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The chemistry of technetium I, II, III and IV
Int. J. Appl. Radiat. Isot. Vol. 33, pp. 867 to 874, 1982
0020-708X/82/100867-08503.00/0 Copyright © 1982 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
The Chemistry of Technetium I, II, III and IV A L U N G. J O N E S a n d A L A N D A V I S O N Department of Radiology, Harvard Medical School and Brigham and Women's Hospital, Boston, MA 02115 and Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.
(Received 5 February 1982) The synthesis, reactivity and molecular structure of technetium in the oxidation states (I), (II), (III) and (IV) are reviewed. The results show that the (III) state has a diverse chemistry and is quite accessible from TcO~ in aqueous solution. There is an emerging chemistry of technetium in the (I) state with n-acceptor ligands.
Introduction IN THE ACCOMPANYINGarticle11) we have reviewed the progress that has been made in the emerging chemistry of technetium in the oxidation state (V). The purpose of this paper is to summarize aspects of the newer chemistry in the lower oxidation states (1) through (IV). The chemistry of technetium has developed fitfully over the last 20 yr. The recent acceleration of effort to elucidate the basic chemistry by several research groups, including our own, has been prompted mainly by the needs of the nuclear medicine community. It has been facilitated by a ready supply of macroscopic quantities of the long lived radionuclide 99Tc, a ,0-particle emitter (tl/2 = 2.13 x 10Syr) by-product of uranium fission. Consequently, a wide range of new coordination complexes has been synthesized in sufficient quantity to allow their isolation and subsequent characterization by chemical and physical methods, including elemental analysis, electronic and vibrational spectroscopy, X-ray crystallography, nuclear magnetic and electron spin resonance, mass spectrometry and other standard techniques. Despite the fact that the chemistry of the element is still poorly developed in comparison to its immediate neighbors Mo, Mn, Ru and Re, there is no shortage of review articles dealing with various aspects of its chemistry/2-29) From what is known it is to be expected that its chemistry will prove to be among the most diverse of the transition elements, certainly rivaling that of its second row neighbors Mo and Ru. Compounds have already been characterized in all oxidation states from ( - I ) d 1°, e.g. [Tc(CO)5]-, ~3°~ to (VII)d°, e.g. [TcO4]- and l-Tell9] 2-(al), in which the element displays coordination numbers which vary from 4 to 9. The most exciting recent development and perhaps the most relevant to nuclear medi867
cine is that a number of the lower valent complexes can be synthesized in high yields in aqueous media at both the carrier and no-carrier-added concentrations. Synthesis of transition metal compounds requires the subtle interplay of thermodynamic and kinetic considerations. Because of the paucity of such information for second and third row transition metals the chemist often has to rely heavily upon experience and the application of periodic correlations as a guide to the elucidation of the chemistry of these metals. The application of single crystal X-ray structure methods is rapidly providing a firm foundation for the development of the chemistry of technetium. This point was made quite succinctly in recent reviews by DEUTSCHJ23'24d9~ Since that time. approximately 17 crystal structures have been completed. Over half of these have dealt with technetium (V) complexes and have been reviewed separately. The additional X-ray structures include a low temperature determination of the hydrogen bonding in NH4TcO4, ts2~ the hexakis(isothiocyanato)technetate(III) ion,~33) hexakis(thiourea)technetium(III)chloridetrihydrate,~34) trans-dichlorobis(bisdimethylphosphinoethane) technetium(III)trifluoromethane sulfonate,~35Jthe metal-metal bonded species [NBu4]2[Tc2CIs(IIIXIII)],(36) Y[Tc2CIs(IIXIII)]"9H20,137) and
cis-dicarbonyltetrakis(diethylphenylphosphonite).tech_ netium(I)perchlorate,t3s) In addition to these, a large number of other isolated compounds have been characterized by spectroscopic methods. The work of Mazzi and his associates vide infra on various phosphines and phosphonite compounds in the oxidation states (I1, (II), (III) and (IV) is particularly noteworthy. It should be noted that occasional ambiguities with respect to oxidation state can occur despite the use of X-ray methods and that sometimes the choice of a suitable single crystal for analysis leads to the characterization of an impurity rather than the material representative of the bulk of the sample.
868
Alun G. Jones and Alan Davison The Reduction of P e r t e c h n e t a t e
With the exception of [TcO4]- and [TcOX4](X = CI, Br and I) the most commonly used starting materials for studying the macroscopic chemistry of technetium are the technetium(IV) compounds, ToO2 and salts of the hexahalometallates [TcX6] 2-. The brown-black insoluble TcO2"xH,O (x = 2) can be prepared by chemical or electrochemical reduction of [TcO4]- in aqueous solution.°9~ For preparative purposes, it is most conveniently prepared from either [NH4]2[TcCI~] or [NHa]2[TcBr6] by hydrolysis with aqueous ammonia: 2 NH~ + [TcX6] 2- + 4 NH3 + 4 H 2 0 = TcO2"2 H20~ + 6 N H ~ + 6X-. It can be separated by centrifugation and washed with water, with little or no loss of technetium. The octahedral, d 3. hexachlorometallates are obtained by heating a pertechnetate salt in the corresponding halo acid (HX): [TcO4]- + 9 H3 O+ q- 9 X- ~ [-TcX6] 2+ 13 H , O + 1.5 X2 The concentrated acid prevents hydrolysis to TcO2. It is interesting to note that the same ratio of reagents leads to the isolation of the oxotechnetium(V) complexes when the reaction is carried out at low temperatures: [TcO,,]- + 6 X- -* [TcOXa]- + 9 H 2 0 + X2. Thus, the interplay between kinetics and thermodynamics is quite evident in these reactions. The difference is that when the reaction occurs at the lower temperature the technetium(V) species becomes kinetically trapped and can be isolated. With extended reaction times or higher temperatures, further reduction to the thermodynamic product [TcX6] 2occurs. (40) When a hexahalometallato salt is used in synthesis, one of four things usually occurs: (i) hydrolysis to TcO2; (ii) the production of a technetium(IV) complex, i.e. no chan#e in oxidation state; (iii) the production of a complex in a lower oxidation state, i.e. reduction of the metal as well as complexation occurs; or (iv) the production of a complex in a higher oxidation state, i.e. adventitious oxidation (usually by air) of the metal occurs as well as coraplexation. Thus, in the absence of a full characterization of the products of a reaction, the assumption that a given reaction involves no change in oxidation state at the metal must be viewed with caution. The use of both [TcO4]- and [TcOX4]- with appropriate ligands and a reducing agent (the ligand itself can often function as a reducing agent) can also lead to Iow-valent complexes of technetium.
field,(3°.41-47) examples being [Tc(CsHs)2]2,(41"42: Tc(CsHsXCO)3, c43~ HTc(Co)5, t3°~ and various halocarbonyl monomers~3°~ and dimers.C44~ Several por. phyrin complexes of the type porphyrin-[Tc(CO)3]2 have been reported ~4a'49) and structurally characterized.(49) Most of these complexes can be considered as derivatives of Tc2(COho, a structurally-character. ized ~5°'5t) complex of technetium in the (O) oxidation state. An exception to this, however, is Ks[Tc(CN)6] which is X-ray isomorphouscs2~ to its Mn(I) and Re(II congeners. The first compound of teehnetium(II) to be reported (s3) was trans-diiodobis(orthophenylenebis. dimethylarsine)technetium(II) in 1959. Subsequently the analogous complexes with bis-l,2-diphenylphos. phinoethane (diphos) as a ligand were obtained.~54~ In recent years MAZZI et al. (39'55-65) have reported on a considerable number of six-coordinate compounds with various combinations of phosphine. halide and carbonyl ligands in oxidation states (I)--(IV). The six-coordinate cisand trans-dicarbonyltetrakis(diethylphenylphosphonite)technetium(I)perchlorates(65~ (the cis compound was characterized by X-ray analysis)~65) (Fig. 1) and the structurally-characterized trans-dichlorotetrakis(diethylphenylphosphonite)technetium(ii)(57)(Fig. 2) were prepared by the slow reduction of the trans-dihalotetrakis(diethylphenylphosphonite)technetium(III) cations(sT'6s~ with excess ligand. The synthesis of the carbonyl complexes required the presence of one atmosphere of carbon monoxide. Under similar conditionst55~ and beginning with trans-[Tc(IV)XaL2] (X = CI, Br; L = P(C6Hs)a or P(CHa),(C6H~) ) only the trans-[Tc(I)X(CO)3L2] are formed. If the starting material used is the crystallographically-characterized mer-[Tc(III)X3(P(CHa)2(C6Hs))3 ] (X = C1, Br), mixtures of trans-[Tc(I)X(CO)3(P(CH3),C6Hs),] and cis-[Tc(I)X(CO)2(P(CH3)2C6Hs)3] are produced. The
7
Complexes of Technetium in Oxidation States (I) and (II) Until quite recently technetium(I) complexes were known to exist mainly in the organometallic
FIG. 1. An ORTEP ofcis-[Tc(IXCO)2(P(OC2Hs)2C6Hs)4]+
Chemistry of technetium I, 11, III and I V
869
a)
0
04
PS LI
JIL~ 06
dpph
FIG. 4. EPR spectrum of [N(C4Hg),]ETc(NO)Br,,] in ethanol 28°C.
FIG. 2. An ORTEP oftrans-[Tc(ll)Cl2(P(OC2Hsl2C6Hs)4].
significance of these results is that they show that there is a rich and as yet largely unexplored chemistry with n-acceptor ligands. Low valent complexes of technetium are expected to be low spin and to be most stable when complexed with ligands that can accept n-electron density from the metal, e.g. phosphines, arsines, carbon monoxide, etc. The complex first isolated by Eakins et al. in 1963 and formulated as a technetium(II) compound "['Tc(NH2OH)z(NH3)3]CI2" appeared t66) to be an exception to this generalization. It was synthesized from [TcCI6] 2- and hydroxylamine. An investigation by ARMSTRONG and TAUBE(67) showed that it was trans-[Tc(NHa)4(NO)(H20)]Cl2, a six-coordinate nitrosyl technetium(I) species. A subsequent X-ray analysis by J. L. HOARD at Cornell University(6a) has verified their proposition (Fig. 3). (The assignment of the oxidation number (I), i.e. spin-paired d 6 results from the observed diamagnetism, the nitric oxide must be considered as bonding as an NO + ligand.) YANGet al. have made several reports (69-72) of
[ t-Tc(NO)(NH3)4(HzO) ] CIz o,
N~ ~
f ' ~ N3
what they consider to be paramagnetic technetium(II) nitrosyls that have been detected by electron spin resonance. Work in our laboratories, performed concurrently,(Ta~ has led to the isolation of several nitrosyltechnetium complexes in the (I) and (II) states. The reaction of TcO2"xH20 with NO gas at 75°C in 4 N HBr gave a blood-red solution from which tetraalkylammonium salts of the [Tc(NO)Br4]- ion can be isolated. The chloro analog was prepared by simple halide substitution on this material. Both complexes as tetrabutylammonium salts show molar conductances in CH3CN consistent with their formulations as 1: 1 electrolytes. The complexes [Tc(NO)X4]- exhibit 10-line electron-spin-resonance spectra at room temperature in methanol solution (e.g. (g) = 2.0795 _+ 0.0015, ( a ) = 133.8 + 0.6G (X = Br)) consistent with their formulations as low spin d 5 technetium(II) complexes. The 10-line pattern (Fig. 4) is the result of electron-technetium nuclear hyperfine interaction (99Tc, I = 9/2). The complexes [Tc(NOXNCS)s] 2- and [Tc(NOXNCS)5] 3- can be synthesized from [Tc(NO)Br4]- and NH4NCS in methanol. The technetium(I) species is formed rapidly by the reduction of the inky-blue solution of [Tc(NO)(NCS)s] 2- with hydrazine. Electrochemical studies in acetonitrile on both of the isolated tetrabutylammonium salts show that a simple reversible one-electron couple (E½ = + 0.14V vs SCE) exists between the technetium(I) and (II) species: [Tc(IIXNOXNCS)5] 2- + e- = [Tc(I)(NOXNCS)s] 3-
N~
"re ow
FIG. 3. An ORTEP of trans-[Tc(NOXNHa)4H20"I "+.
In exploring the utility of hexakisthioureatechnetium(III) salts as precursors for other new reduced technetium species vide infra, we have been able to synthesize a variety of hexakisalkylisonitrile-technetium(I) salts, to°) The reaction of the red thiourea complex with t-butylisonitrile gave a clear solution from which the white [(t-C4H9NC)6Tc(I)][PF6] was obtained in 60~ yield. Aqueous solutions of the complexes are very stable both to substitution and oxidation. The salts have been characterized by field desorption mass spectrometry (Fig. 5) and vibrational
870
Alun G. Jones and Alan Davison CL
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FIG. 7. An ORTEP of mer-ETc(III)Cl3(PMe,Phh] (PMe2Ph = dimethylphenylphosphine).
FIG. 5. Field desorption mass spectrum (FDMS) of [(t-C,,H9NC)t,Tc(I)]I-PFt,] showing the cation at m/z = 597. (III) cation,tas) These cationic complexes can b~ prepared from 1-99mTcO4] at the no-carrier-addec levelst35'76~ and have yielded excellent )'-ray images 0 spectroscopy. The complexes are kinetically inert, diathe normal myocardium in dogs. Complexes contain magnetic and iso-structural to the known Mn and Re ingAhe bidentate phosphine and arsine ligands cat analogs.ta ~ also be prepared from [TcX6] 2- in ethanolic aqueou: HX containing excess ligand. This is a case in whict Complexes of Technetium in the resulting complex is formed by reduction presum the Oxidation States (III) ably by the excess ligand. and (IV) The work of MAZZI et al. c77) shows that the natur~ It is ironic that many of the first coordination com- of the phosphines as well as the conditions 0 plexes of technetium to be prepared three decades ago the reaction are important in the nature of th~ have only recently been shown to contain technetium resulting technetium species. With triphenylphos in the (III) state. The first complex that was isolated phine and NH4TcO4 in aqueous ethanolic HC only trans- (TcCI4(P(C6Hs)3)2], a neutral techne. in 1959, namely the trans-dichlorobis(orthophenylene_bisdimethylarsine)technetium(III)chloride,~53~ has tium(IV) complex, is obtained. With dimethylphenyl been structurally characterized by DEUTSCH et phosphine the green technetium(IV) complex trans a/. ~74'7s) (Fig. 6) as well as the analogous trans- [Tc(IV)CI,,(P(CH3)2C6Hs)2] is produced in th~ dichlorobis(1,2-dimethylphosphinoethane)technetium- cold with a 5:1 molar ratio of the ligand to metal but the yellow-orange technetium(III) complex mer [Tc(III)Cla(P(CHa)2C~Hs)a] (Fig. 7) is produced ol reflux with a 15:l molar ratio. This is ye another example of the interplay of kinetic ant thermodynamic factors in a reaction. The mer [Tc(III)Cla(P(CH3hC6Hs)a] is rapidly oxidized tt trans-[Tc(IV)Cl4(P(CHa)2C~Hs)2] by refluxing il CC1,. We have already noted that trans-[TcX,L2] (X = C1, Br; L = P(C6Hs)3 and P(CH3)2C~Hs) giv the technetium(I) complexes trans-[Tc(I)X(CO)aL2 in boiling ethyleneglycolmethylether under on atmosphere of carbon monoxides while the met ETc(III)X3(P(CH3)2C6Hsh] (X = C1, Br) gives mix tures of trans-[Tc(I)X(CO)a(P(CH3hC6Hsh] and ei~ [Tc(I)X(CO)2(P(CH3),C~Hs)3] under the sarffe con ditions.~55)In refluxing ethanol, however, under CO al one atmosphere mer-[Tc(III)CI3(P(CH3hC6Hs)3] t~l forms a seven-coordinate adduct carbonyltrichlorc t ris.(dimethylphenylphosphine)technetium(iii)/62~Tb FIG. 6. An ORTEP of trans-[Te(III)Cl2(diars)2]. coordination polyhedron of this complex was show (diars = orthophenylenebisdimethylarsine).
Chemistry ~f technetium I. II. III and I V
~P2
871
PI
Cl.
FIG. 8. An ORTEP of [Tc(III)CI3CO(PMe2Phl3]. by an X-ray structure determination to be a distorted face-capped octahedron (Fig. 8). Further studies by the Italian group using technetium(IV) complexes as starting materials demonstrate again tag~ that the nature and the oxidation state of the product depend markedly on the reaction conditions. The reaction of [P(C6Hs)4]2I-TcX6] or trans-[TcX4(P(C6Hs)a)2] with various ratios of acetylacetone (2,4-pentanedione) led to the isolation of a variety of compounds in both the (III) and (IV) oxidation states: [P(C6Hs)4][Tc(IV)X4(acac)], [Tc(IV)Bra(acac)P(C6Hs)3], [Tc(IV)X2(acac)2], [Tc(IIIXacac)3], [Tc(III)X2(acacXP(C6Hs)3)z], and I-Tc(III)X(acac)2P(C6Hs)a], (X = C1, Br). Two crystalline modifications of [Tc(lII)Cl(acac)2P(C6Hs)~] have been characterized by X-ray methods/6a.64~ Both complexes have the same molecular structure with trans-acetylacetone chelate rings (Fig. 9). Among the first ligands used for the colorometric determination of technetium were thiocyanate and thiourea, tTa~ The nature of the species present when acid solutions of pertechnetate were treated with an excess of these ligands has only recently been elucidated by X-ray studies. The thiocyanate system has been unequivocally shown ~aa~ to consist of a reversible redox couple between the purple ~I'c(IVXNCS)6] 2-(,;.m,~ = 500 nm) and the yellow ISI'c(IIIXNCS)6]3-(Am,~ = 4 0 0 n m ) with E~ = + 0.18 V vs SCE in CHaCN. Solutions of the latter complex have yielded crystalline salts with large cations. An X-ray crystal structure of the tetrabutylammonium salt I'N(C4Hg),~]3[Tc(IIIXNCS)6] showed it to contain six equivalent octahedral and virtually linear N-bonded or isothiocyanate ligands (Fig. 10). The significant feature of this determination shows that the radius of Tc(III) is the same as that found {79~ for Fe(III) in [Fe(III)(NCS)6] a(Tc-N -- 204 pm, and 205 pm, Fe-N = 203-206 pro)
FIG. 9. An ORTEP of trans-[Te(llIXacac)2Cl(PPh3)2]. (PPh3 = triphenylphosphine).
implying that, structurally, technetium(III) is capable of closely mimicking iron in its chemistry. If convenient ways could be found to introduce technetium(III) it might substitute for Fe(III) in biological systems. The red solutions ~6) that result by the treatment of [TcO4]- with thiourea in a variety of acids have been shown to contain the hexakisthioureatechnetium(III) ion. ts°~ An X-ray structure determination of [Tc(III)(tu)6]Cla.3H20 shows that the six thiourea molecules are S-bonded in an octahedral arrangemerit, c~4) This complex can also be synthesized from [TcOCI4]- and thiourea in ethanolic HCI. Initial studies show that the thiourea ligands can be displaced e.g. forming trans-[Tc(III)C12(diphos)4], and [Tc(NCS)6] 3-. It has also been used to synthesizeIs°~ a variety of hexakisalkylisonitriletechnetium(I) cations [(RNC)6Tc(I)] + by treatment with an excess of the isonitrile, where the excess ligand possibly functions as the reducing agent.
S11~"
55
N4 143
S
FIG. 10. An ORTEP of I'Tc(III)(NCS)6]~-.
872
Alun G. Jones and Alan Darison
CL8
theoretical study. A qualitative explanation of the probable cause has been proposed (36.a7) by COTTON
CL5 CL7i
~
C L 6 ~ ) ~ C L 4
CL2 CL3 FIG. I1. The eclipsed Tc2CIs structure found for both [Tc2CIs] 2- and [Tc2CIs] 3-.
The reaction of ammonium hexaiodotechnetate(IV) with potassium cyanide in refluxing aqueous methanol, under nitrogen, gives potassium heptacyanotechnetate(llI)dihydrate K4Tc(CN)7'2H20. Infrared and Raman spectroscopy indicate that it has a pentagonal-bipyramidal structure (Dsh) in both the solid and solution,is2) It is presumably isostructural with the known rhenium complex,tsa) The isolation of the Tc(III) cyano-complex in this reaction is a reflection of the reducing power of the displaced ligand (I-) under the reaction conditions. Aqueous solutions of [Tc(III)(CN)7]4- are air sensitive, decomposing to the technetium(V) species [Tc(V)O(CN)~] 2- which in turn can be hydrolyzedts2~ to trans-dioxotetracyanotechnetate(V), [Tc(V)O2(CN)4] 3-. A number of structural studies have appeared on a variety of the octahedral hexahalometallate species,~s*-94) including one on K2Tc(OH)CI 5 which ~95) is isomorphous with K2TcCI6, that are unexceptional. The anhydrous halide (TcCI4), has been structurally characterized. It comprises chains of edge-shared octahedral TcCI 6 units. There are two unshared cis-Cl atoms in each of the distorted octahedra. There are a number of technetium metal-metal bonded species that have been structurally characterized: [ N H 4 ] 3[-Tc2Cls] " n 2 0 , (96) Ka [ Tc2CIs ] " H20,197) [Y(H20)s][Tc2CIs]'H20, (371 (Fig. 11) [N(C4H9),t]2[Tc2CIs], (36) Tc2CI2(OCCMe3),,, tgs) [Tc 2(OCsH4N),,]CI. t99~ In all of the structures there is a short Tc-Tc bond, and eight of the tigands have the eclipsed MX4-MX4 structure first found (too.~ot) in [Re2Cla] 2-, indicative of strong metal-metal multiple bonding (quadruple bonding) tr2rt't62 for the (III,III) species or 0"27t4626"1 for the mixed oxidation state (II,III) compounds. These compounds are proving to be of fundamental importance in elucidating the fine details of multiple metal-metal bonding interactions. The fact that the Tc-Tc bond lengths (211.7,~2~ 210.5 pm)"~ in three structurally characterized salts of the mixed valent (III,II) anion [Tc2Cls] 3- (formal bond-order 3.5 a2n'*di26*~) are significantly shorter than that found in [N(C4Hg),t]2[Tc2Cls] (214.7pm) (III,III; bond order 4.0, a2n't62) warrants a further
et al.
There are a number of technetium complexes with EDTA as a ligand. The complexes are often difficult to obtain reproducibly and to isolate pure in crystalline form. However, one of the complexes recently characterized "°2) (H2EDTA)Tc(IVX/t-O)2Tc(IVXH2EDTA)-5H20 has been shown to contain a four membered Tc(/~-O)2 Tc ring with a short Tc-Tc distance of 233 pm. An extended Hiickel molecular orbital calculation suggests that this may represent a formal Tc-Tc bond (a2n26*2) rather than the expected triple bond (a2n262). Each EDTA coordinates to one technetium via two nitrogen and two oxygen atoms. The remaining two carboxylates in each EDTA ligand are protonated and non-coordinating.
Conclusions The molecular nature of the technetium species in already existing radiopharmaceuticals is not known with certainty, and it has been suggested that some of them contain the element in either the (IV) or the (lid state. For example, the family of hepato-biliary agents discovered by Loberg has been proposed, "°3~ on the basis of electrophoretic studies, to be a series of sixcoordinate bis-ligand technetium(Ill) complexes. Also, the polarographic work of RUSSELLet al. tI°4~ has indicated the presence of technetium(III), (IV) and (V) species in preparations of the bone-seeking agents. Although neither this review nor the accompanying article on technetium(V)(1) have addressed such problems, it is hoped that both have given an indication of the complexity and versatility of the chemistry and thereby provided a stimulus for further basic studies. In turn these would then give a firmer basis for the design of new diagnostic agents. Acknowledgements--We would like to thank Drs
E. DEUrSCH, U. M^ZZl, and C. J. L. LOCKand their research groups for providing us with structural information and Dr C. E. COSTELLOfor the mass spectral studies. Support from the National Institutes of Health is gratefully acknowledged.
References I. DAVtSONA. and JONESA. G. Int. J. Appl. Radiat. Isot. 33, 875 (1982). 2. BoYD G. E. J. Chem. Educ. 36, 3 (1959). 3. TRIBL^TS S. In Nouveau Traite de Chimie Minerale (Ed. PASCALP.) Vol. 16, pp. 1081. (Masson, Paris, 1960). 4. ENDrRSE. Ann. Rec. Nucl. Sci. 9, 203 (1959). 5. SCHWOCHAUK. Angew. Chem. 76, 9 (1964). 6. COBBLEJ. W. In Treatise on Analytical Chemistry (Eds KOLTHO~ I. M. and ELVI~GP. J.) Part 2, Vol. 6, pp. 407. (Interscience, New York, 1964). 7. COLTONR. The Chemistry of Rhenium and Technetium (Interscience, London, 1965). 8. PEACOCK R. D. The Chemistry of Technetium and Rhenium (Elsevier. Amsterdam. 1966).
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77. MAZZl U., DE PAOLI G., DIBERNARDO P. and MAC,ON L. J. lnorg. Nucl. Chem. 38, 721 (1976). 78. LAVRUKHINAA. K. and POZDNYAKOV A. A IN Analytical Chemistry of Technetium Promethium Astatine and Francium, p.I (Ann-Arbor-Humphrey Scientific, Ann-Arbor, 1970). 79. MULLER V. V. Acta Crystallogr. B33, 2197 (1977). 80. ABRAMS M. J., DAVISON A. and JONES A. G. (unpublished results). 81. COTTON F. A. and ZINGALES F. J. AnL Chron. Soc. 83, 341 (1961); SACCO A. Ann. Chim. (Italy) 48, 225 (1958); FRENI M. Gazz, Chim. Ital. 91, 1352 (1961). 82. TROP. H. S., JONES A. G. and DAVlSON A. lnorg. Chem. 19, 1993 (1980). 83. MANOLI J. M., POTVIN C., BREGEAULTJ. M. and GRIFFITHS W. P. J. Chem. Soc. Dalton 192 (1980). 84. EDWARDS A. J., HUGILL D. and PEACOCK R. D. Nature 200, 672 (1963). 85. Peacock R. D. J. Chem. Soc. 1291 0956). 86. COWIE M., LOCK C. J. L. and OZOG J. Can. J. Chem. 48, 3760 (1970). 87. SCHWOCHAU K. Z. Naturforsch. 19a, 1237 (1964). 88. DALZIEL J., GILL N. S., NYHOLM R. S. and PEACOCK R. D. J. Chem. Soc. 4012 (1958). 89. ZAITSEVAL. L., KONAREV M. I., KOZHEVNIKOV P. B., VINOGRADOV I. V., KRUGLOW A. A. and CHEBOTAREV N. T. Zh. Neorg. Khim. 17, 2411 (1972). 90. KUZINA A. F'., KOZ'MIN P. A. and KOVlTSKAYAG. N. Zh. Neor.o. Khim. 18, 841 (1973). 91. ZAITSEVA L. L., KONAREV M. 1., IL'YASHENKO V. S., VINOGRADOV I. V., SHEPEL'KOV S. V., KRUGLOV A. A.,
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and CHEBORATEV N. T. Zh. Neorg. Khim. 18, 241( (1973). ZAITSEVA L. L., KONAREV M. I., VINOORADOV I. V. KOZHINOV E. G., KROUGLOV A. A. and CHEBOTAREX N. T. Zh. Neorg. Khim. 19, 976 (1974). SHEPEL'KOVS. V., KONAREV M. I., CHEaOTAREV N. T. ZAITSEVA L. L. and VINOGRAOOV I. V. Zh. Neory Khim. 20, 3310 (1975). ELDER R. C., ESTESG. W. and DEUTSCH E. Acta Crys. talloyr. B35, 135 (1979). ELDER M., FERGUSSONJ. E., GAINSFORD G. J., HICK. FORD J. H. and PENFOLD B. R. J. Chem. Soc. A 142 ~' (1967). BRATTON W. K. and COTTON F. A. lnor~l. ChenL 9. 789 (1970). COTTON F. A. and Salve L. W. lnor~l. Chem. 14, 203,~ (1975). COTTON F. A. and GAGE L. D. Nouv. J. ChinL !, 441 (1977). COTTON F. A., FANWlCK P. E. and GAGE L. D. J. Am. Chem. Soc. i02, 1570 (1980). COTTON F. A. and HARRIS C. B. lnorg. Chem. 4, 33( (1965). COTTON F. A., FRENZ B. A., STULTS B. R. and WEaL T. R. J. Am. Chem. Soc. 98, 2768 (1976). BURGI H. B., ANDEREGG G. and BLAUENSTEIN P Inory. Chem. 20, 3829 (1981). LOBERG M. D. and FIELDS A. T. Int. J. Appl. Radiat Isot. 29, 167 (1978). RUSSELLC. D. and CASH A. G. J. Nucl. Med. 20, 53~ (1979).
0020-708X/82/100867-08503.00/0 Copyright © 1982 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
The Chemistry of Technetium I, II, III and IV A L U N G. J O N E S a n d A L A N D A V I S O N Department of Radiology, Harvard Medical School and Brigham and Women's Hospital, Boston, MA 02115 and Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.
(Received 5 February 1982) The synthesis, reactivity and molecular structure of technetium in the oxidation states (I), (II), (III) and (IV) are reviewed. The results show that the (III) state has a diverse chemistry and is quite accessible from TcO~ in aqueous solution. There is an emerging chemistry of technetium in the (I) state with n-acceptor ligands.
Introduction IN THE ACCOMPANYINGarticle11) we have reviewed the progress that has been made in the emerging chemistry of technetium in the oxidation state (V). The purpose of this paper is to summarize aspects of the newer chemistry in the lower oxidation states (1) through (IV). The chemistry of technetium has developed fitfully over the last 20 yr. The recent acceleration of effort to elucidate the basic chemistry by several research groups, including our own, has been prompted mainly by the needs of the nuclear medicine community. It has been facilitated by a ready supply of macroscopic quantities of the long lived radionuclide 99Tc, a ,0-particle emitter (tl/2 = 2.13 x 10Syr) by-product of uranium fission. Consequently, a wide range of new coordination complexes has been synthesized in sufficient quantity to allow their isolation and subsequent characterization by chemical and physical methods, including elemental analysis, electronic and vibrational spectroscopy, X-ray crystallography, nuclear magnetic and electron spin resonance, mass spectrometry and other standard techniques. Despite the fact that the chemistry of the element is still poorly developed in comparison to its immediate neighbors Mo, Mn, Ru and Re, there is no shortage of review articles dealing with various aspects of its chemistry/2-29) From what is known it is to be expected that its chemistry will prove to be among the most diverse of the transition elements, certainly rivaling that of its second row neighbors Mo and Ru. Compounds have already been characterized in all oxidation states from ( - I ) d 1°, e.g. [Tc(CO)5]-, ~3°~ to (VII)d°, e.g. [TcO4]- and l-Tell9] 2-(al), in which the element displays coordination numbers which vary from 4 to 9. The most exciting recent development and perhaps the most relevant to nuclear medi867
cine is that a number of the lower valent complexes can be synthesized in high yields in aqueous media at both the carrier and no-carrier-added concentrations. Synthesis of transition metal compounds requires the subtle interplay of thermodynamic and kinetic considerations. Because of the paucity of such information for second and third row transition metals the chemist often has to rely heavily upon experience and the application of periodic correlations as a guide to the elucidation of the chemistry of these metals. The application of single crystal X-ray structure methods is rapidly providing a firm foundation for the development of the chemistry of technetium. This point was made quite succinctly in recent reviews by DEUTSCHJ23'24d9~ Since that time. approximately 17 crystal structures have been completed. Over half of these have dealt with technetium (V) complexes and have been reviewed separately. The additional X-ray structures include a low temperature determination of the hydrogen bonding in NH4TcO4, ts2~ the hexakis(isothiocyanato)technetate(III) ion,~33) hexakis(thiourea)technetium(III)chloridetrihydrate,~34) trans-dichlorobis(bisdimethylphosphinoethane) technetium(III)trifluoromethane sulfonate,~35Jthe metal-metal bonded species [NBu4]2[Tc2CIs(IIIXIII)],(36) Y[Tc2CIs(IIXIII)]"9H20,137) and
cis-dicarbonyltetrakis(diethylphenylphosphonite).tech_ netium(I)perchlorate,t3s) In addition to these, a large number of other isolated compounds have been characterized by spectroscopic methods. The work of Mazzi and his associates vide infra on various phosphines and phosphonite compounds in the oxidation states (I1, (II), (III) and (IV) is particularly noteworthy. It should be noted that occasional ambiguities with respect to oxidation state can occur despite the use of X-ray methods and that sometimes the choice of a suitable single crystal for analysis leads to the characterization of an impurity rather than the material representative of the bulk of the sample.
868
Alun G. Jones and Alan Davison The Reduction of P e r t e c h n e t a t e
With the exception of [TcO4]- and [TcOX4](X = CI, Br and I) the most commonly used starting materials for studying the macroscopic chemistry of technetium are the technetium(IV) compounds, ToO2 and salts of the hexahalometallates [TcX6] 2-. The brown-black insoluble TcO2"xH,O (x = 2) can be prepared by chemical or electrochemical reduction of [TcO4]- in aqueous solution.°9~ For preparative purposes, it is most conveniently prepared from either [NH4]2[TcCI~] or [NHa]2[TcBr6] by hydrolysis with aqueous ammonia: 2 NH~ + [TcX6] 2- + 4 NH3 + 4 H 2 0 = TcO2"2 H20~ + 6 N H ~ + 6X-. It can be separated by centrifugation and washed with water, with little or no loss of technetium. The octahedral, d 3. hexachlorometallates are obtained by heating a pertechnetate salt in the corresponding halo acid (HX): [TcO4]- + 9 H3 O+ q- 9 X- ~ [-TcX6] 2+ 13 H , O + 1.5 X2 The concentrated acid prevents hydrolysis to TcO2. It is interesting to note that the same ratio of reagents leads to the isolation of the oxotechnetium(V) complexes when the reaction is carried out at low temperatures: [TcO,,]- + 6 X- -* [TcOXa]- + 9 H 2 0 + X2. Thus, the interplay between kinetics and thermodynamics is quite evident in these reactions. The difference is that when the reaction occurs at the lower temperature the technetium(V) species becomes kinetically trapped and can be isolated. With extended reaction times or higher temperatures, further reduction to the thermodynamic product [TcX6] 2occurs. (40) When a hexahalometallato salt is used in synthesis, one of four things usually occurs: (i) hydrolysis to TcO2; (ii) the production of a technetium(IV) complex, i.e. no chan#e in oxidation state; (iii) the production of a complex in a lower oxidation state, i.e. reduction of the metal as well as complexation occurs; or (iv) the production of a complex in a higher oxidation state, i.e. adventitious oxidation (usually by air) of the metal occurs as well as coraplexation. Thus, in the absence of a full characterization of the products of a reaction, the assumption that a given reaction involves no change in oxidation state at the metal must be viewed with caution. The use of both [TcO4]- and [TcOX4]- with appropriate ligands and a reducing agent (the ligand itself can often function as a reducing agent) can also lead to Iow-valent complexes of technetium.
field,(3°.41-47) examples being [Tc(CsHs)2]2,(41"42: Tc(CsHsXCO)3, c43~ HTc(Co)5, t3°~ and various halocarbonyl monomers~3°~ and dimers.C44~ Several por. phyrin complexes of the type porphyrin-[Tc(CO)3]2 have been reported ~4a'49) and structurally characterized.(49) Most of these complexes can be considered as derivatives of Tc2(COho, a structurally-character. ized ~5°'5t) complex of technetium in the (O) oxidation state. An exception to this, however, is Ks[Tc(CN)6] which is X-ray isomorphouscs2~ to its Mn(I) and Re(II congeners. The first compound of teehnetium(II) to be reported (s3) was trans-diiodobis(orthophenylenebis. dimethylarsine)technetium(II) in 1959. Subsequently the analogous complexes with bis-l,2-diphenylphos. phinoethane (diphos) as a ligand were obtained.~54~ In recent years MAZZI et al. (39'55-65) have reported on a considerable number of six-coordinate compounds with various combinations of phosphine. halide and carbonyl ligands in oxidation states (I)--(IV). The six-coordinate cisand trans-dicarbonyltetrakis(diethylphenylphosphonite)technetium(I)perchlorates(65~ (the cis compound was characterized by X-ray analysis)~65) (Fig. 1) and the structurally-characterized trans-dichlorotetrakis(diethylphenylphosphonite)technetium(ii)(57)(Fig. 2) were prepared by the slow reduction of the trans-dihalotetrakis(diethylphenylphosphonite)technetium(III) cations(sT'6s~ with excess ligand. The synthesis of the carbonyl complexes required the presence of one atmosphere of carbon monoxide. Under similar conditionst55~ and beginning with trans-[Tc(IV)XaL2] (X = CI, Br; L = P(C6Hs)a or P(CHa),(C6H~) ) only the trans-[Tc(I)X(CO)3L2] are formed. If the starting material used is the crystallographically-characterized mer-[Tc(III)X3(P(CHa)2(C6Hs))3 ] (X = C1, Br), mixtures of trans-[Tc(I)X(CO)3(P(CH3),C6Hs),] and cis-[Tc(I)X(CO)2(P(CH3)2C6Hs)3] are produced. The
7
Complexes of Technetium in Oxidation States (I) and (II) Until quite recently technetium(I) complexes were known to exist mainly in the organometallic
FIG. 1. An ORTEP ofcis-[Tc(IXCO)2(P(OC2Hs)2C6Hs)4]+
Chemistry of technetium I, 11, III and I V
869
a)
0
04
PS LI
JIL~ 06
dpph
FIG. 4. EPR spectrum of [N(C4Hg),]ETc(NO)Br,,] in ethanol 28°C.
FIG. 2. An ORTEP oftrans-[Tc(ll)Cl2(P(OC2Hsl2C6Hs)4].
significance of these results is that they show that there is a rich and as yet largely unexplored chemistry with n-acceptor ligands. Low valent complexes of technetium are expected to be low spin and to be most stable when complexed with ligands that can accept n-electron density from the metal, e.g. phosphines, arsines, carbon monoxide, etc. The complex first isolated by Eakins et al. in 1963 and formulated as a technetium(II) compound "['Tc(NH2OH)z(NH3)3]CI2" appeared t66) to be an exception to this generalization. It was synthesized from [TcCI6] 2- and hydroxylamine. An investigation by ARMSTRONG and TAUBE(67) showed that it was trans-[Tc(NHa)4(NO)(H20)]Cl2, a six-coordinate nitrosyl technetium(I) species. A subsequent X-ray analysis by J. L. HOARD at Cornell University(6a) has verified their proposition (Fig. 3). (The assignment of the oxidation number (I), i.e. spin-paired d 6 results from the observed diamagnetism, the nitric oxide must be considered as bonding as an NO + ligand.) YANGet al. have made several reports (69-72) of
[ t-Tc(NO)(NH3)4(HzO) ] CIz o,
N~ ~
f ' ~ N3
what they consider to be paramagnetic technetium(II) nitrosyls that have been detected by electron spin resonance. Work in our laboratories, performed concurrently,(Ta~ has led to the isolation of several nitrosyltechnetium complexes in the (I) and (II) states. The reaction of TcO2"xH20 with NO gas at 75°C in 4 N HBr gave a blood-red solution from which tetraalkylammonium salts of the [Tc(NO)Br4]- ion can be isolated. The chloro analog was prepared by simple halide substitution on this material. Both complexes as tetrabutylammonium salts show molar conductances in CH3CN consistent with their formulations as 1: 1 electrolytes. The complexes [Tc(NO)X4]- exhibit 10-line electron-spin-resonance spectra at room temperature in methanol solution (e.g. (g) = 2.0795 _+ 0.0015, ( a ) = 133.8 + 0.6G (X = Br)) consistent with their formulations as low spin d 5 technetium(II) complexes. The 10-line pattern (Fig. 4) is the result of electron-technetium nuclear hyperfine interaction (99Tc, I = 9/2). The complexes [Tc(NOXNCS)s] 2- and [Tc(NOXNCS)5] 3- can be synthesized from [Tc(NO)Br4]- and NH4NCS in methanol. The technetium(I) species is formed rapidly by the reduction of the inky-blue solution of [Tc(NO)(NCS)s] 2- with hydrazine. Electrochemical studies in acetonitrile on both of the isolated tetrabutylammonium salts show that a simple reversible one-electron couple (E½ = + 0.14V vs SCE) exists between the technetium(I) and (II) species: [Tc(IIXNOXNCS)5] 2- + e- = [Tc(I)(NOXNCS)s] 3-
N~
"re ow
FIG. 3. An ORTEP of trans-[Tc(NOXNHa)4H20"I "+.
In exploring the utility of hexakisthioureatechnetium(III) salts as precursors for other new reduced technetium species vide infra, we have been able to synthesize a variety of hexakisalkylisonitrile-technetium(I) salts, to°) The reaction of the red thiourea complex with t-butylisonitrile gave a clear solution from which the white [(t-C4H9NC)6Tc(I)][PF6] was obtained in 60~ yield. Aqueous solutions of the complexes are very stable both to substitution and oxidation. The salts have been characterized by field desorption mass spectrometry (Fig. 5) and vibrational
870
Alun G. Jones and Alan Davison CL
L 597
FIG. 7. An ORTEP of mer-ETc(III)Cl3(PMe,Phh] (PMe2Ph = dimethylphenylphosphine).
FIG. 5. Field desorption mass spectrum (FDMS) of [(t-C,,H9NC)t,Tc(I)]I-PFt,] showing the cation at m/z = 597. (III) cation,tas) These cationic complexes can b~ prepared from 1-99mTcO4] at the no-carrier-addec levelst35'76~ and have yielded excellent )'-ray images 0 spectroscopy. The complexes are kinetically inert, diathe normal myocardium in dogs. Complexes contain magnetic and iso-structural to the known Mn and Re ingAhe bidentate phosphine and arsine ligands cat analogs.ta ~ also be prepared from [TcX6] 2- in ethanolic aqueou: HX containing excess ligand. This is a case in whict Complexes of Technetium in the resulting complex is formed by reduction presum the Oxidation States (III) ably by the excess ligand. and (IV) The work of MAZZI et al. c77) shows that the natur~ It is ironic that many of the first coordination com- of the phosphines as well as the conditions 0 plexes of technetium to be prepared three decades ago the reaction are important in the nature of th~ have only recently been shown to contain technetium resulting technetium species. With triphenylphos in the (III) state. The first complex that was isolated phine and NH4TcO4 in aqueous ethanolic HC only trans- (TcCI4(P(C6Hs)3)2], a neutral techne. in 1959, namely the trans-dichlorobis(orthophenylene_bisdimethylarsine)technetium(III)chloride,~53~ has tium(IV) complex, is obtained. With dimethylphenyl been structurally characterized by DEUTSCH et phosphine the green technetium(IV) complex trans a/. ~74'7s) (Fig. 6) as well as the analogous trans- [Tc(IV)CI,,(P(CH3)2C6Hs)2] is produced in th~ dichlorobis(1,2-dimethylphosphinoethane)technetium- cold with a 5:1 molar ratio of the ligand to metal but the yellow-orange technetium(III) complex mer [Tc(III)Cla(P(CHa)2C~Hs)a] (Fig. 7) is produced ol reflux with a 15:l molar ratio. This is ye another example of the interplay of kinetic ant thermodynamic factors in a reaction. The mer [Tc(III)Cla(P(CH3hC6Hs)a] is rapidly oxidized tt trans-[Tc(IV)Cl4(P(CHa)2C~Hs)2] by refluxing il CC1,. We have already noted that trans-[TcX,L2] (X = C1, Br; L = P(C6Hs)3 and P(CH3)2C~Hs) giv the technetium(I) complexes trans-[Tc(I)X(CO)aL2 in boiling ethyleneglycolmethylether under on atmosphere of carbon monoxides while the met ETc(III)X3(P(CH3)2C6Hsh] (X = C1, Br) gives mix tures of trans-[Tc(I)X(CO)a(P(CH3hC6Hsh] and ei~ [Tc(I)X(CO)2(P(CH3),C~Hs)3] under the sarffe con ditions.~55)In refluxing ethanol, however, under CO al one atmosphere mer-[Tc(III)CI3(P(CH3hC6Hs)3] t~l forms a seven-coordinate adduct carbonyltrichlorc t ris.(dimethylphenylphosphine)technetium(iii)/62~Tb FIG. 6. An ORTEP of trans-[Te(III)Cl2(diars)2]. coordination polyhedron of this complex was show (diars = orthophenylenebisdimethylarsine).
Chemistry ~f technetium I. II. III and I V
~P2
871
PI
Cl.
FIG. 8. An ORTEP of [Tc(III)CI3CO(PMe2Phl3]. by an X-ray structure determination to be a distorted face-capped octahedron (Fig. 8). Further studies by the Italian group using technetium(IV) complexes as starting materials demonstrate again tag~ that the nature and the oxidation state of the product depend markedly on the reaction conditions. The reaction of [P(C6Hs)4]2I-TcX6] or trans-[TcX4(P(C6Hs)a)2] with various ratios of acetylacetone (2,4-pentanedione) led to the isolation of a variety of compounds in both the (III) and (IV) oxidation states: [P(C6Hs)4][Tc(IV)X4(acac)], [Tc(IV)Bra(acac)P(C6Hs)3], [Tc(IV)X2(acac)2], [Tc(IIIXacac)3], [Tc(III)X2(acacXP(C6Hs)3)z], and I-Tc(III)X(acac)2P(C6Hs)a], (X = C1, Br). Two crystalline modifications of [Tc(lII)Cl(acac)2P(C6Hs)~] have been characterized by X-ray methods/6a.64~ Both complexes have the same molecular structure with trans-acetylacetone chelate rings (Fig. 9). Among the first ligands used for the colorometric determination of technetium were thiocyanate and thiourea, tTa~ The nature of the species present when acid solutions of pertechnetate were treated with an excess of these ligands has only recently been elucidated by X-ray studies. The thiocyanate system has been unequivocally shown ~aa~ to consist of a reversible redox couple between the purple ~I'c(IVXNCS)6] 2-(,;.m,~ = 500 nm) and the yellow ISI'c(IIIXNCS)6]3-(Am,~ = 4 0 0 n m ) with E~ = + 0.18 V vs SCE in CHaCN. Solutions of the latter complex have yielded crystalline salts with large cations. An X-ray crystal structure of the tetrabutylammonium salt I'N(C4Hg),~]3[Tc(IIIXNCS)6] showed it to contain six equivalent octahedral and virtually linear N-bonded or isothiocyanate ligands (Fig. 10). The significant feature of this determination shows that the radius of Tc(III) is the same as that found {79~ for Fe(III) in [Fe(III)(NCS)6] a(Tc-N -- 204 pm, and 205 pm, Fe-N = 203-206 pro)
FIG. 9. An ORTEP of trans-[Te(llIXacac)2Cl(PPh3)2]. (PPh3 = triphenylphosphine).
implying that, structurally, technetium(III) is capable of closely mimicking iron in its chemistry. If convenient ways could be found to introduce technetium(III) it might substitute for Fe(III) in biological systems. The red solutions ~6) that result by the treatment of [TcO4]- with thiourea in a variety of acids have been shown to contain the hexakisthioureatechnetium(III) ion. ts°~ An X-ray structure determination of [Tc(III)(tu)6]Cla.3H20 shows that the six thiourea molecules are S-bonded in an octahedral arrangemerit, c~4) This complex can also be synthesized from [TcOCI4]- and thiourea in ethanolic HCI. Initial studies show that the thiourea ligands can be displaced e.g. forming trans-[Tc(III)C12(diphos)4], and [Tc(NCS)6] 3-. It has also been used to synthesizeIs°~ a variety of hexakisalkylisonitriletechnetium(I) cations [(RNC)6Tc(I)] + by treatment with an excess of the isonitrile, where the excess ligand possibly functions as the reducing agent.
S11~"
55
N4 143
S
FIG. 10. An ORTEP of I'Tc(III)(NCS)6]~-.
872
Alun G. Jones and Alan Darison
CL8
theoretical study. A qualitative explanation of the probable cause has been proposed (36.a7) by COTTON
CL5 CL7i
~
C L 6 ~ ) ~ C L 4
CL2 CL3 FIG. I1. The eclipsed Tc2CIs structure found for both [Tc2CIs] 2- and [Tc2CIs] 3-.
The reaction of ammonium hexaiodotechnetate(IV) with potassium cyanide in refluxing aqueous methanol, under nitrogen, gives potassium heptacyanotechnetate(llI)dihydrate K4Tc(CN)7'2H20. Infrared and Raman spectroscopy indicate that it has a pentagonal-bipyramidal structure (Dsh) in both the solid and solution,is2) It is presumably isostructural with the known rhenium complex,tsa) The isolation of the Tc(III) cyano-complex in this reaction is a reflection of the reducing power of the displaced ligand (I-) under the reaction conditions. Aqueous solutions of [Tc(III)(CN)7]4- are air sensitive, decomposing to the technetium(V) species [Tc(V)O(CN)~] 2- which in turn can be hydrolyzedts2~ to trans-dioxotetracyanotechnetate(V), [Tc(V)O2(CN)4] 3-. A number of structural studies have appeared on a variety of the octahedral hexahalometallate species,~s*-94) including one on K2Tc(OH)CI 5 which ~95) is isomorphous with K2TcCI6, that are unexceptional. The anhydrous halide (TcCI4), has been structurally characterized. It comprises chains of edge-shared octahedral TcCI 6 units. There are two unshared cis-Cl atoms in each of the distorted octahedra. There are a number of technetium metal-metal bonded species that have been structurally characterized: [ N H 4 ] 3[-Tc2Cls] " n 2 0 , (96) Ka [ Tc2CIs ] " H20,197) [Y(H20)s][Tc2CIs]'H20, (371 (Fig. 11) [N(C4H9),t]2[Tc2CIs], (36) Tc2CI2(OCCMe3),,, tgs) [Tc 2(OCsH4N),,]CI. t99~ In all of the structures there is a short Tc-Tc bond, and eight of the tigands have the eclipsed MX4-MX4 structure first found (too.~ot) in [Re2Cla] 2-, indicative of strong metal-metal multiple bonding (quadruple bonding) tr2rt't62 for the (III,III) species or 0"27t4626"1 for the mixed oxidation state (II,III) compounds. These compounds are proving to be of fundamental importance in elucidating the fine details of multiple metal-metal bonding interactions. The fact that the Tc-Tc bond lengths (211.7,~2~ 210.5 pm)"~ in three structurally characterized salts of the mixed valent (III,II) anion [Tc2Cls] 3- (formal bond-order 3.5 a2n'*di26*~) are significantly shorter than that found in [N(C4Hg),t]2[Tc2Cls] (214.7pm) (III,III; bond order 4.0, a2n't62) warrants a further
et al.
There are a number of technetium complexes with EDTA as a ligand. The complexes are often difficult to obtain reproducibly and to isolate pure in crystalline form. However, one of the complexes recently characterized "°2) (H2EDTA)Tc(IVX/t-O)2Tc(IVXH2EDTA)-5H20 has been shown to contain a four membered Tc(/~-O)2 Tc ring with a short Tc-Tc distance of 233 pm. An extended Hiickel molecular orbital calculation suggests that this may represent a formal Tc-Tc bond (a2n26*2) rather than the expected triple bond (a2n262). Each EDTA coordinates to one technetium via two nitrogen and two oxygen atoms. The remaining two carboxylates in each EDTA ligand are protonated and non-coordinating.
Conclusions The molecular nature of the technetium species in already existing radiopharmaceuticals is not known with certainty, and it has been suggested that some of them contain the element in either the (IV) or the (lid state. For example, the family of hepato-biliary agents discovered by Loberg has been proposed, "°3~ on the basis of electrophoretic studies, to be a series of sixcoordinate bis-ligand technetium(Ill) complexes. Also, the polarographic work of RUSSELLet al. tI°4~ has indicated the presence of technetium(III), (IV) and (V) species in preparations of the bone-seeking agents. Although neither this review nor the accompanying article on technetium(V)(1) have addressed such problems, it is hoped that both have given an indication of the complexity and versatility of the chemistry and thereby provided a stimulus for further basic studies. In turn these would then give a firmer basis for the design of new diagnostic agents. Acknowledgements--We would like to thank Drs
E. DEUrSCH, U. M^ZZl, and C. J. L. LOCKand their research groups for providing us with structural information and Dr C. E. COSTELLOfor the mass spectral studies. Support from the National Institutes of Health is gratefully acknowledged.
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