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Mono- and Binuclear Molybdenum Complexes Incorporating 4-(4-Hydroxyphenyl)pyridine MetalЦMetal Interactions Across an Asymmetric Bridging Ligand.

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leads to a slight but hardly significant improvement of the
statistical index. In any case, the E value never exceeded
1 cm- ' and g, never differed from g, by more than 0.1. On
the other hand, a small J value was found and fixing J to 0
induced a significantly poorer fit. Accordingly, the final calculations were done with five parameters: J; D, g, = gy, g,.
The solid lines in Figure 2 were calculated with the following set of parameters:['8J g, = 1.63(5), gy = 1.63(5), g, =
2.56(5), d = 0.3(2) cm-', and D = 9.8(5) cm-'. The value
estimated for the axial zero-field splitting is comparable to
the one observed for deoxyhemerythrin derivatives,[8.'I1 but
the most remarkable feature is the extreme weakness of the
exchange interaction J. In the case of reduced hemerythrin
- J is 12-38cm-', in agreement with the presence of a
hydroxo bridge.C8]A further protonation to an aqua bridge
is presumed to occur with the binding of some exogeneous
anions and results in the decrease of the exchange interaction. When the counterion is azide, the coupling even
changes sign ( J = 1.7 cm-')["] and the interaction becomes
ferromagnetic, which leads to an EPR-active S = 4 ground
state of the deoxyHrN, protein. In its reduced form methane
monooxygenase (MMO), another diiron protein, presents
the same kind of EPR signal characterized by a high g value
(g 16).[19'It is worth mentioning that reduced ribonucleotide reductase from E. coli is EPR silentr2'] for conditions
under which deoxyHrN, and reduced MMO exhibit the high
g value spectrum. This observation might suggest that a water molecule is not the ligand bridging the iron atoms in
reduced ribonucleotide reductase, but which bridging ligands could then be present in reduced ribonucleotide reductase? During reduction of the oxidized enzyme, a weakly
binding aqua bridge could conceivably be formed upon protonation of the 0x0 bridge and subsequently be replaced by
the carboxylate group from glutamate 238. This residue is
terminal and monodentate in oxidized R2 and would become bridging and bidentate in the reduced protein. From
the structural analysis of a wealth of carboxylato metal complexes, Lippard et al.["I have shown that such a change in
the coordination mode of a carboxylate residue from terminal to bridging is a viable possibility. Moreover, the carboxylate group from aspartate 84 would probably become
monodentate resulting in the hypothetical situation illustrated in Figure 3. Interestingly, a similar structure has been
[3] a) J. Sanders-Loehr in Iron Curriers und Iron Proteins, (Ed.: T. Loehr),
VCH, New York, 1989, p. 373; b) L. Que, Jr., A. E. True, Prog. Inorg.
Chem. 1990, 38,97.
[4] L. Petersson, A. Graslund, A. Ehrenberg, B.-M. Sjoberg, P. Reichard, J.
Biol. Chem. 1980, 255, 6706.
[5] a) M. Fontecave, R. Eliasson, P. Reichard, J. Biol. Chem. 1989,264,9164;
b) M. Fontecave, C. Gerez. D. Mansuy, P. Reichard, ibid. 1990, 265,
[6] J. B. Lynch, C . Juarez-Garcia, E. Munck, L. Que, Jr., J. B i d . Chem. 1989,
264, 8091.
[7] M. Sahlin, A. Graslund, L. Petersson, A. Ehrenherg, B.-M. Sjoherg, Biochemisfry 1989,28, 2618.
[8] R. C. Reem, E. I. Solomon, .
Am. Chem. Soc. 1987, 109, 1216.
[9] D. M. Kurtz, Chem. Rev. 1990, YO, 585.
[lo] W. A. Armstrong, S . J. Lippard, J. Am. Chem. Soc. 1984, 106, 4632.
[111 M. P. Hendrich, L. L. Pearce, L. Que, Jr., N. D. Chasteen, E. P. Day, J.
Am. Chem. Soc. 1991, 113, 3039.
[I21 Proteins R2 and apoR2 were prepared in Tris buffer (0.1 M, pH 7.5, 20%
glycerol). The solvent of both solutions was then exchanged for D,O by
successive concentration followed by dilution with D,O in an Amicon
YM-30 centricon. Finally the protein solution was concentrated
(1 x
M, 125 1L) and deaerated on a manifold argon/vacuum line.
Reduced R2 was prepared in an anaerobic glove box by adding four
equivalents of ammonium iron(l1) sulfate to an apoR2 solution in D,O.
That iron was correctly incorporated was shown from the appearance of
the tyrosyl radical upon exposure to air. This is observed both by EPR and
UV/VIS spectroscopy. Moreover, no free iron(rl1) could be detected in the
reoxidized sample by EPR spectroscopy (signal with g = 4.3).
[13] E. P. Day, T. A. Kent, P. A. Lindahl, E. Miinck, W. H. Orme-Johnson, H.
Roder, A. Roy, Biophys. J. 1987, 52, 837.
[14] S. M. Gorun, S. J. Lippard, Inorg. Chrm. 1991, 30, 1625.
[15] B. M. Sjoberg, T. M.Loehr, J. Sanders-Loehr, Biochemistry 1982, 21, 96.
[16] R. E. Stenkamp, L. C. Sieker, L. H. Jensen, J. Am. Chem. Soc. 1984, 106,
(171 a) Also note that the single-ion ZFS tensors are assumed to be colinear.
This simplification places very little restriction on a frozen solution when
the J value is small, as found. The average molar magnetization m,,(H)
along the direction of the magnetic field H is calculated numerically from
the partial derivative of the free energy [17 b]. The six parameters 4 D,E,
9,. gr, and g. of the Hamiltonian are obtained through a least-squares fit
to the experimental values of the product m,, ( H ) T for all the magnetic
fields and temperatures; b) N. W. Ashcroft, N. D. Mermin, Solid Slate
Physics, Saunders, Philadelphia, 1976, p. 643.
[18] The average value for g is 1.992, too low for an iron(@ derivative. We
believe that the uncertainty in protein quantitation is responsible for this
result, as outlined by Day et al. [Ill. The other parameters (4 D ,E) are
[19] M. P. Hendrich, E. Munck, B. G. Fox, J. D. Lipscomb, J. Am. Chem. SOC.
1990, 112, 5861.
[20] Que, Jr. [6] reported that reduced ribonucleotide reductase exhibits a
broad EPR signal at low field, hut does not rule o u t that it is associated to
adventitious Fe(ir) ions. We have never detected such a signal in our preparations of the reduced enzyme.
[21] R. L. Rardin, W. B. Tolman, S. J. Lippard, New J. Chem. 1991, 15, 417.
(221 M. Atta, P. Nordlund, A. h e r g , H. Eklund, M. Fontecave. J. B i d . Chem.
in press.
Glu 238
His 241
Glu 115
Fig. 3. Structural hypothesis for the diiron center of reduced protein R2. It is
highly likely that the coordination of the iron(n) ions is completed by water
molecules as found for oxidized R2
found recently by X-ray crystallography for a new form of
protein R2 in which the iron centers have been replaced by
divalent manganese ions.r221This observation gives additional support to the above hypothesis.
Received: May 12, 1992 [Z 5342 I€]
German version: Angew. Chem. 1992, 104, 1558
[l] M. Fontecave, P. Nordlund, H. Eklund, P. Reichard, Adv. Enzymol. 1992,
65, 147.
[2] P. Nordlund, H. Eklund, B.-M. Sjoberg, Nature 1990, 345, 593.
Angrw. Chem. l n t . Ed. Engl. 1992, 31, No. 11
Mono- and Binuclear Molybdenum Complexes
Incorporating 4-(4-Hydroxyphenyl)pyridine:
Metal-Metal Interactions Across an Asymmetric
Bridging Ligand""
By Amitava Das, John C. Jeffery, John P . Maher,
Jon A . McCleverty,* Erik Schatz, Michael D. Ward,*
and Gerd Wollermann
The study of binuclear complexes whose metal centers are
linked by an unsaturated bridging ligand is of relevance to
[*] Prof. .
A. McCleverty, Dr. M. D. Ward, Dr. A. Das, Dr. J. C. Jeffery,
Dr. J. P. Maher, E. Schatz, G. Wollermann
School of Chemistry
Cantock's Close, GB-Bristol BS8 ITS (United Kingdom)
We thank the Science and Engineering Research Council (SERC) for a
postdoctoral fellowship (A.D.) and for a grant for the purchase of an EPR
Veriagsgesellschaft mbH. W-6940 Weinheim, 1992
intramolecular electron transfer in mixed-valence systems[']
and the study of magnetic interactions between paramagnetic centers. The most extensively studied such complex is the
Creutz-Taube ion;[21more recently 4,4-bipyridine-bridged
binuclear complexes have received much attention.[3,41
The synthesis of the symmetrical binuclear complex
[(Mo(NO)L*Cl),(p-bpy)] [L* = tris(3,5-dimethylpyrazolyl)hydridoborate, bpy = 4,4'-bipyridine] was recently reported,r4] in which a strong electronic coupling between the
[Mo(NO)L*Cl] centers caused a difference of 765 mV between the reductions of the first and the second metal centers; EPR spectroscopy showed rapid site exchange of the
two unpaired electrons at room temperature. We now report
the syntheses, and electrochemical and EPR spectroscopic
properties of asymmetric, mixed-valent (17e-16e) and homovalent (17e-17e) (e = valence electrons) binuclear molybdenum complexes containing a 4-(4-hydroxyphenyl)pyridine
bridge, and present evidence that metal-metal interactions
are assisted by a nearly planar conformation of the bridging
The precursor L2 was prepared by extensions of known
procedures,I5 - 71 and demethylated with pyridinium chloride['] to give HL' (Scheme 1). Syntheses of the complexes
[Mo(NO)L*CI,]. Attachment of the pyridyl terminus to
molybdenum could only be achieved if the phenol group was
protected, as in L2; thus, reaction of [Mo(NO)L*CI,] with
L2 produced 4 which has a pendant methoxyphenyl group.
This complex has 17 valence electrons; attachment of the
pyridyl ligand i s accompanied by a one-electron reduction of
the molybdenum center, probably by the Et,N, which is
known to act as a hydridic reducing agent.[']
Methylation of the pendant pyridyl group in 1 gave 2 + ,
isolated as its PF; salt. The electrochemical data (Table 1)
show that the metal-centered reduction is anodically shifted
Table 1. Electrochemical and EPR data for the new complexes.
Electrochemical processes [a]
1 - [d]
- 0.70 (78) [c]
- 0.58 (80) [c]
2 [dl
- 0.68 (90) [c]
EPR parameters
g [bl
- 1.88 (100) [el
+ 0.12 (80) [ f j
- 2.00 (180) [g]
3- [d]
-1.87 (100) [g]
Cul, THF
Scheme 1. Syntheses of the ligands Lz [6] and HL'.
are outlined in Scheme 2.['] Reaction of [MoL*(NO)Cl,]
with HL' afforded 1, in which the phenoxide terminus of L'
is attached to the 16-electron Mo center and the pyridyl
group is pendant. This reaction is apparently controlled by
the difference in reaction rate between the anionic phenolate
and the neutral pyridine with the electron-deficient
L2 + [Mo(NO)L'Cl,]
+ 0.11 (70) [fl
[a] Potentials quoted in V vs. the ferrocene/ferrocenium couple, with peak-peak
separations [mV] in parentheses. Electrochemical measurements were made
with Pt-bead working and auxiliary electrodes and a saturated calomel electrode as reference electrode in a 0.1 M solution of (Bu,N)PF, in CH,CI,. In all
cases ferrocene was used as an internal reference. [b] Average value of the g
factor. [c] Potentials for the 16e-17e reduction of the [Mo(NO)L*CI(phenolate)] moiety. [d] 17e complex produced by in situ reduction of the parent
16e complex with cobaltocene. [el Potentials for the reduction of the
methylpyridinium moiety. [ f j Potentials for the 17e-16e oxidation of the
[Mo(NO)L*Cl(pyridyl)] moiety. [g] Potentials for the 17e- 18e reduction of the
[Mo(NO)L*Cl(pyridyl)] moiety.
by 120 mV on N-methylation of the pendant pyridyl group.
This is in marked contrast to the behavior of polypyridineruthenium(r1) complexes containing 4,4'-bipyridyl groups,
where N-methylation of the pendant pyridyl groups results
redox couples by just
in an anodic shift of the Ru"/Ru"'
20 mV per methylpyridinium group.["] The large interaction in 2 + is due to charge recombination in the deprotonated, methylated ligand resulting in a quinonoidal structure
(Fig. 1).This constrains the ligand to be planar, prevents free
rotation about the central C-C bond normally associated
with biphenyls, and causes a decrease of electron density at
the metal center by removal of the phenolate negative
charge, thereby facilitating the 16e-1 7e reduction.
Reaction of 1 with further [Mo(NO)L*Cl,] produced the
mixed-valence complex 3,[' 'I in which the N-coordinated
M e O ~ - - M o ( N O ) L ' C I
Schenie2. Syntheses of the complexes 1-4 under an N, atmosphere.
a) Toluene, Et,N, 4 h under reflux; b) [Mo(NO)L*CI,], toluene, Et,N, 6 h under reflux; c) MeI, CH,CI,, 18 h under reflux.
8 VCH Vrrlagsgesellschafr mbH,
W-6940 Weinheim,1992
Fig. I . Resonance structures of the deprotonated. N-methylated ligand HL'.
0570-0833~92jllll-1516$3.50+ .25/0
Angew. Chem. In!. Ed. Engl. 1992, 31, No. I 1
molybdenum atom has 17 valence electrons, whereas the 0coordinated molybdenum atom has only 16. Thus, here also
the addition of the pyridyl ligand is accompanied by a reduction of the molybdenum center. Electrochemical measurements (Table 1) confirm the presence of an electronic interaction between the two metal centers; whereas the difference
between the reductions at the metal centers in 1 and 4 is
1.17 V. the presence of a direct link between them in 3 results
in a A(E1,*)value of 1.35 V, that is, an increase of 180 mV.
Again, this may be accounted for by a quinonoidal contribution to the structure of the bridging ligand which results in
decreased electron density at the 0-terminus and increased
electron density at the N-terminus. This contribution will be
more significant in 3 than in 1, due to stabilization of the
electron-rich pyridyl N atom by attachment to a 17-electron
Mo center; the pyridyl group thus functions as a x donor
rather than as a TC acceptor, which is its more common role.
The EPR spectra of 4 and the reduced complexes 1- and
2 (produed by in situ reduction of the 16-electron species 1
and 2' with cobaltocene) all show hyperfine coupling constants of about 5.0 mT due to coupling to a single molybdenum center (Table 1). The coupling constant is similar for 3;
this indicates that the unpaired electron is trapped at the
pyridyl-coordinated molybdenum center, which is to be expected considering the difference in the 16e-17e redox potentials for the two molybdenum sites. The localization of the
unpaired electron in 3 is confirmed by virtually identical
values of g for 3 (1.979) and 4 (1.978).
One-electron reduction of 3 with cobaltocene affords the
17e-17e species 3 - , a biradical where the two electrons are in
redox levels 0.77 V apart. Despite this energy gap, the EPR
spectrum (Fig. 2) indicates a change from localized to delo-
electron-exchange frequency," 1' a reasonable value since
studies of electron transfer across saturated hydrocarbon
bridges have shown transfer rates in the region lo6 to
lo9 s - ' . [ ' ~ ]Also the hyperfine splitting is halved to 2.6 mT.
This confirms that, as with [ ( M O ( N O ) L * C I ) , ( ~ - ~ ~ ~ ) ] , [ ~ ~
there is fast electron exchange between the two metal centers
with 1 J ( % A , (1 J ( is the exchange integral, A , is the hyperfine
i n t e r a ~ t i o n ) ; "in
~ ~such cases the hyperfine splitting is predicted to be 0.5 (A,/gp), that is, half the value found in the
spectra of the mononuclear components. No half-field resonance was observed either at room temperature or 77 K,
which may be ascribed to the intermetallic ~eparation.'~]
Although the complex is asymmetric, there is no barrier to
electron exchange arising from reorganization of the metals'
coordination spheres, since both metal centers possess
17 valence electrons before and after any exchange. To our
knowledge this is the first example of fast exchange in an
asymmetric biradical, bimetallic system.
The crystal structure of 4 is shown in Figure 3.['*] The
coordination geometry is approximately octahedral. The
most interesting feature is the nearly coplanar aromatic rings
Fig. 3. Molecular structure of 4. Selected bond lengths [A] and angles ["I:
Mo(1)-Cl(1a) 2.432(6), Mo(l)-N(8a) 1.85(2),Mo(1)-N(1) 2.204(5), Mo(l)-N(3)
2.246(5), MO(l)-N(5) 2.163(5), MO(l)-N(7) 2.196(5); N(l)-Mo(l)-N(7) 87.4(2),
N(l)-Mo(l)-N(3) 86.9(2), N(8a)-Mo(l)-N(l) 92.0(6), Cl(1a)-Mo(1)-N(1)
93.6(2), Cl(la)-Mo(l)-N(Ea) 91.2(6), Cl(la)-Mo(l)-N(3) 89.7(2), Cl(1a)-Mo( 1)N(5) 95.4(2), N(8a)-Mo(l)-N(S) 96.5(6), N(fh)-Mo(l)-N(7) 94.4(6).
Fig. 2. Room-temperature EPR spectra of 3 and 3-
calized electrons on the EPR timescale. A single signal is
observed at g = 1.973, midway between the values for the
mononuclear components 1- and 4 (g = 1.969 and 1.978,
respectively); this places a lower limit of 4 x 10' Hz on the
Angew. Chem. Inf. Ed. Engl. 1992, 31, N o . 11
of L2 (dihedral angle of 5"). By contrast, in the isoelectronic
complex [M0(N0)L*(ppy)~]+(ppy = 4-phenylpyridine, an
analogue of L2 without the electron-donating methoxy
group), the torsion angles between the rings are 30 and
41".['81 This is consistent with our view that planarity of L2
in 4, and of L' in the other complexes, is due to partial
quinonoidal character, although the central C-C bond
length in 4 [1.476(9)& is close to the average value for
biphenyl-type structures" 91 and not significantly different
from the corresponding distances in [Mo(NO)L*(ppy),]+
[1.430(15) and 1.497(13) A]. Although near-planarity in the
crystal structures of biphenyl-type species may often be ascribed to favorable stacking interactions between the aromatic rings,["' this does not appear to apply in 4; the aromatic rings are not especially close to any other groups, so it
is less likely that the ligand is being forced into planarity by
crystal packing effects.
Verlagsgesellschafl mbH, W-6940 Weinheim, 1992
Received: June 30,1992 [Z 5440 IE]
German version: Angew. Chem. 1992, 104, 1554
0570-0833!92jll11-1S1? $3.50+.25jO
CAS Registry numbers'
1, 144018-72-4; I - , 144018-79-1; 2, 144018-77-9; Z'PF;, 144018-74-6; 3,
144018-75-7;3 - , 144018-78-0;4, 144018-76-8;4.CH2CI,, 144018-80-4;HL',
77409-99-5; L2, 5938-16-9;[MoL*(NO)CI,], 24981-81-5;4-bromoanisole, 10492-7; methyl chloroformate, 79-22-1 ; pyridine, 110-86-1.
[l] N. S. Hush, Prog. Inorg. Chem. 1967, 8, 391.
[2] C. Creutz, Prog. Inorg. Chem. 1983, 30, 1.
(31 D. J. Salmon, M. J. Powers, R. W Callahan, T. J. Meyer, Inorg. Chem.
1976, is, 894; J. E. Sutton, H. Taube, ibid. 1981, 20. 3125; M. M. Zulu,
A. J. Lees, ibid. 1989, 28, 8 5 ; A. J. Downard, G. E. Honey, L. F. Phillips,
P. J. Steel, ibid. 1991,30,2259; R. L. Blackburn, J. T. Hupp, J Phys. Chem.
1988,92,2817; B. L. Loeb, G. A. Neyhart, L. A. Worl, E. Danielson, B. P.
Sullivan, T. J. Meyer, ibid. 1989, 93, 717; E. C. Constable, M. D. Ward, J
Chem. SOC.Dalton Trans. 1990, 1405.
[4] S. L. W. McWhinnie, C. J. Jones, J. A. McCleverty, D. Collison, F. E.
Mabbs, J. Chem. SOC.Chem. Commun. 1990,940.
[S] D. L. Comins, A. H. Abdullab, J Org. Chem. 1982, 47, 4375.
[6] Preparation of L2: To an ice-cold, stirred suspension of anhydrous CuI
(0.1 g,0.53mmol)anddrypyridine(l.l9 g, 15mmol)indryTHF(2Scm3)
under N, was added methyl chloroformate (0.95 g, 10mmol); a white
precipitate appeared. The mixture was cooled to - 30 "C, and a solution of
the Grignard reagent prepared from 4-bromoanisole (2.06 g, 11 mmol)
and Mg turnings (0.34 g, 14 mmol) in T HF (20 cm3) was added dropwise
over 15 min. The mixture was stirred for 15 min at - 30 "C and then at
room temperature for 1 h. After hydrolysis with aqueous NH,CI, the intermediate dihydropyridine was extracted with ether. concentrated, and
rearomatized with KMnO, in acetone at 0°C. The product L2 was obtained as a white solid in 65 % yield after chromatography on alumina with
[7] All new compounds gave satisfactory elemental analyses. Data for HL':
M.p. 250-252°C. ' H NMR (270 MHz, CD30D, TMS): 6 = 6.91 (d,
J = 8.6 Hz, 2 H ; phenyl-H2, -H6), 7.63 (d, J = 8.6 Hz, 2 H ; phenyl-H',
-H5)+7.65 (d, J = 5.6 Hz, 2 H ; pyridyl-H', -H5),8.49 (d, J = 5.6 Hz, 2 H ;
100%). DataforL,: M.p.95-96°C.
pyridyl-H2,-H6);MS:m/z 185(M+,
'H N M R (270 MHz, CD,OD, TMS): 6 = 3.84 (s, 3H; OMe); 7.06 (d,
J = 8.6 Hz, 2 H ; phenyl-H2, -H6), 7.69 (d, J = 8.6 Hz, 2H; phenyl-H3,
8.50 (d, J = 5.6 Hz, 2 H ,
-H5),7.71 (d, J = 5.6 Hz, 2H; pyridyl-H3, -H5),
pyridyl-HZ, -H6); MS: mjz 171 ( M i , 100%). FAB mass spectra of
complexes 1-4 show molecular ion peaks at m / r 630, 644, 1088,
and 644, respectively (based on the masses of the most abundant isotopes).
[S] C. Dietrich-Buchecker, J.-P. Sauvage, Tetrahedron 1990, 46, 503.
[Y] G . S. Daitmazova, N. P. Gambaryan, E. M. Rokhlin, Russian Chem. Rev.
1989, 58, 1145.
[lo] M. A. Hayes, C. Meckel, E. Schatz, M. D. Ward, 1 Chem. Soc. Dalton
Pans. 1992, 703; B. P. Sullivan, H. Abruna, H. 0. Finklea, D. J. Salmon,
J. K. Nagle, T. J. Meyer, H. Sprintschnik, Chem. Phgs. Lelt. 1978, 58,
1111 In addition to mass spectrometry and elemental analysis, the molybdenum
complexes have characteristic <(NO)stretching frequencies. 1: ?(NO) =
1686cm-'; 4: ?(NO) =1607cm-'; 3, which contains one Mo center of
each type, ;(NO) = 1611 and 1687 cm- '.
[12] The difference between the signals at g =3.978 and g =1.969 is 15 G. At
an average magnetic field of approximately 3400 G and a microwave frequency of approximately 9 GHz, the rate ofelectron exchange necessary to
give an averaged signal is simply given by u,,, = (15/3400) x (9 x lo9) z
4 x 107 H ~ .
[13] J. R. Miller, L. 7. Calcaterra, G. L. Closs, J Am. Chem. SOC.1984, 106,
3047; G. L. Closs, P. Piotrowiak, J. M. MacInnis, S. R. Fleming, ibid.
1988, 110, 2652.
[14] D. C. Reitz, S . I. Weissmann, J Chem. Phys. 1960, 33, 700.
[I51 4 : C,,H,,BCIMoN80, . CH,CI,, M =728.7, triclinic, space group P i ,
a = 10.641(4), h = 10.893(4), c = 14.865(7) A, (L = 96.43(3), @ = 98.00(3),
y = 98.56(3)", V = 1672(1) A', Z = 2, ecSicd
=1.45 g ~ m - F(OO0)
p(Moy,) = 6.7 cm-', R = 0.066 (R, = 0.067) for 4403 unique data
1293 K, Wyckoff-w-scans,20 5 50", F 2 5rs(F)];Data were collected with
a Siemens R 3 m p diffractometer (Mo,, radiation, graphite monochromator, i= 0.71069 A). The data were corrected for Lorentz, polarization,
and X-ray absorption effects. The structure was solved by conventional
heavy-atom methods and successive difference Fourier syntheses were
used to locate all non-hydrogen atoms. The CI and NO ligands are mutually disordered (relative site occupancies 76:24), and only the major sites
for these ligands are shown in Figure 3 . There was no evidence of disorder
in the remainder of the complex. Final refinements by full matrix leastsquares procedures were performed on a Micro Vax computer with the
SHELXTL system of programs [16]. Scattering factors with corrections
for anomalous dispersion were taken from ref. (171. Further details of the
crystal structure investigation are available on request from the Director
of the Cambridge Crystallographic Data Centre, University Chemical
Laboratory, Lensfield Road, Cambridge CB2 IEW (UK), on quoting the
full journal citation.
Verlagsgesellschajt mbH, W-6940 Wernheim, 1992
G. M. Sheldrick, SHELXTLprograms for use with a Nicolet X-ray System,
Cambridge, 1976, updated Gottingen, 1981.
Znternalional Tables for X-Ray Crystallography, Vol. 4, Kynoch Press,
Birmingham, 1974.
F. McQuillan, C. J. Jones, J. A. McCleverty, T. A. Hamor, unpublished
C. P. Brock, R. P. Minton, J. Am. Chem. SOC.1988, 111, 4586.
Se,N,Cl,, a Novel Selenium-Nitrogen Chloride:
Reinvestigation of "Se,N," **
By Jari Siivari, Tristram Chivers,* and Risto Laitinen
The development of selenium-nitrogen (Se-N) chemistry
has been impeded by the lack of readily available Se-N
reagents."] For example, the selenium analogue of the useful
synthon S3N,CI,[21 (la) is unknown, although the mixed
sulfur-selenium halides S,SeN,CI, (lb)I3] and SSe,N,Cl,
( 1 ~ have
) ~been
~ ~structurally characterized recently.
In 1991 Dehnicke reported the preparation of tetraselenium dinitride obtained as a black powder [Eq. (a)].I5.61 The
2 Se,CI,
+ 4 Me,SiN,
+ 5 N, + 4 Me,SiCI
identification of Se,N, relied solely on a comparison of the
IR spectrum with that of the structurally characterized sulfur
analogue cyclo-1,3-S4N,, which is a six-membered ring
in the half-chair conformation,['. '1 and with the calculated
(3-21G*) vibrational frequencies.15' The formation of 2: 1
and 1 : 1 adducts of Se,N, with SnCI, and TiCI,, respectively,
has also been reported.['] These adducts were characterized
by IR spectroscopy and by chemical analyses.
The reported stability of Se4N, up to 80 0C[51appeared to
us to be incompatible with the facile thermal decomposition
of diiminoselanes RN=Se=NR (R = tBu,["'] SiMe31111)
above 0 "C. These observations, and our interest in heterocycles of the type Se,S,_,N, (x = 1-4),"*' prompted us to
reinvestigate reaction (a). We report here that the compound
formulated by Dehnicke et al. as Se,N, is, in fact, Se,N,CI,
(Id) and describe an alternative route to this novel seleniumnitrogen halide.
The reaction of trimethylsilyl azide with diselenium
dichloride was carried out according to the literature proce[*I
Prof. T. Chivers, J. Siivari
Department of Chemistry
The University of Calgary
Calgary, Alberta T2N 1N4 (Canada)
Prof. R. Laitinen
Department of Chemistry
University of Oulu (Finland)
[**I This work was funded by the Natural Sciences and Engineering Research
Council (NSERC) (Canada) and the Academy of Finland. J. Siivari is an
exchange Ph.D. student at the University of Calgary supported by Neste
Oy Foundation, Espoo, Finland.
$3.50+ ,2510
Angew. Chem. Int Ed. Engl. 1992.31, No. 11
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asymmetric, interactions, acros, bridging, mono, complexes, hydroxyphenyl, ligand, binucleata, pyridin, metalцmetal, incorporation, molybdenum
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