Three-Coordinate and Four-Coordinate Cobalt Hydride Complexes That React with Dinitrogen

Abstract

We report the formation and N 2 reactivity of novel cobalt hydride complexes supported by bulky β-diketiminate ligands (L). Addition of KHBEt 3 to LCoCl gives [LCo(μ-H)] 2 (1) or K 2 [LCoH] 2 (2), depending on the amount of borohydride used. Compound 2 is the first example of a crystallographically characterized hydride complex in which a transition metal is three-coordinate. Both 1 and 2 react with N 2 at room temperature to give dinuclear N 2 complexes with loss of H 2 .Complexes of a transition metal (defined here as a metal with a partially filled d shell) having only three bonds to the metal are interesting because the orbital energies, spin states, and reaction pathways can be different than traditional complexes. 1 Isolating three-coordinate complexes invariably depends on the use of extremely bulky supporting ligands, which protect the metal center through steric effects. However, this strategy might seem to be incompatible with three-coordinate hydride complexes, because H -is the smallest possible ligand. Accordingly, no three-coordinate hydride complexes are known. 2 New kinds of hydride complexes are desired because of the many roles for hydrides in organometallic chemistry and catalysis. 3 Using iron complexes, we recently introduced the use of the very bulky bidentate β-diketiminate ligand L (L = 2,2,6,6-tetramethyl-3,5-bis(2,4-diisopropylphenylimido)hept-4-yl) for enabling the isolation of three-coordinate complexes in which one of the three ligands is small (e.g. halide, CH 3 ). 4 Here, we report that L can be used to stabilize the first crystallographically characterized three-coordinate hydride complex of any transition metal. We also report that this cobalt-hydride complex and another related complex react with N 2 at room temperature and atmospheric pressure, through the bimetallic reductive elimination of H 2 . Figure 1 shows the syntheses and structures of dimeric cobalt(II) hydride (1) and cobalt(I) hydride (2) complexes, which come from adding different amounts of KHBEt 3 to toluene solutions of the three-coordinate cobalt(II) complex LCoCl under Ar. 5 Addition of 1 equiv of KHBEt 3 to LCoCl gives [LCo(μ-H)] 2 (1) in 72% yield. Although we were not able to completely free 1 from impurities (see Supporting Information), several forms of characterization have been possible. The X-ray crystal structure of 1 reveals that it has two hydride ligands (located in the difference Fourier map) that bridge diketiminate-bound cobalt centers. The metal coordination and geometry closely resemble those for the iron(II) complex [LFe(μ-H)] 2 6 and a dimeric β-diketiminate nickel(II) hydride complex recently reported by Limberg. 7 The cobalt atoms in 1 are separated by 2.476 (5) the metal-metal distances in the iron (2.624(2) Å) and nickel (2.3939(6) Å) analogues. The presence of the hydride ligands in 1 was confirmed through the reaction of a toluene solution of 1 with 2 equiv of cyclohexene to give the cobalt(II) cyclohexyl product LCo(C 6 H 11 ), which results from [1,2]-addition of the cobalt h...

Coordination chemistry insights into the role of alkali metal promoters in dinitrogen reduction

“…37 and 38 have slightly shorter N-N bond distances and higher ν NN than their Fe analogs, which is due to lower energy d-orbitals on Co compared to Fe. As with the Fe analogs, the identity of the alkali cation has little effect on the extent N 2 activation [111, 112]. …”

Characterization of the FeH Bond in a Three‐Coordinate Terminal Hydride Complex of Iron(I)

“…Its structure is similar to the previously reported cobalt(I) analogue. [17] The compound is a dimer, and the two crystallographically equivalent halves are held together by potassium cations that associate with aryl groups of the β̃diketiminate ligands with K-centroid distances of 3.02 Å and 2.96 Å. The two diketiminate-iron planes are parallel but not coplanar, and the hydrides are 2.98(4) Å apart.…”

“…The presence of impurities discouraged us from using solid-state magnetic susceptibility, but the solution magnetic moments of 4.4(2) μ B and 3.5(2) μ B in THF- d 8 are similar to the values for related S = 3/2 diketiminate-cobalt(II) complexes. [17, 22, 23] …”

Mild and Homogeneous Cobalt‐Catalyzed Hydrogenation of CC, CO, and CN Bonds

“…While previous efforts to design cobalt catalysts have focused primarily on the synthesis of cobalt(I)-hydride and cobalt(I)-dihydrogen complexes, [16,[33][34][35] the reactivity of cobalt(II)-hydride complexes has been largely unexplored. [36][37][38][39][40][41][42] The high catalytic activities reported herein and potential involvement of cobalt(II) suggest a new frontier in catalyst design. Future work will focus on further understanding the reactivity of these rare 15-electron alkyl and hydride complexes of cobalt(II).…”