Multiple Bonds Between Metal Atoms / 07-Technetium Compounds

7

Technetium Compounds

Alfred P. Sattelberger,

Los Alamos National Laboratory

7.1Synthesis and Properties of Technetium

Technetium was the first man-made element and isotopes 95Tc and 97Tc were obtained by Perrier and Segré in 1937 by bombarding molybdenum with deuterons.1,2 Today, 21 isotopes of element 43 are known with mass numbers from 90-111 and all are radioactive. The longest-lived isotope is 98Tc (t1/2 = 4.2×106 years), but the most readily available isotope is 99Tc (t1/2 = 2.1×105 years). The latter is isolated in large quantities from spent nuclear fuel and constitutes approximately 6% of the fission product yield.3 Ammonium pertechnetate is readily available in gram quantities with a radiopurity of >99% from Oak Ridge National Laboratory.4 All other starting materials, including technetium metal, trace their origins to ammonium pertechnetate.

The 99Tc isotope is a weak `-emitter (Emax = 0.292 MeV). The decay properties of 99Tc allow handling of the isotope during normal chemical operations in quantities up to c. 1 g. With

this mass limitation, special shielding precautions are not necessary since the low energy ` radiation is completely absorbed by ordinary glassware. It is prudent to remember that 99Tc, like all radionuclides, is a potential health hazard and protective gloves, lab coats, and safety glasses are essential at all times when working with 99Tc compounds. Additional care must be exercised with volatile compounds such as Tc2O7, Me3SiOTcO3, and Tc2(CO)10 to avoid inhalation and the unwanted spread of radioactivity. The author’s collaborators have safely carried out numerous synthetic reactions and spectroscopic characterizations in laboratories designed for low-level radioactivity using efficient fume hoods and Schlenk and glove box techniques and following, in our case, Department of Energy approved handling and monitoring procedures.

Because technetium bears a close electronic relationship to rhenium, the occurrence of analogous compounds, including those containing metal–metal multiple bonds, is to be expected, but the radioactive nature of technetium has served to limit the development of Tc chemistry relative to its heavier congener Re. As one striking comparison, thirteen binary halides have been reported for rhenium, but only three binary halides of technetium (TcF6, TcF5, and TcCl4) are reasonably well characterized.5 Logic predicts a plethora of exciting Tc chemistry yet to be discovered. Several recent review articles have been published that contain accounts of the chemistry, properties, and structures6-9 of dinuclear and polynuclear technetium compounds. These sources should be consulted for additional details.

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7.2Preparation of Dinuclear and Polynuclear Technetium Compounds

A few words on the methods of synthesis employed in the preparation of dinuclear and polynuclear technetium compounds seem appropriate. Synthetic methodologies fall into one of three categories: (1) moderate temperature (100-250 °C) reduction of higher-valent mononuclear Tc precursors in concentrated aqueous hydrohalic acid solution using molecular hydrogen (30-50 atm) as the reductant; (2) reduction of higher-valent mononuclear precursors using chemical reductants other than H2, either in aqueous acid or non-aqueous solvents; (3) substitution and/or redox reactions involving pre-formed dinuclear complexes. Russian chemists have been advocates for the first method while American and European chemists have traditionally opted for the latter two strategies. Each has its attendant advantages and disadvantages. The hydrogen reductions do require the use of high-pressure stainless steel autoclaves. The use of glass test tubes inside the autoclave minimizes corrosion of the stainless steel. Russian chemists have isolated a wide variety of dinuclear and polynuclear Tc compounds by systematically varying the experimental parameters (such as time, Tc and acid concentration, temperature, pressure, cool-down rate). Structurally characterized technetium compounds containing Tc–Tc multiple bonds are presented in Table 7.1.

7.3Bonds of Order 4 and 3.5

The original entry into the chemistry of Tc–Tc multiple bonds was afforded by the work of Eakins, Humphreys, and Mellish10 who discovered that the reaction of (NH4)2TcCl6 or MgTcCl6 with zinc in concentrated hydrochloric acid at roughly 100 °C gave a mixture which could be used to prepare the deeply colored salts (NH4)3Tc2Cl8·2H2O, YTc2Cl8·9H2O and K3Tc2Cl8·2H2O:

4(NH		)	TcCl		+ 3Zn	12 M HCl	2(NH		)	Tc	Cl	·2H		O + 2NH		Cl + 3ZnCl
	4			6				4					2		4		2
		2							3	2	8

The average Tc oxidation state of c. +2.5 was established via oxidative titrations using ceric sulfate or basic peroxide. In dilute hydrochloric acid or water, the compounds decompose rapidly by oxidation and hydrolysis. The British work10 was published shortly before the structural characterization of K2Re2Cl8·2H2O and, accordingly, the authors’ conclusions were limited to the observation that “the stoichiometry of the [Tc2Cl8]3- ion is unusual, and it seems to have no analogs.” Cotton and Pedersen11 published an improvement in the original synthetic procedures some years ago.

Following the completion of the original structural work on K2Re2Cl8·2H2O, the full structural characterization of a salt containing the [Tc2Cl8]3- anion became an important objective. Black crystalline (NH4)3Tc2Cl8·2H2O was chosen for this study and a structure solution revealed the presence of the [Tc2Cl8]3- anion having the same non-bridged, eclipsed M2Cl8 structure as [Re2Cl8]2-.12,13 The very short Tc–Tc distance of 2.13(1) Å was indicative of a strong metal–metal bond. The paramagnetism of the ammonium and yttrium salts (µeff = 1.78±0.03 B.M.)11,12,14,15 is consistent with the anion possessing a μ2/4β2β*1 electronic configuration, a conclusion supported by SCF-X_-SW calculations (see Chapter 16).16,17 Frozen solution EPR spectral measurements on YTc2C18·9H2O at X- and Q-band frequencies revealed the expected coupling of one unpaired electron to two equivalent Tc nuclei each with a nuclear spin of 9/2.11 The spectrum was analyzed to afford g˺ = 1.912 and g = 2.096. The values are consistent with the odd electron occupying the b1u β* orbital.18 Every indication is that [Tc2Cl8]3- contains a Tc–Tc bond of order 3.5 in contrast with the recognition of the first Re–Re multiple bond as being one of order four.

Unlike [Re2Cl8]3-, the stability of the [Tc2Cl8]3- anion has been demonstrated on a number of occasions since the original synthesis and structural characterization. However, some later work is confusing and contradictory. Glinkina et al.19 described the reduction of solutions of ammonium or potassium pertechnetate in concentrated hydrochloric acid by hydrogen under

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Table 7.1. Structurally characterized technetium compounds with Tc–Tc multiple bonds.

Compound		Tc–Tc (Å)	ref.
		Bonds of order 4.0
(Bu4N)2Tc2Cl8		2.147(4)	30
Tc2(O2CCMe3)4Cl2		2.192(1)	39
[Tc2(O2CMe)4](TcO4)2		2.149(1)	51,52
[Tc2(O2CCH3)2Cl4(dma)2]		2.1835(7)	42
K2[Tc2(SO4)4]·2H2O		2.155(1)	7
		Bonds of order 3.5
K3Tc2Cl8·nH2O		2.117(2)	20,21
(NH4)3Tc2Cl8·2H2O		2.13(1)	12,13
Y[Tc2Cl8]·9H2O		2.105(2)	22
(C5H5NH)3Tc2Cl8		2.1185(5)	23
Tc2(hp)4Cl		2.095(1)	53
Tc2(O2CCH3)4Cl		2.117(1)	46
K[Tc2(O2CCH3)4Cl2]		2.1260(5)	45
Tc2(O2CCH3)4Br		2.112(1)	49
[Tc2Cl4(PMe2Ph)4]PF6 (orthorhombic)		2.109(1)	57
[Tc2Cl4(PMe2Ph)4]PF6 (monoclinic)		2.106(1)	57
[Tc2Cl4(PMe2Ph)4]PF6·0.5THF		2.107(1)	57
Tc2Cl5(PMe2Ph)3		2.109(1)	57
Tc2(DPhF)4Cl·C7H8		2.119(2)	54
Tc2(DTolF)3Cl2		2.094(1)	54
		Bonds of order 3.0
K2[Tc2Cl6]		2.044(1), 2.047(1), 2.042(1)	59,60
Tc2Cl4(PEt3)4		2.133(3)	61
Tc2Cl4(PMe2Ph)4		2.127(1)	61
Tc2Cl4(PMePh2)4·C6H6		2.138(1)	61
_-Tc2Cl4(dppe)2		2.15(1)	58
`-Tc2Cl4(dppe)2		2.117(1)	58
`-Tc2Cl4(dppm)2		2.1126(7)	65
[Tc2(NCCH3)8(CF3SO3)2](BF4)4·CH3CN		2.122(1)	66
	Hexanuclear cluster compounds
[(CH3)4N]3{[Tc6(µ-Cl)6Cl6]Cl2}		2.16(1), 2.69(1)	72
[(CH3)4N]2[Tc6(µ-Cl)6Cl6]		2.22(1), 2.57(1)	73
[(C2H5)4N]2{[Tc6(µ-Br)6Br6]Br2}		2.188(5), 2.66(2)	74
[(CH3)4N]3{[ Tc6(µ-Br)6Br6]Br2}		2.154(5), 2.702(2)	74
	Octanuclear cluster compounds
{[Tc8(µ-Br)8Br4]Br}·2H2O		2.146(2), 2.521(2), 2.687(23)	77
[H(H2O)2]{[Tc8(( -Br)8Br4]Br}		2.155(3), 2.531(2), 2.70(2)	79
[H(H2O)2]2{[Tc8(( -Br)8Br4]Br2}		2.152(9), 2.520(9), 2.69(1)	78
[(C4H9)4N]2{[Tc6(	-Br)4( -I)4Br2I2]I2}	2.162(9), 2.507(2), 2.704(10)	81
[Fe(C5H5)2]3{Tc6(	-I)6I6]I2}	2.17(1), 2.67(1)	52

254Multiple Bonds Between Metal Atoms Chapter 7

pressure at 170 °C to produce dark blue solutions from which salts with the compositions K8(Tc2Cl8)3·4H2O, (NH4)8(Tc2Cl8)3·2H2O, or Cs8(Tc2Cl8)3·2H2O could be isolated. In spite of these complexes having spectral and magnetic properties clearly in accord with the presence of the [Tc2Cl8]3- anion, these workers describe the oxidation number of technetium as being 2.67 on the basis of oxidation state titrations. Furthermore, they cited the results of an X-ray crystallographic study20 that purportedly showed “that technetium exists as the binuclear anionic octachloroditechnetate complex {[Tc2Cl8]3}8-, in which technetium has an average valency of 2.67”. Actually, the cited report20 describes no such result. Rather, the publication discusses the structures of K3Tc2Cl8·2H2O and Cs3Tc2Cl8·2H2O using crystals provided by Glinkina and Kuzina.19 The potassium and cesium salts were described20 as being isostructural with (NH4)3Tc2Cl8·2H2O, and for K3Tc2Cl8·2H2O a Tc–Tc distance of 2.10 Å was obtained. Since this structure determination was of relatively poor quality, a further structural study was carried out on a sample of K3Tc2Cl8·nH2O prepared by cation exchange from YTc2Cl8·9H2O.21 As before, the [Tc2Cl8]3- anion was found to have virtual D4h symmetry and to be very similar in structure to the [Re2Cl8]2- anion (Fig. 7.1). The Tc–Tc distance of 2.117(2) Å was determined with greater precision than before. The structure of the yttrium salt Y[Tc2Cl8]·9H2O was determined several years later.22 The Tc–Tc distance is 2.105(1) Å, and the counter cation proved to be [Y(H2O)8]3+.

Fig. 7.1. Structure of the [Tc2Cl8]3- anion in K3Tc2Cl8·nH2O.

The ease of conversion of technetium(IV) to [Tc2Cl8]3- has also been demonstrated by the high pressure hydrogen reduction (30 atm H2, 160 °C, 5 h) of the pyridinium and quinolinium salts of [TcCl6]2- in 11 M HCl to (pyH)3Tc2Cl8·2H2O described as forming dark-brown crystals,23 and (quinH)3Tc2Cl8·2H2O which is olive colored.24 Similar reductions of Tc(VII) or Tc(IV) species in hydrobromic acid have been used to obtain brown M3[Tc2Br8]·2H2O (M = NH4 or K).25

The ease of producing [Tc2Cl8]3-, rather than [Tc2Cl8]2-, was long considered a rather curious result. Shown experimentally11 in 1975, [Tc2Cl8]3- (as its yttrium salt) is reversibly oxidized to [Tc2Cl8]2- at +0.14 V versus SCE in mixtures of hydrochloric acid and ethanol (1:9 v/v). The resulting product gave no EPR signal and is likely diamagnetic. With a lifetime in solution of at least 5 min, it seemed reasonable to conclude11 that a “suitably designed effort to isolate (it) might be successful.” Accordingly, in 1977, a communication by Schwochau et al.26 describing their isolation and characterization of (Bu4N)2Tc2Cl8 was received with considerable interest. An olive-green complex of this stoichiometry was described as being prepared by the hypophosphorous acid (H3PO2) reduction of [TcO4]- in hydrochloric acid followed by the addition of Bu4NCl. The synthetic details presented were minimal, i.e., quantities of reactants, HCl concentration, temperature, and the duration of the reaction were not provided in the report.

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The diamagnetic product was said to be isomorphous with (Bu4N)2Re2Cl8 and to possess an electronic absorption spectrum similar to that of the latter complex with its βΑβ* transition located at about 700 nm. Thus the authors concluded ‘that there seems to be no more doubt about the existence of a stable dinegative octachloroditechnetate(III) which closely resembles the analogous rhenium complex in magnetic, structural and spectroscopic properties.’

In order to establish definitely the structure of (Bu4N)2Tc2Cl8 by X-ray crystallography this system was reinvestigated in 1979.27 However, an attempt to reproduce the hypophosphorous acid reduction procedure of Schwochau et al.26 afforded dark-green (Bu4N)TcOCl4 that was easily converted to the bis(triphenylphosphine)iminium salt, [(Ph3P)2N]TcOCl4.27 The infrared spectra of both salts revealed the characteristic ι(Tc=O) mode at c. 1020 cm-1 and an X-ray crystallographic analysis of [(Ph3P)2N]TcOCl4 confirmed the presence of the distorted square pyramidal [TcOCl4]- anion.27 A second report on the synthesis of (Bu4N)2Tc2Cl8 via H3PO2 reduction of pertechnetate was published by Schwochau in 1981.28 In this report, the synthetic details were provided, as well as the fact that the desired compound was isolated in only 10% yield starting from NH4TcO4.28 A note in a 1995 review article by Kryutchkov7 claims that the Schwochau28 procedure can be optimized to obtain much higher yields of (Bu4N)2Tc2Cl8.

In between the two Schwochau publications,26,28 Preetz and Peters29 reported a successful preparation of (Bu4N)2Tc2Cl8, together with grey-blue (Bu4N)3Tc2Cl8, by a procedure that involved the mossy zinc reduction of (NH4)2TcCl6 in aqueous HCl followed by cation exchange using Bu4NCl. The green complex (Bu4N)2Tc2Cl8 can be converted to the carmine-red bromide derivative (Bu4N)2Tc2Br8 by dissolving it in aqueous acetone/HBr.29 Raman and electronic absorption spectral data supported the proposed formulations, but the successful completion of an X-ray crystal structure determination on (Bu4N)2Tc2Cl8 provided the incontrovertible proof as to the structure of the [Tc2Cl8]2- anion.30 (Bu4N)2Tc2Cl8 is isostructural with (Bu4N)2Re2Cl8 and, like the latter, possesses a quadruply bonded dimetal unit with an eclipsed rotational geometry. While there is disorder associated with the orientation of the [Tc2Cl8]2- ions, the structure is of high precision; the Tc–Tc distance is 2.147(4) Å; the weighted average of Tc–Tc distances of 2.151(1) Å and 2.133(3) Å for the major and minor orientations.30 Actually, the Tc–Tc distance of 2.147(4) Å poses an interesting dilemma since it is longer than the Tc–Tc distances in the NH4+, K+, and Y3+ salts of [Tc2Cl8]3- (see above). This trend is, of course, the opposite expected based upon a simple bond length/bond order correlation argument, but the explanation is probably similar to that advanced to explain metal–metal bond length changes in the series [Re2Cl4(PMe2Ph)4]n+ (n = 0, 1 or 2),31 namely, as the formal bond order increases (and the metal core charge increases) there is some decrease in the strength of the μ- and/or /-bonding contributions to the Tc–Tc bond because of orbital contraction.

One result not readily explained concerns the electrochemical redox characteristics of the [Tc2Cl8]2-/[Tc2Cl8]3- couple. In a mixed hydrochloric acid-ethanol solvent (1:9 by volume) the [Tc2Cl8]3- ion is reversibly oxidized to [Tc2Cl8]2- at +0.14 V versus SCE.11 Solutions of

(Bu4N)2Tc2Cl8 in 0.1 M Bu4NClO4/CH2Cl2 are characterized by E1/2 = -0.13 V versus SCE at a rotating platinum electrode,30 demonstrating the solvent dependence of the electrochemical

potential for this process. However, partial solvolysis of the [Tc2Cl8]3- ion is likely to occur in HCl-EtOH solutions. In this context, it should be noted that spectrophotometric methods have been used to investigate the stability of solutions of the [Tc2X8]3- anions (X = Cl or Br) in hydrohalic acids as a function of metal and acid concentration both in the presence and absence of air.32-34 At HCl concentrations below ~3 M in the absence of air, [Tc2Cl8]3- hydrolyzes to mixed aquo-chloro species of the type [Tc2Cl8-n(H2O)n](3-n)-.32

In 1994, Preetz and coworkers published a definitive synthetic/spectroscopic paper that provided new information on the solution behavior of the octachloroand octabromoditechnetate

256Multiple Bonds Between Metal Atoms Chapter 7

anions and described high yield syntheses for all four [Tc2X8]n-dimers (X = Cl, Br; n = 2, 3).35 Their improved preparative route for (Bu4N)2Tc2Cl8 starts with the tetrabutylammonium salt of [TcO4]- which is first reduced to (Bu4N)TcOCl4 via treatment with concentrated aqueous HCl. The compound (Bu4N)TcOCl4 is then dissolved in THF and treated dropwise with a THF solution containing 2 equiv of (Bu4N)BH4. The latter step provides a brown intermediate (not characterized) that is isolated and dried, dissolved in methylene chloride, and then treated with gaseous HCl and air. The green (Bu4N)2Tc2Cl8 is crystallized by adding ether and cooling to c. -30 °C:

	12 M HCl		THF
(Bun4N)TcO4		(Bun4N)TcOCl4		brown intermediate
			+ 2(Bun4N)BH4
			- H2, -B2H6

CH2Cl2

brown intermediate (Bun4N)2Tc2Cl8 HCl(g), air

The overall yield of (Bu4N)2Tc2Cl8, starting from (NH4)TcO4, is nearly 80%. A similar procedure, via (Bu4N)TcOBr4 and bromine-free HBr(g), provides (Bu4N)2Tc2Br8 in comparable yield. The [Tc2X8]2- anions can be interconverted by dissolution in methylene chloride and treatment with the appropriate gaseous hydrogen halide:

CH2Cl2/HBr(g)

[Tc2Cl8]2- [Tc2Br8]2- CH2Cl2/HCl(g)

Green (Bu4N)2Tc2Cl8 and carmine-red (Bu4N)2Tc2Br8 are both diamagnetic crystalline solids that contain Tc–Tc quadruple bonds. A structure determination of (Bu4N)2Tc2Br8 has yet to be performed. The compounds are stable in dry air and can be stored under argon in the dark for several years without signs of decomposition. Solutions of either complex are stable in dry methylene chloride or acetone for several days; extended exposure to air results in oxidation to the corresponding hexahalogenotechnetate(IV) ions, [TcX6]2-. The (Bu4N)2Tc2X8 salts are only sparingly soluble in concentrated aqueous HX and on warming disproportionate:

conc. HX

3[Tc2X8]2- + 4X- 2[Tc2X8]3- + 2[TcX6]2-

The ready availability of (Bu4N)2Tc2X8 should pave the way for further elaborations of Tc–Tc quadruple bond chemistry.

Grey-blue (Bu4N)3Tc2Cl8 and golden (Bu4N)3Tc2Br8 can be prepared, in good yield, from the corresponding (Bu4N)2Tc2X8 salts by dissolution of the latter in acetone and treatment with one equivalent of (Bu4N)BH4:35

(CH3)2CO

(Bun4N)2Tc2Cl8 + (Bun4N)BH4 (Bun4N)3Tc2Cl8 + 0.5H2 + 0.5B2H6

Both salts are paramagnetic with Tc–Tc bond orders of 3.5.

Neither of the (Bu4N)3Tc2X8 salts has been characterized by X-ray crystallography. The solids are very sensitive to air and water but can be stored for several weeks in a dry argon atmo-

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sphere in the absence of light. Both salts are readily soluble in methylene chloride but the solutions are photo-labile and decompose rather rapidly. On the other hand, both salts are readily soluble and stable in the respective airand halogen-free, constant-boiling aqueous hydrohalic acid. Addition of KCl, RbCl or CsCl to these solutions results in the precipitation of the alkali metal salt, M3Tc2X8. The synthesis and the low energy optical spectrum (βΑβ* transition) of Cs3Tc2Br8 have been described in considerable detail.36

As can be gleaned from the foregoing paragraphs, the solution stability of the [Tc2X8]2- and [Tc2X8]3- anions is very much solvent dependent. With rigorous exclusion of air and water, the [Tc2X8]2- anions are stable in organic solvents and unstable in concentrated aqueous hydrohalic acid. In contrast, the [Tc2X8]3- anions are stable in concentrated aqueous HX and unstable in organic solvents. These properties have undoubtedly contributed to some of the difficulties encountered in earlier chemical and physical studies of these systems.

In addition to the aforementioned structural studies and measurements of the EPR spectra and magnetic properties of salts of the [Tc2Cl8]3- anions, other physicochemical investigations have included the X-ray photoelectron spectrum of K3Tc2Cl8·2H2O; as part of a larger investigation devoted to the measurement of the Tc 3d binding energies.37 Normal coordinate analyses have been performed on the [Tc2X8]2-/3- (X = Cl, Br) ions. The calculated force constants for the Tc–Tc multiple bonds range from 2.67 mdyne/Å for [Tc2Br8]2- to 4.86 mdyne/Å for [Tc2Cl8]3-.35 The thermal decomposition of (NH4)3Tc2Cl8.2H2O has been found to yield technetium metal via the intermediacy of (NH4)2TcCl6, TcNCl, and TcN.38

7.4Tc26+ and Tc25+ Carboxylates and Related Species with Bridging Ligands

While quadruply bonded, carboxylate-bridged Re26+ complexes of the type Re2(O2CR)4X2 are well known and easily prepared, comparable Tc26+ carboxylate compounds are still quite rare and were, until the development of reliable routes to (Bu4N)2Tc2X8, difficult to isolate. The first such example, for which there was definitive structural proof, was the pivalate Tc2(O2CCMe3)4Cl2.39 The compound was prepared in very low yield, as red crystals, by the reaction of (NH4)3Tc2Cl8 with molten pivalic acid in a nitrogen atmosphere. The structure of Tc2(O2CCMe3)4Cl2 resembles closely that of its rhenium analog (Fig. 7.2); the Tc–Tc bond length of 2.192(1) Å is longer than in (Bu4N)2Tc2Cl8 (2.147(4) Å),30 a complex that does not contain axial Tc–ligand bonds that weaken the Tc–Tc bond. Subsequently, the diamagnetic acetate complex Tc2(O2CCH3)4Cl2 was prepared as cherry-red crystals from the reaction between KTcO4, hydrochloric acid, and acetic acid in a hydrogen atmosphere.40 The reaction of (Bu4N)2[Tc2X8] with acetic acid/acetic anhydride provides Tc2(O2CCH3)4Cl2 and orangered Tc2(O2CCH3)4Br2 in excellent yield.41 By analogy with known rhenium chemistry, other carboxylic acid/acid anhydride reactions could be a source of as yet unknown Tc2(O2CR)4X2 derivatives. A thorough analysis of the low temperature (80 K) IR and Raman spectra of the Tc2(O2CCH3)4X2 complexes allowed assignments of the metal–metal, metal–ligand and intraligand vibrations. The Tc–Tc stretching vibration is found at 319 cm-1 for the chloro compound, and at 310 cm-1 for the bromo derivative. A normal coordinate analysis provided a Tc–Tc force constant of 4.08 mdyne/Å for the chloride and 3.99 mdyne/Å for the bromide.41

The deep green complex [Tc2(O2CCH3)2Cl4(H2O)2] has been prepared by reaction of acetic anhydride and HBF4 with [Tc2Cl8]2-.42 Subsequent treatment with Lewis bases such as dmf, dma, dmso, Ph3P=O, or pyridine results in substitution of the water ligands providing complexes of general composition [Tc2(O2CCH3)2Cl4L2].42 The X-ray crystal structure of Tc2(O2CCH3)2Cl4(dma)2 reveals a cis arrangement of the bridging acetate ligands and the terminal chlorides. The dma ligands are axial and coordinate via the amido oxygen atoms (Fig. 7.3). The Tc–Tc distance is 2.1835(7) Å, significantly longer than in [Tc2Cl8]2-. The elongation of the Tc–Tc bond is due to the presence

258Multiple Bonds Between Metal Atoms Chapter 7

of strongly bound axial ligands that weaken the Tc–Tc bond. Electronic spectra show only minor dependence on the Lewis base. The βΑβ* transitions are found in the range of 648-652 nm for all adducts.43 The correlation between the donor strength of the axial bases and the Tc–Tc vibrational mode was studied,43 and a linear relationship between the donor number and ιTc-Tc was discovered. For the strongest donor pyridine, the Tc–Tc stretching vibration is at 282 cm-1; for the weakest donor, H2O, it is at 311 cm-1.

Fig. 7.2. Structure of Tc2(O2CCMe3)4Cl2.

Fig. 7.3. Structure of cis-Tc2(O2CCH3)2Cl4(dma)2.

Reaction of K3Tc2Cl8·2H2O and glacial acetic acid in an atmosphere of argon or hydrogen at 120 °C and 30 atm in an autoclave has been used to prepare the crystalline Tc25+ derivatives Tc2(O2CCH3)4Cl (green) and K[Tc2(O2CCH3)4C12] (pale brown), admixed with K2TcCl6 (argon atmosphere) or a material speculated to be a Tc2+ complex (hydrogen atmosphere).44 The complexes Tc2(O2CCH3)4Cl and K[Tc2(O2CCH3)4Cl2] are clearly authentic derivatives of the Tc25+ core. Both compounds are paramagnetic and EPR-active, and possess magnetic moments in accord with the presence of a μ2/4β2β*1 ground state electronic configuration.44 A comparison of their X-ray photoelectron spectra has been made; the Tc 3d5/2 binding energy is 255.8 eV for both compounds.37 X-ray crystal structure determinations on K[Tc2(O2CCH3)4Cl2] and Tc2(O2CCH3)4Cl have been completed.45,46 The former salt contains the dinuclear [Tc2(O2CCH3)4Cl2]- anion with Tc–Tc and Tc–Cl distances of 2.126(1) Å and 2.589(1) Å, respectively.45 The complex Tc2(O2CCH3)4Cl has a structure with chains of [Tc2(O2CCH3)4]+ units linked by bridging chloride ligands.46 Note that there is a longer Tc–Tc distance in Tc2(O2CCMe3)4Cl239 compared to [Tc2(O2CCMe3)4Cl2]-.45,47 A related green bromide compound, Tc2(O2CCH3)4Br, has been prepared48 from the reaction of

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M2Tc2Br6·2H2O (M = NH4 or K; see below), and acetic acid at 230-250 °C under argon. The structure of Tc2(O2CCH3)4Br is quite similar to that of Tc2(O2CCH3)4Cl. The Tc–Tc separation is 2.112(1) Å.49 The magnetic susceptibilities and frozen solution (MeOH) EPR of Tc2(O2CCH3)4Cl,

K[Tc2(O2CCH3)4Cl2] and Tc2(O2CCH3)4Br have been measured.50 The values of µeff are 1.78±0.05 B.M. for the first two compounds and ~2.0 B.M. for the bromide, the higher value apparently due

to the presence of K2TcBr6 as an impurity. The EPR spectral parameters coincide within experimental error, viz., g˺ = 1.85±0.03 and g = 2.13±0.03 for all three compounds.

Aerial oxidation of solutions of [Tc2(O2CCH3)4Cl2]- provided a low yield of red crystals which proved to be the Tc26+ complex, [Tc2(O2CCH3)4](TcO4)2.51,52 The structure is similar to Tc2(O2CCMe3)4Cl2 with a paddlewheel [Tc2(O2CCH3)4]2+ core axially ligated by a single oxygen of each pertechnetate anion. The Tc–Tc distance of 2.149(1) Å is 0.04 Å shorter than that in Tc2(O2CCMe3)4Cl2 and the Tc–Oax distances average 2.153(5) Å.51,52

A compound that bears a close structural relationship to Tc2(O2CCH3)4Cl is the dark green complex Tc2(hp)4Cl, which is prepared by reacting (NH4)3Tc2Cl8 with molten 2-hydroxypyridine.53 It is paramagnetic (g = 2.046 from the EPR spectrum) and exhibits a Raman active ι(Tc–Tc) mode at 383 cm-1. The parent ion has been detected in the mass spectrum, while in the solid-state the structure resembles Tc2(O2CCH3)4Cl and consists of infinite chains of [Tc2(hp)4]+ units (the Tc–Tc distance is 2.095(1) Å) symmetrically linked by bridging chloride ligands.53 Perhaps the most interesting feature of the compound is the visible absorption spectrum measured on single crystals at 5 K (see Chapter 16). The lowest energy transition at 12,194 cm-1 is z polarized and consistent with the assignment as a βΑβ* transition.

As discussed in Chapter 1, the use of aryl amidinate ligands, [ArNC(R)NAr]−, relatives of more common carboxylate ligands, has become increasingly prominent in the field of metal– metal multiple bond research. The success of these ligands derives, at least in part, from their enhanced /-basicity relative to carboxylate ligands. Seeking examples of this class of compound in technetium chemistry, Cotton and coworkers examined reactions of Tc24+ compounds of the type Tc2Cl4(PR3)4 (see below), and reasoned that treatment of Tc2Cl4(PR3)4 with molten aryl formamidines, ArN(H)C(H)NAr, might liberate volatile PR3 and HCl(g), produced by the transfer of H+ from the formamidine to the Cl- ligands, and drive the reaction to equilibrium and concomitant formation of Tc2(ArNC(H)NAr)4. Instead, the reactions produced two types of higher-valent formamidinate complexes in low to moderate yield:54

Tc2Cl4(PR3)4 + HDPhF	140-160 °C, vacuum	+ Tc2(DPhF)4Cl
	Tc2(DPhF)3Cl2

HCl + PR3

Both reddish-purple Tc2(DTolF)3Cl2 and red-orange Tc2(DPhF)4Cl were structurally characterized.54 The structure of Tc2(DTolF)3Cl2 can be described as a variant of the familiar paddlewheel variety in which one of the bridging formamidinate ligands has been replaced by two chloride anions. At 2.0937(6) Å, the metal–metal bond length in Tc2(DTolF)3Cl2 is among the shortest known Tc–Tc bonds. The structure of a related complex, Tc2(DPhF)4Cl, consists of four bridging formamidinate ligands in the traditional lantern motif (Fig. 7.4). The Tc–Tc bond length of 2.119(2) Å is more typical of structurally characterized complexes with a Tc25+ core. The chloride ligand occupies an axial position along the four-fold axis at a rather short distance of 2.450(4) Å from one of the Tc atoms. Unlike the situation in Tc2(hp)4Cl, there are no bridging chloride interactions in solid Tc2(DPhF)4Cl. The electrochemistry of the Tc25+

260Multiple Bonds Between Metal Atoms Chapter 7

formamidinate complexes, measured in methylene chloride/0.1 M (Bu4N)PF6, is rich with a reversible one-electron oxidation and a one-electron reduction for each complex. The potentials for Tc2(DTolF)3Cl2 occur at -0.2 V and -1.5 V; those for Tc2(DPhF)4Cl are -0.46 and -1.73 V (vs. Fc+/Fc). Based on the electrochemistry, it is reasonable to postulate that compounds of the type Tc2[ArNC(H)NAr]4Cl2 and Tc2[ArNC(H)NAr]4 might be isolable.54

Fig. 7.4. Structure of Tc2(DPhF)4Cl.

Spin-restricted SCF-X_-SW calculations were performed on the model complexes Tc2(HNCHNH)4Cl and Tc2(HNCHNH)3Cl2. For both systems the HOMO is the β* orbital, and the (primarily) metal-based orbital ordering was calculated to be μ</<β<β*</*<μ*.54

A green µ-sulfato complex of composition K2Tc2(SO4)4·2H2O has been obtained55 by treating K3Tc2Cl8.2H2O with 8 M sulfuric acid at ~100 °C for 1 h followed by slow cooling to room temperature. The structure of K2Tc2(SO4)4·2H2O has been determined by P. A. Koz’min and coworkers,7 and is similar to that of [Re2(SO4)4(H2O)2]2-.56 The Tc–Tc distance is 2.155(1) Å.7

A penultimate set of Tc25+ species to discuss in this section are the [Tc2Cl4(PR3)4]+ cations and the related neutral Tc2Cl5(PR3)4 complexes.57 Mild chemical oxidation of purple Tc2Cl4(PMe2Ph)4 (see Section 7.5) with [Fc]PF6 in acetonitrile provides green [Tc2Cl4(PMe2Ph)4]PF6 in 82% yield. If the same reaction is carried out in the presence of additional chloride ion, one of the phosphines is substituted to give neutral, orange Tc2Cl5(PMe2Ph)3. Both complexes are paramagnetic, consistent with a ground state μ2/4β2β*1 electronic configuration. The X-ray crystal structure shows that the cation [Tc2Cl4(PMe2Ph)4]+ adopts an eclipsed structure with an arrangement of ligands similar to that observed for the neutral counterpart (Fig. 7.5). The Tc–Tc bond length is 2.1074(9) Å compared to a Tc–Tc separation of 2.127(1) Å in the neutral Tc24+ counterpart Tc2Cl4(PMe2Ph)4. In this case, the bond length change is consistent with an increase in bond order (3 to 3.5) due to the removal of an electron from the β* orbital.57 The complex Tc2Cl5(PMe2Ph)3 is structurally similar to [Tc2Cl4(PMe2Ph)4]PF6, and consists also of two eclipsed ML4 fragments with a relatively short Tc–Tc separation of 2.1092(4) Å. Both [Tc2Cl4(PMe2Ph)4]PF6 and Tc2Cl5(PMe2Ph)3 exhibit well resolved EPR spectra in frozen solution as expected for molecules with μ2/4β2β*1 ground states.57

Another cationic Tc25+ complex has recently been mentioned in the literature. Treatment of a pink methylene chloride solution containing `-Tc2Cl4(dppe)2 with 1 equiv of the oneelectron oxidant NOPF6 provides a green solution from which green crystals of (presumably) [Tc2Cl4(dppe)2]PF6 are isolated.58 No additional details are available at this time.