Synthesis and reactivity of titanium ‘POCOP’ pincer complexes

Abstract

Titanium ‘POCOP’ complexes have been made, and their ability to support further reactivity investigated, giving a rare isolable titanium chlorohydride.

PCP Complexes of Titanium in the +3 and +4 Oxidation State

“…Very recently the deployment of PCP-type ligands onto early transition metals has been described by us and others. [15][16][17][18] Reacting n BuLi with C6H3-1-Br-2,6-(CH2P t Bu2)2 resulted in the formation of [ tBu PCP]Li. Others have described forming this in-situ but it can be isolated as a highly-reactive white solid in 81% yield.…”

“…This regular structure of 1 is in direct contrast to the equivalent 'POCOP' complex (C6H3-2,6-(O2P t Bu2)2TiCl3) which has an extremely distorted octahedron. 15 In order to probe the electronic structure of these two analogous complexes DFT calculations were performed. The optimised geometry of 1 reproduces the crystallographic data with good accuracy (Figure S1), the Ti-P bond lengths are 2.62 and 2.67 Å (vs. 2.679(2) and 2.675(2) Å in Figure 1), while the Ti-Cl bonds are 2.29-2.31 Å vs. their crystallographic counterparts of 2.268(2), 2.318(2), and 2.304(2) Å.…”

“…The single unpaired electron is predominantly located on the metal centre (evidenced by g0 = 1.9505 at room temperature, significantly reduced from that of a carbon-centred radical and similar to that found for the analogous POCOP complex). 15 There is however some spin-density on the P centres, which gives rise to the triplet form of the room temperature spectrum. Upon freezing the spectrum becomes rhombic, giving three overlapping triplets (Figure 2, right).…”

“…The bond lengths to the titanium centre are in line with those observed for 2, other 'PCP'Ti complexes, and other Ti(III) methyl complexes. 15,38 4 has a solution phase magnetic moment of 1.60 µB (Evans method). The continuous wave X-band EPR spectrum, as well as the simulated spectrum, is shown in Figure 6.…”

“…1 We recently synthesised a library of ( R POCOP)Ti complexes ( R POCOP = C6H32,6-(OPR2)2; R = tert-butyl, iso-propyl) and used this pincer framework to form early transition metal hydride complexes. 15 Others have recently reported ( R POCOP)Ti(CH2SiMe3)2 and ( R PCP)Ti(CH2SiMe3)2 ( R PCP = C6H32,6-(CH2PR2)2; R = tert-butyl) complexes as catalysts for styrene polymerisation. 16 Here we build on this work to further explore the ( R PCP)Ti motif, expanding it to Ti(IV) complexes, as well as presenting EPR data for known and new Ti(III) complexes.…”