https://doi.org/10.1595/205651317X696621 Johnson Matthey Technol. Rev., 2018, 62, (1), 89–106 www.technology.matthey.com Coordination Compounds of Hexamethylenetetramine with Metal Salts: A Review Properties and applications of a versatile model ligand Reviewed by Jia Kaihua* and Ba chemical industry. It has been used in many fields Shuhong including as a curing agent for phenolic resins Military Chemistry and Pyrotechnics, Shenyang (1), as an accelerant in vulcanisation (2), in food Ligong University, Shenyang, 110186, China preservatives (3) and explosives (4) because of its useful properties including high solubility in water *Email: [email protected] and polar organic solvents (5). In addition, hmta can act as a multifunctional ligand, using its N atom to form coordination complexes with many Hexamethylenetetramine (hmta) was chosen as transition metals (6–10). It has been employed a model ligand. Each of the four nitrogen atoms to prepare complexes with metals, and has been has a pair of unshared electrons and behaves increasingly applied in chemical synthesis where it like an amine base, undergoing protonation and has received increasing attention due to its simple N-alkylation and being able to form coordination operation, mild conditions and environmental compounds with many inorganic elements. The friendliness (11, 12). ligand can be used as an outer coordination sphere modulator of the inner coordination sphere and as 1. Coordination with Metal Salts a crosslinking agent in dinuclear and multinuclear coordination compounds. It can also be used as A large number of complexes of hmta and a model for bioactive molecules to form a great metal salts have been studied and reports on number of complexes with different inorganic their synthesis, preparation, structure analysis salts containing other molecules. Studies of hmta and applications in medicine and military are coordination compounds with different metal summarised below. salts have therefore attracted much attention. The present review summarises the synthesis, 1.1 Coordination with Metal Salts of preparation, structure analysis and applications Main Group Elements of coordination compounds of hmta with different metal salts. The magnesium dichromate hmta complex was first crystallised by Debucquet and Velluz with 5H2O Introduction in 1933 (13), but subsequent analysis showed the presence of six water molecules instead of Hexamethylenetetramine as a reagent has five and on the basis of former study, Dahan been used as a source of –CH=N– and –CH= put forward the topic of “the crystal structure of functions. It has a cage-like structure and is the magnesium dichromate hmta hexahydrate considered to be a crucial basic material for the complex” in 1973 (14). In Dahan’s study, the 89 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696621 Johnson Matthey Technol. Rev., 2018, 62, (1) structure of the Mg dichromate hmta hexahydrate lead to new materials such as polyoxometalates complex can be regarded as composed of two CrO4 (POMs) with different properties and more stable tetrahedra joined through a shared oxygen atom, structures. a slightly distorted octahedron around Mg and Chen et al. synthesised two new extended two hmta molecules. There were no coordination frameworks based on two different sandwich-type bonds between these groups. They are linked by polytungstoarsenates under routine conditions in hydrogen bonds, thus determining the packing and 2009. They found an advance on the sandwich- controlling the stability. type POMs, which had larger volumes and more Sieranski and Kruszynski (15) studied complexes negative charges than commonly used POMs, of Mg and hmta in 2011. Sieranski chose hmta allowing the formation of higher coordination and 1,10-phenanthroline as ligands complexed numbers with metal cations. Thus, sandwich- with Mg sulfate. Coordination compounds type POMs should be excellent building blocks for 2+ 2− [Mg(H2O)6] •2(hmta)•SO4 •5(H2O) and constructing extended networks (28). Mg(C12H8N2)(H2O)3SO4 were synthesised and characterised by elemental and thermal analysis, 1.3 Coordination with Different Metal infrared (IR) spectroscopy, ultraviolet-visible Salts of Subgroup B Elements (UV-vis) spectroscopy, fluorescence spectroscopy and X-ray crystallography. The compounds were Allan et al. (30) studied transition metal halide found to be air stable and well soluble in water. complexes of hmta in 1970. Complexes of Mg and calcium have been most studied but hmta have been prepared with the halides of heavier alkaline earth metals have also been manganese(II), cobalt(II), nickel(II), zinc(II), investigated. For instance, after the development cadmium(II), iron(II) and copper(II) and also with of the strontium-based drug, strontium renelate, the thiocyanates of Co(II), Ni(II) and Zn(II). These which reduces the incidence of fractures in complexes have been characterised by elemental osteoporotic patients, there has been an increasing analysis, vibrational and electronic spectra and awareness of this metal’s role in humans. magnetic moments. Khandolkar et al. (16) studied the synthesis, Ahuja et al. (31) studied hmta complexes with crystal structure, redox characteristics and Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) photochemistry of a new heptamolybdate thiocyanates. As a potentially tetradentate ligand, supported coordination compound hmta complexes with Mn(II), Co(II), Ni(II), Cu(II), (hmta)2[{Mg(H2O)5}2{Mo7O24}]•3H2O in 2015. Zn(II) and Cd(II) thiocyanates were prepared and In their study, the synthesis, characterisation characterised by their elemental analyses, magnetic and photochemistry of a new heptamolybdate susceptibilities, electronic and IR spectral studies supported Mg-aqua coordination complex using down to 200 cm–1, 611 cm–1 as well as X-ray powder hmta as structure directing agent was reported. diffraction patterns in the solid state. It was shown Tanco Salas (17) studied a complex of aluminium that hmta, though a potentially tetradentate ligand, and hmta (‘Al-hmta’) in 2004. The Al-hmta complex acts only as a terminally bonded monodentate was found to have deodorant and therapeutic ligand or a bidentate ligand bridging between two properties with potential applications in cosmetic metal atoms, retaining the chair configuration of and pharmaceutical compositions. Furthermore, the uncoordinated molecule in all these complexes. the Al-hmta complex had the capacity to trap The tentative stereochemistries of the complexes substances which could subsequently be released, were discussed. therefore being useful as a carrier of those Agwara et al. (32) studied the physicochemical substances. properties of hmta complexes with Mn(II), Co(II) and Ni(II) in 2004, and in 2012 these complexes 1.2 Coordination with Salts of Main had been synthesised in water and ethanol (33). All the complexes were hydrogen-bonded, except the Group Elements and Subgroup B Co complex [Co(hmta) (NO ) (H O) ] which was Elements 2 3 2 2 2 polymeric. These complexes were characterised Different metal salts have different coordination by elemental analysis, IR and visible spectroscopy abilities and properties. Usually, the complex is as well as conductivity measurements. The results made up of the main group element as the central suggest octahedral coordination in which the atom, and the subgroup elements appear in the central metal ion is bonded to aqua ligands and ligands. The combination of two metals may the hmta is bonded to the aqua ligands through 90 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696621 Johnson Matthey Technol. Rev., 2018, 62, (1) 91 Table I Major Crystal Data and Refinement for Compounds of hmta Salts with Main Group Elements Serial Crystal system, Z, d, R indices CCDC Reference Formula (M:hmta) F (000) No. space group Mg m–3 (all data) No. No. + – Orthorhombic, R1 = 0.0384, [Li(H2O)4] •2(hmta)•ClO4 (1:2) 1a 4, 1.416 976 1003176 (19) Pna21 (No. 33) wR2 = 0.0964 Orthorhombic, R1 = 0.0284, [Na(ClO4)(H2O)(hmta)]n (1:1) 1b 4, 1.625 584 1003177 (19) Pnma (No. 62) wR2 = 0.0867 2+ – Monoclinic, R1 = 0.0378, [Na(hmta)(H2O)4]2 •2SCN (1:1) 2b 2, 1.366 624 802360 (20) P21/c (No. 14) wR2 = 0.0900 Monoclinic, R1 = 0.0339, [K2(hmta)(SCN)2]n (2:1) 1c 4, 1.559 688 833296 (20) C2/c (No. 15) wR2 = 0.0877 Trigonal, R1 = 0.0268, [NaNO3•(hmta)]n (1:1) 3b 6 1212 266846 (8) R3c (No. 161) wR2 = 0.0777 + – Trigonal, R1 = 0.0339, [Li(H2O)4] •(hmta)•Cl (1:1) 2a 18, 1.289 2448 859820 (21) R32 (No. 155) wR2 = 0.0881 + – Monoclinic, R1 = 0.0546, [Li(H2O)4] •(hmta)•I (1:1) 3a 4, 1.629 688 859821 (21) P21/c (No. 14) wR2 = 0.1321 2+ – Orthorhombic, R1 = 0.0388, [Na(H2O)4•(hmta)]2 •2H2O•2Br (1:1) 4b 4, 1.522 1376 859822 (21) Pbca (No. 61) wR2 = 0.0806 2+ – Orthorhombic, R1 = 0.0450, [Na(H2O)4•(hmta)2] •2H2O•2I (1:2) 5b 4, 1.649 1520 859823 (21) Pbca (No. 61) wR2 = 0.1192 Monoclinic, R1 = 0.0308, [K(H2O)(hmta)I]n (1:1) 2c 4, 1.913 632 859824 (21) C2/m (No. 12) wR2 = 0.0716 Monoclinic, R1 = 0.0234, [Rb(H2O)(hmta)I]n (1:1) 1d 4, 2.095 704 859825 (21) C2/m (No. 12) wR2 = 0.0639 (hmta) [{Mg(H O) } Monoclinic, R = 0.0382, 2 2 5 2 1e 4, 2.493 1 3152 1049780 (16) {Mo7O24}]•3H2O (1:2) C2/c (No. 15) wR2 = 0.0829 Triclinic MgCr O •2(hmta)•6H O (1:2) 2e 2, 1.595 – 654 – (14) 2 7 2 P1 (No. 2) 2+ [Mg(H2O)6] •2(hmta) Triclinic, R1 = 0.0317, © 2018 Johnson Matthey © 2018Johnson 2− 3e 1, 1.446 322 813464 (15) •SO4 •5(H2O) (1:2) P1 (No. 1) wR2 = 0.0761 [Mg(H O) ]2+•2(hmta)•2NO 2−•4H O Monoclinic, R = 1.0538, 2 6 3 2 4e 4, 1.3782 1 1304 865230 (18) (1:2) P21/c (No.