Unravelling the Molecular Origin of the Regiospecificity in Extradiol Catechol Dioxygenases.

Many factors have been suggested to control the selectivity for extradiol or intradiol cleavage in catechol dioxygenases. The varied selectivity of model complexes and the ability to force an extradiol enzyme to do intradiol cleavage indicate that the problem may be complex. In this paper we focus on the regiospecificity of the proximal extradiol dioxygenase, homoprotocatechuate 2,3-dioxygenase (HPCD), for which considerable advances have been made in our understanding of the mechanism from an experimental and computational standpoint. Two key steps in the reaction mechanism were investigated: (1) attack of the substrate by the superoxide moiety and (2) attack of the substrate by the oxyl radical generated by O-O bond cleavage. The selectivity at both steps was investigated through a systematic study of the role of the substrate and the first and second coordination spheres. For the isolated native substrate, intradiol cleavage is calculated to be both kinetically and thermodynamically favored, therefore nature must use the enzyme environment to reverse this preference. Two second sphere residues were found to play key roles in controlling the regiospecificity of the reaction: Tyr257 and His200. Tyr257 controls the selectivity by modulating the electronic structure of the substrate, while His200 controls selectivity through steric effects and by preventing alternative pathways to intradiol cleavage.

Reversible C-C bond formation between redox-active pyridine ligands in iron complexes.

This manuscript describes the formally iron(I) complexes L(Me)Fe(Py-R)(2) (L(Me) = bulky ß-diketiminate; R = H, 4-tBu), in which the basal pyridine ligands preferentially accept significant unpaired spin density. Structural, spectroscopic, and computational studies on the complex with 4-tert-butylpyridine ((tBu)py) indicate that the S = 3/2 species is a resonance hybrid between descriptions as (a) high-spin iron(II) with antiferromagnetic coupling to a pyridine anion radical and (b) high-spin iron(I). When the pyridine lacks the protection of the tert-butyl group, it rapidly and reversibly undergoes radical coupling reactions that form new C-C bonds. In one reaction, the coordinated pyridine couples to triphenylmethyl radical, and in another, it dimerizes to give a pyridine-derived dianion that bridges two iron(II) ions. The rapid, reversible C-C bond formation in the dimer stores electrons from the formally reduced metal as a C-C bond in the ligands, as demonstrated by using the coupled diiron(II) complex to generate products that are known to come from iron(I) precursors.

Substitution reactions in dinuclear Ru-Hbpp complexes: an evaluation of through-space interactions.

The synthesis of new dinuclear complexes of the general formula in,in-{[Ru(II)(trpy)(L)](µ-bpp)[Ru(II)(trpy)(L')]}(3+) [bpp(-) is the bis(2-pyridyl)-3,5-pyrazolate anionic ligand; trpy is the 2,2':6',2″-terpyridine neutral meridional ligand, and L and L' are monodentate ligands; L = L' = MeCN, 3a(3+); L = L' = 3,5-lutidine (Me(2)-py), 3c(3+); L = MeCN, L' = pyridine (py), 4(3+)], have been prepared and thoroughly characterized. Further, the preparation and isolation of dinuclear complexes containing dinitrile bridging ligands of the general formula in,in-{[Ru(II)(trpy)](2)(µ-bpp)(µ-L-L)}(3+) [µ-L-L = 1,4-dicyanobutane (adiponitrile, adip), 6a(3+); 1,3-dicyanopropane (glutaronitrile, glut), 6b(3+); 1,2-dicyanoethane (succinonitrile; succ), 6c(3+)] have also been carried out. In addition, a number of homologous dinuclear complexes previously described, containing the anionic bis(pyridyl)indazolate (bid(-)) tridentate meridional ligand in lieu of trpy, have also been prepared for comparative purposes. In the solid state, six complexes have been characterized by X-ray crystallography, and in solution, all of them have been spectroscopically characterized by NMR and UV-vis spectroscopy. In addition, their redox properties have also been investigated by means of cyclic voltammetry and differential pulse voltammetry and show the existence of two one-electron waves assigned to the formation of the II,III and III,III species. Dinitrile complexes 6a(3+), 6b(3+), and 6c(3+) display a dynamic behavior involving their enantiomeric interconversion. The energy barrier for this interconversion can be controlled by the number of methylenic units between the dinitrile ligand. On the other hand, pyridyl complexes in,in-{[Ru(II)(T)(py)](2)(µ-bpp)}(n+) (T = trpy, n = 3, 3b(3+); T = bid(-), n = 1, 3b'(+)) and 3c(3+) undergo two consecutive substitution reactions of their monodentate ligands by MeCN.The substitution kinetics have been monitored by (1)H NMR and UV-vis spectroscopy and follow first-order behavior with regard to the initial ruthenium complex. For the case of 3b(3+), the first-order rate constant k(1) = (2.9 ± 0.3) × 10(-5) s(-1), whereas for the second substitution, the k obtained is k(2) = (1.7 ± 0.7) × 10(-6) s(-1), both measured at 313 K. Their energies of activation at 298 K are 114.7 and 144.3 kJ mol(-1), respectively. Density functional theory (DFT) calculations have been performed for two consecutive substitution reactions, giving insight into the nature of the intermediates. Furthermore, the energetics obtained by DFT calculations of the two consecutive substitution reactions agree with the experimental values obtained. The kinetic properties of the two consecutive substitution reactions are rationalized in terms of steric crowding and also in terms of through-space interactions.

Achieving C-N bond cleavage in dinuclear metal cyanide complexes.

Cleavage of cyanide is more difficult to achieve compared to dinitrogen and carbon monoxide, even though these species contain triple bonds of greater strength. In this work, we have used computational methods to investigate thermodynamic and mechanistic aspects of the C-N bond cleavage process in [L(3)M-CN-M'L(3)] systems consisting of a central cyanide unit bound in an end-on fashion to two terminal metal tris-amide complexes. In these systems, [M] is a d(3) transition metal from the 3d, 4d, 5d, or 6d series and groups 4 through 7, and [L] is either [NH(2)], [NMe(2)], [N(i)PrPh], or [N(t)BuAr]. A comparison of various models for the experimentally relevant [L(3)Mo-CN-MoL(3)] system has shown that while the C-N cleavage step appears to be an energetically favourable process, a large barrier exists for the dissociation of [L(3)Mo-CN-MoL(3)]((-)) into [L(3)Mo-C]((-)) and [N-MoL(3)], which possibly explains why C-N bond scission is not observed experimentally. The general structural, bonding, and thermochemical trends across the transition metal series investigated, indicate that the systems exhibiting the greatest degree of C-N activation, and most favourable energetics for C-N cleavage, also possess the most favourable electronic properties, namely, a close match between the relevant π-like orbitals on the metal-based and cyanide fragments. The negative charge on the cyanide fragment leads to significant destabilization of the π* level which needs to be populated through back-donation from the metal centres in order for C-N bond scission to be achieved. Therefore, metal-based systems with high-lying d(π) orbitals are best suited to C-N cleavage. In terms of chemical periodicity, these systems can be identified as the heavier members within a group and the earlier members within a period. As a consequence, Mo complexes are not well suited to cleaving the C-N bond, whereas the Ta analogues are the most favourable systems and should, in principle, be capable of cleaving cyanide under relatively mild conditions. An important conclusion from this work is that a successful strategy for achieving cleavage of multiply-bonded, and relatively unreactive, molecular fragments, may simply lie in tuning the electronic structures and orbital interactions by judicious choice of metal sites and ligand groups.

Mechanistic study of amine to imine oxidation in a dinuclear Cu(II) complex containing an octaaza dinucleating ligand.

Density functional theory (DFT) calculations have been carried out to elucidate the mechanism of self-oxidation of a Cu(II) complex octaaza dinucleating macrocyclic ligand. The reaction is bimolecular and spontaneous, in which amine groups of one macrocycle are oxidized and the Cu(II) centers of a second macrocylic complex are reduced. No additional oxidation or external base agents are required. DFT calculations predict the reaction to proceed via a two-step mechanism, in which the first step is proton transfer between two reactant complexes. This is followed by a second transfer step in which an electron and proton are transferred together between the two complexes. Concurrent with this external transfer there is also an internal electron transfer in which the ligand reduces the metal center to give the imine product bound to Cu(I). The complexity of this final step differs from the generally accepted mechanisms for transition metal catalyzed amine to imine oxidation in which protons and electrons are transferred individually.

Dinitrogen activation by Fryzuk's [Nb(P(2)N(2))] complex and comparison with the Laplaza-Cummins [Mo{N(R)Ar}(3)] and Schrock [Mo(N(3)N)] systems.

The reaction profile of N(2) with Fryzuk's [Nb(P(2)N(2))] (P(2)N(2)=PhP(CH(2)SiMe(2)NSiMe(2)CH(2))(2)PPh) complex is explored by density functional calculations on the model [Nb(PH(3))(2)(NH(2))(2)] system. The effects of ligand constraints, coordination number, metal and ligand donor atom on the reaction energetics are examined and compared to the analogous reactions of N(2) with the three-coordinate Laplaza-Cummins [Mo{N(R)Ar}(3)] and four-coordinate Schrock [Mo(N(3)N)] (N(3)N=[(RNCH(2)CH(2))(3)N](3-)) systems. When the model system is constrained to reflect the geometry of the P(2)N(2) macrocycle, the N--N bond cleavage step, via a N(2)-bridged dimer intermediate, is calculated to be endothermic by 345 kJ mol(-1). In comparison, formation of the single-N-bridged species is calculated to be exothermic by 119 kJ mol(-1), and consequently is the thermodynamically favoured product, in agreement with experiment. The orientation of the amide and phosphine ligands has a significant effect on the overall reaction enthalpy and also the N--N bond cleavage step. When the ligand constraints are relaxed, the overall reaction enthalpy increases by 240 kJ mol(-1), but the N(2) cleavage step remains endothermic by 35 kJ mol(-1). Changing the phosphine ligands to amine donors has a dramatic effect, increasing the overall reaction exothermicity by 190 kJ mol(-1) and that of the N--N bond cleavage step by 85 kJ mol(-1), making it a favourable process. Replacing Nb(II) with Mo(III) has the opposite effect, resulting in a reduction in the overall reaction exothermicity by over 160 kJ mol(-1). The reaction profile for the model [Nb(P(2)N(2))] system is compared to those calculated for the model Laplaza and Cummins [Mo{N(R)Ar}(3)] and Schrock [Mo(N(3)N)] systems. For both [Mo(N(3)N)] and [Nb(P(2)N(2))], the intermediate dimer is calculated to lie lower in energy than the products, although the final N-N cleavage step is much less endothermic for [Mo(N(3)N)]. In contrast, every step of the reaction is favourable and the overall exothermicity is greatest for [Mo{N(R)Ar}(3)], and therefore this system is predicted to be most suitable for dinitrogen cleavage.

Oxidative dehydrogenation of an amine group of a macrocyclic ligand in the coordination sphere of a Cu(II) complex.

The spontaneous oxidation of an amine group to an imine has been observed experimentally in an octa-aza macrocyclic dinucleating ligand LH(4) coordinated to Cu(II). The reaction is bimolecular and spontaneous in which amine groups of one macrocycle are oxidised and the Cu(II) centres of a second macrocyclic complex are reduced. No additional oxidating or external base agents are required. DFT calculations are carried out to compare the reaction with that recently reported for a ligand coordinated to an Fe(III) centre, but which requires an external base as proton acceptor. The computational results show that the copper and iron catalysed amine to imine reactions proceed via different mechanisms.

The influence of peripheral ligand bulk on nitrogen activation by three-coordinate molybdenum complexes--a theoretical study using the ONIOM method.

Electronic structure methods have been combined with the ONIOM approach to carry out a comprehensive study of the effect of ligand bulk on the activation of dinitrogen with three-coordinate molybdenum complexes. Calculations were performed with both density functional and CCSD(T) methods. Our results show that not only is there expected destabilization of the intermediate on the pathway due to direct steric interactions of the bulky groups, but also there is significant electronic destabilization as the size of the ligand increases. This latter destabilization is due to the inability of the molecule to accommodate a rotated amide group bound to the molybdenum once the amide reaches a certain size. This destabilization also leads to a clear preference for the triplet intermediate (rather than the singlet intermediate) for bulky substituents which is in agreement with experiment. Overall, the calculated reaction profile for the bulky substituents shows a good correlation with the available experimental data.

Activation and cleavage of the N-O bond in dinuclear mixed-metal nitrosyl systems and comparative analysis of carbon monoxide, dinitrogen, and nitric oxide activation.

The activation and scission of the N-O bond in nitric oxide using dinuclear mixed-metal species, comprising transition elements with d(3) and d(2) configurations and trisamide ligand systems, have been investigated by means of density functional calculations. The [Cr(iii)-V(iii)] system is analyzed in detail and, for comparative purposes, the [Mo(iii)-Nb(iii)], [W(iii)-Ta(iii)], and (mixed-row) [Mo(iii)-V(iii)] systems are also considered. The overall reaction and individual intermediate steps are favourable for all systems, including the case where first row (Cr and V) metals are exclusively involved, a result that has not been observed for the related dinitrogen and carbon monoxide systems. In contrast to the cleavage of dinitrogen by three-coordinate Mo amide complexes where the dinuclear intermediate possesses a linear [Mo-NN-Mo] core, the [M-NO-M'] core must undergo significant bending in order to stabilize the dinuclear species sufficiently for the reaction to proceed beyond the formation of the nitrosyl encounter complex. A comparative bonding analysis of nitric oxide, dinitrogen and carbon monoxide activation is also presented. The overall results indicate that the pi interactions are the dominant factor in the bonding across the [M-L(1)L(2)-M'] (L(1)L(2) = N-O, N-N, C-O) moiety and, consequently, the activation of the L(1)-L(2) bond. These trends arise from the fact that the energy gaps between the pi orbitals on the metal and small molecule fragments are much more favourable than for the corresponding sigma orbitals. The pi energy gaps decrease in the order [NO < N(2) < CO] and consequently, for each individual pi orbital interaction, the back donation between the metal and small molecule increases in the order [CO < N(2) < NO]. These results are in accord with previous findings suggesting that optimization of the pi interactions plays a central role in increasing the ability of these transition metal systems to activate and cleave small molecule bonds.

A comparison of N2 cleavage in Schrock's Mo[N3N] and Laplaza-Cummins' Mo[N(R)Ar]3 systems.

The four-coordinate Mo[N(3)N] complex, [N(3)N] = [{RNCH(2)CH(2)}(3)N], R = 3,5-(2,4,6-iPr(3)C(6)H(2))(2)C(6)H(3) (HIPT), which is capable of converting N(2) to ammonia catalytically, reacts with N(2) in a similar manner to Mo[N(R)Ar](3) (R = tBu, Ar = 3,5-C(6)H(3)Me(2)) to form a dinitrogen-bridged dimer intermediate, but unlike its three-coordinate counterpart, N(2) cleavage is not observed. To rationalise these differences, the reaction of N(2) with the model Mo[NH(2)](3)[NH(3)] and full ligand Mo[N(3)N] systems was explored using density functional theory and compared with the results of an earlier study involving the model three-coordinate Mo[NH(2)](3) system. Although the overall reaction is exothermic, the final N-N cleavage step is calculated to be endothermic by 75 kJ mol(-1) for the model system when the Mo-amine cap bond length is fixed to mimic the constraints of the ligand straps, but exothermic by 14 kJ mol(-1) for the full ligand system. In the latter case, the slightly exothermic cleavage step can be attributed to the destabilization of the N(2) bridged dimer relative to the nitride product owing to the steric effects of the bulky R groups. The activation barrier for N-N cleavage is estimated at 151 kJ mol(-1) for the model system, more than twice the calculated value for Mo[NH(2)](3), and even greater, 213 kJ mol(-1), for the full ligand [N(3)N]Mo system. A bonding analysis shows that although the binding of the amine cap helps to stabilize the intermediate dimer, at the same time it destabilizes the metal d-orbitals involved in backbonding to the pi* orbitals on N(2). As a result, backdonation is less efficient and N-N activation reduced compared to the three-coordinate system. Thus, the increased stability of the intermediate dimer on binding of the amine cap combined with the reduced level of N-N activation and higher kinetic barrier, explain why N-N cleavage has not been observed experimentally for the four-coordinate Mo[N(3)N] system.

Investigating CN- cleavage by three-coordinate M[N(R)Ar]3 complexes.

Three-coordinate Mo[N((t)Bu)Ar]3 binds cyanide to form the intermediate [Ar((t)Bu)N]3Mo-CN-Mo[N((t)Bu)Ar]3 but, unlike its N2 analogue which spontaneously cleaves dinitrogen, the C-N bond remains intact. DFT calculations on the model [NH2]3Mo/CN- system show that while the overall reaction is significantly exothermic, the final cleavage step is endothermic by at least 90 kJ mol(-1), accounting for why C-N bond cleavage is not observed experimentally. The situation is improved for the [H2N]3W/CN- system where the intermediate and products are closer in energy but not enough for CN- cleavage to be facile at room temperature. Additional calculations were undertaken on the mixed-metal [H2N]3Re+/CN- /W[NH2]3 and [H2N]3Re+/CN-/Ta[NH2]3 systems in which the metals ions were chosen to maximise the stability of the products on the basis of an earlier bond energy study. Although the reaction energetics for the [H2N]3Re+/CN /W[NH2]3 system are more favourable than those for the [H2N]3W/CN- system, the final C-N cleavage step is still endothermic by 32 kJ mol(-1) when symmetry constraints are relaxed. The resistance of these systems to C-N cleavage was examined by a bond decomposition analysis of [H2N]M-L1[triple bond]L2-M[NH2]3 intermediates for L1[triple bond]L2 = N2, CO and CN which showed that backbonding from the metal into the L1[triple bond]L2 pi* orbitals is significantly less for CN than for N2 or CO due to the negative charge on CN- which results in a large energy gap between the metal d(pi), and the pi* orbitals of CN-. This, combined with the very strong M-CN- interaction which stabilises the CN intermediate, makes C-N bond cleavage in these systems unfavourable even though the C[triple bond]N triple bond is not as strong as the bond in N2 or CO.

Rationalizing the different products in the reaction of N2 with three-coordinate MoL3 complexes.

The reaction of N2 with three-coordinate MoL3 complexes is known to give rise to different products, N-MoL3, L3Mo-N-MoL3 or Mo2L6, depending on the nature of the ligand L. The energetics of the different reaction pathways are compared for L = NH2, NMe2, N((i)Pr)Ar and N((t)Bu)Ar (Ar = 3,5-C6H3Me2) using density functional methods in order to rationalize the experimental results. Overall, the exothermicity of each reaction pathway decreases as the ligand size increases, largely due to the increased steric crowding in the products compared to reactants. In the absence of steric strain, the formation of the metal-metal bonded dimer, Mo2L6, is the most exothermic pathway but this reaction shows the greatest sensitivity to ligand size varying from significantly exothermic, -403 kJ mol(-1) for L = NMe2, to endothermic, +78 kJ mol(-1) for L = N((t)Bu)Ar. For all four ligands, formation of N-MoL3 via cleavage of the N2 bridged dimer intermediate, L3Mo-N-N-MoL3, is strongly exothermic. However, in the presence of excess reactant MoL3, formation of the single atom-bridged complex L3Mo-N-MoL3 from N-MoL3 + MoL3 is both thermodynamically and kinetically favoured for L = NMe2 and N((i)Pr)Ar, in agreement with experiment. In the case of L = N((t)Bu)Ar, the greater steric bulk of the (t)Bu group results in a much less exothermic reaction and a calculated barrier of 66 kJ mol(-1) to formation of the L3Mo-N-MoL3 dimer. Consequently, for this ligand, the energetically and kinetically favoured product, consistent with the experimental data, is the nitride complex L3Mo-N.

Breaking chemistry's strongest bond: can three-coordinate [M{N(R)Ar}3] complexes cleave carbon monoxide?

The reaction pathway for the interaction of CO with three-coordinate TaIII, WIII and ReIII complexes (modelled on the experimental [M{N(tBu)Ar}3] system) has been explored by using density functional methods. Calculations show that CO binds without a barrier to [Re(NH2)3], forming the encounter complex [OC--Re(NH2)3], which is stabilized by approximately 280 kJ mol-1 relative to the reactants. The binding of [Ta(NH2)3] to the oxygen terminus of CO is inhibited by a barrier of only 20 kJ mol-1 and is followed by spontaneous cleavage of the C--O bond to form the products [C--Re(NH2)3] and [O--Ta(NH2)3]. The salient features of the potential energy surface are more favourable to CO cleavage than the analogous N2 cleavage by [Mo(NH2)3], which is less exothermic (335 vs. 467 kJ mol-1) and is impeded by a significant barrier (66 kJ mol-1). The ReIII/TaIII/CO system therefore appears to be an excellent candidate for cleaving the exceptionally strong C--O bond under mild laboratory conditions. The related WIII/TaIII dimer, which significantly weakens but does not cleave the CO bond, may be a suitable alternative when the chemistry is to be performed on activated CO rather than on the strongly bound oxide and carbide cleavage products.

Optimizing small molecule activation and cleavage in three-coordinate M[N(R)Ar]3 complexes.

The sterically hindered, three-coordinate metal systems M[N(R)Ar]3 (R = tBu, iPr; Ar = 3,5-C6H3Me2) are known to bind and activate a number of fundamental diatomic molecules via a [Ar(R)N]3M-L-L-M[N(R)Ar]3 dimer intermediate. To predict which metals are most suitable for activating and cleaving small molecules such as N(2), NO, CO, and CN(-), the M-L bond energies in the L-M(NH2)3 (L = O, N, C) model complexes were calculated for a wide range of metals, oxidation states, and dn (n = 2-6) configurations. The strongest M-O, M-N, and M-C bonds occurred for the d2, d3, and d4 metals, respectively, and for these d(n) configurations, the M-C and M-O bonds were calculated to be stronger than the M-N bonds. For isoelectronic metals, the bond strengths were found to increase both down a group and to the left of a period. Both the calculated N-N bond lengths and activation barriers for N2 bond cleavage in the (H2N)3M-N-N-M(NH2)3 intermediate dimers were shown to follow the trends in the M-N bond energies. The three-coordinate complexes of Ta(II), W(III), and Nb(II) are predicted to deliver more favorable N2 cleavage reactions than the experimentally known Mo(III) system and the Re(III)Ta(III) dimer, [Ar(R)N]3Re-CO-Ta[N(R)Ar]3, is thermodynamically best suited for cleaving CO.

Ligand rotation in [Ar(R)N]3M-N2-M'[N(R)Ar]3(M, M' = MoIII, NbIII; R = iPr and tBu) dimers.

Earlier calculations on the model N2-bridged dimer (micro-N2)-{Mo[NH2]3}2 revealed that ligand rotation away from a trigonal arrangement around the metal centres was energetically favourable resulting in a reversal of the singlet and triplet energies such that the singlet state was stabilized 13 kJ mol(-1) below the D(3d) triplet structure. These calculations, however, ignored the steric bulk of the amide ligands N(R)Ar (R =iPr and tBu, Ar = 3,5-C6H3Me2) which may prevent or limit the extent of ligand rotation. In order to investigate the consequences of steric crowding, density functional calculations using QM/MM techniques have been performed on the Mo(III)Mo(III) and Mo(III)Nb(III) intermediate dimer complexes (mu-N(2))-{Mo[N(R)Ar]3}2 and [Ar(R)N]3Mo-(mu-N2)-Nb[N(R)Ar]3 formed when three-coordinate Mo[N(R)Ar]3 and Nb[N(R)Ar]3 react with dinitrogen. The calculations indicate that ligand rotation away from a trigonal arrangement is energetically favourable for all of the ligands investigated and that the distortion is largely electronic in origin. However, the steric constraints of the bulky amide groups do play a role in determining the final orientation of the ligands, in particular, whether the ligands are rotated at one or both metal centres of the dimer. Analogous to the model system, QM/MM calculations predict a singlet ground state for the (mu-N2)-{Mo[N(R)Ar]3}2 dimers, a result which is seemingly at odds with the experimental triplet ground state found for the related (mu-N2)-{Mo[N(tBu)Ph]3}2 system. However, QM/MM calculations on the (mu-N2)-{Mo[N(tBu)Ph]3}2 dimer reveal that the singlet-triplet gap is nearly 20 kJ mol(-1) smaller and therefore this complex is expected to exhibit very different magnetic behaviour to the (mu-N2)-{Mo[N(R)Ar]3}2 system.

Nitrogen activation via three-coordinate molybdenum complexes: comparison of density functional theory performance with wave function based methods.

Obtaining an accurate theoretical model for the activation of dinitrogen by three-coordinate molybdenum amide complexes (e.g. Mo(NH2)3) is difficult due to the interaction of various high- and low-spin open-shell complexes along the reaction coordinate which must be treated with comparable levels of accuracy in order to obtain reasonable potential energy surfaces. Density functional theory with present-day functionals is a popular choice in this situation; however, the dinitrogen activation reaction energetics vary substantially with the choice of functional. An assessment of the reaction using specialized wave function based methods indicates that although current density functionals in general agree qualitatively on the mechanistic details of the reaction, a variety of high-level electron correlation methods (including CCSD(T), OD(T), CCSD(2), KS-CCSD(T), and spin-flip CCSD) provide a consistent but slightly different representation of the system.

Activation and cleavage of dinitrogen by three-coordinate metal complexes involving Mo(III) and Nb(II/III).

Density functional calculations have been employed to rationalize why the heteronuclear N(2)-bridged Mo(III)Nb(III) dimer, [Ar((t)Bu)N](3)Mo(mu-N(2))Nb[N((i)Pr)Ar](3)(Ar = 3,5-C(6)H(3)Me(2)), does not undergo cleavage of the dinitrogen bridge in contrast to the analogous Mo(III)Mo(III) complex which, although having a less activated N-N bond, undergoes spontaneous dinitrogen cleavage at room temperature. The calculations reveal that although the overall reaction is exothermic for both systems, the actual cleavage step is endothermic by 144 kJ mol(-1) for the Mo(III)Nb(III) complex whereas the Mo(III)Mo(III) system is exothermic by 94 kJ mol(-1). The reluctance of the Mo(III)Nb(III) system to undergo N(2) cleavage is attributed to its d(3)d(2) metal configuration which is one electron short of the d(3)d(3) configuration necessary to reductively cleave the dinitrogen bridge. This is confirmed by additional calculations on the related d(3)d(3) Mo(III)Nb(II) and Nb(II)Nb(II) systems for which the cleavage step is calculated to be substantially exothermic, accounting for why in the presence of the reductant KC(8), the [Ar((t)Bu)N](3)Mo(mu-N(2))Nb[N((i)Pr)Ar](3) complex was observed to undergo spontaneous cleavage of the dinitrogen bridge. On the basis of these results, it can be concluded that the level of activation of the N-N bond does not necessarily correlate with the ease of cleavage of the dinitrogen bridge.

Dinitrogen activation in sterically-hindered three-coordinate transition metal complexes.

Dinuclear metal systems based on sterically-hindered, three-coordinate transition metal complexes of the type ML3 where the ancillary ligands L comprise bulky organic substituents, hold great promise synthetically for the activation and scission of small, multiply-bonded molecules such as N2, NO and N2O. In this study we have employed density functional methods to identify the metal/ligand combinations which achieve optimum activation and/or cleavage of N2. Strong pi donor ligands such as NH2 and OH are found to produce the greatest level of activation based on N-N bond lengths in the intermediate dimer complex, L3Mo(mu-N2)MoL3, whereas systems containing the weak or non-pi donor ligands NH3, PH3, OH2 and SH2 are found to be thermodynamically unfavourable for N2 activation. In the case of the Mo-NH2 and W-NH2 systems, a fragment bonding analysis reveals that the orientation of the amide ligands around the metal is important in determining both the spin state and the extent of dinitrogen activation in the intermediate dimer. For both systems, an intermediate dimer structure where one of the NH2 ligands on each metal is rotated 90 degrees relative to the other ligands, is more activated than the structure in which the NH2 ligands are trigonally disposed around the metals. The level of activation is found to be very sensitive to the electronic configuration of the metal with d3 metal ions delivering the best activation along any one transition series. In particular, strong activation or cleavage of N2 was calculated for the third row d3 metals systems involving Ta(II), W(III) and Re(IV), with the level of activation decreasing as the nuclear charge on the metal increases. This trend in activation reflects the size of the valence 5d orbitals and consequently, the capacity of the metal to back donate into the dinitrogen pi* orbitals.