Reductive nitrosylation of tetraoxometallates. Part 16. Generation and reactivity of the Os(NO)3+ moiety. Synthesis, characterisation, and electrochemistry of oxalato- and halogeno-nitrosyl derivatives of osmium; crystal and molecular structure of bis(1,10-phenanthrolinium) pentabromonitrosylosmate dihydrate

Abstract

Osmium tetraoxide smoothly reacts with NH₂OH·HCl in the presence of oxalate ion furnishing solid complexes A[Os(NO)(C₂O₄)₂]·H₂O (A = K, 1a; PPh₄, 1b; or AsPh₄, 1c). The complex anion possesses a square-pyramidal structure, as is evident from analytical, spectroscopic (IR, UV/VIS), and thermoanalytical data. The complexes react with concentrated HX (X = Cl, Br or I) forming besides [Os(NO)(C₂O₄)Cl₃]^2–3, [Os(NO)Br₄(H₂O)]^–4a and [Os(NO)Br₄]^–4b; [Os(NO)X₅]^2–(X = Cl, 2a; Br, 2b; or I, 2c) as the main product. The latte, (X = Cl or I) reacts with phen (1,10-phenanthroline) producing [Os(NO)X₃(phen)], whereas the bromo complex furnishes [Hphen]₂[Os(NO)Br₅]·2H₂O, 5c. The crystal and molecular structure of 5c has been determined by Patterson and Fourier methods and refined to R= 0.052 for 1221 unique reflections with I > 3σ(I). The crystals are monoclinic, space group P2₁/c, a= 7.371(1), b= 13.646(1), c= 14.001(2)Å, β= 99.88(1)° and Z= 2. The structure consists of [Hphen]⁺ cations, disordered [Os(NO)Br₅]^2– anions, and two water molecules of crystallisation; the anions have approximate octahedral geometry with a near linear nitrosyl ligand in the axial positions. Cyclic voltammograms of compounds 2a and 2b show a reversible oxidative response in MeCN at E°₂₉₈+0.94 and +0.96 V respectively, while 2c shows an irreversible oxidation peak at +1.08 V in dimethylformamide, indicating the ease of oxidation of Os(NO)³⁺→ Os(NO)⁴⁺(confirmed by ESR study), increasing in the series Cl^– < Br^– < I^–.