Synthesis and Electronic Ground-State Properties of Pyrrolyl-Based Iron Pincer Complexes: Revisited.

The pyrrolyl-based iron pincer compounds [(tBuPNP)FeCl] (1), [(tBuPNP)FeN2] (2), and [(tBuPNP)Fe(CO)2] (3) were prepared and structurally characterized. In addition, their electronic ground states were probed by various techniques including solid-state magnetic susceptibility and zero-field 57Fe Mössbauer and X-band electron paramagnetic resonance spectroscopy. While the iron(II) starting material 1 adopts an intermediate-spin (S = 1) state, the iron(I) reduction products 2 and 3 exhibit a low-spin (S = 1/2) ground state. Consistent with an intermediate-spin configuration for 1, the zero-field 57Fe Mössbauer spectrum shows a characteristically large quadrupole splitting (ΔEQ ≈ 3.7 mm s-1), and the solid-state magnetic susceptibility data show pronounced zero-field splitting (|D| ≈ 37 cm-1). The effective magnetic moments observed for the iron(I) species 2 and 3 are larger than expected from the spin-only value and indicate an incompletely quenched orbital angular momentum and the presence of spin-orbit coupling in the ground state. The experimental findings are complemented by density functional theory computations, which are in good agreement with the experimental data. Most notably, these calculations reveal a low-lying (S = 2) excited state for complex 1; furthermore, the computed Mössbauer parameters for all complexes studied herein are in excellent agreement with the experimental findings.

Homolytic H2 cleavage by a mercury-bridged Ni(I) pincer complex [{(PNP)Ni}2{µ-Hg}].

Reduction of the pincer nickel(ii) complex [(PNP)NiBr] with sodium amalgam (Na/Hg) forms the mercury-bridged dimer [{(PNP)Ni}2{µ-Hg}], which homolytically cleaves dihydrogen to form [(PNP)NiH]. Reversible CO2 insertion into the Ni-H bond is observed for [(PNP)NiH], forming the monodentate κ(1)O-formate complex [(PNP)NiOC(O)H].

Coordination chemistry of sterically encumbered pyrrolyl ligands to chromium(II): mono(pyrrolyl)chromium and diazachromocene formation.

A series of diazachromocenes with sterically demanding pyrrolyl ligands, 2,5-(Me3C)2C4H2N (1), 2,5-(Me3C)2-3,4-Me2C4N (2) and 2,3,5-(Me3C)3C4HN (3), was prepared and investigated by various spectroscopic techniques, and in some cases by X-ray diffraction and magnetic susceptibility studies. The diazachromocenes exhibit an S = 1 ground state; no indication of a spin-equilibrium was obtained. With the same ligands mono(pyrrolyl)chromium(II) complexes are accessible, [{(κN-2,5-(Me3C)2C4H2N)Cr(thf)}2(µ-Cl)2] (1-CrCl(thf)), [(κN-2,5-(Me3C)2-3,4-Me2-C4H2N)Cr(Cl)(tmeda)] (2-CrCl(tmeda) and [(η(5)-2,3,5-(Me3C)3C4HN)Cr(µ-Cl)2] (3-CrCl), which show either η(5)- or η(1)-(κN) coordination depending on the substitution pattern. (1)H NMR spectroscopy serves as a valuable tool to distinguish between these coordination modes. The Cr(II) atoms in the mono(pyrrolyl) complexes adopt a high spin configuration (S = 2) and in dimeric species antiferromagnetic coupling between the spin carriers was observed. However, none of these mono(pyrrolyl)chromium complexes is an effective or selective ethylene oligomerization catalyst on activation with MAO or AlMe3, supporting the importance of a Cr(I)/Cr(III)-based catalytic cycle.

Phosphine-substituted sterically encumbered pyrrolyl ligands - synthesis and reactivity studies.

The sterically encumbered pyrrolyl KPyr(tBu2) (1-K, Pyr(tBu2) = 2,5-(Me3C)2C4H2N) reacts with PCl3, (iPr)2PCl and Ph2PCl exclusively at the C3-position to yield a series of phosphine-substituted pyrroles HPyr(tBu2R) (R = PCl2 (2-H), (iPr)2P (4-H) and Ph2P (5-H)). Pyrrole 2-H can further be functionalized with MeLi (3 equiv.) to yield LiPyr(tBu2PMe2) (3-Li). The coordination chemistry of these phosphine-substituted pyrroles was explored and the resulting complexes were characterized by various spectroscopic techniques and in some cases using single crystal X-ray diffraction. The neutral pyrroles 4-H and 5-H react cleanly with [Mo(CO)6] to form the κP-coordinate complexes 4-H-Mo(CO)5 and 5-H-Mo(CO)5, respectively. IR spectroscopy of these complexes shows that C3-substitution barely changes the π-acceptor properties of the phosphine moiety. The η(5)-coordination was achieved when pyrrolides 4-K and 5-K were reacted with [Mn(CO)5Br] to give 4-Mn(CO)3 and 5-Mn(CO)3, respectively. The energy of the HOMOs in these three legged-piano stool complexes decreases on PPh2-substitution (5-Mn(CO)3) stabilizing the Mn-CO bonds, whereas the inverse effect is noticed for P(iPr)2-substitution (4-Mn(CO)3).

Sterically encumbered pyrrolyl ligands and their incorporation into the cycloheptatrienyl zirconium coordination sphere.

An improved synthesis of 2,5-di(tert-butyl)-3,4-dimethylpyrrole (2-H, HPyr(tBu(2)Me(2))) and a subsequent reaction with KH yield K(Pyr(tBu(2)Me(2))) (2-K) in multi-gram quantities. Four different pyrrolyl (Pyr) and imidazolyl (Im) ligands were used in salt metathesis reactions with [(η(7)-C(7)H(7))ZrCl(tmeda)] (7) to afford a series of azatrozircenes: [(η(7)-C(7)H(7))Zr(η(5)-Pyr(tBu(2)))] (1-Zr), [(η(7)-C(7)H(7))Zr(η(5)-Pyr(tBu(2)Me(2)))] (2-Zr), [(η(7)-C(7)H(7))Zr(η(5)-Pyr(tBu(3)))] (3-Zr) and [(η(7)-C(7)H(7))Zr(η(5)-Im(tBu(3)))] (4-Zr), which were characterized by NMR spectroscopy and elemental analysis. In addition, the molecular structures of 2-H, 2-K·18-crown-6, 1-Zr, 2-Zr and 4-Zr were determined by X-ray diffraction analysis, revealing η(5)- rather than κ(1)-N-coordination of the N-heterocyclic ligands. Cone angle measurements on the sandwich complexes 1-Zr­4-Zr showed that their nitrogencontaining ligands belong to the class of very sterically encumbered π-ligands, but DFT calculations suggest lower stabilities compared to their all-carbon analogues.