Metal-Promoted Aromatic Ring Amination and Deamination Reactions at
a Diazo Ligand Coordinated to Rhodium and Ruthenium
Chayan Das,† Amit K. Ghosh,† Chen-Hsiung Hung,‡Gene-Hsiang Lee,§ Shie-Ming Peng,§and Sreebrata Goswami*,†
Department of Inorganic Chemistry, Indian Association for the CultiVation of Science,
Kolkata 700 032, India, Department of Chemistry, National Changhua UniVersity of Education, Changhua, Taiwan 500, Republic of China, and Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan, Republic of China
Received June 24, 2002
Reactions of MCl3‚3H2O (M ) Rh and Ru) with the ligand 2-[(2-N-arylamino)phenylazo]pyridine [HL1; NH4C5Nd NC6H4N(H)C6H4(H) (HL1a), NH4C5NdNC6H4N(H)C6H4(CH3) (HL1b), and NH4C5NdNC6H4N(H)C5H4N (HL1c)] in the presence of dilute NEt3afforded multiple products. In the case of rhodium, two green compounds, viz. [Rh(L1)2]+ ([2]+) and [RhCl(pap)(L1)]+ ([3]+), where L1 and pap stand for the conjugate base of [HL1] and 2-(phenylazo)-pyridine, respectively, were separated on a preparative thin layer chromatographic plate. The reaction of RuCl3‚ 3H2O, on the other hand, produced two brown compounds, viz. [RuCl(HL1)(L1)] (4) and [RuCl(pap)(L1)] (5), respectively, as the major products. The X-ray structures of the representative complexes are reported. Except for complex 2, and 4, the products are formed due to the cleavage of an otherwise unreactive C(phenyl)−N(amino) bond. In complex 4, one of the tridentate ligands (HL1) does not use its maximum denticity and coordinates as a neutral bidentate donor. Plausible reasons for the differences in their modes of coordination of the ligands as in 2 and 4 have been discussed. The ligand pap in the cationic mixed ligand complex [3]+reacts instantaneously with ArNH2 to produce an ink-blue compound, [RhCl(HL2)(L1)]+([6]+) in a high yield. The ligand HL2is formed due to regioselective fusion of ArNH2residue at the para carbon of the phenyl ring (with respect to the azo fragment) of pap in [3]+. The above complexes are generally intensely colored and show strong absorptions in the visible region, which are assigned to intraligand charge transfer transitions. These complexes undergo multiple and successive one-electron-transfer processes at the cathodic potentials. Electrogenerated cationic complexes of ruthenium(III), [4]+and [5]+, showed rhombic EPR spectra at 77 K.
Introduction
The chemical reactions that are associated1 with bond-making and bond-breaking processes in organic molecules have been of immense interest. In this respect, the reactions of coordinated ligands are important.2In fact, these provide facile synthetic routes for the synthesis of many novel
molecules, which are otherwise difficult, or in some cases even impossible, to synthesize following conventional syn-thetic procedures. In the recent years we have noted a number of metal-promoted C-N bond forming processes3-12
includ-* Author to whom correspondence should be addressed. Fax: (+91) 33-473 2805. E-mail: [email protected].
†Indian Association for the Cultivation of Science. ‡National Changhua University of Education. §National Taiwan University.
(1) See, for example: (a) Hartwig, J. F. Acc. Chem. Res. 1998, 31, 852. (b) Lockwood, M. A.; Fanwick, P. E.; Einentein, O.; Rothwell, I. P.
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(3) Mitra, K. N.; Goswami, S. Inorg. Chem. 1997, 36, 1322.
(4) Mitra, K. N.; Majumdar, P.; Peng, S.-M.; Castin˜eiras, A.; Goswami, S. Chem. Commun. 1997, 1267.
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Inorg. Chem. 2002, 41, 7125
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7135
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ing the fusion of aromatic amines at a coordinated diazo ligand, 2-(phenylazo)pyridine (pap). We note here that amination of an aromatic ring that occurs with C-N bond formation is difficult but is a synthetically useful reaction. In the course of our investigations on the amination of pap ligands in M-pap complexes, we were concerned with the site selectivity of the fusion of ArNH2 at the pendant phenyl ring of the coordinated diazo ligand (Scheme 1). We recently have noted9that one of the two pap ligands in the rhodium complex [RhCl2(pap)2]+([1]+) undergoes amination at the ortho carbon to result in a tridentate ligand, while the second pap ligand in the above rhodium complex is aminated at the para carbon and thus produces a bidentate ligand. These results have prompted us to look further into the above fusion reactions. To understand the reasons for the site selection of the above amination reaction, we have designed and carried out the chemical reactions starting from some chosen rhodium substrates. In this context, we found it worthwhile to compare the results with the ruthenium analogues. Bivalent ruthenium is isoelectronic with trivalent rhodium; however, the properties of these two are vastly different, which has been useful for understanding the above fusion reactions of coordinated pap ligands.
The primary aim of this paper is to look into the aromatic amine fusion reactions of metal-coordinated pap ligand. These have been followed by the isolation of the intermedi-ates as well as the final products. In all cases, identifications of the products are made, beyond doubt, by use of single-crystal X-ray structure analysis.
Results and Discussion
Reaction of Preformed HL1and RhCl
3‚3H2O. We first consider the products obtained from the reaction of the preformed tridentate ligand HL1with hydrated RhCl
3. The
ligand HL1 was obtained13 by the regioselective ortho-aromatic amination of coordinated pap ligand to a cobalt(II) center. The reaction (Scheme 2) in boiling methanol produced a green solution in about 4 h. Two green products 2 and 3 were isolated from the crude mixture on a preparative silica gel thin-layer chromatographic (TLC) plate. The presence of dilute NEt3is necessary for the above reaction; otherwise the yields of the products 2 and 3 become negligible. These cationic compounds were obtained as their chloride salts, and the complexes 2c and 3a were found14to be suitable for X-ray structural work.
Compound 2 was eluted as the first green band with chloroform-acetonitrile (2:1) mixture as the eluent and was obtained in ca. 35% yield. The ORTEP plot and atom numbering scheme for the cationic part of 2c are shown in Figure 1. Selective bond distances are collected in Table 1. The structural analysis of the cationic [Rh(L1c)
2]+ reveals the presence of two [L1c]-ligands, each of which acts as an N,N,N-tridentate donor with deprotonation of the amine nitrogens, viz. N(2) and N(7). The pyridyl nitrogens N(1) and N(6) remain uncoordinated. The geometry of the cationic complex is meridional, and the Rh atom sits on an imposed C2 axis bisecting the angles N(5)-Rh-N(10) and N(2)-Rh-N(7). The two azo nitrogen atoms of the anionic tridentate ligands approach the metal center more closely [Rh-N(3)/N(8) average 1.962(4) Å] than the other four Rh-N bonds. There is an indication of significant backbone conjugation in the coordinated [L1c]
-ligand. Thus the Nd N azo distances (average 1.291(5) Å) in this complex are appreciably elongated. The main features of the structure of
2c are similar to those for the complexes11,13 of [L1a] -involving 3d-metal ions, [M(L1)
2] +
[M ) Co(III), Fe(III)]. The second green band was eluted by the same solvent mixture and was obtained in ca. 30% yield. The color of this 3 is similar to that of compound 2. The compound 3a formed suitable X-ray quality crystals for structure deter-mination. Figure 2 shows the ORTEP plot and atom (7) Ghosh, A. K.; Mitra, K. N.; Mostafa, G.; Goswami, S. Eur. J. Inorg.
Chem. 2000, 1961.
(8) Majumdar, P.; Falvello, L. R.; Tomas, M.; Goswami, S. Chem. Eur.
J. 2001, 7, 5222.
(9) Saha, A.; Ghosh, A. K.; Majumdar, P.; Mitra, K. N.; Mondal, S.; Rajak, K. K.; Falvello, L. R.; Goswami, S. Organometallics 1999, 18, 3372. (10) Ghosh, A. K.; Majumdar, P.; Falvello, L. R.; Mostafa, G.; Goswami,
S. Organometallics 1999, 18, 5086.
(11) Saha, A.; Majumdar, P.; Peng, S.-M.; Goswami, S. Eur. J. Inorg.
Chem. 2000, 2631.
(12) (a) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buchwald, S. L. Acc.
Chem. Res. 1998, 31, 805. (b) Hartwig, J. F. Angew. Chem., Int. Ed. 1998, 37, 2046.
(13) Saha, A.; Majumdar, P.; Goswami, S. J. Chem. Soc., Dalton Trans.
2000, 1703.
(14) After repeated trials we could grow X-ray quality crystals of 2c. The compounds 2a and 2b are crystalline but did not produce suitable X-ray quality crystals.
Scheme 1 Scheme 2
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numbering scheme for the cationic part of the salt 3a. Analysis of the crystal structure indeed authenticated the formation of a mixed ligand compound, [RhCl(pap)(L1)]+
, from the reaction as shown in Scheme 2. One of the two HL1ligands here has undergone a deamination reaction with the cleavage of a C-N bond to form the bidentate neutral
pap ligand. Notably, the relative geometry of the pairs of azo nitrogens in 2 is trans while that in 3 is cis. Finally, we wish to note that the N-N distance in the anionic extended ligand [L1a]
-is appreciably longer [N(2)-N(3), 1.298(4) Å] than that [N(6)-N(7), 1.266(4) Å] in the neutral pap ligand. This is attributed to the strong electron delocalization in [L1a]-along the ligand backbone. Selected bond distances are collected in Table 1. Moreover, we wish to note here that the M-N distances with the middle nitrogen of coordinated terpyridyl and other related N, N, N donors are usually shorter than the other two M-N lengths.15,16 How-ever, there is no indication of π-interactions between rhodium(III) and the neutral pap ligand.
During chromatographic purification, a small dark green brown band remained uneluted near the spot, which could not be purified so far.
Reaction of Preformed HL1 and RuCl
3‚3H2O. The reaction of hydrated RuCl3with HL1in boiling methanol in the presence of dilute NEt3produced a brown solution in about 5 h. Two new brown compounds 4 and 5 and three isomers of the known [RuCl2(pap)2] were isolated from the crude product using a preparative TLC separation technique (Scheme 3). Compound 4 was eluted as the first brown band (15) (a) Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Mastroianni, S.; Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J. J.
Chem. Soc., Dalton Trans. 2001, 1639. (b) Cadierno, V.; Gamasa, M.
P.; Gimeno, J.; Iglesias, L.; Garcı´a-Granda, S. Inorg. Chem. 1999,
38, 2874. (c) Pramanik, N. C.; Pramanik, K.; Ghosh, P.; Bhattacharya,
S. Polyhedron 1998, 17, 1525.
(16) Jahng, Y.; Thummel, R. P.; Bott, S. G. Inorg. Chem. 1997, 36, 3133. Figure 1. ORTEP plot and atom numbering scheme for the cationic part
of the complex 2c. Hydrogen atoms are not shown for clarity. Table 1. Selected Bond Distances (Å)
2c Rh(1)-N(5) 2.043(4) C(12)-N(4) 1.397(6) C(28)-N(9) 1.393(7) Rh(1)-N(3) 1.965(4) N(4)-N(3) 1.288(5) N(9)-N(8) 1.293(5) Rh(1)-N(2) 2.036(4) N(3)-C(11) 1.366(6) N(8)-C(27) 1.348(6) Rh(1)-N(10) 2.053(4) C(11)-C(6) 1.436(6) C(27)-C(22) 1.447(6) Rh(1)-N(8) 1.958(4) C(6)-N(2) 1.346(5) C(22)-N(7) 1.335(6) Rh(1)-N(7) 2.037(4) N(2)-C(5) 1.421(5) N(7)-C(21) 1.419(6) N(5)-C(12) 1.377(6) N(10)-C(28) 1.374(6) 3a Rh(1)-N(1) 2.047(3) N(1)-C(5) 1.366(4) N(4)-C(12) 1.423(4) Rh(1)-N(3) 1.941(3) C(5)-N(2) 1.394(4) N(5)-C(22) 1.356(4) Rh(1)-N(4) 2.045(3) N(2)-N(3) 1.298(4) C(22)-N(6) 1.379(4) Rh(1)-N(5) 2.052(3) N(3)-C(6) 1.366(4) N(6)-N(7) 1.266(4) Rh(1)-N(7) 2.011(3) C(6)-C(11) 1.416(5) N(7)-C(23) 1.433(4) Rh(1)-Cl(1) 2.3337(9) C(11)-N(4) 1.342(4) 4a Ru-N(1) 2.091(2) C(22)-N(6) 1.372(3) C(5)-N(2) 1.382(3) Ru-N(3) 1.940(2) N(6)-N(7) 1.316(3) N(2)-N(3) 1.308(3) Ru-N(4) 2.044(2) N(7)-C(23) 1.438(3) N(3)-C(6) 1.378(3) Ru-N(5) 2.065(2) C(27)-C(28) 1.404(3) C(6)-C(11) 1.423(3) Ru-N(7) 1.950(2) C(28)-N(8) 1.379(3) C(11)-N(4) 1.355(3) Ru-Cl 2.3923(9) N(8)-C(29) 1.415(4) N(4)-C(12) 1.428(3) N(5)-C(22) 1.354(3) N(1)-C(5) 1.365(3) 5a Ru-N(1) 2.086(6) N(1)-C(5) 1.357(8) N(4)-C(12) 1.421(8) Ru-N(3) 1.933(5) C(5)-N(2) 1.364(8) N(5)-C(22) 1.368(8) Ru-N(4) 2.038(6) N(2)-N(3) 1.312(7) C(22)-N(6) 1.360(8) Ru-N(5) 2.068(5) N(3)-C(6) 1.372(8) N(6)-N(7) 1.314(7) Ru-N(7) 1.933(5) C(6)-C(11) 1.406(9) N(7)-C(23) 1.432(7) Ru-Cl 2.391(2) C(11)-N(4) 1.353(8) 6d Rh(1)-N(4) 2.048(5) C(14)-N(3) 1.378(9) N(5)-C(23) 1.360(9) Rh(1)-N(2) 1.937(6) N(3)-N(2) 1.313(7) C(23)-N(6) 1.408(9) Rh(1)-N(1) 2.042(5) N(2)-C(13) 1.351(8) N(6)-N(7) 1.278(8) Rh(1)-N(5) 2.055(6) C(13)-C(8) 1.421(9) N(7)-C(24) 1.427(8) Rh(1)-N(7) 2.056(6) C(8)-N(1) 1.359(9) C(27)-N(8) 1.364(9) Rh(1)-Cl(1) 2.3427(18) N(1)-C(5) 1.415(8) N(8)-C(30) 1.436(9) N(4)-C(14) 1.391(8)
Figure 2. ORTEP plot and atom numbering scheme for the cationic part of the complex 3a. Hydrogen atoms are not shown for clarity.
Scheme 3
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with benzene-chloroform mixture (3:2) as the eluent and was obtained in ca. 10% yield. The unsubstituted compound
4a formed suitable X-ray quality crystals for structure
determination. An ORTEP plot along with atom numbering scheme for 4a is shown in Figure 3. It shows that the coordination modes of attachment of the two ligands to the central ruthenium(II) ion are different. While one of these acts as a monoanionic tridentate ligand and chelates through N(1), N(3), and N(4), the other one acts as a neutral tridentate ligand and chelates to the metal ion via pyridine nitrogen N(5) and the azo nitrogen N(7). The neutral ligand thus bears a pendant arylamino group. The sixth coordination site of the central metal ion is occupied by a chloride ion providing a distorted octahedral [RuN5Cl] coordination environment. The relative positions of the pairs of pyridyl nitrogen and azo nitrogen in this complex are both cis. In this geometry, the amine N(8) is far from the metal center for coordination. Notably, the relative orientations of the coordinated atoms in 4 are different from that in the rhodium complex 2. The extensiveπ-bonding from metal to ligand in 4a is clear from the bond distance data of the compound (Table 1). Among the five N bonds the two N(azo) bonds, viz. Ru-N(3) and Ru-N(7), are shorter compared to other three Ru-N bonds. The N-N distances [1.308(3) Å (N(2)-N(3)) and 1.316(3) Å (N(6)-N(7))] are longer17 than those in [papH]ClO4. These facts are indications of (a) delocalization of electronic charge of the anionic ligand, [L1]
-, along the ligand backbone and (b) strongπ-interactions between the metal d-orbitals and the neutral ligand [HL1] via its azo functionality.
The color of compound 5 is also brown; 5 was separated from the first brown band 4 on a preparative silica gel TLC plate using same solvent mixture as the eluent. These two bands overlap each other and were separated only upon allowing sufficient time for the separation. This was obtained in ca. 20% yield. Elemental analysis of compound 5 and its
1H NMR spectra indicated the loss of an aromatic amine group from the ligand. The structural analysis of 5a revealed that its composition corresponds to a mixed ligand complex, [RuCl(pap)(L1a)]. In this compound, the ligand pap is formed due to deamination of HL1a. The physicochemical properties of this compound are similar to those of compound 4. Also, bond distances data (Table 1) show similar strong metal-ligandπ-interactions. The ORTEP plot and atom numbering scheme for 5a are shown in Figure 4. It may be noted here that the geometry of the molecular complex 5 is identical to that of the cationic rhodium complex 3. It may be relevant to compare the bond lengths of the two related complexes
3a and 5a. In these two complexes, the metal centers are
surrounded by a distorted octahedral coordination environ-ment with one chloride ligand, the bidentate neutral chelating ligand, pap, and the tridentate monoanionic bischelating ligand, [L1a]-. The bond lengths of both 3a and 5a, along the ligand backbone of the coordinated [L1a]-, indicate extensive delocalization. As a result, (a) the NdN lengths are elongated, (b) azo nitrogen to phenyl carbon lengths are shorter than those in pap, and (c) the bond lengths of amine nitrogens to the phenyl group of pap are shorter than a C-N single bond. However, such delocalization was not observed in the neutral pap ligand. Interestingly, the Ru-N (azo, pap) distance, in 5a, is notably shorter than the Ru-N (py, pap) distance. Moreover, the N-N distance (pap) is elongated appreciably. This is attributed to strong dπ-pπ interactions between Ru(II) and π* (azo) of pap. Such metal-ligand interaction was, however, absent in 3a. In this compound Rh-N (azo, pap) and Rh-N (py, pap) lengths are compa-rable and consequently the N-N distance (pap) retains its double bond character.17
Besides the aforementioned two brown compounds, the three known isomers of [RuCl2(pap)2], viz. tcc-[RuCl2(pap)2], ctc-[RuCl2(pap)2], and ccc-[RuCl2(pap)2], were also formed. These were separated on a silica gel TLC plate using chloroform as the eluent. The yields of these compounds varied between 5% and 10%. Their spectroscopic and (17) Saha, A.; Das, C.; Peng, S.-M.; Goswami, S. Indian J. Chem., Sect.
A 2001, 40, 198.
Figure 3. ORTEP plot and atom numbering scheme for the molecular
complex 4a. Hydrogen atoms are not shown for clarity. Figure 4. ORTEP plot and atom numbering scheme for the molecular complex 5a. Hydrogen atoms are not shown for clarity.
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voltammetric data exactly corresponded to those of the authentic samples of the known isomeric [RuCl2(pap)2]. We also noted a few more minor bands on the TLC plate; none of these produced any detectable pure compound. In the above synthetic reaction involving RuCl3‚3H2O and HL1, the starting trivalent ruthenium salt underwent one-electron reduction in methanol. The mechanism and the byproducts of the above reduction [Ru(III) f Ru(II)] are not established. However, we believe that complex formation followed by reduction18would be a plausible path for the formation of bivalent ruthenium complexes.
Except for the first brown compound 4, the rest of the ruthenium compounds, described above, are obtained due to the cleavage of at least one C(phenyl)-N(amine) bond in HL1. This organic transformation was not observed in the absence of metal ion.
Comparison between Schemes 2 and 3. The two types
of products of undissociated HL1/[L1]-were obtained from the above two reactions. While in the case of ruthenium one of the two coordinated ligands does not use its maximum denticity, the ligands are symmetrically chelated in the rhodium complex, [Rh(L1)
2] +
. It is known16that the substitu-tion reacsubstitu-tions in [MCl3‚3H2O] (M ) Ru, Rh) proceed stepwise. Thus the product {[MCl3L1]m-} may be viewed as the intermediate complex for the above reactions (Scheme 4). This intermediate complex has one equatorial and two axial leaving groups (Cl), which are substituted by another HL1/[L1]-to form the final product. An axial attack by the terminal pyridyl(N) at the intermediate complex would produce a bischelate [M(L1)
2]n+, whereas an equatorial attack would lead to the mixed ligand complex [MCl(HL1)(L1)]n+. Thus it may be concluded that while axial attack is favored in the case of rhodium, the equatorial attack is preferred for ruthenium. It may be recalled here that in all its known complexes11,13with 3d-metal ions, the ligand, under
consid-eration, binds as an anionic tridentate donor to yield metal complexes of the general formula [M(L1)
2]n+. In order to look for the differences, we also note that trans-N(azo), N(azo) pair orientation19,20has never been observed in the ruthenium(II) complexes of pap ligands. The trans-M[N(azo), N(azo)] grouping is however, an essential prerequisite for the formation of [M(L1)
2]n+ complex. Unlike the 3d-metal ions, the ruthenium(II) ion is known to enter into π-interac-tions effectively with the low-lyingπ* (ligand) orbitals. Thus, very strong dπ-azo(π*) back-bonding is a persistent feature in Ru(II)-pap chelates. In the trans-N(azo), N(azo) pair geometry, theπ*(azo) orbitals compete for the same metal dπ orbital, which weakens back-bonding. The rhodium(III) ion, on the other hand, is not a goodπ-donor and behaves like 3d-metal ions. Notably, osmium(II) also behaves21like ruthenium(II) in its reaction with HL1.
Compounds 3 and 5 and the three isomers of [RuCl2(pap)2] are formed due to cleavage22 of at least one C(phenyl)-N(amine) bond. The mechanism for this bond cleavage reaction has not been elucidated. We note here that the ligand HL1reacts spontaneously with 3d-metal ions, where no such C-N bond cleavage was observed.11,13
Amination Reaction. The mixed ligand complexes
[MCl-(pap)(L1)]n+(M ) RhIII, 3 (n ) 1); M ) RuII, 5 (n ) 0)) contain coordinated pap ligand, and it was anticipated9that these might be suitable substrates for amination reactions. In line with our strategy, both the complexes 3a and 5a were reacted with ArNH2neat and in the presence of air. While compound 3a reacted instantaneously to produce an ink blue product, the corresponding ruthenium complex 5a is unre-active to ArNH2. Compounds 3a and 5a are similar except that the former is cationic while the latter is a molecular complex. Although [5a]+ can be generated in solution by the exhaustive electrolysis of 5a, we could not isolate [5a]+ in its pure state. Thus it is concluded that amination reaction is facilitated23in a cationic complex, whose electrophilicity is higher than that for a molecular complex. Interestingly, the ink blue product, obtained from the reaction of 3a and ArNH2, is similar to the product obtained directly from the reaction of [RhCl2(pap)2]+and ArNH2. It was proposed9,24 that dissociation of a chloride ion from [RhCl2(pap)2]+creates a vacant site and is hence responsible for ortho-amination of one of the two pap ligands in the starting rhodium dichloro complex. The second chloride ion could not be dissociated under our experimental conditions, and hence the amination
(18) (a) Goswami, S.; Chakravarty, A. R.; Chakravorty, A. Inorg. Chem.
1981, 20, 2246. (b) Choudhury, S.; Kakoti, M.; Deb, A. K.; Goswami,
S. Polyhedron 1992, 11, 3183.
(19) (a) Seal, A.; Ray, S. Acta Crystallogr., Sect. C 1984, 40, 929. (b) Krause, K.; Krause, R. A.; Larsen, S.; Rasmussen, B. Acta Chem.
Scand. Ser. A 1985, 39, 375. (c) Majumdar, P.; Peng, S.-M.; Goswami,
S. J. Chem. Soc., Dalton Trans. 1998, 1569. (d) Velders, A. H.; Kooijman, H.; Spak, A. L.; Haasnoot, J. G.; Vos, D. de; Reedijk, J.
Inorg. Chem. 2000, 39, 2966.
(20) Ghosh, B. K.; Mukhopadhyay, A.; Goswami, S.; Ray, S.; Chakravorty, A. Inorg. Chem. 1984, 23, 4633.
(21) Das, C.; Peng, S.-M.; Lee, G.-H.; Goswami, S. New J. Chem. 2002,
26, 222.
(22) (a) Bhattacharyya, S.; Weakley, T. J. R.; Chaudhury, M. Inorg. Chem.
1999, 38, 5433. (b) Gray, S. D.; Weller, K. J.; Bruck, M. A.; Briggs,
P. M.; Wigley, D. E. J. Am. Chem. Soc. 1995, 117, 10678. (23) Kamar, K. K.; Saha, A.; Castin˜eiras, A.; Hung, C.-H.; Goswami, S.
Inorg. Chem. 2002, 41, 4531. Das, C.; Saha, A.; Hung, C.-H.; Lee,
G.-H.; Peng, S.-M.; Goswami, S. Inorg. Chem. 2003, 42, 0000. (24) Deb, A. K.; Goswami, S. J. Chem. Soc., Dalton Trans. 1989, 1635.
Scheme 4
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of the second pap in the said complex occurs at the para position as the second choice. Our present results have indeed confirmed our proposition. Compound 3, in fact, is the proposed intermediate for the transformation [RhCl2(pap)2]
+ f [RhCl(HL2)(L1)]+([1]+f [6]+). The results have been
summarized in Scheme 5.
The reaction of 3a with three different aromatic amines, ArNH2 (ArNH2 ) aniline, p-toluidine, and p-anisidine) produced a deep blue mixture, which on chromatographic purification yielded the ink blue crystalline compound 6 in
>75% yield. The compound [RhCl(HL2b)(L1b)](PF 6) (6d) formed crystals suitable for the determination of its X-ray structure. In this complex, the extended ligand HL2bis neutral and is formed due to fusion of p-toluidine fragment to the coordinated pap ligand in 3b at the para position (relative to the azo fragment) of the phenyl group of pap. This acts as a bidentate ligand and chelates to rhodium through the pyridine nitrogen N(5) and the azo nitrogen N(7). The amine nitrogen atom N(8) of this bears a hydrogen atom and remains uncoordinated. The bidentate HL2b and anionic tridentate [L1b]
-along with one chloride ion complete a distorted octahedral environment about rhodium.9 A com-parison of bond distance data in this molecule (Table 1)
indicates different levels of conjugation in the two coordi-nated ligands. For example, the N-N distance for the neutral HL2b, N(6)-N(7), is 1.278(8) Å while the corresponding distance in the anionic tridentate ligand [L1b]-, N(2)-N(3), is 1.313(7) Å. Taken together, the lengthening of the azo NdN distance in [L1b]-and the foreshortening of the N-C distances alongside the azo moiety, with respect to the corresponding distances in HL2b itself, indicate greater delocalization of π-electron density in [L1b]-. The central Rh-N bond of the anionic ligand, Rh(1)-N(2) ) 1.937(6) Å, is significantly shorter than the other four Rh-N lengths, which are all near their average value of 2.050(5) Å.
Furthermore, the mixed rhodium(III) complex [RhCl(pap)-(L1)]+(3) is substitutionally inert, from which chloride ion dissociation was not possible. The amination reaction in this substitutionally inert complex occurs exclusively at the para carbon of the pendant phenyl ring of coordinated pap.
Spectral Properties. Selected spectral data are collected
in Table 2. The features of the IR spectra of the complexes are closely similar to those of the reported examples.9,11,13 The NdN stretching frequencies are lowered. The1H NMR spectra of the bis chelated compounds, 2, are resolved. Notably, each kind of proton of the coordinated ligand [L1] -gave rise to one signal confirming the presence of a 2-fold symmetry axis. The rest of the compounds 3-6 are all mixed ligand complexes that are coordinated by different ligands. As a result, these gave rise to complex spectral patterns due to the presence of large numbers of unique protons. While the methyl resonances of HL1b and HL2b appeared in the range 2.15-2.35δ, in HL2c (OCH
3) it appeared near 3.90 δ.
Figure 6 shows the solution spectra of the representative examples. Multiple low-energy transitions are the charac-teristics of all the spectra (Table 2). The solution color of the two rhodium complexes, viz. 2 and 3, are green while that for 6 is ink blue. The spectral patterns of the compounds
2 and 3 are similar except that the intensities of the
low-energy transitions (900-650 nm) for compound 2 are roughly double the intensities of similar transitions for compound, 3. It is to be noted here that while compound 2 is coordinated by the two anionic tridentate ligands [L1]
-, compound 3, on the other hand, has only one such ligand. Table 2. Spectral Data
IR,acm-1 compd ν(N-H) ν(CdN) ν(NdN) absorptionbλ max, nm (, M-1cm-1) 2a 1600 1310 890 (7370), 795 (11 350), 725 (11 240), 655,c440,c350 (19 890), 255d(36 160) 2b 1595 1325 900 (6270), 800 (9710), 735 (9520), 660,c435,c355 (16 120), 265d(30 930) 2c 1595 1310 885 (5670), 790 (8960), 715 (8240), 645,c430,c350 (15 800), 275 (23 560) 3a 1610 1320, 1435 895 (3870), 800 (6840), 730 (6330), 660,c360 (18 320), 265d(25 250) 3b 1610 1320, 1430 900 (3280), 800 (5770), 735 (5360), 665,c365 (15 250), 265d(21 640) 3c 1600 1310, 1435 890 (3090), 790 (5610), 720 (4790), 650,c350 (13 140), 275 (16 390) 4a 3390 1595 1233, 1295 950d(3310), 830 (4380), 555d(6190), 470 (9410), 375,c290 (29 180) 4b 3405 1595 1230, 1275 960d(3150), 830 (4340), 555d(6350), 470 (9420), 390,c290 (29 330) 5a 1595 1230, 1275 970d(3450), 850 (4450), 555 (7050), 475 (10 280), 300 (22 030) 5b 1595 1225, 1270 990d(3620), 850 (5060), 550 (7400), 475 (10 980), 300 (23 080) 6a 3410 1585 1310, 1435 910 (3020), 805 (5500), 605 (28 910), 395,c355d(11 620), 270 (25 230) 6b 3400 1585 1310, 1440 915 (2820), 810 (5150), 605 (25 500), 400,c350d(10 500), 280 (21 600) 6c 3405 1585 1315, 1440 910 (2910), 805 (5500), 605 (24 860), 400,c360 (11 830), 275 (26 560) 6d 3410 1590 1315, 1440 910 (2720), 805 (4940), 610 (27 660), 395,c340d(16 260), 270 (26 920) aIn KBr disk.bIn acetonitrile.cIll-defined shoulder.dShoulder.
Scheme 5
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These results tempt us to propose that these low-energy transitions in 2 and 3 involve orbitals that are predominantly ligand in character and originated primarily from the coordinated [L1]
-ligand part. We have performed extended Hu¨ckel MO calculations with the crystallographic parameters
of 2c, 3a and 6d, using the CACAO program25 by Mealli and Proserpio. Due to the obvious limitations, we note that the results are qualitative in nature. However, the calculations have indicated that the HOMO and HOMO - 1 orbitals in these complexes are predominantly ligand orbitals with only small contributions from the metal orbitals (Supporting Information Figure S1). The acceptor orbital LUMO is also a ligand orbital with ca. 20% contribution from the metal d-orbitals. Interestingly, on moving from 3 to 6 the spectral nature is completely changed (Figure 6a). Two low-energy transitions are observed in 6. The lowest energy transitions in the range (900-800 nm) are similar, in both intensity and energy, to that observed in 3. The transition near 730 nm (in 3), however, is replaced by a highly intense transition (
> 25 000 M-1cm-1) near 605 nm, which is responsible for the intense ink blue color of compound 6. In fact, the blue color is characteristic23of all the transition metal complexes that contain HL2 as a ligand. In the ligand, HL2, a donor amine group is separated from an acceptor azo function by a conjugated spacer. The acceptor ability of the azo chro-mophore is expected to be augmented on coordination. Hence the transition near 605 nm in the complexes 6 is assigned to an intraligand transition localized at the coordinated HL2 ligand. We wish to note here that such low-energy transitions in rhodium(III) complexes are virtually unknown.26
The spectra of the ruthenium complexes consist of similar two low-energy transitions in the range (1000-820 nm), which are red shifted compared to those of the rhodium(III) complexes. Notably, two additional strong transitions near 550 and 475 nm were also observable (Figure 6b). The ruthenium(II) complexes containing acceptor ligands in general show27intense metal-to-ligand charge transfer (MLCT) in the visible region. The above transitions may be ascribed to MLCT transitions. Notably, in addition to the multiple-step ligand reductions, metal oxidation redox processes were observable only in the ruthenium complexes (vide infra).
Redox Properties. Both the free ligands HL1 and HL2 are known to undergo11 two-step reductions in the range -1.20 to -2.00 V. These also show an irreversible anodic
wave near 1.05 V due to their oxidation.
The cyclic voltammograms of the rhodium complexes are dominated by the multiple step reduction processes (Table 3, Figure 7). The anodic responses, on the other hand, are irreversible. All the redox processes in the rhodium com-plexes involve24primarily ligand orbitals.
The voltammograms of the ruthenium complexes 4 and 5 are almost identical. In fact, the two ruthenium complexes are similar with the exception that in 4 an ortho-substituted ligand HL1 replaces the pap ligand. The o-arylamino substitution on pap appears to have a very minor effect on the redox and the spectral properties of the reference complexes. The cathodic responses in the ruthenium com-(25) Mealli, C.; Proserpio, D. M. J. Chem. Educ. 1990, 67, 399. (26) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; Wiley: New York, 1999; p 1048.
(27) (a) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3967. (b) Goswami, S.; Mukherjee, R.; Chakravorty, A. Inorg. Chem. 1983, 22, 2825.
Figure 5. ORTEP plot and atom numbering scheme for the cationic part of the complex 6d. Hydrogen atoms are not shown for clarity.
Figure 6. Electronic spectra of (a) 2a (s), 3a (---), and 6a (- ‚‚ -); and (b) 4a (s) and 5a (---).
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plexes are also due to27-29the reductions of the coordinated ligands. Notably, these responses shift anodic appreciably in the corresponding complexes of rhodium(III) (Table 3). This shift is due to higher electrophilicity of the cationic rhodium(III) complexes10over the neutral ruthenium com-plexes. The first reversible oxidative anodic responses in the ruthenium(II) complexes near 0.45 V are due to the RuII/ RuIIIredox process.
Low potentials of RuII/RuIIIcouples, in 4 and 5, persuaded us to try to isolate the corresponding ruthenium(III) com-pounds in their pure states. Accordingly, attempts were made to oxidize them both chemically and electrochemically.
Unfortunately, the oxidized complexes were not stable enough and revert to the parent bivalent complexes. How-ever, we were successful at generating two representatives of the cationic complexes, viz. [4a]+and [5a]+, in solution by the controlled potential bulk electrolysis of 4a and 5a. Well-resolved rhombic spectra of the above ruthenium complexes [4a]+and [5a]+were obtained in frozen (77 K) dichloromethane-toluene solution. The details are given in the Experimental Section.
The EPR data are collected in Table 4, and the representa-tive spectra are shown in Figure 8. The observed rhombicity of the EPR spectra in the present cases is understandable in terms of the gross molecular symmetry of these complexes, containing three nonequivalent axes.30The rhombic distortion can be thought of as a combination of axial distortion (∆,
which splits t2into a and e) and rhombic distortion (V, which splits e). Spin-orbit coupling causes further changes in the energy gaps. Thus, two electronic transitions of transition energies ∆E1 and ∆E2 (∆E1 < ∆E2) are, in principle, probable within these three levels. All these energy param-eters for the above low spin ruthenium(III) complexes have been computed31 using the observed g-values. The axial distortion is much stronger than the rhombic one (Table 4). The analyses of the EPR spectral data thus indicate that both [4a]+ and [5a]+ are significantly distorted from ideal octahedral arrangements, which indeed is in agreement with the observed structures of the parent bivalent complexes.
The ∆E1and∆E2transitions have been observed in the expected near-IR region and visible region, respectively. In view of approximations of the theory, the agreement between the calculated and observed transition energies is satisfactory.
Conclusion
Some examples of fascinating metal-promoted transforma-tions, viz. amination and deamination at the pendant phenyl ring of coordinated 2-(phenylazo)pyridine, have been fol-lowed by the careful design of chemical reactions. The multiple products, from each of the reactions, have been purified and completely characterized. Electronic as well as kinetic reasons for the formation of the mixed ligand ruthenium complex [RuCl(HL1)(L1)] (4) as against the bis chelate, [Rh(L1)
2]Cl (2) have been noted. The deaminated products, particularly in the cases of rhodium, are found to be useful for the study of amination reactions. Regioselec-tivity of the amination reaction has been proven. The complexes thus formed show rich spectral as well as redox properties, which have been thoroughly investigated. Most notable is the appearance of intense intraligand charge-transfer transitions in the low-energy part of the visible region. The aminated organic ligands, in general, have both donor and acceptor chromophores, which are separated by the conjugated spacers. The complexes of these are under active scrutiny and might be useful in the development32of nonlinear optically active materials.
(28) Krejcˇı´k, M.; Za´lisˇ, S.; Klı´ma, J.; Sy´kora, D.; Matheis, W.; Klein, A.; Kaim, W. Inorg. Chem. 1993, 32, 3362.
(29) Ghosh, B. K.; Chakravorty, A. Coord. Chem. ReV. 1989, 95, 239.
(30) Reiger, P. H. Coord. Chem. ReV. 1994, 135/136, 203.
(31) Bhattacharya, S.; Chakravorty, A. Proc. Indian Acad. Sci., Chem. Sci.
1985, 95, 159 and references therein. Table 3. Cyclic Voltammetric Dataa
compd E1/2,bV (∆Ep, mV) -E1/2,bV (∆Ep, mV)
2a 1.42c 0.48 (70), 0.81 (100), 1.45 (120), 1.75 (120) 2b 1.33c 0.50 (70), 0.81 (90), 1.43 (100), 1.73 (100) 2c 1.49c 0.43 (60), 0.71 (80), 1.35 (100), 1.73 (100) 3a 1.32c 0.07 (90), 0.69 (100), 1.43 (100), 1.83 (100) 3b 1.25c 0.21 (90), 0.75 (90), 1.18 (100), 1.41 (110) 3c 1.39c 0.10 (70), 0.62 (100), 1.32 (100), 1.71 (120) 4a 0.46 (90), 1.42d 0.91,e1.17,e1.72e 4b 0.45 (70), 1.40d 0.90,e1.14,e1.70e 5a 0.46 (80), 1.25d 0.90 (150), 1.87e 5b 0.45 (100), 1.19 (100) 0.89,d1.85e 6a 1.33c 0.25 (80), 0.58 (120), 0.82 (70), 1.48 (110), 1.80 (130) 6b 1.30c 0.25 (90), 0.54 (120), 0.82 (80), 1.42 (120), 1.76 (140) 6c 1.26c 0.25 (90), 0.55 (110), 0.83 (90), 1.45 (100), 1.75 (120) 6d 1.27c 0.25 (90), 0.52 (100), 0.80 (70), 1.50 (90), 1.78 (120)
aConditions: solvent, acetonitrile; supporting electrolyte, NEtClO 4, (0.1
M); working electrode, platinum for oxidation and glassy carbon for reduction processes; reference electrode SCE; solute concentration, ca. 10-3 (M); scan rate, 50 mV s-1.bE1/2, the redox processes are reversible. E
1/2is
calculated as the average of anodic (Epa) and cathodic (Epc) peak potentials.
∆Ep) Epa- Epcin mV.cEpa, irreversible anodic response.d
Quasirevers-ible.eEpc, irreversible cathodic response.
Figure 7. Segmented cyclic voltammograms of (a) 3a and (b) 5a in CH3
-CN (0.1 M TEAP); scan rate 50 mV s-1.
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Experimental Section
Materials. The starting metal salts RhCl3‚3H2O and RuCl3‚3H2O
were digested thrice with concentrated HCl before use. The ligand HL1 and [RhCl
2(pap)2]ClO4 were prepared following the
re-ported13,24procedure. Solvents and chemicals used for the syntheses
were of analytical grade.
Physical Measurements. Microanalytical data (C, H, N) were collected from a Elmer 240C elemental analyzer. A Perkin-Elmer 783 spectrophotometer was used for the measurement of IR spectral data. The 1H NMR spectral data were recorded with a
Bruker Avance 300 spectrophotometer using SiMe4as an internal
standard. Electrochemical measurements were performed at 298 K under a dry nitrogen atmosphere on a PC-controlled PAR Model 273A electrochemistry system. A platinum disk, a platinum wire auxiliary electrode, and an aqueous saturated calomel reference electrode (SCE) were used in a three-electrode configuration. The
E1/2for the ferrocenium-ferrocene couple under our experimental condition was 0.39 V. Electronic spectral data were recorded on a JASCO V-570 spectrophotometer. EPR spectral measurements were done on a Varian Model 109C E-line X-band spectrometer fitted with a quartz Dewar for measurement at 77 K, and all spectral data were calibrated against the spectrum of DPPH.
Reactions of RhCl3‚3H2O with HL1. The reactions of RhCl3‚
3H2O with different substituted ligands (HL1a-HL1c) were
per-formed similarly to synthesize the compounds [Rh(L1)
2]Cl 2 and
[RhCl(pap)(L1)]Cl 3, where [L1]-is the conjugate base of HL1. A
detail of a representative reaction with HL1a is elaborated below.
Isolation of [Rh(L1a)
2]Cl (2a) and [RhCl(pap)(L1a)]Cl (3a).
To a reddish brown mixture of RhCl3‚3H2O (0.23 g, 0.88 mmol)
and HL1a(0.48 g, 1.76 mmol) in 60 mL of methanol, 2 drops of
NEt3was added. The mixture was refluxed on a steam bath. The
color of the solution gradually turned green in 1/2 h. The reaction was continued for 4 h for completion. The resultant solution was cooled and filtered. On evaporation of the green filtrate a dark mass was obtained, which was then loaded on a preparative silica gel TLC plate for separation. The solvent mixture chloroform-acetonitrile (2:1) was used as the eluent. Two green bands 2a and 3a were separated. A dark greenish brown band was observed at the spot, which remained uneluted even with acetonitrile. The isolated green compounds were finally recrystallized from dichlo-romethane-hexane solvent mixture.
2a. Yield: 0.21 g, 35%. Anal. Calcd for C34H26N8ClRh: C,
59.62; H, 3.83; N, 16.36. Found: C, 59.57; H, 3.61; N, 16.25. MS: m/z 649 [M - Cl]+.
3a. Yield: 0.16 g, 29%. Anal. Calcd for C28H22N7Cl2Rh: C,
53.35; H, 3.52; N, 15.55. Found: C, 53.53; H, 3.72; N, 15.61. MS: m/z 594 [M - Cl]+.
The yields and the analytical data of 2b, 3b and 2c, 3c are given below:
2b. Yield: 35%. Anal. Calcd for C36H30N8ClRh: C, 60.64; H,
4.24; N, 15.71. Found: C, 60.51; H, 4.30; N, 15.55.
3b. Yield: 30%. Anal. Calcd for C29H24N7Cl2Rh: C, 54.05; H,
3.75; N, 15.22. Found: C, 54.15; H, 3.80; N, 15.32.
2c. Yield: 32%. Anal. Calcd for C32H24N10ClRh: C, 55.95; H,
3.52; N, 20.39. Found: C, 55.84; H, 3.48; N, 20.31.
3c. Yield: 28%. Anal. Calcd for C27H21N8Cl2Rh: C, 51.37; H,
3.35; N, 17.75. Found: C, 51.32; H, 3.19; N, 17.64.
Reactions of RuCl3‚3H2O with HL1. The reactions of RuCl3‚
3H2O with HL1aand HL1bwere performed by following a similar
procedure. A detail of a representative is elaborated below. Isolation of [RuCl(HL1a)(L1a)] (4a) and [RuCl(pap)(L1a)] (5a).
To a reddish mixture of RuCl3‚3H2O (0.23 g, 0.89 mmol) and HL1a
(0.49 g, 1.78 mmol) in 60 mL of methanol, 2 drops of NEt3was
added. The mixture was refluxed on a steam bath for 5 h. The initial reddish color changed to dark brown during this period. The resultant solution was cooled and filtered. On evaporation of the filtrate a dark crude mass was obtained. It was washed thoroughly with diethyl ether. The crude mass was recrystallized several times from dichloromethane-hexane mixture. Then it was subjected to chromatography on a preparative silica gel TLC plate using a benzene-chloroform mixture (3:2) as the eluent. Two brown bands 4a and 5a were separated gradually after allowing sufficient time. The other byproducts, the three isomers of [RuCl2(pap)2], formed
in this reaction were also isolated as distinct bands using chloroform as the eluent. Some minor brownish violet bands were also observed on the TLC plate, which could not be isolated in pure form.
4a. Yield: 0.07 g, 11%. Anal. Calcd for C34H27N8ClRu: C,
59.69; H, 3.98; N, 16.38. Found: C, 59.48; H, 4.12; N, 16.23. 5a. Yield: 0.12 g, 23%. Anal. Calcd for C28H22N7ClRu: C,
56.71; H, 3.74; N, 16.53. Found: C, 56.53; H, 3.90; N, 16.39. The yields and analytical data of 4b and 5b are as follows: 4b. Yield: 12%. Anal. Calcd for C36H31N8ClRu: C, 60.71; H,
4.39; N, 15.73. Found: C, 60.47; H, 4.19; N, 15.45. (32) (a) Lacroix, P. G. Eur. J. Inorg. Chem. 2001, 339. (b) Bozec, H. L.;
Remouazd, T. Eur. J. Inorg. Chem. 2000, 229. (c) Dhenaut, C.; Ledoux, I.; Samuel, I. D. W.; Zyss, J.; Bourgault, M.; Bozec, H. L.
Nature 1995, 374, 339. (d) Zyss, J.; Ledoux, I. Chem. ReV. 1994, 94,
77.
Table 4. Electronic Spectra and EPR Data of the Cationic Compounds [4a]+and [5a]+
EPR g valuescand derived energy params,dcm-1 compd electronic spectraaλ
max, cm-1 g1 g2 g3 ∆ V ∆E1 ∆E2
[4a]+ 7930, 11 900, 15 430,b19 530, 27 320, 35 460 2.1437 2.0792 1.9699 10 330 4800 7970 12 850
[5a]+ 8200, 11 620, 19 450, 27 620, 32 470b 2.1367 2.0726 1.9729 10 950 5430 8280 13 780
aQualitative spectra in acetonitrile.bShoulder.cIn 1:1 dichloromethane-toluene solution at 77 K.dTaking the value of spin-orbit coupling constant
(λ) for low-spin ruthenium(II) as equal to 1000 cm-1.
Figure 8. EPR spectra of (a) 4a and (b) 5a in 1:1 dichloromethane-toluene solution at 77 K, also showing computed splittings of t2orbitals.
DPPH ) diphenylpicrylhydrazyl.
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5b. Yield: 22%. Anal. Calcd for C29H24N7ClRu: C, 57.38; H,
3.98; N, 16.15. Found: C, 57.17; H, 4.11; N, 16.01.
Amination Reactions. Two different routes of amination reac-tions on coordinated pap ligand have been explored. In the first route we have described the amination reaction of coordinated pap in [RhCl(pap)(L1)]Cl (3). The second method involves the reaction
of [RhCl2(pap)2]ClO4 (1) with different ArNH2 compounds. The
products, obtained from the two routes, are identical but the yields are different.
Method I. [3]+f [6]+, a Representative Example: Synthesis
of [RhCl(HL2a)(L1a)]+, [6a]+. A sample of [RhCl(pap)(L1a)]Cl (3a)
(0.20 g, 0.32 mmol) was mixed with 0.5 mL of aniline, and the mixture was heated on a steam bath. The green mixture turned blue within 15 min. The reaction was continued for 3 h for the completion of the reaction. The mixture was then cooled and thoroughly washed with diethyl ether. A blue compound was purified on a preparative TLC plate (silica gel) using chloroform-acetonitrile mixture (3:1) as the eluent. Finally the compound, as its chloride salt, was recrystallized from dichloromethane-hexane solvent mixture. Yield: 0.18 g, 78%. Anal. Calcd for C34H27N8
-Cl2Rh: C, 56.60; H, 3.77; N, 15.53. Found: C, 56.47; H, 3.63; N,
15.42. MS: m/z 685 [M - Cl]+.
The yields and analytical data of 6b, 6c, and 6d are given below. 6b. Yield: 78%. Anal. Calcd for C35H29N8Cl2Rh: C, 57.16; H,
3.97; N, 15.23. Found: C, 57.41; H, 4.12; N, 15.39. MS: m/z 699 [M - Cl]+.
6c. Yield: 75%. Anal. Calcd for C35H29N8Cl2ORh: C, 55.94;
H, 3.89; N, 14.91. Found: C, 55.85; H, 3.80; N, 14.98.
6d. Yield: 80%. Anal. Calcd for C36H31N8Cl2Rh: C, 57.69; H,
4.17; N, 14.95. Found: C, 57.52; H, 4.01; N, 14.85.
Method II. [1]+f [6]+, a Representative Example: Synthesis of [RhCl(HL2a)(L1a)]+, [6a]+. A mixture of [RhCl
2(pap)2]ClO4
(0.20 g, 0.31 mmol) and aniline (0.5 mL) was heated on a steam bath for 2 h. The cooled blue mixture was thoroughly washed with diethyl ether. The crude mass was purified on a preparative TLC plate (silica gel) using chloroform-acetonitrile solvent mixture (3: 1) as the eluent. Recrystallization of the product from a dichlo-romethane-hexane solvent mixture yielded dark blue cationic compound 6a as its perchlorate salt. Yield: 0.13 g, 55%. Anal. Calcd for C34H27N8Cl2O4Rh: C, 51.99; H, 3.46; N, 14.26. Found:
C, 51.73; H, 3.33; N, 14.02. MS: m/z 685 [M - ClO4]+.
The cationic compound [6a]+obtained following method II is identical to that obtained from method I. We, however, note that
the two ligands in the product 6 (method II) carries identical substitution, R, depending on the starting ArNH2. The starting
rhodium complex 3 is, in fact, the intermediate for the reaction 1
f 6 (vide supra). Hence the two ligands in the products, obtained
from 3, may not necessarily bear identical substitutions. Electrochemical Generation of [4a]+and [5a]+. The complexes [4a]+and [5a]+were generated in solution by constant potential coulometric oxidation of solutions of 4a and 5a, respectively. A detail of a representative example ([4a]+) is noted below.
A solution of 9.0 mg of 4a in 30 mL of dichloromethane solvent containing 35 mg of tetrabutylammoniumperchlorate was oxidized coulometrically. The oxidation was performed at 0.65 V; n ) 1.22/ 1.27 ) 0.96, n ) Q/Q′, where Q is the coulomb count found after exhaustive electrolysis and Q′is the calculated coulomb count for one-electron transfer. A part of the electrogenerated solution (5 mL) of [4a]+ was mixed with an equal volume of toluene, and the mixture was quickly frozen at 77 K and then used for an EPR measurement. The oxidized complex [4a]+was not stable at room temperature. It underwent reduction to produce 4a almost quanti-tatively. To date, we were not successful in isolating the trivalent ruthenium compound [4a]+in its pure state.
The trivalent complex [5a]+was generated in solution similarly as described above.
Crystallography. Crystallographic data for the compounds 2c, 3a, 4a, 5a, and 6d are collected in Table 5. The suitable X-ray crystals of 2c and 3a were obtained by slow diffusion of a dichloromethane solution of chloride salt of 2c and 3a into hexane. The X-ray crystals of 6d were obtained by slow diffusion of a dichloromethane solution of hexaflourophosphate salt of 6d into hexane. Intensity data of all these three compounds (2c, 3a, and 6d) were collected on a Bruker SMART diffractometer, equipped with graphite monochromated Mo KR radiation,λ ) 0.710 73 Å.
These were corrected for Lorentz polarization effects. All three structures were solved by employing SHELXS-9733 package of
programs and refined by full matrix least squares based on F2
(SHELXL-97).34The X-ray quality crystals of 4a were obtained
by the slow diffusion of a dichloromethane solution of 4a into hexane. Intensity data were collected on a NONIUS CAD4 diffractometer, equipped with graphite monochromated Mo KR,λ (33) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467. (34) Sheldrick, G. M. SHELXL97, Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997. Table 5. Crystal Data for Compound 2c, 3a, 4a, 5a and 6d
2c‚4H2O 3a‚4H2O 4a 5a 6d
empirical formula C32H32ClN10O4Rh C28H30Cl2N7O4Rh C34H27ClN8Ru C28H22ClN7Ru C36H31ClF6N8PRh
formula weight 759.04 702.40 684.16 593.05 859.02
temp [K] 293(2) 293(2) 295(2) 293(2) 150(2)
cryst syst monoclinic monoclinic monoclinic monoclinic triclinic
space group P2(1)/n P2(1)/c P2(1)/n P2(1)/n P1h a [Å] 11.8962(7) 19.7142(11) 10.998(2) 13.481(7) 8.9645(8) b [Å] 22.2032(14) 7.8744(4) 20.067(4) 14.377(9) 12.4699(12) c [Å] 13.4009(8) 20.4434(11) 14.530(3) 13.543(7) 17.0283(17) R [deg] 90 90 90 90 107.483(2) β [deg] 105.7390(10) 101.5580(10) 110.26(2) 91.83(4) 104.519(2) γ [deg] 90 90 90 90 92.834(2) Z 4 4 4 4 2 Dcalcd[mg/m3] 1.480 1.501 1.511 1.502 1.638 cryst dimen [mm3] 0.30× 0.12 × 0.08 0.24× 0.23 × 0.21 0.60× 0.50 × 0.40 0.50× 0.40 × 0.40 0.34× 0.21 × 0.07
θ range for data collection [deg] 1.83-27.51 2.03-27.53 1.81-25 2.07-24.01 1.31-27.52
GOF 0.895 0.864 1.064 1.047 0.819
reflns collected 21 424 19 165 5295 4470 11 255
unique reflns 7746 7097 5295 4027 7761
final R indices [I > 2σ(I)] R1 ) 0.0485 R1 ) 0.0378 R1 ) 0.0252 R1 ) 0.0512 R1 ) 0.0506
wR2 ) 0.1343 wR2 ) 0.0895 wR2 ) 0.0645 wR2 ) 0.1151 wR2 ) 0.1071
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) 0.710 73 Å. The data were corrected for Lorentz polarization
effects. The X-ray quality crystals of 5a were grown by slow diffusion of a toluene solution of 5a into hexane. Intensity data were collected on a Sieman R3m/V diffractometer with graphite monochromated Mo KR radiation,λ ) 0.710 73 Å. All the data
were corrected for Lorentz polarization effects. The structures of compounds 4a and 5a were solved by using the SHELXS-86 package35of programs and refined by full matrix least squares based
on F2 (SHELXL-93).36 All the hydrogen atoms were added in
calculated positions.
Acknowledgment. Financial support received from the
Department of Science and Technology and The Council of Scientific and Industrial Research (New Delhi) is gratefully acknowledged. We are thankful to Prof. Animesh Chakra-vorty and Dr. Samaresh Bhattacharya for help. Thanks are due to RSIC, Lucknow, for providing mass spectra.
Supporting Information Available: X-ray crystallographic details in CIF format of the five compounds 2c, 3a, 4a, 5a, and 6d; molecular orbital diagram of 3a: (a) LUMO, (b) HOMO, and (c) HOMO - 1. These materials are available free of charge via the Internet at http://pubs.acs.org.
IC020421C
(35) Sheldrick, G. M. SHELXS86, Program for the Solution of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1990.
(36) Sheldrick, G. M. SHELXL93, Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1993.
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