Atom Transfer as a Preparative Tool in Coordination Chemistry. Synthesis and Characterization of Cr(V) Nitrido Complexes of Bidentate Ligands

Article abstract of DOI:10.1021/ic034777f

The transfer of a terminal nitrido ligand from Mn^V(N)(salen) to Cr(III) complexes is explored as a new preparative route to Cr^V nitrido complexes. Reaction of Mn^V(N)(salen) with labile CrCl ₃(THF)₃ in acetonitrile solution precipitates [Mn(Cl)(salen)]·(CH₃CN) and yields a solution containing a mixture of Crv nitrido species with only labile auxiliary ligands. From this solution Cr^V nitrido complexes with bidentate monoanionic ligands can be obtained in high yields. Five coordinate complexes of 8-hydroxoquinolinate (quin), 1,3-diphenylpropane-1,3-dionate (dbm), and pyrrolidinedithiocarbamate (pyr-dtc) have been structurally characterized: Cr(N)(quin)₂ (1) crystallizes as compact orange prisms in the triclinic space group P1 with cell parameters a = 7.2450(6) A, b = 8.1710(4) A, c = 13.1610(12) A, α = 80.519(6)°, β = 75.721(7)°, γ = 75.131 (5)°, V = 725.47(10) A³, Z = 2. Cr(N)(dbm)₂ (2) crystallizes as green rhombs in the orthorhombic space group Pbca with cell parameters a = 14.6940(6) A, b = 16.4570(18) A, c = 19.890(3), A, V = 4809.8(8) A³, Z = 8. Cr(N)(pyr-dtc)₂ (3) crystallizes as orange prisms in the monoclinic space group P21/c with cell parameters a = 14.8592(14) A, b = 8.5575(5) A, c = 11.8267(12) A, β = 106.528(7)°, V = 1441.7(2) A³, Z = 4. Complexes 2 and 3 represent new coordination environments for first row transition metal nitrido complexes. The d-orbital energy splitting in these systems with relatively weak equatorial donors differs significantly from the pattern in vanadyl and the previously known first row transition metal nitrido complexes. The d_x2-y2 orbital in 2 and 3 is lower in energy and well resolved from the M-N π* orbitals {d_zx,d_yz}.

Full text of DOI:10.1021/ic034777f

Inorg. Chem. 2003, 42, 7608−7615
Atom Transfer as a Preparative Tool in Coordination Chemistry.
Synthesis and Characterization of Cr(V) Nitrido Complexes of Bidentate
Ligands
Torben Birk and Jesper Bendix*
Department of Chemistry, UniVersity of Copenhagen, UniVersitetsparken 5, DK-2100, Denmark
Received July 7, 2003
The transfer of a terminal nitrido ligand from MnV(N)(salen) to Cr(III) complexes is explored as a new preparative
route to CrV nitrido complexes. Reaction of MnV(N)(salen) with labile CrCl3(THF)3 in acetonitrile solution precipitates
[Mn(Cl)(salen)]‚(CH CN) and yields a solution containing a mixture of CrV nitrido species with only labile auxiliary
3
ligands. From this solution CrV nitrido complexes with bidentate monoanionic ligands can be obtained in high
yields. Five coordinate complexes of 8-hydroxoquinolinate (quin), 1,3-diphenylpropane-1,3-dionate (dbm), and
pyrrolidinedithiocarbamate (pyr-dtc) have been structurally characterized: Cr(N)(quin)2 (1) crystallizes as compact
orange prisms in the triclinic space group P1h with cell parameters a ) 7.2450(6) Å, b ) 8.1710(4) Å, c ) 13.1610(12)
3
Å, R ) 80.519(6)°, â ) 75.721(7)°, γ ) 75.131(5)°, V ) 725.47(10) Å , Z ) 2. Cr(N)(dbm)2 (2) crystallizes as
green rhombs in the orthorhombic space group Pbca with cell parameters a ) 14.6940(6) Å, b ) 16.4570(18) Å,
3
c ) 19.890(3) Å, V ) 4809.8(8) Å , Z ) 8. Cr(N)(pyr-dtc)2 (3) crystallizes as orange prisms in the monoclinic
space group P21/c with cell parameters a ) 14.8592(14) Å, b ) 8.5575(5) Å, c ) 11.8267(12) Å, â ) 106.528(7)°,
3
V ) 1441.7(2) Å , Z ) 4. Complexes 2 and 3 represent new coordination environments for first row transition
metal nitrido complexes. The d-orbital energy splitting in these systems with relatively weak equatorial donors
differs significantly from the pattern in vanadyl and the previously known first row transition metal nitrido complexes.
2
2
The d
orbital in 2 and 3 is lower in energy and well resolved from the M−N π* orbitals {d ,d }.
x -y
zx yz
Introduction
ligands. This area was further developed by Woo et al. and
the mechanistic details of the transfer reaction were clarified.4
The nitrogen atom transfer reaction was extended to Schiff-
base complexes of manganese by Gray et al.5 Recently,
nitrogen atom transfer chemistry has received considerable
attention fueled by the facile preparation of [Mn(N)(salen)]
developed by Carreira and the demonstration that this
compound when activated by (CF3CO)2O allows stoichio-
metric aziridination of electron-rich olefins.6,14 With a few
exceptions the studies of nitrogen atom transfer reactions
have been fundamental, aiming at elucidating the mechanism
and thermodynamics of the reactions and not exploring the
preparative potential of this type of reactions.
Atom- and group-transfer reactions are central to the
chemistry of metal complexes with multiply bonded ligands.
This is true for biological oxo-transfer reactions catalyzed
by metalloenzymes1 as well as for the reactivity of simple
oxo-complexes toward organic substrates exemplified by the
epoxidation with manganese(III) catalysts developed by
Jacobsen and others.2 The possibility of transferring a
nitrogen atom between metal centers was discovered some
20 years ago by Bottomley and Neely, who found that the
transfer of a nitrido ligand between Mn(V) and Cr(III) as
well as between Mn(V) and Mn(III) is possible.3 All of the
systems investigated by these authors contained porphyrin
Common to the known nitrido complexes of Cr(V) is that
they feature polydentate ligands (cyclam,7 tacn,8 porphyrins,9
* Author to whom correspondence should be addressed. E-mail:
Bendix@kiku.dk.
(1) Holm, R. H. Chem. ReV. 1987, 87, 1401.
(2) (a) Zhang, W.; Loebach, J. L.; Wilson, S. R.; Jacobsen, E. N. J. Am.
Chem. Soc. 1990, 112, 2801. (b) Srinivasan, K.; Michaud, P.; Kochi,
J. K. J. Am. Chem. Soc. 1986, 108, 2309.
(4) Woo, L. K.; Goll, J. G.; Czapla, D. J.; Hays, J. A. J. Am. Chem. Soc.
1991, 113, 8478.
(5) Chang, C. J.; Low, D. W.; Gray, H. B. Inorg. Chem. 1997, 36, 270.
(6) Du Bois, J.; Tomooka, C. S.; Hong, J.; Carreira, E. M. Acc. Chem.
Res. 1997, 30, 364.
(3) (a) Bottomley, L. A.; Neely, F. L. Inorg. Chem. 1997, 36, 5435. (b)
Bottomley, L. A.; Neely, F. L. J. Am. Chem. Soc. 1989, 111, 5955.
7608 Inorganic Chemistry, Vol. 42, No. 23, 2003
10.1021/ic034777f CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/21/2003

Cr(V) Nitrido Complexes of Bidentate Ligands
Scheme 1. Schematic Representation of the Synthetic Routes to the Nitrido Chromium(V) Complexes and the Labeling Scheme
salen10), the only exceptions being the tetra- and pentacy-
anonitridochromates.11,12 This is a consequence of the
methods of preparation which have been either photolysis
of Cr(III) azido complexes9 or ClO- oxidation in the presence
of NH3.5 These harsh conditions prevent synthesis of Cr(N)
complexes which do not have a robust auxiliary coordination
sphere. In this paper we present a protocol involving nitrogen
atom transfer from Mn(N)(salen) to CrCl3(THF)3 yielding
(a mixture of) CrV(N) species with a very labile coordination
sphere, from which a wide variety of chromium(V) nitrido
complexes can be obtained. Here we exemplify the synthetic
usefulness of the method by synthesizing and characterizing
complexes with bidentate auxiliary ligands including the
novel coordination environments Cr(N){O}4 and Cr(N){S}4.
methylformamide >99% (Merck-Schuchardt), were HPLC grade
and were used without further purification.
Atom Transfer Reaction. The following procedure (cf. Scheme
1) was used for preparing the solution containing {CrtN}2+
complexed by labile auxiliary ligands: CrCl3(THF)3 (0.906 g; 2.42
mmol) was dissolved in acetonitrile (20 mL) to give a purple
solution. Addition of solid Mn(N)(salen) (0.810 g; 2.42 mmol) was
accompanied by an immediate color change to yield a dark brown
solution with suspended solid material. The reaction mixture was
stirred for 30 min under a N2 atmosphere and filtered to give a
dark brown crystalline precipitate and a yellow-brown solution (sol.
A). The brown product was isolated and analyzed showing it to be
essentially pure Mn(Cl)(salen)‚CH3CN (Anal. Calcd for C18H17N3O2-
ClMn C, 54.36; H, 4.31; N, 10.56; Mn, 13.81; Cl, 8.92. Found C,
53.88; H, 4.23; N, 10.75; Mn, 12.99; Cl, 8.42). MS, FAB+: m/z
321 ([Mn(salen]+). In some preparations, improved yields were
obtained by using an excess of manganese nitrido complex. In those
cases, a 1:2 stoichiometry of Cr to Mn was realized by using 1.21
mmol of CrCl3(THF)3 as the only difference to the above procedure
(sol. B).
Experimental Section
Materials. The compounds CrCl3(THF)3,13 Mn(N)(salen),14 N,N′-
ethylenebis(salicylidenamine)15 (salen), and 2-benzyliminomethyl-
phenol were prepared by literature or standard procedures. Mn(N)-
(salen) was purified by Soxhlet extraction with CH2Cl2 before use.
Dibenzoylmethane (1,3-diphenylpropane-1,3-dione) 98% (Aldrich),
8-hydroxyquinoline 99+% (Aldrich), and ammonium-pyrro-
lidinedithiocarbamate (Merck) were used as received. All solvents
used, acetonitrile (Lab-Scan), diethyl ether (Lab-Scan), and N-
Synthesis of Nitridobis(8-hydroxoquinolinate)chromium(V)
(1). A solution of 8-hydroxoquinoline (0.720 g; 4.832 mmol) in
acetonitrile (6 mL) was added to sol. B, resulting in the immediate
precipitation of an orange-brown crystalline product. The reaction
mixture was stirred for 15 min and the product collected by
filtration. The product was washed with H2O (5 × 5 mL), H2O/
NMF (3:2; 1 × 5 mL), methanol (6 × 5 mL), and diethyl ether (1
× 5 mL). Yield: 0.318 g (75%). The product was recrystallized
from boiling acetonitrile (200 mL) to give large orange-red crystals.
Anal. Calcd for C18H12N3O2Cr: C, 61.02; H, 3.41; N, 11.86; Cr,
14.68. Found: C, 61.01; H, 3.27; N, 11.88; Cr, 13.96. MS,
FAB+: m/z 355 ({M + H}+, rel intensity 15%). EI: m/z 354 (M+,
rel intensity 100%). IR: ν(Cr-N) 1015 cm-1(vs). UV-vis (CH2Cl2,
room temperature), λmax [nm] (ꢀ) [M-1 cm -1]: 403 (4080), 334
(1270), 315 (1670), 304 (1690), 264 (36000).
(7) Meyer, K.; Bendix, J.; Bill, E.; Weyhermu¨ller, T.; Wieghardt, K. Inorg.
Chem. 1998, 37, 5180.
(8) Niemann, A.; Bossek, U.; Haselhorst, G.; Wieghardt, K.; Nuber, B.
Inorg. Chem. 1996, 35, 906.
(9) Groves, J. T.; Takahashi, T.; Butler, W. M. Inorg. Chem. 1983, 22,
884.
(10) Arshankov, S. I.; Poznjak, A. L. Z. Anorg. Allg. Chem. 1981, 481,
201.
(11) Bendix, J.; Meyer, K.; Weyhermu¨ller, T.; Bill, E.; Metzler-Nolte, N.;
Wieghardt, K. Inorg. Chem. 1998, 37, 1767.
(12) Bendix, J.; Deeth, R. J.; Weyhermu¨ller, T.; Bill, E.; Wieghardt, K.
Inorg. Chem. 2000, 39, 930.
(13) Herweg, W.; Zeiss, H. H. J. Org. Chem. 1958, 23, 1404.
(14) Du Bois, J.; Hong, J.; Carreira, E. M.; Day, M. W. J. Am. Chem. Soc.
1996, 118, 915.
Synthesis of Nitridobis(1,3-diphenylpropane-1,3-dionate)-
chromium(V) (2). A solution of dibenzoylmethane (0.542 g, 2.42
mmol) in acetonitrile (4 mL) was added slowly to stirred sol. B,
with immediate formation of a green precipitate. The reaction
(15) Mason, A. T. Chem. Ber. 1887, 20, 267.
Inorganic Chemistry, Vol. 42, No. 23, 2003 7609

Birk and Bendix
mixture was stirred for further 15 min, and the product was collected
by filtration and washed with diethyl ether (2 × 5 mL), methanol
(1 × 5 mL), and diethyl ether (2 × 5 mL). Yield: 0.315 g (51%).
The product was recrystallized from a boiling mixture (240 mL)
of toluene and n-heptane (2:1) giving green crystals (0.226 g, overall
37%). Anal. Calcd for C30H22NO4Cr: C, 70.31; H, 4.33; N, 2.73;
Cr, 10.15. Found: C, 70.08; H, 4.22; N, 2.69; Cr, 10.09. MS,
FAB+: m/z 513 ({M + H}+, rel intensity 3%). EI: not observed.
IR: ν(Cr-N) 1007 cm-1(s). UV-vis (CH2Cl2, room temperature),
changed from green to orange. Cooling of the filtered solution
afforded orange crystals. Yield: 21 mg (100%). Anal. Found: C,
60.92; H, 3.25; N, 11.76.
Physical Measurements. UV/vis spectra were recorded on a
Perkin-Elmer, Lambda 2 UV/vis spectrophotometer. EPR spectra
were recorded on a Bruker ESP 300 equipped with a EIP 538B
frequency counter and a ER035M NMR Gauss-meter. MCD spectra
were recorded at room temperature on a Jasco J-710 spectropola-
rimeter equipped with an electromagnet providing a magnetic field
of 1.6 T. The samples were measured with the field parallel and
antiparallel to the light propagation direction, and the differences
of these measurements are reported, thus giving an effective applied
field of 3.2 T. The magnetic susceptibilities of powdered samples
were measured by the Faraday method in the temperature range
50-300 K at a field strength of 1.3 T. The susceptibility data were
corrected for diamagnetism by Pascal’s constants. The magnetic
field was calibrated with Hg[Co(NCS)].16 A more detailed descrip-
tion of the equipment has been published elsewhere.17 Fast atom
bombardment (FAB, Xe ions, accelerated by 6 kV) and direct inlet
(EI) mass spectra were recorded on a JEOL JMS-HX/HX110A
tandem mass spectrometer (positive ion detection). Matrix for
FAB: m-nitrobenzyl alcohol (m-NBA).
X-ray Crystallography. Crystal structure and refinement data
for 1, 2, and 3 are summarized in Table 1. Details of the structure
determinations are available in the Supporting Information. For all
three compounds, all non-hydrogen atoms were refined with
anisotropic temperature factors. The molecular structure diagrams
were made with the ORTEP-II program.18 Crystals suitable for
X-ray diffraction were obtained by the following procedures: slow
evaporation of a filtered unsaturated solution of 1 in CH2Cl2
afforded orange prismatic X-ray quality crystals in 24 h. 2 (20 mg)
was dissolved in a boiling mixture of toluene (10 mL) and heptane
(5 mL). Slow cooling and partial evaporation afforded green block
shaped crystals in 5 days. Complete dissolution of 3 in CH2Cl2
and slow evaporation over 24 h gave orange-brown prismatic
crystals.
λ
max [nm] (ꢀ) [M-1 cm -1]: 595 (26.4), 469 sh (93.3), 359 (37100),
271 (31500).
Synthesis of Nitridobis(pyrrolidinedithiocarbamate)chromium-
(V) (3). A solution of ammoniumpyrrolidinedithiocarbamate (0.986
g, 6.00 mmol) in methanol (70 mL) was added to stirred sol. A,
resulting in immediate precipitation. The brown product was filtered
off and washed repeatedly with cold methanol. The crude product
was dissolved in CH2Cl2 (50 mL) and reprecipitated by dropwise
addition of methanol (100 mL). Overall yield: 0.612 g (71%) of
golden-brown plates. Anal. Calcd for C16H10N3S4Cr: C, 33.50; H,
4.50; N, 11.72; Cr, 14.50. Found: C, 33.46; H, 4.44; N, 11.52%.
Cr, 13.96%. MS, FAB+: not observed; EI: m/z ) 344 ({M -
N}+, rel intensity 30%). IR: ν(Cr-N) 991 cm-1(vs). UV-vis (CH2-
Cl2, room temperature), λmax [nm] (ꢀ) [M-1 cm -1]: 548 (54.5),
446 (221), 432 sh (212), 276 (17400). The procedure is applicable
to a range of common dialkyldithiocarbamate ligands.
Synthesis of Nitridobis(2-benzyliminomethylphenolato)-
chromium(V) (4). To stirred sol. A was added dropwise a solution
of 2-benzyliminomethyl-phenol (2.114 g, 10.0 mmol) in acetonitrile
(3 mL). After a few minutes crystallization commenced. The
reaction mixture was cooled to 5 °C and the product consisting of
red-brown needles was collected by filtration and washed repeatedly
with methanol. Yield 0.81 g (69%). Anal. Calcd for C28H24N3O2-
Cr: C, 69.13; H, 4.97; N, 8.64; Cr, 10.69. Found: C, 69.20; H,
4.97; N, 8.75; Cr, 10.33. MS, EI: m/z 486 (M+, rel intensity 25%).
IR: ν(Cr-N) 1016 cm-1 (s). UV-vis (CH2Cl2, room temperature),
λ
max [nm] (ꢀ) [M-1 cm -1]: 552 (26.1), 362 (8400), 269 sh (24500).
Synthesis of Nitrido[N,N′-ethylenebis(salicylideneaminato)]-
Results and Discussion
chromium(V) (5). Stirred sol. A was heated to 70 °C and a solution
of salen (0.648 g; 2.42 mmol) in acetonitrile (30 mL) was added
dropwise. The hot solution was filtered and placed at 5 °C for 12
h. Precipitation of red crystals commenced after 30 min. The product
was filtered off and washed by the mother liquor and methanol (4
× 10 mL). Yield: 0.147 g (18%). Anal. Calcd for C16H14N3O2Cr:
C, 57.83; H, 4.25; N, 12.65. Found: C, 57.60; H, 4.15; N, 12.60.
IR: ν(Cr-N) 1011 cm-1(s). The EPR spectrum was identical with
the one reported in the literature.10
Synthesis of [Chloronitrido(8-hydroxoquinolinate)chromium-
(V)] Di- or Polymer (6). Dropwise addition over 15 min of a
solution of 8-hydroxoquinoline (175 mg, 1.21 mmol) in acetonitrile
(6 mL) to sol B resulted in formation of a green precipitate. The
product was filtered off and washed repeatedly with H2O, methanol,
diethyl ether, and CH2Cl2. Yield: 37 mg (12%). Anal. Calcd for
C18H12N4O2Cl2Cr2: C, 44.01; H, 2.46; N, 11.41. Found: C, 43.73;
H, 2.39; N, 10.88. IR: ν(Cr-N) 1023 cm-1(vs).
Synthesis of 1 from 6. To a solution of 8-hydroxoquinoline (82
mg, 0.56 mmol) and acetonitrile (10 mL) was added solid 6 (11
mg, 0.022 mmol). The reaction mixture was boiled for ca. 5 min,
converting the green suspension into an orange solution. The
solution was filtered hot and cooled to deposit orange crystals. Yield
7.0 mg (44%). Anal. Found: C, 60.75; H, 3.24; N, 11.74.
Synthesis of 1 from 2. To a solution of 8-hydroxoquinoline (132
mg, 0.91 mmol) and acetonitrile (10 mL) was added 2 (30 mg,
0.059 mmol). Upon reflux for 5 min the color of the solution
Synthesis. In agreement with the finding of Woo et al.
that nitrogen atom transfer from Mn(V) to Cr(III) proceeds
via a dissociative mechanism, we have found that a labile
Cr(III) substrate is a prerequisite for turning N-atom transfer
into a preparatively useful reaction. Labile Cr(III) complexes
are scarce, but CrCl3(THF)3 was found to be well suited for
our purpose. Addition of green [Mn(N)(salen)] to a violet
solution of CrCl3(THF)3 in acetonitrile results in an instan-
taneous color change giving a yellow-brown solution and a
brown precipitate of Mn(Cl)(salen)‚(CH3CN). Other Cr(III)
substrates were investigated, but all were inferior to CrCl3-
(THF)3 with regard to the rate of reaction and/or the solubility
in suitable solvents. The yellow-brown solution contains
{CrtN}2+ complexed only by labile ligands and is an
excellent precursor for the synthesis of Cr(V) nitrido
complexes (Scheme 1). Here we have demonstrated the
general applicability of this protocol by synthesizing the new
compounds 1-4. In addition, we have confirmed that this
thermal route extends to polydentate ligand systems by
(16) Figgis, B. N.; Nyholm, R. S. J. Chem. Soc., 1958, 4190.
(17) Pedersen, E. Acta Chem. Scand. 1972, 26, 333.
(18) Johnson, C. K. ORTEP-II; Report ORNL-5138, Oak Ridge National
Laboratory, Oak Ridge, TN, 1976.
7610 Inorganic Chemistry, Vol. 42, No. 23, 2003

Cr(V) Nitrido Complexes of Bidentate Ligands
Table 1. Crystallographic Data for Compounds
1
2
3
empirical formula
fw (g/mol)
temp (K)
wavelength (Å)
space group
crystal size (mm3)
unit cell dimensions
a (Å)
C18H12CrN3O2
354.31
122(1)
0.71073
P1h (No. 2)
C30H22CrNO4
512.49
122(1)
C10H16CrN3S4
358.51
122(1)
0.71073
0.71073
Pbca (No. 62)
0.261 × 0.233 × 0.223
P21/c (No. 14)
0.28 × 0.15 × 0.14
0.351 × 0.20 × 0.120
7.2450(6)
8.1710(4)
13.1610(12)
80.519(6)
75.721(7)
75.131(5)
725.47(10)
2
14.6940(6)
16.4570(18)
19.890(3)
14.8592(14)
8.5575(5)
11.8267(12)
b (Å)
c (Å)
R (deg)
â (deg)
106.528(7)
γ (deg)
V (Å3)
4809.8(8)
8
1.415
0.514
2120
1441.7(2)
4
1.652
1.357
740
Z
d
calc (g/cm3)
1.622
0.805
362
µ (mm-1
F(000)
)
θ range for data collection (deg)
no. reflns collected
2.59-40.00
69245
2.05-30.00
193667
1.43-34.95
53523
no. of unique reflns, Rint
no. of data/restraints/params
final R1, wR2 [I > 2σ(I)]a
final R1, wR2 (all data)a
goodness of fit on F2
8981, 0.0315
8264/0/217
0.0246, 0.0709
0.0287, 0.0745
1.106
7007, 0.0878
5608/0/325
0.0334, 0.0805
0.0475, 0.0872
0.873
6327, 0.0374
5383/0/163
0.0308, 0.0727
0.0416, 0.0793
1.052
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑{w(|Fo| - |Fc| )2}/∑{w(|Fo| )2}]1/2
.
preparing the previously reported [Cr(N)(salen)] (5), which
had previously only been obtained by photolysis of the
corresponding chromium(III) azide complex.
Using a 1:1 stoichiometry between Cr(V) and 8-hydroxo-
quinoline allows for the isolation of a product of composition
[Cr(N)(quin)Cl]n in place of 1. This product could be
formulated as either a dimer or a chain polymer. Charac-
terization of this species is hampered by its insolubility and
a clear distinction between the di- and polymeric structures
has not been achieved. However, vibrational spectroscopy
shows clearly the presence of a terminal nitrido ligand (Table
4 and Supporting Information), which rules out involvement
of the nitrido ligand in bridging. This finding agrees with
the previously reported low nucleophilicity of the nitrido
ligand in CrV(N) and MnV(N) complexes.11,19 The formulation
of 6 as a di- or polymer is supported by its magnetic
properties; there is a clear antiferromagnetic interaction in 6
(cf. Figure 1). We tentatively formulate 6 as a chloride
bridged dimer (cf. Scheme 1). This accounts for the anti-
ferromagnetism by direct overlap of the dxy orbitals of the
Cr centers. However, considering the low solubility and the
intermediate magnitude of the antiferromagnetic interaction,
a chloride bridged chain structure remains a possibility.
X-ray Crystallography. The molecular structures of the
Cr(V)nitrido complexes 1, 2, and 3 have been determined
from single-crystal X-ray diffraction. The structures are
shown in Figures 2-4. Selected bond lengths and angles
are listed in Table 2. The structures are similar, consisting
of square-pyramidal five-coordinate Cr(V) centers with a
short bond to the apical nitrido ligand. The equatorial
coordination sphere in 1 consists of the same donor atoms
({O}2{N}2) as those in the known Cr(V) nitrido complexes
Figure 1. Temperature variation of magnetic susceptibility (O) and
magnetic moment (+) of 6. The solid line is a fit to the susceptibility (g )
1.97 fix; J ) -21 cm-1; H ) -2J(S1‚S2)). The irregularity around 60 K is
due to an oxygen leak in the setup.
of tetradentate Schiff-base complexes. However, 1 differs
in nature from the known systems by having the two
8-hydroxoquinolinate ligands trans oriented relative to each
other leading to a system with an expected higher degree of
rhombicity. The coordination environments of the chromium
nitrido moiety in 2 and 3 are unprecedented in the nitrido
chemistry of first row transition metals.
Crystal packing of the molecules in all three compounds
is determined by shape and π-stacking of the equatorial
ligands (cf. Figure 2). There are no close contacts involving
the nitrido ligands. This is the most common situation,
although examples exist where five-coordinate molecular
nitrido complexes stack head-to-tail to form infinite chains.20
The notion of isolated molecular units agrees with the
completely temperature independent (50-300 K) magnetic
moments measured for 1-3 (cf. Supporting Information).
(19) Meyer, K.; Bendix, J.; Metzler-Nolte, N.; Weyhermu¨ller, T.; Wieghardt,
(20) Tsuchimoto, M.; Yoshioka, N.; Ohba, S. Eur. J. Inorg. Chem. 2001,
K. J. Am. Chem. Soc. 1998, 120, 7260-7270.
1045.
Inorganic Chemistry, Vol. 42, No. 23, 2003 7611

Birk and Bendix
Figure 2. Top: Molecular structure and atom labeling for 1 with thermal ellipsoids (50% probability). Cf. Table 2 for metrical details. Bottom: Crystal
packing in 1.
Figure 3. Molecular structure and atom labeling for 2 with thermal
ellipsoids (50% probability). Cf. Table 2 for metrical details.
The Cr-N bond length in five-coordinate Cr(V) nitrido
complexes is at (1.555 ( 0.011 Å), almost independent of
the nature of the remaining coordination sphere (cf. Table
3), making it justified to consider {CrtN}2+ as an integrated
moiety. With the large variation of the equatorial ligators
encompassed by 1-3, this is a much stronger statement than
Figure 4. Molecular structure and atom labeling for 3 with thermal
ellipsoids (50% probability). Cf. Table 2 for metrical details. The top view
emphasizes the pronounced pyramidalization of the molecule with Cr
what could be concluded from existing data. Comparison
with the six-coordinate systems in Table 3 shows that
coordination of a sixth ligand trans to the nitrido ligand
invariably elongates the Cr-N bond.
In 1 and 2 the Cr ion is displaced out of the plane of the
equatorial ligands toward the nitrido ligand by ca. 0.5 Å.
This is in line with findings for other five-coordinate nitrido
complexes (cf. Table 3) and a distinctly larger displacement
than found in six-coordinate complexes. In 3 there is an even
larger degree of pyramidalization (cf. Figure 4) leaving the
Cr center more than 0.72 Å out of the plane of the sulfur
donor atoms. This is by far the largest out-of-plane distance
for any first row transition metal nitrido complex, but it is
almost identical in magnitude to the 0.727 Å found for
displaced 0.725 Å out of the least-squares plane of the four sulfur donor
atoms.
[Re(N)(S2CN(Et)2)2]21 and appears thus to be property
governed by the dithiocarbamate ligands. The bond lengths
to the equatorial ligands in 1-3 are invariably shorter than
those found in Cr(III) complexes of the same ligands:
CrIII(quin) (Cr-N, 1.947-1.964 Å; Cr-O, 2.082-2.093
Å),22 CrIII(dbm) (1.958-1.961 Å),23 and CrIII(pyr-dtc) (2.392-
(21) Fletcher, S. R.; Skapski, A. C. J. Chem. Soc., Dalton Trans. 1972,
1079.
(22) Nagi, M. K.; Harton, A.; Donald, S.; Lee, Y.-S.; Sabat, M.; O’Connor,
C. J.; Vincent, J. B. Inorg. Chem. 1995, 34, 3813.
(23) Gibson, V. C.; Newton, C.; Redshaw, C.; Solan, G. A.; White, A. J.
P.; Williams, D. J. Eur. J. Inorg. Chem. 2001, 1895.
7612 Inorganic Chemistry, Vol. 42, No. 23, 2003

Cr(V) Nitrido Complexes of Bidentate Ligands
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Compounds
1, 2, and 3
Table 4. IR Data for Cr(V) Nitrido Complexes
complex
n
Cr-N (cm-1
)
ref
Cr(N)(quin)2 (1)
1
2
3
4
1015
1007
991
1016
1023
1012
1017
972
a
a
a
a
a
10
9
11
11
7
Cr1-N3
1.5549(7)
1.9300(5)
1.9303(5)
2.0380(6)
2.0399(6)
81.67(2)
Cr1-N1
N3-Cr1-N1
N3-Cr1-N2
N3-Cr1-O1
N3-Cr1-O2
N1-Cr1-O2
O1-Cr1-O2
102.86(3)
103.39(3)
106.85(3)
107.85(3)
89.01(2)
Cr1-N2
Cr1-O1
6
Cr1-O2
[Cr(N)(salen)]
[Cr(N)(tpp)]
Cs2Na[Cr(N)(CN)5]
[PPh4]2[Cr(N)(CN)4(py)]
trans-[Cr(N)(cylam)(CH3CN)](ClO4)2
N1-Cr1-O1
N1-Cr1-N2
153.73(2)
145.25(3)
Cr(N)(dbm)2 (2)
995
996
Cr1-N6
1.5491(14)
1.9319(10)
1.9450(10)
1.9222(11)
1.9283(10)
88.60(4)
Cr1-O1
N6-Cr1-O1
N6-Cr1-O2
N6-Cr1-O3
N6-Cr1-O4
O1-Cr1-O3
O2-Cr1-O3
103.69(6)
105.06(6)
103.32(6)
103.65(6)
83.53(5)
a This work.
Cr1-O2
Cr1-O3
Cr1-O4
O1-Cr1-O2
O1-Cr1-O4
152.60(5)
151.59(5)
Cr(N)(S2CNC4H8)2 (3)
Cr1-N1
1.5490(13)
2.3811(4)
2.3763(5)
2.3689(5)
2.3842(4)
Cr1-S1
N1-Cr1-S1
N1-Cr1-S2
N1-Cr1-S3
N1-Cr1-S4
S1-Cr1-S3
S2-Cr1-S3
106.38(5)
109.00(5)
107.14(5)
108.52(5)
94.513(16)
143.853(17)
Cr1-S2
Cr1-S3
Cr1-S4
S1-Cr1-S2
S1-Cr1-S4
75.209(15)
145.099(16)
Table 3. Structurally Characterized Cr(V) Nitrido Complexes
Cr out of
complex
C.N. Cr-N (Å) plane (Å) ref
1
2
3
5
5
5
5
5
5
6
6
6
6
1.5549(7)
1.5491(14) 0.4652(6)
1.5490(13) 0.7251(3)
1.544(3)
1.565(6)
1.560(2)
1.575(9)
1.561(3)
1.570(3)
1.594(9)
0.5193(3)
a
a
a
34
9
35
8
7
11
12
[Cr(N)(salen)]
[Cr(N)(ttp)]
[Cr(N)(bpb)]
[(Me3tacn)Cr(N)(acac)](ClO4)
trans-[Cr(N)(cyclam)(NCCH3)]2+
trans-[Cr(N)(CN)4(py)]2-
[Cr(N)(CN)5]3-
0.499(3)
0.42
0.51
0.30
0.265
0.261(2)
0.255
a This work.
2.412 Å)24 (dbm ) 1,3-diphenylpropane-1,3-dionate, pyr-
dtc ) pyrrolidinedithiocarbamate). However, the difference
is in most cases quite small, only ca. 0.02 Å, despite a
difference of two units in formal oxidation state of the
chromium central ion. The small difference can be taken as
evidence of the high degree of charge leveling within the
{CrtN}2+ unit found previously in calculations. The previ-
ous data are supplemented by a recent calculation on
[Cr(N)Cl4]2-, which places charges of +0.70 and -0.34 on
Cr and N, respectively.25
Vibrational Spectroscopy. Cr-N stretching frequencies
of the new compounds and some reference systems are given
in Table 4. All new compounds, except 3, have ν(Cr-N) in
the normal range 1000-1025 cm-1 for five-coordinate Cr(V)
nitrido systems. In 3 ν(Cr-N) is significantly lower and in
the range normally observed for six-coordinate systems. This
is counter-intuitive when considering the relatively weak
ligand field from the dithiocarbamates (vide supra), but may
be correlated to the pyramidal geometry.
Figure 5. Experimental and simulated EPR spectra of 1 and 3. Cf. Table
5 for simulation parameters.
(cf. Figure 5). The spectra have an intense central resonance
line due to the chromium isotopes with I ) 0 (90.5%),
surrounded by a weak hyperfine quartet arising from coupling
to 53Cr (9.5%, I ) 3/2). In addition, superhyperfine coupling
to coordinated nitrogens can be observed for all systems.
The spectra of 2 (not shown) and 3 have a simple appearance
with all lines split into triplets due to coupling to the nitrido
ligand (14N, ≈100%, I ) 1). In 1 and 4 the superhyperfine
coupling results in a septet with intensities corresponding to
almost equally strong coupling to the nitrido ligand and the
EPR Spectroscopy. In solution at room temperature
complexes 1-4 exhibit sharp isotropic signals with g ≈ 1.98
(24) Sinn, E. Inorg. Chem. 1976, 15, 369.
(25) Bendix, J. J. Am. Chem. Soc. Submitted for publication.
Inorganic Chemistry, Vol. 42, No. 23, 2003 7613

Birk and Bendix
Table 5. EPR Parameters for Cr(V)Nitrido Complexes
complex
giso
g|
g
AisoN [mT]
A
N [mT]
AisoCr [mT]
A
Cr [mT]
ref
1
2
3
4
1.976
1.971
1.985
1.977
1.990
1.9856
1.987
1.9825
1.948
1.935
1.966
1.944
1.9755
1.965
1.951
1.9583
1.992
1.989
1.993
1.993
1.9990
1.997
1.991
1.9945
0.25
0.25
0.27
0.25
0.29
0.23
0.25
0.277
2.70
3.03
2.60
2.70
2.49
2.80
2.83
2.827
1.8
1.9
1.9
1.8
1.5
1.6
1.98
2.24
a
a
a
a
12
7
10
9
0.45
0.45
[Cr(N)(CN)5]3-
0.36
trans-[Cr(N)(cyclam)(CH3CN)]2+
[Cr(N)(salen)]
[Cr(N)(tpp)]
0.37b
a This work. b Determined by ENDOR.
equatorial ligands. This near degeneracy of the Fermi contact
interactions is a general feature of Cr(V) nitrido complexes
and has been discussed before.26,27 The EPR spectra observed
in a frozen glass at 77 K are characteristic of axially
symmetric systems. In the solid-state spectra of 2 and 3, the
anisotropies of the hyperfine and superhyperfine couplings
are directly observable. This is a uniquely advantageous
situation, brought about by the simple coordination sphere
without ligators with nuclear spin. Normally it requires dilute
single-crystal spectra or ENDOR to obtain the anisotropy
of the superhyperfine interaction. EPR parameters from
simulations of the solution and solid-state EPR spectra are
collected and compared with literature data in Table 5. All
the parameters lie in the ranges of previously reported
data.7,9,10,12
UV/vis Spectroscopy. UV/vis and MCD spectra of 2 and
3 are shown in Figure 6. In both cases, a weak absorption is
located between 540 and 600 nm. A similar absorption (λ
) 552 nm, ꢀ ) 26.1 M-1 cm-1) is found for 4. In the
spectrum of 3, a second (slightly split) band at λ ) 446,
432 (sh) nm is observed. In all cases we assign the low-
2
2
energy band to be the dx -y r dxy transition. The band system
at ca. 440 nm in 3 is the transition {dzx, dyz} r dxy, slightly
split due to the large deviation from 90° bite angles in the
dithiocarbamate ligands. In these assignments, the relative
band intensities in the spectrum of 3 agree with the selection
2
2
rules in (idealized) C4V symmetry, where the dx -y r dxy is
the only electrical dipole forbidden d-d transition and is
thus expected to be fairly weak. The assignment of the
second band system in 3 as {dzx, dyz} r dxy is further
corroborated by the MCD spectrum where, in agreement with
previous findings, this band is clearly observed. On the basis
of the experimental coordination geometry of 3, the following
AOM expression28 is obtained for the position of the first
Figure 6. UV-vis and MDC spectra of 3 (λmax [nm] (ꢀ) [M-1 cm-1]:
548 (54.5), 446 (221), 432 sh, (212), 276 (17 400)) and 2 (λmax [nm] (ꢀ)
[M-1 cm-1]: 595 (26.4), 469 sh, (93.3), 359, (37 100), 271 (31 500)).
expressions and the experimental position of the first band
one can calculate in a σ-only model a splitting of the {dzx,
dyz} set of 1400 cm-1. Since the experimental value of this
splitting is less (although quite uncertain), and since it is
S
band: E(dx -y ) - E(dxy) ) 2.31eσS - 3.40eπ|S + 0.34 eπ
.
2
2
And for the splitting of the {dzx, dyz} set of orbitals E(dyz) -
S
S
S 29
E(dzx) ) 0.18eσ - 0.07eπ| + 0.50eπ
.
From these
S
S
normal from bonding considerations to assume eπ > eπ|
(26) Hori, A.; Ozawa, T.; Yoshida, H.; Imori, Y.; Kuribayashi, Y.; Nakano,
E.; Azuma, N. Inorg. Chim. Acta 1998, 281, 207.
in planar bidentate ligands, it follows that π-donation from
the dithiocarbamates is small. Considering that the sulfur
ligand has to π-donate to the same orbitals as the nitrido
ligand, the result of this simple analysis appears reasonable.
The first spin-allowed bands in Cr(III) complexes provide
(for ortho-axial ligation) a direct measure of ∆o ) 3eσ -
4eπ ) 3eσEquatorial - 4eπEquatorial. The positions of the first band
in the spectra of 2 and 3 can thus be compared to data for
Cr(III) complexes of the same ligands. In the spectrum of
(27) It is possible to distinguish the nitrido ligand from the equatorial
nitrogen donors in simulations of the EPR spectra of 1 and 4. A
complete reproduction of the intensity distribution in the septet is only
N
possible if a slightly smaller Aiso ) 0.24 mT is employed for the
equatorial N-donors.
(28) (a) Scha¨ffer, C. E. Struct. Bonding 1968, 5, 93. (b) Bendix, J. In
ComprehensiVe Coordination Chemistry II; Meyer, T. J., McCleverty,
J. A., Ed.; Elsevier, in press Chapter 9.4.
(29) Parallel and perpendicular refers to the plane of the dithiocarbamate
ligand and not to the Cr-N axis, where the designations would have
been interchanged.
7614 Inorganic Chemistry, Vol. 42, No. 23, 2003

Cr(V) Nitrido Complexes of Bidentate Ligands
Cr(acac)3 the 4A2 f 4T2 transition is located at 17 860 cm-1,30
and in the spectrum of Cr(S2CN(Et)2)3 it is found at 15 500
cm-1.31 Again the very small difference between Cr(III) and
Cr(V) is noteworthy and emphasizes the usefulness of
thinking in terms of a {CrtN}2+ unit. The data also show
that there is a reversal of the spectrochemical ordering of
the O and S donor ligands upon going from Cr(III) to Cr(V).
Possible explanations could be that the low ∆o for dithio-
carbamate coordinated to Cr(III) reflects a relatively large
π-donation, which is suppressed in the nitrido complexes in
line with the above discussion. However, the reversal may
also be due to a slightly better fit of the smaller Cr(V) center
to the narrow bite angle of the dithiocarbamate ligands.
The first Cr(V) nitrido complexes to be prepared all
featured conjugated organic ligand systems such as salen and
porphyrins. Accordingly, d-d spectra were not observed and
d-orbital splittings had to be inferred indirectly from EPR
data. Later, systems with spectroscopically more innocent
ligands such as cyclam, tacn, and cyanide were prepared and
their d-d spectra were analyzed. Common to these systems
were thus a coordination sphere completed by strong donor
ligands. The outcome of these studies was an orbital splitting
diagram reminiscent of the one devised for vanadyl systems
early on by Ballhausen and Gray32 and independently
Jørgensen33 (cf. Figure 7). Conceptually, the splitting pattern
is normally conceived as a strong tetragonal compression of
an octahedral reference system. In the nitrido complexes the
π* orbitals were found to be higher in energy than in {VO}2+
Figure 7. d-orbital energy diagrams illustrating the variation in splitting
pattern going from vanadyl systems via {CrtN}2+ with strongly donating
auxiliary ligands to (pseudolinear, see text) {CrtN}2+ complexes with weak
equatorial donors.
Conclusion
A new general route to nitrido complexes of Cr(V) based
on nitrogen atom transfer from Mn(N)(salen) to labile
CrCl3(THF)3 has been developed. By this approach, four
members of the hitherto unknown class of Cr(N) nitrido
complexes with bidentate auxiliary ligands have been
synthesized and characterized. The mild conditions of this
protocol allow for the first time for the introduction of weak
donor ligands in {Cr(N)}2+ coordination chemistry. Conse-
quently, the orbital energies of these systems become
completely dominated by the nitrido ligand. Work in progress
has established the present procedure to be applicable also
to the synthesis and isolation of Cr(V) nitrido complexes
with a wide variety of monodentate ligands including
phosphines, pyridines, thiocyanate, and chloride.
2
2
and nearly degenerate with the dx -y orbital. Of the systems
presented here, 2 and 3 are fairly spectroscopically transpar-
ent and represent with their relatively weakly donating
equatorial ligands a qualitatively different d-orbital splitting
2
2
pattern where the dx -y orbital lies well below the π* orbitals
(cf. right hand side of Figure 7). This splitting pattern with
Acknowledgment. We thank Flemming Hansen (CCS)
and Magnus Magnussen for assistance in the crystallographic
work. Access to software for EPR simulation and fitting of
magnetic data written by H. Weihe is greatly appreciated.
J.B. acknowledges a grant (SNF No. 1266) from the Danish
Research Council.
2
2
the two δ orbitals (dx -y and dxy, quantization along the Cr-N
axis) lowest in energy approaches the situation for a linear
system, where the two δ orbitals are degenerate and lowest
in energy. It may be conceptually useful to consider these
new complexes not as strongly perturbed octahedral systems,
but rather as less strongly perturbed linear systems.
Supporting Information Available: Crystallographic data (CIF
format), magnetic moment data for 1-3, and experimental and
simulated EPR spectra of 1 (77 K), 2 (77, 293 K), and 4 (77, 293
K). This material is available free of charge via the Internet at
http://pubs.acs.org.
(30) Fatta, A. M.; Lintvedt, R. L. Inorg. Chem. 1971, 10, 478.
(31) Jørgensen, C. K. J. Inorg. Nucl. Chem. 1962, 24, 1571.
(32) Ballhausen, C. J.; Gray, H. B. Inorg. Chem. 1962, 1, 111.
(33) Jørgensen, C. K. Acta Chem. Scand. 1957, 11, 73.
(34) Bendix, J.; Wilson, S. R.; Prussak-Wiechowska, T. Acta Crystallogr.
1998, C53, 923.
(35) Che, C.-M.; Ma, J.-X.; Wong, W.-T.; Lai, T.-F.; Poon, C.-K. Inorg.
Chem. 1988, 27, 2547.
IC034777F
Inorganic Chemistry, Vol. 42, No. 23, 2003 7615