A Novel Oxidative Addition Product from [ Pt 2 ( dba ) 3 ] : Synthesis and Spectroscopic Characterization of a Bimetallic Pt – η 2-Tetracyanoethylene Complex Containing 2 , 3 , 5 , 6-Tetrakis ( α-pyridyl ) pyrazine

{[Pt(η-TCNE)] 2 (μ-TPP)} (1) foi preparado a partir da reação entre o composto de Pt(0) [Pt 2 (dba) 3 ] (dba = dibenzilidenoacetona), tetracianoetileno (TCNE) e 2,3,5,6-tetraquis(αpiridil)pirazina (TPP) na razão estequiométrica 1:2:1, respectivamente. 1 foi caracterizado por análise elementar, espectroscopia no infravermelho, RMN de H em solução e RMN de C no estado sólido (CP/MAS). De acordo com os espectros de RMN, o ligante TPP em 1 se coordena a cada centro metálico por dois átomos de nitrogênio piridínicos, segundo um modo de coordenação bis-bidentado já relatado em outros complexos de TPP com Pt. Além dos sinais atribuídos aos átomos de carbono do TPP (δ 128-155) e aos átomos de carbono dos grupos nitrila (δ 115,1), o espectro de RMN de C-CP/MAS de 1 exibe na região de menor freqüência um duplo dupleto [δ 1 –2,6; J(Pt-C) = 223 Hz e δ 2 1,6; J(Pt-C) = 188 Hz]. Estes últimos sinais são atribuídos aos átomos de carbono etilênicos do TCNE ligados a platina e confirmam que a ligação nos fragmentos Pt-η-TCNE é melhor descrita segundo o modelo metalaciclopropano.

A well-known strategy to synthesize Pt-η 2 -alkene complexes with N,N donor ligands, such as α-diimines or aromatic nitrogen heterocycles, involves a "one-pot" reaction between a zerovalent platinum reagent {e.g.3] Usually, the isolated products have the general formula [Pt(η 2 -alkene)(N,N)] and the bonding at the Pt-η 2 -alkene moiety is described according to the metalacyclopropane model. 24Indeed, in olefin complexes of platinum, the bonding at the Pt-η 2alkene moiety can be understood in terms of either the classical Dewar-Chatt-Duncanson model 24,25 or the metalacyclopropane model. 243][24][25][26] In this work we have employed the aforementioned strategy to prepare a bimetallic Pt-η 2 -TCNE complex containing TPP as a bis-bidentate ligand, namely [{Pt(η 2 -TCNE)} 2 (μ-TPP)} (1).Solid-state 13 C (CP/ MAS) NMR spectroscopy has been used to examine the bonding at the Pt-η 2 -TCNE moieties as well as the mode adopted by TPP upon coordination to the metallic centers.

General details
The reaction and the work-up of the reaction mixture were carried out under argon using Schlenk-type glassware and vacuum-inert gas line manipulation techniques.THF was dried over sodium-benzophenone ketyl and deoxygenated according to standard techniques.[Pt 2 (dba) 3 ] 27 and TPP 28 were synthesized according to the literature methods.TCNE was purchased from Acros and used as received.CD 3 CN (Cambridge Isotopes, Inc.) was commercially obtained and stored over 3 Å molecular sieves.

Physical measurements
Elemental analysis (C, H, and N) was performed on a Perkin-Elmer 2400 CHN Elemental Analyzer.IR spectra were recorded on a Magna-IR760 FTIR Nicolet spectrometer.The spectra were obtained using Nujol mulls between CsI windows (4000-200 cm -1 ) and a resolution of 4 cm -1 . 1 H (CD 3 CN) and solid-state 13 C (CP/MAS) NMR spectra were acquired from Bruker DPX200 ( 1 H) and DRX300 ( 13 C) instruments operating at 200.08 MHz ( 1 H) and at 75.47 MHz ( 13 C), respectively.All NMR studies were performed at 293 K, and the chemical shift (δ / ppm) values were determined in relation to SiMe 4 .In the 13 C-CP/MAS NMR spectra the samples were spun at 6-8 kHz in 4 mm diameter ZrO 2 rotors with Kel-F caps.A satisfactory 13 C-CP/MAS NMR spectrum for 1 was obtained using a relaxation delay of 20 s and a contact time of 1 ms.A 13 C-CP/MAS NMR spectrum of TPP with good signal to noise ratio was obtained with a contact time of 0.5 ms and a relaxation delay of 60 s.

Results and Discussion
Selected FT-IR data for [Pt 2 (dba) 3 ], TCNE, TPP, and 1 are listed in Table 1.The FT-IR spectrum of 1 does not exhibit bands in the expected carbonyl region, thereby indicating that the dba ligands in the zerovalent platinum reagent were displaced in the reaction conditions.Moreover, the bands assigned to the nitrile groups of 1 were shifted to lower wavenumbers with respect to free TCNE.Table 1 also shows that some TPP internal vibrations were shifted to lower wavenumbers (e.g.1457 and 1388 cm -1 ) upon complexation; others, however, were shifted to higher wavenumber values (e.g.1605 cm -1 ), whereas some other bands remained unchanged (e.g.1568 and 1487 cm -1 ).In the low-frequency region of the FT-IR spectrum of 1, the bands at 385, and 375 cm -1 are assigned to the ν (Pt-C) stretching modes, whereas the bands at 280 and 246 cm -1 are ascribed to the ν (Pt-N) vibration modes. 29elected 1 H NMR data for TPP and for 1 are summarized in Table 2.According to the number of 1 H signals and their integration (see Experimental Section), it is possible to suggest that there are no uncoordinated pyridine rings in 1.This result implies that in 1 TPP is coordinated to the platinum centers according to the bisbidentate(A) chelating mode.The structure proposed for 1 is illustrated in Figure 2. In Table 2 the ∆δ values show  that the signals for H4, H5, and H6 were shifted to higher frequencies on coordination, whereas the signal for H3 exhibited only a slight shift to lower frequency.
The 13 C-CP/MAS NMR spectrum of 1 is depicted in Figure 3.The signals in the δ 128-155 ppm range are assigned to the TPP carbon atoms.In the 13 C-CP/ MAS NMR spectrum of free TPP, these signals appear in the region from 124 to 160 ppm. 20The signal at δ max 115.1 (Figure 3) is attributed to the nitrile carbon atoms.In the solution (CDCl 3 ) 13 C NMR spectrum of free TCNE, a single nitrile signal can be seen at 117 ppm.Also in the solution 13 C NMR spectrum of free TCNE, the signal for the olefinic carbon atoms lies at δ 101.In the 13 C NMR spectrum of 1 (Figure 3) no signals were observed in the expected alkene C(sp 2 ) region. 30nstead, a double doublet appears at lower frequencies, e.g. in the alkane C(sp 3 ) region. 30These signals, δ 1 -2.6 [ 1 J( 195 Pt-13 C) = 223 Hz] and δ 2 1.6 [ 1 J( 195 Pt-13 C) = 188 Hz] (C a in Figure 3), are assigned to the TCNE carbon atoms bound to platinum.The presence of these two isotropic signals, δ 1 and δ 2 , is likely to indicate that the asymmetric unit of 1 bears two crystallographically distinct coordinated carbon atoms of TCNE. 31Moreover, the low values for the 1 J( 195 Pt-13 C) coupling constants in 1 are in agreement with literature data for related compounds. 26he very low δ 1 and δ 2 values (Figure 3) imply that the TCNE carbon atoms attached to platinum exhibit an approximate sp 3 character.This confirms that the bonding at the Pt-η 2 -TCNE moieties is best described in terms of the metalacyclopropane model, 24 as illustrated in Figure 2. Hence, to each platinum in 1 one might assign a 2+ formal oxidation state.Indeed, the reaction described herein can be thought of as resulting from the oxidative addition of the olefinic C=C bond of TCNE to Pt(0) in the presence of TPP.
a sh = shoulder, w = weak, m = medium, s = strong; b spectra obtained in nujol mull between CsI windows; c not uniquely assignable.

a
spectra obtained in CD 3 CN; b dd = double doublet, m = multiplet.

Figure 3 .
Figure 3.The 13 C-CP/MAS NMR spectrum of 1. Spectrum recorded at a spinning frequency of 7.2 kHz.

Table 2 .
Selected solution 1 H NMR data (δ / ppm) for TPP and for 1 a,b