Copper(I) Complexes

So far, only the coordination of phosphorylanthracenes to gold(I) has been described in this thesis. Although several interesting features have been discovered in this class of metal complexes, the linear coordination geometry of the gold(I) complexes is also somewhat limiting. To expand the spectrum of complexed metals in order to investigate the coordination of phosphanylanthracenes to metals in other coordination geometries, the synthesis of copper(I) complexes was peered.

To synthesize a copper(I) complex, the corresponding metal ions must be dissolved in sufficient quantity, which is often a limiting factor. The precursor complex [Cu(PPh3)2BH4] shows very good solubility even in weakly or non-donating solvents and the PPh3 donors can be easily replaced which makes it a very potent starting material in complex synthesis. The un-oxidized MeAnP(NMe2)2 (33) was reacted with 0.5 equivalents of [Cu(PPh3)2BH4] in DCM at ambient temperature, then the mixture was heated to 35°C for 5 min. and then stirred at room temperature for 8 h. Storage of the reaction mixture at –30°C yielded [(MeAnP(NMe2)2)2CuBH4] (54) as yellow octahedral crystals (Scheme 3-32).

Scheme 3-33: Synthesis of [(MeAnP(NMe2)2)2CuBH4] (54).

Though an incomplete reaction under exchange of only one PPh3 ligand by the phosphanylanthracene is also thinkable, both PPh3 donors were replaced, yielding a dimer complex. The perfectly octahedral shape of the crystals is visible in Figure 3-80, which shows a crystal of 54 mounted on a glass fibre for diffraction experiments.

Although the precursor molecule MeAnP(NMe2)2 (33) is a highly viscous oil which could not be crystallized, the coordination to Cu(I) affords a solid metal complex. Due to the fixed orientation of the coordinated phosphanyl substituents, an ordered arrangement suitable for crystallization is generated.

The second driving force for crystallization can be derived from the crystal structure of 54. The formation of a dimer complex brings the π-systems of the donating phosphanyl anthracenes into close proximity to one another (Figure 3-80, left). The

resulting π-stacked arrangement appears to be energetically favored towards crystallization. Although the π systems show parallel orientation at a distance of 3.53 Å, the anthracene moieties are rotated against each other, rather than assuming a completely ecliptic conformation (Figure 3-80, right). This way an overlap of only ~40%

is achieved.

Figure 3-79: Crystal structure of

[(MeAnP(NMe₂)₂)₂CuBH₄] (54), hydrogen atoms are omitted for clarity.

Table 3-30: selected bond lengths [Å] and angles [°] of 54.

P1-Cu1 2.2589(4)

P1-C9 1.8499(12) Cu1-H101 1.769(17) P1-Cu1-P1’ 118,48(1) C9-P1-Cu1 112.60(4)

Folding 11.4

Twist 6.8

Figure 3-2: Crystal of 54.

Figure 3-81: π-π overlap in the crystal structure of 54, side view (left) and top view (right); amine bound methyl groups, borate ion and hydrogen atoms are omitted for clarity.

The asymmetric unit contains one half of the dimer complex, the other half is produced by symmetry operations. Both the copper atom and the borate anion are located on a mirror plane and are only present to ½ in the asymmetric unit. 54 crystallizes in the orthorhombic space group Pbcn. The angles of the P-bound substituents including the coordinated copper(I) ion range from 99.7° to 113.7°

indicating distorted tetrahedral geometry. The P-Cu bond exhibits the clearly highest spatial demand, as all angles to the P-Cu bond are larger than 112°, while the angles between all other substituents are smaller than 110°. The P-Cu bond measures 2.259 Å, which is very close to the average value of P-Cu bonds (Figure 3-81).

Moreover, the P1-Cu1-P1’ angle is close to 120° at 118.5° which is a major deviation from the 109.45° expected for tetrahedral coordination. This can be attributed to the large steric bulk of the donating phosphanylanthracenes, whose repulsion induces the observed widening of the P1-Cu1-P1’ angle. A further factor which may explain the large angle is the absence of further donors to fill the coordination sphere of the copper ion to its preferred coordination number of 4. Instead of coordination of two additional donors, there is only an interaction between two hydrogen atoms of the borate anion and the copper cation is present. Both Cu-H distances are symmetric and measure 1.769 Å, which indicates the strength of these interactions. Similar interactions have repeatedly been observed between borate anions and transition metal cations. Besides copper(I), interactions of this kind have also been found

between hydrogen and various early transition metal (Fe, Cr, Ni, Co, Zn) and late transition metals (Rh, Ir, Os, Nb, Ru, Re). As Figure 3-81 shows, the Cu-H distance found in 54 is distinctly shorter than the average non-hydride M-H bond. The H101-Cu1-H101’ angle is very small at only 63.9°, which deviates extremely from the tetrahedral angle preferred in the coordination of Cu(I). This phenomenon probably also contributes to the large P1-Cu1-P1’ angle. The fluorophore deformation is moderate and can be ranked between the deformations of the sulfur and selenium oxidation products MeAnPS(NMe2)2 (21) and MeAnPSe(NMe2)2 (25).

Figure 3-82: left: CSD statistics on P-Cu bond distances, value of 54 is marked (blue) (total average = 2.259 Å), right: CSD statistics on non-hydride transition metal/hydrogen interactions, value of

54 is marked (blue) (total average = 1.822 Å).

Overall, the change from gold(I) to copper(I) has made a completely different mode and geometry of metal coordination accessible, which afforded a π stacked dimer complex. A similar arrangement has been reported by Kubiak et al. who were able to synthesize a platinum(II) complex of their 9-anthrylphosphirane.^[76] The orientation of the fluorophores to one another is comparable to the one observed in 54, while the π-π distance of 3.40 Å is slightly shorter than the 3.53 Å found in 54. Additionally, the P-Pt-P angle of their cis-dichlorobis[1-(9-anthracene)phosphirane]platinum(II) is by approximately 20° smaller than the P-Cu-P angle of 54. This can be assigned to the preferred square-planar coordination of platinum(II), which produces angles close to 90° around the platinum atom. Moreover, the preferred coordination number of four is achieved by coordination of two chloride ions, which also strongly contrasts the

mere two Cu-H interactions found in 54. The smaller P-M-P angle is most likely also the reason for the smaller observed π-π distance in Kubiak’s platinum complex.