Petr Kilian

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.

Isolable Phosphanylidene Phosphorane with a Sterically Accessible Two-Coordinate Phosphorus Atom**

Phosphanylidene-σ4-phosphoranes (RP=PR′3) are phosphorus analogues of alkylidene-σ4-phosphoranes (R2C=PR′3), better known as Wittig reagents. Phosphanylidene-σ4-phosphoranes are synthetically accessible in the free form (RP=PR′3) and also in the transition-metal-stabilized form (LnM←P(R)=PR′3).1 The latter complexes are commonly used in both P=C bond generation and as a source of the phosphinidene (R-P) moiety in the continuing pursuit of new terminal phosphinidene complexes R-P=MLn. In marked contrast, free phosphanylidene-σ4-phosphoranes have received little attention because isolable (i.e., thermally stable) examples remain rare.2–4 Herein we report the synthesis and structure of the stable cyclic phosphanylidene-σ4-phosphorane 3 (see Scheme 1). Compound 3 possesses a sterically nonhindered phosphanylidene moiety, yet it is thermally stable enough to be isolated and stored at room temperature. The potential diversity of the coordination chemistry of 3 is illustrated by its transition-metal (Pd0) complex and bis(borane) adduct.

Intramolecular phosphine-phosphine donor-acceptor complexes.

The reaction of 5-diphenoxyphosphanyl-6-diisopropylphosphinoacenaphthene 12 with chlorotrimethylsilane unexpectedly gave a phosphonium-phosphine compound 13, containing the structural motif of four phosphorus atoms connected in a chain. To explain the mechanism of this complex transformation, a proposed intermediate 5-dichlorophosphino-6-diisopropylphosphinoacenaphthene 14 was synthesized by an alternative method. The two (formally) phosphine environments in 14 form an intramolecular donor-acceptor (phosphonium-phosphoranide) complex, stable at room temperature in the solid state and as a solution in certain solvents. A (31)P NMR mechanistic study showed that, despite the presence of a rigid acenaphthene backbone, 14 is unstable in the presence of nucleophiles and disproportionates into 13 and other phosphorus containing products. Both 13 and 14 have been crystallographically characterized.

Prediction of a New Delocalised Bonding Motif between Group 15 or Group 16 Atoms

The nature of the chemical bond never ceases to intrigue chemists. Whether or not two atoms are linked by a bond is one of the most fundamental insights ingrained in the electronic structure of a molecule. There are tools of theoretical chemistry available to address this question, but because they are not always unambiguous, heated debates concerning this issue can ensue. In particular when there is electron delocalisation over atoms that are close in space, but not linked by a regular covalent bond, the question to what extent this delocalisation contributes to a direct bond between these atoms can be quite delicate. One class of compounds that falls into this category are peri1,8-disubstituted naphthalenes (Scheme 1), the chemistry of which is well developed. In particular, peri-substituents from the third or fourth row, aided by their propensity for hyperva-

Synthesis and characterisation of four- and six-membered P–Se heterocycles

The preparation, spectroscopic characterisation and crystal structures of [FcP(mu-Se)Se]2, [FcP(mu-Se2)Se]2 and [PhP(mu-Se2)Se]2 are reported. Crystallographic data reveal planar four-membered PSePSe and skewed six-membered P2Se4 rings, respectively, in all cases with trans arrangement of organic substituents and exo selenium atoms. Whilst stable at room temperature in solid state, NMR data suggest the six-membered rings of both the ferrocenyl and phenyl compounds decompose in the solution with loss of red selenium, forming PSe2PSe five-membered rings.

Geometrically enforced donor-facilitated dehydrocoupling leading to an isolable arsanylidine-phosphorane

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.

Reactivity Profile of a Peri-Substitution-Stabilized Phosphanylidene-Phosphorane: Synthetic, Structural, and Computational Studies

The reactions of 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 extents, 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 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 11 species were isolated. The bonding in the chalcogeno derivatives is discussed using DFT (B3LYP) and natural bond orbital analysis, which indicate a contribution from 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.

Synthesis and full characterisation of 6-chloro-2-iodopurine, a template for the functionalisation of purines

A simple and efficient synthesis of 6-chloro-2-iodopurine from hypoxanthine has been achieved. This strategy relied on a regiospecific lithiation/quenching sequence of 6-chloro-9-(tetrahydropyran-2-yl)purine using Harpoons base and tributyltin chloride. HMBC NMR studies on the product and intermediates confirmed the regioselectivity of this methodology. The molecular structures of the final dihalogenopurine and its 9-protected precursor were determined by single crystal X-ray diffraction.

Preparation and structures of 1,2-dihydro-1,2-diphosphaacenaphthylenes and rigid backbone stabilized triphosphenium cation

The effect of the special peri-geometry of rigid naphthalene-1,8-diyl backbone in phosphenium formation reaction was investigated. 1,8-Bis(diphenylphosphino) naphthalene and P2I4 afforded triphosphenium iodide in a clean reaction. The reaction of 1,8-bis(dimethylaminophosphino) naphthalene with P2I4 is complex, it afforded four products, all containing the 1,2-dihydro-1,2-diphosphaacenaphthylene motif and heterophosphonium functionalities. Two examples of the rare structural motif of two acenaphthylene units connected head to head and thus a contiguous chain of four phosphorus atoms were also isolated, one compound showing diastereomerization in the solution. All new compounds were fully characterised including single crystal X-ray diffraction.

Novel Condensed Thionated Bis(phosphonic) Acid Salts with a Rigid Naphthalene-1,8-diyl Backbone

New salts of thionated (naphthalene-1,8-diyl)bis(phosphonic) acid monoanhydrides [PPh4+]2[C10H6PS2(μ-S)PS22–] (2) and [K+]2[C10H6PS2(μ-S)POS2–]·H2O (3) both containing the C3P2S ring, were prepared in high yields by the reaction of 2,4-(naphthalene-1,8-diyl)-1,3,2,4-dithiadiphosphetane 2,4-disulfide (1) with NaHS and PPh4Cl, or KOH, respectively, in water. The derivative 3, containing a P=O terminal bond as well as P–S–P bridge, undergoes, in acid conditions, a rearrangement reaction leading to the O,S-symmetrically substituted derivative [K+]2[C10H6PS2(μ-O)PS22–] (4) containing a C3P2O heterocycle. Dipotassium salt 4 was converted into the bis(tetraphenylphosphonium) salt 5 by the treating with PPh4Cl. Hydrolysis of [PyH+][C10H6P(S)(NHMe)(μ-S)PS22–] (15) led to [CH3NH3+][PyH+][C10H6POS(μ-O)POS2–]·1.5 Py (6). New compounds were characterised by 31P{1H}-, 1H- and 13C{1H}-NMR, FT-IR, ES-MS and in the cases 2–6 by X-ray structure determination. The 31P{1H}-NMR spectra of these mixed O,S-derivatives are briefly discussed.