Exploring the reactivity of four-coordinate PNPCoX with access to three-coordinate spin triplet PNPCo

Article abstract of DOI:10.1021/ic701171p

The compounds (PNP)CoX, where PNP is (^tBu₂PCH ₂SiMe₂)₂N^- and X is Cl, I, N ₃, OAr, OSO₂CF₃ and N(H)Ar, are reported. Some of these show magnetic susceptibility, color, and ¹H NMR evidence of being in equilibrium between a blue, tetrahedral S = 3/2 state and a red, planar S = 1/2 state; the equilibrium populations are influenced by subtle solvent effects (e.g., benzene and cyclohexane are different), as well as by temperature. Attempted oxidation to Co(III) with O₂ occurs instead at phosphorus, giving [P(O)NP(O)]CoX species. The single O-atom transfer reagent PhI=O likewise oxidizes P. Even I₂ oxidizes P to give the pendant phosphonium species (^tBu₂P(I)CH₂SiMe ₂-NSiMe₂CH₂P^tBu₂)CoI ₂ with a tetrahedral S = 3/2 cobalt; the solid-state structure shows intermolecular PI...ICo interactions. Attempted alkyl metathesis of PNPCoX inevitably results in reduction, forming PNPCo, which is a spin triplet with planar T-shaped coordination geometry with no agostic interaction. Triplet PNPCo binds N₂(weakly) and CO (whose low CO stretching frequency indicates strong PNP → Co donor power), but not ethene or MeCCMe.

Full text of DOI:10.1021/ic701171p

Inorg. Chem. 2007, 46, 10321−10334
Exploring the Reactivity of Four-Coordinate PNPCoX with Access to
Three-Coordinate Spin Triplet PNPCo
Michael J. Ingleson, Maren Pink, Hongjun Fan, and Kenneth G. Caulton*
Department of Chemistry, Indiana UniVersity, Bloomington, Indiana 47405
Received June 15, 2007
The compounds (PNP)CoX, where PNP is (tBu2PCH2SiMe2)2N- and X is Cl, I, N3, OAr, OSO2CF3 and N(H)Ar, are
1
reported. Some of these show magnetic susceptibility, color, and H NMR evidence of being in equilibrium between
a blue, tetrahedral S
) 3/2 state and a red, planar S ) 1/2 state; the equilibrium populations are influenced by
subtle solvent effects (e.g., benzene and cyclohexane are different), as well as by temperature. Attempted oxidation
to Co(III) with O2 occurs instead at phosphorus, giving [P(O)NP(O)]CoX species. The single O-atom transfer reagent
PhI
NSiMe2CH2PtBu2)CoI2 with a tetrahedral S
d
O likewise oxidizes P. Even I2 oxidizes P to give the pendant phosphonium species (tBu2P(I)CH2SiMe2-
)
3/2 cobalt; the solid-state structure shows intermolecular PI‚‚‚ICo
interactions. Attempted alkyl metathesis of PNPCoX inevitably results in reduction, forming PNPCo, which is a spin
triplet with planar T-shaped coordination geometry with no agostic interaction. Triplet PNPCo binds N2(weakly) and
CO (whose low CO stretching frequency indicates strong PNP
f Co donor power), but not ethene or MeCCMe.
Introduction
polymerization, cyclopropanation/aziridination/oxidation, and
group- or atom-transfer reactivity.10,11 Maintainence of a low
metal-coordination number (vs dimerization or oligomeriza-
tion) generally relies on bulky ligand substituents, which in
turn influence reaction regiochemistry and substrate selectiv-
ity. Among the ligands studied, pincer ligands have appeal
for relative ease of synthesis, modular character which permit
systematic optimization, and a variety of spectroscopic
probes, thus being especially “communicative”. Pincer
ligands which are conjugated, either electron rich or electron
poor, also show evidence of delocalization of their frontier
orbitals when the metal/ligand complex is brought to low or
high oxidation states, thus threatening the simple concept
of metal oxidation state.12-15
Interest in the developement of multidentate ligands
originates from the fact that ligand characteristics, as much
as the behavior inherent to the metal itself, dictate the
chemical reactivity of transition metal complexes. One goal
in ligand choice is to furnish multiple available coordination
sites, resulting in space for more than one substrate molecule
and, thus, in the substrate being coordinated in an essentially
“barrierless” Lewis acid/base reaction. For the later transition
elements, the question of a “high-spin” versus “low-spin”
ground state arises, and therefore, the possible influence of
spin state on chemical reactivity (both rate and thermody-
namics) is also in question. For a variety of reasons, there is
interest in the later 3d elements, with a goal of characterizing
compounds with singly (M-R), doubly (MdCR2, MdNR,
and MdO), and triply bonded (M≡CR, MtN) ligands.1-9
These are of interest as possible intermediates in olefin
(5) Hawrelak, E. J.; Bernskoetter, W. H.; Lobkovsky, E.; Yee, G. T.; Bill,
E.; Chirik, P. J. Inorg. Chem. 2005, 44, 3103.
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2006, 128, 5302.
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(8) Matsunaga, P. T.; Hess, C. R.; Hillhouse, G. L. J. Am. Chem. Soc.
1994, 116, 3665.
(9) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 10782.
(10) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283.
(11) Amisial Latasha, D.; Dai, X.; Kinney, R. A.; Krishnaswamy, A.;
Warren Timothy, H. Inorg. Chem. 2004, 43, 6537.
(12) Knijnenburg, Q.; Hetterscheid, D.; Kooistra, T. M.; Budzelaar, P. H.
M. Eur. J. Inorg. Chem. 2004, 1204.
* To whom correspondence should be addressed. E-mail: caulton@
indiana.edu.
(1) Jenkins, D. M.; Di Bilio, A. J.; Allen, M. J.; Betley, T. A.; Peters, J.
C. J. Am. Chem. Soc. 2002, 124, 15336.
(2) Schebler, P. J.; Riordan, C. G.; Guzei, I. A.; Rheingold, A. L. Inorg.
Chem. 1998, 37, 4754.
(3) Knijnenburg, Q.; Horton, A. D.; van der Heijden, H.; Kooistra, T.
M.; Hetterscheid, D. G. H.; Smits, J. M. M.; de Bruin, B.; Budzelaar,
P. H. M.; Gal, A. W. J. Mol. Catal. A 2005, 232, 151.
(4) Bart, S. C.; Hawrelak, E. J.; Lobkovsky, E.; Chirik, P. J. Organome-
tallics 2005, 24, 5518.
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Q.; Budzelaar, P. H. M.; Gal, A. W. Inorg. Chim. Acta 2004, 357,
2945.
10.1021/ic701171p CCC: $37.00
Published on Web 10/23/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 24, 2007 10321

Ingleson et al.
1
Figure 1. (Left) H NMR (298 K) of complex 1 in C6D6 and in C6D12; impurity peaks observed in both spectra can be assigned to residual solvent or
silicone grease. (Right) ORTEP view (50% probabilities) of complex 1 (hydrogens are omitted for clarity). Selected bond lengths (Å) and angles (deg):
Co-N ) 1.9223(11), Co-Cl ) 2.2074(4), Co-P1 ) 2.2898(4), Co-P2 ) 2.2802(4), P1-Co-P2 ) 177.414(15), N-Co-Cl ) 178.26(4), P1-Co-Cl )
90.671(14).
The pincer ligand chosen in this work (tBu2PCH2SiMe2)2N-1
has been previously demonstrated to carry enough bulk on
phosphorus that it prohibits dimerization of (PNP)RuX by
µ-X behavior, and thus enables the study of exceptionally
low coordination numbers.16-18 In this sense, PNP shares
some advantages of â-diketiminates, HC(CRNR′)2-1, when
R and R′ are bulky.19 However, because amide is one of the
most potent π-donor ligands, PNP also makes a metal to
which it is bound a stronger π-base, and indeed, highly
unsaturated (PNP)RuX has demonstrated exceptional reduc-
ing ability for a d6, RuII species. Indeed, it can even be
oxidized by 1-electron transfer to form isolable RuIII spe-
cies.16 It thus becomes of interest to study the physical
properties and reactivity characteristics of 3d metals when
bound to PNP.20,21 These metals are less expensive than Ru,
they frequently show more rapid kinetics than 4d or 5d
analogs, they often disregard the 18-electron rule, they have
smaller d-orbital splittings, which often yield non-singlet spin
states, they can prefer 1-electron (vs 2e for 4d and 5d
analogs) redox changes, and their smaller size should
aggravate the effects of steric bulk. The PNP ligand was thus
anticipated to be a sufficiently potent donor that 1) 3-coor-
dinate Co could be achieved and isolated and 2) the resulting
species, even if far short of an 18 valence electron count,
would be a reducing agent. In the work presented here, each
of these has been Verified, but with obvious subtle variations.
Presented here is a general view of the chemistry of (PNP)-
CoX species. A portion of this work has been previously
communicated.22,23
in saturated hydrocarbon solvents (pentane or cyclohexane)
resulted in red solutions. SolVatochromism is a common
indicator of a change in coordination number at the metal
center, often caused by solvent coordination (and less
frequently dimerization);24-26 to establish if this was the
situation with this system, an excess (5-10 equiv) of THF
(or C6H6) was added to a red pentane solution. This resulted
in no significant color change, precluding solvent coordina-
tion. Dimerization was disfavored because of the substantial
t
steric bulk provided by the Bu groups. A spin crossover
between tetrahedral high-spin and square planar low-spin
forms is an alternative, albeit a less common, phenomenon
to account for the observed solvatochromism. Spin-state
crossover has been previously suggested to be the origin of
thermochromism in the closely related compound PhPNPNiCl
(PhPNP ) [(Ph2PCH2SiMe2)2N]), and square planar cobalt
complexes ligated by the PhPNP ligand are known, although
they require stronger alkyl σ-donors to enforce spin pairing.27
An X-ray diffraction study on the red crystals formed from
cooling of a blue toluene solution revealed a monomeric
complex, PNPCoCl, 1 (Figure 1) in a square planar environ-
(14) Enright, D.; Gambarotta, S.; Yap, G. P. A.; Budzelaar, P. H. M. Angew.
Chem., Int. Ed. 2002, 41, 3873.
(15) Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky,
E.; Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2006,
128, 13901.
(16) Ingleson, M. J.; Pink, M.; Huffman, J. C.; Fan, H.; Caulton, K. G.
Organometallics 2006, 25, 1112.
(17) Walstrom, A.; Pink, M.; Tsvetkov, N. P.; Fan, H.; Ingleson, M.;
Caulton, K. G. J. Am. Chem. Soc. 2005, 127, 16780.
(18) Ingleson, M. J.; Yang, X.; Pink, M.; Caulton, K. G. J. Am. Chem.
Soc. 2005, 127, 10846.
(19) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. ReV. 2002,
Results and Discussion
102, 3031.
(20) Ozerov, O. V.; Guo, C.; Fan, L.; Foxman, B. M. Organometallics
2004, 23, 5573.
(21) Ozerov, O. V.; Guo, C.; Foxman, B. M. J. Organomet. Chem. 2006,
691, 4802.
(22) Ingleson, M. J.; Pink, M.; Caulton, K. G. J. Am. Chem. Soc. 2006,
128, 4248.
(23) Ingleson, M.; Fan, H.; Pink, M.; Tomaszewski, J.; Caulton, K. G. J.
Am. Chem. Soc. 2006, 128, 1804.
(24) El-Ayaan, U.; Murata, F.; Fukuda, Y. Monatsh. Chem. 2001, 132,
1279.
(25) Bhadbhade, M. M.; Srinivas, D. Inorg. Chem. 1993, 32, 6122.
(26) Heinze, K.; Huttner, G.; Zsolnai, L. Chem. Ber./Recl. 1997, 130, 1393.
(27) Fryzuk, M. D.; MacNeil, P. A.; Rettig, S. J.; Secco, A. S.; Trotter, J.
Organometallics 1982, 1, 918.
Synthesis and Solid-State Structures of PNPCoX (X )
Cl, I, N3, NH(2,6-Me2-C6H3)). The combination of equimo-
lar quantities of PNPMgCl/dioxane and anhydrous CoCl2 in
THF results, after 18 h, in the formation of a deep-blue
solution, characteristic of a high-spin CoII center in a
tetrahedral environment. Recrystallization from deep-blue
THF (or toluene) solutions, however, yielded red crystalline
solids that upon being redissolved in THF or benzene became
deep-blue solutions again; this process could be repeated
numerous times. In contrast, dissolution of these red crystals
10322 Inorganic Chemistry, Vol. 46, No. 24, 2007

ReactiWity of Four-Coordinate PNPCoX
Figure 2. Graphical representation of the variation of solution magnetic moment with temperature for 1.
ment (angles around cobalt equal 360°), indicating that spin
crossover is the origin of the two observed species/colors.
Structurally characterized square planar complexes of CoII-
Cl are rare, generally requiring stronger field ligands. A
Cambridge Structural Database search revealed only one
other square planar Co(II) complex with terminal chlorides,
[Co2Cl2(C8H4O4)(C12H8N2)2], which is geometrically very
similar to 1.28 The only major point of note in the solid-
state structure of 1 is the twisting of the Si-N-Si backbone
(dihedral angles P1-Co-N-Si1, 28.70(6)°; P2-Co-N-
Si2, 23.80(7)°) to orient the amide lone pair away from an
occupied cobalt d-orbital. Comparison of the structural
metrics between the square planar complexes 1 and PhPNPCo-
(CH2Ph) (PhPNP ) [Ph2PCH2SiMe2]2N-), where the benzyl
group is unambiguously η1, reveals a number of minor
structural differences:29 (i) The Co-P bond distances in 1
(2.2802(4) and 2.2898(4) Å) are longer than those in
PhPNPCo(CH2Ph) (2.2127 and 2.2162 Å), possibly resulting
from the increased steric requirements of the bulkier phos-
the spin-only value (1.73 µB) and the values reported29 for
the square planar PhPNPCoR series (R ) Me, CH2Ph, CH2-
SiMe3; 1.9-2.2 µB) is comparable to the solution magnetic
moment recorded31 for monomeric low-spin PhB(CH2PPh2)3-
CoI (2.87 µB). The value of 2.85 µB suggests that the low
spin form of 1 is, at least, the dominant species in cyclo-
hexane solutions, in sharp contrast to that in C6H6 solutions.
1
A comparison of the H NMR spectra of 1 in C6D12 and
C6D6 (Figure 1) reveals a drastic change in the chemical shifts
of the three resonances, with the CH2 protons (proximal to
the paramagnetic cobalt) moving by more than 14 ppm
t
because of the change in solvent; the SiMe2 and Bu
resonances shift to a lesser degree. The stepwise addition of
an increasing volume of THF to a C6D12 solution of 1 leads
1
to a gradual shift of all the resonances of 1 in the H NMR
spectra toward the spectra observed in neat benzene (Figure
1 top), and ultimately, at a volume ratio of 2:1 (THF/C6D12),
1
the H NMR spectra is effectively identical to that of 1 in
pure THF-d8 (and it is also blue).
t
phine Bu substituents. The P-Co bond distances are also
Surprisingly, the solution magnetic moment of 1 in
toluene-d8 (3.21 µB) was significantly lower than that found
in benzene (3.96 µB), implying a higher concentration of the
low-spin isomer (the solution magnetic moments were
consistent over a number of independently prepared samples).
At present, we are unsure why solvents with such similar
properties (benzene and toluene) should stabilize different
spin states to such a degree. Cooling of a sample of 1 in
toluene-d8 in 10° steps from 298 to 210 K resulted in the
expected thermochromism, with a color change from blue
to red observed and a gradual decrease in the solution
magnetic moment to a minimum of 2.9 µB at 210 K (Figure
2). This variation in magnetic moment is fully reversible.
The chemical shift of the CH2 protons (most sensitive to
change in metal spin state) over this temperature range travels
upfield significantly from the initial position (27.1 ppm at
298 K). At 210 K, the slow-exchange regime is not reached,
although the resonances have all broadened dramatically,
with the CH2 resonance is no longer visible. No spin
crossover is observed in the solid state: heating of a red
powdered sample of 1 in a sealed capillary results in no color
change; it decomposes at 110 °C to an intractable mixture.
It is important to note that the phenyl derivative PhPNPCoCl
is rigorously high spin,29 while the tBu derivative 1 appears
notably longer in 1 than those in (PEt2Ph)2Co(mesityl)2
(2.232(4) Å).30 (ii) Compound 1 is significantly closer to
the ideal square planar geometry (P-Co-P ) 177.41(1) and
N-Co-Cl ) 178.26(4)°) than PhPNPCoCH2Ph (P-Co-P
) 162.93(3), N-Co-C ) 173.6(1)°), although the deforma-
tion may be caused by the bulkier benzyl group rather than
any specific effects from the change of the PNP ligand. The
Co-N distances vary as expected because of the respective
ligand field strength of the anionic substituents. Unfortu-
nately, despite repeated attempts under various conditions,
crystals of the blue, presumably the high-spin tetrahedral
form of PNPCoCl were not obtainable.
Solution studies (298 K, C6D6) revealed a compound that,
on the NMR time scale, has C2V symmetry (as determined
by only three distinct well-defined, paramagnetically shifted,
1
and broadened resonances in H NMR spectrum; Figure 1)
and a solution magnetic moment (Evans method) at 3.96 µB
supporting the high-spin (S ) 3/2) assignment suggested by
the color. In contrast, the solution magnetic moment in C6D12
is 2.85 µB. This value, while significantly higher than both
(28) Xiao, H. P.; Hu, M. L.; Li, X. H. Acta Crystallogr., Sect. E 2004,
E60, m336.
(29) Fryzuk, M. D.; Leznoff, D. B.; Thompson, R. C.; Rettig, S. J. J. Am.
Chem. Soc. 1998, 120, 10126.
(30) Falvello, L.; Gerloch, M. Acta Crystallogr., Sect. B 1979, 35, 2547.
(31) Jenkins, D. M.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 7148.
Inorganic Chemistry, Vol. 46, No. 24, 2007 10323

Ingleson et al.
four tBu groups, with the structural differences reducing the
larger steric repulsion between tBu groups. The assumption
of a similar structure for high-spin 1 leads to the conclusion
that the increased stability of the low-spin isomer observed
for 1 compared to that of PhPNPCoCl is dominated by the
distal phosphine steric bulk, with the square planar structure
stabilized because it enables a strict trans-phosphine disposi-
t
tion, thereby minimizing the steric repulsion between Bu
groups (corroborated by the significantly smaller minimum
t
C-C contact observed between Bu carbons of “trans”-
disposed phosphines in high-spin 2 (3.785 Å) compared to
that in low-spin 1 (4.545 Å). This effect more than offsets
the contraction of the P-Co bonds inherent in moving from
high-spin Co(II) to low-spin Co(II). The effects of the
increased steric pressure present in 2 (and by inference in
the high-spin isomer of 1) may also prevent the high-spin
structure from approaching the minimum energy conforma-
tion around cobalt, in comparison to the sterically unhindered
phenyl derivative, resulting in a higher-energy quartet state.
An electronic effect caused by the presence of the stronger
σ-donor alkyl phosphine in 1 may also contribute to
stabilization of the square planar isomer, with an increased
orbital splitting arising from the stronger-field phosphine
ligand (analogous to square planar (PCy3)2NiCl2 and tetra-
hedral (PPh3)2NiCl2).5
The solution magnetic moment at 298 K for 2 is
unambiguously high spin in both C6D6 (4.54 µB) and C6D12
(5.05 µB); furthermore, cooling of a toluene-d8 solution of 2
resulted in no observable thermochromism. The magnetic
moments of 2 compare closely to other high-spin Co(II)
complexes previously reported, for example, Tp′′CoI (5.0
µB, where Tp′′ ) tris(3-iPr, 5Me pyrazolylborate))32 and PhB-
(CH2StBu)3CoCl (4.5 µB).2 Therefore, substitution of the
chloride in 1 for the weaker-field iodide ligand destabilizes
the low-spin form enough for it to be no longer detectably
populated. Furthermore, the solution magnetic moments for
PNPCoI in C6D6 (and in C6D12) are consistently higher than
that for PNPCoCl, in contrast to the PhPNP ligated series,
PhPNPCoX (X ) Cl, Br, and I), where the solution magnetic
moments are all within 4.2 ( 0.1 µB;29 this implies that there
is a non-negligible amount of the low-spin 1 present upon
dissolution in C6D6/THF and is consistent with the crystal-
lization of low-spin 1 from THF solutions.
Figure 3. (Above) ORTEP view (50% probabilities) of complex 2. (Below)
Selected bond lengths and angles for 2 and the corresponding values for
the related phenyl substituted phosphine complex previously synthesized
by Fryzuk et al.
to be on an energetic cusp between high spin and low spin,
with subtle solvent changes (benzene to toluene) sufficient
to significantly perturb the equilibrium position.
DFT calculations, using the continuum solvent model,
show that the two spin states are very similar in free energy.
While the low-spin state is favored (∆Gï) by 4.9 kcal/mol
in the gas phase or by 4.0 kcal/mol in cyclohexane, this
decreases to 2.9 kcal/mol in THF (still favoring low spin).
In an attempt to produce crystalline material analogous to
high-spin 1 that may afford insight into the contrasting
behavior of 1 compared to its phenyl derivative, PNPCoI, 2
was synthesized in reasonable yield as a turquoise crystalline
solid. Isolated yields for 2 (and other PNP ligated complexes)
are consistently lower than optimal because of the extremely
high solubility imparted by the lipophilic PNP ligand. An
X-ray diffraction study revealed (Figure 3) monomeric
PNPCoI, with an environment around cobalt significantly
distorted from ideal tetrahedral toward a cis-divacant octa-
hedron (Figure 3). The bond lengths to cobalt are consistently
longer in 2 than in 1, as expected with the change in spin
state. The structural characterization of 2 allows for an
examination into the effect that the change of the phosphine
substituent on the PNP ligand has on the solid-state structure
by comparison with the previously determined29 structure
of PhPNPCoI. Upon inspection, there are three significant
differences: (i) The Co-P bond lengths in 2 are significantly
longer than that for the phenyl derivative. (ii) The P-Co-P
angle in 2 is larger (by ∼15°). (iii) The N-Co-I angle in
complex 2 is smaller (by 8.3°). The first two effects are a
direct result of the increased steric bulk generated by the
(PNP)Co(N3). It seemed prudent for comparison purposes
(especially of solution magnetic moments) to synthesize a
related complex that was exclusively low spin.33 This was
achieved by the reaction of 1 with 10 equiv of NaN3, which
resulted in a color change from deep blue to terracotta, after
the mixture was stirred overnight in THF, consistent with a
CoII spin change from high spin to low spin. PNPCoN3, 3,
was isolated in reasonable yield as a microcrystalline red
powder that possessed time-averaged C2V symmetry and a
solution magnetic moment of 2.47 µB (C6D6, 298 K).
Solutions of 3 behave as strictly low spin, independent of
(32) Reinaud, O. M.; Rheingold, A. L.; Theopold, K. H. Inorg. Chem. 1994,
33, 2306.
(33) Rheingold, A. L.; Liable-Sands, L. M.; Golan, J. A.; Trofimenko, S.
Eur. J. Inorg. Chem. 2003, 2767.
10324 Inorganic Chemistry, Vol. 46, No. 24, 2007

ReactiWity of Four-Coordinate PNPCoX
range of 123-130°), thereby increasing amido ortho-methyl-
t
phosphine Bu separation. (3)The P-Co bonds in 4 (Co-
P1, 2.4966(9) Å; Co-P2, 2.5209(10) Å) are longer than those
in 2 (Co-P1, 2.4237(5) Å; Co-P2, 2.4601(5) Å). Thus the
PNP ligand has some degree of flexibility and can undergo
minor structural rearrangements, as opposed to spin isomer-
ism, to reduce steric crowding.37 The final noteworthy point
is from comparison of the two Co-amido distances in 4,
which reveals a slightly longer PNP backbone Co-N
distance; this we attribute to the constraint inherent in the
two fused five-membered rings of the PNP ligand rather than
any multiple bonding between Co-N2 (further supported
by the effectively identical distances of Co-N2 in 4 (1.916-
(2) Å) and Co-N1 in 2 (1.9391(5) Å)).
Figure 4. Synthesis of low-spin PNPCoN3 (3) from complex 1 by addition
of excess NaN3.
The 1H NMR of complex 4 reveals solution C2V symmetry
and displays isotropically shifted resonances for SiMe, tBu,
CH2, and phenyl ortho-methyls; despite numerous attempts,
the resonance for the phenyl protons and N-H were not
detectable. Furthermore, the N-H stretch was not observable
in the infrared spectrum. The similarity of the solid-state
structure of 4 to that found for complex 2 confirms its identity
as a Co(II) amido complex, with a possible Co(III) imido
complex expected to have a significantly different structure
and magnetic moment. It is also noteworthy that despite the
reducing ability of lithium amides, there is no evidence for
any cobalt reduction, with salt metathesis the only reaction
observed here.
Figure 5. ORTEP view (50% probabilities) of complex 4 (most hydrogens
are omitted for clarity). Selected bond lengths (Å) and angles (deg): Co-
N1 ) 1.995(2), Co-N2 ) 1.916(2), Co-P1 ) 2.4966(9), Co-P2 ) 2.5209-
(10), P1-Co-P2 ) 141.61(3), N1-Co-N2 ) 119.46(10), Co-N2-C23
) 139.6(2), N1-Co-P1 ) 91.84(7), N2-Co-P1 ) 106.44(8)
solvent and temperature (to 80 °C). The solution magnetic
moment of 3 at 2.47 µB, while still greater than the spin-
only value (a common occurrence for low-spin Co(II)
compounds), is consistent with the presence of only low-
spin 3. Comparison to the value for 1 in C6D12 (2.85µB),
suggests that cyclohexane solutions of 1 are only predomi-
nated by low-spin 1; the amount of high-spin isomer is non-
negligible. Attempts to produce 3 from the reaction of 1 and
stoichiometric or excess TMSiN3 (TMS ) trimethylsilyl) in
benzene failed, with 1 recovered unreacted.
With the full characterization and elucidation of the
solution behavior of these basic PNPCoX complexes in hand,
their chemistry was explored.
Oxidation Reactions of PNPCoX. Formation of P(O)-
NP(O)CoX. Compounds 1, 2, and 3 all proved to be highly
oxygen sensitive (and to a lesser degree moisture sensi-
tive).38-40 The addition of dry O2 to a degassed benzene
solution of 1 resulted in an immediate color change from
deep blue to light blue. Inspection of the 1H NMR spectrum
revealed complete consumption of 1 and the appearance of
six new paramagnetic signals that, by their relative intensities
(18:18:6:6:2:2), can be assigned to a single new Cs symmetric
product. It is noteworthy that nonplanar (PNP)CoX com-
pounds show apparent C2V symmetry because of the rapid
(at 25 °C) achievement of a planar structure in a fluxional
process. In [P(O)NP(O)]CoX species, any such inversion has
a higher barrier because all diastereotopic substituents show
separate signals at 25 °C. This is attributed to the larger ring
size, making nonplanar structures much more favorable than
planar structures. Mass spectroscopy confirmed the addition
of two atoms of oxygen, while the solution magnetic moment
(4.50 µB) supported the presence of three unpaired electrons,
(PNP)Co[NH(2,6-Me2-C6H3)]. It was also of interest to
examine the effect that the substitution of halide in 1 for a
more sterically demanding, yet weaker ligand field (i.e., a
high-spin directing ligand), anionic ligand would have on
the resultant structure. Would sterics or electronics dominate
the energetics of the structure? The amido complex, PNP-
CoN(H)-2,6-Me2-C6H3, 4, is synthesized by reaction of 1
with a stoichiometric equivalent of LiN(H)-2,6-Me2-C6H3 in
THF, with a rapid color change from deep blue to purple
observed. Standard workup and recrystallization from mini-
mum pentane at -40 °C allowed for unambiguous charac-
terization as the high-spin isomer (Figure 5). The gross
structure around cobalt is comparable for complexes 2 and
4, implying similar electronic structures; this is supported
by the observed high-spin solution magnetic moment (4.28µB,
298 K, Evans method). There are a number of subtle changes
in the ligand geometry that act to reduce the steric impact
(34) Brown, S. D.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 4538.
(35) Cowan, R. L.; Trogler, W. C. J. Am. Chem. Soc. 1989, 111, 4750.
(36) Villanueva, L. A.; Abboud, K. A.; Boncella, J. M. Organometallics
1994, 13, 3921.
(37) Hope, H.; Olmstead, M. M.; Murray, B. D.; Power, P. P. J. Am. Chem.
Soc. 1985, 107, 712.
(38) Hu, X.; Castro-Rodriguez, I.; Meyer, K. J. Am. Chem. Soc. 2004, 126,
13464.
(39) Egan, J. W., Jr.; Haggerty, B. S.; Rheingold, A. L.; Sendlinger, S. C.;
Theopold, K. H. J. Am. Chem. Soc. 1990, 112, 2445.
(40) Betley, T. A.; Peters, J. C. Inorg. Chem. 2003, 42, 5074.
t
between the amido ortho-methyls and ligand butyl groups:
(1) An increase in the P-Co-P angle in 4 (141.61(3)°)
compared to 2 (135.557(18)°) to increase interphosphine
spacing. (2) The Co-N2-C23 angle (139.6(2)°) is greater
than systems34-36 with low steric crowding (angles in the
Inorganic Chemistry, Vol. 46, No. 24, 2007 10325

Ingleson et al.
Figure 6. (Left) Synthesis of compounds 5-7 by action of O2 on precursors 1-3. (Right) ORTEP view (50% probabilities) of one of the molecules of
complex 7 found in the asymmetric unit (hydrogens are omitted for clarity). Selected bond lengths (Å) and angles (deg): Co-N1 ) 1.981(2), Co-N2 )
1.979(2), Co-O1 ) 1.9797(18), Co-O2 ) 1.9906(18), P1-O1 ) 1.5144(18), N2-N3 ) 1.196(3), N3-N4 ) 1.161(3), O1-Co-O2 ) 109.01(8), N1-
Co-O1 ) 104.53(8), N1-Co-N2 ) 127.50(10), Co-N2-N3 ) 125.5(2), N2-N3-N4 ) 176.5(3).
consistent with a high-spin, S ) 3/2 Co(II) center. This leads
to a product assignment where the PNP ligand has undergone
a four-electron oxidation to give (tBu2P(dO)CH2SiMe2)2-
NCoCl, 5 (Figure 6). Analogous reactivity was observed for
2 and 3 to yield (tBu2P(dO)CH2SiMe2)2NCoI, 6, and
(tBu2P(dO)CH2SiMe2)2NCoN3, 7, respectively. Single crys-
tals suitable for an X-ray diffraction study were obtained by
the slow evaporation of a benzene solution of 7 and
confirmed a Co(II) tetrahedral complex with both phosphines
oxidized. The asymmetric unit contains two independent
molecules of 7 that are effectively identical; the structural
metrics of only one will be discussed here. The greater
flexibility of the two fused six-membered rings (vs five in
3), combined with the weaker ligand field experienced by
cobalt in 7, allows for a geometry around cobalt closely
approaching ideal tetrahedral, considerably more so than that
observed in 2 (O1-Co-O2 ) 109.01(8)° in 7 compared to
P1-Co-P2 ) 135.557(18)° in 2); the only significant
deviation from ideal tetrahedral geometry is in the N1-Co-
N2 angle, 127.50(10)°. The Co-N(azide) bond in 7 is
elongated (Co-N2 ) 1.979(2) Å) compared to those of the
other structurally characterized four-coordinate cobalt(II)
azide complexes, Tp′CoN3 (Co-N ) 1.911(2) Å)33 and
[(TIMEN)CoN3][BPh4]41 (Co-N ) 1.938(2) Å). The re-
maining bond distances and angles are unremarkable when
compared to other tridentate phosphine-oxide cobalt(II)
complexes.
of benzyl halides,17 we investigated the reactivity of 1 toward
benzyl halides. No reaction was observed between 1 and
stoichiometric or excess PhCH2X (X ) Cl or Br), even after
prolonged periods (96 h) or at raised temperatures (18 h at
60 °C), with both 1 and benzyl halide recovered unreacted.
This lack of reactivity is in sharp contrast to the phenyl
derivative and is paradoxical with the convention that the
superior σ-donor phosphine present in 1 would be able to
better stabilize the higher oxidation state of CoIII. The
difference in the observed reactivity might be attributable
to distinct ground spin states in the respective CoIII products
because disparate ground-state electronic structures are found
for the cobalt(II) chloride starting materials. It is doubtful
that the negative result is caused by the enhanced steric
t
shielding provided upon moving from phenyl to Bu sub-
stituents on the phosphine, as will be demonstrated later
where reactivity occurs with equally bulky reagents. An
equally counterintuitive trend was observed by Peters et al.31
for the oxidation potentials of CoII in PhB(CH2PR2)3CoI (R
i
) Ph or Pr): the less electron-donating phenyl derivative
proved to be significantly easier to oxidize than the more
i
electron-rich Pr analogue.
Despite the above, chloride 1 rapidly reacted with 1 equiv
of I2 in benzene to give a number of new paramagnetic
1
compounds (by H NMR). Suspecting that the multiple
products observed would be in part caused by mixed halide
isomers, we attempted the reaction between 2 and I2. This
produced one new paramagnetic product with Cs symmetry
on the NMR time scale. This complex proved to be only
sparingly soluble in benzene, with light blue crystals
spontaneously precipitating out of the synthetic solution. An
X-ray diffraction study revealed the unanticipated formation
of a zwitterionic, tetrahedral, four-coordinate CoII complex,
[κ2-(tBu2P(I)CH2SiMe2NSiMe2CH2PtBu2)CoI2, 8 (Figure 7).
In 8, one phosphine has been oxidized to a phosphonium
cation and is no longer coordinated to cobalt; the resultant
cobalt coordination sphere is now occupied by one phos-
phine, one amide, and two iodides, carrying an overall
negative charge to balance the phosphonium cation. There-
fore, I2 has successfully oxidized 2, but the locus of oxidation
is at phosphorus, not cobalt. In 2, therefore, phosphorus is
In an attempt to observe any intermediates in the reaction
between 1 and O2, the reagents were combined in toluene-
d8 at -78 °C, resulting in an immediate reaction (judged by
a rapid color change). The 1H NMR even at this low
temperature showed complete consumption of 1, with only
resonances attributable to 5 visible. Complex 5 can alterna-
1
tively be synthesized quantitatively (by H NMR) from 1
by the addition of 2 equiv of iodosobenzene (PhIdO),
producing iodobenzene as a byproduct (by 1H NMR).
Complex 1 however does not react with N2O (18 h, 25 °C).
Complexes 5-7 undergo no reaction upon standing under
an atmosphere of dry O2 for 24 h. Therefore O2 is not a
strong enough oxidant to oxidize cobalt(II) in 5-7.
Oxidation of PNPCoX with R-X and I2. Following the
work of Fryzuk et al., where PhPNPCo(II) halides are
successfully oxidized to PhPNPCo(III) dihalides by the action
(41) Hu, X.; Meyer, K. J. Am. Chem. Soc. 2004, 126, 16322.
10326 Inorganic Chemistry, Vol. 46, No. 24, 2007

ReactiWity of Four-Coordinate PNPCoX
Figure 7. (Left) ORTEP view of complex 8 (hydrogens and a disordered solvent molecule omitted for clarity). (Right) The intermolecular I‚‚‚I interaction
between I3 and I2′. Selected bond lengths (Å) and angles (deg): N1-Co ) 1.957(4), Co-I1 ) 2.5973(9), Co-I2 ) 2.6279(9), Co-P1 ) 2.3594(14),
P2-I3 ) 2.4178(14), I3-I2 ) 3.48, P1-Co-N1 ) 94.34(14), I1-Co-P1 ) 112.32(5), I1-Co-I2 ) 107.25(3), I2-Co-N1 ) 109.56(13).
more reducing than CoII. This divergence in phosphine
reactivity compared to that of the phenyl derivative is clearly
t
caused by the electron-rich Bu substituents making the
phosphorus in 1 (and 2) significantly more reducing. The
structural metrics of 8 are unremarkable, with the geometry
around cobalt close to ideal tetrahedral with only minor
distortions arising from the constraints of the remaining five-
Figure 8. Reaction of complex 8 with a stoichiometric equivalent of I2,
membered ring. The only observed anomaly is an elongation
of the Co-I2 bond (Co-I1, 2.5973(9) Å; Co-I2, 2.6279-
(9) Å), further highlighted by comparison to the Co-I bond
length in 2 (2.5955(3) Å). The source of this becomes clear
on examination of the extended structure of 8, which reveals
a close PI‚‚‚ICo intermolecular contact which, at 3.48 Å, is
within the combined van der Waals radii of two iodines. This
interaction propagates complex 8 into a one-dimensional
polymer in the solid state (Figure 7). Similar I‚‚‚I interactions
are well documented42 and arise from the different formal
charges associated with each iodine, with I3 formally
positively charged, while the I2 bonded to the d7 center is
unambiguously δ-.
leading to the probable insoluble product [(I-PtBu2CH2SiMe2)2NCoI3].
extended structure through extensive intermolecular Iδ+‚‚‚Iδ-
interactions.
Oxidation of 1 (or 2) in THF with an outer-sphere oxidant,
ferrocinium, Cp2Fe+ (as the PF6 and BArF salts) also failed,
with the starting materials unchanged after 24 h at 20 °C.
Oxidation of 1 by reaction with TEMPO or Galvinoxyl free
radicals was also unsuccessful, with complex 1 unchanged
after 24 h at 25 °C in the presence of a stoichiometric
equivalent of either of these reagents. Finally oxidation of 1
was also attempted with stoichiometric AgOTf in THF. This
led to a number of new paramagnetic products, along with
a diamagnetic “PNPAg” complex (as determined by the 31P-
{1H} NMR spectra, exhibiting the characteristic two-doublet
pattern for coupling of phosphorus to 107Ag and 109Ag). This
reaction mixture proved to be intractable; furthermore,
attempts to produce a single product by use of excess AgOTf
were equally unsuccessful. Thus Co(III) halide/pseudo-halide
complexes of the PNP ligand would appear to be either
totally inaccessible or transient, in contrast to Fryzuk’s PhPNP
ligated cobalt.29
It was hypothesised that the complex mixture observed
above may have arisen from the bifunctionality of the AgOTf
reagent, with it able to function via salt metathesis or
oxidation pathways. The metathesis reaction was indepen-
dently investigated using the redox-innocent cations Li+, Tl+,
and Na+; all of these triflates failed to react with 1 in THF
even after extended periods (3 days). Reaction of 1 with
equimolar TMSOTf resulted in the partial conversion to a
new paramagnetic product with C2V symmetry in the 1H NMR
spectrum (this product was evident in the reaction between
AgOTf and 1). The yield of this product could be enhanced
by the use of a large excess of TMSOTf (∼100 equiv),
although unreacted 1 still remained.
NMR studies in THF-d8 (in which 8 is significantly more
soluble) confirms the expected high-spin nature of 8 with a
solution magnetic moment of 4.7 µB, corresponding to a Co-
(II) center with three unpaired electrons (S ) 3/2). Further-
more a 31P{1H} resonance for the oxidized phosphine, not
coordinated to cobalt, is observed as a broadened singlet at
+269 ppm; the larger than expected downfield shift is caused
by the long-range paramagnetic influence of the high-spin
cobalt. At no stage is a five-coordinate trivalent PNPCoI2
complex, analogous to the fully characterized PhPNPCoX2
(X ) Cl, Br),17 observed. Complex 8 is indeed one of the
1
products observed (by H and 31P(1H} NMR spectroscopy)
in the reaction mixture generated by 1 and I2, confirming
that a similar phosphine oxidation has occurred.
The addition of 1 equiv of I2 to complex 8 in THF-d8
produces a single new paramagnetic C2V symmetric product
that within minutes proceeds to precipitate out as a micro-
crystalline green powder. The lack of solubility in suitable
solvents has frustrated further identification of this complex,
but it is reasonable to postulate that iodination of the second
phosphine has occurred, opening the second five-membered
ring, to generate a bis-phosphonium salt (Figure 8), with the
poor solubility then attributable to the formation of an
Recrystallization of a sample of 1 in the presence of 100
equiv of TMSOTf solution yielded two sets of crystals, red
crystals of 1 and dark blue crystals of sufficient quality for
(42) McAuliffe, C. A.; Godfrey, S. M.; Mackie, A. G.; Pritchard, R. G.
Angew. Chem. 1992, 104, 932.
Inorganic Chemistry, Vol. 46, No. 24, 2007 10327

Ingleson et al.
Figure 9. (Left) Equilibrium established between complexes 1 and 9 in the presence of a 100-fold excess of TMSOTf. (Right) ORTEP view (50% probabilities)
of complex 9 (only one of the two disordered sites for one tBu and the triflate are shown, hydrogens are also omitted for clarity). Selected bond lengths (Å)
and angles (deg): Co-N1 ) 1.928(3), Co-P1 ) 2.4182(10), Co-P2 ) 2.3942(10), Co-O1 ) 2.035(2), P1-Co-P2 ) 135.70(4), P1-Co-N1 ) 93.43(9),
P2-Co-N1 ) 97.94(9), N1-Co-O1 ) 118.95(11), P1-Co-O1 ) 106.78(8), P2-Co-O1 ) 104.60(8).
1
Figure 10. (Left) H NMR (C6D6, 298 K) for complex 10. (Right) ORTEP view (50% probabilities) of complex 10 (hydrogens are omitted for clarity).
Selected bond lengths (Å) and angles (deg): Co-N1 ) 1.9735(15), Co-P1, 2.2281(6), Co-P2 ) 2.2237(6), P1-Co-P2 ) 170.56(2), P1-Co-N1 )
94.65(5), P2-Co-N1 ) 94.75(5).
an X-ray diffraction study. This revealed the product to be
the anticipated PNPCo(OTf), 9. The structure of 9 (Figure
9) is analogous to that observed for 2, with minimal variation
in bond angles (e.g., for complex 9, P1-Co-P2 ) 135.70-
(4)° and N-Co-O ) 118.95(11)°, while for complex 2, P1-
Co-P2 ) 135.557(18)° and N-Co-I ) 117.45(4)°),
precluding any structural distortion caused by the increased
steric profile of triflate. The bond lengths to cobalt in 9
exhibit a minor contraction compared to the analogous bonds
in 2, presumably to compensate for the poorer donor ability
of triflate. The cobalt-triflate bond length in 9 is consider-
ably longer (Co-O1 ) 2.035(2) Å) than that reported43 for
another high-spin distorted tetrahedral CoII-OTf complex,
TpPh,MeCo-OTf (Co-O ) 1.963(3) Å). This relative length-
ening is presumably influenced by the superior electron
donating properties of the PNP ligand. The excellent solubil-
ity of complex 9 in C6H6 confirms that the triflate is
coordinated to cobalt in this nonpolar environment. The
difficulties in obtaining any useful quantities of pure complex
9 frustrated further characterization and any reactivity studies.
Alkylation/Reduction of PNPCoX. Thermally stable
four-coordinate cobalt(II) alkyl complexes using other tri-
dentate monoanionic ligands are well documented29,44,45 and
following the divergence in reactivity observed between 1
and PhPNPCoCl, we next investigated the alkylation of 1 with
a variety of alkyl lithiums. The phenyl-substituted phosphine
derivative readily forms square-planar CoII alkyl complexes,
and we were interested in determining if the better σ-donor
alkyl phosphine would allow for the formation of analogous
PNPCoR complexes. The addition of stoichiometric MeLi,
PhMgCl, or LiCH2SiMe3 to 1 at -78 °C in THF resulted in
a rapid color change to green. The 1H NMR spectra revealed
an identical new product from all three reactions, each
displaying three new paramagnetically shifted resonances,
confirming C2V symmetry. The reagent independence coupled
with a significant reduction in the peak width at half-height
1
(Figure 10) observed in the H NMR spectra (compared to
CoII S ) 1/2 and 3/2 PNPCoX complexes) suggested a
different spin state; therefore a redox change at cobalt (the
line widths of equivalent groups in paramagnetic complexes
is highly dependent on the number of unpaired electrons at
the metal center).46,47 At no time in the attempted alkylation
(44) Jewson, J. D.; Liable-Sands, L. M.; Yap, G. P. A.; Rheingold, A. L.;
Theopold, K. H. Organometallics 1999, 18, 300.
(45) DuPont, J. A.; Coxey, M. B.; Schebler, P. J.; Incarvito, C. D.;
Dougherty, W. B.; Yap, G. P. A.; Rheingold, A. L.; Riordan, C. G.
Organometallics 2007, 26, 971.
(43) Uehara, K.; Hikichi, S.; Akita, M. J. Chem. Soc., Dalton Trans. 2002,
3529.
10328 Inorganic Chemistry, Vol. 46, No. 24, 2007

ReactiWity of Four-Coordinate PNPCoX
of 1 are any intermediates observed; thus if a compound
PNPCoR is formed, it is extremely short-lived. Attempted
alkylations leading to a reduction of a first-row transition
metal complex (including those of cobalt) are well prece-
assume the amide is a one-electron donor). The two SOMOs
of (PNP)Co are x2-y2 and z2.58 Three-coordinate cobalt
complexes are documented,37,59,60 although only in the
oxidation states +2 and +3; to the best of our knowledge,
complex 9 is the first three-coordinate CoI complex to be
structurally characterized. A number of three-coordinate d8
Ni complexes are known,61-63 and all assume a geometry
best described as trigonal planar in contrast to 10. Two bona
fide (i.e., without agostic or other weak donor interactions)
three-coordinate d8 rhodium complexes have been structurally
determined,64 one65 with a nonchelate analog of the PNP
ligand, Rh(PEt3)2[N(SiMePh2)2]. Interestingly this nonchelate
restrained complex assumes a distorted trigonal planar
geometry (angles around rhodium sum to 360°), implying
that the T-shaped geometry observed for 10 is not the most
stable electronic configuration for a bisphosphine amido
ligated group nine metal. Furthermore, the amide plane in
Rh(PEt3)2[N(SiMePh2)2] is orthogonal to the RhP2 plane,
while the constraints of the PNP chelate prevent this
conformation in 10. Care must be taken in this comparison
because 10 has a triplet ground state compared to the singlet
ground state observed for the 4d metal complex Rh(PEt3)2-
[N(SiMePh2)2], and this may have a significant effect because
the spin state and geometry are interrelated. The second d8
three-coordinate complex64 (â-diketiminate)RhCOE, assumes
a distorted T-shaped geometry analogous to that of 10 and
again is diamagnetic, as expected for the second-row
rhodium, in contrast to the cobalt center in 10. Also of note
is a three-coordinate Rh pincer complex, [C10H5-(CH2PiPr2)2-
Rh]-, recently reported by Milstein et al.66 (not structurally
characterized), which although anionic is still Rh(I) and can
be expected to be geometrically identical to complex 10.
Complex 10 is indefinitely stable in the solid state under
inert atmosphere and stable for weeks in solution (even upon
heating to 60 °C for 24 h in C6D6). The stability and the
absence of any weak donor coordination in the fourth site
of 10 can be attributed to its high-spin nature, with the energy
needed to induce spin pairing (to generate a low-lying vacant
orbital and bind and subsequently activate a fourth ligand)
not recouped by weak donors. It should however be noted
that, with the existence of two singlet ground state, three-
coordinate, rhodium d8 complexes, a high-spin metal center
is not a prerequisite for the formation of three-coordinate
MI d8 complexes. The reluctance of 10 to bind weak donors
is further highlighted by the lack of formation of any new
products upon cooling of a THF-d8 (or toluene-d8) solution
1
dented,48-51 and the observation of an identical H NMR
spectrum in the reaction of 1 (or 2) with excess magnesium
powder (or stoichiometric sodium naphthalide) confirmed
that reduction of cobalt indeed occurred. The 1:1 reaction
stoichiometry suggested a one-electron reduction of cobalt
to a d8 CoI species, confirmed by a solution magnetic moment
measurement (at 298 K, C6D6) of 3.2 µB, consistent with a
triplet (S ) 1) ground state. The deviation above the spin-
only value (2.83 µB) is common for cobalt complexes (e.g.,
52
TpCo(C2H4) ) 3.8 µB and (TIMENxyl)Co+ ) 3.65 µB)38
and can be attributed to a substantial contribution from the
orbital angular momentum of cobalt. An X-ray diffraction
study on green crystals formed from cooling a concentrated
toluene solution to -40 °C overnight revealed the formation
of three coordinate PNPCo, 10 (Figure 10), with no agostic
or weak donor (e.g., THF or toluene) occupying the fourth
site trans to the amide nitrogen.
The geometry around cobalt in complex 10 is planar
(angles ) 360°) and closely approximates a T-shape, with a
slight expansion of the two five-membered rings of 10 (N1-
Co-P1 ) 94.65(5)° and N1-Co-P2 ) 94.75(5)°), resulting
in an improved encapsulation of the low-coordinate cobalt
center (compared to N1-Co-P1 ) 89.46(3)° and N1-Co-
P2 ) 89.73(3)° in 1). Thus, complex 10 is the analog of the
three-coordinate skeletal pincer complex (pincer)M (M )
Rh or Ir) that has been previously postulated53-57 as a
intermediate in a number of challenging small molecule
activations, but hitherto not observed. Inspection of the bond
lengths around cobalt in 10 reveal shortened Co-P bonds
and a lengthened N-Co bond (by comparison to 1); it is
difficult to ascribe this to any one factor because it can be
attributed to the change in spin state at cobalt, the change in
sterics around cobalt on reduction in coordination number
and/or the increased electron deficiency at cobalt in 10
(complex 10 is a formally 14-electron compound if we
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Inorganic Chemistry, Vol. 46, No. 24, 2007 10329

Ingleson et al.
Figure 11. (Left) DFT geometry optimized structure for complex 10. (Center) Essentially thermoneutral binding of N2 to PNPCo at 298 K. (Right) DFT
geometry optimized structure for singlet (S ) 0), PNPCo(N2), 11. Selected bond lengths (Å) and angles (deg) for the calculated structure of 11: Co1-P2
) 2.29, Co1-P3 ) 2.29, Co-N4 ) 1.94, Co1-N5 ) 1.72, N5-N6 ) 1.15, P2-Co1-P3 ) 176.4, P2-Co1-N4 ) 88.9, P2-Co1-N5 ) 91.3, N4-
Co1-N5 ) 115.3.
1
to -70 °C (by H NMR); at no point are any diamagnetic
66.3 ppm along with a 1H NMR spectrum displaying ligand
resonances in the expected diamagnetic region. Returning
the solution to 25 °C produced a NMR spectrum displaying
only resonances attributable to 10. Repeating the low-
temperature NMR study under an argon atmosphere resulted
in no observable diamagnetic products at -50 °C, verifying
that the new diamagnetic product is a dinitrogen compound
of PNPCo. The time-averaged C2V symmetry combined with
the single NtN stretching frequency attests to a monoadduct,
presumably PNPCo(N2), 11. A dinitrogen bridged complex
(PNP)Co-NtN-Co(PNP) is an alternative (as previously
reported67 with a related, albeit sterically more compact, PNP
ligand); we disfavor this possibility because of the extreme
products (i.e., PNPCo(solvent)) observed. The distal steric
bulk of the phosphine Bu groups, in addition to enforcing
the unusual planar T-shaped geometry, prevent the dimer-
ization observed for a closely related PNP ligand [(iPr2P(m-
C6H4)2N]- that forms a dimer with an anilido bridged
[Co2N2] core.67
t
Again the performance of a simple reaction (alkylation of
a metal halide) has resulted in a distinct outcome for the
tBu derivative of the PNP ligand compared to the phenyl
derivative. A possible explanation, based on the observed
product 10, is that the replacement of halide by a strong
σ-donor ligand (alkyl/aryl) would result in a metal center
that is extremely electron-rich (overloaded) and thus prone
to undergo reduction by a formal Co-R bond homolysis.
The success of Fryzuk et al. in synthesising PhPNPCoR is
then attributed to the presence of poorer donor bis-aryl
phosphines and consequently a less electron-rich metal
center. A similar trend (i.e., an increased ease of metal
reduction on phenyl for alkyl exchange on phosphine
substituent) to that hypothesised here, has been observed in
the Fe(II) complexes, PhB(CH2PR2)3FeCl (R ) Ph and iPr).7,9
Binding of Neutral Two Electron Donors to Complex
10. The reluctance of complex 10 to bind weak donor
solvents (even at low temperature) led us to probe the
reactivity of 10 with respect to small neutral two-electron
donors.
t
steric crowding associated with the Bu phosphine substit-
uents and on the basis of DFT calculations (see below). We
would expect such a bridged dinitrogen complex to have a
weak or even symmetry-forbidden N/N stretch. There is no
evidence for the coordination of a second molecule of N2.
Complex 10 therefore reversibly binds N2 to generate 11,
an equilibrium that favors 10 at room temperature, but upon
cooling (thereby reducing the entropic penalty), complex 11
dominates. Comparison of the NtN stretch for 11 (2004
cm-1) with other four-coordinate CoI dinitrogen complexes
(e.g., pseudotetrahedral Tp’CoN2, 2046 cm-1)52 exemplifies
the strong effect of the amide π-donor.
To probe the energetics of this conversion (which has to
be essentially thermoneutral, i.e., ∆G ) 0 ( 3 kcalmol-1),
DFT calculations were performed on complexes 10 and 11
and on the energy profile of their interconversion (Figure
11). The computation proved to be extremely density-
functional-dependent, as previously found by Harvey and
co-workers68-70 for other 3d metal systems, with the widely
used hybrid functionals that include “exact” exchange leading
to an artificially more stable high-spin state. As a result, pure
nonlocal functionals are a better choice for our reactions
involving a change in spin state. The choice of functional
for all our DFT studies of (PNP)CoLn was calibrated by its
ability to successfully predict the equilibrium of (PNP)Co
(triplet) + N2 f (PNP)Co(N2) (singlet) (to agree with the
(a) N2. The charging of a degassed toluene solution of 10
with 1 atm of N2 resulted in no observable color change and
a H NMR spectrum consistent with the presence of only
1
10. However, the infrared spectrum in a N2-saturated pentane
solution revealed one low-intensity peak in the expected
range for ν(N2) (2004 cm-1). This observation implied the
presence of a low concentration of a coordinated dinitrogen
complex. This was confirmed by a low-temperature NMR
study in toluene-d8, which at temperatures below -10 °C,
revealed the growth of a new diamagnetic C2V product. When
it reached -50 °C, the solution was a terracotta color, and
the diamagnetic product had increased in concentration (10
is still observed, although now the minor constituent). A 31P-
{1H} NMR resonance was observed as a broad singlet at
(68) Harvey, J. N. Struct. Bonding (Berlin, Germany) 2004, 112, 151.
(69) Carreon-Macedo, J.-L.; Harvey, J. N. J. Am. Chem. Soc. 2004, 126,
5789.
(67) Fout, A. R.; Basuli, F.; Fan, H.; Tomaszewski, J.; Huffman, J. C.;
(70) Broenstrup, M.; Schroeder, D.; Kretzschmar, I.; Schwarz, H.; Harvey,
J. N. J. Am. Chem. Soc. 2001, 123, 142.
Baik, M.-H.; Mindiola, D. J. Angew. Chem., Int. Ed. 2006, 45, 3291.
10330 Inorganic Chemistry, Vol. 46, No. 24, 2007

ReactiWity of Four-Coordinate PNPCoX
Figure 12. (Left) ORTEP view (50% probabilities, hydrogens omitted for clarity) of complex 12. (Right) Equilibrium between the mono- and dicarbonyl
complexes under 1 atm of CO. Selected bond lengths (Å) and angles (deg) for 12: N1-Co ) 1.9478(15), P1-Co ) 2.2322(5), P2-Co ) 2.2348(5),
Co-C23 ) 1.692(2), C23-O1 ) 1.167(3), P1-Co-P2 ) 178.49(2), N1-Co-C23 ) 179.46(10), N1-Co-P1 ) 89.75(5), C23-Co-P2 ) 90.37(1).
NMR studies). On the basis of this initial calibration, the
PBE functional was selected as optimal and was used
throughout this work. The calculated structure of complex
10 correlates well with the solid-state structure and confirms
a triplet ground state, with an energy gap of +24.8 kcal/mol
to the singlet state. The singlet ground state of 11 is also
correctly predicted (though in this case the triplet state is
only 3 kcal/mol higher in energy) and displays the expected
square planar geometry around cobalt, with a short (es-
sentially unactivated) N-N bond length (1.15 Å). The
calculated ∆G° (at 298 K) for the equilibrium shown in
Figure 11, +2.96 kcal/mol (including solvent effects), is
entirely consistent with the reversible N2 binding observed
by NMR studies. This essentially thermoneutral value found
by calculations lends support to a monomeric structure for
11. The positive ∆G°298 observed for the formation of 11 is
in sharp contrast to the calculated71 binding energy of N2 to
a closely related 4d complex, (PCP)Rh (∆G°298 ) -15.8
kcal/mol; PCP ) κ3-[2,6-(iPr2PCH2)2C6H3]-). The large
difference in the binding of N2 to (PCP)Rh and 10 can be
partly attributed to the energetic penalty paid to enforce spin-
pairing in the cobalt system ((PCP)Rh has a singlet ground
state).
(b) CO. Following the partial success in binding N2 to
the cobalt center in 10, we next tried with CO, the binding
of which should be significantly more thermodynamically
favored (CO is a superior σ-donor and π-acid, compared to
N2). The addition of one atmosphere of CO to a degassed
C6D6 solution of 10 resulted in a rapid color change from
green to red. Recrystallization yielded red crystals that X-ray
diffraction analysis revealed to be the square planar mono-
carbonyl adduct PNPCo(CO), 12 (Figure 12). The solid-state
structure of 12 is unremarkable, being closely similar to that
of the d7 square planar complex 1. Complex 12 possesses a
1H NMR spectrum consistent with C2V symmetry, a resonance
in the 31P{1H} NMR at 79.3 ppm and a broad singlet (from
the quadrupolar, I ) 7/2 cobalt) in the 13C{1H} NMR for
coordinated CO at 203.1 ppm. The infrared spectrum of
crystalline 12 dissolved in pentane exhibits one sharp intense
CO stretch, νCO ) 1885 cm-1. This low CO stretching
frequency (by comparison to that of both PNPRhCO,22 1932
cm-1, and [(iPr2P-o-C6H4)2N]CoCO, 1901 cm-1)67 confirms
a significant degree of back-bonding from cobalt (again
enhanced by the push-pull effect).
Interestingly, an examination of the NMR spectra of a
sample of 12 under an atmosphere of CO reveals a number
of differences from that recorded under argon, the most
apparent being the broadening of all the resonances (to such
a degree that no spin coupling is observable). The chemical
shifts are unchanged from those of 10. In addition, no
resonance is observed in the 13C{1H} NMR for free CO (even
when isotopically enriched 13CO is used); these findings
combined imply a fluxional process involving the coordina-
tion of CO. Dissociation of CO as source of the fluxionality
(as found for the binding of N2 to 10) can be discounted
because placing 12 under vacuum for extended periods (1
1
× 10-3 Torr, 3 days) results in no loss of CO (by H and
31P{1H} NMR). The fluxionality therefore must involve the
association of a second molecule of CO, forming PNPCo-
(CO)2, analogous to [(iPr2P-o-C6H4)2N]Co(CO)2.67 The in-
frared spectrum of 12 in a CO-saturated pentane solution
indeed confirms the presence of a second CO-containing
molecule, PNPCo(CO)2, in low concentration with two (albeit
weak) stretching frequencies observed at 1840 and 1931
cm-1. These CO stretches are again extremely low, especially
when compared to other Co(I) dicarbonyls, for example,
40
PhB(CH2PiPr3)3Co(CO)2 (1990 and 1904 cm-1), further
supporting the strongly electron-releasing properties of the
PNP ligand. PNPCo(CO)2 is observable at low temperature
by NMR spectroscopy, and cooling of a sample of 12 under
∼4 atm of 13CO to 213 K reveals a second 31P{1H} NMR
signal at 94.4 ppm, along with a new coordinated CO
resonance in the 13C{1H} NMR spectrum at 205.5 ppm (free
13CO is also observed at this temperature as a sharp singlet
at 184.1 ppm). It is interesting that there is such a different
propensity for the binding of CO between 12 and [(iPr2P-
o-C6H4)2N]CoCO (which under a CO atmosphere exists only
as the dicarbonyl). Presumably this is the result of the
superior electron-donating ability of the silyl amido, tBu PNP
(71) Cohen, R.; Rybtchinski, B.; Gandelman, M.; Rozenberg, H.; Martin,
i
J. M. L.; Milstein, D. J. Am. Chem. Soc. 2003, 125, 6532.
ligand versus the anilido Pr PNP ligand. Two other four-
Inorganic Chemistry, Vol. 46, No. 24, 2007 10331

Ingleson et al.
halide species (in comparison to the PhPNP derivative) was
elucidated and also shown to be dominated by ligand steric
bulk, with a square-planar structure maximizing phosphine
substituent separation. Increasing the steric bulk of the fourth
ligand substantially does not enforce a low-spin structure,
with the high-spin structure being consistently observed. The
Co(II) high-spin structure instead displays a large degree of
ligand flexibility, with two major structural changes alleviat-
ing the steric crowding: (1) a significant opening of the
P-Co-P bond (approaching the sterically preferred planar
geometry) and (2) a drastic lengthening (up to 0.16 Å
compared to PNPCoCl) of the P-Co bonds.
Figure 13. Reactivity of complex 2 with respect to excess CO, producing
12 and 8 (by H NMR; reaction is not balanced).
1
coordinate monocarbonyl adducts of Co(I) have been re-
ported, Tp′CoCO (νCO ) 1950 cm-1)52 and [TIMENCo-
CO]+38 (1927 cm-1). Both are in a distorted tetrahedral
geometry, resulting in a triplet electronic ground state in
contrast to square planar diamagnetic 12, making a direct
comparison of the ligand electron donating ability invalid.
Compound 12 is also produced by the reaction of excess
CO with 2, producing 8 as the only iodine-containing
byproduct detected. The reduction of the cobalt center is
balanced by the oxidation of a phosphine in compound 8
and not of cobalt. A similar process has been previously
reported40 with PhB(CH2PiPr2)3CoI reacting with excess CO
to generate PhB(CH2PiPr2)3Co(CO)2; no iodine-containing
products were reported.
Attempts to reach stable Co(III) complexes ligated by PNP
met, counterintuitively, with limited success; simple logic
dictates that the superior electron-donating phosphine with
tBu substituents (compared to the phenyl derivative of
Fryzuk) should allow greater stabilization of this higher
oxidation state. Experimentally, the opposite was ascertained,
with PNPCoIIIX2 complexes proving elusive, obtainable only
when ligated with low electronegativity ligands (hydrides/
silanes),23 where the oxidation state formalism has less
meaning. This behavior may in part be attributable to ligand
t
non-innocence, with the presence of readily oxidized Bu
The reaction energy calculated (DFT) for the addition of
the first CO to (PNP)Co is -41.0 kcal/mol, while that for
the addition of CO to (PNP)Co(CO) is only -13.3 kca/mol,
which is comparable to T∆S at 298 K and thus is in
agreement with experiment. The calculated geometry of
(PNP)Co(CO)2 is square pyramidal with CO apical and
equatorial.
phosphines that result in phosphorus repeatedly being the
locus of oxidation. This is exemplified by the reaction of
PNPCoIIX with O2, which even at -78 °C yields a product
involving phosphine oxidation and not a superoxo or peroxo
species. Furthermore, the low energetic barrier (full conver-
sion by -78 °C) to phosphine oxidation repeatedly observed,
despite the coordinated PIII having no available lone pair,
implies a labile CoII-P bond.72
(c) Other Donors. Following the success with CO, the
reactivity of 10 with potential ligands was investigated.46 The
addition of excess ethene to 10 was attempted, but no ethene
coordination is observed even on cooling to -60 °C, and
10 is recovered after heating in the presence of excess ethene
for prolonged periods (70 °C, C6D6 72 h). Ethene therefore
cannot be a strong enough donor to impose spin pairing on
the cobalt center. Correspondingly, no reaction was observed
between 10 and excess 2-butyne. Complex 10 does react with
(rigorously dried) phenyl acetylene; the major phosphorus-
containing product is the free amine PN(H)P, possibly
generated by ligand protonation by the weakly acidic phenyl
acetylene. Unfortunately the paramagnetic cobalt-containing
The CoI oxidation state is easily obtained from PNPCoX
and any number of reducing agents with the resultant three-
coordinate PNPCo complex shown to be T-shaped and have
a triplet ground state. Weak nucleophiles show no or limited
propensity to bind in the vacant fourth site, implying that
any new Co-L bonds will have to provide significant
binding energy to overcome that required to induce electron
pairing. The cobalt center in PNPCo was found to be
extremely electron rich (by IR spectroscopy of its carbonyl);
in comparison to other pincer ligands, PNP builds the most
reducing power into (pincer)M yielding23 dihydride PNPCoIII-
(H)2, not dihydrogen. The redox properties of the electron-
rich PNPCo are however paradoxical; while one electron
oxidation of cobalt readily occurred, there was a reluctance
for full two electron-transfer that would result in CoIII. To a
degree this failure (and the related inability to produce
PNPCoH from reaction with H atom donors) is caused by
the reducing power of PNPCo being strongly dependent on
not only the redox potential of cobalt but also, equally
importantly, on the energy recouped in the formation of new
Co-X bonds.
1
products (observed in the H NMR) could not be separated
or purified, thereby frustrating unambiguous identification.
Conclusions
In summary, we have demonstrated that the PNP ligand
set allows access to a range of low-coordinate cobalt
complexes that are rigorously monomeric because of the
distal steric bulk provided by four tBu groups, in contrast to
other related, low-coordinate Co(I) and Co(II) complexes
with sterically demanding ligand environments. PNPCoCl
exists in equilibrium between planar (S ) 1/2) and nonplanar
(S ) 3/2) species of energies so similar that solvent properties
(not coordination to Co) can reverse the dominant species,
planar (S ) 1/2) or nonplanar (S ) 3/2). The origin of the
energetic preference for the low-spin isomer in this simple
Overall this work with O2 and I2 highlights a current
common theme, that of ligand non-innocence, which in the
tBuPNP ligand set is evident at both the labile, oxidizable
(72) Fryzuk, M. D.; Carter, A.; Rettig, S. J. Organometallics 1992, 11,
469.
10332 Inorganic Chemistry, Vol. 46, No. 24, 2007

ReactiWity of Four-Coordinate PNPCoX
singlet), 19.49 (12H, broad singlet) 1.94 (36H, broad singlet).
Magnetic moment (C6D6): 4.54 µB. H NMR (298 K, C6D12): δ
59.57 (4H, broad singlet), 18.30 (12H, broad singlet), 1.66 (br
singlet, accurate integration was impossible because of the overlap
of the signal with residual protio cyclohexane). Magnetic moment
(C6D12): 5.05 µB.
phosphine functionalities. These ligand vulnerabilities need
to be considered when attempting to install any electrophilic
reactive functionality at the metal center. Certainly every
ligand has its vulnerabilities.73-75
1
Experimental Section
PNPCoN3 (3). A Schlenk flask was charged with PNPCoCl
(0.400 g, 0.74mmol) and 15 mL of THF; 10 equiv of NaN3 (0.481
g, 7.4mmol) was added as a solid. After it was stirred overnight,
the solution changed color from deep blue to terracotta; the THF
was removed in vacuo, and the resultant oil was redissolved in
pentane and filtered. The pentane was reduced to minimum volume.
Storage of the mixture at -40 °C for 3 days yielded red crystals
(0.303 g, 0.55mmol). Yield: 74%. 1H NMR (298 K, C6D6): δ 6.5
(4H, broad singlet), 2.4 (36H, broad singlet) -4.0 (12H, broad
singlet). Magnetic moment (C6D6): 2.47 µB. IR (cm-1, pentane):
ν 2081.
PNPCoNH(2,6-Me2-C6H3) (4). A Schlenk flask was charged
with PNPCoCl (0.126 g, 0.23 mmol) and 10 mL of THF to yield
a deep blue solution; 1.05 equiv of LiNH(2,6-Me2-C6H3) (0.031 g,
0.24 mmol) was then added as a solid, resulting in a rapid color
change to purple. The solution was stirred for a further 4 h before
the THF was removed in vacuo. The resultant purple oil was
redissolved in toluene and filtered; the toluene was removed in
vacuo, and the oil was redissolved in a minimum amount of pentane
and cooled to -40 °C for 18 h to yield purple crystals (0.090 g,
0.14mmol). Yield: 62%. 1H NMR (298 K, C6D6): δ 12.49 (4H br
singlet), 10.07 (48H, br singlet of 4 tBu and 4 SiMe), -88.72 (6H,
br singlet of anilido methyls). The phenyl ring protons are not
observed in the region of +150 to -150 ppm. Magnetic moment
(C6D6): 4.28 µB.
General Considerations. All manipulations were performed
using standard Schlenk techniques or in an argon-filled glovebox.
Solvents were distilled from Na/benzophenone, CaH2, or 4 Å
molecular sieves, degassed prior to use and stored in airtight vessels.
All reagents were used as received from commercial vendors or
synthesized by published routes. PNPMgCl/dioxane (PNP )
[(tBu2PCH2SiMe2)2N]-) was prepared according to literature meth-
ods. 1H NMR chemical shifts are reported in ppm relative to protio
impurities in the deuterated solvents. 31P{1H} spectra are referenced
to external standards of 85% H3PO4 (at 0 ppm); 29Si and 29Si{1H}
spectra are referenced to external standards of (CH3)4Si (at 0 ppm).
NMR spectra were recorded with a Varian Gemini 2000 (300 MHz,
1H; 121 MHz, 31P; 75 MHz, 13C), a 400 MHz Varian Unity Inova
(400 MHz, 1H; 162 MHz, 31P; 101, MHz 13C), or a 500 MHz Varian
Inova instrument (500 MHz, 1H; 99 MHz, 29Si). Solution magnetic
moments were recorded at 25 °C (unless otherwise stated) using
the Evans method. Consistently for these paramagnetic complexes,
no 31P{1H} was observable. Infrared spectra were recorded on a
Nicolet 510P FT-IR spectrometer.
PNPCoCl (1). Compound 1 was synthesized in a closely related
manner to that reported for the phenyl derivative, (Ph2PCH2SiMe2)2-
NCoCl.27 PNPMgCl/dioxane (1.6 g, 2.7mmol) was dissolved in 40
mL of THF, and 1.05 equiv of anhydrous CoCl2 (0.365 g, 2.8mmol)
was added as a solid. The solution rapidly turned deep blue. After
the mixture was stirred for 18 h, the THF was removed in vacuo;
the resulting oil was dissolved in toluene and filtered, and the
volume was reduced to ∼5 mL. Cooling of the mixture to -40 °C
overnight yielded 0.800 g of a red crystalline product. The filtrate
was reduced to dryness in vacuo, redissolved in minimum pentane,
and cooled overnight at -40 °C to yield a second crop (0.260 g)
of red crystals. Combined yield: 73% (1.06 g, 1.9mmol). 1H NMR
(298 K, C6D6): δ 30.4 (4H, v. broad singlet), 8.2 (12H, v. broad
singlet) 2.6 (36H, v. broad singlet). Magnetic moment (Evans
method, 298 K, C6D6): 3.96 µB. 1H NMR (298 K, C6D12): δ 15.98
(4H, broad singlet), 3.01 (36H, broad singlet) 1.08 (12H, broad
P(O)NP(O)CoCl (5). In a standard reaction, a Youngs NMR
tube was charged with 0.015 g of PNPCoCl and 0.45 mL of C6D6
and transferred to a gas line. The deep blue solution was degassed
twice (freeze/pump/thaw) and refilled with an atmosphere of O2.
When it thawed, a rapid color change was observed from deep blue
1
to royal blue, and the H NMR spectrum showed the quantitative
conversion to a single new Cs symmetric paramagnetic product.
Alternatively P(O)NP(O)CoCl can be synthesized quantitatively by
the addition of 2.1 equiv of PhIdO (iodosobenzene) to PNPCoCl
in C6D6, with PhI observed as the byproduct. Yield: Quantitative
1
by NMR. H NMR (298 K, C6D6): δ 24.30 (6H, singlet), 11.61
1
singlet). Magnetic moment (Evans method, C6D12): 2.85 µB. H
(2H, singlet) 0.11 (18H, singlet), -3.53 (18H singlet), -14.10 (6H,
singlet), -21.65 (2H, singlet). Magnetic moment (C6D6): 4.50 µB.
MS (ESI+) Calcd for C22H52Si2P2N1O2Co1Cl1: 574.2. Found (M
+ 1): 575.1.
NMR (298K, C7D8): δ 27.0 (4H, broad singlet), 7.1 (broad singlet,
overlapped with protio toluene making an accurate integration
impossible), 2.6 (36H, broad singlet). Magnetic moment (C7D8):
3.21 µB.
Attempts to record a mass spectrum of Complex 1 repeatedly
failed, with the major molecular ion observed corresponding to P-
(O)NP(O)CoCl [M + 1]+.
PNPCoI (2). PNPMgCl/dioxane (1.5 g, 2.5 mmol) was dissolved
in 30 mL of THF, and 1.05 equiv of anhydrous CoI2 (0.825 g,
2.6mmol) was added as a solid. The solution rapidly turned
turquoise. After the mixture was stirred for 18 h, the THF was
removed in vacuo; the resulting oil was dissolved in toluene and
filtered, and the volume was reduced to ∼5 mL. Cooling to -40
°C overnight yielded 0.850 g (1.4 mmol) of turquoise crystalline
product. Yield: 54%. 1H NMR (298 K, C6D6): δ 63.73 (4H, broad
P(O)NP(O)CoI (6). In a standard reaction, a Youngs NMR tube
was charged with 0.010 g of PNPCoI and 0.45 mL of C6D6 and
transferred to a gas line. The turquoise solution was degassed twice
(freeze/pump/thaw) and refilled with an atmosphere of O2. When
thawed, a rapid color change was observed from deep blue to royal
blue, and the 1H NMR spectrum showed the quantitative conversion
to a single new Cs symmetric paramagnetic product. Alternatively
P(O)NP(O)CoI can be synthesized quantitatively by the addition
of 2.1 equiv of PhIdO to PNPCoI in C6D6, with PhI observed as
1
the byproduct. Yield: Quantitative by NMR. H NMR (298 K,
C6D6): δ 24.05 (6H, singlet), 11.09 (2H, singlet) -0.23 (18H,
singlet), -2.15 (18H singlet), -15.13 (6H, singlet) and -23.57
(2H, singlet). MS (ESI+) Calcd for C22H52Si2P2N1O2Co1I1: 666.1.
Found (M + 1): 667.3.
P(O)NP(O)CoN3 (7). In a standard reaction, a Youngs NMR
tube was charged with 0.015 g of PNPCoN3 and 0.45 mL of C6D6
and transferred to a gas line. The red solution was degassed twice
(73) Ben-Ari, E.; Leitus, G.; Shimon, L. J. W.; Milstein, D. J. Am. Chem.
Soc. 2006, 128, 15390.
(74) Scott, J.; Gambarotta, S.; Korobkov, I.; Budzelaar, P. H. M. J. Am.
Chem. Soc. 2005, 127, 13019.
(75) Collins, T. J. Acc. Chem. Res. 1994, 27, 279.
Inorganic Chemistry, Vol. 46, No. 24, 2007 10333

Ingleson et al.
(freeze/pump/thaw cycles) and refilled with an atmosphere of O2.
When thawed, a rapid color change was observed from red to blue,
and H NMR spectrum showed the quantitative conversion to a
and reduced to dryness to give a green solid. Redissolution in
minimum toluene and cooling to -40 °C yielded, upon isolation,
0.310 g of green crystals. The remaining filtrate was reduced in
volume and cooled to -40 °C for 24 h to yield a second crop of
green crystals (0.065 g). Combined Yield: 76% (0.375 g, 0.74
1
single new Cs symmetric paramagnetic product, crystals of which
could be obtained by the slow evaporation of a benzene solution.
1
1
Yield: Quantitative by NMR. H NMR (298 K, C6D6): δ 25.24
mmol). H NMR (C6D6, 298 K): δ 12.5 (36H, s), 0.30 (12H, s),
-24.2 (4H, s). 13C{1H} NMR (C6D6, 298 K): δ 272 (s), 253 (s),
234 (br s), 101 (br s). Magnetic Moment (C6D6): 3.2 µB.
(6H, singlet), 11.55 (2H, singlet) 1.14 (18H, singlet), -4.59 (18H
singlet), -14.01 (6H, singlet), -21.87 (2H, singlet). IR (cm-1
,
KBr): 2058.
PNPCo(N2) (11). A J. Youngs NMR tube was charged with
PNPCo (0.020 g, 0.04 mmol) that was dissolved in 0.45 mL of
toluene-d8. On a gas line, the green solution was degassed three
times by freeze/pump/thaw cycles and charged with N2 at 77 K
(∼4 atm). 1H NMR (223 K, toluene-d8): δ 1.39 (s, 36H), 1.12 (s,
4H), 0.32 (s 12H). 31P{1H} NMR (223 K, toluene-d8): δ 66.3 (s).
IR (298 K, cm-1, N2 saturated pentane solution): 2004 (s).
PN(P-I)CoI2 (8). A Schlenk flask was charged with PNPCo
(0.050 g, 0.1 mmol) and dissolved in 3 mL of C6H6; 2 equiv of I2
(0.025 g, 0.2 mmol) dissolved in C6H6 was then added dropwise
while stirring. The solution rapidly changed color from green to
pale blue. After the mixture was left standing for several minutes
a pale blue microcrystalline solid precipitated from solution (0.068
g, 0.077 mmol) An alternative synthesis starting from PNPCoI and
1.1 equiv of I2 quantitatively yields 8 (by NMR). Yield: 77%. 1H
NMR (298 K, THF-d8): δ 56.35 (2H, br s), 27.58 (6H, br s), 15.26
(6H, br s), 1.12 (18H br s), -6.46 (18H, s), -72.36 (2H, v. br s).
31P{1H} NMR (298 K, THF-d8): δ 269.5 (br s). Magnetic moment
(THF-d8): 4.78 µB. MS (ESI-): Molecular ion not observed,
instead [M - I]- (calcd for C22H52NP2Si2CoI2, 761.1; found, 761.2)
and [M - tBu]- (calcd for C18H43NP2Si2CoI3, 830.9; found, 831.0).
PNPCoCl + TMSOTf, forming PNPCo(OTf) (9). A J. Youngs
tube was charged with 15 mg of PNPCoCl and 1 mL of C6H6, and
100 equiv of TMSOTf was then added. Slow evaporation of
benzene yielded two distinct sets of crystals: red crystals (confirmed
as 1 by X-ray crystallography) and dark blue crystals. Mechanical
separation of the crystals leads to a predominantly pure sample of
9 with only minor contamination from 1. Attempts to record a
magnetic moment (Evans method) for this sample were frustrated
by the inability to obtain useful analytically pure amounts. 1H NMR
(298 K, C6D6): δ 43.3 (4H, broad singlet), 17.7 (36H, broad
singlet), -5.4 (12H, broad singlet).
PNPCo(CO) (12). A flask with a Teflon cap was charged with
PNPCo (0.550 g, 1.1 mmol) and 10 mL of pentane and transferred
to a gas line. The solution was degassed three times (freeze/pump/
thaw cycles) and backfilled with 1 atm of CO. When thawed, the
solution immediately turned from green to red. The pentane was
removed in vacuo, and the resultant red crystalline solid (0.464 g,
0.87mmol) was analytically pure. The CO is not removable after
1
evacuation at 25 °C (at 5 Torr) after 18 h. Yield: 80%. H NMR
(298 K, C7D8): δ 1.34 (36H, t, 6.3 Hz), 0.84 (4H t, 4.5 Hz), 0.25
(12H, br s). 31P{1H} NMR (298 K, C7D8): δ 79.3 (s). 13C{1H}
NMR (298 K, C7D8): δ 203.1 (br s), 35.5 (s), 29.8 (s), 10.7 (s),
5.92 (s). IR (cm-1, pentane): 1885.
Acknowledgment. This work was supported by the NSF
(CHE-0544829). Professors Mu-Hyun Baik and Daniel
Mindiola are thanked for useful discussions.
Supporting Information Available: Computational details,
optimized structures of (PNP)Co, (PNP)Co(N2), (PNP)Co(CO),
(PNP)Co(CO)2, (PNP)CoCl, selected bond distances and angles,
SOMOs, and full crystallographic details. This material is available
free of charge via the Internet at http://pubs.acs.org.
PNPCo (10). PNPCoCl (0.500 g, 0.92 mmol) was loaded into a
Schlenk flask and dissolved in 30 mL of THF. Magnesium powder
(25 equiv, 0.560 g, 23 mmol) was added as a solid to the deep
blue solution and stirred for 18 h. The resulting green solution was
reduced to dryness in vacuo, redissolved in toluene (20 mL) filtered,
IC701171P
10334 Inorganic Chemistry, Vol. 46, No. 24, 2007