Heteronuclear NMR Applications (La–Hg)

718 HETERONUCLEAR HETERONUCLEAR NMR NMR APPLICATIONS APPLICATIONS (LA–HG) (La–Hg)

Heteronuclear NMR Applications (La–Hg) Trevor G Appleton, The University of Queensland, Brisbane, Australia

MAGNETIC RESONANCE Applications

Copyright © 1999 Academic Press

Introduction NMR spectroscopy for this group of metals is dominated by the three nuclei: 183W, 195Pt and 199Hg. Each of these will be dealt with in turn, followed by brief mention of other nuclei with non-zero values of the nuclear spin quantum number, I.

Tungsten,

183

W

The only tungsten isotope with a non-zero value of I is 183W, I = , (14.4% natural abundance). Its receptivity relative to that of 1H is low (1.06 × 10–5). Its resonance frequency in a magnetic field of 2.35 T, where protons resonate at 100 MHz, is 4.17 MHz. This nucleus is radioactive, an α-particle emitter with a long half-life (> 1 × 1017 y). ‘Satellite’ peaks from coupling with 183W are readily observed in the 31 P NMR spectra of tertiary phosphine complexes, the 1H NMR spectra of hydrides, the 19F NMR spectra of fluoro complexes and the 13C NMR spectra of carbonyl complexes. The earliest measurements of tungsten chemical shifts were indirect measurements through INDOR (internuclear double resonance) experiments. Direct measurement is now commonplace, but large sensitivity enhancements are possible with two-dimensional indirect spectroscopy, using the highly receptive 100% abundant nuclei 31P, 1H or 19F. A solution of Na2[WO4] (1 M in D2O, pD 9) is now becoming accepted as the standard reference for 183 W NMR in preference to WF6. Relaxation times for 183W are frequently long. Since a major relaxation mechanism is through chemical shift anisotropy (CSA), shorter, more favourable relaxation times are obtained when the environment about the tungsten nucleus is less symmetric. Addition of a paramagnetic substance, such as [Cr(acac)3] can be used to shorten relaxation times which are inconveniently long. The tungsten chemical shift is very sensitive to the environment about the nucleus. The overall range is large (~6000 ppm). Shifts for some representative compounds are given in Table 1. Variations in chemical shifts for heavy atom nuclei are dominated by changes in σp, the paramagnetic contribution to total nuclear screening. For a

transition metal complex, a simplified expression for σp is

where r is an average distance of valence d-electrons from the nucleus, Ek and Ej are energies of unoccupied and occupied molecular orbitals, respectively, and C is a sum containing the coefficients of the metal s, p and d orbitals used in the summation to develop the various molecular orbitals. It will be noted that the tungsten nucleus in the series [W(CO)3(Cp)X] becomes progressively more shielded as the halide is changed from chloride to bromide to iodide, which is the ‘normal halide dependence’ order for transition metal complexes. This is opposite to the order expected from the effect of X– on excitation energies, which must therefore be

Table 1 pounds

183

W chemical shifts for some representative com-

Compound

δW (ppm)

W(0) W(CO)6

−3505

W(CO)5(PMePh2)

−3324

W(I) [{W(CO)3(Cp)}2]

−4040

W(II) [W(CO)3(Cp)CI]

−2406

[W(CO)3(Cp)Br]

−2584

[W(CO)3(Cp)l]

−2996

[W(CO)3(Cp)Me]

−3549

[W(CO)3(Cp)H]

−4017

W(IV) [W(Cp)2H2]

−4671

W(VI) WF6 [W10O22]4– [WO4]2– [W6O19]2– [WCl6] Cp = η5-cyclopentadienyl.

−1120 −43 0 +59 +2181

HETERONUCLEAR HETERONUCLEAR NMR NMR APPLICATIONS APPLICATIONS (LA–HG) (La–Hg) 719

outweighed by the effects of metal–ligand covalency on r and the coefficients C. For the W(VI) complexes WF6 and WCl6, however, the order expected from excitation energies is observed. One area where 183W NMR has made a major contribution is in the study of iso- and heteropolytungstate ions. Since the tungsten atoms are usually in highly distorted octahedral environments, the relaxation times are short enough to allow convenient direct observation. The sensitivity of δw to the tungsten environment, and the observation of W–O–W coupling patterns, facilitate structural assignments. Paramagnetic heteropolytungstate ions have been included in these studies. Where similar series of tungsten and molybdenum compounds have been studied by 183W and 95Mo NMR, respectively, there are close parallels between the two series. 95Mo (I = ) is quadrupolar, and lines are relatively broad in unsymmetrical environments. Magic-angle spinning (MAS) solid-state 183W NMR spectra may be obtained, but long spectrometer times are usually required because of long spin– lattice relaxation times.

Platinum,

195

Pt

The only platinum nucleus with magnetic properties is 195Pt, I = , (33.7% abundance). The resonance frequency in a magnetic field of 2.35 T is approximately 21.4 MHz. ‘Satellite’ peaks from coupling with 195Pt were observed in 1H and 31P NMR spectra in the 1960s, and much of the early work on 193 Pt detection used INDOR methods. Direct one-dimensional observation of 195Pt NMR spectra is now routine. Because 195Pt relaxation times are short for most compounds, there need be only a very short delay between pulses, allowing rapid accumulation. Two-dimensional inverse detection methods are also being increasingly used. The most commonly used reference for 195Pt NMR is an aqueous solution of Na2[PtCl6]. As a number of authors have pointed out, there are some disadvantages in using this substance. At high magnetic fields, peaks due to different combinations of chlorine isotopes may be resolved, and the resonance frequency is significantly dependent on temperature. This leads to some uncertainty (up to ± 5 ppm) in reported chemical shifts, but this is not very significant in the context of the overall chemical shift range for platinum (~15 000 ppm). However, this reference does have the great advantage of convenience, and is therefore unlikely to be supplanted. Other suggestions have been [Pt(CN)6]2– (– 3863 ppm) and ΞPt, a frequency of exactly 21.4 MHz in a magnetic field in which the resonance frequency of tetramethylsilane

(TMS) is exactly 100 MHz (–4533 ppm). An aqueous solution of K2[PtCl4] (δPt –1624 ppm) is sometimes used as a secondary reference. Of greater concern than the small uncertainty in the reference frequency is the possibility, with a very wide chemical shift range for 195Pt, of observing ‘folded’ peaks. If peaks are not carefully ‘checked for folding’, reported shifts may be in error by hundreds of ppm! Chemical shifts for some representative platinum compounds are given in Table 2. The order of ligands in their effect on δPt is similar to that on other well-studied transition metals (e.g. 59Co, 103 Rh). In particular, ligands with heavy donor atoms tend to cause increased shielding of the metal nucleus so that, for example, the shielding increases from Cl− to Br− to I−. As discussed above for W(II) compounds, this may be understood in terms of the heavier donor atoms having a greater effect on r and C in Equation [1]. While δPt depends primarily on the donor atom set, the nature of the whole ligand does have an important effect (compare, for example, [Pt(NH3)4]2+ and [Pt(NO2)4]2–). Geometric isomers will also have different shifts, which are greatest when the different ligands bound to the metal are very different in their trans influence (compare the cis and trans pairs [PtCl2(NH3)2] and [Pt(NH3)2(H2O)2]2+). There are also ‘ring-size’ effects, evident in the difference between the shifts for [{Pt(NH3)2(µ-OH)}n]n– with n = 2 (four-membered ring) and n = 3 (six-membered ring), and for cis-[Pt(NH3)2(H2O)2]2+ and [Pt(en)(H2O)2]2+ (five-membered chelate ring). Chemical-shift anisotropy relaxation is important for platinum complexes, especially for square planar complexes, and at high magnetic field strengths. It is also enhanced when there is significant hydrogenbonding between the solvent and solute (as with diammineplatinum compounds in water), and when platinum is coordinated to bulky ligands (both of which will decrease the rate of tumbling in solution). Fast CSA relaxation leads to broadening of peaks and loss of resolved splittings from couplings. It can also effectively decouple platinum from another nucleus which is being observed. Coupling constants involving 195Pt [e.g. 1J(Pt–31P), 1J(Pt–15N), 1J(Pt–1H), 1 J(Pt–13C), 2J(Pt–CH3), 2J(Pt–CF3)] can provide useful structural information (e.g. through the trans influence of the trans ligand on these couplings) which may therefore be lost at high magnetic fields. In compounds containing platinum–platinum bonds, the magnitude of J(Pt–Pt) does not appear to be related simply to bond length or bond strength, although J(Pt–Pt) is affected in the same way as other coupling constants by the trans influence of the ligands trans to the metal–metal bond.

720 HETERONUCLEAR HETERONUCLEAR NMR NMR APPLICATIONS APPLICATIONS (LA–HG) (La–Hg)

195

Table 2 pounds

Pt chemical shifts for some representative comδPt

Compound Pt (0) [Pt(PPh3)3

−4583

[Pt(1,5-cyclooctadiene)2]

−4636

[Pt(PMe2Ph)4]

−4728

[Pt(PCy3)2]

−6501

When platinum is bound by ligands with donor atoms having quadrupolar nuclei (e.g. 14N, 75As), NMR peaks are frequently broad, because the rate of quadrupole-induced relaxation is such that 195Pt is only partially decoupled from the quadrupolar nucleus. Solid state 195Pt NMR spectra, including CP-MAS spectra, have been obtained. Much higher spinning rates are required for platinum(II) compounds than for most platinum(lV) complexes, because of their much greater shielding anisotropy.

Pt(I) (Pt—Pt bond) −4162

[{PtCl2(CO)}2]2− Pt(II)

+31

[Pt(H2O)4]2+ [{Pt(NH3)2(µ-OH)}2]2−

−1153

[PtCl3(H2O)]−

−1180

trans-[Pt(NH3)2(H2O)2]2+ [{Pt(NH3)2(µ-OH)}3]

3−

cis-[Pt(NH3)2(H2O)2]2+

−1374 −1505 −1584 −1620

2−

[PtCI4]

[PtCl3(NMe3)]−

−1715

[Pt(en)(H2O)2]

−1914

trans-[PtCI2(NH3)2] (indmf)

−2101

cis-[PtCl2(NH3)2] (indmf)

−2104

[Pt(NO2)4]2−

−2166

[Pt(NH3)4]2+

−2580

[PtBr4]2−

−2690

[PtCI3(SMe2)]−

−2757

[PtCI3(PMe3)]−

−3500

[Pt(CN)4]2−

−4746

2+

Pt(III) (Pt—Pt bond) [{(H2O)Pt(µ-SO4)2}2]2−

+1753

[{ClPt(µ-P(O)2O(O)2P)2}2]4−

−4236

Pt(IV) [PtF6]2−

+7314

[Pt(OH)6]2−

+3277

[PtCl6]

2−

fac-[PtMe3(H2O)3]+

0 −1794

[PtBr6]

−1860

[Pt(CN)6]2−

−3866

2−

Mercury,

199

Hg

Mercury has two isotopes with I > 0, 199Hg (I = ; 16.8% abundance) and 201Hg (I = ; 13.2% abundance). Because of the large quadrupole moment of 201 Hg, this nucleus is not readily observed, and only 199 Hg is of significance for NMR spectroscopy. Its resonance frequency in a magnetic field with strength 2.35 T is 17.9 MHz. There is general agreement on the use of neat dimethylmercury as reference, despite the high toxicity of this substance. An aqueous solution of Hg(ClO4)2 (–2253 relative to Me2Hg) has been suggested as an alternative but its shift is dependent on concentration and temperature. Common coordination behaviour for Hg(II) involves strong linear coordination by two ligands, often with additional weaker interactions from solvent molecules, counter-ions, etc. In some cases, (e.g. halides X−) higher coordination numbers are possible (e.g. tetrahedral [HgX4]2–). There is considerable ligand lability, so that averaged signals are often observed. There is therefore often some uncertainty about the precise nature of the species being observed. Chemical shift anisotropies are also large (except for highly symmetrical [HgX4]2–), leading to efficient relaxation, and some line broadening at high magnetic fields. Two-dimensional NMR spectra may be readily obtained. Some 199Hg chemical shift data are given in Table 3 for some representative compounds whose structures appear to be fairly well defined in solution. As with typical transition metals, such as 195 Pt, and 113Cd, the nuclear shielding increases in halide complexes as the halide ion becomes heavier (‘normal halide dependence’) Coupling constants between 199Hg and other nuclei [e.g 13C, 31P and 1H(2J, 3J)] have been extensively studied. In a series of complexes (e.g. CH3HgX) the coupling constants [2J(Hg–CH3) in this instance] may be used to establish a trans influence series which is similar to that obtained from coupling constants involving 195Pt.

HETERONUCLEAR HETERONUCLEAR NMR NMR APPLICATIONS APPLICATIONS (LA–HG) (La–Hg) 721

199

Table 3

Hg shifts for some representative compounds

Compound Hg(I) [Hg2]2+(ClO4)2 /H2O(satd.)

δHg (ppm) −1614

Hg(II)

Hafnium

Hf (I = ; 18.5% abundance) and 179Hf (I = ; 13.8% abundance) have large quadrupole moments, causing severe line broadening.

171

Tantalum

Me2Hg

0

MeHgCl

−813

MeHgBr

−915

[MeHg(NH3)]BF4

−943

Ta (I = ; 99.99% abundance) also possesses a large quadrupole moment so that, even in octahedral species such as [TaCl6]−, lines are broad. 181

MeHgI

−1097

Rhenium

[MeHg(OH2)]+

−1150

185

[HgCl4]2–

−1200

[HgBr4]2–

−1810

[Hg(H2O)6]ClO4)2

−2253

[HgI4]

2–

−3510

Re (I = ; 37.1% abundance) and 187Re (I = ; 62.9% abundance) also have large quadrupole moments, so that the line widths are large even for the symmetric species [ReO4]−.

Osmium

Os has I = , but its low natural abundance (1.64%) and its very low receptivity make it difficult to observe. 189Os (I = ; 16.1% abundance) has a large quadrupole moment, leading to very broad lines, except in highly symmetric environments. In some cases, 187Os spectra may be obtained by indirect detection, using more sensitive nuclei such as 31 P. Coupling constants between 187Os and other nuclei (e.g. 31P, 13C) may be easily observed, and provide structural information. 187

Other elements, La–Hg Lanthanum

Nuclei with I > 0 are 139La (I = ; 99.91% abundance) and 138La (I = 5; 0.09% abundance). Because of its low natural abundance, 138La is of no practical importance for NMR spectroscopy. The common oxidation state, La(III), is diamagnetic but co-ordination complexes are usually labile, and quadrupole-induced relaxation causes broad line widths, except when the environment is highly symmetric, as in [LaX6]3– complexes, which are presumably octahedral. Both the chemical shift and line width can provide information about the average environment of the La3+ ion. As expected for a d0 ion, the metal nucleus is less shielded in [LaBr6]3– (+1090 ppm from aqueous La3+) than in [LaCl6]3– (+851 ppm). Other lanthanides

Diamagnetic oxidation states tend to be unstable in aqueous solution, except for Yb(II) and Lu(III). Since 171 Yb (14.3% abundance) has I = , there have been some solution studies on Yb(II) complexes. 169Tm (100% abundance) also has I = , but diamagnetic compounds are not stable in solution. 175Lu (I = , 97.4% abundance) has a large quadrupole moment, which limits its use. Solid state spectra have been obtained on paramagnetic compounds using nuclei with relatively low quadrupole moments: 141Pr (I = ; 100% abundance), 151Eu (I = ; 47.8% abundance), 159 Tb (I = ; 100% abundance), 165Ho (I = ; 100% abundance) and with 169Tm (I = ; 100% abundance).

Iridium

Ir (I = ; 37.3% abundance) and 193Ir (I = ; 62.7%) have large quadrupole moments, low receptivity and low resonance frequencies, which make observation difficult.

191

Gold

Au (I = ; 100% abundance) has a large quadrupole moment. With the preferred linear [Au(I)] and square planar [Au(III)] geometries, quadrupole-induced relaxation is very fast, and signals have not been observed in solution. 197

List of symbols I = nuclear spin quantum number; J = coupling constant; σp = paramagnetic contribution to total nuclear screening. See also: Fourier Transformation and Sampling Theory; Heteronuclear NMR Applications (Sc–Zn); Heteronuclear NMR Applications (Y–Cd); NMR Relaxation Rates; NMR Spectrometers.

722 HETERONUCLEAR HETERONUCLEAR NMR NMR APPLICATIONS APPLICATIONS (O, (O, S, S, SE Se AND TE) Te)

Pregosin PS (1982) Platinum-195 nuclear magnetic resonance. Coordination Chemistry Reviews 44: 247–291. Pregosin PS (1986) Platinum NMR spectroscopy. Annual Reports on NMR Spectroscopy 17: 285–349. Pregosin PS (1991) Group 10 (nickel to platinum). In: Pregosin PS (ed) Transition Metal Nuclear Magnetic Resonance, pp 216–263. Amsterdam: Elsevier. Rehder D (1987) Early transition metals, lanthanides and actinides. In: Mason J (ed) Multinuclear NMR, pp 479–519. New York: Plenum. Rehder D (1991) Groups 3–5 scandium to tantalum. In: Pregosin PS (ed) Transition Metal Nuclear Magnetic Resonance, pp 1–58. Amsterdam: Elsevier. Wrackmeyer B and Contreras R (1992) 199Hg NMR parameters. Annual Reports on NMR Spectroscopy 24: 267–329.

Further reading Appleton TG, Clark HC and Manzer LE (1973) The trans influence: its measurement and significance. Coordination Chemistry Reviews 10: 335–422. Brevard C, Pregosin PS and Thouvenot R (1991) Group 6 chromium to tungsten. In: Pregosin PS (ed) Transition Metal Nuclear Magnetic Resonance, pp 59–89. Amsterdam: Elsevier. Goodfellow RJ (1987) Group VIII transition metals. In: Mason J (ed) Multinuclear NMR, pp 521–561. New York: Plenum. Goodfellow RJ (1987) Post-transition metals, copper to mercury. In: Mason J (ed) Multinuclear NMR, pp 563– 589. New York: Plenum. Granger P (1991) Groups 11 and 12 copper to mercury. In Pregosin PS (ed) Transition Metal Nuclear Magnetic Resonance, pp 265–346. Amsterdam: Elsevier.

Heteronuclear NMR Applications (O, S, Se and Te) Ioannis P Gerothanassis, University of Ioannina, Greece

MAGNETIC RESONANCE Applications

Copyright © 1999 Academic Press

majority of tellurium NMR investigations owing its significantly better receptivity.

NMR spectroscopic properties and techniques for 17O, 33S, 77Se and 123,125 Te

Experimental techniques Nuclear properties

The stringent requirements in 17O and 33S NMR studies of compounds at natural abundance are the high concentrations and extensive signal averaging needed. Recording of spectra can be greatly facilitated by the use of enriched samples. Synthesis with oxygen isotopes involves rather straightforward organic reactions. The quadrupolar moments of 17O

Both 17O and 33S are quadrupolar nuclei with very low natural abundance (Table 1). The NMR active isotopes of selenium and tellurium are spin nuclei. The receptivity of 77Se is about three times larger than that of 13C. The element tellurium has two active isotopes, 123,125Te. 125Te has been used in the Table 1

Isotope

Nuclear properties of 17O, 33S, 77Se, 123Te and 125Te

Spin

Natural abundance (%)

NMR frequency a (MHz)

Receptivity Absolute c

Relative b

17

O

0.037

54.227

2.91 × 10–2

1.08 × 10–5

33

S

0.76

30.678

2.26 × 10–3

1.71 × 10–5

77

Se



7.58

76.270

6.93 × 10–3

5.26 × 10–4

Te



0.87

104.831

1.80 × 10–2

1.57 × 10–4

Te



126.387

3.15 × 10

2.91 × 10–3

123 125

a b c

6.99

At 9.395 T (1H frequency: 400 MHz). Relative to proton, at constant field, for equal number of nuclei. Product of relative receptivity and natural abundance.

–2