Upon the Chemical Origin of Exchange Couplings in d2 Systems

Sabo-Étienne, Sylviane; Chaudret, Bruno; Makarim, Hassna Abou El; Barthelat, Jean‐Claude; Daudey, J. P.; Ulrich, Stefan; Limbach, Hans−Heinrich; Moı̈se, Claude

doi:10.1021/ja00151a035

Cited by 55 publications

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“…Rotation of the dihydrogen ligand around the M−H 2 bond axis must be rapid on the chemical shift time scale. This is quite reasonable, since reported barriers to hydrogen rotation are very low, except in d 2 systems. , A second process exchanges hydrogen nuclei between the dihydrogen and the hydride ligands. Since our complexes are positively charged, it seems reasonable to speculate that a proton transfer may be involved, although it has been reported that the hydride ligands of the closely related ruthenium complexes Tp Me 2 Ru(PR 3 )(H 2 )H also exchange rapidly on the NMR time scale.…”

Section: Discussionmentioning

confidence: 56%

Synthesis and Characterization of Hydrotris(pyrazolyl)borate Dihydrogen/Hydride Complexes of Rhodium and Iridium

1997

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Protonation of TpM(PR 3 )H 2 (M ) Rh, Ir) complexes with HBF 4 ‚Et 2 O or [H(Et 2 O) 2 ][B(Ar) 4 ] (Ar ) 3,5-(CF 3 ) 2 C 6 H 3 ) affords cationic complexes which exhibit a single hydride resonance at all accessible temperatures in the 1 H NMR spectrum. Formulation as fluxional dihydrogen/hydride complexes is indicated by short T 1 (min) values of ca. 22 ms (Ir) and 7 ms (Rh). The relaxation times are consistent with H-H bond lengths of 0.88-1.11 Å in the iridium complexes and 0.73-0.92 Å in the rhodium complexes depending on the relative rate of the dihydrogen rotational motion. In the case of the iridium complexes, partial substitution of the hydride positions with deuterium or tritium results in large temperature-dependent isotope shifts and resolvable J H-D or J H-T coupling constants. Analysis of the chemical shift and coupling constant data as a function of temperature is consistent with a preference for the heavy hydrogen isotope to occupy the hydride rather than the dihydrogen site. This analysis also provides the limiting chemical shifts of the dihydrogen and hydride ligands as well as the 1 J H-D coupling constant (ca. 25 Hz) in the bound dihydrogen ligand.Since the first report of a stable molecular hydrogen complex by Kubas, 1 the possibility that a fluxional polyhydride complex might also contain a dihydrogen ligand has been actively investigated. 2 In general, transition metal polyhydride complexes are characterized by high coordination numbers (CN 7-9) and high formal oxidation states. 3 Because several structures of nearly equivalent energy are available to seven-, eight-, and nine-coordinate complexes, rapid permutation of the hydride positions is often observed by 1 H NMR spectroscopy. As a result, structural characterization in solution depends upon indirect methods, in which the observed NMR parameters are a population-weighted average of all the hydride environments. For example, Crabtree and co-workers have employed T 1 measurements to detect short H-H contacts in a range of polyhydride complexes, including [Ir(PCy 3 ) 2 H 6 ] + and Fe-(PEtPh 2 ) 3 H 4 . 4 A quantitative treatment of relaxation in polyhydride complexes has been developed which allows useful structural information to be obtained from T 1 (min) data. [4][5][6] We have previously reported the structure and properties of cationic iridium complexes of the form [CpIr(L)H 3 ]BF 4 (L ) various PR 3 ), which have been shown to adopt iridium(V) trihydride structures in the solid state. 7,8 These complexes undergo a rapid hydride rearrangement which leads to a single hydride resonance in the 1 H NMR spectrum above ca. 220 K. However, at very low temperatures, spectra consistent with the solid state structure are obtained.In this paper we investigate the effect of substituting the Cp ligand of [CpIr(L)H 3 ]BF 4 complexes with the hydrotris(1-pyrazolyl)borate (Tp) 9 ligand. The new Tp complexes, [TpIr-(L)(H 2 )H]BF 4 (L ) PMe 3 , PPh 3 ), are formulated as dihydrogen/ hydride complexes, although only a single hydride resonance is observed...

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Section: Discussionmentioning

confidence: 56%

Synthesis and Characterization of Hydrotris(pyrazolyl)borate Dihydrogen/Hydride Complexes of Rhodium and Iridium

1997

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“…However, when the H 2 is rotated 90°, the σ* orbital does not find any occupied d orbital in the metal to interact so that the back-donation is completely lost in complex 3 , which therefore has a remarkably high energy. Recent experimental findings support this analysis . In these works it has been observed that the H 2 rotation in the [(Cp) 2 M(η 2 -H 2 )L] (M = Ta, Nb; L = several π acceptor ligands) complexes has a large rotational barrier (ca.…”

Section: Resultsmentioning

confidence: 75%

Theoretical Study of the Hydrogen Exchange Coupling in the Metallocene Trihydride Complexes [(C₅H₅)₂MH₃]ⁿ⁺ (M = Mo, W, n = 1; M = Nb, Ta, n = 0)

et al. 1996

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The effect on the hydrogen exchange coupling in metallocene [(C5H5)2MH3] n + complexes under the substitution of the transition metal M is theoretically analyzed using a simple methodology that requires a modest number of ab initio electronic energy calculations that are used in a one-dimensional tunneling model within a basis set method. Concretely, the cases M = Mo, W (n = 1) and M = Nb, Ta (n = 0) whose hydrogen exchange couplings have been experimentally measured through the corresponding 1H NMR spectra are considered. Our results of the exchange couplings at different temperatures for the considered cases are in satisfactory agreement (within the correct order of magnitude) with experimental results. This agreement seems to confirm that the mechanism we previously established for some formally d4 iridium complexes that involved a dihydrogen-like transition state is also operative in the case of d0 transition metal trihydride complexes. As a matter of fact, it is the stability of the η2-H2 structure relative to the minimum energy trihydride that is the main parameter governing the magnitude of the exchange coupling.

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“…H NMR spectra for the hydride region of a mixture of 1 and 1-d 1 at various temperatures as reported by SaboEtienne and coworkers. 18 The signal designated as c is due to 1 and is invariant with temperature. The signals designated as b and c are due to 1-d 1 .…”

Section: Dihydrogen Complexes With An Adjacent Hydride Ligandmentioning

confidence: 99%

“…In contrast the HD complex 1-d 1 exhibits decoalescence at low temperature, allowing the two ends of the bound HD ligand to give distinct resonances (Scheme 1). 18 This profound change in the NMR spectrum upon isotope substitution is not the result of a conventional KIE on the rotation process. Rather, Chaudret and coworkers attribute this to the quenching of QMEC by isotope substitution.…”

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confidence: 99%

Transition metal dihydrogen complexes: isotope effects on reactivity and structure

Heinekey

2007

Labelled Comp Radiopharmac

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Complexation of dihydrogen to transition metal centers was discovered by Kubas and coworkers in 1984. The notion that the simplest molecule in chemistry can act as a ligand to form relatively stable transition metal complexes has led to a paradigm shift in coordination chemistry. Crucial to the exploration of this intriguing new chemistry has been the use of isotope substitution. Several aspects of the coordination chemistry of dihydrogen have revealed fascinating isotope effects on reactivity, spectroscopy and in some cases structure. Complexation of HD has been used to diagnose bond distances from measurements of J HD and isotope effects for D 2 versus H 2 binding have been evaluated. Examples of quantum mechanical exchange coupling in H 2 complexes have been described. These effects disappear when one of the H atoms is replaced by D. In molecules with bound hydrogen adjacent to a hydride ligand, non-statistical occupancy of hydrogen versus hydride sites by deuterons has been observed. In some cases, isotope-dependent structures have been established by the study of HD, HT and DT complexes using NMR spectroscopy.

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Upon the Chemical Origin of Exchange Couplings in d2 Systems

Cited by 55 publications

References 0 publications

Synthesis and Characterization of Hydrotris(pyrazolyl)borate Dihydrogen/Hydride Complexes of Rhodium and Iridium

Synthesis and Characterization of Hydrotris(pyrazolyl)borate Dihydrogen/Hydride Complexes of Rhodium and Iridium

Theoretical Study of the Hydrogen Exchange Coupling in the Metallocene Trihydride Complexes [(C₅H₅)₂MH₃]ⁿ⁺ (M = Mo, W, n = 1; M = Nb, Ta, n = 0)

Transition metal dihydrogen complexes: isotope effects on reactivity and structure

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Upon the Chemical Origin of Exchange Couplings in d2 Systems

Cited by 55 publications

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Synthesis and Characterization of Hydrotris(pyrazolyl)borate Dihydrogen/Hydride Complexes of Rhodium and Iridium

Synthesis and Characterization of Hydrotris(pyrazolyl)borate Dihydrogen/Hydride Complexes of Rhodium and Iridium

Theoretical Study of the Hydrogen Exchange Coupling in the Metallocene Trihydride Complexes [(C5H5)2MH3]n+ (M = Mo, W, n = 1; M = Nb, Ta, n = 0)

Transition metal dihydrogen complexes: isotope effects on reactivity and structure

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Theoretical Study of the Hydrogen Exchange Coupling in the Metallocene Trihydride Complexes [(C₅H₅)₂MH₃]ⁿ⁺ (M = Mo, W, n = 1; M = Nb, Ta, n = 0)