Cis Oxo Groups Research Articles

Complexes of cis-dioxomolybdenum(VI) with a chiral tetradentate tripodal-like ligand system: Syntheses, structures and catalytic activities

Racemic complexes with the general formula cis-[MoO2(bzacLn)] (1–4) (H2bzacLn=2-((4/5-R-2-hydroxyphenylamino)(pyridin-2-yl)methyl)-1-phenylbutane-1,3-dione, where n=1–4 for R=H, 5-Me, 5-Cl and 4-Me, respectively and 2Hs represent the dissociable phenolic proton and the active tertiary CH proton) have been synthesized in 75–82% yields by reacting [MoO2(bzac)2] (Hbzac=benzoylacetone) with the potentially N2O-donor 5,5-membered fused chelate rings forming Schiff bases 4/5-R-2-(2-pyridylaldimine)phenols (HLn; n=1–4 for R=H, 4-Me, 4-Cl and 5-Me, respectively) in hot methanol. The chiral ligand system (bzacLn)2− in 1–4 is formed via metal assisted Mannich-type addition of benzoylacetonate methine to the azomethine fragment of HLn. All four complexes have been characterized by elemental (CHN) analysis, solution conductivity, magnetic susceptibility, spectroscopic (IR, UV–Vis and NMR) and electrochemical measurements. The molecular structures of 1–3 have been established by single crystal X-ray crystallography. In each complex, the chiral (bzacLn)2− acts as a tetradentate, N2O2-donor, tripodal-like ligand system and along with the two mutually cis oxo groups forms a distorted octahedral N2O4 coordination environment around the molybdenum(VI) center. All four complexes are diamagnetic and non-electrolytic. The infrared spectra are generally consistent with the structural formulas of 1–4. The electronic spectra of 1–4 in dimethylformamide display two strong absorption bands in the range 245–300nm. The cyclic voltammograms of 1–4 in dimethylformamide exhibit a metal centered one-electron reduction response within −0.64 to −0.74V. All these complexes (1–4) and the analogous cis-[MoO2(acacL1–4)] (5–8) synthesized from [MoO2(acac)2] (Hacac=acetylacetone) and HL1–4 have been evaluated for their bromoperoxidase activities.

Cis‐Dioxomolybdenum(VI) complexes with unsymmetric linear tetradentate ligands: syntheses, structures and bromoperoxidase activities

Reactions of [MoO2(acetylacetonate)2], 2‐((2‐(2‐hydroxyethylamino)ethylamino)methyl)‐4‐R‐phenols (H2Ln, n = 1–5 for R = H, Me, OMe, Cl and Br, respectively) and KOH in 1:1:2 mole ratio in methanol afford a series of complexes having the general formula cis‐[MoO2(Ln)] (1, 2, 3, 4, 5) in 81–86% yields. The complexes have been characterized using elemental analysis, spectroscopy (infrared, UV–visible, and 1H NMR, 13C NMR and 13C‐DEPT NMR) and electrochemical measurements. The molecular structures of 1, 2, 3, 4 have been determined using single‐crystal X‐ray crystallography. In each of 1, 2, 3, 4, the ONNO‐donor 6,5,5‐membered fused chelate rings forming (Ln)2− and the two mutually cis oxo groups assemble a distorted octahedral N2O4 coordination sphere around the metal centre. In the crystal lattice, each of 1, 2, 3, 4 forms a one‐dimensional infinite chain structure via intermolecular NH⋅⋅⋅O hydrogen bonding interactions. In cyclic voltammograms, the diamagnetic complexes display an irreversible metal‐centred reduction in the potential range −0.73 to −0.88 V (vs Ag/AgCl). The physicochemical data are consistent with a very similar gross molecular structure for all of 1, 2, 3, 4, 5. All the complexes exhibit decent bromoperoxidase activities and are also able to effectively catalyse benzoin and methyl(phenyl)sulfide oxidation reactions. Copyright © 2015 John Wiley & Sons, Ltd.

Structure and spectroscopy of a bidentate bis-homocitrate dioxo-molybdenum(VI) complex: Insights relevant to the structure and properties of the FeMo-cofactor in nitrogenase

Direct reaction of potassium molybdate (with natural abundance Mo or enriched with 92Mo or 100Mo) with excess hydrolyzed homocitric acid-γ-lactone in acidic solution resulted in the isolation of a cis-dioxo bis-homocitrato molybdenum(VI) complex, K2[*MoO2(R,S-H2homocit)2]·2H2O (1) (*Mo=Mo, 1; 92Mo, 2; 100Mo, 3; H4homocit=homocitric acid-γ-lactone·H2O) and K2[MoO2(18O-R,S-H2homocit)2]·2H2O (4). The complex has been characterized by elemental analysis, FT-IR, solid and solution 13C NMR, and single crystal x-ray diffraction analysis. The molybdenum atom in (1) is quasi-octahedrally coordinated by two cis oxo groups and two bidentate homocitrate ligands. The latter coordinates via its α-alkoxy and α-carboxy groups, while the β- and γ-carboxylic acid groups remain uncomplexed, similar to the coordination mode of homocitrate in the Mo-cofactor of nitrogenase. In the IR spectra, the MoO stretching modes near 900cm−1 show 2–3cm−1 red- and blue-shifts for the 92Mo-complex (2) and 100Mo-complex (3) respectively compared with the natural abundance version (1). At lower frequencies, bands at 553 and 540cm−1 are assigned to νMo–O vibrations involving the alkoxide ligand. At higher frequencies, bands in the 1700–1730cm−1 region are assigned to stretching modes of protonated carboxylates. In addition, a band at 1675cm−1 was observed that may be analogous to a band seen at 1677cm−1 in nitrogenase photolysis studies. The solution behavior of (1) in D2O was probed with 1H and 13C NMR spectra. An obvious dissociation of homocitrate was found, even though bound to the high valent Mo(VI). This suggests the likely lability of coordinated homocitrate in the FeMo-cofactor with its lower valence Mo(IV).

Enantiomeric and mesomeric mandelate complexes of molybdenum – on their stereospecific formations and absolute configurations

The stereospecific formation and absolute configuration of R-homocitrate coordinated FeMo-co in nitrogenase was mimicked through the structural analyses of a collection of enantiomeric and mesomeric mandelato molybdenum complexes, i.e., (NH 4) 2[Mo ΔO 2( R-mand) 2] · 3H 2O ( 1a), (NH 4) 2[Mo ΛO 2( S-mand) 2] · 3H 2O ( 1b), (NH 4) 4[Mo ΔO 2( RS-mand) 2][Mo ΛO 2( RS-mand) 2] · 8H 2O ( 2), (NH 4) 2[W ΔO 2( R-mand) 2] · 2H 2O ( 3a), (NH 4) 2[W ΛO 2( S-mand) 2] · 2H 2O ( 3b) (H 2mand = mandelic acid, C 8H 8O 3), which have been characterized by elemental analyses, optical rotation, circular dichroism, IR, NMR spectroscopes and X-ray single crystal studies. The R and S chiral mandelic acids induce the formations of the enantiomeric pair of chiral complexes, which are supported by the characterizations of optical rotation and circular dichroism. The configuration of the resulted metal center could be assigned as Δ or Λ. While the RS racemic reagent yields only mesomeric compound. The Δ R, R -complexes 1a and 3a are enantiomers of Λ S, S - 1b and 3b, respectively. Of the five complexes, Mo and W atoms are all hexa-coordinated by two cis-oxo groups and two bidentate mandelate ligands through the deprotonated α-alkoxyl and α-carboxyl groups, forming a stable five-membered chelated rings. The average Mo(VI)–O bond distances with α-alkoxyl and α-carboxyl are 1.944 and 2.210 Å, respectively. Further comparison indicates that bonds of α-alkoxyl groups in the hydroxycarboxylato molybdenum complexes are much sensitive to the change in the oxidation state of molybdenum, which support the possible Mo activation model in FeMo-co through the protonation and cleavage of α-alkoxyl group in homocitrate ligand.

Synthesis, spectroscopic and structural properties of hexavalent molybdenum complexes with thio- and seleno-ether ligands.

The highly unusual Mo(VI) thioether complexes [MoO(2)X(2)(L-L)][space](X = Cl or Br; L-L = MeS(CH(2))(2)SMe or EtS(CH(2))(2)SEt) were obtained by reaction of MoO(2)X(2) with L-L in rigorously anhydrous CH(2)Cl(2) solution. Similar reaction of MoO(2)Cl(2) with the diselenoether MeSe(CH(2))(2)SeMe gives the very reactive [MoO(2)Cl(2)[MeSe(CH(2))(2)SeMe]] as a yellow solid. These compounds are very moisture sensitive and were characterised by IR, diffuse reflectance UV-vis and multinuclear ((1)H, (13)C[(1)H], (77)Se and (95)Mo) NMR spectroscopy. The data are consistent with distorted 6-coordination at Mo(vi)viatrans X ligands, mutually cis oxo groups and a chelating dithio- or diseleno-ether ligand. Variable temperature (1)H and (13)C[(1)H] NMR data indicate fast pyramidal inversion at the coordinated chalcogen atoms occurs at room temperature, but cooling slows this process to reveal resonances consistent with the meso and dl forms. The (95)Mo NMR spectra are single resonances in the region 200-300 ppm, as expected for Mo(vi) complexes, and show inverse dependence of the chemical shifts upon both halide and chalcogen type. Crystal structures of three of the dithioether complexes are described and provide unequivocal evidence for Mo(vi) thioether coordination, confirming chelation of the dithioether through long Mo-S interactions of ca. 2.7 [Angstrom]. Attempts to extend the range of compounds by using other chalcogenoether ligands failed, indicating that to obtain complexes involving these extremely mis-matched metal ligand combinations requires both the favourable 5-membered chelate ring and small terminal alkyl substituents on the chalcogen.

Synthesis and characterization of homochiral polymeric S-malato molybdate(VI): toward the potentially stereospecific formation and absolute configuration of iron-molybdenum cofactor in nitrogenase

Reaction of sodium or potassium molybdate and excess malic acid in a wide range of pH values (pH 4.0–7.0) resulted in the isolation of two cis-dioxo-bis(malato)-Mo(VI) complexes, viz. Na 3[MoO 2H( S-mal) 2] and K 3[MoO 2H( S-mal) 2]·H 2O (H 3mal=malic acid). The sodium complex is also characterized by an X-ray structure analysis, showing that the mononuclear Mo units are linked together via very strong symmetric CO 2···H··· O 2C-hydrogen bond [2.432(5) Å], forming a polymeric chain. The molybdenum atoms are quasi-octahedrally coordinated by two cis-oxo groups and two bidentate malate ligands via its alkoxy and α-carboxyl groups, while the β-carboxylic and carboxylate groups remain uncomplexed, as the coordination of vicinal carboxylate and alkoxide of homocitrate in FeMo cofactor of nitrogenase. The absolute configuration of the metal center in this S-malato complex is assigned as Λ and the homochirality within the chain is established as a homochiral form ···Λ S–Λ S–Λ S–Λ S···. It is proposed that the chiral configuration of the metal center in wild-type FeMo-co biosynthesis might be induced by the early coordination of the chiral R-homocitric acid, while a mixture of raceme might be obtained in the biosynthesis of NifV − FeMo-cofactor. The absolute configuration of wild-type FeMo-cofactor is assigned as Δ R.

Tungsten–malate interaction. Synthesis, spectroscopic and structural studies of homochiral S-malato tungstate(VI), Λ-Na 3[WO 2H( S-mal) 2

Reaction of the sodium tungstate and malic acid at 1 to 2 ratio in weak acidic solution (pH 3.0–6.0) resulted in the first isolation and structural characterization of a tungsten(VI) malato complex Λ-Na 3[WO 2H( S-mal) 2] ( 1) (malic acid=H 3mal). The complex has been characterized by elemental analysis, UV and IR spectroscopy. The IR spectra are consistent with an oxo-coordinated mononuclear structure as revealed by single crystal X-ray diffraction study. The tungsten atoms are quasi-octahedrally coordinated by two cis-oxo groups and two bidentate malate ligands via its alkoxy and α-carboxyl groups, while the β-carboxylic and carboxylate groups remain uncomplexed. The latter are bonded each other through strong hydrogen bonding, forming a catenarian chain, and the anions are connected in homochiral form. It is proposed that the chiral configuration of metal center in wild-type FeMo-co biosynthesis might be induced by the coordination of the chiral R-homocitric acid, while a mixture of raceme might be obtained in the biosynthesis of NifV–FeMo-cofactor.

Rhenium and molybdenum oxo-complexes containing ligands related to N-hydroxyiminodipropionic acid

The reaction of N-hydroxy-2,2′-iminodipropionic acid with [PPh 4 ][ReOBr 4 ], [ReOCl 3 (PPh 3 ) 2 ], [ReOCl 2 (OEt)(PPh 3 ) 2 ] or [PPh 4 ][MoOCl 4 (H 2 O)] has produced four novel complexes: [ReOBr 3 (C 6 H 8 NO 5 )] - , [ReOBr 3 (C 3 H 3 NO 2 )] - , [ReOCl 2 (R,S-idpa)] - (H 3 idpa = 2,2′-iminodipropionic acid) and [MoO 2 Cl 2 (C 3 H 4 NO 3 )] - . All have been successfully crystallised as their [PPh 4 ] + salts which have been characterised by single-crystal X-ray diffraction. The anions [ReOBr 3 (C 6 H 8 NO 5 )] - and [ReOBr 3 (C 3 H 3 NO 2 )] - involve tetragonally distorted octahedral Re V OBr 3 (N,O) centres with the bromides cis to the oxo-group; [ReOCl 2 (R,S-idpa)] - involves a tetragonally distorted octahedral Re V OCl 2 (O,N,O) centre with the 2Cl cis and O trans to the oxo-group; [MoO 2 Cl 2 (C 3 H 4 NO 3 )] - involves a skew-trapezoidal Mo VI O 2 Cl 2 (N,O) centre with essentially mutual cis oxo-groups and trans chloro-groups. The dimensions of each of these co-ordination spheres are generally typical of related complexes; however, [ReOBr 3 (C 3 H 3 NO 2 )] - involves an unusually short Re–O oxo bond of 1.45(4) A. The organic moieties are considered to arise from rearrangement and/or fragmentation of N-hydroxy-2,2′-iminodipropionic acid, presumably under the influence of the metal centre.

Structure, characterization and photoreactivity of monomeric dioxovanadium(V) Schiff-base complexes of trigonal-bipyramidal geometry

Five mononuclear cis-dioxovanadium(V) complexes of tridentate Schiff bases derived from salicylaldehyde and its derivatives and 8-aminoquinoline have been synthesized and characterized. Single-crystal X-ray analyses were performed with [VO2L1]1 and [VO2L2]2(L1 and L2 denote the [1 + 1] Schiff-base anions derived from salicylaldehyde or naphthaldehyde and 8-aminoquinoline). While the structure of 2 was refined to a final R= 0.074 (R′= 0.055), that of 1 was refined only to R= 0.134 (R′= 0.139) due to its poor diffraction quality. The complexes contain cis-oxo groups in the equatorial plane and a trigonal-bipyramidal geometry around the vanadium at which the Schiff base binds meridionally. Photoirradiation of these complexes in CH2Cl2 yielded chloride-bound VO3+ species, as studied using absorption and 51V NMR spectroscopy. These species are convertible back to the dioxovanadium(V) complexes upon addition of water to the photoirradiated solution.

Dioxomolybdenum(VI) Complexes Stabilized by Polydentate Ligands with NO3, N2O2, and NS2 Donor-Atom Sets

Dioxomolybdenum(VI) complexes of the general formula LMoO 2 (L=(OCH 2 CH 2 ) 2 N(CH 2 CH 2 OH) (1), O(CH 2 ) 2 -NMe(CH 2 ) 2 NMe(CH 2 ) 2 O (2), HN(CH 2 CH 2 S) 2 (3)) have been synthesized and characterized. The structures of all compounds have unambiguously been determined in solution with the use of NMR spectroscopy. Complexes 1 and 2 have a distorted octahedral coordination about Mo with cis oxo groups and two alkoxo oxygen atoms in the apical positions. The coordination sphere is completed by amino and alkoxo oxygen atoms in 1 and by two N atoms in 2. Dissolution of 1 in H 2 O promotes hydrolysis of the remaining hydroxy arm of the ligand to give the anionic species H[MoO 2 (OCH 2 CH 2 ) 3 N] in which all the alkoxo arms are equivalent

Synthesis and crystal and molecular structure of [(nC 4H 9) 4N] 2[MoO 2(C 5O 5) 2]; confirmation of exclusively η 2-coordination by a croconate ligand

The reaction of [(nC 4H 9) 4N] 2[Mo 2O 7] with BaC 5O 5 in acetonitrile yields red crystals of [(n C 4H 9) 4N] 2[MoO 2(C 5O 5) 2]. The compound crystallizes in the monoclinic space group C2/ c with a=36.502(11), b=14.681(3), c=18.109(5) Å, β=93.84(1)°, V=9682.5(15) Å 3, D calc=1.22 g cm −3 for Z=8. Structure solution and refinement based on 3593 reflections with F o⩾6σ(F o) Mo Kα, λ= 0.71073 Å) converged at 0.0693. The structure of the anion consists of a mononuclear Mo(VI) center in ar distorted octahedral environment of the cis-oxo groups and the oxygen donors of two bidentate croconate ligands. The 1,2-chelating geometry of the croconate is unusual in that all previously structurally characterized examples of croconate coordination exhibit additional bonding to a second metal site via the oxygen donor in the 4 position.

The crystal and molecular structures of dioxo mo(VI) complexes of tripodal, tetradentate N,S-donor ligands

The crystal and molecular structures of the complexes MoO 2((SCH 2CH 2) 2NCH 2CH 2SCH 3), I and MoO 2((SCH 2CH 2) 2NCH 2CH 2N(CH 3) 2), II, have been determined from X-ray intensity data collected by counter methods. Compound I crystallizes in two forms, Ia and Ib. In form Ia the space group is P2 1/n with cell parameters a = 7.235(2), b = 7.717(2), c = 24.527(6) Å, β = 119.86(2)°, V = 1188(1) Å 3, Z = 4. In form Ib the space group is P2 1/c with cell parameters a = 14.945(5), b = 11.925(5), c = 14.878(4) Å, β = 114.51(2)°, V = 2413(3) Å 3, Z = 8. The molecules of I in Ia and Ib are very similar having an octahedral structure with cis oxo groups, trans thiolates ( cis to both oxo groups) and N and thioether sulfur atoms trans to oxo groups. Average ditances are MoO = 1.70, MoS (thiolate) = 2.40, MoN = 2.40 and MoS (thioether) = 2.79 Å. Molecule II crystallizes in space group P2 12 12 1 with a = 7.188(1), b = 22.708(8), c = 7.746(2) Å, V = 1246(1) Å 3 and Z = 4. The coordination about Mo is octahedral with cis oxo groups, trans thiolates and N atoms trans to oxo. Distances in the first coordination sphere are MoO = 1.705(2), 1.699(2), MoS = 2.420(1), 2.409(1) and MoN = 2.372(2), 2.510(2) Å. The conformational features of the complexes are discussed. Complex I displays MoO and MoS distances which are very similar to those found by EXAFS in sulfite oxidase. This similarity is discussed.

Structural comparison of octahedral MoO 22+ complexes of bidentate and linear tetradentate N,S-donor ligands

The structures of MoO 2[NH 2C(CH 3) 2CH 2S] 2 and MoO 2[SC(CH 3) 2CH 2NHCH 2CH 2NHCH 2C(CH 3) 2S] have been determined using X-ray diffraction intensity data collected by counter techniques. MoO 2[NH 2C(CH 3) 2CH 2S] 2 crystallizes in space group Pbca with a = 11.234(3), b = 11.822(3) and c = 20.179(5) Å, V = 2680(2) Å 3 and Z = 8. Its structure is derived from octahedral coordination with cis oxo groups [MoO = 1.705(3) and 1.705(3)], trans thiolate donors cis to the oxo groups [MoS = 2.416(1) and 2.402(1) and N donors trans to oxo [MoN = 2.325(3) and 2.385(4) Å]. MoO 2[SC(CH 3) 2CH 2NHCH 2CH 2NHCH 2C(CH 3) 2S] crystallizes in the space group P2 1/c with a = 10.798(5), b = 6.911(2), c = 20.333(9) Å, β = 95.20°, V = 1511(2) Å 3 and Z = 4. Its structure is very similar to that of MoO 2[NH 2C(CH 3) 2CH 2S] 2 with MoO = 1.714(2) and 1.710(2), MoS = 2.415(1) and 2.404(1) and MoN = 2.316(3) and 2.362(3). The small differences in the geometries of the two compounds are attributed to the constraints of the extra chelate ring in the complex with the tetradentate ligand. The structures in this paper stand in contrast to those reported for complexes of similar ligands wherein steric hindrance produces complexes with a skew trapezoidal bipyramidal structure.