Isomerization of Cyclooctadiene to Cyclooctyne with a Zinc/Zirconium Heterobimetallic Complex

Abstract Reaction of a zinc/zirconium heterobimetallic complex with 1,5‐cyclooctadiene (1,5‐COD) results in slow isomerization to 1,3‐cyclooctadiene (1,3‐COD), along with the formation of a new complex that includes a cyclooctyne ligand bridging two metal centers. While analogous magnesium/zirconium and aluminum/zirconium heterobimetallic complexes are competent for the catalytic isomerization of 1,5‐COD to 1,3‐COD, only in the case of the zinc species is the cyclooctyne adduct observed.

The coalescence temperature of the observed exchange processes of the metal hydrides appears dependent on the trace impurity profile, and different batches of Mg•Zr showed the same chemical exchange process but operating at lower temperature. We report here the upper limit at which the chemical exchange between Ht and H µ' is observed. Figure S4. VT NMR of Zn•Zr between 193-373 K: the chemical exchange process between the H t and H µ' leads to broadening above room temperature. T C = 373 K, with a concomitant collapse of the H µ doublet to form a triplet. Broadening and splitting out of the Nacnac ligand aryl substituents occurs below 323 K.
-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 VT NMR data was treated by line shape analysis with the DNMR programme in Topsin v3.1. The hydride resonances of Zn•Zr were fitted over the 303 to 353 K range with an initial line broadening factor of 2 Hz. Fits for k were optimized to the experimental data and the modelled data are presented below. The activation parameters for the exchange of H t with H µ' are as follows:

S16
Based on the assignment of H µ and H µ ' using DFT calculations (GIAO method) on optimized structures of Mg•Zr and Zn•Zr a mechanism can be proposed for the observed chemical exchange process between H t and H µ '. These data are attributed to an additional fluxional process operating well below 298 K and attributed to hindered rotation around the C ipso -N bond; the shorter metal---metal distance in Zn•Zr may contribute to the apparently higher activation energy of this processes in this analogue.

C A T A L Y T I C I S O M E R I S A T I O N O F C Y C L O O C T A -1 , -D I E N E , A N D T H E S Y N T H E S I S O F 1
Scheme S16. Catalysis of cycloocta-1,5-COD isomerisation with M•Zr (M = Mg, Al, Zn and Zr) and its by-products.
The solid was dissolved in 500 µL C 6 D 6 . Roughly 2-3 mg of ferrocene was added to the vial (as internal standard for quantification). The reaction mixture was transferred to an NMR tube and cycloocta-1,5-diene (0.185 mmol, 20 equiv.) added directly by calibrated pipette.
The reactions were heated to 80 °C in an oil bath. The products were confirmed by spiking product mixtures.

Catalytic 1,5-COD isomerisation experiment -Al•Zr and Zr•Zr:
In a glovebox, 500 µL C 6 D 6 was transferred to an NMR tube containing 2-3 mg of ferrocene (as internal standard for quantification). The cycloocta-1,5-diene (0.185 mmol, 20 equiv.) was added to the tube by calibrated pipette and a 1 H NMR spectrum measured. After returning the NMR tube to the glove box, the required amount of bimetallic (0.005 mmol Zr•Zr) was added to the tube. The reaction progress was monitored at room-temperature by 1 H NMR spectroscopy.

D F T S T U D I E S
Calculations were conducted in Gaussian09. All minima were confirmed by frequency calculations and solid-state data were used as an input for the atom coordinates. NBO calculations were run using NBO v5.9 within g09. A series of functionals (B3LYP, M-062x, wB97x, wB97xD) and basis-sets (6,311G+(d,p) / 6,31G+(d,p) / LanL2DZ / SDD) were investigated. Bader analysis was conducted on optimized geometries in the AIMALL package. In all cases the geometries were compared against the solid-state data.      (14) 56.43 (6) 102.81 Zn H C C Zr C C 94.69 (14) 57.13 (6) 102.10 28.60(9),51.40°L θ, φ:

X -R A Y D A T A
Crystal structures were obtained from the (colourless) single crystals of M•Zr grown from hexane at -20 °C. Both of the crystals gave a unit cell containing two very slightly different heterobimetallic structures, whose vital dimensions are within error. Both X-ray diffraction studies that reveal roughly flat over-all shapes (ignoring the Dipp substituents), with intermetallic areas that are less sterically crowded on one face. This opening is most likely to be the location of the H t . Notably, the cleft seems to arise from a slight (ca. 18.3°) twisting of one Cp centroid away from the (Nacnac)Mg plane in Mg•Zr. This is not the case Zn•Zr, whose (Nacnac)Zn metallacycle adopts a shallow boat structure with Zn and C β bow and The crystal of Mg·Zr that was studied was found to be severely twinned. The best results were obtained from a three component twin model in a ca. 61:33:6 ratio (though at least one more component could be seen in the data), with the two major lattices related by the approximate twin law [-1.00 0.00 0.00 0.00 1.00 0.00 0.00 -0.05 -1.00]. The structure was found to contain two independent complexes, Mg·Zr-A and Mg·Zr-B, and the presumed three Zr-H and Zr-H-Mg hydrogen atoms could not be located, so the atom list for the asymmetric unit is low by 6 hydrogen atoms, and that for the unit cell is low by 12 hydrogen atoms. The C40-and C50-based included toluene solvent molecules were both found to be disordered, and in each case two orientations were identified, of ca. 74:26 and 58:42% occupancy respectively. The geometries of all four orientations were optimised, the thermal parameters of adjacent atoms were restrained to be similar, and only the non-hydrogen atoms of the major occupancy orientations were refined anisotropically (those of the minor occupancy orientations were refined isotropically).

1454104.
The structure of 1 was found to contain two independent complexes, 1-A and 1-B. For each complex the Zr-H-Zn bridging hydrogen atom was located from a ΔF map and refined freely. The standard calculated position for the C2-H hydrogen atom in each complex was found to differ significantly from the observed electron density in the ΔF map, and so these hydrogen atoms were placed in the found positions and then refined freely subject to a C-H distance constraint of 0.950 Å (the distance that would have been used in the standard treatment).

Figure S29
The structure of one (Zn·Zr-A) of the two independent C S -symmetric complexes present in the crystal of Zn·Zr (50% probability ellipsoids).

Figure S30
The structure of one (Zn·Zr-B) of the two independent C S -symmetric complexes present in the crystal of Zn·Zr (50% probability ellipsoids). Figure S31. The structure of one (1-A) of the two independent complexes present in the crystal of 1 (50% probability ellipsoids). Figure S32. The structure of one (1-B) of the two independent complexes present in the crystal of 1 (50% probability ellipsoids).