Coordination-induced O-H/N-H bond weakening by a redox non-innocent, aluminum-containing radical.

Several renewable energy schemes aim to use the chemical bonds in abundant molecules like water and ammonia as energy reservoirs. Because the O-H and N-H bonds are quite strong (>100 kcal/mol), it is necessary to identify substances that dramatically weaken these bonds to facilitate proton-coupled electron transfer processes required for energy conversion. Usually this is accomplished through coordination-induced bond weakening by redox-active metals. However, coordination-induced bond weakening is difficult with earth's most abundant metal, aluminum, because of its redox inertness under mild conditions. Here, we report a system that uses aluminum with a redox non-innocent ligand to achieve significant levels of coordination-induced bond weakening of O-H and N-H bonds. The multisite proton-coupled electron transfer manifold described here points to redox non-innocent ligands as a design element to open coordination-induced bond weakening chemistry to more elements in the periodic table.

Recent advances in cooperative activation of CO2 and N2O by bimetallic coordination complexes or binuclear reaction pathways.

The gaseous small molecules, CO2 and N2O, play important roles in climate change and ozone layer depletion, and they hold promise as underutilized reagents and chemical feedstocks. However, productive transformations of these heteroallenes are difficult to achieve because of their inertness. In nature, these gases are cycled through ecological systems by metalloenzymes featuring multimetallic active sites that employ cooperative mechanisms. Thus, cooperative bimetallic chemistry is an important strategy for synthetic systems, as well. In this Perspective, recent advances (since 2010) in cooperative activation of CO2 and N2O are reviewed, including examples involving s-block, p-block, d-block, and f-block metals and different combinations thereof.

Cooperative Activation of CO2 and Epoxide by a Heterobinuclear Al-Fe Complex via Radical Pair Mechanisms.

Activation of inert molecules like CO2 is often mediated by cooperative chemistry between two reactive sites within a catalytic assembly, the most common form of which is Lewis acid/base bifunctionality observed in both natural metalloenzymes and synthetic systems. Here, we disclose a heterobinuclear complex with an Al-Fe bond that instead activates CO2 and other substrates through cooperative behavior of two radical intermediates. The complex Ldipp(Me)AlFp (2, Ldipp = HC{(CMe)(2,6-iPr2C6H3N)}2, Fp = FeCp(CO)2, Cp = η5-C5H5) was found to insert CO2 and cyclohexene oxide, producing LdippAl(Me)(µ:κ2-O2C)Fp (3) and LdippAl(Me)(µ-OC6H10)Fp (4), respectively. Detailed mechanistic studies indicate unusual pathways in which (i) the Al-Fe bond dissociates homolytically to generate formally AlII and FeI metalloradicals, then (ii) the metalloradicals add to substrate in a pairwise fashion initiated by O-coordination to Al. The accessibility of this unusual mechanism is aided, in part, by the redox noninnocent nature of Ldipp that stabilizes the formally AlII intermediates, instead giving them predominantly AlIII-like physical character. The redox noninnocent nature of the radical intermediates was elucidated through direct observation of LdippAl(Me)(OCPh2) (22), a metalloradical species generated by addition of benzophenone to 2. Complex 22 was characterized by X-band EPR, Q-band EPR, and ENDOR spectroscopies as well as computational modeling. The "radical pair" pathway represents an unprecedented mechanism for CO2 activation.

Base-stabilized formally zero-valent mono and diatomic molecular main-group compounds.

Various compounds are known for transition metals in their formal zero-oxidation state, while similar compounds of main-group elements are recently realized and limited to only a few examples. Lewis-base-stabilized mono and diatomic molecular species (B2, C, C2, Si, Si2, Ge, Ge2, Sn, P2, As2, Sb2) represent groundbreaking examples of main-group compounds with formally zero-oxidation state. In recent years, the isolation of low-valent main-group compounds has attracted increasing attention of both experimental and theoretical chemists. This is not only due to their fascinating electronic structures and exceptional reactivities, but also their use as valuable precursors for the synthesis of exotic yet important chemical species. This has led to a better understanding of the intricate balance of the donor-acceptor properties of the ligand(s) used to stabilize elements in a formally zero-oxidation state. Owing to the unusual oxidation state of the central element, many compounds containing formally zero-valent elements can efficiently activate otherwise inert small molecules. This review describes the synthesis, characterization, and reactivity of reported mono and diatomic formal zero-oxidation state main-group compounds. This review also emphasizes the comparative description of systems where different ligands are used to stabilize an element in its formal zero-oxidation state.

A W/Cu Synthetic Model for the Mo/Cu Cofactor of Aerobic CODH Indicates That Biochemical CO Oxidation Requires a Frustrated Lewis Acid/Base Pair.

Constructing synthetic models of the Mo/Cu active site of aerobic carbon monoxide dehydrogenase (CODH) has been a long-standing synthetic challenge thought to be crucial for understanding how atmospheric concentrations of CO and CO2 are regulated in the global carbon cycle by chemolithoautotrophic bacteria and archaea. Here we report a W/Cu complex that is among the closest synthetic mimics constructed to date, enabled by a silyl protection/deprotection strategy that provided access to a kinetically stabilized complex with mixed O2-/S2- ligation between (bdt)(O)WVI and CuI(NHC) (bdt = benzene dithiolate, NHC = N-heterocyclic carbene) sites. Differences between the inorganic core's structural and electronic features outside the protein environment relative to the native CODH cofactor point to a biochemical CO oxidation mechanism that requires a strained active site geometry, with Lewis acid/base frustration enforced by the protein secondary structure. This new mechanistic insight has the potential to inform synthetic design strategies for multimetallic energy storage catalysts.

The unique ß-diketiminate ligand in aluminum(i) and gallium(i) chemistry.

Over the past few decades, ß-diketiminate ligands have been widely used in coordination chemistry and are capable of stabilizing various metal complexes in multiple oxidation states. Recently, the chemistry of aluminum and gallium in their +1 oxidation state has rapidly emerged. NacNacM(i) (M = Al, Ga; NacNac = ß-diketiminate ligand) shows a two coordinate metal center comparable with singlet carbene-like species. The metal center also possesses a formally vacant p-orbital. In this article we present an overview of the last 10 years for aluminum(i) and gallium(i) stabilized by ß-diketiminate ligands that have been widely explored in bond breaking and forming species.

Germylene stabilized group 12 metal complexes and their reactivity with chalcogens.

This manuscript reports the first examples of germylene stabilized cadmium complexes {[{(i-Bu)2ATIGe(i-Pr)}2(CdI2)] (3, monomeric), [{(i-Bu)2ATIGe(i-Pr)(CdCl2)}2] (6, dimeric), [{(i-Bu)2ATIGe(i-Pr)(CdI2)}2] (7, dimeric)} and novel germylene zinc complexes {[{(i-Bu)2ATIGe(i-Pr)}2(ZnCl2)] (2, monomeric), [{(i-Bu)2ATIGe(i-Pr)(ZnI2)}2] (5, dimeric)} (ATI = aminotroponiminate). The reactions of germylene zinc complex [{(i-Bu)2ATIGe(i-Pr)(ZnCl2)}2] (4) with elemental sulphur and selenium resulted in the first examples of germathione and germaselenone stabilized ZnCl2 complexes [{(i-Bu)2ATIGe(i-Pr)(S)(ZnCl2)}2] (8) and [{(i-Bu)2ATIGe(i-Pr)(Se)(ZnCl2)}2] (9), respectively. Compound 4 was obtained through the reaction of compound 2 with ZnCl2. Interconversions between the monomeric and dimeric zinc/cadmium complexes (2 ↔ 4/3 ↔ 7) are shown. Compounds 2-3 and 5-9 are characterized by multinuclear NMR spectroscopy and single crystal X-ray diffraction studies are performed on compounds 2-3, 5-7, and 9. To understand the nature of bonding in the first examples of germylene cadmium complexes, ab initio calculations are also carried out on compounds 3 and 7.

Treatment of Silylene-Phosphinidene with Chalcogens Resulted Exclusively in the Formation of Silicon-Bonded Chalcogens.

Chalcogen-bonded silicon phosphinidenes LSi(E)-P-Me cAAC (E=S (1); Se (2); Te (3); L=PhC(NtBu)2 ; Me cAAC=C(CH2 )(CMe2 )2 N-2,6-iPr2 C6 H3 )) were synthesized from the reactions of silylene-phosphinidene LSi-P-Me cAAC (A) with elemental chalcogens. All the compounds reported herein have been characterized by multinuclear NMR, elemental analyses, LIFDI-MS, and single-crystal X-ray diffraction techniques. Furthermore, the regeneration of silylene-phosphinidene (A) was achieved from the reactions of 2-3 with L'Al (L'=HC{(CMe)(2,6-iPr2 C6 H3 N)}2 ). Theoretical studies on chalcogen-bonded silicon phosphinidenes indicate that the Si-E (E=S, Se, Te) bond can be best represented as charge-separated electron-sharing σ-bonding interaction between [LSi-P-Me cAAC]+ and E- . The partial double-bond character of Si-E is attributed to significant hyperconjugative donation from the lone pair on E- to the Si-N and Si-P σ*-molecular orbitals.

Stable cyclic (alkyl)(amino)carbene (cAAC) radicals with main group substituents.

Isolation and characterization of stable radicals has been a long-pursued quest. While there has been some progress in this field particularly with respect to carbon, radicals involving heavier p-block elements are still considerably sparse. In this review we describe our recent successful efforts on the isolation of stable p-block element radicals particularly those involving aluminum, silicon, and phosphorus.

Donor-acceptor-stabilised germanium analogues of acid chloride, ester, and acyl pyrrole compounds: synthesis and reactivity.

Germaacid chloride, germaester, and N-germaacyl pyrrole compounds were not known previously. Therefore, donor-acceptor-stabilised germaacid chloride (i-Bu)2ATIGe(O)(Cl) → B(C6F5)3 (1), germaester (i-Bu)2ATIGe(O)(OSiPh3) → B(C6F5)3 (2), and N-germaacyl pyrrole (i-Bu)2ATIGe(O)(NC4H4) → B(C6F5)3 (3) compounds, with Cl-Ge[double bond, length as m-dash]O, Ph3SiO-Ge[double bond, length as m-dash]O, and C4H4N-Ge[double bond, length as m-dash]O moieties, respectively, are reported here. Germaacid chloride 1 reacts with PhCCLi, KOt-Bu, and RLi (R = Ph, Me) to afford donor-acceptor-stabilised germaynone (i-Bu)2ATIGe(O)(CCPh) → B(C6F5)3 (4), germaester (i-Bu)2ATIGe(O)(Ot-Bu) → B(C6F5)3 (5), and germanone (i-Bu)2ATIGe(O)(R) → B(C6F5)3 (R = Ph 6, Me 7) compounds, respectively. Interconversion between a germaester and a germaacid chloride is achieved; reaction of germaesters 2 and 5 with TMSCl gave germaacid chloride 1, and 1 reacted with Ph3SiOLi and KOt-Bu to produce germaesters 2 and 5. Reaction of N-germaacyl pyrrole 3 with thiophenol produced a donor-acceptor-stabilised germaacyl thioester (i-Bu)2ATIGe(O)(SPh) → B(C6F5)3 (10). Furthermore, the attempted syntheses of germaamides and germacarboxylic acids are also discussed.

Ge(ii) cation catalyzed hydroboration of aldehydes and ketones.

Well-defined germylene cations [(i-Bu)2ATI]GeOTf (4) and [(i-Bu)2ATIGe][GaCl4] (5) are isolated, and the catalytic utility of compound 4 for the hydroboration of a variety of aldehydes and ketones is reported (ATI = aminotroponiminate).

Synthesis of cAAC stabilized biradical of "Me2Si" and "Me2SiCl" monoradical from Me2SiCl2 - an important feedstock material.

The cyclic alkyl(amino) carbene (cAAC) coordinated biradical of dimethylsilicon was isolated as (cAAC)2Me2Si (1), (cAAC = C(CH2)(CMe2)2N-2,6-i-Pr2C6H3), synthesized from the reduction of Me2SiCl2 using two equivalents of KC8 in the presence of two equivalents of cAAC. The reduction of Me2SiCl2 by one equivalent of KC8 in the presence of one equivalent of cAAC resulted in the stable dimethylsiliconchloride monoradical (cAAC)Me2SiCl (2).

Isolation of Transient Acyclic Germanium(I) Radicals Stabilized by Cyclic Alkyl(amino) Carbenes.

Despite the notable progress in the stabilization of main group radicals by NHCs and cAACs, no germanium radicals have been isolated so far due to synthetic challenges. Stabilization of neutral [:EIR]• (E = Si, Ge) radicals is an uphill task, as these reactive transient species are highly susceptible to dimerization. Herein, we report the synthesis of acyclic neutral germanium(I) radicals Cy-cAAC:GeN(SiMe3)Dip (1) and Me-cAAC:GeN(SiPh3)Mes (2) obtained by the reduction of [Ar(SiR3)NGeCl3] with KC8 in the presence of cAAC. Compounds 1 and 2 are well characterized by single crystal X-ray structural analysis, cyclic voltammetry, and EPR spectroscopy. Furthermore, the structure and bonding of compounds 1 and 2 have been investigated by theoretical methods.

Comparison of Two Phosphinidenes Binding to Silicon(IV)dichloride as well as to Silylene.

The cyclic alkyl(amino) carbene (cAAC) anchored silylene with two phosphinidenes was isolated as (cAAC)Si{P(cAAC)}2 (3) at room temperature, which was synthesized from the reduction of (Cl2)Si{P(cAAC)}2 (2) using 2 equiv of KC8. Compound 2 resulted from the reaction of 2 equiv of (cAAC)PK (1) with 1 equiv of SiCl4. Compounds 2 and 3 are the first examples where two terminal phosphinidenes are binding each to a silicon center characterized by single crystal X-ray structural analysis. Furthermore, the structure and bonding of compounds 2 and 3 have been investigated by theoretical methods for comparison.

Silanylidene and Germanylidene Anions: Valence-Isoelectronic Species to the Well-Studied Phosphinidene.

The reduction of TipMCl3 (Tip=2,4,6-triisopropylphenyl) (M=Si, Ge) with KC8 in the presence of cyclic alkyl(amino) carbene (cAAC) afforded the acyclic silanylidene and germanylidene anions in the form of potassium salt [K(cAAC)MTip]2 (M=Si (1); Ge (2)). The silanylidene and germanylidene anions are valence-isoelectronic to the well-studied phosphinidene and are a new class of acyclic anions of Group 14. Compounds 1 and 2 were isolated and well characterized by NMR and single-crystal X-ray structure analysis. Furthermore, the structure and bonding of compounds 1 and 2 was investigated by computational methods.

Pseudohalogenogermylenes versus Halogenogermylenes: Difference in their Complexation Behavior towards Group 6 Metal Carbonyls.

Pseudohalogenogermylenes [(iBu)2 ATI]GeY (Y=NCO 4, NCS 5) show different coordination behavior towards group 6 metal carbonyls in comparison to the corresponding halogenogermylenes [(iBu)2 ATI]GeX (X=F 1, Cl 2, Br 3) (ATI=aminotroponiminate). The reactions of compounds 4-5 and 1-3 with cis-[M(CO)4 (COD)] (M=Mo, W, COD=cyclooctadiene) gave trans-germylene metal complexes {[(iBu)2 ATI]GeY}2 M(CO)4 (Y=NCO, M=Mo 6, W 11; Y=NCS, M=Mo 7) and cis-germylene metal complexes {[(iBu)2 ATI]GeX}2 M(CO)4 (M=Mo, X=F 8, Cl 9, Br 10; M=W, X=Cl 12), respectively. Theoretical studies on compounds 7 and 9 reveal that donor-acceptor interactions from Mo to Ge atoms are better stabilized in the observed trans and cis geometries than in the hypothetical cis and trans structures, respectively.

Reagent for Introducing Base-Stabilized Phosphorus Atoms into Organic and Inorganic Compounds.

The cyclic alkyl(amino) carbene (cAAC) stabilized monoanionic phosphorus atom in the form of lithium phosphinidene [cAACPLi(THF)2]2 (1) has been isolated as a molecular species and characterized by single crystal X-ray structure analysis. Furthermore, the structure and bonding of compound 1 has been investigated by theoretical methods. The utilization of the lithium phosphinidene as a phosphorus transfer reagent for a wide range of organic and inorganic substrates has been investigated. Herein, we report on the preparation of fascinating compounds containing P-C, P-Si, P-Ge, and P-P bonds using a single step with a base-stabilized phosphorus atom.

A Route to Base Coordinate Silicon Difluoride and the Silicon Trifluoride Radical.

Silicon difluoride (SiF2 ) is highly unstable at room temperature and condenses at this temperature rapidly to a polymeric material of unknown structure. Therefore, the isolation of a stable monomeric silicon difluoride species is a challenging task. The cyclic alkyl(amino) carbene (cAAC) coordinated silicon difluoride was isolated as (cAAC)2 SiF2 (2), synthesized from the reduction of cAAC-SiF4 (1) by using two equivalents of KC8 in the presence of one equivalent of cAAC. In the solid state, compound 2 is stable at room temperature for a long time under inert conditions. The reduction of compound 1 in the presence of one equivalent KC8 resulted in the first stable silicon trifluoride monoradical (cAAC)SiF3 (3).

Synthesis and characterization of Lewis base stabilized mono- and di-organo aluminum radicals.

Two cyclic (alkyl)(amino)carbene (cAAC) stabilized mononuclear neutral radicals of aluminum have been synthesized. They contain an ethyl [(cAAC)2AlClEt (1)] and as well a diethyl group [(cAAC)2AlEt2 (2)], and have been prepared from the reduction of EtAlCl2 and Et2AlCl, respectively, with KC8. Compounds 1 and 2 are monoradicals, which were confirmed by EPR measurements to have the spin located on the carbene carbon of one of the cAAC ligands.

Organosilicon Radicals with Si-H and Si-Me Bonds from Commodity Precursors.

The cyclic alkyl(amino) carbene (cAAC) stabilized biradicals of composition (cAAC)2SiH2 (1), (cAAC)SiMe2-SiMe2(cAAC) (2), and (cAAC)SiMeCl-SiMeCl(cAAC) (3) have been isolated as molecular species. All the compounds are stable at room temperature for more than 6 months under inert conditions in the solid state. All radical species were fully characterized by single-crystal X-ray structure analysis and EPR spectroscopy. Furthermore, the structure and bonding of compounds 1-3 have been investigated by theoretical methods. Compound 1 contains the SiH2 moiety and this is the first instance, where we have isolated 1 without an acceptor molecule.