om7b00164_si_002.xyz (13.65 kB)
Geometric and Electronic Structure Analysis of the Three-Membered Electron-Transfer Series [(μ‑CNR)2[CpCo]2]n (n = 0, 1–, 2−) and Its Relevance to the Classical Bridging-Carbonyl System
dataset
posted on 2017-05-25, 19:46 authored by Charles
C. Mokhtarzadeh, Alex E. Carpenter, Daniel P. Spence, Mohand Melaimi, Douglas W. Agnew, Nils Weidemann, Curtis E. Moore, Arnold L. Rheingold, Joshua S. FigueroaThe dimeric bridging carbonyl complexes
[(μ‑CO)2[CpCo]2]n (n = 0, 1−) have occupied
a central position in the
understanding of metal–metal
bonding interactions when bridging ligands are present. Based on simple
electron-counting formalisms, these dimers have been proposed to possess
formal Co–Co bond orders of 2 and 1.5, respectively. However,
this simple bonding scheme has been contrasted by molecular orbital
theory considerations, as well as spectroscopic data that probes M–M
bonding interactions generally. While this system has received considerable
attention, there has been a long-standing synthetic limitation in
that the doubly reduced dianionic dimer, [(μ‑CO)2[CpCo]2]2–, has not been
amenable to isolation, thereby precluding an analysis of ostensible
full-integer reduction in a homologous series. Accordingly, herein
is presented the synthesis of a homologous, three-membered series
of bridging-isocyanide [(μ‑CNAr)2[CpCo]2]n dimers, including the dianionic
member. Structural and spectroscopic analyses of these [(μ‑CNAr)2[CpCo]2]n dimers,
which feature the m-terphenyl isocyanide CNArMes2 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3), reveal that
this series possesses similar overall properties to the bridging carbonyl
counterparts. However, high-resolution X-ray crystallographic studies
have revealed important structural differences that were not discernible
in older studies of the carbonyl complexes. Also presented is the
synthesis of the bridging-isocyanide/η6-arene dimers
[Co2((η6-Mes)(μ‑CNArMes))2]n (n = 0, 1+), which are valence isoelectronic to the mono- and dianionic
[(μ‑CNAr)2[CpCo]2]n derivatives. Structural and spectroscopic studies
of these η6-arene complexes, as well as the related
neutral nickel dimer (μ‑CNArMes)2[CpNi]2 provide evidence for an electronic structure
environment dominated by M→(CN)π* back-bonding interactions,
rather than direct M–M bonding. This conclusion is supported
by DFT-derived molecular orbital analysis on the bridging-isocyanide
[(μ‑CNArMes2)2[CpCo]2]n dimers.
