Ligand Rearrangement Reaction Research Articles

Internal and External Influences on Stability and Ligand Exchange Reactions in Bromido[3-ethyl-4-aryl-5-(2-methoxypyridin-5-yl)-1-propyl-1,3-dihydro-2H-imidazol-2-ylidene]gold(I) Complexes.

The ligand scrambling reaction of gold(I) complexes is a phenomenon occurring primarily in L–AuI–X (L = phosphine, N-heterocyclic carbene (NHC), and thiol; X = halide and thiol) complexes and has been observed among others for e.g., the bromido[3-ethyl-4-(4-methoxyphenyl)-5-(2-methoxypyridin-5-yl)-1-propyl-1,3-dihydro-2H-imidazol-2-ylidene]gold(I) complex (7a), which underwent ligand rearrangement in aqueous solutions. In this study, we investigated the influence of substituents on the 4-aryl ring of the related (NHC)AuIBr complexes (1a–9a) in terms of the conversion to the [(NHC)2AuI]+ (1b–9b) and [(NHC)2AuIIIBr2]+ (1c–9c) species. Furthermore, the influence of external factors such as solvent, temperature, concentration, and presence of halides (Cl–, Br–, and I–) or hydroxyl ions was studied to gain a deeper understanding of the ligand rearrangement reaction. The substituent on the 4-aryl ring has a marginal impact on the scrambling reaction. Out of the investigated organic solvents (dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol (EtOH), methanol (MeOH), and acetonitrile (ACN)), only ACN separates single complex molecules. In all other solvents, relatively stable ((NHC)AuIBr)2 dimers are present. The addition of water to ACN solutions forces the formation of such dimeric units, starting the transformation to [(NHC)2AuI]+ and [(NHC)2AuIIIBr2]+. The rate-determining step is the release of Br– from a T-shape intermediate because an excess of KBr terminates this reaction. Furthermore, it is obvious that only single molecules react with halides. The aurophilic interactions between two (NHC)AuIBr molecules are too strong in the presence of water and largely impeded reaction with halides. As a single molecule, the reaction with Cl– (e.g., in a 0.9% NaCl solution) is notable, while I– even leads to a fast and quantitative conversion to (NHC)AuII and finally to [(NHC)2AuI]+.

Reactivity of N-Phosphinoguanidines of the Formula (HNR)(Ph2PNR)C(NAr) toward Main Group Metal Alkyls: Facile Ligand Rearrangement from N-Phosphinoguanidinates to Phosphinimine-Amidinates.

We report the reactivity of N-phosphinoguanidines of the formula (HNR)(Ph2PNR)C(NAr) (R = iPr and Ar = 2,6-iPr2C6H3 [Dipp] for 1a, R = iPr and Ar = 2,4,6-Me3C6H2 [Mes] for 1b, and R = Cy and Ar = Dipp for 1c), prepared in high yields from the corresponding trisubstituted guanidines, toward main group metal alkyls AlMe3, ZnEt2, MgnBu2, and nBuLi to obtain novel phosphinoguanidinato and phosphinimine-amidinato compounds. Reactions of 1a-c with AlMe3 at room temperature led to the kinetic phosphinoguanidinato products [Al{κ2-N,N'-(NR)C(NAr)(NRPPh2)}Me2] (2a-c), whereas the mild heating (60-80 °C) of solutions of 2a-c give the thermodynamic phosphinimine-amidinato products [Al{κ2-N,N'-(NR)C(NAr)(PPh2NR)}Me2] (3a-c) after ligand rearrangement. The reactions of equimolar amounts of 1a-c and ZnEt2 initially give solutions containing unstable phosphinoguanidinato compounds [Zn{κ2-N,P-(NR)C(NAr)(NRPPh2)}Et] (4a-c), which rearrange upon mild heating to the phosphinimine-amidinato derivatives [Zn{κ2-N,N'-(NR)C(NAr)(PPh2NR)}Et] (6a-c). Bis(phosphinoguanidinato) compounds [Zn{κ2-N,P-(NR)C(NAr)(NRPPh2)}2] (5a-c) can be obtained under mild conditions (<45 °C) in THF, whereas bis(phosphinimine-amidinato) compounds [Zn{κ2-N,N'-(NR)C(NAr)(PPh2NR)}2] (7a-c) are also accessible under more forcing conditions (55-100 °C) from (i) ZnEt2 and 1b,c (2 equiv), (ii) 6a and 1a, or (iii) 5b,c. Equimolar mixtures of MgnBu2 and 1a-c in THF at room temperature give unstable phosphinimine-amidinato monoalkyl products [Mg{κ2-N,N'-(NR)C(NAr)(PPh2NR)}nBu(THF)2] (8a-c), whereas 2 equiv of 1a,b are required to reach the bischelate compounds [Mg{κ2-N,N'-(NiPr)C(NAr)(PPh2NiPr)}2] (9a,b). Finally, phosphinoguanidinato compounds [Li{κ2-N,P-(NR)C(NDipp)(NRPPh2)}(THF)2] (10a,c) were obtained in the reactions of 1a,c with nBuLi in THF under ambient conditions. The removal of the solvent from solutions of 10a,c under partial vacuum leads to the dinuclear compounds [Li2{μ-κ2-N,N':κ1-N-(NR)C(NDipp)(NRPPh2)}2(THF)2] (11a,c) after the decoordination of one of the THF molecules in 10a,c and dimerization. Heating solutions of 10a,c at 60 °C triggers ligand rearrangement to give phosphinimine-amidinato compounds [Li{κ2-N,N'-(NR)C(NDipp)(PPh2NR)}(THF)2] (12a,c). We also propose a mechanism for the ligand rearrangement reaction from 10a to give 12a, supported by DFT calculations, which fits nicely with our experimental results. It essentially involves a carbodiimide deinsertion reaction followed by a [3 + 2] cycloaddition between the resulting lithium phosphino-amide and the carbodiimide.

Nucleophile induced ligand rearrangement reactions of alkoxy- and arylsilanes

The ligand-redistribution reactions of aryl- and alkoxy-hydrosilanes can potentially cause the formation of gaseous hydrosilanes, which are flammable and pyrophoric. The ability of generic nucleophiles to initiate the ligand-redistribution reaction of commonly used hydrosilane reagents was investigated, alongside methods to hinder and halt the formation of hazardous hydrosilanes. Our results show that the ligand-redistribution reaction can be completely inhibited by common electrophiles and first-row transition metal pre-catalysts.

A Mono-, Di-, and Trivanadocene Phosphorus Oxide Series: Synthesis, Magnetism, and Chemical/Electrochemical Properties.

In this Article, we outline the synthesis of B(C6F5)3-coordinated mono-, di-, and trivanadocene phosphorus oxide complexes, Cp2VOP(OB(C6F5)3)Ph2 (2), (Cp2VO)2P(OB(C6F5)3)Ph (3), and (Cp2VO)3P(OB(C6F5)3) (4), respectively (Cp = η5-cyclopentadienyl). The complexes were synthesized from the known reagents, Cp2VF and Ph2P(O)OSiMe3 (for 2) or PhP(O)(OSiMe3)2 (for 3) or (Me3SiO)3PO (for 4), via Me3SiF elimination and in the presence of B(C6F5)3. The multimetallic complexes (3 and 4) could not be synthesized without the capping B(C6F5)3 Lewis acid, whereas the uncapped version of 2, Cp2VOP(O)Ph2 (1), has previously been reported by us. Spectroscopic and crystallographic analyses of 2-4 support an increasingly Lewis basic P═O bond upon substitution of -Ph for -OVCp2 fragments (2-4). The increased metal nuclearity also results in increasingly reducing complexes as evidenced by cyclic voltammetry (CV). Magnetic measurements (SQUID) further revealed high-spin complexes with negligible magnetic exchange between V centers. Chemical oxidation of 2 with 0.5 equiv of [Ag][B(C6F5)4] resulted in a ligand rearrangement reaction producing the VIV product, Cp2V(OP(OB(C6F5)3)Ph2)2 (7). In contrast, the oxidation of 4 with the trityl salt, [Ph3C][B(C6F5)4], resulted in the isolation of a mixed-valent VIII/VIV dimetallic species, (Cp2VO2P(OB(C6F5)3)OVCp2 (9). Both oxidations likely produce the [Cp2V][B(C6F5)4] byproduct and evidence for its formation is presented. The synthesis and characterization of the mono- and dimetallic species, Cp2VOP(OBPh3)Ph2 (8) and (Cp2VO)2P(OB(C6F5)3)OSiMe3 (5), is also reported.

Monitoring Photochemical Reaction Pathways of Tungsten Hexacarbonyl in Solution from Femtoseconds to Minutes

Metal-organic complexes are widely used across disciplines for energy and biological applications, however, their photophysical and photochemical reaction coordinates remain unclear in solution due to pertaining molecular motions on ultrafast time scales. In this study, we apply transient absorption and tunable femtosecond stimulated Raman spectroscopy (FSRS) to investigate the UV photolysis of tungsten hexacarbonyl and subsequent solvent binding events. On the macroscopic time scale with UV lamp irradiation, no equilibrated intermediate is observed from W(CO)6 to W(CO)5(solvent), corroborated by vibrational normal mode calculations. Upon 267 nm femtosecond laser irradiation, the excited-state absorption band within ∼400-500 nm exhibits distinct dynamics in methanol, tetrahydrofuran, and acetonitrile on molecular time scales. In methanol, solvation of the nascent pentacarbonyl-solvent complex occurs in ∼8 ps and in tetrahydrofuran, 13 ps which potentially involves the associative oxygen-donating ligand rearrangement reaction. In contrast, a stimulated emission feature above 480 nm emerges after ∼1 ps in acetonitrile with a nitrogen-donating ligand. These structural dynamics insights demonstrate the combined resolving power of ultrafast electronic and stimulated Raman spectroscopy to elucidate photochemistry of functional organometallic complexes in solution. The delineated reaction pathways in relation to ligand nucleophilicity and solvent reorientation time provide the rational design principles for solution precursors in nanowrite applications.

Allosteric regulation of supramolecular oligomerization and catalytic activity via coordination-based control of competitive hydrogen-bonding events.

Herein, we demonstrate that the activity of a hydrogen-bond-donating (HBD) catalyst embedded within a coordination framework can be allosterically regulated in situ by controlling oligomerization via simple changes in coordination chemistry at distal Pt(II) nodes. Using the halide-induced ligand rearrangement reaction (HILR), a heteroligated Pt(II) triple-decker complex, which contains a catalytically active diphenylene squaramide moiety and two hydrogen-bond-accepting (HBA) ester moieties, was synthesized. The HBD and HBA moieties were functionalized with hemilabile ligands of differing chelating strengths, allowing one to assemble them around Pt(II) nodes in a heteroligated fashion. Due to the hemilabile nature of the ligands, the resulting complex can be interconverted between a flexible, semiopen state and a rigid, fully closed state in situ and reversibly. FT-IR spectroscopy, (1)H DOSY, and (1)H NMR spectroscopy titration studies were used to demonstrate that, in the semiopen state, intermolecular hydrogen-bonding between the HBD and HBA moieties drives oligomerization of the complex and prevents substrate recognition by the catalyst. In the rigid, fully closed state, these interactions are prevented by steric and geometric constraints. Thus, the diphenylene squaramide moiety is able to catalyze a Friedel-Crafts reaction in the fully closed state, while the semiopen state shows no reactivity. This work demonstrates that controlling catalytic activity by regulating aggregation through supramolecular conformational changes, a common approach in Nature, can be applied to man-made catalytic frameworks that are relevant to materials synthesis, as well as the detection and amplification of small molecules.

Reactivity of Ruthenium Phosphido Species Generated through the Deprotonation of a Tripodal Phosphine Ligand and Implications for Hydrophosphination

The fragmentation of the 1,1,2-tris(diphenylphosphino)ethane ligand in [RuCp*((Ph2P)2CHCH2PPh2)][PF6] (1) was explored through treatment with base under aprotic conditions. The neutral phosphido complex RuCp*(PPh2CH═CHPPh2)(PPh2) (2) with a (Z)-1,2-bis(diphenylphosphino)ethene (dppen) ligand was generated through a base-facilitated dehydrophosphination reaction. Installation of a bis(p-tolyl)phosphido ligand was attempted by combining bis(p-tolyl)phosphine with RuCp*(dppen)Cl in the presence of KOtBu, but surprisingly, the unsymmetrical diphenylphosphido compound RuCp*(Ph2PCHCHP(p-tol)2)(PPh2) (5) was generated instead. The ligand rearrangement reaction was driven by the greater electron density on the bis(p-tolyl)phosphido moiety. Density functional theory calculations showed that fragmentation to the 1,2-disubstituted ligand was thermodynamically favored over the 1,1-disubstituted ligand and that intramolecular phosphido exchange was kinetically accessible at room temperature. The greater basicity of th...

General Strategy for the Synthesis of Rigid Weak-Link Approach Platinum(II) Complexes: Tweezers, Triple-Layer Complexes, and Macrocycles

Air-stable, heteroligated platinum(II) weak-link approach (WLA) tweezer and triple-layer complexes that possess P,X-Aryl hemilabile ligands (P^ = Ph2PCH2CH2-, X = chalcoethers or amines) have been synthesized via the halide-induced ligand rearrangement (HILR) reaction, using a one-pot, partial chloride-abstraction method. The approach is general and works with a variety of phosphine-based hemilabile ligands; when a P,S-Ph ligand is used as the relatively strongly chelating ligand, heteroligated complexes are formed cleanly when an ether- (P,O-Ph), amine- (P,N-Ph2), or fluorinated thioether-based (P,S-C6F4H) hemilabile ligand is used as the weakly chelating counterpart. The HILR reaction has also been used to synthesize bisplatinum(II) macrocycles free of oligomeric material without having to resort to the high-dilution conditions typical for macrocycle synthesis. This approach is complementary to the traditional WLA to the synthesis of macrocyclic complexes which typically proceeds via fully closed, chloride-free intermediates. The structures of the complexes may be toggled between semiopen (with only one chelating ligand) and fully closed (with both ligands chelating) via the abstraction and addition of chloride.

Heteroligated PtII Weak-Link Approach complexes using hemilabile N-heterocyclic carbene–thioether and phosphino–thioether ligands

Herein we demonstrate a stepwise synthesis of heteroligated PtII Weak-Link Approach complexes with hemilabile N-heterocyclic carbene–thioether (NHC,S) and phosphino-thioether (P,S) ligands. These complexes, with both tweezer and triple-layer geometries, can be toggled between open, semiopen, and condensed states through the abstraction or introduction of Cl−. All species were fully characterized by multinuclear NMR spectroscopy and, in many cases, by single-crystal X-ray diffraction studies. Because the condensed tweezer and triple-layer species exhibit dynamic behavior at room temperature, the tweezer complex was studied by variable temperature NMR spectroscopy, revealing two species at low temperatures thought to be diastereomers resulting from thioether inversion. The relevant thermodynamic parameters for this exchange were determined. Competition experiments were performed to probe the lability of the P,S ligand, and the ligand scrambling that occurs in WLA systems with entirely P,S ligands was not observed in these NHC,S/P,S heteroligated complexes. In contrast to the halide-induced ligand rearrangement reaction, in which heteroligated complexes form when the electron donating abilities of the “weak links” are different, the stepwise assembly strategy described here does not require different electron-donating abilities of the “weak links”, as it makes use of a non-labile N-heterocyclic carbene–metal interaction instead.

Elucidating the Mechanism of the Halide-Induced Ligand Rearrangement Reaction

The formation of heteroligated Rh(I) complexes containing two different hemilabile phosphinoalkyl ligands, (κ(2)-Ph(2)PCH(2)CH(2)S-Aryl)(κ(1)-Ph(2)PCH(2)CH(2)O-C(6)H(5))RhCl, through a halide-induced ligand rearrangement (HILR) reaction has been studied mechanistically. The half-life of this rearrangement reaction depends heavily on the Rh(I) precursor used and the chelating ability of the phosphinoalkyl thioether (PS) ligand, while the chelating ability of the phosphinoalkyl ether (PO) ligand has less of an effect. An intermediate complex which contains two PO ligands, (nbd)(κ(1)-Ph(2)PCH(2)CH(2)O-C(6)H(5))(2)RhCl (nbd = norbornadiene), converts to (nbd)(κ(1)-Ph(2)PCH(2)CH(2)O-C(6)H(5))RhCl resulting in a free PO ligand. The free PO ligand can then react with a homoligated PS complex [(κ(2)-Ph(2)PCH(2)CH(2)S-Aryl)(2)Rh](+)Cl(-) producing the heteroligated product. The PS ligand generated during the reaction pathway can be trapped by the monoligated PO complex (nbd)(κ(1)-Ph(2)PCH(2)CH(2)O-C(6)H(5))RhCl, leading to the formation of the same heteroligated product. In this study, some of the key intermediates and reaction steps underlying the HILR reaction have been identified by variable temperature (31)P{(1)H} NMR spectroscopy and in two cases by single-crystal X-ray diffraction studies. Significantly, this work provides mechanistic insight into the HILR process, which is a key reaction used to prepare a large class of highly sophisticated three-dimensional metallosupramolecular architectures and allosteric catalysts.

Tris-phenyl substituted tris(pyrazolyl)methane: Victim of a novel rearrangement in a cobalt(II) complex

Abstract The bisfacial cobalt(II) complex [(HC(3-Phpz)2(5-Phpz))2Co][BF4]2 (1) was prepared by reaction of the tris(pyrazolyl)methane HC(3-Phpz)3 with Co(BF4)2·6H2O. A remarkable ligand rearrangement reaction occurs resulting in incorporation of the new ligand HC(3-Phpz)2(5-Phpz). Complex 1 was characterised by X-ray crystallography, mass spectrometry, IR spectroscopy and elemental analysis. DFT studies on both ligands as well as on complex 1 complement the experimental investigation.

Chelating Effect as a Driving Force for the Selective Formation of Heteroligated Pt(II) Complexes with Bidentate Phosphino-Chalcoether Ligands

The halide-induced ligand rearrangement reaction (HILR) has been employed to provide selective and exclusive in situ formation of heteroligated Rh(I), Pd(II), and Pt(II) complexes with bidentate phosphino-chalcoether ligands. To gain insights on the nature of this unique reaction, we explored this process via the stepwise addition of bidentate phosphino-chalcoether (P, X; X = S or Se) and relevant monodentate phosphine ligands with a Pt(II) metal precursor. The corresponding monoligated complexes were obtained in quantitative yields by reacting 1 equiv of a P, X bidentate ligand with Pt(II) and were fully characterized via single crystal X-ray diffraction studies and heteronuclear ((31)P, (77)Se, and (195)Pt) NMR spectroscopy in solution. These species were further reacted with a second equivalent of either a bidentate ligand or the monodentate ethyl diphenylphosphine ligand, resulting in the clean formation of the heteroligated species or, in the case of the monodentate ligand with an electron-withdrawing bidentate ligand, a mixture of products. On the basis of competitive exchange reactions between these heteroligated, homoligated, and monoligated complexes, we conclude that ligand chelation plays a crucial role in the Pt(II) HILR. The in situ preferable formation of the stable monoligated complex allows for ligand sorting to occur in these systems. In all cases where the heteroligated product results, the driving force to these species is ligand chelation.

Synthesis and Characterization of Pentaphosphino Zero-Valent Iron Complexes and Their Corresponding Iron(II)-Chloride and -Hydride Complexes

A pentaphosphino iron(II)-chloride species [(t)SiP(3)(dmpm)FeCl][Cl] (1-Cl) ((t)SiP(3) = (t)BuSi(CH(2)PMe(2))(3), dmpm = Me(2)PCH(2)PMe(2)) was prepared from [((t)SiP(3)Fe)(2)(mu-Cl)(3)][Cl] and dmpm. This species was reduced to give the corresponding iron(0) complex, (t)SiP(3)(dmpm)Fe (3), in near quantitative yield. Analogous complexes [SiP(3)(dmpe)FeCl][Cl] (2-Cl) and SiP(3)(dmpe)Fe (4) (SiP(3) = MeSi(CH(2)PMe(2))(3), dmpe = Me(2)PCH(2)CH(2)PMe(2)) were prepared in the same manner as 1 and 3 but with lower yields because of competitive ligand rearrangement reactions that gave byproduct of trans-(dmpe)(2)FeCl(2) and (dmpe)(5)Fe(2) (5). [(t)SiP(3)(dmpm)FeH][A] (6) was prepared from the reaction of 3 with weak acids (HA), and the pK(a) of 6 was established to be approximately 25. Attempts to prepare pentaphosphino-iron(0) complexes of the form SiP(3)(PR(3))(2)Fe using PPh(3) and PMe(3) resulted in cyclometalated products, SiP(3)FeH((o-C(6)H(4))PPh(2)) (7) and SiP(3)FeH(CH(2)PMe(2)) (8). Synthesis and characterization of these complexes, including crystal structures of 1-5, are reported.

Selective Formation of Heteroligated Pt(II) Complexes with Bidentate Phosphine-Thioether (P,S) and Phosphine-Selenoether (P,Se) Ligands via the Halide-Induced Ligand Rearrangement Reaction

Bidentate phosphine-selenoether (P,Se) ligands were synthesized, and their heteroligated Pt(II) complexes were made and studied. The unique "P,S/P,Se" ligand coordination to Pt(II) can be realized via the halide-induced ligand rearrangement reaction. In all cases, the exclusive formation of semi-open heteroligated complexes was achieved as shown by (31)P and (77)Se NMR spectroscopy and from single crystal X-ray diffraction studies. This is the first example of the use of (77)Se NMR spectroscopy to characterize these types of structures through direct observation of the weak-link interaction with the metal center. Heteroligated structure formation is believed to be driven by the relative electron-donating ability of the substituent groups on the seleno or thioether moieties. This effect is studied by comparing the structures of corresponding "P,SMe" and "P,SeMe" complexes bearing a hemilabile "P,SCH(2)CF(3)" group, which is less sterically demanding than "P,SPh" but is similar in terms of electron withdrawing ability.

Heteroligated supramolecular coordination complexes formed via the halide-induced ligand rearrangement reaction.

Supramolecular coordination chemistry allows researchers to synthesize higher-order structures that approach the nanoscale dimensions of small enzymes. Frequently, such structures have highly symmetric macrocyclic square or cage shapes. To build functional structures that mimic the complex recognition, catalytic, and allosteric properties of enzymes, researchers must do more than synthesize highly symmetric nanoscale structures. They must also simultaneously incorporate different functionalities into these structures and learn how to regulate their relative arrangement with respect to each other. Designing such heteroligated coordination complexes remains a significant challenge for supramolecular chemists. This Account focuses on the discovery and development of a novel supramolecular reaction known as the halide-induced ligand rearrangement (HILR) reaction. Two hemilabile ligands with different binding strengths combine with d(8) transition metal precursors that contain halide ions. The reaction spontaneously results in heteroligated complexes and is highly modular and general. Indeed, it not only can be used to prepare tweezer complexes but also allows for the rapid and quantitative formation of heteroligated macrocyclic triple-decker/step and rectangular box complexes from a variety of different ligands and transition metal ions. The relative arrangement between functional groups A and B in these structures can be regulated in situ using small ancillary ligands such as halides, CO, and nitriles. Based on this reaction, zinc- and magnesium-porphyrin moieties can be incorporated into heteroligated macrocyclic or tweezer scaffolds. These examples demonstrate the convergent and cofacial assembly of functional sites that are known to be involved in numerous processes in enzymes. They also show how the relative spatial and lateral distances of these sites can be varied, in many cases reversibly. Researchers can use such complexes to study a wide range of enzymatic processes, including catalysis, molecular recognition, electron transfer, and allosteric signal transfer.

A Highly Modular and Convergent Approach for the Synthesis of Stimulant-Responsive Heteroligated Cofacial Porphyrin Tweezer Complexes

The synthesis of new hemilabile phosphine ligands and their reaction with [Rh(COE)2Cl]2 to form dissymmetric heteroligated tweezer complexes using a halide-induced ligand rearrangement reaction are reported. These complexes can undergo reactions with small-molecule ligands and elemental anions quantitatively in situ, which serve to regulate the porphyrin-porphyrin distances and interactions within the assembly.

A Convergent Coordination Chemistry-Based Approach to Dissymmetric Macrocyclic Cofacial Porphyrin Complexes

We report a highly convergent and modular approach for the synthesis of dissymmetric cofacial porphyrin complexes, which is based upon the weak-link approach to supramolecular coordination chemistry. Specifically, we have utilized a halide-induced ligand rearrangement reaction, which is capable of providing heteroligated mixed-metal porphyrin complexes in quantitative yield. Significantly, the adoption of a coordination chemistry based approach for the synthesis of these complexes allows for facile in situ regulation of the porphyrin-porphyrin interactions through the addition of external chemical stimuli.

Ligand Rearrangement Reactions of Cr(CO)6 in Alcohol Solutions: Experiment and Theory

The ligand rearrangement reaction of Cr(CO)6 is studied in a series of alcohol solutions using ultrafast infrared spectroscopy and Brownian dynamics simulations. Excitation with 266 nm light gives Cr(CO)5 which is quickly solvated by a ligand from the bath. In alcohol solutions, solvation by an alkyl or hydroxyl site can occur; all alkyl bound complexes eventually rearrange to hydroxyl bound complexes. This rearrangement has been described using both an intermolecular (stochastic) and intramolecular (chainwalk) mechanism. Experiments alone do not allow for characterization of the mechanism, and therefore, theoretical calculations were carried out for the first time by modeling the ligand rearrangement as a diffusive walk along a potential defined by the different interaction possibilities. Experiments and simulations were carried out for Cr(CO)6 in 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutanol, 1-pentanol, 2-pentanol, 2-methylbutanol, and 3-methylbutanol. The trends in the theoretical and experimental rearrangement times are similar for all simulations carried out indicating that the two mechanisms have very similar ensemble behavior when bath effects are taken into account. The nature of the mechanism responsible for motion along the alcohol chain is not of primary importance in isolating the kinetic behavior because of the highly diffusive nature of the reaction. Future experimental and theoretical work will be directed at identifying a definitive assignment of the reaction mechanism.

Femtosecond infrared studies of ligand rearrangement reactions: silyl hydride products from Group 6 carbonyls

The ultrafast dynamics of the SiH bond activation reaction by the Group 6 d 6 organometallic compounds M(CO) 5 (M=Cr, Mo, and W) have been studied in neat tri-substituted silanes under ambient conditions. The ultrafast spectral evolutions of the CO stretching bands were monitored following UV photolysis using femtosecond pump–probe spectroscopic methods. It was found that the coordinatively unsaturated species, which is formed following CO photolysis from the parent molecule, is quickly solvated (<2 ps) via the CH bonds of the solvent. These species then rearranged to the silyl hydride product on a timescale of a few nanoseconds. These results were augmented by rearrangement studies in neat ethanol, propanol and hexanol solutions in which the initially formed metal CH complex rearranged to the metal hydroxyl complex. The mechanism of this rearrangement was discussed by comparison of the data with various models in the literature. It was found that a mechanism that is primarily dissociative in nature provided the best description of the experimental data.

Structural and Kinetic Study on the Ligand Rearrangement Reaction of 2-Aminomethylpyridine on a Di(μ-acetato)dirhodium(II) Center

Abstract Three new dirhodium(II) complexes, [Rh2(μ-O2CCH3)2(O2CCH3)(amp)2]ClO4 (1) (amp = 2-aminomethylpyridine), [Rh2(μ-O2CCH3)2(amp)2](ClO4)2 (2) and [Rh2(μ-O2CCH3)2(amp)2(py)2](PF6)2 (3) were prepared and characterized by X-ray structural analysis except for 2. The first order rate constant of the conversion of 1 to 2 was (2.5 ± 0.1) × 10−4 s−1 at 75 °C in aqueous solution (I = 0.075 (LiClO4), pH = 6.5).