Coordination chemistry of the manganese and rhenium fragments (C5H5)M(CO)2

Half-sandwich manganese complexes Cp(CO)2Mn(NHC) as redox-active organometallic fragments

Oxidation of the half-sandwich Mn^I complexes Cp(CO)₂Mn(NHC) bearing dialkyl-, arylalkyl- and diarylsubstituted N-heterocyclic carbene ligands (NHC = IMe, IMeMes, IMes) affords the corresponding stable Mn^II radical cations [Cp(CO)₂Mn(NHC)](BF₄) isolated in 92-95% yield. Systematic X-ray diffraction studies of the series of Mn^I and Mn^II NHC complexes revealed the expected characteristic structural changes upon oxidation, namely the elongation of the Mn-CO and Mn-NHC bonds as well as the diminution of the OC-Mn-CO angle. ESR spectra of [Cp(CO)₂Mn(IMes)](BF₄) in frozen solution (CH₂Cl₂/toluene 1 : 1, 70 K) allowed the identification of two conformers for this complex and their structural assignment using DFT calculations. The stability of these NHC complexes in both metal oxidation states, moderate oxidation potentials and the ease of detection of Mn^II species by a variety of spectroscopic techniques (UV-Vis, IR, paramagnetic ¹H NMR, and ESR) make these compounds promising objects for applications as redox-active organometallic fragments.

Crystal and molecular structures of some phosphane-substituted cymantrenes [(C5H4X)Mn(CO)LL'] (X = H or Cl, L = CO, L' = PPh3 or PCy3, and LL' = Ph2PCH2CH2PPh2)

UV irradiation of tetrahydrofuran solutions of [CpMn(CO)3] (Cp = π-C5H5 or π-C5H4Cl) in the presence of the phosphanes PPh3 or PCy3 (Cy = cyclohexyl) and Ph2PCH2CH2PPh2 yields the substitution products [CpMn(CO)2PR3] (R = Ph or Cy) and [CpMn(CO)(Ph2PCH2CH2PPh2)], namely, dicarbonyl(η5-cyclopentadienyl)(triphenylphosphane-κP)manganese(I), [Mn(C5H5)(C18H15P)(CO)2], 1a, dicarbonyl(η5-1-chlorocyclopentadienyl)(triphenylphosphane-κP)manganese(I), [Mn(C5H4Cl)(C18H15P)(CO)2], 1b, dicarbonyl(η5-cyclopentadienyl)(tricyclohexylphosphane-κP)manganese(I), [Mn(C5H5)(C18H33P)(CO)2], 2a, dicarbonyl(η5-1-chlorocyclopentadienyl)(tricyclohexylphosphane-κP)manganese(I), [Mn(C5H4Cl)(C18H33P)(CO)2], 2b, carbonyl(η5-cyclopentadienyl)[1,2-bis(diphenylphosphanyl)ethane-κ2P,P']manganese(I), [Mn(C5H5)(C26H24P2)(CO)], 3a, and carbonyl(η5-1-chlorocyclopentadienyl)[1,2-bis(diphenylphosphanyl)ethane-κ2P,P']manganese(I), [Mn(C5H4Cl)(C26H24P2)(CO)], 3b, The crystal structure determinations show a very small influence of the chlorine substitution and a moderate influence of the phosphane substitution on the bond lengths. The PR3 groups avoid being eclipsed with the C-Cl bonds. All the compounds employ weak C-H...O interactions for intermolecular association, which are enhanced by C-H...Cl contacts in the chlorinated products.

Enantioselective Preparation of Planar-Chiral Transition Metal Complexes by Asymmetric Olefin-Metathesis Reactions in Metal Coordination Spheres

Planar-chiral transition metal complexes are useful chiral auxiliaries in organic and organometallic chemistry, and they have been utilized as chiral ligands, chiral catalysts, or chiral building blocks, etc. Despite the importance of such planar-chiral species in asymmetric synthesis, their preparation in optically active forms is still a challenging problem. Indeed, reported examples of catalytic enantioselective synthesis of planar-chiral complexes have been rare, and this has been a developing area in this field. In this personal account, recent results from our research group on the catalytic asymmetric synthesis of various planar-chiral transition metal complexes are summarized. The asymmetric ring-closing metathesis reactions catalyzed by the well-defined molybdenum-alkylidene species are powerful methods to control the planar chirality in ferrocenes, ruthenocenes, (η⁶ -arene)chromium complexes, and (η⁵ -cyclopentadienyl)manganese(I) complexes. Application of the enantiomerically enriched complexes obtained by our methods is described as well.

The mechanistic investigations of photochemical carbonyl elimination and oxidative addition reactions of (η5-C5H5)M(CO)3, (M = Mn and Re) complexes

We used computational methods to explore the mechanisms of the photochemical decarbonylation and the Si–H bond activation reaction of the group 7 organometallic compounds, η⁵-CpM(CO)₃ (M = Mn and Re). The energies of both conical intersections and the intersystem crossings, which play a decisive role in these photo-activation reactions, are determined. Both intermediates and transition states in either the singlet or triplet states are also computed to furnish a mechanistic interpretation of the whole reaction paths. In the case of Mn, four types of reaction pathways (path I–path IV) that lead to the final insertion product are examined. The theoretical findings suggest that at the higher-energy band (295 nm) the singlet-state channel is predominant. As a result, the conical intersection mechanism (i.e., path I) prevails. However, at the lower-energy band (325 nm) the triplet-state channel occurs. In such a situation, the intersystem crossing mechanism (i.e., path IV) can successfully explain its CO-photodissociation mechanism. In the case of Re, on the other hand, the theoretical evidence reveals that only the singlet state-channel is superior. In consequence, the conical intersection mechanism (i.e., path V) can more effectively explain its photochemical decarbonylation mechanism. These theoretical analyses agree well with the available experimental observations.

The theoretical works suggest that under UV photoirradiation, η⁵-CpMn(CO)₃ follows either the conical intersection mechanism or the intersystem crossing mechanism to obtain the final oxidative addition product. However, η⁵-CpRe(CO)₃ proceeds only the singlet state channel.

Cymantrenyl-Nucleobases: Synthesis, Anticancer, Antitrypanosomal and Antimicrobial Activity Studies

The synthesis of four cymantrene-5-fluorouracil derivatives (1-4) and two cymantrene-adenine derivatives (5 and 6) is reported. All of the compounds were characterized by spectroscopic methods and the crystal structure of two derivatives (1 and 6), together with the previously described cymantrene-adenine compound C was determined by X-ray crystallography. While the compounds 1 and 6 crystallized in the triclinic P-1 space group, compound C crystallized in the monoclinic P2₁/m space group. The newly synthesized compounds 1-6 were tested together with the two previously described cymantrene derivatives B and C for their in vitro antiproliferative activity against seven cancer cell lines (MCF-7, MCF-7/DX, MDA-MB-231, SKOV-3, A549, HepG2m and U-87-MG), five bacterial strains Staphylococcus aureus (methicillin-sensitive, methicillin-resistant and vancomycin-intermediate strains), Staphylococcus epidermidis, and Escherichia coli, including clinical isolates of S. aureus and S. epidermidis, as well as against the protozoan parasite Trypanosoma brucei. The most cytotoxic compounds were derivatives 2 and C for A549 and SKOV-3 cancer cell lines, respectively, with 50% growth inhibition (IC50) values of about 7 µM. The anticancer activity of the cymantrene compounds was determined to be due to their ability to induce oxidative stress and to trigger apoptosis and autophagy in cancer cells. Three derivatives (1, 4 and 5) displayed promising antitrypanosomal activity, with GI50 values in the low micromolar range (3-4 µM). The introduction of the 5-fluorouracil moiety in 1 enhanced the trypanocidal activity when compared to the activity previously reported for the corresponding uracil derivative. The antibacterial activity of cymantrene compounds 1 and C was within the range of 8-64 µg/mL and seemed to be the result of induced cell shrinking.

Chemistry of Mangana- and Rhenatricarbadecaboranyl Tricarbonyl Complexes: Evidence for an Associative Mechanism of Ligand Substitution Involving an η6−η4 Cage-Slippage Process Analagous to η5−η3-Cyclopentadienyl Ring-Slippage

The reaction of the tricarbadecaboranyl anion, 6-Ph-nido-5,6,9-C(3)B(7)H(9)(-), with M(CO)(5)Br [M = Mn, Re] or [(eta(6)-C(10)H(8))Mn(CO)(3)(+)]BF(4)(-) yielded the half-sandwich metallatricarbadecaboranyl analogues of (eta(5)-C(5)H(5))M(CO)(3) [M = Mn, Re]. For both 1,1,1-(CO)(3)-2-Ph-closo-1,2,3,4-MC(3)B(7)H(9) [M = Mn (2) and Re (3)], the metal is eta(6)-coordinated to the puckered six-membered open face of the tricarbadecaboranyl cage. Reactions of 2 and 3 with isocyanide at room temperature produced complexes 8-(CNBu(t))-8,8,8-(CO)(3)-9-Ph-nido-8,7,9,10-MC(3)B(7)H(9) [M = Mn (4), Re (5)], having the cage eta(4)-coordinated to the metal. Photolysis of 4 and 5 then resulted in the loss of CO and the formation of 1-(CNBu(t))-1,1-(CO)(2)-2-Ph-closo-1,2,3,4-MC(3)B(7)H(9) [M = Mn, Re (6)], where the cage is again eta(6)-coordinated to the metal. Reaction of 2 and 3 with 1 equiv of phosphine at room temperature produced the eta(6)-coordinated monosubstituted complexes 1,1-(CO)(2)-1-P(CH(3))(3)-2-Ph-closo-1,2,3,4-MC(3)B(7)H(9) [M = Mn (7), Re (9)] and 1,1-(CO)(2)-1-P(C(6)H(5))(3)-2-Ph-closo-1,2,3,4-MC(3)B(7)H(9) [M = Mn (8), Re (10)]. NMR studies of these reactions at -40 degrees C showed that substitution occurs by an associative mechanism involving the initial formation of intermediates having structures similar to those of the eta(4)-complexes 4 and 5. The observed eta(6)-eta(4) cage-slippage is analogous to the eta(5)-eta(3) ring-slippage that has been proposed to take place in related substitution reactions of cyclopentadienyl-metal complexes. Reaction of 9 with an additional equivalent of P(CH(3))(3) gave 8,8-(CO)(2)-8,8-(P(CH(3))(3))(2)-9-Ph-nido-8,7,9,10-ReC(3)B(7)H(9) (11), where the cage is eta(4)-coordinated to the metal. Photolysis of 11 resulted in the loss of CO and the formation of the disubstituted eta(6)-complex 1-CO-1,1-(P(CH(3))(3))(2)-2-Ph-closo-1,2,3,4-ReC(3)B(7)H(9) (12).

Direct Crystallographic Observation of a Coordinatively Unsaturated Transition-Metal Complex in situ Generated within a Self-Assembled Cage

In situ crystallographic observation of an extremely labile coordinatively unsaturated transition-metal complex, Cp'Mn(CO)2 (Cp' = methylcyclopentadienyl), was achieved by generating and trapping it within the confined cavity of a self-assembled cage in the crystalline state at a low temperature. The labile target Cp'Mn(CO)2 was formed by the photodissociation of the CO ligand from stable Cp'Mn(CO)3, which was accommodated within the cavity of the coordination cage. Being trapped in the cage at a low temperature, Cp'Mn(CO)2 showed sufficient stability to be analyzed by X-ray crystallography. The diffraction study revealed that the 16-electron unsaturated Mn complex adopted a pyramidal geometry rather than a planar geometry.

A Favorable Case Where an Experimental Electron Density Analysis Offers a Lead for Understanding a Specific Fluxional Process Observed in Solution

A topological analysis of the electron density in the ketene complex (eta(5)-MeC(5)H(4))(CO)(2)Mn[eta(2)-O=C=C((mu-eta(2)-CCPh)Co(2)(CO)(6))Ph] indicates a predisposition for the carbene component of the ketene ligand to bind the neighboring C atom of the adjacent CO ligand.

Redox Site Confinement in Highly Unsymmetric Dimanganese Complexes

A set of highly preorganized pyrazolate-bridged dimanganese complexes L(Mn)MnX have been prepared and structurally characterized. They can be described as hybrid organometallic/Werner-type systems that consist of a low-spin CpMn(I)(CO)2 subunit (Mn1) and a proximate tripodal tetradentate {N4} binding pocket accommodating a high-spin Mn(II) ion (Mn2), with Mn...Mn distances of approximately 4.3 A and different coligands bound to Mn2. Density functional theory (DFT) calculations (both the hybrid B3LYP and the pure BP86 functionals and the all-electron basis sets 6-311G and 6-311G*) confirm that the valence alpha and beta Kohn-Sham molecular orbitals (MOs) of these mixed-valent Mn(I)Mn(II) compounds have predominant Mn(3d) character and an almost perfectly localized nature: all five unpaired electrons are essentially localized at the Werner-type Mn2, whereas Mn1 possesses an effective closed-shell structure with the MOs of highest energy centered there. One-electron oxidation occurs in a clean process at approximately E(1/2) = -0.6 V (versus ferrocene/ferrocinium), giving the low-spin/high-spin Mn(II)Mn(II) species. UV/vis and IR spectroelectrochemistry as well as a detailed theoretical analysis reveal that the redox process takes place with strict site control at the organometallic subunit, while it does not significantly influence the spin and charge distribution on the Werner-type site. Positions and shifts of the nu(C[triple bond]O) absorptions are largely reproduced by the DFT calculations. These systems thus represent an exceptional example of the effect the unsymmetry of a dinucleating ligand scaffold has on the spin and charge distribution in homobimetallic complexes and might offer interesting prospects for the study of the cooperative effects of bimetallic arrays.

Subtle reactivity patterns of non-heteroatom-substituted manganese alkynyl carbene complexes in the presence of phosphorus probes

The non-heteroatom-substituted manganese alkynyl carbene complexes (eta5-MeC5H4)(CO)2Mn=C(R)C[triple bond]CR'(3; 3a: R = R'= Ph, 3b: R = Ph, R'= Tol, 3c: R = Tol, R'= Ph) have been synthesised in high yields upon treatment of the corresponding carbyne complexes [eta5-MeC5H4)(CO)2Mn[triple bond]CR][BPh4]([2][BPh4]) with the appropriate alkynyllithium reagents LiC[triple bond]CR' (R'= Ph, Tol). The use of tetraphenylborate as counter anion associated with the cationic carbyne complexes has been decisive. The X-ray structures of (eta5-MeC5H4)(CO)2Mn=C(Tol)C[triple bond]CPh (3c), and its precursor [(eta5-MeC5H4)(CO)2Mn=CTol][BPh4]([2b](BPh4]) are reported. The reactivity of complexes toward phosphines has been investigated. In the presence of PPh3, complexes act as a Michael acceptor to afford the zwitterionic sigma-allenylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R)=C=C(PPh3)R' (5) resulting from nucleophilic attack by the phosphine on the remote alkynyl carbon atom. Complexes 5 exhibit a dynamic process in solution, which has been rationalized in terms of a fast [NMR time-scale] rotation of the allene substituents around the allene axis; metrical features within the X-ray structure of (eta5-MeC5H4)(CO)2MnC(Ph)=C=C(PPh3)Tol (5b) support the proposal. In the presence of PMe3, complexes undergo a nucleophilic attack on the carbene carbon atom to give zwitterionic sigma-propargylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R)(PMe3)C[triple bond]CR' (6). Complexes 6 readily isomerise in solution to give the sigma-allenylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R')=C=C(PMe3)R (7) through a 1,3 shift of the [(eta5-MeC5H4)(CO)2Mn] fragment. The nucleophilic attack of PPh2Me on 3 is not selective and leads to a mixture of the sigma-propargylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R)(PPh(2)Me)C[triple bond]CR' (9) and the sigma-allenylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R)=C=C(PPh(2)Me)R' (10). Like complexes 6, complexes 9 readily isomerize to give the sigma-allenylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R')=C=C(PPh2Me)R'). Upon gentle heating, complexes 7, and mixtures of 10 and 10' cyclise to give the sigma-dihydrophospholium complexes (eta5-MeC5H4)(CO)2MnC=C(R')PMe2CH2CH(R)(8), and mixtures of complexes (eta5-MeC5H4)(CO)2MnC=C(Ph)PPh2CH2CH(Tol)(11) and (eta5-MeC5H4)(CO)2MnC=C(Tol)PMe2CH2CH(Ph)(11'), respectively. The reactions of complexes 3 with secondary phosphines HPR(1)(2)(R1= Ph, Cy) give a mixture of the eta2-allene complexes (eta5-MeC5H4)(CO)2Mn[eta2-{R(1)(2)PC(R)=C=C(R')H}](12), and the regioisomeric eta4-vinylketene complexes [eta5-MeC5H4)(CO)Mn[eta4-{R(1)(2)PC(R)=CHC(R')=C=O}](13) and (eta5-MeC5H4)(CO)Mn[eta4-{R(1)(2)PC(R')=CHC(R)=C=O}](13'). The solid-state structure of (eta5-MeC5H4)(CO)2Mn[eta2-{Ph2PC(Ph)=C=C(Tol)H}](12b) and (eta5-MeC5H4)(CO)Mn[eta4-{Cy2PC(Ph)=CHC(Ph)=C=O}](13d) are reported. Finally, a mechanism that may account for the formation of the species 12, 13, and 13' is proposed.