Terminal Parent Phosphanide and Phosphinidene Complexes of Zirconium(IV)

Abstract The reaction of [Zr(TrenDMBS)(Cl)] [Zr1; TrenDMBS=N(CH2CH2NSiMe2But)3] with NaPH2 gave the terminal parent phosphanide complex [Zr(TrenDMBS)(PH2)] [Zr2; Zr−P=2.690(2) Å]. Treatment of Zr2 with one equivalent of KCH2C6H5 and two equivalents of benzo‐15‐crown‐5 ether (B15C5) afforded an unprecedented example (outside of matrix isolation) of a structurally authenticated transition‐metal terminal parent phosphinidene complex [Zr(TrenDMBS)(PH)][K(B15C5)2] [Zr3; Zr=P=2.472(2) Å]. DFT calculations reveal a polarized‐covalent Zr=P double bond, with a Mayer bond order of 1.48, and together with IR spectroscopic data also suggest an agostic‐type Zr⋅⋅⋅HP interaction [∡ZrPH=66.7°] which is unexpectedly similar to that found in cryogenic, spectroscopically observed phosphinidene species. Surprisingly, computational data suggest that the Zr=P linkage is similarly polarized, and thus as covalent, as essentially isostructural U=P and Th=P analogues.

Well-defined transition-metal phosphinidene complexes (L n M = PR) are of interest owing to ad esire to better understand their bonding and PR-group transfer chemistry. [1,2] However, although such complexes were first reported three decades ago, [3] they remain ar elatively rare class of metal-ligand multiple bond. This relative paucity reflects the inherent nature of the phosphinidene functional group,which as af ree moiety is very reactive due to the P-triplet ground state and unsaturated valence shell. [4] Stabilization of aphosphinidene by metal-coordination is an attractive strategy, [1] but normally also demands as terically bulky group at phosphorus to kinetically stabilize the M=PR linkage.
Indeed, it is notable that under ambient conditions all isolable transition-metal phosphinidene complexes exhibit sterically demanding Rg roups to kinetically protect these vulnerable M=PR bonds; [3,[5][6][7][8][9] in abroader sense the only exceptions are where fundamental, elegant species such as H 2 M = PH (M = Ti,Z r, and Hf) have been prepared and spectroscopically observed under cryogenic conditions. [10] Early transitionmetal phosphinidene complexes are perhaps the most developed of all metal-phosphinidenes,s oi ti ss urprising that an early transition-metal parent phosphinidene has not yet been realized under ambient conditions.
Recently,a spart of our work on actinide-ligand multiple bonds, [11] we reported uranium and thorium phosphinidene complexes using the parent phosphinidene (HP) 2À , [12] despite the large triplet-singlet energy gap of approximately 22 kcal mol À1 for free PH, [4g] which had previously only been seldom observed as af leeting spectroscopic intermediate or probed theoretically. [4] Those two actinide complexes are the only two M = PH complexes yet isolated outside cryogenic spectroscopic experiments,a nd were supported by the very sterically demanding triamidoamine ligand N(CH 2 CH 2 NSiPr i 3 ) 3 (Tren TIPS ). Noting that it is unusual for key metal-ligand linkages to be realized in d-block chemistry subsequent to fblock ones,r ather than the other way around, we wondered whether Group 4a nalogues could be prepared, which would provide abasis from which to make d-f bonding comparisons with ap hosphinidene moiety that remains exceedingly rare under any circumstance.
Herein, we report at erminal parent zirconium-phosphinidene,w hich represents the first example of as tructurally authenticated transition-metal complex of the parent phosphinidene group.D espite very different zirconium coordination environments,a na gostic-type Zr···HP interaction is found in the phosphinidene complex reported herein as has been suggested from matrix isolation data on H 2 Zr=PH. [10] Surprisingly,q uantum chemical calculations suggest that the Zr = Pbond reported herein is nearly as covalent as essentially isostructural U = Pa nd Th = Pb onds.T his is contrary to expectations,g iven the general view that d-block metals,o n al ike-for-like basis,s hould be expected to engage in more covalent bonding than f-block elements.
Thes ynthesis of Zr2 is notable in that, unlike Ua nd Th congeners, [12] it does not require a[M(Tren)][BPh 4 ]separated ion-pair formulation to install the pnictide.T he 1 HNMR spectrum of Zr2 is consistent with ap seudo-C 3 symmetric species,a nd the phosphanide hydrogen atoms resonate as adoublet centered at 1.63 ppm (J PH = 166.6 Hz);the 31 PNMR spectrum likewise exhibits at riplet centered at À175.3 ppm. TheA TR-IR spectrum of Zr2 exhibits ab road feature at approximately 2288 cm À1 ,w hich is ac omposite of two overlapping absorptions;this compares well to computed stretching frequencies of 2313 and 2332 cm À1 from an analytical frequencies calculation.
Thes olid-state structure of Zr2 was confirmed by X-ray crystallography (see Supporting Information) which revealed aZ r-P distance of 2.690(2) ;t his is slightly longer than the sum of the single bond covalent radii of Zr and P(2.65 ), [13] but shorter than the ZrÀPd istance of 2.725 (2) in [Zr-{N(CH 2 CH 2 NSiMe 3 ) 3 }(PHPh)] [16] where the phosphanide may be aw eaker donor from charge delocalization into the P-phenyl ring.
Tr eatment of Zr2 with one and two molar equivalents of KCH 2 Ph and benzo-15-crown-5 ether (B15C5), respectively, afforded the zirconium terminal parent phosphinidene complex [Zr(Tren DMBS )(PH)][K(B15C5) 2 ]( Zr3), isolated as orange crystals in 24 %yield after work-up and recrystallization. 1 HNMR spectroscopy of freshly prepared reaction mixtures suggests that Zr3 is the major product, and so the low crystalline yield reflects the high solubility and inherently oily nature of this separated ion pair. However,wenote that Zr3 is clearly on the cusp of stability,because its solutions in aromatic solvents completely decompose to unknown products within hours.L ike Zr2,t he 1 HNMR spectrum of 3 is indicative of ap seudo-C 3 symmetric zirconium species,a nd the phosphinidene hydrogen resonates as ad oublet at 8.53 ppm (J PH = 173.4 Hz). The 31 PNMR spectrum of Zr3 exhibits ab road resonance at + 246.75 ppm, shifted approximately 422 ppm from that of Zr2,where the PÀHcoupling is obscured by the broad linewidth (full-width at half maximum = 256 Hz). TheA TR-IR spectrum of Zr3 exhibits one very broad and thus overall weak absorption, tentatively attributed to the P À Hs tretch at approximately 2100 cm À1 , which compares well to ac alculated PÀHs tretching frequencyo f2 140 cm À1 from an analytical frequencies calculation.
Thesolid-state molecular structure of Zr3 was determined by X-ray crystallography,F igure 1, confirming the separated ion pair nature,and thus terminal phosphinidene assignment of Zr3.The Zr = Pdistance is found to be 2.4723 (17) ,which represents acontraction of around 0.22 (ca. 9%)compared to Zr2.However,the Zr = Pdistance in Zr3 is longer than the sum of the covalent double-bond radii of Zr and Po f 2.29 ; [13] this may be due to the electron-rich anionic formulation of the phosphinidene portion of Zr3,a nd/or due to the phosphinidene being located trans to the trialkylamine donor of the Tr en DMBS ligand. Tw oo bservations consistent with those notions are that the Zr À N amine distance in Zr2 (2.516 (5) )isshorter than the corresponding distance in Zr3 (2.586(4) ), and the Zr À N amide distances in Zr3 (average of 2.124 (8) )a re considerably longer than the analogous distances in Zr2 (2.062 (9) ). However,t he Zr=P distance in Zr3 is shorter than the Zr=Pd istance in [Zr(h 5 -C 5 H 5 ) 2 (P{2,4,6-Bu t 3 C 6 H 2 })(PMe 3 )] (2.505(4) ), [5q] and compares well to the computed Zr = Pdistance of 2.324 in H 2 Zr = PH, [10] which is considerably less sterically encumbered, has al ower coordination number at Zr than in Zr3,a nd is also neutrally charged overall. Interestingly,although there are no obvious interactions between the phosphanide hydrogen atoms and zirconium center in Zr2,inZr3 the phosphinidene hydrogen, located and refined by acombination of crystallographic difference Fourier map data and DFT calculations, appears to be engaged in aw eak agostic-type Zr···HP interaction (Zr···H = 2.322(19) ; ] ZrPH = 66.7 (8)8 8). The ] ZrPH for Zr3 is very similar to that computed for H 2 Zr=PH (] ZrPH = 63.88 8), despite the very different zirconium coordination numbers and geometries of the two complexes. [10] The Zr···H distance in Zr3 is considerably longer than the sum of the single bond covalent radii of Zr and H(1.86 ), [13] and the analogous Zr···H distance of 2.13 in H 2 Zr = PH, [10] which can be rationalized by the aforementioned steric and charge differences between Zr3 and H 2 Zr=PH. [10] In order to gain ag reater understanding of the nature of the bonding in the Zr=PH unit in Zr3,w ec arried out DFT calculations on the full anion component of Zr3, Zr3 À ,and for comparison the full molecule of Zr2,T able 1. Thegeometryoptimized gas-phase structures of Zr2 and Zr3 À closely match the experimental solid-state structures,t ow ithin 0.05 and 28 8 of corresponding bond lengths and angles,a nd so we conclude that the calculations provide aqualitative picture of the electronic structures of these molecules.Inorder to make wider comparisons,w ea lso compile the computed data for the closely related phosphinidene anions [U(Tren TIPS )(PH)] À (U3 À )a nd [Th(Tren TIPS )(PH)] À (Th3 À )i nT able 1. [12] As expected, the ZrÀPbond order virtually doubles upon moving from phosphanide Zr2 to phosphinidene Zr3 À .F or comparison, the ZrÀN amide /ZrÀN amine bond orders are 0.68/ 0.22 and 0.53/0.14 for Zr2 and Zr3 À ,r espectively,w hich suggests that as the Zr = Pd ouble bond develops,t he Zr À N interactions diminish, as suggested by the crystallographic data. Likewise,the charge on Zr decreases as the Zr=Pbond is established, and the negative charge on Pi ncreases more than two times,i nl ine with the formal mono-and di-anionic charges on H 2 P À and HP 2À ,respectively.
Inspection of the Kohn-Sham molecular orbitals (KSMOs) of Zr3 À reveals the anticipated Zr = P p (HOMO) and s (HOMOÀ1) bonds,F igure 2. Unfortunately,t hese KSMOs are mixed with amide N-lone pair orbital coefficients. Therefore,i no rder to inspect the Zr=Pd ouble bond in isolation we used NBO (natural bond orbital) analysis,which also finds discrete Zr = P s-a nd p-bonding combinations.O n moving from Zr2 to Zr3 À the Zr contributions to the Zr À P bonds increase markedly,and although there is no p-bonding combination to compare between Zr2 and Zr3 À the Zr contribution to the ZrÀP s bond more than doubles on moving from the former to the latter.
To gain further insight into the nature of the Zr=Pbond in Zr3 À ,w eu sed QTAIM (atoms-in-molecules) to analyze the Zr = Pbond topology.This reveals,despite asuperficial picture of covalence from the orbital based methods,am ore ionic bonding picture,where the 1(r)term is in the region typically associated with predominantly ionic bonding (< 0.1). However,t he presence of spherical and non-spherical electron density,w ith respect to density along the ZrÀPi nternuclear vector, for Zr2 and Zr3 À ,r espectively,i sc onsistent with the presence of formal single-and double-bonding interactions in these complexes,though of course in Zr3 À the Zr + À P À dipolar resonance form will contribute to the bonding overall. [17] Thesignificantly smaller PÀHstretching frequency of Zr3 compared to Zr2,along with crystal structure data for Zr3,are certainly suggestive of an agostic-type Zr···HP interaction in Zr3. [10] Interestingly,t he DFT calculations return aZ r À H bond order of 0.14 in support of an agostic-type Zr···HP interaction. However, close inspection of the QTAIM data does not reveal aZ r ÀHb ond-critical point in Zr3 À ,a nd so although the presence of aw eak agostic-type Zr···HP interaction is likely,iti snot unequivocally confirmed.
Theionic-bonding picture of Zr3 À is perhaps unexpected, and surprisingly in line with computed data for 3U À and Th3 À . [12] Indeed, the data for these three complexes are remarkably similar overall, with the exceptions of the Mayer bond orders that are surprisingly higher for Ua nd Th compared to Zr. Theb ond orders follow the trend U > Th >  Zr,and the %Mcontributions to the M=Pbonds for Uand Zr are around twice that of Th.W ethus conclude that the Zr = P bond reported herein is essentially as covalent as its U = Pand Th = Pc ounterparts,a nd may be even less covalent;t his challenges traditional views of the levels of covalencyi nt he chemical bonding of the transition-metals,even early d-block ions,versus the f-block.
To conclude,w eh ave prepared at erminal parent zirconium-phosphanide complex, which is ar are example of ap arent d-block phosphanide.W eh ave used this phosphanide complex to prepare the first example of as tructurally authenticated transition-metal terminal parent phosphinidene complex under ambient conditions on bulk scale,adding to the generally rare family of early transition-metal phosphinidene compounds,a nd very rare occurrences of an isolable parent phosphinidene outside of spectroscopic experiments.The zirconium phosphinidene complex reported herein appears to exhibit aw eak agostic-type Zr···HP interaction that has also been suggested to be present in cryogenic matrix isolation data on H 2 Zr=PH. Since the parent phosphinidene is free from sterically demanding substituents that may dictate the geometry of this unit, this suggests that this agostic interaction may be an intrinsic feature for Group 4metals.Q uantum chemical calculations suggest that the Zr=Pb ond is qualitatively about as covalent as isostructural U=Pa nd Th=Pb onds,a nd may even be less covalent. This is surprising,b ecause it runs against expectations of the general view that d-block metals,o nal ike-for-like basis,a re usually expected to engage in more covalent bonding in metal-ligand complexes than for f-block elements.