Redox potential and substituent effects in ferrocene derivatives: II

The anodic behavior of 31 ferrocene derivatives (FcX) (four of which have been prepared for the first time) of the types shown below has been investigated by cyclic voltammetry (and, in some cases, also by controlled-potential electrolysis and differential pulse polarography) at a Pt electrode, in an aprotic solvent: Fc---C(Y)=Z (class A; Y = R (hydrogen, alkyl or unsaturated-carbon moiety) or Cl; Z = CRR′, derived functionalized groups or {W(CO)₅}); Fc---C(Y)=O (class B; Y --- R or OH); other types (class C, including azide, other unsaturated N moieties, acyloxy or phosphino); disubstituted ferrocenes (class D, with alkyl, aminoalkyl, phosphino, B(OH)₂ or −HGCl substituents, or the S---S---S bridge). All these substituted ferrocenes exhibit a one-electron reversible oxidation centered at iron and the effect of the substituents on the half-wave oxidation potential is discussed in terms of electronic properties. Using the accepted linear correlations between the half-wave oxidation potential and the Hammett constant σ_p of the substituent, σ_p was estimated for the first time for 29 of the substituents in the ferrocenes listed above. The anomalous behavior of complexes with a hydroxy or a carbonyl group is discussed. No correlation was observed between the oxidation potential and either the energy of the electronic absorption bands or the ¹H NMR chemical shift of the unsubstituted cyclopentadienyl ring, although there are rough correlations for class A compounds. This behaviour is compared with those of [Fc-CHYZ]. kw]Iron; Redox potentials; Ferrocenes; Electrochemistry; Substituent effects     

Spectroelectrochemical Properties and Catalytic Activity in Cyclohexane Oxidation of the Hybrid Zr/Hf-Phthalocyaninate-Capped Nickel(II) and Iron(II) tris-Pyridineoximates and Their Precursors

The in situ spectroelectrochemical cyclic voltammetric studies of the antimony-monocapped nickel(II) and iron(II) tris-pyridineoximates with a labile triethylantimony cross-linking group and Zr(IV)/Hf(IV) phthalocyaninate complexes were performed in order to understand the nature of the redox events in the molecules of heterodinuclear zirconium(IV) and hafnium(IV) phthalocyaninate-capped derivatives. Electronic structures of their 1e-oxidized and 1e-electron-reduced forms were experimentally studied by electron paramagnetic resonance (EPR) spectroscopy and UV−vis−near-IR spectroelectrochemical experiments and supported by density functional theory (DFT) calculations. The investigated hybrid molecular systems that combine a transition metal (pseudo)clathrochelate and a Zr/Hf-phthalocyaninate moiety exhibit quite rich redox activity both in the cathodic and in the anodic region. These binuclear compounds and their precursors were tested as potential catalysts in oxidation reactions of cyclohexane and the results are discussed.     

Supramolecular assemblies of trinuclear triangular copper(II) secondary building units through hydrogen bonds. Generation of different metal-organic frameworks, valuable catalysts for peroxidative oxidation of alkanes

Formation of the trinuclear triangular copper derivative [Cu3(mu3-OH)(mu-pz)3(EtCOO)2(H2O)] x H2O, 1b (Hpz = pyrazole), has been simply achieved by addition of Hpz to a water solution of Cu(EtCOO)2 x H2O and leaving the resulting solution to crystallize at ca. 12 degrees C. When the reaction and crystallization were carried out at a slightly higher temperature (18-22 degrees C), the compound [Cu3(mu3-OH)(mu-pz)3(EtCOO)2(H2O)], 1c, formed. Single-crystal X-ray molecular structure determinations show that both compounds have analogous trinuclear triangular structures, but very different supramolecular assemblies, due mainly, but not only, to the crystallization molecule of H2O in 1b. In particular, contrarily to the previously reported, strictly related, [Cu3(mu3-OH)(mu-pz)3(EtCOO)2(EtOH)], 1a, the propionate ions in 1b and 1c do not bridge different triangular units, whereas they are involved in intra- and intermolecular H-bonds, generating complex supramolecular 2-D MOFs. Compounds 1a and 1c act as remarkably active and selective catalysts or catalyst precursors for liquid biphasic (MeCN/H2O) peroxidative oxidation of cyclohexane and cyclopentane to the corresponding alcohols and ketones.     

Kinetic and mechanistic study of the Pt(II) versus Pt(IV) effect in the platinum-mediated nitrile-hydroxylamine coupling

The metal-mediated coupling between coordinated EtCN in the platinum(II) and platinum(IV) complexes cis- and trans-[PtCl(2)(EtCN)(2)], trans-[PtCl(4)(EtCN)(2)], a mixture of cis/trans-[PtCl(4)(EtCN)(2)] or [Ph(3)PCH(2)Ph][PtCl(n)(EtCN)] (n = 3, 5), and dialkyl- and dibenzylhydroxylamines R(2)NOH (R = Me, Et, CH(2)Ph, CH(2)C(6)H(4)Cl-p) proceeds smoothly in CH(2)Cl(2) at 20-25 degrees C and the subsequent workup allowed the isolation of new imino species [PtCl(n){NH=C(Et)ONR(2)}(2)] (n = 2, R = Me, cis-1 and trans-1; Et, cis-2 and trans-2; CH(2)Ph, cis-3 and trans-3; CH(2)C(6)H(4)Cl-p, cis-4 and trans-4; n = 4, R = Me, trans-9; Et, trans-10; CH(2)Ph, trans-11; CH(2)C(6)H(4)Cl-p, trans-12) or [Ph(3)PCH(2)Ph][PtCl(n){NH=C(Et)ONR(2)}] (n = 3, R = Me, 5; Et, 6; CH(2)Ph, 7; CH(2)C(6)H(4)Cl-p, 8; n = 5, R = Me, 13; Et, 14; CH(2)Ph, 15; CH(2)C(6)H(4)Cl-p, 16) in excellent to good (95-80%) isolated yields. The reduction of the Pt(IV) complexes 9-16 with the ylide Ph(3)P=CHCO(2)Me allows the synthesis of Pt(II) species 1-8. The compounds 1-16 were characterized by elemental analyses (C, H, N), FAB-MS, IR, (1)H, (13)C{(1)H}, and (31)P{(1)H} NMR (the latter for the anionic type complexes 5-8 and 13-16) and by X-ray crystallography for the Pt(II) (cis-1, cis-2, and trans-4) and Pt(IV) (15) species. Kinetic studies of addition of R(2)NOH (R = CH(2)C(6)H(4)Cl-p) to complexes [Ph(3)PCH(2)Ph][Pt(II)Cl(3)(EtCN)] and [Ph(3)PCH(2)Ph][Pt(IV)Cl(5)(EtCN)] by the (1)H NMR technique revealed that both reactions are first order in (p-ClC(6)H(4)CH(2))(2)NOH and Pt(II) or Pt(IV) complex, the second-order rate constant k(2) being three orders of magnitude larger for the Pt(IV) complex. The reactions are intermolecular in nature as proved by the independence of k(2) on the concentrations of added EtC triple bond N and Cl(-). These data and the calculated values of Delta H++ and Delta S++ are consistent with the mechanism involving the rate-limiting nucleophilic attack of the oxygen of (p-ClC(6)H(4)CH(2))(2)NOH at the sp-carbon of the C triple bond N bond followed by a fast proton migration.     

Synthesis, structure and reactions of organometallic compounds of Groups 1–3 containing bulky silicon-substituted alkyl groups

Complexes trans-[PtX(L)(PPh₃)₂]A [1: X = CF₃; A = BF₄; L = NCNH₂, NCNMe₂, NCNEt₂, or NCNC(NH₂)₂. 2: X = Cl; A = BPh₄; L = NCNMe₂ or NCNEt₂] and cis-[PtCl(L)(PPh₃)₂][BPh₄] [3: L = NCNH₂ or NCNC(NH₂)₂], which appear to be the first cyanamide or cyanoguanidine complexes of platinum to be reported, have been prepared by treatment of trans-[PtBr(CF₃)(PPh₃)₂] (in CH₂Cl₂/acetone and in the presence of Ag[BF₄]) or of cis-[PtCl₂(PPh₃)₂] (in THF and in the presence of Na[BPh₄]), respectively, with the appropriate substrate. In KBr pellets or in solution 1 (L = NCNMe₂ or NCNEt₂) undergoes ready replacement of the organocyanamide (under the trans influence of CF₃) by bromide to regenerate trans-(PtBr(CF₃)(PPh₃)₂]. The X-ray structure of 1 (X = CF₃, A = BF₄, L = NCNEt₂) is also reported, and shows the presence of two apical intramolecular contacts of the metal with two ortho-hydrogen atoms of the phosphines, whereas the amine N atom of the diethylcyanamide is trigonal planar in the linear NCN framework with a delocalized π system.     

A Mixed Valence CoIICoIII2 Field-Supported Single Molecule Magnet: Solvent-Dependent Structural Variation

One-pot reaction of the Schiff base N,N’-ethylene bis(salicylaldimine) (H₂L), CoCl₂.6H₂O, and [Ph₂SnCl₂] in acetone produces the mixed valence Co^IICo^III₂ compound [Co^IICo^III₂(μ-L)₂(Ph)₂(μ-Cl)₂]·(CH₃)₂CO·H₂O (1). Our recent study already revealed that the same reaction mixtures in methanol or ethanol produced a heterometallic Sn^IVCo^III (2) or monometallic Co^III complex (3), respectively. Comparison of these organometallic systems shows that the 2,1-intermetallic Ph shift occurs in any of those solvents, but their relevant structural features (mononuclear, dinuclear-heterometallic, and trinuclear mixed valence) are solvent dependent. Geometrical structural rotation is also discussed among the related organometallic Co^IICo^III₂ systems. The AC magnetic susceptibility measurements indicate that 1 is a single molecule magnet (SMM), exhibiting a field-induced slow magnetic relaxation with two modes. The relaxation time for the low-frequency channel is as slow as τ~0.6 s at T = 2.0 K and B_DC = 1.0 T.     

Facile Ni(II)/ketoxime-mediated conversion of organonitriles into imidoylamidine ligands. Synthesis of imidoylamidines and acetyl amides

Treatment of alkyl nitriles with NiX(2).6H(2)O (X = Cl, NO(3)) and 2-propanone oxime, followed by (X = Cl) addition of [i-Pr(4)N](NO(3)) for precipitation of the product, resulted in the formation of amidinium nitrates [RC([double bond]NH(2))NH(2)](NO(3)) (R = Me, Et, n-Pr). The reaction went to another direction with NiX(2).2H(2)O, i.e., the reaction between neat RCN (R = Me, Et, n-Pr, i-Pr, n-Bu, CH(2)Cl, CH(2)C(6)H(4)OMe-p) and NiCl(2).2H(2)O/2-propanone oxime (other ketoximes can also be used) gave the (imidoylamidine)Ni(II) complexes [Ni[N(H)[double bond]C(R)NHC(R)[double bond]NH](2)](2+) (1(2+)-7(2+)). The latter were isolated in good yields (65-91%) as the bis-chloride salts 1.Cl(2)-6.Cl(2) and the mixed salt 7.(Cl)(p-MeOC(6)H(4)CH(2)CO(2)). Remarkably, the latter transformation does not proceed at all if NiCl(2).2H(2)O or the ketoxime are taken alone. Liberation of imidoylamidines was performed for one alkyl-containing complex [2.Cl(2)] and one benzyl-containing complex [7.(Cl)(p-MeOC(6)H(4)CH(2)CO(2))], by (i) addition of HBF(4).Et(2)O to the acetonitrile solution of the complexes to yield [N(H)[double bond]C(R)NHC(R)[double bond]NH].2HBF(4) (R = Et 8 and R = CH(2)C(6)H(4)OMe-p 9) or (ii) substitution for ethanediamine (en) with following precipitation of the complex [Ni(en)(3)]Cl(2) with formation of free N(H)[double bond]C(R)NHC(R)[double bond]NH (R = Et 10 and R = CH(2)C(6)H(4)OMe-p 11). In contrast to the liberation in nonaqueous media, treatment of 2.Cl(2) and 7.(Cl)(p-MeOC(6)H(4)CH(2)CO(2)) with Na(2)EDTA.2H(2)O in water-methanol solutions led to substitution and hydrolysis to furnish the acyl amides [EtC([double bond]O)](2)NH (12) and [p-MeOC(6)H(4)CH(2)C([double bond]O)](2)NH (13). Alternatively, 12 and 13 were obtained by hydrolysis of 10 and 11 in water at pH ca. 8.5. It was shown that the oxime complexes trans-[NiCl(2)(C(4)H(8)C[double bond]NOH)(4)] (14) or cis-[Ni(O,O-NO(3))(2)(C(4)H(8)C[double bond]NOH)(2)] (15) can be intermediates in the formation of amidines and imidoylamidines. The sequence of the Ni(II)/oxime mediated formation of (imidoylamidine)Ni complexes and liberation (or hydrolytic liberation) of the ligands opens up a novel, facile and environmentally benign route to imidoylamidines and acyl amides.