Synthesis and Spectroscopic Characterization of Tin ( II ) and Tin ( IV ) Complexes Containing 2 , 3 , 5 , 6-Tetrakis ( α-pyridyl ) pyrazine as a Bridging Ligand #

Neste trabalho foram investigadas as reações entre o ligante heterocíclico nitrogenado 2,3,5,6tetraquis(α-piridil)pirazina, TPP, e seis precursores de estanho, a saber: SnCl 2 , SnX 4 (X = Cl ou Br), SnRCl 3 (R = Ph ou Me) e SnPh 2 Cl 2 . Os produtos foram caracterizados por microanálise (C, H, N e Sn), espectroscopia no infravermelho (4000-200 cm), RMN de H, C{H}, C-CP/MAS, Sn e Sn-MAS, bem como por espectroscopia Mössbauer de Sn. Todas as reações levaram ao isolamento de complexos bimetálicos, nos quais o TPP se comportou como um ligante bis-bidentado em ponte, ligando-se a cada centro metálico através de dois átomos de nitrogênio piridínicos. Este modo de coordenação é ainda raro na literatura e, pela primeira vez, é observado em complexos de TPP com um metal representativo.

Owing to this paucity of information, we decided to investigate the reactivity and coordinating behavior of TPP towards Sn(II) and Sn(IV) compounds, namely SnCl 2 , SnX 4 (X = Cl or Br), SnRCl 3 (R = Ph or Me), and SnPh 2 Cl 2 .The various complexes thus produced were characterized by a number of different spectroscopic methods.
8][19] However, this work shows that the stannylated products described herein are all bimetallic adducts and exhibit a similarity of structural behavior.Regardless of the stannylated reagent, TPP coordinates each tin atom through two pyridine nitrogen atoms, in the bis-bidentate 1 chelating mode (Figure 1).This sort of TPP coordination mode was first reported for the cationic part of the salt {[Pt(PEt 3 )Cl] 2 -μ-TPP}[Pt(PEt 3 )(SnCl 3 ) 4 ] 12 and is reproduced in the present study.

General procedures
All reactions were carried out under an Ar or N 2 atmosphere using Schlenk glassware and vacuum/inertgas line manipulation techniques.Solvents were dried by standard procedures and distilled immediately prior to use.2,3,5,6-tetrakis(α-pyridyl)pyrazine (TPP) was synthesized according to Goodwin & Lions's method. 20All other chemicals were obtained from commercial sources (Strem or Aldrich) and used as received.

Physical measurements
Decomposition points were determined on a Mel-Temp II (Lab.Devices, Inc.) apparatus.Elemental analyses (C, H, and N) were conducted on a Perkin-Elmer 2400 CHN Elemental Analyzer.Tin was analyzed on a Hitachi Z-8200 atomic absorption spectrophotometer.IR spectra were recorded either on a 283B Perkin-Elmer or on a Magna-IR760 FTIR Nicolet spectrometer.The IR spectra were obtained in the 4000-200 cm -1 range using CsI pellets. 1 H and 13 C{ 1 H} NMR spectra in solution were acquired from a Bruker DRX200 instrument operating at 200.00 and 50.30MHz, respectively.The solution 119 Sn NMR spectra were obtained from a Bruker DRX400 spectrometer operating at 149.17 MHz.Both 13 C-CP/MAS and 119 Sn-MAS spectra were recorded on a Bruker DRX300 spectrometer operating at 75.47 and 111.92 MHz, respectively.In order to obtain the isotropic chemical shift values, the sample was spun at 6 and 7.5 kHz in the 13 C-CP/MAS spectra and also at 8 kHz in the 119 Sn-MAS spectra.All NMR studies were performed at room temperature, and the chemical shift values were determined in relation to SiMe 4 for the 1 H and 13 C nuclei and to an external reference of SnMe 4 for the 119 Sn nucleus. 119Sn Mössbauer spectra were obtained from a constant acceleration spectrometer moving a CaSnO 3 source at room temperature.The samples were analyzed at liquid N 2 temperature, and the isomer shift values are given with respect to that source.All Mössbauer spectra were computerfitted assuming Lorentzian lineshapes.

Synthesis of [(SnCl 2 ) 2 -μ-TPP] (6)
Solid TPP (0.36 g, 0.93 mmol) was added under vigorous magnetic stirring to a solution of anhydrous SnCl 2 (0.41 g, 2.17 mmol) in EtOH (25 mL).A yellow solid immediately precipitated.The reaction mixture was kept under stirring for two additional days at room temperature.The yellow solid was then isolated by filtration, washed with EtOH and MeOH, and dried under vacuum.After drying, product 6 became orange and was stored under argon.The yield of 6 was 0.50 g (70%).
Because of their importance for the present study, the 119 Sn NMR and Mössbauer spectroscopic data are separately listed in Table 1.

Results and Discussion
Elemental analyses agree with the supposition that all products are bimetallic adducts of general formula [(SnL n ) 2 TPP], in which SnL n stands for the different stannylated precursors.The analytical and spectroscopic data point to the fact that complexes 1 to 5 have two hexacoordinate Sn(IV) sites, whereas complex 6 has two tetracoordinate Sn(II) centers.
In the IR spectra, the ν(CC/CN) bands were shifted from 1590-1390 cm -1 in free TPP to higher wavenumbers in the products.Additionally, new ν(Sn-N) bands appeared in the low-frequency region of the spectra, as expected.Moreover, the ν(Sn-X) (X = Cl or Br) bands were shifted to lower wavenumber values in relation to the free stannylated reagents.
The coordination of TPP is also indicated by the 1 H and 13 C{ 1 H} (or 13 C-CP/MAS in the case of 6) NMR data.With no exception, the NMR signals assigned to the H and C atoms of TPP in the products moved to higher δ values with respect to those of the free ligand.Moreover, the number and integration of the 1 H signals imply that there are no uncoordinated pyridine rings in 1 to 6, suggesting that TPP may be symmetrically attached to both metallic centers in each adduct.Thus, on the basis of the 1 H and 13 C NMR spectra, only the bis-bidentate 1 or bis-tridentate chelating modes (Figure 1) can occur in 1 to 6.
Table 1 shows the 119 Sn NMR and 119 Sn Mössbauer spectral data collected for the products and for other tin compounds.Our six adducts exhibit only one sharp 119 Sn NMR signal.This indicates that both tin atoms in each bimetallic product are chemically and magnetically equivalent in the NMR time scale.Except for 2, the 119 Sn chemical shift values, δ( 119 Sn), for the products are significantly lower than those for their precursors.An important feature of 119 Sn chemical shifts is that an increase in coordination number normally leads to lower values of δ( 119 Sn) due to shielding effects. 199][30][31] If one takes SnCl 4 , its values of δ( 119 Sn) are -150 and -601 ppm as the neat compound and in its MeOH solution, respectively (Table 1).This difference suggests that in MeOH the metal is hexacoordinate due to solvation.In fact, the value of δ( 119 Sn) observed for SnCl 4 in MeOH is analogous to that measured for 1 (Table 1).A more striking case arises when one compares 1 with [SnCl 4 (DPS)] 31 (DPS = di(2-pyridyl)sulfide), in which δ( 119 Sn) is -636 ppm in DMF-d 7 and the ligand DPS forms an N,N-bonded chelate.The similarity between the δ( 119 Sn) values for both [(SnCl 4 ) 2 -μ-TPP] and [SnCl 4 (DPS)] suggests that in the former each Sn(IV) center is hexacoordinate and thus, TPP binds to each stannylated site through two pyridine nitrogen atoms, according to the bis-bidentate 1 coordination mode (Figure 1).The same interpretation can be made by comparing δ( 119 Sn) of 2 (-485 ppm) with that of [SnBr 4 (DPS)] (-502 ppm). 31Curiously, the value of δ( 119 Sn) of SnBr 4 (Table 1) is lower than that measured for 2. It has been pointed out that, for adducts obtained from SnBr 4 , there appears to be no obvious relationship between the coordination number of tin and the value of δ( 119 Sn). 28n 3 to 5 the upfield shift in the values of δ( 119 Sn) (Table 1), in relation to their precursors, also suggests the formation of hexacoordinate Sn(IV) compounds.
Summing up the discussion in terms of the NMR results, of the seven coordinating possibilities so far reported for TPP (Figure 1), only the symmetrical bis-bidentate 1 and bis-tridentate are possible for 1 to 6.Both afford equivalent tin centers; nevertheless, the former leads to hexacoordinate Sn(IV) and tetracoordinate Sn(II) centers, whereas the latter leads to heptacoordinate Sn(IV) and pentacoordinate Sn(II) products.Heptacoordinate Sn(IV) complexes, however, have much lower 119 Sn chemical shifts than those obtained for 1 to 5. 30,32 Therefore, the 119 Sn NMR results point to hexacoordinate tin centers in the Sn(IV) products.With respect to complex 6, since only a few data about 119 Sn-MAS NMR spectra of Sn(II) compounds are available for comparison with our result, a discussion concerning the coordination mode of TPP in 6 will be centered on its 119 Sn Mössbauer spectrum.In turn, the 119 Sn Mössbauer data of the Sn(IV) adducts corroborate the conclusions from the 119 Sn NMR spectra.This shows that TPP acts as a bisbidentate 1 ligand in both solution and the solid state and leads to the conclusion that no dissociation of complexes 1 to 5 in coordinating solvents (DMF-d 7 or CD 3 CN) occurred.Table 1 gives the 119 Sn Mössbauer parameters for the precursors and products.Both isomer shifts (δ) and quadrupole splittings (∆E Q ) show great variation upon complexation.Invariably, the isomer shift values of 1 to 6 are lower than those of their precursors.Furthermore, the presence of only one doublet in the 119 Sn Mössbauer spectra also indicates the existence of just one type of tin atom in each dinuclear adduct.19]21 Thus, an increase in the coordination number of tin leads to a decrease in the δ value. 119Sn Mössbauer spectroscopy can also be reliably used to distinguish between the 2+ and 4+ oxidation states of tin.Usually, δ values in the interval from -0.5 to 2.1 mm s -1 are expected for Sn(IV) compounds, and Sn(II) complexes present δ values ranging from +2.5 to 5.0 mm s -1 . 2119]21 The three inorganic precursors have ∆E Q values of zero (SnCl 4 and SnBr 4 ) or very low (SnCl 2 ).After complexation, the ∆E Q values of 1, 2, and 6 became higher than those of their precursors, showing an increase in electron density asymmetry around the Sn centers.A similar increase in ∆E Q values also takes place upon coordination of the three organotin reagents.
In solid anhydrous tin(II) chloride, the tin atom is surrounded by nine chlorine atoms, leading to a facially capped trigonal prismatic arrangement. 21,35Of the nine Sn-Cl distances in solid SnCl 2 , three are very short and the others are longer and of variable length.In view of this, it is not possible to clearly state the coordination number of tin in solid SnCl 2 .Thus, it is difficult to discuss the coordination number of tin in 6 solely from a comparison between its Mössbauer parameters and those of SnCl 2 .On the other hand, in the related compound [SnCl 2 (pyridine) 2 ] δ is 3.02 mm s -1 . 36The δ value of 6, 3.44 mm s -1 , is very similar to that of [SnCl 2 (pyridine) 2 ], suggesting similar tin environments in both complexes.Thus, we propose that in 6 TPP also coordinates each stannylated center at two pyridine nitrogen atoms, according to the bis-bidentate 1 chelating mode.
It is important to mention that the bis-bidentate 1, as well as the mono-bidentate 1 chelating modes of TPP do not lead to planar metallacycles, as Figure 1 might suggest.In fact, due to rotation about the single bonds uniting the pyridine rings and the central pyrazine ring, the adjacent pyridine rings of TPP acquire a non-planar configuration upon coordination through the mono-or bis-bidentate 1 bonding modes. 12,13

Conclusions
The present work describes an unprecedented case of the reactivity of TPP towards several inorganic and organometallic stannylated species.Six different adducts were prepared, and all of them are bimetallic species, with TPP bridging two tin centers.From spectroscopic data we conclude that TPP coordinates both Sn(IV) and Sn(II) centers in the same mode, namely the rare bis-bidentate 1 chelating mode.

Table 1 .
119Sn NMR and Mössbauer spectral data neat; b in MeOH; c in CS 2 ; d in DMSO; e in CH 2 Cl 2 /C 6 D 6 ; f in CD 2 Cl 2 ; g in CDCl 3 ; h in DMF-d 7 ; i in CD 3 CN; j solid state. a