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Addition of NaEt3BH to (Ph2PPrDI)CoCl2 affords the corresponding monohydride, (Ph2PPrDI)CoH. X-ray diffraction and DFT calculations indicate that this compound possesses a radical monoanion α-DI chelate and a Co(ii) centre. Notably, (Ph2PPrDI)CoH catalyzes the hydroboration of alkynes and dihydroboration of nitriles under mild conditions.

Reduction of 6-coordinate ((Ph2PPr)DI)FeBr₂ under N2 results in formation of the terminal dinitrogen complex, ((Ph2PPr)DI)FeN2. Heating this product to 75 °C allows for C-H and C-P activation of the chelate to generate the cisoid and transoid isomers of [(μ-PrPPh-κ(5)-P,N,N,Cγ,P-(Ph2PPr)DI(PrPPh))Fe]2. Mechanistic possibilities for this transformation are discussed.

A series of pyridine(diimine)iron tetrazene compounds, (iPrPDI)Fe[(NR)NN(NR)] [iPrPDI = 2,6-(ArN = CMe)2C5H3N; Ar = 2,6-iPr2C6H3] has been prepared either by the addition of 2 equiv of an organic azide, RN3, to the corresponding iron bis(dinitrogen) compound, (iPrPDI)Fe(N2)2 or by the addition of azide to the iron imide derivatives, (iPrPDI)FeNR. The electronic structures of these compounds were determined using a combination of metrical parameters from X-ray diffraction, solution and solid-state magnetic measurements, zero-field 57Fe Mossbauer and 1H NMR spectroscopies, and density functional theory calculations. The overall electronic structure of the iron tetrazene compounds is sensitive to the nature of the tetrazene nitrogen substituent with three distinct classes of compounds identified: (i) overall diamagnetic (S = 0) compounds arising from intermediate-spin iron(II) centers (SFe = 1) engaged in antiferromagnetic coupling with both pyridine(diimine) and tetrazene radical anions (SPDI = −1/2 and Ste...

Attempts to prepare low-valent molybdenum complexes that feature a pentadentate 2,6-bis(imino)pyridine (or pyridine diimine, PDI) chelate allowed for the isolation of two different products. Refluxing Mo(CO)6 with the pyridine-substituted PDI ligand, (PyEt)PDI, resulted in carbonyl ligand substitution and formation of the respective bis(ligand) compound ((PyEt)PDI)2Mo (1). This complex was investigated by single-crystal X-ray diffraction, and density functional theory calculations indicated that 1 possesses a Mo(0) center that back-bonds into the π*-orbitals of the unreduced PDI ligands. Heating an equimolar solution of Mo(CO)6 and the phosphine-substituted PDI ligand, (Ph2PPr)PDI, to 120 °C allowed for the preparation of ((Ph2PPr)PDI)Mo(CO) (2), which is supported by a κ(5)-N,N,N,P,P-(Ph2PPr)PDI chelate. Notably, 1 and 2 have been found to catalyze the hydrosilylation of benzaldehyde at 90 °C, and the optimization of 2-catalyzed aldehyde hydrosilylation at this temperature afforded turnover frequencies of up to 330 h(-1). Considering additional experimental observations, the potential mechanism of 2-mediated carbonyl hydrosilylation is discussed.

The electronic structures of the complexes [M((t)bpy)(3)](0,1-) (M = Nb, Ta; (t)bpy = 4,4'-di-tert-butyl-2,2'-bipyridine) have been investigated using a combination of UV-vis spectroscopy, EPR spectroscopy, and XAS. Furthermore, the crystal structure of [Na(THF)(5)][Ta((t)bpy)(3)] has been determined. These studies were supplemented by density functional theory (DFT) and the calculations extended to include the series [Y(bpy)(3)](m) (m = 0, 1-, 2-, 3-), [Ti(bpy)(3)](n) (n = 1+, 0, 1-, 2-, 3-), [Zr(bpy)(3)](p), and [Hf(bpy)(3)](p) (p = 0, 1-, 2-). This has allowed us to define the correct electronic structures of these early transition metal tris(2,2'-bipyridine) complexes. It is shown that in the [Y(bpy)(3)](m) series the central ion possesses an invariant +III oxidation state and that the three successive one-electron redox processes that comprise the series are solely ligand-based, yielding three (bpy(•))(1-) radical anions in the neutral complex through to three diamagnetic dianions (bpy(2-))(2-) in the trianion. The same is true for the [Ti(bpy)(3)](n) series where the neutral complex contains 3(bpy(•))(1-) and the trianion 3(bpy(2-))(2-) anions. Hence, the central ion always possesses a central Ti(III) (d(1)) ion that intramolecularly antiferromagnetically couples to any (bpy(•))(1-) ligands present. In contrast, the central metal ions in the series [Zr(bpy)(3)](p) and [Hf(bpy)(3)](p) always possess a +IV oxidation state; hence, the dianions contain three (bpy(2-))(2-) ligands and yield an S = 0 ground state. The electronic structures of the neutral Nb and Ta analogues possessing S = (1)/(2) ground states are best described as [Nb(IV)(bpy(2-))(2)(bpy)(0))](0) and [Ta(V)(bpy(•))(bpy(2-))(2)](0), and their S = 0 monoanions as [Nb(IV)(bpy(•))(bpy(2-))(2)](1-) and [Ta(V)(bpy(2-))(3)](1-). The central metal ion in the Nb series maintains a +IV oxidation state, while in the Ta series the central metal ion displays a +V oxidation state throughout.

Three new N-alkyl substituted bis(imino)pyridine iron imide complexes, ((iPr)PDI)FeNR ((iPr)PDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)-N═CMe)(2)C(5)H(3)N; R = 1-adamantyl ((1)Ad), cyclooctyl ((Cy)Oct), and 2-adamantyl ((2)Ad)) were synthesized by addition of the appropriate alkyl azide to the iron bis(dinitrogen) complex, ((iPr)PDI)Fe(N(2))(2). SQUID magnetic measurements on the isomeric iron imides, ((iPr)PDI)FeN(1)Ad and ((iPr)PDI)FeN(2)Ad, established spin crossover behavior with the latter example having a more complete spin transition in the experimentally accessible temperature range. X-ray diffraction on all three alkyl-substituted bis(imino)pyridine iron imides established essentially planar compounds with relatively short Fe-N(imide) bond lengths and two-electron reduction of the redox-active bis(imino)pyridine chelate. Zero- and applied-field Mössbauer spectroscopic measurements indicate diamagnetic ground states at cryogenic temperatures and established low isomer shifts consistent with highly covalent molecules. For ((iPr)PDI)FeN(2)Ad, Mössbauer spectroscopy also supports spin crossover behavior and allowed extraction of thermodynamic parameters for the S = 0 to S = 1 transition. X-ray absorption spectroscopy and computational studies were also performed to explore the electronic structure of the bis(imino)pyridine alkyl-substituted imides. An electronic structure description with a low spin ferric center (S = 1/2) antiferromagnetically coupled to an imidyl radical (S(imide) = 1/2) and a closed-shell, dianionic bis(imino)pyridine chelate (S(PDI) = 0) is favored for the S = 0 state. An iron-centered spin transition to an intermediate spin ferric ion (S(Fe) = 3/2) accounts for the S = 1 state observed at higher temperatures. Other possibilities based on the computational and experimental data are also evaluated and compared to the electronic structure of the bis(imino)pyridine iron N-aryl imide counterparts.

The synthesis and electronic structure of reduced aryl-substituted bis(imino)pyridine manganese compounds have been explored. Stirring a THF slurry of [(iPrPDI)MnCl2] {iPrPDI = 2,6-(2,6-iPr2–C6H3N=CMe)2C5H3N} with excess Na and catalytic (0.5 mol-%) naphthalene furnished the bis(THF) compound [(iPrPDI)Mn(THF)2]. Performing the reduction with excess Na(Hg) in toluene furnished the bis(chelate) manganese compound [(iPrPDI)2Mn]. For both compounds, a combination of EPR spectroscopy, magnetic measurements and metrical parameters determined from X-ray diffraction established high-spin MnII compounds with reduced, redox-active bis(imino)pyridine ligands. Substitution of the THF ligands with carbon monoxide yielded [(iPrPDI)Mn(CO)2], a low-spin MnI, d6 compound with an experimentally observed bis(imino)pyridine-centred radical. Oxidation and reduction of this compound furnished [(iPrPDI)Mn(CO)3]+ and [(iPrPDI)Mn(CO)2]–, respectively, and provided a series of three manganese carbonyl compounds over three oxidation states. Elucidation of the electronic structure of these compounds established that oxidation events within the series are ligand- rather than manganese-based, most likely a result of the stable low-spin MnI, d6 electron configuration imparted by the strong-field carbonyl ligands.

The stepwise 1-3 electron reduction of the N-alkyl substituted bis(imino)pyridine cobalt dichloride complexes, ((R)APDI)CoCl(2), was studied where (R)APDI = 2,6-(RN=CMe)(2)C(5)H(3)N, R = C(6)H(11) (Cy), CHMe(2) ((i)Pr). One electron reduction with either zinc metal or NaBEt(3)H furnished the bis(imino)pyridine cobalt monochloride compounds, ((R)APDI)CoCl. X-ray diffraction on the ((iPr)APDI)CoCl derivative established a distortion from square planar geometry where the chloride ligand is lifted out of the idealized cobalt-chelate plane. Superconducting Quantum Interference Device (SQUID) magnetometry on both compounds established spin crossover behavior with an S = 1 state being predominant at room temperature. Computational studies, in combination with experimental results, establish that the triplet spin isomer arises from a high spin Co(II) center (S(Co) = 3/2) antiferromagnetically coupled to a bis(imino)pyridine chelate radical anion, [PDI](-) (S(PDI) = 1/2). At lower temperatures, the Co(II) ion undergoes a spin transition to the low spin form (S(Co) = 1/2) and antiferromagnetic coupling gives rise to the observed diamagnetic ground state. Replacing the chloride ligand with a methyl group, namely ((R)APDI)CoCH(3), also yielded distorted compounds, albeit less pronounced, that are diamagnetic at room temperature. Two electron reduction of the ((R)APDI)CoCl(2) derivatives with excess 0.5% sodium amalgam or 2 equiv of NaBEt(3)H furnished the bis(chelate)cobalt complexes, ((R)APDI)(2)Co, while three electron reduction with 3 equiv of sodium naphthalenide yielded the cobalt dinitrogen anions, [Na(solv)(3)][((R)APDI)CoN(2)] (solv = THF, Et(2)O). Both bis(chelate) compounds were crystallographically characterized and determined to have S = 3/2 ground states by SQUID magnetometry and electron paramagnetic resonance (EPR) spectroscopy. Computational studies, in combination with metrical parameters determined from X-ray diffraction, establish a high spin (S(Co) = 3/2) cobalt(II) center with two bis(imino)pyridine chelate radical anions. Antiferromagnetic coupling between the two chelate centered radicals is mediated by a doubly occupied t(2g) cobalt orbital and gives rise to the observed overall quartet ground state.

Sodium amalgam reduction of the aryl-substituted bis(imino)pyridine cobalt dihalide complexes ((Ar)PDI)CoCl(2) and ((iPr)BPDI)CoCl(2) ((Ar)PDI = 2,6-(2,6-R(2)-C(6)H(3)N=CMe)(2)C(5)H(3)N (R = (i)Pr, Et, Me); (iPr)BPDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)N=CPh)(2)C(5)H(3)N) in the presence of an N(2) atmosphere furnished the corresponding neutral cobalt dinitrogen complexes ((Ar)PDI)CoN(2) and ((iPr)BPDI)CoN(2). Magnetic measurements on these compounds establish doublet ground states. Two examples, ((iPr)PDI)CoN(2) and ((iPr)BPDI)CoN(2), were characterized by X-ray diffraction and exhibit metrical parameters consistent with one-electron chelate reduction and a Co(I) oxidation state. Accordingly, the toluene solution EPR spectrum of ((iPr)PDI)CoN(2) at 23 degrees C exhibits an isotropic signal with a g value of 2.003 and hyperfine coupling constant of 8 x 10(-4) cm(-1) to the I = 7/2 (59)Co center, suggesting a principally bis(imino)pyridine-based SOMO. Additional one-electron reduction of ((iPr)PDI)CoN(2) was accomplished by treatment with Na[C(10)H(8)] in THF and yielded the cobalt dinitrogen anion [((iPr)PDI)CoN(2)](-). DFT calculations on the series of cationic, neutral, and anionic bis(imino)pyridine cobalt dinitrogen compounds establish Co(I) centers in each case and a chelate-centered reduction in each of the sequential one-electron reduction steps. Frequency calculations successfully reproduce the experimentally determined N[triple bond]N infrared stretching frequencies and validate the computational methods. The electronic structures of the reduced cobalt dinitrogen complexes are evaluated in the broader context of bis(imino)pyridine base metal chemistry and the influence of the metal d electron configuration on the preference for closed-shell versus triplet diradical dianions.

The steric, electronic, and synthetic characteristics of 3,4-diazaphospholanes are reported. Crystallographic structures of free and metal-complexed 3,4-diazaphospholanes provide steric metrics (cone angle, solid angle, etc.). Diazaphospholanes span a wide range of sizes with cone angles varying from 135 to 188°. The electron-donating abilities of diazaphospholanes have been estimated using the carbonyl infrared stretching frequencies, ν(CO), of [trans-Rh(diazaphospholane)2(CO)Cl] complexes. Frequencies for the CO stretches range from 1975 to 2011 cm-1, thus indicating that 3,4-diazaphospholanes may be as electron rich as dialkylarylphosphines or as electron deficient as trialkyl phosphites. Reduction of N,N‘-phthalamido-3,4-diazaphospholanes with BH3·SMe2 yields diazaphospholanes that not only are more electron rich but also show a reorientation of the phospholane substituents that may affect catalytic properties. Diazaphospholanes readily react with many Rh and Pd catalyst precursors to form complexes. ...

Addition of the [NH] transfer reagent Hdbabh (dbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene) to the bis(imino)pyridine iron bis(dinitrogen) complex, (((i)Pr)PDI)Fe(N(2))(2) (((i)Pr)PDI = 2,6-(2,6-(i)Pr(2)C(6)H(3)N horizontal lineCMe)(2)C(5)H(3)N), furnished the corresponding iron amide and ammonia compounds, resulting from cleavage of the strained amine. Isotopic labeling studies support N-H bond activation by a transient, parent imide, [(((i)Pr)PDI)FeNH].

The electron transfer series of complexes [V((t)bpy)(3)](z) (z = 3+, 2+, 0, 1-) has been synthesized and spectroscopically characterized with the exception of the monocationic species. Magnetic susceptibility measurements (4-290 K) establish an S = 1 ground state for [V((t)bpy)(3)](3+), S = (3)/(2) for [V((t)bpy)(3)](2+), S = (1)/(2) for [V((t)bpy)(3)], and an S = 0 ground state for [V((t)bpy)(3)](1-). The electrochemistry of this series recorded in tetrahydrofuran solution exhibits four reversible one-electron transfer steps. Electronic absorption, X-band electron paramagnetic resonance (EPR), and V K-edge X-ray absorption (XAS) spectra were recorded. All complexes have been studied computationally with density functional theory (DFT) using the B3LYP functional. It is unequivocally shown that the electronic structure of complexes is best described as [V(III)((t)bpy(0))(3)](3+), [V(II)((t)bpy(0))(3)](2+), [V(II)((t)bpy(•))(2)((t)bpy(0))](0), and [V(II)((t)bpy(•))(3)](1-), where ((t)bpy(0)) represents the neutral form of the ligand and ((t)bpy(•))(1-) is the one-electron reduced mononanionic radical form. In the neutral and monoanionic members, containing two and three ((t)bpy(•))(1-) ligands, respectively, the ligand spins are strongly antiferromagnetically coupled to the spins of the central V(II) ion (d(3); S = (3)/(2)) affording the observed ground states given above.

A series of planar aryl-substituted bis(imino)pyridine cobalt azide complexes were prepared and evaluated as synthetic precursors for the corresponding cobalt nitrido compounds. Thermolysis or photolysis of two examples resulted in intramolecular C-H activation of the benzylic positions of the aryl substituents. For the mesityl-substituted compound, C-H activation by the putative nitride resulted in formation of a neutral imine ligand and modification of the chelate by hydrogen transfer to the imine carbon.

Treatment of the intermediate spin ferrous bis(dinitrogen) complex, (iPrPDI)Fe(N2)2 (iPrPDI = 2,6-(2,6-iPr2−C6H3NCMe2)2C6H3N), with 1 equiv of N2CHSiMe3 afforded the corresponding iron diazoalkane compound, (iPrPDI)Fe(N2CHSiMe3). Spectroscopic studies in conjunction with the metrical parameters from the solid-state structure support an intermediate spin ferrous complex with a doubly reduced chelate and a neutral diazoalkane ligand. In solution at 23 °C, (iPrPDI)Fe(N2CHSiMe3) promotes the intramolecular C−H activation of an isopropyl aryl substituent with loss of SiMe4. Exposure of (iPrPDI)Fe(N2CHSiMe3) to 1 atm of H2 resulted in hydrogenative cleavage of the N−N and N−C bonds to yield (iPrPDI)Fe(NH3) and SiMe4. Similar N−N bond cleavage chemistry was observed upon addition of hydrazine to (iPrPDI)Fe(N2)2.

The bis(imino)pyridine iron bis(dinitrogen) complex, (iPrPDI)Fe(N2)2 (iPrPDI = 2,6-(2,6-iPr2C6H3NCR)2C5H3N), serves as an efficient precursor for the catalytic [2pi + 2pi] cycloaddition of alpha,omega-dienes to yield the corresponding bicycles. For amine substrates, the rate of catalytic turnover increases with the size of the nitrogen substituents, demonstrating competing heterocycle coordination and product inhibition. In one case, a bis(imino)pyridine iron azobicycloheptane product was characterized by X-ray diffraction. Preliminary mechanistic studies highlight the importance of the redox activity of the bis(imino)pyridine ligand to maintain the ferrous oxidation state throughout the catalytic cycle.