Solid-state structures of non-alkylated nickel complexes

3.4 X-ray crystallographic characterisation of nickel complexes

3.4.1 Solid-state structures of non-alkylated nickel complexes

Crystals of non-alkylated nickel Schiff base complexes suitable for X-ray crystallographic investigation were obtained by slowly adding DMSO to suspensions of the complexes in small volumes (~ 2 mL) of solvents that they had limited solubility in, such as MeOH or water (Table 3.4). The suspensions were stirred at room temperature on a hot plate until sufficient DMSO had been added so that the nickel complexes dissolved. High quality crystals were obtained from the solutions by allowing them to slowly evaporate over a period of several days at room temperature.

H2O Acetone CHCl3

H23,24,25

H3 H22,26

H5 H16

H8,13

H6H15

H28,37

H29,38

H1H20

H31,35, 40,44

H32,34, 41,43

H33,42

134 Table 3.4 Conditions used to obtain crystals of non-alkylated nickel Schiff base complexes suitable for X-ray crystallography.

Complex

Mass of complex used

(mg)

Solvent(s) used to crystallise the nickel

complex

Time of crystallisation

(days)

(72) 21.2 MeOH/DMSO 7

(74) 21.8 MeOH/DMSO 14

(80) 20.3 MeOH/DMSO 14

(94) 25.5 MeOH/DMSO 19

Table 3.5 summarises the results of data collection and structure refinement processes performed on four complexes. Figure 3.22 presents ORTEPs for each of the complexes, and shows the atom numbering schemes used, while Table 3.6 shows selected bond lengths and bond angles. Solvent molecules were observed in the unit cells of each complex. Complexes (74) and (94) crystallised in the monoclinic crystal system and were assigned to the P21/n space group. In contrast, while (80) also gave monoclinic crystals, its space group was P21/c, and (72) crystallised in the triclinic crystal system, and was found to belong to the space group P1. All complexes crystallised in an asymmetric unit with four molecules per unit cell (Z = 4), with the exception of complex (72), for which Z = 2.

135 Table 3.5 Crystallographic data for non-alkylated nickel Schiff base complexes.

(72) (74) (80) (94)

Formula C29H24N2NiO4. 2(C2H6OS).H2O

C29H24N2NiO4.

(C2H6OS).3H2O C28H22N2NiO4.2(C2H6OS) C22H18N2NiO4.2(H2O)

Mr 697.52 655.41 665.47 469.14

Crystal system Triclinic Monoclinic Monoclinic Monoclinic

Crystal colour Orange Green Orange Orange

Space group P1 P21/n P21/c P21/n

a (Å) 9.9443 (2) 12.8742 (1) 13.1465 (1) 7.9887 (1)

b (Å) 10.8213 (2) 9.3175 (1) 14.9679 (1) 20.7141 (2)

c (Å) 15.0907 (3) 25.5985 (2) 16.4141 (2) 12.4586 (2)

 90.9180 (16) ---- ---- ----

 95.2472 (16) 98.4790 (7) 107.3576 (11) 98.920 (1)

 94.9377 (15) ---- ---- ----

V (Å3) 1610.64 (5) 3037.11 (5) 3082.81 (5) 2036.70 (5)

Dx (Mg m-3) 1.438 1.433 1.434 1.530

Z 2 4 4 4

(h,k,l)

-11<h<12 -13<k<13 -17<l<18

-16<h<15 -11<k<11 -31<l<28

-16<h<16 -18<k<18 -20<l<16

-9<h<9 -25<k<25

-15<l<15 Number of unique

reflections 6529 6155 6213 4095

Refinement Rint= 0.019 R[F2 > 2σ(F2)] = 0.038

Rint= 0.027 R[F2 > 2σ(F2)] = 0.030

Rint = 0.037 R[F2 > 2σ(F2)] = 0.036

Rint = 0.032 R[F2 > 2σ(F2)] = 0.038

136 Figure 3.22 Molecular structures of (72), (74), (80) and (94),with anisotropic displacement ellipsoids drawn at the 30% probability level. Colours:

white = hydrogen, black = carbon, blue = nitrogen, red = oxygen, green = nickel. The dotted line for (72) shows the position of atoms for the enantiomeric complex.

(72)

(74)

(80) (94)

137 The nickel atom in all four complexes adopted a square planar coordination geometry, with Ni–O and Ni–N bond distances and bond angles subtended at the metal centre generally close to expected values and those reported previously for similar complexes.[126, 127, 130] The one exception to this was for (74), which exhibited two O-Ni-N angles of ~ 172°, and slightly longer Ni–N bond distances.

These differences may be the reason for the green colour of (74), which contrasts with the red or orange colours of most other non-alkylated complexes, and may be a consequence of (74) being the only complex in Figure 3.22 to feature a diamine moiety that is part of a six-membered chelate ring.

One aspect of the structures of all novel complexes that was of particular interest was the degree of co-planarity between the different aromatic ring systems.

This was because it was expected that alkylated complexes which show a high degree of co-planarity might be able to -stack more effectively onto G-tetrads of G- quadruplex structures. In order to examine this further, the ring notation system illustrated in Figure 3.23 was used. This enabled the coplanar angles for different pairs of aromatic rings also included in Table 3.6 to be determined. Inspection of the data obtained shows that there was, as expected, a high degree of coplanarity between each of the ring systems in the bottom parts of the molecules. For example, the largest coplanar angle of this type was that observed between rings A and B in complex (80), which was 21.30°. For all of the other complexes in Table 3.6 the Ring A/Ring B coplanar angle ranged from 4.14° to 12.27°. In addition, aromatic rings C and D were found to exhibit a high degree of coplanarity with rings A and B, respectively, with the coplanar angles all < 11°.

138 Figure 3.23 Structures of complexes (71) and (80) illustrating the lettering system (A-F) used to discuss the degree of coplanarity of different rings in the solid state structures of the complexes.

In contrast to the above, Table 3.6 shows the aromatic rings in the top halves of the complexes (E and F) did not show a high degree of coplanarity with those in the bottom halves (C and D).

Table 3.6 Selected bond lengths (Å), bond angles (°) and coplanar ring angles (°) for (72), (74), (80) and (84).

Geometric parameters

Nickel complexes

(72) (74) (80) (94)

Ni1–O1 (Å) 1.8434 (12) 1.8385 (9) 1.8352 (12) 1.8536 (14) Ni1–O2 (Å) 1.8232 (13) 1.8313 (9) 1.8357 (12) 1.8313 (14) Ni1–N1 (Å) 1.8563 (17) 1.8761 (11) 1.8564 (14) 1.8396 (17) Ni1–N2 (Å) 1.8541 (16) 1.8944 (12) 1.8543 (14) 1.8602 (17) O1–Ni1–O2 (°) 83.37 (6) 82.72 (4) 85.25 (5) 84.37 (6) O2–Ni1–N2 (°) 95.00 (6) 92.48 (5) 93.87 (6) 94.10 (7) O1–Ni1–N2 (°) 178.33 (6) 171.65 (5) 175.57 (6) 178.22 (7) O2–Ni1–N1 (°) 177.62 (7) 171.67 (5) 175.46 (6) 178.07 (8) O1–Ni1–N1 (°) 94.55 (6) 92.46 (5) 93.83 (6) 94.57 (7) N1–Ni1–N2 (°) 87.09 (7) 93.08 (5) 87.38 (6) 86.99 (8)

N1—C8—C9—N2 (°) 52.1 (4) ---- 36.63 (17) 41.7 (2)

Ring A/Ring B (°) 4.14 13.27 21.30 6.65

Ring A/Ring C (°) 3.07 9.83 7.84 5.51

Ring B/Ring D (°) 1.59 10.13 5.67 5.56

Ring C/Ring E (°) 86.49 79.65 81.59 87.86

Ring D/Ring F (°) 87.34 73.68 84.18 ----

139 The crystal lattices of all four non-alkylated complexes were composed of molecules assembled in pairs, with three of the complexes ((72), (74) and (94)) possessing a crystallographic inversion centre. Only in the case of (72) were pairs of molecules packed in a slipped co-facial configuration, resulting in a separation of 4.535 Å between the average planes of the pairs (Figure 3.24). The crystal lattices of (72), (74) and (94) were supported by hydrogen bonds between the protons of water molecules and the oxygen atoms of the complexes, or between the H atoms of the OH groups on the complexes and the oxygens of water molecules. The interatomic O–H⋯O distances ranged from 1.76 to 2.32 Å. Since (80) had no water molecules in its crystal lattice, the hydrogen bond present instead involved the H atoms of the OH groups of one molecule of the complex, and O atoms of neighbouring (CH3)2SO molecules.

Figure 3.24 Packing of molecules of (72) in the crystal lattice.

4.535 Å

140