Formation of a Hexacarbonyl Diiron Complex Having a Naphthalene-1,8-bis(phenylphosphido) Bridge and the Electrochemical Behavior of Its Derivatives

The P–P bond of the <i>cis</i>-<b>1</b> ligand in (μ-<i>cis</i>-<b>1</b>)­[Fe­(CO)₄]₂ (<i>cis</i>-<b>1</b> = naphthalene-1,8-diphenyldiphosphine) was cleaved by the two iron centers after CO dissociation from the iron centers, although the P–P bond of <i>cis</i>-<b>1</b> was stereochemically stabilized with a robust naphthalene group, unlike the usual diphosphines, which lack such support. The resulting (μ-nabip)­[Fe­(CO)₃]₂ (<b>3</b>; nabip = naphthalene-1,8-bis­(phenylphosphido)) had the diiron core linked by the bisphosphido bridge. Since the trans isomer (μ-<i>trans</i>-<b>1</b>)­[Fe­(CO)₄]₂ was stable under ambient conditions, the <i>cis</i> disposition of the two Fe­(CO)₄ fragments was responsible for the cleavage of the P–P bond. The one or two terminal CO ligands of <b>3</b> can be replaced by MeCN and a range of phosphine ligands: i.e., PMe₃, PPh₃, <i>cis</i>-<b>1</b>, and <i>trans</i>-<b>1</b>. Interestingly, it was found that the diphosphine <i>cis</i>-<b>1</b> could coordinate the iron center in an unusual κ² fashion to form a three-membered ring, which was confirmed by NMR spectra as well as X-ray analysis. These diiron complexes can be protonated with the strong acid TfOH in CH₂Cl₂ to form cationic complexes having a μ-H bridge between the two iron centers. The parent hexacarbonyl complex <b>3</b> could act as a proton reduction catalyst at −2.0 V in the presence of TsOH as the proton source in CH₂Cl₂. When protonated complexes having MeCN or phosphine ligands were used, the proton reduction potentials catalyzed by these complexes were shifted to a more positive range of around −1.77 to −1.37 V, depending on the terminal ligand

Synthesis and Crystal Structures of <i>P</i>-Iron-Substituted Phosphinoborane Monomers

A family of <i>P</i>-iron-substituted phosphinoboranes, Cp­(CO)₂Fe­{P­(Ar)­BMes₂} (Ar = Ph, Mes, Tipp, Mes*), have been prepared from the reaction of Cp­(CO)₂FeCl and (Li)­(Ar)­PBMes₂. All the complexes have been characterized successfully by ¹H, ¹¹B, and ³¹P NMR; IR spectroscopy; and X-ray crystallography. In the IR spectra, all the complexes display similar carbonyl stretching frequencies that are remarkably higher than those of closely related phosphide complexes. These observations indicate that a repulsive interaction between the filled d orbital on the iron and the lone pair on the phosphorus is less severe in the studied iron-phosphinoboranes, which is most likely because of the P→B π interaction that occurs in them. The ³¹P­{¹H} NMR chemical shifts of the phosphinoborane phosphorus move upfield with the increasing steric bulk of the Ar groups in the order Ph (−51.4 ppm) < Mes (−68.8 ppm) < Tipp (−84.9 ppm). However, the phosphorus bearing the most sterically demanding Mes* group appears at an unexpectedly downfield value of −44.9 ppm, which is probably reflective of its structural peculiarities. The ¹H NMR spectrum of each complex displays two sets of signals, assignable to inequivalent Mes groups on the boron atom, as a consequence of a hindered rotation around the P–B bond. This high rotational barrier most likely results from the significant double-bond character in the P–B bond. The X-ray diffraction studies have confirmed the iron-phosphinoboranes considered herein to be monomeric species. Each molecule consists of a nearly planar phosphinoborane fragment with a short P–B bond. The Fe–P bond is notably elongated as the Ar group becomes larger, demonstrating its somewhat vulnerable nature with respect to steric congestion. In contrast, the variation in the P–B bond distance is relatively small throughout the series of iron-phosphinoboranes, suggesting that the PB double-bond character is balanced by steric and electronic effects of the substituents around the phosphorus

Carbon(0)-Bridged Pt/Ag Dinuclear and Tetranuclear Complexes Based on a Cyclometalated Pincer Carbodiphosphorane Platform

A carbon(0)-bridged Pt₂Ag₂ cluster was synthesized from the reaction of a cyclometalated pincer carbodiphosphorane platinum complex with AgOTf, by forming Pt­(II)←C(0)→Ag­(I) dative bonds along with Pt­(II)–Ag­(I) and Ag­(I)–Ag­(I) metal–metal interactions. X-ray diffraction analysis reveals that the cluster adopts an antiparallel sandwich structure with a ladder-shaped PtC/AgAg/CPt core. The coordination plane of the platinum unit is highly distorted due to the in-plane steric repulsion between the PEt₃ ligand on the platinum and the nearest proton on each of the two cyclometalated phenyl rings in the pincer carbodiphosphorane framework. The cluster is very labile and displays different reactivity patterns toward trivalent phosphorus ligands. In the reaction with bulky PPh₃, a dinuclear complex was formed because of coordination of PPh₃ to the silver atom upon cleavage of the tetranuclear core. In contrast, replacement of the PEt₃ on the platinum center by P­(OPh)₃, which is sterically less demanding, led to a dinuclear complex where the eliminated PEt₃ ligand recoordinated to the silver atom

Carbon(0)-Bridged Pt/Ag Dinuclear and Tetranuclear Complexes Based on a Cyclometalated Pincer Carbodiphosphorane Platform

A carbon(0)-bridged Pt₂Ag₂ cluster was synthesized from the reaction of a cyclometalated pincer carbodiphosphorane platinum complex with AgOTf, by forming Pt­(II)←C(0)→Ag­(I) dative bonds along with Pt­(II)–Ag­(I) and Ag­(I)–Ag­(I) metal–metal interactions. X-ray diffraction analysis reveals that the cluster adopts an antiparallel sandwich structure with a ladder-shaped PtC/AgAg/CPt core. The coordination plane of the platinum unit is highly distorted due to the in-plane steric repulsion between the PEt₃ ligand on the platinum and the nearest proton on each of the two cyclometalated phenyl rings in the pincer carbodiphosphorane framework. The cluster is very labile and displays different reactivity patterns toward trivalent phosphorus ligands. In the reaction with bulky PPh₃, a dinuclear complex was formed because of coordination of PPh₃ to the silver atom upon cleavage of the tetranuclear core. In contrast, replacement of the PEt₃ on the platinum center by P­(OPh)₃, which is sterically less demanding, led to a dinuclear complex where the eliminated PEt₃ ligand recoordinated to the silver atom