Crystal structure and synthesis of the bis(anthracene)dicuprate dianion as the dipotassium salt, [K(tetrahydrofuran)2]2[{Cu(9,10-η2-anthracene)}2], the first anionic arene complex of copper

An unprecedented example of an anionic (arene)copper complex has been prepared and structurally characterized in the salt [K(THF)2]2[Cu2(anthracene)2] (THF is tetrahydrofuran), which contains the first homoleptic anthracene complex of a d- or f-block metal bound only to the central ring of anthracene.


Introduction
Our interest in the stabilization of 'naked' atomic anions of d-block elements as homoleptic (arene)metallates (Ellis, 2019), where the arene is often a polycyclic aromatic hydrocarbon or polyarene, especially naphthalene or anthracene, led to an examination of 'anionic copper' (Rieke et al., 1990) or 'Cu 1À ' (Stack et al., 1993), reported more than 30 years ago.In these studies, the assumed, but never isolated or characterized, cuprate species was generally prepared in tetrahydrofuran (THF) at subambient temperatures, ca 170 K, by addition of soluble Cu I halide complexes to two equivalents of lithium naphthalene (LiNp).Although the cuprate was originally speculated to be a copper analog of the monoatomic gold anion, Au 1À , established to be present in cesium auride, CsAu (Knecht et al., 1978), and subsequently observed in single crystals of [Me 4 N][Au], wherein Au 1À has about the same ionic radius as Br 1À (Dietzel & Jansen, 2001), the bona fide atomic copper anion has only been identified in the gas phase (Hotop et al., 1973).Indeed, to our knowledge, no substance containing copper in a formal negative oxidation state is known in a condensed phase, possibly except for the cryogenic species, [Cu(CO) n ] 1À (n = 1, 2, or 3), which have been proposed to exist in solid neon at 4-10 K (Zhou & Andrews, 1999).Based on prior studies of compounds containing naphthalene-stabilized transition-metal anions (Ellis, 2006), we believed that these highly thermolabile solutions might contain presently unknown homoleptic (naphthalene)cuprates (Davies, 2011).Owing to the extreme thermal instability of these solutions above 170 K, our attempts to isolate or characterize products from these reactions have failed to date.However, recognition of the usual greater thermal stability in solution and/or the solid state of (anthracene)metallates of d-block elements, compared to formally analogous (naphthalene)metallates (Kucera et al., 2022), led to conducting previously unreported reactions of copper(I) halides with alkali metal anthracene radical anions, MAn (M = Li, Na, or K; An = anthracene), in THF.Also, because stoichiometrically analogous naphthalene and anthracene complexes of a given d-block element may possess very similar molecular structures, particularly in the solid state (Ellis, 2019), identification of an (anthracene)cuprate would be of substantial interest in shedding light on the possible nature of the previously reported 'anionic copper' (Rieke et al., 1990) or 'Cu À 1 ' (Stack et al., 1993).
Although 'anionic copper' was often prepared by the reaction of the lithium naphthalene radical anion with triorganophosphane adducts of copper(I) halides, [CuX(PR 3 )] n (X = Cl, Br, or I; R = phenyl or n-butyl) (Rieke et al., 1990), owing to the particularly good solubility in THF and accessibility of [CuCl(PCy 3 )] 2 (Cy = cyclohexyl) (Churchill & Rotella, 1979), this was the only copper precursor employed in our initial study reported herein.Also, we elected not to use triphenylphosphane (PPh 3 ) adducts of copper(I) halides due to the ease with which coordinated or free PPh 3 and mixed tertiary aryl-alkyl phosphanes undergo reductive cleavage of P-C(aryl) bonds, unlike trialkylphosphanes (Chou et al., 1986).Although reactions of [CuCl(PCy 3 )] 2 with four equivalents of MAn (M = Li, Na, or K) in THF appear by NMR spectra to afford similar products in solution, owing to the facile isolation and crystallization of the potassium salt, only the latter will be described now.Thus, the addition of a colorless solution of [CuCl(PCy 3 )] 2 in THF to a dark-blue solution of KAn (molar ratio: 2 KAn/Cu) in THF at 200 K, led to the formation of an air-sensitive but thermally stable (at 293 K) yellow-brown solution, from which yellow crystalline blocks of [K(THF) 2 ] 2 [{Cu(9,10-� 2 -C 14 H 10 )} 2 ], 1, were isolated (Fig. 1).
Single-crystal X-ray diffraction (SCXRD) characterization of 1 confirmed the presence of an (anthracene)cuprate, the first anionic arene complex of copper.However, unlike all previously known homoleptic (anthracene)metallates of the d-block elements (Ellis, 2019), including the recently reported bis(anthracene)divanadate(1À ) (Kucera et al., 2022), which contain metals coordinated only to terminal rings, the organocuprate unit in 1 contains Cu atoms bound to the 9,10carbons of the central ring of anthracene.Mononuclear heteroleptic complexes of d-and f-block elements containing M(9,10-� 2 -anthracene) moieties have been structurally authenticated for the Group 3 elements scandium (Ellis et al., 2018;Ghana et al., 2020;Zhu et al., 2023) and lutetium (Roitershtein et al., 1992(Roitershtein et al., , 1993)), and the f-block elements thulium (Fedushkin et al., 2001) and thorium (Yu et al., 2020).Unique uranium anthracene complexes containing individual metals bound to both the terminal and central rings of different anthracene ligands have been published recently (Murillo et al., 2021(Murillo et al., , 2022)).Dinuclear complexes of scandium (Huang et al., 2011(Huang et al., , 2014)), yttrium (Fryzuk et al., 2000), and iron (Hatanaka et al., 2012) containing bridging anthracenes, in which one metal binds to the terminal ring and the other anti to the central ring of anthracene, are also known.However, none of these species are of particular relevance to 1 and will not be discussed further.Although d-block complexes with � 6 -coordination of the central ring of untethered specially substituted anthracenes, for example, 2,3,6,7-tetramethoxy-9,10-dimethylanthracene, have been confirmed recently (Karslyan et al., 2017;Kuchuk et al., 2019), no d-or f-block metal is known to afford an anthracene or other arene complex having the architecture of the organodicuprate present in 1.
The structure of 1 (Fig. 1) consists of a contact ion-pair complex in which two anthracene ligands, distinctly folded about the 9,10-carbons [fold angle of 36.06 (6) � , calculated from all C atoms in the anthracene ligand] are present as centrosymmetric anthracene dianions bridging two equivalent Cu atoms in a near linear fashion across the 9,10-carbons of symmetry-related rings.Equivalent [K(THF) 2 ] 1+ counter-ions interact weakly with essentially planar terminal exo-benzene units (mean deviations from planarity of 0.011 and 0.006 A ˚for rings C1-C4/C14/C13 and C5-C8/C12/C11, respectively) on 1.Details of these interactions will be presented in Section 3.
To the best of our knowledge, only the long-known dimeric organoalanate complexes [{(AlR 2 )(arene)} 2 ] 2À [R = methyl; arene = naphthalene, 2 (Brauer & Stucky, 1970), and anthracene, 3 (see Fig. 2; Brauer & Stucky, 1972)] possess molecular structures in the solid state very similar to that observed for the organocuprate dimer in 1, vide infra.In particular, the coordinated anthracenes in both the cuprate, 1, and the alanate, 3, are effectively functioning as 9,10-dihydro-9,10anthrylene dianionic units with about one negative charge on each of the mixed sp 2 /sp 3 -hybridized 9,10 bridgehead carbons.Thus, the respective copper and aluminium anthracene complexes contain formally Cu 1+ and Al 3+ , both closed-shell ions.One major difference in the molecular structures of the anionic components of 1, 2, and 3 is the coordination environment about the metals.Whilst the four-coordinated Al atoms in 2 and 3 adopt distorted tetrahedral geometries, the dimeric structure observed for organocuprate 1 contains identical two-coordinate Cu atoms with a nearly linear C-Cu-C angle of 174.74 (7) � .The substantially different steric impacts of the dimethylaluminium groups in 3 compared to the Cu atoms in 1 are responsible for some dramatic and surprising differences in the molecular structures of these two stoichiometrically similar contact ion-pair complexes (see Section 3).It should also be pointed out that dimeric neutral nonmetal or metalloid adducts of anthracene are known, which have molecular structures similar to the dimeric anion in 3, including anthracene-based macrocyclic diphosphanes, [{(PX 2 )(9,10-� 2 -An)} 2 ], with X = Cl (Velian et al., 2014) and phenyl (Riu et al., 2020).Indeed, the oldest example, now known as the photodimer of anthracene, di-para-anthracene, was first reported nearly 160 years ago (see Bouas-Laurent et al., 2000) and forms as poorly soluble microcrystals when solutions of anthracene in benzene, toluene, etc., are exposed to sunlight/UV radiation.Structural characterization by SCXRD showed that the labile dimer arises by symmetrical coupling of the 9,10-carbons on two anthracenes, where the bridgehead C-C distance [1.624 (3) A ˚] is long (Choi & Marinkas, 1980), resulting in a low dissociation energy of the dimer, calculated to be 9 � 3 kcal mol À 1 (Grimme et al., 2006).Interestingly, 1 may be considered formally to be a molecule in which atomic copper anions have been inserted into each of the two bridgehead C-C bonds of the anthracene dimer, thereby increasing the separation of the bridgehead C atoms from 1.624 (3) to 3.98 (1) A ˚.
Unperturbed mononuclear homo-diorganocuprate complexes, [CuR 2 ] 1À (R = alkyl, aryl, etc.), also have close to linear C-Cu-C units and are formulated to contain two carbanions bound to Cu I by quite polar Cu-C � bonds, where the Cu-C distance in 1 [1.989 (2) A ˚] is well within the range of 1.83-2.05A ˚observed previously for homoleptic diorganocuprates (Davies, 2011).Although Cu I commonly adopts coordination numbers 2-4 in mononuclear complexes, organodicuprate 1 contains two quite bulky dianionic hydrocarbyl units, which appear to favor the formation of the observed structure containing two-coordinated Cu I with essentially linear C-Cu-C units, as expected for d 10 Cu I (Cotton et al., 1999).Owing to the latter, the separation of the 9,10-carbons on individual anthracene groups in 1 effectively defines the Cu� � �Cu distance of 2.6172 (7) A ˚. Prior studies suggest that d 10 Cud 10 Cu interactions are very weak to non-existent when the Cu� � �Cu separation is greater than �2.50A ˚in molecular entities containing Cu I complexes (Mehrotra & Hoffmann, 1978;Cotton et al., 1988;Merz & Hoffmann, 1988).
Although d 10 Cu-d 10 Cu or 'cuprophilic' interactions (Harisomayajula et al., 2019) appear unlikely to stabilize the dimeric structure adopted by 1 in the solid state, the significant contact ion-pairing and resulting increase in its crystalline lattice stabilization may play a key role in the formation of the observed bis(anthracene)dicuprate(2À ) complex, relative to unknown monomeric monoanions, i.e. [{Cu(THF) x }(9,10-� 2 -An)] 1À (x = 0-2), which would likely be strained metallacycles, vide infra.In the case of the structurally related organoaluminium complexes 2 and 3, it was proposed that the dimeric, rather than plausible monomeric structures, 'are favored by smaller deviations from tetrahedral angles about the aluminum atoms in the dianions rather than about the aluminum atom in the hypothetical monoanions.'However, surprisingly, the possible importance of contact ion-pairing in contributing to the stabilization of the dimers in the solid state, and possibly also in solution, was not considered (Brauer & Stucky, 1972).Most interestingly, the bis(naphthalene)dialanate(2À ) salt, 2, was originally prepared and proposed to be a monomer, [{AlMe 2 }(1,4-� 2 -Np)] 1À (Np = naphthalene), based only on a proton NMR spectrum in THF and identification of 1,4-dihydronaphthalene as a key hydrolysis product (Lehmkuhl, 1966).Later, the identical salt, as crystalline 2, dissolved in THF, was found to afford a proton NMR spectrum in good agreement with that of the purported monomer (Brauer & Stucky, 1970).In both SCXRD studies on 2 and 3, the possibility that they could be present as monomers in solution appears to have been implicitly rejected (Brauer & Stucky, 1970, 1972).However, the later structural authentication of an (anthracene)aluminium monomer, the neutral metallacycle [{AlEt(THF)}(9,10-� 2 -9,10-bis(trimethylsilyl)anthracene)], 4, strongly suggests that the natures of 2 and 3 in solution merit reinvestigation.It should also be emphasized that presently we cannot rule out the possibility that monomeric forms of 1 could be present in THF or other solvents (see Section 2).
The structure of the alane monomer 4 is consistent with that expected of a strained metallacycle, vide infra, i.e. a sharp C9-Al-C10 angle of 81.5 � for four-coordinated aluminium The molecular plot of 3.
and an average Al-C(bridgehead) distance of 2.057 (4) A ˚, which is significantly longer than the normal Al-C(Et) distance of 1.966 (4) A ˚present in 4 (Lehmkuhl et al., 1985).Structurally similar (anthracene)alane and (anthracene)alanate(1À ) monomers have been prepared recently by novel additions of anthracene to an Al I complex (Bakewell et al., 2020) and a dialkylaluminyl anion (Sugita et al., 2020), respectively.
A more well-known related complex is the monomeric (anthracene)Mg solvate in crystalline [{Mg(THF) 3 }(9,10-� 2 -An)], 5, possessing a C9-Mg-C10 angle of 71.4 � .This angle is significantly sharper than the corresponding angle in 4, likely owing in part to the higher coordination number of Mg in 5.The latter also contains a rather long Mg-C distance of 2.30 (2) A ˚ (Engelhardt et al., 1988) compared to that present in [(9-anthracenyl)Mg(�-Br)•di-n-butyl ether] 2 , a dimeric Grignard reagent with bridging bromides and an essentially unstrained Mg-C bond with a distance of 2.132 (2) A ˚ (Bock et al., 1996).For related reasons, 5 appears to be the first (anthracene)metal complex to be recognized as a strained metallacycle (Bogdanovic ´et al., 1987).In solution, 5 is a metastable species, which following facile loss of THF in hexane or diethyl ether, readily eliminates free anthracene and is proposed to initially form 'quasi-atomic' magnesium under strictly anaerobic conditions (Bo ¨nnemann et al., 1983).The latter rapidly forms a mirror of magnesium metal in the absence of other reactants (Alonso et al., 1987).Compound 5 has been found by the Cummins group (Velian & Cummins, 2012) and others to be a highly useful precursor to numerous new strained monomeric main group element adducts of anthracene, including the formal Ge IV complex [{GeMe 2 }-(9,10-� 2 -An)], 6, or 2,3:5,6-dibenzo-7-dimethylgermanorbornadiene.X-ray structure characterization of 6, Me 2 GeAn, revealed a sharp C9-Ge-C10 angle of 77.72 (5) � , and Ge-C(9,10 or bridgehead) distances [average 2.030 (1) A ˚] which are significantly longer than the Ge-C(Me) distance [average 1.943 (4) A ˚] (Velian et al., 2015).The latter is statistically identical to that observed for Me 4 Ge [1.945 (3) A ˚; Hencher & Mustoe, 1975].Compound 6 suffers thermal loss of anthracene at 373 K in toluene to produce in good yield an intriguing dimethylgermylene, Me 2 Ge, adduct [{Ge 2 Me 4 }-(9,10-� 2 -An)], 7, which may arise via insertion of highly reactive Me 2 Ge into a strained Ge-C(bridgehead) bond of 6.However, also possible is an initial dimerization of Me 2 Ge to the digermene, Me 4 Ge 2 , followed by its facile [4 + 2] cycloaddition to free anthracene, which was established previously to afford 7 (Sakurai et al., 1982).Also noteworthy is that the thermolysis of two equivalents of 6 to form the less strained 7 and free anthracene has been calculated to have a quite favorable free energy change (�G) of ca À 37 kcal mol À 1 of 7 (Velian et al., 2015).Finally, under milder conditions, 6 has been shown to function as an Me 2 Ge group transfer reagent (Geeson et al., 2019).
To the best of our knowledge, none of the reported formally strained metallacyclic monomers, [{ML n }(9,10-� 2 -An)] z (z = 0 or À 1), are presently known to form dimers (or oligomers) related to 1, 2, or 3. Calculations on the structures and relative stabilities of the monomeric forms of 1, 2, and 3, compared to the respective dimers, would be of considerable interest.Inclusion of contact ion-pairing could be of key importance in such a study, but might prove to be a nontrivial extension.

Synthesis and crystallization
All manipulations were carried out under argon in a standard glove-box and/or using Schlenk techniques to maintain strictly anaerobic conditions (Shriver, 1969;Wayda & Darensbourg, 1987).Solvents were dried by standard methods, as described previously (Brennessel & Ellis, 2012).Reagentgrade anthracene (99%) was sublimed in vacuo and [CuCl(PCy 3 )] 2 (Cy = cyclohexyl) was prepared as described previously (Churchill & Rotella, 1979).NMR samples were sealed under argon into 5 mm tubes and analyzed on a Varian Unity 500 MHz or a Bruker Avance III 400 MHz spectrometer. 1 H and 13 C chemical shifts are given with reference to residual 1 H and 13 C solvent resonances relative to tetramethylsilane.

[K(THF) 2
] 2 [{Cu(C 14 H 10 )} 2 ] (1).Sublimed anthracene (0.939 g, 5.27 mmol) and shiny pieces of potassium metal (0.211 g, 5.40 mmol) were transferred in an argon-filled glovebox to a round-bottomed Schlenk flask, along with a glassenclosed magnetic stirrer bar.Subsequently, THF (100 ml) was added and the mixture was stirred vigorously in the dark for 6 h at 293 K to afford a deep-blue solution of potassium anthracene (KAn), which is susceptible to photo-oxidation by visible light.This solution/slurry was cooled to 200 K with stirring and to it was transferred via cannula a cold (200 K) colorless solution of [CuCl(PCy 3 )] 2 (1.000 g, 1.318 mmol) in THF (50 ml) and stirring continued for 12 h at 200 K.The resulting yellow-brown solution was warmed over a ca 6 h period to near 290 K (room temperature) and filtered (medium or P4 porosity frit) to remove KCl.After careful evaporation of all but about 20 ml of solvent in vacuo at 273-293 K, pentane (100 ml) was added with stirring.The resulting slurry was filtered, washed vigorously with pentane (2 � 10 ml) and dried in vacuo to afford 0.59 g (53% yield, based on the copper precursor) of homogeneous yellow solid [K-(THF) 2 ] 2 [{Cu(C 14 H 10 )} 2 ], 1.No elemental analysis was conducted on the bulk solid 1, so its composition is based exclusively on the SCXRD study and NMR spectra.X-ray-quality yellow blocks of 1 were grown from a pentane-layered saturated solution in THF at 240 K over a 6 d period.
It is interesting that the first structurally authenticated (by SCXRD) monomeric (anthracene)metal complex, [{Mg-(THF) 3 }(9,10-� 2 -An)], 5 (Engelhardt et al., 1988), was originally formulated to be a monomer based only on its NMR spectra in THF.Noteworthy is that the latter spectra, without THF, exhibit 1 H and 13 C{ 1 H} resonances for the coordinated anthracene in 5 with quite similar values to those observed for 1; i.e. 1 H NMR (400 MHz, 293 K, THF-d 8 , �, ppm): 3.51 (s, 1H, H9), 5.95 (m, 2H, H1), 6.01 (m, 2H, H2); 13 C{ 1 H} NMR (75.4 MHz, 293 K, THF-d 8 , �, ppm): 57.7 (s, C9), 114.1 (s, C1), 118.1 (s, C2), 145.9 (s, C11) (Bogdanovic ´et al., 1984).However, based on available data, it would now be premature to suggest that 1 may also be present as a monomer in THF.Noteworthy is that the corresponding NMR spectra for the aluminium dimer 3 in THF apparently have not been reported for comparison.Also, studies of 1 in other solvents would be of interest, as well as further NMR spectral analyses of this cuprate.Unfortunately, this latter work must be carried out independently in another laboratory because we are no longer able to examine this intriguing species.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1.H atoms on anthracene C atoms were found from difference Fourier maps and refined freely.H atoms on the tetrahydrofuran ligands were placed geometrically and treated as riding atoms, with C-H = 0.99 A ånd U iso (H) = 1.2U eq (C).
However, in 3, the considerably smaller C9-Al-C10A angle [120.2 (4) � ], resulting from the distorted tetrahedral geometry of four-coordinated Al III , causes the Al� � �Al distance to be much longer (Fig. 2).Of particular interest is that the relatively uncrowded linear Cu atoms in 1 permit both solvated potassium ions to approach the cuprate centers more closely than the smaller solvated sodium ions can with the bulkier distorted tetrahedral aluminate centers in 3. Whereas the Na� � �Al distances in 3 exhibit a small difference [0.107 (7) A ˚] and, as a result, its molecular structure, without the THF groups, is fairly symmetrical and deviates only slightly from D 2h symmetry (Brauer & Stucky, 1972), the two K� � �Cu distances in 1 differ by 0.287 (2) A ˚(Fig.1), with one potassium ion having a surprisingly short K� � �Cu distance of 3.3762 (9) A ˚, close to the sum of the covalent radii of K and Cu (3.35A ˚) (Cordero et al., 2008).As a consequence, 1 differs from 3 in having a slightly more compact but a much less symmetrical contact ion-pair structure, owing largely to the different coordination numbers of copper and aluminium, in otherwise quite similar species.Another key difference between 1 and 3 are the nonbonded bridgehead C9� � �C10A distances [3.974 (3) and 3.57 (1) A ˚, respectively], which show that the two formal 9,10-coordinated anthracene dianion units are slightly further apart in 1 than they are in 3, owing also to the different coordination numbers of copper and aluminium in these remarkable compounds, both of which are worthy of additional study.
Despite the aforementioned differences, the geometries of the central anthracene ring of 1 and 3 are similar.Thus, for 1, the bridgehead angles C11-C10-C14 [113.3 ( 2) � ] and C12-C9-C13 [113.1 (2) � ] (Table 2) do not differ significantly from the corresponding angles reported for 3 [114.1 (9) and  112 ( 2], are statistically identical to corresponding C-C distances reported for 3 [average 1.49 (2) A ˚], which are both close to the value of 1.51 (5) A ˚observed previously for C(sp 2 )-C(sp 3 ) distances (Jolly, 1976).Perhaps surprisingly, the geometry of the coordinated central ring of anthracene in the first reported structurally characterized anionic metallacyclic monomer of this type, [{Lu(� 5 -Cp) 2 }(9,10-� 2 -An)] 1À , is structurally nearly identical, without the metal, to those observed in the dianions of 1 and 3, with an average bridgehead C-C distance of 1.485 (9) A ˚.The lutetium anion also has an average bridgehead C-C-C angle of 111.8 (5) A ˚, which is very close to the corresponding angles of 1 and 3. A key structural difference in the lutetium anion is the sharp C9-Lu-C10 angle of 67.1 (2) � , owing to the metallacyclopropane character of the complex and, likely also the large bulk of the bis(cyclopentadienyl)lutetium moiety, which may well be responsible for its stability towards dimerization or oligomerization in solution (THF) and in the solid state as a crystalline [Na(diglyme) 2 ] 1+ salt (diglyme = diethylene glycol dimethyl ether) at ca 293 K (Roitershtein et al., 1993).
Interactions of the alkali metal cations with the exobenzene C atoms are consistent with the overall molecular structures of 1 and 3.In the less symmetrical cuprate 1, the K� � �C distances fall into a distinct 1:1 pattern, with the six C atom closest to the short K� � �Cu contact ion pairing (C1, C2, C5, C6, C11, and C13) having the shortest distances [average 3.14 (13) A ˚], whereas the K� � �C distances for the six C atoms closest to the long K� � �Cu contact (C3, C4, C7, C8, C12, and C14) are longer [average 3.38 (5) A ˚] (see Figs. 1 and 3).In contrast, for the aluminate 3, the Na-C distances for the eight outer C atoms are rather similar and range from 2.94 (1) to 3.24 (1) A ˚, but the Na-C distances of the four more crowded ring-junction C atoms are mostly longer [average 3.28 (4) A ˚; Brauer & Stucky, 1972].Finally, in 1, the essentially planar exo-benzene units possess C-C distances [average 1.396 (3) A ˚] and C-C-C angles [average 119.9 (2) � ] that are quite similar to those found in uncharged free benzene (Mitchell & Cross, 1965), indicating that the contact ion pairing is sufficiently weak to have no significant influence on the structure of the exo-benzene groups and that the negative charge on each anthracene ligand is essentially localized on the central ring and the appended metal center.Similar features were found for the exo-benzene groups in 3 (Brauer & Stucky, 1972).However, in both 1 and 3, the interactions of the counter-ions in these contact ion-paired complexes may play an important role in defining the structures of the anionic components and perhaps their existence in the solid state.In this respect, the isolation of the bis(anthracene)dicuprate(2À ) with relatively weakly interacting cations, such as [K([2.2.2.]cryptand)] 1+ , tetraalkylammonium(1+), etc., would be of considerable interest.As our laboratory will be irreversibly shuttered in 2023, others are encouraged to examine these possibilities and related issues, vide infra.
Since our initial report on tris(1,2,3,4-� 4 -naphthalene)zirconate(2À ), the first confirmed homoleptic (polyarene) metallate of a d-block metal (Jang & Ellis, 1994), which was remarkably labile and functioned formally as a 'masked' naked source of low-spin atomic Zr 2À in its facile reaction with carbon monoxide to afford [Zr(CO) 6 ] 2À , our research group has been interested in discovering what other d-block elements, throughout the periodic table, would afford related and hopefully labile complexes.Thus began our exploration in a systematic fashion of the synthesis and reactivity patterns of transition-metal compounds containing metals in formal negative oxidation states (Ellis, 2006).Early on we wondered whether f-block elements would also 'succumb' to this strategy, but unfortunately never examined these elements (Ellis, 2019).In this regard, we would like to point to the exciting recent results of Skye Fortier and co-workers in highly challenging uranium chemistry (Murillo et al., 2021(Murillo et al., , 2022)).As mentioned in Section 1, we were also intrigued by early reports of 'anionic copper' (Rieke et al., 1990) and this led to our synthesis and structural characterization of the totally unexpected dicuprate salt, 1, described herein.Owing to the early departure of a group member, extension of this study was not possible.For example, we had hoped to examine the reactivity of 1 with good acceptor ligands, such as CO, PF 3 , and P(OR) 3 , and particularly organic isocyanides, to determine whether the anthracenes would be displaced to produce new formal Cu(0,1À ) complexes.Noteworthy is that although isolable Cu 1À complexes remain unknown, very recently, the first unambiguous Cu 0 complex was isolated, thoroughly characterized, and structurally authenticated (Graziano et al., 2022).Also, examination of Rieke's thermally unstable alleged cuprates, derived from naphthalene radical anion reductions remain of great interest because the possible 'naphthalene stabilized cuprate' may be a more labile source of 'naked Cu(1À )' than one derived from the anthracene radical anion.Extension of these studies to silver and gold promise to uncover exciting results.For example, could a bona fide gold anion (Jansen, 2008) be 'tamed' by naphthalene or anthracene to provide labile complexes, enabling the study of new aurate chemistry?We mention these possible extensions because none of this research will be carried out by us at the University of Minnesota.
Refinement.The H atoms on the metal-coordinating C atoms were found from the difference Fourier map and refined freely.

Figure 3
Figure 3End views of 1 (left) and 3 (right) that highlight the much greater asymmetry of the dicuprate.Representative atom-atom distances are shown (A ˚).In 1, the six closest C atoms to the shorter Cu� � �K contact have an average K� � �C distance of 3.14 (13) A ˚, while those closest to the longer Cu� � �K contact have an average K� � �C distance of 3.38 (5) A ˚.

Table 1
Experimental details.
Computer programs: SMART