Phase Transitions,Prominent Dielectric Anomalies,and Negative Thermal Expansion in Three High Thermally Stable Ammonium Magnesium–Formate Frameworks

We present three Mg–formate frameworks, incorporating three different ammoniums: [NH₄][Mg(HCOO)₃] ( 1 ), [CH₃CH₂NH₃][Mg(HCOO)₃] ( 2 ) and [NH₃(CH₂)₄NH₃][Mg₂(HCOO)₆] ( 3 ). They display structural phase transitions accompanied by prominent dielectric anomalies and anisotropic and negative thermal expansion. The temperature‐dependent structures, covering the whole temperature region in which the phase transitions occur, reveal detailed structural changes, and structure–property relationships are established. Compound 1 is a chiral Mg–formate framework with the NH₄⁺ cations located in the channels. Above 255 K, the NH₄⁺ cation vibrates quickly between two positions of shallow energy minima. Below 255 K, the cations undergo two steps of freezing of their vibrations, caused by the different inner profiles of the channels, producing non‐compensated antipolarization. These lead to significant negative thermal expansion and a relaxor‐like dielectric response. In perovskite 2 , the orthorhombic phase below 374 K possesses ordered CH₃CH₂NH₃⁺ cations in the cubic cavities of the Mg–formate framework. Above 374 K, the structure becomes trigonal, with trigonally disordered cations, and above 426 K, another phase transition occurs and the cation changes to a two‐fold disordered state. The two transitions are accompanied by prominent dielectric anomalies and negative and positive thermal expansion, contributing to the large regulation of the framework coupled the order–disorder transition of CH₃CH₂NH₃⁺. For niccolite 3 , the gradually enhanced flipping movement of the middle ethylene of [NH₃(CH₂)₄NH₃]²⁺ in the elongated framework cavity finally leads to the phase transition with a critical temperature of 412 K, and the trigonally disordered cations and relevant framework change, providing the basis for the very strong dielectric dispersion, high dielectric constant (comparable to inorganic oxides), and large negative thermal expansion. The spontaneous polarizations for the low‐temperature polar phases are 1.15, 3.43 and 1.51 μC cm^?2 for 1 , 2 and 3 , respectively, as estimated by the shifts of the cations related to the anionic frameworks. Thermal and variable‐temperature powder X‐ray diffraction studies confirm the phase transitions, and the materials are all found to be thermally stable up to 470 K.     

An A‐Site Mixed‐Ammonium Solid Solution Perovskite Series of [(NH2NH3)x(CH3NH3)1−x][Mn(HCOO)3] (x=1.00–0.67)

The A‐site mixed‐ammonium solid solutions of metal–organic perovskites [(NH₂NH₃)_x(CH₃NH₃)_1?x][Mn(HCOO)₃] (x=1.00–0.67) exhibit para‐ to ferroelectric diffuse phase transitions with lowered transition temperatures from x=1.00 to 0.67. These properties are due to the decreased framework distortion and polarization in their low temperature ferroelectric phases caused by the increased CH₃NH₃⁺ concentration.     

Solvent‐Dependent Delamination,Restacking, and Ferroelectric Behavior in a New Charge‐Separated Layered Compound: [NH4][Ag3(C9H5NO4S)2(C13H14N2)2]⋅8 H2O

A new anionic coordination polymer, [NH₄][Ag₃(C₉H₅NO₄S)₂(C₁₃H₁₄N₂)₂] ⋅ 8 H₂O, with a two‐dimensional structure, has been synthesized by a reaction between silver nitrate, 8‐hydroxyquinoline‐5‐sulfonic acid (HQS), and 4,4′‐trimethylene dipyridine (TMDP). The compound stabilizes in a noncentrosymmetric space group, and the lattice water molecules and the charge‐compensating [NH₄]⁺ group occupy the inter‐lamellar spaces. The lattice water molecules can be fully removed and reinserted, which is accompanied by a crystalline–amorphous–crystalline transformation. This transformation resembles the collapse/delamination and restacking of the layers. To the best of our knowledge, this is the first observation of delamination and restacking in an inorganic coordination polymer that contains silver. The presence of a natural dipole (the anionic framework and cationic ammonium ions) along with the noncentrosymmetric space group gives rise to the room‐temperature ferroelectric behavior of the compound. The ferroelectric behavior is also water‐dependent and exhibits a ferroelectric–paraelectric transformation. The temperature‐dependent dielectric measurements indicate that the ferroelectric/ paraelectric transformation occurs at 320 K. This transformation has also been investigated by using in‐situ IR spectroscopy and PXRD studies. The second‐harmonic generation (SHG) study indicated values that are comparable to some of the known SHG solids, such as potassium dihydrogen phosphate (KDP) and urea.     

Neon‐Bearing Ammonium Metal Formates: Formation and Behaviour under Pressure

The incorporation of noble gas atoms, in particular neon, into the pores of network structures is very challenging due to the weak interactions they experience with the network solid. Using high‐pressure single‐crystal X‐ray diffraction, we demonstrate that neon atoms enter into the extended network of ammonium metal formates, thus forming compounds Ne_x[NH₄][M(HCOO)₃]. This phenomenon modifies the compressional and structural behaviours of the ammonium metal formates under pressure. The neon atoms can be clearly localised within the centre of [M(HCOO)₃]₅ cages and the total saturation of this site is achieved after ～1.5 GPa. We find that by using argon as the pressure‐transmitting medium, the inclusion inside [NH₄][M(HCOO)₃] is inhibited due to the larger size of the argon. This study illustrates the size selectivity of [NH₄][M(HCOO)₃] compounds between neon and argon insertion under pressure, and the effect of inclusion on the high‐pressure behaviour of neon‐bearing ammonium metal formates.     

Metal–Organic Perovskites: Synthesis,Structures, and Magnetic Properties of [C(NH2)3][MII(HCOO)3] (M=Mn,Fe, Co,Ni, Cu,and Zn; C(NH2)3= Guanidinium)

We report the synthesis, crystal structures, and spectral, thermal, and magnetic properties of a family of metal–organic perovskite ABX₃, [C(NH₂)₃][M^II(HCOO)₃], in which A=C(NH₂)₃ is guanidinium, B=M is a divalent metal ion (Mn, Fe, Co, Ni, Cu, or Zn), and X is the formate HCOO^?. The compounds could be synthesized by either diffusion or hydrothermal methods from water or water‐rich solutions depending on the metal. The five members (Mn, Fe, Co, Ni, and Zn) are isostructural and crystallize in the orthorhombic space group Pnna, while the Cu member in Pna2₁. In the perovskite structures, the octahedrally coordinated metal ions are connected by the anti–anti formate bridges, thus forming the anionic NaCl‐type [M(HCOO)₃]^? frameworks, with the guanidinium in the nearly cubic cavities of the frameworks. The Jahn–Teller effect of Cu²⁺ results in a distorted anionic Cu–formate framework that can be regarded as Cu–formate chains through short basal Cu? O bonds linked by the long axial Cu? O bonds. These materials show higher thermal stability than other metal–organic perovskite series of [AmineH][M(HCOO)₃] templated by the organic monoammonium cations (AmineH⁺) as a result of the stronger hydrogen bonding between guanidinium and the formate of the framework. A magnetic study revealed that the five magnetic members (except Zn) display spin‐canted antiferromagnetism, with a Néel temperature of 8.8 (Mn), 10.0 (Fe), 14.2 (Co), 34.2 (Ni), and 4.6 K (Cu). In addition to the general spin‐canted antiferromagnetism, the Fe compound shows two isothermal transformations (a spin‐flop and a spin‐flip to the paramagnetic phase) within 50 kOe. The Co member possesses quite a large canting angle. The Cu member is a magnetic system with low dimensional character and shows slow magnetic relaxation that probably results from the domain dynamics.     

Theoretical study of the electronic spectra of bi‐ and tri‐heteronuclear platinum complexes

The electronic structure and the spectroscopic properties of [Pt(NH₃)₄][Au(CN)₂]₂, [Pt(NH₃)₄][Ag(CN)₂]₂, [Pt(CNCH₃)₄][Pt(CN)₄], and [Pt(CNCH₃)₄][Pd(CN)₄] were studied at the HF, MP2, B3LYP, and PBE levels. In all the complexes, it was found that the nature of the intermetal interactions is consistent with the presence of a high‐ionic contribution (90%) and a dispersion‐type interaction (10%). The absorption spectra of these complexes were calculated by the single‐excitation time‐dependent (TD) method at the HF, B3LYP, and PBE levels. The [Pt(NH₃)₄][M(CN)₂]₂ (M ? Au, Ag) complexes showed a ¹(dσ* → pσ) transition associated with a metal–metal charge transfer. On the other hand, the [Pt(CNCH₃)₄][M(CN)₄] (M ? Pt, Pd) complexes showed a ¹(dσ* → π*) transition associated with a metal‐to‐metal and ligand charge transfer. The values obtained theoretically are in agreement with the experimental range. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Characterization of Zn‐Containing Metal–Organic Frameworks by Solid‐State 67Zn NMR Spectroscopy and Computational Modeling

Metal–organic frameworks (MOFs) are an extremely important class of porous materials with many applications. The metal centers in many important MOFs are zinc cations. However, their Zn environments have not been characterized directly by ⁶⁷Zn solid‐state NMR (SSNMR) spectroscopy. This is because ⁶⁷Zn (I=5/2) is unreceptive with many unfavorable NMR characteristics, leading to very low sensitivity. In this work, we report, for the first time, a ⁶⁷Zn natural abundance SSNMR spectroscopic study of several representative zeolitic imidazolate frameworks (ZIFs) and MOFs at an ultrahigh magnetic field of 21.1 T. Our work demonstrates that ⁶⁷Zn magic‐angle spinning (MAS) NMR spectra are highly sensitive to the local Zn environment and can differentiate non‐equivalent Zn sites. The ⁶⁷Zn NMR parameters can be predicted by theoretical calculations. Through the study of MOF‐5 desolvation, we show that with the aid of computational modeling, ⁶⁷Zn NMR spectroscopy can provide valuable structural information on the MOF systems with structures that are not well described. Using ZIF‐8 as an example, we further demonstrate that ⁶⁷Zn NMR spectroscopy is highly sensitive to the guest molecules present inside the cavities. Our work also shows that a combination of ⁶⁷Zn NMR data and molecular dynamics simulation can reveal detailed information on the distribution and the dynamics of the guest species. The present work establishes ⁶⁷Zn SSNMR spectroscopy as a new tool complementary to X‐ray diffraction for solving outstanding structural problems and for determining the structures of many new MOFs yet to come.     

Thermal Ammonia Activation by Cationic Transition‐Metal Hydrides of the First Row – Small but Mighty

The thermal reactions of cationic 3d transition‐metal hydrides MH⁺ (M=Sc–Zn, except V and Cu) with ammonia have been studied by gas‐phase experiments and computational methods. There are three primary reaction channels: 1) H₂ elimination by N? H bond activation, 2) ligand exchange under the formation of M(NH₃)⁺, and 3) proton transfer to yield NH₄⁺. Computational studies of these three reaction channels have been performed for the couples MH⁺/NH₃ (M=Sc–Zn) to elucidate mechanistic aspects and characteristic reaction patterns of the first row. For N? H activation, σ‐bond metathesis was found to be operative.     

Infrared and Raman studies of phase transitions in metal–organic frameworks of [(CH3)2NH2][M(HCOO)3] with M=Zn,Fe

We report on temperature-dependent infrared (IR) and Raman studies of [(CH₃)₂NH₂][M(HCOO)₃] metal–organic frameworks (MOFs) with M=Zn, Fe. Based on Raman and IR data, an assignment of the observed modes to respective vibrations of atoms is proposed. Temperature-dependent studies revealed abrupt changes below 160 K that are attributed to the onset of first-order structural phase transition. The most pronounced changes are observed for the modes corresponding to the dimethylammonium cation, especially those involving motion of hydrogen atoms. This behavior proves that the phase transition has an order–disorder character and is associated with the ordering of protons. The abrupt splitting of some modes related to the formate ion indicates that this transition is also associated with significant distortion of the metal-formate framework.     

[Si4]4– and [Si9]4– Clusters Crystallized from Liquid Ammonia Solution – Synthesis and Characterization of K8[Si4][Si9]·(NH3)14.6

The extraction of the silicide K₁₂Si₁₇ with liquid ammonia in the presence of a sequestering agent and AuPPh₃Cl or Zn(Cp*)₂ led to crystals of the solvate compound K₈[Si₄][Si₉] · (NH₃)_14.6, which was characterized by single‐crystal X‐ray diffraction. It is the first compound with an isolated and ligand‐free [Si₄]^4– cluster obtained from solution. It also contains one [Si₉]^4– cluster per formula unit, whereas the precursor K₁₂Si₁₇ is built from [Si₄]^4– and [Si₉]^4– clusters with a 2:1 ratio.     

Zinc‐Diluted Magnetic Metal Formate Perovskites: Synthesis,Structures, and Magnetism of [CH3NH3][MnxZn1−x(HCOO)3] (x=0–1)

The preparation, structures, and magnetic properties of a series of metal formate perovskites [CH₃NH₃][Mn_xZn_1?x(HCOO)₃] were investigated. The isostructural solid solution can be prepared in the complete range of x=0–1. The metal–organic perovskite structures consist of an anionic NaCl type [Mn_xZn_1?x(HCOO)₃^?] framework with CH₃NH₃⁺ templates located in the nearly cubic cavities and forming hydrogen bonds to the framework. When the proportion of Mn increased (i.e., x changed from 0 to 1), the lattice dimensions and metal–oxygen and metal–metal distances show a slight, nonlinear increase because of the increased averaged metal ionic radius and the local structure distortion. Through the series, the magnetism changes from the long‐range ordering of spin‐canted antiferromagnetism for x≥0.40 to paramagnetism when x≤0.30, and the percolation limit was estimated to be x_P=0.31(2) for this simple cubic lattice. In the low‐temperature region, enhancement of magnetization and the gradual decrease and final disappearance of coercive field, remnant magnetization, and spin‐flop field upon dilution were observed through this isotropic Heisenberg magnetic series. IR spectroscopic and thermal properties were also investigated.     

Pressure‐Induced Bond Rearrangement and Reversible Phase Transformation in a Metal–Organic Framework

Pressure‐induced phase transformations (PIPTs) occur in a wide range of materials. In general, the bonding characteristics, before and after the PIPT, remain invariant in most materials, and the bond rearrangement is usually irreversible due to the strain induced under pressure. A reversible PIPT associated with a substantial bond rearrangement has been found in a metal–organic framework material, namely [tmenH₂][Er(HCOO)₄]₂ (tmenH₂²⁺=N,N,N′,N′‐tetramethylethylenediammonium). The transition is first‐order and is accompanied by a unit cell volume change of about 10 %. High‐pressure single‐crystal X‐ray diffraction studies reveal the complex bond rearrangement through the transition. The reversible nature of the transition is confirmed by means of independent nanoindentation measurements on single crystals.     

Competition in the formation of ion–neutral complexes. Expulsion of methane from the molecular ion of acetamide

An ion–neutral complex is a non-covalently bonded aggregate of an ion with one or more neutral molecules in which at least one of the partners rotates freely (or nearly so) in all directions. A density-of-states model is described, which calculates the proportion of ion–neutral complex formation that ought to accompany simple bond cleavages of molecular ions. Application of this model to the published mass spectrum of acetamide predicts the occurrence of ions that have not hitherto been reported. Relative intensities on the order of 0.1 (where the abundance of the most intense fragment ion = 1) ere predicted for [M – HO]⁺ and [M – CH₄]⁺˙ ions, which have the same nominal masses as the prominent [M – NH₃]⁺˙ and [M – NH₂]⁺ fragments. High-resolution mass spectrometric experiments confirm the presence of the predicted fragment ions. The [M – HO]⁺ and [M – CH₄]⁺˙ fragments were observed with relative abundances of 0.02 and 0.04, respectively. Differences between theory and experiment may be ascribed to effects of competing distonic ion pathways.     

The Protonation of Acetamide and Thioacetamide in Super­acidic Solutions: Crystal Structures of [H3CC(OH)NH2]+AsF6– and [H3CC(SH)NH2]+AsF6–

Acetamide and thioacetamide react with the superacid solutions HF/MF₅ (M = As, Sb) under formation of the corresponding salts [H₃CC(OH)NH₂]⁺MF₆^– and [H₃CC(SH)NH₂]⁺MF₆^– (M = As, Sb), respectively. The reaction of DF/AsF₅ with acetamide and thioacetamide lead to the corresponding deuterated salts [H₃CC(OD)ND₂]⁺AsF₆^– and [H₃CC(SD)ND₂]⁺AsF₆^–, respectively_. The salts are characterized by vibrational and NMR spectroscopy, and in the case of [H₃CC(OH)NH₂]⁺AsF₆^– and [H₃CC(SH)NH₂]⁺AsF₆^– also by single‐crystal X‐ray analyses. The [H₃CC(OH)NH₂]⁺AsF₆^– ( 1 ) salt crystallizes in the triclinic space group P$\bar{1}$ with two formula units per unit cell, and the [H₃CC(SH)NH₂]⁺AsF₆^– ( 2 ) salt crystallizes in the monoclinic space group P2₁/c with four formula units per unit cell. In both crystal structures three‐dimensional networks are observed which are formed by intra‐ and intermolecular N–H ··· F and O–H ··· F or S–H ··· F hydrogen bonds, respectively. For the vibrational analyses, quantum chemically calculated spectra of the cations [H₃CC(OH)NH₂ · 3HF]⁺ and [H₃CC(SH)NH₂ · 2HF]⁺ are considered.     

Vibrational properties and DFT calculations of formamidine-templated Co and Fe formates

Experimental Raman and IR spectra of [NH₂-CH-NH₂][M(HCOO)₃] (M = Co, Fe), containing formamidinium cations [NH₂-CH-NH₂]⁺ (FMD⁺) were recorded at room temperature. In order to assign the vibrational modes corresponding to the FMD⁺ cation, the three-parameter hybrid B3LYP density functional method has been used with the 6-311G(2d,2p) basis to derive the vibrational wavenumbers (harmonic and anharmonic), infrared intensities and Raman scattering activities of formamidine molecule and FMD⁺ cation. The performed calculations revealed that protonation should affect most significantly the ν(CH), ρ(NH₂), ω(NH₂) and τ(NH₂) modes, which are expected to shift towards higher wavenumbers after protonation.     

A Copper–Formate Framework Showing a Simple to Helical Antiferroelectric Transition with Prominent Dielectric Anomalies and Anisotropic Thermal Expansion,and Antiferromagnetism

We present here the compound [NH₄][Cu(HCOO)₃], a new member of the [NH₄][M(HCOO)₃] family. The Jahn–Teller Cu²⁺ ion leads to a distorted 4⁹?6⁶ chiral Cu–formate framework. In the low‐temperature (LT) orthorhombic phase, the Cu²⁺ is in an elongated octahedron, and the ${{\rm NH}{{+\hfill \atop 4\hfill}}}$ ions in the framework channel are off the channel axis. From 94 to 350 K the ${{\rm NH}{{+\hfill \atop 4\hfill}}}$ ion gradually approaches the channel axis and the related modulation of the framework and the hydrogen‐bond system occurs. The LT phase is simple antiferroelectric (AFE). The material becomes hexagonal above 355 K. In the high‐temperature (HT) phase, the Cu²⁺ octahedron is compressed, and the ${{\rm NH}{{+\hfill \atop 4\hfill}}}$ ions are arranged helically along the channel axis. Therefore, the phase transition is one from LT simple AFE to HT helical AFE. The temperature‐dependent structure evolution is accompanied by significant thermal and dielectric anomalies and anisotropic thermal expansion, due to the different status of the ${{\rm NH}{{+\hfill \atop 4\hfill}}}$ ions and the framework modulations, and the structure–property relationship was established based on the extensive variable‐temperature single‐crystal structures. The material showed long range ordering of antiferromagnetism (AFM), with low dimensional character and a Néel temperature of 2.9 K. Therefore, within the material AFE and AFM orderings coexist in the low‐temperature region.     

Ion–molecule reaction in the gas phase 4—Stereospecific nucleophilic substitution under ammonia chemical ionization conditions from diastereomeric methyl and benzyl ethers of 3-hydroxy steroids

The ammonia chemical ionization (CI/[NH₄⁺]) mass spectra of a series of diastereomeric methyl and benzyl ethers derived from 3-hydroxy steroids (unsaturated in position 5 and saturated) have been studied. The adduct ions [M+NH₄]⁺ and [MH]⁺ and the substitution product ions [M+NH₄? ROH]⁺ (thereafter called [M_sH]⁺) are characterized by an inversion in their relative stabilites in relation to their initial configuration. [M+NH₄]_α⁺ and [MH]_α⁺ formed from the α-Δ⁵-steroid isomers are stabilized by the presence of a hydrogen bond which is not possible for the β-isomers. This stereochemical effect has also been observed in the mass analysed ion kinetic energy (MIKE) spectra of [M+NH₄]⁺ and [MH]⁺. The MIKE spectra of [M_sH]⁺ indicate that those issued from the β-isomers are more stable than the one originating from the α-isomers. This behavior is also observed in the first field free region (HV scan spectra) for [MH]⁺, [M_sH]⁺ and [M+NH₄]⁺ which are precursors of the ethylenic carbocations (base peak in the conventional CI/[NH₄]⁺ spectra). Mechanisms, such as S_N1 and S_Ni, have been ruled out for the formation of [M_sH]⁺, but instead the data support an S_N2 mechanism during the ion-molecule reaction between [M+NH₄]⁺ and NH₃.     

The Influence of the Counterions [AsF6]– and [GeF6]2– on the Structure of the [ClSO2NH3]+ Cation

Chlorosulfonamide reacts in the superacidic solutions HF/GeF₄ and HF/AsF₅ under the formation of ([ClSO₂NH₃]⁺)₂[GeF₆]^2– and [ClSO₂NH₃]⁺[AsF₆]^–, respectively. The chlorosulfonammonium salts were characterized by X‐ray single crystal structure analysis as well as vibrational spectroscopy and discussed together with quantum chemical calculations. ([ClSO₂NH₃]⁺)₂[GeF₆]^2– crystallizes in the triclinic space group P1 with one formula unit in the unit cell. [ClSO₂NH₃]⁺[AsF₆]^– crystallizes in the monoclinic space group P2₁/n with four formula units in the unit cell. Dependent on the counterion, [AsF₆]^– or [GeF₆]^2–, considerable structural differences of the [ClSO₂NH₃]⁺ cation are observed. Furthermore, the hitherto unknown X‐ray single crystal structure of chlorosulfonamide is determined in the course of this study. Chlorosulfonamide crystallizes in the orthorhombic space group Pmc2 with four formula units per unit cell.     

Metallampullen als Mini‐Autoklaven: Synthese und Kristallstrukturen der Ammoniakate [Al(NH3)4Cl2][Al(NH3)2Cl4] und (NH4)2[Al(NH3)4Cl2][Al(NH3)2Cl4]Cl2

Metal Ampoules as Mini‐Autoclaves: Syntheses and Crystal Structures of [Al(NH₃)₄Cl₂][Al(NH₃)₂Cl₄] and (NH₄)₂[Al(NH₃)₄Cl₂][Al(NH₃)₂Cl₄]Cl₂ The salts [Al(NH₃)₄Cl₂]⁺[Al(NH₃)₂Cl₄]^–≡AlCl₃ · 3 NH₃ ( 1 ) and (NH₄⁺)²[Al(NH₃)₄Cl₂]⁺[Al(NH₃)₂Cl₄]^–(Cl^–)₂≡ AlCl₃ · 3 NH₃ · (NH₄)Cl ( 2 ) have been obtained as single crystals during the reactions of aluminum and aluminum trichloride, respectively, with ammonium chloride in sealed Monel metal containers. The crystal structure of 1 was determined again [triclinic, P‐1; a = 574.16(10); b = 655.67(12); c = 954.80(16) pm; α = 86.41(2); β = 87.16(2); γ = 84.89(2)°], that of 2 for the first time [monoclinic, I2/m; a = 657.74(12); b = 1103.01(14); c = 1358.1(3) pm; β = 103.24(2)°].     

A Three‐Dimensional MnII‐[MoIII(CN)7]4– Ferrimagnet Containing Formate as a Second Bridging Ligand

Self‐assembly of the [Mo(CN)₇]^4– anion and the Mn²⁺ ion in the aqueous solution containing ammonium formate results in a new coordination polymer, {(NH₄)₃[(H₂O)Mn₃(HCOO)][Mo(CN)₇]₂·4H₂O}_n. Single crystal X‐ray analysis revealed a very complicated three‐dimensional (3D) framework, where both the [Mo(CN)₇]^4– and the formate anions act as bridges between the Mn^II centers. Magnetic measurements revealed that this compound displays ferrimagnetic ordering below 70 K. Competing antiferromagnetic interactions between the spin carriers might lead to spin frustration and non‐linear alignment of the magnetic moments. Specifically, this compound is the first mixed [Mo(CN)₇]^4–/HCOO^– bridged molecule magnet.