Aryls represent a
class of both σ- and π-donating electron-rich
axial ligands that is underexplored in the field of metal–metal
multiply bonded paddlewheel species. By a decrease in the steric bulk
around the diruthenium axial sites and an increase in the basicity
of the bridging ligands, the first examples of bisaryl diruthenium(III)
paddlewheel complexes have been isolated. Compounds of the form Ru2(DMBA)4Ar2 (DMBA = N,N′-dimethylbenzamidinate, Ar = C6H4-4-
t
Bu (1), C6H5 (2), C6H3-3,5-(OCH3)2 (3)) were synthesized via a lithium–halogen
exchange reaction between Ru2(DMBA)4Cl2 and excess LiAr and characterized using mass spectrometry, electronic
absorption spectroscopy, cyclic and differential pulse voltammetry,
and 1H NMR spectroscopy. The molecular structures of compounds 1–3 were established using single-crystal
X-ray diffraction analysis and their electronic structures (both ground
and excited state) analyzed using density functional theory calculations.
Reported herein are
the first examples of the formation of Ru–Csp2
bonds in paddlewheel diruthenium species. A series of six
Ru2(ap)4(C6H4-4-X) type compounds (ap = 2-anilinopyridinate;
X = NMe2 (1), N,N-(C6H4-4-OMe)2 (2),
t
Bu (3), H (4), Br (5), CF3 (6)) was synthesized by employing
the lithium–halogen exchange reaction with a variety of para-substituted
aryl halides. These compounds have been characterized via electronic
absorption spectroscopy, cyclic voltammetry, mass spectrometry, and
magnetism studies, and their molecular structures have been established
by single-crystal X-ray diffraction studies. All compounds are in
the Ru2
5+ oxidation state, with a ground-state
electronic configuration of σ2π4δ2(π*δ*)3. Crystal structures
of 1–6 confirm this, indicating a
Ru–Ru bond order of 2.5. Electrochemical data suggest that
the σ-aryls are stronger donors than the σ-alkynyls. A
range of electronically different substituents allows for a closer
inspection of the extent of electronic conjugation across the diruthenium
paddlewheel core and the axial aryl ligand.
Reported herein is the use of aryls as axial ligands to manipulate reactivity at the distal metal site through metal− metal−ligand interactions in diruthenium paddlewheel complexes. The vacant ruthenium site in Ru 2 (ap) 4 (Ar) (1; ap = 2anilinopyridinate and Ar = C 6 H 4 -4-NMe 2 ), thus rendered reactive, is able to bind a series of isoelectronic ligands to afford three complexes of the form (Y)[Ru 2 (ap) 4 ](Ar) [Y = CN − (2), HCC − (3), CO (4)], each of which exhibits a distinct electronic structure. While reactions with anionic ligands subsequently result in oxidation of the diruthenium core from Ru 2 (II,III) to Ru 2 (III,III), the reaction with CO yields a rare example of a Ru 2 (II,III)-CO axial adduct. The latter reaction is particularly interesting in its completely reversible change of the ground state from S = 3 / 2 in 1 to S = 1 / 2 in 4, the first of its kind seen in Ru 2 (II,III) species. In general, this work sheds light on the modulation of the electronic structure of diruthenium paddlewheel complexes using distinct coordination environments around each of the ruthenium centers.
Two new nickel(II) complexes of cyclams bearing C‐alkyl groups, Ni(MEC)OTf2 (1, MEC = 5,12‐diethyl‐7,14‐dimethyl‐1,4,8,11‐tetraazacyclotetradecane) and Ni(CTMC)OTf2 (2, CTMC = 5,7,12,14‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane), were prepared, and their similarity to NiII(MPC) (MPC = 5,12‐dimethyl‐7,14‐diphenyl‐1,4,8,11‐tetraazacyclotetradecane) in ring conformation was revealed through single‐crystal X‐ray diffraction studies. Solution electronic absorption spectroscopy indicates the retention of octahedral coordination mode for both 1 and 2 in 20 % aqueous acetonitrile. Cyclic voltammetry studies of 1 and 2 under CO2 in 20 % aqueous acetonitrile revealed significantly increased catalytic currents compared to previously studied NiII(cyclam) and NiII(MPC). Controlled potential electrolysis studies of 2 revealed a 250 % increase in CO turn over frequency from that of Ni(cyclam)OTf2 and a 40 % increase from that of Ni(MPC)OTf2. Such improvements establish the benefit of electronically donating substituents that minimize steric interference around the axial catalytic sites.
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