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.

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.

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).

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.

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).

Potassium, zinc, and magnesium complexes of a bulky OOO-tridentate bis(phenolate) ligand: synthesis, structures, and studies of cyclic ester polymerisation.

Reaction of the OOO-coordinating tridentate bis(phenolate) protio-ligand 2,2'-{oxybis(methylene)}bis{4,6-di(1-methyl-1-phenylethyl)phenol} (L(O3)-H2), with 1 equiv. of KN(SiMe3)2 in toluene or THF yielded [K(L(O3)-H)] (1) or [K(L(O3)-H)(THF)] (2), respectively. Single-crystal X-ray diffraction studies of 1 and 2 revealed mononuclear structures with the phenyl rings of the bulky ligand displaying stabilising π-interactions to the potassium centre. L(O3)-H2 also reacts with 1 equiv. of ZnEt2 or Mg(n)Bu2 to give [M2(L(O3))2] (M = Zn (3) or Mg (4)) in good yield. The molecular structures of complex 3 and 4 reveal dinuclear species in which the metal centres are tetra-coordinated to the three oxygen atoms of one L(O3) ligand, and to the bridging oxygen atom of one phenolate group of another. Complexes 1-4 are catalysts for ring-opening polymerisation of ε-caprolactone and L- and rac-lactide in the presence of benzyl alcohol (BnOH) and also other initiators to give the corresponding polyesters. Kinetic studies for the ROP of ε-caprolactone using 3 and BnOH gives an unusual rate expression R(p) = -d[CL]/dt = k(p)[BnOH]0[3]0(0.5) for which a tentative kinetic model is proposed.

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.

Heterobimetallic complexes containing Ca-Fe or Yb-Fe bonds: synthesis and molecular and electronic structures of [M{CpFe(CO)2}2(THF)3]2 (M = Ca or Yb).

Reaction of calcium or ytterbium amalgam with [CpFe(CO)(2)](2) (Fp(2)) gave the isostructural heavy alkaline earth or divalent rare earth compounds [MFp(2)(THF)(3)](2) (M = Ca or Yb) containing two direct Ca-Fe (3.0185(6) Å) or Yb-Fe (2.9892(4) Å) bonds. Density functional theory supports experiment in finding shorter Yb-Fe than Ca-Fe distances, and Ziegler-Rauk, molecular orbital, and atoms-in-molecules analyses find the M-Fe bonding to be predominantly electrostatic in nature. The Yb-Fe interaction energy and bond critical point electron density are slightly larger than for Ca-Fe, in agreement with the shorter M-Fe bond in the former. The corresponding reaction for magnesium gave MgFp(2)(THF)(4) with two O-bound Fp moieties and no Mg-Fe bond.