X₄TCNQ²⁻ dianions: versatile building blocks for supramolecular systems

Abstract

In 2008 a new approach to generating tetracyanoquinodimethane (TCNQ)-based materials was described which involved the use of the diprotonated, reduced form of TCNQ (TCNQH₂) as a reactant. Since the initial work, the dianionic forms of TCNQH₂ and F₄TCNQH₂ have been incorporated into a wide assortment of coordination polymers in which the ligand, with four potential donor atoms, binds to a variety of metal centres. The structures of neutral 1D, 2D and 3D coordination polymers are described, in addition to the structures of anionic networks. Not surprisingly, the oxidation state of the metal ion as well as its preference for certain coordination geometries has a major influence upon the topology and geometry of the polymeric material. In addition to the identity of the metal centre, the type of structure obtained depends upon the nature of the co-ligand in the case of neutral polymers. For anionic networks the shape, charge and size of the counter-cation impacts upon the network connectivity. The large number of metal compounds formed with the dianions is in contrast with the relatively small number of metal complexes involving TCNQ and F₄TCNQ in the 0 and −1 oxidation states. In addition to coordination polymers, organic salts of TCNQ²⁻ and F₄TCNQ²⁻ have also been investigated. The packing within these crystalline salts has been categorised into four types. In both the case of the coordination polymers and the organic salts, charge transfer interactions are common with the electron-rich TCNQ²⁻ and F₄TCNQ²⁻ dianions often serving as electron donors. The presence of various species in the crystal that can act as electron acceptors normally leads to intensely coloured crystals. Whilst TCNQ²⁻ and F₄TCNQ²⁻ dianions have been shown to be versatile building blocks capable of yielding a variety of unusual and aesthetically appealing structures, the redox activity of these dianions offers the prospect of creating materials that possess fascinating electronic properties. An overview of the types of structures obtained since 2008 using the TCNQH₂/F₄TCNQH₂ synthetic approach is presented.

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Introduction

Over the last fifty years there has been considerable interest in tetracyanoquinodimethane (TCNQ, I, X = H) and its derivatives because their presence in both inorganic and organic materials leads to interesting magnetic and electronic properties.^1–3 The charge transfer complex, TTF–TCNQ, is widely regarded as the first “organic metal” (TTF = tetrathiafulvalene).^4–6 Although the complex is formally a semi-conductor, it exhibits a level of conductivity comparable to that of metal conductors. The interesting physical properties commonly found with TCNQ complexes are associated with the ready accessibility of the relatively stable radical anionic state, TCNQ˙⁻ (II). Whilst TCNQ is able to exist, unambiguously, as both a neutral molecule and a monoanionic radical, mixed valence states are common.^1,2,7 For example, in the aforementioned charge transfer complex, TTF–TCNQ, in which TCNQ is considered to be an electron acceptor and TTF an electron donor, the charge on the TCNQ has been calculated to be −0.59 whilst the TTF carries a positive charge of the same magnitude.⁸ There is a sizeable structural library of organic TCNQ-based compounds within the literature, many of which involve some form of charge transfer. In the vast majority of cases in which the radical monoanion is present, it exists as π-dimers, (TCNQ˙⁻)₂ or conglomerates comprised of TCNQ˙⁻ and TCNQ units.

TCNQ has four N-donor atoms directed towards the vertices of a rectangle and thus it offers the prospect of acting as a ligand in the formation of discrete and polymeric metal complexes. The first structurally characterised radical-derived coordination polymer, Ag^ITCNQ˙⁻, was reported by Shields⁹ and consists of two independent interpenetrating 3D networks. In this structure, both the Ag(I) centres and the TCNQ˙⁻ ligands serve as 4-connecting nodes within a 3D network which possesses the PtS topology; the Ag(I) centre and the TCNQ˙⁻ unit correspond to the tetrahedral S²⁻ anion and the planar 4-connecting Pt²⁺ cation respectively. Dunbar and co-workers later described two semi-conducting forms (phase I and II) of CuTCNQ.¹⁰ The crystal structure of the interpenetrating phase I shows close contacts between TCNQ˙⁻ units in neighbouring networks, whereas phase II possesses a 3D structure with a distinctly different topology. AgTCNQ, CuTCNQ phase II and some substituted derivatives have been investigated as candidates for electronic devices due to their resistance-state switching,^11–17 field emission^18–20 and optical properties.^21–24

Whilst there has been considerable interest in the generation of metal–TCNQ˙⁻ compounds^25–38 there are surprisingly few examples of structurally characterised extended framework materials in the literature^39–56 with many of the compounds being inhomogeneous and exhibiting poor crystallinity.^43,57–66 As an example, the family of compounds formulated as M^II(TCNQ)_n (M^II = Mn, Fe, Co, Ni) were found to display interesting magnetic behaviours at low temperatures, however the exact composition of the materials is unclear.⁶¹ In the 1990’s we undertook synthetic investigations with the aim of producing novel metal–TCNQ coordination polymers, however, after failing to produce the targeted compounds we abandoned this area. We suspect that other workers in this area may have had similar experiences in trying to incorporate the radical form of TCNQ into coordination networks.

The two-electron reduction of X₄TCNQ (I) generates the spin-paired dianion, X₄TCNQ²⁻ (III), which possesses an aromatic core. In contrast to the neutral and radical forms, which are π-acceptors, the dianion is a π-donor. Siedle et al. reported air-sensitive transition metal derivatives of the TCNQ dianion in 1979.⁶⁷ These almost black compounds, which were not structurally characterised, were obtained by the reduction of neutral TCNQ with organometallic species. In 2006 the structures of divanadium–TCNQ²⁻ complexes were reported by Choukroun and co-workers⁶⁸ and in the same year Kitagawa et al. reported a coordination network in which neutral parallel 2D sheets of composition Zn(TCNQ) are linked by 4,4′-bipyridine to form a beautiful 3D network containing solvent-filled voids.⁶⁹ In the paper reporting this structure the authors suggest that the dianionic form of TCNQ is a product from the disproportionation of the radical monoanion, TCNQ˙⁻. Dunbar and co-workers also proposed that the disproportionation of F₄TCNQ˙⁻ led to the formation of F₄TCNQ²⁻ which served as a bridging ligand within a Mn(II) coordination network.⁷⁰ The first structurally characterised compound, involving the uncoordinated TCNQ²⁻ dianion was the charge transfer salt [Co^III(C₅Me₅)₂]₂(TCNQ), obtained via the reduction of TCNQ, which was reported in 1987.⁷¹

The TCNQ and F₄TCNQ dianions have the potential to participate in charge transfer interactions with suitable acceptors. The electron rich X₄TCNQ dianions are colourless in the absence of an acceptor but when combined with appropriate electron acceptors the resulting charge transfer complexes are often intensely coloured. The compounds reported in this highlight range from colourless to black, indicating various degrees of charge transfer.

Up until 2008 there were relatively few examples of crystal structures incorporating X₄TCNQ in its dianionic form.^{68–70,72,73} In each case the dianion was formed either via in situ reduction of TCNQ or TCNQ˙⁻ or the disproportionation of the radical anion. Whilst the dianion, X₄TCNQ²⁻, readily undergoes oxidation, its doubly protonated form, X₄TCNQH₂ (IV), is much more stable and far easier to handle.⁷⁴ In 2008 we proposed that it may be possible to generate TCNQ-based coordination networks in which metal centres are linked by the dianion, by combining TCNQH₂ with metal ions in the presence of a weak base.⁷⁵ This work succeeded in demonstrating that although the dianion is susceptible to reaction with oxygen,⁷⁶ it can be stabilised by coordination to metal centres. In addition, by starting with TCNQH₂ it has been shown that the dianion can be stabilised in the presence of appropriate counter-cations.⁷⁵ Similar work with F₄TCNQ²⁻ has resulted in analogous products.

The redox activity of TCNQ and F₄TCNQ has been studied using cyclic voltammetry.⁷⁴ Two reversible, one-electron couples were found for TCNQ as indicated:

E°(vs. Ag/Ag⁺)

TCNQ + e⁻ → TCNQ˙⁻ −0.05 V

TCNQ˙⁻ + e⁻ → TCNQ²⁻ −0.59 V

Similar reversible reduction processes occur for F₄TCNQ, however the reduction potentials are each shifted ∼0.36 V to higher potentials as a result of the electron withdrawing F atoms providing stabilisation of the anionic forms.

E°(vs. Ag/Ag⁺)

F₄TCNQ + e⁻ → F₄TCNQ˙⁻ 0.31 V

F₄TCNQ˙⁻ + e⁻ → F₄TCNQ²⁻ −0.22 V

These data indicate that TCNQ²⁻ is more easily oxidised than F₄TCNQ²⁻ and in general it has been found in synthetic investigations that compounds involving the fluorinated dianion are more stable upon exposure to the atmosphere. Nevertheless, compounds involving either type of dianion exhibit reasonable stability either upon coordination to metal centres or in the formation of charge–transfer complexes.

Since our initial work in 2008 we have employed X₄TCNQH₂ (X = H, F) (IV) in the synthesis of a wide range of novel materials that incorporate the dianionic species, X₄TCNQ²⁻. The use of the acid form represents a new synthetic approach in the generation of crystalline X₄TCNQ-based compounds. It was anticipated that the increased negative charge on the anion, relative to the radical monoanion form, would allow the ligands to bind more strongly to metal centres thus yielding more robust networks. Thus far, X₄TCNQ²⁻ has shown a propensity to form coordination polymers in which face-to-face intra-framework TCNQ–TCNQ interactions are not common, which is presumably a consequence of electrostatic repulsion associated with the formal charge of each TCNQ²⁻ unit. One of our longer term aims is to intercalate oxidants into porous X₄TCNQ²⁻-networks in order to generate mixtures of X₄TCNQ^0/−1/−2 oxidation states. The presence of intra-framework voids, in which one or both sides of the ligand is accessible, makes this an eminently reasonable objective. In this regard, we note that Dunbar and co-workers have reported the compound [Cd₂(TCNQ)_3.5H₂O], which contains TCNQ in a variety of oxidation states from −0.66 to −1.57 (ref. 77) and also the compound [M(TCNQ)₂(4,4bipy)(MeOH)₂]·TCNQ·0.5H₂O (M = Mn, Zn), in which the TCNQ species have a charge of −2/3.⁵¹

This highlight is a structural review describing our recent work using the X₄TCNQH₂ synthetic approach, which has resulted in a series of crystalline compounds including:

1) Electrically neutral coordination polymers of general formula [(M^y+)_a(X₄TCNQ²⁻)_b(co-ligand)_c], wherein the choice of metal centre and co-ligand has a marked effect on the network geometry and topology (Table 1);

Table 1 A selection of coordination polymers derived from X₄TCNQ²⁻

Compound	Cp'd no.	CSD ref code	Colour	Type
(1) Products formed from the same reaction mixture in approximately equal proportions; (2a) is the minor product from the same reaction mixture as (2); (3a) is the minor product from the same reaction mixture as (3); (4a) is the minor product from the same reaction mixture as (4); (5) products formed from the same reaction mixture in approximately equal proportions; (6) products formed from the same reaction mixture in approximately equal proportions.
[Mn(TCNQ)(en)2]	1	DACCES	Colourless	1D
[Mn(F4TCNQ)(en)2]	2	DABLUQ	Orange	1D
[Zn(F4TCNQ)(2,2bipy)2]·MeOH	3	DABXOW	Orange	1D
[Cd(F4TCNQ)(TACN)]·MeOH	4	DABXIQ	Yellow	1D double layer
[Zn(F4TCNQ)(TACN)]·MeOH	5	DABLOK	Yellow	1D double layer
[Zn(F4TCNQ)(2,2bipy)(MeOH)]	6	DABMAX	Yellow	1D double layer
[Zn3(TCNQ)3(MeOH)5(DMF)]·MeOH·3DMF	7	CUTFII	Yellow	2D
[Mn3(TCNQ)3(MeOH)5(DMF)]·1MeOH·3DMF	8	CUTCAX	Colourless	2D
[Zn(TCNQ)(DMSO)2]·0.41pyrazine·½MeOH	9	CUTDIG	Orange	2D
Mn(F4TCNQ)(DMSO)2]·pyz	10	CUTBUQ	Pale yellow	2D
[Mn(F4TCNQ)(MeOH)2]·½(tmpyz)·1½(MeOH)	11	CUTMOV	Yellow	2D H-bonding
[Zn(F4TCNQ)(MeOH)2]·Me2pyz	12	CUTDOM	Yellow	2D H-bonding
[Zn(TCNQ)(MeOH)2]·tmpyz^(1)	13	CUTDUS01	Orange	2D H-bonding
[Zn(TCNQ)(MeOH)2]·tmpyz^(1)	14	CUTDUS	Yellow	2D H-bonding
[Cd(TCNQ)(MeOH)2]·tmpyz	15	CUSZUN	Yellow	2D H-bonding
[Mn(F4TCNQ)(MeOH)2]·phenazine	16	CUTCIF	Red/orange	2D H-bonding
[Mn(F4TCNQ)(MeOH)2]·2acridine	17	CUTCEB	Light orange	2D
[Zn(TCNQ)(pyridine)2]· ½MeOH	18	QUQQAV	Colourless	2D interdigitating
[Co(TCNQ)(quinoline)2]·DMF	19	NAJGEM	Green	2D interdigitating
[Zn(TCNQ)(quinoline)2]·DMF	20	QUQQEZ	Green	2D interdigitating
[Zn(TCNQ)(4-phenylpyridine)2]·2MeOH^(2)	21	QUQQID	Pink	2D interdigitating
[Zn(TCNQ)(4-phenylpyridine)2]^(2a)	22	CUTFEE	Orange	2D interdigitating
[Mn(TCNQ)(pyridine)2]·2MeOH	23	QUQQUP	Purple	2D interdigitating
[Cd(TCNQ)(4-picoline)2]·2MeOH	24	QUQRAW	Colourless	2D interdigitating
[Zn(TCNQ)(nicotinamide)2]·DMF	25	QUQQOJ	Pale yellow	2D H-bonding
[Cd(F4TCNQ)(DMSO)2]	26	DABMEB	Colourless	3D
[Cd(TCNQ)(2,2bipy)]·DMF	27	SUMTUR	Light orange	2D corrugated
[Mn(TCNQ)(2,2bipy)]·DMF	28	SUMVAZ	Light orange	2D corrugated
[Mn(F4TCNQ)(2,2bipy)]	29	CUTBAW	Orange	2D corrugated
[Mn(F4TCNQ)(2,2bipy)]·½(2,2bipy)	30	CUSZOH	Yellow	2D corrugated
[Cd(TCNQ)(phen)]·DMF	31	SUMVED	Yellow/orange	2D corrugated
[Mn(TCNQ)(phen)]·DMF	32	SUNJOC	Yellow/orange	2D corrugated
[Cd(F4TCNQ)(phen)]	33	CUTMEL	Orange	2D corrugated
[Zn(F4TCNQ)(phen)]·½H2O	34	CUTFOO	Yellow	2D interdigitating
[Co(TCNQ)(phen)]	35	SUMVIH	Purple	3D
[Fe(TCNQ)(4,4bipy)]·solvate	36	NAJFOV	Red	3D pillared
[Co(TCNQ)(4,4bipy)]·solvate	37	NAJFUB	Red	3D pillared
[Cd(TCNQ)(4,4bipy)]·solvate	38	NAJGAI	Yellow	3D pillared
[Zn(TCNQ)(4,4bipy)]·solvate	39	VACJEQ	Yellow	3D pillared
[Mn(TCNQ)(4,4bipy)]·solvate	40	QUQPEY	Yellow	3D pillared
[Cd(TCNQ)(bpe)]·solvate	41	QUQPIC	Orange	3D pillared
[Zn(TCNQ)(bpe)]·solvate	42	QUQPOI	Orange	3D pillared
[Zn(TCNQ)(Obip)½(MeOH)]·DMF	43	QUQPUO	Red	3D pillared
[Cu^I2(TCNQ)(2-picoline)2]·EtOH	44	BUKDOC	Colourless	1D
[Cu^I2(TCNQ)(isoquinoline)2]	45	BUKDUI	Pale yellow	1D
[Cu^I2(TCNQ)(4-phenylpyridine)2]	46	BUKFAQ	Colourless	1D
[Cu^I2(F4TCNQ)(2-picoline)2]·MeCN	47	BUKFEU	Pale yellow	1D
[Cu^I2(F4TCNQ)(2,6-lutidine)2]	48	BUKFIY	Pale yellow	1D
[Cu^I2(F4TCNQ)(quinoline)2]·DMF	49	BUKHAS	Pale yellow	1D
[Cu^I2(F4TCNQ)(propionitrile)2]	50	QIXQAR	Colourless	1D
[Cu^I2(TCNQ)(2,2bipy)2]·2H2O	51	BUKGEV	Red	1D
[Cu^I2(F4TCNQ)(2,2bipy)2]·2MeOH	52	BUKJIC	Red	1D
[Cu^I2(TCNQ)(phen)2]·MeOH	53	BUKGIZ	Red	1D
[Cu^I2(TCNQ)(Me2bipy)]·2EtOH	54	BUKGOF	Red	1D
[Cu^I2(TCNQ)(terpyridine)2]	55	BUKGAR	Red	1D
[Cu^I2(F4TCNQ)(PPh3)3]^(5)	56	BUKFOE	Pale yellow	1D
[Cu^I2(TCNQ)(Me4en)2]·solvate^(6)	57	BUKGUL	Colourless	1D
[Cu^I2(TCNQ)(Me4en)2]·2MeCN^(6)	58	BUKHOG	Colourless	2D
[Cu^I2(F4TCNQ)(PPh3)4]^(5)	59	BUKHIA	Pale yellow	2D
[Cu^I2(TCNQ)(quinuclidine)2]	60	BUKHEW	Colourless	2D
[Cu^I2(TCNQ)(Me2pyz)]	61	BUKHUM	Red	2D
[Cu^I2(TCNQ)(apyz)2]·5MeOH	62	BUKJAU	Orange	3D
(NEt4)[Cu^I(F4TCNQ)]^(4a)	63	CUTMIP	Pale yellow	2D
(MePPh3)2[Cd2(TCNQ)3]	64	KOBFIR	Yellow	3D
(MePPh3)2[Fe2(TCNQ)3]	65	TONTIB	Yellow	3D
(MePPh3)2[Co2(TCNQ)3]	66	TONTOH	Red	3D
(MePPh3)2[Ni2(TCNQ)3]	67	TONTUN	Green	3D
(MePPh3)2[Zn2(TCNQ)3]^(3)	68	TONVAV	Yellow	3D
(PPh3Me)2[Zn3(TCNQ)3Br2]^(3a)	69	CUTDEC	Yellow	2D
[Mn(DMSO)4(H2O)2][Mn2(TCNQ)3]	70	FAKLIO	Pale green	3D
[Cd(DMSO)4(H2O)2][Mn2(TCNQ)3]	71	FAKLOU	Pale yellow	3D
[Zn(DMSO)4(H2O)2][Mn2(TCNQ)3]	72	FAKLUA	Pale yellow	3D
[Mn(2,2bipy)3][Mn2(TCNQ)3]	73	FAKMAH	Red	3D
(NMe4)[Cu^I(F4TCNQ)]·H2O	74	KINMUR	Pale green	3D PtS
(NMe2Pr2)[Cu^I(F4TCNQ)]·3EtOH	75	KINNAY	Pale yellow	3D PtS
(NMe2Bu2)[Cu^I(F4TCNQ)]·2EtOH	76	KINNEC	Pale yellow	3D PtS
(NEt4)[Cu^I(F4TCNQ)]·DMSO^(4)	77	KINNIG	Pale yellow	3D PtS
(PPh4)[Cu^I(F4TCNQ)]·½DMF	78	KINNOM	Dark green	3D PtS
(MePPh3)[Cu^I(F4TCNQ)]·DMF	79	KINNUS	Yellow/green	3D PtS
(NPr4)[Cu^I(F4TCNQ)]·½MeCN	80	DABMUR	Yellow	3D
(NMe4)[Cu^I(TCNQ)]	81	YEVKAO	Colourless	3D PtS
(NEt4)[Cu^I(TCNQ)]	82	CUTMAH	Colourless	3D PtS
(NMe2pent2)[Cu^I(TCNQ)]	83	CUSZIB	Colourless	3D PtS
(Nspiro)[Cu^I(TCNQ)]·3DMSO	84	YEVKES	Colourless	3D PtS
(NPr4)[Cu^I(TCNQ)]	85	CUSYIA	Colourless	3D PtS
(Mepyz)[Cu^I(TCNQ)]·DMF	86	CUSYUM	Black	3D PtS
(Meiq)[Cu^I(TCNQ)]·MeCN	87	CUSZEX	Black	3D PtS
(pnpyz)[Cu^I(F4TCNQ)]	88	CUXLEO	Red	3D PtS
(dpx)1/2[Cu^I(F4TCNQ)]·H2O	89	CUYDIL	Red	3D PtS
(AsPh4)[Cu^I(TCNQ)]·solvate	90	CUSZAT	Orange	3D PtS
(PPh4)[Cu^I(TCNQ)]·MeCN	91	CUSYOG	Red	3D PtS

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2) Anionic coordination polymers of general formula [(M^y+)_a(X₄TCNQ²⁻)_b]^z− in which the counter-cation plays a crucial structure directing role and (Table 1);

3) Organic charge transfer salts of X₄TCNQ²⁻ (Table 2).

Table 2 Organic salts formed with the X₄TCNQ²⁻ anion

Compound	Cpd no.	CSD ref no.	Colour	Packing type	Est. charge^a	Interaction
a Calculated from the Kistenmacher relationship.
[(enH)2(en)]TCNQ	92	MOYBUY	Colourless	Discrete	−2.00(6)	H-bonding
(HNEt3)2TCNQ	93	MOYCOT	Colourless	Discrete	−2.04(9)	H-bonding
(TEAH)2TCNQ	94	MOYCUZ	Colourless	Discrete	−1.97(10)	H-bonding
[(dabco·H^+)2(dabco)]TCNQ	95	MOYBAE	Colourless	Discrete	−2.00(6)	Electrostatic
(dbdab)F4TCNQ	96	BODZEB	Pale yellow	Pairs	−2.14(17)	Electrostatic
					−2.29(17)
(Me2dab)F4TCNQ	97	YEVJER	Colourless	Stacks	−2.04(10)	Electrostatic
(Mev)F4TCNQ	98	BODZIF	Dark green	V	−1.93(6)	Face-to-face
(Mev)TCNQ	99	MOYCAF	Black	V	−2.01(9)	Face-to-face
(Etv)F4TCNQ	100	DABMOL	Green	V	−1.90(7)	Face-to-face
(bv)TCNQ	101	MOYCEJ	Black	V	−1.94(6)	Face-to-face
(dpmv)F4TCNQ	102	BODZOL	Black	V	−1.94(17)	Face-to-face
(ddabx)F4TCNQ	103	BODZUR	Green	V	−2.14(11)	Face-to-face
(diquat)TCNQ	104	MOYBEI	Black	V	−1.86(6)	Face-to-face
					−1.76(13)
(phv)F4TCNQ	105	YEVJIV	Black	V	−1.63(12)	Face-to-face
(phv)F4TCNQ·2MeOH	106	YEVJUH	Red	V	−1.95(6)	Face-to-face
(dpbene)F4TCNQ	107	YEVHOZ	Red	V	−1.77(10)	Face-to-face
(dpbene)TCNQ	108	MOYBIM	Black	V	−1.95(5)	Face-to-face
[Pt(2,2bipy)2]TCNQ	109	MOYCIN	Black	V	−1.76(6)	Face-to-face
(dpx)TCNQ	110	MOYBOS	Black	V	−1.94(5)	Edge-to-face
(dpe)F4TCNQ	111	DABMIF	Red	V	−1.94(13)	Face-to-face
(mnbp)2F4TCNQ	112	BOFBAB	Black	VI	−1.86(7)	Face-to-face
(dnbp)2F4TCNQ	113	BODMEO	Black	VI	−2.05(8)	Face-to-face
(Meiq)2F4TCNQ	114	BODNAL	Black	VI	−1.97(6)	Face-to-face
(mnbiq)2F4TCNQ	115	BODYEA	Dark purple	VI	−2.02(7)	Face-to-face
(dnbdab)F4TCNQ	116	BODYIE	Black	VI	−1.90(8)	Face-to-face
(dcpx)F4TCNQ	117	DABXUC	Dark purple	VI	−1.79(13)	Face-to-face
(phv)F4TCNQ·MeOH	118	YEVJOB	Brown	VI	−1.96(13)	Face-to-face
(pnbphp)F4TCNQ	119	BODYOK	Dark purple	VII	−1.88(5)	Edge-to-face
(pnbphp)TCNQ	120	BODYUQ	Dark purple	VII	−1.91(5)	Edge-to-face
(p-spongeH)F4TCNQ	121	BODZAX	Pale yellow	VII	−1.99(8)	Edge-to-face
[Zn(2,2bipy)2(OAc)]TCNQ	122	MOYDAG	Red	VII	−1.96(15)	Edge-to-face
[Fe(phen)3]F4TCNQ	123	YEVHUF	Red	VII	−2.02(8)	Edge-to-face
(dbipx)F4TCNQ	124	DACCIW	Dark green	VII	−1.87(12)	Edge-to-face
(dpx)F4TCNQ	125	BODMIS	Red	VIII	−2.09(6)	Segregated
(dpicx)F4TCNQ	126	BODMOY	Red	VIII	−1.86(17)	Segregated
(diqx)F4TCNQ	127	BODMUE	Dark red	VIII	−2.01(6)	Segregated

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1. Electrically neutral coordination polymers

This section describes two types of coordination polymers. The first type includes networks in which divalent metal ions are linked by X₄TCNQ²⁻ ligands and the second type focuses solely on neutral Cu(I) coordination polymers.

Neutral coordination polymers with divalent metal centres

The 1 : 1 combination of a 4-coordinate TCNQ dianion with divalent metal centres within a network may be expected to yield metal centres in which a maximum of four coordination sites of the metal centre are occupied. Given that divalent metal centres often prefer higher coordination numbers, there is the possibility of introducing neutral co-ligands that can occupy additional coordination sites around the metal centre. As indicated by the examples presented below, the nature of the co-ligand has been found to have a major impact on the topology and geometry of the resulting structure. A range of structures are described in which a variety of co-ligands have been employed to generate 1D, 2D and 3D coordination polymers.

1D coordination polymers (1–6) have been formed in cases where some of the N atoms of the X₄TCNQ dianionic ligands are uncoordinated. The combination of TCNQ²⁻ with Mn(II) and the bidentate co-ligand ethylenediamine (en) results in the formation of the 1D chain-like polymer, [Mn(TCNQ)(en)₂] (1), wherein the TCNQ²⁻ anion coordinates to two Mn^II centres through nitrile groups on opposite sides of the aromatic ring. Each metal centre is bound by two TCNQ²⁻ units in a cis arrangement (Fig. 1a). When F₄TCNQ²⁻ is employed under similar reaction conditions, the metal centres are bound to the ligands by nitriles from the same –C(CN)₂ unit, with each Mn(II) centre coordinated by two F₄TCNQ²⁻ ligands in a trans fashion (2). The resulting linear polymers interdigitate, as shown in Fig. 1b. Compound 3, [Zn(F₄TCNQ)(2,2bipy)₂]·MeOH, adopts a structure with similar connectivity to that of 1.

Fig. 1 1D coordination polymers in which divalent metal ions are linked by X₄TCNQ²⁻ ligands: a) a single chain of [Mn(en)₂(TCNQ)] (1) b) interdigitating chains of [Mn(en)₂(F₄TCNQ)] (2) c) a “double-layered” polymer of [Cd(TACN)(F₄TCNQ)] (4) and d) a pair of “double-layered” polymers of [Zn(F₄TCNQ)(2,2bipy)(MeOH)] (6) with hydrogen bonding between the chains indicated by striped connections.

Linear “double-layered” polymers can be formed when TCNQ²⁻ ligands bind to three metal centres. The compounds [Cd(F₄TCNQ)(TACN)] (4) and [Zn(F₄TCNQ)(TACN)] (5) have a 1D chain-like polymer in which the F₄TCNQ²⁻ ligand is bound to three metal centres whilst the octahedral metal centre is coordinated by three F₄TCNQ²⁻ anions and a 1,4,7-triazacyclononane (TACN) ligand in a fac arrangement (Fig. 1c). The compound [Zn(F₄TCNQ)(2,2bipy)(MeOH)] (6) displays the same double layered strip motif as 4 and 5 but with a bidentate 2,2′-bipyridine (2,2bipy) ligand and a coordinated methanol molecule in place of the tridentate TACN ligand. In this case the non-coordinated nitrile group of the F₄TCNQ²⁻ ligand forms a hydrogen bond to a coordinated methanol molecule from an adjacent strip, leading to a 2D hydrogen bonded network (Fig. 1d).

A series of electrically neutral 2D coordination polymers with the general formula [M^II(X₄TCNQ^−II)(co-ligand)₂]·solvate have been structurally characterised (7–25). In the majority of cases, the X₄TCNQ²⁻ ligand is coordinated to four metal centres and each metal centre is bound to four X₄TCNQ²⁻ anions, to give a network which has the topology of a 4,4-net i.e. both the metal centres and the X₄TCNQ²⁻ ligands serve as four connecting nodes and the smallest circuit in the net contains four nodes. The octahedral metal centres are also coordinated by co-ligands which sit above and/or below the sheets. Two types of connectivity are observed within the sheets, wherein the C–X₄C₆–C axes are either: i) alternating in orientation (type A), with a square-type array of metal centres of edge ∼8.5 Å (Fig. 2a), or ii) parallel (type B), with the metal centres sitting at the corners of a rectangle of long edge ∼11.3 Å and short edge ∼7.4 Å (Fig. 2b). Due to the steric effects of the fluorine atoms, networks involving F₄TCNQ²⁻, thus far, have only been found to adopt the type B arrangement.

Fig. 2 The arrangement of metal centres; a) in a square grid (type A) in which the C–X₄C₆–C axes of the TCNQ²⁻ ligands alternate in orientation, and b) in a rectangular grid (type B) with C–X₄C₆–C axes of the X₄TCNQ²⁻ units parallel.

The compounds [M^II(X₄TCNQ^−II)(MeOH)₂]·G (M = Zn, Mn or Cd; G = pyrazine, Me₂pyz, tmpyz, acridine, phenazine) (11–17) have slightly undulating 2D sheets that, apart from one exception (see below), adopt the type B geometry. The uncoordinated pyrazine-based molecules are able to form hydrogen bonds with the coordinated methanol ligands of neighbouring sheets to form 3D networks. The representative compound [Zn(TCNQ)(MeOH)₂]·tmpyz (14) is shown in Fig. 3a. Compound 13, which has the same composition as 14, has the square grid, type A arrangement. The phenazine molecule in compound 16, in addition to forming an H-bonded bridge between adjacent sheets, participates in a stacking interaction with the rings of the F₄TCNQ²⁻ ligands to either side (Fig. 3b). This facilitates a charge–transfer interaction that gives the crystals a red colour. In the structure of [Mn(F₄TCNQ)(MeOH)₂]·2acridine (17) each acridine molecule forms a hydrogen bond with one coordinated methanol and participates in aromatic stacking interactions with an F₄TCNQ²⁻ ligand and a symmetry-related acridine molecule which has a hydrogen bond to an adjacent sheet (Fig. 3c).

Fig. 3 a) The structure of [Zn(TCNQ)(MeOH)₂]·tmpyz (14) in which sheets are linked via H-bonding between coordinated MeOH ligands and tetramethylpyrazine units (H-bonds represented by striped connections). b) The structure of [Mn(F₄TCNQ)(MeOH)₂]·phenazine (16) showing the H-bonding and aromatic interactions of the phenazine molecule. c) The H-bonding and stacking in [Mn(F₄TCNQ)(MeOH)₂]·2acridine (17).

The combination of M^II centres (M = Mn, Zn, Co and Cd) and TCNQ²⁻ with pyridine-based co-ligands (pyridine, 4-methylpyridine, quinoline and 4-phenylpyridine) (18–24) results in interdigitating 2D polymers that adopt the type B geometry.⁷⁸ The separation of co-ligands within a single sheet (∼7.4 Å) corresponds to the short edge of a rectangle of metal centres (Fig. 2b), which appears ideal to promote interdigitation. Face-to-face contacts, half that of the short edge (∼3.7 Å), occur between co-ligands of adjacent sheets. When pyridine (18) is substituted with 4-phenylpyridine (21), there is a marked increase in separation between sheets, resulting in larger solvent filled channels throughout the crystal structure as is apparent upon inspection of Fig. 4. The compound [Zn(TCNQ)(nicotinamide)₂] (25) differs in that sheets are linked directly via complementary amide-amide double hydrogen bonds between nicotinamide ligands of neighbouring sheets. The resulting 3D H-bonded network is shown in Fig. 5. The orientation of the nicotinamide ligands results in intra-framework channels that are occupied by disordered solvent molecules.

Fig. 4 Two interdigitating type B sheets in a) [Mn(TCNQ)(pyridine)₂] (18) and b) the increased sheet-sheet separation in [Zn(TCNQ)(phenylpyridine)₂] (21).

Fig. 5 The 3D hydrogen bonded network [Zn(TCNQ)(nicotinamide)₂] (25) (amide–amide H-bonds represented by striped connections).

The inclusion of chelating co-ligands such as 2,2bipy and 1,10-phenanthroline (phen) results in sheets with the 4,4-topology (27–33). In contrast to the gently undulating sheets formed when the co-ligands are trans, the occupation of the cis sites by the chelating ligands means that the sites available to the TCNQ ligand can only be occupied if the 4,4-networks adopt a highly corrugated structure.⁷⁹ An example of this corrugation which is apparent in [Cd(TCNQ)(phen)] (31) is presented in Fig. 6. The metal centres deviate significantly from the mean plane of the sheets, with the co-ligands sitting above or below the sheets. The compounds incorporating the TCNQ²⁻ anion generally adopt the type A connectivity, whereas the type B connectivity is observed with the F₄TCNQ²⁻ analogues 29, 30 and 33.

Fig. 6 The highly corrugated sheet structure of [Cd(TCNQ)(phen)] (31).

The non-chelating ligands 4,4′-bipyridine (4,4bipy) and 1,2-bis(4-pyridyl)ethylene (bpe) are able to bridge metal centres in adjacent type A sheets (except for 42 which adopts the type B geometry) to form 3D coordination polymers of general composition [M^II(TCNQ)(4,4bipy)] (M = Fe, Co, Cd, Zn and Mn) (36–40) and [M^II(TCNQ)(bpe)] (M = Cd and Zn) (41, 42).⁷⁸ The structure of [Mn^II(TCNQ)(4,4bipy)] (40) is shown in Fig. 7. The sheets are slightly undulating and are linked together such that the 4,4bipy and bpe ligands act as ‘pillars’, resulting in large solvent filled intra-framework voids. The pale-yellow compound [Zn^II(TCNQ)(4,4bipy)]·solvate was first reported by Kitagawa and co-workers, which was obtained via the disproportionation of TCNQ˙⁻ in the presence of ascorbic acid.^69,80 Recently, Dunbar and co-workers reported that the compound [Fe^II(TCNQ)(4,4bipy)]·solvate displayed solvent induced changes in magnetic behaviour at low temperature.⁸¹

Fig. 7 The 3D pillared structure of [Mn^II(TCNQ)(4,4bipy)] (40). Disordered solvent molecules occupy the large intra-framework voids.

The co-ligand 4,4′-bipyridine-1,1′-dioxide (Obip) bridges Zn(TCNQ) sheets in [Zn(TCNQ)(Obip)_1/2(MeOH)] (43), as shown in Fig. 8. In contrast to the compounds discussed above, in which the 4,4bipy and bpe ligands are orientated perpendicular to the mean planes of the sheets, a bend at the oxygen atom of the Obip (N–O–Zn ca. 118°), enables a face-to-face interaction between the electron deficient aromatic rings of Obip with the electron rich TCNQ²⁻ anions. The resulting charge transfer interaction leads to a deep red colour.

Fig. 8 The structure of [Zn(TCNQ)(Obip)_1/2(MeOH)] (43). The Obip ligands are inclined with respect to the sheets to form close charge transfer interactions with the TCNQ²⁻ ligands. For clarity, the coordinated MeOH ligands have been omitted.

Neutral coordination polymers with Cu^I centres

As stated in the introduction, Cu^ITCNQ exists in two distinct phases each of which possesses interesting electrical properties. In each of these phases the Cu(I) centre is 4-coordinate and the TCNQ˙⁻ ligand binds to four Cu(I) centres. When the dianionic form of TCNQ is employed, twice as many Cu(I) centres per TCNQ unit are required for electrical neutrality. Under these circumstances, in which there are potentially twice as many coordination sites, there is the opportunity for including co-ligands into the structure. This section describes a series of neutral [Cu^I₂X₄TCNQ^−II] structures in which co-ligands also coordinate to the Cu(I) centres.

The combination of Cu^I and X₄TCNQ²⁻ with various co-ligands forms the series of neutral coordination polymers with the general formula [Cu^I₂(X₄TCNQ^−II)(co-ligand)_y] (y = 1–4) (44–62).^82,83 The use of propionitrile co-ligands⁸² or the planar co-ligands, 2-picoline, 2,6-lutidine, quinoline, isoquinoline and 4-phenylpyridine, results in the formation of interdigitating strip-like coordination polymers (44–50).⁸³ Within each strip, the C–X₄C₆–C axes of the X₄TCNQ²⁻ ligands are parallel, with the X₄TCNQ²⁻ ligands binding to four Cu^I centres, which lie at the corners of a rectangle. Each Cu^I centre has a trigonal planar geometry, coordinated by two X₄TCNQ²⁻ ligands and a terminal co-ligand. Close face-to-face interactions occur between co-ligands from neighbouring strips. The compound [Cu^I₂(TCNQ)(4-phenylpyridine)₂] (46) which is representative of this class of compounds is presented in Fig. 9a. Similar interdigitating strips are formed with the chelating co-ligands 2,2bipy (51, 52), phen (53) and 5,5′-dimethyl-2,2′-bipyridine (Me₂bipy) (54), in which the Cu^I centres adopt a tetrahedral geometry, in addition to terpyridine (55) in which the Cu^I centre is 5-coordinate. The compound [Cu^I₂(F₄TCNQ)(PPh₃)₃] (56) also forms a strip, with a combination of Cu^I centres, in which trigonal Cu^I centres are each bound to a single PPh₃ ligand and tetrahedral Cu^I centres, are each bound to a pair of PPh₃ ligands.

Fig. 9 a) Two interdigitating strips in [Cu^I₂(TCNQ)(4-phenylpyridine)₂] (46). b) The 2D sheet structure of [Cu^I₂(TCNQ)(quinuclidine)₄] (60).

The chelating ligand N,N,N′N′-tetramethylethlenediamine (Me₄en) when combined with Cu^I and TCNQ²⁻ generates slightly undulating strips of composition [Cu^I₂(TCNQ)(Me₄en)₂]·solvate (57). In contrast to the structures discussed above, in which the co-ligands are ostensibly planar, the Me₄en ligands are too bulky to allow interdigitation. Within the same reaction mixture which produced 57, a 2D coordination polymer (58), similar in composition, is also formed. The sheet structure is topologically identical to that observed in the compounds [Cu^I₂(F₄TCNQ)(PPh₃)₄] (59) and [Cu^I₂(TCNQ)(quinuclidine)₄] (60), which is represented in Fig. 9b.

There are two examples to date in which Cu^I₂(TCNQ)-type structures include bridging co-ligands. The compound [Cu^I₂(TCNQ)(Me₂pyz)] (61) comprises near planar 2D networks. Although 61 may be considered to possess the same Cu^I₂(TCNQ) topology as compounds 58–60, the overall connectivity arising from the participation of bridging 2,5-dimethylpyrazine (Me₂pyz) ligands yields the 2D network depicted in Fig. 10a in which the Cu(I) centres are trigonal. The compound [Cu^I₂(TCNQ)(apyz)₂] (62), contains [Cu^I₂TCNQ] strips which are linked to neighbouring strips by disordered 2-aminopyrazine (apyz) bridges, to form the 3D coordination polymer shown in Fig. 10b which possesses channels with approximately hexagonal cross-section.

Fig. 10 a) The 2D sheet structure of [Cu^I₂(TCNQ)(Me₂pyz)₂] (61) (TCNQ²⁻ represented in blue; Me₂pyz represented in red). b) The 3D coordination polymer [Cu^I₂(TCNQ)(2-aminopyrazine)₂] (62); for clarity, only one orientation of the disordered 2-aminopyrazine ligands is shown (TCNQ²⁻ represented in blue; 2-aminopyrazine represented in red).

2. Anionic coordination polymers

This section describes coordination polymers that carry a net negative charge. As with the previous section this is divided into two parts, 1) structures in which the X₄TCNQ²⁻ ligands are linked by divalent metal ions and 2) structures which contain Cu(I) centres.

Anionic networks of general formula [M^II₂(X₄TCNQ^−II)₃]²⁻

A series of TCNQ²⁻ derived anionic coordination polymers, of general formula [M^II₂(TCNQ^−II)₃]²⁻, have been reported in which both the choice of counter-cation and metal centre has had a significant impact on the resulting network topologies.^75,84,85 The reaction of Cd(NO₃)₂, TCNQH₂, LiOAc and MePPh₃Br leads to the formation of a network material with formula, (MePPh₃)₂[Cd^II₂(TCNQ)₃] (64).⁷⁵ The compound crystallises in the space group R3̄. The 3D network comprises slightly distorted octahedral Cd^II centres, which adopt a pseudo-primitive-cubic arrangement (Fig. 11). The TCNQ²⁻ ligands bind to four Cd^II centres, with each octahedral Cd^II centre coordinated by six planar TCNQ²⁻ anions. For simplicity, the framework can be divided into “octants”, as illustrated in Fig. 11, in which the MePPh₃⁺ counter-cations reside. Within each octant, only half of the six faces are occupied by TCNQ²⁻ ligands.

Fig. 11 A representation of (MePPh₃)₂[Cd^II₂(TCNQ)₃] (64) illustrating the pseudo-cubic arrangement of Cd centres. For clarity, only two MePPh₃⁺ cations, which occupy “octants” within the framework are shown.

The family of isostructural compounds (MePPh₃)₂[M^II₂(TCNQ)₃] (M = Zn, Ni, Co, Fe) (65–68),⁸⁴ formed under very similar reaction conditions to the Cd derivative, form 3D coordination polymers with a markedly different network topology (Fig. 12). In contrast to the planar TCNQ²⁻ ligands present in the [M^II₂(TCNQ)₃]²⁻ framework, the NC–C–CN moieties are rotated out of the plane of the central aromatic ring. Pairs of MePPh₃ cations, which interact via a hexaphenyl embrace, occupy the intra-framework voids. Magnetic studies reveal long-range magnetic ordering within the networks.

Fig. 12 The structure of (MePPh₃)₂[M^II₂(TCNQ)₃] (M^II = Zn, Ni, Co, Fe) (65–68) in which there is a significant twist in the TCNQ²⁻ ligands. Pairs of MePPh₃⁺ cations (green) occupy the intra-framework cavities.

The compounds (M^II(DMSO)₄(H₂O)₂)[M^II₂(TCNQ)₃] (M = Mn, Zn, Cd) (70–72)⁸⁵ consist of 3D honeycomb-type networks, in which disordered metal complexes serve as counter-cations. The [Cd₂(TCNQ)₃]²⁻ (71) framework is shown in Fig. 13a. Large parallel hexagonal channels, ca. 15 Å in diameter, are present throughout the structure, with the network atoms occupying less than half of the crystal volume. The TCNQ²⁻ units are oriented such that the C–H₄C₆–C axes of the TCNQ²⁻ units are inclined relative to the channel walls (Fig. 13b). The [Mn₂(TCNQ)₃]²⁻ network, in which the metal complex [Mn(2,2bipy)₃]²⁺ serves as the counter-cation, is topologically identical, however the orientation of the TCNQ²⁻ anions is noticeably different. In the case of (Mn^II(2,2bipy)₃)[Mn^II₂(TCNQ)₃] (73), the C–H₄C₆–C axes are oriented parallel to the channel walls, with the TCNQ²⁻ anions located above and below the mean plane of the walls in an alternating fashion (Fig. 13c). Crystals of (Mn^II(2,2bipy)₃)[Mn^II₂(TCNQ)₃] are a deep red, suggestive of charge transfer interactions between the cations and the network.

Fig. 13 a) The hexagonal [Cd₂(TCNQ)₃]²⁻ network in (Cd(DMSO)₄(H₂O)₂)[Cd₂(TCNQ)₃] (71). b) and c) The orientation of the coordinated TCNQ²⁻ ligands with respect to the mean plane of the metal centres within (M^II(DMSO)₄(H₂O)₂)[M^II₂(TCNQ)₃] (M^II = Mn, Zn, Cd) (70–72) and (Mn^II(2,2bipy)₃)[Mn^II₂(TCNQ)₃] (73) respectively.

Anionic networks of general formula [Cu^I(X₄TCNQ^−II)]⁻

One of the most important networks targeted in the generation of coordination polymers is the PtS net, which consists of 4-connecting tetrahedral centres and 4-connecting square planar centres in equal proportions, as depicted in Fig. 14. In its most symmetrical form the network is tetragonal and possesses channels of square and hexagonal cross-section which run perpendicular to the c-axis. As previously discussed, the electrically neutral coordination polymers [M^I(TCNQ˙⁻)] (M = Ag, Cu) adopt the PtS topology, with both [Ag^I(TCNQ˙⁻)] and phase II of [Cu^I(TCNQ˙⁻)] each existing as interpenetrating neutral networks. With the general tendency of Cu^I to adopt a tetrahedral geometry, combined with the ability of X₄TCNQ²⁻ anions to bind to four metal centres in a planar arrangement, it was anticipated that an anionic coordination polymer of composition [Cu^I(X₄TCNQ^−II)]⁻ may also adopt the PtS topology. The negative charge on the network resulting from the incorporation of a dianionic ligand in place of the radical monoanionic ligand requires the incorporation of an appropriately sized counter-cation which would occupy the intraframework voids. The presence of a cation reduces the space available for a second network, thus preventing interpenetration. The following discussion relates to structures composed of an anionic [Cu^I(X₄TCNQ)]⁻ network with various cations located in the framework channels.

Fig. 14 The platinum sulphide network (Pt, purple; S, blue).

A family of F₄TCNQ²⁻ derived compounds with the general formula [Cu^I(F₄TCNQ)]⁻ (74–79) comprising networks with the PtS topology was reported in 2013.⁸⁶ Similar compounds have been subsequently synthesised (81–91). Compound 80 adopts a related structure with a complex topology.

The colourless compound (NMe₄)[Cu^I(TCNQ)] (Fig. 15) (81) adopts the PtS-type network in which tetrahedral Cu^I centres are bound to four TCNQ²⁻ ligands and each planar ligand is coordinated to four Cu^I centres at the corners of a rectangle. The C–H₄C₆–C axes of the TCNQ²⁻ anions are oriented parallel to the c-axis, with ligands linked along this direction alternating in orientation by 90°. The elongated hexagonal channels are occupied by NMe₄⁺ counter-cations. The “floor” and “ceiling” of the channels are separated by a distance corresponding to the short edge of a rectangular ‘Cu^I₄TCNQ’ unit (∼7.5 Å). The compound is highly symmetrical, crystallising in the tetragonal space group P4₂/mmc, as does PtS itself. The compounds (cation)[Cu^I(F₄TCNQ)] (cation = NMe₄⁺, NMe₂Pr₂⁺ and NMe₂Bu₂⁺) (74–76) have very similar tetragonal networks, however, unlike the TCNQ²⁻ anions in (NMe₄)[Cu^I(TCNQ)], rotational disorder of the ‘F₄C₆’ cores with respect to CN–C–CN moieties are observed due to steric factors associated with the larger fluorine atoms.

Fig. 15 The high symmetry [Cu^I(TCNQ)]⁻ network in (NMe₄)[Cu^I(TCNQ)] (81) viewed a) down the a axis and b) along the unique tetragonal axis. The NMe₄⁺ cations have been omitted.

The presence of the counter-cations NEt₄⁺, NMe₂Pent₂⁺ and Nspiro⁺77, 82–84 results in a lowering of network symmetry, with the Cu^I centres adopting a distorted tetrahedral geometry. A reduction of network symmetry is also apparent in (Mepyz)[Cu^I(TCNQ)] (86) (Fig. 16). The concertina-like collapse of the network, most evident when viewed down the pseudo-tetragonal axis (Fig. 16b), presumably occurs to maximise the TCNQ²⁻-cation interactions (closest contact ca. 3.1 Å). The hexagonal channels within the network are occupied by the cations and DMF solvent molecules (Fig. 16c). The compound is almost black, indicative of charge–transfer between the cations and the electron-rich TCNQ²⁻ anions. Comparable networks with very close donor–acceptor contacts are observed in compounds (Meiq)[Cu^I(TCNQ)] (87), (pnpyz)[Cu^I(F₄TCNQ)] (88), and (dpx)_½[Cu^I(F₄TCNQ)] (89) which are also darkly coloured.

Fig. 16 The distorted [Cu^I(TCNQ)]⁻ network in (Mepyz)[Cu^I(TCNQ)] (86) viewed a) slightly off the b axis and b) slightly off the pseudo-tetragonal axis. The Me-pyrazinium cations have been omitted. c) View along a single hexagonal channel in (Mepyz)[Cu^I(TCNQ)] occupied by Mepyz⁺ cations (red) and DMF molecules (green).

The compounds (AsPh₄)[Cu^I(TCNQ)] (90), (PPh₄)[Cu^I(F₄TCNQ)] (78), (PPh₄)[Cu^I(TCNQ)] (91) and (MePPh₃)[Cu^I(F₄TCNQ)] (79) adopt the PtS-type topology, however significant differences exist between these structures and those described above. The C–X₄C₆–C axes of the X₄TCNQ²⁻ ligands are oriented differently (see Fig. 17), with the separation of the TCNQ²⁻ units occupying the floor and ceiling of the channels corresponding to the long axis of a “Cu^I₄TCNQ” rectangle (∼11.6 Å). The intra-framework channels, which now adopt an approximately rectangular cross-section, are large enough to accommodate the bulkier cations. The structure of (AsPh₄)[Cu^I(TCNQ)] is shown in Fig. 17. In each case, a phenyl group of the cation is ‘sandwiched’ between two X₄TCNQ²⁻ ligands, resulting in the formation of infinite X₄TCNQ²⁻/H₄C₆ stacks, to give highly coloured compounds.

Fig. 17 The structure of (AsPh₄)[Cu^I(TCNQ)] (79). The intra-framework voids are large enough to accommodate the bulky AsPh₄⁺ cations (represented in blue).

3. Organic charge transfer salts: cation – X₄TCNQ²⁻ complexes

A wide range of “organic complexes” involving the X₄TCNQ˙⁻ radical species have been reported to date.^3,7 Such complexes are intensely coloured and almost always involve close π–π interactions between TCNQ˙⁻ units that often are present within the crystal structure as closely associating pairs. In contrast, complexes involving the dianionic species (Table 2), have a propensity to exist as discrete anions (with a few exceptions which are discussed below) (92–127).^87,88 As previously discussed, the neutral forms of X₄TCNQ act as acceptors in charge transfer complexes, whereas the radical monoanion, X₄TCNQ˙⁻ has the potential to act as both a donor and acceptor. The electron-rich dianionic species, X₄TCNQ²⁻ can serve only as a donor. Compounds involving the dianion with an assortment of cations range from colourless to black, indicative of varying degrees of charge transfer between X₄TCNQ²⁻ donors and acceptor cations. The cation–anion Coulombic interaction between cations and X₄TCNQ²⁻ anions is commonly supported by directional secondary interactions including hydrogen bonding, as well as edge-to-face and face-to-face van der Waals interactions involving planar units. The compounds described in this section represent solid-state examples of cases in which the X₄TCNQ²⁻ dianion is crystallised with appropriate cations. The cations reported here are depicted in Scheme 1.

Scheme 1

The combination of the spin-paired TCNQ²⁻ dianion with “innocent” cations, which are inherently unable to act as π-acceptors, largely results in the formation of colourless compounds. The complexes [(enH⁺)₂(en)]TCNQ, (Et₃NH⁺)₂TCNQ and (TEAH)₂TCNQ (92–94) are comprised of TCNQ²⁻ units hydrogen bonded, via the nitrile groups, to protonated amines. The structure of (Et₃NH⁺)₂TCNQ (93) is shown in Fig. 18a. The colourless compound [(dabco·H⁺)₂(dabco)]TCNQ (95), shown in Fig. 18b, is composed of strips of parallel TCNQ²⁻ anions that interact electrostatically with the internally hydrogen bonded (dabcoH⁺–dabco–⁺Hdabco) trimers. The very pale yellow compound dbdab–F₄TCNQ (96) is unusual in that it contains discrete pairs of F₄TCNQ²⁻ dianions, which interact electrostatically with the surrounding cations (Fig. 18c).

Fig. 18 a) Two HNEt₃⁺ cations H-bonding to a TCNQ²⁻ anion in (HNEt₃)₂TCNQ (93). b) A view of [(dabco·H⁺)₂(dabco)]TCNQ (95). The internally H-bonded dabcoH⁺–dabco–⁺Hdabco trimers (purple) interact electrostatically with strips of TCNQ²⁻ anions (blue). c) The structure of dbdab–F₄TCNQ (96) showing a pair of F₄TCNQ²⁻ units (orange) surrounded by dbdab cations (purple).

The crystal packing arrangements of complexes that involve either edge-to-face or face-to-face π- interactions can be broadly divided into four categories (V–VIII), shown schematically in Fig. 19. The category V crystal packing arrangement, observed in compounds 98–111, consists of infinite “stacks” of alternating dications and X₄TCNQ²⁻ dianions. A representative example is the compound (Mev)F₄TCNQ (98), shown in Fig. 20a. Within a column, Mev²⁺ cations and F₄TCNQ²⁻ units are co-planar and associate through close face-to-face π-interactions. The orientation of the C–F₄C₆–C axes of the dianions is offset slightly from the long axes of the cations. Subtle variations are observed in other type V packing compounds. The compound (bv)TCNQ (101), comprises co-planar cations and anions associating in a face-to-face manner, however, the mean planes of both entities are significantly tilted with respect to the stacking direction. The steric imposition of the phenyl groups in (dpmv)F₄TCNQ (102) results in a perpendicular orientation of the C–F₄C₆–C axis of the F₄TCNQ²⁻ anion and the long axis of the cation. The compound (dpx)TCNQ (110) is the only compound with the type V packing to date in which edge-to-face interactions between cations and anions are present (Fig. 20b). It is notable that the majority of the type V packing family of compounds are black, consistent with strong charge–transfer interactions.

Fig. 19 The four packing arrangements of X₄TCNQ²⁻ anions (blue) and cations (yellow).

Fig. 20 A single type V stack in: a) (Mev)F₄TCNQ (98) and b) (dpx)TCNQ (110).

Type VI packing consists of infinite stacks in which discrete F₄TCNQ²⁻ anions are separated by pairs of cations, with face-to-face interactions between the donors and acceptors (112–118). The compound (Meiq)₂F₄TCNQ (114) is shown in Fig. 21a. The cations and anions are co-planar, with the mono-cations making close contact on both sides of the F₄TCNQ²⁻ anion. Similar structures are observed for compounds (dnbp)₂F₄TCNQ (113) and (mnbiq)₂F₄TCNQ (115), however, interesting variations in donor–acceptor interactions are observed. The structurally similar cations each possess two potential components able to associate with the F₄TCNQ²⁻ anion, with the isoquinolinium moiety of mnbiq⁺ and the di-nitrobenzyl component of dnbp⁺ preferentially participating in close face-to-face contacts with the anion. The dication containing compound (dnbdab)F₄TCNQ (116) comprises linked stacks in which the singly charged moieties associate with the F₄TCNQ²⁻ anions in neighbouring stacks, as illustrated in Fig. 21b.

Fig. 21 a) A single type VI stack in (Meiq)₂F₄TCNQ (114), in which pairs of Meiq⁺ cations (red) make face-to-face contacts with the F₄TCNQ²⁻ anions (purple). b) A view of the crystal packing of (dnbdab)F₄TCNQ (116) in which type VI stacks are linked by dnbdab cations.

Type VII packing is very similar to the type VI packing, however the anions and the cations associate via edge-to-face interactions (119–124). The compound (p-sponge·H⁺)₂F₄TCNQ (121) consists of stacks in which pairs of cations associate with each other in a face-to-face manner and edge-to-face with the anions, as shown in Fig. 22a. A similar packing arrangement is observed within the compound (dbipx)F₄TCNQ (124), wherein the stacks are linked by the dications, with bipyridylium units from separate dications forming edge-to-face contacts with the anions (Fig. 22b).

Fig. 22 a) A single type VII stack in (p-spongeH⁺)₂F₄TCNQ (121) in which pairs of cations (purple) make edge-to-face contact with the F₄TCNQ²⁻ anions (green). b) The type VII packing of (dbipx)F₄TCNQ (124) in which stacks are linked by the dications (red).

The type VIII packing arrangement comprises discrete stacks of F₄TCNQ²⁻ dianions and xylene-based dications (125–127) – an unexpected arrangement, given that it would be anticipated that molecules possessing the same higher charge of 2+ or 2−, would result in significant Coulombic repulsions. The positive charges on the cations, however, are located on the pyridyl rings and therefore well separated, minimising the repulsion. The compound (dpx)F₄TCNQ (125), which has a markedly different packing arrangement to the TCNQ²⁻ analogue (110), is shown in Fig. 23. The C–F₄C₆–C axes of the F₄TCNQ²⁻ anions are orientated in an alternating fashion along an anionic column, with similar alternation present within a column of cations.

Fig. 23 The type VIII packing arrangement involving segregated stacks of dpx²⁺ cations and F₄TCNQ²⁻ anions in (dpx)F₄TCNQ (125).

Confirmation of 2- charge

As a general rule, an increase in negative charge on X₄TCNQⁿ (n = 0, −1 or −2) results in a shortening in b and d bond lengths whereas the c bond length increases (Scheme 2). The Kistenmacher equation, q = A[c/(b + d)] + B, where q is the estimated charge and A and B are coefficients, allows for an estimation of charge based upon the relevant bond lengths.⁸⁹ Originally, the empirical values calculated for A and B were based on neutral and radical forms. We have now calculated revised values for A and B that take into account the dianionic form (for TCNQ A = −41.667, B = 19.818; for F₄TCNQ A = −45.756, B = 21.846).⁸⁸ The IR nitrile stretches provide another indicator of the oxidation state, with an increase of charge resulting in a lowering of CN stretching frequencies (ν_CN stretching frequencies are typically in the range of 2170–2133 for F₄TCNQ²⁻ and 2176–2100 for TCNQ²⁻).⁹⁰ Both methods have been used to confirm the 2- charge of the X₄TCNQ moieties in the structures reported in this highlight. The estimated charge from the Kistenmacher relation are presented in Table 2.

Scheme 2

Concluding remarks

The TCNQ²⁻ and F₄TCNQ²⁻ dianions, obtained using the X₄TCNQH₂ synthetic approach, are versatile building blocks for the generation of a significant number of new supramolecular systems. A sizeable family of easily obtained, highly crystalline X₄TCNQ-based 1D, 2D and 3D coordination polymers and charge–transfer salts have been synthesised and structurally characterised. Whilst the choice of metal centre, counter-cation and reaction conditions all impact upon the type of structure obtained, the high variability in geometry and topology of X₄TCNQ²⁻ based coordination polymers can also be attributed to the variability in the binding of the dianion to metal centres; X₄TCNQ dianions can bind to various numbers of metal centres, from 2 to 4 for the examples presented in the highlight. Furthermore, when acting as a ligand binding to four metal centres there is considerable variability in the location of the metal centres to which the dianions are bound. This is illustrated by the existence of both the square and rectangular arrangement of metal centres apparent in the type A and B geometries respectively (see Fig. 2). Inspection of these two contrasting structural types reveal relatively small deviations from linearity at the C–N–M angle can have a major influence on the type of structure obtained. Furthermore, the CN–C–CN moieties are able to rotate out of the plane of the C₆X₄ ring (Fig. 24) and as a consequence of this conformational flexibility, the four metal centres bound to an X₄TCNQ dianion are not required to be co-planar.

Fig. 24 A schematic representation highlighting the conformational flexibility associated with free rotation around the formally single C–C bonds of the X₄TCNQ dianion.

The combination of TCNQ and F₄TCNQ dianions with cations has led to the formation of several organic salts. In the case when accepting cations are employed, charge transfer complexes are formed, which result in intensely coloured crystals. The nature of the cation employed has been found to have a significant impact on the crystal packing arrangements.

In many cases, contrary to the majority of radical derived compounds, open frameworks have been generated in which the X₄TCNQ²⁻ ligands are accessible. Such networks open up the very real possibility of generating redox active coordination polymers, via oxidative intercalation, in which some or all of the X₄TCNQⁿ/X₄TCNQⁿ moieties (n = 0 to −2) have been oxidised. Work in this area is currently ongoing.

Abbreviations

Ligands:

apyz	2-Aminopyrazine
2,2bipy	2,2′-Bipyridine
4,4bipy	4,4′-Bipyridine
DABCO	1,4-Diazabicyclo[2.2.2]octane
en	Ethylenediamine
Me4en	N,N,N′N′-tetramethylethylenediamine
Me2pyz	2,5-Dimethylpyrazine
phen	1,10-Phenanthroline
Obip	4,4′-Bipyridine-1,1′-dioxide
TACN	1,4,7-Triazacyclononane
tmpyz	2,3,5,6-Tetramethylpyrazine

Cations:

bv	Benzyl-viologen
dbdab	(Dibenzyl)dabco
dnbp	((3,5-Dinitro)benzyl)pyridinium
ddabx	(Didabco)xylene
diquat	1,1′-Ethylene-2,2′-bipyridylium
dcpx	Di-(4-cyanopyridinium)xylene
dnbdab	(Di-(4-nitrobenzyl))-dabco
dpbene	(Dipyridinium)butene
dpe	(Dipyridinium)ethane
dpicx	Di(picolinium)xylene
dbipx	Di-(4,4′-bipyridylium)xylene
dpmv	Diphenylmethane-viologen
dpx	(Dipyridinium)xylene
Etv	Ethyl-viologen
me2dab	N,N′-dimethyldabco
Meiq	N-methyl-isoquinolinium
Mepyz	N-methyl-pyrazinium
Mev	Methyl-viologen
mnbiq	(m-Nitrobenzyl)isoquinolinium
mnbp	(m-Nitrobenzyl)pyridinium
Nspiro	1,1′-Spirobipiperidinium
phv	Phenylviologen
pnbphp	p-Nitrobenzyl-4-phenylpyridinium
pnpyz	(p-Nitrobenzyl)pyrazinium
p-spongeH^+	8-(Dimethylamino)-N,N-dimethylnaphthalen-1-ammonium

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial support of the Australian Research Council. A. L. S. and R. W. E. gratefully acknowledge the support of the Australian Government with respect to the provision of research scholarships. Several data collections were performed on the macromolecular crystallography beamlines at the Australian Synchrotron, Victoria, Australia.

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