Extending motifs in lithiocuprate chemistry: unexpected structural diversity in thiocyanate complexes

Abstract

The new area of lithio(thiocyanato)cuprates has been developed. Using inexpensive, stable and safe CuSCN for their preparation, these complexes revealed Lipshutz-type dimeric motifs with solvent-dependent point group identities; planar, boat-shaped and chair shaped conformers are seen in the solid state. In solution, both Lipshutz-type and Gilman structures are clearly seen. Since the advent in 2007 of directed ortho cupration, effort has gone into understanding the structure-reactivity effects of amide ligand variation in and alkali metal salt abstraction from Lipshutz-type cuprates such as (TMP)2Cu(CN)Li2(THF) 1 (TMP = 2,2,6,6-tetramethylpiperidide). The replacement of CN(-) with SCN(-) is investigated presently as a means of improving the safety of lithium cuprates. The synthesis and solid state structural characterization of reference cuprate (TMP)2Cu(CN)Li2(THP) 8 (THP = tetrahydropyran) precedes that of the thiocyanate series (TMP)2Cu(SCN)Li2(L) (L = OEt29, THF 10, THP 11). For each of 9-11, preformed TMPLi was combined with CuSCN (2 : 1) in the presence of sub-stoichiometric Lewis base (0.5 eq. wrt Li). The avoidance of Lewis basic solvents incurs formation of the unsolvated Gilman cuprate (TMP)2CuLi 12, whilst multidimensional NMR spectroscopy has evidenced the abstraction of LiSCN from 9-11 in hydrocarbon solution and the in situ formation of Gilman reagents. The synthetic utility of 10 is established in the selective deprotometalation of chloropyridine substrates, including effecting transition metal-free homocoupling in 51-69% yield.

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

“…Of most interest is the observation that whereas the cores of 8a 2 and 8b 2 are essentially identical, the geometries of the 6-membered N 3 MLi 2 rings vary significantly. In 8a (M = Cu) the obtuse angle of 167.3(4)° at Cu1 results in a non-bonding N3···Cu1 distance of 2.93(1) Å. Insofar as this distance is consistent with a dicoordinate (lower) order Cu centre this bonding pattern is consistent with the recent characterization of Lipshutz cuprates 1a–c , 17 , 18 , 21 halogenocuprates 2–4 ( ref. 19 and 20 ) and thiocyanatocuprates 5a–c .…”

“…This core is supported by two 6-membered N 3 MLi 2 rings, within which the coordination sphere of Li3 is completed by THF. However, in contrast to the previously noted THF-solvated thiocyanate 5b 2 , 21 8 2 reveals not a boat conformation but an essentially planar arrangement, with only TMP and THF deviating significantly from a plane defined by the three conjoined metallacycles and the O(THF)–Li bonds (ESI, Fig. S1 † ).…”

“…19 and 20 ) and thiocyanatocuprates 5a–c . 21 In contrast, Li1 in the N 3 Li 3 metallacycle of 8b is located significantly closer to N3 than is Cu1 in 8a , resulting in a reflex N1–Li1–N2 angle of 186.6(5)°. The observation of a N3–Li1 2.597(11) Å distance suggests a weak interaction between these two centres and yields a motif reminiscent of intercepted ladder-type structures reported in the past for amidolithium aggregates.…”

“… 32 The ability to prepare 7 offers the possibility of analyzing the cyanate analogue of recently reported (TMP) 2 Cu(SCN)Li 2 (THF). 21 A hexane solution of TMPLi was therefore reacted with CuOCN in a 2 : 1 ratio in the presence of THF (1 eq. wrt Cu).…”

“…In these structures, significant solvent effects were noted through the adoption of either planar, boat or chair conformers for the 8-membered core depending on whether the cuprate was additionally stabilized by OEt 2 , THF or THP, respectively. 21 …”