Titanium-Mediated Catalytic Hydrogenation of Monocyclic and Polycyclic Arenes

Two electron-reduction of the Ti^IV guanidinate complex (Im^DippN)(^Xyketguan)TiCl₂ gives (η⁶-Im^DippN)(^xyketguan)Ti ( 1^intra ) and (Im^DippN)(^Xyketguan)Ti(η⁶-C₆H₆) ( 1^inter ) (^Xyketguan=[(tBuC=N)C(NXylyl)₂]⁻, Xylyl=2,5-dimethylphenyl) in the absence or presence of benzene, respectively. These complexes have been found to hydrogenate monocyclic and polycyclic arenes under relatively mild conditions (150 psi, 80 °C)—the first example of catalytic, homogeneous arene hydrogenation with TON >1 by a Group IV system.     

Reactivity of titanium imidazolin-2-iminato complexes with 2,6-diisopropylaniline and 2-{(2,6-diisopropylphenyl)-iminomethyl}pyrrole

Abstract

We report the reactions of imidazolin-2-iminato titanium complexes [(Im^RN)Ti(NMe₂)₃] (R = Mes, 2b; R = Dipp, 2c; Mes = mesityl, Dipp = 2,6-diisopropylphenyl) with 2,6-diisopropylaniline in a 1:3 molar ratio to yield the titanium imido complexes of composition [(Im^RNH)Ti = N(^Dipp)(HN^Dipp)₂] (R = Mes, 3b; R = Dipp, 3c) in good yield by the Ti-Niminato bond cleavage at 60 °C. In contrast, the reaction of [(Im^RN)Ti(NMe₂)₃] with 2,6-diisopropylaniline in a 1:1 molar ratio afforded mono-substituted products [(Im^RN)Ti(NMe₂)₂(HN^Dipp)] (R = Mes, 4b; R = Dipp, 4c) in good yield. The reaction of [(Im^RN)Ti(NMe₂)₃] with the iminopyrrole ligand [2-(2,6-iPr₂C₆H₃-N = CH)C₄H₃NH] (N^DippPyH) in a 1:1 ratio afforded mixed ligands, titanium complexes [(Im^RN)Ti(NMe₂)₂(N^Dipp-Py)] (R = tBu, 5a; R = Dipp, 5c) with imidazolin-2-iminato and iminopyrrolide ligands. Molecular structures of 3b, 3c, 4c, 5a, and 5c were determined by single-crystal X-ray analysis. The solid-state structures of 3b and 3c clearly indicate the formation of true Ti = N double bonds, measuring 1.730(2) Å and 1.727(1) Å, respectively. The solid-state structures of 5a and 5c reveal the formation of five-coordinate titanium complexes.     

Synthesis and EPR/UV/Vis‐NIR Spectroelectrochemical Investigation of a Persistent Phosphanyl Radical Dication

The reaction of the bis(imidazoliumyl)‐substituted P^I cation [(2‐Im^Dipp)P(4‐Im^Dipp)]⁺ ( 10 ⁺) (2‐Im=imidazolium‐2‐yl; 4‐Im=imidazolium‐4‐yl; Dipp=2,6‐di‐isopropylphenyl) with trifluoromethanesulfonic acid (HOTf) or methyl trifluoromethylsulfonate (MeOTf) yields the corresponding protonated [(2‐Im^Dipp)PH(4‐Im^Dipp)]²⁺ ( 11 ²⁺) and methylated [(2‐Im^Dipp)PMe(4‐Im^Dipp)]²⁺ ( 12 ²⁺) dications, respectively. EPR/UV/Vis‐NIR spectroelectrochemical investigation of the low‐coordinated P^I cation 10 ⁺ predicted a stable and “bottleable” P‐centered radical dication [(2‐Im^Dipp)P(4‐Im^Dipp)]^2+. ( 13 ^2+.). The reaction of 10 ⁺ with the nitrosyl salt NO[OTf] yields the persistent phosphanyl radical dication 13 ^2+. as triflate salt in crystalline form. Quantum chemical investigation revealed an exceptional high spin density at the P atom.     

Site‐Selective Ligand Exchange on a Heteroleptic TiIV Complex Towards Stepwise Multicomponent Self‐Assembly

A series of heteroleptic [Ti 1 ₂X]^? complexes have been selectively constructed from a mixture of Ti^IV ions, a pyridyl catechol ligand (H₂ 1 ; H₂ 1 =4‐(3‐pyridyl)catechol), and various bidentate ligands (HX) in the presence of a weak base, in addition to a previously reported [Ti 1 ₂(acac)]^? (acac=acetylacetonate) complex. Comparative studies of these Ti^IV complexes revealed that [Ti 1 ₂(trop)]^? (trop=tropolonate) is much more stable than the [Ti 1 ₂(acac)]^? complex, which allows the replacement of acac with trop on the [Ti 1 ₂(acac)]^? complex. This Ti^IV‐centered site‐selective ligand exchange reaction also takes place on a heteronuclear Pd^II? Ti^IV ring complex with the preservation of the Pd^II‐centered coordination structures. Intra‐ and intermolecular linking between two Ti^IV centers with a flexible or a rigid bis‐tropolone bridging ligand provided a tetranuclear and an octanuclear Pd^II? Ti^IV complex, respectively. These higher‐order structures could be efficiently constructed only through a stepwise synthetic route.     

A Flexible Photoactive Titanium Metal–Organic Framework Based on a [TiIV3(μ3‐O)(O)2(COO)6] Cluster

The synthesis of titanium–carboxylate metal–organic frameworks (MOFs) is hampered by the high reactivity of the commonly employed alkoxide precursors. Herein, we present an innovative approach to titanium‐based MOFs by the use of titanocene dichloride to synthesize COK‐69, the first breathing Ti MOF, which is built up from trans‐1,4‐cyclohexanedicarboxylate linkers and an unprecedented [Ti^IV₃(μ₃‐O)(O)₂(COO)₆] cluster. The photoactive properties of COK‐69 were investigated in depth by proton‐coupled electron‐transfer experiments, which revealed that up to one Ti^IV center per cluster can be photoreduced to Ti^III while preserving the structural integrity of the framework. The electronic structure of COK‐69 was determined by molecular modeling, and a band gap of 3.77 eV was found.     

Adduct Formation,B−H Activation and Ring Expansion at Room Temperature from Reactions of HBcat with NHCs

We report the reactions of catecholborane (HBcat; 1 ) with unsaturated and saturated NHCs as well as CAAC^Me. Mono‐NHC adducts of the type HBcat?NHC (NHC=nPr₂Im, iPr₂Im, iPr₂Im^Me, and Dipp₂Im) were obtained by stoichiometric reactions of HBcat with the unsaturated NHCs. The reaction of CAAC^Me with HBcat yielded the B?H activated product CAAC^Me(H)Bcat via insertion of the carbine‐carbon atom into the B?H bond. The saturated NHC Dipp₂SIm reacted in a 2:2 ratio yielding an NHC ring‐expanded product at room temperature forming a six‐membered ?B?C=N?C=C?N? ring via C?N bond cleavage and further migration of the hydrides from two HBcat molecules to the former carbene‐carbon atom.     

The Nature of the UC Double Bond: Pushing the Stability of High‐Oxidation‐State Uranium Carbenes to the Limit

Treatment of [K(BIPM^MesH)] (BIPM^Mes={C(PPh₂NMes)₂}^2?; Mes=C₆H₂‐2,4,6‐Me₃) with [UCl₄(thf)₃] (1 equiv) afforded [U(BIPM^MesH)(Cl)₃(thf)] ( 1 ), which generated [U(BIPM^Mes)(Cl)₂(thf)₂] ( 2 ), following treatment with benzyl potassium. Attempts to oxidise 2 resulted in intractable mixtures, ligand scrambling to give [U(BIPM^Mes)₂] or the formation of [U(BIPM^MesH)(O)₂(Cl)(thf)] ( 3 ). The complex [U(BIPM^Dipp)(μ‐Cl)₄(Li)₂(OEt₂)(tmeda)] ( 4 ) (BIPM^Dipp={C(PPh₂NDipp)₂}^2?; Dipp=C₆H₃‐2,6‐iPr₂; tmeda=N,N,N′,N′‐tetramethylethylenediamine) was prepared from [Li₂(BIPM^Dipp)(tmeda)] and [UCl₄(thf)₃] and, following reflux in toluene, could be isolated as [U(BIPM^Dipp)(Cl)₂(thf)₂] ( 5 ). Treatment of 4 with iodine (0.5 equiv) afforded [U(BIPM^Dipp)(Cl)₂(μ‐Cl)₂(Li)(thf)₂] ( 6 ). Complex 6 resists oxidation, and treating 4 or 5 with N‐oxides gives [{U(BIPM^DippH)(O)₂‐ (μ‐Cl)₂Li(tmeda)] ( 7 ) and [{U(BIPM^DippH)(O)₂(μ‐Cl)}₂] ( 8 ). Treatment of 4 with tBuOLi (3 equiv) and I₂ (1 equiv) gives [U(BIPM^Dipp)(OtBu)₃(I)] ( 9 ), which represents an exceptionally rare example of a crystallographically authenticated uranium(VI)–carbon σ bond. Although 9 appears sterically saturated, it decomposes over time to give [U(BIPM^Dipp)(OtBu)₃]. Complex 4 reacts with PhCOtBu and Ph₂CO to form [U(BIPM^Dipp)(μ‐Cl)₄(Li)₂(tmeda)(OCPhtBu)] ( 10 ) and [U(BIPM^Dipp)(Cl)(μ‐Cl)₂(Li)(tmeda)(OCPh₂)] ( 11 ). In contrast, complex 5 does not react with PhCOtBu and Ph₂CO, which we attribute to steric blocking. However, complexes 5 and 6 react with PhCHO to afford (DippNPPh₂)₂C?C(H)Ph ( 12 ). Complex 9 does not react with PhCOtBu, Ph₂CO or PhCHO; this is attributed to steric blocking. Theoretical calculations have enabled a qualitative bracketing of the extent of covalency in early‐metal carbenes as a function of metal, oxidation state and the number of phosphanyl substituents, revealing modest covalent contributions to U?C double bonds.     

Di‐μ‐chlorido‐bis[dichloridobis(methylamido‐κN)bis(methylamine‐κN)titanium(IV)]

The title compound, [Ti₂Cl₆(C₂H₆N)₂(C₂H₇N)₂], is a binuclear octahedral complex lying about an inversion centre. There are four different chloride environments, two terminal [Ti—Cl = 2.2847 (5) and 2.3371 (5) Å] and two bridging [Ti—Cl = 2.4414 (5) and 2.6759 (5) Å], with the Ti—Cl distances being strongly influenced by both the ligand trans to the chloride and whether or not the chloride anion is bridging between the two Ti^IV centres. The compound forms a two‐dimensional network in the solid state, with weak intermolecular C—H...Cl interactions giving rise to a planar network in the (10) plane.     

Tris(dimethylamido)bis(dimethylamine)titanium(IV) chloridobis(dimethylamine)[tris(pentafluorophenyl)boron–amido][tris(pentafluorophenyl)boron–nitrido]titanate(IV) toluene solvate

The title ionic solid, [Ti(C₂H₆N)₃(C₂H₇N)₂][Ti(C₁₈BF₁₅N)(C₁₈H₂BF₁₅N)Cl(C₂H₇N)₂]·C₇H₈, (I), comprises a cation with three dimethylamide ligands in the equatorial plane and two dimethylamine ligands positioned axially in a trigonal–bipyramidal geometry about the central Ti^IV atom. The anion has a highly distorted octahedral structure. The two dimethylamine ligands are coordinated mutually trans. The chloride is trans to the tris(pentafluorophenyl)boron–amide, while the sixth coordination site is occupied by an ortho‐F atom of the tris(pentafluorophenyl)boron–amide group in a trans disposition with respect to the tris(pentafluorophenyl)boron–nitride ligand. The most significant feature of the anion is the presence of an unprecedented terminal Ti[triple‐bond]N moiety [1.665 (2) Å], stabilized by coordination to B(C₆F₅)₃, with a Ti[triple‐bond]N—B angle of 169.50 (19)°.     

Catalytic Dinitrogen Reduction to Ammonia at a Triamidoamine–Titanium Complex

Catalytic reduction of N₂ to NH₃ by a Ti complex has been achieved, thus now adding an early d‐block metal to the small group of mid‐ and late‐d‐block metals (Mo, Fe, Ru, Os, Co) that catalytically produce NH₃ by N₂ reduction and protonolysis under homogeneous, abiological conditions. Reduction of [Ti^IV(Tren^TMS)X] (X=Cl, 1A ; I, 1B ; Tren^TMS=N(CH₂CH₂NSiMe₃)₃) with KC₈ affords [Ti^III(Tren^TMS)] ( 2 ). Addition of N₂ affords [{(Tren^TMS)Ti^III}₂(μ‐η¹:η¹‐N₂)] ( 3 ); further reduction with KC₈ gives [{(Tren^TMS)Ti^IV}₂(μ‐η¹:η¹:η²:η²‐N₂K₂)] ( 4 ). Addition of benzo‐15‐crown‐5 ether (B15C5) to 4 affords [{(Tren^TMS)Ti^IV}₂(μ‐η¹:η¹‐N₂)][K(B15C5)₂]₂ ( 5 ). Complexes 3 – 5 treated under N₂ with KC₈ and [R₃PH][I], (the weakest H⁺ source yet used in N₂ reduction) produce up to 18 equiv of NH₃ with only trace N₂H₄. When only acid is present, N₂H₄ is the dominant product, suggesting successive protonation produces [{(Tren^TMS)Ti^IV}₂(μ‐η¹:η¹‐N₂H₄)][I]₂, and that extruded N₂H₄ reacts further with [R₃PH][I]/KC₈ to form NH₃.     

Synthesis,Structures and DFT Studies of Imido‐bridged and (Bis)ligand‐coordinated Titanium Complexes

Syntheses and structures of five imido‐bridged dinuclear titanium complexes and two (bis)ligand‐coordinated mononuclear titanium complexes are reported. Addition of 1 or 2 equiv. of Schiff base ligand (((1H‐pyrrol‐2‐yl)methylene)amino)‐2,3‐dihydro‐1H‐inden‐2‐ol (H₂L) to Ti(NMe₂)₄ resulted in transamination with 4 equiv. of dimethylamides generating a (bis)ligand‐coordinated complex Ti(L)₂ ( 1 ). Treatment of Ti(NMe₂)₄ with 1 equiv. of ^tBuNH₂ followed by addition of 1 equiv. of H₂L afforded an imido‐bridged complex [Ti(L)(N^tBu)]₂ ( 2 ). 1:1:1:1 reaction of Ti(NMe₂)₄/RNH₂/H₂L/py(or phen) produced imido‐bridgedcomplexes [Ti(L)(NPh)(py)]₂ ( 3 ), [Ti(L)(4‐F‐PhN)(py)]₂·Tol ( 4 ·Tol), [Ti(L)(4‐Cl‐PhN)(py)]₂·Tol·THF ( 5 ·Tol·THF), [Ti(L)(4‐Br‐PhN)(py)]₂·Tol ( 6 ·Tol) and a (bis)ligand‐coordinated complex Ti(L)₂·phen ( 7 ) (py = pyridine, phen = 1,10‐phenanthroline). Attempts to prepare the monomeric titianium imido complexes were unsuccessful. DFT studies show that the assumed compound which contains Ti = N species is less stable than imido‐bridged Ti‐N(R)‐Ti complexes, providing the better understanding of the experimental results.     

Cyaarside (CAs−) and 1,3‐Diarsaallendiide (AsCAs2−) Ligands Coordinated to Uranium and Generated via Activation of the Arsaethynolate Ligand (OCAs−)

Reaction of the trivalent uranium complex [((^Ad,MeArO)₃N)U(DME)] with one molar equiv [Na(OCAs)(dioxane)₃], in the presence of 2.2.2‐crypt, yields [Na(2.2.2‐crypt)][{((^Ad,MeArO)₃N)U^IV(THF)}(μ‐O){((^Ad,MeArO)₃N)U^IV(CAs)}] ( 1 ), the first example of a coordinated η¹‐cyaarside ligand (CAs^?). Formation of the terminal CAs^? is promoted by the highly reducing, oxophilic U^III precursor [((^Ad,MeArO)₃N)U(DME)] and proceeds through reductive C?O bond cleavage of the bound arsaethynolate anion, OCAs^?. If two equiv of OCAs^? react with the U^III precursor, the binuclear, μ‐oxo‐bridged U₂^IV/IV complex [Na(2.2.2‐crypt)]₂[{((^Ad,MeArO)₃N)U^IV}₂(μ‐O)(μ‐AsCAs)] ( 2 ), comprising the hitherto unknown μ:η¹,η¹‐coordinated (AsCAs)^2? ligand, is isolated. The mechanistic pathway to 2 involves the decarbonylation of a dimeric intermediate formed in the reaction of 1 with OCAs^?. An alternative pathway to complex 2 is by conversion of 1 via addition of one further equiv of OCAs^?.     

Hydrosilylation in Aryliminopyrrolide‐Substituted Silanes

A range of silanes was synthesized by the reaction of HSiCl₃ with iminopyrrole derivatives in the presence of NEt₃. In certain cases, intramolecular hydrosilylation converts the imine ligand into an amino substituent. This reaction is inhibited by factors such as electron‐donating substitution on Si and steric bulk. The monosubstituted (^DippIMP)SiHMeCl (^DippIMP=2‐[N‐(2,6‐diisopropylphenyl)iminomethyl]pyrrolide), is stable towards hydrosilylation, but slow hydrosilylation is observed for (^DippIMP)SiHCl₂. Reaction of two equivalents of ^DippIMPH with HSiCl₃ results in the hydrosilylation product (^DippAMP)(^DippIMP)SiCl (^DippAMP=2‐[N‐(2,6‐diisopropylphenyl)aminomethylene]pyrrolide), but the trisubsitituted (^DippIMP)₃SiH is stable. Monitoring the hydrosilylation reaction of (^DippIMP)SiHCl₂ reveals a reactive pathway involving ligand redistribution reactions to form the disubstituted (^DippAMP)(^DippIMP)SiCl as an intermediate. The reaction is strongly accelerated in the presence of chloride anions.     

Bonding in titanocenyl complexes containing O,O′‐cyclic ligands

Density functional theory calculations show that the formal 16‐electron count of d⁰ [Cp₂Ti^IV(O,O′‐BID)]^0/1 complexes containing a O,O′‐chelated bidentate ligand O,O′‐BID of different ring size, is increased via Ti←O π bonding when both the O donor atoms carry a formal negative charge. The Ti←O π bonding occurs by symmetry lowering of the complex by either symmetrical (C_s) or unsymmetrical (C₂) folding of the O,O′‐BID ligand round the O···O axis. An NBO analysis confirms the Ti←O π charge transfer via back‐bonding. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Dianionic Titanyl and Vanadyl (Cation+)2[MIVO(Pc4−)]2− Phthalocyanine Salts Containing Pc4− Macrocycles

In this study, the titanyl and vanadyl phthalocyanine (Pc) salts (Bu₄N⁺)₂[M^IVO(Pc^4?)]^2? (M=Ti, V) and (Bu₃MeP⁺)₂[M^IVO(Pc^4?)]^2? (M=Ti, V) with [M^IVO(Pc^4?)]^2? dianions were synthesized and characterized. Reduction of M^IVO(Pc^2?) carried out with an excess of sodium fluorenone ketyl in the presence of Bu₄N⁺ or Bu₃MeP⁺ is exclusive to the phthalocyanine centers, forming Pc^4? species. During reduction, the metal +4 charge did not change, implying that Pc is an non‐innocent ligand. The Pc negative charge increase caused the C?N(pyr) bonds to elongate and the C?N(imine) bonds to alternate, thus increasing the distortion of Pc. Jahn–Teller effects are significant in the [eg(π*)]² dianion ground state and can additionally distort the Pc macrocycles. Blueshifts of the Soret and Q‐bands were observed in the UV/Vis/NIR when M^IVO(Pc^2?) was reduced to [M^IVO(Pc ^{. 3?})] ^{. ?} and [M^IVO(Pc^4?)]^2?. From magnetic measurements, [Ti^IVO(Pc^4?)]^2? was found to be diamagnetic and (Bu₄N⁺)₂[V^IVO(Pc^4?)]^2? and (Bu₃MeP⁺)₂[V^IVO(Pc^4?)]^2? were found to have magnetic moments of 1.72–1.78 μ_B corresponding to an S=1/2 spin state owing to V^IV electron spin. As a result, two latter salts show EPR signals with V^IV hyperfine coupling.     

Intramolecular Gas‐Phase Reactions of Synthetic Nonheme Oxoiron(IV) Ions: Proximity and Spin‐State Reactivity Rules

The intramolecular gas‐phase reactivity of four oxoiron(IV) complexes supported by tetradentate N₄ ligands ( L ) has been studied by means of tandem mass spectrometry measurements in which the gas‐phase ions [Fe^IV(O)( L )(OTf)]⁺ (OTf=trifluoromethanesulfonate) and [Fe^IV(O)( L )]²⁺ were isolated and then allowed to fragment by collision‐induced decay (CID). CID fragmentation of cations derived from oxoiron(IV) complexes of 1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane (tmc) and N,N′‐bis(2‐pyridylmethyl)‐1,5‐diazacyclooctane ( L ⁸Py₂) afforded the same predominant products irrespective of whether they were hexacoordinate or pentacoordinate. These products resulted from the loss of water by dehydrogenation of ethylene or propylene linkers on the tetradentate ligand. In contrast, CID fragmentation of ions derived from oxoiron(IV) complexes of linear tetradentate ligands N,N′‐bis(2‐pyridylmethyl)‐1,2‐diaminoethane (bpmen) and N,N′‐bis(2‐pyridylmethyl)‐1,3‐diaminopropane (bpmpn) showed predominant oxidative N‐dealkylation for the hexacoordinate [Fe^IV(O)( L )(OTf)]⁺ cations and predominant dehydrogenation of the diaminoethane/propane backbone for the pentacoordinate [Fe^IV(O)( L )]²⁺ cations. DFT calculations on [Fe^IV(O)(bpmen)] ions showed that the experimentally observed preference for oxidative N‐dealkylation versus dehydrogenation of the diaminoethane linker for the hexa‐ and pentacoordinate ions, respectively, is dictated by the proximity of the target C? H bond to the oxoiron(IV) moiety and the reactive spin state. Therefore, there must be a difference in ligand topology between the two ions. More importantly, despite the constraints on the geometries of the TS that prohibit the usual upright σ trajectory and prevent optimal σ_CH–σ* overlap, all the reactions still proceed preferentially on the quintet (S=2) state surface, which increases the number of exchange interactions in the d block of iron and leads thereby to exchange enhanced reactivity (EER). As such, EER is responsible for the dominance of the S=2 reactions for both hexa‐ and pentacoordinate complexes.     

Facile and Reversible Formation of Iron(III)–Oxo–Cerium(IV) Adducts from Nonheme Oxoiron(IV) Complexes and Cerium(III)

Ceric ammonium nitrate (CAN) or Ce^IV(NH₄)₂(NO₃)₆ is often used in artificial water oxidation and generally considered to be an outer‐sphere oxidant. Herein we report the spectroscopic and crystallographic characterization of [(N4Py)Fe^III‐O‐Ce^IV(OH₂)(NO₃)₄]⁺ ( 3 ), a complex obtained from the reaction of [(N4Py)Fe^II(NCMe)]²⁺ with 2 equiv CAN or [(N4Py)Fe^IV=O]²⁺ ( 2 ) with Ce^III(NO₃)₃ in MeCN. Surprisingly, the formation of 3 is reversible, the position of the equilibrium being dependent on the MeCN/water ratio of the solvent. These results suggest that the Fe^IV and Ce^IV centers have comparable reduction potentials. Moreover, the equilibrium entails a change in iron spin state, from S =1 Fe^IV in 2 to S =5/2 in 3 , which is found to be facile despite the formal spin‐forbidden nature of this process. This observation suggests that Fe^IV=O complexes may avail of reaction pathways involving multiple spin states having little or no barrier.     

Efficient Hydroboration of Esters and Nitriles Using a Quinazolinone-Supported Titanium(IV) Multitasking Catalyst

We report catalytic hydroboration of esters as well as nitriles under solvent-free and mild conditions using single titanium(IV) metal complex, [{κ²-C₆H₄C(O)N(ⁱPr)C(N-ⁱPr)=N}{κ³-(ⁱPr)N=C(O)−C₆H₄−NC(NMe₂)N(ⁱPr)}TiNMe₂] 1 as a sustainable, economical, and efficient pre-catalyst. The molecular structure of the Ti^IV complex in the solid state reveals the unique coordination of Ti^IV metal with N, N, and O atoms of one quinazolinone unit via in-situ rearrangement, while another quinazolinone moiety coordinates in bidentate fashion via both N atoms only. The Ti^IV complex demonstrates excellent activity as a pre-catalyst towards the hydroboration of a wide array of esters and nitriles with pinacolborane (HBpin) to afford alkoxyboranes and diboryl amines in high yield (up to 99 %) with greater tolerance to a variety of electron-withdrawing and electron-donating functional groups. A most plausible mechanism of hydroboration of esters is also proposed based on kinetics and NMR studies, which suggests the formation of titanium-hydride species as an active catalyst.     

A Titanium(IV)‐Based Metal–Organic Framework Featuring Defect‐Rich Ti‐O Sheets as an Oxidative Desulfurization Catalyst

While titanium‐based metal–organic frameworks (MOFs) have been widely studied for their (photo)catalytic potential, only a few Ti^IV MOFs have been reported owing to the high reactivity of the employed titanium precursors. The synthesis of COK‐47 is now presented, the first Ti carboxylate MOF based on sheets of Ti^IVO₆ octahedra, which can be synthesized with a range of different linkers. COK‐47 can be synthesized as an inherently defective nanoparticulate material, rendering it a highly efficient catalyst for the oxidation of thiophenes. Its structure was determined by continuous rotation electron diffraction and studied in depth by X‐ray total scattering, EXAFS, and solid‐state NMR. Furthermore, its photoactivity was investigated by electron paramagnetic resonance and demonstrated by catalytic photodegradation of rhodamine 6G.     

The Weakly Coordinating Tris(trichlorosilyl)silyl Anion

Closely following the procedure for the preparation of the base‐stabilized dichlorosilylene complex NHC^Dipp⋅SiCl₂ reported by Roesky, Stalke, and co‐workers (Angew. Chem. Int. Ed . 2009 , 48 , 5683–5686), a few crystals of the salt [NHC^Dipp−H⋅⋅⋅Cl⋅⋅⋅H−NHC^Dipp]Si(SiCl₃)₃ were isolated, aside from the reported byproduct [NHC^Dipp−H⁺⋅⋅⋅Cl⁻], and characterized by X‐ray crystallography (NHC^Dipp=N,N‐di(2,6‐diisopropylphenyl)imidazo‐2‐ylidene). They contain the weakly coordinating anion Si(SiCl₃)₃⁻, which was also obtained in high yields upon deprotonation of the conjugate Brønsted acid HSi(SiCl₃)₃ with NHC^Dipp or PMP (PMP=1,2,2,6,6‐pentamethylpiperidine). The acidity of HSi(SiCl₃)₃ was estimated by DFT calculations to be substantially higher than those of other H‐silanes. Further DFT studies on the electronic structure of Si(SiCl₃)₃⁻, including the electrostatic potential and the electron localizability, confirmed its low basicity and nucleophilicity compared with other silyl anions.