Three-coordinate β-diketiminato nickel nitrosyl complexes from nickel(I)-lutidine and nickel(II)-alkyl precursors

Article abstract of DOI:10.1021/om034041q

The nickel(I) β-diketiminates [Me₂NN]Ni(2,4-lutidine) and [2,4,6-Me₃NN]Ni(2,4-lutidine) react with NO to form diamagnetic, three-coordinate Ni-NO complexes, and [Me₂NN]Ni(NO) undergoes insertion with a nitrosobenzene to form the N-aryl-N-nitrosohydroxy-laminato species [Me₂NN]Ni(η²-O₂N₂Ar). Addition of 2 equiv of NO to the Ni(II)-monoalkyl species [Me₂NN]-Ni(Et)(2,4-lutidine) at room temperature provides an alternate route to [Me₂NN]Ni(NO), while NO addition at -78°C favors the formation of [Me₂NN]Ni(η²-O₂N₂-Et).

Full text of DOI:10.1021/om034041q

3974
Organometallics 2003, 22, 3974-3976
Th r ee-Coor d in a te â-Dik etim in a to Nick el Nitr osyl
Com p lexes fr om Nick el(I)-Lu tid in e a n d Nick el(II)-Alk yl
P r ecu r sor s
Simona C. Puiu and Timothy H. Warren*
Georgetown University, Department of Chemistry, Box 571227, Washington, D.C. 20057-1227
Received J uly 15, 2003
Summary: The nickel(I) â-diketiminates [Me2NN]Ni(2,4-
lutidine) and [2,4,6-Me3NN]Ni(2,4-lutidine) react with
NO to form diamagnetic, three-coordinate Ni-NO com-
plexes, and [Me2NN]Ni(NO) undergoes insertion with a
nitrosobenzene to form the N-aryl-N-nitrosohydroxy-
laminato species [Me2NN]Ni(η2-O2N2Ar). Addition of 2
equiv of NO to the Ni(II)-monoalkyl species [Me2NN]-
Ni(Et)(2,4-lutidine) at room temperature provides an
alternate route to [Me2NN]Ni(NO), while NO addition
at -78 °C favors the formation of [Me2NN]Ni(η2-O2N2-
Et).
[Ir(NO)(PPh3)]2O.9 While most nickel nitrosyls are
tetrahedral {MNO}10 complexes (following the clas-
sification of Enemark and Feltham)10 of the types [Ni-
(NO)L3]+ 7,11,12 and [Ni(NO)XL2]12,13 (L ) neutral donor;
X ) halogen or pseudo-halogen) that contain a linear
nitrosyl ligand, the cylopentadienyl complexes Cp′Ni-
(NO)14-16 are probably the best studied Ni-NO species.
Both CpNi(NO)14 and Cp*Ni(NO)16 have been shown to
have a linear Ni-NO ground state, and a side-bound
η2-NO photoexcited state has been characterized by
X-ray crystallography for Cp*Ni(NO).16 Our recent
synthesis of the d9, three-coordinate â-diketiminato Ni-
(I) complexes [Me2NN]Ni(2,4-lutidine) (1) and [2,4,6-
Me3NN]Ni(2,4-lutidine) (2) by dihydrogen reduction17
of the corresponding Ni(II)-ethyl complexes [Me2NN]-
Ni(Et)(2,4-lutidine)18 (3) and [2,4,6-Me3NN]Ni(Et)(2,4-
lutidine)17 (4) sparked our interest to prepare related
nickel nitrosyl complexes.19 For instance, addition of NO
The chemistry of transition-metal nitrosyl complexes
has received increased attention in recent years, due
to the important role that nitric oxide plays as a
signaling molecule in biological systems.1 For instance,
binding of NO to a heme iron is believed to activate the
enzyme guanylate cyclase toward formation of cyclic
GMP,2 which regulates a variety of physiological func-
tions, including neurotransmission, vasodilation, inhibi-
tion of platelet aggregation, and cell adhesion.3 NO is
also involved in respiration rate regulation via its
reversible binding to the heme a3 active site of the
respiratory enzyme cytochrome c oxidase.4 Many heme5
as well as non-heme1,2,6,7 nitrosyl complexes of Fe-Cu
have been synthesized and provide insight into the
active sites of such enzymes.
to the Ni(I) dimer {(tBu2PCH2CH2PBut )Ni(µ-Cl)}2 re-
2
sults in the mononuclear (tBu2PCH2CH2PBut )Ni(NO)-
2
Cl.13b
Addition of NO gas (1.1 equiv) to the Ni(I) â-diketimi-
nates 1 and 2 in ether provides [Me2NN]Ni(NO) (5) and
[2,4,6-Me3NN]Ni(NO) (6), which may be isolated as air-
stable, green crystals from pentane in 65-75% yield
(Scheme 1). The X-ray structure of 6 (Figure 1) contains
Transition-metal nitrosyl complexes with a coordina-
tion number of 3, however, are rare. Examples charac-
terized by X-ray are the pyrazolate-bridged nickel
nitrosyls [(pz)Ni(NO)]2 and [(pz)2Ni(NO)]2Ni (pz ) 3,5-
Me2N2C3H)8 as well as the dinuclear iridium complex
(8) Chong, K. S.; Rettig, S. J .; Storr, A.; Trotter, J . Can. J . Chem.
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15, 1555. (b) Kourkine, I. V.; Maslennikov, S. V.; Ditchfield, R.; Glueck,
D. S.; Yap, G. P. A.; Liable-Sands, L. M.; Rheingold, A. L. Inorg. Chem.
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Acta 2003, 345, 299.
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37, 1519.
(17) Kogut, E.; Wiencko, H. L.; Zhang, L.; Warren, T. H. Submitted
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2003, 345, 199.
* To whom correspondence should be addressed. E-mail: thw@
georgetown.edu.
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10.1021/om034041q CCC: $25.00 © 2003 American Chemical Society
Publication on Web 08/30/2003

Communications
Organometallics, Vol. 22, No. 20, 2003 3975
F igu r e 1. ORTEP diagram of one of the two independent
molecules of [2,4,6-Me3NN]Ni(NO) found in the solid-state
structure of 6. Selected bond distances (Å) and angles
(deg): Ni(1)-N(1) ) 1.883(2), Ni(1)-N(2) ) 1.885(2), Ni-
(1)-N(3) ) 1.610(3), N(3)-O(1) ) 1.170(3); N(1)-Ni(1)-
N(2) ) 95.79(10), Ni(1)-N(3)-O ) 171.0(3).
F igu r e 2. ORTEP diagram of the solid-state structure of
7. Selected bond distances (Å) and angles (deg): Ni-N(1)
) 1.866(3), Ni-N(2) ) 1.873(3), Ni-O(1) ) 1.863(3), Ni-
O(2) ) 1.867(3), O(1)-N(3) ) 1.340(5), O(2)-N(4) ) 1.320-
(5), N(3)-N(4) ) 1.237(5); N(1)-Ni-N(2) ) 94.43(15),
O(1)-Ni-O(2) ) 82.00(14).
Sch em e 1. Syn th esis of Ni-NO Com p lexes
Sch em e 2. Rea ction of 3 w ith 2 Equ iv of NO
from a monoalkyl complex. Treating an ether solution
of [Me2NN]Ni(Et)(2,4-lutidine) (3) with 2 equiv of NO
(1 atm) at room temperature results in the formation
of both 5 and the N-alkyl-N-nitrosohydroxylaminato
complex [Me2NN]Ni(η2-O2N2Et) (7) in a 0.6:0.4 ratio
(determined by 1H NMR analysis), from which a sample
of 5 (contaminated with ca. 10% of 7) could be isolated
in 40% yield after recrystallization (Scheme 2). Perform-
ing the reaction at -78 °C, however, allows for the
selective formation of 7 and its isolation as light green
crystals from pentane in 46% yield (Scheme 2). Reaction
of [2,4,6-Me3NN]Ni(Et)(2,4-lutidine) (4) with 2 equiv of
NO at -78 °C in ether similarly results in the isolation
of [2,4,6-Me3NN]Ni(η2-O2N2Et) (8) in 57% yield. Such
double NO insertion behavior is typical for early-
transition-metal alkyl or aryl complexes such as Cp2-
Zr(CH2Ph)2, CpW(NO)(CH2SiMe3)2, and WMe6, which
react with NO to form Cp2Zr(CH2Ph)(η2-O2N2CH2Ph),21
CpW(NO)(CH2SiMe3)(η2-O2N2CH2SiMe3),22 and WMe4-
(η2-O2N2Me)2,23 respectively. The X-ray structure of 7
reveals a square-planar Ni(II) center chelated by the
N-ethyl-N-nitrosohydroxylaminato ligand with nearly
identical Ni-O bond distances of 1.863(3) and 1.867(3)
Å (Figure 2). The long N-O distances (1.340(5) and
1.320(5) Å) and the short N(3)-N(4) distance (1.237(5)
Å) indicate a high degree of delocalization along the η2-
O2N2Et moiety, a common feature for N-organo-N-
nitrosohydroxylaminato ligands.24-26
two independent molecules in the asymmetric unit with
nearly identical metrical parameters and reveals trigo-
nal-planar coordination at nickel as well as a linear
nitrosyl ligand (Ni1-N3-O1 ) 171.0(3)° and Ni2-N6-
O2 ) 174.3(3)°). The Ni-NO distances (Ni1-N3 )
1.610(3) Å and Ni2-N6 ) 1.616(3) Å) are similar to that
found in the pyrazolate-bridged nickel complex [(pz)Ni-
(NO)]2 (1.616(4) Å),8 and 6 possesses an N-O bond (N3-
O1 ) 1.170(3) Å and N6-O2 ) 1.161(3) Å) slightly
longer than that of free NO (1.154 Å).20 Consistent with
a lower coordination number at nickel, the â-diketimi-
nate Ni-N distances (1.883(2)-1.885(2) Å) in 6 are
shorter as compared to those in [Me2NN]Ni(Et)(2,4-
lutidine) (1; 1.915(3) and 1.987(3) Å).18 1H NMR spectra
of 5 and 6 in benzene-d6 show significantly upfield
shifted â-diketiminato backbone-CH resonances (δ 4.351
and 4.398 ppm, respectively) as compared to the corre-
sponding Ni(II)-ethyl complexes 3 and 4 (δ 5.03 and
4.99 ppm, respectively, in toluene-d8), suggestive of a
more electron-rich metal center in the nitrosyl deriva-
tives. Solid-state IR spectra of 5 and 6 display sharp
bands at 1792 and 1785 cm-1, respectively, which are
at the lower end of the range of νNO values (1817-1779
cm-1) found in the family of neutral pyrazolato nickel
nitrosyls [(pz)Ni(NO)]2 and [(pz)2Ni(NO)]2Ni.8 The some-
what lower value of νNO for 6 as compared to 5 may be
attributed to a slightly more electron-donating character
of the [2,4,6-Me3NN]- ligand as compared to [Me2NN]-.
Since the Ni(I)-lutidine precursors 1 and 2 employed
in the synthesis of 5 and 6 are prepared by H2 reduction
of the corresponding Ni(II)-ethyl species 3 and 4,17 we
explored the synthesis of a nitrosyl derivative directly
(21) Fochi, G.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J . Chem.
Soc., Dalton Trans. 1986, 445.
(22) Legzdins, P.; Rettig, S. J .; Sanchez, L. Organometallics 1988,
7, 2394.
(23) Fletcher, S. R.; Skapski, A. C. J . Organomet. Chem. 1973, 59,
299.
(24) Hrabie, J . A.; Keefer, L. K. Chem. Rev. 2002, 102, 1135.
(25) Yi, G. B.; Khan, M. A.; Richter-Addo, G. B. Inorg. Chem. 1995,
34, 5703.
(20) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements;
Pergamon Press: Oxford, U.K., 1993.

3976 Organometallics, Vol. 22, No. 20, 2003
Communications
Sch em e 3. Ra p id In ter con ver sion betw een
Con for m er s in 7 or 8
Sch em e 4. Rea ction of 5 w ith Ar NdO (Ar )
3,5-(CH3)2C6H3)
The most notable feature in room-temperature 1H
NMR spectra of 7 and 8 (300 MHz, benzene-d6) is the
presence of single resonances each for the â-diketimi-
nato N-aryl and backbone methyl groups, which sug-
gests that conformers of the N-ethyl-N-nitrosohydroxyl-
aminato group rapidly interconvert within the coordina-
tion wedge of the [â-diketiminato]Ni fragment (Scheme
3). Low-temperature 1H NMR spectra of 7 at -75 °C
(300 MHz, toluene-d8) also show sharp, single reso-
nances for these methyl groups, indicating that the
symmetrization is quite facile. While this process could
take place via a tetrahedral27 η2-O2N2Et or a trigonal
η1-O2N2Et intermediate, it is intramolecular. The 1H
NMR spectrum of a 1:1 mixture of 7 and 8 at room
temperature in benzene-d6 shows sharp, overlapping
multiplets at δ 2.616 and 2.621 ppm as well as at δ 0.247
and 0.263 ppm, corresponding to the CH2 and CH3
resonances of the [O2N2Et]- anion in 7 and 8, respec-
tively.
In the reaction of NO with [Me2NN]Ni(Et)(2,4-luti-
dine) (3), 1 equiv of (bound) EtNdO is likely produced
either by direct insertion of NO into the Ni-ethyl
bond28,29 or by reductive nitrosylation (reduction to Ni(I)
by NO followed by insertion of NO+ into the resulting
Ni-alkyl bond).30 We explored the reaction of [Me2NN]-
Ni(NO) (5) with a nonvolatile organonitroso compound31
to obtain additional insight into the formation of the
N-ethyl-N-nitrosohydroxylaminato complexes 7 and 8.
Addition of the nitrosobenzene derivative ArNdO (Ar
) 3,5-Me2C6H3) to a benzene solution of 5 results in the
immediate formation of [Me2NN]Ni(η2-O2N2Ar) (9),
which may be isolated as yellow crystals from pentane
1
in ca. 70% yield (Scheme 4). The H NMR spectrum of
9 at room temperature is similar in form to that of 7
and 8, indicating a related, facile symmetrization pro-
cess. Although diazeniumdiolates [O2N2X]- (X ) R2N,
Ph; R ) alkyl) serve as a source of NO under a wide
variety of conditions24 and have been employed as
nitrosylating reagents in the synthesis of nitrosylmet-
alloporphyrins,25,32 reaction of 7 or 9 with Lewis acids25
such as BF3‚OEt2 did not result in RNO loss to form 5.
In summary, the addition of NO to both Ni(I)-lutidine
and Ni(II)-alkyl â-diketiminato precursors results in
the formation of three-coordinate Ni-NO complexes,
which react with organonitroso compounds to give
N-organo-N-nitrosohydroxylaminato species. Efforts are
underway to extend this methodology to the synthesis
of other low-coordinate metal nitrosyl complexes.
Ack n ow led gm en t. T.H.W. thanks Georgetown Uni-
versity for generous start-up funding, the donors of the
Petroleum Research Fund, administered by the Ameri-
can Chemical Society (Type-G), and the NSF for a
CAREER award (Grant No. CHE-0135057).
(26) Ahmed, M. A.; Edwards, A. J .; J ones, C. J .; McCleverty, J . A.;
Rothin, A. S.; Tate, J . P. J . Chem. Soc., Dalton Trans. 1988, 257.
(27) Crystallographically characterized four-coordinate â-diketimi-
nato Ni(II)-halide complexes are tetrahedral: {[Me2NN]Ni(µ-Cl)}2;18
{[2,6-iPr2NN]Ni(µ-Cl)}2 and {[2,6-iPr2NN]NiCl2}Li(THF)2;19b [Me2NN]-
Ni(Cl)(2,4-lutidine) and [Me2NN]Ni(I)(2,4-lutidine) (Kogut, E.; Warren,
T. H. Unpublished observations).
Su p p or tin g In for m a tion Ava ila ble: Text giving details
on the syntheses of 5-9 with relevant analytical and spectro-
scopic data and CIF files giving crystallographic information
for 6 and 7. This material is available free of charge via the
Internet at http://pubs.acs.org.
(28) CO inserts into the Ni-Et bond of 3 to give the corresponding
Ni-acyl species [Me2NN]Ni(C(O)Et)(2,4-lutidine): Wiencko, H. L.;
Warren, T. H. Manuscript in preparation.
OM034041Q
(29) For examples of NO insertion into metal-alkyl bonds to give
coordinated nitrosoalkanes, see: (a) Weiner, W. P.; White, M. A.;
Bergman, R. G. J . Am. Chem. Soc. 1981, 103, 3612. (b) Weiner, W. P.;
Bergman, R. G. J . Am. Chem. Soc. 1983, 105, 3922. (c) Seidler, M. D.;
Bergman, R. G. J . Am. Chem. Soc. 1984, 106, 6110. (d) Diel, B. N. J .
Organomet. Chem. 1985, 284, 257. (e) Seidler, M. D.; Bergman, R. G.
J . Am. Chem. Soc. 1984, 106, 6110. (f) Chang, J .; Seidler, M. D.;
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M. B. J . Am. Chem. Soc. 1997, 199, 3077.
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Liable-Sands, L. M.; Rheingold, A. L. J . Am. Chem. Soc. 2002, 124,
12068. We thank a reviewer for pointing out this possibility.
(31) Lee, J .; Chen, L.; West, A. H.; Richter-Addo, G. B. Chem. Rev.
2002, 102, 1019.
(32) Piciulo, P. L.; Scheidt, W. R. Inorg. Nucl. Chem. Lett. 1975, 11,
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