Dalton Transactions Blog

In their recent paper in Dalton Transactions, Stasch, Jones and co-workers describe the use of bulky ring-expanded N-heterocyclic carbenes (NHCs) for the stabilisation of Group 15 trichlorides, ECl₃; E = P, As, Sb.

The authors show that mixing 1:1 solutions of the ring-expanded, 2,6-diisopropylphenyl-subsitutied NHC “6-Dipp” with ECl₃ (E = P, As, and Sb) affords satisfactory yields of the corresponding [(6-Dipp)ECl₃] adduct, noting that the use of the alternatively-substituted mesityl-substituted carbene (6-Mes) led to a mixture of products thus highlighting the importance of the 6-Dipp ligand for stabilising the adducts.

The group characterised each adduct using standard means, including NMR spectroscopy and electron-impact (EI) mass spectrometry. In the cases of P and Sb, they were able to use X-ray crystallography to determine the solid state structures of those compounds and observed that each pnictogen centre adopted a saw-horse geometry.

When the researchers tried to reduce these adducts using either KC₈ or a Mg(I) derivative, they were mostly unsuccessful, however they were able to reduce [(6-Dipp)PCl₃] using the former reagent to form a unique dicationic carbene-stabilized P₄ unit. Once again, the authors attributed the stability of this species to the steric profile of the 6-Dipp ligand framework. The X-ray crystal structure of the complex shows a P₄-butterfly geometry stabilised by two carbene moieties.

During the course of their reactivity studies, the group identified (6-MesH)₂ as a byproduct resulting from reaction of in-situ generated [(6-Mes)PCl₃] with KC₈. With further optimisation, they revealed that this moiety could be accessed from treatment of [6-MesH]Br with KC₈ – marking the first successful reductive coupling of cyclic amidinium ions. Along with NMR and X-ray crystal data, a cyclic voltammetry (CV) study was also performed on (6-MesH)₂ to fully characterise this unique species.

The successful syntheses of all these p-block NHC complexes pave the way for new discoveries in fundamental reactivity, bonding, and catalysis employing main group elements, further demonstrating the potential of these elements to perform exciting chemistry.

Read the full article to find out more:

Expanded Ring N-Heterocyclic Carbene Adducts of Group 15 Element Trichlorides: Synthesis and Reduction studies
Anastas Sidiropoulos, Brooke Osborne, Alexandr Simonov, Deepak Dange, Alan Bond, Andreas Stasch and Cameron Jones
Dalton Trans., 2014, DOI: 10.1039/C4DT02074J

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.

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.

Posted on behalf of Ian Mallov, web writer for Dalton Transactions

Reading the chemical literature as a synthetic chemist, I can often empathize with the story of the practical challenges which underlies the research story in a paper.  This article from Gunnoe and co-workers certainly left me with an appreciation for the gutsy way in which the researchers overcame significant challenges to the usual synthetic and analytical techniques of the inorganic chemist to present some hard-won gains on the reductive elimination chemistry of rhodium.

The power of reductive elimination – inducing two chemical moieties bonded to a central atom to leave without one of their electrons, resulting in a formal gain of electrons by this central atom – lies in its ability to fuse together the two groups leaving. In this way chemical bonds which are difficult or impossible to form by other means can be created.  When the elimination of one group breaks a metal-carbon bond, the possibility to form a carbon-X bond with the other group reductively-eliminated, and thus functionalize a carbon centre, is particularly attractive.

Such is the technique the Gunnoe group present here.  Much more commonly used in platinum chemistry, they prove this approach to be applicable to rhodium chemistry also, inducing reductive elimination of CH₃ and a range of halides or pseudo-halides.  While the CH₃-X products formed are useful only as a proof of principle, the proof was a result of overcoming significant challenges. 

The Rh-terpyridine complex ultimately coaxed to undergo reductive CH₃-X elimination was so insoluble that they were unable to obtain a NMR spectrum, much less an x-ray crystal structure, and had to trust that a combustion analysis supporting their hypothesized product was evidence enough to proceed.  Then, the hoped-for reductive elimination did not occur until electron-withdrawing NO₂ groups were installed on the terpyridine backbone, the reaction was heated, and the solvent changed to CD₃NO₂ (certainly not the first solvent one would have tried).  Moreover, the CH₃-X products formed were gases, making it very difficult to quantify the amount produced.

Nonetheless, their persistent tweaks of the organometallic complex itself, aided by computational thermodynamic data and the use of second-choice analytical techniques when necessary yielded insight into Rh reductive elimination.

Find out more and download the article now:

Reductive functionalization of a rhodium(III)–methyl bond by electronic modification of the supporting ligand
M. E. O’Reilly, D. R. Pahls, J. R. Webb, N. C. Boaz, S. Majumdar, C. D. Hoff, J. T. Groves, T. R. Cundari and T. B. Gunnoe
Dalton Trans., 2014, DOI: 10.1039/C4DT00234B

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.

Posted on behalf of Philip Mountford, Chair, Dalton Transactions Editorial Board

I am writing on behalf of the Editorial Board of Dalton Transactions to let you know that Dr Jamie Humphrey has accepted a new position within the Royal Society of Chemistry as Publisher of Dalton Transactions and as such will no longer be Editor. However, we are delighted to report the appointment of the new Editor for Dalton Transactions, Sarah Ruthven.

Sarah has been a member of Royal Society of Chemistry editorial staff since 2005. Since her time at the organisation, Sarah has overseen the successful development of a number of journals, including Green Chemistry and Food & Function, and in 2011 she launched the Royal Society of Chemistry’s innovative journal, RSC Advances, which in the three years since its launch has grown to be the largest journal that the Royal Society of Chemistry publishes.

As the Journal’s Editor, Sarah brings extensive journal and publishing experience to Dalton Transactions, together with tremendous enthusiasm and a reputation for getting things done. I have every surety that Dalton Transactions will thrive and prosper under her editorship. Sarah will lead the Dalton Transactions editorial team based in Cambridge, UK: Deputy Editor, Fiona McKenzie and Development Editor, Guy Jones, and Editorial Production Manager Andrew Shore and his team of Publishing Editors.

Together with all the authors and readers, and the editorial and advisory board members of Dalton Transactions I am sure you would wish to join me in thanking Jamie for his hugely important role at the Journal during the past 11 years. From 2003 to 2014, the Journal has seen the number of published articles grow substantially from 689 to 1709 per year, with a subsequent increase in issue frequency from 24 to 48 issues per year—making Dalton Transactions the first weekly inorganic journal. Jamie also introduced topic-based themed issues and increased the number of Associate Editors to 7, based in 6 countries worldwide.

During Jamie’s tenure, Dalton Transactions also has seen its impact factor grow from 3.02 to 3.81, maintaining its highly competitive position in comparison to its international counterparts. Jamie has been tireless in promoting and representing the Journal at meetings and conferences throughout the world, and many of us have enjoyed his company in both a professional and personal setting.

We thank you Jamie for all of this and wish you all the best for the future!

Orignally published in Dalton Transactions as an Editorial article