![]() ![]() In turn, the three-spin-model requires antiferromagnetic coupling of the Cr II ions with the diazenido ligand that is still quite strong ( J AF≤−450 cm −1). The temperature independence of the magnetic moment even at high temperatures indicates a very strong magnetic exchange interaction that is rather unlikely for purely covalent bonding with a diamagnetic N 2 bridging ligand. b) Covalent bonding model.Ĭharacterization of 5 by SQUID magnetometry (Figure 3, top left) shows nearly constant χ M T value of 6.06 cm 3 ⋅ K ⋅ mol −1 above T=12 K, corresponding to the spin-only value of 6.00 cm 3 ⋅ K ⋅ mol -1 for a S T=3 spin state. ![]() Simplified descriptions for bonding within the Cr 2N 2-core of complex 5: a) Ionic three-spin model with antiferromagnetically coupled hs-Cr II and N 2 2− ions. 23- 27 In fact, full splitting of linearly bridging N 2 was attributed to an electronic configuration with 10 electrons in the M−N−N−M π-MO manifold, 2b as is the case for 5. The five-coordinate chromium(III) complex [CrCl 2 core (Figure 4b), as frequently applied to rationalize the degree of N 2 activation and N 2-centered reactivity, such as dissociation into terminal nitrides. Structural, spectroscopic, magnetic, and computational characterization shows distinct differences in the degree of covalent bonding with the bridging ligands. Stepwise replacement of N 2 by CO enabled the characterization of unprecedented complexes with linearly bridging N 2 and CO, respectively, in otherwise identical coordination environments. We here present the synthesis of a dinitrogen bridged dichromium(II) complex. CO bonding in such linearly bridged species has not been systematically compared as a basis to address strategies for CO activation.Įxamples for transition metal complexes with linearly bridging isocarbonyl ligands. 15 This was attributed to σ-bonding and π-backbonding interactions of the Cr(CO) 5 fragments with both ends of the bridge, yet only about one fourth in magnitude at the O-terminus. predicted that the hypothetical C 4v symmetric, isocarbonyl bridged complex is stable vs. 14 This common binding motif for N 2 can lead to strong activation up to full N−N dissociation. ![]() 13 In some rare cases, transition metal isocarbonyl complexes with distinctly linearly bridging μ 2-CO-κ C:κ O ligands were reported (Figure 1). 12 CO is the stronger ligand that readily displaces N 2, and a wide range of CO binding modes are known. 11 However, while N−N dissociation via multinuclear N 2-bridged complexes is well established for a wide variety of transition metals, the full cleavage of CO ligands remains rare, despite relevance for Fischer-Tropsch model chemistry. ![]() Chemical transformations of coordinated carbon monoxide (e.g., the Hieber base reaction) 10 were known long before the first N 2 complex had been reported. The isoelectronic analogy of N 2 and CO provides another intriguing comparison. 7, 8 Multinuclear N 2 activation might be instrumental to overcome these challenges, 9 but further studies are required to better understand N 2 bonding to multiple chromium ions. 5, 6 Compared with isostructural 4/5d analogues, reduced backbonding and overstabilization of high spin states were pointed out to hamper strong binding of N 2. 4 In contrast, Cr-mediated N 2 functionalization is significantly less well developed. 2 This includes both the first and the currently most active homogeneous catalysts (Mo), 3 as well the as the initial examples for thermally (Mo) and photochemically (W) driven splitting of N 2 into well-defined nitrido complexes. 1 Group 6 model complexes were instrumental for detailed mechanistic examinations of N 2 activation, all the way to full cleavage of the extraordinarily strong N−N triple bond. The current interest in electrocatalytic nitrogen fixation has fuelled a renaissance of N 2 coordination chemistry. ![]()
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