Fluxional Process Research Articles

Bidentate and tridentate coordination modes of bis(3-methylindolyl)-2-pyridylmethane in complexes of aluminum and gallium: Structural characterization of bridging N-indolide in a dialuminum complex

Reactions of a slight excess of trialkylaluminum or tri-tert-butylgallium with bis(3-methylindolyl)-2-pyridylmethane (1) produced monomeric, tripodal complexes (2-C5H4N)HC(3-CH3C8H4N)2MR (M = Al, R = Me (2a), Et (2b), iBu (2c), tBu (2d); M = Ga, R = tBu (3)). Under more mild reaction conditions and with a stoichiometric amount of tri-tert-butylaluminum or tri-tert-butylgallium, complexes (2-C5H4N)HC(3-CH3C8H4N)(3-CH3C8H4NH)MtBu2 (M = Al (4), Ga (5)) were isolated. Whereas di-deprotonated 1 adopts a tridentate coordination mode in 2a-2d and 3, mono-deprotonated 1 adopts a bidentate coordination mode in 4 and 5. Reaction of 1 with two equivalents of trimethylaluminum or triethylaluminum yields dialuminum complexes (2-C5H4N)HC(3-CH3C8H4N)2Al2R4 (R = Me (6a), Et (6b). In addition to NMR (1H, 13C) spectroscopy, compounds 1, 2b·0.25C6H6, 4, and 6a·C7H8 were further characterized by X-ray crystallography. The solid-state structure for 6a·C7H8 confirms the presence of a bridging μ2-η1:η1-N-indolide moiety. Variable-temperature NMR spectra of toluene solutions of 6a and 6b are consistent with retention of a bridging μ2-η1:η1-N-indolide group at low temperature, which becomes fluxional in solution at higher temperatures. A mechanism for the fluxional process is proposed.

Coordinatively Unsaturated [RhCp*Rf2] (Cp* = C5Me5; Rf = C6F3Cl2-3,5), General Precursor to Cp*-Diaryl and Cp*-Halo-Aryl RhIII Complexes. Observing and Testing the Effect of Cp* as Electronic Buffer

The pentacoordinated [RhCp*Rf2] (Rf = C6F3Cl2-3,5) and the octahedral (μ-Cl)2[RhCp*Rf]2, obtained by stoichiometric rearrangement with (μ-Cl)2[RhCp*Cl]2, are general precursors of [RhCp*RfXL] (X = Rf, Cl; L = ligand) complexes, which were studied by NMR (L dissociation and fluxional processes) and X-ray diffraction (structural effects affecting the Rh–Cp* distances) techniques. The Rh–Cp*centroid distances decrease markedly for identical L in the order [RhCp*Rf2L] > [RhCp*RfClL] > [RhCp*Cl2L] and are further influenced regularly within each family by the trans influence of L (longer distances for higher trans influence of L). The structural effects observed reveal a remarkable capability of Cp* to act as an electron-density buffer, which attenuates the Rh electron density variations induced by the substituents in front of Cp* by releasing toward Rh or polarizing toward Cp*, on demand, the electron density of the Rh–Cp* bonds. This buffer effect explains the easy L dissociation from [RhCp*Rf2L] and the acc...

The missing agostomer in the fluxionality of cyclohexenylmanganese tricarbonyl

Fluxionality of the cyclohexenylmanganese tricarbonyl complex [(C6H9)Mn(CO)3], which has an Mn-H-C 3-center 2-electron agostic interaction, have been analyzed with density functional theory (DFT) computations. The fluxionality of this complex is a classic in organometallic chemistry (as highlighted by its inclusion as a study problem in the textbook by Kegley and Pinhas, Problems and Solutions in Organometallic Chemistry), and we provide new insights into its chemistry. Besides the three previously reported fluxional processes [1,3-endo fast exchange (TS-1), CO ligand equivalence (TS-2), and 1,2-metal migration via Cs η4 diene hydride species (TS-3)], two additional processes were identified: 1,2-agostic isomerization (TS-1-2), and a low-energy hydride transfer process (TS-1-9). This investigation uncovered a previously unidentified agostic interaction in the fluxionality of (C6H9)Mn(CO)3: the 1,2-agostic isomerization leads to a previously uncharacterized “closed” Cs agostomer (structure 2) with a Mn-H distance of 2.04 Å, a Mn-H-C bond angle of 104.3°, and the 1H NMR chemical shift of −4.7 ppm. Energetically-similar agostic isomers have previously been defined as “agostomers”. The variable-temperature 1H NMR spectra were also simulated. Excellent agreement is observed (R2 = 0.9949) when comparing the computed and experimental 1H NMR chemical shifts. Reasonable agreements between the computed activation energies (6.1, 12.9, and 13.8 kcal mol−1) and experimental activation energies (8.3 ± 0.3, 13.1 ± 0.1, and 15.4 ± 0.3 kcal mol−1) were found.

Fluxional Behavior of cis-Fe(DMeOPrPE)2(H)2 (DMeOPrPE = 1,2-[bis(dimethoxypropyl)phosphino]ethane); Implications for the Pressure Swing Purification of Natural Gas

Materials such as trans-Fe(DMeOPrPE)2(X)(H2O)+ (DMeOPrPE = 1,2-[bis(dimethoxypropyl)phosphino]ethane; X = Cl or H) show considerable promise as sorbents for the removal of N2 from natural gas in a pressure swing process. One problem, however, is that in long-term tests these sorbents slowly degrade over a period of many weeks. This study probed the degradation mechanism for the purpose of finding ways to stabilize the sorbents. To investigate the likelihood of degradation initiated by Fe–P bond dissociation, the fluxional behavior of the related cis-Fe(DMeOPrPE)2(H)2 complex was investigated by variable temperature 1P{1H} NMR spectroscopy. Analysis of the spectra gave the following activation parameters for the fluxional process: ΔH‡ = 11.6 kcal/mol and ΔS‡ = −9.2 cal/mol K. These results suggest that Fe–P bond dissociation (20–45 kcal/mol) is not occurring in the fluxional process and by implication that Fe–P bond dissociation is not initiating the decomposition of the trans-Fe(DMeOPrPE)2(X)(H2O)+ sorbents. Consequently, macrocycle ligands may not be the best approach for stabilizing the sorbents, as we have previously proposed. A “tetrahedral jump” mechanism is proposed for the fluxional process in cis-Fe(DMeOPrPE)2(H)2.

Ultrafast fluxional exchange dynamics in electrolyte solvation sheath of lithium ion battery

Lithium cation is the charge carrier in lithium-ion battery. Electrolyte solution in lithium-ion battery is usually based on mixed solvents consisting of polar carbonates with different aliphatic chains. Despite various experimental evidences indicating that lithium ion forms a rigid and stable solvation sheath through electrostatic interactions with polar carbonates, both the lithium solvation structure and more importantly fluctuation dynamics and functional role of carbonate solvent molecules have not been fully elucidated yet with femtosecond vibrational spectroscopic methods. Here we investigate the ultrafast carbonate solvent exchange dynamics around lithium ions in electrolyte solutions with coherent two-dimensional infrared spectroscopy and find that the time constants of the formation and dissociation of lithium-ion···carbonate complex in solvation sheaths are on a picosecond timescale. We anticipate that such ultrafast microscopic fluxional processes in lithium-solvent complexes could provide an important clue to understanding macroscopic mobility of lithium cation in lithium-ion battery on a molecular level.

Solid-state molecular organometallic chemistry. Single-crystal to single-crystal reactivity and catalysis with light hydrocarbon substrates.

Single-crystal to single-crystal solid/gas reactivity and catalysis starting from the precursor sigma-alkane complex [Rh(Cy2PCH2CH2PCy2)(η2η2-NBA)][BArF 4] (NBA = norbornane; ArF = 3,5-(CF3)2C6H3) is reported. By adding ethene, propene and 1-butene to this precursor in solid/gas reactions the resulting alkene complexes [Rh(Cy2PCH2CH2PCy2)(alkene) x ][BArF 4] are formed. The ethene (x = 2) complex, [Rh(Cy2PCH2CH2PCy2)(ethene)2][BArF 4]-Oct, has been characterized in the solid-state (single-crystal X-ray diffraction) and by solution and solid-state NMR spectroscopy. Rapid, low temperature recrystallization using solution methods results in a different crystalline modification, [Rh(Cy2PCH2CH2PCy2)(ethene)2][BArF 4]-Hex, that has a hexagonal microporous structure (P6322). The propene complex (x = 1) [Rh(Cy2PCH2CH2PCy2)(propene)][BArF 4] is characterized as having a π-bound alkene with a supporting γ-agostic Rh···H3C interaction at low temperature by single-crystal X-ray diffraction, variable temperature solution and solid-state NMR spectroscopy, as well as periodic density functional theory (DFT) calculations. A fluxional process occurs in both the solid-state and solution that is proposed to proceed via a tautomeric allyl-hydride. Gas/solid catalytic isomerization of d3-propene, H2C[double bond, length as m-dash]CHCD3, using [Rh(Cy2PCH2CH2PCy2)(η2η2-NBA)][BArF 4] scrambles the D-label into all possible positions of the propene, as shown by isotopic perturbation of equilibrium measurements for the agostic interaction. Periodic DFT calculations show a low barrier to H/D exchange (10.9 kcal mol-1, PBE-D3 level), and GIPAW chemical shift calculations guide the assignment of the experimental data. When synthesized using solution routes a bis-propene complex, [Rh(Cy2PCH2CH2PCy2)(propene)2][BArF 4], is formed. [Rh(Cy2PCH2CH2PCy2)(butene)][BArF 4] (x = 1) is characterized as having 2-butene bound as the cis-isomer and a single Rh···H3C agostic interaction. In the solid-state two low-energy fluxional processes are proposed. The first is a simple libration of the 2-butene that exchanges the agostic interaction, and the second is a butene isomerization process that proceeds via an allyl-hydride intermediate with a low computed barrier of 14.5 kcal mol-1. [Rh(Cy2PCH2CH2PCy2)(η2η2-NBA)][BArF 4] and the polymorphs of [Rh(Cy2PCH2CH2PCy2)(ethene)2][BArF 4] are shown to be effective in solid-state molecular organometallic catalysis (SMOM-Cat) for the isomerization of 1-butene to a mixture of cis- and trans-2-butene at 298 K and 1 atm, and studies suggest that catalysis is likely dominated by surface-active species. [Rh(Cy2PCH2CH2PCy2)(η2η2-NBA)][BArF 4] is also shown to catalyze the transfer dehydrogenation of butane to 2-butene at 298 K using ethene as the sacrificial acceptor.

Selective C–H Activation at a Molecular Rhodium Sigma-Alkane Complex by Solid/Gas Single-Crystal to Single-Crystal H/D Exchange

The controlled catalytic functionalization of alkanes via the activation of C-H bonds is a significant challenge. Although C-H activation by transition metal catalysts is often suggested to operate via intermediate σ-alkane complexes, such transient species are difficult to observe due to their instability in solution. This instability may be controlled by use of solid/gas synthetic techniques that enable the isolation of single-crystals of well-defined σ-alkane complexes. Here we show that, using this unique platform, selective alkane C-H activation occurs, as probed by H/D exchange using D2, and that five different isotopomers/isotopologues of the σ-alkane complex result, as characterized by single-crystal neutron diffraction studies for three examples. Low-energy fluxional processes associated with the σ-alkane ligand are identified using variable-temperature X-ray diffraction, solid-state NMR spectroscopy, and periodic DFT calculations. These observations connect σ-alkane complexes with their C-H activated products, and demonstrate that alkane-ligand mobility, and selective C-H activation, are possible when these processes occur in the constrained environment of the solid-state.

Cationic indenylnickel complexes bearing a 1,5-cyclooctadiene ligand: Synthesis and characterization

The reaction of [Ni(η-1-R-Ind)2] (R=H, Me) with one equivalent of 1,5-cyclooctadiene (COD), in the presence of a stoichiometric amount of HBF4 (in Et2O), at low temperature, afforded the corresponding cationic [Ni(η-1-R-Ind)(η4-COD)]BF4 in high yields. In the solid state, these complexes showed a slightly distorted pseudo-square-planar geometry around the metal centre. In solution, 1H NMR spectra showed the existence of a fluxional process involving the 1,5-cyclooctadiene ligand. This process was investigated by DFT calculations and the results exclude the possibility of η5-indenyl rotation.

H2S Reactivity on Oxygen-Deficient Heterotrimetallic Cores: Cluster Fluxionality Simulates Dynamic Aspects of Surface Chemical Reactions.

Understanding the mechanistic aspects of heterogeneous reactions on supported metal catalysts is challenging due to several disparate factors, among which the dynamic nature of the surface is a major contributor. In this study, the dynamic aspect of a surface has been probed by choosing small metal clusters as illustrative models. Two anionic hetero-trimetallic clusters, namely, W2TcO6(-) and W2OsO6(-), were reacted with H2S gas to exhibit splitting of the gas molecule and complete oxygen-sulfur exchange in the metal core. During this atom-exchange process, the core exhibits remarkable fluxionality to augment a thiol proton migration from one metal center to another, as well as a rapid interchange of the terminal and bridging oxygens. The fluxional nature of the core is further evidenced by two oppositely oriented oxo groups working in concert to accomplish the proton transfer, upon introduction of sulfur inside the core. These fluxional processes in the small hetero-trimetallic cores closely resemble the dynamic nature of the surface in a heterogeneous reaction. Throughout the fluxional processes investigated in this study, two-state reactivity and multiple instances of spin crossover are observed in our computational studies. Interestingly, the neutral hetero-trimetallic cores can also undergo complete oxygen-sulfur exchange reaction with H2S. The investigated metal clusters are promising materials, since they not only can liberate dihydrogen from water (reported in J. Phys. Chem. A, 2014, 118, 11047) but also can completely strip the sulfur from environmentally hazardous H2S gas.

ChemInform Abstract: The Ligand Polyhedral Model Approach to the Mechanism of Complete Carbonyl Exchange in [Rh4(CO)12] and [Rh6(CO)16

AbstractReview: 30 refs.

ChemInform Abstract: A Theoretical Analysis of Ambivalent and Ambiphilic Lewis Acid/Bases with Symmetry Signatures

AbstractReview: 115 refs.

Diphosphorus-bridged heterometallic anions and hydrides derived from reactions of complex [Mo2Cp2(μ-PCy2)(μ-κ2:κ2-P2)(CO)2]− with precursors of 16-electron metal carbonyl fragments

Reaction of the Li+ salt of the title anion with [Fe2(CO)9] or tetrahydrofuran adducts of formula [MLn(THF)] gave the trinuclear species Li[Mo2MCp2(μ-PCy2)(μ-κ2:κ2:κ1-P2)(CO)2Ln], (MLn = Fe(CO)4, MnCp′(CO)2, Mo(CO)5, W(CO)5; Cp′ = η5-C5H4Me), some of which could be isolated as the corresponding PPN+ salts (PPN = N(PPh3)2). These products followed from further coordination of the P2 ligand to the incoming 16-electron MLn fragment via the lone electron pair at its basal P atom, and displayed characteristically high P–P couplings (1JPP ca. 500Hz) indicative of retention of a strong P–P bond. In addition, these products underwent a fluxional process derived from the swing of the P2 unit around the Mo–Mo axis with concomitant exchange of the MLn fragment between P atoms. Reaction of the title anion with excess of the above carbonyl complexes involved further addition of a second MLn fragment to the P atom still bearing a lone electron pair, thus leading to the tetranuclear anionic derivatives Li[Mo2M2Cp2(μ-PCy2)(μ-κ2:κ2:κ1:κ1-P2)(CO)2L2n], most of which could be isolated as the corresponding PPN+ salts. For the Mo and W derivatives, this reaction also involved a trans to cis isomerisation at the Mo2(CO)2 moiety of the parent anion. All tetranuclear anions displayed fluxional behaviour in solution, comparable to that of the parent anion in the case of the Fe and Mn derivatives, but likely involving rotation of the P2 ligand in the case of the cisoid Mo and W derivatives. Protonation of the above trinuclear anions with [NH4]PF6 led to decomposition except for the Mn complex, this yielding a poorly characterized diphosphenyl derivative [MnMo2Cp2Cp′(μ-κ1,η2:κ2:κ1-HP2)(μ-PCy2)(CO)4] displaying an agostic-like P–H–Mo interaction, eventually decomposing to the known phosphide-bridged complex [MnMo2Cp2Cp′(μ3-P)(μ-PCy2)(CO)4]. In contrast, protonation of the tetranuclear anions yielded the unsaturated hydride derivatives [Mo2M2Cp2(H)(μ-PCy2)(μ-κ2:κ2:κ1:κ1-P2)(CO)L2n], (M = Mn, Mo, W), with a formal intermetallic double bond (Mo–Mo = 2.7412(6) Å for the tungsten complex) as a result of loss of a CO ligand from the Mo2(CO)2 moiety after protonation, probably favoured by the presence of excess of adducts [MLn(THF)] in these reaction mixtures.

The ligand polyhedral model approach to the mechanism of complete carbonyl exchange in [Rh4(CO)12] and [Rh6(CO)16

The Ligand Polyhedral Model (LPM) is applied to the carbonyl scrambling observed on the NMR timescale for the two cluster carbonyls [Rh4(CO)12] and [Rh6(CO)16]. For [Rh4(CO)12] the NMR data is completely consistent with the libration of the Rh4 central core about a C3 - rotational axis within the icosahedral shell of twelve CO ligands leading to a single-step concerted exchange process. Rotation of the Rh4 unit is also found to lead to the formation of a new all terminally bonded isomer which retains the icosahedral shell of CO ligands. Importantly, this new isomer does not have the [Ir4(CO)12] structure which is known to possess an cube-octahedral ligand shell. With [Rh6(CO)16] and its substituted derivative [Rh6(CO)15L] exchange is considered to take place via a polyhedral inter-conversion in which the external tetra-capped truncated tetrahedral shell of carbonyl ligands observed in the ground-state undergoes rearrangement first to the 2 : 6 : 6' : 2 complementary geometry and then returns to the ground-state structure. This leads to a single-step fully concerted CO exchange. The LPM approach also provides an understanding of the variation of activation energies observed in going from [Rh4(CO)12] to [Rh6(CO)16] and of the driving force to the fluxional process.

Ortho Derivatization of Phenols through C–H Nickelation: Synthesis, Characterization, and Reactivities of Ortho-Nickelated Phosphinite Complexes

Reported here are the synthesis and characterization of ortho-nickelated complexes derived from phosphinite ligands and investigated as model compounds in the development of C–H functionalization strategies for arenol substrates. Reaction of i-Pr2POPh with 0.6 equiv of [(i-PrCN)NiBr2]n and 0.8 equiv of NEt3 in toluene (100 °C, 36 h) gave the yellow, monomeric cyclometalated complex trans-{κ2P,C-C6H4OP(i-Pr)2}Ni(i-Pr2POPh)Br (3a) in 93% yield. The closely related yellow-orange dimeric species [{κ2P,C-C6H4OP(i-Pr)2}Ni(μ-Br)]2 (4a) was obtained in 70% yield when i-Pr2POPh was treated with 2 equiv each of the Ni precursor and NEt3. These complexes have been characterized fully and shown to interconvert in the presence of excess ligand (4a → 3a) or excess Ni precursor (3a → 4a). Treatment of 3a or 4a with benzyl bromide at 90 °C over extended periods led to benzylation of the Ni–aryl moiety in these complexes. Examination of the cyclometalation pathway for i-Pr2POPh has shown that the first species formed from its ambient-temperature reaction with [(i-PrCN)NiBr2]n is trans-(i-Pr2POPh)2NiBr2 (2a). NMR studies showed that 2a undergoes a rapid ligand exchange at room temperature, which can be slowed down at −68 °C; this fluxional process shifts in the presence of NEt3, implying the partial formation of an amine adduct. Heating toluene mixtures of 2a and NEt3 at 90 °C for 38 h led to the formation of 3a via C–H nickelation. That phosphinite dissociation from 2a precedes the C–H nickelation step is implied by the observation that the formation of 3a is hindered in the presence of excess i-Pr2POPh. The impact of phenol ring substituents on the C–H nickelation rate was probed by preparing substituted derivatives of 2a, trans-(4-R-C6H4OP(i-Pr2)}2NiBr2 (R = OMe (2b), Me (2c), COOMe (2d)), and measuring their relative rates of C–H nickelation. These studies showed that the formation of cyclonickelated products is favored in the order COOMe < Me < OMe, which is consistent with an electrophilic nickelation mechanism. Studying the C–H nickelation of 3-F-C6H4OP(i-Pr2) allowed us to establish that metalation is favored at the para position with respect to F (85:15).

Rationalizing and Disrupting Fluxional Processes in Agostically Stabilized 14‐Electron Alkyliridium Hydride Complexes

AbstractWe have investigated the fundamental patterns of reactivity for the cationic IrIII complexes [(6‐Mes)(6‐Mes′)Ir(H)]+ and [(7‐Mes)(7‐Mes′)Ir(H)]+ [6‐Mes, 7‐Mes = ring‐expanded N‐heterocyclic carbene (NHC) ligands, ′ indicates that NHC is tethered through an iridium‐bound CH2 group], each of which undergoes degenerate fluxional exchange between an iridium‐bound agostic C–H bond and the discrete alkyl/hydride ligands resulting from C–H activation. A comparison between the activation parameters ΔH‡ and ΔS‡ determined by variable‐temperature NMR (VT‐NMR) spectroscopy line‐shape analysis and those obtained from DFT calculations on model systems offers a basis to discriminate between different possible exchange mechanisms, and an oxidative addition pathway gives the lowest barrier and also the best agreement between experiment and theory. The addition of neutral Lewis base donors to [(6‐Mes)(6‐Mes′)Ir(H)]+ resulted in the displacement of the weak agostic C–H···Ir interaction and the generation of static (alkyl)iridium(III) hydride complexes such as [(6‐Mes)(6‐Mes′)Ir(H)(CNtBu)2]+. The analogous carbonyl complex is more labile, with the enhanced π‐acceptor properties of the ancillary ligands leading to C–H reductive elimination and to the formation of the IrI complex [(6‐Mes)2Ir(CO)2]+, which incorporates unactivated NHC donors and has the ability to reversibly take up H2.

DFT study and dynamic NMR evidence for cis-trans conformational isomerism in square planar Ni(II) thioselenophosphinate, Ni(SeSPPh2)2

Theoretical (DFT) and experimental (dynamic NMR) study of cis-trans conformational isomerism in Ni(II) square planar thioselenophosphinate, Ni(SeSPPh2)2, have been carried out. The DFT investigation [B3LYP/6-311++G(d,p), gas] of this complex reveals that its cis-to-trans isomerization occurs through two minimal energy crossing points (MECPs) located at the intersection of the lowest singlet and triplet PESs (potential energy surfaces). The calculated relative energies of the MECPs, 5.6 and 3.6 kcal/mol, correspond to low-energy barriers of “cis-Ni(SeSPPh2)2 [S] ⇌ pseudoterahedral-Ni(SeSPPh2)2 [T]” and “trans-Ni(SeSPPh2)2 [S] ⇌ pseudoterahedral-Ni(SeSPPh2)2 [T]” spin crossover rearrangements. The dynamic 31P NMR study of Ni(SeSPPh2)2 is fully confirms these computations: below 208 K, the cis- and trans-isomers are in a dynamic equilibrium. The free energy of activation for this fluxional process calculated from the coalescence temperature is ΔG≠ = 11.5 kcal/mol.

Conformational Mobility and Pendent Base Effects on Electrochemistry of Synthetic Analogues of the [FeFe]-Hydrogenase Active Site

Dynamic NMR (13C and 1H) studies of (μ-SCH2XCH2S)[Fe(CO)3]2 complexes (X = CR2, NR) were utilized to examine the fluxional processes that are important in the [FeFe]-hydrogenase active site models, where an open site for proton/hydrogen binding, achieved by configurational mobility of the Fe(CO)3 unit, is required for electrocatalysis of proton reduction. In order to interrogate the effects of fluxional mobility on electrochemical response to added acid, energy barriers for the CO site exchange in Fe(CO)3 rotors were determined for nitrogen- and carbon-based bridgehead complexes. The effect of the methyl substituent in both the NH/NCH3 and CH2/C(CH3)2 cases is to lower the Fe(CO)3 rotational activation barrier relative to the NH or CH2 analogues. Although the C(CH3)2 case has the lowest Fe(CO)3 rotational barrier, its performance as a proton reduction electrocatalyst is 2-fold less than that for the X = NR species, indicating the proton-directing effect of the pendent base on catalytic efficiency.

A new convenient synthetic route to metal diselenophosphinates: Synthesis and characterization of [M2(Se2PPh2)4] (M = Zn, Cd and Hg) complexes

An efficient synthesis for group 12 metal diselenophosphinates has been elaborated based on original reaction between easily available ammonium diselenophosphinates, NH4[Se2PPh2] or Et2NH2[Se2PPh2] and corresponding metal(II) oxides (0–25 °C, acetone, 1–4 h). The Zn(II), Cd(II), and Hg(II) diselenophosphinates have been first fully characterized to reveal dimers [M2(Se2PPh2)4] in the solid state. These dinuclear species contain an eight-membered chair-like ring, (MSePSe)2, formed by two metal atoms connecting via Se–P–Se chains. The distorted tetrahedral environment of each metal center is defined by four Se atoms belonging to one chelating (η2) and two bridging (μ2, η2) diselenophosphinato ligands. A special feature for Cd(II) complex is the existence of a solvent- and temperature-dependent fluxional process (determined by VT 31P NMR spectroscopy) which was tentatively assigned for the exchange between bridging and chelating “dsepi” (diselenophosphinato) ligands. The free energy of activation for this equilibrium calculated from the coalescence temperature being ΔG≠(233K) = 41.9 kJ mol−1.

A combined experimental and computational study of fluxional processes in sigma amine–borane complexes of rhodium and iridium

A combined experimental and computational study on the fluxional processes involving the M-H and B-H positions in the sigma amine-borane complexes [M(PR3)2(H)2(η(2)-H3B·NMe3)][BAr(F)4] (M = Rh, Ir; R = Cy for experiment; R = Me, Cy for computation; Ar(F) = 3,5-(CF3)2C6H3) is reported. The processes studied are: B-H bridging/terminal exchange; reaction with exogenous D2 leading to exchange at M-H; and intramolecular M-H/B-H exchange. Experimentally it was found that B-H bridging/terminal exchange is most accessible and slightly favoured for Rh; D2/M-H exchange occurs at qualitatively similar rates for both M = Rh and Ir, while M-H/B-H exchange is the slowest overall, with the Ir congener having a lower barrier than Rh. Evidence for the isotopic perturbation of equilibrium is also reported for the BH/BD isotopologues of [Ir(PCy3)2(H)2(η(2)-H3B·NMe3)][BAr(F)4]. DFT calculations using model complexes (R = Me) qualitatively reproduce the relative rates of the various exchange processes for both M = Rh and Ir, i.e. barriers for B-H bridging/terminal exchange are less than those for M-H/H2 exchange, which in turn are less than those for M-H/B-H exchange. Which metal promotes these processes more effectively depends upon the nature of the rate-limiting transition state, which can change between Rh and Ir. Computational analysis of the full experimental system (R = Cy) reveals similar overall trends in terms of the relative ease of the various exchange processes. However, there are differences in the details, and these are discussed.

Synthesis, structure and dynamics of NHC-based palladium macrocycles

A series of macrocyclic CNC pincer pro-ligands based on bis(imidazolium)lutidine salts with octa-, deca- and dodecamethylene spacers have been prepared and their coordination chemistry investigated. Using a Ag2O based transmetallation strategy, cationic palladium(II) chloride complexes [PdCl{CNC-(CH2)n}][BAr(F)4] (n = 8, 10, 12; Ar(F) = 3,5-C6H3(CF3)2) were prepared and fully characterised in solution, by NMR spectroscopy and ESI-MS, and in the solid-state, by X-ray crystallography. The smaller macrocyclic complexes (n = 8 and 10) exhibit dynamic behaviour in solution, involving ring flipping of the alkyl spacer across the Pd-Cl bond, which was interrogated by variable temperature NMR spectroscopy. In the solid-state, distorted coordination geometries are observed with the spacer skewed to one side of the Pd-Cl bond. In contrast, a static C2 symmetric structure is observed for the dodecamethylene based macrocycle. For comparison, palladium(II) fluoride analogues [PdF{CNC-(CH2)n}][BAr(F)4] (n = 8, 10, 12) were also prepared and their solution and solid-state structures contrasted with those of the chlorides. Notably, these complexes exhibit very low frequency (19)F chemical shifts (ca. -400 ppm) and the presence of C-H···F interactions ((2h)J(FC) coupling observed by (13)C NMR spectroscopy). The dynamic behaviour of the fluoride complexes is largely consistent with the smaller ancillary ligand; [PdF{CNC-(CH2)8}][BAr(F)4] exceptionally shows C(2v) time averaged symmetry in solution at room temperature (CD2Cl2, 500 MHz) as a consequence of dual fluxional processes of the pincer backbone and alkyl spacer.