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, ‡ and Maheswaran Shanmugam* ,† † Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai - 400076, Maharashtra, India ‡ Department of Bio-Sciences and Bio-Engineering, Indian Institute of Technology Bombay, Powai, Mumbai - 400076, Maharashtra, India * S Supporting Information ABSTRACT: Three structurally analogous hexanuclear ruthenium(III) complexes were isolated with the general molecular formula of [Ru 6 III (O) 2 (μ 4 -η 2 -η 2 -CH 2 O 2 )(t- BuCO 2 ) 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 I4̅ c2 and P4 1 2 1 2 space group, respectively, while 2 crystallized in the monoclinic system with P2 1 /c space group. In all three complexes, two oxo-centered Ru(III) triangles were bridged by a unique and a rare methylenediolate (CH 2 O 2 ) 2- ) ligand. This (CH 2 O 2 ) 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 (CH 2 O 2 ) 2- is unknown at the moment, as no precursor was used to form this intermediate. The presence of (CH 2 O 2 ) 2- identi fied through X-ray di ffraction was further unambiguously confirmed by various 1D ( 1 H and 13 C) 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 1 H nuclei of the (CH 2 O 2 ) 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 (Ru 3 O-) 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. ■ INTRODUCTION The preformed oxo-centered transition metal based triangular complexes are used as a precursor to reveal fascinating coordination complexes with excellent electronic and magnetic properties. For example, several oligonuclear iron, manganese, chromium, and ruthenium complexes ranging from 3 to 84 metal centers containing complexes were isolated using their respective oxo-centered triangular homovalent or mixed valent metal precursor. 1 Among the numerous metal clusters isolated, common bridging ligands such as oxo, hydroxo, methoxo, phenoxo, carboxylates, etc. tend to template the metal cluster, and the peripheral ligands employed usually prevent the formation of extended networks. Apart from the usual bridging ligand mentioned above, precedents are known with unusual bridging ligands such as methylene diolate [(CH 2 O 2 ) 2- ], which could template the metal clusters. For example, when an iron(III) oxo-centered triangle reacts with pyridine or THF, it results in formation of a Fe 6 cluster in which two Fe 3 O units are bridged by methylene diolate and the peripheral coordination sites are decorated by various carboxylates (see the magenta trace in Figure 1A). 2 A similar scenario, observed when a [Mn 12 (O) 12 (OAc) 16 (H 2 O) 4 ] cluster is recrystallized in CH 2 Cl 2 /nitromethane solvents, leads to a Mn 6 cluster which is structurally similar to the Fe 6 cluster bridged by a methylene diolate ligand. 3 In an unrelated report, however, methylene diolate ligand was trapped within the coordination sphere of transition metal clusters either by reduction of CO 2 (Figure 1B) 4 or by reacting ammonium molybdate with formaldehyde (Figure 1C). 5 To the best of our knowledge these are the only five transition metal clusters with the (CH 2 O 2 ) 2- moiety were structurally characterized, and other metal ion complexes with this rare bridging ligand are not known yet, particularly the more amenable Fe(III) congener, ruthenium ions. A great deal of focus has been paid for the past two decades to link the oxo- centered ruthenium triangles using various bridging ligands to better understand the mechanism of electron transfer and fine- tune the electron transfer rate between the two triangles. 6 Received: September 29, 2018 Article pubs.acs.org/IC Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.8b02780 Inorg. Chem. XXXX, XXX, XXX-XXX Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 11/13/18. For personal use only.