Philip Mountford

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.

NEW SYNTHETIC PATHWAYS INTO THE ORGANOMETALLIC CHEMISTRY OF GALLIUM

Abstract Ultrasonic irradiation of gallium metal with di-iodine and alkyl iodides, RI, rapidly gives high yields of [RGaI2] via a reactive intermediate mate

Dibenzotetraaza[14]annulenes: versatile ligands for transition and main group metal chemistry

This article presents an overview of the chemistry of metal complexes of dibenzotetraaza[14]annulene ligands and highlights most of the recent developments. The title tetraazamacrocycles are related to the porphyrins but have a smaller N4 coordination cavity ‘hole size’ and typically possess a non-planar, saddle-shaped conformation. The similarity of the dibenzotetraaza[14]annulenes to the porphyrins means that these synthetic macrocycles are of bioinorganic relevance, while their distinctive individual characteristics make them interesting ligands in their own right. In early transition metal chemistry, for example, the dibenzotetraaza[14]annulenes have been studied as potential alternatives to the ubiquitous bis(η-cyclopentadienyl) ligand set, while later transition metal derivatives can mimic certain biological systems and act as precursors to electroactive polymeric films. The dibenzotetraaza[14]annulenes have also recently allowed advances in structural and reactivity studies of main group organometallic and coordination compounds, including the stabilisation of new metal–chalcogenide multiple bonds.

Zwitterionic bis(phenolate)amine lanthanide complexes for the ring-opening polymerisation of cyclic esters

The reaction of Sm{N(SiMe3)2}3 with the bis(phenol)amines H2O2N(R) (H2O2N(R) = RCH2CH2N(2-HO-3,5-C6H2(t)Bu2)2; R = OMe, NMe2 or Me) gave exclusively zwitterions Sm(O2N(R))(HO2N(R)). For R = OMe or NMe2 these were efficient catalysts for the ring-opening polymerisation of epsilon-caprolactone and D,L-lactide with a tendency to form cyclic esters; in contrast, no polymerisation was observed for R = Me.

New transition metal imido chemistry with diamido-donor ligands

Abstract This short review presents an account of the synthesis and chemistry to date of new Groups 4, 5 and 6 imido complexes derived from the diamido–pyridine or diamido–amine ligand precursors MeC(2-C5H4N)(CH2NHSiMe2R)2 (abbreviated as H2N2Npy for R=Me or H2N2Npy* for R=But) and Me3SiN(CH2CH2NHSiMe3)2 (abbreviated as H2N2Nam). Reaction of the dilithium salts Li2N2Npy, Li2N2Npy* or Li2N2Namwith metal imido synthons of the general type [M(NR)xCly(LB)z] (M=Groups 4–6 transition metals; R=alkyl or aryl substituents; LB=Lewis base) gives a diverse range of products among which are the following complexes: [Ti(NR)(N2Npy)], [M(NR)(LN2N)(LB)] (M=Ti or Zr; LN2N=a general diamido-donor ligand), [M(NR)(N2Npy)Cl(py)] (M=Nb or Ta), [Nb(μ-NBut)(N2Nam)(μ-Cl)]2, [W(NR)(LN2N)Cl2] (LN2N=N2Npy or N2Nam), [Mo(NR)2(LN2N)] (LN2N=N2Npy or N2Nam). The diamido–pyridine supported titanium imido complexes [Ti(NR)(N2Npy)] and [Ti(NR)(N2Npy)(py)] in particular have reactive Ti=NR linkages and these undergo a wide range of coupling reactions with the following unsaturated organic substrates: RNC, MeCN, ButCP, ArNCO, RC2Me, and RCHCCH2. Many of these transformations are the first, or among the first, of their type in transition metal imido chemistry. The combined diamido–pyridine–imido donor set also forms a useful supporting ligand environment for new chemistry at Groups 5 and 6 metal centres, especially in the Group 5 complexes [M(NR)(N2Npy)Cl(py)] (M=Nb or Ta) in which the chloride can be substituted by a range of N-, O- and C-donor ligands.

Group 3 and Lanthanide Boryl Compounds: Syntheses, Structures, and Bonding Analyses of Sc−B, Y−B, and Lu−B σ-Coordinated NHC Analogues

Reaction of [Ln(CH(2)SiMe(3))(2)(THF)(n)][BPh(4)] (Ln = Sc, Y, Lu ; n = 3, 4) with Li{B(NArCH)(2)}(THF)(2) (Ar = 2,6-C(6)H(3)(i)Pr(2)) formed the first group 3 and lanthanide boryl compounds, Sc{B(NArCH)(2)}(CH(2)SiMe(3))(2)(THF) and Ln{B(NArCH)(2)}(CH(2)SiMe(3))(2)(THF)(2) (Ln = Y, Lu), which contain two-center, two-electron Ln-B σ bonds. All of these systems were crystallographically characterized. Density functional theory analysis of the Ln-B bonding found it to be predominantly ionic, with covalent character in the σ-bonding Ln-B HOMO.

Sulfonamide-Supported Group 4 Catalysts for the Ring-Opening Polymerization of ε-Caprolactone and rac-Lactide

Reaction of RCH(2)N(CH(2)CH(2)NHSO(2)Tol)(2) (R = 2-NC(5)H(4) (8, H(2)L(py)) or MeOCH(2) (9, H(2)L(OMe))) with Ti(NMe(2))(4) at room temperature afforded Ti(L(py))(NMe(2))(2) (10) or Ti(L(OMe))(NMe(2))(2) (11), respectively, which contain tetradentate bis(sulfonamide)amine ligands. The corresponding reactions with Ti(O(i)Pr)(4) or Zr(O(i)Pr)(4) x HO(i)Pr required more forcing conditions to form the homologous bis(isopropoxide) analogues, M(L(R))(O(i)Pr)(2) (M = Ti, R = py (12) or OMe (14); M = Zr, R = py (13) or OMe (15)). Reaction of Ti(NMe(2))(2)(O(i)Pr)(2) with H(2)L(R) formed 12 or 14 under milder conditions. The X-ray structures of 10-15 have been determined revealing C(s) symmetric, 6-coordinate complexes except for 13 which is 7-coordinate with one kappa(2)(N,O) bound sulfonamide donor. Compounds 10-15 are all catalysts for the ring-opening polymerization (ROP) of epsilon-caprolactone, with the isopropoxide compounds being the fastest and best controlled, especially in the case of zirconium. In addition, Zr(L(OMe))(O(i)Pr) (2) (15) was an efficient catalyst for the well-controlled ROP of rac-lactide both in toluene at 100 degrees C and in the melt at 130 degrees C, giving atactic poly(rac-lactide). The polymerization rates and control achieved for 13 and 15 are comparable to those of the well-established bis(phenolate)amine-supported Group 4 systems reported recently.

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.

A remarkable inversion of structure–activity dependence on imido N-substituents with varying co-ligand topology and the synthesis of a new borate-free zwitterionic polymerisation catalyst

Ethylene polymerisation productivities of tris(pyrazolyl)methane-supported catalysts [Ti(NR){HC(Me2pz)3}Cl2] show a dramatically different dependence on the imido R-group compared to those of their TACN analogues, attributed to differences in fac-N3 donor topology; when treated with AliBu3, the zwitterionic tris(pyrazolyl)methide compound [Ti(N-2-C6H4tBu){C(Me2pz)3}Cl(THF)] also acts as a highly active, single site catalyst (TACN = 1,4,7-trimethyltriazacyclononane).

A Generic One-Pot Route to Acyclic Two-Coordinate Silylenes from Silicon(IV) Precursors: Synthesis and Structural Characterization of a Silylsilylene

Si in sight: a one-pot, single-step synthesis of an acyclic silylsilylene, Si{Si(SiMe(3))(3)}{N(SiMe(3))Dipp} (Dipp=2,6-iPr(2)C(6)H(3)), from a silicon(IV) starting material is reported, together with evidence for a mechanism involving alkali metal silylenoid intermediates.