition to modifying the ring itself. This latter study involved the reaction of substrates like tertbutyl substituted 1,3 butadiyne to create novel zirconacyclocumulene complexes. In 2018, Staubitz and coworkers used the pyridine complex in combination with dialkyne complexes to form the zirconacyclopentadiene after loss of the pyridine and the bis(trimethylsilyl)acetylene. These zirconium metallacycles can then be transmetalated to create functionalized stannoles which Staubitz later used in Stille cross coupling reactions to form polymers with thiophene groups.

Staubitz’s group followed this with a reactivity comparison between Cp2Zr(btmsa)(py) and Negishi’s reagent with respect to forming zirconacyclopentadienes. They found that this reaction took place quicker and more efficiently than with Negishi’s reagent. In 2019, Ye and coworkers further extended the scope of the pyridine Rosenthal reagent reactivity, demonstrating its reaction with bis(alkylnyl)boranes in an attempt to create compounds capable of activating small molecules. The product of this reaction has resonance structures including a boron zirconium(IV) 6-member heterocycle and a zirconium(II) donating into the boron stabilized by the two alkynes. Zirconocene bis(trimethylsilyl)acetylene pyridine was also shown to react with other zirconocene derivatives containing alkyne substituents with Lindenau et. al. showing the creation of a bimetallic transition metal hydride. This was achieved by the reaction of Rosenthal’s reagent with Zr(Cp)2(CH3)(CCSiMe3) to create a methyl bridged complex which could be converted to the hydride upon the addition of BH3•NHMe2. Tonks and colleagues looked into the reactivity of this Rosenthal reagent as a potential ring opening complex, but instead formed new zirconocene heterocycles. Upon addition of the zirconocene bis(trimethylsilyl) acetylene pyridine to cyclopropyl methyl ketone, a zirconium oxygen bond formed simultaneously forming a new carbon-carbon bond from the cyclopropene and the carbonyl carbon.