60th Annual Report on Research 2015

We have achieved significant progress in both research directions outlined in the original proposal.  

1)      Activation of challenging bonds on Al(I) center

We have previously shown that the Al(I) compound NacNacAl (1, NacNac = [ArNC(Me)CHC(Me)NAr]⁻ and Ar = 2,6-Prⁱ₂C₆H₃) can easily add H-X (X = H, Si, B, Al, C, N, P, O) s bonds, thus mimicking the key bond activation step in transition metal catalysis.[1] We have now extended these oxidative additions to the activation of particularly challenging bonds, such as C-O and C-F. Thus, hexafluorobenzene reacts with 1 occurs already at room temperature and affords NacNacAlF(C₆F₅) (2) in only 20 minutes. Pentafluorobenzene also adds  to give regiospecifically the aryl fluoride NacNacAlF(3-C₆F₄) (3), with no evidence of compromising C-H bond addition.  The 1,2,3,4 isomer of tetrafluorobenzene requires a much longer reaction time (16 h) but only the product of insertion into the 2-position is formed. This regioselectivity is controlled by the increasing number of ortho C-F groups in the product which strengthens the Al-C bond formed. In contrast, addition of 1,2,4,5-C₆H₂F₄ occurs only at elevated temperature (60° C) to give exclusively the product of C-F activation, NacNacAlF(5-C₆H₂F₃-1,2,4) (5). Further decrease of the number of fluorine atoms in the starting arene increases the reaction temperature significantly (Scheme 1). Prolonged heating of 1 with fluorobenzene (days at 100 °C) resulted in decomposition without formation of any aluminum fluoride species. The products of C-F bond oxidative addition were completely characterized by NMR spectroscopy and by X-ray diffraction of 5 (Figure 1).

Scheme 1. Oxidative addition of C-F bond in polyfluorinated aromatics to 1

Moreover, we found that these addition reactions can be extended to the activation of the more challenging C(sp³)-F bond. Thus, 1-fluorohexane adds to compound 1 readily at room temperature affording the fluoride derivative NacNacAlF(C₆H₁₃) (8). Oxidative addition of the secondary C-F bond of fluorocyclohexane also takes place under very mild conditions to give the aluminum fluoride NacNacAlF(C₆H₁₁) (9). Remarkably, NMR monitoring revealed that these additions take place even at temperatures down to −60 °C! The formations of compounds 8 and 9 represent the first examples of alkyl-fluorine bond oxidative addition to any metal center. The structures of these products were studied by NMR spectroscopy and X-ray diffraction analyses (Figure 2).

Figure 1. Molecular structure of 5.

Figure 2. Molecular structure of 9.

The C-O bond (358 kJ/mol) can be similarly activated. Initial experiments with diethyl ether showed no reaction. But the less bulky substrate tetrahydrofuran adds to 1 at room temperature to give the cyclic alkoxide derivative NacNacAl(-OCH₂CH₂CH₂CH₂-) (10, Figure 3).

Figure 3. Molecular structure of 10.

Kinetic studies on the addition of 1,2,3,4-tetrafluorobenzene to 1 established the first order dependence in the aluminum compound  and the first order dependence in the reagent as well. The overall reaction is therefore second order, consistent with a simple one-step C-F oxidative addition process. The large negative entropy of activation, ΔS^≠ = −108.9(3) J/K^.mol, points to a highly ordered transition state, supporting the concerted oxidative addition mechanism.

2)      Preparation of mononuclear Zn(I) and the first Zn(0) compounds.

We have previously reported a Ge(0) compound stabilized by the non-innocent diiminopyridine (dimpyr) ligand.[2] We have now extended this chemistry to the Zn(I) and Zn(0) dimpyr complexes. The reaction of compound dimpyrZnCl₂ (11) with an equivalent of KC₈ in benzene afforded the neutral complex dimpyrZnCl (12) in the formal Zn(I) oxidation state (Scheme 2). Similar reduction of 2 in the presence of DMAP (4-dimethylaminopyridine) results in the Zn(I) derivative dimpyrZnCl(DMPA) (4). EPR spectra 3 and 4 (Figure 4) are consistent with the presence of a single electron delocalized over the dimpyr ligand.

Scheme 2. Preparation of compounds 12, 13, and 14. Ar=2,6-ⁱPr₂C₆H₃, DMAP = 4-dimethylaminopyridine

Figure 4. EPR spectrum of compound 12

Further reduction of compound 13 in the presence of another equivalent of DMAP results in the formal Zn(0) species dimpyrZn(DMAP)₂ (14) (Scheme 2). The structures of complexes 12, 13 and 14 were established by X-ray diffraction analysis. The molecular structure of complex 12 (Figure 5) shows a nearly planar geometry around the zinc center (the mean deviation from the Zn1-N1-N2-N3-Cl1 least square plane is 0.146 Å). Such a flat coordination environment is very unusual for a four-coordinate zinc center and finds the only precedents in zinc porphyrines.

Figure 5. Molecular structure of 12.

The coordination of DMAP to the zinc(I) center results in a five-coordinate, square base pyramidal geometry of 13 featuring elongated bonds around zinc (Figure 2, right). Thus, the Zn(1)-N1 distance to the central pyridine ring elongates from 1.9400(16) Å in 12  to 1.9967(18) in 13 and the Zn(1)-Cl(1) distance elongates from 2.1566(6) to 2.2255(7) Å, respectively. The Zn-imine distances at 2.2459(19) and 2.3262(19) Å are also noticeably longer than the corresponding bonds in 12. The C=N(imine) distances, however, remain virtually undisturbed (1.294(3) and 1.304(3) Å).

Figure 6. Molecular structure of 13.

X-ray structure determination from a single crystal of 14 revealed the presence of two DMAP ligands, one trans to the central pyridine and the other approximately orthogonal to the dimpyr plane (Figure 7). In contrast to the parent compounds 12 and 13, the zinc atom occupies a very asymmetric position, with one Zn-imine distance being very short at 2.0265(14) Å and the other one being very long (2.585 (1) Å), so that the zinc atom adopts an unprecedented see-saw geometry. The asymmetric coordination of zinc results in more pronounced differences between two imine fragments. Thus, the imine moiety only loosely coordinated to zinc has the C=N bond of 1.3005(18) Å, whereas the tightly coordinated arm shows a much longer C-N distance of 1.3893(16) Å. The latter bond is very close to the N-aryl distance of 1.4096(19) Å and is consistent with a nearly complete reduction of the C=N double bond.

Figure 7. Molecular structure of 14.

span>[1] T. Chu, I. Korobkov, G.I. Nikonov, J. Am. Chem. Soc. 2014, 136,  9195−9202.

span>[2] T. Chu, L. Belding, A. van der Est, T. Dudding, I. Korobkov, G.I. Nikonov, Angew. Chem. Int. Ed. 2014, 53, 2711 – 2715.