Pyridine Donors Research Articles

Zirconium and Titanium Complexes Supported by Tridentate LX2 Ligands Having Two Phenolates Linked to Furan, Thiophene, and Pyridine Donors: Precatalysts for Propylene Polymerization and Oligomerization

Zirconium and titanium complexes with tridentate bis(phenolate)−donor (donor = pyridine, furan and thiophene) ligands have been prepared and investigated for applications in propylene polymerization. The ligand framework has two “X-type” phenolates connected to the flat heterocyclic “L-type” donor at the 2,6- or 2,5- positions via direct ring−ring (sp2−sp2) linkages. The zirconium and titanium dibenzyl complexes have been prepared by treatment of the neutral bis(phenol)−donor ligands with M(CH2Ph)4 (M = Ti, Zr) with loss of 2 equiv of toluene. Titanium complexes with bis(phenolate)pyridine and -furan ligands and zirconium complexes with bis(phenolate)pyridine and -thiophene ligands have been characterized by single-crystal X-ray diffraction. The solid-state structures of the bis(benzyl)titanium complexes are roughly C2-symmetric, while the zirconium derivatives display Cs and C1 symmetry. The bis(phenolate)pyridine titanium complexes are structurally affected by the size of the substituents (CMe3 or CEt3) ortho to the oxygens, the larger group leading to a larger C2 distortion. Both titanium and zirconium dibenzyl complexes were found to be catalyst precursors for the polymerization of propylene upon activation with methylaluminoxane (MAO). The activities observed for the zirconium complexes are particularly notable, exceeding 106 g polypropylene/mol Zr·h in some cases. The bis(phenolate)pyridine titanium analogues are about 103 times less active, but generate polymers of higher molecular weight. When activated with MAO, the titanium bis(phenolate)furan and bis(phenolate)thiophene systems were found to promote propylene oligomerization.

Energy Transfer Dynamics in the Presence of Preferential Hydrogen Bonding: Collisions of Highly Vibrationally Excited Pyridine-h5, -d5, and -f5 with Water

The relaxation of highly vibrationally excited pyridine-h5, pyridine-d5, and pyridine-f5 (E approximately 38,000 cm(-1)) through collisions with water was investigated by high resolution transient IR absorption spectroscopy to investigate how preferential hydrogen bonding interactions impacts the energy transfer dynamics. Nascent rotational and translational energy gain profiles for scattered H2O(000) molecules with E(rot) > 1000 cm(-1) are reported. H2O(000) molecules scattered from pyridine-h5 and pyridine-d5 have rotational distributions with T(rot) = 890 K. Less rotational energy is found in H2O(000) scattered from pyridine-f5 for which T(rot) = 530 K. The recoil energy distributions are similar for the three donors with T(rel) = 400-700 K. To explain the results, a torque-inducing mechanism is proposed that involves directed movement of water between sigma and pi-hydrogen bonding interactions with the pyridine donors. The experimental results are consistent with this mechanism, and with effects due to the state-density energy dependence of the highly excited donor molecules. Differences in vibrational mode frequencies of the hot donor molecules do not appear to explain the experimental results.

Reactions of Copper(II)-H2O2 Adducts Supported by Tridentate Bis(2-pyridylmethyl)amine Ligands: Sensitivity to Solvent and Variations in Ligand Substitution

The copper(II) complexes 1(H) and 1(Ar(X)), supported by the N,N-di(2-pyridylmethyl)benzylamine tridentate ligand (L(H)) or its derivatives having m-substituted phenyl group at the 6-position of pyridine donor groups (L(Ar(X))), have been prepared, and their reactivity toward H2O2 has been examined in detail at low temperature. Both copper(II) complexes exhibited a novel reactivity in acetone, giving 2-hydroxy-2-hydroperoxypropane (HHPP) adducts 2(H) and 2(Ar(X)), respectively. From 2(Ar(X)), an efficient aromatic ligand hydroxylation took place to give phenolate-copper(II) complexes 4(Ar(X)). Detailed spectroscopic and kinetic analyses have revealed that the reaction proceeds via an electrophilic aromatic substitution mechanism involving copper(II)-carbocation intermediates 3(Ar(X)). Theoretical studies at the density functional theory (DFT) level have strongly implicated conjugate acid/base catalysis in the O-O bond cleavage and C-O bond formation steps that take the peroxo intermediate 2(Ar(X)) to the carbocation intermediate 3(Ar(X)). In contrast to the 2(Ar(X)) cases, the HHPP-adduct 2(H) reacted to give a copper(II)-acetate complex [Cu(II)(L(H))(OAc)](ClO4) (6(H)), in which one of the oxygen atoms of the acetate co-ligand originated from H2O2. In this case, a mechanism involving a Baeyer-Villiger type 1,2-methyl shift from the HHPP-adduct and subsequent ester hydrolysis has been proposed on the basis of DFT calculations; conjugate acid/base catalysis is implicated in the 1,2-methyl shift process as well. In propionitrile, both 1(H) and 1(Ar(X)) afforded simple copper(II)-hydroperoxo complexes LCu(II)-OOH in the reaction with H2O2, demonstrating the significant solvent effect on the reaction between copper(II) complexes and H2O2.

Synthesis and properties of guanidine-pyridine hybridligands and structural characterisation of their mono- and bis(chelated) cobalt complexes

The synthesis of four guanidine-pyridine hybridligands and their spectroscopic features in MeCN are described. In order to demonstrate their coordinating properties, the corresponding cobalt(II)chloride complexes have been prepared and completely characterised by means of X-ray structure analysis, UV/Vis spectroscopy and mass spectrometry. The neutral complexes {1,1,3,3-tetramethyl-2-(quinolin-8-yl)guanidine}cobalt(II)-dichloride [Co(TMGqu)Cl 2] and { N-(1,3-dimethylimidazolidin-2-yliden)pyridin-8-amine}cobalt(II)-dichloride [Co(DMEGpy)Cl 2] exhibit a tetrahedral coordination of the cobalt atom, whereas in bis[chlorobis{ N-(1,3-dimethylimidazolidin-2-yliden)quinolin-8-amine}cobalt(II)]tetrachlorocobaltate [Co(DMEGqu) 2Cl] 2[CoCl 4] and chlorobis{1,1,3,3-tetramethyl-2-((pyridin-2-yl)methyl)guanidine}cobalt(II)chloride [Co(TMGpy) 2Cl]Cl, the cobalt atom is coordinated in a trigonal pyramidal environment. These trigonal pyramidal complex cations represent the first bis(chelated) guanidine cobalt complexes in which the pyridine donor resides on the apical position and the guanidine donor forms with the chlorine atom the base of the pyramid. Besides the structural characterisation, the quenching effect of the cobalt(II) ion ( d 7) on the ligand fluorescence has been studied.

Stable Five‐ and Six‐Coordinate Cobalt(III) Complexes with a Pentadentate Bispidine Ligand

I am ligand: A pentadentate bispidine ligand with two tertiary amine and three pyridine donors stabilizes the uncommon intermediate-spin electronic configuration of CoIII (S=1; see picture: C gray, N blue, O red, Co violet). Dissociation of the monodentate coligand yields a catalytically active pentacoordinate complex.

Halogen bonding in substituted cobaloximes

Aquabis(dimethylglyoximato)nitrocobalt(III) reacts with halogen-substituted pyridines to give nitro complexes in which the pyridine donors substitute the aqua ligand; reaction with halogen-substituted pyridinium chlorides directly displaces both the coordinated water molecule and the nitro ligand in a one-pot reaction and affords the analogous chloro complexes. Ten crystal structures of eight new compounds are reported: among these, the nitro complex bis(dimethylglyoximato)nitro(4-chloropyridine)cobalt(III) is characterized by a remarkably long bond between the metal and the nitro ligand whereas the analogous chloro complex chlorobis(dimethylglyoximato)(4-chloropyridine)cobalt(III) features a very long Co–Cl distance. The structures communicated comprise three isomorphous pairs which are particularly suited for the comparative study of chloro and bromo derivatives. The most relevant intermolecular interactions in these compounds are due to very short oxygen⋯halogen contacts of ca. 2.9 Å whereas shortest interhalogen distances are slightly longer than the sum of the van der Waals radii. For both types of interactions, contact distances involving bromine are shorter than those associated with chlorine.

Reactivity of Mononuclear Alkylperoxo Copper(II) Complex. O−O Bond Cleavage and C−H Bond Activation

A detailed reactivity study has been carried out for the first time on a new mononuclear alkylperoxo copper(II) complex, which is generated by the reaction of copper(II) complex supported by the bis(pyridylmethyl)amine tridentate ligand containing a phenyl group at the 6-position of the pyridine donor groups and cumene hydroperoxide (CmOOH) in CH3CN. The cumylperoxo copper(II) complex thus obtained has been found to undergo homolytic cleavage of the O-O bond and induce C-H bond activation of exogenous substrates, providing important insights into the catalytic mechanism of copper monooxygenases.

Tetraalkoxyaluminates of Nickel(II), Copper(II), and Copper(I)

The syntheses and structural details of tetraisopropoxyaluminates and tetra-tert-butoxyaluminates of nickel(II), copper(I), and copper(II) are reported. Within the nickel series, either Ni[Al(OiPr)4]2.2HOiPr, with nickel(II) in a distorted octahedral oxygen environment, or Ni[Al(OiPr)4]2.py, with nickel(II) in a square-pyramidal O4N coordination sphere, or Ni[(iPrO)(tBuO)3Al]2, with Ni(II) in a quasi-tetrahedral oxygen coordination, has been obtained. Another isolated complex is Ni[(iPrO)3AlOAl(OiPr)3].3py (with nickel(II) being sixfold-coordinated), which may also be described as a "NiO" species trapped by two Al(OiPr)3 Lewis acid-base systems stabilized at nickel by three pyridine donors. Copper(I) compounds have been isolated in three forms: [(iPrO)4Al]Cu.2py, [(tBuO)4Al]Cu.2py, and Cu2[(tBuO)4Al]2. In all of these compounds, the aluminate moiety behaves as a bidentate unit, creating a tetrahedrally distorted N2O2 copper environment in the pyridine adducts. In the base-free copper(I) tert-butoxyaluminate, a dicopper dumbbell [Cu-Cu 2.687(1) A] is present with two oxygen contacts on each of the copper atoms. Copper(II) alkoxyaluminates have been characterized either as Cu[(tBuO)4Al]2, {Cu(iPrO)[(iPrO)4Al]}2, and Cu[(tBuO)3(iPrO)Al]2 (copper being tetracoordinated by oxygen) or as [(iPrO)4Al]2Cu.py (pentacoordinated copper similar to the nickel derivative). Finally, a copper(II) hydroxyaluminate has been isolated, displaying pentacoordinate copper (O4N coordination sphere) by dimerization, with the formula {[(tBuO)4Al]Cu(OH).py}2. The formation of all of these isolated products is not always straightforward because some of these compounds in solution are subject to decomposition or are involved in equilibria. Besides NMR [copper(I) compounds], UV absorptions and magnetic moments are used to characterize the compounds.

The NCN-Type Pincer Complex of Palladium(II) with a 2,6-Bis(2-pyridyl)-4-tert-butylbenzene Tridentate Ligand: Synthesis, Structure and Catalytic Activity in the Mizoroki-Heck Reaction

The reaction of 1-bromo-4-tert-butyl-2,6-diiodobenzene with 2-pyridylzinc chloride in the presence of catalytic Pd(PPh 3 ) 4 afforded an NCN-type pincer palladium(II) complex with two pyridine donors. The structure of the complex was determined by X-ray crystallography. The complex was found to catalyze the Mizoroki-Heck reaction at 250 °C under aerobic conditions.

A one-dimensional copper(ii) coordination polymer containing [Fe(pytpy)2]2+(pytpy = 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine) as an expanded 4,4′-bipyridineligand: a hydrogen-bonded network penetrated by rod-like polymers

The one-dimensional coordination polymer [{(H2O)(NO3)2CuFe(1)2}2(NO3)4·2.15MeCN·5.85H2O]n assembles when the pendant pyridine donors in [Fe(1)2]2+ (1 = 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine) are treated with Cu(NO3)2·3H2O, exemplifying [Fe(1)2]2+ as an expanded 4,4′-bipyridine ligand. In the solid state, the polymer chains are threaded through hydrogen-bonded macrocycles comprising coordinated and non-coordinated water molecules and nitrate ions, the macrocycles being motifs within interconnected sheets in the structure.

Epoxidation and 1,2-Dihydroxylation of Alkenes by a Nonheme Iron Model System − DFT Supports the Mechanism Proposed by Experiment

The FeII complexes of two isomeric pentadentate bispidine ligands in the presence of H2O2 are catalytically active for the epoxidation and 1,2-dihydroxylation of cyclooctene (bispidine = 3,7-diazabicyclo[3.3.1]nonane; the two isomeric pentadentate bispidine ligands discussed here have two tertiary amine and three pyridine donors). The published spectroscopic and mechanistic data, which include an extensive set of 18O labeling experiments, suggest that the FeIV=O complex is the catalytically active species, which produces epoxide as well as cis- and trans-1,2-dihydroxylated products. Several observations from the published experimental study are addressed with hybrid density functional methods and, in general, the calculations support the proposed, for nonheme iron model systems novel mechanism, where the formation of a radical intermediate emerges from the reaction of the FeIV=O oxidant and cyclooctene. The calculations suggest that the S = 1 ground state of the FeIV=O complex reacts with cyclooctene in a stepwise reaction, leading to the formation of a carbon-based radical intermediate. This radical is captured by O2 from air to produce the majority of the epoxide products in an aerobic atmosphere. Under anaerobic conditions, the produced epoxide product is due to the cyclization of the radical intermediate. Several possible spin states (ST = 3, 2, 1, 0) of the radical intermediate are close in energy. As a result of the substantial energy barrier, calculated for the ST = 3 spin ground state, a spin-crossover during the cyclization step is assumed, and a possible two-state scenario is found, where the S = 2 state of the FeIV=O complex participates in the catalytic mechanism. The 1,2-dihydroxylation proceeds, as suggested by experiment, via an unprecedented pathway, where the radical intermediate is captured by a hydroxyl radical, the source of which is FeIII-OOH, and this reaction is barrierless. The calculations suggest that dihydroxylation can also occur by a direct oxidation pathway from FeIII-OOH. The strikingly different reactivities observed with the two isomeric bispidine FeII complexes are rationalized on the basis of structural and electronic differences.

Synthetic routes to mixed-ligand cobalt(III) dithiocarbamato complexes containing imidazole, amine and pyridine donors and the X-ray crystal structure of a cobalt(III) bis(dithiocarbamato) histamine complex

The binuclear cobalt complex [Co2(Me2dtc)5]+ reacts with a range of nitrogen donor ligands L′ or L″ to form an equimolar mixture of Co(Me2dtc)3 and the mixed-ligand complexes [Co(Me2dtc)2(L′)2]+ or [Co(Me2dtc)2(L″)]+, where (L′)2 is two monodentate ligands and (L″) is one bidentate ligand. The complexes prepared by this route contain the monodentate ligands L′=1-methyl-imidazole, 1-methyl-5-nitro-imidazole and benzimidazole, all of which coordinate to cobalt through an imidazole nitrogen atom. Symmetrical bidentate ligand complexes contain the bisimidazole L″=2,2′-bis(4,5-dimethylimidazole), the diamine L″=1,2-diaminobenzene and the pyridine donors L″=2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine and 1,10-phenanthroline. Two examples of complexes with unsymmetrical bidentate imidazole-amine donors were prepared in which L″=4-(2-aminoethyl)imidazole (histamine) and 2-aminomethylbenzimidazole. All new complexes were fully characterised, and the X-ray crystal structure of the histamine complex [Co(Me2dtc)2(hist)]ClO4 is also reported.

Tetranuclear supramolecular structures containing phthalocyanine cores

A metal-free phthalocyanine with four pyridyl donor groups, bound through ethylthio ester bridges on the periphery, have been prepared. The pyridine donors were quaternized with iodomethane to a water-soluble tetracationic phthalocyanine. The tetranuclear supramolecular phthalocyanine was prepared by the coordination of peripheral pyridine donors with VO(acac)2. The paramagnetic tetranuclear structure was studied in powder and solution forms by the electron paramagnetic resonance (EPR) technique. Electron paramagnetic resonance studies, together with the other spectral data confirmed the presence of identical pyridine-coordinated VO(acac)2paramagnetic centers attached to the peripheral positions of the phthalocyanine core. The X-band EPR signals recorded from powder and solution forms of supramolecules have a characteristic line shape that proves the presence of axial symmetry around the paramagnetic vanadium ions. The anisotropic Lande splitting factors were calculated as g∥< g⊥< ge= 2.0023. Orbital energy levels for magnetic electrons were determined from theoretically fitted Spin Hamiltonian parameters.

Theoretical study on electronic structures and luminescent properties of phenol–pyridyl boron complexes

Electronic properties, such as HOMO and LUMO energies, band gaps and the ionization potentials ( I P) of five boron complexes containing mixed phenol–pyridine donor set, were studied by the density functional theory with Becke–Lee–Young–Parr composite exchange correlation functional (B3LYP). The absorption spectra of these complexes were also investigated by time dependent density functional theory (TDDFT) with 6-31G ∗ basis set. The calculated results indicated that the HOMO and LUMO of the complexes are both destabilized by electron-donating substituents (CH 3 and NH 2). Seen from I Ps, they all exhibit good hole-transporting ability, and their correlations with HOMO energies is linear. The TDDFT calculations showed that different substituents on the phenol–pyridyl unit lead to nominal change on the absorptions of these boron complexes. The calculations on the structural, electronic and optical properties of the compounds are in good agreement with the experimental data.

Preparation and Coordination Complex of the First Imine-Bridged Tetrathiafulvalene−Pyridine Donor Ligand

The first imine-bridged pyridyltetrathiafulvalene building block (TTF-CH=N-Py, 1) has been synthesized via the Schiff base condensation of formyltetrathiafulvalene and 2-aminopyridine. The preparation, X-ray crystal structure, electrochemical and magnetic characterization of a 1:1 copper complex [CuII(hfac)2(TTF-CH=N-Py)] (2) are reported. The crystal structure reveals that the imine N atom participates in chelation to the paramagnetic center, thus making this ligand an attractive precursor for the assembly of pi-d systems.

Hydrogen‐Atom Transfer in Open‐Shell Organometallic Chemistry: The Reactivity of RhII(cod) and IrII(cod) Radicals

A series of new metalloradical rhodium and iridium complexes [M(II)(cod)(N-ligand)](2+) in the uncommon oxidation state +II were synthesized by one-electron oxidation of their [M(I)(cod)(N-ligand)](+) precursors (M=Rh, Ir; cod=(Z,Z)-1,5-cyclooctadiene; and N-ligand is a podal bis(pyridyl)amine ligand: N,N-bis(2-pyridylmethyl)amine (dpa), N-(2-pyridylmethyl)-N-(6-methyl-2-pyridylmethyl)amine (pla), or N-benzyl-N,N-bis(6-methyl-2-pyridylmethyl)amine (Bn-dla). EPR spectroscopy, X-ray diffraction, and DFT calculations reveal that each of these [M(II)(cod)(N-ligand)](2+) species adopts a square-pyramidal geometry with the two cod double bonds and the two pyridine fragments in the basal plane and the N(amine) donor at the apical position. The unpaired electron of these species mainly resides at the metal center, but the apical N(amine) donor also carries a considerable fraction of the total spin density (15-18 %). Density functional calculations proved a valuable tool for the analysis and simulation of the experimental EPR spectra. Whereas the M(II)(olefin) complexes are quite stable as solids, in solution they spontaneously transform into a 1:1 mixture of M(III)(allyl) species and protonated M(I)(olefin) complexes (in the forms [M(I)(olefin)(protonated N-ligand)](2+) for M=Rh and [M(III)(H)(olefin)(N-ligand)](2+) for M=Ir). Similar reactions were observed for the related propene complex [M(II)(propene)(Me(2)tpa)](2+) (Me(2)tpa=N,N,N-tris(6-methyl-2-pyridylmethyl)amine). The decomposition rate of the [M(II)(cod)(N-ligand)](2+) species decreases with increasing N-ligand bulk in the following order: dpa>pla>Bn-dla. Decomposition of the most hindered [M(II)(cod)(Bn-dla)](2+) complexes proceeds by a second-order process. The kinetic rate expression v=k(obs)[M(II)](2) in acetone with k(obs)=k'[H(+)][S], where [S] is the concentration of additional coordinating reagents (MeCN), is in agreement with ligand-assisted dissociation of one of the pyridine donors. Solvent coordination results in formation of more open, reactive species. Protonation of the noncoordinating pyridyl group increases the concentration of this species, and thus [H(+)] appears in the kinetic rate expression. The kinetic data are in agreement with bimolecular hydrogen-atom transfer from M(II)(cod) to another M(II) species (DeltaH( not equal)=11.5+/-2 kcal mol(-1), DeltaS( not equal)=-27+/-10 cal K(-1) mol(-1), and DeltaG( not equal)(298 K)=19.5+/-5 kcal mol(-1)).

Effect of Electron Donors on the Radical Polymerization of Vinyl Acetate Mediated by [Co(acac)2]: Degenerative Transfer versus Reversible Homolytic Cleavage of an Organocobalt(III) Complex

The molecular structure of bis(acetylacetonate)cobalt(II) ([Co(acac)2]) in solution and in the presence of the electron donors (ED) pyridine (py), NEt3, and vinyl acetate (VOAc) was investigated using 1H NMR spectroscopy in C6D6. The extent of formation of ligand adducts, [Co(acac)2(ED)x], varies in the order py>NEt3>VOAc (no interaction). Density functional theory (DFT) calculations on a model system agree with Co--ED bond strengths decreasing in the same order. The effect of electron donors on the [Co(acac)2]-mediated radical polymerization of VOAc was examined at 30 degrees C by the addition of excess py or NEt3 to the complex in the molar ratio [VOAc]0/[Co]0/[V-70]0/[py or NEt3]0=500:1:1:30 (V-70=2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile)). As previously reported by R. Jerome et al., the polymerization showed long induction periods in the absence of ED. However, a controlled polymerization without an induction period took place in the presence of ED, though the level of control was poorer. The effective polymerization rate decreased in the order py>NEt3. A similar behavior was found when these electron donors were added to an ongoing [Co(acac)2]-mediated radical polymerization of VOAc. On the basis of the NMR and DFT studies, it is proposed that the polymerization is controlled by the reversible homolytic cleavage of an organocobalt(III) dormant species in the presence of ED. Conversely, the faster polymerization after the induction period in the absence of ED is due to a degenerative transfer process with the radicals produced by the continuous decomposition of the excess initiator. Complementary experiments provide additional results in agreement with this interpretation.

Synthesis and Oxidation of Vanadyl Complexes Containing Bispidine Ligands

The reaction of 3-methyl-9-oxo-7-picolyl-2,4-di(2-pyridyl)-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylate (L2) with vanadyl sulfate in methanol yields the purple complex[VIV=O(L2′)](ClO4)2, where L2 is hydrated to the corresponding 9-diol L2′. The crystal structure of [VIV=O(L2′)](ClO4)2 has a saturated coordination sphere with a long bond to the tertiary amine N7 trans to the oxido group (2.36 Å). Oxidation of [VIV=O(L2′)](ClO4)2 with an excess of aqueous hydrogen peroxide yields the orange complex [VV=O(O2)(L2′)]ClO4. This can be further oxidized with Ce4+ in MeCN at –40 °Cto the unstable vanadium(V)-oxidosuperoxido complex [VV=O(O2)(L2′)]2+, as shown by EPR spectroscopy. The X-ray crystal structure of [VV=O(O2)(L2′)]ClO4 shows that the N7-pendant pyridine donor is dissociated and the V–N7 bond is elongated (2.56 Å). The O–O distance of the side-on bound peroxide in [VV=O(O2)(L2′)]ClO4 is unexpectedly short (1.28 Å) and is therefore in the range which is typical for a superoxido group. However, the IR spectrum of [VV=O(O2)(L2′)]ClO4 shows 18O isotope shifts, which support the assignment as a peroxido complex, and this is also supported by EPR and UV/Vis spectroscopic analysis. The short O–O bond is attributed to a static disorder rather than to libration of the “non-rigid” peroxido group.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Co-crystals of iodopentafluorobenzene with nitrogen donors: 2-D molecular assemblies through halogen bonding and aryl–perfluoroaryl interactions

Electron deficient iodopentafluorobenzene forms heterodimers with electron rich pyridine donors stabilized by halogen bonding, that are further assembled into polar brick-wall type 2-D network via π–π stacking interactions between aryl and perfluoroaryl moieties.

Stability Constants: A New Twist in Transition Metal Bispidine Chemistry

Transition metal complexes with 2,4-substituted tetradentate, 2,3,4- and 2,4,7-substituted pentadentate, and 2,3,4,7-substituted hexadentate bispidine ligands (bispidine = 3,7-diazabicyclo[3.3.1]nonane) with two tertiary amine and two, three, or four pyridine donors are relatively stable (10 < log K(CuL) < 18). Interestingly, the two isomeric pentadentate ligands have very different stabilities with a variety of metal ions and, depending on the metal ion, one of the isomers leads to more stable complexes than the hexadentate and the other to less stable complexes than the tetradentate ligand. Another interesting observation is that the complex stabilities of all bispidine ligands reported here do not follow the Iriving-Williams series since the stability constants of the cobalt(II) complexes are up to 4 log units larger than those of the corresponding nickel(II) complexes. All these observations are analyzed on the basis of subtle distortions of the coordination geometries, and these have been related previously to Jahn-Teller-derived distortions for the copper(II) complexes. However, similar but less pronounced structural properties are observed with other metal centers, as shown, e.g., with the experimental structures of the two zinc(II) complexes with the isomeric pentadentate ligands reported here. The structural properties and the related stabilities are also discussed on the basis of force field calculations.