Synthesis and Characterisation of Dimolybdenum and Dirhenium Quintuple Bond Precursors

The year 1964 saw the isolation of the first quadruply bonded complex which sparked the desire to prepare more of these compounds. It was in 1979 that theoretical scientists first proposed that a quintuple bond could exist with a D3h geometry of the formula M2L6 the desire to isolate these compounds was set in motion, it was later actualised in 2005 when the first quintuple bonded dichromium complex was isolated. This discovery led to the generation of more quintuple bond complexes using the group VI elements; chromium and molybdenum and a host of bidentate N-donor ligands however, no such complexes exist that use the group VII elements (which could theoretically form a quintuple bond as well) or a different type of ligand, such as a P-donor.

A number of quadruply bonded dimolybdenum and dirhenium complexes have been synthesised as precursors for the production of new quintuple bonded dimolybdenum complexes and the first dirhenium quintuple bond. The employment of bulky ligands, such as bidentate phosphine, formamidinate and guanidinate ligands were used to stabilise the quadruply bonded precursors and improve the stability of the target quintuply bonded complexes. Reactions between Mo2(O2CCH3)4 [1] and bis(diphenylphosphino)amine (Hdppa) and (NH4)4Mo2Cl8 [2] with 1,2-bis(diphenylphosphino)ethane (dppe) in THF and ethanol respectively successfully yielded the bis-substituted products: Mo2(Hdppa)2Cl4 [3] and Mo2(dppe)2Cl4 [4] respectively. Meanwhile reacting (TBA)2Re2Cl8 [5] with the respective formamidine or guanidine in DCB yielded Re2(dipf)2Cl4 [6] (dipf = N,N’-bis(2,6-diisopropylphenyl)formamidinate), Re2(tmpf)2Cl4 [7] (tmpf = N,N’-bis(2,4,6-trimethylphenyl)formamidinate) and Re2(tpg)2Cl4 [8] (tpg = 1,2,3-triphenylguanidinate). Attempts were made to prepare the bis substituted dimolybdenum complex Mo2(tpg)2Cl4, however it could not be fully characterised. The preparation of other dirhenium complexes utilising dicyclohexylbenzamidinate (DCyBA), diphenylformamidine (dpfH), an iminophosphonamide ligand and Hdppa as bridging ligands were attempted but yielded unsuccessful results.

UV-Vis spectra, IR spectra and cyclic voltammograms (CV) for complexes [3], [4], [6], [7] and [8] are reported and discussed in this thesis and why they have the possibility to form a metal-metal quintuple bond. The UV-vis spectra of the molybdenum complexes, [3] and [4] show δ → δ* transitions at 700 nm and 680 nm, respectively. The dirhenium complexes, [6], [7] and [8], exhibit δ → δ* transitions at 621, 615 and 628 nm, respectively.

The CV of [3] displays a broad irreversible reduction at an E1/2 value of -1.534, meanwhile, complexes [4] has two reversible one-electron reductions at E1/2 = -1.829 V and -1.943 V. For the dirhenium complex [6], two reversible one-electron reductions at E1/2 = -1.348 V and -1.623 V are seen. Complex [7] displays one broad, irreversible reduction at E1/2 = -1.898 V, meanwhile complex [8] had two one-electron reductions with one at E1/2 = -1.374 V being irreversible and the second reduction being reversible at an E1/2 value of -1.549 V.

Reductions using KC8 and potassium hexafluorophosphate (as a halide abstractor) were attempted on complexes [3], [4], [6], [7] and [8], but were all unsuccessful resulting in unresolvable 1H NMR spectra.

This thesis, while unsuccessful in isolating a quintuply bonded dimetal complex, has produced quadruply bonded dimolybdenum and dirhenium complexes that may still have the potential to be chemically reduced with cyclic voltammograms showing they can be electrochemically reduced.