An anionic zerovalent nickel carbonyl complex supported by a triphosphine borate ligand: An Ni-C≡O-Li isocarbonyl

Article abstract of DOI:10.1021/om049601f

An anionic zerovalent nickel carbonyl complex, [Ni(PhBP₃)(CO)] [Li(TMED)] (1), supported by a triphosphine borate ligand was prepared and structurally characterized. The solid state structure shows the coordination of Li⁺ to the O atom of the CO ligand. The isocarbonyl structure indicates strong electron donor ability of the [PhBP₃] ligand and charge redistribution to the carbonyl oxygen. Results of a DFT calculation support this view. Complex 1 represents the first example of a Lewis acid O atom adduct of a Ni(0) carbonyl complex.

Full text of DOI:10.1021/om049601f

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Organometallics 2005, 24, 231-233
231
An Anionic Zerovalent Nickel Carbonyl Complex
Supported by a Triphosphine Borate Ligand: An
Ni-CtO-Li Isocarbonyl
Hongyi Hou, Arnold L. Rheingold,† and Clifford P. Kubiak*
Department of Chemistry and Biochemistry, University of California at San Diego,
9500 Gilman Drive, La Jolla, California 92093-0358
Received June 3, 2004
An anionic zerovalent nickel carbonyl complex, [Ni(PhBP3)(CO)][Li(TMED)] (1), supported
by a triphosphine borate ligand was prepared and structurally characterized. The solid state
structure shows the coordination of Li+ to the O atom of the CO ligand. The isocarbonyl
structure indicates strong electron donor ability of the [PhBP3] ligand and charge redistribu-
tion to the carbonyl oxygen. Results of a DFT calculation support this view. Complex 1
represents the first example of a Lewis acid O atom adduct of a Ni(0) carbonyl complex.
Scheme 1. Synthesis of Complex
[Ni(PhBP3)(CO)][Li(TMED)] (1)
Introduction
The anionic transition metal carbonyls (carbonylates)
have been extensively studied with regard to their
interactions with main group metal cations and Lewis
acids in solid state and solution.1,2 Besides their syn-
thetic and catalytic applications, the transition metal
carbonylates have served as models for the biological
systems that exhibit bifunctional (acidic-basic) activa-
tion of small molecules such as CO, NO, N2, and CO2.3
A better understanding of the basic principles behind
the solvent, cation, and ancillary ligand effects on
organometallic ion pairing is thus an important goal.
Burlitch and co-workers reported studies of the CO
stretching frequencies in the tungsten carbonyl anion
[CpW(CO)3]- system, in which both the metal center
and the carbonyl oxygen could be sites of Lewis basicity,
depending on the nature of the Lewis acid added.4-6
Shriver and co-workers demonstrated the coordination
of alkyl aluminum reagents to the bridging carbonyl
oxygen in an iron carbonyl complex.7 Darensbourg et
al. have examined in detail the factors including solvent
polarity, cation electrostatic potential, cation solvation,
and the phosphine ligand cone angle that affect ion
contact site selection of alkali cation contact ion pairs
with anionic transition metal hydrides and carbon-
ylates.8-10
Scheme 2. Valence Bond Structures of Complex
[Ni(PhBP3)(CO)][Li(TMED)] (1)
The anionic tridentate phosphine ligand [PhB-
(CH2PPh2)3]- (herein abbreviated as [PhBP3]) was
reported in 1999.11,12 The incorporation of a negative
charge into the ligand framework adjacent to the “soft”
phosphine donor groups attracted our attention. A
question of particular interest is how the donating
ability of the ligand is affected by charge redistribution
after binding to a metal center. Here we report the
preparation and structure of an Ni-CtO-Li isocarbon-
yl formed by coordination of Li+ to the O atom of the
CO ligand of an anionic nickel(0) carbonyl complex with
the [PhBP3] ligand.
* To whom correspondence should be addressed. E-mail: ckubiak@
ucsd.edu.
† Address correspondence pertaining to crystallographic studies to
this author. E-mail: arheingold@ucsd.edu.
Results and Discussion
(1) Edgell, W. F. Ions Ion Pairs Org. React. 1972, 1, 153-176.
(2) Darensbourg, M. Y. Prog. Inorg. Chem. 1985, 33, 221-274.
(3) Floriani, C. Pure Appl. Chem. 1983, 55, 1-10.
(4) Burlitch, J. M.; Petersen, R. B. J. Organomet. Chem. 1970, 24,
C65-C67.
(5) Burlitch, J. M.; Petersen, R. B.; Stezowski, J. J.; Wan, C. n.;
Hughes, R. E. J. Am. Chem. Soc. 1971, 93, 3532-3533.
(6) Ulmer, S. W.; Skarstad, P. M.; Burlitch, J. M.; Hughes, R. E. J.
Am. Chem. Soc. 1973, 95, 4469-4471.
(7) Nelson, N. J.; Kime, N. E.; Shriver, D. F. J. Am. Chem. Soc. 1969,
91, 5173-5174.
The complex [Ni(PhBP3)(CO)][Li(TMED)] (1) was
obtained straightforwardly by reaction of [Li(TMED)]-
13
[PhB(CH2PPh2)3]12 with Ni(COD)2 (COD ) 1,5-cyclo-
octadiene) under 1 atm CO at room temperature (Scheme
1). This contrasts with the apparent difficulty in prepar-
(10) Ash, C. E.; Delord, T.; Simmons, D.; Darensbourg, M. Y.
Organometallics 1986, 5, 17-25.
(11) Barney, A. A.; Heyduk, A. F.; Nocera, D. G. Chem. Commun.
1999, 2379-2380.
(8) Darensbourg, M. Y.; Jimenez, P.; Sackett, J. R.; Hanckel, J. M.;
Kump, R. L. J. Am. Chem. Soc. 1982, 104, 1521-1530.
(9) Kao, S. C.; Darensbourg, M. Y.; Schenk, W. Organometallics
1984, 3, 871-876.
(12) Peters, J. C.; Feldman, J. D.; Tilley, T. D. J. Am. Chem. Soc.
1999, 121, 9871-9872.
(13) Schunn, R. A. Inorg. Synth. 1974, 15, 5-9.
10.1021/om049601f CCC: $30.25 © 2005 American Chemical Society
Publication on Web 12/08/2004

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232 Organometallics, Vol. 24, No. 2, 2005
Hou et al.
Table 2. Selected Bond Distances for Mononuclear
Ni(0) Carbonyls
complex
Ni-C(O), Å C-O, Å
ref
[Ni(PhBP3)(CO)][Li(TMED)] 1.702(2)
1.175(3) this work
Ni[N(CH2CH2PPh2)3](CO)
Ni(C8H18N4)(CO)2
Ni[PhPH(CH2)3PPhH](CO)2
Ni(PPh3)2(CO)2
1.74(2)
1.18(2)
1.06
20
21
22
23
24
1.85(6,1)
1.746(9,2) 1.129
1.763(3)
1.72(5,4)
1.817(2)
1.142
1.185
Ni(CO)3P(CMe3)3
Ni(CO)4
1.127(3) 25
bond distance is 1.702(2) Å, while the C-O distance is
1.175(3) Å. Table 2 lists selected bond distances of
mononuclear Ni(0) carbonyls for comparison.
The most striking feature of the crystal structure is
the coordination of the carbonyl oxygen to the lithium
cation with a Li-O bond distance of 1.924(5) Å. The
coordination geometry around lithium is pseudo-tetra-
hedral, with the CO and three THF oxygen atoms
occupying the apical sites. It should be noted that the
Li-Ocarbonyl distance is within the range of the distances
found between lithium and the oxygen atoms of the THF
molecules. As discussed by Darensbourg,8,10 THF, a
ubiquitous solvent used in carbonylate chemistry, is
evidently of O atom basicity similar to anions such as
Co(CO)4-, Mn(CO)5-, and CpFe(CO)2-. The isocarbonyl
bond angle, C(1)-O(1)-Li(1), is 151.1(2)°. The bent
structure in conjunction with the relatively short Ni(1)-
C(1) distance, long C(1)-O(1) distance, and similar
carbonyl and THF O atom-Li+ bond distances under-
score the importance of valence bond structure A
(Scheme 2), a result of substantial back-donation from
the nickel atom to the carbonyl ligand.
Recent research results on the [PhBP3] ligand by
Peters et al. showed strong ligand-field donor strength
of [PhBP3], suggesting appreciable communication be-
tween the anionic phosphine borate ligand and the
metal center.26,27 This appears to result from an elec-
trostatic polarization of the dπ orbitals of the metal by
the negatively charged [PhBP3] ligand.
Results of a density functional theory (DFT) electronic
structure calculation support this view of electronic
charge delocalization to the CO ligand.28 The structure
of the anion of 1, [Ni(PhBP3)(CO)]-, was theoretically
determined by performing a geometry optimization
based on the crystal structure of 1 as a starting point.
The structure calculation converged with reasonable
Figure 1. ORTEP view of [Ni(PhBP3)(CO)][Li(THF)3] (1)
(50% probability). The H atoms are omitted for clarity.
Selected bond distances (Å) and angles (deg) are as fol-
lows: Ni(1)-C(1), 1.702(2); Ni(1)-P(2), 2.1913(6); Ni(1)-
P(1), 2.1949(6); Ni(1)-P(3), 2.1974(6); O(1)-C(1), 1.175(3);
C(1)-Ni(1)-P(2), 127.28(8); C(1)-Ni(1)-P(1), 121.37(8);
P(2)-Ni(1)-P(1), 93.88(2); C(1)-Ni(1)-P(3), 117.51(8);
P(2)-Ni(1)-P(3), 94.58(2); P(1)-Ni(1)-P(3), 94.60(2); C(1)-
O(1)-Li(1), 151.1(2); O(1)-C(1)-Ni(1), 175.7(2).
Table 1. CO Stretching Frequencies and Ni-C
Bond Distances for L3Ni(0)CO Complexes
entry
complex
νCO (cm-1
)
ref
1
2
3
4
5
6
[Ni(PhBP3)(CO)][Li(THF)3]
Ni[N(CH2CH2PPh2)3](CO)
Ni[MeC(CH2PPh2)3](CO)
Ni[PPh(PPh2CH2CH2)2](CO)
Ni(DPB)2(CO)
1812
1878
1901
1903
1910
1996
this work
15
16
this work17
18
19
Ni[P(OC6H5)3]3(CO)
ing the corresponding isocyanide complexes, encoun-
tered by us and Peters.14 The 31P{1H} NMR spectrum
of 1 in THF showed a singlet at 26.7 ppm. The solid
state IR spectrum showed an intense ν(CO) stretching
band at 1812 cm-1, while the solution IR in THF showed
the band shifting to 1869 cm-1. This is typical behavior
for the transformation of an alkali metal ion isocarbonyl
to a solvent-separated ion pair in solution, where the
alkali metal ion becomes associated with solvent THF
molecules.8,10
The CO stretching frequencies of several nickel(0)
monocarbonyl complexes with tridentate phosphine
ligands are listed in Table 1 for comparison. Complexes
2-6 exhibit the normal inverse correlation of ν(CO) with
ligand donor ability. In comparison, complex 1 has an
anomalously low ν(CO) value.
(15) Bianchini, C.; Zanobini, F.; Aime, S.; Gobetto, R.; Psaro, R.;
Sordelli, L. Organometallics 1993, 12, 4757-4763.
(16) Chatt, J.; Hart, F. A. J. Chem. Soc. Abstr. 1965, 812-813.
(17) See Experimental Section.
(18) Corain, B.; De Nardo, L.; Favero, G. J. Organomet. Chem. 1977,
125, 105-114.
(19) Olechowski, J. R. J. Organomet. Chem. 1971, 32, 269-271.
(20) Ghilardi, C. A.; Sabatini, A.; Sacconi, L. Inorg. Chem. 1976,
15, 2763-2767.
(21) Hausen, H. D.; Krogmann, K. Z. Anorg. Allg. Chem. 1972, 389,
247-253.
Single crystals of 1 were obtained by slow diffusion
of diethyl ether into a THF solution of 1. The crystal
structure of 1 (Figure 1) shows the pseudo-tetrahedral
nickel center, expected for a nickel(0) complex. The three
phosphorus atoms bind to the nickel atom facially, with
all three P-Ni-P angles approximately 90°. The Ni-C
(22) Baacke, M.; Morton, S.; Stelzer, O.; Sheldrick, W. S. Chem. Ber.
1980, 113, 1343-1355.
(23) Krueger, C.; Tsay, Y. H. Cryst. Struct. Commun. 1974, 3, 455-
458.
(24) Pickardt, J.; Roesch, L.; Schumann, H. Z. Anorg. Allg. Chem.
1976, 426, 66-76.
(25) Braga, D.; Grepioni, F.; Orpen, A. G. Organometallics 1993,
12, 1481-1483.
(26) Jenkins, D. M.; Di Bilio, A. J.; Allen, M. J.; Betley, T. A.; Peters,
J. C. J. Am. Chem. Soc. 2002, 124, 15336-15350.
(27) Betley, T. A.; Peters, J. C. Inorg. Chem. 2003, 42, 5074-5084.
(28) The calculation was performed on Titan1,0,8, with the LACVP*
basis set.
(14) In our attempts to prepare Ni(0)[BP3](CNxylyl)[Li(TMED)], we
were only able to isolate a mixture of Ni(I) and Ni(0) complexes. The
same observations were obtained by the Peters group (Jonas C. Peters,
personal communication).

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Anionic Zerovalent Nickel Carbonyl Complex
Organometallics, Vol. 24, No. 2, 2005 233
DFT calculation shows significant charge redistribution
from the nickel atom to the carbonyl oxygen, consistent
with this view. Complex [Ni(PhBP3)(CO)][Li(TMED)] (1)
represents the first example of Lewis acid O atom
complexation to a Ni(0) carbonyl ligand. These results
illustrate the powerful electron-donating nature of the
[PhBP3] ligand.
Experimental Section
General Methods. Unless noted otherwise, all manipula-
tions were performed under N2 atmosphere either by Schlenk
techniques or in an inert atmosphere glovebox. Solvents were
HPLC grade and dried over a two-column solid state purifica-
tion system. Triphos (triphos ) bis(2-diphenylphosphinoethyl)-
phenylphosphine) was purchased from Strem and was used
as received. Ni(COD)2 and [Li(TMED)][PhB(CH2PPh2)3] were
prepared by literature methods. 1H and 31P NMR spectra were
obtained using a Varian 400 spectrometer. NMR solvents were
1
obtained from Cambridge Isotope Laboratories. H chemical
shifts were referenced to internal solvent peaks. 31P chemical
shifts were referenced with respect to external 85% H3PO4.
Solid state FT-IR spectra were collected using a Mattson
Research series infrared spectrometer. Crystallography data
were collected by a Bruker SMART APEX CCD X-ray diffrac-
tometer. Elemental analyses were performed by H. Kolbe
Microanalytical Laboratories, Mu¨lheim.
Figure 2. Space-filling representation for complex [Ni-
(PhBP3)(CO)][Li(THF)3] (1).
[Ni(PhBP3)(CO)][Li(TMED)] (1). A toluene suspension of
Ni(COD)2 (138 mg, 0.5 mmol) was added slowly to the toluene
solution (30 mL) of [Li(TMEDA)][BPh(CH2Ph2P)3] (405 mg, 0.5
mmol). CO was then bubbled into the reaction mixture. The
color of the solution changed from orange-yellow to yellow. The
reaction mixture was allowed to stir for 12 h, and the resulting
yellow precipitate was collected by filtration and washed with
diethyl ether three times. The product was dried under
vacuum to give 195 mg of yellow powder. Yield: 44%.
Solid state IR showed intensive ν(CO) at 1812 cm-1, while
solution IR in THF showed intensive ν(CO) at 1869 cm-1
.
31P{1H} NMR (THF): δ 26.7 ppm. H NMR (d8-THF) δ 6.8-
7.6 (35H, br, C6H5); 1.1 (6H, br, BCH2); 2.3 (4H, s, -CH2- in
TMED); 2.2 (12H, s, -CH3 in TMED). 13C NMR (d8-THF) δ
210.6 (br, Ccarbonyl). Anal. Calcd for C52H57BLiN2NiOP3: C,
69.75; H, 6.42; N, 3.13; P, 10.37. Found: C, 69.88; H, 6.37; N,
3.17; P, 10.23.
1
Figure 3. Depiction of the highest occupied molecular
orbital (HOMO) of the anion [Ni(PhBP3)(CO)]- of complex
1 computed in a DFT calculation.
Vapor diffusion of diethyl ether into the THF solution of
complex 1 yielded single crystals suitable for X-ray crystal-
lography.
Ni(triphos)(CO) (4). A toluene suspension of Ni(COD)2 (138
mg, 0.5 mmol) was added slowly to the toluene solution (30
mL) of triphos (267 mg, 0.5 mmol). CO was then bubbled into
the reaction mixture. The yellow reaction solution was allowed
to stirr for 12 h, and the resulting yellow precipitate was
collected by filtration and washed with diethyl ether three
times. The product was dried under vacuum to give 165 mg of
yellow powder. Yield: 53%.
agreement between the theoretical structure of 1 and
the crystal structure that was determined experimen-
tally.29 The calculated atomic charge on the carbonyl
oxygen is -0.52. The calculated charge on the nickel
atom is +0.75, suggesting significant back-bonding,
which corroborates well with the short Ni-C bond
distance and low CO stretching frequency determined
experimentally. The electronic structure of 1 determined
by DFT also suggests that the HOMO orbital is mainly
composed of the CO antibonding orbital and the nickel
Solid state IR of complex 4 showed intensive ν(CO) at 1903
1
cm-1
.
31P{1H} NMR (THF): δ 48.9 (d), 64.9 (t). H NMR (d8-
THF) δ 7.0-7.9 (25H, br, C6H5); 1.6-2.4 (8H, br, PCH2CH2P).
13C NMR (d8-THF) δ 206.9 (m, Ccarbonyl). Anal. Calcd for C35H33-
NiOP3: C, 67.73; H, 5.36; P, 14.96. Found: C, 67.61; H, 5.40;
P, 14.95.
2
2
dπ orbitals consisting of dx -y , dxy, and dxz, which back-
bond strongly to the CO π* orbital (Figure 3).
Conclusions
Acknowledgment. We gratefully acknowledge the
DOE (DE-FG03-99ER14992) for support.
An anionic zerovalent nickel carbonyl complex with
a [PhBP3] backbone was synthesized and characterized.
The crystal structure shows a lithium cation binding to
the carbonyl oxygen atom in an isocarbonyl linkage. A
Supporting Information Available: Tables of selected
bond distances and angles of calculated and experimental
results, crystallographic data for 1. This material is available
free of charge via the Internet at http://pubs.acs.org.
(29) See Supporting Information: Table 1 Selected bond distances
and angles of calculated and experimental results.
OM049601F