A. Guy Orpen

Mechanochemistry: opportunities for new and cleaner synthesis

The aim of this critical review is to provide a broad but digestible overview of mechanochemical synthesis, i.e. reactions conducted by grinding solid reactants together with no or minimal solvent. Although mechanochemistry has historically been a sideline approach to synthesis it may soon move into the mainstream because it is increasingly apparent that it can be practical, and even advantageous, and because of the opportunities it provides for developing more sustainable methods. Concentrating on recent advances, this article covers industrial aspects, inorganic materials, organic synthesis, cocrystallisation, pharmaceutical aspects, metal complexes (including metal-organic frameworks), supramolecular aspects and characterization methods. The historical development, mechanistic aspects, limitations and opportunities are also discussed (314 references).

Tables of bond lengths determined by X-ray and neutron diffraction. Part 1. Bond lengths in organic compounds

The average lengths of bonds involving the elements H, B, C, N, O, F, Si, P, S, Cl, As, Se, Br, Te, and l in organic compounds are reported.

Retrieval of Crystallographically-Derived Molecular Geometry Information

The crystallographically determined bond length, valence angle, and torsion angle information in the Cambridge Structural Database (CSD) has many uses. However, accessing it by means of conventional substructure searching requires nontrivial user intervention. In consequence, these valuable data have been underutilized and have not been directly accessible to client applications. The situation has been remedied by development of a new program (Mogul) for automated retrieval of molecular geometry data from the CSD. The program uses a system of keys to encode the chemical environments of fragments (bonds, valence angles, and acyclic torsions) from CSD structures. Fragments with identical keys are deemed to be chemically identical and are grouped together, and the distribution of the appropriate geometrical parameter (bond length, valence angle, or torsion angle) is computed and stored. Use of a search tree indexed on key values, together with a novel similarity calculation, then enables the distribution matching any given query fragment (or the distributions most closely matching, if an adequate exact match is unavailable) to be found easily and with no user intervention. Validation experiments indicate that, with rare exceptions, search results afford precise and unbiased estimates of molecular geometrical preferences. Such estimates may be used, for example, to validate the geometries of libraries of modeled molecules or of newly determined crystal structures or to assist structure solution from low-resolution (e.g. powder diffraction) X-ray data.

Innovation in crystal engineering

The first CrystEngComm discussion meeting on crystal engineering has demonstrated that the field has reached maturity in some areas (for example: design strategies, characterization of solid compounds, topological analysis of weak and strong non-covalent interactions), while the quest for novel properties engineered at molecular and supramolecular levels has only recently begun and the need for further research efforts is strongly felt. This Highlight article aims to provide a forward look and a constructive discussion of the prospects for future developments of crystal engineering as a bridge between supramolecular and molecular materials chemistry.

Supplement. Tables of bond lengths determined by X-ray and neutron diffraction. Part 2. Organometallic compounds and co-ordination complexes of the d- and f-block metals

Average lengths for metal–ligand bonds are reported, together with some intraligand distances, for complexes of the d- and f-block metals. Mean values are presented for 325 different bond types involving metal atoms bonded to H, B, C, N, O, F, Si, P, S, Cl, As, Se, Br, Te, or I atoms of the ligands.

β‐Diiminato Complexes of VIII and TiIII – Formation and Structure of Stable Paramagnetic Dialkylmetal Compounds

(Mono-β-diiminato)titanium(III) and -vanadium(III) dichlorides LMCl2 [L = ArNC(R)CHC(R)NAr–] are easily accessible from the metal trichlorides and LLi in THF. The crystal structures of LVCl2 (Ar = 2,6-iPr2C6H3, R = Me) and LTiCl2 (Ar = 2,4,6-Me3C6H2, R = Me and Ar = 2,4,6-Me3C6H2, R = tBu) reveal tetrahedral metal environments. Treatment of LVCl2 with alkyllithium reagents affords surprisingly stable dialkylvanadium(III) compounds; the structure of LV(nBu)2 (Ar = 2,6-iPr2C6H3, R = Me) is similar to that of the dichloride. The corresponding dialkyltitanium(III) compounds are less stable; only the dimethyl derivatives could be obtained in pure form (from LTiCl2 and MeMgI), and only for ligands bearing 2,6-disubstituted aryl groups. The structure of LTiMe2 (Ar = 2,4,6-Me3C6H2, R = Me) is also similar to that of the dichloride. Reaction of LTiMe2 with B(C6F5)3 produces a catalyst for α-olefin polymerization, but the corresponding VIII derivatives are inactive.

OXIDATIVE ADDITION OF BORON-BORON, BORON-CHLORINE AND BORON-BROMINE BONDS TO PLATINUM(0)

Abstract The synthesis and spectroscopic characterisation of the new diborane(4) compounds B2(1,2-O2C6Cl4)2 and B2(1,2-O2C6Br4)2 are reported together with the diborane(4) bis-amine adduct [B2(calix)(NHMe2)2] (calix=Butcalix[4]arene). B–B bond oxidative addition reactions between the platinum(0) compound [Pt(PPh3)2(η-C2H4)] and the diborane(4) compounds B2(1,2-S2C6H4)2, B2(1,2-O2C6Cl4)2 and B2(1,2-O2C6Br4)2 are also described which result in the platinum(II) bis-boryl complexes cis-[Pt(PPh3)2{B(1,2-S2C6H4)}2], cis-[Pt(PPh3)2{B(1,2-O2C6Cl4)}2] and cis-[Pt(PPh3)2{B(1,2-O2C6Br4)}2] respectively, the former two having been characterised by X-ray crystallography. In addition, the platinum complex [Pt(PPh3)2(η-C2H4)] reacts with XB(1,2-O2C6H4) (X=Cl, Br) affording the mono-boryl complexes trans-[PtX(PPh3)2{B(1,2-O2C6H4)}] as a result of oxidative addition of the B–X bonds to the Pt(0) centre; the chloro derivative has been characterised by X-ray crystallography.

Solid state synthesis of coordination compounds from basic metal salts

The preparation of the complex salts [H2im]2[MCl4] (H2im = imidazolium, M = Co, 1; Zn, 2; Cu, 3) and coordination compounds [MCl2(Him)2] (M = Co, 4; Zn, 5; Cu, 6) by a range of solid-state and solid-gas reactions is reported. Compounds 4–6 and the related [{MCl2(4,4′-bipy)}n] (M = Co, 7; Zn, 8) were prepared by the solid state reactions of metal hydroxide or carbonate salts (or their equivalent) with the hydrochloride salt of the appropriate ligand (imidazole or 4,4′-bipy).

Water Chains in Hydrophobic Crystal Channels: Nanoporous Materials as Supramolecular Analogues of Carbon Nanotubes

The behavior of water in narrow apolar pores has attracted much recent interest. Although it might seem that such channels should repel water, it transpires that they can be hydrated and moreover that their “hydrophobic” nature promotes rapid water flow. The phenomenon is observed in biology, where the pores of aquaporins (water-transporting proteins) are composed largely of hydrophobic amino acids. It is also seen for carbon nanotubes (CNTs) which, according to both theoretical and experimental studies, allow rapid passage of water molecules. Water flow through CNTs can be selective and controllable, suggesting applications in water-purification and nanofluidic devices. To understand these systems, it is important to gather structural information on water in hydrophobic environments. Especially relevant is the “water wire”, a one-dimensional hydrogen-bonded chain of water molecules. The water in aquaporin channels adopts this motif, as does the water in the narrowest and best-understood CNTs. However, while there are many crystal structures which show water molecules in single file, there is limited information on water wires in purely hydrophobic environments. In particular there seem to be no structures in which water wires are surrounded exclusively by p systems. We now report the crystal engineering of channels bounded by nonpolar aromatic units, and the structural characterization of linear chains of water molecules within these pores. We have previously shown that steroidal bisphenylureas 1 (Figure 1a) crystallize as monohydrates to form structures with hexagonal P61 symmetry, containing one-dimensional channels of unusually large diameter (Figure 1b,c). Groups R and R are directed into the channels, so that in principle they can be varied substantially without disrupting the crystal packing. Potentially, this should allow a broad scope for tuning of channel size and properties. Indeed, our original report described three structures 1a–c, in which group R was altered to give pore diameters of 14.3, 12.3, and 11.6 , respectively. Having shown that R could be varied, we sought to confirm that ester group CO2R 2 could also be used to tune the properties of the crystals. Included among our targets were steroids 2–4, featuring large aromatic groups linked to C24 O via two-carbon tethers. Modeling suggested that this arrangement would allow the aromatic groups to lie roughly parallel to the channel axis, lining the walls and reducing the diameters to about 6 to 7 . We could thus create an environment not dissimilar to that inside a CNT. Esters 2–4 Figure 1. a) Previously described steroidal bis-phenylureas forming nanoporous crystals. b) The structure of 1b in the crystal, showing the bound water molecule. c) Packing diagram for 1b viewed along the axis of the channels. A single steroid is highlighted in the centre of the diagram, showing how the ester CH3 groups form part of the surface of one channel while the C3-terminal NHPh groups protrude into another. The ester CH3 and C3 NHPh groups are shown in space-filling mode. Also highlighted are the water molecules bound to each unit of 1b.

1,8-bis(diphenylphosphino)naphthalene : a rigid chelating, diphosphine analogue of proton sponge

Abstract The synthesis of the new diphosphine, 1,8-bis(diphenylphosphino) naphthalene ( 1a ), and its X-ray crystal structure are reported. Protonation of 1a gives a fluxional species to which a PP bonded structure is assigned. Despite the strain apparent in the solid state due to the proximity of the diphenylphosphino groups, it appears that 1a has a normal coordination chemistry with platinum(II) and palladium(II).