Synthesis of Ni(II) Complexes Supported by Tetradentate Mixed-Donor Bis(amido)/Phosphine/Phosphido Ligands by Phosphine Substituent Elimination

Kappa what? Insights into the coordination modes of N2P2 ligands

While first synthesized more than three decades ago, complexes supported by N2P2 ligands have seen renewed interest due to the synthesis of new ligands, expansion of their reactivity, and catalytic applications. Possessing both soft phosphines and hard nitrogen donors, N2P2 ligands can accommodate various metal geometries and coordination modes thanks to their capability to act as bidentate, tridentate or tetradentate ligands. This short review will explore how metals bind to these ligands and also highlight the complexes' reactivity and catalytic abilities.

Dipyrromethane-diphosphine: the effect of meso substituents on the formation of nickel complexes and on their performance in the transfer hydrogenation of ketones

Three dipyrromethane-diphosphine ligands containing phenyl (L1H2), ethyl (L2H2) and cyclohexyl (L3H2) groups at their meso positions and their nickel complexes were synthesized and structurally characterized. Treatment of Ph2C(C4H3N)2-1,9-(CH2PPh2)2 (L1H2) with [NiCl2(DME)] gave complex [NiCl2(κ2-P,P-L1H2)] 2a. Conversely, the analogous reactions of L2H2 and L3H2 with [NiCl2(DME)] showed a mixture of products containing both a pyrrolide nitrogen coordinated complex of type [Ni(κ4-P,N,N,P-L)] 3 without an exogenous base and a chelated complex of type 2a. In addition, all three ligands react with [NiCl2(DME)] in the presence of a strong base to give a complex of type 3. Furthermore, a novel binuclear Ni(0) complex bearing L1H2 was characterized by X-ray crystallography. Both complexes 2a and 3 (0.5 mol% of loading) catalyze the transfer hydrogenation of a series of aromatic and aliphatic ketones (20 substrates) to their corresponding secondary alcohols using iPrOH as a hydrogen source in the presence of KOH at 100 °C in 6 h. The kinetic trace of the catalytic reaction shows that the meso-phenyl substituted diphosphine coordinated nickel complexes perform better than the other two ligand coordinated nickel complexes.

B-substituted group 1 phosphides: synthesis and reactivity

1-Boryl-8-phosphinonaphthalenes 1-BCy2-8-PCl2-C10H6 (1) and 1-BCy2-8-PPhCl-C10H6 (2) were prepared and used as starting materials for the synthesis of B-substituted phosphides. The reduction of 1 and 2 by Mg provided neutral compounds [1-BCy-8-PCy-C10H6]2 (3) and [1-BCy2-8-PPh-C10H6]2 (4). Compound 3 represents the dimer of phosphinoborane 1-BCy-8-PCy-C10H6 while complex 4 is a rare example of a discrete B ← P coordinated diphosphine. The reduction of 2 by Na or K in THF yielded B-substituted group 1 phosphides [Na(THF)3]+[1-BCy2-8-PPh-C10H6]- (5) and {[K(THF)2]+[1-BCy2-8-PPh-C10H6]-}∞ (6), which structurally resembled bulky group 1 phosphides. Complex 5 showed easy activation of elemental chalcogens E (E = O, S, Se) to give B-substituted chalcogenophosphinites {[Na(THF)2]+[1-BCy2-8-P(E)Ph-C10H6]}2 (E = O (7), S (8), Se (9)) as the products of chalcogen insertion into the P-Na bond. Importantly no oxidation to dichalcogenophosphinates was observed. Compound 5 is tolerant of the CO polar bonds in organic substrates and the reactions of 5 with 2,3-butanedione or an acyl chloride provided {[Na(THF)2]+[1-BCy2-8-P{CHC(O)C(Me)O}Ph-C10H6]-}2 (10) and [1-BCy2-8-P{C(O)tBu}Ph-C10H6] (11). Finally, B-coordinated phosphatetrylenes [1-BCy2-8-P(SnL)Ph-C10H6] (12) and [1-BCy2-8-P(PbL)Ph-C10H6] (13) (L is {2,6-(Me2NCH2)C6H3}-) were also prepared by substitution reactions of 5.

Desulfurization and N2 Binding at an Iron Complex Derived from the C-S Activation of Benzothiophene

Metal insertion into the C-S bonds of thiophenes is a facile route to interesting polydentate ligand scaffolds with C and S donors. Here, we describe iron-mediated C-S activation of a diphenylphosphine-functionalized benzothiophene proligand. Metalation of the proligand with "tetrakis(trimethylphosphine)iron" gives an initial five-coordinate, diamagnetic iron(II) species with two PMe3 ligands and a dianionic PCS pincer ligand. Upon one-electron reduction, a reactive anionic iron(I) complex is formed. This species then undergoes deep-seated changes, notably cleavage of C-S and C-P bonds in the supporting ligand. Substantial coordination sphere alterations accompany the ligand C-S bond activation, including loss of a sulfur anion from the S-Fe-C metallacycle and reorganization of the two PMe3 ligands. The resulting desulfurized six-coordinate PCC iron complex also has an N2 ligand trans to the vinyl C. Reducing this complex then cleaves a C-P bond in the appended diphenylphosphine, giving a phosphido arm. These ligand transformations demonstrate novel approaches to pincers with thiolates and phosphides, which would be difficult to synthesize using typical methods through free ligand salts.

Nickel-Templated Replacement of Phosphine Substituents in a Tetradentate Bis(amido)bis(phosphine) Ligand

The replacement of phosphine substituents in nickel-bound PNNP ligands is reported as an alternative method for preparing multidentate phosphine ligands with alkyl substituents. Treatment of the previously reported bis(phosphide) complex {K(THF)_x}₂^2Ph[PNNP]Ni (2) with 2 equiv of MeI, ⁱPrI, and 1,3-dibromoethane formed alkyl-substituted complexes ^2Ph,2Me[PNNP]Ni (3), ^2Ph,2iPr[PNNP]Ni (4), and ^{2Ph,propylene}[PNNP]Ni (5), respectively. The stereoselectivity (racemic vs meso) of these reactions can be controlled by varying the reaction temperature. The racemic mixtures of products with the new alkyl substituents in an anti configuration were favored at lower temperatures, whereas a larger proportion of meso compounds was acquired at higher temperatures. Further treatment of 3 with KH resulted in selective elimination of the remaining phenyl groups rather than the methyl substituents, affording bis(methylphosphide) complex {K(THF)_x}₂^2Me[PNNP]Ni (6). Subsequent treatment of 6 with additional MeI formed ^4Me[PNNP]Ni (7), in which all four phenyl groups were replaced with methyl substituents. As a proof of concept, demetalation of the ligand from 7 was achieved using aqueous KCN to form a free dimethylphosphine-substituted ligand H₂^4Me[PNNP] (8), and 8 was subsequently coordinated to a different metal, using PdCl₂ to form ^4Me[PNNP]Pd (9). Unlike the clean elimination of phenyl substituents from 3, the reactions of KH with 4 and 5 exhibited competitive elimination of both alkyl and phenyl substituents and/or attenuated reactivity.

[Ni(NHC)2 ] as a Scaffold for Structurally Characterized trans [H-Ni-PR2 ] and trans [R2 P-Ni-PR2 ] Complexes

The addition of PPh2 H, PPhMeH, PPhH2 , P(para-Tol)H2 , PMesH2 and PH3 to the two-coordinate Ni0 N-heterocyclic carbene species [Ni(NHC)2 ] (NHC=IiPr2 , IMe4 , IEt2 Me2 ) affords a series of mononuclear, terminal phosphido nickel complexes. Structural characterisation of nine of these compounds shows that they have unusual trans [H-Ni-PR2 ] or novel trans [R2 P-Ni-PR2 ] geometries. The bis-phosphido complexes are more accessible when smaller NHCs (IMe4 >IEt2 Me2 >IiPr2 ) and phosphines are employed. P-P activation of the diphosphines R2 P-PR2 (R2 =Ph2 , PhMe) provides an alternative route to some of the [Ni(NHC)2 (PR2 )2 ] complexes. DFT calculations capture these trends with P-H bond activation proceeding from unconventional phosphine adducts in which the H substituent bridges the Ni-P bond. P-P bond activation from [Ni(NHC)2 (Ph2 P-PPh2 )] adducts proceeds with computed barriers below 10 kcal mol-1 . The ability of the [Ni(NHC)2 ] moiety to afford isolable terminal phosphido products reflects the stability of the Ni-NHC bond that prevents ligand dissociation and onward reaction.

Synthesis of a carborane-substituted bis(phosphanido) cobaltate(i), ligand substitution, and unusual P4 fragmentation

Oxidative addition of the P-P single bond of an ortho-carborane-derived 1,2-diphosphetane (1,2-C2(PMes)2B10H10) (Mes = 2,4,6-Me3C6H2) to cobalt(-i) and nickel(0) sources affords the first heteroleptic complexes of a carborane-bridged bis(phosphanido) ligand. The complexes also incorporate labile ligands suitable for further functionalisation. Thus, the cobalt(i) complex [K([18]crown-6)][Co{1,2-(PMes)2C2B10H10}(cod)] (cod = 1,5-cyclooctadiene) bearing a labile cyclooctadiene ligand undergoes facile ligand exchange reactions with isonitriles and tert-butyl phosphaalkyne with retention of the bis(phosphanido) ligand. However, in the reaction with one equivalent of P4, the electron-rich bis(phosphanido) moiety abstracts a single phosphorus atom with formation of a new P3 chain, while the remaining three P atoms derived from P4 form an η3-coordinating cyclo-P3 ligand. In contrast, when the same reaction is performed with two equivalents of the cobalt(i) complex, a dinuclear product is formed which features an unusual P4 chain in its molecular structure.