Synthesis of 14-oxa-1,4,8,11-tetraazabicyclo[9.5.3]nonadecane (L1) and a Spectroscopic and Structural Study of [Ni(L1)(ClO4)](ClO4) and of the Macrobicyclic Precursor Diamide Complex, [Ni(HL2)](ClO4); Chloride Substitution Kinetics of the Corresponding [Ni(III)(L1)]3+ Species

• Published in: Inorganic chemistry Vol. 40; no. 12; pp. 2737 - 2742
• Main Authors: Rodopoulos, M, Rodopoulos, T, Bridson, J. N, Elding, L. I, Rettig, S. J, McAuley, A
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 04.06.2001
• Subjects: Chemical Sciences; cyclam based ligands; Inorganic Chemistry; Kemi; Kinetics; Natural Sciences; Naturvetenskap; Nickel complexes; Oorganisk kemi; structure
• DOI: 10.1021/ic001163r
• ISSN: 0020-1669; 1520-510X

The pentadentate ligand 14-oxa-1,4,8,11-tetraazabicyclo[9.5.3]nonadecane (L1) has been synthesized by the high dilution cyclization of 1-oxa-4,8-diazacyclododecane ([10]aneN2O) (1) with 1,3-bis(α-chloroacetamido)propane (2) and subsequent reduction of the diamide intermediate. The structure [Ni(L1)(ClO4)](ClO4) (P21/c (no. 14), a = 8.608(3), b = 16.618(3), c = 14.924(4) Å, β = 91.53(3)° converged at R = 0.050 (R w = 0.046) for 307 parameters using 2702 reflections with I > 2σ(I). For the nickel(II) complex of the (monodeprotonated) precursor diamide ligand 14-oxa-1,4,8,11-tetraazabicyclo[9.5.3]nonadecane-3,9-dione (H2L2), [Ni(HL2)](ClO4) (Pbca (no. 61), a = 15.1590(3), b = 13.235(2), c = 18.0195(6) Å), the structure converged at R = 0.045 (R w = 0.038) for 265 parameters using 1703 reflections with I > 3σ(I). In the reduced system, the cyclam-based ligand adopts a trans-III configuration. The [Ni(L1)(ClO4)]2+ ion is pseudooctahedral with the Ni−O(ether) 2.094(3) Å distance shorter than the Ni−O(perchlorate) 2.252(4) Å. The nickel(II) and nickel(III) complexes are six-coordinate in solution. Oxidation of [Ni(L1)(OH2)]2+ with K2S2O8 in aqueous media yielded an axial d7 Ni(III) species (g ⊥ = 2.159 and g ⊥ = 2.024 at 77 K). The [Ni(L1)(solv)]2+ ion in CH3CN showed two redox waves, NiII/I (an irreversible cathodic peak, E p,c = −1.53 V) and NiIII/II (E 1/2 = 0.85 V (reversible)) vs Ag/Ag+. The complex [Ni(HL2)](ClO4) displays square-planar geometry with monodeprotonation of the ligand. The ether oxygen is not coordinated. Ni−O(3) = 2.651(6) Å and Ni−O(3a) = 2.451(12) Å, respectively. The NiIII/II oxidation at E 1/2 = 0.24 V (quasi-reversible) vs Ag/Ag+ is considerably lower than the saturated system. The kinetics of Cl- substitution at [Ni(L1)(solv)]3+ are pH dependent. Detachment of the ether oxygen atom is proposed, with insertion of a protonated water molecule which deprotonates at a pK a more acidic than in the corresponding cyclam complex. Mechanistic implications are discussed.