- Email: [email protected]
Spectroscopic studies of temperature induced phase transitions in metal-organic complex trans-PtCl2(PEt3)2
Journal Pre-proof Spectroscopic studies of temperature induced phase transitions in metal-organic complex trans-PtCl2(PEt3)2 Naini Bajaj, Himal Bhatt, H.K. Poswal, M.N. Deo PII:
S1386-1425(19)31018-2
DOI:
https://doi.org/10.1016/j.saa.2019.117628
Reference:
SAA 117628
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: 11 February 2019 Revised Date:
17 September 2019
Accepted Date: 6 October 2019
Please cite this article as: N. Bajaj, H. Bhatt, H.K. Poswal, M.N. Deo, Spectroscopic studies of temperature induced phase transitions in metal-organic complex trans-PtCl2(PEt3)2, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2019), doi: https://doi.org/10.1016/ j.saa.2019.117628. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Spectroscopic Studies of Temperature Induced Phase Transitions in Metal-Organic Complex trans-PtCl2(PEt3)2 Naini Bajaj,1,2 Himal Bhatt,1,* H.K. Poswal,1 and M.N. Deo1,2 1
High Pressure & Synchrotron Radiation Division, Bhabha Atomic Research Centre, 2Homi Bhabha National Institute, Anushaktinagar, Mumbai *Email: [email protected]
Graphical Abstract
1
Spectroscopic Studies of Temperature Induced Phase Transitions in Metal-Organic Complex trans-PtCl2(PEt3)2 Naini Bajaj,1,2 Himal Bhatt,1,* H.K. Poswal,1 and M.N. Deo1,2 1
High Pressure & Synchrotron Radiation Division, Bhabha Atomic Research Centre, 2Homi Bhabha National Institute, Anushaktinagar, Mumbai *Email: [email protected]
Abstract. Tuning of molecular and electronic properties of Pt(II)-organic complexes have a profound effect on their applications in the fields of technology, pharmaceuticals and crystal engineering. Here, we present combined infrared and Raman spectroscopic investigations on trans-PtCl2(PEt3)2 systematically carried out at various temperatures from 300 – 4.2 K in a wide spectral range. The studies suggest drastic orientational changes of different moieties around 180 K and 130 K in the ligand groups attached to the central Pt atom. This is accompanied by a systematic strengthening of C-H---Cl hydrogen bonds in the 180 – 130 K temperature range. A discontinuous change in intensity, peak variations of modes and emergence of new modes across 180 K and 130 K in the lattice region are suggestive of a possible structural phase transition. It is interesting to note that the spectral signatures of the low temperature phase are different from those reported recently for the high pressure phase in this compound. These studies will be useful in better understanding the physico-chemical properties of metal-organic complexes in order to exploit their applications in various bio-chemical and technological fields.
Keywords: Pt(II)-organic complex, infrared, Raman spectroscopy, low temperature, hydrogen bonding 1
1. Introduction Metal-organic complexes have applications in the fields of catalysis,1 drug delivery,2-7 gas storage,8, 9 pharmacy
2, 9-12
etc. due to their porous structure formed by organic linkers, thus
attracting researchers in the fields of materials science and crystal engineering. Since their discovery, there have been several efforts to tune their structural properties by various methods in order to further improve/ optimize their capabilities. This tuning can be performed by modifying the size of the linking group, changing the metal salt to which linking (organic) group is attached and by altering the experimental conditions like thermodynamic parameters. Changing the thermodynamic parameters such as pressure/ temperature offers the cleanest way as it does not involve the addition of any external impurity. Such changes cause modifications in the inter-atomic separations, which may sometimes lead to phase transitions. Though both high pressure and low temperature result in compression/ contraction of the system, the mechanism of possible phase transitions or structural relaxations in the square planar metal-organic complexes may be similar or different.13-15 For example, in the complex [(CH2)3NH2][Zn(HCOO)3], phase transitions have been observed at low temperature as well as high pressure due to the ring-puckering motion of the cationic groups and the framework rearrangement resulting from rotations of the HCOO- ions respectively.16 In contrast, the compound trans-Pt(II) dithiocarbamate, which shows small variations in metal-ligand stretching Raman modes and no variations in deformation modes at low temperatures, depicts drastic and different pressure induced changes, where a large variation of the deformation Raman modes was noted as compared to the stretching modes.17,
18
The high pressure studies on the title
compound trans-PtCl2(PEt3)2 (trans-dichloro bis(triethylphosphine) platinum(II)), have shown that preferred molecular orientations trigger a structural phase transition at 0.8 GPa.19 Our 2
preliminary Low temperature (LT) -FT Raman study on the structural isomers of this compound (cis- and trans-) at two temperatures of 300 K and 77 K in the spectral range 60 – 600 cm-1 showed drastic alterations in the skeletal Raman modes of trans-isomer upon cooling, whereas relatively no change was detected in the spectrum of cis-isomer.20 The increase in intensity of deformation/ stretching Raman modes of phosphine ligands upon cooling trans-PtCl2(PEt3)2 provided the impression of new modes appearing close to the positions where the corresponding IR modes are expected.20 Such a phenomenon indeed happens under high pressure at 0.8 GPa, where the structure changes from centrosymmetric to a non-centrosymmetric phase.19 The systematic variable temperature studies are therefore essential to confirm if such a phase transformation takes place upon cooling. In addition, given the importance of organic ligands in the structural stability, low temperature studies of metal organic complexes are important in light of the reports that freezing of methyl groups is associated with a dynamical phase transition.21 In the present paper, detailed low temperture studies on trans-PtCl2(PEt3)2 have been reported using combined Fourier transform infrared (FTIR) and Raman spectroscopy in a wide spectral range to understand the molecular structure. Interestingly, these studies reveal that a phase transition indeed takes place across 180 K and 130 K, but the low temperature phase is not similar to the high pressure phase transition.
2. Experimental details 2.1. Low temperature IR measurements: Infrared spectroscopic studies have been carried out on the powder sample of transPtCl2(PEt3)2 at various temperatures from room temperature down to 4.2 K in 140 – 3200 cm-1 spectral range. A continuous flow liquid Helium (LHe) cryostat was used for in-situ low 3
temperature investigations. The sample, dispersed in CsI matrix, was pelletized and mounted on the cold finger of the cryostat; the temperature was varied from 300 – 4.2 K at steps of 10 K (± 2 K). The cryostat was mounted in the sample compartment of a Bruker Vertex 80V Fourier transform infrared spectrometer, installed in the experimental station of IR beamline at Indus-1 Synchrotron facility in India. The far infrared (FIR) spectra in the range 140 – 650 cm-1 were recorded using Hg source, Mylar beamsplitter and FIR-DTGS detector at a resolution of 2 cm-1, whereas the mid infrared (MIR) spectra (500 – 3500 cm-1) were recorded using thermal source, KBr beamsplitter and LN2 cooled MCT detector at a resolution of 1 cm-1. For each spectrum, a total of 100 scans were coadded at all temperatures. The flow of liquid helium (inlet) and helium vapour (outlet) were controlled using a gas flow controller calibrated for helium gas and an oil free diaphragm pump. The measurements were carried out in the transmission mode in vacuum condition and the background spectra were also recorded in the same temperature points.
2.2. Low temperature Raman measurements: The Raman spectra of trans-PtCl2(PEt3)2 were recorded on triple stage Raman spectrograph (JobinYvon T64000) equipped with LN2 cooled CCD detector. The Raman signal was excited using diode pump laser of 532 nm wavelength. The data were recorded in the subtractive mode in the 10 – 3000 cm-1 spectral range at a resolution of 4 cm-1. Temperature dependent measurements from room temperature down to 77 K were carried out using a heating and cooling microscope stage (Linkam THMS 600).
3. Results and discussion
4
3.1. Structure and Vibrational mode assignments: The ambient structure of trans-PtCl2(PEt3)2 consists of a central platinum (Pt) atom which is connected to two chlorine (Cl) atoms and two phosphine triethyl ligands P(C2H5)3 in transposition to each other as shown in Figure 1. Its crystal structure was first described by Messmer et al.,22 which suggested a centre of inversion symmetry at the Platinum atom. The changes associated with the metal – ligand bonds and hence the crystal symmetry can be probed by studying the lattice and skeletal vibrational modes which are generally observed in the low frequency regions of the infrared (IR)/ Raman spectrum, whereas the deformation and stretching vibrational modes of organic ligands are observed at higher frequencies.
Figure 1. trans-PtCl2(PEt3)2 (Et = C2H5, ethyl group) molecule with central Platinum (Pt) Silver, Chlorine (Cl) - Blue, Phosphorous (P) - Orange and Carbon (C) - Grey atoms. Hydrogen atoms are omitted for clarity. Numbers are the C-C bond lengths in the Angstrom (Å) unit.19
The vibrational spectra of trans-PtCl2(PEt3)2 can be classified into different regions in the infrared and Raman spectra at ambient conditions, accordingly the vibrational mode assignments have been carried out by various workers.23-29 For mode assignments in our spectra, we compared the observed frequencies with reported works in literature for the same as well as 5
similar compounds,
23, 26-30
i.e. metal (Pt, Ni, Pd etc.) organic compounds, with same as well as
different ligand chains, viz. triethylphosphine (PEt3),31 trimethylphosphine (PMe3),31 TeP(CH3)3,32 Pt(PEt3)2(CF=CF2)Cl,33 C2H5SiH2F,34 (C2H5)3P=S etc. The observed IR and Raman peak positions have been listed in Tables 1 and 2 respectively alongside the reported values and assignments. The spectral region 10 – 500 cm-1 contains lattice modes and the skeletal modes of Pt-Cl2 and PtP2 units. The stretching vibrations of Pt-Cl and Pt-P are observed at 334 cm−1 (341 cm-1) and 433 cm−1 (415 cm-1) respectively in the Raman (and IR) spectra. At lower frequencies, various deformation modes: δ(P–Pt–Cl) (157 cm−1 - Raman), δ(Pt–Cl2) (167 cm−1 - Raman, 168 cm-1 IR), and γ(P–Pt–Cl) (228 cm−1 - Raman, 228 cm-1 - IR ) (γ – out of plane, δ – in plane bending) and lattice modes are observed. The variations in geometry, viz. bond lengths and angles exhibited by the three ethyl groups linked to Phosphorous atom19 is well reflected by the complex spectral profiles in ligand deformation and stretching regions. The region 500 – 1600 cm-1 is the fingerprint region. This region consists of internal modes of triethylphosphine ligands attached to the central platinum atom. The P-C stretching vibration is observed at 634 cm-1 in the IR spectra and at 642 cm-1 in the Raman spectra. Since there are various ethyl groups attached to the P atom with varying C-C bond lengths (as shown in Figure 1), the spectra show populated profiles in the regions 700 – 1100 cm-1 range, with prominent absorption features due to skeletal modes of PEt3 and CH2 rock followed by C-C stretching vibrations. The peak around 1030 cm-1 (in IR) and 1047 cm-1 (in Raman) corresponds to C-CH3 rocking vibration. The region from 1200 – 1500 cm-1 consists of different deformation modes of organic ligand: CH2 wagging (IR – 1257 cm−1, Raman – 1243 cm−1), CH2 scissoring (IR – 1410 cm−1, Raman – 1425 cm−1), CH3 symmetric deformation (IR – 6
1376 cm-1, Raman – 1384 cm-1) and CH3 asymmetric deformation (IR – 1449 cm-1, Raman – 1460 cm-1). The region from 2800 – 3200 cm-1 consists of CH2 and CH3 stretching vibrations along with various other overtone and combination modes. The symmetric modes lie at lower frequency than the asymmetric modes. These modes are observed in the infrared and Raman spectra at: νsCH3 – 2880 cm−1 (Raman) and 2874 cm−1 (IR); νasCH2 – 2932 cm−1 (Raman) and 2931 cm−1 (IR); νasCH3 – 2964 cm−1 (Raman) and 2964 cm−1 (IR); and νsCH2 – 2912 cm−1 (Raman) and 2913 cm-1 (IR). Our observations of temperature induced variations of these IR and Raman modes have been sequentially listed in sections 3.2 and 3.3 respectively, which is followed by a discussion on these changes.
Table 1: Infrared vibrational modes of trans-PtCl2(PEt3)2 from various references and this study in the spectral region 150 – 3000 cm-1. Values in parenthesis are the fitting errors. Infrared modes
Ref. 23
Ref. 24
Ref. 28
Ref. 31
Ref. 35
transPtCl2(PEt3)2 (cm-1)
transPtCl2(PEt3)2 (cm-1)
transPtCl2(PEt3)2 (cm-1)
transPtCl2(PEt3)2 (cm-1)
transPtCl2(PE t3)2 (cm-1)
Lattice mode
168 w
Ligands (or internal modes of PEt3)
185 wm
νs(Pt-Cl2) νs(Pt-P2) ν(P-C) Skeletal modes of PEt3 & CH2 rock
228 w 275 w 384 w 341 vs 415 m
Ref. 25 (PEt3) (cm-1)
384 341 415
339 s 413 w 382 w 632 m
632 s 610 w
710 m
7
655 657 670 690
This Study trans-PtCl2(PEt3)2 Peak Width Peak Position (cm-1) (cm-1) 167.4(9) w + weak shoulder at 178 180.8(3) wm + weak shoulder at 192 230.1(2) 278.8(3) 384.2(4) 341.4(1) 415.5(2)
10.1(3)
13.2(8) 19.5(1) 2.2(3) 8.5(2) 12.7(6)
634.5(7)
8.1(2)
8.0(2)
717 733 s
νs(C-C)
748 765 975 1003 1023 1043 1098 1185 1231 m 1245 m,sh
1005 w 1033 s
C-CH3 rock
CH2 wag.
1255 w
CH3 sym. def. Combinat ion CH2 scissoring Overtone CH3 asym. def. 1465=73 5+735 CH3 sym. str. CH2 sym str. CH2 asym. str. CH3 asym str.
1375 w
1380 s
711.5(2) 734.9(1)
3.9(5) 8.7(3)
767.6(9) 1007.4(2) 1031.2(8) 1041.0(1)
13.6(3) 3.9(8) 4.3(3) 15.1(4)
1257.2(1)
15.4(7)
1376.8(1)
4.8(4)
1384.2(3)
4.0(1)
1418 m
1423 s
1410.5(3)
5.5(8)
1452 m
1468 s
1424.5(4) 1449.0(3)
12.3(1) 5.4(8)
1468.0(3)
14.2(7)
2845
2874.5(2)
7.8(7)
2900 s
2913.4(3)
13.4(1)
2931.5(1)
10.4(7)
2963.8(1)
9.7(3)
2998 s
Table 2: Raman vibrational modes of trans-PtCl2(PEt3)2. Values in parenthesis are the fitting errors. Raman modes
In plane def. Other low freq. and lattice modes
Ref. 23
Ref. 36
transPtCl2(PE t3)2 (cm-1) 153 m 360 w 274 m 233 ms 195 m
transPtCl2(PE t3)2 (cm-1) 154 m 365 w
164 m Pt-Cl2 str Pt-P2 str. P-C str. CH2 rock C-C sym str.
330 vs 432 ms
Ref. 35
Ref. 25
transPtCl2(PEt3)2 (cm-1)
(PEt3) (cm-1)
233 m 178 sh 164 m 114 sh 334 s 432 m
333 443 757 982
8
This Study trans-PtCl2(PEt3)2 Peak Position Peak Width (cm-1) (cm-1)
152.9(2)
10.1(4)
268.1(4) 229.2(3)
13.2(3) 16.3(7)
164.5(1) 120.9(4) 329.9(1) 429.7(2) 636.6(1)
6.7(2) 11.0(2) 6.9(2) 9.9(4) 7.1(2)
1003.0(6)
8.1(3)
C-CH3 rock CH2 wag CH3 sym def. CH2 sym def. CH2 scissoring CH3 asym def. CH3 sym str CH2 sym str CH2 asym str CH3 asym str
1041 1243 1384 1425 1464 2821 2873 2896 2926
1012.5(3) 1045.2(2)
7.3(2) 6.7(5)
1417.2(7)
22.2(6)
1460.5(2) 2880.5(4) 2913.6(6) 2931.9(3) 2964.2(1)
15.8(6) 14.6(7) 11.8(1) 16.3(5) 8.9(5)
3.2. Low temperature infrared spectroscopy: a) Low frequency spectral Region (below 500 cm-1):
Figure 2. Low temperature IR spectra of trans-PtCl2(PEt3)2 in the (a) 150-500 cm-1 spectral region. Dashed lines indicate new peaks emerging at low temperatures. (b) Variation of IR peak positions at low temperatures in the 160 – 640 cm-1 spectral range. Assignment of few peaks
9
directly influencing the skeletal motion have been marked. Open circles represent new modes appearing upon cooling.
Figures 2a represents low temperature IR spectra in the 150 – 500 cm-1 spectral range. Figure 2b shows the corresponding frequency versus temperature plots in the 160 – 640 cm-1 spectral range. The IR mode corresponding to ν(Pt-Cl) (341 cm-1) and ν(Pt-P) (417 cm-1) stretching vibrations show small but monotonous stiffening upon cooling, with a small change in the rate of variations across 130 K, up to the lowest temperature measured, implying systematic strengthening of the skeletal bonds (Figures 2b). The shoulder (333 cm-1) developing on the lower frequency side of the ν(Pt-Cl) (341 cm-1) IR mode transforms to a clear peak below 180 K and it softens upon further cooling (Figure 2a). The deformation mode γ(P-Pt-Cl), which is observed at 228 cm-1 in the IR spectra shows systematic stiffening upon cooling down to 130 K (Figures 2b). In addition, there are some weak IR absorption features below 300 cm-1 which show small stiffening upon cooling and reach a plateau at nearly 130 K. A weak feature is observed to be emerging at ~ 147 cm-1 across 130 K (marked with dotted line in Figure 2a). The weak and broad band which spans the region 150 – 200 cm-1, consists of δ(Pt–Cl2) mode at 167 cm-1 and other deformation and lattice modes,23, 25 is completely resolved upon cooling down to 180 K and transforms to four clear peaks at further lower temperatures (Figure 2a). A new mode is clearly seen to appear around 250 cm-1 in the IR spectra below 130 K (marked with dotted line in Figure 2a), which strengthens at lower temperatures.
b) Region 500 – 1600 cm-1:
10
(b)
Figure 3: Low temperature IR spectra of trans-PtCl2(PEt3)2 in the (a) 620 – 650 cm-1 (b) 700 – 1100 cm-1 spectral region. Dashed lines are guide to the eye. (c) Variation of IR peak positions at low temperatures in the 600 – 1100 cm-1 spectral range. Filled squares – ambient modes, filled circles – represent new modes appearing upon cooling.
The deformation modes of triethylphosphine ligands are observed in the 600 – 1600 cm-1 fingerprint region. The systematic IR spectra at various temperatures from 4.2 K to 300 K, shown in Figures 3 reveal that the orientational changes associated with the organic ligand groups take place at temperatures of 180 and 130 K. In the phosphine ligand, the ν(P-C) IR mode at 634 cm-1 in Figure 3a, shows an increase in the rate of stiffening (~ 0.028 cm-1/K below 180 K) upon cooling below 180 K with a new shoulder appearing on the low frequency side (~ 633 cm-1 at 180 K), which shows small softening upon cooling, as shown in Figure 2b. The spectral region 700 – 1500 cm-1, describing the various deformation vibrational motions of organic group is highly populated because there are in all three ethyl groups on either side of the Pt atom, with varying bond parameters as well as geometrical orientations (see Figure 1).19 The IR spectral region 700 – 800 cm-1 (skeletal modes of PEt3 and CH2 rock), becomes further complex upon 11
cooling due to splitting and appearance of new modes across 180 K and 130 K as shown in Figure 3b. A new mode also appears around 1003 cm-1 (near ν(C-C) IR mode) below 180 K which softens and shows a large increase in the relative intensity upon cooling, as shown in Figure 3b. The C-CH3 rocking IR mode at 1032 cm-1 stiffens in the 180 – 130 K temperature range, with the appearance of new shoulder (~1030 cm-1) on its lower frequency side below 180 K. This new mode softens with an increase in the relative intensity upon cooling upto 4.2 K as shown in Figures 3a and b. Below 130 K, these modes show no change upon lowering the temperature.
(a)
Figure 4. (a) Low temperature IR spectra of trans-PtCl2(PEt3)2 in the 1220 – 1480 cm-1 spectral region. Dashed lines are guide to the eyes. (b) Variation of IR peak positions at low temperatures in the 1220 – 1480 cm-1 spectral range. The primary IR mode assignments in various vibrational band regions of ethyl groups with varying bond parameters and geometry, have been marked. Open circles represent new modes appearing upon cooling. 12
The wagging IR mode ω(CH2) at 1260 cm-1 stiffens upon cooling upto 130 K as shown in Figure 4a. The symmetric deformation ds(CH3) (~1260 cm-1) and asymmetric deformation das(CH3) (~1450 cm-1) IR modes soften upon cooling upto 130 K, with the appearance of a new mode on the lower wavenumber side of ds(CH3) below 180 K as shown in Figures 4a and 4b. Upon further lowering the temperature below 130 K, these modes show no variation. CH2 scissoring (1410 cm-1) mode shows stiffening behavior upto 130 K and is constant with further lowering of temperature upto 4.2 K.
c) Region 2800 – 3200 cm-1:
Figure 5. (a) Low temperature IR spectra of trans-PtCl2(PEt3)2 in the 2850 – 3050 cm-1 spectral range. Spectral plot at 300 K and 180 K show the deconvoluted peaks (Green color deconvoluted peaks at 180 K are the new peaks emerging at low temperature). (b) Variation of CH2 and CH3 stretching IR modes at low temperature in the 2800 – 3000 cm-1 spectral range (the combination/ 13
overtone and new modes at lower temperatures are excluded in plot for clarity). Symbols: νs symmetric stretch, νas - asymmetric stretch.
The region 2800 – 3200 cm-1 consists of stretching vibrations of CH2 and CH3 groups (plotted in Figure 5b) along with various overtone and combination modes (not plotted in Figure 5b for clarity). At low temperatures, the symmetric and asymmetric IR stretching vibrational peaks of CH3 group (2874 cm-1 and 2963 cm-1 respectively) show small softening in the 300 – 4.2 K temperature range, with a noticeable increase in the rate of softening in 180 – 130 K temperature range, as shown in Figure 5b. A new shoulder peak emerges close to the CH3 asymmetric stretching mode, showing a similar behavior (softening at ~ - 0.100 cm-1/K) and intensification relative to other modes in the 180-130 K temperature range, as shown in Figure 5a.
3.3. Low temperature Raman Spectroscopy: a) Low frequency spectral region (below 500 cm-1):
14
Figure 6. (a) Low temperature Raman spectra of trans-PtCl2(PEt3)2 in the 5 – 650 cm-1 region. Inset: zoomed 10-70 cm-1 spectral region at low temperatures. (b) Variation of Raman active modes at low temperatures in the 10 – 650 cm-1 spectral range.
Figure 6a represents low temperature Raman spectra of trans-PtCl2(PEt3)2 in the low frequency region 5 – 650 cm-1. Figure 6b shows temperature induced variation of the corresponding Raman modes. The lattice modes at 20 and 23 cm-1 in the Raman spectra show initial softening upto 173 K, followed by stiffening up to 133 K as shown in Figures 6a and 6b. Also, at temperatures close to 130 K, a sudden increase in intensity of lattice modes is observed as shown in Figure 6a. Below this temperature, the peak positions as well as intensities do not show any appreciable changes (Figures 6a and 6b). New modes also appear in the Raman spectra at ~ 71 and 147 cm-1 across 180 K and 130 K respectively, which stiffen upon cooling (Figure 6a). Among the skeletal modes, the δ(Pt-Cl2) Raman mode at 165 cm-1 remains almost unaltered with temperature. The deformation mode γ(P-Pt-Cl), which is observed at 233 cm-1 in the Raman 15
spectra, initially shows systematic stiffening upon cooling down to 130 K (Figures 6b). Its position reaches a plateau at nearly 130 K. The Raman modes corresponding to ν(Pt-Cl) (329 cm1
) and ν(Pt-P) (429 cm-1) stretching vibrations show small but monotonous stiffening upon
cooling, with a small change in the rate of variations across 130 K, up to the lowest temperature measured, implying systematic strengthening of the skeletal bonds (Figures 6b). The ν(P-C) Raman mode at 632 cm-1 shows stiffening behavior upon cooling in the 173 – 133 K temperature range (Figure 6b). Across 130 K, it shows discontinuous shift to a higher value. Further, most of the vibrational modes in the region 10 – 650 cm-1 show slope change in the frequency versus temperature plots across 180 K and 130 K. In summary, the features like appearance of new modes in the lattice and skeletal regions, abrupt changes in the rate of variation of frequencies and change in the relative intensity of some vibrational modes in the low frequency regions of Raman spectra are clear indications of phase transition in trans-PtCl2(PEt3)2 across 180 K and 130 K. The changes occurring in the organic ligands groups can provide further insights into the nature of these transitions.
b) Deformation region (changes associated with organic ligands):
16
Figure 7. Low temperature Raman spectra of trans-PtCl2(PEt3)2 in the 650 – 1500 cm-1 spectral region.
The internal vibrational modes of the organic ligands also show similar changes under temperature induced contraction as observed in IR spectroscopy. The distinctive feature in the Raman spectra in this region is the sudden increase in the relative intensity of C-CH3 rocking mode at 1055 cm-1 across 173 K as shown in Figure 7. In the 1250 – 1500 cm-1 spectral range, the band lying in the region of asymmetric deformation vibrations of CH3 unit (1450 – 1500 cm1
) shows splitting below 180 K in the Raman spectra (Figure 7). The CH2 scissoring mode (~
1413 cm-1) shows a relatively large rate of stiffening (~ 0.020 cm-1/K) compared to other modes upto 130 K and below this temperature it shows no variation as shown in Figure 7. This spectral region becomes complex upon cooling due to splitting of several modes. A drastic increase in the intensity of modes, which otherwise are weak at ambient conditions, has been observed at low temperatures. Hence, the Raman spectra in lattice/ skeletal as well as ligand deformation regions 17
show marked changes across 180 K and 130 K, which can be related to temperature induced phase transitions in trans-PtCl2(PEt3)2. The stretching vibrational modes may further provide important information on the hydrogen bonding network in the structure in the low temperature phase.
c) Region 2800 – 3200 cm-1:
Figure 8. (a) Low temperature Raman spectra of trans-PtCl2(PEt3)2 in the 2850 – 3000 cm-1 spectral region. (b) Variation of Raman modes in the 2860 – 2990 cm-1 spectral region at low temperature.
The ligand stretching region 2850 – 3050 cm-1 is particularly important, as it provides information on weak non-covalent interactions like hydrogen bonds formed via C-H groups. At ambient conditions, trans-PtCl2(PEt3)2 forms two short H---Cl hydrogen bonds. One is 18
intramolecular hydrogen bond with H---Cl = 2.719 Å, C---Cl = 3.328 Å and ∠CHCl=114.37°and the other is intermolecular hydrogen bond with H---Cl = 2.657 Å, C---Cl = 3.684 Å and ∠CHCl = 155.66° 19. The softening behavior of CH3 symmetric and asymmetric stretching modes in the 180 – 130 K temperature range, observed in IR spectroscopy, is also verified from the Raman spectra (2875 cm-1 and 2962 cm-1 respectively) shown in Figure 8b. The CH2 symmetric and asymmetric Raman (2913 cm-1 and 2941 cm-1 respectively) stretching modes show comparatively less changes in the complete range, except for small discontinuous shifts across 130 K (Figure 8b). These observations imply preferred strengthening of the C-H---Cl hydrogen bonds through CH3 groups in trans-PtCl2(PEt3)2 in the low temperature phase between 180 – 130 K temperature range.
3.4. Comparison of low temperature infrared and Raman spectra: Thus, based on the spectroscopic observations in the complete frequency range, upon cooling trans-PtCl2(PEt3)2, most of the vibrational modes show change in slope around two temperatures of 180 K and 130 K. The changes around 180 K are associated with the orientational dynamics of organic ligands (phosphine triethyl), which initiate strengthening of CH---Cl hydrogen bonds through CH3 groups upto 130 K. Across 130 K, appearance of new modes as well as drastic increase in the intensity of some modes is observed in both infrared and Raman spectra. At further low temperatures, no new features or intensity/ peak position variations are noticed. In the skeletal modes, small changes are observed in the frequency variations of Pt-Cl deformation modes, whereas no significant effect is noticed in the stretching modes. Both the IR and Raman modes corresponding to Pt-Cl and Pt-P stretching vibrations show monotonous stiffening upon cooling with a slight change in the rate of variations across 19
130 K. But a shoulder developing on the lower frequency side of Pt-Cl stretching vibration in IR spectra transforms to a clear peak upon cooling below 180K, whereas, no such feature observed in Raman spectra around Pt-Cl stretching vibration. In the organic ligand deformation region, the significant information provided by IR spectra includes further modifications in geometrical conformation of organic ligands as indicated by enhanced complexity of spectral profiles related to skeletal modes of PEt3, including C-C stretching modes and CH2 rock. The Raman spectra distinctly show intensification of C-CH3 rocking vibration upon cooling. It is interesting to note that at high pressures also, this compound shows phase transitions as reported recently,19 where the pressure induced orientational changes in the ethyl groups bring them in close proximity of the chlorine atoms. This leads to the systematic strengthening of interand intra-molecular C-H---Cl hydrogen bonds and results in hydrogen bonded stitched stair case supramolecular assembly.19 During this phase transition at 0.8 GPa, the IR active modes were found to appear in the Raman spectra and vice versa, implying that the principle of mutual exclusion becomes invalid across the high pressure phase transition.19 However, the low temperature observations, reported here, are in contrast to these results. Although phase transition is indicated at low temperatures across 180 K and 130 K, but features pointing to loss of inversion symmetry have not been observed. For example, in the high pressures IR spectra, ν(P-C) Raman mode appears on the high frequency side of ν(P-C) IR mode at 0.8 GPa and vice versa in the Raman spectra,19 whereas at low temperatures the new mode appears on the low frequency side of ν(P-C) in the IR spectra (Figure 3) and no new mode is seen to emerge in the Raman spectra close to the position of ν(P-C) mode (Figure 6). A one to one correspondence of all the spectral changes confirms that the changes associated with phase transitions at low temperature and high pressure are dissimilar. Hence, the studies of metal20
organic square planar complexes under varying thermodynamic conditions are essential to understand molecular conformational flexibility which governs their technological and catalytic applications.
4. Summary Systematic low temperature studies of trans-PtCl2(PEt3)2 have been carried out using complementary FTIR and Raman spectroscopic techniques, which show significant variations in the vibrational modes associated with organic ligand moieties across 180 K and 130 K. An increase in the C-H---Cl hydrogen bonding strengths, associated with CH3 groups, is noticed in the intermediate temperature range of 180 – 130 K. Softening of the lattice modes and appearance of new Raman modes in the lattice region across 180 K and 130 K are suggestive of phase transitions in this compound. However, unlike in the reported high pressure phase, no signatures of loss of inversion symmetry across the phase transitions have been observed at low temperatures.
Conflicts of interest There are no conflicts to declare.
REFERENCES (1).Ruano, D.; Díaz‐García, M.; Alfayate, A.; Sánchez‐Sánchez, M., Nanocrystalline m–mof‐74 as heterogeneous catalysts in the oxidation of cyclohexene: Correlation of the activity and redox potential. ChemCatChem 2015, 7, 674‐681. (2).Allan, P. K.; Wheatley, P. S.; Aldous, D.; Mohideen, M. I.; Tang, C.; Hriljac, J. A.; Megson, I. L.; Chapman, K. W.; De Weireld, G.; Vaesen, S., Metal–organic frameworks for the storage and delivery of biologically active hydrogen sulfide. Dalton Transactions 2012, 41, 4060‐4066.
21
(3).Cattaneo, D.; Warrender, S. J.; Duncan, M. J.; Castledine, R.; Parkinson, N.; Haley, I.; Morris, R. E., Water based scale‐up of cpo‐27 synthesis for nitric oxide delivery. Dalton Transactions 2016, 45, 618‐ 629. (4).Chavan, S.; Bonino, F.; Valenzano, L.; Civalleri, B.; Lamberti, C.; Acerbi, N.; Cavka, J. H.; Leistner, M.; Bordiga, S., Fundamental aspects of h2s adsorption on cpo‐27‐ni. The Journal of Physical Chemistry C 2013, 117, 15615‐15622. (5).Chavan, S.; Vitillo, J. G.; Groppo, E.; Bonino, F.; Lamberti, C.; Dietzel, P. D.; Bordiga, S., Co adsorption on cpo‐27‐ni coordination polymer: Spectroscopic features and interaction energy. The Journal of Physical Chemistry C 2009, 113, 3292‐3299. (6).Dietzel, P. D.; Panella, B.; Hirscher, M.; Blom, R.; Fjellvåg, H., Hydrogen adsorption in a nickel based coordination polymer with open metal sites in the cylindrical cavities of the desolvated framework. Chemical Communications 2006, 959‐961. (7).Glover, T. G.; Peterson, G. W.; Schindler, B. J.; Britt, D.; Yaghi, O., Mof‐74 building unit has a direct impact on toxic gas adsorption. Chemical Engineering Science 2011, 66, 163‐170. (8).McKinlay, A.; Eubank, J.; Wuttke, S.; Xiao, B.; Wheatley, P.; Bazin, P.; Lavalley, J.‐C.; Daturi, M.; Vimont, A.; De Weireld, G., Nitric oxide adsorption and delivery in flexible mil‐88 (fe) metal–organic frameworks. Chemistry of Materials 2013, 25, 1592‐1599. (9).McKinlay, A. C.; Allan, P. K.; Renouf, C. L.; Duncan, M. J.; Wheatley, P. S.; Warrender, S. J.; Dawson, D.; Ashbrook, S. E.; Gil, B.; Marszalek, B., Multirate delivery of multiple therapeutic agents from metal‐ organic frameworks. APL Materials 2014, 2, 124108. (10).Hinks, N. J.; McKinlay, A. C.; Xiao, B.; Wheatley, P. S.; Morris, R. E., Metal organic frameworks as no delivery materials for biological applications. Microporous and Mesoporous Materials 2010, 129, 330‐ 334. (11).Miller, S. R.; Alvarez, E.; Fradcourt, L.; Devic, T.; Wuttke, S.; Wheatley, P. S.; Steunou, N.; Bonhomme, C.; Gervais, C.; Laurencin, D., A rare example of a porous ca‐mof for the controlled release of biologically active no. Chemical Communications 2013, 49, 7773‐7775. (12).Rojas, S.; Wheatley, P. S.; Quartapelle‐Procopio, E.; Gil, B.; Marszalek, B.; Morris, R. E.; Barea, E., Metal–organic frameworks as potential multi‐carriers of drugs. CrystEngComm 2013, 15, 9364‐9367. (13).Choudhury, R. R.; Poswal, H.; Chitra, R.; Sharma, S. M., Pressure induced structural phase transition in triglycine sulfate and triglycine selenate. The Journal of chemical physics 2007, 127, 154712. (14).Boldyreva, E. V.; Kolesnik, E. N.; Drebushchak, T. N.; Ahsbahs, H.; Beukes, J. A.; Weber, H.‐P., A comparative study of the anisotropy of lattice strain induced in the crystals of l‐serine by cooling down to 100 k or by increasing pressure up to 4.4 gpa. Zeitschrift für Kristallographie-Crystalline Materials 2005, 220, 58‐65. (15).Boldyreva, E. V.; Kolesnik, E. N.; Drebushchak, T. N.; Sowa, H.; Ahsbahs, H.; Seryotkin, Y. V., A comparative study of the anisotropy of lattice strain induced in the crystals of dl‐serine by cooling down to 100 k, or by increasing pressure up to 8.6 gpa. A comparison with l‐serine. Zeitschrift für Kristallographie-Crystalline Materials 2006, 221, 150‐161. (16).Mączka, M.; da Silva, T. A.; Paraguassu, W.; Ptak, M.; Hermanowicz, K., Raman and ir studies of pressure‐and temperature‐induced phase transitions in [(ch2) 3nh2][zn (hcoo) 3]. Inorganic chemistry 2014, 53, 12650‐12657. (17).Suffren, Y.; Rollet, F.‐G.; Reber, C., Raman spectroscopy of transition metal complexes: Molecular vibrational frequencies, phase transitions, isomers, and electronic structure. Comments on Inorganic Chemistry 2011, 32, 246‐276. (18).Genre, C.; Levasseur‐Thériault, G.; Reber, C., Emitting‐state properties of square‐planar dithiocarbamate complexes of palladium (ii) and platinum (ii) probed by pressure‐dependent luminescence spectroscopy. Canadian Journal of Chemistry 2009, 87, 1625‐1635. 22
(19).Bajaj, N.; Bhatt, H.; Pandey, K. K.; Poswal, H. K.; Arya, A.; Ghosh, P.; Deo, M., Phase transition in metal‐organic complex trans‐ptcl2 (pet3) 2 under pressure–insights of molecular and crystal structure. CrystEngComm 2018. (20).Bhatt, H.; Deo, M.; Vishwakarma, S.; Bajaj, N.; Sharma, S. M. In Ft-raman spectroscopy of structural isomers of pt (ii) complex ptcl2 (pet3) 2, AIP Conference Proceedings, AIP Publishing: 2015; p 090045. (21).Besara, T.; Jain, P.; Dalal, N. S.; Kuhns, P. L.; Reyes, A. P.; Kroto, H. W.; Cheetham, A. K., Mechanism of the order–disorder phase transition, and glassy behavior in the metal‐organic framework [(ch3) 2nh2] zn (hcoo) 3. Proceedings of the National Academy of Sciences 2011. (22).Messmer, G.‐G.; Amma, E., Crystal structures and multiple bonding in trans‐pt [p (c2h5) 3] 2cl2 and trans‐pt [p (c2h5) 3] 2br2. Inorganic Chemistry 1966, 5, 1775‐1781. (23).Duddell, D.; Goggin, P.; Goodfellow, R.; Norton, M.; Smith, J., Vibrational spectra of some gold (i), palladium (ii), and platinum (ii) complexes with trialkylphosphines and trialkylarsines. Journal of the Chemical Society A: Inorganic, Physical, Theoretical 1970, 545‐564. (24).Wood, J., Far‐infrared spectroscopy. Quarterly Reviews, Chemical Society 1963, 17, 362‐381. (25).Kaesz, H.; Stone, F., Infra‐red spectra of some vinyl and ethyl compounds of mercury, cadmium, zinc, tin and phosphorus. Spectrochimica Acta 1959, 15, 360‐377. (26).Goodfellow, R.; Evans, J.; Goggin, P.; Duddell, D., Infrared spectra (800–200 cm.–1) of trimethylphosphine and trimethylarsine complexes of platinum and palladium mx 2 l 2 and m 2 x 4 l 2 (x= cl, br, or i). Journal of the Chemical Society A: Inorganic, Physical, Theoretical 1968, 1604‐1609. (27).Allkins, J. R.; Obremski, R. J.; Brown, C. W.; Lippincott, E. R., Effect of high pressure on the vibrational spectra of square‐planar coordination compounds. Inorganic Chemistry 1969, 8, 1450‐1454. (28).Adams, D.; Chatt, J.; Shaw, B., 415. The infrared spectra of some alkylplatinum complexes, and the influence of various ligands on the pt–c bond strength. Journal of the Chemical Society (Resumed) 1960, 2047‐2053. (29).Adams, D.; Chatt, J.; Gerratt, J.; Westland, A., 145. Far‐infrared spectra of some square‐planar complexes [ptx 2 l 2](x= cl or br). The influence of l upon the platinum–halogen stretching frequency and its relation to the trans‐effect. Journal of the Chemical Society (Resumed) 1964, 734‐739. (30).Coates, G.; Parkin, C., 64. Tertiary phosphine complexes of trimethylgold: Infrared spectra of complexes of gold and some other metals. Journal of the Chemical Society (Resumed) 1963, 421‐429. (31).JENSEN, K. A.; NIELSEN, H.; PEDERSEN, C. T., Stereochemistry of 5‐co‐ordinated compounds. ACTA CHEMICA SCANDINAVICA 1963, 17, 1115‐1125. (32).Watari, F., Vibrational spectra of trimethylphosphine telluride and the phosphorus‐tellurium stretching force constant. Inorganic Chemistry 1981, 20, 1776‐1779. (33).Clark, H.; Tsang, W., Chemistry of metal hydrides. Ii. Spectroscopic studies of fluorovinylplatinum (ii) derivatives. Journal of the American Chemical Society 1967, 89, 533‐539. (34).Durig, J.; Nashed, Y.; Tao, J.; Guirgis, G., Conformational and structural studies of ethyl fluorosilane from temperature dependent ft‐ir spectra of krypton solutions and ab initio calculations. Journal of Molecular Structure 2001, 560, 39‐55. (35).Goggin, P.; Goodfellow, R., Identification of the stereochemistry of square planar bis‐ triethylphosphine complexes by infrared spectroscopy. Journal of the Chemical Society A: Inorganic, Physical, Theoretical 1966, 1462‐1466. (36).Chatt, J.; Leigh, G.; Mingos, D., Raman spectra of some tertiary phosphine complexes of rhodium, iridium, palladium, and platinum. Journal of the Chemical Society A: Inorganic, Physical, Theoretical 1969, 2972‐2975.
23
Spectroscopic Studies of Temperature Induced Phase Transitions in Metal-Organic Complex trans-PtCl2(PEt3)2 Naini Bajaj,1,2 Himal Bhatt,1,* H.K. Poswal,1 and M.N. Deo1,2 1
High Pressure & Synchrotron Radiation Division, Bhabha Atomic Research Centre, 2Homi Bhabha National Institute, Anushaktinagar, Mumbai *Email: [email protected]
Highlights 1. Systematic low temperature infrared and Raman spectroscopic studies on Pt(II)organic complex trans-PtCl2(PEt3)2 have been reported from ambient to 4.2 K, in a wide spectral range. 2. Drastic orientational changes occur in the different molecular units across 180 K and 130 K. 3. Strengthening of C-H---Cl hydrogen bonds have been observed in the 180-130 K temperature range. 4. Change in the intensity, discontinuous shift in the peak positions and appearance of new modes in the lattice region across 180 K and 130 K point towards structural phase transitions.
1
S1386-1425(19)31018-2
DOI:
https://doi.org/10.1016/j.saa.2019.117628
Reference:
SAA 117628
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: 11 February 2019 Revised Date:
17 September 2019
Accepted Date: 6 October 2019
Please cite this article as: N. Bajaj, H. Bhatt, H.K. Poswal, M.N. Deo, Spectroscopic studies of temperature induced phase transitions in metal-organic complex trans-PtCl2(PEt3)2, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2019), doi: https://doi.org/10.1016/ j.saa.2019.117628. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Spectroscopic Studies of Temperature Induced Phase Transitions in Metal-Organic Complex trans-PtCl2(PEt3)2 Naini Bajaj,1,2 Himal Bhatt,1,* H.K. Poswal,1 and M.N. Deo1,2 1
High Pressure & Synchrotron Radiation Division, Bhabha Atomic Research Centre, 2Homi Bhabha National Institute, Anushaktinagar, Mumbai *Email: [email protected]
Graphical Abstract
1
Spectroscopic Studies of Temperature Induced Phase Transitions in Metal-Organic Complex trans-PtCl2(PEt3)2 Naini Bajaj,1,2 Himal Bhatt,1,* H.K. Poswal,1 and M.N. Deo1,2 1
High Pressure & Synchrotron Radiation Division, Bhabha Atomic Research Centre, 2Homi Bhabha National Institute, Anushaktinagar, Mumbai *Email: [email protected]
Abstract. Tuning of molecular and electronic properties of Pt(II)-organic complexes have a profound effect on their applications in the fields of technology, pharmaceuticals and crystal engineering. Here, we present combined infrared and Raman spectroscopic investigations on trans-PtCl2(PEt3)2 systematically carried out at various temperatures from 300 – 4.2 K in a wide spectral range. The studies suggest drastic orientational changes of different moieties around 180 K and 130 K in the ligand groups attached to the central Pt atom. This is accompanied by a systematic strengthening of C-H---Cl hydrogen bonds in the 180 – 130 K temperature range. A discontinuous change in intensity, peak variations of modes and emergence of new modes across 180 K and 130 K in the lattice region are suggestive of a possible structural phase transition. It is interesting to note that the spectral signatures of the low temperature phase are different from those reported recently for the high pressure phase in this compound. These studies will be useful in better understanding the physico-chemical properties of metal-organic complexes in order to exploit their applications in various bio-chemical and technological fields.
Keywords: Pt(II)-organic complex, infrared, Raman spectroscopy, low temperature, hydrogen bonding 1
1. Introduction Metal-organic complexes have applications in the fields of catalysis,1 drug delivery,2-7 gas storage,8, 9 pharmacy
2, 9-12
etc. due to their porous structure formed by organic linkers, thus
attracting researchers in the fields of materials science and crystal engineering. Since their discovery, there have been several efforts to tune their structural properties by various methods in order to further improve/ optimize their capabilities. This tuning can be performed by modifying the size of the linking group, changing the metal salt to which linking (organic) group is attached and by altering the experimental conditions like thermodynamic parameters. Changing the thermodynamic parameters such as pressure/ temperature offers the cleanest way as it does not involve the addition of any external impurity. Such changes cause modifications in the inter-atomic separations, which may sometimes lead to phase transitions. Though both high pressure and low temperature result in compression/ contraction of the system, the mechanism of possible phase transitions or structural relaxations in the square planar metal-organic complexes may be similar or different.13-15 For example, in the complex [(CH2)3NH2][Zn(HCOO)3], phase transitions have been observed at low temperature as well as high pressure due to the ring-puckering motion of the cationic groups and the framework rearrangement resulting from rotations of the HCOO- ions respectively.16 In contrast, the compound trans-Pt(II) dithiocarbamate, which shows small variations in metal-ligand stretching Raman modes and no variations in deformation modes at low temperatures, depicts drastic and different pressure induced changes, where a large variation of the deformation Raman modes was noted as compared to the stretching modes.17,
18
The high pressure studies on the title
compound trans-PtCl2(PEt3)2 (trans-dichloro bis(triethylphosphine) platinum(II)), have shown that preferred molecular orientations trigger a structural phase transition at 0.8 GPa.19 Our 2
preliminary Low temperature (LT) -FT Raman study on the structural isomers of this compound (cis- and trans-) at two temperatures of 300 K and 77 K in the spectral range 60 – 600 cm-1 showed drastic alterations in the skeletal Raman modes of trans-isomer upon cooling, whereas relatively no change was detected in the spectrum of cis-isomer.20 The increase in intensity of deformation/ stretching Raman modes of phosphine ligands upon cooling trans-PtCl2(PEt3)2 provided the impression of new modes appearing close to the positions where the corresponding IR modes are expected.20 Such a phenomenon indeed happens under high pressure at 0.8 GPa, where the structure changes from centrosymmetric to a non-centrosymmetric phase.19 The systematic variable temperature studies are therefore essential to confirm if such a phase transformation takes place upon cooling. In addition, given the importance of organic ligands in the structural stability, low temperature studies of metal organic complexes are important in light of the reports that freezing of methyl groups is associated with a dynamical phase transition.21 In the present paper, detailed low temperture studies on trans-PtCl2(PEt3)2 have been reported using combined Fourier transform infrared (FTIR) and Raman spectroscopy in a wide spectral range to understand the molecular structure. Interestingly, these studies reveal that a phase transition indeed takes place across 180 K and 130 K, but the low temperature phase is not similar to the high pressure phase transition.
2. Experimental details 2.1. Low temperature IR measurements: Infrared spectroscopic studies have been carried out on the powder sample of transPtCl2(PEt3)2 at various temperatures from room temperature down to 4.2 K in 140 – 3200 cm-1 spectral range. A continuous flow liquid Helium (LHe) cryostat was used for in-situ low 3
temperature investigations. The sample, dispersed in CsI matrix, was pelletized and mounted on the cold finger of the cryostat; the temperature was varied from 300 – 4.2 K at steps of 10 K (± 2 K). The cryostat was mounted in the sample compartment of a Bruker Vertex 80V Fourier transform infrared spectrometer, installed in the experimental station of IR beamline at Indus-1 Synchrotron facility in India. The far infrared (FIR) spectra in the range 140 – 650 cm-1 were recorded using Hg source, Mylar beamsplitter and FIR-DTGS detector at a resolution of 2 cm-1, whereas the mid infrared (MIR) spectra (500 – 3500 cm-1) were recorded using thermal source, KBr beamsplitter and LN2 cooled MCT detector at a resolution of 1 cm-1. For each spectrum, a total of 100 scans were coadded at all temperatures. The flow of liquid helium (inlet) and helium vapour (outlet) were controlled using a gas flow controller calibrated for helium gas and an oil free diaphragm pump. The measurements were carried out in the transmission mode in vacuum condition and the background spectra were also recorded in the same temperature points.
2.2. Low temperature Raman measurements: The Raman spectra of trans-PtCl2(PEt3)2 were recorded on triple stage Raman spectrograph (JobinYvon T64000) equipped with LN2 cooled CCD detector. The Raman signal was excited using diode pump laser of 532 nm wavelength. The data were recorded in the subtractive mode in the 10 – 3000 cm-1 spectral range at a resolution of 4 cm-1. Temperature dependent measurements from room temperature down to 77 K were carried out using a heating and cooling microscope stage (Linkam THMS 600).
3. Results and discussion
4
3.1. Structure and Vibrational mode assignments: The ambient structure of trans-PtCl2(PEt3)2 consists of a central platinum (Pt) atom which is connected to two chlorine (Cl) atoms and two phosphine triethyl ligands P(C2H5)3 in transposition to each other as shown in Figure 1. Its crystal structure was first described by Messmer et al.,22 which suggested a centre of inversion symmetry at the Platinum atom. The changes associated with the metal – ligand bonds and hence the crystal symmetry can be probed by studying the lattice and skeletal vibrational modes which are generally observed in the low frequency regions of the infrared (IR)/ Raman spectrum, whereas the deformation and stretching vibrational modes of organic ligands are observed at higher frequencies.
Figure 1. trans-PtCl2(PEt3)2 (Et = C2H5, ethyl group) molecule with central Platinum (Pt) Silver, Chlorine (Cl) - Blue, Phosphorous (P) - Orange and Carbon (C) - Grey atoms. Hydrogen atoms are omitted for clarity. Numbers are the C-C bond lengths in the Angstrom (Å) unit.19
The vibrational spectra of trans-PtCl2(PEt3)2 can be classified into different regions in the infrared and Raman spectra at ambient conditions, accordingly the vibrational mode assignments have been carried out by various workers.23-29 For mode assignments in our spectra, we compared the observed frequencies with reported works in literature for the same as well as 5
similar compounds,
23, 26-30
i.e. metal (Pt, Ni, Pd etc.) organic compounds, with same as well as
different ligand chains, viz. triethylphosphine (PEt3),31 trimethylphosphine (PMe3),31 TeP(CH3)3,32 Pt(PEt3)2(CF=CF2)Cl,33 C2H5SiH2F,34 (C2H5)3P=S etc. The observed IR and Raman peak positions have been listed in Tables 1 and 2 respectively alongside the reported values and assignments. The spectral region 10 – 500 cm-1 contains lattice modes and the skeletal modes of Pt-Cl2 and PtP2 units. The stretching vibrations of Pt-Cl and Pt-P are observed at 334 cm−1 (341 cm-1) and 433 cm−1 (415 cm-1) respectively in the Raman (and IR) spectra. At lower frequencies, various deformation modes: δ(P–Pt–Cl) (157 cm−1 - Raman), δ(Pt–Cl2) (167 cm−1 - Raman, 168 cm-1 IR), and γ(P–Pt–Cl) (228 cm−1 - Raman, 228 cm-1 - IR ) (γ – out of plane, δ – in plane bending) and lattice modes are observed. The variations in geometry, viz. bond lengths and angles exhibited by the three ethyl groups linked to Phosphorous atom19 is well reflected by the complex spectral profiles in ligand deformation and stretching regions. The region 500 – 1600 cm-1 is the fingerprint region. This region consists of internal modes of triethylphosphine ligands attached to the central platinum atom. The P-C stretching vibration is observed at 634 cm-1 in the IR spectra and at 642 cm-1 in the Raman spectra. Since there are various ethyl groups attached to the P atom with varying C-C bond lengths (as shown in Figure 1), the spectra show populated profiles in the regions 700 – 1100 cm-1 range, with prominent absorption features due to skeletal modes of PEt3 and CH2 rock followed by C-C stretching vibrations. The peak around 1030 cm-1 (in IR) and 1047 cm-1 (in Raman) corresponds to C-CH3 rocking vibration. The region from 1200 – 1500 cm-1 consists of different deformation modes of organic ligand: CH2 wagging (IR – 1257 cm−1, Raman – 1243 cm−1), CH2 scissoring (IR – 1410 cm−1, Raman – 1425 cm−1), CH3 symmetric deformation (IR – 6
1376 cm-1, Raman – 1384 cm-1) and CH3 asymmetric deformation (IR – 1449 cm-1, Raman – 1460 cm-1). The region from 2800 – 3200 cm-1 consists of CH2 and CH3 stretching vibrations along with various other overtone and combination modes. The symmetric modes lie at lower frequency than the asymmetric modes. These modes are observed in the infrared and Raman spectra at: νsCH3 – 2880 cm−1 (Raman) and 2874 cm−1 (IR); νasCH2 – 2932 cm−1 (Raman) and 2931 cm−1 (IR); νasCH3 – 2964 cm−1 (Raman) and 2964 cm−1 (IR); and νsCH2 – 2912 cm−1 (Raman) and 2913 cm-1 (IR). Our observations of temperature induced variations of these IR and Raman modes have been sequentially listed in sections 3.2 and 3.3 respectively, which is followed by a discussion on these changes.
Table 1: Infrared vibrational modes of trans-PtCl2(PEt3)2 from various references and this study in the spectral region 150 – 3000 cm-1. Values in parenthesis are the fitting errors. Infrared modes
Ref. 23
Ref. 24
Ref. 28
Ref. 31
Ref. 35
transPtCl2(PEt3)2 (cm-1)
transPtCl2(PEt3)2 (cm-1)
transPtCl2(PEt3)2 (cm-1)
transPtCl2(PEt3)2 (cm-1)
transPtCl2(PE t3)2 (cm-1)
Lattice mode
168 w
Ligands (or internal modes of PEt3)
185 wm
νs(Pt-Cl2) νs(Pt-P2) ν(P-C) Skeletal modes of PEt3 & CH2 rock
228 w 275 w 384 w 341 vs 415 m
Ref. 25 (PEt3) (cm-1)
384 341 415
339 s 413 w 382 w 632 m
632 s 610 w
710 m
7
655 657 670 690
This Study trans-PtCl2(PEt3)2 Peak Width Peak Position (cm-1) (cm-1) 167.4(9) w + weak shoulder at 178 180.8(3) wm + weak shoulder at 192 230.1(2) 278.8(3) 384.2(4) 341.4(1) 415.5(2)
10.1(3)
13.2(8) 19.5(1) 2.2(3) 8.5(2) 12.7(6)
634.5(7)
8.1(2)
8.0(2)
717 733 s
νs(C-C)
748 765 975 1003 1023 1043 1098 1185 1231 m 1245 m,sh
1005 w 1033 s
C-CH3 rock
CH2 wag.
1255 w
CH3 sym. def. Combinat ion CH2 scissoring Overtone CH3 asym. def. 1465=73 5+735 CH3 sym. str. CH2 sym str. CH2 asym. str. CH3 asym str.
1375 w
1380 s
711.5(2) 734.9(1)
3.9(5) 8.7(3)
767.6(9) 1007.4(2) 1031.2(8) 1041.0(1)
13.6(3) 3.9(8) 4.3(3) 15.1(4)
1257.2(1)
15.4(7)
1376.8(1)
4.8(4)
1384.2(3)
4.0(1)
1418 m
1423 s
1410.5(3)
5.5(8)
1452 m
1468 s
1424.5(4) 1449.0(3)
12.3(1) 5.4(8)
1468.0(3)
14.2(7)
2845
2874.5(2)
7.8(7)
2900 s
2913.4(3)
13.4(1)
2931.5(1)
10.4(7)
2963.8(1)
9.7(3)
2998 s
Table 2: Raman vibrational modes of trans-PtCl2(PEt3)2. Values in parenthesis are the fitting errors. Raman modes
In plane def. Other low freq. and lattice modes
Ref. 23
Ref. 36
transPtCl2(PE t3)2 (cm-1) 153 m 360 w 274 m 233 ms 195 m
transPtCl2(PE t3)2 (cm-1) 154 m 365 w
164 m Pt-Cl2 str Pt-P2 str. P-C str. CH2 rock C-C sym str.
330 vs 432 ms
Ref. 35
Ref. 25
transPtCl2(PEt3)2 (cm-1)
(PEt3) (cm-1)
233 m 178 sh 164 m 114 sh 334 s 432 m
333 443 757 982
8
This Study trans-PtCl2(PEt3)2 Peak Position Peak Width (cm-1) (cm-1)
152.9(2)
10.1(4)
268.1(4) 229.2(3)
13.2(3) 16.3(7)
164.5(1) 120.9(4) 329.9(1) 429.7(2) 636.6(1)
6.7(2) 11.0(2) 6.9(2) 9.9(4) 7.1(2)
1003.0(6)
8.1(3)
C-CH3 rock CH2 wag CH3 sym def. CH2 sym def. CH2 scissoring CH3 asym def. CH3 sym str CH2 sym str CH2 asym str CH3 asym str
1041 1243 1384 1425 1464 2821 2873 2896 2926
1012.5(3) 1045.2(2)
7.3(2) 6.7(5)
1417.2(7)
22.2(6)
1460.5(2) 2880.5(4) 2913.6(6) 2931.9(3) 2964.2(1)
15.8(6) 14.6(7) 11.8(1) 16.3(5) 8.9(5)
3.2. Low temperature infrared spectroscopy: a) Low frequency spectral Region (below 500 cm-1):
Figure 2. Low temperature IR spectra of trans-PtCl2(PEt3)2 in the (a) 150-500 cm-1 spectral region. Dashed lines indicate new peaks emerging at low temperatures. (b) Variation of IR peak positions at low temperatures in the 160 – 640 cm-1 spectral range. Assignment of few peaks
9
directly influencing the skeletal motion have been marked. Open circles represent new modes appearing upon cooling.
Figures 2a represents low temperature IR spectra in the 150 – 500 cm-1 spectral range. Figure 2b shows the corresponding frequency versus temperature plots in the 160 – 640 cm-1 spectral range. The IR mode corresponding to ν(Pt-Cl) (341 cm-1) and ν(Pt-P) (417 cm-1) stretching vibrations show small but monotonous stiffening upon cooling, with a small change in the rate of variations across 130 K, up to the lowest temperature measured, implying systematic strengthening of the skeletal bonds (Figures 2b). The shoulder (333 cm-1) developing on the lower frequency side of the ν(Pt-Cl) (341 cm-1) IR mode transforms to a clear peak below 180 K and it softens upon further cooling (Figure 2a). The deformation mode γ(P-Pt-Cl), which is observed at 228 cm-1 in the IR spectra shows systematic stiffening upon cooling down to 130 K (Figures 2b). In addition, there are some weak IR absorption features below 300 cm-1 which show small stiffening upon cooling and reach a plateau at nearly 130 K. A weak feature is observed to be emerging at ~ 147 cm-1 across 130 K (marked with dotted line in Figure 2a). The weak and broad band which spans the region 150 – 200 cm-1, consists of δ(Pt–Cl2) mode at 167 cm-1 and other deformation and lattice modes,23, 25 is completely resolved upon cooling down to 180 K and transforms to four clear peaks at further lower temperatures (Figure 2a). A new mode is clearly seen to appear around 250 cm-1 in the IR spectra below 130 K (marked with dotted line in Figure 2a), which strengthens at lower temperatures.
b) Region 500 – 1600 cm-1:
10
(b)
Figure 3: Low temperature IR spectra of trans-PtCl2(PEt3)2 in the (a) 620 – 650 cm-1 (b) 700 – 1100 cm-1 spectral region. Dashed lines are guide to the eye. (c) Variation of IR peak positions at low temperatures in the 600 – 1100 cm-1 spectral range. Filled squares – ambient modes, filled circles – represent new modes appearing upon cooling.
The deformation modes of triethylphosphine ligands are observed in the 600 – 1600 cm-1 fingerprint region. The systematic IR spectra at various temperatures from 4.2 K to 300 K, shown in Figures 3 reveal that the orientational changes associated with the organic ligand groups take place at temperatures of 180 and 130 K. In the phosphine ligand, the ν(P-C) IR mode at 634 cm-1 in Figure 3a, shows an increase in the rate of stiffening (~ 0.028 cm-1/K below 180 K) upon cooling below 180 K with a new shoulder appearing on the low frequency side (~ 633 cm-1 at 180 K), which shows small softening upon cooling, as shown in Figure 2b. The spectral region 700 – 1500 cm-1, describing the various deformation vibrational motions of organic group is highly populated because there are in all three ethyl groups on either side of the Pt atom, with varying bond parameters as well as geometrical orientations (see Figure 1).19 The IR spectral region 700 – 800 cm-1 (skeletal modes of PEt3 and CH2 rock), becomes further complex upon 11
cooling due to splitting and appearance of new modes across 180 K and 130 K as shown in Figure 3b. A new mode also appears around 1003 cm-1 (near ν(C-C) IR mode) below 180 K which softens and shows a large increase in the relative intensity upon cooling, as shown in Figure 3b. The C-CH3 rocking IR mode at 1032 cm-1 stiffens in the 180 – 130 K temperature range, with the appearance of new shoulder (~1030 cm-1) on its lower frequency side below 180 K. This new mode softens with an increase in the relative intensity upon cooling upto 4.2 K as shown in Figures 3a and b. Below 130 K, these modes show no change upon lowering the temperature.
(a)
Figure 4. (a) Low temperature IR spectra of trans-PtCl2(PEt3)2 in the 1220 – 1480 cm-1 spectral region. Dashed lines are guide to the eyes. (b) Variation of IR peak positions at low temperatures in the 1220 – 1480 cm-1 spectral range. The primary IR mode assignments in various vibrational band regions of ethyl groups with varying bond parameters and geometry, have been marked. Open circles represent new modes appearing upon cooling. 12
The wagging IR mode ω(CH2) at 1260 cm-1 stiffens upon cooling upto 130 K as shown in Figure 4a. The symmetric deformation ds(CH3) (~1260 cm-1) and asymmetric deformation das(CH3) (~1450 cm-1) IR modes soften upon cooling upto 130 K, with the appearance of a new mode on the lower wavenumber side of ds(CH3) below 180 K as shown in Figures 4a and 4b. Upon further lowering the temperature below 130 K, these modes show no variation. CH2 scissoring (1410 cm-1) mode shows stiffening behavior upto 130 K and is constant with further lowering of temperature upto 4.2 K.
c) Region 2800 – 3200 cm-1:
Figure 5. (a) Low temperature IR spectra of trans-PtCl2(PEt3)2 in the 2850 – 3050 cm-1 spectral range. Spectral plot at 300 K and 180 K show the deconvoluted peaks (Green color deconvoluted peaks at 180 K are the new peaks emerging at low temperature). (b) Variation of CH2 and CH3 stretching IR modes at low temperature in the 2800 – 3000 cm-1 spectral range (the combination/ 13
overtone and new modes at lower temperatures are excluded in plot for clarity). Symbols: νs symmetric stretch, νas - asymmetric stretch.
The region 2800 – 3200 cm-1 consists of stretching vibrations of CH2 and CH3 groups (plotted in Figure 5b) along with various overtone and combination modes (not plotted in Figure 5b for clarity). At low temperatures, the symmetric and asymmetric IR stretching vibrational peaks of CH3 group (2874 cm-1 and 2963 cm-1 respectively) show small softening in the 300 – 4.2 K temperature range, with a noticeable increase in the rate of softening in 180 – 130 K temperature range, as shown in Figure 5b. A new shoulder peak emerges close to the CH3 asymmetric stretching mode, showing a similar behavior (softening at ~ - 0.100 cm-1/K) and intensification relative to other modes in the 180-130 K temperature range, as shown in Figure 5a.
3.3. Low temperature Raman Spectroscopy: a) Low frequency spectral region (below 500 cm-1):
14
Figure 6. (a) Low temperature Raman spectra of trans-PtCl2(PEt3)2 in the 5 – 650 cm-1 region. Inset: zoomed 10-70 cm-1 spectral region at low temperatures. (b) Variation of Raman active modes at low temperatures in the 10 – 650 cm-1 spectral range.
Figure 6a represents low temperature Raman spectra of trans-PtCl2(PEt3)2 in the low frequency region 5 – 650 cm-1. Figure 6b shows temperature induced variation of the corresponding Raman modes. The lattice modes at 20 and 23 cm-1 in the Raman spectra show initial softening upto 173 K, followed by stiffening up to 133 K as shown in Figures 6a and 6b. Also, at temperatures close to 130 K, a sudden increase in intensity of lattice modes is observed as shown in Figure 6a. Below this temperature, the peak positions as well as intensities do not show any appreciable changes (Figures 6a and 6b). New modes also appear in the Raman spectra at ~ 71 and 147 cm-1 across 180 K and 130 K respectively, which stiffen upon cooling (Figure 6a). Among the skeletal modes, the δ(Pt-Cl2) Raman mode at 165 cm-1 remains almost unaltered with temperature. The deformation mode γ(P-Pt-Cl), which is observed at 233 cm-1 in the Raman 15
spectra, initially shows systematic stiffening upon cooling down to 130 K (Figures 6b). Its position reaches a plateau at nearly 130 K. The Raman modes corresponding to ν(Pt-Cl) (329 cm1
) and ν(Pt-P) (429 cm-1) stretching vibrations show small but monotonous stiffening upon
cooling, with a small change in the rate of variations across 130 K, up to the lowest temperature measured, implying systematic strengthening of the skeletal bonds (Figures 6b). The ν(P-C) Raman mode at 632 cm-1 shows stiffening behavior upon cooling in the 173 – 133 K temperature range (Figure 6b). Across 130 K, it shows discontinuous shift to a higher value. Further, most of the vibrational modes in the region 10 – 650 cm-1 show slope change in the frequency versus temperature plots across 180 K and 130 K. In summary, the features like appearance of new modes in the lattice and skeletal regions, abrupt changes in the rate of variation of frequencies and change in the relative intensity of some vibrational modes in the low frequency regions of Raman spectra are clear indications of phase transition in trans-PtCl2(PEt3)2 across 180 K and 130 K. The changes occurring in the organic ligands groups can provide further insights into the nature of these transitions.
b) Deformation region (changes associated with organic ligands):
16
Figure 7. Low temperature Raman spectra of trans-PtCl2(PEt3)2 in the 650 – 1500 cm-1 spectral region.
The internal vibrational modes of the organic ligands also show similar changes under temperature induced contraction as observed in IR spectroscopy. The distinctive feature in the Raman spectra in this region is the sudden increase in the relative intensity of C-CH3 rocking mode at 1055 cm-1 across 173 K as shown in Figure 7. In the 1250 – 1500 cm-1 spectral range, the band lying in the region of asymmetric deformation vibrations of CH3 unit (1450 – 1500 cm1
) shows splitting below 180 K in the Raman spectra (Figure 7). The CH2 scissoring mode (~
1413 cm-1) shows a relatively large rate of stiffening (~ 0.020 cm-1/K) compared to other modes upto 130 K and below this temperature it shows no variation as shown in Figure 7. This spectral region becomes complex upon cooling due to splitting of several modes. A drastic increase in the intensity of modes, which otherwise are weak at ambient conditions, has been observed at low temperatures. Hence, the Raman spectra in lattice/ skeletal as well as ligand deformation regions 17
show marked changes across 180 K and 130 K, which can be related to temperature induced phase transitions in trans-PtCl2(PEt3)2. The stretching vibrational modes may further provide important information on the hydrogen bonding network in the structure in the low temperature phase.
c) Region 2800 – 3200 cm-1:
Figure 8. (a) Low temperature Raman spectra of trans-PtCl2(PEt3)2 in the 2850 – 3000 cm-1 spectral region. (b) Variation of Raman modes in the 2860 – 2990 cm-1 spectral region at low temperature.
The ligand stretching region 2850 – 3050 cm-1 is particularly important, as it provides information on weak non-covalent interactions like hydrogen bonds formed via C-H groups. At ambient conditions, trans-PtCl2(PEt3)2 forms two short H---Cl hydrogen bonds. One is 18
intramolecular hydrogen bond with H---Cl = 2.719 Å, C---Cl = 3.328 Å and ∠CHCl=114.37°and the other is intermolecular hydrogen bond with H---Cl = 2.657 Å, C---Cl = 3.684 Å and ∠CHCl = 155.66° 19. The softening behavior of CH3 symmetric and asymmetric stretching modes in the 180 – 130 K temperature range, observed in IR spectroscopy, is also verified from the Raman spectra (2875 cm-1 and 2962 cm-1 respectively) shown in Figure 8b. The CH2 symmetric and asymmetric Raman (2913 cm-1 and 2941 cm-1 respectively) stretching modes show comparatively less changes in the complete range, except for small discontinuous shifts across 130 K (Figure 8b). These observations imply preferred strengthening of the C-H---Cl hydrogen bonds through CH3 groups in trans-PtCl2(PEt3)2 in the low temperature phase between 180 – 130 K temperature range.
3.4. Comparison of low temperature infrared and Raman spectra: Thus, based on the spectroscopic observations in the complete frequency range, upon cooling trans-PtCl2(PEt3)2, most of the vibrational modes show change in slope around two temperatures of 180 K and 130 K. The changes around 180 K are associated with the orientational dynamics of organic ligands (phosphine triethyl), which initiate strengthening of CH---Cl hydrogen bonds through CH3 groups upto 130 K. Across 130 K, appearance of new modes as well as drastic increase in the intensity of some modes is observed in both infrared and Raman spectra. At further low temperatures, no new features or intensity/ peak position variations are noticed. In the skeletal modes, small changes are observed in the frequency variations of Pt-Cl deformation modes, whereas no significant effect is noticed in the stretching modes. Both the IR and Raman modes corresponding to Pt-Cl and Pt-P stretching vibrations show monotonous stiffening upon cooling with a slight change in the rate of variations across 19
130 K. But a shoulder developing on the lower frequency side of Pt-Cl stretching vibration in IR spectra transforms to a clear peak upon cooling below 180K, whereas, no such feature observed in Raman spectra around Pt-Cl stretching vibration. In the organic ligand deformation region, the significant information provided by IR spectra includes further modifications in geometrical conformation of organic ligands as indicated by enhanced complexity of spectral profiles related to skeletal modes of PEt3, including C-C stretching modes and CH2 rock. The Raman spectra distinctly show intensification of C-CH3 rocking vibration upon cooling. It is interesting to note that at high pressures also, this compound shows phase transitions as reported recently,19 where the pressure induced orientational changes in the ethyl groups bring them in close proximity of the chlorine atoms. This leads to the systematic strengthening of interand intra-molecular C-H---Cl hydrogen bonds and results in hydrogen bonded stitched stair case supramolecular assembly.19 During this phase transition at 0.8 GPa, the IR active modes were found to appear in the Raman spectra and vice versa, implying that the principle of mutual exclusion becomes invalid across the high pressure phase transition.19 However, the low temperature observations, reported here, are in contrast to these results. Although phase transition is indicated at low temperatures across 180 K and 130 K, but features pointing to loss of inversion symmetry have not been observed. For example, in the high pressures IR spectra, ν(P-C) Raman mode appears on the high frequency side of ν(P-C) IR mode at 0.8 GPa and vice versa in the Raman spectra,19 whereas at low temperatures the new mode appears on the low frequency side of ν(P-C) in the IR spectra (Figure 3) and no new mode is seen to emerge in the Raman spectra close to the position of ν(P-C) mode (Figure 6). A one to one correspondence of all the spectral changes confirms that the changes associated with phase transitions at low temperature and high pressure are dissimilar. Hence, the studies of metal20
organic square planar complexes under varying thermodynamic conditions are essential to understand molecular conformational flexibility which governs their technological and catalytic applications.
4. Summary Systematic low temperature studies of trans-PtCl2(PEt3)2 have been carried out using complementary FTIR and Raman spectroscopic techniques, which show significant variations in the vibrational modes associated with organic ligand moieties across 180 K and 130 K. An increase in the C-H---Cl hydrogen bonding strengths, associated with CH3 groups, is noticed in the intermediate temperature range of 180 – 130 K. Softening of the lattice modes and appearance of new Raman modes in the lattice region across 180 K and 130 K are suggestive of phase transitions in this compound. However, unlike in the reported high pressure phase, no signatures of loss of inversion symmetry across the phase transitions have been observed at low temperatures.
Conflicts of interest There are no conflicts to declare.
REFERENCES (1).Ruano, D.; Díaz‐García, M.; Alfayate, A.; Sánchez‐Sánchez, M., Nanocrystalline m–mof‐74 as heterogeneous catalysts in the oxidation of cyclohexene: Correlation of the activity and redox potential. ChemCatChem 2015, 7, 674‐681. (2).Allan, P. K.; Wheatley, P. S.; Aldous, D.; Mohideen, M. I.; Tang, C.; Hriljac, J. A.; Megson, I. L.; Chapman, K. W.; De Weireld, G.; Vaesen, S., Metal–organic frameworks for the storage and delivery of biologically active hydrogen sulfide. Dalton Transactions 2012, 41, 4060‐4066.
21
(3).Cattaneo, D.; Warrender, S. J.; Duncan, M. J.; Castledine, R.; Parkinson, N.; Haley, I.; Morris, R. E., Water based scale‐up of cpo‐27 synthesis for nitric oxide delivery. Dalton Transactions 2016, 45, 618‐ 629. (4).Chavan, S.; Bonino, F.; Valenzano, L.; Civalleri, B.; Lamberti, C.; Acerbi, N.; Cavka, J. H.; Leistner, M.; Bordiga, S., Fundamental aspects of h2s adsorption on cpo‐27‐ni. The Journal of Physical Chemistry C 2013, 117, 15615‐15622. (5).Chavan, S.; Vitillo, J. G.; Groppo, E.; Bonino, F.; Lamberti, C.; Dietzel, P. D.; Bordiga, S., Co adsorption on cpo‐27‐ni coordination polymer: Spectroscopic features and interaction energy. The Journal of Physical Chemistry C 2009, 113, 3292‐3299. (6).Dietzel, P. D.; Panella, B.; Hirscher, M.; Blom, R.; Fjellvåg, H., Hydrogen adsorption in a nickel based coordination polymer with open metal sites in the cylindrical cavities of the desolvated framework. Chemical Communications 2006, 959‐961. (7).Glover, T. G.; Peterson, G. W.; Schindler, B. J.; Britt, D.; Yaghi, O., Mof‐74 building unit has a direct impact on toxic gas adsorption. Chemical Engineering Science 2011, 66, 163‐170. (8).McKinlay, A.; Eubank, J.; Wuttke, S.; Xiao, B.; Wheatley, P.; Bazin, P.; Lavalley, J.‐C.; Daturi, M.; Vimont, A.; De Weireld, G., Nitric oxide adsorption and delivery in flexible mil‐88 (fe) metal–organic frameworks. Chemistry of Materials 2013, 25, 1592‐1599. (9).McKinlay, A. C.; Allan, P. K.; Renouf, C. L.; Duncan, M. J.; Wheatley, P. S.; Warrender, S. J.; Dawson, D.; Ashbrook, S. E.; Gil, B.; Marszalek, B., Multirate delivery of multiple therapeutic agents from metal‐ organic frameworks. APL Materials 2014, 2, 124108. (10).Hinks, N. J.; McKinlay, A. C.; Xiao, B.; Wheatley, P. S.; Morris, R. E., Metal organic frameworks as no delivery materials for biological applications. Microporous and Mesoporous Materials 2010, 129, 330‐ 334. (11).Miller, S. R.; Alvarez, E.; Fradcourt, L.; Devic, T.; Wuttke, S.; Wheatley, P. S.; Steunou, N.; Bonhomme, C.; Gervais, C.; Laurencin, D., A rare example of a porous ca‐mof for the controlled release of biologically active no. Chemical Communications 2013, 49, 7773‐7775. (12).Rojas, S.; Wheatley, P. S.; Quartapelle‐Procopio, E.; Gil, B.; Marszalek, B.; Morris, R. E.; Barea, E., Metal–organic frameworks as potential multi‐carriers of drugs. CrystEngComm 2013, 15, 9364‐9367. (13).Choudhury, R. R.; Poswal, H.; Chitra, R.; Sharma, S. M., Pressure induced structural phase transition in triglycine sulfate and triglycine selenate. The Journal of chemical physics 2007, 127, 154712. (14).Boldyreva, E. V.; Kolesnik, E. N.; Drebushchak, T. N.; Ahsbahs, H.; Beukes, J. A.; Weber, H.‐P., A comparative study of the anisotropy of lattice strain induced in the crystals of l‐serine by cooling down to 100 k or by increasing pressure up to 4.4 gpa. Zeitschrift für Kristallographie-Crystalline Materials 2005, 220, 58‐65. (15).Boldyreva, E. V.; Kolesnik, E. N.; Drebushchak, T. N.; Sowa, H.; Ahsbahs, H.; Seryotkin, Y. V., A comparative study of the anisotropy of lattice strain induced in the crystals of dl‐serine by cooling down to 100 k, or by increasing pressure up to 8.6 gpa. A comparison with l‐serine. Zeitschrift für Kristallographie-Crystalline Materials 2006, 221, 150‐161. (16).Mączka, M.; da Silva, T. A.; Paraguassu, W.; Ptak, M.; Hermanowicz, K., Raman and ir studies of pressure‐and temperature‐induced phase transitions in [(ch2) 3nh2][zn (hcoo) 3]. Inorganic chemistry 2014, 53, 12650‐12657. (17).Suffren, Y.; Rollet, F.‐G.; Reber, C., Raman spectroscopy of transition metal complexes: Molecular vibrational frequencies, phase transitions, isomers, and electronic structure. Comments on Inorganic Chemistry 2011, 32, 246‐276. (18).Genre, C.; Levasseur‐Thériault, G.; Reber, C., Emitting‐state properties of square‐planar dithiocarbamate complexes of palladium (ii) and platinum (ii) probed by pressure‐dependent luminescence spectroscopy. Canadian Journal of Chemistry 2009, 87, 1625‐1635. 22
(19).Bajaj, N.; Bhatt, H.; Pandey, K. K.; Poswal, H. K.; Arya, A.; Ghosh, P.; Deo, M., Phase transition in metal‐organic complex trans‐ptcl2 (pet3) 2 under pressure–insights of molecular and crystal structure. CrystEngComm 2018. (20).Bhatt, H.; Deo, M.; Vishwakarma, S.; Bajaj, N.; Sharma, S. M. In Ft-raman spectroscopy of structural isomers of pt (ii) complex ptcl2 (pet3) 2, AIP Conference Proceedings, AIP Publishing: 2015; p 090045. (21).Besara, T.; Jain, P.; Dalal, N. S.; Kuhns, P. L.; Reyes, A. P.; Kroto, H. W.; Cheetham, A. K., Mechanism of the order–disorder phase transition, and glassy behavior in the metal‐organic framework [(ch3) 2nh2] zn (hcoo) 3. Proceedings of the National Academy of Sciences 2011. (22).Messmer, G.‐G.; Amma, E., Crystal structures and multiple bonding in trans‐pt [p (c2h5) 3] 2cl2 and trans‐pt [p (c2h5) 3] 2br2. Inorganic Chemistry 1966, 5, 1775‐1781. (23).Duddell, D.; Goggin, P.; Goodfellow, R.; Norton, M.; Smith, J., Vibrational spectra of some gold (i), palladium (ii), and platinum (ii) complexes with trialkylphosphines and trialkylarsines. Journal of the Chemical Society A: Inorganic, Physical, Theoretical 1970, 545‐564. (24).Wood, J., Far‐infrared spectroscopy. Quarterly Reviews, Chemical Society 1963, 17, 362‐381. (25).Kaesz, H.; Stone, F., Infra‐red spectra of some vinyl and ethyl compounds of mercury, cadmium, zinc, tin and phosphorus. Spectrochimica Acta 1959, 15, 360‐377. (26).Goodfellow, R.; Evans, J.; Goggin, P.; Duddell, D., Infrared spectra (800–200 cm.–1) of trimethylphosphine and trimethylarsine complexes of platinum and palladium mx 2 l 2 and m 2 x 4 l 2 (x= cl, br, or i). Journal of the Chemical Society A: Inorganic, Physical, Theoretical 1968, 1604‐1609. (27).Allkins, J. R.; Obremski, R. J.; Brown, C. W.; Lippincott, E. R., Effect of high pressure on the vibrational spectra of square‐planar coordination compounds. Inorganic Chemistry 1969, 8, 1450‐1454. (28).Adams, D.; Chatt, J.; Shaw, B., 415. The infrared spectra of some alkylplatinum complexes, and the influence of various ligands on the pt–c bond strength. Journal of the Chemical Society (Resumed) 1960, 2047‐2053. (29).Adams, D.; Chatt, J.; Gerratt, J.; Westland, A., 145. Far‐infrared spectra of some square‐planar complexes [ptx 2 l 2](x= cl or br). The influence of l upon the platinum–halogen stretching frequency and its relation to the trans‐effect. Journal of the Chemical Society (Resumed) 1964, 734‐739. (30).Coates, G.; Parkin, C., 64. Tertiary phosphine complexes of trimethylgold: Infrared spectra of complexes of gold and some other metals. Journal of the Chemical Society (Resumed) 1963, 421‐429. (31).JENSEN, K. A.; NIELSEN, H.; PEDERSEN, C. T., Stereochemistry of 5‐co‐ordinated compounds. ACTA CHEMICA SCANDINAVICA 1963, 17, 1115‐1125. (32).Watari, F., Vibrational spectra of trimethylphosphine telluride and the phosphorus‐tellurium stretching force constant. Inorganic Chemistry 1981, 20, 1776‐1779. (33).Clark, H.; Tsang, W., Chemistry of metal hydrides. Ii. Spectroscopic studies of fluorovinylplatinum (ii) derivatives. Journal of the American Chemical Society 1967, 89, 533‐539. (34).Durig, J.; Nashed, Y.; Tao, J.; Guirgis, G., Conformational and structural studies of ethyl fluorosilane from temperature dependent ft‐ir spectra of krypton solutions and ab initio calculations. Journal of Molecular Structure 2001, 560, 39‐55. (35).Goggin, P.; Goodfellow, R., Identification of the stereochemistry of square planar bis‐ triethylphosphine complexes by infrared spectroscopy. Journal of the Chemical Society A: Inorganic, Physical, Theoretical 1966, 1462‐1466. (36).Chatt, J.; Leigh, G.; Mingos, D., Raman spectra of some tertiary phosphine complexes of rhodium, iridium, palladium, and platinum. Journal of the Chemical Society A: Inorganic, Physical, Theoretical 1969, 2972‐2975.
23
Spectroscopic Studies of Temperature Induced Phase Transitions in Metal-Organic Complex trans-PtCl2(PEt3)2 Naini Bajaj,1,2 Himal Bhatt,1,* H.K. Poswal,1 and M.N. Deo1,2 1
High Pressure & Synchrotron Radiation Division, Bhabha Atomic Research Centre, 2Homi Bhabha National Institute, Anushaktinagar, Mumbai *Email: [email protected]
Highlights 1. Systematic low temperature infrared and Raman spectroscopic studies on Pt(II)organic complex trans-PtCl2(PEt3)2 have been reported from ambient to 4.2 K, in a wide spectral range. 2. Drastic orientational changes occur in the different molecular units across 180 K and 130 K. 3. Strengthening of C-H---Cl hydrogen bonds have been observed in the 180-130 K temperature range. 4. Change in the intensity, discontinuous shift in the peak positions and appearance of new modes in the lattice region across 180 K and 130 K point towards structural phase transitions.
1