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

Understanding the relationship between intracellular motion and macromolecular structure remains a challenge in biology. Macromolecular structures are assembled from numerous molecules, some of which cannot be labeled. Most techniques to study motion require potentially cytotoxic dyes or transfection, which can alter cellular behavior and are susceptible to photobleaching. Here we present a multimodal label-free imaging platform for measuring intracellular structure and macromolecular dynamics in living cells with a sensitivity to macromolecular structure as small as 20 nm and millisecond temporal resolution. We develop and validate a theory for temporal measurements of light interference. In vitro, we study how higher-order chromatin structure and dynamics change during cell differentiation and ultraviolet (UV) light irradiation. Finally, we discover cellular paroxysms, a near-instantaneous burst of macromolecular motion that occurs during UV induced cell death. With nanoscale sensitive, millisecond resolved capabilities, this platform could address critical questions about macromolecular behavior in live cells.

Addition of 4 equiv of Li(N=CtBu2) to VCl3 in THF, followed by addition of 0.5 equiv I2, generates the homoleptic V(IV) ketimide complex, V(N=CtBu2)4 (1), in 42% yield. Similarly, reaction of 4 equiv of Li(N=CtBu2) with NbCl4(THF)2 in THF affords the homoleptic Nb(IV) ketimide complex, Nb(N=CtBu2)4 (2), in 55% yield. Seeking to extend the series to the tantalum congener, a new Ta(IV) starting material, TaCl4(TMEDA) (3), was prepared via reduction of TaCl5 with Et3SiH, followed by addition of TMEDA. Reaction of 3 with 4 equiv of Li(N=CtBu2) in THF results in a isolation of a Ta(V) ketimide complex, Ta(Cl)(N=CtBu2)4 (5), which can be isolated in 32% yield. Reaction of 5 with Tl(OTf) yields Ta(OTf)(N=CtBu2)4 (6) in 44% yield. Subsequent reduction of 6 with Cp*2Co in toluene generates the homoleptic Ta(IV) congener Ta(N=CtBu2)4 (7), although the yields are poor. All three homoleptic Group 5 ketimide complexes exhibit squashed tetrahedral geometries in the solid state, as determined by X-ray crystallography. This geometry leads to a dx2−y21 (2B1 in D2d) ground state, as supported by DFT calculations. EPR spectroscopic analysis of 1 and 2, performed at X- and Q-band frequencies (~9 and 35 GHz, respectively), further supports the 2B1 ground state assignment, while comparison of 1, 2, and 7 with related Group 5 tetra(aryl), tetra(amido) and tetra(alkoxo) complexes shows a higher M-L covalency in the ketimide-metal interaction. In addition, a ligand field analysis of 1 and 2 demonstrates that the ketimide ligand is both a strong π-donor and strong π-acceptor, an unusual combination found in very few organometallic ligands.

The tetrametallic Fe ketimide cluster, [Fe4(NCPh2)6], exhibits a thermally persistent S = 7 ground-state along with single molecule magnet behavior.