Mononuclear Carbonyl Complexes Research Articles

Dynamics and Coordination of a P2N2 Ligand - from Twisted Conformation to Chelation.

Based on their general spacial flexibility, their Lewis and Brønsted basicity, and ability to mimic second sphere effects the 1,5-diaza-3,7-diphosphacyclooctane ligand family and their complexes have regained substantial scientific interest. It was now possible to structurally analyze a recently reported member of this family with p-tolyl and t-butyl substituents on P and N, respectively, (P2 p-tolN2 tBu). Notably, the ligand crystallizes with a 'twisted' backbone. This compound is the very first of its kind to have been unambiguously characterized with regard to its chemical and molecular structure as being in this conformation. A temperature-dependent NMR study provides insight into the molecular dynamics of two isomers in solution, which are most likely also both twisted, as judged by the observed limited reactivity. Despite the in principle unfavorable conformation of the free ligand, it was successfully chelated to tungsten and molybdenum centers in mononuclear carbonyl complexes. The ligand, a derivative thereof and four new complexes were comprehensively characterized and analyzed in comparison. This includes single crystal XRD molecular structures of P2 p-tolN2 tBu and all four complexes. P2 p-tolN2 tBu, regardless of its twisted conformation, is able to coordinate to metal centers given that enough energy (heat) for a conformational change is provided.

Syntheses and crystal structures of new mononuclear and binuclear ruthenium complexes supported by salicylaldimine ligands

Treatment of Ru3(CO)12 with salicylaldimines [2-HOC6H4-CH =N–C6H4-4-R] [R = Me; Cl; Br; OMe; CF3] in refluxing toluene gave three novel binuclear ruthenium carbonyl complexes {[µ-ƞ2-2-OC6H4-CH=N-C6H4-4-R)][µ-ƞ2-2-CH2-OC6H4][µ-ƞ-NH-C6H4-4-R]}Ru2(CO)4 [R = Me (1), Cl (2), Br (3)] and three mononuclear carbonyl complexes [2-OC6H4-CH=N-C6H4-4-R][2-OC6H4-CH2NH-C6H4-4-R]Ru(CO)2 [R = Me (4), OMe (5), CF3 (6)], respectively. The structures of 1–6 were fully characterized using IR and NMR spectroscopy, elemental analysis and single-crystal X-ray diffraction. These results suggest that the substituent group on the phenyl of salicylaldimine has a significant effect on the structure of the Ru complex.

Direct conversion of methane to oxygenates on porous organic polymers supported Rh mononuclear complex catalyst under mild conditions

Converting CH4 to value-added products has been attracting more and more attention. However, most routes for CH4 catalytic conversion are performed at high temperature. Here we report a direct catalytic conversion process of CH4 on a Cu species promoted Rh mononuclear complex catalyst embedded on porous organic polymers (Rh1-Cu/POPs) under the atmosphere of CH4, CO and O2 at 150 °C in the aqueous solution. Through the combined characterization, mononuclear complex of (CH3)RhX(CO)(PPh3)2 was considered as the stay species, X denoted Cl or I. The iodine species participated the coordination of Rh mononuclear carbonyl complex and further promoted the transformation of CH3 to produce more oxygenated products, especially for the acceleration of acetic acid production. The space time yield of total oxygenated products, including CH3OH, HCOOH, HOCH2OH and CH3COOH, on iodine species promoted Rh1-Cu/POPs catalyst reached as high as 289.3 g/(kgcat ⋅h), recycled for five times without obvious decay.

Syntheses, structures and catalytic activity for Friedel-Crafts reactions of substituted indenyl rhenium carbonyl complexes

The complexes [(η5-C9H6R)Re(CO)3] [R = nBu (8), tBu (9), CH(CH2)4 (10), Ph (11), Bz (12), 4-methoxyphenyl (13), 4-chlorophenyl (14)] were synthesized by refluxing substituted indenyl ligands [C9H7R] [R = nBu (1), tBu (2), CH(CH2)4 (3), Ph (4), Bz (5), 4-methoxyphenyl (6), 4-chlorophenyl (7)], and Re2(CO)10 in decalin. The molecular structures of 9, 10, 12, and 13 were determined by X-ray diffraction analysis. These four crystals have similar molecular structures of the mononuclear carbonyl complex. In each of these molecules, Re is η5-coordinated to the five-membered ring of the indenyl group. Complexes 8–14 have catalytic activity for Friedel-Crafts reactions of aromatic compounds with a variety of alkylation and acylation reagents. Compared with traditional catalysts, these mononuclear metal carbonyl complexes have obvious advantages such as high activities, mild reaction conditions, high selectivity, and environmentally friendly chemistry.

Synthesis and catalytic activity of rhenium carbonyl complexes containing alkyl-substituted tetramethylcyclopentadienyl ligands

A series of six alkyl-substituted tetramethylcyclopentadienyl mononuclear metal carbonyl complexes [(η 5-C5Me4R)Re(CO)3] [R = allyl (1), i-Pr (2), n-butyl (3), t-butyl (4), benzyl (5), CH(CH2)4 (6)] have been synthesized by treating the corresponding ligands (C5Me4R) [R = allyl, i-Pr, n-butyl, t-butyl, benzyl, CH(CH2)4] with Re2(CO)10 in refluxing xylene. The six new complexes were characterized by elemental analysis, IR, 1H NMR and 13C NMR spectroscopy. The crystal structures of all six complexes were determined by X-ray crystal diffraction analysis, showing that they have similar molecular structures, being mononuclear carbonyl complexes. In each of these complexes, the Re atom is η 5 -coordinated to the cyclopentadienyl ring. Complexes 1–5 have significant catalytic activity in Friedel–Crafts reactions of aromatic compounds with alkylation reagents. Compared with traditional catalysts, these mononuclear rhenium carbonyl complexes have obvious advantages such as lower amounts of catalyst, mild reaction conditions and environmentally friendly chemistry.

Mechanisms of the water–gas shift reaction catalyzed by carbonyl complexes M(CO)6 (M = Mo, W)

The reduction of water by CO to form CO2 and H2 (Water–Gas Shift Reaction (WGSR)) is an important process catalyzed in homogeneous phase by metal carbonyls. Here we report a theoretical study on four mechanisms of the gas-phase mononuclear carbonyl complexes M(CO)6 (M = Mo, W) catalyzed WGSR. The energetic span model (ESM) proposed by Shaik et al. has been applied to reveal the kinetic behavior of the four catalytic cycles. Our calculation results indicate that W(CO)6 is a promising candidate for an improved WGSR catalyst. An accessible reaction mechanism for the binuclear species M2(CO)10 is also proposed. Mechanistic studies on the formation of and elimination from M2(CO)9COOH− suggest a stepwise process with addition of water to the binuclear hydride species as the key step prompting elimination. Both bimetallic catalysts W2(CO)10 and Mo2(CO)10 show higher activity than mononuclear metal-based catalysts with W2(CO)10 considering as the most efficient catalyst for WGSR. To confirm whether the activity trends in WGSR can be understood in terms of the Hammer–Nørskov d-band model in combination with the linear energy relations, we studied correlations between the turnover frequency (TOF) and the position of the d-band center (εd) relative to the Fermi energy (EF) of various metal complex catalyst. The results indicate that the εd − EF value is useful as a qualitative activity descriptor in catalysis of WGSR. The conclusions will be useful to design of the WGSR catalyst with high performance.

DFT calculations of 1H and 13C NMR chemical shifts in transition metal hydrides

Transition metal hydrides are of great interest in chemistry because of their reactivity and their potential use as catalysts for hydrogenation. Among other available techniques, structural properties in transition metal (TM) complexes are often probed by NMR spectroscopy. In this paper we will show that it is possible to establish a viable methodological strategy in the context of density functional theory, that allows the determination of 1H NMR chemical shifts of hydride ligands attached to transition metal atoms in mononuclear systems and clusters with good accuracy with respect to experiment. 13C chemical shifts have also been considered in some cases. We have studied mononuclear ruthenium complexes such as Ru(L)(H)(dppm)2 with L = H or Cl, cationic complex [Ru(H)(H2O)(dppm)2]+ and Ru(H)2(dppm)(PPh3)2, in which hydride ligands are characterized by a negative 1H NMR chemical shift. For these complexes all calculations are in relatively good agreement compared to experimental data with errors not exceeding 20% except for the hydrogen atom in Ru(H)2(dppm)(PPh3)2. For this last complex, the relative error increases to 30%, probably owing to the necessity to take into account dynamical effects of phenyl groups. Carbonyl ligands are often encountered in coordination chemistry. Specific issues arise when calculating 1H or 13C NMR chemical shifts in TM carbonyl complexes. Indeed, while errors of 10 to 20% with respect to experiment are often considered good in the framework of density functional theory, this difference in the case of mononuclear carbonyl complexes culminates to 80%: results obtained with all-electron calculations are overall in very satisfactory agreement with experiment, the error in this case does not exceed 11% contrary to effective core potentials (ECPs) calculations which yield errors always larger than 20%. We conclude that for carbonyl groups the use of ECPs is not recommended, although their use could save time for very large systems, for instance in cluster chemistry. The reliance of NMR chemical shielding on dynamical effects, such as intramolecular rearrangements or trigonal twists, is also examined for H2Fe(CO)4, K+[HFe(CO)](-), HMn(CO)5 and HRe(CO)5. The accuracy of the theory is also examined for complexes with two dihydrogen ligands (Tp*RuH(H2)2 and [FeH(H2)(DMPE)2]+) and a ruthenium cluster, [H3Ru4(C6H6)4(CO)]+. It is shown that for all complexes studied in this work, the effect of the ligands on the chemical shielding of hydrogen coordinated to metal is suitably calculated, thus yielding a very good correlation between experimental chemical shifts and theoretical chemical shielding.

The synthesis and behaviour of pyrazine mononuclear carbonyl complexes of Rh(I), Ir(I), Ru(II) and Os(II)

The synthesis in high yields and the dissociative behaviour in the solid state and in solution of the mononuclear complexes [ cis-M(CO) 2Cl(pyz)] (M=Rh, Ir; pyz=pyrazine) and [ fac-M(CO) 3Cl 2(pyz)] (M=Ru, Os) are reported. The mononuclear complexes of Rh and Ir are relatively labile with respect to pyrazine release. Particularly in the case of rhodium they generate even in the solid state the corresponding dinuclear complexes [ cis-Cl(CO) 2M(pyz) cis-M(CO) 2Cl] (M=Rh, Ir). The 1H NMR spectra of these mononuclear Rh and Ir complexes in CHCl 3 solution show, at 25 and 60 °C, respectively, a fast and reversible dissociation of metal coordinated pyrazine, which is hindered by lowering the temperature. Crystallographic aspects of [ cis-Ir(CO) 2Cl(pyz)] have been investigated via single crystal X-ray diffraction. The mononuclear complexes of Ru and Os are more stable. In the solid state they do not rearrange, with release of pyrazine, to generate the related dimeric complexes with pyrazine as bridge. In solution, at room temperature, they do not dissociate quickly, although a mixture of monomeric and dimeric pyrazine complexes (ratio monomer to dimer 9:1 and 15:1 for Ru and Os, respectively) is slowly formed by a process which is reverted by addition of excess pyrazine, as expected for a dissociative equilibrium.

Deprotonation of Cp2Ti(SH)2 with Mononuclear Rhodium and Iridium Compounds: A New Route to Trinuclear Early−Late Heterobimetallic Complexes

The reaction of Cp2Ti(SH)2 with [M(acac)(diolefin)] (acac = acetylacetonate) yields the complexes [Cp(acac)Ti(μ3-S)2{M(diolefin)}2] (M = Rh, diolefin = 1,5-cyclooctadiene (cod) (1); diolefin = tetrafluorobenzobarrelene (tfbb) (2); M = Ir, diolefin = cod (3)). The formation of the trinuclear heterobimetallic complexes results from a complex reaction involving the deprotonation of the hydrosulfido ligands along with the addition of the late-metal fragment(s), followed by the release of cyclopentadiene and the coordination of acetylacetonate to the titanium center. Similarly, the reaction of Cp2Ti(SH)2 with [M(quinol)(diolefin)] (quinol = 8-oxyquinolinate) gives the complexes [Cp(quinol)Ti(μ3-S)2{M(diolefin)}2] (M = Rh, diolefin = cod (4); tfbb (5); M = Ir, diolefin = cod (6)), which incorporate the 8-oxyquinolinate ligand. The d0−d8 trinuclear early−late heterobimetallic complexes exhibit triangular TiRh2 and TiIr2 cores doubly capped by two μ3-sulfido ligands. Deprotonation of Cp2Ti(SH)2 with the mononuclear carbonyl complexes [M(acac)(CO)2] (M = Rh, Ir) yields the ion-pair complexes [Cp2Ti(acac)][M3(μ3-S)2(CO)6] (M = Rh (7); M = Ir (8)), which result from complete transference of the sulfido ligands to the d8 metal centers. Carbonylation of the diolefin complexes [Cp(acac)Ti(μ3-S)2{M(diolefin)}2] or [Cp(quinol)Ti(μ3-S)2{M(diolefin)}2] yields the compounds [Cp(acac)Ti(μ3-S)2{M(CO)2}2] (M = Rh (9); M = Ir (10)) and [Cp(quinol)Ti(μ3-S)2{M(CO)2}2] (M = Rh (11); M = Ir (12)), respectively. Replacement reactions on the carbonyl complexes with triphenylphosphine give the disubstituted complexes [Cp(acac)Ti(μ3-S)2{M(CO)(PPh3)}2] (M = Rh (13); M = Ir (14)) or [Cp(quinol)Ti(μ3-S)2{M(CO)(PPh3)}2] (M = Rh (15); M = Ir (16)), which are obtained as mixtures of cis and trans isomers. The molecular structures of complexes 1 and 8 have been determined by X-ray diffraction methods.

Cyclometallation of bis-benzimidazole derivatives with rhodium(III) halides

Treatment of rhodium(III) halides with the N-heterocycles (LH), 1,3-bis(benzimidazolyl)benzene (bBzlH 2bzH; Ia) and its N-methyl derivative (bBzlMe 2bzH; Ib) in methanol gave halobridged binuclear cyclometallated products of the composition [RhX 2L] 2 (X=Cl, Br or I). The chloro complex undergoes halobridge cleavage reactions to yield several new mononuclear complexes of the types RhCl 2(bBzlH 2bz)(AsPh 3), RhCl(bBzlH 2bz)(OClO 3)(L′/N-N) (L′=AsPh 3; N-N=2,2′-bipyridine or 1,10-phenanthroline) and the heterocycle bridged binuclear complexes of the composition [RhCl 2(bBzlH 2bz)] 2( μ-N-N) (N-N=pyrazine or 4,4′-bipyridine). Passage of CO through [RhCl 2(bBzlH 2bz)] 2 in DMF yielded mononuclear carbonyl complex RhCl 2(CO)(bBzlH 2bz)·2H 2O. Treatment of carbonylated solution of rhodium trichloride with Ia produced non-cyclometallated mononuclear complex of the type [Rh(CO) 2(bBzlH 2bzH)]Cl. The complexes are characterised by 1H, 13C NMR, IR, Far-IR, electronic and FAB-mass spectral studies.

Mono- and Dinuclear Carbonyl Complexes of (1,4,7-Trimethyl-1,4,7-triazacyclononane)rhodium(I): Facile Migration of a C(O)OMe Ligand at a Dinuclear Rh(μ-CO)2Rh Core

The reaction of 1,4,7-trimethyl-1,4,7-triazacyclononane (Cn*) with [{(CO)2Rh(μ-Cl)}2] in methanol at 0°C, followed by the addition of NH4PF6, results in the formation of the mononuclear carbonyl complex [Cn*Rh(μ-CO)2]PF6 ([1]PF6), the dinuclear carbonyl complex [Cn*Rh(μ-CO)3RhCn*](PF6)2 {[2](PF6)2} and the dinuclear methoxycarbonyl complex [Cn*Rh(C(O)OMe)(μ-CO)2RhCn*]PF6 ([3]PF6). The reaction of [2](PF6)2 with NaOMe results in formation of [3]PF6 by the attack of the methoxide at a bridging carbonyl. Treatment of [3]PF6 with NH4PF6 regenerates [2](PF6)2, thus showing that the methoxylation of [2](PF6)2 is reversible. The structures of [1]PF6, [2](PF6)2 and [3]PF6 have been determined by single-crystal X-ray diffraction. For [3]PF6 an unprecendented facile, reversible, migration of the C(O)OMe ligand between the two rhodium centres of the Rh(μ-CO)2Rh core is observed.