Rearrangements, Intramolecular Exchanges, and Isomerizations of Organometallic Compounds

Twigg, M. V.

doi:10.1007/978-1-4613-2479-9_13

Cited by 2 publications

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“…NMR is one of the primary techniques to probe chemical or conformational exchanges. ,− EXSY (exchange spectroscopy) has proven useful to observe exchanges slow on the NMR timescale. − , EXSY and NOESY are the same experiment except that, for compounds of low molecular weights, EXSY and NOESY cross-peaks are easily recognized as they have opposite phases . Cross-peaks of EXSY have the same phase of the diagonal peaks, while those of NOESY have the opposite phase.…”

Section: Resultsmentioning

confidence: 99%

Syntheses of Group 5 Amide Amidinates and Their Reactions with Water: Different Reactivities of Nb(V) and Ta(V) Complexes

et al. 2022

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Chemistries of Nb(V) and Ta(V) compounds are essentially identical as a result of lanthanide contraction. Hydrolysis of M(NMe 2 ) 5 (M = Nb, Ta), for example, yields [M(μ 3 -O)(NMe 2 ) 3 ] 4 (M = Nb, 1; Ta, 2) reported earlier. The similar reactivities of Nb(V) and Ta(V) compounds make it challenging, for example, to separate the two metals from their minerals. We have found that the reactions of H 2 O with amide amidinates M(NMe 2 ) 4 [MeC(N i Pr) 2 ] (M = Nb, 3; Ta, 4) show that the niobium and tantalum analogues take different principal paths. For the Nb(V) complex 3, the amidinate and one amide ligand are liberated upon treatment with water, yielding [Nb(μ 3 -O)(NMe 2 ) 3 ] 4 (1). For the Ta(V) complex 4, the amide ligands are released in the reaction with H 2 O, leaving the amidinate ligand intact. [Ta(μ 3 -O)(NMe 2 ) 3 ] 4 (2), the analogue of 1, was not observed as a product in the reaction of 4 with H 2 O. To our knowledge, this is the first example of the formation of two different complexes that maintain the (V) oxidation state in both metals. The new complexes M(NMe 2 ) 4 [MeC(N i Pr) 2 ] (M = Nb, 3; Ta, 4) have been prepared by the aminolysis of M(NMe 2 ) 5 (M = Nb, Ta) with i PrN(H)C(Me)=N i Pr (5). The hydrolysis of 3 and 4 has been investigated by DFT electronic structure calculations. The first step in each hydrolysis reaction involves the formation of a hydrogen-bonded complex that facilitates a proton transfer to the amidinate ligand in 3 and protonation of an axial dimethylamide ligand in 4. Both proton transfers furnish an intermediate metal-hydroxide species. The atomic charges in 3 and 4 have been computed by Natural Population Analysis (NPA), and these data are discussed relative to which of the ancillary ligands is protonated initially in the hydrolysis sequence. Ligand exchanges in 3 and 4 as well as the exchange in i PrN(H)C(Me)=N i Pr (5) were probed by EXSY NMR spectroscopy, giving rate constants of the exchanges: 0.430(13) s −1 (3), 0.033(6) s −1 (4), and 2.23(7) s −1 (5), showing that the rate of the Nb complex Nb(NMe 2 ) 4 [MeC(N i Pr) 2 ] (3) is 13 times faster than that of its Ta analogue 4.

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