Elsevier

Polyhedron

Volume 52, 22 March 2013, Pages 1081-1089
Polyhedron

Ligand-centered vibrational modes as a probe of molecular and electronic structure: Raman spectroscopy of cis-Fe(1,10-phenanthroline)2(NCS)2 and trans-Fe(pyridine)4(NCS)2 at variable temperature and pressure

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Abstract

Raman spectra of cis-Fe(1,10-phenanthroline)2(NCS)2 and trans-Fe(pyridine)4(NCS)2 are recorded at variable temperature and pressure and used to characterize these complexes via the vibrational modes of the [NCS] ligands. Vibrational frequencies of the [NCS] anion in KNCS are 2052, 748 and 484 cm−1 for the ν(C–N) stretching, ν(C–S) stretching and δ(NCS) bending modes, respectively. The two stretching frequencies show average increases of 20 and 60 cm−1 upon coordination to high-spin iron(II) ions in both complexes. For low-spin iron(II), observed for cis-Fe(1,10-phenanthroline)2(NCS)2 at 83 K or 15 kbar, the ν(C–N) stretching frequency is 2114 cm−1, an increase by approximately 40 cm−1 compared to the high-spin complex. This frequency can therefore be used as a sensitive probe, even if C–N bond lengths as determined by X-ray diffraction are identical within experimental precision for all conditions. The frequencies of [NCS] ligands show pressure-induced changes of +0.16 and +0.20 cm−1/kbar for the ν(C–N) modes of high-spin and low-spin cis-Fe(1,10-phenanthroline)2(NCS)2, and +0.31 cm−1/kbar for high-spin trans-Fe(pyridine)4(NCS)2.

Graphical abstract

The Raman-active ν(C–N) stretching modes of thiocyanato ligands in cis and trans-FeN4(NCS)2 complexes are a sensitive probe for structure and spin-state at variable pressure and temperature.

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Introduction

Stereoisomerism of six-coordinate, octahedral transition metal complexes is one of the experimental concepts at the origin of coordination chemistry and was one of the foundations of Alfred Werner’s pioneering experiments on isomers [1], leading to correct molecular structures before modern spectroscopic and structural techniques were available. Over the last century, a wide variety of quantitative measurements have provided additional insight on both the molecular and electronic structures of coordination compounds, of high relevance for many different fields. Examples include bioinorganic chemistry, where significant new insight is gained from Raman spectra [2] or the use of optimized isomers in solar cells [3], with the six-coordinate cis-RuL2(NCS)2 complex, where L denotes 2,2′-bipyridyl-4,4′-dicarboxylic acid, used as one of the most promising dyes in photovoltaic cells, an illustrative application of a complex with thiocyanato ligands. Detailed insight can be gained from the correlation of structural data with spectroscopic information, both in the ground and excited electronic states [4], [5].

cis/trans isomers of octahedral complexes are among the earliest documented examples of stereoisomerism, as illustrated by the first schemes of reference [1]. In addition to the two isomers, some complexes show spin transitions, providing a particularly attractive field for comparisons of structures and spectra. Spin-crossover phenomena are observed for six-coordinate first-row metal ions with 3d4, 3d5, 3d6 and 3d7 electron configurations, and transitions between the high-spin and low-spin forms are often induced by temperature changes [6], [7], [8], [9]. The two spin states have different magnetic moments, and their measurements have been extensively used to characterize spin transitions, but the two forms also have distinctly different structures that can be characterized by X-ray diffraction [10], [11], [12], [13], [14], [15], [16], revealing, e.g. significant metal–ligand bond length differences. Many spectroscopic methods, such as Mössbauer [17], [18], [19], [20], [21], [22], UV–Vis absorption [23], IR [24], [25], [26], [27], [28], Raman [26], [27], [29], [30], [31], [32], [33], nuclear magnetic resonance [34], electronic paramagnetic resonance [35], and XANES [36] techniques, have also been used to characterize spin-crossover systems. Spin transitions at variable temperature can be gradual [15], [37], abrupt [10], [38] or occur in steps [39], [40], [41], depending on interactions and cooperative effects in the solid state [42]. The structural changes induced by a spin transition can be characterized through shifts of vibrational frequencies. Typical metal–ligand bond lengths for iron(II) complexes change on the order of 0.1–0.2 Å, leading to a shift of up to 200 cm−1 for metal–ligand stretching frequencies [43], [44], [45]. The frequency changes of ligand-centered vibrational modes are smaller than those for metal–ligand modes, but are often better resolved and easier to identify in the vibrational spectra. We illustrate this in the following using the vibrational frequencies of thiocyanato ligands in two iron(II) complexes. Their ν(C–N) stretching frequencies are used as a probe for metal–ligand bonding, stereoisomerism and spin state [25], [31], [45], [46].

The spin transition in Fe(1,10-phenanthroline)2(X)2 complexes, where X denotes a halide or pseudo-halide such as [NCS], has been extensively characterized over the past half century [10], [15], [25], [27], [31], [34], [47], [48], [49], [50], [51], [52], [53]. cis-Fe(1,10-phenanthroline)2(NCS)2 1, schematically shown in Fig. 1, is one of very few complexes for which crystal structures have been reported at variable temperature and pressure [10], [15], [16]. In the following, we correlate aspects of these structures with vibrational Raman data. The ν(C–N) stretching frequency of [NCS] ligands in 1 is approximately 2070 cm−1 for the high-spin form and shifts to approximately 2110 cm−1 for the low-spin form, a significant frequency shift of approximately 40 cm−1 [31], and a sharp spin transition is observed at 176 K [10], [31] and in the pressure range between 4.5 and 13 kbar [36]. Vibrational frequencies for this complex have been calculated by a variety of DFT techniques with geometry optimization [26], [27], [54], [55]. trans-Fe(pyridine)4(NCS)2 2, schematically shown in Fig. 1, has not been studied by Raman spectroscopy. Its structures at room temperature and low temperature are published [14], [20] and do not show evidence for a spin transition. At variable pressure, XANES spectroscopy indicates the onset of a continuous spin transition starting at approximately 30 kbar [36].

We present Raman spectroscopy at variable temperature and pressure for both compounds illustrated in Fig. 1. Spectra recorded at variable conditions reveal characteristics specific to cis/trans isomerism, to metal–ligand bonding and to spin-crossover characteristics.

Section snippets

Spectroscopic methods

Raman spectra were recorded with a Renishaw inVia imaging microscope system. A 782 nm diode laser was used as excitation source. The excitation light was focused through the microscope onto a spot of approximately 1 μm in diameter. The scattered light was collected by the microscope and detected with a Peltier cooled CCD camera. Sample temperatures were controlled between 83 and 298 K with a Linkam microscope cryostat, the sample is placed in a nitrogen gas atmosphere preventing oxidation. All

Raman spectra at variable temperature

Variable-temperature Raman spectra for complexes 1 and 2 are presented in Fig. 2, Fig. 3, respectively. The Raman spectra between 180 and 2200 cm−1 show many obvious changes with temperature for complex 1. Significant variations of Raman intensities at low and high temperature and shifts of band maxima are illustrated in Fig. 2, indicative of a spin transition. The spin transition for 1 has been characterized previously by many methods [10], [15], [16], [26], [27], [31], [34], [48], [51], [52],

Vibrational frequencies of the [NCS] ligands

The frequencies of the three vibrational modes of [NCS] ligands are easily measured by Raman spectroscopy. They are given in Table 1 for different conditions and are, in decreasing order of frequency, ν(C–N) stretching, ν(C–S) stretching and δ(NCS) bending. All three modes show characteristic frequency changes upon coordination to a transition metal center, as determined by the comparison to uncoordinated [NCS] in KNCS and for coordinated ligands in Table 1. The ν(C–N) stretching frequency

Conclusion

The Raman spectra of cis-Fe(1,10-phenanthroline)2(NCS)2 and trans-Fe(pyridine)4(NCS)2 at variable temperature and pressure are presented and analyzed. The vibrational frequencies of the [NCS] ligands can be used to characterize metal–ligand bonding, cis/trans isomerism and spin state of the complexes. Published crystal structures at ambient conditions, low temperature and high pressure are used to correlate the ν(C–N) stretching frequencies of the [NCS] ligands and the Fe–NCS bond lengths.

Acknowledgments

Financial support from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. We thank Karim Zarkout for helping to complete this study and Dr. Tomáš Bučko, Comenius University, Bratislava, for sharing calculated vibrational frequencies from Ref. [54].

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    Dedicated to Alfred Werner on the 100th Anniversary of his Nobel Prize in Chemistry in 1913.

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