Synthetic Strategies towards Substituted Pentalenides with Tailored Electronic Properties
: (Alternative Format Thesis)

  • Niko A. Jenek

Student thesis: Doctoral ThesisPhD


The focus of this thesis is the development of facile access towards designed and purposely tuned pentalenide ligands (parent framework C8H62-; Pn2-) with the potential to introduce any desired substitution pattern – an approach which hasn’t existed in the 60 years of pentalenide research, until today.

Chapter One is introducing the reader to the subject of pentalenide complex synthesis. This review evaluates a collection of reports which have established the unique structures and electronic properties of mono- and dinuclear pentalenide complexes of s-, p-, d- and f-block elements. It is being shown that access to these intriguing compounds is restricted by synthetic challenges. Various strategies for the synthesis, functionalisation and (trans)metalation of pentalenide complexes from a practical point of view is being reviewed, pointing out promising avenues for future research that may allow wider access to novel pentalenide complexes for application in many different areas. Published in: Dalton Transactions, 2019, 48, 5107─5124.

Chapter Two addresses the aims and objectives of this thesis as reflection of the cul-de-sacs presented in the preceding introductory chapter. The reader is introduced to the main aims and objectives of this collection of investigations: the development of synthesis methods towards substituted pentalenide ligands a) in few reaction steps, b) under mild conditions, c) without the need of special equipment and d) total control of substituent patterns as well as their position in the bicyclic framework to demonstrate versatility.

Chapter Three demonstrates an investigation towards a mild solution phase synthesis of the first reported tetra-substituted pentalenide ligand 1,3,4,6-tetraphenylpentalenide (Ph4Pn2-). The literature known compound 1,3,4,6-tetraphenyl-dihydropentalene (Ph4PnH2) is being formed via a base-promoted high-yielding solution phase synthesis based on a simple annulation reaction of 1,4-diphenylcyclopenta-1,3-diene with chalcone. Deprotonative metalation of Ph4PnH2Ph with alkali metal bases (Li, Na, K) of moderate basicity (pKa >15) cleanly yields the corresponding hydropentalenide complexes, which exist as solvent-separated ion pairs in coordinating solvents such as THF, pyridine, and DMSO. A second deprotonative metalation with stronger alkali metal bases (pKa >25) yields double-anionic, 10 π aromatic tetraphenylpentalenide complexes, which are of low solubility in the case of Li2, Na2, and K2. In the solid state the two metals are bound η5 to each anionic ring in anti- configuration as in the unsubstituted pentalenide Li2[C8H6]. Mixed-metal tetraphenylpentalenide complexes of Li/Na/K display remarkably enhanced solubility, allowing for solution phase characterization via NMR and UV−vis spectroscopy and application in transmetalation reactions. Published in: Organometallics, 2022, 41, 211─225.

Chapter Four
evaluates whether the formation of Ph4PnH2 was an isolated incident in the search of precursors of novel pentalenide ligands for use in organometallic chemistry and homogeneous catalysis. Therefore, the scope of a straightforward base-promoted Michael annulation of substituted cyclopentadienes with α,β-unsaturated ketones that allows the introduction of symmetrical as well as unsymmetrical aryl and alkyl substitution patterns including electron-donating as well as electron-withdrawing substituents is being investigated. As a result, more than 16 examples of various isomers of 1,3,4,6-tetraarylated dihydropentalenes have been synthesised in isolated yields of up to 78%, representing a substantial expansion of the range of dihydropentalene scaffolds known to date, allowing even the implementation of four different substituents. Double bond isomerisation between the two pentacyclic rings in 1,2-dihydropentalenes (PnH2) with electronically different substituents occurred depending on the polarisation of the molecule. The melting points of the air-stable PnH2 decrease, and their solubilities in organic solvents improve with increasing substitution and decreasing symmetry of the molecule. A competitive pseudo-retro-aldol pathway produces 1,3,6-triarylated monocyclic pentafulvenes as side products in yields of 9─68%, which can be cleanly isolated (8 new examples) and used for other synthetic purposes, including separate cyclisation to other PnH2. Published in: Journal of Organic Chemistry, 2022, 87, 13790─13802.

Chapter Five
reports the first example of systematic tuning of the electronic properties of dianionic pentalenides through the controlled variation of substituents in the 1,3,4,6-positions. Both electron-withdrawing as well as electron-donating aromatics have been incorporated to achieve different polarisations of the bicyclic 10π aromatic core as indicated by characteristic 1H NMR shifts and supported by DFT calculations including nucleus-independent chemical shift (NICS) scans and anisotropy of the induced current density (ACID) calculations. 10 novel arylated pentalenide species were synthesised in a two-step protocol, namely 1,3,4,6-tetra-p-tolylpentalenide p-Tol4Pn2-, 1,3,4,6-tetrakis(3,5-dimethylphenyl)pentalenide m-Xyl4Pn2-, 1,3-diphenyl-4,6-di-p-tolylpentalenide Ph2(p-Tol)2Pn2-, 1,3-bis(3,5-dimethylphenyl)-4,6-diphenylpentalenide Ph2(m-Xyl)2Pn2-, the first partially oxygenated pentalenide 1,3-bis(4-methoxyphenyl)-4,6-diphenylpentalenide (p-MeOPh)2Ph2Pn2-, the first partially fluorinated 1,3-bis(4-fluorophenyl)-4,6-di-p-tolylpentalenide (p-FPh)2(p-Tol)2Pn2-, the disubstituted 1,3-diphenylpentalenide Ph2Pn2- and 1,3-bis(4-fluorophenyl)pentalenide (p-FPh)2Pn2- and the regioisomers 1-(3,5-dimethylphenyl)-3-(4-fluorophenyl)-6-phenyl-4-(p-tolyl)pentalenide (m-Xyl)(p-FPh)(Ph)(p-Tol)Pn2- and 1-(3,5-dimethylphenyl)-3-(4-fluorophenyl)-4-phenyl-6-(p-tolyl)pentalenide (m-Xyl)(p-FPh)(p-Tol)(Ph)Pn2-. The introduction of methyl substituents was also investigated and required positional control in the dihydropentalene precursor to avoid exocyclic deprotonation during the metalation, resulting in another pentalenides species 1-methyl-3,4,6-triphenylpentalenide MePh3Pn2-. Manuscript in preparation.

Chapter Six
follows up the question whether yet another condensation approach could be developed to fulfil all requirements of a versatile pentalenide ligand synthesis protocol. Consequently, more attention was laid on the utilisation of dialkylated cyclopentadienes/cyclopentadienides (CpH/Cp-) towards dialkylated-diarylated PnH2. Base-promoted reactions between tBu2CpH and chalcone were screened and showed that performing the reaction in MeCN/tBuOH as solvent pair and KOtBu as base led to the isolation of diphenyl-di-tert-butyl substituted dihydropentalenes. However, this substitution pattern led to the most challenging scenario in PnH2 isolation yet, since a tetrasubstituted 1,5-dihydropentalene joined the isomerisation scheme between two mirrored tetrasubstituted PnH2. Attempted double deprotonation of isomer mixtures with heterogenous and therefore kinetically hindered bases like NaNH2 or KH led to yet another isomerisation and further deprotonation failed. This result implied the demand for a selective synthesis of tailored isomers, which was accomplished when utilising (un)substituted sodium cyclopentadienides (scheme 7.1). Subsequent deprotonation of the outcome with LiNEt2 led indeed to the synthesis of 1,3-diphenyl-4,6-di-tert-butylpentalenide (Ph2tBu2Pn2-). While anticipating a high polarisation of the core, the 1H NMR chemical shift difference between both hydrogen wingtip atoms showed only a value of 0.18 ppm, indicating a loss of coplanar cross-aromaticity between the phenyl groups and pentalenide core. The 1,3-diphenyl-4,6-dimethylpentalenide (Ph2Me2Pn2-) species, which was formed by double deprotonation of a tailored PnH2 isomer with LiNEt2 or via redox metalation with Fe2CO9, outperformed all other ligands in core polarisation by showing a 1H NMR chemical shift wingtip difference of 1.12 ppm (dilithium species) or 1.28 ppm (diiron pentacarbonyl species). The corresponding 1,3-diphenyl-4,6-diisopropylpentalenide (Ph2iPr2Pn2-) species was synthesised in an analogous way and showed 1H NMR resonances in between Ph2Me2Pn2- and Ph2tBu2Pn2-. Unsubstituted and monosubstituted sodium cyclopentadienides were tested as nucleophiles as well and the protocol allowed the synthesis of di- and trisubstituted dihydropentalenes, the latter forming exclusively in 1,3,5-/2,4,6-substitution pattern, which were easily further deprotonated towards the trisubstituted pentalenides Ph2MePn2- and Ph2iPrPn2-. As a final proof of concept of the modularity of the synthesis protocol towards full regiocontrol, the electrophiles were varied by utilising α,β-unsaturated aldehydes instead of enones, furnishing PnH2 species with 1,4,6- and 1,3,5-substitution pattern, with the former being deprotonated into the first reported 1,3,4-trisubstituted pentalenide species (tBu2PhPn2-).

Chapter Seven
summarises and concludes the investigations which are presented in this work and offers thoughts of the author of this thesis towards potential avenues of pentalenide chemistry, which the investigations of all preceding chapters evidently enable.

Chapter Eight
as a collection of appendices contains supplementary information for chapters 3─6, which might be useful for the reader.
Date of Award28 Jun 2023
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorUlrich Hintermair (Supervisor) & Michael Hill (Supervisor)

Cite this