Studies on the chemical reduction of polynuclear titanium(IV) nitrido complexes

The redox chemistry of cube-type titanium(IV) nitrido complexes [{Ti4(η5-C5Me5)3(R)}(μ3-N)4] (R = η5-C5Me5 (1), N(SiMe3)2 (2), η5-C5H4SiMe3 (3), and η5-C5H5 (4)) was investigated by electrochemical methods and chemical reactions. Cyclic voltammetry studies indicate that 1-4 undergo a reversible one-electron reduction at ca. -1.8 V vs. ferrocenium/ferrocene. Thus, complex 1 reacts with sodium sand in tetrahydrofuran to produce the highly reactive ionic compound [Na(thf)6][{Ti(η5-C5Me5)}4(μ3-N)4] (5). The treatment of complexes 1-4 in toluene with one equivalent of [K(C5Me5)] in the presence of macrocycles (L) leads to C10Me10 and the formation of more stable derivatives [K(L)][{Ti4(η5-C5Me5)3(R)}(μ3-N)4] (R = η5-C5Me5, L = 18-crown-6 (6), crypt-222 (7); R = N(SiMe3)2, L = 18-crown-6 (8), crypt-222 (9); R = η5-C5H4SiMe3, L = 18-crown-6 (10), crypt-222 (11); R = η5-C5H5, L = crypt-222 (12)). However, the analogous reaction of 4 with [K(C5Me5)] and 18-crown-6 affords [{(18-crown-6)K}2(μ-η5:η5-C5H5)][{Ti4(η5-C5Me5)3(η5-C5H5)}(μ3-N)4] (13) via abstraction of one cyclopentadienide group from a putative intermediate [(18-crown-6)K(μ-η5:η5-C5H5)Ti4(η5-C5Me5)3(μ3-N)4]. In contrast to the cube-type nitrido systems 1-4, the cyclic voltammogram of the trinuclear imido-nitrido titanium(IV) complex [{Ti(η5-C5Me5)(μ-NH)}3(μ3-N)] (14) does not reveal any reversible redox event and 14 readily reacts with [K(C5Me5)] to afford C5Me5H and the diamagnetic derivative [{K(μ4-N)(μ3-NH)2Ti3(η5-C5Me5)3(μ3-N)}2] (15). The treatment of 15 with two equiv. of 18-crown-6 polyethers produces the molecular species [(L)K{(μ3-N)(μ3-NH)2Ti3(η5-C5Me5)3(μ3-N)}] (L = 18-crown-6 (16), dibenzo-18-crown-6 (17)). Complex 17 further reacts with one equiv. of dibenzo-18-crown-6 to yield the ion-separated compound [K(dibenzo-18-crown-6)2][Ti3(η5-C5Me5)3(μ3-N)(μ-N)(μ-NH)2] (18) similar to the ion pair [K(crypt-222)][Ti3(η5-C5Me5)3(μ3-N)(μ-N)(μ-NH)2] (19) obtained in the treatment of 15 with cryptand-222.

Anions featuring an aluminium-silicon core with alumanyl silanide and aluminata-silene characteristics

Molecular species containing multiple bonds to aluminium have long been challenging synthetic targets. Despite recent landmark discoveries in this area, heterodinuclear Al-E multiple bonds (where E is a group-14 element) have remained rare and limited to highly polarized π-interactions (Al=E ↔ +Al-E-). Here we report the isolation of three alumanyl silanide anions that feature an Al-Si core stabilized by bulky substituents and a Si-Na interaction. Single-crystal X-ray diffraction studies, spectroscopic analysis and density functional theory calculations show that the Al-Si interaction possesses partial double bond character. Preliminary reactivity studies support this description of the compounds through two resonance structures: one that displays a predominant nucleophilic character of the sodium-coordinated silicon centre in the Al-Si core, as shown by silanide-like reactivity towards halosilane electrophiles and the CH-insertion of phenylacetylene. Moreover, we report an alumanyl silanide with a sequestered sodium cation. Cleavage of the Si-Na bond by [2.2.2]cryptand increases the double bond character of the Al-Si core to produce an anion with high aluminata-silene (-Al=Si) character.

29Si NMR Spectroscopy as a Probe of s- and f-Block Metal(II)-Silanide Bond Covalency

We report the use of ²⁹Si NMR spectroscopy and DFT calculations combined to benchmark the covalency in the chemical bonding of s- and f-block metal-silicon bonds. The complexes [M(Si^tBu₃)₂(THF)₂(THF)_x] (1-M: M = Mg, Ca, Yb, x = 0; M = Sm, Eu, x = 1) and [M(Si^tBu₂Me)₂(THF)₂(THF)_x] (2-M: M = Mg, x = 0; M = Ca, Sm, Eu, Yb, x = 1) have been synthesized and characterized. DFT calculations and ²⁹Si NMR spectroscopic analyses of 1-M and 2-M (M = Mg, Ca, Yb, No, the last in silico due to experimental unavailability) together with known {Si(SiMe₃)₃}^--, {Si(SiMe₂H)₃}^--, and {SiPh₃}^--substituted analogues provide 20 representative examples spanning five silanide ligands and four divalent metals, revealing that the metal-bound ²⁹Si NMR isotropic chemical shifts, δ_Si, span a wide (∼225 ppm) range when the metal is kept constant, and direct, linear correlations are found between δ_Si and computed delocalization indices and quantum chemical topology interatomic exchange-correlation energies that are measures of bond covalency. The calculations reveal dominant s- and d-orbital character in the bonding of these silanide complexes, with no significant f-orbital contributions. The δ_Si is determined, relatively, by paramagnetic shielding for a given metal when the silanide is varied but by the spin-orbit shielding term when the metal is varied for a given ligand. The calculations suggest a covalency ordering of No(II) > Yb(II) > Ca(II) ≈ Mg(II), challenging the traditional view of late actinide chemical bonding being equivalent to that of the late lanthanides.

Lewis-Base Stabilization of the Parent Al(I) Hydride under Ambient Conditions

Aluminum(III) is inherently electron deficient and therefore acts as a prototypical Lewis acid. Conversely, Al(I) is a rare, nucleophilic variant of aluminum that is thermodynamically unstable under ambient conditions. While attempts to stabilize and isolate Al(I) species have become increasingly successful, the parent Al(I) (i.e, Al-H) remains accessible only under extreme temperatures/pressures or matrix conditions. Here, we report the isolation of the parent Al(I) hydride under ambient conditions via the reduction of a Lewis-base-stabilized alkyldihaloalane. Computational and spectroscopic analyses indicate that the ground-state electronic configuration of this monomeric aluminum species is best described as an Al(I) hydride with non-negligible open-shell Al(III) singlet diradical character. These findings are also supported by reactivity studies, which reveal both the p-centered lone pair donating ability and the hydridic nature of the parent aluminene.

Group 4 Metal and Lanthanide Complexes in the Oxidation State +3 with Tris(trimethylsilyl)silyl Ligands

A number of paramagnetic silylated d¹ group 4 metallates were prepared by reaction of potassium tris(trimethylsilyl)silanide with group 4 metallates of the type K[Cp₂MCl₂] (M = Ti, Zr, Hf). The outcomes of the reactions differ for all three metals. While for the hafnium case the expected complex [Cp₂Hf{Si(SiMe₃)₃}₂]⁻ was obtained, the analogous titanium reaction led to a product with two Si(H)(SiMe₃)₂ ligands. The reaction with zirconium caused the formation of a dinuclear fulvalene bridged complex. The desired [Cp₂Zr{Si(SiMe₃)₃}₂]⁻ could be obtained by reduction of Cp₂Zr{Si(SiMe₃)₃}₂ with potassium. In related reactions of potassium tris(trimethylsilyl)silanide with some lanthanidocenes Cp₃Ln (Ln = Ce, Sm, Gd, Ho, Tm) complexes of the type [Cp₃Ln Si(SiMe₃)₃]⁻ with either [18-crown-6·K]⁺ or the complex ion [18-crown-6·K·Cp·K·18-crown-6] as counterions were obtained. Due to d¹ or fⁿ electron configuration, unambiguous characterization of all obtained complexes could only be achieved by single crystal XRD diffraction analysis.

Reactions of (Me₃Si)₃SiK with K[Cp₂MCl₂] (M = Ti, Zr, Hf) gave different paramagnetic silylated d¹ Group 4 metal ate complexes. For Hf the complex [Cp₂Hf{Si(SiMe₃)₃}₂]⁻ was formed, but for Ti a product with two Si(H)(SiMe₃)₂ ligands was isolated. With Zr a dinuclear fulvalene bridged complex formed but [Cp₂Zr{Si(SiMe₃)₃}₂]⁻ could be obtained by reduction of the neutral complex. In reactions of (Me₃Si)₃SiK with Cp₃Ln (Ln = Ce, Sm, Gd, Ho, Tm) complexes of the type [Cp₃LnSi(SiMe₃)₃]⁻ were observed.

Isolation of reactive Ln(ii) complexes with C5H4Me ligands (CpMe) using inverse sandwich countercations: synthesis and structure of [(18-crown-6)K(μ-CpMe)K(18-crown-6)][CpMe3LnII] (Ln = Tb, Ho)

We report that crystallographically-characterizable Ln(ii) complexes of Tb and Ho can be isolated by reducing Cp^Me₃Ln(THF) with KC₈ in THF in the presence of 18-crown-6 (18-c-6).

[K(18-crown-6)][FeCp*(CO)2]

The title compound, (1,4,7,10,13,16-hexaoxacyclooctadecane-κ6O)potassium dicarbonyl(η5-pentamethylcyclopentadienyl)ferrate(II), [K(C12H24O6)][Fe(C10H15)(CO)2], consists of K+cations embedded in 18-crown-6 molecules and [FeCp*(CO)2]−anions. Cations and anions form ion pairs which are linked by weak C—H...O interactions.

Carbene-induced synthesis of the first borironium cations using the [(η5-C5Me5)Fe(CO)2]−anion as an unlikely leaving group

The first examples of borironium cations are prepared by a remarkably mild carbene-induced process. The cations are three-membered unsaturated boron-containing rings with two Lewis donors bound to the boron atom.

The first example of a two-coordinated Au(I) atom bonded to an Fe(II) atom and an N-heterocyclic carbene (NHC) ligand

The aurophilicity exhibited by Au(I) complexes depends strongly on the nature of the supporting ligands present and the length of the Au-element (Au-E) bond may be used as a measure of the donor-acceptor properties of the coordinated ligands. A binuclear iron-gold complex, [1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene-2κC(2)]dicarbonyl-1κ(2)C-(1η(5)-cyclopentadienyl)gold(I)iron(II)(Au-Fe) benzene trisolvate, [AuFe(C5H5)(C27H36N2)(CO)2]·3C6H6, was prepared by reaction of K[CpFe(CO)2] (Cp is cyclopentadienyl) with (NHC)AuCl [NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]. In addition to the binuclear complex, the asymmetric unit contains three benzene solvent molecules. This is the first example of a two-coordinated Au atom bonded to an Fe and a C atom of an N-heterocyclic carbene.

Redetermination of (acetonitrile-κN)dicarbon-yl(η(5)-cyclo-penta-dien-yl)iron(II) tetra-fluoridoborate

The crystal structure of the title compound, [Fe(C₅H₅)(CH₃CN)(CO)₂]BF₄, of which only the coordinates of the non-H atoms of the cation have previously been reported [Fadel et al. (1979 ▶). Z. Anorg. Allg. Chem.453, 98–106] has been redetermined. The Fe^II atom in the complex cation is coordinated by a cyclo­penta­dienyl ring, two carbonyl ligands and an acetonitrile mol­ecule displaying a three-legged piano stool structure. Three of the four F atoms of the BF₄⁻ anion are disordered over two sets of sites, with a site-occupancy factor of 0.709 (10) for the major occupied site.