Lewis Acidic Metal Center Research Articles

Cobalt Coordination Induced Functionalized Molecular Clefts: Isolation of {CoIII−ZnII} Heterometallic Complexes and Their Applications in Beckmann Rearrangement Reactions

The present work shows that the Co3+ coordination to the deprotonated pyridine-amide ligands orients the noncoordinated or hanging pyridine rings, thus furnishing a cleft in which Zn2+ ions coordinate. The building block approach points out a strategy to incorporate a Lewis acidic metal center in the periphery. This strategy has been used to synthesize Co3+-centered-Zn2+-peripheral heterobimetallic complexes. These heterobimetallic complexes have been thoroughly characterized including structural studies and have been successfully shown to catalyze the Beckmann rearrangement of the aldoximes and ketoxime to their respective amides.

Recognition of Anions by Synthetic Receptors in Aqueous Solution

Natural anion binding systems achieve high substrate affinity and selectivity most often by arranging converging binding sites inside a cavity or cleft that is well shielded from surrounding solvent molecules by the folded peptide chain. Types of interactions employed for anion recognition are electrostatic interactions, hydrogen-bonding, and coordination to a Lewis-acidic metal center. In this review, successful strategies aimed at the development of synthetic receptors active in water or aqueous solvent mixtures are described. It is shown that considerable progress has been made during recent years in the development of potent anion receptors and that for every type of interaction used in nature for anion binding, corresponding synthetic models exist today. Representative examples of these systems are presented with a special emphasis on synthetic receptors whose characterization involved a detailed thermodynamic analysis of complex formation to demonstrate the important interplay between enthalpy and entropy for anion recognition in water.

Reactions of (imidazol-2-ylidene)silver(I) chlorides with group 4 metal containing Lewis acids

The reactivity of the monomeric N-heterocyclic carbene silver(I) complexes, 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene-silver(I) chloride ([Ag(IMes)Cl], 1) and 1,3-bis-(4-bromo-2,6-dimethylphenyl)imidazol-2-ylidene-silver(I) chloride ([Ag(IMes Br)Cl], 2), toward the group 4 metal containing Lewis acids, TiCl 4 and (η 5-C 5H 5)ZrCl 3, in dichloromethane was investigated. Instead of the expected transfer of the N-heterocyclic carbene to the Lewis acidic metal centers with accompanying precipitation of AgCl, chloride transfer occurred leading to the formation of the salts, [Ag(IMes) 2] +[(TiCl 3) 2(μ 2-Cl) 3] − ( 3) and [Ag(IMes Br) 2] +[{(η 5-C 5H 5)ZrCl} 2(μ 2-Cl) 3] − ( 4). The structure of the [Ag(IMes Br) 2] + cation in 4 is significantly distorted in the solid state by interactions between the para-Br atoms of the IMes Br ligands and chloride ligands of the anions.

Gas-Phase and Solution-Phase Polymerization of Epoxides by Cr(salen) Complexes: Evidence for a Dinuclear Cationic Mechanism

The gas-phase reactions of a series of mass-selected mononuclear and dinuclear Cr(salen) complexes with propylene oxide suggest that the enhanced reactivity of the dinuclear complexes in gas-phase and in solution may derive from a dicationic mechanism in which the alkoxide chain is mu(2)-coordinated to two Lewis acidic metal centers. The double coordination is proposed to suppress backbiting, and hence chain-transfer in the gas-phase homopolymerization of epoxides.

Intermolecular hydroamination of alkynes catalyzed by zinc-exchanged montmorillonite clay

Intermolecular hydroamination of phenyl acetylene (PhAc) by aniline to give phenyl-(1-phenylethylidene) amine has been investigated using Zn2+ exchanged K-10 montmorillonite clay (hereafter Zn/K-10) catalysts. Under optimized reaction conditions, aniline to PhAc mole ratio 2, toluene solvent, catalyst Zn/K-10 (0.5 g) and at 130 °C gave 77 wt.% conversion of PhAC after 20 h. The activities of Zn/K-10, Zn/H-beta and Zn/silica having same amount of Zn2+ were compared. Zn/H-beta and Zn/K-10 showed nearly the same activity at the end of 10 h and others followed in the order Zn/silica, H-beta, K-10 and silica. The role played by Lewis and Brönsted acidities of the catalysts in influencing the catalytic activity for hydroamination reaction has been discussed. It is suggested that Brönsted acidity acts as a co-catalyst for Lewis acidic metal center. Zn/K-10 is the inexpensive but efficient catalyst for intermolecular hydroamination of alkynes, which can be utilized as an alternative to Zn/H-beta catalyst.

Reactivity of 2-pyridine–aldehyde and 2-acetyl–pyridine coordinated to [Re(CO) 3] + with alcohols and amines: metal mediated Schiff base formation and dimerization

The coordination of an aldehyde or a ketone to Lewis acidic metal centers activates the carbonyl carbon atom towards nucleophilic attack. We studied the coordination chemistry of [ReBr 3(CO) 3] 2− ( 1) in water and in organic solvents with 2-acetyl–pyridine ( 2) and 2-pyridine–aldehyde ( 3) and investigated its reactivity towards aromatic and aliphatic amines. Rapid imine formation in water between coordinated organic carbonyls and pendant amine groups of biomolecules is useful to directly link the latter with the fac-[Re(CO) 3] + or fac-[ 99mTc(CO) 3] + moiety. The reaction of [ReBr 3(CO) 3] 2− with 2 in methanol produced the mononuclear complex [ReBr(CO) 3(C 5H 4NCOCH 3)] ( 4), in which the carbonyl group is coordinated to the Re(I) center. The reaction of 1 with 3 is different and gave the dinuclear complex [Re 2(μ-OCH(OCH 3)C 5H 4N) 2(CO) 6] ( 5) in which a semiacetal is formed. The coordinated aldehyde in the mononuclear complex [ReBr(CO) 3(C 5H 4NCHO)] ( 6) is highly electrophilic and reacts with methanol to form the dinuclear complex 5. The reaction of 4 with aniline affords very rapid formation of [ReBr(CO) 3(C 5H 4NC(CH 3)NC 6H 5)] ( 7). In situ preparation of 6 and reaction with aniline or 1-aminopropane gave the corresponding Schiff base complexes [ReBr(CO) 3(C 5H 4NCHNC 6H 5)] ( 8) and [ReBr(CO) 3(C 5H 4NCHNCH 2CH 2CH 3)] ( 9), respectively. Compounds 4, 5, 8 and 9 are characterized by X-ray structure analysis. The Schiff base formation occurs in water at a very high rate. The rate constant for the formation of 7 was determined at −50 °C in methanol. The rate law is second-order with k=1.36±0.19×10 −2 M −1 s −1. In the absence of rhenium, no formation of Schiff base is observed at 20 °C over the same time period. The formation of the 99mTc analogues of complexes 8 and 9 was confirmed by HPLC comparison with the corresponding Re complexes.

Synthesis and Reactivity of Bi-, Tri-, and Tetrametallic Aluminum Tetraphenolate Complexes

A series of ligands, each with four phenoxide arms, have been prepared with the intent to place two Lewis acidic metal center in close proximity and thus foster cooperative binding of external Lewis bases. The ligands 1−5 incorporate a pair of 2,2‘-methylenebis(4-tert-butyl-6-alkylphenol) groups linked by three different spacers: anthracene (1H4, 5H4), dibenzofuran (2H4, 4H4), and xanthene (3H4). The reaction of 1H4 and 2H4 with trimethylaluminum led to the formation of the C2-symmetric, tetranuclear aluminum compounds [(1)Al4Me8], 6, and [(2)Al4Me8], 7, respectively. In contrast, when 3H4 was treated with trimethylaluminum, a binuclear aluminum species, [(3)Al2Me2], 8, containing an Al2O2 bridging unit, was isolated. Heating solutions of the tetranuclear complexes 6 and 7 in the presence of excess ligand induced the elimination of 2 equiv of trimethylaluminum and afforded the binuclear aluminum complexes [(1)Al2Me2], 9, and [(2)Al2Me2], 10. Addition of NEt4Cl to solutions of 9 and 10 resulted in the iso...

Linear trimer analogues of calixarene as chiral coordinating ligands: X-ray crystallographic and NMR spectroscopic characterization of chiral and achiral trisphenolates complexed to titanium(IV) and aluminum(III).

Achiral and chiral linear trisphenol analogues of calixarene (HOArCH(2)Ar'(OH)C(R)HArOH, Ar = 4,6-di-tert-butylphenyl; Ar' = 4-tert-butylphenyl; R = H (achiral), Me (chiral)) were prepared in anticipation of their adoption of a chiral conformation upon coordination to Lewis acidic metal centers. The trisphenols react with 1 equiv of Ti(OR')(4) (R' = i-Pr or t-Bu) to yield complexes with molecular formula Ti(2)(OArCH(2)Ar'(O)C(R)HArO)(2)(OR')(2) (R = H, Me; R' = i-Pr or t-Bu). An X-ray crystal structure of the titanium complex of the achiral trisphenol (R = H; R' = t-Bu) reveals that the trisphenolate ligand adopts an unsymmetrical (and therefore chiral) conformation, with eta(2)-coordination to one metal center and eta(1)-coordination to the second metal center. The chiral trisphenol, which contains a stereogenic center (indicated as C in the shorthand notation used above), coordinates titanium in an analogous fashion to produce only one diastereomer (out of four possible); therefore, the configuration of the stereogenic center controls the conformation adopted by the bound ligand. The reaction of achiral trisphenol with AlMe(3) produces a compound with molecular formula Al(2)(OArCH(2)Ar'(O)CH(2)ArO)(2). (1)H NMR spectroscopy and X-ray crystallography reveal that the trisphenolate ligand adopts an asymmetric, C(2) conformation in this complex, where the central phenolate oxygen bridges the aluminum centers and the terminal phenolate oxygens each coordinate a separate aluminum center. Because these trisphenolate ligands adopt chiral conformations when coordinated to metal centers, they may be useful for developing diastereo- or enantioselective catalysts and reagents.

ChemInform Abstract: Highly Polar Metal-Metal Bonds in “Early-Late” Heterodimetallic Complexes

Metal-metal bond polarity in its extreme form involving transition elements is found in di- or polynuclear complexes in which molecular fragments containing metal atoms from the two ends of the d block in the periodic table are combined. This linkage by direct metal-metal bonds of metal centers having very different oxidation states has been a challenge to the synthetic chemist. The suppression of degradative reaction channels caused by intramolecular single-electron transfer and the protection of the highly Lewis acidic early transition metal center by an appropriately designed ligand shell have opened up the systematic investigation of such systems. Concomitant with this development, advances in the conceptual framework for the quantitative description of bond polarity have led to a refined understanding of the nature of this type of metal-metal bonding. The greatest stimulus for the development of this field of research is the investigation of the cooperative reactivity of two or more coordination centers in their interaction with and transformation of organic substrates. This cooperativity, which is characterized by the different functions adopted by the metal centers in these conversions offers attractive perspectives in stoichiometric or even catalytic transformations.

Highly Polar Metal-Metal Bonds in "Early-Late" Heterodimetallic Complexes.

Metal-metal bond polarity in its extreme form involving transition elements is found in di- or polynuclear complexes in which molecular fragments containing metal atoms from the two ends of the d block in the periodic table are combined. This linkage by direct metal-metal bonds of metal centers having very different oxidation states has been a challenge to the synthetic chemist. The suppression of degradative reaction channels caused by intramolecular single-electron transfer and the protection of the highly Lewis acidic early transition metal center by an appropriately designed ligand shell have opened up the systematic investigation of such systems. Concomitant with this development, advances in the conceptual framework for the quantitative description of bond polarity have led to a refined understanding of the nature of this type of metal-metal bonding. The greatest stimulus for the development of this field of research is the investigation of the cooperative reactivity of two or more coordination centers in their interaction with and transformation of organic substrates. This cooperativity, which is characterized by the different functions adopted by the metal centers in these conversions offers attractive perspectives in stoichiometric or even catalytic transformations.

Metal centered molecules and their application in chemistry and physics

Abstract

Hydrolysis of Zirconium and Hafnium Siloxide Complexes. Isolation and Characterization of the Intermediate Aqua Complexes M[OSi(OtBu)3]4(H2O)n (M = Zr, Hf; n = 1, 2)

Hydrolysis of the tris(tert-butoxy)siloxy complexes M[OSi(OtBu)3]4 (1, M = Zr; 2, M = Hf) was studied. Careful addition of 1 or 2 equiv of water to THF solutions of 1 and 2 produced the isolable aqua complexes M[OSi(OtBu)3]4(H2O) (3, M = Zr; 4, M = Hf) and M[OSi(OtBu)3]4(H2O)2 (5, M = Zr; 6, M = Hf). These complexes are stable for weeks in solution and are stable indefinitely in the solid state. In solution, the aqua ligands in 3−6 are labile. Crystals of 4 are monoclinic (C2/c) with a = 62.2352(4) Å, b = 13.3362(2) Å, c = 27.0940(4) Å, β = 103.535(1)°, V = 21836.0(4) Å3, and Z = 4. Addition of excess water to solutions of 5 and 6 at room temperature results in rapid hydrolysis to form a MOx(OH)y(OH2)z gel and HOSi(OtBu)3. Below 10 °C in tetrahydrofuran-d8, however, hydrolysis does not occur at an appreciable rate. At low temperatures, the rate of exchange between the aqua ligands of 5 and excess free water was found to depend on the water concentration. This indicates that the aqua ligand exchange occurs by an associative mechanism involving a seven-coordinate intermediate. On the basis of this observation, we postulate that the hydrolysis of 5 also proceeds via the tris(aqua) intermediate Zr[OSi(OtBu)3]4(H2O)3. Compounds 3−6 are more thermally stable than 1 and 2, indicating that the elimination of isobutene from the latter complexes is facilitated by a free coordination site on the Lewis acidic metal center.

Neutral anion receptors: design and application

After the development of synthetic cation receptors in the late 1960s, only in the past decade has work started on the development of synthetic neutral anion receptors. Combination and preorganization of different anion binding groups, like amides, urea moieties, or Lewis acidic metal centers lead to receptor molecules that strongly bind inorganic anions with high selectivity. Combined with neutral cation ligands, ditopic receptors were obtained for the complexation of inorganic salts. The molecular complexation properties of anion receptors has been transduced into macroscopic properties in membrane separation processes and in sensors for selective anion detection.

(Hexafluoroacetylacetonato-O,O')(1-hydroxy-2,2,6,6-tetramethylpiperidinato-O,N)palladium(II), [Pd(C5HF6O2)(C9H18NO)], a metal complex containing a reduced nitroxylradical

[ PtC12(C6 H 13N)(CsH 11P)] The shortest intermolecular contact [3.46 (2)A], observed between N and Cl(2)[~+x, ~--y, - z ] , is probably due to an N - H . . . C 1 hydrogen bond. The packing arrangement is shown in Fig. 3. Thanks are due to Dr G. Consiglio (ETH, Zfirich) who kindly prepared the compound. The drawings were made by Mr M. Viola. References AMBUEHL, J., PREGOSIN, P. S., VENANZI, L. M., CONSIGLIO, G., BACHECHI, F. ~ ZAMBONELLI, L. (1979). J. Organomet. Chem. Fig. 3. Packing arrangement of the molecules viewed down a. It can be observed that the conformation of the unsubstituted pyrrolidine ring in (II) may be considered intermediate between the conformation of the ring in (IV) and that in (III), perhaps closer to the confor- mation in (IV) which has an envelope-like form with C(4) at the flap. Therefore the five structures seem to represent five phases of the pseudorotation of the pyrrolidine ring in the sequence (IV)--,(II)--,(III)--, (V)-,(I) or, to stress the influence of the methyl sub- stituents, (IV),-- (II)-,(III)--,(V) ~(I). BACHECHI, F., MURA, P. & ZAMBONELLI, L. (1980). Acta Cryst. B36, 2604-2608. BACHECHI, F., ZAMBONELLI, L. & MARCOTRIGIANO, G. (1977). J. Cryst. MoL Struct. 7, 11-20. CERRINI, S. & SPAGNA, R. (1977). 4th Eur. Crystallogr. Meet., Oxford. Abstracts, p. 7. CHATT, J. & VENANZI, L. M. (1955). J. Chem. So¢. pp. 3858-3864. CO'rTIN, H. (1966). Bull. Soe. Ch#n. Fr. p. 2731. International Tables for X-ray Crystallography (1974). Vol. 1V, Tables 2.2B and 2.3.1 Birmingham: Kynoch Press. (Present distributor D. Reidel, Dordrecht.) MCCANDLISH, L. E., STOUT, G. H. & ANDREWS, L. C. (1975). Aeta Cryst. A31, 245-249. NORTH, A. C. T., PHILLIPS, D. C. & MATHEWS, F. S. (1968). Acta Cryst. A24, 351-359. SPAGNA, R. & ZAMBONELLI, L. (1971). J. Chem. Soc. A, pp. Acta Cryst. (1985). C41, 838-840 (Hexafluoroacetylacetonato-O,O')(l-hydroxy-2,2,6,6-tetramethylpiperidinato- O,N)palladium(II), [Pd(C5 HF602) (C9H lsN O)], a Metal Complex Containing a Reduced Nitroxyl Radical BY LEIGH C. PORTER AND ROBERT J. DOEDENS* Department of Chemistry, University o f California, Irvine, California 92717, USA (Received 28 September 1984; accepted 10 December 1984) M r=469.7, monoclinic, P21/n, a= b=20.511(8), c=11.655(3)A, t= V = 1812 (1) A 3, Z = 4, Dx= 1.722 Mg m -3, 2(Mo Kct) = 0,71073 A, /t = 1.08 mm -1, F(000) = 936, T = 297 K, final R = 0.042 for 2498 observed reflections. The Pd atom is bound to a chelating hexafluoroacetylacetonato ligand and to a reduced nitroxyl radical coordinated in an r/2 fashion through its N and O atoms. Principal bond distances include P d - O ( F 6 a c a c ) = 2.058 (4), 2.078 (4), P d - O(nitroxyl) = 2.009 (4), P d - N = 1.985 (4), N - O = 1.346 (5) A. Abstract. * To whom correspondence should be addressed. Introduction. The title compound was prepared in the course of exploratory studies of the reactions of stable nitroxyl radicals with transition-metal complexes con- taining a strongly Lewis-acidic metal center. Previous studies in this series have yielded examples of various types of interactions of the nitroxyl group, including metal coordination of the nitroxyl radical through O (Dickman & Doedens, 1981; Porter, Dickman & Doedens, 1983; Porter & Doedens, 1985), bidentate N,O binding of the reduced radical (Dickman & Doedens, 1982), hydrogen bonding to coordinated water (Dickman & Doedens, 1983), and formation of a co-crystallized solid with no specific interactions involv- ing the nitroxyl group (Porter & Doedens, 1984). We © 1985 International Union of Crystallography

Co-crystallized bis(hexafluoroacetylacetonato)palladium(II) and 3-cyano-2,2,5,5-tetramethyl-2,5-dihydropyrrolyl-1-oxyl, C10H2F12O4Pd.2C9H13N2O

[Li(CaHloO2)3] [Zr2ClaO(C4H4N)2(C lo H 15)2].C4H 1002 The CsMe 5 ring shows r:-coordination: the Z r - C lengths range from 2.49 (1) to 2.57 (2)A. The bond lengths and angles within the ligands themselves are normal (Bynum, Hunter, Rogers & Atwood, 1980). The two independent L i - O lengths, 2.09 (1) and 2.20 (3) A, have large e.s.d.'s associated with them due to the high thermal motion of the cation. However, the distances are long when compared to the L i - O lengths of 2.064(9) and 2 . 1 1 2 ( 4 ) A found for the five- coordinate Li in [Li(dme)2lBr (Rogers & Atwood, 1983, unpublished results). An uncoordinated dme molecule was found highly disordered about a crystallographic twofold axis. Refinement of the parameters of this moiety could not be carried out, and the non-hydrogen atoms were included with fixed contributions. The rather high R value resulted from this difficulty. We are grateful to the National Science Foundation for support of this work. References BYNUM, R. V., HUNTER, W. E., ROGERS, R. D. & ATWOOD, J. L. (1980). Inorg. Chem. 19, 2368-2374. BYNUM, R. V., ZHANG, H., HUNTER, W. E. & ATWOOD, J. L. (1984). Can. J. Chem. Submitted. CROMER, D. T. & LmERMAN, D. (1970). J. Chem. Phys. 53, CROMER, D. T. & WABER, J. T. (1965). Acta Cryst. 18, 104-109. FACHINETTI, G., FLORIAN1, C., CHIES1-VILLA, A. & GUASTINI, C. (1979),J. Am. Chem. Soc. 101, 1767-1775. HOLTON, J., LAPPERT, M. F., BALLARD, D. G. H., PEARCE, R., ATWOOD, J. L. & HUNTER, W. E. (1979). J. Chem. Soc. Dalton Trans. pp. 45-53. HUNTER, W. E., HRNC1R, D. C., BYNUM, R. V., PENTTILA, R. A. & ATWOOD, J. L. (1983). Organometallics, 2, 750-755. LAPPERT, M. F., RILEY, P. I., YARROW, P. I. W., ATWOOD, J. L., HUNTER, E. W. & ZAWOROTKO, M. J. (1981). J. Chem. Soc. Dalton Trans. pp. 814-821. PETERSEN, J. L. (1979). J. Organomet. Chem. 166, 179-192. SALDARRIAGA-MOLINA, C. H., CLEARFIELD, A. & BERNAL, I. (1974). Inorg. Chem. 13, 2880-2885. WOLCZANSKI, P. T., THRELKEL, R. S. & SANTARSlERO, B. D. (1983). Acta Co'st. C39, 1330-1333. Acta Cryst. (1984). C40, 1814-1817 Co-crystallized Bis(hexafluoroacetylacetonato)palladium(II) and 3-Cyano-2,2,5,5- tetramethyl-2,5-dihydropyrrolyl- l-oxyl, C 10H2F 12 O4Pd.2C9H13 N20 BY LEIGH C. PORTER AND ROBERT J. DOEDENS* Department o f Chemistry, University o f California, Irvine, California 92717, USA (Received 16 April 1984; accepted 28 June 1984) M r=850.9, monoclinic, P2x/n, a = b=19.139(7), c = 1 0 . 9 2 8 ( 3 ) A, f l = v = 1772 (1) A 3, z = 2, Dx= 1.595 Mg m -3, 2(Mo Ka) = 0.71073 A, g = 0.62 mm -l, F ( 0 0 0 ) = 8 5 2 , T = 2 9 6 K , final R = 0 . 0 4 5 for 2498 observed reflections. The co-crystallized species are arranged in alternating layers perpendicular to the b axis, with only van der Waals type interactions between layers. Magnetic susceptibility data (6-300 K) are consistent with the presence of one free spin on each nitroxyl radical. Abstract. I n t r o d u c t i o n . Several complexes in which a transition- metal ion is directly bound to a nitroxyl group have recently been prepared and characterized by crystal structure analyses (Anderson & Kuechler, 1980; Dickman & Doedens, 1983; Porter, Dickman & Doedens, 1983; Dickman, 1983). These molecules constitute a relatively rare example of a series of metal * To whom correspondence should be addressed. complexes containing a coordinated flee-radical ligand which retains its radical character. They also show other interesting features, including unusual metal- ligand binding geometries and various types of mag- netic behavior. One factor which favors formation of a metal-nitroxyl bond is a strongly Lewis acidic metal center. Most frequently this has been achieved by the presence of electron-withdrawing groups on other ligands. The nature of the nitroxyl ligand is also a factor, though this has not been thoroughly investigated. The title compound was prepared in the course of exploratory studies designed to define the conditions under which a transition-metal-nitroxyl bond will be formed. From bis(hexafluoroacetylacetonato)- palladium(II) [Pd(F6acac) 2] and 3-cyano-2,2,5,5-tetra- methyl-2,5-dihydropyrrolyl-1-oxyl (tmpCN) in pentane a compound of stoichiometry Pd(F6acac)2(tmpCN)2 was obtained in good yield. A crystal structure analysis was performed to establish the mode of binding of the tmpCN in this material. © 1984 International Union of Crystallography