Songsheng Zhang

RuIII(hedta) as an oxygen atom transfer catalyst in the epoxidation of stilbenes

Abstract RuIII(hedta) and RuIII((CH3)2edda)+ (hedta3−=N-hydroxyethylethylenediaminetriacetate; (CH3)2edda2−=N, N− dimethethylethylenediamine-N, N−diacetate) catalyze the epoxidation of cis-stilbene and trans-stilbene using tert- butylhydroperoxide (t-BuOOH) as the oxygen source. Prior spin-trapping studies have documented the existence of LRuIIIO ↔ LRuIVO·− ↔ LRuVO2− character in the species obtained from RuIIIL and to-BuOOH (L=polyam- inopolycarboxylate ligands related to edta4−). The O-atom complex, LRuIIIO, appears responsible for the epoxidation of stilbenes. Yields as high as 63.5% cis-stilbene oxide plus 11.0% trans-stilbene oxide from cis-stilbene and 65.1% cis-stilbene oxide from trans-stilbene (with no trans-stilbene oxide) are formed in the epoxidation reactions. Secondary oxidations of the epoxide products produce between 4 to 8% benzaldehyde depending on conditions. The product distribution using the RuIIIL/t-BuOOH catalyst requires at least three epoxidation pathways: (i) concerted transfer of the oxenoid oxygen to the stilbene nucleophile; this process is favored for cis-stilbene; (ii) an outer-sphere electron transfer from the stilbene to LRuIIIO forming a carbon-centered cation radical adjacent to LRuIIIO·−; this radical pair may couple directly for cis-stilbene or after a rapid isomerization of the trans- stilbene radical; (iii) an acyclic pathway which has both free radical and carbocation resonant character; this allows for isomerism of cis-stilbene to trans-stilbene oxide products. RuIIIO(hedta) is also observed to cleanly oxidize benzaldehyde to benzoic acid, sec-phenetyl alcohol to acetophenone, and benzyl alcohol to benzaldehyde and benzoic acid. Cyclohexene is hydroxylated and further oxidized to 2-cyclohexene-1-one.

Five coordination modes of 4-aminopyrimidine with N-hydroxy-ethylethylenediaminetriacetatoruthenate(II)

SummaryThe complex [RuII(hedta)(4NH2pym)]−, hedta3− = N-hydroxyethylethylenediaminetriacetate, 4NH2pym = 4-aminopyrimidine, exists at pH 7 as five different coordination isomers, which are most readily distinguished by their electrochemical waves in comparison with the 2-aminopyridine (2NH2py) complex. The 2NH2py complex exhibits N(1) (pyridine bound), exo-NH2 (amine bound) and N(1), NH2-chelated species. The 4NH2pym complex forms N(1), exo-amine and N(3), NH2-chelated isomers analogues to the 2NH2py species, but also engages in η2 (olefin bound) coordination of the dearomatized 4NH2pym ring in C(5)–C(6), and another η2 type of complex involving electron density between N(1) and N(3) of the ring (η3 form). N(1), η2 and η3 isomers have also been detected for unsubstituted pyrimidine (pym), 4-methylprimidine (4CH3pym) and 2-aminopyrimidine (2NH2pym). Electrochemical waves (V versus NHE) for the five isomers are assigned as follows: (RuII/III) exo-NH2 (0.06 V), N(1) (0.29 V), η2 (0.49 V); (RuII/III) η3 (0.76 V); N(3), NH2-chelated (1.09 V).

Formation constants of RuII(hedta)− complexes of olefins and η2-coordinated pyrimidines related to cytidine and uridine

The association constants for the formation of RuII(hedta)L−, where L=an olefin or an η2-coordinated pyrimidine nucleobase derivative of C or U, were determined for 16 ligand structures. The formation constants Kf are a sensitive function of the substituent of the α-carbon of the olefin unit. Effective Kfs range over 5 orders of magnitude (from Kf=7.51, L=3-deazauracil; Kf=6.33×103, L=3-cyclohexen-1-methanol; to Kf=2.04×106, L= methyl vinyl ketone) depending on the electronic influence of the α substituents. π-Withdrawing groups such as a keto functionality increase Kf by 102 to 103 in magnitude. π-Donating units (RNH2, OH) reduce Kf by 10 to 102 in magnitude. Steric factors are of less importance for determining the overall Kf value. The sensitivity to electronic versus steric factors places RuII mid-range on a scale deduced previously by Tolman from low-oxidation state complexes of lower coordination number than RuII(hedta): Ni0>Pt0>RhI>RuII>PtII>CuI>AgI. (The position of RuII is defined by the current study.) The combined influence of substituents and functionalities of uracil/uridine rings cancel each other; the net affinity (Kf=1.2×103, L=uridine) is virtually the same as a simple olefin without electronically active α substituents. This promotes η2 coordination of uridine nucleobases to RuII(hedta)−. The largest value of Kf for an olefin was found for methyl vinyl ketone, MVK. This olefin coordinates in the η2 mode to all three stereochemical isomers of RuII(hedta)− as shown by the 1H NMR spectrum. Two forms with the olefinic unit trans to the nitrogen, either the one substituted by the N-hydroxyethyl functionality or the nitrogen doubly substituted by glycinate units, provide unhindered coordination to MVK. The third isomer of Ru(hedta)(MVK)− is structurally more hindered and the coordinated ligand exhibits distinguishable Ha and Hb terminal MVK protons. Protons of the olefin region and the methyl group of MVK all exhibit large upfield 1H resonance shifts upon coordination. The highest measured binding constant for Ru(hedta)(MVK)− parallels the highest stability for η2 olefin complexes of RuII as determined by the E12 values of RuIII/III(hedta)L0/−; (E12=0.80 V versus NHE for the MVK complex). It is shown, however, that Kf is not simply related to the E12 values and Kf may be a function of changing solvation of olefins upon coordination.

Protonation of coordinated 2-methylpyrazine and 4,4′-bipyridine as a probe of π-donor potential of ruthenium(II) polyaminopolycarboxylate complexes

SummaryThe proton dissociation pKa data for RuII(hedta)(LH) and Ru2II(ttha)(LH)2 complexes were measured by the spectrophotometric titration mid-point method using the difference in the MLCT spectra for the protonated and deprotonated complexes with L=2-methylpyrazine (2-Mepz) and 4,4′-bipyridine (4,4′-bpy). The pKa′s were as follows at 22° C, μ=0.10 (NaCl): Ru(hedta)(2-MepzH), 3.45; Ru2(ttha)(2-MepzH)2, 3.25; Ru(hedta)(4,4′-bpyH), 4.5; Ru2(ttha)(4,4′-bpyH)2, 4.51. The two series (2-Mepzversus 4,4′-bpy) lead to a divergent prediction of the ability of RuII (polyaminopolycarboxylates) to serve as π-donor metal centers in comparison with RuII(NH3)52+. The results with 4,4′-bpy ligation, where the site of protonation is more isolated from electrostatic effects of the ruthenium center, appear to give the more correct interpretation, consistent with their capacity to make good π-complexes with olefins and pyrimidine nucleobases, that RuII(polyaminopolycarboxylates) are better π-donor metal centers than Ru(NH3)52+·ΔpK*, the difference between ground state and excited state pKa′s, are used to establish the π-donating order of RuII(polyaminopolycarboxylates) as compared to Ru(NH3)52+ and Ru(CN)53−. The π-donor effect of the ruthenium(II) center in Ru2II(ttha)(1,3-butadiene)22− is observed to influence the ligand electronically up to five bonds away from the site of metallation.

Electrochemical detection of the oxidation of alcohols byN-hydroxyethylethylenediamminetriacetatoruthenate(IV)

SummaryThe oxidation of [RuIII(hedta)(H2O)]=(1) to its RuIV monomeric complex at a glassy carbon electrode is abserved to promote oxidation of alcohols bearing an a-hydrogen (i-PrOH benzyl alcohol,sec-phenethyl alcohol). Tertiary substitution blocks the oxidation (t-BuOH). The oxidation of the alcohols is detected by an enhancement in the current of the RuIV/III waves at potentials above 0.96V, caused by scavenging (reduction) of RuIV by the alcohols. Binuclear complexes which possess RuIV bridged by oxo to either a second RuIV or to RuIII in species of composition [LRuORuL]n−, L=hedta3−, fail to oxidize the alcohols. The terminal oxo moiety attached to RuIV is postulated to facilitate the oxidation of primary and secondary alcohols in a manner analogous to Meyers [RuO(trpy)(bpy)]2+ catalyst. The dissociation of the (III,IV) binuclear complex into its monomers provides a pathway which increases catalytic activity at the expense of the inactive (III, IV) binuclear complexs concentration.

A comparison of the solution behavior of Pt(II) complexes of N,N′- and N,N-ethylenediaminediacetate (edda and uedda)

Abstract Pt(II) complexes of N , N ′-ethylenediaminediacetate (edda) and N , N -ethylenediaminediacetate (uedda) have been prepared from K 2 PtCl 4 by stepwise addition of the nitrogen backbone donors at pH ∼ 2.9 (50–60 °C, 60 h) and further coordination of the deprotonated carboxylate donors at pH ≥ 4 (65–75 °C, 24 h). Coordination of the glycinato donors was shown by 1 H and 13 C NMR and IR methods. The symmetrical edda ligands form 38.3% ( R , R )/( S , S )-[Pt(edda)] isomers and 61.7% meso ( R , S )/( S , R )-[Pt(edda)] isomers. All four forms of [Pt(edda)] undergo aquation of one in-plane glycinato donor in 72 h as detected by the appearance of a 13-line 1 H NMR pattern which may be deconvoluted into four AB glycinato sets. These results are indicative of pendant or ion-paired glycinato donor for [Pt(edda) (H 2 O)] which is placed either on the same side, or the opposite side, of the PtN 2 O 2 plane and coordinated glycinato donor. 195 Pt NMR shows that H 2 O is actually replaced by Cl − , i.e.[Pt(edda)Cl] − . The unsymmetrical [Pt II (uedda) X] (X = H 2 O, Cl − , OH − ) complex exhibits no major change over long time intervals (≥ 10 days, pD ∼ 6). The presence of a minor species at 15% abundance may be a similarly structured species as for [Pt(edda)(H 2 O)] with a pendant glycinato functionality. The major complex in solution is shown by the 1 H NMR with [NaCl] and [NaClO 4 ]-dependence studies to be [Pt(uedda)(H 2 O)] at low [Cl − ] and [Pt(uedda)Cl] − at 1.0 M Cl − . 195 Pt NMR confirms the formulation of X = H 2 O at low [Cl − ]. 1 H and 13 C NMR evidence supports one axially associated and one in-plane coordinated glycinato donor each for the major [Pt(uedda)(H 2 O)] complex. The 13 C NMR shows only one type of glycinato donor with a chemical shift of 189.3 ppm for the major species, and two types for the 15% species (185.6 and 170.5 ppm). The major species of [Pt(uedda)(H 2 O)] has only one type of carboxylate stretch in the IR spectra (1661 cm −1 ; shoulder feature at 1639 cm −1 ) which compares favorably with the fully-coordinated pair of glycinato donors of [Pt(edda)] (1640 cm −1 ). It is proposed that the structures of [Pt(uedda)(H 2 O] and [Pt(uedda)Cl] − are pseudo-square pyramids which illustrates the capacity of Pt(II) to adopt five-coordinate, 18-electron complexes when a suitable chelate ligand offers a fifth associable donor. These species are similar to the five-coordinate intermediates of ligand substitution reactions of typical square-planar Pt(II) complexes.

η2 complexes of [RuII (hedta)]−: sensitivity of the 1H NMR chemical shifts of the olefinic protons to alpha ring substituents

Abstract The η2 coordination of olefinic units with RuII centers with amine and oxygen donors has generally been associated with signature 1H and 13C NMR parameters. An upfield 1H NMR shift of between + 1.0 and + 2.0 ppm for protons of the η2 unit and an upfield 13C NMR shift of +40 to +80 ppm for the η2 carbons has been noted previously. A series of [RuII(hedta)L]− complexes, hedta3− = N-hydroxyethyl-ethylenediaminetriacetate (L=3-cyclohexene-1-methanol (1), 2-cyclohexene-1-one (2), pyrithyldione (3), 1,3-dimethyluracil (4). 3-deazauracil (5), 3-methylcytosine (6), have been examined by 1H NMR and electrochemical methods. The η2 complexes of [RuII(hedta)L]− (L=1–6) exhibit the anticipated RuII/III wave in the range of 0.45 to 0.71 V versus NHE. Isolated olefinic cyclic units such as 1 exhibit a reduced upfield 1H NMR shift of between +0.39 and +0.80 ppm. Olefinic protons of the η2 units with adjacent withdrawing carbonyl moieties exhibit more positive upfield 1H NMR shifts (> +0.80 ppm). The presence of an alpha NR(R′) functionality of the ring promotes a downfield 1H NMR shift in the range of −0.02 to −0.46 ppm for [RuII(hedta)L]− complexes that is not observed for [RuII(NH3)5L]2+ complexes. The 1H NMR shifts of the η2-coordinated olefinic units are both a function of chemical groups adjacent to the olefinic binding site and the secondary ligand system of the Ru(II) center. The 13C NMR shifts of the η2 carbons of the [RuII(hedta)L]− series remain in the range of +40 to +80 ppm upfield of the free ligand for complexes 1–6.

μ-(Pyrimidine) (triethylenetetraaminehexaacetato)diruthenate(II, III); nearly class I behaviour for a mixed-valence complex with polyaminopolycarboxylate ligand donors

SummaryThe pyrimidine-bridged [Ru2II(ttha)]2− complex [2, pym, 2] has been prepared and characterized in solution by several spectral methods. The mixed-oxidation state species [2, pym, 3] undergoes dissociation of the RuIII-pym bond. However, electrochemical methods have been used to determine a corrected comproportionation constant K′c=5.7, for [2, pym, 3] prior to dissociation. Similar evaluations have been carried out with [Ru(hedta)]2(pz)− (K′c=190) and [Ru(hedta)]2(pym)− (K′c=22.7). Comparison with literature data reveals a decrease in stability for the mixed-valence species in a polyaminopolycarboxylate ligand environment compared to a pentaammine ligand environment. This effect is attributed to the role of the carboxylate anionic donors in stabilizing RuIII, giving virtually isolated valence properties for these mixed-oxidation state binuclear ruthenium polyaminopolycarboxylate complexes (virtual class I behaviour).

η2 and η4 olefin complexation by S, S-[RuII(Me2edda)]: a structural change to accommodate bidentate dienes

[RuII(Me2edda)(H2O)2] (1), Me2edda2− = N,N′-dimethylethylenediaminediacetate, exhibits a sterically-controlled molecular recognition in forming η2 and η4 olefin complexes. 1 exists with an N2O2 in-plane set of chelate donors and axial H2O ligands. The two CH3 functionalities of Me2edda2− are poised above and below the N2O2 plane of the glycinato rings. Studies herein of the 2,2′-bipyridine complex, [RuII(Me2edda)(bpy)], with bidentate bpy chelation as established via 1H NMR and electrochemical methods show 1 to be ligated in the S,S configuration with the glycinato rings in-plane as a cis-O form. 1 is sterically discriminating in forming η2 complexes with smaller olefins (ethylene, 2-propene, cis-2-butene, methyl vinyl ketone and 3-cyclohexene-1-methanol), but rejects larger decorated ring structures and branched olefins (1,2-dimethyluracil, cyclohexene-1-one 2-methyl-2-propene). η2 complexes of 1 have characteristic RuII/III DPP waves near 0.55 V which vary slightly with olefin structure. Potentially bidendate dienes (1,3-butadiene, 1,3-cyclohexadiene and 2,5-norbornadiene (nbd) form η4 complexes as shown by RuII/III waves between 0.94 and 1.30 V, indicate of a highly stabilized RuII center by π-backboning. An η2⇌η4 ‘equilibrium’ with apparent K = 22 at 25 °C is observed for nbd coordinated to 1. (The η2 and η4 distribution may be a kinetic one and not a thermodynamic one). To allow formation of the cis η4 complexes, 1 must undergo a shift of one or both glycinato donors from the N2O2 plane into the axial site away from the dimethyl functionalities. η4 chelation by 1,3-butadiene has been confirmed by 1H NMR spectral assignments of two [RuII(Me2edda)] isomers, one in the axial rans-O glycinato configuration, e.g. 1,3-butadiene is bidentate in the original N2O2 plane and a second unsymmetrical glycinato arrangement with in-plane and axial glycinato as well as in-plane and axial η4-1,3-butadiene coordination. [RuII(hedta)(H2O)]− (2), hedta3− = N-hydrpxyethylenediaminetriacetate, is less discriminating for olefin structures, forming η2 complexes with all eleven olefins and dienes mentioned for studies with 1. However, 2 does not undergo displacement of a carboxylate donor by the second olefin unit of a diene [RuII(hedta)(diene)]− complexes possess a pendant non-coordinated olefin and on η2-bound olefin in the complex, indicated by a normal RuII(pac)(olefin)RuII/III wave near 0.55 V.

Effect of a thio group on simplified analogs of bleomycin

Abstract Two new series of ligands, SAPH ( 1–3 ) and DAPHS ( 4–6 ) related to the metal chelating portion of the anti-tumor drug bleomycin (BLM) have been synthesized and evaluated. These series differ from the related simplified models, PYML and AMPHIS by modifications of the primary amine of PYML and AMPHIS to a thio functionality (-SR) (R = CH 3 , 1 ; R = CH 2 C 6 H 4 OCH 3 , 2 ; R = H, 3 ) in the SAPH ( 1–3 ) series and by the additional change of the secondary amino functionality of SAPH to an amido unit in the DAPHS ( 4–6 ) compounds. SAPH ( 1–3 ) and DAPHS ( 4–6 ) are, therefore, potentially five-coordinate N 4 S donor ligands possessing imidazole, amide, pyridine, amine, and sulfhydryl moieties for SAPH-3 and imidazole, amide, pyridine, amide, and sulfhydryl for DAPHS-6. One:one metal:ligand complexes were prepared with Cu II , Fe II and Fe III . Frozen glass ESR spectra and electronic spectra at room temperature of the Cu II complexes show that the resultant coordination geometry and choice of ligand donor set is strongly influenced by pH. Cu II (SAPH-3) exhibits imidazole, pyridine, amine and sulfhydryl (N 3 S) coordination in a trigonal bipyramidal complex at 2.80. The coordination changes to imidazole, deprotonated amide, pyridine and amine (N 4 ) at pH⩾ 5.80; the ESR spectra require rhombically distorted square- planar coordination similar to Cu II (AMPHIS) and Cu II (BLM) in this range. Cu II (DAPHS-6) exhibits ESR spectra indicative of 2N (imidazole, pyridine), 3N (imidazole, first amide, pyridine) and 4N (imidazole, pyridine, and both amides) coordination at pHs of 3.18, 5.62 and 8.00, respectively. Sulfhydryl coordination for Fe III (SAPH-3) is indicated by the presence of charge transfer bands (>400 nm) which are absent for the SCH 3 analog, Fe III (SAPH-1), and by the electrochemical behavior of Fe II (SAPH-3) above pH 6.9 where N 4 S coordination is observed. Fe II (SAPH-1) reacts with molecular O 2 but possesses one-tenth the activity of Fe II (AMPHIS), in forming HO· or HO 2 · radicals which escape the solvent cage of the Fe III complex for trapping by DMPO or PBN spin-trap scavengers. Fe II (SAPH-3) shows no free HO· trappable by DMPO during autoxidation, indicative of 2e − reduction steps. Fe II (SAPH-3) exhibits no reversible 1e − electrochemical wave from −0.64 to +1.24 V for forms having sulfhydryl coordination, but the same complex reacts rapidly with 2e − chemical oxidants (O 2 , H 2 O 2 , Ru IV ). Diamide analogs (DAPHS) also lack HO· or O 2 − generating activity.