The equilibria which exist between the potentially tridentate cyclic amine 1,4,7-triazacyclononane, [9]aneN3, and Ni(II), Cu(II), and Zn(II) ions have been investigated in 0.1 KNO3 at 25.0 °C. The bis complexes Cu([9]aneN3)2X2 and Ni([9]aneN3)2X2 as well as Ni([9]aneN3)X2 (where X-= NO3and/or Cl") have also been isolated and characterized. The unexpectedly large formation constants determined for the 1:1 metal-ligand complexes and the large value of Dq for Ni ([9]aneN3)22+ suggest that this ligand imposes rather severe steric constraints on coordination.

greatly reduced (e.g. dimethyl sulfoxide (log Kx = 1.04)27 or carbon monoxide (no reaction observed)).28 The relatively low equilibrium constants observed for good -acceptor ligands such as P(OPh)3 and CO indicate that the contribution of -back-bonding interactions to the stability of the Rh-L bond is relatively insignificant. These findings are consistent with theoretical and experimental indications of a lack of -stabilization in dirhodium(II) tetraacetate complexes.24"26 Drago and co-workers, however, have observed that the properties of dirhodium(II) butyrate14 and perfluorobutyrate15 complexes with axial -acceptor ligands cannot be interpreted on the basis of contributions alone and must include some metal-to-ligand -back-bonding interactions. The

No abstract

No abstract

Complexes of protonated hexacyclen ( C I Z H~~N~) with NO,-, CI-, C104-, and Br-are found by conductometry and pH potentiometry to have the compositions H4LX3+ (where L = hexacyclen, X = anion). Formation constants with these anions at 25 OC are determined by pH potentiometry. A H and AS for the NO3-and C1-complexes as well as the acid dissociation of H4L4+ are derived from the temperature-dependent equilibrium constants. The results indicate that although the NO,complex is more stable the CIapparently forms stronger bonds to H4L4+. Bonding in these complexes is discussed in terms of ionic H bonding and solvent ordering effects. X-ray crystallographic study of H4L(NO3),Cl2.2H20 supports the proposed bonding mechanism.The selective binding of inorganic and organic anions by macrocyclic ligands is a recently reported phenomenon.'s2 In many cases the host ligand species feature protonated amine groups of the type (RR'R''N+H), (where y > 2), and it has generally been assumed that electrostatic and ionic hydrogen-bonding interactions provide the bonding forces in these complexes. The shape and size of the macrocycle cavity are presumed to influence the bonding interactions and so to account for selectivity.In the present work we wish to elucidate the factors which influence these complexation reactions by means of a study of the cyclic hexamine 1,4,7,10,13,16-hexaazacyclooctadecane (hexacyclen, L) complexes of nitrate, perchlorate, chloride, and bromide ions. The study utilizes pH-potentiometric and conductometric measurements designed to determine the nature, formation constants, and thermodynamic properties of the complexes. Also an X-ray crystallographic determination provides information about structure and bonding. Experimental MethodsMaterials. All chemical reagents and solutions were purified or were reagent grade.Hexacyclen hemihydrate, C12H,oN6-0.5H,0, was prepared from the commercial trisulfate salt purchased from either Aldrich or Parish Chemical Co. Weighed portions of the salt were used to prepared 0.5 M solutions of the polyamine and these were treated with an equivalent amount of solid BaCO,. After the resultant slurry was mixed for 2 h at 80 OC, the filtered hot solution was dried with a rotary evaporator. The waxy product purified by vacuum sublimation yielded pure white crystalline hexacyclen-hemihydrate (L.0.5H20): NMR HN,S 2.02 (l), H C ,~, 2.70 (4); mass spectrum, m / e parent peak 259. Anal. Calcd C, 53.70; H, 11.64; N, 31.68. Found: C, 53.20; H, 11.29; N, 31.54. Hexacyclen hexahydrochloride and hexahydrogennitrate salts were prepared directly from the concentrated filtrate solution above by addition of a large excess of the appropriate acid. The product was recrystallized from a fresh portion of the 6 M acid and then air-dried. The crystals were Ci,H3N6.6HCI.3H20 (H&C16.3H20) and Ci2H30N6.6H-Registry No. L, 296-35-5; L.6HC1, 58105-91-2; L.6HN03, 81294-Supplementary Material Available: Table of thermal parameters (1 page). Ordering information is given on any current masthead page.17-9; L.2HNO...