Syntheses and crystal structures of new naphthalene- and anthracene-vanadate salts and an unprecedented dimetallabis(anthracene) sandwich complex: [Na(tetrahydrofuran)3][V2(anthracene)2].

Reduction of bis(naphthalene)vanadium(0) by potassium naphthalene (KNp) in tetrahydrofuran (THF) provides a highly reactive, thermolabile, and so far unisolable brown substance, which affords the first reported derivatives of bis(naphthalene)vanadates. From these solutions, thermally stable (298 K) and structurally characterized compounds have been obtained, including dark-red rods of catena-poly[bis(µ3-η4:η6:η4-naphthalene)tetrakis(tetrahydrofuran)dipotassiumvanadium], [K2V(C4H8O)4(C10H8)2]n or [K(THF)2]2[V(C10H8)2] (3), and red plates of (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane)potassium [1,2-bis(dimethylphosphanyl)ethane]bis(η4-naphthalene)vanadium tetrahydrofuran monosolvate, [K(C18H36N2O6)][V(C10H8)2(C6H16P2)]·C4H8O or [K([2.2.2]cryptand)][V(C10H8)2(dmpe)]·THF [dmpe is 1,2-bis(dimethylphosphanyl)ethane] (4b). Notably, [V(C10H8)2]2- is the only example of a structurally authenticated homoleptic bis(arene)metallate dianion and was obtained by further reduction of the brown material by KNp in THF, in the presence of trimethylphosphane (PMe3). Addition of anthracene (An) to the brown material in THF afforded deep-violet and paramagnetic crystalline (1,4,7,10,13,16-hexaoxacyclooctadecane)bis(tetrahydrofuran)potassium [(η4-anthracene)(tetrahydrofuran)vanadium]-µ-η4:η2-anthracene-[(1,4,7,10,13,16-hexaoxacyclooctadecane)potassium]-µ-η2:η4-anthracene-[(η4-anthracene)(tetrahydrofuran)vanadium] tetrahydrofuran disolvate, [K(C12H24O6)(C4H8O)2][KV2(C12H24O6)(C4H8O)2(C14H10)4]·2C4H8O or [K(18-crown-6)][K(18-crown-6)(THF)2][V(C14H10)2(THF)]2·2(THF) (5), which readily reacted with PMe3 and dmpe to give new vanadate salts. These were structurally characterized as (1,4,7,10,13,16-hexaoxacyclooctadecane)bis(tetrahydrofuran)potassium bis(η4-anthracene)(trimethylphosphane)vanadium tetrahydrofuran monosolvate, [K(C12H24O6)(C4H8O)2][V(C14H10)2(C3H9P)]·C4H8O or [K(18-crown-6)(THF)2][V(C14H10)2(PMe3)]·THF (6), and tetrakis(1,2-dimethoxyethane)potassium bis(η4-anthracene)[1,2-bis(dimethylphosphanyl)ethane]vanadium, [K(C4H10O2)4][V(C14H10)2(C6H16P2)] or [K(DME)4][V(C14H10)2(dmpe)] (DME is 1,2-dimethoxyethane) (7b). The last three structures contain the first known bis(anthracene)vanadates and are thereby derivatives of the unknown bis(anthracene)vanadium(0). Attempts to obtain the sodium salt analog of 5 in THF resulted instead in the formation of a unique substance, (µ3-η6:η6:η6-anthracene)(µ2-η6:η6-anthracene)tris(tetrahydrofuran)sodiumdivanadium, [NaV2(C14H10)2(C4H8O)3] or [Na(THF)3][V2(C14H10)2] (8), containing the first reported dimetallabis(anthracene) sandwich compound.

Niobium isocyanide complexes, Nb(CNAr)6, with Ar = 2,6-dimethylphenyl (Xyl), a diamagnetic dimer containing four reductively coupled isocyanides, and Ar = 2,6-diisopropylphenyl (Dipp), a paramagnetic monomer analogous to the highly unstable hexacarbonylniobium(0).

Treatment of bis(mesitylene)niobium(0) with 6-7 equivalents of 2,6-dimethylphenyl isocyanide (CNXyl) affords two products with the empirical formula Nb(CNXyl)n (n = 7 or 6), which have been shown to be the diamagnetic dimers bis[µ-N,N',N'',N'''-tetrakis(2,6-dimethylphenyl)squaramidinato(2-)]bis[pentakis(2,6-dimethylphenyl isocyanide)niobium(I)], [Nb2(C9H9N)10(C36H36N4)] or [Nb(CNXyl)5]2[µ-C4(NXyl)4]·xSolvent, 1, and bis[µ-N,N',N'',N'''-tetrakis(2,6-dimethylphenyl)squaramidinato(2-)]bis[tetrakis(2,6-dimethylphenyl isocyanide)niobium(I)] tetrahydrofuran trisolvate, [Nb2(C9H9N)8(C36H36N4)]·3C4H8O or [Nb(CNXyl)4]2[µ-C4(NXyl)4]·3THF (THF = tetrahydrofuran), 2. Each contains NbI bound to either five or four terminal isocyanides, respectively, and to an unprecedented bridging tetraarylsquaramidinate(2-) unit, coordinated as a bidentate ligand to each niobium center, symmetrically due to the crystallographic inversion center that coincides with the centroid of the central C4 unit. Thus, in the presence of CNXyl, the bis(mesitylene)niobium(0) is oxidized to niobium(I), resulting in the facile loss of both mesitylene groups and the reductive coupling of two CNXyl groups per niobium to provide the first examples of tetraarylsquaramidinate(2-) ligands, [cyclo-C4N4Ar4]2-, coordinated to metals. In contrast, bis(mesitylene)niobium(0) reacts with the more crowded 2,6-diisopropylphenyl isocyanide (CNDipp) to afford the paramagnetic monomer hexakis(2,6-diisopropylphenyl isocyanide)niobium(0), [Nb(C13H17N)6] or Nb(CNDipp)6, 3, the first zero-valent niobium isocyanide analog of the highly unstable Nb(CO)6, which is presently only known to exist in an argon matrix at 4.2 K.

Crystal structures and spectroscopic characterization of MBr2(CNXyl)n (M = Fe and Co, n = 4; M = Ni, n = 2; Xyl = 2,6-dimethylphenyl), and of formally zero-valent iron as a cocrystal of Fe(CNXyl)5 and Fe2(CNXyl)9.

Structures and spectroscopic characterization of the divalent complexes cis-dibromidotetrakis(2,6-dimethylphenyl isocyanide)iron(II) dichloromethane 0.771-solvate, [FeBr2(C9H9N)4]·0.771CH2Cl2 or cis-FeBr2(CNXyl)4·0.771CH2Cl2 (Xyl = 2,6-dimethylphenyl), trans-dibromidotetrakis(2,6-dimethylphenyl isocyanide)iron(II), [FeBr2(C9H9N)4] or trans-FeBr2(CNXyl)4, trans-dibromidotetrakis(2,6-dimethylphenyl isocyanide)cobalt(II), [CoBr2(C9H9N)4] or trans-CoBr2(CNXyl)4, and trans-dibromidobis(2,6-dimethylphenyl isocyanide)nickel(II), [NiBr2(C9H9N)2] or trans-NiBr2(CNXyl)2, are presented. Additionally, crystals grown from a cold diethyl ether solution of zero-valent Fe(CNXyl)5 produced a structure containing a cocrystallization of mononuclear Fe(CNXyl)5 and the previously unknown dinuclear [Fe(CNXyl)3]2(µ2-CNXyl)3, namely pentakis(2,6-dimethylphenyl isocyanide)iron(0) tris(µ2-2,6-dimethylphenyl isocyanide)bis[tris(2,6-dimethylphenyl isocyanide)iron(0)], [Fe(C9H9N)5][Fe2(C9H9N)9]. The (M)C-N-C(Xyl) angles of the isocyanide ligand are nearly linear for the metals in the +2 oxidation state, for which the ligands function essentially as pure donors. The νCN stretching frequencies for these divalent metal isocyanides are at or above that of the free ligand. Relative to FeII, in the structure containing iron in the formally zero-valent oxidation state, the Fe-C bond lengths have shortened, the C[triple-bond]N bond lengths have elongated, the (M)C-N-C(Xyl) angles of the terminal CNXyl ligands are more bent, and the νCN stretching frequencies have shifted to lower energies, all indicative of substantial M(dπ)→π* backbonding.

Crystal Structures of DOTMA Chelates from Ce3+ to Yb3+ : Evidence for a Continuum of Metal Ion Hydration States.

The crystal structures of chelates formed between each stable paramagnetic lanthanide ion and the octadentate polyamino carboxylate ligand DOTMA are described. A total of 23 individual chelates structures were obtained; in each chelate the coordination geometry around the metal ion is best described as a twisted square antiprism (torsion angle -25.0°--31.4°). Despite the uniformity of the general coordination geometry provided by the DOTMA ligand, there is a considerable variation in the hydration state of each chelate. The early Ln3+ chelates are associated with a single inner sphere water molecule; the Ln-OH2 interaction is remarkable for being very long. After a clear break at gadolinium, the number of chelates in the unit cell that have a water molecule interacting with the Ln3+ decreases linearly until at Tm3+ no water is found to interact with the metal ion. The Ln-OH2 distance observed in the chelates of the later Ln3+ ions are also extremely long and increase as the ions contract (2.550-2.732 Å). No clear break between hydrated and dehydrated chelates is observed; rather this series of chelates appear to represent a continuum of hydration states in which the ligand gradually closes around the metal ion as its ionic radius decreases (with decreased hydration) and the metal drops down into the coordination cage.

Bis(pyrene)metal complexes of vanadium, niobium and titanium: isolable homoleptic pyrene complexes of transition metals.

Reduction of VCl3(THF)3 (THF is tetrahydrofuran) and NbCl4(THF)2 by alkali metal pyrene radical anion salts in THF affords the paramagnetic sandwich complexes bis[(1,2,3,3a,10a,10b-η)-pyrene]vanadium(0), [V(C16H10)2], and bis[(1,2,3,3a,10a,10b-η)-pyrene]niobium(0), [Nb(C16H10)2]. Treatment of tris(naphthalene)titanate(2-) with pyrene provides the isoelectronic titanium species, isolated as an (18-crown-6)potassium salt, namely catena-poly[[(18-crown-6)potassium]-µ-[(1,2-η:1,2,3,3a,10a,10b-η)-pyrene]-titanate(-I)-µ-[(1,2,3,3a,10a,10b-η:6,7-η)-pyrene]], {[K(C12H24O6)][Ti(C16H10)2]}n. The first two compounds have very similar packing, with neighboring molecules arranged orthogonally to one another, such that aromatic donor-acceptor interactions are likely responsible for the specific arrangement. The asymmetric unit contains a half-occupancy metal center η(6)-coordinated to one pyrene ligand, with the full M(pyrene)2 molecule generated by a crystallographic inversion center. In the titanium compound, the cations and anions are in alternating contact throughout the crystal structure, in one-dimensional chains along the [101] direction. As in the other two compounds, the asymmetric unit contains a half-occupancy Ti atom η(6)-coordinated to one pyrene ligand. Additionally, the asymmetric unit contains one half of an (18-crown-6)potassium cation, located on a crystallographic inversion center coincident with the K atom. The full formula units are generated by those inversion centers. In all three structures, the pyrene ligands are eclipsed and sandwich the metals in one of two inversion-related sites. These species are of interest as the first isolable homoleptic pyrene transition metal complexes to be described in the scientific literature.

Properties, solution state behavior, and crystal structures of chelates of DOTMA.

The chemistry of polyamino carboxylates and their use as ligands for Ln(3+) ions is of considerable interest from the point of view of the development of new imaging agents. Of particular interest is the chemistry of the macrocyclic ligand 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and its derivatives. Herein we report that the tetramethylated DOTA derivative, DOTMA, possess several properties that, from an imaging agent development point of view, are more advantageous than those of the parent DOTA. In particular, the Ln(3+) chelates of DOTMA exhibit a marked preference for the monocapped twisted square antiprismatic coordination isomer which imparts more rapid water exchange kinetics on the chelates; τ(M)(298) was determined to be 85 ns for GdDOTMA. Differential analysis of the (17)O R(2ρ) temperature profiles of both GdDOTA and GdDOTMA afforded the τ(M)(298) values for the square (SAP) and twisted square antiprismatic (TSAP) isomers of each chelate that were almost identical: 365 ns (SAP) and 52 ns (TSAP). The origin of this accelerated water exchange in the TSAP isomer appears to be the slightly longer Gd-OH(2) bond distance (2.50 Å) that is observed in the crystal structure of GdDOTMA which crystallizes in the P(2) space group as a TSAP isomer. The Ln(3+) chelates of DOTMA also exhibit high thermodynamic stabilities ranging from log K(ML) = 20.5 for CeDOTMA, 23.5 for EuDOTMA and YbDOTMA comparable to, but a shade lower than, those of DOTA.

Analysis of the conformational behavior and stability of the SAP and TSAP isomers of lanthanide(III) NB-DOTA-type chelates.

Controlling the water exchange kinetics of macrocyclic Gd(3+) chelates, a key parameter in the design of improved magnetic resonance imaging (MRI) contrast media, may be facilitated by selecting the coordination geometry of the chelate. The water exchange kinetics of the mono- capped twisted square antiprism (TSAP) being much closer to optimal than those of the mono capped square antiprism (SAP) render the TSAP isomer more desirable for high relaxivity applications. Two systems have been developed that allow for selection of the TSAP coordination geometry in 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-type Gd(3+) chelates, both based upon the macrocycle nitrobenzyl cyclen. In this paper we report investigations into the stability and formation of these chelates. Particular focus is given to the production of two regioisomeric chelates during the chelation reaction. These regioisomers are distinguished by having the nitrobenzyl substituent either on a corner or on the side of the macrocyclic ring. The origin of these two regioisomers appears to stem from a conformation of the ligand in solution in which it is hypothesized that pendant arms lie both above and below the plane of the macrocycle. The conformational changes that then result during the formation of the intermediate H(2)GdL(+) chelate give rise to differing positions of the nitrobenzyl substituent depending upon from which face of the macrocycle the Ln(3+) approaches the ligand.

New binuclear Mn(II) and Fe(II) complexes supported by 1,4,8-triazacycloundecane.

Two new binuclear metal complexes supported by 1,4,8-triazacycloundecane (tacud) are reported. [Fe(2)(tacud)(2)(µ-Cl)(2)Cl(2)] (1) and [Mn(2)(tacud)(2)(µ-Cl)(2)Cl(2)] (2) are isomorphs consisting of bis(µ-chloro) bridged metal centers along with terminal chloro groups and tacud ligands. Both compounds 1 and 2 crystallize in the P1 space group. For 1, a = 7.7321(12) Å, b = 7.8896(12) Å, c = 11.4945(17) Å, α = 107.832(2)°, ß = 107.827(2)°, γ = 92.642(2)°, V = 627.85(17) Å(3) and Z = 1. For 2, a = 7.7607(12) Å, b = 7.9068(12) Å, c = 11.6111(18) Å, α = 108.201(2)°, ß = 108.041(2)°, γ = 92.118(3)°, V = 636.47(17) Å(3) and Z = 1. Variable-temperature and variable-field magnetic susceptibility studies on 1 indicate the presence of weak ferromagnetic interactions between the high-spin iron(ii) centers in the dimer (J = + 1.6 cm(-1)) and the crystalline field anisotropy of the ferrous ion (D = - 2.8, E = - 0.1 cm(-1)). Variable temperature magnetic susceptometry studies on 2 indicate that weak antiferromagnetic coupling exists between the manganese(ii) centers (J = - 1.8 cm(-1)). Compounds 1 and 2 retain their dinuclearity in weakly coordinating or low polarity solvents, while both become mononuclear in solvents such as methanol.

Iron(II) complexes of sterically bulky alpha-ketocarboxylates. structural models for alpha-ketoacid-dependent nonheme iron halogenases.

Reaction of the sterically hindered alpha-ketocarboxylate 2,6-di(mesityl)benzoylformate (MesBF) with the iron(II) complexes LFeCl 2 [L = N, N, N', N'-tetramethylpropylenediamine (Me 4pda) or 6,6'-dimethyl-2,2'-bipyridine (dmby)] yielded LFe(Cl)(MesBF) ( 1 or 2). X-ray crystal structures of these complexes showed that they closely model the active site structure of the nonheme iron halogenase enzyme SyrB2. A similar synthetic procedure using benzoylformate with L = dmby yielded (dmby)Fe[(O 2CC(O)Ph)] 2 ( 3) instead, demonstrating the need for the sterically hindered alpha-ketocarboxylate to assemble the halogenase model compounds. In order to make reactivity comparisons among the structurally related iron(II) complexes of benzoylformates of varying steric properties, the complexes [LFe(O 2CC(O)Ar)] n ( 4- 6) were prepared, where L' = tris(pyridylmethyl)amine (tpa) and Ar = 2,6-dimesitylphenyl, 2,6-di p-tolylphenyl, or 2,4,6-trimethylphenyl, respectively. X-ray structures for the latter two cases ( 5 and 6) revealed dinuclear topologies ( n = 2), but UV-vis and (1)H NMR spectroscopy indicated that all three complexes dissociated in varying degrees to monomers in CH 2Cl 2 solution. Although compounds 1- 6 were oxidized by O 2, oxidative decarboxylation of the alpha-ketocarboxylate ligand(s) only occurred for 3. These results indicate that the steric hindrance useful for structural modeling of the halogenase active site prohibits functional mimicry of the enzyme.

Modular syntheses of multidentate ligands with variable N-donors: applications to tri- and tetracopper(I) complexes.

A general method for the preparation of multidentate ligands comprised of a multi-imine platform derived from 1,1,1-tris(aminomethyl)ethane or tris(aminoethyl)amine connected to bi- and tridentate N-donor chelates has been developed. The feasibility of the method has been demonstrated through the synthesis and characterization of a large set of these ligand types. Complexation to Cu(I) was accomplished for several cases, yielding tri- and tetracopper(I) complexes that have been characterized in solution by NMR spectroscopy and conductivity, and in the solid state by elemental analysis, mass spectrometry, and/or X-ray crystallography. These complexes are potentially useful for modeling multicopper protein active sites.

N,N'-dimethylformamide-derived products from catalytic oxidation of 3-hydroxyflavone.

The biomimetic conversion of 3-hydroxyflavone in the presence of a copper(II) catalyst, dioxygen, and N,N'-dimethylformamide to oxidation products as well as two previously unreported solvent-derived products is seen. The two solvent-derived products were characterized, and their crystal structures were determined.

Homoleptic isocyanidemetalates of 4d- and 5d-transition metals: [Nb(CNXyl)(6)](-), [Ta(CNXyl)(6)](-), and derivatives thereof.

Treatment of [M(CO)(6)](-), M = Nb, Ta, with Ag(+), I(2) or NO(+) in the presence of CNXyl provided [M(CNXyl)(7)](+), M(CNXyl)(6)I, or cis-[M(CNXyl)(4)(NO)(2)](+), which are isocyanide analogues of the unknown carbonyl complexes [M(CO)(7)](+), M(CO)(6)I, or cis-[M(CO)(4)(NO)(2)](+), respectively. Reduction of M(CNXyl)(6)I by cesium graphite gave the respective Cs[M(CNXyl)(6)], which have been structurally characterized and represent the first isolable homoleptic isocyanidemetalates for second or third row transition metals. Nitrosylation of [Ta(CNXyl)(6)](-) affords a rare example of a mononitrosyl tantalum complex, Ta(CNXyl)(5)NO, which is an isocyanide analogue of the unknown Ta(CO)(5)NO. This study emphasizes, inter alia, the remarkable versatility of the CNXyl ligand compared to CO in stabilizing various electronic environments at heavier group 5 metal centers.

Hydrido and chloro gallium and aluminium complexes with the tridentate bis(2-dimethylaminoethyl)amide ligand.

Five-coordinate gallium and aluminium dihydrides, H2Ga[N(CH2CH2NMe2)2] () and H2Al[N(CH2CH2NMe2)2] (), were synthesized and found to be volatile and thermally stable. and reacted with H3Ga(NMe3) and H3Al(NMe3), respectively, to form H2Ga[N(CH2CH2NMe2)2]GaH3 () and H2Al[N(CH2CH2NMe2)2]AlH3 (), in which the amido nitrogen bridged between the MH2 and MH3 groups (M=Ga or Al). A mixed metal complex, H2Al[N(CH2CH2NMe2)2]GaH3 () was obtained from the reaction of with H3Al(NMe3) or with H3Ga(NMe3), and a crystal consisting of a mixture of and was structurally characterized. The five-coordinate chloro derivative, Cl2Ga[N(CH2CH2NMe2)2] (), was synthesized and characterized.

Self-assembly of the 2-His-1-carboxylate facial triad in mononuclear iron(II) and zinc(II) models of metalloenzyme active sites.

A synthetic strategy involving the use of sterically hindered N-donor and terphenylcarboxylate ligands has been used to prepare complexes of iron(II) and zinc(II) that feature N2(carboxylate) donors. X-ray crystallographic and NMR data show that the 2-His-1-carboxylate facial triad found in metalloenzyme active sites is closely modeled by the mononuclear complexes. In addition, by virtue of the flexibility of the ligands used, the geometries and coordination environments of the complexes display carboxylate binding mode differences such as those seen in the enzymes.

Generation of dimethylsilylene and allylidene holmium complexes from trimethylsilylated allyl ligands.

The reaction of K[1,3-(SiMe3)2C3H3] with partially hydrated holmium triflate leads to a dimeric complex (1) in which hydrogen abstraction from a trimethylsilyl group has occurred on two allyl ligands, forming dimethylsilylene units that bridge the holmium atoms. When the reaction time is prolonged, a different product (2) is isolated, in which in addition to two dimethylsilylene bridges, the metal centers are joined with a mu-eta1,eta3-allylidene ligand. Both crystallographic and computational studies provide evidence for delocalized bonding in the allylidene fragment.

Mono- and digallane complexes of a tridentate amido-diamine ligand.

Bis(2-dimethylaminoethyl)amido gallane, H2GaN(CH2CH2NMe2)2, that melts at 27 degrees C and remains stable upon heating at 55 degrees C for two days, was synthesized either from the reaction of the quinuclidine adduct of monochlorogallane with the lithium salt of the corresponding amine, or from the reaction of trimethylamine gallane and the amine; the latter affords an unusual co-product with both GaH2 and GaH3 bonded to the same amido nitrogen.