Reversible catenation of coordination rings with Pd(II) and/or Pt(II) metal centers: Chirality induction through catenation

A Pd(II)-linked coordination ring is reversibly transformed into its catenanted dimer at room temperature through the efficient organic stacking of the component rings. An analogous Pt(II)-linked ring is also catenated only at high temperature (100 °C), but not at room temperature because of the kinetic inertness of Pt(II)-ligand interaction. Interestingly, the combination of the Pd(II)- and the Pt(II)-linked coordination rings selectively gives a Pd(II)/Pt(II) cross-catenane, because the kinetically inert Pt(II) ring can be catenated only via the dissociation of the kinetically labile Pd(II) ring. Planer conformation of the monomer rings is twisted upon catenation, inducing helical chirality in the catenated structure. Thus, induced circular dichroism (ICD) is observed in the catenation when chiral-1,2-cyclohexandiamine is attached as a chiral auxiliary on the metal centers. The ICD decreases with increasing temperature due to less effective chiral aromatic stacking at higher temperature. The Pd(II) ring shows higher ICD than the Pt(II) ring, probably due to the more flexible conformation of the Pd(II) ring that can adopt chiral orientation easily. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3478–3485, 2003     

Formation of bis- and tris[2]catenanes via the cross-catenation of Pd(II)- and Pt(II)-linked coordination rings

We have demonstrated the self-assembly of linear oligo[2]catenanes via selective cross-catenation. A Pd(II)-linked double looped molecule 1 was transformed into the cyclic trimer c-(1)₃ through the catenation. When 1 was treated with a kinetically inert Pt(II)-linked single looped molecule 2a in aqueous media (1:1.2 DMSO/H₂O) at room temperature, linear oligo[2]catenanes of 2a-(1)_n-2a (n=1 and 2) were selectively obtained, because the kinetically inert Pt(II)-linked ring 2a is allowed to thread on only kinetically labile Pd(II)-linked ring of 1. The distribution of the oligomers depends on the monomer ratio of 1 to 2a. When the ratio of 1 to 2a was 1:2, bis[2]catenane 3a (2a-1-2a) was quantitatively assembled. When the ratio of 1 to 2a was 1:1, not only 3a but also tris[2]catenane 4 (2a-1-1-2a) was assembled. The ratio of 3a to 4 was carefully determined to be 1:1 by NMR. The lengths of 3a and 4 in an extended conformation were estimated by MD/MM2 simulation to be 3.6 and 5.4 nm, respectively.     

Selective cross-catenation of Pd(II) and Pt(II) coordination rings

The reversible cross-catenation of two different coordination rings (Pd(II)- and Pt(II)-linked rings) has been achieved by using the labile nature of the Pd-N interaction and efficient hydrophobic contact between the rings.     

The neglected Pt-N(sulfonamido) bond in Pt chemistry. New fluorophore-containing Pt(II) complexes useful for assessing Pt(II) interactions with biomolecules

Treatment of cis-Pt(Me2SO)2Cl2 with DNSH-tren afforded [Pt(DNSH-tren)Cl]Cl and with DNSH-dienH, under increasingly more basic conditions, led to Pt(DNSH-dienH)Cl(2), Pt(DNSH-dien)Cl, and Pt(DNS-dien). (DNSH = 5-(dimethylamino)naphthalene-1-sulfonyl, linked via a sulfonamide group to tris(2-aminoethyl)amine (DNSH-tren) and diethylenetriamine (DNSH-dienH); the H's in DNSH-dienH designate protons sometimes lost upon Pt binding, i.e., sulfonamide NH for the dienH moiety and H8 for the DNSH moiety). Respectively, the three neutral DNSH-dienH-derived complexes are difunctional, monofunctional, and nonfunctional and exhibit decreasing fluorescence in this order as the dansyl group distance to Pt decreases. 2D NMR data establish that Pt(DNS-dien) has a Pt-C8 bond and a Pt-N(sulfonamido) bond. Pt(DNSH-dien)Cl and [Pt(DNSH-tren)Cl]Cl bind to N7 of 6-oxopurines (e.g., 5'-GMP, 3'-IMP, and 9-ethylguanine) and sulfur of methionine (met). Competition and challenge reactions for Pt(II) with met and 5'-GMP typically reveal that met binding is favored kinetically but that 5'-GMP binding is favored thermodynamically. This common type of behavior was found for [Pt(DNSH-tren)Cl]Cl. In contrast, Pt(DNSH-dien)Cl had reduced kinetic selectivity for met. This unusual behavior undoubtedly arises as a consequence of the bound Pt-N(sulfonamido) group, which donates strongly to Pt (as indicated by relatively upfield dien NH signals) and which places the bulky DNSH moiety close to the monofunctional reaction site. The decrease in the relatively upfield shifts of the DNSH group signals indicates that this group stacks with the purine. This stacking could explain the unprecedented, relatively low reactivity of a Pt complex bearing a dien-type ligand toward met vs 5'-GMP.     

Insertion of molecular oxygen into a palladium(II) hydride bond

Insertion of molecular oxygen into a palladium(II) hydride bond to form an (eta1-hydroperoxo)palladium(II) complex is reported. The hydroperoxo palladium(II) product has been crystallographically characterized. A second-order rate law (first-order in palladium and first-order in oxygen) is observed for the reaction and a large kinetic isotope effect implicates Pd-H bond cleavage in the rate-determining step. The results of studies with radical inhibitors and light suggest that the reaction does not proceed by a radical chain mechanism.     

Pt(II) as a proton shuttle during C-H bond activation in the Shilov process

The C-H activation in Shilov's system on cis- and trans-PtCl(2)(H(2)O)(CH(4)) was investigated by ab initio molecular dynamics in water. Simulations revealed an easy C-H bond cleavage forming a transient 5-coordinated species Pt(H)Cl(2)(H(2)O)(CH(3)) that spontaneously releases a proton to the bulk solution.     

Relationships between Hg(II)-S bond distance and Hg(II) coordination in thiolates

Extended X-ray absorption fine structure (EXAFS) spectroscopy is a sensitive structural probe of the coordination environments of Hg(II) with thiol (sulfhydryl) groups, and is equally applicable to solid and aqueous organic or inorganic matter. Information on the number and geometric arrangement of S ligands can be derived from metal-ligand distances because the distances vary with Hg stereochemistry and can be accurately measured by the EXAFS technique. To improve the reliability of determining coordination structures of Hg with thiol groups, correlations among Hg-S bond distance, Hg coordination, and S-Hg-S angle in homoleptic Hg(II)-thiolates were calculated from analysis of the structures of the 162 Hg(SR)n motifs (n = 2, 3, 4) contained in the Cambridge Structural Database v. 5.28. Graphical correlations of bond distance with coordination number and with bond angle show distinct ranges of values characteristic of specific structural configurations.     

Pt(IV)-catalyzed cyclization of arene-alkyne substrates via C-H bond functionalization

We herein report that PtCl₄ has proven to be a hydroarylation catalyst with an efficiency and substrate scope superior to previously known methods. This catalyst demonstrated consistent performance with arene-yne substrates of diverse structural features, including propargyl ethers, propargylamines, and alkynoate esters, providing good to excellent yields of the 6-endo products (chromenes, dihydroquinolines, and coumarins). In contrast, Pt(II), Pd(II), and Ga(III) salts were shown to be sensitive to the substitution on the alkyne moiety. PtCl₄ is compatible with both terminal and disubstituted alkynes, as well as with various functionalities on the arene ring, including methyl, methoxyl, hydroxyl, protected amine, and halide.     

A luminescent Pt(II) complex with a terpyridine-like ligand involving a six-membered chelate ring

The ligand 2-(8'-quinolinyl)-1,10-phenanthroline (1) was prepared in 79% yield by the Friedlander condensation of 8-amino-7-quinolinecarbaldehyde and 8-acetylquinoline. The complex [Pt(1)Cl]+ was prepared and compared with the isomeric 2-(2'-quinolinyl)-1,10-phenanthroline (2) complex. An X-ray analysis indicated that the six-membered chelate ring in the tridentate complex resulted in a relief of angle strain as well as some non-planarity in the bound ligand 1. The control system for photophysical studies is [Pt3Cl]+ where denotes 2-(2'-pyridyl)-1,10-phenanthroline. Relative to the complex of 3, in dichloromethane solution [Pt(1)Cl]+ exhibits noticeably higher energy charge-transfer absorption but slightly lower energy emission. The gap between the onset of absorption and emission is larger because the emission from [Pt(1)Cl]+ originates from a triplet excited state with substantial intra-ligand character. At room temperature in deoxygenated dichloromethane, [Pt(1)Cl]+ has an excited-state lifetime of 310 ns vs. 230 ns for [Pt(1)Cl]+. Within the series, [Pt(1)Cl]+ also exhibits the largest activation barrier for thermally induced quenching at 2730 cm(-1) in fluid dichloromethane solution. However, the barrier is only about 50% larger than that found for [Pt(1)Cl]+. There is reduced ring strain in [Pt(1)Cl]+, but inter-ligand steric interactions weaken the ligand field.     

Pt(II)- and Pt(IV)-bridged cofacial diporphyrins via carbon-transition metal sigma-bonds

A directly Pt(IV)-bridged cofacial diporphyrin has been synthesized by the cyclometalation reaction of beta-pyridylporphyrin with a Pt(IV) salt. Upon treatment with methylhydrazine, the Pt(IV) bridge is reduced to the Pt(II) center, resulting in a Pt(II)-bridged cofacial dimer with a helicity inversion of the complex as well as change in electronic communication through the metal bridge.     

A new class of self-assembly multinuclear Pt(II) coordination cages by a modular approach

The formation of three examples of a new class of self-assembly Pt(II) cage molecules of general formula [Pt_m(ligand)_n](NO₃)_2m is achieved from Pt(II) and ligands. The compounds are observed in solution state and confirmed from the similarity of their proton NMR behavior as compared with that of reported Pd(II) compounds of the formula [Pd_m(ligand)_n](NO₃)_2m.     

Molecular encapsulation via metal-to-ligand coordination in a Cu(I)-folded molecular basket

A molecular basket, composed of a semirigid C3v symmetric tris-norbornadiene framework and three pyridine flaps at the rim, has been shown to coordinate to a Cu(I) cation and thereby fold in a multivalent fashion. The assembly was effective (Ka = 1.73 +/- 0.08 x 10(5) M(-1)) and driven by enthalpy (DeltaH(o) = -7.2 +/- 0.1 kcal/mol, DeltaS(o) = -0.25 eu). Variable temperature (1)H NMR studies, assisted with 2D COSY and ROESY investigations, revealed the existence of Cu(I)-folded basket 10b with a molecule of acetonitrile occupying its interior and coordinated to the metal. Interestingly, 10b is in equilibrium with Cu(I)-folded 10a , whose inner space is solvated by acetone or chloroform. The incorporation of a molecule of acetonitrile inside 10a was found to be driven by enthalpy (DeltaH(o) = -3.3 +/- 0.1 kcal/mol), with an apparent loss in entropy (DeltaS(o) = -9.4 +/- 0.4 eu); this is congruent with a complete immobilization of acetonitrile and release of a "loosely" encapsulated solvent molecule during 10a/b interconversion. From an Eyring plot, the activation enthalpy for incorporating acetonitrile into 10a was found to be positive (DeltaH(double dagger) = 6.5 +/- 0.5 kcal/mol), while the activation entropy was negative (DeltaS(double dagger) = -20 +/- 2 eu). The results are in agreement with an exchange mechanism whereby acetonitrile "slips" into an "empty" basket through its side aperture. In fact, DFT (BP86) calculations are in favor of such a mechanistic scenario; the calculations suggest that opening of the basket's rim to exchange guests is energetically demanding and therefore less feasible.     

Mechanism of direct molecular oxygen insertion in a palladium(II)-hydride bond

The mechanism of the direct insertion of molecular oxygen into a palladium hydride bond has been elucidated using quantum mechanics (B3LYP/LACVP** with the PBF continuum solvent model). The key step is found to be the abstraction of the hydrogen atom resulting in the formation of a PdI/HO2 (triplet) radical pair, which then proceeds to form a singlet palladium hydroperoxo species. Potential palladium(0) pathways were explored and were found to be inaccessible. The results are in agreement with recent experimental results and are consistent with our previously predicted mechanism for an analogue system.     

Tuning of redox potentials by introducing a cyclometalated bond to bis-tridentate ruthenium(II) complexes bearing bis(N-methylbenzimidazolyl)benzene or -pyridine ligands

A series of asymmetrical bis-tridentate cyclometalated complexes including [Ru(Mebib)(Mebip)](+), [Ru(Mebip)(dpb)](+), [Ru(Mebip)(Medpb)](+), and [Ru(Mebib)(tpy)](+) and two bis-tridentate noncyclometalated complexes [Ru(Mebip)(2)](2+) and [Ru(Mebip)(tpy)](2+) were prepared and characterized, where Mebib is bis(N-methylbenzimidazolyl)benzene, Mebip is bis(N-methylbenzimidazolyl)pyridine, dpb is 1,3-di-2-pyridylbenzene, Medpb is 4,6-dimethyl-1,3-di-2-pyridylbenzene, and tpy is 2,2':6',2″-terpyridine. The solid-state structure of [Ru(Mebip)(Medpb)](+) is studied by X-ray crystallographic analysis. The electrochemical and spectroscopic properties of these ruthenium complexes were studied and compared with those of known complexes [Ru(tpy)(dpb)](+) and [Ru(tpy)(2)](2+). The change of the supporting ligands and coordination environment allows progressive modulation of the metal-associated redox potentials (Ru(II/III)) from +0.26 to +1.32 V vs Ag/AgCl. The introduction of a ruthenium cyclometalated bond in these complexes results in a significant negative potential shift. The Ru(II/III) potentials of these complexes were analyzed on the basis of Lever's electrochemical parameters (E(L)). Density functional theory (DFT) and time-dependent DFT calculations were carried out to elucidate the electronic structures and spectroscopic spectra of complexes with Mebib or Mebip ligands.     

Irreversible but noncovalent Ru(II)-pyridine bond: its use for the formation of [2]-catenanes

The reaction between ligand 1, which consists of two terminal pyridines attached to a central 1,10-phenanthroline (phen), and the complex Ru(phen)2(CH3CN)2(PF6)2 has been studied. A new ruthenium containing metallamacrocycle has been obtained and fully characterized. Despite the relatively poor yield for the cyclization process involving the ruthenium center (20%), this strategy led to the synthesis of two different kinds of [2]-catenane. The first example reported in this article is a bimetallic Cu(I)/Ru(II) catenane 5(3+) consisting of a purely organic ring interlocked with the ruthenium(II)-incorporating metallacycle. Complex 5(3+) was selectively demetalated at the Cu(I) center to lead to the free Ru(II)-containing catenane. A trimetallic Ru(II)/Cu(I)/Ru(II) catenane 8(5+) was also synthesized showing that this approach is reliable and promising for the elaboration of photoactive multicomponent systems.     

Wavelength-dependent photofragmentation of a mixed-ligand cyclometalated platinum(II) coordination compound in a molecular beam

The photofragmentation of (3-Me-4',6'-dfppy)Pt(dpm) (dfppy = difluorophenylpyridinato; dpm = dipivaloylmethyl or 2,2,6,6,-tetramethyl-3,5-heptanedionato- O, O) in a molecular beam is reported. Time-of-flight mass spectra (TOF-MS) and resonance-enhanced multiphoton ionization (REMPI) data are presented and discussed. The dissociation patterns are strongly wavelength-dependent. With 355 nm excitation, the heaviest mass platinum-containing fragments are Pt(+) and diatomic PtC(+). The formation of PtC(+) is the result of an intramolecular rearrangement on the ligand. During irradiation with 410-500 nm light, the fragmentation pattern changes such that the parent ion and platinum-containing fragments of the parent are formed in abundant yield. The (3-Me-4',6'-dfppy) ligand remains intact and coordinated to platinum, but coordinated (dpm) successively breaks apart. A spin-forbidden charge-transfer absorption band centered at around 460 nm plays an important role in the gas-phase photoexcitation of the parent molecule; it is observed in the REMPI spectrum of the parent ion.     

Thiocyanate bond mode determination on solid manganese(II) mixed ligand coordination compounds

Synthesized and in solid state characterized compounds correspond to the following types [Mn(NCS)₂L_mL'_n] · x D, [Mn(NCS)₂L_y]_z, and [MnX₂(CH₃SCN)(CH₃OH)] · x D (L=CH₃OH or O-donors; L=CH₃SCN or N-donors; D=solvent molecules; x=0, 1 or 2; m=0, 1 or 2; n=1, 2 or 3; y=0, 1, 2, 3 or 4; z2; X=Cl or Br). Derivatographic investigations confirm the formation of intermediates, especially for L=H₂O the existence of thermic stable partially dehydrated (hydrate poor) species (y=1/2 or 1/3) and -NCS-bonding with different ligating atoms. Solid state FT-IR analysis were carried out for band shapes, measurements of the integrated intensity, and correlations between located absorption maxima for v(CN), v(CS), (NCS), v(MnN), and v(MnS) and the thiocyanate bond mode.