Isolation of two agostic isomers of an organometallic cation: Different structures and colors

Article abstract of DOI:10.1002/anie.201305032

Two of a kind: Two agostic isomers of [CpMo(CO)₂(PiPr ₃)]⁺ B(C₆F₅)₄^- have been isolated. Both were characterized in the solid state by X-ray crystallography and spectroscopic techniques, and also by DFT calculations. Significantly different LUMO energies cause the difference in color (blue versus orange, see picture) of these isomers. Copyright

Full text of DOI:10.1002/anie.201305032

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Chemie
DOI: 10.1002/anie.201305032
Agostic Isomers
Isolation of Two Agostic Isomers of an Organometallic Cation:
Different Structures and Colors**
Edwin F. van der Eide, Ping Yang,* and R. Morris Bullock*
Agostic interactions are of paramount importance in organ-
ometallic chemistry and homogeneous catalysis.[1,2] These
been simultaneously characterized using low-temperature
NMR spectroscopy.[8,9] Interconversion between agostomers
would presumably be hampered in the solid state, but there
are no previous reports of complexes for which agostomers
have been isolated or characterized as solids. Herein we
report crystal structures for b- and g-agostomers (cf. B in
Scheme 1) of a Mo phosphine complex, and we show that
these isomers have distinct colors.
ꢀ
intramolecular 3-center/2-electron C H···M interactions can
stabilize electronically and coordinatively unsaturated com-
plexes, and are involved in key steps of catalytic processes
such as Ziegler–Natta polymerization.[3] Similar C H···M
ꢀ
interactions occur in metal–alkane s-bond complexes[4]—very
[5]
ꢀ
reactive intermediates in C H bond activation that are
often modeled by agostic complexes.
The chemistry reported herein results from our interest in
isolating “16-electron” piano-stool complexes [CpMo(CO)2-
(PR3)]+ (Cp = h-C5H5), because of their intermediacy in ionic
hydrogenation catalysis.[10] Earlier, we found for R = Ph that
The energetic stabilization associated with an agostic
interaction is modest to low (ꢁ 15 kcalmolꢀ1).[2,6] Conse-
ꢀ
quently, when more than one C H bond is available for
coordination, energetically similar agostic isomers may form
(Scheme 1;[7] we call these isomers “agostomers”). In solution,
fast interconversion between agostomers often complicates
efforts to compare their properties, and for only a handful of
Nb and Ta alkyls have agostomers (a and b, cf. A in Scheme 1)
the phosphine can act as a four-electron donor by binding in
3
ꢀ
an h fashion (through P and a phenyl C C p bond), which
emphasizes the tendency of Mo to attain an 18-electron
count.[11] We therefore expected the phosphine ligand in
trialkylphosphine analogues to behave as a four-electron
donor as well, by engaging in an agostic interaction.
Abstraction of hydride from the pale yellow, 18-electron
complex 1[12] (Scheme 2), using Ph3C+ B(C6F5)4 in fluoro-
ꢀ
benzene at room temperature,[13] proceeded smoothly to give
a violet solution. NMR and IR spectroscopy confirmed the
Scheme 1. Agostic isomerism in metal alkyl (A) and phosphine (B)
complexes. The complexes may be neutral or positively charged.
[*] Dr. E. F. van der Eide, Dr. R. M. Bullock
Physical Sciences Division
Pacific Northwest National Laboratory
P.O. Box 999, Richland, WA 99352 (USA)
E-mail: morris.bullock@pnnl.gov
Dr. P. Yang
Environmental Molecular Sciences Laboratory
Pacific Northwest National Laboratory
P.O. Box 999, Richland, WA 99352 (USA)
E-mail: ping.yang@pnnl.gov
Scheme 2. Synthesis of the agostomers 2g and 2b (Aꢀ =B(C6F5)4ꢀ).
Photographs of representative crystals also shown.
[**] We thank the U.S. Department of Energy (DOE), Office of Basic
Energy Sciences, Division of Chemical Sciences, Geosciences and
Biosciences for support of this work. Pacific Northwest National
Laboratory (PNNL) is a multiprogram national laboratory operated
for DOE by Battelle. The computational studies were performed
using EMSL, a national scientific user facility sponsored by the
DOE’s Office of Biological and Environmental Research and located
at PNNL. We thank Dr. Birgit Schwenzer for assistance with
recording solid-state Vis/NIR spectra, and Nat Saenz and Shelley J.
Carlson for assistance with optical microscopy. We also thank Dr.
Jonathan M. Darmon for designing the Frontispiece.
clean production of (diamagnetic) [CpMo(CO)2(PiPr3)]+ [B-
(C6F5)4]ꢀ (2). Conclusive support for the occurrence of agostic
interactions was not obtained in solution,[12,14] but character-
ization in the solid state was definitive in this regard. Layering
of the fluorobenzene solution of 2 with hexanes provided two
types of crystals, growing in the same vessel. Blue/violet
crystals formed at the solvent interface, while orange crystals
formed near the bottom of the vessel. Under selective
crystallization conditions,[12] both forms were also individually
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201305032.
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obtained as pure materials, each in 80–90% yield. Redis-
solution of either the blue or orange crystals in fluorobenzene
gave violet solutions with identical spectroscopic signatures
(and identical to freshly generated 2), suggesting an isomeric
relationship between the two forms, with a low barrier to
isomerization in solution.
Single-crystal X-ray diffraction analyses established the
blue and orange forms of 2 to be agostomers 2g and 2b,
respectively (Figure 1 and Scheme 2).[15] Short Mo···Cag (ca.
2.7 ꢀ, Table 1) and Mo···Hag (ca. 2.0 ꢀ)[12,16] distances confirm
Solid-state IR spectra show carbonyl stretching bands at
~
similar frequencies for both agostomers (nCO ¼2002 and
1929 cmꢀ1 for 2g; 2006 and 1942 cmꢀ1 for 2b), with only
one set of bands observable in fluorobenzene solution (2006
and 1936 cmꢀ1). However, we cannot assign with enough
confidence[12] a band to nCH of a weakened, agostic C H bond,
ꢀ
in either the solid state or in solution. The electronic spectrum
of 2 (Figure 2) in fluorobenzene displays a broad band with
lmax = 535 nm (emax = 370mꢀ1 cmꢀ1), while 1 does not signifi-
Figure 1. Structures of cationic fragments of 2g and 2b · C6H5F,
thermal ellipsoids set at 50% probability. Hydrogen atoms belonging
to the agostic isopropyl group are also shown (drawn with arbitrary
radius).
Figure 2. Vis/NIR spectra. Artifacts at 800 nm in the solid-state spectra
are due to a combined detector and grating changeover. The sticks
mark the transitions calculated by TD-DFT; their heights are propor-
tional to the calculated oscillator strengths.
cantly absorb in the visible.[12] Although the agostomers are
not resolved in solution, the magnitude of the extinction
coefficient does inform that a d–d transition is responsible for
the absorption. We do observe significantly different spectra
when the individually isolated agostomers are measured as
solids (measured in total reflectance mode). The absorption
maximum shifts from approximately 450 nm for 2b to about
580 nm for 2g, which is in qualitative agreement with the
orange versus blue appearance of these isomers.
The availability of isolated samples of both agostomers,
obtained under the same conditions, suggests that they are
nearly isoenergetic. However, because they are not resolved
in solution by either NMR, IR, or Vis spectroscopy, we cannot
quantify this experimentally. DFT calculations (see the
Experimental Section for details) were employed to provide
insights into the energetics involved and the origin of the
color difference. Fully optimized geometries of the cationic
fragments show good agreement with experiment (Figures 1
and 3, Table 1),[12] and the theoretical energy difference
Table 1: Selected experimental [theoretical] bond angles [8] and
lengths [ꢀ].
Parameter
2g
2b
1
](Mo-P-C8) 95.3(1)
[94.0]
74.57(4)
[74.7]
2.4070(3)
[2.448]
2.612(1)
[2.662]
[1.971]
114.6
[114.4][a]
d(Mo-P)
2.4862(4)
2.464(1)/2.456(1)[b]
[2.483]
[2.535]
2.737(1)
[2.890]
d(Mo···Cag)
–
d(Mo···Hag)[c] [2.114]
[1.724] (Mo-H)
–
d(C-Hag)[c,d]
[1.129]
[1.165]
[a] Average of three Mo-P-C angles. [b] Two independent molecules.
[c] Only theoretical values shown; see Ref. [16]. [d] 1.093–1.099 ꢀ for
3
ꢀ
non-agostic sp C H.
that an agostic interaction occurs in both isomers.[1] The
agostic H atoms approximately take the position of the
hydride in 1, thus retaining the four-legged piano-stool
geometry around Mo. Severe distortions in the phosphines
are also apparent: the Mo-P-C angle involving the agostic iPr
group is 95.38 for 2g and only 74.68 for 2b, while the Mo-P-C
angles involving the non-agostic iPr groups fall in the range
117–1238 (114–1158 for 1). Notably, the Mo–P distances in 2g
and 2b differ by 0.0791(5) ꢀ, being longer and shorter,
respectively, than the approximately 2.46 ꢀ in 1.
For the overwhelming majority of structurally character-
ized complexes with an agostic alkylphosphine, g-agostic
bonding is found, as in the related [W(CO)3(PR3)2] (R = iPr,
ꢀ
Cy)[17] and [M(CO)3(PCy3)2]+ B(3,5-(CF3)2C6H3)4 (M =
Mn,[18] Re[19]). We are aware of only one reported precedent
Figure 3. Theoretical energy diagram. Mo blue, P orange, C gray, O red,
and H white (only agostic H shown; agostic interaction shown as
a black line).
for a b-agostic phosphine, as found for [Rh(PiPr3)3]+ B-
ꢀ
[20]
(3,5-(CF3)2C6H3)4
.
2
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between 2g and 2b is only 0.6 kcalmolꢀ1, corroborating the
calculated position of the more intense HOMO-1!LUMO
ꢀ
validity of the experimental results. The agostic C H bonds
transition is in excellent agreement with the maxima obtained
in the experimental spectra. Overall, the experimental and
theoretical results suggest that the b-agostic bonding mode of
the phosphine induces a stronger ligand field than the g-
agostic bonding mode does.
are longer than the non-agostic ones: the elongation is
approximately 0.03 ꢀ for 2g and approximately 0.07 ꢀ for
ꢀ
~
2b. Additionally, the calculated nCH for the agostic C H bond
is 2560 cmꢀ1 for 2g and 2150 cmꢀ1 for 2b compared to 2960–
3050 cmꢀ1 for the non-agostic ones.[21] Weakening of the
To estimate the total stabilization energy associated with
the agostic interactions (DEagostic, Figure 3), hydride 1 was also
optimized. Subsequent computational removal of the molyb-
denum-bound hydride then gives 2vacant (Figure 3) as a plau-
sible model for a non-agostic structure, having a vacant
ꢀ
agostic C H bond clearly occurs in both complexes, its extent
being the highest in 2b.
The molecular orbital (MO) energy diagram and popula-
tion analysis[12] show that the compositions and energies of the
occupied MOs of both agostomers are quite similar
(Figure 4). The HOMO and HOMOꢀ1 are both largely
metal-based, harboring the four d-electrons. MOs involved
with the agostic interactions (HOMOꢀ12 for 2g; HOMOꢀ13
for 2b) are low in energy and of s symmetry with respect to
coordination site at Mo. A single-point calculation on 2vacant
,
which relaxes to 2g when fully optimized, revealed it to be
approximately 12 kcalmolꢀ1 higher in energy than 2g and 2b.
This provides an upper bound for DEagostic; its magnitude is in
agreement with estimates of strengths of no more than
15 kcalmolꢀ1 for agostic interactions.[2,6]
The weak agostic interactions are readily displaced
(Scheme 3). Not surprisingly, 2 binds acetonitrile essentially
irreversibly to form orange 2-MeCN, but it also binds the
ꢀ
the Mo···H axis; the contribution from the C H moiety is
more significant (21% for 2g; 25% for 2b) than that from Mo
(less than 5%).
Scheme 3. Displacement of the agostic interactions by MeCN and N2.
poorer ligand dinitrogen to give 2-N2. The extent of N2
coordination, under an atmosphere of N2 in fluorobenzene
solution,[22] is only 5–10% at room temperature.[12] Upon
cooling, the equilibrium shifts toward the N2 adduct, and at
ꢀ408C an approximately 2:1 ratio of 2-N2:2 is detected by
NMR spectroscopy. Warming to room temperature again
establishes 2 as the major species, showing that formation of
2-N2 is reversible. 15N and 31P NMR spectra[23] collected for
the isotopomer 2-15N2 provide evidence for the end-on
coordination of N2. Additional reactivity studies of 2 toward
small molecules are currently underway.
In summary, we have described the first example of an
organometallic complex for which two agostomers have been
individually isolated and characterized. These results illus-
trate the versatility of agostic bonding, and have led to the
unanticipated insight that agostomers may be differently
colored. Clearly, one crystal structure will not always provide
the complete “agostic picture”.
Figure 4. Energy level diagram for 2g and 2b, with selected MOs
depicted (isosurface value is 0.03). The double occupancies are only
illustrated for the HOMOs. Only the atoms of the agostic Mo-P-C(-C)-
H moieties are drawn as balls.
An important difference between the agostomers is that
the LUMO of 2b is 0.64 eV higher in energy than that of 2g.
This is probably caused by a higher contribution from the P
atom to the LUMO in 2b (10%) than in 2g (4%), owing to
Experimental Section
ꢀ
the significantly shorter Mo P bond length in 2b (D[d(Mo-
Synthetic details and characterization data, as well as more complete
computational details and atomic coordinates, are given in the
Supporting Information. First principles calculations were performed
using the DFT-D3(BJ) dispersion-corrected PBE exchange-correla-
tion functional[24] implemented in the Amsterdam Density Functional
(ADF 2013.01) program.[25] Scalar relativistic effects were taken into
account by the ZORA formalism to the Dirac equation.[26] TZ2P basis
sets with small cores were used for geometry optimization; all-
electron basis sets were used for property calculations.[27] The time-
P)] = 0.087 ꢀ, Table 1). The resulting different HOMO–
LUMO gaps in 2g and 2b provide the explanation for the
different colors observed, as further quantitatively investi-
gated by time-dependent DFT. Two transitions in the visible
region were found: HOMO!LUMO (2g: 698 nm; 2b:
517 nm) and HOMO-1!LUMO (2g: 574 nm; 2b: 472 nm),
drawn as sticks in Figure 2. For both agostomers, the
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dependent DFT approach was used to calculate the energies and
oscillator strengths of excited states.[28]
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Krossing, I. Raabe, Angew. Chem. 2004, 116, 2116 – 2142;
Angew. Chem. Int. Ed. 2004, 43, 2066 – 2090; b) W. Beck, K.
Sꢂnkel, Chem. Rev. 1988, 88, 1405 – 1421.
Received: June 11, 2013
[14] Owing to the highly fluxional behavior of agostic complexes,
particularly with agostic PiPr3 or PCy3 groups, NMR spectros-
copy is often not definitive in characterizing the agostic
interaction (see also Refs. [17–20]).
[15] CCDC 938519 (1), 938520 (2g), and 938521 (2b·C6H5F) contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[16] The X-ray datasets were of high quality, and all H atoms were
clearly visible in the difference Fourier map. They were placed at
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refined as rotating groups).
[17] H. J. Wasserman, G. J. Kubas, R. R. Ryan, J. Am. Chem. Soc.
1986, 108, 2294 – 2301.
Published online: && &&, &&&&
Keywords: agostic interactions ·
.
density functional calculations · isomers · molybdenum ·
structure elucidation
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ꢀ
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[23] 15N NMR (233 K, 51 MHz): d = 12.9 (br, 1N, Ndistal), ꢀ86.1 ppm
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[13] It is crucial to use non-coordinating solvents and weakly
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4
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Agostic Isomers
Two of a kind: Two agostic isomers of
[CpMo(CO)2(PiPr3)]+ B(C6F5)4ꢀ have been
isolated. Both were characterized in the
solid state by X-ray crystallography and
spectroscopic techniques, and also by
DFT calculations. Significantly different
LUMO energies cause the difference in
color (blue versus orange, see picture) of
these isomers.
E. F. van der Eide, P. Yang,*
R. M. Bullock*
&&&& — &&&&
Isolation of Two Agostic Isomers of an
Organometallic Cation: Different
Structures and Colors
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!