Synthesis and redox reactivity of a phosphine-ligated dichromium paddlewheel

Abstract

A pentadentate ligand platform, bis[2-(diispropylphosphino-methylamino)phenyl]ether (1), abbreviated as H 2 (PNONP), is introduced that enables the isolation of homodinuclear chromium complexes. In a one-step metalation using Cr(III) and Cr(II) chloride reagents, the bimetallic compounds, [Cr(μ−Cl)(PNONP)] 2 (2) and [Cr(PNONP)] 2 (3), respectively, were synthesized. These complexes have been characterized by X-ray diffraction, NMR spectroscopy, cyclic voltammetry, magnetometry, UV−vis−NIR spectroscopy, combustion analysis, and computational methods. Complex 3 has a reasonably short Cr−Cr bond length of 2.1342(5) Å. Quantum chemical calculations support a diradical singlet ground-state with a formal triple bond between the chromium centers. By cyclic voltammetry, 3 exhibits two reversible oxidations at E ½ = −472 and −753 mV versus FeCp 2 0/+. The one-and two-electron oxidized analogues, 3 + and 3 2+ , were generated in situ via chemical oxidation using ferrocenium. Based on in situ characterization of 3 + and 3 2+ , we hypothesize the oxidations are metal-based to yield Cr 2 5+ and Cr 2 6+ cores, respectively.

Chromium Diamino-bis(phenolate) Complexes as Catalysts for the Ring-Opening Copolymerization of Cyclohexene Oxide and Carbon Dioxide

“…Several bond angles such as Cl­(1*)–Cr(1)–Cl(1) and N(2)–Cr(1)–N(1) with angles of 83.15(4) and 75.50(12)°, respectively, show distortions from ideal octahedral geometry. The Cr–Cl bond distances were observed to be 2.3805(12) and 2.4244(12) Å and are similar to those of related chloride-bridged Cr­(III) dimers. ,,− The solid-state structure of 1′ contains one trans dichloride-containing chromium center with a [O 2 N 2 ] ligand completing the coordination sphere. One of the phenolate oxygen donors bridges the second chromium ion, which possesses a cis -β-[O 2 N 2 ] ligand.…”

Computational Spectroscopy of the Cr–Cr Bond in Coordination Complexes

“…Over the last 60 years, multiple bonds between a wide variety of transition metals have been assigned bond orders ranging from single to sextuple. − Nowadays, synthetic chemists seek out new metal–metal bonds for as catalysts, building blocks in metal–organic frameworks, photosensitizers, and molecular conductors. ,− Group 6 metal–metal bonds have been broadly studied for their interesting bonding and unique photophysical properties. , Of these, Cr–Cr multiple bonds are notable because of their rich electronic structure and unique spectroscopic properties. Starting with Cotton’s work in the 1970s, a variety of dichromium complexes have been synthesized; characterization by diffraction has shown that the bond distances range from 1.7 to 2.3 Å. ,− Although in the early days formal bond orders were estimated based on distances, computational work using multiconfigurational methods has established that the electronic structure of these complexes is multiconfigurational due to the small energy splitting between the σ, π, and δ orbitals. The concept of an effective bond order (EBO), which accounts for the partial occupation of low-lying antibonding orbitals, has been used to quantify the nature of the overall Cr–Cr bond that is composed of multiple partial bonds. , Furthermore, these computations highlight that the multiconfigurational electronic structure of Cr 2 bonds is more nuanced compared to Mo 2 and W 2 analogues .…”

“…For the computation of bond length, several procedures have been proposed to empirically correct DFT simulations. For example, DFT geometry optimizations can be performed keeping the Cr–Cr bond distance fixed at the experimental value with subsequent analysis of the electronic structure with CASPT2 . Alternatively, a series of constrained DFT optimizations can be performed to determine the Cr–Cr bond distance point wise with CASPT2 yielding good agreement with the experiment. − These hybrid approaches, however, are not applicable to the computation of the vibrational spectra.…”