Deepak Dange

A Stable Two-Coordinate Acyclic Silylene

Simple two-coordinate acyclic silylenes, SiR(2), have hitherto been identified only as transient intermediates or thermally labile species. By making use of the strong σ-donor properties and high steric loading of the B(NDippCH)(2) substituent (Dipp = 2,6-(i)Pr(2)C(6)H(3)), an isolable monomeric species, Si{B(NDippCH)(2)}{N(SiMe(3))Dipp}, can be synthesized which is stable in the solid state up to 130 °C. This silylene species undergoes facile oxidative addition reactions with dihydrogen (at sub-ambient temperatures) and with alkyl C-H bonds, consistent with a low singlet-triplet gap (103.9 kJ mol(-1)), thus demonstrating fundamental modes of reactivity more characteristic of transition metal systems.

Stable GaX2, InX2 and TlX2 radicals

The chemistry of the Group 13 metals is dominated by the +1 and +3 oxidation states, and simple monomeric M(II) species are typically short-lived, highly reactive species. Here we report the first thermally robust monomeric MX2 radicals of gallium, indium and thallium. By making use of sterically demanding boryl substituents, compounds of the type M(II)(boryl)2 (M = Ga, In, Tl) can be synthesized. These decompose above 130 °C and are amenable to structural characterization in the solid state by X-ray crystallography. Electron paramagnetic resonance and computational studies reveal a dominant metal-centred character for all three radicals (>70% spin density at the metal). M(II) species have been invoked as key short-lived intermediates in well-known electron-transfer processes; consistently, the chemical behaviour of these novel isolated species reveals facile one-electron shuttling processes at the metal centre.

Contrasting reductions of group 14 metal(II) chloride complexes: synthesis of a β-diketiminato tin(I) dimer

Reductions of the β-diketiminato group 14 metal(II) chloride complexes, [((But)MesNacnac)ECl] ((But)MesNacnac = [(MesNCBu(t))(2)CH](-); Mes = mesityl; E = Ge, Sn or Pb), with a magnesium(I) dimer have led to differing outcomes, which include the formation of the first β-diketiminato group 14 metal(I) dimer, [{((But)MesNacnac)Sn}(2)].

Extremely Bulky Amido and Amidinato Complexes of Boron and Aluminium Halides: Synthesis and Reduction Studies

An extremely bulky secondary amine, HN(Ar†)(SiPr3i) HL† (Ar† = C6H2{C(H)Ph2}2Pri-2,6,4) has been synthesised and deprotonated with KH in toluene, to afford the potassium amide [KL†(η6-toluene)], which was structurally authenticated. Reaction of this with BBr3 and AlBr3, reproducibly gave the crystallographically characterised amido bromo-borane, [L†B(H)Br], and aluminacycle, [AlBr2{κ2-C,N-N(H)(SiPr3i){C6H2[CPh2][C(H)Ph2]Pri-2,6,4}}], respectively, via ligand C–H activation processes. The known secondary amines, HN(Dip)(Mes) (HLMes) and HN(Dip)(Trip) (HLTrip) (Dip =2,6-diisopropylphenyl, Mes = mesityl, Trip = 2,4,6-triisopropylphenyl), have been structurally characterised, and deprotonated to give the in situ generated lithium amides, [Li(LMes)] and [Li(LTrip)]. Reaction of these with BBr3 and AlBr3 has given the amido group 13 element halide complexes, [LMesBBr2] and [LAlBr2(THF)] (L = LMes or LTrip), the crystal structures of all of which have been determined. Synthetic routes to two new bulky amidine pro-ligands, ArN = C(But)-N(H)Ar, Ar = C6H2{C(H)Ph2}2Me-2,6,4 (Piso*H) or C6H2Pr2i(CPh3)-2,6,4 (Piso″H), have been developed, and the compounds crystallographically characterised. Deprotonation of Piso″H gave the potassium amidinate, [K(Piso″)], which was reacted with BBr3 to give [(Piso″)BBr2]. Reaction of Piso″H with AlMe3 afforded [(Piso″)AlMe2], which, when treated with I2 yielded [(Piso″)AlI2], the crystal structure of which was determined. Reductions of all of the prepared amido and amidinato group 13 element(iii) halide complexes were attempted using a variety of reducing reagents, with a view to prepare boron(i) or aluminium(i) complexes. While these were not successful, this study does offer synthetic inorganic chemists a variety of new very bulky anionic N-donor ligands, and boron/aluminium halide complexes thereof, for use in their own research.

Experimental Charge Density Analysis of a Gallium(I) N-Heterocyclic Carbene Analogue

The experimental electron density of the only known example of a four-membered Ga(I) N-heterocyclic carbene analogue has been determined by multipole modeling of 90 K X-ray diffraction data and compared to theoretical data. In order to obtain a satisfactory model, it is necessary to modify the radial dependency of the core electrons of Ga using two separate scaling parameters for s,p- and d-electrons. Evidence for significant lone-pair density on Ga is found in the electron density and derived properties despite the partial positive charge of this atom. Static deformation density and molecular electrostatic potential clearly show a directional lone pair on Ga, whereas the Laplacian of the total electron density does not; this feature is, however, present in the Laplacian of the valence-only density. The Ga center also acts as an acceptor in four intramolecular C-H···Ga contacts, whose nature is probed by density properties. Substantial covalent character is apparent in the Ga-N bonds, but no sign of donation from filled N p-orbitals to empty Ga p-orbitals is found, whereas π-delocalization over the organic ligand is evident. This study highlights the utility of experimental charge density analysis as a technique to investigate the unusual bonding and electronic characteristics of low oxidation state/low coordinate p-block complexes.

Circumventing redox chemistry: synthesis of transition metal boryl complexes from a boryl nucleophile by decarbonylation.

The very strong reducing capabilities of the boryllithium nucleophile (THF)2Li{B(NDippCH)2} (1, Dipp = 2,6-iPr2C6H3) render impractical its use for the direct introduction of the {B(NDippCH)2} ligand via metathesis chemistry into the immediate coordination sphere of transition metals (d(n), with n ≠ 0 or 10). Instead, 1 typically reacts with metal halide, amide and hydrocarbyl electrophiles either via electron transfer or halide abstraction. Evidence for the formation of M-B bonds is obtained only in the case of the d(5) system [{(HCDippN)2B}Mn(THF)(μ-Br)]2. Lower oxidation state metal carbonyl complexes such as Fe(CO)5 and Cr(CO)6 react with 1 via nucleophilic attack at the carbonyl carbon atom to give boryl-functionalized Fischer carbene complexes Fe(CO)4{C(OLi(THF)3)B(NDippCH)2} and Cr(CO)5{C(OLi(THF)2)B(NDippCH)2}. Although C-to-M boryl transfer does not occur for these formally anionic systems, more labile charge neutral bora-acyl derivatives of the type LnM{C(O)B(NDippCH)2} [LnM = Mn(CO)5, Re(CO)5, CpFe(CO)2] can be synthesized, which cleanly lose CO to generate M-B bonds. From a mechanistic standpoint, an archetypal organometallic mode of reactivity, carbonyl extrusion, has thus been shown to be applicable to the boryl ligand class, with (13)C isotopic labeling studies confirming a dissociation/migration pathway. These proof-of-methodology synthetic studies can be extended beyond boryl complexes of the group 7 and 8 metals (for which a number of versatile synthetic routes already exist) to provide access to complexes of cobalt, which have hitherto proven only sporadically accessible.

Oxidative Bond Formation and Reductive Bond Cleavage at Main Group Metal Centers: Reactivity of Five-Valence-Electron MX2 Radicals

Monomeric five-valence-electron bis(boryl) complexes of gallium, indium, and thallium undergo oxidative M-C bond formation with 2,3-dimethylbutadiene, in a manner consistent with both the redox properties expected for M(II) species and with metal-centered radical character. The weaker nature of the M-C bond for the heavier two elements leads to the observation of reversibility in M-C bond formation (for indium) and to the isolation of products resulting from subsequent B-C reductive elimination (for both indium and thallium).

Boryl substituted group 13 metallylenes: complexes with an iron carbonyl fragment

The first examples of boryl substituted aluminylene and gallylene complexes, [(DAB)B(THF)Al{Fe(CO)3(μ-CO)}]2 and [(DAB)BGa{μ-Fe(CO)4}]2 (DAB = {(C6H3Pri2-2,6)NCH}2) have been prepared by reduction of MX2(THF){B(DAB)} (M = Al or Ga, X = Cl or Br) with K2[Fe(CO)4]. Spectroscopic and crystallographic analyses of the compounds show them to be structurally distinct dimers, the latter of which possesses a close GaGa separation that computational analyses reveal has negligible bonding character.