Magnesium, calcium and zinc [N2N'] heteroscorpionate complexes.

A new family of sterically demanding N2N' heteroscorpionate pro-ligands (HC(tBu2pz)2SiMe2N(H)R (R = iPr, tBu, Ph, Xyl)) has been prepared via a straightforward modular synthetic route. An extensive study into the synthesis and characterisation of lithium, magnesium, calcium and zinc complexes supported by both 3,5-tBu and 3,5-Me substituted N2N' ligand families has been conducted. Attempted deprotonation of the pro-ligands with nBuLi afforded the corresponding lithium salts Li{HC(tBu2pz)2SiMe2NR} (R = iPr (1), tBu (2), Ph (3) and Xyl (4)) but air- and thermal-sensitivity limited the yields of these potentially useful precursors; only the sterically encumbered ligand system allowed clean reactivity. Magnesium methyl complexes Mg{HC(tBu2pz)2SiMe2NR}Me (R = iPr (5) and R = Ph (6)) were prepared using an excess of the Grignard reagent MeMgCl. Magnesium butyl complexes were synthesised in good yields using the dialkyl precursor MgnBu2 to afford Mg{HC(R'2pz)2SiMe2NR}nBu (R' = Me; R = iPr (7), tBu (8), Ad (9), Ph (10). R' = tBu; R = iPr (11), Ph (12)). Protonolylsis reactions were used to synthesise magnesium and calcium amide complexes Mg{HC(R'2pz)2SiMe2NR}{N(SiHMe2)2} (R' = Me; R = iPr (13), tBu (14), Ph (15). R' = tBu; R = Ph (16)) or Mg{HC(R'2pz)2SiMe2NR}{N(SiMe3)2} (R' = Me; R = iPr (17), tBu (18), Ph (19). R' = tBu; R = Ph (20)), and Ca{HC(R'2pz)2SiMe2NR}{N(SiMe2)2} (L) (R' = Me; L = thf; R = iPr (21), tBu (22), Ph (23). R' = tBu; L = none; R = Ph (24). Zinc methyl complexes Zn{HC(R'2pz)2SiMe2NR}Me (R' = Me; R = iPr (25), tBu (26), Ph (27). R' = tBu; R = Ph (28)) were prepared by reaction of the N2N' heteroscorpionate pro-ligands with ZnMe2. In preliminary studies, magnesium amide complexes 16 and 20 were evaluated as initiators for the ring-opening polymerisation (ROP) of ε-caprolactone (ε-CL) and rac-lactide (rac-LA). Although the overall polymerisation control was poor, 16 and 20 were found to be active initiators.

Electronic Delocalization in Two and Three Dimensions: Differential Aggregation in Indium "Metalloid" Clusters.

Reduction of indium boryl precursors to give two- and three-dimensional M-M bonded networks is influenced by the choice of supporting ligand. While the unprecedented nanoscale cluster [In68 (boryl)12 ]- (with an In12 @In44 @In12 (boryl)12 concentric structure), can be isolated from the potassium reduction of a bis(boryl)indium(III) chloride precursor, analogous reduction of the corresponding (benzamidinate)InIII Br(boryl) system gives a near-planar (and weakly aromatic) tetranuclear [In4 (boryl)4 ]2- system.

New Titanium Borylimido Compounds: Synthesis, Structure, and Bonding.

We report a combined experimental and computational study of the synthesis and electronic structure of titanium borylimido compounds. Three new synthetic routes to this hitherto almost unknown class of Group 4 imide are presented. The double-deprotonation reaction of the borylamine H2NB(NAr'CH)2 (Ar' = 2,6-C6H3iPr2) with Ti(NMe2)2Cl2 gave Ti{NB(NAr'CH)2}Cl2(NHMe2)2, which was easily converted to Ti{NB(NAr'CH)2}Cl2(py)3. This compound is an entry point to other borylimides, for example, reacting with Li2N2pyrNMe to form Ti(N2pyrNMe){NB(NAr'CH)2}(py)2 and with 2 equiv of NaCp to give Cp2Ti{NB(NAr'CH)2}(py) (23). Borylamine-tert-butylimide exchange between H2NB(NAr'CH)2 and Cp*Ti(NtBu)Cl(py) under forcing conditions afforded Cp*Ti{NB(NAr'CH)2}Cl(py), which could be further substituted with guanidinate or pyrrolide-amine ligands to give Cp*Ti(hpp){NB(NAr'CH)2} (16) and Cp*Ti(NpyrNMe2){NB(NAr'CH)2} (17). The Ti-Nim distances in compounds with the NB(NAr'CH)2 ligand were comparable to those of the corresponding arylimides. Dialkyl- or diaryl-substituted borylamines do not undergo the analogous double-deprotonation or imide-amine exchange reactions. Reaction of (Cp″2Ti)2(µ2:η1,η1-N2) with N3BMes2 gave the base-free, diarylborylimide Cp″2Ti(NBMes2) (26) by an oxidative route; this compound has a relatively long Ti-Nim bond and large Cp″-Ti-Cp″ angle. Reaction of 16 with H2NtBu formed equilibrium mixtures with H2NB(NAr'CH)2 and Cp*Ti(hpp)(NtBu) (ΔrG = -1.0 kcal mol-1). In contrast, the dialkylborylimide Cp*Ti{MeC(NiPr)2}(NBC8H14) (2) reacted quantitatively with H2NtBu to give the corresponding tert-butylimide and borylamine. The electronic structures and imide-amine exchange reactions of half-sandwich and sandwich titanium borylimides have been evaluated using density functional theory (DFT), supported by quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis, and placed more generally in context with the well-established alkyl- and arylimides and hydrazides. The calculations find that Ti-Nim bonds for borylimides are stronger and more covalent than in their organoimido or hydrazido analogues, and are strongest for alkyl- and arylborylimides. Borylamine-tert-butylimide exchange reactions fail for H2NBR2 (R = hydrocarbyl) but not for H2NB(NAr'CH)2 because the increased strength of the new Ti-Nim bond for the former is outweighed by the increased net H-N bond strengths in the borylamine. Variation of the Ti-Nim bond length over short distances is dominated by π-interactions with any appropriate orbital on the Nim atom organic substituent. However, over the full range of imides and hydrazides studied, overall bond energies do not correlate with bond length but with the Ti-Nim σ-bond character and the orthogonal π-interaction.

New Scandium Borylimido Chemistry: Synthesis, Bonding, and Reactivity.

We report a combined synthetic, mechanistic, and theoretical study of the first borylimido complex of a rare earth metal, (NacNacNMe2)Sc{NB(NAr'CH)2} (25, Ar' = 2,6-C6H3iPr2, NacNacNMe2 = Ar'NC(Me)CHC(Me)NCH2CH2NMe2). Thermolysis of the methyl-borylamide (NacNacNMe2)Sc(Me){NHB(NAr'CH)2} (18) generated transient imide 25 via rate-determining, first-order methane elimination (KIE ≈ 8.7). In the absence of external substrate, 25 underwent a reversible cyclometalation reaction (sp3 C-H bond addition to Sc═Nimide) with a methyl group of the NacNacNMe2 ligand forming {MeC(NC6H3iPrCH(Me)CH2)CHC(Me)NCH2CH2NMe2}Sc{NHB(NAr'CH)2} (21). In the presence of pyridine or DMAP, reversible sp2 C-H bond activation occurred, forming orthometalated complexes (NacNacNMe2)Sc{NHB(NAr'CH)2}(η2-4-NC5H3R) (R = H or NMe2). In situ reaction of 25 with HCCTol gives irreversible sp C-H bond activation under kinetic control, and with MeCCPh [2+2] cycloaddition to Sc═Nimide takes place. These reactions represent the first substrate activation processes for any metal-bound borylimide. The bonding in 25 and the mechanism and thermodynamics of the reactions have been studied using density functional theory (DFT), supported by quantum theory of atoms in molecules and natural bond orbital analysis. Although the borylimido and arylimido dianions studied here are formally isoelectronic and possess comparable frontier molecular orbitals, the borylimido ligand is both a better π-donor and σ-donor, forming stronger and shorter metal-nitrogen bonds with somewhat reduced ionicity. Despite this, reactions of these types of borylimides with C-H or C≡C bonds are all more exothermic and more strongly activating than for the corresponding arylimides. DFT calculations on model systems of differing steric bulk unpicked the underlying thermodynamic factors controlling the reactions of 25 and its reaction partners, and a detailed comparison was made with the previously described arylimido homologues.

Enabling and Probing Oxidative Addition and Reductive Elimination at a Group 14 Metal Center: Cleavage and Functionalization of E-H Bonds by a Bis(boryl)stannylene.

By employing strongly σ-donating boryl ancillary ligands, the oxidative addition of H2 to a single site Sn(II) system has been achieved for the first time, generating (boryl)2SnH2. Similar chemistry can also be achieved for protic and hydridic E-H bonds (N-H/O-H, Si-H/B-H, respectively). In the case of ammonia (and water, albeit more slowly), E-H oxidative addition can be shown to be followed by reductive elimination to give an N- (or O-)borylated product. Thus, in stoichiometric fashion, redox-based bond cleavage/formation is demonstrated for a single main group metal center at room temperature. From a mechanistic viewpoint, a two-step coordination/proton transfer process for N-H activation is shown to be viable through the isolation of species of the types Sn(boryl)2·NH3 and [Sn(boryl)2(NH2)](-) and their onward conversion to the formal oxidative addition product Sn(boryl)2(H)(NH2).

Probing the limits of alkaline earth-transition metal bonding: an experimental and computational study.

Reduction of Fp2 (Fp = CpFe(CO)2) or [Co(CO)3(PCy3)]2 (15) with Mg-mercury amalgam gave [Mg{TM(L)}2(THF)]2 (TM(L) = Fp or Co(CO)3(PCy3) (19)) in which the TM is bonded to two Mg atoms. Reduction of 15 with Ca-, Sr-, Ba-, Yb-, Eu- and Sm-mercury amalgam gave a series of compounds "M{Co(CO)3(PCy3)}2(THF)n" (M = Ae or Ln) in which the M-Co bonding varies with the charge-to-size ratio of M. For M = Ca or Yb (24), each metal forms one M-Co bond and one M(µ-OC)Co η(1)-isocarbonyl linkage. With M = Sr (21) or Eu (25), a switch from M-Co bonding to side-on (η(2)) CO ligand coordination is found. Sm(II){Co(CO)3(PCy3)}2(THF)3 disproportionates in pentane to form Sm(III){Co(CO)3(PCy3)}3(THF)3 containing two Sm(III)-Co bonds, in contrast with 25, showing the importance of the Ln charge on Ln-TM bonding. Diffusion NMR spectroscopy found that in solution, 21 and 24 are dimeric compounds [M{Co(CO)3(PCy3)}2(THF)3]2 that, according to DFT calculations, contain either one (Ae = Ca) or two (Ae = Sr) Ae-Co bonds per Co atom. DFT calculations in combination with Ziegler Rauk energy decomposition and atoms in molecules analysis were used to assess the nature and energy of Ae-Co bonding in a series of model compounds. The Ae-Co interaction energies decrease from Be to Sr, and toward the bottom of the group, side-on (η(2)) CO ligand coordination competes with Ae-Co bonding. The PCy3 ligand plays a pivotal role by increasing solubility in nondonor solvents and the Ae-Co interaction energy.

Reactions of Titanium Hydrazides with Silanes and Boranes: N-N Bond Cleavage and N Atom Functionalization.

Reaction of Ti(N2(iPr)N)(NNPh2)(py) with Ph(R)SiH2 (R = H, Ph) or 9-BBN gave reductive cleavage of the N(α)-N(ß) bond and formation of new silyl- or boryl-amido ligands. The corresponding reactions of Cp*Ti{MeC(N(i)Pr)2}(NNR2) (R = Me or Ph) with HBPin or 9-BBN gave borylhydrazido-hydride or borylimido products, respectively. N(α) and N(ß) atom transfer and dehydrogenative coupling reactions are also reported.

Bis(phenolate)amine-supported lanthanide borohydride complexes for styrene and trans-1,4-isoprene (co-)polymerisations.

New bis(phenolate)amine-supported neodymium borohydride complexes and their previously reported samarium analogues were tested as catalysts for the polymerisation of styrene and isoprene. Reaction of Na2O2N(L) (L = py, OMe, NMe2) with Nd(BH4)3(THF)3 afforded the borohydride complexes Nd(O2N(L))(BH4)(THF) (L = py (1-Nd), OMe (2-Nd), NMe2 (3-Nd)). Complex 1-Nd has shown a propensity to form phenolate-O-bridged dimer [Nd(µ-O2N(py))(BH4)]2 (1'-Nd) as previously observed with the samarium analogues Sm(O2N(L))(BH4)(THF) (L = py or Pr). X-ray structures of 1'-Nd and 2-Nd were determined and are presented. The neodymium borohydride complexes 1-Nd to 3-Nd and their samarium analogues Sm(O2N(L))(BH4)(THF)x (L = py (1-Sm), OMe (2-Sm), NMe2 (3-Sm), Pr (4-Sm)) were tested as catalysts for the polymerisation of isoprene and styrene in the presence of n-butylethylmagnesium (Mg((n)Bu)(Et)). All complexes were found to be active for the polymerisation of isoprene in these conditions, leading to polyisoprene up to 95.1% trans-1,4 stereoregular. They were also found to be active for the polymerisation of styrene leading to atactic polystyrene in all cases. Interestingly, samarium-based complexes were found to be more active than the neodymium ones toward this latter monomer, in sharp contrast to what is usually observed with rare earth borohydride complexes. The structure of both trans-polyisoprenes and polystyrenes obtained were studied in detail by MALDI-ToF analysis in order to better understand the polymerisation mechanisms. The coordinative chain transfer polymerisation (CCTP) of both monomers was further conducted using Mg((n)Bu)(Et) as transfer agent. Finally, the statistical copolymerisation of isoprene and styrene was examined using these catalytic systems, leading to the formation of poly[(trans-1,4-isoprene)-co-styrene] with up to 39% of styrene moieties inserted in a highly trans-1,4-stereoregular polyisoprene.

Synthesis, molecular and electronic structure, and reactions of a Zn-Hg-Zn bonded complex.

Reaction of ((Ar')NacNac)ZnI with potassium/mercury amalgam gave the trimetallic compound {((Ar')NacNac)Zn}2Hg (1) containing a Zn-Hg-Zn unit and the first example of a bond between two different Group 12 metals; DFT and QTAIM analyses suggest that 1 is best described as two formally Zn(I) atoms with a Hg(0) atom positioned between them; reactions of 1 with stoichiometric I2, FpI or Fp2 gave addition products of the type ((Ar')NacNac)ZnX (X = I, Fp) and Hg. (Ar')NacNac = HC{C(Me)N(2,6-C6H3(i)Pr2)}2; Fp = CpFe(CO)2.

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).

Heavy metal boryl chemistry: complexes of cadmium, mercury and lead.

Synthetic routes to the first boryl complexes of cadmium and mercury are reported via transmetallation from boryllithium; the syntheses of related group 14 systems highlight the additional factors associated with extension to more redox-active post-transition elements.

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.

Chiral lanthanide complexes: coordination chemistry, spectroscopy, and catalysis.

The coordination chemistry and catalytic applications of organometallic and related lanthanide complexes bearing chiral oxazoline ligands is an area that has been largely underdeveloped, in comparison to complexes based upon lanthanide triflates for use in Lewis acid catalysis. In this article we report on the coordination chemistry of the bis(oxazolinylphenyl)amide (BOPA) ligand with lanthanide alkyl and amide co-ligands (Ln = Y, La, Pr, Nd, Sm). Their structural and spectroscopic characterisation are reported, including an assessment of their photophysical properties using luminescence spectroscopy, and are supported by density functional calculations. The application of these complexes in the hydroamination/cyclisation of aminoalkenes, and in the ring-opening polymerisation of rac-lactide is reported.

Group 5 hydride and borohydride complexes supported by cyclopentadienyl-imido ligand sets.

Reactions of Cp*Ta(NAr)Cl2 and CpM(NAr)Cl2 (M = Nb, Ta; Ar = 2,6-C6H3(i)Pr2) with NaBH4 in the presence of an excess of PMe3 provided facile access to the corresponding dihydride derivatives Cp(R)M(NAr)H2(PMe3) (Cp(R) = Cp, Cp*). Reaction of Cp*Nb(NAr)Cl2 with NaBH4 in the absence of phosphine gave the Nb(+5) borohydride-hydride complex Cp*Nb(NAr)H(η(2)-BH4). When the corresponding reactions for CpM(NAr)Cl2 (M = Nb, Ta) were carried out in the absence of an excess of phosphine, dimeric M(IV) products [CpM(µ-NAr)(η(2)-BH4)]2 containing M-M single bonds were formed.

Synthesis and structures of calcium and strontium 2,4-di-tert-butylphenolates and their reactivity towards the amine co-initiated ring-opening polymerisation of rac-lactide.

Calcium and strontium metals react with Hg(C6F5)2 and 2,4-di-tert-butylphenol (H-DBP) in tetrahydrofuran (THF) and 1,2-dimethoxyethane (DME) to give [Ca(DBP)2(THF)4] (1), [Ca2(DBP)4(DME)4(µ-DME)] (2), [Sr3(µ-DBP)6(THF)6] (3), and [Sr2(DBP)(µ-DBP)3(DME)3] (4). Compound 1 is a six coordinate trans-octahedral monomer, whereas in binuclear 2 two seven-coordinate Ca centres are bridged by a DME ligand. In 3 a central Sr is connected by three bridging DBP groups to each of two terminal Sr(THF)3 moieties, all metal atoms being six coordinate. Compound 4 has one six- and one seven-coordinate Sr, bridged by three DBP ligands, the former Sr also having a terminal DBP and a bidentate DME ligand and the latter two DME ligands. Complexes 2 and 4 act as ring-opening polymerisation (ROP) catalysts for the benzyl alcohol or benzylamine co-initiated ROP rac-lactide forming atactic alcohol- or amine-terminated polylactide H-[PLA]-XBn (X = O or NH) with reasonable control of molecular weight via an activated monomer propagation mechanism. Kinetic studies for BnNH2 found the unusual rate expression -d[LA]/dt = k(p(Ae))[2 or 4]0[rac-LA](2)[BnNH2]0(2.5) (k(p(Ca)) ≈ 1.7 ×k(p(Sr))). Preliminary studies suggest that [Y(DBP)3(THF)2] also catalyses amine or alcohol co-initiated ROP by an activated monomer mechanism without loss of a phenoxide ligand.

Synthesis and reactions of ß-diketiminate-supported complexes with Mg-Fe or Yb-Fe bonds.

Reaction of [M(NacNac)I(THF)]n (M = Mg, Ca or Yb; n = 1 or 2) with KFp gave Mg(NacNac)Fp(THF) (, Mg-Fe = 2.6326(4) Å) or [M(NacNac)(µ-Fp)(THF)]2 (M = Ca or Yb (10), no M-Fe bonds); reaction of with [YbFp2(THF)3]2 gave [{Yb(NacNac)(THF)}2(µ-YbFp4)] (Yb-Fe = 2.9758(5)-3.1702(5) Å) containing the first example of a lanthanide bound solely to four transition metals; reaction of with TolNCNTol gave Mg(NacNac){(NTol)2CFp} via the first net insertion of an unsaturated substrate into an alkaline earth-transition metal bond (NacNac = HC{C(Me)N(2,6-C6H3(i)Pr2)}2; Fp = CpFe(CO)2; Tol = 4-C6H4Me).

Synthesis, bonding and reactivity of a terminal titanium alkylidene hydrazido compound.

We report a detailed study of the reactions of the Ti=NNCPh2 alkylidene hydrazide functional group in [Cp*Ti{MeC(NiPr)2}(NNCPh2)] (8) with a variety of unsaturated and saturated substrates. Compound 8 was prepared from [Cp*Ti{MeC(NiPr)2}(NtBu)] and Ph2CNNH2. DFT calculations were used to determine the nature of the bonding for the Ti=NNCPh2 moiety in 8 and in the previously reported [Cp2Ti(NNCPh2)(PMe3)]. Reaction of 8 with CO2 gave dimeric [(Cp*Ti{MeC(NiPr)2}{µ-OC(NNCPh2)O})2] and the "double-insertion" dicarboxylate species [Cp*Ti-{MeC(NiPr)2}{OC(O)N(NCPh2)C(O)O}] through an initial [2+2] cycloaddition product [Cp*Ti{MeC(NiPr)2}{N(NCPh2)C(O)O}], the congener of which could be isolated in the corresponding reaction with CS2. The reaction with isocyanates or isothiocyanates tBuNCO or ArNCE (Ar = Tol or 2,6-C6 H3 iPr2 ; E = O, S) gave either complete NNCPh2 transfer, [2+2] cycloaddition to Ti=Nα or single- or double-substrate insertion into the Ti=Nα bond. The treatment of 8 with isonitriles RNC (R = tBu or Xyl) formed σ-adducts [Cp*Ti{MeC(NiPr)2}(NNCPh2)(CNR)]. With Ar(F5)CCH (Ar(F5)=C6F5) the [2+2] cycloaddition product [Cp*Ti{MeC(NiPr)2}{N(NCPh2)C(Ar(F5))C(H)}] was formed, whereas with benzonitriles ArCN (Ar = Ph or Ar(F5)) two equivalents of substrate were coupled in a head-to-tail manner across the Ti=Nα bond to form [Cp*Ti{MeC(NiPr)2}{N(NCPh2)C(Ar)NC(Ar)N}]. Treatment of 8 with RSiH3 (R = aryl or Bu) or Ph2SiH2 gave [Cp*Ti{MeC(NiPr)2}{N(SiHRR')N(CHPh2)}] (R' = H or Ph) through net 1,3-addition of Si-H to the N-N=CPh2 linkage of 8, whereas reaction with PhSiH2X (X = Cl, Br) led to the Ti=Nα 1,2-addition products [Cp*Ti{MeC(NiPr)2}(X){N(NCPh2)SiH2Ph}].