Dinickel I Compounds

Reactivity studies of the dinickel(I) compound 13 were carried out (Scheme 10.6) [10]. The reaction between 13 and CO gas produces the dinickel(I) carbonyl compound 15, with the Ni(I)-Ni(I) bond length elongated from 2.2908(11) in 13 to 2.437(1) A in 15. When 13 was treated with trimethylsilyl azide (MejSiNj), a dinickel(II) azide 16 was obtained. The two Ni(II) centers... 

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. 

After the pioneering work of Zagarian and coworkers, a handful of related syK-q q -coordinated arene-bridging dinickel(I) compounds were reported and are summarized in Scheme 10.10, and bond lengths of these Ni(I) - Ni(I) bond can be found both in Scheme 10.10 and Table 10.2 (Entries 6 -10)... 

Scheme 10.11 Synthesis and reactivity of an arene-bridged dinickel(i) compounds 27 and 28.

A mixed-valent dinickel monohydride 49 was found to react with bases ( BuOK, LiN(SiMe3)j) to produce dinickel(I) compounds 50 and 51, both of which feature a Ni(I)-Ni(I) bond (Scheme 10.21) [22]. Complex 50 features a triangular Ni P core with Ni(I)-Ni(I) bond length of 2.515(1) A (Entry 21, Table 10.2) while 51 features a fused Ni PC and NiPC bicyclic core, with a significantly shorter Ni(I)-Ni(I) bond (2.408(2) A) (Entry 22, Table 10.2) in comparison with that in 50. These reactions involve sequences of deprotonation, C-H/C-P bond activation, and C-H bond formation, although the mechanism is still ambiguous. 

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]. 

Another example of this type of neutrtil NHC-supported dinickel(I) compound is dinickel(I) p-iodide p-nitrosyl compound 57, in which the two Ni(I) centers were bridged by an iodide and a nitrosyl, and capped by the NHCs. Here, the Ni(I)-Ni(I) bond length is 2.314(1)A (Entry 25, Table 10.2). Complex 57 was made from the reduction of the Ni(II) nitrosyl iodide monomer [Ni(II)( PPr)(NO)(l)] by sodium amalgam (Scheme 10.24) [24]. 

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... 

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... 

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. 

In 2006, it was found that [NiBr2(PEt3)j can act as catalyst for the transformation of a nickel aryne compound into the dinickel(I) biarylyl 44 in the presence of sodium amalgam (Scheme 10.17) [20]. A detailed study revealed that [NiBr2(PEt3)j was first reduced into [Ni(0)(PEt3)2], which subsequently formed a dinickel aryne complex and acted as the true catalyst. After oxidative... 

Ni(0) catalyzed ethyne cyclization to afford cyclooctatetraene (COT) and related biphenylene compounds, which involved a series of C-C bond couplings, were studied in the last decade, and dinickel(I) species were postulated as possible catalytic intermediates [31]. However, the mechanistic... 

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. 

Dithiolate compounds are similar to the carboxylates, with sulfur atoms taking the place of the oxygen atoms in the polar head. Each of the tetrakis(alkyldithiolato)dinickel(II) complexes ((101) R = C H2 +i, n = 4- ) shows a monotropic lamellar mesophase between 80and 90°C, which has been described as discotic lamellar (Dl) phase (see Figure 22). In the case where R is a branched chain, an enantiotropic Colh and a monotropic Dl mesophases have been found for tetrakis( 1 -ethylpentyldithiolato)dinickel(II) complex. ... 

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... 

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