Dinuclear Cobalt Complexes Research Articles

The photodecarboxylation of [N,N-bis(2-pyridylmethyl)amino acidato]phenanthrolinecobalt(III) complexes: – formation and decomposition of metallacyclic species †

Photolysis of [N,N-bis(2-pyridylmethyl)amino acidato]phenanthrolinecobalt(III) complexes (amino acidato = glycinato (dpg) or alaninato (dpa) or 2-cyclopropylglycinato (dpc)) with UV light led to elimination of carbon dioxide from the amino acidato chelate and the formation of cobalt(III) complexes which contain a Co–C–N metallacycle. These photolysis products were characterised by NMR and UV-vis spectroscopy and, in the case of dpg, by X-ray crystallography. The eventual decomposition reactions of the photolysis products were monitored by NMR spectroscopy in both D2O and dilute DCl. The decomposition products included bis(2-pyridylmethyl)amine (bpa) and an aldehyde derived from the carbon atom of the metallacycle and its alkyl substituents. A μ-peroxo dinuclear cobalt(III) complex, [(bpa)(phen)(Co(O2)Co(phen)(bpa)]4+, was formed from the decomposition products and its structure determined by X-ray crystallography. Experiments with a cyclopropyl derivative demonstrate that any intermediates with a radical centre on what was the α-carbon of the amino acid must be very short-lived.

Synthesis, Crystal Structure, Magnetic Properties and Cyclic Voltammetry of the Unsymmetric (μt-Oxalato)-[N, N′, N″, N″′-Tetrakis-(2-Pyrii)Ylmethyl)-1, 4, 8, 11-Tetraazacyclotetfw-Decane]Dicobalt(II) Pekchlokate Trihydkate

A new dinuclear cobalt(II) complex, [Co2(ox)tpmc](ClO4)2 3H2O, (tpmc = N, N′, N″, N″′-tetrakis(2-pyridylmethyl)-1, 4, 8, 11-tetraazacyclotetradecane, ox2 -oxalate ion), has been synthesized and its crystal structure solved by single crystal X-ray diffraction studies. It crystallizes in the monoclinic system, space group P21, with a = 97558(8) A, b = 21.455(2) A, c - 21.241(2) A, β=100.069(8)°, V=4391.0(8) A3, Z =4,R = 0.083, Rw = 0.086. Its structure consists of two dinuclear, crystallographically unique molecules in which the oxalate ion bridges two cobalt(II) ions unsymmetrically. E:ach cobalt is hexa-coordinated with four macrocyclic nitrogens and two oxygens in an octahedral arrangement of an “em” coordination of a macrocyclic ligand. The third oxygen is simultaneously bonded to both cobalt ions and the last one remains uncoordinated, which is the unique way of oxalato ion coordination. Intramolecular antiferromagnetic coupling between the two cobalt(II) ions is observed and yielded J = -9.3 an−1, g - 5.64 and g = 1.21. The variable temperature (4.2-192 K) inagiietic susceptibility nieasureinents are interpreted on the basis of the spin lamiltonian H 25/9JSASB + goβHz(SAZ - SBZ). Cyclic voltanllnetiy suggests that the complex is electrochcmically stable up to the potential of the cobalt reduction.

Metal–Metal Interactions in Dinuclear (triphos)Cobalt Complexes Exhibiting Mixed Valency

The spectroscopic and electrochemical properties of a series of dinuclear mixed-valence complexes containing two (triphos)Co units are reported: [(triphos)Co(L)Co(triphos)]+ (L = C6O4X2; × = H, Cl, Br, I, Me: 2a–e+; L = C14H4O4Me2: 4+). Complexes 2a–e+ are bridged by tetraoxybenzene ligands and exhibit very strong metal–metal interaction leading to delocalized class-III behaviour while in 4+ the extended tetraoxoanthracence bridging ligand leads to a partial electron localization. Additionally, the different oxidation behaviour of the parent dicationic complexes 1a–e2+ and 32+ have been investigated and are explained on the basis of a qualitative MO model.

A Structural and Functional Model of Dinuclear Metallophosphatases

Hydrolysis of four substituted phenyl phosphate monoesters, each coordinated to a dinuclear Co(III) complex, was studied ([Co2(tacn)2(OH)2{O3P(OAr)}]2+; tacn = 1,4,7-triazacyclononane; substituent m-F, p-NO2 (5a); p-NO2 (5b); m-NO2 (5c); unsubstituted (5d)). Crystallographic data reveal that 5b is an excellent structural model of the active sites of several phosphatases: protein phosphatase-1, kidney bean purple acid phosphatase, and calcineurin-α. All of these structures consist of two octahedral metal complexes connected by two oxygen bridges, forming a four-membered-ring diamond core. The pH−rate profile and the 18O labeling experiment for the hydrolysis of 5b indicates that the oxide bridging the two metal centers in the diamond core is acting as an intramolecular nucleophile for cleaving the coordinated phosphate monoester. The phosphate monoesters in this model system are hydrolyzed more rapidly than those in previously reported model systems. Hence, the dinuclear cobalt complexes 5 appear to be excellent structural and functional models of the above-mentioned phosphatases. The rate of hydrolysis of 5 is highly sensitive to the basicity of the leaving group (βlg = −1.10). Detailed analysis of the leaving group dependence for the hydrolysis of 5 indicates only a partial negative charge on the leaving group oxygen at the transition state, further supporting the nucleophilic mechanism.

Metal–Metal Interactions in Dinuclear (triphos)Cobalt Complexes Exhibiting Mixed Valency

The spectroscopic and electrochemical properties of a series of dinuclear mixed-valence complexes containing two (triphos)Co units are reported: [(triphos)Co(L)Co(triphos)]+ (L = C6O4X2; × = H, Cl, Br, I, Me: 2a–e+; L = C14H4O4Me2: 4+). Complexes 2a–e+ are bridged by tetraoxybenzene ligands and exhibit very strong metal–metal interaction leading to delocalized class-III behaviour while in 4+ the extended tetraoxoanthracence bridging ligand leads to a partial electron localization. Additionally, the different oxidation behaviour of the parent dicationic complexes 1a–e2+ and 32+ have been investigated and are explained on the basis of a qualitative MO model.

Structure and Reactivity of Dinuclear Cobalt(III) Complexes with Peroxide and Phosphate Diester Analogues Bridging the Metal Ions

Structure and Reactivity of Dinuclear Cobalt(III) Complexes with Peroxide and Phosphate Diester Analogues Bridging the Metal Ions

Reaction of the dinuclear cobalt complex [Co 2{μ-PPh 2CHCHC(O)CHCHPPh 2}(CO) 4] with two-electron donor ligands; the stabilisation of cobalt-alkene bonds by incorporation of alkenes into a bridging diphosphine ligand

The thermal reaction of [Co 2{μ-PPh 2CHCHC(O)CHCHPPh 2}(CO) 4] ( 2) with two-electron donor ligands, L, leads to the mono- bis-substituted complexes [Co 2{μ-PPh 2CHCHC(O)CHCHPPh 2}(CO) 3(L)] (L = PPhMe 2 ( 3a), PPh 2H ( 3b), P(OMe) 3 ( 3c). CN tBu ( 3d) and [Co 2{μ-PPh 2CHCHC(O)CHCHPPh 2}(CO) 2(L) 2] (L = PPhMe 2 ( 4a), PPh 2H ( 4b), P(OMe) 3, ( 4c), CN tBu ( 4d)), in which CO groups rather than bound alkenes have been displaced from the metal centres. Single crystal X-ray diffraction has been used to determine the structure of [Co 2{μ-PPh 2CHCHC(O)CHCHPPh 2}(CO) 2(PPhMe 2) 2]·0.25[C 6H 14] ( 4a) (which crystallises i monoclinic space group P2 t / c with a = 15.932(2), b = 12.608(4), c = 23.847(3) A ̊ , β = 98.87(3)°, Z = 4 ).

Reactivity of Phosphate Diesters Doubly Coordinated to a Dinuclear Cobalt(III) Complex: Dependence of the Reactivity on the Basicity of the Leaving Group

Reactivities of eight phosphate diesters each coordinated to a dinuclear Co(III) complex were investigated ([Co2(tacn)2(OH)2{O2P(OR)2}]3+; tacn = 1,4,7-triazacyclononane). Four of the complexes were coordinated by substituted phenyl methyl phosphates (substituent m-F, p-NO2 (1a); p-NO2 (1b); m-NO2 (1c); unsubstituted (1d)) and two by substituted phenyl 2-hydroxypropyl phosphates (substituent p-NO2 (2a); unsubstituted (2b)). Reactivities of dialkyl phosphates coordinated to the dinuclear Co(III) complex (1,2-propylene phosphate (3); dimethyl phosphate (4)) were also investigated. Hydrolysis of the phosphate diesters in 1a to 1d takes place by intramolecular oxide attack on the bridging phosphate while hydrolysis of 3 principally takes place by intermolecular hydroxide attack on the bridging phosphate. The diester in 2a cleaves by intramolecular oxide attack while that in 2b cleaves by intramolecular transesterification. Dimethyl phosphate dissociates from 4 without any observable cleavage of the diester. These results can be understood in terms of the More O'Ferrall−Jencks energy diagram, and they indicate that there will be considerable cooperativity between the oxide activation and leaving group activation.

Synthesis and Structure of a Dinuclear Cobalt Complex Bridged by Nonsubstituted Borylene−Trimethylphosphine

Synthesis and Structure of a Dinuclear Cobalt Complex Bridged by Nonsubstituted Borylene−Trimethylphosphine

Structural, Magnetic, and Electrochemical Properties of Dinuclear Triple Helices: Comparison with Their Mononuclear Analogues

Comparison of mononuclear and dinuclear triple helical complexes of iron and cobalt (right) shows that metal ions in the dinuclear species have electronic environments essentially similar tothe mononuclear complexes. The formation of the complexes, however, occurs in a stepwise manner for the mononuclear complexes, but in a single self-assembly step for the dinuclear complexes.

Syntheses and structure of dinuclear cobalt(II) complexes of the polypyridine ligand 6,6′-bis[bis(2-pyridylmethyl)aminomethyl]-2,2′-bipyridine

A series of dinuclear cobalt(II) complexes of the octadentate polypyridine ligand 6,6′-bis[bis(2-pyridylmethyl)aminomethyl]-2,2′-bipyridine (L) has been prepared. The crystal structure of the complex [{Co(NCS)} 2L] · 0.4H 2O has been determined by X-ray diffraction methods. The cobalt atoms, bridged by L ( CoCo = 4.901(4) A ̊ ) , are trigonal bipyramidally coordinated by one nitrogen atom of the thiocyanato group and four nitrogen atoms of L. Magnetic measurements show that no magnetic interaction between the metal centers occurs.

Tuning the metal–metal separation in pyrazolate-based dinuclear complexes by the length of chelating side arms ‡

Six dinuclear cobalt(II) complexes of pyrazolate ligands with multidentate chelating side arms {3,5-(R2NCH2)2C3N2H2; R2N = [Me2N(CH2)3]2N (HL1) or (Et2NCH2CH2)2N (HL2)} have been prepared and characterised. The reaction of L1 and L2 with 2 equivalents of [Co(MeCN)6][BF4]2 and NaBPh4 proceeds via the isolable compounds [Co2L1(BF4)][BPh4]2 1a and [Co2L2(BF4)][BPh4]2 2a finally to afford the dinuclear complexes [Co2L1F][BPh4]2 1b and [Co2L2(F)(H2O)][BPh4]2 2b, respectively, where a fluoride has been abstracted from the BF4– starting material in both cases. While the longer side arms in 1b allow for an exogenous bridging position of the fluoride, an additional water molecule is incorporated in 2b to form an FHO(H) moiety with an unusually short F–H–O bridge between the two cobalt centres. Evidence is reported for the reversible extrusion of the water molecule from 2b. Treatment of 2b with KBr or NaN3 yielded the respective bromide- and azide-bridged complexes 3 and 4, where in the latter case the azide adopts a µ-1,3-bridging mode. The magnetic properties of 3 and 4 have been studied in the temperature range 8–200 K. Complexes 1b, 2b, 3 and 4 were also characterised by means of X-ray crystallography.

Aminoguanidinium Hydrolysis Effected by a Hydroxo-Bridged Dicobalt(II) Complex as a Functional Model for Arginase and Catalyzed by Mononuclear Cobalt(II) Complexes

The dinuclear complex [Co2(μ-OH)(μ-XDK)(bpy)2(EtOH)](NO3), where XDK is the dinucleating dicarboxylate ligand m-xylylenediamine bis(Kemp's triacid imide) and bpy = 2,2‘-bipyridine, was prepared as a functional model for arginase. The substrate aminoguanidinium nitrate was hydrolyzed to urea in ethanol by the complex but not by free hydroxide ion under the same conditions. The amino group of the substrate binds to cobalt, as demonstrated by UV−vis spectroscopic studies. The syntheses of related dinuclear cobalt(II) complexes [Co2(μ-XDK)(NO3)2(CH3OH)2(H2O)], [Co2(μ-Cl)(μ-XDK)(bpy)2(EtOH)2](NO3), and [Co2(μ-XDK)(py)3(NO3)2] are described. Mononuclear complexes [Co(XDK)(bpy)(H2O)] and [Zn(XDK)(bpy)(H2O)] were also prepared and characterized. The former catalytically hydrolyzes aminoguanidinium nitrate to urea in basic 1:1 methanol/water solutions, whereas the latter does not promote this reaction. Hydrolysis of aminoguanidinium ion is effected by [Co(CH3COO)2] and [Cu(CH3COO)2] in the presence of bpy, but not by [Zn(CH3COO)2], [Ni(CH3COO)2], or [Mn(CH3COO)2] in the presence of bpy in 1:1 methanol/water solution. In all cases, coordination of the amino group of the substrate to the metal center under the reaction conditions may activate the leaving group and orient the guanidinium moiety close to the attacking nucleophile, metal-bound hydroxide ion, to promote the hydrolysis reaction.

Unusual Mono- and Dinuclear Cobalt and Platinum Complexes of Rearranged Boriranylideneborane and Diboretanylideneborane Ligands: Dicobalt-μ-diborylcarbene Complexes as Derivatives of Planar Methane

Unusual Mono- and Dinuclear Cobalt and Platinum Complexes of Rearranged Boriranylideneborane and Diboretanylideneborane Ligands: Dicobalt-μ-diborylcarbene Complexes as Derivatives of Planar Methane

Synthesis and Properties of Di-μ-superoxo Dinuclear Cobalt(II, III) Complex

Abstract A novel di-μ-superoxo mixed-valence dicobalt(II, III) complex, [(Me3tacn)2CoIICoIII(μ-OH)(μ-O2)2]2+ has been prepared from a μ-peroxo complex, [(Me3tacn)2(acac)2CoIII2(μ-O2)]2+.Electrochemical studies demonstrated the reversible redox behavior among the oxidation states of Co(III, III), Co(III, II), and Co(II, II).

Preparation, characterization and redox properties of o,o′‐geometrical isomers of μ‐α and μ‐β‐aminoisobuty‐rato co(ii) complexes with n,n′,n′,n‴‐tetrakis(2‐pyridylmethyl)‐1,4,8,11‐tetraazacyclotetradecane

Two new dinuclear cobalt(II) complexes which, besides macrocyclic ligand N′,N′,N″,N‴-tetrakis (2-pyridylmethyl)-l,4,8,11-tetraazacyclotetradecane (tpmc) contain α- or β-aminoisobutyrato ions (α-aibu − and β-aibu −, respectively) were synthesized. The complexes having general formula [Co2(aibu)tpmc](CIO4)3 were characterized by elemental analysis, electronic and 1R spectroscopy, magnetic measurements and cyclic voltammetry. It was assumed that in both complexes an exo coordination of the macrocyclic ligand is achieved. Cobalt(II) ions are bridged with O,O′-aminoisobutyrato ligand. Both complexes are stable against air oxidation, whereas the oxidation of the complex containing the α-aibu− ion by hydrogen-peroxide gave the coresponding mixed-valence Co11/Co111 complex. The complexes are also electrochemically stable in the potential range from-0,6V to 0,4V vs SCE.

Synthesis and characterization of μ-peroxo dinuclear cobalt(III) complexes containing sugars

Synthesis and characterization of μ-peroxo dinuclear cobalt(III) complexes containing sugars

Electron Transfer in Dinuclear Cobalt Complexes with “Non‐innocent” Bridging Ligands

AbstractReaction of tripod cobalt(II) templates [{CH3C(CH2PAr2)3}CoII] with potentially bridging ligands L generates the dinuclear compounds [(tripod)‐Co‐L‐Co(tripod)]2+. With L = oxalate (C2O2‐4) a biscobalt(II) complex (1) is formed, while with L = C6H2O2‐4, the dianion derived from 2,5‐dihydroxy‐1,4=benzoquinone (anilic acid), two‐electron transfer within the dimetallic unit occurs and a biscobalt(III) charge distribution results (2a), as shown by X‐ray structural analyses of 1 and 2a, NMR spectroscopy, and theoretical investigations by the INDO method. Complex 2a exhibits an unusually intense, low‐energy absorption in its electronic spectrum; this is explained with a simple MO model. One‐electron reduction of 2a generates the corresponding mixed‐valence complex, which is highly stabilised through extensive electron delocalisation. Substituents at the 3,6 positions of the bridging ligand (Cl, Br, I, NO2, Me, iPr, Ph; 2b‐h) as well as alkyl substitution at the aromatic rings of the tripod ligands (3, 4) influence the optical and electrochemical properties consistent with the proposed model of charge distribution. Formal replacement of one [(tripod)CoIII]3+ moiety by [CH2]2+ leads to the mononuclear complex 6, which is shown to be a typical [(tripod)CoIII=(catecholato)]+ complex. Therefore the substantially different optical and electrochemical properties of the dinuclear complexes with respect to those of 6 result from strong metal‐metal interactions mediated by the bridging ligand.

A Comparison of the Lability of Mononuclear Octahedral and Dinuclear Triple-Helical Complexes of Cobalt(II)

The lability of the mononuclear octahedral complex tris(5-methyl-2-(1‘-methylbenzimidazol-2-yl)-pyridine)cobalt(II), [Co(2)3]2+, is compared with the dinuclear triple-helical complex tris[bis[1-met...

Quantum‐Chemical Investigations of Stabilizing Interactions in μ‐Diborylcarbene Dicobalt Complexes with a Planar Tetracoordinate Carbon Atom

Quantum-chemical methods have been employed to study the nature of stabilization in dinuclear cobalt complexes of the general formula [{(C5 H5 )Co}2 (μ-CR(1) 2 BCBR(2) R(3) )] (6) as well as the "antivan't Hoff-Le Bel" configuration of the planar tetracoordinate carbon (ptC) atom of the bridging diborylcarbene ligand 9. Extended Hückel and ab initio Hartree-Fock calculations have been carried out for the model compounds 6b (R(1) = R(2) = R(3) = H) and 6c (R(1) = R(2) = H; R(3) = C6 H5 ). Ab initio electron deformation density maps and natural population analysis calculations show that complexes 6 are stabilized through push-pull effects by which the ptC experiences π electron density delocalization and σ electron density accumulation. The calculated electronic configuration of the ptC in the free ligand 9b is σ(2.978) π(1.501) , and in 6b σ(3.944) π(1.356) . Electron density donation from one cobalt atom to an aryl group on the bridging ligand further contributes to the stabilization of the complexes 6.