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Final Report Summary - OXRDSN (E-H Bond Functionalization Using Group 14 Based Catalysts)

The Marie Curie project ‘E-H bond functionalization using group 14 based catalysts’ was concluded 6 months earlier than the original schedule by mutual agreement between the fellow (Dr Arnab Rit) and the host (Prof Simon Aldridge) after the former was successful in obtaining a faculty position at IIT Madras. The training acquired by the fellow both in technical/scientific aspects of the project and in more general management, organizational, communication and planning aspects were very important in Dr Rit’s successful application for this highly competitive position as a junior chemistry professor at one of India’s most prestigious institutions.

Dr Rit’s work during his time in Oxford has given rise to three publications to date, including papers in the top-ranking chemistry journals Angewandte Chemie (one) and Journal of the American Chemical Society (two). Two further papers are under consideration currently (one with Nature Chemistry, another with Chemistry-A European Journal) and at least two others will be submitted in the coming six-month period. In addition, Dr Rit was afforded the opportunity to present his work at a number of national and international meetings, including conferences organized by the Royal Society of Chemistry (2015 RSC Coordination and Organometallic Chemistry Discussion Group Meeting, presented poster), and Mike Lappert Memorial Symposium, and the 14th International Symposium on Inorganic Ring Systems (IRIS) in Regensburg, Germany (July 2015; presented poster).

Scientifically Dr Rit’s output from the project has been prodigious, including the synthesis of some completely unprecedented types of compound, and establishing a series of fundamental bases for bond activation at low-valent germanium centres. This work – while fundamental in its nature offers enormous potential going forward in the developing area of bond activation and catalysis by main group compounds. Catalysis is central to wealth creation in modern chemicals/materials manufacture, and the use of new, environmentally friendly and cost-effective catalysts based on main group elements offers a potentially transformative alternative to the continuing use of expensive and toxic heavy transition metal systems.

From a technical perspective, We have isolated an acyclic two-coordinate mono-cationic germylene of type [alkyl(NHC)Ge:]+ which features essentially no π-donor stabilization of the metal center and hence has a very small HOMO-LUMO gap (187 kJ mol-1). This facilitates versatile oxidative reaction chemistry including C-Cl insertion and [2+1] cycloaddition. Most interestingly, it allows for the synthesis of the first examples of heavier Group 14 element cations containing M=E multiple bonds (E = C, N) via oxidative addition. From a synthetic viewpoint, the use of an electrostatic (rather than wholly steric) approach to discourage aggregation allows access to a system containing a very rare primary (i.e. CH-containing) alkylidene fragment. Reactions with different oxo-transfer reagents were also examined; in the case of electron-deficient oxo-transfer compound this led to the formation of a GeIV compound in which the presumed germanone fragment, [alkyl(NHC)Ge(O)]+, is trapped by a second molecule of the pyridine-N-oxide. This work was recently published in Angewandte Chemie (2016, 55, 378-382). Steric and electronic tuning of the substituents at germanium allows control of aggregation. Thus, combination of smaller NHC with strongly σ-donating boryl ligand result in dimerization to give a dicationic digermene [X(NHC)Ge=Ge(NHC)X]2+ (X = boryl) which dissociates into monomeric counterpart in presence of suitable ligand and activates E-H bonds at ambient temperature.

Various boryl-stabilized low-valent Ge species were successfully synthesized. Use of a milder Mg(I) reducing agent gave mono-NHC-coordinated dinuclear Ge(I) species, while the use of forcing conditions (excess KC8) generates the more highly reduced, formally Ge0 compound [{boryl}GeGe{boryl}]2- (boryl = {(HCDippN)2B}). Chemical oxidation of the Ge0 compound gave access to a compound of composition (boryl)2Ge2. It turned out to be more stable in its 1,1-form, {boryl}2GeGe (digermavinylidene), rather than the expected 1,2 system (boryl)GeGe(boryl). Compound {boryl}2GeGe is the first structurally characterized compound of the type R2EE (R = anionic ligand) which is stable at room temperature for any of the Group 14 elements. Initial reactivity studies show that both the mono-NHC-coordinated dinuclear Ge(I) species and the digermavinylidene are able to activate H2 under mild condition in accord with the proposed reactivity behavior. This work is currently under consideration for publication in Nature Chemistry.

Transformation of this stoichiometric reactivity to viable catalytic processes has not yet been achieved, but represents one of the major goals of the project going forward, and one which will be enabled by the significant ground work established by Dr Rit during the period of the Fellowship.

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Gill Wells, (Head of European Team)
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