Synthesis, Structures and Reactivity of Group 4 Hydrazido Complexes Supported by Calix[4]arene Ligands

Electronic Structures of an [Fe(NNR2)]+/0/- Redox Series: Ligand Noninnocence and Implications for Catalytic Nitrogen Fixation

The intermediacy of metal-NNH₂ complexes has been implicated in the catalytic cycles of several examples of transition-metal-mediated nitrogen (N₂) fixation. In this context, we have shown that triphosphine-supported Fe(N₂) complexes can be reduced and protonated at the distal N atom to yield Fe(NNH₂) complexes over an array of charge and oxidation states. Upon exposure to further H⁺/e^- equivalents, these species either continue down a distal-type Chatt pathway to yield a terminal iron(IV) nitride or instead follow a distal-to-alternating pathway resulting in N-H bond formation at the proximal N atom. To understand the origin of this divergent selectivity, herein we synthesize and elucidate the electronic structures of a redox series of Fe(NNMe₂) complexes, which serve as spectroscopic models for their reactive protonated congeners. Using a combination of spectroscopies, in concert with density functional theory and correlated ab initio calculations, we evidence one-electron redox noninnocence of the "NNMe₂" moiety. Specifically, although two closed-shell configurations of the "NNR₂" ligand have been commonly considered in the literature-isodiazene and hydrazido(2-)-we provide evidence suggesting that, in their reduced forms, the present iron complexes are best viewed in terms of an open-shell [NNR₂]^•- ligand coupled antiferromagnetically to the Fe center. This one-electron redox noninnocence resembles that of the classically noninnocent ligand NO and may have mechanistic implications for selectivity in N₂ fixation activity.

New Titanium Borylimido Compounds: Synthesis, Structure, and Bonding

We report a combined experimental and computational study of the synthesis and electronic structure of titanium borylimido compounds. Three new synthetic routes to this hitherto almost unknown class of Group 4 imide are presented. The double-deprotonation reaction of the borylamine H₂NB(NAr'CH)₂ (Ar' = 2,6-C₆H₃ⁱPr₂) with Ti(NMe₂)₂Cl₂ gave Ti{NB(NAr'CH)₂}Cl₂(NHMe₂)₂, which was easily converted to Ti{NB(NAr'CH)₂}Cl₂(py)₃. This compound is an entry point to other borylimides, for example, reacting with Li₂N₂^pyrN^Me to form Ti(N₂^pyrN^Me){NB(NAr'CH)₂}(py)₂ and with 2 equiv of NaCp to give Cp₂Ti{NB(NAr'CH)₂}(py) (23). Borylamine-tert-butylimide exchange between H₂NB(NAr'CH)₂ and Cp*Ti(N^tBu)Cl(py) under forcing conditions afforded Cp*Ti{NB(NAr'CH)₂}Cl(py), which could be further substituted with guanidinate or pyrrolide-amine ligands to give Cp*Ti(hpp){NB(NAr'CH)₂} (16) and Cp*Ti(N^pyrN^Me₂){NB(NAr'CH)₂} (17). The Ti-N_im distances in compounds with the NB(NAr'CH)₂ ligand were comparable to those of the corresponding arylimides. Dialkyl- or diaryl-substituted borylamines do not undergo the analogous double-deprotonation or imide-amine exchange reactions. Reaction of (Cp″₂Ti)₂(μ₂:η¹,η¹-N₂) with N₃BMes₂ gave the base-free, diarylborylimide Cp″₂Ti(NBMes₂) (26) by an oxidative route; this compound has a relatively long Ti-N_im bond and large Cp″-Ti-Cp″ angle. Reaction of 16 with H₂N^tBu formed equilibrium mixtures with H₂NB(NAr'CH)₂ and Cp*Ti(hpp)(N^tBu) (Δ_rG = -1.0 kcal mol^-1). In contrast, the dialkylborylimide Cp*Ti{MeC(NⁱPr)₂}(NBC₈H₁₄) (2) reacted quantitatively with H₂N^tBu to give the corresponding tert-butylimide and borylamine. The electronic structures and imide-amine exchange reactions of half-sandwich and sandwich titanium borylimides have been evaluated using density functional theory (DFT), supported by quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis, and placed more generally in context with the well-established alkyl- and arylimides and hydrazides. The calculations find that Ti-N_im bonds for borylimides are stronger and more covalent than in their organoimido or hydrazido analogues, and are strongest for alkyl- and arylborylimides. Borylamine-tert-butylimide exchange reactions fail for H₂NBR₂ (R = hydrocarbyl) but not for H₂NB(NAr'CH)₂ because the increased strength of the new Ti-N_im bond for the former is outweighed by the increased net H-N bond strengths in the borylamine. Variation of the Ti-N_im bond length over short distances is dominated by π-interactions with any appropriate orbital on the N_im atom organic substituent. However, over the full range of imides and hydrazides studied, overall bond energies do not correlate with bond length but with the Ti-N_im σ-bond character and the orthogonal π-interaction.

Molecular titanium nitrides: nucleophiles unleashed

In this contribution we present reactivity studies of a rare example of a titanium salt, in the form of [μ2-K(OEt2)]2[(PN)2Ti[triple bond, length as m-dash]N]2 (1) (PN- = N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide) to produce a series of imide moieties including rare examples such as methylimido, borylimido, phosphonylimido, and a parent imido. For the latter, using various weak acids allowed us to narrow the pK a range of the NH group in (PN)2Ti[triple bond, length as m-dash]NH to be between 26-36. Complex 1 could be produced by a reductively promoted elimination of N2 from the azide precursor (PN)2TiN3, whereas reductive splitting of N2 could not be achieved using the complex (PN)2Ti[double bond, length as m-dash]N[double bond, length as m-dash]N[double bond, length as m-dash]Ti(PN)2 (2) and a strong reductant. Complete N-atom transfer reactions could also be observed when 1 was treated with ClC(O)tBu and OCCPh2 to form NCtBu and KNCCPh2, respectively, along with the terminal oxo complex (PN)2Ti[triple bond, length as m-dash]O, which was also characterized. A combination of solid state 15N NMR (MAS) and theoretical studies allowed us to understand the shielding effect of the counter cation in dimer 1, the monomer [K(18-crown-6)][(PN)2Ti[triple bond, length as m-dash]N], and the discrete salt [K(2,2,2-Kryptofix)][(PN)2Ti[triple bond, length as m-dash]N] as well as the origin of the highly downfield 15N NMR resonance when shifting from dimer to monomer to a terminal nitride (discrete salt). The upfield shift of 15Nnitride resonance in the 15N NMR spectrum was found to be linked to the K+ induced electronic structural change of the titanium-nitride functionality by using a combination of MO analysis and quantum chemical analysis of the corresponding shielding tensors.

Redox Non-Innocent Behavior of a Terminal Iridium Hydrazido(2-) Triple Bond

The synthesis of the first terminal Group 9 hydrazido(2-) complex, Cp*IrN(TMP) (6) (TMP=2,2,6,6-tetramethylpiperidine) is reported. Electronic structure and X-ray diffraction analysis indicate that this complex contains an Ir-N triple bond, similar to Bergman's seminal Cp*Ir(Nt Bu) imido complex. However, in sharp contrast to Bergman's imido, 6 displays remarkable redox non-innocent reactivity owing to the presence of the Nβ lone pair. Treatment of 6 with MeI results in electron transfer from Nβ to Ir prior to oxidative addition of MeI to the iridium center. This behavior opens the possibility of carrying out facile oxidative reactions at a formally IrIII metal center through a hydrazido(2-)/isodiazene valence tautomerization.

Crystal structure of 2,2-di-phenyl-hydrazinium chloride

In the title mol­ecule, the phenyl rings are inclined to one another by 78.63 (17)°. In the crystal, mol­ecules are linked via N—H⋯Cl hydrogen bonds, forming chains along [10-1], which enclose two adjacent (6) ring motifs.

In the title compound, C₁₂H₁₃N₂⁺·Cl⁻, the chloride salt of 1,1′-di­phenyl­hydrazine, the phenyl rings are inclined to one another by 78.63 (17)°. The N—⁺NH₃ bond lengths is 1.445 (3) Å, and the N—C_phen­yl bond lengths are 1.435 (3) and 1.447 (4) Å. In the crystal, mol­ecules are linked via N—H⋯Cl hydrogen bonds, forming chains along [10-1], which enclose two adjacent R⁴₂(6) ring motifs. The chains are reinforced by C—H⋯Cl hydrogen bonds.

(E)-1-(2,4-Di-nitro-benzyl-idene)-2,2-di-phenyl-hydrazine

In the crystal of the title compound, C19H14N4O4, the asymmetric unit consists of two discrete mol-ecules. The C=N bonds in both mol-ecules show an E conformation. The dihedral angles between the C atoms of the 2,4-dinitrobenzene rings and the C=N-N planes are 13.52 (9) and 13.82 (9)° for the two mol-ecules. In the crystal, C-H⋯O hydrogen bonds, mainly between the phenyl ring and the nitro group along the b axis.

Coordination chemistry of N,N,N',N'-tetrakis(3,5-substituted benzyl-2-oxide)-2,2'-(ethylenedioxy)diethanamine modified Group 4 metal alkoxides

The coordination behavior of a set of (ethylenedioxy)diethanamine-based tetraphenol ligands with a series of Group 4 metal alkoxides ([M(OR)(4)]) was determined. The ligands were synthesized from a modified Mannich reaction and fully characterized as N,N,N',N'-tetrakis(3,5-tert-butyl-benzyl-2-hydroxy)-2,2'-(ethylenedioxy)diethanamine, termed H(4)-OEA-DBP(4) (1), and N,N,N',N'-tetrakis(3,5-chloro-benzyl-2-hydroxy)-2,2'-(ethylenedioxy)diethanamine, termed H(4)-OEA-DCP(4) (2). The reaction of 1 with a set of [M(OR)(4)] [M = Ti, Zr, or Hf; OR = iso-propoxide (OPr(i)), neo-pentoxide (ONep), or tert-butoxide (OBu(t))] precursors led to the isolation of [(OPr(i))(2)Ti](2)(μ-OEA-DBP(4)) (3), [(ONep)(2)Ti](2)(μ-OEA-DBP(4)) (4), and [(OBu(t))(2)M](2)(μ-OEA-DBP(4)) where M = Ti (5), Zr (6), or Hf (7). In addition, the [(ONep)(2)Ti](2)(μ-OEA-DCP(4)) (4a) derivative was isolated from the reaction of 2 and [Ti(ONep)(4)], demonstrating the similarity of coordination behavior between the two OEA-R(4) ligands. For 3-7, the metal center adopts a slightly distorted octahedral geometry by binding the two O atoms of the phenoxide moiety, as well as one N and one O atom from the OEA moieties, while retaining two of the original OR ligands. Solution NMR demonstrates inequivalent protons for the majority of the bound OEA ligands, which argues for retention of structure in solution. The synthesis and characterization of these compounds are presented in detail.

(E)-1-Benzyl-idene-2,2-diphenyl-hydrazine

The asymmetric unit of the title compound, C₁₉H₁₆N₂, contains two independent mol­ecules, both of which show an E configuration with respect to the C=N bond. The dihedral angles between the phenyl rings bonded to the hydrazine group are 81.00 (10) and 88.34 (8)° in the two mol­ecules. Inter­molecular C—H⋯π inter­actions are observed in the crystal structure.

(1E,2E)-1,2-Bis(2,2-diphenylhydrazin-1-ylidene)ethane

In the crystal structure of the title compound, C₂₆H₂₂N₄, the mol­ecule is located on an inversion centre and shows an E configuration with respect to each C=N bond. The dihedral angle between the phenyl rings in the diphenyl­hydrazone group is 83.69 (11)°. These two rings make dihedral angles of 30.53 (15) and 84.53 (16)° with the central N—N=C—C=N—N dihydrazonoethane plane. Inter­molecular C—H⋯π inter­actions are observed.

N′-[5-(4-Nitrophenyl)furan-2-ylmethylidene]-N,N-diphenylhydrazine

The title compound, C₂₃H₁₇N₃O₃, has an E configuration with respect to the C=N bond. The dihedral angle between the two phenyl rings bonded to the hydrazine group is 86.45 (13)°. The furan ring makes dihedral angles of 3.4 (2) and 7.06 (13)°, respectively, with the methyl­idenehydrazine C=N—N plane and the benzene ring.