Dinickel compounds ligand

The preparation of pentadienylsodium was being studied at about the same time as ferrocene was discovered. It was not until 1968, however, that the first binary pentadienyl complex of a transition element, bis(pentadienyl)-chromium, was obtained from PINa and CrCl2 (121). This compound forms green, air-sensitive crystals, and like chromocene it has two unpaired electrons (Ht = 2.74 BM). This discovery was shortly followed by that of the curious complex, bis(pentadienyl)dinickel (12) which was prepared from NiCl2 and triethylaluminium in 1,4-pentadiene (122). The pentadienyl ligands in 12... 

Complex 13, a bulky guanidinato analog of 11, was produced from arene ligand elimination reaction of an dinickel(I) arene compound 12 in presence of alkane solvent, with its isomeric form 14 as co-product (Scheme 10.5) [10]. Compound 14 can be fully converted into 13 in hexane solution in 1 month at room temperature, thus 13 is the thermodynamic product while 14 is the kinetic... 

A dinickel(I) compound 17 was made from the reaction between metallacyclic Ni(n) carboxylate ( nickelalactone ) and bis(diphenylphosphino)methane (dppm) (Scheme 10.7) [11]. The Ni(I)-Ni(I) bond length in 17 is 2.563(1) A (Entry 4, Table 10.2), and features three different bridging ligands (dppm, carboxylate, diphenylphosphido). The formation of 17 was proposed to proceed via the mechanism depicted in Scheme 10.8, and is remarkable because it acts as a model for the key step in the formation of acrylic acid from COj and ethylene. 

Compounds with Aryl and Aryne Ligands C-H and C-C bond activation/formation has become the most exciting field of research in recent decades. The dinickel(I) aryl and aryne compounds were usually studied as key intermediates in Ni(0) mediated C-H and C-C bond activation/formation on phenyl or other aromatic rings. 

Other bis(phosphane) ligands have found use in supporting this type of dinickel(I) compound. Starting from a known dinickel(I) dihydride [Ni2(dippe)2( i-H)2] [32], the dinickel(I) hydride thiolate compound [Ni2(dippe)2( i-H)( i-S-2-biphenyl)] (52) was separated as an intermediate of hydrodesulfurization of biphenyl-2-thiol [33], and was further converted into the dinickel(I) sulfido compound [Ni2(dippe)2( i-S)] 53 (Scheme 10.22 ), with concomitant formation of biphenyl [34]. 

An elusive example of anionic NHC ligand in dinickel(I) chemistry can be found in Scheme 10.25 [25]. Compound 58 features both neutral and anionic NHC ligands, and the two nickel centers are bridged by two [i-N,C coordinated NHCs, with Ni(I)-Ni(I) bond length of 2.4354(9) A (Entry 26, Table 10.2). Complex 58 was produced from the reaction between [Nilcodlj] and an excess amount of NHC in greaseless apparatus for a long period (14 days) with a relatively low but reproducible yield ( 30%). Obviously, a N-C bond here is cleaved. It is noteworthy that with the presence of silicon grease, a Si=0 bond-activated product was obtained. 

Multidentate N,P-donor ligands were also used to support Ni(I)-Ni(I) bonds. A dinickel(I) compound 59 bearing a Dewar-benzene-type Ni2P2N2 core was reported [26]. In this example, a... 

Scheme 10.27 Pyrrole-based PNP-pincer ligand precursor, its dinickel(l) compound 60, and a related dinickel(l) diradical compound 61.

Scheme 10.28 Synthesis of an amidinate ligand-supported paddlewheel dinickel(ll) compound 62.

Related dinickel(II) p-N bridging compounds with NHC ligands were reported by Hillhouse et al. in 2009 (Scheme 10.31) [40]. [Ni(I)(p-Cl)(l Pr)]2 underwent one-electron oxidation by MesNj to afford a dinickel(II) bridging imide [ Ni(ll)(l Pr)(Cl) 2(p-NMes)] 66, in which one of the chlorides can be abstracted by NaBAr, to produce the dinickel(ll) ion pair compound 67. The Ni(ll)-Ni(ll) bond lengths in 66 and 67 are 2.5767(15) and 2.2911(8) A, respectively (Entries 5 and 6, Table 10.3). Complex 66 can also undergo one-electron reduction of the [(Ni(ll)Cl 2(p-NMes)] core, to afford a mixed-valent Ni(I)-Ni(II) compound 68, which upon treating with a one-electron oxidant gives 67 as the product. It is noteworthy that 67 and 68 bear similar core structures, but the Ni-Ni bond... 

Two dinickel(ll) paddlewheel compounds with amidinate (77) or guanidinate (78) ligands were synthesized and their one-electron oxidation was studied (Scheme 10.36) [45]. In the Ni2 compounds 79 and 80, the Ni-Ni distance was found to be 0.lA shorter than the corresponding dinickel(ll) precursor (2.476(1) A [77] vs 2.3703(4) A [79] 2.4280(5) A [78] vs 2.3298(6)A [80]) (Entries 4 and 5, Table 10.5). Based on this observation, it was proposed that upon oxidation one electron was removed from a metal-based a orbital to give an overall Ni-Ni bond order of 1/2 in the Nij species. Furthermore, a single-point calculation with no simplified solid-state structure of 79 suggested that the unpaired electron in the Ni2 species is in a metal-based a orbital, and this was also verified by a solution EPR spectrum. 

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