Correlation of one-bond 15N 13C, 15NM, 29SiX (M = 11B, 29Si, 119Sn, X = 1H, 13C) nuclear spin-spin coupling constants with the structure of metallatranes in solution

Abstract

The 15N 13C, 15N 11B, 15N 29Si, 29Si 1H and 29Si 13C one-bond coupling constants in 15N-labelled boratranes, silatranes and germatranes were measured and considered together with the literature data on 15N 119Sn and 119Sn 13C couplings in stannatranes. The quantitative pattern of structural variations in metallatranes revealed by analysis of X-ray data was used for the interpretation of the results. 1 J( 15N 13C) coupling constants quantitatively describe the increase in nitrogen pyramidality with the strengthening of the donor—acceptor (DA) bond N → M in solution. The coupling constants of the DA bond N → M were interpreted in terms of the Fermi contact interaction. 1 J( 15N 29Si) values were used to predict the bond order and the length of the N → Si bond in silatranes. In silocanes, bicyclic analogues of silatranes, a linear correlation was found between the 1 J( 15N 29Si) values and the free activation energy for DA bond cleavage in solution. The latter was used to estimate the energy of the DA bond N → Si in silatranes (50–70 kJ/mol). The coupling constant 2J( 15NSi 1H) depends greatly on the size of the corresponding valence angle, whose increase from 90 to 180° results in a sharp reduction of the coupling constant 2 J( 15NSi 1H) from ∼ 8 to 0 Hz. The effect of the substituents' electronegativity on the coupling constant through the covalent and DA bonds was found to be the opposite.