1574 Organometallics, Vol. 17, No. 8, 1998
Konze et al.
exhibits a 31P NMR spectrum that is consistent with
the structural data. A proton-coupled 31P NMR spec-
trum shows the peak at δ 38.0 as a sharp doublet of
triplets, which allows assignment of this peak to the
CdPR phosphorus P(x), while the other two peaks are
broadened due to coupling with phenyl protons and are
assigned as PPh3 groups. The characteristic doublet-
of-triplets splitting pattern for P(x) allows for unam-
biguous assignments of the peaks; the doublet arises
from splitting by the carbon-bound phosphine P(a) at δ
C- and P-substituents in IIa and Va , Cl2CdPN(SiMe3)2
(δ 251.7 ppm)23 and (Cl)(H)CdPMes* (δ 245 ppm),30
shows that changes in these particular substituents do
not necessarily impart large changes on the 31P NMR
chemical shifts. A better explanation for the large
differences in chemical shifts between IIa and Va is that
the configuration around the CdP bond in IIa (Z) is
different than that which was determined by X-ray
diffraction for Va (E). It is well-documented that the
E and Z configurations of phosphaalkenes can have a
large effect on the 31P chemical shift of the CdP
phosphorus, although it is not possible to predict the
relative shifts of the isomers.31 An illustrative example
is the phosphaalkene compound [(Ph)(Me3Si)N]C(Ph)dP-
(Ph) in which the E isomer has a chemical shift of δ
225 ppm and the Z isomer is at δ 144 ppm.32 Further
evidence for the different configurations in IIa and Va
2
22.3 with J ) 57.8 Hz, and the triplet is caused by the
2
two equivalent nickel phosphines P(b) at δ 20.0 with J
) 41.2 Hz. Although it is not possible to compare the
31P NMR properties of IIIa with uncoordinated R3Pd
CdPR ligands since they are unknown, some compari-
sons with similar compounds can be made. The chemi-
cal shift of δ 38.0 for P(x) in IIIa is significantly upfield
from that of the phosphaalkene Cl2CdPN(SiMe3)2 (δ
251.7 ppm)23 and the phosphavinyl phosphonium salt
[(Ph3P)(H)CdPN(i-PR)2]+ (δ 303.5 ppm for CdP).7 This
is consistent with similar dramatic upfield shifts which
occur upon η2-coordination of CdP double bonds; for
example, Ni(PMe3)2[η2-(Me3Si)2CdPCH(SiMe3)2] (δ 23.4
ppm for CdP) is 380 ppm upfield from the free phos-
phaalkene (Me3Si)2CdPCH(SiMe3)2 (δ 404 ppm).28 The
2
is the appreciably smaller value of J P(x)P(a) ) 60.5 Hz
in IIa than that (2J P(x)P(a) ) 82.5 Hz) in Va . According
to the cis rule in phosphaalkenes, substituents that are
located cis to the phosphorus lone pair show larger
couplings to the CdP phosphorus atom,33 as is the case
2
in the E configuration in Va . The value of J P(x)P(a) in
IIa (60.5 Hz) is also quite similar to that in IIIa (57.8
Hz), in which the carbon-bound PPh3 group and the
N(SiMe3)2 group are arranged in a Z configuration as
well.
2
coupling constant of J ) 57.8 Hz between the CdP
phosphorus P(x) and the carbon-bound phosphine P(a)
in IIIa is smaller than values found for free phosphavi-
nyl phosphonium salts, e.g., 2J P-P ) 124.6 Hz in [(Ph3P)-
(H)CdPN(i-PR)2]+, which also contains an R3P-CdPR
linkage.7 However, in IIIa , the CdP phosphorus P(x)
has sp3-like character, which allows for less s-character
in the bonding to carbon and would then result in a
smaller coupling constant between P(x) and P(a). An
In contrast to the reactions above with triphenylphos-
phine as the ligand, when a 1:2 Ni(COD)2/PEt3 mixture
was reacted with Cl2CdPN(SiMe3)2, complex Ib formed
(Scheme 1). This compound did not react further to
form the triethylphosphine analogues of IIa or IIIa ,
even when 2 equiv of the Ni(0) reagent were added.
However, all attempts to isolate Ib resulted in decom-
position to unidentified products. The 31P NMR spec-
trum of compound Ib, δ 221.7 (t, 3J PP ) 27.5 Hz, CdP-
2
even smaller J P-P value of 10.9 Hz was reported in the
η2-diphosphaallene complex [(Ph3P)2Pt(η2-RPdCdPR)]29
(R ) tri-tert-butylbenzene) which contains an RPdCdPR
unit with one of the CdP bonds coordinated. The
3
R), 27.7 (d, J PP ) 27.5 Hz, Ni-PEt3), is quite charac-
teristic of a phosphavinyl structure (Scheme 1) and is
very similar to that of a platinum analogue Cl(Et3P)2-
2
coupling constant of J ) 41.2 Hz in IIIa between the
3
CdP phosphorus P(x) and the two equivalent nickel
Pt[C(Cl)dPMes*], δ 234.2 (t, J PP ) 24.7 Hz, CdP-R),
3
phosphines P(b) that are located cis to P(x) is slightly
15.0 (d, J PP ) 24.7 Hz, Pt-PEt3), that was character-
2
ized previously by X-ray diffraction studies.14
larger than the coupling constant of J P-P ) 28.6 Hz
between P(x) and the PMe3 group that is cis to it in
Ni(PMe3)2[η2-(Me3Si)2CdPCH(SiMe3)2],28 most likely
because the P(x) lone pair is involved in bonding to the
nickel atoms in IIIa , which allows for more s-character
from phosphorus in the P(x)-Ni bonds.
Rea ction s of Ni(0) Com p lexes w ith X2CdP Mes*
(X)Cl, Br ). The reactions (eq 4) of 0.5 equiv of
X2CdPMes* (X ) Cl, Br) with 1:2 Ni(COD)2/PPh3,
Ni(PPh3)4, or (Ph3P)2Ni(C2H4) in toluene at -78 °C
produce X(Ph3P)Ni[η2-C(H)(PPh3)dP(Mes*)] (Va , X )
Cl; Vb, X ) Br) in moderate yields, along with a roughly
equimolar amount of Ni(PPh3)3X. This Ni(I) compound
was characterized by X-ray diffraction studies as the
acetone solvate (Ph3P)3ClNi‚(Me2CdO), but the struc-
ture of a toluene solvate of the same compound was
reported previously.34 The preparation of Va using Ni-
(COD)2 and PPh3 (Method A) is preferred because of the
higher yield. The 2:1 metal complex to X2CdPMes*
stoichiometry is necessary to optimize the yield of
The 31P NMR spectrum of IIa is similar to that of the
analogous compound Va (eq 4). The peak at δ 103.7
ppm is assigned to the CdPR phosphorus P(x), since a
proton-coupled 31P NMR spectrum showed this peak as
a sharp doublet of doublets, while the peaks at δ 22.4
and 19.0 ppm were broadened by proton coupling,
indicative of PPh3 groups. The peak for P(x) in IIa is
82.1 ppm downfield from the corresponding peak for P(x)
in Va . This may be partially due to the different R
group on phosphorus or the proton on the carbon in Va
instead of a chloride in IIa . However, a comparison of
the 31P NMR spectra of two phosphaalkenes with
different substituents that correspond to the different
(30) Bickelhaupt, F. Pure Appl. Chem. 1993, 65, 621-624.
(31) Fluck, E.; Heckmann, G. Phosphorus 31 NMR Spectroscopy in
Stereochemical Analysis: Organic Compounds and Metal Complexes;
Verkade, J . G., Quin, L. D., Eds.; VCH: Deerfield Beach, FL, 1987; pp
74-76.
(32) Issleib, K.; Schmidt, H.; Meyer, H. J . Organomet. Chem. 1980,
160, 47.
(33) Knaap, T. A. v. d.; Bickelhaupt, F. Chem. Ber. 1984, 117, 915.
(34) Cassidy, J . M.; Whitmire, K. H. Acta Crystallogr., Sect. C 1991,
47, 2094.
(28) Cowley, A. H.; J ones, R. A.; Stewart, C. A.; Stuart, A. L.;
Atwood, J . L.; Hunter, W. E.; Zhang, H. M. J . Am. Chem. Soc. 1983,
105, 3737-3738.
(29) Akpan, C. A.; Meidine, M. F.; Nixon, J . F.; Yoshifuji, M.; Toyota,
K.; Inamoto, N. J . Chem. Soc., Chem. Commun. 1985, 946.