Nacnac Ligand Research Articles

Modification of a Common β-diketiminate NacNac Framework via Sequential Lithiation and Small Molecule Insertion.

A widely utilised class of ligands in synthesis and catalysis, β-diketiminate (BDI) or NacNac compounds were initially considered innocent in the sense that they remained intact in all their applications. That changed when the γ-C-H unit of their NCCCN backbone was found to engage in reactions with electrophiles. Here, we show that this special reactivity can be used advantageously to prepare tripodal modifications of the common NacNac ligand derived from 2,6-diisopropylphenyl-β-methyldiketimine [NacNacH (Me, Dipp)]. Lithiation to give NacNacLi, followed by reactions with isocyanates, isothiocyanates and a carbodiimide, have afforded a series of tripodal NacNac variants having N,N,N,O; N,N,N,S; or N,N,N,N potential dentation sites, many of which have been crystallographically characterised. Distinct ligating modes of these new ligands have been elucidated through the crystal structures of their lithiated derivatives.

A series of four-coordinate heteroleptic copper(I) complexes with diimine and β-diketiminate ligands

In this paper, a series of heteroleptic bis-chelate copper(I) complexes of the general formula Cu(N^N)(NacNacR), where N^N is the diimine ligand and NacNacR is the β-diketiminate ligand are introduced. The complexes are easily prepared by reacting NacNac ligands with CuOtBu in the presence of diimine ligands. The molecular structures of all complexes are characterized by NMR spectroscopy and X-ray crystallography. The studies of cyclic voltammetry and UV–vis absorption spectra show that the HOMO and the LUMO energy level can be tailored by changing the substitution pattern of each ligand. The oxidation potential is dependent on the NacNac ligand, and the reduction potential shows the dependence on the diimine ligand. Tunable redox potentials and a wide range of visible absorption can make these copper(I) complexes promising candidates for applications in solar light harvesting.

Sterically Encumbered Heteroleptic Copper(I) β-Diketiminate Complexes with Extended Excited-State Lifetimes.

One of the main challenges in developing effective copper(I) photosensitizers is their short excited-state lifetimes, usually attributed to structural distortion upon light excitation. We have previously introduced copper(I) charge-transfer chromophores of the general formula Cu(N^N)(ArNacNac), where N^N is a conjugated diimine ligand and ArNacNac is a substituted β-diketiminate ligand. These chromophores were promising regarding their tunable redox potentials and intense visible absorption but were ineffective as photosensitizers, presumably due to short excited-state lifetimes. Here, we introduce sterically crowded analogues of these heteroleptic chromophores with bulky alkyl substituents on the N^N and/or ArNacNac ligand. Structural analysis was combined with electrochemical and photophysical characterization, including ultrafast transient absorption (UFTA) spectroscopy to investigate the effects of the alkyl groups on the excited-state lifetimes of the complexes. The molecular structures determined by single-crystal X-ray diffraction display more distortion in the ground state as alkyl substituents are introduced into the phenanthroline or the NacNac ligand, showing smaller τ4 values due to the steric hindrance. UFTA measurements were carried out to determine the excited-state dynamics. Sterically encumbered Cu5 and Cu6 display excited-state lifetimes 15-20 times longer than unsubstituted complex Cu1, likely indicating that the incorporation of bulky alkyl substituents inhibits the pseudo-Jahn-Teller (PJT) flattening distortion in the excited state. This work suggests that the steric properties of these heteroleptic copper(I) charge-transfer chromophores can be readily modified and that the excited-state dynamics are strongly responsive to these modifications.

Unusual formation of Ir(III) complexes with non‐symmetrical NacNac ligands: Synthesis, characterization, and evaluation of catalytic activity in transfer hydrogenation reduction reactions

A new type of trans‐dichloro‐Ir(III) complexes derived from non‐symmetrical NacNac‐type ligands was unexpectedly obtained as the main reaction product from the former ligands and the dimeric species [Ir(COD)Cl2]. One equivalent of LH was reacted with an excess of [Ir(COD)Cl]2 in dichloromethane or toluene as solvent and at room temperature. The general formula of the product is [IrCl2(COD)L] and was isolated as a sole product instead of the expected Ir(I) compound [Ir(COD)L]; all the new Ir(III) were prepared in high yields as microcrystalline solids. They were all stable under laboratory atmosphere, lasting for weeks in solution and for months in solid state. The structure of each compound was examined by 1D and 2D nuclear magnetic resonance (NMR) and high‐resolution mass spectrometry (HRMS). Complex 1k was selected for an X‐ray diffraction study. Finally, an evaluation was made of the catalytic activity of all complexes in the transfer hydrogenation reaction of ketones and imines.

Chiral‐at‐Molybdenum – A Computational Study on N‐Substituted cis‐MoO2(nacnac)2

AbstractCis‐MoO2(nacnac)2 (nacnac=β‐diketiminate) can be a viable chiral‐at‐metal catalyst when its two β‐diketiminate (nacnac) ligands are N‐substituted to prevent Λ⇌Δ racemization. Even simple methyl groups raise the barriers for racemization significantly for the Bailar, Rây‐Dutt, Conte‐Hippler, and Dhimba‐Muller‐Lammertsma (DML) twists to a respectable 28.5, 37.8, 30.5, and 25.1 kcal/mol, respectively, at ωB97X‐D/6‐311+G(2d,p) (LANL2DZ for Mo) including acetonitrile solvation. The lowest energy DML barrier increases to ΔGo=27.4 kcal/mol with N‐substituted t‐butyl groups. The nacnac ligands’ electronic and steric components to the racemization barriers are discussed.

Structural Diversity and Multielectron Reduction Reactivity of Samarium(II) Iodido-β-diketiminate Complexes Dependent on Tetrahydrofuran Content.

The molecular structures of complexes [Sm(Nacnac)I(thf)n] (Nacnac = HC(C(Me)Ndipp)2-, dipp = 2,6-diisopropylphenyl, thf = tetrahydrofuran) depending on the number of thf ligands are studied. The complete removal of thf from a known complex [Sm(Nacnac)I(thf)2] leads to a tetranuclear product [Sm(Nacnac)I]4 (4). The partial removal of thf results in mixtures of dinuclear [Sm2(Nacnac)2I2(thf)] (2), trinuclear [Sm3(Nacnac)3I3(thf)] (3), and tetranuclear [Sm4(Nacnac)4I4(thf)2] (4*) complexes and 4, depending on the conditions. The reaction of solvent-free SmI2 with 1 equiv of K(Nacnac) results mainly in [Sm(Nacnac)2] (1), while the interaction of 4 with certain amounts of thf allows obtaining pure 2 and 3 (with the admixture of 4*). Complex 4* is the exact dimer of 2, and both compounds are stable in solutions. Reactions with 3 and 4 as reductants are studied. 4 is oxidized by I2 to stoichiometrically yield two products, mixed-valent tetranuclear [Sm4(Nacnac)4I5] (5) and binuclear [Sm(Nacnac)I2]2 (6) complexes. In the reaction of 4 with nBu3PTe, a trinuclear complex [Sm3(Nacnac)3(μ-I)3(μ3-E)2] (8, E = I or Te) is formed in small amounts, with the formation of 6 as the second product. 3 serves as a two-electron reductant in the reaction with nBu3PTe to yield a trinuclear complex [Sm3(Nacnac)3I3(μ-Te2)] (7). Complexes 2, 4, 4*, 5, 6, and 8 possess a unique flat SmxIy core of heavy atoms, which is assumed to be a consequence of the Nacnac ligand geometry.

Crystallographic Characterisation of Organolithium and Organomagnesium Intermediates in Reactions of Aldehydes and Ketones

AbstractWe report reactions of LiNacNac and NacNacMg(TMP) with common organic substrates (TMP=2,2,6,6‐tetramethylpiperidide). Using bulky β‐diketiminate compound NacNac (Me, Dipp), we have trapped metalated intermediates amenable to X‐ray crystallographic study. LiNacNac and acetone produced the NacNac‐free hexameric diacetone alkoxide [{LiO(MeC(=O)CH2C(Me)2}6]; whereas pinacolone gave the simple monomeric donor–acceptor complex [Li{(MeCN‐2,6‐iPr2C6H3)2CH}{O=C(Me)tBu}]. Benzaldehyde produced dimeric [Li{(MeCN‐2,6‐iPr2C6H3)2CC(=O)Ph}{O=C(Ph)OCH2Ph}]2, where one benzaldehyde molecule has inserted in to the γ‐carbon position of the NacNac ligand, with a molecule of benzyl benzoate derived from a Tishchenko reaction of the aldehyde acting as a terminal donor to lithium. Tetranuclear, mixed dimer [{LiO(MeCN‐2,6‐iPr2C6H3)2CH‐CH(Ph)}2{LiO(PhC(=O)}2] containing the same modified NacNac ligand was the fortuitous product from repeating the reaction of LiNacNac and benzaldehyde in a solution presumably contaminated with benzoic acid. Combining NacNacMg(TMP) with pinacolone affords monomeric heteroleptic [Mg{(MeCN‐2,6‐iPr2C6H3)2CH}{OC(tBu)(Me)CH2C(=O)tBu}] with an aldolate constituting the second anion. Completing the set is dimeric [Mg{(MeCN‐2,6‐iPr2C6H3)2CH}{O(C=CH2)Ph}2], where the enolate derived from acetophenone bridges the magnesium centres.

Group 13 Heavier Carbene Analogues Stabilized by the Bulky Bis(4-benzhydryl-benzoxazol-2-yl)methanide Ligand.

On the basis of the bulky bis(4-benzhydryl-benzoxazyl-2-yl)methane ligand (4-BzhH2Box2CH2), neutral monovalent group 13 complexes [M13(4-BzhH2Box2CH)] [M13 = Tl (1), In (2), or Ga (3)] have been synthesized by salt metathesis reaction of the corresponding potassium or sodium precursor and TlOTf, InOTf, or "GaI". The diiodido gallium species [GaI2(4-BzhH2Box2CH)] (3a) was realized as a byproduct once the synthesis of 3 was carried out at higher temperatures. The synthesis of [AlI2(4-BzhH2Box2CH)] (6) as a potential precursor for an aluminum(I) congener was accomplished by two alternative synthetic routes. During one of those procedures, [AlMe2(4-BzhH2Box2CH)] (4) was synthesized in good yields by deprotonation with an AlMe3 solution (method A). Subsequently, 4 was converted to the monoiodinated species [AlMeI(4-BzhH2Box2CH2)] (5) using 1 equiv of I2 or to 6 by iodination with 2 equiv of I2 at 70 °C for 4 days. As an alternative, complex 6 could be prepared by iodination of 1 equiv of I2 and [AlH2(4-BzhH2Box2CH)] (7), which was previously obtained by facile reaction of 4-BzhH2Box2CH2 and AlH3NMe2Et. All main products 1-7 were completely characterized by nuclear magnetic resonance spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray structure determination. Alane 7 was additionally analyzed by solid-state fluorescence spectroscopy. Density functional theory calculations on [M13(4-BzhH2Box2CH)] [M13 = Tl (1), In (2), Ga (3), or Al] revealed that the complexes consist of monovalent group 13 cations coordinated by an anionic (4-BzhH2Box2CH) ligand similar to metallacycles incorporating a NacNac ligand.

Expected and Unexpected Reactivities of Homoleptic LiNacNac and Heteroleptic NacNacMg(TMP) β-Diketiminates toward Various Small Unsaturated Organic Molecules.

Homoleptic LiNacNac forms simple donor–acceptor complexes with N,N′-dicyclohexylcarbodiimide (CyN=C=NCy), triphenylphosphine oxide (Ph3P=O), and benzophenone (Ph2CO). These crystallographically characterized compounds could be regarded as model intermediates en route to reducing the N=C, P=O, and C=O bonds of unsaturated substrates. Heteroleptic NacNacMg(TMP) intriguingly functions as a TMP nucleophile both with t-BuNCO and t-BuNCS, producing a urea or thiourea derivative respectively attached to Mg, though the NacNac ligand in the former reaction also engages noninnocently with a second t-BuNCO molecule via insertion at the reactive NacNac backbone γ-carbon site.

Selective Cross-Coupling of Unsaturated Substrates on AlI.

The AlI compound NacNacAl (1, NacNac = [ArNC(Me)CHC(Me)NAr]- , Ar = 2,6-iPr2 C6 H3 ) serves as a template for the chemoselective coupling between carbonyls (benzophenone, fenchone, isophorone, p-tolyl benzoate, N,N-dimethylbenzamide, (1-phenylethylidene)aniline) and pyridine. With the CH-acidic ketone (1R)-(+) camphor, the reaction affords a hydrido alkoxide compound of Al, formed as the result of enolization, whereas an enolizable imine, (1-phenylethylidene)aniline, and the bulky ketone isophorone, still chemoselectively couple with pyridine. In contrast, reaction with the ester p-tolyl benzoate results in cleavage of the ester bond together with replacement of the alkoxy group by a hydrogen atom of the pyridine moiety. This study demonstrates that for carbonyl substrates featuring phenyl substituents, the reaction proceeds via intermediate formation of η2 (C,X)-coordinated (X = O, N) carbonyl adducts, whereas the reaction of 1 with (R)-(-)-fenchone in the absence of pyridine leads to CH activation in the pendant isopropyl group of the Ar substituent of the NacNac ligand.

Synthesis and luminescence studies of lanthanide complexes (Gd, Tb, Dy) with phenyl- and 2-pyridylthiolates supported by a bulky β-diketiminate ligand. Impact of the ligand environment on terbium(iii) emission

Structure-dependent photoluminescence was studied for lanthanide complexes with widespread Nacnac ligands, supplemented by thiolates; notable antenna properties of the latter are revealed.

Backbone Reactivity of Lithium β-Diketiminate (NacNac) Complexes with CO2 , tBuNCO and iPrNCO.

Though alkali metal NacNac (β-diketiminate) complexes have been utilised in synthesis as NacNac-transfer agents, studies of them in their own right with small molecules are exceptionally rare. Here, the lithium compound of the common 2,6-diisopropylphenyl-β-methyldiketiminate [NacNac(Dipp, Me)] ligand is investigated with carbon dioxide and isocyanates. In all four cases reaction occurs at the backbone γ-C atom of the NacNac ligand, which redistributes electronically into a diimine. Insertion of CO2 gives an eight-atom carboxylate (Li2 O4 C2 ) ring at the γ-C site in a dimer. Insertion of tBuNCO gives a secondary amide at the γ-C site in a monomer with TMEDA chelating lithium. Double insertion of tBuNCO and (adventitious) oxygen gives a dimer with a (LiO)2 central core involving the latter source. Insertion of less bulky (iPrNCO) gives a dimer with dimerisation through the C=O bonds of the emergent secondary amide function.

Synthesis and Characterization of Strong Cyclometalated Iridium Photoreductants for Application in Photocatalytic Aryl Bromide Hydrodebromination

A series of potent bis-cyclometalated iridium photoreductants with electron-rich β-diketiminate (NacNac) ancillary ligands is described. Structure–property analysis reveals that substituent modification of the NacNac ligands has a large effect on the ground-state IrIV/IrIII potential, which shifts cathodically as the NacNac is made more electron-rich. In addition, the excited-state IrIV/*IrIII potentials are ca. 300–500 mV more negative than that of fac-Ir(ppy)3 (ppy = 2-phenylpyridine), indicating that these compounds have much more reducing excited states. Rate constants for excited-state electron transfer between these photosensitizers and benzophenone are ∼2–3 times faster than fac-Ir(ppy)3, demonstrating that these complexes are both kinetically and thermodynamically more potent for excited-state electron transfer. We use these photosensitizers to optimize a simple reaction procedure for photocatalytic debromination of aryl bromide substrates, which requires only the photosensitizer, blue light, and an amine base, without silanes or other additives that are used in previously reported methods.

Bis(benzoxazol‐2‐yl)methanes Hounding NacNac: Varieties and Applications in Main Group Metal Coordination

Inspired by the well‐established class of β‐diketiminate or NacNac ligand scaffolds, the systematic building of ligand platforms mimicking its structural features and shielding abilities represents an exciting challenge. One of our focuses in the past decade has been the development of bis(benzheterocyclo)methanes, ‐amines and –phosphanes featuring additional donor‐sites in the ligand backbone as well as increasing steric bulk adjacent to the coordination pocket. This article addresses the variety of obtained ligand scaffolds, highlights some applications in 1, 2 and 13 metal coordination and looks to future developments of even more steric demanding systems to finally complement the famous Dipp‐substituted NacNac system.

Aluminum(III) Halide Complexes based on a Bis‐(benzoxazol‐2‐yl)methane Ligand

Low‐valent aluminum(I) heterocycles that are specified as metal carbenes can be applied as catalysts and in small molecule activation. Thus, the synthesis of new types of metal carbenes is a rewarding task we set out to contribute. The reactive aluminum compounds should be obtained by reduction of an aluminum(III) halide precursor coordinated to a suitable ligand. The bis(benzoxazol‐2‐yl)methanide ligand HCBOX2 (BOX = benzoxazol‐2‐yl) mimics the beneficial features of the omni‐present nacnac ligand. Two new aluminum halide complexes [(HCBOX2)AlX2] (X = Cl, I) were synthesized in a salt elimination reaction with the corresponding metal halide and the lithiated ligand [(HCBOX2)Li(Et2O)2], derived from H2CBOX2 [bis‐(benzoxazol‐2‐yl)methane]. The complexes [(HCBOX2)AlCl2] (1) and [(HCBOX2)AlI2] (2) were characterized by NMR spectroscopy from solution, mass spectrometry and single‐crystal X‐ray diffraction. 1 crystallizes in the monoclinic space group P21/n and crystals of complex 2 were obtained in the triclinic crystal system with space group P1. Especially [(HCBOX2)AlI2] (2) is a promising precursor for the synthesis of new low‐valent aluminum(I) compounds because especially the Al–I bond is prone to reduction.

Lutetium functionalities supported by a sterically encumbered β-diketiminate ligand

The sterically encumbered NacNac ligand, [HC(MeCNAr)2]− (Ar = 2,6-i-Pr2C6H3), was investigated as a platform for supporting Lu-halide complexes, sought for their potential capability of being further converted into hydrocarbyl derivatives via metathetical chemistries with alkali metal alkyls. These substituted analogs were targeted as potentially viable candidates for alkane elimination chemistries, with an eye towards the formation of an isolable Lu-alkylidene fragment.

Introducing NacNac-Like Bis(4,6-isopropylbenzoxazol-2-yl)methanide in s-Block Metal Coordination.

Within this work, the field of bulky methanides in metal coordination is exceeded by the synthesis of the versatile and promising bis(4,6-isopropylbenzoxazol-2-yl)methane (7) ligand platform. As an enhancement in this class of ligands, isopropyl (iPr) substituents as steric-demanding groups have been successfully introduced in proximity to the coordination pocket, mimicking the shielding abilities of the ubiquitous NacNac ligand scaffold to improve the steric protection of a coordinated s-block metal cation. A percent buried volume (% Vbur) calculation as well as an electronic structure analysis shades light onto the shielding and electronic abilities of the ligand in comparison to other selected methanides and diketiminates. Upon deprotonation with a variety of different group 1 and 2 metalation agents, a row of novel s-block metal complexes of the parent deprotonated monoanionic ligand 7 was obtained and structurally, as well as spectroscopically, characterized. In particular, in this context, the alkali-metal precursor complexes [Li(THF)2{(4,6-iPr-NCOC6H2)2CH}] (8) and [K{μ-(4,6-iPr-NCOC6H2)2CH}]∞ (9) as well as the alkaline-earth-metal compounds [MgCl(THF)2{(4,6-iPr-NCOC6H2)2CH}] (10) and [M(THF)n{(4,6-iPr-NCOC6H2)2CH}2] [M = Mg, n = 0 (11); M = Ca, n = 1 (12); M = Sr, n = 1 (13); M = Ba, n = 1 (14)] were successfully synthesized. Especially, the latter four exhibit interesting trends in the solid state as well as in solution within the metal series.

Oxidative Cleavage of the C═N Bond on Al(I).

Reaction of the cyclic guanidine TolN═SIMe with the aluminum(I) compound NacNacAl (1) results in the unprecedented cleavage of the C-N multiple bond to give, after rearrangement, the carbene-ligated Al(III) amide, NacNac'Al(NHTol)(SIMe) (6). DFT calculations revealed that these reactions proceed via a bimolecular mechanism in which either the basic Al(I) center or the transient Al═NTol species deprotonates the methyl group of the NacNac ligand.

Why Low Valent Lead(II) Hydride Complex Would be a Better Catalyst for CO2 Activation than Its 14 Group Analogues?

A low valent Pb(II) hydride complex with NacNac ligand (NacNac = [ArNC(Me)CHC (Me)NAr]−, with Ar = 2,6–iPr2C6H3) is predicted to be the best catalyst for CO2 activation compared to its Ge(II) and Sn(II) analogues, which have been experimentally reported (Jana, A., et al. J. Am. Chem. Soc. 2009, 131, 1288;Jana, A. et al. Angew. Chem. Int. Ed. 2009, 48, 1106). The CO2 activation mechanism mediated by Ge(II), Sn(II) and Pb(II) catalysts was studied in toluene using density functional theory calculations. The results show that the activation can be carried out through two reaction pathways, giving rise to two different conformers which have been computationally predicted for the first time. In all the cases, the activation process was thermodynamically favored, in addition, the Pb(II) catalyst exhibited the lowest activation energy, compared with Ge(II), Sn(II), and even Si(II). Thus, it was found that, going down in group 14, the reactivity of the NacNac based complexes toward CO2 activation increases consid...

Unusual Reactions of NacNacAl with Urea and Phosphine Oxides.

The reaction of cyclic urea 1,3-dimethyl-2-imidazolidinone with the aluminum(I) compound NacNacAl (1) gives an unexpected adduct of urea with the isomerized aluminum(III) hydride NacNac'AlH(O═SIMe) (3). A related reaction of 1 with phosphine oxides results in cleavage of the P═O bond and formation of hydroxyl derivatives NacNac'Al(OH)(O═PR3) [R = Ph (5) and Et (6)]. Density functional theory calculations revealed that these reactions proceed via a bimolecular mechanism in which either the basic aluminum(I) center or the transient Al═O species deprotonate the methyl group of the NacNac ligand.