Weak Te,Te Interactions through the Looking Glass of NMR Spin–Spin Coupling

Abstract

Most of the tools for quantifying the extent of chemical bonding between two atoms are quantum-chemical in nature. None of them are unambiguous, however, and different analyses can lead to conflicting interpretations, even concerning the most fundamental question of whether or not atoms are linked by a chemical bond. [1,2] One of the indicators that can be probed experimentally is the indirect spin-spin coupling constant (SSCC). For instance, observation of spinspin coupling across hydrogen bonds [3] has been taken as evidence for covalent contributions to this kind of bonding. For atoms within the same molecule that are close in space, but not linked through a direct formal bond, the question arises, how much of the observed coupling is transmitted through a succession of bonds that eventually links them ("through-bond coupling"), and how much is due to interaction through the overlap of lone pairs ("through-space coupling"). Again, a variety of quantum-chemical tools have been developed to address this question.A rigid scaffold that is used to achieve such spatial proximity is the naphthalene framework, in which substituents in the peri (1,8) positions have a typical separation of around 3 . [4] Through-space J( 19 F, 19 F) SSCCs, long known for their distance dependence, [5a] have been studied in some detail in peri-difluoronaphthalenes. [5b,c] Similarly, J( 31 P, 31 P) values in peri-bis(phosphino)naphthalenes have been attributed to through-space coupling, [6] and J( 77 Se, 77 Se) values in peri-bis(seleno) derivatives have been analyzed in detail through quantum-chemical computations. [7] In systematic studies of naphthalene (N) and acenaphthene (A) derivatives with pnictogen and chalcogen atoms in the peri positions, it became apparent that for the heavier congeners, steric repulsion is partly counterbalanced by attractive interactions. In particular with Te substituents, formally nonbonded, "across-the-bay" distances are significantly shorter than the sum of the van-der-Waals radii, which has been traced back to weak donor-acceptor interactions and the onset of 3-center-4-electron (3c4e) bonding. [8] For instance in N1 and A1 (Scheme 1), Te···Te distances of around 3.3 are observed in the solid state (ca. 0.7 below the sum of the vdW radii), and Wiberg bond indices [9] (WBIs, a measure for the covalent character of a bond, approaching WBI = 1 for a true single bond) of around 0.15 have been computed. Slightly larger WBIs of approximately 0.18 have been obtained for cationic methylated species N2 and A2, despite a slightly longer Te-Te separation (ca. 3.4 ). [10] These unsymmetrical systems show remarkably large J( 125 Te, 125 Te) SSCCs, formally 4 J values, of 1093.0 Hz and 945.8 Hz in N2 and A2, respectively. [10] We now report even larger couplings in N1 and A1, along with computational conformational analysis underlining the potential of this property as a structural and interpretative method.The computed (ZORA-SO/BP//B3LYP level) [11] J( 125 Te, 125 Te) SSCCs in N2 and A2 (1490 and 1377 Hz, respec...

Structural, Spectroscopic and Computational Examination of the Dative Interaction in Constrained Phosphine–Stibines and Phosphine–Stiboranes

“…This value is comparable to those of related systems such as Acenap(P i Pr 2 )(SnPh 3 ) or Acenap(TePh) 2 (Acenap=acenaphthene‐5,6‐diyl), where the onset of the 3c–4e bonding between the formally non‐bonded atoms has been shown through the sizable J ( 31 P– 119 Sn) or J ( 125 Te– 125 Te) coupling constants 32. 34 Unfortunately, the unfavourable properties of the NMR‐active antimony isotopes ( 121 Sb and 123 Sb), in particular their high quadrupolar moments, prevent the use of direct PSb NMR probe in this series. On the other hand, compound 4 displays large magnitude (40.1 Hz) of a 31 P 13 C coupling involving the ipso ‐carbon from a phenyl group on the antimony atom (atom C13 in Figure 2, hence formally 5ts J CP ).…”

Conformational Dependence of Through‐Space Tellurium–Tellurium Spin–Spin Coupling in Peri‐Substituted Bis(Tellurides)

“…In their study of corresponding selenium derivatives, Nakanishi and colleagues concluded that this small energy difference must result in an equilibrium existing between the AB and CC t conformers in solution 9. This hypothesis was supported in our own study, in which the experimentally observed J value for Acenap(TePh) 2 (2110 Hz; G , Figure 1) was found to lie intermediate between the predicted 4 J ( 125 Te, 125 Te) SSCCs for the two conformers (CC t 2604 Hz; AB 1543 Hz) 11…”

“…Our own detailed conformational analyses of the related bis(tellurium) compound Nap(TeMe) 2 ( F , Figure 1) revealed a similar conformational dependence of 4 J (Te,Te) through‐space coupling, with a dramatic change in the magnitude of 4 J (Te,Te) SSCCs predicted upon subtle changes to the structural conformation 11. The optimised CC t conformer (vide infra) is the global minimum and is predicted to have 4 J ( 125 Te, 125 Te) values around 2500 Hz, whilst a conformer in the AB region is predicted to have a much lower J value (ca.…”

“…1500 Hz). A two‐dimensional (2D) plot of potential energy as a function of the two C‐C‐Te‐C Me dihedral angles displayed a vast area within just 1 kcal mol −1 which extended from the global minimum (CC t ) into the region classified as AB 11. In their study of corresponding selenium derivatives, Nakanishi and colleagues concluded that this small energy difference must result in an equilibrium existing between the AB and CC t conformers in solution 9.…”

“…Interestingly, when the phenyl moiety was replaced by a much larger mesityl group ( H , Figure 1), a conformational change was observed in the solid from AB to CC t , and despite only a 0.03 Å reduction in the Te⋅⋅⋅Te internuclear distance, this was accompanied by a significant increase in the 4 J ( 125 Te, 125 Te) through‐space coupling (3398 Hz) 11. This implies that the effective overlap of the interacting lone‐pairs and hence the size of the SSCCs, depends not only on the internuclear distance, but also on the orientation of the lone‐pairs and hence the conformation of the molecule.…”