Structural and isotopomeric isomerism of hydrogen- bonded complexes

Two kinds of hydrogen-bonded complex isomerism, structural and isotopomeric, are the subject of contemporary research. Structural isomerism represents an extension of the phenomenon known from organic chemistry. Isotopomeric isomerism can be considered with any complex containing, in addition to the hydrogen involved in the hydrogen bond, at least one other hydrogen atom. Then, two different, isomeric species can be created by mono-deuteration of the complex. An example is given by the water-dimer gas-phase isotopomers HOD·OHD and DOH·OHD. The isomeric systems of both types can exhibit interchanges in the relative stabilities of the structures with changing temperature. Moreover, the temperature changes can be quite pronounced so that values of temperature derivatives are obtained which are not frequently observed with other types of isomeric systems. It can be well manifested in thermodynamic terms (standard changes of enthalpy, entropy and heat capacity upon cluster formation), or at least in isomerism contributions to the latter terms, especially so for heat capacity.     

Rotational isomerism inbis carbene MOL4 complexes: A theoretical study

LCAO-MO-SCF calculations are reported for the different stereoisomers of the Mo(CO)₄(CH₂)₂ and Mo(CO)₄[C(NH₂)₂]₂ systems. The substitution of the hydrogen atoms by the amino groups in the carbene ligands leads to an almost zero rotational barrier. Steric interactions are therefore expected to govern the barrier for diaminocarbene ligands which are more bulky than C(NH₂)₂. The rotational isomerism in thesebis carbene MoL₄ systems is also discussed in connection with the isolobal analogy between CH₂ and C₂H₄.     

Sequential C-F activation and borylation of fluoropyridines via intermediate Rh(I) fluoropyridyl complexes: a multinuclear NMR investigation

The C-F bond activation of fluoropyridines by [Rh(SiPh3)(PMe3)3] afforded Rh(I) fluoropyridyl complexes of the type [Rh(Ar(F))(PMe3)3] with concomitant formation of fluorotriphenylsilane; subsequent treatment with bis-catecholatodiboron yielded fac-[Rh(Bcat)3(PMe3)3] and the free fluoropyridyl boronate esters (Ar(F)Bcat).     

Structural and rotational isomerism in diaminocarbene complexes of iron

Primary amines react with a variety of cationic and neutral iron isocyanide complexes to yield structural and rotational diaminocarbene isomers characterized by variable temperature ¹H and ¹³C NMR spectra. Structural isomers result from the conversion of amines to isocyanide ligands via the base-catalyzed nucleophilic attack of initially formed diaminocarbenes on $\text{cis}$ isocyanides substituents, the effect on isomer populations is often marked. Factors influencing structural and rotational isomeric preferences are discussed.     

Linkage isomerism,kinetics and electrochemistry of ruthenium-edta complexes of benzotriazole

The [RuIII(edta)(benzotriazole)]− complex has been investigated in aqueous solution, by means of cyclic voltammetry, stopped-flow kinetics and spectroelectrochemistry. The formation reaction, starting from [RuIII(edta)H2O]− and benzotriazole, leads essentially to the N(1) coordinated isomer. In the case of the RuII species, the products exhibit an intramolecular isomerization equilibrium involving the N(1) and N(2) binding sites, with K12=0.33. A complete set of acid/base and association constants has been obtained for the RuII and RuIII complexes. The data are coherent with theoretical calculations, reflecting the importance of π-backbonding interactions in the reduced species. This revised version was published online in June 2006 with corrections to the Cover Date.     

Geometric isomerism of copper(II) with monochlorobenzhydrazide chelate complexes

Summary Chelate complexes having a 12 Cu^II ion:(singly deprotonated ligand ratio, formed from Cu^II including 2-, 3- and 4-chlorobenzhydrazides, (ClC₆H₄C(O)NHNH₂) have been studied in the pH > 10 region. With the 2-chloro substituted derivative the cis-isomer is formed, whereas with the 3- and 4-chloro substituted derivatives trans-isomers are formed. The complexing schemes are described.     

Hydride-phosphoniodithiocarboxylate/ phosphonium-betaine isomerism in Cy3PCS2 complexes of ruthenium

Reaction of Cy₃PCS₂ (Cy = cyclohexyl) with the hydrido complexes [RuClH(CA)(PPh₃)₃] (A  O, S), [RuH(CO)(NCMe)₂(PPh₃)₂]⁺, and [RuH(OClO₃)(CO)(CN^tBu)(PPh₃)₂] leads to the complex cations [RuH(CA)(PPh₃)₂(η²-S₂CPCy₃)]⁺, [Ru(η²-S₂CHPCy₃)(CO) (PPh₃)₂]⁺, [RuH(η¹-S₂CPCy₃)(CO)(CN^tBu)(PPh₃)₂]⁺. The σ-vinyl complex [Ru(CHCHC₆H₄Me-4)Cl(CO)(PPh₃)₂] reacts with Cy₃PCS₂ to give the cationic complex [Ru(CHCHC₆H₄Me-4) (CO)(PPh₃)₂(η²-S₂CPCy₃)]⁺, but this complex is not formed by hydroruthenation of HCCC₆H₄Me-4 by [RuH(CO)(PPh₃)₂(η²-S₂CPCy₃)]⁺. The inter-relationships between the above complexes are discussed.     

Nucleation of self-associating fluids: free versus activated association

We use the density functional theory of statistical mechanics in a square gradient approximation to analyze the structure, size, and work of formation of critical nuclei in self-associating fluids where association reduces the strength of the interactions between bonded particles. This effect is expected in systems of strongly dipolar particles that associate into chains. In this work we analyze the nucleation behavior of two types of self-associating fluids: a system comprised of particles that can freely associate, and a system in which the association process involves a thermally activated initiation step. For the first case, we explore the properties of critical nuclei in fluids that exhibit a metastable critical point between a vapor phase and a highly associated liquid phase. In fluids where the association dynamics involves an initiation step, we investigate the nucleation behavior in the vicinity of the polymerization transition. In both cases critical nuclei undergo a structural transition that shares many of the features of the coil-globule transition reported in Monte Carlo simulations of strongly dipolar Stockmayer fluids. Our results suggest that the sharp structural transition observed in these simulations is evidence of the existence of a second-order or nearly second-order association transition in these model fluids.     

A simple synthesis of thioketonemolybdenum carbonyl complexes. An example of geometric isomerism

Thioketones react with molybdenum hexacarbonyl in tetrahydrofuran (60–65°C, 2–3 h) to give molybdenum pentacarbonyl complexes in 52–80% yield. Geometric isomers were isolated in the case of thiocamphor.     

Single-molecule magnets: Jahn-Teller isomerism and the origin of two magnetization relaxation processes in Mn12 complexes

Several single-molecule magnets with the composition [Mn12O12(O2CR)16(H2O)x] (x = 3 or 4) exhibit two out-of-phase ac magnetic susceptibility signals, one in the 4-7 K region and the other in the 2-3 K region. New Mn12 complexes were prepared and structurally characterized, and the origin of the two magnetization relaxation processes was systematically examined. Different crystallographic forms of a Mn12 complex with a given R substituent exist where the two forms have different compositions of solvent molecules of crystallization and this results in two different arrangements of bound H2O and carboxylate ligands for the two crystallographically different forms with the same R substituent. The X-ray structure of cubic crystals of [Mn12O12(O2CEt)16(H2O)3]. 4H2O (space group P1) (complex 2a) has been reported previously. The more prevalent needle-form of [Mn12O12(O2CEt)16(H2O)3] (complex 2b) crystallizes in the monoclinic space group P2(1)/c, which at -170 degrees C has a = 16.462(7) A, b = 22.401(9) A, c = 20.766(9) A, beta = 103.85(2) degrees, and Z = 4. The arrangements of H2O and carboxylate ligands on the Mn12 molecule are different in the two crystal forms. The complex [Mn12O12-(O2)CC6H4-p-Cl)16(H2O)4].8CH2Cl2 (5) crystallizes in the monoclinic space group C2/c, which at -172 degrees C has a = 29.697(9) A, b = 17.708(4) A, c = 30.204(8) A, beta = 102.12(2) degrees, and Z = 4. The ac susceptibility data for complex 5 show that it has out-of-phase signals in both the 2-3 K and the 4-7 K ranges. X-ray structures are also reported for two isomeric forms of the p-methylbenzoate complex. [Mn12O12(O2CC6H4-p-Me)16(H2O)4]. (HO2CC6H4-p-Me) (6) crystallizes in the monoclinic space group C2/c, which at 193 K has a = 40.4589(5) A, b = 18.2288(2) A, c = 26.5882(4) A, beta = 125.8359(2) degrees, and Z = 4. [Mn12O12(O2CC6H4-p-Me)16(H2O)4].3(H2O) (7) crystallizes in the monoclinic space group I2/a, which at 223 K has a = 29.2794(4) A, b = 32.2371(4) A, c = 29.8738(6) A, beta = 99.2650(10) degrees, and Z = 8. The Mn12 molecules in complexes 6 and 7 differ in their arrangements of the four bound H2O ligands. Complex 6 exhibits an out-of-phase ac peak (chi(M)' ') in the 2-3 K region, whereas the hydrate complex 7 has a chi(M)' ' signal in the 4-7 K region. In addition, however, in complex 6, one Mn(III) ion has an abnormal Jahn-Teller distortion axis oriented at an oxide ion, and thus 6 and 7 are Jahn-Teller isomers. This reduces the symmetry of the core of complex 6 compared with complex 7. Thus, complex 6 likely has a larger tunneling matrix element and this explains why this complex shows a chi(M)' ' signal in the 2-3 K region, whereas complex 7 has its chi(M)' ' peak in the 4-7 K region, i.e., the rate of tunneling of magnetization is greater in complex 6 than complex 7. Detailed 1H NMR experiments (2-D COSY and TOCSY) lead to the assignment of all proton resonances for the benzoate and p-methyl-benzoate Mn12 complexes and confirm the structural integrity of the (Mn12O12) complexes upon dissolution. In solution there is rapid ligand exchange and no evidence for the different isomeric forms of Mn12 complexes seen in the solid state.     

Fragmentation of protonated amides through intermediate ion-neutral complexes: Neighboring group participation

It has been shown that neighboring group participation plays an important role in the fragmentation of protonated amides; the attachment of an adjacent functional group capable of accepting a proton provides alternative pathways of low energy for the formation of the inevitable N-protonated species in the fragmentation of the amide bond. Under methane chemical ionization (CI) conditions, protonated aniline (m/z 94) is only 1. 6% of the base peak MH⁺ ion for acetanilide; the abundance of the m/z 94 ion is increased to 15% for acetoacetanilide and protonated o-methoxyaniline reaches a relative intensity of 49% for N-acetyl-o-methoxyaniline. A more striking difference in ease of the formation of protonated anilines is found for acetanilides bearing a nitro group at different positions. Protonated nitroaniline (m/z 139) is the base peak in the methane CI spectrum of N-acetyl-o-nitroaniline; the m/z 139 ion drops to only 0.7% for the para isomer, and this ion is increased to 31.5% in the spectrum of N-acetoaceto-p-nitroaniline. By employing low energy collision-induced dissociation, it has been found that the fragmentation of protonated amides proceeds by way of ion-neutral complexes. In the case of acetanilide, for example, the cleavage of the amide bond gives rise to an acetylium ion and neutral aniline, which are bound together as a complex. An α-hydrogen of the acetylium ion, which is activated by the positive charge, is captured by aniline due to its higher proton affinity as compared with ketene. For those compounds having mobile protons other than the amidic hydrogen, it is indicated that such proton has the priority to be transferred in the reaction. Thus, the proton on the free carboxyl group of N-phenyl succinic and maleic monoamides is transferred in the fragmentation, leading to anhydrides as the neutral species in the formation of protonated aniline.     

Solvatochromism of heteroaromatic compounds: XXIX. Configurational isomerism of H complexes of phenols with amides and esters

Phenol and amides and esters in carbon tetrachloride form an H complex with an OH n bond, whose configurational isomers differ from each other by the direction of the hydrogen bond and, as a result, by the value of the spectral shift. Structural details of such configurational isomers and solvatochromism of their IR absorption bands are discussed.     

De novo prediction of ground state multiplicity and structural isomerism for transition metal complexes

Prediction of the ground state geometries and multiplicities for 33 transition metal tetrachlorides has been carried out using two different levels of quantum mechanics: semiempirical and density functional theory. All data regarding geometry and spin state provided by both computational methods were compared with experimental data when available. The calculations were performed for all possible spin multiplicities. The most important geometries for coordination number four (tetrahedral, square-planar, dodecahedral, and disphenoidal), as well as less symmetric structural isomers, were evaluated. A match between both computational methods in terms of predicted ground state multiplicity and geometry was found for 26 species, which translated into almost 80% agreement. Even though the PM3(tm) geometry prediction protocol involved more steps for isolating a feasible global minimum, the aggregate of these calculations was still orders of magnitude faster than DFT calculations using extended basis sets. The calculations indicate that caution is needed in the application of the PM3(tm) method to very high-spin transition metal complexes, but point to the suitability of very rapid semiempirical methods for reliable prediction of structural and ‘spin’ isomers, and hence their use in an efficient de novo design protocol for transition metal complexes.     

Mononuclear and one-dimensional manganese(II) complexes constructed by dithioethers: Effect of flexibility and positional isomerism

Interactions of dithioether ligands L², L⁴ and L⁵ (L² = 1,3-bis(4-(3-pyridyl) pyrimidin-2-ylthio) propane; L⁴ = 1,3-bis[4-(3-pyridyl) pyrimidinyl thiomethyl]benzene; L⁵ = 1,4-bis[4-(3-pyridyl)pyrimidinylthiomethyl] benzene) with Mn(II) ions and NH₄SCN in an analogous way led to the formation of two discrete mononuclear complexes and a one-dimensional chain, respectively, which may be attributed to the different flexibility and positional isomerism of the ligands.     

Structural diversity of silver(I) 4,6-dipyridyl-2-aminopyrimidine complexes: effect of counteranions and ligand isomerism

Using two ligands, 4,6-bis(2-pyridyl)-2-aminopyrimidine (L1) with two N,N'-chelating sites and 4-(2-pyridyl)-6-(4-pyridyl)-2-aminopyrimidine (L2) (as the isomer of L1) containing one chelating site and one bridging unit, a series of novel Ag(I) complexes varying from zero- to two-dimensions have been prepared and their crystal structures determined via single-crystal X-ray diffraction. The two ligands are employed for the first time in coordination chemistry. The structures of compounds 1-3 are directed by the counteranions adopted in the reaction system: The reaction of L1 with AgNO3 yielded a dimer [Ag2L12](NO3)2 (1). The reaction of L1 with AgCF3SO3 led to a one-dimension "V-shaped" chain {[AgL1](CF3SO3)}n (2). When AgSCN was used, a one-dimension ladder {[Ag2L1(SCN)2].H2O}n (3) was obtained. While ligand L2 reacted with AgNO3, a two-dimension {[Ag2(L2)2](NO3)2.H2O}n (4) was prepared with the help of an argentophilic interaction. Compounds 1-4 display room-temperature photoluminescence.