Synthesis and crystal structures of oxo pyridinemethanolate technetium(V) complexes

ELSEVIER

Inorganica ChimicaActa254 (1997) 303-307

Synthesis and crystal structures of oxo pyridinemethanolate technetium(V) complexes Fernande D. Rochon *, Robert Melanson, Pi-Chang Kong D~partemenl de Chimie, Universitdda Qu~becf~Montrdal, C.P 8888. Succ. Centre-~ille.Montreal, Que. H3C 3PS, Canada

Received7 September1995;revised 19March 1996

Abstract Oxo Tc(V) complexes with 2-(hydmxymethyl)pyridine and 2,6-di(hydmxymethyl)pyridine were ~ynthesized from the reaction of n-Bu4[Tc(O)X4] (X=CI or Br) with the ligand in methanol or ethanol. The three compounds Tc(O)(OCHzPyCH2OH)2CI (1), Tc(O)(OCH2PyCH2OH)2Br (2) and Tc(O)(OCH2Py)2CI (3) (Py=pyridine) were studied by crystallographic n'~thocls. Cryshal 1 is triclinic, Pl, a =7.479(2), b= 8.043(2), c= 14.940(4) A, a = 93.66(2),/3= 102.16(2), 7 = 117.18(2) °, Z = 2 and R=O.050. The bromo analogue 2 is also triclinic, PI, a=7.416(4), b=8.115(4), c = 15.!51(10) A, ~=94.36(5), t = 102.09(5), 7= 116.77(4) ~, Z = 2 and R = 0.060. Compound 3 crystallizes in the monoclinic P2t Ic space group with a = 12.861(4), b = 7. I l ! (2), c =- 14.623(6) A,/3~ 92.86(3) °, Z= 4 and R = 0.043. The geometry around the Tc atom is a very distorted octahedron with the oxo and halide ligands in c/s position to each other. Crystal 3 is disordered on two positions. The two organic ligands are deprotonated and the disubstituted molecule 2,6di(hydroxymethyl)pyridine in I and 2 is bidentate with one free --CH2OHgroup. Keywords: Technetiumcomplexes:(Hydroxymethyl)pyridieecomplex~;"Oxo complexes:Crystalstructures

1. Introduction During the last decade, several transition metal complexes containing the bidentate ligand 2-(hydroxymethyl)pyridine or 2-(hydroxyethyl)pyridine al~d the tridentate 2,6di(hydroxymethyl)pyridine have been reported [I-9]. Some of these complexes were made directly from the reaction of the starting metallic compounds with the iigands or with lithium alkoxide [ 1-6], while others were obtained by an indirect method, using unsubstituted pyridine with the insertion of carbon monoxide [7,8 ] or insertion of acylpyridine into the hydride-metal bond [9]. These compounds present different interests especially as catalysts or as models for several enzymes. No such compound has been reported yet for technetium. The chemistry of Tc complexes has recently become very important, especially because of the wide use of the isotope '~'JmTcin diagnostic radiopharmaceutical studies. A large proportion of the technetium radiopharmaceutical preparations currently used in nuclear medicine involves compounds containing the [TrY-O] 3÷ core. Oxo To(V) compounds with

* Corresponding author. Tel: (514) 987-4896: fax: (514~ 987-4054; e-mail: rochon.femand¢@aqam.ca.

polydentate N-O ligands have been used as brain agents for several years, but the search for more specific imaging agents is still an active field of research. Recent advances in this area have recently been published in two books edited by Nicolini et al. [ 10]. Our main interest in Tc chemistry is in the development of new methods to prepare cationic or neutral species of the stable isotope ~Tc for the potential radiopharma:eutical application to heart or brain imaging. We are especially interested in the development of methods to synthesize mixed-ligand compounds since the literature in this area is quite scarce. Furthermore, the physical and chemical properties of the complexes can be slightly altered by varying the pair of ligands, thus providing a way of fine-tuning the biodistribution properties of these complexes, in order to design and develop more specific imaging agents. We have now synthesized a few neutral Tc (V) complexes with 2-(hydroxymethyl)pyridine and 2,6-di(hydroxymethyl)pyridine. Although these compounds arc very interesting by themselves, they might also be good intermediate molecules for synthesizing mixed-ligand compounds. We have determined the crystal su'uctures of three complexes, Tc(O)(OCH2PyCH2OH)2CI (1), Tc(O)(OCH2I~CH2 OH)2Br (2) and Tc(O)(OCHzPY)2CI (3) (Py =pyridine), and the results of these studies arc described below.

0020-1693/97/$17.00Copyright© 1997ElsevierScienceS.A. All rightsreserved PIl S0020-1693 (96) 05 ! 76-6

304

F.D. .r~ochonet aL/lnorganica ChimicaActa254 (1997)303-307

2. Experimental Ammonium pertechnetate (NI-I499TcO4) was purchased from Oak Ridge National Laboratory. It was recrystatlized in nitric acid (Caution.* Ammonium pertechnetate in acid medium will produce some radioactive volatile compound) and dissolved in water. A 0.286 M solution was prepared. All manipulations were made in a laboratory approved for lowlevel radioactive material (99Tc is a fl-emitter with a particle energy of 0.292 MeV and a half-life of 2.13 x 105 years). The ligands were bought from Aldrich. n-Bu,,[Tc(O)CI4] was synthesized according to the published procedure [ 11 ] and n.Bu4[Tc(O)Br4] was prepared as already reported [ 12]. 2.1. Tc(O)(OCH2 pyCHeOH):Cl (1)

2,6-(OHCH2)zPy (0.22 g, 0.5 mmol) dissolved in 14 ml of ethanol was added to a solution ofn-Bu4[Tc(O) C14] (0.08 g, 0.16 mmol) dissolved in I0 ml of ethanol. A blue mixture resulted and after 30 min the sob~don became clear and the dark precipate was filtered out at~d washed first with ethanol, then with diethyl ether. Yield: quantitative. Anal. Found: C, 39.99; H, 3.93. Calc.: C, 39.50; H, 3.79%. IR: v ( T c - O ) = 930cm-1; other main bands: 1620, 1575, 1165, 1070, 1015, 795, 780, 660, 565, 320, 340 cm- 1. The procedure was repeated in methanol and the mixture was left covered at room temperature for 3 days. The light blue solution was decanted and single crystals were obtained. 2.2. Tc(O)(OCH2pyCH2OH)2Br (2)

A quantity of 0.1 g (0.15 mmol) of n-Bu,,[Tc(O)Br4] was dissolved in 6 ml of methanol and added to another solution consisting of 2,6-(OHCH2)2Py (0.16 g) dissolved in 6 ml of methanol. The mixture became deep blue after 30 min; it was left standing at room temperature overnight. The next day, the solution was decanted and the crystals were washed with methanol. Yield: 60%. IR: v ( T c - O ) = 9 3 0 cm- i. 2.3. Tc(O)tOCn2py)pC (3)

A quantity of 8 drops of 2-(OHCH2)Py was added to a solution of n-Bu4[Tc(O)X4] (0.22 mmol) dissolvet~ in 10 ml of ethanol. A blue mixture resulted and after 30 min it became clear with a dark precipitate. The latter was filtered out and washed first with ethanol, then with diethyl ether. Yield: 80%. Tc(O)(OCH2py)2Cl: IR: v ( T c - O ) = 9 1 8 era-i; other main bands: 1620, 1095, 1040, 1030, 765, 715, 670, 580, 575, 320, 305 em-I. Anal. Found: C, 38.69; H, 3.28. Calc.: C, 39.40; Ho 3.31%. Tc (O) (OCH2py)2Br: Anal. Found: C, 35.57; H, 3.12. Calc.: C, 35.14; H, 2.95%. The synthesis was repeated in methanol and the blue mixture was sealed and left at room temperature overnight. The next day, the solution was decanted and the crystals were washed with

ethanol and ether. The crystals with X = C! were adequate for crystallographic studies. 2.4. Crystallographic measurements and structure resolution

The brownish crystals were selected after examination under a polarizing microscope for homogeneity. The unit cell parameters were obtained by least-squares refinement of the angles 20(16-30°), ¢o and X for 15 (25 for crystal 3) wellcentered reflections on a Syntex PI (Siemens P4 for 3) diffractometer using graphite-monochromatized Mo Ka radiation. The 20/0 scan technique was used for the data collections. The crystal data and the experimental details of the X-ray diffraction studies are shown in Table 1. The coordinates of the Tc atom were determined by direct methods and the positions of all the other non-hydrogen atoms were found by the usual Fourier methods. A very intense residual peak was found in the difference map of crystal 3. This peak, located at 0.62/~ from Tc, was attributed to disorder of the Tc atom. Refinement of the occupancy factors (keeping the thermal factors constant) led to occupancy factors of 0.83 for Tc and 0.17 fur To'. Further refinement led to the observation of another residual peak of low intensity which was assigned to a disorder of C(6). The occupancy factors were again 0.83 for C (6) and 0.17 for C ( 6' ). A data collection was made on a second crystal of 3 and similar results were obtained. The refinement of the structures was done by full-matrix leastsquares analysis minimizing Ew( 1Fol - IF¢ I ) 2. The H atoms on the C atoms were fixed at their calculated positions with U=0.08 ,~2 (riding model) for 3 and = U(C) +0.01/k for 1 and 2. The hydroxy H atoms ( 1 and 2) could not be located. A face-indexed numerical absorption correction was made for crystal 3 (min.-max. transmission: 0.715--0.887). For crystals 1 and 2, the NRCVAX programs [ 13] were used, while the SHELXTL system [ 14] was used for crystal 3.

3. Results and discussion The compounds were synthesized from the reaction of n-Bu4[Tc(O)X4] (X =Ci or Br) with 2-(hydroxymethyl)pyridine or 2,6-di(hydroxymethyl)pyridine in methanol or ethanol solution. After about 30 min, the reaction was complete and the neutral compounds could be filtered off. Quantitative yields were obtained when the solvent was ethanol and about 60% when the reaction was performed in methanol. Crystals, adequate for crystallographic studies, were obtained directly from the synthesis in methanol. The following three complexes were studied by X-ray diffraction methods: Tc(O)(OCHzPyCH2OH)zCI (1), Tc(O)(OCH2PyCH2OH)2Br (2) and Tc(O) (OCH2Py)2CI (3) (Py = pyridine). The vibration v(Tc---O) was observed between 918 and 930 cm- t. The results of the crystallographic studies have shown that the two ligands in the three Tc complexes are bidentate. This

F.D. Rochon et al. I Inorganica Chimica Acta 254 (1997) 303-307

305

Table I Crystaldataand experimentaldetailsof the X-raydiffractionstudies Crystal Compound ?,4.

1 C14H,,N20~CITc

426.64 P] 7.479(2) 8.043(21

Space group a (/~) b (~) c (~) (°1 (°)

y (°) Volume (A '~)

2 C,4HI~I:O~BrTc 471.09 P] 7AI6(4) 8.i 15(4)

3

Ct2H~,N203CITc 366.6 P2D/c

12.861(4) 7.111(2)

14.940(4)

15.151(I0)

14.623(6)

93.66(2) 102.16(2)

94.36(5)

90 92.86(3)

117.18(2) 768.3( 3 )

116.77(4)

I02.09(5)

90 1335.6(9 )

780.7 (8 ) 2

Z F(000) Pca,~(Mg m-~ tz(Mo Ka) (mm- t) Size of crystal (mm) 20 max. (°)

2 428 1.844

464

I.12

3.45

1.282

0.13×0.21 x0.48 58

0.I0×0.12× 0,44

h, L I

--I0-+ 8, O~ I0, - 20-,20

-8--,9, - I0---,O, - 18---,18

Weightingscheme(w- ~) NO.independentreflections(R~,,) No. observedreflections R max. A/0. Rw Largestdifferencepeak (e A-3) Goodness-of-fit

or?(F) + 0.00008F2 3719 (0.009) 3338, I> 2.50.(I) 0.050

0.2(F) + 0.00008F2 3084 (0.018) 2!81, I> 3o'(I) 0.060 0.002 0.063 0.61 1.50

0.09x0.28 x 0.53 55 0--* 16, 0--+9. - 1~--, 18 o~(F) + 0.0C07F2 3062 (0.019) 2438. F>4.60.(F) 0.043

2.004

0.004

0.057 0.55 1.67

might seem surprising since all the reported metallic structures [ 1-9] with 2,6-di(hydroxymethyl/ethyl)pyridine exhibit a tridentate ligand. The refined atomic parameters of the three structures are listed in Table 2. Labelled diagrams of the molecules are shown in Figs. 1-3. Bond distances and angles are shown in Table 3. Crystals 1 and 2 are isomorpbous. Crystal 3 is disordered on Tc and C(6), leading to two different conformations of the molecule (occupation factors of 0.83 and 0.17). This also implies that other atoms like the chloride ligand, O( 1) and 0 ( 2 ) are also disordered, but the pairs of atoms are too close to resolve the two different positions. In Fig. 3, the minor components Te' and C(6') are clg)

C(13)

4 728 1.823

52

0.04

0.048 0.60 1.64

shown, and the bonds have been drawn with dashes. The disorder will probably affect some of the bond d/stances in structure 3, even if the standard deviations are very reasonable. The structures are approximately octahedral, but there are important distortions partly due to the bidenmte ligands. The oxo ligand is in cis position to the C! or Br ligand. The chelate angles are 74.85(12), 81.40(13) ° for 1; 74.7(3), 80.8(3) ° for 2; 74.9(1), 82.1(!) ° for 3. The Tc--O bonds are 1.675(3), 1.666(6) and 1.645(3) ~ for 1, 2 and 3, respectively, similar to those reported for related octahedral oxo Te(V) compounds [15-18]. The bond distance Tc-C! is 2.385(2) ,~ for i and 2.432(2) A for 3, while Tc-Br is 2.524(3) A for 2. The Tc-N bond distances vary ~ t w e e n 2.194 (3) and 2.219(7) A for the two crystals containing di(hydroxymethyl)pyridine (1 and 2), while for crystal 3, the distances are 2.220(4) and 2.094(4)

C(H)

~ Nf2IF Cll2}

0(5)

c('o

0(4)

0(i)~C04 ) [ 0(4)

~

C161

~

-

(:13) Fig. !. Labeleddiagramof To(O )(OCH:PyCH.,OH),CI( 11.

(:(4)

CO)

Fig. 2. Labeleddiagramof Tc(O)(OCH,.i~CH,_OH)2Br(2).

306

F.D. Rachon et al. / Inorganica Chimica Acta 254 (1997) 303-307

Table 2 Positional parameters ( x 10a for 3) with their e.s.d.s and temperature

Table 2 (continued )

factors~

Te(O) (OCH2PyCHaOH)2CI ( 1) Atom x y

z

U~.q

Tc (O) (OCH2PyCH-OH) 2CI (1 )

Atom

x

y

z

B~,,

Tc CI O(l) 0(3) 0(4) 0(5) N(I) N(2) C(I) C(2) C(3) C14) C(5) C(6) C(7) C(8) C(9) C(10) C(ll) C(12)

0.09119~5) -0.27272(19) 0.081415) 0.185815) 0.0772(5) --0.2075(5) 0.6892(6) 0.118915) 0.420215) 0.2331t7) 0.2865(9) 0.2156(9) 0.0940(8) 0.0442(6) 0.3016(7) -0.0941~7) 0.4547(7) 0.6444(8) 0.8022(7) 0.7668(7) 0,5723(6)

0.39275(4) 0.20793117) 0.5384(4) 0.3094(4) 0.172514) 0.810515) 1.035714) 0.5904(4) 0.5t6014) 0.5860(6) 0.7093(7) 0.8424(8) 0.8447(7) 0.7181(6) 0.4369(6) 0.7161(6) 0.3751(5) 0.4132(6) 0.5965(7) 0.7419(6) 0.6980(5)

0.26460(2) 0.1889119) 0.3464(2) 0.168612) 0.3164(2) 0.187913) 0.3877(2) 0.165612) 0.3428(2) 0.t98513) 0.046113) 0.0422(4) 0.098113) 0.159413) 0.1186(3) 0.2205(3) 0.3743(3) 0.4315(3) 0.4582(3) 0.4274(3) 0.3711(3)

2.746115) 4,38(6) 3.27(15) 3.23(14) 3.74116) 4.81120) 4.34(18) 2.76115) 2.50115) 3.20121) 4.22(27) 4,80132) 4.07(25) 3.07119) 3.45(22) 3.61(23) 2.99(21) 3.61(25) 3.68(24) 3.37(21) 2.67(19)

C(13)

0.276118)

0.177416)

0.3427(3)

3.74(24)

C(14)

0.5250(7)

0.8536(6)

0.34t6(3)

3.62(24)

z

Btso

026654{5) 0.1873518) 0.3470(4) 0.171414) 0.316614) 0.1930(6) 0.3930(5) 0.169315) 0.3460(4) 0.1125(6) 0.0483(7)

2.52(3) 4.48(6)

0(2)

Tc(O)(OCH2PyCHaOH)2Br(2) Atom x y Tc Br O(I) 0(2) 0(3) N(I) N(2) C(I) C12) C(3)

0.08942(13) -0.29828(18) 0.0819(10) 0.183619) 0.0724110) -0.2049(11) 0.6993(11) 0.1172111) 0.4214(11) 0.2320(15) 0.2766117) 0.2082118)

0.39909(10) 0.20792115) 0.5456(8) 0.314918) 0.180718) 0.8184(10) 1.0327(8) 0.5946(10) 0.516419) 0.5874112) 0.7041(15) 0.8342(14)

C(4) C(5) C(6) C(7)

0.0827117) 0.0411(14) 0.3013(15) -0.0920115)

0.8412114) 0.7206(12) 0.4406(13)

0(4) 0(5)

0.0442(7) 0,0999(7) 0.]63016) 0.1234(6) 0.2249(7)

C(8) C(9) C(10) C(ll)

0.4523114) 0.6431115) 0.8013(15) 0.7697114)

0.7240113) 0.3756(tl) 0.3744(6) 0.4117113) 0.4308(6) 0.5912113) 0.4588(6) 0.7375112) 0.4309(6)

C(12) C(13) C(14)

0.5759(14) 0.2712(16) 0.5285(16)

0.6968111) 0.1817(12) 0.8528112)

0.3744(6) 0.341717) 0.3473(7)

3.114) 2.8(3) 3.4(4) 5.0(5) 4.014) 2.7(4) 2.4(4)

3.115) 4.0(6) 4.4(7) 4.2(6) 3.1(5) 3.1(5) 3.5(6) 2.6(5) 3.2(6) 3.2(6) 3.0(5) 2.4(4) 3.6(6) 3.3(6)

Tc(O) (OCH~.Py)=Ct (3)

Atom

x

y

Tc O(1) 0(2)

221311) 2709(I) 291713) t82613)

177(I) 1759(2; - 172214) 2654(4)

2195(I) 3626(I) 2401(3) 169312)

0(3) NIl)

868(3) 3661(3)

-329(4) 1289(5)

2603(2) 1629(3)

CI

U,,, 3211) 58(I) 5811) 49(I) 44(I) 49(1)

(continued)

N(2) C(I) C12) C13) C(4) C(5) C(6) C(8) C(9) C(10) C(II) C(12) C(13) Tc' C(6')

155113) 3656(4) 4552(5) 5456(4) 542114) 451714) 2579(5) 617(4) 53(4) 496(4) 148714) 1979(4) 199(4) 216412) 3992(9)

-90115) 3048(7) 3875(7) 2822(7) 984(7) 278(6) 3892(6) -161015) -2489(6) -2668(6) -191417) - 103617) -137416) 102613) - 130819)

967(3) 134613) 102914) 995(4) 126414) 1587(4) 136814) I076(3) 356(3) -47113) -586(3) 14413) 199813) 210212) 166918)

42(I) 6211) 7711) 70(I) 6411) 60(I) 6011) 3911) 5111) 5811) 6211) 5711) 5511) 2911) 45(I)

' Bl,, ( × !02) is the meanof the principalaxes of the thermalellipsoid. U~.q

(XlO~)=ll3E,E~U;p,*aj*a:aj. C(3) } C(2}

C14)

~ • Off}

CI5)

C(6')

C161 0121

C(

C1111 0(11 C1101 013} ell3)

CI91

Fig. 3. Labeled diagram of To(O)(OCH2Py).,CI(3), showing the major (bold lines) and the minor (dashed lines) components. A. The difference observed for crystal 3 is not real and is probably a result of the disorder observed for this crystal. The distances observed for crystals 1 and 2 are slightly longer than those reported for t r a n s - [ T c O 2 ( 4 - t - b u t y l p y r i d i n e ) 4 ] + (2.12917)-2.15817) ,~) [ 191, probably due to a slight strain inside the chelate rings of our compounds. The average bond distances T c - O (melhanolate) are 1.941 (3), 1.93916) and 1.92814) ,~, for 1, 2 and 3, respectively. For crystals I and 2 containing two hydroxymethyl groups on the pyridine ligand, one of the group is free and oriented away from the Tc atom. The two crystals are stabilized by intermolecular hydrogen bonds involving the free hydroxy groups. For crystal 1, 0 ( 4 ) is hydrogen bonded to 0 ( 3 ) with a distance of 2.933(4) ~ and an angle C 1 7 ) -

F.D. Rochon el al. I lnorganica Chimica Acta 254 (1997) 303-307

Table 3 Selectedbond distances CA) and angles (°) 1

2

3

2.524(3) 1.666(6)

2.432(2) 1.645(3)

1.935(6) 1.943(6)

1.963{3)

Tc-O(3) Tc-N( I ) Tc-N(2)

2.385(2) 1.675(3) 1.936(3) 1.946(3) 2.211(3) 2.194(3)

O(2)--C(6) O(3)-C(13)

1,385{5) 1.437{6)

O(4)-C(7)

1.413(5) 1.414(5)

Tc-X To-O( I ) Tc-O(2)

O(5~C(14)

X-Tc-O ( ! ) X-Tc-O(2) X-Tc--O(3) X-Tc-N( 1)

2.219(71 2.212(7) 1.382( 11) 1.440(12) 1.409(! I )

1.892(4) 2.220(4) 2.094(4) 1.409(7) 1.415(6)

t.424( I1 )

99.1 ( I ) 96.2( I ) 88,4( I ) 89.58(9)

99.3(2) 96,3(2) 88.0(2) 89.5(2)

99.1( I ) 96.2( I ) 88,4( 1) 89.58(9)

X-Tc-N(2) O( I)-Tc-O(2) O( I)-Tc--O(3) O( I)-Tc-N( I) O( I )-Tc-N(2) O(2)-Tc-O(3) O(2)-Tc-N(I) O(2)-Tc-N(2) O( 3)-Tc-N( I) O(3)-Tc-N(2)

169.74(9) 159,5( I) 108.0( I) 91.7( I) 84.6( I ) 85.9( 1) 74.9( I) 82.7( I) 160.3( l) 81.4( I)

168.8(2) 158.9(3) 108.2(3) 91.3(3)

169.74(9) 159.5( l) 108.0( I) 91.7( I)

N(I)-Tc-N(2) Tc-O(2)-C(6) Tc-O(3)-C(13)

99.9(1) 120.6(2) 113.0(21

84.4(3)

84.6( I )

86,3{3) 74.7(3) 83.0(3) 160.5(3) 80.8(3) 10l.I (3) 120.5(5) 113.0(5)

85.9( 1) 74.9( I) 82.7( I) 1603( 1) 82.1( I) 99.9(I) 120.6(2) 113.0(2)

O ( 4 ) . . "O(3) = 95.8(3) °, while 0 ( 5 ) is hydrogen bonded to the chloro ligand with a distance 0 ( 5 ) . - . C i = 3.367(4) A and angle C ( 1 4 ) - O ( 5 ) - - - C i = 9 3 . 7 ( 2 ) °. The atom 0 ( 5 ) is also close to 0 ( 3 ) with a distance of 3.050(5) A and angle C ( 1 4 ) - - O ( 5 ) - - . 0 ( 3 ) = 114.5(3) °. For the bromo analogue, 0 ( 4 ) is hydrogen bonded to 0 ( 3 ) with a distance of 2.913(9) A and angle C ( 7 ) - 0 ( 4 ) - - . O ( 3 ) = 97.8( 5)°, while 0 ( 5 ) is involved with either Br with O ( 5 ) , , . B r = 3.522(7) A and C( 1 4 ) - 0 ( 5 ) , - - B r = 9 2 . 4 ( 5 ) ° or with 0 ( 3 ) (distance O(5)...O(3) =3.001(I0) ,/~, C ( 1 4 ) - O ( 5 ) . . , O ( 3 ) = 116.8(5)°). In crystal 3, no hydrogen bonds are possible and the molecules are held together only by van der Waals forces.

4. Supplementary material

Lists of anisotropic temperature factors (Table S ! ), hydrogen coordinates (Table $2), bond distances and angles

307

(Table $3), and observed and calculated structure amplitudes (Table $4) are available from the authors on request. These have also been deposited at the Cambridge Crystallographic Data Centre, University Chemical Laboratory, Cambridge, UK.

Acknowledgements The authors are grateful to the Natural Sciences and Engineering Reseach Council of Canada and DuPont for financial support.

References

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