Matthew L.
Bracken
*a,
Manuel A.
Fernandes
a,
Daniel
Wamwangi
bc and
Orde Q.
Munro
ad
aMolecular Sciences Institute, School of Chemistry, University of the Witwatersrand, PO Wits 2050, Johannesburg, South Africa. E-mail: matthewbracken125@gmail.com
bSchool of Physics, University of the Witwatersrand, Wits 2050, Johannesburg, South Africa
cDSI-NRF Centre of Excellence in Strong Materials, University of the Witwatersrand, Wits 2050, Johannesburg, South Africa
dSchool of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
First published on 8th May 2024
NNN bis-aryl amide pincer ligands may be designed to meet the structural requirements for Cu(II) metallocycle hexamer formation, giving supramolecular crystals containing solvent-accesible voids.
The amide CO that forms part of the chelate ring, but is not a bridging group, has an average bond length of 1.24 ± 0.01 Å. However, when the amide C
O acts as bridging group and forms a Cu–Ocarbonyl bond, the bond length increases to an average of 1.27 ± 0.01 Å for Cu1–Cu3. This increase in C
O bond length is due to back-donation of electron density from the 3dx2−y2 into a vacant σ* molecular orbital on the carbonyl, thereby decreasing Coulombic repulsion within the metal-based orbital. There is additional overlap between the atomic 3dxy orbital and π* molecular orbitals of the carbonyl, resulting in further bond length elongation for the bridging moiety. The orbital analysis has been presented in Fig. 2. The inner coordination sphere of Cu1 is shown in Fig. 2a, and the only symmetry element is a vertical mirror plane, σV. The mirror plane defines the Cartesian z-axis as the principal z-axis is collinear with the highest symmetry element. As a result, the x-axis is perpendicular to the z-axis and collinear with highest number of identical atoms, i.e., the two Namide. That leaves the y-axis to pass through Npyr and Ocarbonyl. The bridging carbonyl interaction is then shown in Fig. 2b. The bonding involves predominately σ-overlap of the metal-based 3dx2−y2 atomic orbital and σ* molecular orbital of the carbonyl. There is additional bonding involving π-overlap of the metal-based 3dxy atomic orbital and the π* molecular orbitals of the carbonyl. This donation of electron density from the metal-based orbitals into antibonding orbitals of the carbonyl causes the bond length to increase for the bridging moiety. This was confirmed with DFT shown in Fig. S2.1–S2.4 (ESI†), as the spin-density plot shows significant 3dx2−y2 character as well as σ-overlap between copper and the carbonyl. The antibonding π-overlap between carbonyl π* molecular orbitals and the atomic 3dxy orbital was observed for the LUMOs of the asymmetric unit for Cu1–Cu3.
Our DFT calculations show significant 3dx2−y2 character in the square plane of the Cu(II) centre as well as spin density localized on the bridging carbonyl which is crucial to metallocycle formation. The delocalization of the paramagnetic electron density leads to the emergence of exchange interactions over the three bridging atoms between the metal centres. Cu1 shows low temperature antiferromagnetic coupling at T < 9.3 K at a field strength of 0.1 T. Cu2 shows competing antiferromagnetic and paramagnetic phases below the Néel temperature (T < 8 K). Cu3 is diamagnetically ordered under an applied magnetic field at room temperature. At 100 K, Cu3 undergoes a magnetic moment reversal from diamagnetic to paramagnetic with complete paramagnetism at 2 K. These magnetic measurements are presented in Fig. S3.1–S3.6 (ESI†). Exchange interactions and magnetic ordering have been reported to influence coordination-based solid-state architecture.10–12 Particularly, hexanuclear copper-based exchange coupling has been compared to the bonding in benzene.13 The DFT spin-density plot is consistent with magnetic ordering and shows that paramagnetic electron density is delocalized into the aromatic rings of the ligand and reflects the energy lowering of the metal-based orbitals as a result of the nephelauxetic effect. The coordination pattern Cu(II)–COamide–Cu(II) that drives metallocycle formation is dominated by 3dx2−y2 character as shown in S3 (ESI†). The Cu(II) based atomic 3dx2−y2 orbital is instrumental to metallocycle formation and the resulting coordinating forces impart significant tensional integrity to the cyclic assembly.
The metallocycle hexamer is held together by a synergistic combination of intramolecular forces that facilitate the tensegrity observed for the supramolecular assembly. The term “tensegrity” was originally coined by the architect, Buckminster Fuller. These forces include coordinative bonding through the cyclic structure, magnetic ordering, van der Waals forces, C–H⋯π14 and d⋯π15 interactions. The tensegrity of the copper metallocycles has been highlighted in Fig. 3 as well as the supramolecular packing. The strongest intramolecular forces holding the cyclic structure together are the coordinating bonds that result in the pattern Cu(II)–N–COamide–Cu(II). The short bond lengths Cu–Ocarbonyl indicate that the negative charge from the amidate nitrogen has been delocalized into the carbonyl oxygen making it a better σ-donor. Significant C–H⋯π and d⋯π interactions are observed to stabilize the cyclic assembly and impart remarkable tensegrity. This self-recognition and intramolecular stabilisation have been shown in Fig. S4.1–S4.4 (ESI†). The bonding exists between the Cu(II) triad that lies above the mean plane of the metallocycle, and these C–H⋯π and d⋯π interactions are inverted about the centroid of the hexamer and exist between the Cu(II) triad that lies below the mean plane. The aromatic rings that serve as d⋯π acceptors on one face of the aromatic ring, function as C–H⋯π acceptors on the reflected face. Notably, the atom-to-plane distances for the d⋯π interactions average 2.88 ± 0.09 Å which indicates that although the bonding may be weak, it is significant. Also noteworthy is the fact that d⋯π bond angles vary between about 50° and 60°, suggesting that atomic 3dxz and 3dyz orbitals are principally involved with d–π overlap. Hence, numerous forces stabilize the supramolecular assembly through intramolecular interactions. There are significantly fewer intermolecular interactions that facilitate packing of the cyclic hexamers in the lattice that result in void space (Fig. S5.1–S5.3, ESI†).
The Cu(II) NNN amide pincers presented here all adopt hexameric cyclic coordination in the solid-state. A CSD16 search of the general amide pincer scaffold yielded 78 unique results, and these have been presented in Table S1 (ESI†). The table shows the search query submitted to the CSD, the coordination number at the metal centre, the charge on the copper ion, and the substitution on the aryl rings. The search filtered out structures with R1 > 7.5%. The 78 copper NNN amide pincers contain only 5 structures (6.4%) where the aryl ring has been substituted on the 3-position. All 5 of these structures contain dinucleating ligands. Most structures (90%) contain ligands where the aryl ring has been substituted on the 2- and 6-positions or 2-,4-and 6-positions. None of the structures adopt the metallocycle architecture observed for the Cu(II) NNN amide pincers presented in this work. It is suspected that metallocycle formation requires the aryl ring to be substituted in the 3- or 4-position so that the substituents do not sterically block metallocycle formation. The stereoelectronic requirements for hexamer formation are presented in Fig. 4. The metallocycle follows the bonding pattern Cu(II)–N–COamide–Cu(II) and so all 78 CSD structures possess the necessary coordination pattern yet none of them form cyclic supermolecules. It is suggested here that this is because 90% of structures are substituted in the 2-,6-position and the substituents sterically block coordination from one copper pincer to another. When only the 2-position is substituted, the CSD structures do not adopt a hexameric geometry because there is intramolecular self-recognition between the substituent and the Cu(II) centre, or alternatively, the complex is anionic and electrostatic forces dominate the solid-state architecture. The structures that possess no substitution on the aryl rings do not form cyclic supermolecules because they are all anionic. The CSD structures with aryl rings substituted in the 3-position are dinucleating structures and hence the flexibility of these ligands is reduced such that metallocycle formation does not occur.
The coordination mode of the Cu(II) NNN amide pincer has been presented in Fig. 4 with the general requirements that permit metallocycle hexamer formation. Firstly, the ligand must be suitably functionalized to avoid steric repulsions. The copper complex must also be neutral and have an open face that is available for coordination by a neighbouring monomer. The variable geometry of the copper ion permits supramolecular coordination as bridging groups may adopt favourable conformations around the metal centre. The metallocycles discussed here contain the Cu(II) ion with square pyramidal,6,17 trigonal bipyramidal3 and octahedral4 geometries depending on the ligand's steric requirements and the metal centre's electronic requirements.
The pincers presented here coordinate Cu(II) via anionic amidate donors which increase electron density at the metal centre through σ-donation. The interelectronic repulsion is relieved at the metal centre upon coordination of a σ*- or π*-acceptor that functions as a bridging group. Therefore, electronic requirements of the metal facilitate metallocycle formation as electron density is balanced through a synergistic interplay of σ- and π-effects. Hence, a general coordination pattern for the copper hexamers may be presented as Cu(II)–Xnπ*–Cu(II), where X represents n number of any bridging atoms and the bridging group must be a π-acceptor. This depends on the electronics at the Cu(II) centre, as seen in the structure reported by Fu et al.3 When the ligand coordinates Cu(II) through only one anionic donor, and the ligand is a good π-acceptor, the electron deficient metal centre may coordinate to a bridging σ-donor moiety. Hence, the general coordination pattern may be presented as Cu(II)–Xnσ–Cu(II), where X represents n number of any bridging atoms that contains a good σ-donor. Ultimately, electron density at the metal centre must be appropriately balanced through σ- and π-effects for the general Cu(II)–Xn–Cu(II) cyclic coordination pattern to form. The size of the metallocycle follows the simple arithmetic sequence 6 + 6n, where n is the number of bridging atoms. Hence, an 18-membered metallocycle is formed when two bridging atoms are present, and 24-membered when three bridging atoms are present, and so on.
1H NMR: (400 MHz, DMSO-d6, 300 K) [δ, ppm] 11.27 (s, 2H), 8.48–8.42 (m, 4H), 8.36 (dd, J = 8.6, 6.9 Hz, 1H), 8.24 (dq, J = 7.8, 2.7 Hz, 2H), 7.75–7.65 (m, 4H). 13C NMR: (100 MHz, DMSO-d6, 300 K) [δ, ppm] 162.53, 148.78, 140.80, 139.35, 130.80, 128.40, 126.32, 126.15, 124.12, 119.15, 112.10. FTIR (cm−1): 3288.66 (m, asymmetric N–H stretch), 3086.81 (w, aromatic C–H stretch), 2230.18 (m, symmetric CN stretch), 1667.75 (s, asymmetric C
O stretch; N–H wag), 1534.30 (s, asymmetric N–H wag).
1H NMR: (400 MHz, DMSO-d6, 300 K) [δ, ppm] 11.26 (s, 2H), 8.41 (d, J = 8.0 Hz, 2H), 8.31 (t, J = 7.9 Hz, 1H), 8.17 (d, J = 8.8 Hz, 4H), 7.91 (d, J = 8.7 Hz, 4H). 13C NMR: (100 MHz, DMSO-d6, 300 K) [δ, ppm] 160.40, 146.58, 140.65, 138.56, 131.59, 124.27, 118.98, 117.32, 116.39. FTIR (cm−1): 3340.51 (m, asymmetric N–H stretch), 3099.14 (w, aromatic C–H stretch), 2217.87 (m, symmetric CN stretch), 1693.08 (s, asymmetric C
O stretch; N–H wag), 1581.22 (s, asymmetric N–H wag).
1H NMR: (400 MHz, DMSO-d6, 300 K) [δ, ppm] 11.12 (s, 2H), 8.41 (d, J = 8.2 Hz, 2H), 8.31 (t, J = 8.0 Hz, 1H), 7.96 (d, J = 8.0 Hz, 4H), 7.46 (d, J = 7.5 Hz, 4H), 7.20 (t, J = 7.4 Hz, 2H). 13C NMR: (100 MHz, DMSO-d6, 300 K) [δ, ppm] 162.19, 149.40, 140.40, 138.56, 129.22, 125.80, 124.85, 121.64. FTIR (cm−1): 3268.21 (m, asymmetric N–H stretch), 2803.26 (w, aromatic C–H stretch), 1671.83 (s, asymmetric CO stretch; N–H wag), 1599.41 (s, asymmetric N–H wag).
Cu1: H2L1 (101 mg, 0.275 mmol) was dissolved in 50 mL of tetrahydrofuran (THF) before adding Cu(OAc)2·H2O (186 mg, 0.929 mmol) to the mixture. Instantly, a light-green precipitate began to form. The reaction was refluxed at 50 °C open to the air for 24 hours. The mixture was then allowed to cool to room temperature before filtering under vacuum. The isolated green powder was then washed with 10 mL of THF to yield Cu1·H2O, (59.8 mg, 49%). The copper chelate may be crystalized by evaporation from a mixture of methanol (MeOH) and ethyl acetate (EtOAc) over 3 weeks at ambient temperatures. Similar crystals can be grown by vapour diffusion of diethyl ether (Et2O) into a saturated MeOH solution containing Cu1 over 2–4 weeks at ambient temperature, 4 °C and −20 °C.
FTIR (cm−1): 3626.76 (w, O–H stretch), 3066.06 (w, aromatic C–H stretch), 2226.17 (s, nitrile stretch), 1626.57 (s, CO stretch). Crystal data: C128H78Cu6N30O16, 1.5[CH3OH] (M = 2721.50 g mol−1): triclinic, space group P
(no. 2), a = 12.8621(7) Å, b = 14.6778(8) Å, c = 17.1404(9) Å, α = 76.085(2)°, β = 70.053(2)°, γ = 81.249(2)°, V = 2943.8(3) Å3, Z = 1, T = 173.0 K, μ(MoKα) = 1.147 mm−1, Dcalc = 1.535 g cm−3, 343
382 reflections measured (4.23° ≤ 2Θ ≤ 51.992°), 11
552 unique (Rint = 0.1174, Rsigma = 0.0282) which were used in all calculations. The final R1 was 0.0361 (I > 2σ(I)) and wR2 was 0.0873 (all data). CCDC 2340637.†
Cu2: H2L2 (113 mg, 0.308 mmol) was dissolved in 50 mL of THF before adding Cu(OAc)2·H2O (154 mg, 0.771 mmol) to the mixture. Instantly, a green precipitate began to form. The reaction was refluxed at 50 °C open to the air for 24 hours. The mixture was then allowed to cool to room temperature before filtering under vacuum. The isolated green powder was then washed with 10 mL of THF to yield Cu2·H2O, (59.1 mg, 45%). The copper chelate was crystalized by vapour diffusion of Et2O into a saturated MeCN solution containing Cu2 over 3 weeks at ambient temperature.
FTIR (cm−1): 3396.60 (w, O–H stretch), 3090.05 (w, aromatic C–H stretch), 2223.05 (s, nitrile stretch), 1625.64 (s, CO stretch). Crystal data: C130H76Cu6N32O14, 4[C2H3N] (M = 2855.68 g mol−1): triclinic, space group P
(no. 2), a = 14.4764(11) Å, b = 15.1477(11) Å, c = 16.4380(10) Å, α = 106.667(2)°, β = 95.920(3)°, γ = 110.424(3)°, V = 3152.1(4) Å3, Z = 1, T = 173.0 K, μ(MoKα) = 1.075 mm−1, Dcalc = 1.504 g cm−3, 162
869 reflections measured (4.062° ≤ 2Θ ≤ 56.86°), 15
772 unique (Rint = 0.1107, Rsigma = 0.0649) which were used in all calculations. The final R1 was 0.0482 (I > 2σ(I)) and wR2 was 0.1261 (all data). CCDC 2340638.†
Cu3: H2L3 (112 mg, 0.353 mmol) was dissolved in 50 mL of THF before adding Cu(OAc)2·H2O (176 mg, 0.882 mmol) to the mixture. Instantly, a green precipitate began to form. The reaction was refluxed at 50 °C open to the air for 24 hours. The mixture was then allowed to cool to room temperature before filtering under vacuum. The isolated green powder was then washed with 10 mL of THF to yield Cu3·H2O, (40.0 mg, 30%). The copper chelate was crystalized by vapour diffusion of Et2O into a saturated MeOH solution containing Cu3 and a drop of ethylene glycol over 2 weeks at ambient temperature.
FTIR (cm−1): 2854.32 (w, aromatic C–H stretch), 1769.46 (w, aqua O–H wag), 1629.90 (s, CO stretch). Crystal data: C114H86Cu6N18O16, C2H6O2, 5[CH3OH] (M = 2567.52 g mol−1): triclinic, space group P
(no. 2), a = 10.8153(11) Å, b = 17.0091(16) Å, c = 17.4196(16) Å, α = 106.568(5)°, β = 100.629(5)°, γ = 107.750(5)°, V = 2793.0(5) Å3, Z = 1, T = 173.0 K, μ(MoKα) = 1.204 mm−1, Dcalc = 1.526 g cm−3, 94
743 reflections measured (2.554° ≤ 2Θ ≤ 45°), 7314 unique (Rint = 0.3031, Rsigma = 0.1282) which were used in all calculations. The final R1 was 0.0681 (I > 2σ(I)) and wR2 was 0.1446 (all data). CCDC 2340639.†
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2340637–2340639. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4nj01330a |
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2024 |