Mechanistic elucidation of the yttrium(III)-catalysed intramolecular aminoalkene hydroamination: DFT favours a stepwise σ-insertive mechanism

A comprehensive computational mechanistic study regarding intramolecular hydroamination (HA) of aminoalkenes mediated by a recently reported class of highly active cyclopentadienyl-bis(oxazolinyl)borate {Cpo}Y(III) alkyl compounds is presented. Two distinct mechanistic pathways of catalytic HA mediated by rare earth and alkaline earth compounds have emerged over the years, describing amidoalkene → cycloamine conversion proceeding through a stepwise σ-insertive mechanism or a concerted non-insertive N-C/C-H bond forming pathway. Notably, both mechanisms account equally for reported distinct process features. Non-competitive kinetic demands revealed for the concerted amino proton transfer associated with N-C ring closure, which commences from a {Cpo(M)}Y(NHR)·(NH(2)R) substrate adduct and evolves through a six-centre TS structure, militates against a proton-triggered non-insertive pathway to promote HA for the rare earth catalyst at hand. A stepwise σ-insertive pathway, featuring rapid and reversible olefin insertion into the Y-N amido σ-bond, linked to a less facile and irreversible intramolecular Y-C azacycle tether aminolysis, is found to prevail energetically. The assessed effective barrier for turnover-limiting aminolysis matches the empirically determined Eyring parameter well and the computationally estimated primary KIE is close to the observed values. A recent computational study revealed a similar scenario for an analogous tris(oxazolinyl)borate {To(M)}Mg system. Valuable insights into the catalytic structure-reactivity relationships have been unveiled by a comparison of {Cpo(M)}Y(NHR)- and {To(M)}Mg(NHR)-catalysed hydroaminations.     

Does a Concerted Non‐Insertive Mechanism Prevail over a σ‐Insertive Mechanism in Catalytic Cyclohydroamination by Magnesium Tris(oxazolinyl)phenylborate Compounds? A Computational Study

The present study comprehensively explores alternative mechanistic pathways for intramolecular hydroamination of 2,2-dimethyl-4-penten-1-amine (1) by [{To(M)}MgMe] (To(M)=tris(4,4-dimethyl-2-oxazolinyl)phenylborate) (2) with the aid of density functional theory (DFT) calculations. A single-step amidoalkene → cycloamine conversion through a concerted proton transfer associated with N-C ring closure has been explored as one possible mechanism; its key features have been described. This non-insertive pathway evolves via a six-centre TS structure featuring activation of the olefin unit towards nucleophilic amido attack outside the immediate vicinity of the metal centre by amino proton delivery and describes a viable mechanistic variant for alkaline-earth metal-mediated aminoalkene hydroamination. However, herein is presented sound evidence for the operation of the Mg-N amido σ-bond insertive mechanism, its turnover-limiting activation barrier is found to be 5.0 kcal mol(-1) lower than for the non-insertive mechanism, for the cyclohydroamination of 2,2-disubstituted 4-aminoalkenes by a [{To(M)}Mg-NHR] catalyst. The operative mechanism involves rapid equilibria of the {To(M)}Mg-amidoalkene resting state 3 with its amine adduct, easily accessible and thermodynamically disfavoured, hence reversible, 1,2-olefin insertion into the Mg-N amido σ-bond with ring closure at 3, linked to turnover-limiting Mg-C azacycle tether aminolysis by an adduct substrate molecule, followed by facile cycloamine liberation to regenerate the active catalyst species 3. The following aspects are in support of this scenario: 1) the derived rate law is consistent with the experimentally obtained empirical rate law; 2) the reasonable agreement between the computationally estimated and the observed value of the primary KIE; 3) the assessed effective activation barrier for turnover-limiting aminolysis matches empirically determined Eyring parameters remarkably well; and 4) the observed resistance of isolated 3 to undergo amidoalkene cycloamine/cycloamido transformation until further quantities of substrate is added is consistently explained. The herein unveiled insights into the structure-reactivity relationships will undoubtedly govern the rational design of alkaline-earth metal-based catalysts and likely facilitate further advances in the area.     

Experimental and Computational Mechanistic Studies of the β-Diketiminatoiron(II)-Catalysed Hydroamination of Primary Aminoalkenes

A comprehensive mechanistic study by means of complementary experimental and computational approaches of the exo-cyclohydroamination of primary aminoalkenes mediated by the recently reported β-diketiminatoiron(II) complex B is presented. Kinetic analysis of the cyclisation of 2,2-diphenylpent-4-en-1-amine ( 1 a ) catalysed by B revealed a first-order dependence of the rate on both aminoalkene and catalyst concentrations and a primary kinetic isotope effect (KIE) (k_H/k_D) of 2.7 (90 °C). Eyring analysis afforded ΔH^≠=22.2 kcal mol⁻¹, ΔS^≠=−13.4 cal mol⁻¹ K⁻¹. Plausible mechanistic pathways for competitive avenues of direct intramolecular hydroamination and oxidative amination have been scrutinised computationally. A kinetically challenging proton-assisted concerted N−C/C−H bond-forming non-insertive pathway is seen not to be accessible in the presence of a distinctly faster σ-insertive pathway. This operative pathway involves 1) rapid and reversible syn-migratory 1,2-insertion of the alkene into the Fe−N_amido σ bond at the monomer {N^N}Fe^II amido compound; 2) turnover-limiting Fe−C σ bond aminolysis at the thus generated transient {N^N}Fe^II alkyl intermediate and 3) regeneration of the catalytically competent {N^N}Fe^II amido complex, which favours its dimer, likely representing the catalyst resting state, through rapid cycloamine displacement by substrate. The collectively derived mechanistic picture is consonant with all empirical data obtained from stoichiometric, catalytic and kinetics experiments.     

Cyclam functionalization through isocyanate insertion in Zr-N bonds

The insertion of isocyanates in (Bn(2)Cyclam)ZrX(2) is regioselective; (Bn(2)Cyclam)Zr(OR)(2) produces urea-like moieties by the insertion of RN═C═O in the Zr-N(amido) bonds of the cyclam ring. Depending on the bulkiness of the isocyanate R groups, O- and N-bound ureates are formed. (Bn(2)Cyclam)Zr(NH(t)Bu)(2) reacts with MesN═C═O at the terminal Zr-N bonds.     

Mechanistic elucidation of intramolecular aminoalkene hydroamination catalyzed by a tethered bis(ureate) complex: evidence for proton-assisted C-N bond formation at zirconium

A broad mechanistic investigation regarding hydroamination reactions catalyzed by a tethered bis(ureate) zirconium species, [ureate(2-)]Zr(NMe(2))(2)(HNMe(2)), is described. The cyclization of both primary and secondary aminoalkene substrates gives similar kinetic profiles, with zero-order dependence on substrate concentration up to ～60-75% conversion, followed by first-order dependence for the remainder of the reaction. The addition of 2-methylpiperidine changes the observed substrate dependence to first order throughout the reaction, but does not act as a competitive inhibitor. The reactions are first order in precatalyst up to loadings of ～0.15 M, indicating that a well-defined, mononuclear catalytic species is operative. Several model complexes have been structurally characterized, including dimeric imido and amido species, and evaluated for catalytic performance. These results indicate that imido species need not be invoked as catalytically relevant intermediates, and that the mono(amido) complex [ureate(2-)]Zr(NMe(2))(Cl)(HNMe(2)) is much less active than its bis(amido) counterpart. Structural evidence suggests that this is due to differences in coordination geometry between the mono- and bis(amido) complexes, and that an equatorial amido ligand is required for efficient catalytic turnover. On the basis of the determination of kinetic isotope effects and stoichiometric reactivity, the catalytic turnover-limiting step is proposed to be a concerted C-H, C-N bond-forming process with a highly ordered, unimolecular transition state (ΔS(?) = -21 ± 1 eu). In addition to this key bond-forming step, the catalytic cycle involves an on-cycle pre-equilibrium between six- and seven-coordinate intermediates, leading to the observed switch from zero- to first-order kinetics.     

C-N bond formation via ligand-induced nucleophilicity at a coordinated triamidoamine ligand

Reaction of (N(3)N)ZrX complexes (X = amido, Cl(-), CH(3)(-)) with carbodiimide substrates results in insertion into an Zr-N bond of the triamidoamine ligand rather than the Zr-X bond as has been observed for related (N(3)N)ZrX complexes (X = PR(2)(-), AsR(2)(-)).     

Mechanism of Al(III)-catalyzed transamidation of unactivated secondary carboxamides

The carbon-nitrogen bond of secondary carboxamides is generally thermodynamically and kinetically unreactive; however, we recently discovered that the trisamidoaluminum(III) dimer Al2(NMe2)6 catalyzes facile transamidation between simple secondary carboxamides and primary amines under moderate conditions. The present report describes kinetic and spectroscopic studies that illuminate the mechanism of this unusual transformation. The catalytic reaction exhibits a bimolecular rate law with a first-order dependence on the Al(III) and amine concentrations. No rate dependence on the carboxamide concentration is observed. Spectroscopic studies (1H and 13C NMR, FTIR) support a catalyst resting state that consists of a mixture of tris-(kappa2-amidate)aluminum(III) complexes. These results, together with the presence of a significant kinetic isotope effect when deuterated amine substrate (RND2) is used, implicate a mechanism in which the amine undergoes preequilibrium coordination to aluminum and proton transfer to a kappa2-amidate ligand to yield an Al(kappa2-amidate)2(kappa1-carboxamide)(NHR) complex, followed by rate-limiting intramolecular delivery of the amido ligand (NHR) to the neutral Al(III)-activated kappa1-carboxamide. Noteworthy in this mechanism is the bifunctional character of Al(III), which is capable of activating both the amine nucleophile and the carboxamide electrophile in the reaction.     

Primary amido substituted diborane4 compounds and imidodiborate4 anions

Treatment of the diborane(4) compound B(2)(NMe(2))(4) with aniline or 2,6-dimethylaniline results in the primary amido compounds B(2)(NHR)(4)(R = Ph, 2,6-Me(2)C(6)H(3)); subsequent treatment with n-BuLi in toluene in each case affords the first examples of anionic imidodiborates namely Li(4)(thf)(6)B(2)(NPh)(4) and Li(4)(thf)(4)B(2)(N-2,6-Me(2)C(6)H(3))(4); all complexes have been characterised crystallographically.     

Is an M?N σ‐Bond Insertion Route a Viable Alternative to the MN [2+2] Cycloaddition Route in Intramolecular Aminoallene Hydroamination/Cyclisation Catalysed by Neutral Zirconium Bis(amido) Complexes? A Computational Mechanistic Study

This study examines alternative reaction channels for intramolecular hydroamination/cyclisation (IHC) of primary 4,5-hexadien-1-ylamine aminoallene (1) by a neutral [Cp(2)ZrMe(2)] zirconocene precatalyst (2) by using the density functional theory (DFT) method. The first channel proceeds through a [Cp(2)Zr(NHR)(2)] complex as the reactive species and relevant steps including the insertion of an allenic C=C linkage into the Zr--NHR sigma-bond and ensuing protonolysis. This is contrasted to the [2+2] cycloaddition mechanism involving a [Cp(2)Zr=NR] transient species. The salient features of the rival mechanisms are disclosed. The cycloaddition route entails the first transformation of the dormant [Cp(2)Zr(NHR)(2)] complex 3 B into the transient [Cp(2)Zr=NR] intermediate 3 A', which is turnover limiting. This route features a highly facile ring closure together with a substantially slower protonolysis (k(cycloadd)>k(protonolysis)) and can display inhibition by high substrate concentration. In contrast, protonolysis is the more facile step for the channel proceeding through the [Cp(2)Zr(NHR)(2)] complex as the catalytically active species. Here, C=C insertion into the Zr--C sigma-bond of 3 B, which represents the catalyst resting state, is turnover limiting and substrate concentration is unlikely to influence the rate. The regulation of the selectivity is elucidated for the two channels. DFT predicts that five-ring allylamine and six-ring imine are generated upon traversing the cycloaddition route, thereby comparing favourably with experiment, whereas the cycloimine should be formed solely along the sigma-bond insertion route. The mechanistic analysis is indicative of an operating [2+2] cycloaddition mechanism. The Zr--NHR sigma-bond insertion route, although appearing not to be employed for the reactants studied herein, is clearly suggested as being viable for hydroamination by charge neutral organozirconium compounds.     

Reactions of (PCP)Ru(CO)(NHPh)(PMe3) (PCP = 2,6-(CH2PtBu2)2C6H3) with substrates that possess polar bonds

The Ru(II) amido complex (PCP)Ru(CO)(PMe(3))(NHPh) (1) (PCP = 2,6-(CH(2)P(t)Bu(2))(2)C(6)H(3)) reacts with compounds that possess polar C=N, C triple bond N, or C=O bonds (e.g., nitriles, carbodiimides, or isocyanates) to produce four-membered heterometallacycles that result from nucleophilic addition of the amido nitrogen to an unsaturated carbon of the organic substrate. Based on studies of the reaction of complex 1 with acetonitrile, the transformations are suggested to proceed by dissociation of trimethylphosphine, followed by coordination of the organic substrate and then intramolecular N-C bond formation. In the presence of ROH (R = H or Me), the fluorinated amidinate complex (PCP)Ru(CO)(N(Ph)C(C(6)F(5))NH) (6) reacts with excess pentafluorobenzonitrile to produce (PCP)Ru(CO)(F)(N(H)C(C(6)F(5))NHPh) (7). The reaction with MeOH also produces o-MeOC(6)F(4)CN (>90%) and p-MeOC(6)F(4)CN (<10%). Details of the solid-state structures of (PCP)Ru(CO)(F)(N(H)C(C(6)F(5))NHPh) (7), (PCP)Ru(CO)[PhNC{NH(hx)}N(hx)] (8), (PCP)Ru(CO){N(Ph)C(NHPh)O} (9), and (PCP)Ru(CO){OC(Ph)N(Ph)} (10) are reported.     

Mechanistic investigation of intramolecular aminoalkene and aminoalkyne hydroamination/cyclization catalyzed by highly electrophilic, tetravalent constrained geometry 4d and 5f complexes. Evidence for an M-N sigma-bonded insertive pathway

A mechanistic study of intramolecular hydroamination/cyclization catalyzed by tetravalent organoactinide and organozirconium complexes is presented. A series of selectively substituted constrained geometry complexes, (CGC)M(NR2)Cl (CGC = [Me2Si(eta5-Me4C5)(tBuN)]2-; M = Th, 1-Cl; U, 2-Cl; R = SiMe3; M = Zr, R = Me, 3-Cl) and (CGC)An(NMe2)OAr (An = Th, 1-OAr; An = U, 2-OAr), has been prepared via in situ protodeamination (complexes 1-2) or salt metathesis (3-Cl) in high purity and excellent yield and is found to be active precatalysts for intramolecular primary and secondary aminoalkyne and aminoalkene hydroamination/cyclization. Substrate reactivity trends, rate laws, and activation parameters for cyclizations mediated by these complexes are virtually identical to those of more conventional (CGC)MR2 (M = Th, R = NMe2, 1; M = U, R = NMe2, 2; M = Zr, R = Me, 3), (Me2SiCp' '2)UBn2 (Cp' ' = eta5-Me4C5; Bn = CH2Ph, 4), Cp'2AnR2 (Cp' = eta5-Me5C5; R = CH2SiMe3; An = Th, 5, U, 6), and analogous organolanthanide complexes. Deuterium KIEs measured at 25 degrees C in C6D6 for aminoalkene D2NCH2C(CH3)2CH2CHCH2 (11-d2) with precatalysts 2 and 2-Cl indicate that kH/kD = 3.3(5) and 2.6(4), respectively. Together, the data provide strong evidence in these systems for turnover-limiting C-C insertion into an M-N(H)R sigma-bond in the transition state. Related complexes (Me2SiCp' '2)U(Bn)(Cl) (4-Cl) and Cp'2An(R)(Cl) (R = CH2(SiMe3); An = Th, 5-Cl; An = U, 6-Cl) are also found to be effective precatalysts for this transformation. Additional arguments supporting M-N(H)R intermediates vs M=NR intermediates are presented.     

Mechanistic investigation of organolanthanide-mediated hydroamination of aminoallenes: a comprehensive computational assessment of various routes for allene activation

The present mechanistic study comprehensively explores alternative scenarios for activation of the amine-linked allene C=C linkage toward nucleophilic amido attack in the intramolecular hydroamination of a prototypical 1,3-disubstituted aminoallene by a well-characterised samarocene-amido catalyst. Firstly, the non-insertive mechanism by Scott featuring C-N ring closure with concomitant amino proton delivery onto the allene unit has been explored and its key features have been defined. This scenario has been compared and contrasted with the classical stepwise Ln-N σ-bond insertive mechanism that involves rapid substrate association/dissociation equilibria for the Ln-amido-substrate resting state and also for Ln-azacycle intermediates, facile and reversible exocyclic ring closure through migratory allene insertion into the Ln-N amido σ-bond, linked to turnover-limiting Ln-C azacycle aminolysis. The Ln-N σ-bond insertive mechanism prevails for the studied intramolecular hydroamination of 4,5-heptadien-1-ylamine 1 by [Cp*(2)Sm-CH(TMS)(2)] starting material 2. The following aspects are in support of this mechanism: 1) the derived rate law is consistent with the observed empirical rate law; 2) the assessed effective barrier for turnover-limiting aminolysis does agree reasonably well with empirically determined Eyring parameters; 3) the ring-tether double bond selectivity is consistently elucidated. On the other hand, this study reveals that the non-insertive mechanism, which features a prohibitively large barrier, is unachievable. Spatial demands around the lanthanide centre effect the two mechanisms differently. A sufficiently accessible lanthanide is a crucial requirement of the Ln-N σ-bond insertive mechanism and enhanced encumbrance renders the cyclisation step less accessible kinetically. This contrasts with the non-insertive mechanism, where greater lanthanide protection has a rather modest influence. The present study indicates that the non-insertive mechanism would prevail if the lanthanide centre were to be protected effectively against C=C bond approach, whilst ensuring a high polarity of the Ln-N σ-bond together with a sufficiently acidic amino proton.     

Intramolecular hydroamination/cyclisation of aminoallenes mediated by a neutral zirconocene catalyst: a computational mechanistic study

The complete catalytic cycle for the intramolecular hydroamination/cyclisation (IHC) of 4,5-hexadien-1-ylamine (1) by a prototypical [ZrCp(2)Me(2)] precatalyst (2) has been scrutinized by employing a reliable DFT method. The present study conducted by means of a detailed computational characterisation of structural and energetic aspects of alternative pathways for all of the relevant elementary steps complements the mechanistic insights revealed from experimental results. The operative mechanism entails an initial transformation of precatalyst 2 into the thermodynamically prevalent, but dormant, bis(amido)-Zr compound in the presence of aminoallene 1. This complex undergoes a reversible, rate-determining alpha-elimination of 1 to form the imidoallene-Zr complex. The substrate-free form, which contains a chelating imidoallene functionality, is the catalytically active species and is rapidly transformed into azazirconacyclobutane intermediates through a [2+2] cycloaddition reaction. This highly facile process does not proceed regioselectively because the alternative pathways for the formation of five- and six-membered azacycles have comparable probabilities. Degradation of cyclobutane intermediates by following the most feasible pathway occurs through protonolysis of the metallacycle moiety and subsequent proton transfer from the Zr-NHR moiety onto the azacycle. The five-membered allylamine is generated through protonation at carbon atom C(6) followed by alpha-hydrogen elimination, whereas protonolysis of the cyclobutane moiety at the Zr-N bond followed by proton transfer onto carbon atom C(5) is the dominant route for the six-membered product. Of the two consecutive proton transfer steps, the second one determines the overall kinetics of the entire protonation sequence. This process is predicted to be substantially slower than the cycloaddition reaction. The factors that regulate the composition of the cycloamine products have been elucidated.     

"Oxidative addition" to a Zirconium(IV) redox-active ligand complex

A strategy to enable reactivity analogous to oxidative addition is presented for d(0) transition-metal complexes. The reaction of the redox-active ligand 2,4-di-tert-butyl-6-tert-butylamidophenolate (ap) with ZrCl(4)(THF)(2) affords the new complex Zr(IV)(ap)(2)(THF)(2). This compound is formally zirconium(IV) and contains no d electrons; however, exposure of Zr(IV)(ap)(2)(THF)(2) to chlorine gas results in swift chlorine addition at the zirconium metal center via one-electron oxidation of each ap ligand. The diradical product, Zr(IV)Cl(2)(isq)(2) (isq = 2,4-di-tert-butyl-6-tert-butyliminosemiquinone), has been characterized by X-ray crystallography, electron paramagnetic resonance spectroscopy, and SQUID magnetometery.     

Metal-ligand cooperation in catalytic intramolecular hydroamination: a computational study of iridium-pyrazolato cooperative activation of aminoalkenes

The present study comprehensively explores diverse mechanistic pathways for intramolecular hydroamination of prototype 2,2-dimethyl-4-penten-1-amine by Cp*Ir chloropyrazole (1; Cp*=pentamethylcyclopentadienyl) in the presence of KOtBu base with the aid of density functional theory (DFT) calculations. The most accessible mechanistic pathway for catalytic turnover commences from Cp*Ir pyrazolato (Pz) substrate adduct 2?S, representing the catalytically competent compound and proceeds via initial electrophilic activation of the olefin C=C bond by the metal centre. It entails 1) facile and reversible anti nucleophilic amine attack on the iridium-olefin linkage; 2) Ir-C bond protonolysis via stepwise transfer of the ammonium N-H proton at the zwitterionic [Cp*IrPz-alkyl] intermediate onto the metal that is linked to turnover-limiting, reductive, cycloamine elimination commencing from a high-energy, metastable [Cp*IrPz-hydrido-alkyl] species; and 3) subsequent facile cycloamine liberation to regenerate the active catalyst species. The amine-iridium bound 2?a?S likely corresponds to the catalyst resting state and the catalytic reaction is expected to proceed with a significant primary kinetic isotope. This study unveils the vital role of a supportive hydrogen-bonded network involving suitably aligned β-basic pyrazolato and cycloamido moieties together with an external amine molecule in facilitating metal protonation and reductive elimination. Cooperative hydrogen bonding thus appears pivotal for effective catalysis. The mechanistic scenario is consonant with catalyst performance data and furthermore accounts for the variation in performance for [Cp*IrPz] compounds featuring a β- or γ-basic pyrazolato unit. As far as the route that involves amine N-H bond activation is concerned, a thus far undocumented pathway for concerted amidoalkene → cycloamine conversion through olefin protonation by the pyrazole N-H concurrent with N-C ring closure is disclosed as a favourable scenario. Although not practicable in the present system, this pathway describes a novel mechanistic variant in late transition metal-ligand bifunctional hydroamination catalysis that can perhaps be viable for tailored catalyst designs. The insights revealed herein concerning the operative mechanism and the structure-reactivity relationships will likely govern the rational design of late transition metal-ligand bifunctional catalysts and facilitate further conceptual advances in the area.     

Investigation of Zr-C,Zr-N,and potential agostic interactions in an organozirconium complex by experimental electron density analysis

The crystal structure and electron density (ED) distribution of an imine coupling product with an open zirconocene, Zr(2,4-C(7)H(11))[(i-Pr)NCHPhCH(2)CMe=CHCMe=CH(2)] (C(7)H(11) = dimethylpentadienyl), have been derived from accurate synchrotron X-ray diffraction measurements. The molecular structure reveals asymmetric coordination of Zr by the pentadienyl (2,4-C(7)H(11)) ligand ( = 2.56(6) A), the butadiene fragment ( = 2.43(5) A), and the amide nitrogen atom (Zr-N = 2.0312(5) A) of the second ligand. The study of the ED and its topological analysis affords new insight into the bonding and electronic structure of the title zirconium complex. The interactions between the metal center and the ligands are represented by a Zr-N bond path and one Zr-C bond path with each of the pentadienyl and butadiene moieties, contrary to the usually depicted global metal-ligand bonding. The butadiene and pentadienyl groups exhibit a polarization of the corresponding pi-like ED in the C-Zr directions, indicating that the whole conjugated systems are nonetheless involved. The 4d atomic orbitals of Zr exhibit unusual populations according to ligand field considerations, which reveal a high degree of sigma-donation from the conjugated pi systems of both ligands. As deduced from numerical integration over the topologically defined atomic basins, the Zr to ligand charge transfer is 1.48 e to the C(17)NH(24) ligand and 0.68 e to dimethylpentadienyl. Topological analysis of a short intramolecular Zr.(C,H) contact provides no indication of the presence of agostic interactions, despite a small Zr-N-C angle of 102.87(4) degrees. Thus, no bond path and BCP (bond critical point) of the ED are found in the Zr.(C,H) region, which would have provided evidence for such direct interactions, nor is there any evidence for charge accumulation between the Zr and H atoms, or for lengthening of the C-H bond involved in the putative interaction. These characteristics, similar to those in other distorted situations, may be common for other electron-deficient d(0) complexes.     

A crystalline σ complex of copper

Over the last decades, our understanding of σ-bond activation at transition metals has progressed considerably from both fundamental and synthetic points of view thanks to the preparation and characterization of a variety of σ complexes. Here we report the synthesis and structural analysis of the first σ complex involving a coinage metal. The copper(I) complex 2 derived from the diphosphine-disilane [Ph(2)P(C(6)H(4))Me(2)Si-SiMe(2)(C(6)H(4))PPh(2)] (1) has been isolated and crystallographically characterized. The coordination of the Si-Si σ bond to copper was thoroughly analyzed by quantum-chemical methods.     

Chiral biarylamido/anisole complexes of yttrium in enantioselective aminoalkene hydaroamination/cyclisation

A group of chiral, dibasic, biaryl-bridged amido proligands containing peripheral methoxyphenyl (anisole) ligation are developed for the synthesis of new amide complexes of yttrium and lanthanum. A potentially tetradentate bis(amidoanisole) system gives, on reaction with [Y[N(SiMe(2)H)(2)](3)(THF)] a crystallographically-characterised bis complex [Y(H)] presumably as a result of low steric demand, since a more bulky version gives the target [Y[N(SiMe(2)H)(2)](THF)]. The molecular structure of the latter reveals a similar cis-alpha structure to our recently reported Schiff-base analogue. Variable-temperature NMR studies are consistent with low rigidity in the molecular structure. A potentially tridentate, amidoanisolyl/amido proligand gives complexes [M[N(SiMe(2)H)(2)](THF)(n)](M = Y, n= 1; M = La, n= 2). Chiral non-racemic versions of the above complexes were tested in the hydroamination/cyclisation of 2,2'-dimethylaminopentane to the corresponding pyrrolidine. Activities were relatively low compared to recently reported examples, and ee values were in the range 20-40% despite the well-expressed chirality of the catalysts.     

Structure and dynamics of dinuclear zirconium(IV) complexes

We have determined by X-ray crystallography the structures of three dinuclear zirconium(IV) complexes containing the heptadentate ligand dhpta (where H(5)dhpta = 1,3-diamino-2-propanol-N,N,N',N'-tetraacetic acid, 1) and different countercations: K(2)[Zr(2)(dhpta)(2)].5H(2)O (2.5H(2)O), Na(2)[Zr(2)(dhpta)(2)].7H(2)O.C(2)H(5)OH (3.7H(2)O.C(2)H(5)OH), and Cs(2)[Zr(2)(dhpta)(2)].H(5)O(2).Cl.4H(2)O (4.H(5)O(2).Cl.4H(2)O). In the K(I) complex 2, crystallized from water, the two Zr(IV) ions are 3.5973(4) A apart and bridged via two alkoxo groups (average Zr-O 2.165 A). Each Zr(IV) is eight-coordinate and also bound to two N atoms (average Zr-N 2.448 A), and four carboxylate O atoms (average Zr-O 2.148 A). The two dhpta ligands in the dinuclear unit have different conformations. One face of the complex contains an array of 14 oxygen atoms and interacts strongly with the two K(I) ions, one of which is 6-coordinate, the other 8-coordinate, which are 3.922(4) A apart and bridged by a carboxylate O and by two water molecules. The structures of the dinuclear anion [Zr(2)(dhpta)(2)](2-) in the Na(I) complex 3 and in the Cs(I) complex 4 are essentially identical to that found in complex 2, although the alkali metal ions coordinate differently to the oxygen-rich face. All Zr(IV) ions have a distorted triangulated dodecahedral geometry. Although the crystal structure of complex 2 does not indicate the presence of acidic protons, in 4 an [H(5)O(2)](+) unit is strongly H-bonded to an oxygen atom of a coordinated carboxylate group. 1D and 2D (1)H and (13)C NMR spectroscopic and potentiometric studies reveal two deprotonations with pK(a) values of 9.0 and 10.0. At low pH, two carboxylate groups appear to undergo protonation accompanied by chelate ring-opening, and the complex exhibits dynamic fluxional behavior in which the two magnetically nonequivalent dhpta ligands exchange at a rate of 11 s(-1) at pH 3.30, 298 K, as determined from 2D EXSY NMR studies. Ligand interchange is not observed at high pH (>11). The same crystals of complex 2 were obtained from solutions at pH 3 or 12. The dynamic configurational change is therefore mediated by the aqueous solvent.     

Substrate‐Dependent H/D Kinetic Isotope Effects and the Role of the Di(μ‐oxo)diiron(IV) Core in Soluble Methane Monooxygenase: A Theoretical Study

Soluble methane monooxygenase (sMMO) is an enzyme that converts alkanes to alcohols using a di(μ‐oxo)diiron(IV) intermediate Q at the active site. Very large kinetic isotope effects (KIEs) indicative of significant tunneling are observed for the hydrogen transfer (H‐transfer) of CH₄ and CH₃CN; however, a relatively small KIE is observed for CH₃NO₂. The detailed mechanism of the enzymatic H‐transfer responsible for the diverse range of KIEs is not yet fully understood. In this study, variational transition‐state theory including the multidimensional tunneling approximation is used to calculate rate constants to predict KIEs based on the quantum‐mechanically generated intrinsic reaction coordinates of the H‐transfer by the di(μ‐oxo)diiron(IV) complex. The results of our study reveal that the role of the di(μ‐oxo)diiron(IV) core and the H‐transfer mechanism are dependent on the substrate. For CH₄, substrate binding induces an electron transfer from the oxygen to one Fe^IV center, which in turn makes the μ‐O ligand more electrophilic and assists the H‐transfer by abstracting an electron from the C?H σ orbital. For CH₃CN, the reduction of Fe^IV to Fe^III occurs gradually with substrate binding and H‐transfer. The charge density and electrophilicity of the μ‐O ligand hardly change upon substrate binding; however, for CH₃NO₂, there seems to be no electron movement from μ‐O to Fe^IV during the H‐transfer. Thus, the μ‐O ligand appears to abstract a proton without an electron from the C?H σ orbital. The calculated KIEs for CH₄, CH₃CN, and CH₃NO₂ are 24.4, 49.0, and 8.27, respectively, at 293 K, in remarkably good agreement with the experimental values. This study reveals that diverse KIE values originate mainly from tunneling to the same di(μ‐oxo)diiron(IV) core for all substrates, and demonstrate that the reaction dynamics are essential for reproducing experimental results and understanding the role of the diiron core for methane oxidation in sMMO.