A Catalyst with Two‐Coordinate Nickel: Theoretical and Catalytic Studies

Abstract

The bisadduct (cAAC)2NiIICl2 [1; cAAC = cyclic (alkyl)(amino)carbene] was directly synthesized by treating cAAC with NiCl2. Compound 1 was reduced to (cAAC)2Ni0 (2) by using lithium diisopropylamide or KC8. Crystals of 2 were stable under an inert gas for several months and decomposed upon heating above 165 °C. On the basis of the calculated natural bond orbital charge values of the nickel atom in 2, the oxidation state of nickel was determined to be between NiI and Ni0 (+0.34). Theoretical calculations suggested a closed‐shell singlet electronic configuration of 2 with little biradical character. Ab initio multiconfigurational C(R)ASSCF/CASPT2 calculations predicted a closed‐shell singlet electronic configuration (Ni0), whereas excited spin states possessed NiI character with unpaired electrons on neighboring carbon atoms. The catalytic activity of complex 2 was investigated for the homocoupling of various unactivated aryl chlorides/fluorides. The biaryls were obtained in good yields at moderate temperature. Theoretical studies showed that an intermediate containing NiIII was more favored than one with NiIV.

Cyclic (Alkyl)(Amino)Carbene Complexes of Rhodium and Nickel and Their Steric and Electronic Parameters

“…The molecular structure of 8 was determined by single‐crystal X‐ray diffraction (Figure ) and reveals the expected tetrahedral geometry around the metal atom, with bond angles between 107 and 111°. The Ni1−C1 distance to the carbene carbon atom of 1.9629(13) Å in 8 is significantly (0.114 Å) longer than the Ni−C CAAC bond length of 1.849(4) Å observed for the more electron‐rich, two‐coordinate complex [Ni(CAAC methyl ) 2 ] ( 9 ) . It is, however, in good agreement with those of other NHC‐containing nickel tricarbonyl complexes, such as [Ni(CO) 3 (SIPr)] (1.962(4) Å) and [Ni(CO) 3 (SIMes)] (1.960(2) Å) .…”

“…Interestingly, [Ni(CO) 3 (CAAC methyl )] ( 8 ) can also be synthesized by the reaction of [Ni(CAAC methyl ) 2 ] ( 9 ) with an excess of CO with concomitant carbonylation of the released carbene ligand. Treatment of the mono‐CAAC‐ligated complex 8 with additional carbene 1 , however, resulted in the substitution of two carbonyl ligands and the formation of [Ni(CO)(CAAC methyl ) 2 ] ( 10 ), the formal product of coordination of carbon monoxide to bis‐carbene complex 9 (Scheme ).…”

Enhanced π‐Back‐Donation as a Way to Higher Coordination Numbers in d10 [M(NHC)n] Complexes: A DFT Study

“…However, Wolters and Bickelhaupt found that π‐back‐donation can stabilize smaller angles in d 10 [ML 2 ] complexes . We note that there is experimental evidence for this statement, as the complex [Ni(CAAC) 2 ] (CAAC=cyclic alkyl(amino)carbene), bearing a much stronger π* acceptor than NHC ligands, exhibits a bent structure in contrast to its linear NHC analogues (Table S3 in the Supporting Information) . The H 2 Im ligand in anionic Group 9 complexes acts as a strong π* acceptor, and thus allows the formation of bent d 10 [M(H 2 Im) 2 ] complexes and distorted tetrahedral d 10 [M(H 2 Im) 4 ] structures (see Table S1 in the Supporting Information).…”

Synthesis, Characterization, and Theoretical Investigation of Two‐Coordinate Palladium(0) and Platinum(0) Complexes Utilizing π‐Accepting Carbenes

“…There are strong π back‐donations from metal to carbene carbon atoms which might be a possible reason (M=Fe, Co, Ni, Pd, Pt). Note, that there is no radical electron on the carbene carbon atom for M=Fe, Co, Ni,12b–d Pd, Pt. The orientation of two cAACs in (cAAC) 2 M has huge effect on the spin ground state and fine electronic structure.…”