Triple benzylic dehydrogenation by osmium in an amide ligand environment

Article abstract of DOI:10.1021/om050916k

The three benzylic methyl hydrogens of p-isopropy-Itoluene (cymene) are cleaved by ligating N(SiMe₂CH₂P^tBu ₂)₂^- to [(cymene)OsCl₂]₂, to give a carbyne complex; its chemical reactivity with H₂, with CO, and with HC≡CH are described.

Full text of DOI:10.1021/om050916k

802
Organometallics 2006, 25, 802-804
Triple Benzylic Dehydrogenation by Osmium in an Amide Ligand
Environment
Joo-Ho Lee, Maren Pink, and Kenneth G. Caulton*
Department of Chemistry, Indiana UniVersity, Bloomington, Indiana 47405
ReceiVed October 21, 2005
Summary: The three benzylic methyl hydrogens of p-isopropy-
ltoluene (cymene) are cleaVed by ligating N(SiMe2CH2PtBu2)2
-
to [(cymene)OsCl2]2, to giVe a carbyne complex; its chemical
reactiVity with H2, with CO, and with HCtCH are described.
Precisely because of the predictive power of the periodic table,
deviations from chemical periodicity are exceptionally impor-
tant. For example, are 4d and 5d transition metals (here Ru and
Os) merely carbon copies of each other?
The surprising character of the reaction of ClMgN(SiMe2-
CH2PtBu2)2 (ClMg(PNP)) with [(cymene)RuCl2]2 is that cymene
(p-CH3C6H4 Pr) is expelled, even though what is left behind,
i
Figure 1. ORTEP drawing (50% probability) of (PNP)Os(H)2-
(CC6H4-p-iPr) showing selected atom labeling. Only the hydride
hydrogens are shown, and unlabeled atoms are carbon. Selected
bond lengths (Å) and angles (deg): Os-C23, 1.746(3); Os-P1,
2.3899(7); Os-H1, 1.46(4); Os-H2, 1.52(4); C23-Os-N1, 175.86-
(10); P1-Os-N1, 83.72(6); P1-Os-P2, 167.10(2); C23-Os-P1,
95.64(8).
(PNP)RuCl, is very electron deficient and has an unusually low
coordination number: 4. Further consequences of this low-
coordinate and π-basic (i.e. amide) ligand environment are as
follows: (PNP)RuCl is planar, has S ) 1 for a 4d element with
six d electrons, and has no agostic interactions.1,2 In our previous
work3 with monodentate phosphines and with hydride and
chloride ligands, systematic comparison of the 4d and 5d
analogues Ru and Os revealed that Os (vs Ru) preferred to be
saturated (18 valence electrons), was in a higher oxidation state,
had more metal-ligand bonds (at the expense of intraligand
atom-atom′ bonds), and had a higher coordination number (cf.
A and B).4 The underlying cause of this difference could be
metal, the consumption of 2 mol of amide per Os, and the
retention of cymene on Os. One role of the amide reagent is
thus to carry away one of the three H atoms which originates
from the cymene methyl group. Since “extra” (vs Ru) chloride
is also removed from Os, the combined effect of this equivalent
of amide is dehydrohalogenation. In contrast to the case for
Ru, an 18-electron, chloride-free osmium product results. The
product for osmium is a dihydride (one hydride triplet at -1.02
ppm), and the cymene, while it is retained in the product
complex, has been triply dehydrogenated exclusively at this ring
methyl to produce a carbyne ligand (13C chemical shift 259
ppm). The full 1H and 31P NMR spectra indicate the product to
have C2V symmetry. This was confirmed by X-ray diffraction
(Figure 1), which shows the molecule to be six-coordinate and
octahedral. Noteworthy features include the long Os-N distance
(2.226(2) Å), consistent with the molecule having an 18-valence-
electron configuration without any NfOs lone pair π-donation
(hence, a single Os-N bond). The presence of a lone pair on
the R-atom of the ligand has been termed “π-loaded,”6 and
VI
RuIVCl2(dCHR)(PR3)2 Os HCl2(tCR)(PR3)2
vs
A
B
rationalized by saying either that the 5d metal is generally more
reducing than the 4d analogue or that the 5d metal forms
stronger metal-ligand bonds. Against a background of these
general principles, we report here an initial look at osmium
reaction chemistry with the strongly electron donating, monoan-
ionic PNP ligand. This shows surprising differences from the
chemistry with ruthenium that, in hindsight, serve to strengthen
the trends summarized above as one goes down the group from
Ru to Os.
Reaction of [(cymene)OsCl2]2 with a source5 of [tBu2PCH2-
SiMe2]2N- in benzene proceeds to completion in 18 h at 22 °C
according to Scheme 1. Differences here from the analogous
reaction with ruthenium include loss of both chlorides from the
(PNP)Os(H)2(tCAr) (Ar ) C6H4 Pr) shows an unusual degree
i
of twist of the NSi2 plane with respect to the P-Os-P plane
(transoid dihedral angles P-Os-N-Si ) 143.2 and 144.7°),
which might serve to diminish the four-electron repulsion with
a filled dπ orbital. If the synthetic reaction is carried out in neat
toluene, there is no trace of (PNP)Os(H)2(tCC6H5), indicating
that the cymene is tightly held to osmium during its dehydro-
genation (i.e. the dehydrogenation is intramolecular).
* To whom correspondence should be addressed. E-mail: caulton@
indiana.edu.
(1) Watson, L. A.; Ozerov, O. V.; Pink, M.; Caulton, K. G. J. Am. Chem.
Soc. 2003, 125, 8426-8427.
(2) Walstrom, A. N.; Watson, L. A.; Pink, M.; Caulton, K. G. Organo-
metallics 2004, 23, 4814-4816.
Despite the absence of empty orbitals on osmium in (PNP)-
Os(H)2(tCAr), this molecule is completely consumed (Scheme
1) in less than 1 h at 22 °C in benzene under 1 atm (i.e. excess)
(3) Caulton, K. G. J. Organomet. Chem. 2001, 617-618, 56-64.
(4) See also: Gusev, D. G.; Lough, A. J. Organometallics 2002, 21,
2601-2603.
of H2. The resulting product, (PNP)Os(H)3, shows (1H and 31
P
(5) The lithium salt employed has the formula Li3Cl(PNP)2 (i.e. is ether-
free), and it also forms (PNP)RuCl from [(cymene)RuCl2]2: Fryzuk. M.
D.; Giesbrecht. G. R.; Tettig, S. J. Organometallics 1997, 16, 725.
(6) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239-482.
10.1021/om050916k CCC: $33.50 © 2006 American Chemical Society
Publication on Web 01/13/2006

Communications
Organometallics, Vol. 25, No. 4, 2006 803
Scheme 1
NMR) C2V symmetry, with the added feature of dynamic
averaging of the three hydrides to a single triplet. Free cymene
is also detected. Under 1 atm of H2 in benzene the hydride NMR
signal has an unchanged chemical shift but is broadened (36
Hz full width at half-height) at 22 °C, reflecting the operationally
unsaturated character of (PNP)Os(H)3 but the low Keq for
binding H2 to this six-coordinate d4 OsIV center. Consistent with
this, under 1 atm of D2 at 22 °C, the hydride resonance is gone
from the H NMR spectrum within 30 min; D NMR of the
resulting sample shows the Os(D)3 signal but negligible
deuterium at any site of the coordinated PNP ligand, even after
24 h under 1 atm of D2 at 22 °C.
now appears as a triplet (P) of doublets (from Os-H) at +19.5
ppm, a chemical shift generally indicative of H directly bound
to a carbene carbon. Carbene (269.6 ppm) and CO (194.9 ppm)
chemical shifts are also evident. The high rate of this addition
to saturated osmium in (PNP)Os(H)2(tCAr) suggests that there
is some preequilibrium to produce a kinetically significant
concentration of unsaturated (PNP)OsH[dC(H)Ar], and this is
what actually is attacked by CO. This rearrangement also
increases the d electron count (i.e. reduces Os), enhancing the
back-donating potential toward CO.
1
2
Over 4 h at 65 °C in benzene solution under argon (Scheme
1), the carbene complex transforms completely into a product
1
(PNP)Os(H)3 is completely consumed (Scheme 1) by excess
(1 atm) HCCH in less than 30 min in benzene at 22 °C. Two H
atoms have been lost to 1 equiv of alkyne, producing ethylene
(1H NMR assay) together with a primary product, (PNP)Os-
(CdCH2)H, with mirror symmetry and a unit intensity hydride
triplet. The 13C{1H} NMR of the product from H213C2 supports
the vinylidene ligand, via chemical shifts of 285.6 (dt, CR) and
with no hydride or carbene dCH H NMR signal, but with an
intensity 2 1H NMR signal at 4.76 ppm. In this species, (PNP)-
Os(CH2Ar)(CO), the CO stretching frequency has decreased to
1865 cm-1 from the value in the carbene carbonyl (1991 and
1963 cm-1).
With similar facility, operationally unsaturated (PNP)Os(H)3
is consumed by 1 atm (i.e. excess) of CO in benzene at 22 °C
within 30 min to liberate H2 (1H NMR detection) and form
(PNP)OsH(CO)2, identified by a hydride triplet, by diaste-
90.7 ppm (Câ, d from JC -C ); the 31P{1H} NMR of this
a
b
isotopomer is a doublet of doublets (coupling to CR and Câ),
and the hydride signal is a triplet of doublets (10.8 Hz coupling
to CR). The deuterium NMR of the product from DCCD shows
peaks at 0.4 ppm (dCdCD2) and -16 ppm (Os-D), indicating
scrambling of hydride with acetylene protons at some point in
the reaction. The preferred vinylidene structure for Os (C) is
t
reotopic Bu, CH2, and SiMe proton NMR signals, and by CO
vibrational frequencies at 1963 and 1898 cm-1 (Scheme 1). The
31P{1H} NMR singlet becomes a doublet on selective hydride
coupling. Elimination of H2 is a typical consequence of binding
a π-acidic ligand (CO) to a polyhydride due to depletion of
metal electron density needed to keep H at the monoatomic H-
(vs H2) oxidation state. It is thus interesting that this does not
occur for CO reacting with PNPOs(H)2(tCAr).
Reaction of (PNP)Os(13CO)(CH2Ar) with 1 atm of H2
(excess) in benzene produces (PNP)OsH(13CO) and free cymene
within 30 min at 22 °C (Scheme 1). The selectively hydride
coupled 31P NMR spectrum (dd, coupled to 13CO and OsH)
shows the product to be a monohydride. In addition, the very
negative chemical shift (-29.4 ppm) of the monohydride
indicates that there is a vacancy trans to the hydride. The full
1H and 31P NMR spectra show that the resulting product has Cs
symmetry. Under 1 atm of H2, the signal at 4.1 ppm (dissolved
H2) is broadened, as is the hydride signal, but without change
in chemical shifts. Addition of excess isotopically normal CO
(1 atm, 22 °C) to this monocarbonyl forms (PNP)OsH(13CO)-
(CO) (Scheme 1). The change in the hydride chemical shift to
-4.8 ppm indicates that one CO is now trans to the hydride.
thus in contrast with that for Ru2 (D), a distinction which is
also reported for the phenyl-based pincer ligand 2,6-(CH2-
PR2)2C6H3 on these two metals.7
(PNP)Os(H)2(tCAr) reacts completely with 1 atm of CO
within 10 min at 22 °C in benzene (Scheme 1). The product,
(PNP)OsH(dCHAr)(CO), has mirror symmetry (e.g. 2 equiv
of P) but no second symmetry element: two tBu virtual triplets
1
and two SiMe singlets by H NMR. One hydride triplet of
doublets has unit intensity, and the second former hydride proton
1
The H and 31P NMR chemical shifts show that the resulting
(7) Wen, T. B.; Cheung, Y. K.; Yao, J.; Wong, W.-T.; Zhou, Z. Y.; Jia,
G. Organometallics 2000, 19, 3803-3809.
product is the same as that from the reaction of (PNP)Os(H)3

804 Organometallics, Vol. 25, No. 4, 2006
Communications
with CO. 13C NMR (187.4 ppm as a natural-abundance doublet
from Os to the carbyne C. In addition, all of these results support
the generalizations at the beginning of this paper regarding
contrasts between Ru and Os. The cleavage of H-C(sp3) bonds
effected here by Os suggests there might be potential for use
of operationally unsaturated Os complexes for nonphotochemical
alkane dehydrogenation at or near 20 °C.10-12
1
of triplets and 183.5 ppm as a strong, 13C-enriched triplet), H
NMR (Os-H, doublet of triplets), and 31P{1H} NMR (one
doublet) show that there is no scrambling of CO. Comparison
of the 13CO coupling constants (JCH) of (PNP)OsH(13CO) (10.8
Hz)and (PNP)OsH(13CO)(CO) (6 Hz) indicates that 13CO
occupies the site cis to hydride in the latter compound (Scheme
1); thus, the reaction is stereoselective.
Acknowledgment. This work was supported by the NSF.
The reactions of (PNP)Os(H)2(tCAr) and of (PNP)Os(H)3
reported here all occur unusually quickly for the normally
kinetically sluggish six-coordinate osmium. We suggest that this
originates from a combination of operational unsaturation8,9
(from the amide N lone pair) and a facile hydride migration
Supporting Information Available: Crystallographic details,
as a CIF file, for the carbyne complex and text and figures giving
full experimental details and characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM050916K
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