Coordination Chemistry of Polynitriles, Part XII— Serendipitous Synthesis of the Octacyanofulvalenediide Dianion and Study of Its Coordination Chemistry with K + and Ag +

: The reaction of diazotetracyanocyclopentadiene with copper powder in the presence of NEt 4 Cl yields, unexpectedly, besides the known NEt 4 [C 5 H(CN) 4 ] (3), the NEt 4 salt of octacyanoful-valenediide (NEt 4 ) 2 [C 10 (CN) 8 ] ( 5 ), which can be transformed via reaction with AgNO 3 to the corresponding Ag + salt ( 4 ), which in turn can be reacted with KCl to yield the corresponding K + salt 6 . The molecular and crystal structures of 4 – 6 could be determined, and show a signiﬁcantly twisted aromatic dianion which uses all its nitrile groups for coordination to the metals; 4 and 6 form three-dimensional coordination polymers with fourfold coordinated Ag + and eightfold coordinated K + cations.


Introduction
Diazotetracyanocyclopentadiene (1) was first reported by Webster in 1965.Reactivity studies showed that it "is a diazonium rather than a diazo compound", and that the mechanism of its reactions "is likely free radical".The presumed tetracyanocyclopentadienyl radical intermediate could be "generated polarographically at 0.23 V vs. sce in acetonitrile or by mild reducing agents" [1].Radical reactions are in general typical of arenedediazonium ions and allow "an easy entry into the chemistry of the aryl radical".For the generation of the radical, a reducing agent is needed, and Cu(I) appeared to be the most popular one.Typical reactions of the produced aryl radicals include H atom abstractions, formal halogen atom additions (so-called "Sandmeyer reactions") and homoand hetero-aryl coupling reactions [2].Although this type of chemistry is very old, it is still being studied intensively, and new synthetic applications are being developed [3,4].In the chemistry of the diazotetracyanocyclopentadiene molecule, one important difference compared to the just mentioned arenediazonium ions is that this compound is neutral, and therefore one-electron reduction generates a radical anion instead of a neutral radical.A combined theoretical/electrochemical study showed that this radical anion can be protonated to give the [C 5 H(CN) 4 ] radical, which in turn can be reduced to the [C 5 H(CN) 4 ] anion, or vice versa [5].Calculations showed that the neutral radicals [C 5 X(CN) 4 ] (X = H, CN) are strong oxidizers and can be termed "superhalogens" [6] or "hyperhalogens" [7], which is by definition an atom or atom group that has an electron affinity higher than a halogen atom.We could show recently that the radicals [C 5 X(CN) 4 ] can be isolated and structurally characterized, when X = NH 2 [8], while for X = NO 2 only electrochemical and EPR characterization was possible [9].Treatment of the NH 2 or NO 2 -substituted anions with Fe(III) generated Fe(II), and another group showed that when trying to prepare a Cu(II) complex of the [C 5 (CN) 5 ] anion, a "yet unknown oxidation of the [C 5 (CN) 5 ] ligand" occurred [10].The fate of the supposed intermediate [C 5 X(CN) 4 ] radicals remained unclear.For halogen radicals, dimerization to obtain the dihalogen X 2 molecules is common textbook knowledge.Thus, a dimerization of "superhalogens" might be expected as well.For radical anions such as [TCNQ] -, a σ-dimerization to give the [TCNQ-TCNQ] 2− dianion has been reported [11], while for [DDQ] -• a π dimer with formation of "pancake bonds" was observed [12,13].
Although there are many hints to the formation of radicals in the chemistry of the diazotetracyanocyclopentadiene molecule, one should not forget that many diazocyclopentadienes show the typical carbene chemistry.Thus, low-temperature photolysis of [C 5 Br 4 N 2 ] in an argon matrix generates the tetrabromocyclopentadienylidene carbene, which reacts with CO to give a ketene.Dimerization to octabromofulvalene is not observed at this temperature [14].Room-temperature laser flash photolysis of [C 5 Cl 4 N 2 ] in several solvents, such as alcohols, pyridine or THF, produced ylidic products derived from a triplet carbene [15], while low-temperature irradiation in an argon matrix in the presence of CF 3 I produced an ylidic iodonium ion [16].Thermolysis of [C 5 Cl 4 N 2 ] in the absence of solvent produced octachloronaphthalene, while treatment of hexane solutions of [C 5 X 4 N 2 ] (X = Br, Cl) with bis(µ-chloro-π-allyl palladium) at 0 • C generated the octahalofulvalenes.Both product types were regarded as products of carbene intermediates [17].Reaction of these tetrahalodiazocyclopentadienes with some Ni(0), Pt(0) and Ru(0) coordination compounds produced complexes of the type "L n M(N 2 -C 5 X 4 )", where the diazo compound is coordinated by one or both diazo-nitrogens [18].A high-temperature-high-pressure study of compound 1 in hexafluoro-isopropanol solution, devoted to the study of possible dinitrogen exchange, lead to the conclusion that the dediazoniation of (1) leads to an 1 A 2 singlet carbene [19].Formation of a dimerization product such as octacyanofulvalene or octacyanonaphthalene was not reported.However, thermal decomposition of Ag[C 5 H(CN) 2 (OMe) 2 ] produced the corresponding fulvalene [C 10 (CN) 4 (OMe) 4 ], which was also shown to undergo a stepwise two-electron reduction to the corresponding fulvalene dianion [20,21].
During the course of our studies on the coordination chemistry of [C 5 X(CN) 4 ] ions, we wanted to also look at the bromo and chloro derivatives (X = Br, Cl).For this purpose, we had to prepare these anions first, and we decided to employ the original synthetic pathway described by Webster in 1966.Here, we describe our experiences with this literature procedure and the unexpected results we obtained.

Reaction of Diazotetracyanocyclopentadiene with Chloride and Bromide
The thermal reaction of diazotetracyanocyclopentadiene (1) with NEt 4 + Cl − in the presence of copper powder or with NEt 4 + Br − without additives was reported by Webster to yield the halo-tetracyanocyclopentadienides 2a/b in high yields ( [1], Scheme 1).
icals, dimerization to obtain the dihalogen X2 molecules is common textbook know Thus, a dimerization of "superhalogens" might be expected as well.For radical such as [TCNQ] -, a σ-dimerization to give the [TCNQ-TCNQ] 2− dianion has been re [11], while for [DDQ] -• a π dimer with formation of "pancake bonds" was observed [ Although there are many hints to the formation of radicals in the chemistry diazotetracyanocyclopentadiene molecule, one should not forget that many diazo pentadienes show the typical carbene chemistry.Thus, low-temperature photoly [C5Br4N2] in an argon matrix generates the tetrabromocyclopentadienylidene ca which reacts with CO to give a ketene.Dimerization to octabromofulvalene is n served at this temperature [14].Room-temperature laser flash photolysis of [C5Cl several solvents, such as alcohols, pyridine or THF, produced ylidic products d from a triplet carbene [15], while low-temperature irradiation in an argon matrix presence of CF3I produced an ylidic iodonium ion [16].Thermolysis of [C5Cl4N2] absence of solvent produced octachloronaphthalene, while treatment of hexane sol of [C5X4N2] (X = Br, Cl) with bis(µ -chloro-π-allyl palladium) at 0 °C generated t tahalofulvalenes.Both product types were regarded as products of carbene interme [17].Reaction of these tetrahalodiazocyclopentadienes with some Ni(0), Pt(0) and coordination compounds produced complexes of the type "LnM(N2-C5X4)", where azo compound is coordinated by one or both diazo-nitrogens [18].A high-temper high-pressure study of compound 1 in hexafluoro-isopropanol solution, devoted study of possible dinitrogen exchange, lead to the conclusion that the dediazonia (1) leads to an 1 A2 singlet carbene [19].Formation of a dimerization product such a cyanofulvalene or octacyanonaphthalene was not reported.However, thermal dec sition of Ag[C5H(CN)2(OMe)2] produced the corresponding fulvalene [C10(CN)4(O which was also shown to undergo a stepwise two-electron reduction to the correspo fulvalene dianion [20,21]. During the course of our studies on the coordination chemistry of [C5X(CN)4 we wanted to also look at the bromo and chloro derivatives (X = Br, Cl).For this pu we had to prepare these anions first, and we decided to employ the original syn pathway described by Webster in 1966.Here, we describe our experiences with thi ature procedure and the unexpected results we obtained.

Reaction of diazotetracyanocyclopentadiene with chloride and bromide
The thermal reaction of diazotetracyanocyclopentadiene (1) with NEt4 + Cl - presence of copper powder or with NEt4 + Br -without additives was reported by W to yield the halo-tetracyanocyclopentadienides 2a/b in high yields ( [1], Scheme 1).When we tried to repeat the synthesis of 2a, we obtained a product mixture.The 1 H NMR spectrum (see Figure S1 of the Supporting Information) showed, besides the two NEt 4 multiplets, one singlet at δ = 6.93.The 13 C NMR spectrum (Figure S2) showed (besides the two NEt 4 carbon atoms) 10 signals, hinting to two different substances A and B of the type [C 5 X(CN) 4 ] (three signals for the ring carbons and two for the cyano groups each).Comparison with the NMR data of Ag[C 5 H(CN) 4 ], which we had prepared before on a different route [22], suggests that one of these compounds (A) is the tetraethylammonium salt of tetracyanocyclopentadienide, NEt 4 + [C 5 H(CN) 4 ] − (3).Our attempt to reproduce the synthesis of compound 2b also yielded a product mixture.The 1 H-NMR spectrum (Figure S3) looked very similar to the one obtained with NEt 4 Cl, suggesting that 3 had also been formed.This interpretation was supported by the 13 C NMR spectrum (Figure S4), which showed (besides the two NEt 4 signals) 15 signals, suggesting three different substances A, B and C all of the type [C 5 X(CN) 4 ].Two of the signal sets were identical to the ones obtained with NEt 4 Cl, and therefore the formation of compound 3 can be assumed for this reaction as well.The identity of compound B remained unclear, as its formation in both reactions excluded the presence of a halide.Since recrystallization attempts did not turn out successful in both cases (but see Section 2.1.3),both products were treated with AgNO 3 in acetone.
From the original NEt 4 Cl product, a mixture was obtained again which could be separated by column chromatography.A first fraction turned out to be the already known Ag[C 5 H(CN) 4 ] (Figures S5 and S6), while the second was obviously the silver analogue B' of the second product of the NEt 4 starting material because of the similarity with the 13 C NMR data (Figure S7).Recrystallization of this product from toluene/acetonitrile yielded X-ray quality crystals.The crystal structure determination identified the product B' as the complex Ag 2 [C 10 (CN) 8 ] (4) which contains the so-far unknown octacyanofulvalenediide dianion.It is therefore very likely that the unidentified product B is the NEt 4 analogue 5 of the silver complex 4.

Synthesis of K 2 [C 10 (CN) 8 ] (6)
Salt metathesis of the silver complex 4 with KCl in MeOH yielded the corresponding potassium complex salt K 2 [C 10 (CN) 8 ] (6) as a colorless solid.Its purity was confirmed by 1 H-and 13 C-NMR spectra (Figures S8 and S9).Recrystallization allowed the isolation of X-ray quality crystals.Our results are summarized in Scheme 2. It was mentioned in the Introduction that treatment of C 5 X 4 N 2 (X = Br, Cl) with a palladium complex yielded the corresponding octahalofulvalenes.We reasoned that compound 1 might also react with catalytically active metals.Just by chance, we chose the ruthenium complex [Ru(C 5 H 5 )Cl(PPh 3 ) 2 ] for our study.We studied the reaction of 1 both with stoichiometric or catalytic amounts of this ruthenium compound in MeCN as the solvent, using either reflux temperature (100 • C) or 0 • C.After addition of NEt 4 Cl or NEt 4 Br and chromatographic work-up, the main product was compound 3.However, MS analysis (ESI or FAB) showed that octacyanofulvalenediide had also been formed, i.e., compound 5.This was confirmed by crystal structure analysis.

Cyclovoltammetry
Cyclic voltammetry (CV) of crystalline 6 reveals one oxidation step at 1.25 V during oxidative scanning and a broad shoulder at 1.4 V in the reverse scan, which could be interpreted as partial reversibility similar to observations in several other tetracyanocyclopentadienides (Figure 1).Since 6 takes the form of a dianion in its initial stage, oxidation leads to a radical mono-anion.While the possibility of a two-electron transfer to a neutral compound (analogous to a fulvalene compound) cannot be excluded due to the broad shoulders of the voltagramm, it would not be supported by results from the preparative part of this work, which gave no indication of such a compound being present.The lack of a neutral fulvalene analogue in CV might indicate that the neutral form of this compound is highly unfavorable, while the radical and dianionic forms are better stabilized.This would fit the electron-deficient nature of the system, which in theory should favor the charged states.
Inorganics 2023, 11, x FOR PEER REVIEW Scheme 2. Summary of the reactions described in Sections 2.1.1 and 2.1.2.The formation of not observed in our study, but was reported in the literature.

Reaction of diazotetracyanocyclopentadiene with [Ru(C5H5)Cl(PPh3)2]
It was mentioned in the Introduction that treatment of C5X4N2 (X = Br, Cl) palladium complex yielded the corresponding octahalofulvalenes.We reasoned th pound 1 might also react with catalytically active metals.Just by chance, we ch ruthenium complex [Ru(C5H5)Cl(PPh3)2] for our study.We studied the reaction of with stoichiometric or catalytic amounts of this ruthenium compound in MeCN solvent, using either reflux temperature (100 °C) or 0 °C.After addition of NE NEt4Br and chromatographic work-up, the main product was compound 3. Howev analysis (ESI or FAB) showed that octacyanofulvalenediide had also been form compound 5.This was confirmed by crystal structure analysis.Scheme 2. Summary of the reactions described in Sections 2.1.1 and 2.1.2.The formation of 2a was not observed in our study, but was reported in the literature.

EPR Spectroscopy
EPR measurements show signals in both the compounds 4 and 6 with g values of 2.001 and 2.002 when subjected to UV or sunlight, similar to measurements of other 1,2,3,4 tetracyanocyclopentadienide derivatives, indicating free radical formation by photoinduced electron transfer (Figure 2).While the signals of 6 are short-lived, 4 shows less degradation over time, a property also observed in other derivatives of this ligand class.
of a neutral fulvalene analogue in CV might indicate that the neutr pound is highly unfavorable, while the radical and dianionic forms a This would fit the electron-deficient nature of the system, which in the charged states.

Molecular and Crystal Structure of Compound 1
Compound 1 crystallizes in the orthorhombic space group Pbca with one the asymmetric unit.Figure 3 shows an ORTEP3 view of the molecular unit.Ta some important metric parameters of the molecule.Although there is still som in the C-C bond lengths of the ring (two "short" bond lengths of ca.1.393(1) "long" ones with ca.1.418(1) Å ), they are definitely more delocalized than in t  Compound 1 crystallizes in the orthorhombic space group Pbca with one molecule in the asymmetric unit.Figure 3 shows an ORTEP3 view of the molecular unit.Table 1 collects some important metric parameters of the molecule.Although there is still some variation in the C-C bond lengths of the ring (two "short" bond lengths of ca.1.393(1) Å and three "long" ones with ca.1.418(1) Å), they are definitely more delocalized than in the structure of diazo-tetraphenylcyclopentadiene [23].In this compound, the "short" bonds are at 1.373(3) Å and the "long" bonds average at 1.450(3) Å.In addition, the N-N bond of this compound is with 1.121(3) Å, which is substantially longer, while the C-N 2 bond is significantly shorter (1.316(3) Å).Unfortunately, there are no more diazocyclopentadiene structures available for comparison.However, there are some DFT calculations on the parent compound, which yield C-C bond lengths of 1.369 and 1.447 Å and a C-N 2 bond length of 1.312 Å [24].

Crystal and Molecular Structure of Compound 4, Toluene So
Compound 4 crystallizes as a bis-toluene solvate Ag2[C10(CN)8 oclinic space group C2/c with half a molecule in the asymmetric twinned and were refined against a HKL5 dataset; in addition, the t  When looking at intermolecular interactions, there are several short N. . .N contacts, particularly between atoms N1 and N5' (1/2 +x, 1.5-y, 1-z) which are closer by 0.23 Å than the sum of their Van der Waals radii.Thus, an infinite 1D chain forms with the base vector [1 0 0] (Figure S10).

Crystal and Molecular Structure of Compound 4, Toluene Solvate
Compound 4 crystallizes as a bis-toluene solvate Ag 2 [C 10 (CN) 8 ] × 2(C 7 H 8 ) in the monoclinic space group C2/c with half a molecule in the asymmetric unit.All crystals were twinned and were refined against a HKL5 dataset; in addition, the toluene molecules were disordered (50:50) across an inversion center.Two crystals were measured, one at 298 K and one at 100 K. Since the results of both refinements did not produce significant differences, here only the low temperature results are discussed.The results of the room-temperature determination are reported in the Supplementary Information.
Figure 4 shows an ORTEP3 view of the asymmetric unit (Figure S11 shows the corresponding plot of the r.t.determination), Figure 5 shows the coordination sphere of the unique silver atom and Figure 6 displays the anion with all coordinated silver ions.
Figure 4 shows an ORTEP3 view of the asymmetric unit (Figure S11 shows the cor sponding plot of the r.t.determination), Figure 5 shows the coordination sphere of t unique silver atom and Figure 6 displays the anion with all coordinated silver ions.Figure 4 shows an ORTEP3 view of the asymmetric unit (Figure S11 shows the corre sponding plot of the r.t.determination), Figure 5 shows the coordination sphere of the unique silver atom and Figure 6 displays the anion with all coordinated silver ions.Figure 4 shows an ORTEP3 view of the asymmetric unit (Figure S11 shows the corr sponding plot of the r.t.determination), Figure 5 shows the coordination sphere of th unique silver atom and Figure 6 displays the anion with all coordinated silver ions.The silver ion is tetrahedrally coordinated by four different octacyanofulvalenediide anions, while each dianion is coordinated to eight silver ions using all of its eight cyano nitrogen atoms.The two halves of the dianion are joined by a single bond between C4 and C4_ii (symm.op.: 1-x, y, 1.5-z) and are twisted by ca.54 • (which is an equivalent description of the torsion angle calculated as −126.1 • ).Other important bond parameters are collected in Table 2.The symmetry operators given in the Ag-N distances refer to Figure 5, while the torsion angle refers to Figure 6.The silver ion is tetrahedrally coordinated by four different octacyanofulvalenediid anions, while each dianion is coordinated to eight silver ions using all of its eight cyano nitrogen atoms.The two halves of the dianion are joined by a single bond between C4 and C4_ii (symm.op.: 1-x, y, 1.5-z) and are twisted by ca.54° (which is an equivalent descrip tion of the torsion angle calculated as −126.1°).Other important bond parameters are col lected in Table 2.The symmetry operators given in the Ag-N distances refer to Figure 5, while the torsion angle refer to Figure 6.
Compound 4 forms a three-dimensional polymer structure, with base vectors [1 0 0] [0 1 0] and [0 0 1]. Figure 7 shows a packing plot watched along the crystallographic b axis and Figure 8 a packing plot along a.Although we were not able to remove the toluene chemically, we just removed it virtually from the crystal structure.Its exclusion reveals a network of pores with a diameter of 5.8 Å at the bottleneck.The calculation of solvent accessible voids shows a volume of 1664 Å 3 per cell, corresponding to 56% of cell volume.The space takes the form of helical channels running parallel to the crystallographic b axis (Figure S12).At the same time, exclusion of the toluene guests from the packing diagrams shows that there exist a series of fused [Ag 2 (NC-(C n )-CN) 2 ] ring systems, as they are quite often observed in the structures of silver polycyanocyclopentadienides. Figure S13 shows the common 14-membered rings (involving nitrile nitrogens N2 and N3) and two different 20-membered rings involving either nitrogens N1 and N3 or N2 and N4.In addition, there are helices winding down the b axis involving nitrogens N1 and N2.Ag. . .Ag distances across the rings are 7.023 and 8.799 Å, respectively, and across the helix the distance is 7.363 Å.Although we were not able to remove the toluene chemically, we just removed virtually from the crystal structure.Its exclusion reveals a network of pores with a diam eter of 5.8 Å at the bottleneck.The calculation of solvent accessible voids shows a volum of 1664 Å 3 per cell, corresponding to 56% of cell volume.The space takes the form of helic channels running parallel to the crystallographic b axis (Figure S12).At the same tim exclusion of the toluene guests from the packing diagrams shows that there exist a serie of fused [Ag2(NC-(Cn)-CN)2] ring systems, as they are quite often observed in the stru tures of silver polycyanocyclopentadienides. Figure S13 shows the common 14-membere rings (involving nitrile nitrogens N2 and N3) and two different 20-membered rings in volving either nitrogens N1 and N3 or N2 and N4.In addition, there are helices windin down the b axis involving nitrogens N1 and N2.Ag…Ag distances across the rings ar 7.023 and 8.799 Å , respectively, and across the helix the distance is 7.363 Å .

Crystal and Molecular Structure of Compound 5
Compound 5 crystallizes in the orthorhombic space group Pbcn with two independ ent half molecules in the unit cell.Figure 9 shows an ORTEP3 plot of the asymmetric unit

Crystal and Molecular Structure of Compound 5
Compound 5 crystallizes in the orthorhombic space group Pbcn with two independent half molecules in the unit cell.Figure 9 shows an ORTEP3 plot of the asymmetric unit.Although we were not able to remove the toluene chemically, we ju virtually from the crystal structure.Its exclusion reveals a network of pore eter of 5.8 Å at the bottleneck.The calculation of solvent accessible voids sh of 1664 Å 3 per cell, corresponding to 56% of cell volume.The space takes the channels running parallel to the crystallographic b axis (Figure S12).At t exclusion of the toluene guests from the packing diagrams shows that ther of fused [Ag2(NC-(Cn)-CN)2] ring systems, as they are quite often observe tures of silver polycyanocyclopentadienides. Figure S13 shows the common rings (involving nitrile nitrogens N2 and N3) and two different 20-memb volving either nitrogens N1 and N3 or N2 and N4.In addition, there are he down the b axis involving nitrogens N1 and N2.Ag…Ag distances across 7.023 and 8.799 Å , respectively, and across the helix the distance is 7.363 Å .

Crystal and Molecular Structure of Compound 5
Compound 5 crystallizes in the orthorhombic space group Pbcn with t ent half molecules in the unit cell.Figure 9 shows an ORTEP3 plot of the asy  The half molecules are completed via single bonds between atoms C101-C101 i and C201-C201 i , respectively, created by a twofold rotation axis through the middle of these bonds (symmetry operator: 1-x, y, 1.5-z).The complete octacyanofulvalenediide dianion of molecule A is shown in Figure 10.The two rings are twisted by ca.54 • .The bond parameters of the two independent molecules are very similar and are collected in Table 3.
The unit cell contains small cavities at the corners and the center, making up for approximately 1.1% of the volume.They are, however, too small to accommodate solvent molecules.As there is no metal ion in the structure, and no "classical" H-bond donor, all anions are isolated and separated from each other.Figure 11 shows a packing plot of this structure.
C201-C201 i , respectively, created by a twofold rotation axis throug bonds (symmetry operator: 1-x, y, 1.5-z).The complete octacyanofu of molecule A is shown in Figure 10.The two rings are twisted by rameters of the two independent molecules are very similar and are The unit cell contains small cavities at the corners and the cent proximately 1.1% of the volume.They are, however, too small to a molecules.As there is no metal ion in the structure, and no "classic anions are isolated and separated from each other.Figure 11 shows structure.The aqua complex 6a crystallizes in the orthorhombic space group Ccca with a quarter molecule in the asymmetric unit (Figure 12).
The potassium ions are coordinated by six nitrile nitrogens (from five different dianions) and two water oxygens (Figure 13).Pairs of K + ions related by [x, y, z] and [-x-1/2, -y, z] form zig-zag chains of face-fused (distorted) octahedra along the x direction, with metal-metal distances of 3.99 Å (Figure S14).The dianion is coordinated to ten K + ions, of which two are bridging two nitrile functions of the same dianion (Figure 14).Important bond parameters are collected in Table 4.
Pairs of independent zig-zag chains of potassium ions (viewed in superposition in Figure 15, or displayed separately in Figure S15), as also shown for a single chain in Figure S14, run along the a axis at y = 0 and y = 0.5, and are bridged by octacyanofulvalenediides via nitrogens N1 in the b direction and N2 in the c direction (Figure S16).The closest approach of two K + ions in the b direction is 8.756 Å, while in the direction of the ab diagonal it is 7.601 Å.The aqua complex 6a crystallizes in the orthorhombic space group Ccca with a qu ter molecule in the asymmetric unit (Figure 12).The potassium ions are coordinated by six nitrile nitrogens (from five different d ions) and two water oxygens (Figure 13).Pairs of K + ions related by [x,y,z] and [-x-½ ,  The aqua complex 6a crystallizes in the orthorhom ter molecule in the asymmetric unit (Figure 12).The potassium ions are coordinated by six nitrile n    The symmetry operators in the distances and angles involving K1 refer to Figure 13, while th involving only the anion refer to Figure 14.The symmetry operators in the distances and angles involving K1 refer to Figure 13, while th involving only the anion refer to Figure 14.The symmetry operators in the distances and angles involving K1 refer to Figure 13, while the ones involving only the anion refer to Figure 14.Pairs of independent zig-zag chains of potassium ions (viewed in superp Figure 15, or displayed separately in Figure S15), as also shown for a single chain S14, run along the a axis at y = 0 and y = 0.5, and are bridged by octacyanofulval via nitrogens N1 in the b direction and N2 in the c direction (Figure S16).The c proach of two K + ions in the b direction is 8.756 Å , while in the direction of the ab it is 7.601 Å .

Discussion
Diazotetracyanocyclopentadiene had been reported to behave as an aroma nium ion, based on its reactivity [1] and its 13 C-and 15 N-NMR data [25].Our crys ture determination supports this view.Its bond length alternation parameter  0.018 Å , much smaller than the 0.078 Å calculated for C5H4N2, which was already as small and as having a strong indication of aromaticity [24].We also perform DFT calculations (B3LYP/6-311G(dp) level) using the program CrystalExplorer ures S17-S19 show HOMO/LUMO contours, an electron density visualization views of the electrostatic potential.In addition, these results support the high ar of the cyclopentadienyl unit.With this background, the typical reactivity of are nium ions towards metals and metal salts, i.e., radical reactions, should be exp

Discussion
Diazotetracyanocyclopentadiene had been reported to behave as an aromatic diazonium ion, based on its reactivity [1] and its 13 C-and 15 N-NMR data [25].Our crystal structure determination supports this view.Its bond length alternation parameter ∆ R is only 0.018 Å, much smaller than the 0.078 Å calculated for C 5 H 4 N 2 , which was already regarded as small and as having a strong indication of aromaticity [24].We also performed some DFT calculations (B3LYP/6-311G(dp) level) using the program CrystalExplorer [26].Figures S17-S19 show HOMO/LUMO contours, an electron density visualization and two views of the electrostatic potential.In addition, these results support the high aromaticity of the cyclopentadienyl unit.With this background, the typical reactivity of arenediazonium ions towards metals and metal salts, i.e., radical reactions, should be expected for compound 1.Unfortunately, the major reaction of these radicals is H abstraction to give the [C 5 (CN) 4 H] anion.The usual main products of the Cu(I)-catalyzed coupling of aromatic diazonium ions, i.e., biaryls and azo compounds [27,28], were formed only in small yields or not at all.(It should be mentioned, however, that the original publication on the synthesis and reactivity of compound 1 reports the formation of an azo compound in the thermal reaction of 1 with CuCN in MeCN [1].)Although our study was not a true mechanistic one, we assume that the low yields of coupling products are the consequence of both steric and electronic repulsions: the two "ortho" cyano groups and the negative charges inhibit the mutual approach of the two radical anions, and thus the H abstractions dominate the reaction kinetics.The fact that rather high yields of coupling products can be obtained from the reaction of 1 with [RuCp(PPh 3 ) 2 Cl] remains rather mysterious.The only comparable reaction in the literature is the reaction of [RuCp*(P(OR) 3 ) 2 Cl] with 1,1-diaryldiazomethanes, which was reported to give cationic N-coordinated Ru diazoalkane complexes [29] with no indication of any coupling products.This result is reminiscent of the N-coordinated C 5 Cl 4 N 2 complexes mentioned above [18].Now, the only "explanation" might be that the aromaticity of compound 1 drives the reaction into another direction compared with "true" diazoalkanes and less aromatic diazocyclopentadienes.
The structure of compound 5 can be regarded as the structure of the "free" octacyanofulvalenediide anion because there are no coordinating cations or strong H-bond donors (there are, however, very weak interactions with aliphatic C-H bonds, but their influence on bond parameters can be neglected).The cyclopentadienyl rings are aromatic, as can be derived from the bond alteration parameter ∆ R , which amounts to 0.019 Å in molecule A and 0.031 Å in molecule B. In addition, the bonds to the nitrile carbon atoms have the same lengths (within 2σ) as the bonds within the ring.The central C-C bond between the two cyclopentadienyl halves is a typical single bond with ca.1.47 Å.The two cyclopentadienyl rings are twisted by ca.55 • .In the Ag + complex 4 (low temperature), these bond parameters hardly change.∆ R amounts to 0.018 Å and the bonds within the ring and between the ring and nitrile carbons are identical within 2σ.Both halves of the fulvalenediide anion are bound via a single bond and are twisted by ca.54 • .In the K + complex 6, the bond alteration parameter also amounts to 0.018 Å.The exocyclic C-C bonds are slightly longer than the endocyclic ones.The single bond between the two cyclopentadienyl rings is ca.1.48 Å long, with the two halves being twisted by ca.40 • .This significantly smaller twist angle is a consequence of the fact that the potassium ion bridges two of the "ortho" nitriles.
The different sizes of Ag + ("effective ionic radius" 1.00 Å for CN 4, [30]) and K + (1.51 Å for CN 8, [30]) lead to significant differences in the coordination spheres.While the Ag + ion is tetrahedrally coordinated by four nitrile nitrogens with bond distances ranging from 2.25-2.31Å, the K + ion has coordination number 8, KN 6 O 2 polyhedra, with bond distances between 2.84 and 2.92 Å.The Ag-N-C bonds deviate only slightly (11-16 • ) from linearity, but the K-N-C bonds can be regarded as "bent", particularly at atoms N1.Both structures might be compared with the structures of [AgC 5 (CN) 5 ] and [KC 5 (CN) 5 ], respectively [31,32].These compounds show different coordination polyhedra dependent on the solvent the crystals came from.Crystals of the Ag + salt from EtOH show severely distorted AgN 4 tetrahedra, with Ag-N distances ranging from 2.236(5) to 2.378(6) Å, N-Ag-N angles between 88.0(2) and 129.7(2) • , and Ag-N-C angles between 149.7(6) and 169.9(5) • .The pentacyanocyclopentadienide anion uses only four of its nitrile groups for coordination.The compound forms an interwoven 3D network with very large pores, similar to the situation found in 4 [31].[KC 5 (CN) 5 ], when grown from isopropanol/pentane, shows an octahedral metal ion with K-N bonds ranging from 2.777(1) to 2.9537(6) Å and a pentacyanocyclopentadienide using all five nitrile groups for coordination, with one bridging two K + ions in 4.975 Å distance, and K-N-C angles ranging from 122.Octacyanofulvalenediide uses, in both compounds 4 and 6, all of its eight nitrile groups for coordination; in compound 6, four of them coordinate to two symmetry-related K + ions each, which is similar to the situation found in [KC 5 (CN) 5 ].Therefore, both compounds form three-dimensional coordination polymers, with the potassium structure being more "complicated".
CV measurements were performed with an Autolab potentiostat/galvanostat (PG-STAT302N) with a FRA32M module operated with Nova 1.11 software and a conventional three-electrode setup.Two platinum wires were used as the working and counter electrode, respectively.A Ag/0.01M AgNO 3 electrode was utilized as reference electrode.

Reaction of 1 with Cu and NEt 4 Cl in MeCN
A suspension of powdered copper (0.57 g, 9.0 mmol) and NEt 4 Cl (1.58 g, 9.5 mmol) in acetonitrile (30 mL) was heated to 90 • C. A solution of 1 (1.13 g, 5.9 mmol) in MeCN (19 mL) was added to the boiling mixture within one hour.After cooling down to room temperature and filtering, the solvent was removed in vacuo.A brown powder was obtained (2.18 g). 1 H-and 13 C{ 1 H}-NMR spectra (Figures S1 and S2) suggested the presence of two compounds: A (=3) and B (=5).

Reaction of 1 with NEt 4 Br in MeCN
A solution of 1 (3.00 g, 15.6 mmol) and NEt 4 Br (10.10 g, 48.1 mmol) in MeCN (160 mL) was heated at 50 • C for 30 min.Then, the solvent was evaporated in vacuo, and water (150 mL) was added and the resulting suspension was filtered.The residue on the filter was washed with water (50 mL) and dried in vacuo.A brown powder was obtained (3.06 g). 1 Hand 13 C{ 1 H}-NMR spectra (Figures S3 and S4) showed the formation of three compounds: A (=3), B (=5) and C (=2b).

Figure 2 .
Figure 2. EPR spectra of compounds 4 and 6 after being subjected to sunlight.

Figure 2 .
Figure 2. EPR spectra of compounds 4 and 6 after being subjected to sunlight.

Figure 3 .
Figure 3. ORTEP3 view of the molecular unit of compound 1.

Figure 3 .
Figure 3. ORTEP3 view of the molecular unit of compound 1.

Figure 4 .Figure 5 .
Figure 4.The asymmetric unit of compound 4 (only one orientation of the toluene solvent shown

Figure 4 .
Figure 4.The asymmetric unit of compound 4 (only one orientation of the toluene solvent shown).

Figure 4 .
Figure 4.The asymmetric unit of compound 4 (only one orientation of the toluene solvent shown).

Figure 4 .Figure 5 .
Figure 4.The asymmetric unit of compound 4 (only one orientation of the toluene solvent shown

Figure 7 .
Figure 7. Packing plot (MERCURY) of compound 4, watched along b.Color coding: green is the coor dination polymer, while blue/red are the disordered toluene molecules.

Figure 7 .
Figure 7. Packing plot (MERCURY) of compound 4, watched along b.Color coding: green is the coordination polymer, while blue/red are the disordered toluene molecules.

Figure 8 .
Figure 8. Packing plot (MERCURY) of compound 4, watched along a. Same color coding as in Figure 7.

Figure 10 . 3 .
Figure 10.The complete octacyanofulvalenediide dianion of molecule A of

Figure 11 .
Figure 11.Packing plot (MERCURY) of compound 5, watched along the crystallographic c axis.C coding: blue and yellow are the NEt4 cations, yellow and red the [C10(CN)8] dianions.

Figure 12 .
Figure 12.ORTEP3 view of the asymmetric unit of 6a.

Figure 11 .
Figure 11.Packing plot (MERCURY) of compound 5, watched along the crystallographic c axis.Color coding: blue and yellow are the NEt 4 cations, yellow and red the [C 10 (CN) 8 ] dianions.

Figure 11 .
Figure 11.Packing plot (MERCURY) of compound 5, watched a coding: blue and yellow are the NEt4 cations, yellow and

Figure 12 .
Figure 12.ORTEP3 view of the asymmetric unit of 6a.

Figure 12 .
Figure 12.ORTEP3 view of the asymmetric unit of 6a.

Figure 15 .
Figure 15.Packing plot of compound 6a, watched along the crystallographic c axis.Color coding "by symmetry operation" (default settings of MERCURY).
1 to 138.0 • [32].Comparison of these compounds with our compounds 4 and 6 shows a wider range of metal-nitrogen distances in the [C 5 (CN) 5 ] salts and also a higher degree of bending in the M-N-C groups.Alternatively, one might use for comparison the structures of the tricyanomethanides M[C(CN) 3 ].While the silver salt has a coordination number of three with Ag-N distances at 2.16 and 2.27 Å and Ag-N-C angles of 172.8 • and 153.7 • [33], the potassium salt has seven nitrile nitrogens coordinated at 2.86-2.98Å and K-N-C angles ranging from 114 to 159 • [34].The K[C(CN) 3 ] contains triply N-bridged K + ions with a K. . .K distance of 3.89 Å.

Table 1 .
Important bond parameters of compound 1.
1Ccp are the carbon atoms of the ring, while CCN are the carbon atoms of th

Table 1 .
Important bond parameters of compound 1.