Peak Performance– Illustrating Selected Physical Methods

There are a number of techniques that rely on applying specific protocols that lead to excitation and a resonance condition reported experimentally as the appearance of a peak(s) in a spectrum by using appropriate instrumentation. It is beyond the scope of this textbook to develop the theory behind these many methods, but it is appropriate to illustrate two of the most common techniques– IR and NMR spectroscopy. NMR relies on change in nuclear spin through absorption of energy in the MHz range, where each atom of a particular element in a compound in a unique molecular environment will yield a signal in a unique position reported as a chemical shift (δ).Moreover atoms may interact with close neighbours in many cases to produce a splitting pattern superimposed on the gross chemical shift that is characteristic of this neighboring environment. This makes the technique powerful as a weapon in determining structure, particularly in solution. The majority of elements can produce an NMR spectrum, but the traditional element targeted is hydrogen, because 1H is of high isotopic abundance, has simple spin characteristics, and the highest sensitivity. Another popular element used, 13C, has a relative sensitivity of only ∼1.6% compared with 1H, and a significantly lower isotopic abundance (only ∼1%) so that its detection is consequently more demanding. Even the metals in complexes can be examined directly but suffer from low sensitivities and/or inappropriate isotopic abundance as well as other limitations that can lead to very broad peaks and extreme chemical shifts. Never the less modern NMR instruments offer adequate to excellent determination of spectra of a vast number of elements although 1H, 13C, 19F and 31P remain most commonly available in commercial instruments, and dominantly the former two are used. This means that, for coordination complexes, it is usually the organic ligands that are being probed by this technique rather than the metal or even the ligand heteroatoms. The MRI instrument used for whole-body scanning in medicine is a type of NMR instrument but focussed on variation of the properties of water molecules in different environments as the basis of its operation. To illustrate the NMR technique very simply, we shall draw on some simple complexes. The NMR method is most applicable to diamagnetic metal complexes, and we shall re strict examples to low-spin d6 Co(III) which has no unpaired d electrons and is therefore diamagnetic. To remove strong signals from the solvent H atoms, spectra are routinely measured in a deuterated solvent in which D atoms replace all H atoms; further, by using an aprotic solvent like CD3CN any H/D exchange issues met for molecules with readily exchangeable centres like NH and OH in the common NMR solvent D₂O are re moved. If we consider highly symmetrical [Co (NH₃)6]³⁺ all six ammonia ligands are in equivalent environments, and so the 1H NMR spectrum should yield a single peak with one specific chemical shift. If we turn to [CoCl(NH₃)₅]²⁺ the four ammonia ligands around the plane of the metal are equivalent, but the one trans to the chloride ion is unique, so two peaks of different chemical shifts should result, in a ratio of 4:1. For [CoCl₂(NH₃)₄]⁺ the trans isomer has all four ammonia ligands equivalent and thus one peak whereas the cis isomer has two different types two opposite chloride ions and two opposite other ammonia molecules, so two peaks in a ratio of 1:1 will result (Figure 7.1). For fac- and mer-[CoCl₃(NH₃)₃], the former will yield a single peak, the latter two peaks in a 2:1 ratio. Thus you may see how molecular symmetry is being defined by the NMR pattern. If we were to replace the NH3 ligands by CH₃NH₂ ligands, we would get two sets of peaks

Figure 7.1

Molecular shapes and 'H chemical shift patterns for chloro-ammine cobalt(III) compounds. Different environments for ammonia molecules in some complexes are defined by eq and ax subscripts, with the two types opposite different types of ligand leading to different magnetic environments and chemical shifts (8).

in different chemical shift regions due to HN and Hc types of protons, but the patterns for each in the absence of any coupling between centres would be identical. Were we to record the 13C NMR spectra (rather than 'H) of the coordination complexes, the pattern would remain the same, but in this case report the different environments of just the carbon centres, spanning a different chemical shift range (~200 ppm) than observed for protons (~10 ppm). For coordination complexes including organic molecules as ligands, the 'H and 13C NMR spectra in combination present a powerful technique for probing complex stereochemistry and ligand character. In a simple sense, every unique carbon will have a unique chemical shift, so that a simple count of peaks (discounting any fine structure from coupling to any neighbouring protons) provides a good start to structural assignment, draw- ing on symmetry considerations to assist. The NMR technique is highly advanced, with modern spectrometers offering a range of methodologies designed to assist in structural elucidation; many of these are more often met in organic chemistry.

Infrared spectroscopy is based on absorption of IR radiation associated with molecular vibrations, such as bond stretching and bond angle deformation. Again, these specific processes lead to resonances and tied absorbance peaks in specific positions characteristic of the molecule or its components. IR spectra in modern Fourier-transform instruments may be reported as absorbance spectra or the inverse transmittance spectra. For simple inorganic complexes, the molecular symmetry allows definition of expected vibrations based on the whole molecule. For more complex molecules, it is the parts of the whole, particularly organic functional groups, that are more likely to be detected and identified. For example, [Co(NH₃)₆]³⁺, of Oh, symmetry, and [CoCl(NH₃)s]²⁺, of C₄, symmetry, will produce inherently different spectra, but it is really the peaks associated with motions of the ammonia molecules themselves (asymmetric and symmetric stretching, twisting, scissoring, rotating and wagging) that dominate and are reported in the region accessible in most IR spectrometers (4000-400 cm-1). When nitrite ion is introduced as a ligand, it can be O-bound or N-bound, and the Co-O-N-O isomer is of different bonding mode and symmetry to the Co-NO2 isomer, and thus not surprisingly produces a different IR spectrum (Figure 7.2). Once more, as for NMR, it is the molecular components present as ligands that are commonly probed in this technique. Since the technique relies on vibrations single atoms or ions (like Cl) cannot themselves give an IR band; however, vibrations involving the Co-Cl bond will occur (though below 500 cm well away from most

Figure 7.2

Shapes and IR spectra patterns for chloro, O-nitrito and N-nitrito cobalt(III) ammine compounds. The thick lines represent the vibrations associated with the coordinated ammonia molecule whereas the thin lines represent the vibrations associated with the linkage isomer of nitrite (two and three bands for N- and O-bound isomers respectively).

ligand vibrations). Basically the heavier the atomic masses of two atoms joined in a bond, the higher energy will be required to cause a stretching motion, for example. This means that the position of vibrational bands reflects the type of atom (particularly atomic mass) involved in a bond, so, for example O-H N-H and C-H stretching vibrations will occur at slightly different positions.

Infrared spectroscopy can also provide information about ligand binding character. This is readily illustrated with carbon monoxide as a ligand, since upon coordination to a transition metal the CO bond is weakened, and this is seen as a shift in the position of the stretching vibration (Table 7.5). Two sets of data illustrate the effect: on the left the effect of increasing the number of metals of one type to which the CO is bound is seen, with the bond becoming progressively weaker as the number of metals bound rises; on the right, the effect of increasing the formal charge on the central metal is shown with again a relationship between metal oxidation state and coordinated ligand bond strength apparent. Electrospray ionization mass spectrometry (ESI-MS is a tandem technique; it employs an electrospray ionization device (ESI) to produce 'bare' ions in the gas phase from a supplied (usually aqueous) solution, and supplies these ions as a very dilute gaseous stream to the high vacuum chamber of a mass spectrometer, which is the analyser. This analyser 'sorts' and detects ions by mass/charge ratio (m/z), as cations or as anions and will report either on request. For coordination complexes, the ESI process can be sufficiently 'soft'

Figure 7.3

Schematic representation of the ESI-MS spectrum of compounds present in a copper(II) ion and 1.2-ethanediamine (1 : 2) mixture in water. Peaks are defined in terms of m/z (mass/charge ratio).

so as to allow cations and anions of complex species in solution to travel into the analyser in their original complex form, providing a method for detecting the mass of complex species that exist in the solution environment, which can assist in defining their structure. This capacity to define the mass of complex ions, and identify the presence of several different complexes in an originally usually dilute aqueous solution, is a key application of ESI-MS. Moreover, where ESI conditions lead to some decomposition of complex species, the pattern of products observed can assist in identifying the character of the parent species and the way it undergoes change. Where a metal exists as a mixture of several isotopes, a set of peaks for a particular complex species related in pattern to the different isotopic metal centres will result, and this characteristic pattern helps confirm the presence of the metal in a complex species, and even of several metals in an oligomer.

To illustrate the ESI-MS technique, we'll look at a simple example, involving complexes formed in solution between Cu²⁺ aq and 1,2-ethanediamine (en). We know from potentio- metric and spectrophotometric titrations that two complex species form dominantly during reaction, the 1:1 Cu(en)2+aq and 1:2 Cu(en)₂²⁺ aq complex ions, and even know the stability constants for these species. In fact, careful and slow addition of en to Cu2+aq in a beaker allows one to see the colour changes as we step from dominantly Cu²⁺ aq to successively the 1:1 and then 1:2 M:L species. Thus this is a well understood system, and suited to illustration and evaluation of the ESI-MS method. When a dilute solution of a 1:2 mixture of copper ion and en ligand is passed into the ESI-MS tandem instrument, a set of cations of different mass are detected (Figure 7.3). From the masses of these ions, we can assign the peaks to (en)H+, Cu²⁺.xH₂O, Cu(en)²⁺.xH₂O and Cu(en)₂²⁺.xH₂O (with peaks for various x values). In addition, as a nitrate salt of copper(II) was employed, an ion-paired species {Cu(en)²⁺.(NO₃)}⁺ is observed. As well, reduced complexes Cu(en)+.xH₂O and Cu(en)²⁺.xH₂O formed by reduction that can occur at electrodes in the ESI unit occur as complicating peaks. What this spectrum tells us is that the solution contains vari- ous amounts of free ligand (detected as (en)H+), free copper ion (detected as Cu²⁺.xH₂O), 1:1 complex (detected as Cu(en)²⁺.xH₂O) and 1 : 2 complex (detected as Cu(en)₂²⁺.xH₂O), and an ion-paired species ({Cu(en)₂²⁺ (NO₃)}⁺). This is consistent with our interpretation of the chemistry from observations and from titration experiments. Thus the ESI-MS has provided additional firm evidence for these species occurring in solution. Moreover it does not detect any Cu(en)₃²⁺ consistent with the extremely low stability of this due to Jahn-Teller distortion, nor does it find any dinuclear bridged species, even for different M: L ratios that could support such speciation. The shortcomings of the ESI-MS tech- nique are first, that it can only detect ions and thus any neutral species remain un-sensed, and secondly, the peak height is not a direct measure of relative amount of a species, so that it is not strictly valid for defining relative concentrations of species present. However, it does provide a very simple method for probing solution species, based on molecular mass/charge. There is one additional caveat; because the method 'strips' solvent molecules from droplets to form ions, it can produce and hence detect 'bare' species such as Cu(en)²⁺ which obviously would exist as Cu(en)²⁺ aq in solution. Solvent 'stripping' may be incomplete so a sequence of solvated species, with peaks separated by 18/z (the mass of water divided by the overall charge on the ion), may be seen in some cases. Lastly there is one additional bonus; where the metal has several significant isotopes (as occurs with Cu) the presence of the metal will, in a high-resolution instrument lead to a characteristic pattern associated with that isotopic ratio whenever the metal is present, allowing easy assignment of metal-containing and metal-free species. Overall it is a useful addition to the armoury of coordination chemists.

The above examples are designed to be merely illustrative of some key techniques. Details of these and other techniques may be found in advanced textbooks on physical inorganic chemistry and/or analytical chemistry. What should shine through the above, at least, is how molecular size, shape and symmetry relate to spectroscopic analysis.

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Pretty in Red? - Colour and the Spectrochemical Series

Probing the Life of Complexes– Using Physical Methods

Organometallic Synthesis

Reactions of Coordinated Ligands

Metal-directed Reactions

Reactions Involving the Metal Oxidation State

Catalysed Reactions

Chiral Complexes

Substitution Reactions without using a Solvent

Substitution Reactions in Nonaqueous Solvents

Reactions Involving the Coordination Shell

Block f : Inner Transition Metals (Lanthanoids and Actinoids)