7858
J. Phys. Chem. C 2008, 112, 7858–7865
Complexes of Osmium with the 2-[(Diphenylphosphanyl)-methyl]-pyridine Ligand† Brenden Carlson,*,‡ Bruce E. Eichinger,‡ Werner Kaminsky,‡ and Gregory D. Phelan‡,§ Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195, and Department of Chemistry and Biochemistry, Seattle Pacific UniVersity, Seattle, Washington 98119 ReceiVed: December 6, 2007; ReVised Manuscript ReceiVed: February 6, 2008
We report new osmium complexes based upon the 2-[(diphenylphosphanyl)-methyl]-pyridine ligand. Hexafluorophosphate salts of bis(3,4,7,8-tetramethyl-1,10-phenantholine)(2-[(diphenylphosphanyl)-methyl]pyridine)osmium(II) (1), and bis(2-[(diphenylphosphanyl)-methyl]-pyridine)(4,7-diphenyl-1,10-phenantholine)osmium(II) (2) have been synthesized, and X-ray structures have been obtained. Indexing and unit cell refinement of both 1 and 2 indicated a monoclinic P lattice. The space groups were found to be P1j (No. 2) and P21/c, respectively. Both 1 and 2 were found to have the same emission at 645 nm despite the differences in structure. Through density functional theory (DFT) calculations we were able to conclude that the highest occupied molecular orbital (HOMO) of the complexes is the dxy orbital on the osmium, while the lowest unoccupied molecular orbital (LUMO) is the b1(Ψ)π* system of the phenanthroline. However, we found that dxy did not have the correct orientation to enable charge transfer to the phenanthroline, and the actual metalto-ligand charge transfer (MLCT) transition is dxz (HOMO-1) to the π* LUMO of the phenanthroline. Introduction Phosphorescence from metal complexes has been shown to be useful in a variety of applications. The long-lived triplet states that constitute phosphorescence allow for increased efficacy of sensors for the detection and/or measurement of a variety of analytes such as oxygen and nitro compounds, and in monitoring oxidation processes.1 Transition metal complexes have been used in a variety of organic electronics such as organic light emitting devices (OLEDs), electrochemical cells, and solar cells. Metal complexes utilized in organic electronics have resulted in increased efficiency and brightness while increasing device stability when incorporated into OLEDs.2 Because of usefulness of the properties of the transition metal complexes in these applications, there is continuing investigation into ligands and metal–ligand combinations to improve performance. The purpose of the research into ligands and metal–ligand combinations is to create, for example, OLEDs that are brighter, more efficient, and have greater longevity, or to make sensors that are more quantitative. The third row position of osmium gives the possibility of synthesizing dyes for use as sensors or as energy-harvesting moeities that are resistant to photodegradation, thermally stable, and give emission that is consistent over a wide temperature range. The desired properties are due to the large crystal field splitting (∆o) that occurs in complexes of osmium, which increases the energy separation of the dπ from the dσ*. In addition to increased stability, the many absorption bands found in osmium complexes may be advantageous for photovoltaic applications. We report osmium complexes of 2-[(diphenylphosphanyl)-methyl]-pyridine (PyP) (Figure 1). PyP was first reported by Uhlig et al.,3 but has been rarely used.4 The attraction of the PyP ligand is that its structure gives opportunity to create a wide range of variants. Alvarez et al.,5 and later Brunner et al.,6 reported the derivation of the methylene bridge † Part of the “Larry Dalton Festschrift”. * Corresponding author. E-mail:
[email protected]. ‡ University of Washington. § Seattle Pacific University.
Figure 1. Synthesis scheme to produce 2-[(diphenylphosphanyl)methyl]-pyridine ligand.
between the pyridyl and the phosphorus. The methylene connection between the phosphorus and the pyridine in PyP gives a synthetic pathway to place coordinating dendrimers for site isolation or energy-harvesting moieties on PyP to improve quantum efficiency. Here we report complexes of osmium with 3,4,7,8-tetramethyl-1,10-phenanthroline, 4,7-diphenyl-1,10-phenanthroline (1), and PyP (Figures 2and 3) that are being developed for sensor, OLED, and photovoltaic applications. In order to understand the PyP-osmium complexes, we used the density functional theory (DFT) code DMol3 to calculate the orbital structure of the complexes and compared the calculations to optical and electrochemical data. Results and Discussion Synthesis and X-ray Structures. Two procedures were attempted to synthesize complexes with PyP (Figures 2and 3). In the first procedure, (NH4)2OsCl6 was reacted with a phenanthroline (N-N) to form the intermediate (N-N)2OsCl2. PyP was then reacted with (N-N)2OsCl2 under Schlenk conditions to give complex 1 (Figure 4). In the second scheme, K2OsCl6 was reacted with PyP to form an intermediate (PyP)2OsCl2. (PyP)2OsCl2 was then reacted with either another equivalent of PyP or with an N-N type ligand. Products were successfully formed with N-N-type ligands as in complex 2 (Figure 5); however, a third PyP did not coordinate as a bidentate ligand.
10.1021/jp711522d CCC: $40.75 2008 American Chemical Society Published on Web 05/01/2008
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J. Phys. Chem. C, Vol. 112, No. 21, 2008 7859
Figure 2. Schematic synthesis sequence resulting in complex 1.
Figure 3. Schematic synthesis resulting in complex 2.
The X-ray structure gives insight as to why a third PyP ligand did not coordinate using the synthetic procedures employed. The X-ray structures of complexes 1 and 2 are illustrated in Figures 4 and 5, and the crystallographic data is summarized in Tables 2 and 3. The complexes are octahedral in structure. Complex 1 has five nitrogen atoms bonded to osmium (four from phenanthroline and one from pyridine) and one phosphorus atom bonded to osmium. The Os-Nphenanthroline bond lengths range from 204.0(9) pm to 209.3(9) pm. The longest of the bonds is the N trans to the P. The pyridyl N bond length was measured to be 211.1(10) pm. The Os-P bond length was measured to be 226.9(3) pm. Complex 2 has four nitrogen atoms bonded to osmium in the xy plane and two phosphorus atoms bonded along the z axis. The two phenanthroline Os-N lengths are 206.4(5) pm and 207.3(6) pm. The two pyridyl N-Os bond
lengths are 209.7(6) pm and 213.2(5) pm. The P-Os bond lengths are 232.93(17) and 233.56(18), respectively. The observed bond lengths to osmium are consistent with the ligand field strengths: pyridine < phenanthroline < phosphine. The X-ray structure of 2 (Figure 5) indicates why a third PyP ligand did not coordinate. The structure of 2 is an octahedral coordination sphere around osmium. The connectivity has both phosphorus atoms bonding to osmium on the z-axis and the four nitrogen atoms bonding in the xy plane. The connectivity of the (PyP)2OsCl2 intermediate is such that the pyridyl groups and chlorine atoms are cis to each other, and the phosphorus atoms are trans to each other. With the phosphorus trans to each other, the connecting phenyl groups hinder a third PyP from connecting as a bidentate ligand. The phenanthroline ligand, being planar, is able to fit between the four phenyl groups
7860 J. Phys. Chem. C, Vol. 112, No. 21, 2008
Carlson et al. TABLE 1: Refinement Data for the X-ray Structures Provided
Figure 4. Crystal structure of complex 1, [(3,4,7,8-tetramethyl-1,10phenanthroline)2 2-[(diphenylphosphanyl)-methyl]-pyridine osmium]2+, with 50% probability spheres. Solvent, hydrogens, and hexafluorophosphate were removed for clarity.
property
1
2
empirical formula formula weight temperature K wavelength Å crystal/color crystal system, space group unit cell dimensions a, Å b, Å c, Å R, deg β, deg γ, deg volume (Å3) density Mg/m3 reflections collected/ unique final R indices [I > 2sigma(I)] R1 wR2 R indices (all data) R1 wR2
C53H54Cl6F12N5OsP3 1484.82 130(2) 0.71073 prism/red
C62H52Cl4F12N4OsP4 1536.96 130(2) 0.71073 plate/red
triclinic, P1j
Monoclinic, P21/c
13.4040(8) 15.0950(11) 15.8460(11) 94.787(4) 101.839(4) 107.937(4) 2948.2(3) 1.673 14757/9938
15.9160(3) 16.2920(3) 23.8490(6) 90 94.4670(8) 90 6165.3(2) 1.656 26673/14525
0.0760 0.1331
0.0558 0.0994
0.1851 0.1646
0.1513 0.1189
TABLE 2: Bond Lengths (pm) to Osmium from X-ray Structure and DFT Calculations
Figure 5. Crystal structure of complex 2, [(2-[(diphenylphosphanyl)methyl]-pyridine)2 4,7-diphenyl-1,10-phenanthroline osmium]2+, with 50% probability spheres. Solvent, hydrogens, and hexafluorophosphate were removed for clarity.
extending from the phosphorus atoms and thereby coordinate to the osmium. It is plausible that higher reaction temperatures, or other synthetic schemes, could result in a geometry that allows a third PyP to coordinate. Spectral Properties. The absorption and emission properties of complexes 1 and 2 are summarized in Table 3 and illustrated in Figure 6. The complexes exhibit a variety of absorption bands due to various transitions. Ligand-centered (LC) π-π* transitions are observed at 4.532 eV for complex 1 and 4.505 eV for complex 2, which are typical of the π-π* transition of the phenanthroline. Another LC transition was observed at ∼4.1 eV for both complexes and is typical of the π-π* transition of the pyridine ring. The extinction coefficients of the LC transi-
bond length
1
2
1 calculated
2 calculated
Os-N1 Os-N2 Os-N3 Os-N4 Os-N5 Os-P1 Os-P2
207.3(8) 209.3(9) 206.4(10) 204.0(9) 211.1(10) 226.9(3) NA
209.7(6) 206.4(5) 207.3(6) 213.2(5) NA 232.93(17) 233.56(18)
211.8 214.1 209.5 210.4 214.7 232.3 NA
215.4 210.2 210.8 216.5 NA 238.1 239.3
tions are 87 000 L mol-1 cm-1 for complex 1 and 42 000 L mol-1 cm-1 for complex 2. The difference between complex 1 and 2 can be attributed to the longer conjugated system for complex 1, resulting in increased absorptive power. To the red of the LC transitions are additional transitions due to metal-toligand charge transfer (MLCT), where an electron is transferred from a d level on the metal to the π system on the ligand. The maximum of the 1MLCT transition is 3.085 eV for complex 1 and 3.171 eV for complex 2. The 1MLCT band of complex 2 is not as symmetric as the band observed for complex 1. Deconvolution of the 1MLCT band for complex 2 indicates two overlapping Gaussian peaks (r2 ) 0.99995, Supporting Information), with one centered at 3.177 eV ( ) 21000 L mol-1 cm-1) and the other at 2.884 eV ( ) 3000 L mol-1 cm-1), separated by 0.293 eV from the former. Unlike complex 2, a single Gaussian (r2 ) 0.9994, Supporting Information) at 3.109 eV fit the 1MLCT band for complex 1. Direct population of the triplet state from the singlet ground-state due to spin–orbit coupling of the osmium atom is observed. The 3MLCT transition is at 2.479 eV ( ) 5800 L mol-1 cm-1) for complex 1 and 2.437 eV (ε ) 2000 L mol-1 cm-1) for complex 2 by deconvolution of the spectra. Complexes 1 and 2 have nearly identical phosphorescence properties. The two feature similar emission maxima and full width at half-maximum (fwhm). The emission lifetime is 1740 ns for complex 1 and 1240 ns for complex 2. The emission quantum yield was measured to be 0.027 for complex 1 and 0.019 for complex 2.
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TABLE 3: Spectral Properties of Complexes 1 and 2
a
complex
LC (ε)
1 2
4.532 eV (87,000) 4.505 eV (42,000)
1
MLCT (ε)
3.109 eV (22,000) 3.177 eV (21,000)
MLCTa (ε)
emission
τ (ns)
Φ
2.479 eV (6,000) 2.437 eV (2,000)
1.887 eV 1.905 eV
1740 1240
0.027 0.019
3
in units of L mol-1 cm-1.
TABLE 4: Electrochemistrya E1/2(mv vs Ag/AgCl) compound
oxidation
first reduction
second reduction
1 2
986 1240
-1506 -1326
-1718 N/C
a
Figure 6. Absorbance (-1, -2) spectra of complexes 1 and 2 in deoxygenated acetonitrile at a temperature of 25 °C.
Figure 7. Cyclic voltammogram of an approximately 1 mM solution of complex 1 (blue line) and complex 2 (red line) in CH3CN. The scan was initiated in the positive direction.
Electrochemistry. The highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap can be estimated from cyclic voltammetry (Figure 7). The energy values were calculated using the ferrocene (FOC) value of 4.8 eV with respect to the vacuum level, which is defined as zero,7 with use of the following correlations:
EHOMO ) -(Eox - EFOC + 4.8) ELUMO ) -(Ered - EFOC + 4.8) The absorbance spectra for either complex do not shift when dissolved in methylene chloride, methanol, or acetonitrile (Supporting Information). In this aspect, the complexes behave as if they are in the gas phase. Thus, peak values and not the onset values are used to estimate the HOMO–LUMO gap. The oxidation peak for complex 1 is 986 mV, while the peak
Scan rate ) 50 mV/s.
for complex 2 is 1240 mV (Table 4). The first reduction peaks for complexes 1 and 2 are -1506 mV and -1326 mV, respectively. The reduction and oxidation peaks give a HOMO–LUMO separation of 2.492 eV for complex 1 and 2.566 eV for complex 2. The HOMO–LUMO energy gap for complex 2 is estimated to be slightly larger than that for complex 1 from the electrochemical potentials; however, these estimates are 0.617 eV (for complex 1) and 0.611 eV (for complex 2) lower in energy than what is observed in the absorption spectra. Background Information. While complexes 1 and 2 are different in structure, the two complexes are similar in absorption and emission properties. The use of 3,4,7,8-tetramethyl-1,10phenanthroline as a ligand for osmium has resulted in blueshifted absorption and emission energies and an increased emission lifetime when compared to other phenanthrolines.8 Thus, there is an expectation that complexes 1 and 2 would feature different emission properties. There are four possible transitions that would result in luminescence from complexes 1 and 2; the two π-π* LC transitions of the pyridyl and phenanthroline, and the MLCT transitions of dπOs-π*pyridyl, and dπOs-π*phenanthroline. It is reasonable to expect that complexes 1 and 2 share a common HOMO–LUMO transition because of the similarity of the emission properties. The most similar orbital system shared between the two complexes is the π system of the pyridine. Thus, the similarity in emission wavelength, quantum yield, and lifetime between the two complexes could be due to a dπOs-π*pyridyl transition. The π-π* transition of the pyridyl occurs at ∼4.1 eV, while the phenanthroline π-π* transition occurs at ∼4.5 eV. With the transitions occurring at lower energy for the pyridyl system, it is plausible that the π* of the pyridine is lower in energy than the π* of the phenanthroline, and the luminescence occurs as a result of charge transfer from the Os d to the pyridyl π-system. We used DFT calculations to provide an explanation to these conjectures. DFT Calculations. Calculations were performed on optimized geometries of complexes 1 and 2 to map the orbital structure of the complexes. The basis sets in DMol3 cover the heavy transition metals, as seen in previous work on platinum systems.9 Geometries were optimized using the PBE functional and high accuracy DNP basis set with DSPP pseudopotential relativistic corrections on Os. The computed geometries and bond lengths (Table 2) are in good agreement with the X-ray structures. Orbital pictures of complex 2 are shown in Figure 8 (complex 1 in Figure 8, Supporting Information), and MO diagrams for both complexes 1 and 2 in Figure 9. The calculated energies of the HOMO and LUMO levels are summarized in Table 5. The HOMO level of both complex 1 and 2 is primarily a dxy orbital at -9.658 eV for complex 1 and -9.850 eV for complex 2. The relative d-orbital energies between complexes 1 and 2 are in good agreement with the electrochemical
7862 J. Phys. Chem. C, Vol. 112, No. 21, 2008
Figure 8. HOMO-1, HOMO, LUMO, and LUMO+2 orbital representations calculated from DMol3 of complex 2. The HOMO-1 represents the dxz, and the HOMO respresents the dxy. The LOMO consists of the b1 π* 4,7-diphenyl-1,10-phenanthroline orbital, and the LUMO+2 consists of the b1 π* pyridine orbital. HOMO-1 overlaps with the LUMO to form a backbond, and the HOMO overlaps with the LUMO+2 to form a backbond. MLCT transitions take place between the HOMO-1 and LUMO levels, and between the HOMO and LUMO+2 levels.
Figure 9. Molecular orbital diagram of the charge transfer transitions of complexes 1 and 2 constructed from DMol3 computations and from optical data deconvoluted values.
TABLE 5: Energy of HOMO and LUMO Orbitals from DFT Calculations orbital LUMO+4 LUMO+3 LUMO+2 LUMO+1 LUMO HOMO HOMO-1 HOMO-2
complex 1 (eV) -6.169 -6.562 -6.571 -6.631 -6.698 -9.658 -9.913 -9.963
b1 pyridyl π* a2 phen π* b1 phen π* a2 phen π* b1 phen π* dxy dxz dyz
complex 2 (eV) -6.153 -6.176 -6.667 -6.763 -9.850 -9.967 -9.973
b1 pyridyl π* b1 pyridyl π* a2 phen π* b1phen π* dxy dxz dyz
experiments and the spectroscopic series of ligands. The phosphine moieties are well-known to be stronger field ligands and thus lead to the lowering energy of the occupied d orbitals. Complex 2, with two phosphine binding points to the osmium, would be predicted to have a lowering of the d orbital energy with respect to complex 1. With the lowering of the d orbital energy it would be more difficult to remove an electron from the d orbital, which is exactly the result from the electrochemical experiments. The LUMO was calculated to be at -6.698 eV
Carlson et al. for complex 1 and -6.763 eV for complex 2. Both the LUMO and LUMO+1 for complexes 1 and 2 have large participation of the π-system of the 1,10-phenanthroline, with the symmetry of the LUMO being b1(Ψ) and the LUMO+1 as a2(χ).10 The energy difference between the HOMO–LUMO levels for complex 1 and complex 2, respectively, is 2.960 and 3.087 eV. The LUMO+2 and LUMO+3 of complex 1 are also shown to be π* phenanthroline orbitals, and the π system of the pyridine is not observed until the LUMO+4 of complex 1 (Supporting Information). Complex 2 is different from complex 1 in that the LUMO+2 and LUMO+3 are the pyridyl π* (Figure 8). The pyridine levels are calculated to be 0.529 and 0.587 eV higher in energy than the phenanthroline LUMO level for complexes 1 and 2, respectively. We are able to assign the majority of optical absorption transitions from the DFT calculations. The DFT orbital representations (Figures 8 and 9, and Figure 7 in the Supporting Information) indicate that the dxy HOMO level of both complexes do not have the correct orientation to backbond with the b1 π-system of the phenanthroline. The phenanthroline ligands are coplanar with the dxy and the π*-system at the nitrogens and do not overlap well with the HOMO orbital. The dxz (HOMO-1) has the correct orientation to interact with the phenanthroline b1 π-system (LUMO). Thus, the 1MLCT absorption occurring between 2.75 and 3.75 eV is due the HOMO-1 to LUMO (dxzOs-π*phenanthroline) transition and not due to a HOMO–LUMO transition. The result has been referred to as a discrepancy between the “redox π* orbital” and the “optical π* MO” to which the most intense (and only detectable) lowenergy MLCT transition occurs and thus provides an explanation as to the lower energy gap predicted in the electrochemical experiments.11 It is the pyridyl π*-system that has the proper orientation for backbonding and to enable the MLCT transition to take place with the HOMO level. The plane of the pyridyl ring bonds to osmium perpendicular to the xy plane allowing for good overlap of the dxy with the pyridyl π*. While the dxyOs-π*pyridyl transitions overlapped significantly with the π-π* transitions, the HOMO to LUMO+4 of complex 1 and HOMO to LUMO+2 of complex 2 transitions were visible in the absorption spectra between 3.5 and 4.0 eV. Through deconvolution the dxyOs-π*pyridyl transitions are found to occur at 3.942 eV for complex 1 and 3.902 eV for complex 2. The 1MLCT transition in the optical spectrum of complex 2 is structured with a shoulder appearing at an energy that is 0.293 eV lower than the main transition. The shoulder does not appear in the spectrum of complex 1. Structured absorption is often observed in bathophenanthroline complexes12 (examples are given in the Supporting Information) and is absent when other phenanthroline derivatives are used. Watts and Crosby have previously characterized the absorption spectra of phenyl substituted bipyridine and phenanthroline complexes.13 In our case, the two Gaussian peaks, separated by 0.293 eV, are distant to the 0.587 eV DFT-calculated energy difference between the LUMO π* of the phenanthroline and the π* of the pyridine. With the exception of the HOMO–LUMO transition (which does not have the correct orientation), which is 0.117 eV lower in energy than the HOMO-1 to LUMO, none of the other possible transitions can be justified energetically, as all of them would be higher, not lower, in energy. Furthermore, given orientation constraints on the orbitals, it does not seem plausible that the shoulder occurring on the 1MLCT band for complex 2 is due to two different dπOs-π* transitions. We investigated other potential reasons for the shoulder’s occurrence. One phenyl group was rotated to an in-plane
Osmium-Pyp Complexes orientation with the phenanthroline, as suggested by the work of Damrauer et al.14 on the excited-state chemistry of phenylsubstituted phenanthroline. The optimized structure was found to be 0.2996 eV higher in energy than the ground-state molecule, which is nearly identical to the 0.293 eV difference between the two 1MLCT peaks. However, Boltzmann distribution implies that it is very sparsely populated (1.013 × 10-5) at 300 K and should not affect the absorption spectrum. We also modeled four different out-of-plane low energy rotamers of the phenyl groups and found the energy of the HOMO-1 to LUMO gap to vary by less than 0.1 eV, which is too little to account for the shoulder. On the basis of the DFT calculations, we are unable to determine the transition that results in the shoulder on the 1MLCT absorption of complex 2. More experimentation is needed to determine whether the shoulder peak occurrence is due to a rotamer structure, enhancement of the HOMO–LUMO transition due to the presence of the phenyl moieties, or an electronic phenomenon imparted by the phenyl groups such as vibronic coupling. Vibronic coupling of phenyl moieties has been observed in the absorption spectra for several organic compounds.15 The DFT calculations indicate that emission from both complexes 1 and 2 results from relaxation of charge from the phenanthroline π* back to the d-orbital on the metal. Emission occurs from lower energy states that are largely triplet in character, which behave kinetically as a single state at room temperature.16 The DFT model indicates that the pyridine levels are populated at ∼10-10 and are negligible for both complexes, and as such emission occurs with little contribution from the pyridine π*. While 1MLCT absorption is a HOMO-1 to LUMO transition, it is unclear whether the emission transition is the result of the same transition or whether internal conversion takes place, resulting in emission from the LUMO to HOMO level. Presumably, the 3MLCT absorption is also HOMO-1 to LUMO, but with simultaneous change in spin states. There is a significant Stokes shift between the 3MLCT absorption and emission of 0.521 and 0.512 eV for complexes 1 and 2, respectively. The calculated energy difference between the dxz and dxy levels is 0.255 eV for complex 1 and 0.117 eV for complex 2. As such, internal conversion between the HOMO and HOMO-1 levels provide a partial explanation of the energy separation between triplet absorption and triplet emission. While the emission properties from the complexes are similar and originate from transitions involving the d and the phenanthroline π*, the energy of the states leading to the emission properties are different between the complexes. Conclusion We report the synthesis and optical properties of osmium complexes with the 2-[(diphenylphosphanyl)-methyl]-pyridine ligand. The photophysical and electrochemical properties of the complexes were measured, and the X-ray structures were determined. It is found that the emission properties of the complexes are very similar, even though the structure for the two complexes is different. Contrary to the hypothesis that the similar emission properties are due to a dπOs-π*pyridyl transition shared between the two complexes, it was found through DFT calculations that the intense MLCT transitions are dπOs-π*phenanthroline in character. The charge transfer transitions that take place to the π* of the pyridyl are of higher energy and are observable in the absorption spectra between the π-π* and dπOs-π*phenanthroline transitions. Emission from the complexes comes from a dπOs-π*phenanthroline transition and not from a dπOs-π*pyridyl transition.
J. Phys. Chem. C, Vol. 112, No. 21, 2008 7863 Experimental Proceedures for synthesizing 2-[(diphenylphosphanyl)-methyl]-pyridine and the two complexes may be viewed in the Supporting Information. Characterization of complex 1 as the hexafluorophosphate salt: Elemental analysis: Calculated: C, 48.82; H, 3.93; N, 5.69 for C50H48F12N5OsP3. Found: C, 48.45; H, 3.79; N, 5.44. Mass Spectrometry (MALDI, high resolution): 1080.2874 (2.1%), 1081.2900 (4.9%), 1082.2860 (29.3%), 1083.2870 (50.9%), 1084.2865 (83.3%), 1085.2938 (34.4%), 1086.2897 (100.0%, C50H48F6N5OsP2), 1087.2963 (48.9%), 1088.3062 (10.4%), 1089.3121 (1.5%). Yield: 42%. Characterization of complex 2 as the hexafluorophosphate salt: Elemental analysis: Calculated: C, 52.71; H, 3.54; N, 4.10 for C60H48F12N4OsP4. Found: C, 52.31; H, 3.39; N, 4.18. Mass Spectrometry (MALDI, high resolution): 1072.119 (1.52%), 1073.143 (16.26%), 1074.154 (42.38%), 1075.162 (81.90%), 1076.175 (54.24), 1077.182 (100.0%, C60H48N4OsP2). 1078.191 (72.43%), 1079.2 (21.44%), 1080.217 (2.76%). Yield: 51%. X-Ray Diffraction. Crystals of complexes 1 and 2 were grown by the slow diffusion of hexane into a solution of the complexes in methylene chloride. The crystals were mounted in a random orientation on a glass fiber on a KAPPA CCD diffractometer using Mo KR (λ)0.71073 Å) radiation. Measurements were performed at 130(2) K for complexes 1 and 2. For complex 1, the crystal-to-detector distance was 30 mm, and exposure time was 20 s/deg for all sets. The scan width was 2°. Data collection was 88.6% complete to 25.68° and 90.4% complete to 25° in ϑ. A total of 35 911 partial and complete reflections were collected covering the indices h ) -16 to 16, k ) -15 to 18, and l ) -18 to 19. A total of 9938 reflections were symmetry independent, and the Rint ) 0.1214 indicated that the data was of slightly less than average quality (0.07). Indexing and unit cell refinement indicated a monoclinic P lattice. The space group was found to be P1j (No. 2). Three solvent molecules, CH2Cl2, were found in the unit cell of complex 1. Thermal parameters for carbon atoms were constrained to a small degree to prevent nonpositive definite matrices. For complex 2, the crystal-to-detector distance was again 30 mm, and exposure time was 20 s/deg for all sets. The scan width was 1.2°. Data collection was 94.6% complete to 28.31° and 98.9% complete to 25° in ϑ. A total of 94 730 partial and complete reflections were collected covering the indices h ) -20 to 21, k ) -18 to 21, and l ) -30 to 30. A total of 14 525 reflections were symmetry independent and the Rint ) 0.0955 indicated that the data was of slightly less than average quality (0.07). Indexing and unit cell refinement indicated a monoclinic P lattice. The space group was found to be P21/c. Two disordered phenyl groups and interleaved CH2Cl2 solvent molecules characterize the structure of complex 2. In fact, the unit cell is best seen as two molecules of complex together with solvent molecules distributed statistically throughout, leading to the observed disorder. The structures were solved by direct methods using SIR97 and DIRDIFF, provided by the refinement package MaXus.17 Missing atoms were found by difference-Fourier synthesis. The non-hydrogen atoms were refined with anisotropic temperature factors. Scattering factors are from Waasmaier and Kirfel.18 The structures were refined with SHELXL-97, and ORTEP plots were generated with ORTEP32.19 Table 1 summarizes the crystal data, collection information, and refinement data for these structures.
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Absorbance and Emission Measurements. Absorbance was measured on a Shimadzu UV-1601 spectrophotometer. Emission of deoxygenated solutions of the complexes was measured at 22 °C on a Perkin-Elmer LS50B fluorescence spectrophotometer. The wavelength sensitivity of the instrument was calibrated prior to measurements using a standard 20 W tungsten lamp of known output. Photoluminescence (PL) quantum yields accurate to (10% were measured on the Os complexes (ΦOs) in deoxygenated (by argon purge) acetonitrile solutions. Standard Ru(II) tris(2,2′-bipyridine) dihexafluorophosphate having the known quantum yield of 0.042 was used for calibration, and the yield of the complexes was determined from20
ΦOs )
absRu areaOs × × 0.042 areaRu absOs
(1)
Samples were excited through the LC state at 280 nm with an absorption of 0.070 for the standard and samples, and were excited again at an absorption of 0.050 for standard and samples. The temperature for the measurements was 22 ( 2 °C. Emission lifetimes were measured using a Photon Technologies, Inc. pulse nitrogen dye laser system. The complexes were excited at 375 nm by the lasing of a 4 × 10-3 M solution of 2-[1,1′-biphenyl]-4-yl-5-phenyl-1,3,4-oxadiazole in a mixture of seven parts toluene to three parts ethanol. Acetonitrile solutions of the complexes were deoxygenated by argon purge prior to use. The temperature for the measurements was 25 °C. The solutions of the complex were prepared to have an absorbance of 0.1 at 375 nm after purging. Electrochemical. Electrochemical experiments were performed at room temperature in 0.1 M solutions of tetrabutylammonium hexafluorophosphate (Fluka) in acetonitrile (Aldrich, HPLC grade) using a Princeton Applied Research model 362 scanning potentiostat equipped with a glassy carbon working electrode and a Ag/AgCl reference (E1/2value of the ferrocene/ ferrocenium couple in acetonitrile was 435 mV with a peak separation of 95 mV at 250 mV/s). Electrolyte solutions were prepared from freshly distilled and dried acetonitrile prior to use. Working solutions were purged with nitrogen presaturated with dried solvent. Computational Methods. We performed DFT calculations with the program package DMol3 in Materials Studio (version 2.2) of Accelrys, Inc. on personal computers. This code uses numerical basis sets, which have been tuned with atomic calculations. Double-numeric quality basis set with polarization functions (DNP) was used to map the orbital structure of complexes 1 and 2 using the PBE functional.21 Two different relativistic methods were applied, giving closely similar results. Pseudopotentials22 on Os were used for geometry optimization, and subsequently the orbital energies and contours were calculated with an all-electron relativistic method.23 The medium convergence criteria for geometry optimization were satisfied for both molecules, with energies, gradients, or displacements (two out of three) converged to 2.0 × 10-5 Ha, 0.004 Ha/Å, or 0.005 Å, respectively. Acknowledgment. B.C. wishes to thank the National Science Foundation Science and Technology Center for use of facilities, Ross Lawerence for measurement of mass spectrometry, and Arumagasamy Elangovan for his enlightening discussions. Supporting Information Available: Experimental procedures for the synthesis of PyP and complexes 1 and 2, and CIF files of the data obtained in this study. This material is available free of charge via the Internet at http://pubs.acs.org.
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