Cation Adaptive Structures Based on Manganese Cyanide Prussian Blue Analogues: Application of Powder Diffraction Data to Solve Complex, Unprecedented Stoichiometries and New Structure Types.

A Mn(II) salt and A+ CN- under anaerobic conditions react to form 2-D and 3-D extended structured compounds of Am MnII n (CN)m+2n stoichiometry. Here, the creation and characterization of this large family of compounds, for example AMnII 3 (CN)7 , A2 MnII 3 (CN)8 , A2 MnII 5 (CN)12 , A3 MnII 5 (CN)13 , and A2 MnII [MnII (CN)6 ], where A represents alkali and tetraalkylammonium cations, is reviewed. Cs2 MnII [MnII (CN)6 ] has the typical Prussian blue face centered cubic unit cell. However, the other alkali salts are monoclinic or rhombohedral. This is in accord with smaller alkali cation radii creating void space that is minimized by increasing the van der Waals stabilization energy by reducing ∠Mn-N≡C, which, strengthens the magnetic coupling and increases the magnetic ordering temperatures. This is attributed to the non-rigidity of the framework structure due the significant ionic character associated with the high-spin MnII sites. For larger tetraalkylammonium cations, the high-spin Mn sites lack sufficient electrostatic A+ ⋅⋅⋅NC stabilization and form unexpected 4- and 5-coordinated Mn sites within a flexible, extended framework around the cation; hence, the size, shape, and charge of the cation dictate the unprecedented stoichio-metry and unpredictable cation adaptive structures. Antiferromagnetic coupling between adjacent MnII sites leads to ferrimagnetic ordering, but in some cases antiferromagnetic coupling of ferrimagnetic layers are compensated and synthetic antiferromagnets are observed. The magnetic ordering temperatures for ferrimagnetic A2 MnII [MnII (CN)6 ] with both octahedral high- and low-spin MnII sites increase with decreasing ∠Mn-N≡C. The crystal structures for all of the extended structured materials were obtained by powder diffraction.

Artificial Antiferromagnets Possessing Extended Zero-, One-, Two-, and Three-Dimensional Structures.

Layered (2D) artificial (or synthetic) antiferromagnets are fabricated by atom deposition techniques and possess very thin, nanometer-scale, magnetically ordered layers separated by a very thin nonmagnetic layer that antiferromagnetically couples the magnetic layers. Artificial antiferromagnets were crucial in the discovery of the giant magnetic effect (GMR), which had an incredible impact on the evolution of computer memory and its applications, and nucleated the dawn of spintronics (magnetoelectrics). The fundamental structural motif has been more recently achieved by using synthetic chemical methods that led to insulating artificial antiferromagnets. Examples of magnetically ordered layers that are antiferromagnetic coupled to form artificial antiferromagnets have been extended to isolated ions (0D) as well as extended chain (1D) and extended network 3D structures, and new phenomena and applications are anticipated as insulating antiferromagnets are more effective at propagating spin currents with respect to dielectric materials.

Pressure Dependence of the Magnetic Ordering Temperature (Tc) for the Na2Mn[Mn(CN)6] Noncubic Prussian Blue Analogue.

The pressure dependence of the magnetic properties of rhombohedral Na2Mn[Mn(CN)6] up to 10 kbar has been studied. The magnetic ordering temperature, Tc, for Na2Mn[Mn(CN)6] reversibly increases with increasing applied hydrostatic pressure, P, by 9.0 K (15.2%) to 68 K at 10 kbar with an average rate of increase, dTc/dP, of 0.86 K/kbar. The magnetization at 50 kOe and remanent magnetization, Mr(H), remain constant with an average value of 13,100 ± 200 and 8500 ± 200 emuOe/mol. The coercive field Hcr increases by 12% from 13,400 to 15,000 Oe. The increase and rate of increase of Tc for rhombohedral Na2Mn[Mn(CN)6] are reduced with respect to monoclinic A2Mn[Mn(CN)6] (A = K and Rb), but they are still greater than those of cubic Cs2Mn[Mn(CN)6]. This is attributed to the compression of the MnNC framework bonding without decreasing ∠MnII-N≡C, maintaining the unit cell in accord with cubic A = Cs at lower applied pressures, and not due to reduction in ∠MnII-N≡C, which correlates with increasing Tc that is reported for A = K and Rb as well as Cs at higher applied pressures.

Ferrimagnetic Ordering and Anomalous Stoichiometry Observed for the Cubic, Extended 3D Prussian Blue Analogues (NEt3 Me)2 MnII 5 (CN)12 and (NEt2 Me2 )2 MnII 5 (CN)12 : A Cation-Adaptive Structure.

The reactions of MnII (O2 CCH3 )2 with NEt3 Me+ CN- and NEt2 Me2 + CN- form (NEt3 Me)2 MnII 5 (CN)12 (1) and (NEt2 Me2 )2 MnII 5 (CN)12 (2), respectively. Structure model-building and Rietveld refinement of high-resolution synchrotron powder diffraction data revealed a cubic [a=24.0093 Š(1), 23.8804 Š(2)] 3D extended structural motif with adjacent tetrahedral and octahedral MnII sites in a 3:2 ratio. Each tetrahedral MnII site is surrounded by four low-spin octahedral MnII sites, and each octahedral MnII site is surrounded by six high-spin tetrahedral MnII sites; adjacent sites are antiferromagnetically coupled in 3D. Compensation does not occur, and magnetic ordering as a ferrimagnet is observed at Tc =13 K for 2 based on the temperature at which remnant magnetization, Mr (T)→0. The hysteresis has an unusual constricted shape with inflection points around 50 and 1.2 kOe with a 5 K coercivity of 16 Oe and remnant magnetization, Mr , of 2050 emuOe mol-1 . The unusual structure and stoichiometry are attributed to the very ionic nature of the high-spin N-bonded MnII ion, which enables the maximization of the attractive van der Waals interactions through minimization of void space via a reduced ∠ MnNC. This results in an additional example of the Ax MnII y (CN)x+2y (x=0, y=1; x=1, y=3; x=2, y=1; x=2, y=2; x=2, y=3; x=3, y=5; and x=4, y=1) family of compounds possessing an unprecedented stoichiometry and lattice motif that are cation adaptive structured materials.

Solid-State 13 C†NMR Evidence for Long Multicenter Intradimer Bonding in Zwitterion-like Structures.

The principal values of the 13 C chemical shift tensor for the ß and δ polymorphs of π-[TTF⋅⋅⋅TCNE] (TTF=tetrathiafulvalene; TCNE=tetracyanoethylene) have been analyzed to understand the abnormally long intra-dimer bonding of singlet π-[TTFδ+ ⋅⋅⋅TCNEδ- ]. These structures possess 12 intradimer contacts <3.40 Å, with the shortest intra π-[TTF⋅⋅⋅TCNE] separations involving 2-center (2c) C-S and 3c C-C-C orbital overlap contributions between the [TTF]δ+ and [TCNE]δ- . This solid-state NMR study compares the [TTF⋅⋅⋅TCNE] 13 C tensor data against previously reported π-[TTF]2 2+ and π-[TCNE]2 2- homo-dimers to determine how the tensor principal values change as a function of electronic structure for both TTF and TCNE moieties. In the ß and δ phases of [TTF⋅⋅⋅TCNE], the TCNE ethylenic 13 C shift tensors predict TCNE oxidation states of -0.46 and -0.73, respectively. The TTF sites are less similar to benchmark 13 C data with the ß-phase differing primarily in the ethylenic π-electrons. The δ form differs significantly from the homo-dimer data at all principal values at both the ethylenic and CH sites, indicating changes to both the π-electrons and σ-bonds. In both hetero-dimer phases, the NMR changes supports long bond formation at nitrile and CH sites not observed in homo-dimers.

Anomalous Stoichiometry, 3-D Bridged Triangular/Pentagonal Layered Structured Artificial Antiferromagnet for the Prussian Blue Analogue A3MnII5(CN)13 (A = NMe4, NEtMe3). A Cation Adaptive Structure.

The size of the organic cation dictates both the composition and the extended 3-D structure for hybrid organic/inorganic Prussian blue analogues (PBAs) of A aMnII b(CN) a+2 b (A = cation) stoichiometry. Alkali PBAs are typically cubic with both MC6 and M'N6 octahedral coordination sites and the alkali cation content depends on the M and M' oxidation states. The reaction of MnII(O2CCH3)2 and A+CN- (A = NMe4, NEtMe3) forms a hydrated material of A3MnII5(CN)13 composition. A3MnII5(CN)13 forms a complex, 3-D extended structural motif with octahedral and rarely observed square pyramidal and trigonal bipyramidal MnII sites with a single layer motif of three pentagonal and one triangular fused rings. A complex pattern of MnIICN chains bridge the layers. (NMe4)3MnII5(CN)13 possesses one low-spin octahedral and four high-spin pentacoordinate MnII sites and orders as an antiferromagnet at 11 K due to the layers being bridged and antiferromagnetically coupled by the nonmagnetic cyanides. These are rare examples of intrinsic, chemically prepared and controlled artificial antiferromagnets and have the advantage of having controlled uniform spacing between the layers as they are not physically prepared via deposition methods. A3Mn5(CN)13 (A = NMe4, NEtMe3) along with [NEt4]2MnII3(CN)8, [NEt4]MnII3(CN)7, and Mn(CN)2 form stoichiometrically related A aMnII b(CN) a+2 b ( a = 0, b = 1; a = 2, b = 3; a = 1, b = 3; and a = 3, b = 5) series possessing unprecedented stoichiometries and lattice motifs. These unusual structures and stoichiometries are attributed to the very ionic nature of the high-spin N-bonded MnII ion that enables the maximization of the attractive van der Waals interactions via minimization of void space via a reduced ∠MnNC. This A aMnII b(CN) a+2 b family of compounds are referred to as being cation adaptive in which size and shape dictate both the stoichiometry and structure.

Organic-based magnon spintronics.

Magnonics concepts utilize spin-wave quanta (magnons) for information transmission, processing and storage. To convert information carried by magnons into an electric signal promises compatibility of magnonic devices with conventional electronic devices, that is, magnon spintronics 1 . Magnons in inorganic materials have been studied widely with respect to their generation2,3, transport4,5 and detection 6 . In contrast, resonant spin waves in the room-temperature organic-based ferrimagnet vanadium tetracyanoethylene (V(TCNE) x (x ≈ 2)), were detected only recently 7 . Herein we report room-temperature coherent magnon generation, transport and detection in films and devices based on V(TCNE) x using three different techniques, which include broadband ferromagnetic resonance (FMR), Brillouin light scattering (BLS) and spin pumping into a Pt adjacent layer. V(TCNE) x can be grown as neat films on a large variety of substrates, and it exhibits extremely low Gilbert damping comparable to that in yttrium iron garnet. Our studies establish an alternative use for organic-based magnets, which, because of their synthetic versatility, may substantially enrich the field of magnon spintronics.

Viewpoint: Metalloids-An Electronic Band Structure Perspective.

Segment of a periodic table depicting the elements that are molecular/atomic (blue) and that have extended network structures (gray) at STP.

Anomalous Stoichiometry and Antiferromagnetic Ordering for the Extended Hydroxymanganese(II) Cubes/Hexacyanometalate-Based 3D-Structured [MnII 4 (OH)4 ][MnII (CN)6 ](OH2 )6 ⋠H2 O.

The reaction of MnII (O2 CMe)2 and NaCN or LiCN in water forms a light green insoluble material. Structural solution and Rietveld refinement of high-resolution synchrotron powder diffraction data for this unprecedented, complicated compound of previously unknown composition revealed a new alkali-free ordered structural motif with [MnII 4 (µ3 -OH)4 ]4+ cubes and octahedral [MnII (CN)6 ]4- ions interconnected in 3D by MnII -N≡C-MnII linkages. The composition is {[MnII (OH2 )3 ][MnII (OH2 )]3 }(µ3 -OH)4 ][MnII (µ-CN)2 (CN)4 ]⋅H2 O=[MnII 4 (µ3 -OH)4 (OH2 )6 ][MnII (µ-CN)2 (CN)4 ]⋅H2 O, which is further simplified to [Mn4 (OH)4 ][Mn(CN)6 ](OH2 )7 (1). 1 has four high-spin (S=5/2) MnII sites that are antiferromagnetically coupled within the cube and are antiferromagnetically coupled to six low-spin (S=1/2) octahedral [MnII (CN)6 ]4- ions. Above 40 K the magnetic susceptibility, χ(T), can be fitted to the Curie-Weiss expression, χ ∝(T-θ)-1 , with θ=-13.4 K, indicative of significant antiferromagnetic coupling and 1 orders as an antiferromagnet at Tc =7.8 K.

Structures of a Complex Hydrazinium Lead Iodide, (N2 H5 )15 Pb3 I21 , Possessing [Pb2 I9 ]5- , [PbI6 ]4- , and I- Ions and α- and ß-(N2 H5 )PbI3.

Photovoltaic perovskites, most notably methylammonium lead triiodide, (NH3 Me)PbI3 , have recently attracted considerable attention, and based upon the modified "Goldschmidt" as well as a "revised" tolerance factors, hydrazinium should be able to occupy the same cation site as methylammonium, and form a cubic unit cell. The reaction of N2 H5+ I- with PbI2 in dimethylformamide results in three types of yellow crystals; hexagonal, needle-like, and rod-like, the structures of which were determined at 100 K. The hexagonal (P63 /m: a=10.8906(10) Å; b=37.845(5) Å) crystals possess isolated face-sharing octahedral [Pb2 I9 ]5- , [PbI6 ]4- , and I- ions. IR spectroscopy indicates the presence of hydrogen-bonded N2 H5+ and the composition was determined by single-crystal X-ray diffraction, density measurements, combustion elemental analysis, and thermogravimetric analysis to be (N2 H5 )15 Pb3 I21 , which is photoluminescent at 50 K, but not at room temperature. The needle and rod crystals have an orthorhombic (Pnma: a=11.1385(7) Å; b=4.4806(3) Å; c=17.6241(11) Å) and hexagonal (P63 /mmc: a=8.7386(9) Å; b=8.2006(9) Å) unit cells, respectively, possessing the perovskite ABX3 composition of (N2 H5 )PbI3 , but neither exhibits the cubic Perovskite structure type. The structures of α- and ß-(N2 H5 )PbI3 possess parallel ribbons of Pb2 I4 and chains of PbI2 , respectively. Strong inter-hydrazinium hydrogen bonding due to it possessing both hydrogen bonding donor and acceptor sites (unlike NH3 Me+ ) appear to stabilize the observed extended ribbon motif for (N2 H5 )15 Pb3 I21 and α-(N2 H5 )PbI3 . (N2 H5 )15 Pb3 I21 has a band-gap-like absorption of 2.34 eV, and both α- and ß-(N2 H5 )PbI3 have a 2.70 eV band-gap-like absorptions.

Cation Dependence of the Dimerization Enthalpy for A2 [tetracyanoethylene]2 (A=NMe4 , Mepy, NEt4 ) Possessing a Long, Multicenter Bond.

[TCNE].- (TCNE=tetracyanoethylene) has been isolated as D2h π-[TCNE]22- possessing a long, 2.9 Šmulticenter 2-electron-4-center (2e- /4c) C-C bond, and as C2 π-[TCNE]22- possessing a longer, 3.04 Šmulticenter 2e- /6c (4 C+2 N atoms) bond. Temperature-dependent UV/Vis spectroscopic measurements in 2-methyltetrahydrofuran (MeTHF) has led to the determination of the dimerization, 2[TCNE].- ⇌π-[TCNE]22- , equilibrium constants, Keq (T), [[TCNE]22- ]/[[TCNE].- ]2 , enthalpy, ΔH, and entropy, ΔS, of dimerization for [Mepy]2 [TCNE]2 (Mepy=N-methylpyridinium, H3 CNC5 H5+ ) possessing D2h π-[TCNE]22- and [NMe4 ]2 [TCNE]2 possessing C2 π-[TCNE]22- conformations in the solid state; however, both form D2h π-[TCNE]22- in MeTHF solution. Based on ΔH=-3.6±0.1 kcal mol-1 (-15.2 kJ mol-1 ), and ΔS=-11±1 eu (-47 J mol-1 K-1 ) and ΔH=-2.4±0.2 kcal mol-1 (-10.2 kJ mol-1 ), and ΔS=-8±1 eu (-32 J mol-1 K-1 ) in MeTHF for [NMe4 ]2 [TCNE]2 and [Mepy]2 [TCNE]2 , respectively, the calculated Keq (298 K) are 1.6 and 1.3 m-1 , respectively. The observed Keq (145 K) are 3 and 2 orders of magnitude greater for [NMe4 ]2 [TCNE]2 and [Mepy]2 [TCNE]2 , respectively. The Keq (130 K) is 4470, 257, ≈0.8, and ≪0.1 m-1 for [NMe4 ]2 [TCNE]2 , [Mepy]2 [TCNE]2 , [NEt4 ]2 [TCNE]2 , and [N(nBu)4 ]2 [TCNE]2 , respectively, decreasing with increasing cation size. At standard conditions and below ambient temperature the equilibrium favors the dimer for the NMe4+ and Mepy+ cations. From the decreasing enthalpy, NMe4+ >Mepy+ , along with the decrease in dimer formation Keq (T) as NMe4+ >Mepy+ >NEt4+ >N(nBu)4+ , the dimer bond energy decreases with increasing cation size in MeTHF. This is attributed to a decrease in the [A]+ ⋅⋅⋅[TCNE]- attractive interactions with increasing cation size. Solid state UV/Vis spectroscopic determinations of [NMe4 ]2 [TCNE]2 are reported and compared to D2h π-[TCNE]22- conformers. The feasibility and limitations of temperature-dependent electron paramagnetic resonance (EPR) measurements for the determination of Keq (T) are also discussed.

Increase in the Magnetic Ordering Temperature (Tc) as a Function of the Applied Pressure for A2Mn[Mn(CN)6] (A = K, Rb, Cs) Prussian Blue Analogues.

Magnetization measurements under pressure reveal that the external hydrostatic pressure significantly increases in the ferrimagnetic transition temperature, Tc, for A2Mn[Mn(CN)6] (A = K, Rb, Cs). In the case of monoclinic A = K and Rb, dTc/dp values are 21.2 and 14.6 K GPa-1, respectively, and Tc increases by 53 and 39%, respectively, from ambient pressure to 1.0 GPa. The cubic A = Cs compound also shows a monotonous increase with an initial rate of 4.22 K GPa-1 and about 11.4 K GPa-1 above 0.6 GPa, and an overall Tc increase by 26% at 1.0 GPa. The increase in Tc is attributed to deformation of the structure such that the MnII-N≡C angle decreases with increasing pressure. The smaller the alkali cation, the greater the decrease in the MnII-N≡C angle induced by pressure and the larger the increase of dTc/dp. This is in accordance with the ambient-pressure structures for A2Mn[Mn(CN)6] (A = K, Rb, Cs), which have decreasing MnII-N≡C angles that correlate to the observed increasing Tcs as K > Rb > Cs. The large increase in Tc for the A = K compound is the highest class among several cyano-bridged metal complexes. The tuning of the transition temperature by such a weak pressure may lead to additional applications such as switching devices.

The Tetracyanopyridinide Dimer Dianion, σ-[TCNPy]2 (2.).

The reaction of 2,3,5,6-tetracyanopyridine (TCNPy) and Cr(C6 H6 )2 forms diamagnetic σ-[TCNPy]2 (2-) possessing a 1.572(3) Šintrafragment sp(3) -sp(3) bond. This is in contrast to the structurally related 1,2,4,5-tetracyanobenzene and 1,2,4,5-tetracyanopyrazine that form π-dimer dianions possessing long, multicenter bonds.

Characterization of Tetracyanopyridine (TCNPy)-Based Magnets: V[TCNPy]2 ⋠z (CH2 Cl2 ) (Tc =111†K) and V[TCNPy]3 ⋠z (CH2 Cl2 ) (Tc =90†K).

The reaction of 2,3,5,6-tetracyanopyridine (TCNPy) with V(CO)6 in CH2 Cl2 forms new organic-based magnets of V[TCNPy]x ⋅z (CH2 Cl2 ) (x=2, 3) composition. Analysis of the IR spectra suggests that the TCNPy is reduced and coordinated to V(II) sites through the nitriles. V[TCNPy]x order as ferrimagnets with 111 and 90 K Tc values for V[TCNPy]2 and V[TCNPy]3 , respectively. Their respective remanent magnetizations and coercive fields are 1260 and 250 emuOe mol(-1) and 9 and 6 Oe at 5 K, and they exhibit some spin-glass behavior.

Hexacyanobutadienide-Based Frustrated and Weak Ferrimagnets: M(HCBD)2·zCH2Cl2 (M = V, Fe).

Hexacyanobutadiene (HCBD) and M(CO)x (M = V, x = 6; Fe, x = 5) react in CH2Cl2 to form new organic-based magnets of M[HCBD]2·z(CH2Cl2) composition. Analysis of the IR spectrum [M = V: ν(CN) 2193 and 2116 cm(-1) (fwhh ∼400 cm(-1)); Fe: 2196 and 2145 (fwhh ∼150 cm(-1))] suggests that HCBD is reduced to the radical anion, [HCBD](•-), and the broadness suggests multiple and variable nitriles sites are coordinated to the V(II), leading to a complex mixture of magnetic couplings and behaviors that deviate from paramagnetic behavior below ∼150 K, and a frustrated magnet with Tc ≈ 9 K is observed for V[HCBD]2, the first cyanocarbon-based frustrated magnet. Fe[HCBD]2 behaves as a weak ferromagnet (canted antiferromagnet) with some spin glass behavior with a 10 K Tc.

Intrinsic organic-based synthetic/artificial antiferromagnets.

Mn(TCNE)[C4(CN)8]1/2 (TCNE = tetracyanoethylene) and [NEt4]Mn(II)3(CN)7 have extended layers with nearest neighbor intralayer S = 5/2 and S = 1/2 spin sites that couple antiferromagnetically forming ferrimagnetic layers. These layers are uniformly connected via diamagnetic (nonmagnetic) bridging µ4-[(C4(CN)8](2-) (8.77 Å) or µ-CN (5.48 Å) ligands, respectively, that antiferromagnetic couple the ferrimagnetic layers resulting in an antiferromagnet. The Jinter/kB is -1.0 and -1.8 K (H=-JSi⋅Sj) for Mn(TCNE)[C4(CN)8]1/2 and [NEt4]Mn(II)3(CN)7, respectively. Albeit intrinsically multilayered, these antiferromagnets have the same motif as that for artificial/synthetic antiferromagnets that exhibit giant magnetoresistance (GMR) and are commercially used in many magnetic memory applications.

Long, Multicenter Bonding--A New Concept for Supramolecular Materials.

A new concept for constructing supramolecular architectures is discussed. In addition to van der Waals and hydrogen-bonding intermolecular interactions that primarily account for supramolecular structures for all materials lacking extended 3D network structures, directional, long, multicenter bonding that can occur for anionic, cationic, neutral, and zwitterionic radicals and can direct intermolecular recognition through π interactions and form extended network supramolecular structural motifs.

Orientational preference of long, multicenter bonds in radical anion dimers: a case study of π-[TCNB]2 (2-) and π-[TCNP]2 (2.).

The similar shape and electronic structure of the radical anions of 1,2,4,5-tetracyanopyrazine (TCNP) and 1,2,4,5-tetracyanobenzene (TCNB) suggest a similar relative orientation for their long, multicenter carbon-carbon bond in π-[TCNP]2 (2-) and in π-[TCNB]2 (2-) , in good accord with the Maximin Principle predictions. Instead, the two known structures of π-[TCNP]2 (2-) have a D2h (θ=0°) and a C2 (θ=30°) orientation (θ being the dihedral angle that determines the rotation of one radical anion relative to the other along the axis that passes through center of the two six-membered rings). The only known π-[TCNB]2 (2-) structure has a C2 (θ=60°) orientation. The origin of these preferences was investigated for both dimers by computing (at the RASPT2/RASSCF(30,28) level) the variation with θ of the interaction energy (Eint ) and the variation of the Eint components. It was found that: 1) a long, multicenter bond exists for all orientations; 2) the Eint (θ) angular dependence is similar in both dimers; 3) for all orientations the electrostatic component dominates the value of Eint (θ), although the dispersion and bonding components also play a relevant role; and 4) the Maximin Principle curve reproduces well the shape of the Eint (θ) curve for isolated dimers, although none of them reproduce the experimental preferences. Only after the (radical anion)(.-) ⋅⋅⋅cation(+) interactions are also included in the model aggregate are the experimental data reproduced computationally.

A New Conformation With an Extraordinarily Long, 3.04†ŠTwo-Electron, Six-Center Bond Observed for the π-[TCNE]2 (2-) Dimer in [NMe4 ]2 [TCNE]2 (TCNE=Tetracyanoethylene).

[NMe4 ]2 [TCNE]2 (TCNE=tetracyanoethenide) formed from the reaction of TCNE and (NMe4 )CN in MeCN has νCN IR absorptions at 2195, 2191, 2172, and 2156 cm(-1) and a νCC absorption at 1383 cm(-1) that are characteristic of reduced TCNE. The TCNEs have an average central CC distance of 1.423 Šthat is also characteristic of reduced TCNE. The reduced TCNE forms a previously unknown non-eclipsed, centrosymmetric π-[TCNE]2 (2-) dimer with nominal C2 symmetry, 12 sub van der Waals interatomic contacts <3.3 Å, a central intradimer separation of 3.039(3) Å, and comparable intradimer C⋅⋅⋅N distances of 3.050(3) and 2.984(3) Å. The two pairs of central C⋅⋅⋅C atoms form a ∢C-C⋅⋅⋅C-C of 112.6° that is substantially greater than the 0° observed for the eclipsed D2h π-[TCNE]2 (2-) dimer possessing a two-electron, four-center (2e(-) /4c) bond with two C⋅⋅⋅C components from a molecular orbital (MO) analysis. A MO study combining CAS(2,2)/MRMP2/cc-pVTZ and atoms-in-molecules (AIM) calculations indicates that the non-eclipsed, C2 π-[TCNE]2 (2-) dimer exhibits a new type of a long, intradimer bond involving one strong C⋅⋅⋅C and two weak C⋅⋅⋅N components, that is, a 2e(-) /6c bond. The C2 π-[TCNE]2 (2-) conformer has a singlet, diamagnetic ground state with a thermally populated triplet excited state with J/kB =1000 K (700 cm(-1) ; 86.8 meV; 2.00 kcal mol(-1) ; H=-2 JSa ⋅Sb ); at the CAS(2,2)/MBMP2 level the triplet is computed to be 9.0 kcal mol(-1) higher in energy than the closed-shell singlet ground state. The results from CAS(2,2)/NEVPT2/cc-pVTZ calculations indicate that the C2 and D2h conformers have two different local metastable minima with the C2 conformer being 1.3 kcal mol(-1) less stable. The different natures of the C2 and D2h conformers are also noted from the results of valence bond (VB) qualitative diagram that shows a 10e(-) /6c bond with one C⋅⋅⋅C and two C⋅⋅⋅N bonding components for the C2 conformer as compared to the 6e(-) /4c bond for the D2h conformer with two C⋅⋅⋅C bonding components.

A New Conformation With an Extraordinarily Long, 3.04†ŠTwo-Electron, Six-Center Bond Observed for the π-[TCNE]2 (2-) Dimer in [NMe4 ]2 [TCNE]2 (TCNE=Tetracyanoethylene).

Invited for the cover of this issue are the groups of Joel S. Miller at the University of Utah and Juan J. Novoa at the University of Barcelona. The image depicts a visualization of the energy difference between the new 2e(-) /6c and the established 2e(-) /4c bond for C2 and D2h conformations, respectively, for π-[TCNE]2 (2-) . Read the full text of the article at 10.1002/chem.201501963.