Mono- and Dinuclear Macrocyclic Calcium Complexes as Platforms for

Dec 28, 2015 - Synopsis. Mono- and dinuclear calcium complexes of Schiff-base pyrrole macrocycles have been prepared and characterized, forming bowl- ...
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Mono- and Dinuclear Macrocyclic Calcium Complexes as Platforms for Mixed-Metal Complexes and Clusters Emma A. Connolly, James W. Leeland, and Jason B. Love* EaStCHEM School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh, U.K. EH9 3FJ S Supporting Information *

ABSTRACT: Mono- and dinuclear calcium complexes of the Schiff-base macrocycles H4L have been prepared and characterized spectroscopically and crystallographically. In the formation of Ca(THF)2(H2L1), Ca2(THF)2(μ-THF)(L1), and Ca2(THF)4(L2), the ligand framework adopts a bowl-shaped conformation instead of the conventional wedge, Pacman-shaped structure as seen with the anthracenyl-hinged complex Ca2(py)5(L3). The mononuclear calcium complex Ca(THF)2(H2L1) reacts with various equivalents of LiN(SiMe3)2 to form calcium/alkali metal clusters and dinuclear transition metal complexes when reacted subsequently with transition metal salts. The dinuclear calcium complex Ca2(THF)2(μ-THF)(L1), when reacted with various equivalents of NaOH, is shown to act as a platform for the formation of calcium/alkali metal hydroxide clusters, displaying alternate wedged and bowl-shaped conformations.

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INTRODUCTION

facilitate the high-oxidation state chemistry involved in oxygen evolution. We have shown previously that the Schiff-base pyrrole macrocycles H4L (Chart 1) can act as platforms for well-

The organometallic chemistry of calcium is often overlooked due to the poor solubility of its complexes, high Ca−C bond reactivity compared to the poor reactivity of calcium metal, and its perceived similarities to magnesium chemistry.1 This is surprising as calcium is the fifth most abundant element in the earth’s crust,2 has a relatively low cost stemming from an ease of isolation and reduction, and is biocompatible, so follows the current trend of using less toxic metals in chemical processes.3 Organocalcium complexes of bulky ligands have been applied in a number of catalytic reactions. In particular, complexes of Schiff-base, aryloxide or cyclen ligands have been shown to catalyze the ring-opening polymerization of cyclic monomers,4 and both β-diketamine- and oxazolinylphenyl-containing complexes act as hydroamination catalysts.5 Calcium hydride clusters have been used to catalyze both ketone, and activated alkene hydrosilylation reactions;6 other similar bimetallic calcium compounds undergo single-site catalysis in styrene polymerization,7 and L-lactide ring-opening polymerization.8 Lastly, calcium−alkali metal (M = Li, K) complexes display synergistic advantages in their properties, and can also assist in the generation of mixed-metal enolates from ketones.9 Significantly, the study of calcium compounds can provide insight into the formation and operation of the oxygen-evolving complex (OEC) found in the protein matrix of Photosystem II (PSII).10 Several molecular Mn−Ca model complexes have been described, including a Mn−(μ-OH)−Ca complex capable of activating dioxygen,11 a Mn3Ca multimetal cubane core supported by tripodal ligands,12 and various high nuclearity clusters.13 All of these structurally characterized species demonstrate the understanding that properties of the calcium ion, such as its redox inertness and Lewis acidity, are thought to © XXXX American Chemical Society

Chart 1. Schiff-Base Pyrrole Macrocycles Used in This Study

defined mono- and multimetallic complexes of various s-, d-, and f-block elements, and that these complexes adopt wedged “Pacman” or bowl-shaped structural motifs that facilitate catalytic chemistry.14 Application of the macrocycle H4L1 in the chemistry of the Group 2 metals found that the magnesium Pacman complex Mg2(L1) reorganized upon hydrolysis to form the cluster {Mg4(OH)8}(H4L1)2 in which a unique magnesium hydroxide cubane is stabilized through encapsulation by orthogonally oriented, bowl-shaped macrocycles.15 This use of an organic framework to limit the growth of Mg(OH)2 to the cubane structure is reminiscent of the role played by the protein environment in the formation of the OEC in PSII.16 The relevance of this chemistry to PSII and the role of calcium in the OEC and organometallic chemistry in general has led us Received: October 7, 2015

A

DOI: 10.1021/acs.inorgchem.5b02289 Inorg. Chem. XXXX, XXX, XXX−XXX

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nonadjacent pyrrolic nitrogen atoms, and two THF O-donors. The 1H NMR spectrum of (1) displays equivalent, but broad, resonances for the meso-ethyl groups at approximately 2.3 and 0.9 ppm, indicative of a higher degree of macrocycle flexibility in solution than seen in related “Pacman”-structured complexes, and demonstrates that this bowl-shaped conformation is also adopted in the solution; the Pacman conformation would display nonequivalent ethyl group protons. This structure is also supported by the presence of only one meso-quaternary resonance at 45.9 ppm in the 13C{1H} NMR spectrum. The 1H NMR spectrum of 1 at 50 °C shows resolution of the single meso-CH3 groups into a sharp triplet, but the two imine resonances are retained at this temperature, implying that the calcium cation does not exchange between the two N4-donor pockets. The presence of the empty N4-donor pocket in (1) allows access to mixed-metal complexes of L1. In the first instance, reaction of (1) with 1 equiv of LiN(SiMe3)2 at room temperature (Scheme 1) forms CaLi(THF)(μ-THF)(HL1) (2) in good yield. The 1H NMR spectrum of (2) shows separate proton environments of the macrocycle, suggesting complete desymmetrization of the ligand framework, as well as a single pyrrole NH peak at 10.5 ppm. This implies that the additional lithium ion bonds to only one pyrrole nitrogen in the N4 pocket in solution, which is supported by the X-ray crystal structure of (2) (Figure 1). In the solid state, the lithium ion adopts a distorted tetrahedral geometry described by a N3donor pocket of one pyrrole and two imine nitrogen atoms. The lithium and calcium metal ions are bridged by a THF molecule, resulting in a similarly sized cleft angle to (1) of 89.87°. The Li ion is highly distorted out of the Li···N3 plane at a distance of 0.516 Å, with Li−Npy and average Li−Nim bond lengths that fit within the range of comparable literature values (for example, in imino-functionalized lithium−pyrrolyl complexes).17 In a similar reaction of (1) with 2 equiv of LiN(SiMe3)2, a second lithium cation is coordinated by the macrocycle to form the calcium−dilithium complex CaLi2(THF)n(L1) (3). Its formulation is supported by 1H and 13C{1H} NMR spectroscopy in which the pyrrole NH peak is absent, and the symmetry of the complex is restored. Both of these calcium/alkali metal complexes are potential synthons for the incorporation of a transition metal cation thus forming calcium−transition metal complexes. The reactions of the monometallic calcium complex (1) with various transition metal salts were undertaken with a view to forming calcium−transition metal complexes; however, these reactions ultimately result in the substitution of the calcium cation. For example, reaction of (1) with FeBr2 in benzene at room temperature forms a mixture of NMR-silent ironcontaining complexes. Analysis of crystals obtained from the reaction mixture by X-ray crystallography shows that two ironcontaining compounds form, the bimetallic iron complex Fe2(L1) (4), and the monometallic bis(ligand) iron complex Fe(H3L1)2 (5) (Scheme 1). In contrast to the calciumcontaining complexes (1−3) described above, the X-ray crystal structure of (4) shows that the iron cations occupy the alternative, and standard for transition metals,14 N4-donor compartments of the compartmental macrocycle, resulting in hinged ortho-phenylene rings and a “Pacman”-shaped molecule overall (Figure 2). However, unlike previously reported FeIII2(μ-O)(L1) complexes featuring similar H4L macrocycles,18 no oxidation of either iron cation has taken place. This results in two square planar iron cations in (4) with no capping solvent

to explore the calcium chemistry of this macrocycle, as well as a selection of other reported macrocycles. Herein, we describe the synthesis of mono- and dinuclear calcium complexes of L1−3 and their use as platforms to construct mixed-metal complexes and clusters. Synthesis and Reactivity of the Mononuclear Calcium Complex Ca(THF)2(H2L1) (1). The transamination reaction between Ca(THF)2(N{SiMe3}2)2 and H4L1 at −80 °C in THF furnishes Ca(THF)2(H2L1) (1) in good yield after workup (Scheme 1). The solid-state structure of (1) was determined by Scheme 1. Synthesis and Reactions of the Monometallic Calcium Complex Ca(THF)2(H2L1) (1)

Figure 1. Solid-state structures of Ca(THF)2(H2L1) (1) (top) and [CaLi(THF)(μ-THF)(HL1)] (2) (bottom). For clarity, all solvents of crystallization and hydrogen atoms except for NHs are omitted (displacement ellipsoids are drawn at 50% probability).

X-ray crystallography (Figure 1) and shows a bowl-shaped topology instead of the expected “Pacman” structure, likely due to the large ionic radius of the Ca2+ cation; the meso-carbons act as hinges with a dihedral angle between the metalated and empty N4-donor compartments of 87.10°. The calcium ion is bound in a distorted octahedral geometry by two imine nitrogen atoms, both attached to the same aryl-backbone, two B

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Scheme 2. Synthesis of Bimetallic Calcium Complexes Ca2(THF)2(μ-THF)(L1) (6), Ca2(THF)4(L2) (7), and Ca2(py)5(L3) (8)

Figure 2. Solid-state structure of Fe2(L1) (4). For clarity, all solvents of crystallization and hydrogen atoms are omitted (displacement ellipsoids are drawn at 50% probability).

motifs or bridging molecules. The Fe···Fe separation of 3.8516(4) Å is 0.709 Å longer than that reported for the μoxo analogue, and the bite angle of 58.3° fits into the range of wedged, Pacman-shaped literature complexes (range: 52.1− 64.8°), including 67.5° for FeIII2(μ-O)(L). Also, in contrast to FeIII2(μ-O)(L1), (4) exhibits a significant lateral twist angle of 23.6°, similar to other MII2(L1) complexes.19 The X-ray structure of (5) features two bowl-shaped frameworks with three protonated pyrrole NHs in each, encapsulating a single iron cation in a distorted tetrahedral geometry (Figure S4B, Supporting Information). Both ligands display mean Fe−Nim and Fe−Npy distances of 2.06 Å and 1.99 Å, respectively, that are consistent with those seen previously in the range of 1.84−2.51 Å, and large dihedral angles of 122.8° and 173.8° to accommodate the iron cation and pyrrole NH interactions. The isolation of both (4) and (5) from the same reaction mixture suggests a reaction stoichiometry of

terminal THF molecule in the exo axial site and a bridging THF molecule within the dinuclear cleft. To accommodate the second calcium ion, the dihedral angle is increased from 87.10° in (1) to 99.38° in (6), leading to a Ca1···Ca2 separation of 4.4752(8) Å; when compared to the wide number of bimetallic calcium complexes (range: 3.123−8.38 Å), this value sits in mid-range of reported distances.20 The effect of the substitution pattern of the macrocyclic meso-carbons on the formation and structure of dinuclear calcium complexes was probed using H4L2, which contains Hand C6F5-meso-substituents. Reaction between H4L2 and 2 equiv of Ca(THF)2(N{SiMe3}2)2 gave Ca2(THF)4(L2) (7) in good yield. The 1H and 13C{1H} NMR spectra of (7) show significant shifts upon metalation, and its 19F NMR spectrum suggests a bowl-shaped structure due to its equivalent 19F environments. The X-ray crystal structure of (7) was determined (Figure 3) and is similar to (6) in which the bowl-shaped complex has each calcium cation bound in a distorted octahedral coordination geometry. In contrast to (6), however, (7) accommodates endo- and exo-THF molecules at each calcium center instead of a bridging THF motif, leading to a large bite angle of 140.4° and a subsequently increased Ca1··· Ca2 separation of 5.677(2) Å; this value lies in the upper range of reported bimetallic calcium complexes.20 It is, therefore, evident that macrocycle L2 is able to access a more planar topology than L1 due to the decreased steric hindrance experienced at the meso-carbon in L2 (hydrogen substituent) compared to L1 (ethyl substituent); the perfluorinated phenyl substituents are also involved in a face-to-face π-stacking interaction This modification of the bite angle and calcium− calcium separation by straightforward variation of the mesosubstitutent could be useful in both synthetic modeling and catalytic applications. To further manipulate the calcium···calcium separation in dinuclear complexes, the anthracenyl-hinged macrocycle H4L3 was used. This macrocycle generally facilitates the formation of transition metal complexes that adopt cofacial “Pacman” topologies that exhibit structures and reaction chemistry most closely related to the diporphyrinic analogues.21 As such,

3Ca(H 2L1) + 3FeBr2 → 3CaBr2 + Fe2(L1) + Fe(H3L1)2

Similar reactions of (1) with the transition metal salts MX2 (M = Fe, Co, Cu; X = Cl, Br, OAc) were undertaken, with, in each case, the corresponding bimetallic transition metal complex being formed and identified by X-ray diffraction and NMR spectroscopy, where applicable. Therefore, while not a suitable synthon for mixed Ca−TM complexes, (1) can be exploited as a suitable starting material for the formation of desirable or synthetically inaccessible, dinuclear metal complexes. Formation and Reactivity of Dinuclear Calcium Complexes of H4L1−3. The dinuclear calcium complex Ca2(THF)2(μ-THF)(L1) (6) was accessible in good yields through a number of synthetic routes: by initial deprotonation of H4L1 by KH and salt metathesis with 2 equiv of CaI2; through reaction of H4L1 with 2 equiv of Ca{N(SiMe3)2}2(THF)2; and by transamination of (1) with a further equivalent of Ca{N(SiMe3)2}2(THF)2 (Scheme 2). The 1H NMR spectrum of (6) displays more resolved ethyl meso-substituents than in (1), with triplets at 1.37 and 1.04 ppm and quartets at 2.83 and 2.69 ppm for dissimilar CH3 and CH2 protons that occupy endo and exo sites in the molecular cleft. The solid-state structure of (6) was determined by X-ray crystallography and is similar to that of (1) (Figure 3). For (6), the calcium ions each occupy N4-donor pockets of two imine nitrogen atoms and two nonadjacent pyrrolic nitrogen atoms, forming an overall bowlshaped complex with each cation bound in a distorted octahedral coordination geometry. Each calcium cation has a C

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Figure 3. Solid-state structures of Ca2(THF)2(μ-THF)(L1) (6) (left), Ca2(THF)4(L2) (7) (middle), and Ca2(py)5(L3) (8) (right). For clarity, all solvents of crystallization and hydrogen atoms are omitted (displacement ellipsoids are drawn at 50% probability).

transamination of H4L3 with Ca(THF)2{N(SiMe3)2}2 in pyridine at room temperature results in the formation of the dinuclear calcium complex (8), and its solid structure was determined by X-ray crystallography (Figure 3). As with previous reported examples of complexes of this macrocycle, the favored cofacial “Pacman” shape is observed for (8). Each calcium cation is bound to a pyrrole-imine N4-donor set, along with axial and equatorial pyridine solvent molecules; Ca1 adopts a distorted pentagonal pyramidal geometry, whereas the accommodation of a molecule of pyridine within the cleft provides a pentagonal bipyramidal geometry at Ca2. The accommodation of pyridine within the cleft results in a bent Ca2−N11−C76 angle of 141.19° and distortion of the Ca1− N4-donor set, resulting in a long Ca1···Ca2 separation of 6.2088(8) Å, one of the largest bimetallic calcium−calcium distances reported, and the largest reported metal−metal separation of this macrocyclic ligand. Even with these distortions, the cleft remains cofacial, with a bite angle of 0.61° and a “twist” angle of 9.54°. Formation of Calcium/Alkali Metal Hydroxide Clusters. The relatively constrained bimetallic cleft environment in (6), when compared to the larger inner clefts of (7) and (8), has the potential for use in catalytic applications, so its reactions with small molecules such as water and hydroxide were explored. Initially, (6) was hydrolyzed with 4 equiv of water to discover if a calcium hydroxyl-cubane similar to the Mg analogue described above could be formed. However, these reactions, even when substoichiometric amounts of water were used, were unsuccessful with only H4L1 formed, so highlighting the contrasting chemistry of magnesium and calcium complexes. An alternative reaction between (6) and varying equivalents of NaOH in THF was found to result in the retention of the calcium cations within the macrocycle and the buildup of Group 1/Group 2 mixed-metal hydroxide clusters. The initial 1H NMR spectrum of a reaction mixture of (6) and 4 equiv of NaOH shows the presence of (6) and new calcium/ alkali metal complexes in solution. While the identity of these complexes could not be unequivocally determined from the 1H NMR spectrum, crystallization of the reaction mixture led to the isolation of new Ca/Na complexes identified as Ca2(THF)2(μ-OH)(Na{THF}2)(L1) (9) and Ca2(THF)2(μO)(μ-OH)2(Na{THF})2(L1) (10) (Scheme 3, Figure 4). In the solid-state structure of (9), the bowl-shaped conformation

Scheme 3. Reactions of the Bimetallic Calcium Complex (3) with Various Equivalents of NaOH

of (6) is maintained with the calcium ions in distorted octahedral environments with exo bound THF molecules. The endo-THF molecule in (6) has been substituted by the hydroxide anion O1 in (9), with the attendant sodium cation bound by two THF molecules, the hydroxide, and through πinteractions with the macrocycle within the cleft. The NaOH molecule is disordered across the cleft with an 80:20 occupancy. The Ca1···Ca2 separation is shorter than in (9) at 4.1565(9) Å, and the dihedral angle of 90.54° in (9) is in between that of (1) and (6). In contrast to (9), the X-ray structure of (10) was synonymous to those reported for bimetallic transition metal complexes of H4L1, as the calcium cations in (10) occupy the alternative N4-donor compartments of the compartmental macrocycle. This results in hinged ortho-phenyl rings and a “Pacman”-shaped molecule overall. Each calcium cation has distorted pentagonal bipyramidal geometry, in which THF and one bridging hydroxide are axial, with the N4-donor set and the second bridging hydroxide equatorial. Though there is a change in conformation from (9) to (10), the calcium cations are only slightly distorted out of the N4 plane in (10) (Ca···N4 = 0.080 Å) to accommodate the bridging ligands. Interestingly, the D

DOI: 10.1021/acs.inorgchem.5b02289 Inorg. Chem. XXXX, XXX, XXX−XXX

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Unfortunately, no pure bulk sample of (11) could be isolated due to the inclusion of Ca(OH)2/NaOH impurities, as identified in the MALDI-MS (Figure S10, SI). X-ray analysis of (11), crystallized from the reaction medium, shows that the bowl-shaped macrocyclic conformation is maintained but with the calcium cations substituted by sodium ions. Each sodium ion is bound in a distorted trigonal bipyramidal geometry to a single pyrrolide nitrogen atom and two imine nitrogen atoms with the alternate pyrrole nitrogen atoms reprotonated. The cation coordination sphere is completed with a THF molecule bound in the apical site, giving a Na1···Na2 separation of 3.996 Å and a cleft bite angle of 90.62°, a decreased angle when compared to (9). Similar to (9), the inner cleft contains a hydroxide anion, and its ancillary sodium cation is bound by two THF molecules and tethered through interactions with the macrocycle.

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CONCLUSIONS A mononuclear calcium complex [Ca(THF)2(H2L1)] (1) was synthesized by transamination of the Schiff-base macrocycle H4L1 with Ca(THF)2(N{SiMe3}2)2; reaction of this complex with 1 or 2 equiv of Li(N(SiMe3)2) led to the formation of calcium−lithium complexes (2) and (3). Unfortunately, when these mixed-metal complexes are reacted with transition metal amides and salts (M = Fe, Co, Cu; X = Cl, Br, OAc), (1) dinuclear transition metal complexes are formed in preference to the desired Ca−TM compounds. These reactions highlight the relative lability of the calcium cation compared to transition metals. The synthesis of a series of dinuclear calcium complexes was achieved for a number of Schiff-base macrocycles, through transamination and salt metathesis reactions in some cases. For the anthracenyl H4L3 ligand (8), the conventional wedgedshaped, cofacial complex is seen. However, the smaller H4L1 and H4L2 macrocycles yield the rare bowl-shaped complexes (6) and (7) to accommodate the large calcium ionic radii. These latter dinuclear complexes were found to act as platforms for the mixed Ca/NaOH complexes (9)−(10), although ultimately complete substitution of calcium occurred, resulting in the sodiuim-containing complex (11). This study demonstrates the difficulty in controlling calcium reaction and substitution chemistry, even when macrocyclic ligands are used which can adopt different bowl and Pacman structural modes to accommodate a range of metal cations.

Figure 4. Solid-state structures of Ca2(THF)2(μ-OH)(Na{THF}2)(L1) (9) (top) and Ca2(THF)2(μ-O)(μ-OH)2(Na{THF})2(L1) (10) (bottom left and right). For clarity, all solvents of crystallization, hydrogen atoms, and THF solvent bound to Na1 and Na2 in (10) are omitted (displacement ellipsoids at 50% probability).

Ca1···Ca2 separation has shortened to 3.466(1) Å and the bite angle is decreased to 63.78° compared to those in (6), likely due to the presence of the two bridging O ligands. This requirement for a shorter Ca1···Ca2 separation to accommodate these bridging ligands is the probable cause of the change in conformation of the complex. As with (9), the macrocyclic π-framework also interacts with the alkali metals. In an attempt to rationalize the nature of kinetic vs thermodynamic products of the reactions between (6) and NaOH in THF, reactions at elevated temperature were undertaken, yielding the single soluble product Na2(μ-OH)(Na{THF}2)(H2L1) (11) (Figure 5). The 1H NMR spectrum of (11) displays the number of resonances associated with a symmetric product, with the emergence of a broad feature at 10.8 ppm that is best assigned to NH resonances.

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EXPERIMENTAL SECTION

General Procedure. Experiments were carried out using standard Schlenk line or glovebox techniques under an atmosphere of dinitrogen. Solvents (THF and hexane) were degassed and dried by passage through Vacuum Atmospheres solvent drying towers and stored over activated 4 Å molecular sieves. Deuterated C6D6 solvent was boiled over potassium, trap-to-trap vacuum distilled, and freeze− pump−thaw degassed three times prior to use. LiN(SiMe3)2 was purchased from Sigma-Aldrich and purified by recrystallization from hexane. The syntheses of H4L1,19a H4L2,19b H4L3,21 and Ca(THF)2(N{SiMe3}2)27 were carried out as described in the literature. 1 H and 13C{1H} NMR spectra were recorded at 298 K on a Bruker AVA 500 spectrometer at operating frequencies of 500.12 and 125.76 MHz, respectively. All NMR spectra were referenced internally to residual protio solvent (1H) or solvent (13C{1H}) resonances and are reported relative to tetramethylsilane (δ = 0 ppm). Chemical shifts are quoted in δ (ppm) and coupling constants in hertz. MALDI mass spectra and high-resolution mass spectra were recorded using a Bruker Ultraflex extreme MALDI TOF/TOF instrument. Elemental analyses

Figure 5. Solid-state structure of Na2(μ-OH)(Na{THF}2)(H2L1) (11). For clarity, all solvents of crystallization are omitted (displacement ellipsoids are drawn at 50% probability). E

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Inorganic Chemistry were carried out by Mr. Stephen Boyer at London Metropolitan University. IR data were collected on a Jasco FT/IR 410 spectrometer. Data for all crystals were collected at 170 K using an Oxford Cryosystems low temperature device attached to an Oxford Diffraction Xcalibur Eos diffractometer equipped with an Eos detector and operating graphite monochromated MoKα radiation (λ = 0.71073 Å). Using the Olex2 suite of programs,22 the structure was solved with the ShelXT structure solution program using Direct Methods and refined with the SHELXL refinement package using Least Squares minimization.23 Non-hydrogen atoms (with some exceptions − see below) were refined with anisotropic displacement parameters while hydrogen atoms were placed at calculated positions and included as part of a riding model. The structures of Ca(THF)2(H2L1) (1), CaLi(THF)(μ-THF)(HL 1 ) (2), Ca 2 (THF) 2 (μ-THF)(L 1 ) (6), Ca2(THF)4(L2) (7), Ca2(THF)2(μ-NaOH{THF}2)(L1) (9), and Ca2(THF)2(μ-OH)2(Na{THF})2(L1) (10) contained diffuse and disordered solvent molecules of crystallization which could not be modeled satisfactorily, so the SQUEEZE routine of PLATON was applied to each of these data sets.24 All crystal structures except Fe2L1 (4) have solvent masks applied to data for full modeling. Synthesis of [Ca(THF)2(H2L1)] 1. A solution of Ca(THF)2(N{SiMe3}2)2 (0.21 g, 0.42 mmol) in THF (20 mL) was added dropwise to a stirring solution of H4L1 (0.30 g, 0.42 mmol) in THF (20 mL) at −80 °C. The resulting orange solution was allowed to warm to room temperature and stirred for 4 h, after which the solvent volume was reduced and the product precipitated by addition of hexanes (10 mL). The solids were isolated by filtration, giving 1 as a yellow powder (0.33 g, 0.36 mmol, 87%). 1H NMR (C6D6): δ 9.24 (s, 2H, NH), 8.37 (s, 2H, imine), 7.88 (s, 2H, imine), 7.12 (d, J = 3.5 Hz, 2H, pyrrole β-H), 7.08 (s, 2H, Ar-H), 6.69 (s, 2H, Ar-H), 6.57 (d, J = 3.5 Hz, 2H, pyrrole β-H), 6.48 (d, J = 3.5 Hz, 2H, pyrrole β-H), 6.36 (d, J = 3.4 Hz 2H, pyrrole β-H), 3.59 (br s, THF), 2.51 (m, 2H, Et-CH2), 2.25 (m, 6H, Et-CH2), 2.10 (s, 6H, Ar-CH3), 2.03 (s, 6H, Ar-CH3), 1.18 (br s, THF), 0.961 (m, 12H, Et-CH3); 13C{1H} NMR (C6D6): δ 158.04 (s, quaternary), 149.28 (s, CH, imine), 148.62 (s, CH, imine), 145.09 (s, quaternary), 143.56 (s, quaternary), 141.99 (s, quaternary), 138.95 (s, quaternary), 133.39 (s, quaternary), 132.40 (s, quaternary), 130.72 (s, quaternary), 124.38 (s, CH, pyrrole), 120.14 (s, CH, Ar-H), 116.04 (s, CH, Ar-H), 115.72 (s, CH, pyrrole), 111.60 (s, CH, pyrrole), 107.89 (s, CH, pyrrole), 68.81 (s, CH2, THF), 45.86 (s, quaternary), 29.76 (s, CH2, Et-CH2), 28.18 (s, CH2, Et-CH2), 25.47 (s, CH2, THF), 19.77 (s, CH3, Ar-CH3), 19.40 (s, CH3, Ar-CH3), 8.69 (s, CH3, Et-CH3); Analysis: Found: C, 72.07; H, 7.49; N, 12.37; C54H66CaN8O2 requires: C, 72.13; H, 7.40; N, 12.46%; MS(MALDI): m/z = 756.0 ([M − 2THF]+, 100%); IR (nujol): ν 3444 (N-H), 1599 (CN), 1560 cm−1 (CC). Synthesis of [CaLi(THF)(μ-THF)(HL1)] 2. To a solution of 1 (0.50 g, 0.55 mmol) in THF (20 mL) was added a solution of Li(N{SiMe3}2)2 (0.093 g, 0.55 mmol) in THF (20 mL) at room temperature. An immediate color change from light to deep orangered solution was noted. The resulting solution was concentrated by removal of the volatiles under vacuum, and 2 precipitated from the saturated solution upon the addition of hexane as an orange solid (0.37 g, 0.48 mmol, 86%). 1H NMR (C6D6): δ 8.69 (s, 1H, pyrrole NH), 8.39 (s, 1H, imine-H), 8.38 (s, 1H, imine-H), 8.11 (s, 1H, imine-H), 7.64 (s, 1H, imine-H), 7.15 (d, J = 3.6 Hz, 1H, pyrrole β-H), 7.13 (s, 1H, Ar-H), 7.10 (d, J = 3.6 Hz, 1H, pyrrole H), 7.08 (s, 1H, Ar-H), 7.06 (d, J = 3.5 Hz, 1H, pyrrole H), 6.89 (s, 1H, Ar-H), 6.84 (d, J = 3.6 Hz, 1H, pyrrole H), 6.72 (d, J = 3.5 Hz, 1H, pyrrole H), 6.44 (t, J = 3.4 Hz, 2H), 6.32 (dd, J = 3.6, 2.4 Hz, 1H), 6.27 (s, 1H, Ar-H), 3.54 (br s, THF), 2.95 (dd, J = 11.6, 7.2 Hz, 1H, Et-CH2), 2.85 (dd, J = 14.2, 7.3 Hz, 1H, Et-CH2), 2.51 (dd, J = 13.2, 7.4 Hz, 1H, Et-CH2), 2.34 (dd, J = 12.5, 7.0 Hz, 2H, Et-CH2), 2.27 (dd, J = 14.1, 6.9 Hz, 3H, Et-CH2), 2.09 (s, 3H, Ar-CH3), 2.09 (s, 3H, Ar-CH3), 2.05 (s, 3H, Ar-CH3), 2.02 (s, 3H, Ar-CH3), 1.13 (br s, THF), 1.10 (m, ∼3H, Et-CH3), 1.07 (t, J = 6.1 Hz, 3H, Et-CH3), 0.92 (t, J = 7.1 Hz, 3H, Et-CH3), 0.87 (t, J = 7.2 Hz, 3H, Et-CH3); 13C{1H} NMR (C6D6): 162.25 (s, quaternary), 158.46 (s, quaternary), 157.46 (s, quaternary), 151.24 (s, CH, imine), 150.83 (s, CH, imine), 148.97 (s, CH, imine), 147.56 (s, CH, imine), 146.73 (s, quaternary), 144.95 (s, quaternary), 142.38

(s, quaternary), 142.35 (s, quaternary), 140.72 (s, quaternary), 139.69 (s, quaternary), 139.58 (s, quaternary), 139.05 (s, quaternary), 135.34 (s, quaternary), 133.21 (s, quaternary), 131.47 (s, quaternary), 130.76 (s, quaternary), 130.64 (s, quaternary), 128.06 (s, quaternary), 124.70 (s, CH, Ar-H), 124.63 (s, CH, pyrrole), 121.59 (s, CH, pyrrole), 120.62 (s, CH, Ar-H), 118.91 (s, CH, pyrrole), 116.39 (s, CH, Ar-H), 115.71 (s, CH, Ar-H), 115.56 (s, CH, pyrrole), 111.60 (s, CH, pyrrole), 110.86 (s, CH, pyrrole), 110.63 (s, CH, pyrrole), 108.69 (s, CH, pyrrole), 69.22(s, CH2, THF), 47.87 (s, quaternary), 45.58 (s, quaternary), 29.46 (s, CH2, Et-CH2), 28.74 (s, CH2, Et-CH2), 28.15 (s, CH2, Et-CH2), 27.95 (s, CH2, Et-CH2), 25.40 (s, CH2, THF), 19.86 (s, CH3, Ar-CH3), 19.78 (s, CH3, Ar-CH3), 19.65 (s, CH3, ArCH3), 19.38 (s, CH3, Ar-CH3), 9.30 (s, CH3, Et-CH3), 8.86 (s, CH3, Et-CH3), 8.51 (s, CH3, Et-CH3), 8.21 (s, CH3, Et-CH3); 7Li NMR (C6D6): Li 2.67 (Li in complex), 1.68 (residual LiN(SiMe3)2. No accurate elemental analyses could be determined − attempted analysis 1: Found: C, 59.02; H, 5.81; N, 8.93; attempted analysis 2: Found: C, 63.79; H, 6.72; N, 10.55 C46H49Ca2LiN8 requires: C, 72.61; H, 6.49; N, 14.73%; MS(MALDI): m/z = 759.8 ([M − 2THF]+, 79%), 753.8 ([M − Li − 2THF]+, 100%). IR (nujol): ν 3351 (N-H), 2922 (C-H), 1619 (CN), 1557 cm−1 (CC). Synthesis of [CaLi2(THF)n(L1)] 3. To a solution of 1 (0.30 g, 0.33 mmol) in THF (20 mL) was added a solution of LiN(SiMe3)2 (0.11 g, 0.66 mmol) in THF (20 mL) at room temperature. An immediate color change from light orange to deep orange-red solution was noted. The resulting solution was concentrated by removal of the volatiles under vacuum, and 3 precipitated from the saturated solution upon the addition of hexane as an orange-brown solid. (0.42 g, 0.55 mmol, 82%). Alternative method: To a solution of CaLi(THF)(μ-THF)(HL) in THF (5 mL) was added a solution of LiN(SiMe3)2. The resulting solution was concentrated by removal of the volatiles under vacuum, and 3 precipitated from the saturated solution upon the addition of hexane as a red/brown solid. Spectroscopic analysis of this material was in agreement with that from the above method. 1H NMR (C6D6/ H8-THF) δ 8.20 (s, 2H, imine), 8.00 (s, 2H, imine), 7.03 (s, 2H, ArH), 6.80 (s, 2H, Ar-H), 6.67 (br s, 2H, pyrrole H), 6.54 (br s, 2H, pyrrole H), 6.21 (br s, 2H, pyrrole H), 6.07 (br s, 2H, pyrrole H), 3.50 (br s, THF), 2.37 (m, 2H, Et-CH2), 2.14 (m, 2H, Et-CH2), 2.00 (s, 12H, Ar-CH3), 1.57 (br s, THF), 0.96 (m, 2H, Et-CH3), 0.80 (m, 8H, Et-CH3), 0.68 (m, 2H, Et-CH3); 13C{1H} NMR (C6D6/H8-THF) δ 161.95 (s, quaternary), 160.71 (s, quaternary), 152.17 (s, CH, imine), 147.85 (s, CH, imine), 143.83 (s, quaternary), 142.14 (s, quaternary), 141.88 (s, quaternary), 139.28 (s, quaternary), 139.10 (s, quaternary), 131.43 (s, quaternary), 131.21 (s, quaternary), 131.97 (s, quaternary), 122.73 (s, CH, pyrrole H), 120.74 (s, CH, pyrrole H), 118.41 (s, CH, Ar-H), 115.51 (s, CH, Ar-H), 112.44 (s, CH, pyrrole H), 112.11 (s, CH, pyrrole H), 67.82 (s, CH2, THF), 47.97 (s, quaternary), 25.32 (s, CH2, THF), 30.67 (s, CH2, Et-CH2), 19.57 (s, CH3, Ar-CH3), 19.37 (s, CH3, Ar-CH3), 9.37 (s, CH3, Et-CH3). MS(MALDI): m/z = 759.3 ([M − Li − nTHF]+, 100%), 753.3 ([M − 2Li − nTHF]+, 78%). IR (nujol): ν 2924 (C-H), 1618 (CN), ∼1560 cm−1 (CC). General Syntheses of M2L1 (M = Fe, Co, Cu); Formation of Fe2L1 4 and Fe(H3L1)2 5. A solution of 1 (0.055 mmol) in C6H6 (5 mL) was added dropwise to a stirred mixture of the selected transition metal salt (0.055 mmol) in C6H6 (5 mL) at room temperature and left to stir for 48 h. After filtration, the solvent was removed giving powder product. FeBr2 example: 1 (0.020 g, 0.022 mmol) and FeBr2 (0.0047 g) gave dark red powder (0.0067 g, ∼37% yield to 4). The MALDI data (see the SI) showed minimal peaks; however, the presence of 4 and 5 in this mixture was implied by subsequent crystallization and Xray crystallographic analysis (see the SI for crystal structure of 5). Similar reactions with CoII and CuII salts resulted in the formation of the known complexes Co2(L1) and Cu2(L1) that were identified crystallographically.19a Synthesis of [Ca2(THF)2(μ-THF)(L1)] 6. A mixture of H4L1 (0.94 g, 1.32 mmol) and excess KH (0.42 g, 10.53 mmol) was treated with THF (20 mL). Gas evolved immediately, and, once effervescence had ceased, the mixture was filtered onto a stirring suspension of CaI2 (0.86 g, 2.91 mmol) in THF (20 mL) at −80 °C. A color change from yellow to orange was observed. The reaction was warmed to room F

DOI: 10.1021/acs.inorgchem.5b02289 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

11.96%; MS(MALDI): m/z = 953.2 ([M + OH − 5py]+, 100%); IR (nujol, KBr): ν 2926 (C-H), 1609 (CN), 1584 cm−1 (CC). Syntheses of Ca2(THF)2(μ-OH)(Na{THF}2)(L1) 9 and Ca2(THF)2(μ-O)(μ-OH)2(Na{THF})2(L1) 10. A solution of 2 (0.11 g, 0.107 mmol) in THF (5 mL) was added dropwise to a stirring mixture of NaOH (0.018 g, 0.450 mmol) in THF (5 mL) at room temperature and left to stir overnight at 40 °C. After filtration, the solvent was removed, giving a dark orange powder (0.056 g, 50%). The 1H NMR spectrum (C6D6/H8-THF; see the SI) showed a mixture of products to be present; however, the presence of 3 and 4 in this mixture was implied by subsequent crystallization and X-ray crystallographic analysis. Synthesis of Na2(THF)2(μ-OH)(Na{THF}2)(H2L) 11. A solution of 2 (0.11 g, 0.10 mmol) in THF (5 mL) was added dropwise to a stirring mixture of NaOH (0.017 g, 0.40 mmol) in THF (5 mL) at room temperature, heated to 80 °C, and left to stir at this temperature for 48 h. After cooling and filtration, the solvent was removed, giving 5 as a brown powder (0.064 g, 0.063 mmol, 60%). 1H NMR (C6D6/H8THF): δ 10.23 (br s, 3H, NH and μ-OH(Na) peaks), 8.13 (s, 4H, imine), 6.79 (s, 4H, Ar-H), 6.36 (d, J = 3.4 Hz, 4H, pyrrole H), 5.95 (d, J = 3.4 Hz, 4H, pyrrole H), 3.49 (br s, THF, masked by presaturation), 2.52 (q, J = 6.8 Hz, 6H, Et-CH2), 2.20 (q, J = 6.8 Hz, 6H, Et-CH2) 2.04 (s, 12H, Ar-CH3), 1.62 (br s, THF, masked by presaturation), 0.94 (t, 6H, Et-CH3), 0.70 (t, 6H, Et-CH3); 13C{1H} NMR (C6D6/H8-THF) δ 150.10 (s, CH(imine) and quaternary), 143.80 (s, quaternary), 136.48 (s, quaternary), 133.10 (s, quaternary), 120.07 (s, CH, pyrrole), 118.14 (s, CH, Ar-H), 109.75 (s, CH, pyrrole), 67.75 (s, CH2, THF), 45.43 (s, quaternary), 28.93 (s, CH2, Et-CH2), 25.64 (s, CH2, THF), 25.27 (s, CH2, Et-CH2), 19.54 (s, CH3, Ar-CH3), 9.10 (s, CH3, Et-CH3), 8.70 (s, CH3, Et-CH3); MS(MALDI): m/z = 783.8 ([M − NaOH − 4THF]+, 71%), 753.9 ([L + NaOH]+, 100%). No accurate elemental analyses could be determined due to contamination with NaOH and Ca(OH)2, as seen in MALDI-TOF MS.

temperature and left to stir overnight, after which it was filtered. The resulting solution was concentrated by removal of the volatiles under vacuum, and 2 precipitated from the saturated solution as an orange solid (0.99 g, 0.98 mmol, 75%). Alternative method: To a solution of H4L1 (0.11 g, 0.15 mmol) in THF (10 mL) was added a solution of Ca(THF)2(N{SiMe3}2)2 (0.15 g, 0.29 mmol) in THF (10 mL) at −80 °C. The solution was allowed to warm to room temperature and stirred overnight. The resulting solution was concentrated by removal of the volatiles under vacuum, and 2 precipitated from the saturated solution as an orange solid (0.11 g, 0.12 mmol, 80%). Spectroscopic analysis of this material was in agreement with that from the above method. 1H NMR (C6D6) δ 8.34 (s, 4H, imine), 7.11 (d, J = 3.6 Hz, 4H, pyrrole H), 7.06 (s, 4H, Ar-H), 6.81 (d, J = 3.6 Hz, 4H, pyrrole H), 3.48 (br s, THF), 2.82 (q, J = 7.0 Hz, 4H, Et-CH2), 2.68 (q, J = 7.1 Hz, 4H, Et-CH2), 2.06 (s, 12H, Ar-Me), 1.37 (t, J = 7.1 Hz, 6H, EtCH3), 1.15 (br s, THF), 1.04 (t, J = 7.3 Hz, 6H, Et-CH3); 13C{1H} NMR (C6D6) δ 161.77 (s, quaternary), 148.65 (s, CH, imine), 141.88 (s, quaternary), 138.97 (s, quaternary), 131.97 (s, quaternary), 125.01 (s, CH, pyrrole), 115.58 (s, CH, Ar-H), 111.89 (s, CH, pyrrole), 69.04 (s, CH2, THF), 47.73 (s, quaternary), 29.39 (s, CH2, Et-CH2), 27.80 (s, CH2, Et-CH2), 25.32 (s, CH2, THF), 19.77 (s, CH3, Ar-CH3), 9.18 (s, CH3, Et-CH3), 8.99 (s, CH3, Et-CH3); Analysis: Found: C, 68.86; H, 7.37; N, 10.86; C58H72Ca2N8O3 requires: C, 69.01; H, 7.19; N, 11.10%; MS(MALDI): m/z = 791.8 ([M − 3THF]+, 36%), 763.8 ([M − 2Me − 3THF]+, 100%); IR (nujol, KBr): ν 2955 (C-H), 1603 (C N), 1571 cm−1 (CC). Synthesis of [Ca2(THF)4(L2)] 7. To a solution of H4L2 (0.20 g, 0.21 mmol) in THF (10 mL) was added a solution of Ca(THF)2(N{SiMe3}2)2 (0.22 g, 0.45 mmol) in THF (5 mL) at −80 °C. The solution was allowed to warm to room temperature and stirred overnight. The resulting solution was concentrated by removal of the volatiles under vacuum, and 7 precipitated from the saturated solution as a dark yellow/green solid (0.15 g, 0.15 mmol, 67%). 1H NMR (500 MHz, C6D6/H8-THF) δ 8.34 (s, 4H, imine), 7.13 (s, 4H, Ar-H), 6.70 (d, J = 3.4 Hz, 4H, pyrrole H), 6.66 (d, J = 2.9 Hz, 4H, pyrrole H), 5.90 (s, 2H, meso-H), 2.10 (s, 12H, Ar-CH3) 13C{1H} NMR (C6D6/ H8-THF): δ 149.20 (s, CH, imine), 149.08 (s, quaternary), 141.55 (s, quaternary), 139.14 (s, quaternary), 133.08 (s, quaternary), 122.28 (s, CH, pyrrole), 116.20 (s, CH, Ar-H), 111.82 (s, CH, pyrrole), 67.80 (s, CH2, THF), 44.71 (s, quaternary), 25.94 (s, CH2, THF), 19.50 (s, CH3, Ar-CH3). 19F{1H}-NMR (C6D6/H8-THF): F −143.04 (d, 4F, J = 16 Hz, ortho-F), −162.17 (t, 2F, J = 22 Hz, para-F), −164.87 (t, 4F, J = 19 Hz, meta-F). Analysis: Found: C, 59.04; H, 3.06; N, 10.96; C50H30Ca2N8F10 requires: C, 59.29; H, 2.99; N, 11.06% MS(MALDI): m/z = 1011.3 ([M + OH − 4THF]+, 100%), 1031.3 ([M + OH − F − 4THF]+, 65%); IR (nujol, KBr): ν 2924 (C-H), ∼1600 (CN), 1578 cm−1 (CC). Synthesis of [Ca2(py)5(L3)] 8. To a solution of H4L3 (0.50 g, 0.58 mmol) in pyridine (20 mL) was added a solution of Ca(THF)2(N{SiMe3}2)2 (0.58 g, 1.16 mmol) in pyridine (20 mL) at room temperature. The solution was allowed to warm to room temperature and stirred overnight at 40 °C. The resulting solution was concentrated by removal of the volatiles under vacuum, and (8) precipitated from the saturated solution as a dark orange solid (0.34 g, 0.36 mmol, 63%). 1H NMR (py-d5) δ 9.50 (s, 2H, Ar-H), 8.26 (s, 4H, imino), 8.14 (s, 2H, Ar-H), 7.46 (d, J = 8.1 Hz, 4H, Ar-H), 7.00 (s, 4H, Ar-H), 6.96 (m, 4H, Ar-H), 6.61 (m, 4H, pyrrole H), 6.41 (m, 4H, pyrrole H), 3.68 (br s, THF), 2.45 (dd, J = 14.2, 7.1 Hz, 1H, Et-CH2), 2.06 (br m, 6H, Et-CH2), 1.78 (q, J = 6.9 Hz, 1H, Et-CH2), 1.65 (br s, THF), 1.43 (m, 3H, Et-CH3), 1.25 (m, 3H, Et-CH3), 1.08 (m, 6H, EtCH3); 13C{1H} NMR (C6D6/H8-THF): δ 160.41 (s, CH, imine), 158.17 (s, quaternary), 154.42 (s, quaternary), 138.40 (s, quaternary), 133.54 (s, quaternary), 128.59 (s, quaternary), 127.56 (s, CH, s, CH, Ar-H), 125.93 (s, CH, s, CH, Ar-H), 122.45 (s, CH, s, CH, Ar-H), 120.05 (s, CH, s, CH, Ar-H), 118.24 (s, CH, pyrrole), 111.75 (s, CH, pyrrole), 68.32 (s, CH2, THF), 49.09 (s, quarternary), 26.30 (s, CH2, THF), 11.81 (s, CH3, Ar-CH3), 10.24 (s, CH3, Ar-CH3). No accurate elemental analyses could be determined − attempted analysis 1: Found: C, 59.29; H, 4.95; N, 7.91; attempted analysis 2: Found: C, 62.45; H, 4.90; N, 10.94 C58H48Ca2N8 requires: C, 74.33; H, 5.16; N,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02289. 1 H, 13C{1H}, 7Li, and 19F NMR spectra; MALDI-TOF spectra, solid-state structures, and table of X-ray crystallographic data (PDF) X-ray crystallographic data (ZIP)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +44(0)131 650 4762. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the EPSRC (U.K.), EaStCHEM and the University of Edinburgh for support. REFERENCES

(1) Harder, S. Chem. Rev. 2010, 110, 3852−3876. (2) Emsley, J. Nature’s Building Blocks: An A-Z Guide to the Elements, 2nd ed.; Oxford University Press: Oxford, U.K., 2002. (3) Anastas, P. T.; Crabtree, R. H. Handbook of Green Chemistry; Wiley-VCH: Weinheim, Germany, 2009; Vol. 1. (4) (a) Darensbourg, D. J.; Choi, W.; Ganguly, P.; Richers, C. P. Macromolecules 2006, 39, 4374−4379. (b) Darensbourg, D. J.; Choi, W.; Richers, C. P. Macromolecules 2007, 40, 3521−3523. (c) Darensbourg, D. J.; Choi, W.; Karroonnirun, O.; Bhuvanesh, N. Macromolecules 2008, 41, 3493−3502. (d) Sarazin, Y.; Howard, R. G

DOI: 10.1021/acs.inorgchem.5b02289 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b02289 Inorg. Chem. XXXX, XXX, XXX−XXX