Cyclopentadienylcobalt dinitrosyl is an organometallic molecule. It is a reactive intermediate in the formation of dinitrosoalkane cobalt complexes. While cyclopentadienylcobalt dinitrosyl has not been isolated and characterized, the preparation of this reactive intermediate in the presence of olefins results in the isolable dinitrosoalkane cobalt complexes. The dinitrosyl intermediate is known for its alkene binding capability. The resulting dinitrosoalkane cobalt complexes are capable of stoichiometric and catalytic C-H bond functionalization.
Discovery
This nitrosyl cobalt complex was first discovered in 1967 by Henri Brunner at the Technical University of Munich. By reacting cyclopentadienylcobalt dicarbonyl with NO in hexane at room temperature, he obtained a dimer (CpCoNO)2. The dimer was formed by bridging nitrosyl (NO) ligands, which connected the two CpCo units. Based on a dipole moment of 1.61 D observed in hexane, Brunner suggested that the nitrosyl ligands extended above the plane of the cobalt atoms. However, crystal structures of the dimer have shown that the nitrosyl ligands and the cobalt atoms lie in the same plane. Subsequently, in 1973, Brunner discovered that the dimer could react with NO and an olefin to form a monomeric cyclopentadienylcobalt dinitrosoalkane, in which the protons of the olefin assumed the endo position. However, the scope of such reactions was limited to norbornene-type olefins. Ethylene and cyclohexene also reacted but the resulting products could not be purified for structure-determination. Notably, this transformation was ligand-based rather than occurring at the metal center, which is unusual for an organometallic transformation. Brunner's initial investigations of the cyclopentadienylcobalt nitrosyl dimer suggested that an olefin could be activated through interaction with the dimer, but the synthetic utility and identification of the monomeric reactive intermediate, cyclopentadienylcobalt dinitrosyl, were yet to be explored. Furthermore, the origin of the stereoselectivity had not been determined.
Brunner's findings were verified by a later crystallographic study conducted by Bernal and coworkers. They confirmed that the hydrogen atoms did occupy the endo positions of norbornene, and the nitroso ligands occupy the exo positions. The crystallographic data of the Co(NO)2C2 fragment closely resembles data from molecules containing free nitroxyl radicals, which led Bernal to suggest that the fragment behaves like a nitroxyl free radical. Specifically, the Co-N-C bond angle of 118.2(2)° closely resembles that of 2,2,5,5-tetramethyl-3-carbamidopyrroline-1-oxyl and other molecules containing free nitroxyl radicals. The N-O bond distances, as well as stretching frequencies closely agree too.
Early synthetic applications
Olefin binding Whereas in Brunner's work the dinitrosoalkane was not further functionalized, Becker and Robert Bergman demonstrated in the 1980s that the functionalized organic fragment resulting from alkene activation could be released from the dinitrosoalkane complex, and that other unstrained olefins could be made to react with the dimer. They hypothesized that the reaction of the dimer [CpCoNO]2 with NO and olefin proceeded via formation of the cyclopentadienylcobalt dinitrosyl intermediate. Specifically, they suggested that simple olefins could not trap this proposed intermediate fast enough. This led them to perform the reaction with excess amounts of olefin so that the trapping rate would increase and correspondingly improve the yield of the reaction. Simple aliphatic olefins could thus be reacted with the dimer to produce the corresponding dinitrosoalkane. Brunner had demonstrated that the reaction of the dimer with norbornene-type olefins resulted in the protons occupying the endo position. Bergman showed that the reaction was completely stereospecific for unrestrained aliphatic olefins by reacting the dimer with (E)- and (Z)-3-methyl-2-pentene. These reactions gave isomerically pure products, which showed that the stereospecificity was not exclusive to ring-strained substrates like norbornene.
1,2-Diamination
Bergman attempted to release the dinitrosoalkane ligand from cobalt via ligand substitution with CO or phosphine, and direct oxidation of the substrate. These unsuccessful experiments suggested the presence of π-backbonding from the cobalt to the nitrosyl ligands. With the presence of π-backbonding, the ligands would resemble nitroxides or nitroxyl radicals rather than nitroso ligands, as suggested by Bernal's XRD study. Despite this shortcoming, Bergman and Becker were successful when they reduced the dinitrosoalkane with LiAlH4. This reaction resulted in the net conversion of alkenes to 1,2-diamines and was applied to a variety of olefins with good yields. The amination reaction however could not be performed with a high degree of stereoselectivity, suggesting that the reaction involves epimerization during reduction. This idea was supported by deuterium studies employing LiAlD4, in which there was a substantial amount of product which had deuterium incorporated at the carbon alpha to the amine. Thus, Becker and Bergman's work demonstrated that the cyclopentadienylcobalt nitrosyl dimer could be used for 1,2-diamination of alkenes. Moreover, they correctly suggested that alkene activation occurred via the cyclopentadienylcobalt dinitrosyl intermediate.
Cylcopentadienylcobalt dinitrosyl reactive intermediate Mechanistic investigation
Becker and Bergman published a detailed mechanistic investigation of the reaction between CpCo(NO)2 and olefins shortly after the above study. In this study they reveal that the cyclopentadienylcobalt dinitrosoalkane complex undergoes reversible exchange with alkenes. Kinetic and spectroscopic investigations allowed them to suggest the mechanism of alkene activation and reveal the monomeric cylcopentadienylcobalt dinitrosyl reactive intermediate.
Preparing a dinitrosoalkane complex with a simple unstrained olefin and introducing norbornene-type olefins, which allow for much more stable dinitrosoalkane complexes, led to the exchange of the olefins. The observation of this exchange reaction led to the simple mechanistic proposal wherein the reactive intermediate could reversibly bind with the acyclic olefin, or irreversibly bind with norbornene to form the more stable alkane complex. The corresponding rate expression was experimentally verified by varying the concentrations of norbornene and acyclic olefin. Bergman also showed that olefin exchange could occur photochemically.
When [CpCoNO]2 was placed in benzene it formed a dark green solution. However, upon introduction of NO gas, the solution turned into a lighter, brighter green color. Infrared spectroscopy revealed that this brighter solution has stretching frequencies of 1609 and 1690 cm−1, as opposed to the dimer's frequencies at 1540 and 1590 cm−1. The greater stretching frequencies supported the assignment of that species as the monomeric CpCo(NO)2 reactive intermediate. These IR stretches disappeared when the solution of the intermediate was treated with norbornene, further supporting its assignment.
The CpCo(NO)2 intermediate could be directly synthesized by treating Co((NO)2-μ-Cl)2 with lithium cyclopentadienide in dimethoxyethane. CpCo(NO)2 still decomposed slowly, but was relatively stable under dilute conditions, as evinced by the UV/Vis spectrum remaining unchanged over 20 minutes. This solution was treated directly with olefin and resulted in the dinitrosoalkane complex. Additional studies with this reaction scheme showed that the rate was insensitive to the polarity of the solvent. Substitutions on the cyclopentadienyl did not affect the reactivity of the intermediate, but the permethylated ring reduced the thermal stability while ester substituents improved the stability. Overall, these studies suggested that the reaction between [CpCoNO]2 dimer and olefin is bimolecular. Mechanistically, since tetrasubstituted olefins were found to react with the dimer, Becker and Bergman concluded that the reaction cannot occur via olefin coordination to the cobalt, and instead occurs directly with nitrogen atoms of the nitrosyl group and the π bond of the olefin in a concerted fashion. Moreover, since exchange of the alkene results in the stereospecific release of the starting alkene, olefin exchange was suggested to occur via concerted C-N bond cleavage to produce diradicals which quickly combine to form the alkene.
Molecular orbital theory perspective Bergman's work on CpCo(NO)2 suggested a likely mechanism of alkene binding, but it was still unusual that an organometallic transformation should proceed via ligand-based activation. Moreover, the structure of the reactive intermediate could not yet be determined. Roald Hoffmann provided a molecular orbital point of view to describe this reactivity and the possible structure.
Hoffmann produced a wavefunction of the highest occupied molecular orbital (HOMO) and found that the HOMO is largely located on the NO π* orbitals, and that these π* orbitals are able to engage in π back-bonding with Co. He identified several plausible structures of CpCo(NO)2. One was a 20-electron species in which the NO ligands are linear and can be considered as LX-type because of the donation of the lone pair and the nitrogen-centered radical. Another was an 18-electron species in which one NO ligand behaves as the aforementioned LX ligand, and the other behaves as an X-type ligand and adopts the bent shape. Finally, if both NO ligands behave as X type ligands and adopt the bent conformation then the CpCo(NO)2 moiety has 16 electrons.
The conformation of the NO ligand and the corresponding electronic behavior can be understood in context of the nitric oxide molecular orbital diagram which has a nitrogen-centered radical residing in the degenerate px/py based molecular orbital and a lone pair residing in the pz based orbital. These orbitals are the HOMO and HOMO-1 respectively. When the NO ligand only uses the radical in bonding with the Co center and behaves as an X ligand there is no need for the Co-N-O to be linear since the px/py orbitals are orthogonal to the nuclear axis of N-O. When the NO ligand uses the pz-based lone pair in bonding then the nuclear axis of N-O must coincide with the Co-N nuclear axis.
The 18-electron species, while it satisfies the 18-electron rule, has less symmetry than the 16-electron species wherein both nitrosyl ligands are bent. Indeed, a Walsh diagram detailing the conformational change from linear NO ligands to bent NO ligands shows that the LUMO and HOMO centered on the NO π* orbitals decreases in energy. Hoffmann attributed this observation to the loss of antibonding overlap between the metal and nitrosyl orbitals. A contour plot of the wavefunction of the CpCo(NO)2 HOMO and LUMO in the bent conformation shows that the HOMO is antisymmetric with respect to the plane perpendicular to the N-Co-N plane, and the LUMO is symmetric with respect to this axis. The electronic structure of the HOMO and LUMO can have the appropriate symmetry to interact with an olefin. The π* orbital of the olefin can interact with the antisymmetric HOMO, and the π orbital can interact with the symmetric LUMO for an overall stabilizing interaction. This HOMO of the resulting dinitrosoalkane then becomes the dxz orbital of Co, and the LUMO becomes the π* NO orbitals. This description of the LUMO explained why the reduction of the dinitrosoalkane complex lead to diamination, the LUMO was centered on the nitrosyl ligands and thus they underwent reduction Finally, while Hoffman showed that the 16-electron doubly-bent nitrosyl conformation was more stable than the linear 20-electron complex, the 18-electron complex in which one nitrosyl is bent and one is linear was put forth as another probable conformation.
Molecular orbital theory of [CpCoNO]2