Nitridomanganese(V) and -(VI) Complexes Containing Macrocyclic Amine Ligands

Photolysis of trans-[(cyclam)MnIII(N3)2](ClO4) (1) in methanol at −35 °C with ∼350 nm light
produces blue [{trans-[(cyclam)MnV(N)]}2(μ-N3)](ClO4)3·3H2O (2) and dinitrogen (cyclam = 1,4,8,11-tetraazacyclotetradecane). A series of six-coordinate complexes trans-[(cyclam)MnV(N)Y]n+, where Y represents
Cl, n = 1 (3); CH3CN, n = 2 (4); ClO4-, n = 1 (5); CF3CO2-, n = 1 (6), was also synthesized. From a
methanol solution of 2 and addition of NaCN the species cis-[(cyclam)MnV(N)(CN)](ClO4) (7) was obtained.
Photolysis of [LMnIII(N3)3], where L represents the macrocycle 1,4,7-trimethyl-1,4,7-triazacyclononane, in
CH3CN at 20 °C with 253.7 nm light produces colorless crystals of [{LMnII(N3)}2(μ-N3)2] (8) (photoreduction),
whereas photolysis with 350 nm light at −35 °C gives the blue photooxidation product [LMnV(N)(N3)2] (9).
Complexes 1−7 were characterized by X-ray crystallography. Compounds 2−7 contain the nitridomanganese(V) unit (Mn⋮N 1.51−1.54 Å). All nitridomanganese(V) complexes display significant temperature-independent
paramagnetism indicative of a low-spin d2 electron configuration. In the electronic spectra four d−d transitions
have been identified for the first time which were unambiguously assigned by single crystal polarized UV/vis/NIR and magnetic circular dichroism (MCD) spectroscopy. The spectra were satisfactorily analyzed in
the frame of ligand-field theory (angular overlap method). 15N NMR spectroscopy revealed an enormous
deshielding of the nitrido group in 4, 6, and 7 (δ = 693−699 ppm referenced to CH3NO2 at 0 ppm).
Electrochemically, complex 4 undergoes a reversible one-electron oxidation generating a stable nitridomanganese(VI) species (d1) which has been characterized by UV−Vis and EPR spectroscopy.