Symmetry Breaking and Cooperative Spin Crossover in a Hofmann-Type Coordination Polymer Based on Negatively Charged {FeII(µ2-[MII(CN)4])2}n2n- Layers (MII = Pd, Pt).

We report herein the synthesis and characterization of two unprecedented isomorphous spin-crossover two-dimensional coordination polymers of the Hofmann-type formulated {FeII(Hdpyan)2(µ2-[MII(CN)4])2}, with MII = Pd, Pt and Hdpyan is the in situ partially protonated form of 2,5-(dipyridin-4-yl)aniline (dpyan). The FeII is axially coordinated by the pyridine ring attached to the 2-position of the aniline ring, while it is equatorially surrounded by four [MII(CN)4]2- planar groups acting as trans µ2-bidentate ligands defining layers, which stack parallel to each other. The other pyridine group of Hdpyan, being protonated, remains peripheral but involved in a strong [MII-C≡N···Hpy+] hydrogen bond between alternate layers. This provokes a nearly 90° rotation of the plane defined by the [MII(CN)4]2- groups, with respect to the average plane defined by the layers, forcing the observed uncommon bridging mode and the accumulation of negative charge around each FeII, which is compensated by the axial [Hdpyan]+ ligands. According to the magnetic and calorimetric data, both compounds undergo a strong cooperative spin transition featuring a 10-12 K wide hysteresis loop centered at 220 (Pt) and 211 K (Pd) accompanied by large entropy variations, 97.4 (Pt) and 102.9 (Pd) J/K mol. The breaking symmetry involving almost 90° rotation of one of the two coordinated pyridines together with the large unit-cell volume change per FeII (ca. 50 Å3), and subsequent release of significantly short interlayer contacts upon the low-spin → high-spin event, accounts for the strong cooperativity.

Order-Disorder, Symmetry Breaking, and Crystallographic Phase Transition in a Series of Bis(trans-thiocyanate)iron(II) Spin Crossover Complexes Based on Tetradentate Ligands Containing 1,2,3-Triazoles.

We report herein a series of neutral trans-thiocyanate mononuclear spin crossover (SCO) complexes, [FeIIL(NCS)2] (1-4), based on tetradentate ligands L obtained by reaction of N-substituted 1,2,3-triazolecarbaldehyde with 1,3-propanediamine or 2,2-dimethyl-1,3-diaminopropane [L = N1,N3-bis((1,5-dimethyl-1H-1,2,3-triazol-4-yl)methylene)propane-1,3-diamine/-2,2-dimethylpropane-1,3-diamine, 1/2 and N1,N3-bis((1-ethyl/1-propyl-1H-1,2,3-triazol-4-yl)methylene)-2,2-dimethylpropane-1,3-diamine, 3/4]. The thermal-induced SCO behavior is characterized by abrupt transitions with an average critical temperature (ΔT1/2)/hysteresis loop width (ΔThyst) in the range 190-252/5-14 K, while the photo-generated metastable high-spin (HS) phases are characterized by TLIESST temperatures in the range 44-59 K. Single crystal analysis shows that except 1, all compounds experience reversible symmetry breaking coupled with the thermal SCO. Furthermore, 4 experiences an additional phase transition at ca. 290 K responsible for the coexistence of two HS phases quenched at 10 K through LIESST and TIESST effects. The molecules form hexagonally packed arrays sustained by numerous weak CH···S and C···C/S···C/N···C bonds involving polar coordination cores, while non-polar pendant aliphatic substituents are segregated inside, occupying hexagonal channels. Energy framework analysis of complexes with one step SCO transition (1, 2, and 4) shows a correlation between the cooperativity and the amplitude of changes in the molecule-molecule interactions in the lattice at the SCO transition.

A Cu-BOX catalysed enantioselective Mukaiyama-aldol reaction with difluorinated silyl enol ethers and acylpyridine N-oxides.

A Cu(II)/BOX complex catalyses the enantioselective addition of difluorinated silyl enol ethers to acylpyridine N-oxides. The reaction provides difluorinated chiral tertiary alcohols of great interest in medicinal chemistry. These compounds are obtained in moderate to excellent yields and with high enantioselectivities. The stereochemical outcome of the reaction has been explained by DFT calculations.

105 K Wide Room Temperature Spin Transition Memory Due to a Supramolecular Latch Mechanism.

Little is known about the mechanisms behind the bistability (memory) of molecular spin transition compounds over broad temperature ranges (>100 K). To address this point, we report on a new discrete FeII neutral complex [FeIIL2]0 (1) based on a novel asymmetric tridentate ligand 2-(5-(3-methoxy-4H-1,2,4-triazol-3-yl)-6-(1H-pyrazol-1-yl))pyridine (L). Due to the asymmetric cone-shaped form, in the lattice, the formed complex molecules stack into a one-dimensional (1D) supramolecular chain. In the case of the rectangular supramolecular arrangement of chains in methanolates 1-A and 1-B (both orthorhombic, Pbcn) differing, respectively, by bent and extended spatial conformations of the 3-methoxy groups (3MeO), a moderate cooperativity is observed. In contrast, the hexagonal-like arrangement of supramolecular chains in polymorph 1-C (monoclinic, P21/c) results in steric coupling of the transforming complex species with the peripheral flipping 3MeO group. The group acts as a supramolecular latch, locking the huge geometric distortion of complex 1 and in turn the trigonal distortion of the central FeII ion in the high-spin state, thereby keeping it from the transition to the low-spin state over a large thermal range. Analysis of the crystal packing of 1-C reveals significantly changing patterns of close intermolecular interactions on going between the phases substantiated by the energy framework analysis. The detected supramolecular mechanism leads to a record-setting robust 105 K wide hysteresis spanning the room temperature region and an atypically large TLIESST relaxation value of 104 K of the photoexcited high-spin state. This work highlights a viable pathway toward a new generation of cleverly designed molecular memory materials.

Catalytic Diastereo- and Enantioselective Synthesis of Tertiary Trifluoromethyl Carbinols through a Vinylogous Aldol Reaction of Alkylidenepyrazolones with Trifluoromethyl Ketones.

A diastereo- and enantioselective organocatalytic aldol reaction between alkylidenepyrazolones and trifluoromethyl ketones leading to chiral tertiary alcohols bearing a trifluoromethyl group is presented. The methodology is based on the use of a bifunctional organocatalyst in order to activate the γ-hydrogen atoms of the alkylidenepyrazolone nucleophile and the carbonyl group of the trifluoromethylarylketone providing highly functionalized trifluoromethyl alcohols with moderate yields, excellent diastereoselectivity, and moderate to good enantioselectivity. Experiments monitoring the conversion by 1H NMR and the enantiomeric excess by HPLC with the reaction time showed that full conversion of the starting materials is not achieved and that the enantiomeric excess decreases upon extended times, probably due to the reversibility of the reaction.

Halobenzene Clathrates of the Porous Metal-Organic Spin-Crossover Framework [Fe(tvp)2(NCS)2]n. Stabilization of a Four-Step Transition.

Here we show that the porous metal-organic spin crossover (SCO) framework [Fe(tvp)2(NCS)2]@4(CH3CN·H2O) [1@4(CH3CN·H2O)] is an excellent precursor material for the systematic synthesis, via single-crystal to single-crystal transformation, of a series of halobenzene clathrates. Immersion of samples constituted of single crystals of 1@4(CH3CN·H2O) in the liquid halobenzenes PhXn, X = F (n = 1-6), X = Cl (n = 1, 2), and X = Br (n = 1) at room temperature induces complete replacement of the guest molecules by PhXn to afford 1@2PhXn. Single-crystal analyses of the new clathrates confirm the integrity of the porous framework with the PhXn guests being organized by pairs via π-stacking filling the nanochannels. The magnetic and calorimetric data confirm the occurrence of practically complete SCO behavior in all of the clathrates. The characteristic SCO equilibrium temperatures, T1/2, seem to be the result of a subtle balance in the host-guest interactions, which are temperature- and spin-state-dependent. The radically distinct supramolecular organization of the PhCl2 guests in 1@2PhCl2 affords a rare example of four-step SCO behavior following the sequence [HS1:LS0] ↔ [HS2/3:LS1/3] ↔ [HS1/2:LS1/2] ↔ [HS1/4:LS3/4] ↔ [HS0:LS1], which has been structurally characterized.

Enhanced Interplay between Host-Guest and Spin-Crossover Properties through the Introduction of an N Heteroatom in 2D Hofmann Clathrates.

Controlled modulation of the spin-crossover (SCO) behavior through the sorption-desorption of invited molecules is an extensively exploited topic because of its potential applications in molecular sensing. For this purpose, understanding the mechanisms by which the spin-switching properties are altered by guest molecules is of paramount importance. Here, we show an experimental approach revealing a direct probe of how the interplay between SCO and host-guest chemistry is noticeably activated by chemically tuning the host structure. Thus, the axial ligand 4-phenylpyridine (4-PhPy) in the 2D Hofmann clathrates {Fe(4-PhPy)2[M(CN)4]} (PhPyM; M = Pt, Pd) is replaced by 2,4-bipyridine (2,4-Bipy), resulting in the isomorphous compounds {Fe(2,4-Bipy)2[M(CN)4]} (BipyM; M = Pt, Pd), which basically differ from the former in that they have a noncoordinated N heteroatom in the ancillary aromatic substituent, i.e., 2-pyridyl instead of phenyl. Our chemical, magnetic, calorimetric, and structural characterizations demonstrate that this subtle chemical composition change provokes outstanding modifications not only in the capability to adsorb small guests as water or methanol but also in the extent to which these guests affect the SCO characteristics.

Bistable Hofmann-Type FeII Spin-Crossover Two-Dimensional Polymers of 4-Alkyldisulfanylpyridine for Prospective Grafting of Monolayers on Metallic Surfaces.

Aiming at investigating the suitability of Hofmann-type two-dimensional (2D) coordination polymers {FeII(Lax)2[MII(CN)4]} to be processed as single monolayers and probed as spin crossover (SCO) junctions in spintronic devices, the synthesis and characterization of the MII derivatives (MII = Pd and Pt) with sulfur-rich axial ligands (Lax = 4-methyl- and 4-ethyl-disulfanylpyridine) have been conducted. The thermal dependence of the magnetic and calorimetric properties confirmed the occurrence of strong cooperative SCO behavior in the temperature interval of 100-225 K, featuring hysteresis loops 44 and 32.5 K/21 K wide for PtII-methyl and PtII/PdII-ethyl derivatives, while the PdII-methyl derivative undergoes a much less cooperative multistep SCO. Excluding PtII-methyl, the remaining compounds display light-induced excited spin-state trapping at 10 K with TLIESST temperatures in the range of 50-70 K. Single-crystal studies performed in the temperature interval 100-250 K confirmed the layered structure and the occurrence of complete transformation between the high- and low-spin states of the FeII center for the four compounds. Strong positional disorder seems to be the source of elastic frustration driving the multistep SCO observed for the PdII-methyl derivative. It is expected that the peripheral disulfanyl groups will favor anchoring and growing of the monolayer on gold substrates and optimal electron transport in the device.

Spin Crossover in a Series of Non-Hofmann-Type Fe(II) Coordination Polymers Based on [Hg(SeCN)3]- or [Hg(SeCN)4]2- Building Blocks.

Self-assembly of [Hg(SeCN)4]2- tetrahedral building blocks, iron(II) ions, and a series of bis-monodentate pyridyl-type bridging ligands has afforded the new heterobimetallic HgII-FeII coordination polymers {Fe[Hg(SeCN)3]2(4,4'-bipy)2}n (1), {Fe[Hg(SeCN)4](tvp)}n (2), {Fe[Hg(SeCN)3]2(4,4'-azpy)2}n (3), {Fe[Hg(SeCN)4](4,4'-azpy)(MeOH)}n (4), {Fe[Hg(SeCN)4](3,3'-bipy)}n (5) and {Fe[Hg(SeCN)4](3,3'-azpy)}n (6) (4,4-bipy = 4,4'-bipyridine, tvp = trans-1,2-bis(4-pyridyl)ethylene, 4,4'-azpy = 4,4'-azobispyridine, 3,3-bipy = 3,3'-bipyridine, 3,3'-azpy = 3,3'-azobispyridine). Single-crystal X-ray analyses show that compounds 1 and 3 display a two-dimensional robust sheet structure made up of infinite linear [(FeL)n]2n+ (L = 4,4'-bipy or 4,4'-azpy) chains linked by in situ formed {[Hg(L)(SeCN)3]2}2- anionic dimeric bridges. Complexes 2 and 4-6 define three-dimensional networks with different topological structures, indicating, in combination with complexes 1 and 3, that the polarity, length, rigidity, and conformation of the bridging organic ligand play important roles in the structural nature of the products reported here. The magnetic properties of complexes 1 and 2 show the occurrence of temperature- and light-induced spin crossover (SCO) properties, while complexes 4-6 are in the high-spin state at all temperatures. The current results provide a new route for the design and synthesis of new SCO functional materials with non-Hofmann-type traditional structures.

Thermochromic Meltable Materials with Reverse Spin Transition Controlled by Chemical Design.

We report a series of meltable FeII complexes, which, depending on the length of aliphatic chains, display abrupt forward low-spin to high-spin transition or unprecedented melting-triggered reverse high-spin to low-spin transition on temperature rise. The reverse spin transition is perfectly reproducible on thermal cycling and the obtained materials are easily processable in the form of thin film owing to their soft-matter nature. We found that the discovered approach represents a potentially generalizable new avenue to control both the location in temperature and the direction of the spin transition in meltable compounds.

Single-Crystal X-Ray Diffraction Study of Pressure and Temperature-Induced Spin Trapping in a Bistable Iron(II) Hofmann Framework.

High-pressure single-crystal X-ray diffraction has been used to trap both the low-spin (LS) and high-spin (HS) states of the iron(II) Hofmann spin crossover framework, [FeII (pdm)(H2 O)[Ag(CN)2 ]2 ⋅H2 O, under identical experimental conditions, allowing the structural changes arising from the spin-transition to be deconvoluted from previously reported thermal effects.

Enantioselective Synthesis of 5-Trifluoromethyl-2-oxazolines under Dual Silver/Organocatalysis.

The first enantioselective formal [3 + 2] cycloaddition between α-isocyanoesters and trifluoromethylketones to give 5-trifluoromethyl-2-oxazolines bearing two contiguous stereogenic centers, one of them being a quaternary stereocenter substituted with a CF3 group, has been developed. The reaction is based upon a multicatalytic approach that combines a bifunctional Brønsted base-squaramide organocatalyst and Ag+ as Lewis acid. The reaction could be achieved with a range of aryl and heteroaryl trifluoromethyl ketones, and the resulting oxazolines were obtained with good to excellent diastereo- and enantioselectivity.

Influence of Host-Guest and Host-Host Interactions on the Spin-Crossover 3D Hofmann-type Clathrates {FeII(pina)[MI(CN)2]2}·xMeOH (MI = Ag, Au).

The synthesis, structural characterization and magnetic properties of two new isostructural porous 3D compounds with the general formula {FeII(pina)[MI(CN)2]2}·xMeOH (x = 0-5; pina = N-(pyridin-4-yl)isonicotinamide; MI = AgI and x ∼ 5 (1·xMeOH); MI = AuI and x ∼ 5 (2·xMeOH)) are presented. The single-crystal X-ray diffraction analyses have revealed that the structure of 1·xMeOH (or 2·xMeOH) presents two equivalent doubly interpenetrated 3D frameworks stabilized by both argentophilic (or aurophilic) interactions and interligand C═O···HC H-bonds. Despite the interpenetration of the networks, these compounds display accessible void volume capable of hosting up to five molecules of methanol which interact with the host pina ligand and establish an infinite lattice of hydrogen bonds along the structural channels. Interestingly, the magnetic studies have shown that solvated complexes 1·xMeOH and 2·xMeOH display two- and four-step hysteretic thermally driven spin transitions, respectively. However, when these compounds lose the methanol molecules, the magnetic behavior changes drastically giving place to gradual spin conversions evidencing the relevant influence of the guest molecules on the spin-crossover properties. Importantly, since the solvent desorption takes place following a single-crystal-to-single-crystal transformation, empty structures 1 and 2 (x = 0) could be also determined allowing us to evaluate the correlation between the structural changes and the modification of the magnetic properties triggered by the loss of methanol molecules.

Organocatalytic enantioselective aminoalkylation of pyrazol-3-ones with aldimines generated in situ from α-amido sulfones.

Herein, an efficient asymmetric aminoalkylation of pyrazolones with α-amido sulfones catalyzed by a quinine-derived squaramide in dichloromethane/aqueous media has been established. A variety of chiral amines were obtained with high yields (up to 98%) and excellent enantioselectivities (up to 99% ee). The corresponding products are transformed into optically active acetylated pyrazoles after treatment with Ac2O/Et3N, because of the instability of some adducts. The reaction tolerates a wide range of α-amido sulfones and different pyrazolones.

Enantioselective Synthesis of 2-Amino-1,1-diarylalkanes Bearing a Carbocyclic Ring Substituted Indole through Asymmetric Catalytic Reaction of Hydroxyindoles with Nitroalkenes.

An asymmetric catalytic reaction of hydroxyindoles with nitroalkenes leading to the Friedel-Crafts alkylation in the carbocyclic ring of indole is presented. The method is based on the activating/directing effects of the hydroxy group situated in the carbocyclic ring of the indole providing nitroalkylated indoles functionalizated at the C-4, C-5, and C-7 positions with high yield, regio-, and enantioselectivity. The optically enriched nitroalkanes were transformed efficiently in optically enriched 2-amino-1,1-diarylalkanes bearing a carbocyclic ring substituted indole.

Switchable Spin-Crossover Hofmann-Type 3D Coordination Polymers Based on Tri- and Tetratopic Ligands.

FeII spin-crossover (SCO) coordination polymers of the Hofmann type have become an archetypal class of responsive materials. Almost invariably, the construction of their architectures has been based on the use of monotopic and linear ditopic pyridine-like ligands. In the search for new Hofmann-type architectures with SCO properties, here we analyze the possibilities of bridging ligands with higher connectivity degree. More precisely, the synthesis and structure of {FeII(LN3)[MI(CN)2]2}·(Guest) (Guest = nitrobenzene, benzonitrile, o-dichlorobenzene; MI = Ag, Au) and {FeII(LN4)[Ag2(CN)3][Ag(CN)2]}·H2O are described, where LN3 and LN4 are the tritopic and tetratopic ligands 1,3,5-tris(pyridin-4-ylethynyl)benzene and 1,2,4,5-tetrakis(pyridin-4-ylethynyl)benzene. This new series of Hofmann clathrates displays thermo- and photoinduced SCO behaviors.

{[Hg(SCN)3]2(µ-L)}2-: An Efficient Secondary Building Unit for the Synthesis of 2D Iron(II) Spin-Crossover Coordination Polymers.

We report an unprecedented series of two-dimensional (2D) spin-crossover (SCO) heterobimetallic coordination polymers generically formulated as {FeII[(HgII(SCN)3)2](L)x}·Solv, where x = 2 for L = tvp (trans-(4,4'-vinylenedipyridine)) (1tvp), bpmh ((1E,2E)-1,2-bis(pyridin-4-ylmethylene)hydrazine) (1bpmh·nCH3OH; n = 0, 1), bpeh ((1E,2E)-1,2-bis(1-(pyridin-4-yl)ethylidene)hydrazine) (1bpeh·nH2O; n = 0, 1) and x = 2.33 for L = bpbz (1,4-bis(pyridin-4-yl)benzene) (1bpbz·nH2O; n = 0, 2/3). The results confirm that self-assembly of FeII, [HgII(SCN)4]2-, and ditopic rodlike bridging ligands L containing 4-pyridyl moieties favors the formation of linear [Fe(µ-L)]n2n+ chains and in situ generated binuclear units {[HgII(SCN)3]2(µ-L)}2-. The latter act as bridges between adjacent chains generating robust 2D layers. The [FeIIN6] centers are equatorially surrounded by four NCS- groups and two axial N atoms of the organic ligand L. The compound 1tvp and the unsolvated form of 1bpmh undergo complete SCO centered at T1/2 = 177 and 226 K, characterized by the enthalpy and entropy variations ΔH = 12.3 and 10.5 kJ mol-1 and ΔS = 69.4 and 48 J K-1 mol-1, respectively. The almost complete SCO of the unsolvated form of 1bpeh occurs at ca. T1/2 = 119 K and exhibits a complete LIESST effect. Regardless of the degree of solvation, a half-spin conversion at T1/2 < 100 K occurs for 1bpbz·nH2O, which becomes almost complete at p = 0.65 GPa. The labile solvent molecules present in 1bpmh·CH3OH and 1bpeh·H2O have a dramatic influence on the corresponding SCO behavior.

Catalytic Asymmetric Formal [3+2] Cycloaddition of 2-Isocyanatomalonate Esters and Unsaturated Imines: Synthesis of Highly Substituted Chiral γ-Lactams.

Unlike their isocyano and isothiocyanato analogues, isocyanato esters remain almost unexplored as formal 1,3-dipoles in asymmetric catalytic reactions. The first asymmetric formal [3+2] cycloaddition involving isocyanato esters and electrophilic alkenes is reported. Diisopropyl 2-isocyanatomalonate reacts with α,ß-unsaturated N-(o-anisidyl) imines in the presence of a Mg(OTf)2 -BOX complex to give highly substituted chiral pyrrolidinones featuring a conjugate exocyclic double bond with excellent yields and enantiomeric excesses up to 99 %. Several transformations of the resulting heterocycles, including the synthesis of a pyroglutamic acid derivative, have been carried out.

Chiral and Racemic Spin Crossover Polymorphs in a Family of Mononuclear Iron(II) Compounds.

Understanding the origin of cooperativity and the equilibrium temperature of transition (T1/2) displayed by the spin-crossover (SCO) compounds as well as controlling these parameters are of paramount importance for future applications. For this task, the occurrence of polymorphism, presented by a number of SCO complexes, may provide deep insight into the influence of the supramolecular organization on the SCO behavior. In this context, herein we present a novel family of mononuclear octahedral FeII complexes with formula cis-[Fe(bqen)(NCX)2], where bqen is the chelating tetradentate ligand N,N'-bis(8-quinolyl)ethane-1,2-diamine and X = S, Se. Depending on the preparation method, these compounds crystallize in either the orthorhombic or the trigonal symmetry systems. While the orthorhombic phase is composed of a racemic mixture of mononuclear complexes (polymorph I), the trigonal phase contains only one of the two possible enantiomers (Λ or Δ), thereby generating a chiral crystal (polymorph II). The four derivatives undergo SCO behavior with well-differentiated T1/2 values occurring in the interval 90-233 K. On one hand, T1/2 is about 110 K (polymorph I) and 87 K (polymorph II) higher for the selenocyanate derivatives in comparison to those for their thiocyanate counterparts. These differences in T1/2 are ascribed not only to the higher ligand field induced by the selenocyanate anion but also to a remarkable difference in the structural reorganization of the [FeN6] coordination core upon SCO. Likewise, the higher cooperativity observed for the thiocyanate derivatives seems to be related to their stronger intermolecular interactions within the crystal. On the other hand, T1/2 is about 53 K (thiocyanate) and 29 K (selenocyanate) higher for the trigonal polymorph II in comparison to those for the orthorhombic polymorph I. These differences, and the small changes observed in cooperativity, stem from the slightly different hetero- and homochiral crystal packing generated by the cis-[Fe(bqen)(NCX)2] molecules, which determines subtle adaptations in the intermolecular contacts and the FeII coordination core.

Guest Induced Strong Cooperative One- and Two-Step Spin Transitions in Highly Porous Iron(II) Hofmann-Type Metal-Organic Frameworks.

The synthesis, crystal structure, magnetic, calorimetric, and Mössbauer studies of a series of new Hofmann-type spin crossover (SCO) metal-organic frameworks (MOFs) is reported. The new SCO-MOFs arise from self-assembly of FeII, bis(4-pyridyl)butadiyne (bpb), and [Ag(CN)2]- or [MII(CN)4]2- (MII = Ni, Pd). Interpenetration of four identical 3D networks with α-Po topology are obtained for {Fe(bpb)[AgI(CN)2]2} due to the length of the rod-like bismonodentate bpb and [Ag(CN)2]- ligands. The four networks are tightly packed and organized in two subsets orthogonally interpenetrated, while the networks in each subset display parallel interpenetration. This nonporous material undergoes a very incomplete SCO, which is rationalized from its intricate structure. In contrast, the single network Hofmann-type MOFs {Fe(bpb)[MII(CN)4]}·nGuest (MII = Ni, Pd) feature enhanced porosity and display complete one-step or two-step cooperative SCO behaviors when the pores are filled with two molecules of nitrobenzene or naphthalene that interact strongly with the pyridyl and cyano moieties of the bpb ligands via π-π stacking. The lack of these guest molecules favors stabilization of the high-spin state in the whole range of temperatures. However, application of hydrostatic pressure induces one- and two-step SCO.