Dalton Transactions
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Conversion to the hydroxo derivatives was accomplished by
treatment with an excess of CsOH in degassed, wet THF. In
addition to ligand resonances, isolated hydroxo compounds 2R
exhibit characteristic signals at 4.73–4.84 ppm for the benzylic
proton on the PCP ligand and triplets at –2.94 to –3.22 ppm
1
(3JHP ≈ 5–6 Hz) for the O–H protons in the H NMR spectra.
The latter resonances broaden in the presence of small
amounts of water, and disappear when D2O is added; the
benzylic protons do not exchange with the deuterium in D2O,
indicating that PCcarbeneP species11 are not formed via elimin-
ation of water from compounds 2R. However, when 17O
enriched 2iPr was treated with H2O, rapid depletion of
label from the Ni–17OH moiety was observed (Fig. S3†). This
demonstrates that the hydroxo group in these compounds is
labile under these conditions, presumably mediated by the cat-
ionic species IR (Scheme 1) perhaps formed upon protonation
of the hydroxo group. It is also conceivable that direct dis-
sociation of the OH ligand occurs; the Ni–O bond distance of
1.978(2) Å found for 2iPr 11 is longer than any reported in
related Ni–OH compounds12–17 and the Csp3 anchoring group
Scheme 2 Hydration of benzonitrile using 2iPr; catalyst resting state.
of the PCP pincer ligand should have
influence.18
a strong trans
In either case, the lability of the hydroxo group in com-
pounds 2 in polar solvent environments suggested to us that
nitriles might undergo facile hydration in the presence of a
cationic species such as IR. When 2iPr was treated with an
excess of benzonitrile in THF, no reaction was observed either
visually or spectroscopically; however, addition of water to this
solution resulted in rapid conversion of hydroxo 2iPr to a new
compound characterized by a broadened peak in the 31P NMR
spectrum at ≈45 ppm, shifted downfield from that observed
for 2iPr at 39.3 ppm. Furthermore, slow conversion of benzo-
nitrile to benzamide was observed over the course of several
hours. The spectroscopic features of this mixture were difficult
to interpret because of broadening and the presence of the
various components in excess, but crystals deposited from
solutions left standing for >24 hours and were subjected to
diffraction analysis. In this way, the new compound was identi-
fied as the κ1 amidate 3iPr·H2NC(O)Ph (Scheme 2). The mole-
cular structure of this compound is shown in Fig. 1, along
with selected metrical parameters. The broadness in the NMR
spectra of this compound is no doubt attributable to dynamic
processes involving the hydrogen bonded benzamide mole-
cules in the product, further complicated by the presence of
excess H2O in the solutions. Amidate 3iPr can be prepared free
of hydrogen bonding partners by treating the known
(PCcarbeneP)Ni(NCtBu) complex11 with one equivalent of dry
benzamide. The N–H bond adds across the NivC linkage to
cleanly deliver 3iPr; the X-ray structure of this species has also
been determined (Fig. S4†) and the numbers in parentheses in
the caption for Fig. 1 indicate the distances and angles found
for this species as a comparison. The lack of hydrogen
bonding partners for 3iPr sharpens the NMR resonances, and
in the 1H NMR spectrum a signal at 3.50 ppm may be assigned
to the N–H moiety, and a sharp resonance at 45.8 ppm is
observed in the 31P NMR spectrum. Addition of an excess of
Fig. 1 Molecular structure of 3iPr·H2NC(O)Ph. Most hydrogen atoms are
omitted for clarity; a second molecule of benzamide H-bonded through
N(2) is also not shown. Selected bond distances, non-bonded distances
(Å), angles and dihedral angles (°); numbers in square brackets are the
metrical parameters for 3iPr
, included for comparison: Ni(1)–C(1),
1.990(4) [1.973(4)]; Ni(1)–P(1), 2.1550(12) [2.1544(10)]; Ni(1)–P(2),
2.1992(12) [2.2165(11)]; Ni(1)–N(1), 1.914(3) [1.902(3)]; N(1)–C(26),
1.308(5) [1.313(6)]; C(26)–O(1), 1.272(4) [1.232(5)]; N(1)–O(2), 3.128;
O(1)–N(2), 2.870; P(1)–Ni(1)–N(1), 93.69(10) [95.01(11)]; N(1)–Ni(1)–P(2),
95.26(10) [96.94(11)]; C(1)–Ni(1)–P(1), 85.66(13) [85.77(12)]; C(1)–Ni(1)–
P(2), 85.27(13) [83.62(12)]; P(1)–Ni(1)–N(1)–C(26), 120.26 [123.35]; P(2)–
Ni(1)–N(1)–C(26), 73.59 [72.80].
benzamide in wet THF to this sample reproduces the NMR fea-
tures of 3iPr·H2NC(O)Ph.
Species 3iPr·H2NC(O)Ph likely forms from a cation IiPr via
displacement of the aquo ligand by benzonitrile and nucleo-
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Dalton Trans., 2015, 44, 12082–12085 | 12083