Four-coordinate Complexes Research Articles

Spontaneous resolution of four-coordinate Zn(ii) complexes in the formation of three-dimensional metal–organic frameworks

The three-dimensional chiral helical framework of the four-coordinate zinc complexes, [Zn(bdt)]n (1a and 1b) (H2bdt = 1,4-benzeneditetrazol-5-yl), has been prepared via hydrothermal reaction. At the chiral centers, the tetrahedral zinc centers link with bdt ligands to form a chiral unc-type network, which exhibits an intense blue emission and spontaneous chiral resolution. The enantiomers of 1a and 1b have been confirmed by solid-state circular dichroism spectra.

Low coordinate lanthanide(ii) complexes supported by bulky guanidinato and amidinato ligands

The preparation of a series of homoleptic, four-coordinate lanthanide(ii) complexes, [Ln(Priso)(2)] (Ln = Sm, Eu or Yb) incorporating the bulky guanidinate ligand Priso(-) ([(ArN)(2)CNPr(i)(2)](-), Ar = 2,6-diisopropylphenyl) is described. X-ray crystallography shows the complexes to be isostructural and to exhibit coordination geometries midway between tetrahedral and planar. Comparisons between the geometries of the complexes and those of the bulkier systems, [Ln(Giso)(2)] (Giso(-) = [(ArN)(2)CNCy(2)](-), Cy = cyclohexyl) are discussed. Attempts to prepare the less hindered amidinate complexes, [Ln(Piso)(2)] (Piso(-) = [(ArN)(2)CBu(t)](-)), were not successful, but did give rise to the heteroleptic complex, [(kappa(1)-N,eta(6)-Piso)Sm(THF)(mu-I)(2)Sm(kappa(1)-N,eta(6)-Piso)]. Whereas the amidinate ligands in this complex chelate the samarium centre in an kappa(1)-N,eta(6)-Ar-fashion, in the closely related complex, [{(kappa(2)-N,N'-Priso)Yb(THF)(mu-I)}(2)], the ytterbium atoms are kappa(2)-N,N'-chelated by the guanidinate ligand. The facility of the planar four-coordinate complex, [Sm(Giso)(2)], to act as a one-electron reducing agent towards a variety of unsaturated substrates has also been explored. The complex has been shown to selectively reductively couple CS(2), via a C-S bond formation, to give the samarium(iii) dimer, [(Giso)(2)Sm(mu-eta(2)-:eta(2)-S(2)CSCS)Sm(Giso)(2)].

Tertiary Amine and N‐Heterocyclic Carbene Coordinated Haloalanes—Synthesis, Structure, and Application

The preparation of several tertiary amine and N-heterocyclic carbene coordinated chloro- and bromoalanes has been studied and routes to their gram-scale synthesis optimized. This provides a catalogue of well-characterized, thermally stable haloalanes for future application. All complexes have been investigated by spectroscopy (IR, NMR) and, where possible, single-crystal X-ray diffraction structure determination. A particular focus of this article is the relative thermal stabilities of the complexes, which provides a useful handle for the aerobic stability of Group 13 hydride complexes. These thermal data have been elucidated in full and rationalized relative to one another on the basis of Lewis base donation, steric shielding, and relative inductive halide strengthening of the aluminum hydride bonds by halides. All of the four-coordinate complexes reported exist as distorted tetrahedra in the solid state with aluminum to N/C-donor bonds that shorten with the increasing Lewis acidity of the aluminum Lewis acid. The five-coordinate complexes [AlBrH(2)(Quin)(2)] and [AlBr(2)H(Quin)(2)] (Quin = quinuclidine) exist in a trigonal-bipyramidal geometry in the solid state with the amine donors situated in the apical positions. Five chloroalanes; [AlClH(2)(Quin)], [AlClH(2)(Quin)(2)], [AlCl(2)H(Quin)(2)], [AlClH(2)(IMes)], and [AlCl(2)H(IMes)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), the latter two of which are aerobically stable, have been applied to the hydroalumination of carbonyl and heterocycle substrates and their chemo-, regio-, and stereoselectivities compared to those of Group 13 hydride reagents cited in the literature. Overall, the reactivities of these species are comparable to non-halogenated alane complexes with the additional benefit of aerobic stability, non-pyrophoricity, and enhanced regioselectivity borne out of greater Lewis acidity.

Three-Coordinate and Four-Coordinate Cobalt Hydride Complexes That React with Dinitrogen

We report the formation and N(2) reactivity of novel cobalt hydride complexes supported by bulky beta-diketiminate ligands (L). Addition of KHBEt(3) to LCoCl gives [LCo(mu-H)](2) (1) or K(2)[LCoH](2) (2), depending on the amount of borohydride used. Compound 2 is the first example of a crystallographically characterized hydride complex in which a transition metal is three-coordinate. Both 1 and 2 react with N(2) at room temperature to give dinuclear N(2) complexes with loss of H(2).

Synthesis and spectroscopic investigations of four-coordinate nickel complexes supported by a strongly donating scorpionate ligand

Two four-coordinate nickel complexes, HB( t BuIm) 3NiBr and HB( t BuIm) 3NiNO, were prepared by reaction of a bulky tris(carbene)borate ligand with NiBr 2(PPh 3) 2 and NiBr(NO)(PPh 3) 2, respectively, and structurally and spectroscopically characterized. In addition to standard techniques, high-frequency and -field electron paramagnetic resonance (HFEPR) was employed to understand the spin triplet ( S = 1) ground state of the bromo complex. HFEPR, combined with electronic absorption spectroscopy allows comparison of this novel complex with other paramagnetic four-coordinate Ni(II) species. The tris(carbene)borate ligand is a stronger σ-donor than corresponding tris(pyrazolyl)borates (traditional “scorpionate” ligands). The tris(carbene)borate ligand may also act as a π-acceptor, in contrast to tris(pyrazolyl)borates, which show relatively little π-bonding interactions. The influence of tris(carbene)borate substituents on the donor strength of the ligand have been elucidated from IR spectroscopic investigations of {NiNO} 10 derivatives. HFEPR spectra of HB( t BuIm) 3NiBr exhibit hyperfine coupling from Br, which indicates the strong electronic interaction between Ni(II) and this halide ligand, consistent with studies on tris(pyrazolyl)borate Ni(II) complexes.

A novel polymeric Hg[(CN)2N2] central core four-coordinate complex: Synthesis, X-ray structure and 199Hg, 13C and 15N CP MAS NMR characterization of catena((μ2-1,4-diaminobutane-N,N′)dicyanomercury(II))

Mercury(II) cyanide reacts with 1,4-diaminobutane (bn) at a 1:1 ratio forming a polymeric complex [(bn)Hg(CN)2]∞ in which (bn) acts as a bridge between the Lewis acid Hg(CN)2 moieties. The metal complex coordination sphere Hg[(CN)2N2] is best described as distorted trigonal pyramidal. The 13C and 15N CP MAS NMR data show two chemically different CN groups consistent with the hydrogen bonding scheme involving only one of the cyano ligands.

Trapping of a Four‐Coordinate Zinc Salphen Complex Inside a Crystal Matrix

A remarkable twosome: Four-coordinate square-planar complexes based on [Zn(salen)] derivatives are conveniently captured inside a crystal matrix using a supramolecular protecting strategy (see figure). The four-coordinate species is accompanied by a five-coordinate complex that binds an axial ligand able to hydrogen bond to an N-heterocycle positioned in the axial region of the coordinative unsaturated derivative. The size of the N-heterocycle is a decisive stabilization parameter.

Synthesis and Structural Characterization of Substituted Salicylate Titanocene Complexes: Three Supramolecular Frameworks Determined by Weak Interactions

Based on the reaction between Cp2TiCl2 and substituted salicylic acids in the presence of β-cyclodextrin polymer (β-CDP), three four-coordinated titanocene complexes [(η5-C5H5)2Ti(S,O′)(OCC6H4)·(C6H6)0.5] (1), [(η5-C5H5)2Ti{(O,O′)(3,5-Cl2-OCC6H2)}] (2) and [(η5-C5H5)2Ti{(O,O′)(3,5-(NO2)2-OCC6H2)}] (3) were synthesized in high yields and their crystal structures have been determined by single-crystal X-ray diffraction. The structure of 1 has a Monoclinic space group P21/c with a = 8.313(3) A, b = 9.960(4) A, c = 22.330(8) A, β = 111.856(11)° and Z = 4. The structure of 2 has a Monoclinic space group P21/c with a = 8.0577(13) A, b = 8.9022(14) A, c = 21.977(4) A, β = 96.298(3)° and Z = 4. The structure of 3 has a Triclinic space group P-1 with a = 8.1687(11) A, b = 8.3027(11) A, c = 12.7164(17) A, α = 102.930(2)°, β = 100.479(2)°, γ = 95.458(2)° and Z = 2. Each of the complexes exhibits a three-dimensional framework constructed through weak interactions, which are hydrogen bonding, π–π stacking and C–H···π interactions. It was found that the variation of the substituted salicylate ligands affect the weak interactions as well as the specific framework structure that forms. Three four-coordinated titanocene complexes [(η5-C5H5)2Ti(S,O′)(OCC6H4)·(C6H6)0.5] (1), [(η5-C5H5)2Ti{(O,O′)(3,5-Cl2-OCC6H2)}] (2) and [(η5-C5H5)2Ti{(O,O′)(3,5-(NO2)2-OCC6H2)}] (3) were synthesized in high yields, each of the complexes exhibits a three-dimensional framework constructed through weak interactions. It was found that simple variation of the substituted salicylate ligands affect the weak interactions as well as the specific framework structure that forms. .

Oxidation states, atomic charges and orbital populations in transition metal complexes

The differences and similarities between calculated atomic charge and formal oxidation state in transition metal complexes are discussed in the light of density functional theory calculations on a variety of four-coordinate complexes. It is shown that the oxidation state formalism provides a framework for the classification of families of compounds related by ligand substitution or redox processes, and can neither be replaced by nor deduced from net atomic charges.

Synthesis and spectroscopic studies of some cadmium(II) and mercury(II) complexes of an asymmetrical bidentate Schiff base ligand

Synthesis and spectroscopic studies on four-coordinate complexes of cadmium(II) and mercury(II) halides with a new asymmetrical bidentate Schiff base ligand of N,N′-bis[α-methylcinamaldehydene]propane-1,2-diamine(L) are described. The ligand and its complexes were characterized by elemental analysis, molar conductance, UV–visible spectra, FT-IR spectra, MS, 1H NMR and 13C NMR spectra. The complexes are non-electrolytes in DMF. The electronic spectra of the complexes were recorded in DMF solution. 1H and 13 C NMR spectra been studied in CDCl3. The molar conductance as well as spectral properties indicated the complexes do not dissociate in DMF and retain their coordination. FT-IR and NMR spectra of the complexes exhibit downfield as well as upfield shifts of the free ligand resonances that show change in geometry during the coordination. The suggested structure of the complexes is pseudo-tetrahedral. Molecular structures of the complexes have been optimized by MM+ calculations that supported pseudo-tetrahedral geometry around the metal (II) ions.

Low-spin complexes of Ni 2+ with six NH 3 and H 2O ligands: A DFT–RX3LYP study

The gas-phase energy minimization of six-coordinate Ni 2+ octahedral structures ( S = 1) with H 2O and NH 3 using the DFT/RX3LYP/6-311++G(d,p) level of theory and S = 0 leads to the formation of square planar complexes of the type [Ni(NH 3) n(H 2O) m] 2+·sH 2O·(2 − s)NH 3, n + m = 4, 0 ≤ n, m ≤ 4, 0 ≤ s ≤ 2. The H-bonding between inner- and outer-sphere (IS and OS) ligands results in the formation of six-member rings characterized by ring critical points. If OS H 2O, H-bonded to IS H 2O, is changed to NH 3, it deprotonates the latter with hydroxide strongly H-bonded to OS NH 4 +, a phenomenon not observed in the modeling of equivalent complexes of Zn 2+ and Cu 2+. The four-coordinate hexa complexes are found to be over 50 kcal mol −1 more stable than the equivalent octahedral structures; this is twice as large as reported for Cu 2+, and much larger than for Zn 2+ complexes. The stability of complexes with two OS H 2O increases by 13 kcal mol −1 as IS H 2O is replaced by NH 3; this is twice as much as found for the analogous octahedral complexes. The deprotonation of IS H 2O by OS NH 3 leads to a stabilization by 10 kcal mol −1; interchanging IS H 2O with OS NH 3 increases stability by a further 4–8 kcal mol −1. The increased stability of these four-coordinate hexa complexes is associated with greater charge density ρ bcp at the bond critical points of Ni O and Ni N bonds and a greater charge transfer to the metal (0.708–1.067 e).

Tuning the Oxidation Level, the Spin State, and the Degree of Electron Delocalization in Homo- and Heteroleptic Bis(α-diimine)iron Complexes

The four-coordinate heteroleptic complex [Fe(III)((F)pda(2-))((F)dad*-)] (1) and its homoleptic analogue [Fe(II)((F)dad*-)2] (2), where (F)pda(2-) is the closed-shell ligand N,N'-bis(pentafluorophenyl)-o-phenylenediamide(2-) and (F)dad*- is the singly reduced N,N'-bis(pentafluorophenyl)-2,3-dimethyl-1,4-diazabutadiene pi-radical anion, have been synthesized. X-ray crystallographic studies reveal a twisted tetrahedral geometry of the FeN4 coordination polyhedron in both 1 and 2. The electronic structures of 1 and 2 were probed by magnetic susceptibility measurements, 57Fe Mössbauer and electronic spectroscopy, and density functional theory (DFT) calculations. In spite of their similar geometries and a common triplet ground state (S(t) = 1), the electronic structures of 1, 2, and the previously reported homoleptic analogue [Fe(III)((F)pda(2-))((F)pda*-)] (3), where (F)pda*- is a one-electron-oxidized form of (F)pda(2-), differ. The electronic structure of 2 consists of two (F)dad*- radicals coupled antiferromagnetically to a high-spin Fe(II) center, whereas in 3, only one (F)pda*- radical is coupled antiferromagnetically to an intermediate-spin Fe(III) ion. This ligand mixed-valent species exhibits class-III behavior. Heteroleptic 1 contains a single (F)dad*- radical coupled antiferromagnetically to an intermediate-spin Fe(III) center but behaves as a class-II ligand mixed-valent species. The observed diversity in the electronic structures of 1-3 is ascribed to the difference in the redox potentials of the ligands. Analysis of reduced orbital charges and spin densities obtained from DFT calculations also suggests that the electronic structures of 1-3 are best described as either a high-spin Fe(II) ion coordinated to two radical monoanions (2) or as an intermediate-spin Fe(III) ion coordinated to one radical monoanion and one closed-shell dianion (1, 3).

Characterization of Three Members of the Electron‐Transfer Series [Fe(pda)2]n (n=2−, 1−, 0) by Spectroscopy and Density Functional Theoretical Calculations [pda=Redox Non‐innocent Derivatives of N,N′‐Bis(pentafluorophenyl)‐o‐phenylenediamide(2−, 1.−, 0)

The four-coordinate iron complexes, [Fe(III)(pda(2-))(pda(.-))] (1) and [AsPh(4)](2)[Fe(II)(pda(2-))(2)] (2) were synthesized and fully characterized; pda(2-) is the closed-shell ligand N,N'-bis(pentafluorophenyl)-o-phenylenediamido(2-), and pda(.-) represents its one-electron-oxidized pi-radical anion. Single-crystal X-ray diffraction studies of 1 and 2 performed at 100(2) K reveal a distorted tetrahedral coordination environment at the iron centers, as a result of the intramolecular pi-pi interactions between C(6)F(5) rings. The electronic structures of 1 and 2 were unambiguously determined by a combination of (57)Fe Mössbauer and electronic spectroscopy, magnetic susceptibility measurements, X-ray crystallography, and DFT calculations. Compound 1 contains an intermediate-spin Fe(III) ion (S(Fe)=3/2) strongly antiferromagnetically coupled to a pi-ligand radical (S(R)=1/2) yielding an S(t)=1 ground state. Complex 2 possesses a high-spin Fe(II) center (S(Fe)=2) with two closed-shell dianionic ligands. Complexes 1 and 2 are members of the redox series [Fe(pda)(2)](n) with n=0 for 1 and n=2- for 2. The anion n=1- has been reported previously in the coordination salt [Fe(dad)(3)][Fe(pda)(2)] (3; dad=N,N'-bis(phenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene). A complicated temperature-dependent electronic structure has been observed for this salt. Here, DFT calculations performed on 3 confirm the previous assignments of spin- and oxidation-states. Thus, [Fe(pda)(2)](n) (n=0, 1-, 2-) constitutes an electron-transfer series, which has also been established by cyclic voltammetry; the mono- and dications (n=1+ and 2+) are also accessible in solution, but have not been further investigated. The (57)Fe Mössbauer spectra of [Fe(pda)(2)](n) species in 1 and 3 show extremely large quadrupole splitting constants due to addition of the valence and covalence contributions that have been confirmed by DFT calculations.

Phosphorus and Olefin Substituent Effects on the Insertion Chemistry of Nickel(II) Hydride Complexes Containing Amido Diphosphine Ligands

A series of nickel(II) hydride complexes supported by amido diphosphine ligands, including symmetrical [N(o-C6H4PR2)2]− ([R-PNP]−; R = Ph, iPr, Cy) and unsymmetrical [N(o-C6H4PPh2)(o-C6H4PiPr2)]− ([Ph-PNP-iPr]−), have been prepared for the investigation of olefin insertion chemistry. The unsymmetrical ligand precursor H[Ph-PNP-iPr] (1d) that features different substituents (phenyl and isopropyl) at the two phosphorus donors was prepared in 53% yield as colorless crystals. Treatment of Ni(COD)2 (COD = cycloocta-1,5-diene) with H[R-PNP] (R = Ph (1a), iPr (1b), Cy (1c)) or 1d produced the corresponding four-coordinate nickel hydride complexes 2a−d. Attempts to isolate 2a led instead to the cyclooct-4-en-1-yl complex [Ph-PNP]Ni(η1-C8H13) (3a) as a consequence of COD insertion into the Ni−H bond of 2a. The reactions of 2a,d with ethylene, 1-hexene, and norbornene, respectively, generated cleanly the corresponding ethyl (4a,d), n-hexyl (5a,d), and 2-norbornyl (6a,d) complexes. The quantitative formation of 5a,d is indicative of exclusive 1,2-insertion of 1-hexene. In contrast, styrene inserts into the Ni−H bond of 2d in an exclusively 2,1-manner to afford [Ph-PNP-iPr]NiCH(Me)Ph (7d) quantitatively. The selective 2,1-insertion products [Ph-PNP]NiCH(Me)CO2Me (8a), [iPr-PNP]NiCH(Me)CO2Me (8b), [Cy-PNP]NiCH(Me)CO2Me (8c), and [Ph-PNP-iPr]NiCH(Me)CO2Me (8d) were also isolated from the reactions of methyl acrylate with the corresponding nickel hydride complexes 2a−d. The effects of the phosphorus and olefin substituents on the reactivity and regioselectivity of the olefin insertion reactions are discussed. In addition to solution NMR spectroscopic data for all new compounds, X-ray structures of 1d, 2b−d, 3a, and 5d−7d are reported.

Synthesis, crystal structure and thermal behavior of dimethylgold(III) derivatives of salicylaldimine Schiff bases – Novel precursors for gold MOCVD applications

Dimethylgold(III) complexes with salicylaldimine Schiff bases of general formula Me 2Au(Sal N–R) (R = methyl ( 1), iso-propyl ( 2), or cyclohexyl ( 3)) have been synthesized in a single step reaction from dimethylgold(III) iodide [Me 2AuI] 2 and N-substituted salicylaldimines. Single-crystal X-ray studies provide evidence of mononuclear four-coordinated complexes with square-planar coordination geometry. The temperature dependences of saturated vapour pressure of complexes have been studied by the Knudsen effusion method with mass spectrometric analysis of the composition of the gas phase. The thermodynamic parameters of the sublimation process Δ H o T and Δ S o T have been calculated. Thermal decomposition of the vapour of compounds has been studied by means of high temperature mass spectrometry in a vacuum, and by-products have been determined. All of these complexes show good volatility and thermal stability. For complex 2, which is more volatile and low-melting, pulse MOCVD experiments have been carried out at substrate temperatures lower than 200 °C. The deposited gold nanoparticles (5–15 nm) have been investigated by means of SEM and AFM.

An Iron Nitride Complex

Coordination compounds of iron in high oxidation states have been invoked as reactive intermediates in biocatalyses. Iron(IV) ferryl species are examples of such highly reactive species that have long been known to be at the catalytic centers of oxygenases. Supported by X-ray diffraction studies on nitrogenase, the iron nitride moiety has recently been suggested to be present at the site of biological nitrogen reduction. As a result, well-characterized high-valent iron complexes have been sought as biomimetic models for transformations mediated by iron-containing enzymes. To gain understanding of iron nitride reactivity and the possible role of such species in biocatalysis, insight into the molecular structure of complexes stabilizing the [FeN] synthon is highly desirable. Whereas significant progress has been made in the synthesis and spectroscopic elucidation of Fe=NR and Fe N species, X-ray crystallographic characterization of a complex with a terminal Fe N functionality has not been accomplished. The first mononuclear Fe=O entity crystallographically characterized was stabilized in an octahedral environment provided by a macrocyclic tetra-N-methylated cyclam ligand. Similar cyclam derivatives also allow the stabilization and detailed spectroscopic characterization of octahedral Fe and Fe nitride complexes in unusually high oxidation states. 10] Recently, Peters and Betley developed a stunningly redox-rich iron system employing the tripodal tris(phosphino)borate ligand system (PhBP3 ), which stabilizes tetrahedral L3Fe=Nx species in oxidation states ranging from + I to + IV. Remarkably, this ligand system enabled the first room-temperature spectroscopic characterization of a terminal Fe nitride species. Concentration-dependent coupling to the Fe-N2-Fe I dinuclear product, however, prevents crystallization of this nitride species. We herein present the synthesis, spectroscopy, and most significantly, the X-ray diffraction analysis of a discrete iron nitride complex stabilized by the sterically encumbering Nanchored tris(carbene) ligand, tris[2-(3-aryl-imidazol-2-ylidene)ethyl]amine (TIMEN, R= xylyl (xyl), mesityl (mes)). Structurally and electronically related to the tetrahedral phosphinoborate ligand system by Peters and Betley, this tripodal N-heterocyclic carbene (NHC) system coordinates a high-spin Fe center in a trigonal-planar fashion, thus forming four-coordinate complexes with the metal ion in trigonalpyramidal environments. Under inert atmosphere, treatment of TIMEN with one equivalent of anhydrous ferrous chloride in pyridine at room temperature yields the four-coordinate Fe complexes [(TIMEN)Fe(Cl)]Cl (1, 1) as analytically pure, white powders in 80% yield (Scheme 1).

Mesomorphism of Four-Coordinated Four-Chained Metal Complexes Based on Pyrazolylpyridine Derivatives

A homologous series of four-chained silver(I) and zinc(II) metal complexes based on the coordination of two N N′-bidentate ligands pzR2py 2-[3,5-bis(4-alkyloxy)phenyl]pyrazol-1-yl]pyridine (R = C6H4OCnH2n+1, n = 16, 18) have been synthesised. All compounds [M(pzR2py)2][A]m (M = Ag, m = 1, n = 16, 18, (1,2), (3,4), CF3 (OTf–) (5,6), (7,8); M = Zn, m = 2, n = 16, 18, (9,10)) exhibit liquid crystal properties showing enantiotropic SmA phases, independently of the starting pzR2py ligands (non-mesomorphism for n = 18 or monotropic liquid crystal for n = 16), as well as the coordination geometry. The silver(I) derivatives [Ag(pzR2py)2]+ containing and as counteranions (1–4) are four-coordinated cationic complexes, but those containing OTf– and (5–8) exhibit a five-coordinated metal environment with distorted base-square pyramidal/trigonal bipyramidal geometry involving the counteranion. For the zinc(II) related compounds [Zn(pzR2py)2][NO3]2 (9–10), the tetrahedral metal environment is also distorted by the presence of coordinative interactions with the groups. Neutral tetrahedral compounds [ZnCl2(pzR2py)] (R = C6H4OCnH2n+1, n = 16 (11), 18 (12), 1 (13)) have also been investigated. Crystal structure of the first member of this series with n = 1, [ZnCl2(pzAn2py)] (13), has been characterised by single crystal X-ray diffraction. The molecules adopt the tetrahedral coordination environment with a planar ZnNNCN core. That structure for the related compounds containing longer alkyloxy chains (11–12) suggests a rod-like molecular shape in agreement with their observed liquid crystal lamellar phases.

Reduction of CO2 to CO Using Low-Coordinate Iron: Formation of a Four-Coordinate Iron Dicarbonyl Complex and a Bridging Carbonate Complex

A reduced diiron(I) complex reacts with CO(2) to give two iron-containing products. One product has a carbonate bridge, which isomerizes rapidly at -70 degrees C and may be derived from an oxodiiron intermediate. The formation of this product releases free CO, which leads to a four-coordinate iron dicarbonyl complex. This product is the first crystallographically characterized example of a four-coordinate iron dicarbonyl species, a moiety that may be present in the active site of Hmd ("iron-sulfur cluster free") hydrogenase.

Zinc and enolato-magnesium complexes based on bi-, tri- and tetradentate aminophenolate ligands

The coordination chemistry of a series of potentially tetra-, tri- and bi-dentate aminophenol pro-ligands ([L1]H–[L5]H) with magnesium and zinc derivatives has been studied. Reactions of the pro-ligands [L1]H, [L2]H and [L3]H with one equiv. of ZnEt2 or Zn(N(SiMe3)2)2 in toluene at room temperature afford cleanly, viaethane or amine elimination, the ethyl- and amido-zinc complexes 1–4. Complexes [L4]ZnEt (5) and [L4]Zn(N(SiMe3)2)2 (6) derived from the tridentate pro-ligand [L4]H were prepared in 84% and 76% yields, following similar alkane and amine elimination protocols, respectively. The 1 : 1 reactions of bidentate pro-ligand [L5]H with ZnEt2 or Zn(N(SiMe3)2)2 in toluene using different reaction protocols systematically yielded mixtures of the bis(ligand) complex [L5]2Zn (7) and the corresponding ethyl-{[L5]ZnEt}n (8; n = 1 or 2) or amido- {[L5]Zn(N(SiMe3)2)}n complexes (9; n = 1 or 2). The synthesis of magnesium-enolate complexes 10 and 11 was carried out in a one-pot, two-step procedure by first reacting pro-ligands [L1]H and [L4]H with one equiv. of Mg(n,sBu)2 to generate the corresponding {[Ln]Mg(n,sBu)}n species, which were further reacted with one equiv. of 2,4,6-Me3C6H2COMe. All complexes have been characterized by multinuclear NMR, elemental analysis, and by single-crystal X-ray diffraction studies for five-coordinate magnesium complex 10, and four-coordinate Zn complexes 3–7. Preliminary studies indicate that magnesium species 10 and 11 as well as zinc complexes 1 and 5 are not active initiators for the polymerization of methyl methacrylate, even in combination with one equiv. of Li[OC(CH2)(2,4,6-Me3C6H2)], while zinc complexes 2, 4 and 6 are effective initiators for the ring-opening polymerization of ε-caprolactone and rac-lactide at 20 °C.

Mechanistic Insight into the Diastereoselective Rhodium‐Catalyzed Pauson–Khand Reaction: Role of Coordination Number in Stereocontrol

How much CO? Theoretical analysis of the origin of diastereocontrol in the rhodium-catalyzed Pauson–Khand reaction (see scheme) provides two mechanistic scenarios in which optimum selectivity can be attributed to a five- rather than a four-coordinate organorhodium complex. The relative population of these complexes is related to carbon monoxide concentration, a finding which is in contrast to phosphine-containing rhodium(I) complexes.