Open access peer-reviewed chapter - ONLINE FIRST
Abstract
In light of the diverse chemical properties inherent in adamantane and its derivatives, this section shows an overview on adamantane-derived Schiff bases. Adamantane compounds, known for their diverse therapeutic applications, play a crucial role in drug discovery due to their hydrophobic nature, enabling modifications across different drug classes. The rigid cage structure of adamantane shields adjacent functional groups from metabolic cleavage, thereby augmenting drug stability and distribution in blood plasma. These derivatives and their complexes have exhibited the capability to disrupt various enzymes, showcasing a wide array of therapeutic activities, including anti-inflammatory, antiviral, antibacterial, antimicrobial, anticancer, anti-Parkinson, and antidiabetic effects. Highlighting the distinctive chemical attributes of the adamantyl scaffold, the significance of synthesizing and characterizing novel adamantane-derived compounds is underscored. The overview and comparative structural analysis of Schiff bases with adamantane moiety and their complex compounds contribute valuable insights to the realms of chemical synthesis, materials science, and medicinal chemistry.
Keywords
- Schiff bases
- adamantane
- metal complexes
- crystal structure
- CSD search
1. Introduction
Adamantane, the smallest representative of diamondoids, is a highly symmetrical polycyclic cage molecule with unique properties. Аmantadine (Figure 1), 1-aminoadamantane, the first adamantane compound used in medicinal chemistry, exhibited potent anti-influenza A activity, initiating the era of adamantane-based drug discovery [1]. This compound has later been recognized for its antibacterial and antifungal properties as well [2]. Adamantane and its derivatives, recognized for their diverse pharmacological effects [3], are frequently employed in drug discovery due to adamantane’s hydrophobic nature, modifying properties across drug classes [4, 5, 6]. These derivatives have been discovered to disrupt various enzymes, showcasing diverse therapeutic activities such as anti-inflammatory [7], anti-viral [8], and anti-Parkinson [9]. They serve as antimicrobial-anticancer DHFR inhibitors [10, 11, 12], and they are also known as antidiabetic agents [13]. The incorporation of the adamantane moiety enhances CNS penetration, making it valuable for targeting CNS drugs [3]. Notably, some derivatives have demonstrated potent anticonvulsant activity, including NMDA-activated channel blockade [14, 15, 16, 17, 18]. Additionally, bananins, a class of antiviral compounds featuring a trioxa-adamantane moiety bound to a pyridoxal derivative, exhibit efficacy against both HCoV-OC43 and SARS-CoV-1 [19, 20], while spiroadamantane amine has proven effective against the coronavirus strain 229E [21].
Figure 1.
The structure of the amantadine.
It is interesting to mention that three adamantane-based COFs with varying pore sizes were synthesized using adamantane as a monomer and nitrogen-containing compounds as building blocks through a Schiff base reaction. The inclusion of rigid adamantane enhances the mechanical and thermal stability of the COFs, while the nitrogen-containing building blocks are anticipated to improve adsorption capacity [22].
Various metal complexes of amantadine-based ligands, including platinum(II) and platinum(IV) complexes known for their anticancer efficacy, have been synthesized and documented in the literature. Silver complexes with amantadine and memantine have been reported as potent antibacterial agents against both Gram-positive and Gram-negative strains [13].
The distinctive attributes of the adamantyl scaffold for biological applications stem from its inherent lipophilicity and its capacity to enhance drug stability, leading to improved pharmacokinetics of modified drug candidates. The rigid cage structure shields adjacent functional groups from metabolic cleavage, thereby augmenting drug stability and distribution in blood plasma [1].
Consequently, the preparation and testing of new derivatives of adamantane continue to be a highly pertinent focus in medicine, as well as in other fields. In this part, the structures of the adamantane-derived Schiff bases described so far, as well as their complex compounds, will be presented.
2. Adamantane-derived Schiff bases and their complexes
The Cambridge Structural Database search revealed a total of 224 structures of adamantane-derived Schiff bases, of which 131 complex compounds with
Figure 2.
The molecular structures of Ag(I) complex with 2,6-pyridyl-iminodiadamantane (refcode HAVJEW).
The most common donor sets in examples of bidentate coordination are ON (42 complexes) and N2 (28 complexes), followed by ON donor set from one part and NC donor set from the other part of the bis-condensed structure (three complexes), OS and NC donor sets (two complexes each), and NS, S2, and O2 donors in one complex each. Regarding tridentate coordination, donor sets ON2 (six complexes), O2N and N3 (four complexes each), ONS (three complexes), ONP and N2C (two complexes each), and ONC (one complex) are represented. Tetradentate donor sets in the examined structures include N4 (two complexes), O2N2 (one complex), and ON3 (one complex). It is noteworthy that dinuclear complexes (six structures), trinuclear and tetranuclear complexes (one structure each), and four structures of polynuclear complexes are structurally characterized [23].
It is interesting to note that among the investigated structures of Schiff bases with an adamantane ring, there are 8 structures with a pyrrole ring as well (refcodes EWOMEI, EWOMIM, LUHXIY, LUHZIA, PIJYAL, AVECUB, PEMDAQ , YARKUA), six with a piperidine/piperazine ring (refcodes HAWCUH, HAWDAO, IDOCIP, LANXEF, TANHAU, GOQJEE), three with a thiazole ring (refcodes PAWJEE, LANXEF, REJZIT), one with a furan ring (refcode FIBWEV), one with an imidazole ring (refcode BOQKOJ), one with a morpholine ring (refcode PIMHIF), and one with a nitrothiophene ring (refcode ZUQDAT) [23].
The structure containing the pyrrole ring, 2-(1-adamantylimino)methyl-1H-pyrrole, which is presented in Figure 3, serves as a robust indicator for the convenient and highly accurate quantitative analysis of trace amounts of hydrochloric acid in various chlorinated organic solvents through UV-VIS absorption spectroscopy [25].
Figure 3.
Molecular structure of the 2-(1-adamantylimino)methyl-1H-pyrrole (refcode YARKUA).
There are eight structures in which the adamantane-derived Schiff base ligand coordinates in the form of a zwitterion and one structure of a complex salt where the protonated form of Schiff base ([(E)-N-(adamantan-1-yl)(2H-1,3-benzodioxol-5-yl)methanimine]) appears as the cation, and the picrate ion serves as the anion (refcode WIRQEX, Figure 4) [23].
Figure 4.
The picrate salt containing a protonated Schiff base (E)-N-(adamantan-1-yl) (2H-1,3-benzodioxol-5-yl)methanimine with hydrogen bonds shown.
The ligand of interest, 2-[(E)-{3-[(E)-(adamantan-1-ylimino)methyl]-4-hydroxyphenyl}diazenyl]benzoic acid, in the work [26], has been utilized to synthesize two polymeric organotin complexes, with the ligand coordinating as a zwitterion. These adamantyl-substituted ligand derivatives demonstrate selective toxicity against HeLa cancer cells while being less toxic to healthy HEK 293 cells. These compounds induce apoptosis in HeLa cells, with the apoptotic effect increasing at higher concentrations, as confirmed by specialized biochemical assays. The presence of biocompatible adamantyl groups in compounds presented in Figure 5 enhances the lipophilicity and stability of the molecular tin complexes, making them promising for future triorganotin antitumor therapeutics [26].
Figure 5.
The molecular structures of two polymeric organotin complexes where the ligand is 2-[(E)-{3-[(E)-(adamantan1-ylimino)methyl]-4-hydroxyphenyl}diazenyl]benzoic acid (refcode OMETAE – above, refcode OMETOS – below).
There is one adamantane-derived metallomacrocycle described in the work [27]. The work emphasizes the efficacy of bis(bidentate)-bis(N-acylamidines), incorporating various spacer units, in self-assembling supramolecular architectures. The ligand of interest features a sterically fixed, curved 1,3-adamantanediyl spacer with a tetrahedral spatial orientation, facilitating bis(coordination) in a parallel orientation. This ligand leads to the formation of a Ni(II) metallomacrocycle due to its specific curved structure. Two nickel atoms coordinate with ligands, creating square-planar
Figure 6.
The molecular structures of Ni(II) metallomacrocycle with adamantane-derived Schiff base (refcode TEYHEM).
3. Complexes of the adamantane-1-carbohydrazide derivatives
There are 30 structures of hydrazones and thiohydrazones Schiff bases containing an adamantane ring.
In the study of Leovac et al. the structural characterization of Cu(II) complexes with hydrazonic derivatives of adamantane was undertaken for the first time. Specifically, this paper describes the synthesis and characterization of two ligands and their corresponding five-coordinated square–pyramidal complexes [28].
The first ligand (Adpy) was obtained through the condensation reaction of 2-acetylpyridine and adamantane-1-carbohydrazide. Three complexes were characterized with this ligand, namely [CuCl2(Adpy)], [Cu2(μ-Cl)2(Adpy-H)2], and [Cu(NCS)2(Adpy)]. The structure for the first mentioned complex was validated through elemental analysis and infrared spectra, due to the unsuitability of the obtained crystals for X-ray, while the molecular structures of the remaining two complexes are presented in Figure 7. Compound [Cu2(μ-Cl)2(Adpy-H)2] is a centrosymmetrical dinuclear neutral complex with Cu(II) ions in deformed square-pyramidal environments. It features an ON2-coordinated monoanionic organic ligand (Adpy-H) in the basal plane, and the remaining sites are occupied by coligands. In its crystal structure, two chlorido anions fill the fourth basal and fifth apical sites, forming a nearly rectangular Cu2(μ-Cl)2 core. In the coordination sphere of monomeric neutral complex [Cu(NCS)2(Adpy)], there are two thiocyanato ions with distinct coordination modes. One is situated in the basal plane, coordinated through a nitrogen atom, while the other occupies the apical position of the square pyramid, coordinated through a sulfur atom. This work states that this compound stands out as an instance of a rare category of mononuclear complexes characterized by the formula [CuL(NCS)(SCN)], where L denotes a tridentate ligand [28].
Figure 7.
The molecular structures of the [Cu2(μ-Cl)2(Adpy-H)2] (refcode WOTDAM) – above, and [Cu(NCS)2(Adpy)] (refcode WOTDEQ ) – below.
The second ligand (Addpy) was obtained through the condensation reaction of di(2-pyridyl) ketone and adamantane-1-carbohydrazide. With this ligand, an additional two centrosymmetrical dinuclear neutral complexes were characterized, namely [Cu2(μ-Cl)2(Addpy-H)2] and [Cu2(NCS)2(μ-Addpy-H)2] (Figure 8). For [Cu2(μ-Cl)2(Addpy-H)2], the fourth basal and fifth apical positions in square – pyramide polyhedron are occupied by two chlorido anions that are crystallographically related through an inversion operation. Consequently, this complex also displays a nearly rectangular Cu2(μ-Cl)2 core, resembling the structural pattern found in other copper(II) complexes utilizing ON2-tridentate hydrazones. For [Cu2(NCS)2(μ-Addpy-H)2], the fourth basal position is filled by an N-coordinated thiocyanato ion, while the apical site is occupied by the N4 pyridinic nitrogen atom of a tetradentate μ-Addpy ligand. In this work, it is stated that this behavior mirrors what has been reported for di(2-pyridyl) ketone Schiff bases [28].
Figure 8.
The molecular structures of the [Cu2(μ-Cl)2(Addpy-H)2] (refcode WOTDIU) – Left, and [Cu2(NCS)2(μ-Addpy-H)2] (refcode WOTDOA) – Right.
The anticancer potential, as well as the DNA binding and cleavage activity, of these five complexes has been assessed. These complexes displayed strong cytotoxicity, triggering apoptosis as the primary method of cell death. DNA-binding activities were investigated through UV/Vis absorption spectroscopy and fluorescence emission measurements, indicating an intercalative mode of interaction. Additionally, the complexes caused significant double-strand cleavage of supercoiled DNA [28].
With the mentioned ligand Addpy, Rodić and collaborators synthesized and characterized three additional complexes in their study, with the formulas [CuII2CuI2(Addpy)2Br2(μ-Br4)],
Figure 9.
The molecular structures of the [CuII2CuI2(Addpy)2Br2(μ-Br4)] (refcode EKANEL) – The first,
In the centrosymmetrical tetranuclear complex [CuII2CuI2(Addpy)2Br2(μ-Br4)], copper atoms have distinct coordination environments and oxidation states. Copper atom with oxidation state +2 (Cu1) is placed in a deformed square-pyramidal environment. In this polyhedron, ON2 donor atoms of the neutral ligand and bromide ion are in the basal plane, and a second bromide ion is in the apical position (this bromide is a linkage between Cu1 and Cu2). Copper atom with the oxidation state +1 (Cu2) is placed in a trigonal-planar geometry by three bridging bromide ions [29].
In the polymeric complex from Rodić and collaborators’ work, the Addpy coordinates to Cu1 in its neutral form as tetradentate ON3-ligand, using two nitrogen atoms and one oxygen atom to bind with Cu1 and one nitrogen atom from the second pyridine ring to bind with Cu2. The chloride anion and the ON3-coordinated Addpy play bridging roles between two copper atoms [29].
In the mononuclear complex [Cu(Addpy)(NCS)2], copper(II) atom is placed in a deformed square-pyramidal environment of ON2 donor atoms of the Addpy ligand and one thiocyanate ion in the basal plane and another thiocyanate ion in apical position. For this coordination polyhedron, a noteworthy deviation of the Cu(II) ion from the basal donor plane is observed – 0.3894(5) Å [29].
Outperforming cisplatin in terms of
The study of Đorđević et al. describes the synthesis and crystal structure of the 2-(diphenylphosphino)benzaldehyde 1-adamantoylhydrazone (HL) (Figure 10) and its two complexes [30].
Figure 10.
Molecular structure of the 2-(diphenylphosphino) benzaldehyde-1-adamantoylhydrazone (refcode SIVXUS).
This ligand reacts with K2[MCl4] (M = Pd(II), Pt(II)) in ethanol, at 50°C for 1 h, yielding square-planar neutral complexes [M(L)Cl] (Figure 11). These complexes feature tridentate ONP coordination from the monoprotonated Schiff base and a chloride ligand. Their antimicrobial and cytotoxic properties were assessed against human larynx carcinoma cells (Hep-2) and non-cancerous human lung fibroblast cells (MRC-5). The platinum(II) complex exhibits cytotoxicity toward Hep-2 cells comparable to oxaliplatin, with enhanced selectivity for cancer cells [30].
Figure 11.
The molecular structures of the [Pd(L)Cl] (refcode SIVXOM) – Left, and [Pt(L)Cl] (refcode SIVXIG) – Right (L = 2-(diphenylphosphino)benzaldehyde-1-adamantoylhydrazone).
In the reaction of 2,6-diacetylpyridine and adamantane-1-carbohydrazide in ethanol, in malor ratio 1:2, under the reflux, a Schiff base was obtained – (Ad)2dap (Figure 12) [31].
Figure 12.
The molecular structure of the (ad)2dap.
A dinuclear complex of Zn(II), [Zn2((Ad)2dap–H)2]∙EtOH (Figure 13), was synthesized by reacting a warm suspension of this ligand in ethanol with a warm ethanolic solution of Zn(CH3COO)2 in a 1:1 molar ratio. The Schiff base in the synthesized complex compound acts as a pentadentate ligand with O2N3 coordination. Consequently, six five-membered metallocycles were formed. Zinc occupied a slightly deformed square-pyramidal environment (τ5 = 0.24), with donor nitrogen atoms and one oxygen atom situated in the equatorial plane and another oxygen atom positioned in the apical position [31].
Figure 13.
The molecular structure of the [Zn2((ad)2dap–H)2]∙EtOH ((ad)2dap = 2,6-diacetylpyridine-adamantane-1-carbohydrazide).
In the reaction of a warm acetronitrile suspension of this ligand and a warm acetone solution of cobalt(II) chloride, in a molar ratio 1:2, green crystals of a complex compound of the formula [Co(Ad)2(CH3CN)2dap][CoCl4]∙CH3CN (Figure 14) were obtained. The ligand, in its neutral form, is coordinated to the central Co(II) as O2N3 pentadentate, resulting in the formation of four five-membered metallocycles. Cobalt(II) is placed in pentagonal bipyramidal environment with the ligand’s donor atoms at the base of this polyhedron and acetonitrile nitrogen in apical positions [32].
Figure 14.
The molecular structure of the[Co(ad)2dap(CH3CN)2][CoCl4]∙CH3CN ((ad)2dap = 2,6-diacetylpyridine-adamantane-1-carbohydrazide).
4. Complexes of the adamantyl semicarbazone and benzoyl derivatives
In the study [33], D. Palanimuthu and collaborators detail the synthesis and characterization of two groups of adamantane-based compounds – adamantyl semicarbazones and adamantyl benzoyl hydrazones, as well as some Cu(II) and Fe(III) complexes formed with them (12 structures).
This research focuses on developing novel adamantane-based semicarbazones and hydrazones targeting multiple pathological hallmarks associated with Alzheimer’s disease (AD). Compound derived from 2-pyridinecarboxaldehyde (N-adamantan-1-yl)benzoyl-4-amidohydrazone emerged as a lead compound, demonstrating efficacy in iron chelation, attenuation of CuII-mediated β-amyloid aggregation, low cytotoxicity, inhibition of oxidative stress, and favorable blood–brain barrier permeation. The study suggests these multi-functional agents present a promising therapeutic strategy for AD treatment [33].
The molecular structure of the ferric complex from this work is illustrated in Figure 15. The adamantane-derived ligand, with both parts deprotonated at the phenol O-atom, coordinates in a tridentate meridional manner, forming a monocationic distorted octahedral complex. This represents the first crystallographically characterized phenolic semicarbazone complex of Fe(III). The Fe▬O bond lengths and Fe▬N bond lengths align with a high-spin
Figure 15.
The molecular structure of the Fe(III) complex with salicylaldehyde-N-adamantan-1- ylsemicarbazide (refcode LEYGII).
The copper complex published within the same research displays a distorted square pyramidal geometry (Figure 16). The asymmetric unit contains two complexes and 1.5 MeOH molecules. This complex features a basal plane occupied by the tridentate ligand’s O2N donor atoms, along with a chlorido ligand, while weakly coordinated chlorido ligands occupy the axial sites. The ligand is coordinated as a zwitterion, with protonated pyridoxal N atom and deprotonated pyridoxal OH group. The presence of partially occupied MeOH correlates with the disorder of the hydroxymethyl substituent on the pyridyl ring. The axially elongated square pyramidal (pseudo-square planar) coordination geometry found in this structure is also observed in other mononuclear copper complexes of tridentate semicarbazone ligands with phenolic O donors [33].
Figure 16.
The molecular structure of the Cu(II) complex with pyridoxal-N-adamantan-1-yl-semicarbazone (refcode LEYGAA).
The other copper(II) complex (Figure 17) crystallizes as a centrosymmetric dimer with two copper ions bridged by two phenolate O-donors. Each copper(II) ion adopts a square pyramidal geometry, with the tridentate O2N semicarbazone ligand and the phenolate O donor atom of the adjacent complex occupying the equatorial plane, and the axial site is occupied by a chlorido ligand. Intermolecular hydrogen bonding was also observed, with the chlorido ion strongly hydrogen bonding to two NH groups of an adjacent molecule. This bis-phenolate bridged di-CuII coordination mode has been noted in various analogous complexes of semicarbazone ligands [33].
Figure 17.
The molecular structure of the Cu(II) complex with pyridoxal-N-adamantan-1-yl-semicarbazone (refcode LEYGEE).
This study introduces the first instances of adamantane-based semicarbazone and hydrazone chelators showcasing multifunctional activity for Alzheimer’s disease (AD) treatment. The adamantoyl semicarbazone and adamantoyl benzoyl hydrazone analogues, along with the pyridil derivative outlined in this research, collectively exhibit advantageous multi-functional properties aimed at addressing the various hallmarks of AD [33].
5. Conclusion
In conclusion, the investigation of adamantane-derived Schiff bases and their complexes, conducted through the Cambridge Structural Database search, sheds light on a promising avenue for drug discovery and therapeutic development. The unique chemical properties of adamantane, including its hydrophobic nature and metabolic stability, make it an attractive scaffold for modifying drug molecules across various therapeutic classes. The demonstrated ability of these derivatives to disrupt enzymes and exhibit diverse therapeutic activities underscores the importance of synthesizing and characterizing novel adamantane-based compounds. This overview not only contributes to the advancement of medicinal chemistry but also provides valuable insights into chemical synthesis and materials science. These insights highlight the potential for future innovations in drug design and development, informed by a robust understanding of adamantane chemistry and its therapeutic implications.
Acknowledgments
The authors gratefully acknowledge the financial support of the Ministry of Science, Technological Development and Innovation of the Republic of Serbia (Grants No. 451-03-66/2024-2103/200125 & 451-03-65/2024-2103/200125).
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