Dimolybdenum complexes containing bridging acetate ligands and anions of N,N′-di(2-methoxyphenyl)formamidine

Article abstract of DOI:10.1016/S0020-1693(02)00850-2

The syntheses, structures and NMR spectra of quadruply bonded complexes containing three, two or one bridging acetate ligands and anions of N,N′-di(2-methoxyphenyl)formamidine (o-HDMophF), of the types Mo₂(O₂CCH₃)₃

Full text of DOI:10.1016/S0020-1693(02)00850-2

Inorganica Chimica Acta 336 (2002) 71ꢁ79
/
www.elsevier.com/locate/ica
Dimolybdenum complexes containing bridging acetate ligands and
anions of N,N?-di(2-methoxyphenyl)formamidine
Ying-Yann Wu a, Jhy-Der Chen a,*, Lin-Shu Liou b, Ju-Chun Wang b
a Department of Chemistry, Chung-Yuan Christian University, Chung-Li, Taiwan, ROC
b Department of Chemistry, Soochow University, Taipei, Taiwan, ROC
Received 20 December 2001; accepted 13 February 2002
Abstract
The syntheses, structures and NMR spectra of quadruply bonded complexes containing three, two or one bridging acetate ligands
and anions of N,N?-di(2-methoxyphenyl)formamidine (o-HDMophF), of the types Mo2(O2CCH3)3(o-DMophF) (1), trans-
Mo2(O2CR)2(o-DMophF)2 (Rꢀ
CH3, 2; CF3, 3; Prn, 4) and Mo2(O2CCH3)(o-DMophF)Cl2(PMe3)2 (5), are discussed. Complex 1
/
was prepared by reaction of Mo2(O2CCH3)4 with excess o-HDMophF in THF. The reactions of Mo2(O2CCH3)4 and Mo2(O2CPrn)4
with excess lithiated formamidine afforded complexes 2 and 4, respectively. Complex 3 was prepared by reaction of Mo2(O2CCF3)4
with o-HDMophF in THF and complex 5 was prepared by reaction of 1 with (CH3)3SiCl and PMe3 in THF/CH2Cl2. Their UVꢁ
and NMR spectra have been recorded and their structures have been determined by X-ray crystallography. The o-DMophFꢂ
ligands show two different conformations, s-trans,s-trans and s-cis,s-trans, in compounds 1ꢁ
5. The study on 31P{1H} NMR
1 Hz. The position
/Vis
/
spectrum of 5 concluded that the through metalꢁ
/
metal quadruple bonding coupling ½3JPꢀMoꢀMoꢀP½ is about 169
/
of the monodentate phosphine ligand, i.e. cis or trans to the acetate or similar ligands, can be determined by evaluating the ½3JCꢀP
coupling constant. # 2002 Elsevier Science B.V. All rights reserved.
½
Keywords: Dimolybdenum complexes; Bridging acetate; Formamidine; 31P NMR
1. Introduction
Mo2(O2CR)4 to form Mo2(form)4, has been reported.
The only dimolybdenum analogues reported are the
acetamidinate compound trans-Mo2(O2CCH3)2(DXy-
lA)2(THF)2 [7a], and the benzamidinate compounds
The coordination chemistry of formamidinate com-
pounds has been investigated extensively during recent
years [1ꢁ5]. Many efforts have been concentrated in
/
trans-Mo2(O2CCH3)2[m-N,N?-PhC(NSiMe3)2)]2
[7b]
their ability to form bridges between metal atoms.
Preparations, structures and spectroscopic properties
of tetrakis(m-diarylformamidinato)dimolybdenum com-
plexes of the type Mo2(form)4, where form is the generic
formamidine, were the subjects of several studies [6].
Crystalline Mo2(form)4 can be prepared by stoichio-
metric ligand metathesis between the dimolybdenum
tetraacetate Mo2(O2CR)4 and the lithiated formamidine.
and trans-Mo2(O2CPh)2[m-N,N?-PhC(NSiMe3)2)]2 [7c].
Some square and triangular arrays of Mo2 units that
4ꢃ
are bridged by formamidinate and dicarboxylate ligands
have also been reported [7d]. We report here the first
mixed formamidinateꢁ
lear types Mo2(O2CCH3)3(o-DMophF), trans-Mo2(O2-
CR)2(o-DMophF)2 (Rꢀ
CH3, CF3 and Prn) and
/acetate complexes of the dinuc-
/
Mo2(O2CCH3)(o-DMophF)Cl2(PMe3)2, which contain
three, two or one bridging acetate ligands and
anions of N,N?-di(2-methoxyphenyl)formamidine (o-
HDMophF). The syntheses, structures and NMR
spectra of these complexes form the subject of this
report.
To our knowledge, no mixed formamidinateꢁacetate
/
dimolybdenum species, which can be envisaged as the
intermediates of the stepwise decarboxylation of
* Corresponding author. Fax: ꢃ886-3-456 310
/
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 0 8 5 0 - 2

72
Y.-Y. Wu et al. / Inorganica Chimica Acta 336 (2002) 71ꢁ79
/
2. Experimental
by ether and then dried under reduced pressure to give
the yellow product. Yield: 1.31 g (68%). UVꢁVis: 459
1617 Mꢂ1 cmꢂ1). H NMR (CDCl3,
/
1
2.1. General procedures
nm (CH2Cl2, oꢀ
/
ppm): 8.99 (s, 2H, CH), 7.26 (d, 4H, Hortho), 6.88 (t, 4H,
Hmeta), 6.92 (t, 4H, Hpara), 6.69 (d, 4H, Hmeta), 3.15 (s,
12H, OCH3), 2.53 (s, 6H, CH3). 13C NMR (CDCl3,
ppm): 179.50, 156.72, 150.94, 139.50, 123.42, 121.39,
120.35, 112.13, 55.71, 23.87. Calc. for C34H36Mo2N4O8
(MW 820.56): C, 49.77; H, 4.42; N, 6.83. Found: C,
49.37; H, 4.37; N, 6.45%. IR (KBr disk): 1560(m),
1493(s), 1420(m), 1323(m), 1247(m), 1206(s), 1178(s),
1052(s), 1060(m), 745(m), 669(m), 625(s), 437(m).
All manipulations were carried out under dry, oxy-
gen-free nitrogen by using Schlenk techniques, unless
otherwise noted. Solvents were dried and deoxygenated
by refluxing over the appropriate reagents before use.
Hexanes, THF and ether were purified by distillation
from sodiumꢁbenzophenone, acetonitrile from CaH2
/
and dichloromethane from P2O5. The visible absorption
spectra were recorded on a Hitachi U-2000 spectro-
photometer. NMR spectra were measured on a Bruker
Avance 300 MHz spectrometer. 85% H3PO4 and CFCl3
were used as standards for 31P{1H} and 19F{1H} NMR,
respectively. IR spectra were obtained from a JASCO
FT/IR-460 plus spectrometer. Elemental analyses were
obtained from a PE 2400 series II CHNS/O analyzer.
2.5. Preparation of trans-Mo2(O2CCF3)2(o-DMophF)2
Mo2(O2CCF3)4 (1.0 g, 1.55 mmol) and o-HDMophF
(1.2 g, 4.66 mmol) was placed in a flask containing 10 ml
CH2Cl2. The mixture was then stirred at room tempera-
ture (r.t.) for 20 h to yield a yellow solution and a yellow
solid. The solid was filtered, washed by ether and then
dried under reduced pressure to give the yellow product.
2.2. Starting material
The complexes Mo2(O2CCH3)4 [8], Mo2(O2CCF3)4
[9], Mo2(O2CPrn)4 [10] and the ligand N,N?-di(2-meth-
oxyphenyl)formamidine (o-HDMophF) were prepared
according to previously reported procedures [11].
Yield: 1.23 g (85%). UVꢁ
/
Vis: 445 nm (CH2Cl2, oꢀ855
/
Mꢂ1 cmꢂ1). 1H NMR (CDCl3, ppm): 9.18 (s, 2H, CH),
7.33 (d, 4H, Hortho), 6.95 (t, 4H, Hmeta), 6.99 (t, 4H,
Hpara), 6.70 (d, 4H, Hmeta), 3.09 (s, 12H, OCH3). 13C
2
NMR (CDCl3, ppm): 160.94 (q, JCꢀF
158.10, 150.77, 138.50, 127.80, 121.50, 119.97, 113.37
ꢀ39.10 Hz),
/
2.3. Preparation of Mo2(O2CCH3)3(o-DMophF)
1
(q, JCꢀF
ꢀ
/
283.65 Hz), 111.72, 55.38. 19F{1H} NMR
2
Mo2(O2CCH3)4 (1.0 g, 2.34 mmol) and o-HDMophF
(1.8 g, 7.02 mmol) was placed in a flask containing 15 ml
THF. The mixture was then refluxed for 2 days to yield
a brown solution and a yellow solid. The solid was
filtered, washed by ether and then dissolved in 10 ml
CH2Cl2 to give a yellow solution. The solvent was
removed under vacuum to leave the yellow product.
(CDCl3, ppm): ꢂ
/
72.30 (1JCꢀF
ꢀ
/
283.25 Hz, JCꢀF
ꢀ
/
38.84 Hz). Calc. for C34H30F6Mo2N4O8 (MW 928.50):
C, 43.98; H, 3.26; N, 6.03. Found: C, 43.39; H, 3.47; N,
6.34%. IR (KBr disk): 1609(m), 1534(m), 1502(m),
1465(s), 1350(s), 1330(m), 1237(s), 1212(s), 1196(m),
1172(s), 1144(m), 1120(m), 1044(s), 1059(s), 1012(m),
936(s), 755(s), 745(m), 728(m), 492(s), 444(s).
Yield: 0.70 g (48%). UVꢁ
/
Vis: 448 nm (CH2Cl2, oꢀ1966
/
Mꢂ1 cmꢂ1). 1H NMR (CDCl3, ppm): 9.58 (s, 1H, CH),
7.55 (d, 2H, Hortho), 6.97 (t, 2H, Hmeta), 6.99 (t, 2H,
Hpara), 6.68 (d, 2H, Hmeta), 3.26 (s, 6H, OCH3), 2.77 (s,
3H, CH3trans), 2.58 (s, 6H, CH3cis). 13C NMR (CDCl3,
ppm): 181.34, 152.74, 149.10, 138.11, 123.57, 121.90,
115.95, 112.02, 56.05, 23.60. Calc. for C21H24Mo2N2O8
(MW 624.30): C, 40.40; H, 3.87; N, 4.49. Found: C,
40.44; H, 3.57; N, 4.66%. IR (KBr disk): 1658(m),
1585(m), 1537(s), 1425(s), 1336(s), 1242(s), 1171(m),
1119(s), 1016(s), 748(m), 673(m).
2.6. Preparation of trans-Mo2(O2CPrn)2(o-DMophF)2
o-HDMophF (0.71 g, 2.77 mmol) was placed in a
flask containing 10 ml THF. n-BuLi (1.7 ml, 1.6 M per
hexane) was then added. The mixture was stirred at
0 8C for 30 min to yield an orange solution.
Mo2(O2CPrn)4 (0.5 g, 0.93 mmol) was then added and
the mixture refluxed for another 20 h. The solid was
filtered, washed by ether and then dried under reduced
pressure to give the yellow product. Yield: 0.71 g (87%).
1
UVꢁ
/
Vis: 460 nm (CH2Cl2, oꢀ
/
2415 Mꢂ1 cmꢂ1). H
2.4. Preparation of trans-Mo2(O2CCH3)2(o-
DMophF)2
NMR (CD2Cl2, ppm): 8.99 (s, 2H, CH), 7.25 (d, 4H,
Hortho), 6.92 (t, 4H, Hmeta), 6.96 (t, 4H, Hpara), 6.73 (d,
4H, Hmeta), 3.16 (s, 12H, OCH3), 2.79 (t, 4H, CH2), 1.78
(q, 4H, CH2), 0.95 (t, 6H, CH3). 13C NMR (CD2Cl2,
ppm): 182.80, 156.80, 151.05, 139.56, 123.75, 121.62,
120.48, 112.21, 55.78, 39.30, 20.68, 13.75. Calc. for
C38H44Mo2N4O8 (MW 876.56): C, 52.06; H, 5.06; N,
6.39. Found: C, 51.96; H, 4.76; N, 6.29%. IR (KBr disk):
2964(m), 1581(s), 1558(m), 1536(s), 1492(s), 1446(s),
o-HDMophF (1.8 g, 7.02 mmol) was placed in a flask
containing 10 ml THF. n-BuLi (4.4 ml, 1.6 M per
hexane) was then added. The mixture was stirred at
0 8C for 30 min to yield an orange solution. Mo2(OAc)4
(1.0 g, 2.34 mmol) was then added and the mixture
refluxed for another 20 h. The solid was filtered, washed

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73
Table 1
Crystal data for compounds 1ꢁ
/
5
Compound
1
2×2CH2Cl2
3
4
5
Formula
C
624.30
21H24Mo2N2O8
C36H40Cl4Mo2N4O8
990.40
C34H30F6Mo2N4O8
928.50
triclinic
C38H44Mo2N4O8
876.65
triclinic
C23H36Cl2Mo2N2O4P2
729.26
triclinic
Formula weight
Crystal system
Space group
monoclinic
P21/c
monoclinic
P21
¯
¯
¯
P/
1
/
P/
1
/
P/ /
1
˚
a (A)
9.816(1)
20.784(1)
11.781()
90
12.910(2)
8.506(2)
19.699(4)
90
9.069(8)
9.712(9)
12.165(11)
72.345(8)
76.084(7)
66.923(7)
930(2)
8.546(1)
9.785(1)
12.266(1)
77.678(6)
80.516(5)
67.038(6)
918.91(13)
1
9.836(1)
10.941(1)
16.069(2)
102.186(7)
105.421(6)
103.016(8)
1555.0(3)
2
˚
b (A
˚
c (A)
a (8)
b (8)
97.453(1)
90
105.383(2)
90
g (8)
3
V (A )
˚
2383.2(2)
4
1.740
2085.8(6)
2
1.577
Z
1
1.658
464
Dcalc (g cmꢂ3
F(000)
)
1.584
448
1.558
736
1248
1000
Crystal size (mm)
m (Mo Ka) (mmꢂ1
Data collection
instrument
0.4ꢄ0.6ꢄ0.6
1.100
Siemens CCD
0.35ꢄ0.4ꢄ0.5
0.910
Siemens CCD
0.12ꢄ0.6ꢄ0.85
0.759
Siemens CCD
0.1ꢄ0.1ꢄ0.2
0.740
Bruker AXS
0.1ꢄ0.1ꢄ0.4
1.110
Bruker AXS
)
Radiation monochro- 0.71073
mated in incident beam
0.71073
0.71073
0.71073
0.71073
˚
(l (Mo Ka) (A))
Range (2u) for data
4.0052u 550.04
4.2852u 549.92
4.6852u 549.80
4.5852u 550.00
4.0052u 550.00
collection (8)
Temperature (8C)
Limiting indices
25
25
25
25
25
ꢂ115h 511,
ꢂ245k 517,
ꢂ135l 514
11 776
ꢂ155h 515,
ꢂ105k 54,
ꢂ135l 523
6477
ꢂ55h 510,
ꢂ115k 511,
ꢂ105l 514
3914
ꢂ15h 510,
ꢂ105k 511,
ꢂ145l 514
3915
ꢂ15h 511,
ꢂ125k 512,
ꢂ195l 518
6456
Reflections collected
Independent
reflections
4180 [Rintꢀ0.0319]
4604 [Rintꢀ0.0296]
2835 [Rintꢀ0.0421]
3214, [Rintꢀ0.0657] 5420, [Rintꢀ0.0278]
Refinement method
Full-matrix least-
squares on F2
4180/0/394
Full-matrix least-
squares on F2
4604/1/464
Full-matrix least-
squares on F2
2835/0/304
Full-matrix least-
squares on F2
3214/0/323
Full-matrix
least-squares on F2
5420/0/430
Data/restraints/
parameters
Quality-of-fit indica-
1.153
1.065
1.160
1.049
1.031
c
tor
Final R indices
R1ꢀ0.0265,
wR2ꢀ0.0682
R1ꢀ0.0279,
wR2ꢀ0.0694
R1ꢀ0.0489,
wR2ꢀ0.1225
R1ꢀ0.0515,
wR2ꢀ0.1265
1.052 and ꢂ0.931
R1ꢀ0.0521,
wR2ꢀ0.1181
R1ꢀ0.0609,
wR2ꢀ0.1298
1.295 and ꢂ1.118
R1ꢀ0.0341,
wR2ꢀ0.0701
R1ꢀ0.0538,
wR2ꢀ0.0759
0.456 and ꢂ1.079
R1ꢀ0.0414,
wR2ꢀ0.0862
R1ꢀ0.0677,
wR2ꢀ0.0983
1.022 and ꢂ0.596
a,b
[I ꢀ2s(I)]
R indices (all data)
Largest difference peak 0.274 and ꢂ0.712
ꢂ3
˚
and hole (e A
)
a
R1ꢀaj½Fo½ꢂ½Fc½½/a½Fo½.
b
2
wR2ꢀ[aw(Fo2ꢂFc2)2/aw(Fo2)2]1/2, wꢀ1/[s2(Fo2)ꢃ(ap)2ꢃ(bp)], Pꢀ[max(Fo or 0)ꢃ2(Fc2)]/3, aꢀ0.0342, bꢀ1.3641 (1); aꢀ0. 0588,
bꢀ6.6239 (2); aꢀ0.0333, bꢀ5.3917 (3); aꢀ0.0295, bꢀ0.4070 (4); aꢀ0.0369, bꢀ2.0448 (5).
c
2
2
Quality-of-fitꢀ[aw(½Fo ½ꢂ½Fc ½)2/NobservedꢂNparameters)]1/2
.
1410(m), 1242(s), 1209(m), 1174(m), 1116(m), 1047(w),
1012(m), 744(m), 642(m), 526(w), 443(w).
added and the mixture refluxed for another 16 h. The
solvent was removed under vacuum to leave a purple red
solid. The solid was washed by ether and then dried
under reduced pressure. Yield: 0.12 g (32%). UVꢁ
531 nm (CH2Cl2, oꢀ
1334 Mꢂ1 cmꢂ1). 1H NMR
(CDCl3, ppm): 9.52 (d, 1H, CH, JHꢀPꢀ4.09 Hz),
/
Vis:
2.7. Preparations of Mo2(O2CCH3)(o-
DMophF)Cl2(PMe3)2
/
4
/
7.67 (d, 1H, Hortho), 7.38 (d, 1H, Hortho), 7.01 (t, 1H,
Hmeta), 6.99 (t, 1H, Hmeta), 6.94 (t, 1H, Hpara), 6.97 (t,
1H, Hpara), 6.61 (d, 1H, Hmeta), 6.73 (d, 1H, Hmeta), 3.23
Mo2(O2CCH3)2(o-DMophF)2 (0.41 g, 0.50 mmol)
was placed in a flask containing 5 ml THF and 5 ml
CH2Cl2, followed by addition of Me3SiCl (0.38 ml, 3.0
mmol). The mixture was stirred at r.t. for 1 h to yield a
brown solution. PMe3 (1 ml, 1 M per THF) was then
(s, 3H, OCH3), 3.40 (s, 3H, OCH3), 1.57 (d, 9H, PMe3,
2JHꢀP
2
ꢀ
/
8.38 Hz), 1.21 (d, 9H, PMe3, JHꢀP
ꢀ8.50 Hz).
/

74
Y.-Y. Wu et al. / Inorganica Chimica Acta 336 (2002) 71ꢁ
/79
3
13C NMR (CDCl3, ppm): 182.38 (d, JCꢀP
ꢀ
/
3.5 Hz),
O-demethylation reaction has been found in the reac-
tion between Ru2(OAc)4Cl and o-DMophF, which was
attributed to the electron deficiency at the Ru2 center
3
156.30 (d, JCꢀP
ꢀ2.2 Hz), 150.68, 148.25, 139.34(d,
/
3JCꢀP
120.38, 115.48, 112.37, 111.24, 55.88, 54.15, 23.69, 12.42
(d, 1JCꢀP 13.4 Hz), 12.11 (d, 1JCꢀP 11.6 Hz). 31P{1H}
NMR (CDCl3, ppm): ꢂ 15.2 Hz), ꢂ8.37
3.58 (d, 3JPꢀP
15.2 Hz). Calc. for C23H36Cl2Mo2N2O4P2
ꢀ
/
2.1 Hz), 137.36, 124.79, 123.84, 121.61, 121.31,
(formal oxidation stateꢀ
/
5ꢃ) [14].
/
ꢀ
/
ꢀ
/
Yellow crystals of 1 conform to the space group P21/c
with four molecules in a unit cell. Fig. 1 shows the
ORTEP diagram for 1. Selected bond distances and
angles are listed in Table 2. The molecular structure of
/
ꢀ
/
/
3
(d, JPꢀP
ꢀ
/
(MW 729.26): C, 37.88; H, 4.97; N, 3.84. Found: C,
37.66; H, 4.84; N, 3.89%. IR (KBr disk): 2964(m),
2906(m), 2835(w), 1685(s), 1535(m), 1500(m), 1460(w),
1427(w), 1261(s), 1115(s), 1020(s), 953(m), 802(s),
750(m), 675(w), 488(w).
1 consists of two molybdenum atoms [Moꢀ
/
Moꢀ2.0933
/
˚
(3) A] which are spanned by three bridging acetate
ligand and one o-DMophFꢂ ligand. The o-DMophFꢂ
ligand coordinates to the molybdenum centers through
the two central nitrogen atoms and is arranged in a s-
trans,s-trans conformation, following the conforma-
tional descriptor for di(m-aloxylphenyl)formamidine
proposed by Ren and co-authors [15]. Such conforma-
tion results in two short Moꢀ ꢀ ꢀO distances. Complex 1 is
of significance, because, it is the first dimolybdenum
paddlewheel species which contains three acetate ligands
and one formamidinate ligand, although dimolybdenum
complex containing three acetate and other ligands, such
3. X-ray crystallography
The diffraction data of 1ꢁ5 were collected on a
/
Siemens CCD or Bruker AXS diffractometer, which
was equipped with a graphite-monochromated Mo Ka
˚
0.71073 A) radiation. Data reduction was carried
(laꢀ
/
by standard methods with use of well-established
computational procedures [12]. A colorless crystal of 1
was mounted on the top of a glass fiber with epoxy
cement. The hemisphere data collection method was
as
Mo2(O2CCH3)3(BAII) (BAIIꢀ
line) [17] are known.
Yellow crystals of 2×
[Mo2(O2CCH3)3(S2CPEt3)]BF4
[16],
bis(arylimino)isoindo-
and
/
used to scan the data points at 4.00B
/
2u B50.048. The
/
/2CH2Cl2 conform to the space
structure factors were obtained after Lorentz and
polarization corrections. The positions of some of the
heavier atoms were located by the direct method. The
remaining atoms were found in a series of alternating
difference Fourier maps and least-square refinements.
The final residuals of the refinement were R1ꢀ
and wR2ꢀ0.0682. The X-ray crystallographic proce-
dures for 2ꢁ5 were similar to those for 1. Basic
group P21 with two molecules in a unit cell. Yellow
crystals of both complexes 3 and 4 conform to the space
¯
group P
/
1 with one molecule in a unit cell. Figs. 2ꢁ
/
4
show the ORTEP diagram for 2ꢁ
bond distances and angles for 2ꢁ
The molecular structures of 2ꢁ
molybdenum atoms [MoꢀMoꢀ2.108 (1) for 2, 2.133
/
4, respectively. Selected
4 are listed in Table 2.
4 consists of two
/
/
0.0265
/
/
/
/
/
˚
(2) for 3 and 2.1088 (6) A for 4] which are spanned by
two trans bridging acetate ligands and two trans
bridging o-DMophFꢂ ligands. This trans configuration
information pertaining to crystal parameters and struc-
ture refinement is summarized in Table 1.
4. Results and discussion
4.1. Synthesis and structures
The yellow complex Mo2(O2CCH3)3(o-DMophF) (1)
was prepared by reaction of Mo2(O2CCH3)4 with excess
o-HDMophF in THF. The reactions of lithiated for-
mamidine with Mo2(O2CCH3)4, Mo2(O2CCF3)4 and
Mo2(O2CPrn)4 afforded the complexes trans-Mo2(O2C-
CH3)2(o-DMophF)2
(2),
trans-Mo2(O2CCF3)2(o-
DMophF)2 (3), and trans-Mo2(O2CPrn)2(o-DMophF)2
(4), respectively. The reaction of 2 with (CH3)3SiCl and
PMe3 in THF gave the complex Mo2(O2CCH3)(o-
DMophF)Cl2(PMe3)2 (5). Their structures have been
determined by spectroscopic methods and by X-ray
crystallography. Although the tetrakis(m-N,N?-di(2-
methoxyphenyl)formamidine) dichromium complex
[13] has been reported, attempts to prepare the dimo-
lybdenum analogue were not successful. It is noted that
Fig. 1. ORTEP drawing for complex 1. Thermal ellipsoids shown at
50% probability level.

Y.-Y. Wu et al. / Inorganica Chimica Acta 336 (2002) 71ꢁ
/79
75
Table 2
Selected bond distances (A) and angles (8) for 1ꢁ
˚
/
4
1
2
3
4
Bond distances
Mo(1)ꢀMo(2)
Mo(1)ꢀO(5)
Mo(1)ꢀO(7)
Mo(1)ꢀO(3)
Mo(1)ꢀN(1)
Mo(2)ꢀO(8)
Mo(2)ꢀO(6)
Mo(2)ꢀO(4)
Mo(2)ꢀN(2)
Mo(1)ꢀ ꢀ ꢀO(1)
Mo(2)ꢀ ꢀ ꢀO(2)
2.0933(3)
2.109(2)
2.135(2)
2.139(2)
2.144(2)
2.114(2)
2.125(2)
2.129(2)
2.136(2)
2.681(2)
2.667(2)
Mo(1)ꢀMo(2)
Mo(1)ꢀN(3)
Mo(1)ꢀO(7)
Mo(1)ꢀO(5)
Mo(1)ꢀN(1)
Mo(2)ꢀO(8)
Mo(2)ꢀO(6)
Mo(2)ꢀN(2)
Mo(2)ꢀN(4)
Mo(1)ꢀ ꢀ ꢀO(1)
Mo(2)ꢀ ꢀ ꢀO(4)
2.108(1) MoꢀMo(A)
2.116(7) MoꢀN(2A)
2.127(7) MoꢀO(4A)
2.141(8) MoꢀO(3)
2.165(7) MoꢀN(1)
2.112(7) Mo(A)ꢀO(4)
2.144(7) Mo(A)ꢀN(2)
2.148(7) Mo—O(1)
2.201(7)
2.133(2) MoꢀMo(A)
2.134(5) MoꢀN(2A)
2.138(4) MoꢀO(4A)
2.154(5) MoꢀO(3)
2.171(5) MoꢀN(1)
2.138(4) Mo(A)ꢀO(4)
2.134(5) Mo(A)ꢀN(2)
2.576(5) Mo—O(1)
2.1088(6)
2.130(3)
2.123(2)
2.124(2)
2.177(3)
2.123(2)
2.130(3)
2.602(2)
2.600(7)
2.615(6)
Bond angles
O(5)ꢀMo(1)ꢀN(1)
O(7)ꢀMo(1)ꢀN(1)
O(3)ꢀMo(1)ꢀN(1)
O(4)ꢀMo(2)ꢀN(2)
O(8)ꢀMo(2)ꢀN(2)
O(6)ꢀMo(2)ꢀN(2)
92.79(8)
95.85(8)
175.96(7)
174.60(8)
92.16(8)
96.34(8)
N(3)ꢀMo(1)ꢀO(7)
N(3)ꢀMo(1)ꢀO(5)
O(7)ꢀMo(1)ꢀN(1)
N(3)ꢀMo(1)ꢀN(1)
O(5)ꢀMo(1)ꢀN(1)
O(8)ꢀMo(2)ꢀN(2)
O(6)ꢀMo(2)ꢀN(4)
O(6)ꢀMo(2)ꢀN(2)
O(8)ꢀMo(2)ꢀN(4)
N(2)ꢀMo(2)ꢀN(4)
89.1(3) N(2A)ꢀMoꢀO(4A)
90.6(3) N(2A)ꢀMoꢀO(3)
90.1(3) O(4A)ꢀMoꢀN(1)
175.1(3) O(4A)ꢀMoꢀO(3)
89.9(3) N(2A)ꢀMoꢀN(1)
90.1(3) O(3)ꢀMoꢀN(1)
89.8(3)
89.1(2) O(4A)ꢀMoꢀO(3)
91.0(2) O(4A)ꢀMoꢀN(2A)
90.5(2) O(3)ꢀMoꢀN(1)
176.9(2) O(3)ꢀMoꢀN(2A)
175.3(2) O(4A)ꢀMoꢀN(1)
89.2(2) N(2A)ꢀMoꢀN(1)
176.9(1)
90.1(1)
90.5(1)
89.5(1)
89.6(1)
175.0(1)
90.5(3)
89.3(3)
174.9(3)
Fig. 2. ORTEP drawing for complex 2. Thermal ellipsoids shown at
50% probability level.
Fig. 3. ORTEP drawing for complex 3. Thermal ellipsoids shown at
50% probability level.
is in marked contrast to that found in the dichromium
analogue Cr2(O2CCH3)2(o-DMophF)2 [18], in which the
two chromium atoms are spanned by two cis bridging
acetate ligands and two cis bridging o-DMophFꢂ
with one of the molybdenum atoms. Complexes 2ꢁ4 are
/
of significance, because they are the first mixed ligands
complexes which contain a pair of bridging acetate and
a pair of bridging formamidinate ligands, although
acetamidinate compound trans-Mo2(O2CCH3)2(DXy-
lA)2(THF)2 7a, and the benzamidinate compounds
trans-Mo2(O2CCH3)2[m-N,N?-PhC(NSiMe3)2)]2 7b and
ligands. The o-DMophFꢂ ligands of complexes 2ꢁ
4
/
are arranged in the s-cis,s-trans conformation [15]. The
OCH3 group, which is in an orientation opposite to that
of the methine hydrogen, forms a short Moꢀ ꢀ ꢀO distance

76
Y.-Y. Wu et al. / Inorganica Chimica Acta 336 (2002) 71ꢁ
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Table 3
˚
Selected bond distances (A) and angles (8) for 5
Bond distances
Mo(1)ꢀMo(2)
Mo(1)ꢀO(3)
Mo(1)ꢀP(1)
Mo(2)ꢀN(2)
Mo(2)ꢀP(2)
2.1239(6)
2.163(3)
2.507(2)
2.125(5)
2.537(2)
Mo(1)ꢀN(1)
Mo(1)ꢀCl(1)
Mo(2)ꢀO(4)
Mo(2)ꢀCl(2)
Mo(1)ꢀ ꢀ ꢀO(1)
2.131(4)
2.449(1)
2.099(3)
2.442(1)
2.562(4)
Bond angles
N(1)ꢀMo(1)ꢀO(3)
N(1)ꢀMo(1)ꢀCl(1)
N(1)ꢀMo(1)ꢀP(1)
O(4)ꢀMo(2)ꢀN(2)
O(4)ꢀMo(2)ꢀP(2)
N(2)ꢀMo(2)ꢀP(2)
92.7(2)
O(3)ꢀMo(1)ꢀP(1)
169.03(10)
89.80(10)
85.53(5)
153.84(12) O(3)ꢀMo(1)ꢀCl(1)
87.24(11) Cl(1)ꢀMo(1)ꢀP(1)
91.6(2)
O(4)ꢀMo(2)ꢀCl(2)
151.34(11)
90.34(13)
85.98(5)
85.46(11) N(2)ꢀMo(2)ꢀCl(2)
166.45(12) Cl(2)ꢀMo(2)ꢀP(2)
phosphine ligand and one chloride atom are trans to the
acetate ligand and the others trans to the o-DMophFꢂ
ligand. The Moꢀ
/
P and Moꢀ
/
Cl bonds, 2.507 (2) and
˚
2.442 (1) A, respectively, that are trans to the acetate
ligand have shorter bond distances than the correspond-
Fig. 4. ORTEP drawing for complex 4. Thermal ellipsoids shown at
50% probability level.
˚
ing bonds, 2.537 (2) and 2.449 (1) A, respectively, that
are trans to the o-DMophFꢂ ligand, indicating that the
acetate ligand has smaller trans influence than the o-
trans-Mo2(O2CPh)2[m-N,N?-PhC(NSiMe3)2)]2 [7c] have
been reported.
Purple red crystals of 5 conform to the space group P
DMophFꢂ ligand. The Moꢀ
/
O distances of complex 5,
˚
2.163 (3) and 2.099 (3) A, which are trans to the
trimethylphosphine ligand and chloride atom, respec-
tively, are typical for complexes which are bridged by a
/
¯
1 with one molecule in a unit cell. Fig. 5 shows the
ORTEP diagram for 5. Selected bond distances and
angles for are listed in Table 3. The molecular structure
single acetate ligand, i.e. Mo2(OAc)[(PhN)2CCH3]3 [19]
˚
2.162 (5) A, Mo2(OAc)(ambt)3(ambtꢀanion of 2-ami-
/
consists of an asymmetric dimolybdenum unit [Moꢀ
/
˚
no-4-methylbenzothizole) [20], 2.16 (1) A, Mo2(O2-
˚
Moꢀ2.1239 (6) A] which are spanned by one bridging
/
acetate ligand and one o-DMophFꢂ ligand. A pair of
chloride atoms and a pair of trimethylphosphine ligands
occupy the remaining coordination sites. As a result, one
˚
CCH3)Cl3(PMe3)3 [21], 2.10 (4) A, (C3N2H5)[Mo2-
˚
A
(OAc)[CH3Ga(C3N2H3)O]4] [22], 2.102(8)
and
P(CH2-
Mo2(OAc)Cl3(3h - tetraphos - 2)(tetraphos - 2ꢀ
CH2PPh2)3) [23], 2.10(1) and 2.13(1) A. The two
/
˚
different MoꢀO distances in complex 5 indicate that
/
trimethylphosphine ligand has a stronger trans influence
than the chloride atom. In this complex, the o-
DMophFꢂ ligand is arranged in the s-cis,s-trans
conformation and the OCH3 group, which is in an
orientation opposite to that of the methine hydrogen
forms a short Moꢀ ꢀ ꢀO distance with one of the
molybdenum atoms.
4.2. Structural comparisons
A comparison of distances and angles observed for 1ꢁ
/
5 is listed in Table 4. The MoꢀMo distances are typical
/
for complexes containing a quadruple bond. In com-
plexes, 1, 2 and 5, which contain the same acetate ligand,
the one in which more acetate ligands are present the
Moꢀ
/
Mo distance is shorter. It seems that the acetate
ligand has slight shortening effect over the o-DMophFꢂ
ligand. A comparison of complexes 2, 3 and 4 shows
that the one with the electronegative atom on the acetate
Fig. 5. ORTEP drawing for complex 5. Thermal ellipsoids shown at
50% probability level.
ligand has the longer Moꢀ/Mo distance. The length of

Y.-Y. Wu et al. / Inorganica Chimica Acta 336 (2002) 71ꢁ
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77
Table 4
Structural comparisons for complex 1ꢁ5
/
Compound
1
2
3
4
5
˚
MoꢀMo (A)
2.0933(3)
2.681(2)
2.667(2)
152.97(5)
154.33(5)
2.108(1)
2.600(7)
2.615(6)
159.2(2)
159.6(1)
2.133(2)
2.576(5)
2.1088(6)
2.602(2)
2.1239(6)
2.562(4)
˚
Moꢀ ꢀ ꢀO (A)
ꢁMoꢀMoꢀ ꢀ ꢀO (8)
159.5(1)
158.75(6)
154.0(1)
the alkyl chain has no influence on the distance of Moꢀ
Mo bond. The distance of the Moꢀ ꢀ ꢀO axial interaction
is dependent on the strength of the MoꢀMo bond. The
stronger is the MoꢀMo bond, the longer is the Moꢀ ꢀ ꢀO
distance. It is noted that although complex 1 has two
Moꢀ ꢀ ꢀO axial interactions, its MoꢀMo bond distance is
similar to that of Mo2(O2CCH3)4, which is 2.0934 (8) A.
This is not unusual since it is well known that MoꢀMo
quadruple bond is less sensitive to the axial interaction
than the CrꢀCr quadruple bond [24].
/
15.2 Hz. To further verify this coupling constant, we
have measured the 31P{1H} NMR spectrum of
Mo2(OAc)Cl3(PMe3)3 [21], shown as I, in which two
monodentate phosphine ligands are positioned cis to the
acetate ligand and one monodentate phosphine ligand
trans to the acetate ligand. The 31P{1H} NMR spectrum
of this complex consists of one doublet and one triplet
/
/
/
˚
/
centered at ꢂ
/
10.20 and ꢂ1.50 ppm, respectively, with a
/
coupling constant of 16.3 Hz, which can be assigned to
½3JPꢀMoꢀMoꢀP½. Thus, we could probably conclude that,
assuming the Karplus-type angular dependence of the
coupling constants to be minimal, the ½3JPꢀMoꢀMoꢀP½ is
/
4.3. NMR spectroscopy
169
/
1 Hz. The 4 Hz difference as compared with 2091
/
Hz derived from Mo2(OAc)Cl3(3h-tetraphos-2) and
Mo2(OAc)Cl3(3h-etp) is presumably due to the chelating
and bridging bondings formed by the polydentate
phosphine ligands to the metals.
1
The variable-temperature H NMR spectra of com-
plexes 1ꢁ4 show only a sharp singlet for the protons of
/
OCH3 groups, indicating that the solid state conforma-
tions of the o-HDMophF ligands in these complexes are
not retained in CDCl3 solution from room temperature
to ꢂ60 8C. Similar situation was seen in the complexes
/
Cr2(o-DMophF)4 [13] and Cr2(O2CCH3)2(o-DMophF)2
[16]. However, due to the asymmetry of the molecules of
complex 5, two singlets for the two methoxy hydrogen
atoms were observed, which appear at d 3.23 and 3.40,
respectively. The 31P{1H} NMR spectrum of complex 5
I
In addition to the ½3JPꢀMoꢀMoꢀP½ coupling constant,
the ½3JCꢀOꢀMoꢀP½ and ½3JCꢀNꢀMoꢀP½ coupling constants
derived in the 13C{1H} NMR spectrum of complex 5
provide an opportunity to investigate the relative
position of the monodentate phosphine ligand, i.e. cis
or trans to the bridging acetate or formamidinate
ligand. Fig. 6 shows the 13C{1H} NMR spectrum of
also consists of two doublets centered at ꢂ
/
3.58 and ꢂ
/
8.37 ppm, respectively. The most striking information of
this spectrum is the coupling constant of the 2 inequiv.
phosphorus atoms, which is 15.2 Hz.
In a previous paper [23], we reported the structures
and 31P{1H} NMR spectra of two dimolybdenum
complexes of the types Mo2(OAc)Cl3(3h-tetraphos-2)
(tetraphos-2ꢀ
Cl3(3h-etp) (etpꢀ
important aspect about the observed Pꢀ
/
P(CH2CH2PPh2)3)
and
Mo2(OAc)-
/
Ph2PCH2CH2(Ph)CH2CH2PPh2). An
/
P coupling
constants in these two complexes is the relative weight-
ing of the contributions via the metal and via the ligand
‘backbone’. Assuming the Karplus-type angular depen-
dence of the coupling constants to be minimal, and the
coupling constant through backbone in the complex is
similar to that in the free ligand, we concluded that the
through metalꢀ
/
metal quadruple bonding coupling
1 Hz. In complex 5, no
½3JPꢀMoꢀMoꢀP½ is about 209
/
influence from the backbone has to be considered and,
assuming the Karplus-type angular dependence of the
coupling constants to be minimal, the j JPꢀMoꢀMoꢀPj is
3
Fig. 6. 13C{1H} NMR spectrum for complex 5.

78
Y.-Y. Wu et al. / Inorganica Chimica Acta 336 (2002) 71ꢁ79
/
5. Concluding remarks
Table 5
Selected dihedral angle 8 (8) for 5
Five complexes, which contain o-DMophFꢂ ligands
and one, two or three acetate ligands have been
P(1)ꢀMo(1)ꢀN(1)ꢀC(1)
P(1)ꢀMo(1)ꢀO(3)ꢀC(2)
101.9 P(2)ꢀMo(2)ꢀN(2)ꢀC(1)
1.8 P(2)ꢀMo(2)ꢀO(4)ꢀC(2)
170.0
98.8
structurally characterized. The Moꢀ
/
Mo bond length is
P(1)ꢀMo(1)ꢀN(1)ꢀC(11) 104.2 P(2)ꢀMo(2)ꢀN(2)ꢀC(21) 173.1
dependent on the number of the acetate ligands and on
the electronegativity of the substituent on the acetate
ligand. Two different conformations, s-trans,s-trans
and s-cis,s-trans, are seen for the o-HDMophFꢂ
ligand. In complexes 1ꢁ5, which conformation to take
is dependent on the number of the methoxyl group each
ligand has to provide to accommodate the two axial sites
/
complex 5 in CDCl3. The two peaks centered at 182.38
and 156.30 ppm can be assigned the central carbon
atoms, C(1) and C(2), of the formamidinate and acetate
ligands, respectively. A heteronuclear shift-correlated 2-
D NMR spectrum for 13C and 1H nuclei shows that the
peaks centered at 156.30 ppm is correlated with the
methine hydrogen and can be assigned to C(1) of the
formamidinate ligand. The peak at 182.38 ppm is then
assigned to C(2). Both C(1) and C(2) atoms experience
3J coupling from two phosphorus atoms, but each peak
of the dimolybdenum unit, although the Moꢀ
/
Mo bond
length is not sensitive to such axial interactions. Com-
plex 5 provides an opportunity to study the through
metalꢀ
without considering the ligand ‘backbone’ contribution
and the result is 1691 Hz. The position of the
monodentate phosphine ligand can be determined by
/
metal quadruple bonding coupling ½3JPꢀMoꢀMoꢀP
½
/
evaluating the ½3JCꢀP½ coupling constant. Complexes 1ꢁ
can be envisaged as the intermediates of the stepwise
decarboxylation of Mo2(O2CR)4 to form Mo2(form)4.
/4
only shows as a doublet with coupling constants
3
of j JC(1)ꢀP(1)ꢃ3
/
JC(1)ꢀP(2)½ꢀ
/
2.2 and ½3JC(2)ꢀP(1)
3.5 Hz for C(1) and C(2), respectively.
ꢃ
/
3JC(2)ꢀP(2)½ꢀ
/
The two peaks centered at 139.34 and 137.36 ppm can
be assigned either to C(11) or C(21) of the formamidi-
3
nate ligand. Each of the carbon atoms experiences J
6. Supplementary material
coupling from one of the two phosphorus atoms and
½3JC(11)ꢀP(1)½ and ½3JC(21)ꢀP(2)½ are found to be 0 (due to
the limit of the instrument resolution) and 2.1 Hz,
Crystallographic data (excluding structure factors) for
the structures in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplemen-
respectively. The three-bond 13Cꢀ
/
31P coupling constants
tary publication numbers CCDC 178806ꢁ178810. Co-
/
(3JCP) can be analyzed in terms of the generalized
Karplus equation which relates the coupling constant
to the dihedral angle (8) in the four-atom fragment
involved. The study generally resulted a Karplus curve
pies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (fax: ꢃ44-1223-336 033; or e-mail: depos-
/
it@ccdc.cam.ac.uk).
showing maxima for J at 8ꢀ
minimum at 8ꢀ908 (J ꢁ0 Hz) [25]. Table 5 lists some
/
0 and 1808, with the usual
/
/
of the important dihedral angles for complex 5. Based
on the definite relation of Karplus equation, we can
easily assign the peak at 137.37 ppm to C(11) which
shows 0 Hz coupling constant, since the dihedral angle
Acknowledgements
We wish to thank the National Science Council of the
Republic of China for support.
of O(1)ꢀ
/
Mo(1)ꢀ
/
N(1)ꢀC(11) is close to 908. We thus
/
conclude that in the dimolybdenum complexes the
phosphine ligand cis to the acetate or formamidinate
ligands does not contribute to ½3JCꢀP½ while the trans
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/
1 Hz. The minimum 8 of
½
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