Radical (NO) and nonradical (N2O) reagents convert a ruthenium(IV) nitride to the same nitrosyl complex

Article abstract of DOI:10.1021/ic700789y

The ruthenium(IV) nitride complex (PNP)RuN (PNP = (Bu₂PCH ₂-SiMe₂)₂N^-) reacts rapidly with 2NO to form (PNP)Ru(NO) and N₂O, via no detectable intermediate. The linear nitrosyl complex has a planar structure. In a slower reaction, (PNP)RuN reacts with N₂O by O-atom transfer (established by ¹⁵N labeling) to give the same nitrosyl complex and N₂. Density functional theory (B3LYP) calculations show both reactions to be very thermodynamically favorable. Analysis of possible intermediates in each reaction shows that radical (PNP)RuN(NO) has much spin density on nitride N (hence, N^2-), while one 2 + 3 metallacycle, (PNP)RuN₃O, has the wrong connectivity to form a product. Instead, an intermediate with a doubly bent N₂O (hence, a two-electron reduced N-nitrosoimide form) brings the O atom in proximity to the nitride N on the path to a product.

Full text of DOI:10.1021/ic700789y

Inorg. Chem. 2007, 46, 7704−7706
Radical (NO) and Nonradical (N2O) Reagents Convert a Ruthenium(IV)
Nitride to the Same Nitrosyl Complex
Amy Walstrom, Maren Pink, Hongjun Fan, John Tomaszewski, and Kenneth G. Caulton*
Department of Chemistry, Indiana UniVersity, Bloomington, Indiana 47405
Received April 25, 2007
The ruthenium(IV) nitride complex (PNP)RuN (PNP
)
(tBu2PCH2-
SiMe2)2N-) reacts rapidly with 2NO to form (PNP)Ru(NO) and N2O,
via no detectable intermediate. The linear nitrosyl complex has a
planar structure. In a slower reaction, (PNP)RuN reacts with N2O
by O-atom transfer (established by 15N labeling) to give the same
nitrosyl complex and N2. Density functional theory (B3LYP) calcu-
lations show both reactions to be very thermodynamically favor-
able. Analysis of possible intermediates in each reaction shows
that radical (PNP)RuN(NO) has much spin density on nitride N
Figure 1. ORTEP drawing (50% probability) of the non-H atoms of
(PNP)Ru(NO) showing labels of the non-C atoms. Selected structural
parameters: Ru-N1, 2.0695(16) Å; Ru-N2, 1.7213(19) Å; Ru-P1, 2.3834-
(5) Å; N2-O2, 1.185(3) Å; N1-Ru-N2, 179.21(9)°; P1-Ru-P2, 173.043-
(19)°; Ru-N2-O2, 179.2(2)°.
(hence, N2-), while one 2
+ 3 metallacycle, (PNP)RuN3O, has
the wrong connectivity to form a product. Instead, an intermediate
with a doubly bent N2O (hence, a two-electron reduced N-nitroso-
imide form) brings the O atom in proximity to the nitride N on the
path to a product.
the structure6 of a coordinated N2O, and the present work
makes some contribution to that subject as well as to the
surface chemistry of metal nitrides with nitrogen oxides.
Reaction7 of (PNP)RuIVN with NO (∼1:1 mole ratio to
avoid degradation of the product by a local excess of NO)
in benzene at 22 °C is complete in time of mixing to give a
The recent synthesis1 of (PNP)RuIVN opens the question
of the reactivity of this RuN unit: nucleophilic or electro-
philic? Learning to control the reactivity of such a terminal
nitride will be an essential step in the conversion of N2 to
amines and heterocycles, for example. We report here high-
yield reaction chemistry which shows the ability of this
molecule to react with two different nitrogen oxides, one a
radical (NO) and the other an even-electron species (N2O).
Both nitrogen oxides yield the same (and diamagnetic) metal-
containing product but by reactions involving very different
processes. The discovery of two paths for production of the
same ruthenium product enables some interesting thermo-
dynamic conclusions to be drawn. In contrast to N2O
reactions exhibited by oxophilic early transition metals,2,3
the results reported here show no transfer of oxygen to the
metal.4,5 Finally, there is no experimental determination of
1
new product whose H and 31P NMR spectra indicate C2V
symmetry and display an NO stretch in the IR at 1691 cm-1
(an Et2O solution). An electrospray ionization mass spectrum
gives a peak consistent with formula (PNP)Ru(NO), and an
X-ray diffraction structure determination of crystals grown
from Et2O (Figure 1) shows the molecule to be a Ru0
complex with a linear RuNO unit. The other product in this
balanced reaction (eq 1) is nitrous oxide (detected by IR in
(PNP)RuN + 2NO f (PNP)RuNO + N2O
(1)
toluene or in Et2O at 2217 cm-1), so the first NO effects a
three-electron reduction (eq 2). When the reagents are
N
3- + NO f N2O + 3e-
(2)
* To whom correspondence should be addressed. E-mail: caulton@
indiana.edu.
(1) Walstrom, A.; Pink, M.; Yang, X.; Tomaszewski, J.; Baik, M.-H.;
Caulton, K. G. J. Am. Chem. Soc. 2005, 127, 5330.
combined below -78 °C in toluene, no intermediate is
detected by 31P or 1H NMR. One sees only unreacted (PNP)-
RuN and product (PNP)Ru(NO), with conversion to the latter
increasing as the temperature is slowly raised. No paramag-
(2) Cherry, J.-P. F.; Johnson, A. R.; Baraldo, L. M.; Tsai, Y.-C.; Cummins,
C. C.; Kryatov, S. V.; Rybak-Akimova, E. V.; Capps, K. B.; Hoff, C.
D.; Haar, C. M.; Nolan, S. P. J. Am. Chem. Soc. 2001, 123, 7271.
(3) Conry, R. R.; Mayer, J. M. Inorg. Chem. 1990, 29, 4862.
(4) For leading references, see: Gorelsky, S. I.; Ghosh, S.; Solomon, E.
I. J. Am. Chem. Soc. 2006, 128 (1), 278.
(6) Paulat, F.; Kuschel, T.; Naether, C.; Praneeth, V. K. K.; Sander, O.;
Lehnert, N. Inorg. Chem. 2004, 43, 6979.
(7) See the Supporting Information.
(5) Pamplin, C. B.; Ma, E. S. F.; Safari, N.; Rettig, S. J.; James, B. R. J.
Am. Chem. Soc. 2001, 123, 8596.
7704 Inorganic Chemistry, Vol. 46, No. 19, 2007
10.1021/ic700789y CCC: $37.00
© 2007 American Chemical Society
Published on Web 08/18/2007

COMMUNICATION
Scheme 1. Reaction Electronic Energies (kcal/mol)
1
netically shifted H NMR signals due to a 1:1 intermediate
gives the more thermodynamically favorable intermediate
and forms the good leaving group N2O. This intermediate
has C2V symmetry (η1-linear N2O), and thus [(PNP)Ru]0 is
unable to reduce N2O; 97% of the spin density in this radical
is on Ru, so this is RuI coordinating neutral N2O. We have
sought minima where η1-N2O is singly bent at either the
terminal or the central N, or doubly bent at both N atoms,
and find that these all optimize to the C2V structure. The
second NO oxidizes this RuI, becoming bent NO- ( RuNO
) 124.4°) in this radical-quenching step.7
The radical complex 1 from the less exothermic step a
(Figure 2) has remarkable spin densities 1a (0.00 at N6, 0.02
at O7, and only 0.06 at Ru and 0.01 at PNP amide N); the
spin density (0.84) is mainly on the nitride N, where it is
thus prepared to couple to the second arriving NO radical;
the alternative Lewis structure 1b contributes negligibly. This
spin density at nitride N, creating N2- radical character,
shows that coordination of NO to Ru effects redox chemistry
not so much at the metal but that an electron-rich nitride
ligand is especially vulnerable to oxidation by electron-poor
NO•. The bent NO in (PNP)RuN(NO) becomes linear in step
c (Scheme 1),7 promoting the loss of N2O. Adding a ligand
to an unsaturated metal is expected to have no major barrier,
so we located transition states (TS1 and TS2) for forming
N-N bonds. TS2 (Scheme 1) has the lower free energy (by
“(PNP)RuNNO”, or any subsequent species “(PNP)RuN3O2”
reach detectable concentration. Thus, the rate-determining
step occurs at the first encounter of (PNP)RuN with NO.
Reaction of (PNP)Ru15N with isotopically normal NO shows
(15N and 31P NMR and IR evidence) (15N)NO and isotopically
normal (PNP)Ru(NO). The reaction is thus N atom transfer
from Ru to NO, not O atom transfer to nitride.8
Remarkably, the reaction7 of (PNP)RuN with N2O (excess;
∼2 atm) in benzene at 22 °C also produces (PNP)Ru(NO)
in a reaction which shows a different (i.e., nonradical) aspect
of the reactivity of this nitride complex. The reaction is much
slower than that with NO, requiring 2 days to go to
completion to form this single metal complex. The balanced
reaction (eq 3) could be accomplished by either O atom
transfer to RuN (i.e., N2-O bond cleavage) or N atom
transfer to RuN (i.e., N-NO bond cleavage). When the
(PNP)RuN + N2O f (PNP)RuNO + N2
(3)
reaction is carried out with labeled (PNP)Ru(15N) and
isotopically normal N2O, 15N NMR shows the ruthenium
product to be (PNP)Ru(15NO) (344 ppm, t, JNP ) 6.5 Hz).
The reaction is thus O atom transfer.9
Density functional theory (DFT; B3LYP) calculations7
provide energetic and structural information on several
potential intermediates in the mechanisms of both reactions.
As shown in Scheme 1, the overall reactions with both NO
and N2O are highly favorable energetically. Consistent with
the rapid rate observed experimentally, the reaction with NO
begins (a or b in Scheme 1) with very energetically favorable
1:1 adduct formation. Does the first NO attack Ru (a) or
nitride (b)? The deeper minimum is b, which is also sterically
better, where the first NO reduces Ru by three electrons (eq
2). Thus, although the reagent Ru is four-coordinate and it
is exergonic to bind NO at Ru, attack at the nitride ligand
(8) McCarthy, M. R.; Crevier, T. J.; Bennett, B.; Dehestani, A.; Mayer,
J. M. J. Am. Chem. Soc. 2000, 122, 12391.
Figure 2. DFT (B3LYP)-optimized structures. Right, 1, (PNP)RuN-
(NO): Ru-N2, 2.17 Å; Ru-N5, 1.74 Å. Left, 2, (PNP)RuN(N2O): Ru-
N2, 2.14 Å; Ru-N5, 1.64 Å; Ru-N6, 2.16 Å; N6-N7, 1.20 Å; N7-O8,
1.26 Å; Ru-N6-N7, 126.7°; N6-N7-O8, 133.6°.
(9) For conversion of (trpy)OsNCl2+ to (trpy)Os(NO)Cl2+ using Me3NO,
see: Williams, D. S.; Meyer, T. J.; White, P. S. J. Am. Chem. Soc.
1995, 117, 823.
Inorganic Chemistry, Vol. 46, No. 19, 2007 7705

COMMUNICATION
2.8 kcal/mol), with a long N-N distance (1.85 Å) and a
strongly bent N‚‚‚NO (112.1°), as these radical centers
couple in the faster paths a and c.
O atom transfer has a reasonable mechanism due to a very
plausible metallacycle transition state. One mechanism of
the radical reaction (NO) involves the initially bound NO
creating radical character on the nitride in (PNP)RuN(NO),
1a, by partial oxidation of that center, followed by radical-
radical annihilation [i.e., (Ru)N-N(O) coupling] to form a
transient containing the good leaving group, N2O. The
surprisingly favorable thermodynamics for both reactions can
be attributed to the stability of electron-rich (PNP)Ru- bound
to π-acidic NO+, as well as the favorable coproducts N2 and
N2O. Given the large swing of the Ru oxidation state in the
reaction, this is not merely the reactivity of N3- but of the
entire reagent RuN+ unit.
Because νNO of (PNP)Ru(NO) is among the lowest
recorded for a neutral molecule containing a linear MNO
unit [surpassed only by TpW(NO)(PMe3)(η2-olefin)11 Cp*W-
(NO)R2,12 and WX3(NO)(pyridine),13 with X ) OR or NR2,
all near 1550 cm-1], the (PNP)Ru fragment has strong
reducing power. This enables recognition here of the two-
electron-reduced N-nitrosoimide bonding to a metal (2),
whose conformational flexibility facilitates O atom transfer.
The N2O reaction can begin with adduct formation (Figure
2). Species 2 is bent7 to near sp2 hybridization at both N6
and N7, consistent with Lewis structure 2 (an N-nitrosoimide
of RuVI), which leads naturally to the metallacycle 3 (RuIV),
where N-O bond formation has begun; 3 is found to be the
transition state associated with O atom transfer. The energy
of binding N2 to (PNP)Ru(NO) in species (PNP)Ru(NO)-
(N2),7 11.1 kcal/mol, is calculated to be comparable to the
unfavorable T∆S for adduct formation, so N2 release is
spontaneous. The energy of 2 is too high to reach an observ-
able concentration. We also sought alternative structures.
Remarkably, the species 4 from 1,3-addition of NdNdO
across the RutN bond with an orientation opposite from
that of 3 is slightly more stable (electronic energy) than the
separated particles, so the ∆G° for its formation10 will be 0
( 5 kcal/mol. Because both five-membered rings have
satisfactory Lewis structures, it is remarkable that 3 and 4
differ in energy by 22.9 kcal/mol. Clearly, 4 lacks the
connectivity needed to form RuNO, and therefore, 4 is a
dead end, not an intermediate on the reaction path.
Acknowledgment. This work was supported by the NSF.
Returning to the question which begins this contribution,
these results show a versatility of (PNP)RuN to participate
in both radical and nonradical reactions. The nonradical
reaction (N2O) relies on the nucleophilic character of the
nitride (N3-), in (PNP)RuN(N2O), together with a new
“doubly bent” binding mode (2) of N2O to a metal vulnerable
to oxidation to give the N-nitrosoimide form (hence, a large
reorganization of the N2O electronic structure), which
simultaneously brings its O atom close to the nitride N. The
Supporting Information Available: Full computational and
experimental details and spectroscopic data (including DFT calcula-
tions), together with applicable CIF files. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC700789Y
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7706 Inorganic Chemistry, Vol. 46, No. 19, 2007