Turnstile Rotation Research Articles

A rotational isomerization of diiron complexes of ferrole type involved in the catalytic hydroboration of aldehydes under mild conditions

In this work, two new diiron(I) complexes of ferrole or ferracyclopentadiene type, [Fe2(CO)6{RC2(CH2)4C2R}] (R = H, 1; CH2CH3, 2) were synthesized by reacting the [Fe2(CO)9] with diynes (1,7-octadiyne and 3,9-dodecanediyne). The ferrole-type complexes were fully characterized, wherein the absolute structure of 2 was determined by single-crystal X-ray diffraction analysis. Interestingly, a rotational isomerization of the diiron(I) complexes (1 and 2) between the sawhorse conformer and the non-sawhorse conformer is verified, originated from the switch of CO coordination. Additionally, the sawhorse conformer with the terminal CO is dominant in solution due to the tricarbonyl iron units undergo a drastic geometrical turnstile rotation, whereas the non-sawhorse conformer with a semi-bridging CO is rigid in a solid state. Particularly, the isomerism of the diiron(I) complexes of ferrole type leads to a unique catalytic activity for hydroboration of aldehydes with pinacolborane (HBpin). Notably, twelve aldehydes (3a-3l) were exampled to be converted quantitatively into the corresponding organoboronates (4a-4l) mediated by complex 2 under mild conditions. Mechanistically, the 1H NMR and FTIR spectroscopic results proved that the ferrole-type complex 2 bound with the aldehyde to activate the carbonyl of the substrate rather than to cleave the H-B bond of pinacolborane. Collectively, a plausible mechanism for the catalytic hydroboration is proposed, with the rotational isomerization of the diiron(I) complexes involved in.

Synthesis, Spectroscopy, and Structure of [FeRu(μ-dithiolate)(CN)2(CO)4]2.

The salt [K(18-crown-6)]2[Ru(CN)2(CO)3] ([K(18-crown-6)]2[1]) was generated by the reaction of Ru(C2H4)(CO)4 with [K(18-crown-6)]CN. An initial thermal reaction gives [Ru(CN)(CO)4]-, which, upon ultraviolet (UV) irradiation, reacts with a second equiv of CN-. Protonation of [1]2- gave [HRu(CN)2(CO)3]- ([H1]-), which was isolated as a single isomer with mutually trans cyanide ligands. The complex cis,cis,cis-[Ru(pdt)(CN)2(CO)2]2- ([2]2-) was prepared by the UV-induced reaction of [1]2- with propanedithiol (pdtH2). The corresponding iron complex cis,cis,cis-[Fe(pdt)(CN)2(CO)2]2- ([3]2-) was prepared similarly. The pdt complexes [2]2- and [3]2- were treated with Fe(benzylideneacetone)(CO)3 to give, respectively, [RuFe (μ-pdt)(CN)2(CO)4]2- ([5]2-) and [Fe2(μ-pdt)(CN)2(CO)4]2- ([4]2-). The pathway from [3]2- to Fe2 complex [4]2- implicates intermetallic migration of CN-. In contrast, the formation of [5]2- leaves the Ru(CN)2(CO) center intact, as confirmed by X-ray crystallography. The structure of [5]2- features a "rotated" square-pyramidal Fe(CO)2(μ-CO) site. NMR measurements indicate that the octahedral Ru site is stereochemically rigid, whereas the Fe site dynamically undergoes turnstile rotation. 57Fe Mössbauer spectral parameters are very similar for rotated [5]2- and unrotated Fe2 complex [4]2-, indicating the insensitivity of that technique to both the geometry and the oxidation state of the Fe site. According to cyclic voltammetry, [5]2- oxidizes at E1/2 ∼ -0.8 V vs Fc+/0. Electron paramagnetic resonance (EPR) measurements show that 1e- oxidation of [5]2- gives an S = 1/2 rhombic species, consistent with the formulation Ru(II)Fe(I), related to the Hox state of the [FeFe] hydrogenases. Density functional theory (DFT) studies reproduce the structure, 1H NMR shifts, and infrared (IR) spectra observed for [5]2-. Related homometallic complexes with both cyanides on a single metal are predicted to not adopt rotated structures. These data suggest that [5]2- is best described as Ru(II)Fe(0). This conclusion raises the possibility that for some reduced states of the [FeFe]-hydrogenases, the [2Fe]H site may be better described as Fe(II)Fe(0) than Fe(I)Fe(I).

Unusual Intramolecular Motion of ReH92- in K2ReH9 Crystal: Circle Dance and Three-Arm Turnstile Mechanisms Revealed by Computational Studies.

The nonahydridorhenate dianion ReH92- is a unique rhenium polyhydride complex due to its remarkably high coordination number; however, its detailed polytopal rearrangement process in either solution or crystal is so far unclear. In this work, our quantum chemical calculations have identified two previously unreported fluxional mechanisms for the ReH92- dianion in the K2ReH9 crystal: three-arm turnstile rotation and circle dance mechanism. These two polytopal rearrangements in the crystal offer an alternative interpretation to the pulse and wide-line NMR spectra (Farrar et al. J. Chem. Phys. 1969, 51, 3595). The previously postulated hindered rotation of the whole ReH92- dianion in K2ReH9 (White et al. J. Chem. Soc., Faraday Trans. 2 1972, 68, 1414) turns out to be a combination of the above-mentioned two elementary fluxional processes. In addition, our calculations have confirmed the Muetterties' D3h⇌C4v rearrangement as the intramolecular motion for the ReH92- dianion in solution.

Describing Polytopal Rearrangement Processes of Octacoordinate Structures. I. Renewed Insights into Fluxionality of the Rhenium Polyhydride Complex ReH5(PPh3)2(Pyridine)

Hydride ligands of transition metal polyhydride complexes with a high coordination number are prone to fluxionality leading to interesting structural dynamics. However, the underlying polytopal rearrangement pathways have been rarely studied. Based on quantum chemical calculations carried out in this work with density functional theory and coupled-cluster theory, two new fluxional mechanisms have been identified for the rhenium polyhydride complex ReH5(PPh3)2(pyridine) to jointly account for two consecutive coalescence events in the variable-temperature NMR spectra upon heating: lateral and basal three-arm turnstile rotation. The frequently cited pseudorotation in ReH5(PPh3)2(pyridine) (Lee et al. Inorg. Chem. 1996, 35, 695) turns out to be a three-step process including two lateral three-arm turnstile steps and one basal turnstile step in between. The new fluxional mechanisms discovered in this work may also exist in other transition metal polyhydrides.

Synthesis, structures and stereodynamic behavior of pentacoordinate (O→Si)-Chelate Difluoro(methyl)silylmethyl derivatives of amides and imides

The structures and stereodynamic behavior of (O→Si)-chelate methyldifluorosilanes in solutions were studied by multinuclear (1H, 13C, 19F, 29Si) and 2D (COSY, HETCOR 1H, 13C) NMR spectroscopy. The range of studied complexes included amides RC(O)N(R')CH2SiMeF2 [R = Ph, R' = H (1a); R = R' = Me (1b); R = Me, R' = CHMePh (1c)], lactams LCH2SiMeF2 (L = 2-oxoazepan-1-yl (2a), 2,2-dimethyl-4-oxo-2H-benzo[1,3]oxazin-3-yl (2b) or 4-methyl-2-oxoquinolin-1-yl (2c)] and the six-membered imide Im6CH2 [2,6-dioxopiperidin-1-yl (3)]. The pentacoordination of silicon atoms in all studied compounds was confirmed by NMR study in solutions and single-crystal X-ray studies of complexes 1c, 2a, 2c and 3. The temperature dependency observed for the 19F NMR signals in solution was explained by the ligand exchange in the coordination environment of silicon atoms (permutational isomerization) and used for determining the activation parameters of this process by 19F DNMR. Quantum-chemical calculations for various isomers of complexes 1c, 2a, 2c and 3 suggested the "turnstile rotation" as the most likely mechanism for the observed stereodynamic processes.

Chirality of Hypervalent Chalcogenuranes

This review presents synthetic procedures applied to the preparation of chiral (mainly optically active) hypervalent chalcogenuranes. The stereoisomerization mechanisms of hypervalent derivatives of sulfur, selenium and tellurium and their selected interconversions are also presented. Keywords: Hypervalency, chirality, sulfurane, selenurane, tellurane, Berry pseudorotation, turnstile rotation.

Diiron Azamonothiolates via Scission of Dithiadiazacyclooctanes by Iron Carbonyls.

The reaction of Fe3(CO)12 with the dithiadiazacyclooctanes [SCH2N(R)CH2]2 affords Fe2[SCH2N(Me)CH2](CO)6 (R = Me, Bn). The methyl derivative 1Me was characterized crystallographically (Fe-Fe = 2.5702(5) Å). Its low symmetry is verified by variable temperature 13C NMR spectroscopy which revealed that the turnstile rotation of the S(CH2)Fe(CO)3 and S(NMe)Fe(CO)3 centers are subject to very different energy barriers. Although 1Me resists protonation, it readily undergoes substitution by tertiary phosphines, first at the S(CH2)Fe(CO)3 center, as verified crystallographically for Fe2[SCH2N(Me)CH2](CO)5(PPh3). Substitution by the chelating diphosphine dppe (Ph2PCH2CH2PPh2) gave Fe2[SCH2N(Me)CH2](CO)4(dppe), resulting from substitution at both the S(CH2)Fe(CO)3 and S(NMe)Fe(CO)3 sites.

Mechanistic Studies on the Stereoisomerization between Two Stereoisomeric, Isolable Five-Coordinate Borylpalladium(II) Complexes Bearing a Phenylene-Bridged PSiP-Pincer Type Ligand

The mechanism of the stereoisomerization of isolable five-coordinate borylpalladium(II) complexes bearing a phenylene-bridged PSiP-pincer ligand is investigated. The trigonal-bipyramidal borylpalladium(II) complexes undergo facile stereoisomerization under heating conditions from trans(B,Si) isomers having boryl and silyl ligands at apical positions to cis(B,Si) isomers in which boryl and silyl ligands are located in equatorial and apical positions, respectively. Kinetic studies and theoretical calculations clarified that the isomerization proceeds through a turnstile rotation mechanism via a five-coordinate transition state without dissociation of a monophosphine ligand. This is a quite rare example of stereoisomerization between two stereoisomeric five-coordinate group 10 metal complexes that are stable enough to be isolated.

Stereomutation in Trigonal‐Bipyramidal Systems: A Unified Picture

Same difference: Berry pseutorotation (BPR) and Ugi turnstile rotation, which are generally treated as two distinctly different mechanisms for rearrangement of trigonal-bipyramidal structures, have been shown to be equivalent. Alternative mechanisms consist of sequences of pseudorotations proceeding in a single step.

Reaction of bis(o-phosphinophenyl)silane with M(PPh3)4 (M = Ni, Pd, Pt): synthesis and structural analysis of η2-(Si–H) metal(0) and pentacoordinate silyl metal(ii) hydride complexes of the Ni triad bearing a PSiP-pincer ligand

Reactions of bis(o-(diphenylphosphino)phenyl)methylsilane with M(PPh(3))(4) (M = Ni, Pd, Pt) were investigated. When M = Ni or Pd, synthesis and isolation of η(2)-(Si-H) complexes of mononuclear Ni(0) and Pd(0) were achieved for the first time as frozen intermediates for oxidative addition of the Si-H bond. Structural analysis by X-ray and NMR spectroscopy disclosed that their η(2)-(Si-H) structures were maintained in both solid and solution states and coordination of the Si-H bond to the metal center was relatively weak. On the other hand, reaction with a platinum(0) complex afforded two kinds of pentacoordinate silyl platinum(II) hydride complexes having a PSiP-pincer ligand, which underwent unique thermal isomerization from a square-pyramidal cis-H-Pt-Si to a trigonal-bipyramidal trans-H-Pt-Si isomer. Mechanistic investigations revealed that this isomerization proceeded via an intramolecular rearrangement process probably through a turnstile rotation.

Stereomutation of Pentavalent Compounds: Validating the Berry Pseudorotation, Redressing Ugi’s Turnstile Rotation, and Revealing the Two- and Three-Arm Turnstiles

A general reaction mechanism describes the qualitative change in chemical topology along the reaction pathway. On the basis of this principle, we present a method to characterize intramolecular substituent permutation in pentavalent compounds. A full description of the geometry around five-coordinate atoms using internal coordinates enables the analysis of the structural changes along the stereomutational intrinsic reaction coordinate. The fluxional behavior of experimentally known pentavalent phosphoranes, silicates, and transition-metal complexes has been investigated by density functional theory calculations, and three principal mechanisms have been identified: Berry pseudorotation, threefold cyclic permutation, and half-twist axial-equatorial interchange. The frequently cited turnstile rotation is shown to be equivalent to the Berry pseudorotation. In combination with graph theory, this approach provides a means to systematically investigate the stereomutation of pentavalent molecules and potentially identify hitherto-unknown mechanisms.

Pentacoordinate Organoantimony Compounds That Isomerize by Turnstile Rotation

Two stereoisomers of pentacoordinate antimony compounds (stiboranes) with a new rigid tridentate ligand were synthesized, and the isomerization was found to proceed by the turnstile rotation (TR) mechanism with the tridentate ligand as a trio and the two monodentate ligands as a pair.

Reaction of an oxaruthenacycle with DMAD. Stoichiometric transformations of 2,6-xylenol to allylic phenols and benzopyrans via sp3 C–H bond cleavage reaction

Insertion of a dimethyl acetylenedicarboxylate (DMAD) into the Ru-C bond in a cycloruthenated complex Ru[OC(6)H(3)(2-CH(2))(6-Me)-kappa(2)O,C](PMe(3))(4) () has been achieved to give a seven-membered oxaruthenacycle Ru[OC(6)H(3){2-CH(2)C(CO(2)Me)[double bond, length as m-dash]C(CO(2)Me)}(6-Me)-kappa(2)O,C](PMe(3))(3) () in 47% yield. The molecular structure of by X-ray analysis shows an agostic interaction between the ruthenium and one of the benzylic methylene protons. Complex shows fluxional behaviour in solution and the variable temperature NMR studies suggest this fluxionality to be responsible for the turnstile rotation of three PMe(3) ligands and the rotation of the alpha-methoxycarbonyl group. Heating of a toluene solution of at 100 degrees C for 2 h results in the 1,3-H shift reaction in to give a kappa(1)O,eta(3)-C,C',C'' allylic complex Ru[OC(6)H(3){2-CHC(CO(2)Me)CH(CO(2)Me)}(6-Me)-kappa(1)O,eta(3)C,C',C''](PMe(3))(3) () (80-90%), whose molecular structure is revealed by X-ray analysis. Acidolyses of and give 2-[(Z)-2',3'-bis(methoxycarbonyl)allyl]-6-methylphenol () (88%) and 2-[(Z)-2',3'-bis(methoxycarbonyl)propenyl]-6-methylphenol () (47%), respectively, and iodolyses of and produce 2,3-bis(methoxycarbonyl)-8-methyl-4H-benzopyran () (24%) and 2,3-bis(methoxycarbonyl)-8-methyl-2H-benzopyran () (48%), respectively.

Frontside versus Backside SN2 Substitution at Group 14 Atoms: Origin of Reaction Barriers and Reasons for Their Absence

We have theoretically studied the gas-phase nucleophilic substitution at group-14 atoms (S(N)2@A) in the model reactions of Cl(-)+AH(3)Cl (A=C, Si, Ge, Sn, and Pb) using relativistic density functional theory (DFT) at ZORA-OLYP/TZ2P. Firstly, we wish to explore and understand how the reaction coordinate zeta, and potential energy surfaces (PES) along zeta, vary as the center of nucleophilic attack changes from carbon to the heavier group-14 atoms. Secondly, a comparison between the more common backside reaction (S(N)2-b) and the frontside pathway (S(N)2-f) is performed. The S(N)2-b reaction is found to have a central barrier for A=C, but none for the other group-14 atoms, A=Si-Pb. Relativistic effects destabilize reactant complexes and transition species by up to 10 kcal mol(-1) (for S(N)2-f@Pb), but they do not change relative heights of barriers. We also address the nature of the transformation in the frontside S(N)2-f reactions in terms of turnstile rotation versus Berry-pseudorotation mechanism.

Synthesis, structure and dynamic stereochemistry of (O → Si)-chelate N-(trifluorosilylmethyl)-[ N-( S)-(1-phenylethyl)]acetamide and 1-(trifluorosilylmethyl)-2-oxoperhydroazepine: Retention of the O → Si coordination in the adduct with KF and 18-crown-6

The novel compounds, N-(trifluorosilylmethyl)-[ N-( S)-(1-phenylethyl)]-acetamide ( 1a) and 1-(trifluorosilylmethyl)-2-oxoperhydroazepine ( 1b) have been prepared from the corresponding NH-compounds using ClCH 2SiCl 3/Et 3N or ClCH 2SiCl 3/(Me 3Si) 2NH followed by methanolysis or hydrolysis of the reaction mixture in the presence of Lewis bases, and then BF 3 etherate. Potassium-(18-crown-6)-(2-oxoperhydroazepinomethyl)tetrafluorosilicate ( 2) was synthesized by reaction of the trifluoride ( 1b) with KF in the presence of 18-crown-6. Using 19F, 29Si NMR and X-ray diffraction techniques it was established that the silicon atom is pentacoordinate in the trifluorides ( 1a, b) and hexacoordinate in the adduct 2. Thus the internal coordination of the O → Si bond present in the trifluoride ( 1b) is retained in the adduct 2. The stereochemical non-rigidity of the trifluorides ( 1a, b) and the N-(trifluorosilylmethyl)- N-methylacetamide ( 1c) was investigated using dynamic 19F NMR spectroscopy. The activation barriers for permutational isomerization are in the range 9.5–10 kcal mol −1. Lower values of Δ G # for permutation of trifluorides ( 1a– c) compared to the monofluorides with the coordination core OSiC 3F together with small negative values for the activation entropy implies a non-dissociative mechanism. Quantum-chemical analysis suggests a mechanism involving a turnstile rotation.

Desymmetrized Diiron Azadithiolato Carbonyls: A Step Toward Modeling the Iron-Only Hydrogenases

Condensation of Fe(2)(SH)(2)(CO)(6), acetaldehyde, and (NH(4))(2)CO(3) affords the methyl-substituted azadithiolate Fe(2)[(SCHMe)(2)NH](CO)(6) (1). The complex exists mainly (~95%) as the meso diastereomer, but the d,l diastereoisomers could be detected. DFT calculations predict that the meso isomer would be 2.5 kcal/mol more stable than the d,l isomer due to conventional nonbonding interactions between the methyl groups and the ring hydrogen atoms. Crystallographic analysis of meso-1 confirms that the two methyl groups are equatorial, constraining the diferraazadithiolate bicycle to a conformation that desymmetrizes the diiron center. The lowered symmetry is confirmed by the observation of two (13)C NMR signals in the FeCO region under conditions of fast turnstile rotation at the Fe(CO)(3) groups. The pK(a) value of the amine in 1 is 7.89 (all pK(a)'s determined in MeCN solution), which is similar to a redetermined value for Fe(2)[(SCH(2))(2)NH](CO)(6) (2, pK(a) = 7.98) and only slightly less basic than the tertiary amine Fe(2)[(SCH(2))(2)NMe](CO)(6) (pK(a) = 8.14). Substitution of 1 with PMe(3) proceeded via the intermediacy of two isomers of Fe(2)[(SCHMe)(2)NH](CO)(5)(PMe(3)), affording Fe(2)[(SCHMe)(2)NH](CO)(4)(PMe(3))(2) (3). (31)P NMR spectra confirm that the two PMe(3) ligands in 3 are nonequivalent, consistent with the desymmetrizing effect of the dithiolate. The pK(a) value of the amine in 3 was found to be 11.3. Using triphenylphosphine, we prepared Fe(2)[(SCHMe)(2)NH](CO)(5)(PPh(3)) as a single regioisomer.

Structural Properties and Dynamics of Five-Coordinate Nickel(II)−π-Allyl Complexes Containing Monodentate Phosphorus Ligands

The novel ionic pentacoordinate NiII−π-allyl complexes [(η3-C3H5)Ni(PMe3)3]Y (1a−d) and [(η3-C3H5)Ni{P(OMe)3}3]Y (2a−d) (Y = SO3CF3 (a), PF6 (b), Br (c), I (d)) have been synthesized and investigated by DSC, solid-state NMR, and X-ray single-crystal and powder diffractometry. Apart from 1a, all complexes are polymorphic. The PMe3 complexes 1b−d undergo monoclinic-monoclinic phase transitions. The solid-state structures of 1a and 1b-II are rigid at low temperature but fluxional at ambient temperature, whereas those of 1c-II and 1d-II are almost rigid, while somewhat faster dynamics occur for 1c-I and 1d-I. Similarly, the structures of 2c-II and 2d-II are fully or almost fully rigid at ambient temperature. The properties of the P(OMe)3-ligated OTf and PF6 salts 2a,b are completely different. 2a crystallizes in the triclinic phase 2a-II, which transforms at −21 °C into the highly mobile, plastically crystalline mesophase 2a-I with a cubic primitive cell. 2b crystallizes from solution in the monoclinic phase 2b-III, having a rigid structure, and transforms at ambient temperature reversibly into the similarly crystalline phase 2b-II, which undergoes slow dynamics. Heating either phase to 53 °C gives rise to the plastically crystalline mesophase 2b-I. For complexes 1a−d (all phases) and the phases 2a-II and 2b-II the solid-state dynamics start with turnstile rotation of the three phosphorus ligands. For the mesophases 2a-I and 2b-I additional π-allyl ligand mobility has been established and anion mobility must also be assumed. DFT MO calculations on the cations 1 and 2 and the parent PH3 complex 3 gave low rotational barriers of up to 3.5 kcal/mol in the gas phase. Complexes 2a,b are rare examples of metallorganic plastic crystals (dynamically disordered mesophases).

Oxidative addition of silanes R3SiH to the unsaturated cluster [Os3(μ-H){μ3-Ph2PCH2PPh(C6H4)}(CO)8]: Evidence for reversible silane formation in the dynamic behaviour of [Os3(μ-H)(SiR3)(CO)9(μ-dppm)

Oxidative addition of the silanes R(3)SiH (R(3)= Ph(3), Et(3), EtMe(2)) to the unsaturated cluster [Os(3)(micro-H)[micro(3)-Ph(2)PCH(2)PPh(C(6)H(4))](CO)(8)] leads to the saturated clusters [Os(3)(micro-H)(SiR(3))(CO)(9)(micro-dppm)](SiR(3)= SiPh(3) 1, SiEt(3) 2 and SiEtMe(2)3) and the unsaturated clusters [Os(3)(micro -H)(2)(SiR(3))[micro(3)-Ph(2)PCH(2)PPh(C(6)H(4))](CO)(7)](SiR(3)= SiPh(3) 4, SiEt(3) 5 and SiEtMe(2)6). Structures are based on spectroscopic evidence and a XRD structure of [Os(3)(micro-H)(SiPh(3))(CO)(9)(micro-dppm)] 1 in which all non-CO ligands are coordinated equatorially and the hydride and the silyl groups are mutually cis. From variable-temperature (1)H NMR spectra of the SiEt(3) compound 2, exchange of the P nuclei is clearly apparent. Simultaneous migrations of the SiEt(3) group and of the hydride from one Os-Os edge to another generate a time-averaged mirror plane in the molecule. VT (1)H NMR spectra of the somewhat less bulky compound [Os(3)(micro-H)(SiMe(2)Et)(CO)(9)(micro-dppm)] 3 have been analysed. Two isomers 3a and 3b are observed with the hydride ligand located on different Os-Os edges. Synchronous migration of the hydride and SiMe(2)Et groups is faster than the observed interconversion of isomers which occurs by hydride migration alone. The synchronous motion of H and SiR(3)only occurs when these ligands are mutually cis as in the major isomer 3a and we propose that this process requires the formation of a transient silane complex of the type [Os(3)(eta(2)-HSiR(3))(CO)(9)(micro-dppm)]. Turnstile rotation within an Os(CO)(3)(eta(2)-HSiR(3)) group leads to the observed exchange within the major isomer 3a without exchange with the minor isomer. This process is not observed for the minor isomer 3b because the hydride and the silyl group are mutually trans. Protonation to give [Os(3)(micro-H)(2)(SiR(3))(CO)(9)(micro-dppm)](+) totally suppresses the dynamic behaviour because there are no edge vacancies.

Slow epimerization of stereochemically rigid diastereomers of the equatorially substituted cluster [Os3H2(mu 3-S)(CO)8((S)-PhCHMeNH2)

Turnstile rotation is suppressed in the equatorially substituted cluster [Os3(mu-H)2(mu 3-S)(CO)8((S)-PhCHMeNH2)] which was separated by HPLC into two diastereomers which do not interconvert at room temperature and epimerize only slowly at 90 degrees C.

Issues of microscopic reversibility and an isomeric intermediate in ligand substitution reactions of five-coordinate oxorhenium(V) dithiolate complexes.

Ligand substitution reactions between five-coordinate oxorhenium(V) dithiolates, [CH(3)ReO(SCH(2)C(6)H(4)S)X], or MeReO(mtp)X, and entering ligands Y have been studied; Y is a phosphine and X is a phosphine (usually) or a pyridine. Many of them occur in two distinct stages, and other two-stage reactions merge to a single kinetic term when the successive rate constants are quite different in value. An intermediate can be detected directly by electronic and NMR spectroscopy. Just for phosphines, the range of rate constants is remarkably large; in the first stage, k spans the range 10(-)(4)-10(1) L mol(-)(1) s(-)(1) at 25 degrees C in benzene; in the second, which also shows a first-order dependence on the concentration of the entering ligand, the range is 10(-)(4)-10(3) L mol(-)(1) s(-)(1). Spectroscopic evidence shows that the intermediate has the same composition as the product; the metastable form is designated as MeReO(mtp)Y. The structures of all the isolated products MeReO(mtp)Y have a single stereochemistry: Me and -SCH(2) lie in trans positions, as do Y and -SAr. This structure is believed to be reversed in the transient, Y and -SCH(2) occupying trans positions. Further support for this assignment comes from the (31)P splitting of the (1)H NMR spectrum, where additional coupling indicates unusual four-bond coupling from a W-pattern of the hydrogen and phosphorus atoms. The intermediate does not undergo an intramolecular rearrangement to the final product; instead, it reacts with a ligand of the same type in an intermolecular reaction leading to rearrangement. The activation parameters were determined for selected reactions, and the results support a mechanism with considerable associative character; DeltaS() values are ca. -125 J K(-)(1) mol(-)(1). Because ligand Y must enter the coordination sphere from the vacant coordination position trans to the Re=O group, a means must be devised for the leaving group X to gain that position. To account for the intervention of the isomer while honoring the principle of microscopic reversibility, two mechanisms are proposed. One involves a C(3) ("turnstile") rotation of a specific group of three ligands in the six-coordinate transition state. Turnstile rotation of the groups X, Me, and Y can accomplish the needed transposition; the transition state passes through an approximate trigonal prismatic configuration, giving rise to a different and less stable isomer. The alternative mechanism, which may more easily accommodate data for Y = Me(2)bpy, involves rearrangement of the common octahedral intermediate to a pentagonal pyramid. The arrangement of ligands in the intermediate, governed by their sizes, determines that isomerization accompanies product formation. Following either rearrangement, a second reaction, between MeReO(SCH(2)C(6)H(4)S)Y and Y, then ensues by the same mechanism. The second rearrangement process then generates the more stable isomer of the product. Results are also presented from a study of monomerization of the dimeric rhenium species, [MeReO(mtp)](2), with phosphines(X) of various size and basicity. The results support a mechanism with two intermediates on the pathway to MeReO(mtp)X.