Synthesis, Characterisation and Reactivity of Cobalt(II) and Cobalt(I) Cyclopentadienyl, Diphosphine and α-Diimine Complexes

Abstract

This thesis presents the synthesis and characterisation of cobalt(II) and (I) complexes bearing cyclopentadienyl, α-diimine and diphosphine ligands, to provide an understanding of their physico-chemical properties to enable their future use as pro-catalysts in, e.g. selective linear α-olefin (LAO) to LAO oligomerisation. There has been considerable recent interest of (P^P)cobalt(I) complexes in a range of catalytic transformations, however the mechanism of formation of the cobalt(I) species has been little studied.
Chapter 2 describes the attempted synthesis of a series of analogues to the reported linear α-olefin (LAO) to LAO dimerisation pro-catalyst [(η5-Cp*)Co(η2-C2H4)(P(OMe)3)] (1), which only exhibits very low activity (TOF 3.7 h–1). Analogues of complex 1 with general formula (L2X)Co(CO)2, where L2X is Cp or Cp* were synthesised (2.17a, 2.17b). Attempts to prepare derivatives where L2X was indenyl or fluorenyl, led to the formation of η1 or η2 complexes (Ind)2Co2(CO)6 (2.21), (Ind)2Co2(CO)4(THF)2 (2.25) and FluCo(CO)4 (2.26). To enhance the stability of pre-catalysts such as 1, Cp-based ligands with a pendant L-type donor (PR2, NR2), a series of (L2X)-CH2CH2-L ligands (L2X = Cp, Cp*, Ind, Flu, L = PR2, NR2) were synthesised. Cp and Cp* derivatives of these “tethered” ligands were derived from spiro[cyclopropane]precursors (2.16a,b); Ind and Flu derivatives were prepared from ClCH2CH2PR2 (2.7a-c). Subsequent complexation of Cp^PPh2 (2.15) to produce a cobalt half-sandwich complex (2.29) revealed the flexibility of 2.15 to coordinate in, e.g., mono-, bi-dentate or bridging modes.
In chapter 3 a series of variously-substituted α-diimine (3.2a-d) and diphosphine (3.3-3.10) cobalt(II) complexes are described and their electronic and steric properties probed by structural (XRD, %Vbur) and spectroscopic (UV-Vis, Raman, IR) methods. The donor capacity of the ligands was found to have a significant impact on the structural and electronic properties of the resulting complexes.
Chapter 4 explores the reduction of (P^P)CoX2 complexes (3.3-3.10) with Zn and the characterisation of the resulting cobalt(I) derivatives (4.1-4.8) of general formula [(P^P)Co(μ2-X)]2. This study showed that the outcome of the Zn-mediated reduction is governed by steric and electronic effects imposed by the P^P ligand. The reaction of (P^P)CoX2 (where phosphine substituent is not Ph) with Zn resulted in formation of (P^P)ZnX2 complexes (Zn-5.1-Zn-5.9). The mechanism by which Co-Zn diphosphine ligand exchange occurs was found to proceed via disproportionation of “(P^P)CoX.” The lability of the P^P ligand was found to play a key role in the formation of (P^P)ZnX2 complexes.
Chapter 5 describes development of an inert atmosphere approach for mass spectrometric analysis of air-/moisture-sensitive solid materials using the Atmospheric Solid Analysis Probe (ASAP), which is applied throughout this work. Chapters 6 and 7 contain all experimental and supporting information.