Isolation and characterization of main group and late transition metal complexes using orthometallated imine ligands

Several late transition metal and main group orthometallated imine complexes were synthesized by utilizing ortholithiated imine precursors. Magnesium, aluminum, zinc, copper(I), and tin(IV) complexes were isolated and characterized. Subsequent reactions with electrophiles such as Ph(2)PCl, MeI and I(2) yielded several functionalized products, including a new iminophosphine ligand and its corresponding copper(I) complex. The coordination modes of the orthometallated imine ligands, as well as the structures of the metal complexes, were studied in the solid state using small molecule X-ray diffraction when possible.     

Catalysis using transition metal complexes featuring main group metal and metalloid compounds as supporting ligands

Recent development in catalytic application of transition metal complexes having an M–E bond (E = main group metal or metalloid element), which is stabilized by a multidentate ligand, is summarized. Main group metal and metalloid supporting ligands furnish unusual electronic and steric environments and molecular functions to transition metals, which are not easily available with standard organic supporting ligands such as phosphines and amines. These characteristics often realize remarkable catalytic activity, unique product selectivity, and new molecular transformations. This perspective demonstrates the promising utility of main group metal and metalloid compounds as a new class of supporting ligands for transition metal catalysts in synthetic chemistry.

Recent development in catalytic application of transition metal complexes having an M–E bond (E = main group metal or metalloid element), which is stabilized by a multidentate ligand, is summarized.     

Synthesis and characterisation of group nine transition metal complexes containing new mesityl and naphthyl based azaindole scorpionate ligands

Two novel boron-based flexible scorpionate ligands based on 7-azaindole, Li[HB(azaindolyl)(2)(1-naphthyl)] and Li[HB(azaindolyl)(2)(mesityl)] {Li[(Naphth)Bai] and Li[(Mes)Bai] respectively}, have been prepared (mesityl = 2,4,6-trimethylphenyl). These salts have been isolated in two forms, either as dimeric structures which contain bridging hydride interactions with the lithium centres or as crystalline material containing mono nuclear bis-acetonitrile solvates. The newly formed ligands have been utilised to prepare a range of group nine transition metal complexes with the general formula [M(COD){κ(3)-NNH-HB (azaindolyl)(2)(Ar)}] (where M = rhodium, iridium; Ar = 1-naphthyl, mesityl; COD = 1,5-cyclooctadiene) and [Rh(NBD){κ(3)-NNH-HB (azaindolyl)(2)(Ar)}] (where NBD = 2,5-norbornadiene; Ar = 1-naphthyl, mesityl). These new complexes have been compared to the previously reported compounds which contain the related scorpionate ligands Li[HB(azaindolyl)(2)(phenyl)] and K[HB(azaindolyl)(3)] {Li[(Ph)Bai] and K[Tai] respectively}. Structural characterisation of the complexes [Rh(COD){κ(3)-NNH-HB (azaindolyl)(2)(mesityl)}], [Ir(COD){κ(3)-NNH-HB (azaindolyl)(2)(mesityl)}] and [Rh(NBD){κ(3)-NNH-HB (azaindolyl)(2)(naphthyl)}] confirm the expected κ(3)-NNH coordination mode for these new ligands. Spectroscopic analysis suggests strong interactions of the B-H functional group with the metal centres in all cases.     

Anionic triazacyclononanes: new supporting ligands in main group and transition metal organometallic chemistry

In the course of developing new ligands to support chemistry with main-group, transition and lanthanide elements, a number of research groups have focused attention on functionalized triazacyclononanes; this article provides a summary of the more recent findings with an emphasis on the organometallic chemistry of one particular class of tacn ligands, namely those involving the anionic tacn moiety.     

Neutral and zwitterionic group 4 metal alkyls with ancillary boroxide ligands

The boroxide anion, [OB(mes)(2)](-), has been applied as an ancillary ligand to generate electron-deficient group 4 metal alkyls; however, the enhanced electrophilicity results in formation of tight ion-pairs in solution.     

Synthesis and structure of metal complexes containing zwitterionic N-hydroxyimidazole ligands

Cu(II) and Zn(II) complexes of N-hydroxyimidazoles were synthesised by reacting simple metal perchlorate salts with the imidazole ligand in alcohol and formulated with a metal:ligand ratio of 1:2. The X-ray crystal structures of five complexes (four Cu(II) and one Zn(II)) were obtained and each showed the two trans, N-hydroxyimidazole ligands forming six-membered, chelate rings with the metal. Both of the NO chelating, neutral N-hydroxyimidazole ligands are in the zwitterion form, with the uncoordinated imidazole imine N atom being protonated and the oxime O atom deprotonated. In the solid state the complexes form hydrogen-bonded supramolecular structures.     

Alternatives to pyridinediimine ligands: syntheses and structures of metal complexes supported by donor-modified alpha-diimine ligands

This report describes the synthesis and characterization of metal halide complexes (M = Mn, Fe, Co) supported by a new family of pendant donor-modified alpha-diimine ligands. The donor (N, O, P, S) substituent is linked to the alpha-diimine by a short hydrocarbon spacer forming a tridentate, mer-coordinating ligand structure. The tridentate ligands are assembled from monoimine precursors, the latter being synthesized by selective reaction with one carbonyl group of the alpha-dione. While attempts to separately isolate tridentate ligands in pure form were unsuccessful, metal complexes supported by the tridentate ligand are readily synthesized in-situ, by forming the ligand in the presence of the metal halide, resulting in a metal complex which subsequently crystallizes out of the reaction mixture. Metal complexes with NNN, NNO, NNP and NNS donor sets have been prepared and examples supported by NNN, NNP and NNS ligands have been structurally characterized. In the solid state, NNN and NNP ligands coordinate in a mer fashion and the metal complexes possess distorted square pyramidal structures and high spin (S = 2) electronic configurations. Compounds with NNS coordination environments display a variety of solid state structures, ranging from those with unbound sulfur atoms, including chloride bridged and solvent ligated species, to those with sulfur weakly bound to the metal center. The extent of sulfur ligation depends on the donor ability of the crystallization solvent and the substitution pattern of the arylthioether substituent.     

Uniqueness and versatility of iminopyrrolyl ligands for transition metal complexes

Nitrogen-based ligands containing an iminopyrrole unit have recently attracted attention because of their flexible complexation to transition metals. Since steric and electronic demands can be readily introduced to the iminopyrrole unit, a wide variety of ligand has been designed and synthesized. In this contribution, we briefly review synthetic and structural features of transition metal complexes with the multidentate iminopyrrolyl ligands and their catalytic activity, especially polymerization of α-olefins.     

Absorption spectra of transition group complexes of sulphur-containing ligands

Nickel(II), rhodium(III), palladium(II), iridium(III) and other central atoms are combined with diethyldithiophosphate (dtp⁻) diethyldithiocarbamate (dtc⁻) 2,2′-di(aminoethyl)sulphide (daes), thiosemicarbazide (tscaz) and other sulphur-containing ligands. The position in the spectrochemical series dtp⁻ < dtc⁻ < daes < SO₃²⁻ is related to the number of lone-pair electrons available on the sulphur atom. The nephelauxetic effect is very conspicuous in Rh dtp₃ and Ir dtp₃. The square-planar low-spin Ni dtp₂ and nickel(II) xanthate form high-spin pyridine addition compounds such as Ni(dtp)₂py₂, while Ni daes₂²⁺ is high-spin. In dtp₃ and Bi dtp₃ can be precipiatated from acid solutions.     

Redox noninnocence of nitrosoarene ligands in transition metal complexes

Studies on the coordination of nitrosoarene (ArNO) ligands to late-transition metals are used to provide the first definition of the geometric, spectroscopic, and computational parameters associated with a PhNO electron-transfer series. Experimentally, the Pd complexes PdCl(2)(PhNO)(2), PdL(2)(PhNO)(2), and PdL(2)(TolNO) (L = CNAr(Dipp2); Ar(Dipp2) = 2,6-(2,6-(i)Pr(2)C(6)H(3))(2)-C(6)H(3)) are characterized as containing (PhNO)(0), (PhNO)(?1-), and (TolNO)(2-) ligands, respectively, and the structural and spectroscopic changes associated with this electron transfer series provide the basis for an extensive computational study of these and related ArNO-containing late-transition metal complexes. Most notable from the results is the unambiguous characterization of the ground state electronic structure of PdL(2)(PhNO)(2), found to be the first isolable, transition metal ion complex containing an η(1)-N-bound π-nitrosoarene radical anion. In addition to the electron transfer series, the synthesis and characterization of the Fe complex [Fe(TIM)(NCCH(3))(PhNO)][(PF(6))(2)] (TIM = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) allows for comparison of the geometric and spectroscopic features associated with metal-to-ligand π-backbonding as opposed to (PhNO)(?1-) formation. Throughout these series of complexes, the N-O, M-N, and C-N bond distances as well as the N-O stretching frequencies and the planarity of the ArNO ligands provided distinct parameters for each ligand oxidation state. Together, these data provide a delineation of the factors needed for evaluating the oxidation state of nitrosoarene ligands bound to transition metals in varying coordination modes.     

Preparation of transition metal complexes of strongly basic ligands by thermal decomposition

The usual stoichiometry of metal salicylates, 2-oxy-3-naphtholates, anthranilates and salicylaldoximates is M²⁺(HL⁻)₂.xH₂O. Heating such a solid in an inert atmosphere causes proton transfer between the two HL⁻-ligands and the following reaction takes place: M(HL)₂(s) → ML(s) + H₂L(g). The new complex ML(s) (e.g. zincsalicylate) reacts with solid or dissolved monoprotic ligands HL' (e.g. 8-hydroxyquinoline) to form the mixed complex (e.g. zinc-salicylate-oxyquinolate) in excellent yield.     

Uranyl complexes as chelate ligands toward transition metal ions

The synthesis and the characterization of some uranyl complexes containing macrocyclic Schiff base ligands are reported. These cyclic complexes may act as ligands towards trasition metal ions and the synthesis of mixed uranyl complexes containing a transition metal ion is reported.     

Titanium and zirconium complexes supported by dipyrrolide ligands

The reactions between meso-disubstituted dipyrromethanes and titanium and zirconium amides and alkyls have generated the first examples of dipyrrolide complexes of Group 4 metals.     

Electrochemistry of transition metal complexes of Schiff base compartmental ligands

Summary The electrochemical behaviour of a series of mononuclear and dinuclear complexes of dioxouranium(VI), nickel(II) and copper(II) ions with the Schiff base, H₄fsalacen, derived from the condensation of 3-formylsalicylic acid and 1,2-diaminoethane, is reported.The potentially hexadentate compartmental ligand H₄fsalacen has an outer O₂O₂ and an inner N₂O₂ coordination site. The redox properties of the metal ions in these two different and adjacent chambers have been investigated and compared with those of the analogous complexes with the ligand H₄ aapen, obtained by reaction ofo-acetoacetylphenol and 1,2-diaminoethane.A preliminary report was presented at the 1st International Conference on the Chemistry and Technology of the Lanthanides and Actinides, Venice, 5 September, 1983, Italy.     

Synthesis and reactivity of transition metal complexes containing halogenated boryl ligands

The synthesis and characterization of iron and manganese complexes containing the tetrachlorocatecholboryl (BO₂C₆Cl₄) ligand are reported. Crystallographic study of the methylcyclopentadienyl derivative (η⁵-C₅H₄Me)Fe(CO)₂BO₂C₆Cl₄ allows comparison of structure and bonding with related complexes of the type (η⁵-C₅R₅)Fe(CO)₂B(OR)₂ and reveals that the relative orientation of (η⁵-C₅H₄Me)Fe(CO)₂ and BO₂C₆Cl₄ moieties is influenced by intramolecular CH?O hydrogen bonding. Additionally, an alternative route to catecholboryl complexes from dilithiocatechol is reported.     

Coordination behaviour in transition metal complexes of asymmetric NPN ligands

Two bicyclic, chiral aminophosphine ligands, namely 4R, 9R-1,3-bis(pyridin-2-ylmethyl)-2-(2-propyl)octahydro-1H-1,3,2-benzodiazaphosphole (1) and 4R, 9R-1,3-bis(pyridin-2-ylmethyl)-2-(2-ethoxy)octahydro-1H-1,3,2-benzodiazaphosphole (2) have been prepared from 1R, 2R-diaminocyclohexane and the appropriate dichlorophosphine and the nature of their coordination to a number of transition metals explored. Ligand 1 coordinates to Pd(II) and Pt(II) as a terdentate donor to give complexes of the type [M(κ³-N,P,N-1)Cl]⁺ whereas ligand 2 favours bidentate κ²-P,N coordination to give the complexes M(κ²-P,N-2)Cl₂. The study of the coordination chemistry of the NPN ligand 1 is frustrated by its ready decomposition to an unknown species which appears to be promoted by transition metals. The ligand 2 does not undergo such a transformation and its metal chemistry is more readily examined. Aside from the Pt(II) and Pd(II) complexes above, 2 has been coordinated to Cr(0) and Mo(0) in the octahedral complexes M(κ²-P,N-2)(CO)₄ and Au(I) in linear Au(κ¹-P-2)Cl. All the complexes have been fully characterised by spectroscopic and analytical techniques including a single-crystal X-ray structure analysis of [Pt(κ³-N,P,N-1)Cl]Cl, 3.     

Tuning main group redox chemistry through steric loading: subvalent Group 13 metal complexes of carbazolyl ligands

The ability of substituted carbazol‐9‐yl systems to ligate in σ fashion through the amido N‐donor, or to adopt alternative coordination modes through the π system of the central five‐membered ring, can be tuned by systematic variation in the steric demands of substituents in the 1‐ and 8‐positions. The differing affinities of the two modes of coordination for hard and soft metal centres can be shown to influence not only cation selectivity, but also the redox properties of the metal centre. Thus, the highly sterically sterically demanding 1,3,6,8‐tetra‐tert‐butylcarbazolyl ligand can be used to generate the structurally characterised amido‐indium(I) complex, [{(tBu₄carb)In}_n], (together with its isostructural thallium counterpart) in which the metal centre interacts with the central pyrrolyl ring in η³ fashion [d(In? N)=2.679(3) Å; d(In? C)=2.819(3), 2.899(3) Å]. By contrast, the smaller 3,6‐di‐tert‐butylcarbazolyl system is less able to restrict the metal centre from binding at the anionic nitrogen donor in the plane of the carbazolyl ligand (i.e. in σ fashion). Analogous chemistry with In^I precursors therefore leads to disproportionation to the much harder In^II [and In⁰], and the formation of the mixed‐valence product, [In₂{In₂(tBu₂carb)₆}], a homoleptic molecular [In₄(NR₂)₆] system. This chemistry reveals a flexibility of ligation for carbazolyl systems that contrasts markedly with that of the similarly sterically encumbered terphenyl ligand family.     

Pyridazine- versus pyridine-based tridentate ligands in first-row transition metal complexes

A series of first-row transition metal complexes with the unsymmetrically disubstituted pyridazine ligand picolinaldehyde (6-chloro-3-pyridazinyl)hydrazone (PIPYH), featuring an easily abstractable proton in the backbone, was prepared. Ligand design was inspired by literature-known picolinaldehyde 2-pyridylhydrazone (PAPYH). Reaction of PIPYH with divalent nickel, copper, and zinc nitrates in ethanol led to complexes of the type [Cu(II)(PIPYH)(NO(3))(2)] (1) or [M(PIPYH)(2)](NO(3))(2) [M = Ni(II) (2) or Zn(II) (3)]. Complex synthesis in the presence of triethylamine yielded fully- or semideprotonated complexes [Cu(II)(PIPY)(NO(3))] (4), [Ni(II)(PIPYH)(PIPY)](NO(3)) (5), and [Zn(II)(PIPY)(2)] (6), respectively. Cobalt(II) nitrate is quantitatively oxidized under the reaction conditions to [Co(III)(PIPY)(2)](NO(3)) (7) in both neutral and basic media. X-ray diffraction analyses reveal a penta- (1) or hexa-coordinated (2, 3, and 7) metal center surrounded by one or two tridentate ligands and, eventually, κ-O,O' nitrate ions. The solid-state stoichiometry was confirmed by electron impact (EI) and electrospray ionization (ESI) mass spectrometry. The diamagnetic complexes 5 and 6 were subjected to (1)H NMR spectroscopy, suggesting that the ligand to metal ratio remains constant in solution. Electronic properties were analyzed by means of cyclic voltammetry and, in case of copper complexes 1 and 4, also by electron paramagnetic resonance (EPR) spectroscopy, showing increased symmetry upon deprotonation for the latter, which is in accordance with the proposed stoichiometry [Cu(II)(PIPY)(NO(3))]. Protic behavior of the nickel complexes 2 and 5 was investigated by UV/vis spectroscopy, revealing high π-backbonding ability of the PIPYH ligand resulting in an unexpected low acidity of the hydrazone proton in nickel complex 2.     

Mesoionic thiazol-5-ylidenes as ligands for transition metal complexes

The first examples of thiazol-5-ylidene complexes featuring group 9, 10 and 11 metal centers, have been prepared by deprotonation of a series of 2,3,4-triaryl-susbtituted thiazolium salts in the presence of the corresponding transition metal precursor.