Protonation studies of a mono-dinitrogen complex of chromium supported by a 12-membered phosphorus macrocycle containing pendant amines.

The reduction of fac-[CrCl3(P(Ph)3N(Bn)3)], (1(Cl3)), (P(Ph)3N(Bn)3 = 1,5,9-tribenzyl-3,7,11-triphenyl-1,5,9-triaza-3,7,11-triphosphacyclododecane) with Mg in the presence of dmpe (dmpe = 1,2-bis(dimethylphosphino)ethane) affords the first example of a monodinitrogen Cr(0) complex, Cr(N2)(dmpe)(P(Ph)3N(Bn)3), (2(N2)), containing a pentaphosphine coordination environment. 2(N2) is supported by a unique facially coordinating 12-membered phosphorus macrocycle containing pendant amine groups in the second coordination sphere. Treatment of 2(N2) at -78 °C with 1 equiv of [H(OEt2)2][B(C6F5)4] results in protonation of the metal center, generating the seven-coordinate Cr(II)-N2 hydride complex, [Cr(H)(N2)(dmpe)(P(Ph)3N(Bn)3)][B(C6F5)4], [2(H)(N2)](+). Treatment of 2((15)N2) with excess triflic acid at -50 °C afforded a trace amount of (15)NH4(+) from the reduction of the coordinated (15)N2 ligand (electrons originate from Cr). Electronic structure calculations were employed to evaluate the pKa values of three protonated sites of 2(N2) (metal center, pendant amine, and N2 ligand) and were used to predict the thermodynamically preferred Cr-NxHy intermediates in the N2 reduction pathway for 2(N2) and the recently published complex trans-[Cr(N2)2(P(Ph)4N(Bn)4)] upon the addition of protons and electrons.

How does Nishibayashi's molybdenum complex catalyze dinitrogen reduction to ammonia?

Recently, Nishibayashi et al. reported a dimolybdenum-dinitrogen complex that is catalytic for complete reduction of dinitrogen to ammonia. This catalyst is different from the Schrock molybdenum catalyst in two fundamental aspects: it contains two metal centers, and the oxidation state is Mo(0) instead of Mo(III). We show that a remarkable feature of the bimetallic complex is the bond-mediated delocalized electronic states, resulting from the two metal centers bridged by a dinitrogen ligand. Using first-principles calculations, we found that this property makes the bimetallic complex the effective catalyst, as opposed to the originally postulated monometallic fragment. A favorable reaction pathway is identified, and the nature of the intermediates is examined. Furthermore, studies of the intermediate states led us to propose possible deactivation processes of the catalyst. The finding that the central bimetallic unit (Mo-N2-Mo) is relevant for catalytic activity may provide a guideline for the development of more efficient dinitrogen-reducing catalysts.

Synthetic approaches to (smif)2Ti (smif = 1,3-di-(2-pyridyl)-2-azaallyl) reveal redox non-innocence and C-C bond-formation.

Attempted syntheses of (smif)(2)Ti (smif =1,3-di-(2-pyridyl)-2-azaallyl) based on metatheses of TiCl(n)L(m) (n = 2-4) with M(smif) (M = Li, Na), in the presence of a reducing agent (Na/Hg) when necessary, failed, but several apparent Ti(II) species were identified by X-ray crystallography and multidimensional NMR spectroscopy: (smif){Li(smif-smif)}Ti (1, X-ray), [(smif)Ti](2)(µ-κ(3),κ(3)-N,N(py)(2)-smif,smif) (2), (smif)Ti(κ(3)-N,N(py)(2)-smif,(smif)H) (3), and (smif)Ti(dpma) (4, dpma = di-2-pyridylmethyl-amide). NMR spectroscopy and K-edge XAS showed that each compound possesses ligands that are redox noninnnocent, such that d(1) Ti(III) centers AF-couple to ligand radicals: (smif){Li(smif-smif)(2-)}Ti(III) (1), [(smif(2-))Ti(III)](2)(µ-κ(3),κ(3)-N,N(py)(2)-smif,smif) (2), [(smif(2-))Ti(III)](κ(3)-N,N(py)(2)-smif,(smif)H) (3), and (smif(2-))Ti(III)(dpma) (4). The instability of (smif)(2)Ti relative to its C-C coupled dimer, 2, is rationalized via the complementary nature of the amide and smif radical dianion ligands, which are also common to 3 and 4. Calculations support this contention.

Pt(II) and Pt(IV) amido, aryloxide, and hydrocarbyl complexes: synthesis, characterization, and reaction with dihydrogen and substrates that possess C-H bonds.

The Pt(II) amido and phenoxide complexes ((t)bpy)Pt(Me)(X), ((t)bpy)Pt(X)(2), and [((t)bpy)Pt(X)(py)][BAr'(4)] (X = NHPh, OPh; py = pyridine) have been synthesized and characterized. To test the feasibility of accessing Pt(IV) complexes by oxidizing their Pt(II) precursors, the previously reported ((t)bpy)Pt(R)(2) (R = Me and Ph) systems were oxidized with I(2) to yield ((t)bpy)Pt(R)(2)(I)(2). The analogous reaction with ((t)bpy)Pt(Me)(NHPh) and MeI yields the corresponding ((t)bpy)Pt(Me)(2)(NHPh)(I) complex. Reaction of ((t)bpy)Pt(Me)(NHPh) and phenylacetylene at 80 °C results in the formation of the Pt(II) phenylacetylide complex ((t)bpy)Pt(Me)(C≡CPh). Kinetic studies indicate that the reaction of ((t)bpy)Pt(Me)(NHPh) and phenylacetylene occurs via a pathway that involves [((t)bpy)Pt(Me)(NH(2)Ph)][TFA] as a catalyst. The reaction of H(2) with ((t)bpy)Pt(Me)(NHPh) ultimately produces aniline, methane, (t)bpy, and elemental Pt. For this reaction, mechanistic studies reveal that 1,2-addition of dihydrogen across the Pt-NHPh bond to initially produce ((t)bpy)Pt(Me)(H) and free aniline is catalyzed by elemental Pt. Heating the cationic complexes [((t)bpy)Pt(NHPh)(py)][BAr'(4)] and [((t)bpy)Pt(OPh)(py)][BAr'(4)] in C(6)D(6) does not result in the production of aniline and phenol, respectively. Attempted synthesis of a cationic system analogous to [((t)bpy)Pt(NHPh)(py)][BAr'(4)] with ligands that are more labile than pyridine (e.g., NC(5)F(5)) results in the formation of the dimer [((t)bpy)Pt(µ-NHPh)](2)[BAr'(4)](2). Solid-state X-ray diffraction studies of the complexes ((t)bpy)Pt(Me)(NHPh), [((t)bpy)Pt(NH(2)Ph)(2)][OTf](2), ((t)bpy)Pt(NHPh)(2), ((t)bpy)Pt(OPh)(2), ((t)bpy)Pt(Me)(2)(I)(2), and ((t)bpy)Pt(Ph)(2)(I)(2) are reported.

Net hydrogenation of Pt-NHPh bond is catalyzed by elemental Pt.

Synthesis and characterization of the monomeric complex ((t)bpy)Pt(Me)(NHPh) ((t)bpy = 4,4'-di-tert-butyl-2,2'-dipyridyl) has been accomplished. Mechanistic studies reveal that 1,2-addition of dihydrogen across the Pt-anilido bond to initially produce ((t)bpy)Pt(Me)(H) and free aniline is catalyzed by elemental Pt rather than through a pathway that involves direct activation of H(2) by Pt and 1,2-addition across the Pt-NHPh bond.

Computational study of methane C-H activation by first-row late transition metal L(n)M=E (M: Fe, Co, Ni) complexes.

Methane functionalization via L(n)M=E active species (L(n) = beta-diketiminate, dihydrophosphinoethane; M = Fe-Ni, E = NCF(3), NCH(3), O) through a hydrogen atom abstraction (HAA)/radical rebound (RR) mechanism is calculated to be thermodynamically and kinetically feasible. The enthalpies of each reaction decrease in the order Fe > Co > Ni and with the proximity of CF(3) supporting ligand substituents ("fluorination") to the metal center. For HAA, lower abstraction enthalpies were calculated for L(n) = beta-diketiminate and E = NCF(3) rather than dhpe and NCH(3), respectively, whereas the opposite trend was found for RR enthalpies. The overall functionalization thermodynamics were optimal for L(n) = beta-diketiminate and E = NCH(3), with similar enthalpies for E = O when M = Ni. The HAA kinetics further implicate fluorinated (beta-diket)Ni=O as the most promising methane functionalization complexes, with calculated activation barriers as low as 8.1 kcal mol(-1).

Cobalt-dinitrogen complexes with weakened N-N bonds.

Reported N(2) complexes of cobalt do not have substantial weakening of the N-N bond. Using diketiminate ligands to enforce three-coordinate geometries, we have synthesized several novel CoNNCo complexes. In formally univalent complexes, cobalt is poorer than iron at weakening the N-N bond, but in formally zerovalent complexes, cobalt and iron give similar N-N weakening. The weakening is due to cobalt-to-N(2) pi-backbonding, and potassium cations pull more electron density into N(2). These results show that the low coordination number of a trigonal-planar geometry is impetus enough to make even the electronegative cobalt weaken the N-N bond of N(2).

Evidence for strong tantalum-to-boron dative interactions in (silox)3Ta(BH3) and (silox)3Ta(eta2-B,Cl-BCl2Ph) (silox = tBu3SiO)1.

Treatment of (silox)3Ta (1, silox = tBu3SiO) with BH3.THF and BCl2Ph afforded (silox)3Ta(BH3) (2) and (silox)3Ta(eta2-B,Cl-BCl2Ph) (3), which are both remarkably stable Ta(III) compounds. NMe3 and ethylene failed to remove BH3 from 2, and no indication of BH3 exchange with BH3.THF-d8 was noted via variable-temperature 1H NMR studies. Addition of BH3.THF to (silox)3TaH2 provided the borohydride-hydride (silox)3HTa(eta3-BH4) (5), and its thermolysis released H2 to generate 2. Exposure of 2 to D2 enabled the preparation of isotopologues (silox)3Ta(BH3-nDn) (n = 0, 2; 1, 2-D; 2, 2-D2; 3, 2-D3) for isotopic perturbation of chemical shift studies, but these failed to distinguish between "inverse adduct" (i.e., (silox)3Ta-->BH3) or (silox)3Ta(eta2-B,H-BH3) forms of 2. Computational models (RO)3Ta(BH3) (R = H, 2'; SiH3, 2SiH SiMe3, 2SiMe, and SitBu3, 2SiBu) were investigated to assess the relative importance of steric and electronic effects on structure and bonding. With small R, eta2-B,H structures were favored, but for 2SiMe and 2SiBu, the dative structure proved to be similar in energy. The electonic and vibrational features of both structure types were probed. The IR spectrum of 2 was best matched by the eta2-B,H conformer of 2SiBu. In related computations pertaining to 3, small R models favored the oxidative addition of a BCl bond, while with R = SitBu3 (3SiBu), an excellent match with its X-ray crystal structure revealed the critical steric influence of the silox ligands.

Chemistry surrounding monomeric copper(I) methyl, phenyl, anilido, ethoxide, and phenoxide complexes supported by N-heterocyclic carbene ligands: reactivity consistent with both early and late transition metal systems.

Monomeric copper(I) alkyl complexes that possess the N-heterocyclic carbene (NHC) ligands IPr, SIPr, and IMes [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] react with amines or alcohols to release alkane and form the corresponding monomeric copper(I) amido, alkoxide, or aryloxide complexes. Thermal decomposition reactions of (NHC)Cu(I) methyl complexes at temperatures between 100 and 130 degrees C produce methane, ethane, and ethylene. The reactions of (NHC)Cu(NHPh) complexes with bromoethane reveal increasing nucleophilic reactivity at the anilido ligand in the order (SIPr)Cu(NHPh) < (IPr)Cu(NHPh) < (IMes)Cu(NHPh) < (dtbpe)Cu(NHPh) [dtbpe = 1,2-bis(di-tert-butylphosphino)ethane]. DFT calculations suggest that the HOMO for the series of Cu anilido complexes is localized primarily on the amido nitrogen with some ppi(anilido)-dpi(Cu) pi-character. [(IPr)Cu(mu-H)]2 and (IPr)Cu(Ph) react with aniline to quantitatively produce (IPr)Cu(NHPh)/dihydrogen and (IPr)Cu(NHPh)/benzene, respectively. Analysis of the DFT calculations reveals that the conversion of [(IPr)Cu(mu-H)]2 and aniline to (IPr)Cu(NHPh) and dihydrogen is favorable with DeltaH approximately -7 kcal/mol and DeltaG approximately -9 kcal/mol.