Metal exchange in lithiocuprates: implications for our understanding of structure and reactivity

New reagents have been sought for directed $\textit{ortho}$ cupration in which the use of cyanide reagents is eliminated. CuOCN reacts with excess TMPLi (TMP = 2,2,6,6-tetramethylpiperidide) in the presence of limited donor solvent to give crystals that are best represented as (TMP)$_2$Cu$_{0.1}$Li$_{0.9}$(OCN)Li$_2$(THF) 8, whereby both Lipshutz-type lithiocuprate (TMP)$_2$Cu(OCN)Li$_2$(THF) 8a and trinuclear (TMP)$_2$(OCN)Li$_3$(THF) 8b are expressed. Treatment of a hydrocarbon solution of TMP$_2$CuLi 9a with LiOCN and THF gives pure 8a. Meanwhile, formation of 8b is systematized by reacting (TMPH$_2$)OCN 10 with TMPH and $n$BuLi to give (TMP)$_2$(OCN)Li$_3$(THF)$_2$ 11. Important to the attribution of lower/higher order bonding in lithiocuprate chemistry is the observation that in crystalline 8, amide-bridging Cu and Li demonstrate clear preferences for di- and tricoordination, respectively. A large excess of Lewis base gives an 8-membered metallacycle that retains metal disorder and analyses as (TMP)$_2$Cu$_{1.35}$Li$_{0.65}$ 9 in the solid state. NMR spectroscopy identifies 9 as a mixture of (TMP)$_2$CuLi 9a and other copper-rich species. Crystals from which the structure of 8 was obtained dissolve to yield evidence for 8b coexisting in solution with $\textit{in situ}$-generated 9a, 11 and a kinetic variant on 9a ($i$-9a), that is best viewed as an agglomerate of TMPLi and TMPCu. Moving to the use of DALi (DA = diisopropylamide), (DA)$_2$Cu$_{0.09}$Li$_{0.91}$(Br)Li$_2$(TMEDA)$_2$ 12 (TMEDA = $N$,$N$,$N^\prime$,$N^\prime$ -tetremethylethylenediamine) is isolated, wherein (DA)$_2$Cu(Br)Li$_2$(TMEDA)$_2$ 12a exhibits lower-order Cu coordination. The preparation of (DA)$_2$Li(Br)Li$_2$(TMEDA)$_2$ 12b was systematized using (DAH$_2$)Br, DAH and $n$BuLi. Lastly, metal disorder is avoided in the 2 : 1 lithium amide : Lipshutztype monomer adduct (DA)$_4$Cu(OCN)Li$_4$(TMEDA)$_2$ 13.This work was supported by the U.K. EPSRC through grants EP/ J500380/1 and EP/K039520/1