Multi-phosphine-chelated iron-carbide clusters via redox-promoted ligand exchange on an inert hexa-iron-carbide carbonyl cluster, [Fe 6 (μ 6 -C)(μ 2 -CO) 4 (CO) 12 ] 2

• Published in: Chemical science (Cambridge) Vol. 15; no. 29; pp. 11455 - 11471
• Main Authors: Cobb, Caitlyn R, Ngo, Ren K, Dick, Emily J, Lynch, Vincent M, Rose, Michael J
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 24-07-2024; The Royal Society of Chemistry
• Subjects: Active control; Bends; Benzene; Carbides; Carbonyls; Chelation; Chemistry; Clusters; Crystallization; Electrons; Iron; Ligands; Organometallic compounds; Oxidation; Phosphines; Reagents; Substitution reactions
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d4sc01370k

We report the reactivity, structures and spectroscopic characterization of reactions of phosphine-based ligands (mono-, di- and tri-dentate) with iron-carbide carbonyl clusters. Historically, the archetype of this cluster class, namely [Fe
(μ
-C)(μ
-CO)
(CO)
]
, can be prepared on a gram-scale but is resistant to simple ligand substitution reactions. This limitation has precluded the relevance of iron-carbide clusters relating to organometallics, catalysis and the nitrogenase active site cluster. Herein, we aimed to derive a simple and reliable method to accomplish CO → L (where L = phosphine or other general ligands) substitution reactions without harsh reagents or multi-step synthetic strategies. Ultimately, our goal was ligand-based chelation of an Fe
(μ
-C) core to achieve more synthetic control over multi-iron-carbide motifs relevant to the nitrogenase active site. We report that the key intermediate is the PSEPT-non-conforming cluster [Fe
(μ
-C)(CO)
] (2: 84 electrons), which can be generated
by the outer-sphere oxidation of [Fe
(μ
-C)(CO)
]
(1:
, 86 electrons) with 2 equiv. of [Fc]PF
. The reaction of 2 with excess PPh
generates a singly substituted neutral cluster [Fe
(μ
-C)(CO)
PPh
] (4), similar to the reported reactivity of the substitutionally active cluster [Fe
(μ
-C)(CO)
] with monodentate phosphines (Cooke & Mays, 1990). In contrast, the reaction of 2 with flexible, bidentate phosphines (DPPE and DPPP) generates a wide range of unisolable products. However, the rigid bidentate phosphine bis(diphenylphosphino)benzene (bdpb) disproportionates the cluster into non-ligated Fe
-carbide anions paired with a bdpb-supported Fe(ii) cation, which co-crystallize in [Fe
(μ
-CH)(μ
-CO)(CO)
]
[Fe(MeCN)
(bdpb)
] (6). A successful reaction of 2 with the tripodal ligand Triphos generates the first multi-iron-chelated, authentic carbide cluster of the formula [Fe
(μ
-C)(κ3-Triphos)(CO)
] (9). DFT analysis of the key (oxidized) intermediate 2 suggests that its (μ
-C)Fe
framework remains fully intact but is distorted into an axially compressed, 'ruffled' octahedron distinct from the parent
cluster 1. Oxidation of the cluster in non-coordinating solvent allows for the isolation and crystallization of the CO-saturated, intact
-analogue [Fe
(μ
-C)(CO)
] (3), indicating that the intact (μ
-C)Fe
motif is retained during initial oxidation with [Fc]PF
. Overall, we demonstrate that redox modulation beneficially 'bends' Wade-Mingo's rules
the generation of electron-starved (non-PSEPT) intermediates, which are the key intermediates in promoting facile CO → L substitution reactions in iron-carbide-carbonyl clusters.