Christopher J. Adams

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).

Iron(I) in Negishi Cross-Coupling Reactions

Herein we demonstrate both the importance of Fe(I) in Negishi cross-coupling reactions with arylzinc reagents and the isolation of catalytically competent Fe(I) intermediates. These complexes, [FeX(dpbz)(2)] [X = 4-tolyl (7), Cl (8a), Br (8b); dpbz = 1,2-bis(diphenylphosphino)benzene], were characterized by crystallography and tested for activity in representative reactions. The complexes are low-spin with no significant spin density on the ligands. While complex 8b shows performance consistent with an on-cycle intermediate, it seems that 7 is an off-cycle species.

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).

Crystal engineering of lattice metrics of perhalometallate salts and MOFs.

The synthesis of the salt 3 and metallo-organic framework (MOF) [{(4,4′-bipy)CoBr2}n] 4 by a range of solid state (mechanochemical and thermochemical) and solution methods is reported; they are isostructural with their respective chloride analogues 1 and 2. 3 and 4 can be interconverted by means of HBr elimination and absorption. Single phases of controlled composition and general formula [4,4′-H2bipy][CoBr4-xClx] 5x may be prepared from 2 and 4 by solid—gas reactions involving HBr or HCl respectively. Crystalline single phase samples of 5x and [{(4,4′-bipy)CoBr2-xClx}n] 6x were prepared by solid-state mechanochemical routes, allowing fine control over the composition and unit cell volume of the product. Collectively these methods enable continuous variation of the unit cell dimensions of the salts [4,4′-H2bipy][CoBr4-xClx] (5x) and the MOFs [{(4,4′-bipy)CoBr2-xClx}n] (6x) by varying the bromide to chloride ratio and establish a means of controlling MOF composition and the lattice metrics, and so the physical and chemical properties that derive from it.

Synthesis, Reactivity, and Electronic Structure of [n]Vanadoarenophanes: An Experimental and Theoretical Study

An optimized procedure for the selective dimetalation of [V(eta (6)-C 6H 6) 2] by BuLi/tmeda allowed for the isolation and characterization of [V(eta (6)-C 6H 5Li) 2].tmeda. X-ray diffraction of its thf solvate [V(eta (6)-C 6H 5Li) 2].(thf) 7 revealed an unsymmetrical, dimeric composition in the solid state, in which both subunits are connected by three bridging lithium atoms. Treatment with several element dihalides facilitated the isolation of [ n]vanadoarenophanes ( n = 1, 2) with boron and silicon in the bridging positions. In agreement with the number and covalent radii of the bridging elements, these derivatives exhibit molecular ring strain to a greater or lesser extent. The B-B bond of the [2]bora species [V(eta (6)-C 6H 5) 2B 2(NMe 2) 2] was readily cleaved by [Pt(PEt 3) 3] to afford the corresponding oxidative addition product. Subsequently, [V(eta (6)-C 6H 5) 2B 2(NMe 2) 2] was employed as a diborane(4) precursor in the diboration of 2-butyne under stoichiometric, homogeneous, and heterogeneous catalysis conditions. This transformation is facilitated by the reduction of molecular ring strain, which was confirmed by a decrease of the tilt angle alpha observed in the corresponding solid-state structures. EPR spectroscopy was used to probe the electronic structure of strained [ n]vanadoarenophanes and revealed an obvious correlation between the degree of molecular distortion and the observed hyperfine coupling constant a iso. State-of-the-art DFT calculations were able to reproduce the measured isotropic vanadium hyperfine couplings and the coupling anisotropies. The calculations confirmed the decrease of the absolute isotropic hyperfine couplings with increasing tilt angle. Closer analysis showed that this is mainly due to increased positive contributions to the spin density at the vanadium nucleus from the spin polarization of doubly occupied valence orbitals of vanadium-ligand sigma-antibonding character. The latter are destabilized and thus made more polarizable in the bent structures.

Cluster chemistry: XC. Some complexes obtained from reactions between M3(CO)12 (M = Ru or Os) or Ru3(μ-dppm)(CO)10 and 2-substituted triphenylphosphines and related keto-phosphine ligands☆

Several complexes of the type M3Ln{PR2(C6H4X)} [M3Ln= (Ru/Os3(CO)11 or RU3(μ-dppm)(CO)9; X=NH2, NHCOPh, N=CHPh, CHO or CH=NNHAr, but not all combinations] have been prepared and their reactions studied. Predominant were H-migrations from the ary] substituent X to cluster; less facile were CC bond cleavage reactions, and PC bond cleavages were not observed. Under the conditions used, the complexes Ru3(CO)11{PPh2(C6H4X-2)} were transient intermediates in the formation of RU3(μ-H) {μ-PPh2(C6H4(XH)-2)}(CO)9; the analogous Ru3(μ-dppm) and Os3(μ-dppm) complexes were more robust. Similar reactions were found for clusters made by reaction of RU3(CO)12 and related complexes with PPh{2{CH2C(O)Ph}}, in which H-migration from the ligand to the cluster results in formation of a phosphino-enolate system. X-ray structures are reported for the complexes Os3(CO)11{PPh2(C6H4X-2)} [X = NH2, NHC(O)Ph, CHO, CH=NNHC6H3(NO2)2-2,4], Ru3(μ-dppm) (CO)9{PPh 2(C6H4X-2)} [X = NHC(O)Ph, CHO], Ru3(μ-H)(μ-PPh2(C6H4Y-2)(CO)9 [Y = NH, NC(Ph)O, N=CPh], Ru3(μ-H) (μ-) PPh2[C6H4NC(Ph)O-2](μ-dppm)(CO)7, Ru3(μ-H)(μ-dppm)(μ-PPh2(C6H4NH-2)(CO)7 and Os3(μ-H){(μ-PPh) 2(C6H4 CO-2)}(CO)9.

Coordination chemistry of platinum and palladium in the solid-state: synthesis of imidazole and pyrazole complexes.

Solid-state reactions of palladium(II) and platinum(II) chloride complexes with imidazole (Him) and pyrazole (Hpz) or their hydrochloride salts are shown to produce metal complex salts and coordination compounds. Thus, K(2)[MCl(4)] or MCl(2) can be ground with imidazolium chloride ([H(2)im]Cl) to produce the salts [H(2)im](2)[MCl(4)] (M = Pd, 1; Pt, 5), which can then be dehydrochlorinated in the solid state to produce the coordination compounds trans-[PdCl(2)(Him)(2)] 3 or cis-[PtCl(2)(Him)(2)] 6. The complex cis-[PdCl(2)(Him)(2)] 2 is produced when Pd(OAc)(2) is ground with [H(2)im]Cl. Reaction of platinum chloride reagents with imidazole (Him) also produces cis-[PtCl(2)(Him)(2)] 6, but reaction of imidazole with analogous palladium chloride reagents first produces [Pd(Him)(4)]Cl(2) 4 which then slowly converts to trans-[PdCl(2)(Him)(2)] 3. Grinding pyrazolium chloride with K(2)[MCl(4)] produces [H(2)pz](2)[MCl(4)] (M = Pd, 7; Pt, 10), which may also be dehydrochlorinated in the solid state to produce the coordination compounds trans-[PdCl(2)(Hpz)(2)] 8 or cis-[PtCl(2)(Hpz)(2)] 11. Grinding K(2)[PdCl(4)] or PdCl(2) with pyrazole gives [Pd(Hpz)(4)]Cl(2) 9, which is then slowly converted into trans-[PdCl(2)(Hpz)(2)] 8. Grinding PtCl(2) with Hpz generates [Pt(Hpz)(4)]Cl(2) 12, but using K(2)PtCl(4) as the metal source does not generate the same product. The single-crystal structures of 8, a new polymorph of 11 and [H(2)pz](2)[PtCl(6)].2H(2)O (isolated as a decomposition product) are reported for the first time, and the structures of 5 and 10 have been solved ab ibitio from XRPD data.

Cluster chemistry. LXXXVII: Some homo- and hetero-nuclear complexes derived from C2(PPh2)2 : crystal structures of Re3(μ-H)3(μ-dppa)(CO)10 {dppa=C2(PPh2)2}, Re3(μ-H)3(CO)11{PPh2[μ-C2Ru2(μ-PPh2)(CO)6]} and Os3Ru2(μ5-C2PPh2)(μ-PPh2)(CO)13

Abstract Reaction of two equivalents of Re 3 (μ-H) 3 (CO) 11 (NCMe) with dppa produces the complex {Re 3 (μ-H) 3 (CO) 11 } 2 (μ-dppa) ( 2 ). When an excess of dppa is used, mono-substituted Re 3 (μ-H) 3 (CO) 11 (dppa) ( 3 ) and Re 3 (μ-H) 3 (μ-dppa)(CO) 10 ( 4 ) were produced. When 3 is treated with Ru 3 (CO) 12 , the complex {Re 3 (μ-H) 3 (CO) 11 }(μ-dppa){Ru 3 (CO) 11 } ( 5 ) is produced in high yield. When 5 is heated, the complex Re 3 (μ-H) 3 (CO) 11 {PPh 2 [μ-C 2 Ru 2 (μ-PPh 2 )(CO) 6 ]} ( 6 ) is produced. Using the same methodology, {Os 3 (CO) 11 }(μ-dppa){Ru 3 (CO) 11 } (bd8} and {Os 3 (CO) 11 }(μ-dppa){Re 3 (μ-H) 3 (CO) 11 } ( 11 ) were prepared. When 8 is heated, the heteronuclear M 5 cluster Os 3 Ru 2 (μ 5 -C 2 PPh 2 )(μ-PPh 2 )(CO) 13 ( 10 ) is produced. Complexes 4 , 6 and 10 were identified by single crystal X-ray studies.

Synthesis of a Paramagnetic Polymer by Ring‐Opening Polymerization of a Strained [1]Vanadoarenophane

During the last three decades it has been impressively demonstrated that the molecular ring-strain present in bridged [n]metallocenophanes can be exploited in a variety of useful ways. The strained character of [n]metallocenophanes, and related [n]metalloarenophanes, is reflected by enhanced reactivity of either the bond between the carbocyclic ligands and the bridging element(s), that between the bridging elements or those between the metal and one of the carbocyclic ligands. Both insertion and substitution chemistry have been demonstrated to afford ring-expanded or ringopened products, respectively. In particular, the discovery of the ring-opening polymerization (ROP) of strained [n]ferrocenophanes in 1992 to yield high-molecular-weight polyferrocenes represented a landmark moment in both organometallic chemistry and materials science. As a consequence of this advance, the last 15 years has seen tremendous interest in the syntheses of compounds containing this structural motif and their propensity to undergo ring-opening reactions. Whereas ROP techniques enabled the successful polymerization of [n]ferrocenophanes with varying bridging elements, the preparation of polymers containing transition-metal centers other than iron still remains a challenging area of research. In fact, their number is limited to a few examples of titanium, chromium, or ruthenium-based materials. To date, no controlled access to well-defined organometallic polymers containing repeat units with unpaired electrons has been reported using ROP, even though these polymers could potentially exhibit highly interesting physical properties such as intramolecular electroand magnetocommunication. It was shown that even dinuclear sandwich derivatives exhibit strong electronic and magnetic communication between the paramagnetic d-vanadium centers, which are transmitted by different silicon or carbon spacers. Combining these properties with the ease of processing commonly associated with polymers may therefore lead to desirable materials. Besides the attempted thermally induced ROP of silicon-bridged [1]trovacenophanes reported by Elschenbroich and co-workers in 2004, the polymerization behavior of strained [n]metalloarenophanes derived from the paramagnetic sandwich complexes trovacene, [V(h-C5H5)(h C7H7)], and bis(benzene)vanadium, [V(h -C6H6)2], has not yet been investigated in detail. However, the molecular distortion of [n]vanadoarenophanes resembles that found in the corresponding [n]ferrocenophanes, and hence suggests their suitability as monomers in ROP processes. Virtually all of the paramagnetic polymers prepared to date are either coordination polymers or have been obtained by polycondensation routes, whereby the isolated materials are usually either of low molecular weight, insoluble, or of poorly defined structure. Consequently, the application of a chain-growth polymerization, such as ROP, appears to be much more suitable for the preparation of well-defined paramagnetic polymers. Herein we report on the synthesis and characterization of [1]boraand [1]sila derivatives of bis(benzene)vanadium, as well as on their polymerization behavior. The ROP of the silicon-bridged species leads to a soluble organometallic polymer containing paramagnetic repeat units, whereas the [1]boravanadoarenophane undergoes a stoichiometric ring-opening reaction under similar conditions. Stoichiometric salt-elimination reactions of dilithiated bis(benzene)vanadium, [V(h-C6H5Li)2]·tmeda (1), with Cl2BN(SiMe3)2 or Cl2SiiPrMe in nonpolar solvents at 78 8C resulted in the formation of the strained ansa complexes 2 and 3, respectively. Both [1]vanadoarenophanes were isolated after recrystallization as deep red or orange crystalline solids in good yields (75 and 61%, respectively; Scheme 1). The identity of 2 and 3 was unambiguously confirmed by mass spectrometry, elemental analysis, and EPR spectroscopy. Both complexes proved to be thermally very robust, which, for example, allowed the observation of the molecular ion as the most intense peak in the mass spectrum of 2. Further evidence for the formation of ansa complexes was derived from NMR spectroscopy. Whereas the aromatic protons of the C6H5 moieties are not observed in the NMR spectra of 2 and 3 due to paramagnetic line broadening, controlled decompostion of C6D6 solutions by exposure to air [*] Prof. Dr. H. Braunschweig, Dr. T. Kupfer Institut f#r Anorganische Chemie Julius-Maximilians-Universit.t W#rzburg Am Hubland, 97074 W#rzburg (Germany) Fax: (+49)931-888-4623 E-mail: h.braunschweig@mail.uni-wuerzburg.de

Towards polymorphism control in coordination networks and metallo-organic salts

[{(4,4′-bipy)ZnCl2}n] (4) is known in three different polymorphs, in the space groups C2/c (4a), Pnma (4b) and Pban (4c). Solution synthesis produces a mixture of 4a and 4b, but solid-state synthesis, either by grinding together 4,4′-bipy and ZnCl2 or by heating [4,4′-H2bipy][ZnCl4] 3 to thermally eliminate HCl, generates only the 4b form, demonstrating that solid-state synthesis can exert control over polymorph formation; other solid-state preparations of 4 such as reaction of basic zinc carbonate with [4,4′-H2bipy]Cl2 or reaction of [4,4′-H2bipy][ZnCl4] 3 with KOH also increase the amount of 4b generated relative to the solution synthesis. It is also possible to form the mixed-metal phases [{(4,4′-bipy)Co1−xZnxCl2}n] 4x as effectively homogeneous solid-solutions by the same techniques, and when x 0.5, the amount of this polymorph is still greater than would be expected for a predominantly zinc-containing phase.