Diffusion coefficients of zirconocene–borate ion pairs studied by pulsed field-gradient NMR—evidence for ion quadruples in benzene solutions

Beck, Stefan; Geyer, Armin; Brintzinger, Hans‐Herbert

doi:10.1039/a908203d

Cited by 91 publications

(72 citation statements)

References 15 publications

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“…Beck et al have claimed on the basis of diffusion NMR experiments that the formation of ion quadruples (B) from simple ion pairs (A), as shown in Scheme 6, is probably more important than previously thought in zirconocene-based catalytic systems. [105] This was concluded by measuring the diffusion coefficient of the zirconocenes 73-77 (Scheme 6), and by comparing the findings with the diffusion coefficient obtained for a series of ion pairs such as 73 a/b and 77 a/b with the diffusion coefficient of 73 c, for which the binuclear structure was previously established by X-ray crystallogra- …”

Section: Diffusion Nmr Spectroscopymentioning

confidence: 97%

Diffusion NMR Spectroscopy in Supramolecular and Combinatorial Chemistry: An Old Parameter—New Insights

2005

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Intermolecular interactions in solution play an important role in molecular recognition, which lies at the heart of supramolecular and combinatorial chemistry. Diffusion NMR spectroscopy gives information over such interactions and has become the method of choice for simultaneously measuring diffusion coefficients of multicomponent systems. The diffusion coefficient reflects the effective size and shape of a molecular species. Applications of this technique include the estimation of association constants and mapping the intermolecular interactions in multicomponent systems as well as investigating aggregation, ion pairing, encapsulation, and the size and structure of labile systems. Diffusion NMR spectroscopy can also be used to virtually separate mixtures and screen for specific ligands of different receptors, and may assist in finding lead compounds.

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Section: Diffusion Nmr Spectroscopymentioning

confidence: 97%

Diffusion NMR Spectroscopy in Supramolecular and Combinatorial Chemistry: An Old Parameter—New Insights

2005

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“…Recently, diffusion data on dendrimers [15 ± 20] and peptides [21 ± 24] as well as on molecules in various environments, e.g., in porous silica [25], and zeolites [26], have been obtained. However, there are very few applications of PGSE methods in coordination and/or organometallic chemistry [27 ± 35].In an interesting and recent application, Beck et al [27] have studied the polymerization catalyst precursors 1 ± 5. Their results prompted the authors to suggest that these zirconium complexes can exist as ion-quadruples in the presence of a boronbased cocatalyst.…”

mentioning

confidence: 99%

“…In an interesting and recent application, Beck et al [27] have studied the polymerization catalyst precursors 1 ± 5. Their results prompted the authors to suggest that these zirconium complexes can exist as ion-quadruples in the presence of a boronbased cocatalyst.…”

mentioning

confidence: 99%

Applications of Pulsed-Gradient Spin-Echo (PGSE) Diffusion Measurements in Organometallic Chemistry

Valentini¹,

Rüegger²,

Pregosin³

2001

HCA

123

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With personal pleasure, we dedicate this Perspective to the memory of Luigi M. Venanzi, who, apart from his many scientific contributions, was a major promotor of NMR spectroscopy within the Swiss and international Inorganic Chemistry communities.Diffusion data from pulsed-field gradient spin-echo (PGSE) methods are shown to be qualitatively useful in the investigation of problems involving unknown molecular aggregation and/or the nature of inter-ionic interactions in metal complexes. For charged species possessing anions such as PF À 6 , BF À 4 , CF 3 SO À 3 or BArF À , both 19 F-and 1 H-PGSE methods offer a valid alternative and, sometimes, unique view of gross and subtle solution molecular structure and dynamics. Problems associated with solvents, concentration, and reproducibility are discussed.1. Introduction. ± Although organometallic chemistry (and especially homogeneous catalysis) continues to move from strength to strength [1 ± 5], the applications of modern NMR methods in these areas have lagged somewhat behind. Slowly, but surely, three-dimensional structures are being solved with NOE-and ROE-NMR methods [6 ± 9]; nevertheless, there are areas, e.g., determining molecular size, aggregation, and/ or the nature of interionic interactions, where NMR spectroscopic possibilities have not been sufficiently explored.A promising NMR method involves the use of pulsed-field gradient spin-echo (PGSE) experiments [10], which can measure the diffusion coefficients of molecules and thus provide information on particle size. PGSE Methods were introduced in 1965 by Stejskal and co-workers [11] [12] and, since then, have been widely used. In the 1970s, this approach was used to determine diffusion coefficients of organic molecules [13]. In the following decade, variants of this technique have been applied to problems in polymer chemistry [14]. Recently, diffusion data on dendrimers [15 ± 20] and peptides [21 ± 24] as well as on molecules in various environments, e.g., in porous silica [25], and zeolites [26], have been obtained. However, there are very few applications of PGSE methods in coordination and/or organometallic chemistry [27 ± 35].In an interesting and recent application, Beck et al. [27] have studied the polymerization catalyst precursors 1 ± 5. Their results prompted the authors to suggest that these zirconium complexes can exist as ion-quadruples in the presence of a boronbased cocatalyst. In their construction of novel Pt-molecules, Olenyuk et al. [28] employed diffusion data to support a self-assembled dodecahedron structure of the product of the reaction shown in the Scheme. In a bio-inorganic application, Gorman et al.[15] estimated the hydrodynamic radii of the iron-sulphur based dendrimers, abbreviated below, using PGSE studies. These three examples are impressive as much for their scarcity as for their elegance.

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“…The bimolecular mechanism might involve the aggregation of two 1 b ion pairs in a transition state which should resemble the ion quadruples described by Brintzinger et al [6,36] However, this would not explain the observed variation of the bandwidth of the Me-B signal, which can arise only from the presence of an exchange partner, that is, a different ion pair containing a [MeB(C 6 F 5 ) 3 ] À anion. Such partner has been detected by 2D EXSY experiments at 300 K ( Figure S9, Supporting Information), that showed an exchange crosspeak between Me-B of 1 b and a very weak and broad resonance, undetectable in the normal 1D spectrum; the d value shifted from 0.84 to % 1.10 ppm on decreasing the concentration.…”

mentioning

confidence: 84%

Solution Conformation and Dynamics of the Ion Pairs Originating from the Reaction of B(C₆F₅)₃ with Bisindenyl Dimethyl Zirconium Complexes

Beringhelli¹,

D’Alfonso²,

Maggioni³

et al. 2004

Chemistry A European J

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The two ion pairs [(4,7‐Me2indenyl)2ZrMe]+[MeB(C6F5)3]− (1 b) and [(indenyl)2ZrMe]+ [MeB(C6F5)3]− (2 b) have been generated in situ by reaction of stoichiometric B(C6F5)3 with the corresponding dimethyl zirconocenes. It has been shown that molecular mechanics computations, guided by experimental 1H/1H NOE correlations, can provide information on the conformers present in solution. The dynamics of the ion pairs has also been investigated, showing the occurrence of both the processes previously characterized for this class of compounds, namely the B(C6F5)3 migration between the two methyl groups and dissociation–recombination of the whole [MeB(C6F5)3]− anion, the latter process being much faster than the first one (about three order of magnitude). Moreover, it has been shown that in certain conditions intermolecular processes can occur, which mimic the above‐mentioned dissociative exchanges. In particular, the presence of species containing loosely bound [MeB(C6F5)3]− anion fastens the exchange of this anion, while the presence of free B(C6F5)3 accelerates its exchange between the two methyl sites.

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Diffusion coefficients of zirconocene–borate ion pairs studied by pulsed field-gradient NMR—evidence for ion quadruples in benzene solutions

Abstract: Diffusion coefficients for ion pairs containing zirconocene cations and fluorinated tetraphenyl-or methyltriphenylborate anions in benzene solutions are substantially smaller than those expected for monomeric species and indicate that ion quadruples are present in these solutions.

Cited by 91 publications

References 15 publications

Diffusion NMR Spectroscopy in Supramolecular and Combinatorial Chemistry: An Old Parameter—New Insights

Diffusion NMR Spectroscopy in Supramolecular and Combinatorial Chemistry: An Old Parameter—New Insights

Applications of Pulsed-Gradient Spin-Echo (PGSE) Diffusion Measurements in Organometallic Chemistry

Solution Conformation and Dynamics of the Ion Pairs Originating from the Reaction of B(C₆F₅)₃ with Bisindenyl Dimethyl Zirconium Complexes

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Diffusion coefficients of zirconocene–borate ion pairs studied by pulsed field-gradient NMR—evidence for ion quadruples in benzene solutions

Abstract: Diffusion coefficients for ion pairs containing zirconocene cations and fluorinated tetraphenyl-or methyltriphenylborate anions in benzene solutions are substantially smaller than those expected for monomeric species and indicate that ion quadruples are present in these solutions.

Cited by 91 publications

References 15 publications

Diffusion NMR Spectroscopy in Supramolecular and Combinatorial Chemistry: An Old Parameter—New Insights

Diffusion NMR Spectroscopy in Supramolecular and Combinatorial Chemistry: An Old Parameter—New Insights

Applications of Pulsed-Gradient Spin-Echo (PGSE) Diffusion Measurements in Organometallic Chemistry

Solution Conformation and Dynamics of the Ion Pairs Originating from the Reaction of B(C6F5)3 with Bisindenyl Dimethyl Zirconium Complexes

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Solution Conformation and Dynamics of the Ion Pairs Originating from the Reaction of B(C₆F₅)₃ with Bisindenyl Dimethyl Zirconium Complexes