During the past decade, Raman spectroscopy has moved out of the shadow of IR spectroscopy and has become a routine tool for characterization. The cost of the instrumentation is decreasing rapidly. With its ease of operation, lack of sample preparation, and rapid analysis time, high- quality FT-Raman spectra have been reported for many colored, main-group metal, transition- metal coordination, and organometallic complexes. Air-sensitive compounds can easily be analyzed in sealed ampoules, because as mentioned earlier, glass is a weak Raman scatter and so does not interfere with the resulting spectra. As the field of inorganic chemistry continues to grow, so do the uses of FT-Raman spectroscopy for analysis. Below are three examples of current uses of the FT-Raman technique in inorganic field in inorganic chemistry.
2.8.3.2 Active Homogeneous Catalysis Identification
Heterogeneous catalysis is an important area, both in academic and industrial settings, with the characterization of the catalytic process continuing to be a subject of considerable interest. Often these catalysts are metals, such as palladium or rhodium, or metal oxides, such as vanadium oxide. Vibrational spectroscopy, and in particular FT-Raman spectroscopy, has emerged as a powerful tool for the identification and characterization of the active catalyst. And, in cases where the catalyst is adsorbed on a heterogeneous substance, the effect that this solid phase has
100 80 60 40 20 0 80 60 40 20 0 80
800 60
600 40
400 20
200 0
647.1 nm
647.1
568.2 nm
568.2
514.5 nm
514.5
5υ1 4υ1 3υ1 2υ1 υ1 hυ1
(cm–1)
(a) (b)
υ = 5 υ = 4 υ = 3 υ = 2 υ = 1 υ = 0
400.0 500.0 600.0 λ (nm)
Optical Density
Intensity (arbitrary units)
Resonance Raman Preresonance Raman Normal Raman
Figure 2 Raman spectra of TiI4 in cyclohexane solution excited with different laser lines. Adapted from Butler, I. S.; Harrod, J. F.Inorganic Chemistry: Principals and Applications, Figure 6.15 (reproduced by permission
of The American Chemical Society from Clark, R. J. H.; Mitchell, R. D.J. Am. Chem. Soc.1973,95, 8300).
106 Raman and FT-Raman Spectroscopy
on the active catalyst and the catalytic cycle can be determined. The feasibility of FT-Raman spectroscopy for this purpose was recently demonstrated by van der Polet al.10who characterized the 2,20-bis-(inden-2-yl)-biphenyl-zirconium dichloride catalyst (A), supported on silica, and in the presence of a co-catalyst, methylaluminoxane (MAO). Metallocenes are particularly useful in the industrial-scale polymerization of alkenes and are unique in this application because the active catalyst is comprised of only one metal atom. In the catalysis of olefin polymerization by the simpler system, Cp2ZrCl2/MAO (Cpẳ5-C5H5), the co-catalyst activates the metal center of the precatalyst by methylation, yielding the dimethyl metal fragmentC, followed by further reaction with MAO to give the cationic monomethyl complexD(Scheme 1).
Zr Cl Cl
(A)
Zr CH3 CH3
Zr CH3 Zr
Cl
Cl MAO MAO
+
(B) (C) (D) Scheme 1
In characterizing the mechanistic intermediates involving complexA, peak assignments for the ZrCl, ZrCH3(neutral dimethyl metal fragment), and ZrCH3(cationic monomethyl metal frag- ment) stretching vibrations were made by analogy to those involving complexBusing a combina- tion of FT-Raman spectroscopy andab initioquantum mechanics calculations. After assignment of these vibrations, which occur in the spectroscopic region below 500 cm1, the calculated stretching frequencies were compared with the experimental values for the pure complexes B andCin order to validate this method. When the spectra of complexAadsorbed onto the MAO/
silica support were collected, useful data on the vibrations of the active site of the metallocene (i.e., ZrCl and/or ZrCH3vibrations) were obtained even though the transition metal was only present at a concentration of about 0.12–0.40 wt.% (Table 1). To ease the analysis of the spectrum of the supported catalyst system, individual spectra of MAO, MAO on silica, the unsupported catalyst, the supported catalyst, and biph[2-Ind]2ZrCH3were recorded (Figure 3). Two key results were obtained using FT-Raman spectroscopy to characterize this system. The first was that the catalyst loading on the solid could be quantitated because the ratio of the peak integration for the Zr complexes (spectral region 1,517–1,489 cm1) to the integration of the peaks representing total MAO loading (spectral region 3,021–2,746 cm1) correlated very well to the actual ratio loading of the Zr and MAO loading (R2ẳ0.9991). Finally, the catalytic yield of the catalyst in the olefin polymerization correlated very well with the intensity of the ZrCH3 stretching frequency at 454 cm1. Thus, the quality of the catalyst in terms of the catalytic yield can be determined using FT-Raman spectroscopy.
2.8.3.3 Metal–Ligand Bond Strengths in Tellurium(II) Thiourea Complexes
Structural information, such as bond strength and intermolecular bonding, can be obtained with the aid of vibrational spectroscopy. Many compounds, which have been thoroughly studied by IR spectroscopy, are being now examined by the FT-Raman technique. Remaining ambiguous or uncertain assignments from the IR experiments can be elucidated with the use of FT-Raman spectroscopy. An interesting example of this application is the recent FT-Raman spectroscopic investigation of a series of cis-bis(thiourea)tellerium(II) halide and thiocyanate compounds (E) by Alı´aet al.11These compounds have a distorted square–planar configuration with identical ligands in acisconfiguration.12,13At first glance, the analysis of these compounds by FT-Raman spectros- copy would seem to be a routine task. For example, analogous to the IR studies, the 3,500–3,000 cm1 region of the spectra is dominated by the thiourea NH stretching vibrations.
However, the power of FT-Raman spectroscopy as an analytical tool for characterization lies in the analysis of the low-frequency region of the spectra (i.e.,<400 cm1). This region is relatively simple and consists mainly of the Te–S, Te–X, and lattice mode vibrations (Figure 4andTable 2).
S C(NH2)2 Te
S
X X
C(H2N)2
(E) X = Cl, Br, I or SCN Intensity (a.u.)
Zr Cl2 complex 9141,001
1,056 1,369
1,463 1,503
1,593
455
Zr (CH3)2complex MAO/silica
Zr Cl2 complex on MAO/silica
(a)
(b) (c)
(d) 200 400 600 800 1,000 1,200 1,400 1,600
Raman shift (cm–1)
Figure 3 FT-Raman spectra of biph[2-Ind]2ZrCl2 (a) biph[2-Ind]2Zr(CH3)2, (b) MAO/silica, (c) and of biph[2-Ind]2ZrCl2on MAO/silica (12.8 wt.% Al and 0.4 wt.% Zr) (d). Many peaks due to the indenyl ligand are insensitive to the silica support; the strongest peaks of the indenyl ligand have been labeled in trace a. In the spectrum of the Zr(CH3)2complex (b) the peak due to the symmetric ZrCH3stretching vibration has been labeled (455 cm1) (reproduced by permission of Elsevier fromJ. Organomet. Chem.2002,651, 80–89).
Table 1 Assignmentof MCl and MCH3vibrations.
Wavenumber (cm1) Assignment
1,105 CH3specific, ‘umbrella mode’
794 Cl specific
454 (ZrCH3)
321 asym(ZrCl)
308 sym(ZrCl)
108 Raman and FT-Raman Spectroscopy
The assignments reported in this study for the Te–S and Te–X stretching vibrations are the reverse of those published earlier for the chloride compound, where the higher frequency vibra- tion was assigned as the Te–X stretching vibration and the Te–S stretching vibration was attributed to the lower frequency vibration.14 In the compounds examined in this study, the TeX bonds are considerably longer than are the TeS bonds (Table 3). This observation led the authors to assign the higher frequency vibration to the symmetric Te–S stretching mode in the decreasing order Cl>SCN>Br>I, corresponding to the increasing TeX bond lengths from X-ray crystallographic data. A comparison of the FT-Raman spectrum of thiourea with those of
Iodide
Bromide
Thiocynate
Chloride
300 200 100
Wavenumber (cm–1)
Figure 4 FT-Raman spectra of thiourea and its complexes with tellurium(II) in the 350–60 cm1region.
Table 2 FT-Raman spectral assignments of tellurium(II)–thiourea complexes in the 350–60 cm1region.
Thiourea Assignment Thiocyanate Chloride Bromide Iodide
(TeS) 266 276 258 232
(TeS) 253 262 250
(STeS) 178 192 184 175
(TeX) 162 162 150 139
(XTeX) 143 144 137
Lattice Mode 124 124 127 124
117 Lattice Mode 117 118
Lattice Mode 111 109 111 111
101 Lattice Mode 103 98 101 103
Lattice Mode 92 91 93 92
84(sh) Lattice Mode 83 84 79 83
the complexes facilitated the assignment of the lattice modes. In this work, the use of FT-Raman spectroscopy has greatly complemented the existing IR data, but it also has provided new insight into the metal–ligand vibrational modes. As mentioned earlier, the low-frequency vibrational region is of particular interest to the synthetic inorganic chemist.
2.8.3.4 Metal–Carbon Bond Strengths in Metal–Alkene Complexes
The v(CO) stretching modes of metal carbonyls have often been used to compare the donor strengths of ligands coordinated to the metal center in such complexes. However, little effort has been made to examine, by FT-Raman spectroscopy, the effect of donor ligands on the strengths of the corresponding M—C bonds. This dearth of data has prompted Moigno and co-workers15 to investigate the trans effectobserved for a series of rhodium halide complexes, where a potentially labile ligand is located trans to the halide, which is varied in the series. The trans effect is defined as ‘‘the effect of a coordinated ligand on a metal center upon the rate of ligand substitution involving ligands positioned trans to the first ligand’’. As part of this study, a comparison of the FT-Raman spectra of trans-[RhF(CẳCH2)(PiPr3)2] and trans- [RhF(13Cẳ13CH2)(PiPr3)2] was undertaken, which resulted in the assignment of the v(RhẳC) and v(Rhẳ13C) vibrations and showed thatv(Rhẳ13C) is of higher intensity and isolated from other vibrations in complexes of this type, whereas the v(RhẳC) frequency is overlapped with vibrations of the tertiary phosphine ligand.15 Therefore, Moigno and co-workers measured the FT-Raman spectra of a series of 13C-labeled complexes of the general formula, trans- [RhX(13Cẳ13CH2)(PiPr3)2], where XẳF, Cl, Br, and I.16 This particular series of complexes facilitated an investigation of the electronic influence of the ligand X trans-disposed to the
13Cẳ13CH2 ligand on the strength of the Rhẳ13C bond, since the vibrational mode for a bond is related to its force constant, which is in turn closely related to its bond strength. The corresponding phenyl-substituted vinylidene and carbonyl rhodium(I) complexes were also examined Table 3 Te—S and Te—X (XẳCl, Br, I, or SCN) bond lengths obtained from X-ray crystallographic data
(from refs.12 and13).
Bond Type Bond Length(A˚)
Te(tu)2Cl2a Te(tu)2Br2 Te(tu)2I2 Te(tu)2SCN2
Te—S 2.457 2.476 2.521 2.458
Te—X 2.936 3.038 3.162 3.039
a tuẳthiourea
Table 4 (Rhẳ13C) or(RhẳC) frequencies for complexes 1–12.
(Rhẳ13C) (cm1)
(RhẳC) (cm1) trans-[RhF(13Cẳ13CH2)(PiPr3)2](1) 559
trans-[RhCl(13Cẳ13CH2)(PiPr3)2](2) 551 trans-[RhBr(13Cẳ13CH2)(PiPr3)2](3) 548 trans-[RhI(13Cẳ13CH2)(PiPr3)2](4) 540
trans-[RhF(ẳCẳCHPh)(PiPr3)2](5) 579
trans-[RhCl(ẳCẳCHPh)(PiPr3)2](6) 574
trans-[RhBr(ẳCẳCHPh)(PiPr3)2](7) 573
trans-[RhI(ẳCẳCHPh)(PiPr3)2](8) 572
trans-[RhF(CO)(PiPr3)2](9) 573
trans-[RhCl(CO)(PiPr3)2](10) 559
trans-[RhBr(CO)(PiPr3)2](11) 555
trans-[RhI(CO)(PiPr3)2](12) 546
110 Raman and FT-Raman Spectroscopy
to explore the trans effect on other potentially labile ligands. The v(Rhẳ13C) or v(RhẳC) frequencies are listed in Table 4. A number of interesting conclusions can be drawn from the trends observed in the FT-Raman data. First, for the vinylidine complexes (1–4), the frequency of thev(Rhẳ13C) vibration increases as the ligand is varied in the order F>ClBrI, which implies that the bond strength increases in the same direction. Throughout this study, the experimental results and the conclusions drawn were also supported by density functional theory (DFT) calculations. The data derived from the calculations showed that the trends observed were primarily due to the change in the Rhẳ13C bond lengths and not due to a mass effect or to coupling of the Rh–X and Rhẳ13C modes. In the related phenyl-substituted vinylidine complexes (5–8), the trend in bond strength as X is varied is essentially the same. Thus, it was concluded from the FT-Raman data for complexes (1–8) that the order of increasingtranseffectof the coordinat- ing ligand followed the sequence I>Br>Cl>F. Interestingly, when the CO complexes (9–12) were compared with the isoelectronic complexes (1–4) and also complexes (5–8), an unexpected observation was made. While thev(Rh-C) frequencies in the series (9–12) increased in the same manner as for compounds (1–8), when thetransligand was varied, thev(CO) frequency increased in compounds (9–12), while that ofv(13Cẳ13C) in compounds (1–4) (or thev(CẳC) vibration in complexes (5–8)) decreased as the trans ligand was varied in the order F!Cl!Br!I. These results are explained by an increased-donor capability of X as one goes up the periodic table in group 17, and supporta push–pull-interaction as an explanation for why the strongest Rh—C bond is formed with fluorine, the most electronegative substituent. Taken together with the DFT calculations, which are not discussed here, the work conducted by Moigno and co-workers is an excellentexample of justhow FT-Raman spectroscopy can be an invaluable source of experi- mental physical data to help explain chemical phenomena.