Unusual Coordination of Tetraphenylborate to Silver ( I ) : Synthesis and X-ray Crystal Structure of the Complex N , N '-bis [ 3-( 2-nitrophenyl ) allylidene ] ethane-1 , 2-diamine ( tetraphenylborato ) silver ( I )

O complexo N,N’-bis[3-(2-nitrofenil)alilideno]etano-1,2-diamino(tetrafenilborato)prata(I), [Ag(nca 2 en)(BPh 4 )], foi sintetizado e caracterizado por análise elementar e por espectroscopias de RMN de H, de absorção no infravermelho e UV-vis. Sua estrutura cristalina e molecular foi determinada por difratometria de raios X (monocristal). O centro de prata(I) no complexo está em um ambiente tetraédrico distorcido e faz ligações covalentes com os dois átomos de nitrogênio da base de Schiff e com dois átomos de carbono do tetrafenilborato. No complexo, o tetrafenilborato apresenta um comportamento não-usual, agindo como um ligante bidentado.


Introduction
Many notable recent advances in the chemistry of silver(I) complexes have been achieved through the use of diimine ligands.4][5][6] Cation-interactions between metal ions and polycyclic aromatic hydrocarbons have also received considerable attention in the synthesis of coordination compounds, mainly due to the possibility of introducing a variety of useful electric and electrochemical properties and of giving rise to novel molecular architectures.

Experimental
All chemicals were reagent grade and used as received.Solvents were purified by literature methods. 20N,N'-bis [3-(2-nitrophenyl)allylidene]ethane-1,2-diamine, nca 2 en, was prepared according to literature procedures. 21Elemental analyses were performed by using a Heraeus CHN-O-RAPID elemental analyzer.Infrared spectra were recorded on a Bruker Tensor 27 instrument.Electronic absorption spectra were recorded on a JASCO V-570 spectrophotometer.NMR spectra were obtained on a BRUKER AVANCE DRX500 (500 MHz, 11.75 T) spectrometer.Hydrogen chemical shifts are reported in parts per million (ppm) relative to Me 4 Si as internal standard.

X-ray crystallography
Diffraction data for 1 were collected on a STOE-IPDSII diffractometer using graphite-monochromated Mo-K radiation ( = 0.71073 Å).Data were collected and integrated using the Stoe X-AREA software package. 22A numerical absorption correction was applied using X-RED and X-SHAPE softwares. 23The structure was solved by direct methods (SHELXS-97). 33The structure refinement was performed by a full-matrix least-squares method against F 2 (SHELXL-97). 24All non-H atoms were refined anisotropically and all H atoms were inserted in calculated positions.

Results and Discussion
The IR spectrum of the proligand exhibits the characteristic band of the imine group, which appears at 1605 cm -1 .This band is shifted to lower frequencies in the IR spectra of the corresponding complexes due to the coordination of the imine nitrogen. 25ince no d-d transitions are expected for a d 10 complex, the UV-Vis bands are assigned to metal to ligand charge transfer (MLCT) or ligand-centered * transitions. 26The absorption spectrum of [Ag(nca 2 en)(BPh 4 )] (1) in chloroform features a clear shoulder at 320 nm (log = 3.73).Additional absorption bands are also observed in the spectra of 1 in chloroform in the UV region [ max = 280 nm (log = 4.61)].The intensity of these bands is consistent with ligand-centered * and/or charge-transfer transitions. 1 H NMR peak assignments, based on the splitting of resonance signals, spin coupling constants, and the literature, are presented in the experimental section.These assignments are in accordance with the molecular structure determined by X-ray crystal structure analysis.
The 1 H resonances of the coordinated nca 2 en are clearly observed in complex 1; however, the aromatic H atoms of the coordinated Ph 4 B overlap to some extent with those of the phenyl H atoms of nca 2 en.Aside from the aromatic-H atoms, which appear at 7.38-7.45ppm, the two imine hydrogens appear as a doublet at ca. 8.35 ppm in 1.The shift of the iminic hydrogens to high frequency relative to the proligand can be attributed to the deshielding effect resulting from coordination.The singlet at 4.02 ppm in 1 is assigned to the H atoms of the CH 2 CH 2 group.In principle, geometrical isomers are possible with respect to the C=N bond.However, the appearance of only one signal for each type of proton in CDCl 3 solution indicates that the symmetry of the molecule is retained in solution, and only one isomer is present.
A view of complex 1, including the atom numbering scheme, is illustrated in Figure 1.Crystallographic data are summarized in Table 1 and selected bond distances and angles are listed in Table 2. Compound 1 crystallizes with two molecules per asymmetric unit, probably due to some small conformational differences (1A, 1B).The coordination environment around the metal ion in this complex is pseudotetrahedral, with a large angular distortion arising from the low intraligand N-Ag-N chelate angle [75.64 (13) o in 1A and 76.44 (13) o in 1B] and the intraligand C-Ag-C chelate angle [99.71( 16) o in 1A and 98.86 (15) o in 1B].The other angles around the Ag center are larger than those of a non-distorted tetrahedral complex.1][32][33] The coordination environment around the boron atom is approximately tetrahedral, since the average of six angles involving B is 109.5 o , although the angles C21-B1-C27 in 1A and C65-B1-C71 in 1B have values of 103.5(3) o and 103.9(3) o respectively.The Ag center is approximately perpendicular to the coordinated phenyl rings of tetraphenylborate (for example, the angles Ag1-C22-C23 in 1A and Ag2-C72-C73 in 1B have values of 91.9(4) o and 90.0(3) o , respectively).
Despite the fact that the N-donor atoms are sp 2 -hybridized, the chelate ring is significantly puckered in this complex, and some strain in the chelate ring is suggested by deviation from the 120 o angle about the N-atom, as for C9−N2−C10 [119.

Figure 1 .
Figure 1.ORTEP view of the crystal structure of [Ag(nca 2 en)(BPh 4 )] (1A), with the atom labeling scheme.Thermal ellipsoids are drawn at the 50% probability level.Hydrogen atoms are omitted for clarity.