Research Article pubs.acs.org/acscatalysis
Catalytic Dinuclear Nickel Spin Crossover Mechanism and Selectivity for Alkyne Cyclotrimerization Doo-Hyun Kwon,† Matthew Proctor,† Sergio Mendoza,† Christopher Uyeda,‡ and Daniel H. Ess*,† †
Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602, United States Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
‡
S Supporting Information *
ABSTRACT: Homodinuclear transition-metal catalysts with a direct metal−metal bond have the potential to induce novel reaction mechanisms and selectivity compared with mononuclear catalysts. The dinuclear (i‑PrNDI)Ni2(C6H6) (NDI = naphthyridine-diimine) complex catalyzes selective cyclotrimerization of monosubstituted alkynes, whereas mononuclear Ni catalysts generally give cyclotetramerization of alkynes. Density functional theory calculations reveal that the homodinuclear Ni−Ni catalyst induces a spin crossover mechanism that involves metallacyclopentadiene and nonclassical bridging metallacycloheptatriene intermediates. The cis configuration of the nonclassical bridging metallacycloheptatriene Ni−vinyl bonds results in alkyne cyclotrimerization by fast reductive elimination. This dinuclear mechanism differs from previously reported mononuclear Ni mechanisms and provides an explanation for cyclotrimerization versus cyclotetramerization selectivity and arene regioselectivity. KEYWORDS: dinuclear, catalysis, density functional theory, nickel, alkyne cyclotrimerization
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INTRODUCTION AND MOTIVATION FOR COMPUTATIONAL STUDY Homodinuclear metal−ligand catalysts with a single covalent1 or dative interaction offer the potential to induce novel intermediates,2 reactivity, and selectivity that might not be available to mononuclear catalysts.3−5 Dinuclear effects can result from unique oxidation states, spin states, and ligand architecture.6 A prominent example is the popular homodinuclear dirhodium catalysts that induce unique reactivity and selectivity for alkene hydroformylation,7 cyclopropanation,8 aziridination,9 and C−H functionalization reactions.10 Our group uses density functional theory (DFT) calculations to discover the influence of dinuclear catalysts on mechanisms, reactivity, and selectivity.11 We are especially interested in homodinuclear catalysts based on first-row transition metals. Uyeda recently reported the homodinuclear Ni−Ni naphthyridinediimine (NDI) catalyst 1 [(i‑PrNDI)Ni2(C6H6)] that induces cyclotrimerization of several monosubstituted alkynes (Scheme 1a).12,13 For example, phenylacetylene undergoes selective cyclotrimerization to yield only terphenyl aromatics. The formation of 1,2,4substituted benzene isomer (4′-phenyl-1,1′:2′,1″-terphenyl) is © XXXX American Chemical Society
preferable to that of the 1,3,5-substituted isomer (5′-phenyl1,1′:3′,1″-terphenyl). This dinuclear catalyst contrasts many mononuclear Ni catalysts that generally give cyclooctatetraenes as the major product, especially with N-donor ligands.12 However, addition of phosphine or NHC ligands to some mononuclear catalysts can result in cyclotrimerization.14 The different selectivity (cyclotrimerization vs cyclotetramerization) for dinuclear catalyst 1 compared with mononuclear Ni catalysts suggests the possibility of a unique reaction mechanism and catalytic cycle influenced by the Ni−Ni interaction and NDI ligand scaffold. Scheme 1b shows the previously proposed catalytic cycle.12 This mechanism involves alkyne coordination and C−C bond coupling at a single Ni metal center with a metallacyclopentadiene intermediate. Evidence of the metallacyclopentadiene is based on stoichiometric reactions between trialkylsilyl alkynes and complex 1 that result in isolation and characterization of the corresponding 2,4-disubstituted Received: March 25, 2017 Revised: May 29, 2017 Published: June 1, 2017 4796
DOI: 10.1021/acscatal.7b00978 ACS Catal. 2017, 7, 4796−4804
Research Article
ACS Catalysis
Scheme 1. (a) Dinuclear Nickel-Catalyzed Alkyne Cyclotrimerization and (b) Previously Proposed Dinuclear Catalysis Mechanism12
Figure 1. M06L/def2-TZVP//M06L/6-31G(d,p)[LANL2DZ] orbital energies for (a) 1 and (b) (i‑PrNDI)Ni2(Et2SiH2) (values in electronvolts).
metallacyclopentadiene. It was proposed that trisubstituted arenes result from outer-sphere regioselective [4 + 2] cycloaddition between the metallacyclopentadiene and alkyne. While this [4 + 2] cycloaddition mechanism is reasonable, there are several viable alternative mechanisms to evaluate, especially considering that likely intermediates may access multiple electronic spin states. This proposal is based on the isolated (i‑PrNDI)Ni2(Et2SiH2) complex that has a singlet−triplet energy gap of 20 kcal/mol lower in energy than quintet structures. This indicates that catalysis begins with intersystem crossing to the triplet-spin-state energy surface (Scheme 2b). M06 and ωB97X-D functionals also show this spin crossover on the energy surface. Formation of Metallacyclopentadiene. From the diacetylene complex 3, there are several closed-shell and open-shell mechanisms for C−C bond formation. Stepwise zwitterionic and diradical pathways involving formation of a C−C σ bond and only one Ni−C bond are significantly unfavorable (see the Supporting Information). The lowest-energy pathway on the triplet-spin surface involves a one-step oxidative coupling transition state (TS1) to form the metallacyclopentadiene 4 (Figure 2 and Scheme 3). The ΔG⧧ for TS1 is 15.4 kcal/mol and exergonic by 32.3 kcal/mol relative to 3. While TS1 is somewhat similar to related metallacyclopentadiene-forming transition states that concertedly form a C−C σ bond and two metal−vinyl carbon bonds,19,20 in this case there is significant participation by the second Ni metal center. The forming C−C partial bond length is 2.18 Å, and the two forming Ni−C vinyl bond lengths are 2.05 and 2.09 Å. In the resulting metallacyclopentadiene 4, one of the Ni−vinyl bonds unsymmetrically bridges both Ni metal centers. In TS1, C2 and C3 have short interaction distances with the second Ni metal center of 1.92 and 2.00 Å, respectively, and these distances increase substantially in 4 because of the bridging vinyl group.
energy. No spin-polarized unrestricted singlet M06L wave function has an energy lower than that of the restricted wave function. The M06L orbitals of (i‑PrNDI)Ni2(Et2SiH2) are qualitatively similar to a restricted singlet spin ground state (Figure 1b). However, the HOMO−LUMO gap decreases from 0.8 eV for 1 to
