Raju Prakash

Iron carbonyl, nitrosyl, and nitro complexes of a tetrapodal pentadentate amine ligand: Synthesis, electronic structure, and nitrite reductase-like reactivity

The tetrapodal pentaamine 2,6-C5H3N[CMe(CH2NH2)2]2 (pyN4, 1) forms a series of octahedral iron(II) complexes of general formula [Fe(L)(1)]Xn with a variety of small-molecule ligands L at the sixth coordination site (L = X = Br, n = 1 (2); L = CO, X = Br, n = 2 (3); L = NO, X = Br, n = 2 (4); L = NO+, X = Br, n = 3 (5); L = NO2-, X = Br, n = 1 (6)). The bromo complex, which is remarkably stable towards hydrolysis and oxidation, serves as the precursor for all other complexes, which may be obtained by ligand exchange, employing CO, NO, NOBF4, and NaNO2, respectively. All complexes have been fully characterised, including solid-state structures in most cases. Attempts to obtain single crystals of 6 produced the dinuclear complex [Fe2[mu 2-(eta 1-N: eta 1-O)-NO2](1)2]Br2PF6 (7), whose bridging NO2- unit, which is unsupported by bracketing ligands, is without precedent in the coordination chemistry of iron. Compound 2 has a high-spin electronic configuration with four unpaired electrons (S = 2), while the carbonyl complex 3 is low-spin (S = 0), as are complexes 5, 6 and 7 (S = 0 in all cases); the 19 valence electron nitrosyl complex 4 has S = 1/2. Complex 4 and its oxidation product, 5 ([Fe(NO)]7 and [Fe(NO)]6 in the Feltham-Enemark notation) may be interconverted by a one-electron redox process. Both complexes are also accessible from the mononuclear nitro complex 6: Treatment with acid produces the 18 valence electron NO+ complex 5, whereas hydrolysis in the absence of added protons (in methanolic solution) gives the 19 valence electron NO. complex 4, with formal reduction of the NO2- ligand. This reactivity mimicks the function of certain heme-dependent nitrite reductases. Density functional calculations for complexes 3, 4 and 5 provide a description of the electronic structures and are compatible with the formulation of iron(II) in all cases; this is derived from the careful analysis of the combined IR, ESR and Mössbauer spectroscopic data, as well as structural parameters.

A non-heme dinuclear iron(II) complex containing a single, unsupported hydroxo bridge.

Complexation of the tetrapodal pentadentate NN4 ligand 2,6-C5H3N[CMe(CH2NH2)2]2 (I) with iron(II) perchlorate hydrate in methanol, in the presence of N-methylimidazole, produces a diferrous complex with a single, unsupported mu-OH ligand between two {(I)FeII} coordination modules.

Highly Soluble Sulfur-Rich [Ni(L)(siS3)] Complexes Containing the New Ligand Bis(2-mercapto-3-trimethylsilylphenyl) Sulfide(2−) (siS32−)

The new organosulfur ligands siS3-H2 [siS32− = bis(2-mercapto-3-trimethylsilylphenyl) sulfide(2−)] and caS3-H2 [caS32− = bis(2-mercapto-3-carboxyphenyl) sulfide(2−)] were synthesized from HS3-H2 [HS32− = bis(2-mercaptophenyl) sulfide(2−)], n-butyllithium, and Me3SiCl or CO2/H+. Reaction of siS3-H2 with Ni(ac)2·4H2O gave the air-stable and well-soluble trinuclear complex [Ni(siS3)]3 (1) whose structure was determined by X-ray crystallography. Reactions of complex 1 with nucleophiles L [L = PR3 (R = nPr, Ph, Cy), N2H4, StBu−, and Cl−] yielded the corresponding neutral or anionic complexes, which were isolated as [Ni(PR3)(siS3)] [R = nPr (2), Ph (3), Cy (4)], Bu4N[Ni(Cl)(siS3)] (5), Bu4N[Ni(StBu)(siS3)] (6), and [Ni(N2H4)(siS3)] (8). The azido complex Et4N[Ni(N3)(siS3)] (7) was prepared from Me3SiN3 and the precursor chloro complex Et4N[Ni(Cl)(siS3)]. Reaction of 1 with NH3 yielded labile [Ni(NH3)(siS3)] (9), which was characterized in solution by 1H and 13C NMR spectroscopy. Analogously, 1 reacts with nicotinamide (NA) or diethylnicotinamide (NAEt2) to give, from equilibrium reactions, the corresponding mononuclear [Ni(L)(siS3)] complexes with L = NA, NAEt2. X-ray structure determinations showed that 1, 3, 4, 5, and 7 all exhibit tetrahedrally distorted planar [Ni(L)(siS3)] fragments. Complex 7 is the first structurally characterized azidonickel complex with a coligand having exclusively sulfur donors. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Protonation and H/D exchange reactions promoted by a sulfur-rich osmium hydride complex: identification of a labile dihydrogen complex

The sulfur-rich osmium nitrosyl complexes Bu(4)N[Os(NO)((bu)S(2))(2)] (1) [(bu)S(2)(2-) = 3,5-tert-butyl-1,2-benzenedithiolate(2-)] and [Os(NO)(py(bu)S(4))]Br ()[py(bu)S(4)(2-) = 2,6-bis(2-sulfanyl-3,5-di-tert-butylphenylthio)dimethylpyridine(2-)] have been synthesized. The molecular structure of 1 exhibits a square-pyramidal geometry with the NO group at the apical position. A pseudo-octahedral geometry with two thiolate and two thioether donors in trans configuration is found for 2. Compound 2 shows two quasi-reversible one-electron redox waves at E(1/2) = 0.51 and -0.46 V vs. NHE for the redox couples [Os(NO)(py(bu)S(4))](+1/0) and [Os(NO)(py(bu)S(4))](0/-1), respectively. 2 reacts with NaAlH(4) to produce Na[Os(H)(py(bu)S(4))] (3), which exhibits a typical hydride resonance in the (1)H NMR spectrum at delta =-15.03 ppm. Protonation of 3 with HBF(4)/CD(3)OD at 20 degrees C rapidly releases H(2)/HD to afford the dinuclear complex [Os(py(bu)S(4))](2) (4). Low temperature (1)H and (2)H NMR spectra of in [D(8)]THF with CH(3)OH or CD(3)OD at -80 degree C allow the observation of the formation of [Os(H(2)/HD)(py(bu)S(4))]. A 1 : 1 : 1 triplet at delta = -7.84 ppm [J(HD) = 31.2 Hz] and a relaxation time of T(1)(min) = 6 ms (-65 degrees C, 270 MHz) firmly establish the presence of eta(2)-H(2)/HD ligand. At room temperature, 3 interacts with D(2) (1 atm) and undergoes heterolytic D(2) cleavage followed by H/D exchange to form Na[Os(D)(py(bu)S(4))] (3a). A plausible cyclic mechanism has been proposed.