It was found by us that the P–C
coupling reaction of >P(O)H
reagents with PhX (X = I and Br) in the presence of NiCl
2
/Zn as the precursors for the assumed Ni(0) complexant together with
2,2′-bipyridine as the ligand took place only with PhI at 50/70
°C. M06-2
X
/6-31G(d,p)//PCM(MeCN) calculations
for the reaction of Ph
2
P(O)H and PhX revealed a favorable
energetics only for the loss of iodide following the oxidative addition
of PhI on the Ni(0) atom. However, the assumed transition states with
Ni(II) formed after P-ligand uptake and deprotonation could not undergo
reductive elimination meaning a “dead-end route”. Hence,
it was assumed that the initial complexation of the remaining Ni
2+
ions with 2,2′-bipyridine may move the P–C
coupling forward via a Ni(II) → Ni(IV) transition. This route
was also confirmed by calculations, and this mechanism was justified
by preparative experiments carried out using NiCl
2
/bipyridine
in the absence of Zn. Hence, the generally accepted Ni(0) →
Ni(II) route was refuted by us, confirming the generality of the Ni(II)
→ N(IV) protocol, either in the presence of bipyridine, or
using the excess of the >P(O)H reagent as the
P
-ligand.
The results of the calculations on the complex forming ability of
Ni(0) and Ni(II) with 2,2′-bipyridine or the
P
-reagents were in accord with our mechanistic proposition.