A systematic investigation of fluoride anion binding properties as a function of chelate backbone has been carried out for ferrocene functionalised boronic esters of the types FcB(OR)2 and fc[B(OR)2]2 [Fc = ferrocenyl = (eta5-C5H5)Fe(eta5-C5H4); fc = ferrocendiyl = Fe(eta5-C5H4)2]. Cyclic boronic esters containing a saturated five- or six-membered chelate ring are readily synthesized from ferrocene, and selectively bind fluoride via Lewis acid/base chemistry in chloroform solution. The resulting complexes are characterized by relatively weak fluoride binding (e.g.K = 35.8 +/- 9.8 M(-1) for FcBO2C2H2Ph2-S,S), and by cathodic shifts in the ferrocene oxidation potential that form the basis for electrochemical or colorimetric fluoride detection. The fluoride selectivity of these systems is attributed to relatively weak Lewis acidity, resulting in weak F- binding, and essentially no binding of potentially competitive anions. By contrast, more elaborate Lewis acid frameworks based on calix[4]arene (calixH4), such as (FcB)2calix or fcB2calix, do not survive intact exposure to standard fluoride sources (e.g. [nBu4N]F.xH2O solutions in chloroform or acetonitrile). Instead B-O bond cleavage occurs yielding the parent calixarene; the differences between alkoxo- and aryloxo-functionalised derivatives can be rationalised, at least in part, by consideration of the differences in electron donating capabilities of RO- (R = alkyl, aryl).

While N(2) and CO have played central roles in developing models of electronic structure, and their interactions with transition metals have been widely investigated, the valence isoelectronic diatomic molecules EX (E = group 13 element, X = group 17 element) have yet to be isolated under ambient conditions, either as the "free" molecule or as a ligand in a simple metal complex. As part of a program designed to address this deficiency, together with wider issues of the chemistry of cationic systems [L(n)M(ER)](+) (E = B, Al, Ga; R = aryl, amido, halide), we have targeted complexes of the type [L(n)M(GaX)](+). Halide abstraction is shown to be a viable method for the generation of mononuclear cationic complexes containing gallium donor ligands. The ability to isolate tractable two-coordinate products, however, is strongly dependent on the steric and electronic properties of the metal/ligand fragment. In the case of complexes containing ancillary pi-acceptor ligands such as CO, cationic complexes can only be isolated as base-trapped adducts, even with bulky aryl substituents at gallium. Base-free gallylene species such as [Cp*Fe(CO)(2)(GaMes)](+) can be identified only in the vapor phase by electrospray mass spectrometry experiments. With bis(phosphine) donor sets at the metal, the more favorable steric/electronic environment allows for the isolation of two-coordinate ligand systems, even with halide substituents at gallium. Thus, [Cp*Fe(dppe)(GaI)](+)[BAr(f)(4)](-) (9) can be synthesized and shown crystallographically to feature a terminally bound GaI ligand; 9 represents the first experimental realization of a complex containing a valence isoelectronic group 13/group 17 analogue of CO and N(2). DFT calculations reveal a relatively weakly bound GaI ligand, which is confirmed experimentally by the reaction of 9 with CO to give [Cp*Fe(dppe)(CO)](+)[BAr(f)(4)](-). In the absence of such reagents, 9 is stable for weeks in fluorobenzene solution, presumably reflecting (i) effective steric shielding of the gallium center by the ancillary phosphine and Cp* ligands; (ii) a net cationic charge which retards the tendency toward dimerization found for putative charge neutral systems; and (iii) (albeit relatively minor) population of the LUMOs of the GaI molecule through pi overlap with the HOMO and HOMO-2 of the [Cp*Fe(dppe)](+) fragment.

The thermodynamic factors underlying the use of ferroceneboronic esters as electrochemical or colorimetric fluoride ion sensors have been investigated through the synthesis of a range of systematically related derivatives differing in the number/nature of the boronic ester substituents and in the nature of ancillary ligands. Thus, if the shift in electrochemical potential associated with the conversion of one (or more) boronic ester group(s) to anionic boronate(s) on fluoride binding is sufficient to allow oxidation of the resulting host/guest complex by dioxygen, colorimetric sensing is possible. In practice, while monofunctional systems of the type CpFe[eta(5)-C(5)H(4)B(OR)(2)] offer selectivity in fluoride binding, electrochemical shifts in chloroform solution are insufficient to allow for a colorimetric response. Two chemical modification strategies have been shown to be successful in realizing a colorimetric sensor: (i) the use of the more strongly electron-donating Cp(*) ancillary ligand (which shifts the oxidation potential of both the free receptor and the resulting fluoride adduct cathodically by ca. -400 mV) and (ii) receptors featuring two or more binding sites and consequently a larger fluoride-induced electrochemical shift. Thus, in the case of [eta(5)-C(5)H(4)B(OR)(2)](2)Fe [(OR)(2) = OC(H)PhC(H)PhO, 2(s)], the binding of 2 equiv of fluoride gives an electrochemical shift (in chloroform) of -960 mV (cf. -530 mV for the corresponding monofunctional analogue, 1(s)). Related tris- and tetrakis-functionalized systems are also shown to be oxidized as the bis(fluoride) adducts, presumably because of fast oxidation kinetics, relative to the rate of the (electrostatically unfavorable) binding of a third equivalent of fluoride. Furthermore, the rate of sensor response (as measured by UV/vis spectroscopy) is found to be strongly enhanced by the presence of pendant (uncomplexed) three-coordinate boronic ester functions (e.g., a rate enhancement of 1-2 orders of magnitude for 3(s)/4(s) with respect to 2(s)) and/or delocalized aromatic substituents.

Transition metal complexes containing ligands featuring three-coordinate, halide-functionalized gallium or indium donors represent key precursors to unsaturated cationic species of the type [L n M(EX)] + via halide abstraction chemistry. Two routes to these three-coordinate systems have been demonstrated: (i) salt elimination, such as the reaction between Na[Cp*Fe(CO) 2 ] and Mes*GaCl 2 or Mes*InBr 2 (Mes* ) C 6 H 2 t Bu 3 -2,4,6, supermesityl) to generate Cp*Fe(CO) 2 E(Mes*)X (3a, E ) Ga, X ) Cl; 5, E ) In, X ) Br), and (ii) insertion of a gallium(I) or indium(I) halide into a metal-halogen or metal-metal bond followed, where necessary, by substitution by a sterically bulky anionic nucleophile. Crystallographic studies have confirmed the presence of the target trigonal planar ligand systems both in gallyl/ indyl complexes of the type L n M-E(Aryl)X and in halide-functionalized gallane-and indanediyl systems of the type (L n M) 2 EX.

Insertion reactions of dicyclohexylcarbodiimide with aminoboranes and with aminoboryl and -borylene transition metal complexes have been examined as potential routes to new boron-containing ligand systems. Reactions with systems containing two-coordinate boron centres are found to be significantly more facile than those with three-coordinate substrates. Thus, reaction of (dicyclohexylamino)boron dichloride () with dicyclohexylcarbodiimide over 36 h at 50 degrees C generates the (structurally authenticated) guanidinate complex Cy(2)NC(NCy)(2)BCl(2) () via insertion into the BN bond. By contrast, the corresponding reaction with the cationic aminoborylene complex [CpFe(CO)(2)(BNCy(2))](+)[BAr(f)(4)](-) () proceeds rapidly at ca.-30 degrees C, via initial insertion into the FeB bond to give [CpFe(CO)(2)C(NCy)(2)BNCy(2)](+)[BAr(f)(4)](-) (). Consistent with related studies, a key factor in facilitating such insertion chemistry is thought to be the formation of an initial donor/acceptor complex between the diimide and the group 13 centre. Thus, DFT studies suggest that [CpFe(CO)(2)B(NCy(2))(CyNCNCy)](+)[BAr(f)(4)](-) is a potential intermediate in the reaction of with CyNCNCy, and that further reaction to give the observed product, , is strongly exergic (-183 kJ mol(-1)). By contrast, DFT calculations for the alternative isomer [CpFe(CO)(2)B(CyN)(2)CNCy(2)](+)[BAr(f)(4)](-) (), formed by BN insertion, suggest that it is 112 kJ mol(-1) less stable than . Such experimental and computational findings imply that under reaction conditions where a suitable isomerisation pathway is available, cationic complexes such as , which contain a four-membered boron-donor heterocycle are likely to be disfavoured with respect to alternative C-bound isomers.

Halide abstraction chemistry offers a viable synthetic route to the cationic two-coordinate complexes [{Cp*Fe(CO) 2 } 2 (µ-E)] + (7, E ) Ga; 8, E ) In) featuring linear bridging gallium or indium atoms. Structural, spectroscopic, and computational studies undertaken on 7 are consistent with appreciable Fe-Ga π-bonding character; in contrast, the indium-bridged complex 8 is shown to feature a much smaller π component to the metal-ligand interaction. Analogous reactions utilizing the supermesityl-substituted gallyl or indyl precursors of the type (η 5 -C 5 R 5 )Fe(CO) 2 E(Mes*)X, on the other hand, lead to the synthesis of halide-bridged species of the type [{(η 5 -C 5 R 5 )Fe(CO) 2 E(Mes*)} 2 (µ-X)] + , presumably by trapping of the highly electrophilic putative cationic diyl complex [(η 5 -C 5 R 5 )Fe(CO) 2 E(Mes*)] + .

A simple, one-step synthesis of multinuclear Lewis acids can be driven with high selectivity towards either macrocyclic or polymeric arrays by appropriate choice of backbone framework.

Metathesis reactions constitute a key component of modern synthetic chemistry; olefin metathesis, for example, provides a versatile and widely exploited carbon-carbon bond-forming methodology.[1] Such reactions are typically catalyzed by organometallic complexes that contain M=C bonds.[2] The synthesis of analogous complexes that contain M=Si bonds, for example, has led to an in-depth investigation of their reactivity towards unsaturated substrates. [3] Synthetic approaches that lead to the isolation of related systems with M = B bonds have been developed only recently: [4][5][6][7] for example, halide-abstraction chemistry gives access to cationic terminal borylene complexes, [L n M= BX][7] Consequently, reports of the fundamental chemistry of M=B bonds are somewhat limited (predominantly to metal-metal transfer reactions and addition/substitution reactivity towards nucleophiles). [4,6,7] Thus, the chemistry of [Cp*Fe(CO) 2 (BMes)] + (Mes = mesityl = 2,4,6-Me 3 C 6 H 2 , Cp* = pentamethylcyclopentadienyl), for example, is dominated by the electrophilic character at both the Fe and B centers. [7] In an attempt to tune the reactivity of these highly unsaturated complexes we investigated the synthesis of cationic aminoborylene systems, [L n M = BNR 2 ]+ .[5] Chloride abstraction from [CpFe(CO) 2 {B(NiPr 2 )Cl}] (2) (Cp = cyclopentadienyl) by Na(BAr À . The steric bulk of the amino substituents is a key point: the use of the smaller NMe 2 group results in the formation of a thermally fragile borylene product in the subsequent halide-abstraction step, [7b, 8] whereas [CpFe(CO) 2 {B(tmp)Cl}] (tmp = tetramethylpiperamino) was inaccessible from tmpBCl 2 . Compound 2 is a pale yellow sublimable crystalline solid, which was characterized by multinuclear NMR and IR spectroscopy, mass spectrometry, and X-ray crystallographic analysis (Figure 1).The reaction of 2 with Na(BAr f 4 ) in dichloromethane results in a quantitative conversion (determined by 1 H and 11 B NMR spectroscopy) into 3. The latter product (along with the C 5 H 4 Me analogue) is a colourless oil at (or close to) room temperature, but its formulation can be definitively established from spectroscopic and reactivity data. The measured 11 B chemical shift for 3 (d B = 93.5 ppm) is very close to that reported by Braunschweig et al. for neutral terminal-aminoborylene systems of the type L n M=BN(SiMe 3 ) 2 (d B = 86.6-98.3 ppm).[5] The downfield shift upon chloride abstraction (Dd B = 38.1 between 2 and 3) mirrors that found for and 88.0 ppm, respectively). [7,8] The 1 H and 13 C NMR data are