Unusual Methylenediolate Bridged Hexanuclear Ruthenium(III) Complexes: Syntheses and Their Application

Jitendrasingh Rajpurohit; Apoorva Upadhyay; Chinmoy Das; Richa Dubey; Shefali Vaidya; Vinoth Krishnan; Ashutosh Kumar; Maheswaran Shanmugam

doi:10.1021/acs.inorgchem.8b02780

Unusual Methylenediolate Bridged Hexanuclear Ruthenium(III) Complexes: Syntheses and Their Application

Inorg Chem. 2018 Dec 3;57(23):14967-14982. doi: 10.1021/acs.inorgchem.8b02780. Epub 2018 Nov 12.

Authors

Jitendrasingh Rajpurohit¹, Apoorva Upadhyay¹, Chinmoy Das¹, Richa Dubey², Shefali Vaidya¹, Vinoth Krishnan¹, Ashutosh Kumar², Maheswaran Shanmugam¹

Affiliations

¹ Department of Chemistry , Indian Institute of Technology Bombay , Powai, Mumbai - 400076 , Maharashtra , India.
² Department of Bio-Sciences and Bio-Engineering , Indian Institute of Technology Bombay , Powai, Mumbai - 400076 , Maharashtra , India.

PMID: 30418745
DOI: 10.1021/acs.inorgchem.8b02780

Abstract

Three structurally analogous hexanuclear ruthenium(III) complexes were isolated with the general molecular formula of [Ru₆^III(O)₂(μ₄-η²-η²-CH₂O₂)( t-BuCO₂)₁₂(L)₂] where L = pyridine (1) or 4-dimethylamino pyridine (DMAP; 2) or 4-cyanopyridine (3). Complexes 1 and 3 were solved in the tetragonal I4̅c2 and P4₁2₁2 space group, respectively, while 2 crystallized in the monoclinic system with P2₁ /c space group. In all three complexes, two oxo-centered Ru(III) triangles were bridged by a unique and a rare methylenediolate (CH₂O₂)^2-) ligand. This (CH₂O₂)^2- group is reported to be an intermediate, which is not isolated in its metal-free form, to date, as it is unstable. Control experiments performed evidently reveal that the unique reaction condition followed is mandatory to isolate 1-3 and the origin of (CH₂O₂)^2- is unknown at the moment, as no precursor was used to form this intermediate. The presence of (CH₂O₂)^2- identified through X-ray diffraction was further unambiguously confirmed by various 1D (¹H and ¹³C) and 2D-NMR (HSQC, TOCSY, NOESY, and DEPT) spectroscopies. Direct current (dc) magnetic susceptibility measurements performed on 1 and 2 reveal the predominant antiferromagnetic exchange interaction between the Ru(III) centers result in a diamagnetic ground state at 2.0 K. The paramagnetic influence of 1-3 at room temperature evidently felt by the ¹H nuclei of the (CH₂O₂)^2- unit predominates compared to other NMR active nuclei in the complexes. The presence of an electron donating or withdrawing substituent on the terminal pyridine results in significant change in the dihedral angle of two oxo-centered triangular (Ru₃O-) planes. The change in the structural parameters of 1-3 due to the substituents markedly reflected on the absorption profile and redox behavior, which are systematically investigated. Preliminary galvanostatic charge/discharge cycling experiments performed on a representative complex (3) suggest that 3 can be a promising candidate to employ as an effective multiple electron charge carrier in a nonaqueous redox flow battery.