Monomeric phosphido and phosphinidene complexes of nickel

Article abstract of DOI:10.1021/ja017787t

An unusual family of three-coordinate, d⁸ and d⁹ nickel phosphido and phosphinidene complexes containing the chelating 1,2-bis(di-tert-butylphosphino)ethane (dtbpe) ligand and a terminal PR₂- or PR²- ligand have been prepared. The complexes (dtbpe)Ni{P(t-Bu)₂} (2), [(dtbpe)Ni{=P(t-Bu)₂}⁺][PF₆-] (3), [(dtbpe)Ni{=P(H)(dmp)}⁺][PF₆-] (5), and (dtbpe)Ni{=P(dmp)} (6) have been structurally characterized by single-crystal X-ray diffraction methods. The three-coordinate d8 complexes exhibit Ni-P bond lengths and ligand geometries that indicate they participate in symmetry-allowed ligand-to-metal π bonding involving phosphorus p-electrons and a metal-orbital of π symmetry that lies in the Ni coordination plane. Compound 6 is a rare example of a late-transition-metal terminal phosphinidene complex. Copyright

Full text of DOI:10.1021/ja017787t

Published on Web 03/21/2002
Monomeric Phosphido and Phosphinidene Complexes of Nickel
Rory Melenkivitz, Daniel J. Mindiola, and Gregory L. Hillhouse*
Searle Chemistry Laboratory, Department of Chemistry, The UniVersity of Chicago, Chicago, Illinois 60637
Received December 17, 2001
Examples of transition-metal and actinide complexes with ter-
minal-phosphinidene (PR2-) ligands are rare, limited to Group 8
and earlier metals.1 These species can exhibit either electrophilic2
or nucleophilic1f reactivity at phosphorus. Later metals, particularly
those of Group 10, have received less attention in this regard as
evidenced by the limited number of nickel3 and platinum4,5 terminal
phosphido compounds. Our recent report of the synthesis of a three-
coordinate nickel imido complex stimulated the pursuit of an
analogous nickel phosphinidene.6 Herein we report the preparation
of stable Ni(I) and Ni(II) phosphido derivatives containing the bulky
1,2-bis(di-tert-butylphosphino)ethane (dtbpe) ligand and the conver-
sion of the phosphido ligand of one such complex to a terminal,
multiply bonded phosphinidene ligand.
Figure 1. Perspective views of the molecular structures of 2 (left) and the
complex cation of 3 (right). H-atoms have been omitted for clarity. See the
text and Table 1 for selected metrical parameters.
Reaction of lithium di-tert-butylphosphide7 with the Ni(I) chloro
dimer {(dtbpe)Ni(µ-Cl)}2 (1)6 affords the paramagnetic Ni(I)
phosphide (dtbpe)Ni{P(t-Bu)2} (2) as turquoise crystals in 71%
Scheme 1
1
isolated yield (Scheme 1). 2 has been characterized by H NMR
and IR spectroscopy, its magnetic moment, elemental analysis, and
single-crystal X-ray diffraction.8 Oxidation of 2 in diethyl ether by
ferrocenium hexafluorophosphate gives the diamagnetic salt [(dtbpe)-
Ni{P(t-Bu)2}+][PF6-] (3) as green crystals in 91% yield (Scheme
1). Compound 3 was fully characterized by 1H, 13C, and 31P NMR
and IR spectroscopy, elemental analysis, and a single-crystal X-ray
diffraction study.8 The most distinctive spectroscopic signature of
the phosphido ligand of 3 is its resonance as a triplet at δ 348 in
the 31P NMR spectrum. There are a few well-characterized terminal
phosphido complexes of nickel, such as (C5H5)(PPh3)Ni{P(SiMe3)2}
and (Cy2PCH2CH2PCy2)Ni{P(SiMe3)2}, and the latter has been
shown to possess a pyramidal phosphido ligand with Ni-P ) 2.225-
(2) Å.3b,c
The solid-state structures of 2 and 3, shown in Figure 1, exhibit
interesting features associated with the Ni-PR2 moieties (sum-
marized in Table 1). Both complexes are planar at Ni. In the d9
complex 2, the phosphido P(1) is substantially pyramidalized with
a Ni-P(1) bond length of 2.2077(12) Å. The structure of the
cationic d8 complex 3 reveals a significantly shorter Ni-P(1) bond
length of 2.098(2) Å (∆ ) 0.11 Å) and a planar P(1). These values
can be compared to Pauling’s predicted bond lengths of 2.23 and
2.13 Å for Ni-P single and double bonds, respectively, based on
their covalent radii corrected for electronegativity differences.9
While these bond lengths may be partially explained by charge
effects, we interpret the structural data as providing strong evidence
of double-bond character for the Ni-PR2 bond in 3, with the p-bond
arising from overlap of a P(1) electron-pair with the empty d-orbital
of π symmetry in the Ni-coordination plane (which could have
p-character as p-d mixing is allowed).
Table 1. Selected Spectroscopic and Structural Dataa
b
compd
δ
31P(JPP
)
Ni−P(1) (Å)
Ni−P(1)−C (deg)
∑ P (deg)
2
3
5
6
c
2.2077(12)
2.098(2)
2.0540(11)
2.0772(9)
116.7(1)d
124.1(2)d
147.9(1)
344
360
360
348 (175)
125 (182)e
970 (134)
130.78(11)
a Data for nonchelate P. b In Hz. c Not observed. d Average value. e Non-
first-order spin system, value from simulation.
(Scheme 1).8 Paramagnetic 4 has been characterized by H NMR
1
and IR spectroscopy, its magnetic moment, and elemental analysis.
Oxidation of 4 with tropylium hexafluorophosphate (THF, -35 °C)
gives dark green crystals of [(dtbpe)Ni{P(H)(dmp)}+][PF6-] (5)
in 63% yield (Scheme 1).8 Complex 5 has been characterized by
1H, 13C, and 31P NMR and IR spectroscopy, elemental analysis,
and single-crystal X-ray diffraction. Its characteristic PH resonance
appears as a doublet-of-triplets at δ 8.06 (1JPH ) 280 Hz, JPH
)
3
1
8 Hz) in the H NMR spectrum. As observed for 3, the structure
of 5 (Figure 2; Table 1) exhibits a short Ni-P(1) bond (2.0540-
(11) Å) and a phosphido ligand planar at P(1), oriented in the
geometry required for in-plane Ni-P π-bonding. The Ni-P(1)-
C(11) angle is opened considerably from the idealized 120° of an
sp2 hybrid, perhaps in part as a consequence of the steric demands
Ether solutions of lithium 2,6-dimesitylphenylphosphide (LiP-
(H)(dmp))10 react cleanly with 1 at -35 °C to give green blocks of
the primary phosphide (dtbpe)Ni{P(H)(dmp)} (4) in 93% yield
* Corresponding author. E-mail: g-hillhouse@uchicago.edu.
9
3846
J. AM. CHEM. SOC. 2002, 124, 3846-3847
10.1021/ja017787t CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
In summary, we have prepared a family of three-coordinate, d8
and d9 nickel phosphido and phosphinidene complexes, and have
structurally characterized several of them. As was found for related
amido and imido complexes,6 the three-coordinate d8 species par-
ticipate in symmetry-allowed π bonding involving ligand p-electrons
and an empty in-plane metal orbital of π symmetry. We are
currently exploring the reactivity of these unusual phosphido and
phosphinidene species, and examining their electronic structures
through calculations.
Acknowledgment. G.L.H. thanks the National Science Founda-
tion for financial support of this research and Prof. John Prota-
siewicz for a generous gift of 2,6-dimesitylphenyl phosphine.
Helpful insight was provided by Prof. Protasiewicz and Prof. David
Glueck. R.M. is grateful to the Department of Education for a
GAANN Fellowship, and D.J.M. acknowledges postdoctoral fel-
lowship support from the Ford Foundation.
Figure 2. A perspective view of the complex cation of 5. H-atoms, except
that on P(1), have been omitted for clarity. See the text and Table 1 for
selected metrical parameters.
Supporting Information Available: Experimental, spectroscopic,
and analytical details; crystallographic details; atomic coordinates; bond
angles and distances; anisotropic thermal parameters; hydrogen atom
coordinates; least-squares planes; torsion angles (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
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Figure 3. A perspective view of the molecular structure of 6. H-atoms
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of the bulky 2,6-dimesitylphenyl substituent. (The hydrogen atom
attached to P(1) was located in a difference map and refined
isotropically.) A distortion from planarity is observed at Ni, where
the sum of the P-Ni-P angles is 352°.
Reaction of cationic 5 with NaN(SiMe3)2 (THF, -35 °C) effects
deprotonation of the phosphide to afford bright green crystals of
(dtbpe)Ni{P(dmp)} (6) in 86% isolated yield (Scheme 1). Char-
acterization of 6 as a terminal Ni(II) phosphinidene complex
followed from 1H, 13C, and 31P NMR and IR spectroscopy,
elemental analysis, and crystallography. The phosphinidene ligand
of 6 resonates as a triplet at δ 970 (2JPP ) 134 Hz) in the 31P NMR
spectrum (Table 1).8
The solid-state structure of 6 boasts several interesting features
(see Figure 3). A distortion from planarity is also observed for the
Ni in 6, where the sum of the P-Ni-P angles is 353°. The Ni-
P(1) bond length (2.0772(9) Å) is similar to those found in 3 and
5 and is consistent with the expected Ni-P(1) double bond (Table
1). The Ni-P(1)-C(11) angle is significantly bent at 130.8(1)°,
which is in contrast to the more linear imido ligand found in the
related complex (dtbpe)Ni{N(2,6-di-iso-Pr-C6H3)} (7; Ni-N-C )
162.8(2)°).6 The direction of the bending, out of the Ni-coordination
plane, gives the geometry required for in-plane Ni-P(1) π overlap.
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orientation in 5; similar changes in ring orientations were observed
in the structures of 7 and its cationic amido precursor [(dtbpe)Ni-
{NH(2,6-di-iso-Pr-C6H3)}+][PF6-].6 It is noteworthy that the solu-
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the temperature range of +23 to -83 °C (THF-d8), although in the
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