Change In Coordination Geometry Research Articles

Metal ion induced single-crystal-to-single-crystal transformation and luminescent sensing properties of resorcin[4]arene-based metal–organic frameworks

In this work, two new resorcin[4]arene-based metal–organic frameworks (MOFs), namely, [(CH3)2NH2][Zn3(HL)(L)(DMF)(H2O)2]·2H2O (1-Zn) and [(CH3)2NH2]2[Cd3(L)2(H2O)4]·2DMF·2H2O (2-Cd), have been synthesized under solvothermal conditions (H4L represents the resorcin[4]arene-based ligand). 1-Zn exhibits a layer structure, while 2-Cd shows a 3D motif. Remarkably, by immersing 1-Zn in an aqueous solution of Cu2+ ions, a new Cu(II)-MOF [Cu2(H2L)2(H2O)2]·3.5H2O (1-Cu) was achieved via a metal ion induced single-crystal-to-single-crystal (SCSC) transformation manner, involving generation of new metal-O bond and change of metal ion coordination geometry. Moreover, luminescent detection properties for Fe3+ ions and Cr2O72− ions were also investigated with 1-Zn and 2-Cd as luminescent sensors.

Tuning two-electron transfer in terpyridine-based platinum(ii) pincer complexes.

An important factor in obtaining reversible multi-electron transfer is overcoming large changes in coordination geometry. One strategy is to use ligands that can support the geometries favored before and after the electron transfer. Pip2NCN− pincer and terpyridine ligands are used to support square planar Pt(ii) and octahedral Pt(iv). For the Pt(ii) complexes, [Pt(Z-pip2NCN)(R-tpy)]+ (Z = NO2, MeO, H; R = H, tertyl butyl, tolyl), 1H NMR spectroscopy shows that the Z-pip2NCN− ligand is monodentate whereas the R-terpyridyl ligand is tridentate. The availability of flanking piperidyl groups of the monodentate pincer ligand is essential for the stabilization of the metal center upon oxidation. Pt(Z-pip2NCN)(R-tpy)+ complexes undergo two-electron platinum centered oxidation near 0.4 V and two Pt(tpy) centered reductions near −1.0 V and −1.5 V. An estimate of nox/nred = 1.8 is consistent with an oxidation that involves two-electron transfer per Pt center. Variation in the pincer-(Z) and terpyridine-(R) substituents allows for tuning of the oxidation process over a 260 mV range and the two reduction processes over ranges of 230 mV (first reduction) and 290 mV (second reduction step).

Tetranuclear dysprosium single-molecule magnets: tunable magnetic interactions and magnetization dynamics through modifying coordination number.

The study of mononuclear lanthanide-based systems, where the observed Single Molecule Magnets (SMMs) properties originate from the local magnetic anisotropy of the single lanthanide ion, has been extensively investigated in the literature. The case for polynuclear lanthanide SMMs becomes more challenging both experimentally and theoretically due to the complexity of such architectures involving interactions between the magnetic centers. Much interest was devoted to the study of the structural effect on the magnetic interactions and relaxation dynamics. However, the understanding of the structural influence on those two factors remains a difficult task. To address this issue, a system containing two structurally related tetranuclear Dy(iii) SMMs, namely [Dy4(L)4(OH)2(DMF)4(NO3)2]·2(DMF)·(H2O) (1) and [Dy4(L)4(OH)2(DMF)2(NO3)2] (2) (H2L = 2-(2-hydroxy-3-methoxybenzylideneamino)phenol), is introduced and investigated. Through modifying the ligands on the changeable coordination sites, the intramolecular magnetic interactions and relaxation dynamics in these two Dy(iii)4 SMMs can be tuned. Both complexes exhibit slow relaxation of their magnetization with a relaxation barrier of 114 K for complex 2 while a blocking temperature below 2 K is observed for complex 1. Ab initio calculations reveal that changes in coordination numbers and geometries on the Dy(iii) sites can significantly affect the magnetic interactions as well as single-ion anisotropy. The combination of experimental work and ab initio calculations offers insight into the relationship between structures and magnetic properties and sheds light on the rational design of future polynuclear lanthanide SMMs with enhanced magnetic properties.

Impact of Chloride Concentration on Ligand Substitution Reactions of Zinc(II) Complexes with Biologically Relevant Nitrogen Nucleophiles

The mole-ratio method was used to determine the metal–ligand stoichiometry between [ZnCl2(en)] and [ZnCl2(terpy)] (where en = 1,2-diaminoethane or ethylenediamine and terpy = 2,2′:6′,2″-terpyridine) and imidazole at pH 7.2 in the presence of different chloride concentrations. The results indicated step-wise formation of 1:1 and 1:2 complexes in the presence of 0.010 M NaCl and 1:1 complexes in the presence of 0.001 M NaCl for the [ZnCl2(en)] complex. These results are correlated with additional coordination of chlorides in the first coordination sphere and with changes in coordination geometry. In the presence of 0.001 M NaCl the five-coordinate complex anion [ZnCl3(en)]- is formed initially and then a substitution reaction with imidazole occurs. In the presence of 0.010 M NaCl the octahedral complex anion [ZnCl4(en)]2- is formed. Additional coordination of chloride in the [ZnCl2(terpy)] complex is not found and the metal–ligand stoichiometry is 1:2. The kinetics of ligand substitution reactions of zinc(II) complexes and biologically relevant nitrogen nucleophiles such as imidazole, 1,2,3-triazole and L-histidine were investigated at pH 7.2 as a function of nucleophile concentration in the presence of 0.001 M and 0.010 M NaCl. The reactions were followed under pseudo first-order conditions by UV-Vis spectrophotometry. The substitution reactions included two steps of consecutive displacement of chlorido ligands with changes only in the coordination geometry of the [ZnCl2(en)] complex. The order of reactivity of the investigated nucleophiles for the first reaction step towards both complexes was L-histidine > 1,2,3-triazole > imidazole, while in the presence of 0.010 M NaCl the most reactive ligand was 1,2,3-triazole towards the [ZnCl2(en)] complex.

Clarifying the Copper Coordination Environment in a de Novo Designed Red Copper Protein.

Cupredoxins are copper-dependent electron-transfer proteins that can be categorized as blue, purple, green, and red depending on the spectroscopic properties of the Cu(II) bound forms. Interestingly, despite significantly different first coordination spheres and nuclearity, all cupredoxins share a common Greek Key β-sheet fold. We have previously reported the design of a red copper protein within a completely distinct three-helical bundle protein, α3DChC2. (1) While this design demonstrated that a β-barrel fold was not requisite to recapitulate the properties of a native cupredoxin center, the parent peptide α3D was not sufficiently stable to allow further study through additional mutations. Here we present the design of an elongated protein GRANDα3D (GRα3D) with Δ Gu = -11.4 kcal/mol compared to the original design's -5.1 kcal/mol. Diffraction quality crystals were grown of GRα3D (a first for an α3D peptide) and solved to a resolution of 1.34 Å. Examination of this structure suggested that Glu41 might interact with the Cu in our previously reported red copper protein. The previous bis(histidine)(cysteine) site (GRα3DChC2) was designed into this new scaffold and a series of variant constructs were made to explore this hypothesis. Mutation studies around Glu41 not only prove the proposed interaction, but also enabled tuning of the constructs' hyperfine coupling constant from 160 to 127 × 10-4 cm-1. X-ray absorption spectroscopy analysis is consistent with these hyperfine coupling differences being the result of variant 4p mixing related to coordination geometry changes. These studies not only prove that an Glu41-Cu interaction leads to the α3DChC2 construct's red copper protein like spectral properties, but also exemplify the exact control one can have in a de novo construct to tune the properties of an electron-transfer Cu site.

Towards Ni(II) complexes with spin switches for 19F MR-based pH sensing.

Our aim was to demonstrate the potential of exploiting simultaneous changes in coordination geometry and spin state in fluorinated Ni(II) complexes as an avenue for 19F magnetic-resonance (MR)-based pH sensing. Crystal structures were studied using an Agilent Technologies SuperNova Dual Source diffractometer. Solution magnetic moment was determined using Evan's method. MR images were collected on a 7.0-T MR scanner equipped with a quadrature 19F volume coil. NiL1 and NiL2 were synthesized; crystallographic and spectroscopic data supported NiL1 as being diamagnetic and NiL2 as being paramagnetic. In aqueous solution, ligand dissociation from Ni(II) center was observed for both complexes at around pH 6, precluding their use as reversible pH sensors. The two complexes have distinct 19F nuclear magnetic resonance (NMR) signals in terms of both chemical shift and relaxation times, and selective imaging of the two complexes was achieved with no signal interference using two 19F MRI pulse sequences. The significant difference in the chemical shift and relaxation times between NiL1 and NiL2 allowed selective imaging of these species using 19F MRI. While NiL1 and NiL2 were not stable to acidic environments, this report lays the framework for development of improved ligand scaffolds that stably coordinate Ni(II) in acidic aqueous solution and act as agents for ratiometric pH mapping by 19F MRI.

Thermochromic Solid-State Emission of Dipyridyl Sulfoxide Cu(I) Complexes

Copper(I) complexes (Cu-DPSO and Cu-Me-DPSO) utilizing sulfoxide-bridged dipyridyl ligands are reported. Cu-DPSO demonstrates photophysical properties typical with [Cu(disphosphine)(diimine)]+ complexes, however with additional steric bulk in the 6- and 6′-positions of the diimine ligand to give complex Cu-Me-DPSO; the photophysics are greatly altered. This species is isolated as an amorphous powder (a-Cu-Me-DPSO) which emits yellow light; upon heating to 180 °C, a crystalline powder is formed (c-Cu-Me-DPSO) which shows a large bathochromic shift (>100 nm) in emission, and shows orange luminescence attributed to a flattening distortion of the complex away from a tetrahedral geometry. On cooling to −196 °C, c-Cu-Me-DPSO displays a reversible thermochromic solid-state emission, from orange at room temperature to yellow at low temperatures. Using solid-state variable temperature excitation and absorption data, this phenomenon is attributed to a change in coordination geometry about the copper atom in the exc...

Assembly of Lanthanide(III) Cubanes and Dimers with Single-Molecule Magnetism and Photoluminescence.

Discrete lanthanide(III) tetranuclear cubane-like clusters seldom occur throughout the LnIII series and behave as single-molecule magnets (SMMs). Herein, a series of cubanes, [Ln4(μ3-OH)4(μ-tfa)4(hfa)4(phen)4] (1-9, Ln = La-Dy (except Pm), tfa = trifluoroacetate, hfa = hexafluoroacetylacetonate, phen = 1,10-phenanthroline), and dinuclear clusters, [Ln2(μ-OH)2(hfa)4(phen)2] (10-16, Ln = Tb-Lu), were synthesized and characterized. Two types of clusters were formed due to the change of preferred coordination geometry for lighter and heavier LnIII ions which favor nine-coordinated cubanes and eight-coordinated dimers, respectively. A magnetic study shows that 8-Tb4 and 9-Dy4 are ferromagnetically coupled and SMM in nature because of the larger Ln···Ln distance compared to other discrete cubanes. The anisotropic barriers, Ueff, of 9-Dy4 are determined to be 67.0 K. In addition, the photophysical properties of 6-Eu4, 8-Tb4, and 10-Tb2 owing to tfa, hfa, and phen sensitization and O-H quenching are discussed.

Probing Cytochrome c Folding Transitions upon Phototriggered Environmental Perturbations Using Time-Resolved X-ray Scattering.

Direct tracking of protein structural dynamics during folding-unfolding processes is important for understanding the roles of hierarchic structural factors in the formation of functional proteins. Using cytochrome c (cyt c) as a platform, we investigated its structural dynamics during folding processes triggered by local environmental changes (i.e., pH or heme iron center oxidation/spin/ligation states) with time-resolved X-ray solution scattering measurements. Starting from partially unfolded cyt c, a sudden pH drop initiated by light excitation of a photoacid caused a structural contraction in microseconds, followed by active site restructuring and unfolding in milliseconds. In contrast, the reduction of iron in the heme via photoinduced electron transfer did not affect conformational stability at short timescales (<1 ms), despite active site coordination geometry changes. These results demonstrate how different environmental perturbations can change the nature of interaction between the active site and protein conformation, even within the same metalloprotein, which will subsequently affect the folding structural dynamics.

Kinetic Studies of the Reactions between Dichlorido(1,2-Diaminoethane)Zinc(II) and Biologically Relevant Nucleophiles in the Presence of Chloride

The mole-ratio method was used for determining metal–ligand stoichiometry for the reaction between [ZnCl2(en)] (where en = 1,2-diaminoethane or ethylenediamine) and chloride ion at pH 7.2. The results have shown step-wise formation of 1:1 and 1:2 complexes and indicate additional coordination of chloride ions in the first coordination sphere. The kinetics of ligand substitution reactions of the zinc(II) complex and biologically relevant nucleophiles such as inosine-5′-monophosphate (5′-IMP), guanosine-5′-monophosphate (5′-GMP), L-methionine (L-Met), glutathione (GSH) and DL-aspartic acid (DL-Asp) were investigated at pH 7.2 as a function of nucleophile concentration in the presence of 0.010 M NaCl. The reactions were followed under pseudo first-order conditions by UV-Vis spectrophotometry. The substitution reactions include two steps of consecutive displacement of chlorido ligands and changes in coordination geometry. In the presence of an excess of chloride, the octahedral complex anion [ZnCl4(en)]2- formed. The first step of the substitution reactions could be interpreted as substitution of the axial chlorido ligands in the cis position to bidentate ethylenediamine by the biologically relevant nucleophiles, while the second step could be interpreted as substitution of the equatorial chlorido ligand. The order of reactivity of the investigated nucleophiles for the first reaction step is 5′-IMP &gt; GSH &gt; L-Met &gt; DL-Asp &gt; 5′-GMP, while for the second reaction step the order of reactivity is GSH &gt; L-Met &gt; 5′-IMP &gt; DL-Asp &gt; 5′-GMP.

Metal-centred azaphosphatriptycene gear with a photo- and thermally driven mechanical switching function based on coordination isomerism

Metal ions can serve as a centre of molecular motions due to their coordination geometry, reversible bonding nature and external stimuli responsiveness. Such essential features of metal ions have been utilized for metal-mediated molecular machines with the ability to motion switch via metallation/demetallation or coordination number variation at the metal centre; however, motion switching based on the change in coordination geometry remain largely unexplored. Herein, we report a PtII-centred molecular gear that demonstrates control of rotor engagement and disengagement based on photo- and thermally driven cis–trans isomerization at the PtII centre. This molecular rotary motion transmitter has been constructed from two coordinating azaphosphatriptycene rotators and one PtII ion as a stator. Isomerization between an engaged cis-form and a disengaged trans-form is reversibly driven by ultraviolet irradiation and heating. Such a photo- and thermally triggered motional interconversion between engaged/disengaged states on a metal ion would provide a selector switch for more complex interlocking systems.

Reactivities study of titanium sites in titanosilicate frameworks by in situ XANES

This study focused on the study of titanium sites, their local structure and reactivities in nanoporous titanosilicates frameworks by using in situ X-ray absorption spectroscopy. XANES provides the information on the oxidation state of the titanium species as well as their change in coordination geometry during de- and re-hydration. Results from in situ XANES studies shows that TS-1 is much more hydrophobic than TiAl or TiFe systems. Pre-edge features of Ti K-edge XANES spectra of these materials also suggested that TiIV is the dominant Ti coordination in dehydrated system whilst TiVI is the dominant in the re-hydrated one.

Variation of Desolvation Behavior in Two Isostructural Metal–Organic Frameworks Based on a Flexible, Racemic Bifunctional Organic Linker

A racemic mixture of the chiral ligand 4,4′‐(1,2‐dihydroxyethane‐1,2‐diyl)dibenzoic acid was used to prepare two isostructural metal–organic frameworks, CPO‐49‐Zn and CPO‐49‐Mn, which contain coordinated solvent molecules at the metal site. The compounds showed different behavior upon desolvation. The dissociation of the solvent molecule from the metal site leads to a single‐crystal‐to‐single‐crystal transformation. In CPO‐49‐Zn, a change of coordination geometry from trigonal bipyramidal to tetrahedral occurs at the zinc atom. In CPO‐49‐Mn, a rearrangement of coordination mode of a carboxylate group occurs instead, leading to a 4+1 coordination of the manganese cation in the form of a capped distorted tetrahedron. N2 gas adsorption confirms that both desolvated structures are permanently porous. The behavior of the compounds upon heating has also been studied using variable temperature powder X‐ray diffraction. The presence of a coordinated solvent molecule in the as‐synthesized structures indicates the possibility to access the metal cation with reactive substrates. Both materials were evaluated in the catalytic oxidation of styrene. CPO‐49‐Mn showed significantly higher conversion than the CPO‐49‐Zn material.

Bonding, Luminescence, Metallophilicity in Linear Au3 and Au2Ag Chains Stabilized by Rigid Diphosphanyl NHC Ligands.

The heterofunctional and rigid ligand N,N'-diphosphanyl-imidazol-2-ylidene (PCNHCP; P = P(t-Bu)2), through its phosphorus and two N-heterocyclic carbene (NHC) donors, stabilizes trinuclear chain complexes, with either Au3 or AgAu2 cores, and dinuclear Au2 complexes. The two oppositely situated PCNHCP (L) ligands that "sandwich" the metal chain can support linear and rigid structures, as found in the known tricationic Au(I) complex [Au3(μ3-PCNHCP,κP,κCNHC,κP)2](OTf)3 (OTf = CF3SO3; [Au3L2](OTf)3; Chem. Commun. 2014, 50, 103-105) now also obtained by transmetalation from [Ag3(μ3-PCNHCP,κP,κCNHC,κP)2](OTf)3 ([Ag3L2](OTf)3), or in the mixed-metal tricationic [Au2Ag(μ3-PCNHCP,κP,κCNHC,κP)2](OTf)3 ([Au2AgL2](OTf)3). The latter was obtained stepwise by the addition of AgOTf to the digold(I) complex [Au2(μ2-PCNHCP,κP,κCNHC)2](OTf)2 ([Au2L2](OTf)2). The latter contains two dangling P donors and displays fluxional behavior in solution, and the Au···Au separation of 2.8320(6) Å in the solid state is consistent with metallophilic interactions. In the solvento complex [Au3Cl2(tht)(μ3-PCNHCP,κP,κCNHC,κP)](OTf)·MeCN ([Au3Cl2(tht)L](OTf)·MeCN), which contains only one L and one tht ligand (tht = tetrahydrothiophene), the metal chain is bent (148.94(2)°), and the longer Au···Au separation (2.9710(4) Å) is in line with relaxation of the rigidity due to a more "open" structure. Similar features were observed in [Au3Cl2(SMe2)L](OTf)·2MeCN. A detailed study of the emission properties of [Au3L2](OTf)3, [Au3Cl2(tht)L](OTf)·MeCN, [Au2L2](OTf)2, and [Au2AgL2](OTf)3 was performed by means of steady state and time-resolved photophysical techniques. The complex [Au3L2](OTf)3 displays a bright (photoluminescence quantum yield = 80%) and narrow emission band centered at 446 nm with a relatively small Stokes' shift and long-lived excited-state lifetime on the microsecond timescale, both in solution and in the solid state. In line with the very narrow emission profile centered in the violet-blue region, fabrication of organic light-emitting devices (OLEDs) comprising the [Au3L2](OTf)3 complex demonstrated its usefulness as a deep-blue emitter in solution-processed OLEDs. Electrochemical and Raman spectroscopic studies were also performed on [Au3L2](OTf)3. Experimental results were rationalized by means of Wave-Function Theory (WFT) and Density Functional Theory (DFT). MP2 calculations gave a satisfactory description of the structures of the cationic complexes [Au3L2](3+) and [Au2L2](2+) and pointed to Au···Au interactions having an electrostatic component owing to the dissimilar charge distribution in the chain caused by the heterofunctional ligand. The nature of the emitting states and their geometric distortions relative to the ground states in [Au3L2](3+) and [Au2L2](2+) was studied by DFT, revealing contraction of the Au···Au distances and coordination geometry changes by association of the dangling P donor, respectively.

Probing Framework-Restricted Metal Axial Ligation and Spin State Patterns in a Post-Synthetically Reduced Iron-Porphyrin-Based Metal-Organic Framework.

An iron-porphyrin-based metal organic framework PCN-222(Fe) is investigated upon postsynthetic reduction with piperidine. Fe K-edge X-ray absorption and Kβ mainline emission spectroscopy measurements reveal the local coordination geometry, oxidation, and spin state changes experienced by the Fe sites upon reaction with this axially coordinating reducing agent. Analysis and fitting of these data confirm the binding pattern predicted by a space-filling model of the structurally constrained pore environments. These results are further supported by UV-vis diffuse reflectance, IR, and resonance Raman spectroscopy data.

ChemInform Abstract: Water‐Induced Reversible SCSC or Solid‐State Structural Transformation in Coordination Polymers

AbstractReview: 58 refs.

Water-dependent charge-transfer-induced spin transition of Prussian blue analogues.

Cobalt-iron Prussian blue analogues are magnetic functional compounds that exhibit charge-transfer-induced spin transition (CTIST), however, the role of guest species such as water in influencing the magnetic behaviour has remained unclear. Here, we report the water-dependent CTIST behaviour of Na0.46Co[Fe(CN)6]0.78(H2O)1.31 together with its single crystal X-ray structural analysis, for the first time.

Probing the Limits of Tetraazamacrocycle‐Glyoxal Condensates as Bidentate Ligands for Cu2+

AbstractTetracyclic bisaminals formed by condensation of tetraazamacrocycles and glyoxal have rarely been utilized as ligands for transition metal ions. Five novel Cu2+ complexes are presented to expand this set. Glyoxal derivatives of pyruvic aldehyde and butanedione were expected to further rigidify the bisaminal skeleton because of the steric requirements of the methyl groups, while the previously unutilized homocyclam was expected to increase flexibility due to the presence of a seven‐membered ring. Additionally, a nitrato complex was prepared to examine the effect of more bulky monodentate ligands on the bisaminal framework. The new complexes showed similar four‐coordination of Cu2+, but with significant changes in coordination geometry caused by substitution of the glyoxal bridge by one or two methyl groups. The homocyclam derivative produced a completely different coordination mode due to the flexibility of the larger macrocycle ring. Both structural and electronic properties were affected by the use of nitrato in place of the typical chlorido monodentate ligands. Overall, the electronic properties (magnetic moment, electronic spectra, and cyclic voltammetry) were only subtly affected. However, larger changes were observed when the larger homocyclam macrocycle was used or the nitrato anion was used in place of the chlorido ligand to complete the coordination. Additionally, the recently published geometric index τδ was evaluated by using this series of closely related Cu2+ complexes.

Impact of Coordination Geometry, Bite Angle, and Trans Influence on Metal-Ligand Covalency in Phenyl-Substituted Phosphine Complexes of Ni and Pd.

Despite the long-standing use of phosphine and diphosphine ligands in coordination chemistry and catalysis, questions remain as to their effects on metal-ligand bonding in transition metal complexes. Here we report ligand K-edge XAS, DFT, and TDDFT studies aimed at quantifying the impact of coordination geometry, diphosphine bite angle, and phosphine trans influence on covalency in M-P and M-Cl bonds. A series of four-coordinate NiCl2 and PdCl2 complexes containing PPh3 or Ph2P(CH2)nPPh2, where n = 1 (dppm), 2 (dppe), 3 (dppp), and 4 (dppb), was analyzed. The XAS data revealed that changing the coordination geometry from tetrahedral in Ni(PPh3)2Cl2 (1) to square planar in Ni(dppe)Cl2 (2) more than doubles the intensity of pre-edge features assigned to Ni-P and Ni-Cl 1s → σ* transitions. By way of comparison, varying the diphosphine in Pd(dppm)Cl2 (4), Pd(dppp)Cl2 (6), and Pd(dppb)Cl2 (7) yielded Pd-P 1s → σ* transitions with identical intensities, but a 10% increase was observed in the P K-edge XAS spectrum of Pd(dppe)Cl2 (5). A similar observation was made when comparing Ni(dppe)Cl2 (2) to Ni(dppp)Cl2 (3), and DFT and TDDFT calculations corroborated XAS results obtained for both series. Comparison of the spectroscopic and theoretical results to the diphosphine structures revealed that changes in M-P covalency were not correlated to changes in bite angles or coordination geometry. As a final measure, P and Cl K-edge XAS data were collected on trans-Pd(PPh3)2Cl2 (8) for comparison to the cis diphosphine complex Pd(dppe)Cl2 (5). Consistent with phosphine's stronger trans influence compared to chloride, a 35% decrease in the intensity of the Pd-P 1s → σ* pre-edge feature and a complementary 34% increase in Pd-Cl 1s → σ* feature was observed for 8 (trans) compared to 5 (cis). Overall, the results reveal how coordination geometry, ligand arrangement, and diphosphine structure affect covalent metal-phosphorus and metal-chloride bonding in these late transition metal complexes.

Probing the structural changes accompanying a spin crossover transition in a chiral [Fe(ii)(N3O2)(CN)2] macrocycle by X-ray crystallography

The preparation and characterization of a novel chiral macrocycle S,S-[FeII(PhCH2OCH2)2(N3O2)(CN)2] (4) is presented. Magnetic susceptibility studies reveal that this complex undergoes a gradual spin crossover in the solid state with no thermal hysteresis. Variable temperature X-ray diffraction studies on single crystals of 4 reveal that at 150 K the complex is comprised of two crystallographically independent octahedral Fe(II) centres. Warming the crystal to 300 K results in a subtle change in coordination geometry of one of the octahedral Fe(II) centers towards a 7-coordinate, pentagonal bipyramidal geometry that is accompanied by a conformation change in one of the PhCH2OCH2 substituents.