Rhenium chemistry of azooximes: oxygen atom transfer, azoimine chelation, and imine-oxime contrast.

The concerned azooximes (L1OH, 1) are of type p-X-C6H4C(N2Ph)(NOH) (X = H, Me, Cl). The reaction of [Re(MeCN)Cl3(PPh3)2] with [Ag(L1OH)(L1O)] in cold dichloromethane-acetonitrile solvent has furnished the green colored ionized azoimine complex [ReV(O)Cl(PPh3)2(L1)](PF6), 2. In effect L1O- has undergone oxidative addition, the oxygen atom being transferred to the metal site. Upon treatment of [ReV(NPh)Cl3(PPh3)2] with L1OH in solution, the neutral azoimine complex [ReV(NPh)Cl3(L1H)], 3, resulted due to the spontaneous transfer of the oxime oxygen atom to a PPh3 ligand, which is eliminated as OPPh3. In contrast, the oxime of 2-acetylpyridine (L2OH, 4) did not undergo oxygen atom transfer and simply afforded the imine-oxime complex [ReV(NC6H4Y)Cl2(PPh3)(L2O)], 5, upon reacting with [ReV(NC6H4Y)Cl3(PPh3)2] (Y = H, Me, Cl). The spectral and electrochemical properties of 2, 3, and 5 and the structures of three representative compounds are reported. In the cation of 2 (X = H) the two PPh3 ligands lie trans to each other and the equatorial plane is defined by the five-membered azoimine chelate ring and the oxo and chloro ligands. The oxo ligand which forms a model triple bond (Re-O length 1.616(6) A) lies cis to the imine-N atom. In 3 (X = Cl) the ReCl3 fragment has meridional geometry and the imido nitrogen lies trans to the imine nitrogen of the planar azoimine chelate ring. In 5 x H2O (Y = Me), the Cl, oximato-N, and P atoms define an equatorial plane and the pyridine-N lies trans to the imido-N. The water of crystallization is hydrogen bonded to the oximato oxygen atom (O...O, 2.829(5) A). Reaction models in which chelation of the azooxime precedes oxygen atom transfer are proposed on the basis of oxophilicity of trivalent rhenium, Lewis acid activity of pentavalent rhenium, electron withdrawal by the azo group, and observed relative disposition of ligands in products.

Rhenium chemistry of diazabutadienes and derived iminoacetamides spanning the valence domain II-VI. Synthesis, characterization, and metal-promoted regiospecific imine oxidation.

The reaction of diazabutadienes of type R'N=C(R)-C(R)=NR', L (R = H, Me; R' = cycloalkyl, aryl) with Re(V)OCl(3)(AsPh(3))(2) has furnished Re(V)OCl(3)(L), 1, from which Re(III)(OPPh(3))Cl(3)(L), 2, and Re(V)(NAr)Cl(3)(L), 3, have been synthesized. Chemical oxidation of 2(R = H) by aqueous H(2)O(2) and of 3(R = H) by dilute HNO(3) has yielded Re(IV)(OPPh(3))Cl(3)(L'), 5, and Re(VI)(NAr)Cl(3)(L'), 4, respectively, where L' is the monoionized iminoacetamide ligand R'N=C(H)-C(=O)-NR'(-). Finally, the reaction of Re(V)O(OEt)X(2)(PPh(3))(2) with L has furnished bivalent species of type Re(II)X(2)(L)(2), 6(X = Cl, Br). The X-ray structures of 1 (R = Me, R' = Ph), 3 (R = H, R' = Ph, Ar = Ph), and 4 (R = H, R' = cycloheptyl, Ar = C(6)H(4)Cl) are reported revealing meridional geometry for the ReCl(3) fragment and triple bonding in the ReO (in 1) and ReNAr (in 3 and 4 ) fragments. The cis geometry (two Re-X stretches) of ReX(2)(L)(2) is consistent with maximized Re(II)-L back-bonding. Both ReX(2)(L)(2) and Re(NAr)Cl(3)(L') are paramagnetic (S = (1)/(2)) and display sextet EPR spectra in solution. The g and A values of Re(NAr)Cl(3)(L') are, respectively, lower and higher than those of ReX(2)(L)(2). All the complexes are electroactive in acetonitrile solution. The Re(NAr)Cl(3)(L) species display the Re(VI)/Re(V) couple near 1.0 V versus SCE, and coulometric studies have revealed that, in the oxidative transformation of 3 to 4, the reactive intermediate is Re(VI)(NAr)Cl(3)(L)(+) which undergoes nucleophilic addition of water at an imine site followed by induced electron transfer finally affording 4. In the structure of 3 (R = H, R' = Ph, Ar = Ph), the Re-N bond lying trans to the chloride ligand is approximately 0.1 A shorter than that lying trans to NPh. It is thus logical that the imine function incorporating the former bond is more polarized and therefore subject to more facile nucleophilic attack by water. This is consistent with the regiospecificity of the imine oxidation as revealed by structure determination of 4 (R = H, R' = cycloheptyl, Ar = C(6)H(4)Cl).

A family of vanadate esters of monoionized and diionized aromatic 1,2-diols: synthesis, structure, and redox activity.

The concerned diols (general abbreviation, H(2)L) are catechol (H(2)L(1)) and its 3,5-Bu(t)(2) derivative (H(2)L(2)). Esters of the type VO(xsal)(HL), 2, are obtained by reacting H(2)L with VO(xsal)(H(2)O) or VO(xsal)(OMe)(HOMe), where xsal(2-) is the diionized salicylaldimine of glycine (x = g), L-alanine (x = a), or L-valine (x = v). The reaction of VO(acac)(2) with H(2)L and the salicylaldimine (Hpsal) of 2-picolylamine has furnished VO(psal)(L), 3. In the structures of VO(gsal)(HL(1)), 2a, and VO(vsal)(HL(2)), 2f, the HL(-) ligand is O,O-chelated, the phenolic oxygen lying trans to the oxo oxygen atom. The xsal(2-) coligand has a folded structure and the conformation of 2f is exclusively endo. In both 2a and 2f the phenolic oxygen atom is strongly hydrogen bonded (O...O, 2.60 A) to a carboxylic oxygen atom of a neighboring molecule. In VO(psal)(L(2)).H(2)O, 3b, the diionized diol is O,O-chelated to the metal and the water molecule is hydrogen bonded to a phenoxidic oxygen atom (O.O, 2.84 A). The C-O and C-C distances in the V(diol) fragment reveal that 2 is a pure catecholate and 3 is a catecholate-semiquinonate hybrid. In solution each ester gives rise to a single (51)V NMR signal (no diastereoisomers), which generally shifts downfield with a decrease in the ester LMCT band energy. The V(V)/V(IV) and catecholate-semiquinonate reduction potentials lie near -0.75 and 0.35, and 1.10 and 0.70 V vs SCE for 2 and 3, respectively. Molecular oxygen reacts smoothly with 2 quantitatively furnishing the corresponding o-quinone, and in the presence of H(2)L the reaction becomes catalytic. In contrast, type 3 esters are inert to oxygen. The initial binding of O(2) to 2 is proposed to occur via hydrogen bonding with chelated HL(-).

Pyridylazole chelation of oxorhenium(V) and imidorhenium(V). Rates and trends of oxygen atom transfer from Re(V)O to tertiary phosphines.

The concerned azoles are 2-(2-pyridyl)benzoxazole (pbo) and 2-(2-pyridyl)benzthiazole (pbt). These react with ReOCl(3)(PPh(3))(2) in benzene, affording Re(V)OCl(3)(pbo) and Re(V)OCl(3)(pbt), which undergo facile oxygen atom transfer to PPh(2)R (R = Ph, Me) in dichloromethane solution, furnishing Re(III)(OPPh(2)R)Cl(3)(pbo) and Re(III)(OPPh(2)R)Cl(3)(pbt). The oxo species react with aniline in toluene solution, yielding the imido complexes Re(V)(NPh)Cl(3)(pbo) and Re(V)(NPh)Cl(3)(pbt). The X-ray structures of pbt, ReOCl(3)(pbt), Re(OPPh(3))Cl(3)(pbt), and Re(NPh)Cl(3)(pbo) are reported. The lattice of pbt consists of stacked dimers. In all the complexes the azole ligand is N,N-chelated and the ReCl(3) moiety is meridionally disposed. In ReOCl(3)(pbt) the metal-oxo bond length is 1.607(9) A. The second-order rates and the associated activation parameters of the oxygen atom transfer reactions of the Re(V)O chelates with PPh(2)R are reported. The large and negative entropy of activation (approximately -24 eu) is consistent with an associative pathway involving nucleophilic phosphine attack. The rate increases with phosphine basicity (PPh(2)Me > PPh(3)) and azole heteroatom electronegativity (O(pbo) > S(pbt)). Logarithmic rate constants for ReOCl(3)(pbo), ReOCl(3)(pbt), and ReOCl(3)(pal) are found to correlate linearly with Re(VI)O/Re(V)O reduction potentials (pal is pyridine-2-(N-p-tolyl)aldimine). The relatively low rate constant of ReOCl(3)(pbt) compared to that of ReOCl(3)(pal) is consistent with the observed shortness of the metal-oxo bond in the former. Crystal data are as follows: (pbt) empirical formula C(12)H(8)N(2)S, crystal system orthorhombic, space group Pca2(1), a = 13.762(9) A, b = 12.952(8) A, c = 11.077(4) A, V = 1974(2) A(3), Z = 8; (ReOCl(3)(pbt)) empirical formula C(12)H(8)Cl(3)N(2)OSRe, crystal system monoclinic, space group P2(1)/c, a = 11.174(7) A, b = 16.403(10) A, c = 7.751(2) A, beta = 99.35(4) degrees, V = 1401.8(13) A(3), Z = 4; (Re(NPh)Cl(3)(pbo)) empirical formula C(18)H(13)Cl(3)N(3)ORe, crystal system monoclinic, space group P2(1)/c, a = 9.566(6) A, b = 16.082(8) A, c = 11.841(5) A, beta = 94.03(4) degrees, V = 1817(2) A(3), Z = 4.

Carbohydrate Binding to VO(3+). Sugar Vanadate Esters Incorporating L-Amino Acid Schiff Bases as Coligands.

The glycosides methyl-2,6-dimethoxy-beta-D-galactopyranoside (beta-D-H(2)Me(3)GP), methyl-4,6-dimethoxy-alpha-D-mannopyranoside (alpha-D-H(2)Me(3)MP), and methyl-5-methoxy-beta-D-ribofuranoside (beta-D-H(2)Me(2)RF) have been synthesized, and their reaction with VO(L-Asal)(OMe)(OHMe) in dichloromethane has afforded esters of the type VO(beta-D-HMe(3)GP)(L-Asal), VO(alpha-D-HMe(3)MP)(L-Asal), and VO(beta-D-HMe(2)RF)(L-Asal) as dark-colored solids (red in solution). Here, L-Asal(2)(-) is the deprotonated salicylaldimine of L-alanine (A = a), L-valine (A = v), and L-phenylalanine (A = p). The X-ray structures of VO(beta-D-HMe(3)GP)(L-vsal).H(2)O and VO(alpha-D-HMe(3)MP)(L-psal).H(2)O have revealed five-membered (O,O)-chelation by monoionized carbohydrates, the undissociated hydroxyl group lying trans to the oxo oxygen atom. In the carbohydrate frame, the alkoxidic oxygen atom is axial in the former ester and equatorial in the latter. The V-O(alkoxidic) and V-O(alcoholic) distances are, respectively, approximately 1.80 and approximately 2.30 Å. The ONO coordinating tridentate salicylaldimine ligand is folded (by approximately 35 degrees ) along a C-N bond. The chiral configuration of the metal site corresponds exclusively to the endo disposition of the V=O and the amino acid C-R (R = Me, CHMe(2), CH(2)Ph) bonds. In VO(beta-D-HMe(3)GP)(L-vsal).H(2)O two ester molecules constitute the asymmetric unit and these along with the two water molecules form a macrocyclic supramolecule (diameter, approximately 6.1 Å) held by hydrogen bonds involving alcohol and an OMe function as well as water (O.O distance, 2.59-2.86 Å). On the other hand, in VO(alpha-D-HMe(3)MP)(L-psal).H(2)O the water molecule bridges two symmetry-related ester molecules via alkoxide.water and alcohol.water hydrogen bonds forming an infinite chain structure (O.O lengths, 2.65 and 2.85 Å). The molecular structures observed in the solid state are preserved in solution ((1)H and (51)V NMR). No isomerization is detectable either at the metal site or at the anomeric carbon atom, and the V-O(alkoxidic) and V-O(alcoholic) sites and the metal-carbohydrate binding remain in tact. The VO(beta-D-HMe(2)RF)(L-Asal) species did not afford single crystals but NMR results are consistent with (O,O)-chelation by the ribose fragment, the alkoxidic carbon being C3. Crystal data are as follows. VO(beta-D-HMe(3)GP)(L-vsal).H(2)O: chemical formula, C(21)H(32)NO(11)V; crystal system, orthorhombic; space group, P2(1)2(1)2(1); a = 13.146(7) Å, b = 15.142(5) Å, c = 25.631(9) Å; Z = 8. VO(alpha-D-HMe(3)MP)(L-psal).H(2)O: chemical formula, C(25)H(32)NO(11)V; crystal system, monoclinic; space group, P2(1); a = 13.645(4) Å, b = 7.022(2) Å, c = 15.500(4) Å; beta = 113.98(2) degrees; Z = 2.

Regiospecific Oximato Coordination at the Oxygen Site: Ligand Design and Low-Spin Mn(II) and Fe(II/III) Species.

The (pyridylazo)oxime ligand (C(5)H(4)N)N=NC(=NOH)C(6)H(4)R(p), HRL (R = H, Me), has been synthesized with the objective of promoting the very rare mononuclear oximato-O coordination. The synthesis involves hydrazone nitrosation. HRL reacts with M(ClO(4))(2).6H(2)O, affording [M(RL)(2)] (M = Mn, Fe), and X-ray structure determinations have indeed revealed the presence of the desired oximato-O coordination mode in the distorted octahedral MN(4)O(2) sphere characterized by a 2-fold axis of symmetry relating the two tridentate ligands, all chelate rings being five-membered. The M-N and M-O distances are relatively short (1.84-1.99 Å), consistent with spin-pairing (M = Mn, S = (1)/(2); M = Fe, S = 0). The M-N bond length trend, Mn(II) > Fe(II), is in accord with the larger radius of the low-spin Mn(II) atom. The M-O length order is, however, Mn(II) < Fe(II). This reversal occurs because of the rigidity of the ligand skeleton. Significant M-azo back-bonding is present (N=N length approximately 1.30 Å). [Mn(RL)(2)] displays hyperfine-split axial EPR spectra (g( parallel) approximately 1.98, g( perpendicular) approximately 2.06, A( parallel) approximately 170 G, A( perpendicular) approximately 65 G) in frozen solution as well as in polycrystalline [Fe(RL)(2)] lattices. The quasireversible Fe(III)/Fe(II) couple of [Fe(RL)(2)] has E(1/2) approximately 0.7 V. The oxidized complex [Fe(RL)(2)]PF(6) has been electrosynthesized. It is low-spin (S = (1)/(2)) and displays axial EPR spectra (g( parallel) approximately 1.98, g( perpendicular) approximately 2.14) in frozen solution. The Mn(II) --> Mn(III) oxidation ( approximately 1.0 V) in [Mn(RL)(2)] is irreversible. Crystal data for [Mn(HL)(2)].CH(2)Cl(2): crystal system monoclinic, space group I2/a, a = 14.142(7) Å, b = 10.721(7) Å, c = 16.933(10) Å, beta = 93.01(4) degrees, Z = 4. Crystal data for [Fe(HL)(2)].CH(2)Cl(2): crystal system monoclinic, space group I2/a, a = 14.254(6) Å, b = 10.742(6) Å, c = 16.719(8) Å, beta = 92.35(4) degrees, Z = 4.

Variable-Valent Re&tbd1;NAr Species. A Family of Re(VI)NAr Amide Complexes and Their Re(V)NAr Imine Precursors Related by Oxygen Atom Transfer.

Imide complexes of type Re(V)Cl(3)(X-SB)(NC(6)H(4)Y(p)), with X, Y= H, Me, OMe, Cl have been synthesized where X-SB is the Schiff base of pyridine-2-carboxaldehyde (the corresponding complex is 4), 2-acetylpyridine (5), 2-benzoylpyridine (6), and anilines, p-XC(6)H(4)NH(2). Treatment of 4 or 5 (but not 6) with aqueous nitric acid in acetonitrile afforded Re(VI)Cl(3)(X-PA)(NC(6)H(4)Y(p)), 7, via oxygen atom transfer (X-PA = monoanionic picolinamide). In the structures of 5(X=Cl,Y=Cl), 6(OMe,OMe), and 7(Me,Me), the chlorine atoms are meridionally disposed in a ReCl(3)N(3) coordination sphere. The trans influence of the imide nitrogen considerably lengthens the Re-N(pyridine) bond. The ReNC(6)H(4)Y(p) group has the triple-bonded linear moiety, Re&tbd1;N-C. The amide group in 7(Me,Me) is planar. In 6(OMe,OMe) the two aryl rings on the imine function block water attack and hence amide formation. The rhenium(VI)-rhenium(V) E(1/)(2) values for 4-6 (0.7-1.0 V vs SCE) are much higher than that for 7 (E(1/2) approximately 0.15 V), which displays the rhenium(VII)-rhenium(VI) couple near 1.6 V. Six EPR hyperfine lines are observed for solutions of 7 at room temperature (g(iso) approximately 1.91; A(av) approximately 490 G). Crystal data for the complexes are as follows: 5(Cl,Cl), empirical formula C(19)H(15)Cl(5)N(3)Re, crystal system monoclinic, space group P2(1)/c, a = 13.360(6) Å, b = 12.110(3) Å, c = 14.954(9) Å, beta = 111.41(4) degrees, V = 2252.4(1.7) Å(3), Z = 4; 6(OMe,OMe), empirical formula C(26)H(23)Cl(3)N(3)O(2)Re, crystal system orthorhombic, space group Pbca, a= 12.079(5) Å, b = 17.083(9) Å, c = 26.049(9) Å, V = 5375.4(4.0) Å(3), Z = 8; 7(Me,Me), empirical formula C(20)H(18)Cl(3)N(3)ORe, crystal system monoclinic, space group P2(1)/c, a = 7.071(2) Å, b = 17.541(6) Å, c = 16.857(8) Å, beta = 100.59(3) degrees, V = 2055.3(1.3) Å(3), Z = 4.

A Triad of Variable-Valent Rhenium Aldimine and Amide Systems Interrelated by Successive Oxygen Atom Transfer.

The title systems are Re(V)OCl(3)(RA), 1, Re(III)(OPPh(3))Cl(3)(RA), 2, and Re(IV)(OPPh(3))Cl(3)(RB), 3, where RA is a 2-pyridinecarboxaldimine, p-RC(6)H(4)N=CHC(5)H(4)N, and RB(-) is the corresponding 2-picolinamide, p-RC(6)H(4)NC(=O)C(5)H(4)N(-) (R = H, Me, OMe, Cl). Controlled reaction of ReOCl(3)(PPh(3))(2) with RA affords 1, which is converted to 2 upon reaction with PPh(3). The oxidation of 2 in aqueous media by Ce(4+) or H(2)O(2) furnishes 3. The X-ray structure of 1 (R = Me) has revealed meridional ReCl(3) geometry, the oxo atom lying trans to the pyridine nitrogen of chelated MeA. The metal atom is displaced by 0.35 Å toward the oxo atom from the ReCl(3)N(aldimine) plane. The couples 2(+)/2 (E(1/2) approximately 0.3 V vs SCE), 3(+)/3 (E(1/2) approximately 1.3 V), and 3/3(-) (E(1/2) approximately -0.5 V) are observable electrochemically. The conversion 1--> 2 follows a second-order rate law with a large and negative entropy of activation ( approximately -40 eu). The reaction is proposed to proceed via nucleophilic attack by the phosphine on Re=O; the observed effects of R and phosphine variation on the rate are consistent with this. In the conversion of 2 to 3, the active species is 2(+), the stoichiometry of the reaction being 32(+) + H(2)O --> 22 + 3 + 3H(+). The amide oxygen in 3 originates from the water molecule. The reaction follows a second-order law, and the entropy of activation is large and negative, approximately -30 eu. The rate-determining addition of water to the aldimine function is believed to afford an alpha-hydroxy amine intermediate which undergoes induced electron transfer and associated changes, affording 3. Crystal data for ReOCl(3)(MeA) are as follows: empirical formula C(13)H(12)Cl(3)N(2)ORe; crystal system triclinic; space group P&onemacr;; a = 7.136(4) Å, b = 8.329(5) Å, c = 14.104(9) Å, alpha = 73.88(5) degrees, beta = 76.35(5) degrees, gamma = 85.64(5) degrees; V = 782.5(8) Å(3); Z = 2.

A Bis(azo-imine)palladium(II) System with 10 Ligand pi Electrons. Synthesis, Structure, Serial Redox, and Relationship to Bis(azooximates) and Other Species.

The first azo-imine chelate system, Pd(N(H)C(R)NNPh)(2) (Pd(RA)(2)), has been isolated in the form of diamagnetic solids by the 6e(-)-6H(+) reduction of bis(phenylazooximato)palladium(II), Pd(N(O)C(R)NNPh)(2) (abbreviated Pd(RB)(2)), with ascorbic acid in a mixed solvent (R = Ph, alpha-naphthyl). Selected spectral features are described. The X-ray structures of Pd(PhA)(2) and Pd(PhB)(2) have revealed trans-planar geometry consistent with metal oxidation state of +2. Bond length trends within the chelate rings are rationalized in terms of steric and electronic factors. In Pd(PhA)(2) a total of 10 ligand pi electrons are present, each formally monoanionic ligand contributing five. Model EHMO studies have revealed that the filled HOMO (a(u)) in Pd(RA)(2) is a bonding combination of two ligand pi orbitals with large azo contributions. The LUMO (b(g)) is roughly the corresponding antibonding combination. The outer pi-electron configuration of Pd(RA)(2) is (a(u))(2)(b(g))(0). Four successive voltammetric responses, two oxidative and two reductive, are observed. The E(1/2) range is -1.3 to +0.8 V vs SCE for Pd(PhA)(2) in a 1:9 MeCN-CH(2)Cl(2) mixture (Pt electrode). EPR and electronic spectra of the electrogenerated one-electron-oxidized complex Pd(PhA)(2)(+) are described. The azo-imine system is compared with imine-imine and azo-azo systems. Crystal data for the complexes are as follows. Pd(PhA)(2): crystal system monoclinic; space group C2/c; a = 18.167(5) Å, b = 7.420(3) Å, c = 16.527(6) Å; beta = 92.70(3) degrees; V = 2225(1) Å(3); Z = 4; R = 2.61%, R(w) = 3.58%. Pd(PhB)(2): crystal system monoclinic; space group P2(1)/n; a = 5.735(5) Å, b = 10.797(6) Å, c = 18.022(11) Å; beta = 97.73(6) Å; V = 1105(1) Å(3); Z = 2; R = 3.37%; R(w) = 3.40%.