A Metallocene Bis(phosphoranocarbene) of Uranium and a Probe of Its Reactivity with Alcohols.

The addition of 2 equiv of the phosphaylide H2C═PPh3 to the dimethyl uranium metallocene Cp*2UMe2 (Cp* = η5-C5Me5) in toluene with gentle heating at 40 °C generates the phosphorano-stabilized bis(carbene) Cp*2U[C(H)PPh3]2 (1) in good yield. Characterization of 1 by X-ray crystallographic analysis reveals two short uranium-carbon bonds, ranging from 2.301(5) to 2.322(5) Å, consistent with the presence of U═C carbene-type bonds. Monitoring the reaction by NMR spectroscopy suggests that it proceeds through the intermediate formation of the methyl carbene complex Cp*2U[C(H)PPh3](Me) (1Int); however, prolonged heating of these solutions leads to the ortho-cyclometalated carbene species Cp*2U{κ2-[C(H)PPh2(C6H4)]} (2) via intramolecular C-H activation. Rapid conversion from 1 to 2 occurs within hours upon heating its toluene solutions to 100 °C. Preliminary reactivity studies of 1 show that it readily reacts with alcohols, such as HODipp (Dipp = 2,6-diisopropylphenyl) and HOC(CF3)3, to give the mixed carbene alkoxide compounds Cp*2U[C(H)PPh3](OR) (R = Dipp (4Dipp), C(CF3)3 (5CF3)). In one case, the reaction of 1 with HODipp in the presence of adventitious water led to the formation of a few crystals of the terminal U(IV) oxo complex, [Ph3PCH3][Cp*2U(O)(ODipp)] (3oxo). The isolation of 1 marks the first instance of an unchelated, heteroatom-stabilized bis(carbene) complex of uranium that also provides an entryway to the synthesis of its monocarbene derivatives through protonolysis.

Photoinduced Cleavage of a Strained N-C Bond in an Iron Complex Supported by Super-Bulky Amidinate and Guanidinate Ligands.

The reaction of Fe2(mes)4 with the super-bulky amidines and guanidines HLAr*-R (LAr*-R = [(Ar*N)2C(R)]-, Ar* = 2,6-bis(diphenylmethyl)-4-tert-butylphenyl), R = Me (LAr*-Me), tBu (LAr*-tBu), Ph (LAr*-Ph), NiPr2 (LAr*-iPr2N), and Pip (LAr*-Pip)) gives access to the three-coordinate iron-mesityl complexes (LAr*-R)Fe(mes) only where LAr*-R = LAr*-Me, LAr*-Ph, or LAr*-Pip. Subsequent protonolysis with the N-atom transfer reagent Hdbabh (Hdbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene) is limited in success, providing in one instance a few crystals of four-coordinate (LAr*-Me)Fe(dbabh)(Hdbabh), while three-coordinate (LAr*-Pip)Fe(dbabh) is synthesized reproducibly. Complexes (LAr*-Me)Fe(dbabh)(Hdbabh) and (LAr*-Pip)Fe(dbabh) are thermally insensitive in solution to temperatures of up to 100 °C. On the other hand, both (LAr*-Me)Fe(dbabh)(Hdbabh) and (LAr*-Pip)Fe(dbabh) show sensitivity to blue LED light (395 nm), undergoing photochemical transformations. For instance, the photolysis of (LAr*-Me)Fe(dbabh)(Hdbabh) leads to N-C bond scission and C-C bond coupling across the -dbabh moieties to give four-coordinate (LAr*-Me)Fe(N=dbabh-dbabhNH2). Photolyzing pyridine-d5 (py-d5) solutions of (LAr*-Pip)Fe(dbabh) at -5 °C produces a new paramagnetic photoproduct, [P]. Due to the thermal sensitivity of compound [P], it has eluded structural characterization; yet, Evans' method measurements suggest that the iron(II) oxidation state is maintained, thereby pointing to the -dbabh moiety as the locus of chemical change. In line with this assessment, addition of excess Me3SiCl to solutions of [P] produces the iron(II) complex (LAr*-Pip)FeCl(py-d5) as shown by 1H NMR spectroscopy. Gas chromatography/mass spectrometry analysis of the solutions of [P] shows a peak in the chromatogram with a molecular mass corresponding to a formulation of C14H11N that cannot be attributed to Hdbabh. This provides evidence for the photochemical-induced isomerization of the -dbabh ligand, revealing a heretofore unknown photochemical sensitivity of this N atom transfer reagent.

Synthesis and Characterization of U≡C Triple Bonds in Fullerene Compounds.

Despite decades of efforts, the actinide-carbon triple bond has remained an elusive target, defying synthesis in any isolable compound. Herein, we report the successful synthesis of uranium-carbon triple bonds in carbide-bridged bimetallic [U≡C-Ce] units encapsulated inside the fullerene cages of C72 and C78. The molecular structures of UCCe@C2n and the nature of the U≡C triple bond were characterized through X-ray crystallography and various spectroscopic analyses, revealing very short uranium-carbon bonds of 1.921(6) and 1.930(6) Å, with the metals existing in their highest oxidation states of +6 and +4 for uranium and cerium, respectively. Quantum-chemical studies further demonstrate that the C2n cages are crucial for stabilizing the [UVI≡C-CeIV] units through covalent and coordinative interactions. This work offers a new fundamental understanding of the elusive uranium-carbon triple bond and informs the design of complexes with similar bonding motifs, opening up new possibilities for creating distinctive molecular compounds and materials.

Molecular Capacitors: Accessible 6- and 8-Electron Redox Chemistry from Dimeric "Ti(I)" and "Ti(0)" Synthons Supported by Imidazolin-2-Iminato Ligands.

Reduction of the diamagnetic Ti(III)/Ti(III) dimer [Cl2Ti(µ-NImDipp)]2 (1) (NImDipp = [1,3-bis(Dipp)imidazolin-2-iminato]-, Dipp = C6H3-2,6-iPr2) with 4 and 6 equiv of KC8 generates the intramolecularly arene-masked, dinuclear titanium compounds [(µ-N-η6-ImDipp)Ti]2 (2) and {[(Et2O)2K](µ-N-µ-η6:η6-ImDipp)Ti}2 (3), respectively, in modest yields. The compounds have been structurally characterized by X-ray crystallographic analysis, and inspection of the bond metrics within the η6-coordinated aryl substituent of the bridging imidazolin-2-iminato ligand shows perturbation of the aromatic system most consistent with two-electron reduction of the ring. As such, 2 and 3 can be assigned respectively as possessing metal centers in formal Ti(III)/Ti(III) and Ti(II)/Ti(II) oxidation states. Exploration of their redox chemistry reveal the ability to reduce several substrate equivalents. For instance, treatment of 2 with excess C8H8 (COT) forms the novel COT-bridged complex [(ImDippN)(η8-COT)Ti](µ-η2:η3-COT)[Ti(η4-COT)(NImDipp)] (4) that dissociates in THF solutions to give mononuclear (ImDippN)Ti(η8-COT)(THF) (5). Addition of COT to 3 yields heterometallic [(ImDippN)(η4-COT)Ti(µ-η4:η5-COT)K(THF)(µ-η6:η4-COT)Ti(NImDipp)(µ-η4:η4-COT)K(THF)2]n (6). Compounds 4 and 5 are the products of the 4-electron oxidation of 2, while 6 stands as the 8-electron oxidation product of 3. Reduction of organozides was also explored. Low temperature reaction of 2 with 4 equiv of AdN3 gives the terminal and bridged imido complex [(ImDippN)Ti(═NAd)](µ-NAd)2[Ti(NImDipp)(N3Ad)] (7) that undergoes intermolecular C-H activation of toluene at room temperature to afford the amido compound [(ImDippN)Ti(NHAd)](µ-NAd)2[Ti(C6H4Me)(NImDipp)] (8-tol). These complexes are the 6-electron oxidation products of the reaction of 2 with AdN3. Furthermore, treatment of 3 with 4 equiv of AdN3 produces the thermally stable Ti(III)/Ti(III) terminal and bridged imido [K(18-crown-6)(THF)2]{[(ImDippN)Ti(NAd)](µ-NAd)2K[Ti(NImDipp)]} (10). Altogether, these reactions firmly establish 2 and 3 as unprecedented Ti(I)/Ti(I) and Ti(0)/Ti(0) synthons with the clear capacity to effect multielectron reductions ranging from 4 to 8 electrons.

Chemical Control of Spin-Orbit Coupling and Charge Transfer in Vacancy-Ordered Ruthenium(IV) Halide Perovskites.

Vacancy-ordered double perovskites are attracting significant attention due to their chemical diversity and interesting optoelectronic properties. With a view to understanding both the optical and magnetic properties of these compounds, two series of RuIV halides are presented; A2 RuCl6 and A2 RuBr6 , where A is K, NH4 , Rb or Cs. We show that the optical properties and spin-orbit coupling (SOC) behavior can be tuned through changing the A cation and the halide. Within a series, the energy of the ligand-to-metal charge transfer increases as the unit cell expands with the larger A cation, and the band gaps are higher for the respective chlorides than for the bromides. The magnetic moments of the systems are temperature dependent due to a non-magnetic ground state with Jeff =0 caused by SOC. Ru-X covalency, and consequently, the delocalization of metal d-electrons, result in systematic trends of the SOC constants due to variations in the A cation and the halide anion.

Isolation of a Bimetallic Cobalt(III) Nitride and Examination of Its Hydrogen Atom Abstraction Chemistry and Reactivity toward H2.

Room temperature photolysis of the bis(azide)cobaltate(II) complex [Na(THF)x][(ketguan)Co(N3)2] (ketguan = [(tBu2CN)C(NDipp)2]-, Dipp = 2,6-diisopropylphenyl) (3a) in THF cleanly forms the binuclear cobalt nitride Na(THF)4{[(ketguan)Co(N3)]2(µ-N)} (1). Compound 1 represents the first example of an isolable, bimetallic cobalt nitride complex, and it has been fully characterized by spectroscopic, magnetic, and computational analyses. Density functional theory supports a CoIII═N═CoIII canonical form with significant π-bonding between the cobalt centers and the nitride atom. Unlike other group 9 bridging nitride complexes, no radical character is detected at the bridging N atom of 1. Indeed, 1 is unreactive toward weak C-H donors and even cocrystallizes with a molecule of cyclohexadiene (CHD) in its crystallographic unit cell to give 1·CHD as a room temperature stable product. Notably, addition of pyridine to 1 or photolyzed solutions of [(ketguan)Co(N3)(py)]2 (4a) leads to destabilization via activation of the nitride unit, resulting in the mixed-valent Co(II)/Co(III) bridged imido species [(ketguan)Co(py)][(ketguan)Co](µ-NH)(µ-N3) (5) formed from intermolecular hydrogen atom abstraction (HAA) of strong C-H bonds (BDE ∼ 100 kcal/mol). Kinetic rate analysis of the formation of 5 in the presence of C6H12 or C6D12 gives a KIE = 2.5 ± 0.1, supportive of a HAA formation pathway. The reactivity of our system was further probed by photolyzing benzene/pyridine solutions of 4a under H2 and D2 atmospheres (150 psi), which leads to the exclusive formation of the bis(imido) complexes [(ketguan)Co(µ-NH)]2 (6) and [(ketguan)Co(µ-ND)]2 (6-D), respectively, as a result of dihydrogen activation. These results provide unique insights into the chemistry and electronic structure of late 3d metal nitrides while providing entryway into C-H activation pathways.

Unusual Dinitrogen Binding and Electron Storage in Dinuclear Iron Complexes.

A rare example of a dinuclear iron core with a non-linearly bridged dinitrogen ligand is reported in this work. One-electron reduction of [(tBupyrr2py)Fe(OEt2)] (1) (tBupyrr2py2- = 2,6-bis((3,5-di-tert-butyl)pyrrol-2-yl)pyridine) with KC8 yields the complex [K]2[(tBupyrr2py)Fe]2(µ2-η1:η1-N2) (2), where the unusual cis-divacant octahedral coordination geometry about each iron and the η5-cation-π coordination of two potassium ions with four pyrrolyl units of the ligand cause distortion of the bridging end-on µ-N2 about the FeN2Fe core. Attempts to generate a Et2O-free version of 1 resulted instead in a dinuclear helical dimer, [(tBupyrr2py)Fe]2 (3), via bridging of the pyridine moieties of the ligand. Reduction of 3 by two electrons under N2 does not break up the dimer, nor does it result in formation of 2 but instead formation of the ate-complex [K(OEt2)]2[(tBupyrr2py)Fe]2 (4). Reduction of 1 by two electrons and in the presence of crown-ether forms the tetraanionic N2 complex [K2][K(18-crown-6)]2(tBupyrr2py)Fe]2(µ2-η1:η1-N2) (5), also having a distorted FeN2Fe moiety akin to 2. Complex 2 is thermally unstable and loses N2, disproportionating to Fe nanoparticles among other products. A combination of single-crystal X-ray diffraction studies, solution and solid-state magnetic studies, and 57Fe Mössbauer spectroscopy has been applied to characterize complexes 2-5, whereas DFT studies have been used to help explain the bonding and electronic structure in these unique diiron-N2 complexes 2 and 5.

Titanium-Mediated Catalytic Hydrogenation of Monocyclic and Polycyclic Arenes.

Two electron-reduction of the TiIV guanidinate complex (ImDipp N)(Xyket guan)TiCl2 gives (η6 -ImDipp N)(xyket guan)Ti (1intra ) and (ImDipp N)(Xyket guan)Ti(η6 -C6 H6 ) (1inter ) (Xyket guan=[(tBuC=N)C(NXylyl)2 ]- , Xylyl=2,5-dimethylphenyl) in the absence or presence of benzene, respectively. These complexes have been found to hydrogenate monocyclic and polycyclic arenes under relatively mild conditions (150 psi, 80 °C)-the first example of catalytic, homogeneous arene hydrogenation with TON >1 by a Group IV system.

Coordination of Uranyl to the Redox-Active Calix[4]pyrrole Ligand.

Reaction of [Li(THF)]4[L] (L = Me8-calix[4]pyrrole]) with 0.5 equiv of [UVIO2Cl2(THF)2]2 results in formation of the oxidized calix[4]pyrrole product, [Li(THF)]2[LΔ] (1), concomitant with formation of reduced uranium oxide byproducts. Complex 1 can also be generated by reaction of [Li(THF)]4[L] with 1 equiv of I2. We hypothesize that formation of 1 proceeds via formation of a highly oxidizing cis-uranyl intermediate, [Li]2[cis-UVIO2(calix[4]pyrrole)]. To test this hypothesis, we explored the reaction of 1 with either 0.5 equiv of [UVIO2Cl2(THF)2]2 or 1 equiv of [UVIO2(OTf)2(THF)3], which affords the isostructural uranyl complexes, [Li(THF)][UVIO2(LΔ)Cl(THF)] (2) and [Li(THF)][UVIO2(LΔ)(OTf)(THF)] (3), respectively. In the solid state, 2 and 3 feature unprecedented uranyl-η5-pyrrole interactions, making them rare examples of uranyl organometallic complexes. In addition, 2 and 3 exhibit some of the smallest O-U-O angles reported to date (2: 162.0(7) and 162.7(7)°; 3: 164.5(5)°). Importantly, the O-U-O bending observed in these complexes suggests that the oxidation of [Li(THF)]4[L] does indeed occur via an unobserved cis-uranyl intermediate.

Electronic Structure and Magnetic Properties of a Titanium(II) Coordination Complex.

Stable coordination complexes of TiII (3d2) are relatively uncommon, but are of interest as synthons for low oxidation state titanium complexes for application as potential catalysts and reagents for organic synthesis. Specifically, high-spin TiII ions supported by redox-inactive ligands are still quite rare due to the reducing power of this soft ion. Among such TiII complexes is trans-[TiCl2(tmeda)2], where tmeda = N,N,N',N'-tetramethylethane-1,2-diamine. This complex was first reported by Gambarotta and co-workers almost 30 years ago, but it was not spectroscopically characterized and theoretical investigation by quantum chemical theory (QCT) was not feasible at that time. As part of our interest in low oxidation state early transition metal complexes, we have revisited this complex and report a modified synthesis and a low temperature (100 K) crystal structure that differs slightly from that originally reported at ambient temperature. We have used magnetometry, high-frequency and -field EPR (HFEPR), and variable-temperature variable-field magnetic circular dichroism (VTVH-MCD) spectroscopies to characterize trans-[TiCl2(tmeda)2]. These techniques yield the following S = 1 spin Hamiltonian parameters for the complex: D = -5.23(1) cm-1, E = -0.88(1) cm-1, (E/D = 0.17), g = [1.86(1), 1.94(2), 1.77(1)]. This information, in combination with electronic transitions from MCD, was used as input for both classical ligand-field theory (LFT) and detailed QCT studies, the latter including both density functional theory (DFT) and ab initio methods. These computational methods are seldom applied to paramagnetic early transition metal complexes, particularly those with S > 1/2. Our studies provide a complete picture of the electronic structure of this complex that can be put into context with the few other high-spin and mononuclear TiII species characterized to date.

Werner-Type Complexes of Uranium(III) and (IV).

Transmetalation of the ß-diketiminate salt [M][MenacnacPh] (M+ = Na or K; MenacnacPh- = {PhNC(CH3)}2CH-) with UI3(THF)4 resulted in the formation of the homoleptic, octahedral complex [U(MenacnacPh)3] (1). Green colored 1 was fully characterized by a solid-state X-ray diffraction analysis and a combination of UV/vis/NIR, NMR, and EPR spectroscopic studies as well as solid-state SQUID magnetization studies and density functional theory calculations. Electrochemical studies of 1 revealed this species to possess two anodic waves for the U(III/IV) and U(IV/V) redox couples, with the former being chemically accessible. Using mild oxidants, such as [CoCp2][PF6] or [FeCp2][Al{OC(CF3)3}4], yields the discrete salts [1][A] (A = PF6-, Al{OC(CF3)3}4-), whereas the anion exchange of [1][PF6] with NaBPh4 yields [1][BPh4].

A Terminal Iron(IV) Nitride Supported by a Super Bulky Guanidinate Ligand and Examination of Its Electronic Structure and Reactivity.

Utilizing the bulky guanidinate ligand [LAr*]- (LAr* = (Ar*N)2C(R), Ar* = 2,6-bis(diphenylmethyl)-4-tert-butylphenyl, R = NCtBu2) for kinetic stabilization, the synthesis of a rare terminal Fe(IV) nitride complex is reported. UV irradiation of a pyridine solution of the Fe(II) azide [LAr*]FeN3(py) (3-py) at 0 °C cleanly generates the Fe(IV) nitride [LAr*]FeN(py) (1). The 15N NMR spectrum of the 115N (50% Fe≡15N) isotopomer shows a resonance at 1016 ppm (vs externally referenced CH3NO2 at 380 ppm), comparable to that known for other terminal iron nitrides. Notably, the computed structure of 1 reveals an iron center with distorted tetrahedral geometry, τ4 = 0.72, featuring a short Fe≡N bond (1.52 Å). Inspection of the frontier orbital ordering of 1 shows a relatively small HOMO/LUMO gap with the LUMO comprised by Fe(dxz,yz)N(px,y) π*-orbitals, a splitting that is manifested in the electronic absorption spectrum of 1 (λ = 610 nm, ε = 1375 L·mol-1·cm-1; λ = 613 nm (calcd)). Complex 1 persists in low-temperature solutions of pyridine but becomes unstable at room temperature, gradually converting to the Fe(II) hydrazide product [κ2-(tBu2CN)C(η6-NAr*)(N-NAr*)]Fe (4) upon standing via intramolecular N-atom insertion. This reactivity of the Fe≡N moiety was assessed through molecular orbital analysis, which suggests electrophilic character at the nitride functionality. Accordingly, treatment of 1 with the nucleophiles PMe2Ph and Ar-N≡C (Ar = 2,6-dimethylphenyl) leads to partial N-atom transfer and formation of the Fe(II) addition products [LAr*]Fe(N═PMe2Ph)(py) (5) and [LAr*]Fe(N═C═NAr)(py) (6). Similarly, 1 reacts with PhSiH3 to give [LAr*]Fe[N(H)(SiH2Ph)](py) (7) which Fukui analysis shows to proceed via electrophilic insertion of the nitride into the Si-H bond.

[U(bipy)4 ]: A Mistaken Case of U0 ?

After more than 50 years, the synthesis and electronic structure of the first and only reported "U0 complex" [U(bipy)4 ] (1) has been reinvestigated. Additionally, its one-electron reduced product [Na(THF)6 ][U(bipy)4 ] (2) has been newly discovered. High resolution crystallographic analyses combined with magnetic and computational data show that 1 and its derivative 2 are best described as highly reduced species containing mid-to-high-valent uranium ligated by redox non-innocent ligands.

Advances in Guanidine Ligand Design: Synthesis of a Strongly Electron-Donating, Imidazolin-2-iminato Functionalized Guanidinate and its Properties on Iron.

Imidazolin-2-imines (ImRN-), derived from N-heterocylic carbenes, have been shown to be strong electron donors when directly coordinated to metals or when used as a substituent in larger ligand frameworks. In an attempt to enhance the electron-donating properties of the popular guanidine ligand class, the effect of appending an ImRN- backbone onto a guanidinate scaffold was investigated. Addition of 1 equiv of [Li(Et2O)][Im tBuN] to the aryl carbodiimide (dippN)2C (dipp = 2,6-diisopropylphenyl) cleanly affords the lithium Im tBuN-functionalized guanidinate [Li(THF)2][(Im tBuN)C(Ndipp)2] (1). Subsequent metalation of the ligand with FeBr2 gives the yellow Fe(II) complex {[(Im tBuN)C(Ndipp)2]FeBr}2 (4) in good yield. Solid-state structural analyses of both 1 and 4 shows the Im tBuN- group acts as a non-coordinating backbone substituent. Direct structural comparison of 4 to the closely related guanidinate and ketimine-guanidinate complexes {[(X)C(Ndipp)2]FeBr}2 (X = t Bu2C=N (5); N( i Pr)2 (6)), differing only in their backbone, reveals a detectable resonance contribution of the Im tBuN- group to the guanidinate ligand electronic structure. Moreover, the Fe(II)/Fe(III) redox couple of 4 (E1/2 = -0.67 V) is cathodically shifted by greater than 200 mV from the oxidation potentials of 5 (E1/2 = -0.42 V) and 6 (E1/2 = -0.45 V), demonstrating the [(Im tBuN)C(Ndipp)2]- system to be a quantifiably superior electron donor.

C(sp3 )-H Oxidative Addition and Transfer Hydrogenation Chemistry of a Titanium(II) Synthon: Mimicry of Late-Metal Type Reactivity.

Two-electron reduction of the TiIV compound (ket guan)(ImDipp N)Ti(OTf)2 (3) gives the arene-masked complex (ket guan)(η6 -ImDipp N)Ti (1) in excellent yield. Upon standing in solution, 1 converts to a TiIV metallacycle (4) through dehydrogenation of a pendant isopropyl group. Spectroscopic evidence shows this transformation initially proceeds via the oxidative addition of a C(sp3 )-H bond and can be reversed upon exposure of 4 to H2 . Interestingly, treatment of 1 with cyclohexene gives cyclohexane and 4 via a titanium-mediated transfer hydrogenation reaction, a process that can be extended to catalytically hydrogenate other unsaturated hydrocarbons under mild conditions. These results, rare for the early-metals, suggest 1 possesses chemical characteristics reminiscent of noble, late-metals.

Cyclo-P3 Complexes of Vanadium: Redox Properties and Origin of the ³¹P NMR Chemical Shift.

The synthesis and characterization of two high-valent vanadium-cyclo-P3 complexes, (nacnac)V(cyclo-P3)(Ntolyl2) (1) and (nacnac)V(cyclo-P3)(OAr) (2), and an inverted sandwich derivative, [(nacnac)V(Ntolyl2)]2(µ2-η(3):η(2)-cyclo-P3) (3), are presented. These novel complexes are prepared by activating white phosphorus (P4) with three-coordinate vanadium(II) precursors. Structural metrics, redox behavior, and DFT electronic structure analysis indicate that a [cyclo-P3](3-) ligand is bound to a V(V) center in monomeric species 1 and 2. A salient feature of these new cyclo-P3 complexes is their significantly downfield shifted (by ∼300 ppm) (31)P NMR resonances, which is highly unusual compared to related complexes such as (Ar[(i)Pr]N)3Mo(cyclo-P3) (4) and other cyclo-P3 complexes that display significantly upfield shifted resonances. This NMR spectroscopic signature was thus far thought to be a diagnostic property for the cyclo-P3 ligand related to its acute endocyclic angle. Using DFT calculations, we scrutinized and conceptualized the origin of the unusual chemical shifts seen in this new class of complexes. Our analysis provides an intuitive rational paradigm for understanding the experimental (31)P NMR spectroscopic signature by relating the nuclear magnetic shielding with the electronic structure of the molecule, especially with the characteristics of metal-cyclo-P3 bonding.

Donor Properties of a New Class of Guanidinate Ligands Possessing Ketimine Backbones: A Comparative Study Using Iron.

Addition of 1 equiv of LiN═C(t)Bu2 or LiN═Ad (Ad = 2-adamantyl) to the aryl carbodiimide C(NDipp)2 (Dipp = 2,6-diisopropylphenyl) readily generates the lithium ketimine-guanidinates Li(THF)2[(X)C(NDipp)2] (X = N═C(t)Bu2 (1-(t)Bu), N═Ad (1-Ad)) in excellent yields. These new ligands can be readily metalated with iron to give the N,N'-bidentate chelates [{(X)C(NDipp)2}FeBr]2 (X = N═C(t)Bu2 (5-(t)Bu), N═Ad (5-Ad)), in which the ketimines behave as noncoordinating backbone substituents. In an effort to understand the potential electronic contributions of the ketimine group to the ligand architecture, a thorough structural and electronic study was conducted comparing the features and properties of 5-(t)Bu and 5-Ad to their guanidinate and amidinate analogues [{(X)C(NDipp)2}FeBr]2 (X = (i)Pr2N (6), (t)Bu (7)). Solid-state structural analyses indicate little electronic contribution from the N-ketimine nitrogen atom, while solution-phase electronic absorption spectra of 5-(t)Bu and 5-Ad are qualitatively similar to the amidinate complex 7. Yet, electrochemical measurements do show the donor properties of the ketimine-guanidinate in 5-(t)Bu to be intermediate between its guanidinate and amidinate counterparts in 6 and 7. Preliminary reactivity studies also show that the reduction chemistry of 5-(t)Bu diverges significantly from that of 6 and 7. Treatment of 5-(t)Bu with excess magnesium or 1 equiv of KC8 leads to the formation of the Fe(I)-Fe(I) complex [{µ-((t)Bu2C═N)C(NDipp)2}2Fe2] (11), which possesses an exceedingly short Fe═Fe bond (2.1516(5) Å), while neither 6 nor 7 forms dinuclear complexes upon reduction. This result demonstrates that ketimine-guanidinates do not simply behave as amidinate variants but can contribute to distinctive metal chemistry of their own.

Electronic Structure and Reactivity of a Well-Defined Mononuclear Complex of Ti(II).

A facile and high-yielding protocol to the known Ti(II) complex trans-[(py)4TiCl2] (py = pyridine) has been developed. Its electronic structure has been probed experimentally using magnetic susceptibility, magnetic circular dichroism, and high-frequency and high-field electron paramagnetic resonance spectroscopies in conjunction with ligand-field theory and computational methods (density functional theory and ab initio methods). These studies demonstrated that trans-[(py)4TiCl2] has a (3)Eg ground state (dxy(1)dxz,yz(1) orbital occupancy), which, as a result of spin­orbit coupling, yields a ground-state spinor doublet that is EPR active, a first excited-state doublet at ∼60 cm(­1), and two next excited states at ∼120 cm(­1). Reactivity studies with various unsaturated substrates are also presented in this study, which show that the Ti(II) center allows oxidative addition likely via formation of [Ti(η(2)-R2E2)Cl2(py)n] E = C, N intermediates. A new Ti(IV) compound, mer-[(py)3(η(2)-Ph2C2)TiCl2], was prepared by reaction with Ph2C2, along with the previously reported complex trans-(py)3Ti═NPh(Cl)2, from reaction with Ph2N2. Reaction with Ph2CN2 also yielded a new dinuclear Ti(IV) complex, [(py)2(Cl)2Ti(µ2:η(2)-N2CPh2)2Ti(Cl)2], in which the two Ti(IV) ions are inequivalently coordinated. Reaction with cyclooctatetraene (COT) yielded a new Ti(III) complex, [(py)2Ti(η(8)-COT)Cl], which is a rare example of a mononuclear "piano-stool" titanium complex. The complex trans-[(py)4TiCl2] has thus been shown to be synthetically accessible, have an interesting electronic structure, and be reactive toward oxidation chemistry.

Synthesis of a "super bulky" guanidinate possessing an expandable coordination pocket.

Friedel-Crafts alkylation of 4-tert-butylaniline with 2 equiv of benzhydrol affords bulky 2,6-bis(diphenylmethyl)-4-tert-butylaniline (Ar*NH2) in good yield, which can be readily synthesized on a tens of grams scale. The reaction of 6 equiv of Ar*NH2 with triphosgene generates the symmetric urea (Ar*NH)2CO, which, upon dehydration with a P2O5/Al2O3 slurry in pyridine, produces the sterically encumbered carbodiimide (Ar*N)2C as an air-stable white solid. The treatment of (Ar*N)2C with LiN═C(t)Bu2 in tetrahydrofuran cleanly gives the monomeric lithium guanidinate Li[(Ar)*ketguan], free of coordinating solvent, in 85% yield. Protonation of Li[(Ar)*ketguan] with lutidinium chloride produces the guanidine (Ar)*ketguanH (MW = 1112.60 g/mol), which is easily derivatized to give the monomeric alkali metal complexes M[(Ar)*ketguan] (M = K, Cs) in 94% and 51% yield, respectively. The solid-state molecular structures of M[(Ar)*ketguan] (M = Li, K, Cs) show formally two-coordinate alkali metal cations encapsulated within a hydrophobic coordination pocket formed by the peripheral diphenylmethyl substituents of the guanidinate. Remarkably, percent buried volume analyses (% V(Bur)) of M[(Ar)*ketguan] [M = Li (94.8% V(Bur)), K (92.1% V(Bur)), Cs (81.7% V(Bur))] reveal a coordination cavity that adjusts to individually accommodate the variously sized metal ions despite the highly encumbering nature of the ligand. This demonstrates a flexible ligand framework that is able to stabilize low-coordinate metal centers within a "super bulky" coordination environment.