Nucleophilic Activation of Red Phosphorus for Controlled Synthesis of Polyphosphides.

Reactions between red phosphorus (Pred) and potassium ethoxide in various organic solvents under reflux convert this rather inert form of the element to soluble polyphosphides. The activation is hypothesized to proceed via a nucleophilic attack by ethoxide on the polymeric structure of Pred, leading to disproportionation of the latter, as judged from observation of P(OEt)3 in the reaction products. A range of solvents has been probed, revealing that different polyphosphide anions (P73-, P162-, P213-, and P5-) can be stabilized depending on the combination of the boiling point and dielectric constant (polarity) of the solvent. The effectiveness of activation also depends on the nature of nucleophile, with the rate of reaction between Pred and KOR increasing in the order t-Bu < n-Hex < Et < Me, which is in agreement with the increasing order of nucleophilic strength. Thiolates and amides were also examined as potential activators, but the reaction with these nucleophiles were substantially slower; nonetheless, all reactions between Pred and NaSR yielded exclusively P162- as a soluble polyphosphide product.

Radical Dimerization in a Plastic Organic Crystal Leads to Structural and Magnetic Bistability with Wide Thermal Hysteresis.

The nitroxyl radical 1-methyl-2-azaadamantane N-oxyl (Me-AZADO) exhibits magnetic bistability arising from a radical/dimer interconversion. The transition from the rotationally disordered paramagnetic plastic crystal, Me-AZADO, to the ordered diamagnetic crystalline phase, (Me-AZADO)2, has been conclusively demonstrated by crystal structure determination from high-resolution powder diffraction data and by solid-state NMR spectroscopy. The phase change is characterized by a wide thermal hysteresis with high sensitivity to even small applied pressures. The molecular dynamics of the phase transition from the plastic crystal to the conventional crystalline phase has been tracked by solid-state (1H and 13C) NMR and EPR spectroscopies.

Dicyanometalates as Building Blocks for Multinuclear Iron(II) Spin-Crossover Complexes.

A synthetic strategy featuring dicyanometalates [M(CN)2]- (M = Ag, Au) as N-coordinating ditopic linkers connecting partially blocked FeII centers has been employed to produce heterometallic hexanuclear complexes, which exhibit spin-crossover (SCO) behavior at the FeII sites. The reaction between tris(2-pyridylmethyl)amine (tpma)-capped FeII ions and [Ag(CN)2]- proceeded with partial decomposition of the dicyanoargentate and led to the formation of {[Fe(tpma)]4(µ-CN)2[µ-Ag(CN)2]2}(ClO4)4·3H2O (1), in which both [Ag(CN)2]- and CN- act as bridging ligands, and the opposite [Ag(CN)2]- bridges are engaged in a pronounced argentophilic d10-d10 interaction. In an analogous synthesis, the more stable [Au(CN)2]- species remained intact and furnished the complex {[Fe(tpma)]2[µ-Au2(CN)4]2} (2), which features two FeII centers bridged by two [Au2(CN)4]2- dimers. The use of S,S'-bis(2-pyridylmethyl)-1,2-thioethane (bpte) as a mixed-donor, N2S2-coordinating capping ligand yielded {[Fe(bpte)]2[µ-Au2(CN)4]2} (3), with a structure analogous to that of 2. Variable-temperature magnetic susceptibility measurements revealed that complexes 1-3 exhibit an onset of SCO above 350 K. Measurements above 400 K further confirmed the occurrence of a gradual spin-state conversion for complex 2.

Heteroleptic Fe(II) Complexes with N4S2 Coordination as a Platform for Designing Spin-Crossover Materials.

Heteroleptic complexes [Fe(bpte)(bim)]X2 and [Fe(bpte)(xbim)]X2 (bpte = S,S'-bis(2-pyridylmethyl)-1,2-thioethane, bim = 2,2'-biimidazole, xbim = 1,1'-(α,α'-o-xylyl)-2,2'-biimidazole, X = ClO4-, BF4-, OTf-) were prepared by reacting the corresponding Fe(II) salts with a 1:1 mixture of the ligands. All mononuclear complexes exhibit temperature-induced spin crossover (SCO) with the onset above room temperature. The SCO is rather gradual, due to low cooperativity of interactions between the cationic complexes, as revealed by crystal structure analyses. These complexes expand the range of the recently discovered Fe(II) SCO materials with {N4S2} coordination environment.

Synthetic, Structural, and Spectroscopic Characterization of a Novel Family of High-Spin Iron(II) [(ß-Diketiminate)(phosphanylphosphido)] Complexes.

This work describes a series of iron(II) phosphanylphosphido complexes. These compounds were obtained by reacting lithiated diphosphanes R2PP(SiMe3)Li (R = t-Bu, i-Pr) with an iron(II) ß-diketiminate complex, [LFe(µ2-Cl)2Li(DME)2] (1), where DME = 1,2-dimethoxyethane and L = Dippnacnac (ß-diketiminate). While the reaction of 1 with t-Bu2PP(SiMe3)Li yields [LFe(η1-Me3SiPP-t-Bu2)] (2), that of 1 with equimolar amounts of i-Pr2PP(SiMe3)Li, in DME, leads to [LFe(η2-i-Pr2PPSiMe3)] (3). In contrast, the reaction of 1 with (i-Pr2N)2PP(SiMe3)Li provides not an iron-containing complex but 1-[(diisopropylamino)phosphine]-2,4-bis(diisopropylamino)-3-(trimethylsilyl)tetraphosphetane (4). The structures of 2-4 were determined using diffractometry. Thus, 2 exhibits a three-coordinate iron site and 3 a four-coordinate iron site. The increase in the coordination number is induced by the change from an anticlinal to a synclinal conformation of the phoshpanylphosphido ligands. The electronic structures of 2 and 3 were assessed through a combined field-dependent 57Fe Mössbauer and high-frequency and -field electron paramagnetic resonance spectroscopic investigation in conjunction with analysis of their magnetic susceptibility and magnetization data. These studies revealed two high-spin iron(II) sites with S = 2 ground states that have different properties. While 2 exhibits a zero-field splitting described by a positive D parameter (D = +17.4 cm-1; E/D = 0.11) for 3, this parameter is negative [D = -25(5) cm-1; E/D = 0.15(5)]. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations provide insights into the origin of these differences and allow us to rationalize the fine and hyperfine structure parameters of 2 and 3. Thus, for 2, the spin-orbit coupling mixes a z2-type ground state with two low-lying {xz/yz} orbital states. These interactions lead to an easy plane of magnetization, which is essentially parallel to the plane defined by the N-Fe-N atoms. For 3, we find a yz-type ground state that is strongly mixed with a low-lying z2-type orbital state. In this case, the spin-orbit interaction leads to a partial unquenching of the orbital momentum along the x axis, that is, to an easy axis of magnetization oriented roughly along the Fe-P bond of the phosphido moiety.

Spin Crossover in Fe(II) Complexes with N4S2 Coordination.

Reactions of Fe(II) precursors with the tetradentate ligand S,S'-bis(2-pyridylmethyl)-1,2-thioethane (bpte) and monodentate NCE(-) coligands afforded mononuclear complexes [Fe(bpte)(NCE)2] (1, E = S; 2, E = Se; 3, E = BH3) that exhibit temperature-induced spin crossover (SCO). As the ligand field strength increases from NCS(-) to NCSe(-) to NCBH3(-), the SCO shifts to higher temperatures. Complex 1 exhibits only a partial (15%) conversion from the high-spin (HS) to the low-spin (LS) state, with an onset around 100 K. Complex 3 exhibits a complete SCO with T1/2 = 243 K. While the γ-2 polymorph also shows the complete SCO with T1/2 = 192 K, the α-2 polymorph exhibits a two-step SCO with the first step leading to a 50% HS → LS conversion with T1/2 = 120 K and the second step proceeding incompletely in the 80-50 K range. The amount of residual HS fraction of α-2 that remains below 60 K depends on the cooling rate. Fast flash-cooling allows trapping of as much as 45% of the HS fraction, while slow cooling leads to a 14% residual HS fraction. The slowly cooled sample of α-2 was subjected to irradiation in the magnetometer cavity resulting in a light-induced excited spin state trapping (LIESST) effect. As demonstrated by Mössbauer spectroscopy, an HS fraction of up to 85% could be achieved by irradiation at 4.2 K.

Facile Conversion of Red Phosphorus into Soluble Polyphosphide Anions by Reaction with Potassium Ethoxide.

Soluble polyphosphide anions were successfully generated in a number of organic solvents by the reaction between shelf-stable red phosphorus and potassium ethoxide. The species were identified by (31)P NMR spectroscopy in solution and by X-ray crystal-structure determination of (Bu4N)2P16 in the solid state. The reaction was scaled up to gram quantities by using a flow-chemistry process.

Incorporation of coinage metal-NHC complexes into heptaphosphide clusters.

Cu(i) and Au(i) ions, capped with an N-heterocyclic carbene (NHC), react with (TMS)3P7 (TMS = trimethyl-silyl) to afford an η4-coordinated anion [NHCDippCu-P7(TMS)]- and a neutral trinuclear complex (NHCDippAu)3P7. Protecting the P7 cage with the TMS groups is instrumental in controlling the course of these reactions.

Homoleptic mono-, di-, and tetra-iron complexes featuring phosphido ligands: a synthetic, structural, and spectroscopic study.

We report the first series of homoleptic phosphido iron complexes synthesized by treating either the ß-diketiminato complex [(Dippnacnac)FeCl2Li(dme)2] (Dippnacnac = HC[(CMe)N(C6H3-2,6-iPr2)]2) or [FeBr2(thf)2] with an excess of phosphides R2PLi (R = tBu, tBuPh, Cy, iPr). Reaction outcomes depend strongly on the bulkiness of the phosphido ligands. The use of tBu2PLi precursor led to an anionic diiron complex 1 encompassing a planar Fe2P2 core with two bridging and two terminal phosphido ligands. An analogous reaction employing less sterically demanding phosphides, tBuPhPLi and Cy2PLi yielded diiron anionic complexes 2 and 3, respectively, featuring a short Fe-Fe interaction supported by three bridging phosphido groups and one additional terminal R2P- ligand at each iron center. Further tuning of the P-substrates bulkiness gave a neutral phosphido complex 4 possessing a tetrahedral Fe4 cluster core held together by six bridging iPr2P moieties. Moreover, we also describe the first homoleptic phosphanylphosphido iron complex 5, which features an iron center with low coordination provided by three tBu2P-P(SiMe3)- ligands. The structures of compounds 1-5 were determined by single-crystal X-ray diffraction and 1-3 by 1H NMR spectroscopy. Moreover, the electronic structures of 1-3 were interrogated using zero-field Mössbauer spectroscopy and DFT methods.

Square-planar Co(iii) in {O4} coordination: large ZFS and reactivity with ROS.

Oxidation of distorted square-planar perfluoropinacolate Co compound [CoII(pinF)2]2-, 1, to [CoIII(pinF)2]1-, 2, is reported. Rigidly square-planar 2 has an intermediate-spin, S = 1, ground state and very large zero-field splitting (ZFS) with D = 67.2 cm-1; |E| = 18.0 cm-1, (E/D = 0.27), g⊥ = 2.10, g‖ = 2.25 and χTIP = 1950 × 10-6 cm3 mol-1. This Co(iii) species, 2, reacts with ROS to oxidise two (pinF)2- ligands to form tetrahedral [CoII(Hpfa)4]2-, 3.

Properties of Prussian blue materials manifested in molecular complexes: observation of cyanide linkage isomerism and spin-crossover behavior in pentanuclear cyanide clusters.

Pentanuclear, cyanide-bridged clusters [M(tmphen)2]3[M'(CN)6]2 (M/M' = Zn/Cr (1), Zn/Fe (2), Fe/Fe (3), Fe/Co (4), and Fe/Cr (5); tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) were prepared by combining [M'III(CN)6]3- anions with mononuclear complexes of MII ions with two capping tmphen ligands. The clusters consist of a trigonal bipyramidal (TBP) core with three MII ions in the equatorial positions and two M'III ions in the axial positions. Compounds 1-4 are isostructural and crystallize in the monoclinic space group P21/c. Complex 5 crystallizes in the enantiomorphic space group P3221. The magnetic properties of compounds 1 and 2 reflect the contributions of the individual [CrIII(CN)6]3- and [FeIII(CN)6]3- ions. The FeII ions in compounds 3 and 4 exhibit a gradual, temperature-induced spin transition between high spin (HS) and low spin (LS), as determined by the combination of Mössbauer spectroscopy, magnetic measurements, and single-crystal X-ray studies. The investigation of compound 5 by these methods and by IR spectroscopy indicates that cyanide linkage isomerism occurs during cluster formation. The magnetic behavior of 5 is determined by weak ferromagnetic coupling between the axial CrIII centers mediated by the equatorial diamagnetic FeII ions. Mössbauer spectra collected in the presence of a high applied field have allowed, for the first time, the direct experimental observation of uncompensated spin density at diamagnetic metal ions that bridge paramagnetic metal ions.

A charge-transfer-induced spin transition in a discrete complex: the role of extrinsic factors in stabilizing three electronic isomeric forms of a cyanide-bridged co/fe cluster.

A series of bimetallic, trigonal bipyramidal clusters of type {[Co(N-N)(2)](3)[Fe(CN)(6)](2)} are reported. The reaction of {Co(tmphen)(2)}(2+) with [Fe(CN)(6)](3)(-) in MeCN affords {[Co(tmphen)(2)](3)[Fe(CN)(6)](2)} (1). The cluster can exist in three different solid-state phases: a red crystalline phase, a blue solid phase obtained by exposure of the red crystals to moisture, and a red solid phase obtained by desolvation of the blue solid phase in vacuo. The properties of cluster 1 are extremely sensitive to both temperature and solvent content in each of these phases. Variable-temperature X-ray crystallography; (57)Fe Mossbauer, vibrational, and optical spectroscopies; and magnetochemical studies were used to study the three phases of 1 and related compounds, Na{[Co(tmphen)(2)](3)[Fe(CN)(6)](2)}(ClO(4))(2) (2), {[Co(bpy)(2)](3)[Fe(CN)(6)](2)}[Fe(CN)(6)](1/3) (3), and {[Ni(tmphen)(2)](3)[Fe(CN)(6)](2)} (4). The combined structural and spectroscopic investigation of 1-4 leads to the unambiguous conclusion that 1 can exist in different electronic isomeric forms, {Co(III)(2)Co(II)Fe(II)(2)} (1A), {Co(III)Co(II)(2)Fe(III)Fe(II)} (1B), and {Co(II)(3)Fe(III)(2)} (1C), and that it can undergo a charge-transfer-induced spin transition (CTIST). This is the first time that such a phenomenon has been observed for a Co/Fe molecule.

A charge-transfer-induced spin transition in the discrete cyanide-bridged complex [[Co(tmphen)2]3[Fe(CN)6]2].

A charge-transfer-induced spin transition (CTIST) is observed in the discrete cyanide-bridged complex, {[Co(tmphen)2]3[Fe(CN)6]2}. Single-crystal X-ray diffraction, 57Fe Mössbauer spectroscopy, and magnetic susceptibility were used collectively to describe the oxidation states of the Co and Fe ions in this cluster as a function of temperature. This pentanuclear complex represents the first example of a CTIST at the discrete molecular level.