Halo, Alkyl, Aryl, and Bis(imido) Complexes of Niobium Supported by the beta-Diketiminato Ligand

Metallaaromaticity involving a d0 early transition metal centre: synthesis, structure, and aromaticity of tantallapyridinazirine complexes

Though late transition metal aromatic metallabenzenes and related heteroatom-containing analogues have been well studied, the corresponding aromatic early transition metal complexes remain elusive. Herein, we demonstrate the synthesis of aromatic, planar, and delocalised organotantallapyridinium complexes via a simple one-pot process by sequential treatment of tantalum methyl complex [η5:σ-Me2C(C5H4)(C2B10H10)]TaMe3 with alkynes and isocyanide. Single-crystal X-ray analyses, NMR spectroscopic data and DFT calculations suggest that they are aromatic tantallapyridinium complexes, a class of long-sought-after molecules. This work would shed some light on the preparation of metallaaromatics involving early transition metals.

Observing Similarities and Differences in the Properties of Isostructural Niobium(V)/Tantalum(V) Coordination Compounds with Strong Pi-Donor Ligands

While niobium and tantalum are found together in their mineral ores, their respective applications in technology require chemical separation. Nb/Ta separations are challenging due to the similar reactivities displayed by these metals in the solution phase. Coordination complexes of these metals have been studied in the contexts of catalysis, small-molecule activation, and functional group insertion reactivity; relatively few studies exist directly comparing the properties of isostructural Nb/Ta complexes. Such comparisons advance the development of Nb/Ta separation chemistry through the potential for differential reactivity. Here, we explore fundamental physicochemical properties in extensively characterized Nb/Ta coordination complexes [Na(DME)3][MClamp], (Clamp6- = tris-(2-(3',5'-di-tert-butyl-2'-oxyphenyl)amidophenyl)amine; M = Nb, Ta) to advance the understanding of the different electronic, optical, and excited-state properties that these metals exhibit in pi-loaded coordination complexes.

Expanding the Utility of β-Diketiminate Ligands in Heavy Group VI Chemistry of Molybdenum and Tungsten

We report the synthesis of 17 molybdenum and tungsten complexes supported by the ubiquitous BDI ligand framework (BDI = β-diketiminate). The focal entry point is the synthesis of four molybdenum and tungsten(V) BDI complexes of the general formula [MO(BDI^R)Cl₂] [M = Mo, R = Dipp (1); M = W, R = Dipp (2); M = Mo, R = Mes (3); M = W, R = Mes (4)] synthesized by the reaction between MoOCl₃(THF)₂ or WOCl₃(THF)₂ and LiBDI^R. Reactivity studies show that the BDI^Dipp complexes are excellent precursors toward adduct formation, reacting smoothly with dimethylaminopyridine (DMAP) and triethylphosphine oxide (OPEt₃). No reaction with small phosphines has been observed, strongly contrasting the chemistry of previously reported rhenium(V) complexes. Additionally, the complexes 1 and 2 are good precursors for salt metathesis reactions. While 1 can be chemically reduced to the first stable example of a Mo(IV) BDI complex 15, reduction of 2 resulted in degradation of the BDI ligand via a nitrene transfer reaction, leading to MAD (4-((2,6-diisopropylphenyl)imino)pent-2-enide) supported tungsten(V) and tungsten(VI) complexes 16 and 17. All reported complexes have been thoroughly studied by VT-NMR and (heteronuclear) NMR spectroscopy, as well as UV-vis and EPR spectroscopy, IR spectroscopy, and X-ray diffraction analysis.

Reactivity and Structure of a Bis-phenolate Niobium NHC Complex

We report the facile synthesis of a rare niobium(V) imido NHC complex with a dianionic OCO-pincer benzimidazolylidene ligand (L ¹ ) with the general formula [NbL ¹ (N ^t Bu)PyCl] 1-Py. We achieved this by in situ deprotonation of the corresponding azolium salt [H ₃ L ¹ ][Cl] and subsequent reaction with [Nb(N ^t Bu)Py ₂ Cl ₃ ]. The pyridine ligand in 1-Py can be removed by the addition of B(C₆F₅)₃ as a strong Lewis acid leading to the formation of the pyridine-free complex 1. In contrast to similar vanadium(V) complexes, complex 1-Py was found to be a good precursor for various salt metathesis reactions, yielding a series of chalcogenido and pnictogenido complexes with the general formula [ NbL ¹ (N ^t Bu)Py(EMes)] (E = O (2), S (3), NH (4), and PH (5)). Furthermore, complex 1-Py can be converted to alkyl complex (6) with 1 equiv of neosilyl lithium as a transmetallation agent. Addition of a second equivalent yields a new trianionic supporting ligand on the niobium center (7) in which the benzimidazolylidene ligand is alkylated at the former carbene carbon atom. The latter is an interesting chemically "noninnocent" feature of the benzimidazolylidene ligand potentially useful in catalysis and atom transfer reactions. Addition of mesityl lithium to 1-Py gives the pyridine-free aryl complex 8, which is stable toward "overarylation" by an additional equivalent of mesityl lithium. Electrochemical investigation revealed that complexes 1-Py and 1 are inert toward reduction in dichloromethane but show two irreversible reduction processes in tetrahydrofuran as a solvent. However, using standard reduction agents, e.g., KC₈, K-mirror, and Na/Napht, no reduced products could be isolated. All complexes have been thoroughly studied by various techniques, including ¹H-, ¹³C{¹H}-, and ¹H-¹⁵N HMBC NMR spectroscopy, IR spectroscopy, and X-ray diffraction analysis.

Engendering reactivity at group 5-heteroatom multiple bonds via π-loading

In this Perspective, we discuss the strategy of π-loading, i.e., coordination of two or more strongly π-donating ligands to a single metal center, as it applies to promoting reactivity at group 5 transition metal-imido groups. When multiple π-donor ligands compete to interact with the same symmetrically-available metal dπ orbitals, the energy of the imido-based frontier molecular orbitals increases, leading to amplified imido-based reactivity. This strategy is of particular relevance to group 5 metals, as mono(imido) complexes of these metals tend to be inert at the imido group. Electronic structure studies of group 5 bis(imido) complexes are presented, and examples of catalytically and stoichiometrically active group 5 bis(imido) and chalcogenido-imido complexes are reviewed. These examples are intended to encourage future work exploring π-loaded bis(imido) systems of the group 5 triad.

[3 + 2] Cycloadditions and Retrocycloadditions of Niobium Imido Complexes: An Experimental and Computational Mechanistic Study

We demonstrate reactivity between a β-diketiminate-supported niobium(III) imido complex and alkyl azides to form niobatetrazene complexes (BDI)Nb(N^tBu)(RNNNNR) (BDI = N,N-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate; R = cyclohexyl (1), benzyl (2)). Intriguingly, niobatetrazene complexes 1 and 2 can be interconverted via addition of an appropriate alkyl azide, likely through a series of concerted [3 + 2] cycloaddition and retrocycloaddition reactions in which π-loaded bis(imido) intermediates are formed. The bis(imido) intermediates were trapped upon addition of alkyl isocyanides to yield five-coordinate bis(imido) complexes (BDI)Nb(N^tBu)(NCy)(CNR) (R = tert-butyl (4a), cyclohexyl (4b)). Two computational methods─density functional theory and density functional tight binding (DFTB)─were employed to calculate the lowest energy pathway across the potential energy surface for this multistep transformation. Reaction path calculations for individual cycloaddition or retrocycloaddition processes along the multistep reaction pathway showed that these transformations occur via a concerted, yet highly asynchronous mechanism, in which the two bond-breaking or -making events do not occur simultaneously. The use of the DFTB method in this work highlights its advantages and utility for studying transition metal systems.

A Diverse Array of C-C Bonds Formed at a Tantalum Metal Center

We demonstrate the formation of a diverse array of organic and organometallic products containing newly formed C-C bonds via successive methyl transfers from di-, tri-, and tetramethyl Ta(V) precursors to unsaturated small molecule substrates under mild conditions. The reactions of Ta(V) methyl complexes 1-X [H₂B(^MesIm)₂]TaMe₃X (X = Me, Cl; Im = imidazole, Mes = 2,4,6-trimethylphenyl) with CO led to oxo enolate Ta(V) products, in which the enolate ligands were constructed from Ta-Me groups and two equivalents of CO. Similarly, the reaction of 1-Me with CNXyl yielded an imido enamine Ta(V) product. Surprisingly, 1-Cl reacted with CNXyl (1 equiv) at the borate backbone of the [H₂B(^MesIm)₂] ligand with concomitant methyl transfer from the metal center to form a new, dianionic scorpionate ligand that supported a Ta(V) dimethyl chloro complex (6). Treatment of 1-Cl with further CNXyl led to an azaallyl scorpionate complex, and an imido isocyanide scorpionate complex, along with propene and xylyl ketenimine. Complex 6 reacted with CO to yield a pinacol scorpionate complex 10-a new reaction pathway in early transition metal chemistry. Mechanistic studies revealed that this proceeded via migratory insertion of CO into a Ta-Me group, followed by methyl transfer to form an η²-acetone intermediate. Elimination of acetone furnished a CO-stabilized Ta(III) intermediate capable of rebinding and subsequently coupling two equivalents of CO-derived acetone to form the pinacol ligand in 10.

1,2-Addition and cycloaddition reactions of niobium bis(imido) and oxo imido complexes

The bis(imido) complexes (BDI)Nb(N t Bu)2 and (BDI)Nb(N t Bu)(NAr) (BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate; Ar = 2,6-diisopropylphenyl) were shown to engage in 1,2-addition and [2 + 2] cycloaddition reactions with a wide variety of substrates. Reaction of the bis(imido) complexes with dihydrogen, silanes, and boranes yielded hydrido-amido-imido complexes via 1,2-addition across Nb-imido π-bonds; some of these complexes were shown to further react via insertion of carbon dioxide to give formate-amido-imido products. Similarly, reaction of (BDI)Nb(N t Bu)2 with tert-butylacetylene yielded an acetylide-amido-imido complex. In contrast to these results, many related mono(imido) Nb BDI complexes do not exhibit 1,2-addition reactivity, suggesting that π-loading plays an important role in activating the Nb-N π-bonds toward addition. The same bis(imido) complexes were also shown to engage in [2 + 2] cycloaddition reactions with oxygen- and sulfur-containing heteroallenes to give carbamate- and thiocarbamate-imido complexes: some of these complexes readily dimerized to give bis-μ-sulfido, bis-μ-iminodicarboxylate, and bis-μ-carbonate complexes. The mononuclear carbamate imido complex (BDI)Nb(NAr)(N( t Bu)CO2) (12) could be induced to eject tert-butylisocyanate to generate a four-coordinate terminal oxo imido intermediate, which could be trapped as the five-coordinate pyridine or DMAP adduct. The DMAP adducted oxo imido complex (BDI)NbO(NAr)(DMAP) (16) was shown to engage in 1,2-addition of silanes across the Nb-oxo π-bond; this represents a new reaction pathway in group 5 chemistry.

Facile Activation of Triarylboranes by Rhenium(V) Oxo Imido Complexes

We report the synthesis and reactivity studies of a pair of rhenium(V) oxo imido complexes. Oxidation of the rhenium(III) terminal oxo ORe(η²-DHF)(BDI) (DHF = dihydrofulvalene, BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) with organic azides R-N₃ (R = ^tBu, 2,6-diisopropylphenyl) yields the title complexes. Computational studies confirm that the rhenium oxo moieties of these complexes are polarized and correspondingly nucleophilic, owing to the preferential π bonding of the imido ligand to the Re center. This asymmetry in the metal-ligand multiple bond electronic structure facilitates the ready activation of B-C bonds in triarylboranes (BPh₃ and B(C₆F₅)₃), yielding rhenium(V) aryl borinate complexes. In the case of BPh₃, subsequent cyclometalation of the 1,2-addition products was found to take place upon heating, ejecting benzene to form bidentate diphenylborinate complexes.

Tungsten-Ligand Bond Strengths for 2p Elements Including σ- and π-Bond Strength Components, A Density Functional Theory and ab Initio Study

Three W^VI crystal structures with multifarious metal-ligand bond types are used to theoretically predict homolytic metal-element bond enthalpies with 11 popular DFT functionals, MP2 wave function methods, and four common valence basis set/pseudopotentials in order to evaluate the accuracy and precision of the resultant bond enthalpy data. To our knowledge, for the first time, estimates of component metal-ligand σ- and π-bond strengths are computed. The WE (E = C, N, O) bond enthalpies have the consistent trend σ > second π > first π. In contrast, the element-element BDE trend for the 2p homologues is second π > first π > σ for nitrogen and oxygen, and σ > first π > second π for carbon. These differences may underpin the differences in stability trends and thus reactivity behavior for metal-element multiple bonds as compared to the element-element multiple bonds, and metal-element triple bonds versus their corresponding double bonded counterparts. For example, Odom et al. show that MeI nucleophilically attacks at the imide (M═N) rather than the nitride (M ≡ N) ligand; the relative π-bond strengths derived herein provide a thermodynamic rationalization for this site preference. In this study, it is deduced from the calculated thermodynamics that the W-oxo ligand is more congruous with a triple bond than a double bond, consistent with the bonding model set forth in the seminal 1961 Ballhausen-Gray paper.

Modern applications of low-valent early transition metals in synthesis and catalysis

Low-valent early transition metals are often intrinsically highly reactive as a result of their strong propensity toward oxidation to more stable high-valent states. Harnessing these highly reducing complexes for productive reactivity is potentially powerful for C-C bond construction, organic reductions, small-molecule activation and many other reactions that offer orthogonal chemoselectivity and/or regioselectivity patterns to processes promoted by late transition metals. Recent years have seen many exciting new applications of low-valent metals through building new catalytic and/or multicomponent reaction manifolds out of classical reactivity patterns. In this Review, we survey new methods that employ early transition metals and invoke low-valent precursors or intermediates in order to identify common themes and strategies in synthesis and catalysis.

Ubiquity of cis-Halide → Isocyanide Direct Interligand Interaction in Organometallic Complexes

We recently reported a density functional theory (DFT) analysis of the Nb(V)-C bond in various NbCl₅(L) complexes, discovering that the carbon ligand L receives electronic density from the metal (classical back-donation) and from the chlorides in the cis position (direct interligand interaction). Here we report the synthesis and the structural characterization of two new coordination compounds of niobium pentahalides, i.e., NbX₅(CNXyl) (X = Cl, Br; Xyl = 2,6-C₆H₃Me₂), and the corresponding DFT analyses of the Nb(V)-C bond using the Natural Orbitals for Chemical Valence-Charge Displacement (NOCV-CD) approach, confirming the presence of a cis-halide → isocyanide direct interligand interaction. To verify whether the latter is limited to Nb complexes or not, we performed a NOCV-CD analysis on a series of several organometallic complexes based on Ti(IV), Nb(V), Ta(V), Rh(III), Pd(II), and Au(III), all of which bear one halide ligand and m-xylyl-isocyanide in a mutual cis position, revealing that the cis-halide → isocyanide interaction is always present.

Synthesis of (Arylmido)niobium(V) Complexes Containing Ketimide, Phenoxide Ligands, and Some Reactions with Phenols and Alcohols

Reactions of Nb(NAr)(N=C t Bu2)3 (3a, Ar = 2,6-Me2C6H3) with 1.0, 2.0, or 3.0 equiv of Ar'OH (Ar' = 2,6- i Pr2C6H3) afforded Nb(NAr)(N=C t Bu2)2(OAr'), Nb(NAr)(N=C t Bu2)(OAr')2, or Nb(NAr)(OAr')3, respectively (at 25 °C), whereas the reaction with 2.0 equiv of 2,6- t Bu2C6H3OH afforded Nb(NAr)(N=C t Bu2)2(O-2,6- t Bu2C6H3) upon heating (70 °C) without the formation of bis(phenoxide) and the reaction of 3a with 2.0 equiv of 2,4,6-Me3C6H2OH afforded Nb(NAr)(N=C t Bu2)(O-2,4,6-Me3C6H2)2(HN=C t Bu2). Similar reactions of 3a with 1.0 equiv of (CF3)3COH or 2.0 equiv of (CF3)2CHOH afforded Nb(NAr)(N=C t Bu2)2[OC(CF3)3](HN=C t Bu2) or Nb(NAr)(N=C t Bu2)[OCH(CF3)2]2(HN=C t Bu2), respectively. On the basis of their structural analyses and the reaction chemistry, it was suggested that these reactions proceeded via coordination of phenol (alcohol) to Nb and the subsequent proton (hydrogen) transfer to the ketimide (N=C t Bu2) ligand. The reaction of Nb(NAr)(N=C t Bu2)2(OAr') with 1.0 equiv of 2,4,6-Me3C6H2OH gave the disproportionation products Nb(NAr)(N=C t Bu2)(OAr')2 and Nb(NAr)(N=C t Bu2)(O-2,4,6-Me3C6H2)2(HN=C t Bu2) with 1:1 ratio, clearly indicating the presence of the above mechanism and the fast equilibrium (between the ketimide and the phenoxide). The reaction of 3a with 1.0 or 2.0 equiv of C6F5OH afforded Nb(N=C t Bu2)2(OC6F5)3(HN=C t Bu2) as the sole isolated product, which was formed from once generated Nb(NAr)(N=C t Bu2)2(OC6F5)(HN=C t Bu2) by treating with C6F5OH.

Olefin-Supported Rhenium(III) Terminal Oxo Complexes Generated by Nucleophilic Addition to a Cyclopentadienyl Ligand

The reactivity of the oxo Re^V β-diketiminate, OReCl₂ (BDI), with various cyclopentadienide (Cp) sources has been investigated. As a result, we have developed a route to a new class of terminal oxo complexes of Re^III supported by olefin moieties of substituted cyclopentadienes. The success of this pathway is due to the electrophilic nature of the Cp ligand in the cation, [ORe(η⁵ -Cp)(BDI)]⁺ (3⁺ ), which allows for nucleophilic attack by a variety of reagents under mild conditions. In contrast, ^t BuNC was found to attack at the oxo moiety to produce isocyanate by oxygen atom transfer.

Redox-Initiated Reactivity of Dinuclear β-Diketiminatoniobium Imido Complexes

High-valent dichloride and dimethylniobium complexes 1 and 2 bearing tert-butylimido and N,N'-bis(2,4,6-trimethylphenyl)-β-diketiminate (BDI^Ar) ligands were prepared. The dimethyl complex reacted with dihydrogen to release methane and generate the hydride-bridged diniobium(IV) complex 3 in high yield. One-electron oxidation of 3 with silver salts resulted in the release of dihydrogen and conversion to a mixed-valent Nb^III-Nb^IV complex, 4, that displayed a frozen-solution X-band electron paramagnetic resonance signal consistent with a slight dissymmetry between the two Nb centers. Spectroscopic and computational analysis supported the presence of Nb-Nb σ-bonding interactions in both 3 and 4. Finally, one-electron reduction of 4 resulted in conversion to the highly dissymmetric Nb^V-Nb^V dimer 5 that formed from the reductive C-N bond cleavage of one of the BDI^Ar supporting ligands.

Photo-activation of d(0) niobium imido azides: en route to nitrido complexes

We report the synthesis and photo-reactivity of d(0) niobium imido azido complexes supported by β-diketiminate ligands, which leads to the unprecedented formation of nitrides through a photo-assisted intramolecular rearrangement. This provides a new entry to metal nitrides that does not require low-valent metal centers and is a rare example in which the metal-imido moiety in group 5 complexes participates in reactivity.

Unusual κ1 coordination of a β-diketiminate ligand in niobium complexes

Reaction of (BDI)Nb(N(t)Bu)Cl2py with NaCp results in the κ(1)-coordination of the BDI ligand in the complex (κ(1)-N,BDI)CpNb(N(t)Bu)Cl (2). Via chloride abstraction from 2, we develop synthetic routes to structurally analogous cationic and Nb(IV) pseudo-four coordinate complexes where the BDI ligand returns to the κ(2)-coordination mode. We compare these to our previously reported tetrahedral niobium bis(imido) complexes to investigate the effects of the Cp ligand on the properties of Nb-BDI-imido systems. Substitution of the chloride in 2 with a hydride also causes return to bidentate binding of the BDI ligand. The X-ray crystal structures of these complexes have been determined, and the structural parameters reflecting the consequences of the electronic differences are discussed.

Coordination complexes of niobium and tantalum pentahalides with a bulky NHC ligand

The reactivity of niobium and tantalum pentahalides with a bulky NHC ligand is described, including the first crystallographic characterization of a Ta complex with a monodentate NHC ligand.

Group 5 chemistry supported by β-diketiminate ligands

β-Diketiminate (BDI) ligands are widely used supporting ligands in modern organometallic chemistry and are capable of stabilizing various metal complexes in multiple oxidation states and coordination environments.

On the non-innocence of “Nacnacs”: ligand-based reactivity in β-diketiminate supported coordination compounds

While β-diketiminate (BDI or ‘nacnac’) ligands have been widely adopted to stabilize a wide range of metal ions in multiple oxidation states and coordination numbers, in several occurrences these ligands do not behave as spectators and participate in reactivity.

Regioselective [2+2] and [4+2] cycloaddition reactivity in an asymmetric niobium(bisimido) moiety towards unsaturated organic molecules

The asymmetric bis-imido structure and the lability of the diethyl ether linkage in complex 1 provide a niobium complex that undergoes regioselective [4+2] cycloaddition reactions with an α,β-unsaturated ketone and cycloaddition reactions that involve bond formation to the MAD ligand (MAD = monoazabutadiene).

A computationally designed titanium-mediated amination of allylic alcohols for the synthesis of secondary allylamines

A direct amination on allylic alcohols under mild conditions was enlightened by computational investigations and implemented in secondary allylamines synthesis.

Activation of white phosphorus by low-valent group 5 complexes: formation and reactivity of cyclo-P4 inverted sandwich compounds

We report the synthesis and comprehensive study of the electronic structure of a unique series of dinuclear group 5 cyclo-tetraphosphide inverted sandwich complexes. White phosphorus (P4) reacts with niobium(III) and tantalum(III) β-diketiminate (BDI) tert-butylimido complexes to produce the bridging cyclo-P4 phosphide species {[(BDI)(N(t)Bu)M]2(μ-η(3):η(3)P4)} (1, M = Nb; 2, M = Ta) in fair yields. 1 is alternatively synthesized upon hydrogenolysis of (BDI)Nb(N(t)Bu)Me2 in the presence of P4. The trinuclear side product {[(BDI)NbN(t)Bu]3(μ-P12)} (3) is also identified. Protonation of 1 with [HOEt2][B(C6F5)4] does not occur at the phosphide ring but rather involves the BDI ligand to yield {[(BDI(#))Nb(N(t)Bu)]2(μ-η(3):η(3)P4)}[B(C6F5)4]2 (4). The monocation and dication analogues {[(BDI)(N(t)Bu)Nb]2(μ-η(3):η(3)P4)}{B(Ar(F))4}n (5, n = 1; 6, n = 2) are both synthesized by oxidation of 1 with AgBAr(F). DFT calculations were used in combination with EPR and UV-visible spectroscopies to probe the nature of the metal-phosphorus bonding.

Electron localization in a mixed-valence diniobium benzene complex

One electron oxidation of a neutral diniobium benzene complex leads to a mixed-valence species. Single crystal X-ray diffraction, EPR, L_3,2-edge XANES, and DFT indicate that the unpaired electron is localized on one metal center.

Reaction of the neutral diniobium benzene complex {[Nb(BDI)N^tBu]₂(μ-C₆H₆)} (BDI = N,N′-diisopropylbenzene-β-diketiminate) with Ag[B(C₆F₅)₄] results in a single electron oxidation to produce a cationic diniobium arene complex, {[Nb(BDI)N^tBu]₂(μ-C₆H₆)}{B(C₆F₅)₄}. Investigation of the solid state and solution phase structure using single-crystal X-ray diffraction, cyclic voltammetry, magnetic susceptibility, and multinuclear NMR spectroscopy indicates that the oxidation results in an asymmetric molecule with two chemically inequivalent Nb atoms. Further characterization using density functional theory (DFT) calculations, UV-visible, Nb L_3,2-edge X-ray absorption near-edge structure (XANES), and EPR spectroscopies supports assignment of a diniobium complex, in which one Nb atom carries a single unpaired electron that is not largely delocalized on the second Nb atom. During the oxidative transformation, one electron is removed from the δ-bonding HOMO, which causes a destabilization of the molecule and formation of an asymmetric product. Subsequent reactivity studies indicate that the oxidized product allows access to metal-based chemistry with substrates that did not exhibit reactivity with the starting neutral complex.

Iron and chromium complexes containing tridentate chelates based on nacnac and imino- and methyl-pyridine components: triggering C-X bond formation

Nacnac-based tridentate ligands containing a pyridyl-methyl and a 2,6-dialkyl-phenylamine (i.e., (2,6-R2-C6H3N═C(Me)CH═C(Me)NH(CH2py); R = Et, {Et(nn)PM}H; R = (i)Pr, {(i)Pr(nn)PM}H) were synthesized by condensation routes. Treatment of M{N(TMS)2}THFn (M = Cr, n = 2; M = Fe, Co, n = 1; TMS = trimethylsilane; THF = tetrahydrofuran) with {(i)Pr(nn)PM}H) afforded {(i)Pr(nn)PM}MN(TMS)2 (1-M(iPr); M = Cr, Fe); {Et(nn)PM}MN(TMS)2 (1-M(Et); M = Fe, Co) was similarly obtained. {R(nn)PM}FeBr (R = (i)Pr, Et; 2-Fe(R)) were prepared from FeBr2 and {R(nn)PM}Li, and alkylated to generate {R(nn)PM}Fe(neo)Pe (R = (i)Pr, Et; 3-Fe(R)). Carbonylation of 3-Fe(R) provided {(i)Pr(nn)PM}Fe(CO(neo)Pe)CO (4-Fe(iPr)), and carbonylations of 1-Fe(R) (R = Et, (i)Pr) and 1-Cr(iPr) induced deamination to afford {R(nn)PI}Fe(CO)2 (R = (i)Pr, 5-Fe(iPr); Et, 5-Fe(Et)), where PI is pyridine-imine, and {κ(2)-N,N-pyrim-pyr}Cr(CO)4 (6-Cr(iPr)), in which the aryl-amide side of the nacnac attacked the incipient PI group. Carbon-carbon bonds were formed at the imine carbon of the {R(nn)PI} ligand. Addition of [{(i)Pr(nn)PI}(2-)](K(+)(THF)x)2 to FeCl3 generated {(i)Pr(nn)CHpy}2Fe2Cl2 (7-Fe(iPr)), and TMSN3 induced the deamination of 1-Fe(Et), but with disproportionation to provide {[Et(nn)CHpy]2}Fe (8-Fe(Et)). Ph2CN2 induced C-C bond formation with 1-Fe(iPr) via its thermal degradation to ultimately afford {(i)Pr(nn)CHpy}2(FeN═CPh2)2 (9-Fe(iPr)). The compounds were examined by X-ray crystallography (1-M(iPr), M = Cr, Fe; 1-Co(Et); 2-Fe(iPr); 4-Fe(iPr); 5-Fe(iPr); 6-Cr(iPr); 7-Fe(iPr); 8-Fe(Et); 9-Fe(iPr)), Mössbauer spectroscopy, and NMR spectroscopy. Structural parameters assessing redox noninnocence are discussed, as are structural and mechanistic consequences of the various electronic environments.

Iron complexes derived from {nacnac-(CH2py)2}- and {nacnac-(CH2py)(CHpy)}n ligands: stabilization of iron(II) via redox noninnocence

Nacnac-based tetradentate chelates, {nacnac-(CH2py)2}(-) ({nn(PM)2}(-)) and {nacnac-(CH2py)(CHpy)}(n) ({nn(PM)(PI)}(n)) have been investigated in iron complexes. Treatment of Fe{N(TMS)2}2(THF) with {nn(PM)2}H afforded {nn(PM)2}FeN(TMS)2 [1-N(TMS)2], which led to {nn(PM)2}FeCl (1-Cl) from HCl and to {nn(PM)2}FeN3 (1-N3) upon salt metathesis. Dehydroamination of 1-N(TMS)2 was induced by L (L = PMe3, CO) to afford {nn(PM)(PI)}Fe(PMe3)2 [2-(PMe3)2] and {nn(PM)(PI)}FeCO (3-CO). Substitution of 2-(PMe3)2 led to {nn(PM)(PI)}Fe(PMe3)CO [2-(PMe3)CO], and exposure to a vacuum provided {nn(PM)(PI)}Fe(PMe3) (3-PMe3). Metathesis routes to {nn(PM)(PI)}FeL2 (2-L2; L = PMe3, PMe2Ph) and {nn(PM)(PI)}FeL (3-L; L = PMePh2, PPh3) from [{nn(PM)(PI)}(2-)]Li2 and FeBr2(THF)2 in the presence of L proved feasible, and 1e(-) and 2e(-) oxidation of 2-(PMe3)2 afforded 2(+)-(PMe3)2 and 2(2+)-(PMe3)2 salts. Mössbauer spectroscopy, structural studies, and calculational assessments revealed the dominance of iron(II) in both high-spin (1-X) and low-spin (2-L2 and 3-L) environments, and the redox noninnocence (RNI) of {nn(PM)(PI)}(n) [2-L2, 3-L, n = 2-; 2(+)-(PMe3)2, n = 1-; 2(2+)-(PMe3)2, n = 0]. A discussion regarding the utility of RNI in chemical reactivity is proffered.