Complexes of Tp and Tp *

Synthesis Of [Tp*Cu(PPh

3

3

been reported that this is not the case above room temperature.^® This Cu(l) phosphine complex was prepared according to the method utilised by Kitajima.®^ CuCI, KTp* and PPhs were stirred together in CH2CI2, the desired product being

isolated as a white crystalline solid by filtration and recrystallisation from CH2CI2.

[Tp*Cu(PPh3)] also appeared to be light sensitive in the crystalline form under

nitrogen. Exposure to light caused the crystals to turn orange, with partial reversal of this phenomenon appearing to occur on subsequent exposure to air. However, since this effect did not appear to be relevant to aziridination, it was not investigated further. The unsubstituted pyrazolylborate analogue is structurally similar to the Tp* complex and is prepared in an identical manner. However, this complex did not appear to show the light-sensitivity observed in the Tp* analogue.

Synthesis Of [Tp*Cu

]2

]2

extensively dissociated in benzene solution and with the ^H NMR spectrum showing only three resonances, it is possible to suggest that dissociation and association are relatively rapid on the NMR timescale.^® However, the solid state structure shows that the pyrazolylborate group bridges the two copper atoms with two pyrazoles coordinating to one copper centre and the third pyrazole coordinating to the other copper centre (see Figure 2.1.2 (b)). In contrast to [Tp*Cu]2, however, [TpCu]2 is dimeric both in solution and in the solid state.®®

There are significant differences between the Tp and Tp* analogues of [TpCu^ and [TpCu(PPh3)] which will make the cause of any difference in aziridination

capability difficult to ascribe. In the case of the dimeric species, the Tp complex remains dimeric in solution, whereas the Tp* complex dissociates. The large difference between the two B-H frequencies (2510 cm'^ for [TpCu]2 compared

with 2441 cm'^ for [Tp*Cu]2) also suggests that the electron density on the latter

is likely to be higher, consistent with the introduction of electron-donating substituents on the ligand. For the phosphine complexes, the steric demand at copper will be greater for [Tp*Cu(PPh3)] than for [TpCu(PPh3)]. The B-H

stretching frequencies, however, suggest that there is also an electronic difference between the two complexes. The effect on the B-H frequency of coordination to a CuPPh3 unit for Tp* is almost double that for Tp. This suggests

greater o-donation from PPh3 in the latter complex as evidenced by a shorter

Cu-P bond.“ ’^°

Synthesis of fTp*Cu(CO)] Pyrazolylborate complexes were the first copper complexes to be able to bind carbon monoxide in a stable fashion and to be isolated in a pure state.®° [CpCu(CO)] had previously been synthesised, but it co-distilled with pentane.^^ Many copper pyrazolylborate carbonyl complexes have since been synthesised using variously substituted pyrazolylborate ligands.^^ [Tp*Cu(CO)] was synthesised by bubbling CO through a THF solution of CuCI and KTp* and was purified by recrystallisation from CH2CI2 and 40/60

petrol. The product was characterised by IR and ^H NMR spectroscopy, FAB mass spectrometry and elemental analysis.

Attempted Synthesis of [Tp*Cu(C

2

4

attempted according to the procedure outlined by Brookhart and Templeton, which involved adding KTp* slowly to a slurry of CuCI in CH2CI2 under a

saturated atmosphere of ethylene.®^ Although the complex could be identified from it's ^H NMR spectrum and its B-H stretch in the IR spectrum, it could not be isolated. The ethylene appears to be so weakly bound that exposure to either heat or vacuum causes dissociation of the ethylene followed by formation of the

dimeric complex [Tp*Cu] 2 mentioned above, and it is for these reasons that all

attempts at recrystallisation failed. The ethylene complex [Tp*Cu(C2H4)] could

also not be identified by NMR after bubbling ethylene through a solution of [Ip*C u]2. From this and observations by others on the relative reactivities of the

[Tp*CuL] complexes (L=PPh3, CO, G ulp*, C2H4), an order of Cu-L bond strength

can be proposed (Figure 2 .1.4 ).^®’®°®^®^

PPhs < CO ~ CuTp* < C2H4

Preparations of all Cu(l) trispyrazolylborate complexes were performed under an atmosphere of nitrogen since oxidation by atmospheric oxygen occurs easily, [Tp*Cu(PPh3)] apart. The relative air-sensitivity of the complexes can be used

empirically to support the order of bond strength proposed above. At room temperature in solution, [Tp*Cu(PPh3)] is air-stable whereas a solution of 50%

[Tp*Cu(C2H4)], 50% [Tp*Cu]2 became coloured almost instantaneously on

exposure to air. The oxidation products included an unidentified green complex, thought possibly to be [Tp*Cu(C0 3)CuTp*], and [CuTp*2]. Both [Tp*Cu(CO)] and

[Tp*Cu]2 have intermediate air-sensitivity.

Synthesis Of [C ulp *

2

cooling of the reaction mixture. The large blue crystals thus isolated were characterised by IR spectroscopy, El mass spectrometry and elemental analysis. The complex is described as an elongated rhombic octahedron, with the steric bulk of Tp* also causing distortions of the natural threefold symmetry of the ligands.^® Similar complexes with bulkier pyrazolylborate ligands are very rare and this steric saturation is often cited as the cause of the low reactivity at copper of this complex.®®