All of our current projects involve the syntheses, characterization and molecular dynamics of organometallic molecules.  These systems generally exhibit interesting fluxional behaviour and so require an investigation not merely of the nature of the rearrangements and the barriers associated with them, but also of the underlying electronic reasons for the ease or difficulty of the processes.  Structures and dynamics are elucidated principally by means of NMR spectroscopy, mass spectrometry and X-ray crystallography.  Molecular orbital calculations are used to probe the nature of the frontier orbital interactions, and all members of the group are encouraged to become proficient in the use of each of these techniques.  We have long-standing collaborations with many other groups both at McMaster University, and elsewhere in Europe and North America, and regularly exchange graduate students and post-doctoral research associates in both directions. 

The following brief summaries give an overview of some of our recent and current activities.  Note that reference numbers in the text correspond to those in the publication list.

Sterically crowded organo-transition metal complexes pose interesting problems in terms of their molecular dynamics.

For example, the observation that the interconversion of proximal and distal ethyl groups in (C₆Et₆)Cr(CO)₃ can be slowed on the NMR time-scale prompted us to ask whether ethyl rotation is correlated with that of the tripod [63].  To probe this phenomenon, Dr. Bavani Mailvaganam synthesized the cation [(C₆Et₆)Cr(CO)(CS)(NO)]⁺ whose variable-temperature ¹³C NMR spectrum exhibits at low temperature eighteen carbon environments (6 methyl, 6 methylene, 6 ring carbons) for the hexaethylbenzene ligand [107].  However, she found that the barriers to tripodal rotation (9.5 kcal/mol) and ethyl rotation (11.5 kcal/mol) are different, showing that these processes are not correlated.

X-ray crystal structure, and variable-temperature 125.8 MHz 13C NMR spectra of [(C6Et6)Cr(CO)(CS)(NO)]+

Moreover, complexes containing C₅Ph₅, C₆Ph₆ or C₇Ph₇ ligands exhibit conformations in which the polyphenylated fragments adopt chiral propeller-type structures.  The peripheral rings in these molecules adopt propeller-like structures as a compromise between a planar, but sterically inaccessible, geometry and the completely orthogonal orientation in which all conjugation with the central ring would be lost.  Such molecules can be locked into chiral conformations and the barriers to the interconversion of such enantiomers (or diastereomers) can be probed by a variety of NMR techniques [126,103,136].   Concurrent with the experimental work, computational studies are under way in a collaborative venture with Professors Kim Baldridge and Jay Siegel (University of California, San Diego). 

Side view & space-fill of (C5Ph5)Fe(CO)(CHO)PMe3 emphasising the propeller conformation of the C5Ph5 group
X-ray crystal structure of ferrocenyl-pentaphenylbenzene
X-ray crystal structure of heptaphenylcycloheptatriene	X-ray crystal structure of  C7Ph6FcH

As the ring size increases, steric crowding between peripheral phenyl rings becomes so severe that the 6- Hückel system [C₇Ph₇]⁺ is non-planar.  The cation was synthesized by Dr. Hari Gupta, and the X-ray crystal structure was solved by Stacey Brydges [162].

 

In an attempt to incorporate a probe to monitor the barrier to peripheral phenyl rotation in cyclopentadienyl systems, the ligand C₅Ph₄(C₆F₅)OH has recently been prepared [170].  The ¹⁹F spectra of this and related molecules exhibit five fluorine environments at room temperature; the pentafluorophenyl rotational barrier exceeds 20 kcal/mol.

 

X-Ray crystal structure and 282 MHz ¹⁹F NMR spectrum of C₅Ph₄(C₆F₅)OH

Sigmatropic migrations, such as those shown for S and R trimethylsilylindene, interconvert stereoisomers (enantiomers or diastereomers, depending on the molecule under investigation), and these rearrangement mechanisms have attracted much interest. In the particular case depicted, the process occurs via successive [1,5]-suprafacial migrations and the intermediate iso-indene can be intercepted as a Diels-Alder adduct [150].

Interconversion of enantiomers of TMS-indene, and trapping of the intermediate iso-indene by either maleic anhydride or TCNE	X-ray crystal structure of the TCNE adduct

The stereochemical complexity of these systems can become relatively severe and, in the tris-indenyl-silane series, investigated by Dr. Mark Stradiotto, the pathways for NMR site exchange can be mapped on a hypercube [150].

Interconversion of the eight different indenyl ring environments in the isomers of tris(1-indenyl)silanes

 

In cases where the molecule is temperature-sensitive, simple variable-temperature NMR using line-broadening techniques may be inapplicable and more sophisticated methods, such as 2D-EXSY or single selective inversion methods, may be required.  We have an ongoing collaboration with our colleague Professor Alex Bain to study such systems.

Typically, (eta-1-indenyl)Fe(CO)₂(C₅H₅) was for many years believed to be non-fluxional; moreover, it readily loses two carbonyl ligands to generate benzoferrocene. However, the 2D-EXCHANGE map shown below clearly reveals the existence of a fluxional process even at a relatively low temperature. Once again, the intermediate iso-indene has been trapped as a TCNE cycloadduct [159].

Interconversion of the enantiomers of (eta-1-indenyl)- Fe(CO)2(C5H5), and decarbonylation to yield benzoferrocene	X-ray crystal structure of the iso-indene-TCNE adduct

2-D EXSY spectrum of (eta-1-indenyl)Fe(CO)₂(C₅H₅) showing exchange between H(1) and H(3), and between H(4) and H(7) sites

 

Occasionally, iso-indenes are sufficiently stabilized, by incorporation of multiple aromatic rings, that they can react with their progenitor to yield a Diels-Alder adduct. Typically,  Dr. Suzie Rigby showed that cyclopenta[l]phenanthrene and its corresponding iso-indene give rise to the dimer (illustrated below) that has been characterized by NMR spectroscopy and X-ray crystallography [161].

Diels-Alder dimerization of cyclopenta[l]phenanthrene with its own iso-indene isomer	500 MHz  1H-1H COSY NMR spectrum of the dimer showing the connectivity between the protons in the four different aromatic rings
X-ray crystal structure of the Diels-Alder adduct, emphasising the endo nature of the dimer

For more examples of silatropic shifts in polycyclic systems, see references 139, 143, 148, 160, 167 and 171.

Haptotropic shifts involve the migration of an organometallic fragment (such as Mn(CO)₃) between rings, as exemplified by the interconversion of the eta-6 and eta-5 isomers of (cyclopenta[def]phenanthrenyl)ML_n, which represents 25% of the C₆₀ framework. Crystallographically characterized molecules depicting "before" and "after" situations are shown below [122].  This work is taken from the thesis of Dr. Andreas Decken.

As previously discussed for indenyl, naphthalene and related systems (Albright, T.A.; Hofmann, P.; Hoffmann, R.; Lillya, C.P.; Dobosh, P.A. J.Am. Chem. Soc. 1983, 105, 3396), the organometallic moiety does not execute a "least-motion" trajectory across the common bond between the rings, but instead follows a rather circuitous pathway along the molecular periphery so as to optimise the aromatic character of the transition state.

The most favored trajectory can be elucidated by use of molecular orbital calculations to generate an energy hypersurface whereby the organometallic fragment is systematically moved over the ligand framework so as to find the local minima, and the pathways that connect them. The examples shown illustrate the migration of (a) a manganese tricarbonyl fragment and (b) a cyclopentadienyl-iron unit over the surface of a cyclopenta[def]phenanthrenyl ligand; energy minima are indicated by increasing red coloration, while blue and purple represent regions of relative instability [151].  In both the Mn and Fe cases (upper and lower pictures, respectively), note the high barrier where the organometallic fragment crosses the common bond between the six- and five-membered rings. [The energy scale is in eV.]  These calculations were carried out by using the program CACAO, written by Dr. Carlo Mealli.

Current work is aimed at synthesizing metal complexes of non-planar polycyclics, e.g. sumanene, C₂₁H₁₂, that are fragments of fullerenes, and elucidating their haptotropic behaviour [154,155].

Transition Metal Stabilized Reaction Intermediates

The use of organometallic moieties or metal clusters to allow the isolation and characterization of short-lived species not only yields structural data on otherwise unavailable systems, but also provides a method of storing these reactive intermediates until required for synthetic purposes.

Benzylic anions and cations are readily generated as chromium tricarbonyl complexes and their geometries and molecular dynamics have been investigated both experimentally (by NMR) and theoretically [58,118]. Propargylic cations are particularly well stabilized as dicobalt hexacarbonyl clusters (Nicholas, K.M. Acc. Chem. Res. 1986, 20, 207). Although they are valuable synthetic intermediates, and have been intensively investigated by a variety of spectroscopic and theoretical methods, crystallographic characterization remains a serious challenge (Melikyan et al. Angew. Chem, Int Ed. Engl. 1998, 37, 161). However, isolobal replacement of a Co(CO)₃⁺ vertex by a neutral Fe(CO)₃ group [124], as in the fluorenyl cluster depicted below (synthesized and crystallographically characterized by Dr. James Dunn), has proven an excellent method of modelling the structures of the analogous cobalt-stabilized cations [168].

X-ray crystal structure of the mixed iron-cobalt fluorenyl cluster

A related project, in collaboration with our colleague Professor M.A. Brook [132,156,164] involves the development of metal cluster stabilized silicon cations, which are currently isolable only when partnered by counter-ions of exceptionally low nucleophilicity.

Sometimes, serendipity plays a role, as when Dr. Luc Girard treated 1,2,4,5 tetra(bromomethyl)benzene with Na₂Fe(CO)₄ and produced the first metal complex of the long-sought disjoint diradical tetramethylenebenzene [129].

X-ray crystal structure of  (1,2,4,5-tetramethylenebenzene)tris-irontricarbonyl

Organometallic Derivatives of Natural Products:

(a) Metal Complexes of Hormonal Steroids

Complexation of organometallic fragments to steroids serves many purposes: the placement of an Fe(CO)₃ unit on the alpha-face of ergosteryl acetate (or dehydrocholesteryl acetate, shown below), protects the B-ring from attack and permits functionalization of the side-chain.  The X-ray crystal structure was solved by Dr. Richard Perrier [115].

More recently, hormonal steroids have been synthesized that bear ruthenium  [98] or rhenium [114,140] moieties and have a high affinity for estradiol receptor sites. The longer term goal is to incorporate appropriate radio-isotopes (e.g. ¹⁰⁶Ru, ¹⁸⁶Re, ¹⁸⁸Re) with which to image or eradicate cancerous tumours. The structures of two typical molecules are shown below:

X-ray crystal structure of  the ß-[CpRu(estrone 3-methyl ether)] cation	X-ray crystal structure of 11ß-(chloromethyl)-17-[(CC-C5H4)Re(CO)3]-estradiol

This is a collaborative project with Professor Gérard Jaouen and Dr. Anne Vessières in Paris. More information is available on the website of their laboratory, the Laboratoire de Chimie Organométallique, ENSCP, Paris.

(b) Terpenes and "Non-classical" Cations.

Wagner-Meerwein rearrangements of terpenes have held a particular fascination for chemists for more than a hundred years. Indeed, the question of "classical" versus "non-classical" carbocations has been an exceedingly contentious topic. In light of the known ability of transition metal clusters to stabilize cations, we chose to investigate the chemistry of organometallic derivatives of the bornyl and fenchyl systems. While cobalt-stabilized cations can still undergo skeletal rearrangements, incorporation of (C₅H₅)Mo(CO)₂ vertices into tetrahedral clusters led to stable, crystalline salts containing the 2-bornyl or 2-fenchyl cations [125,138].

X-ray crystal structure of a Co-Mo alkyne cluster possessing a molybdenum-stabilized endo-2-bornyl cation	X-ray crystal structure of a Co-Mo alkyne cluster possessing a molybdenum-stabilized exo-2-fenchyl cation

Current work is directed towards the synthesis of metal-stabilized 7-bornyl cations via Diels-Alder additions to alkynylcyclopentadienols, which occasionally yield surprising results.  Thus, the reaction with benzyne is normal, but the Diels-Alder adduct with dimethyl acetylenedicarboxylate undergoes a multi-step skeletal rearrangement in the presence of traces of base [173].