Quantum Mechanical Exchange Coupling in Polyhydride and

Aug 13, 1998 - Sylviane Sabo-Etienne was born in 1956 in Tarbes, France. She studied at Université Paul Sabatier, Toulouse, and obtained her “Thès...
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Volume 98, Number 6

September/October 1998

Quantum Mechanical Exchange Coupling in Polyhydride and Dihydrogen Complexes Sylviane Sabo-Etienne and Bruno Chaudret*,† Laboratoire de Chimie de Coordination CNRS, 205 Route de Narbonne, F-31077 Toulouse Cedex 4, France Received December 23, 1997 (Revised Manuscript Received June 8, 1998)

Contents I. Introduction II. Description and Quantum Mechanical Origin of the Phenomenon A. Description B. The Quantum Mechanical Origin III. Complexes Displaying Quantum Mechanical Exchange Couplings A. Sandwich and Half-Sandwich Trihydrides 1. Cp2NbH3 and Related Complexes 2. Cp*RuH3(PR3) and Related Complexes 3. [CpIrH3(PR3)]+ and Related Complexes 4. [Cp2MoH3]+ and [(η6-C6H6)OsH3(PR3)]+ B. Sandwich Dihydrides C. Octahedral Polyhydrides IV. Modification of Exchange Couplings A. Addition of Lewis Acidic Coinage Cations B. Formation of H‚‚‚H Hydrogen Bonds C. Pressure Effects V. Exchange Couplings and Fluxionality VI. Conclusion VII. Acknowledgments VIII. References

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I. Introduction Transition metal hydride complexes represent a fascinating class of compounds, both because of their importance in homogeneous catalysis and because of their spectroscopic properties. The first organometallic polyhydride, FeH2(CO)4, was reported by Hieber as early as 1931 whereas the first non carbonyl hydride was Cp2ReH, synthesized by Wilkinson in 1955. In this complex, the measurement by 1H NMR †

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of the resonance of the hydride ligand at high field, outside the normal range of chemical shifts for organic derivatives, was an unexpected result. This high field signal, specific for the hydride ligands, has been of wide use for the structural characterization of polyhydride complexes. These findings opened the way to a new area of intense research concerning the chemistry and spectroscopy of transition metal polyhydrides.1 Then the demonstration by Kubas in 1984 that dihydrogen could coordinate to a transition metal without dissociation of the H-H bond had a tremendous influence in the field of inorganic chemistry.2,3 This discovery, which originally concerned molybdenum and tungsten derivatives, was rapidly followed by the identification of similar iridium, ruthenium, and iron complexes respectively by Crabtree and Morris.4 This discovery also stimulated quantum mechanical calculations in order first to understand the electronic structure of these novel species and second to foresee the existence of more extended polyhydrogen ligands (H3-, H3+, H4, ...).5 In this context, the “rediscovery” of polyhydride complexes displaying large H-H couplings by the group of Bercaw,6 our group,7 and the group of Heinekey8 in the late eighties led to speculation about the real structure of these species and the origin of the NMR phenomena. Thus, in the early 70’s, during the course of reactivity studies, Tebbe observed that addition of AlEt3 to Cp2NbH3 (1) led to the observation of large H-H coupling constants (>100 Hz) which were not explained at that time.9 Furthermore, whereas Cp2TaH3 showed an expected highfield 1H NMR spectrum characterized by an AB2 pattern and a JA-B coupling constant of ca. 10 Hz, the spectroscopic properties of the analogous niobium complex Cp2NbH3 characterized by two broad peaks in a 2:1 ratio appeared more puzzling. One hypothesis given for explaining the strange behavior of this

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Sabo-Etienne and Chaudret

Sylviane Sabo-Etienne was born in 1956 in Tarbes, France. She studied at Universite´ Paul Sabatier, Toulouse, and obtained her “The`se de Troisie`me Cycle” in 1980 working under the supervision of Pr. Danie`le Gervais. She then entered the CNRS in 1980 and obtained her “Doctorat d’Etat” in 1984. In 1985 she went to Universite´ de Bretagne Occidentale in Brest to work with Pr. Herve´ des Abbayes. After a year working with Pr. Maurice Brookhart (Chapel Hill, NC) she returned to the Laboratoire de Chimie de Coordination where she is currently “Directrice de Recherche”. Her research interests concern the chemistry and spectroscopic properties of polyhydride, dihydrogen, silane, and more generally σ-bond complexes, with particular focus on catalytic applications.

Bruno Chaudret was born in 1953 in Ferolles, France. He received his degree from “Ecole Nationale Supe´rieure de Chimie de Paris” in 1975, his Ph.D. from Imperial College of Science and Technology, London, working under the direction of Pr. Sir Geoffrey Wilkinson in 1977, and his “Doctorat d’Etat” from Universite´ Paul Sabatier, Toulouse, working under the direction of Pr. Rene´ Poilblanc in 1979. He entered the CNRS in 1977 and was promoted to the position of “Directeur de Recherche” in 1988. His research interests concern the chemistry and spectroscopic properties of polyhydride, dihydrogen, and more generally σ-bond complexes as well as the synthesis, chemistry, and physical properties of nanoparticles of metal, alloys, and oxides.

complex was the existence of a large quadrupolar moment on niobium.10 Another trihydride complex displaying a large, although not so anomalous, H-H coupling constant (56 Hz), [Cp*IrH3(PMe3)]+ (2), was described by Bergman et al. in the early eighties.11 This compound was obtained by protonation of Cp*IrH2(PMe3), the precursor for alkane C-H bond activation reactions. All these anomalous observations were clarified and are now understood in terms of quantum mechanical exchange. We will discuss hereafter the origin of the phenomenon and focus the review on the current knowledge of the properties of polyhydrides displaying quantum mechanical exchange couplings.

measurements). (ii) The second class contains complexes exhibiting quantum mechanical exchange couplings. As stated above, this phenomenon was first discovered in trihydride complexes,6-8 and the question arose whether it could also be observed within dihydride complexes.12 In this section, we will briefly present some general features common to all systems exhibiting exchange couplings; a more detailed analysis will be reported in section III. The quantum mechanical origin will be discussed in section II.B. The exchange couplings algebraically add to magnetic couplings which can be of the same sign or of opposite sign. This can lead to a resulting value close to zero in selected cases. They are in general characterized by large couplings. The most clearly detectable couplings are found in the range 50-1000 Hz, but very small couplings of a few Hz as well as very large ones, up to 106 Hz, have been estimated. The presence of very small exchange couplings in the system results in a broadening of the hydride signal, as can be seen for Cp2NbH3. In the case of very large couplings (J/∆ν . 1), a singlet is observed for dihydrides whereas a 1:10:1 triplet appears for trihydrides. These features remain invariant as a function of temperature, even if the couplings continue to increase. The most characteristic aspect of the phenomenon is however that these “coupling constants” vary as a function of temperature according to an exponential law. Figure 1 shows a typical variation in a trihydride: at low temperature a firstorder AB2 pattern is observed. When the temperature raises, the J/∆ν ratio increases leading to a second-order AB2 pattern. Finally, at higher temperature, a coalescence is observed. In general, it will however be difficult to distinguish a coalescence from the presence of very large exchange couplings without performing isotopic exchange experiments. In addition, these isotopic exchange experiments have shown that in these systems the JH-D(T)/JH-H ratio does not

II. Description and Quantum Mechanical Origin of the Phenomenon A. Description Transition metal polyhydrides (except d0 metals) display in 1H NMR a signal for the hydrides outside the normal range of observation of organic protons. The signal appears at high field (δ < 0) which was interpreted as resulting from a shielding of the hydride by the d-orbitals of the metal. This signal shows characteristic couplings with the other magnetic nuclei present in the molecule, typically the metal and phosphorus. Polyhydrides may also display a magnetic coupling between the hydrides. This coupling is generally weak, typically below 20 Hz. Among the polyhydrides two classes of compounds deserve special attention: (i) The first group contains dihydrogen complexes sometimes called “nonclassical polyhydrides”.2,3 In this case, a simple analysis of the 1H NMR spectra does not allow one to distinguish the dihydrogen formulation M(η2-H2)(H)x from the polyhydride M(H)x+2 and complementary experiments need to be performed (in particular T1 and JH-D

Polyhydride and Dihydrogen Complexes

Figure 1. High-field 1H NMR spectra of Cp′2NbH3 (4) at various temperatures.

follow the γD(T)/γH ratio but is dramatically decreased.13 Finally, most complexes displaying exchange couplings are fluxional, like more generally most polyhydride complexes. However, so far, no kinetic isotope effect has been reported for the classical exchange processes.

B. The Quantum Mechanical Origin Given the novelty of the observations and the previous discovery of dihydrogen coordination, the phenomenon has first been attributed by the synthetic chemists to the presence of bonding between several hydrogens in the coordination sphere of the transition metal, whether a trihydrogen ligand8 or a nonclassical interaction between a hydride and coordinated dihydrogen.7a However, the discovery of coupling constants far larger than the magnetic ones expected for dihydrogen led to the proposal of more elaborate models, quantum mechanical in origin. The first model was proposed simultaneously by Zilm et al.13 and by Weitekamp et al.14 They showed that the problem can be described as a pairwise quantum mechanical exchange of two protons in a double-well potential. This accounts for the temperature dependence of the couplings and for the isotopic effects concerning these couplings. This model requires a low-energy bending vibration as liftoff of the exchange phenomenon and proposes for the temperature dependence an explanation based on the thermal population of higher vibrational states. It is not the purpose of this review to discuss in detail the theoretical aspects of exchange couplings, but we will give some milestones, which make it possible to understand the debate of the differing models and the agreement that has been reached to a large extent, and in the end present the actual understanding of the mechanism of the exchange. A first refinement upon this model was brought by Limbach et al.15 They demonstrated the necessity to invoke the presence of a rotational process for the pairwise exchange of hydrides. This could occur through access to a dihydrogen excited state in which rotational tunneling of dihydrogen could occur. This raised the problem of the temperature dependence of the couplings. To account for this effect, the authors proposed the existence of an equilibrium between a dihydride configuration and a dihydrogen configuration in an excited state. This proposal was

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in agreement with the known chemistry of these complexes and with the dependence of the couplings upon the electronic properties of the ligands. Theoretical calculations on several complexes showed indeed the presence of thermally accessible dihydrogen states in iridium and niobium trihydride complexes. However, Clot et al.16 showed that it is not necessary to invoke the presence of a dihydrogen state but that the phenomenon is governed by a low barrier to hydrogen exchange through a pseudorotational process. These authors pointed out the similarity between the rotational tunneling observed in NMR and that observed for dihydrogen complexes in inelastic neutron scattering experiments (INS). In this case, it represents a direct measurement of the tunnel splitting of dihydrogen when coordinated to a transition metal complex. Eckert showed that this phenomenon is observable by INS when the barrier of rotation of dihydrogen is low (1600 (130) 45 (187) 150 (134) 4600 (155) 110 (177) 280 (173) 920 (173) 20 (125) 20 (125) 30 (125) 37 (125) 47 (125) 43 (125) 72 (125) 160 (125)

69 39 51 38 39 40 37 47 48 44 42 40 41 39 39

classical exchange barrier, and the classical one (incoherent), which follows the potential energy surface. Table 1 displays a summary of data concerning complexes discussed in this review. A good characterization of the process is the measurement of the barrier to classical exchange ∆Gq(coalescence). This value is generally found around 40 kJ‚mol-1 for the complexes displaying exchange couplings. However the table makes clear that the magnitude of the couplings is not only related to the height of the barrier. Thus in [Cp2Ta(H2)(CO)]+, the barrier to classical exchange is 39 kJ‚mol-1 and the minimum value for exchange couplings is greater than 1600 Hz at 130 K,33 whereas, in Cp*RuH3(PCy3)‚(CF3)3COH in toluene, the barrier is 37 kJ‚mol-1 and the H-H coupling is 125 Hz at 183 K.55 The height of the barrier is therefore not the only criterion to determine the magnitude of exchange couplings, but the length of the tunneling path also plays an important role. Barriers between 69 kJ‚mol-1 for the highest (in cis[Cp2TaH2P(OMe)3]+)33 and 34 kJ‚mol-1 (in Cp*RuH3(Ppy3)) were measured in our group.32 It is conceivable that in Cp2NbH3 which displays very low exchange couplings the barrier could be even higher. Some complexes may display quantum exchange which fails to be recognized, in particular if the couplings are too large. In this case, the distinction between exchange couplings and fluxionality can only be achieved by isotopic labeling. Furthermore, in some transition metal complexes such as ReH6(PPh3)2(SiPh3) or ReH5(PPh3)2(pyridine),63 Crabtree found barriers for hydride exchange of magnitudes comparable to those of complexes exhibiting quantum exchange, namely 44 and 36 kJ‚mol-1, but failed to observe quantum mechanical couplings. The reason for this failure could be a rearrangement of the other ligands present in the complex hence impeding the tunneling process. Finally some dihydrogen complexes contain a stretched H-H bond.45,64 This is due to the fact that

JHH (max) (Hz) (T (K)) 181 (237) 3000 (180) 20 (303) 50 (318) 89 (318) 16 (310) 220 (365) 544 (287) 2500 (247) 20000 (184) 340 (207) 24 (201) 23 (201) 59 (201) 99 (201) 94 (201) 95 (201) 168 (201) 533 (201)

ref 31 55 31 32 22 7b 7b 33 33 33 50 50 50 46 42 42 34 34 34 34 34 34 34 34

the back-bonding from the metal to the σ* antibonding orbitals of dihydrogen is important. The same effect should increase the rotation barrier of coordinated dihydrogen. Some complexes have been shown to contain dihydrogen molecules with very large H-H bonds or slowly rotating dihydrogen molecules at the NMR time scale, but except in the d2 systems mentioned above,33 no other case of quantum exchange has been observed, probably as a result of the presence of a too low rotation barrier.

VI. Conclusion We have shown in this review a variety of complexes displaying exchange couplings. These complexes can adopt a dihydride, trihydride, dihydrogen, hydrido-dihydrogen, or even trihydrido-dihydrogen structure. The common feature of these compounds is therefore not structural but is the presence of an exchange barrier for two hydrogens of ca. 40 kJ‚mol-1. This barrier is low for polyhydrides and high for dihydrogen compounds. In the case of dihydrogen complexes, the origin of this barrier is the absence of back-bonding in the transition state of the dihydrogen rotation due to the absence of electrons in the d-orbitals. It can therefore only concern d0 or d2 systems, but it has only been observed in the latter case. In the case of the polyhydrides, the low barrier is due to the narrowing of the H‚‚‚H distance between two hydrides because of the geometry of the structure. This can be due to the presence of Cp ligands which will force hydrides to be in close contact in the equatorial plane as is found in most complexes, or this can be due to more subtle electronic factors creating a Y shape again forcing a close H‚‚‚H contact. The complex RuH(H2)(ph-py)(PiPr3)2 is atypical since it displays a priori an octahedral structure. However, it is possible that a cis hydride-hydrogen interaction be present which would favor the hydride/ hydrogen exchange.

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Theoretical progress on the understanding of the exchange process has pointed out to the fundamental similarity between the rotational tunneling phenomenon evidenced through INS experiments by Eckert and the one observed by NMR. The first type of experiment measures the ortho-hydrogen/para-hydrogen transition and applies for compounds displaying a very low rotation barrier for coordinated H2 whereas the second type of experiment applies for stable polyhydride or compounds displaying high rotation barriers. This establishes from the spectroscopic point of view a continuum between free H2 and a classical dihydride. Typically, as stated herein above, the barriers to pairwise incoherent exchange observed in the complexes displaying exchange couplings in NMR vary from 35 to 70 kJ‚mol-1. By comparison, the rotational tunneling of dihydrogen can be observed by INS for compounds exhibiting rotation barriers up to ca. 12 kJ‚mol-1. It is clear that numerous compounds, in particular stretched dihydrogen complexes, will display barriers which will fall between 12 and 35 kJ‚mol-1. At the present time, no method is available to study these tunneling processes. This similarity between INS and NMR experiments suggests that exchanging hydrogens act as a potential dihydrogen molecule. Thus, the ease of pairwise exchange in these systems is related to the accessibility of a dihydrogen state and to the narrowing of the H‚‚‚H distance on the potential energy surface. This factor can favor further reactivity, through H2 loss for example. Besides these fundamental aspects, it is tempting to look for applications of complexes displaying exchange couplings. Considerable changes in the magnitude of exchange couplings are observed in a homogeneous series of compounds which correspond to very little or no modification of the structure of the core of the complexes and little variation in the barrier to classical exchange of hydrogens. This indicates that no correlation can be established between the existence and magnitude of exchange couplings in a given complex and its reactivity. However, exchange couplings being very sensitive to subtle changes of the electronic or geometric environment of the metal center, they might find useful applications as sensors of very weak interactions in solution. They could also be used as temperature probes in NMR since the couplings vary very reproducibly as a function of temperature. Finally, the presence or absence of exchange couplings gives an indication on the behavior of the molecule since the other ligands must not rearrange during the exchange process for the tunneling to take place. In conclusion, as stated in the text, the presence of quantum mechanical exchange couplings is probably general for all transition metal cis-dihydride complexes when the chemical path to exchange does not involve rearrangement of the heavy ligands. However such couplings can only be observed in the NMR “window”, i.e., when the barrier to classical exchange is found between 35 and 70 kJ‚mol-1. This phenomenon is the most likely to occur when the tunneling path of the hydrogens is reduced, i.e., when the hydrogens are located close to one another either

Sabo-Etienne and Chaudret

as a result of the structure of the complex or as a result of an attractive interaction.

VII. Acknowledgments Support of this work by the CNRS, SHELL International Chemicals BV (Amsterdam), and the European Union (Brussels) via the Human Capital & Mobility Network “Localization and Transfer of Hydrogen” is gratefully acknowledged. The authors warmly thank Pr. H.-H. Limbach, J. C. Barthelat, and J. P. Daudey for a fruitful collaboration in this field.

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