Zr-E π-bonding in complexes derived from E-H additions to Cp2Zr(P(C6H2-2,4,6-t-Bu3))(PMe 3)

Article abstract of DOI:10.1021/om960314o

Additions of PhEH (E = O, S), PhEH₂ (E = N, P), MesPH₂, and Ph₂PH to Cp₂Zr(PR*)-(PMe₃) (2) provide a facile route to complexes of the form Cp₂Zr(PHR*)(ER) (R* = 2,4,6-t-Bu-C₆H₂; ER = OPh (3), SPh (4), NHPh (5), PHPh (6), PHMes (7), PPh₂ (8)). Variabletemperature NMR studies are consistent with facile metal-mediated inversion at phosphorus as well as rapid rotation about the Zr-E bonds at room temperature.

Full text of DOI:10.1021/om960314o

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Organometallics 1996, 15, 4223-4227
4223
Zr -E π-Bon d in g in Com p lexes Der ived fr om E-H
Ad d ition s to Cp 2Zr (P (C6H2-2,4,6-t-Bu 3))(P Me3)
Tricia L. Breen and Douglas W. Stephan*,†
Department of Chemistry and Biochemistry, University of Windsor,
Windsor, ON, Canada N9B 3P4
Received April 25, 1996X
Additions of PhEH (E ) O, S), PhEH2 (E ) N, P), MesPH2, and Ph2PH to Cp2Zr(PR*)-
(PMe3) (2) provide a facile route to complexes of the form Cp2Zr(PHR*)(ER) (R* ) 2,4,6-t-
Bu-C6H2; ER ) OPh (3), SPh (4), NHPh (5), PHPh (6), PHMes (7), PPh2 (8)). Variable-
temperature NMR studies are consistent with facile metal-mediated inversion at phosphorus
as well as rapid rotation about the Zr-E bonds at room temperature.
A feature common to early metal complexes of the
Exp er im en ta l Section
form Cp2MX2 (M ) Ti, Zr, Hf; X ) OR, SR, NHR, PHR)
is some degree of M-E π-bonding1 arising from an
interaction between lone pairs on E and the vacant 1a1
orbital of the Cp2M fragment.2 Perhaps one of the most
striking examples of such π-bonding is illustrated by the
structural study of Cp2Hf(PEt2)2 by Baker et al.3 This
species contains both pyramidal and planar phosphorus
atoms consistent with the attainment of an 18-electron
configuration at Hf via a Hf-P π-bond.
Gen er a l Da ta . All preparations were done under an
atmosphere of dry, O2-free N2 by employing either Schlenk line
techniques or a Vacuum Atmospheres inert-atmosphere glove-
box. Solvents were reagent grade, distilled from the appropri-
ate drying agents under N2 and degassed by the freeze-thaw
method at least three times prior to use. All organic reagents
were purified by conventional methods. 1H and 13C{1H} NMR
spectra were recorded on a Bruker AC-300 operating at 300
and 75 MHz, respectively. 31P and 31P{1H} NMR spectra were
recorded on a Bruker AC-200 operating at 81 MHz. Trace
amounts of protonated solvents were used as references, and
chemical shifts are reported relative to SiMe4 and 85% H3-
PO4, respectively. Combustion analyses were performed by
Galbraith Laboratories Inc., Knoxville, TN, or Schwarzkopf
Laboratories, Woodside, NY. PH2(C6H2-t-Bu3) was purchased
from the Quantum Design Chemical Co. All other reagents
were purchased from the Aldrich Chemical Co. Cp2ZrMe-
(PHR*) (1) and Cp2Zr(PR*)(PMe3) (2) were prepared by
literature methods.4 In all instances, R* and Ar refer to the
2,4,6-t-Bu-C6H2 group.
This localized bonding was also found in solution by
variable-temperature NMR studies of the analogous
Cp2Hf(PCy2)2 complex. Although numerous studies
have invoked arguments based on such π-interactions
to account for both solution behavior and solid-state
structures of early metal alkoxide, thiolate, amide, and
phosphide complexes, related mixed-ligand systems
have drawn little attention.1 The absence of such
studies is due in part to synthetic difficulties associated
with the preparation of complexes of the form Cp2M-
(X)(Y), where X and Y are potential π-donors. Recently,
we reported the synthesis of Cp2Zr(PR*)(PMe3) (2) and
demonstrated that the Zr-P double bond of 2 is highly
reactive and readily undergoes cycloadditions, phos-
phinidene transfer reactions, and additions of group 14
halides.4 Herein we show that additions of PhEH (E )
O, S), PhEH2 (E ) N, P), MesPH2, and Ph2PH to 2
provide a facile route to complexes of the form Cp2Zr-
(PHR*)(ER) and Cp2Zr(PHR*)(EHR). Variable-temper-
ature NMR studies of these species are reported, and
the implications regarding Zr-P and Zr-E π-bonding
are discussed.
1H and 13C NMR spectra of 1 acquired at 25 °C agreed with
published data.4 1H NMR (-80 °C, CD3C6D5): δ 7.74 (br s,
1H, Ar-H), 7.66 (br s, 1H, Ar-H), 5.57 (s, 5H, Cp), 5.30 (s, 5H,
Cp), 1.86 (br s, 9H, o-tBu), 1.58 (br s, 9H, o-tBu), 1.33 (s, 9H,
p-tBu), -0.07 (br s, 3H, Me). 31P NMR (-80 °C, CD3C6D5): δ
67.7 (d, |J P-H| ) 261.5 Hz).
Syn th esis of Cp 2Zr (P HR*)(OP h ) (3), Cp 2Zr (P HR*)-
(NHP h ) (5), a n d Cp2Zr (P HR*)(P P h 2) (8). These compounds
were prepared in a similar manner; thus, only one representa-
tive preparation is given. To a benzene solution of 2 (114 mg,
0.4 mmol) was added phenol (18 mg, 0.4 mmol). The reaction
mixture stood 1 h, after which time the solvent was removed
in vacuo and the product dissolved in pentane. Yellow-brown
crystals of 3 formed after this solution was allowed to stand
for 12 h and were isolated by filtration.
4
3. Yield: 52%. 1H NMR (25 °C, C6D6): δ 7.60 (d, | J P-H| )
1.8 Hz, 2H, Ar-H), 7.20 (m, 2H, Ph-H), 6.85 (m, 1H, Ph-H),
6.61 (m, 2H, Ph-H), 5.66 (br s, 10, Cp), 5.29 (d, |J P-H| ) 221.2
Hz, 1H, P-H), 1.74 (br s, 18H, o-tBu), 1.35 (s, 9H, p-tBu). 13C-
{1H} NMR (25 °C, C6D6): δ 165.3 (s, quat), 146.0 (s, quat),
145.0 (d, |J | ) 36.8 Hz, quat), 129.3 (s, arom. C-H), 121.0 (br
s, arom C-H), 119.6 (s, arom C-H), 118.0 (s, arom C-H),
111.3 (s, Cp), 38.3 (br s, o-C(CH3)), 34.6 (s, p-C(CH3)), 33.1 (br
s, o-C(CH3)), 31.3 (s, p-C(CH3)). 31P NMR (25 °C, C6D6): δ
-20.0 (d, |J P-H| ) 220.6 Hz). 1H NMR (-70 °C, CD3C6D5): δ
7.63 (br, 2H, Ar-H), 7.22-6.89 (m, 3H, Ph-H), 6.52 (m, 2H,
Ph-H), 5.61 (s, 5H, Cp), 5.40 (s, 5H, Cp), 1.89 (s, 9H, o-tBu),
1.68 (s, 9H, o-tBu), 1.36 (s, 9H, p-tBu). 31P NMR (-70 °C,
CD3C6D5): δ -22.4 (d, |J P-H| ) 222.8 Hz).
† E-mail: Stephan@UWindsor.ca.
X Abstract published in Advance ACS Abstracts, September 1, 1996.
(1) For leading references see: (a) Hey-Hawkins, E. Chem. Rev.
1994, 94, 1661. (b) Stephan, D. W. Nadasdi, T. T. Coord. Chem. Rev.
1996, in press.
(2) Lauher, J ., Hoffmann, R. J . Am. Chem. Soc. 1976, 98, 1729.
(3) Baker, R. T.; Whitney, J . F.; Wreford, S. S. Organometallics 1983,
2, 1049.
4
5. Yield: 78%. 1H NMR (25 °C, C6D6): δ 7.60 (d, | J P-H| )
(4) Breen, T. L.; Stephan, D. W. J . Am. Chem. Soc. 1995, 117, 11914.
1.7 Hz, 2H, Ar-H), 7.25-6.85 (m, 5H, Ph-H), 6.59 (s, 1H, N-H),
S0276-7333(96)00314-7 CCC: $12 00 © 1996 American Chemical Society

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4224 Organometallics, Vol. 15, No. 20, 1996
Breen and Stephan
Ta ble 1. Cr ysta llogr a p h ic Da ta
5.51 (br s, 10H, Cp), 5.22 (d, |J P-H| ) 223.2 Hz, 1H, P-H),
1.68 (s, 18H, o-tBu), 1.36 (s, 9H, p-tBu). 13C{1H} NMR (25 °C,
C6D6): δ 156.1 (s, quat), 155.8 (s, quat), 145.8 (s, quat), 144.6
(d, |J | ) 36.5 Hz, quat), 120.9 (s, arom C-H), 120.6 (s, arom
C-H), 120.5 (s, arom C-H), 119.4 (s, arom C-H), 109.7 (s,
Cp), 38.2 (s, o-C(CH3)3), 34.6 (s, p-C(CH3)3), 32.7 (s, o-C(CH3)3),
formula
fw
C34H46PNZr
590.94
cryst color, form
cryst size
a (Å)
blocks
0.30 × 0.25 × 0.25
9.627(4)
31.3 (s, p-C(CH3)3). 31P NMR (25 °C, C6D6): δ -11.1 (d, |J P-H
|
b (Å)
d (Å)
â (deg)
21.063(7)
16.227(4)
106.30(3)
) 223.4 Hz). 1H NMR (-60 °C, CD3C6D5): δ 7.72 (br s, 1H,
Ar-H), 7.67 (br s, 1H, Ar-H), 7.37-6.75 (m, 5H, Ph-H), 6.60
(br s, 1H, N-H), 5.63 (s, 5H, Cp), 5.28 (d, |J P-H| ) 223.0 Hz,
1H, P-H), 5.22 (s, 5H, Cp), 1.79 (br s, 9H, o-tBu), 1.70 (br s,
9H, o-tBu), 1.41 (s, 9H, p-tBu). 31P NMR (-60 °C, CD3C6D5):
δ -13.6 (d, |J P-H| ) 222.3 Hz).
cryst syst
space group
V (Å3)
monoclinic
P21/n
3158(2)
1.24
Dcalcd (g cm-3
)
Z
4
4.20
abs coeff, µ, cm-1
radiation (λ (Å))
temp (°C)
8. Yield: 85%. 1H NMR (25 °C, C6D6): δ 7.64-6.98 (m,
12H, Ar-H and Ph-H), 6.67 (d, |J P-H| ) 276.3 Hz, 1H, P-H),
5.50 (s, 10H, Cp), 1.70 (br s, 18H, o-tBu), 1.36 (s, 9H, p-tBu).
13C{1H} NMR (25 °C, C6D6): δ 149.9 (s, quat), 148.8 (d, |J | )
28.3 Hz, quat), 135.1 (d, |J | ) 8.0 Hz, quat), 134.0 (d, |J | )
15.2 Hz, arom C-H), 134.0 (d, |J | ) 16.9 Hz, arom C-H), 124.9
(s, arom C-H), 121.7 (br s, arom C-H), 106.9 (s, Cp), 38.8 (s,
o-C(CH3)3), 34.8 (s, p-C(CH3)3), 32.9 (s, o-C(CH3)3), 31.2 (s,
Mo KR (0.710 69)
24
scan speed, deg/min
scan range (deg)
bkgd/scan ratio
data collcd
16 (θ/2θ) (1-3 scans)
1.0 above KR1, 1.0 below KR2
0.5
5747
4.5-50
(h,k,l
2075
284
2θ range (deg)
index range
data Fo > 3σ(Fo
variables
2
2
p-C(CH3)3). 31P NMR (25 °C, C6D6): δ 153.2 (dd, | J P-P| ) 21.9
2
)
2
Hz, |J P-H| ) 278.9 Hz), 17.4 (d, | J P-P| ) 21.9 Hz). 1H NMR
transm factors
0.818-0.996
7.3
(-70 °C, CD3C6D5): δ 7.75-7.00 (m, 12H, Ar-H and Ph-H),
5.54 (s, 5H, Cp), 5.40 (s, 5H, Cp), 2.00 (br s, 9H, o-tBu), 1.47
(br s, 9H, o-tBu), 1.32 (s, 9H, p-tBu). 31P NMR (-70 °C,
R (%)a
Rw (%)a
5.5
largest ∆/σ
goodness of fit
0
2.76
2
CD3C6D5): δ 164.6 (br d, |J P-H| ) 292.4 Hz), 5.0 (d, | J P-P| )
20.7 Hz).
a
2
R ) ∑||Fo|-|Fc||/∑|Fo|; Rw ) [∑(|Fo| - |Fc|)2|/∑|Fo| ]0.5
.
Gen er a tion of Cp 2Zr (P HR*)(SP h ) (4), Cp 2Zr (P HR*)-
(P HP h ) (6), a n d Cp 2Zr (P HR*)(P HMes) (7). These com-
pounds were generated in a similar manner; thus, only one
representative preparation is given. To a benzene solution of
2 (114 mg, 0.4 mmol) was added thiophenol (20.4 µL, 0.4
mmol). The reaction mixture stood 10 min, after which time
the solvent was removed in vacuo. The residue was character-
ized by NMR, although the unstable product degraded after
24 h in solution.
Hz, quat), 133.4 (s, quat), 121.6 (br s, arom C-H), 119.7 (s,
arom C-H), 107.2 (s, Cp), 38.6 (br s, o-C(CH3)), 34.8 (s,
p-C(CH3)), 32.8 (br s, o-C(CH3)), 31.2 (s, p-C(CH3)), 24.8 (d,
| J P-C| ) 9.4 Hz, o-Me), 20.9 (s, p-Me). 31P NMR (25 °C,
3
2
C6D6): δ 130.6 (ddd, |J P-H| ) 282.5 Hz, | J P-P| ) 36.6 Hz,
3
2
| J P-H| ) 14.3 Hz), -60.4 (d, |J P-H| ) 206.1 Hz, | J P-P| ) 36.6
Hz). 1H NMR (-70 °C, CD3C6D5): δ 7.78 (br s, 2H, Ar-H),
7.69 (br s, 2H, Ar-H), 5.96 (d, |J P-H| ) 286.1 Hz, 1H, P-H),
5.29 (s, 5H, Cp), 5.23 (s, 5H, Cp), 3.42 (br dd, |J P-H| ) 204.2
4. Yield: 76% (by 1H NMR). 1H NMR (25 °C, C6D6): δ
7.84-6.98 (m, 7H, Ph-H and Ar-H), 5.96 (d, |J P-H| ) 259.3 Hz,
1H, P-H), 5.56 (s, 10, Cp), 1.70 (br s, 18H, o-tBu), 1.32 (s, 9H,
p-tBu). 13C{1H} NMR (25 °C, C6D6): δ 154.2 (d, |J | ) 7.5 Hz,
quat), 148.1 (s, quat), 147.4 (s, quat), 139.5 (d, |J | ) 12.1 Hz,
quat), 133.8 (s, arom C-H), 124.3 (s, arom C-H), 122.0 (s,
arom C-H), 121.4 (br s, arom C-H), 109.2 (s, Cp), 38.6 (br s,
o-C(CH3)), 34.7 (s, p-C(CH3)), 32.9 (br s, o-C(CH3)), 31.3 (s,
p-C(CH3)). 31P NMR (25 °C, C6D6): δ 64.4 (d, |J P-H| ) 259.3
Hz). 1H NMR (-50 °C, CD3C6D5): δ 7.93-7.07 (m, 7H, Ph-H
and Ar-H), 5.54 (s, 5H, Cp), 5.45 (s, 5H, Cp), 1.99 (br s, 9H,
o-tBu), 1.53 (br s, 9H, o-tBu), 1.32 (s, 9H, p-tBu). 31P NMR
(-70 °C, CD3C6D5): δ 63.9 (d, |J P-H| ) 259.3 Hz).
3
Hz, | J P-H| ) 15 Hz, 1H, P-H), 2.55 (br s, 6H, o-Me), 2.40 (s,
3H, p-Me), 1.92 (br s, 9H, o-tBu), 1.54 (br s, 9H, o-tBu), 1.34
(s, 9H, p-tBu). 31P NMR (-70 °C, CD3C6D5): δ 142.2 (br d,
2
|J P-H| ) 286.8 Hz), -69.7 (dd, |J P-H| ) 204.1 Hz, | J P-P| ) 24.4
Hz).
X-r a y Da ta Collection a n d Red u ction . X-ray-quality
crystals of 5 were obtained directly from the preparation as
described above. The crystal was manipulated and mounted
in capillaries in a glovebox, thus maintaining a dry, O2-free
environment for each crystal. Diffraction experiments were
performed on a Rigaku AFC6 diffractometer equipped with
graphite-monochromatized Mo KR radiation. The initial
orientation matrix was obtained from 20 machine-centered
reflections selected by an automated peak search routine.
These data were used to determine the crystal system.
Automated Laue system check routines around each axis were
consistent with the crystal system. Ultimately, 25 reflections
(20° < 2θ < 25°) were used to obtain the final lattice
parameters and the orientation matrix. Crystal data are
summarized in Table 1. The observed extinctions were
consistent with the space groups. The data were collected in
three shells (4.5° < 2θ < 50.0°), and three standard reflections
were recorded every 197 reflections. Fixed scan rates were
employed. Up to 4 repetitive scans of each reflection at the
respective scan rates were averaged to ensure meaningful
statistics. The number of scans of each reflection was deter-
mined by the intensity. The intensities of the standards
showed no statistically significant change over the duration
of data collection. The data were processed using the TEXSAN
6. Yield: 92% (by 1H NMR). 1H NMR (25 °C, C6D6): δ
7.67-6.98 (m, 7H, Ar-H and Ph-H), 6.60 (d, |J P-H| ) 291.4 Hz,
3
1H, P-H), 5.43 (s, 10, Cp), 3.36 (dd, |J P-H| ) 188.0 Hz, | J P-H
|
) 15.4 Hz, 1H, P-H), 1.65 (br s, 18H, o-tBu), 1.28 (s, 9H, p-t-
Bu). 13C{1H} NMR (25 °C, C6D6): δ 150.3 (s, quat), 149.9 (s,
quat), 133.2 (d, |J | ) 12.7 Hz, arom C-H), 128.4 (s, arom
C-H), 123.7 (s, arom C-H), 121.7 (s, arom C-H), 107.1 (s,
Cp), 38.7 (br s, o-C(CH3)), 34.8 (s, p-C(CH3)), 32.8 (br s,
o-C(CH3)), 31.2 (s, p-C(CH3)). 31P NMR (25 °C, C6D6): δ 151.9
(d, |J P-H| ) 291.2 Hz), -43.6 (d, |J P-H| ) 187.7 Hz). 1H NMR
(-80 °C, CD3C6D5): δ 7.73-6.96 (m, 7H, Ar-H and Ph-H), 6.38
(d, |J P-H| ) 291.6 Hz, 1H, P-H), 5.38 (s, 5H, Cp), 5.23 (s, 5H,
3
Cp), 3.20 (br dd, |J P-H| ) 180 Hz, | J P-H| ) 16 Hz, 1H, P-H),
1.77 (br s, 9H, o-tBu), 1.45 (br s, 9H, o-tBu), 1.31 (s, 9H, p-t-
Bu). 31P NMR (-80 °C, CD3C6D5): δ 155.1 (d, |J P-H| ) 294.9
Hz), -46.6 (d, |J P-H| ) 183.1 Hz).
7. Yield: 94% (by 1H NMR). 1H NMR (25 °C, C6D6): δ 7.63
(s, 2H, Ar-H), 7.01 (s, 2H, Ar-H), 6.38 (d, |J P-H| ) 283.0 Hz,
crystal solution package operating on a SGI Challenger
mainframe with remote X-terminals. The reflections with Fo
> 3σ(Fo2) were used in the refinements.
Str u ctu r e Solu tion a n d Refin em en t. Non-hydrogen
atomic scattering factors were taken from the literature
3
2
1H, P-H), 5.36 (s, 10H, Cp), 3.53 (dd, |J P-H| ) 205.7 Hz, | J P-H
|
) 12.8 Hz, 1H, P-H), 2.51 (s, 6H, o-Me), 2.30 (s, 3H, p-Me),
1.66 (br s, 18H, o-tBu), 1.35 (s, 9H, p-tBu). 13C{1H} NMR (25
°C, C6D6): δ 149.4 (s, quat), 144.7 (s, quat), 138.4 (d, |J | ) 9.9

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Zr-E π-Bonding
Organometallics, Vol. 15, No. 20, 1996 4225
Sch em e 1
tabulations.5,6 The Zr atom position was determined using
direct methods. The remaining non-hydrogen atoms were
located from successive difference Fourier map calculations.
The refinement was carried out by using full-matrix least-
squares techniques on F, minimizing the function w(|Fo|-|Fc|)2,
where the weight w is defined as 4Fo2/2σ(Fo2) and Fo and Fc
are the observed and calculated structure factor amplitudes.
In the final cycle of refinement, all the non-hydrogen atoms
were assigned anisotropic temperature factors. An empirical
absorption correction was applied to the data sets based on
ψ-scan data. C-H hydrogen atom positions were calculated
and allowed to ride on the carbon to which they are bonded
assuming a C-H bond length of 0.95 Å. The N-H and P-H
hydrogen atoms were located in a difference Fourier map and
their contributions included but not refined in subsequent
least-squares calculations. Hydrogen atom temperature fac-
tors were fixed at 1.10 times the isotropic temperature factor
of the carbon atom to which they are bonded. The hydrogen
atom contributions were calculated but not refined. The final
values of R, Rw, and the maximum ∆/σ on any of the
parameters in the final cycle of refinement are given in Table
1. The locations of the largest peak in the final difference
Fourier map calculation as well as the magnitude of the
residual electron density were of no chemical significance.
Positional parameters, hydrogen atom parameters, thermal
parameters, and bond distances and angles have been depos-
ited as Supporting Information.
F igu r e 1. ORTEP drawing of 5 with 30% thermal el-
lipsoids shown. Distances (Å) and angles (deg): Zr-P
2.707(4), Zr-N 2.09(1), P-C 1.86(1), N-C 1.38(1); P-Zr-N
99.0(3).
instability of compounds 4 and 6-8 resulted in either
disproportionation or elimination of organophosphine
over a 24 h period at 25 °C. The latter degradation
pathway is consistent with established reactivity of
zirconocene bis(organophosphides).7
The formulation of 5 was verified by a single-crystal
diffraction study (Figure 1). As expected, the geometry
about the zirconium atom of 5 is pseudo-tetrahedral,
with two η5-cyclopentadienyl ligands, a supermesi-
tylphosphide, and a phenylamido ligand completing the
coordination sphere. The location of both the amido and
phosphido hydrogen atoms confirmed a pseudo-trigonal
planar geometry at nitrogen, and a pseudo-tetrahedral
geometry at phosphorus. The Zr-P distance (2.707(4)
Å) in 5 is considerably longer than the Zr-P bonds in
Cp2Zr(PHR*)Cl (2.558(7) Å),8 (MeCp)2Zr(PH(2,4,6-i-
Pr3C6H2)Cl (2.6381(8) Å),9 and Cp2Zr(PHR*)2 (2.681(5),
2.682(5) Å).10 Moreover, the Zr-N bond length of 2.09-
(1) Å is shorter than the Zr-N bonds in Cp2Zr(NC4H4)2
(2.171(2), 2.167(2) Å) and Cp2Zr(NCO)2 (2.117(7) Å).11
The P-Zr-N angle of 99.0(3)° in 5 compares with the
Cl-Zr-P angles of 97.1(2) and 93.55(2)° in Cp2Zr-
(PHR*)Cl8 and (MeCp)2Zr(PH(2,4,6-i-Pr3C6H2)Cl9, re-
spectively. The larger angle in 5 may be credited to the
greater steric demands of the substituents on the two
ancillary ligands. These data are consistent with a
formal single Zr-P bond and strong π-character in the
Zr-N bond of 5.
Resu lts a n d Discu ssion
Syn th esis. We have previously reported that elimi-
nation of methane from Cp2ZrMe(PHR*) (1) affords,
upon trapping with PMe3, the terminal phosphinidene
complex Cp2Zr(PR*)(PMe3) (2) (Scheme 1).4 Protic
reagents such as phenol, aniline, thiophenol, and pri-
mary or secondary phosphines react rapidly with 2 to
yield a series of compounds of the form Cp2Zr(PHR*)-
(ER) (ER ) OPh (3), SPh (4), NHPh (5), PHPh (6),
PHMes (7), PPh2 (8)). In each case, 31P NMR spectra
were consistent with E-H addition across the ZrdP
double bond, as evidenced by the P-H coupling in the
resulting supermesitylphosphide ligands as well as the
P-P coupling in 7 and 8. 1H NMR data also supported
the formulations above. Although these compounds
could be generated and characterized spectroscopically,
they proved difficult to isolate due to their extreme
solubility in organic solvents. In addition, the thermal
Tem p er a tu r e-Dep en d en t NMR Da ta . NMR spec-
tra for compounds 1 and 3-8 were recorded over a
temperature range 30 to -80 °C. Marked temperature
dependences were observed in each case. In each of
these compounds, the resonances attributable to the
ortho-tert-butyl groups of the supermesityl substituents
are broad at 25 °C and split into two resonances upon
(7) (a) Hou, Z.; Stephan, D. W. J . Am. Chem. Soc. 1992, 114, 10088.
(b) Hou, Z.; Breen, T. L.; Stephan, D. W. Organometallics 1993, 12,
3158.
(5) (a) Cromer, D. T.; Mann, J . B. Acta Crystallogr., Sect. A: Cryst.
Phys., Theor. Gen. Crystallogr. 1968, A24, 324. (b) Cromer, D. T.;
Mann, J . B. Acta Crystallogr., Sect. A: Cryst. Phys., Theor. Gen.
Crystallogr. 1968, A24, 390.
(6) Cromer, D. T.; Waber, J . T. International Tables for X-ray
Crystallography, Knoch Press: Birmingham, England, 1974.
(8) Hey, E.; Lappert, M. F.; Atwood, J . L.; Bott, S. G. Polyhedron
1988, 7, 2083.
(9) Hey-Hawkins, E.; Kurz, S. Z. Naturforsch. 1995, 50b, 239.
(10) Hey-Hawkins, E.; Kurz, S. J . Organomet. 1994, 479, 125.
(11) Vann Bynum, R.; Hunter, W. E.; Rogers, R. D.; Atwood, J . L.
Inorg. Chem. 1980, 19, 2368.

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4226 Organometallics, Vol. 15, No. 20, 1996
Breen and Stephan
experimentally and via computational studies. The
barriers for the dynamic processes observed herein are
in accord with those previously reported for transition
metal mediated P inversion and are significantly lower
than the barrier to inversion for organophosphines.21,22
Zr-P π-bonding accounts for this reduction in the
energy barrier and is accomplished by alignment of the
P p-orbital with the LUMO or 1a1 orbital of the Cp2M
fragment.2 This orbital is oriented in the plane which
lies between the cyclopentadienyl ligands, hence in the
plane of the ancillary donor atoms (Figure 3). The
aforementioned Zr-P π-bonding is maximized by a
planar geometry at P, which has been observed in the
solid-state structure of Cp2Hf(PEt2)2.3 In the diphos-
phide complexes 6-8, the disparate nature of the 31P
NMR resonances is consistent with pyramidal and
planar geometries at the phosphorus atoms. The pres-
ence of this dative Zr-P bond reflects the Lewis acidity
of the Zr center, which achieves a formal 18-electron
count. For compound 8, the P-H coupling of 292 Hz
associated with the low-field resonance indicates that
it is the supermesitylphosphide ligand that participates
in π-bonding. NOE experiments designed to probe this
question for 6 and 7 were inconclusive. Compound 5
exhibits a relatively high-field 31P NMR resonance
consistent with a pyramidal geometry at phosphorus,
while Zr-N π-bonding is confirmed by the crystal-
lographic data (vide supra). Similarly, the 31P NMR
data for compounds 1, 3, and 4 imply pyramidal P
geometries. Although unconfirmed, Zr-O and Zr-S
π-bonding analogous to that seen in 5 is anticipated in
3 and 4, respectively. Structural studies of complexes
of the form Cp2Zr(ER)2 (E ) O, S)20 support this notion.
It should be noted, however, that other related studies
of early metal chalcogenide species by the groups of
Parkin and Rothwell conclude that structural data does
not support M-E π bonding.23 Furthermore, the upfield
chemical shift of the cyclopentadienyl rings for 3 and 4
are consistent with a formal 18-electron configuration
at the metal center. The comparatively low barrier to
inversion observed for 1 is ascribed to strong σ-donation
by the methyl group to Zr, while the rotational barriers
about the Zr-P bonds in 6-8 are consistent with the
greater steric demands of the ancillary phosphide sub-
stituents. In comparison, the steric inhibition in Cp2-
F igu r e 2. Variable-temperature 1H NMR spectrum
for 5.
F igu r e 3. Schematic depiction of the (a) 2a1, (b) b2, and
(c) 1a1 orbitals of the Cp2M fragment, (d) σ-interaction of
the b2 orbital of the Cp2M fragment with the ligand σ
orbitals, and (e) π-interaction of the ligand p-orbital with
the 1a1 orbital.
Sch em e 2
cooling. This temperature dependence results from
sterically inhibited rotation about the P-C bond, a
feature common to systems with sterically demanding
substituents.10 In addition, a second dynamic process
may be distinguished for these compounds. As the
temperature is lowered, the broad resonance due to the
cyclopentadienyl rings splits and eventually sharpens
into two lines (Figure 2). ∆Gcq values pertaining to this
process were determined to be 11.8, 13.1, 13.1, 13.4,
12.5, 12.4, and 12.4, and 12.4 ( 1 kcal mol-1 for com-
pounds 1 and 3-8.
Inequivalent cyclopentadienyl groups in these com-
plexes arise as a result of the incorporation of a PHR*
ligand which provides a chiral phosphorus center and,
consequently, diastereotopic cyclopentadienyl ligands.
The dynamic process which results in a single cyclo-
pentadienyl ring environment at room temperature
must therefore involve facile inversion at P in addition
to rapid rotation about the Zr-E bond (Scheme 2).
Related metal-mediated epimerization of transition
metal phosphide complexes,12-17 as well as rotational
barriers about early metal π-bonds to amide,18,19 alkox-
ide, and thiolate ligands,20 have been studied both
(14) (a) Crisp, G. T.; Salem, G.; Wild, S. B. Organometallics 1989,
8, 2360. (b) Crisp, G. T.; Salem, G.; Stephens, F. S.; Wild, S. B. J .
Chem. Soc., Chem. Commun. 1987, 600.
(15) (a) Zwick, B. D.; Dewey, M. A.; Knight, D. A.; Buhro, W. E.;
Arif, A. M.; Gladysz, J . A. Organometallics 1992, 11, 2673. (b) Buhro,
W. E.; Zwick, B. D.; Georgiou, S.; Hutchinson, J . P.; Gladysz, J . A. J .
Am. Chem. Soc. 1988, 110, 2427.
(16) Rogers, J . R.; Wagner, T. P. S.; Marynick, D. S. Inorg. Chem.
1994, 33, 3104.
(17) J org, K.; Malisch W.; Reich, W.; Meyer, A.; Schubert, U. Angew.
Chem., Int. Ed. Engl. 1986, 25, 92.
(18) (a) Hillhouse, G. L.; Bulls, A. R.; Santarsiero, B. D.; Bercaw, J .
E. Organometallics 1988, 7, 1309. (b) Hillhouse, G. L.; Bercaw, J . E.
Organometallics 1982, 1, 1025.
(19) (a) Chrisholm, M. H.; Cotton, F. A.; Extine, Stults, B. R. J . Am.
Chem. Soc. 1976, 98, 4477. (b) Chisholm, M. H.; Cotton, F. A.; Frenz,
B. A.; Reichert, W. W.; Shive, L. W.; Stults, B. R. J . Am. Chem. Soc.
1976, 98, 4469.
(20) Stephan, D. W.; Nadasdi, T. T. Coord. Chem. Rev. 1996, 147,
147.
(21) Macdonell, G. D.; Berlin, K. D.; Baker, J . R.; Ealick, S. E.; van
der Helm, D.; Marsi, K. L. J . Am. Chem. Soc. 1978, 100, 4535.
(22) Schmidbaur, H.; Schier, A.; Lauteschlager, S.; Reide, J .; Muller,
G. Organometallics 1984, 3, 1906.
(12) Roddick, D. M.; Santarsiero, B. D.; Bercaw, J . E. J . Am. Chem.
Soc. 1985, 107, 4670.
(13) Bohra, R.; Hitchcock, P. B.; Lappert, M. F.; Leung, W.-P. J .
Chem. Soc., Chem. Commun. 1989, 728.
(23) For a leading reference, see: (a) Howard, W. A.; Parkin, G. J .
Am. Chem. Soc. 1994, 116, 606. (b) Profilet, R. D.; Fanwick, P. E.;
Rothwell, I. P. Angew. Chem., Int. Ed. Engl. 1990, 29, 664.

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Zr-E π-Bonding
Organometallics, Vol. 15, No. 20, 1996 4227
Hf(PCy2)2 and Cp*2HfH(NHR)18 is substantially re-
duced, and the energies required to average the
cyclopentadienyl rings are diminished to 6 and 10 kcal
mol-1, respectively.3
In summary, the reactivity of the Zr-P double bond
of 2 affords the facile synthesis of complexes of the form
Cp2Zr(PHR*)(ER) and Cp2Zr(PHR*)(EHR). Data pre-
sented herein illustrate that Zr-E π-interactions are
generally weak and yet provide avenues for both metal-
mediated pyramidal inversion at P and quenching of the
Lewis acidity of electron deficient Zr centers. In addi-
tion, the strengths of such Zr-E π-bonds are dependent
on the nature of both the substituents and the other
ancillary ligands in the complex.
Ack n ow led gm en t. Support from the NSERC of
Canada is acknowledged. T.L.B. is grateful for the
award of an NSERC postgraduate scholarship.
Su p p or tin g In for m a tion Ava ila ble: Tables of positional
parameters, hydrogen atom parameters, thermal parameters,
and bond distances and angles (10 pages). Ordering informa-
tion is given on any current masthead page.
OM960314O