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

The pentadentate ligand 14-oxa-1,4,8,11-tetraazabicyclo[9.5.3]nonadecane (L 1) has been synthesized by the high dilution cyclization of 1-oxa-4,8-diaza cyclododecane ([10]aneN(2)O) (1) with 1,3-bis(alpha -chloroacetamido)propan e (2) and subsequent reduction of the diamide intermediate. The structure [ Ni(L1)(ClO4)](ClO4) (P2(1)/c (no. 14), a = 8.608(3), b = 16.618(3), c = 14. 924(4) Angstrom, beta = 91.53(3)degrees converged at R = 0.050 (R-w = 0.046 ) for 307 parameters using 2702 reflections with I > 2 sigma (I). For the n ickel(II) complex of the (monodeprotonaled) precursor diamide ligand 14-oxa -1,4,8,11-tetraazabicyclo[9.5.3]nonadecane-3,9-dione (H(2)L2), [Ni(HL2)](Cl O4) (Pbca (no. 61), a = 15.1590(3), b = 13.235(2), c = 18.0195(6) Angstrom, the structure converged at R = 0.045 (R-w = 0.038) for 265 parameters usin g 1703 reflections with I > 3 sigma (I). In the reduced system, the cyclam- based ligand adopts a trans-III configuration. The [Ni(L1)(ClO4)](2+) ion i s pseudooctahedral with the Ni-O(ether) 2.094(3) Angstrom distance shorter than the Ni-O(perchlorate) 2.252(4) Angstrom. 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 d(7) Ni(III) species (gi = 2. 159 and g(perpendicular to) = 2.024 at 77 K). The [Ni(L1)(solv)](2+) ion in CH3CN showed two redox waves, Ni-II/I (an irreversible cathodic peak, E-p, E-c = -1.53 V) and Ni-III/II (E-1/2 = 0.85 V (reversible)) vs Ag/Ag+. The c omplex [Ni(HL2)]-(ClO4) displays square-planar geometry with monodeprotonat ion of the ligand. The ether oxygen is not coordinated. Ni-O(3) = 2.651(6) Angstrom and Ni-O(3a) = 2.451(12) Angstrom, respectively. The Ni-III/II oxi dation at E1/2 = 0.24 V (quasireversible) vs Ag/Ag+ is considerably lower t han the saturated system. The kinetics of Cl- substitution at [Ni(L (L1)(so lv)](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 implica tions are discussed.