Binding of Nitriles and Isonitriles to V(III) and Mo(III) Complexes: Ligand vs Metal Controlled Mechanism.

The synthesis and structures of nitrile complexes of V(N[tBu]Ar)3, 2 (Ar = 3,5-Me2C6H3), are described. Thermochemical and kinetic data for their formation were determined by variable temperature Fourier transform infrared (FTIR), calorimetry, and stopped-flow techniques. The extent of back-bonding from metal to coordinated nitrile indicates that electron donation from the metal to the nitrile plays a less prominent role for 2 than for the related complex Mo(N[tBu]Ar)3, 1. Kinetic studies reveal similar rate constants for nitrile binding to 2, but the activation parameters depend critically on the nature of R in RCN. Activation enthalpies range from 2.9 to 7.2 kcal·mol-1, and activation entropies from -9 to -28 cal·mol-1·K-1 in an opposing manner. Density functional theory (DFT) calculations provide a plausible explanation supporting the formation of a π-stacking interaction between a pendant arene of the metal anilide of 2 and the arene substituent on the incoming nitrile in favorable cases. Data for ligand binding to 1 do not exhibit this range of activation parameters and are clustered in a small area centered at ΔH‡ = 5.0 kcal·mol-1 and ΔS‡ = -26 cal·mol-1·K-1. Computational studies are in agreement with the experimental data and indicate a stronger dependence on electronic factors associated with the change in spin state upon ligand binding to 1.

Dihydrogen cleavage by a dimetalloxycarbene-borane frustrated Lewis pair.

A frustrated Lewis pair of dititanoxycarbene [(Ti(N[tBu]Ar)3)2(µ-CO2)] (Ar = 3,5-Me2C6H3) and B(C6F5)3 cleaved dihydrogen under ambient conditions to give the zwitterionic formate [(Ti(N[tBu]Ar)3)2(µ-OCHO-ηO:ηO')(B(C6F5)3)] and the hydrido borate [Ti(N[tBu]Ar)3][HB(C6F5)3].

Countercation Effect on CO2 Binding to Oxo Titanate with Bulky Anilide Ligands.

This work focuses on nucleophilic activation of CO2 at the anionic terminal oxo titanium tris(anilide) complexes [(Solv)n M][OTi(N[t Bu]Ar)3 ]m with M=Li, Na, K, Mg, MgMe, AlCl2 , AlI2 ; Ar=3,5-Me2 C6 H3 ; Solv=Et2 O, THF, 12-crown-4, 2,2,2-cryptand; n, m=1-2. The CO2 binding strength to the terminal oxo ligand of [OTi(N[t Bu]Ar)3 ]- ([1]- ) and the stability of the resulting carbonate moiety [O2 COTi(N[t Bu]Ar)3 ]- ([2]- ) are highly dependent on the Lewis acidity of the countercation. We report herein on CO2 binding as a function of countercation and countercation coordination environment, and comment in this respect on the bottom and upper limits of the cation Lewis acidity. Thermodynamic parameters are provided for oxo complexes with lithium as countercation, that is, [(Et2 O)2 Li][1] and [(12-c-4)Li][1].

A Dimetalloxycarbene Bonding Mode and Reductive Coupling Mechanism for Oxalate Formation from CO2.

We describe the stable and isolable dimetalloxycarbene [(TiX3 )2 (µ2 -CO2 -κ(2) C,O:κO')] 5, where X=N-(tert-butyl)-3,5-dimethylanilide, which is stabilized by fluctuating µ2 -κ(2) C,O:κ(1) O' coordination of the carbene carbon to both titanium centers of the dinuclear complex 5, as shown by variable-temperature NMR studies. Quantum chemical calculations on the unmodified molecule indicated a higher energy of only +10.5 kJ mol(-1) for the µ2 -κ(1) O:κ(1) O' bonding mode of the free dimetalloxycarbene compared to the µ2 -κ(2) C,O:κ(1) O' bonding mode of the masked dimetalloxycarbene. The parent cationic bridging formate complex [(TiX3 )2 (µ2 -OCHO-κO:κO')][B(C6 F5)4], 4[B(C6 F5)4], was simply deprotonated with the strong base K(N(SiMe3 )2 ) to give 5. Complex 5 reacts smoothly with CO2 to generate the bridging oxalate complex [(TiX3 )2 (µ2 -C2 O4 -κO:κO'')], 6, in a C-C bond formation reaction commonly anticipated for oxalate formation by reductive coupling of CO2 on low-valent transition-metal complexes.

Experimental and computational studies on the formation of cyanate from early metal terminal nitrido ligands and carbon monoxide.

An important challenge in the artificial fixation of N2 is to find atom efficient transformations that yield value-added products. Here we explore the coordination complex mediated conversion of ubiquitous species, CO and N2, into isocyanate. We have conceptually split the process into three steps: (1) the six-electron splitting of dinitrogen into terminal metal nitrido ligands, (2) the reduction of the complex by two electrons with CO to form an isocyanate linkage, and (3) the one electron reduction of the metal isocyanate complex to regenerate the starting metal complex and release the product. These steps are explored separately in an attempt to understand the limitations of each step and what is required of a coordination complex in order to facilitate a catalytic cycle. The possibility of this cyanate cycle was explored with both Mo and V complexes which have previously been shown to perform select steps in the sequence. Experimental results demonstrate the feasibility of some of the steps and DFT calculations suggest that, although the reduction of the terminal metal nitride complex by carbon monoxide should be thermodynamically favorable, there is a large kinetic barrier associated with the change in spin state which can be avoided in the case of the V complexes by an initial binding of the CO to the metal center followed by rearrangement. This mandates certain minimal design principles for the metal complex: the metal center should be sterically accessible for CO binding and the ligands should not readily succumb to CO insertion reactions.

Thermodynamic and kinetic study of cleavage of the N-O bond of N-oxides by a vanadium(III) complex: enhanced oxygen atom transfer reaction rates for adducts of nitrous oxide and mesityl nitrile oxide.

Thermodynamic, kinetic, and computational studies are reported for oxygen atom transfer (OAT) to the complex V(N[t-Bu]Ar)3 (Ar = 3,5-C6H3Me2, 1) from compounds containing N-O bonds with a range of BDEs spanning nearly 100 kcal mol(-1): PhNO (108) > SIPr/MesCNO (75) > PyO (63) > IPr/N2O (62) > MesCNO (53) > N2O (40) > dbabhNO (10) (Mes = mesityl; SIPr = 1,3-bis(diisopropyl)phenylimidazolin-2-ylidene; Py = pyridine; IPr = 1,3-bis(diisopropyl)phenylimidazol-2-ylidene; dbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene). Stopped flow kinetic studies of the OAT reactions show a range of kinetic behavior influenced by both the mode and strength of coordination of the O donor and its ease of atom transfer. Four categories of kinetic behavior are observed depending upon the magnitudes of the rate constants involved: (I) dinuclear OAT following an overall third order rate law (N2O); (II) formation of stable oxidant-bound complexes followed by OAT in a separate step (PyO and PhNO); (III) transient formation and decay of metastable oxidant-bound intermediates on the same time scale as OAT (SIPr/MesCNO and IPr/N2O); (IV) steady-state kinetics in which no detectable intermediates are observed (dbabhNO and MesCNO). Thermochemical studies of OAT to 1 show that the V-O bond in O≡V(N[t-Bu]Ar)3 is strong (BDE = 154 ± 3 kcal mol(-1)) compared with all the N-O bonds cleaved. In contrast, measurement of the N-O bond in dbabhNO show it to be especially weak (BDE = 10 ± 3 kcal mol(-1)) and that dissociation of dbabhNO to anthracene, N2, and a (3)O atom is thermodynamically favorable at room temperature. Comparison of the OAT of adducts of N2O and MesCNO to the bulky complex 1 show a faster rate than in the case of free N2O or MesCNO despite increased steric hindrance of the adducts.

Facile synthesis of zero-, one-, and two-dimensional vanadyl pyrophosphates.

Crystallization of Na(2)VOP(2)O(7) from its aqueous solution results in formation of a one-dimensional inorganic polymer {Na(2)VO(H(2)O)P(2)O(7)·7H(2)O}(n) (1). When this polymer is dehydrated at elevated temperatures this polymer undergoes a phase transition to form the two-dimensional framework ß-Na(2)VOP(2)O(7), which although previously reported had been difficult to access. Exchanging lithium for sodium via ion-exchange chromatography results in formation of a discrete, cyclic, tetramer species, Li(8)[VOP(2)O(7)(H(2)O)·4H(2)O](4) (2). Isolation of crystalline ß-Li(2)VOP(2)O(7) using a dehydration procedure analogous to the one employed for the sodium derivative was unsuccessful. In contrast, we show that ß-K(2)VOP(2)O(7) can be obtained from the amorphous phase K(2)VOP(2)O(7)·nH(2)O (n = 0-7) upon thermal dehydration.

Ligand-based reduction of CO(2) to CO mediated by an anionic niobium nitride complex.

The terminal nitride anion complex [Na][N[triple bond]Nb(N[(t)Bu]Ar)(3)] ([Na][1], Ar = 3,5-Me(2)C(6)H(3)) reacts quantitatively with CO(2) to give the carbamate complex [Na][O(2)CN[triple bond]Nb(N[(t)Bu]Ar)(3)] ([Na][O(2)C-1]). The structure of [Na][O(2)C-1] as the bis-12-crown-4 solvate, as determined by X-ray crystallography, displays a unique N-bound carbamate ligand without any metal-oxygen interactions. When treated with organic acid anhydrides or acid chlorides, complex [Na][O(2)C-1] reacts via salt elimination to give the five-coordinate complexes (RC(O)O)(OCN)Nb(N[(t)Bu]Ar)(3) (R-2, R = Me, (t)Bu, F(3)C). We show that complexes R-2 yield the starting complex [Na][1] with concomitant release of CO upon two-electron reduction. This series of reactions constitutes a closed cycle for the conversion of CO(2) to CO mediated by a terminal nitride anion complex.

Two-electron reduction of a vanadium(V) nitride by CO to release cyanate and open a coordination site.

We report herein that the terminal nitride complex Na[NV(N[t-Bu]Ar)(3)] (Na[1-VN], Ar = 3,5-Me(2)C(6)H(3)) reacts with CO over the course of 24 h to generate V(N[t-Bu]Ar)(3) (1-V) and sodium cyanate in an isolated yield of 77%. The reaction products were identified using a combination of NMR and IR spectroscopy, and cyanate formation was further confirmed by performing a (13)C labeling experiment. In addition, we report on the synthesis and structural characterization of the isocyanate complex (OCN)V(N[t-Bu]Ar)(3) (1-V(NCO)) to compare its redox chemistry with that of (OCN)Nb(N[t-Bu]Ar)(3) (1-Nb(NCO)). Electrochemical studies of 1-Nb(NCO) and 1-V(NCO) revealed large differences in the M(5+)/M(4+) and M(4+)/M(3+) reduction potentials. Complex 1-V(NCO) displays two redox events in the cyclic voltammogram: a quasireversible event -0.11 V vs Fc/Fc(+) and a reversible event at -1.56 V vs Fc/Fc(+). Complex 1-Nb(NCO) displays two redox events as well: a quasireversible event at -1.1 V and an irreversible event at -2.9 V vs Fc/Fc(+).

Pyridyl 'ring-flipping' in the dimers [Me2E(2-py)]2 (E=B, Al, Ga; 2-py=2-pyridyl).

The boron and aluminium dimers [Me2E(micro-py)]2 [E=B (1); Al (2)] are formed as mixtures of two isomers in which the group 13 centres are coordinated by the bridging 2-py ligands in a cis or trans manner, however, in contrast to previous studies, we find that simply heating the mixtures of these isomers of and gives the more thermodynamically stable (synthetically useful) trans isomers exclusively (the trans isomer being the only product in the case of the gallium analogue ).