O-protonation of a terminal nitrosyl group to form an η1-hydroxylimido ligand [14]

Full text of DOI:10.1021/ja002738m

J. Am. Chem. Soc. 2001, 123, 8143-8144
8143
O-Protonation of a Terminal Nitrosyl Group To
Form an η1-Hydroxylimido Ligand
W. Brett Sharp, Peter Legzdins,* and Brian O. Patrick
Department of Chemistry
The UniVersity of British Columbia
VancouVer, British Columbia, Canada V6T 1Z1
ReceiVed April 18, 2001
Nitric oxide (NO) has a rich chemistry within living organisms
and continues to gain ever increasing recognition for the physi-
ological roles it plays in biological systems.1 Since some of its
chemistry in these environments is mediated by transition metals,
studies of the chemistry and biochemistry of NO bound to a metal
center as the nitrosyl ligand are also experiencing a renaissance.
For instance, Shiro and co-workers have recently studied the nitric
oxide reductase (Nor) enzyme isolated from the denitrifying
fungus Fusarium oxysporum.2 By utilizing resonance Raman and
crystallographic techniques, they have concluded that for this
cytochrome P450-type heme enzyme (P450nor) it is probably the
protonation of a terminal heme-nitrosyl oxygen by water that
constitutes one of the key mechanistic steps during its catalysis
of the reduction of NO to N2O. Such complexation of H+ has
been unequivocally demonstrated for the more basic doubly and
triply bridging NO groups,3 but to date it has only been
inferred,4a,5a,b sometimes incorrectly,4b,5c for terminal NO ligands.
We now wish to report the first definitively characterized
examples of O-protonated terminal nitrosyl ligands and the
changes to the metal-NO bonding interactions that occur within
the resulting η1-hydroxylimido-metal linkages.
Figure 1. The solid-state molecular structure of cation 2b in 2b[BArf4];
50% probability thermal ellipsoids are shown.
1). Similarly, reaction of 1b with triflic acid generates the
unsolvated complex [Cp*WBz2(NOH‚OTf)] (3). Crystallographic
and spectral data indicate that the WNOH linkages in these
complexes are best viewed as involving hydroxylimido ligands,
i.e., WtN-OH.
A single-crystal X-ray crystallographic analysis of 2b[BArf4]9
(Figure 1) reveals that the metrical parameters of the Cp*W(η1-
CH2Ph)(η2-CH2Ph) fragment of cation 2b are generally similar
to those exhibited by its neutral precursor 1b (W(1)-C(19) )
2.509 Å).8 However, the W-N bond in 2b is significantly shorter
than that extant in 1b (W(1)-N(1) ) 1.716(5) (2b), 1.752(3) Å
(1b)) and within the range typical of WtN(imido) bond lengths,10
whereas the N-O bond is significantly longer (N(1)-O(1) )
1.339(6) (2b), 1.239(5) Å (1b)). The proton of the hydroxylimido
ligand in 2b was refined isotropically, thereby permitting an
estimation of its distance from the nitrosyl oxygen atom (i.e.
0.90(7) Å) and the N(1)-O(1)-H(52) bond angle (103(4)°).
Finally, a molecule of Et2O within the crystal lattice is hydrogen
bonded to the hydroxylimido proton with a O(2)-H(52) contact
of 1.71(8) Å.
Cp*WR2(NO) + [H(OEt2)2]+ f
[Cp*WR2(NOH‚OEt2)]+ + Et2O (1)
Reaction of 1 equiv of the oxonium acids [H(OEt2)2][B(3,5-
(CF3)2C6H3)4] (HBArf4)6 or [H(OEt2)2][B(C6F5)4] (HBφf4),7 with
the bis(hydrocarbyl) nitrosyl complexes, Cp*WR2(NO) (1, R )
CH2SiMe3 (a), CH2Ph (b)),8 results in quantitative formation of
the corresponding cations [Cp*WR2(NOH‚OEt2)] 2a and 2b (eq
The ν(NO) of the hydroxylimido ligand is obscured by
counterion absorptions in the IR spectra of 2a[BArf4], 2b[BArf4],
and 3 as KBr pellets. However, a band at 1303 cm-1 that shifts
under a counterion band at 1270 cm-1 in the IR spectrum (KBr)
of 2b[Bφf4]-15N can be assigned to the N-O stretch of 2b[Bφf4].
This absorption is ca. 250 cm-1 lower in energy than the ν(NO)
of 1b (1556 cm-1).8 Such a change is indicative of a highly
reduced N-O bond order and reinforces the description of these
complexes as containing terminal hydroxylimido ligands.11
The 1H and 13C NMR spectra of complexes 2 and 3 are
qualitatively similar to those of their precursor nitrosyl complexes
1 and indicate that they retain their Cp*W(η1-CH2Ph)(η2-CH2-
Ph) cores in solutions. Resonances due to the coordinated Et2O
molecule are evident in the NMR spectra of 2 but not in the
spectra of 3, thereby indicating that in 3 the triflate anion probably
remains hydrogen bonded to the hydroxylimido proton.12 A signal
due to the -NOH proton is evident between 12.5 and 14.5 ppm
in the 1H NMR spectra of 2 and 3 obtained in Et2O-d10 or
chlorinated solvents.13 Interestingly, the ambient-temperature 1H
(1) (a) Wink, D. A.; Mitchell, J. B. Free Radical Biol. Med. 1998, 25,
434. (b) Adams, D. R.; Brochwicz-Lewinski, M.; Butler, A. R. Nitric Oxide:
Physiological Roles, Biosynthesis and Medical Uses. In Progress in the
Chemistry of Organic Natural Products; Herz, W., Falk, H., Kirby, G. W.,
Moore, R. E., Tamm, Ch., Eds.; Springer: New York, 1999; No. 76.
(2) (a) Obayashi. E.; Takahashi, S.; Shiro, Y. J. Am. Chem. Soc. 1998,
120, 12964. (b) Shimizu, H.; Obayashi. E.; Gomi, Y.; Arakawa, H.; Park,
S.-Y.; Nakamura, H.; Adachi, S.; Shoun, H.; Shiro, Y. J. Biol. Chem. 2000,
275, 4186.
(3) (a) Stevens, R. E.; Gladfelter, W. L. J. Am. Chem. Soc. 1982, 104,
6454. (b) Gladfelter, W. L.; Stevens, R. E.; Guettler, R. D. Inorg. Chem. 1990,
29, 451. (c) Hash, K. R.; Rosenberg, E. Organometallics 1997, 16, 3593. (d)
Delgado, E.; Jeffery, J. C.; Simmons, N. D.; Stone, F. G. A. J. Chem. Soc.,
Dalton. Trans. 1986, 869. (e) Legzdins, P.; Nurse, C. R.; Rettig, S. J. J. Am.
Chem. Soc. 1983, 105, 3727.
(4) (a) Roberts, R. L.; Carlyle, D. W.; Blackmer, G. L. Inorg. Chem. 1975,
14, 2739. (b) Bottomley, F. Reactions of Nitrosyls. In Reactions of Coordinated
Ligands; Braterman, P. S., Ed.; Plenum: New York, 1989; Vol. 2, pp 115-
222.
(5) (a) Nast, R.; Schmidt, J. Angew. Chem., Int. Ed. Engl. 1969, 8, 383.
(b) van Voorst, J. D. W.; Hemmerich, P. J. Chem. Phys. 1966, 45, 3914. (c)
Bowden, W. L.; Bonnar, P.; Brown, D. B.; Geiger, W. E. Inorg. Chem. 1977,
16, 41.
(9) Crystal data for 2b[BArf4]: C60H52NO2F24WB, T ) 173 K, Mr
)
1469.70, yellow, block, orthorhombic, Pna21 (No. 33), a ) 26.696(1) Å, b )
12.9115(3) Å, c ) 17.5937(8) Å, V ) 6064.3(9) Å3, Z ) 4, R ) 0.049, Rw
) 0.066, GOF ) 0.65.
(6) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992, 11,
3920.
(7) Jutzi, P.; Mu¨ller, C.; Stammler, A.; Stammler, H.-G. Organometallics
2000, 19, 1442.
(10) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239.
(11) 15N labeling studies revealed no bands attributable to WtN stretches
in the IR spectra of these compounds.
(8) 1a: Veltheer, J. E.; Legzdins, P. In Synthetic Methods of Organometallic
and Inorganic Chemistry; Herrmann, W. A., Ed.; Thieme Verlag: New York,
1997; Vol. 8, pp 79-85. 1b: Legzdins, P.; Jones, R. H.; Phillips, E. C.; Yee,
V. C.; Trotter, J.; Einstein, F. W. B. Organometallics 1991, 10, 986.
(12) In this regard the ν(SO) stretch of the triflate anion is found at 1306
cm-1 in the IR spectrum (KBr) of 3. This value is slightly higher than that
expected for a nonassociated triflate anion. See: Lawrance, G. A. Chem. ReV.
1986, 86, 17.
10.1021/ja002738m CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/28/2001

8144 J. Am. Chem. Soc., Vol. 123, No. 33, 2001
Communications to the Editor
NMR spectra of equimolar mixtures of either 2 or 3 and the
corresponding free acid exhibit a single broad resonance situated
in a position intermediate to those of the hydroxylimido proton
and the free acid. This observation indicates that in acidic solution
the -NOH proton exchanges with free acid. Decoalescence of
these signals occurs upon cooling of the solutions.14 The 15N NMR
spectra of isotopically labeled 2a/b-15N exhibit signals that are
shifted ca. 30 ppm upfield relative to those of 1, but these values
of δ are still within the range associated with the sp-hybridized
N atoms of linear, terminal nitrosyls.15 No 15N-1H coupling
between the hydroxylimido proton and the labeled nitrosyl
nitrogen is evident even at -70 °C.
The hydroxylimido protons in 2 and 3 are acidic and can be
readily abstracted by basic reagents such as pyridine to reform
the neutral nitrosyl species. Furthermore, these complexes are
unstable with respect to protonolysis of a hydrocarbyl ligand under
appropriate conditions. For instance, dissolution of 2b and 3 in
acetonitrile for 24 h leads to the clean formation of the known
cationic complex [Cp*W(Bz)(NO)(NCMe)]+.16 This observation
suggests that in MeCN solution the coordination of H+ to the
nitrosyl is reversible and ultimately leads to protonolysis.
The reaction of 1b with HCl generates the known benzyl chloro
complex, Cp*W(Bz)Cl(NO), with no observable hydroxylimido
intermediate.17 It thus appears that the counterion of the acid plays
a key role in permitting the isolation of the product resulting from
the kinetic site of protonation, namely the nitrosyl ligand.
Surprisingly, a different mode of reactivity occurs during the
reaction of the oxonium acid, HBF4‚OEt2, with complexes 1. In
this instance the nitrosyl-boron trifluoride adducts, Cp*WR2-
(NOBF3) (4, R ) CH2SiMe3 (a), CH2Ph (b)) (eq 2 ) are
Figure 2. The solid-state molecular structure of 4a; 50% probability
thermal ellipsoids are shown.
is sharply bent at an angle of 118.0(2)°. These distortions to the
bond lengths and angles are similar to those reported for the two
other nitrosyl-borane adducts that have been structurally char-
acterized and, consistent with the IR spectroscopic evidence,
display less alteration than that evident in the structure of
2b[BArf4].19
The complexes 4 are probably formed via a reaction pathway
that transiently involves analogues of 3. The BF4-counterion is
known to hydrogen bond to the proton of an η3-hydroxylimido
proton.3e It is therefore not unreasonable that an initially formed
hydroxylimido species such as [Cp*WR2(NOH‚FnBF4-n)] de-
composes via HF elimination to afford the neutral isonitrosyl
complexes 4.
In summary, we have presented here the first well-characterized
examples of compounds containing the biologically relevant η1-
hydroxylimido ligand. While reasonably thermally stable, the
ligand itself is acidic and prone to transfer its proton to basic
substrates. Furthermore, the stabilization of this ligand is depend-
ent on the nature of the acid used to effect the protonation of the
nitrosyl precursor. Our investigations of the reactivity of terminal
nitrosyl ligands with other important electrophiles are currently
in progress.
Cp*WR2(NO) + HBF4 f Cp*WR2(NOBF3)
(2)
immediately and quantitatively formed. The complexes 4 display
physical properties and spectroscopic features akin to those
exhibited by the hydroxylimido complexes. The NO-derived IR
stretching frequencies for 4a (1389 cm-1) and 4b (1378 cm-1) in
KBr have been definitively assigned by comparison of their
spectra with those of isotopically labeled 4a/b-15N. As expected,
there is a decrease in ν(NO) upon complexation of the nitrosyl
ligand, but the softer Lewis acid BF3 effects less of a decrease
than does a proton. A single-crystal X-ray crystallographic
analysis of 4a has been performed, and the results are shown in
Figure 2.18
A comparison of the solid-state molecular structure of 4a with
those of other crystallographically characterized Cp*W(hydro-
carbyl)2(NO) species reveals that the nitrosyl-derived ligand is
again distorted with a slightly shorter W(1)-N(1) distance
(1.740(2) Å) and longer N(1)-O(1) distance (1.303(3) Å) as
compared to an uncomplexed nitrosyl ligand. The N-O-B angle
Acknowledgment. We are grateful to the Natural Sciences and
Engineering Research Council of Canada for support of this work in the
form of grants to P.L. and to the University of British Columbia for
postgraduate scholarships to W.B.S. We also thank Dr. P. J. Daff for
stimulating and helpful discussions.
Note Added after ASAP: The Received date posted July 28, 2001
was in error. The corrected Received date was posted August 15, 2001.
Supporting Information Available: Experimental procedures and
complete characterization data for complexes 2, 3, and 4 and full details
of the crystal structure analyses of 2b[BArf4] and 4a including associated
tables (PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
(13) 1H NMR (NOH) δ: 2a[BArf4] 13.97 (Et2O-d10), 2b[BArf4] 13.60 (Et2O-
d10), 2b[Bφf4] 12.82 (CD2Cl2), 3 14.32 (CDCl3). 2a[Bφf4] has not yet been
isolated analytically pure.
(14) For instance, decoalescence of the signals attributable to the NOH
proton and the protons of free HBArf4 occurs at -20 °C in Et2O-d10.
(15) Mason, J. Chem. ReV. 1981, 81, 205.
JA002738M
(19) CpRe(SiMe2Cl)(PPh3)(NOBCl3): (a) Lee, K. E.; Arif, A. M.; Gladysz,
J. A. Inorg. Chem. 1990, 29, 2885. (b) Lee, K. E.; Arif, A. M.; Gladysz, J. A.
Chem. Ber. 1991, 124, 309. Re(H)(PiPr3)(NO)(NOBF3): (c) Gusev, D.;
Llamazares, A.; Artus, G.; Jacobsen, H.; Berke, H. Organometallics 1999,
18, 75. The bond lengths exhibited in the M-N-O-B fragment reported in
refs 19a,b are more drastically distorted from those of the parent nitrosyl than
are those presented here or in ref 19c. The authors of ref 19c suggest that this
may be due to the relatively low quality data set that was used for the X-ray
analysis of CpRe(SiMe2Cl)(PPh3)(NOBCl3).
(16) Dryden, N. H.; Legzdins, P.; Sayers, S. F.; Trotter. J.; Yee, V. C.
Can. J. Chem. 1995, 73, 1035.
(17) Dryden, N. H.; Legzdins, P.; Trotter, J.; Yee, V. C. Organometallics
1991, 10, 2857.
(18) Crystal data for 4a: C18H37NOSi2WBF3, T ) 173 K, Mr ) 591.32,
orange, block, monoclinic, P21/n (No. 14), a ) 9.5887(4) Å, b ) 17.2976(6)
Å, c ) 14.7882(6) Å, â ) 97.574(3)°, V ) 2431.4(1) Å3, Z ) 4, R ) 0.036,
Rw ) 0.052, GOF ) 0.94.