Atta M. Arif

Isopestacin, an isobenzofuranone from Pestalotiopsis microspora, possessing antifungal and antioxidant activities.

Isopestacin is an isobenzofuranone obtained from the endophytic fungus Pestalotiopsis microspora. While a few other isobenzofuranones are known from natural sources, isopestacin is the only one having a substituted benzene ring attached at the C-3 position of the furanone ring. The compound was isolated from culture broths of the fungus and crystallized and its structure was determined by X-ray crystallography. Both proton and carbon NMR spectral assignments are also reported for isopestacin. This compound possesses antifungal activity and, as measured by electron spin resonance specroscopy, it also behaves as an antioxidant scavenging both superoxide and hydroxy free radicals.

Pestacin: a 1,3-dihydro isobenzofuran from Pestalotiopsis microspora possessing antioxidant and antimycotic activities

Abstract Pestalotiopsis microspora, an endophytic fungus native to the rainforest of Papua New Guinea, produces a 1,3-dihydro isobenzofuran. This product, pestacin, is 1,5,7-trisubstituted and exhibits moderate antifungal properties and antioxidant activity 11 times greater than the vitamin E derivative trolox. Antioxidant activity is proposed to arise primarily via cleavage of an unusually reactive C–H bond and, to a lesser extent, through O–H abstraction. Isolation of pestacin was achieved by extraction of culture fluid with methylene chloride followed by silica gel chromatography. Structure was established by X-ray diffraction and 13C and 1H NMR. The X-ray data demonstrate that pestacin occurs naturally as a racemic mixture. A mechanism for post-biosynthetic racemization is proposed.

Template and Guest Effects on the Self‐Assembly of a Neutral and Homochiral Helix

Coordination-driven assembly based on manganese(II) centers and flexible 1,3-bis(4-pyridyl)propane leads to the solid-state formation of electronically neutral, self-templated homochiral helices, closed ring structures, and racemic mixtures of helices depending upon the presence or absence of guests such as benzene and 1,2-diphenylethane.

Metal complexes of anionic 3-borane-1-alkylimidazol-2-ylidene derivatives

Addition of BH 3 ·thf to 1-alkylimidazoles (alkyl=methyl, butyl) and 1-methylbenzimidazole leads to BH 3 adducts, which are deprotonated by BuLi to yield the organolithium compounds (L)Li + ( 1b – d ) − . In the solid state (thf)Li + 1b − is dimeric. The acyl–iron complexes (thf) 3 Li + ( 3b , d ) − are formed from (thf)Li + ( 1b , d ) − and Fe(CO) 5 . (L)Li + ( 1a – c ) − react with [CpFe(CO) 2 X], however, the only complex obtained is [CpFe(CO) 2 1a ] (5a ). The analogous reaction of (L)Li + 1a − with the pentadienyl complex [(C 7 H 11 )Fe(CO) 2 Br] yields the corresponding iron compound 6a . Their compositions follow from spectroscopic data. Treatment of Cp 2 TiCl with (L)Li + 1a − leads to [Cp 2 Ti 1a ] ( 7a ), which could not be oxidized with PbCl 2 to give the corresponding Ti(IV) complex. The compounds [Li(py) 4 ] + 9a − and [Li(L) 4 ] + ( 10b – d ) − are obtained when (L)Li + 1 − are reacted with VCl 3 and ScCl 3 . The X-ray structure analysis of the vanadium complex reveals a distorted tetrahedron of the anion [V( 1a ) 4 ] − with two smaller and four larger CVC angles. The scandium compound [Li(dme) 2 + 10c − ] has a different structure: the distorted tetrahedron of the anion [Sc( 1c ) 4 ] − contains two larger (140.2 and 142.9°) and four smaller CScC angles (93.9–98.7°). This arrangement allows the formation of four bridging BHSc 3c,2e bonds to give an eight-fold coordination. The anion 10c − is formally a 16e complex.

Crystal structure and magnetic properties of [Fe{N(CN)2}2(MeOH)2] a 2-D layered network consisting of hydrogen-bonded 1-D chains

A novel 2-D layered network structure [Fe{N(CN) 2 } 2 (MeOH) 2 ] was synthesized and characterized by X-ray crystallography, vibrational, and magnetic susceptibility. The neutral 2-D stair-like framework consists of hydrogen-bonded infinite 1-D {Fe[N(CN) 2 ] 2 } ribbons that pack in a staggered arrangement where nearest-neighboring chains are slipped a/2 relative to one another. Two methanol molecules are coordinated to the Fe II center via the oxygen atoms in a trans configuration resulting in a compressed FeN 4 O 2 octahedron. Hydrogen-bond interactions occur via N(2)‥H(1)-O(1) where N(2) is the amide nitrogen atom of a nearby ribbon. The magnetic susceptibility was interpreted according to an S=2 expression which includes the Weiss constant and zero-field splitting giving g=2.04, θ=–2.0 K, and D=–1.7 K. Intrachain exchange interactions were determined from a fit to an S=2 antiferromagnetic chain model leading to g=2.04 and J/k B =–0.23 K. Further interchain interaction via the hydrogen bond was determined by incorporation of a molecular-field correction term yielding J′/k B =–0.02 K indicating very weak antiferromagnetic coupling between chains.

A dinuclear iron(II) complex, [(TPyA)FeII(THBQ2–)FeII(TPyA)](BF4)2 [TPyA = tris(2-pyridylmethyl)amine; THBQ2– = 2,3,5,6-tetrahydroxy-1,4-benzoquinonate] exhibiting both spin crossover with hysteresis and ferromagnetic exchange

Dinuclear [(TPyA)FeII(THBQ(2-))FeII(TPyA)](BF4)2 (1) possesses hydrogen bonding interactions that form a 1-D chain, and pi-pi interactions between the 1-D chains that give rise to a 2-D supramolecular-layered structure, inducing hysteresis in the spin crossover behavior; 1 has shown spin crossover behavior around 250 K with thermal hysteresis and ferromagnetic interactions at low temperature.

Reversible carboxylation of N-heterocyclic carbenes

Spectroscopic analysis, thermogravimetric analysis, and crossover experiments performed on a series of imidazolium carboxylates revealed carboxylation was reversible with N-aryl substituted adducts.

Charge Density Analysis of the (C−C)→Ti Agostic Interactions in a Titanacyclobutane Complex

The experimental electron density study of Ti(C(5)H(4)Me)(2)[(CH(2))(2)CMe(2)] provides direct evidence for the presence of (C-C)-->Ti agostic interactions. In accord with the model of Scherer and McGrady, the C(alpha)-C(beta) bond densities no longer show cylindrical symmetry in the vicinity of the Ti atom and differ markedly from those of the other C-C bonds. At the points along the C(alpha)-C(beta) bond where the deviation is maximal the electron density is elongated toward the metal center. The distortion is supported by parallel theoretical calculations. A calculation on an Mo complex in which the agostic interaction is absent supports the Scherer and McGrady criterion for agostic interactions. Despite the formal d(0) electron configuration for this Ti(IV) species, a significant nonzero population is observed for the d orbitals, the d orbital population is largest for the d(xy) orbital, the lobes of which point toward the two C(alpha) atoms. Of the three different basis sets for the Ti atom used in theoretical calculations with the B3LYP functional, only the 6-311++G** set for Ti agrees well with the experimental charge density distribution in the Ti-(C(alpha)-C(beta))(2) plane.

Modeling carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ACS): a trinuclear nickel complex employing deprotonated amides and bridging thiolates

Carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ACS) utilizes a unique Ni-M bimetallic site in the biosynthesis of acetyl-CoA, where a square-planar Ni ion is coordinated to two thiolates and two deprotonated amides in a Cys-Gly-Cys motif. The identity of M is currently a matter of debate, although both Cu and Ni have been proposed. In an effort to model ACS’s unusual active site and to provide insight into the mechanism of acetyl-CoA formation and the role of each of the metals ions, we have prepared and structurally characterized a number of Ni(II)–peptide mimic complexes. The mononuclear complexes Ni(II) N,N’-bis(2-mercaptoethyl)oxamide (1), Ni(II) N,N′-ethylenebis(2-mercaptoacetamide) (2), and Ni(II) N,N′-ethylenebis(2-mercaptopropionamide) (3) model the Ni(Cys-Gly-Cys) site and can be used as synthons for additional multinuclear complexes. Reaction of 2 with MeI resulted in the alkylation of the sulfur atoms and the formation of Ni(II) N,N′-ethylenebis(2-methylmercaptoacetamide) (4), demonstrating the nucleophilicity of the terminal alkyl thiolates. Addition of Ni(OAc)2·4H2O to 3 resulted in the formation of a trinuclear species 5, while 2 crystallizes as an unusual paddlewheel complex (6) in the presence of nickel acetate. The difference in reactivity between the similar complexes 2 and 3 highlights the importance of ligand design when synthesizing models of ACS. Significantly, 5 maintains the key features observed in the active site of ACS, namely a square-planar Ni coordinated to two deprotonated amides and two thiolates, where the thiolates bridge to a second metal, suggesting that 5 is a reasonable structural model for this unique enzyme.

First Row Transition Metal(II) Thiocyanate Complexes, and Formation of 1-, 2-, and 3-Dimensional Extended Network Structures of M(NCS)2(Solvent)2 (M = Cr, Mn, Co) Composition

The reaction of first row transition M(II) ions with KSCN in various solvents form tetrahedral (NMe4)2[M(II)(NCS)4] (M = Fe, Co), octahedral trans-M(II)(NCS)2(Sol)4 (M = Fe, V, Ni; Sol = MeCN, THF), and K4[M(II)(NCS)6] (M = V, Ni). The reaction of M(NCS)2(OCMe2)2 (M = Cr, Mn) in MeCN and [Co(NCMe)6](BF4)2 and KSCN in acetone and after diffusion of diethyl ether form M(NCS)2(Sol)2 that structurally differ as they form one-dimensional (1-D) (M = Co; Sol = THF), two-dimensional (2-D) (M = Mn; Sol = MeCN), and three-dimensional (3-D) (M = Cr; Sol = MeCN) extended structures. 1-D Co(NCS)2(THF)2 has trans-THFs, while the acetonitriles have a cis geometry for 2- and 3-D M(NCS)2(NCMe)2 (M = Cr, Mn). 2-D Mn(NCS)2(NCMe)2 is best described as Mn(II)(μ(N,N)-NCS)(μ(N,S)-NCS)(NCMe)2 [= Mn2(μ(N,N)-NCS)2(μ(N,S)-NCS)2(NCMe)4] with the latter μ(N,S)-NCS providing the 2-D connectivity. In addition, the reaction of Fe(NCS)2(OCMe2)2 and 7,7,8,8-tetracyanoquino-p-dimethane (TCNQ) forms 2-D structured Fe(II)(NCS)2TCNQ. The magnetic behavior of 1-D Co(NCS)2(THF)2 can be modeled by a 1-D Fisher expression (H = -2JS(i)·S(j)) with g = 2.4 and J/kB = 0.68 K (0.47 cm(-1)) and exhibit weak ferromagnetic coupling. Cr(NCS)2(NCMe)2 and Fe(II)(NCS)2TCNQ magnetically order as antiferromagnets with Tcs of 37 and 29 K, respectively, while Mn(NCS)2(NCMe)2 exhibits strong antiferromagnetic coupling. M(NCS)2(THF)4 and K4[M(NCS)6] (M = V, Ni) are paramagnets with weak coupling between the octahedral metal centers.