Intramolecular Site Exchange Research Articles

Coordination sphere flexibility of active-site models for Fe-only hydrogenase: studies in intra- and intermolecular diatomic ligand exchange.

A series of dinuclear complexes, (mu-SRS)Fe(2)(CO)(6) (R = -CH(2)CH(2)-, -CH(2)CH(2)CH(2)-, -CH(2)-C(6)H(4)-CH(2)-; edt, pdt, and o-xyldt, respectively) has been examined for specific characteristics that might relate to structural similarity with the active site of Fe-only hydrogenases. Variable-temperature proton NMR studies display the fluxionality of the iron-dithiocyclohexane unit in (mu-pdt)Fe(2)(CO)(6) while in the (mu-o-xyldt)Fe(2)(CO)(6) compound, the bridge is fixed. Temperature-dependent (13)C NMR spectral studies establish intramolecular CO site exchange localized on discrete Fe(CO)(3) units in all complexes, which is influenced by steric effects of the mu-SRS unit. Kinetic studies of intermolecular CO/CN(-) ligand-exchange reactions establish associative or I(a) mechanisms in sequential steps to form the dicyano dianion, (mu-SRS)[Fe(CO)(2)(CN)](2)(=) with 100% selectivity. Theoretical calculations (DFT) of transition states in the intramolecular site-exchange processes lead to a rationale for the interesting cooperativity in the CN(-)/CO intermolecular ligand-exchange process. The hinge motion of the three light atom S-to-S bridge is related to a possible heterolytic H(2) activation/production process in the enzyme.

Structural distortions in mer-M(H)3(NO)L2 (M = Ru, Os) and their influence on intramolecular fluxionality and quantum exchange coupling.

Molecules of the type mer-M(H)3(NO)L2 [M = Ru (1), Os (2); L = PR3] are characterized on the basis of 1H NMR T1min values and IR spectra as pseudo-octahedral trihydrides significantly distorted by compression of the cis H-M-H angles to approximately 75 degrees. The distortion, uncharacteristic of six-coordinate d6 complexes, is rationalized with DFT (B3LYP) calculations as being driven by increased H-to-M sigma donation and by the exceptional pi-accepting ability of linear NO+. In both 1 and 2, hydrides undergo intramolecular site exchange with delta HHH++(1) = 10-11 kcal/mol and delta HHH++(2) = 16-20 kcal/mol, depending on L, whereas for mer-Ru(H)3(NO)(PtBu2Me)2 (1b), moderate exchange couplings (up to 77 Hz) are featured in the low-temperature 1H NMR spectra, in addition to chemical exchange. On the basis of experimental and theoretical results, a dihydrogen intermediate is suggested to mediate hydride site exchange in 1. The cis H-M-H distortion shortens the tunneling path for the exchanging hydrides in 1, thereby increasing the tunneling rate; diminishes the "conflict" between trans hydrides in the mer geometry; and decreases the nucleophilicity of the hydrides. The generality of the observed structural distortion and its dependence on the ligand environment in late transition metal tri- and dihydrides are discussed. A less reducing metal center is generally characterized by greater distortion.

Solution equilibrium between classical and nonclassical polyhydride tautomers [ReH4(CO)L3]+ and [ReH2(.eta.2-H2)(CO)L3]+ (L = PMe2Ph). Equilibrium isotope effects and an intermediate trihydrogen complex in intramolecular site exchange of dihydrogen and hydride ligands

Treatment of ReCl 3 (CO)(PMe 2 Ph) 3 with LiAlH 4 in refluxing Et 2 O gives ReH 3 (CO)(PMe 2 Ph) 3 (1). Protonation of 1 with HBF 4 .OEt 2 in CD 2 Cl 2 at -78 o C results in an equilibrium mixture of [ReH 4 ()(PMe 2 Ph) 3 ] + (2a) and its nonclassical tautomer [ReH 2 (η 2 -H 2 )(CO)(PMe 2 Ph) 3 ] + .

The origin of structural variety of alkyne complexes of d8 metals. An example of structural isomerism

The X-ray structures of three metal alkyne complexes are presented: Ir(PMe 2 Ph) 3(MeC 2Me) +, M(CO) 4(F 3CC 2CF 3) (M = Ru, Os). The iridium(I) complex adopts an unusual structure (“CS”) which differs notably from that expected for four-coordinate d 8 ML 4 square-planar complexes. The structure resembles that of a square pyramid, with essentially C s symmetry, with a short apical MP bond (2.236 vs 2.31 Å for the basal MP bonds), short MC (2.01 Å) and long CC bonds (1.306 Å), which indicate a strong metal—alkyne interaction. The ruthenium(O) and osmium(O) complexes adopt the expected pseudo-octahedral structure. The unusual CS structures, previously reported for cobalt(I) and iron(O) complexes, contrast with the square-planar structure of platinum(II) alkyne complexes. Fluxionality is present in the iridium complex, as well as in those of cobalt, and is attributed to a rapid intramolecular site exchange among inequivalent phosphorus ligands. The iridium complex reacts with additional ligands like alkynes and H 2. Extended Hückel calculations are presented to rationalize the structural and reactivity aspects of the whole family of d 8 ML 3(alkyne) species (M = Pt II, Ir I, Co I, Fe O). It is shown that the four-electron donation of an alkyne ligand is responsible for the unusual CS structure of certain of these ML 3(alkyne) species. The alkyne moieties induce a four-electron destabilization in the square-planar structure which is large for diffuse d orbitals such as those of iridium(I), cobalt(I), iron(O), but not of platinum(II). The fluxionality of the complexes is shown to be associated with an easy rotation of the alkyne about the metal—alkyne midpoint, accompanied by a breathing motion of the ML bonds of the ML 3 metal fragment. It is shown that unsymmetrical alkynes stabilize a conformation of the complex in which the ligand is turned by 90° with respect to the one presented here. The reactivity towards additional ligands is shown to be associated with the presence of a low-lying LUMO pointing away from the apical bond. Effects of substituents at the alkyne and of ligands at the metal are discussed. It is shown that it is residual four-electron destabilization in M(CO) 4(alkyne) complexes which is responsible for the lability of CO ligands and thus for the ligand exchange process.

The heteronuclear cluster chemistry of the Group 1 B metals. Part 12. Variable temperature nuclear magnetic resonance spectroscopic studies of the dynamic behaviour of the mixed-metal cluster compounds [M2Ru4(µ3-H)2(µ-L)(CO)12][M = Cu or Ag, L = Ph2P(CH2)nPPh2(n= 1–6) or cis-Ph2PCHCHPPh2; M = Cu, L = Ph2As (CH2)nPPh2(n= 1 or 2)] in solution

The free energies of activation (ΔG‡) for the intramolecular metal core rearrangements which the clusters [M2Ru4(µ3-H)2(µ-L)(CO)12][M = Cu or Ag, L = Ph2P(CH2)nPPh2(n= 1–6); M = Cu, L = Ph2 As (CH2)nPPh2(n= 1 or 2)] undergo in solution have been calculated by bandshape analysis of variable temperature 31P-{1H} or 1H n.m.r. spectra or estimated from the coalescence temperature in 31P-{1H} n.m.r. spectra. Changes in the nature of the bidentate ligands attached to the coinage metals cause relatively large alterations of up to ca. 13 and ca. 1O kJ mol–1 in the values of ΔG‡ for the copper- and silver-containing species, respectively. The formal replacement of two PPh3 groups in [Ag2Ru4(µ3-H)2(CO)12(PPh3)2] by a Ph2PCH2PPh2 or Ph2P(CH2)2PPh2 ligand reduces the value of ΔG‡ by ca. 8 and ca. 4 kJ mol–1, respectively. These decreases, together with the previously reported distortions in metal framework geometry from capped trigonal bipyramidal to or towards capped square-based pyramidal, which occur for the same formal change of ligands in the analogous gold-containing species, provide some indirect evidence to support a restricted Berry pseudo-rotation mechanism for the Group 1 B metal site-exchange process in the clusters studied. The new heteronuclear cluster compounds [M2Ru4H2(µ-cis- Ph2PCHCHPPh2)(CO)12][M = Cu (2O), Ag (21), or Au (22)] have been synthesized and the effect of the cis-Ph2PCHCHPPh2 ligand on the value of ΔG‡ for the metal core rearrangement of (21) and on the skeletal geometry adopted by (22) is also consistent with the restricted Berry pseudorotation mechanism. The magnitude of ΔG‡ for the silver-containing species (21) is ca. 7 kJ mol–1 lower than that for the analogous PPh3-ligated cluster and the metal framework geometry of the gold-containing compound (22) is altered from capped trigonal bipyramidal to capped squarebased pyramidal when two PPh3 groups are formally replaced by the cis-Ph2PCHCHPPh2 ligand. In addition, variable temperature 1H and 31P-{1H} n.m.r. spectroscopic studies show that the hydrido ligands in the copper and silver clusters containing Ph2E(CH2)nPPh2(E = P, n= 1–6; E = As, n= 1 or 2) are rendered equivalent by dynamic behaviour which involves conformational changes in the methylene backbones of the bidentate ligands and that the cis-Ph2PCHCHPPh2 ligand in (21) undergoes a novel fluxional process involving intramolecular site exchange of the two phosphorus atoms between the two silver atoms.

The heteronuclear cluster chemistry of the Group 1B metals. Part 2. Synthesis, X-ray crystal structures, and dynamic behaviour of the bimetallic hexanuclear Group 1B metal cluster compounds [M2Ru4H2(CO)12(PPh3)2](M = Cu, Ag, or Au)

Treatment of dichloromethane solutions of the salt [N(PPh3)2]2[Ru4(µ-H)2(CO)12] with two equivalents of the complex [M(NCMe)4]PF6(M = Cu or Ag) at –30°C, followed by the addition of two equivalents of PPh3, affords the mixed-metal cluster compounds [M2Ru4(µ3-H)2(CO)12(PPh3)2][M = Cu (1) or Ag(2)] in ca. 75% yield. The analogous gold species [Au2Ru4(µ3-H)(µ-H)(CO)12(PPh3)2](5)(ca. 60% yield) and reduced yields (ca. 50%) of (1) and (2) can be obtained by treating acetone solutions of the salt [N(PPh3)2]2[Ru4(µ-H)2(CO)12] with a dichloromethane solution containing two equivalents of the appropriate complex [MX(PPh3)](M = Cu or Au, X = Cl; M = Ag, X = I), in the presence of TlPF6. Single-crystal X-ray diffraction studies on each member of this series of Group 1B metal congeners show that all three clusters adopt the same metal core structure. Each metal skeleton consists of a tetrahedron of ruthenium atoms capped by a M(PPh3)(M = Cu, Ag, or Au) moiety, with one of the MRu2 faces of the MRu3 tetrahedron so formed further capped by a second M(PPh3) fragment to give a capped trigonal bipyramidal metal core geometry. For (1) and (2), the other two faces of the MRu3 tetrahedron are each capped by triply bridging hydrido ligands but, in the case of (5), one of these hydrido ligands bridges a ruthenium–ruthenium edge only. In all three clusters, each ruthenium atom is ligated by three terminal carbonyl groups. Variable-temperature n.m.r. studies show that, at ambient temperature in solution, the coinage metals in all three clusters are exchanging between the two distinct sites in the capped trigonal bipyramidal metal cores, the CO ligands all exhibit dynamic behaviour involving complete intramolecular site exchange and, in addition, the PPh3 groups of (2) are undergoing intermolecular exchange between clusters.

The heteronuclear cluster chemistry of the group 1B metals. Part 4. Synthesis, structures, and dynamic behaviour of group 1B metal–ruthenium cluster compounds containing sulphur ligands. X-Ray crystal structures of [M2Ru3(µ3-S)(µ-Ph2PCH2PPh2)(CO)9](M = Cu or Au)

Treatment of a tetrahydrofuran solution of the salt K2[Ru3(µ3-S)(CO)9] with the complex [MX(PPh3)](M =Cu or Au, X =Cl; M = Ag, X = I), in the presence of TIPF6, affords a mixture of cluster compounds, [MRu3(µ-H)(µ3-S)(CO)9(PPh3)][for M = Cu (2), Ag (3), or Au (4)], [MRu3(µ-H)-(µ3-S)(CO)8(PPh3)2][for M = Cu (5) or Ag (6)], and [M2Ru3(µ3-S)(CO)9(PPh3)2][for M = Cu (8), Ag (9), or Au (10)]. In addition, the pentanuclear clusters [M2Ru3(µ3-S)(µ-Ph2PCH2PPh2)-(CO)9][M = Cu (12) or Au (13)] can be synthesized from the reaction between K2[Ru3(µ3-S)-(CO)9] and [M2(µ-Ph2PCH2PPh2)Cl2], using TIPF6. Infrared and n.m.r. spectroscopic data imply that the tetranuclear copper- or silver-containing species (2), (3), (5), and (6) all adopt the same ‘butterfly’ MRu3 metal-core geometry as that previously established for the Au cluster (4) and that the pentanuclear Cu and Ag species (8) and (9) exhibit the same trigonal-bipyramidal M2Ru3 framework as that determined for the Au cluster (10). However, X-ray diffraction studies on the bidentate phosphine-containing pentanuclear Cu2Ru3 species (12) and its Au2Ru3 analogue (13) reveal two different metal-corestructures. The Cu cluster (12) has a trigonal-bipyramidal metal skeleton [Cu–Cu 2.515(3), Cu–Ru 2.552(2)–2.794(2), Ru–Ru 2.814(2)–2.856(2)A], with one of the two phosphine-bridged Cu atoms occupying an equatorial site and the other an axial site. The S atom caps the Ru3 face of the trigonal-bipyramidal unit. In the Au species (13), the metal framework structure is intermediate between a trigonal bipyramid and a square-based pyramid [Au–Au 2.802(1), Au–Ru 2.742(l)–2.836(1), Ru–Ru 2.773(2)–2.968(2)A]. Variable-temperature n.m.r. studies show that, at ambient temperature in solution, the coinage metals in each of the pentanuclear clusters (8)–(10), (12), and (13) are exchanging between the two distinct sites in the cluster skeleton, the PPh3 groups in the copper–ruthenium species (2) and the silver–ruthenium species (3), (6), and (9) are undergoing intermolecular exchange between clusters, and the CO ligands in all of the clusters exhibit dynamic behaviour involving intramolecular site-exchange.

Intramolecular Carbonyl Group Site Exchange in Symmetrically Substituted (n‐1, 3‐Diene)tricarbonyliron Complexes Short Communication

AbstractActivation parameters have been determined for the intramolecular site exchange of the carbonyl groups in tricarbonyliron complexes of 1,4‐ and 2,3‐disubstituted 1,3‐butadienes, of 1,3‐cycloalkadienes and of 1,2‐dimethylidene‐cycloalkanes by temperature‐dependent 13C‐NMR. spectroscopy (see Table 1). The variations of the activation parameters are discussed in terms of a turnstile‐type mechanism for the exchange process and of the differences in the ground state and transition state stability of the complexes. A VB‐model of the complexes is used to explain the difference in the stabilities.

Proton magnetic resonance study of inter- and intra-molecular site exchange on the dioxopentakis(tetramethylurea) uranium(VI) ion

The species [UO2(tmu)5]2+ is shown from 1H n.m.r. studies to be the predominant dioxuranium (VI) species existing in tetramethylurea (tmu) solutions in CD2Cl2 and CD3CN solvents. The kinetic parameters characterizing rotation about the carbonyl carbon–nitrogen bond in the tmu ligand are k(195 K)= 126 ± 7 s–1, ΔH‡= 29.0 ± 0.5 kJ mol–1, and ΔS‡=–52 ± 3 J K–1 mol–1. The rate law for tmu exchange is rate = 5kex.[UO2(tmu)52+] over a 59.3- and 23.7-fold variation of [tmu] in CD2Cl2 and CD3CN solvents respectively, consistent with a dissociative exchange mechanism. The kinetic parameters in the two solvents show no significant differences, a typical data set being kex.(273 K)= 73 ± 7 s–1, ΔH‡= 80 ± 2 kJ mol–1, and ΔS‡= 85 ± 7 J K–1 mol–1 in a solution in which [UO2(tmu)52+], [tmu], and [CD2Cl2]= 0.130, 0.404, and 12.91 mol dm–3 respectively. In general, the kinetic parameters for tmu exchange are consistent with the surface charge density of the uranium(VI) centre seen by the equatorial ligands being between that on aluminium(III) and the bivalent first-row transition-metal ions.

Inter- and intramolecular ligand exchange reactions of ruthenium(II) carbonyl porphine complexes with nitrogen bases

To investigate both inter- and intramolecular site exchange in coordinated nitrogenous bases the complexes of tetra( p-isopropylphenyl)porphinato ruthenium carbonyl with4-tert-butylpyridine, 2-methylpyridine, 3,5-dimethylpyrazole, 4,5-dimethylpyridazine, and 3,6-dimethylpyridazine have been studied by total line shape analysis of the variable temperature PMR spectra. Both types of exchange mechanisms have been found in these complexes. All of the complexes undergo intermolecular ligand exchange at rates ranging from ∼0.09 sec −1 (Δ G ≠ ∼ 19 kcal/mol) to 2 × 10 4 sec − (Δ G ≠ ∼ 12 kcal/mol) at 25° for the various complexes, independent of the concentration of excess ligand. Thus the rate determining step is dissociation. The two pyridazine complexes provide the first examples of intramolecular ligand exchange with coordinated nitrogenous bases. The rates at 25° for intramolecular site exchange (∼10 6 and 66 sec −1) are 20–85 times faster than the rates for intramolecular ligand exchange in the same complexes. Within experimental uncertainty there is no intramolecular ligand site exchange in the 3,5-dimethylphyrazole complex. Full kinetic and mechanistic details are discussed.