Synthesis and coordination chemistry of tri-substituted benzamidrazones

Article abstract of DOI:10.1039/c0dt01267j

A series of N¹,N¹,N³-tri-substituted benzamidrazones of the general formula [PhC(NHR)NNMe₂] (R = Me, n-Pr, i-Pr, n-Bu, Bn, Ph; 1a-f) was synthesized via condensation of 1,1-dimethylhydrazine with the correspondi

Full text of DOI:10.1039/c0dt01267j

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PAPER
Synthesis and coordination chemistry of tri-substituted benzamidrazones†
Mark R. Crimmin,a Denise A. Colby,a Jonathan A. Ellman*b and Robert G. Bergman*a
Received 20th September 2010, Accepted 19th October 2010
DOI: 10.1039/c0dt01267j
A series of N1,N1,N3-tri-substituted benzamidrazones of the general formula [PhC(NHR) NNMe2]
(R = Me, n-Pr, i-Pr, n-Bu, Bn, Ph; 1a–f) was synthesized via condensation of 1,1-dimethylhydrazine with
the corresponding imidoyl chloride, [PhC(Cl) NR]. Multinuclear NMR data, and zero-point energy
DFT calculations conducted with the B3LYP functional and 6–31G+(d,p) basis set, suggest that these
compounds exist as a single tautomer in solution; possessing a weak intramolecular hydrogen bond and
a structure dominated by the localised resonance structure ArC(NHR) N–NMe2. An X-ray
crystallographic study upon PhC(NHPh) NNMe2 (1f) demonstrated that this compound adopts an
identical tautomer in the solid state. Reactions of [PhC(NHMe) NNMe2] (1a) with [LMCl2]2 (M =
Ru, L = cymene; M = Rh, Ir, L = Cp*) results in the stoichiometric formation of products of the
formula [LM{PhC( NMe)NHNMe2}Cl]+Cl- (2a–c) in which the amidrazone chelates the metal in a
2
k -N1,N3-coordination mode. Formation of this five-membered chelate occurs with a concomitant
tautomerisation of the amidrazone ligand to an alternative tautomer, i.e. [PhC( NMe)NHNMe2], the
latter tautomer is expected to be readily energetically accessible based upon the aforementioned DFT
calculations. This series of salts may be deprotonated with lithium hexamethyldisilazide to form the
corresponding charge neutral complexes [LM{PhC(NMe) NNMe2}] (3a–c). In contrast, the reaction
of N1,N1,N3-tri-substituted benzamidrazones with [(cymene)RuCl2]2 in the presence of NaOAc yielded
a mixture of cyclometallation (C–H activation) and amidrazone chelation/deprotonation (N-H
activation) products. Reaction of 1a yielded an inseparable mixture of products, whilst the reaction of
1c resulted in formation of the cyclometallated product [LM{C6H5C( NiPr)NHNMe2}] (L =
cymene, M = Ru; 4a) in a modest 62% yield. This latter complex could be isolated as a crystalline
orange solid, full characterisation including single crystal X-ray diffraction demonstrated that the
2
amidrazone coordinates in a k -N2,C-coordination mode.
such as tetramethylethylene diamine (TMEDA) have a pedi-
gree in the stabilisation of reactive s-block complexes, more
recently these ligands have been applied to the stabilisation of
Introduction
Neutral and mono-anionic N,N-chelating ligands that form five-
bimetallic metallation reagents.7 Along with these ethylene and
ortho-phenylene bridged systems (I–III), a number of ligands
containing additional heteroatoms within the back-bone have
been reported, including tetrazacyclopentadienes,8 azoimines,9
pyridinine-phosphinimes and imidazole-phosphinimines.10
Perhaps the least well studied of existing N,N-chelating lig-
ands that have the potential to form 5-membered metallocycles
are amidrazones (IV). Amidrazones, sometimes referred to as
hydrazidines or hydrazide-hydrazones, are unsaturated open-
chain nitrogen containing compounds of the general formula
[R1C(NR2R3)(N-NR4R5)].11 Whilst the parent amidrazone (R1 =
R2 = R3 = R4 = R5 = H) has been shown to form upon metal tem-
plated condensation of hydrazine and hydrogen cyanide within the
coordination sphere of octahedral phosphite-substituted Group
VIII complexes12 and a number of keto-substituted amidrazones
have been shown to form complexes with gold and vanadium
metals,13 assignment of the coordination chemistry of IV in
membered metallacycles upon binding to metal centres have
found extensive applications in transition metal mediated pro-
cesses. Whilst potentially redox active, a-diimine based ligands
(I) have been employed in alkene polymerisation catalysis,1–4
mono-anionic ligands derived from chiral diamines (II) have
been used in the catalytic asymmetric hydrogenation of un-
saturated substrates.5–6 Although simple diamine ligands (III)
aDepartment of Chemistry, University of California, Berkeley, California,
94720, USA. E-mail: rbergman@berkeley.edu; Fax: +1 510-642-7714;
Tel: +1 510-642-1548
bDepartment of Chemistry, Yale University, New Haven, Connecticut, 06520,
USA. E-mail: jonathan.ellman@yale.edu; Tel: +1 203-432-3647
† Electronic supplementary information (ESI) available: Full experimental
details including multinuclear NMR data and cif files for compounds
1f, 2a, 3c and 4a. CCDC reference numbers 794635–794638. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt01267j
514 | Dalton Trans., 2011, 40, 514–522
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The Royal Society of Chemistry 2011
©

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existing metal complexes remains ambiguous. Of perhaps the
most relevance to the current work, Krajete and co-workers have
demonstrated the coordination of nickel(II) bromide to an N1,N1-
dimethyl-N3-aryl benzamidrazone.14
detailed characterisation data are provided in the supporting
information.20
Representative procedure for synthesis of
N1,N2,N3-benzamidrazones
Directing group mediated C–H activation of aryl rings of
organic substrates, either via oxidative addition of low-valent
transition metal fragments or base-mediated electrophilic addition
of organometallic complexes, may be considered a key reaction
in the controlled and selective functionalisation of carbon–
hydrogen bonds.15 Although numerous functional groups, includ-
ing ketones, imines, pyridines, diazocompounds, hydroxylimines
and nitriles, have been shown to direct this C–H activation
reaction, examples of directing groups containing more than two
heteroatoms with the potential to bind the organometallic reagent
through chelation remain scarce.16 Indeed, despite a number of
tridentate cyclometallated complexes having been prepared, the
vast majority of these contain symmetrical pincer type ligands
with the cyclometallated carbon as the central atom of the array.17
A number of cyclometallated complexes with oxygen, sulfur or
nitrogen atom containing side chains coordinated to the metal
centre have also been reported.18
On a Schlenk-line, a Schlenk-tube was flame-dried under vacuum
and N-methylbenzamide (6.25 g, 46.6 mmol) was added under a
purge of argon. Dry toluene (30 mL) was added via cannula, and
solid PCl5 (9.72 g, 46.6 mmol) was added to the resultant slurry.
The reaction mixture was warmed until homogeneous and stirred
for 2 h at room temperature, after which the solvent was removed
in vacuo. Additional dry toluene (2 ¥ 20 mL) was added and
removed under reduced pressure to distil any remaining POCl3.
N-Methylbenzimidoyl chloride was obtained as a slightly yellow
oil and used directly. A solution of the imidoyl chloride in dry
toluene (20 mL) was added to a mixture of triethylamine (7.1 g,
69.9 mmol) and 1,1-dimethylhydrazine (2.80 g, 46.6 mmol) in
toluene solution (20 mL) in a separate Schlenk-tube. Upon
addition, an exothermic reaction was observed, and noticeable
warming of the Schlenk tube occurred along with the precipitation
of a colourless solid from solution. The tube was sealed and
after 48 h at room temperature the reaction was quenched by
dilution with toluene (100 mL) and H2O (100 mL). The phases
were separated and the aqueous layer back-extracted with DCM
(100 mL), the combined organics were dried over magnesium
sulfate, and the solvent was removed under vacuum to give the
crude product as viscous yellow oil. Purification by short-path
distillation 100–105 ◦C at 0.25 mmHg gave pure 1a (5.13 g,
We now report the synthesis and solution conformation of
a series of N1,N2,N3-benzamidrazones along with coordination
studies with late-transition metal complexes [LMCl2]2. These
studies demonstrate that although the latter substrates may act
2
as neutral or mono-anionic k -N,N-chelating ligands, under
certain reaction conditions benzamidrazones may act as directing
2
groups for electrophilic C–H activation forming k -N,C-chelated
complexes.
1
29.0 mmol, 62%) as a yellow oil. H NMR (300 MHz, 298 K,
CD2Cl2) d 2.41 (s, 6H, -NMe2), 2.71 (d, 3H, J = 5.4 Hz, -
NHMe), 6.02 (broad s, 1H, -NHMe), 7.38–7.45 (m, 5H, ArH);
13C (75.5 MHz, 298 K, CD2Cl2) d 31.0, 46.9, 128.3, 128.6, 129.3,
134.3, 162.4; Infrared (thin film, cm-1) 3327, 3059, 2976, 2945,
2853, 1603, 1570; m/z (ESI, +ve) 178 (100%, [M + H]+), 163 (20%,
[M + H - CH3]+); HR-MS (ESI, +ve) calcd. for C10H16N3 178.1339
found 178.1337.
Experimental
General experimental
Unless otherwise indicated, operations were performed under
anhydrous conditions and inert atmosphere employing standard
Schlenk-line and glovebox techniques. Glassware was dried in
an oven at 160 ◦C overnight or flame-dried prior to use. NMR
spectra were acquired using Bruker AV-300, AVQ-400 and AVB-
400 spectrometers. Chemical shifts are reported as part per million
Compound 1b
Isolation by distillation gave 1b (5.40 g, 26.3 mmol, 86%) as
a viscous light yellow oil b.p. 140–143 ◦C at 0.25 mmHg.
1H NMR (400 MHz, 298 K, C6D6) d 0.63 (t, 3 H, J =
7.2 Hz, -NHCH2CH2CH3), 1.15 (tq, 2 H, J = 7.2, 6.8 Hz, -
NHCH2CH2CH3), 2.57 (s, 6H, -NMe2), 2.80 (dt, 2H, J = 6.9,
6.8 Hz, -NHCH2CH2CH3), 6.20 (broad s, IH, -NHCH2CH2CH3),
7.12–7.16 (m, 3H, ArH), 7.62–7.64 (m, 2H, ArH); 13C NMR
(100 MHz, 298 K, C6D6) d 11.5, 25.3, 46.3, 47.5, 128.6, 129.3,
129.6, 135.5, 162.5; Infrared (solid, cm-1) 3307, 2944, 2856, 1600,
1570; m/z (ESI, +ve) 206 (100%, [M + H]+), 161 (5%, [M - C3H8]+);
HR-MS (ESI, +ve) calcd. for C12H20N3 206.1652 found 206.1650.
1
(ppm, d) and H and 13C chemical shifts are referenced to the
corresponding residual protic solvent resonance. Mass spectral
data were obtained at the QB3 Mass Spectrometry Facility
operated by the College of Chemistry, University of California,
Berkeley. Fast atom bombardment mass spectra were recorded
on a Micromass ZAB2-EQ magnetic sector instrument. Infrared
spectral data were recorded on a Thermo Scientific Nicolet iS10
spectrometer fitted with a Smart OMNI-transmission or Smart
iTR device as either KBR discs, neat solids or thin films. Elemental
analyses were recorded by the UC Berkeley micro-mass facility.
Solvents were dried through a push-still system via passage
through alumina. [(cymene)RuCl2]2 was purchased from Sigma-
Aldrich and used without further purification. [Cp*MCl2]2 (M =
Rh, Ir) were synthesised by literature methods.19 NaOAc was
purchased from Sigma-Aldrich and used as received and lithium
hexamethyldisilazide (LiHMDS) was freshly prepared from n-
BuLi and hexamethyldisilazane and recrystallised before use.
Whilst compounds 1e and 1f are literature known, for completion,
Compound 1c
Isolation by distillation gave 1c (2.13 g, 10.4 mmol, 68%) as
a viscous light yellow oil b.p. 135–137 ◦C at 0.25 mmHg. 1H
NMR (400 MHz, 298 K, C6D6) d 0.83 (d, 6H, J = 6.4 Hz, -
NHCHMe2), 2.54 (s, 6H, -NMe2), 3.43 (d hept, 1H, J = 6.4, 5.1 Hz,
-NHCHMe2), 6.03 (d, 1H, J = 5.1 Hz, -NHCHMe2), 7.15–7.19
(m, 3H, ArH), 7.63–7.65 (m, 2H, ArH); 13C NMR (100 MHz,
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298 K, C6D6) d 24.6, 45.5, 47.5, 129.1, 129.7, 135.8, 162.3; Infrared
(solid, cm-1) 3289, 2944, 2855, 1597, 1569; m/z (ESI, +ve) 206
(100%, [M + H]+), 164 (80%, [M - C3H5]+); HR-MS (ESI, +ve)
calcd. for C12H20N3 206.1652 found 206.1647.
-NMe2), 7.35–7.37 (m, 2H, ArH), 7.45–7.51 (m, 3H, ArH), 11.75
(broad s, 1H, -NH); 13C NMR (100 MHz, 298 K, CD2Cl2) d
1
103
13
9.7, 41.0, 95.9 (d, J
= 8.2 Hz), 127.5, 129.1, 129.2, 131.5,
Rh–
C
164.9 (remaining 13C resonances could not be observed); Infrared
(solid, cm-1) 3376, 3111, 2984, 1626, 1600, 1578; m/z (ESI, +ve)
450 (100%, [M - Cl]+), 414 (25%, [M - H - 2Cl]+); HR-MS (ESI,
+ve) calcd. for C20H30N335Cl103Rh 450.1178 found 450.1170.
Compound 1d
Isolation by distillation gave 1d (4.25 g, 19.3 mmol, 69%) as
a viscous light yellow oil b.p. 158–160 ◦C at 0.25 mmHg. 1H
NMR (400 MHz, 298 K, C6D6) d 0.70 (t, 3H, J = 7.2 Hz, -
NH(CH2)3CH3), 1.02–1.18 (m, 4H, -NHCH2(CH2)2CH3), 2.58 (s,
6H, -NMe2), 2.85 (dt, 2H, J = 6.9, 6.8 Hz, -NHCH2(CH2)2CH3),
6.19 (broad s, 1H, -NH(CH2)3CH3), 7.15–7.20 (m, 3H, ArH),
7.64–7.67 (m, 2H, ArH); 13C NMR (100 MHz, 298 K, C6D6)
d 14.2, 20.3, 34.2, 44.4, 47.5, 128.6, 129.3, 129.6, 135.5, 162.5;
Infrared (solid, cm-1) 3310, 2944, 2855, 1600, 1570; m/z (ESI,
+ve) 220 (100%, [M + H]+), 205 (5%, [M + H - CH3]+), 177 (15%,
[M - C3H7]+); HR-MS (ESI, +ve) calcd. for C13H22N3 220.1808
found 220.1802.
Compound 2c
1
Isolated as a yellow solid (123 mg, 0.214 mmol, 86%). H NMR
(400 MHz, 298 K, CD2Cl2) d 1.65 (s, 15H, C5Me5), 3.21 (s, 3H,
-NMe), 3.54 (broad s, 3H, -NMe2), 3.73 (broad s, 3H, -NMe2),
7.39–7.41 (m, 2H, ArH), 7.47–7.50 (m, 3H, ArH), 12.10 (broad s,
1H, -NH); 13C NMR (100 MHz, 298 K, CD2Cl2) d 9.6, 41.8, 87.7,
126.7, 129.1, 129.2, 131.6, 166.8 (remaining 13C resonances could
not be observed) Infrared (solid, cm-1) 3368, 3109, 2916, 1624,
1600, 1540; m/z (ESI, +ve) 540 (80%, [M - Cl]+), 504 (100%,
[M - H - 2Cl]+); HR-MS (ESI, +ve) calcd. for C20H30N335Cl191Ir
538.1729 found 538.1742.
Synthesis of [LM{PhC( NMe)NHNMe2}Cl]+Cl- (2a–c)
Synthesis of [LM{PhC( NMe)NNMe2}Cl] (3a–c)
In a glovebox, 1a (57 mg, 0.32 mmol, 2 equiv.) and the corre-
sponding metal complex [LMCl2]2 (L = cymene, M = Ru; L =
Cp*, M = Rh, Ir; 0.16 mmol, 1 equiv.) were weighed separately,
each dissolved in 2.5 mL of dichloromethane and the solutions
combined. The resulting reaction mixture was transferred to a
20 mL vial and stirred for 48 h at room temperature. The solvent
volume was then concentrated to ca. 1 mL under reduced pressure,
this mixture was then added to 15–20 mL of pentane upon which
point the product precipitated from solution (the mixture may
then be triturated for a further 4 h if oiling out of solution occurs).
Isolation by filtration, followed by washing with pentane (2 ¥
5 mL) yielded 2a–c as yellow, orange or red solids which are
tolerant of both moisture and air.
In a glovebox, to a stirred suspension of 2a–c (0.20 mmol,
1 equiv.) in toluene (10 mL) was added dropwise a solution of
lithium hexamethyldisilazide (0.20 mmol, 1 equiv.) as a solution
in the same solvent (5 mL). The resulting reaction mixture quickly
changed appearance, becoming homogeneous and changing in
colour. After 2 h at room temperature the mixture was filtered,
the solid was washed with toluene (5 mL) and the resulting filtrate
concentrated to approximately 3 mL. The product was precipitated
from solution by addition of 15–20 mL of pentane. Isolation by
filtration, followed by washing with pentane (2 ¥ 5 mL) yielded
3a–c.
Compound 3a
Compound 2a
Isolated as a red solid (9.7 mg, 0.021 mmol, 53% based upon
0.040 mmol of 2a) 1H NMR (300 MHz, 298 K, CD2Cl2) d 1.34 (d,
6H, J = 6.9 Hz, ArCHMe2), 2.29 (s, 6H, ArMe), 2.99 (hept, 1H,
J = 6.9 Hz, ArCHMe2), 3.13 (s, 6H, -NMe2), 3.21 (s, 3H, -NMe),
5.00 (m, 2H, ArH), 5.32 (m, 2H, ArH), 7.16–7.19 (m, 2H, ArH),
7.27–7.29 (m, 2H, ArH); 13C NMR (75.5 MHz, 298 K, CD2Cl2) d
18.7, 22.7, 31.9, 45.2, 81.7, 85.9, 94.7, 104.5, 128.3, 128.4, 129.0,
134.4, 170.9 (remaining 13C resonances not observed); Infrared
(solid, cm-1) 2978, 2916, 1540, 1449, 1384; m/z (FAB, +ve) 448
(60%, [M]+), 412 (100%, [M - H - Cl]+); HR-MS (FAB, +ve)
calcd. for C20H29N335Cl104Ru 450.1148 found 450.1104.
Isolated as an orange solid (103 mg, 0.208 mmol, 66%). X-Ray
quality crystals were grown from slow diffusion of pentane into a
dichloromethane solution. 1H NMR (400 MHz, 298 K, CD2Cl2)
d 1.36 (d, 6H, J = 8.0 Hz, ArCHMe2), 2.35 (s, 6H, ArMe), 3.00
(hept, 1H, J = 8.0 Hz, ArCHMe2), 3.37 (s, 3H, -NMe2), 3.51 (s,
3H, -NMe2), 3.81 (s, 3H, -NMe), 5.32 (d, 1H, J = 6.0 Hz, ArH),
5.39 (d, 1H, J = 6.0 Hz, ArH), 5.51 (d, 1H, J = 6.0 Hz, ArH),
5.60 (d, 1H, J = 6.0 Hz, ArH), 7.27–7.29 (m, 2H, ArH), 7.44–7.48
(m, 3H, ArH), 11.40 (broad s, 1H, NH); 13C NMR (100 MHz,
298 K, CD2Cl2) d 18.8, 22.5, 22.7, 32.0, 45.1, 56.8, 62.3, 81.3, 82.9,
85.5, 87.2, 96.7, 107.0, 126.8, 129.0, 129.1, 131.6, 165.2; Infrared
(solid, cm-1) 3370, 3109, 2963, 2855, 1620, 1600, 1578; m/z (ESI,
+ve) 448 (70%, [M - Cl]+), 412 (10%, [M - H - 2Cl]+); HR-MS
(ESI, +ve) calcd. for C20H29N335Cl102Ru 448.1088 found 448.1103;
Elemental analysis calcd. for C20H29N3Cl2Ru: C 49.69, H 6.00, N
8.69 found: C, 48.85, H 5.95, N 8.37.
Compound 3b
Isolated as a deep-red solid (70.9 mg, 0.158 mmol, 77%). 1H NMR
(400 MHz, 298 K, CD2Cl2) d 1.65 (s, 15H, C5Me5), 2.92 (s, 3H,
-NMe), 2.99 (s, 6H, -NMe2), 7.22–7.30 (m, 5H, ArH); 13C NMR
1
103
13
(100 MHz, 298 K, CD2Cl2) d 9.6, 41.3, 57.0, 94.0 (d, J
C = 7.7
Rh–
Compound 2b
Hz), 128.2, 128.3, 129.1, 134.9, 171.3; Infrared (solid, cm-1) 2908,
1542, 1445, 1374; m/z (FAB, +ve) 450 (45%, [M + H]+), 414 (65%,
[M - H - Cl]+); HR-MS (FAB, +ve) calcd. for C20H30N335Cl103Rh
450.1178 found 450.1189.
1
Isolated as a brown-red solid (113 mg, 0.232 mmol, 72%). H
NMR (400 MHz, 298 K, CD2Cl2) d 1.68 (s, 15H, C5Me5), 3.13
(s, 3H, -NMe), 3.38 (broad s, 3H, -NMe2), 3.57 (broad s, 3H,
516 | Dalton Trans., 2011, 40, 514–522
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Table 1 Selected X-ray diffraction acquisition data
1f
2a
3c
4a
Molecular formula
Formula weight (g mol-1)
Crystal system
C15H17N3
239.32
C20H29Cl2N3Ru·CH2Cl2
568.34
Orthorhombic
C20H29ClN3Ir
539.11
Orthorhombic
C22H32ClN3Ru
475.03
Triclinic
Triclinic
¯
¯
Space group
P1
P212121
P212121
P1
˚
a (A)
8.8213(14)
9.2284(15)
9.3398(15)
80.219(2)
75.043(2)
62.591(2)
650.94(18)
2
10.3261(7)
13.8183(9)
17.4025(12)
90
9.7105(11)
12.4005(13)
16.6567(18)
90
9.437(5)
˚
b (A)
10.703(5)
11.940(5)
110.278(5)
90.291(5)
106.338(5)
1078.5(9)
2
0.862
1.463
1.83–25.37
0.0257, 0.0976
0.0265, 0.0997
23597/3909/0.0237
˚
c (A)
a (◦)
b (◦)
g (◦)
90
90
2483.1(3)
4
1.075
90
90
2005.7(4)
4
6.798
3
˚
V (A )
Z
m (mm-1)
0.074
1.221
2.49–25.34
0.0377, 0.1155
0.0548, 0.1280
2340/1837/0.0161
r (g cm-3)
1.514
1.785
H range (◦)
1.88–25.38
0.0515, 0.1053
0.0638, 0.110
4566/3933/0.0549
2.05–25.35
0.0301, 0.0599
0.0350, 0.0624
27219/3660/0.0655
R1, wR2 [I > 2s(I)]
R1, wR2 (all data)
Measured/independent reflections/Rint
Compound 3c
1569, 1462; LR-MS (FAB, +ve) 475 (60%, [M]+), 440 (100%, [M -
Cl]+); HR-MS (FAB, +ve) calcd. For C22H32N335Cl102Ru 475.1323
found 475.1338.
Isolated as an orange solid (67.2 mg, 0.125 mmol, 64%). X-Ray
quality crystals were grown from slow diffusion of pentane into a
toluene solution at -35 ◦C. 1H NMR (400 MHz, 298 K, CD2Cl2)
d 1.66 (s, 15H, C5Me5), 3.14 (s, 3H, -NMe), 3.31 (s, 6H, -NMe2),
7.18–7.32 (m, 5H, ArH); 13C NMR (100 MHz, 298 K, CD2Cl2)
d 11.3, 43.0, 60.2, 87.4, 130.0, 130.1, 131.0, 135.9, 176.4; Infrared
(solid, cm-1) 2967, 2911, 1544, 1445, 1388; m/z (FAB, +ve) 540
(40%, [M]+), 504 (100%, [M - H - Cl]+); HR-MS (FAB, +ve)
calcd. for C20H30N335Cl193Ir 540.1743 found 540.1758.
X-Ray crystallographic data
All single crystal X-ray diffraction experiments were conducted
at UC Berkeley CheXray facility using a SMART APEX diffrac-
tometer equipped with a fine-focus sealed tube, Mo K/a source
and Bruker APEX-I CCD detector. Structure solution, followed
by full-matrix least squares refinement was performed using the
WinGX-1.70 suite of programs. A multi-scan absorption correc-
tion was applied, whilst structures were solved with SHELXS-97
and refined in SHELXL-97.21
Synthesis of [(cym)Ru{PhC(NHiPr) NMe2}Cl] (4a)
In a glovebox, a solution of 1c (26.0 mg, 0.127 mmol) in CD2Cl2
(2 mL) was added to [(cym)RuCl2]2 (50 mg, 0.063 mmol). In
a separate 20 mL vial, sodium acetate (25.9 mg, 0.316 mmol,
5 equiv.) was weighed out and the reaction mixture added.
The reaction mixture was stirred for 4 days and the reaction
monitored by removing aliquots and analysing by 1H NMR
spectroscopy. The product 4a (0.078 mmol, 62%) was observed
and the yield recorded against tetrakis(trimethylsilyl)silane as
an internal standard. Although unstable on the benchtop for
extended periods, 4a could be isolated in pure form, albeit
with significant decomposition occurring during isolation, by
chromatography upon silica gel using a 10 cm column with 2.5 cm
diameter employing a 10 : 1 mixture of hexanes : ethylacetate. X-
Ray quality crystals were grown by recrystallisation from hexanes.
1H NMR (400 MHz, 298 K, CDCl3) d 0.59 (d, 3H, J = 6.8 Hz,
ArCHMe2), 1.06 (d, 3H, J = 6.8 Hz, ArCHMe2), 1.13 (d, 3H, J =
6.4 Hz, -NHCHMe2), 1.29 (d, 3H, J = 6.4 Hz, -NHCHMe2), 2.18
(s, 3H, ArMe), 2.38 (hept, 1H, J = 6.8 Hz, ArCHMe2), 2.57 (s, 3H,
-NMe2), 2.74 (s, 3H, -NMe2), 4.23 (d hept, 1H, J = 6.8 and 6.4 Hz),
4.80 (d, 1H, J = 6.0 Hz, ArH), 4.94 (d, 1H, J = 5.6 Hz, ArH), 5.65
(d, 1H, J = 6.0 Hz, ArH), 6.16 (d, 1H, J = 5.6 Hz, ArH), 6.92–694
(m, 2H, ArH + -NH), 7.13 (dd, 1H, J = 7.5 and 7.1 Hz), 7.50 (d, 1H,
J = 7.7 Hz, ArH), 8.16 (d, 1H, J = 7.5 Hz); 13C NMR (100 MHz,
298 K, CDCl3) d 18.6, 19.9, 24.4, 24.7, 25.0, 30.5, 44.8, 45.4,
47.4, 73.4, 82.0, 90.3, 90.5, 104.8, 104.9, 121.4, 126.4, 128.8, 137.2,
139.5, 171.4, 183.1; Infrared (solid, cm-1) 3247, 2958, 2866, 1585,
DFT calculations
Calculations were implemented in Gaussian03 using the restricted
B3LYP functional and 6,31G+(d,p) basis set.22 All minima were
confirmed by frequency calculations and, for 1f, metrical outputs
were compared against the solid-state data.
Results and discussion
Synthesis and conformation of tri-substituted amidrazones
Whilst the structurally related, but diverse, series of compounds
referred to by the generic term amidrazone have numerous meth-
ods of preparation and applications in synthesis,11 reports of the
synthesis and study of N1,N1-dimethyl-N3-alkylbenzamidrazones
and N1,N1-dimethyl-N3-arylbenzamidrazones remain limited to a
handful of examples.
Thus, in 1971 Smith and co-workers reported that the re-
action of N-phenylbenzimidoyl chloride with an excess of
1,1-dimethylhydrazine yielded the corresponding amidrazone.20a
In 1972, Walter and Weiss provided an analysis of N1,N3-
disubstituted and N1,N1,N3-trisubstituted amidrazones by in-
frared spectroscopy and concluded that these compounds exist in
solution as a single tautomer containing an intramolecular hydro-
gen bond (Fig. 2, tautomer A).23 In contrast, Smith and co-workers
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Table 2 Relative sum of thermal and zero-point energies (kcal mol-1)
for the possible tautomers of 1a, 1c and 1f calculated using the B3LYP
functional and 6,31G+(d,p) basis set
Confomer
A
B
C
D
R = Me
R = iPr
R = Ph
0
0
0
+4.0
+3.5
+5.2
+6.9
+1.8
+5.1
+7.4
+2.8
+5.8
Fig. 1 Representative N,N-chelating ligands (the numbering scheme in
IV is used throughout).
◦
˚
Table 3 Selected bond angles ( ) and bond lengths (A) within 1f, 2a, 3c
and 4a
1f
2a
3c
4a
N1–M
—
—
—
—
2.180(5)
—
2.081(5)
—
1.449(7)
1.337(7)
1.295(7)
119.1(6)
115.9(5)
75.80(19)
2.162(6)
—
2.065(6)
—
1.474(8)
1.302(9)
1.339(9)
124.8(6)
110.5(6)
75.8(2)
—
2.106(2)
—
2.039(3)
1.446(3)
1.315(4)
1.357(4)
122.4(2)
112.7(2)
—
N2–M
Fig. 2 Four possible tautomers of N1,N2,N3-benzamidrazones 1a–f.
N3–M
Cipso–M
argued that, although N1,N1-dimethyl-N3-alkylbenzamidrazones
exist in this conformation, based upon resonance arguments,
tautomerisation to an alternative hydrazide imide form may occur
for N1,N1-dimethyl-N3-phenylbenzamidrazones (Fig. 2, tautomer
C or D).20a,b
N1–N2
1.4496(15)
1.2942(16)
1.3690(17)
123.23(13)
112.49(11)
—
C–N2
C–N3
N3–C–N2
N1–N2–C
N1–M–N3
Following literature precedent,20 the reaction of 1,1-
dimethylhydrazine with N-alkyl or N-aryl benzimidoyl chlorides
in the presence of two equivalents of triethylamine proceeded
to yield the corresponding amidrazones (Scheme 1). The latter
compounds could be purified by vacuum distillation or flash
column chromatography upon silica gel using a mixture of
dichloromethane, methanol and ammonium hydroxide as the
eluent (for experimental details see ESI†).
Scheme 1 Synthesis of N1,N2,N3-benzamidrazones.
In line with the expectations23 furnished by Walter and Weiss,
1H NMR experiments upon compounds 1a–f, in either CDCl3
3
or d8-toluene solution, demonstrated significant JH–H coupling
constants (5.1–6.9 Hz) between the N–H proton and the a-methyl,
methylene, or a-methine protons of the group at the N3-position.
In the case of the N1,N1,N3-trimethylbenzamidrazone 1a the N3-
Fig. 3 ORTEP representation of 1f, thermal ellipsoids at 50% probability.
Whilst the N1–N2, N3–C and N2–C bond lengths of 1.4496(15),
methyl resonance, apparent as a sharp doublet at 2.71 ppm (3JH–H
=
1.3690(17) and 1.2942(16) A, respectively, are consistent with the
˚
6.2 Hz), collapsed to a broadened singlet upon addition of a single
drop of D2O. These data are suggestive of the predominance of
tautomer A in solution.
Due to a lack of a-protons, the structural assignment of
compound 1f is more convoluted (vide supra). Whilst the literature
precedent provides two contrasting views of the bonding in this
molecule, we now provide a more comprehensive account of the
ground-state tautomer of these compounds.
Our understanding of the solution structures of 1a–f was further
enhanced by a single crystal X-ray diffraction experiment upon
1f in combination with gas phase DFT calculations. Readily
crystallised from a concentrated hexanes solution, compound 1f
exists as tautomer A in the solid state. Important bond lengths and
angles are listed in Table 3 and the structure is represented in Fig. 3.
localised resonance structure of tautomer A, the presence of a
weak intramolecular hydrogen bond is suggested by the short N3–
1
˚
H distance of 2.259 A and the degree of pyramidalisation at N
(DP = 39%).24
As the solid state data are not necessarily representative of the
solution structure, a series of DFT studies were conducted using
the B3LYP functional employing the 6,31G+(d,p) basis set. The
results of this study are presented in Table 2 and the competency
of the model was confirmed by comparison of the calculated bond
angles and bond lengths of tautomer A of compound 1f to that
acquired from the X-ray crystallography study (see ESI, Fig. S1†).
In all instances, tautomers B–D proved energetically un-
favourable in comparison to tautomer A. Despite some relatively
i
small energy differences (R = Pr) it is noteworthy that for the
518 | Dalton Trans., 2011, 40, 514–522
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series of compounds characterised (1a–f) both solution and solid
state data support this finding. Whilst it has been previously
suggested that for R = Ph the tautomeric species C or D may
become energetically favourable due to resonance considerations,
calculations put these tautomers some 5–6 kcal mol-1 higher in
energy than A. Although gas-phase computational data match
experimental (R = Ph, see ESI, Fig. S1†), it is noteworthy that,
despite being reasonably well modelled, the calculated N1–H
˚
intramolecular hydrogen bond is some 0.1 A longer than recorded
experimentally.
Reactions of 1a with [(cymene)RuCl]2 and [Cp*MCl2]2 (M = Rh, Ir)
Fig. 4 ORTEP representation of 2a. Thermal ellipsoids at 50% probabil-
ity. H-atoms with the exception of the N–H proton and dichloromethane
solvent molecules omitted for clarity.
N1,N1,N3-Trimethylbenzamidrazone 1a reacts stoichiometrically
with [(cymene)RuCl2]2, [Cp*RhCl2]2 or [Cp*IrCl2]2 in CDCl3
2
solution to yield the corresponding k -N1,N3-chelate complexes
in which a chloride ion has been displaced from the metal
coordination sphere (Scheme 2). Salts 2a–c were readily isolated by
precipitation from chloroform solutions by addition of a mixture
of diethyl ether and hexanes and were isolated in 67–86% yield
as orange, red or yellow solids (see experimental section for
details). Whilst reactions in CDCl3 solution were most notably
characterised by the formation of a low-field broad resonance
between 10 and 12 ppm, assigned to the N–H of the chelated
amidrazone, upon binding, the quarternary 13C NMR resonance
of the amidrazone shifts downfield (2a, 165.2 ppm; 2b, 164.9 ppm;
2c, 166.8 ppm) relative to that of the free ligand (1a, 162.3 ppm).
Thus, multinuclear NMR data are suggestive of a change in the
tautomer of the ligand within complexes 2a–c.
of the amidrazone ligand with the H(2a)–Cl(2) bond length
˚
being 2.540 A. The metal–nitrogen bond lengths (2.081(5)–
˚
˚
2.180(5) A) and metal–chlorine bond length (2.4132(17) A) of
2a are consistent with literature known 6-coordinate ruthenium
amidinate complexes containing similar coordination geometry
at the metal.25 Most importantly, the tautomerisation of the
amidrazone to C is apparent by N2–C and N3–C bond lengths
˚
of 1.337(7) and 1.295(7) A, respectively.
Despite complexes 2a–c displaying limited solubility in hexane
or toluene solutions, they could be deprotonated in the presence
of one equivalent of lithium bis(trimethylsilyl)amide (LiHMDS)
in toluene solution to give the neutral analogues 3a–c (Scheme 3).
These latter compounds demonstrated improved solubility in
hydrocarbon solvents and, in all cases, the progress of the reaction
was evidenced by the disappearance of n(N–H) in the infrared
spectrum and the disappearance of the heavily deshielded N–H
resonance in the 1H NMR spectrum.
Scheme 2 Reaction of 1a with [LMCl2]2.
In all instances, the cationic fragments [LM{PhC( NMe)-
NHNMe2}Cl]+ (L = cym, M = Ru; L = Cp*, M = Rh, Ir) could
be observed by electrospray ionisation mass spectrometry with
further fragmentation occurring with loss of chloride from the
parent ion. Although infrared spectroscopy demonstrated the
expected N–H stretch (3350–3380 cm-1) implied by the NMR
data, ultimately the coordination mode of the amidrazone 1a was
confirmed by a single crystal X-ray diffraction study upon 2a.
Represented in Fig. 4 with important bond angles and bond
lengths listed in Table 3, compound 2a crystallises with a single
molecule of dichloromethane in the unit cell. Most importantly
Scheme 3 Deprotonation of 2a–c with [Li{N(SiMe3)2}] (LiHMDS).
2
these data demonstrate that 1a provides a k -N1,N3-chelate to
Scheme 4 Cyclometallation of 1c with [(cym)RuCl2]2 mediated by
NaOAc.
ruthenium and that coordination occurs with a concomitant
tautomerisation of the ligand to the hydrazide imide resonance
form, i.e. tautomer C, a structure that is expected to be some 6–
7 kcal mol-1 less stable than the ground-state A based upon DFT
calculations.
For 2a the coordination sphere of the metal is completed by
coordination to the p-system of cymene and a single chloride
ion. The additional chloride counter-ion exists in the secondary
coordination sphere and forms a close contact with the proton
Compared to the parent amidrazone (1a, 162.4 ppm) and
cationic metal complexes 2a–c (vide supra), 13C NMR reso-
nances of the quaternary carbon centre of the ligand shift
downfield significantly upon deprotonation (3a, 170.9 ppm; 3b,
171.3 ppm; 3c, 176.4 ppm). The former data compare well to
those reported for ruthenium amidinate complexes of the formula
6
2
[(h -C6H5R)Ru{h -tBuNC(Ph) NtBu}X] (R = H, Me, OMe, F;
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X = Cl, PF6). These complexes contain a similar monoanionic
ligand coordinated to a Group VIII metal, albeit via a four-
membered N,N-chelate, and demonstrate characteristic reso-
nances in the 13C NMR between 170 and 175 ppm.25
metal and intramolecular deprotonation by the acetate group.28 In
related studies, Jones and co-workers have expanded the scope
of this electrophilic C–H cyclometallation reaction to include
electron-rich and electron-poor imines.29
Once again the coordination mode of the chelated amidrazone
was provided by an X-ray diffraction experiment. Thus, single
crystals of 3c grown from slow diffusion of pentane into a toluene
solution proved suitable for a single crystal diffraction experiment,
and selected bond angles and bond lengths are listed in Table 3
whilst the structure is represented in Fig. 5. Although structural
features of the iridium(III) centre warrant little discussion, it is
important to note that upon deprotonation the amidrazone ligand
readily tautomerises back to a monoanionic version of tautomer
A, as evidenced by the N1–N2, N2–C and C–N3 bond lengths of
Of particular relevance to the current study, it was found that
the reaction of [Cp*IrCl2]2, NaOAc and pyrrole imine substrates
containing acidic N–H protons yielded a mixture of N–H and C–
H activation products.27b The N–H activation product 3a derived
2
from k -N1,N3-chelation observed during the reaction 1a with
[(cymene)RuCl2]2 and NaOAC could be avoided by employing an
amidrazone substrate possessing a bulky group at the N3-terminus.
Thus, reaction of 1c under the same reaction conditions gave the
cyclometallated product 4a in 62% yield as evidence by NMR
spectroscopy. The selectivity in this reaction is consistent with the
idea that the bulky iso-propyl group disfavours coordination of N3
to the hindered metal centre and favours instead coordination to
N2 followed by electrophilic cyclometallation of the ortho-position
of the phenyl ring.
˚
1.474(8), 1.302(9) and 1.339(9) A with the metal effectively taking
the place of the proton in the structure represented in Fig. 2. It
is noteworthy that, despite the N2–C bond length being approxi-
mately equal to that recorded for the free ligand 1f, not only is the
N1–N2 distance significantly longer but the N3–C bond length is
significantly shorter than observed in 1f (see Table 3). These data
are consistent with effective charge localisation across the N3–C–
N2 moiety of the chelated, deprotonated amidrazone ligand.
Multinuclear NMR spectra of isolated samples of 4a provided
diagnostic data. Thus, whilst the four proton spin system of the
metallated aromatic apparent from the in situ NMR data of
reactions of 1c was readily observable in the aromatic region of
the 1H NMR spectrum in CD2Cl2 6.92–694 (m, 2H), 7.13 (dd, 1H,
J = 7.5 and 7.1 Hz), 7.50 (d, 1H, J = 7.7 Hz), 8.16 (d, 1H, J =
7.5 Hz), resonances consistent with the intact amidrazone ligand
were also apparent. Most notably the methine position of the iso-
propyl group could be observed as a doublet of heptets at 4.23 ppm
(3JH–H = 6.8 and 6.4 Hz) with coupling occurring to the acidic N–
H of the amidrazone moiety. These data are consistent with those
observed for the parent organic fragment which demonstrates a
similar resonance at 3.43 ppm (3JH–H = 6.4 and 5.1 Hz). The ipso-
carbon of the cyclometallated ligand also provided a characteristic
resonance in the 13C NMR at 183.1 ppm, which being shifted
some 30 ppm downfield of that in the free ligand, compares well
with values reported for cyclometallated complexes containing the
same “[(cymene)RuCl]” fragment. For instance [(p-cymene)RuCl-
{(C6H4)C(H) N(CH2)2OCH3}], a complex which contains a
Fig. 5 ORTEP representation of 3c. Thermal ellipsoids at 50% probabil-
ity. H-atoms omitted for clarity.
2
cyclometallated imine ligand exhibiting a k -N,C-coordination
Reactions of 1a and 1c with in situ generated [(cymene)Ru(OAc)Cl]
mode, demonstrates a 13C NMR chemical shift at 188.5 ppm
assigned to the carbon directly attached to the metal centre.27a
Whilst infrared spectroscopic data are consistent with the
proposed formulation with n(N–H) observed as a sharp peak
at 3247 cm-1 (cf. 1c, 3289 cm-1), fast-atom bombardment mass
spectrometry provided additional support for the cyclometallated
product and M+ was observed at 475 and 477 m/z with fragmen-
tation occurring with loss of chloride to form the corresponding
cation at 440 m/z.
In contrast, the reaction of amidrazone 1a with [(cymene)RuCl2]2
in the presence of sodium acetate provided a mixture of products.
Despite the complex expected from coordination and deproto-
nation being present, i.e. 3a, among the mixture a new ruthe-
nium containing species was readily apparent from 1H NMR
spectroscopic data. The latter complex was characterised by a
diagnostic four proton spin system (characterised by 1D and
2D-experiments), characteristic of an ortho-metallated aromatic
ring and, based upon comparison to similarly cyclometallated
ruthenium complexes of benzylamines reported by Pfeffer and
co-workers, was assigned as the product derived from cyclometal-
lation of the amidrazone ligand.26
Davies, MacGregor and co-workers have recently published a
series of papers demonstrating and rationalising electrophilic C–
H activation of imines, 2-phenylpyridines and oxazolines by a
[LMCl2]2 (L = cymene, M = Ru; L = Cp*, M = Rh, Ir) and sodium
acetate reaction mixture.27–28 Through both experimental and
computational studies it has been hypothesised that the reaction
proceeds by in situ formation of the metal acetate [LM(OAc)Cl],
with subsequent simultaneous activation of the C–H bond by the
Despite strong evidence for the proposed formulation, the
coordination geometry of compound 4a was ultimately elucidated
by a single crystal X-ray diffraction experiment. The results of this
experiment are represented in Fig. 6 and important bond angles
and bond lengths are listed in Table 3. Whilst the N1–N2 and
3
˚
C–N bond lengths of 1.446(3) and 1.357(4) A, respectively, are
consistent with those in the free ligand 1f, the C–N2 bond length of
˚
1.315(4) A is significantly longer due to the augmented coordina-
tion number of the N2-position. These data are consistent with the
isolation of a cyclometallation product in which the amidrazone
exists as tautomer A. In addition, the N2–Ru and Caryl–Ru bond
˚
lengths of 2.106(2) and 2.039(3) A, respectively, are similar to
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Fig. 6 ORTEP representation of 4a. Thermal ellipsoids at 50% probabil-
ity. H-atoms, with the exception of the N–H proton, omitted for clarity.
˚
those of 2.080(2) and 2.043(2) A, respectively, reported by Davies
and co-workers for the aforementioned cyclometallated complex
2
[(p-cymene)RuCl{(C6H4)C(H) N(CH2)2OCH3-k C,N}].27a
Summary and conclusions
The synthesis and solution conformation of a series of N1,N2N3-
benzamidrazones along with coordination studies with late-
transition metal complexes [LMCl2]2 has been investigated. These
studies demonstrate that although the latter substrates may
2
act as neutral or mono-anionic k -N,N-chelating ligands, un-
der suitable reaction conditions benzamidrazones may act as
2
directing groups for electrophilic C–H activation forming k -N,C-
coordinated complexes. We are continuing to study the applica-
tions of amidrazones as directing groups for C–H activation and
functionalisation.
Acknowledgements
M.R.C. acknowledges the Royal Commission for the Exhibition
of the 1851 for the generous provision of a research fellow-
ship. M.R.C. is also indebted to Antonio diPasquale, Casey
Brown and the graduate students associated with UC Berkeley
X-ray crystallography course 2009 for assistance with single
crystal diffraction experiments. R.G.B. acknowledges support
from DOE, Office of Basic Energy Sciences, Chemical Sciences
Division, U.S. Department of Energy, under Contract DE-AC03-
76SF00098, and J.A.E. acknowledges support from the NIH
(GM069559).
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