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Paul S. Pregosin

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Herausgeber: Wiley-VCH
Kategorie: Wissenschaft und neue Technologien
Sprache: Englisch

Beschreibung

The first and ultimate guide for anyone working in transition organometallic chemistry and related fields, providing the background and
practical guidance on how to efficiently work with routine research problems in NMR.
The book adopts a problem-solving approach with many examples taken from recent literature to show readers how to interpret the data.
Perfect for PhD students, postdocs and other newcomers in organometallic and inorganic chemistry, as well as for organic chemists
involved in transition metal catalysis.

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Leseprobe

Contents

Preface

Abbreviations

1 Introduction

References

2 Routine Measuring and Relaxation

2.1 Getting Started

2.2 Relaxation

References

3 COSY and HMQC 2-D Sequences

3.1 Tactics

3.2 COSY

3.3 HMQC and HMBC

References

4 Overhauser Effects and 2-D NOESY

4.1 Background

4.2 Assigning Signals via NOEs and General Applications

4.3 X,1 H Overhauser Effects

4.4 2-D NOESY

4.5 HOESY

4.6 Metal Complexes and NOEs. ROESY vs NOESY

References

5 Diffusion Constants via NMR Measurements

References

6 Chemical Shifts

6.1 1HNMR

6.2 Introduction to Heavy Atom NMR

6.3 13C

6.4 15N NMR

6.5 19F NMR

6.6 31P NMR

6.7 Transition Metals

References

The Author

Prof. Dr. Paul S. Pregosin

ETHZ HCI Hönggerberg

Lab. für Anorganische Chemie

Hönggerberg HCI/G139

8093 Zürich

Switzerland

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Library of Congress Card No.: applied for

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

Bibliographic information published by the Deutsche Nationalbibliothek

The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.

All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.

Print ISBN: 978-3-527-33013-3

This book is dedicated, first and foremost, to my wife Carole-Joyce, without whose support and understanding, it would never have appeared and also to the memory of Prof. L. M. Venanzi.

Preface

Multinuclear NMR spectroscopy is without doubt a major contributor to elucidating molecular structure in solution. Coordination and organometallic chemists routinely measure hundreds (if not thousands) of NMR spectra every day. Nevertheless, there are very few books devoted to the NMR characteristics of these metal complexes. Further, although many of the NMR details connected with these measurements are closely related to those associated with the 1H and 13C characteristics of organic or biological molecules, there are some important and unique differences arising due to complexation of an organic molecule to a transition metal.

Somewhere in an old cookbook I remember reading ‘‘the first thing one needs to do in order to make rabbit stew is to catch the rabbit”. First one needs to obtain the various spectra so that these NMR techniques make up the first part of the book. Although many young researchers measure NMR spectra, it is important to think not only about ‘‘routine measuring’’ but also a bit about how one might improve the quality of the spectra obtained. In my experience, we are a little spoiled, in that modern NMR spectrometers often deliver good quality spectra without much effort on the part of the user. Of course once the spectrum is present, there is the question of assignment. Since it may prove necessary to assign modestly complicated spectra, a few words on current two-dimensional methods are appropriate. Some of these 2-D methods are fairly straightforward, whereas others require more effort on the part of the user. All of the NMR techniques presented are fairly standard (and available on all modern spectrometers) so that I will not review the theory associated with these methods.

A major part of this text, chapter 11, is devoted to solving structural and other NMR problems concerned with transition metal coordination and organometallic compounds. The reader will be shown a reaction followed by the kind of NMR data that one normally finds in a preparative experimental section. Using these chemical shifts and coupling constants one is asked to propose a structure. In chapter 12 the solutions to the problems will be provided, together with a literature citation and some subjective commentary.

In order to prepare the reader for these problems, a schematic very brief introduction to selected organometallic reaction mechanisms and the syntheses of several classes of metal complex will be given. This will hopefully direct the reader’s thinking when confronted with the problems.

This text is not designed to be comprehensive, but rather to emphasize-as briefly as possible-what one needs to know in order to obtain the most useful spectra and then using the NMR data that one can derive from them, to solve routine research problems in coordination and organometallic chemistry.

August 2011Paul S. Pregosin

Abbreviations

methyl

ethyl

-Pr (or Pr

)

iso-propyl

-Bu (or Bu

)

tert

-butyl

cyclohexyl

-Tol

para

-methyl phenyl

Mes

2,4,6-trimethyl phenyl

Triflate

CF3SO3

−

anion

OAc

acetate anion

acac

acetyl acetonate anion

DFT

density functional theory

DMSO

dimethyl sulfoxide

DMF

dimethyl formamide

1 Introduction

Just as in organic chemistry or biochemistry, it is now routine to measure 1H, 13C, and, often, 31P NMR spectra of diamagnetic organometallic and coordination compounds. Many NMR spectra are measured simply to see if a reaction has taken place as this approach can take sometimes take <5min. Having determined that something has happened, the most common reasons for continuing to measure are usually associated with

1) Confirmation that a reaction has taken place and, by simply counting the signals, deciding how to proceed

2) The recognition of new and/or novel structural features via marked changes in chemical shifts and/or J-values, and

3) The need for a unique probe with sufficient “structural resolution” to follow the kinetics or the development of a reaction.

When a P atom is present, proton-decoupled 31P NMR often represents one of the simplest analytical tools available as the spectra can be obtained quickly and do not normally contain many lines.

Figure 1.131P NMR spectra recorded on the same solution after 10 cycles between pH 4 and 13: (a) recorded at pH 13, showing the Pt(0) complex, D, Pt(TPPTS)3, and (b) recorded at pH 4, showing Pt(H)(TPPTS)3 cation, A. Traces of the hydroxide-bridged dinuclear complex, C, as well as the phosphine oxide, OTPPTS, are marked [1].

Apart from recognizing the number of different chemical environments, many times the important clue(s) with respect to the nature/and or source of the reaction products stem from specific chemical shifts.

Reaction of Ru(OAc)2(Binap) with 2 equivalents of the strong acid CF3SO3H affords the product 1.2 in high yield. Superficially, complex 1.2 appears to arise as a result of the addition of H2O across a Binap P-C bond. But what is the water source? The 13C spectrum of the reaction solution, see Figure 1.2, reveals that acetic anhydride is produced (and thus water) from the two molecules of HOAc produced from the protonation. Further, the spectrum shows a C=O signal for the novel intermediate 1.1.

This reaction represents an example of point 2, in that the product reveals an unexpected feature.

Figures 1.3 and 1.4 demonstrate point 3. The 1H NMR spectrum of the deuterated rhodium pyrazolylborate isonitrile complex, RhD(CH3)(Tp′)(CNCH2But), in the methyl region, slowly changes to reveal the isomer in which the deuterium atom in now incorporated in the methyl group to afford RhH(CH2D)(Tp′)(CNCH2But). In this chemistry, the deuterium isotope effect on the 1H methyl chemical shift is sufficient to allow the resolution of the two slightly different methyl groups and thus allow the 1H(2H) exchange to be followed.

To be fair, a unique structural assignment cannot usually be made by counting the number of 1H, 13C, or 31P signals and/or measuring their chemical shifts. X-ray crystallography remains the acknowledged ultimate structure proof. However, for monitoring reactions, identifying mixtures of products and detailed mechanistic studies involving varying structures, NMR has proven to be a flexible and unique methodology. Apart from 1H, 13C, 15N, 19F, or 31P, already mentioned, there are many other possibilities, including 2H, 29Si, one of the Sn isotopes, and 195Pt, to mention only a few.

Figure 1.3 Methyl region as a function of time (minutes) of the 1H NMR spectrum from the rearrangement of RhD(CH3)(Tp′)(CNCH2But), to RhH(CH2D)(Tp′)(CNCH2But) in benzene-d6 at 295 K [3].

Figure 1.4133Cs NMR spectra of human erythrocytes suspended in a buffer containing 140 mM NaCl and 10 mM CsCl. The origin of the chemical shift scale is arbitrary [4, 5].

References

1. Helfer, D.S. and Atwood, J.D. (2002) Organometallics,21, 250.

2. Geldbach, T.J., den Reijer, C.J., Worle, M., and Pregosin, P.S. (2002) Inorg.Chim. Acta,330, 155.

3. Wick, D.D., Reynolds, K.A., and Jones, W.D. (1999) J. Am. Chem. Soc.,121, 3974.

4. Davis, D.G., Murphy, E., and London, R.E. (1988) Biochemistry,27, 3547.

5. Ronconi, L. and Sadler, P.J. (2008) Coord. Chem. Rev.,252, 2239.

1) Although not always specified, the 13C and 31P NMR spectra that follow throughout this text (and in the literature) are almost always measured with broad band 1H decoupling, so that nJ(1H,X) coupling constants are not present and this helps to simplify the spectra. Occasionally, this will be indicated as “13C(1H)” or “31P{1H}.” Unless otherwise specified, the reader should assume broad-band proton decoupling.

2 Routine Measuring and Relaxation

2.1 Getting Started

Preparing the sample may not be trivial (many organometallic complexes are air and water sensitive); but assuming that one has prepared circa 0.7 ml of a clear solution containing 5–10mg of sample,1) in the usual 5 mm NMR tube, one is ready to measure a spectrum.

Most beginning researchers place the sample in the magnet, stabilize the magnetic field via a 2H lock (frequently no longer necessary), call up a simple measuring program, and type, “go.” For a standard one-dimensional proton 1H NMR measurement, a few minutes (or less) of accumulation time are often sufficient to obtain a 1H free induction decay (FID) that, after Fourier transformation, affords a spectrum with sufficient signal-to-noise (S/N) ratio. After a phase correction, the spectrum is plotted.2)

Typically, these operations are followed by an integration procedure to determine the relative number of protons in the various groupings of resonances, and this may be where a problem arises. The integration obtained may indicate that, instead of, for example, a 2 : 1 ratio, one finds a 1.7 : 1 ratio or 2.3 : 1 ratio. There may be a structural reason for the observed results, but sometimes, the problem is simply one of “relaxation.”

The NMR program may already contain a “recommended” 1H pulse length (or it may simply be what the last researcher found to be optimal for his or her chemistry). If the chosen pulse length and/or the acquisition time (the time used by the computer to collect the FID) have not been properly considered, the integrals may not (and usually do not) correctly reflect the relative populations. Moreover, the S/N ratio may not be optimal.

Scheme 2.1

Scheme 2.1 shows a cartoon of a routine measurement (note that the horizontal axis is not to scale). We will assume that the pulse length chosen (in microseconds) corresponds to a 90° pulse. A 90° pulse is defined as that pulse length that tips the magnetization vector 90° from its equilibrium position on the z-axis (A) and affords the maximum signal after one pulse (B). Less than 90° will afford a vector whose projection on (for example) the x-axis, will not be quite so large (C).

Before discussing relaxation phenomena, it is useful to think, briefly, about the subject of spectral resolution and especially the accuracy of measured coupling constants. If one selects, for example, a spectral width of 20000 Hz and places the spectrum (after transformation of the FID) into 32 K data points, the resolution cannot be better than circa 0.62 Hz. If we assume that an error of ±1 channel is reasonable, rather than just 1 channel, then the J-values will not be better than about 1.2 Hz. Of course, one can use 64 K points, but this will still not allow the J-values to be determined to ±0.1 Hz. The student might keep this in mind when reading the literature.

2.2 Relaxation

Returning to relaxation, what happens after the first 90° pulse? It is often the case that the various protons (or carbons) to be measured have not had sufficient time to completely relax to their equilibrium positions during the acquisition time chosen. Consequently, in a multipulse experiment, the amount of signal obtained for each of the individual protons of the molecules under consideration will vary with their 1H relaxation characteristics. It is sufficient to note that, to obtain a spectrum with the correct relative integrals, one either reduces the length of the pulse and/or one adds a relaxation delay at the end of the FID accumulation. An alternative to a relaxation delay involves the use of more data points during the accumulation of the FID. This adds more time in the gathering of the data (not always desirable) and improves the spectral resolution. It is not normally necessary to add a paramagnetic relaxation reagent.

Usually, a smaller pulse angle, perhaps, between 30° and 45°, will most likely afford correct integrals. But how do we know whether 30° or 45° corresponds to the correct choice?4) Generally speaking, if one plans to work on a given set of molecules for a prolonged period (for example, for the length of a Ph.D. research project), it is advisable to measure the spin lattice relaxation times, T1 for the protons of the class of compounds to be studied, and then to set up the experiment with a suitable acquisition time and/or postdelay.

Measuring T1, for example, via the inversion-recovery sequence (Eq. (2.1)):

(2.1)

is a straightforward process and affords a series of spectra as a function of the different waiting times, τ. The 180° pulse inverts the magnetization. After short τ values, the magnetization is still inverted and, after the 90° pulse, transformation of the FID affords a “negative” signal. As the waiting time τ increases, the spins relax more and more toward their original equilibrium positions and the 90° pulse results in a “positive” signal.

Figure 2.1a shows an inversion-recovery T1 measurement for the two types of hydride ligand in (2.1). Note that with a τ value of 2000ms, the central proton is “positive” whereas the outer two hydride signals have close to zero intensity. For the two separate 195Pt measurements on Pt{P(t-Bu)3}2 at 310 K, given in Figure 2.1b, one sees the effect on the signal intensities of altering the waiting time and of changing the magnetic field strength and we shall come back to this field dependence shortly. The T1 values can be calculated from the experimental data via a well-known regression analysis. The waiting time that corresponds to “zero” signal is circa T1(ln2) and although this may prove a useful relation for estimating T1 it is not usually very accurate.

An understanding of the factors affecting the spin–lattice relaxation times, T1’s, will help to solve the 1H integration problem, and also can be of value in connection with optimizing S/N ratios and/or mixing times in 2-D 1H,1H nuclear Ovehauser effect (NOE) experiments. Moreover, there are some subtle problems concerned with integrals and intensities for 13C and 31P, involving T1 so that a brief discussion on NMR relaxation is useful.

One can summarize the longitudinal relaxation rate of a nucleus, R1(= 1/T1), as resulting from the sum of a number of contributions and these are shown in Eq. (2.2).

(2.2)

The various contributions [3] to the overall relaxation rate are defined as follows: DD, dipole–dipole; CSA, chemical shift anisotropy; SR, spin rotation; SC, scalar coupling; Q, quadrupole; and EN, electron–nuclear. For our purposes, it is useful to discuss only two of these contributions, and .

Figure 2.1 (a) Inversion-recovery experiments to determine the relaxation times of the hydride ligands for in CF3CO2H at 298 K. The waiting times are in milliseconds. The weak resonances surrounding the main bands stem from 183W [1]. (b) 195Pt T1 measurements on Pt{P(t-Bu)3}2 at 310 K at 7.0 T (left) and 9.4 T (right) using the inversion-recovery pulse sequence. The variable delays between the 90” and 180” pulses were varied between 0.001 and 0.4 s. The spectral width corresponds to 12 000 Hz [2].

2.2.1 Dipole–Dipole Relaxation

Dipole–dipole relaxation, in which one of the two dipoles is 1H, represents an important (and usually dominating) contributor to the relaxation of the two

(2.3)

important nuclei 1H, 13C, and occasionally for 15N and 31P. In the equation above, γ is the gyromagnetic ratio and is directly proportional to the magnitude of the magnetic moment of the nuclei A and 1H, S is the spin quantum number, h is Planck’s constant, τ is a molecular correlation time, and r is the distance between the two dipoles. Often, a number of proximate dipoles (for example, a set of protons in an aliphatic chain and/or the protons of an aromatic moiety) will contribute to the relaxation of a given 13C, 15N, or (sometimes) 31P. The values of τ and r represent two very important factors that vary with molecular structure. Large molecules move slowly (large τ) and some 13C or 15N spins have the protons directly bonded (short r-values).

Specifically for 13C

(2.4)

where N in Eq. (2.4) represents the number of protons attached to the carbon in question. Clearly, a short r-value (e.g., a hydrogen atom directly bound to the 13C in question) will afford a shorter T1. However, the τ value can be quite important. Anything that changes molecular motions (different solvents, via their viscosities, variable temperature experiments – again through a change in viscosity –, large vs small molecular size, or perhaps steric crowding) can affect τ and thus T1.

The 13C T1 values given below (in seconds) for 1-decanol, represent a classical example of how τ can be important for relaxation

Changing nucleus, Table 2.1 shows some 31P T1 values for P(Aryl)3 complexes of iridium and gold, plus T1’s for several alkyl phosphine complexes of palladium, in CDCl3 solution. The T1 values for the Ir and Au complexes decrease as the molecular weight of the P(Aryl)3 increases due to the differing τ-values. Note that PPh3 itself (not complexed, and thus presumably with a shorter τ-value5)) has a relatively long T1, 26.0 seconds. The same trend for the 31P T1’s is found when the R group of the trans-PdCl2(PR3)2 is made larger by increasing the chain length. Since dipole–dipole relaxation is important for these alkyl phosphines (the methylene protons on the adjacent carbon are not too far away), the observed changes in the TI’s are reminiscent of what we have seen for the 13C T1 values shown above, for trans-PdCl2L2, that is, an increasing correlation time leading to a shorter 13C T1.

Table 2.131P T1 for PPh3 and some phosphine complexes in CDCl3.

Where dipole–dipole relaxation is important (as in 1H and 13C NMR), NOEs, that develop due to relaxation effects, can result in signal enhancement. The theoretical maximum is 50% for 1H and almost a factor of 2 for 13C. Although not widely recognized, there is a dipole–dipole contribution to the 31P relaxation in metal phosphine complexes, and especially in alkyl phosphine complexes. Indeed, the dipole–dipole contribution can amount to between 70 and 100% of the relaxation [5]. Since a substantial dipole–dipole contribution exists, Overhauser effects between 31P and 1H can have a marked effect on the 31P signal intensity when the phosphorus spectrum is measured with 1H decoupling.

Assume that one is interested in studying a reaction involving P-donor exchange

(2.5)

such as that indicated in Eq. (2.5). Simple integration of the 31P spectra will most likely not lead to the correct relative populations, unless both T1’s and NOE’s have been considered. Measuring and using integrals in 31P spectra can be quite challenging!

Although technically important, there are not too many areas of organometallic chemistry where measuring T1’s is necessary to understand the chemistry. However, in the discussion of the NMR parameters of molecular hydrogen complexes, to follow later on, 1H T1 data for both the hydrogen and hydride ligands are suggested to afford a diagnostic tool for these complexes in solution. The short H–H distance in the complexed η2-H2 ligand (short r-values) results in extremely short 1H T1 values, (usually <20ms) relative to those T1’s for terminal hydrides (often several hundred milliseconds). A review by Morris [6] includes more than 100 representative examples of these very short T1’s.

2.2.2 Chemical Shift Anisotropy

The CSA mechanism, R1CSA, is an important contributor to the relaxation of heavy nuclei, and particularly for the transition metals. This

(2.6)

Figure 2.2195Pt relaxation effects on the methyl 1H line shapes for (a) PtCl(CH3)(1,5-COD) at 200, 400, 500, and 700 MHz and (b) trans-PtCl2{As(CH3)3}2 at 200, 400, and 700 MHz. Notice the increasing line width as the B0 field is increased. (A. Moreno, P.S. Pregosin 2010, unpublished results.)

In Chapter 7, the reader will find comments concerned with metal-to-proton and metal-to-carbon coupling constants. Sometimes, and especially where high-field magnets are employed, these nJ(M, 1H or 13C) values are not observed. The source of the problem is the B02 dependence of the metal relaxation. Increasing the magnetic field strength, from, for example, 300 MHz (7.04T) to 700 MHz (16.4T), will (i) drastically shorten the metal T1’s and (ii) broaden the proton or carbon line widths (a T2 effect). As noted above, the 195Pt metal T1’s can be of the order of milliseconds. The long-range proton or carbon J-values are often <50 Hz, so that the fast metal relaxation can cause the connected 1H or 13C resonances to almost or indeed completely “disappear.” Figure 2.2 give examples of 1H spectra of two Pt-complexes, PtCl(CH3)(1,5-COD) and trans-PtCl2(As(CH3)3)2, at different field strengths. In both cases, the 195Pt satellite resonances broaden markedly as the field increases.

2.2.3 Passing Comments

3) R1SR relaxation involves molecules or fragments that rotate rapidly with the result that a coupling exists between the nuclei and the magnetic moment generated by rotating charge distribution. This process can be important for 19F and 31P among others.

4) R1EN is responsible for the rapid relaxation of the nuclei in a paramagnetic complex and depends strongly on the number and relaxation rate of the unpaired electrons.

2.2.4 Useful Tips

1) It will be easier to set up an NMR experiment correctly, and obtain good integrals, if the T1’s are known.

2) For low-temperature measurements, if dipole–dipole relaxation dominates, the S/N ratios can sometimes be improved by using a larger pulse angle, since the T1’s will be shorter (viscosity effects).

3) One should be careful when using 31P integrals to determine equilibrium constants.

4) Sometimes, it is wiser to measure 1J(Metal,1H) values at lower field.

References

1. Henderson, R.A. and Oglieve, K.E. (1993) J. Chem. Soc. Dalton Trans., 3431.

2. Benn, R., Reinhardt, R.D., and Rufinski, A. (1985) Magn. Reson. Chem.,23, 259.

3. See (a) Liu, M. and Lindon, J.C. (1996) Concepts Magn. Reson.,8, 161, for a review on NMR relaxation; See also (b) Bakhmutov, V.I. (2004) PracticalNMR Relaxation for Chemists, John Wiley & Sons, Ltd.

4. (a) Jans-Burli, S. and Pregosin, P.S. (1985) Magn. Reson. Chem.,23, 198; (b) Bosch, W. and Pregosin, P.S. (1979) Helv. Chim. Acta,62, 838.

5. Burling, S., Haller, L.J.L., Mas-Marza, E., Moreno, A., Macgregor, S.A., Mahon, M.F., Pregosin, P.S., and Whittlesey, M.K. (2009) Chem.-Eur. J.,15, 10912.

6. Morris, R.H. (2008) Coord. Chem. Rev.,252, 2381, and references therein.

7. Jindal, A.K., Merritt, M.E., Hyun Suh, E., Malloy, C.R., Sherry, A.D., and Kovacs, Z. (2010) J.Am. Chem. Soc.,132, 1784.

1) The amount necessary to obtain excellent signal-to-noise in a short period of time, will depend on the molecular weight and of course the nucleus to be measured amongst other parameters.

2) No longer carried out at the NMR console, but rather at some remote PC station so that others can efficiently utilize the machine time.

3) For Bruker instruments.

4) This problem has been studied at length. The optimum angle (the so-called Ernst angle) depends on T1. For T1 values between 0.4 and 4.0 s this angle changes from 53° to 86° assuming an acquisition time of 1 s.

5) Dipole–dipole is not the only relaxation mechanism for PPh3.

3 COSY and HMQC 2-D Sequences

3.1 Tactics

Assuming that one has measured a routine 13C and/or 1H spectrum, these spectra now need to be properly assigned and interpreted. For a routine 1H spectrum of a simple complex, it is often sufficient to inspect the chemical shifts (as these may be diagnostic for a structure type), the individual integrals, and the various nJ(1H,1H) interactions.

A somewhat lengthy strategy to assign spectra and subsequently solve the structure for complicated problems might involve

1-D

H and

C and, when present,

P, measurements to obtain routine chemical shifts and coupling constants.

A 2-D

H correlation (COSY) to recognize proton–proton coupling constants over two, three, or four bonds. This may not always be required.

A 2-D

H correlation (heteronuclear multiple quantum correlation, HMQC and/or HMBC). These measurements connect the

C resonances to the protons via one-bond and long-range interactions and can be relatively important measurements.

A 2-D

P(or X),

H correlation (HMQC or related). This will reveal phosphorus (or X) coupling to the protons. Very useful, but only in special cases.

A NOESY or ROESY 2-D

H correlation. These methods reveal through-space correlations of proximate protons. They can also reveal unexpected (slow) exchange processes. The exchange aspect is under appreciated and will be dealt with in the section on dynamics.

A 2-D X,

H correlation (HOESY) – this method involves a through-space correlation of, for example,

F, to the proton spins. This is useful not only for fluorocarbons but also for anion–cation interactions where the anion has

F spins. This is something of a specialty technique.

One rarely needs all the above, especially as the entire set can easily consume 24 h or more of machine time; however, for some particularly tricky problems, many (or all) of the above will prove necessary. How does one decide which of these methods to use?

3.2 COSY

Occasionally, the 1H spectrum of an unknown is sufficiently complicated such that one cannot easily find which spins are coupled to one another, perhaps due to signal overlap. This is often the case where structurally complicated phosphine ligands (such as a tricyclohexyl phosphine, PCy3, derivative) and/or large organic substrates (perhaps as part of a chiral auxiliary) are in play. In these cases one might wish to measure one or more 2-D NMR spectra, the simplest of these being a 1H,1H COSY spectrum. Equation (3.1) shows the basic two-pulse sequence (there are many variations on this theme), and Figure 3.1 provides a trivial example concerned with the CH3O-Biphep salt, [Pt(CH3CN)2(P,P)](CF3SO3)2.

(3.1)

Normally, the diagonal (whose projection on the axes affords the chemical shifts) runs from the top right to the bottom left and the cross-peaks arising from the proton-proton coupling constants, appear as off-diagonal signals. The COSY spectrum for this simple Pt-dication contains cross-peaks connecting H3 and H4, H4 and H5 plus H3, and H5. The H3 and H5 cross-peaks are relatively weak due to the small value of 4J. Although COSY represents a valuable tool, many times the proton spectra of the ligands employed are sufficiently simple so that this measurement is not necessary. Alternatively, the 1H assignment can be made via an equally useful method, for example, one or more 13C,1H correlations.1)

Figure 3.1 Section of the COSY spectrum for the three biaryl protons H3–H5 in [Pt(CH3CN)2(P, P)](CF3SO3)2 [1].

3.3 HMQC and HMBC

3.3.1 Methods

The ability to recognize how many H atoms are attached to a given carbon, together with the corresponding 1H and 13C chemical shifts, provides powerful tools with respect to assigning both spectra and structure. Given the extensive literature 13C chemical shift database (there are commercially available programs that calculate 13C chemical shifts in organic compounds), it is useful to correlate the measured 1H signals to their corresponding 13C (or 31P or 15N) absorptions. For 13C, one-dimensional methods such as attached proton test (APT) fulfill this function [3].

However, a variety of two-dimensional HMQC pulse sequences [4], based on using one, two, or three-bond coupling constants, not only can achieve the goal but also allow the X-nuclei data to be obtained efficiently. Obviously, the success of this technique depends on the presence of a spin–spin interaction between the X resonances and a suitable proton. For 13C [5], these coupling constants are usually present in either the ligands and/or from a reagent that has entered the coordination sphere. Fortunately, there is a substantial literature concerned with nJ(13C,1H) over one, two, and three bonds.

Given that related HMQC pulse sequences are also in use for measuring 1H correlations to main-group nuclei, such as 29Si, 77Se, and 117,119Sn, as well as transition metal nuclei such as 183W or 103Rh, Figures 3.2 and 3.3 give pulse sequences for a selection of these methods [5].

The basic sequence, given in Figure 3.2a, involves four pulses: two pulses, a 90° and 180°, on the I-channel and two 90° pulses on the S-channel, followed by collecting the free induction decay. These sequences represent sensitive, routinely used tools, and involve double-polarization transfer (I → S → I), with the I spins representing a high-sensitivity nucleus, most often 1H, but occasionally 19F or 31P, and the less sensitive, S-spin (13C or 29Si, etc.). The time, Δ, in Figure 3.2a, is critical and should be set close to 1/2{nJ(S, I)}, whereas the time t1 represents the time variable for the second dimension and is much shorter. The less sensitive nucleus data are detected using the proton signals (the two nuclei share common energy levels because of the coupling constant), and the spectra are usually presented as contour plots. Other 13C,1H, correlation methods are available, but currently, the HMQC approach is favored due to the S/N advantage provided by these methods. This signal enhancement is proportional to the ratio (γI/γS)5/2 where γ is the gyromagnetic ratio of the nucleus (and is directly related to the nuclear magnetic moment). Enhancement factors for a few other nuclei are indicated in Table 3.1.

Figure 3.2 Heteronuclear multiple quantum correlation (HMQC) pulse sequences. (a) Sequence for small J(I,S) values, (b) for larger, resolved J(I,S) values and phase-sensitive presentation, (c) zero- or double-quantum variant for the determination of the I-spin-multiplicity, and (d) with refocusing and optional S-spin decoupling. The broad bars indicate 180°, and the narrower bars indicate 90° pulses.

Figure 3.3 Heteronuclear single-quantum correlation (HSQC) pulse sequences with optional decoupling of the S-spin. (a) Standard sequence and (b) modified for the I-spin-multiplicity determination. The broad bars indicate 180°, and the narrower bars indicate 90° pulses.

Since the potential signal-to-noise gain can be of the order of 103 or more, for very insensitive nuclei, the use of HMQC methods allows one to obtain relatively insensitive “X” nuclei chemical shift data in a reasonable period of time. Practically speaking, for about 10–20 mg of a complex of molecular weight circa 500, one can obtain reasonable quality data for many (but not all) less sensitive nuclei in 1 h or less. This methodology is not new. Early on, Benn, Brevard, and von Philipsborn, among others, have advocated their use in the measurement of, for example, 57Fe, 103Rh, and 183W spectra. The reader is recommended to consult the literature [7–9] for an explanation of why and when one would use the more complicated sequences.

3.3.2 One-Bond 2-D 13C,1H Correlations

Table 3.1 Enhancement factors, via HMQC methods, for selected nuclei [6, 7].

Having established the method of choice, a few examples involving both assigning carbon resonances and recognizing structure are illustrative. The dialkyl Ru-arene complex (3.1), in Figure 3.4, and the dinuclear Pd(I) diene salt (3.2), in Figure 3.5, represent examples where one-bond 13C,1H correlations helped to assign the carbon spectra. Only a section of the total spectrum is shown in both cases. In the Ru complex (3.1), the three protons of the coordinated arene, H3–H5 (already assigned via spin-spin multiplicities and NOE effects), are correlated to their 13C signals via 1J(13C,1H). The separation of the two cross-peaks in the horizontal direction for each carbon represents the one-bond J-value. These cross-peaks have different phases (indicated by the open and closed circles) and knowing the phases can be helpful when the structure in question is much more complicated and there are many cross-peaks in the map, some of which overlap. Note that these three carbons are found at relatively low frequency, due to the complexation of the arene. Further, both H3 and C3 do not appear at the lowest frequency so that an assignment based on literature “organic substituent effects” would not be correct.

Figure 3.5 Section of the 13C,1H HMQC spectrum of the isoprene complex (3.2). The intense triplet stems from CDCl3 [11].

Figure 3.5 shows a section of the one-bond 13C,1H HMQC spectrum containing the five 1H resonances of the complexed isoprene for the bridging Pd(I) dimer (3.2). This 1-D 1H spectrum is not trivial due to a number of long-range 31P,1H coupling constants. Once again, the open and filled-in cross-peaks indicate the phases for each of the two 13C spin states. Note that isoprene carbon C3 correlates to a single-proton H3 at 2.96 ppm, whereas the CH2 carbons C1 and C4 each show cross-peaks (near 2.8 ppm and 4.9 ppm, for C1 and 2.8 ppm and 4.2 ppm for C4) for the two attached protons. Apart from connecting the protons to their carbons, this C,H correlation is helpful as it confirms the overlap near 2.8 ppm and pinpoints the methine isoprene CH proton, thereby providing a starting point for the 1H assignment. The distinction between the protons of the two isoprene =CH2 groups (and thus the two 13C resonances) can be made via an analysis of the 3J(1H,1H) values since H3 will couple to the H4 protons, together with NOE results. The spectrum also contains a cross-peak connecting H1E (that trans to the C2-C3 single bond) to C3, and this is due to a long-range correlation. These can also be quite useful as we will see shortly.

Figure 3.6 Section of the 13C,1H HMQC spectrum of the rhodium 1,5-COD oxazoline cation shown below. The anion is BF4. THF solution [12].

3.3.3 One-Bond 2-D 15N,1H Correlations

The same type of HMQC sequence can be used in connection with one-bond coupling constants involving N-H protons. The 1J(15N,1H) values are quite substantial and for sp3 and sp2 N atoms often fall in the range circa 65–80 and 85–100 Hz, respectively. The bridging amide ligand in palladium complex (3.3), in Figure 3.7, is representative, and in this example, one would like to distinguish between the two broad OH and NH absorptions. If the NH proton does not exchange rapidly, it can be correlated to the 15N resonance. The observed 70–75 Hz 1J(15N,1H) value falls in the expected range and assigns the single-proton signal at 0.69 ppm as that bound to nitrogen.

3.3.4 Two and Three Long-Range Bond 13C,1H Correlations

For aliphatic compounds, 3J(13C,1H) shows a dihedral angle (Karplus-type) dependence with maxima at circa 7–9 Hz. For routine aryl sp2 carbons, the values for 2J(13C,1H) are often <5 Hz and those for 3J(13C,1H) a little larger between 6 and 10 Hz; however, there are exceptions.

Figure 3.8 The long-range HMQC 13C,1H correlation for the Pd–carbene complex (3.4) with the intense cross-peak due to the N-CH proton between 7 and 7.5 ppm. Note also the interaction of the carbene carbon with the anti-allyl proton trans to the carbene donor [14].

Although carbene complexes are increasingly in use in organometallic chemistry, the 13C signal intensity from a fully substituted carbene of an N-heterocyclic carbene can be weak, due to a relatively long T1. However, several possible long-range J-values can be used to detect this carbene absorption using HMBC methods. In the Pd-carbene complex (3.4), the carbene carbon, at 186–187ppm, couples to the heterocyclic =CH protons via three bonds and this correlation is often strong, as seen in Figure 3.8.

Figure 3.9 Section of the 13C,1H HMBC spectrum for Ru complex 3.5 arising from the three-bond correlation between the methine protons and the carbene carbon atom (CD2Cl2, 400 MHz, 298 K) [15].

Long-range 13C,1H correlations are especially useful for finding the fully substituted aryl ipso carbons. In the Pd-aryl complex (3.6a) (Figure 3.10), the three-bond correlation from the aryl methyl 1H signal to the meta carbon assigns C3 (and thus Hm, via the one-bond correlation). In Figure 3.10, this meta proton, Hm, gives rise to two cross-peaks due to the three-bond spin–spin interactions: one to the very weak fully substituted ipso carbon, C1, and a second to the much more intense meta carbon, C3. This latter three-bond route involves the coupling pathway (shown in the structure 3.6b) via the para carbon. The figure also reveals two three-bond correlations from the H° proton, close to 7.1 ppm. One of these assigns C4, the fully substituted aryl carbon bearing the methyl group (the weak 13C signal immediately adjacent to C3), and the second assigns C2. This spectrum is relatively simple as the 2J interactions are absent.

Figure 3.10 HMBC spectrum of Pd complex (3.6) showing long-range correlations to the 13C signals from the two aryl protons, Ho and Hm. C2 and C4 are not labeled. C1, the ipso carbon, has the highest frequency (CDCl3, 250 MHz) [16].

Figure 3.11 Section of the 13C,1H long-range correlation for the cationic Ru(phosphoramidite) complex shown. Only the coordinated carbons C1 and C6 are shown [17].

When a coordination position becomes available, the phosphoramidite ligand of the Ru-arene complex (3.7) can function as a chelate ligand in that, in addition to the P donor, one double bond from one of the two phosphoramidite phenyl rings can coordinate giving an olefin complex. Support for this type of η2-olefin structure comes from the 13C,1H long-range correlation, a section of which is given in Figure 3.11. The barely visible ipso carbon, C1, on the vertical axis is identified via the cross-peaks from H3 and H5. The protonated carbon C6 reveals strong cross-peaks from H2 and H4 and a weaker cross-peak from H5 due to a two-bond coupling. Both C1 and C6 are shifted 20–30 ppm to lower frequency relative to the aryl carbon positions of the second NCH(CH3)Ph fragment, and these 13C chemical shifts strongly support the proposed structure.

Figure 3.1213C,1H long-range correlation for complex 3.8, showing (among others) the low-frequency positions of the complexed C1 and the analogous noncomplexed C1′ (arrows). This pseudo-coordination chemical shift difference indicates that C1 is indeed complexed. The chemical shift of C1′ is typical for a fully substituted aromatic Binap carbon resonance [18].

Figures 3.8–3.11 represent slices out of the 2-D spectrum. In reality, the complete 2-D long-range maps are often much more complicated, so that a little practice is worthwhile. For example, the long-range 13C,1H correlation for the π-arene complex (3.8a) (Figure 3.12) with its two different naphthyl aromatic fragments reveals rather a lot of cross-peaks. Just as with the Ru-arene complex (3.7), coordinated olefins and arenes have their 13C chemical shifts at lower frequency relative to conventional uncoordinated organic arenes, and this exercise concentrates on assigning a few of the 13C resonances from this organometallic ligand.

The experimental cross-peaks arising from the three aryl protons of the coordinated ring, H4–H6, appear between 5.5 and 6.5 ppm (see 3.8b). The very weak signal for the fully substituted carbon, C1 of the coordinated ring, indicated with an arrow on the vertical axis, appears close to 100 ppm, and can be identified via the correlations to H5–H7. In addition to the correlations connecting C1 to H5 and H7, the two-bond correlation to H6 is also visible. The related carbon, C1′, appears at much higher frequency (see the second arrow in the spectrum). The cross-peaks from H6 and H10 assign C4, and the two fairly strong cross-peaks from the pseudo-para proton, H4, assign the fully substituted complexed carbon C2 in addition to C6. The three-bond correlation from H6 points to C4 and carbon C3 is assigned via a correlation to H5. The resonance for C5 can be obtained via a one-bond interaction. With a few expansions and some practice, it gets easier!

3.3.5 X,1H Correlations

Long-range HMBC correlations are especially well suited for detecting main-group resonances. A valuable, routinely used long-range correlation involves 31P. For this spin, finding the signal is no problem and obtaining an overview with respect to the presence (or absence) of 31P,1H coupling constants can be very useful, especially since assigning the aryl 1H signals of complicated phosphorus ligands can be a chore. Occasionally, the 31P,1H correlation can be used as a sort of filter and Figure 3.13 gives an example. A slice through the 2-D map of the 31P,1H correlation in which only the cross-peaks from the PPh(OH)(OCH2CH3) ligand are shown narrows the problem and allows one to pinpoint specific, often overlapped signals. In this example, one can assign the strong cross-peaks at about 6.6–6.7 ppm to the ortho PPh resonances. Further, the methylene proton resonance of the ethoxy group, close to 3.5 ppm, is readily identified (despite half of it being overlapped with an arene proton from the complexed ring). These protons afford easily recognized strong cross-peaks because of the large 3J(P-O-C-H) value. There is also a strong cross-peak arising from the P(OH) moiety near 9.7 ppm.

Figure 3.1431P,1H correlation spectrum of complex 3.10 in THF solution showing the connectivity between the two diastereotopic phosphorus atoms and (a) the four methyl groups (indicated at low frequency by the boxed cross-peaks) and (b) the two terminal allyl protons (1H, 500 MHz, RT) [20].

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