The distribution of the electron density around covalently bonded atoms is anisotropic, and this determines the presence, on atoms surface, of areas of higher and lower electron density where the electrostatic potential is frequently negative and positive, respectively. The ability of positive areas on atoms to form attractive interactions with electron rich sites became recently the subject of a flurry of papers. The halogen bond (HaB), the attractive interaction formed by halogens with nucleophiles, emerged as a quite common and dependable tool for controlling phenomena as diverse as the binding of small molecules to proteinaceous targets or the organization of molecular functional materials. The mindset developed in relation to the halogen bond prompted the interest in the tendency of elements of groups 13-16 of the periodic table to form analogous attractive interactions with nucleophiles. This Account addresses the chalcogen bond (ChB), the attractive interaction formed by group 16 elements with nucleophiles, by adopting a crystallographic point of view. Structures of organic derivatives are considered where chalcogen atoms form close contacts with nucleophiles in the geometry typical for chalcogen bonds. It is shown how sulfur, selenium, and tellurium can all form chalcogen bonds, the tendency to give rise to close contacts with nucleophiles increasing with the polarizability of the element. Also oxygen, when conveniently substituted, can form ChBs in crystalline solids. Chalcogen bonds can be strong enough to allow for the interaction to function as an effective and robust tool in crystal engineering. It is presented how chalcogen containing heteroaromatics, sulfides, disulfides, and selenium and tellurium analogues as well as some other molecular moieties can afford dependable chalcogen bond based supramolecular synthons. Particular attention is given to chalcogen containing azoles and their derivatives due to the relevance of these moieties in biosystems and molecular materials. It is shown how the interaction pattern around electrophilic chalcogen atoms frequently recalls the pattern around analogous halogen, pnictogen, and tetrel derivatives. For instance, directionalities of chalcogen bonds around sulfur and selenium in some thiazolium and selenazolium derivatives are similar to directionalities of halogen bonds around bromine and iodine in bromonium and iodonium compounds. This gives experimental evidence that similarities in the anisotropic distribution of the electron density in covalently bonded atoms translates in similarities in their recognition and self-assembly behavior. For instance, the analogies in interaction patterns of carbonitrile substituted elements of groups 17, 16, 15, and 14 will be presented. While the extensive experimental and theoretical data available in the literature prove that HaB and ChB form twin supramolecular synthons in the solid, more experimental information has to become available before such a statement can be safely extended to interactions wherein elements of groups 14 and 15 are the electrophiles. It will nevertheless be possible to develop some general heuristic principles for crystal engineering. Being based on the groups of the periodic table, these principles offer the advantage of being systematic.

The Chalcogen Bond in Crystalline Solids: A World Parallel to Halogen Bond

In this manuscript we analyze the Cambridge Structural Database (CSD) to compare the relative importance of halogen, chalcogen and pnicogen bonding. The three interactions can be explained in terms of electrostatic effects, considering the halogen, chalcogen or pnicogen as a Lewis acid due to the presence of a sigma hole (σ-hole). We have studied the behaviour of the three interactions considering two types of Lewis bases: amines and arenes. Combining the CSD search and a comprehensive theoretical study (DFT-D3) we conclude that the halogen bonding interaction is the energetically most favourable when the electron donor is an amine. In contrast, the pnicogen bond is the most favourable if the Lewis base is benzene (pnicogen–π interaction).

Halogen bonding versus chalcogen and pnicogen bonding: a combined Cambridge structural database and theoretical study

Alternative Motifs for Halogen Bonding

Small inorganic rings in the 21st century: from fleeting intermediates to novel isolable entities.

A systematic investigation into the Lewis base catalyzed, asymmetric, intramolecular selenoetherification of olefins is described. A critical challenge for the development of this process was the identification and suppression of racemization pathways available to arylseleniranium ion intermediates. This report details a thorough study of the influences of the steric and electronic modulation of the arylselenenyl group on the configurational stability of enantioenriched seleniranium ions. These studies show that the 2-nitrophenyl group attached to the selenium atom significantly attenuates the racemization of seleniranium ions. A variety of achiral Lewis bases catalyze the intramolecular selenoetherification of alkenes using N-(2-nitrophenylselenenyl)succinimide as the electrophile along with a Bronsted acid. Preliminary mechanistic studies suggest the intermediacy of ionic Lewis base-selenium(II) adducts. Most importantly, a broad survey of chiral Lewis bases revealed that 1,1'-binaphthalene-2,2'-diamine (BINAM)-derived thiophosphoramides catalyze the cyclization of unsaturated alcohols in the presence of N-(2-nitrophenylselenenyl)succinimide and methanesulfonic acid. A variety of cyclic seleno ethers were produced in good chemical yields and in moderate to good enantioselectivities, which constitutes the first catalytic, enantioselective selenofunctionalization of unactivated olefins.

Preparative and Mechanistic Studies Toward the Rational Development of Catalytic, Enantioselective Selenoetherification Reactions

The synthesis and characterisation of Ph4Se4Br4 (1) directly from the reaction of Ph2Se2 with dibromine is reported. The solid-state structure of 1 consists of four PhSeBr units linked by weak selenium–selenium bonds [3.004(2)–3.051(2) A] into a Se4 square, and is structurally analogous to the previously reported Ph4Te4I4. The reactions of Ph4Se4Br4 with a variety of tertiary phosphines have been undertaken, resulting in the formation of compounds of formula R3PSe(Ph)Br. X-Ray crystallographic analysis of three of the compounds reveals different structural isomers. Ph3PSe(Ph)Br (2) is a charge-transfer (CT) compound [Se–Br 3.0020(8) A], with an essentially linear P–Se–Br bond angle, 172.15(4)° and T-shaped geometry at selenium. Me3PSe(Ph)Br (5) also contains the selenium atom in a T-shaped geometry, consistent with a CT formulation, although the Se–Br distance of 3.327(3) A is considerably longer than observed for 2. In contrast, Cy3PSe(Ph)Br (6) is an ionic phosphonium salt, [Cy3PSePh]Br with no short Se–Br interactions. Geometry at selenium is bent, as expected for an ionic compound. These results are discussed with reference to the previously reported iodo-compounds Ph3PSe(Ph)I and [(Me2N)3PSe(Ph)]I.

Reactions of Ph4Se4Br4 with tertiary phosphines. Structural isomerism within a series of R3PSe(Ph)Br compounds

A comparison of the solid-state structures of a series of phenylseleno-halogen and pseudohalogen compounds, PhSeX (X = Cl, CN, SCN)

The reaction of (Ph(2)Se(2)I(2))(2) with a wide variety of tertiary phosphines possessing different steric and electronic properties has been studied, leading in most cases to R(3)PSe(Ph)I adducts; [R(3)P = (p-CH(3)C(6)H(4))(3)P (1), (m-CH(3)C(6)H(4))(3)P (2), (o-OCH(3)C(6)H(4))(3)P (4), Ph(2)MeP (6), Me(2)PhP (7), Me(3)P (8), Cy(3)P (9)]. All of the products formed were characterised by elemental analysis, Raman and multinuclear NMR spectroscopy. Both steric and electronic factors are important in determining the structural motif (CT vs. ionic) observed in the solid-state. In general, highly basic phosphines result in a lengthening of the Se-I interaction, and a preference for an ionic structure. The reaction with (o-CH(3)C(6)H(4))(3)P does not yield a stable R(3)PSe(Ph)I adduct, and instead (o-CH(3)C(6)H(4))(3)PI(2) (3) is formed. The unusually long P-I bond, [2.5523(12) A], and short I-I bond, [3.0724(4) A], exhibited by is a result of the high steric requirements of this phosphine. The similarly bulky (o-SCH(3)C(6)H(4))(3)P yields a mixture of (o-SCH(3)C(6)H(4))(3)PSe(Ph)I (5a) and [(o-SCH(3)C(6)H(4))(3)PSePh]I(3) (5b). The crystal structures of (m-CH(3)C(6)H(4))(3)PSe(Ph)I, 2, (o-CH(3)C(6)H(4))(3)PI(2), 3, [(o-OCH(3)C(6)H(4))(3)PSePh]I.CH(2)Cl(2), 4, [(o-SCH(3)C(6)H(4))(3)PSePh]I(3), 5b, two pseudo-polymorphs of Ph(2)MePSe(Ph)I, 6a/6b, and [Me(3)PSe(Ph)I](2).CH(2)Cl(2), 8, are reported. The R(3)PSe(Ph)I adducts formed exhibit one of four types of behaviour. Type I products, (such as 2) are CT in the solid-state and display fluxionality in solution. Type II products (such as 6a/6b) lie close to the CT/ionic structural borderline, displaying long Se-I bonds, and are more appropriately classified as [R(3)PSePh] (acceptor)/I(-) (donor) CT complexes. Type II complexes ionise in solution to [R(3)PSePh]I. Type III products, such as 8, are ionic in solution, but frequently show cation-anion, or cation-solvent interactions in the solid-state, although these interactions are weak and the linear P-Se-I motif is lost. Type IV products (such as 4) are ionic and feature bulky phosphines. They display no short cation-anion interactions in the solid-state.

The reaction of tertiary phosphines with (Ph2Se2I2)2--the influence of steric and electronic effects.

The reactions of the tris-dialkylamino phosphines (Et2N)3P and (nPr2N)3P, and the pyrrolidinyl substituted phosphines (C4H8N)3P and tBuP(NC4H8)2, with I2 and (Ph2Se2I2)2, have been reported. The reactions with diiodine lead to the formation of [R3PI]I adducts, which are essentially ionic, but show a tendency to display long, soft–soft, I⋯I interactions in the solid state. The crystal structures of [(Et2N)3PI]I, (1), [(nPr2N)3PI]I, (2), and [(C4H8N)3PI]I, (3), have all been determined, and display I⋯I interactions varying between 3.5170(6) and 3.6389(14) A. The analogous reactions with (Ph2Se2I2)2 lead to the formation of phenylseleno-phosphonium salts, [R3PSePh]I. The structures of [(C4H8N)3PSePh]I, (6) and [(C4H8N)2tBuPSePh] I, (7), have been determined and do not display any soft–soft interactions between the selenium and iodine atoms. All of the phosphonium salts represent examples of structures containing tris-dialkylamino phosphine fragments which show no “special” nitrogen atom, i.e. all three nitrogen atoms are planar. This type of arrangement is usually observed when a C3 symmetric conformation is observed, (which is the case for 1 and 2), but not for the (C4H8N)3P adducts 3 and 6, where the conformation is closer to Cs, although the nitrogen atoms are still essentially planar. The P–N bonds in all the compounds reported herein are short, ranging between 1.599(12) A and 1.643(12) A, and are consistent with the previously reported short P–N bonds in phosphonium salts featuring tris-dialkylamino substituted phosphines.

Ruth T. A. Halton

Papers

Reactions of Ph4Se4Br4 with tertiary phosphines. Structural isomerism within a series of R3PSe(Ph)Br compounds

A comparison of the solid-state structures of a series of phenylseleno-halogen and pseudohalogen compounds, PhSeX (X = Cl, CN, SCN)

The reaction of tertiary phosphines with (Ph2Se2I2)2--the influence of steric and electronic effects.

The reactions of alkylamino substituted phosphines with I2 and (Ph2Se2I2)2: structural features of alkylamino phosphonium cations.

Formation of a Dicyanotriorganophosphorane from the Reaction of Triphenylphosphane with Phenylselenocyanate