A Two‐in‐one Pincer Ligand and its Diiron(II) Complex Showing Spin State Switching in Solution through Reversible Ligand Exchange

Abstract

A novel pyrazolate-bridged ligand providing two {PNN} pincer-type compartments has been synthesized. Its diiron(II) complex LFe 2 (OTf) 3 (CH 3 CN) (1; Tf= triflate) features, in solid state, two bridging triflate ligands, with a terminal triflate and a MeCN ligand completing the octahedral coordination spheres of the two high-spin metal ions. In MeCN solution, 1 is shown to undergo a sequential, reversible, and complete spin transition to the low-spin state upon cooling. Detailed UV/Vis and 19 F NMR spectroscopic studies as well as magnetic measurements have unraveled that spin state switching correlates with a rapid multistep triflate/ MeCN ligand exchange equilibrium. The spin transition temperature can be continuously tuned by varying the triflate concentration in solution.

Sequential Double Dearomatization of the Pyrazolate‐Based “Two‐in‐One” Pincer Ligand in a Dinuclear Rhodium(I) Complex

“…When we first reported the pyrazolate‐based dinucleating scaffold [L] – we described it as having two fused pincer‐type PNN compartments and thus dubbed it “two‐in‐one” pincer ligand . Following the initial report of its diiron(II) complex, the present study now introduces a first dinuclear 4d metal complex of [L] – and demonstrates its close similarity with related mononuclear PNN pincer systems, thus validating the view that [L] – provides two closely spaced pincer sites.…”

“…As such, L – is distinct from other more flexible dinucleating ligands that feature two fused pincer‐type binding sites . In the initial report it was described that a diiron(II) complex of L – shows spin state switching in solution via reversible ligand exchange processes . Here we demonstrate that dinuclear complexes of L – can undergo sequential double deprotonation and dearomatization at the two chelate arms, which is a prerequisite for achieving MLC in the way it has been successfully exploited for the mononuclear systems …”

“…Commercially available reagents were used as received. The proligand HL was prepared according to a literature procedure …”

“…With the aim of combining both concepts and to enable MLC and MMC in a single system, we recently designed a pyrazolate‐based ligand scaffold HL (see Scheme below) that is comprised of two pincer‐type PNN sites and can be viewed as having two fused PNN subunits akin to the PNN ligand in II ; we coined this the “two‐in‐one” pincer ligand . In dinuclear complexes of L – the two active coordination sites trans to the pyridine‐N are directed into the bimetallic cleft, which is believed to potentially favor MMC.…”

How to Switch Spin‐Crossover Metal Complexes at Constant Room Temperature

“…Ligand exchange in solution is usually suggested on the basis of variable temperature spectroscopic studies or titration experiments . However, SCO at room temperature that is undoubtedly confirmed by magnetic measurements is rarely reported . Brooker et al.…”