Published: October 14, 2011 r2011 American Chemical Society 18058 dx.doi.org/10.1021/ja207638h | J. Am. Chem. Soc. 2011, 133, 18058–18061 COMMUNICATION pubs.acs.org/JACS CarbonÀCarbon Bond Formation from Azaallyl and Imine Couplings about MetalÀMetal Bonds Elliott B. Hulley, Peter T. Wolczanski,* and Emil B. Lobkovsky Department of Chemistry & Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, United States b S Supporting Information ABSTRACT: Typical CÀC bond-forming processes fea- ture oxidative addition, insertion, and reductive elimination reactions. An alternative strategy toward CÀC bond forma- tion involves the generation of transient radicals that can couple at or around one or more metal centers. Generation of transient azaallyl ligands that reductively couple at CH positions possessing radical character is described. Two CÀC bonds form, and the redox non-innocence of the resulting pyridine-imines may be critical to formation of a third CÀC bond via dinuclear di-imine oxidative coupl- ing. Unique metalÀmetal bonds are a consequence of the chelation. C arbonÀcarbon bond-making is one of the most important transformations mediated by transition metals. Processes typically utilize oxidative addition, insertion, and reductive elimi- nation sequences that directly involve metalÀcarbon bonds. In certain circumstances, ligands that possess radical character, perhaps due to a redox exchange with the metal, may be induced to form new CÀC bonds. Recent investigations into azaallyl-based ligands, i.e., smif = {(2-py)CH} 2 N À , 1À4 suggested that CÀC bonds might be constructed from coupling of the -HCNCH- backbone, whose nonbonding highest occupied molecular orbital (HOMO) pos- sesses diradical character. As shown in Scheme 1, treatment of metal diamides M{N- (SiMe 3 ) 2 } 2 (THF) n (M = Cr (n = 2), 5 Co (n = 1)) 6 and 1 / 2 [Ni(NPh 2 ) 2 ] 2 7 with the bis-pyridine-imine chelate precursor Me 2 C(CHdNCH 2 py) 2 afforded diamagnetic {Me 2 C(CHNCH- py) 2 M} 2 (1 -M; M = Cr, Co, Ni) dimers that feature three new CÀC bonds. Coupling from positions 1, 2, and 3 of one chelate to related 2, 3, and 4 positions of the adjacent ligand render the molecules C 2 symmetric. In an NMR tube experiment, 1-Ni was heated to test the stability of the bicyclic ring scaffold, and demetalation, i.e., loss of “NiNCpy”, occurred to provide a diamagnetic bis-pyridine-imine complex 2-Ni, with the ring structure intact. Complete demetala- tion was envisaged as a combination of amide protonation and Ni oxidation events; hence, dimethylglyoxime (dmgH 2 ) was em- ployed. Exposure of 1-Ni to 4 equiv of dmgH 2 afforded Ni- (dmgH) 2 , a small amount of combustible gas, presumably H 2 (11%), and free ligand and its mono- and bis-reduction products in a 1:1:0.4 ratio. The CÀC bonds appear to be produced irreversibly, and the bicyclic skeleton can be harvested. The metalÀmetal bonding in each dinuclear complex is unique. In 1-Cr, the imine d(CÀN) av = 1.339(3) Å and C im ÀC py (av) distance of 1.387(3) Å are consistent with single electron occupancy of the pyridine-imine π* orbital; 8 hence, the oxidation state of each Cr is +2, and the potential for quadruple CrÀCr bonding exists. As Figure 1 illustrates, one N 4 Cr plane is rotated by 52.4(2)° relative to the other, a staggered orientation that mitigates δ-bonding. 9,10 From the temperature dependence of five chemical shifts, 11 a ΔE(S-T) of 630(70) cm À1 shows that the d(CrÀCr) of 2.5515(3) Å is best construed as representing a triple bond. Broken-symmetry (BS) calculations 12 portray anti- ferromagnetically (AF) coupled pyridine-imine ligands and AF- coupled high-spin (HS) d 4 chromous centers. While the former is reasonable, inspection of the magnetic orbitals of the BS solution suggests that the CrÀCr interaction is best considered (dσ) 2 (dπ) 4 , i.e., a long, weak, but conventional triple bond. Imine (1.332(3) Å) and C im ÀC py (1.395(4) Å) distances of 1-Ni implicate a reduced pyridine-imine framework, 8 consistent with Ni(II) centers. While configuration interactions derived from nd/(n + 1)p mixing are responsible for weak metalÀmetal bonds in d 8 Àd 8 systems in the second and third rows of the transition elements, 13À15 the ΔE(3d/4p) is considered too great for the first row, 16 yet d(NiÀNi) = 2.8164(6) Å, clearly a significant bond. The N 4 Ni planes are rotated relative to one another by 58.6°, permitting d z 2 orbitals of each Ni(II) to slip one another, as shown by the 6.8° cant of the NiÀNi vector relative to the central CÀC bond (Figure 2). Its framework is thereby compressed similarly to 1-Cr, with an angle between the N 4 M planes of 12.6° for 1-Ni vs 13.8° for 1-Cr. Figure 3 is a truncated orbital diagram of 1-Ni obtained from a BS[1,1] calculation. 12 Due to σ/π mixing, the antibonding character of the (d z 2Àd z 2) orbital (i.e., Ni(2)), is sufficiently nullified that a NiÀNi bond exists; its primary component is the (d z 2+d z 2) orbital (Ni(5)). A set of py π b orbitals (red) is intermingled with Ni-based dπ b (Ni(6), Ni(7)), dπ* (Ni(3), Ni(4)), dδ b (Ni(9)), and dδ* (Ni(8)) orbitals, and an amide- based set (green) is close to the HOMO of the system. The pair of electrons in the pyridine-imine π* orbitals is depicted by BS[1,1] as an AF-coupled pair of α and β spins (blue); a closed- shell singlet solution is calculated to be 3.2 kcal/mol higher in energy. Preparation of 1-Co was predicated on the idea that a d 7 Àd 7 configuration akin to the d 8 Àd 8 electronic structure of 1-Ni would lack significant MÀM σ* character, but its d(CoÀCo) of 2.9111(7) Å belies this logic, although its Mayer bond order 17 of 0.1221 is slightly greater than that of 1-Ni (0.1018; cf. 0.4095 for 1-Cr). Its imine (1.316(3) Å) and C im ÀC py (1.408(3) Å) Received: August 12, 2011