Catalytic Hydrogen/Deuterium Exchange of Unactivated Carbon–Hydrogen Bonds by a Pentanuclear Electron‐Deficient Nickel Hydride Cluster

Abstract

The activation and catalytic functionalization of CÀH bonds has gained importance as both a green and economical synthetic approach. [1] A modern goal is the use of less expensive first-row metals, such as nickel, in lieu of the expensive second-and third-row metals currently used for most catalytic C À H functionalization processes. Although there are examples of catalytic CÀH bond functionalization using nickel, which include unprecedented reactions such as CÀH bond stannylation, [2] these transformations are typically limited to activated substrates like fluorinated aromatics. [3] An alternative approach is to use cooperative bond activation [4] by dinuclear or polynuclear complexes in unusual oxidation states; for example, dinuclear Ni I complexes have been reported to mediate the rearrangement of activated CÀ H bonds. [5] Polynuclear clusters of the first-row transition metals may have an advantage over their heavier congeners in catalytic reactivity, because of the smaller HOMO-LUMO gap and weaker metal-metal bonds that render these complexes more reactive (HOMO/LUMO = highest occupied/ lowest unoccupied molecular orbital). [6] The reaction of [(iPr 3 P) 2 Ni] 2 (m-N 2 ) [7] and dihydrogen with the loss of N 2 according to Scheme 1 provides the pentanuclear cluster (iPr 3 P)Ni(m 3 -H) 2 [(iPr 3 P)Ni(m 2 -H)] 4 (1). Cooling a pentane solution of the product to À34 8C led to the precipitation of dark-brown rhombic crystals of 1 in 26 % yield; the low yield reflects the high solubility of the product. The infrared spectrum confirmed the presence of bridging hydride ligands and the absence of terminal hydrides. A broad band attributed to n(Ni À H) is found at 1235 cm Àl . The solidstate structure of 1 was determined by X-ray diffraction, and two views are given in Figure 1. [8] Complex 1 consists of a distorted square pyramid of Ni atoms, with the base atoms exhibiting marked differences in the Ni-Ni distances (2.3891(4)-2.6310(4) ); the bond lengths and angles associated with these distortions are shown in Figure 2. The electron densities associated with the six hydride ligands were located in a difference map, and the hydride positions were refined. Each basal Ni-Ni edge is spanned by a m 2 -hydride ligand. Hydrogen atoms H(4) and Scheme 1. Synthesis of pentanuclear complex 1. Figure 1. Solid-state molecular structure of [(iPr 3 P)Ni] 5 H 6 (1) as determined by X-ray crystallography, shown with 50 % probability ellipsoids. Hydrogen atoms not attached to Ni are omitted for clarity. Selected bond distances [] and angles [8]:

Syntheses and Properties of Triruthenium Polyhydrido Complexes Composed of 1,2,4-tri-tert-butylcyclopentadienyl and p-Cymene Ruthenium Units

“…The conversion reached ca. 85% after 1 h, and the rate of H/D scrambling with C 6 D 6 was comparable to that for the pentanuclear nickel complex [{Ni­(P i Pr 3 )} 5 (μ-H) 4 (μ 3 -H) 2 ] reported by Johnson and co-workers …”

“…85% after 1 h, and the rate of H/D scrambling with C 6 D 6 was comparable to that for the pentanuclear nickel complex [{Ni(P i Pr 3 )} 5 (μ-H) 4 (μ 3 -H) 2 ] reported by Johnson and co-workers. 49 Treatment of 5 with 1 atm of H 2 at 25 °C resulted in the formation of pentahydrido complex 9 (eq 7). The hydrides of 9 were observed to be equivalent, similarly to those in 5, and resonated at δ −13.71 ppm.…”

Photo‐Initiated Cobalt‐Catalyzed Radical Olefin Hydrogenation

“…Interestingly, the 1 H NMR spectrum of the deuteride isotopologue 1‐D exhibit a noticable isotopic shift of up to 1 ppm for one set of methyne protons in the pincer backbone. Pronounced NMR isotope effects on remote nuclei, sometimes called paramagnetic isotope effect on chemical shift (PIECS), [22f] were previously reported for several paramagnetic molecules, [22] including some multinuclear, hydride bridged complexes [22f–i] . In that case, PIECS was attributed to the slightly shorter M−D bond lengths, resulting in increased exchange coupling of the open‐shell metal ions.…”

Binuclear complexes of Ni(i) from 4-terphenyldithiophenol

“…Whereas the 1 H and 13 C NMR chemical shifts determined at low temperatures account for the respective structural features observed in the solid state structures of 3-5, the continuous shift of δ H to lower fields at higher temperatures, especially δ H (8/9-H) being progressively greater than 11 ppm, suggests thermal population of a paramagnetic biradical (S = 1) state 2,9 which would add contact and dipolar shift contributions to the observed chemical shift. 28,59,60 An independent parameter to probe for variable net electron spin moments is the trend of the temperature dependence of the 1 H spin-lattice relaxation time, T 1 , which is expected to be positive for protons in diamagnetic environments. Values of T 1 for 8/9-H of 3-5 were found to decrease markedly with increasing temperatures, e.g.…”