Rational Synthesis and Characterization of Dimolybdenum(II) Compounds Bearing Ferrocenyl-Containing Ligands toward Modulation of Electronic Coupling

Three novel cis-to-trans-converted dimolybdenum­(II) complexes, <i>trans</i>-[Mo₂(O₂C-Fc)₂(DPPX)₂]­[BF₄]₂ {<b>2a</b>–<b>2c</b>; DPPX = DPPA [<i>N</i>,<i>N</i>-bis­(diphenylphosphino)­amine], DPPM [1,1-bis­(diphenylphosphino)­methane], and DPPE [1,2-bis­(diphenylphosphino)­ethane], respectively}, were synthesized through the insertion of bulky diphosphine ligands, which force a permanent trans arrangement, as evidenced by X-ray crystallography and density functional theory calculations. All compounds were characterized by means of NMR, UV–vis, and IR spectroscopy as well as thermogravimetry–mass spectrometry measurements. Interestingly, uncommon UV–vis transitions and oxidation sequences were observed compared to previously reported ones. As verified by electrochemical measurements, all synthesized complexes show two separate one-electron-redox processes assigned to subsequent oxidations of the two redox-active ferrocenecarboxylate ligands, with a split of ca. 70 mV. This behavior reveals electronic interaction between the two equatorially trans-positioned ferrocenyl units. The presented work provides new insights into the rational synthesis of electronically coupled trans-coordinated Mo₂ systems, paving the way toward the design of linear multicenter redox-active oligomers

Direct Synthesis and Bonding Properties of the First μ²‑η²,η²‑Allyl-Bridged Diiridium Complex

The direct synthesis of the first μ²-η²,η²-allyl-bridged diiridium complex ([<b>2</b>]⁺), bearing the uncommon counterion [IrCl₂(COD)]⁻ ([<b>3</b>]⁻), is described. Both bridging moieties in [<b>2</b>]⁺, namely, allyl and acetate, are introduced in a single reaction step from [{IrCl­(COD)}₂] (<b>1</b>) and allyl acetate. A combination of X-ray crystallography and density functional theory calculations reveals pronounced metal–allyl π-back-bonding

Platinum Catalysis RevisitedUnraveling Principles of Catalytic Olefin Hydrosilylation

Hydrosilylation of C–C multiple bonds is one of the most important applications of homogeneous catalysis in industry. The reaction is characterized by its atom-efficiency, broad substrate scope, and widespread application. To date, industry still relies on highly active platinum-based systems that were developed over half a century ago. Despite the rapid evolution of vast synthetic applications, the development of a fundamental understanding of the catalytic reaction pathway has been difficult and slow, particularly for the industrially highly relevant Karstedt’s catalyst. A detailed mechanistic study unraveling several new aspects of platinum-catalyzed hydrosilylation using Karstedt’s catalyst as platinum source is presented in this work. A combination of ²H-labeling experiments, ¹⁹⁵Pt NMR studies, and an in-depth kinetic study provides the basis for a further development of the well-established Chalk–Harrod mechanism. It is concluded that the coordination strength of the olefin exerts a decisive effect on the kinetics of the reaction. In addition, it is demonstrated how distinct structural features of the active catalyst species can be derived from kinetic data. A primary kinetic isotope effect as well as a characteristic product distribution in deuterium-labeling experiments lead to the conclusion that the rate-limiting step of platinum-catalyzed hydrosilylation is in fact the insertion of the olefin into the Pt–H bond rather than reductive elimination of the product in the olefin/silane combinations studied

On the Concept of Hemilability: Insights into a Donor-Functionalized Iridium(I) NHC Motif and Its Impact on Reactivity

Novel iridium­(I) complexes bearing N-donor-functionalized N-heterocyclic carbene ligands were synthesized. Although hemilabile coordination of the attached donor is considered beneficial in catalysis, no detailed study of this phenomenon in these systems is available to date. The present report provides insight into the hemilabile bonding properties of a <i>N</i>,<i>N</i>′-bis­(pyridin-2-yl)-imidazolylidene (NCN) ligand motif on iridium­(I). In most cases, the presented compounds exhibit rare fluxional hemilabile coordination of the N donor, and remarkable performance in catalytic transfer hydrogenation is observed. Further, extensive reactivity studies often led to unexpected products

Synthesis and Characterization of Novel Iron(II) Complexes with Tetradentate Bis(N-heterocyclic carbene)–Bis(pyridine) (NCCN) Ligands

Three novel iron­(II) complexes bearing tetradentate ligands of the type pyridine–bis­(N-heterocyclic carbene)–pyridine (NCCN) have been synthesized. The compounds <i>trans</i>-diacetonitrile­[bis­(<i>o</i>-imidazol-2-ylidenepyridine)­alkane]­iron­(II) hexafluorophosphate (alkane = methane (<b>2a</b>), ethane (<b>2b</b>)) and <i>cis</i>-diacetonitrile­[1,3-bis­(<i>o</i>-imidazol-2-ylidenepyridine)­propane]­iron­(II) hexafluorophosphate (<b>2c</b>) have been characterized by single-crystal X-ray diffraction (XRD), nuclear magnetic resonance spectroscopy (NMR), and infrared spectroscopy (IR). Cyclic voltammetry (CV) measurements show reversible oxidation of Fe­(II) to Fe­(III). The rotational barrier of the bridge has been determined via variable-temperature NMR (VT-NMR) studies of <b>2b</b>. In all complexes, the Fe centers coordinatein addition to the NCCN ligandstwo acetonitrile ligands in the solid state as well as in solution. The alkylene bridge connecting the two NHCs has an influence on the coordination mode of the NCCN ligands. Whereas the methylene- and ethylene-bridged NHC moieties lead to a nearly planar geometry of the NCCN ligand and two trans-positioned acetonitrile ligands, the propylene-bridged complex <b>2c</b> exhibits a sawhorse-type coordination mode, with two cis-oriented acetonitrile ligands. The reactivity of the synthesized complexes toward substitution of the solvent ligands was investigated, showing that acetonitrile is readily substituted by benzonitrile. Upon addition of carbon monoxide, one acetonitrile ligand is replaced by CO to yield complexes <b>3a</b>–<b>c</b>, as shown by NMR and IR spectroscopy, as well as by XRD in the case of compound <b>3c</b>

Synthesis and Characterization of Novel Iron(II) Complexes with Tetradentate Bis(N-heterocyclic carbene)–Bis(pyridine) (NCCN) Ligands

Three novel iron­(II) complexes bearing tetradentate ligands of the type pyridine–bis­(N-heterocyclic carbene)–pyridine (NCCN) have been synthesized. The compounds <i>trans</i>-diacetonitrile­[bis­(<i>o</i>-imidazol-2-ylidenepyridine)­alkane]­iron­(II) hexafluorophosphate (alkane = methane (<b>2a</b>), ethane (<b>2b</b>)) and <i>cis</i>-diacetonitrile­[1,3-bis­(<i>o</i>-imidazol-2-ylidenepyridine)­propane]­iron­(II) hexafluorophosphate (<b>2c</b>) have been characterized by single-crystal X-ray diffraction (XRD), nuclear magnetic resonance spectroscopy (NMR), and infrared spectroscopy (IR). Cyclic voltammetry (CV) measurements show reversible oxidation of Fe­(II) to Fe­(III). The rotational barrier of the bridge has been determined via variable-temperature NMR (VT-NMR) studies of <b>2b</b>. In all complexes, the Fe centers coordinatein addition to the NCCN ligandstwo acetonitrile ligands in the solid state as well as in solution. The alkylene bridge connecting the two NHCs has an influence on the coordination mode of the NCCN ligands. Whereas the methylene- and ethylene-bridged NHC moieties lead to a nearly planar geometry of the NCCN ligand and two trans-positioned acetonitrile ligands, the propylene-bridged complex <b>2c</b> exhibits a sawhorse-type coordination mode, with two cis-oriented acetonitrile ligands. The reactivity of the synthesized complexes toward substitution of the solvent ligands was investigated, showing that acetonitrile is readily substituted by benzonitrile. Upon addition of carbon monoxide, one acetonitrile ligand is replaced by CO to yield complexes <b>3a</b>–<b>c</b>, as shown by NMR and IR spectroscopy, as well as by XRD in the case of compound <b>3c</b>