Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 2 / Section H / 32 / c

C and the dimer of D have the same composition, $\mathrm{C}{\mathrm{u}}_2\mathrm{L}{\mathrm{i}}_2(\mathrm{TMP}{)}_4$, but different metal ordering around an eight-membered metal–nitrogen ring. C has adjacent Cu sites and adjacent Li sites; in the conventional Gilman dimer D${}_2$, Cu and Li alternate around the four metal positions. D is represented by $\mathrm{Li}[\mathrm{Cu}(\mathrm{TMP}{)}_2]$ per formula unit. The salt by-product is LiCl.

A TMP anion has formula ${\mathrm{C}}_9{\mathrm{H}}_{18}\mathrm{N}$ and mass about 140.25. Therefore the mass per empirical unit is $2(140.25)+63.55+6.94\simeq 350.99$, giving $M_r\simeq 702$ for the dimer, consistent with $699\pm 5$. A monomer would be near 351 and is excluded. In C, inequivalent Cu/Cu, Cu/Li and Li/Li amide surroundings, combined with the constrained TMP orientations, permit the three methyl environments in the observed 1:2:1 integral ratio. D${}_2$ has the higher exchange-averaged methyl equivalence needed for one methyl signal. One signal need not imply that all instantaneous solid-state methyl environments are identical.

The remaining formulae follow from atom and charge balance:E can itself aggregate; its formula here specifies the copper-amide unit. F is a mixed organyl/amido Gilman cuprate, and G combines one F unit with one D unit. The distinct metal-ordering isomers and these mixed aggregates have experimental precedent.^[ref]

F${}_2$ is an eight-membered ring with sequence $\mathrm{Cu}-\mathrm{N}-\mathrm{Li}-{\mathrm{C}}_{\mathrm{ipso}}-\mathrm{Cu}-\mathrm{N}-\mathrm{Li}-{\mathrm{C}}_{\mathrm{ipso}}$, closed back to the first Cu. Each N belongs to TMP; each ipso carbon belongs to a phenyl group. Each Cu is approximately linear between one TMP nitrogen and one phenyl ipso carbon. Each Li contacts a TMP nitrogen and the phenyl group bound to the other Cu, so the dimer is assembled head-to-tail rather than from two independent separated ion pairs.

G has the eight-membered sequence $\mathrm{C}{\mathrm{u}}_1-{\mathrm{N}}_1-\mathrm{L}{\mathrm{i}}_2-{\mathrm{N}}_2-\mathrm{C}{\mathrm{u}}_3-{\mathrm{N}}_3-\mathrm{L}{\mathrm{i}}_4-{\mathrm{C}}_{\mathrm{ipso}}$, closed to Cu${}_1$. The three N atoms belong to three TMP groups and the carbon belongs to the single phenyl group. Both copper sites remain approximately two-coordinate. The Li next to phenyl can also interact with its aromatic $\pi$ surface; this secondary contact need not have the same hapticity as in F${}_2$. The figure gives a connectivity projection, not idealized square bond angles or an X-ray metric reconstruction.