Indirect Nonbonded Nuclear SpinâSpin Coupling: A Guide for the

Review pubs.acs.org/CR

Indirect Nonbonded Nuclear Spin−Spin Coupling: A Guide for the Recognition and Understanding of “Through-Space” NMR J Constants in Small Organic, Organometallic, and Coordination Compounds Jean-Cyrille Hierso* Institut de Chimie Moléculaire (ICMUB, UMR-CNRS 6302), Université de Bourgogne and Institut Universitaire de France (IUF), 9 Avenue Alain Savary, BP 47870 Dijon F-21078, France 5. Structural Determination in Organometallic and Coordination Compounds Using Nonbonded Coupling 5.1. Nonbonded JFF Coupling in Organometallic and Coordination Complexes 5.2. Nonbonded JPP Coupling in Organometallic and Coordination Complexes 5.3. Metals Centers Involved in Nonbonded Spin Coupling 6. Conclusions and Outlook Author Information Corresponding Author Notes Biography Acknowledgments Dedication Acronyms References

CONTENTS 1. Introduction 2. Basic Principles of Spin Coupling NMR Detection 2.1. Nuclei Equivalence 2.2. Direct Dipolar and Indirect Scalar Spin−Spin CouplingThe Coupling Constant J 3. Phenomenon of Nonbonded Nuclear Spin Coupling 3.1. First Descriptions of Nonbonded Spin−Spin Coupling in NMR 3.2. Theoretical Approaches in Transmission Mechanisms for TS Coupling 4. Structural Perspective of Nonbonded Coupling in Small Organic Molecules 4.1. Through-Space Spin Coupling in 19F NMR 4.1.1. Nonbonded JFF Coupling 4.1.2. Nonbonded JFX Couplings (X = N, P, Se, and C) 4.2. Through-Space Spin Coupling in 31P NMR 4.2.1. Nonbonded JPP Coupling 4.2.2. Nonbonded JPX Coupling (X = Se or C) 4.3. Through-Space Spin Coupling Involving Hydrogen Bonding 4.3.1. General Occurrence of Spin Coupling “Through Hydrogen Bonding” 4.3.2. Hydrogen Bonding in Nonbonded JFH Coupling 4.3.3. Structural Approaches of Spin Coupling Through Hydrogen Bonding 4.4. Other Through-Space Couplings with Miscellaneous Nuclei (C, N, Se, Te)

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1. INTRODUCTION Nuclear magnetic resonance (NMR) is widely employed for investigating a range of important issues from the determination of novel molecular structures in inorganic, organic, and biological compounds to medical imaging for medical therapy applications. In all of these cases, mastering the nature of the high-resolution nuclear magnetic resonance parameters is essential. Among these parameters, that of the indirect nuclear spin−spin coupling involving the most commonly encountered nuclei such as 1H, 13C, 19F, or 31P provides conclusive data for compound characterization in solution. This involves using, as a basis, the determination and interpretation of nuclear spin− spin coupling constants which are commonly abbreviated as J. This electron-mediated coupling, characterized by the J constant, is classically taught as being transmitted by unambiguously covalently bonded atoms. Yet, since the 1960s, both experimental and theoretical NMR studies have highlighted the existence of scalar J spin couplings operating through what are clearly nonbonded interactions. These couplings are often referred to as “through-space” internuclear spin−spin couplings (TS couplings). This review aims at discussing the nonbonded contribution of indirect nuclear spin−spin in nuclei commonly encountered in small organic

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Table 1. State-of-the-Art Reviews and Chapters Addressing TS Couplings title

authors

Nuclear spin−spin coupling Through-space mechanism in spin−spin coupling Mechanisms which produce spin−spin coupling in NMR Advances in theoretical and physical aspects of spin−spin coupling constants Angular dependence of spin−spin coupling constants Advances in theoretical and physical aspects of spin−spin coupling constants Theoretical aspects of spin−spin couplings Advances in theoretical and physical aspects of spin−spin coupling constants Interpretation of indirect nuclear spin−spin coupling constants Recent advances in theoretical calculations of indirect spin−spin coupling constants Comprehensive data on experimental indirect scalar NMR spin−spin coupling constants across hydrogen bonds 19 F NMR in organometallic chemistry −Applications of fluorinated aryls Coupling through space in organic chemistry Fluorine as an NMR probe for structural studies of chemical and biological system. “Through-space” nuclear spin−spin couplings in ferrocenyl polyphosphanes and diphosphino cavitands Palladium complexes of constrained polyphosphinesinvestigation of “through-space” NMR spin−spin couplings Direct detection of hydrogen bonds in biopolymers by NMR spectroscopy. Hydrogen bond scalar couplingsa new tool in biomolecular NMR Insights into biomolecular hydrogen bonds from hydrogen bond scalar couplings

ref

Grinter, R. Hilton, J.; Sutcliffe, L. H. Contreras, R. H. et al. Contreras, R. H.; Facelli, J. C. Contreras, R. H.; Peralta, J. E. Contreras, R. H. et al. Webb, G. A.; Fukui, H.; Baba, T. Contreras, R. H. et al. Malkina, O. L. Krivdin, L. B.; Contreras, R. H. Alkorta, I.; Elguero, J.; Denisov, G. S.

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Espinet, P. et al. Mallory, F. B.; Mallory, C. W. Gakh, Y. G.; Gakh, A. A.; Gronenborn, A. M. Hierso, J.-C. ; Armspach, D.; Matt, D. Hierso, J.-C.

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Gemmecker, G. Grzesiek S.; Cordier, F.; Dingley, A. J. Grzesiek, S.; Cordier, F.; Jaravine, V.; Barfield, M.

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2. BASIC PRINCIPLES OF SPIN COUPLING NMR DETECTION

and even organometallic compounds. The case of large proteins and organics having a molecular mass greater than 2000 g· mol−1, although pertinent to this topic, will not be treated in detail here. Theoretical and fundamental aspects are introduced in relation to experimental spectroscopic and structural data. Current trends in modeling this complex phenomenon with a view toward reaching a better understanding of its origin are also discussed. While some essential basic definitions should be borne in mind, the application of nonbonded spin−spin coupling constants (SSCCs) for structural determination in small organics and organometallic species is of particular interest for the wider community of “nonmathematical” chemists. Therefore, we will address both the theory and the practice where the physical background should not present a barrier to the understanding of the pertinent principles. This review aims particularly at dissipating some prevalent misconceptions and erroneous interpretation of the puzzling spin multiplets which are sometimes encountered by experimental scientists. Numerous reports in the literature still incorrectly consider the nature of SSCCs in an implicit relationship with what is generally assumed to be clear covalent bonding. They omit the reality that this rather fascinating physical interaction is in fact much more complex with respect to the fundamental concepts of chemical bonding. Several accounts and book chapters (as listed in Table 1) largely discuss the theoretical aspects of nonbonded spin coupling in the general framework of spin− spin interactions1−10 or focus on a particular nucleus (fluorine, phosphorus, or hydrogen) or on specific families of compounds.11−13 This is, to my knowledge, the first review which discusses the large body of recognition and understanding in the field of through-space spin coupling known since the 1960s from a structural perspective more familiar to synthetic chemists. The survey encompasses the most frequently encountered coupling nuclei in organic and organometallic chemistry.

2.1. Nuclei Equivalence

In NMR spectroscopy, analysis of molecular structures makes it possible to distinguish chemically equivalent nuclei, magnetically equivalent nuclei, and accidentally equivalent nuclei. Two or more nuclei are chemically equivalent when their chemical environment is identical; hence, they will experience the same shielding and have the same chemical shift. In such cases the term isochronous is also often employed. A group of two or more nuclei is magnetically equivalent when, in addition to their chemical equivalence (they share an identical environment), the set of coupling constants they share with all the other nuclei individually considered is identical for all nuclei of that group. Finally, the combination of factors which determines the chemical shift of a nucleus can eventually lead to a situation in which two nuclei, or groups of nuclei, embedded in different chemical environments eventually experience the same final shielding and chemical shift. Such nuclei are sometimes called accidentally equivalent, although they are clearly nonequivalent from a structural point of view. 2.2. Direct Dipolar and Indirect Scalar Spin−Spin CouplingThe Coupling Constant J

Independent of equivalence considerations, proximate magnetic nuclei experience both direct and indirect spin interactions. The presence of neighboring magnetic nuclei alters the local field and energy states of a nucleus. Because of Brownian motion, the direct dipolar interaction (dipole−dipole interaction), which operates through space directly between nuclei, is averaged to zero in high-resolution liquid-phase NMR. In solid or liquid crystals a direct dipolar interaction has an r−3 dependence (r is the distance separating the nuclei), which means that the dipolar coupling is strong at close approach and falls off steeply with increasing internuclear distances. Unlike this direct dipolar interaction, the tumbling of the molecular framework does not affect the indirect scalar interaction, which is responsible for the multiplet splitting phenomena generated and observed in high4839


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bonded TS couplings which are typical of 19F19F SSCCs in small organic molecules23−25 have also been studied when they occur between fluorine atoms and other nuclei such as 1H, 13C, 14 N, 31P, or 77Se.26−29 Scheme 1 illustrates some representative

resolution NMR spectra. An interpretation of this type of interaction was first given by Ramsey and Purcell,14 who showed that such splittings arise from an indirect coupling mechanism via the electrons in the molecule. Important information on the connectivity within a molecule is then carried by this type of nuclear spin−spin interaction from magnetically nonequivalent nuclei. It is worth noting that the coupling of magnetically equivalent nuclei exists but is not directly observable.15 The energy in the indirect scalar interactions of nuclei A and B has been found to be proportional to the scalar product of the nuclear spin vectors IA and IB according to expression 1.16 E = JAB (IA )(IB)

Scheme 1. Molecular Structures of Small Organics with 1H, 13 C, 14N, 19F, 31P, and 77Se Spin-Active Nucleia

(1)

The spin−spin coupling constant JAB which quantifies this interaction is usually expressed in Hertz, and it is characterized by both its magnitude and its sign. The observed splittings are independent of the applied magnetic field as well as the specific methods, such as double-irradiation methods, that are employed. This demonstrates that the coupling constant is real and can be either positive or negative, but this sign is not normally evident in the NMR spectrum.17,18 Pople, Schneider, and Bernstein,16 considered the description of a single covalent bond between two atoms and concluded that the interaction of nucleus A with the electron of its atom will make the electron spin lie more frequently antiparallel than parallel to the nuclear spin. According to the Pauli principle two electron spins in a covalent bond are antiparallel. Since the electron of the second atom magnetically interacts also with its nucleus B their spins tend also to be antiparallel (Figure 1). The combination of these magnetization effects provides a spin interaction between nuclei A and B.

a

SSCCs range between 1.5 and 174 Hz for interconnection > 3 covalent bonds.

examples that appeared in the literature during the period 1970−2000. The unusually high value of SSCCs in each of these cases was related to the nonbonded proximity in space of the nuclei involved. This is in turn directly related to the structure of these small organic molecules. These examples encompass both rigid (Scheme 1, 1b, 1d and 1e) and fairly flexible backbones (Scheme 1, 1a, 1c, and 1f). This appears to be irrespective of the nature of the nuclei or of the hybridization state of the atoms involved. Indeed, molecules with single or double bonds and incorporating alkyl, aryl, and heteroaryl (etc.) moieties are seen to exhibit such couplings. Fundamental interest in this TS coupling phenomenon extends to interpretation of structures in complex biological molecules as demonstrated by Feeney and co-workers.30 In a protein they observed the occurrence of nuclear spin couplings between fluorinated amino acid chains separated by some 127 residues. In a dihydrofolate reductase-NADPH-MTX complex from Lactobacillus casei, a JFF SSCC value has been measured at 17 Hz between two [6-F]-tryptophan residues [6-F]Trp5 and [6-F]Trp133. This coupling is not a through-bond (TB) 398JFF coupling but rather a direct TS interaction between two spatially proximate residues. This is clearly indicative of a folded structure for the protein. Oldfield and co-workers later confirmed this important structural information by modeling a F···F distance of about 2.98 Å from a molecular mechanics energy minimization of the entire protein.31 The origin of spin−spin coupling constants of the type above described has been early on analyzed by Mallory and coworkers using a simple but meaningful perturbation model (Figure 2).22,32 According to this proposal, these couplings, mainly nonbonded in origin, result from overlap interactions between lone-pair orbitals. For instance, in the case of the particular constrained geometry of the compounds sketched in Figure 2 (top), the C− F/C−F (left) and C−F/CN (right) bonds are coplanar and approximately parallel. As a consequence, the nonbonding

Figure 1. Coupling of nuclei A and B via electron spin polarization.

Following this principle, indirect nuclear spin coupling of two nuclei transmitted via polarization of the intervening electron spins depends upon the electron density at each nucleus.

3. PHENOMENON OF NONBONDED NUCLEAR SPIN COUPLING 3.1. First Descriptions of Nonbonded Spin−Spin Coupling in NMR

In the late 1950s, Saika and Gutowski reported the existence of unusual multiplet signals in the NMR spectroscopy of an organic fluorine-containing compound.19 In addition, Roberts et al. reported several examples of spin−spin coupling constants which were recognized as being transmitted via the spin polarization of nonbonded electron pairs. These were later named “across-space” or through-space couplings.20 Following these seminal observations, it was established that organic compounds containing two fluorine atoms which are separated by four or more bonds but are still spatially close exhibit large 19 19 F F nuclear spin coupling constants JFF.21,22 These non4840


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nuclei can present TS couplings which are directly measurable on NMR spectra if they are not magnetically equivalent. (ii) The participating nuclei must at a particular instant in time be in close proximity in solution. Therefore, either congested structures with very rigid similar topologies in solution and in the solid state, or very flexible spacercontaining systems can undergo the TS coupling phenomenon. (iii) The nuclei interaction can be driven by electron lone pairs. Previously, two lone pairs were believed necessary for transmitting TS SCCCs, but our group34 demonstrated that a unique lone pair can efficiently interact with atoms having all their valence electrons involved in covalent bonds. 3.2. Theoretical Approaches in Transmission Mechanisms for TS Coupling

Figure 2. Orbitals generated by the overlap of two lone-pair orbitals on intramolecularly crowded nitrogen- and fluorine-containing compounds according to Mallory’s model.

Although the Mallory and co-workers orbital model is a convenient basic description, it can be complemented and improved by sound theoretical considerations. These attempted to describe transmission mechanisms for various TS couplings.5,10 Reviewing the vast field of theoretical works devoted to spin−spin coupling calculations is beyond the scope of this review, and already a number of recent pertinent surveys are available on this topic.4−11 However, the more essential well-established methods, and results are worth mentioning. Indirect nuclear spin coupling of two nuclei via polarization of the intervening electron spins depends upon the electron density at each nucleus. Since only s atomic states have a finite electron density at the nuclei, nuclear coupling will depend to some degree on the fraction of s character in the mutual interaction. The corresponding quantum mechanical form was first described by Fermi35 and is called the Fermi contact term. This is due to the fact that this approach considers a finite electron density at the nucleus and is the only term in the wave equation which involves electron density at the nucleus. A complementary mechanism first discussed by Gutowsky et al.36 states that coupling of two nuclei can also occur by means of the orbital motions of the valence electron. Each nuclear magnetic moment will induce currents in the electron cloud, and the resulting magnetic field will be felt by neighboring atoms. This contribution, although unimportant for proton− proton interactions, will be significant for heavier atoms with high electron density. Currently, in the nonrelativistic theory, the four distinct contributions considered as participating to the final SSCCs are as follows:37 (i) the Fermi contact (FC), arising from the s electrons, which is usually the dominant term, even if the other contributions are not neglected,38,39 (ii) the diamagnetic spin−orbit (DSO), (iii) the paramagnetic spin− orbit (PSO), and (iv) the spin dipole (SD). These are the contributions from mechanisms involving interactions mediated by the p, d, and f electrons. The orbital term arises from the perturbation of the magnetic field due to the orbital motion of the electron. On the other hand, the spin−dipolar term arises from the direct interaction of the magnetic dipole of a nucleus with that of the orbital electron. This can then interact with another nucleus and vice versa.18 The consequent formula takes into account the four terms contributing to the global spin− spin coupling constants J(X,X′) as expressed in eq 3.8 This formula is nevertheless insufficient for separating the respective contribution of the TS and TB components.

distances d(F···F) and d(F···N), respectively, are short. In this orientation, two lone-pair orbitals experience a mutual internuclear overlap. As depicted in Figure 2 (bottom), the overlap between these lone-pair orbitals is expected to afford both an in-phase and an out-of-phase combination. Since both bonding and antibonding orbitals are occupied (two orbitals, four electrons) no bonding stabilization is observed. Nevertheless, this interaction provides an adequate pathway for transmitting spin information between the coupled nuclei. Consistent findings from experimental studies confirm that the magnitude of the J constant depends on the extent to which the two lone-pair orbitals interact as a result of their overlap. This simple model is relevant in terms of ease of use and does not require extensive quantum chemistry knowledge for making useful predictive deductions. In the general approach used to describe the origin of nuclear spin−spin coupling constants between nuclei X and X′, J is expressed as the sum of three terms as shown in expression 2 J(X, X′) = TSJ (X, X′) + TB(π )J (X, X′) + TB(σ )J (X, X′) (2) TS

The J(X,X′) term corresponds to the nonbonded coupling. The other two terms arise from through-bond interactions involving either σ or π orbitals.4 In the interpretation of global spin−spin coupling J constants a clear differentiation between the through-space (TS) and the through-bond (TB) contributions of indirect scalar nuclear couplings could prove a very useful tool for the elucidation of molecular structures. Consequently, various modeling approaches have been developed which are specifically devoted to separating out the respective TS and TB contributions (see details in section 3.2).8,10,33 Regardless of the usefulness of these theoretical approaches, the most common method for experimentally evaluating strong TSJ(X,X′) values consists in studying systems where the shortest through-bond pathway is long enough (at least four bonds) for through-bond contributions to nuclear coupling to be considered as negligible. Accordingly, some empiric requirements for detection of through-space coupling constants can be expressed as follows. (i) Chemically similar, interacting nuclei need to be magnetically nonequivalent. Spin-active isochronous 4841


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J(X, X′) = FCJ (X, X′) +SD J(X, X′) + PSOJ (X, X′) + DSOJ (X, X′)

(3)

In the first published modeling paper, a semiempirical intermediate neglect of the differential overlap method (INDO)40 was used to study through-space spin coupling.41,42 Barfield attempted to express coupling constants split into TS and TB components using an approach called neglect of nonbonded interactions (NNBI) on 3JCH.43 Contreras and coworkers, on the other hand, used the partially restricted molecular orbitals approach (PRMO)44 to calculate the TS contribution to some coupling constants 5JHH.45 Since this approach was applied to canonical molecular orbitals (MOs) and not to localized MOs, the PRMO method was modified to create the inner projections of the polarization propagator approach (IPPP). This is a theoretical formulation still based on the semiempirical INDO method.46 Using the IPPP method, nonbonded contributions to JFF TS-SSCCs were then calculated.47,48 Although some experimental trends could be consistently calculated, the INDO approach was found not to be reliable enough for this kind of spin−spin coupling constants calculations. Nonetheless, IPPP-INDO calculations supported the assumptions concerning the efficient TS coupling pathway defined by the overlap of two proximate lone pairs.49 It was shown that SSCCs have a substantial FC through-space component that is larger than the TB contribution. This is consistent with experimental data such as the large 3JPP above 150 Hz found in 1,2-C6H4(PPh2)(PMePh).50 These results are consistent with the coupling energy density visualization model reported later by Malkina and co-workers for a cis-diphosphinoethylene. Indeed, these authors established a powerful tool for “visualization” of coupling pathways useful for the qualitative discrimination of TS and TB influence on spin−spin coupling.9,33 The model, based on real-space functions in three-dimensional space, is appropriate for both localized and delocalized bonding situations. The indirect spin−spin coupling was expressed as the energy splitting between states with parallel and antiparallel nuclear spins. These energies were then written as an integral over an “energy density” called the coupling energy density (CED). The CED integral, over all space, was equal to the reduced coupling constant. CED is a real-space function and can be visualized easily in three-dimensional space. The CED has been calculated using a double finite perturbation theory for a range of molecules. Using a cis-diphosphinoethylene model derivative [C2H2(PH2)2] (Figure 3), CED visualization established that the most important pathways for the Fermi contact of the JPP couplings was a through-space component transmitted by the direct overlap of the lone pairs of the phosphorus atoms. Thus, the TS interaction between the two phosphorus nuclei dominates over the through-bond pathway, and the CED integration made it possible to estimate a JPP coupling value of about 180 Hz. In such coupling energy density plots the positive and negative function values are observed as isosurfaces. Their relative importance is indicative of the different pathways and judged by the volumes of the isosurfaces. Figure 3 shows the dominating positive CED located at the phosphorus atoms as well as the across-space strong negative CED contribution between them. The sum of these isosurfaces is greater than the CED contribution which specifically results from the P−C−C−P covalent bonding network.

Figure 3. Visualization of the 3JPP coupling energy density (CED) for C2H2(PH2)2. Isosurfaces are shown with blue and red colors corresponding to the positive and negative function values, respectively. The relative importance of the different pathways can be judged by the intensities and volumes of the colored surfaces. Reprinted with permission from ref 107. Copyright 2009 Académie des Sciences.

A complementary modeling approach was proposed by Soncini and Lazzeretti.51 It should be noted that while accurate electronic wave functions contain all the information needed for understanding the physical properties of molecules, their degree of sophistication may render them more difficult to handle for simple interpretation of the role of the electronic distribution. Thus, Soncini and Lazzeretti introduced a density function which depends on the position in 3D space for nuclear spin−spin coupling.52 The combined use of Fermi contact coupling density plots (as an indication of the electron cloud in which the coupling process occurs) and Fermi hole density maps (as a measure of electron delocalization)53 allows a better rationalization of NMR coupling depending on the molecular structure for one- and two-bond spin coupling. The authors report that in situations where the coupling is intricate and difficult to apprehend such density function plots could help clarify, for example, long-range interactions, the falloff on increasing internuclear bonded separation, as well as throughspace effects.51 These results also suggest that overlap between other types of nonbonding electron pairs could also constitute an efficient coupling pathway for transmission of the spin coupling dominant FC terms. The only requirement for spin coupling would be an overlap between the electron clouds. Subsequently, exchange interactions (or closed-shell interactions) transmit the spin information associated with the FC term between both coupling nuclei.54 As a result, SSCCs involving TS effects, hydrogen bonding, and mixed TS plus TB are included in these pathways. Results related to this work later showed that electron delocalization interactions are the main vehicle for transmitting the FC term for long-range SSCCs, nJXY for n > 3.55 Rationalization of the general pathways for the FC contribution has been reported recently.56 These were based on the analysis of charge-transfer interactions using a natural bond orbitals (NBOs) approach.57 A new convenient approach for identifying coupling pathways for the FC term of SSCCs was introduced by Contreras et al. This is based on the analysis of the spatial distribution of canonical molecular orbitals.58 In this approach, which can be easily applied using current quantum chemistry tools, the Fermi hole was circulated through the whole region spanned by each canonical molecular orbital through exchange interactions. This method has been applied to analysis of the TS transmission of the FC term of JPP SSCCs by the overlapping of the P lone pairs. For the models chosen (2JPP), this study clearly established an exponential 4842


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the magnitude of the coupling constant between 4-F and 5-F remains equal at ±3%. These JFF coupling constants are in agreement with the related short F···F spatial distances, which are almost invariant in these constrained molecules. These features mainly led to the conclusion that any through-bond electronic effect due to the presence of substituents on the rings of the phenanthrene derivatives had no effect on the 19F SSCCs. Analogously, in larger difluoro benzenoid fused rings (Scheme 3), decreasing values of JFF are observed with an

decrease of the (P,P) lone-pair overlap as a function of the P···P distance,58 This is consistent with the experimental data reported.34 The calculations strongly support the intuitive conclusion that the TS lone-pair overlap of proximate phosphorus atoms is the main transmission mechanism.

4. STRUCTURAL PERSPECTIVE OF NONBONDED COUPLING IN SMALL ORGANIC MOLECULES Several molecular systems display very large intramolecular (or sometimes intermolecular) nonbonded spin coupling. Since this phenomenon is intimately linked to the structure adopted by the molecular systems in the liquid-phase (solution) NMR, the main aim of this section is to describe a representative view of these couplings and their usefulness in the structural determination of low-weight organic compounds (less than 2000 g·mol−1). Both very rigid as well as fairly flexible structures can show nonbonded spin−spin coupling constants in solution. For the latter, a notable solvent as well as temperature dependence of the magnitude of the SSCCs can be occasionally seen. In this review, the notation nJXY (or alternatively nTSJXY) is used for spin coupling constants with n ≥ 4, n being the smallest number of covalent bonds separating nuclei X and Y. Nevertheless, in most of the cases reported here nonbonded coupling is clearly dominant. 4.1. Through-Space Spin Coupling in

Scheme 3. JFF TS Couplings Decrease in the Series of Difluoro Benzenoid Fused Rings 3a−3c with the Increase of Helical Character of the Structure

19

F NMR

19

F is 100% abundant with a spin of 1/2, and in addition, it has a high sensitivity and receptivity. In this respect, its NMR is easy to observe and fairly comparable to that of protons. However, a much greater range of chemical shifts and homonuclear spin−spin coupling constants is observed.59 In addition to these analytical advantages, introduction of fluorine atoms into compounds for biomedical applications generally improves their membrane transport, general bioavailability, solubility, and metabolic stability compared to their nonfluorinated analogues. As previously mentioned, the 19F atom is historically not only the first but also the most studied nuclei with regard to nonbonded NMR couplings. This is due to the large number of organofluorine compounds which were first observed demonstrating these typical features. The sign of the TS coupling constants and the methods used to determine them have been previously discussed in the literature.60 However, most of the studies only report the absolute SSCC values. 4.1.1. Nonbonded JFF Coupling. Development of fluorine-substituted aromatic benzenoid fused rings has led to a better understanding of the structure relationships in nonbonded JFF coupling. Studies focused on rigid 4,5difluorophenanthrenes by Servis and Fang61 and Ricker and Mallory (Scheme 2)25 highlight the fact that while the 19F chemical shift of unsymmetrically substituted 4,5-difluorophenanthrenes varies within a fairly large range of about 20 ppm,

increase in the helicity of the relevant molecules.62 With respect to the Mallory model (Figure 1), in the series J(1b) > J(3a) > J(3b) > J(3c), the helical out-of-plane distortion gives rise to both increasing F···F distances and loss of coplanarity for σ overlap of the in-plane 2p orbitals of the fluorine atoms.25 Due to this distortion, the π overlap of the out-of-plane 2p lone pairs is expected to also make a contribution: one even less efficient. Restoration of the structural rigidity and fluorine atom proximity (with lone-pair coplanarity) in the benzo[ghi]perylene derivative 3d consistently leads to a JFF of the same high value as the one found for the phenanthrene derivative 1b. In the related 1,8-difluoronaphthalene molecules (Scheme 4), JFF values range around 68 Hz. In these molecules, contrary Scheme 4. 1,8-Difluoronaphthalene Molecules with SSCCs JFF = 68 ± 8 Hz

Scheme 2. Rigid 4,5-Difluorophenanthrenes Which Display Large SSCCs JFF = 170 ± 5 Hz

to what is observed in the phenanthrenes and higher analogues, no helical distortion exists and thus the magnitude of the SSCCs should depend more on the σ-overlap interactions of the in-plane lone pairs of fluorine atoms than on the π overlap of the other lone pairs. The spatial distances separating the fluorine nuclei have been computed using ab initio methods to give values ranging from 2.533 (4a, JFF = 59 Hz) to 2.479 Å (4g, JFF = 76 Hz).28,63 4843


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The general relationship existing between the JFF coupling intensity and the nuclei internuclear distances has been confirmed for the family of constrained five-membered 4,5bridged derivatives of 1,8-difluoronaphthalene (Scheme 5).28

Scheme 6. JFF TS Coupling and Related d(F···F) Value in Difluorocyclophane Molecules

Scheme 5. JFF TS Couplings and Related d(F···F) in Bridged 1,8-Difluoronaphthalene Molecules

also by the relative spatial orientation of the interacting parts of the molecule. In compounds 6a−i (Scheme 6) the excellent correlation between the distances and the magnitude of the SSCCs is directly related to their small C−F···C−F torsion angle which is close to 0 °C. Conversely, for 6j in which a large deviation from coplanarity of the C−F bonds is observed, the fairly short calculated distance of 3.017 Å found for d(F···F) corresponds to a weak JFF, below 2 Hz. The structural features established for the difluorocyclophane and 1,8-difluoronaphthalene families have also been documented in a related class of derivatives which incorporates two fluorine atoms in a variety of positions, namely, the 1-(fluorophenyl)-8-(fluorophenyl)naphthalenes.69 Besides such difluorinated constrained compounds, many other examples of polyfluorinated molecules which exhibit large 19 F SSCCs arising from spatially proximate fluorine atoms separated by several nonconjugated bonds have been reported. Scheme 7 illustrates some typical examples in which spin coupling has been related to structural features.

For this class of molecules, larger spatial F···F distances separating the fluorine nuclei have been computed. Accordingly, smaller JFF = 30 ± 3 Hz are found for d(F···F) around 2.68 Å (5c−g).63 In the case of the comparatively relaxed sixmembered 4,5-bridged derivatives of 1,8-difluoronaphthalene 5i, a JFF = 62 Hz is found for d(F···F) = 2.540 Å. In the case of this family, unsaturated two-carbon bridges 5a, 5b, and 5h have larger values attributed to TB(π)J(X,X′) through-bond additional contributions (see eq 2). These examples clearly illustrate the difficulty in the fine distinguishing of the contributions of spin− spin coupling propagation across space or via clearly established covalent bonds and hence also the challenges in progressing relevant models and computational approaches.39,64 Fluorinated cyclophanes constitute another class of compounds for which TS 19F19F spin couplings have been studied in detail.65−68 In these species (Scheme 6), generally seven covalent bonds or more separate the fluorine nuclei. In the syn-dithia[3.3]-m-cyclophane molecule 6a, the aromatic rings are tilted such that the C−F bonds point toward each other. According to molecular mechanics computation the TS distance between the fluorine atoms is estimated to be d(F···F) = 2.781 Å. The measured JFF is about 42 Hz. In this family of [3.3]-m-cyclophanes, a decrease in the d(F···F) distance can be effected by introducing hindering bulky t-Bu groups in the para position to the fluorine atoms (6b and 6c). An increase of the JFF coupling, which can reach about 60 Hz, is observed when two t-Bu groups are introduced. In agreement this shortens the distance to 2.597 Å. On closer contact by shortening the m-cyclophanes bridges, d(F···F) values ranging between 2.500 and 2.400 Å are obtained. This is shown in Scheme 6 (6d−g). As a result SSCCs of between 89 and 110 Hz are observed. In the related p-cyclophanes 6h and 6i, the d(F···F) distances are longer (2.998 and 3.180 Å, respectively). Accordingly, the JFF coupling values are lowered to between 14 and 7 Hz. The magnitude of the TS coupling is influenced not only by the spatial proximity of the nuclei but

Scheme 7. JFF TS Coupling Constant and d(F···F) in Selected Polyfluorinated Molecules

In polycyclic organic molecules such as 7a70 or in the molecules 7b and 7c which contain the CF3 group,71 only the more proximate fluorine atoms (2.560−3.014 Å) exhibit SSCCs ranging approximately from 4 to 40 Hz. Bridge-fluorinated bicyclo[1.1.1]pentane derivatives with strain-rich geminal proximate (or W-related) arrangements of fluorine atoms 4844


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have shown a clear dependence on the calculated F···F distance, in the range 2.58−2.43 Å, for 4JFF constants between 50 and 100 Hz, respectively.72 Several studies have early addressed the issue of establishing a mathematical correlation between the magnitude of the JFF spin−spin coupling constants and the nonbonded spatial distances d(F···F).73−77 Ernst and Ibrom did this for difluorocyclophanes (Scheme 6), while Mallory and co-workers did it for the difluoronaphthalenes (Scheme 4). They disclosed the existence of an inverse exponential dependence between the JFF spin−spin coupling constants and the related nonbonded spatial distances d(F···F).66,28 The set of data points for the difluorocyclophanes 6a−6i (Scheme 6) makes it possible to derive a function (Figure 4) that describes the correlation between JFF and d(F···F). This follows expression 4 with a fairly good correlation regression coefficient r2.66 JFF = 867 911e−0.037d(F···F) (r 2 = 0.9876)

Figure 5. Dependence of the JFF coupling constant (Hz) on the nonbonding distances (pm) in difluoronaphthalenes 4a−g (and related compounds) and 5e−g. Reprinted with permission from ref 28. Copyright 2000 American Chemical Society.

orbitals) and the side-by-side systems in difluoronaphthalenes (overlap of locally in-plane 2p fluorine orbitals). In addition, the d(F···F) values were estimated using different methods. MM2 calculations for data related to eq 4 and ab initio calculations for the difluoronaphthalenes data which gives eq 5. On the basis of Mallory’s orbital model, the magnitude of the TS coupling constant JFF depends on the extent to which the σFF and σ*FF molecular orbitals differ in the spatial distribution of their electron densities. The energy difference ΔEpσ between these two molecular orbitals is a direct measure of the extent of the overlap interaction between the fluorine lone pairs and thus the difference in the electronic density distribution. Therefore, this predicted dependence of JFF on the extent of lone-pair overlap is consistent with JFF falling off exponentially with the distance between the fluorine atoms,78 in other words, consistent with the reported expressions 4 and 5. The spin−spin couplings of some of the few difluoronaphthalene compounds which did not fit the exponential relationship 528 were analyzed by DFT with a view to calculating their values for the FC, SD, DSO, and PSO terms.39 Interestingly, the magnitude of the deviation with respect to the expression could be attributed to the existence of significant TB contributions in the FC and PSO terms which are apparently unevenly distributed across the structures. This study clearly demonstrates the difficulty of discriminating between the TS and the TB contributions in the simpler models. A number of examples have been reported in the literature in which, despite a nuclei separation of several covalent bonds existing, the through-space origin of the couplings is debatable. Pertinent examples include those structures in which spatial distances are largely above the sum of van der Waals radii (about 2.7 Å for two fluorine atoms). Because, in solution, some conformations may contribute to a closer positioning of the affected nuclei, a distance of twice the van der Waals radii seems to be a reasonable value for questioning the TS coupling (above ca. 5.5 Å for fluorine atoms). In such cases where typical direct through-space coupling is unlikely, the term “long-range” spin−spin coupling is more suitable. Scheme 8 illustrates some cases. In 8a, for instance, the (19Fa,19Fb) spin transmission has been tentatively attributed to the overlap of the 2p orbitals of each fluorine atom with some two-electron-filled π orbitals of the phenyl group.79 Thus, this interpretation can potentially constitute a pertinent example of alternant TS···TB···TS spin−spin coupling. In the group of cycloalkanes such as the

(4)

Figure 4. Dependence of the JFF coupling constant (Hz) on the nonbonding distances (pm) in difluorocyclophanes 6a−i. Reprinted with permission from ref 66. Copyright 1995 Wiley-VCH.

In this relationship, MM2 molecular mechanics computation methods have been used for determination of d(F···F). These are assumed to be better suited to describing geometries in solution. The use of distances from solid-state X-ray data has been avoided since different values were present in the asymmetric unit of some compounds. This indicates that the packing forces in the solid state have a non-negligible influence in this particular case. The usefulness of such an empirical correlation was checked against 1,8-difluoro-4-methylnaphthalene, which has a calculated d(F···F) = 258.7 pm and a JFF of 65.6 Hz (experimental). Despite the clear differences in structure, expression 4 predicts an acceptable value of 60.5 Hz. Mallory and co-workers constructed the plot shown in Figure 5 using the data associated with compounds 4a−g (Scheme 4) and 5e−g (Scheme 5).28 The best fit for these data, which span a narrow range of distances and SSCCs, corresponds to the exponential expression 5. JFF = (2.0 × 107)e−0.05d(F···F) (r 2 = 0.9913)

(5)

These two sets of data are fitted to the exponential expressions 4 and 5, in which the exponents are rather similar (−0.037d and −0.05d, respectively). The quantitative differences of these distance dependencies are due to structural differences in the compounds: the face-to-face systems in the difluorocyclophanes (overlap of locally out-of-plane 2p fluorine 4845


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couplings are typically dominated by Fermi contact (FC) coupling mechanisms, in these long-range couplings spin− dipole coupling mechanisms (SD) have been proposed as being the dominate ones.85 For the FC mechanism, spin polarization of the electron system is caused by the strongly localized magnetic field inside the first coupling nucleus (perturbing nucleus) and probed by the second (responding) nucleus. Analysis of SDs indicates that the π and π* orbitals of the F nuclei provides an effective transfer of spin information into the global electronic system without the need for σ orbitals as intermediates for a large spin polarization. In contrast to the FC mechanism, the π(F) orbital in addition to the adjacent π(CC) orbitals and 3pz(F) Rydberg orbital provide a SD(π) coupling mechanism at long bonded distances that is more effective because it does not include mediating σ orbitals.85 In studies devoted to polyenes, the conformational flexibility of the structure is assumed to fluctuate by no more than about 10°. The cis conformation is considered to be present at less than 1% concentration at room temperature. Acyclic perfluorinated alkanes were also studied in which JFF scalar couplings greater than three bonds between proximate fluorine atoms suggested TS couplings occur via extended delocalization of the p electrons from the fluorine atoms.87 4.1.2. Nonbonded JFX Couplings (X = N, P, Se, and C). 4.1.2.1. JFN Couplings. Spin-active nuclei which have one or more lone pairs available for overlap with the fluorine lone pairs (such as 15N, 31P, or 77Se) are susceptible to generating nonbonded JFX couplings. These lone-pair effects were detected and identified in 2-fluoropyridines (10a, Scheme 10) with

Scheme 8. Long-Range Spin−Spin Coupling with Unusual Transmission Pathways

4,9-difluorodiamantane 8b and the 1,4-difluorocubane 8c, the interacting nuclei are pointing away from each other (see also section 4.1.2.4). For this type of compounds, different spin− spin transmission mode mechanisms have been discussed, for example, the overlap of the rear lobes of the aligned C−F bonds.80 However, it has also been calculated that on the center of the cube the minimum electron density was lower than that on the surface. This makes it unlikely that the coupling transmission occurs through an electronically deficient space.80 In 8b and 8c, a common structural feature of the compounds is the existence of six-membered cycloalkane rings with C−H bonds in the plane of the rings. This is a required condition for the σ aromaticity of the Möbius type (conjugated closed molecular loops are called Möbius bands).81 Accordingly, Gronenborn and co-workers proposed the possibility that spin−spin information in cubanes and related structures is transmitted through the system of σ bonds as well as the Möbius type σ-aromatic ring.80 Acyclic molecules are also likely to display unusual longrange spin−spin couplings. In fluoropropenes, the four-bond 4 JFF couplings have been analyzed theoretically. In particular, the angular dependence of the FC, SO, and SD terms was taken into account.48,82 These couplings were found to be critically dependent on the F···F distances and are larger wherever there are more electronegative fluorine atoms on the α-carbon (CF3 groups). In extended unsaturated molecules such as polyene (Scheme 9)83 and polyyne derivatives,84 both experimental and

Scheme 10. JFN Through-Space Couplings in Both Constrained (10a,10b) and Flexible Molecules (10c)

Scheme 9. Fluorinated Acyclic Polyenes with Long-Range Spin−Spin Couplings

abnormally large 2JFN values above 50 Hz.88 The 15N-enriched oxime derivative of fluoronaphthalene 10b27,32 confirms that intramolecularly crowded F and N atoms generate large nonbonded SSCCs. From the solid-state structure studies of 10b, it was found that d(F···N) = 2.595(4) Å is short and the torsion angle C−F···CN = 11° is small. As a corollary of Mallory’s theory, a 2sp2 hybrid lone-pair orbital of nitrogen is suited to interaction with the 2p lone-pair orbitals of fluorine for the transmission of nuclear spin information. An even greater efficient spin−spin coupling is expected as a result of the s character of the hybrid orbital which allows direct Fermi contact participation. Conversely, the approaching unconstrained oxime compound 10c (Scheme 10), in which two conformers can coexist, leads to a SSCC of low magnitude (3.2 Hz). This is despite the same connectivity as is found in 10b. This is attributable to the (F,N) nuclei which are spatially remote from one another in the lower energy conformation.

theoretical results suggest that the delocalized π-electron system may provide measurable SSCCs over nine bonds.85 Thus, in 9a, a 5JFF of about 36 Hz was measured;83 in addition, a 7JFF in the 20 Hz range was calculated for 9b.85 Provasi and co-workers also predicted such long-range spin− spin coupling constants for fluorinated polyynes, cumulenes, and polyenes. The latter have 19F19F SSCCs ≥ 9 Hz across 11 bonds and a spatial distance of more than 11 Å.86 While TS 4846


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Hz, respectively.90 The magnitude of this JFP is related to the electron-withdrawing effect of the two fluorine atoms directly bonded to the phosphorus. Up until 2000, nonbonded JFP coupling constants had been only sporadically mentioned in the literature and then only for a limited number of compounds such as the interesting naphthalene derivatives 12e and 12f. Here the short d(F···P) < 2.9 Å induces large JFP above 130 Hz.28 Large JFP SSCCs (7TSJFP = 15 Hz) have been reported for compounds based on the rigid 1,8-bis(diphosphinonaphthalene) backbone91 in which the phosphino groups were fluoroaryl substituted.91a More recently through-space (19F,31P) spin couplings have been experimentally analyzed for C1-symmetric flexible triflone-bearing phospharamidites92 as well as biaryl diphosphines (Scheme 13, 13a and 13b,c, respectively).93

Other molecules with constrained structures and containing nitrogen and fluorine atoms display a large JFN coupling. This is particularly true for the nitrogen derivatives of fluorinated benzenoid compounds as discussed in section 4.1.1 and illustrated in Scheme 11.32 From the JFN values in compounds Scheme 11. JFN Through-Space Couplings in Constrained Benzenoid Fused Rings 11a−g

Scheme 13. JFP TS Coupling Values in C1-Symmetric Phospharamidite and Biaryl Diphosphines

11d−g the spin−spin coupling constants are essentially independent of the nature of the substitution on the oxime oxygen atom. This therefore suggests that these substituents have little steric and electronic influence on the properties of the nitrogen atom lone pair. 4.1.2.2. JFP Coupling. The magnitude of the JFP spin constants, greater than 50 Hz, found in o-trifluoromethylsubstituted triphenylphosphine derivatives has been compared to the null spin−spin JFP SSCCs in their meta- and parasubstituted analogues. This indicates that a through-space mechanism is primarily responsible for the large coupling observed in the ortho-substituted family of compounds 12a−c (Scheme 12).89 For (F,P) nuclei pairs the intensity of the coupling is sensitive at internuclear distances larger than the (F,F) pairs since the van der Waals radius of phosphorus is 1.8 Å compared to 1.35 Å for fluorine. In 12a−c, the SSCCs JFP ranges between 50 and 55 Hz for short d(F···P) distances around 3.02 Å in the solid state. Consistently, the SSCCs 4TSJFP and 5TSJFF in 12d were found to be on the order of 68.3 and 8.3

Only one of the two diastereomers of 13a has 19F nuclei of the triflone group which couple to the 31P nucleus with a JFP of about 5 Hz, while a six-bond internuclear separation exists between F and P nuclei. The three nuclei shielding is time averaged by free rotation of the CF3 moiety. In this flexible structure, the dependence of the TS SSCCs on the configuration adopted was calculated using a simplified DFT model for a Me2NP(OH)2···CF4 pair (Scheme 13, top right). The CF4 molecule was positioned across a range of P···C through-space distances 8 ≥ d(P···C) ≥ 2 Å, and the θ angle between the lone pair and the P···C axis was varied from 0° to 55°. The calculated values are of comparable magnitude (4−6 Hz) to the experimental ones for a narrow window of geometries for which d(P···C) ranges between 5.7 and 6.1 Å and 35° > θ > 10°. These parameters allow attribution of the SSCCs to the SS-13a diastereoisomer. As an additional evidence of flexibility in 13a, a notable solvent/temperature dependence of the spin coupling constant was also noted.92 Bis(diphenylphosphanyl)biphenyl compounds 13b and 13c substituted with rigid cyclic fluorinated ether groups (Scheme 13) present both large nonbonded JPP and JFP SSCCs.93 (P,P) nuclei (see also section 4.2.1) and (F,P) nuclei are separated by at least five and seven covalent bonds, respectively. In these

Scheme 12. JFP TS Couplings in o-CF3-Substituted PPh3 Derivatives 12a−c and Naphthalene Derivatives 12e and 12f

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compounds with C1 symmetry, at room temperature a slow internal rotation about the C1−C1′ carbon bond is seen. This most certainly allows proximity in space of coupling nuclei otherwise precluded within a fully rigid conformation. While only a single heteroannular JFP value of 2.3 Hz is observed in the difluorinated compound 13b, in the corresponding tetrafluorinated 13c, two heteroannular JFP SSCCs of 2.5 and 5.1 Hz are distinguishable. In these compounds all fluorine atoms are anisochronous, and the attribution of couplings to the (P,F) nuclei pairs in the syn positions has been proposed, with a JFP of higher magnitude for the more proximate P···Fa. In comparison with JFF and JFN, the nonbonded spin coupling constants JFP evidently span a wider range of magnitude, up to 140 Hz for nonbonded distances, as well as d(F···P) distances ranging from 2.8 to ca. 4.8 Å. In the framework of the orbitals overlap model, this feature which is very relevant for structure and conformation determination is attributable to the extent of the phosphorus 3p lone-pair orbitals or more precisely their hybrid derivatives incorporating fractional s character (see also the discussion in section 4.2). 4.1.2.3. JFSe Coupling. 77Se with a 7.63% naturally abundance and which is a low sensitivity spin 1/2 nucleus is also resonant over a very wide range of chemical shifts (Δδ > 3000 ppm). Selenium 77 nuclei often show heterocouplings to other nuclei such as 1H, 13C, 15N, 19F, 31P, etc. With respect to TS spin coupling they are of special interest because of the extent of their van der Waals radius (1.9 Å) as well as their populated 4s and 4p orbitals. The latter should lead, according to lone-pair overlap theory, to large JFSe values detectable at d(F···Se) distances of around 3.25 Å or less. This corresponds to the sum of the fluorine and selenium van der Waals radii. Accordingly, nonbonded F···Se interactions as evidenced by large spin coupling constants over a four-covalent-bond separation have been reported for o-selenobenzyl fluoride (14a−h, Scheme 14)94 and aryl- or alkyl-selanylnaphthalene compounds (15a and 15b, Scheme 15).95 Scheme 14.

Scheme 15. High-Magnitude Alkyl-Selanylnaphthalenes

4TS

JFSe SSCCs in Aryl- and

effect on the JFSe values of Se (cyano and chloro substituents, 14a and 14b). This is consistent with an enhancement of the Fermi contact and results from an increased electron s density at the Se. Molecular modeling led to an estimated d(F···Se) of about 3.0 Å for 14g and 14h. Here the rapid rotation of the trifluoromethyl group could not be frozen at −90 °C. The strong steric constraints of the naphthalene derivatives 15a and 15b (Scheme 15) lead to very large, above 280 Hz, JFSe SSCCs .95 On the other hand, for the analogous compounds 4a (Scheme 4) and 12e seen in Scheme 12 JFF and JFPs, of respectively, 59 and 144 Hz have been reported for nonbonded internuclei distances in the same range as the one reported from the X-ray structure of 15a (d(F···Se) = 2.75 Å). The reported crystal structure for 15a also indicates a remarkable F···Se−CPhOMe alignment characterized by an angle θ (F···Se− C) = 175°. In 15a and 15b, the fluorine, selenium, and carbon atoms which are linearly arranged also lead to 5TSJFC values of around 15−18 Hz. In comparison to 15a and 15b, considerably weaker interactions F···Se are found in molecules with structures approaching that of o-fluoro-benzylselenide (Scheme 16). Here the flexibility is ensured by a spacer between the aryl moiety and the selenium atom.96 Scheme 16. Low-Magnitude benzylselenide

4TS

JFSe SSCCs in o-Seleno-benzylfluoride

4TS

JFSe SSCCs in o-Fluoro-

4.1.2.4. JFC Coupling. The 13C and 1H spin-active nuclei which are most common in organic compounds are also susceptible to interaction with 19F via assumed nonbonding mechanisms. This is most notable in those specific cases without any available lone pairs for through-space overlap with the fluorine lone pairs. Therefore, it appears obvious that the electron pairs from the existing C−H bonds play a very important intermediate role in the nuclear spin transmission.78 Quite early related calculations using the INDO method supported this view for various fluorinated ketones, propenes, and hydroxynaphthalenes.97−99 Bicycloalkanes were the first family of compounds in which nonbonded interactions involving both 13C and 19F nuclei were identified.100,101 In the polycycloalkanes 17a−c (Scheme 17), the origin of the detectable spin−spin coupling between remote (F,C) nuclei of at least four covalent bonds has been discussed in terms of through-space interactions arising as a result of the

For compounds 14a−f, two stable rotational conformations have been identified. These conformers are depicted in Scheme 14, and their corresponding JFSe values, found in solution to lie between 22.7 and 84.2 Hz, obviously result from a rapid equilibrium between this kind of rotational isomers.94,26 Some electron-withdrawing substituents induce a strong amplifying 4848


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Scheme 17. 4JFC Spin−Spin Coupling in Polycycloalkanes

Scheme 19. Fluorinated Nucleosides with JFC SSCCs at Six Covalent-Bond Separation

proximity of the bridgehead carbons. Also involved is hypothetically the number of different through-bond pathways available for spin−spin transmission.100 Large 3JCFs for vicinal atoms in related 1-fluorobicycloalkanes such as 17d and 17e (23−43 Hz) have been attributed to the positive contribution of TS coupling between the bridgehead carbon atoms (Ca,Cb). These are separated by 1.844−2.172 Å and have a θ angle Ca··· Cb−H of 180−175°.101 The interpretation of the possible transmission mechanisms for NMR JFC scalar couplings in such polycycloalkanes and parent cubanes (Scheme 8) is still controversial.80 The overlap of rear orbital lobes is assumed to be an efficient pathway for transmitting the TS contribution associated with the FC term (Barfield “transmission effect”).8,10,102 On the other hand, data from DFT calculations has suggested the important influence of transmission mechanisms in covalently bonded alkane edifices. Examples are found in the 1-F,4-X-cubanes (X = halogen atom) where the strong σ-hyperconjugative interactions involve the cage C−C bonds as donors.55,103 JFC coupling between nuclei separated by more than four bonds has been observed in a range of organic and biological small molecules (Schemes 18 and 19, respectively). In the family of constrained fluoro-naphthalenes, 4-substituted-1acetyl-8-fluoronaphthalenes 18a−i (Scheme 18) exhibit large JFC values at a five covalent-bond separation.104 This family of

compounds exhibited such JFC SSCCs because of their general structural features which also led to TS spin−spin coupling as previously discussed for JFF (Schemes 4 and 5), JFN (Schemes 10 and 11), and JFP (Scheme 12) SSCCs. The nature of the substituent R induces a variation in the JFC of less than 30%, ca. 10 Hz. The same range of change was observed with variation of the solvent for naphthalene derivative 18d, R = F, JFC 6.78 Hz in DMSO, 9.06 Hz in CHCl3. Molecular conformation information of the fluorescent boraindacene dyes (BODIPY derivatives) 18l−n (Scheme 18) in solution has been revealed by specific SSCCs JFCa.105 Data indicates the presence of TS coupling of the two fluorine atoms to Ca as evidenced by a clear triplet in the 13C NMR spectra. No other JFC SSCCs were detected, even with connections of less than five bonds. In compound 18l, the average C−H···F and C···F distances were estimated, respectively, at 2.5(2) and 3.04(8) Å. These are clearly less than the van der Waals radii estimated for (H,F) of about 2.62 Å and for (C,F) about 3.19 Å. In the species 18l−m, the ethylene bridges (E = CH2CH2, JFCa ≈ 11 Hz) force the benzene rings inward toward the fluorine atoms. Conversely, in 18m (E = S, JFCa ≈ 5 Hz), the conformation is less congested with the benzene rings directed outward. JFC values of notable intensities above a six covalent-bond separation were observed in a family of synthetic fluorinated nucleosides.29 In representative examples such as 19a and 19b (Scheme 19), spatial distances d(F···C6) below 2.8 Å were estimated using energy minimization models. In addition, (C6− H···F) angles within the range 145−159° were found. An attractive interaction between F and H on C6 is consistent with an intramolecular hydrogen bond (see section 4.3.2). In these biomolecules, such couplings were exclusively detected for the α-glycoside anomers. Directed irradiation NMR experiments confirmed the spatial proximity of the nuclei, thus supporting a nonbonded spin transmission. JFC coupling for nuclei separated by up to six covalent bonds has been also used to determine the Z/E configuration for some substituted trifluoromethylvinyl compounds of the CF3CRCR′R″ type. Here fluorine atoms from the CF3 interact with the remote carbon atoms from R′ or R″ depending on the short internuclear TS distances provided by one stereoconfiguration and absent in the other.106 The reported JFC SSCCs remain low (below 3 Hz) above a fourbond separation. However, they are still detectable for up to a six nonconjugated bond separation. Chemistry and structural biology often involves a F NMR probe; therefore, TS spin coupling can be an additional valuable tool in pursuing structural questions which may be difficult to answer by other methods. Only relevant examples of small

Scheme 18. JFC Spin−Spin Couplings at Five Covalent-Bond Separation

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Table 2 summarizes the X-ray and 31P NMR data for typical 1,2-substituted ferrocenyl diphosphines. Consistent with the lone-pair overlap theory, JPP couplings ranging between 15 and 120 Hz are obtained when the phosphorus lone pairs of the molecules point toward each other. This is illustrated in entry 1 for the molecular structure named EJAKAB in the Cambridge Structural Database, CSD. According to X-ray structural data, these spin coupling constants correspond to d(P···P) nonbonded distances ranging from 3.35 to 3.90 Å. The sum of the van der Waals radii for (P,P) is estimated at 3.60 Å. Interestingly, singlet NMR resonances with no apparent spin coupling have been observed for each of the phosphorus atoms in the only two examples for which the lone pairs have been found pointing in different directions. Again, this is according to the X-ray analysis (entries 15 and 16, YIFPOS and YOJKUD). Accordingly, longer P···P through-space distances have also been found with values above 4.8 Å in the corresponding X-ray structures. These typical features probably indicate a time-averaged preferred conformation in solution fairly similar to those found in the solid state. These nonbonded large couplings can sometimes result from a blocked conformation of the ferrocene backbone due to ansa bridged rings (entries 1, 12, and 13) or from very sterically hindered lateral substituent moieties (entries 10 and 11). In addition to the 1,2-substituted ferrocenyl diphosphines, some other specific ferrocenyl polyphosphines which incorporate more than two phosphorus donor atoms have provided a set of species for which the proximity of the heteroannular phosphorus results in the existence of diverse and large unambiguous through-space nuclear spin couplings (Scheme 21).107,108 This class of compounds also differs from the ferrocenyl 1,2-diphosphines because the phosphorus atoms involved in the JPP SSCCs are attached to the two different cyclopentadienyl rings which are linked by the iron metal center. In the tetraphosphine compound 21a (Scheme 21), the occurrence of a large nonbonded nuclear spin coupling TSJAA′ (60 Hz) between the heteroannular phosphorus atoms A and A′ (magnetically nonequivalent atoms from an AA′BB′ spin system) is demonstrated by the zero value of the analogous 4 JAB′, 4JA′B, and 4JBB′ even though the covalent bond separation scheme is exactly identical.34 The X-ray structure of this conformationally rigid molecule confirms a close distance d(PA···PA′) of about 3.7 Å. Our group then developed the conformationally constrained parent ferrocenyl triphosphines (21b−f, Scheme 21). Here the less congested environment at the phosphorus atoms led to a weaker TSJAM from the ABM spin systems: values spanning the range 6−23 Hz were obtained. This correlated to a conformation rotation of these compounds around the iron center and their C−P bonds to a certain extent.112 In the X-ray structure of hexaphosphine 21g (Scheme 21) a piano-stool arrangement for each of the two sets of three P atoms facing the same direction is observed.113 A strong coupling constant value for JPP of 37 Hz between the heteroannular phosphorus atoms was identified on slowing down the fluxional behavior of the molecule by cooling it to −60 °C. In the solid state, distances between the phosphorus atoms ranging between 3.9121(6) and 3.9756(6) Å were observed. A model of the origin of JPP spin transmission inspired by the lone-pair overlap theory has been proposed.34 Thus, the spatial proximity of the two heteroannular phosphorus P and P′ (Figure 6, left) and the prerequisite spatial orientation of their lone pairs leads to an 3sp3−3sp3

organic molecules are presented in the present section. However, a major perspective of TS spin couplings involving fluorine atoms can be expected from the analysis of 19F coupling in fluorinated amino acids, proteins, nucleic acids, and even nucleic acid−protein complexes.80 Questions that can be answered are expected to include the differentiation of rotamers and diastereoisomers, estimation of internuclear distances, computation of tertiary and quaternary structures, as well as greater understanding of the dynamic processes involved in solution and hence in vivo. Therefore, incorporation of 19Flabeled amino acids into proteins may be able to provide additional information regarding protein structure determination in the future. Also, the capacity of fluorine to undergo TS coupling with many important nuclei, in particular, carbon and nitrogen, is not negligible and remains to be exploited. It should be emphasized that these concerns are intimately connected also to the existence of spin−spin coupling transmitted through hydrogen bonding (as will be discussed in section 4.3). 4.2. Through-Space Spin Coupling in

31

P NMR

31

The P nucleus is essentially 100% abundant with a spin of 1/2 and a high receptivity. The chemical shift of phosphorus is very sensitive to its environment, and phosphorus nuclei coupling is sometimes characterized by complex multiplets. As discussed below, these can be very informative with respect to the molecular structures under study. As previously shown, 19F interactions with phosphorus were early recognized as generating TS spin couplings because of the large number of organofluorine compounds which were described. Conversely, although the experimental occurrence of unusually large spin− spin coupling constants observed in 31P NMR between atoms separated by four chemical bonds or more is fairly frequent, it has been only rarely suggested that this is an abnormal or even simply a curious phenomenon with regard to fundamental NMR knowledge. Experimental and theoretical studies are now advanced enough to illustrate the significant impact of nonbonded JPP SSCCs on the structural determination of organophosphorus molecules as well as for metallocenic phosphines and other related coordination compounds.33,34,58,107−109 4.2.1. Nonbonded JPP Coupling. 1,2-Substituted ferrocenyl diphosphines of the type 20a are compounds which present large 31P−31P nonbonded SSCCs (Scheme 20). Scheme 20. Ferrocenyl Homoannular Diphosphines (R1, R2, R3 = aryl or alkyl)

Significant interest in these species is related to their usefulness in transition metal coordination and catalysis.110,111 For instance, in 2010 a survey of the relevant solid-state structures reported yielded more than 15 structures in which large TSJPP couplings are clearly observed between homoannular phosphorus separated by four nonconjugated covalent bonds or more. The term homoannular stands for atoms attached to the same cyclopentadienyl ring.108 4850


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Table 2

Scheme 21. TSJPP and d(P···P) in Ferrocenyl Hexa-, Tetra-, and Triphosphines

Figure 6. Orbitals generated by the overlap of the two lone-pair orbitals on intramolecularly crowded phosphorus atoms.

orbital overlap of the same nature as the ones assumed for 2p− 2p orbitals in the F/F pairs or for 2p−2sp2 orbitals in F/N pairs (Figure 1). The overlap of the phosphorus lone-pair atomic 4851


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Scheme 23. 3TSJPP in a Polykis(phosphino)arene 23a (left), and Conservation of a Detectable JPP′ SSCCs in 23b,b′ Despite Oxidation of One Phosphine (right)

orbital can be formulated as generating two molecular orbitals occupied by four electrons. That overlap interaction does not lead to a net chemical bonding between the phosphorus atoms but rather ensures the transmittance of nuclear spin information via the electronic spin polarization (Figure 6, right). As detailed previously for fluorine atoms, the overlap between the electron clouds generates a significant Fermi contact interaction between the P nuclei. This is amplified by the s character of 3sp3 hybrid orbitals. The larger size of phosphorus lone pair allows prediction of some significant spin couplings JPP at distances longer than for JFF and above the sum of the van der Waals radii of P,P′ pairs. To support this view, several other examples of ferrocenyl polyphosphines, for which lone-pair overlap is structurally unlikely, have been synthesized. Subsequently, no JPP coupling was detected.114,115 The TSJPP SSCCs observed in the aforementioned metallocenic ferrocenyl phosphines and related to short spatial separations between the phosphorus nuclei are also present in other families of purely organic compounds. For example, the enantiomerically pure phosphinated calix[4]arenes 22a and 22b (Scheme 22) in which two proximal phenoxy rings are

Scheme 24. Fused Carbocyclic Structures with Proximate −PR2 Moieties Having Lone-Pair Overlap

Scheme 22. 10TSJPP SSCCs in Chiral Diphosphino-calixarenes

(24a) and fused diarene−heteroarene (24b and 24c) are good examples.126,127 TSJPP SSCCs can also be expected to be found in phosphametallocenes,128,129 in constrained bis(ferrocenyl) phosphines,130 and even possibly in polyphosphoro catenates.131 The chemical differentiation of proximate phosphorus nuclei has been also achieved by synthesizing and studying a variety of interacting P(III)−P(V) compounds (phosphite, phosphinite, and phosphonite).132 The electron-withdrawing effect of oxygen increases the s character in the resulting P···P(O) mutual interaction. As a result large J values are to be expected. In the diastereoisomeric forms of the aminophosphine-

substituted by a flexible −OCH2PPh2 group display a typical AB spin system pattern with a SSCCs JPP of about 8.0 Hz.116−118 In the solid state, the separation between the two phosphorus atoms was found to be on the order of 5.333(1) Å, with an approximate alignment of the two phosphorus lone pairs. It appears likely that in solution the P···P separation becomes even shorter temporarily. The calixarene core of 22b displays a high flexibility with the nonsubstituted phenolic ring flipping rapidly through the calixarene annulus. This fluxional behavior, combined with the mobility of the −OCH2PPh2 arms, increases the possibility of interaction of the phosphorus lone pairs. This JPP value increases up to 10.5 Hz on lowering the temperature down to −90 °C. Significant TSJPP SSCCs are also detectable in the constrained (diphosphino)naphthalenes,119−122 which are analogous to the (difluoro)naphthalenes which are discussed above and also in the polykis(phosphino)arenes which have vicinal phosphorus atoms with properly oriented lone pairs (Scheme 23, 23a). 12 3 ,1 2 4 Interestingly, in the partially oxidized (diphosphino)naphthalene 23b (Scheme 23),125 the two interchanging conformers appear to conserve a detectable 4TS JPP (about 4−5 Hz) despite the fact that only one lone pair is available for nuclear spin transmission. This may very well be an interesting example of spin transmission between one lone pair of electrons and another localized electron pair involved in a covalent bond: either P−Cl or PO, while P−C is unlikely since no resonance splitting is observed in the 13C NMR. The rigidity of other fused carbocycles has also led to significant JPP SSCCs for which the magnitude of the coupling can be attributed to the overlap of well-oriented phosphorus lone pairs (Scheme 24): phosphino-functionalized triptycenes

Scheme 25.

4TS

JPP in Arylphosphine-phosphonites

phosphinophenols 25a,a′ (Scheme 25) the magnitude of the 4TS JPP (151 and 238 Hz) are comparable to the 1JPP values of P−P single covalent bonds.133 This example clearly shows that fairly flexible structures not embedded in fused aromatic rings can also display significantly large nuclear spin couplings. It could be assumed that over the time scale of the NMR the average position of the lone-pair electrons involves a significant overlap, thus allowing a very effective nuclear spin transmission. It is very likely that variable-temperature NMR experiments 4852


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heteroatom bonds (P = E) are involved (26c, E = O, S, or Se), probably because the phosphorus P electron pair is no longer available and more localized toward E atom. The role of lone-pair orbitals in the nonbonded P···Se interactions of compounds such as 26b and 26c has been analyzed using ab initio MO calculations on [H3PX···SeH2] basic models (X = O, null).136 Such models are, however, accepted as being definitely too rudimentary for sound predictions. 4.2.3.2. JPC Coupling. Although carbon nuclei are ubiquitous in organic compounds, nonbonded spin−spin nuclear coupling between phosphorus and carbon atoms have, to date, not been commonly reported. This is probably in part due to the fact that 13C NMR is often considered more as a routine characterization method, and hence, its potential (and interpretation) is sometimes overlooked. JPC coupling is more easily detected from a 13C NMR spectrum but may require broad band 31P decoupling for confirmation. An early identification of such a coupling was made by Pascal and coworkers.137 They observed JPC couplings which involved the phosphorus atom of the cyclophane phosphine 27a and the six carbon atoms of an aromatic ring facing the phosphorus lone pair (Scheme 27).

(using low temperature to slow down the dynamics) conducted more systematically on such flexible systems could very well validate this hypothesis. In particular, by freezing a conformation this would then show intense J coupling. The collection of phosphorus-containing compounds previously described demonstrates the existence of TS couplings in a great variety of both strongly constrained and more flexible organic structures. In many of these cases the nonbonding repulsive interactions, attributable to the phosphorus lone pair(s), obviously contribute to the transmission of nuclear spin information. Hence, the magnitude of the resulting SSCCs can be of the same order as (and even sometimes greater than) those observed from well-established “short range” through covalent bonding. 4.2.2. Nonbonded JPX Coupling (X = Se or C). As is found for fluorine nuclei, phosphorus atoms also display strong and directly detectable nonbonded coupling with other spinactive atoms such as fluorine (see section 4.1.2.2) but also with selenium, carbon, and even hydrogen. Due to the extent of the phosphorus orbitals (3p, 3d) such couplings can even be expected at fairly long distances even greater than 3.0 Å. 4.2 .3 .1 . J P S e Coupling. Treat men t of 1,8-bis(diphenylphosphino)naphthalene derivatives with stoichiometric amounts of selenium afford the corresponding diphosphine monochalcogenides. An example is 26a, which is shown in

Scheme 27. Phosphorus−Arene Interactions Involving the P Lone Pair as Evidenced by TS SSCCs

Scheme 26. 5TSJPSe and 4TSJPP Values in Selenated (Phenylphosphino)naphthalene Derivatives

5TS

134

31

JPCn SSCCs ranging from 3.5 to 7.5 Hz were observed in solution, while an X-ray diffraction study showed a d(P···Ct) phosphorus ring center distance of 2.90 Å. Conversely, no coupling was observed between the phosphorus atom and the bridging methylene carbons C′3 despite their greater proximity to the phosphorus through covalent bonding. This is strong evidence for the interaction TS of the phosphorus lone pair with the arene ring current. The proximity of the two cyclopentadienyl rings in metallocenes can give rise to J(P,C) spin−spin coupling between atoms connected to the distinct cyclopentadienyl rings. The ferrocenyl triphosphines 28a (Scheme 28) is an example of a molecule in which such a coupling occurs.138 As demonstrated by selective 31P decoupling experiments, the 13C NMR spectroscopic data revealed a through-space spin−spin nuclear coupling J(P,C) = 5.5 Hz between the methyl carbon atoms of the t-Bu group (with fast rotating methyl groups) and the phosphorus atom. Indirect evidence supporting a throughspace coupling mechanism came from the absence of any other 4 J(P,C) or 5J(P,C) SSCCs for 28a. A single-crystal X-ray diffraction study confirmed the spatial proximity of the t-Bu and

1

Its P{ H} NMR spectrum showed a large Scheme 26. 5TS JPSe SSCCs (54.0 Hz), which indicates a significant throughspace coupling component. Interestingly, a strong JPP (53.0 Hz) was also observed. One possibility is that this is transmitted via a mechanism combining both TS(P,Se) + 1(Se,P) in which the lone pairs from selenium would be expected to be involved. Alternatively, the three-center couplings with P and P′−Se, as described above, and an interaction between a lone pair and the localized electrons of the PSe bond may be responsible. Clearly, it would be highly interesting to apply the Malkina et al. method of visualizing CED to this example (see section 3.2),9 with a view to resolving the indeterminacy of this particular coupling pathway. In the mixed phosphorus−chalcogen peri-substituted system (8-phenylselanylnaphth-1-yl)diphenylphosphine 26b (Scheme 26) a very large JPSe of 391 Hz is observed.135 In the analogous sulfanyl compound d(P···S) = 3.0330(7) Å, here also a fairly short P···Se distance is expected with, in addition, a more effective lone-pair overlap in comparison to the case of 26a. Such high values of SSCCs are not seen when phosphorus− 4853


Chemical Reviews Scheme 28.

TS

Review

Scheme 30. 5TSJPC and 6TSJPC SSCCs within a Rigid 1,8Bis(alkylphosphino)naphthalene

JPC Couplings within Ferrocenyl Phosphine

reasonable pathway would combine a TS(P,P) + 1(P,Ca) and a (P,P) + 2(P,Cb), respectively. The spin transmission should then basically involve both a lone-pair repulsive interaction and a classical P−C covalent bonding. Again, modeling for visualization33 of the contributions involved in such situations is still lacking and would be highly interesting for understanding these intriguing mixed spin−spin transmission pathways. It may in fact constitute a priority objective in this field of research. Owing to the current interest in P-containing molecules used as ligands for transition metals in the field of homogeneous catalysis as well as in the fundamental reactivity of organometallic compounds, there is much interest, both basic and applied, in the observation of TS coupling involving P nuclei. In particular, such coupling can provide structural information in solution that is not systematically identical (or even attainable) in the solid state. This idea is further developed in section 5.2. Therefore, structural rigidity in solution and some specific internuclear proximity aspects can be easily assessed by understanding the SSCCs observed. This is particularly true of chelating diphosphines and polyphosphines. From a modeling perspective, naphthalene and cyclophane phosphinated derivativesif further synthetically developedmay prove to be complementary species for theoretical studies of the different coupling transmission pathways due to their very different arrangements of cyclic σ and π covalent bonding. TS

P(i-Pr2) groups with a shortest d(P···C) separation of about 3.64 Å. In the 13C NMR spectra of the azaphosphole ferrocene 28b (Scheme 28),139 triplets are detected for C1 and C2 at 123.2 and 65.8 ppm. This shows that these carbons that are part of the same cyclopentadienyl ring display a through-bond coupling with their homoannular phosphorus (2JCP and 3JCP, respectively). Additionally, through-space JCP coupling between 2.5 and 7.5 Hz with the available lone pair of the phosphorus of the other ring has also been seen by 13C NMR. In good agreement is an X-ray diffraction study which clearly showed the spatial proximity of the heteroannular P and C atoms with internuclear distances ranging between 3.68 and 3.80 Å. Nonbonded JPC spin coupling was also shown in the rigid C2symmetrical cyclodextrin derivative 29a synthesized by Matt and Armspach (Scheme 29).140 A spin−spin coupling J(P,C) of Scheme 29.

JPC in a Cyclodextrin-Based Diphosphinea

8TS

4.3. Through-Space Spin Coupling Involving Hydrogen Bonding

4.3.1. General Occurrence of Spin Coupling “Through Hydrogen Bonding”. Indirect scalar coupling can be frequently observed across the hydrogen bonds of biomolecules as well as smaller chemical compounds.143,144 The values of the SSCCs provide a very sensitive measure of the hydrogenbonding geometries in proteins145−147 as well as in nucleic acids.148 This is because of their strong dependence on the A− H···B bond distances and angles. This topical subject related to the characterization of biomolecules and high-resolution multiparameter NMR has already been thoroughly discussed.149,150 The designations nhJAB (or alternatively hnJAB) are currently employed for SSCCs between nuclei A and B through hydrogen bonding. This convention emphasizes that one of the n bonds connecting the two nuclei in the chemical structure is actually a hydrogen bond. In this review hydrogen bonding will refer to the weak attractive forces between an electronegative acceptor atom and a hydrogen atom covalently attached to a different electronegative atom. Thus, the activation energy for formation of hydrogen bonding as well as the resulting bond energy are small compared to covalent bonding.151,152 Similar to the through-space couplings discussed earlier in this review (sections 4.1 and 4.2) the most common through

a

Reprinted with permission from ref 107. Copyright 2009 Académie des Sciences.

3.5 Hz between the C-6C atom (equivalent to C-6F) and both phosphorus atoms was detected in the 13C NMR. The throughbond contribution to the J(P,C) spin coupling is negligible given the eight-bond separation existing between the C-6 atoms of the glucose units C and F and the two phosphorus atoms. Molecular mechanics calculations (MM2) as well as X-ray crystal structures of metal complex141 confirmed that the C-6C,F atoms are proximate to both phosphorus atoms (averaged d(P···C) 4.8 Å). The rigid structure of a series of 1,8-bis(phosphino)naphthalene compounds also led to interesting JPC phenomena. For all of these a P···P interaction through space can be reasonably postulated. In compounds 30a (Scheme 30) pseudotriplets are found for the 13C NMR signals corresponding to the tert-butyl carbons.142 TS SSCCs at 3.6 Hz for the Ca atoms and 9.0 Hz for the Cb atoms are observed for which a 4854


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hydrogen-bonding coupling involves interactions with the 1H hydrogen nucleus as well as the nuclei 13C, 15N, 19F, and 31P. Again, yet another feature common to through-space and through hydrogen-bonding spin coupling is that the widely accepted mechanism for nuclear spin−spin transmission via the spin polarization pathway [nucleus/electrons/nucleus] is apparently independent of the concept of covalent bonding. This is despite an earlier belief,143 and it still gives rise to controversial discussions.153,154,144,151 This assumption seems reasonable when it is considered that even strong repulsive interactions (from noncoordinated lone pairs for instance, as demonstrated above) may efficiently transmit spin−spin information. Grzesiek, Cordier, Barfield, and co-workers collected the most commonly encountered cases of nhJAB.150 These can be combined with some other less usual examples as shown in Figure 7. The sign of the SSCCs is accessible through

F/H−O) coupling are due to the existence of F···O−H hydrogen bonds or alternatively due to overlap of electronic clouds from fluorine and an O−H bond (this question is pertinent also for C−F/H−C couplings).176 Regardless of the mechanisms involved, the use of JFH values for ascertaining the proximity in space of (F,H) nucleiand thus the specific conformation in solution of the molecules involvedhas been experimentally proven for a range of structures (Scheme 31).These include cyclophanes (31a, 9hJFH ≈ 6 Hz),174 oScheme 31. JFH SSCCs from (C−F/H−O) and (C−F/H−C) Interactions in Various Structures

Figure 7. Type and intensity of spin−spin couplings through hydrogen bonding (depicted in red).

specific NMR correlation experiments.155 However, in the interest of simplicity (and with regard with available data) the present discussion will focus only on the magnitude of the J constants. Intramolecular indirect scalar coupling mediated by hydrogen bonding was described early on in molecular hexachloride halfcages as 2hJHH = 1.1 Hz (Figure 7, case j).156 In the most recent theoretical studies, such (1H,1H) coupling has been found to be governed by the DSO term and not the FC term (despite a strong distance and conformational dependence).This is a very rare situation which emphasizes a clear difference with covalent bonding.157 Experimental observation of JHH thereafter has been mainly related to biomolecules. Depending on the atoms involved, coupling constants which arise through hydrogen bonding can occur in very different situations and span a range of values from 0.14 Hz (4hJNN′, case e)158 to values around 150 Hz (2hJFF, case l):159 a,148,160,161 b,145−147 c,162−165 d,143,166 f,29,167 g,168 h,169 i,170 k,169 m,171 n.172 To date, in contrast to the N−H···N situation of their amino analogues, there is no evidence of the existence of intermolecular P−H···P hydrogen bonding.173 4.3.2. Hydrogen Bonding in Nonbonded JFH Coupling. The study of through-space JFH SSCCs is closely associated with controversial discussions174,175 concerning the elusive conception of hydrogen bonding.151,152 For instance, it has been debated whether or not some F/H interactions from (C−

fluorophenyldiphenylmethanol (31b, 5hJFH = 9.2 Hz),177 8fluoro-4-methyl-1-naphthol (31c,5hJFH = 28.4 Hz),178 trifluoromethylated dienoates, aryldienoates, and trienoates (31d−f 5TS JFH ≈ 2 Hz),179 and fluoromethyl-uracil derivatives (31g, 7TS JFH ≈ 2 Hz).167 In such compounds, the solvent dependency may be fairly strong with higher JFH values found in the less polar solvents (chlorocarbons, dielectric constant 2 < ε < 9) and correspondingly much weaker values in the more highly polar solvent (DMSO, ε ≈ 47). This may be attributed to the strong [OH···solvent] hydrogen bonds perturbing the F···H interactions. Theoretical studies have treated the effects of the solvent on F···H−N−C intramolecular hydrogen bonding and the corresponding hJFX (X = H, N, C) in 2-fluorobenzamide.180 DFT calculations justified the existence of a weak hydrogen bond in the absence of solvent, whereas solvents that act as hydrogen-bond acceptors break down the intramolecular hydrogen bond of 2-fluorobenzamide. Both atoms in molecules (AIM) analyses and Steiner-Limbach plots were used to analyze the structure of these compounds. Earlier studies reported that a family of rigid 9-alkyl-1,2,3,4tetrafluorotriptycenes present 1-F1 coupling with some of the α and β protons in the 9 substitutent (32a−h in Scheme 32), 4855


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range of N−(H)−O distances (2.8−3.3 Å), several values of JNC′ were determined and used to extrapolate eq 6, as shown in Figure 8.

Scheme 32. JFH SSCCs in Tetrafluorotriptycene Derivatives

3h

|JNC | = (5.9 × 104)e−4d(N···O)

showing 6TSJFH values of up to 9.0 Hz. This is in contrast to other analogous (F,H) couplings which reach at most 0.2 Hz.181 Assumptions from NMR studies and molecular models led to the conclusion that two stereochemical situations can provide significant TS coupling: first, the close spatial proximity of F and the atom bearing the proton, and second, a quasilinear arrangement of the F atom and the C−H bond in which the fluorine lone-pair orbital can interact with the rear lobe of the C−H bond. This latter case is related to the earlier discussion concerning polycycloalkanes and parent cubanes (see in sections 4.1.1 and 4.1.2, Schemes 8 and 17). 2-Fluorobenzamide and its N-methyl derivative also show SSCCs between the fluorine and the nitrogen and carbon atoms of their amide groups.182 Such couplings are, on the other hand, absent in the corresponding N,N-dimethylamide. The 2-fluoro-N-methyl-benzamide presents a hydrogen bond F···H−N and also shows fluoro coupling to the nuclei of the methyl group. This is solvent dependent with weaker values in polar solvents consistent with the existence of H bonding. Calculations conducted at the INDO level (IPPP method, see also section 3.2)46 are in reasonable agreement with the magnitude of the observed 4J(F,N) values. It was confirmed also that Fermi contact contributions are dominant with spin information transmitted mainly through the hydrogen bond.182 Most recent studies have discussed the nature of the transmission mechanism of the FC term of 4TSJFH by analyzing the canonical molecular orbitals in 2-fluorophenol and derivatives.176,183 No bond critical point was found between the OH group and the fluorine atom. This is indicative of the absence of H bonding. Instead, the FC term originates in exchange interactions between the overlapping electronic clouds surrounding the proximate (F,H) nuclei involved. This is consistent with Fermi hole transmission. In order to clarify the sign of TS JFHsometimes reported as negative and sometimes as positiveTormena and co-workers conducted a critical analysis focusing on FC and PSO terms calculated using DFT.184 These studies, conducted on the 5TS JFH values in fluorinated aldehydes as well as a dichlorotetrafluorotriptycene, confirmed that the sign of JFH is not indicative of the existence (or not) of F···H hydrogen bonding. Moreover, besides the existence of TS transmission of the FC term due to either positive or negative exchange interactions, some direct and long-range charge transfer interactions were identified which make positive contributions to the FC term. 4.3.3. Structural Approaches of Spin Coupling Through Hydrogen Bonding. A significant correlation between the intensity of hJAB couplings and the geometrical parameters of the hydrogen bond has been demonstrated in several cases. For instance, for the streptococcal protein G, Bax and co-workers analyzed the distance dependence of the 3hJNC (N−H···OC) coupling using a correlation of the NMR solution data with a precise X-ray structure.147 Over a small

(6)

Figure 8. Dependence of the 3hJNC coupling constant (Hz) on the nonbonding distances N−(H)−O (Å) for the streptococcal protein G. Reprinted with permission from ref 147. Copyright 1999 American Chemical Society.

As pointed out by Oldfield and Arnold, this exponential dependence is consistent with the correlations observed for TS couplings involving fluorine lone pairs (see section 4.1).153 In Figure 8, point deviation from eq 6 can be mainly attributed to differences in the protein conformation between the solid state and in solution. In addition, the angular dependence of SSCCs h JAB is not taken into account in this approach. The computed results for 2hJNN′ in situations involving N− H···N−H are also believed to show an exponential dependence of the SSCCs on the d(N···N) distance for Watson−Crick nucleic acid base pairs.185 By means of DFT, Barfield investigated structural dependences in 3hJNC′(N−H···OC′) using formamide molecule models. The internuclear distances and angles that were analyzed are depicted in Figure 9.186 The most influential

Figure 9. Geometric dependence of computation and formamide models.

3h

JNC′ as shown using DFT

parameters are clearly the distance d(O···H) and the angle θ2. The other contributions are negligible (θ1) in comparison or at least less pertinent (d(N···O) and d(N···C′)). The final relationship derived for quantifying JNC′ is expressed in eq 7 |JNC ′| = (360)e−3.2d(O···H)cos

2

θ2

(± 0.04Hz)

(7)

Concerning small organic structures, DFT and ab initio calculations predicted the existence of intermolecular throughspace JHH and JCH spin−spin coupling constants through CH/π interactions. Those interactions were established in van der 4856


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predict a value of 18.72 Hz composed mainly of transannular contributions (16.96 Hz) and residual TB relay (1.76 Hz). Nakanishi and co-workers studied the interactions of selenium with many other atoms in detail. They did this within the framework of naphthalenes which were substituted in the peri positions such as, for example, in 33b (Scheme 33).189,190 In such compounds, the values of 4J(Se,Se) are very high, ranging from 203 to 340 Hz. This is due to the extent of the selenium-occupied orbitals as well as the close internuclear distances. Tomoda and co-workers consistently found that in the 2-selenobenzylamine derivatives 33c−h (Scheme 33) the 4 J(Se,N) SSCCs were smaller, lying between 13.0 and 52.0 Hz.191 Here the lone-pair overlap efficiency is probably weaker because of orbital symmetry and for energy-related reasons. The IPPP-INDO method was applied for the study of the transmission mechanisms of 3J(Se,Se) SSCCs in cis- and trans1,2-bis(methylseleno)ethane.192 In the lower energy conformation of the cis compound, the selenium atom lone pairs experience some overlap. Accordingly, a large TS transmission of the FC term, which depends on the Se···Se distances, was reported. Conversely, (77Se,77Se) coupling in the trans compound was found to be dominated by the orbital interaction-transmitted TB occurring via the vinyl π-electron system. The IPPP-INDO method was also used to analyze the cis and trans 3J(13CH3,77Se) values in the selenoimidates MeSe−CHNMe. Experimentally it was found that J = 34 ± 5 Hz for the cis isomer, which has a Se···H distance of about 1.95 Å, and J < 10 Hz for trans isomer.193 The theoretical study showed that for cis J(C,Se) couplings a large through-space FC term operates via a Se···H−C transmission pathway. More recently, Knight and Bühl and co-workers used a combination of NMR, X-ray analysis, and DFT techniques to study the interactions between formally nonbonded but spatially close Te···Te atoms embedded in the peri(1,8)naphthalene framework. This is analogous to 33b shown in Scheme 33.194,195 In each case, the distortion of some rigid naphthalene and acenaphthene backbones away from the “ideal” was investigated. These were correlated with the steric bulk of the interacting atoms located at the proximal peri positions. Weak donor−acceptor interactions in the perinaphthalene system reinforce the R1Te···TeR2 couplings and lead to 4TSJ(125Te,125Te) large values of around 900−1100 Hz. For the symmetrical analogues (R1 = R2 = Ph or Me), even larger values of around 2000 Hz (not directly observable except via the 123Te satellites) are found for Te···Te distances around 3.4 Å in the solid state. The van der Waals radius of Te = 2.06 Å. 4TSJ data is informative not only with respect to the electronic structure underlying the actual bonding situation but also for determining the particular conformation associated with it. This strongly resembles the examples discussed in section 4.2.1 with respect to 31P···31P spin coupling in metallocene frameworks.112,115 The conformation of the aromatic rings and the subsequent location of the p-type lone pairs have a significant impact on the geometry of the peri region. There are, however, anomalies in peri separations which are correlated to the ability of the frontier orbitals to engage in either attractive or repulsive interactions. The greater atomic size of the heavier chalcogen Se and Te atoms compared to F and P should definitely allow very long range TS spin coupling between spatially remote nuclei above 4 Å to be observed in future studies. In addition, both relevant experimental and theoretical studies will be needed in order to achieve a better understanding of noncovalent interactions at

Waals dimer models involving methane/benzene and benzene/ benzene edifices (0.1−0.3 Hz and smaller values).187 Several convergent studies devoted to hydrogen bonding in biological molecular and supramolecular constructions concluded that a strong correlation between the structures of proteins and nucleic acid and the magnitude of hJ does in fact exist. Thus, it can be confidently stated that these noncovalent interactions are relevant indicators of biomolecular structures. Essentially, because of their dependence on H-bond distances and angles, the values of such SSCCs provide a very sensitive measure of hydrogen-bonding geometries. These together with X-ray structural data and computational studies are most promising for achieving a better structural and dynamic understanding of biological interactions. From the above studies, it is also clear that while some proximate F···H interactions cannot be considered as “hydrogen bonds”, in other cases clearly they are. It should be mentioned that this may be not exclusive to F···H interactions. The signs of the SSCCs and the related internuclear distances do not, however, reveal the nature of the transmission pathway nor the existence of true H bonding. Hence, in order to achieve further progress in this respect more studies are needed. 4.4. Other Through-Space Couplings with Miscellaneous Nuclei (C, N, Se, Te)

In addition to the pairs of nuclei for which TS coupling is the most frequently observed (F, P, and H), some other pairs of nuclei are involved in significant TS spin coupling. The selected examples shown in Scheme 33 include carbon, selenium, and nitrogen atoms with large to very large and typical nonbonded SSCCs of the type J(C,H), J(Se,Se), and J(Se,N). Scheme 33. Other Nuclei Pairs That Can Also Engage in TS Spin−Spin Nonbonded Coupling

J(C,H) spin coupling between remote nuclei which are not involved, either in highly delocalized π systems or in hydrogen bonding, have been reported for polycycloalkanes (see also Schemes 8 and 17 in the sections 4.1.1 and 4.1.2).188 A compelling illustration of the importance of the effect of such through-space interactions is the magnitude of 4TSJCH (14.2 Hz) observed in compound 33a (Scheme 33). MO calculations 4857


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Scheme 35. 19F−19F Inter-ring SSCCs for Palladium Complexes Bearing the Asymmetric Fluorinated Aryl Groups m-C6BrF4 and o-C6BrF4

such long distances. Some key parameters which need to be examined have already been identified. These include the location of the lone pairs of the chalcogen and the ability of the frontier orbitals to take part in attractive or repulsive interactions. The structural deformation of the platform bearing the chalcogen would be also relevant for consideration in priority.

5. STRUCTURAL DETERMINATION IN ORGANOMETALLIC AND COORDINATION COMPOUNDS USING NONBONDED COUPLING A number of the molecules examined in the previous sections for their TS spin coupling properties can also be involved in the building of bigger constructs such as coordination compounds and organometallic complexes. For the purposes of this review, such compounds are considered as species in which there is at least one shared bond between a metal and an organic fragment. This may occur via a carbon atom or alternatively another donor atom. The case of metallocenic phosphines (which are clearly organometallic compounds) has been arbitrarily treated in section 4.2, which is devoted to organic molecules due to the dominant applications of these species as ligands for transition metals. Coordination complexes can present nonbonded throughspace coupling from the ligands bonded to the metal and more rarely, to date, from the metal itself. Thus, interligand, intraligand, and metal−ligand nonbonded SSCCs are all observed.196 These relate principally to nuclei such as F, P, C, and hydrogen. Such couplings usually provide relevant information regarding the structure of the coordination complexes and their dynamic or static behavior in solution. Such structural data is highly relevant since organometallic reactivity in solution relates in practice to most of the homogeneous catalysis studies and applications.

A mixture of atropisomers (syn-Br,Br and anti-Br,Br) is found for most bis(aryl) complexes with various ratios of syn:anti atropisomers.199 In these palladium complexes, the JFF values have been determined, and this proved very useful in identifying the atropisomers formed as well as for detecting distortions in the structure of the complexes themselves. (19F,19F) inter-ring SSCCs have been measured in cisbis(bromotetrafluorophenyl)palladium(II) complexes, and the values with respect to the proximate ortho,ortho-(F,F), range from 9.9 to 52.0 Hz. This data has been related to the throughspace nuclei distances which range from 3.20 to 2.65 Å, respectively. These distances are consistent with the geometry of the complexes. They do, however, reveal the existence of distortions from an ideal square-planar coordination with the two aryl rings perpendicular to the coordination plane. Therefore, the syn-Br,Br and anti-Br,Br atropisomers of the cis-[Pd(o-C6BrF4)2L2] (L = PMe3, CNMe, and bidentate OPPy2Ph) derivatives are easily distinguishable due to the fact that the former shows a large inter-ring (F6,F6′) coupling while the latter does not. The same applies to compounds such as the cis-[Pd(m-C6BrF4)2L2] derivatives, 35b and 35b′ (Scheme 35). The anti-Br,Br atropisomer 35b shows strong inter-ring coupling between the chemically nonequivalent syn F2−F6′ atoms. On the other hand, the syn atropisomer 35b′, with chemically equivalent syn (F2,F2′) or (F6,F6′) atoms, lacks this coupling. In complexes of the type 35a, the highest coupling values are observed for the combination of syn-Br,Br atropisomers and nonplanar ligands L. This can be explained by assuming that the crowding in the z axis forces a tilt of the aryl groups in order to increase the distance between the Br atoms. This brings the syn-F-ortho atoms closer to each other. Therefore, the through-space JFF inter-ring coupling in cisbis(fluoroaryl) coordination complexes is a convenient tool for both their characterization and other structural studies in solution. Differentiating several isomers can be achieved, and the restriction to free rotation of the arene rings can be successfully appraised.200 This mode of structural identification used for fluorinated arene ligands was extended to palladium and platinum complexes of 3,5-dichlorotrifluorophenyl ligands (complexes

5.1. Nonbonded JFF Coupling in Organometallic and Coordination Complexes

Some early reports mentioned the existence of interligand TS coupling in titanium complexes (34a and 34b in Scheme 34).197,198 However, the potential of such TS coupling Scheme 34.

TS

JFH and

TS

JPP in Titanium Complexes

information for determining the structures of organometallic species was only revealed later by Espinet and co-workers. They used asymmetric fluorinated aryl groups coordinated to metals to investigate atropisomeric issues in the related complexes.199,12 Palladium complexes bearing the asymmetric fluorinated aryl groups m-C6BrF4 and o-C6BrF4, such as is found in complexes 35a and 35a′ (Scheme 35), have been successfully synthesized. 4858


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of type 36a and 36b, Scheme 36).201 The existence of throughspace coupling makes it possible to study the dynamic

Scheme 37. Boat-Shaped Structure of Dinuclear AzolatoBridged Complexes (right), and a Representation of the Spatial Arrangement of the ortho-F Atoms Leading to Intricate Eight Nuclei Spin Systems (left)

Scheme 36. Polyfluoroarene Complexes of the Group 9 and 10 Transition Metals: TSJ SSCCs Are Indicative of the Preferred Conformations in Solution and the Existence of Dynamic Processes

revealed the existence of several dynamic processes in solution. These include aryl group rotation, boat−boat inversion, as well as dinuclear/mononuclear Pd(3,5-C6Cl2F3) exchanges.204 Arenes containing trifluoromethyl groups, such as the fluoromesityl group, are also useful ligands in palladium complexes. In these the observation of TSJ(F,F) values around 6.0−10.0 Hz is indicative of steric crowding and slow dynamic exchange processes as shown in complex 38a (Scheme 38). Here the proximate ortho-CF3 groups are facing each other.206

processes in which the internal motion does not change the chemical shift but rather changes the spin system. This is the case found in the rotation of nonequivalent 3,5-dichlorotrifluorophenyl groups each bearing chemically equivalent orthoF, as in cis-[M(3,5-C6Cl2F3)(LL′)] complexes (LL′ is a planar or fast averaged planar ligand). In these complexes the only way to detect the rotation of the fluoroaryl group is the change in spin systems from AA′XX′ to A2X2 when going from the lower rate to the higher rate limits. The through-space J coupling constants are typically about 15 Hz in cis-[M(3,5-C6Cl2F3)(LL′)] complexes. Consequently, the range of rotation rates observable by a change in the spin system is rather narrow.202 In spite of this the method is valid since exchange processes otherwise unobservable are in fact detected. The interest in polyfluorinated arenes for clarifying structural and dynamic features in transition metal complexes was also explored by Hughes and Rheingold for group 9 rhodium and iridium complexes.203 In complexes of the type 36c and 36d (Scheme 36), the favored conformation both in the solid state and in solution is with one of the fluorine atoms oriented in the ortho position of the arene group closest to the PMe3 group. 4TS JFP values of around 19−25 Hz and 6TSJHF = 1.5 Hz for H atoms from the PMe3 groups are detected. These results indicate an unpredicted slow (or even zero) rotation of the arene group over the time scale and temperature of the NMR measurement. A detailed analysis of the 19F NMR spectra of the boatshaped dinuclear palladium complexes (NBu4)2[Pd2(μ-azolate)2(3,5-C6Cl2F3)4] of the type 37a−d (Scheme 37) and which bear 3,5-dichlorotrifluorophenyl groups has also been conducted (azolate = pyrazolate, pz; dimethylpyrazolate, dmpz; 3-methylpyrazolate, mpz; indazolate, indz). Results delivered important structural information about these dimers.204,12 The (19F,19F) inter-ring coupling between the endo ortho-F atoms of the different Pd(3,5-C6Cl2F3) fragments (F2a, F2b, F2c, and F2d in Scheme 37) are sensitive to the distortion produced by the presence of substituents in the bridging N,N-azolate ligands.205 These substituents reduce the dihedral angle between the coordination planes of the two metals. As a result, endo ortho-F atoms get closer. The ortho-F signals result from an eight nuclei spin system involving the four 3,5dichlorotrifluorophenyl groups. Careful investigation of theses spin systems, which included through-space JFF couplings,

Scheme 38. Trifluoromethylated Arene Complexes of Palladium and Group 4 Transition Metals Showing TSJFX (X = F, C, H)

The group 4 transition metals zirconium and titanium complexes which contain trifluoromethylated arene-substituted pyridinyl groups (38b and 38c, Scheme 38) have also shown significant J(F,H) or J(F,C) SSCCs both inter- and intraligand. For such spin-active nuclei, these have been attributed to noncovalently bonded SSCCs related to short internuclear distances below 2.7 Å.207,208 Trifluoromethyl groups attached to donor ligands such as the constrained tris(pyrazolyl)borates196 or corroles209 have also shown inter- and intraligand TS coupling in their coordination complexes. The silver tris(pyrazolyl)borato-phosphino complex 39a (Scheme 39) is a good example since it shows both 4859


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theoretical DFT calculations for mapping the transmission pathways.

Scheme 39. Trifluoromethylated Tris(pyrazolyl)borato and Corrole Complexes of Ag and Cu

5.2. Nonbonded JPP Coupling in Organometallic and Coordination Complexes

It has been shown that constrained polyphosphines which display TS JPP SSCCs (see section 4.2.1) can serve as efficient ligands for transition metals. Thus, it was recently discovered that under certain conditions nonbonded JPP coupling could involve a proximate metal center in the absence of proper bonding interaction. Our group developed a set of ferrocenyl polyphosphine coordination complexes in order to investigate, using NMR spectroscopy in solution, the 31P nuclear spin−spin interactions in the ferrocenyl ligands. A great variety of 31P TS couplings has been observed in coordination complexes of the type 40a and 40b (Scheme 40)34,210 with ABMX spin systems of the type shown in Figure 10. Scheme 40. Coordination Complexes of the Type 40a and 40b of the Tetraphosphine 21a (X = Cl, Br, I and M = Pd, Ni)a

intraligand JFH and interligand JFP TS SSCCs. In addition, a rare nonbonded metal−ligand JAgF coupling, as evidenced by the (107,109Ag,19F) interaction, has been observed (see also section 5.3 for other metal centers M involved in TS couplings).196 Compounds such as these with M···F−C interactions are of particular interest due to their significance in C−F bond activation processes.91b The solid-state structure of 39a shows that the fluorine atoms of one of the CF3 groups are in close proximity to both P and Ag. For instance, the d(F···M) with M = Ag or Cu is about 3.0−3.5 Å. This strongly supports the TS spin coupling mechanism. However, the strong 1JPAg of 759 Hz in the complexwith a P−Ag bond of 2.376 Å, which is shorter than the average values reported for similar bonds could also suggest the presence of a (117,119Ag,19F) spin coupling transmitted via a TS(F···P) + 1(P,Ag) pathway. Certainly in this case also, further data from DFT studies as well as a visualization of the transmission pathway would be of very high interest in clarifying the actual situation. The coordination of Cu(III) to a corrole ligand as seen in complex 39b has a strong deformation effect on the macrocycle.209 The corrole is so severely saddled that vicinal trifluoromethyl groups display TSJFF SSCCs ranging from 9.0 to 14.0 Hz. Conversely, no intra-CF3 2JFF was observed. This indicates that the CF3 groups, although congested, are, on the NMR time scale, still rotating freely at room temperature. As summarized above, the use of fluorinated ligands in organometallic and coordination chemistry is fairly well developed. Espinet and co-workers reviewed in some detail the many advantages of using 19F NMR in organometallic chemistry in general and particularly in the case of fluorinated aryls.12 The degree of congestion in the metal structures can be assessed by considering the intensity of TS coupling. An additional advantage is the possibility of determining the conformers or unusual coordination modes. The TS coupling phenomena are also closely related to fundamental organometallic reactions and structures. This is especially true for C−F bond activation and C···H agostic bonds in structures, which are a topic wherein TS SSCCs investigation may help. Finally, the complexity of the mechanisms involved in nonbonded coupling between F or P nuclei with spin-active transition metals such as Ag, Pt, and Rh, etc., remains mostly unclear. This would be a most suitable subject for further studies: for example, using fluorinated arenes and congested ligands such as fluorinated tris(pyrazolyl)borates and cyclophanes as well as

a

40a and 40b display the spin system ABMX with large and varied JPP coupling.

4TS

These complexes were thoroughly examined, and it was established that to observe through-space nuclear spin coupling in the complexes, it is not essential that two lone-pair orbitals are present. One lone-pair orbital with the appropriate orientation can in fact successfully interact with a bonding electron pair shared between a phosphorus atom and a metal. Hence, through-space nuclear spin (P,P′) information can be transmitted. Such through-space interaction involves a threecenter system composed of P′ and P−M (M = Pd, Ni). Figure 11 presents an important corollary to the model developed for purely organic fluorinated compounds. The three localized orbitals involved in the through-space interaction were proposed. The metal−ligand bond between P and M is considered as arising from the σ overlap between a 3sp3 and a metallic d orbital. To account for the fact that nuclear spin information is transmitted between the phosphorus atoms, two filled molecular orbitals which incorporate a contribution from the metal were constructed. The qualitative orbital energy ordering as well as the symmetry of the interacting orbitals make this schematic view possible. Accordingly, a semiquantitative correlation of the distance dependence of 31P−31P coupling in such coordination complexes has been established from solid-state structural data and NMR data in solution (Figure 12).34,210 An excellent fit was obtained for an exponential curve expressed by eq 8, in which JPP is in Hertz and d(P···P) in Angstroms (regression coefficient r2 = 0.987). Notably, the fitted data all correspond to experimentally determined values. These span a wide range of coupling constants (from 1.9 to 76.0 Hz) as well as distances (from 2.90 to 5.40 Å). 4860


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Figure 10. 31P NMR spectrum of a complex of the type 40a (main spectrum): [(κ2-21a)PdBr2] (see Scheme 27), δA (ddd) = 39.89 ppm, JAB = 20.6 Hz, JAM = 25.6 Hz, JAX = 1.9 Hz; δB (dd) = 37.38 ppm, JBA = 20.6 Hz, JBM = 5.8 Hz, δM (ddd) = 25.09 ppm, JMA = 25.6 Hz, JMB = 5.8, JMX = 11.5 Hz; δX (dd) = 27.32 ppm, JXA = 1.9 Hz, JXM = 11.5 Hz. Simulated spectra computed by G-NMR program confirmed the values for the coupling constants at ±0.1 Hz (mirror spectrum).

couplings involving phosphorus atoms, longer through-space distances are observed in comparison to that of fluorine atoms. This is consistent with the presence of more extended occupied orbitals for the P atoms. TS JPP values were also experimentally and theoretically studied with relation to the “noncovalent” ionic clusters alkali metal tetraphosphine-1,4-diides depicted in Scheme 41.211 Scheme 41. Two Types of Alkali Metal Tetraphosphine-1,4diides Have a Strong TS Component in J(PB,PB′), J(PA,PB′), and J(PA,PA′)a

Figure 11. Simplified extension of the Mallory lone-pair overlap interaction model applied to coordination complexes involving a TM orbital contribution and donating phosphorus lone pairs.

a

Structures 41a and 41b were determined for [Na(thf)2]2[P4Mes4] and [K(thf) 3] 2 [P4 Mes 4]. Structure 41c was found for [Na(thf)2,5]2[P4Ph4]; THF ligated on cations are not depicted. Figure 12. Plot of JPP as a function of d(P···P) through-space distance. Data from complex [(κ2-21a)PdBr2] (40a) are encircled. The plot is matched by the exponential relationship defined by eq 8. Dotted line corresponds to the first formula in the initial studies.34 These were later confirmed by complementary data.210

JPP = (8.8591 × 103)e − 1.5884d(P···P)(r 2 = 0.987)

In the anionic (P4R4)2− ligands which are involved in several ion pairs of the type 41a, 41b, and 41c (P,P) couplings related to the A, A′, B, and B′ nuclei exhibit a large through-space component which is strongly dependent on the relative orientation of the nonbonding electron pairs of the phosphorus atoms. This was shown by visualizing the coupling pathways using the coupling energy density (CED) parameter9,33 and supported by calculations using the electron localization function (ELF). In such species the influence of the metal cations on the JPP SSCCs is apparently largely related to a structuring role. This is due to the fact that the conformation within the anion depends partly on interactions with the cations. Additional electrostatic effects of the cations on the coupling are of secondary importance. Any contributions to the coupling through covalent P−M bonding have not been identified. This is an important result that may also have a bearing on many other contact ion pairs. Interestingly, such results might also account for the interrelations between spin− spin coupling across hydrogen bonds and the controversial covalent character of these hydrogen bonds.

(8)

Interestingly, in the formula found, the record [5.398 Å; 1.9 Hz] is consistent with eq 8 despite the unusually long internuclear separation. This has been attributed as possibly resulting from the extent of the palladium 4d orbitals. Another equally valid hypothesis is that a combination of TS and through-bond transmission exists: with JAX resulting from a mechanism combining a TS(A,M) + 3(M,X) pathway. In comparison with the related relationships in fluorine chemistry (Figures 4 and 5) we noted a less steep exponential inverse dependence on the internuclear distances. We attributed this to a less efficient orbital overlap due to the involvement of orbitals from three different nuclei, hence inducing significant energy and symmetry differences. It is also noteworthy that for TS 4861


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5.3. Metals Centers Involved in Nonbonded Spin Coupling

stabilizing a metal center) congested spin-active donors (P, F, or N, etc.) will generate more examples of such couplings. It is very possible that the alliance of experimental and more recent theoretical studies such as CED visualization33 or Fermi contact coupling density/Fermi hole mapping51,52 could prove a promising perspective for an improved conception of spin− spin transmission in such complex systems.

Direct involvement of metals in spin−spin coupling constants between two other nuclei was reported by Johnson and coworkers in the case of trinuclear magnesium complexes of the type 42a−c (Scheme 42). For these, a somewhat puzzling Scheme 42. Trinuclear Magnesium(II) Complex Based on Two Triamidophosphine Ligands

6. CONCLUSIONS AND OUTLOOK Indirect nuclear spin−spin coupling involving very commonly encountered nuclei such as 1H, 13C, 19F, or 31P provides definitive data for characterization of molecules in solution. Yet, this fairly complex electron-mediated coupling which is generally assumed to be directly relating to covalent bonding is still not fully understood with respect to the physical mechanisms involved. The existence of scalar J spin coupling operating via clearly nonbonded interactions, also called through-space couplings, has been recognized since the 1960s. Nonbonded spin coupling characterized by highmagnitude values of JFF, JFX (X = N, P, Se, C and H) and JPP and JPX (X = Se and C) have been repeatedly authenticated for a range of compounds such as functionalized cyclophanes, naphthalenes, coordination complexes of polyphosphines, as well as fluorinated arene complexes. Semiempirical quantitative relationships have been discovered which show an apparent exponential dependence of the spin-coupling intensity on the spatial internuclear distance. In noncovalently bonded interactions, indirect scalar couplings can be frequently observed across the hydrogen bonding of biomolecules and smaller chemical compounds. The related spin-coupling constants through hydrogen-bonding nhJHX and nhJXY (X = H, N, C, P, F; Y = N, C, F, P) provide very sensitive measures of the hydrogen-bonding geometries in proteins as well as in nucleic acid. In these cases they have also been successfully related to structural parameters such as the H···X distances and the Z− H···X angles. Therefore, it appears that nonbonded spin−spin coupling is in fact an additional NMR tool for determining molecular and supramolecular structures in the liquid phase. As a consequence, several future perspectives are envisioned which are related to the various nuclei and the experimental and theoretical studies conducted over recent decades. (i) The range of applications in chemistry and structural biology involving a 19F NMR probe proves that TS spin coupling with this nucleus can be a valuable additional tool in pursuing structural questions. Analysis of TS 19F coupling in fluorinated organic molecules, but also in amino acids, proteins, nucleic acids, and nucleic acid− protein complexes can inform us regarding the differentiation of rotamers and diastereoisomers, estimation of internuclear distances, computation of tertiary and quaternary structures, as well as understanding of dynamic processes involved in solution and hence in vivo. Therefore, incorporation of 19F-labeled amino acids into proteins may very well provide additional important information for protein structure determination in the future. The capacity of fluorine to undergo TS coupling with many important nuclei and particularly carbon and nitrogen should not be neglected and remains to be fully exploited. (ii) In relation to the previous point, several convergent studies devoted to hydrogen bonding in biological molecular and supramolecular structures concluded that

nonbonded JPP spin coupling was observed.212 This SSCC is conceptually different and rather intriguing since it was found that it possibly operates “through metal”. In 42a−c, the phosphorus atoms of the two triamidophosphine ligands are nonequivalent and separated at least by six covalent bonds. In addition, the Mg−N interactions are largely ionic in character. Yet, a clear JPP of around 15 Hz has been observed. The large d(P···P) distance prohibits direct through-space coupling, but DFT calculations have predicted the role of an orbital extending from P1 to P2 (FC contact). This creates a through-space amido−magnesium contribution relayed by an orbital overlap involving N−CH2(p)−P bonds. This example is typical of the complexity that the spin−spin coupling transmission pathway can apparently achieve with the involvement of lone pairs from phosphorus and several electron pairs involved in covalent bonding with carbon, nitrogen, and magnesium atoms. In addition to the above-mentioned Ag···F interactions observed in a coordination complex containing a pyrazolate− borato ligand (see section 5.1),196 using 1H and 13C NMR as very sensitive probes, metal−arene interactions were also observed in diamagnetic Zn(II), Cd(II), and Hg(II) benziporphyrin complexes.213 Thus, SSCCs nTSJXY were obtained in the range 4−46 Hz (n = 5, 6, X = 1H, 13C, Y = 111 Cd, 113Cd, 199Hg). This is in agreement with the concept of a “weak agostic bond” proposed by Pregosin and co-workers for JHPt and JCPt which were found in the range 8−39 Hz within through-space C−H···Pt interactions.214 Such kinds of rare interactions were also found in cationic dithallium cryptate with a 5TSJ(1H,203/205Tl) of 17 Hz corresponding to a distance d(Tl··· H) = 3.82 Å as observed in the solid state by XRD.215 Some polynuclear organometallics present spin−spin coupling constants between two heavy metal nuclei, such as 183W, 195 Pt, 199Hg, 205Tl, and 207Pb. Here, with respect to TS spin coupling this type of bonding is relevant. These systems have been studied by DFT, and this work has been previously reviewed.216 Only a few cases of TS coupling involving metals have been authenticated. This is mainly due to its transmission mode via the lone-pair electrons, which is obviously not compatible with the electron-pair bonding transfer dominating most often in metal/ligand interactions. Nevertheless, development of molecules having multiple (i.e., more than that necessary for 4862


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could be studied and would most certainly contribute to progress in this field. Since virtually all spin-active nuclei can display nonbonded interactions which can be easily investigated using routine NMR experiments, more experimental and theoretical studies will still continue to feed fundamental knowledge on this topic in the coming years. Even more importantly, new nuclei pairs and new transmission pathways will undoubtedly be investigated. This should lead to a more precise understanding of spin−spin transmission bonds in relation to the fundamental concept of atomic bonding.

a strong correlation between protein structures and nucleic acid and the magnitude of hJ can exist. Spin interactions between proximate nuclei often involve nearby hydrogen atoms. While some of these noncovalently bonded interactions could not be considered as hydrogen bonding, in other cases clearly they are. Until now, despite several studies, no simple property of nonbonded SSCCs has been identified as an indicator of the existence of proper H bonding. Thus, further progress in this respect from the alliance of experimental and theoretical work is a clear aim. (iii) Use of fluorinated ligands in organometallic and coordination chemistry has been developed from fluorinated arene ligands. The degree of congestion in the related complexes can be assessed by studying the intensity of 19F TS couplings. The possibility of determining conformers or unusual coordination modes is also a remarkably useful application. TS coupling phenomena are also closely related to fundamental states in organometallic reaction and structures. This is especially true for C−F bond activation and C···H agostic bonds which are prominent literature topics in which TS SSCCs investigations may help. (iv) Owing to the current interest in phosphorus-containing molecules which are used as ubiquitous ligands in coordination complexes, observation of TS coupling in 31 P NMR has direct application to the chemistry of transition metal complexes. Such couplings, due to the proximity of donor atoms, provides structural information in solution (concerning both ligands and complexes) which can be related to fundamental reactivity and in particular to homogeneous catalysis applications,217 especially in the case of congested diphosphine and polyphosphine ligands.218,219 For instance, the proximity of phosphorus atoms, as demonstrated by strong TS coupling, is a means for metal catalytic centers to accommodate various coordination modes during the cycle.217 Therefore, it can be predicted that further congested polyphosphines and even fluorinated phosphines and amines should nourish this important field. Additionally, it should be noted that only a few cases of TS coupling involving metal centers have been authenticated to date. Development of molecules having multiple congested spin-active donors will generate more examples of such couplings. Then, the alliance of experimental and most recent theoretical studies such as CED visualization or Fermi contact coupling density/ Fermi hole map could prove most helpful for an improved understanding of spin−spin transmission in such intricate systems. There is no doubt that these are also among the most interesting to investigate. Clearly, dividing couplings into through-space (TS) and through covalent bonding (TB) transmitted contributions is a major issue and a frontier in this field. Pioneering work by the Contreras and Malkina groups, and others, has been conducted using computational methods. These were concerned with Fermi contact coupling pathways involving canonical molecular orbitals and visualization of coupling energy density (CED) as a real-space function, respectively. The latter is observable in three-dimensional space but has not yet been clearly quantified. On the basis of these methods, numerous genuine systems (nonmodel), developed experimentally by synthetic chemists,

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The author declares no competing financial interest. Biography

Jean-Cyrille Hierso has been Full Professor of Chemistry since 2009 and headed the Department of “Organometallic Chemistry and Catalysis” (OMBC3) of the Institute of Molecular Chemistry at the Université de Bourgogne (Dijon) since 2011. He studied physical chemistry at the Université P. Sabatier (Toulouse), working successively on palladium nanochemistry (M.S. 1994, Dr. B. Chaudret, LCC-CNRS) and chemical vapor deposition for heterogeneous catalysis (Ph.D. 1997, Professor P. Kalck). After postdoctoral positions working on scorpionate tantalum organometallics (Professor M. Etienne, LCC-CNRS) and cobalt oxidation chemistry (Professor J. Reedijk, Leiden, The Netherlands), in 2001 he was appointed Maı̂tre de Conférences in Dijon. He has interest in the fields of organometallic chemistry, ligand design, hetero- and homogeneous catalysis, chemical physics, and material science. He has authored about 80 papers and patents in these domains. His research includes development of polydentate ferrocenyl phosphine ligands and their peculiar coordination chemistry, which can then be applied in palladium-catalyzed cross-coupling reactions for C−C and C−X bond formation (X = N, O, S). In 2011 he was awarded the Prize for Coordination Chemistry from the French Chemical Society (SCF) and in 2012 the EurJIC Young Researcher Award. He was elected a Junior Member of the Institut Universitaire de France (IUF) at the end of 2012.

ACKNOWLEDGMENTS The author thanks the “Région Bourgogne” and “Université de Bourgogne” for funding some aspects of the work discussed in this review. Beneficial support from the CNRS (PICS 4863


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equivalent nuclei would involve transition between symmetric and antisymmetric states which are strictly forbidden. (16) Pople, J. A.; Schneider, W. G.; Bernstein, H. J. High-resolution Nuclear Magnetic Resonance; McGraw-Hill Book Co., Inc.: New York, 1959. (17) Jackman, L. M.; Sternhell, S. Applications of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry, 2nd ed.; Pergamon Press: New York, 1969; p 154 and references therein for determination of relative signs of SSCCs. (18) Jameson, C. J.; Mason, J. In Multinuclear NMR; Mason, J., Ed.; Plenum Press: New York, 1987. (19) Saika, A.; Gutowski, H. S. J. Am. Chem. Soc. 1956, 78, 4818. (20) Roberts, J. D.; Lutz, R. P. J. Am. Chem. Soc. 1961, 83, 246. (21) Petrakis, L.; Sederholm, C. H. J. Chem. Phys. 1961, 35, 1243. (22) Mallory, F. B. J. Am. Chem. Soc. 1973, 95, 7747. (23) Mallory, F. B.; Mallory, C. W.; Fedarko, M. C. J. Am. Chem. Soc. 1974, 96, 3536. (24) Reuben, J.; Shvo, Y.; Demiel, A. J. Am. Chem. Soc. 1965, 87, 3995. (25) Mallory, F. B.; Mallory, C. W.; Ricker, W. M. J. Org. Chem. 1985, 50, 457. (26) Iwaoka, M.; Komatsu, H.; Katsuda, T.; Tomoda, S. J. Am. Chem. Soc. 2002, 124, 1902. (27) Mallory, F. B.; Mallory, C. W. J. Am. Chem. Soc. 1985, 107, 4816. (28) Mallory, F. B.; Mallory, C. W.; Butler, K. E.; Lewis, M. B.; Xia, A. Q.; Luzik, E. D., Jr.; Fredenburgh, L. E.; Ramanjulu, M. M.; Van, Q. N.; Francl, M. M.; Freed, D. A.; Wray, C. C.; Hann, C.; Nerz-Stormes, M.; Carroll, P. J.; Chirlian, L. E. J. Am. Chem. Soc. 2000, 122, 4108. (29) Mele, A.; Salani, G.; Viani, R.; Bravo, P. Magn. Reson. Chem. 1997, 35, 168. (30) Kimber, B. J.; Feeney, J.; Roberts, G. C. K.; Birdsall, B.; Griffiths, D. V.; Burgen, A. S. V.; Sykes, B. D. Nature 1978, 271, 184. (31) Arnold, W. D.; Mao, J.; Sun, H.; Oldfield, E. J. Am. Chem. Soc. 2000, 122, 12164. (32) Mallory, F. B.; Luzik, E. D., Jr.; Mallory, C. W.; Carroll, P. J. J. Org. Chem. 1992, 57, 366. (33) Malkina, O. L.; Malkin, V. G. Angew. Chem., Int. Ed. 2003, 42, 4335. (34) Hierso, J.-C.; Fihri, A.; Ivanov, V. V.; Hanquet, B.; Pirio, N.; Donnadieu, B.; Rebière, B.; Amardeil, R.; Meunier, P. J. Am. Chem. Soc. 2004, 126, 11077. (35) Fermi, E. Z. Phys. 1930, 60, 320. (36) Gutowsky, H. S.; McCall, D. W.; Slichter, C. P. J. Chem. Phys. 1953, 21, 279. (37) Ramsey, N. F. Phys. Rev. 1953, 91, 303. (38) Peralta, J. E.; Barone, V.; RuIź de Azúa, M. C.; Contreras, R. H. Mol. Phys. 2001, 99, 655. (39) Peralta, J. E.; Barone, V.; Contreras, R. H. J. Am. Chem. Soc. 2001, 123, 9162. (40) Pople, J. A. ; Beveridge, D. L. Approximate Molecular Orbital Theory; McGraw-Hill: New York, 1970. (41) Wasylishen, R. E.; Schaefer, T. Can. J. Chem. 1972, 50, 1852. (42) Wasylishen, R. E.; Barfield, M. J. Am. Chem. Soc. 1975, 97, 4545. (43) Barfield, M. J. Am. Chem. Soc. 1980, 102, 1. (44) Engelmann, A. R.; Contreras, R. H.; Facelli, J. C. Theor. Chim. Acta 1981, 59, 17. (45) De Kowalewski, D. G.; Contreras, R. H.; Engelmann, A. R.; Facelli, J. C.; Duran, J. C. Org. Magn. Reson. 1981, 17, 199. (46) Engelmann, A. R.; Contreras, R. H. Int. J. Quantum Chem. 1983, 23, 1033. (47) Natiello, M. A.; Scuseria, G. E.; Contreras, R. H. Chem. Phys. Lett. 1984, 108, 589. (48) Natiello, M. A.; Contreras, R. H.; Gavarini, H. O.; Rae, I. D. Chem. Phys. 1985, 98, 279. (49) Diz, A. C.; Contreras, R. H.; Natiello, M. A.; Gavarini, H. O. J. Comput. Chem. 1985, 6, 647. (50) Bowmaker, G. A.; J. P. Williams, J. P. Aust. J. Chem. 1994, 47, 451. (51) Soncini, A.; Lazzeretti, P. J. Chem. Phys. 2003, 119, 1343.

program), as well as the French ANR agency (ANR CP2D Camelot, ANR-DFG Menolep), and the “Ministère des Affaires Etrangères” (Merlion program) is gratefully acknowledged. The reviewers, the editor and Dr. I. R. Butler are sincerely acknowledged for their suggestions in editing this review.

DEDICATION This review is dedicated to Dr. Igor Tkatchenko for the invaluable gift of his library. ACRONYMS BODIPY boron-dipyrromethene CED coupling energy density DFT density functional theory dmpz dimethylpyrazolate DSO diamagnetic spin−orbit FC Fermi contact INDO intermediate neglect of the differential overlap indz indazolate IPPP inner projections of the polarization propagator approach mpz 3-methylpyrazolate NBO natural bond orbital NMR nuclear magnetic resonance NNBI neglect of nonbonded interactions PRMO partially restricted molecular orbitals PSO paramagnetic spin−orbit pz pyrazolate SD spin dipole SSCCs spin−spin coupling constants TB through bond TM transition metal TS through space REFERENCES (1) Grinter, R. In Nuclear Magnetic Resonance; The Chemical Society, Burlington House: London, 1973; Vol. 2, pp 50−111. (2) Hilton, J.; Sutcliffe, L. H. Prog. Nucl. Magn. Reson. 1975, 10, 27. (3) Contreras, R. H.; Natiello, M. A.; Scuseria, G. E. Magn. Reson. Rev. 1985, 9, 239. (4) Contreras, R. H.; Facelli, J. C. Annu. Rep. NMR Spectrosc. 1993, 27, 255. (5) Contreras, R. H.; Peralta, J. E. Prog. Nucl. Magn. Reson. 2000, 37, 321. (6) Contreras, R. H.; Peralta, J. E.; Giribet, C. G.; RuIź de Azúa, M. C.; Facelli, J. C. Annu. Rep. NMR Spectrosc. 2000, 41, 55. (7) Webb, G. A.; Fukui, H.; Baba, T. In Nuclear Magnetic Resonance; The Royal Society of Chemistry, Thomas Graham House: Cambridge, 2003; Vol. 32, pp 126−145. (8) Contreras, R. H.; Barone, V.; Facelli, J. C.; Peralta, J. E. Annu. Rep. NMR Spectrosc. 2003, 51, 167. (9) Malkina, O. L. In Calculation of NMR and EPR Parameters; Kaupp, M., Bühl, M., Malkin, V., Eds.; Wiley-VCH: Weinheim, 2004; pp 307−323. (10) Krivdin, L. B.; Contreras, R. H. Annu. Rep. NMR Spectrosc. 2007, 61, 133. (11) Alkorta, I.; Elguero, J.; Denisov, G. S. Magn. Reson. Chem. 2008, 46, 599. (12) Espinet, P.; Albéniz, A. C.; Casares, J. A.; Martínez-Ilarduya, J. M. Coord. Chem. Rev. 2008, 252, 2180. (13) See refs 78, 80, 107, 108, 144, 149 and 150 listed in Table 1. (14) Ramsey, N. F.; Purcell, E. M. Phys. Rev. 1952, 85, 143. (15) The coupling of equivalent spins splits the energy level but not the signals as the transitions have the same energy. In quantum mechanical terms the observation of a splitting in the resonance of 4864


Chemical Reviews

Review

(52) Soncini, A.; Lazzeretti, P. J. Chem. Phys. 2003, 118, 7165. (53) Bader, R. F. W.; Stephens, M. E. J. Am. Chem. Soc. 1975, 97, 7391. (54) Alkorta, I.; Elguero, J. Struct. Chem. 2004, 15, 117. (55) Contreras, R. H.; Esteban, A. L.; Díez, E.; Head, N. J.; Della, E. W. Mol. Phys. 2006, 104, 485. (56) Gauze, G. F.; Basso, E. A.; Contreras, R. H.; Tormena, C. F. J. Phys. Chem. A 2009, 113, 2647. (57) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899. (58) Contreras, R. H.; Gotelli, G.; Ducati, L. C.; Barbosa, T. M.; Tormena, C. F. J. Phys. Chem. A 2010, 114, 1044. (59) Akitt, J. W. In Modern NMR Techniques and Their Application in Chemistry; Popov, A. I., Hallenga, K., Eds.; Marcel Dekker, Inc.: New York, 1991. (60) Chambers, R. D.; Sutcliffe, L. H.; Tiddy, G. J. T. Trans. Faraday 1970, 66, 1025 and references therein. (61) Servis, K. L.; Fang, K. N. J. Am. Chem. Soc. 1968, 90, 6712. (62) Mallory, F. B.; Mallory, C. W.; Ricker, W. M. J. Am. Chem. Soc. 1975, 97, 4770. (63) These values are approximately 0.05 Å smaller than the ones found by single-crystal X-ray analysis for representative samples. (64) Bryce, D. L.; Wasylishen, R. E. J. Am. Chem. Soc. 2000, 122, 11236. (65) Ernst, L.; Ibrom, K.; Marat, K.; Mitchell, R. H.; Bodwell, G. J.; Bushnell, G. W. Chem. Ber. 1994, 127, 1119. (66) Ernst, L.; Ibrom, K. Angew. Chem., Int. Ed. Engl. 1995, 34, 1881. (67) Ernst, L.; Ibrom, K. Magn. Reson. Chem. 1997, 35, 868. (68) Ghiviriga, I.; Zhang, L.; Martinez, H.; Contreras, R. H.; Tormena, C. F.; Nodin, L.; Dolbier, W. R., Jr. Magn. Reson. Chem. 2011, 49, 93. (69) Ernst, L.; Sakhaii, P. Magn. Reson. Chem. 2000, 38, 559. (70) Peralta, J. E.; Contreras, R. H.; Snyder, J. P. Chem. Commun. 2000, 2025. (71) Bekiaris, G.; Röschenthaler, G.-V. Heteroat. Chem. 1998, 9, 173. (72) Shtarev, A. B.; Pinkhassik, E.; Levin, M. D.; Stibor, I.; Michl, J. J. Am. Chem. Soc. 2001, 123, 3484. (73) Buckingham, A. D.; Cordle, J. E. J. Chem. Soc., Faraday Trans. 2 1974, 70, 994. (74) Schumann, C.; Cavagna, F. J. Magn. Reson. 1975, 18, 172. (75) Cavagna, F.; Schumann, C. J. Magn. Reson. 1976, 22, 333. (76) Hilton, J.; Sutcliffe, L. H. Spectrochim. Acta, Part A 1976, 32, 201. (77) Bakhmutov, V. I.; Galakhov, M. V.; Fedin, E. I. Magn. Reson. Chem. 1985, 23, 971. (78) Mallory, F. B.; Mallory, C. W. Coupling through space in organic chemistry. In Encyclopedia of Nuclear Magnetic Resonance; Grant, D. M., Harris, R. K., Eds.; J. Wiley & Sons: Chichester, 1996; Vol. 3, pp 1491−1501. Online 2007 John Wiley & Sons, Ltd. (79) Mallory, C. W.; Baker, M. B. J. Am. Chem. Soc. 1990, 112, 2577. (80) Gakh, Y. G.; Gakh, A. A.; Gronenborn, A. M. Magn. Reson. Chem. 2000, 38, 551. (81) Cremer, D. Tetrahedron 1988, 44, 7427. For a description of Möbius aromaticity, see: Rzepa, H. S. Chem. Rev. 2005, 105, 3697. (82) Rae, I. D.; Burgess, D. A.; Bombaci, S.; Baron, M. L.; Woolcock, M. L. Aust. J. Chem. 1984, 37, 1437. (83) Servis, K. L.; Roberts, J. D. J. Am. Chem. Soc. 1965, 87, 1339. (84) Bohlmann, F.; Arndt, C.; Bornowski, H.; Kleine, K.-M. Chem. Ber. 1963, 96, 1485. (85) Gräfenstein, J.; Cremer, D. J. Chem. Phys. 2007, 127, 174704. (86) Provasi, P. F.; Aucar, G. A.; Sauer, S. P. A. J. Phys. Chem. A 2004, 108, 5393. (87) Schwarz, R.; Seelig, J.; Künnecke, B. Magn. Reson. Chem. 2004, 42, 512. (88) Jakobsen, H. J.; Brey, W. S. J. Chem Soc., Chem. Commun. 1979, 478. (89) Miller, G. R.; Yankowsky, A. W.; Grim, S. O. J. Chem. Phys. 1969, 51, 3185.

(90) Schaefer, T.; Marat, K.; Lemire, A.; Janzen, A. F. Org. Magn. Reson. 1982, 18, 90. (91) (a) Karaçar, A.; Freytag, M.; Jones, P. G.; Bartsch, R.; Schmutzler, R. Z. Anorg. Allg. Chem. 2002, 628, 533. (b) Stanek, K.; Czarniecki, B.; Aardoom, R.; Rüegger, H.; Togni, A. Organometallics 2010, 29, 2540. (92) Kruck, M.; Munoz, M. P.; Bishop, H. L.; Frost, C. G.; Chapman, C. J.; Kociok-Köhn, G.; Butts, C. P.; Lloyd-Jones, G. C. Chem.Eur. J. 2008, 14, 7808. (93) Bonnafoux, L.; Ernst, L.; Leroux, F. R.; Colobert, F. Eur. J. Inorg. Chem. 2011, 3387. (94) Iwaoka, M.; Komatsu, H.; Tomoda, S. Chem. Lett. 1998, 969. (95) Nakanishi, W.; Hayashi, S.; Sakaue, A.; Ono, G.; Kawada, Y. J. Am. Chem. Soc. 1998, 120, 3635. (96) Iwaoka, M.; Katsuda, T.; Komatsu, H.; Tomoda, S. J. Org. Chem. 2005, 70, 321. (97) Contreras, R. H.; Giribet, C. G.; Natiello, M. A.; Pérez, J.; Rae, I. D; Weigold, J. A. Aust. J. Chem. 1985, 38, 1779. (98) Natiello, M. A.; Contreras, R. H. Chem. Phys. Lett. 1984, 104, 568. (99) Rae, I. D.; Staffa, A.; Diz, A. C.; Giribet, C. G.; RuIź de Azúa, M. C.; Contreras, R. H. Aust. J. Chem. 1987, 40, 1923. (100) Della, E. W.; Cotsaris, E.; Hine, P. T. J. Am. Chem. Soc. 1981, 103, 4131. (101) Barfield, M.; Della, E. W.; Pigou, P. E.; Walter, S. R. J. Am. Chem. Soc. 1982, 104, 3449. (102) Barfield, M.; Brown, S. E.; Canada, E. D., Jr.; Ledford, N. D.; Marshall, J. L.; Walter, S. R.; Yakali, E. J. Am. Chem. Soc. 1980, 102, 3355. (103) Hyperconjugative interactions were evaluated within the NBO approach using the same level of theory as that employed for calculating spin−spin coupling constants (DFT-B3LYP-6-311-G**/ EPR-III level). (104) Jaime-Figueroa, S.; Kurz, L. J.; Liu, Y.; Cruz, R. Spectrochim. Acta, Part A 2000, 56, 1167. (105) Chen, J.; Reibenspies, J.; Derecskei-Kovacs, A.; Burgess, K. Chem. Commun. 1999, 2501. (106) Matsubara, K.; Oba, A.; Usui, Y. Magn. Reson. Chem. 1998, 36, 761. (107) Hierso, J.-C.; Armspach, D.; Matt, D. C. R. Chim. 2009, 12, 1002. (108) Hierso, J.-C. Curr. Org. Chem. 2011, 15, 3197. (109) Gavarini, H. O.; Natiello, M. A.; Contreras, R. H. Theor. Chim. Acta 1985, 68, 171. (110) Fihri, A.; Meunier, P.; Hierso, J.-C. Coord. Chem. Rev. 2007, 251, 2017. (111) Blaser, H.-U.; Lotz, M. Bidentate 1,2-ferrocenyl diphosphine ligands. In Chiral Ferrocenes in Asymmetric Catalysis-Synthesis and Applications; Dai, L.-X., Hou, X.-L., Eds.; Wiley-VCH: Weinhem, 2010; pp 73−95. (112) Mom, S.; Beaupérin, M.; Roy, D.; Royer, S.; Amardeil, R.; Cattey, H.; Doucet, H.; Hierso, J.-C. Inorg. Chem. 2011, 50, 11592. (113) Smaliy, R. V.; Beaupérin, M.; Mielle, A.; Richard, P.; Cattey, H.; Kostyuk, A. N.; Hierso, J.-C. Eur. J. Inorg. Chem. 2012, 1347. (114) Thomas, D. A.; Ivanov, V. V.; Butler, I. R.; Horton, P. N.; Meunier, P.; Hierso, J.-C. Inorg. Chem. 2008, 47, 1607. (115) Smaliy, R. V.; Beaupérin, M.; Cattey, H.; Meunier, P.; Hierso, J.-C. Organometallics 2009, 28, 3152. (116) Dieleman, C. B.; Matt, D.; Jones, P. G. J. Organomet. Chem. 1997, 545−546, 461. (117) Dieleman, C. B.; Steyer, S.; Jeunesse, C.; Matt, D. J. Chem. Soc., Dalton Trans. 2001, 2508. (118) Kuhn, P.; Jeunesse, C.; Matt, D.; Harrowfield, J.; Ricard, L. Dalton Trans. 2006, 3454. (119) Ernst, L. J. Chem. Soc., Chem. Commun. 1977, 375. (120) Costa, T.; Schmidbaur, H. Chem. Ber. 1982, 115, 1374. (121) Jackson, R. D.; James, S.; Orpen, A. G.; Pringle, P. G. J. Organomet. Chem. 1993, 458, C3. 4865


Chemical Reviews

Review

(122) Karaçar, A.; Thönnessen, H.; Jones, P. G.; Bartsch, R.; Schmutzler, R. Heteroat. Chem. 1997, 8, 539. (123) McFarlane, H. E. C.; McFarlane, W. Polyhedron 1988, 7, 1875. (124) McFarlane, H. E. C.; McFarlane, W. Polyhedron 1999, 18, 2117. (125) Kilian, P.; Milton, H. L.; Slawin, A. M. Z; Woollins, J. D. Inorg. Chem. 2004, 43, 2252. (126) Grossman, O.; Azerraf, C.; Gelman, D. Organometallics 2006, 25, 375. (127) Haenel, M. W.; Fieseler, H.; Jakubik, D.; Gabor, B.; Goddard, R.; Krtiger, C. Tetrahedron Lett. 1993, 34, 2107. (128) Bartsch, R.; Hitchcock, P. B.; Nixon, J. F. J. Chem. Soc., Chem. Commun. 1987, 15, 1146. (129) Hitchcock, P. B.; Nixon, J. F.; Matos, R. M. J. Organomet. Chem. 1995, 490, 155. (130) Sawamura, M.; Hamashima, H.; Sugawara, M.; Kuwano, R.; Ito, Y. Organometallics 1995, 14, 4549. (131) Rheingold, A. L.; Fountain, M. E. Organometallics 1984, 3, 1417. (132) Pouységu, L.; De Jéso, B.; Lartigue, J.-C.; Pétraud, M.; Ratier, M. Magn. Reson. Chem. 2000, 38, 668. (133) Heinicke, J.; He, M.; Kadyrov, R.; Jones, P. G. Heteroat. Chem. 1998, 9, 183. (134) Karaçar, A.; Freytag, M.; Thönnessen, H.; Omelanczuk, J.; Jones, P. G.; Bartsch, R.; Schmutzler, R. Z. Anorg. Allg. Chem. 2000, 626, 2361. (135) Knight, F. R.; Fuller, A. L.; Bühl, M.; Slawin, A. M. Z.; Woollins, J. D. Chem.Eur. J. 2010, 16, 7617. (136) Nakanishi, W.; Hayashi, S. Phosphorus Sulfur Silicon Relat. Elem. 2002, 177, 1833. (137) Pascal, R. A.; West, A. P., Jr.; Van Engen, D. J. Am. Chem. Soc. 1990, 112, 6406. (138) Hierso, J.-C.; Ivanov, V. V.; Amardeil, R.; Richard, P.; Meunier, P. Chem. Lett. 2004, 33, 1296. (139) Marchenko, A.; Hurieva, A.; Koidan, H.; Rampazzi, V.; Cattey, H.; Pirio, N.; Kostyuk, A. N.; Hierso, J.-C. Organometallics 2012, 31, 5986. (140) Engeldinger, E.; Poorters, L.; Armspach, D.; Matt, D.; Toupet, L. Chem. Commun. 2004, 634. (141) Poorters, L.; Armspach, D.; Matt, D.; Toupet, L.; Choua, S.; Turek, P. Chem.Eur. J. 2007, 13, 9448. (142) Karaçar, A.; Thönnessen, H.; Jones, P. G.; Bartsch, R.; Schmutzler, R. Heteroat. Chem. 1997, 8, 539. (143) Blake, P. R.; Park, J. B.; Adams, M. W. W.; Summers, M. F. J. Am. Chem. Soc. 1992, 114, 4931. (144) Gemmecker, G. Angew. Chem., Int. Ed. 2000, 39, 1224. (145) Cordier, F.; Grzesiek, S. J. Am. Chem. Soc. 1999, 121, 1601. (146) Cornilescu, G.; Hu, J.-S.; Bax, A. J. Am. Chem. Soc. 1999, 121, 2949. (147) Cornilescu, G.; Ramirez, B. E.; Frank, M. K.; Clore, G. M.; Gronenborn, A. M.; Bax, A. J. Am. Chem. Soc. 1999, 121, 6275. (148) Dingley, A. J.; Grzesiek, S. J. Am. Chem. Soc. 1998, 120, 1998. (149) Grzesiek, S.; Cordier, F.; Dingley, A. J. Hydrogen bond scalar couplings − a new tool in biomolecular NMR. In Biological Magnetic Resonance: Protein NMR for the Millennium, Krishna, N. R., Berliner, L. J., Eds.; Kluwer Academic Publisher: New York, 2003; Vol 20. (150) Grzesiek, S.; Cordier, F.; Jaravine, V.; Barfield, M. Progr. Nucl. Magn. Reson. 2004, 45, 275. (151) Grabowski, S. J. Chem. Rev. 2011, 111, 2597. (152) In hydrogen bonding a distinction is necessary between systems with weak donors and strong acceptors (C−H···N/O−C), strong donors and weak acceptors (O/N−H···F−C), and weak donors and weak acceptors (C−H···F−C), see: Hyla-Kryspin, I.; Haufe, G.; Grimme, S. Chem.Eur. J. 2004, 10, 3411. (153) Arnold, W. D.; Oldfield, E. J. Am. Chem. Soc. 2000, 122, 12835. (154) Benedict, H.; Shenderovich, I. G.; Malkina, O. L.; Malkin, V. G.; Denisov, G. S.; Golubev, N. S.; Limbach, H. H. J. Am. Chem. Soc. 2000, 122, 1979.

(155) Schwalbe, H.; Schmidt, P.; Griesinger, C. Coupling constants determined by ECOSY. In Encyclopedia of Nuclear Magnetic Resonance 3; Grant, D. M., Harris, R. K. Eds.; Wiley: New York, 1996; p 1473. (156) Anet, F. A. L.; Bourn, A. J. R.; Carter, P.; Winstein, S. J. Am. Chem. Soc. 1965, 87, 5249. (157) Dračínský, M.; Jansa, P.; Bouř, P. Chem.Eur. J. 2012, 18, 981. (158) Liu, A.; Majumdar, A.; Hu, W.; Kettani, A.; Skripkin, E.; Patel, D. J. J. Am. Chem. Soc. 2000, 122, 3206. (159) Shenderovich, I. G.; Smirnov, S. N.; Denisov, G. S.; Gindin, V. A.; Golubev, N. S.; Dunger, A.; Reibke, R.; Kirpekar, S.; Malkina, O. L.; Limbach, H.-H. Ber. Bunsen-Ges Phys. Chem. 1998, 102, 422. (160) Pietrzak, M.; Limbach, H.-H.; Pérez-Torralba, M.; Sanz, D.; Claramunt, R. M.; Elguero, J. Magn. Reson. Chem. 2001, 39, S100. (161) Li, H.; Cukier, R. I.; Bu, Y. J. Phys. Chem. B 2008, 112, 9174. (162) Mishima, M.; Hatanaka, M.; Yokoyama, S.; Ikegami, T.; Wälchli, M.; Ito, Y.; Shirakawa, M. J. Am. Chem. Soc. 2000, 122, 5883. (163) Czernek, J.; Brüschweiler, R. J. Am. Chem. Soc. 2001, 123, 11079. (164) Del Bene, J. E.; Perera, S. A.; Bartlett, R. J.; Elguero, J.; Alkorta, I.; López-Leonardo, C.; Alajarin, M. J. Am. Chem. Soc. 2002, 124, 6393. (165) Gschwind, R. M.; Armbrüster, M.; Zubrzycki, I. Z. J. Am. Chem. Soc. 2004, 126, 10228. (166) Blake, P. R.; Lee, B.; Summers, M. F.; Adams, M. W. W.; Park, J.-B.; Zhou, Z. H.; Bax, A. J. Biomol. NMR 1992, 2, 527. (167) Mele, A.; Vergani, B.; Viani, F.; Meille, S. V.; Farina, A.; Bravo, P. Eur. J. Org. Chem. 1999, 187. (168) Cordier, F.; Barfield, M.; Grzesiek, S. J. Am. Chem. Soc. 2003, 125, 15750. (169) Fierman, M.; Nelson, A.; Khan, S. I.; Barfield, M.; O’Leary, D. J. Org. Lett. 2000, 2, 2077. (170) Löhr, F.; Mayhew, S. G.; Rüterjans, H. J. Am. Chem. Soc. 2000, 122, 9289. (171) Golubev, N. S.; Shenderovich, I. G.; Smirnov, S. N.; Denisov, G. S.; Limbach, H.-H. Chem.Eur. J. 1999, 5, 492. (172) Del Bene, J. E.; Perera, S. A.; Bartlett, R. J.; Alkorta, I.; Elguero, J. J. Phys. Chem. A. 2000, 104, 7165. (173) Reiter, S. A.; Nogai, S. D.; Karaghiosoff, K.; Schmidbaur, H. J. Am. Chem. Soc. 2004, 126, 15833. (174) Takemura, H.; Kotoku, M.; Yasutake, M.; Shinmyozu, T. Eur. J. Org. Chem. 2004, 2019. (175) Cormanich, R. A.; Freitas, M. P.; Tormena, C. F.; Rittner, R. RSC Adv. 2012, 2, 4169. (176) Cormanich, R. A.; Moreira, M. A.; Freitas, M. P.; Ramalho, T. C.; Anconi, C. P. A; Rittner, R.; Contreras, R. H; Tormena, C. F. Magn. Reson. Chem. 2011, 49, 763 and references therein. (177) Takemura, H.; Kaneko, M.; Sako, K.; Iwanaga, T. New J. Chem. 2009, 33, 2004. (178) Takemura, H.; Ueda, R.; Iwanaga, T. J. Fluorine Chem. 2009, 130, 684. (179) Palmas, P.; Thibonnet, J.; Parrain, J.-L.; Abarbri, M.; DavidQuillot, F.; Duchêne, A. Magn. Reson. Chem. 2002, 40, 537. (180) Alkorta, I.; Elguero, J.; Limbach, H.-H.; Shenderovich, I. G.; Winkler, T. Magn. Reson. Chem. 2009, 47, 585. (181) Yamamoto, G.; O̅ ki, M. J. Org. Chem. 1984, 49, 1913 and references therein. (182) Rae, I. D; Weigold, J. A.; Contreras, R. H.; Biekofsky, R. R. Magn. Reson. Chem. 1993, 31, 836. (183) Other pertinent studies of proximate JFH transmitted through the H bond are available, see: Del Bene, J. E.; Perera, S. A.; Bartlett, R. J. J. Am. Chem. Soc. 2000, 122, 3560. (184) Contreras, R. H.; Ducati, L. C.; Tormena, C. F. Int. J. Quantum Chem. 2012, 112, 3158. (185) Barfield, M.; Dingley, A. J; Feigon, J.; Grzesiek, S. J. Am. Chem. Soc. 2001, 123, 4014. (186) Barfield, M. J. Am. Chem. Soc. 2002, 124, 4158. (187) Bagno, A.; Saielli, G.; Scorrano, G. Chem.Eur. J. 2002, 8, 2047. (188) Della, E. W.; Gangodawila, H.; Pigou, P. E. J. Org. Chem. 1988, 53, 592. 4866


Chemical Reviews

Review

(189) Nakanishi, W.; Hayashi, S.; Toyota, S. J. Org. Chem. 1998, 63, 8790. (190) Hayashi, S.; Nakanishi, W. J. Org. Chem. 1999, 64, 6688. (191) Iwaoka, M.; Tomoda, S. J. Am. Chem. Soc. 1996, 118, 8077. (192) Contreras, R. H.; Gavarini, H. O.; Natiello, M. A. J. Comput. Chem. 1987, 8, 265. (193) Contreras, R. H.; RuIź de Azúa, M. C.; Giribet, C. G. Magn. Reson. Chem. 1986, 24, 675. (194) Knight, F. R.; Randall, R. A. M.; Arachchige, K. S. A.; Wakefield, L.; Griffin, J. M.; Ashbrook, S. E.; Bühl, M.; Slawin, A. M. Z.; Woollins, J. D. Inorg. Chem. 2012, 51, 11087. (195) Bühl, M.; Knight, F. R.; Kristkova, A.; Malkin Ondik, I.; Malkina, O. L.; Randall, R. A. M.; Slawin, A. M. Z.; Woollins, J. D. Angew. Chem., Int. Ed. 2013, 52, 2495. (196) Dias, H. V. R.; Jin, W.; Kim, H.-J.; Lu, H.-L. Inorg. Chem. 2003, 35, 2317. (197) Leblanc, J.-C.; Moïse, C. Org. Magn. Reson. 1980, 14, 157. (198) Kless, A.; Lefeber, C.; Spannenberg, A.; Kempe, R.; Baumann, W.; Holz, J.; Börner, A. Tetrahedron 1996, 52, 14599. (199) Albéniz, A. C.; Casado, A. L.; Espinet, P. Organometallics 1997, 16, 5416. (200) For an other application of fluorinated arenes, see: Gómez-de la Torre, F.; de la Hoz, A.; Jalón, F. A.; Manzano, B. R.; Rodríguez, A. M.; Elguero, J.; Martínez-Ripoll, M. Inorg. Chem. 2000, 39, 1152. (201) Alonso, M. A.; Casares, J. A.; Espinet, P.; Martínez-Ilarduya, J. M.; Pérez-Briso, C. Eur. J. Inorg. Chem. 1998, 1745. (202) For a detailed discussion see ref 12. (203) Hughes, R. P.; Laritchev, R. B.; Williamson, A.; Incarvito, C. D.; Zakharov, L. N.; Rheingold, A. L. Organometallics 2002, 21, 4873. (204) De la Cruz, R.; Espinet, P.; Gallego, A. M.; Martín-Á lvarez, J. M.; Martínez-Ilarduya, J. M. J. Organomet. Chem. 2002, 663, 108. (205) In addition, in the case of the asymmetric azolates mpz and indz, head-to-head and head-to-tail arrangement of the two bridges can exist. (206) Bartolomé, C.; Espinet, P.; Martín-Á lvarez, J. M.; Villafañe, F. Eur. J. Inorg. Chem. 2004, 2326. (207) Kui, S. C. F.; Zhu, N.; Chan, M. C. W. Angew. Chem., Int. Ed. 2003, 42, 1628. (208) Chan, M. C. W.; Kui, S. C. F.; Cole, J. M.; McIntyre, G. J.; Shigekazu, M.; Nianyong, Z.; Tam, K.-H. Chem.Eur. J. 2006, 12, 2607. (209) Thomas, K. E.; Wasbotten, I. H.; Ghosh, A. Inorg. Chem. 2008, 47, 10469. (210) Hierso, J.-C.; Evrard, D.; Lucas, D.; Richard, P.; Cattey, H.; Hanquet, B.; Meunier, P. J. Organomet. Chem. 2008, 693, 574. (211) Kaupp, M.; Patrakov, A.; Reviakine, R.; Malkina, O. L. Chem. Eur. J. 2005, 11, 2773. (212) Hatnean, J. A.; Raturi, R.; Lefebvre, J.; Leznoff, D. B.; Lawes, G.; Johnson, S. A. J. Am. Chem. Soc. 2006, 128, 14992. (213) Stępień, M.; Latos-Grażyński, L.; Szeterenberg, L.; Panek, J.; Latajka, Z. J. Am. Chem. Soc. 2004, 126, 4566. (214) Albinati, A.; Pregosin, P. S.; Wombacher, F. Inorg. Chem. 2004, 29, 1812 and references therein. (215) Howarth, O. W.; Nelson, J.; McKee, V. Chem. Commun. 2000, 21. (216) Autschbach, J.; Ziegler, T. Relativistic calculations of spin-spin coupling constants of heavy nuclei. In Calculation of NMR and EPR Parameters; Kaupp, M., Bühl, M., Malkin, V., Eds.; Wiley-VCH: Weinheim, 2004; p 249. (217) Platon, M.; Cui, L.; Mom, S.; Richard, P.; Saeys, M.; Hierso, J.C. Adv. Synth. Catal. 2011, 353, 3403. (218) Roy, D.; Mom, S.; Lucas, D.; Cattey, H.; Hierso, J.-C.; Doucet, H. Chem.−Eur. J. 2011, 17, 6453. (219) Roy, D.; Mom, S.; Beaupérin, M.; Doucet, H.; Hierso, J.-C. Angew. Chem., Int. Ed. 2010, 49, 6650.

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