Synthesis, characterization, DFT and Td-dfT study of the [Fe(mnt)(L)(t-BuNC) 2] octahedral complex (L = phen, bipy)

ARTIGO

Synthesis, characterization, DFT and Td-dfT study of the [Fe(mnt)(L)(t-BuNC)₂] octahedral complex (L = phen, bipy)

Milton K. Morigaki^I,*; Elias M. da Silva^I; Carlos V. P. de Melo^I; Jamile R. Pavan^I; Renzo C. Silva^I; Armando Biondo^II; Jair C. C. Freitas^II; Gilson H. M. Dias^III

^IDepartamento de Química, Universidade Federal do Espírito Santo, Av. Fernando Ferrari, s/n, 29060-900 Vitória - ES, Brasil

^IIDepartamento de Física, Universidade Federal do Espírito Santo, 29060-900 Vitória - ES, Brasil

^IIIInstituto de Química, Universidade Estadual de Campinas, CP 6154, 13084-971 Campinas - SP, Brasil

ABSTRACT

FeBr₂ has reacted with an equivalent of mnt^2- (mnt = cis-1,2-dicyanoethylene-1,2-dithiolate) and the α-diimine L (L = 1,10'-phenantroline, 2,2'-bipyridine) in THF solution, and followed by adding of t-butyl-isocyanide to give [Fe(mnt)(L)(t-BuNC)₂] neutral compound. The products were characterized by infrared, UV-visible and Mössbauer spectroscopy, besides thermogravimetric and conductivity data. The geometry in the equilibrium was calculated by the density functional theory and the electronic spectrum by the time-dependent. The experimental and theoretical results in good agreement have defined an octahedral geometry with two isocyanide neighbours. The π→π^* intraligand electronic transition was not observed for cis-isomers in the near-IR spectral region.

Keywords: iron(II); isocyanide; DFT.

INTRODUCTION

Transition metal complexes with an oxidizing and a reducing ligand have the fascinating physical properties and electronic structures are frequently characterized by low-energy transitions between HOMO/LUMO. In suitable case, the involvement of the dithiolate (mnt) donor-ligand and of the α-diimine acceptor-ligand in the coordination sphere facilitates ready movement of electrons between HOMO and LUMO. For this reason is that some [M(α-diimine)(dithiolate)] square-planar transition metal compounds containing a mutually trans arrangement diimine and dithiolate ligands have multiple accessible redox states¹ with high degree of electron delocalization. Therefore, they can exhibit the common low-energy charge transfer term, which is a condition to be photocatalist chromophores for solar energy conversion and storage.² Iron-sulfur compounds with ancillary isocyanide ligands cause structural systematic changes with a concomitant strong effect on the chemical properties,^3,4 besides have an active center in biochemical processes,^5,6 could also display features of the proteins and [M(α-diimine)(dithiolate)] counterparts.

The work presented herein toward a comprehensive description of novel octahedral products with α-diimine chelating ligands is a continuation of our previous studies on the nature of the iron compounds containing mnt-isocyanide ligands.⁷

EXPERIMENTAL

Experiments were performed under argon atmosphere, using standard Schlenk techniques to avoid the oxidation process. The solvents were dried and distilled under O₂-free argon prior to use. The compounds t-BuNC,⁸ FeBr₂⁹ and Na₂mnt¹⁰ were prepared as previously described. The phen and the bipy were purchased from Aldrich and used as received.

IR spectra were obtained on a Midac Prospect FT-IR instruments, using Nujol mulls. Melting points were recorded on a Büchi 510 apparatus and are uncorrected. Elemental analyses (C, H, and N) were performed on a Perkin Elmer 2400 microanalytical instrument. Optical absorption spectra were obtained on Varian Cary 1, using THF solution. Conductivities were measured at 25 ºC, using a HANNA Instruments HI 8033 conductivity bridge and standard cell. For the Mössbauer measurements, the usual transmission geometry was employed, with an Ortec multichannel system PC board, using 512 channels, as the data counting. The gamma rays source was a nominal 10 mCi ⁵⁷Co in a Pd or Rh matrix and the isomer shift values are quoted relative to α-Fe. Thermogravimetric (TG) curves were recorded with a Shimadzu TGA-50H thermal analyser system. The samples with initial mass around 12 mg were heated in alumina crucible under oxygen flow (20 cm³ min^-1) at a heating rate of 10.0 ºC min^-1.

Preparation of [Fe(mnt)(phen)(t-BuNC)₂] (1)

Small portions of Na₂mnt (0.400 g, 2.15 mmol) and phen (0.387 g, 2.15 mmol) were added to a stirred solution of FeBr₂ (0.464 g, 2.15 mmol) in THF (45 cm³). To the stirring mixture was added t-BuNC (0.58 cm³, 5.2 mmol). After 2h, the resulting solution was filtered through a Florisil column and concentrated to ca. 4 cm³. Dropwise addition of diethyl ether (10 cm³) afforded a brown solid, which was washed with diethyl ether (3 x 3 cm³ portions), and vacuum-dried. Yield 40%. (Found: C, 56.5; H, 5.3; N, 15.5. C₂₄H₂₆S₂FeN₆ requires C, 57.6; H, 4.8; N, 15.5%). IR ν(CN) (Nujol): 2189.0 (m), 2129.2 (vs), 2094.5 (s) cm^-1. The Optic spectrum (10^-4 M, THF) 2.79 (s), 2.49 (sh) eV. The molar conductivity (10^-3 M, acetone) 30.7 Ω^-1 cm² mol^-1.

Preparation of [Fe(mnt)(bipy)(t-BuNC)₂] (2)

Reaction of FeBr₂ (0.263 g; 1.22 mmol) with Na₂mnt (0.227 g, 1.22 mmol) and bipy (0.190 g; 1.22 mmol) and t-BuNC (0.27 cm³; 2.44 mmol) as above gave 2 as a brown solid. Yield 47%. (Found: C, 53.9; H, 4.2; N, 15.9. C₂₂H₂₆S₂FeN₆ requires C, 55.6; H, 5.0; N, 16.2%). IR ν(CN) (Nujol): 2189.0 (m), 2142.7 (vs), 2110.0 (vs) cm^-1. The optic spectrum (2.2 10^-4 M, THF) 2.95 (s) eV. The molar conductivity (10^-3 M, acetone) 16.0 Ω^-1 cm² mol^-1.

Methods and calculations details

The DFT calculations have been performed with the versions ADF2006/2007 program packages of the Amsterdam Density Functional,^11,12 using the combination of exchange functional of Becke¹³ and the correlation functional of Perdew¹⁴ (BP86). Scalar relativistic effects have been considered using the zero-order regular approximation (ZORA).^15,16 The (1s2s2p)¹⁰ frozen core electrons for Fe and S atoms (ADF database TZP), and the (1s)² for C, and N atoms (ADF database DZP) were used in all calculations. In all cases, the subsequent steps after the geometry optimization in gas phase, were performed the electric field gradient (EFG) on the Fe atom and the analytical vibration frequency calculations. The investigations of UV-visible spectroscopic properties have been carried out with the TD-DFT (30 lowest spin-singlet-allowed transitions were taken into account) in gas phase. The metal-ligand energy interactions^17,18 of 1 and 2 with two isocyanides t-BuNC ligands have been proceeded with the ADF suite at the same BP86 level, using geometry optimized in gas phase.

The geometry optimizations and frequency calculations of 1 and 2 with a degree of similitude result with other program were also calculated by the Gaussian 2003W program (G03),¹⁹ employing the spin-restricted formalism of the hybrid functional B3LYP (Becke's three-parameter functional²⁰ with the LYP²¹ correlation functional), in conjunction with LANL2DZ²² basis set for all atoms. The optimized molecular geometries have confirmed the true energy minima by observation of only positive eigenvalues in the Hessian matrixes. The EFG and ρ₀(0) (electron density at the nucleus)²³ of 1 and 2 were carried out with the program package ORCA,²⁴ using for the Fe atom a TZV(P)²⁵ and for the other atoms a DZV(P) basis set.²⁶

RESULTS AND DISCUSSION

The elemental analysis data of the microcrystalline isolated products synthesized correspond to the formulation of 1 and 2, according to Scheme 1.

Molar conductivity values have defined their neutral nature, because are in the typical region of a non-electrolyte compound (30.7 and 16.0 Ω^-1 cm² mol^-1 in acetone for 1 and 2).²⁷ Since they are poor soluble and slowly decomposed in polar solvents, precluded measurements of the solvatochromic of absorption bands.

Mössbauer parameters (Table 1) of compounds have been a good indicator of oxidation state, covalence, and geometrical features.^23,28,29 In particular, ⁵⁷Fe-Mössbauer spectra of various octahedral complexes with the alternative trans configuration show a quadrupole splitting (ΔE_Q) twice as big as the ones given by complexes in the cis configuration.^30,31 Therefore, ΔE_Q are assigned the cis position of two isocyanides in an octahedral environment. Both cis-isomers and that the two forms of cis and trans isomers could not coexist in the material described above.^32,33

The isomer shifts (δ) (Figure 1) have compatible values of a diamagnetic Fe(II) atom, mainly when they are plotted in a described standard curve,²³ or compare simply to the higher value for the FeBr₂ precursor (1.34 mm s^-1).³⁴

Acceptable IR data have been extensively used for understanding of many inherent molecular properties,³⁵ mainly for structure prediction.^36-39 Since IR bands were calculated of the cis-isomer can distinguish from the alternative trans configuration (Table1). The experimental of 1 and theoretical IR spectra in the CN stretching frequency region of trans and cis isomers were compared in Figure 2. Two intense CN stretching bands of near intensity at 2129 and 2095 cm^-1 indicate that isocyanide ligands reside at the cis disposition on the Fe(t-BuNC)₂ molecular fragment in the IR experimental spectra. The another ν(CN) bands at 2189 cm^-1 is near of the values observed of mnt free, and isocyanide (2134 cm^-1) precursors.⁴⁰ Similar trend is observed for 2 with the bipy ligand shifts the two ν(CN) isocyanide bands to higher wavenumber at 2143 and 2110 cm^-1. Since bipy is the better electron-deficient nature than the phen ligand, there is a decreased in the degree of the back *π-bonding of Fe to isocyanides. More complete IR spectra of compounds 1 and 2 (Figures 1S and 2S, respectively) are available in supplementary material.

The thermogravimetric fragmentation of 1 and 2 occurs with a simultaneous loss of the two isocyanides at the same temperature (109.5 and 138.9 ºC, respectively), according to the Scheme 2 and (Table 2). Since the calculation of the metal-ligand interaction energy is useful for describing the nature of the chemical bonding,^41,42 there is reasonable agreement between TG measures with t-BuNC moieties trans to Fe-N and Fe-S bonds in the cis-1 and cis-2 structural models, dissociating from the coordination sphere around the same energy (-68 kcal mol^-1).

Although slight the difference thermodynamic stabilization energy between optimized geometries (Figure 3 and Figure 3S), the data suggests that the pseudo octahedral Fe(II) arrangement has two mutually neighbors isocyanides for 1 and 2 in the cis-isomer (7 kcal mol^-1 for L = phen and 6 kcal mol^-1 for L = bipy). The cis conformation of isocyanides maximizes the π* back-bonding, because are pointed mainly at d_x, dy, and d_z orbitals direction, while the trans-isomer the orbitals are mainly in d_z² and d_x²_-y² direction. The bond lengths average of Fe-C, Fe-N and Fe-S values fit well with those experimentally obtained in the literature^{43-44454647484950} for [Fe(S₂C₆H₄)₂ ]^2- (2.205 Å of Fe-S),⁵¹ [{Fe(mnt)₂}₂]^2- (2.230 Å of Fe-S).⁵² Furthermore, the reaction has proceeded in a short time,⁵³ and the intrinsic reaction coordinate calculation of 1 show some rupture of the ligand in the compounds, what suggesting that the cis-trans isomerism process is difficult to happen.

The UV-visible absorption spectra of 1 (Figure 4) and 2 (Figure 4S) in THF are dominated by intense transitions in the visible region, and undergo a small red shift when bipy (2.95 eV) is replaced by phen (2.79 eV). The results of calculations and experimental data are good. The TD-DFT formalism was used for determining the electronic contributions to MOs, in order to assign the absorption bands and verify preferred isomer formation.The experimental 1 and theoretical cis-1 spectra were combined show two main spectral features with a diffused shoulder of the band I at range 2.53-2.25 eV in the visible region and a more intense band II at 2.79 eV.

Energies, frontier orbitals, and the character of the calculated bands of iron cis-1 and cis-2 isomers are summarized in Table 3 and illustrated in Figure 5. The strongest absorption bands d →π* charge transfer are largely located on Fe orbitals from all frontier orbitals with increased in sizable contributions from sulphur atoms p orbitals on dithiolate donor with a simultaneous decreased in percentages from the Fe-localized d orbitals in two highest-occupied HOMO/HOMO-1 orbitals. May be, the mixture of Fe and dithiolate orbitals force the HOMO orbital nature becomes quasi-nonbonding or antibonding. The back-donation are to three lowest-unoccupied LUMOs that have essentially characters of π* acceptor orbitals on the diimine carbon atoms (L). The allowed back-donation to isocyanides is mainly from HOMO orbital to the highest energies LUMO-3/LUMO-4.

The main difference observed between cis and trans isomers is that the HOMO d → d transitions in the cis-isomer is preferentially dominated by Fe orbitals, reaching to high levels LUMOs. However, the trans-isomer has L and Fe orbitals in degenerate or quasi-degenerate levels in the LUMO/LUMO+1.

CONCLUSION

In conclusion, we have synthesized and characterized novel complexes of formula [Fe(mnt)(L)(t-BuNC)₂] (L = phen, bipy) from the reaction of FeBr₂ with dithiolate, α-diimine L, and isocyanide reagents in THF at room temperature. The electronic, structural and spectroscopy proprieties of the product have been experimentally investigated for a better characterization, providing a good agreement by means of combined DFT theoretical calculations. The main result has defined an octahedral structure with a cis configuration of the two isocyanide ligands around iron centre in [Fe(mnt)(L)(t-BuNC)₂] complexes. Furthermore, the interplay between from mixed dithiolate-to-ligand and metal-to-ligand transitions instead of the typical MLCT transition should be of great fundamental interest.

SUPPLEMENTARY MATERIAL

The Figures 1S to 4S, showing infrared spectrum of 1 and 2 products in 2800-600 cm^-1 region, optimized structure for cis-2 and trans-2 isomers and absorption spectrum of cis-2 isomer are available on http://quimicanova.sbq.org.br.

ACKNOWLEDGMENTS

This work was supported by the CNPq and FAPESP.

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Recebido em 12/9/08; aceito em 8/4/09; publicado na web em 26/8/09

MATERIAL SUPLEMENTAR

Publication Dates

• Publication in this collection
  22 Oct 2009
• Date of issue
  2009

History

• Accepted
  08 Apr 2009
• Received
  12 Sept 2008