DEDICATION: Dedicated to Wolf-Walther du Mont on the occasion of his 70 th birthday.KEYWORDS: peri-substitution, phosphorus-tellurium, through-space spin-spin coupling, acenaphthene 2 ABSTRACT: A series of peri-substituted phosphorus-tellurium systems R'Te-Acenap-PR 2 (R' = Ph, pAn, Nap, Mes, Tip; R = i Pr, Ph) exhibiting large "through space" spin-spin coupling constants and the "onset" of three-centre four-electron type interactions are presented. The influence of the substituents at the phosphorus and tellurium atoms as well as their behavior upon oxidation (with S, Se) or metalcoordination (Pt, Au) is discussed using NMR spectroscopy, single crystal X-ray diffraction, and advanced DFT studies including NBO, AIM and ELI-D analyses.

A proximate Lewis basic group facilitates the mild dehydrogenative P-As intramolecular coupling in the phosphine-arsine peri-substituted acenaphthene 3, affording thermally and hydrolytically stable arsanylidine-phosphorane 4 with a sterically accessible two-coordinate arsenic atom. The formation of 4 is thermoneutral due to the dehydrogenation being concerted with the donor coordination. Reaction of 4 with a limited amount of oxygen reveals arsinidene-like reactivity via formation of cyclooligoarsines, supporting the formulation of the bonding in 4 as base-stabilized arsinidene R3P→AsR.

is distorted, but notably less so than in bis(sulfide) 4-S. All compounds were fully characterized and except for bis(phosphine) 5 crystal structures were determined.

A series of phosphine-stibine and phosphine-stiborane peri-substituted acenaphthenes containing all permutations of pentavalent groups -SbCln Ph4-n (5-9), as well as trivalent groups -SbCl2 , -Sb(R)Cl, and -SbPh2 (2-4, R=Ph, Mes), were synthesised and fully characterised by single crystal diffraction and multinuclear NMR spectroscopy. In addition, the bonding in these species was studied by DFT computational methods. The P-Sb dative interactions in both series range from strongly bonding to non-bonding as the Lewis acidity of the Sb acceptor is decreased. In the pentavalent antimony series, a significant change in the P-Sb distance is observed between -SbClPh3 and -SbCl2 Ph2 derivatives 6 and 7, respectively, consistent with a change from a bonding to a non-bonding interaction in response to relatively small modification in Lewis acidity of the acceptor. In the Sb(III) series, two geometric forms are observed. The P-Sb bond length in the SbCl2 derivative 2 is as expected for a normal (rather than a dative) bond. Rather unexpectedly, the phosphine-stiborane complexes 5-9 represent the first examples of the σ(4) P→σ(6) Sb structural motif.

The reactions of a peri-substitution stabilized phosphanylidene-phosphorane 1 with [AuCl(tht)] or [PtCl2(cod)] afford binuclear complexes [((1)(AuCl)2)2] 2 and [((1)(PtCl2))2] 3, in which four electrons of the ligand are used in bonding to two metal atoms in the bridging arrangement. Reactions of 1 with [Mo(CO)4(nbd)] or (RhCl2Cp*)2 afford mononuclear complexes [(1)2Mo(CO)4] 4 and [(1)RhCl2Cp*] 5, in which two electrons of the ligand are used to form terminal complexes. Formation of these complexes disrupts the negative hyperconjugation at the P-P bond to various extent, which is mirrored by variations in their P-P bond distances (2.179(4)-2.246(4) Å). The P-P bond is ruptured upon formation of the Pd diphosphene complex 6, which is likely to proceed through a phosphinidene intermediate. In air 1 is fully oxidized to phosphonic acid 7. Reactions of 1 with chalcogens under mild conditions generally afford mixtures of products, from which the trithionated 8, dithionated 9, diselenated 10 and monotellurated species 11 were isolated. The bonding in the chalcogeno derivatives is discussed using DFT (B3LYP) and natural bond orbital analysis, which indicates a contribution from a dative bonding in 8-10. The buttressing effect of the peri-backbone is shown to be an essential factor in the formation of the single push-double pull bis(borane) 13. This is demonstrated experimentally through a synthesis parallel to that used to make 13, but lacking the backbone, which leads to different products. The P-P bond distances in the reported products, as well as additional species, are correlated with Wiberg Bond Indices, showing very good agreement for a variety of bonding modes including the negative hyperconjugation.

(4)). The degree of intramolecular P-Sn bonding within the series was investigated by X-ray crystallography, solution and solid-state NMR spectroscopy and density functional theory (DFT/B3LYP/SBKJC/PCM) calculations. All members of the series adopt a conformation such that the phosphorus lone-pair is located directly opposite the tin centre, promoting an intramolecular donor-acceptor P→Sn type interaction. The extent of covalent bonding between Sn and P is found to be much greater in triorganotin chlorides 2-4 compared with triphenyl derivative 1. Coordination of a highly electronegative chlorine atom naturally increases the Lewis acidity of the tin centre, enhancing the lp(P)−σ*(Sn−Y) donoracceptor 3c-4e type interaction, as indicated by conspicuously short Sn-P peri-distances and significant 1 J( 31 P, 119 Sn) spin-spin coupling constants (SSCCs) in the range 740-754 Hz. Evidence supporting the presence of this interaction was also found in solidstate NMR spectra of some of the compounds which exhibit an indirect spin-spin coupling on the same order of magnitude as observed in solution. DFT calculations confirm the increased covalent bonding between P and Sn in 2-4, with notable WBIs of ca. 0.35 obtained, compared to 0.1 in 1.

Here, we highlight the ability of peri-substitution chemistry to promote a series of unique P-P/P-As coupling reactions, which proceed with concomitant C-H bond formation. This dealkanative reactivity represents an interesting and unexpected expansion to the established family of main-group dehydrocoupling reactions. These transformations are exceptionally clean, proceeding essentially quantitatively at relatively low temperatures (70-140 °C), with 100% diastereoselectivity in the products. The reaction appears to be radical in nature, with the addition of small quantities of a radical initiator (azobis(isobutyronitrile)) increasing the rate dramatically, as well as altering the apparent order of reaction. DFT calculations suggest that the reaction involves dissociation of a phosphorus centered radical (stabilized by the peri-backbone) to the P-P coupled product and a free propyl radical, which carries the chain. This unusual reaction demonstrates the powerful effect that geometric constraints, in this case a rigid scaffold, can have on the reactivity of main group species, an area of research that is gaining increasing prominence in recent years.

Tris(acenaphthyl) and bis(acenaphthyl) substituted pnictogens (iPr2P-Ace)3E (2-4) (E = As, Sb, Bi; Ace = acenaphthene-5,6-diyl) and (iPr2P-Ace)2EPh (5 and 6), (E = As, Sb) were synthesised and fully characterised by multinuclear NMR, HRMS, elemental analysis and single crystal X-ray diffraction.The molecules adopt propeller-like geometries with the restricted rotational freedom of the sterically encumbered iPr2P-Ace groups resulting in distinct NMR features. In the tris(acenaphthyl) species (2-4) the phosphorus atoms are isochronous in the 31 P{ 1 H} NMR spectra, and the rotation of the three acenaphthyl moieties around E-Cipso bond is locked. On the other hand, the bis(acenaphthyl) species show a fluxional behaviour, resulting in an AX to A2 spin system transition in the 31 P{ 1 H} VT NMR spectra. This allowed elucidation of remarkable through-space couplings of 8TS JPP 11.5 Hz (for 5) and 25.8 Hz (for 6) at low temperature. In addition, detailed lineshape analysis of the thermodynamic parameters of the restricted rotation of the "propeller blades" in 5 was performed in the intermediate temperature region and also at coalescence. The lone pairs on the pnictogen atoms in 2-6 are oriented such that they form a bowl shaped area which is somehow buried within the molecule.2