Chromium(IV) Siloxide

Chromous siloxides of variable nuclearity and magnetism

Treatment of Cr[N(SiMe₃)₂]₂(thf)₂ with HOSiR₃ (R = Et, iPr) in THF afforded the bridged Cr^II siloxide complexes Cr₃(OSiEt₃)₂(μ-OSiEt₃)₄(thf)₂ and Cr₂(OSiiPr₃)₂(μ-OSiiPr₃)₂(thf)₂ in high yield. Exposure of these compounds to vacuum in aliphatic solvents led to the loss of coordinated THF and to the formation of the homoleptic chromous siloxides Cr₄(μ-OSiEt₃)₈ and Cr₃(OSiiPr₃)₂(μ-OSiiPr₃)₄, respectively, in moderate to high yield. Use of TMEDA as a potentially bidentate donor molecule gave the monomeric cis-coordinated siloxide Cr(OSiiPr₃)₂(tmeda) (tmeda = N,N,N',N'-tetramethylethane-1,2-diamine). Oxidation of Cr₂(OSiiPr₃)₂(μ-OSiiPr₃)₂(thf)₂ with CHI₃ and C₂Cl₆ produced the trigonal bipyramidal chromic compound Cr^III(OSiiPr)₃(thf)₂ and asymmetrically coordinated Cr₂Cl₃(OSiiPr₃)₃(thf)₃, respectively. Magnetic measurements (Evans and SQUID) hinted at (a) antiferromagnetic interactions between the Cr^II centres, (b) revealed higher effective magnetic moments (μ_eff) for cis-coordinated monomeric heteroleptic complexes compared to trans-coordinated ones, and (c) pointed out the highest (μ_eff) for the tetranuclear complex Cr₄(μ-OSiEt₃)₈ (6.26μ_B, SQUID, 300 K; Cr⋯Cr_adjacent avg. 2.535 A).

Progress towards creating optically addressable molecular qubits

The emerging field of quantum information science promises to transform a diverse range of scientific fields, ranging from computation to sensing and metrology. The interdisciplinary scientific community laid the groundwork for the next generation of quantum technologies through key advances in understanding the fundamental unit of quantum information science, the qubit. Electronic spin is a promising platform for qubits, demonstrating suitably long coherence times, optical initialization, and single spin addressability. Herein, we discuss recent accomplishments and future progress from our group targeted at imbuing transition metal complexes with the aforementioned properties, creating a pathway to fusing spatial precision with long coherence times. A strong emphasis of this feature article is progressing towards single spin measurements via a chemical approach for imbuing molecular qubits with an optically-induced spin polarization mechanism.

Structurally characterized terminal manganese(iv) oxo tris(alkoxide) complex

First structurally characterized Mn(iv) oxo.

A Mn(iv) complex featuring a terminal oxo ligand, [Mn^IV(O)(ditox)₃][K(15-C-5)₂] (3; ditox = ^tBu₂MeCO^–, 15-C-5 = 15-crown-5-ether) has been isolated and structurally characterized. Treatment of the colorless precursor [Mn^II(ditox)₃][K(15-C-5)₂] (2) with iodosobenzene affords 3 as a green free-flowing powder in high yields. The X-ray crystal structure of 3 reveals a pseudotetrahedral geometry about the central Mn, which features a terminal oxo (d(Mn–O_term = 1.628(2) Å)). EPR spectroscopy, SQUID magnetometry, and Evans method magnetic susceptibility indicate that 3 consists of a high-spin S = 3/2 Mn(iv) metal center. 3 promotes C–H bond activation by a hydrogen atom abstraction. The [Mn^IV(O)(ditox)₃]^– furnishes a model for the proposed terminal oxo of the unique manganese of the oxygen evolving complex of photosystem II.

Homoleptic transition metal complexes of the 7-azaindolide ligand featuring κ(1)-N1 coordination

Homoleptic complexes of the anion of 7-azaindole (AzaIn) were synthesized and characterized for a series of 3d transition metals. For Mn(II), Fe(II), and Co(II), complexes of formula Na2[M(AzaIn)4]·2L (L = tetrahydrofuran (THF), 2-MeTHF, toluene, or benzene) were isolated by treatment of the corresponding metal chloride salts with 7-azaindole in the presence of sodium hexamethyldisilazide. The complexes adopt tetrahedral geometries with exclusive coordination to the transition metal ion through the pyrrolic N1 nitrogen atoms of the AzaIn ligands. Solid-state structures of the complexes demonstrate that the sodium cations remain tightly associated with the coordination entities through interaction with both the pyrrolic and pyridine nitrogen atoms of the azaindolide ligands. For Fe(II), replacement of the sodium cations by other alkali metal ions (Li or K) generates new complexes that demonstrate similar coordination geometries to the sodium salts. As a means of comparison, the Fe(II) complex of 4-azaindolide was also investigated. Na2[Fe(4-AzaIn)4]·2L adopts a similar solution structure to the 7-azaindolide complexes as judged by NMR spectroscopy and cyclic voltammetry. Density functional theory calculations were performed to investigate the bonding in the 7-azaindolide complexes. Results demonstrate that 7-azaindolide-κ(1)-N1 is a nearly pure sigma donor ligand that features a high degree of ionic character in its bonding to mid 3d transition metal ions.

Heterometallic europium disiloxanediolates: synthesis, structural diversity, and photoluminescence properties

This contribution presents a full account of a structurally diverse class of heterometallic europium disiloxanediolates. The synthetic protocol involves in situ metalation of (HO)SiPh2OSiPh2(OH) (1) with either (n)BuLi or KN(SiMe3)2 followed by treatment with EuCl3 in suitable solvents such as 1,2-dimethoxyethane (DME) or tetrahydrofuran (THF). Reaction of EuCl3 with 2 equiv of (LiO)SiPh2OSiPh2(OLi) in DME afforded the Eu(III) bis(disiloxanediolate) "ate" complex [{(Ph2SiO)2O}2{Li(DME)}3]EuCl2 (2), which upon attempted reduction with Zn gave the tris(disiloxanediolate) [{(Ph2SiO)2O}3{Li(DME)}3]Eu (3). Treatment of EuCl3 with (LiO)SiPh2OSiPh2(OLi) in a molar ratio of 1:2 yielded both the ate complex [{(Ph2SiO)2O}3Li{Li(THF)2}{Li(THF)}]EuCl·Li(THF)3 (4) and the LiCl-free europium(III) complex [{(Ph2SiO)2O}2{Li(THF)2}2]EuCl (5). Compound 5 was found to exhibit a brilliant red triboluminescence. When (KO)SiPh2OSiPh2(OK) was used as starting material in a 3:1 reaction with EuCl3, the Eu(III) tris(disiloxanediolate) [{(Ph2SiO)2O}3{K(DME)}3]Eu (6) was isolated. Attempted ligand transfer between 5 and (DAD(Dipp))2Ba(DME) (DAD(Dipp) = N,N'-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene) afforded the unique mixed-valent Eu(III)/Eu(II) disiloxanediolate cluster [(Ph2SiO)2O]6Eu(II)4Eu(III)2Li4O2Cl2 (7). All new complexes were structurally characterized by X-ray diffraction. Photoluminescence studies were carried out for complex 5 showing an excellent color quality, due to the strong (5)D0→(7)F2 transition, but a weak antenna effect.

Addition of pnictogen atoms to chromium(ii): synthesis, structure and magnetic properties of a chromium(iv) phosphide and a chromium(iii) arsenide

Chromium(ii) chloride reacts with LiCp* (Cp* = C₅Me₅) and LiE (SiMe₃)₂ (E = P or As) to give the chromium(iv) phosphide [(η⁵-Cp*Cr)(μ₃-P)]₄ (1) or the chromium(iii) arsenide [(η⁵-Cp*Cr)₃(μ₃-As)₂] (2), respectively.