Homogeneous CO Hydrogenation: Ligand Effects on the Lewis Acid ...

Re <strong>CO</strong>
OC
OC
2
[hept4N][BF4] Re <strong>CO</strong>
OC
<strong>CO</strong>
B PPh2 O
OC C
B PPh2 H
Re <strong>CO</strong>
OCPh 2P B
O
OC C
B PPh2 H
Re <strong>CO</strong>
OC
Ph2P B
O
O C C
H
B PPh2
Scheme S2.
Re <strong>CO</strong>
OC
Ph2P B
C <strong>CO</strong>
B O
H
Re <strong>CO</strong>
OC
O
OC C
B PPh2 N
H
Variable temperature NMR studies of 2-M2 between -35 °C and +95 °C in C6D5Cl
revealed fluxional processes. The 31 P NMR spectrum of 2-M2 displays two sharp doublets (δ -
2.9, 2 JPP = 99.1 Hz; δ 6.5, 2 JPP = 99.2 Hz) at low temperatures, which coalesce into a singlet (δ
2.3) at ~50 °C. The 1 H NMR spectrum shows similar sharpening upon cooling to -35 °C, with
two sharp doublets at δ 3.07 (JPH = 12.1 Hz) and δ 1.87 (JPH = 13.1 Hz) emerging for two
inequivalent CH2 groups on the ligands. Near 50 °C these doublets coalesce into one broad
singlet, consistent with the fluxional processes that give a more symmetric ligand environment.
Interestingly, the carbene proton triplet collapses into a doublet as it is cooled, δ 13.85, JPH =
11.1 Hz. This is consistent with a static structure with intramolecular coordination of one
pendant borane to the carbene oxygen, in contrast to ethylene-linker-containing complex 2-E2,
which has intermolecular coordination and is dimeric. The restricted rotation of the carbene
affords favorable angles for P-H coupling to the phosphinoborane that is binding the oxygen,
while the other phosphine has an undetectable coupling constant due to the unfavorable coupling
angles. The phosphine resonance at δ -2.9 is assigned as the intramolecularly coordinated
phosphinoborane, due to the slight upield shift consistent with a 6-membered ring structure, and
from 1 H- 31 P gHMBC correlation to the carbene proton, as well as the downfield-shifted
methylene protons of that ligand (also consistent with a constrained ring geometry).
From the coalescence temperature the barrier for the fluxtional process can be estimated,
ΔG ‡ = 13.6 ± 0.2 kcal/mol (323 K). The exact process that creates equivalent ligand
PPh 2
S60