Cooperative redox and spin activity from three redox congeners of sulfur-bridged iron nitrosyl and nickel dithiolene complexes

Site specific redox properties in ligand differentiated di-nickel complexes inspired by the acetyl CoA synthase active site

Bimetallic transition metal complexes with site-specific redox properties offer a versatile platform for understanding electron polarization, intramolecular electron transfer processes, and customizing electronic and magnetic properties that might impact reactivity and catalyst design. Inspired by the dissymmetric nickel sites in the Acetyl CoA Synthase (ACS) Active Site, three new bimetallic Ni(N2S2)-Ni(S2C2R2) complexes based on Ni(N2S2) metalloligand donor synthons, Nid, in mimicry of the nickel site distal to the redox-active iron sulfur cluster of ACS, and nickel dithiolene receiver units, designated as Nip, the nickel proximal to the 4Fe4S cluster, were combined to explore the influence of ligand environment on electronic structure and redox properties of each unit. The combination of synthons gave a matrix of three S-bridged dinickel complexes, characterized by X-ray crystallography, and appropriate spectroscopies. Computational modeling is connected to the electronic characteristics of the nickel donor and receiver units. This study demonstrated the intricacies of identifying sites of electrochemical redox processes, within multi-metallic systems containing non-innocent ligands.

Development of (NO)Fe(N2S2) as a Metallodithiolate Spin Probe Ligand: A Case Study Approach

ConspectusThe ubiquity of sulfur-metal connections in nature inspires the design of bi- and multimetallic systems in synthetic inorganic chemistry. Common motifs for biocatalysts developed in evolutionary biology include the placement of metals in close proximity with flexible sulfur bridges as well as the presence of π-acidic/delocalizing ligands. This Account will delve into the development of a (NO)Fe(N2S2) metallodithiolate ligand that harnesses these principles. The Fe(NO) unit is the centroid of a N2S2 donor field, which as a whole is capable of serving as a redox-active, bidentate S-donor ligand. Its paramagnetism as well as the ν(NO) vibrational monitor can be exploited in the development of new classes of heterobimetallic complexes. We offer four examples in which the unpaired electron on the {Fe(NO)}7 unit is spin-paired with adjacent paramagnets in proximal and distal positions.First, the exceptional stability of the (NO)Fe(N2S2)-Fe(NO)2 platform, which permits its isolation and structural characterization at three distinct redox levels, is linked to the charge delocalization occurring on both the Fe(NO) and the Fe(NO)2 supports. This accommodates the formation of a rare nonheme {Fe(NO)}8 triplet state, with a linear configuration. A subsequent FeNi complex, featuring redox-active ligands on both metals (NO on iron and dithiolene on nickel), displayed unexpected physical properties. Our research showed good reversibility in two redox processes, allowing isolation in reduced and oxidized forms. Various spectroscopic and crystallographic analyses confirmed these states, and Mössbauer data supported the redox change at the iron site upon reduction. Oxidation of the complex produced a dimeric dication, revealing an intriguing magnetic behavior. The monomer appears as a spin-coupled diradical between {Fe(NO)}7 and the nickel dithiolene monoradical, while dimerization couples the latter radical units via a Ni2S2 rhomb. Magnetic data (SQUID) on the dimer dication found a singlet ground state with a thermally accessible triplet state that is responsible for magnetism. A theoretical model built on an H4 chain explains this unexpected ferromagnetic low-energy triplet state arising from the antiferromagnetic coupling of a four-radical molecular conglomerate. For comparison, two (NO)Fe(N2S2) were connected through diamagnetic group 10 cations producing diradical trimetallic complexes. Antiferromagnetic coupling is observed between {Fe(NO)}7 units, with exchange coupling constants (J) of -3, -23, and -124 cm-1 for NiII, PdII, and PtII, respectively. This trend is explained by the enhanced covalency and polarizability of sulfur-dense metallodithiolate ligands. A central paramagnetic trans-Cr(NO)(MeCN) receiver unit core results in a cissoid structural topology, influenced by the stereoactivity of the lone pair(s) on the sulfur donors. This {Cr(NO)}5 radical bridge, unlike all previous cases, finds the coupling between the distal Fe(NO) radicals to be ferromagnetic (J = 24 cm-1).The stability and predictability of this S = 1/2 moiety and the steric/electronic properties of the bridging thiolate sulfurs suggest it to be a likely candidate for the development of novel molecular (magnetic) compounds and possibly materials. The role of synthetic inorganic chemistry in designing synthons that permit connections of the (NO)Fe(N2S2) metalloligand is highlighted as well as the properties of the heterobi- and polymetallic complexes derived therefrom.

Sulfur Lone Pairs Control Topology in Heterotrimetallic Complexes: An Experimental and Theoretical Study

Heterotrimetallic complexes with (N2S2)M metallodithiolates, M = Ni2+, [Fe(NO)]2+, and [Co(NO)]2+, as bidentate chelating ligands to a central trans-Cr(NO)(MeCN) unit were characterized as the first members of a new class, NiCrNi, FeCrFe, CoCrCo. The complexes exhibit a cisoid structural topology, ascribed to the stereoactivity of the available lone pair(s) on the sulfur donors, resulting in a dispersed, electropositive pocket from the N/N and N/S hydrocarbon linkers wherein the Cr-NO site is housed. Computational studies explored alternative isomers (transoid and inverted cisoid) that suggest a combination of electronic and steric effects govern the geometrical selectivity. Electrostatic potential maps readily display the dominant electronegative potential from the sulfurs which force the NO to the electropositive pocket. The available S lone pairs work in synergy with the π-withdrawing ability of NO to lift Cr out of the S4 plane toward the NO and stabilize the geometry. The metallodithiolate ligands bound to Cr(NO) thus find structural consistency across the three congeners. Although the dinitrosyl [(bme-dach)Co(NO)-Mo(NO)(MeCN)-(bme-dach)Co(MeCN)][PF6]2 (CoMoCo') analogue displays chemical noninnocence and a partial Mo-Co bond toward (N2S2)Co'(NCCH3) in an "asymmetric butterfly" topology [Guerrero-Almaraz P.Inorg. Chem.2021, 60(21 (21), ), 15975-15979], the stability of the {Cr(NO)}5 unit prohibits such bond rearrangement. Magnetism and EPR studies illustrate spin coupling across the sulfur thiolate sulfur bridges.

Magnetic coupling between Fe(NO) spin probe ligands through diamagnetic NiII, PdII and PtII tetrathiolate bridges

Reaction of the nitrosylated-iron metallodithiolate ligand, paramagnetic (NO)Fe(N2S2), with [M(CH3CN)n][BF4]2 salts (M = NiII, PdII, and PtII; n = 4 or 6) affords di-radical tri-metallic complexes in a stairstep type arrangement ([FeMFe]2+, M = Ni, Pd, and Pt), with the central group 10 metal held in a MS4 square plane. These isostructural compounds have nearly identical ν(NO) stretching values, isomer shifts, and electrochemical properties, but vary in their magnetic properties. Despite the intramolecular Fe⋯Fe distances of ca. 6 Å, antiferromagnetic coupling is observed between {Fe(NO)}7 units as established by magnetic susceptibility, EPR, and DFT studies. The superexchange interaction through the thiolate sulfur and central metal atoms is on the order of NiII < PdII ≪ PtII with exchange coupling constants (J) of -3, -23, and -124 cm-1, consistent with increased covalency of the M-S bonds (3d < 4d < 5d). This trend is reproduced by DFT calculations with molecular orbital analysis providing insight into the origin of the enhancement in the exchange interaction. Specifically, the magnitude of the exchange interaction correlates surprisingly well with the energy difference between the HOMO and HOMO-1 orbitals of the triplet states, which is reflected in the central metal's contribution to these orbitals. These results demonstrate the ability of sulfur-dense metallodithiolate ligands to engender strong magnetic communication by virtue of their enhanced covalency and polarizability.