The stabilisation of low-coordinate and heteroleptic transition metal complexes

Since the middle of the last decade, many ¬m-terphenyl d-block complexes have been reported in the literature. Some of these have been shown to possess the qualities of single molecule magnets whilst others have been applied in the activation of small molecules and as catalysts in organic and inorganic transformations. Heteroleptic, m-terphenyl, d-block complexes are underrepresented in the literature, despite the potential such complexes could have in the applications detailed above. In the development, described herein, of reliable synthetic and purification procedures toward heteroleptic, transition metal complexes a range of previously reported m-terphenyl ligands has been utilised. In some instances, attempts have been made to combine these ligands with N-heterocyclic carbene ligands such as 1, 3-diisopropyl-4, 5-dimethylimidazol-2-ylidene (IPrMe) and 1, 3-bis (2, 6-diisopropylphenyl) imidazol-2-ylidene (IPr). A series of d-block and Group 12 complexes bearing one m-terphenyl or NHC ligand and a halide has been synthesised and have been characterised in the solid state and in solution. These Grignard analogues have been developed as intermediates in the multistep synthesis of heteroleptic complexes. The complexes 2, 6-Ar2C6H3Hg2, 6-Ar*2C6H3 (Ar = 2, 6-Xyl, Pmp or Dcp; Ar* = Mes or Tmp) have been synthesised, utilising 2, 6-Ar*2C6H3HgBr as an intermediate. The diamagnetic nature of mercury (II) allowed study using NMR techniques. 1H NMR spectroscopy suggests that the opposing ligands interact through space. Application of the synthetic route developed to other d-block and Group 12 metals affords 2, 6-Dcp2C6H3M2, 6-Mes2C6H3 (M = Mn and Zn) in low yields. The homoleptic by-products isolated through from the synthesis of these latter complexes and attempt synthesis of others indicates that ligand exchange occurs over the course of the reaction. This was confirmed through the study of reactions towards heteroleptic cadmium complexes using 1H and 113Cd NMR spectroscopy. Complexes of the type 2, 6-Dcp2C6H3M2, 6-Mes2C6H3 (M = Fe and Co) could not be isolated in our hands. Attempts to isolate the homoleptic complexes 2, 6-Dcp2C6H32M (M = Mn, Fe or Co) result in the desired 2, 6-Dcp2C6H32Mn (M = Mn), the biphenylene 1-2, 6-Dcp2C6H3-7-DcpC12H6 (M = Fe) and the Grignard analogue 2-2, 6- (Dcp2C6H3) 2C6H3-6-DcpC6H3CoBr (M = Co). These results demonstrate that iron and cobalt halides are able to couple the aryl halide flanking groups to the central lithium aryl of the ligand precursor. A series of metal (I) complexes, supported by one m-terphenyl ligand and one NHC ligand has been envisioned. It was hoped that this ligand system would alleviate the issues caused by the exchange of m-terphenyl ligands in the reaction mixture. To this end, complexes of the type (2, 6-Ar2C6H3) MX (IPrMe) n (Ar = Mes, n = 2, M = Mn; Ar = Mes, n = 1, Fe, Co, Zn, Cd, Hg; Ar = Dipp, n = 1, M = Fe, Co, Zn) have been developed. The reaction between 2, 6-Dipp2C6H3CoBrPrMe and KC8 affords the heteroleptic, two-coordinate cobalt (I) complex 2, 6-Dipp2C6H3CoBrPrMe, the first such complex stabilised by two different ligands binding through a carbon. Studies of the complex using Evans’ NMR techniques demonstrate that this complex possessed a magnetic moment at room temperature that is significantly higher than the value of the spin only d8 ion. Preliminary investigations into the ability of the complexes disclosed in this work to promote small molecule reactivity have been conducted and the results are discussed herein.