Directed Selenium–Iodine Halogen Bonding and Se···H–C Contacts in

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Directed Selenium−Iodine Halogen Bonding and Se···H−C Contacts in Solid Iododiisopropylphosphane Selenide Published as part of the Crystal Growth & Design virtual special issue on Halogen Bonding in Crystal Engineering: Fundamentals and Applications Constantin G. Daniliuc, Cristian G. Hrib, Peter G. Jones, and Wolf-W. du Mont* Institut für Anorganische und Analytische Chemie der Technischen Universitaet Braunschweig, D 38023 Braunschweig, Germany S Supporting Information *

ABSTRACT: Solid iododiisopropylphosphane selenide crystallizes in the space group P21/n with one molecule iPr2P(I)Se in the asymmetric unit. Halogen bonds of the type Se···I−P (Se···I, 3.612 Å; Se···I−P, 171°) connect the molecules to form polymeric chains (-Se···I−P-)x. These are further connected by Se···H contacts (2.98 Å) involving the tertiary H atom from one isopropyl group. The extended structure thus formed is a layer parallel to 101̅, and a substructure thereof consists of 10membered (···Se···I−P−C−H···)2 rings.

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Scheme 1. Topologies of Solid Iodophosphonium Iodides3

[R3PI]+ + R3P* ⇌ R3P + [R3P*I]+ This kinetic lability of iodophosphonium ions correlates with their ground state property in the solid state to enter into halogen-bond-like, essentially linear P−I···I “soft−soft” interactions with nucleophilic anions such as iodide and triiodide.2,3 Since the extent of cation−anion I···I interactions correlates with P−I bond lengthening [e.g., tBu3PI···I: d(P−I) 2.461 Å, I···I 3.326 Å; tBu3PI···IWI2(CO)4: d(P−I) 2.401 Å, I···I 3.551 Å],4 the iodine−iodine cation−anion interactions are regarded as weakly bonding by (nI → σI−P*)-type overlap.3 In solid iodophosphonium iodides (R3PI+ I−), these halogen bonding donor−acceptor P−I···I interactions lead to ion pairs; in solid diiodophosphonium iodides the bidentate halogen donor cations R2PI2+ exhibit cyclic or chain-polymeric structures with μ2-bridging iodide ions, and in triiodophosphonium iodides the tridentate halogen donor cations RPI3+ exhibit extended puckered layers (MePI4, iPrPI4) or a three-dimensional network (tBuPI4) together with μ3-bridging iodide anions (Scheme 1).3,5,6 These structures suggest that, as bi- and tridentate acceptors (Lewis acids, that is, halogen bonding donors), di- and triiodophosphonium ions are potential building blocks for supramolecular architectures with soft−soft interactions (i.e., halogen bonding). Iodophosphonium ions behave as halogen bonding donors, whereas phosphane chalcogenides, their iso(valence)electronic

counterparts, are halogen bonding acceptors, as known from numerous adducts of phosphane sulfides and selenides with iodine. According to this consideration, iodophosphane chalcogenides, containing a halogen donor and a halogen bond acceptor function in the same molecule, will represent sof t acid/base functions, related to hard phosphinic or carboxylic acids, which are known to undergo self-assembly as base pairs or as chains.7a Consequently, an iodophosphane selenide (Scheme 2) can be regarded as a tool for supramolecular design of base pairs or of chain structures, as “soft− soft” halogen bonding counterparts of “hard−hard” hydrogen bonding phosphinic acids. Examples of species containing an iodophosphonium and a phosphane selenide function in one molecule are, however, still extremely rare in the literature.7

INTRODUCTION Iodine atoms adjacent to σ4λ5-phosphonium centers in trialkyl(iodo)phosphonium ions [R3PI]+ behave as soft electrophiles, which in solution allow rapid degenerate iodine cation transfer to the corresponding iodophosphanes R3P1−3

© 2011 American Chemical Society

Received: July 18, 2011 Revised: November 9, 2011 Published: November 30, 2011 185

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Scheme 2. Donor and Acceptor Features of Diiodophosphonium Ions, Iodophosphane Selenides, and Phosphinic Acids

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EXPERIMENTAL SECTION

Synthesis of Iododiisopropylphosphane Selenide. To a suspension of 1.49 g (10 mmol) of vacuum-dried sodium iodide in 25 mL of toluene was added 0.76 g (5 mmol) of chlorodiisopropylphosphane. After 1 day of stirring at room temperature, the solids were removed by filtration, the solution was evaporated under reduced pressure to about half the volume, 0.32 g (4 mmol) of gray selenium powder were added, and the mixture was stirred for 1 day. After removal of unconsumed selenium by filtration, toluene was evaporated, and the yellowish residue was recrystallized from pentane; yield 1.10 g (68%). Yellow single crystals suitable for X-ray analysis were obtained from a dichloromethane solution by evaporation. 31 P NMR (CH2Cl2, ref ext. 85% H3PO4). δ = 87.8 ppm; 77SeNMR (CH2Cl2, ref ext. Me2Se): δ = −77.6 ppm; 1JP, Se ± 722 Hz. X-ray Structure Determination of Iododiisopropylphosphane Selenide. Crystal data: C6H14IPSe, M = 323.00, monoclinic, P21/n, a = 7.6111(4), b = 12.8775(8), c = 10.6698(6) Å, β = 94.792(3)°, Z = 4, V = 1042.11(10) Å3, Dx = 2.059 Mg m−3, μ = 6.65 mm−1, T = 133(2) K. Data collection: A crystal 0.25 × 0.25 × 0.20 mm was used to record 21 093 intensities to 2θ 61° on a Bruker Smart diffractometer using monochromated Mo Kα radiation. An absorption correction was performed on the basis of multiscans. Structure refinement: The structure was refined anisotropically on F2 using the program SHELXL-97 (G.M. Sheldrick, University of Göttingen, Germany).8 Hydrogen atoms were included using rigid methyl groups or a riding model. The final wR2 was 0.0426 for 86 parameters and all 3179 unique reflections, with conventional R1 (F > 4σ(F)) 0.0214; max Δρ 1.35 e Å−3; S 1.06.

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RESULTS AND DISCUSSION The solid state structure of the new compound iPr2P(Se)I, crystallizing in the space group P21/n with one molecule in the asymmetric unit, is shown in Figure 1 (top). As a first experimental test of the hypothesis on the selfassembly of iodophosphane selenides, the structures of tBu2P(Se)I and c-HexP(Se)I2 were determined about a decade ago.7a Both compounds can be described as helical chain-like polymers, when intermolecular Se···I halogen bonding through linear P−Se···I moieties is taken into consideration (Table 1).9 Helical packing around a 21 screw axis as a consequence of Se···I−P halogen bonding was also observed in the iminobisphosphane-derived solid compound tBu2P(Se)NPtBu2I (Se···I, 3.797 Å), whereas the less bulky derivative iPr2P(Se)NPiPr2I (Se···I, 3.952 Å) crystallizes as a dimer.7b The most significant difference between the two independent molecules of solid tBu2P(Se)I [(P1−Se1: 2.104(2) Å, P2− Se2: 2.121(2) Å] is the slightly larger P2−Se2 distance in the molecule #2, which is involved in weak interchain Se...Se contacts [Se2···Se2′, 3.594(2) Å].7a,10 Similarly, one of the iodine atoms in c-HexP(Se)I2 participates in linear P−Se···I

Figure 1. Molecular structure of iPr2P(I)Se (top, 50% probability ellipsoids) and packing diagram (bottom) showing I···Se (thick dashed bonds) and H···Se contacts (thin dashed bonds). For selected bond distances, see Table 1. Selected bond angles: C4−P−C1 107.17(7); C4−P−Se 114.76(5); C1−P−Se 113.50(5); C4−P−I 103.18(5); C1− P−I 103.37(5); Se−P−I 113.71(2).

donor−acceptor interactions leading to staircase-like helices. These helices are interconnected through interchain Se···Se contacts involving all c-HexP(Se)I2 molecules (Table 1). The tendency of donating selenium atoms of phosphane selenides to exhibit further intermolecular or cation-pair Se···Se contacts is also documented in a number of iodine complexes of tertiary 186

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Table 1. Interatomic Distances in Iodophosphane Selenides and Related Compounds with Se···I Halogen Bonding iPr2P(Se)Ia

tBu2P(Se)Ib

d(PI)

2.4618(4)

d(PSe)

2.1033(4)

d(Se--I)

3.6120(3)

2.4535(14) 2.446(2) 2.104(2) 2.121(2) 3.6904(9) 3.8438(9) 3.594(2)

d(Se--Se) a

b

tBu2P(Se)I-I2b

cHexP(Se)I2b

2.431(2)

tBu2P(Se)NPtBu2Ic 2.5022(17)

2.173(2)

2.443(2) 2.439(2) 2.092(2)

2.7819(11)

3.4759(8)

3.7975(11)

3.829(2)

3.6123(14)

2.1373(19)

c

This work. Ref 7a. Ref 7b.

Scheme 3. Left: PSe···I Links and Se···Se Crosslinks in tBu2P(Se)I; Right: PSe···I Links and CH···Se Crosslinks in iPr2P(Se)I

phosphane selenides.11 Even the base-pair-like dimeric iodine adduct [tBu2P(Se)I···I−I]2 exhibits additional very weak interdimer Se···Se contacts.7a The solid state structure of the new compound iPr2P(Se)I fulfills the expectations as far as the L-shaped motif P−Se···I−P [linear halogen bond P−I···Se 170.72(1)° and angular moiety I···SeP 120.24(1)°] are concerned.7a,10 In iPr2P(I)Se, the I···Se distance is shorter and the P−I distance is slightly longer than in tBu2P(Se)I and in c-HexP(Se)I2 (Table 1). In contrast to the helical structures of tBu2P(Se)I and cHexP(Se)I2, however, the chains of solid iPr2P(Se)I are connected in a simple zigzag manner parallel to the b axis, and weak interchain Se···Se contacts are not observed (Figure 1, below).10 Unlike the linear structure-building halogen bonding motif with Se···I−P moieties, additional Se···Se contacts are apparently not an essential property of iodophosphane selenide structures. The nature of these Se···Se contacts is comparable to that of very weak chalcogen−chalcogen or halogen−halogen contacts that are regarded as predominantly van der Waalstype.12,13 A search for interchain contacts in solid iPr2P(I)Se reveals that one of the two isopropyl groups directs its tertiary C−H bond toward the selenium atom of a neighboring chain. These Se···H contacts (Se···H 2.98 Å, Se···C 3.952 Å, ∠Se···C− H 166°) lead to a network of 10-membered (···Se···I−P−C− H···)2 units; the extended structure consists of layers parallel to 101̅. Apart from the simple steric differences between t-butyl and i-propyl groups attached to phosphorus, the C−H functions of i-propyl groups are distinguished by their ability to “solvate” heavy atoms such as Se, Br, or I.5b,14 For the comparison of cross-links between chains in iPr2P(Se)I and tBu2P(Se)I, see Scheme 3.

required. In the three now known iodophosphane selenides, the P−I···Se halogen bonds are a structure-generating motif, but additional Se···Se contacts are not essential. The obvious tendency of the selenium atoms of phosphane selenide functions involved in halogen bonding to undergo further van der Waals-type contacts can be satisfied by Se···Se soft−soft contacts or alternatively, as shown in the new compound iPr2P (I)Se, by directed C−H···Se contacts.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

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REFERENCES

S Supporting Information * Crystallographic information files (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *E-mail [email protected]. Phone: (49)531-3915304. Fax (49)531-3915387.

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CONCLUSION To allow future crystal engineering with the help of heavier chalcogen−halogen interactions, the understanding of the interplay of different types of intermolecular interactions is 187

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