Synthesis and Characterization of [M₂(NCtBu₂)₅]⁻ (M=Mn, Fe, Co): Metal Ketimide Complexes with Strong Metal–Metal Interactions

Abstract

The study of metal-metal interactions has provided many important insights into transition-metal bonding and electronic structure. [1] This is perhaps best exemplified by the synthesis of quintuply bonded [Ar'CrCrAr'] (Ar' = C 6 H 3 -2,6(C 6 H 3 -2,6-iPr 2 ) 2 ), which has intrigued both experimentalists and theoreticians since it was first reported in 2005. [2] Complexes with metal-metal bonds also exhibit interesting optoelectronic properties [2d] and intriguing chemical reactivity. [3] Interestingly, a survey of complexes with metal-metal bonds shows a large knowledge gap between the late first-row transition metals and the rest of the transition-metal block. For example, the Cambridge Structural Database contains only a few M 2 4+ complexes with metalmetal bonds for Mn (4 structures), Fe (28 structures), and Co (54 structures), [4] whereas many more structure are known for Cr (> 500 structures), Ru (> 500), and Rh (> 1500 structures). [1] These trends can be rationalized by the contracted nature of the 3d electrons for the later first-row transition metals, [2i] and highlights the challenge of making metal-metal bonds with these elements. In this regard, the development of new ligands that can promote metal-metal bonding would be of significant benefit for the exploration of these interactions and their application in the field of catalysis. Herein we demonstrate the ability of the ketimide ligand, [N = CR 2 ] À , [5] to promote metal-metal interactions, specifically in the ketimide-bridged transition-metal complexes, [M 2 (N = CtBu 2 ) 5 ] À (M = Mn, Fe, Co), which exhibit short metal-metal distances and strong inter-metal magnetic communication.Addition of 2.5 equiv of Li(N=CtBu 2 ) to MCl 2 (M = Mn, Fe, and Co) in THF, followed by addition of 1 equiv of

An iron ketimide single-molecule magnet [Fe4(NCPh2)6] with suppressed through-barrier relaxation

“…Complex 1 crystallizes as brown plates in the triclinic space group P 1 with two independent molecules per unit cell ( 1A with r ¼ 1.05. 21 Finally, the Fe-Fe distances for the mixedvalent Fe(II)/Fe(I) clusters reported by Betley and co-workers are also in good agreement with complex 1, with average Fe-Fe distances ranging from 2.46 to 2.65 A. 14,15,17 In contrast to most other multi-metallic Fe clusters, however, which require specially-designed ligands to control nuclearity, 14,15,17,18,30 complex 1 spontaneously self-assembles using a simple, commercially available ketimide ligand.…”

“…Similarly, Hoffman and co-workers isolated a tetrametallic ketimide-supported Cu cluster, [Cu(N]C t Bu 2 )] 4 , further highlighting the ability of ketimide ligands to bridge metal centers. 22 The ketimide ligand also promotes metal-metal communication via super exchange, as observed for the mixed-valent bimetallic, [Fe 2 (N]C t Bu 2 ) 5 ], 21 which is likely a consequence of the highly covalent metalketimide interaction. 23 Thus, we hypothesized that ketimides, by virtue of their proclivity to promote M-M bonding and communication, could engender the formation of high nuclearity metal clusters with strong M-M exchange.…”

“…In an effort to access a high nuclearity iron cluster, we sought to employ ketimide ( À N]CR 2 ) as the supporting ligand. Previously, we had shown that ketimides can be used to promote metal-metal bonding, as seen in the bimetallic complexes [Li (12- 21 The Fe example is notable, as magnetic data supported the presence of a double bond between the two Fe centres. Similarly, Hoffman and co-workers isolated a tetrametallic ketimide-supported Cu cluster, [Cu(N]C t Bu 2 )] 4 , further highlighting the ability of ketimide ligands to bridge metal centers.…”

Homoleptic Transition Metal Complexes of the 7-Azaindolide Ligand Featuring κ1-N1 Coordination

“…Similar, albeit even lower values for the Fe II/III couple were observed for related four-coordinate homoleptic complexes of iron containing the di- tert -butyl ketimide ligand. 54,55 …”

Reactivity of an All‐Ferrous Iron–Nitrogen Heterocubane under Reductive and Oxidative Conditions

“…There are no comparable molecules known, but similar binding motifs were observed with two Fe II ions or one Fe II and one Fe III ion. In these known compounds, the magnetic interactions between the two metal centers range from weakly ferromagnetic ( J =2.24 cm −1 ) to strongly antiferromagnetic ( J =−235 cm −1 ) 31a,d. The poor solubility of 5 in common organic solvents such as benzene, THF, acetonitrile or DMSO hampered a solution NMR spectroscopic characterization 42…”