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Sandro Gambarotta

Publications and source records attributed to Sandro Gambarotta.

At least 19 recordsLinked to original sources
PubMed2006

Ligand-centred reactivity in diiminepyridine complexes.

The diiminepyridine ligand, made famous by Brookhart and Gibson a decade ago through its use in the first iron-containing olefin polymerization catalyst, also displays an amazing variety of ligand-centred reactions, including electron transfer, alkylation, dimerization and deprotonation. The present Perspective summarizes these reactions and also speculates on the links between ligand- and metal-centred reactivity.

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PubMed2006

Role of the metal oxidation state in the SNS-Cr catalyst for ethylene trimerization: isolation of di- and trivalent cationic intermediates.

The reaction of the highly selective [CySCH2CH2N(H)CH2CH2SCy]CrCl3 catalyst precursor with alkyl aluminum activators was examined with the aim of isolating reactive intermediates. Reaction with Me3Al afforded a cationic trivalent chromium alkyl species {[CySCH2CH2N(H)CH2CH2SCy]CrMe(mu-Cl)}2{(AlMe3)2(m-Cl}2.(C7H8)2 (1a). Although it was not possible to obtain crystalline samples of sufficient quality from the reaction with MAO (the most preferred activator), the near-to-identical EPR spectra indicated a very close structural similarity with 1a. Ethylene oligomerization tests clearly revealed that 1 and other cationic trivalent dimeric complexes {[CySCH2CH2N(H)CH2CH2SCy] CrCl(mu-Cl)}2{AlCl4}2.(C7H8)1.5 (2), monomeric [(CySCH2CH2N(H)CH2CH2SCy)CrCl2 (THF)][AlCl4] (3), and {[CySCH2CH2N(H)CH2CH2SCy]Cr(eta2-AlCl4)}{Al2Cl7} (4) adducts display the same catalyst selectivity as the [CySCH2CH2N(H)CH2CH2SCy]CrCl3 complex and, therefore, are probably all precursors to the same catalytically active species. 2, 3, and 4 were obtained upon treatment of [CySCH2CH2N(H)CH2CH2SCy] CrCl3 with different stoichiometric ratios of AlCl3.. When i-BAO activator was used, reduction of the metal center occurred readily, affording {([CySCH2CH2N(H)CH2CH2S Cy]Cr)(mu-Cl)]2}{(i-Bu)2AlCl2}2 (5). 5 is also a selective catalyst, thus indicating that trivalent species are most probably precursors to a divalent catalytically active complex. Reaction of CrCl2(THF)2 with the ligand afforded the labile divalent adduct [CySCH2CH2N(H)CH2CH2SCy]CrCl2(THF) (6), also catalytically active and selective. Instead, deprotonation of the ligand with n-BuLi followed by reaction with CrCl2(THF)2 gave the dinuclear complex [(mu-CySCH2CH2NCH2CH2SCy)CrCl]2 (7), which did not produce oligomers.

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PubMed2005

Formation of a paramagnetic Al complex and extrusion of Fe during the reaction of (diiminepyridine)Fe with AlR3 (R = Me, Et).

The reaction of the {2,6-[2,6-(iPr)2PhN=C(CH3)]2(C5H3N)}FeCl2 catalyst precursor with R3Al [R = Me, Et] afforded {2,6-[2,6-(iPr)2PhN=C(CH3)]2(C5H3N)}AlMe2 (1) and [eta4-LAl2Et3(mu-Cl)]Fe-(eta6-C7H8) (2), respectively. These paramagnetic species arises from both transmetalation, during which the strong terdentate ligand loses the Fe center, and reduction. The extent of reduction depends on the nature of the Al alkylating agent. The electrons necessary for the reduction are likely to be provided by cleavage of Fe-C bond of transient low-valent organo-Fe species.

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PubMed2005

Metal versus ligand alkylation in the reactivity of the (bis-iminopyridinato)Fe catalyst.

The alkylation of the Brookhart-Gibson {2,6-[2,6-(i-Pr)2PhN=C(CH3)]2(C5H3N)} FeCl2 precatalyst with 2 equiv of LiCH2Si(CH3)3 led to the isolation of several catalytically very active products depending on the reaction conditions. The expected dialkylated species {2,6-[2,6-(i-Pr)2PhN=C(CH3)]2}(C5H3N)Fe(CH2SiMe3)2 (2) was indeed the major component of the reaction mixture. However, other species in which alkylation occurred at the pyridine ring ortho position, {2,6-[2,6-(i-Pr)2PhN=C(CH3)]2-2-CH2SiMe3}(C5H3N)Fe(CH2SiMe3) (1), and at the imine C atom, {2-[2,6-(i-Pr)2PhN=C(CH3)]-6-[2,6-(i-Pr)2PhNC(CH3)(CH2 SiMe3)](C5H3N)}Fe(CH2SiMe3) (3), have also been isolated and fully characterized. In addition, deprotonation of the methyl-imino functions and formation of a new divalent Fe catalyst {[2,6-[2,6-(i-Pr)2PhN-C=(CH2)]2(C5H3N)}Fe(mu-Cl)Li(THF)3 (4) also occurred depending on the reaction conditions. In turn, the formation of 4 might trigger the reductive coupling of two units through the methyl-carbon wings. This process resulted in the one-electron reduction of the metal center, affording a dinuclear Fe(I) alkyl catalyst {[{[2,6-(i-Pr)2C6H5]N=C(CH3)}(C5H3N){[2,6-(i-Pr)26H5]N=CCH2}Fe(CH2SiMe3)]}2 (5). Different from other metal derivatives, complex 5 could not be prepared from the monodeprotonated version of the ligand. Its reaction with a mixture of FeCl2 and RLi afforded instead [{2,6-[2,6-(i-Pr)2PhN-C=(CH2)]2(C5H3N)}FeCH2Si(CH3)3][Li(THF)4] (6) which is also catalytically active. All of these high-spin species have been shown to have high catalytic activity for olefin polymerization, producing polymers of two distinct natures, depending on the formal oxidation state of the metal center.

Alkenes↗
PubMed2005

Dinitrogen partial reduction by formally zero- and divalent vanadium complexes supported by the bis-iminopyridine system.

Reduction of the two trivalent 2,6-{[2,6-(i-Pr)2C6H5]N=C(CH3)}2(C5H3N)VCl3 and {[2,6-{[2,6-(i-Pr)2C6H3]N-C=(CH2)}2(C5H3N)]VCl(THF) complexes with excess NaH afforded two corresponding end-on dinitrogen-bridged complexes [2,6-{[2,6-(i-Pr)2C6H5]N=C(CH3)}2(C5H3N)V]2(m-N2).(hexane) (1) and [{[2,6-{[2,6-(i-Pr)2C6H3]N-C=(CH2)}2(C5H3N)]V]2(m-N2).(hexane) (3). Despite their very close structural similarity, the two species have completely different natures. The first is paramagnetic and may be regarded as generated by the two-electron attack of two formally zerovalent vanadium moieties on the same N2 unit. In the nearly diamagnetic 3 instead, the N2 unit has been reduced by two vanadium atoms, formally divalent. Structural analysis and DFT calculations have indicated that partial reduction of the bridging nitrogen occurred for both complexes while, in the case of 1, substantial metal-to-ligand electron transfer also occurs.

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PubMed2004

Multimetallic cooperative activation of N2.

The impressive number of breakthroughs reported in recent years in the field of dinitrogen activation reiterates the great interest that is still attracted by this molecule as a target for molecular activation studies. In spite of the fact that the discovery of dinitrogen fixation is rapidly approaching its 40th birthday, a thorough understanding of the factors that determine either fixation or activation is yet to be achieved. Nevertheless, substantial progress has recently been made. The aim of this article is to review some of the most recent literature and to assess the necessity of multimetallic attack to dinitrogen as a prerequisite towards further cleavage and functionalization.

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PubMed2004

Preparation, characterization, and magnetic behavior of the ln derivatives (ln = nd, la) of a 2,6-diiminepyridine ligand and corresponding dianion.

An unprecedented Nd[2,6-[[2,6-(i-Pr)(2)C(6)H(5)]N=C(CH(3))](2)(C(5)H(3)N)]NdI(2)(THF) (1) complex was prepared by oxidizing metallic Nd with I(2) in THF and in the presence of 2,6-[[2,6-(i-Pr)(2)C(6)H(5)]N=C(CH(3))](2)(C(5)H(3)N). The magnetic behavior at variable T clearly indicated that the complex should be regarded as a trivalent Nd atom antiferromagnetically coupled to a radical anion. By using the doubly deprotonated form of the diimino pyridine ligand [[2,6-[[2,6-(i-Pr)(2)C(6)H(5)]N-C=CH(2)](2)(C(5)H(3)N)](2-) (2) the corresponding trivalent complexes [[2,6-[[2,6-(i-Pr)(2)C(6)H(5)]N-C=CH(2)](2)(C(5)H(3)N)]Ln (THF)](mu-Cl)(2)[Li(THF)(2)].0.5 (hexane) [Ln = Nd (3), La (4)] were obtained and characterized. Reduction of these species afforded electron transfer to the ligand system which gave ligand dimerization via C-C bond formation through one of the two ene-amido functions of each molecule. The resulting dinuclear [[([2,6-(i-Pr)(2)C(6)H(5)]N-C=(CH(2)))(C(5)H(3)N)([2,6-(i-Pr)(2)C(6)H(5)]N=CCH(2))]Ln(THF)(2)(mu-Cl)[Li(THF)(3)])(2).2(THF) [Ln = Nd (5), La (6)] were isolated and characterized.

Anions↗
PubMed2004

Divalent molybdenum complexes of the dipyrrolide ligand system. Isolation of a Mo2 unit with a 45 degree twist angle.

The preparation of divalent Mo complexes of dipyrrolide dianions was carried out by reacting Mo(2)(acetate)(4) with the dipotassium salts of Ph(2)C(2-C(4)H(3)NH)(2) and 2-[1,1-bis(1H-pyrrol-2-yl)ethyl]pyridine. The two reactions respectively afforded the diamagnetic [[Ph(2)C(C(4)H(3)N)(2)](2)Mo(2)(OAc)(2)[K(THF)(3)][K(THF)]].THF (1) and [[(2-C(5)H(4) N)(CH(3))C(2-C(4)H(3)N)(2)]Mo(OAc)[K(THF)]](2).THF (2). Both compounds retained two acetate units in the dimetallic structure. Conversely, the reaction of Me(8)Mo(2)Li(4)(THF)(4) with Et(2)C(2-C(4)H(3)NH)(2) afforded the paramagnetic dimer [[Et(2)C(C(4)H(3)N)(2)](4)Mo(2)Li(2)][Li(THF)(4)](2).0.5THF (3). The paramagnetism is most likely caused by the 45 degree rotation of the two Mo(dipyrrolide) units with respect to each other and which, in turn, is caused by the presence of two lithium cations in the molecular structure.

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PubMed2003

Dinuclear and octanuclear Mn(II) complexes with mu2-C, mu2-N(pyrrolide), and mu-eta1:eta5-(pyrrolide) bridges: a structural and magnetic study.

Reaction of the dinuclear [(CH2SiMe3)(mu-CH2SiMe3)Mn(THF)]2 (1) with an equivalent amount of 1,1-dipyrrolylcyclohexane afforded two compounds depending on the solvent employed. Reaction carried out in THF afforded the dinuclear ([1,1-(mu-C4H3N)(C4H3N)C6H10]Mn(THF)2)2.2(THF) (2) while reaction in toluene yielded the octanuclear and cyclic cluster ([1,1-(mu,eta1:eta5-C4H3N)2C6H10]Mn)8.4(toluene) (3). The magnetism in all three cases is dominated by intramolecular antiferromagnetic exchange with strong coupling in 1 (J=-85 cm(-1)), and in 2 (J=-23.2 cm(-1)), whereas substantially weaker coupling through the sigma/pi-bonded dipyrrolide bridges (J=-3.3 cm(-1)) was observed within the cyclic and octameric 3.

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PubMed2002

Participation of the alpha,alpha'-diiminopyridine ligand system in reduction of the metal center during alkylation.

The reaction of [[2,6-(i-Pr)(2)PhN=C(Me)](2)(C(5)H(3)N)]MnCl(2) with alkylating agents formed a dinuclear Mn(I) derivative via ligand reductive coupling. In the case of the trivalent Cr analogue, a similar reaction afforded reduction toward Cr(II) but also alkylation at the pyridine ring para position followed by an unprecedented cycloaddition that generated a tricyclic system.

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PubMed1997

Synthesis and Structure of a New Lithium Amide Ligand Precursor: A Tridentate Nitrogen-Based Donor Set of the Formula N(SiMe(2)CH(2)NMe(2))(2). Synthesis and Structure of the Group 4 Amides MCl(3)[N(SiMe(2)CH(2)NMe(2))(2)] (M = Ti, Zr, Hf).

The new lithium amide LiN(SiMe(2)CH(2)NMe(2))(2) was prepared by reaction of NH(3) with the corresponding silylamine Me(2)NSiMe(2)CH(2)NMe(2) followed by addition of butyllithium. This lithium derivative exists as a dimer in the solid state wherein the two lithium ions are bridged by the two amido units with the amine arms of each unit bonded to opposite lithium centers in an overall pseudo D(2) structure; however, in solution, a fluxional process serves to interconvert the enantiomeric forms of the dimer unit. The coordination chemistry of the lithium amide dimer has been investigated; reaction with a series of group 4 starting halides, MCl(4), leads to the corresponding complexes MCl(3)[N(SiMe(2)CH(2)NMe(2))(2)], where M = Ti, Zr, and Hf. The structures of these starting trihalides in solution and in the solid state are presented.

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