Synthesis and Characterization of Two “Tied-Back” Lithium Ketimides and Isolation of a Ketimide-Bridged [Cr₂]⁶⁺ Dimer with Strong Antiferromagnetic Coupling

Abstract

Reaction of 1 equiv of KN(SiMe 3 ) 2 with 9-fluorenone results in the formation of (Me 3 Si)NC 13 H 8 (1) in high yield after work-up. Addition of 1 equiv of phenol to 1 results in rapid desilylation and formation of 9-fluorenone imine, HNC 13 H 8 (2). Subsequent reaction of 2 with 1 equiv of LiN i Pr 2 results in deprotonation and formation of [Li(Et 2 O)] 4 [NC 13 H 8 ] 4 (3) in good yield. Reaction of 1 equiv of KN(SiMe 3 ) 2 with 2-adamantanone for 7 days at room temperature results in the formation of (Me 3 Si)NC 10 H 14 (4) in good yield. Dissolution of 4 in neat MeOH results in rapid desilylation concomitant with formation of 2-adamantanone imine, HNC 10 H 14 (5). Subsequent reaction of 5 with 1 equiv of LiN i Pr 2 results in formation of [Li(THF)] 4 [NC 10 H 14 ] 4 (6). Both 3 and 6 were characterized by X-ray crystallography. Finally, reaction of CrCl 3 with 3.5 equiv of 6 results in formation of the [Cr 2 ] 6+ dimer, [Li][Cr 2 (N C 10 H 14 ) 7 ] (7), which can be isolated in modest yield after work-up. Complex 7 features a Cr−Cr bond length of 2.653(2) Å. Additionally, solid-state magnetic susceptibility measurements reveal strong antiferromagnetic coupling between the two Cr centers, with J = −200 cm −1 .

Measuring Metal–Metal Communication in a Series of Ketimide-Bridged [Fe2]6+ Complexes

“…For comparison, the magnetic coupling constants in 1, 2, and 4 are much smaller than those measured for other ketimide-bridged dimers, such as [Fe 2 (μ-N�C t Bu 2 ) 2 (N� C t Bu 2 ) 3 ] (J = −235 cm −1 ), [Mn 2 (μ-N�C t Bu 2 ) 3 (N� C t Bu 2 ) 2 ] − (J = −78 cm −1 ), and [Li][Cr 2 (μ-N� C 10 H 14 ) 3 (N�C 10 H 14 ) 4 ] (J = −200 cm −1 ). 33,36 In the case of the [Fe 2 ] 5+ complex, [Fe 2 (μ-N�C t Bu 2 ) 2 (N�C t Bu 2 ) 3 ], the shorter Fe−Fe distance [2.547(1) Å] likely contributes to the larger coupling. 82,83 Similarly, the [Fe 2 ] 4+ complex, {(PhCN) 2 (Mes) 2 Fe 2 [μ-N�C(Mes)(Ph)] 2 }, 84 also features larger antiferromagnetic coupling (J = −63.7 cm −1 ) than 1, 2, and 4, likely for the same reason.…”

“…39−54 In this regard, we recently reported the syntheses of two "tied-back" ketimides, 2-adamantyl ketimide and 9fluorenyl ketimide, in an effort to increase this substituent diversity. 33 The latter example, with its conjugated π-system, could be especially good at promoting metal−metal communication. Herein, we evaluate the magnetic properties of a series of [Fe 2 ] 6+ dimers with varying ketimide substituents, including the fluorenyl substituent, in an effort to tune the magnetic interactions between iron centers.…”

“…The ketimide anion, [NCR 2 ] − , is another ligand class that is adept at promoting metal–metal interactions, − as shown by the multimetallic complexes, [Li­(12-crown-4) 2 ]­[M 2 (μ-NC t Bu 2 ) 3 (NC t Bu 2 ) 2 ] (M = Mn, Fe, Co), [Fe 2 (μ-NC t Bu 2 ) 2 (NC t Bu 2 ) 3 ], [Fe 4 (μ-NCPh 2 ) 6 ], and [Fe 4 (μ-Br) 2 (μ-NC t Bu 2 ) 4 ]. − In the case of [Fe 4 (μ-NCPh 2 ) 6 ] and [Fe 4 (μ-Br) 2 (μ-NC t Bu 2 ) 4 ], metal–metal communication occurs via direct exchange, which leads to ferromagnetic coupling between metal centers. , In contrast, for [Fe 2 (μ-NC t Bu 2 ) 2 (NC t Bu 2 ) 3 ], metal–metal communication likely occurs via superexchange, which leads to antiferromagnetic coupling . While the ability of the ketimide ligand to mediate metal–metal communication is reasonably well established, ,, the role that the ketimide substituents play in mediating these interactions is not well understood. In fact, the diversity of known ketimide substituents is relatively low and the most common substituents, by a substantial margin, are Ph and t Bu. − In this regard, we recently reported the syntheses of two “tied-back” ketimides, 2-adamantyl ketimide and 9-fluorenyl ketimide, in an effort to increase this substituent diversity .…”

Metal–Metal-Bonded Fe4 Clusters with Slow Magnetic Relaxation

“…More remarkably, we observe no evidence for throughbarrier relaxation in its AC relaxation measurements. We hypothesized that its good SMM performance was due to 29 which feature varying degrees of magnetic communication as assayed by superconducting quantum interference device (SQUID) magnetometry.…”

“…In fact, the ketimide ligand is known for its ability to promote both metal–metal bonding and magnetic communication, as shown by the isolation of the [Cu­(NC t Bu 2 )] 4 , which is stabilized by cuprophilic interactions, and [Pd 7 (NC t Bu 2 ) 6 ], which features an unusual hexagonal planar Pd 7 core. Also notable are the bimetallic ketimide complexes, [Li­(12-crown-4) 2 ]­[M 2 (NC t Bu 2 ) 5 ] (M = Mn, Fe, Co), [Fe 2 (NC t Bu 2 ) 5 ], and [Li]­[Cr 2 (N=C 10 H 14 ) 7 ], which feature varying degrees of magnetic communication as assayed by superconducting quantum interference device (SQUID) magnetometry.…”