Article pubs.acs.org/Organometallics
Activation of Acetyl Ligands through Hydrogen Bonds: A New Way to Platinum(II) Complexes Bearing Protonated Iminoacetyl Ligands Tim Kluge,† Martin Bette,† Tobias Rüffer,‡ Clemens Bruhn,§ Christoph Wagner,† Dieter Ströhl,† Jürgen Schmidt,⊥ and Dirk Steinborn*,† †
Institute of Chemistry, Martin Luther University Halle-Wittenberg, Kurt-Mothes-Straße 2, D-06120 Halle, Germany Institute of Chemistry, Chemnitz University of Technology, Straße der Nationen 62, D-09111 Chemnitz, Germany § Institute of Chemistry, University of Kassel, Heinrich-Plett-Straße 40, D-34132 Kassel, Germany ⊥ Department of Bioorganic Chemistry, Leibniz Institute of Plant Biochemistry, Weinberg 3, D-06120 Halle, Germany ‡
S Supporting Information *
ABSTRACT: The dinuclear platina-β-diketone [Pt2{(COMe)2H}2(μCl)2] (1) reacted with 2-pyridyl-functionalized monoximes and with dioximes in the presence of NaOMe to yield oxime−diacetyl platinum(II) complexes [Pt(COMe)2(2-pyCRNOH)] (R = H, 4a; Me, 4b; Ph, 4c) and [Pt(COMe)2(HONCR−CRNOH)] (R/R = Me/Me, 5a; Ph/Ph, 5b; (CH2)4, 5c; NH2/NH2, 5d), respectively. The strong intramolecular O−H···O hydrogen bonds in these complexes give rise to an activation of the acetyl ligands for Schiff-base type reactions, thus forming with primary amines iminoacetyl platinum complexes [Pt(COMe)(CMeNHR′)(2-pyCRNO)] (R/R′ = H/ Bn, 6a; Me/Bn, 6b; Ph/Bn, 6c; H/CH2CH2Ph, 6d; H/CH2CHCH2, 6e; Bn = benzyl) and [{Pt(CMeNHR′)2(ONCR−CRNO)}2] (R/R = Me/Me, 7a−d; Ph/Ph, 8a−d; (CH2)4, 9a; R′ = Bn, a; CH2CH2Ph, b; CH2CHCH2, c; CH2CH2OH, d). The intramolecular N−H···O hydrogen bonds in type 6−9 complexes make clear that protonated iminoacetyl ligands (i.e., aminocarbene ligands) and deprotoanted oxime ligands are present. These complexes could also be obtained in reactions of [Pt(COMe)2(NH2R′)2] (3) with pyridyl-functionalized monoximes and with dioximes where type 4/5 complexes were found to be intermediates. In solution, the bis(iminoacetyl) complexes 7−9 were found to be present as dimers (as also 8a in the solid state) with smaller amounts of monomers. The importance of hydrogen bonds for activation of acetyl ligands was further evidenced by synthesis of complexes [Pt(COMe)2(2-pyCHNOMe)] (10) and [Pt(COMe)2(HONCMe−CMeNOMe)] (11) bearing O-methylated oxime ligands and their reactivty toward amines. The hydrogen-bond activated acetyl and iminoacetyl ligands in type 5, 7, and 8 complexes were found to undergo in CD3OD solutions facile H/D exchange reactions resulting in complexes bearing C(CD3)O/C(CD3)NDR′ ligands. The constitution of all complexes was unambiguously confirmed analytically, spectroscopically and in part by single-crystal X-ray diffraction analyses. Structural and NMR parameters as well as DFT calculations gave evidence for relatively strong intramolecular hydrogen bonds. acyl platinum(II) complexes. Thus, the “parent” platina-βdiketone (R = Me, 1) reacted with bidentate N∧N donors such as 2,2′-bipyridine to form thermally extraordinarily stable hydrido−diacetyl platinum(IV) complexes (Scheme 1, reaction path A).3 At elevated temperatures, they were found to undergo a reductive C−H elimination and in the presence of bases a reductive H−Cl elimination yielding mono- and diacetyl platinum(II) complexes, respectively (Scheme 1, B/C).3,4 Reactions of 1 with 2-pyridyl ketoximes and aldoximes proceeded analogously, but the thermal decomposition of the platinum(IV) intermediate complexes resulted in the formation both of mono- and diacetyl platinum(II) complexes bearing 2pyridyl ket-/aldoxime ligands (Scheme 1, D/E).5 In an isolated
1. INTRODUCTION Metalla-β-diketones are formally derived from enol forms of 1,3-diketones by replacing the central CH group by a metal fragment LxM. Thus, they can be understood as hydroxycarbene complexes that are stabilized by intramolecular O−H··· O hydrogen bonds. Due to its electron count (16 VE; VE = valence electrons) and its kinetically labile coordination sphere, the dinuclear platina-β-diketones [Pt2{(COR)2H}2(μ-Cl)2] exhibited a unique reactivity1 that is completely different from Lukehart’s metalla-β-diketones 2 such as [Re{(COMe)2H}(CO)4], which are electronically saturated (18 VE), kinetically inert complexes. A typical reactivity of platinaβ-diketones is a ligand-induced proton shift from the O−H···O bridges, yielding hydrido−acyl platinum(IV) complexes [PtHCl(COR)2L2], which can undergo reductive C−H and/ or H−Cl elimination reactions, resulting in the formation of © 2013 American Chemical Society
Received: August 13, 2013 Published: November 26, 2013 7090
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Scheme 1. Reactivity of the Platina-β-diketone 1 toward N∧N Donors
2. RESULTS AND DISCUSSION 2.1. Diacetyl Platinum(II) Complexes Bearing Monoxime and Dioxime Ligands. 2.1.1. Synthesis and Spectroscopic Characterization. At −78 °C, the dinuclear platina-β-diketone 1 was reacted in methylene chloride with 2pyridyl-functionalized monoximes and with dioximes followed by the addition of a base (NaOMe in MeOH), to yield diacetyl platinum(II) complexes bearing monoxime (4a−c) and dioxime ligands (5a−d), respectively (Scheme 2). Instead of
case, the diacetyl platinum complex 5a, bearing a dioxime ligand, was obtained in the reaction of the 1D-coordination polymeric diacetyl-bridged platinum(II) complex 2 with dimethylglyoxime (H2dmg) (Scheme 1, F/G).6 The concept of ligand reactivity comprises metal-assisted transformations in which upon coordination to a metal the reactivity of a substrate L is changed such that a reaction can proceed that would otherwise not take place.7 Thus, ligands may offer an altered reactivity that allows, for example, the synthesis of organic molecules that are not accessible in a classic way, whereby, as the case may be, metal coordination is required for stabilization (e.g., NHCMe28). Furthermore, transformations of this type also include, for example, hydrolysis9 and condensation reactions,10 the activation of coordinated nitriles11 and isocyanides12 toward nucleophilic or electrophilic attack, and template syntheses to form macrocyclic structures.13 Whereas condensation reactions of aldehydes and ketones are frequently known and, in general, readily proceed, those of acyl ligands have not been often reported yet. Thus, for example, acyl tin compounds were found to react with primary amines in Schiff-base type reactions, yielding iminoacyl tin complexes (e.g., [SnBu3(CMeNBu)]14). Analogously, acyl germanium complexes underwent with O-benzylhydroxylamine and N,N-dimethylhydrazine proton-catalyzed condensation reactions.15 In the case of platinum, such reactions are not known until now, but iminoacyl platinum complexes are accessible via migratory insertion of an isonitrile into a Pt−C bond,16,17 as also for other metals.18 Here, we report on reactions of the platina-β-diketone 1 and of bis(amine)−diacetyl platinum complexes [Pt(COMe)2(NH2R)2] (3) with 2-pyridyl-functionalized monoximes (2-pyCRNOH), which are known for a bidentate κN,κN′ coordination19 and with dioximes (HONCR−CR NOH). In the resulting oxime−diacetyl platinum(II) complexes, the oxime ligands act as H donor and the acetyl ligands as H acceptor for strong intramolecular O−H···O hydrogen bonds. This gives rise to an activation of the acetyl ligands that opens up in reactions with primary amines a new synthetic route for iminoacetyl platinum(II) complexes.
Scheme 2. Synthesis of Diacetyl Platinum(II) Complexes Bearing Monoxime and Dioxime Ligands (4a−c, 5a−d)
NaOMe in MeOH, NaOH in water or NEt3 can be used as base. The complexes were isolated in 35−75% yield. They are stable in air over weeks. The monoxime complexes 4a−c are orange-colored both in the solid state and in solution (CH2Cl2). In methylene chloride, the dioxime complexes 5a− d are orange-colored, whereas in the solid state, except for 5d, they are dark blue (5a/c) and dark green (5b), likely due to closed-shell Pt···Pt (d8···d8) interactions.6,20 The constitution of all complexes was unambiguously identified analytically, by NMR (1H, 13C, 195Pt) and IR spectroscopy, and in the case of the monoxime complexes 4a−c also by single-crystal X-ray diffraction analyses. 7091
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Table 1. Selected NMR Spectroscopic Parameters (δ in ppm, J in Hz; in CDCl3) of the Diacetyl Platinum(II) Complexes [Pt(COMe)2(N∧N)] (4a−c,a 5a−d, 10, 11) ∧
N N 4a 4b 4c 5a 5b 5c 5db 10 11
2-pyCHNOH 2-pyCMeNOH 2-pyCPhNOH HONCMe−CMeNOH HONCPh−CPhNOH 1,2-Cy(NOH)2 HONC(NH2)−C(NH2)NOH 2-pyCHNOMe H(Me)dmg
δH (3JPt,H)
δH (3JPt,H)
δC (2JPt,C)
δC (1JPt,C)
NOH
COCH3
COCH3
CO
15.8 15.5 15.9 14.2 14.6 14.1 12.5
(32) (32) (32) (22) (26)
15.6 (35)
2.27 2.27 2.31 2.39 2.55 2.37 2.30 2.24 2.20
(33.0)/2.53 (20.6) (32.8)/2.52 (21.3) (31.6)/2.58 (18.5) (24.9) (23.4) (25.7) (23.5) (26.8), 2.33 (23.4) (33.0), 2.50 (18.5)
41.6 41.7 41.6 42.9 42.9 42.9 43.0 41.9 40.0
(353.7)/44.6 (319.5) (348.7)/44.8 (321.6) (344.1)/44.8 (319.8) (308.4) (315.4) (300.7) (340.3) (399.2), 44.8 (362.5) (362.1), 44.7 (304.3)
229.8 230.6 231.1 236.8 237.0 237.0 232.7 223.3 227.6
δPt
(1335)/233.3 (1284) (1334)/234.0 (1274) (1332)/233.8 (1277) (1324) (1325) (1319) (1342) (1308), 228.0 (1260) (1381), 231.2 (1271)
−3430 −3421 −3428 −3481 −3487 −3422 −3411 −3433 −3533
a
For complexes 4 the NMR parameters of the acetyl ligands trans to the Npy and the NNOH atoms are given separated by a slash. bMeasured in acetone-d6.
Table 2. Selected NMR Spectroscopic Parameters (δ in ppm, J in Hz; in CDCl3) of the Acetyl−Iminoacetyl Platinum(II) Complexes [Pt(COMe)(CMeNHR′)(2-pyCRNO)] (6a−e) δH (3JPt,H)
6a 6b 6c 6d 6e
R
R′
H Me Ph H H
Bn Bn Bn CH2CH2Ph CH2CHCH2
NOH 15.6 16.2 15.9 15.2 15.2
(78) (73) (76) (78) (71)
δH (3JPt,H)
δH (3JPt,H)
COCH3
C(CH3) NHR′
2.47 2.46 2.51 2.42 2.47
(16.0) (18.1) (14.0) (14.7)
2.20 2.17 2.17 2.00 2.16
(57.4) (56.1) (56.4) (55.2) (55.4)
δC (2JPt,C)
δC (2JPt,C)
COCH3
C(CH3) NHR′
44.5 44.7 44.7 44.5 44.5
Characteristic 1H, 13C, and 195Pt NMR spectroscopic data of the diacetyl platinum(II) complexes with pyridyl-functionalized monoxime (4a−c) and dioxime ligands (5a−d) are compiled in Table 1. The spectra confirm the identities of the complexes; all signals are found in the expected chemical shift ranges with correct intensities in the proton NMR spectra. Upon coordination, in the monoxime complexes 4a−c, the oxime protons (NOH) exert a strong low-field shift by about 6 ppm, resulting in δNOH values of 15.5−15.9 ppm (4a−c). This is a strong indication of the formation of (intramolecular) O− H···O hydrogen bonds because the coordinated induced shifts (CIS) upon N coordination of an oxime (without hydrogen bond formation) in other platinum(II) complexes are much smaller (e.g., CISNOH in [Pt(C6H4CMeNOH-κC,κN)(DMSO)Cl]: 1.3 ppm21). The oxime protons in the dioxime complexes 5a−c were found at 14.1−14.6 ppm, whereby for solubility reasons and/or H/D exchange reactions the CIS could not be determined. Nevertheless, both in complexes 4a− c and in 5a/b the oxime protons show 3JPt,H couplings (22−32 Hz), thus showing unambiguously the N coordination. The diaminoglyoxime ligand (H2dag) in complex 5d may coordinate through the oxime N atoms or through the amino N atoms. The CIS of the oxime and the amine protons of 3.5 and 1.5 ppm (measured in acetone for solubility reasons), respectively, might indicate that the oxime N atoms are coordinated to Pt and N−O−H···O hydrogen bonds are present. Such a coordination of the H2dag ligand was also found in other complexes, for example in [Pt(Hdag)2].22 Furthermore, this is also in accord with DFT calculations of the two possible configuration isomers of 5d, where that with the H2dag ligand coordinated through the NH2 groups was found to be 23.5 kcal/mol higher in standard Gibbs free energy (with consideration of solvent effects, acetone) than the one with the ligand coordinated through the NOH groups.
(247.7) (249.0) (247.7) (247.6) (246.4)
29.3 29.1 29.2 28.6 28.8
(159.0) (157.2) (159.0) (155.3) (158.7)
δC (1JPt,C)
δC (1JPt,C)
COCH3
C(CH3) NHR′
236.6 237.9 237.2 236.2 236.2
(1133) (1124) (1140) (1133)
196.3 196.5 197.4 195.3 196.1
(1358) (1337) (1357) (1351)
δPt
−3384 −3391 −3389 −3391 −3385
Whereas the two acetyl ligands in the dioxime−diacetyl complexes 5a−d are chemically equivalent, this is not the case for the monoxime−diacetyl complexes 4a−c. With complexes 4a/b as examples, in NOESY and, additionally, in 2D NMR experiments (HMQC, HMBC) a correlation of the acetyl protons (δH = 2.53/2.52) with H6 of the pyridine rings allowed an assignment of the acetyl ligands trans to Npy and NNOH atoms (Table 1). This assignment is in accord with the chemical shifts of the acetyl ligands in acetyl−iminoacetyl complexes 6a−d (vide infra, Table 2). The platinum chemical shifts (−3411 to −3487 ppm, Table 1) of the oxime−diacetyl complexes (4/5) are in the range of those of other diacetyl platinum(II) complexes with nitrogen coligands.6,23,24 As expected, in deuterated methanol the hydrogen-bonded protons in the oxime complexes (4/5) are replaced by deuterium. Unexpectedly, in the dioxime complexes 5a/b, additionally, the methyl protons of the acetyl ligands are exchanged by deuterium, as proven by the disappearance of the COCH3 signals in the 1H NMR spectra in CD3OD and also by the detection of the requisite CD3 signals in the 2H NMR spectra in CHCl3 at 2.47/2.32 ppm. At room temperature, this H/D exchange is definitely complete after one day, with (roughly) estimated half-lifes of about 1 (5a) and 2 h (5b). Due to a slow decomposition of the monoxime complex 4a in CD3OD, no clear conclusion regarding the H/D exchange can be drawn in this case. 2.1.2. Structural Characterization. From the complexes [Pt(COMe)2(2-pyCRNOH)] (R = H, 4a; Me, 4b; Ph, 4c) single crystals suitable for X-ray diffraction measurements could be grown. The complexes were found to crystallize in isolated molecules. The asymmetric unit of crystals of complex 4a contains two symmetry-independent molecules that are structurally similar. The molecular structures of the complexes 7092
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4a−c are shown in Figures 1−3; selected structural data are given in the respective figure captions.
Figure 3. Molecular structure of [Pt(COMe)2(2-pyCPhNOH)] (4c, displacement ellipsoids at 30% probability). Selected distances (Å) and angles (deg): Pt−C1 1.974(6), Pt−C3 1.980(6), Pt−N1 2.104(5), Pt−N2 2.121(5), C1−O1 1.201(7), C3−O2 1.240(8), N1− O3 1.364(6), C1−Pt−N1 170.8(2), C3−Pt−N2 170.2(2), O3···O2 2.540(7), O3−H 0.77, O2···H 1.81, O3−H···O2 160.
Figure 1. Molecular structure of one of the two symmetryindependent molecules of [Pt(COMe)2(2-pyCHNOH)] (4a, displacement ellipsoids at 30% probability). Selected distances (Å) and angles (deg); the values for the two symmetry-independent molecules are given separated by a slash: Pt−C1 1.984(4)/1.991(4), Pt−C3 1.977(3)/1.986(4), Pt−N1 2.105(3)/2.116(3), Pt−N2 2.130(3)/2.148(3), C1−O1 1.227(5)/1.215(5), C3−O2 1.242(5)/ 1.232(5), N1−O3 1.361(3)/1.360(4), C1−Pt−N1 172.6(1)/ 173.4(1), C3−Pt−N2 170.8(1)/170.5(1), O3···O2 2.548(4)/ 2.541(4).
upper quartile 1.385/1.408 Å; n = 27, n = number of observations28), as also supported by DFT calculations, see Section 2.4.2. Furthermore, the formation of hydrogen bonds gives rise to a significant elongation of the C3O2 bonds (1.232(5)−1.242(5) Å) compared to the C1O1 bond lengths of the acetyl ligands that are not involved in hydrogen bonds (1.201(7)−1.227(5) Å); see also Section 2.4.2. The Pt− C bond lengths (1.974(6)−1.991(4) Å) are within the range of platinum(II) complexes with acyl ligands trans to N donor ligands (Pt−C: median 1.997; lower/upper quartile 1.981/ 2.010 Å; n = 3128). In crystals of 4a and 4b, intermolecular interactions were found: In 4b the pyridine rings are π stacked29 with a centroid− centroid distance of 3.467(2) Å and an angle between the centroid−centroid vector and the normal to the plane of 13.8°. Furthermore, in crystals of 4a and 4b the five-membered PtNCCN rings are stacked with centroid−centroid distances between 3.457(2) and 3.592(2) Å and angles between the centroid−centroid vectors and the normals to the planes between 12.2° and 17.8°. In no case were closed shell Pt···Pt (d8···d8) interactions observed.20 2.2. Acetyl−Iminoacetyl Platinum(II) Complexes Bearing Monoxime Ligands. 2.2.1. Synthesis and Spectroscopic Characterization. Previous investigations showed that the amine ligands in complexes [Pt(COMe)2(NH2R′)2] (3a−c) are only weakly coordinated, and, thus, they are easily subject to ligand substitution reactions.23,24 Unexpectedly, the bis(amine)−diacetyl platinum complexes 3a−c were found to react with pyridyl-functionalized monoximes not only under ligand substitution but, additionally, under conversion of one of the two acetyl ligands into a protonated iminoacetyl ligand (Pt−CMeO → Pt−CMeNHR′), thus yielding complexes of the type [Pt(COMe)(CMeNHR′)(2-pyCRNO)] (6a− e, Scheme 3, reaction path A). These complexes were isolated in moderate to good yields (35−80%). The yellow complexes were fully characterized analytically, by NMR (1H, 13C, 195Pt) and IR spectroscopy, and by single-crystal X-ray diffraction analyses (6a, 6b·H2O, 6e). To prove whether the previously described type 4 complexes (obtained from reactions of the platina-β-diketone 1 with 2pyCRNOH/NaOMe) are intermediates in the formation of the iminoacetyl complexes 6, the complexes 4a−c were reacted with primary amines (Scheme 3, B). In all cases the iminoacetyl
Figure 2. Molecular structure of [Pt(COMe)2(2-pyCMeNOH)] (4b, displacement ellipsoids at 30% probability). Selected distances (Å) and angles (deg): Pt−C1 1.980(3), Pt−C3 1.978(4), Pt−N1 2.104(3), Pt−N2 2.116(3), C1−O1 1.214(5), C3−O2 1.237(5), N1− O3 1.368(4), C1−Pt−N1 171.1(1), C3−Pt−N2 168.9(1), O3···O2 2.542(4), O3−H 0.84, O2···H 1.72, O3−H···O2 167.
In the three complexes, the platinum centers are squareplanar coordinated (sum of angles around Pt: 360.0/360.1°,25 4a; 360.5°, 4b; 359.9°, 4c) by two nitrogen atoms of the bidentate 2-pyridyl oxime ligand and by two carbon atoms of the acetyl ligands in mutual cis position. As mostly found in acetyl platinum complexes,4,6,23,24 one of the two acetyl ligands is nearly perpendicular to the complex plane (interplanar angle: 86.7/86.1°, 4a; 82.0°, 4b; 85.9°, 4c). In contrast, unusually, the other acetyl ligand lies in the complex plane (2.7/0.6°, 4a; 16.0°, 4b; 1.9°, 4c), which is forced by the formation of an intramolecular O−H···O hydrogen bond to the oxime group (graph set:26 S(6)). The O3···O2 distances (2.548(4)/2.541(4) Å, 4a; 2.542(4) Å, 4b; 2.540(7) Å, 4c) and O3−H···O2 angles (167°, 4b; 160°, 4c; in 4b/c the H atoms were located in the electron density map) are an indication for moderate to strong hydrogen bonds.27 In addition, the N1−O3 bonds (1.360(4)− 1.368(4) Å) are shorter than those in oxime platinum(II) complexes without hydrogen bonds (median 1.393; lower/ 7093
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to about 50% in 1 h), which points to an H/D exchange, but additionally, a decomposition of the complex took place. 2.2.2. Structural Characterization and DFT Calculations. The structures of the complexes [Pt(COMe)(CMeNHR′)(2-pyCRNO)] (R/R′ = H/Bn, 6a; Me/Bn, 6b·H2O; H/ CH2CHCH2, 6e), obtained by X-ray diffraction measurements, are shown in Figure 4−6. Selected structural parameters
Scheme 3. Synthetic Routes to Acetyl−Iminoacetyl Platinum(II) Complexes Bearing Monoxime Ligands (6a−e)
complexes 6a−e were obtained in 30 to 75% yields, which points out that also in reactions according to path A (Scheme 3) type 4 complexes were formed first followed by a Schiff-base type reaction yielding complexes 6. In contrast to type B reactions described above, complex 4a showed no reactivity against the less basic aniline and adenine. Selected 1H, 13C, and 195Pt NMR spectroscopic data of the acetyl−iminoacetyl platinum(II) complexes 6a−e are given in Table 2. The chemical shifts and coupling constants as well as the signal intensities in the 1H NMR spectra are fully consistent with the constitution of the complexes given in Scheme 3. The presence of strong intramolecular N−H···O hydrogen bonds is verified by low-field-shifted broad signals (15.2−16.2 ppm) in the proton NMR spectra. COSY experiments with complexes 6a, 6d, and 6e as examples exhibited a coupling of the hydrogen-bonded proton to the NCH2 group (δH = 3.68− 4.81), thus pointing to the presence of a deprotonated oxime ligand and a protonated iminoacetyl ligand, which is further supported by single-crystal X-ray diffraction analyses and by DFT calculations (vide infra). The relatively large 3JPt,H coupling constants (71−78 Hz) (compared with those in type 4 complexes at 32 Hz) might also be attributed to this fact. Furthermore, two-dimensional NMR experiments (HMBC) showing a correlation between these NCH2 protons and the iminoacetyl C atoms allowed distinguishing between these C atoms (195.3−197.4 ppm) and those of the acetyl ligands (236.2−237.9 ppm). In the same way, the methyl C atoms were assigned to the iminoacetyl and acetyl ligands (Table 2). In all cases the signals of the acetyl ligands are more low-field shifted than those of the iminoacetyl ligands. Furthermore, the 3JPt,H coupling constants (55.2−57.4 Hz vs 14.0−18.1 Hz) of the methyl protons of the iminoacetyl ligands proved to be much higher than those of the acetyl ligands. The same holds for the 1 JPt,C coupling constants (1337−1358 Hz vs 1124−1140 Hz). On the other hand, 2JPt,C couplings showed the opposite (155.3−159.0 Hz vs 246.4−249.0 Hz). The platinum shifts δPt (−3384 to −3391 ppm) are similar to those of the diacetyl platinum(II) complexes 4 and are in a range typical for organo platinum(II) complexes.6,23,24 As exemplified by complex 6a, in deuterated methanol an immediate H/D exchange of the hydrogen-bonded proton was observed, as expected. Furthermore, the intensity of the signal of the iminoacetyl protons (C(CH3)NDBn) decreased (up
Figure 4. Molecular structure of [Pt(COMe)(CMeNHBn)(2pyCHNO)] (6a, displacement ellipsoids at 30% probability). Selected distances (Å) and angles (deg): Pt−C1 1.992(4), Pt−C3 2.007(4), Pt−N1 2.137(3), Pt−N2 2.105(3), C1−O1 1.223(5), C3− N3 1.298(6), N1−O2 1.328(4), C1−Pt−N1 172.0(1), C3−Pt−N2 174.3(1), N3···O2 2.636(5), N3−H 0.83, O2···H 1.84, N3−H···O2 162.
are given in the respective figure captions. The platinum atoms are square-planar coordinated (sum of angles around Pt: 360.0°, 6a; 360.3°, 6b; 360.2°, 6e) by the bidentate 2-pyridylfunctionalized oxime ligand as well as one acetyl and one iminoacetyl ligand (configuration index: SP-4-3). The oxime and the iminoacetyl ligands form intramolecular N3−H···O2
Figure 5. Structure of [Pt(COMe)(CMeNHBn)(2-pyCMe NO)]·H2O in crystals of 6b·H2O (displacement ellipsoids at 30% probability). Selected distances (Å) and angles (deg): Pt−C1 1.994(4), Pt−C3 2.000(4), Pt−N1 2.120(3), Pt−N2 2.082(3), C1− O1 1.212(6), C3−N3 1.294(5), N1−O2 1.327(4), C1−Pt−N1 170.3(2), C3−Pt−N2 173.6(2), N3···O2 2.646(5), N3−H 0.83, O2···H 1.86, N3−H···O2 157, O3···O2 2.786(5), O3−H 0.90, O2···H 1.89, O3−H···O2 175, O3···O1′ 2.817(7), O3−H 0.84, O1′···H 1.99, O3−H···O1′ 169. 7094
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Organometallics
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(1.360(4)−1.368(4) Å) and in platinum(II) complexes with (neutral) oxime ligands that are not involved in hydrogen bonds (median 1.393 Å; lower/upper quartile 1.385/1.408 Å; n = 2728). Both the elongation at the H donor site (N−H) and the shortening at the H acceptor site (N−O) were also observed in DFT calculations (see Section 2.4.2). Other structural parameters are similar to those of complexes 4a−c. Within the 3σ criterion, the Pt−C bonds of the acetyl (1.992(4)−2.01(1) Å) and the protonated iminoacetyl ligands (2.000(4)−2.03(1) Å) are of the same length. Furthermore, as expected, the acetyl ligands lie nearly perpendicular to the complex planes (interplanar angles: 82.9°, 6a; 84.8°, 6b; 85.9°, 6e). Crystals of 6a and 6e consist of molecules without unusual intermolecular interactions. In crystals of 6b·H2O the water molecules act as H donors in intermolecular O−H···O hydrogen bonds (graph set:26 C22(8)). H acceptors are, on one side, O2 atoms of the deprotonated oxime ligands (O3··· O2 2.786(5) Å, O3−H···O2 175°) and, on the other, O1′ atoms of the acetyl ligands (O3···O1′ 2.817(7) Å, O3−H···O1′ 169°); see Figure 5. To further confirm the connectivity of the intramolecular (asymmetric) hydrogen bonds (N−H···O vs O−H···N) in type 6 complexes [Pt(COMe)(CMeNHR′)(2-pyCRNO)], high-level DFT calculations (details see Experimental Section) were performed with a type 6 complex, but for reduction of computational expense associated with substituents R/R′ = Me/Me (6f*31). Despite the different substituents R/R′, the calculated and experimentally found (6a, 6b, 6e) values of bond lengths and angles are in a good agreement, especially the parameters of the hydrogen bonds. In accordance with the experimental findings, the thermodynamically stable configuration of 6f*, both in the gas phase and with consideration of solvent effects (CH2Cl2), is that with an intramolecular N−H··· O hydrogen bond. We failed to localize an equilibrium structure having the opposite connectivity (O−H···N). Relaxed potential energy surface scans (with consideration of solvent effects) indicate that this structure is about 7.2 kcal/mol higher in energy than the equilibrium structure discussed above; for details see the Supporting Information.
Figure 6. Molecular structure of [Pt(COMe)(CMeNHCH2CH CH2)(2-pyCHNO)] (6e, displacement ellipsoids at 30% probability). Selected distances (Å) and angles (deg): Pt−C1 2.01(1), Pt− C3 2.03(1), Pt−N1 2.131(8), Pt−N2 2.122(9), C1−O1 1.20(1), C3− N3 1.31(1), N1−O2 1.31(1), C1−Pt−N1 171.4(4), C3−Pt−N2 173.8(4), N3···O2 2.65(1).
hydrogen bonds. In complexes 6a and 6b, the hydrogenbonded protons could be located in the electron density maps, showing asymmetrical hydrogen bonds. Thus, protonated iminoacetyl ligands (i.e., aminocarbene ligands) and deprotonated oxime ligands are present, which is in accordance with NMR measurements and DFT calculations. Due to the intramolecular N3−H···O2 hydrogen bonds (graph set:26 S(6)) the protonated iminoacetyl ligands lie nearly in the complex plane (interplanar angles: 6.6°, 6a; 20.2°, 6b; 11.1°, 6e). The N3···O2 distances (2.636(5) Å, 6a; 2.646(5) Å, 6b; 2.65(1) Å, 6e) indicate moderate hydrogen bonds.27 The C3N3 bond lengths (1.294(5)−1.31(1) Å) are at the lower end of the range of other platinum(II) complexes bearing protonated iminoacyl ligands in which the NH protons are not involved in hydrogen bonds (1.31(1)−1.346(5) Å; n = 617,30), but longer than those in platinum complexes bearing (nonprotonated) iminoacyl ligands (median 1.276 Å; lower/ upper quartile 1.261/1.288 Å; n = 1428). Furthermore, the deprotonation of the oxime ligands gives rise to a strong shortening of the N1−O2 bonds (1.31(1)−1.328(4) Å) in comparison to the respective bonds both in type 4 complexes
Scheme 4. Synthetic Routes to Dinuclear (7−9) and Mononuclear (7′−9′) Bis(iminoacetyl) Platinum(II) Complexes Bearing Dioxime Ligandsa
a
The assignment of the signal sets (S1/S2) of the dinuclear complexes may be reversed (see text). 7095
dx.doi.org/10.1021/om400812w | Organometallics 2013, 32, 7090−7106
Organometallics
Article
Table 3. Selected NMR (δ in ppm, J in Hz; in CDCl3/CD2Cl2) Spectroscopic Parameters of the Complexes [{Pt(CMe NHR′)2(N∧N)}2] (N∧N = dmg, 7a−d; dpg, 8a−c; 1,2-Cy(NO)2, 9a)
7a 7b 7c 7db 8a 8b 8c 8db 9a
δH (3JPt,H)
δH (3JPt,H)
δC (2JPt,C)a
δC (1JPt,C)a
R′
NH S2/S1
C(CH3)NHR′ S1/S2
C(CH3)NHR′ S1/S2
C(CH3)NHR′ S1/S2
Bn CH2CH2Ph CH2CHCH2 CH2CH2OH Bn CH2CH2Ph CH2CHCH2 CH2CH2OH Bn
13.6 (107)/17.4 (62) 12.5 (105)/17.0 (61) 12.8/17.0
1.39 (44.3)/2.91 (35.1) 1.42 (43.4)/2.77 (36.1) 1.91(45.0)/2.82 (36.1) 2.11(44.0)/2.69 (36.8)d 1.38 (43.6)/2.72 (35.5) 1.39 (43.6)/2.63 (33.6) 1.89 (43.6)/2.69 (34.1) 2.21 (44.6) 2.76 (36.9)d 1.42 (45.6)/2.93 (36.2)
29.6 (118)/35.5 (56) 28.6/35.3 29.8/35.2 29.5/35.2 28.5/34.7 29.5 (108)/34.5 (58)
195.4 (1098)/222.8 (1178) 194.4/222.6 195.1/221.8 199.7/225.2 196.8/223.4 (1271) 194.5/221.9 195.3 (1088)/222.6 (1208)
e
e
29.7/35.7
195.6/222.8
c
12.5 (110)/17.3 (69) 11.9 (111)/16.9 (73) 12.1/17.1 c
13.7 (120)/17.3
e
δPt −3534 −3539 −3539 −3552 −3521 −3515 −3507
a
Due to poor solubility Pt,C couplings could not be observed in all cases. bFor solubility reasons measured in CD3OD. cNH signals were not observed due to H/D exchange. dDue to H/D exchange, CH3 signals could only be observed in freshly prepared solutions. eCD3 and C(CD3)ND signals could not be observed due to bad signal-to-noise ratio.
2.3. Bis(iminoacetyl) Platinum(II) Complexes Bearing Dioxime Ligands. 2.3.1. Synthesis and Spectroscopic Characterization of the Dinuclear Complexes. The bis(amine)−diacetyl platinum(II) complexes 3a−c were reactedanalogously to the synthesis of the platinum complexes 6a−e, bearing 2-pyridyl-functionalized monoxime ligands with dioximes, yielding dinuclear bis(iminoacetyl) platinum(II) complexes of the type [{Pt(CMeNHR′)2(ONCR−CR NO)}2] (R/R = Me/Me, 7a−d, Ph/Ph, 8a−d, (CH2)4, 9a) (Scheme 4, reaction path A). Although two equivalents of primary amines NH2R′ were set free in the initial ligand substitution reaction, to facilitate the subsequent formation of two protonated iminoacetyl ligands (Pt−CMeO → Pt− CMeNHR′), the addition of about two further equivalents of NH2R′ is advantageous. The complexes 7−9 were isolated as yellow powders and microcrystalline substances, respectively, in moderate to good yields (35−90%) and fully characterized analytically, by NMR (1H, 13C, 195Pt) and IR spectroscopy, as well as by single-crystal X-ray diffraction analysis (8a·2CH2Cl2). In solution monomer−dimer equilibria were established (Scheme 4, C); see Section 2.3.3. Reactions of the dioxime−diacetyl complexes 5a−c (obtained from reactions of the platina-β-diketone 1 with HONCR−CRNOH/NaOMe) with four equivalents of a primary amine NH2R′ resulted also in the formation of the bis(iminoacetyl) complexes 7−9 (Scheme 4, B), thus indicating that type 5 complexes are intermediates in reaction A. Furthermore, reaction path B is superior to A because the requisite bis(amine)−diacetyl complexes 3 do not have to be synthesized as precursor complex. Moreover, complexes 7d/8d are only accessible via route B because the precursor complex 3 bearing two ethanolamine ligands could not be synthesized. Notably, the synthesis of bis(iminoacetyl) platinum(II) complexes bearing diaminoglyoxime ligands (HONC(NH2)−C(NH2)NOH) failed in both routes A and B: Via route A only complex 5d was obtained and, moreover, 5d was found not to react with primary amines (route B). Characteristic NMR spectroscopic data of the dinuclear complexes 7−9 are shown in Table 3. In 1H and 13C NMR spectra two sets of signals (S1, S2) were found both for the iminoacetyl ligands and for the dioxime ligands (Scheme 4). This gives proof for a dinuclear structure in solution having (on the NMR time scale) inversion symmetry. Two-dimensional NMR experiments (COSY, HMQC, HMBC) made clear which H and C atoms of the iminoacetyl ligands belong to set S1 and
which ones to set S2 (Scheme 4). With the assumption that the intramolecular N−H···O hydrogen bond is stronger than the hydrogen bonds bridging the mononuclear platinum units, we tentatively assigned the more low-field-shifted signals (δH = 16.9−17.4) to the protons of the intramolecular H bonds (S1) and the other ones (δH = 11.9−13.7) to the bridging H bonds (S2). Nevertheless, this assumption remains to be speculative. The 3JPt,H coupling constants for the hydrogen-bonded protons at higher fields are much greater (105−120 Hz) than the couplings of the signals at lower fields (61−73 Hz). On the basis of the assignment of the N−H···O protons, the 2D NMR experiments made clear that all other signals of the iminoacetyl ligand assigned to the signal set S1 are more high-field shifted than those of signal set S2. The 3JPt,H couplings (Pt−C−C−H) of signal set S2 are slightly smaller than those of S1 (33.6−36.9 Hz vs 43.4−45.6 Hz). As expected, in dinuclear complexes 7a, 8a, and 9a the NCH2 protons show an AB system (as also for complexes 7b−d and 8b/c, but with additional couplings to other protons) with typical geminal H,H couplings (13.1−15.2 Hz). At lower temperature (−30 °C), an additional splitting could be observed due to coupling to the NH protons, which was further confirmed (7a/8a) by COSY experiments. As shown exemplarily for 7a/8a, as for the diacetyl−dioxime complexes 5a/b, in deuterated methanol, additionally to a fast H/D exchange of the hydrogen-bonded protons, an H/D exchange of the protons of the methyl group of the iminoacetyl ligand (CH3 → CD3) took place. For this H/D exchange a halflife of about one hour was estimated. 2.3.2. Structure of [{Pt(CMeNHBn)2(ONCPh−CPh NO)}2] (8a). From CH2Cl2/diethyl ether solutions of [{Pt(CMeNHBn)2(ONCPh−CPhNO)}2] (8a) single crystals of 8a·2CH2Cl2 suitable for X-ray measurements were obtained. No unusual interactions between the platinum complex 8a and solvate molecules were observed. The dinuclear complex 8a exhibits crystallographically imposed inversion symmetry, and its molecular structure is shown in Figure 7 with selected structural parameters given in the figure caption. The platinum centers are square-planar coordinated (sum of angles around Pt: 360.3°) each by a doubly deprotonated bidentate diphenylglyoxime ligand and by two protonated iminoacetyl ligands (the hydrogen-bonded protons could be located in the electron density map). The terminal protonated iminoacetyl ligands are involved in intramolecular, moderate N3−H···O2 hydrogen bonds (N3··O2 2.547(6) Å; N3−H··O2 166°; graph set:26 S(6)). Due to this, the planes of 7096
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of the mononuclear complexes present in solution, not all NMR parameters could be obtained. In general, the 1H and 13C signals of the monomeric complexes lie in between the corresponding signals of the sets S1 and S2 of the dinuclear complexes. In complexes 7c/d, 8d, and 9a smaller signals indicate a content of monomer of
