Fergus R. Knight

Naphthalene and related systems peri-substituted by Group 15 and 16 elements.

Synthetic and bonding aspects of heavier Group 15 (P, As, Sb, Bi) and 16 (S, Se, Te) peri-substituted naphthalenes, are discussed in this review. An important and unifying feature of the chemistry of these systems is the lively discussion about the nature of the interaction between peri-atoms. Are atoms bonded when they are closer than the sum of their van der Waals radii? Is there any (weak) bonding, or just a strained repulsive interaction? Positioning atoms of Group 15 and 16 at the naphthalene 1,8-positions provides leading systems with which to study these bonding issues.

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

Across the bay: J((125)Te, (125)Te) spin-spin coupling is a highly sensitive probe into the electronic and geometric structure of 1,8-peri-substituted naphthalene tellurium derivatives. The coupling is related to the onset of multicenter bonding in these systems.

Exploring hypervalency and three-centre, four-electron bonding interactions: Reactions of acenaphthene chalcogen donors and dihalogen acceptors

Sterically crowded peri-substituted selenium and tellurium acenaphthene donors D1-D7 [Acenap(EPh)(Br) E = Se, Te; Acenap(SePh)(EPh) E = Se, S; Acenap(TePh)(EPh) E = S, Se, Te] react with dibromine and diiodine acceptors to afford a group of structurally diverse addition products 1-12, comparable in some cases to previously reported naphthalene analogues. Tellurium donors D4-D6 react conventionally with the dihalogens to afford insertion adducts 6-11 (X-R(2)Te-X) exhibiting molecular see-saw geometries, characterised by hypervalent X-Te-X quasi-linear fragments. The reactions of selenium donors D1-D3 with diiodine afford expected neutral charge-transfer (CT) spoke adducts 1, 4 and 5 (R(2)Se-I-I) containing quasi-linear Se-I-I alignments. Conversely, treatment of D2 and D3 with dibromine results in the formation of two tribromide salts 2 and 3 containing bromoselanyl cations [R(2)Se-Br](+)···[Br-Br(2)](-), each exhibiting a quasi-linear three-body Br-Se···E (E = Se, S) fragment. The peri-bonding in these species can be thought of as a weak hypervalent G···Se-X three-centre, four-electron (3c-4e) type interaction, closely related to the T-shaped 3c-4e interaction. Density-functional calculations performed on 2 and 3 and their bare cations (2a and 3a) reveal Wiberg bond indices of 0.25-0.37, suggesting substantial 3c-4e character in these systems. The presence of such an interaction operating in 2 and 3 alleviates steric strain within the peri-region and minimises the degree of molecular distortion required to achieve a relaxed geometry. Ditellurium donor D7 reacts with dibromine to afford an unorthodox insertion adduct 12 containing a Te-O-Te bridge and two quasi-linear Br-Te-O fragments, with the central tellurium atoms assuming a molecular see-saw geometry. Whilst DFT calculations indicate 12 is thermodynamically unfavourable, its formation is viable under experimental conditions.

Controlling Cu⋯Cu distances using halides: (8-phenylthionaphth-1-yl)diphenylphosphine copper halide dimers

In the isomorphous binuclear Cu2X2L2 systems (L = (8-phenylthionaphth-1-yl)diphenylphosphine the Cu...Cu separation is reduced as the halide size increases.

Highly Strained Heterocycles Constructed from Boron–Boron Multiple Bonds and Heavy Chalcogens

The reactions of a diborene with elemental selenium or tellurium are shown to afford a diboraselenirane or diboratellurirane, respectively. These reactions are reminiscent of the sequestration of subvalent oxygen and nitrogen in the formation of oxiranes and aziridines; however, such reactivity is not known between alkenes and the heavy chalcogens. Although carbon is too electronegative to affect the reduction of elements with lower relative electronegativity, the highly reducing nature of the B-B double bond enables reactions with Se(0) and Te(0) . The capacity of multiple bonds between boron atoms to donate electron density is highlighted in reactions where diborynes behave as nucleophiles, attacking one of the two Te atoms of diaryltellurides, forming salts consisting of diboratellurenium cations and aryltelluride anions.

Synthetic and Structural Studies of 1‐Halo‐8‐(alkylchalcogeno)naphthalene Derivatives

A series of eight 1-halo-8-(alkylchalcogeno)naphthalene derivatives (1-8; halogen=Br, I; alkylchalcogen=SEt, SPh, SePh, TePh) containing a halogen and a chalcogen atom occupying the peri positions have been prepared and fully characterised by using X-ray crystallography, multinuclear NMR spectroscopy, IR spectroscopy and MS. Naphthalene distortion due to non-covalent substituent interactions was studied as a function of the bulk of the interacting chalcogen atoms and the size and nature of the alkyl group attached to them. X-ray data for 1, 2, 4 and 5-8 were compared. Molecular structures were analysed in terms of naphthalene ring torsions, peri-atom displacement, splay angle magnitude, X...E interactions, aromatic ring orientations and quasi-linear X...E-C arrangements. A general increase in the X...E distance was observed for molecules that contain bulkier atoms at the peri positions. The I...S distance of 4 is comparable with the I...Te distance of 8, and is ascribed to a stronger lone pair-lone pair repulsion due to the presence of an axial S(naphthyl) ring conformation. Density functional theory (B3LYP) calculations performed on 5-8 revealed Wiberg bond index values of 0.05-0.08, which indicate minor interactions taking place between the non-bonded atoms in these compounds.

Probing interactions through space using spin–spin coupling

The series of eight 5-(TeY)-6-(SePh)acenaphthenes (Y = Fp (2), Tol (3), An-p (4), An-o (5), Tp (6), Mes (7), Tip (8), Nap (9)) were prepared and structurally characterised by X-ray crystallography, solution and solid-state NMR spectroscopy and density functional theory (DFT/B3LYP) calculations. All members of the series, except 5, adopt a BA type configuration comparable to the parent compound 1 (Y = Ph), aligning the Te-C(Y) bond along the mean acenaphthene plane and promoting a nonbonded Se···Te-C(Y) 3c-4e type interaction to form to stabilise the molecule (G-dependence). 5 (Y = An-o) adopts a BC type conformation in the solid but DFT calculations show this optimises to BA. Indication of strong through-space peri-interactions between Te and Se are observed in the (77)Se and (125)Te NMR spectra, with J(Te,Se) spin-spin coupling constants (SSCCs) in the range -688 to -748 Hz. Evidence supporting the presence of this interaction was also found in solid-state NMR spectra of some of the compounds which exhibit an indirect spin-spin coupling on the same order of magnitude as observed in solution. In order to quantify the steric bulk of the aryl groups (Y), we introduce the crystallographic steric parameter (θ), the cone angle measured from the furthest H atoms lying on the edges of the cone to the Te atom located at its vertex. Modification to Y has no apparent influence over the conformation of the molecule, the degree of molecular distortion occurring in the acenaphthene backbone or the extent of 3c-4e interaction; peri-distances for all eight compounds are within 0.08 Å and no apparent correlation is observed between the steric bulk of Y (θ) and the (77)Se chemical shifts or J(Te,Se) SSCCs. In contrast, a good correlation is found between θ and (125)Te chemical shifts. DFT calculations performed on all members of the series confirm the comparable covalent bonding between Te and Se in the series, with WBIs of ca. 0.1 obtained. Natural bond orbital analysis shows a noticeable donor-acceptor interaction between a p-type lone pair on Se and a σ*(Te-C) antibonding orbital, confirming the onset of 3c-4e type bonding.

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

Three related series of peri-substituted bis(tellurides) bearing naphthalene, acenaphthene and acenaphthylene backbones (Nap/Acenap/Aceyl(TeY)2 (Nap = naphthalene-1,8-diyl N; Acenap = acenaphthene-5,6-diyl A; Aceyl = acenaphthylene-5,6-diyl Ay; Y = Ph 1; Fp 2; Tol 3; An-p- 4; An-o- 5; Tp 6; Mes 7; Tip 8) have been synthesised and their solid-state structures determined by X-ray crystallography. Molecular conformations were classified as a function of the two C9-C-Te-C(Y) dihedral angles (θ); in the solid all members adopt AB or CCt configurations, with larger Te(aryl) moieties exclusively imposing the CCt variant. Exceptionally large J((125)Te,(125)Te) spin-spin coupling constants between 3289-3848 Hz were obtained for compounds substituted by bulky Te(aryl) groups, implying these species are locked in a CCt-type conformation. In contrast, compounds incorporating smaller Te(aryl) moieties are predicted to be rather dynamic in solution and afford much smaller J values (2050-2676 Hz), characteristic of greater populations of AB conformers with lower couplings. This conformational dependence of through-space coupling is supported by DFT calculations.

Electrochemically informed synthesis : oxidation versus coordination of 5,6-Bis(phenylchalcogeno)acenaphthenes

Chalcogen dications: Facile synthesis of E--E bonded dications can be readily achieved. Radical cations are identified as the intermediates.

1-Bromo-8-(phenylselenyl)naphthalene

We are investigating the structures of 1,8-disubstituted naphthalenes as part of a wider study into steric crowding and hyperconjugation. In the title compound, C16H11BrSe, the Se⋯Br distance is 3.1136 (5) A. The Br and Se atoms lie 0.400 (1) and −0.421 (1) A, respectively, from the mean plane of the naphthalene backbone. The heavy atoms are further accommodated by in-plane distortions in the C—C—C group between the Br and Se atoms. As expected from the heavy atom displacement, the phenylselenyl group lies on one side of the naphthalene plane, the phenyl ring being inclined at 88° to the naphthalene plane.