Elucidation of the Pyridine Ring-Opening Mechanism of 2,2'-Bipyridine or 1,10-Phenanthroline Ligands at Re(I) Carbonyl Complexes.

The cleavage of the C-N bonds of aromatic heterocycles, such as pyridines or quinolines, is a crucial step in the hydrodenitrogenation (HDN) industrial processes of fuels in order to minimize the emission of nitrogen oxides into the atmosphere. Due to the harsh conditions under which these reactions take place (high temperature and H2 pressure), the mechanism by which they occur is only partially understood, and any study at the molecular level that reveals new mechanistic possibilities in this area is of great interest. Herein, we unravel the pyridine ring-opening mechanism of 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen) ligands coordinated to the cis-{Re(CO)2(N-RIm)(PMe3)} (N-RIm= N-alkylimidazole) fragment under mild conditions. Computational calculations show that deprotonation of the pyridine ring, once dearomatized, is crucial to induce ring contraction, triggering extrusion of the nitrogen atom from the ring and cleavage of the C-N bond. It is noteworthy that different products (regioisomers) are obtained depending on whether the ligand used is bipy or phen due to the additional rigidity and stability conferred by the central ring of the phen ligand, an issue also addressed and clarified computationally. Strong support for the proposed mechanism is provided by the characterization and isolation, including three single-crystal X-ray diffraction structures, of several of the proposed reaction intermediates.

Strongly Electron-Donating Triazolylidene Ligands: Cationic Metal Carbonyl Complexes of 1-Methyl-1,2,3-triazole as Triazolium Surrogates.

A new strategy for tuning the electronic properties of 1,2,3-triazol-5-ylidene metal complexes is reported using {Mo(η3-C4H7)(bipy)(CO)2} or {Re(bipy)(CO)3} fragments as substituents at the triazole N3 atom. The reaction of cationic molybdenum(II) and rhenium(I) 1-methyl-1,2,3-triazole compounds with the strong base KN(SiMe3)2 in the presence of electrophilic metal fragments, such as AgOTf (OTf = trifluoromethanesufonate) or [CuCl(IPr)] [IPr = 2,6-(diisopropyl)phenylimidazol-2-ylidene] affords a new type of 1,2,3-triazol-5-ylidene complexes. For silver(I) cationic bis(triazolylidene), [Ag(tzNHCM)2]OTf (M = [Mo], 2; [Re], 4), complexes are obtained, whereas in the case of Cu(I) mixed normal/mesoionic NHC, [Cu(IPr)(tzNHCM)]OTf (M = [Mo], 7; [Re], 8) complexes are formed. This special type of mesoionic N-heterocyclic carbenes bear a metal fragment at the N3 atom of the 1,2,3-triazole moiety, showing notable enhancement of the carbene electron donor ability compared to conventional alkyl-substituted analogues. Transmetalation from cationic silver bis(triazolylidene) complexes 2 and 4, prepared using this methodology, has proven to be very efficient toward [M'Cl(cod)]2 (M' = Rh, Ir; cod = 1,5-cyclooctadiene), affording the corresponding cationic bis(triazolylidene) [M'(cod)(tzNHCM)2]OTf (9-12) complexes. A subsequent reaction with CO(g) easily produces substitution of the diene ligand, affording the corresponding cis-dicarbonyl [M'(CO)2(tzNHCM)2]OTf (13-16) compounds.

Building C(sp3 ) Molecular Complexity on 2,2'-Bipyridine and 1,10-Phenanthroline in Rhenium Tricarbonyl Complexes.

The reactions of [Re(N-N)(CO)3 (PMe3 )]OTf (N-N=2,2'-bipyridine, bipy; 1,10-phenanthroline, phen) compounds with tBuLi and with LiHBEt3 have been explored. Addition to the N-N chelate took place with different site-selectivity depending on both chelate and nucleophile. Thus, with tBuLi, an unprecedented addition to C5 of bipy, a regiochemistry not accessible for free bipy, was obtained, whereas coordinated phen underwent tBuLi addition to C2 and C4. Remarkably, when LiHBEt3 reacted with [Re(bipy)(CO)3 (PMe3 )]OTf, hydride addition to the 4 and 6 positions of bipy triggered an intermolecular cyclodimerization of two dearomatized pyridyl rings. In contrast, hydride addition to the phen analog resulted in partial reduction of one pyridine ring. The resulting neutral ReI products showed a varied reactivity with HOTf and with MeOTf to yield cationic complexes. These strategies rendered access to ReI complexes containing bipy- and phen-derived chelates with several C(sp3 ) centers.

Influence of the Nucleophilic Ligand on the Reactivity of Carbonyl Rhenium(I) Complexes towards Methyl Propiolate: A Computational Chemistry Perspective.

A comparative theoretical study on the reactivity of the complexes [ReY(CO)3(bipy)] (Y = NH2, NHMe, NHpTol, OH, OMe, OPh, PH2, PHMe, PMe2, PHPh, PPh2, PMePh, SH, SMe, SPh; bipy = 2,2'-bipyridine) towards methyl propiolate was carried out to analyze the influence of both the heteroatom (N, O, P, S) and the alkyl and/or aryl substituents of the Y ligand on the nature of the product obtained. The methyl substituent tends to accelerate the reactions. However, an aromatic ring bonded to N and O makes the reaction more difficult, whereas its linkage to P and S favour it. On the whole, ligands with O and S heteroatoms seem to disfavour these processes more than ligands with N and P heteroatoms, respectively. Phosphido and thiolato ligands tend to yield a coupling product with the bipy ligand, which is not the general case for hydroxo, alcoxo or amido ligands. When the Y ligand has an O/N and an H atom the most likely product is the one containing a coupling with the carbonyl ligand, which is not always obtained when Y contains P/S. Only for OMe and OPh, the product resulting from formal insertion into the Re-Y bond is the preferred.

Regiochemistry Control by Bipyridine Substituents in the Deprotonation of ReI and MoII N-Alkylimidazole Complexes.

Compounds containing N-alkylimidazoles (N-RIm) and 4,4'-disubstituted 2,2'-bipyridines (4,4'-R'2 bipy) coordinated to cationic {Mo(η3 -C4 H7 )(CO)2 } and {Re(CO)3 } fragments undergo deprotonation of the C2-H group of the N-RIm ligands in their reactions with KN(SiMe3 )2 . The resulting internal nucleophile adds either to one pyridyl ring, which becomes dearomatized and can undergo ring opening in the subsequent reaction with excess MeOTf, or to the metal center, yielding imidazol-2-yl complexes, which in turn add HOTf or MeOTf, affording N-heterocyclic carbene complexes. Which pathway is followed is dictated by the metal and the nature of the imidazole (R) and bipyridine (R') substituents. For ReI compounds, addition to pyridine is found with R'=tBu and OMe, whereas for R=Me and R'=NMe2 , imidazolyl formation is preferred. Coordination of 4,7-Cl2 -1,10-phenanthroline to MoII favors C-C coupling, in contrast to the analogous parent bipy or phenanthroline complexes, for which formation of the imidazol-2-yl complexes had been found. DFT calculations showed the theoretically expected products in each case, and following their predictions new types of products were obtained experimentally.

Intermolecular C-C Coupling between 1-Methyl-1,2,3-Triazole and 2,2'-Bipyridine or 1,10-Phenanthroline in MoII Complexes.

Unsupported 1-methyl-1,2,3-triazole has been coordinated to {Mo(η3 -methallyl)(CO)2 (N-N)} (N-N=2,2'-bipyridine, bipy; or 1,10-phenanthroline, phen) fragments, yielding cationic complexes that can be regarded as metalated triazolium salts. Their reactivity towards a strong base led to the deprotonation of the C5-H group of the triazole moiety, followed by an intermolecular nucleophilic attack to the ortho CH group of a bipy or phen ligand affording cyclic, bimetallic dearomatized C-C coupling products. The reaction of the neutral bipy derivative with an acid led to the formation of dihydropyridyl units by protonation of a CH group of the dearomatized rings, the dimeric nature of complexes being mantained upon protonation.

Interligand C-C Coupling between α-Methyl N-Heterocycles and bipy or phen at Rhenium Tricarbonyl Complexes.

Intramolecular C-C coupling between N-bonded 1,2-dimethylimidazole, 2-methyloxazoline, or 2-methylpyridine and either 2,2'-bipyridine (bipy) or 1,10-phenanthroline (phen) ligands results from α-methyl group deprotonation in the coordination sphere of Re(CO)3 fragments. The nucleophilic CH2 group generated by the deprotonation attacks the 6 (bipy) or 2 (phen) positions of the diimines, dearomatizing the involved pyridine ring and generating new asymmetric, fac-capping tridentate ligands.

Insights on the Reactivity of Terminal Phosphanido Metal Complexes toward Activated Alkynes from Theoretical Computations.

Herein we present a theoretical study on the reaction of [Re(PPh2) (CO)3(phen)] (phen = 1,10-phenanthroline) and [Re(PPh2) (CO)3(bipy)] (bipy = 2,2'-bipyridine) toward methyl propiolate. In agreement with experimental results for the phen ligand, the coupling of the substituted acetylenic carbon with the nonsubstituted ortho carbon of the phen ligand is the preferred route from both kinetic and thermodynamic viewpoints with a Gibbs energy barrier of 18.8 kcal/mol and an exoergicity of 11.1 kcal/mol. There are other two routes, the insertion of the acetylenic fragment into the P-Re bond and the coupling between the substituted acetylenic carbon and a carbonyl ligand in cis disposition, which are kinetically less favorable than the preferred route (by 2.8 and 1.9 kcal/mol, respectively). Compared with phen, the bipy ligand shows less electrophilic character and also less π electron delocalization due to the absence of the fused ring between the two pyridine rings. As a consequence, the route involving the coupling with a carbonyl ligand starts to be kinetically competitive, whereas the product of the attack to bipy is still the most stable and would be the one mainly obtained after spending enough time to reach thermal equilibrium.

Nucleophilic Additions to Coordinated 1,10-Phenanthroline: Intramolecular, Intermolecular, Reversible, and Irreversible.

KN(SiMe3 )2 reacts with [Re(CO)3 (phen)(PMe3 )]OTf via reversible addition to the phen ligand and irreversible deprotonation of the PMe3 ligand followed by intramolecular attack to phen by the deprotonated phosphane, whereas MeLi irreversibly adds to phen. The addition of MeLi has been shown to be intermolecular, unlike previously known nucleophilic additions to pyridines.

Activation of Aromatic C-C Bonds of 2,2'-Bipyridine Ligands.

4,4'-Disubstituted-2,2'-bipyridine ligands coordinated to MoII and ReI cationic fragments become dearomatized by an intramolecular nucleophilic attack from a deprotonated N-alkylimidazole ligand in cis disposition. The subsequent protonation of these neutral complexes takes place on a pyridine carbon atom rather than at nitrogen, weakening an aromatic C-C bond and affording a dihydropyridyl moiety. Computational calculations allowed for the rationalization of the formation of the experimentally obtained products over other plausible alternatives.

Deprotonation of coordinated phosphanes in a rhenium complex: C-C coupling with diimine coligands.

The reaction of fac-[Re(bipy)(CO)3(PMe3)][OTf] (bipy = 2,2'-bipyridine) with KN(SiMe3)2 affords two neutral products: cis,trans-[Re(bipy)(CO)2(CN)(PMe3)], and a thermally unstable compound, which features a new C-C bond between a P-bonded methylene group (from methyl group deprotonation) and the C6 position of bipy. The solid-state structures of more stable 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene analogs, resulting from the deprotonation of PMe3, PPhMe2, and PPh2Me ligands, are determined by X-ray diffraction.

Influence of the N-N Coligand: C-C coupling instead of formation of imidazol-2-yl complexes at {Mo(η(3)-allyl)(CO)2} fragments. Theoretical and experimental studies.

New N-methylimidazole (N-MeIm) complexes of the {Mo(η(3)-allyl)(CO)2(N-N)} fragment have been prepared, in which the N,N-bidentate chelate ligand is a 2-pyridylimine. The addition of a strong base to the new compounds deprotonates the central CH group of the imidazole ligand and subsequently forms the C-C coupling product that results from the nucleophilic attack to the imine C atom. This reactivity contrasts with that previously found for the analogous 2,2'-bipyridine compounds [Mo(η(3)-allyl)(CO)2(bipy)(N-RIm)]OTf [N-RIm = N-MeIm, N-mesitylimidazole (N-MesIm, Mes= 2,4,6-trimethylphenyl); OTf = trifluoromethanesulfonate) which afforded imidazol-2-yl complexes upon deprotonation. Density Functional Theory (DFT) computations uncover that the reactivity of the imine C atom along with its ability to delocalize electron density are responsible for the new reactivity pattern found for the kind of molybdenum complexes reported herein.

C-C coupling of N-heterocycles at the fac-Re(CO)(3) fragment: synthesis of pyridylimidazole and bipyridine ligands.

A new family of cationic rhenium tricarbonyl complexes with either two N-alkylimidazole (N-RIm) and one pyridine (Py) ligand, or two pyridine and one N-RIm ligand, [Re(CO)3 (N-RIm)(3-x) (Py)x ](+) , has been prepared. The reaction of these complexes with a strong base, followed by an oxidant, selectively afforded 2,2'-pyridylimidazole complexes as the result of intramolecular dehydrogenative CC coupling reactions. For tris(pyridine) complexes [Re(CO)3 (Py)3 ](+) the reaction pattern upon a deprotonation/oxidation sequence is maintained, which allows the generation of complexes with 2,2'-bipyridine ligands. In the particular combination of two different types of pyridine ligand in the cationic fac-Re(CO)3 complexes only the cross-coupling products with asymmetric 2,2'-bipyridine ligands were obtained; the homocoupling products were not observed.

Imidazole-nitrile or imidazole-isonitrile C-C coupling on rhenium tricarbonyl complexes.

Ligand activation: Deprotonation of the nitrile or isonitrile complexes [Re(CO)3(N-RIm)2(L)](+) (N-RIm = N-alkylimidazole; L = N≡CtBu, C≡NtBu) selectively afforded alkylidenamido or iminoacyl derivatives, respectively, in which C-C coupling has occurred. Protonation of the latter complex leads to aminocarbene products.

Intramolecular nucleophilic addition to the 2 position of coordinated 2,2'-bipyridine by a deprotonated dimethyl sulfide ligand.

Deprotonation of the dimethyl sulfide ligand in [Re(bipy)(CO)3(SMe2)][OTf] (1) by KN(SiMe3)2 afforded a mixture of two diastereomers (2M and 2m) in which a C-C bond has been formed between the S-bonded CH2 group and the 2 position of 2,2'-bipyridine. The solid-state structure of the more stable 2,6-(i)Pr-BIAN analogue could be determined by X-ray diffraction.

Re-mediated C-C coupling of pyridines and imidazoles.

Rhenium tricarbonyl complexes with three N-heterocyclic ligands (N-alkylimidazoles or pyridines) undergo deprotonation with KN(SiMe(3))(2) and then oxidation with AgOTf to afford complexes with pyridylimidazole or bipyridine bidentate ligands resulting from deprotonation, C-C coupling and rearomatization.

Imidazole to NHC rearrangements at molybdenum centers: an experimental and theoretical study.

Both manganese and rhenium complexes of the type [M(bipy)(CO)(3)(N-RIm)](+) (bipy=2,2'-bipyridine) undergo deprotonation of the central CH group of the N-alkylimidazole (N-RIm) ligand when treated with a strong base. However, the outcome of the reaction is very different for either metal. For Mn, the addition of the equimolar amount of an acid to the product of the deprotonation affords an N-heterocyclic carbene (NHC) complex, whereas for Re, once the deprotonation of the central imidazole CH group has occurred, the bipy ligand undergoes a nucleophilic attack on an ortho carbon, affording the C-C coupling product. The extension of these studies to pseudo-octahedral [Mo(η(3)-allyl)(bipy)(CO)(2)(N-RIm)](+) complexes has allowed us to isolate cationic NHC complexes (Mn(I)-type behavior), as well as their neutral imidazol-2-yl precursors. Theoretical studies of the reaction mechanisms using DFT computations were carried out on the deprotonation of [Mn(bipy)(CO)(3)(N-PhIm)](+), [Re(bipy)(CO)(3) (N-MesIm)](+), and [Mo(η(3)-C(4)H(7))(bipy)(CO)(2) (N-MesIm)](+) complexes (Mes=mesityl) at the B3LYP/6-31G(d) (LANL2DZ for Mn, Re, and Mo) level of theory. Our results explain why different products have been found experimentally for Mn, Mo, and Re complexes. For Re, the process leading to a C-C coupling product is clearly more favored than those forming an imidazol-2-yl product. In contrast, for Mn and Mo complexes, the lower stabilizing interaction between the central imidazole and ortho bipy C atoms, along with the higher lability of the ligands, make the formation of an NHC-type product kinetically more accessible, in good agreement with experimental findings.

Two different hydrogen bond donor ligands together: a selectivity improvement in organometallic {Re(CO)3} anion hosts.

Rhenium(I) compounds [Re(CO)(3)(Hdmpz)(2)(ampy)]BAr'(4) and [Re(CO)(3)(N-MeIm)(2)(ampy)]BAr'(4) (Hdmpz = 3,5-dimethylpyrazole, N-MeIm = N-methylimidazole, ampy = 2-aminopyridine or 3-aminopyridine) have been prepared stepwise as the sole reaction products in good yields. The cationic complexes feature two different types of hydrogen bond donor ligands, and their anion binding behavior has been studied both in solution and in the solid state. Compounds with 2-ampy ligands are labile in the presence of nearly all of the anions tested. The X-ray structure of the complex [Re(CO)(3)(Hdmpz)(2)(ampy)](+) (2) shows that the 2-ampy ligand is metal-coordinated through the amino group, a fact that can be responsible for its labile character. The 3-ampy derivatives (coordinated through the pyridinic nitrogen atom) are stable toward the addition of several anions and are more selective anion hosts than their tris(pyrazole) or tris(imidazole) counterparts. This selectivity is higher for compound [Re(CO)(3)(N-MeIm)(2)(MeNA)]BAr'(4) (5·BAr'(4), MeNA = N-methylnicotinamide) that features an amido moiety, which is a better hydrogen bond donor than the amino group. Some of the receptor-anion adducts have been characterized in the solid state by X-ray diffraction, showing that both types of hydrogen bond donor ligands of the cationic receptor participate in the interaction with the anion hosts. DFT calculations suggest that coordination of the ampy ligands is more favorable through the amino group only for the cationic complex 2, as a consequence of the existence of a strong intramolecular hydrogen bond. In all other cases, the pyridinic coordination is clearly favored.

Effect of the nature of the substituent in N-alkylimidazole ligands on the outcome of deprotonation: ring opening versus the formation of N-heterocyclic carbene complexes.

Complexes [Re(CO)(3)(N-RIm)(3)]OTf (N-RIm=N-alkylimidazole, OTf=trifluoromethanesulfonate; 1a-d) have been straightforwardly synthesised from [Re(OTf)(CO)(5)] and the appropriate N-alkylimidazole. The reaction of compounds 1a-d with the strong base KN(SiMe(3))(2) led to deprotonation of a central C-H group of an imidazole ligand, thus affording very highly reactive derivatives. The latter can evolve through two different pathways, depending on the nature of the substituents of the imidazole ligands. Compound 1a contains three N-MeIm ligands, and its product 2a features a C-bound imidazol-2-yl ligand. When 2a is treated with HOTf or MeOTf, rhenium N-heterocyclic carbenes (NHCs) 3a or 4a are afforded as a result of the protonation or methylation, respectively, of the non-coordinated N atom. The reaction of 2a with [AuCl(PPh(3))] led to the heterobimetallic compound 5, in which the N-heterocyclic ligand is once again N-bound to the Re atom and C-coordinated to the gold fragment. For compounds 1b-d, with at least one N-arylimidazole ligand, deprotonation led to an unprecedented reactivity pattern: the carbanion generated by the deprotonation of the C2-H group of an imidazole ligand attacks a central C-H group of a neighbouring N-RIm ligand, thus affording the product of C-C coupling and ring-opening of the imidazole moiety that has been attacked (2c, d). The new complexes featured an amido-type N atom that can be protonated or methylated, thus obtaining compounds 3c, d or 4c, d, respectively. The latter reaction forces a change in the disposition of the olefinic unit generated by the ring-opening of the N-RIm ligand from a cisoid to a transoid geometry. Theoretical calculations help to rationalise the experimental observation of ring-opening (when at least one of the substituents of the imidazole ligands is an aryl group) or tautomerisation of the N-heterocyclic ligand to afford the imidazol-2-yl product.

Organometallic complexes with terminal imidazolato ligands and their use as metalloligands.

Compounds [Re(bipy)(CO)(3)(HIm)]OTf (1) and [Mo(η(3)-C(3)H(4)-R-2)(CO)(2)(HIm)(phen)]BAr'(4) [R = Me (2a), H (2b); Ar' = 3,5-bis(trifluoromethyl)phenyl; HIm = 1H-imidazole] were prepared from 1H-imidazole and either [Re(OTf)(bipy)(CO)(3)] or [MoCl(η(3)-C(3)H(4)-R-2)(CO)(2)(phen)]. Compounds 1, 2a, and 2b were deprotonated to afford the terminal κ-N-imidazolate complexes [Re(bipy)(CO)(3)(Im)] (3) and [Mo(η(3)-C(3)H(4)-R-2)(CO)(2)(Im)(phen)] [R = Me (4a), H (4b)], which were fully characterized, including an X-ray structural determination of 3. The topological analysis of the electron density (obtained from the X-ray diffraction study) and its Laplacian were used to characterize the differences in the electron density at the five-membered ring ligand between the imidazole and imidazolate complexes 1 and 3. The reaction of complexes 3, 4a, and 4b with the appropriate organometallic complexes afforded the bimetallic imidazolate-bridged compounds [{Re(bipy)(CO)(3)}(2)(µ-Im)]OTf (5), [{Mo(η(3)-C(4)H(7))(CO)(2)(phen)}(2)(µ-Im)]OTf (6), and [{Mo(η(3)-C(3)H(5))(CO)(2)(phen)}(µ-Im){Re(phen)(CO)(3)}]OTf (7). The reaction of [Mo(η(3)-C(4)H(7))(CO)(2)(Im)(phen)] (4a) with SnClPh(3) led to the formation of the trinuclear complex [{Mo(η(3)-C(4)H(7))(CO)(2)(phen)(µ-Im)}(2){SnPh(3)}]BAr'(4) (8).