Hexanuclear and Trinuclear Metal Complexes of a Giant Octadecaaza

Article Cite This: Inorg. Chem. 2017, 56, 12719-12727

pubs.acs.org/IC

Hexanuclear and Trinuclear Metal Complexes of a Giant Octadecaaza Macrocycle ́ Janusz Gregoliński,* Katarzyna Slepokura, and Jerzy Lisowski* Department of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław, Poland

Downloaded via KAOHSIUNG MEDICAL UNIV on July 1, 2018 at 16:02:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: A large macrocyclic ligand containing six pyridine fragments and six diaminocyclopentane fragments is able to form hexanuclear Zn(II) and Ni(II) complexes as well as a trinuclear Zn(II) complex. X-ray crystal structures of these complexes indicate quite different ligand conformations. In the hexanuclear Zn(II) derivative with chloride counteranions metal ions have a distorted-trigonal-bipyramidal geometry and occupy loop sections formed by the highly folded macrocycle, which adopts a globular shape. In the hexanuclear Ni(II) derivative with nitrate counteranions metal ions exhibit a distorted-octahedral geometry and the ligand conformation is much more open, while in the trinuclear Zn(II) complex the macrocycle wraps around the octahedral metal ions. The last highly entangled conformation of the trinuclear complex is also present in solution, as confirmed by the NOESY spectra. The NMR data indicate that the hexanuclear Zn(II) complex partially dissociates in water solutions to form the trinuclear complex, while the 1H NMR titration of the free macrocycle with zinc(II) chloride indicates that the formation of a trinuclear complex corresponds to cooperative binding of metal ions.

■

INTRODUCTION Macrocycles are of fundamental importance in supramolecular recognition of cations and anions.1 Macrocyclic ligands often allow for precise control over the coordination number, oxidation state, or reactivity of bound metal ion(s). Among macrocyclic systems, aza macrocyles constitute one of the most appealing classes of ligands in coordination chemistry.2 Apart from important tetraaza macrocycles such as porphyrins and cyclam, which are able to bind a single metal ion in the center, larger macrocycles are known, which can bind multiple metal ions.2−6 For instance, a nonaaza 3 + 3 macrocycle derived from trans-1,2-diaminocyclohexane (DACH) and 2,6-diformylpyridine (DFP) can bind three transition-metal ions5 and similar triphenolic 3 + 3 macrocycles form trinuclear complexes with lanthanide(III) or transition-metal ions.6 The application of very large macrocycles possessing multiple metal binding sites allows for controlled simultaneous binding of several metal ions in close proximity, which may lead to unique properties. Thus, large macrocycles may be used to form designed multimetallic complexes to study magnetic interactions or exploit multiple metal centers in cooperative catalysis. Sometimes the macrocyclic ligand is large enough to embrace and often stabilize an entire metal cluster. Macrocyclic polynuclear complexes may also be used for mimicking metalloenzymes, which utilize multiple metal ions, or be used as host molecules for specific binding of large anionic hosts. For instance, the recently reported hexanuclaear Zn(II) complex of a very large macrocycle forms a unique stacked dimeric © 2017 American Chemical Society

structure accompanied by the binding of dicarboxylic acids, and such a sophisticated macrocyclic system was suggested as a promising platform for allosteric catalysis.7 Recently we have reported the synthesis of the giant 6 + 6 macrocyclic amine L (Figure 1) based on racemic trans-1,2diaminocyclopentane (DACP), which contains 18 nitrogen atoms in its main cycle.8 We speculated that formation of this

Figure 1. Octadecaaza macrocycle L. Received: May 9, 2017 Published: October 19, 2017 12719

DOI: 10.1021/acs.inorgchem.7b01173 Inorg. Chem. 2017, 56, 12719−12727

Article

Inorganic Chemistry

sites point to the center of the macrocycle, while in complex 1 of compact conformation the labile coordination sites point outward. The crystal structure of 2 shows a different mode of metal coordination by the macrocycle L in comparison with that of 1 (Figure 3). The Ni(II) ions in this hexanuclear complex have a

macrocycle is based on a Cd(II) template effect leading to a corresponding hexanuclear 6 + 6 macrocyclic imine precursor. In this report we show that macrocycle L is indeed able to form hexanuclear Zn(II) and Ni(II) complexes 1 ([Zn6LCl12]) and 2 ([Ni6L(NO3)12]), respectively, as well as trinuclear Zn(II) complex 3 ([Zn3(L)](NO3)5Cl); we also show that these polynuclear complexes are able to bind additional guest molecules in the central void, which is not occupied by metal ions. While many metalla macrocyclic complexes of high nuclearity are known,9 the analogous macrocyclic complexes are not as common3,4,7 and complexes 1 and 2 represent a rare case of structurally characterized, symmetric hexanuclear macrocyclic complexes.

■

RESULTS AND DISCUSSION Synthesis and X-ray Crystal Structures. The free amine L readily forms complexes when it is combined with 6 equiv of metal salts such as zinc chloride and nickel nitrate in methanol. Upon slow evaporation of water−acetonitrile solutions crystalline derivatives 1 and 2 can be obtained. The zinc(II) complex 1 is practically insoluble in organic solvents such as chloroform and methanol and sparingly soluble in DMSO, and it dissolves slowly in water but with decomposition. The crystal structure of 1 reveals a hexanuclear complex (Figure 2), where

Figure 3. Top and side views of the hexanuclear Ni(II) complex 2. Color code: Ni, green balls, N, light blue sticks; C, gray sticks; O, red sticks. H atoms are omitted for clarity.

highly distorted octahedral geometry. All of them are coordinated by three nitrogen atoms of the macrocycle and one monodentate nitrate anion. The two remaining coordination sites (see details in the Supporting Information) in five Ni(II) cations are occupied by a bidentate nitrate anion and in the sixth cation by acetonitrile and a water molecule. The set of the three coordinating nitrogen atoms of the macrocycle is different from that of the zinc(II) complex. This time the pyridine nitrogen atom is accompanied by the two nitrogen atoms of the same diaminocyclopentane fragment (Figure S1 in the Supporting Information). Yet another mode of metal binding by L is present in the trinuclear Zn(II) complex 3 (with a mixed set of nitrate and chloride anions), which can be obtained by using 3 equiv of metal ion. This time the Zn(II) ions have a distorted-octahedral geometry and are coordinated exclusively by the nitrogen atoms of the macrocycle. The macrocycle is triply twisted in such a way that two crossed N3 sections, described above for complex 1, surround Zn(II) ions in a meridional fashion (Figure 4 and Figure S1 in the Supporting Information). The comparison of the structures of 1−3 illustrates the flexibility of macrocycle L in adjusting its conformation in order to match the coordination requirements of the metal ions. The conformation of the macrocycle in complex 1 is remarkably similar to that observed for the protonated hydrochloride

Figure 2. Top and side views of the hexanuclear Zn(II) complex 1. Color code: Zn, blue balls; N, light blue sticks; C, gray sticks; Cl, green sticks. H atoms are omitted for clarity.

the Zn(II) ions are embedded in six loops (U-shaped compartments) formed by the highly folded macrocycle. Within each loop the metal ion is coordinated by the N3 section of the macrocycle composed of the pyridine nitrogen atom and two nitrogen atoms of the two adjacent diaminocyclopentane fragments (Figure 1 in the Supporting Information). The pentacoordinate Zn(II) ions in complex 1 are of irregular geometry, as indicated by the τ values ranging from 0.30 to 0.43 (τ is the index of trigonality,10 τ = (β − α)/ 60°, where α and β are the two largest angles in the coordination sphere; the limit values of τ equal to 0 and 1 correspond to ideal square-pyramidal and ideal trigonalbipyramidal geometries, respectively). The coordination sphere can be viewed as a severely distorted trigonal bipyramid, where axial positions correspond to diaminocyclopentane nitrogen atoms and equatorial positions correspond to chloride anions and pyridine nitrogen atoms. Complex 1 is similar to the hexanuclear Zn(II) complex of a different giant macrocycle reported in ref 7; in both cases each Zn(II) ion is coordinated by three donor atoms of the macrocycle and by two additional labile donors. However, in the latter complex of open structure the labile coordination

Figure 4. Top and side views of the trinuclear Zn(II) complex 3. Color code: Zn, blue balls; N, light blue sticks; C, gray sticks. H atoms are omitted for clarity. 12720


Article

Inorganic Chemistry

Figure 5. Comparison of the macrocycle conformation (top view) in its protonated form 5, hexanuclear zinc(II) complex 1, hexanuclear nickel(II) complex 2, and trinuclear Zn(II) complex 3 (from left to right, respectively).

a value of 3.33 cm3 K mol−1 at 1.8 K (Figure S5 in the Supporting Information). The lack of sizable magnetic exchange is in agreement with the structure of this complex, which indicates rather long intermetallic distances and lack of bridging anions. The 1H NMR spectrum of 2 also indicates the presence of paramagnetic high-spin Ni(II) ions in this hexanuclear complex (Figure S6 in the Supporting Information). The NMR signals exhibit clear paramagnetic shift and paramagnetic line broadening effects, the broadening being indeed so severe that only 13 out of the expected 15 signals are observed. Unexpectedly, the 1H NMR spectra of D2O or D2O HEPES buffer solutions of Zn(II) complexes 1 and 3 are practically identical (Figure 7 and Figure S7 in the Supporting Information); moreover, a single set of signals is observed after mixing the solutions of the two complexes. The same spectrum is also observed for the trinulear complex 4, [Zn3(L)](Cl)6, which is an all-chloride analogue of 3. These observations point to a partial decomposition of hexanuclear complex 1 upon dissolution in water and formation of a trinuclear complex. In order to confirm the partial dissociation of a hexanuclear Zn(II) complex in water solution and formation of a trinuclear complex, we have performed NMR titrations with EDTA salt, EDTANa4·H2O, a competing strong Zn(II)-binding agent. Gradual addition of up to 3 equiv of EDTA tetrasodium salt to a D2O solution of 1 results in the gradual appearance of 1H NMR signals of [ZnEDTA]2− while the signals of the macrocyclic Zn(II) complex remain intact (Figures S8 and S9 in the Supporting Information). The addition of 3 equiv more of EDTA tetrasodium salt results in gradual decomposition of the macrocyclic complex accompanied by appearance of the signals of the free macrocycle and further increase in the signals of [ZnEDTA]2−. This experiment indicates the presence of 3 equiv of free Zn(II) ions and a trinuclear macrocyclic Zn(II) complex in water solutions of 1. In contrast, the gradual addition of the first 3 equiv of tetrasodium salt of EDTA to a D2O solutions of trinuclear complexes 3 and 4 results in complex decomposition and formation of [ZnEDTA]2− from the very beginning. The same results were obtained when titrations with the tetrasodium salt of EDTA were performed in the presence of HEPES buffer (Figures S10 and S11 in the Supporting Information). The presence of a trinuclear Zn(II) complex in solution is also in accord with the 1H, 13C, and 2D NMR spectra (Figure 7 and Figures S12−S15 in the Supporting Information). The 1H NMR spectrum of complex 3 (500 MHz, D2O) consists of 15 nonexchangeable signals, while the 13C{1H} NMR spectrum consist of 13 signals. These spectra are more complicated (exhibit more signals) in comparison with those of the free

derivative 5 of the free macrocycle8a (Figure 5), which corresponds to a globular container shape. This container can be viewed as a cyclic combination of six alternating U-shaped compartments or loops, which are built from a pyridine fragment and two adjacent cyclopentane diamine fragments each (Figure S1 in the Supporting Information). In 5 these compartments are occupied by hydrogen-bonded chloride anions, whereas in 1 they are occupied by metal ions. The macrocycle in 1 is somewhat more “open” and the container is less compact in comparison with those of 5. In contrast, the conformation of the macrocycle is much more open in nickel(II) complex 2 with pseudo-S6 symmetry (Figure 5 and Figures S2−S4 in the Supporting Information). The overall conformation is less globular, and the U-shaped compartments discussed above for the zinc(II) complex are now widely open (Figure S1). On the other hand, a completely different conformation with three highly twisted parts of L is observed in the trinuclear complex 3. Ligand L binds six metal ions in such a way which leaves a cavity in a center of the complex. In complexes 1 and 2 this cavity is occupied by disordered solvent molecules (Figure 6). Similarly, the center of the cationic complex molecule of 3 is not occupied by a metal ion but by a chloride anion guest via electrostatic and H-bonding interactions (Figure 6).

Figure 6. Space-filling representations of 1 (left), 2 (middle), and 3 (right). Color code: Zn, blue; Ni, light green; N, light blue; C, gray; O, red; H, white.

NMR Spectra and Magnetic Properties. Magnetic data for 2 at 300 K indicate an χMT value of 7.42 cm3 K mol−1, which corresponds to a magnetic moment of individual Ni(II) ions equal to 3.14 μB. These values are in line with the octahedral geometry of Ni(II) ions and orbital contribution to a spin-only value of 6.04 cm3 K mol−1 for six S = 1 metal ions. The χMT value is fairly constant down to ca. 25 K and then decreases with decreasing temperature, indicating a weak antiferromagnetic interactions and/or ZFS effects. This decrease accelerates below ca. 8 K with χMT finally reaching 12721


Article

Inorganic Chemistry

Figure 7. 1H NMR spectra (500 MHz, D2O) of the complex 1, [Zn6Cl12(L)]·7H2O (C = 2.5 mM/dm3, pD 6.43) (a), complex 4, [Zn3(L)](Cl)6· 13.5H2O (C = 2.5 mM/dm3, pD 6.20) (b), complex 3, [Zn3(L)](NO3)5Cl(L)]·3.5H2O (C = 2.5 mM/dm3, pD 6.37) (c), and macrocycle L (saturated solution, pD 9.93) (d). The asterisk denotes a residual solvent signal.

Figure 8. Dynamic averaging of the structures of complex 3 in solution.

macrocycle with an effective D3d symmetry and reveal a different conformation due to metal binding. The aromatic region in the COSY spectrum indicates that the complex contains one type of pyridine ring (only one triplet of pyridine γ protons is observed, assigned as signal a), which are not symmetrictwo different coupled pyridine β protons are observed, which are arbitrarily assigned as b and b′. The HMQC spectrum identifies the signals of the groups of aromatic, secondary, tertiary, and quaternary carbon atoms together with the corresponding protons. In particular, two

different signals e and e′ corresponding to cyclopentane >CHNH tertiary protons can be identified by analyzing the HMQC spectrum. These two signals are not correlated in the COSY spectrum, which indicates that they do not correspond to the same cyclopentane ring but to two different types of cyclopentane fragments. In addition the two different cyclopentane fragments have to be positioned on the symmetry axes (C2), because only four proton signals (two of them overlapped) and only three carbon signals can be identified for each cyclopentane ring on the basis of COSY and HMQC 12722


Article

Inorganic Chemistry

the complicated 1H NMR spectrum measured for this complex at room temperature and by ESI MS results (Figures S18−S21 in the Supporting Information). It seems that the pure hexanuclear Zn(II) complex formed during crystallization is not stable in coordinating solvents (while it is practically insoluble in the common noncoordinating solvents). One can expect that the macrocycle L should be able to form mono- and dinuclear complexes with metal ions. In order to identify such species, we have performed various titration experiments monitored with NMR spectroscopy. Interestingly, the titration of L with zinc(II) chloride in the mixed D2O/ CD3OD solution or in D2O HEPES buffer solution monitored with 1H NMR spectroscopy (Figure 9 and Figures S22 and S23 in the Supporting Information) suggests a cooperative binding of metal ions. When up to 3 equiv of Zn(II) salt is gradually added, one set of signals of the dominant complexed form is observed. This complex is in slow exchange (on the NMR time scale) with the free macrocycle L. The fact that the obtained spectrum is very similar to that of a D2O solution of 1 and that the addition of more than 3 equiv of zinc(II) chloride affects the spectrum substantially only when a large excess is used indicates that the trinuclear cationic complex [Zn3(L)]6+ is the dominant complex form in this solution. Similar behavior was observed for the analogous NMR titration of the macrocycle L with nickel(II) nitrate (Figures S24 and S25 in the Supporting Information). In a general case of a ligand having three binding sites, one expects that K1 > K2 > K3 (for statistical reasons and because of the repulsion between the cations). Thus, for instance, in the presence of 1 equiv of metal ion per ligand, the NMR spectrum should be dominated by a complicated set of signals of a low-symmetry mononuclear complex. In both titration experiments the intermediate mono- or dinuclear complexes were present in concentrations smaller than expected on a statistical basis and the intensity of signals indicates cooperative binding. For instance, the intensity of the signals of the free macrocycle L, the trinuclear complex Zn3L6+, and the sum of intensities of the remaining signals (that likely correspond to the mono- and dinuclear complexes ZnL2+ and Zn2L4+, respectively) indicates that the ratio of concentrations [L]:([ZnL2+] + [ Zn2L4+]):[Zn3L6+] is equal to 0.58:0.18:1 for the NMR spectrum obtained after addition of 1.8 equiv of Zn2+ (Figure 9 and Figure S26 in the Supporting Information). As illustrated by the calculations presented in the Supporting Information, such a ratio is an indication that the value of K3 is larger than the values K1 and K2, which corresponds to the positive cooperativity11 in the binding of the first three metal ions by the macrocycle L. It is likely that the complexation of the first Zn(II) ion and formation of the first twisted N6 section of the macrocycle results in conformational rearrangement of the macrocycle leading to preferential binding of additional two Zn(II) ions. This kind of positive cooperativity has been previously observed in the formation of trinuclear Ln(III) triple-stranded helicates.12 It was observed that the preorganization of the receptor, brought about by the first two Ln(III) ions, more than compensated for the electrostatic repulsion associated with the complexation of the third Ln(III) ion. Similar cooperative binding of three metal ions was also observed in a 1H NMR titration of a linear multicompartment ligand with zinc(II) acetate.13

spectra. The HMQC spectrum also allows identification of the signal of geminal protons of the cyclohexane rings as well as geminal protons of two different types of methylene bridges (denoted as pair c, d and pair c′, d′). These data are consistent with the presence of a C3 axis “perpendicular” to the macrocycle and the presence of three C2 axes, which pass through the cyclohexane rings and which are perpendicular to the C3 axis. Thus, the complex has effective D3 symmetry, which may result from conformational averaging of C3symmetric complexes observed in the solid state. This dynamic process results in averaging of positions of pyridine rings to an averaged effective orientation that is perpendicular to the plane defined by the three metal ions, as illustrated in Figure 8. The pyridine triplet a is NOESY and COSY correlated to signals of aromatic protons b and b′ (Figures S13 and S15 in the Supporting Information). The signal of proton b is in turn NOESY correlated to the signals of the pair of geminal protons c and d. Similarly, the signal of the proton b′ is NOESY correlated to the signals of the pair of geminal protons c′ and d′. Signal e′ corresponding to cyclopentane >CH-NH protons (as identified on the basis of HMQCFigure S14 in the Supporting Information) exhibits NOE-type correlation to signals c′ and d′ and also to signal c. This means that this cyclopentane >CH-NH proton is close to both methylene bridges. An inspection of the close contacts in the crystal structure of 3 identifies the position e′ as the position belonging to the cyclopentane ring which is more inside. On the other hand, the proton e of the more outside cyclopentane ring exhibits NOE-type correlation with only one methylene bridge: i.e., to protons c and d. This distinction of the primed positions (outer cyclopentane, Figure S16 in the Supporting Information) and nonprimed positions (inner cyclopentane) is further confirmed by the observation of a NOESY correlation between signals c and f′. This correlation indicates close contact, which is in line with a characteristic multiply twisted conformation of the macrocycle in complex 3 and the presence of two types of cyclopentane rings. The signal e is also NOESY and COSY correlated to the geminal signals f and g, which are in turn NOESY and COSY correlated to signal h. Similar correlations are observed for the signals e′, the overlapped signals f′ and g′, and the signal of proton h′. The HMQC spectrum in combination with the assignment of 1H NMR signals also enables the assignment of 13C NMR signals of the corresponding carbon atoms (Figure S12 in the Supporting Information). Apart from the expected NOESY cross peaks corresponding to the neighboring protons of L, there are additional cross peaks (Figure S15 in the Supporting Information) corresponding to pairs of signals of protons that are far apart in the untwisted “open” macrocycle presented in Figure 1 or in hexanuclar complex 1. On the other hand, these protons are positioned close in space in the crystal structure of 3 due to substantial entanglement of the macrocycle (Figure S16 in the Supporting Information), thus confirming that the structure of the trinuclear complex in water solution is similar to that observed in the solid state. On the other hand, for the neutral hexanuclear complex 1 and effective D3d symmetry a simpler spectrum with eight 1H NMR signals would be expected. This kind of spectrum is indeed observed at 373 K for DMSO-d6 solutions (Figure S17 in the Supporting Information). It is likely, however, that this simple spectrum results from the dynamic averaging of species of various nuclearities. The mixture of species is suggested by

■

CONCLUSIONS In summary, we have shown that L is a macrocycle predisposed to bind six transition-metal ions in such a way that the center of 12723


Article

Inorganic Chemistry

studies of the complexing abilities of L toward various metal ions are underway.

■

EXPERIMENTAL SECTION

The NMR spectra were recorded on Bruker Avance 500 and Bruker Avance III 600 MHz spectrometers. The elemental analyses were carried out on a PerkinElmer 2400 CHN elemental analyzer. Variabletemperature magnetizations (1.8−300 K) were measured using a Quantum Design SQUID-based MPMSXL-5-type magnetometer with field strength 5 T. The SQUID magnetometer was calibrated with a palladium rod sample. Background corrections for the sample holder and diamagnetic contribution estimated from the Pascal constants were applied. The pD of HEPES buffer was adjusted by adding 10 M NaOH solution in D2O. The pD values of D2O solutions used in NMR measurements were determined by using a glass electrode and calculated according to the formula: pD = pH* + 0.4, where pH* is the pH-meter reading.14 Macrocycle L, C72H102N18·3H2O, has been obtained as previously described.7 Free macrocycle L was prepared by addition of 3 mL of 2 M NaOH water solution to a water solution of 182 mg (0.100 mmol) of its hydrochloride salt, L·12HCl·9H2O, extraction of the resulting mixture three times with 3 mL of dichloromethane, drying the combined organic layers over anhydrous sodium sulfate, and evaporating to dryness using a vacuum pump. Quantitative yield (127 mg). Anal. Calcd (found) for C72H108N18O3: C, 67.89 (67.96); H, 8.55 (8.76); N, 19.79 (19.78). Complex 1, [Zn6Cl12(L)]·7H2O. A solution of 115.6 mg (0.0948 mmol) of the macrocycle L in 3.5 mL of methanol was combined with a solution of 77.3 mg (0.567 mmol) of ZnCl2 in 3.5 mL of methanol. The mixture was refluxed for 1 h, and the obtained white precipitate was filtered, washed with 1 mL of methanol, and dried under vacuum. Yield 108.2 mg (52.8%). Anal. Calcd (found) for C72H116Cl12N18O7Zn6: C, 39.97 (40.10); H, 5.40 (5.14); N, 11.65 (11.40). MS (ESI-TOF) m/z: 2003.0 [Zn6(C72H102N18)(Cl)11]+, 1865.2 [Zn5(C72H102N18)(Cl)9]+, 1729.3 [Zn4(C72H102N18)(Cl)7]+, 914.1 [Zn5(C72H102N18)(Cl)8]2+, 846.2 [Zn4(C72H102N18)(Cl)6]2+. 1H NMR (500 MHz, D2O): δ 8.22 (t, J = 7.8 Hz, 1H, α-pyr); 7.67 (d, J = 7.9 Hz, 1H, β-pyr); 7.30 (d, J = 7.9 Hz, 1H, β-pyr); 4.45, 4.27 (ABq, JAB = 18.0 Hz, 2H, Cγ-pyrCH2NH); 3.91, 3.38 (ABq, JAB = 16.2 Hz, 2H, Cγ-pyrCH2NH); 2.83 (m, 1H, NHCHCH2 (CP)); 2.53 (m, 1H, NHCHCH2 (CP)); 2.23 (m, 1H, CHCH2CH2 (CP)); 1.73 (m, 1H, CH2CH2CH2 (CP)); 1.54 (m, 1H, CH2CH2CH2 (CP)); 1.49 (m, 2H, CHCH2CH2 (CP)); 1.38 (m, 1H, CHCH2CH2 (CP)). Complex 2, [Ni6(NO3)12(L)]·20H2O. A solution of 147.3 mg (0.1207 mmol) of the macrocycle L in 5 mL of methanol was combined with a solution of 231.8 mg (0.7970 mmol) of Ni(NO3)· 6H2O in 5 mL of methanol. The mixture was refluxed for 2 h, and the clear solution was evaporated to dryness. The residue was dissolved in 5 mL of methanol, 10 mL of acetonitrile was added, and the formed blue precipitate was filtered, washed with 1 mL of acetonitrile, and dried under vacuum. Yield 159.6 mg (49.4%). After the concentration of the filtrate to ca. 7−8 mL an additional fraction (27.0 mg, yield 8.4%) of the blue product was obtained. Anal. Calcd (found) for C72H142N30Ni6O56: C, 32.31 (32.49); H, 5.35 (5.09); N, 15.70 (15.80). MS (ESI-TOF) m/z: 1887.6 [Ni4(C72H102N18)(NO3)7]+, 1704.7 [Ni3(C72H102N18)(NO3)5]+, 1095.2 [Ni6(C72H102N18)(NO3)10]2+, 1004.2 [Ni 5 (C 72 H 102 N 18 )(NO 3 ) 9 ] 2+ , 912.3 [Ni 4 (C 72 H 102 N 18 )(NO3)6]2+, 821.3 [Ni3(C72H102N18)(NO3)4]2+. 1H NMR (500 MHz, D2O): δ 181.84; 96.76; 55.08; 49.75; 46.90; 35.86; 16.48; 1.87; 0.27; −1.70; −2.36; −3.55; −7.35. Complex 3, [Zn3(L)](NO3)5Cl·3.5H2O. A 74.8 mg amount (0.250 mmol) of solid Zn(NO3)2·6H2O and 6.8 mg (0.050 mmol) of solid anhydrous ZnCl2 were added to a solution of 122.0 mg (0.100 mmol) of macrocycle L in 15 mL of methanol. The mixture was refluxed for 1 h, and the solution was evaporated to dryness. The residue was recrystallized from a methanol/acetonitrile mixture and dried under vacuum. Yield 100.7 mg of white solid (57.1%). Anal. Calcd (found) for C72H109ClN23O18.5Zn3: C, 47.40 (47.54); H, 6.02 (6.31); N, 17.66

Figure 9. 1H NMR spectra (500 MHz, D2O/CD3OD (1/1 v/v)) corresponding to the titration of the macrocycle L with increasing amounts of ZnCl2.

the complexed macrocyclic system is not occupied by metals. The ligand is flexible enough to adopt considerably different conformations in these types of complexes and to accommodate different metal ion geometries. Moreover, the macrocycle is able to adopt a completely different, multiply twisted conformation and to form a cationic trinuclear Zn(II) complex where the 18 nitrogen atoms of the macrocycle constitute the coordination environment of 3 octahedral metal ions. Likely the formation of the hexanuclear complexes in organic solutions is triggered at least in part by their low solubility. As indicated by the NMR spectra, these complexes decompose in water solutions into the respective trinuclear complexes. Further 12724


Article

Inorganic Chemistry

refined with iso- or anisotropic (for Cl−) thermal parameters, with details given in the CIF files. Some of the nitrate ions and acetonitrile and water molecules in 2 were refined isotropically with fixed SOFs (0.5 for NO3−, CH3CN, and H2O and or 0.25 for H2O). The H atoms in 3 were found in difference Fourier maps or were included using geometrical considerations and initially were refined freely. Those in 1 and 2 were included from geometry. In the final refinement cycles, all C- and N-bound H atoms in all crystals were repositioned in their calculated positions and refined using a riding model, with C−H = 0.94−1.00 Å, N−H = 0.99−1.00 Å, and Uiso(H) = 1.2Ueq(C,N) for CH, CH2, and NH or Uiso(H) = 1.5Ueq(C) for CH3. Water H atoms in 3 were refined with O−H bond lengths and H···H distances restrained to 0.840(2) and 1.380(2) Å, respectively, and with Uiso(H) = 1.5Ueq(O) and were then constrained to ride on their parent atoms (AFIX 3 instruction in Shelxl201417). All positions of water molecules in 2 are partially occupied; their O atoms were refined with isotropic thermal parameters, and their H atoms were not found in the Fourier maps. The finally accepted formula for 2 is {[Ni6(L)(NO3)11(CH3CN)(H2O)]NO3}·6CH3CN·6.5H2O, but the amount of solvent molecules should be treated as a rough approximation. Some geometrical restraints (DFIX, SAME instruction in Shelxl201417), restraints on anisotropic displacement parameters (ISOR), and constraints on the fractional coordinates and anisotropic displacement parameters (EXYZ and EADP instructions) were applied in the refinement procedures if appropriate. Details of structure refinements are given in Table S1 in the Supporting Information, and the crystallographic information files (CIFs) have been deposited at the Cambridge Crystallographic Data Centre (CCDC Nos. 1541936−1541938) and are provided as detailed in Accession Codes.

(17.31). MS (ESI-TOF) m/z: 817.8 [Zn3(C72H102N18)(NO3)3(Cl)]2+, 525.2 [Zn3(C72H102N18)(NO3)2(Cl)]3+. 1H NMR (500 MHz, D2O): δ 8.20 (t, J = 7.8 Hz, 1H, α-pyr); 7.66 (d, J = 7.9 Hz, 1H, β-pyr); 7.30 (d, J = 8.0 Hz, 1H, β-pyr); 4.44, 4.27 (ABq, JAB = 18.1 Hz, 2H, CγpyrCH2NH); 3.91, 3.40 (ABq, JAB = 16.2 Hz, 2H, Cγ-pyrCH2NH); 2.86 (m, 1H, NHCHCH2 (CP)); 2.53 (m, 1H, NHCHCH2 (CP)); 2.23 (m, 1H, CHCH2CH2 (CP)); 1.72 (m, 1H, CH2CH2CH2 (CP)); 1.49 (m, 1H, CH2CH2CH2 (CP)); 1.48 (m, 2H, CHCH2CH2 (CP)); 1.37 (m, 1H, CHCH2CH2 (CP)). 13C NMR (125 MHz, CDCl3): δ 155.2 (γ-pyr); 154.2 (γ-pyr); 141.8 (α-pyr); 123.6 (β-pyr); 122.7 (βpyr); 64.1 (NHCHCH2 (CP)); 58.7 (NHCHCH2 (CP)); 48.4 (CγpyrCH2NH); 42.6 (Cγ-pyrCH2NH); 26.0 CHCH2CH2 (CP)); 24.6 CHCH2CH2 (CP)); 20.1 (CH2CH2CH2 (CP)); 18.8 (CH2CH2CH2 (CP)). Complex 4, [Zn3(L)](Cl)6·13.5H2O. A solution of 250 mg (0.205 mmol) of the macrocycle L in 7.5 mL of methanol was combined with a solution of 83.8 mg (0.615 mmol) of ZnCl2 in 7.5 mL of methanol. The mixture was refluxed for 2 h and then evaporated to dryness. The residue was recrystallized from methanol/acetonitrile mixture and dried under vacuum. Yield 158.9 mg (41.4%). Anal. Calcd (found) for C72H129Cl6N18O13.5Zn6: C, 46.10 (46.20); H, 6.78 (6.95); N, 13.20 (13.47). MS (ESI-TOF) m/z: 1593.4 [Zn3(C72H102N18)(Cl)5]+, 1455.6 [Zn2(C72H102N18)(Cl)3]+, 1317.7 [Zn(C72H102N18)(Cl)]+, 778.2 [Zn3(C72H102N18)(Cl)4]2+, 709.3 [Zn2(C72H102N18)(Cl)2]2+, 506.5 [Zn3(C72H102N18)(Cl)3]3+. 1H NMR (500 MHz, D2O): δ 8.22 (t, J = 7.8 Hz, 1H, α-pyr); 7.68 (d, J = 7.9 Hz, 1H, β-pyr); 7.30 (d, J = 7.9 Hz, 1H, β-pyr); 4.45, 4.27 (ABq, JAB = 18.0 Hz, 2H, CpyrCH2NH); 3.91, 3.38 (ABq, JAB = 16.2 Hz, 2H, Cγ-pyrCH2NH); 2.83 (m, 1H, NHCHCH2 (CP)); 2.53 (m, 1H, NHCHCH2 (CP)); 2.22 (m, 1H, CHCH2CH2 (CP)); 1.73 (m, 1H, CH2CH2CH2 (CP)); 1.54 (m, 1H, CH2CH2CH2 (CP)); 1.49 (m, 2H, CHCH2CH2 (CP)); 1.38 (m, 1H, CHCH2CH2 (CP)). Conversion of Complex 4 into Complex 1. A solution of 93.6 mg (0.050 mmol) of complex 4 in 3 mL of methanol was combined with a solution of 20.5 mg (0.150 mmol) of ZnCl2 in 3 mL of methanol. The mixture was refluxed for 2 h, and the obtained white precipitate was filtered, washed with 1 mL of methanol, and dried under vacuum. Yield 76.6 mg (70.8%). Anal. Calcd (found) for C72H116Cl12N18O7Zn6: C, 39.97 (40.00); H, 5.40 (5.49); N, 11.65 (11.46). X-ray Crystallography. Crystalline forms of complexes 1 and 2 ([Zn6(L)Cl12]·3.2CH3CN and {[Ni6(L)(NO3)11(CH3CN)(H2O)]NO3}·6CH3CN·6.5H2O, respectively) were grown by dissolution of [Zn6(L)Cl12]·7H2O and [Ni6(L)(NO3)12]·20H2O complexes in a 1/1 acetonitrile/water mixture and slow evaporation (almost to dryness) of the respective mixtures. The crystals of complex 3, {[Zn3(L)](NO3)5Cl}·9H2O, were grown by slow diffusion of acetonitrile into a water solution of the [Zn3(L)](NO3)5Cl·3.5H2O complex. Crystals of 2 were extremely delicate and had to be measured in a CryoLoop. The crystallographic measurements were performed at 100(2)− 220(2) K on a κ-geometry Oxford Diffraction Xcalibur PX (ω scans) four-circle diffractometer with graphite-monochromated Cu Kα radiation (see details in Table S1 in the Supporting Information). Data were corrected for Lorentz and polarization effects. Data collection, cell refinement, data reduction, and analysis were carried out with CrysAlis PRO or CrysAlis CCD and CrysAlis RED, respectively. 15 Analytical or empirical (multiscan) absorption correction was applied to the data with the use of CrysAlis PRO or CrysAlis RED. Structures were solved by direct methods using the Shelxs9716 program and refined on F2 by a full-matrix least-squares technique using Shelxl201417 with anisotropic thermal parameters for the ordered and fully occupied non-H atoms. The complex molecule in 1 and complex cation in 2 lie in special positions: on a 2-fold axis (2) or an inversion center (3). The macrocyclic ligand in 1 was found to be partially disordered and was refined isotropically with one of the pyridynyl rings in two sites, with SOF = 0.56(3) and 0.44(3). One of the chloride anions in 1, some of the nitrate anions in 2, and acetonitrile molecules in 1 and 2 are disordered in two positions each (or partially occupied) and were

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01173. Crystallographic and magnetic data, NMR and mass spectra, views of molecular structures, and estimation of the relative binding constants (PDF) Accession Codes

CCDC 1541936−1541938 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.G.: [email protected]. *E-mail for J.L.: [email protected]. ORCID

Jerzy Lisowski: 0000-0002-4793-1748 Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This research was supported by NCN grant (Narodowe Centrum Nauki, Poland) 2011/01/D/ST5/02816. REFERENCES

(1) Liu, Z. C.; Nalluri, S. K. M.; Stoddart, J. F. Surveying macrocyclic chemistry: from flexible crown ethers to rigid cyclophanes. Chem. Soc. Rev. 2017, 46, 2459−2478.

12725


Article

Inorganic Chemistry (2) Selected reviews on macrocyclic complexes: (a) Lindoy, L. F.; Park, K. M.; Lee, S. S. Metals, macrocycles and molecular assemblies macrocyclic complexes in metallo-supramolecular chemistry. Chem. Soc. Rev. 2013, 42, 1713−1727. (b) Mewis, R. E.; Archibald, S. J. Biomedical applications of macrocyclic ligand complexes. Coord. Chem. Rev. 2010, 254, 1686−1712. (c) Rezaeivala, M.; Keypour, H. Schiff base and non-Schiff base macrocyclic ligands and complexes incorporating the pyridine moiety - The first 50 years. Coord. Chem. Rev. 2014, 280, 203−253. (d) Vigato, P. A.; Tamburini, S.; Beltolo, L. The development of compartmental macrocyclic Schiff bases and related polyamine derivatives. Coord. Chem. Rev. 2007, 251, 1311− 1492. (e) Lee, E.; Lee, S. Y.; Lindoy, L. F.; Lee, S. S. Metallacycles derived from metal complexes of exo-coordinated macrocyclic ligands. Coord. Chem. Rev. 2013, 257, 3125−3138. (f) Korybut-Daszkiewicz, B.; Bilewicz, R.; Wozniak, K. Tetraimine macrocyclic transition metal complexes as building blocks for molecular devices. Coord. Chem. Rev. 2010, 254, 1637−1660. (g) Joshi, T.; Graham, B.; Spiccia, L. Macrocyclic Metal Complexes for Metalloenzyme Mimicry and Sensor Development. Acc. Chem. Res. 2015, 48, 2366−2379. (h) Gavey, E. L.; Pilkington, M. Coordination complexes of 15-membered pentadentate aza, oxoaza and thiaaza Schiff base macrocycles ″Old Complexes Offer New Attractions″. Coord. Chem. Rev. 2015, 296, 125−152. (3) Selected recent examples of multinuclear complexes of large Schiff base macrocycles: (a) Arnold, P. L.; Stevens, C. J.; Farnaby, J. H.; Gardiner, M. G.; Nichol, G. S.; Love, J. B. New Chemistry from an Old Reagent: Mono- and Dinuclear Macrocyclic Uranium(III) Complexes from (U(BH4)(3)(THF)(2). J. Am. Chem. Soc. 2014, 136, 10218−10221. (b) Cadenbach, T.; Pankhurst, J. R.; Hofmann, T. A.; Curcio, M.; Arnold, P. L.; Love, J. B. Macrocyclic Platforms for the Construction of Tetranuclear Oxo and Hydroxo Zinc Clusters. Organometallics 2015, 34, 2608−2613. (c) Connolly, E. A.; Leeland, J. W.; Love, J. B. Mono- and Dinuclear Macrocyclic Calcium Complexes as Platforms for Mixed-Metal Complexes and Clusters. Inorg. Chem. 2016, 55, 840−847. (d) Leeland, J. W.; White, F. J.; Love, J. B. Encapsulation of a Magnesium Hydroxide Cubane by a BowlShaped Polypyrrolic Schiff Base Macrocycle. J. Am. Chem. Soc. 2011, 133, 7320−7323. (e) Feltham, H. L. C.; Clerac, R.; Ungur, L.; Vieru, V.; Chibotaru, L. F.; Powell, A. K.; Brooker, S. Synthesis and Magnetic Properties of a New Family of Macrocyclic M(3)(II)Ln(III) Complexes: Insights into the Effect of Subtle Chemical Modification on Single-Molecule Magnet Behavior. Inorg. Chem. 2012, 51, 10603− 10612. (f) Kwit, M.; Zabicka, B.; Gawroński, J. Synthesis of chiral large-ring triangular salen ligands and structural characterization of their complexes. Dalton Trans. 2009, 6783−6789. (g) Tandon, S. S.; Bunge, S. D.; Toth, S. A.; Sanchiz, J.; Thompson, L. K.; Shelley, J. T. Antiferromagnetically Coupled Dimeric Dodecacopper Supramolecular Architectures of Macrocyclic Ligands with a Symmetrical mu(6)BO33- Central Moiety. Inorg. Chem. 2015, 54, 6873−6884. (h) Nabeshima, T.; Yamamura, M. Cooperative formation and functions of multimetal supramolecular systems. Pure Appl. Chem. 2013, 85, 763−776. (i) Salassa, G.; Castilla, A. M.; Kleij, A. W. Cooperative self-assembly of a macrocyclic Schiff base complex. Dalton Trans. 2011, 40, 5236−5243. (j) Givaja, G.; Blake, A. J.; Wilson, C.; Schröder, M.; Love, J. B. Metal-directed ring-expansion in Schiff-base polypyrrolic macrocycles. Chem. Commun. 2005, 4423−4425. (4) Selected recent examples of multinuclear complexes of large polypyridine macrocycles: (a) Gao, C.-Y.; He, X.; Zhao, L.; Wang, M.X. Dual templated synthesis of silver acetylide cluster-encapsulated supramolecular boxes. Chem. Commun. 2012, 48, 8368−8370. (b) Gao, C.-Y.; Zhao, L.; Wang, M.-X. Stabilization of a Reactive Polynuclear Silver Carbide Cluster through the Encapsulation within a Supramolecular Cage. J. Am. Chem. Soc. 2012, 134, 824−827. (c) He, X.; Wang, Y. C.; Gao, C. Y.; Jiang, H.; Zhao, L. A macrocycle-assisted nanoparticlization process for bulk Ag2S. Chem. Sci. 2015, 6, 654−658. (d) Gao, C.-Y.; Zhao, L.; Wang, M.-X. Designed Synthesis of Metal Cluster-Centered Pseudo-Rotaxane Supramolecular Architectures. J. Am. Chem. Soc. 2011, 133 (22), 8448−8451. (e) Gao, C.-Y.; Zhao, L.; Wang, M.-X. Designed Synthesis of Metal Cluster-Centered PseudoRotaxane Supramolecular Architectures. J. Am. Chem. Soc. 2011, 133,

8448−8451. (f) Tian, L.-l.; Wang, C.; Dawn, S.; Smith, M. D.; Krause, J. A.; Shimizu, L. S. Macrocycles with Switchable exo/endo Metal Binding Sites. J. Am. Chem. Soc. 2009, 131 (48), 17620−17629. (g) Stadler, A.-M.; Jiang, J.-J.; Wang, H.-P.; Bailly, C. Large shapepersistent metal-invertible 15-Nsp(2)-donor-atom macrocycles functioning as trinucleating ligands. Chem. Commun. 2013, 49, 3784−3786. (h) Lee, S. G.; Park, K. M.; Habata, Y.; Lee, S. S. Endo- and Exocyclic Supramolecular Complexes of Mixed-Donor Macrocycles via 1:1 and 2:2 Cyclizations. Inorg. Chem. 2013, 52, 8416−8426. (5) (a) Loffler, M.; Gregolinski, J.; Korabik, M.; Lis, T.; Lisowski, J. Multinuclear Ni(II), Cu(II) and Zn(II) complexes of chiral macrocyclic nonaazamine. Dalton Trans. 2016, 45, 15586−15594. (b) González-Alvarez, A.; Alfonso, I.; Cano, J.; Díaz, P.; Gotor, V.; Gotor-Fernández, V.; García-España, E.; Garcia-Granda, S.; Jiménez, H. R.; Lloret, G. A Ferromagnetic Cu-3(OH)(2) (4+) Cluster Formed inside a Tritopic Nonaazapyridinophane: Crystal Structure and Solution Studies. Angew. Chem., Int. Ed. 2009, 48, 6055−6058. (6) (a) Janczak, J.; Prochowicz, D.; Lewiński, J.; Fairen-Jimenez, D.; Bereta, T.; Lisowski, J. Trinuclear Cage-Like Zn-II Macrocyclic Complexes: Enantiomeric Recognition and Gas Adsorption Properties. ́ Chem. - Eur. J. 2016, 22, 598−609. (b) Kobyłka, M. J.; Slepokura, K.; Acebrón Rodicio, M.; Paluch, M.; Lisowski, J. Incorporation of Trinuclear Lanthanide(III) Hydroxo Bridged Clusters in Macrocyclic Frameworks. Inorg. Chem. 2013, 52, 12893−12903. (c) Kobyłka, M. J.; Janczak, J.; Lis, T.; Kowalik-Jankowska, T.; Kłak, J.; Pietruszka, M.; Lisowski, J. Trinuclear Cu(II) complexes of a chiral N6O3 amine. Dalton Trans.s 2012, 41, 1503−1511. (d) Gao, J.; Zingaro, R. A.; Reibenspies, J. H.; Martell, A. E. Direct observation of enantioselective synergism at trimetallic centers. Org. Lett. 2004, 6, 2453−2455. (e) Korupoju, S. R.; Mangayarkarasi, N.; Zacharias, P. S.; Mizuthani, J.; Nishihara, H. Synthesis, structure, and DNA cleavage activity of new trinuclear Zn-3 and Zn2Cu complexes of a chiral macrocycle: Structural correlation with the active center of P1 nuclease. Inorg. Chem. 2002, 41, 4099−4101. (7) Nakamura, T.; Kaneko, Y.; Nishibori, E.; Nabeshima, T. Molecular recognition by multiple metal coordination inside wavystacked macrocycles. Nat. Commun. 2017, 8, 129. ́ (8) (a) Gregoliński, J.; Slepokura, K.; Paćkowski, T.; Lisowski, J. Expansion of a 2 + 2 Macrocycle into a 6 + 6 Macrocycle: Template Effect of Cadmium(II). Org. Lett. 2014, 16, 4372−4375. (b) Gregó liński, J.; Slepokura, K.; Paćkowski, T.; Panek, J.; Stefanowicz, P.; Lisowski, J. From 2 + 2 to 8 + 8 Condensation Products of Diamine and Dialdehyde: Giant Container-Shaped Macrocycles for Multiple Anion Binding. J. Org. Chem. 2016, 81, 5285−5294. (9) For metalla macrocyclic complexes of high nuclearity see for instance: (a) Cook, T. R.; Stang, P. J. Recent Developments in the Preparation and Chemistry of Metallacycles and Metallacages via Coordination. Chem. Rev. 2015, 115, 7001−7045. (c) Shankar, B.; Elumalai, P.; Sathiyashivan, S. D.; Sathiyendiran, M. Spheroid Metallocavitands with Eight Calixarene-Shaped Receptors on the Surface. Inorg. Chem. 2014, 53, 10018−10020. (d) Shankar, B.; Marimuthu, R.; Sathiyashivan, S. D.; Sathiyendiran, M. Spheroid Metallacycles and Metallocavitands with Calixarene- and/or CleftShaped Receptors on the Surface. Inorg. Chem. 2016, 55, 4537−4544. (e) Jin, Y. H.; Wang, Q.; Taynton, P.; Zhang, W. Dynamic Covalent Chemistry Approaches Toward Macrocycles, Molecular Cages, and Polymers. Acc. Chem. Res. 2014, 47, 1575−1586. (10) Addison, A. W.; Rao, T. N.; Reedijk, J.; Vanrijn, J.; Verschoor, G. C. Synthesis, Structure, And Spectroscopic Properties Of Copper(II) Compounds Containing Nitrogen Sulfur Donor Ligands - The Crystal And Molecular-Structure Of Aqua 1,7-Bis(NMethylbenzimidazol-2′-Yl)-2,6-Dithiaheptane Copper(Ii) Perchlorate. J. Chem. Soc., Dalton Trans. 1984, 1349−1356. (11) Perlmutter-Hayman, B. Cooperative Binding to Macromolecules. A Formal Approach. Acc. Chem. Res. 1986, 19, 90−96. (12) Floquet, S.; Ouali, N.; Bocquet, B.; Bernardinelli, G.; Imbret, D.; Bünzli, J.-C. G.; Hopfgartner, G.; Piguet, C. The First Self-Assembled Trimetallic Lanthanide Helicates Driven by Positive Cooperativity. Chem. - Eur. J. 2003, 9, 1860−1875. 12726


Article

Inorganic Chemistry (13) Akine, S.; Taniguchi, T.; Nabeshima, T. Helical Metallohost− Guest Complexes via Site-Selective Transmetalation of Homotrinuclear Complexes. J. Am. Chem. Soc. 2006, 128, 15765−15774. (14) Covington, A. K.; Paabo, M.; Robinson, R. A.; Bates, R. G. Use of the glass electrode in deuterium oxide and the relation between the standardized pD scale and the operational pH in heavy water. Anal. Chem. 1968, 40, 700−706. (15) (a) CrysAlis PRO in Xcalibur PX Software; Agilent Technologies, Yarnton, Oxfordshire, England, 2012. (b) CrysAlis CCD and CrysAlis RED in Xcalibur PX Software; Oxford Diffraction Ltd., Yarnton, Oxfordshire, England, 2010. (16) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (17) Sheldrick, G. M. Crystal structure refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8.

12727


Hexanuclear and Trinuclear Metal Complexes of a Giant Octadecaaza

Recommend Documents