Synthesis and characterization of manganese and iron complexes supported by multidentate [N2P2] ligands

Article abstract of DOI:10.1039/b821954k

The coordination chemistry of mono- and divalent manganese and iron complexes supported by the monoanionic multidentate ligands, [N ₂P₂] (where [N₂P₂] = ^tBuN^(-)SiMe₂N(CH₂CH ₂PⁱPr₂)₂) and [N₂P ₂^tolyl] (where [N₂P₂ ^tolyl] = MeC₆H₄N^(-)SiMe ₂(CH₂CH₂PⁱPr₂) ₂) is presented. The Mn(II) and Fe(II) halide complexes [N ₂P₂]MnCl (1) and [N₂P₂]FeCl (2) serve as precursors to the alkyl and hydride species [N₂P ₂]MnMe (3), [N₂P₂]FeMe (4), [N ₂P₂]FeCH₂SiMe₃ (5), and ([N ₂P₂]Mn)₂(μ-H)₂ (6). Reduction of 1 and 2 results in the formation of the new bridging dinitrogen complexes ([N₂P₂]Mn)₂(μ-N₂) (7) and ([N₂P₂]Fe)₂(μ-N₂) (8), respectively. Upon exposure to vacuum, N₂ is abstracted from 8, resulting in the observed Fe(I) complex, [N₂P₂]Fe (9). The new Fe(II) halide complex [N₂P₂^tolyl]FeCl (10) was isolated following the substitution of [N₂P₂ ^tolyl] for [N₂P₂]. Reduction of 10 in the presence of N₂ resulted in the formation of the dinitrogen free adduct [N₂P₂^tolyl]Fe (11).

Full text of DOI:10.1039/b821954k

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Dalton
Transactions
An international journal of inorganic chemistry
www.rsc.org/dalton
Number 10 | 14 March 2009 | Pages 1677–1872
ISSN 1477-9226
PAPER
PAPER
Arnold and Chomitz
Synthesis and characterization of
manganese and iron complexes
supported by multidentate [N2P2]
ligands
Williams, Záliš et al.
COMMUNICATION
Tipping the balance from localised to
intraligand charge-transfer excited
states in cyclometalated platinum
complexes
Guo, Wang et al.
Fabrication of water soluble and
biocompatible CdSe nanoparticles in
apoferritin with the aid of EDTA

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis and characterization of manganese and iron complexes supported
by multidentate [N2P2] ligands†
Wayne A. Chomitz and John Arnold*
Received 8th December 2008, Accepted 9th January 2009
First published as an Advance Article on the web 26th January 2009
DOI: 10.1039/b821954k
The coordination chemistry of mono- and divalent manganese and iron complexes supported by the
t
monoanionic multidentate ligands, [N2P2] (where [N2P2] = BuN(-)SiMe2N(CH2CH2PiPr2)2) and
[N2P2tolyl] (where [N2P2tolyl] = MeC6H4N(-)SiMe2(CH2CH2PiPr2)2) is presented. The Mn(II) and Fe(II)
halide complexes [N2P2]MnCl (1) and [N2P2]FeCl (2) serve as precursors to the alkyl and hydride
species [N2P2]MnMe (3), [N2P2]FeMe (4), [N2P2]FeCH2SiMe3 (5), and ([N2P2]Mn)2(m-H)2 (6).
Reduction of 1 and 2 results in the formation of the new bridging dinitrogen complexes
([N2P2]Mn)2(m-N2) (7) and ([N2P2]Fe)2(m-N2) (8), respectively. Upon exposure to vacuum, N2 is
abstracted from 8, resulting in the observed Fe(I) complex, [N2P2]Fe (9). The new Fe(II) halide complex
[N2P2tolyl]FeCl (10) was isolated following the substitution of [N2P2tolyl] for [N2P2]. Reduction of 10 in the
presence of N2 resulted in the formation of the dinitrogen free adduct [N2P2tolyl]Fe (11).
Here we report our findings on the application of [N2P2] as a
supporting ligand for organometallic and low-valent species of
First-row middle-transition metal complexes have proven to be
Mn and Fe. The synthesis of a number of metal-alkyl, metal-
Introduction
useful catalysts for a number of important transformations.
Brookhart et al.,1 Gibson et al.,2 and Bennett3 have reported
hydride, and low-valent compounds is described, and the reactivity
of reduced Mn and Fe complexes of [N2P2] with N2 is explored.
Finally, the modified [N2P2] ligand, [N2P2tolyl], is introduced into
the Fe system and the resulting changes in reactivity with N2 are
examined.
highly active ethylene polymerization catalysts incorporating
bulky bis(imino)pyridine ligands with Fe and Co. By reducing the
steric bulk of the bis(imino)pyridine ligand, it was discovered that
selective oligomerization of ethylene to linear a-olefins could be
achieved.4 The utility of bis(imino)pyridine ligands was extended
to include Mn by Gambarotta and co-workers who reported
multi-electron reduction at the metal center following alkylation.5
More recently, Chirik et al. reported the synthesis of reduced
Fe complexes supported by bis(imino)pyridine ligands and their
application for hydrogenation and hydrosilation.6 First-row metal-
hydride species have also been shown to be active in stoichiometric
Experimental
All reactions were performed using standard Schlenk-line tech-
niques or in an MBraun drybox (<1 ppm O2/H2O) unless noted
R
otherwise. All glassware, cannulae, and Celiteꢀ were stored in an
oven at >425 K. Pentane, toluene, diethyl ether, and tetrahydro-
furan were purified by passage through a column of activated alu-
mina and degassed with nitrogen prior to use.27 Deuterated solvent
was vacuum transferred from sodium/benzophenone (benzene).
NMR spectra were recorded at ambient temperature on Bruker
AV-300, AVQ-400, AVB-400, and DRX-500 spectrometers. 1H and
reactions, as shown by Holland et al. who observed azobenzene
7
=
N N bond cleavage by a three coordinate iron hydride.
The activation and functionalization of N2 using transition
metal complexes is also an important research topic.8–13 The
industrial standard for N2 fixation is the Haber–Bosch process,
which converts N2 and H2 to NH3 using an ill-defined Fe catalyst.14
Though extremely good conversion rates (98%) can be achieved,
a tremendous amount of energy is consumed (1% of world energy
consumption).15 In contrast, nature uses nitrogenase enzymes to
catalytically reduce atmospheric N2 to ammonium salts, which
are then used to synthesize useful nitrogen-containing molecules.
Nitrogenase operates under ambient conditions and also contains
an Fe catalyst.16–19 Given the role low-valent middle transition
metals play in the above catalytic reactions, their chemistry with
N2 has become a recent topic of interest.6,20–26
1
13C{ H} chemical shifts are given relative to residual solvent peaks,
1
and coupling constants (J) are given in hertz. 31P{ H} chemical
shifts are referenced to an external standard of P(OMe)3 set to
1.67 ppm. Infrared samples were prepared as Nujol mulls and
taken between KBr disks. Magnetic susceptibility (meff) values
were determined ◦using the solution Evans method at ambient
temperature (22 C).28 Melting points were determined using
sealed capillaries prepared under nitrogen and are uncorrected.
Li[N2P2],29 Li[N2P2tolyl],30 [N2P2]MnCl (1),29 and [N2P2]FeCl (2)29
were prepared using the literature procedures, and unless otherwise
noted, all reagents were acquired from commercial sources.
Elemental analyses and mass spectral data were determined at
the College of Chemistry, University of California, Berkeley. The
X-ray structural determination was performed at CHEXRAY,
University of California, Berkeley.
Department of Chemistry, University of California, Berkeley, CA 94720-
1460, USA. E-mail: arnold@berkeley.edu; Tel: +1 510-643-5381
† CCDC reference numbers 711736 and 711737. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b821954k
1714 | Dalton Trans., 2009, 1714–1720
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[N2P2]MnMe (3)
d 0.24 (s); 0.62 (s); 0.88 (s); 1.08 (s); 1.17 (s); 1.65 (s); 3.23 (s);
8.14 (br s); 16.30 (br s). IR (cm-1): 1790 (w); 1349 (m); 1294 (w);
1238 (m); 1200 (s); 1093 (s); 1066 (s); 938 (m); 908 (m); 879 (w); 836
(s); 824 (m); 795 (m); 760 (s); 722 (m); 689 (w); 665 (w); 610 (w);
564 (m); 527 (w); 502 (m). Anal. Calc. C44H104Mn2N4P4Si2. C:
53.95; H: 10.72; N: 5.72. Observed. C: 53.99; H: 11.01; N: 5.83.
mp = 104–106 ◦C. meff = 3.4 mB per Mn.
Methyl lithium (1.6 M in Et2O, 0.36 mL, 0.57 mmol) was syringed
into ◦a solution of 1 (0.30 g, 0.57 mmol) in 10 mL Et2O at
-40 C. The reaction mixture was warmed to room temperature
and was stirred for 4 h. The solvent was removed under vacuum,
and the crude product was extracted with pentane. Following
concentration and cooling at -40 ◦C, large colorless needles were
isolated in 56% yield (0.16 g). 1H NMR (300 MHz, C6D6): d 0.267
(s); 1.10 (s); 1.67 (s); 3.24 (s); 6.07 (s). IR (cm-1): 1346 (m); 1297 (w);
1245 (m); 1199 (s); 1101 (m); 1056 (s); 1017 (m); 969 (w); 926 (w);
882 (w); 844 (s); 794 (m); 752 (m); 727 (s); 685 (w); 649 (m); 597 (w);
526 (m); 439 (m). Anal. Calc. C23H54MnN2P2Si. C: 54.83; H: 10.83;
([N2P2]Mn)2(l-N2) (7)
A freshly prepared solution of sodium naphthalide (0.57 mmol)
in 5 mL THF was added to a suspension of 1 (300 mg,
0.57 mmol) in 5 mL THF at -40 ◦C. The reaction mixture
turned dark red instantly and was allowed to warm to room
temperature. After stirring for 2 h, the solvent was removed under
vacuum. Naphthalene was removed by sublimation, and the crude
product was extracted with pentane (10 mL). The solution was
concentrated until the precipitation of crystalline material was
observed. Cooling at -40 ◦C resulting in the isolation of large
dark red blocks (69 mg, 24% yield). IR (cm-1): 1298 (w); 1229 (s);
1196 (m); 1111 (m); 1050 (s); 1017 (s); 927 (m); 833 (s); 776 (m);
689 (m); 646 (m); 490 (m). Raman (cm-1): 1685 (nN-N). Anal. Calc.
for C44H102N6P2Si2Mn2. C: 52.56; H: 10.25; N: 8.36. Observed. C:
52.68; H: 10.55; N: 8.31. mp (dec) = 108–111 ◦C. meff = 8.7 mB.
◦
N: 5.56. Observed. C: 54.45; H: 11.08; N: 5.47. mp = 85–86 C.
meff = 5.7 mB.
[N2P2]FeMe (4)
Methyl lithium (1.0 M in Et2O, 0.60 mL, 0.60 mmol) was added
to a solution of 2 (0.30 g, 0.57 mmol) in 7 mL THF at -40 ◦C
via syringe. The reaction mixture was allowed to warm to room
temperature and was stirred for 3 h. The volatile material was
removed under vacuum, and the crude product was extracted
with pentane. Following concentration and cooling at -40 ◦C,
large yellow needles were isolated in 54% yield (0.16 g). 1H NMR
(300 MHz, C6D6): d -2.49 (br s); 0.24 (s); 1.10 (s); 1.65 (s); 3.25
(s); 17.61 (br s); 26.85 (br s); 44.90 (br s); 56.55 (br s); 61.10 (br s);
73.40 (br s). IR (cm-1): 1643 (w); 1306 (w); 1243 (s); 1196 (s); 1113
(s); 1050 (s); 1016 (s); 970 (w); 926 (m); 882 (w); 843 (s); 797 (m);
753 (m); 733 (m); 683 (w); 653 (m); 600 (w); 549 (w); 529 (w);
462 (m). Anal. Calc. C23H54FeN2P2Si. C: 54.74; H: 10.81; N: 5.55.
([N2P2]Fe)2(l-N2) (8)
A suspension of KC8 (0.36 g, 2.7 mmol) in 10 mL THF was added
to a suspension of 2 (1.4 g, 2.7 mmol) in 15 mL THF at -40 ◦C. The
reaction mixture was allowed to warm to room temperature and
was stirred for 3 h. The solvent was removed under vacuum,
and the crude product was extracted with pentane. Following
Observed. C: 54.73; H: 11.01; N: 5.41. mp = 99–101 ◦C. meff
5.0 mB.
=
◦
concentration and cooling at -40 C, large dark red block were
isolated (1.1 g, 80% yield). IR (cm-1): 1417 (w); 1349 (m); 1300 (w);
1239 (m); 1201 (m); 1105 (m); 1077 (w); 1047 (s); 1015 (m); 921
(s); 885 (m); 839 (s); 802 (w); 730 (s); 689 (w); 652 (w); 603 (w);
511 (w); 487 (s). Raman (cm-1): 1760 (nN-N). Anal. Calc. for
C44H102N6P4Si2Fe2: C: 52.46; H: 10.23; N: 8.35. Observed. C: 52.74;
H: 10.50; N: 7.97. mp = 115–116 ◦C. meff = 7.0 mB.
[N2P2]FeCH2SiMe3 (5)
A solution of LiCH2SiMe3 (0.056 g, 0.59 mmol) in 5 mL THF
was added to a solution of 2 (0.31 g, 0.59 mmol) in 5 mL
THF at -40 ◦C. The reaction mixture was allowed to warm to
room temperature and was stirred overnight. Solvent was removed
under vacuum, and the crude product was extracted with pentane.
Following concentration and cooling at -40 ◦C, colorless plates
were collected in 77% yield (0.26 g). 1H NMR (300 MHz, C6D6):
d -5.42 (br s); -2.46 (br s); 0.24 (s); 1.10 (s); 1.65 (s); 2.07 (br s);
3.21 (s); 4.63 (s); 16.77 (br s); 26.04 (br s). IR (cm-1): 1348 (m);
1292 (w); 1235 (s); 1077 (s); 1055 (s)l 1028 (m); 966 (w); 880 (s); 846
(s); 817 (s); 769 (m); 736 (m); 719 (m); 665 (m); 607 (m); 536 (w);
519 (m); 487 (m); 455 (m). Anal. Calc. C26H62FeN2P2Si2. C: 54.13;
H: 10.86; N: 4.86. Observed. C: 54.25; H: 11.22; N: 4.68. mp =
70–72 ◦C. meff = 5.1 mB.
[N2P2]Fe (9)
Method A. A dark red solution of 8 (0.30 g, 0.30 mmol) in
10 mL pentane was exposed to vacuum until dryness. Over the
course of the solvent evacuation, the solution changed color from
dark red to yellow-green. The resulting solid was identified as
[N2P2]Fe based on the absence of any distinguishing features in
the IR spectrum between 1600 and 2700 cm-1 and the isolation
of 8 upon exposure of 9 to an atmosphere of N2. The green color
remained as long as negative pressure or an Ar atmosphere was
maintained.
([N2P2]Mn)2(l-H)2 (6)
Method B. An Ar atmosphere was used instead of N2, and
all solvents were degassed with Ar prior to use. A suspension
of KC8 (77 mg, 0.57 mmol) in 5 mL THF was added to a
suspension of 2 (0.30 g, 0.57 mmol) in 5 mL THF at -40 ◦C. The
reaction mixture was allowed to warm to room temperature
and was stirred overnight. The reduction of 2 was slower in
the absence of N2, so longer reaction times were needed for
the reaction to go to completion. The volatile materials were
R
A solution of Red-Alꢀ (70% in toluene, 0.18 g, 0.57 mmol) in
5 mL THF was added to a solution of 1 (0.30 g, 0.57 mmol) in
5 mL THF at -40 ◦C. The reaction mixture was warmed to room
temperature and was stirred for 30 min. Solvent was removed
under vacuum, and the crude product was extracted with pentane.
Following concentration and cooling at -40 ◦C, colorless crystals
1
were isolated in 57% yield (0.16 g). H NMR (500 MHz, C6D6):
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Table 1 Crystal data and structure refinement for 5–8
Compound number
5
6
7
8
Compound name
Empirical formula
FW
[N2P2]FeCH2SiMe3
C26H62FeN2P2Si2
576.75
160(2)
Triclinic
¯
P1
8.6908(11)
9.9321(13)
21.345(3)
96.779(2)
97.816(2)
104.621(2)
1743.8(4)
2
([N2P2]Mn)2(m-H)2
C22H52MnN2P2Si
979.25
160(2)
Monoclinic
([N2P2]Mn)2(m-N2)
C22H51MnN3P2Si
502.63
151(2)
Orthorhombic
([N2P2]Fe)2(m-N2)
C22H51FeN3P2Si
503.54
154(2)
Monoclinic
Temperature/K
Crystal system
Space group
C2/c
Pbca
P21/c
˚
a/A
49.385(5)
10.9384(11)
22.807(2)
90
107.880(3)
90
11725(2)
8
0.71073
0.611
18.381(2)
15.461(2)
19.772(2)
90
90
90
5619.0(11)
8
0.71073
0.64
11.648(1)
18.056(2)
13.382(1)
90
102.055(1)
90
2752.3(5)
4
0.71073
0.72
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
˚
l/A
0.71073
0.609
4920
m/cm-1
Number of reflectionsobs
7019
4787
5248
R(int)
Robs (%)
wRobs (%)
0.0260
4.86
11.85
0.0722
4.79
9.96
0.1055
5.34
12.63
0.0162
3.19
8.83
removed under vacuum, and the crude product was extracted with
Et2O. Following concentration and cooling at -40 ◦C, yellow-green
crystals of 9 were obtained. The yield was not obtained directly
due to conversion of 9 to 8 following exposure to N2.
for crystal decay was applied. Data were analyzed for agreement
and possible absorption using XPREP.33 An empirical absorption
correction based on comparison of redundant and equivalent
reflections was applied using SADABS.34 The structures were
solved using SHELXS35 and refined on all data by full-matrix
least squares with SHELXL-97.36 Thermal parameters for all non-
hydrogen atoms were refined anisotropically. ORTEP diagrams
were created using ORTEP-3. A summary of the X-ray diffraction
data is presented in Table 1.
[N2P2tolyl]FeCl (10)
A solution of Li[N2P2tolyl] (1.0 g, 2.1 mmol) in 20 mL was added
dropwise to a suspension of FeCl2 (0.27 g, 2.1 mmol) in 20 mL
DME. The reaction mixture was stirred overnight. The solvent
was removed under vacuum, and the crude product was dissolved
Results and discussion
R
in THF and filtered through a bed of Celiteꢀ. An equal volume
Substitution chemistry of 1 and 2
of pentane was added, and the resulting solution was cooled at
-40 ◦C overnight. Bright orange crystals were isolated in 70%
The metal-halides [N2P2]MnCl (1) and [N2P2]FeCl (2) were
synthesized as reported.29 Reaction of 1 and methyl lithium in
Et2O at -40 ◦C resulted in the isolation of large colorless needles
of [N2P2]MnMe (3) in 56% yield following work-up and crystalliza-
tion from pentane at -40 ◦C (Scheme 1a). The solution magnetic
susceptibility of 3 is 5.7 mB (S = 5/2), in agreement with high-spin
Mn(II). A similar reaction of 2 resulted in large yellow needles of
[N2P2]FeMe (4) in 54% yield (Scheme 1b). The solution magnetic
susceptibility of 4 is 5.0 mB, indicating the presence of high-
spin Fe(II). Interestingly, the Co analogue, [N2P2]CoMe, is low-
spin, and the solid-state structure shows a trigonal bipyramidal
geometry about the Co atom.30 No crystallographic data for 3
or 4 could be obtained due to inherent twinning of the crystals.
However, based on magnetic data, we infer that the solid-state
structures of 3 and 4 are likely tetrahedral and similar to those of
1 and 2.
1
yield (0.82 g). H NMR (500 MHz, C6D6): d -70.29 (br s); 1.03
(br s); 9.40 (br s); 13.87 (br s); 29.75 (br s); 33.62 (br s); 40.07 (s);
57.78 (br s); 61.22 (br s); 68.28 (s); 72.86 (br s); 87.59 (br s). IR
(cm-1): 1606 (s); 1503 (s); 1306 (w); 1265 (s); 1239 (m); 1178 (w);
1118 (m); 1095 (m); 1067 (m); 1049 (m); 997 (w); 973 (w); 930 (s);
887 (w); 844 (s); 800 (m); 762 (s); 682 (m); 663 (m); 601 (m); 547 (w);
506 (m); 456 (w). Anal. Calc. C25H49ClFeN2P2Si. C: 53.70; H: 8.85;
N: 5.01. Observed. C: 53.49; H: 9.02; N: 4.93. mp = 208–209 ◦C.
meff = 4.7 mB.
Crystallographic analysis†
Single crystals of 5–8 were coated in Paratone-N oil, mounted on
a Kaptan loop, transferred to a Siemens SMART diffractometer
or a Bruker APEX CCD area detector,31 centered in the beam,
and cooled by a nitrogen flow low-temperature apparatus that
has been previously calibrated by a thermocouple placed at the
same position as the crystal. Preliminary orientation matrices
and cell constants were determined by collection of 60 30-second
frames, followed by spot integration and least-squares refinement.
An arbitrary hemisphere of data was collected and the raw data
were integrated using SAINT.32 Cell dimensions reported were
calculated from all reflections with I > 10s. The data were
corrected for Lorentz and polarization effects, but no correction
Scheme 1
Alkylation of 2 was also accomplished using LiCH2SiMe3,
which afforded [N2P2]FeCH2SiMe3 (5) as colorless plate-like
1716 | Dalton Trans., 2009, 1714–1720
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◦
◦
˚
˚
Table 2 Selected bond lengths (A) and angles ( ) for [N2P2]FeCH2SiMe3
(5)
Table 3 Selected bond lengths (A) and angles ( ) for {([N2P2]Mn)2(m-
H)}2 (6)
Fe(1)–N(1)
Fe(1)–N(2)
1.972(2)
2.315(2)
Fe(1)–C(23)
Fe(1)–P(1)
2.066(3)
2.4560(9)
Mn(1)–N(1)
Mn(1)–Mn(2)
Mn(1)–H(101)
Mn(2)–P(3)
2.029(2)
2.7945(7) Mn(1)–H(100)
1.91(2) Mn(2)–N(3)
2.6244(9) Mn(2)–H(101)
2.04(2)
Mn(1)–P(1)
2.5961(9)
2.07(2)
2.032(3)
2.04(2)
N(1)–Fe(1)–C(23)
C(23)–Fe(1)–N(2)
C(23)–Fe(1)–P(1)
126.78(11)
113.49(10)
117.76(9)
N(1)–Fe(1)–N(2)
N(1)–Fe(1)–P(1)
N(2)–Fe(1)–P(1)
74.28(9)
115.46(7)
81.76(6)
Mn(2)–H(100)
N(1)–Mn(1)–P(1)
112.68(8) N(1)–Mn(1)–Mn(2)
111.00(3) N(1)–Mn(1)–H(100)
136.00(7)
120.3(6)
46.6(7)
109.3(7)
93.4(10)
132.68(7)
118.4(6)
47.6(6)
P(1)–Mn(1)–Mn(2)
P(1)–Mn(1)–H(100)
N(1)–Mn(1)–H(101)
Mn(2)–Mn(1)–H(101)
N(3)–Mn(2)–P(3)
P(3)–Mn(2)–Mn(1)
P(3)–Mn(2)–H(100)
N(3)–Mn(2)–H(101)
Mn(1)–Mn(2)–H(101)
crystals in 77% yield following crystallization from pentane at
98.3(6)
119.5(7)
46.8(7)
Mn(2)–Mn(1)–H(100)
P(1)–Mn(1)–H(101)
H(100)–Mn(1)–H(101)
-40 ◦C. In the related Co system, the binding mode of the [N2P2]
4
3
ligand changed from k -N2P2 to k -N2P when the R group was
changed from methyl to trimethylsilylmethyl.30 An ORTEP dia-
gram of 5 is displayed in Fig. 1 with selected bond lengths and an-
gles provided in Table 2. Like the Co analogue, the Fe center lies in
a mildly distorted tetrahedral geometry with angles of 126.78(11)◦
{N(1)–Fe(1)–C(23)}, 113.49(10)◦ {C(23)–Fe(1)–N(2)}, 115.46(7)◦
{N(1)–Fe(1)–P(1)}, and 117.76(9)◦ {C(23)–Fe(1)–P(1)}. The
114.72(7) N(3)–Mn(2)–Mn(1)
112.47(3) N(3)–Mn(2)–H(100)
100.5(7)
117.8(7)
43.1(7)
Mn(1)–Mn(2)–H(100)
P(3)–Mn(2)–H(101)
H(100)–Mn(2)–H(101)
111.4(7)
90.7(9)
˚
Fe(1)–C(23) bond distance of 2.066(3) A is close to the cor-
responding parameters previously reported for the a-diimine
=
Fe complex [ArN C(CH3)–C(CH3)=NAr]Fe(CH2SiMe3)2, Ar =
37
˚
2,6-diisoproylphenyl, (2.042(3) and 2.072(3) A) and in the
bis(imino)pyridine complex (iPrPDI)Fe(CH2SiMe3)2 (2.062(3) and
2.054(3) A).38 The solution magnetic susceptibility was determined
˚
Scheme 2
to be 5.1 mB, in agreement with a high-spin Fe(II) center.
Unfortunately, efforts to generate the corresponding Mn alkyl
complex were unsuccessful.
Fig. 2 Thermal ellipsoid (50%) plot of 6. Hydrogen atoms (with the
exception of bridging hydrides), iso-propyl methyl, and tert-butyl methyl
groups have been omitted for clarity.
and angles provided in Table 3. In contrast to [N2P2]CoH which
Fig. 1 Thermal ellipsoid (50%) plot of 5. Hydrogen atoms, iso-propyl
methyl, and tert-butyl methyl groups have been omitted for clarity.
4
is monomeric with a k -N2P2 bound ligand,30 the Mn derivative 6
2
is dimeric and displays k -NP bound [N2P2] ligands.39 There is a
˚
short contact between the two Mn centers (2.7945(7) A), which is
Attempts to isolate an [N2P2]Fe hydride from 2 and reducing
R
agents such as Red-Alꢀ, lithium aluminum hydride, and Super-
less than typical Mn–Mn single bonds (ave. 2.842 A) but is very
˚
R
40
Hydrideꢀ resulted in dark orange-brown solutions that did not
close to a Mn Mn double bond (ave. 2.791 A). The existence
of a Mn–Mn interaction in 6 is further supported by magnetic
susceptibility data (see below). The Mn–H bond distances of
˚
=
yield any tractable material. Addition of trapping agents, such as
PMe3, also did not lead to identifiable products. In contrast to 2,
complex 1 is a useful starting material for the bridging dihydride
{([N2P2]Mn)2(m-H)2} (6) (Scheme 2). Reaction of 1 with one
˚
1.91(2), 2.07(2), 2.04(2), and 2.04(2) A are longer than those
observed in the only other structurally characterized Mn dimer
R
equivalent of Red-Alꢀ resulted in the isolation of colorless acicular
with two bridging hydrides [Mn2(m-H)2(CO)6(m-dppm)] (1.59(5),
˚
crystals of 6 in 57% yield. The IR spectrum of 6 shows a weak,
broad absorption at 1790 cm-1, suggestive of a metal-hydride. The
solid-state structure of 6 was determined via X-ray diffraction; the
ORTEP diagram is shown in Fig. 2 with selected bond lengths
1.68(5), 1.76(5), and 1.81(5) A), though little should be drawn from
this observation as the distance between the Mn centers in [Mn2(m-
˚
H)2(CO)6(m-dppm)] (2.699(2) A) is significantly shorter than that
observed in 6.41
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◦
˚
The solution magnetic susceptibility was found to be 3.4 mB per
Mn center, much lower than the theoretical value of 5.5 mB (S =
5/2) for high-spin Mn(II). The difference between the observed
and theoretical meff values indicated an interaction between the
two Mn centers might exist. To probe this interaction, SQUID
data was collected on crystalline 6. The meff versus T plot shows a
temperature dependence on the magnetic susceptibility consistent
with direct communication between the two Mn centers even
at 300 K (Fig. 3).42 The solution meff may be larger than that
observed in the solid-state due to the presence of a paramagnetic
impurity resulting from the formation of monomer ([N2P2]MnH)
in solution.43
Table 4 Selected bond lengths (A) and angles ( ) for ([N2P2]Mn)2(m-N2)
(7)
Mn(1)–N(3)
Mn(1)–P(1)
N(3)–N(3¢)
1.860(3)
2.5514(12)
1.208(6)
Mn(1)–N(1)
Mn(1)–P(2)
2.066(3)
2.5543(12)
N(3)–Mn(1)–N(1)
N(1)–Mn(1)–P(1)
N(1)–Mn(1)–P(2)
128.21(13)
102.08(9)
107.91(9)
N(3)–Mn(1)–P(1)
N(3)–Mn(1)–P(2)
P(1)–Mn(1)–P(2)
105.07(10)
107.33(10)
103.61(4)
increased p back-bonding into the p*-orbital of the N2 moiety as a
result of the low coordination number observed in 7 and the lack of
other p-accepting ligands such as CO. The Mn–N2 bond distance
˚
(1.860(3) A) is significantly shorter than that of the Mn(1)–N(1)
˚
˚
amide bond (2.066(3) A) but still longer than a typical Mn–N
40
imido bond (ave. 1.651 A). The increased activation of the N–N
bond seen in the X-ray structure of 7 is also reflected in the Raman
spectrum where the nN-N is observed at 1685 cm-1 compared to
5
1975 cm-1 previously reported for {h -C5H4CH3Mn(CO)2}2(m-N2)
(free N2: nN-N = 2329 cm-1). The solution magnetic susceptibility
was determined to be 8.7 mB, very close to the spin-only value of
8.9 mB for two high-spin Mn(I) centers.
◦
Reduction of the iron analogue 2 with KC8 in THF at -40 C
under an atmosphere of N2 resulted in an immediate color
change of yellow to dark red. Following removal of solvent under
vacuum, a yellow-green solid was obtained. When redissolved in
N2 degassed pentane, the dark red color returned. Cooling of
the red pentane solution at -40 ◦C resulted in the isolation of
([N2P2]Fe)2(m-N2) (8) as dark red crystals in 80% yield. The solid-
state structure of 8, determined via X-ray diffraction, is shown
as an ORTEP diagram in Fig. 5 with selected bond lengths
and angles provided in Table 5. As with 7, compound 8 is a
bridging dinitrogen complex possessing two tetrahedral metal
centers bridged by a reduced, end-on N2 moiety. The N–N bond
Fig. 3 SQUID data for 6.
Reduction chemistry of 1 and 2
Addition of sodium naphthalide to a suspension of 1 in THF
at -40 ◦C under an atmosphere of N2 resulted in an immediate
color change of colorless to dark red. Following the removal of
solvent under vacuum, extraction with pentane, concentration,
and cooling at -40 ◦C, dark red crystals of ([N2P2]Mn)2(m-N2)
(7) were isolated in 24% yield. The solid-state structure of 7
was determined by X-ray crystallography. An ORTEP diagram
is shown in Fig. 4 with selected bond lengths and angles provided
in Table 4. As in 1, each Mn center is four-coordinate in a
˚
distance in 8 is 1.166(3) A and is less reduced than the N2 in
7. However, the N–N distance is still significantly longer than
tetrahedral environment, and the [N2P2] ligand is bound k -NP2.29
3
In 7, however, the chloride ligand has been replaced by an end-
on bridging N2 moiety. Complex 7 represents only the second
crystallographically characterized bridging dinitrogen manganese
˚
compound. The N–N bond length in 7 is 1.208(6) A, significantly
5
longer than the distance reported for {h -C5H4CH3Mn(CO)2}2(m-
N2)44 (1.118(7) A). The elongation of the N–N bond is likely due to
˚
Fig. 5 Thermal ellipsoid (50%) plot of 8. Hydrogen atoms, iso-propyl
methyl, and tert-butyl methyl groups have been omitted for clarity.
◦
˚
Table 5 Selected bond lengths (A) and angles ( ) for ([N2P2]Fe)2(m-N2)
(8)
Fe(1)–N(3)
Fe(1)–P(1)
N(3)–N(3¢)
1.8510(15)
2.4465(6)
1.166(3)
Fe(1)–N(1)
Fe(1)–P(2)
1.9983(15)
2.3958(5)
N(3)–Fe(1)–N(1)
N(1)–Fe(1)–P(1)
N(1)–Fe(1)–P(2)
120.99(6)
101.34(5)
116.41(5)
N(3)–Fe(1)–P(1)
N(3)–Fe(1)–P(2)
P(1)–Fe(1)–P(2)
113.64(5)
99.97(5)
103.756(19)
Fig. 4 Thermal ellipsoid (50%) plot of 7. Hydrogen atoms, iso-propyl
methyl, and tert-butyl methyl groups have been omitted for clarity.
1718 | Dalton Trans., 2009, 1714–1720
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˚
Addition of Li[N2P2tolyl] to a suspension of FeCl2 in DME
resulted in an immediate color change of colorless to bright orange
(Scheme 3). Following the removal of solvent under vacuum,
extraction with THF, concentration, and cooling at -40 ◦C,
[N2P2tolyl]FeCl (10) was isolated as bright orange crystals in 70%
yield; the solution magnetic susceptibility of this material is
4.7 mB. Given the similarities between the solid-state structures
of [N2P2]CoI and [N2P2]FeCl, it is reasonable to infer that the
structure of 10 is similar to that of [N2P2tolyl]CoI, which is
tetrahedral and has similar metrical parameters to [N2P2]CoI.30
the value observed in free N2 (1.098 A) and is similar to
the related complexes {[PhB(CH2PiPr2)3]Fe}(m-N2)24 (1.138(6) A)
˚
and {([ArNC(tBu)]2CH)Fe}2(m-N2)21 (1.182(5) A), where Ar =
˚
2,6-diisopropylaniline. Similar to 7, the Fe–N2 bond distance
˚
(1.8510(15) A) is shorter than the Fe(1)–N(1) amide bond
distance of 1.9983(15) A and has some imido bond character
(ave. 1.653 A). The Raman spectrum of 8 reveals a nN-N
˚
40
˚
of 1760 cm-1, similar to the corresponding value reported for
{([ArNC(tBu)]2CH)Fe}2(m-N2)21 (1778 cm-1) and further supports
the existence of a higher N–N bond order in 8 as compared to
7. The solution magnetic susceptibility of 8 is 7.0 mB, slightly
higher than the spin-only value of 6.9 mB for six unpaired electrons
(two high-spin Fe(I) centers) but lower than the values seen in
the related compounds {[PhB(CH2PiPr2)3]Fe}(m-N2)25 (8.2 mB)
and{([ArNC(tBu)]2CH)Fe}2(m-N2)22 (8.4 mB).
Though the structural features of 7 and 8 are similar, the binding
strength of the N2 moiety appears to differ significantly, even
though only a small difference is seen in the N–N bond lengths.
When a dark red solution of 7 is exposed to vacuum, no color
change is observed. However, if the same experiment is performed
on a solution of 8, the solution changes color from red to lime
green. This green color remains constant if the solution is held
under vacuum or if an argon atmosphere is established. Once re-
exposed to N2, the solution returns to dark red, reforming 8 as
confirmed by its successful re-isolation.
The green product may also be obtained by direct synthesis.
Reduction of 2 with KC8 under an argon atmosphere results in a
color change of yellow to green. Following the removal of solvent
under vacuum, extraction with Et2O, concentration and cooling
at -40 ◦C, yellow-green crystals were obtained. Unfortunately,
crystals suitable for an X-ray diffraction study have been elusive.
The yield of this reaction has yet to be determined due to
reformation of 8 upon exposure to N2.
Our initial assignment of the identity of this green complex,
[N2P2]Fe (9), is based on a number of experimental observations.
Peters et al. recently reported an unstable Fe(I) complex that
performed an intramolecular C–H bond activation to form an
Fe(III) hydride species.45 Though no structural data was reported,
the IR spectrum of this complex revealed an Fe–H absorption at
2058 cm-1. The solution IR spectrum of 9 lacks any features in the
region typically associated with a M–H or an asymmetric N–N
stretch (1600–2700 cm-1), i.e. 9 does not appear to be an Fe(III)
hydride or a monomeric Fe(I)–N2 complex. It is also relevant to
note that the Co(I) analogue, [N2P2]Co, is known and does not
bind N2.30
Scheme 3
Reduction of 10 with KC8 in THF under an atmosphere
of N2 resulted in a color change of bright orange to dark
orange (Scheme 4). The resulting material, tentatively as-
signed [N2P2tolyl]Fe (11), shows good solubility in toluene and
Et2O. Though crystalline material has been obtained, the ideal
crystallization conditions for obtaining X-ray quality crystals
have yet to be determined. As gauged by both IR and Raman
spectroscopy we found no evidence of N2 coordination in isolated
samples of 11. Given the stability of 9 in the absence of N2, it is
not unreasonable to believe that 11 has a similar structure to 9
(monomeric and N2 free). The p-tolyl group could stabilize the
lone pair of electrons on the amide nitrogen, rendering the Fe
center less electron rich, and thus, perhaps, less amenable to N2
binding. In addition, the lack of donation by the free pair of
electrons on the amide nitrogen might encourage coordination of
the basal nitrogen, which could also discourage N2 coordination.
Both [N2P2]Co and [N2P2tolyl]Co have similar structural parameters
and are monomeric,30 and though 9 and 11 could be dimers, the
formulations of these substances in the solid-state cannot be un-
ambiguously established until X-ray quality crystals are obtained.
Scheme 4
Conclusions
Ligand modification
This study has shown that the [N2P2] motif is a competent
supporting ligand for organometallic derivatives of Mn and Fe.
In doing so, it adds further scope to the chemistry of this ligand
system, which has now been shown to support a wide range of
metal complexes from the main-group and both early and late
metals in the d-transition series. While no Fe-H species could be
observed, a dimeric bridging dihyride of Mn (6) was synthesized,
and strong coupling between the two Mn(II) centers was found
both in solution and the solid-state. The [N2P2] ligand also
stabilizes structurally similar Mn(I) (7) and Fe(I) (8) complexes
possessing bridging dinitrogen moieties.
We recently reported the synthesis of the new ligand [N2P2tolyl],
which was used to support Co(II) and Co(I) complexes.30 Evalua-
tion of the nCO and electrochemical data for Co complexes showed
that [N2P2] is a better donor than [N2P2tolyl], but the difference is
minor. One aspect of the [N2P2tolyl] ligand that was not probed
in the Co system was the difference in steric bulk on complex
reactivity. Since the p-tolyl group penetrates farther away from
the metal center than the tert-butyl group, we were curious about
what effect this might have on the formation of bridging dinitrogen
complexes.
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©