End-on Coordination Mode Research Articles

Photoinduced Isomerization of [N2]2- in a Bimetallic Lutetium Complex.

The first lanthanide dinitrogen photoswitch [(C5Me4H)2(THF)Lu]2(μ-η2:η2-N2), 1, is reported. 1 is a unique example of controlled isomerization between side-on and end-on coordination modes of [N2]2- in a bimetallic lutetium dinitrogen complex that results in photochromism. Near-infrared light (NIR) was used to promote this effect, as evidenced by single X-ray diffraction (XRD) connectivity and Raman data, generating the [N2]2- end-on bound isomer, [(C5Me4H)2(THF)Lu]2(μ-η1:η1-N2), 2. Although different ligands and coordinating solvents were studied to replicate and control the optical properties in 1/2, only the original configuration with C5Me4H ligands and THF as the coordinating solvent worked. Supported by the first-principles calculations, the electronic structures along with the mechanistic details of the side-on to end-on isomerization were unraveled. Preliminary reactivity studies show that 2 formed with NIR light reacts with anthracene, generating dihydroanthracene and anthracene dimers, indicating new redox reaction pathways.

Unlocking Air- and Water-Stable Technetium Acetylides and Other Organometallic Complexes.

The first technetium complexes containing anionic alkynido ligands in an end-on coordination mode have been prepared by using the nonprotic, cationic precursor mer,trans-[Tc(SMe2)(CO)3(PPh3)2]+. This cation acts as a functional analogue of the highly reactive 16-electron metallo Lewis acid {Tc(CO)3(PPh3)2}+ in reactions with alkynes, acetylides, and other organometallic reagents. Such reactions give a variety of organometallic technetium complexes in excellent yields and enable the preparation of [Tc(CH3)(CO)3(PPh3)2], [Tc(Ph)(CO)3(PPh3)2], [Tc(Cp)(CO)2(PPh3)], [Tc(═CCH2CH2CH2O)(CO)3(PPh3)2]+, [Tc(═CCH2CH2CH2CH2O)(CO)3(PPh3)2]+, [Tc(C≡C-H)(CO)3(PPh3)2], [Tc(C≡C-Ph)(CO)3(PPh3)2], [Tc(C≡C-tBu)(CO)3(PPh3)2], [Tc(C≡C-nBu)(CO)3(PPh3)2], [Tc(C≡C-SiMe3)(CO)3(PPh3)2], and [Tc{C≡C-C6H3(CF3)2}(CO)3(PPh3)2]. The bonding situation in the alkynyl complexes is compared to that in corresponding alkyl- and arylnitrile and -isonitrile complexes. [Tc(N≡C-Ph)(CO)3(PPh3)2](BF4), [Tc(C≡N-Ph)(CO)3(PPh3)2](BF4), [Tc(N≡C-tBu)(CO)3(PPh3)2](BF4), and [Tc(C≡N-tBu)(CO)3(PPh3)2](BF4) were prepared in high yields by ligand exchange reactions starting from mer,trans-[Tc(OH2)(CO)3(PPh3)2](BF4). The novel complexes were characterized by single-crystal X-ray diffraction and spectroscopic methods. In particular, 99Tc nuclear magnetic resonance spectroscopy proved to be an invaluable and sensitive tool for the characterization of the complexes. Density functional theory calculations strongly suggest similar bonding situations for the related alkynyl, nitrile, and isonitrile complexes of technetium.

Dual Iron Sites in Activation of N2 by Iron-Sulfur Cluster Anions Fe5S2- and Fe5S3.

Inspired by the fact that the active centers of natural nitrogenases are polynuclear iron-sulfur clusters, the reactivity of isolated iron-sulfur clusters toward N2 has received considerable attention to gain fundamental insights into the activation of the N≡N triple bond. Herein, a series of gas-phase iron-sulfur cluster anions FexSy- (x = 1-8, y = 0-x) were prepared and their reactivities toward N2 were investigated systematically by mass spectrometry. Among the 44 investigated clusters, only Fe5S2- and Fe5S3- showed superior reactivity toward N2. Theoretical results revealed that N2 binds molecularly to the iron sites of Fe5S2,3- in a common end-on coordination mode with an unprecedented back-donation interaction from the localized d-d bonding orbitals of Fe-Fe sites to the π* antibonding orbitals of N2. This is the first example to disclose the significant contribution of the dual metal sites rather than the single metal atom to N2 adsorption in the prevalent end-on binding mode.

Structure and NMR properties of the dinuclear complex di-μ-azido-κ4 N 1:N 1-bis-[(azido-κN)(pyridine-2-carboxamide-κ2 N 1,O)zinc(II)

The new diamagnetic complex, [Zn2(N3)4(C6H6N2O)2] or [Zn2(pca)2(μ1,1-N3)2(N3)2] was synthesized using pyridine-2-carboxamide (pca) and azido ligands, and characterized using various techniques: IR spectroscopy and single-crystal X-ray diffraction in the solid state, and nuclear magnetic resonance (NMR) in solution. The mol-ecule is placed on an inversion centre in space group P . The pca ligand chelates the metal centre via the pyridine N atom and the carbonyl O atom. One azido ligand bridges the two symmetry-related Zn2+ cations in the end-on coordination mode, while the other independent azido anion occupies the fifth coordination site, as a terminal ligand. The resulting five-coordinate Zn centres have a coordination geometry inter-mediate between trigonal bipyramidal and square pyramidal. The behaviour of the title complex in DMSO solution suggests that it is a suitable NMR probe for similar or isostructural complexes including other transition-metal ions. The diamagnetic nature of the complex is reflected in similar 1H and 13C NMR chemical shifts for the free ligand pca as for the Zn complex.

Linear End-On Coordination Modes of CO2.

The second case of linear end-on and evidence for an unprecedented bridging end-on coordination mode of CO2 have been discovered for vanadium aryloxide complexes of the tetradentate ligand system (ONNO)2- (ONNO=2,4-Me2 -2-(OH) C6 H2 CH2 ]2 N(CH2 )2 NMe2 ). The reaction of divalent (ONNO)VII (TMEDA) with CO2 and under the appropriate reaction conditions affords the trivalent (ONNO)VIII (OH)(η1 -CO2 ) resulting from an intermediate CO2 deoxygenation pathway followed by H-atom abstraction from the aromatic solvent, and CO2 fixation. In contrast, the reduction of trivalent (ONNO)VIII Cl(THF) with K, followed by exposure to CO2 in ethereal solvent, afforded the dinuclear [(ONNO)VII ]2 (μ, η1 -CO2 ).

From Mesocates to Helicates: Structural, Magnetic and Chiro-Optical Studies on Nickel(II) Supramolecular Assemblies Derived from Tetradentate Schiff Bases.

The systematic reactions of a family of tetradentate pyridyl/imine and quinolyl/imine racemic or enantiopure Schiff bases with Ni(NO3 )2 or Ni(ClO4 )2 in the presence of sodium azide yielded, as a function of the starting racemic, chiral or achiral base, a set of chiral, meso or achiral complexes. In all cases, the compounds consist of two NiII cations linked by a double azido bridge in its end-on coordination mode. All the dimers exhibit a mesocate supramolecular structure and one of them, the unprecedented mix of helicate and mesocate in 2:1 ratio. The transition from mesocate to helicate conformation has been reached by tuning the flexibility of the central spacers of the Schiff bases and the size of the substituents. Electronic circular dichroism (ECD) studies have been performed for two pairs of enantiomers and interpreted by means of DFT calculations. Susceptibility measurements show a ferromagnetic coupling between the NiII cations mediated by the end-on azido bridges.

Synthesis, characterization and structures of copper(II) and cobalt(II) complexes involving N3S-coordinated tetradentate ligand and azide/ thiocyanate/ nitrite ion

Four mononuclear copper(II) complexes [Cu(bdmpe)X]Y, one mononuclear cobalt(II) complex [Co(bdmpe)(NCS)2] and two binuclear cobalt(II) complexes [Co(bpmpe)(N3)]2Y2, where X = NCS− or NO2, Y = PF6− or ClO4− with N3S-coordinating ligand N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-2-(phenylthio)ethan-1-amine(bdmpe) have been synthesized and characterized by microanalyses, IR, UV–Vis spectra, magnetic study and EPR data. Single crystal X-ray data confirmed that the chiral complex [Cu(bdmpe)(NCS)]ClO4 has orthorhombic P212121 space group with distorted square pyramidal geometry, [Cu(bdmpe)(NO2)]ClO4 has distorted square pyramidal geometry, [Co(bdmpe)(NCS)2] has distorted trigonal bipyramidal geometry and binuclear cobalt(II) complex [Co(bpmpe)(N3)]2(ClO4)2 has distorted octahedral geometry where the two cobalt centers is bridged by two azide ions with end-on coordination mode.

Linear, Zigzag, and Helical Cerium(III) Coordination Polymers

Five novel one-dimensional cerium(III) carboxylate coordination polymers, [Ce(O2CCH2CHMe2)3(EtOH)2]n (1), {[Ce(O2CCH2Me)3(H2O)]·0.5(4,4′-bpy)}n (2; 4,4′-bpy = 4,4′-bipyridine), {[Ce2(O2CCHMe2)6(H2O)3]}n (3), {[Ce3(O2CCHMe2)9(nPrOH)4]}n (4), and {[Ce3(O2CCHMe2)9(HO2CCHMe2)2(H2O)2]·2Me2CHCO2H}n (5), showcase the surprisingly consistent tendency of Ce(III) coordination network structures to adopt one-dimensional connection modes. The type of carboxylate as well as the reaction solvents determines the exact bridging versus end-on coordination modes for the carboxylates and, in turn, discriminate between linear, zigzag, and helical arrangements. Detailed magnetochemical analyses reveal pronounced single-ion effects and the expected weak antiferromagnetic coupling.

On the unprecedented level of dinitrogen activation in the calix[4]arene complex of Nb(iii)

The calix[4]arene niobium(III) complex ([L]Nb-N=N-Nb[L] where [L] = p-tert-butylcalix[4]arene), reported to bind N(2) in a μ(2)-linear dimeric capacity and to activate the N(2) triple bond to 1.39 Å, corresponding to the longest N(2) bond known in the end-on coordination mode, was subjected to a computational investigation involving both density functional and wavefunction based methods to establish the basis for the unprecedented level of activation. Replacement of the calix[4]arene ligand with hydroxide or methoxide ligands reveals that the organic backbone structure of the calix[4]arene ligand exerts negligible electronic influence over the metal centre, serving only to geometrically constrain the coordinating phenoxide groups. A fragment bonding analysis shows that metal-to-dinitrogen π* backbonding is the principal Nb-N interaction, providing a strong electronic basis for analogy with other well-characterised three- and four-coordinate complexes which bind N(2) end-on. While the calculated structure of the metallacalix[4]arene unit is reproduced with high accuracy, as is also the Nb-Nb separation, the calculated equilibrium geometry of the complex under a variety of conditions consistently indicates against a 1.39 Å activation of the N(2) bond. Instead, the calculated N-N distances fall within the range 1.26-1.30 Å, a result concordant with closely related three- and four-coordinate μ(2)-N(2) complexes as well as predictions derived from trends in N-N stretching frequency for a number of crystallographically characterized linear N(2) activators. A number of potential causes for this bond length discrepancy are explored.

Switching End-on into Side-on C⋮N Coordination: A Computational Approach

The stability of the end-on and side-on complexes of acetonitrile with HCo(L)3 [L = CO, PH3, PPh3, PMe3, 1,2-bis(dimethyl)phosphinoethane (dmpe), N-heterocyclic carbene (NHC)] has been computed at the B3LYP density functional level of theory. It is found that the most common end-on coordination mode of nitriles to transition metals can be changed into the side-on coordination via reasonable ligand tailoring. The dmpe- and NHC-modified catalyst, HCo(dmpe)(NHC), prefers the side-on complex over the end-on complex of acetonitrile by 1.1 kcal/mol in free energy. The driving force for this conversion is addressed.

Low-weight coordination polymers derived from the self-assembly reactions of Pd(II) pyrazolyl compounds and azide ion

The low-weight Pd(II) coordination polymers [(N 3)(HL)Pd {Pd 3(μ-N 3)(μ-L) 5} 10(μ-L) 2Pd(L)(HL)] {L = Pz − ( 1); mPz − ( 2), IPz − ( 3)} and [(N 3)(HPz)Pd{Pd 6(μ-N 3) 2(μ-Pz) 5(μ-L) 5} 10(μ-L) 2Pd(Pz)(HPz)] {L = mPz − ( 4), dmPz − ( 5); IPz − ( 6)} {L = pyrazolate (Pz −), 4-methylpyrazolate (mPz −), 4-iodopyrazolate (IPz −), 3,5-dimethylpyrazolate (dmPz −)} have been prepared in this work. IR spectra clearly indicated the exobidentate nature of pyrazolato ligands as well the end-on coordination mode of the azido group. The molecular weight determinations by osmometry indicated that the species have a low degree of polymerization ( n = 10). NMR experiments showed two pyrazolate environments in a 2:1 ratio, being assigned to the six-membered ring Pd(μ-L) 2Pd and the Pd(μ-N 3)(μ-L)Pd metallocycle, respectively. UV–visible spectroscopy gave further evidences for the oligomeric structures of 1– 6. Some alternative structures for the isostructural polymers have been suggested.

Synthesis, structure, and magnetic properties of three new one-dimensional nickel(II) complexes: new magnetic model for the first one-dimensional S = 1 complex with alternating ferro-ferromagnetic coupling.

Three new one-dimensional nickel(II) complexes with the formulas trans-[Ni(N-Eten)2(mu1.3-N3)]n(ClO4)n (1), trans-[Ni(N-Eten)2(mu1.3-N3)]n(PF6)n (2), and cis-[Ni(N-Eten)(mu1.1-N3)2]n (3) (N-Eten = N-Ethylethylenediamine) were synthesized and characterized. Complex 1 has the P2(1)/c space group and consists of a structurally and magnetically alternating one-dimensional antiferromagnetic system with end-to-end azido bridges. Compound 2 has the P1 space group and has alternate units in its structure but consists of a magnetically uniform one-dimensional antiferromagnetic system with end-to-end azido bridges. Complex 3 has the I2/a space group and may be described as a structurally and magnetically alternating one-dimensional ferromagnetic system with double azido bridged ligands in an end-on coordination mode. The chi(M)T versus T plots for compound 3 suggest an intramolecular ferromagnetic interaction between adjacent NiII ions and metamagnetism at low temperature (below 10 K). The magnetization measurements versus applied field confirm this metamagnetic ordering. In order to describe the magnetic data of this compound we developed a general formula for the magnetic susceptibility of the isotropic ferro-ferromagnetic S = 1 Heisenberg chain in terms of the alternation parameter alpha (= J2/J1); this assumed a variation of chi(M)T versus the length N.

Linkage isomerism in exchange coupled dinuclear Ni(II) complexes with three azide bridges; tris(μ-1,1-azido) versus tris(μ-1,3-azido) binding. Crystal structures of [L 2Ni 2(μ-1,1-N 3) 3]ClO 4 and [L 2Ni 2(N 3) 2-(μ-1,3-N 3) 2] (L = 1,4,7-trimethyl-1,4,7-triazacyclononane)

Two linkage isomers of Ni(II), [L 2Ni 2(N 3) 3]ClO 4, containing either three end-to-end (μ-1,3-azido) 1, or three end-on (μ-1,1-azido) 2 bridging azide ligands and 1,4,7-trimethyl-1,4,7-triazacyclononane (L) as the capping ligands have been reported. The compounds have been characterized on the basis of IR, UV-Vis spectroscopy and variable-temperature (2–290 K) magnetic susceptibility measurements. The crystal structure of 1 has been reported earlier ( J. Chem. Soc., Chem. Commun., (1985) 1618). 2 crystallizes in the monoclinic space group P2 1/ c with cell constants a=14.989(1), b=14.610(2), c=14.756(2) A ̊ , β=111.12(1)°, V=3014.5 A ̊ 3 and Z=4 . The structure of 2 consists of a six-coordinated dinickel(II) center with three azide ions linking two Ni(II) ions adopting the end-on coordination mode. Analysis of the susceptibility data yields a moderately strong antiferromagnetic interaction between nickel centers in 1, but a ferromagnetic interaction in 2. The crystal structure of [L 2Ni 2(N 3) 2(μ-1,3-N 3) 2] ( 3) has been determined at 193 K. Crystals of 3·CH 3OH have the following cell parameters: monoclinic, C2/c; a=24.302(2), b=11.276(1), c=11.437(1) A ̊ ; β=109.70(1)°; V=2950.8; Z=4 . The structure of 3 consists of a six-coordinated dinickel(II) center with two end-to-end azide ions linking two Ni(II) ions. Magnetic susceptibility measurements have shown the existence of antiferromagnetic exchange interaction between the nickel centers (2 J = −72.8 cm −1). The magnetic behavior of 1,2 and 3 together with other related complexes is discussed to show the difficulty of rationalizing the magnetostructural trends.

Derivate des Imidazols, XI. (C8H14N2)M(CO)5 (M = Mo, W) ‐ Terminale Koordination eines Olefins in Pentacarbonylmetall‐Komplexen

Imidazole Derivatives, XI. ‐ (C8H14N2)M(CO)5 (M = Mo, W) ‐ Terminal Coordination of an Olefin in Pentacarbonyl Metal ComplexesHerrn Professor Reinhard Schmutzler zum 60. Geburtstag gewidmet. The 2‐methylenimidazoline complexes (C8H14N2)M(CO)5 (M = Mo, W; 6) are obtained from M(CO)6 and 1,3,4,5‐tetramethyl‐2‐methyleneimidazoline (5). Spectroscopic data reveal the strong donor character of the ylidic ligand in 6. The end‐on coordination mode of the olefin is demonstrated by an X‐ray structure analysis of 6a.

Reactivity of acetone oxime towards oxomolybdenum(VI) complexes. Part 1. Syntheses and crystal structures of tetranuclear molybdenum(VI) complexes

Reaction of acetone oxime with [NBun4]4[α-Mo8O26] in refluxing methanol yielded [NBun4]2[Mo4O12(Me2CNO)2]1 while that with [MoO2(acac)2](acac = acetylacetonate) at room temperature resulted in the formation of [Mo4O10(OMe)4(Me2CNHO)2]2. The crystal structures of 1 and 2 have been determined [1, space group P, a= 9.009(1), b= 17.860(1), c= 16.769(1)A, α= 91.624(5), β= 101.158(7), γ= 91.983(7)°, R= 0.037 for 7429 reflections with I > 3σ(I); 2, space group P21/c, a= 9.437(1), b= 8.827(1), c= 13.727(3)A, β= 92.15(1)°, R= 0.029 for 2821 reflections with I > 3σ(I)]. The anion of 1 displays a cyclic (MoO2)4(µ-O)4 ring while 2 displays a MoVI4(µ3-OMe)2 central core. The µ4-κ2N:κ2O co-ordination mode exhibited by the acetone oximate in 1 is unprecedented for such a ligand, so is the end-on co-ordination mode of zwitterionic acetone oxime in 2. The reactivity of 1 and 2 respectively towards hydrochloric acid and tetrabutylammonium hydroxide was studied.

Ab initio Density Functional Calculations on Copper(I)–CO2 Coordinations

Abstract Full geometry optimizations have been carried out for the complex formations of Cu+ and Cu(2,2′-bipyridine)+ with CO2 by using the density functional methods. The calculation demonstrated that the end-on coordination mode was mainly favored for all cases, while the binding of 2,2′-bipyridine (bpy) to Cu(CO2)+ largely labilized the end-on coordination mode and stabilized the in-plane side-on coordination mode. The detailed examination of the population analysis indicate that the coordination of bpy to Cu+ weakens the electron donating character of CO2 to Cu+ and promotes the important participation of π-back-donative interaction from the metal ion to CO2.

Bridge bonding of nitrogen to dinuclear transition metal systems

Hb initio quantum chemical calculations have been performed on two different structures of M 2 N 2 for first-and second=row transition metal systems involving titanium, yttrium, and zirconium. In both structures N 2 is bridging the metal atoms, either in a perpendicular side-on coordination mode, or in a parallel end-on coordination mode leading to a linear M-N-N-M. Both types of structures have been observed experimentally for ligated complexes of the type (LnM) 2 N 2 . Comparisons between experiment and theory show that the geometries of the M 2 N 2 unit in the experimentally observed complexes correspond closely to the calculated geometries for different states of the naked M 2 N 2 systems

Ion–molecule reactions of transition-metal ions with nitriles in the gas phase: competitive formation of both "end-on" and "side-on" coordination

First field-free region B/E linked scan studies of metastable and particularly collision-induced dissociations show that two competitive coordination processes ("end-on" and "side-on") exist in the complexation of metal ions (Ni+, Co+, Fe+, and Mn+) with alkanenitriles in the FAB source of an E/B configuration mass spectrometer. Analysis of the B/E linked scan CA spectra gives approximate percentages of the end-on coordination mode in the reactive complexes. An unusual β-methyl migration, involved in the elimination of CH4 from end-on complexes, may be in competition with β-H migration/elimination of H2 and β-C—C bond cleavage/elimination of alkene. For the side-on complexes, the metal ions can insert into various C—C bonds followed by β-H transfer/competitive elimination of alkene and alkane or by a direct C—M+ bond cleavage to give an alkyl radical. Key words: FAB mass spectroscopy, metal-ion/nitrile reactions, collisional activation, coordination modes, ion–molecule reactions.