Pentacoordinate Compounds Research Articles

Complexes R2Sn(IV)L with O,N,O'-Donor Schiff Bases: Synthesis, Structures, and Redox Properties

New tin(IV) complexes with O,N,O'-donor Schiff bases (L1H2–L4H2) of the (Ln)SnR2 type (R = Ph (I–III), Et (IV–VII)) are synthesized and characterized. The molecular structures of compounds I–III, VI, and VII in the crystalline form are determined by X-ray diffraction (XRD) (CIF files CCDC nos. 2181140 (I), 2181142 (II), 2181143 (III∙CH3CN), 2181141 (VI), and 2181139 (VII)). Tin complexes I–III and VI are mononuclear pentacoordinate compounds. Crystalline complex VII forms dimers via the pairwise bridging coupling between the oxygen and tin atoms of the mononuclear fragments. The redox-active ligand in the synthesized compounds exists as the iminobis(phenolate) dianion. The electrochemical properties of free ligands and complexes I–VII are studied. In the case of compounds I, II, IV, and V with tert-butyl substituents in the redox-active ligand, the formation of relatively stable monocationic and monoanionic species is electrochemically detected for the first time. The presence of the electroactive nitro group results in the destabilization of the oxidized forms of the complexes and induces the appearance of an additional peak in the cathodic range. The energy gaps between the frontier redox orbitals are determined by the electrochemical and spectral methods. The obtained parameters are close and vary in a range of 2.43–2.68 eV.

Theoretical Design of Stable Pentacoordinate Boron Compounds

Through theoretical computations, we found that boron can form thermodynamically stable pentacoordinate compounds. Pentacoordinate boron (penta-B) is just hypercoordinate but not hypervalent because it forms only four covalent bonds, of which at least one is a multicenter bond. Being electron deficient, to be pentacoordinate, at least two of its bonding atoms should have low electronegativity. Penta-B can be formed in HkB(CH3)m(XH3)n (X = Si, Ge, Sn, and n ≥ 2) and BR5 (R = BH2NH3, AsH2, and BeH). Based on a systematic investigation of these model compounds, we designed three thermodynamically stable penta-B compounds that can potentially be synthesized by hydrogenating their tricoordinate counterparts under mild reaction conditions.

New sulfonamide complexes with essential metal ions [Cu (II), Co (II), Ni (II) and Zn (II)]. Effect of the geometry and the metal ion on DNA binding and nuclease activity. BSA protein interaction

Mixed divalent Cu, Co, Ni and Zn complexes containing the new sulfonamide ligand N–(2–(pyridin–2–yl)ethyl)quinoline–8–sulfonamide (HQSEP) were prepared and characterized by physico-chemical techniques. The tetracoordinate [Cu(QSEP)X] [X = Br (1), Cl (2)] compounds present a seesaw geometry (τ4 = 0.56 (1) and 0.50 (2)). The Cu(II) in the [Cu(QSEP)(NO3)(MeOH)] (3) complex is five coordinate with a slightly distorted SP geometry (τ = 0.11). The [M(QSEP)(benz)] [M = Cu(II) (4), Ni(II) (5), Co(II) (6) and Zn(II) (7); benz = benzoate] compounds are configurationally isotypic. The coordination geometries of the M(II) ions can be best described as distorted SP (τ = 0.29, 0.15, 0.34 and 0.18 for 4, 5, 6 and 7, respectively). The interaction of the compounds with CT–DNA was studied by different techniques. Notably, these studies indicated that the tetracoordinate complexes (1 and 2) present higher DNA affinity than pentacoordinate compounds (3–7). In line with the Irving–Williams order of stability, 5 presented higher propensity for DNA binding than 6. Interestingly, the cleavage activity of 1–4 in the presence of ascorbate/H2O2 follows the same trend as that found for DNA binding affinity, being the tetracoordinate 1 and 2 more effective as nucleases than the five coordinate 3 and 4. Also, the DNA cleavage reaction mechanism was investigated. DNA cleavage experiments upon irradiation indicated the important role of the aromatic nature of the coligand in the photocleavage activity of 1–4. Finally, the interaction of the compounds with bovine serum albumin (BSA) was studied and the binding constants were calculated.

Stereocontrol in nucleophilic substitution reactions at silicon: the role of permutation in generating silicon-centered chirality.

Intramolecular isomerization in pentacoordinate compounds can play an essential role for the adjustment of defined stereochemical information. Here, we present a conclusive mechanism of a stereocontrolled reaction on chiral dimethoxysilanes that opens new aspects in understanding the origin of creating silicon-centered chirality during a nucleophilic substitution process. By combining experimental, structural, and quantum chemical methods, we were able to disclose an interconversion process, based on consecutive Berry-type motions, as the most plausible mechanism for describing the stereochemical outcome in suchlike substitution reactions.

Theoretical exploration of stereochemical nonrigidity for R f Co(PF3) x (CO)4−x (R f =CF3, C2F5, C3F7, x=0–4)

The stereochemical nonrigidity of R f Co(PF3) x (CO)4−x (R f =CF3, C2F5, C3F7, x=0–4) was studied at the theoretical level of B3LYP/6-311+G* via Gaussian 09. The intramolecular rearrangements in these penta-coordinated compounds are mainly caused by the vibrations of perfluoroalkyl groups. All the barriers along the reaction coordinate are less than 66.9 kJ/mol, which indicates that the rearrangements are kinetically favorable and hard to elucidate by experiment. Besides, ligand PF3 is a ligand similar to CO, the energy difference between the reactant and product is small.

ChemInform Abstract: First Preparation of a Sulfurane with Four Carbon-Sulfur Bonds: Synthesis and Molecular Structure of Bis(2,2′-biphenylylene)sulfurane.

The first stable pentacoordinated sulfur compound with four carbon–sulfur bonds, bis(2,2′-biphenylylene)sulfurane 1, is prepared and its molecular structure is determined by X-ray crystallographic analysis.

Experimental Evidence for Magnetic Exchange in Di- and Trinuclear Uranium(IV) Ethynylbenzene Complexes

We report the preparation and magnetic property investigations of a structurally related family of mono-, di-, and trinuclear U(IV) aryl acetylide complexes. The reaction between [(NN'(3))UCl] and lithiated aryl acetylides leads to the formation of the hexacoordinate complexes [(NN'(3))U(CCPh)(2)(Li.THF)] (1) and [(NN'(3))(2)U(2)(p-DEB)(THF)] (2) as red-brown and yellow-green crystalline solids, respectively. In contrast, combining the uranacycle [(bit-NN'(3))U] (bit-NN'(3) = [N(CH(2)CH(2)NSi(t)BuMe(2))(2)(CH(2)CH(2)Si(t)BuMeCH(2)]) with stoichiometric amounts of mono-, bis-, and tris(ethynyl) benzenes affords the yellow-green pentacoordinate arylacetylide complexes [(NN'(3))U(CCPh)] (3), [(NN'(3))(2)U(2)(m-DEB)] (4), [(NN'(3))(2)U(2)(p-DEB)] (5), and [(NN'(3))(3)U(3)(TEB)] (6), where NN'(3) = [N(CH(2)CH(2)NSi(t)BuMe(2))(3)]. The measured magnetic susceptibilities for 1-6 trend toward non-magnetic ground states at low temperatures. Nevertheless, the di- and trinuclear pentacoordinate compounds 4-6 appear to display weak magnetic communication between the uranium centers. This communication is modeled by fitting of the direct current (DC) magnetic susceptibility data, using the spin Hamiltonian H = -2J(S(i) x S(j)). These results are consistent with weak ferromagnetic coupling for complexes 4-6 (J = 4.76, 2.75, and 1.11 cm(-1), respectively), while the fit for 2 is consistent with a near-negligible exchange interaction (J = -0.05 cm(-1)). Geometry-optimized Stuttgart/6-31 g* B3LYP hybrid DFT calculations were carried out (spin-orbit coupling omitted) on model complexes of 3-5. The mononuclear complex shows a triplet ground state with singly occupied degenerate f orbitals. The meta- and para-bridged species are computed to show very weak ferro- and antiferromagnetic coupling, respectively. All three complexes show only small net spin density on the acetylide-containing ligands. The monomeric phenylacetylide complex 3 undergoes a reversible redox couple at -1.02 V versus [Cp(2)Fe](+/0), assignable to an oxidation of U(IV) to U(V).

Water-soluble and stable dinitrogen phosphine complexes trans-[ReCl(N2)(PTA-H)n(PTA)4−n]n+(n = 0–4), the first with 1,3,5-triaza-7-phosphaadamantane

The first dinitrogen complexes with the hydrosoluble PTA ligand, or its protonated form PTA-H, trans-[ReCl(N2)(PTA-H)n(PTA)(4-n)]n+ (n = 0-4), are prepared, shown to be soluble and stable in water, interconvertible by stepwise protonation/deprotonation and to form, upon N2 loss, the corresponding penta-coordinate compounds. Dinitrogen displacement by CO affords trans-[ReCl(CO)(PTA)4].

Silicon rehybridization and molecular rearrangements in hypercoordinate silicon dichelates

AbstractHydrazide-based hypercoordinate silicon dichelates are remarkably flexible in terms of geometry and reactivity: this paper demonstrates how rather subtle constitutional changes result in dramatic geometrical and reactivity changes. A change of ligand-donor group from NMe2to N=CMe2changes the solid-state geometry from bicapped tetrahedral to octahedral. These two geometries are shown to coexist in solution in dynamic equilibrium. Hexacoordinate complexes are shown to dissociate to pentacoordinate compounds in two distinct ways: ionic or neutral, depending on substitution. Hexacoordinate dichelates with imino-donor groups undergo a skeletal rearrangement (intramolecular aldol-type condensation of imines), catalyzed by their dissociated halide counterions. However, even in the absence of counterions, silacyclobutane dichelates undergo a similar rearrangement.

Dynamic Properties of Methyl(trimethylphosphane)nickel Complexes Containing (2‐Diphenylphosphanyl)phenylamido Ligands

AbstractP,N‐chelates of nickel NiCl[2‐(Ph2P)C6H4NR](PMe3) (1: R = H; 2: R = SiMe3) have been obtained from NiCl2(PMe3)2 and 2‐(Ph2P)C6H4NRLi. Reaction with MeLi affords methylnickel compounds NiMe[2‐(Ph2P)C6H4NR](PMe3) (3: R = H; 4: R = Me; 5: R = SiMe3). By the same route NiMe[2‐(R1R2P)C6H4NR](PMe3) (6: R1 = R2 = CHMe2) was synthesized. Solutions in THF contain cis and trans isomers of square‐planar complexes, and dynamic NMR spectroscopy shows dissociation of trimethylphosphane to be more rapid in the trans isomers. In a crystal of 5 the molecular structure is cis‐5. Out of these complexes only 3 and 4 add trimethylphosphane to form pentacoordinate compounds NiMe[2‐(Ph2P)C6H4NR](PMe3)2 (7, 8) in the crystal as shown by the molecular structure of 7 while in THF equilibria 7 ⇄ cis‐1 and 8 ⇄ cis‐2 are observed by NMR spectroscopy. The same solution properties are found with NiMe[2‐(R1R2P)C6H4NR](PMe3)2 (9: R1 = CMe3, R2 = Ph). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

A spirocyclic tetraoxyphosphorane with an apically located chlorine in a trigonal bipyramid and its hydrolysis product

The spirocyclic (chloro)tetraoxyphosphorane CH 2(6- t-Bu-4-Me-C 6H 2O) 2P(Cl)(1,2-O 2C 6Cl 4) ( 2) and its hydrolysis product CH 2(6- t-Bu-4-Me-C 6H 2O) 2P(O)(OC 6Cl 4-2-OH) ( 3) have been synthesized and characterized by X-ray crystallography and solution state NMR. Unlike the previously reported spirocyclic chlorophosphoranes where chlorine occupies an equatorial position of a trigonal bipyramid (TBP), in 2 it is apical. The hydrolysis product 3 is a hydrogen bonded dimer. Compound CH 2(6- t-Bu-4-Me-C 6H 2O) 2P(Cl)[1,2-O 2–3,5-( t-Bu) 2C 6H 2] ( 4) was also synthesized in a manner analogous to that of 2. Attempts to isolate structurally characterizable pentacoordinate compounds by reacting CH 2(6- t-Bu-4-Me-C 6H 2O) 2PX [X=Cl ( 1), 2,6-Cl 2C 6H 3O ( 5), cycl-C 6H 11NH ( 6)] with N-chlorodiisopropylamine were not successful, although in some cases a pentacoordinate species was detected; the corresponding oxidized products CH 2(6- t-Bu-4-Me-C 6H 2O) 2P(O)X [X=Cl ( 7), 2,6-Cl 2C 6H 3O ( 8), cycl-C 6H 11NH ( 9)] were isolated by this route. Variable temperature ( 1H, 31P) NMR spectra of 2 have been recorded; at low temperatures, two 31P NMR signals of unequal intensity are seen in the pentacoordinate region. A probable intramolecular process involving equatorial–equatorial↔apical–equatorial (for the eight-membered ring) has been invoked to explain the spectral features.

Penta-coordinate phosphorous compounds and biochemistry

The relationship between penta-coordinate phosphorus compounds and biochemistry is briefly reviewed. Some interesting phenomena such as peptide formation, ester formation, ester exchange on phosphorus and N to O migration occur at room temperature when the amino group of amino acid is associated with phosphoryl group. Serine or threonine in conjugate of nucleo-side-amino acid could recognize different nucleobases. N-phosphoryl Histine and Ser-His dipeptide could cleavage nucleic acid, protein and ester in neutral medium. It is found that the above phenomena all undergo penta-coordinate intermediate of phosphorus atom, which is proposed as the key factor to determine their activities.

Mechanisms of Pentacoordinate Pseudorotation. A Molecular Modeling Study of PF5

This exercise in molecular modeling allows students to compare the two commonly suggested mechanisms for pseudorotation in pentacoordinate compounds--the Berry and turnstile mechanisms. The evidence from single-point energies, vibrational calculations, and transition-state searching favors the Berry mechanism in the case of PF5.

Tripodliganden mit Neopentangrundgerüst und zwei verschiedenen Donorfunktionen CH3C(CH2PPh2)2(CH2SR): Synthese, Struktur, Redoxchemie und Spektroskopie von Komplexen des Typs tripodM{ortho‐(X)(Y)C6H4}

Tripod Ligands with Neopentane Frame and two Different Donor Groups CH3C(CH2PPh2)2(CH2SR): Synthesis, Structure, Redox Chemistry, and Spectroscopy of the Complexes tripodM{ortho‐(X)(Y)C6H4}Prof. Dr. Jörn Müller zum 60. Geburtstag gewidmet. Neopentane‐based tripod ligands CH3C(CH2PPh2)2(CH2Z) (Z = SBn, SH, S−) form pentacoordinate compounds [tripodM{ortho(X)(Y)C6H4}]m 1–4 with ortho‐phenylene‐bridged coligands (X)(Y)C6H4 (X, Y = O−, S−, NH−) and Co(II), Co(III), Fe(II), or Fe(III) as the metal centers. The structures of these complexes are very similar to those observed for CH3C(CH2PPh2)3 as the tripod ligand. The redox potentials, however, for the corresponding one‐electron oxidation and reduction processes are highly affected by the change in the tripod donor groups. Both potentials are shifted by a maximum of 700 mV upon replacement of a PPh2 donor group by a sulfur‐centered donor with the difference between the potential of the oxidation step and the potential of the reduction step staying almost constant for the whole series of compounds. This difference of around 1.7 eV nicely corresponds to the energy of the HOMO‐LUMO chargetransfer bands observed around 2 eV for all of the compounds. It may be inferred therefore that both observations (electron spectroscopy and cyclic voltammetry) refer in a similar way to the HOMO‐LUMO gap of the compounds. It is shown that the formation of [tripodCo(III){ortho‐(NH)2C6H4}]+(BF) from ortho‐phenylenediamine as the source of the coligand involves precoordination of the amine ligand followed by deprotonation of the coordinated ligand. The capability of the tripodCo(II) template to form five‐coordinate compounds with diamines is further corroborated by the characterization of [tripodCo(en)]2+ (52+). In addition to the standard analytical data, EPR, UV/Vis, cyclovoltammetric data and X‐ray structure analyses are presented where appropriate.

On the “pentavalent” nitrogen atom and nitrogen pentacoordination

Abstract Hydrazoic acid (HN3) is an example of a molecule whose bond lengths suggest hat the central nitrogen atom is apparently pentavalent, as indicated in the classical valence bond structure (I). However, unless the nitrogen atom expands its valence shell, the π bonds of this structure are fractional electron-pair bonds. The increased-valence structure (II) with fractional electron-pair π and π′ bonds, and 1-electron π and π′ bonds, also involves an apparent pentavalence. Some of the properties of these two VB structures are used to restate the nature of the origin of the apparent electronic pentavalence for nitrogen, namely appreciable contributions of Dewar-type structures such as (III) to the component Lewis structure resonance scheme. It is shown that although the valence of the central nitrogen atom of structure (II) is able to exceed a value of 4, it can never attain a value of 5. Increased-valence structures for the C2 isomer of N6 are also presented, and the bond lengths that are associated with the most stable of these structures are in accord with those calculated using ab initio techniques. The results of some ab initio VB calculations, with minimal basis sets, are reported for: (a) N2, to demonstrate the effect of variation in σ bond atomic orbital hybridization on the lengths of the N″N‴ bonds of HN3 (as HN′N″N‴) and the NN bond of N2; (b) trigonal bipyramidal NH3F2 and PH3F2, to suggest that the unwillingness of nitrogen to form stable pentacoordinate compounds is associated with some reluctance by nitrogen to participate in the formation of axial 4-electron 3-centre σ bonding units.

Synthesis and molecular structure of new families of iridium(III)-Cp★ and rhodium(III)-Cp★ complexes derived from 1,2-dicyanoethene-1, 2-dithiolate 2,2′-biimidazole or 2,2′-bithiazole. Single crystal structures of [(η 5-Me 5C 5)Ir(biimH 2)Cl]Cl and [(η 5-Me 5C 5)Rh(dcdt)

Reaction of [(η 5-Me 5C 5)MCl 2] 2 (M  Ir or Rh) with 1,2-dicyanoethene-1,2-dithiolate (dcdt 2−), 2,2′-biimidazole (biimH 2), or 2,2′-bithiazole (bith) affords various d 6 complexes in high yield. Pentacoordinate compounds such as [(η 5-Me 5C 5)M(dcdt)] (M  Ir or Rh) and hexacoordinate complexes of the type [(η 5-Me 5C 5)M(biimH 2)Cl]Cl, (M  Ir or Rh) or [(η 5-Me 5C 5)Ir(bith)Cl]Cl have been synthesized and characterized. [(η 5-Me 5C 5)Rh (dcdt)] crystallizes in the orthorhombic space group C222 1 with unit cell parameters a = 11.972 (3), b = 15.072 (4), c = 16.456 (5) Å. Refinement of 859 observed reflections led to the final values of R = 2.31 and R w = 3.46%. [(η 5-Me 5C 5)Ir(biimH 2)Cl]Cl crystallizes in the orthorhombic space group P2 12 12 1 with unit cell parameters a = 14.610 (4), b = 14.556 (6), c = 8654 (3) Å. Refinement of 1630 observed reflections led to a value of R = 186 and R w = 2.65%. The biimH 2-iridium complex exhibits a characteristic three-legged “piano-stool” arrangement and the dcdt-rhodium complex a flattened basket shaped arrangement.

Cyclic voltammetric and electrocrystallization studies of axially substituted biscyanophthalocyaninato metal complexes and related compounds

Cyclic voltammetric studies of several biscyanophthalocyaninato metal complexes and related compounds A[Mc(−2)M(+III)(CN) 2] (A = Na, K, Bu 4N; Mc = macrocycle = phthalocyaninato, tetrabenzoporphyrinato, 2,3-naphthalocyaninato; M = Co, Rh, Fe, Ru, Cr, Mn) in acetone/0.1 M Bu 4NClO 4 indicated that the first oxidation process occurs on the macrocycle leading to an Mc(−1)M(+III)(CN) 2 radical-cation species. By galvanostatic and potentiostatic electrocrystallization of the monomeric compounds A[Mc(−2)M(+III)X 2] in acetone, the corresponding Mc(−1)M-(+III)X 2 compounds could be synthesized. These compounds were characterized by IR, far-IR and UV-V is spectroscopy, as well as elemental analyses and mass spectrometry. In contrast to previous results, penta-coordinated compounds Mc(−2)M(+III)X were not observed in the electrocrystallization experiments.

The structure and stability of neutral pentacoordinated silicon compounds

Ab initio and semi-empirical wavefunctions are used to examine the stability of neutral pentacoordinated compounds with the formula H 3NSiXY 3, where X and Y are the axial and equatorial ligands, respectively, and both ligands can be H, F, Cl. It is found from both levels of theory that a relatively weakly bound ligand (e.g. Cl) in the axial position facilitates complexation of the Si with NH 3 due to a more balanced three-center bond. Very electronegative ligands (e.g. F) are also stabilizing, since they increase the positive charge on Si.

Coordinative interactions in chelated complexes of silicon: XIV. Regular or irregular exchange mechanisms in pentacoordinate silicon compounds: an interpretation based on crystal structures, DNMR-data and molecular modelling

A crystal structure determination on 1-(2-(difluoromethylsilyl))- N, N-dimethylmethanamine (Ib) has revealed a strongly distorted trigonal bipyramidal coordination with N and F at apical positions. A coordinative interaction of 2,346(3) Å is present. Dynamic NMR-data for this complex and related pentacoordinated compounds indicate positional exchange of substituents at silicon with an energy barrier of 7–19 kcal/mol. For the exchange mechanism, regular (pseudorotation, turnstile rotation) and irregular processes, with bond rupture/bond reformation, are discussed. A computer simulation (MNDO-calculations) of the exchange process in 2-((methyl(trifluorosilyl)amino)methyl)pyridine (IIIa), which shows trigonal bipyramidal structure in the crystal and fluxional properties in solution, indicates that any distinction between the mechanisms is unreasonable and can be reduced to the question of which limiting Si-N distance can be regarded as still corresponding to a chemical bond. There is good agreement between the calculated and observed energy barriers. The structure correlation method is used to compare the calculated molecular transformation route with the Berry pseudorotation pathway.

Reactions of theortho-metallated rhodium(I) complex of the formula Rh[P(OC6H4)(OPh)2][P(OPh)3]3 with HX molecules

Theortho-metallated complex [RhP3Pt] [P=P(OPh)3, P′=P(OC6H4)(OPh)2] was obtained in the reaction of [RhP4]ClO4 with KOH. It reacts easily with proton donors HX (X=ClO4, F, Cl, SCN, or acetylacetonate) to produce complexes [RhP3X] when X is a strong donor. If X is a weaker donor (X=ClO4 or F), pentacoordinate compounds of the type [PhP4X] are formed. [RhP3P′] reacts with acetylacetone (Hacac) to produce [Rh(acac)P2].