Dalton Transactions Blog

In their recent paper in Dalton Transactions, Erker and co-workers describe B(C₆F₅)₃ as an unorthodox probe for the detection of σ ligand character and allenoid-type bonding in substituted zirconocenes.

Chemists have studied the strong Lewis acid, B(C₆F₅)₃ for the past decade, particularly for its uses in frustrated Lewis pairs (FLPs). This small molecule has, however, gained popularity in other areas of chemistry. In catalysis, B(C₆F₅)₃ is commonly employed to generate cationic metal centres by alkyl group abstraction (σ-ligand abstraction) to activate molecular pre-catalysts for use in polymerisation.

Expanding the scope beyond alkyl groups, Erker and co-workers showed that B(C₆F₅)₃ can mediate cleavage of Zr-C(sp³) bonds in zirconacycles, creating unique allene coordination complexes. In one instance, they used an unsubstituted zirconacycloallenoid (Zr-CH₂-) to synthesise a zwitterionic (η²-allenyl)zirconocene with an allene bond angle close to linearity.

In a second case, they reacted a 4-phenyl substituted zirconacycloallenoid (Zr-CHPh-) to produce a zwitterionic allene-coordinated zirconocene.

In demonstrating such reactivity, the authors lead the way for B(C₆F₅)₃ to act as a standard probe for detecting latent σ ligand character in other molecules.

Interested in finding out more? Read the full article:

Reaction of Five-membered Zirconacycloallenoids with the Strong Lewis Acid B(C6F5)3
Gerald Kehr, Gerhard Erker, Constantin Gabriel Daniliuc, Birgit Wibbeling and Georg Bender
Dalton Trans. 2014, DOI: 10.1039/C4DT01137F

Marcus Drover is a Ph.D. student, co-supervised by Professors Laurel Schafer and Jennifer Love at the University of British Columbia. His research is focused on the preparation of low-coordinate RhI and IrI complexes for use in small-molecule reactivity. He grew up in St. John’s, Newfoundland and graduated from Memorial University (MUN) before beginning graduate school in 2012.

What we think of as “organic chemistry” doesn’t often focus on the structure and bonding environments of the carbon atom, unlike inorganic chemistry which often does so with heteroatoms.  So I don’t think I’m going far out on a limb when I say that perhaps the key development in the structure and bonding of carbon over the last quarter century has been the isolation and use of the persistent carbene. 

Arduengo carbene

A carbene is a divalent carbon atom possessing two electrons.  Once thought to be unisolable, its history is recent enough that almost all important figures in its development are not only still living to this day, but still working.  Ron Breslow proposed that carbenes could be isolated in 1957, Guy Bertrand isolated a liquid dicarbene in 1989 and, in 1991, Anthony Arduengo reported the first comprehensive solid-state data of the type of carbene that now bears his name (pictured above).  

So why waste the words of a short post blog on historical perspective?

Because the uses of the carbene have fallen almost entirely in one direction.  Primarily, the carbene has been used as a strong s 2-electron donor – attached to many transition metal atoms it forms very stable complexes, and it is relatively easy to vary its size. 

Only recently have the uses of the carbene diverged. 

In a recent Dalton Transactions paper, Arnold, Love and co-workers present work on labile carbenes tethered to rare earth metals by an alkoxy arm.  On rare earth metals – which are much harder Lewis acids than late transition metals – the soft carbene donor and hard metal acceptor are poorer matches.  Thus the carbon-metal bond may break, freeing the carbene for reactivity, while the hard alkoxy arm keeps the ligand tethered to the metal.

Reactivity of tethered alkoxycarbene complexes 

The authors present some examples of reactivity, including co-operative activation of pyrroles and alkynes by the carbene and metal centres, as well as outer-sphere interactions in the form of hydrogen bonding between pyrroles and the metal-bound O atom in solution.   Despite these being small steps, they are nonetheless important if the carbene is to find new roles outside stabilising late transition metal centres.

Interested in finding out more? Read the full paper:

Homo- and heteroleptic alkoxycarbene f-element complexes and their reactivity towards acidic N–H and C–H bonds
Polly L. Arnold, Thomas Cadenbach, Isobel H. Marr, Andrew A. Fyfe, Nicola L. Bell, Ronan Bellabarba, Robert P. Tooze and Jason B. Love
Dalton Trans., 2014, DOI: 10.1039/C4DT01442A

Ian Mallov is currently a Ph.D. student in Professor Doug Stephan’s group at the University of Toronto. His research is focused on synthesizing new Lewis-acidic compounds active in Frustrated Lewis Pair chemistry. He grew up in Truro, Nova Scotia and graduated from Dalhousie University and the University of Ottawa, and worked in chemical analysis in industry for three years before returning to grad school.