Syntheses and coordination studies of 2-(diphenylphosphinomethyl)pyridine N,P dioxide with Co2+, Ni2+, Cu2+ and Zn2+ tetrafluoroborate

Inorganica Chimica Acta 405 (2013) 511–521

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Inorganica Chimica Acta

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Syntheses and coordination studies of2-(diphenylphosphinomethyl)pyridine N,P dioxidewith Co2+, Ni2+, Cu2+ and Zn2+ tetrafluoroborate

0020-1693/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.ica.2013.03.037

⇑ Corresponding author. Tel.: +1 254 710 2665; fax: +1 254 710 4272.E-mail address: [email protected] (K.K. Klausmeyer).

Daniel A. Padron, Kevin K. Klausmeyer ⇑Department of Chemistry and Biochemistry, Baylor University, One Bear Place #97348, Waco, TX, USA

a r t i c l e i n f o

Article history:Received 20 December 2012Received in revised form 13 March 2013Accepted 15 March 2013Available online 29 March 2013

Keywords:N–O/P@O ligandsChelating agentsFirst row transition metalsCoordination chemistrySingle crystal X-ray diffraction

a b s t r a c t

Several new coordination complexes of Co2+, Ni2+, Cu2+ and Zn2+ were synthesized by the reaction of theircorresponding tetrafluoroborate salts with 2-(diphenylphosphinomethyl)pyridine N,P dioxide, L. Allstructures were determined by single-crystal X-ray crystallography showing an octahedral environmentaround the metal centers. Single crystals of M(BF4)2L2(S)2 (M = Co2+, Ni2+ or Zn2+; S = MeOH or CH3CN)were obtained at �5 �C by slow diffusion of ether into solutions of these complexes. When these crystalswere re-dissolved in methanol and allowed to grow again at room temperature using the same techniqueand solvent system, the product obtained presented a 3:1 ratio ligand-to-metal. The complex [Cu(BF4)2-

L2(MeOH)](BF4), exhibits two units of L, one molecule of MeOH and one counter-anion coordinated to theCu2+ center. This can be attributed to the steric restrictions generated by the lower symmetry in this com-plex, a product of significant Jahn–Teller effects, which favors a long range interaction between the metalcenter and a less coordinative counter-anion, compared to a MeOH unit. A 1:1 ligand-to-metal ratio com-plex of Ni2+ was also obtained by allowing the starting materials to react for only a short time. Attemptsto obtain single crystal of complexes of Co2+, Cu2+ or Zn2+ in a 1:1 ratio were not successful. The resulting3:1 ligand-to-metal products were fully characterized by elemental analysis, UV–Vis spectroscopy andFT-IR spectroscopy.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Chelating ligands and their coordination behavior have beenstudied since the inception of coordination chemistry. They havereceived great attention due to their ability to coordinate to transi-tion metals, an important characteristic in many biological pro-cesses and industrial applications. They have been used in metalion extraction [1–5], synthesis of new luminescent and polymericmaterials and homogeneous catalysis, treatment of human dis-eases related to metal overload (e.g. Hemochromatosis [6–8], sat-urnism [9,10], aluminum overload [11,12]), among others [13–20].

It is known that anionic or neutral ligands with strong oxygendonor centers form stable coordination complexes with hard acids.For example, chelating phosphine oxides have been used in thesynthesis of new luminescent materials due to their capability ofstabilizing metal centers with higher oxidation state [21]. Also,they possess an enhanced capacity to improve both photolumines-cent and electroluminescent properties of these complexesthrough the reduction of solvent-induced quenching and improve-ment of intramolecular electron transfer [22]. These ligands have

also been shown to be good extractants for lanthanide and actinideions under specific conditions [23,24]. Recently, Rosario-Amorindescribed the synthesis of monofunctional P@O-containing ligandsand their coordination properties towards several f-block metalcations. In this study they showed that these ligands could be usedfor liquid–liquid solvent extraction of hard acceptor ions [25].

The design of compounds based on N-oxides has been of inter-est due to their dual application in both metal-free catalytic trans-formations and as ligands in metal-based catalysis [26]. Liangsynthesized different monofunctional ligands containing N–Ogroups and investigated their role in the Cu-catalyzed N-arylationof imidazoles using water as a solvent. They found that ligandsbased on pyridine N-oxides were more beneficial to the catalyticprocess than their pyridine analogs. The higher catalytic activitiesobtained were partly attributed to their excellent solubility inwater, and out of three different N–O-containing ligands em-ployed, the bidentate with the greatest flexibility gave the highestyield [27]. In 2009, Zhang developed a novel chiral indium com-plex, based on (S)-pipecolic acid-derived N,N0-dioxide ligands, forthe enantioselective allylation of ketones. Their results showedthat the novel chiral indium complex could be an efficient catalystfor this reaction yielding good enantioselectivity. In a more recentwork, Liu synthesized a new family of C2-symmetric N,N0-dioxide

PN

O

O

Scheme 1. Structure of 2-(diphenylphosphinomethyl)pyridine N,P dioxide (1).

Table 1Crystallographic data for compounds 1–4.

1 2

Empirical formula C54H48B2CoF8N3O6P3 C54H48B2

Formula mass 1160.41 1192.24a (Å) 13.9051(9) 10.5814(b (Å) 23.1098(16) 15.021(2c (Å) 17.9489(11) 17.249(3a (�) 90 90b (�) 95.906(3) 91.786(6c (�) 90 90V (Å3) 5737.2(6) 2740.3(7Z 4 2Crystal System monoclinic monoclinSpace group P2(1)/c P2(1)T (K) 110(2) 110(2)Dcalc (g/cm3) 1.343 1.445l (mm�1) 0.458 0.5242hmaz (�) 28.31 26.75Reflections measured 103376 27498Reflections used 14240 10845Data/restraints/parameters 14240/0/732 10845/1/R1 [I > 2r(I)] 0.0444 0.0582wR2 [I > 2r(I)] 0.1029 0.1392R (F2

o) (all data) 0.0611 0.0701Rw (F2

o) (all data) 0.1104 0.1460Goodness-of-fit (GOF) on F2 1.078 1.043

Table 2Crystallographic data for compounds 5–9.

5 6

Empirical formula C44H44B2CoF8N6O4P2 C39H44BF4N2NiO7P2

Formula mass 1015.34 860.22a (Å) 17.7024(19) 13.175(5)b (Å) 18.9467(17) 13.669(5)c (Å) 14.0724(15) 14.543(4)a (�) 90 67.989(7)b (�) 90 69.059(11)c (�) 90 77.423(7)V (Å3) 4719.9(8) 2257.1(13)Z 4 2Crystal System orthorhombic triclinicSpace group Pbcn P�1T (K) 110(2) 110(2)Dcalc (g/cm3) 1.429 1.266l (mm�1) 0.512 0.5622hmaz (�) 26.44 25.74Reflections measured 37923 41663Reflections used 4857 8273Data/restraints/parameters 4857/0/305 8273/0/514R1 [I > 2r(I)] 0.0515 0.0756wR2 [I > 2r(I)] 0.1218 0.1977R (F2

o) (all data) 0.0643 0.0916Rw (F2

o) (all data) 0.1316 0.2059Goodness-of-fit (GOF) on F2 1.033 1.041

512 D.A. Padron, K.K. Klausmeyer / Inorganica Chimica Acta 405 (2013) 511–521

amide ligands and demonstrated that these compounds could par-ticipate in a wide range of chiral ligand–metal-catalyzed andorganocatalyzed asymmetric reactions under mild reaction condi-tions obtaining excellent enantioselectivity and activity [28].

Siddall suggested that bifunctional carbamoylmethylphospho-nate ligands with the ability to adopt bidentate chelate structuresin solution could be used as improved chelating agents for therecovery of lanthanide and actinide ions, when compared to themonofunctional organophosphoryl and organoamide ligands [29].Several studies on the coordination chemistry of bifunctional li-gands that contain both P@O and N–O donors have demonstratedthat these chelating reagents produce stable complexes with 3d

3 4

F8N3NiO6P3 C54H48B2CuF8N3O6P3 C54H48B2F8N3O6P3Zn1202.38 1198.90

19) 14.1007(10) 10.5390(5)) 22.6498(17) 15.0341(7)) 17.5725(12) 17.2991(8)

90 90) 98.585(4) 92.116(2)

90 90) 5549.4(7) 2739.1(2)

4 2ic monoclinic monoclinic

P2(1)/c P2(1)110(2) 110(2)1.439 1.4540.562 0.61828.35 27.7250561 4111613836 12760

714 13836/4/738 12760/1/7140.0539 0.04380.1163 0.10410.1020 0.05350.1419 0.11081.008 1.023

7 8 9

C37H36B2CuF8N2O5P2 C39H44BF4N2O7P2Zn C28H31B2F8N6NiO2P887.78 866.88 746.8910.844(3) 13.245(9) 11.0974(5)12.839(4) 13.670(10) 25.2713(11)15.666(5) 14.573(10) 12.0411(6)90.874(4) 67.655(13) 90107.544(4) 68.811(11) 97.489(2)113.885(4) 77.101(14) 901877.9(10) 2264(3) 3348.1(3)2 2 4triclinic triclinic monoclinicP�1 P�1 P21/n110(2) 110(2) 110(2)1.570 1.272 1.4820.753 0.674 0.70828.42 25.00 29.5718227 17372 371978720 7792 92988720/0/519 7792/1/517 9298/0/4380.0399 0.0805 0.03840.0938 0.2193 0.08810.0515 0.0974 0.05430.1023 0.2286 0.09661.034 1.048 1.023

Fig. 1. Thermal ellipsoid of 1 with an atomic numbering scheme. Ellipsoids areshown at the 50% level. Hydrogen atoms have been removed for clarity.

Fig. 2. Thermal ellipsoid of 2 with an atomic numbering scheme. Ellipsoids ar

D.A. Padron, K.K. Klausmeyer / Inorganica Chimica Acta 405 (2013) 511–521 513

metal ions as well as with f-elements [30–33]. Rapko andco-workers have studied the coordination ability of several mixedligands containing P@O and N–O towards different lanthanide ions.Their work revealed that these bifunctional ligands produce stablecomplexes with (UO2)2+ and Ln3+ and that modest effectivenesscan be achieved in liquid–liquid extractions of Eu3+, Ce3+, Yb3+

and Am3+ under certain conditions [34–37]. It has been observedthat complexes displaying seven-membered rings, product of thecoordination of metal ions to flexible chelating ligands containingN–O and P@O groups, have superior stability compared to the fiveand six-membered chelating ring analogs [38,39].

Our group has put considerable effort to study the coordinationbehavior of different N,P chelating ligands towards several transi-tion metal ions. This work has shown the versatility of theseligands to stabilize metal ions in different oxidation states [40–45]. Furthermore, we have studied several coordination complexesbased on bifunctional ligands of phosphine oxides containing oneor two pyridyl groups [46–48]. These studies showed that the hardoxygen donor in these ligands would coordinate to borderline acidslike Cu2+ and even to softer acids like Hg2+ yielding stable struc-tures, probably due to large entropic contributions [49,50]. In orderto expand our understanding in the coordination behavior of thesemixed ligands, we have turned our attention to their oxidizedcounterparts, which contain the harder N–O and P@O functions.In a recent work, we studied the coordination properties of the tri-dentate ligand phenylphosphino-bis-2-methylpyridine N,N0,P tri-oxide towards Cu2+ [47], and more recently, the coordinationbehavior of the bifunctional ligand 2-(diphenylphosphino-methyl)pyridine N,P dioxide, (Scheme 1), towards the soft Ag+

e shown at the 50% level. Hydrogen atoms have been removed for clarity.

Fig. 3. Thermal ellipsoid of 3 with an atomic numbering scheme. Ellipsoids areshown at the 50% level. Hydrogen atoms have been removed for clarity.

514 D.A. Padron, K.K. Klausmeyer / Inorganica Chimica Acta 405 (2013) 511–521

ion [48]. This last study showed the expanded coordination ability ofthis ligand compared to its non-oxidized counterpart, and how theresulting molecular structures have a strong dependence on thecounter-anion interacting with the silver centers. In the presentstudy we focus our attention on the coordination mode of this ver-satile ligand towards the borderline acids Co2+, Ni2+, Cu2+ and Zn2+.

2. Experimental

2.1. General remarks

All experiments were performed under nitrogen atmosphereusing Schlenk techniques. All air and light sensitive compoundswere stored and handled in an inert atmosphere glovebox and usedas received. All solvents were reagent-grade and distilled under in-ert atmosphere from the appropriate drying agent right before use.Infrared spectra were recorded on a Thermo Nicolet Nexus 870 (FT-IR E.S.P. System) spectrometer. 1H and 31P NMR spectra were re-corded at 499.78 and 202.31 MHz respectively, at 25.0 �C/298.1 K MHz, using a Varian VNMRS 500 MHz Spectrometer. UV–Vis absorption data were acquired on a Varian Cary Bio spectro-photometer. Solutions were prepared in CH3CN and charged intoquartz cuvettes. Elemental analyses were performed by AtlanticMicrolabs Inc. in Norcross, Georgia. Yields for compounds 1–4 werecalculated with respect to M(BF4)2�6H2O.

Fig. 4. Thermal ellipsoid of 4 with an atomic numbering scheme. Ellipsoids areshown at the 50% level. Hydrogen atoms have been removed for clarity.

2.2. Synthesis

Synthesis of M(BF4)2L3: A solution of L (0.0927 g, 0.300 mmol) inMeOH (5 mL) was added to a solution of M(BF4)2�6H2O, (M = Co2+

for 1, Ni2+ for 2, Cu2+ for 3 and Zn2+ for 4), (0.100 mmol), in MeOH(5 mL). The resulting solution was allowed to stir for 8 h and thendried under vacuum to leave a powder. This was then dissolved ina small amount of MeOH and precipitated with ether (30 mL).

Crystals were obtained by slow diffusion of ether into a MeOHsolution of the corresponding product at room temperature. (1)Yield 79% (0.092 g). Anal. Calc. for C54H48B2CoF8N3O6P3�H2O(1178.46): C, 55.03; H, 4.27; N, 3.56. Found: C, 54.82; H, 4.28; N,3.58%. IR (KBr, cm�1): 1211 (mNO), 1061 (mBF, br). (2) Yield 91%(0.106 g). Anal. Calc. for C54H48B2F8N3NiO6P3�H2O (1178.22): C,55.04; H, 4.27; N, 3.56. Found: C, 54.68; H, 4.24; N, 3.55%. IR(KBr, cm�1): 1213 (mNO), 1062 (mBF, br). (3) Yield 76% (0.089 g). Anal.Calc. for C54H48B2CuF8N3O6P3�H2O (1183.08): C, 54.82; H, 4.26; N,3.55. Found: C, 54.49; H, 3.95; N, 3.42%. IR (KBr, cm�1): 1202(mNO), 1067 (mBF, br). (4) Yield 87% (0.101 g). Anal. Calc. for C54H48-

B2F8N3O6P3Zn (1166.91): C, 55.58; H, 4.15; N, 3.60. Found: C,55.43; H, 4.12; N, 3.53%. 1H NMR (CD3CN, 298.1 K): d = 4.50 (d,2H), 7.05 (d, 12H), 7.44 (m, 1H), 8.09 (d, 1H). 31P NMR (CD3CN,298.1 K): d = 41.43, (s) ppm. IR (KBr, cm�1): 1212 (mNO), 1060(mBF, br).

Synthesis of M(BF4)2S2L2: A solution of 2 equiv. of L in 5 mL ofMeOH was added to a solution of 1 equiv. of M(BF4)2�6H2O(M = Co2+ for 1, Ni2+ for 2, Cu2+ for 3 and Zn2+ for 4) in 5 mL ofMeOH. The resulting solution was allowed to stir for 30 min andthen dried under vacuum to leave a powder. This was then dis-solved in a small amount of MeOH and precipitated with ether(30 mL). Crystals were obtained by slow diffusion of ether into asolution of the corresponding product using MeOH for 5 (Co2+)and CH3CN for 6, 7 and 8 (Ni2+, Cu2+ and Zn2+, respectively) at�5 �C.

Synthesis of Ni(BF4)2(CH3CN)4L: An equimolar amount of L andNi(BF4)2�6H2O were reacted in 10 mL of MeOH for 5 min and thendried under vacuum to leave a light green powder. This was thendissolved in a small amount of MeOH and precipitated with30 mL of ether. Crystals were obtained by slow diffusion of etherinto a solution of CH3CN at �5 �C.

2.3. X-ray crystallography

Crystallographic data was collected on crystals with dimensions0.304 � 0.112 � 0.137 mm for 1, 0.158 � 0.150 � 0.143 mm for 2,

Fig. 5. Thermal ellipsoid of 5 with an atomic numbering scheme. Ellipsoids are shown at the 50% level. Hydrogen atoms have been removed for clarity.

Fig. 6. Thermal ellipsoid of 6 with an atomic numbering scheme. Ellipsoids areshown at the 50% level. Hydrogen atoms have been removed for clarity.

D.A. Padron, K.K. Klausmeyer / Inorganica Chimica Acta 405 (2013) 511–521 515

0.131 � 0.150 � 0.084 mm for 3, 0.241 � 0.273 � 0.285 mm for 4,0.164 � 0.186 � 0.126 mm for 5, 0.199 � 0.180 � 0.170 mm for 6,0.206 � 0.206 � 0.108 mm for 7, 0.322 � 0.175 � 0.150 mm for 8and 0.176 � 0.108 � 0.355 for 9. Data was collected at 110 K ona Bruker X8 Apex using Mo K radiation (k = 0.71073 Å). All struc-tures were solved by direct methods after correction of the datausing SADABS [49,50]. Details of the crystal parameters, data collec-tion, and refinement are summarized in Tables 1 and 2. The molec-ular structures of the compounds are displayed in Figs. 1–9.Summary of selected bond lengths, angles, and interatomic dis-tances are given in Tables 3 and 4. All the data were processedusing the BRUKER AXS SHELXTL software, version 6.10 [51]. Unless other-wise noted, all non-hydrogen atoms were refined anisotropicallyand hydrogen atoms were placed in calculated positions. Two ofthe phenyl rings in compound 1 were disordered over two posi-tions these were refined to have identical displacement parametersand similar bond lengths and angles. Compound 3 contains a disor-dered MeOH and an H2O molecule that is only partially occupied(30%). Compounds 1, 6 and 8 each contained highly disordered sol-vent (1) or solvent and anion (6,8), we were unable to satisfactorilymodel the disordered molecules therefore the SQUEEZE procedureimplemented in PLATON [52] was used to remove the electron den-sity due to these molecules. Volumes of 118 Å3/unit cell for 1,168 Å3/unit cell for 6 and 164 Å3/unit cell volume for 8 wereexcluded.

3. Results and discussion

3.1. General characterizations

3.1.1. Syntheses and characterizationThe ligand, L, was synthesized according to a previously re-

ported procedure [35,46,47]. Compounds 1–4 were synthesizedby the direct reaction of L with M(BF4)2�6H2O in a 3:1 ligand-to-metal ratio. This mixture was allowed to stir for 8 h. The resultingcomplexes were isolated as a pink solid for 1 (Co2+), green solid for2 (Ni2+), blue-green solid for 3 (Cu2+) and white solid for 4 (Zn2+).Single crystals of 1–4 were obtained from diffusion of ether intoa methanolic solution of the corresponding product at room tem-perature (Scheme 2, a), as noted in the experimental section. Thesecompounds were stable under normal conditions, in solid state orin solution, showing no signs of decomposition at room tempera-ture while exposed to light or air during long periods of time. Mul-

tinuclear NMR was performed only on complex 4, since the otherthree are paramagnetic species. TMS was used as internal referencefor 1H NMR and H3PO4 (25%) as external reference for 31P NMR. Asexpected, the 1H NMR for complex 4 looks very similar to the freeligand spectrum showing small variations only in the chemicalshifts. On the other hand, a single signal was observed in the 31PNMR, which presented a chemical shift downfield of 7.28 ppmwith respect to the free ligand, indicating that L remains coordi-nated to the Zn+2 ion in the acetonitrile-d3 solution. Complexes5–8 were obtained by reacting 2 equiv. of L with 1 equiv. of theappropriate M(BF4)2 salt. The mixture was allowed to stir for30 min. Single crystals for all these complexes were obtain at

Fig. 7. Thermal ellipsoid of 7 with an atomic numbering scheme. Ellipsoids are shown at the 50% level. Hydrogen atoms have been removed for clarity.

Fig. 8. Thermal ellipsoid of 8 with an atomic numbering scheme. Ellipsoids areshown at the 50% level. Hydrogen atoms have been removed for clarity.

Fig. 9. Thermal ellipsoid of 9 with an atomic numbering scheme. Ellipsoids areshown at the 50% level. Hydrogen atoms have been removed for clarity.

516 D.A. Padron, K.K. Klausmeyer / Inorganica Chimica Acta 405 (2013) 511–521

�5 �C, using a CH3CN/Ether solvent system for 1 and MeOH/Ethersolvent system for 2, 3 and 4 (Scheme 2b). The products showedcrystal structures with an octahedral environment around the me-tal centers in a 2:1 ligand-to-metal ratio. The hexa-coordinategeometry is completed by the coordination of two CH3CN unitsfor 5, two MeOH units for 6 and 8, and one molecule of MeOH

and one counter-anion for complex 7. The complex 9 was obtainedin a 1:1 ligand-to-metal ratio by reacting 1 equiv. of Ni(BF4)2�6H2Owith 1 equiv. of L. The mixture was allowed to stir for 5 min. Singlecrystals were obtained by slow diffusion of ether into a solution of9 in acetonitrile at �5 �C. These crystals were re-dissolved in 3 mlof MeOH and the resulting solution turned immediately a greencolor. Ether was allowed to slowly diffuse into this solution atroom temperature and crystals with a 2:1 ligand-to-metal ratiowere obtained after 5 days. Attempts to obtain complexes ofCo2+, Cu2+ or Zn2+ in a 1:1 ligand-to-metal ratio was not successful(Scheme 2c).

The resulting ligand-to-metal ratio for the complexes presentedin this work showed dependence with the reaction time. When the

Table 3Selected bond lengths (Å) and angles (�) for compounds 1–4.

1Co(1)–O(4) 2.0441(13) O(4)–Co(1)–O(3) 173.61(5) O(3)–Co(1)–O(2) 90.23(5)Co(1)–O(3) 2.0603(13) O(4)–Co(1)–O(1) 88.60(5) O(1)–Co(1)–O(2) 92.27(5)Co(1)–O(1) 2.0790(13) O(3)–Co(1)–O(1) 86.45(5) O(5)–Co(1)–O(2) 88.85(5)Co(1)–O(5) 2.1053(12) O(4)–Co(1)–O(5) 93.91(5) O(6)–Co(1)–O(2) 175.23(5)Co(1)–O(6) 2.1187(12) O(3)–Co(1)–O(5) 91.12(5) N(1)–O(1)–Co(1) 119.00(10)Co(1)–O(2) 2.1289(12) O(1)–Co(1)–O(5) 177.33(5) P(1)–O(2)–Co(1) 131.78(7)P(1)–O(2) 1.4975(13) O(4)–Co(1)–O(6) 89.96(5) N(2)–O(3)–Co(1) 116.79(10)P(2)–O(6) 1.5016(13) O(3)–Co(1)–O(6) 94.07(5) N(3)–O(4)–Co(1) 120.58(10)P(3)–O(5) 1.4989(13) O(1)–Co(1)–O(6) 90.08(5) P(3)–O(5)–Co(1) 128.51(8)O(1)–N(1) 1.3326(19) O(5)–Co(1)–O(6) 88.99(5) P(2)–O(6)–Co(1) 134.10(8)O(3)–N(2) 1.334(2) O(4)–Co(1)–O(2) 85.94(5)O(4)–N(3) 1.3253(19)

2Ni(1)–O(3) 2.052(3) O(3)–Ni(1)–O(1) 93.20(13) O(1)–Ni(1)–O(2) 91.71(12)Ni(1)–O(1) 2.052(3) O(3)–Ni(1)–O(5) 175.52(12) O(5)–Ni(1)–O(2) 93.96(13)Ni(1)–O(5) 2.053(3) O(1)–Ni(1)–O(5) 82.43(13) O(4)–Ni(1)–O(2) 89.81(13)Ni(1)–O(4) 2.063(3) O(3)–Ni(1)–O(4) 93.88(12) O(6)–Ni(1)–O(2) 174.32(13)Ni(1)–O(6) 2.064(3) O(1)–Ni(1)–O(4) 172.86(13) N(1)–O(1)–Ni(1) 118.7(3)Ni(1)–O(2) 2.072(3) O(5)–Ni(1)–O(4) 90.51(12) P(1)–O(2)–Ni(1) 129.63(19)P(1)–O(2) 1.502(3) O(3)–Ni(1)–O(6) 89.21(14) N(2)–O(3)–Ni(1) 117.8(2)P(2)–O(4) 1.502(3) O(1)–Ni(1)–O(6) 89.33(13) P(2)–O(4)–Ni(1) 129.68(18)P(3)–O(6) 1.489(3) O(5)–Ni(1)–O(6) 91.71(13) N(3)–O(5)–Ni(1) 118.5(2)O(1)–N(1) 1.325(5) O(4)–Ni(1)–O(6) 89.85(12) P(3)–O(6)–Ni(1) 134.3(2)O(3)–N(2) 1.341(5) O(3)–Ni(1)–O(2) 85.16(14)O(5)–N(3) 1.339(4)

3Cu(1)–O(6) 1.9675(18) O(6)–Cu(1)–O(4) 90.19(8) O(4)–Cu(1)–O(3) 91.85(7)Cu(1)–O(4) 1.973(2) O(6)–Cu(1)–O(2) 173.04(8) O(2)–Cu(1)–O(3) 95.25(7)Cu(1)–O(2) 1.9778(19) O(4)–Cu(1)–O(2) 82.86(8) O(5)–Cu(1)–O(3) 85.79(7)Cu(1)–O(5) 1.9864(19) O(6)–Cu(1)–O(5) 95.50(8) O(1)–Cu(1)–O(3) 171.01(7)Cu(1)–O(1) 2.2961(18) O(4)–Cu(1)–O(5) 173.63(8) P(1)–O(1)–Cu(1) 131.12(11)Cu(1)–O(3) 2.4339(18) O(2)–Cu(1)–O(5) 91.45(8) N(1)–O(2)–Cu(1) 116.25(15)P(1)–O(1) 1.488(2) O(6)–Cu(1)–O(1) 86.60(7) P(2)–O(3)–Cu(1) 125.91(11)P(2)–O(3) 1.4864(19) O(4)–Cu(1)–O(1) 92.01(8) N(2)–O(4)–Cu(1) 118.43(15)P(3)–O(5) 1.506(2) O(2)–Cu(1)–O(1) 93.29(7) P(3)–O(5)–Cu(1) 130.23(11)O(2)–N(1) 1.340(3) O(5)–Cu(1)–O(1) 91.19(8) N(3)–O(6)–Cu(1) 118.91(14)O(4)–N(2) 1.335(3) O(6)–Cu(1)–O(3) 85.27(7)O(6)–N(3) 1.334(3)

4Zn(1)–O(6) 2.0680(18) O(6)–Zn(1)–O(4) 176.45(9) O(4)–Zn(1)–O(1) 95.48(9)Zn(1)–O(4) 2.0726(18) O(6)–Zn(1)–O(2) 94.43(8) O(2)–Zn(1)–O(1) 89.94(8)Zn(1)–O(2) 2.090(2) O(4)–Zn(1)–O(2) 82.08(8) O(5)–Zn(1)–O(1) 89.63(8)Zn(1)–O(5) 2.091(2) O(6)–Zn(1)–O(5) 93.06(8) O(3)–Zn(1)–O(1) 173.84(8)Zn(1)–O(3) 2.102(2) O(4)–Zn(1)–O(5) 90.44(8) P(1)–O(1)–Zn(1) 129.73(12)Zn(1)–O(1) 2.118(2) O(2)–Zn(1)–O(5) 172.43(8) N(1)–O(2)–Zn(1) 118.44(17)P(1)–O(1) 1.495(2) O(6)–Zn(1)–O(3) 88.79(9) P(2)–O(3)–Zn(1) 134.56(12)P(2)–O(3) 1.489(2) O(4)–Zn(1)–O(3) 90.65(9) N(2)–O(4)–Zn(1) 118.63(16)P(3)–O(5) 1.503(2) O(2)–Zn(1)–O(3) 91.45(8) P(3)–O(5)–Zn(1) 130.20(13)O(2)–N(1) 1.329(3) O(5)–Zn(1)–O(3) 89.77(8) N(3)–O(6)–Zn(1) 118.30(15)O(4)–N(2) 1.343(3) O(6)–Zn(1)–O(1) 85.12(9)O(6)–N(3) 1.333(3)

D.A. Padron, K.K. Klausmeyer / Inorganica Chimica Acta 405 (2013) 511–521 517

starting materials (M(BF4)2 and L) are allowed to react in methanolfor 8 h the resulting complexes are 3:1 in a ligand-to-metal ratio. Inorder to obtain single crystals the products were re-dissolved inacetonitrile (since they were no longer soluble in methanol) andether was slowly diffused in the resulting solutions at room tem-perature. The isolated products show the 3:1 ligand-to metal-ratioindependent of the ratio of the starting materials (1:1, 2:1 or 3:1).On the other hand, when the reaction time was cut down to 30 minand the crystals were allowed to grow a �5 �C, the resulting com-plexes showed a 2:1 ligand-to-metal ratio. Finally, if the startingmaterials are allowed to stir for 5 min and the crystals are set togrow at �5 �C, the complex obtained (only for complex 9) displaysa 1:1 ligand-to-metal ratio. In contrast to complexes with a ligand-to-metal ratio 3:1 and 2:1, obtaining of complexes with a 1:1 ratiodepends on the ratio of the starting materials. This observation was

supported by running reactions at 1:1 and 2:1 starting material ra-tios, vide infra. When 2 equiv. of L is added to 1 equiv. of Ni(BF4)2

the color of the solution turns immediately from light blue togreen. Instead, when 1 equiv. of L is added to 1 equiv. of Ni(BF4)2

the color of the resulting solution changes from light blue to amore intense blue. All reactions were run in 10 ml of methanolat room temperature. Single crystals of the 3:1 complexes can alsobe obtained at room temperature by slow diffusion of ether in amethanolic solution of the products, regardless of the initial reac-tion stoichiometries, if the time at which the starting materialsare allowed to stir is short (if the reaction is allowed to stir for sev-eral hours the product becomes insoluble in methanol). Furthercharacterization of complexes 5–9 was not performed due to theuncertainty on the presence of products with 2:1 and/or 3:1 li-gand-to-metal ratio.

Table 4Selected bond lengths (Å) and angles (�) for compounds 5 – 9.a

5Co(1)–O(2) 2.0470(18) O(2)#1–Co(1)–O(2) 177.99(10) O(2)–Co(1)–N(2) 87.54(8)Co(1)–O(1) 2.0890(19) O(2)–Co(1)–O(1) 91.71(7) O(1)#1–Co(1)–N(2) 176.24(8)Co(1)–N(2) 2.131(2) O(2)–Co(1)–O(1)#1 89.71(7) O(1)–Co(1)–N(2) 87.85(9)P(1)–O(1) 1.4945(19) O(1)#1–Co(1)–O(1) 89.67(11) N(2)–Co(1)–N(2)#1 94.77(13)O(2)–N(1) 1.336(3) O(2)#1–Co(1)–N(2) 91.10(8) P(1)–O(1)–Co(1) 136.12(11)

N(1)–O(2)–Co(1) 119.05(14)

6Ni(1)–O(2) 2.032(3) O(2)–Ni(1)–O(4) 175.12(15) O(2)–Ni(1)–O(5) 90.93(18)Ni(1)–O(4) 2.034(4) O(2)–Ni(1)–O(3) 90.70(13) O(4)–Ni(1)–O(5) 85.1(2)Ni(1)–O(3) 2.035(3) O(4)–Ni(1)–O(3) 92.18(14) O(3)–Ni(1)–O(5) 90.97(17)Ni(1)–O(6) 2.046(4) O(2)–Ni(1)–O(6) 84.91(15) O(6)–Ni(1)–O(5) 89.67(18)Ni(1)–O(1) 2.046(4) O(4)–Ni(1)–O(6) 92.24(16) O(1)–Ni(1)–O(5) 176.13(17)Ni(1)–O(5) 2.064(5) O(3)–Ni(1)–O(6) 175.57(15) P(1)–O(1)–Ni(1) 136.2(2)P(1)–O(1) 1.485(4) O(2)–Ni(1)–O(1) 92.68(14) N(1)–O(2)–Ni(1) 118.1(3)P(2)–O(3) 1.486(3) O(4)–Ni(1)–O(1) 91.24(15) P(2)–O(3)–Ni(1) 137.0(2)O(2)–N(1) 1.341(5) O(3)–Ni(1)–O(1) 90.38(15) N(2)–O(4)–Ni(1) 116.8(3)O(4)–N(2) 1.327(6) O(6)–Ni(1)–O(1) 89.26(16)

7Cu(1)–O(1) 1.9464(15) O(1)–Cu(1)–O(4) 91.52(7) F(1)–Cu(1)–O(1) 88.28 (6)Cu(1)–O(4) 1.9524(16) O(1)–Cu(1)–O(2) 93.49(7) F(1)–Cu(1)–O(2) 81.21(6)Cu(1)–O(2) 1.9532(16) O(4)–Cu(1)–O(2) 171.52(6) F(1)–Cu(1)–O(3) 170.19(5)Cu(1)–O(5) 1.9747(17) O(1)–Cu(1)–O(5) 170.67(7) F(1)–Cu(1)–O(4) 92.11(6)Cu(1)–O(3) 2.2577(15) O(4)–Cu(1)–O(5) 83.96(7) F(1)–Cu(1)–O(5) 83.76 (6)P(1)–O(1) 1.4986(17) O(2)–Cu(1)–O(5) 90.09(7) P(1)–O(1)–Cu(1) 137.48(9)P(2)–O(3) 1.4885(16) O(1)–Cu(1)–O(3) 99.97(7) N(1)–O(2)–Cu(1) 116.59(12)O(2)–N(1) 1.332(2) O(4)–Cu(1)–O(3) 92.96(7) P(2)–O(3)–Cu(1) 129.44(9)O(4)–N(2) 1.340(2) O(2)–Cu(1)–O(3) 92.90(6) N(2)–O(4)–Cu(1) 119.64(13)Cu(1)–F(1) 2.5289(14) O(5)–Cu(1)–O(3) 88.43(7) C(37)–O(5)–Cu(1) 125.44(16)

8Zn(1)–O(4) 2.061(4) O(4)–Zn(1)–O(2) 176.22(18) O(4)–Zn(1)–O(5) 91.9(2)Zn(1)–O(2) 2.063(4) O(4)–Zn(1)–O(1) 90.53(17) O(2)–Zn(1)–O(5) 85.0(2)Zn(1)–O(1) 2.067(4) O(2)–Zn(1)–O(1) 91.81(17) O(1)–Zn(1)–O(5) 92.2(2)Zn(1)–O(3) 2.076(4) O(4)–Zn(1)–O(3) 92.15(17) O(3)–Zn(1)–O(5) 174.9(2)Zn(1)–O(6) 2.087(5) O(2)–Zn(1)–O(3) 90.77(18) O(6)–Zn(1)–O(5) 88.4(2)Zn(1)–O(5) 2.104(6) O(1)–Zn(1)–O(3) 90.85(18) P(1)–O(1)–Zn(1) 136.8(2)P(1)–O(1) 1.479(4) O(4)–Zn(1)–O(6) 84.62(19) N(2)–O(2)–Zn(1) 116.8(3)P(2)–O(3) 1.490(4) O(2)–Zn(1)–O(6) 93.06(19) P(2)–O(3)–Zn(1) 136.0(2)O(2)–N(2) 1.322(7) O(1)–Zn(1)–O(6) 175.13(18) N(1)–O(4)–Zn(1) 117.8(3)O(4)–N(1) 1.335(6) O(3)–Zn(1)–O(6) 89.0(2)

9Ni(1)–O(2) 2.0293(12) O(2)–Ni(1)–O(1) 92.00(5) N(2)–Ni(1)–N(5) 178.28(6)Ni(1)–O(1) 2.0483(12) O(2)–Ni(1)–N(2) 93.66(5) N(4)–Ni(1)–N(5) 92.33(6)Ni(1)–N(2) 2.0584(15) O(1)–Ni(1)–N(2) 88.78(5) O(2)–Ni(1)–N(3) 177.85(5)Ni(1)–N(4) 2.0679(15) O(2)–Ni(1)–N(4) 90.40(5) O(1)–Ni(1)–N(3) 89.04(5)Ni(1)–N(5) 2.0739(15) O(1)–Ni(1)–N(4) 176.67(5) N(2)–Ni(1)–N(3) 88.24(6)Ni(1)–N(3) 2.0840(15) N(2)–Ni(1)–N(4) 88.77(6) N(4)–Ni(1)–N(3) 88.63(6)P(1)–O(1) 1.5057(12) O(2)–Ni(1)–N(5) 85.02(5) N(5)–Ni(1)–N(3) 93.11(6)N(1)–O(2) 1.3363(18) O(1)–Ni(1)–N(5) 90.17(5) P(1)–O(1)–Ni(1) 137.09(7)

N(1)–O(2)–Ni(1) 119.09(9)

a Symmetry transformations used to generate equivalent atoms: For 5: #1 �x + 1, y, �z + 3/2.

518 D.A. Padron, K.K. Klausmeyer / Inorganica Chimica Acta 405 (2013) 511–521

3.2. Description of the crystal structures

Crystallographic analyses of complexes 1–4 evidenced crystalstructures with a 3:1 ligand-to-metal ratio. Only mer isomerswhere observed. Selected bond lengths and angles for these com-pounds are listed in Table 3. These coordination complexes displayan octahedral geometry, where three units of bidentate L are coor-dinated in a chelating fashion to the M2+ center through the oxygenatoms of the phosphoryl and nitrosyl groups, forming three seven-membered rings (Fig. 1). The axial positions are occupied by thetwo oxygen atoms of two P@O groups, with M–OP@O distances of2.133 and 2.116 Å. The OP@O–M–OP@O axial angle was found tobe nearly linear for all four complexes: 175.0� for 1, 174.3� for 2,171.0� for 3 and 173.8� for 4. The equatorial positions are occupied

by three other oxygen atoms corresponding to three N–O groups(M–ON–O distance range: 1.973–2.090 Å) and one P@O group (M–OP@O distance range: 1.986–2.103 Å). The shortest equatorial bondlengths and the longest axial bond lengths were observed in com-plex 3, Cu(BF4)2L3. This elongation is due to the presence of Jahn–Teller distortion in this d9 complex. In complex 1, Co(BF4)L3, noappreciable distortion in the Z axis is observed suggesting that thisd7 octahedral complex possesses a high-spin system, which agreeswith the ligand L being a rather weak field ligand. The angles O–M–O in the equatorial plane are all close to 90� with an average angleof 90.0�. The bite angles for all three chelating ligand units of L incomplex 1–4 range from 91.23� to 93.53�, with an average bite an-gle of 92.65�. Several hydrogen bonding of the type F� � �H–C wereobserved between the BF4

� fluorine atoms and hydrogen atoms

N

P

O

O + M(BF4)2

Ph Ph

N

P

O

OM

PhPh

N

P

OO

PhPh

N

PO

O

Ph Ph

N

P

OO

M

OCH3

X

PhPh

N

P O

O

X = CH3OH (f or M = Co, Ni and Zn)X = BF4 (for M = Cu)

(a)

(b)

MeOH / Ether(8 Hr)

LH

Ph Ph

N

P

OO

M

NCCH3

NCCH3

H3CCN

H3CCN

MeOH / Ether(30 min)

CH3CN / Ether(5min)

(c)

Scheme 2. Schematic representation of the conditions used to obtain single crystal of complexes 1–4 (a), 5–8 (b) and 9 (c).

D.A. Padron, K.K. Klausmeyer / Inorganica Chimica Acta 405 (2013) 511–521 519

on the phenyl rings and solvent molecules, with H� � �F distancerange of 2.120–2.941 Å. The distances between the metal centersand the boron atoms in the counter-anions range between 6.357and 12.201 Å (Table 5).

Complexes 5–8, with molecular formula M(BF4)2L2S2 (M = Co2+,Ni2+, Cu2+ and Zn2+, respectively, and S = MeOH or CH3CN), areproduct of the reaction between 1 equiv. of the corresponding me-tal salt of M(BF4)2, (M = Co2+, Ni2+, Cu2+ or Zn2+), and 2 equiv. of L.Selected bond lengths and angles for these compounds are listed inTable 4. Complexes 5–8 also present an octahedral geometryaround the metal center M, with two L units coordinated in a che-lating fashion generating two seven-membered rings. The othertwo coordination sites are occupied by two solvent molecules, inthe case of 5, 6 and 8, and one solvent molecule and one counteranion in the case of 7. The axial positions in complex 7 are occu-pied by one OP@O and one of the BF4

� fluorine atoms, which is posi-tioned towards the Cu2+ center.

Complexes 5, 6 and 8 presented M–OP@O and M–ON–O averagebond distances of 2.062 ± 0.021 and 2.047 ± 0.014 Å, respectively,which are within the range of those found for complexes 1–4.The average bite angle OP@O–M–ON–O was also in the range of thatfound for complexes 1–4. Similarly to complex 3, complex 7 was

Table 5Selected averaged parameters for compounds 1–4.

Compound Bite distance(Å) (average)

Bite angle (�)(average)

Axialangles(�)

Equatorial angles(�) (average)

1 3.041 ± 0.012 93.4 ± 0.81 175.0 90.0 ± 3.12 2.973 ± 0.029 92.43 ± 1.27 174.3 90.0 ± 5.23 3.074 ± 0.132 93.53 ± 1.80 171.0 90.0 ± 5.24 2.987 ± 0.026 91.23 ± 1.66 173.8 90.0 ± 5.5

found to experience an elongation of bonds on the z-axis (Z-outdistortion) with Cu–OP2@O3 and Cu–F1 bond lengths of2.2577(15) and 2.5289(14) Å, respectively. The O3–Cu–F1 anglewas found to be 170.2�. The equatorial plane is formed by theinteraction of the Cu center with the OP1@O1 and ON1–O2 atoms ofone L unit, the ON2–O4 atom of the other ligand and the oxygenatom, O5, of one methanol molecule. The bond lengths for theseinteractions presented the following values: 1.9464(15) Å for Cu–O1, 1.9532(16) Å for Cu–O2, 1.9524(16) Å for Cu–O4 and1.9747(17) Å for Cu–O5. The longest value was observed for theinteraction Cu–O5, possibly due to the steric hindrance imposedby the methanolic –CH3 group.

Complex 9 is product of the reaction between 1 equiv. ofNi(BF4)2�6H2O and 1 equiv. of L. The crystallographic analysisshowed a hexa-coordinate environment around the nickel centerwhere one L unit acts in a bidentate fashion and four CH3CN mol-ecules complete the octahedral geometry. The Ni–OP@O and Ni–ON–O distances are 2.048 and 2.029 Å, respectively, with a P@Obond length of 1.506 Å and N–O bond length of 1.336 Å. The O–Ni–O bite distance is 2.933 Å with a bite angle of 92.0�. The averageNi–N distance was found to be 2.071 ± 0.010 Å. All the N–Ni–N an-gles between the CH3CN molecules positioned cis to each other arenearly 90� (average = 90.2�). The N5–Ni–N2 angle (trans CH3CNgroups) is 178.3�. Interactions are observed between the hydrogenatoms in one of the phenyl rings and an oxygen atom on the phos-phoryl group (H� � �OP@O = 2.625 Å; C–H� � �OP@O = 106.0�). Similarly,one hydrogen atom on the ligand methylene group interacts withthe nitrosyl oxygen atom with a H� � �ON–O distance of 2.412 Å anda C–H� � �ON–O angle of 97.0�. Interactions of this type are alsopresent in the crystal structure of the ligand L and all the othercomplexes presented in this work. Selected bond lengths andangles for compound 9 are listed in Table 4. It is interesting to notethat in complexes 2:1 (1–4) only isomers with P@O groups in cis

Table 6FT–IR data for compounds 1–4.

Compound m(B–F) (cm�1) m(N–O) (cm�1)

L – 1266 (s)1 1061 (s) 1211 (s)2 1062 (s) 1213 (s)3 1067 (s) 1202 (s)4 1060 (s) 1212 (s)

Table 7UV–Vis data for compounds 1–3 at room temperature and 4 � 10�4 M in CH3CN.

Compound kmax (nm) e (M�1 cm�1) M(BF4)2

1 386, 593 372.5, 72.5 4912 393 329.6 366, 585, 9673 858 73.4 759

520 D.A. Padron, K.K. Klausmeyer / Inorganica Chimica Acta 405 (2013) 511–521

position and N–O groups in trans positions are formed. Similarly,for complexes with a ratio 3:1 (5–8) only the mer isomer areformed. The second ligand unit coordinates preferentially withN–O groups trans to each other and P@O groups cis to each other,even though a trans positioning between the two P@O would besterically less demanding.

3.3. FT-IR spectra

The FT-IR spectra of complexes 1–4 showed a red shift of the N–O stretching bands relative to the free ligand L (Table 6). Complex 3presents the lowest energy N–O stretching band, 1202 cm�1, whichis consistent with a contraction in the equatorial plane due to theJahn–Teller distortion, leading to a weakening of the N–O bonds.All the other complexes showed nearly the same stretching fre-quencies, with a shift to lower energies of approximately50 cm�1 with respect to L. A broad band, characteristic for the B–F bond in the BF4

� counter-anion, was observed at 1061, 1063,1067 and 1060 cm�1 for complexes 1–4, respectively. These bandsare shifted 25–30 cm�1 to lower energies compared to the M(BF4)2

starting materials. These wavelength differences can be attributedto the increase in the B–F vibration energy due to the weaker inter-action of the BF4

� counter-anions with the M centers after coordi-nation of the L units. The P@O bands were not assigned due tooverlapping with the much broader BF4

� bands.

3.4. UV–Vis spectra

The UV–Vis spectra for compounds 1–3 were obtained atroom temperature from solutions with concentration 4 � 10�4 Min CH3CN. A comparison with the parent starting materialsM(BF4)2�H2O is shown in Table 7. All three complexes showed ared-shift relative to the M(BF4)2 stating materials. Complex 1showed one absorption band at 386 nm, with a small shoulderat 490 nm, corresponding to the d–d electronic transitions4T1g(F)?4T1g(P) and 4T1g(F)?4A2g, and another band at 593 nmcorresponding to the transition 4T1g(F)?4T2g. Complex 2 showedone absorption band at 393 nm, assigned to the electronictransition 3A2g?

3T1g(P). Complex 3 showed a band at 858 nmwhich corresponds to a 2Eg?

2T2g transition. None of thecompounds discussed here show any fluorescence even at liquidnitrogen temperatures.

4. Conclusions

Several new octahedral coordination complexes of Co2+, Ni2+,Cu2+ and Zn2+ were synthesized by the reaction of their corre-

sponding tetrafluoroborate salts with 2-(diphenylphosphino-methyl)pyridine N,P dioxide, L. All crystal structures showed anoctahedral geometry around the metal centers. The ligand L coor-dinates in a chelating fashion to the M+2 centers through the oxy-gen atoms of the phosphoryl and nitrosyl groups, forming threeseven-membered rings. As expected, a strong Jahn–Teller distor-tion is observed in complexes 3 and 7 where the central atom,Cu2+, is a d9 ion. In the case of complex Co(BF4)2L3 (1), a d7 high-spin system, this distortion is not observed. The d–d electronictransitions displayed in the UV–Vis spectra were in the expectedrange of energies. The FT-IR spectra also showed stretching bandsin the range reported in other work. The formation of specific iso-mers is probably driven by both kinetic and entropic effects.

Acknowledgments

This research was supported by funds provided by the Depart-ment of Chemistry & Biochemistry at Baylor University and theRobert A. Welch Foundation (AA-1508). This study was supportedin part by funds from the University Research Committee and theVice Provost for Research at Baylor University.

Appendix A. Supplementary material

CCDC 916359, 916358, 916357, 916356, 916352, 916354,916351, 916355, and 916353 contain the supplementary crystallo-graphic data for 1–9, respectively. These data can be obtained freeof charge from the Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-ated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2013.03.037.

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