Coordination Isomers of Trinuclear Pt2Hg Complex That Differ in Type

Jul 17, 2019 - New type isomers that differ in metal–metal bond type are isolated. One is the Pt–Hg–Pt complex, which has two metal–metal cova...
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Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Coordination Isomers of Trinuclear Pt2Hg Complex That Differ in Type of Metal−Metal Bond Tadashi Yamaguchi* and Kohei Yoshiya Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, Tokyo 169-8555, Japan

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S Supporting Information *

C in Scheme 1). The coordination geometry around both Pt atoms is a six-coordinated octahedron and both Pt−Hg bonds are short covalent-type metal−metal bonds. The other complex is a dative-covalent-type complex that can be denoted as PtII→ HgI−PtIII (type D). The complex can be described that the Hg atom of the Pt−Hg dinuclear complex is coordinated by the other PtII unit through a Pt→Hg dative bond, and one Pt is sixcoordinated octahedral and the other Pt is five-coordinated square pyramidal. The Pt−Hg bond related to the sixcoordinated Pt is a short covalent-type metal−metal bond and that related to the five-coordinated Pt is a longer dative-type bond. If the ligand set is the same, these two complexes are mutual isomers; however, no such pair has been reported. Vicente et al. reported an example of “two-covalent-type” complexes, [Hg{Pt(κ2-Ar)2(O2CCF3)}2] (κ2-Ar = C6(NO2)22,6-(OMe)3-3,4,5-κ2-C,O),36 whereas Puddephatt and co-workers reported “dative-covalent-type” complexes, [PtMe2X(HgX)( t bu 2 bpy){PtMe 2 ( t bu 2 bpy)}] (X − = Br − (1a), Cl − (1b), CF3COO−(1c)).37,38 Here, we report the crystal structures of two isomers of the [PtMe2(bpy)]2-[HgCl2] system, which is based on the types of metal−metal bond, i.e., “two-covalent-type” (Pt−Hg−Pt), [{PtMe2(bpy)Cl}2Hg] (2), and “dative-covalent-type” (Pt→ Hg−Pt), [PtMe2(bpy)Cl(HgCl){PtMe2(bpy)}] (3). We also report that the solution structure of the tbu2bpy derivative 1 at low temperature is a Pt−Hg−Pt type complex, which is different from the estimation of a Pt→Hg−Pt type complex by Puddephatt and co-workers.38 Caution! Mercury complexes usually are toxic and should be handled with great caution. With use of similar system to Puddephat’s trinuclear complex 1 by changing an ancillary ligand on platinum from tbu2bpy to bpy, we obtained two species that are mutual isomers. Slow evaporation of an acetone solution containing [PtMe2(bpy)] and HgCl2 gave brown crystals and yellow crystals simultaneously. The brown color is consistent with the dative-covalent complex 1. The Raman spectrum of the brown crystal shows two peaks at 148 and 123 cm−1, Figure S1. The former peak is similar to that of the corresponding Pt−Hg dinuclear complex, suggesting the existence of a covalent-type Pt−Hg bond, and the latter peak at lower wavenumber is indicative of a weaker metal−metal bond such as a metal−metal dative bond. The coexistence of two peaks suggests that the brown crystals are of the dative-covalent isomer. In contrast, the yellow crystal

ABSTRACT: New type isomers that differ in metal− metal bond type are isolated. One is the Pt−Hg−Pt complex, which has two metal−metal covalent bonds, Pt− Hg = 2.5459(8) and 2.5483(7) Å, and the other is the Pt→Hg−Pt complex, which has one metal−metal covalent bond, Pt−Hg = 2.5744(6) Å, and one metal− metal dative bond, Pt→Hg = 2.6635(7) Å.

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t is well-known that PtII in square-planar complexes with strong field ligands can form metal dative bonds.1−22 When a HgII ion is an acceptor, dative-type Pt→Hg complexes (type A in Scheme 1) are obtained. However, in some cases, covalent-type Scheme 1

Pt−Hg complexes (type B) are obtained.23−40 The major differences between the two types of hetero dinuclear complexes are the Pt−Hg distance and the coordination geometry around the Pt. The Pt−Hg distances are shorter for the covalent-type complexes than the dative-type complexes. The coordination geometry of Pt in dative-type complexes is square pyramidal including a Pt−Hg bond, whereas that in covalent-type complexes is octahedral including a Pt−Hg bond. For the latter complexes, it is thought that an electron transfer from Hg to Pt occurs to form PtIII−HgI (d7-sd10) covalent bond. Which type of the hetero dinuclear complex is obtained depends on the type of the ligands coordinated to the metals. Similarly, there are two types of electron configurations for heterometallic trinuclear Pt2Hg complexes formed from two PtII complexes and one HgII complex. One is a two-covalent-type complex, Pt−Hg−Pt (type © XXXX American Chemical Society

Received: April 4, 2019

A

DOI: 10.1021/acs.inorgchem.9b00978 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry showed only one Raman peak at 104 cm−1, which suggests that the yellow crystals are of the two-covalent isomer. The lower wavenumber relative to the Pt−Hg dinuclear complex is explained as follows. The two Pt−Hg stretchings of the twocovalent isomer are strongly coupled to each other and only symmetric stretching is active in Raman spectroscopy, which is observed at lower wavenumber. The molecular structure of the yellow crystal of [{PtMe2(bpy)Cl}2Hg]·H2O (2·H2O) was confirmed by single crystal X-ray analysis. Figure 1 shows the ORTEP drawing of 2.

Figure 2. ORTEP drawing of 3 in 3·DMSO. Relevant bond lengths (Å) and angles (deg): Pt(1)−Hg(1) = 2.5744(6), Pt(2)−Hg(1) = 2.6635(7), Pt(1)−Cl(1) = 2.470(2), Hg(1)−Cl(2) = 2.649(2); Pt(1)−Hg(1)−Pt(2) = 142.78(2), Pt(1)−Hg(1)−Cl(2) = 110.61(4), Pt(2)−Hg(1)−Cl(2) = 105.85(4), Hg(1)−Pt(1)−Cl(1) = 175.56(5).

Å.14−21 These distances are almost the same as those of Puddephatt’s trinuclear complex, 2.5767(7)−2.5425(3) and 2.6973(7)−2.6283(3) Å, respectively.37,38 The Pt−Hg−Pt angle is 142.78(2)°, and the Hg is also coordinated by Cl−. Although the two isomers are different in the solid state, the DMSO solutions of the two isomers show the same electronic spectrum and the same 1H NMR spectrum that consists of only one methyl signal and half a set of bpy peaks. Moreover, addition of a small amount of [PtMe2(bpy)] to the solution only causes a shift of the peaks without observation of free [PtMe2(bpy)] peaks. These results indicate that rapid equilibrium involving the trinuclear complexes and [PtMe2(bpy)] occurs in the solution, and the spectra only show coalesced peaks. Because of the low solubility of the complex, the 195Pt NMR spectrum of the complexes cannot be measured. However, Puddephatt reported the 195Pt NMR spectrum of the tbu2bpy derivative 1 in low temperature solution.25 The spectrum shows only one singlet with JPt−Hg satellites despite the existence of two environments of Pt. They explained this spectrum as a result of the fluxional behavior involving halide ion migration between Pt and Hg sites. If that is the case, the rate constant required for the migration to show such a sharp coalesced peak should be more than 107 s−1, since a chemical shift difference between PtII and PtIII sites is typically more than 1000 ppm. However, the typical rate constant for substitution of PtIII−X is much less.46−49 The spectrum can also be explained if we assume that the complex exists in two-covalent form instead of the “dative-covalent form” without halide migration. To reveal whether the [{Pt(tbu2bpy)Me2}2{HgX2}] in low temperature solution is in “dative-covalent form” or “twocovalent form”, the 195Pt NMR spectrum of a mixture of [ { P t ( t b u 2 b p y ) M e 2 } 2 { Hg C l 2 }] and [ {Pt( t b u 2 b p y ) Me2}2{HgBr2}] is measured. Hereafter, we denote [{Pt(tbu2bpy)Me2}2HgXY] as {X, Y}. If rapid intermolecular halide migration occurs, the 195Pt NMR spectrum of the mixture should show a single resonance with 199Hg satellites. However, if the halide migration occurs only intramolecularly, the 195Pt NMR spectrum of the mixture shows superposition of the three species, {Cl, Cl}, {Cl, Br}, and {Br, Br}. The spectra of {Cl, Cl} and {Br, Br} are known; thus the remaining peaks are of the {Cl, Br} species. If the complexes exist in “dative-covalent form” with rapid halide migration as proposed by Puddephatt, the {Cl, Br} peaks should be another similar peak set, a single resonance at a slightly different chemical shift with 199Hg satellites since the

Figure 1. ORTEP drawing of 2 in 2·H2O. Relevant bond lengths (Å) and angles (deg): Pt(1)−Hg(1) = 2.5459(8), Pt(2)−Hg(1) = 2.5483(7), Pt(1)−Cl(1) = 2.553(2), Pt(2)−Cl(2) = 2.538(2); Pt(1)−Hg(1)−Pt(2) = 172.15(2), Hg(1)−Pt(1)−Cl(1) = 176.26(4),Hg(1)−Pt(2)−Cl(2) = 177.54(5).

Two Pt atoms are bound to a Hg atom and each Pt is coordinated by one bpy, one Cl−, and two Me ligands. The coordination geometry of Pt is a six-coordinated octahedron and that of Hg is two-coordinated linear geometry, if the Pt−Hg bond is included. Both of the Cl− ligands are located at axial positions of the Pt−Hg−Pt core. The Pt−Hg distances 2.5459(8) and 2.5483(7) Å are slightly shorter than those of Vicente’s complexes, 2.5898(2)−2.6093(3) Å,36 but within the range of the typical distance for covalent-type Pt−Hg bonds, 2.509−2.609 Å.32−40 The coordination geometry of Pt and the short Pt−Hg distance clearly show that the complex is the twocovalent form of the Pt−Hg−Pt complex. Although the Hg atoms of Vicente’s Pt−Hg−Pt complexes are weakly coordinated by carboxylate (Hg−O = 2.714(4)−2.897(5) Å), the Hg atom of the present complex is not coordinated by any ligand except for the Pt atoms. The coordinatively naked Hg resembles a coordinatively naked Pt0 in [Pt3(dppp)2(Me2C6H3CN)2]2+.41 The Cl− is located at an axial position and the Pt−Cl distances are 2.538(2) and 2.554(2) Å, which are slightly elongated from the normal Pt−Cl distance, but similar to that of a Pt−Cl bond trans to a Pt−M bond owing to the trans influence of the Pt−M bond.42−45 Although the crystallinity of the brown crystal was not high enough for X-ray analysis, a similar brown crystal of [PtMe2 (bpy)Cl(HgCl){PtMe2(bpy)}]·DMSO (3·DMSO) with the same Raman spectrum was obtained in good quality from DMSO/acetone mixed solvent. Figure 2 shows an ORTEP drawing of 3. The structure is almost the same as those of Puddephatt’s trinuclear complex. The Pt−Hg bond related to the six-coordinated Pt is shorter, 2.5744(6) Å, which is indicative of a covalent bond, whereas that related to the fivecoordinated Pt is longer, 2.6635(7) Å, which is indicative of a dative bond, whose typical bond lengths are 2.613−2.834 B

DOI: 10.1021/acs.inorgchem.9b00978 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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environment of the two Pt is the same due to rapid halide migration. However, if the complexes exist in “two-covalent form” without halide migration, the {Cl, Br} peaks should be a superposition of isotopomers since the natural abundance of 195 Pt is 33.8%, that is, two single peaks and mutually coupled two doublets, all of which have 199Hg satellites. Figure 3a shows the 195Pt NMR spectrum of the CDCl3 solution of the mixture at −60 °C. The spectrum shows many

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00978. Details on the synthesis, Raman spectra, crystallographic data, bond lengths and angles, 195Pt NMR parameters (PDF) Accession Codes

CCDC 913511−913512 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

Figure 3. (a) 195Pt NMR spectrum of mixture of [{Pt(tbu2bpy)Me2}2{HgCl2}] and [{Pt(tbu2bpy)Me2}2{HgBr2}]. (b) Simulated spectrum for {Cl,Br} (red line), {Cl,Cl} (blue line), and {Br,Br} (green line).

*E-mail: [email protected]. ORCID

Tadashi Yamaguchi: 0000-0002-0434-7066 Funding

This work was supported by Grants-in-Aid for Scientific Research (No. 21550072) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

peaks including those of {Cl, Cl} and {Br, Br}. The rest of the peaks consist of two main peaks and an AB pattern, both accompanied by 199Hg satellites, which is consistent with a strongly coupled unsymmetric diplatinum compound with one Hg.50−52 Thus, the observed spectrum clearly suggests that the Pt−Hg−Pt trinuclear complex exists in “two-covalent form” in low temperature CDCl3 solution. The spectrum of the mixedhalide complex {Cl, Br} can be simulated by using the two chemical (δ(PtCl) and δ(PtBr)) shifts, the coupling constant between the two platinum (2JPt−Pt), and the two coupling constants between the platinum and mercury (1JPt−Hg). The δ(PtCl) and δ(PtBr) are determined from the chemiclal shift of two main peaks, the 2JPt−Pt is determined from the AB pattern peaks, and the two 1 J Pt−Hg are determined from the corresponding satellites. Figure 3b shows the simulated spectrum of a mixture of three species with appropriate intensity ratio. The 2JPt−Pt value of 6080 Hz is rather large for a two-bond coupling. This may be a consequence of a large amount of s character in the metal−metal bond. Similar enlargement of s character is reported for PtIII−PtIII dinuclear complexs with Me ligands whose 1JPt−Pt values range from 10 000 to 12 000 Hz.53 The 1JPt−Hg values of 18 130 Hz for the Cl side and 17 850 Hz for the Br side are almost the same as those of homohalide complexes (Table S7). These values are about half that of the Pt−Hg dinuclear complex.37,38 This is because the contribution of the s orbital of the Hg is reduced by half since it contributes to two Pt−Hg bonds. The peaks became broad when temperature increases, and no peaks are observed at room temperature, which suggests that interconversion between the three species occurs. In summary, we have successfully prepared two isomers of a trinuclear Pt2Hg complex, the “two-covalent type” isomer, Pt− Hg−Pt, and the “dative-covalent type” isomer, Pt→Hg−Pt, based on the difference in the types of metal−metal bonds. To our knowledge, this is the first report of isomerism that differ in type of metal−metal bond.

Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acs.inorgchem.9b00978 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry (42) Yamaguchi, T.; Kubota, O.; Ito, T. An Unbridged Platinum(III) Dimer with Added Chloro Ligands in Equatorial Sites, [Pt2Cl2(phpy)4] (Hphpy = phenylpyridine), Synthesized by an Oxidation with Aurous Complex Chem. Chem. Lett. 2004, 33, 190−191. (43) Yamaguchi, T.; Sasaki, Y.; Ito, T. Unusual C,O-Bridging Coordination of Acetate and Acetylacetonate Ligands in the Platinum Clusters [PtIII2(μ-CH2COO-C,O)2(μ-CH3COO-O,O’)2Cl2]2‑ and PtII4(μ-CH3COO-O,O’)4(μCH3COCHCOCH3-O,C3)4. J. Am. Chem. Soc. 1990, 112, 4038−4040. (44) Bancroft, D. P.; Cotton, F. A. Tetramethyldiplatinum(III) (PtPt) Complexes with 2-Hydroxypyridinato Bridging Ligands. 3. Compounds with Diethyl Sulfide as the Axial Ligand. Inorg. Chem. 1988, 27, 4022−4025. (45) Lippert, B.; Schollhorn, H.; Thewalt, U. Additive Trans Influences of the Axial Ligand and Metal-Metal Bond in a Diplatinum(III) Complex Leading to an Asymmetric Structure with Penta- and Hexacoordination of the Two Metals. J. Am. Chem. Soc. 1986, 108, 525−526. (46) Iwatsuki, S.; Ishihara, K.; Matsumoto, K. Kinetics and mechanisms of the axial ligand substitution reactions of platinum(III) binuclear complexes with halide ions. Sci. Technol. Adv. Mater. 2006, 7, 411−424. (47) Saeki, N.; Nakamura, N.; Ishibashi, T.; Arime, M.; Sekiya, H.; Ishihara, K.; Matsumoto, K. Mechanism of Ketone and Alcohol Formations from Alkenes and Alkynes on the Head-to-head 2Pyridonato-Bridged cis-Diammineplatinum(III) Dinuclear Complex. J. Am. Chem. Soc. 2003, 125, 3605−3616. (48) El-Mehdawi, R.; Bryan, S. A.; Roundhill, D. M. Axial Ligand Anation and Aquation Reactions in Diplatinum(III) Complexes. Comparison of Aquation Rates between PtCl62‑ and Diplatinum(III) Chloro Complexes Having μ-Phosphato or μ-Pyrophosphito Ligands. J. Am. Chem. Soc. 1985, 107, 6282−6286. (49) Camadanli, S.; Deveci, N.; Gokagac, G.; Isci, H. Axial water substitution kinetics of sulphato- and hydrogenphosphato-bridged binuclear platinum(III) complexes. Inorg. Chim. Acta 2003, 351, 1−6. (50) Since the natural abundance of 195Pt (I = 1/2) is 33.8% and all other isotopes have I = 0, the 195Pt NMR spectrum of the unsymmetric platinum dimer is a superposition of the spectra of isotopomers 195 PtA−195PtB, 195PtA−PtB, and PtA−195PtB with abundances of 11.4%, 22.4%, and 22.4%, respectively, and the 195PtA−195PtB spectrum shows an AB pattern when the chemical shift difference is comparable to the coupling constant (Δδ < 5J). See refs 51 and 52. (51) Appleton, T. G.; Hall, J. R.; Neale, D. W. NMR Spectra of Platinum(III) Complexes with Sulfato- and Phosphato-bridges. Inorg. Chim. Acta 1985, 104, 19−31. (52) Yamaguchi, T.; Abe, K.; Ito, T. 195Pt NMR of Tetranuclear Platinum(II) Cluster Complexes That Have Chemically Nonequivalent Nuclei: [Pt4(CH3COO)7(CH3CONH)] and [Pt4(CH3COO)5(CH3CONH)3]. Inorg. Chem. 1994, 33, 2689−2691. (53) Peterson, E. S.; Bancroft, D. P.; Min, D.; Cotton, F. A.; Abbott, E. H. Proton, Carbon-13, and Platinum-195 Nuclear Magnetic Resonance Spectroscopy of Hydroxypyridinate-Bridged Dinuclear Platinum (III) Complexes. Equilibria and Mechanism of Bridging Ligand Rearrangement. Inorg. Chem. 1990, 29, 229−232.

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DOI: 10.1021/acs.inorgchem.9b00978 Inorg. Chem. XXXX, XXX, XXX−XXX