is not only essential for their usability and reactivity but which also underpins their amenity towards structural characterization by enabling crystallization (Scheme 1.24).
It has been recognized for some time that the Cu : Li stoichiometry employed in cuprate formation offers a profound structural impact upon the resulting complex. Two fundamentally different types of cuprate have been recognized in consequence; so‐called ‘lower‐order’ and ‘higher‐order’ forms. The former are characterized by two‐coordinate Cu, the latter by Cu bearing a higher coordination number. Early structural data was gathered largely in the solution‐state, where evidence from vapour pressure depression, 1H NMR spectroscopy and solution X‐ray scattering all lent weight to the dominance of cyclic dimers [128]. It was not until the 1980s that the first reports on the X‐ray structures of lithium cuprates appeared. However, these revealed atypical copper‐rich anions. The synthetic utility of phenylcuprates has been alluded to above, and the first clusters of these species to be characterized, [Ph6Cu5][Li(THF)4] 96 [129], [Ph6CuLi4][Li(Et2O)4] 97 [130] and [Ph6Cu3Li2]2[Li4Cl2(Et2O)10] 98 [131], were obtained by reacting PhLi with CuBr and CuCN, respectively. The cuprate moieties in these SIPs revealed the same fundamental architecture, based upon a compressed trigonal bipyramidal arrangement of metal atoms in which the apical sites could be considered to bridge three [Ph2Cu]− units (Figure 1.11a) [132]. The subsequent isolation of the neutral phenylcuprate dimer of Ph2CuLi(Et2O) 99 (whose structure could be derived from [Ph6Cu3Li2]− by the formal replacement of one [Ph2Cu]− unit by Et2O (Figure 1.11b)) [133] lent support to this interpretation. Similar aggregates have also been reported where dimethyl sulfide replaces Et2O [134].
Scheme 1.24 Synthesis of lithium dimethylcuprate 83.
Figure 1.11 Structures of phenylcuprate species (a) [Ph6Cu3Li2]− in 98 and (b) [Ph2CuLi(Et2O)]2992.
Sources: Adapted from Hope et al. [131]; Lorenzen and Weiss [133].
Structures of complexes such as 96 [129] and 97 [130] exhibit unusual ‘higher‐order’ copper centres that act as bridges towards lower‐order [Ph2Cu]− units. On the other hand, compounds like Ph9Cu4Li5(SMe2)4100 [134] and Ph5Cu2Li3(SMe2)4101 [135] incorporate [Ph3Cu]2− units as one of the primary cuprate moieties, making a straightforward higher‐order description more appropriate (Figure 1.12a). Indeed, recent work has shown that a higher Cu coordination number is attainable in spirocuprate (biph)2CuLi3(THF)6102 (biph = 2,2′‐biphenyl, Figure 1.12b), with Cu now displaying a remarkable distorted tetrahedral geometry [dihedral angle between cuprocycles = 84.1(1)°] [136].
Moving from simple phenylcuprates, the use of aminoaryl ligands capable of providing internal coordination has enabled the isolation of neutral (DMBA)2CuLi 103, whose dimeric structure revealed a near‐planar arrangement of alternating Cu and Li atoms, bridged by aryl ligands and with only the Li centres interacting with the pendant amine functions (Figure 1.13a) [137]. In this case, the bridging mode of the aryl ligand differed from earlier reports on the tetramer of similar organocopper species [2‐(Me2NCH2)C6H4‐Me‐5]Cu 104 [138], its asymmetric nature suggesting primary σ‐type interaction of the C‐based sp2 lone pair with Cu (C–Cu 1.942(3) and C–Li 2.385(6) Å, respectively). A much more pronounced contrast in σ/π‐bonding has been reported in (Mes)2CuLi 105, where dimerization results in each Li centre adopting both η1 and η6‐coordination towards mesityl groups, leaving Cu free to adopt a preferred linear geometry (C–Cu–C = 178.34(7)°, Figure 1.13b) [139]. The dominance of Li…π interaction here can be attributed to the absence of donor solvent in the structure.
Figure 1.12 Selected higher‐order cuprates (a) Ph5Cu2Li3 (SMe2)4101 and (b) (biph)2CuLi3(THF)6102 (biph = 2,2′‐biphenyl).
Sources: Adapted from Olmstead et al. [135]; Liu et al. [136].
Figure 1.13 Molecular structures of the dimers of (a) (DMBA)2CuLi 103 and (b) (Mes)2CuLi 105.
Sources: Adapted from Van Koten et al. [137]; Davies et al. [139].
Simple alkylcuprates of the type routinely used in synthesis have proved difficult to study due to their relatively low thermal stability. In 1984, the crystal structure of SIP [{(Me3Si)3C}2Cu][Li(THF)4] 106 was described, providing the first solid‐state evidence for the structure of a dialkylcuprate (Figure 1.14a) [140]. Crystallography revealed linear, two‐coordinate copper, though the possibility that these features were imposed by the steric bulk of the anion could not be excluded. Other breakthroughs in the alkylcuprate field have included characterization of the polymer of (Me3SiCH2)2CuLi(SMe2) 107 [141], a structure consisting of dimeric units (similar to those seen in the structure of 99 joined by SMe2 ligands. Meanwhile, only two lithium dimethylcuprate structures have been reported for reagents; SIPs [Me2Cu][Li(12‐crown‐4)2] 108 [142] and [Me2Cu][Li(DME)3] 109 [143] (DME = 1,2‐dimethoxyethane). Recently, these have been added to by a possible pre‐reaction π‐complex (fluorenone)CuMe2Li(THF)3110 (Figure 1.14b) [144]. In contrast to the linear cuprate ion geometry observed in the first two cases, the C=O π‐complex in Figure 1.14b reveals a C–Cu–C angle of 104°. Ion separation would appear to be
