Role of Phosphine Sterics in Strained ... - ACS Publications

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Role of Phosphine Sterics in Strained Aminophosphine Chelate Formation Eric G. Bowes, D. Dawson Beattie, and Jennifer A. Love* Department of Chemistry, The University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada

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S Supporting Information *

Scheme 1. (a) Formation of κ1-P Products, (b) Halide Abstraction To Provide Cationic κ2-PN Complexes, and (c) Bulky Phosphines That Permit the Preparation of Strained Chelates

ABSTRACT: The preparation of four-membered aminophosphine (PN) chelates from common metal precursors has largely evaded realization because of the ring strain associated with these species. We report a straightforward approach to the synthesis of such PN metallacycles using simple α-PN ligands analogous to the popular class of small-bite-angle diphosphinomethane ligands. It is demonstrated that bulky phosphine substituents are important to the formation of these chelates.

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ncillary ligand design represents a critical strategy in synthetic chemistry. Metal complexes of ligands with labile donor functionalities have proven invaluable in catalysis. These “hemilabile” ligands enhance reactivity by facilitating the generation of low-coordinate species and have the potential for the decoordinated group to interact with substrates as a Lewis or Brønsted base.1 Hemilability and associated reactivity is governed by the ligand dissociation barrier, and mixed-donor ligands featuring a combination of hard and soft bases enhance reactivity by providing disparate metal−donor interactions. As such, ligands with both nitrogen and phosphorous donors (PN), the largest class of hybrid ligands, have proven to be superior to bidentate diamine (NN) and diphosphine (PP) ligands in a variety of catalytic applications.2 Building on our previous efforts toward hydrocarbon functionalization, we were interested in platinum(II) complexes of PN ligands bearing sp3-hybridized N-atom donors capable of deprotonating platinum(IV) hydride intermediates. 3 We anticipated that strained chelates would afford enhanced hemilability, increasing reactivity toward small molecules such as methane. The coordination chemistry of N(sp3)-hybridized α-PN ligands is well developed and characterized by a distinct preference for κ1-P coordination (Scheme 1a) that limits their utility as hemilabile ligands.4 Examples of transition-metal complexes featuring κ2-PN coordination of these ligands are rare and are typically formed under forced conditions such as ligand abstraction to form cationic products (Scheme 1b).5 PN ligands with N(sp2)-hybridized donor functionalities are well-known to form four-membered chelates,6 but these species often prefer to adopt κ1-P or bridging μ-PN coordination modes with late transition metals.7 We report the preparation of archetypical (κ2-PN)PtX2 complexes via the ligation of simple metal precursors and demonstrate that chelate formation is facilitated by increasing the steric profile of the phosphine group, despite increased repulsion between P and N substituents (Scheme 1c). While © XXXX American Chemical Society

bulky substituents were found to lower ring strain through a Thorpe−Ingold-like effect,8 our key finding was that steric protection of vacant sites adjacent to P was the determining factor in chelate formation. We initially attempted to prepare a four-membered PN chelate via ligation of L1 to [Pt2Me4(μ-SMe2)2], resulting in a mixture of (SMe2)2PtMe2 and two κ1-P products, 1 and 2 (Scheme 2). Notably, the desired κ2-PN complex 3 was not formed in appreciable quantities. Similar observations were made in attempts to prepare 3 from (nbd)PtMe2 (nbd = norbornadiene) and L1 under a number of conditions.9 The treatment of [Pt2Me4(μ-SMe2)2] with the more strongly donating iPr-substituted phosphine L2 resulted in a similar distribution of κ1-P products 4 and 5, but a third Pt-bound phosphine product 6 was detected in small quantities (8%) by NMR spectroscopy.10 Given that the desired chelate was observed using an alkylsubstituted phosphine, we explored a number of other PNs bearing electron-rich P centers. In contrast to what was observed with L1/L2, the reaction of L3 (R = tBu) and L4 (R = Ad = 1Received: December 17, 2018

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DOI: 10.1021/acs.inorgchem.8b03514 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

putative complex 6, with 195Pt−1H coupling to the N−CH3 groups (20−21 Hz) confirming a N → Pt dative interaction. We anticipated that the use of bulky phosphine substituents might be a general strategy for accessing a variety of small-biteangle chelates. Precedent for this hypothesis is provided by observations from Grotjan and co-workers showing that κ2-PN chelates are formed in the oxidation of (κ1-PN)2Pd complexes (PN = imidazolylphosphine derivatives) featuring tBu-substituted phosphines but not with smaller iPr substituents.6a We thus sought to prepare (κ2-PN)PtMe2 complexes featuring an sp2-hybridized donor in place of the −NMe2 group. Gratifyingly, the use of tBu-substituted (L5) and Ad-substituted (L6) pyridylphosphines allowed for the straightforward isolation of N(sp2) derivatives 9 and 10 (Scheme 3b). This is significant because attempts to prepare analogous Ph-substituted derivatives with commercially available (2-pyridyl)PPh2 have been unsuccessful.13 ORTEP diagrams of 7, 9, and 10 are shown in Scheme 3c. To address the structural properties affected by changes in the chelate size, five-membered PN chelates (L7)PtMe2 (11; L7 = t Bu2PCH2CH2NMe2) and (L8)PtMe2 [12; L8 = (2-pyridyl)CH2PtBu2] were prepared by the addition of the appropriate ligand to [Pt2Me4(μ-SMe2)2]. The X-ray structure of 11 was obtained and is presented for comparison with four-membered chelates in Scheme 3c, while the structure of 12 was reported previously by Goldberg and co-workers.14 The data for the fourmembered chelate complexes confirm their monomeric nature and show distorted square-planar geometries (τ4 = 0.13−0.15)15 because of the small ligand bite angles of close to 70° [7, 71.4(2)°; 9, 70.0(2)°; 10, 68.90(7)°]. These angles are significantly smaller than those in related diphosphinomethane complexes (∼74°).16 The most notable structural change upon an increase in the chelate size from four (7/9) to five (11/12) is the ligand bite angle (P−Pt−N), which increases by ∼14° when an additional methylene spacer is added to the ligand backbone. The effects of ring strain in the four-membered rings are evident in the changes in pyramidalization at the N- and P-atom donors as a function of the chelate size. The sum of the C−P−C angles about P in 7 is greater than that in 11 [Δ∑∠C−P−C = +6.7(5)°]. Similar observations were made for changes in the C−N−C angles in 7 and 11, as well as the C−P−C angles in 9 and 12. This is rationalized by considering the effect of ∑∠C−E−C (E = P, N) on the orientation of the E lone pair, which has better alignment with Pt as ∑∠C−E−C becomes larger. In both the N(sp3) and N(sp2) systems, the changes in the Pt−N bond lengths were found to be insignificant. Interestingly, while a modest increase in the Pt−P bond length [0.036(3) Å] occurs upon moving from the fourmembered 9 to five-membered 12, the difference in N(sp3) derivatives 7 and 11 is negligible [0.004(2) Å]. The 195Pt−31P coupling constants in 7/9 are reduced by 350 Hz in comparison to those in the five-membered chelates, highlighting reduced Pt−P interactions despite similar bond lengths. This is also evident in the reduced trans influence of the phosphine in 7/9, which show shorter Pt−C bonds trans to the phosphine in comparison to 11/12 [−0.020(7) and −0.019(13) Å, respectively]. Thorpe−Ingold-type effects have been observed in organometallic chemistry and suggested to promote the formation of C−H agostic complexes17 and cyclometalation of monodentate phosphines.18 Significant differences in the coordination chemistry of tBu- and iPr-substituted diphosphinomethane19

Scheme 2. Reaction of [Pt2Me4(μ-SMe2)2] with L1 (R = Ph) and L2 (R = iPr) and 1H NMR Spectroscopic Yields

Scheme 3. Synthesis of Strained PN Chelates Featuring (a) N(sp3) and (b) N(sp2) Donors and (c) X-ray Structures of Four- and Five-Membered PN Chelates

adamantyl) with [Pt2Me4(μ-SMe2)2] afforded quantitative conversion to the κ2-PN metallacycles 7/8 (Scheme 3a). The 195 Pt−31P coupling constants for 7/8 (JPt−P = 1674 and 1656 Hz, respectively) were 140−190 Hz smaller in magnitude than those reported previously for related PR2R′ ligands trans to hydrocarbonyl donors at PtII.11 Similar observations have been made previously for four-membered PN and PC chelates.12 The 1H NMR spectra for 7/8 exhibited features similar to those for B

DOI: 10.1021/acs.inorgchem.8b03514 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry and imidazolylphosphine6a ligands have also been attributed to the size of alkyl substituents at P. Work by Shaw demonstrated that larger geminal substituents such as tBu are required to observe significant effects for P, resulting in the termed “gem-ditert-butyl” effect.20 To probe the factors influencing the differences in coordination preference of the PN ligands under investigation, a density functional theory (DFT) study on 17 ligands, expanding on the set studied experimentally, was conducted to determine the effects of P substitution and chelate size on the structure and bonding of (κ2-PN)PtMe2 complexes.21 A comparison of the DFT-optimized geometries for 7/9 and 11/12 reproduced the experimentally observed structural differences between four- and five-membered PN chelates (Tables S2). Natural bond orbital22 and atoms-in-molecules23 analyses revealed weaker Pt−N and Pt−P bonding interactions in the small rings. Bond orders were lower by 6−11%, accompanied by less electron density (0.002−0.009 au) at the bond critical points (bcps) and greater concentrations of negative charge on P and N. Partial molecular graphs for 7 and 11 are presented in parts a and b of Figure 1, respectively. The

Although significant changes were found between the fourand five-membered chelates, our computations suggest that the differences in the strain (as evaluated by ∑ΔBP−GB) and M−L bond order metrics are small within the series of four-membered chelates featuring P substituents of varying bulk (Me, iPr, tBu, etc.). While the observed changes were small, they were in agreement with a Thorpe−Ingold-like effect, wherein increased C−P−C angles in bulky phosphines reduce the ring strain by improving the Pt and P orbital alignment. The N dissociation enthalpy for 7 (16.4 kcal·mol−1) was calculated to be greater than that for complex 3 (11.8 kcal· mol−1) or 6 (11.6 kcal·mol−1), indicating greater chelate stability for the larger phosphines. As anticipated, the energetic penalty due to steric repulsion between the PR2 and NMe2 groups in the bound conformation was calculated to be larger for L3/L4 than for L1/L2.26 Evidently, the reduction in the ring strain provided by large R groups compensates for the steric repulsion. The increased stability of chelate rings with large P substituents is in agreement with experimental trends in the coordination preferences for the N(sp3) ligands but does not account for the behavior of the pyridylphosphine derivatives. The enthalpy for ring opening in 9 (9.2 kcal·mol−1) and 10 (8.0 kcal·mol−1) reveals lower chelate stability in comparison with that in 3 and 6, indicating that a factor other than ring strain is influencing our ability to isolate κ2-PN chelates with the bulky phosphines. The calculated ΔG values for chelate opening of all κ2-PN complexes were positive, suggesting that κ2 coordination of L1 and L2 should be feasible in the absence of a competitive donor. Attempts to access κ2-PN chelates using (cod)PtMe2 as an alternative PtII precursor were unsuccessful.26 The treatment of complexes 2 and 5 with [Ph3C][BF4] to generate a vacant coordination site via methyl abstraction did, however, result in chelate formation (Scheme 4a). Cationic complexes 13 and 14 were isolable, demonstrating that L1 and L2 can indeed adopt κ2PN coordination modes when alternative ligation is unfavorable.27 The treatment of 7 with excess free ligand L3 did not yield κ1-P product 15, suggesting that the metal cannot accommodate two Scheme 4. Reactivity Studies

Figure 1. Partial molecular graphs and calculated electron density topologies ρ(r) for (a) 7 and (b) 11 and topologies of ∇2ρ(r) (negative contours, red; positive contours, black) for (c) 7 and (d) 11. Dots: red, (3, −1) bcps; green, (3, +1) rcps.

paths of maximal e− density connecting the Pt,P and Pt,N atoms (bond paths) in 7 are visibly distorted; strained molecules typically have bond paths that exceed the geometric bond length (recall the “banana bonds” of cyclopropane).24 The sum of the differences between the bond path and geometric bond lengths, ∑ΔBP−GB, for the atoms forming the chelate ring in 7 (0.018 Å) was significantly larger than that in 11 (0.004 Å), with the greatest deviation exhibited by the Pt−P bond (0.011 Å). Analysis of the Laplacian of the electron density, ∇2ρ(r), provides information about regions of local concentration and depletion of the electron density and can be used to identify the locations of nonbonding electron pairs.23,25 The P lone pair (red contours) in strained 7 (Figure 1c) clearly suffers from poor alignment with Pt relative to that in 11 (Figure 1d), resulting in reduced stabilization despite similar M−L bond lengths. C

DOI: 10.1021/acs.inorgchem.8b03514 Inorg. Chem. XXXX, XXX, XXX−XXX

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actor ligands as versatile platforms for small molecule activation and catalysis. RSC Adv. 2013, 3, 11432−49. (2) (a) Carroll, M. P.; Guiry, P. J. P,N ligands in asymmetric catalysis. Chem. Soc. Rev. 2014, 43, 819−33. (b) Rong, M. K.; Holtrop, F.; Slootweg, J. C.; Lammertsma, K. 1,3-P,N hybrid ligands in mononuclear coordination chemistry and homogeneous catalysis. Coord. Chem. Rev. 2019, 380, 1−16. (c) Stradiotto, M.; Lundgren, R.; Hesp, K. Design of new ‘DalPhos’ P,N-ligands: applications in transition-metal catalysis. Synlett 2011, 2011, 2443−2458. (d) Schubert, U.; Pfeiffer, J.; Stö hr, F.; Sturmayr, D.; Thompson, S. Transformations of organosilanes by Pt(II) complexes with hemilabile P,N-chelating ligands. J. Organomet. Chem. 2002, 646, 53−58. (e) Zhang, W.-H.; Chien, S. W.; Hor, T. S. A. Recent advances in metal catalysts with hybrid ligands. Coord. Chem. Rev. 2011, 255, 1991− 2024. (f) Helmchen, G.; Pfaltz, A. Phosphinooxazolines - a new class of versatile, modular P,N-ligands for asymmetric catalysis. Acc. Chem. Res. 2000, 33, 336−345. (3) Bowes, E. G.; Pal, S.; Love, J. A. Exclusive Csp3-Csp3 vs Csp2-Csp3 reductive elimination from Pt(IV) governed by ligand constraints. J. Am. Chem. Soc. 2015, 137, 16004−7. (4) (a) Balint, E.; Tajti, A.; Tripolszky, A.; Keglevich, G. Synthesis of platinum, palladium and rhodium complexes of α-aminophosphine ligands. Dalton Trans. 2018, 47, 4755−4778. Examples of μ-PN coordination are also known: (b) Turpin, R.; Dagnac, P.; Poilblanc, R. A rhodium(I) derivative of diethyl(diphenylphosphinomethyl)amine as a ligand for the synthesis of homo- and hetero-dinuclear complexes. J. Organomet. Chem. 1987, 319, 247−255. (c) Dagnac, P.; Turpin, R.; Poilblanc, R. Reactions of homobinuclear platinum and palladium complexes with carbon monoxide. J. Organomet. Chem. 1983, 253, 123−129. (5) (a) Zhang, S.; Bullock, R. M. Molybdenum hydride and dihydride complexes bearing diphosphine ligands with a pendant amine: formation of complexes with bound amines. Inorg. Chem. 2015, 54, 6397−6409. (b) Zhang, S.; Appel, A. M.; Bullock, R. M. Reversible heterolytic cleavage of the H-H bond by molybdenum complexes: controlling the dynamics of exchange between proton and hydride. J. Am. Chem. Soc. 2017, 139, 7376−7387. (c) Hazari, A.; Labinger, J. A.; Bercaw, J. E. A versatile ligand platform that supports Lewis acid promoted migratory insertion. Angew. Chem., Int. Ed. 2012, 51, 8268− 8271. (d) Clarke, M. L.; Slawin, A. M. Z.; Woollins, J. D. Platinum complexes of tertiary amine functionalised phosphines. Polyhedron 2003, 22, 19−26. (e) Payet, E.; Auffrant, A.; Le Goff, X. F.; Floch, P. L. Phosphine- and thiophosphorane-amine ligands: lithiation and coordination to Rh(I). J. Organomet. Chem. 2010, 695, 1499−1506. (f) Clarke, M. L.; Cole-Hamilton, D. J.; Foster, D. F.; Slawin, A. M. Z.; Woollins, J. D. Co-ordination chemistry and metal catalysed carbonylation reactions using 8-(diphenylphosphino)methylaminoquinoline: a ligand that displays monodentate, bidentate and tridentate coordination modes. J. Chem. Soc., Dalton Trans. 2002, 1618−1624. (g) Plotek, M.; Starosta, R.; Komarnicka, U. K.; Skorska-Stania, A.; Jezowska-Bojczuk, M.; Stochel, G.; Kyziol, A. New ruthenium(II) coordination compounds possessing bidentate aminomethylphosphane ligands: synthesis, characterization and preliminary biological study in vitro. Dalton Trans. 2015, 44, 13969−78. (6) For examples, see: (a) Grotjahn, D. B.; Gong, Y.; Zakharov, L.; Golen, J. A.; Rheingold, A. L. Changes in coordination of sterically demanding hybrid imidazolylphosphine ligands on Pd(0) and Pd(II). J. Am. Chem. Soc. 2006, 128, 438−453. (b) Grotjahn, D. B.; Larsen, C. R.; Gustafson, J. L.; Nair, R.; Sharma, A. Extensive isomerization of alkenes using a bifunctional catalyst: an alkene zipper. J. Am. Chem. Soc. 2007, 129, 9592−9593. (c) Jones, N. D.; MacFarlane, K. S.; Smith, M. B.; Schutte, R. P.; Rettig, S. J.; James, B. R. Coordination chemistry of the 2-pyridyldiphosphine ligands, (py)2P(CH(CH2)3CH)P(py)2 and (py)2P(CH2)2P(py)2 (py = 2-pyridyl), with platinum(II) and ruthenium(II). Ruthenium-catalyzed hydrogenation of imines. Inorg. Chem. 1999, 38, 3956−3966. (d) Almeida Leñero, K. Q.; Guari, Y.; Kamer, P. C.; van Leeuwen, P. W.; Donnadieu, B.; Sabo-Etienne, S.; Chaudret, B.; Lutz, M.; Spek, A. L. Heterolytic activation of dihydrogen by platinum and palladium complexes. Dalton Trans. 2013, 42, 6495−

large PR3 substituents in a cis configuration (Scheme 4b). This is further supported by the degree of distortion in 5, with a P−Pt− P angle of 106.82(2)° (Scheme 2). Tellingly, 7 was found to react with the less hindered phosphine tBu2PH to form 16 (43%) along with L3 (23%) and cis-(tBu2PH)2PtMe2 (23%; Scheme 4c). The steric bulk of the PN ligands were quantified by means of buried volume (Vbur) calculations,28 showing that bulky ligands L3−L8 occupy between 30 and 32% of the Pt coordination sphere. Vbur values for L1 and L2 were found to be slighter smaller (27−28%), with a less spherical distribution of bulk, which may limit steric repulsion with cis ligands. While we have not evaluated the influence of hindered rotation in the ligand backbone,20 our data are consistent with a mechanism in which chelate formation is promoted by steric protection of the site cis to P. We have demonstrated that the preparation of fourmembered chelates of N(sp3)-hybridized PN ligands using simple metal precursors is possible when employing sterically encumbered phosphines. The behavior of these complexes toward small-molecule activation and the effects of the chelate size on the reactivity are currently being investigated.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03514. Full computational and experimental details (PDF) Accession Codes

CCDC 1872321−1872325 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Eric G. Bowes: 0000-0003-3605-1814 D. Dawson Beattie: 0000-0002-7909-2416 Jennifer A. Love: 0000-0003-2373-1036 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank The University of British Columbia, NSERC (Discovery and Research Tools and Instrumentation grants), Compute Canada Calcul Canada, and Canada Foundation for Innovation for supporting this research. E.G.B. and D.D.B. are grateful to the Government of Canada and NSERC for Vanier CGS and PGS-D scholarships, respectively. Marcus W. Drover and Yiming Ren are thanked for providing assistance with X-ray crystallography and ligand synthesis.

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REFERENCES

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Communication

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DOI: 10.1021/acs.inorgchem.8b03514 Inorg. Chem. XXXX, XXX, XXX−XXX