Extending the Side-Arm

In order to further enhance the flexibility and adaptability of the new ligand system, the length of the side-arm can be changed. One way would be to use a phosphane with longer alkane substituents like ethane or butane. It is a problem though, that those groups are deprotonated at the CH2 group next to the phosphorus atom as those hydrogen atoms are the most acidic. This was clearly demonstrated by the synthesis of [Li{Et2PCH(Me)S(NSiMe3)2}]2 (7). Therefore, another synthetic route had to be explored. Morrow et al. reported the preparation of the diphenyl-1-alkinylphosphane Ph2PCCCH3 in 1969 (Equation 2-4), which should be a good starting material.^[75] The absence of CH2 groups should prevent side-reactions.

In order to lithiate the phosphane, a solution of tBuLi in pentane was reduced in volume and Ph2PCCCH3 added drop wise at room temperature. The orange precipitate was filtered and washed with pentane. Although the substance was poorly soluble, the NMR spectra showed to expected signals. The equimolar reactions with

S(NtBu)2 or S(NSiMe3)2 did not yield crystals or other uniform products even after months, therefore this route was abandoned.

H

1) nBuLi, rt 2) Ph₂PCl, 0 °C 1) - nBuH 2) - LiCl

Ph₂P

Equation 2-4: Preparation of Ph2PCCCH3.

Another way to elongate the donating side-arm would be to oxidise the phosphorus atom. This reaction is well known in our work group from the field of phosphanyl anthracenes. This substance class can be oxidised at the phosphorus atom with [H2O2 · (NH2)2CO], elemental sulphur and selenium.^[76] The reaction is of importance because some of the resulting phosphoryl anthracenes show solid state fluorescence when aromatic guest molecules are present in the crystal lattice and can therefore be employed as chemosensors.

For the oxidation of [Li{Me2PCH2S(NSiMe3)2}]2 (4) elemental sulphur was suspended in pentane and a solution of 4 in pentane was added slowly at -78 °C.

Figure 2-12: Molecular structure of [Li{Me2P(S)CH2S(NSiMe3)2}]2 (8). Hydrogen atoms are omitted for clarity.

After warming to room temperature and stirring over night, insoluble material was filtered off, the solution reduced in volume and stored at -25 °C for

crystallization. The product [Li{Me2P(S)CH2S(NSiMe3)2}]2 (8) crystallises in the monoclinic space group P21/n as colourless blocks (Figure 2-12). There is one dimer in the asymmetric unit which displays the same geometrical characteristics as all the other structures of that type. The lithium atoms are coordinated by three nitrogen and one sulphur atom, thus the side-arm is indeed elongated. The S4–Li1 bond length is 2.495(7) Å which is slightly shorter than the P1–Li1’ length of 2.665(2) Å in 4. First of all this is due to the fact that the sulphur atom is not as sterically hindered as the phosphorus atom because of the missing methyl groups. Second, it is the better donor for lithium because of its greater HSAB hardness.^[41] In addition, it can get in closer proximity because of the greater flexibility of the side-arm. The side-arm in 8 forms a six-membered ring in a boat conformation when it is coordinating to the lithium cation vs. a five-membered ring in 4, thereby reducing the steric strain in the system. It can also be seen that the central (LiN)2 ring and the diimido moieties intersect at a much smaller angle (42.2°) than all other structures of this type.

The P1–S3 bond of 1.9526(14) Å matches that of related structures like [(thf)Li{SP(NiPr)(NHiPr)2}]2 (P–S: 1.9927(8) Å)^[77], [(tmeda)Li{tBuN(S)P(μ -NtBu)2P(S)NHtBu}] (P–S: 1.978(2) Å)^[78] and the predicted value of 1.92 Å.^[67]

However it is slightly elongated due to the coordination of the lithium atom. Selected bond lengths and angles can be found in Table 2-5.

Table 2-5: Selected bond lengths [Å] and angles [°] in 8 and 9

8 9 8 9

S1–N1 1.599(3) 1.595(2) N1–S1–N2 107.56(16) 107.49(12)

S1–N2 1.602(3) 1.614(2) S1–C7–P1 118.6(2) 119.80(16)

S1–C7 1.828(4) 1.831(3) S3–P1–C7/Se1–P1–C7 116.29(14) 114.75(10) P1–C7 1.819(4) 1.822(3) P1–S3–Li2/P1–Se1–Li1 94.30(16) 91.28(11) P1–S3/Se1 1.9526(14) 2.1196(8) N1–Li1–N2/N1–Li2–N2 67.9(2) 68.25(16)

Li1–N1/N3 1.982(7) 1.994(5) N2–Li1–N4 132.7(4) 99.0(2)

N2–Li1 2.560(7) 2.018(5) Li1–N2–Li2 82.5(3) 80.8(2)

N2–Li2 1.989(7) 2.554(5) S3–Li2–N2/Se1–Li1–N2 104.7(3) 105.6(2) S3–Li2/Se1–Li1 2.470(7) 2.614(5) S3–Li2–N3/Se1–Li1–N3 115.4(3) 115.0(2)

All signals in the ¹H NMR spectrum of 8 show a downfield shift of approx.

0.4 ppm in comparison to the starting material 4. This could be due to the electron withdrawing effect of the sulphur atom at the phosphorus atom and the resulting

deshielding of the hydrogen atoms. Interestingly, a ²JLi–P coupling is not detected.

The ³¹P{¹H} shift of 27.05 ppm is in the expected region for oxidised phosphorus atoms although not very near to the reported value of 83 ppm for the olefin polymerisation catalyst [(C3H5)Ni{tBu2PCH2S(NSiMe2)}] which was already mentioned in the introduction of this chapter.^[63] Regarding the chemical shift of this catalyst, it is thinkable that is was oxidised during the reaction. This finding will be discussed in more detail in chapter 2.4.

The selenium analogue of 8 was synthesised in the same way, using grey selenium as a starting material. [Li{Me2P(Se)CH2S(NSiMe3)2}]2 (9) also crystallises in the monoclinic space group P21/n with the whole molecule in the asymmetric unit (Figure 2-13). The structure is isostructural to 8. The coordination of the lithium cations is similar to 8, with the selenium atoms taking part in the coordination.

Figure 2-13: Molecular structure of [Li{Me2P(Se)CH2S(NSiMe3)2}]2 (9). Hydrogen atoms are omitted for clarity.

The central (LiN)2 four-membered ring shows similar values for the bond lengths and angles. Nevertheless, the coordination of the SePCH2 side-arm is even weaker than in the corresponding sulphur compound with Li–Se bond lengths of 2.614(5) Å (Se1–Li1) and 2.593(5) Å (Se2–Li2) which are quite similar to the values found for [(thf)2Li{tBuN(Se)P(μ-NtBu)2PNHtBu}] (Se–Li: 2.605(10) Å)^[78]. Interestingly,

these bond distances are almost the same as the P1–Li1’ bond of 2.655(2) Å in the non-oxidised starting material [Li{Me2PCH2S(NSiMe3)2}]2 (4).

The ³¹P{¹H} signal of 6.23 ppm is shifted less downfield in comparison to 8 because of the lower electronegativity of selenium compared to sulphur. On the contrary this effect is not visible in the ¹H spectrum where the signals are shifted to even lower fields.

By oxidizing the phosphorus atom it was possible to extend the side-arm by one atom. It could be shown that the oxidation of phosphorus in the NSCP ligands with elemental sulphur or selenium is possible if the right conditions are chosen, which represents an additional possibility for ligand design. By this synthesis the coordinating side-arm can be lengthened which is essential if larger metal atoms are to be coordinated.