Novel Zwitterionic Oxorhenium ( V ) Complexes : Synthesis , Characterization and Crystal Structure of [ ReOX 2 ( Hdhp ) ( PPh 3 ) ] ( X = Cl , Br ; H 2 dhp = 2 , 3-Dihydroxypyridine )

Dois novos complexos zwitteriônicos de oxorrênio(V), [ReOCl 2 (Hdhp)(PPh 3 )] (1) e [ReOBr 2 (Hdhp)(PPh 3 )] (2) (H 2 dhp = 2,3-dihidroxipiridina), foram sintetizados e caracterizados por espectroscopia de absorção no infravermelho, ressonância magnética nuclear de H e P, análise elementar e determinação da estrutura cristalina e molecular por difração de raios X em monocristais. Os complexos apresentam geometria de coordenação octaédrica bastante distorcida, com os dois ligantes haletos arranjados em posições cis equatoriais, o ligante trifenilfosfina em posição trans a um dos haletos e o ligante Hdhp coordenado de forma bidentada através de seus átomos de oxigênio, sendo um em posição trans ao ligante oxo e o outro em posição trans com relação ao outro haleto. Este ligante tem seu átomo de nitrogênio protonado. Os compostos 1 e 2 apresentam empacotamento cristalino bastante diferente, influenciado em ambos os casos por ligações de hidrogênio intermoleculares dos tipos N–H⋅⋅⋅X (X = Cl, Br) e N–H⋅⋅⋅O.


Introduction
The ability of 2,3-dihydroxypyridine (H 2 dhp) to act as a chelating ligand is well known.Previous X-ray diffraction studies showed a bidentate coordination via the oxygen atoms, as Hdhp -, in complexes with the trivalent ions Al 3+ , Cr 3+ and Fe 3+ .The pyridine nitrogen atoms in these examples remain protonated, resulting in zwitterionic structures. 1,2o the best of our knowledge no rhenium complex was reported so far involving the Hdhp -anion as a ligand.In a recent work we described new zwitterionic rhenium complexes containing 2-hydroxypyridine as ligand. 3his work describes the synthesis, characterization and the X-ray crystal structure of novel rhenium complexes with Hdhp -ligands.The zwitterionic [ReOX 2 (Hdhp) (PPh 3 )] (X = Cl, Br) complexes contain monoanionic, chelating Hdhp -ligands, which are coordinated via their oxygen atoms.They formally possess a positive charge at the protonated nitrogen atom and a negative charge at the rhenium(V) center.

Reagents and apparatus
The rhenium starting complexes [ReOX 3 (PPh 3 ) 2 ] (X = Cl, Br) were prepared as previously described. 42,3-Dihydroxypyridine was used as purchased (Aldrich) without further purification.Solvents of analytical grade were degassed by purging with argon for about 15 minutes prior to use.Melting points were measured on a Melt-Temp II apparatus.Microanalytical data for C, H and N were obtained on a CHNS analyzer, FISONS model EA 1108.IR spectra were recorded on a BOMEM MICHELSON FT BM 102 spectrophotometer, using CsI pellets, within the 4000-200 cm -1 range.NMR spectra were recorded at room temperature, in CD 2 Cl 2 solutions, on a Varian Mercury Plus spectrometer, 7.05 T, operating at 300.07 MHz for 1 H and 121.47 MHz for 31 P.The 1 H spectra were internally referenced to TMS and 31 P{ 1 H} spectra were externally referenced to H 3 PO 4 (85%, δ 0).The data collections for crystal structure determinations were performed on a NONIUS KAPPA CCD diffractometer, equipped with Mo-K α radiation (71.073 pm) and graphite monochromator, applying standard procedures.

Crystal structure determinations
Suitable single crystals of 1 and 2 were obtained in the form of green needles by crystallization from dichloromethane.The cell constants were calculated from 33906 reflections for 1 and 77905 reflections for 2. Direct methods 5 were used for the solution of the structures.Except for the hydrogen atoms, which were calculated at idealized positions, all other atoms were refined with anisotropic displacement parameters. 6Complex 1 crystallizes in the trigonal crystal system, space group R -3, with one complex molecule in the asymmetric unit.Complex 2 crystallizes in the triclinic system, space group P -1, with two complexes in the asymmetric unit.This compound presented radiation damage which prevented the acquisition of an ideally complete data set.Additional information on the crystal structure analyses is given in Table 1.The complexes were isolated as green precipitates of good purity and could be obtained also in the crystalline form by crystallization from their dichloromethane solutions.The air stable compounds are moderately soluble in dichloromethane, but only slightly soluble in chloroform.

Spectroscopic characterization
The IR spectra of 1 and 2 are very similar in the range between 4000 and 400 cm -1 , which excludes the absorption bands related to ν(Re-X) (X = Cl in 1 and Br in 2), indicating that they have an analogous coordination sphere.The ν(Re=O) bands appear at 978 cm -1 for both complexes.The O,O-coordination mode of the Hdhp -ligand is indicated by shifts of the ν(C=O) (1627 cm -1 for 1 and 2) and ν(C-O) bands (1186 cm -1 for 1 and 1197 cm -1 for 2) with respect to the values found in uncoordinated H 2 dhp (1663 cm -1 and 1188 cm -1 ).The ν(C=C + C=N) bands are found at 1599 cm -1 and 1542 cm -1 in 1 and at 1605 and 1546 in 2, respectively.The ν(N-H) bands at 3266 cm -1 for 1 and at 3224 cm -1 for 2 are consistent with the zwitterionic structure of the complexes.The presence of the triphenylphosphine ligands in the complexes is evidenced by the presence of characteristic bands at 1482 cm -1 and 1435 cm -1 (ν(C=C)), and 1096 cm -1 and 1097 cm -1 (ν(P-C)) for 1 and 2, respectively.The ν(Re-Cl) band appears at 335 cm -1 for 1, while a band at 273 cm -1 was tentatively assigned for ν(Re-Br) for 2.
The 1 H-NMR spectra of 1 and 2 (for details see the Experimental Section) are consistent with the structures determined in the solid state.The 31 P{ 1 H}-NMR spectra show predictable singlet peaks for the phosphorus donor atoms at -21.06 ppm and -20.60 ppm for 1 and 2, respectively.

Crystal structures of 1 and 2
Complexes 1 and 2 possess a similar arrangement of the ligands around the hexacoordinate rhenium(V) centers.The coordination spheres of the rhenium atoms are distorted octahedra, each one formed by one oxo ligand, two halides (Cl -for 1 and Br -for 2), a triphenylphosphine and two oxygen atoms of the Hdhp -ligand, which coordinates as a O,O-chelate.The pyridine nitrogen atom of the chelating ligands is protonated.The halides have a cis arrangement R 1 = 0.0408; wR 2 = 0.0871 R 1 = 0.0382; wR 2 = 0.0699 Final R-factors (all data) R 1 = 0.0685; wR 2 = 0.0973 R 1 = 0.0594; wR 2 = 0.0763 "Goodness-of-fit, S, on F 2 1.051 1.022 Final peak and hole in the last difference map 1.725 and -0.793 e Å -3 1.070 and -0.958 e Å -3 Software used SHELXS97 5 and SHELXL 5 SHELX97 5 and SHELXL 6 to each other.The triphenylphosphine ligand is in trans position to one of the halides.One of the oxygen donor atoms from Hdhp -is located trans to the oxo ligand, while the other is trans to the second halide.The molecular structure of 1 is shown in Figure 1.The molecular structures of the two independent molecules of 2 are virtually identical and essentially the same as 1, and are therefore not shown here.Selected bond lengths and angles for both compounds are given in Table 2.The molecular labeling scheme of 1 has also been adopted for the bromo complex.
As in 1, in 4 the chlorine atoms are disposed in cis positions, while the structure of 3 presents these ligands in trans positions.No crystallographic data were found in the literature for a complex with a ReO 3 Br 2 P coordination sphere, as observed in 2.
As observed in 4, the Re-Cl bond in 2 is longer for the chloro ligand trans to PPh 3 .This results from the translabilizing effect of the phosphine ligand.The Re-O bonds trans to the oxo ligands are shorter than those trans to the chloro ligands in 1 and in 4.0][11] The Re-O and Re-P bond lengths in 2 are similar to the values observed in 1.The distances between the rhenium atoms and the oxo ligands correspond to values normally found for Re=O double bonds in oxorhenium(V) complexes. 9he bond lengths in the Hdhp -ligands in 1 and 2 do not differ significantly from the values found for the uncoordinated H 2 dhp, which is also protonated at the nitrogen atom. 1 The following resonance structures (a, b and c) should best represent the structure of the Hdhp - ligands in 1 and 2.   14 The molecules are linked by H-bonds of the type N-H⋅⋅⋅Cl, forming channels in the crystal structure, which are parallel to the crystallographic c axis.N(1)-H(1) = 86.0pm, H(1)⋅⋅⋅Cl(1) (x-y, x-1, -z+1) = 232.6 pm, N(1)⋅⋅⋅Cl(1) (x-y, x-1, -z+1) = 317.6 pm, N(1)-H(1)⋅⋅⋅Cl(1) (x-y, x-1, -z+1) = 169.76º.